Natural fibers, biopolymers, and biocomposites

NATURAL FIBERS, BIOPOLYMERS, AND BIOCOMPOSITES

Edited by

Amar K. Mohanty Manjusri Misra Lawrence T. Drzal

Boca Raton London New York Singapore

A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.

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Published in 2005 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2005 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-10: 0-8493-1741-X (Hardcover) International Standard Book Number-13: 978-0-8493-1741-5 (Hardcover) Library of Congress Card Number 2004058580 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

Library of Congress Cataloging-in-Publication Data Natural fibers, biopolymers, and biocomposites/edited by Amar K. Mohanty, Manjusri Misra, Lawrence T. Drzal. p. cm. Includes bibliographical references and index. ISBN 0-8493-1741-X (alk. paper) 1. Polymeric composites. 2. Biopolymers. 3. Fibers. I. Mohanty, Amar K. II. Misra, Manjusri. III. Drzal, Lawrence T. TA418.9.C6N373 2005 620.1’92--dc22

2004058580

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This book is dedicated to the committed community of current and future scientists, engineers, academics, students, civil servants, and entrepreneurs through whose interest, dedication, and preseverance the transition to a sustainable, biobased materials industry is becoming a reality.

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Preface

We are living in an interesting world. Our society has achieved enormous advances in quality of life due to an extensive discovery and availability of plastics derived from petroleum. However, as with any technology, unanticipated negative secondary effects are produced as well. The persistence of plastics in the environment, shortage of landfill space, concerns over emissions resulting from incineration, and hazards to human health as well as hazards to animals, birds, and fish from entrapment or ingestion of these materials have spurred the efforts to find more environment friendly alternative materials. The depletion of petroleum resources coupled with increase in environmental regulations have added to this effort of finding new materials and products that are compatible with the environment and independent of fossil fuels. Industries are developing and manufacturing “greener” materials; government is encouraging biobased product research; academicians are searching for eco-friendly materials; and the public is coming to value the benefit of environment friendly products and processes, but at affordable prices. Biobased materials offer a potential solution to this complex problem. Natural fibers are now emerging as viable alternatives to glass fibers either alone or combined in composite materials for various applications in automotive parts, building structures and rigid packaging materials. The advantages of natural fibers over synthetic or man-made fibers such as glass are low cost, low density, competitive specific mechanical properties, carbon dioxide sequestration, sustainability, recyclability, and biodegradability. Biobased polymers may be obtained from renewable resources and are gaining much importance over petroleum-based biodegradable polymers in recent years. Biopolymers have started migrating into the mainstream and biobased polymers may soon be competing with commodity plastics. Some of the examples for biopolymers include cellulosic biopolymers derived from renewable cellulose, starch plastics, corn-derived plastics, and bacterial polyesters. Biocomposites produced from the combination of biofibers and bioplastics produce the necessary performance either entirely or in combination with petroleum-based polymers and offer a path to achieve ecofriendly materials in the 21st century. However, the need to produce 100% biobased materials as substitutes for petroleum-based materials is not immediate. Biocomposites that contain a significant content of biobased materials can presently achieve this at an affordable cost–performance ratio to compete with petroleum-based materials and still maintain a positive balance among ecology, economy, and technology.

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This publication is intended to provide a comprehensive source for the latest advances in the area of biofibers, biopolymers, and biocomposites that can substitute for and compete with traditional petroleum-based materials and at the same time reduce environmental harm while maintaining the economic viability. We have assembled chapters on topics ranging from natural fibers (e.g., agricultural fibers, grass fibers, straw-based fibers, and traditional wood fibers), biopolymers to biobased composite materials in this book. In addition, we have included a comprehensive chapter on life cycle analysis of biobased polymers and materials that is emerging as the framework upon which sustainability of materials and processes will be established. We hope this book will serve as a guide to (1) government policy makers to encourage more research on the generation and use of biobased materials; (2) industrial personnel to show that high performance, economical, biobased products can be produced; (3) university students and faculty researchers who are striving to advance sustainable materials and practices; and (4) the public to illustrate that materials sustainability, biodegradability, and environmental stewardship can be achieved without economic sacrifice. We thank Susan G. Farmer, the former Editor of Materials Science and Engineering Division of CRC Press, whom we met at the American Society of Composites Annual Meeting in 2001 and who supported our ideas for this book, all the contributing authors who have made this project a reality, and our sincere thanks to our friends, colleagues, and students who have labored diligently on their chapters and responded to all the constructive criticisms that conceptualized this book. Our special thanks to Prasad V. Mulukutla who provided special assistance at various stages of this edition. Amar K. Mohanty Manjusri Misra Lawrence T. Drzal Michigan State University

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About the Editors

Dr. Amar K. Mohanty is an Associate Professor in the School of Packaging at Michigan State University. Prior to this position, he was a Visiting Associate Professor in the Chemical Engineering and Materials Science Department and Composite Materials and Structures Center at Michigan State University. He was an Alexander Von Humboldt Fellow at the Technical University of Berlin, Germany, and also worked as a postdoctoral research associate at Iowa State University. Dr. Mohanty served as a faculty member in Chemistry at three different educational institutions in Orissa, India, for about 10 years. He earned an M.Sc. degree (with gold medal) in chemistry with a specialization in polymer chemistry and a Ph.D. in Chemistry from Utkal University. He was the recipient of a young scientist award in Orissa, India. He has more than 170 publications to his credit including around 90 peer-reviewed journal papers, three textbooks and 6 U.S. patents pending. He is a reviewer of more than 15 international journals and has acted as chair and co-chair in several professional organizations. He has served as a Guest Editor of the journal Composites Part A: Applied Science and Manufacturing. His primary research is focused mainly on nanocomposite materials, biocomposite/green composite materials, biodegradable polymers, materials applications in packaging, and new biobased green materials development.

Dr. Manjusri Misra is a Visiting Associate Professor in the Department of Chemical Engineering and Materials Science and Composite Materials and Structures Center at Michigan State University. She earned her Ph.D. in polymer chemistry from Utkal University, India, in 1988. She served as a faculty member at various educational institutes under Utkal University, Orissa, India, for more than 16 years. During her teaching career in India, Dr. Misra pursued various research projects in the areas of conducting polymers and bio-composites. From April 1998 to December 1998, she worked as a Visiting Scientist at the Fritz Haber–Max Planck Institute and Hahn–Meitner

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Institute, Berlin, Germany, in the area of microemulsion polymerization. From January 1999 to April 1999 she served as a Visiting Scientist at the Technical University of Berlin, Germany, and worked in the areas of polymers and composites. From May 1999 to December 1999 she worked at Iowa State University in the area of biodegradable seed coating. Dr. Misra joined Michigan State University in January 2000 and continues to pursue her research activities in the areas of biobased materials and nanotechnology. She has more than 150 publications to her credit as well as 6 U.S. patents pending. She has acted as chair and co-chair in several professional organizations. Her primary research focuses on biodegradable polymers, and biocomposite and nanocomposite materials.

Professor Lawrence T. Drzal is a University Distinguished Professor in the Department of Chemical Engineering and Materials Science and Director of the Composite Materials and Structures Center. His research interests include surface and interfacial phenomena, adhesion, fiber-matrix bonding, surface modification of polymers, nanocomposites, and biomaterials. Dr. Drzal obtained a B.Ch.E. from the University of Detroit and a Ph.D. in chemical engineering from Case Western Reserve University in 1974. From 1972 until 1985 he was employed as a military and civilian scientist at the Air Force Materials Laboratory at Wright-Patterson Air Force Base, where he worked on surface and interfacial aspects of adhesively bonded joints and the fiber-matrix interphase in composite materials. Dr. Drzal has been elected as a Fellow of the Adhesion Society, American Society for Composites, American Institute of Chemists, and the Society of Plastics Engineers, and has also been elected to membership in the European Academy of Sciences. Dr. Drzal has made over 300 presentations at national and international conferences, published more than 250 research papers, and has been awarded 18 patents. He is listed as one of the top ‘highly cited’ materials researchers by the Information Sciences Institute. Since 1985 he has mentored 33 master’s, 26 doctoral, and 31 postdoctoral students at Michigan State University. He serves on several government committees, technical journal advisory boards, and industrial panels, and he and his students have received several awards for their original research contributions.

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Contributors

Danny E. Akin R. B. Russell Agricultural Research Center, USDA-ARS, Athens, Georgia, U.S.A. Lars Berglund Lightweight Structures, Royal Institute of Technology, Stockholm, Sweden Alexander Bismarck Department of Chemical Engineering & Chemical Technology, Imperial College London, United Kingdom Bor-Sen Chiou United States Department of Agriculture, Agricultural Research Service, Albany, California, U.S.A. Roy B. Dodd Department of Agricultural and Biological Engineering, Clemson University, Clemson, South Carolina, U.S.A. Lawrence T. Drzal Department of Chemical Engineering and Materials Science, and Composite Materials and Structures Center, Michigan State University, East Lansing, Michigan, U.S.A. Jean-Pierre Latere Dwan’Isa Drug Evaluation – Pharmaceutical Sciences, Janssen Pharmaceutica, Beerse, Belgium Mahmoud A. Dweib ACRES Program, Department of Chemical Engineering and Center for Composite Materials, University of Delaware, Newark, Delaware, U.S.A. William J. Evans Materials Research Centre, School of Engineering, University of Wales, Swansea, United Kingdom Christian Fürll

Institute of Agricultural Engineering, Potsdam, Germany

Paul Gatenholm Department of Materials and Surface Chemistry/ Biopolymer Technology, Chalmers University of Technology, Göteborg, Sweden Richard D. Gilbert Wood and Fiber Chemistry Group, North Carolina State University, Raleigh, North Carolina, U.S.A. Gregory M. Glenn United States Department of Agriculture, Agricultural Research Service, Albany, California, U.S.A. Shankar Godavarti Aspen Research Corp., White Bear Lake, Minnesota, U.S.A.

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Patrick Gruber Cargill Dow LLC, Minnetonka, Minnesota, U.S.A. Manorama Gupta

Central Building Research Institute, Roorkee, India

Hiroyuki Hamada Advanced Fibro Science, Kyoto Institute of Technology (KIT), Japan Bruce R. Harte School of Packaging, Michigan State University, East Lansing, Michigan, U.S.A. Heinz Hempel

Institute of Agricultural Engineering, Potsdam, Germany

David E. Henton Cargill Dow LLC, Minnetonka, Minnesota, U.S.A. Pedro J. Herrera Franco Unidad de Materiales, Centro de Investigación Científica de Yucatán, Mérida, Yucatán, México Georg Hinrichsen Technical University of Berlin, Institute of Materials Science and Technology, Berlin, Germany Alma Hodzic Mechanical Engineering, School of Engineering, James Cook University, Townsville, Queensland, Australia Bo Hu Department of Civil and Environmental Engineering, University of Delaware, Newark, Delaware, U.S.A. Syed H. Imam United States Department of Agriculture, Agricultural Research Service, Albany, California, U.S.A. Maria K. Inglesby United States Department of Agriculture, Agricultural Research Service, Albany, California, U.S.A. Zainal A. Mohd Ishak School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Penang, Malaysia Umaru S. Ishiaku Advanced Fibro-Science, Kyoto Institute of Technology, Kyoto, Japan Maya Jacob School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India Seena Joseph School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India John F. Kadla Biopolymer Chemistry Group, University of British Columbia, Vancouver, British Columbia, Canada Kazuo Kitagawa Kyoto Municipal Industrial Research Institute, Chudoji, Simogyo-Ku, Kyoto, Japan Satoshi Kubo Biopolymer Chemistry Group, University of British Columbia, Vancouver, British Columbia, Canada Copyright © 2005 by Taylor & Francis

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Joseph V. Kurian Sorona R&D, E. I. du Pont de Nemours and Company, Wilmington, Delaware, U.S.A. Thomas Lampke Lehrstuhl für Verbundwerkstoffe, Technische Universität Chemnitz, Chemnitz, Germany Richard C. Larock Iowa State University, Ames, Iowa, U.S.A. Fengkui Li Research and Technology Center, TOTAL Petrochemicals, Inc., Deer Park, Texas, U.S.A. Wanjun Liu Composite Materials and Structures Center, Michigan State University, East Lansing, Michigan, U.S.A. Jim Lunt

Cargill Dow LLC, Minnetonka, Minnesota, U.S.A.

Supriya Mishra

OAK House, Manchester, United Kingdom

Manjusri Misra Department of Chemical Engineering and Materials Science, and Composite Materials and Structures Center, Michigan State University, East Lansing, Michigan, U.S.A. Machiko Mizoguchi Advanced Fibro-Science Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto, Japan Xiaoqun Mo Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas, U.S.A. Amar K. Mohanty School of Packaging Science, Michigan State University, East Lansing, Michigan, U.S.A. Friedrich Munder Institute of Agricultural Engineering, Potsdam, Germany Ramani Narayan Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan, U.S.A. Annmarie O’Donnell ACRES Program, Department of Chemical Engineering and Center for Composite Materials, University of Delaware, Newark, Delaware, U.S.A. William J. Orts United States Department of Agriculture, Agricultural Research Service, Albany, California, U.S.A. Martin Patel Department of Science, Technology and Society, Utrecht University, Netherlands David Plackett Danish Polymer Centre, Risø National Laboratory, Frederiksborgvej, Roskilde, Denmark Jed Randall Cargill Dow LLC, Minnetonka, Minnesota, U.S.A. Dipa Ray Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, India Copyright © 2005 by Taylor & Francis

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Jogeswari Rout Department of Chemistry, Padmanava College of Engineering, Orissa, India Hj D. Rozman School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia Brian D. Seiler Cellulose Esters Research, Eastman Chemical Company, Kingsport, Tennessee, U.S.A. Susan E. Selke School of Packaging, Michigan State University, East Lansing, Michigan, U.S.A. Harry W. Shenton III Department of Civil and Environmental Engineering, University of Delaware, Newark, Delaware, U.S.A. Brajeshwar Singh Central Building Research Institute, Roorkee, India Anders Södergård Turku Centre for Biomaterials, University of Turku, Finland Douglas D. Stokke Department of Natural Resource Ecology and Management, and Affiliate of the Center for Crops Utilization Research, Ames, Iowa, U.S.A. Brett C. Suddell Materials Research Centre, School of Engineering, University of Wales, Swansea, United Kingdom Xiuzhi S. Sun Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas, U.S.A. Sabu Thomas School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India Debra Tindall Cellulose Esters Research, Eastman Chemical Company, Kingsport, Tennessee, U.S.A. Guillermo Toriz University of Guadalajara, Mexico Praveen Tummala

QTR, Inc., Evansville, Indiana, U.S.A.

Alex Valadez-González Unidad de Materiales, Centro de Investigación Científica de Yucatán, Mérida, Yucatán, México Donghai Wang Department of Biological & Agriculture Engineering, Kansas State University, Manhattan, Kansas, U.S.A. Delilah F. Wood United States Department of Agriculture, Agricultural Research Service, Albany, California, U.S.A. Richard P. Wool ACRES Program, Department of Chemical Engineering and Center for Composite Materials, University of Delaware, Newark, Delaware, U.S.A.

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Contents

1.

Natural Fibers, Biopolymers, and Biocomposites: An Introduction Amar K. Mohanty, Manjusri Misra, Lawrence T. Drzal, Susan E. Selke, Bruce R. Harte, and Georg Hinrichsen

2.

Plant Fibers as Reinforcement for Green Composites Alexander Bismarck, Supriya Mishra, and Thomas Lampke

3.

Processing of Bast Fiber Plants for Industrial Application Friedrich Munder, Christian Fürll, and Heinz Hempel

4.

Recent Developments in Retting and Measurement of Fiber Quality in Natural Fibers: Pro and Cons Roy B. Dodd and Danny E. Akin

5.

Alternative Low-Cost Biomass for the Biocomposites Industry Douglas D. Stokke

6.

Fiber-Matrix Adhesion in Natural Fiber Composites Pedro J. Herrera Franco and Alex Valadez-González

7.

Natural Fiber Composites in Automotive Applications Brett C. Suddell and William J. Evans

8.

Natural Fiber Composites for Building Applications Brajeshwar Singh and Manorama Gupta

9.

Thermoset Biocomposites Dipa Ray and Jogeswari Rout

10.

Thermoplastic Wood Fiber Composites Shankar Godavarti

11.

Bamboo-Based Ecocomposites and Their Potential Applications Kazuo Kitagawa, Umaru S. Ishiaku, Machiko Mizoguchi, and Hiroyuki Hamada

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12.

Oil Palm Fiber–Thermoplastic Composites Hj D. Rozman, Zainal A. Mohd Ishak, and Umaru S. Ishiaku

13.

−Rubber Composites and Their Applications Natural Fiber− Seena Joseph, Maya Jacob, and Sabu Thomas

14.

Straw-Based Biomass and Biocomposites Xiaoqun Mo, Donghai Wang, Xiuzhi S. Sun

15.

Sorona® Polymer: Present Status and Future Perspectives Joseph V. Kurian

16.

Polylactic Acid Technology David E. Henton, Patrick Gruber, Jim Lunt, and Jed Randall

17.

Polylactide-Based Biocomposites David Plackett and Anders Södergård

18.

Bacterial Polyester-Based Biocomposites: A Review Alma Hodzic

19.

Cellulose Fiber-Reinforced Cellulose Esters: Biocomposites for the Future Guillermo Toriz, Paul Gatenholm, Brian D. Seiler, and Debra Tindall

20.

Starch Polymers: Chemistry, Engineering, and Novel Products Bor-Sen Chiou, Gregory M. Glenn, Syed H. Imam, Maria K. Inglesby, Delilah F. Wood, and William J. Orts

21.

Lignin-Based Polymer Blends and Biocomposite Materials Satoshi Kubo, Richard D. Gilbert, and John F. Kadla

22.

Soy Protein-Based Plastics, Blends, and Composites Amar K. Mohanty, Wanjun Liu, Praveen Tummala, Lawrence T. Drzal, Manjusri Misra, and Ramani Narayan

23.

Synthesis, Properties, and Potential Applications of Novel Thermosetting Biopolymers from Soybean and Other Natural Oils Fengkui Li and Richard C. Larock

24.

Houses Using Soy Oil and Natural Fibers Biocomposites Mahmoud A. Dweib, Annmarie O’Donnell, Richard P. Wool, Bo Hu, and Harry W. Shenton III

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25.

Biobased Polyurethanes and Their Composites: Present Status and Future Perspective Jean-Pierre Latere Dwan’Isa, Amar K. Mohanty, Manjusri Misra, and Lawrence T. Drzal

26.

Cellulose-Based Nanocomposites Lars Berglund

27.

How Sustainable Are Biopolymers and Biobased Products? The Hope, the Doubts, and the Reality Martin Patel and Ramani Narayan

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1 Natural Fibers, Biopolymers, and Biocomposites: An Introduction

Amar K. Mohanty, Manjusri Misra, Lawrence T. Drzal, Susan E. Selke, Bruce R. Harte, and Georg Hinrichsen

CONTENTS 1.1 Introduction 1.2 Motivation: Biobased Materials vs. Environmental Impact 1.3 What Are Biocomposites? 1.4 Natural/Biofibers as Reinforcements in Biocomposites 1.5 Biodegradable/Biobased Polymers as Matrices for Biocomposite Applications 1.5.1 Biodegradable Polymers from Starch and Cellulose 1.5.2 Biobased/Biodegradable Plastics from Soybeans and Other Plant Resources 1.5.3 Biodegradable Polyesters from Renewable Resources and Petroleum Resources 1.5.4 Biobased Polymeric Materials from Mixed Resources (Renewable and Petroleum Resources) 1.6 Biocomposites as Alternatives to Petroleum-Based Composites: Recent Trends and Opportunities for the Future 1.7 Sustainable Biobased Products: New Materials for a New Economy 1.8 Conclusions Acknowledgments References

ABSTRACT Persistence of plastics in the environment, the shortage of landfill space, the depletion of petroleum resources, concerns over emissions during incineration, and entrapment by and ingestion of packaging plastics by fish, fowl and animals have spurred efforts to develop

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biodegradable/biobased plastics. This new generation of biobased polymeric products is based on renewable biobased plant and agricultural stock and form the basis for a portfolio of sustainable, eco-efficient products that can compete in markets currently dominated by products based on petroleum feedstock in applications such as packaging, automotives, building products, furniture and consumer goods. It is not necessary to produce 100% biobased materials as substitutes for petroleum-based materials immediately. A viable solution is to combine petroleum and bioresources to produce a useful product having the requisite cost-performance properties for realworld applications. Biopolymers or synthetic polymers reinforced with natural/biofiber frequently termed 'biocomposites' can be viable alternatives to glass fiber reinforced composites. The combination of biofibers like kenaf, industrial hemp, flax, jute, henequen, pineapple leaf fiber, sisal, wood and various grasses with polymer matrices from both non-renewable (petroleumbased) and renewable resources to produce composite materials that are competitive with synthetic composites such as glass-polypropylene, glassepoxies, etc., is gaining attention over the last decade. This chapter provides a general overview of biopolymers, natural biofibers, biocomposites and the research and applications of these materials. Biobased polymers such as polylactic acid (PLA), polyhydroxybutyrate (PHB), cellulose esters, soybased plastic, starch plastic, poly (trimethylene terephthalate), biobased resins from functionalized vegetable oils and biocomposites are also introduced in the chapter along with petroleum derived biodegradable polymers. Detailed discussions about the chemical nature, processing, testing and properties of these polymers, fibers and composites will be discussed in the remaining chapters of the book.

1.1

Introduction

As a result of a growing awareness of the interconnectivity of global environmental factors, principles of sustainability, industrial ecology, ecoefficiency, and green chemistry and engineering are being integrated into the development of the next generation of materials, products, and processes.1–7 The depletion of petroleum resources coupled with increasing environmental regulations are acting synergistically to provide the impetus for new materials and products that are compatible with the environment and independent of fossil fuels. Composite materials, especially “green composites,” fit well into this new paradigm shift. Simply stated, biobased materials include industrial products for durable goods applications, made from renewable agricultural and forestry feed stocks, including wood, agricultural waste, grasses and natural plant fibers composed of carbohydrates such as sugars and starch, lignin and cellulose, as well as vegetable oils and proteins. Producing chemical products and new materials from renewable Copyright © 2005 by Taylor & Francis

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resources is not a new idea. Most of the chemical products and materials came from renewable resources until the early part of the 20th century.1 The success and tremendous growth of the petrochemical industry in the 20th century, however, slowed the growth of biobased products. Environmental as well as economic factors are now driving the trend toward greater utilization of biobased polymers and materials.2–5 The challenge to scientists and engineers is to develop the technology needed to make the biobased materials revolution a reality. The production of chemicals and materials from biobased feedstocks1 is expected to increase from today’s 5% level to ~12% in 2010, ~18% in 2020, and ~25% in 2030. Expectations are that two-thirds of the $1.5 trillion global chemical industry can eventually be based on renewable resources. The U.S. agricultural, forestry, life sciences, and chemical communities have developed a strategic vision6 for using crops, trees, and agricultural residues to manufacture industrial products, and have identified major barriers7 to its implementation. The Technology Road Map for Plant/Crop-based Renewable Resources 2020, developed by the U.S. Department of Agriculture (USDA) and the U.S. Department of Energy (DOE), has set a target of 10% of basic chemical building blocks arising from plant-derived renewables by 2020, with developed concepts in place by then to achieve a further increase to 50% by 2050. Petroleum transitioned from a single product (kerosene in the early 1900s) to a multiproduct industry (fuel gas, gasoline, jet fuel, naphtha, diesel fuel, asphalt, chemicals, etc.) between the late 19th century and the middle of the 20th century. Research conducted from the 1990s to the present has led to many new biobased products.8–14 Some examples include polylactic acid (PLA) from corn; polyurethane products from soy oil; soy protein adhesives; solvents from soy and corn oil; lubricants from vegetable oil; thermoset and thermoplastic polymers from soy and corn; organic acids from crop sources; and biocomposites from lignocellulosic fibers combined with petroleumbased polymers like polypropylene (PP) and polyethylene (PE), or biopolymers like PLA, cellulose esters, polyhydroxyalkanoates, and vegetable oil-based bioresins. Recent advances in genetic engineering, natural fiber development, and composite science offer significant opportunities for new, improved materials from renewable resources, which can be biodegradable and recyclable but also obtained from sustainable sources at the same time. The persistence of plastics in the environment, the shortage of landfill space, concerns over emissions during incineration, and entrapment and ingestion hazards from these materials have spurred efforts to develop biodegradable plastics. Several of the world’s largest chemical companies, including DuPont, Monsanto, Dow, and Cargill have announced a major shift in their base science and technology from traditional petrochemical processing to life sciences.15 DuPont and Monsanto have invested $12.5 billion to acquire expertise in agricultural biotechnology.16 Biopolymers are now starting to migrate into the mainstream and biobased polymers may soon be competing with commodity plastics. The best examples of biopolymers Copyright © 2005 by Taylor & Francis

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derived from renewable resources are cellulosic plastics like cellulose acetate, starch plastics like starch esters, and corn-derived plastics, i.e., PLA (polylactic acid). Sales growth rates of more than 20–30%/yr are expected along with improved economics as production and sales increase. The challenge in replacing conventional plastics with biodegradable materials is to design materials that exhibit structural and functional stability during storage and use, yet are susceptible to microbial and environmental degradation only upon disposal and without any significant environmental impacts.

1.2

Motivation: Biobased Materials vs. Environmental Impact

The successful transition to a biobased economy challenges the global academia, government, and industry. The world technology (WTEC) panel report17 has reviewed the status of “environmentally benign manufacturing” (EBM) technologies, applications, and policies in Europe and Japan in comparison to those in the United States. In this report, the main focus was given to polymers, electronics, transportation applications, and energy-related issues as well as the broader issues of government policies affecting environmental issues. The Biomass Research and Development Act of 2000 (U.S. Public Law 106-224), presidential executive orders 13134 (calling for tripling America’s use of biobased products by 2010) and 13101 (greening the government through recycling and waste prevention), and the Farm Security and Rural Investment Act of 2002 (Public Law 107-17), also known as the 2002 Farm Bill, together are creating an environment where there is an economic incentive to seriously consider biobased alternatives to petroleumbased materials. Biobased alternatives would play a role in reducing U.S. dependence on foreign oil. Biobased product development would have significant benefits for our citizens and society.

1.3

What Are Biocomposites?

Composite materials are attractive because they combine material properties in ways not found in nature. Such materials often result in lightweight structures having high stiffness and tailored properties for specific applications, thereby saving weight and reducing energy needs. Fiber-reinforced plastic composites began with cellulose fiber in phenolics in 1908, later extending to urea and melamine, and reaching commodity status in the 1940s with glass fiber in unsaturated polyesters. From guitars, tennis racquets, and cars to microlight aircrafts, electronic components, and artificial joints, composites are finding use in diverse fields. The fiber-reinforced composites market Copyright © 2005 by Taylor & Francis

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(Figure 1.1) is a multibillion-dollar business.18 Glass fiber is the dominant fiber and is used in 95% of cases to reinforce thermoplastic and thermoset composites. Current research findings show that in certain composite applications, natural fibers demonstrate competitive performance to glass fibers. Broadly defined, biocomposites (Figure 1.2) are composite materials made from natural fiber and petroleum-derived nonbiodegradable polymers like PP, PE, and epoxies or biopolymers like PLA and PHAs. Composite materials derived from biopolymer and synthetic fibers such as glass and carbon also come under biocomposites. Biocomposites derived from plant-derived fiber (natural/biofiber) and crop/bioderived plastic (biopolymer/bioplastic) are likely more ecofriendly, and such biocomposites are sometimes termed “green composites.” (1%) Aerospace

Miscellaneous

Appliances Consumer products

4%

8%

Automotives

8%

31%

10% Electronic components

12% 26%

Construction Marine

FIGURE 1.1 Fiber-reinforced plastic composites used in 2002 — 2.28
109 lb. (Adapted from Plast. News. August 26, 2002.)

Natural/biofiber composites (biocomposites*)

Partly ecofriendly Biofiberpetroleum-based plastic (polypropylene/polyester, etc.)

Ecofriendly/green Biofiber-renewable resource-based bioplastic (soy plastic/ cellulosic plastic/PLA, etc.)

Hybrid biocomposites (fiber blending/matrix blending) *Composites made from synthetic fiber; like , glass and bioplastic; like PLA can also come under biocomposite ,

FIGURE 1.2 Classification of biobased composites.

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After decades of development of high-performance artificial fibers like carbon, aramid, and glass, natural fibers have gained renewed interest, especially as a glass fiber substitute in automotive industries. Advantages of natural fibers over synthetic or man-made fibers such as glass and carbon are as follows: low cost, low density, acceptable specific strength properties, ease of separation, carbon dioxide sequestration, and biodegradability. Natural fiber composites are now emerging as a realistic alternative to wood-filled and glass-reinforced plastics. Ecofriendly biocomposites have the potential to be the new material of the 21st century and be a partial solution to many global environmental problems.

1.4

Natural/Biofibers as Reinforcements in Biocomposites

The world’s supply of natural resources is decreasing and the demand for sustainable and renewable raw materials continues to rise. In 1997, approximately 25 million metric tons of man-made fibers and about 20 million metric tons of natural fibers were produced worldwide.19 Biofiber-reinforced composites represent a potential nontraditional, value-added source of income to the agricultural community.3 Jute is from India and Bangladesh; coir is produced in the tropical countries of the world, with India accounting for 20% of the total world production; sisal is also widely grown in tropical countries of Africa, the West Indies, and the Far East, with Tanzania and Brazil being the two main producing countries; kenaf is grown commercially in the United States; flax is a commodity crop grown in the European Union as well as in many diverse agricultural systems and environments throughout the world, including Canada, Argentina, India, and Russia. Flax fiber accounts for less than 2% of world consumption of apparel and industrial textiles, despite the fact that it has a number of unique and beneficial properties. Hemp originated in Central Asia, from which it spread to China, and is now cultivated in many countries in the temperate zone. Ramie fibers are the longest and one of the strongest fine textile fibers and mostly available and used in China, Japan, and Malaysia. The price for natural fiber varies depending on the economy of the countries where such fibers are produced. Most plastics by themselves are not suitable for load-bearing applications due to their lack of sufficient strength, stiffness, and dimensional stability. However, fibers possess high strength and stiffness but are difficult to use in load-bearing applications by themselves because of their fibrous structure. In fiber-reinforced composites, the fibers serve as reinforcement by giving strength and stiffness to the structure while the plastic matrix serves as the adhesive to hold the fibers in place so that suitable structural components can be made. A broad classification (nonwood and wood fibers) of natural fibers is represented schematically in Figure 1.3, whereas Figure 1.4 displays images of several natural fibers with reinforcement potential. Copyright © 2005 by Taylor & Francis

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Reinforcing natural/biofibers

Wood fibers

Nonwood natural / biofibers

Straw fibers

Bast

Leaf

Examples: Kenaf, flax, jute, hemp

Grass fibers

Seed/ fruit

Examples Cotton, coir

Examples:

Examples:

Rice/wheat/ corn straws

Henequen, sisal, pineapple leaf fiber

Examples: Soft and hard woods

Examples : Bamboo fiber, switch grass, elephant grass

Recycled wood fibers Examples: Newspaper/ magazine fibers

FIGURE 1.3 Schematic representation of reinforcing natural/biofibers classification.

FIGURE 1.4 Digital photographs of some natural fibers and sources of natural fibers.

Currently several nonwood fibers (e.g., hemp, kenaf, flax, and sisal) are being utilized commercially in biocomposites in combination with polypropylene for automotive applications. More details on various plant Copyright © 2005 by Taylor & Francis

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fibers, the processing of bast fiber plants, and retting of natural fibers can be found in Chapters 2–4 of this book. Native grass fibers also are gaining the attention of scientists as reinforcing fibers for biocomposites applications.20,21 Similarly, recycled wood fibers such as newspaper fibers are another viable potential source of fiber reinforcement.22 Straw fibers from rice, wheat, or corn are widely available in different parts of the world and can also be used as very inexpensive reinforcements for biocomposites.23 Chapter 14 discusses more about straw-based biomass and biocomposites whereas the alternative low-cost biomass for the biocomposite industry is discussed in Chapter 5. Thermoplastic/wood composites have been known for many years.24 The production of wood-plastic composites has grown fourfold25 between the years 1997 and 2000. Thermoplastics like polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are widely used in wood-plastic composite industries.26 Historically, most of these used wood flour to produce filled plastics. The wood flour decreased the cost, but was not usually intended to improve the performance in any substantial way. All natural fibers, whether wood or nonwood types, are cellulosic in nature. The major constituents of natural biofibers are cellulose and lignin. The amount of cellulose, in lignocellulosic systems, can vary depending on the species and age of the plant. Cellulose is a hydrophilic glucan polymer consisting of a linear chain of 1,4-β anhydroglucose units, which contain alcoholic hydroxyl groups (Figure 1.5). These hydroxyl groups form intermolecular and intramolecular hydrogen bonds with the macromolecule itself and also with other cellulose macromolecules or polar molecules. Therefore, all natural fibers are hydrophilic in nature. Although the chemical structure of cellulose from different natural fibers is the same, the degree of polymerization (DP) varies. The mechanical properties of a fiber are significantly dependent on the DP. During the biological synthesis of plant cell walls, polysaccharides such as cellulose and hemicellulose are produced, and simultaneously lignin fills the spaces between the polysaccharide fibers, cementing them together. This lignification process causes a stiffening of cell walls, and the carbohydrate is protected from chemical and physical damage. Lignin is a biochemical polymer that functions as a structural support material in plants. Lignin is a high molecular-weight phenolic compound, generally resistant to microbial degradation. The exact chemical nature of lignin still remains obscure.

CH2OH H HO

H OH H

H

OH

O O H

H

OH

H

OH H

H

CH2OH H

H O

H OH

O H

O CH2OH

H

H

H

OH n

FIGURE 1.5 Cellulose structure.

Copyright © 2005 by Taylor & Francis

H

OH

OH

H

O H O CH2OH

H OH

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The main difficulty in lignin chemistry is that no method has been established by which it is possible to isolate the lignin in its native state from the fiber. The chemical nature of lignin in lignocellulosic materials has been an important subject of study. A probable structure of lignin27 is represented in Figure 1.6. Although the exact structural formula of lignin has not yet been established, most of the functional groups and units, which make up the molecule, have been identified. The high carbon and low hydrogen content of lignin suggest that it is highly unsaturated or aromatic in nature. Lignin is characterized by its associated hydroxyl and methoxy groups. The topology of lignin from different sources may be different but it has the same basic composition. Although the exact mode of linkages of lignin with cellulose in lignocellulosic natural fiber is not well known, lignin is believed to be linked with the carbohydrate moiety through two types of linkages, one alkali sensitive and the other alkali resistant. The alkali-sensitive linkage forms an ester-type combination between lignin hydroxyls and carboxyls of hemicellulose uronic acid. The ether-type linkage occurs through the lignin hydroxyls combining with the hydroxyls of cellulose. The lignin, being polyfunctional, exists in combination with more than one neighboring chain molecule of cellulose and/or hemicellulose, making a cross-linked structure. The tensile strengths as well as Young’s modulus of natural fibers like kenaf, hemp, flax, jute, and sisal are lower than that of E-glass fiber commonly used in composites. However, the density of E-glass is high, ~2.5 g/cc, while that of natural fibers is much lower (~1.4 g/cc). The specific strength and specific moduli of some of these natural fibers are quite comparable to glass fibers.3,4 This becomes particularly important where the weight of the structure needs to be reduced. The chemical compositions as well as properties of different natural fibers are discussed in more detail by Bismarck et al. in Chapter 2. The place of origin and climatic conditions also affect the physicomechanical properties of these natural fibers. To create confidence in the

OH OH

CH 3O HO

OH

OCH 3

O

CH3O

OCH 3 OH

OH

OH OH

OH

O

O

O

CH3O

OH

O

CH3O

OH

OH OH OCH3

HO

O

O

O

O

O

CH3O

OCH3

OH

HO

OH

HO HO

OCH 3

O

CH 3O

OH O CH3O

OH

CH 3O OH

OH OCH 3 OH OH

FIGURE 1.6 Lignin structure. (Adapted from Rouchi, A.M., Chem. Eng. News, Nov. 13, 2000, pp. 29–32.)

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consistent quality of specific biofibers among the industrial users is a serious challenge to researchers. Genetic manipulation, which produces fibers with low lignin content and with consistent properties is poised to produce a new generation of cellulose-rich biofibers for wide-scale industrial uses.

1.5

Biodegradable/Biobased Polymers as Matrices for Biocomposite Applications

The classification of biodegradable/biobased polymers is represented in Figure 1.7. Biobased polymers may or may not be biodegradable, depending on their composition and structure as well as on the environment in which they are placed. Renewable sources of polymeric materials offer an answer to maintaining the sustainable development of economically and ecologically attractive technology. The innovations in the development of materials from biopolymers, the preservation of fossil-based raw materials, complete biological degradability, the reduction in the volume of waste and compostability, reduction of atmospheric carbon dioxide released, as well as increased utilization of agricultural resources for the production of new “green” materials are some of the reasons for the increased public interest. Biodegradable polymers have offered scientists a possible solution to waste disposal problems

Biodegradable/biobased polymers Renewable resource-based
Polyhydroxyalkanoates (PHAs)
Polylactides (PLA)
Cellulose esters
Starch plastics

Petroleum/fossil fuel-based
Aliphatic polyesters
Aliphatic-aromatic polyesters
Poly(ester amide)

Poly(alkyene succinate)s

Mixed resource-based: renewable resources + petroleum resources
SORONATM (condensation polymer of corn-derived 1,3 propane diol & petroleumderived terephthalic acid)
Blendings of: Two/more biodegradable polymers (example: starch plastic + PLA)


Poly(vinyl alcohol) Successful blending: new polymeric materials of desired properties (such materials may be termed as biobased & may or may not be biodegradable)

One biodegradable + one fossil fuel-made polymer (example: starch plastic + polyethylene) Epoxidized soybean oil + petro-based epoxy resin

FIGURE 1.7 Broad classification of biodegradable polymers/biobased polymers.

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associated with traditional petroleum-derived plastics. Rising oil prices helped to stimulate early interest in biodegradables in the 1970s, and concerns over the dwindling availability of landfill sites, environmental regulations, and increasing oil prices are reviving interest in biodegradable materials today. Biodegradable polymers may be defined3 as those which undergo microbially induced chain scission leading to photodegradation, oxidation, and hydrolysis, which can alter the polymer during the degradation process. Another definition states that biodegradable polymers are capable of undergoing decomposition primarily through enzymatic action of microorganisms to carbon dioxide, methane, inorganic compounds, or biomass in a specified period of time. The biopolymers may be obtained from renewable resources and also can be synthesized from petroleum-based chemicals. Blending of two or more biopolymers can produce a new biopolymer designed for specific requirements. Biodegradability is not only a function of origin but also of its chemical structure and degrading environment. When a biodegradable material (neat polymer, blended product, or composite) is obtained completely from renewable resources, it may be termed as a green polymeric material. The life cycle of compostable biodegradable polymers is represented schematically in Figure 1.8.

Sunlight H2O + CO2 Photosynthesis Photosynthesis Natural resources

Biodegradation

Innovation: Polymer production

Use & discard

Processing FIGURE 1.8 Life cycle of biodegradable polymers can maintain CO2 balance in the environment.

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Traditional plastics like polypropylene, polyethylene, polyester, and epoxy have a development history and have attained adequate status in composite applications. Several biodegradable polymers need to be developed so as to make them suitable as matrix polymers for composite applications. Originally biopolymers were intended to be used in packaging, farming, and other industries with minor strength requirements. The performance limitations and high cost of biopolymers are major barriers for their widespread acceptance as substitutes for traditional nonbiodegradable polymers. The high cost of some biopolymers as compared to traditional plastics is not due to the raw material costs for biopolymer synthesis but mainly to the low volume of production. New and emerging applications of biopolymers need to be developed for their high-volume applications. The challenge for development of biodegradable polymers lies in the fact that they should be stable during storage or use and again should degrade only when they are disposed of after their intended lifetime. Biopolymers reinforced with biofibers can produce novel biocomposites to replace and substitute for glass fiber-reinforced composites in various applications.

1.5.1

Biodegradable Polymers from Starch and Cellulose

Biopolymers evolved to function as cellular components of the organisms. In order to produce useful plastics from biopolymers, biopolymers have to be modified. The best-known renewable resources capable of making biodegradable plastics are starch28,29 and cellulose.30 More detailed descriptions of cellulose ester and starch plastics can be found in Chapters 19 and 20, respectively, of this book. Starch and cellulose are not plastics in their native form, but are converted into plastics through various approaches, including extrusion cooking, functionalization, and plasticization. Starch is one of the least expensive biodegradable materials available in the world market today. It is a versatile biopolymer with immense potential for use in non-food industries. Starchbased polymers can be produced from corn, rice, wheat, or potatoes. Starch is produced in plants and is a mixture of linear amylose (poly-α-1,4-D-glucopyranoside) and branched amylopectin (poly-α-1,4-D-glucopyranoside and α-1,6-D-glucopyranoside). Starch can be made thermoplastic through destructurization in the presence of specific amounts of plasticizers (water and/or polyalcohols) under specific extrusion conditions. Three phenomena (i.e., fragmentation of starch granules, hydrogen-bond cleavage between starch molecules leading to loss of crystallinity, and partial depolymerization of the virgin starch polymers) generally occur during conversion of starch to starch plastic under extrusion conditions. Unmodified thermoplastic starch alone can be processed as a traditional plastic; however, its sensitivity to humidity makes it unsuitable for many applications. Unmodified thermoplastic starch is mainly used in soluble compostable foams, loose-fill materials, expanded trays, shape-molded parts and expanded layers, and as

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a replacement for polystyrene. Poly(ε-caprolactone), PCL, a fossil fuel- or petroleum-derived synthetic biodegradable polymer, provides water resistance to starch-based formulations, making them attractive for commercial enterprises. The strength and stiffness of starch plastic is enhanced considerably31 as a result of reinforcement with surface-treated jute fibers, and biocomposites made with 50 wt% jute improve the tensile strength of virgin starch plastic by more than 150%. Cellulose from trees and cotton plants is a substitute for petroleum feedstocks to make cellulosic plastics.30 The structures of cellulose esters including cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) are represented in Figure 1.9. CAB and CAP are now used in a variety of plastic applications. For instance, premium toothbrush handles are typically made of CAP, and screwdriver handles are often made from CAB. Recently, cellulosic plastics have gained importance in biocomposite formulations.32,33 Chapter 19 discusses biobased composite materials from cellulose esters and natural fibers. 1.5.2

Biobased/Biodegradable Plastics from Soybeans and Other Plant Resources

In recent years, engineering of bioplastics and biobased materials from plant-based proteins and oils has gained global attention. Considering the importance of the research and development in this area as well as the commercial value of such protein and vegetable oil-based biobased materials, the editors have included four chapters (Chapters 22–25) describing recent developments. In the United States, soybeans provide over 60% of the fats and oils used for food. Research on applications of soybeans for non-food uses in plastics and composites is under way at various U.S. universities. Soybeans typically contain about 20% oil and 40% protein. Soy protein is available in three different forms as soy flour, soy isolate, and soy concentrate. Soy protein, soy meal, and soy oil from soybean can be converted to plastic resins.

OR

O HO

OR O

OR

CH 2 OR

O

CH 2 OR

OR

O

OR O

OR

CH 2 OR O

O

CH 2OR

OR

OR n

n = 400−750; R = H (cellulose), acetyl (cellulose acetate), acetyl and propionyl (cellulose acetate propionate), or acetyl and butyryl (cellulose acetate butyrate)

FIGURE 1.9 Structures of cellulose esters.

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OH

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Chemically, soy protein is an amino acid polymer or polypeptide while soy oil is a triglyceride. Through extrusion processing and blending technology, soy protein polymers are converted into biodegradable plastics,34 whereas through fuctionalization of soy oil, a matrix resin suitable for natural fiber composites is also reported.35 Soy protein-based bioplastics are thermoplastics and likely to be biodegradable. Soy oil-based resins are usually thermosets and likely to be nonbiodegradable, based on the existing literature. Green composites from soy protein-based bioplastics and natural fibers36–38 show potential for rigid packaging and even for housing and transportation applications.

1.5.3

Biodegradable Polyesters from Renewable Resources and Petroleum Resources

Biodegradability is not only a function of origin but also of chemical structure and degrading environment. Sometimes thermoset bioresins, even if made or derived from bioresources, may not be biodegradable. The chemical structures of some important members of the polyester class of biodegradable polymers are represented in Figure 1.10. PLA as well as PHAs are renewable resource-based biopolyesters, in contrast to PCL, PBS and aliphatic-aromatic polyesters, which are petroleum-based biodegradable polyesters. Aliphatic polyesters are readily biodegradable, whereas aromatic polyesters like poly(ethylene terephthalate) (PET), are nonbiodegradable. However, aliphatic-aromatic copolyesters have been shown to be biodegradable, and recently these polyesters have gained commercial interest, especially for packaging applications.39 Eastman’s Easter Bio® and BASF’s Ecoflex® are two examples of aliphatic-aromatic copolyesters based on butanediol, adipic acid, and terephthalic acid. Eastar Bio is highly linear in structure while Ecoflex has a long-chain branched structure. As early as 1973, the biodegradability of PCL was demonstrated.40 It is a tough and semirigid material at room temperature with a modulus between low-density and high-density polyethylene.3 PCL has a low melting point (~60°C), low viscosity, and can be melt processed easily. It possesses good water, oil, solvent, and chlorine resistance. PCL is widely used as a blending partner with a number of polymers, especially with hydrophilic starch plastic.41,42 Biocomposites from PCL and natural fibers have been developed.43 The tensile strength and Young’s modulus of PCL improved by 450% and 115%, respectively, after reinforcement with 40 wt% wood flour. Poly(alkylene dicarboxylate) biodegradable aliphatic polyesters have been developed by Showa Highpolymer under the trade name Bionolle®. Different grades of Bionolle include polybutylene succinate (PBS), poly(butylene succinate-cobutylene adipate) (PBSA), and poly(ethylene succinate). SK Chemicals also produces aliphatic PBS polyesters. DuPont`s biodegradable Biomax copolyester resin, a modified form of PET, was launched in 1997. Its properties, according to DuPont, are diverse and customizable, but they are generally Copyright © 2005 by Taylor & Francis

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Chemical Structures

O

R

O

CH

C

Examples

R = H, Poly(glycolic acid), PGA R = CH3, Poly(lactic acid), PLA

n

Poly(α-hydroxy acid)

O

R CH

O

R = CH3, Poly(β-hydroxybutyrate), PHB

C

C H2

CH3 CH

C 2H5

O CH 2

C

O

(Hydroxybutyrate, HB)

n

CH

O CH 2

C

O

m

(Hydroxyvalerate, HV)

PHBV copolymer containing HB and HV units

Aliphatic polyesters

R = CH3, C2H5, Poly(β-hydroxybutyrateco-valerate), PHBV copolymer

n

Poly(β-hydroxy acid)

O O

( CH )

C

2 x

x = 5, Poly(∈-caprolactone), PCL n

Poly(ω-hydroxyalkanoate)

O O

( CH ) 2

x

O

C

O

( C H 2) y

C n

x = 4, y = 2; Poly(butylene succinate), PBS x = 4, y = 2,4, Poly(butylene succinate-cobutylene adipate), PBSA

Poly(alkylene dicarboxylate)

O

O

O

C

C

O O

(CH2)4

O C

O (CH 2)4

C

Aliphatic-aromatic polyester

FIGURE 1.10 Structures of some aliphatic and aliphatic-aromatic polyesters (biodegradable polymers of commercial interest).

formulated to mimic polyethylene or polypropylene.44 The biodegradable plastics from polyesters are mostly used in packaging applications. PLA, a polymer of the relatively simple lactide molecule, is not a new polymer. The manufacture of polyester from lactic acid was pioneered in 1932 by Carothers45 and further developed by DuPont46 and Ethicon. The use of PLA in biomedical applications47 began in the 1970s. High production Copyright © 2005 by Taylor & Francis

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costs restricted the applicability of these polymers outside the medical field until the late 1980s. Recent developments in the economical manufacture of monomer of PLA from agricultural products has placed this material at the forefront of the emerging biodegradable plastics industries. Polylactide (PLA) is a highly versatile biopolymer derived from renewable resources like corn.48 The use of PLA as a cost-effective alternative to commodity petroleum-based plastic will increase the demand for agricultural products. A Cargill-Dow plant in Nebraska is capable of producing 300 million pounds of renewable resource-based PLA per year from 40,000 bushels of corn per day. More details on PLA technology are in Chapter 16. Cargill-Dow uses a solvent-free process and a novel distillation process49 in contrast to Mitsui Chemicals, which uses a solvent-based process50 to make high molecularweight PLA. Biocomposites from natural fiber and PLA have useful properties51–53 and are discussed more in Chapter 17. Polyhydroxyalkanoates (PHAs) are biodegradable polyesters, which are produced by bacterial fermentation. The first PHA discovered was polyhydroxybutyrate (PHB). Figure 1.11 shows a photograph of the formation of bacterial polyesters during fermentation by microorganisms. PHA polymers are synthesized in the bodies of bacteria fed with glucose (e.g., from sugarcane) in a fermentation plant. Over one hundred PHA compositions have been reported in the literature. In the late 1980s, ICI Zeneca commercialized PHAs produced by microbial fermentation under the trade name Biopol®. PHB and the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) were also produced by Monsanto and sold under the trade name Biopol.

White patches in microorganism are PHB

FIGURE 1.11 Photograph showing bacterial polyester (PHB) formation during fermentation by microorganisms. (Courtesy: Biomer, Germany.)

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These bacterial polyesters were originally intended as biodegradable substitutes for oil-based polyolefins in films, bottles, and plastic containers.54 The actual and potential uses of PHB and PHBV for containers, films, and papercoating materials have been reviewed.55 In 1990, the manufacture of blowmolded bottles using Biopol for packaging shampoo was started in Germany by Wella AG, Darmstadt. The range of possible uses of Biopol polymers has been summarized by Amass et al.56 PHBVs are highly crystalline polymers with melting points and glass transition temperatures similar to polypropylene (PP).57 Due to their characteristics of biodegradability through nontoxic intermediates and easy processibility, PHBV polymers are being developed and commercialized as ideal candidates for the substitution of nonbiodegradable polymeric materials in commodity applications.58,59 However, their high cost, the small difference between their melting and thermal degradation temperatures, and their low-impact resistance at ambient and subambient temperatures prevented their larger commercial applications. Recent developments in bacterial polyester technology look promising. Different companies such as Metabolix (Cambridge, MA), Proctor & Gamble, Biomer in Germany, and PHB Industrial S. A. in Brazil are pursuing commercialization pathways to make these bacterial polyesters competitive with traditional polyolefins like polyethylene and polypropylene. Metabolix recently produced PHBV in a commercial-scale fermentation plant39 and is pursuing a transgenetic approach to develop PHAs from switchgrass. Proctor & Gamble is looking to commercialize their specific branched PHAs (Nodax®). The longchain branching of such polyesters allows a considerable range for tailoring the crystallinity, stiffness, toughness, and melting point of the Nodax polymers. Green composites from PHAs and natural fibers59–61 are gaining importance in recent years. This area is comprehensively discussed in Chapter 18. The strength of biodegradable polyester may be increased by substituting a fraction of ester links by amide groups. On a call from the Government of Germany for research and development on biodegradable thermoplastics with good performance and processing behavior, Bayer, in 1990, presented two grades of polyester amides,62 namely, BAK 1095 and BAK 2095. BAK 1095 is based on caprolactam (Nylon 6), butanediol, and adipic acid, whereas BAK 2195 is synthesized from adipic acid and hexamethylene diamine (Nylon 6,6) and adipic acid with butanediol and diethylene glycol as ester components. BAK 1095 has mechanical and thermal properties resembling those of polyethylene.63 The resin is also noted for its high toughness and tensile strain at break. It can be processed into film and also into extruded and blow-molded parts. It is suitable for thermoforming and can be colored, printed, hot-sealed, and welded. BAK 2195 resin is an injection-molding grade biodegradable thermoplastic that exhibits greater stiffness. The property profile of BAK 2195 can also be extended through the addition of fillers and reinforcing substances, such as starch, natural fibers, wood flour, and minerals.64 The combined performance of both BAK grades and the compounds open a wide range of applications like disposable plant pots, agricultural films, biowaste bags, plant clips, cemetery decorations, and one-way Copyright © 2005 by Taylor & Francis

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TABLE 1.1 Properties of Some Traditional Polymers and Biodegradable Polymers

Property 3

PHB (P226a)

PHBV

PCL (Tone 787)

Density (g/cm ) 1.21 1.25 1.25 1.145 Melting point (°C) 177–180 168–172 135 60 Tensile stress at 45 24–27 25 41 break (MPa) Tensile modulus 2800 1700–2000 1000 386 (MPa) Elongation 3 6–9 25 900 at break (%) Copyright © 2005 by Taylor & Francis a Plasticized PHB, Source: Biomer, Germany, http://www.biomer.de/ b From technical data sheet, Showa Highpolymer Co. Ltd.

PBS (Bionolle 1001b) Film Grade

PS

LDPE

PP

1.26 115 57

1.04–1.09 — 35–64

0.92 124 8–10

0.90 164 34

2800–3500

100–200

—

150–600

12

BAK 1095

Easter Bio

1.07 125 25

1.22 108 22

220

100

80

—

400

700

820

700

Ecoflex 1.25 110–115 36

1–2.5

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PLA (Eco-PLA)

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dishes. BAK 1095 breaks down into water, carbon dioxide, and biomass under aerobic conditions. The degradation rate is comparable to that of other organic materials that are composted.63 Bayer has recently withdrawn the production and sale of their polyester amide products.65 Table 1.1 represents some comparative properties2,66–71 of a few traditional polymers and biodegradable polymers. Biodegradable polymers will play a prominent role in plastic packaging applications. In the United States alone, over 25 million tons of plastics entered the municipal solid waste stream (MSW), which is over 11% of the total MSW generation in the year 2001 (Figure 1.12, after ref. 72). Plastic packaging materials are everywhere in our daily lives. Around $450 billion worth of packaging materials are used each year across the world. From food wrappings and containers to detergent and soft drink bottles to foam packaging used in shipping delicate goods, many products are surrounded by or contained in polymeric materials. Packaging dominates the plastics use (Figure 1.13, after ref. 73). It is forecasted that in the year 2005 more than $27 billion worth of plastic will be used in packaging (Figure 1.14, after ref. 74). At present most polymeric packaging materials are based on nonrenewable fossil fuel feedstocks. Most notably, polyethylene is currently produced from ethylene gas, a product of cracking of ethane, which comes from nonrenewable petroleum resources. Incineration of these materials makes a net contribution to atmospheric CO2, and plastics currently account for in excess of 20% of the nation’s landfill. In addition, many widely used materials, notably polystyrene and PVC, are made from noxious or toxic monomeric components. Packaging now dominates the plastics industries. Out of 71 billion pounds of plastic use in different areas like building products, consumer products, and transportation, the maximum share (27.3%) goes to packaging. Petroleum-based nonbiodegradable plastic bags have already been restricted in Germany, Ireland, South Africa, and Taiwan due to concerns about their disposability. 1.5.4

Biobased Polymeric Materials from Mixed Resources (Renewable and Petroleum Resources)

In day-to-day life most people are interested in using green materials but do not want to spend more money or use materials having inferior performance than the existing dominant fossil fuel-based polymers and materials. Currently it is difficult to replace petroleum-based materials, from a cost and performance perspective. It is not necessary to make a 100% substitution for petroleum-based materials immediately. A viable solution is to combine the different features and benefits of both petroleum and bioresources to produce a useful product having the requisite cost-performance properties for real-world applications. Biobased products may be categorized as follows: (a) Low biobased content product (20% or less biobased content) (b) Medium biobased content product (21–50% biobased content) (c) High biobased content product (51–90% biobased content) Copyright © 2005 by Taylor & Francis

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Yard trimmings Food scraps 12.2% 11.4% Wood 5.7% Other 3.4% Rubber, leather & textiles 7.1%

Paper 35.7%

Plastics 11.1% Glass 5.5%

Metal 7.9%

FIGURE 1.12 2001 total municipal solid waste (MSW) generation, 229 million tons (before recycling). (Adapted from Municipal Solid Waste in the United States: 2001, U.S. Environmental Protection Agency.)

Exports Industrial/ machinery 12% 1%

Transportation 5%

All other 14%

Adhesives/coatings 2%

Consumer & institutional 14%

Electrical 3%

Packaging 28%

Building & construction 17% Furniture 4%

FIGURE 1.13 Packaging dominates plastic use: 2002 U.S. plastic use areas. Total 86.7 billion lb. (Adapted from American Plastic Council 2002.)

Some examples of biobased polymeric materials are the following: Biobased Sorona®: DuPont Sorona polymer e.g., poly(trimethylene terephthalate), PTT, a 3-carbon glycol terephthalate (3GT), is an example of a condensation polymer that can be made from 1,3-propanediol (derived from renewable corn sugar) and petroleum-derived terephthalic acid (TPA). DuPont received the 2003 Presidential Green Chemistry Award for the successful development of 1,3-propanediol (PDO) by a biological process. More details on Sorona polymer can be found in Chapter 15. Biobased thermoset resins: Petroleum-derived epoxy resins can be blended with epoxidized vegetable oil in the presence of suitable curing agents to make biobased epoxies.75,76 Petroleum-based epoxy resins are known for their superior tensile strength, high stiffness, and exceptional Copyright © 2005 by Taylor & Francis

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Tex 1%

4%

4%

s:

d:

as

: tile

oo

W

Gl

Pa ap per er:: 10 10% %

Paperboard/ molded pulp: 39%

Metal: 16% Plastics: 26%

FIGURE 1.14 U.S. packaging materials consumption, forecast (year 2005), excluding fast-food packaging, plastic food and garbage bags, tubes and cores, reconditioned barrels and drums, gas cylinders, and bulk containers. (Adapted from Plast. News, Sept. 9, 2002.)

solvent resistance. One of the chief drawbacks of such epoxies is their brittleness or very low impact strength. However, the petroleum-based epoxies can show improvement of impact strength when blended with epoxidized vegetable oil, with a reduction in stiffness.76 A balance of stiffness and toughness can be obtained by adjusting the amount of epoxidized vegetable oil and petroleum-based epoxy in the resulting biobased epoxy. The biobased resins are formed by blending ortho unsaturated polyester resins with functionalized vegetable oils; such bioresins on reinforcement with natural fibers make a biobased composite material.14 Using bioresins as polymer matrices can improve the impact strength of the resulting biobased resin and can produce a material with higher biobased content.

1.6

Biocomposites as Alternatives to Petroleum-Based Composites: Recent Trends and Opportunities for the Future

Fiber-reinforced plastic composites began with cellulose fiber-reinforced phenolics in 1908, later extending to urea and melamine, and reaching commodity status in the 1940s with glass fiber-reinforced unsaturated polyesters. The manufacture, use, and removal of traditional composite structures, usually made of glass, carbon, or aramid fibers reinforced with epoxy, unsaturated polyester resins, polyurethanes, or phenolics, is being scrutinized from an environmental and legislative perspective.3 The disposal of composites after their intended life span is becoming critical and expensive. The recycling as well as reuse of composite materials is not easy since they are made from two dissimilar materials; however, we find continued efforts of such practice. Two disposal alternatives are land filling and

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incineration. Landfill space is decreasing due to heavy ongoing waste disposal. In the United States, the number of landfills dropped from 8000 to 2314 between 1988 and 1998.77 Reports suggest that five states have less than 5 years of landfill capacity; close behind are 14 more states with 5 to 10 years of landfill capacity.78 Biocomposites consist of reinforcing biofibers and matrix polymer systems. Since the biofiber is biodegradable and traditional thermoplastics (like polypropylene) and thermosets (like unsaturated polyester) are nonbiodegradable, biocomposites from such fiber-reinforced polymers are classified as the “partially biodegradable” type. If the matrix resin/polymer is biodegradable, the biofiber-reinforced biopolymer composites would come under the “completely biodegradable” category, as represented in Figure 1.2. Two or more biofibers in combination with a polymer matrix result in “hybrid” biocomposites. The purpose of hybrid composites is the manipulation of properties of the resulting biocomposites. The interrelationship between the development and applications of biocomposite materials is schematically represented in Figure 1.15. Some of the growing areas of applications for green/biocomposite materials are in automotive parts, housing products, and packaging. The challenge in replacing conventional glass-reinforced plastics with biocomposites is to design

Substitute of petroleumbased materials

Energy/ environment

Information/ communication

Performance

Transportation (auto parts)

Processing synthesis

Expanding biocomposite materials complexity

Structure composition

Packaging Value-added agricultural products

Interface chemistry

Civil infrastructure (green housing)

Value-added forest products

FIGURE 1.15 Biocomposite materials limits/complexity, interface chemistry, processing, structure/ composition, and performance are to be governed to exploit their uses via value-added forest and agricultural products for various applications in transportation, packaging, green housing panels, etc., from energy/environmentally beneficial perspectives through university-industrial interactions, thus creating effective communication that would enable real-world use of biocomposite materials.

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materials that exhibit structural and functional stability during storage and use, yet are susceptible to microbial and environmental degradation upon disposal without any adverse environmental impact. Automakers will see potential in biocomposites79–83 if these materials can deliver the same performance as conventional composites with lower weight. Moreover, they exhibit nonbrittle fracture on impact, which is another important requirement. In the United States, 10 million to 11 million vehicles reach the end of their useful lives annually. A network of salvage and shredder facilities processes about 96% of these old cars. About 25 wt% of these vehicles, including plastics, fibers, foams, glass, and rubber, remain as waste. The car parts made from green composites would simply be buried after their lifetime and would in time be consumed naturally by bacteria. The use of biocomposites in making interior automotive parts with natural fiber–polypropylene and exterior parts from natural fiber polyester resinbased composites has been reported.80 Johnson Controls has started production81 of door-trim panels from natural fiber and polypropylene. It is estimated that ~75% of a vehicle’s energy consumption is directly related to factors associated with the vehicle’s weight, resulting in a critical need to produce safe and cost-effective lightweight vehicles. Auto companies are seeking materials with sound abatement capability as well as reduced weight for fuel efficiency. Natural fibers possess excellent sound-absorbing efficiency and have excellent energy management characteristics. In automotive parts, compared to glass composites, the composites made from natural fibers reduce the mass of the component and lower the energy needed for production82 by 80%. Table 1.2 (after ref. 84) demonstrates how the weight of materials can be lowered by going from steel to glass fiber-reinforced plastic (GFRP). Natural fiber composites can reduce the mass in a properly designed component. Chapter 7 provides a more detailed description of current trends of natural fiber composites in automotive applications. Ford has a long history of R&D on new materials.83 Henry Ford began experimenting with composites around 1941, initially using compressed TABLE 1.2 Weight Savings of Materials Lightweight Material

Material Replaced

High-strength steel Aluminum Magnesium Magnesium Glass fiberreinforced composites

Mild steel Steel, cast iron Steel or cast iron Aluminum Steel

Percent Reduction Relative Cost of Mass per Part* 10 40–60 60–75 25–35 25–35

1 1.3–2 1.5–2.5 1–1.5 1–1.5

* Cost includes both materials and manufacturing. Source: Adapted from Powers, W.F., Adv. Mater. Process, 38–41, 2000.

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soybeans to produce composite plastic-like components. During that period petroleum-based chemicals were very inexpensive, so soy-based plastic did not grow in importance. New environmental regulations and depletion of and uncertainty about petroleum resources have revived interest in composite materials from soybean-based plastics and natural fibers. North American market studies identify the potential impact and opportunities for natural fiber composites.85 In the year 2000, the North American market for natural fiber composites exceeded $150 million, By 2005, this market is expected to reach nearly $1.4 billion in sales. The future of biobased composite materials for building product applications is bright (Table 1.3, after ref. 86), and several new natural fiber-based building materials are already making their way into the building industry. The majority of resins used in the composite industry are thermosets.87 About 65% of all composites produced currently for various applications use glass fiber and polyester or vinyl ester resins. Unsaturated polyester (USP) resins are widely used, thanks to a relatively low price, ease of handling, and a good balance of mechanical, electrical, and chemical properties. Natural fiber-thermosetbased biocomposites are discussed extensively in Chapters 8 and 9. Although natural fiber-reinforced nonbiodegradable polymer-based composites are gaining interest, the challenge is to replace conventional glassreinforced plastics with biocomposites that exhibit structural and functional stability during storage and use, yet are susceptible to microbial and environmental degradation upon disposal without any adverse environmental impact. A three-cornered approach in designing biocomposites of superior and desired properties include efficient but low-cost natural and biofiber treatment, matrix modification through functionalization and blending, and selection of appropriate and efficient processing techniques (Figure 1.16). Since the significant attraction of natural fibers is their low cost, inexpensive yet effective surface treatments that avoid organic solvents are logical ways of making a reactive natural fiber surface.4,88 Aqueous silane solution and a water emulsion of maleated polypropylene can be mixed together to produce a novel sizing for natural fibers and thus make polypropylene-based biocomposites of superior mechanical properties.89 Through utilization of an TABLE 1.3 North American Market Forecast for NWFCs Market Building products Infrastructure Transportation Consumer Industrial Total

Year 2002 1062 126 63 32 28 1311

Year 2005 2324 163 78 56 45 2667

Year 2010 3375 185 88 89 61 3799

Source: Adapted from Principia Partners, Exton, Pennsylvania, USA. With permission.

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“engineered natural fibers” concept (Figure 1.17), superior strength biocomposites can be obtained.90 This concept is defined as a suitable blend of surface-treated bast fibers (e.g., kenaf and hemp) and leaf fibers (e.g., henequen, sisal, and pineapple leaf fiber). Selection of blends of biofibers is also based on the fact that the correct blend achieves an optimum balance in mechanical properties. The kenaf- or hemp/flax-based composites exhibit excellent tensile and flexural properties, while leaf fiber-based composites give better

Injection Molding molding Biocomposite stampable sheet process Silane treatment Alkali treatment

Low-cost but efficient surface treatments of natural fibers

Water emulsionbased (silane (silane++ maleated coupling agent) sizing

Efficient biocomposite processing

Compression Molding molding

Extrusion Reactive blending

SYNERGISM

Polymer matrix modification Maleated matrix as: as compatibilizer

High-performance biocomposite formulation

Sisal

Flax

Henequen

Low-cost but efficient surface treatment

Bast fibers

Kenaf

Leaf fibers

Natural/biofibers

FIGURE 1.16 Tricorner coordinate approach in designing and engineering of high-performance biocomposites.

Different ratios blends of desired bast and leaf fibers are mixed, in a specific case, 1:1 blend of chopped kenaf and henequen fiber may be chosen.

"Engineered natural/biofibers" ready for biocomposite fabrications

FIGURE 1.17 Concept on design of “engineered natural fiber” for biocomposite formulations.

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impact properties to the composites. The combination of bast and leaf fiber is expected to provide a stiffness-toughness balance in the resulting biocomposites. A strong fiber-matrix interface bond is critical for high mechanical properties of the composites. In polymer matrix composites, there appears to be an optimum level of fiber-matrix adhesion, which can provide the best composite properties. Chapter 6 highlights this important aspect. Since natural fibers are inexpensive, one should approach making a reactive surface of natural fibers through a low-cost but effective surface treatment and/or by using a compatibilizer during biocomposite fabrication.91 Alkali treatment is an effective method to improve fiber-matrix adhesion in natural fiber composites.60,92–94 It is believed that alkali treatment leads to fiber fibrillation, i.e., the breaking down of fibers into smaller thin fibrils, thereby increasing the effective surface area for contact with the matrix resin. Maleated polypropylene (MAPP) has received widespread application as a coupling agent or a compatibilizer in natural fiber-reinforced polypropylene composites.95–98 Maleated polyethylene (MAPE) is used as a compatibilizer for natural fiberpolyethylene composites. The molecular weight and acid number of such maleated coupling agents affect the compatibilization chemistry. Maleated polyolefins of varying molecular weights and acid numbers are now commercially available. There are two ways by which the maleic anhydride compabilization chemistry can be implemented during biocomposite fabrication: First, natural fibers are pretreated with maleated polymer and the treated fibers are then dispersed into the desired polymer matrix during melt processing to obtain the biocomposites. The maleated coupling agent needs to be dissolved in an appropriate concentration of specific organic solvents for fiber treatment. Such processes are not economical and also add volatile organic compounds (VOCs) to the atmosphere. Second, in a reactive extrusion process one can add chopped biofiber, polymer matrix, and maleated coupling agent in one step, processing them into compatibilized biocomposite pellets for further compression/injection molding. Such processing techniques are of commercial importance. In making compatibilized green composites from natural fibers and biopolymers like PLA and PHB, maleated compatibilizers (with appropriate molecular weight and acid number) are being developed at the laboratory scale but are not yet commercialized. In time, with the widespread use of green composites, it is expected that commercial maleated biopolymers will be available. In an industrial process, reducing the number of processing steps reduces the overall cost of the product. Biopolymers like PHAs and PLA are plasticized with citrate or a combination of citrate and functionalized vegetable oil. The commercial polyhydroxybutyrate (PHB) from Biomer is plasticized with a proprietary citrate plasticizer system. On the industrial scale one can target doing the plasticization of the biopolymer along with natural fiber reinforcement in the presence of a compatibilizer in a one-step process (Figure 1.18) to make it more commercially attractive. Copyright © 2005 by Taylor & Francis

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PHA/PLA + maleated compatibilizer

Liquid plasticizer

Addition of chopped engineered biofiber

Peristaltic pump

K-Tron T-20 feeder

Pelletizer

Green composite pellets for further processing

Twin screw extruder

FIGURE 1.18 Schematic representation in fabricating bio/green composite pellets through reactive extrusion processing.

1.7

Sustainable Biobased Products: New Materials for a New Economy

The most common definition of sustainable is the following: sustainable development meets the needs of the present without compromising the needs of future generations. The “sustainability” issues of bioplastics, e.g., polylactic acid (PLA), polyhydroxyalkanoate (bacterial polyesters), and cellulosic plastics (cellulose esters), are the subject of scientific and engineering studies and divergent views.99–105 The evaluation of a bioplastic or biocomposite product’s sustainability is a complex problem. Life cycle assessment (LCA) analysis is a framework being developed that incorporates several parameters to be considered, including the raw materials from which the bioplastic is generated, the energy consumed during bioplastic conversion, and its ultimate disposal or recycle. While comparing the sustainability of a newly emerging bioplastic with a petroleum-based plastic, consideration should also be given to the technology development time gap between petrochemicals (~100 years old) and developing bioplastics (~5 to 10 years old). Detailed descriptions of each of these factors are beyond the scope of Copyright © 2005 by Taylor & Francis

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this chapter, but the interested reader can find contemporary references on the subject. It is encouraging to derive cost-effective biobased products or biocomposites from inexpensive bioplastics through inexpensive natural and biofiber reinforcements. Although most bioplastics cannot compete economically in their present state with petroleum-based plastic, cost-effective biocomposite formulations and designs with natural fiber reinforcements can compete at the economic level. The emergence of new applications of biocomposites will spur large-scale demand for bioplastics, which would help the long-range attainment of sustainability. A detailed understanding of natural and biofibers, bioplastics, and their biocomposite formulations will be the foundation for developing new and emerging biobased composite materials. Sustainability issues of biopolymers and biobased products are discussed in Chapter 27. Since natural fibers are inexpensive reinforcements for use with biopolymers, their use would be to provide new biobased and green materials that can attain sustainability. Green composite formulations incorporating 50–60% inexpensive natural fiber are expected to be cost-effective, with additional environmental benefits. A biobased product derived from renewable resources that can have the attractive attributes of recyclability, triggered biodegradability (i.e., stable in use but biodegradable after disposal), along with commercial viability and environmental acceptability is a sustainable biobased product (schematic representation is given in Figure 1.19). An additional benefit is that eventual decomposition of the biocomposite does not add any new net CO2 to the global environment, since the components came from plant material (overall balance; see Figure 1.8). Product realization of green/biobased/natural fiber composite materials requires a complete LCA. The glass fiber-reinforced plastics (GFRP) are the predominant composites in our industries today. The natural fiber composites have the potential to replace GFRP in many applications. A simplified LCA study approach of green composites vs. traditional glass fiber composites is shown schematically in Figure 1.20. LCA studies comparing natural fiber composites and glass fiber composites have recently been reviewed.102 The

Recyclable/ natural recycling

Renewable/ biobased

Triggered biodegradable

Commercial viability & environmental acceptibility

Sustainable FIGURE 1.19 One concept of sustainable biobased product that can fit well to green composites.

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Green composites from natural fiber & bioplastic

Conventional composites from glass fiber & traditional plastic

Crop production

Petroleum extraction

Fiber extraction

Refining

Bioplastic manufacture

Plastic manufacture

Green composite part manufacture

Glass fiber−plastic composite part manufacture

Composite part use

Composite part use

End of life management (recycling, disposal)

End of life management (recycling, disposal)

Glass fiber manufacture

FIGURE 1.20 Simplified LCA study approach for green composites vs. glass fiber-reinforced composites.

discussion of this review centers mostly on composites from glass and natural fibers using traditional plastics like polypropylene and ethylene propylene diene (EPDM), especially for automotive applications. Natural fiber composites are likely to be environmentally superior to GFRP. Some of the important reasons are the following. (1) the production of natural fiber poses lower environmental impact compared to glass fiber production; (2) natural fiber-traditional plastic composites can have higher biofiber content for equivalent performance of a GFRP, thus reducing more polluting base polymer content, and (3) natural fiber composites can provide lighter weight as compared to GFRP, which, for automotive parts use, can improve fuel efficiency.

1.8

Conclusions

New environmental regulations and societal concerns have triggered the search for new materials, products, and processes that are compatible with Copyright © 2005 by Taylor & Francis

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the environment. The incorporation of bioresources into the materials, products, and processes of today can satisfy these concerns and reduce further dependency on petroleum reserves. Biocomposites can supplement and eventually replace petroleum-based composite materials in several applications, thus offering new agricultural, environmental, manufacturing, and consumer benefits. Several critical issues related to biofiber surface treatment to make it more reactive, bioplastic modification to make it a suitable matrix for composite application, and processing techniques depending on the type of fiber form (chopped, nonwoven/woven fabrics, yarn, sliver, etc.), need to be solved to design biocomposites of commercial interest. Recent advances in genetic engineering, natural fiber development, and composite science offer significant opportunities for improved value-added materials from renewable resources with enhanced support for global sustainability. Thus, the main motivation for developing biocomposites has been and still is to create a new generation of fiber-reinforced plastics competitive with glass fiber-reinforced composites but which are environmentally compatible in terms of production, use, and removal. Natural fibers are biodegradable and renewable resource-based bioplastics can be designed to be either biodegradable or not, according to the specific demands of a given application. Bioplastics and biocomposites based on renewable agricultural and biomass feedstocks can form the basis for a portfolio for sustainable and ecoefficient biobased products that can compete and capture markets currently dominated by products based exclusively on petroleum feedstocks. There is an immense opportunity in developing new biobased products, but the real challenge is to design sustainable biobased products through innovative ideas. Green materials are the wave of the future.

Acknowledgments We acknowledge NSF/EPA (Award #DMI-0124789) under the 2001 Technology for a Sustainable Environment (TSE) Program, EPA–STAR award #RD-83090401 under the 2002 Environmental Futures Research in Nanoscale Science Engineering and Technology, and NSF 2002 Award #DMR-021686, NSF- PREMISE (Product Realization and Environmental Manufacturing Innovative System) 2002 Award #0225925,, NSF-NER 2002 Award #BES- 0210681 under the Nanoscale Science and Engineering (NSE); Nanoscale Exploratory Research (NER) Program, NSF-Partnership for Advancing Technology in Housing (PATH) 2001 Award #0122108, NSF 2002 Award #DMR-0216865, under the Instrumentation for Materials Research (IMR) Program, USDA-NRI (Grant No. 2001-35504-10734) and GREEEN (Generating Research and Extension to meet Economic and Environmental Needs) (Grant Nos. GR01-037 and GR02-066), USDA-MBI Award No. 200234189-12748-S4057 for the project “Bioprocessing for Utilization of

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Agricultural Resources” and Michigan State University’s start-up funding to A.K. Mohanty.

References 1. Wedin, R., Chemistry on a High-Carb Diet, American Chemical Society, Washington, D.C. 2004, pp. 30–35. 2. Gross, R.A. and Karla, B., Biodegradable polymers for environment, Science, 297, 803–807, 2002. 3. Mohanty, A.K., Misra, M., and Hinrichsen, G., Biofibers, biodegradable polymers and biocomposites: an overview, Macromol. Mater. Eng., 276/277, 1–25, 2000. 4. Mohanty, A.K., Misra, M., and Drzal, L.T., Sustainable Bio-Composites From Renewable Resources: Opportunities and Challenges in the Green Materials World, J. Polym. Environ., 10, 19–26, 2002. 5. Netravali, A.N. and Chabba, S., Composites get greener, Mater. Today, April 2003, pp. 22–29. 6. Plant/Crop-based Renewable Resources 2020, URL: http://www.oit.doe. gov/agriculture/pdfs/vision2020.pdf. 7. The Technology Roadmap for Plant/Crop-based Renewable Resources 2020, URL: http://www.oit.doe.gov/agriculture/pdfs/ag25942.pdf. 8. Drumright, R.E., Gruber, P.R., and Henton, D.E., Polylactic acid technology, Adv. Mater., 12, 1841–1846, 2000. 9. Petrovic, Z., Guo, A., and Javani, I., Process for the Preparation of Vegetable Oil Based Polyols and Electroninsulating Casting Compounds Created from Vegetable Oil Based Polyols, U.S. Patent 6,107,433, 2000. 10. Thomas, S.M., Biomass Grass makes for composite Cars, Mater. World, 9, 24, 2001. 11. Berenberg, B., Natural Fibers and Resins Turn Composites Green, Compos. Technol., 13, 2001. 12. Mapleston, P., Automakers see strong promise in natural fiber reinforcements (thermoplastic reinforced with natural fiber), Modern Plast., 76, 73–74, 1999. 13. Mohanty, A.K., Wibowo, A., Misra, M., and Drzal, L.T., Effect of Process Engineering on the Performance of Natural Fiber Reinforced Cellulose Acetate Biocomposites, Compos. Part A: Appl. Sci. Manufac., 35, 363–370, 2004. 14. Mehta, G., Mohanty, A.K., Misra, M., and Drzal, L.T., Biobased resin as a toughening agent for biocomposites, Green Chem., 6, 254–258, 2004. 15. Thayer, A.M., Living and Loving Life Sciences, Chem. Eng. News, 76, Nov. 23, 1998, pp. 17–24. 16. Thayer, A.M., Chasing the Innovation Wave, Chem. Eng. News, 77, Feb. 8, 1999, pp. 17–22. 17. Gutowski, T.G., Murphy, C.F., Allen, D.T., Bauer, D.J., Bras, B., Piwonka, T.S., Sheng, P.S., Sutherland, J.W., Thurston, D.L., and Wolf, E.E., WTEC Panel Report on Environmentally Benign Manufacturing, NSF-DOE Report, April 2001. 18. Fiber reinforced plastics use, Plast. News, August 2002. 19. Barghoorn, P., Stebani, U., and Balsam, M., Trends in Polymer Chemistry 1997. Part 2: Applied Polymer Chemistry, Acta Polymerica, 49, 266–267, 1998.

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20. Thomas, S.M., Biomass Grass makes for composite Cars, Mater. World, 9, 24, 2001. 21. Liu, W., Mohanty, A.K., Drzal, L.T., Askeland, P., and Misra, M., Effect of alkali treatment on the structure, morphology and thermal properties of native grass fibers as reinforcements for polymer matrix composites, J. Mater. Sci., 39, 1051–1054, 2004. 22. Huda, M.S., Mohanty, A.K., Drzal, L.T., Misra, M., and Schut, E., Physicomechanical Properties of "Green" composites from Poly(lactic acid) and Cellulose Fiber, Proceedings of the 10th Annual Global Plastics Environmental Conference (GPEC 2004), Feb. 18 and 19, 2004, Detroit, MI, Paper # 11. 23. Mohanty, A.K., Drzal, L.T., Ferguson, K.J., Dale, B.E., Misra, M., and Schalek, R., Explosion Treatment of Corn Stalk Fibers and Their Characterizations: A New Look At Value Added Applications, Polym. Preprint—Polym. Chem. Div., Am. Chem. Soc., 43, 749–750, 2002. 24. Selke, S.E. and Wichman, I., Woodfiber/polyolefin composites, Compos. Part A: Appl. Sci. Manufac., 35, 321–326, 2004. 25. Smith, P., Proceedings of the 6th International Conference on Woodfiber–Plastic Composites, Forest Products Society, Madison, WI, May 2001, pp. 13–17. 26. Harper, D. and Wolcott, M.P., Wolcott, "Interaction between coupling agents and lubricants in wood-polypropylene composites, Compos. Part A: Appl. Sci. Manufac., 35, 385–394, 2004. 27. Rouchi, A.M., Lignin and Lignan Biosynthesis, Chem. Eng. News, 78, 2000, pp. 29–32. 28. Narayan, R., Commercialization technology: a case study of starch based biodegradable plastics, in Paradigm for Successful Utilization of Renewable Resources, Sessa, D.J. and Willett, J.L., Eds., AOCS Press, Champaign, IL, 1998, p. 78. 29. Hocking, P.J., The classification, preparation, and utility of degradable, J. Macromol. Sci. — Rev. Macromol. Chem. Phys., C32, 35–54, 1992. 30. Wilkinson, S.L., Nature`s Pantry is open for business, Chem. Eng. News, Jan. 22, 2001, pp. 61–62. 31. Misra, M., Mohanty, A.K., and Drzal, L.T., Environmentally-Friendly Composites from Jute and Mater-Bi, SAMPE, Society for the Advancement of Material and Process Engineering, Dearborn, MI, USA, Sept. 12–14, 2000. Full paper published in the Proceedings, 2000, pp. 189–196. 32. Mohanty, A.K., Wibowo, A., Misra, M., and Drzal, L.T., Effect of Process Engineering on the Performance of Natural Fiber Reinforced Cellulose Acetate Biocomposites, Compos. Part A: Appl. Sci. Manufac., 35, 363–370, 2004. 33. Wibowo, A.C., Mohanty, A.K., Misra, M., and Drzal, L.T., Chopped Industrial Hemp Fiber Reinforced Cellulosic Plastic Bio-composites: Thermo-Mechanical and Morphological Characterization, Ind. Eng. Chem. Res., 43, 4883–4888, 2004. 34. John, J. and Bhattacharya, M., Properties of reactively blended soy protein and modified polyesters, Polym. Int., 48, 1165–1172, 1999. 35. Williams, G.I. and Wool, R.P., Composites from natural fibers and soy oil resins, Appl. Compos. Mater., 7, 421–432, 2000. 36. Liu, W., Mohanty, A.K., Misra, M., and Drzal, L.T., Injection molded ‘Green’ composites from native grass and soy based bioplastic, American Society for Composites (18th Annual Technical Conference), Oct. 19–22, 2003, Gainesville, FL (Full Paper Published: Proceedings of the American Society for Composites 2003, Sankar, B.V., Ifju, P.G., and Gates, T.S., Eds.) 37. Tummala, P., Mohanty, A.K., Misra, M., and Drzal, L.T., Eco-composite Materials from Novel Soyprotein-Based Bioplastics and Natural Fibers,

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39. 40. 41. 42.

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46. 47. 48. 49.

50. 51. 52. 53. 54. 55. 56.

57. 58.

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Proceedings of the 14th International Conference on Composite Materials (ICCM–14), San Diego, CA, USA, July 14–18, 2003 (Full paper published in the ICCM–14 Conference Proceedings, #1759). Lodha, P. and Netravali, A.N., Characterization of interfacial and mechanical properties of “green” composite with soy protein isolate and ramie fiber, J. Mat. Sci., 37, 3657–3665, 2002. Leaversuch, R., Biodegaradable polyester: packaging goes green, Plast. Technol., 48, p. 66, 2002. Potts, J.E., Clendinning, R.A., Ackart, W.B., and Niegish, W.D., Biodegradability of synthetic polymers, Polym. Sci. Technol., 3, 61, 1973. Averous, L., Moro, L., Dole, P., and Fringant, C., Properties of thermoplastic blends: starch-polycaprolactone, Polymer, 41, 4157–4167, 2000. Kweon, D.K., Kawasaki, N., Nakayama, A., and Aiba, S., Preparation and characterization of starch/polycaprolactone blend, J. Appl. Polym. Sci., 92, 1716–1723, 2004. Nitz, H., Semke, H., Landers, R., and Mulhaupt, R., Reactive extrusion of polycaprolactone compounds containing wood flour and lignin, J. Appl. Polym. Sci., 81, 1972–1984, 2001. Marshall, D., Eur. Plast. News., Mar. 1998, p. 23. Carothers, W.H., Dorough, G.L., and Van Natta, F.J., Studies of polymerization and ring formation. x. The reversible polymerization of six-membered cyclic esters, J. Am. Chem. Soc., 54, 761–772, 1932. Lowe, C.E., Preparation of High Molecular Weight Polyhydroxy Acetic Ester, U.S. Patent 2,668,162, 1954. Lipinsky, E.S. and Sinclair, R.G., Is lactic acid a commodity chemical? Chem. Eng. Prog., 82, 26–32, 1986. Drumright, R.E., Gruber, P.R., and Henton, D.E., Polylactic acid technology, Adv. Mater. 12, 1841–1846, 2000. Gruber, P.R., Hall, E.S., Kolstad, J.J., Iwen, M.L., Benson, R.D., and Borchardt, R.L., Continuous Process for Manufacture of Lactide Polymers with Controlled Optical Purity, U.S. Patent 5,142,023, 1992 (to Cargill). Enomoto, K., Ajioka, M., and Yamaguchi, A., Polyhydroxycarboxylic Acid and Preparation Process Thereof, U.S. Patent 5,310,865, 1994 (to Mitsui Tuatsu). Lanzillotta, C., Pipino, A., and Lips, D., New functional biopolymer natural fiber composite from agricultural resources, ANTEC-2002, 2002. Plackett, D., Andersen, T.L., Pedersen, W.B., and Nielsen, L., Biodegradable composites based on L-polylactide and jute fibres, Compos. Sci. Technol., 63, 1317, 2003. Oksman, K., Skrivfars, M., and Selin, J.-F., Natural fibres as reinforcement in polylactic acid (PLA) composites, Comp. Sci. Technol., 63, 1317, 2003. Pool, R., In search of Plastic Potato, Science, 245, 1187, 1989. Hocking, P.J. and Marchessault, R.H., Chemistry and Technology of Biodegradable Polymers, Blackie & Son, Glasgow, 1994, p. 48. Amass, W., Amass, A., and Tighe, B., A review of biodegradable polymers: uses, current developments in the synthesis and characterization of biodegradable polyesters, blends of biodegradable polymers and recent advances in biodegradation studies, Polym. Int., 17, 89, 1998. Avella, M., Immirzi, B., Malinconico, M., Martuscelli, E., and Volpe, M.G., Reactive blending methodologies for biopol, Polym. Int., 39, 191, 1996. King, P.P., Biotechnology: an industrial view, J. Chem. Technol. Biotechnol., 32, 2, 1982.

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59. Fernandes, E.G., Pietrini, M., and Chiellini, E., Bio-based polymeric composites comprising wood flour as filler, Biomacromolecules, 5, 1200–1205, 2004. 60. Mohanty, A.K., Khan, M.A., Sahoo, S., and Hinrichsen, G., Effect of chemical modification on the performance of biodegradable Jute yarn–Biopol® composites, J. Mater. Sci., 35, 2589–2595, 2000. 61. Luo, S. and Netravali, A.N., Interfacial and mechanical properties of environment -friendly "green" composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin, J. Mater. Sci., 34, 3709–3719, 1999. 62. Grigat, G., Koch, R., and Timmermann, R., BAK 1095 and BAK 2195: completely biodegradable synthetic thermoplastics, Polym. Degrad. Stab. 59, 223, 1998. 63. Polym. News, 23, 129, 1998. 64. Marshall, D., Eur. Plast. News, Mar. 1998, p. 23. 65. Frankfurter Allegemeine Zeitung, 24 Dec. 2001, p.19. 66. http://www.basell.com/portal/site/basell/index.jsp 67. Bastioli, C., Properties and applications of Mater-Bi starch-based materials, Polym. Degradation Stab., 59, 263–272, 1998. 68. http://www.dow.com/webapps/lit/litorder.asp?filepathtone/pdfs/noreg/ 321-00052.pdf&pdftrue 69. Grigat, E., Koch, R., and Timmermann, R., BAK 1095 and BAK 2195: completely biodegradable synthetic thermoplastics, Polym. Degradation Stab., 59, 223–226, 1998. 70. http://www.eastman.com/Product_Information/ProductHome.asp? Product1469&familyGMN 71. Asrar, J. and Gruys, K.J., Biodegradable polymer (Biopol), Biopolymers, Doi, Yoshiharu, Steinbruchel, Alexander, Eds., 4, 53–90, 2002. 72. Municipal Solid Waste in the United States: 2001 Facts and Figures Executive Summary, U.S. Environmental Protection Agency, http://www.epa.gov/ epaoswer/non-hw/muncpl/pubs/msw-sum01.pdf 73. American Plastic Council 2002 Percentage Distribution of Thermoplastic Resin Sales & Captive use by Major Market. http://www.americanplasticscouncil. org/benefits/economic/PIPSmajmktchart_2002.pdf 74. Impact Marketing Consultants Inc. Manchester Center, VT; Plast. News, Sept. 9, 2002. 75. Miyagawa, H., Mohanty, A.K., Misra, M., and Drzal, L.T., Thermo-physical and Impact Properties of Epoxy Containing Epoxidized Linseed Oil: Part 1: Anhydride-cured Epoxy, Macromol. Mater. Eng., 289, 629–635, 2004. 76. Miyagawa, H., Mohanty, A.K., Misra, M., and Drzal, L.T., Thermo-physical and Impact Properties of Epoxy Containing Epoxidized Linseed Oil: Part 2: Aminecured Epoxy, Macromol. Mater. Eng., 289, 636–641, 2004. 77. Stevens, E.S., Green Plastics, Princeton University Press, Princeton, 2002. 78. Mase, T., Drzal, L.T., and Jurek, B., Development of a Heat/Pressure Formable Wood Fiber Thermoplastic Composite from Recycled Paperboard, Proceedings of the 8th Annual Global Plastics Environmental Conference, GPEC, SPE Environmental Division, 2002, pp. 245–249. 79. Mapleston, P., Automakers see strong promise in natural fiber reinforcements (thermoplastic reinforced with natural fiber), Mod. Plast., pp. 73–74, 1999. 80. Grown to Fit the Part, DaimlerChrysler High Tech. Report, 1999, pp. 82–85. 81. Green door-trim panels use PP and natural fibers, Plast. Technol., 46, 27, 2000. 82. Broge, J.L., Natural fibers in automotive components, Automotive Eng. Int., 108, 120, 2000.

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83. Brouwer, W.D., Natural Fibre Composites: Where Can Flax Compete With Glass, SAMPE J., 36, 18–23, 2000. 84. Powers, W.F., Automotive materials in the 21st century, Adv. Mater. Process, 157, 38–41, 2000. 85. Eckert, C. (Kline & Co, NJ), Opportunities for natural fiber in plastic composites, Oct. 4–6, 2000, Memphis, TN, USA. 86. Personal correspondence with Jim Morton, Principia Partners, Exton, Pennsylvania, USA. The data and projections are based on multiclient report and study released to industry in 2003. 87. Mohanty, A.K. and Misra, M., Studies on Jute Composites–A Literature Review, Polym.—Plast. Technol. Eng., 34, 729–792, 1995. 88. Drzal, L.T., Mohanty, A.K., and Misra, M., Environmentally Benign Powder Impregnation Processing and Role of Novel Water Based Coupling Agents in Natural Fiber-Reinforced Thermoplastic Composites, Polym. Preprint—Polym. Chem. Div. Am. Chem. Soc., 42, 31–32, 2001. 89. Mohanty, A.K., Drzal, L.T., and Misra, M., Novel Hybrid Coupling Agent as Adhesion Promoter in Natural Fiber Reinforced Powder Polypropylene Composites, J. Mater. Sci. Lett., 21, 1885–1888, 2002. 90. Mohanty, A.K., Drzal, L.T., and Misra, M., Engineered Natural Fiber Reinforced Composites: Influence of Surface Modifications and Novel Powder Impregnation Processing, J. Adhes. Sci. Technol., 16, 999–1015, 2002. 91. Mohanty, A.K., Misra, M., and Drzal, L.T., Surface modifications of natural fibers and performance of the resulting biocomposites: an overview, Compos. Interface, 8, 313–343, 2001. 92. Bisanda, E.T.N. and Ansell, M.P., The effect of silane treatment on the mechanical and physical properties of sisal-epoxy composites, Compos. Sci. Technol., 41, 165, 1991. 93. Gassan, J. and Bledzki, A.K., Possibilities for improving the mechanical properties of jute/epoxy composites by alkali treatment of fibres, Compos. Sci. Technol., 59, 1301, 1999. 94. Prasad, S.V., Pavithran, C., and Rohatgi, P.K., Alkali treatment of coir fibers for coir-polyester composites, J. Mater. Sci., 18, 1443, 1983. 95. Felix, J. and Gatenholm, P., The nature of adhesion in composites of modified cellulose fibers and polypropylene, J. Appl. Polym. Sci., 42, 609, 1991. 96. Gassan, J. and Bledzki, A.K., The influence of fiber-surface treatment on the mechanical properties of jute-polypropylene composites, Compos. Part A: Appl. Sci. Manufac., 28A, 1001, 1997. 97. Sanadi, A.R., Caulfield, D.F., Jacobson, R.E., and Rowell, R.M., Renewable Agricultural Fibers as Reinforcing Fillers in Plastics: Mechanical Properties of Kenaf Fiber-Polypropylene Composites, Ind. Eng. Chem. Res., 34, 1889, 1995. 98. Keener, T.J., Stuart, R.K., and Brown, T.K., Maleated coupling agents for natural fibre composites, Compos. Part A: Appl. Sci. Manufac., 35, 357–362, 2004. 99. Gerngross, T.U. and Slater, S.C., How green are green plastics? Sci. Am., August 2000, pp. 37–41. 100. Scott, G. and Wiles, D.M., Programmed-Life Plastics from Polyolefins: A New Look at Sustainability, Biomacromolecules, 2, 615, 2001, Published on web Mar. 8, 2001. 101. Gruber, P.R., Glassner, D., and Vink, E., Comparing the sustainability of petrochemical-based polymers to renewable resources–based polymers, Polym. Mater. Eng., 86, 337, 2002.

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102. Joshi, S.V., Drzal, L.T., Mohanty, A.K., and Arora, S., Are Natural Fiber Composites Environmentally Superior to Glass Fiber Reinforced Composites?, Compos. Part A: Appl. Sci. Manufac., A35, 371–376, 2004. 103. Jimenez–Gonzalez, C., Kim, S., and Overcash, M., Methodology for Developing Gate to Gate Life Cycle Inventory Information, Int. J. Life Cycle Assessment, 5, 153–159, 2000. 104. Kim, S., Hwang, T., and Lee, K., Allocation for Cascade Recycling System, Int. J. Life Cycle Assessment, 2, 217–222, 1997. 105. Kim, S. and Dale, B.E., Cumulative Energy and Global Warming Impact from the Production of Biomass from Biobased Products, J. Ind. Ecol., 7, 147–162, 2004.

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2 Plant Fibers as Reinforcement for Green Composites

Alexander Bismarck, Supriya Mishra, and Thomas Lampke

CONTENTS 2.1 Introduction 2.2 Plant Fiber Composition and Structure 2.3 Fiber Production Chain 2.4 Agricultural Fiber Crop Cultivation 2.4.1 Harvesting Bast Fiber Crops 2.4.2 Fiber Extraction, Separation and Processing 2.4.2.1 Dew or Field Retting 2.4.2.2 Stand-Retting 2.4.2.3 Cold-Water Retting 2.4.2.4 Warm-Water Retting 2.4.2.5 Mechanical or Green Retting 2.4.2.6 Wet-Retting Processes 2.4.2.6.1 Ultrasound Retting 2.4.2.6.2 Steam Explosion Method 2.4.2.6.3 The Duralin Process 2.4.2.6.4 Enzyme Retting 2.4.2.6.5 Chemical and Surfactant Retting 2.5 Impact on Fiber Properties: Different Fiber Separation/Retting Procedures 2.6 Fiber Treatment and Modification 2.7 Bast Fibers 2.7.1 Flax (Linum usitatissimum L., Linaceae) Fibers 2.7.2 Hemp (Cannabis sativa L., Cannabaceae) Fibers 2.7.3 Jute (Corchorus capsularis, Tiliaceae) Fibers 2.7.4 Kenaf (Hibiscus cannabinus L., Malvaceae) Fibers 2.7.5 Ramie (Boehmeria nivea L. and Boehmeria viridis, Urticaceae) Fibers 2.7.6 (Stinging) Nettle (Urtica dioica L., Urticaceae) Fibers

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2.8

Leaf Fibers 2.8.1 Sisal (Agave sisalana, Liliaceae) Fibers 2.8.2 Henequen (Agave fourcroydes, Liliaceae) Fibers 2.8.3 Pineapple (Anannus comosus, Bromeliaceae) Leaf Fibers (PALF) 2.8.4 Abaca (Musa textilis Nee, Musaceae) Fibers 2.8.5 Oil Palm (Elaeis guineensis, Palmacea) Fibers 2.9 Seed Fibers 2.9.1 Cotton (Gossypium spp., Malvaceae) 2.10 Fruit Fibers 2.10.1 Coconut Husk or Coir (Cocos nucifera, Palmae) Fibers 2.11 Stalk Fibers: (Cereal) Straw Fibers 2.12 Conclusion 2.13 Outlook Acknowledgments References

ABSTRACT Plant fiber crops belong to the earliest known cultivated plants. They were cultivated for fiber production and were extensively developed through breeding and selection according to the human needs and values. These fibers used to possess great agricultural importance for the production of textiles until the late 19th century. However, the production of cheap synthetic textile fibers nearly terminated the production of traditional fiber crops, especially in Western Europe and North America. The increasing environmental awareness, growing global waste problems, the continuously rising high crude oil prices motivated governments to increase the legislative pressure; see, for instance, the European Union End-ofLife Vehicles* as well as Waste Electrical and Electronic Equipment (WEEE) Directive.† This in turn prompted researchers, industry and farmers to develop concepts of environmental sustainability and reconsider renewable resources. As a result of new legislation, the composite and polymer manufacturers, the processing industry and end-users but also the local communities will need to move away from traditional materials. New strategies will have to be developed for environmentally and economically viable materials manufacturing and processing, but also reuse and recycling. Composites with moderate strength will perform for many noncritical structural applications in the automotive and electronic, but also for packaging, housing and building industry. Green composites made entirely from renewable agricultural * Directive 2000/53/EC of the European Parliament and of the Council of 18 September 2000 on end-of life vehicles, link in: http://europa.eu.int/eur-lex/pri/en/oj/dat/2000/l_269/ l_26920001021en00340042.pdf accessed on: 01.09.2004. † Directive 2002/96/EC of the European Parliament and of the Council of 27 January 2003 on waste electrical and electronic equipment (WEEE), link in: http://europa.eu.int/eur-lex/pri/ en/oj/dat/2003/l_037/l_03720030213en00240038.pdf accessed on: 27.05.2004.

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resources could offer a unique alternative for these applications. Plant fibers will be used as the reinforcing phase in such composites. They offer a real alternative to the commonly used synthetic reinforcing fibers, such as carbon, glass or aramid, because of their low density, good mechanical properties, abundant availability and problem-free disposal. Farmers might benefit from fiber crops because of their quick turnaround time. Fiber crops often produce very long fibers. The energy consumption for fiber crop cultivation, harvesting and fiber separation is much lower than the energy needed to manufacture synthetic fibers. However, there are also some drawbacks related to the use of plant fibers as reinforcement for polymers. Restrictions for the successful exploitation are their high moisture absorption, low microbial resistance and low thermal stability. Further research is necessary to gain a better understanding to develop sustainable economically viable materials processes for composites and to design innovative products of common interests.

2.1

Introduction

Fiber crops have accompanied human society since the start of our time. In early history, humans collected the raw materials for ropes and textiles from the wild. Later societies learned to cultivate such crops. Plant fiber crops are among the earliest known cultivated plants and humans continued the domestication of these crops over millennia.1 Fiber crop varieties have been extensively developed through breeding and selection, according to the societies’ needs and values. For instance, hemp and linen fragments were found in Neolithic sites in Syria, Turkey, Mesopotamia (present-day Iraq), and Persia (present-day Iran), and have been carbon dated back to 8000−6000 B.C.2–4 The ancient Egyptians wrapped their corpses in linen cloth for thousands of years. Tomb paintings and hieroglyphs show and describe the production of flax, retting, spinning, and weaving as well as the treatment and dyeing of linen cloths. In Central Europe, the Swiss lake dwellers started flax cultivation and the production of linen more than 4000 years ago. Fiber crops have been bred focusing on fiber quality, climatic adaptability, and yield factors. Ingenious fiber crops, such as flax, hemp, and nettle, possessed great agricultural importance for the production of textile fibers until the late 19th century. However, the mechanization of cotton harvest, processing, and development, and the growing demand for and production of cheap synthetic textile fibers destroyed the production of traditional fiber crops. Gradually, they became less significant and almost vanished in Western Europe and North America. More recently, increasing environmental awareness, concern for environmental sustainability, and the growing global waste problem, initiation of ecological regulations and legislation such as the end-of-life vehicles Copyright © 2005 by Taylor & Francis

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regulation,5,6 the depletion of fossil fuels, and the increasingly higher price of crude oil have together created a groundswell of interest in renewable resources. Legislative pressures for greener technologies as well as customers’ demands for more environmentally friendly consumer goods are forcing materials suppliers and manufacturers to consider the environmental impact of their products at all stages of their life cycle, including materials selection processing, recycling, and final disposal. This and the worldwide availability of plant fibers7 and other abundantly accessible agrowaste is responsible for this new research interest 8–10 in the field of polymer science, engineering, and is responsible for a new interest in research in sustainable technology. Research has as its objective the development, processing and manufacturing, recycling, and disposal of “green” plastics, adhesives, polymer composites, blends, and many other industrial products from renewable resources. Consequently, a “cradle-to-grave” approach is emerging (Figure 2.1). Renewable resources11 from agricultural or forestry products form a basis for new industrial products or alternative energy sources. Plant-based fibers are already used in a wide range of products. Plant fibers find applications as textiles and geotextiles, twines and ropes, special pulps, insulating and padding materials, fleece, felts and nonwoven materials, and increasingly as reinforcement for polymers. Final product − eco-composite Production

Intermediate product fiber mats, fleece etc.

Processing

Raw fibers The Sun

Photosynthesis

Compost bio-waste natural CO2 & H2Odecomposition

Fiber extraction & separation Renewable resources fiber crops FIGURE 2.1 Truly green ecocomposites in nature’s circle of life. (Adapted from Fakten & Trends 2002 Zur Situation der Landwitschaft, Eggenfelden, 2002, p. 193.)

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The mechanical properties of plant fibers are much lower when compared to those of the most widely used competing reinforcing glass fibers (Table 2.1). However, because of their low density, the specific properties (propertyto-density ratio), strength, and stiffness of plant fibers are comparable to the values of glass fibers.12–14 The public generally regards products of renewable raw materials as environmentally friendly. A comparison of the ecobalances (see also Figure 2.2) for glass and plant fibers starting with the seed production and/or acquiring the raw material to the finished fleece showed that the energy input is up to 83% lower for mats manufactured from plant fibers.15,16 * Other ecologically relevant parameters provide some distinct advantages to the use of plant fiber crops. These are the lower ecotoxicity, due to the reduced TABLE 2.1 Characteristic Values for the Density, Diameter, and Mechanical Properties of Vegetable and Synthetic Fibers Fiber Flax Hemp Jute Kenaf Ramie Nettle Sisal Henequen PALF Abaca Oil palm EFB Oil palm mesocarp Cotton Coir E-glass Kevlar Carbon

Density (g cm⫺3)

Diameter (µm)

1.5 1.47 1.3–1.49

40–600 25–500 25–200

1.55

—

1.45

50–200 20–80

0.7–1.55

150–500

1.5–1.6 1.15–1.46 2.55 1.44 1.78

12–38 100–460 ⬍ 17 5–7

Tensile Young’s Elongation Strength (MPa) Modulus (GPa) at Break (%) 345–1500 690 393–800 930 400–938 650 468–700

27.6 70 13–26.5 53 61.4–128 38 9.4–22

2.7–3.2 1.6 1.16–1.5 1.6 1.2–3.8 1.7 3–7

413–1627 430–760 248 80 287–800 131–220 3400 3000 3400a–4800b

34.5–82.5

1.6

3.2 0.5 5.5–12.6 4–6 73 60 240b–425a

25 17 7–8 15–40 2.5 2.5–3.7 1.4–1.8

a

Ultra high modulus carbon fibers. Ultra high tenacity carbon fibers. Source: Adapted from Mohanthy, A.K. et al., Macromol. Mater. Engin., 276–277, 1, 2000. Further developed with Kennwerte von Fasern nach KOHLER & WEDLER 1996 and Kennwerte von Faserwerkstoffen nach BOBETH. Link in: http:// www.inaro.de/Deutsch/ ROHSTOFF/industrie/FASER/mechkenn.htm accessed on July 25, 2003, and Sreekala, M.S. et al., J. Appl. Polym. Sci., 66, 821, 1997. b

* This issue is still under debate.10 If plant fibers are to replace glass fibers in composite applications, then it should be made clear that overall energy balance, i.e., the energy involved in harvesting, separating, treating, and processing the fibers as well as their incorporation in composites plus their disposal is less compared to glass and glass fiber composite processing.

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Environment Resources Recycling

Raw material production

Transport Storage

Processing & production

[waste] Transport

Transport Application

Storage

Storage

[harmful]

Transport

[harmful]

Transport

Emissions

Storage

Emissions

Storage

Waste usage or removal

[harmful]

[harmful]

Emissions

Emissions

Environment FIGURE 2.2 Ecological product life cycle. (Adapted from Bennauer, U. and Dyckhoff, H., Katalyse-Institut (Hrsg.), Hanf & Co. Die Renaissance der heimischen Faserpflanzen, Verlag Die Werkstatt, Göttingen, 1995, p. 75.)

need for pesticides and herbicides, and, therefore, lower CO2 emissions from diesel fuel during farming. However, overall energy and ecological balance depends strongly on the final product, its production process, and the circumstances of the journey of life of the product.17 For example, the contribution of hemp long-fiber textiles to emissions of CO2 (greenhouse effect) strongly depends on the efficiency of the fiber processing, the spinning equipment, and the design life of the final product. This dependency prevents simplified conclusions on the ecological superiority of plant fibers.18 Yet, flax and/or hemp fibers are up to 40% cheaper compared to glass fibers.15 Moreover, plant fibers are nonabrasive to mixing and molding equipment, which can contribute to significant cost reductions.13 These are the primary advantages of using such fibers as reinforcements for polymer matrix composites. Additionally, plant fibers are of less concern to occupational health and safety during handling than other fibrous materials. The favorable aspect ratios and high specific properties at low cost make them an ecologically-friendly alternative to conventional reinforcing fibers in composite materials.19 The ecological character, the biodegradability, and the price of plant fibers are very important for their acceptance in large volume engineering markets such as the automotive and construction industry. Innovative plant fiber-reinforced polymers are rapidly finding more and more applications in secondary structural applications20 especially for various applications in the automotive industry.21–25 Plant fibers are currently used in considerable quantities in various applications in the automotive industry only in the interior of passenger cars and truck cabins. For example, fiber crops, such as flax and hemp, currently grown in the European Union (EU) (and in North America) provide an alternative to the overproduction of food crops and land division. The most important end user of natural Copyright © 2005 by Taylor & Francis

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fiber products in Europe is the automobile industry (1999: 14,000 t (Germany/Austria), 2000: 17,000 t natural fibers). Plant fibers are used as trim parts in door panels or cabin linings. Coir fibers bonded with natural latex are used as seat cushions. Plant fibers are increasingly used for thermoacoustic insulation purposes. These insulating materials are mainly based on recycled textiles and have high fiber content exceeding 80 wt%.26 At the present time, there are very few exterior parts made from plant fiber composites. Sustainable biobased ecoproducts are products with commercial viability and environmental acceptability that are derived from renewable resources and that have recycling capabilities and triggered biodegradability.27 By incorporating plant fibers into biopolymer matrices,28 such as derivatives of cellulose, starch, shellac, lactic acid, cashew nut shell liquid, polyhydroxyalkanoates (bacterial polyesters), and soy-based plastics, new “truly green” biodegradable ecocomposites (also called biocomposites) are created. This class of materials is currently under development29–32 and heavily researched.33–42 An increased demand for renewable resources, including plant fibers, as well as sources for energy or raw materials (reactants) for paints, lacquer, varnishes, adhesives, and polymers43 can have an important socioeconomic impact. It will generate a nonfood crop source for the economic development of farming and rural- and agricultural-based areas of the world.44 The increased demand for fiber crops offers alternatives for employment and income.17 Conversely, the recently adopted EU end-of-life vehicles regulation, applying to all passenger cars and light commercial motor vehicles, threatens the most important market for fiber crops. Starting from 2015 the regulation allows only an incineration quota of 5% for disused cars. This particularly concerns composite materials, since a recycling and reutilization of the material is technically and economically impossible.45 Aggravating this situation is the regulation forbidding composite waste landfill by the end of 2004 in most EU member states. The incineration of composite waste will have imposed limits that depend on the level of the energy content of the waste. Consequently, the composite suppliers and users have to agree to accept responsibility for the recycling of the end-of-life waste by providing a “green FRP (fiber-reinforced plastics) recycling label” on their products.46 Fiber crops produced in the EU (and North America) cannot and will not compete on the global market without EU and governmental subsidies in the near future. This is due to the relatively high processing costs as compared to the synthetic reinforcing fiber materials they are supposed to substitute.47,48 The amount of subsidies in the EU is decreasing in the past couple of years (for hemp 1999/2000 €721/ha, 2000/2001 €646/ha,49,50 down to €392/ha in 2002 /200351), which makes the cultivation of fiber crops less interesting for farmers.52 Therefore, a profitable production of fiber crops in Western Europe (and probably North America) can only be realized if the production costs can be recovered without subsidies in the long term.53 One way to compete on the world market is to produce and supply high-quality fibers for high value-added products for niche applications in the textile or composite sector.54–56 Up to now the greatest barrier against increased industrial use of Copyright © 2005 by Taylor & Francis

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European-grown plant fibers is the mismatch between production costs of the fibers and their value added.47,57 Obviously, the profitable production in the long run is not sustainable unless a technological breakthrough in fiber processing can be realized. The price competition between European-grown fibers and fibers cultivated and harvested in developing countries, such as jute from India and Bangladesh, will be severe.47 The use of plant fibers as reinforcement for polymer matrix composites has also some drawbacks. One restriction to the successful exploitation of plant fibers for durable composite applications is their high moisture absorption and their low microbial resistance and susceptibility to rotting. These properties need to be considered, particularly during shipment and long-term storage, as well as during composite processing. The hydrophilic character of the fibers and, therefore, the high moisture uptake affects not only the fiber surface and bulk properties but also the properties of their composites.58–61 The high moisture absorption leads to fiber swelling, which in turn results in changing mechanical and physical properties and reduces the dimensional stability of the composite.62 The predominantly hydrophilic character of plant fibers is also the reason for incompatibility, and poor impregnation behavior of the fibers with some (e.g., polyolefins63) hydrophobic thermoplastic matrices. Another limiting factor for the extended use of lignocellulose fibers in composites is their low thermal stability. In order to avoid degradation of the fibers during processing (Figure 2.3), the temperatures are limited to ca. 200°C (taking into account that the processing time

1000

Tensile strength (MPa)

800

600

400 Treatment 180 °C

200

200 °C 220 °C

0 0

15

30 Time (min)

45

60

FIGURE 2.3 Effect of thermal stress on the mechanical properties of retted flax fibers. The fibers were exposed to air at varying temperatures. (Reprinted from Wielage, B. et al., Thermochim. Acta, 337, 169−177, 1999. With permission from Elsevier.)

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should be as short as possible),64 which further restricts the choice of the polymer matrix materials. A major drawback of plant fibers is their nonuniformity and the variability of their dimensions and of their mechanical properties (even between individual plants in the same cultivation) as compared to synthetic reinforcing fibers. A precondition for increased use of plant fibers in technically challenging applications is the availability of reproducible fiber mechanical and morphological properties. The major task to be solved, in order to boost the acceptance of plant fibers as a quality alternative to conventional reinforcing fibers is to develop fiber quality assurance protocols.65–67 Nevertheless, plant fibers as fillers and reinforcements for thermoplastics and thermosets are the fastest-growing type of polymer additives. Significant markets are emerging in building products, transportation (automotive, railroad, trucking), furniture, and marine. It has been predicted that the demand of the North American market for both wood and agricultural fiber used as plastic additives will grow from 15% to 20% per year in automotive applications, to 50% or more per year in selected building products.68

2.2

Plant Fiber Composition and Structure

Natural fibers are subdivided based on their origins, whether they are derived from plants, animals, or minerals (Figure 2.4). Plant fibers include bast (or stem or soft or sclerenchyma) fibers, leaf or hard fibers, seed, fruit, wood, cereal straw, and other grass fibers.

Natural fibers Vegetable (cellulose or lignocellulose)

Seed Fruit Cotton Coir Kapok Milkweed

Bast Leaf Wood (or stem) (or hard) Flax Hemp Jute Ramie Kenaf

Pineapple (PALF) Abaca (Manila-hemp) Henequen Sisal

Mineral fibers

Animal (protein)

Stalk Cane, grass & reed fibers Wheat Maize Barley Rye Oat Rice

FIGURE 2.4 Classification of natural fibers.

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Bamboo Bagasse Esparto Sabei Phragmites Communis

Wool/hair

Lamb's wool Goat hair Angora wool Cashmere Yak Horsehair etc.

Silk

Tussah silk Mulberry silk

Asbestos Fibrous brucite Wollastonite

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The chemical composition (Table 2.2) as well as the structure of plant fibers is fairly complicated. Plant fibers are a composite material designed by nature. The fibers are basically a rigid, crystalline cellulose microfibrilreinforced amorphous lignin and/or hemicelluloses matrix. Most plant fibers, except for cotton, are composed of cellulose, hemicelluloses, lignin, waxes, and some water-soluble compounds, where cellulose, hemicelluloses, and lignin are the major constituents.69 The major component of most plant fibers is cellulose (α-cellulose). Cellulose is a linear macromolecule consisting of D-anhydroglucose (C6H11O5) repeating units joined by β -1,4-glycosidic linkages (Figure 2.5) with a degree of polymerization (DP) of around 10,000. Each repeating unit contains three hydroxyl groups. These hydroxyl groups and their ability to hydrogen bond play a major role in directing the crystalline packing and also govern the physical properties of cellulose materials. Solid cellulose has a semicrystalline structure, i.e., consists of highly crystalline and amorphous regions. Cellulose forms slender rodlike crystalline

TABLE 2.2 Chemical Composition, Moisture Content, and Microfibrillar Angle of Vegetable Fibers

Fiber

Cellulose (wt%)

Flax 71 Hemp 70–74 Jute 61–71.5 Kenaf 45–57 Ramie 68.6–76.2 Nettle 86 Sisal 66–78 Henequen 77.6 PALF 70–82 Banana 63–64 Abaca 56–63 Oil palm EFB 65 Oil palm mesocarp 60 Cotton 85–90 Coir 32–43 Cereal straw 38–45

Hemicelluloses Lignin Pectin (wt%) (wt%) (wt%) 18.6–20.6 17.9–22.4 13.6–20.4 21.5 13.1–16.7 10–14 4–8 10

2.2 3.7–5.7 12–13 8–13 0.6–0.7

2.3 0.9 0.2 3–5 1.9

10–14 10 13.1 5–12.7 5 12–13 1 19

Moisture Microfibrillar Content Waxes Angle (wt%) (wt%) (deg) 8–12 6.2–12 12.5–13.7

1.7 0.8 0.5

7.5–17 11–17 10–22

0.3

7.5

2

10–22

11.8 10–12 5–10

14

42

11 5.7 0.15–0.25 15–31

40–45 12–20

0–1 3–4 8

5–10 2–6.2 8

7.85–8.5 8

0.6

46 — 30–49

Source: Adapted from Mohanthy, A.K. et al., Macromol. Mater. Engin., 276–277, 1, 2000. Further developed with Sreekala, M.S. et al., J. Appl. Polym. Sci., 66, 821, 1997, and Olesen, P.O. and Plackett, D.V., Natural Fibers Performance Forum, Copenhagen, 1999. Link in: http:// www. ienica.net/fibresseminar/olesen.pdf accessed on May 30, 2003. Lilholt, H. and Lawther, J.M., Vol. 1, Fiber Reinforcements and General Theory of Composites, Chou T.-W., ed., Comprehensive Composite Materials, Kelly, A. and Zweben C., eds., Elsevier Science, Amsterdam, 2000, pp. 303–325.

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H H H HO

H

H O

HO

H H

CH β1 H

H

HO O

H H

H

H H β1

H O O

HO

H O

HO O

OH β1 H H

H

H H

OH H

H

H

O

H

H

FIGURE 2.5 Cellulose.

microfibrils. The crystal structure (monoclinic sphenodic) of naturally occurring cellulose is known as cellulose I. Cellulose is resistant to strong alkali (17.5 wt%) but is easily hydrolyzed by acids to water-soluble sugars. Cellulose is relatively resistant to oxidizing agents. Hemicelluloses are polysaccharides composed of a combination of 5- and 6-ring carbon ring sugars. The polymer chains are much shorter (DP around 50 to 300) and branched, containing pendant side groups giving rise to its noncrystalline nature. Hemicelluloses form the supportive matrix for cellulose microfibrils. Hemicellulose is very hydrophilic and soluble in alkali and easily hydrolyzed in acids. Lignin is the compound that gives rigidity to the plants. It is thought to be a complex, three-dimensional copolymer of aliphatic and aromatic constituents with very high molecular weight. Its chemistry has not yet been precisely established, but most of its functional groups and building units of the macromolecule have been identified. It is characterized by high carbon but low hydrogen content. Hydroxyl, methoxyl, and carbonyl groups have been identified. Lignin has been found to contain five hydroxyl and five methoxyl groups per building unit. It is believed that the structural units of a lignin molecule are derivatives of 4-hydroxy-3-methoxy phenylpropane.70 Lignin is amorphous and hydrophobic in nature. It is a thermoplastic polymer, exhibiting a glass transition temperature of around 90°C and melting temperature at which the polymer starts to flow of around 170°C.71 It is not hydrolyzed by acids, but soluble in hot alkali, readily oxidized, and easily condensable with phenol. Plant fibers are bundles of elongated thick-walled dead plant cells. A single or elementary plant fiber is a single cell typically of a length from 1 to 50 mm and a diameter of around 10−50 µm. Plant fibers are like microscopic tubes (Figure 2.6), i.e., cell walls surrounding the center lumen. The lumen contributes to the water uptake behavior of plant fibers.72 The fibers comprise different hierarchical microstructures (Figure 2.6).73 The cell wall in a fiber is not a homogeneous membrane. It is build up of several layers: the primary cell wall that is the first layer deposited during cell growth, and the secondary cell wall (S), which again consists of three layers (S1, S2, and S3). The cell walls are formed from oriented reinforcing semicrystalline

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S1,2,3: secondary walls

S3 S2

S1

Primary wall (fibrils of cellulose in a lignin/hemicellulose matrix)

(a)

(b)

FIGURE 2.6 Structure of a vegetable (flax) fiber cell. (a) Schematic and (b) a microscopic cross section. (From Bismarck, A. et al., Polym. Composites, 23, 872, 2002. With permission of the Society of Plastics Engineers.)

cellulose microfibrils embedded in a hemicelluloses/lignin matrix of varying composition. Such microfibrils have typically a diameter of about 10−30 nm, are made up of 30 to 100 cellulose molecules in extended chain conformation, and provide mechanical strength to the fiber. The amorphous matrix phase in a cell wall is very complex and consists of hemicellulose, lignin, and in some cases pectin. The hemicellulose molecules are hydrogen bonded to cellulose and act as a cementing matrix between the cellulose microfibrils, forming the cellulose/hemicellulose network, which is thought to be the main structural component of the fiber cell. The hydrophobic lignin network affects the properties of the other network in a way that it acts as a coupling agent and increases the stiffness of the cellulose/hemicellulose composite. The cell walls differ in their composition, the ratio between cellulose and lignin/hemicellulose, and in the orientation of the cellulose microfibrils. In most plant fibers the cellulose microfibrils are oriented at an angle to the normal fiber axis called the microfibrillar angle (Figure 2.7). The characteristic value for this structural parameter varies from one plant fiber to another. The outer cell wall is porous and contains almost all of the noncellulose compounds, except proteins, inorganic salts, and coloring matter, which have been found in the fiber lumen,74 and it is this section that creates the problemspoor absorbency, poor wettability, and other undesirable textile properties.75 In most of today’s plant fiber applications, fiber bundles or strands are used rather than individual fibers. “Technical bast fibers” isolated from the plants, consist of several elementary fibers or cells, and are, therefore, fiber bundles with an average length up to a meter and diameters that are typically between 50 to 100 µm.76–78 Within a fiber bundle, the fiber cells overlap and are bonded together by pectin that gives strength to the bundle as a whole (Figure 2.8). However, the strength of the bundle structure is significantly lower than that of the individual fiber cell. Copyright © 2005 by Taylor & Francis

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Secondary wall S3

Lumen

Secondary wall S2 Helically arranged crystalline microfibrils of cellulose

Spiral angle Secondary wall S1

Primary wall Amorphous region mainly consisting of lignin and hemicellulose

Disorderly arranged crystalline cellulose microfibrils networks

FIGURE 2.7 Structure of an elementary plant fiber (cell). The secondary cell wall, S2, makes up about 80% of the total thickness. (Reprinted from Rong, M.Z. et al., Composites Sci. Technol., 61, 1437−1447, 2001. With permission from Elsevier.)

FIGURE 2.8 Overlapping flax fiber cells cemented together by pectin (optical micrograph).

The structure, microfibrillar angle, cell dimensions and defects, and the chemical composition of the plant fibers are the most important variables that determine the overall properties of the fibers.79,80 Figure 2.9 shows the averaged fiber tensile strength as a function of the cellulose content and the microfibrillar angle. In general, the tensile strength and Young’s modulus of plant fibers increase with increasing cellulose content of the fibers. The orientation of the cellulose microfibrils with respect to the fiber axis determine stiffness of the fibers. Plant fibers are more ductile if the microfibrils have a spiral orientation to the fiber axis. Fibers are inflexible, rigid, and have a high tensile strength if the microfibrils are oriented parallel to the fiber axis.81 Copyright © 2005 by Taylor & Francis

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1200

Averaged fiber tensile strength σ (MPa)

Averaged fiber tensile strength σ (MPa)

1200

1000

800

600

400

200

0

1000

800

600

400

200

0 0

20

40

60

80

Cellulose content (%)

100

0

10

20

30

40

50

60

Microfibrillar angle (°)

FIGURE 2.9 Influence of the cellulose content and the microfibrillar angle on the average tensile strength of vegetable fibers. (Adapted and modified from Flemming, M. et al., Faserverbundbauweisen. Fasern und Matrices. Springer-Verlag, Berlin, 1995, p. 159.)

Satyanarayana et al.82 established a semiempirical relationship to correlate the fiber elongation ε and the microfibrillar angle θ :

ε ⫽2.78⫹7.28⫻10⫺2 θ ⫹7.7⫻ 10⫺3θ 2 and also the tensile strength σ and microfibrillar angle θ with the cellulose content W.

σ ⫽334.005⫺ 2.830θ ⫹12.22W

2.3

Fiber Production Chain

The production chain for fiber crops can be divided into three major steps: (1) agricultural production, (2) fiber processing, and (3) fiber utilization.55 The agricultural production includes breeding, growth, and harvesting of the fiber crops as well as storing the raw material. Afterward, the raw material is processed. First the fibers have to be extracted and separated from the plant. Once the raw fibers are extracted, they need to be cleaned, refined, and

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processed into the required fiber product by spinning or weaving. Different applications require different fiber qualities. When the design life of the fiber product is finally over, it needs to be disposed of. It could be recycled and reused or incinerated to partially regain energy or it could be composted under certain circumstances. During the various steps of fiber crop and plant fiber production (Figure 2.10), a large number of factors influence the final fiber quality.55,83 The fiber crop variety (genotype) and the plants’ biology, climate, location of the origin region (i.e., soil conditions and so on), even the farmer himself, the ripeness at the harvesting time, and the harvesting machinery and fiber extraction and separation process affect the fiber quality.12,84,85 Furthermore, each stage and additional workstep in the production chain reduces the value added of the final product. Many extensive, labor-intensive steps are necessary to manufacture high-quality fibers, which makes the price of some plant fibers, as in case of European-grown flax, noncompetitive as compared to standard E-glass fibers.86 In the end, the final fiber quality will determine its end application and the production and processing costs will determine its economics. The traditional fiber crop production and the fiber processing are composed of many labor-intensive worksteps (Figure 2.11) utilizing production technology that was established shortly after World War II and, is therefore, quite unproductive.87

Species

Plant production

Crop production Location Climate Hollow fiber

Degree of ripeness

Optimum strength Lignification

Decortication

Retting or other fiber extraction methods Fiber yield

Fiber processing

Fiber degradation

Dry/mechanical Steam explosion Wet

Ultrasound

FIGURE 2.10 Factors that influence fiber quality. (Adapted from Katalyse-Institut (Hrsg.), Hanf & Co. Die Renaissance der heimischen Faserpflanzen, Verlag Die Werkstatt, Göttingen, 1995, p. 72.)

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Sowing Swathing, mowing, or pulling Drying Threshing Dew-retting Baling & storing Breaking & scutching Transportation

Hackling Combing Modification

Scutched tow

Degumming & cottonization drafting Wet spinning

Farmer

Further processing Hackled tow

Dry spinning

Fleeces, mats, etc.

Fine yarns

Coarse yarns

End User FIGURE 2.11 Processing steps of bast fiber production: from the farmer to the end user. (Adapted from Fölster, T., Textilveredelung, 30, 2, 1995 and Flemming, M. et al., Faserverbundbauweisen. Fasern und Matrices. Springer-Verlag, Berlin, 1995, p. 162.)

2.4

Agricultural Fiber Crop Cultivation

The farmer’s interest in fiber crops stems primarily from the potential as commercial fiber, the wooden core, and seed oil source. Fiber crops also offer production alternativesthey can be grown as shelter crop and they are effective pollen insulators, since they form impermeable hedges, which minimize outside pollination.88 Fiber crops have excellent crop rotation properties.52 Furthermore, only a small amount of crop residue returns back to the soil after harvesting because the entire top growth is harvested.89 In agricultural production the aim is to improve crop yields in order to improve the farmer’s income. Particularly in the field of fiber crop production, additional emphasis has been invested in agronomic aspects, such as breeding of new varieties, disease and lodging resistance, cultivation, and crop management.55,88,90,91 Fiber crop production efficiency has been and still is improving by the mechanization of soil tillage and harvesting, an optimized use of fertilizers, and the Copyright © 2005 by Taylor & Francis

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efficient use of herbicides and pesticides.55 It can be expected that new bast fiber crop varieties will be generated in the future. Such new plant varieties will contain high-quality fibers that will be easier to extract.91 However, plant fiber crops certainly have merit as an alternative crop, but they are not automatically sustainable. The modern, yield-oriented agricultural techniques for the production of fiber crops (especially cotton) require huge amounts of water, pesticides, fungicides, and herbicides. If fiber crops are grown like any other row crops, as monocrops (see also ref. 92) and even worse without crop rotation, with tillage, commercial fertilizers, herbicides, and pesticides, they are contributing to soil degradation just as other crops are89 (see also ref. 93). These factors tend to disrupt ecological balances (see also Figure 2.2). However, fiber crops as a source for plant fibers and as renewable resources could be of major importance for the world’s environment and economy (e.g., farmers’ income); however, this outcome will be decided through the global market.

2.4.1

Harvesting Bast Fiber Crops

The timing of harvesting bast fiber crops depends on whether the plants are grown for high-quality fibers or for their seeds, or both.94 The technical maturity (defined by the ratio of the cross sections of the walls to that of the whole fiber95) of hemp is reached at around the time the plant has completed producing pollen. The optimal time for harvesting, however, is at the beginning of seed maturity, about three to four weeks after the anthesis (flowering). Figure 2.12 shows increasing formation of bast fibers from the time the flax plants are in full flower (a) to the optimal time for harvesting (b). At this time the secondary fiber cell wall occupies more than 90% of the plant stalk.76 The mechanical strength of the bast fibers increases with increasing fiber ripeness. However, competing with the development of bast fibers is the

(a)

(b)

FIGURE 2.12 Optical micrographs (200:1) of cross sections through flax stalks at the time (a) of full anthesis and (b) of the harvest. (From Lampke, T., Technische Universität Chemnitz, Lehrstuhl für Verbundwerkstoffe, 2001, p. 64. With permission.)

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ever-increasing lignifications of the stalk, i.e., the fibers get coarser. Therefore, the fibers adhere much more strongly to each other and to the wood core.95 The decortication and separation of the fibers gets much more difficult if the plants are left growing well beyond optimal ripeness.95 However, it was also reported that a relatively late harvesting did not affect the tensile strength of the bark and is advantageous for mechanical decortication of green (nonretted and dried) hemp stalks.96 Normally, farmers aim to achieve maximum stem yield at best fiber quality. In the case where plants are grown for both their seeds and fibers, the actual harvesting process starts about six weeks after anthesis at a time when all seeds are properly ripened.97 The fibers obtained from plants grown for seed production are stiffer, coarser, and more brittle. Traditionally, all hemp and flax crops are harvested by pulling the plants with their roots to preserve the full fiber length (in some cases by cutting them near the ground). The plant straw remains for a certain time (to dry or to rett and dry) in swaths on the field. To enhance the efficiency of the harvesting process new harvesting machinery is currently being developed.98–100 2.4.2

Fiber Extraction, Separation and Processing

After mowing or pulling bast fiber crops, the fibers must be separated and extracted from the woody tissue of the fiber crops. The process that causes the separation of the technical fiber bundles from the central stem, which loosens the fibers from the woody tissue, is called retting. Most available methods of retting rely on the biological activity of microorganisms, bacteria, and fungi from the environment to degrade the pectic polysaccharides from the nonfiber tissue and, thereby, separate the fiber bundles. The retting or rotting of the straw is caused with time by exposure to moisture and, sometimes, by the help of a mechanical decorticator. The fiber separation and extraction process has a major impact on fiber yield and final fiber quality.101 It influences the structure, chemical composition,69,102 and properties of the fibers.103 Retting procedures can be divided into biological, mechanical, chemical, and physical fiber separation processes (Figure 2.13).

Retting

Biological Natural

Artificial

Mechanical

Physical

Green retting

STEX ultrasound retting

Dew (or field) retting Hot water or canal Cold-water retting retting FIGURE 2.13 Classification of retting processes.

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Chemical Surfactant retting NaOH retting

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2.4.2.1 Dew or Field Retting Dew or field retting83,104,105 is the most commonly applied retting process in regions that have appropriate moisture and temperature ranges. After being mown, the crops should remain on the field until microorganisms have separated the fibers from the cortex and xylem. The degradation of the cortex primarily occurs due to the action of indigenous fungi. Mycelium grows on the carbohydrate-rich tissue, utilizes the easy to access pectin, and degrades the pectin in the phloem with excreted enzymes. In order to guarantee a homogeneous retting of the plant straw, the plant has to be turned over at least once. After the retting the stalk is dried and baled. The retting process has to be stopped at the right time to prevent overretting, i.e., degradation of the cellulose of the bast fibers by the fungi, thereby lowering their tensile strength. However, underretting results in fibers that are difficult to separate and to further process. Therefore, farmers have to monitor the retting process to ensure the quality of the fibers. The duration of the dew-retting process is between 3 and 6 weeks and is highly dependent on uncontrollable weather conditions. Rainfall and humidity, sun hours and temperature, and the way the fiber crops are spread on the ground affect the retting process and, therefore, the quality of the fiber. The unpredictability of the dew-retting process leads to varying and inconsistent fiber quality and results every so often (statistically every 6 to 7 years) in complete loss of the fiber crop harvest.106 2.4.2.2 Stand-Retting A modified field-retting procedure is the thermally induced stand-retting procedure.107 Open gas flames are used to terminate the plant growth of flax crops. The plant bases are heated up to ~100°C. The plants afterward dry homogeneously within 1 to 2 days, thus allowing replacement of the weather-susceptible dew-retting procedure on the ground. Even though weather dependence and therefore risk of fiber damage decreases, costs for retting increase because additional special machinery is needed. The mechanical properties of the fibers are not affected by thermal treatment. Furthermore, such a retting procedure allows for obtaining desired fiber quality by adjusting the retting parameters. 2.4.2.3 Cold-Water Retting Traditional cold-water retting is mainly used by fiber producers in Eastern Europe. The process utilizes anaerobic bacteria that break down the pectin of plant straw bundles submerged in huge water tanks,108 ponds, hamlets or ditches, rivers, and vats. The process takes between 7 and 14 days and depends on the water type, the temperature of the retting water, and any bacterial inoculum.109 Even though the process produces high-quality fibers, environmental pollution is high due to unacceptable organic fermentation wastewaters.110 Because of that and the malodor of fermentation gases, the procedure was abolished in 1918 in Germany.83 Copyright © 2005 by Taylor & Francis

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2.4.2.4 Warm-Water Retting Warm-water retting is a form of accelerated retting and produces homogeneous and clean fibers of high quality in only 3 to 5 days. Plant bundles are soaked in warm (28° to 40°C) water tanks. The process as such is prohibited in Europe because of the extreme impact on the environment.83 After sufficient retting the bast fibers have to be separated from the woody parts. The shieves or hurds are loosened and extracted from the raw fibers in a breaking and swingle or scutching process. The coarse processed fibers are passed through fluted rolls to break the woody portion into small particles. The scutching is done by mechanically beating the material with metal blades. Afterward the short fibers are separated from the long fibers by hackling or combing. All the consecutive processing stages (retting, scutching, and hackling) affect fiber properties. The physical form of the fibers (Figure 2.14) obtained after different separation steps can range from bundles to elementary fibers or even further opened, smaller fibers.111 Most of the emerging technical and industrial applications of plant fibers require sufficient quantities of constant quality with good mechanical properties at competitive prices. However, this can only be achieved if the varying raw fiber quality can be refined and adjusted using improved and flexible processes.112 In order to supply fibers that fulfill these requirements, several new fiber retting and separation techniques have been developed.

Hacking Technical fiber ∅50−100 µm

Breaking scutching

Elementary fiber ∅20−20 µm

Meso fibril ∅0.5 µm

Bast fiber bundle

Micro fibril ∅4−10 nm Flax stem ∅2−3 mm

FIGURE 2.14 Schematic of flax stalk and fiber structure. (From Bos, H.L. et al., J. Mater. Sci., 37, 1683, 2002. With permission.)

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2.4.2.5 Mechanical or Green Retting Mechanical or green retting is a much simpler and more cost-effective alternative to separate the bast fibers from the plant straw. The raw material for this procedure is either field dried but only slightly retted (2 to 3 days, but 10 days maximum) plant straw113 or technically dried straw. The bast fibers are separated from the woody part by mechanical means. Weather-dependent variations of fiber quality are eliminated. However, the produced green fibers are much coarser and less fine as compared to dew- or water-retted fibers.114 The properties of the resulting green fibers are much less sensitive to variable raw material input.115 2.4.2.6 Wet-Retting Processes Homogeneously fine and clean fibers can only be obtained from wet-retting processes. Wet-retting processes offer a wider range of parameters that allow for modifying the fibers with respect to given applications by adjusting the process to take into account varying properties of the raw material.95 2.4.2.6.1 Ultrasound Retting ECCO Gleittechnik GmbH (Seeshaupt, Germany) has developed an ultrasound-retting process to separate the bast fibers from the rough plant straw which are neither subjected to dew nor to water retting.106,116 After harvest, the stems are broken and washed. The slightly crushed stems are immersed in a hot water bath (~70°C) that contains small amounts of alkali and surfactants, and then exposed to high-intensity (1 kW) ultrasound (40 kHz). This continuous process separates the hurds from the fiber. The process separates the fibers from the woody components to a degree sufficient for technical and nontextile applications. The retting solution containing the hurds can be used to produce fuel or can be used for many other applications. One advantage of the process as compared to traditional retting methods is that green (unretted) raw material is used, thereby avoiding the unreliable dew-retting process. 2.4.2.6.2 Steam Explosion Method The steam explosion method (STEX)87,95,117,118 represents another suitable alternative to the traditional field-retting procedure.119 Under pressure and increased temperature, steam and additives penetrate the fiber interspaces of the bast fiber bundles. The center lamella is thus removed at optimum conditions of reaction. The subsequent sudden relaxation of the steam leads to an effective breaking up of the bast fiber composite which results in an extensive decomposition into fine fibers. The fineness and properties of the produced fibers are comparable to cotton fibers.83 2.4.2.6.3 The Duralin Process Another novel upgrading process for lignocellulose materials has been developed to improve the poor environmental and dimensional stability of Copyright © 2005 by Taylor & Francis

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these materials.120 This upgrading process, which was initially developed for wood, has also proved its applicability to natural fibers and has lent itself to the development of upgraded bast fibers, Duralin® flax and hemp.121 The novel upgrading process developed by Ceres B.V. (Wageningen, The Netherlands) to treat flax and hemp consists of three steps, hydrothermolysis, drying, and curing. Full-rippled (deseeded) straw fibers are used as raw material. The use of straw is beneficial for both strength and reproducibility (no dew retting required) of the treated fibers. In addition, a valuable by-product, the treated flax shives or hemp hurds, is produced from which products such as water-resistant chipboards can be made.122 The Duralin treatment consists of a steam or water-heating step of the straw at temperatures above 160°C for ~30 min in an autoclave. A drying step and a heating (curing) step above 150°C for ~2 h follows the first step. During this treatment, the hemicellulose and lignin are depolymerized into lower molecular- weight aldehyde and phenolic functionalities, which are combined by the subsequent curing reaction into a water-resistant resin that cements the cellulose microfibrils together. After the treatment the fibers can easily be separated from the stem by a simple breaking and scutching operation. The fibers obtained by these procedures are fiber bundles rather than individual fibers. The availability of an upgrading process for natural fibers could remove one of the main restrictions for the successful application of natural fibers in high-quality engineering composites. 2.4.2.6.4 Enzyme Retting Another alternative for producing high and consistent quality fibers is enzyme retting.123,124 This retting procedure uses pectin-degrading enzymes to separate the fibers from the woody tissue. The use of enzymes promotes the controlled retting of the fiber crops through the selective biodegradation of the pectinaceous substances. Crimping, mechanically splitting the stalks of non- or only slightly retted plant straw prior to enzyme retting, enhances enzyme penetration into the tissue and accelerates the degradation process. The actual retting process takes place in a tank. The enzyme activity increases with increasing temperature up to an optimum temperature above which the enzymes start to denature.125 The process lasts between 2 and 24 hours. Enzyme retting can lead to undamaged individual fibers (without kink bands) with much higher intrinsic fiber strength.126 However, thus far this process has only progressed to pilot-scale experiments probably because of the high costs of the enzymes, the equipment, and the wastewater treatment.55 2.4.2.6.5 Chemical and Surfactant Retting Chemical and surfactant retting refers to all retting processes in which the fiber crop straw is submerged in heated tanks containing water solutions of sulfuric acid, chlorinated lime, sodium or potassium hydroxide, and soda ash (sodium

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carbonate) to dissolve the pectin component. The use of surface-active agents in retting allows the simple removal of unwanted noncellulose components adhering to the fibers by dispersion- and emulsion-forming processes.127 These processes reduce the duration of retting, depending on the process conditions, to a few minutes or up to 48 hours. Chemical retting produces highquality fibers but adds costs to the final product.

2.5

Impact on Fiber Properties: Different Fiber Separation/Retting Procedures

Agronomic factors such as crop variety, seed density, soil quality, fertilization, location of the plantation and location of fibers on the stem of the plant, climate and weather conditions, and timing of the harvest (age of the plant) affect final fiber quality and their overall properties.128 Disturbances during plant growth will affect plant structure and are also responsible for the enormous scatter of mechanical plant fiber properties. The number of knobby swellings (Figure 2.15) and growth induced lateral displacement (Figure 2.16) present in plant fibers influence the tensile strength and elongation at break of the fibers obtained. The mechanical properties of plant fibers do not only depend on the factors previously mentioned. More importantly, the fiber separation process significantly determines fiber quality and its mechanical properties. Optimal biological or chemical and physical retting procedures result in a much better and easier separation of the fibers from the woody core (Figure 2.17) and,

FIGURE 2.15 Knobby swellings along flax fibers. (From Lampke, T., Technische Universität Chemnitz, Lehrstuhl für Verbundwerkstoffe, 2001, p. 64. With permission.)

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FIGURE 2.16 Growth induced lateral displacement through a flax fiber bundle. (From Lampke, T., Technische Universität Chemnitz, Lehrstuhl für Verbundwerkstoffe, 2001, p. 64. With permission.)

(a)

(b)

FIGURE 2.17 Optical micrographs (200:1) of cross sections through flax stalks after (a) 2 days of retting and (b) after 14 days of retting. (From Lampke, T., Technische Universität Chemnitz, Lehrstuhl für Verbundwerkstoffe, 2001, p. 65. With permission.)

therefore, minimize the mechanical loading on the fibers (Figure 2.18a). Mechanical loads which affect the mechanical properties of the fibers are experienced during the breaking, scutching, and hackling steps, especially during green (mechanical) retting procedures (Figure 2.18b). Mechanical overstraining of the fibers during the mechanical processing steps of fiber separation can result in the formation of kink bands (Figure 2.19) and splices (Figure 2.20) which dramatically lower the mechanical properties (tensile and compressive strength) of the fibers. Furthermore, mechanical retting results in much shorter fibers, which, in extreme cases, could be disadvantageous for further processing. Copyright © 2005 by Taylor & Francis

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100 µm

100 µm (a)

(b)

FIGURE 2.18 Single (elementary) fiber (a) well prepared and (b) damaged due to excessive mechanical stress during fiber separation. (From Kohler, R. and Kessler, R.W., Proc. of the 5th International Conference on Woodfiber–Plastic Composites, Madison, 1999, p. 33. With permission.)

FIGURE 2.19 Optical micrograph of kink bands (see arrows) introduced in an elementary flax fiber by standard isolating methods. (Adapted from Bos, H.L. and van den Oever, M.J.A., Proc. of the 5th International Conference on Woodfiber-Plastic Composites, Madison, 1999, p. 81.)

Besides the mechanical properties, the chosen retting procedure also affects the fiber morphology, their surface composition and properties,129,130 as well as their water uptake behavior.60 Not only the fiber appearance (Figure 2.21) but also the degree of disintegration, the fineness, and the amount of noncellulose components of the plant (flax) fibers depend on the fiber retting/separation process. These fiber properties can be adjusted within a broad range by the fiber retting and separation process. Copyright © 2005 by Taylor & Francis

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FIGURE 2.20 Splices at the flax fiber surface after excessive mechanical processing. (From Lampke, T., Technische Universität Chemnitz, Lehrstuhl für Verbundwerkstoffe, 2001, p. 65. With permission.)

Figure 2.21 shows several original but differently retted and separated flax fibers. Figure 2.21a shows dew-retted native flax fibers. The fibers are not well separated and are still covered and “glued” together by noncellulose compounds. Green flax fibers were separated from the nonretted raw flax straw using ultrasonification (US) (Figure 2.21b). During the separation procedure the fiber bundles are partially separated. The micrographs reveal partially uncovered structures, comparable to those of the cellulose fibers (Figure 2.21c); however, other areas are still covered by noncellulose compounds (such as waxes or fats). STEX produces relatively smooth fibers (Figure 2.21d). However, the fiber structure is still not completely laid open (e.g., fiber waxes and fats are still attached to the surface). The STEX separation process results in the dissolution of most cementing substances, leading to a better disintegration of the remaining fiber bundles into elementary fibers under the shear stress exerted during compounding in an extruder or an injection molding machine. In contrast, the fiber bundles obtained by purely mechanical separation (green fibers) do not disintegrate during compounding, causing noticeably poorer mechanical properties of the composite.112,131 In general, plant fibers have very small specific surface areas (As ≈0.5 m2/g), just slightly bigger than the calculated geometric surface area (As,geo⫽0.38 m2/g) at a fiber diameter d 苶f ≈14 µm (compare Figure 2.21) and a density of flax fibers132 of 苶 ρ ⫽1.47 g/cm3. Even though plant fibers have a very small specific surface area, the retting/separation process used affects the surface area. The higher the degree of disintegration and fineness, the larger is the

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FIGURE 2.21 Scanning electron micrographs of differently separated flax fibers. (a) Retted flax fibers, native (raw) state. (b) Green flax, separated using ultrasonification. (c) Cellulose fibers. (d) STEX (DDA) flax. (From Bismarck, A. et al., Polym. Composites, 23, 872, 2002. With permission of the Society of Plastics Engineers.)

total specific surface area measured in adsorption experiments applying the Brunauer, Emmet and Teller procedure (BET) (Table 2.3). The water absorption of some flax fibers under different relative humidity was also studied.60 Green flax fibers absorb much more water (averaged

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FIGURE 2.21 (Continued)

MC 苶 苶 ⫽ 43%) than dew-retted flax (M 苶C 苶 ⫽ 27%). However, flax fibers obtained from the novel Duralin separation process absorb much less water (M 苶C 苶 ⫽ 19%). During the Duralin process, the noncellulose components of the bast fibers are depolymerized into low molecular-weight oligomers that react in a curing step to a water-resistant resin. Copyright © 2005 by Taylor & Francis

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TABLE 2.3 Influence of Flax Fiber Retting/Separation Procedures on Fiber Surface Area, Fiber Surface Tension, and Water Uptake Behavior (Moisture Content) Fiber Green flax Dew-retted flax US flax STEX flax Duralin flax

2.6

Surface Area Surface Tension Maximum Moisture Content 苶C 苶 /%) (As /m2 g⫺1) (γf /mN m⫺1) (M 0.31 0.51 0.75 0.88

35.5 ⫾ 1.0 36.3 ⫾ 0.7 38.7 ⫾ 1.0

43 27

19

Fiber Treatment and Modification

It is well known that the performance of composites depends on the properties of the individual components and their interfacial compatibility. For numerous applications plant fibers have to be prepared or modified122,133 with the following considerations in mind: ● ● ● ● ● ●

Homogenization of the properties of the fibers Degree of elementarization and degumming Degree of polymerization and crystallization Good adhesion between fiber and matrix Moisture repellence Flame-retardant properties

These properties can be partly influenced by different fiber separation procedures, but subsequent fiber treatment processes are more influential. Several noncellulose components have to be removed to assure the compatibility of the plant fibers to the surrounding polymer matrix. Alkalization, washing, or boiling of the plant fibers in 2 to 10% sodium, potassium, or lithium hydroxide solutions leads to the removal of unwanted fiber components, which dissolve during the process. Alkalization, depending on the concentration of the alkali and the process temperature, can significantly improve the fiber’s mechanical and surface properties. After naturalizing and thoroughly washing and drying the fibers, the following changes as compared to the raw fiber material can be revealed: ● ● ●

Fibers are purified Unwanted fiber ingredients are largely removed Fiber separation ability is increased

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surfaces strongly influence the surface properties of cellulose-based materials. Most raw plant fibers are covered with hydrophobic noncellulose compounds (like waxes). It is, therefore, necessary to treat original fibers to remove these noncellulose components. Such a treatment increases the accessibility of surface groups (but not necessarily the specific fiber surface). These uncovered, reactive surface groups can be used in further chemical modification steps to increase the compatibility of natural fibers to nonpolar polymer matrices. Such further modification processes can be reactions with compatibilizing agents such as carboxylic anhydrides, isocyanates, vinylsulfone, chlorotriazine systems,134 organo silanes, and compounds that contain methylol groups or grafting of MAH-graft PP.122

2.7

Bast Fibers

The shape and size of the stem of various bast fiber crops* are different but they all contain varying amounts of fiber cells in the phloem (Figure 2.22 and

FIGURE 2.22 A schematic comparison of the stalk cross sections of flax, nettle, and hemp. (From Scheer-Triebel, M. and Léon, J., Pflanzenbauwissenschaften, 4, 26, 2000. With permission of Verlag Eugen Ulmer, Stuttgart, Germany.)

* For more digital photographs, schematics, and illustrations of fiber crops, see: Digital Flora of Texas (DFT), Vascular Plant Image Library. Link in: http:// www.csdl.tamu.edu/FLORA/ gallery.htm accessed on 26.07.2003; Kurt Stüber’s Online Library, link in: http:// caliban. mpiz-koeln.mpg.de/~stueber/stueber_library.html accessed on 26.07.2003, and index of 4246 botanical images taken by Kurt Stüber. Link in: http:// caliban.mpiz-koeln.mpg.de/~stueber/ mavica/index.html accessed on 26.07.2003.

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Figure 2.23). Long individual fibers or long fiber bundles can be obtained from many bast fiber crops at relative low cost. However, bast fibers have a nonhomogeneous cell structure than do the much shorter wood fibers (averaged fiber length 2.7 mm), which are uniform, readily available, and inexpensive. All bast fiber crops have a similar structure (see Figure 2.23).

(a)

(b) FIGURE 2.23 Micrographs of cross sections through the stems of (a) flax with bast fibers situated under the cuticula, (c) nettle (Urtica dioica) with bast fibers (arrow), and (d) hemp stalk cross sections (Micrographs (a), (b) and (d) from Lampke, T., Technische Universität Chemnitz, Lehrstuhl für Verbundwerkstoffe, 2001, p. 22; and for (c) from Dreyling, G., Institut für Angewandte Botanik, Universität Hamburg, Institut für Angewandte Botanik, Abteilung Nutzpflanzenbiologie (Arbeitsgruppe Dreyling, Germany: http: // www.biologie.unihamburg.de/ianb/nb/urtica05.html#Text accessed on June 17, 2003.)

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(c)

(d) FIGURE 2.23 (Continued)

Bast fiber crops have rigid herbaceous stalks containing nodes at regular intervals that are fluted or channeled. From the inside to the outside the stalks have a hollow core, except at joints, followed by the pith, the cambium, the phloem or parenchyma, the cortex, and finally the protective layer epidermis with the waxy cuticula (Figure 2.24). The pith is composed of a thick woody tissue that supports the plant. After harvesting this area produces the hurds or shives that can comprise up to 60 to 75% of the total mass. The cambium is the differentiating layer. Copyright © 2005 by Taylor & Francis

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FIGURE 2.24 The structure of a bast fiber crop stem: a cross section of a flax stem. (Micrograph from Lampke, T., Technische Universität Chemnitz, Lehrstuhl für Verbundwerkstoffe, 2001. With Permission.)

It is the pith on the inside which produces bast fiber cells, and it is bark on the outside. Short chlorophyll-containing cells and long bast fiber cells make up the phloem or parenchyma. The cortex is a thin wall of cells that produce no fibers but contain chlorophyll. The epidermis is the thin outside protective layer of plant cells. 2.7.1

Flax (Linum usitatissimum L., Linaceae) Fibers

Flax is one of the bast fibers grown in temperate regions. Flax for millennia has provided many important products such as fibers for textiles, oil seed, and paper and pulp. Flax has been cultivated for nearly 10,000 years and its fibers, i.e., linen, are reported to be the oldest textile known, dating back to early Mesopotamian times.3,127 Textile flax is primarily grown in Europe, Argentina, India, China, and the Commonwealth of Independent States (the states of the former Soviet Union).135 The flax plant consists of the root, stalk, and branches carrying the seed capsules. The central portion of the stalk is delimited at the bottom by the cotyledonary node and at the top end by the bottom of the branches (Figure 2.25a). Flax is dicotyledonous. In 80 to 110 days the plant grows to heights between 80 and 150 cm (Figure 2.25 and Figure 2.26). Only the central portion (up to 75% of the plant height) can be used to produce bast fibers. The flax fiber Copyright © 2005 by Taylor & Francis

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FIGURE 2.25 (a) Schematic of a flax plant: 1, branches; 2, central portion of stalk; 3, cotyledonary node; and 4, root. (From the Gesamtverband der Deutschen Versicherungswirtschaft e.V.: http:// www.tis-gdv.de/tis_e/ware/f_inhalt0.htm accessed on June 24, 2003. With permission.) (b) A flowering flax plant. (From Seigler, D.S., http:// www.life.uiuc.edu/plantbio/263/ image/Flax_Bs.jpeg accessed on July 5, 2003. With permission.)

bundles are between 60 and 140 cm long and their diameter ranges from 40 to 80 µm. The color of the flax fibers varies from light blond to gray. The fibers are very strong but also flexible. Flax is a particularly inextensible fiber. It stretches only slightly as tension increases. Within the small degree of extension, flax is an elastic fiber. Flax fiber properties are controlled by the molecular fine structure of the fibers which are affected by growing conditions136 and the retting procedure that is applied.137 A nature flax cell wall (for composition see ref. 138) consists of ~70−75% cellulose, 15% hemicelluloses and 10−15% pectic material, about 2% lignin, and 2% waxes. The decomposition temperature of flax fibers is above 200°C,64 as can be seen in Figure 2.27. Flax fibers lose strength gradually on exposure to sunlight. Copyright © 2005 by Taylor & Francis

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FIGURE 2.26 Photograph of flax plantation. Depending upon the flax variety, it can be cultivated for production of flax fibers or linseed oil. (Courtesy of www.inaro.de.)

100

Mass (%)

80 60 40 20 0 200

4 K/min 5 K/min 7 K/min 10 K/min 12 K/min 15 K/min

250

300 350 Temperature (°C)

400

450

FIGURE 2.27 Thermogravimetry STEX (DDA) flax fibers in helium atmosphere at a heating rate of 4−15 K/min.

Flax can withstand dilute weak acids, but is attacked by hot dilute acids or cold concentrated acids. It has good resistance to alkaline solutions. Flax fibers are quite expensive because of the many labor-intensive production steps. Still, flax fibers are used as reinforcement for high value-added Copyright © 2005 by Taylor & Francis

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FIGURE 2.28 (a) Green-Line suitcases for a variety of instruments. (Courtesy of Jakob Winter GmbH, Satzung, Germany and Kompetenzzentrum Strukturleichtbau, TU Chemnitz, Germany. From Odenwald, S. and Griesmann, G., Proc. 10. Internationale Tagung “Stoffliche Verwertung nachwachsender Rohstoffe,” 2002, Chemnitz, p. 157. With permission.) (b) Biobased helmets for various applications. (From Schuberth Helme GmbH, Braunschweig, Germany: http:// www.riko.net/html/produkte/produkte_2.php3 accessed on July 21, 2003.)

composite products in fields where the composites are exposed only to a medium stress range (Figure 2.28a,b),139 especially for interior automotive components (Figure 2.29).140

2.7.2

Hemp (Cannabis sativa L., Cannabaceae) Fibers

Hemp (Figure 2.30) is another famous bast fiber crop that is having an impressive comeback after being legislated out of existence.141 The flowering tops, and to a lesser extent the leaves of hemp and hemp varieties produce resin secretions containing the narcotic 9-∆ tetrahydrocannabinol (THC, a diterpene containing a hydroxyl functionality), for which marijuana and hashish are famous. Hemp cannot be used as a narcotic since it produces virtually no THC (less than 1%), whereas marijuana varieties produce between 3 and 20% THC.142 Copyright © 2005 by Taylor & Francis

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FIGURE 2.29 Photograph of flax fiber composite product. Flax fibers find applications as reinforcement for interior door panelings for passenger cars to replace glass fibers. (Courtesy of www.inaro.de.)

FIGURE 2.30 Schematic (a) and photograph (b) of a hemp plant. (Courtesy of www.inaro.de.)

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Hemp is native to Central Asia and has been cultivated for more than 12,000 years.83 Wild hemp and cannabinoide hemp varieties are typically dioecious. However, examples of monoecious plants, single plants having both male and female inflorescence, can also be found within any given population.143 Planting and producing of hemp crops has many advantages for farmers. Hemp needs almost no or minimal herbicides, pesticides, fungicides, and fertilizer. The plants have impressive growth rates so that they quickly cover the ground and, therefore, suppress weeds and some soil-borne pathogens. Hemp also restores nutrients to the soil, which are then available to the next crop planted in rotation. The plant is very deep rooting and is a beneficial break crop because it cleans the ground and provides a good disease break while helping soil structure. The different hemp varieties grow to heights between 1.2 and 5 m135 and have bast contents in the stalk from 28 to 46%.144 The male plants (fimble hemp) ripen earlier and must be harvested earlier. The female plants are more highly branched and bear denser foliage. All monoecious varieties have the same rate of development. The dioecious male plants yield particularly fine fibers. Dioecious hemp varieties yield better textile fibers, while the monoecious varieties are preferred by the pulp and paper industry.143 Strands of hemp fibers may be 1.8 m or longer. The individual or elementary fibers are on average 13 to 25 mm long. Elementary fibers are thick-walled and polygonal in cross section, with joints, cracks, swellings, and other irregularities on the surface. The whitish to yellow hemp fibers can be up to 50 mm in length, are highly water-resistant, and have good tensile strength. Hemp fibers are coarser as compared to flax and difficult to bleach. The fibers have an excellent moisture resistance and rot only very slowly in water.83 Hemp fibers have high tenacity (about 20% higher than flax) but low elongation at break. The textile industry keeps using hemp fibers because of their excellent properties. Hemp textiles are resistant to water and mechanical stress, insensitive during washing, and moths will not attack them.83 Products made from hemp fibers include specialty paper, textiles, construction materials, plastics and composites, food, medicine, and fuel (Figure 2.31).

2.7.3

Jute (Corchorus capsularis, Tiliaceae) Fibers

Jute fibers are the most important plant fibers alongside cotton. Jute (Figure 2.32) is native to the Mediterranean from where it spread throughout the Near and Far East. Jute was used by humans since prehistoric times. The crops are herbaceous annuals that grow to 2 to 3.5 m in height, with a stalk 2 to 3 cm in diameter. Jute is grown entirely for its fibers. The plants thrive in hot and humid climates. Jute is the most versatile, ecofriendly, natural, durable, and antistatic fiber available. Today most jute is produced in the delta formed by the Ganges River and Bramhaputra River in India and Bangladesh. Corchorus capsularis, known as white jute, and C. olitorius, Copyright © 2005 by Taylor & Francis

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FIGURE 2.31 Hemp products in front of a hemp plantation. (Courtesy of www.inaro.de.)

known as dark jute, are grown in India, Bangladesh, Thailand, and China as well as Brazil. Jute plantations, fiber processing, and spinning and weaving mills all make a very important contribution to the economies of several of these countries. Individual jute fibers have a polygonal cross section and vary in size. Owing to these irregularities in the thickness of the cell walls the fibers vary greatly in strength. Jute fibers are long (1.5 to 3 m), lustrous, and resilient. Jute is sensitive to chemical and photochemical attack, but resistant to microorganisms. The natural color of jute fibers is brown. Jute fibers are strong but brittle and have a low extension to break (about 1.7%). The high lignin content (up to 20%) causes brittleness of the fibers.145 The fibers lose Copyright © 2005 by Taylor & Francis

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FIGURE 2.32 Schematic (a) and photograph (b) of a jute plant. ((a) from the Gesamtverband der Deutschen Versicherungswirtschaft e.V.: http:// www.tis-gdv.de/tis_e/ware/f_inhalt0.htm; photograph (b) from Seigler, D.S.: http: // www.life.uiuc.edu/plantbio/263/image/Corchorus.jpeg accessed on July 5, 2003.)

tensile strength on exposure to sunlight and have little resistance to moisture and acids. Jute is the most hygroscopic plant fiber; however, if kept dry, jute will last indefinitely. The tensile strength of jute fibers is lower compared to flax or hemp. Jute’s silky texture, its biodegradability, and its resistance to heat and fire make it suitable for use in industries as diverse as fashion, travel, luggage, furnishings, and carpets and other floor coverings. It is now possible to visualize jute not only as a major textile fiber, but also as a raw material for nontextile products. Jute fibers are suitable as reinforcement for partitions, paneling, false ceilings, and other furniture. Mohanty and Misra146 have reviewed jute fiber-reinforced thermoset, thermoplastic, and rubberbased composites. Extensive studies have been carried out to fabricate jute/epoxy, jute/polyester, and jute/phenol-formaldehyde composites for applications such as low-cost housing materials, silos for grain storage, and small fishing boats.147 Copyright © 2005 by Taylor & Francis

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Kenaf (Hibiscus cannabinus L., Malvaceae) Fibers

Kenaf (Figure 2.33) is an annual canelike crop originating in Asia and Africa. Kenaf is fast growing and reaches heights up to 2.4 to 6 m in 5 months. Furthermore, it has the highest carbon dioxide absorption of any plant (1 ton kenaf absorbs 1.5 tons of atmospheric CO2), a valuable tool in the prevention of global warming.148 The stems of kenaf are generally round and contain thorns, which, depending on variety, vary in size from small to large. Kenaf contains two fiber types: long fibers situated in the cortical layer and short fibers located in the ligneous zone. Kenaf elementary fibers are short, between 1.5 and 6 mm, and polygonal.149 Their surface is striated and irregular. The lumen varies greatly in thickness at different points in the cell, sometimes disappearing altogether. Kenaf is a pale-colored fiber that contains less noncellulosic materials than jute. Kenaf fibers are coarse, brittle, and difficult to process. They have a breaking strength similar to that of low-grade jute and are weakened only slightly when wet.150 The kenaf whole stalk and outer bast fibers have many potential specific uses, including paper, textiles, and composites. The use of kenaf fibers is also of particular significance from the standpoint of environmental friendliness. Historically, kenaf fibers were used to manufacture rope, twine, and sackcloth. Now, various new applications for kenaf products are emerging, including those for paper products, building materials, absorbents, and feed and bedding for livestock. It is certain that still other options will also emerge. These options will involve issues ranging from basic agricultural production methods to marketing of kenaf products.151 Kenaf fibers are envisioned for applications as reinforcing fibers for polymers.

2.7.5

Ramie (Boehmeria nivea L. and Boehmeria viridis, Urticaceae) Fibers

Ramie (Figure 2.34) is considered one of the oldest cultivated fiber crops of East Asia and is cultivated mainly in Indonesia, China, Japan, and India. Ramie is a hardy perennial crop and can be harvested up to six times per year. The plant grows to heights of 1.2 to 2.5 m.152,153 Technical ramie fibers are long (1.5 m or more). The fibers obtained from the outer part of the stalk are the longest and one of the strongest fine textile fibers. The diameter of the elementary fiber varies from 10 to 25 µm. They are flat and irregular in shape, with a thick cell wall, and taper to rounded ends. The primary cell wall is often lignified, and this aspect is responsible for the low hygroscopic character of the fibers.154 Ramie fibers have been used as a textile fiber for centuries due to their excellent fiber characteristics.154 Ramie fibers are very fine and silk-like, naturally white in color, and have a high luster. Other advantages of ramie fibers are good resistance to bacteria, mildew, and insect attack. The fibers are stable in alkaline media and not harmed by mild acids. The fibers have Copyright © 2005 by Taylor & Francis

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FIGURE 2.33 (a) Kenaf plant. (Courtesy of www.inaro.de.) (b) Cross section of a kenaf stalk. (Courtesy of USDA/American Kenaf Society.)

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FIGURE 2.34 Schematic (a) and photograph (b) of a ramie plant. (Schematic (a) from the Gesamtverband der Deutschen Versicherungswirtschaft e.V.: http:// www.tis-gdv.de/tis_e/ware/f_inhalt0. htm accessed on July 5, 2003; photograph (b) from Seigler, D.S.: http:// www.life.uiuc.edu/ plantbio/263/image/Boehmeria.jpeg accessed on July 5, 2003.)

an exceptional strength that even increases slightly when wet.153 Because of excellent fiber properties, ramie fibers have a high potential as reinforcing fibers for polymer composites.

2.7.6

(Stinging) Nettle (Urtica dioica L., Urticaceae) Fibers

The stinging nettle (Figure 2.35) is a perennial, cosmopolite, dioecious plant that grows to heights of 2.8 m and contains nonlignified bast fibers in the bark (see Figure 2.22 and Figure 2.23).155,156 The fibers of the stinging nettle have a remarkable tensile strength and fineness (Figure 2.36) at an average length of about 4 cm. Growing nettles as a source of fibers, food, and pharmaceuticals dates back to 3000 B.C.157,158 The nettle fiber yield of the wild nettle (3−5%) was increased by breeding and through selection procedures to 14%. During the Middle Ages and the great wars (World Wars I and II), the extremely durable nettle bast fibers were used to produce various textiles (nettle cloth). However, after the war (1945), use of nettle stopped completely. Nowadays, nettle fibers can be used as reinforcing fibers for polymer composites.159,160 Copyright © 2005 by Taylor & Francis

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FIGURE 2.35 The stinging nettle and its fibers. (From Bürstmayr, H.: http:// ifa-tulln.boku.ac.at/BPNEU/ Homepage/Deutsch/BPdeFAN.htm accessed on July 5, 2003.)

2.8 2.8.1

Leaf Fibers Sisal (Agave sisalana, Liliaceae) Fibers

Sisal (Figure 2.37) is native to Mexico and Central America, but is now widely grown in tropical countries in Africa, the West Indies, and the Far East. The plant grows to a height of about 2 m. The Mayans and Aztecs used sisal fibers, extracted from the sword-shaped leaves of the plant, to manufacture crude fabrics.161 The fibers are extracted from the fresh leaves by decorticators, then washed and sun dried. Currently, decorticators are fully automatic, to which the plant leaves are fed crossways on a conveyor belt. The machines remove the leaf tissue by crushing, scraping, and washing.162 Copyright © 2005 by Taylor & Francis

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FIGURE 2.36 Scanning electron microscopy of elementary nettle fibers (Urtica dioica). (From Dreyling, G., Institut für Angewandte Botanik, Universität Hamburg, Institut für Angewandte Botanik, Abteilung Nutzpflanzenbiologie, Arbeitsgruppe Dreyling, Germany: http:// www. biologie.uni-hamburg.de/ianb/nb/urtabb16.html accessed June 17, 2003.)

The sisal fiber (Figure 2.38) is a hard fiber extracted from the leaves of the sisal plant and is one of the four most widely used plant fibers. It accounts for almost half the total production of plant fibers. Nearly 4.5 million tons of sisal fiber are produced every year throughout the world. Tanzania and Brazil are the two main producing countries.163 The length of sisal fiber varies between 0.6 and 1.5 m and its diameters range from 100 to 300 µm. The fiber is actually a bundle of hollow subfibers. Their cell walls are reinforced with spirally oriented cellulose in a hemicelluloses and lignin matrix. The composition of the external surface of the cell wall is a layer of ligninaceous material and waxy substances that bond the cell to its adjacent neighbors.164 Hence, this surface does not form strong bonds with a polymer matrix. Sisal fibers are smooth, straight, yellow, and are easily degraded in salt water. The tensile properties of sisal fiber are not uniform along its length. The fibers extracted from the root or lower part of the leaf have a lower tensile strength and modulus but a higher fracture strain. The fibers become Copyright © 2005 by Taylor & Francis

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FIGURE 2.37 (a) Schematic of a sisal plant, and (b) sisal plants in Cuba. (From the Gesamtverband der Deutschen Versicherungswirtschaft e.V.: http:// www.tis-gdv.de/tis_e/ware/f_inhalt0.htm accessed on July 5, 2003.)

stronger and stiffer at midspan, and the fibers extracted from the tip have moderate properties. The tensile strength, modulus, and toughness of sisal fibers decrease with increasing temperature.165 Copyright © 2005 by Taylor & Francis

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FIGURE 2.38 Original, untreated (standard) sisal fiber, plan view (magnification ⫻400). (From Bismarck, A. et al., Green Chem., 3, 100, 2001. With permission of The Royal Society of Chemistry.)

Sisal fiber-reinforced polymers166 are used for interiors of cars.167 For instance, Georgia Composites developed a product that is made of partially consolidated recycled poly propylene (PP) reinforced with sisal. The composite is produced on a double-belt laminating press. The sisal-reinforced PP can be used to boost the performance of “Woodstock”-type materials. When the two are compression molded together in a laminate, the sisal/PP significantly increases tensile modulus and strength for applications such as doorpanel inserts and trunk liners.25

2.8.2

Henequen (Agave fourcroydes, Liliaceae) Fibers

Henequen (Figure 2.39) is a close relative of the sisal plant. Its fibers are commonly used in the manufacture of textile products. It was used by the Mayans (in ancient Mexico) to make string, hammocks, clothing, and rugs. Today the henequen (and sisal) industry is concentrated in the tropical regions of Africa, Central and South America, and Asia (particularly China). Henequen (and sisal) fibers are produced in some of the poorest areas of the Copyright © 2005 by Taylor & Francis

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FIGURE 2.39 (a) Processed henequen fibers from Mexico. (From Baltazar Y Jimenez. With permission.) (b) Scanning electron microscopy of original, untreated henequen fibers, plan view (magnification ⫻50.)

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world. In many cases the fiber crop production is the only source of income and economic activity in these areas. Therefore, fiber production can contribute significantly to the efforts to reduce poverty and provide rural employment to nearly 6 million people.168 Henequen fibers consist from ~ 60% cellulose, 25% hemicelluloses, 8% lignin, and 2% waxes. The fibers have a variable diameter, being larger at the butt end and smaller at the tip end of the fiber. Also, the diameter changes for different fibers cultivated in other locations. The fiber cross section changes from a beamlike shape at the butt end to a rounded form at the tip end of the fiber. Like sisal, henequen fibers are smooth, straight, yellow, and easily degraded in salt water. Compared to other leaf fibers, henequen has low elongation at break and a low modulus. Henequen fibers have a relatively high tenacity which makes them suitable as reinforcement for polymers. The attraction of henequen fibers is due to toughness and resiliency.169,170

2.8.3

Pineapple (Anannus comosus, Bromeliaceae) Leaf Fibers (PALF)

Pineapple is usually cultivated for its fruit. Pineapple bran, the fruit residue after juicing, is high in vitamin A and is used in cattle feed. Pineapple leaf fibers (PALF) are obtained from the leaves of the pineapple plant (Figure 2.40). The plant is largely cultivated in tropical countries. Its cultivation in India is substantial (about 91,000 ha) and still increasing. The pineapple crop has a very short stem which first produces a rosette of fibrous leaves. The leaves are about 91 cm long, 5 to 7.5 cm wide and sword shaped, dark green in color, and bear spines of claws on their margins. From them strong, white, and silky fibers can be separated. So far, pineapple leaves are mostly being wasted simply because of the lack of knowledge about their economic uses. Traditionally, PALF yarns have been used to manufacture fabrics, carpets, mops, and curtains. PALF is a multicellular lignocellulosic fiber consisting mainly of cellulose (70−82%), polysaccharides and lignin. The fibers have a ribbon-like structure and consist of a vascular bundle system present in the form of bunches of fibrous cells, which are obtained after mechanical removal of all the epidermal tissues. PALF is of fine quality, and unlike jute, its structure is without mesh. The fiber is very hygroscopic. The superior mechanical properties of PALF are associated with its high cellulose content and comparatively low microfibrillar angle (14°).171 Both flexural and torsional rigidity of PALF are comparable with jute fibers. An interesting characteristic of PALF is that the fiber bundle strength of PALF decreases by 50% when wet, but the yarn strength increases by about 13%. The presence of high amounts of lignin and other waxy substances on PALF causes dyes to fade more rapidly when compared to cotton. PALF has an additional problem of difficult dye penetration due to its relatively high coarseness.172 Copyright © 2005 by Taylor & Francis

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FIGURE 2.40 (Top) Photograph of a pineapple plantation in Hawaii. (Courtesy of Regina HessenmüllerLampke.) (Bottom) Pineapple plant. (Courtesy of Dominik Wieder, Schwertberg, Austria.)

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Abaca (Musa textilis Nee, Musaceae) Fibers

The abaca plant (commonly known as Manila hemp), a relative of the banana, grows to heights of 3 to 4 m (see Figure 2.41). It is native to the Philippines, but was also introduced to Indonesia and Central and South America. The Philippines are the world’s biggest supplier of abaca fibers and products. Abaca is a hard fiber obtained from the leaf sheaths. The fibers are separated from the plants in much the same way as sisal fibers. Abaca is considered to be the strongest of all plant fibers.173 Its tensile strength is three times higher than cotton and twice that of sisal. Furthermore, abaca is far more resistant to degradation in salt water than most other plant fibers. Abaca is a lustrous fiber and yellowish white in color. The technical fibers are 2 to 4 m long. The single fibers are relatively smooth and straight and have narrow pointed ends. The single abaca fibers have a length of 4 to 6 mm and diameters between 17 and 21 µm. Abaca fibers are used mainly to manufacture ropes and handicraft goods such as bags, placemats, slippers, and doormats. More recently, the potential applications of abaca fibers as reinforcement for ecocomposites have been explored.174

2.8.5

Oil Palm (Elaeis guineensis, Palmacea) Fibers

The oil palm (Figure 2.42) is a perennial crop native to West Africa and the Congo Basin. In the 20th century the oil palm was the object of one of the most successful cultivated palm crop improvement programs. Still, extensive stands of wild or semiwild oil palms continue to exist throughout Africa. Mesocarp and endosperm oil are major products of oil palms. Other products include palm wine, food, medicine, raw material for potassium fertilizers, boiler fuel, building materials, and a wide range of handicraft items and, therefore, the oil palm is of significant economic value. The African oil palm is a classic multipurpose species, unlike plantation counterparts that focus only on palm oil and palm kernel oil.175 The oil palm industry creates an abundant amount of various oil palm fibers from fronds, trunks, mesocarp, empty fruit bunches (EFB), palm oil mill effluent, palm kernel cake, and palm press. This fibrous waste can create great environmental problems when simply left on the plantation fields. Some of the fiber by-products, especially palm kernel cake and oil palm frond, are used to feed dairy and beef cattle, sheep, and goats.176 In order to develop other applications for oil palm fibers they need to be extracted from the waste using a retting process. The fibers need also to be cleaned of oily and dirty materials. The major component of oil palm fibers is cellulose, but the fibers also contain large amounts of lignin. The fibers are very porous and have a lacuna-like cross section. Their diameters greatly vary. All these factors affect the mechanical properties of the fiber. Oil palm fibers are very ductile but have a lower tensile strength than most other plant fibers.177 Currently, the academic community is searching for applications of oil palm fibers as a reinforcement for polymers.178–180 Copyright © 2005 by Taylor & Francis

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FIGURE 2.41 (a) Schematic and (b) photograph of abaca (Manila hemp) and (c) abaca fibers of varying quality. (Schematic (a) from the Gesamtverband der Deutschen Versicherungswirtschaft e.V.: http:// www.tis-gdv.de/tis_e/ware/f_inhalt0.htm accessed on July 5, 2003; photographs (b and c) courtesy of Celia Tuazon-Concepcion, Abaka Republic, Inc.: http:// www.abaka-plant-oflife.com/or http: // www.abakarepublic.com/.)

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FIGURE 2.41 (Continued)

FIGURE 2.42 Oil palm (Elaeis guineensis, Palmacea) (Courtesy of Sumati Nagrath).

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Seed Fibers Cotton (Gossypium spp., Malvaceae)

The variety Gossypium hirsutum accounts for most of the world’s production of cotton (87% of the overall production), G. barbadense for 8%, the species G. herbaceum and G. arboreum for only 5%.181 Cotton fibers consist of the unicellular seed hairs of the bolls of the cotton plant. The cotton fruit burst when mature, revealing a fist-sized tuft of fibers with a length from 25 to 60 mm and diameters varying between 12 and 45 µm.182 Cotton is of tropical origin but is most successfully cultivated in temperate climates with well-distributed rainfall. Cotton fibers have been used by early societies to produce textiles. The oldest archeological evidence of cultivated cotton is some cottonseed fragments discovered in a cage in Mexican Tehuacán Valley, dated to 5800 B.C.182 Cotton was used to fabricate textiles as early as 3000 B.C. in the Indus Valley in Pakistan and around 2500 B.C. in Peru. Increased use of cotton fibers in Europe was boosted by the development of the first ring-spinning wheel (“Spinning Jenny”), invented by James Hargreaves in 1764 (see also ref. 183), and by the rippling machine (the “cotton gin” invented by Eli Whitney in 1794). Cotton fibers have a pronounced three-walled structure. An outer wax layer protects the primary wall. The secondary cell wall consists mainly of cellulose. The tertiary wall surrounds the lumen. Mature cotton fibers have kidney-bean shaped cross sections. The surface contour of the fibers is flat and twisted, ribbon-like. The color of cotton ranges from creamy white to dirty gray depending upon the conditions under which it is produced. Cotton is hydrophilic and the fibers swell considerably in water. Cotton releases a considerable amount of heat when absorbing moisture. It dries slowly. Cotton has high soil and oil absorption but also high release. It is stable in water and its wet tenacity is higher than its dry tenacity. The toughness and initial modulus of cotton are lower compared to hemp fibers, whereas its elongation at break, its flexibility, and its elastic recovery are higher. The fibers are resistant to alkalis but degraded by acids. The microbial resistance of cotton is low but the fibers are highly resistant to moths and beetles. Cotton burns readily and quickly, can be boiled and sterilized, and does not cause skin irritation or other allergies. Innumerable products are made from cotton, primarily textile and yarn goods, cordage, and automobile tire cords. Cotton fibers are the backbone of the textile trade of the world.

2.10 2.10.1

Fruit Fibers Coconut Husk or Coir (Cocos nucifera, Palmae) Fibers

Coir fibers (Figure 2.43) are obtained from the fibrous husk (mesocarp) encasing the fruit of the coconut palm (Figure 2.44), which is a by-product of the Copyright © 2005 by Taylor & Francis

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FIGURE 2.43 Original, untreated (standard) coir fiber, plan view (⫻250) and magnified plan view (⫻1000). (From Bismarck, A. et al., Green Chem., 3, 100, 2001. With permission of The Royal Society of Chemistry.)

FIGURE 2.44 Photograph (close-up) of a Cocos nucifera palm with coconuts. (Courtesy of Regina Hessenmüller-Lampke.)

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copra extraction process. The term coir is derived from kayar, a rope or cord, and from kayaru, meaning to be twisted. Coconut palms are cultivated throughout tropical countries mostly for the high oil content of the endosperm (copra). The oil is widely used in both the food and nonfood industries (e.g., surfactant production). Large production areas are found along the coastal regions in the wet tropical areas of Asia in the Philippines, Indonesia, India, Sri Lanka, and Malaysia. In these countries millions of people make a living from coconut palm and its many products.184 However, India and Sri Lanka are the main producing countries of coir fibers. On average, from 100 coconuts 7.5 to 8.2 kg of coir fibers can be obtained. After separation of the nut manually from the husk, the fibrous husk can be processed by a variety of retting techniques,185 but traditionally this takes place in ponds of brackish water (for 3 to 6 months), in salt backwaters, or in lagoons. Saltwater retting requires 10 to 12 months. Traditional water-retting procedures result in highly polluted waters and the accumulation of large chucks of coir pith.186,187 During the retting of the husks, the coir fibers are softened and can be easily decorticated and extracted by beating. After hackling, washing, and drying, the fibers are loosened manually and further cleaned.184 Afterward the fibers are separated into bristle fibers (~20 to 40 cm long), by combing, and mattress fibers, which are random fibers having a length of 2 to 10 cm. The diameter of coir fibers varies between 0.1 and 1.5 mm. Better-grade coir fibers are light in color, but the color ranges from golden yellow for fibers obtained from not completely ripened nuts to reddish-brown from ripe nuts. Individual coir fibers are quite short, about 500 µm in length. The cells are thick walled, having an irregular lumen. The fiber surface is covered with cavities (Figure 2.43)38 arising from dried out sieve cells. Coir fibers have high lignin but low cellulose content, as a result of which the fibers are resilient, strong, and highly durable. Coir is one of the toughest plant fibers available. It does not pill and is highly abrasion and rot (fungal and bacterial) resistant.188 Furthermore, coir is naturally insulating and sound absorbing, antistatic, and difficult to ignite. Due to the ability of coir fibers to tolerate water immersion for months without disintegrating, they find many applications as horticultural and erosion control products (geotextiles). Coir fibers are the only fruit fiber that is used in the textile industry. A variety of products such as brushes and brooms, ropes and yarns for nets, bags, and mats, as well as padding for mattresses are manufactured from coir fibers. They are widely used as seat cushions in the automotive industry because of their high rate of moisture absorption. Recently, the academic and industrial R&D communities have begun seeking ways to develop new applications for coir as a reinforcement for polymers. Attempts have been made to combine retted coir fibers with polyester resins to manufacture composite laminates for consumer articles.189–191 The studies suggest that the surface of the fiber should be modified by either coating or leaching out the cuticle layer of the fiber and thereby improving compatibility between the fibers and the polyester matrix. Copyright © 2005 by Taylor & Francis

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Stalk Fibers: (Cereal) Straw Fibers

Straw is produced throughout the world in enormous quantities as a byproduct of cereal cultivation. Half the straw is simply burned on the fields just to get rid of it or else it is buried in greater quantities than what is needed to replenish organic matter in the soil; both of these practices contribute to important environmental problems. Straw plowed into the soil degrades quite slowly, especially under dry conditions, and it can harbor crop stem diseases. Grain straw from rice, wheat, maize, oats, barley, and rye, left after harvesting forms a large source of fibrous biomass for the Western world. Straw is mainly used in cattle and stock feeding, but can also be processed into paper, boards, power, fuel, and other products. Straw consists of stem and leaves branching off the stem at the nodes. The internodal sections of the stem are the technologically desirable stalks. On examination of the molecular structures of the cell wall, straw is found to have less cellulose and lignin but more hemicellulose as compared to wood. Cellulose is the technologically valuable component; hemicelluloses are mere fillers. Compared with wood, straw tissue contains more silica and extractives and has less cell wall substance.192 Moreover, the presence of hydroxycinnamic acids, such as p-coumaric and ferulic acids, in the cell walls of cereal straw makes it more difficult to characterize the composition of straw.193 Individual straw fibers have a much lower tensile strength and lower compressive strength as compared to other plant fibers. Packaging materials and other biobased products from straw could open a new market for farmers in straw. Rice straw has been used to reinforce clay in the making of bricks. It can also be used as a component in constructing drains and pipelines because it is made up of strong fibers that do not easily decay.194 Cereal straw can be a wood substitute as a fiber source for building panels.195 Straw offers some technological advantages over wood because it allows better packing of the fibers or strands and the degree of bonding between them is much better. Straw-reinforced panels can compete with reconstituted wood products, such as particle and fiberboard, in markets for floor underlay, furniture, and cabinet construction.195 Muskoka Cabinet Company (Ottawa, Ontario, Canada) has introduced a new, environmentally friendly melamine panel called “Woodstalk”* into the manufacturing of kitchen and bathroom cabinetry.196 The furniture panels are manufactured from wheat straw, which is advantageous to the environment and the economy. Farmers sell more straw and customers can enjoy furniture made from nontoxic resins reinforced by straw that is formaldehyde-free. Pinnacle Technology, Inc. (Lawrence, Kansas, USA) has developed and commercialized, in a joint effort with the Forest Products

* For photographs see: Muskoka Cabinet Company. Link in: http://www.muskokacabco.com/ what.html accessed on July 26, 2003.

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Laboratory of the U.S. Department of Agriculture, a product called “AgroPlastic,” which is a new straw-filled plastic for sale to producers in the finished plastic goods industry.197 Agro-Plastic consists of polypropylene or polyethylene, up to 50% wheat straw, pigments, bonding agents, and chemical modifiers. The palletized material is suitable for injection molding and extrusion. Wheat straw reportedly provides much lower density and significantly higher tensile and flexural strengths and flexural modulus as compared to mineral-filled polyolefins.25 The German company Retrupor Verpackungs- und Dämmstoffwerk GmbH (Glindenberg, Germany)198 has developed a truly green (eco) composite by combining wheat, barley, or rye straw with cellulose from waste paper. The Retrupor composite mass consists of almost 80% cereal straw and about 20% cellulose plus about 0.1% of additives such as starch, wax, a fungicide, and natural latex. In 2002 the company began manufacturing shockproof, robust, and sturdy packaging materials mainly for mechanical engineering companies.† The material is used to properly pack and transport heavy engines or passenger car cylinders with weights up to 150 kg. The ecostraw composite competes with cardboard, fiberboard, and foamed plastics, such as polystyrene foams, on the market. After use, the products can be incinerated just like retired timber or lumber, or it can simply be composted. When burned the material sets free only the amount of carbon dioxide that the plants once adsorbed during photosynthesis. Therefore, the disposal of this packaging material does not add further greenhouse gases to the atmosphere.199

2.12

Conclusion

Plant fibers have many significant advantages over synthetic fibers. Some plant fibers are low cost, low density, have high specific strength and Young’s modulus, and possess ease of formability. The specific properties of plant fibers are comparable to glass fibers. During processing, natural fibers are nonabrasive to processing equipment such as its compounding parts. A further advantage is that plant fibers cause no skin irritation during handling and use. These properties are especially advantageous when comparing plant fibers with E-glass fibers that are used extensively as reinforcement in polymer composites. Also, the amount of energy necessary for the production of plant fiber textiles and fabrics was estimated to be ~80% lower than for the production of glass fibers. As a consequence, plant-based fibers are used in a wide range of products for textile and nontextile applications. The bast

†

For photographs see: Retrupor Verpackungs- und Dämmstoffwerk GmbH. Link in: http://www. retrupor.de/ or http://www.verpackungsformteile.de/ both accessed on July 26, 2003.

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fibers, flax, hemp, and jute, and leaf fibers, sisal, henequen are reestablishing themselves in the world market because of their excellent specific properties. The textile industry uses mainly cotton but also flax, jute, ramie, hemp, and sisal fibers, while the use of fibers obtained from straw is limited mainly to the paper industry. Many plant fibers are also used as reinforcement for green composites. ‘Green’ composite materials have found applications that do not involve very high stresses, for instance, as non-structural parts for the automotive and railway industry. The main disadvantage of plant fibers is their high moisture absorption, which leads to swelling of the fibers and subsequently causes dimensional variations of the final material. Because of irregular fiber geometry, modeling of discrete volumes of ‘green’ composites is lacking. Plant fiber products are environmentally friendly. Their use is also increasingly popular, as consumers are becoming more ecologically conscious. Additionally, plant fibers have a positive impact on the income of farmers, fiber producers, and material manufacturers. Some ecological advantages of these biofibers are the reduction of CO2 emission, biodegradability, and recycling. Increased applications of plant fibers will therefore also benefit future generations. The availability of plant fibers with the required qualities is the key point to fulfill the demand and supply cycle for their application as reinforcement in green composites. Currently, national and international markets can be satisfied by the fiber production in Asia (jute, hemp, ramie, kenaf, and coir), Africa (kenaf and sisal), North America (kenaf), South America (sisal), and Europe (flax and hemp). Local fiber production chains meet the required quality standards for textile applications as well as the requirements for lowbudget mass products. Fiber production chains of plant fibers have been reviewed with regard to agroindustrial factors. Still there is a need to develop plant fibers that meet high-quality standards and have homogeneous properties as required for high-value added, high-tech applications. The origin of the fibers, seed quality, weather conditions during growth and harvesting, fiber retting, and treatment procedures influence the final materials performance. The premise is to develop a complete line of carefully controlled actions starting with economically and ecologically viable crop production and harvesting of the raw material, to a novel but gentle fiber separation procedure and clever fiber treatments adapted to supply fibers that meet the quality standards for textile and nontextile applications. Unfortunately, the higher the required fiber quality, the higher the costs resulting from extensive fiber treatments. It can be concluded that fibers for high-tech applications have to be less expensive than synthetic fibers. Otherwise, they will not be considered as substitutes for the production of conventional composites. Intensive cooperation among farmers, respective governments, consumer organizations, local research institutes, and fiber industries is necessary to make the added-value products of these “old” to “new” fibers in green composites. Consumers, governments, and industry alike need to become aware of the importance of sustainable technology and those products that help Copyright © 2005 by Taylor & Francis

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preserve nonrenewable resources, protect the environment, and respect indigenous populations and cultures when making decisions about the future.

2.13

Outlook

The European “end-of-life vehicles regulation (ELV),” the “European Composite Recycling Concept,” and other waste management legislations in industrialized countries are gearing the fiber-reinforced plastic (FRP) market toward plant fiber-reinforced composites. However, the developing countries must be supported in dealing with the problems related to plastic waste disposal. Green-labeled products benefit present and future humankind by preserving limited natural resources and reducing greenhouse gas emissions. Waste management guidelines such as biodegradability, recycling, and reuse need to be developed extensively alongside appropriate allowances for nonprivileged countries to adjust. An alternative is to convince governments to accept incineration of products made of renewable resources and classify them as recycling. This will become the key factor to support the present research in plant-based, truly green composites. Plant fibers emit less CO2 when they are broken down than they absorb during growth; therefore a positive ecobalance can be realized using such fibers. The plant fiber market is increasing since its renaissance in the 1980s. On the way to producing fuel-efficient, low-polluting vehicles, the automotive industry is anticipated to be the main user of these fibers followed by the commodity sector as well as in railways, textiles, furniture, etc. Composite applications are increasing rapidly in Asia compared to its present market in Europe and North America, which might lead to even cheaper green composites because of the profound local cultivation of plant fibers. Research in genetics, biotechnology, and new processing methods of fiber crops aiming to produce fibers with constant quality needs to be highlighted. The infrastructure needs to be provided and supported by low taxes and subsidies to motivate and encourage farmers to produce renewable fibers and other biomass for renewable resources. Furthermore, research is necessary to gain a better understanding of ‘green’ materials and to develop sustainable and economically viable materials processing and design. There is an urgent need to develop suitable biomatrices for truly ‘green’ composites to guarantee that these materials can compete with conventional materials, including composites such as glass/epoxy systems. There is a new market to be explored and conquered by plant fiber composites. These materials have the potential to replace commodity plastics if the biobased products are lightweight with high performance at comparable costs.

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Acknowledgments The authors would like to acknowledge the help of many people who provided valuable input, comments, assistance, samples, figures, and photographs. Without their help the text you read would have been pretty boring. Many thanks to Alexis Baltazar Y Jimenez (Department of Chemical Engineering, Imperial College London, UK), Jonny J. Blaker (Department of Materials, Imperial College London, UK), Dr. Harriette Bos (Agrotechnological Research Institute, Wageningen, The Netherlands), Bettina Bues (Schuberth Helme GmbH, Braunschweig, Germany), Prof. Hermann Bürstmayr (Institute for Agrobiotechnology in Tulln (IFA-Tulln, Abteilung Biotechnologie in der Pflanzenproduktion, Austria), Björn von Diepenbrock (Gesamtverband der Deutschen Versicherungswirtschaft e.V., Transport, Berlin, Germany), Prof. Gisela Dreyling (Institut für Angewandte Botanik, Abteilung Nutzpflanzenbiologie (Arbeitsgruppe Dreyling), Universität Hamburg, Germany), Prof. Robert Kohler (Institut für Angewandte Forschung, Hochschule Reutlingen, Reutlingen, Germany), Martina Mulder (Informations system Nachwachsende Rohstoffe, www.inaro.de and IfUL, Institut für umweltgerechte Landbewirtschaftung, Müllheim, Germany), Sumati Nagrath (School of Social Studies, University College Northampton, UK), Dr. Stephan Odenwald (Kompetenzzentrum Strukturleichtbau, Technische Universität (TU) Chemnitz, Germany), Prof. David S. Seigler (Department of Plant Biology, University of Illinois at Urbana-Champaign, USA), Dr. Artemis Stamboulis (School of Electrical and Mechanical Engineering, University of Ulster, UK), Celia Tuazon-Concepcion (ABAKA REPUBLIC, INC., Carson, CA, USA), USDA/American Kenaf Society (McAllen, TX, USA), and Dominik Wieder (Schwertberg, Austria) for supplying and granting the kind permission to use figures, photographs, and images to illustrate our text. Special thanks to Prof. Bernhard Wielage (Lehrstuhl für Verbundwerkstoffe, TU Chemnitz, Germany), who initiated the project on flax and hemp-reinforced polymer composite materials in Chemnitz. Last, but not least, we would like to thank Ramesh Babu Adusumalli (Lehrstuhl für Verbundwerkstoffe, TU Chemnitz, Germany) and Dr. Katharine Sarikakis (Institute of Communication Studies, University of Leeds, UK) for their most helpful comments and assistance.

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113. Fölster, Th. and Michaeli, W., Flachs–eine nachwachsende Verstärkungsfaser für Kunststoffe? Kunststoffe, 83, 687, 1993. 114. Müssig, J., Bäumer, R., and Hasenpath, J., Mechanischer Aufschluß von Hanfstroh, Melliand Textilber., 5, 302, 1996. 115. Lampke, T., Beitrag zur Charakterisierung naturfaserverstärkter Verbundwerkstoffe mit hochpolymerer Matrix, Schriftenreihe Werkstoffe und werkstofftechnische Anwendungen, Vol. 9, Wielage, B., Ed., Technische Universität Chemnitz, Lehrstuhl für Verbundwerkstoffe, 2001. 116. Zimmer, H. and Kloß, K.D., Ultraschallaufschluß von Hanf Ziele–Technologie –Anwendung –Resultate –Qualitätsmanagement, Proceedings Biorohstoff Hanf – Resource Hemp, Reader zum Technologisch-wissenschaftlichen Symposium, NovaInstitut, Frankfurt, Mar. 2–5, 1995, p. 317. 117. D’Agostino, D., Richard, A., and Taylor, J. D., Continuous Steam Explosion Pulping: A Viable Alternative for Pulping of Non-Woody Fibers. Link in: http: // www.steamexplosion.com/pulp/tappi96.html accessed on 03.07.2003. 118. Kessler, R.W., Becker, U., Kohler, R., and Goth, B., Steam explosion of flax–a superior technique for upgrading fibre value, Biomass Bioenergy, 14, 237, 1998. 119. Schenek, A., Reutlinger Flachs-Dampfaufschlußverfahren erfolgreich im Großversuch, Melliand Textilber., 10, 806, 1991. 120. Stamboulis, A., Baillie, C.A., and Peijs, T., Effects of environmental conditions on mechanical and physical properties of flax fibers, Composites A, 32, 1105, 2001. 121. Pott, G.T., Pilot, R.J., and van Hazendonk, J.M., Upgraded Natural Fibres for Polymer Composites, Proceedings of the 5th European Conference on Advanced Materials and Processes and Applications (EUROMAT 97), Vol. 2, Polymers and Ceramics, Maastricht, Apr. 21–23, 1997, p. 107. 122. Bledzki, A.K. and Gassan, J., Natural Fiber Reinforced Plastics, in Handbook of Engineering Polymeric Materials, Cheremisinoff, N.P., Ed., Marcel Dekker, New York, 1998, pp. 787–810. 123. Van Sumere, C.F., Retting of Flax with Special Reference to Enzyme-Retting, in The Biology and Processing of Flax, Sharma, H.S.S. and Van Sumere, C.F., Eds., M. Publications, Belfast, 1992, p. 157. 124. Akin, D.E., Foulk, J.A., Dodd, R.B., and McAlister III, D.D., Enzyme-retting of flax and characterization of processed fibers, J. Biotechnol., 89, 193, 2001. 125. Foulk, J.A., Akin, D.E., and Dodd, R.B., Processing techniques for improving enzyme-retting of flax, Ind. Crop Prod., 13, 239, 2001. 126. Mooney, C., Stolle-Smits, T., Schols, H., and de Jong, E., Analysis of retted and non retted flax fibres by chemical and enzymatic means, J. Biotechnol., 89, 205, 2001. 127. Wurster, J. and Daul, D., Flachs, eine durch Forschung moderene alte Kulturpflanze, Melliand Textilber., 12, 851, 1988. 128. Müssig, J., Martens, R., and Harig, H., Der Einfluß von Ernte-, Aufbereitungsund Verarbeitungstechnik auf die Eigenschaften hanfnadelfilzverstärkter Polymerwerkstoffe, Proceedings 2nd International Wood and Natural Fibre Composites Symposium, Kassel, June 28–29, 1999, pp. 21-1 to 21-13. 129. Bismarck, A., Aranberri-Askargorta, I., Springer, J., Lampke, T., Wielage, B., Stamboulis, A., Shenderovich, I., and Limbach, H.-H., Surface characterization of flax, hemp and cellulose fibers; surface properties and the water uptake behavior, Polym. Compos., 23, 872, 2002. 130. Aranberri-Askargorta, I., Lampke, T., and Bismarck, A., Wetting behavior of flax fibers as reinforcement for polypropylene, J. Colloid Interface Sci., 263, 580, 2003.

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131. Keller, A., Compounding and mechanical properties of biodegradable hemp fibre composites, Compos. Sci. Technol., 63, 1307, 2003. 132. Data from the Sächsisches Textilforschungsinstitut e.V. (STFI) Chemnitz (see also Lampke, T., Möglichkeiten zur Charakterisierung der Einzelkomponenten (Ausgangsmaterial) und Interfaces naturfaserverstärkter Composite mit hochpolymerer Matrix. Diplomarbeit, TU-Chemnitz, 1997). 133. George, J., Sreekala, M.S., and Thomas, S., A review on interface modification and characterization of natural fiber reinforced plastic composites, Polym. Eng. Sci., 41, 1471, 2001. 134. Joly, C., Kofman, M. and Gauthier, R., Polypropylene/cellulosic fiber composites: chemical treatment of the cellulose assuming compatibilization between the two materials, J. M. S. - Pure Appl. Chem., A33, 1981, 1996. 135. Lloyd, E.H., A Review of Building Materials from Bast Plants. Link in: http: // www.olywa.net/when/WSUtoc.html accessed on 24.06.2003. 136. Jähn, A., Schröder, M.W., Füting, M., Schenzel, K., and Diepenbrock, W., Characterization of alkali treated flax fibres by means of FT Raman spectroscopy and environmental scanning electron microscopy, Spectrochim. Acta A, 58, 2271, 2002. 137. Kessler, R.W. and Kohler, R., New strategies for exploiting flax and hemp, Chemtech, 12, 34, 1996. 138. Koch, P.-A., Flachs sowie andere Bast- und Hartfasern, Faserstoff-Tabelle, Chemiefasern/Textilindustrie, 44, 96, 1994. 139. Mieck, K.-P., Lützkendorf, R., and Reußmann, T., Needle-punched hybrid nonwovens of flax and PP-Fibers-Textile semiproducts for manufacturing of fiber composites, Polym. Compos., 17, 873, 1996. 140. Foulk, J.A., Akin, D.E., Dodd, R.B., and McAlister III, D.D., Flax Fiber: Potential for a New Crop in the Southeast, in Trends in New Crops and New Uses, Janick, J. and Whipkey, A., Eds., ASHS Press, Alexandria, 2002, p. 361. 141. Roulac, J.W., Hemp Horizons: The Comeback of the World’s Most Promising Plant, Chelsea Green Publishing Co., White River Junction, 1997. 142. NAIHC, Hemp Defined. Link in: http://naihc.org/hemp_information/ hemp_facts.html accessed on 05.07.2003. 143. Mignoni, G., Cannabis as a Licit Crop: Recent Developments in Europe, in Bulletin on Narcotics–1997 Issue 1. Link in: http://www.unodc.org/unodc/ bulletin/bulletin_1997-01-01_1_page003.html#2 accessed on 03.07.2003. 144. Mediavilla, V., Bassetti, P., Leupin, M., and Mosimann, E., Agronomische Eigenschaften von Hanfsorten, Agarforschung, 6, 393, 1999. 145. Rowell, R.M. and Stout, H.P., Jute and Kenaf, in Handbook of Fiber Chemistry, 2nd ed., Lewin, M. and Pearce, E.M., Eds., Marcel Dekker, New York, 1998, p. 465. 146. Mohanty, A.K. and Misra, M., Studies on jute composites – a literature review, Polym. Plast. Technol. Eng., 34, 729, 1995. 147. Biswas, S., Srikanth, G., and Nangia, S., New Developments of Natural Fibre Composites in India, in Composites 2001, Technical Papers and Presentations, Composite Fabricators Association, Tampa, 2001. Link in: http://www.cfa-hq. org/members/composites2001/ResourceCenter.pdf accessed on 03.07.2003. 148. NEC Press release 03.02.2003, NEC develops high-strength reinforced bioplastic, –Featuring polylactic acid reinforced with kenaf fiber–link in: http:// www. nec.co.jp/press/en/0302/0301.html and in http: // www.globalcomposites. com/news/news_fiche.asp?id⫽675& accessed on 25.06.2003.

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149. Meister, E., Mediavilla, V., Vetter, R., and Konermann, M., Prüfung des Anbaus und der Möglichkeiten einer Markteinführung von neuen Faserpflanzen (Hanf, Kenaf, Miscanthus), Grenzüberschreitenden Instituts zur rentablen umweltgerechten Landbewirtschaftung (ITADA), Colmar, 1999. 150. Tao, W.Y., Calamari, T.A., Shih, F.F., and Cao, C.Y., Characterization of kenaf fiber bundles and their nonwoven mats, Tappi J., 80, 162, 1997. 151. Webber III, C.L., Bledsoe, V.K., and Bledsoe R.E., Kenaf Harvesting and Processing, Trends in New Crops and New Uses, Janick, J. and Whipkey, A., Eds., ASHS Press, Alexandria, 2002. 152. Jarman, C.G., Canning, A.J., and Mykoluk, S., Cultivation, extraction and processing of Ramie fibre: a review, Trop. Sci., 202, 91, 1978. 153. Ramie Yarns, Link in: http: // www.swicofil.com/products/007ramie. html#Properties accessed on 15.06.2003. 154. Angelini, L.G., Lazzeri, A., Levita, G., Fontanelli, D. and Bozzi, C. Ramie (Boehmeria nivea (L.) Gaud.) and Spanish Broom (Spartium junceum L.) fibres for composite materials: agronomical aspects, morphology and mechanical properties, Ind. Crop Prod., 11, 145, 2000. 155. Die Große Brennessel. Link in: http: // www.biologie.uni-hamburg.de/ianb/ nb/urtica02.html accessed on 25.06.2003. 156. Faserasergehalt und Faserqualität von Urtica dioica L. Link in: http: // www. biologie.uni-hamburg.de/ianb/nb/urtica05.html accessed on 25.06.2003. 157. Dreyer, J., Die Fasernessel als nachwachsender Rohstoff, Leistungsprüfung von Fasernesseln (Urtica dioica L., Großer Brennessel) unter besonderer Berücksichtigung der phänotypischen Differenzierung anbauwürdiger Klone, Verlag Dr. Kova , 1999. 158. Dreyer, J., Ertrags- und Faserqualitätsversuche mit der Fasernessel an verschiedenen Standorten in Niedersachsen. Link in: http: // www.faserinstitut. de/nessel.htm accessed on 15.06.2003. 159. Dreyer, J., Dreyling, G., and Feldmann, F., Fibre Nettle (Urtica dioica L.) as an Industrial Fibre Crop for Composites (PMC’s)?, Proceedings “Biomass for Energy and Industry” 10th European Conference and Technology Exhibition, Carmen, e.V., Ed., June 8–11, 1998, Würzburg, pp. 516–519. 160. Lützkendorf, R., Mieck, K.-P., Reußmann, T., Dreyling, G., Dreyer, J., and Lück, M., Nesselfaser-Verbundwerkstoffe für Fahrzeuginnenteile – Was können sie?, Techn. Textilien/Techn. Textiles, 43, 30, 2000. 161. Seigler, D.S., Plant Biology 263, Plants and their Uses. Link in: http: // www.life.uiuc.edu/plantbio/263/FIBERS.html accessed on 25.06.2003. 162. Tansport Information Service, GDV, Sisal Hemp. Link in: http: // www. tis-gdv.de/tis_e/ware/f_inhalt0.htm accessed on 25.06.2003. 163. Hartemink, A.E. and Wienk, J.F., Sisal production and soil fertility decline in Tanzania, Outlook Agr., 24, 91, 1995. 164. Mukherjee, P.S. and Satyanarayana, K.G., Structure and properties of some vegetable fibers. 1 Sisal fiber, J. Mater. Sci., 19, 3925, 1984. 165. Chand, N. and Hashmi S.A.R., Mechanical properties of Sisal fiber at elevated temperatures, J. Mater. Sci., 28, 6724, 1993. 166. Li, Y., Mai, Y.-W., and Ye, L., Sisal fibre and its composites: a review of recent developments, compos. Sci. Technol., 60, 2037, 2000. 167. Brouwer, W.D., Natural Fibre Composites in structural components: alternative applications for Sisal? in Common Fund for Commodities – Alternative Applications for Sisal and Henequen, Technical Paper No. 14, Proceedings of a Seminar, held by the

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Food and Agriculture Organization of the UN (FAO) and the Common Fund for Commodities (CFC), Rome, Dec. 13, 2000. Link in: http: // www.fao.org/ DOCREP/004/Y1873E/y1873e00.htm#Contents accessed on 15.06.2003. Shamte, S., Overview of the Sisal and Henequen Industry: A Producers’ Perspective, in Common Fund for Commodities – Alternative Applications for Sisal and Henequen, Technical Paper No. 14, Proceedings of a Seminar, held by the Food and Agriculture Organization of the UN (FAO) and the Common Fund for Commodities (CFC), Rome, Dec. 13, 2000, Amsterdam, 2001. Link in: http:// www.fao.org/DOCREP/004/Y1873E/y1873e05.htm#bm05 accessed on 25.06.2003. Cazauranc-Martinez, M.N., Herrera-Franco, P.J., Gonzalez-Chi, P.I., and Aguilar-Vega, M., Physical and mechanical properties of Henequen fibers, J. Appl. Polym. Sci., 43, 749, 1991. Aguilar-Vega, M. and Cruzramos, C.A., Properties of Henequen cellulosic fibers, J. Appl. Polym. Sci., 56, 1245, 1995. Uma Devi, L., Bhagawan, S.S., and Thomas, S., Mechanical properties of pineapple leaf fiber-reinforced polyester composites, J. Appl. Polym. Sci., 64, 1739, 1997. Saha, S.C., Das, B.K., Ray, P.K., Pandey, S.N, and Goswami, K., Some physicalproperties of pineapple leaf fiber (PALF) influencing its textile behavior, J. Appl. Polym. Sci., 50, 555, 1993. Isarog Pulp & Paper Co., The Abaca Fiber and The Pulp. Link in: http://www.isarogpulp.com/origin.html accessed on 03.07.2003. Shibata, M., Ozawa, K., Teramoto, N., Yosomiya, R., and Takeishi, H., Biocomposites made from short abaca fiber and biodegradable polyesters, Macromol. Mater. Eng., 288, 35, 2003. Non-Wood Forest Products 10: Tropical Palms, FAO–Food and Agriculture Organization of the United Nations, 1995. Link in: http://www.fao.org/ docrep/x0451e/x0451e00.htm#Contents accessed on 04.07.2003. Malaysian Agricultural Research and Development Institute (MARDI), Processing and Utilization of Oil Palm By-Products as Livestock Feed. Link in: http:// www.agnet.org/library/article/rh2001011a.html accessed on 04.07.2003. Sreekala, M.S., Kumaran, M.G., and Thomas, S., Oil palm fibers: morphology, chemical composition, and mechanical properties, J. Appl. Polym. Sci., 66, 821, 1997. Sreekala, M.S., Kumaran, M.G., and Thomas, S., Utilization of short oil palm empty fruit bunch fiber (OPEFB) as a reinforcement in phenol-formaldehyde resins: Studies on mechanical properties, J. Polym. Eng., 16, 265, 1997 Rozman, H.D., Saad, M.J., and Mohd Ishak, Z.A., Flexural and impact properties of oil palm empty fruit bunch (EFB)-polypropylene composites-the effect of maleic anhydride chemical modification of EFB, Polym. Testing, 22, 335, 2003. Sreekala, M.S., Kumaran, M.G., Joseph, S., Jacob, M., and Thomas, S., Oil palm fibre reinforced phenol formaldehyde composites: influence of fibre surface modifications on the mechanical performance, Appl. Compos. Mater., 7, 295, 2000. Müssig, J. Baumwolle, Leitfaden Nachwachsende Rohstoffe: Anbau-VerarbeitungProdukte. KATALYSE Institut für angewandte Umweltforschung, C.F.Mueller Verlag, Heidelberg,1998. Baumwolle. Link in: http: // fortbildung.geo.bildungszentrum-markdorf.de/ pages/BAUMWOLLE.HTM accessed on 29.06.2003. Bellis, M., Spinning Jenny - James Hargreaves. Link in: http:// inventors.about. com/library/inventors/blspinningjenny.htm accessed on 29.06.2003.

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184. van Dam, J.E.G., Coir Processing Technologies–Improvements of Drying, Softening, Bleaching and Dyeing Coir Fibre/Yarn and Printing Coir Floor Coverings, Technical Paper No. 6, COMMON FUND FOR COMMODITIES, Amsterdam, 2002. Link in: http: // www.fao.org/DOCREP/005/Y3612E/ y3612e00.htm#Contents accessed on 19.06.2003. 185. Ravindranath, A.D. and Bhosle, S. Development of a bacterial consortium for coir retting, J. Sci. Ind. Res., 59, 140, 2000. 186. Abbasi, S.A. and P.C. Nipaney, Environmental impact of retting of coconut husk and directions for the development of alternative retting technology, Pollut. Res., 12, 117, 1993. 187. Umayorubhagan, V., Albert, G.M.I., and Ray, C.I.S., Physico-chemical analysis of the water of Pottakulam Lake at Thengapattanam in Kanyakumari district (Tamil Nadu). Asian J. Chem Rev., 6, 7, 1995 188. Goulart Silva, G., De Souza, D.A., Machado, J.C., and Hourston, D.J., Mechanical and thermal characterization of native Brazilian coir fiber, J. Appl. Polym. Sci., 76, 1197, 2000. 189. Rout, J., Misra, M., Tripathy, S.S., Nayak, S.K., and Mohanty, A.K., The influence of fiber surface modification on the mechanical properties of coir–polyester composites, Polym. Compos. 22, 468, 2001. 190. Rout, J., Misra, M., Tripathy, S.S., Nayak, S.K., and Mohanty, A.K., The influence of fibre treatment on the performance of coir–polyester composites, Compos. Sci. Technol., 61, 1303, 2001. 191. Agrawal, S.P. and Dolui, S.K., A new alternative to wood-based on renewable materials, Res. Ind., 40, 1, 1995. 192. Rowell R.M., Han, J.S., and Rowell, J.S., Characterization and Factors Effecting Fiber Properties, in Natural Polymers and Agrofibers Composites, Frollini, E., Leão, A. L. and Mattoso, L. H. C., Eds., Embrapa-S, Sao Carlos, 2000, pp. 115–134. 193. Sun, R.C., Sun, X.F., Wang, S.Q., Zhu, W., and Wang, X.Y., Ester and ether linkages between hydroxycinnamic acids and lignins from wheat, rice, rye, and barley straws, maize stems, and fast-growing poplar wood, Ind. Crops Prod., 15, 179, 2002. 194. US Rice Producers Association, 6th Grade - Social Studies LESSON PROBLEM: Economics: How Can Rice Straw be Disposed of in an Economical and Safe Way? Link in: http: // www.riceromp.com/teachers/lessonContent.cfm? pId⫽79 accessed on 29.06.2003. 195. Goldboard Development Corporation (GDC), The Goldboard Vision. Link in: http:// www.goldboard.com/corporate/corp.htm accessed on 29.06.2003. 196. Muskoka Cabinet Company, Mukoka goes Green with Leftover Wheat Straw. Link in: http:// www.muskokacabco.com/what.html accessed on 29.06.2003. 197. Pinnacle Technology, Agro-Plastic Market Analysis Final Report. Link in: http:// www.westbioenergy.org/studies/study1.htm accessed on 29.06.2003. 198. Retrupor Verpackungs- und Dämmstoffwerk GmbH, Mit Zukunft verpacken. Link in: http:// www.retrupor.de/ accessed on 29.06.2003. 199. Boeckh, M. Wissenschaft in Unternehmen: Stroh statt Plastik, Spektrum der Wissenschaft, März, 76, 2003.

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3 Processing of Bast Fiber Plants for Industrial Application

Friedrich Munder, Christian Fürll, and Heinz Hempel

CONTENTS 3.1 Introduction 3.2 Present Growing of Bast Fiber Plants Worldwide—Volume and Yield 3.3 Composition of Bast Fiber Plants 3.4 Demands on Bast Fibers for Various Industrial Applications 3.5 Harvesting 3.5.1 Cutting and Drying 3.5.2 Retting 3.5.3 Baling and Storage 3.6 Processing of Natural Fiber Plants 3.6.1 Process Overview and Description 3.6.2 Opening of the Bales and Shortening of the Stalks 3.6.3 Elimination of Metallic Impurities and Stones 3.6.4 Metering of Plant Straw 3.6.5 Decortication 3.6.5.1 Definition and Principles of Decortication 3.6.5.2 Breaking Rollers 3.6.5.3 Hammer Mills without Integrated Cleaning Effect 3.6.5.4 Hammer Mills with Integrated Cleaning Effect 3.6.6 Fiber Cleaning 3.6.6.1 Working Principles and Tools 3.6.6.2 Scutching Turbine/Tambour 3.6.6.3 Multiple Ultracleaner 3.6.6.4 Comb Shaker 3.6.7 Opening of the Fibers 3.6.7.1 Object of Fiber Opening

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3.6.7.2 Opener 3.6.7.3 Carding Machine 3.6.8 Cutting of Fibers into a Defined Length 3.6.9 Baling of Long Fibers 3.6.10 Cleaning of Waste Air 3.7 Components Arising from Processing and Fiber Yields 3.8. Mechanical Properties of Processed Fibers 3.8.1 Fiber Length 3.8.2 Fineness 3.8.3 Tensile Strength 3.8.4 Elongation 3.8.5 Cleanness 3.9 Comparative Valuation of Technologies and Fiber Data 3.9.1 Operational Data 3.9.2 Fiber Data 3.10. Economic Aspects References

ABSTRACT Natural fibers are gaining progressive account as renewable, environmentally acceptable, and biodegradable starting material for industrial applications, technical textiles, composites, pulp and paper, as well as for civil engineering and building activities. The fibers of the plants, such as flax, hemp, linseed, jute, sisal, kenaf, yucca, abaca, or ramie, have outstanding mechanical properties. The tensile strength of the natural fibers, for example, is comparable to the strength of high-tensile steel. However, the properties of the natural product “bast fiber” depend on the variety grown, the growing conditions, and the technology of processing. Basically, there are two working principles to separate the bast fibers from the wood. The conventional method uses breaking rollers, which alternating bend, buckle, and soften the stalks. This method requires an intensive retting of the stalks before processing. The retting is effected by microorganisms which dissolve the lignin and pectin of the stalk. Modern technologies use swing hammer mills in most cases. The fiber decortication is effected by impact stress of the hammers directly on the surface of the straw stalks. This working principle ensures a complete separation of the fibers from the wood even when processing freshly harvested, nonretted plants. The effective mechanical separation of the fibers and the wood inside the decorticator simplifies the subsequent fiber cleaning. The processed fibers have a fineness ranging between 2.5 and 15 tex. The fiber length varies in an adjustable range from 50 to 200 mm after processing. This length meets the requirements of many industrial applications. The optimal fiber length for processing of composites is between 2 and 4 mm only. Such lengths are cut after the fiber cleaning using special fiber-cutting machines.

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Introduction

Natural fibers are gaining progressive account as renewable, environmentally acceptable, and biodegradable starting material for industrial applications, technical textiles, pulp and paper, as well as for civil engineering and building activities. The best-known bast fiber plants are flax, hemp, linseed, jute, sisal, kenaf, yucca, abaca, and ramie. These fiber plants have been grown, processed, and used in different climatic zones of the world since about 5000 B.C. Nowadays, the application of these fiber plants is an important contribution to a sustainable development in the course of the current globalization of the industry. Growing and application of natural fiber plants contribute to recycling of carbon dioxide from the atmosphere and to holding it well on the earth biologically. Actually, the estimated worldwide production of bast fiber raw materials is about 25 million t,1 which is equal to approximately 6 million t of pure fibers. The first-rate mechanical characteristics of these natural fibers permit the substitution of synthetic, glass, and carbon fibers in a wide range of industrial products. The weight of the natural fibers is about two thirds and the consumption of primary energy for their production is only one third that of glass fibers at a comparable strength. Therefore, natural fibers embedded in plastics will soon compete strongly with conventional reinforcing fibers. Remarkable strength, high stiffness, and dimensional stability of light constructional elements make it possible to essentially reduce the weight of aircraft, trains, trucks, and cars. Technical designers have long been talking about the “end of the metal age,” and a silent revolution will take place in the construction of aircraft and vehicles during the next ten years. The industrial application of natural fibers requires making highquality fibers continuously available in large quantities at competitive prices and independently of weather conditions and annual yields. Conventional processing technologies cannot meet the strict demands of modern industries. Consequently, new technologies have to be developed in order to successfully set up powerful process plants for natural bast fiber. The processing needed for bast fibers depends on the quality of the input material (the straw), the type of end-product being produced, and the specific capabilities of the machinery on which it will be produced. The following chapter will show aspects, trends, data, and the possibilities for such processing from the present point of view. For reasons of economic efficiency, all components of the bast fiber plants have to be processed and utilized. The main emphasis of this book, however, is on the utilization of the fibers themselves. The treatment of the accompanying shives/hurds, however, will not be discussed in detail.

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Present Growing of Bast Fiber Plants Worldwide—Volume and Yield

The growing of bast fiber plants is spread across all five continents. Qualities and yield depend on the kind of plant, the grown variety, the soil, and the climatic conditions. The total amount of growing biomass of these plants is huge (Table 3.1). However, only a small part of these fibers is used for industrial applications up to now. This shows that the natural potential of the existing bast plants is not yet exhausted and huge natural resources are still available.

3.3

Composition of Bast Fiber Plants

The stalks of the bast fiber plants contain the main part of the fibers. Therefore, the processing of the plants is focused on the stalks. The stalks consist of a peripheral bast ring below the bark and a central wood part (Figure 3.1). The fibers themselves are mainly embedded in the peripheral bast ring and are connected to strong bundles. The bundles consist of 10 to 25 elementary fibers. The elementary fibers have a length of 2 to 5 mm and a diameter of only 0.01 to 0.05 mm. The bast fibers consist of cellulose (~ 65%) and hemicellulose (~ 16%). This medium brings about the high strength of the elementary fibers.3

TABLE 3.1 Estimated Volume and Yield of Grown Bast Fiber Plants, Year 2000/2001 Cultivated Area (ha) Bast Plant Hemp Flax Linseed Jute Kenaf Abaca Ramie Sisal

Total EU countries

Total world

Mean Straw Yield (t/ha)

Mean Yield of Long Fibers (%)

Annual Worldwide Production (Mt)

20,404 94,000 598,111 — — — — —

82,178 614,626 3,489,786 — — — — —

8.0 2.7 1.5 — — — — —

20 to 27 26 to 30 16 to 23 — — — — —

157,800 464,650 942,240 2,900,000 470,000 98,000 170,000 380,000

Sources: From Information Bulletin of the FAO European Cooperative Research Network on Flax and other Bast Plants, No. 2(16), 2001; Kozlowski, R., Proceedings of the International symposium, Instiute of Agricultural Engineering, Bornim, Potsdam, Germany, 2002.

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Cuticula Epidermis Bark fabric Fiber bundle Elementary fiber Inner bark fabric

Cambium Wood body

FIGURE 3.1 Section through a typical stalk.4 (From Ruschmann, G., Grundlagen der Röste—eine wissenschaftlich, technische Einführung für Bakteriologen, Landwirte, Röster, Spinner und Fachschüler, 1st ed. S. Hirzel Verlag GmbH & Co, Leipzig, Germany, 1923. With permission.)

The bundles are connected by lateral ramifications which form a threedimensional network. This network effects the high strength of the fiber bundles. The elementary fibers and bundles are cemented by an intercellular substance, which mainly consists of lignin and pectin. This cementation of the fibers must be demolished or dissolved during the processing to extract the fibers, which can be done mechanically, chemically, thermally, biologically, or by ultrasonic machining. The fiber content of the stalks increases with the length growth and age of the plants. The thickness of the cell walls qualifies the fiber content. Normally, the cell walls are thickest during the blooming period. After this stage a productive fiber harvest can be started.

3.4

Demands on Bast Fibers for Various Industrial Applications

The demands on the fibers and their properties arise both from the technology of the fabrication and the expected properties of the final products. The natural fibers are used in a wide range of products. Present requirements of the frequent applications are shown in the overview given in Table 3.2. These industrial demands on the natural fibers are very strict. Often a fiber type alone cannot meet the demands. Furthermore, a certain natural tolerance Copyright © 2005 by Taylor & Francis

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TABLE 3.2 Examples of Demands on the Fibers of the Manufacturing Industry Required Data/Criteria of Acceptance

Final Product Internal boards and mats of cars, vehicles, trains, aircraft Heat insulation mats for buildings Yarn (spinning machines) Coarse yarn Composites: Injection molding Compression molding Pulp and paper Air-placed concrete Adsorption material for various industries a

Fiber length (mm)

Tensile Shive Moisture Fineness strength content content Putrid Fungal (tex)a (cN/tex) (%) (%) infection attack

70 to 90

3 to 18

ⱖ 40

ⱕ2

ⱕ 10

No

No

50 to 70

3 to 10

ⱖ 30

ⱕ4

ⱕ 12

No

No

30 to 40

2 to 4

ⱖ 40

ⱕ 0.5

ⱕ 12

No

No

30 to 60

2 to 8

ⱖ 40

ⱕ 1.0

ⱕ 12

No

No

4 to 16

ⱖ 40

ⱕ4

ⱕ 12

—

—

2 to 6 ⬍ 18 2 to 20

ⱖ 25 ⬎ 20 —

ⱕ2 ⬍ 12 ⱕ 15

ⱕ 12 ⱕ 14 ⱕ8

— — —

— — —

1 to 2 2 to 4 2 to 4 2 to 8 ⱕ2

1 tex is equal to 1 g/1000 m of a fiber.

of the fibers caused by local growing and climatic influences cannot be avoided. Therefore, different fibers and fiber qualities are blended to meet the industrial requirements and to ensure a lasting and constant quality of the final product, e.g., 50% flax, 25% kenaf, 25% hemp.

3.5 3.5.1

Harvesting Cutting and Drying

The height of the plants and the content of the strong fibers in the stalks, in particular at the ground level zone of kenaf, jute, and hemp, require special harvesters with knives of high-tensile steel, which ensure an acceptable lifetime without service. The fibers are liable to wind around the rotating elements of the machines. Furthermore, the long stalks should be shortened to a length of ⬍ 600 mm to make their handling easier during the next process stages. Therefore, ordinary mowers cannot normally be used, and special harvesters have been developed for fiber plants. Powerful harvesting practices are the following: 1. Hempflax harvest technique: The stalks are cut and shortened by a chopper drum to a length of about 300 mm.5 Copyright © 2005 by Taylor & Francis

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2. Two-level double-knife mower: The stalks are cut twice, near the ground, and at a height of 1.25 m.6 3. Drum mower: Two vertical drums with integrated, flexibly fastened knives cut and shorten the stalks into a length of 600 mm (Figure 3.2). The machine forms a loose swath behind, which is advantageous for the drying and retting processes that follow.7 The drying period of the straw depends on the moisture content during cutting and on local climatic conditions, mainly temperature, wind, and air humidity, as well as the thickness of the stalks. In general, drying takes up to 14 days after cutting in autumn in order to reduce the moisture content to storing properties of ⬍ 14%.8 The next stages depend on the following process technologies. Highspeed decorticators can process the dried plants without any further pretreatment (see Sections 3.6.5.1 and 3.6.5.4). Other decorticating principles, e.g., breaking rollers, require an intensive retting before processing.

3.5.2

Retting

The retting of the fiber plants takes place on the fields. The biological or fermentative process of retting starts under the alternating influence of temperature and moisture caused by dew or rain. Aerobic and anaerobic fungus

FIGURE 3.2 Drum mower for natural fiber plants. (From Kranemann Gartenbaumaschinen GmbH, 2002, Blücherhof, Germany. With permission.)

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and bacteria cultures dissolve the lignin and pectin of the stalk, which causes a separation of the wood body from the bark ring, a separation of the fibers from the bark fabric, as well as the loosening of the elementary fibers inside the fiber bundles. By this, the stalks become mellow and the later mechanical extraction of the fibers is made easier. The fibers themselves resist the fermentative process and remain relatively unchanged during this procedure. Only a certain aging of the fibers takes place, by which the mechanical properties and the color of the fibers are changed (see Sections 3.8.2 and 3.8.3). The retting process depends on moisture and temperature and can take up to 4 weeks. However, local climatic conditions for retting vary. Thus, retting is possible in humid coastal areas but is difficult in the aridity of continental climate zones. After the retting process, the fiber plants have to be dried again, which means a high weather risk, certain losses, and additional expenditures.

3.5.3

Baling and Storage

The harvest period of the fiber plants is limited to only 3 or 4 weeks. The fiber plant straw, processed all year round for economic operation of the plant, has to be stored under a closed roof to ensure lasting constant fiber qualities. For that, the straw is baled into round bales (diameter 1.5 or 1.8 m, length 1.2 m) or rectangular bales (2.5⫻1.2⫻0.8 m) (Figure 3.3). It is advantageous to cut the straw stalks during baling. Thereby, the density of the bales is increased up to 210 kg/m³. Thus, the bales have a total weight up to 500 kg each, which improves the economy of transportation and storage, especially in cases of long distances.

3.6 3.6.1

Processing of Natural Fiber Plants Process Overview and Description

In the present practice mechanical processes are used only for the treatment of fiber plants. The basic technology of all processing plants is similar and independent of the kind of fiber plants processed or the working machines used. The process starts with the opening of the stored bales and ends with the cleaned final products that are technically long fibers, short fibers, and shives (Figure 3.4). Stage 1 (Figure 3.4) is the reception and the opening of the bales inclusive of the elimination of the baler twine. If the straw was not precut during baling, it would be advantageous to shorten the straw stalks to a length of 300 to 400 mm in the course of bale opening in order to facilitate their further handling and metering. Copyright © 2005 by Taylor & Francis

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FIGURE 3.3 Round and rectangular bales of flax.

9 Cleaning of waste air 1 Opening of bales Fiber plants

2 Elimination of metallic impurities and stones

3 Metering of long straw stalks

4 Decortication of fibers

5 Cleaning of fibers

Waste air

6 Opening of fibers Technical long fibers

7 Separation of short fibers/tows and shives 8 Fractionation of shives

Short fibers/tows Shives

Several fraction of shives

FIGURE 3.4 Process flow sheet of fiber plant processing.

The fiber plant straw often contains metallic impurities, stones, sand, leaves, and seeds which have to be eliminated before its processing in stage 2 in order to protect the highly sensitive working machines and to prevent sparks and fire. Copyright © 2005 by Taylor & Francis

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The mass flow of stages 1 and 2 is interrupted by changing bale feeding and breakup of the bales. Therefore, in stage 3, a metering system with a technological buffer is inserted into the machine line which converts the incoming straw into a continuous mass flow to ensure stable working effects for machine processing that follows. After this preparatory work, the extraction of the fibers follows, stage 4 – also called decortication. The object of decortication is: ● ●

● ●

to demolish the stalks to completely loosen and cancel the natural adhesion between fibers and wood/shives to extensively open, disaggregate, and thin the fiber bundles, and to separate the fibers from the shives as far as possible.

The first step of decortication is mechanical in most cases. Other methods, such as steam explosion and chemical or biological disintegration, are possible. They are preferably used, however, for the decortication of pretreated fibers in a second step to improve, for instance, fiber fineness for textiles. The discharge of the decorticators is always a mixture of technically long fibers and a certain percentage of remaining shives, as well as shives containing a certain amount of short fibers and tows. The fibers can be easily cleaned from the wood particles and shives if the adhesion is dissolved completely and no material linkages between the fibers and shives exist. The fibers are cleaned by mechanically loosening up and shaking the mixture – stage 5. The heavy particles are accelerated and separated by gravity. Due to their rough surface, the cleaned fibers link together during processing into an undefined random structure. This structure is improved by fiber openers or by carding systems, which draw and parallel the fibers – stage 6. After this procedure, the fibers are loose and have only a very small bulk density. Therefore, the fibers are compacted by big balers up to a density of 350 kg/m³ for temporary storage or transportation. During processing of the fiber plants, shives arise from decortication and fiber cleaning, stages 4 and 5 (Figure 3.4). Especially the shives coming from the cleaning machines contain up to 20% of short fibers and tows, which are deliberately separated from the technically long fibers during processing. These short fibers have a length of 8 to 20 mm and can be recovered from the shives. Revolving screens and zigzag sifters are mainly used for this purpose. The particle sizes of the shives range widely from fine dust up to 12 mm. Normally, the shives are fractionated into different particle size classes to get a defined particle size distribution for different applications. The fiber plants are processed in a moisture range from 9 to 14%. Thus, a lot of dust arises, especially in the process stages with fast-running working tools, e.g., stages 3 to 6. The amount of dust can be up to 2% of the total mass processed. Powerful suction plants with integrated modern bag house filters are used to suck off the dust, to clean the waste air, and to prevent the dust Copyright © 2005 by Taylor & Francis

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from accumulating around the machines. Modern filtering systems meet the emission limits of 30 mg/m³ of organic dust.9

3.6.2

Opening of the Bales and Shortening of the Stalks

There are three different basic processes used for opening the bales. They are the guillotine, the hopper feeder, and the bale cutter. The guillotine is a very robust and efficient machine. A static knife tears the bales into segments under the high pressure of hydraulic cylinders. The stepwise portioning of the straw at adjustable sizes is done in the same machine. The advantages and disadvantages of this principle are presented in Table 3.3. TABLE 3.3 Advantages and Disadvantages of the Bale Openers Opening Principle

Advantages

Disadvantages

Guillotine

Simple robust working tools Unequal and discontinuous Precut of stalks not required mass flow of discharge Suitable for round and rectangular Short lifetime of the knife, bales frequent maintenance High capacity up to 4 t/h required Stalks are shortened to a Big floor space required defined length Bale twine needs not be removed before cutting, if it is of natural fibers Insensitive to impurities and stones No dust generation

Hopper feeder

Even and continuous mass flow of discharge Long lifetime of the working tools

Precut of stalks required Suitable for rectangular bales only Capacity up to 800 kg/h only Bale twine has to be removed before cutting Sensitive to impurities and stones High dust generation, dust extraction set required

Bale cutter

Precut of stalks not required Suitable for round and rectangular bales Capacity up to 2 t/h Stalks are shortened to a defined length Bale twine has not to be removed before cutting, if it is from natural fibers No dust generation

Bale fixing and turning complicated Sensitive to impurities and stones Uneven and discontinuous mass flow of discharge Short lifetime of the knife, frequent service required

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The hopper feeder is suitable for rectangular bales only. The bales are slowly moved by an endless floor conveyor but at an adjustable controlled speed against a fast-running steep belt conveyor or rotating drums (Figure 3.5 and Figure 3.6). Both are equipped with special spring steel tines having form-closure for slaving the straw stalks. However, the stalks must be precut into a maximum length of 400 mm to avoid winding and functional malfunctions. The bale cutter consists of three interlocking controlled structural units, the bale holding and turning device, the cutting mechanism, and the discharging conveyor (Figure 3.7). The straw is fed into the bale cutter via a feed conveyor that also serves as a buffer. The bale holding and turning device takes a bale from the feed conveyor and locates it in a centered cutting position. A driven knife disk moves along a surface line of the bale, cutting the straw out strip by strip. The width and the thickness of the strips are adjustable to meet the requirements of the machines following. The capacity of the whole bale opener, however, is essentially determined by these dimensions. The knife disks are of a special tool steel and have a special duplicated cutting edge with a self-sharpening effect in order to ensure a maximum running time without maintenance. The cut strips fall down on a belt conveyor leading horizontally out of the machine.

FIGURE 3.5 Hopper feeder with steep belt conveyor.

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FIGURE 3.6 Hopper feeder with rotating drums.

3.6.3

Elimination of Metallic Impurities and Stones

It cannot be ignored that the straw bales partly contain stones and metallic impurities. In order to protect the sensitive machines later used in processing, an electromagnetic metal detector warns of steel parts and stops the conveyor. The sensitivity of the system is adjustable so that each small metal piece bigger than a 6 mm screw nut can be detected in the straw. A single drop between two conveyors is especially designed to discharge stones by gravity. Simultaneously, sand, dust, leaves, and loose seeds fall out. Up to 70% of the stones can be separated in this way. Stones completely embedded in straw stalks are carried over the drop. These stones can be detected electronically. Such a device is expensive, however, and often is not used. 3.6.4

Metering of Plant Straw

The straw strips coming from the bale opener must be converted into a defined continuous output, thus ensuring a consistent working effect for the machines in the process. The metering systems used (see Figure 3.8) consist of an intermediate bunker (a), the metering device itself (b), and a discharging conveyor (c). Copyright © 2005 by Taylor & Francis

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FIGURE 3.7 Bale cutter.

a

d

b

c FIGURE 3.8 Working principle of a metering system.

The intermediate bunker is equipped with an endless floor, controlling the conveying of the loose straw strips against the discharging metering system at an adjustable speed. This bunker acts simultaneously as a Copyright © 2005 by Taylor & Francis

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technological buffer of the complete machine line to balance variations of the mass flow. The discharging metering system consists of a steep belt conveyor (b) and a retaining rake (d). Both are equipped with special spring steel tines having form-closure for slaving of the straw. The speed of the steep belt conveyor controls the volumetric displacement. The distance between the steep belt conveyor and the retaining rake limits the maximum output. The steep belt conveyor can be replaced by three rotating drums with a diameter of more than 600 mm. The operation of both the steep belt conveyor and the rotating drums requires precut straw with a length of maximally 400 mm to ensure an output tolerance of less than ± 20%. 3.6.5

Decortication

3.6.5.1 Definition and Principles of Decortication Decortication means the disaggregating of the stalks, the loosening of the adhesion between fibers and wood, and the extraction of the fibers. Simultaneously, the fiber bundles should be opened to get thinner bundles or elementary fibers, as far as possible. For this purpose the wooden elements have to be bruised many times without altering the natural fiber structure. Nowadays, three methods are practiced, in contrast to lengthy manual labor procedures used in former times: breaking rollers, swing-hammer mills without an integrated cleaning effect, and swing-hammer mills with an integrated cleaning effect. The advantages and disadvantages of the methods are presented in Table 3.4. 3.6.5.2 Breaking Rollers The fiber plant stalks pass through multiple paired rollers with a shaped cross section (Figure 3.9). The profiled rollers are in gear and buckle and crush the stalks. Decortication happens by alternating bending, buckling, breaking, bruising, and softening of the stalks. The brittle wood breaks, but the thin, elastic, and insensible fibers resist this stress undamaged. Part of the broken wood—the shives—falls out of the fibers. The pitch of the rollers is reduced from one roller pair to the next corresponding to the fiber stream becoming thinner. The capacity of the breaking rollers is limited by the maximum thickness of the layer running through the profile of the rollers. Thicker layers reduce the intensity of the decortication. The capacity of such machines cannot exceed 1 t/h if an efficient decortication effect has priority. Furthermore, the decortication effect depends on the moisture and thickness of the stalks, the degree of retting, as well as the position of the stalk opposite the roller shaft. Usually, unretted fiber plants cannot be efficiently processed by breaking rollers because the stalks are too elastic and the adhesion between the fibers and the wood is still too strong. Copyright © 2005 by Taylor & Francis

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TABLE 3.4 Advantages and Disadvantages of Various Decortication Methods Decortication Method

Advantages

Disadvantages

Breaking rollers

Precutting of stalks not required Long fibers are possible Big shives Less dust generation Simple robust working tools Long lifetime of the working tools Shives are partly discharged during the processing

Retting of the plants required Poor separation of fibers and shives No loosening of fiber bundles Poor fiber fineness Fibers are hard to clean and have a high percentage of remaining shives Capacity limited to 1 t/h Controlled charging required Sensitive to variable and higher moisture content, limit 12% Shortening of the fibers impossible Hard fiber cleaning in the following stages at a remaining shive content of finally ⬎ 12%

Hammer mill without integrated cleaning effect

Suitable for retted and unretted fiber plants, retting of the plants not required Insensitive to unstable moisture content High capacity up to 6 t/h Good loosening of fibers and shives Shortening of the fibers possible

Precutting of stalks required Fibers and shives are not separated completely No loosening of fiber bundles Poor fiber fineness Maximum fiber length 200 mm No discharging of shives while processing High dust generation, dust extraction set required Limited lifetime of the working tools Possible fiber cleaning in the following stages at a remaining shive content of finally ⬎ 6%10

Hammer mill with integrated cleaning effect

Suitable for retted and unretted fiber plants, retting of the plants not required Insensitive to variable moisture content up to 18%11 Proven capacity up to 3 t/h12 Complete loosening of fibers and shives Good loosening of fiber bundles Best fiber fineness by mechanical processing13 Discharging of up to 60% of shives while processing Simplified fiber cleaning at the following stage at a remaining shive content of ⬍ 2%12

Precutting of stalks required Mean fiber length of max. 150 mm Wide particle size distribution High dust generation, dust extraction set required Limited lifetime of the working tools

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Breaking rollers, synchronous

Fibers with remaining shives

Shives

FIGURE 3.9 Principle of decortication by breaking rollers.

3.6.5.3 Hammer Mills without Integrated Cleaning Effect Decorticating hammer mills operate according to the principle of a swinghammer mill. These mills consist of a cylindrical mill space with interior guidance plates and a fast-running rotor inside. The rotor is equipped with swinging beaters—the hammers. The special hammers used for the decortication of fiber plants have a thickness of only 2 to 3 mm. The operating peripheral speed of the hammers is in the range of 40 to 60 m/s and has to be adjusted according to the mechanical and specific properties of the stalks. The straw is vertically charged directly into the working area between the hammers and the machine case. The fiber decortication is effected by impact stress of the hammers at the surface of the straw stalks. The interior guidance plates lead the decorticated fibers—the mixture of fibers and shives—in an axial direction through the machine to the discharge pipe. Such hammer mills operate at a capacity of up to 6 t/h. 3.6.5.4 Hammer Mills with Integrated Cleaning Effect The latest development of a decorticator is a hammer mill with an integrated cleaning effect (Figure 3.10).14 The basic structure and the working principle are the same as those of known hammer mills. However, the bottom of the case is equipped with a screen and provides for a permanent sorting of the material. The disintegrated pieces of wood—the shives—pass the discharging area of the screen at the bottom and are sucked off from the machine pneumatically for further separate cleaning or treatment.15 Up to 60% of the shives are separated in this way. The separation of the shives during the process intensifies the action of the hammers on the remaining fiber bundles. Furthermore, the holes of the screen are additional impact cants for the running fibers. Both the separation of the shives and the Copyright © 2005 by Taylor & Francis

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Precut straw Roof baffle plate Finger plate

Shear plate Steel beaters

v

Screen Fibers with remaining shives

Shives

FIGURE 3.10 Principle of a hammer mill with integrated cleaning effect.

additional impact cants improve the decortication effect and the fineness of the fibers. The decorticated fibers and the remaining part of the shives run about the screen bottom and are pneumatically axially discharged to the fiber cleaning machines following.15 The excellent decorticating effect of these machines makes the following fiber cleaning simpler. Thus, the intensive and expensive cleaning process is at the same time simplified. First prototypes of such machines have been calculated and tested for a permanent capacity of 3 t/h. Both retted and unretted fiber plants can be processed with these mills. 3.6.6

Fiber Cleaning

3.6.6.1 Working Principles and Tools Fibers have to be separated from shives and dust at the cleaning stage. Often, the short fibers of a length ⬍ 20 mm also must be eliminated from the technically long fibers. The bast fibers are cleaned in several steps that are usually the coarse, fine, and final cleaning. How many cleaning steps are used depends on the quality of the previous decortication and the purity requested. For instance, when using a hammer mill with integrated cleaning effect, which ensures a complete loosening of fibers and shives, only two cleaning steps are needed to reduce the remaining shive content of the fibers below 2%.11 Copyright © 2005 by Taylor & Francis

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Three working principles are used for cleaning of the bast fibers: the scutching turbine or tambour, the multiple ultracleaner, and the comb shaker. Further known cleaning machines are straight-pin, needle, and sawtooth rollers in different shapes and combinations. These machines are often used in the textile industry for the secondary afterpurification of opening rooms.

3.6.6.2 Scutching Turbine/Tambour The fibers are charged by feed rollers continuously into the beating cycle of the tambour, which is hard faced with beater blades or pins (Figure 3.11). The fibers become arched by the beats and are conveyed between the beaters and the bar screen. Shives and dust pass the screen. At the fibers adhesive shives are abraded. These tambours have a high capacity of up to 500 kg of fibers/m of working width and an excellent separating capacity. Therefore, they are mainly used for coarse cleaning of the fibers. 3.6.6.3 Multiple Ultracleaner This fiber cleaner is also called a step cleaner because up to six drums are placed like stairs (Figure 3.11). In opposite direction, working drums are hard faced with beater arms which loosen up the fiber stream intensively and generate an airflow. The extreme loosening, relaxation and mixing of the fiber volume brings about an excellent selection of fibers and shives. The airflow carries the fibers upward between the drums and the floor grids and prevents fiber material from being misplaced. Simultaneously, the airflow frees the fibers from dust, which is an essential advantage over other principles. Only a few short fibers are discharged through the floor grid. However, the shives pass the floor grid, which has an adjustable gap width for different particle sizes. Multiple ultracleaners are used for fiber fine cleaning. The remaining shive content is less than 3%, mostly after the fibers have passed a tambour and a multiple ultra cleaner.11

Feed rollers

Tambour

Beater arm

Oscillating teeth rows

Floor grid, adjustable

Screen, adjustable Tambour

Multiple ultracleaner

FIGURE 3.11 Principles of bast fiber cleaning mainly used.

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Floor grid, adjustable Comb shaker

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Multiple ultracleaners are very robust machines with a capacity of up to 700 kg/m of working width.

3.6.6.4 Comb Shaker The comb shaker consists of a floor grid with an ascending gradient and oscillating rows of teeth which are placed like a comb (Figure 3.11). The fiber layer is reciprocated upward. The shives fall out of the loosened fibers by gravity and pass the grid. Normally, comb shakers are used for the final cleaning of the fibers. However, the reciprocating movement and low speed limit their capacity.

3.6.7

Opening of the Fibers

3.6.7.1 Object of Fiber Opening Natural fibers grow in the stalks of plants in the form of bundles (Figure 3.1). The decorticated fibers mainly consist of such bundles and contain a certain amount of bark fabric— the bast—as well as remaining shives. Opening means to unravel the fiber bundles mechanically into thinner units or elementary fibers. This opening improves fiber fineness, whereas the remaining bark fabric and the shives can be simultaneously separated. For instance, the fineness of hemp fibers is in the range of 8 to 15 tex after decortication, which can be increased by mechanical opening to 3 to 5 tex.16 Further fiber opening is possible by steam, enzymatic, chemical, or thermal treatment. 3.6.7.2 Opener The fibers are fed by feed rollers to one or several opening cylinders arranged in tandem. The opening cylinders are equipped with needle, pin, or a sawtooth garniture with different pitches (Figure 3.12) and run with a rotational speed of more than 30 m/s. The saw teeth draw the incoming fibers over several passive carding segments, plates, and knives. The fibers are combed, stretched, drawn out, expended, and paralleled. This procedure unravels the fiber bundles and improves the fiber fineness as well as their structure. The opened fibers are sucked off pneumatically. The knives strip the bast residues from the fibers and discharge these residues together with thick bundles and shives. The capacity of the opener is up to 650 kg/m of working width for processing bast fibers. 3.6.7.3 Carding Machine The heart of each carding machine is a fast-running drum equipped with fine hook-type pin bars of spring steel. Several working and turning drums are arranged in pairs at the periphery of the main drum. They are also hard Copyright © 2005 by Taylor & Francis

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FIGURE 3.12 Feed rollers and opening cylinder with a sawtooth garniture of an opener.

faced with hook-type pin bars. These active working and turning drums run in the same direction as the main drum, but the peripheral speed of the main drum is higher than the speed of the working drums. The fibers have to be fed continuously into the machine by feed rollers. By the ganging of main and working drums the fibers are flattened, compressed, and paralleled to form a thin card sliver or fleece. Simultaneously, the opening and refining of the fibers takes place. Small impurities are eliminated. A stripper roller of the card lifts the sliver from the main drum and moves it out of the machine. The capacity of card machines is up to 500 kg/m of working width. The carding is costly and is used only if the technical bast fibers are applied for manufacturing of fleeces.

3.6.8

Cutting of Fibers into a Defined Length

Fiber plants are cut before the decortication, which also shortens the fibers. A further undefined tearing by mechanical stress takes place during decortication and opening of the fibers. This shortening is more or less undefined. Therefore, the fibers have a wide length range between a few millimeters and Copyright © 2005 by Taylor & Francis

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250 mm and more after processing (Figure 3.16). However, special applications require defined fiber lengths (Table 3.2). A defined cut of the very thin but high-strength fibers makes great demands on the machines, their working tools, and the operating parameters. An exact cutting without stress and damage of the fiber structure is only possible when using a very accurate adjusted blade and counterblade. In principle, a guillotine or rotating knives of drum or disk wheel choppers can be used. In general, the guillotine is preferred for very short fibers with a smaller fiber length distribution (Figure 3.13). The adjustable theoretical cut length starts at 2 mm. The real cut length, however, depends on the feed position of the fibers relative to the blade. Fibers in parallel position to the blade are not cut. This causes the typical length distribution (Figure 3.17). The fibers have to be paralleled before cutting, e.g., by carding, to narrow the length distribution.

3.6.9

Baling of Long Fibers

The bulk density of processed dry natural fibers is in the range of 20 to 30 kg/m³ only. Therefore, the fibers have to be baled for handling, transportation, and temporary storage.

FIGURE 3.13 Guillotine cutting flax fibers, theoretical cut length 8 mm.

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Hydraulic, crank, and knuckle-joint bale presses are used to compact the fibers up to a bale density of 150 to 200 kg/m³. The bale sizes are well matched to the dimensions of ISO containers and modern trucks. Usual dimensions are: 0.7 ⫻0.7⫻1.2 m and 0.8⫻1.0 ⫻1.2 m. 3.6.10 Cleaning of Waste Air Large amounts of dust arise when processing dry natural fiber plants. Machines with high-speed working tools generate a lot of dust and airflows. These airflows and the waste air of the pneumatic conveying systems have to be cleaned. The dust is mainly fine organic plant material, which also contains mineral components. The dust has to be eliminated from the fibers and shives and must not enter into the room and the environment. Furthermore, industrial hygiene regulations and emission standards must be met. Consequently, the dust has to be eliminated by sucking off large volume flows. There are two kinds of dust in a processing line which have to be collected and treated separately. The dust arising before the decortication at the bale opener and the metering systems contains mainly plant particles, leaves, and mineral dirt. This dust can be separated from the air by ordinary bag house filters and discharged into barrels for compost preparation or disposal. The dust arising at the decortication and the fiber cleaning stages contains up to 60% of valuable short fibers at a length of 1 to 4 mm and 40% of fine dust of organic plant material. It is advantageous to separate the short fibers by a filtering screen at a first cleaning stage. This screen has to be equipped with an automatic stripping brush roller to ensure a stable differential pressure. The separated short fibers are used directly for the production of composites or as an adsorption material. At a second cleaning stage, a downstream bag house filter separately eliminates the fine dust of the waste air that has passed the screen in front.

3.7

Components Arising from Processing and Fiber Yields

When processing natural fiber plants like hemp, flax, or linseed and using a hammer mill with an integrated cleaning effect, a tambour, a multiple ultracleaner, and a fiber opener, components arise in different qualities and quantities (see Figure 3.14). These data, however, mainly depend on the respective processed variety as well as on the quality of the input material. The separate discharging of the components is an essential advantage for the best possible application in the industry for the production of special final products. For instance, the long fibers used for the production of interior panels for the automotive industry and for isolating mats should not contain any fibers shorter than 20 mm. The practical ranges of fiber yields are presented in Table 3.5. Copyright © 2005 by Taylor & Francis

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70

66 61

Percentage by mass (%)

60

56

50 Hemp Flax Linseed

40 29

30 24 20

18

10 4 3 3

3 3 3

1 2 1

2 2 1

5 4 6

0 Long fibers Short fibers, Short fibers, Short fibers, Shives, 8 to 20 mm 4 to 8 mm 1 to 4 mm different sizes

Dust

Loss

Components of processing

FIGURE 3.14 Mean percentage by mass of components arising at the processing of hemp, flax, and linseed.11,12,17

TABLE 3.5 Range of Practical Fiber Yields Range of Fiber Yield (%)

Hemp Flax Linseed

3.8 3.8.1

Fiber length 20 to 200 mm

Fiber length ⬍ 20 mm

23 to 27 27 to 30 17 to 23

4 to 8 4 to 9 3 to 7

Mechanical Properties of Processed Fibers Fiber Length

The effective length of the technically long fibers after processing is based on the type of plant, the processing technology, the precut length of the stalks, and the operation data of the decorticator. A typical length mixture of processed fibers for technical application is shown in Figure 3.15. However, the length of natural fibers varies in a wide range from 10 to 250 mm. Typical fiber length distributions are shown in Figure 3.16. Copyright © 2005 by Taylor & Francis

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FIGURE 3.15 Cleaned flax fibers, mean fiber length 70 mm.

The average fiber length required for many industrial applications is approximately 80 mm, which can be met by adjusted operation parameters, accordingly. Fibers shortened by defined cutting have a comparable length distribution on a lower level. (Figure 3.17). The distribution curves show that mean fiber length can be adjusted to approximately 4 or 8 mm. The range of tolerance is about ⫾ 5 mm. Only 5% of the fibers are longer than 13 mm. Such a distribution should be suitable for the production of composites in most cases.

3.8.2

Fineness

Fiber fineness is a measure of the degree of loosening up the natural fiber bundles into thinner bundles or elementary fibers. Modern decortication methods, e.g., the hammer mill with integrated cleaning effect, provide a high fiber fineness of less than 15 tex (Table 3.6). Pretreatment of fiber bundles by retting improves the fineness of fibers after processing. The fibers of flax and linseed, which have thinner natural fiber bundles, show a higher fineness after processing. Copyright © 2005 by Taylor & Francis

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Percentage of fiber in length class (%)

30

25 Hemp Flax Linseed 20 Mean fiber length Hemp: 118 mm Flax: 72 mm Linseed: 55 mm

15

10

5

0 10

30

50

70

90

110 130 150 170 190 210 230 250 270 290

Mean fiber length of class (mm)

FIGURE 3.16 Typical fiber length distributions after processing,13,17 measured according to DIN 53808.

Percentage of fiber in length class (%)

80 70 Hemp, theoretical cut length 4 mm Hemp, theoretical cut length 8 mm Flax, theoretical cut length 4 mm Flax, theoretical cut length 8 mm

60 50

Mean fiber length Hemp, 4 mm: 4.43 mm Hemp, 8 mm: 6.28 mm Flax, 4 mm: 5.35 mm Flax, 8 mm: 9.43 mm

40 30 20 10 0 3

8

13 18 23 28 33 Mean fiber length of class (mm)

38

FIGURE 3.17 Length distribution of cut fibers.13

The fineness of hemp fibers meets the requirements of boards, mats, and fleeces, ⬍ 18 tex in each case (compare Table 3.2). Retted hemp as well as unretted flax and linseed meet the requirements of heat insulation mats, coarse yarn, and injection and compression molding. Copyright © 2005 by Taylor & Francis

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TABLE 3.6 Fineness of Natural Fibers After Their Processing in a Hammer Milla Fineness (tex) Mean Diameter (equal to g/1000 m of Fiber Bundle of fiber length) (µm)

Fiber Hemp, unretted Hemp, less retted Hemp, retted Flax, unretted Flax, unretted, 2 passages through the hammer mill Linseed, unretted

14.9 8.1 7.4 4.1 3.3

114 84 81 60 54

2.7

49

a

Measured according to DIN EN ISO 1973. Source: From Blacha, J. and Rawluk, M., Research Laboratories of the Institute of Natural Fibres, Poznan, Poland, 2003. With permission.

The requirements of fine yarn and pulp and paper are only met by flax and linseed fibers. Hemp fibers must be treated and opened further for such purposes. The substance density of natural fibers amounts to 1.45 g/cm³. On the basis of this density, the mean diameter of a fiber bundle can be calculated for the measured fineness using the following equation (1) (Table 3.6): 4·f·10 ᎏ 冪莦 π·ρ

d⫽

3

(3.1)

where d is the diameter (in microns), f is fineness (in tex), and ρ is substance density 1.45 (in g/cm³).

3.8.3

Tensile Strength

The mechanical properties are influenced by several external effects, which can result in wide ranges of tolerance of their parameters. Due to this fact, harvest, storage, and processing have to be supervised and controlled as far as possible in order to eliminate the loss of high natural fiber strength and to get stable qualities. Under normal conditions, the forces and strengths as presented in Table 3.7 can be deemed to be measured and ensured. The retting process increases the mean tensile strength of the hemp fibers. That means the fibers become stronger and harder. Simultaneously, elasticity and elongation decrease (Table 3.8). The finer fibers of flax and linseed have approximately the same tensile strength as hemp fibers. This high performance at excellent fineness favors the flax and linseed fibers for application in composites. Tensile strength of natural fibers is comparable to the strength of high-tensile steel which is classified by a strength of ⬎ 450 N/mm² (65,000 lb/sq. inch). Copyright © 2005 by Taylor & Francis

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TABLE 3.7 Measured Tensile Forces and Strengths of Natural Fibersa Tensile Force (N)b

Fiber Hemp, unretted Hemp, less retted Hemp, retted Flax, unretted Linseed, unretted

5.0 3.1 3.2 1.8 1.0

Tensile Strength Tensile Strength (N/mm²) (cN/tex) 490 560 620 640 550

33.8 38.9 43.2 44.3 38.4

a

Measured according to DIN EN ISO 5079, free clamping length 15 mm, tensile speed 10 mm/min. b Calculated for a single fiber or a fiber bundle. Source: From Blacha, J. and Rawluk, M., Research Laboratories of the Institute of Natural Fibres, Poznan, Poland, 2003. With permission.

TABLE 3.8 Examples of Measured Elongationsa Fiber Hemp, unretted Hemp, retted Flax, unretted

Modulus of Elasticity (kN/mm²) 30 to 50 40 to 60 24 to 40

a

Measured according to DIN EN ISO 5079. Sources: From Eastern Thuringia Company of Material Test for Textiles and Plastics, Rudolstadt-Schwarza, Germany, 1999; Brüning, H., Diploma thesis, Technical University of Berlin, 2002.

3.8.4

Elongation

The single natural fiber is a three-dimensional body of cells forming a chain. In other words, the natural fiber is a three-dimensional biopolymer.19 Single fibers are interlinked and twisted into stronger bundles. This structure brings about an excellent expansibility at a high performance. The modulus of elasticity of natural fibers depends on the types of plants and the degree of retting. The measured data vary in the range of from 24 to 60 kN/mm² (Table 3.8). The modulus of elasticity is about one seventh of the modulus of steel. This modulus marks the outstanding tensile stress of fibers. Because of this, natural fibers are predestined for application in composites and construction components requiring high package crushability, e.g., bumpers of cars. 3.8.5

Cleanness

The impurities of natural fibers are shives and dust. Cleanness of the fibers mainly depends on the efficiency of decortication. Normally, required fiber Copyright © 2005 by Taylor & Francis

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cleanness is reached by two cleaning steps thanks to the excellent decortication process of modern hammer mills with an integrated cleaning effect (Table 3.9). The limit of ⬍ 2% of remaining shives is easily reached when processing retted or unretted hemp. Unretted flax and linseed have very elastic and flexible stalks. Up to now such plant straws have had to pass the decorticator twice to meet the limit of cleanness. Currently, work is going forward to improving the cleanness in one passage by increasing the working intensity and the dwelling time inside the machine. Dust elimination works in parallel with sucking off the airflow from the multiple ultracleaner or the tambour.

3.9 3.9.1

Comparative Valuation of Technologies and Fiber Data Operational Data

The decortication machines used determine the operational data of a plant. Therefore, these machines are the key machines of the whole process, the capacity, the fiber data, and the economic data. Currently, breaking rollers and hammer mills are primarily in use. The choice has to cover the goals of the process18 (Table 3.10). 3.9.2

Fiber Data

The valuation of the fibers has to take place in relation to the individual application. A general overview with regard to the decortication process is given in Table 3.11. TABLE 3.9 Remaining Shives Content of the Fibers After a Two-Step Cleaning Process Fiber Hemp, unretted Hemp, retted Flax, unretted Flax, unretted, 2 passages through the hammer mill Linseed, unretted Linseed, unretted, 2 passages through the hammer mill

Average Remaining Shives Content (%) 1.2 0.85 3.5 ⬍ 0.5 2.3 ⬍ 0.8

Sources: From Munder, F., Hochschule Magdeburg, Sterdal, 2003; Test Report No. 03/02/3, 2003; Test Report No. 02/12/3, Institute of Agricultural Engineering, Potsdam, Germany, 2002. With permission.

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TABLE 3.10 Operational Data of Processing Plants Currently Operated Operational Data

Criteria Capacity Previous retting of plants required Maximum of moisture content Loosening of fibers and shives Fiber yield Shortening of fibers to prevent malfunctions of the following machines Discharging of shives during decortication Percentage of short fibers in the shives Simplified fiber cleaning in 2 steps following the decortication Power consumptionb Available machine time Wear of working tools a b

3.10

Breaking rollers

Hammer mills Hammer mills without cleaning with cleaning effect effect

ⱕ 1 t/h Yes

ⱕ 6 t/h No

ⱕ 3 t/h No

14%

18%

18%

Incomplete

Complete

Complete

High Not possible

Reduceda Possible

Reduceda Possible

Partly

No

Partly

⬍ 5%

⬍ 10%

⬍ 10%

No

No

Yes

20 kWh/t 80% Less

50 kWh/t 80% Yes

35 kWh/t 80% Yes

Reduced by a certain amount of short fibers discharged with the shives. For decortication only.

Economic Aspects

Natural fibers have to be grown and processed very economically and at competitive prices, as compared with man-made fibers produced by modern industries. Therefore, the first processing stage of the low-level product—the fiber plants—has to take place locally and be done by powerful processing plants in the vicinity of the farms. The plants should be grown in a circular area around the processing plant within a radius of maximally 30 km in order to keep transportation costs low. A proportional crop rotation has to be considered. After the first processing stage, the higher added-value products—the components of processing (see Section 3.7)—can be transported separately to the industry for special applications. The machine lines of processing have to be configured individually for each special case. Consequently, the needed investment of a complete processing plant depends essentially on the bast plant itself to be processed and the quality demands on the fibers of the manufacturing industry. Copyright © 2005 by Taylor & Francis

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TABLE 3.11 Fiber Properties after Processing Operational Data Breaking rollers

Criteria Fiber length adjustable Remaining shive content Fineness Tensile strength Opening and loosening of fiber bundles Rough surface Branched fiber ends Knot formation, pilling Misplacing of short fiber

a

No ⬎ 8% Coarse Close to the natural strength Little opening effect, strong fiber bundles still Surface unchanged No No Little

Hammer mills without cleaning effect

Hammer mills with cleaning effect

Yes ⬍ 5% Middle Slightly reduced

Yes ⬍ 2% Fine Slightly reduced

Opened partly

Good opening and loosening effect

Partly roughened Yes Partly possible A certain percentage has to be considereda

Partly roughened Yes Partly possible A certain percentage has to be considereda

Separation of short fibers from the technical long fibers mostly requested.

TABLE 3.12 Recommendations for the Economic Operation of Processing Plants Capacity Operating time Available machine time Fiber plant consumption Fiber yield Minimum operation period

ⱖ 3 t/h ⱖ 2,000 h/y ⱖ 80 % ⱖ 4,800 t/y ⱖ 100 t/month 10 y

From an economic point of view, the requirements and recommendations for the successful engineering, establishment, and operation of processing plants are given in Table 3.12. There is a large and increasing demand for high-quality upgraded natural fibers. Flexible processing and stable fiber qualities are the criteria for industrial application.

References 1. Kozlowski, R., Green Fibers and their Potential in Diversified Applications, Common Fund for Commodities, Technical Paper No. 14, Proceedings of a FAO Seminar, Rome, Dec. 13, 2000.

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2. Anonym, Information Bulletin of the FAO European Cooperative Research Network on Flax and other Bast Plants, No. 2(16), Institute of Natural Fibres Coordination Centre, Poznan, Poland, Dec. 2001. 3. Kozlowski, R., Future Directions in Usage of Natural Renewable Raw Materials, Proceedings of the International Symposium, Institute of Agricultural Engineering, Bornim, Potsdam, Germany, Sept. 2002. 4. Ruschmann, G., Grundlagen der Röste–eine wissenschaftlich, technische Einführung in für Bakteriologen, Landwirte, Röster, Spinner und Fachschüler, S. Hirtzel Verlag, Leipzig, 1923. 5. Mastel, K. et al., Untersuchungen zu pflanzenbaulichen, erntetechnischen und ökonomischen Fragen des Anbaues von Faser- u. Körnerhanf. Landesanstalt für Pflanzenbau Forchheim, Informationen für die Pflanzenproduktion 7, 1998. 6. Ehlert, D., Zweiebenen-Schneidwerk für die Hanfernte, Landtechnik 52, 1, pp. 25–26, 1997. 7. Kranemann, H., Tests results of the drum mower Blücher 02, personal information, Dec. 1999. 8. Gusovius, H-J., Stoffwandlungen und Umwelteinflüsse in Verfahrensketten für Faserhanf, Dissertation Humboldt-Universität Berlin, Cuvillier Verlag Göttingen 2002, Forschungsbericht Agrartechnik, Max-Eyth-Gesellschaft Agrartechnik im VDI 390. 9. Anonym, Technical Administration Regulation for Keeping Clean of the Air – TA-Luft, German Federal Ministry of Environment, Protection of Nature and Reactor Safety, Edition July 24, 2002. 10. Betrencourt, Y., La Chanvriere De L`Aube, Bar sur Aube, personnal message, Mar. 2003. 11. Munder, F., Ergebnisbericht zum Faseraufschluss von Hanf Forschungs- und Entwicklungsprojekt zur Hanffasergewinnung des BMBF, Hochschule Magdeburg, Stendal, Bericht-Nr. 03/01/1, Feb. 2003. 12. Munder, F., Test Report about Processing of Hemp, Test Report No. 03/02/3, La Chanvriere De L`Aube, France, Feb. 2003. 13. Blacha, J. and Rawluk, M., Report of fibre testing No. LW/08/03, Research Laboratories of the Institute of Natural Fibres, Poznan, Poland, 2003, 12pp. 14. Fürll, Ch. and Hempel, H., Gewinnung von Naturfasern durch Prallbeanspruchung, Chem. Technik, 51, 6, 1999. 15. Fürll, Ch. and Hempel, H., Optimieren einer neuen Maschine zum Faseraufschluss durch Prallbeanspruchung, Freiberger Forschungsreihe A 852 Ökologie, 1999. 16. Anonym, Measurement of material properties of hemp samples, Test Report No. 3.5 P/23/99 and 3.5/73/99, Eastern Thuringia Company of Material Test for Textiles and Plastics, Rudolstadt-Schwarza, Germany, 1999. 17. Munder, F., Processing of linseed straw to cleaned fibers and shives, Test Report No. 02/12/3, Institute of Agricultural Engineering, e.V. Potsdam, Germany, 2002. 18. Brüning, H., Methodische Entwicklung eines Anlagenkonzeptes zum Faseraufschluss in der Hanfverarbeitung (Methodical development of a plant concept for the processing of hemp), Diploma thesis, Department of Engineering, Technical University of Berlin, 2002. 19. Rowell, R.M., Chemical Modification of Natural Fibers to Improve Performance, USDA, Forest Products Laboratory, Madison, WI, USA, Presentation at the ICFPAM, Bucharest, June 2003.

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4 Recent Developments in Retting and Measurement of Fiber Quality in Natural Fibers: Pro and Cons

Roy B. Dodd and Danny E. Akin

CONTENTS 4.1 Introduction 4.1.1 Dew Retting with Glyphosate 4.1.2 Enzyme Retting 4.1.3 Chemical/Mechanical Retting 4.2 Standards for Flax Fibers 4.2.1 Color 4.2.2 Fineness 4.2.3 Trash (Nonfiber) Components 4.3 Conclusions References

ABSTRACT Natural fibers have always experienced a large variation in quality parameters such as strength, fineness, color, and trash content. These variations have been a barrier to large-scale applications because of their impact on the quality of the final product. Retting of the fibers is a process that causes much of this variation. Dew retting of flax fibers produces the majority of the commercial fiber. Because this is based upon weather conditions, large differences can occur from year to year. The use of glyphosate to desiccate a standing crop and start the retting process of all plants in the field has potential to increase the uniformity of the retting processing. Recent tests show that fiber equal to or better than traditional dew retting can be achieved. However, the cost of application and the increased time for the retting process will require further research. Enzyme retting replaces dew retting in the field by applying an enzyme formulation at the central processing facilities. Research into the formulation and application process indicates

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that commercial enzymes and chelators are available that give satisfactory results. Viscozyme L at 0.05% and Mayoquest 200 at 18 mM have developed into a standard formulation which gives consistent quality fibers. Whatever processing is used to produce the fiber, there is no universally accepted process to measure fiber properties such as strength, fineness, color, and trash content. A committee, D13.17 Flax and Linen, was formed in ASTM I. Subcommittees were formed for each quality parameter with the goal of writing an ASTM standard. A terminology standard has been approved and is published as ASTM D6798. The color standard has been approved and will be published soon. A fineness standard is ready for balloting at the subcommittee level. Current research indicates that NIR technology can determine the trash content in the fiber. When in place, these standards will allow the trading of fibers with known parameters that can be verified with standardized procedures.

4.1

Introduction

One characteristic of natural fibers that is always given as negative is the wide variation in quality of the fibers. Variation within a single bale has been known to give problems during processing. Because the fiber is natural, this variation is often cited as unavoidable and one that cannot be corrected or measured. While some variation is caused by the variety and growing conditions, the largest factor is how the material is processed to achieve a usable fiber. There are several different processes involved for the different types of natural fibers. Of the natural fibers, flax is considered to be the premiere fiber having the highest quality. The applications for flax extend from luxury apparel to paper, with new applications in composites. Considerable resources have been spent on processing flax. The majority of recent research has been with flax fiber, and this will be the primary subject for this chapter. Retting is a major problem in processing flax. Retting, which is usually microbial in nature, is the separation or loosening of bast fibers from shive and other nonfiber fractions. Plant development and weather influence the quality of retting, which determines both fiber yield and quality.37 Underretted flax results in coarser fibers heavily contaminated with shive and cuticular fragments; overretting reduces fiber strength due to excessive thinning of bundles or microbial attack on fiber cellulose. Water retting and dew retting have been used traditionally to extract fibers for textile and other commercial applications. Despite the fact that the highest quality flax fiber is produced by water retting, this practice has been largely discontinued in Western Europe due to the high cost and the pollution and stench arising from fermentation of the plant material. Further, fermentation products absorbed by the fibers during water retting impose an unpleasant odor. Dew retting is now the most common practice for separating flax fibers.37 Copyright © 2005 by Taylor & Francis

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Dew retting is reportedly the oldest method of retting. Even though the flax fiber is of lower quality than that from water retting, lower labor costs and high fiber yields make dew retting attractive. Stalks are pulled, spread in uniform and thin, nonoverlapping swaths, and left in the field where the moisture and temperature encourage colonization and partial degradation of flax stems, primarily by saprophytic fungi. Dew retting suffers from several disadvantages. In addition to poor and inconsistent fiber quality since dew retting replaced water retting, dew retting requires occupation of agricultural fields for several weeks and is restricted to geographic regions with appropriate moisture and temperature for effective microbial growth.14,37 In Western Europe, which reportedly produces the highest quality linen due to a favorable climate for dew retting,20 substantial crop losses frequently occur. Too much rain and lack of sufficient time for drying further contribute to failed harvests of flax fiber. In other regions, dry weather after harvest prevents microbial growth and therefore retting.

4.1.1

Dew Retting with Glyphosate

Work was conducted in Western Europe during the 1980s with glyphosate used as an agent to assist in the controlled retting of flax.15,32 In 1996 and 1998 field tests were conducted in Northern Ireland using trimesium salts of glyphosate to desiccate the flax plant as a method to improve the uniformity of retting.16 The results were compared to dew retting and water retting of the same crop. The factors investigated were the effects of varieties, time of application, and rate of application. Application rates of 2, 4, or 6 L/ha were applied at 10, 20, 30, and 40 days after the midpoint of flowering (MPF). Applications at 10 days after MPF and 40 days after MPF were found to be too early in the development of the plant and too late in the harvest season, respectively. The application rate of 2 L/ha did not give complete desiccation. The quality and quantity of fibers from the glyphosate-treated crop compared well those that were dew retted and water retted. The time to ret the standing crop was longer with glyphosate and could impact the commercial applications because of weather and second crop planting. Easson and Cooper16 reported the optimum application rate would be 6 L/ha applied at 30 days MPF. 4.1.2

Enzyme Retting

Enzymes have been considered for some time as a potential replacement for dew retting of flax. Early work with water- and dew-retting microorganisms indicated that pectinases were required for effective retting.37 Other matrixdegrading enzymes are often considered to give an appropriate balance of enzymes for degrading flax polysaccharides.33,37 Earlier efforts in Europe using a commercial pectinase-rich enzyme mixture, Flaxzyme, resulted in enzyme-retted flax fibers of higher yield and equal quality to water-retted fibers in pilot plant studies.38 Fiber strength, however, can be problematic with

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enzyme retting due to continuing action of enzymes on fiber, requiring extensive rinsing or further oxidative treatments to denature the enzymes.37 In this work, enzyme retting was used essentially as a replacement for water retting in which flax stems were immersed in an enzyme solution under controlled conditions for traditional linen production. Flax samples were obtained from a variety of sources, including fiber flax harvested for optimal fiber, mature plants, and linseed straw. All samples were grown in commercial operations or in test plots using standard methods for production. Fiber flax included “Ariane” from van de Bilt Zaden and Vlas grown near Sluiskil, Netherlands, in 1995 (Ariane NL 95). Ariane was grown as a winter crop in the coastal plains of South Carolina, United States, in 1998–1999 and harvested for optimal fiber (Ariane SC 99 early) or for seed and fiber (Ariane SC 99 mature). This material was produced in a field experiment and was drum-mowed rather than pulled. Some of this material was dried inside to prevent weathering and other samples were dew retted. Ariane was grown in test plots in North Dakota, United States, and stored inside without weathering (Ariane ND 00). “Natasja” was grown as winter crop in South Carolina and dew retted and commercially cottonized (Natasja SC 95). “Omega” seed flax was grown to maturity in South Carolina as a winter crop and stored inside without weathering (Omega SC 00). Linseed straw was produced from typical oilseed operations in North Dakota, United States (Seed flax ND 99). Tow from commercial linen processors in Europe was included for some comparisons. Enzymes included several commercial products. Flaxzyme, obtained in the mid-1990s from Novozymes (Copenhagen, Denmark) and SP 249 and Viscozyme L from Novozymes North America, Inc. (Franklinton, North Carolina, United States) were pectinase-rich mixtures used in a volumetric dilution as supplied. Lyvelin, from Aspergillus niger and containing an endopolygalacturonase with activity of 11,000 U/g, is marketed for flax retting by Lyven SA (Gagny, France). BioPrep 3000L is a commercial pectate lyase produced by a genetically modified Bacillus species with a reported activity of 3000 alkaline pectinase standard units (APSU)/g from Novozymes North America, Inc. and marketed by Dexter Chemical L.L.C. (Bronx, New York) under the trade name Dextrol Bioscour 3000. PGase I was polygalacturonase from A. niger (product number 31660, Serva Feinbiochemica GmbH, Germany). PGase II was polygalacturonase from Rhizopus species (product number 76285, Fluka Chemie GmbH, Switzerland). EPG was an experimental endopolygalacturonase from R. oryzae isolated from dew-retted flax in South Carolina and purified using standard methods.6,21,23 Purified enzymes or other mixtures used with pectinases to assess retting potential included pectin methyl esterase, pectate lyase, xylanase, and endoglucanase obtained commercially or experimentally as described.6 Chelators used include EDTA and oxalic acid as pure products. Mayoquest 200 is a commercial product with about 36–38% sodium EDTA and about 40% total dissolved salts and was used as a percentage as supplied. Other chelators included for comparison are described.1,2 Copyright © 2005 by Taylor & Francis

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Multiple tests were carried out for retting and for fiber yield and properties. The Fried Test was used to score the degree of retting from 0 for no fiber separation up to 3 for total fiber separation, based on standard images, from intact stems incubated with a series of enzyme formulations.22,37 An expanded Fried Test, from 0 to 5 or 6, was used to give more sensitive scoring for fiber separation6,29 in some studies. Control and retted flax samples were evaluated by scanning electron microscopy in the secondary electron mode7 and by standard bright-field, phase contrast, and polarized light microscopy. Retted flax was cleaned in laboratory studies using a handcarder (Friche Enterprises, Granite Falls, Washington, United States) to separate fiber from shives. In commercial cleaning, retted flax was processed through the Unified Line and the La Roche cottonizing system, formerly at Ceskomoravsky len (Humpolec, Czech Republic). The Unified Line, now produced by Czech Flax Machinery (Merin, Czech Republic), uses a series of crushing rollers, upper and lower shakers, and a scutching wheel to separate total fiber from stems. The La Roche cottonizing system involved cutting the fibers to specified lengths and then cleaning and refining by processing through fiber-opening equipment. Fine fiber yield was determined by fibers that were separated by the Shirley Analyzer (SDL America, Charlottsville, North Carolina, United States), given as a percentage of the original straw weight. Strength and elongation were determined by Stelometer12 and airflow fineness by a modified cotton micronaire system11 as described.7 Elemental composition of flax tissues was determined by inductive coupling plasma (ICP) emission spectrometry. In preliminary work, several pectinase-rich enzyme mixtures effectively separated fiber from core tissues.5 Henriksson et al.22 showed that chelators such as oxalic acid and EDTA included with enzyme reduced the level of enzyme by about 50-fold for extraction of fibers from flax stems. Sharma24 showed that EDTA at high pH separated fibers in a chemical retting process, and a patent exists for use of chelators for retting flax. Calcium bridges pectin molecules and stabilizes cell wall. In flax the cuticularizede epidermis is particularly high in calcium. Therefore, chelators likely remove calcium, destabilize the cell wall, and improve action by pectinanses. Various chelators were tested for their efficiency in enzyme retting formulation. EDTA was found to be the most effective at pHs 5 through 6, which is the optimum for the pectinase employed.1,2,21,22 Field-retting by intact microorganisms occurs in part by penetration of the hyphae through the protective barrier of the flax stem, and extracellular enzymes from hyphae are then delivered to sites of available polysaccharides for the retting of flax. For use of free enzyme in retting, results indicated that crimping stems was most effective in increasing the liquid absorbance by flax; increased pressure was more effective than vacuum treatments in noncrimped samples.18 Based on the preliminary tests for crimping of the stems and including calcium-binding chelators with pectinases, a method was developed3 with the intent of optimizing the use of enzyme formulations to ret flax from diverse Copyright © 2005 by Taylor & Francis

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and nontraditional sources. This method, designated spray enzyme retting (SER), later modified to soak flax by a brief immersion, consisted of the following steps: (1) crimp flax stems through fluted, steel rollers to disrupt stem integrity, (2) use a formulation of pectinase-rich enzyme mixture (at pH and temperature optimal for pectinase activity) plus Ca

chelator, (3) spray to soaking, or briefly immerse (ca. 2 min), crimped flax stems in enzyme-chelator formulations, (4) incubate under controlled conditions for optimal enzyme activity, (5) wash to remove enzyme, and (6) dry the fibers. Application of the enzyme formulation in this manner resulted in a liquid:fiber ratio of about 2:1 or less. Adjusting the aqueous chelator solution to pH 5 or 6 (depending on the enzyme used), was shown to work as well as buffer solution. Further, we have seen the need to disinfect flax stem prior to enzyme application. Viscozyme L is a commercially available enzyme mixture that effectively retted flax with the SER method. Pure EDTA has been replaced by a commercial solution Mayoquest 200, with 36–38% active EDTA. The SER method, using brief immersion in Viscozyme L and Mayoquest 200, has been used as our “standard” for further laboratory and pilot scale tests toward optimizing the procedure based on costs and fiber yield and properties. To investigate SER on a broader level, a variety of flax sources and two levels of enzyme and chelator were evaluated in a pilot scale study (ca. 10 kg samples), and the retted flax was commercially cleaned and cottonized through the Unified Line and La Roche systems.4 Fine fiber yield, as determined by the fiber collected in the Shirley Analyzer, and fiber strength and fineness by airflow8 were determined (Table 4.1). SER effectively separated fiber flax of different maturities and linseed straw, with a higher yield of fine fiber for fiber flax at both maturities in this test. Fiber strength and fineness were assessed within a few weeks after retting and again after about 30 months of storage. No loss in quality occurred with storage up to this time, indicating that enzyme TABLE 4.1 Properties of Enzyme-Retted and Commercially Cleaned and Cottonized Flax Fiber Retting Treatment Samplea

EDTA (mM)

Viscozyme (%)

Seed flax straw Seed flax straw Ariane (early) Ariane (early) Ariane (early) Ariane (early) Ariane (early) Ariane (late)

50 25 50 25 50 25 Dewretted 50

0.05 0.05 0.05 0.05 0.3 0.3 3 weeks 0.05

a

Finenessb (airflow) 5.9  0.1 bc 6.0  0.2 b 5.8  0.1 cd 5.7  0.1 d 3.9  0.1 g 4.6  0.1 f 5.3  0.1 e 6.7  0.1 a

Strength (g tex1)b

Fine Fiber Yield (%)

25.9  2.9 bc 19.6  1.1 de 24.0  2.0 c 20.9  1.3 d 13.0  1.3 g 15.8  1.8 f 36.2  2.3 a 26.8  3.4 b

25.3  1.0 ef 23.6  1.0 f 30.7  8.8 de 37.9  0.2 bc 61.4  0.7 a 58.7  1.1 a 43.0  1.1 b 32.3  0.3 cd

Seed flax straw is Seed flax ND 99 and Ariane is SC 99. a–g, within columns and for samples tested Aug., 1999, values with different lowercase letters differ, p  0.05. Source: Data adapted from Akin, D.E. et al., J. Biotechnol., 89, 193–203, 2001. With permission. b

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activity had abated and enzyme denaturing was not required. The SER method then appeared to provide a framework for future evaluation of enzyme retting where random-length fibers from a variety of flax sources could be obtained with relatively low amounts of enzymes. These data indicated that the higher levels of Viscozyme L improved fiber yield and fineness but reduced strength. Dew-retted flax from the same young fiber flax was significantly stronger than any SER fiber. Light microscopic comparison of fibers from the 0.05% and 0.3% level of Viscozyme L, both with chelator, suggested that the higher enzyme level weakened the e fiber. Likely nodes or kink bands in the fibers were especially susceptible to the cellulases26 in the enzyme formulation. This susceptibility resulted in strength loss with increased enzyme levels. Viscozyme L and Mayoquest 200 continued to be used more or less as the commercial “standard” for other comparisons in our work. Tests based on fiber yield, strength, and fineness showed properties for various levels of components in the formulation (Table 4.2). Fiber strength was inversely related to enzyme level (as noted above), and higher enzyme levels resulted TABLE 4.2 Yields and Fiber Properties of Flax Fiber Enzyme, Retted with Viscozyme L and Mayoquest 200 Fine Fiber Yieldb (%)

Strengthc (g/tex)

Finenessd,e (airflow)

0/0 0.05/0.4 0.05/0.7 0.05/1.8

4.3  1.7 f 5.4  2.2 ef 7.0  1.8 bcde 8.5  0.6 abc

26.9  0.8 a 24.0  1.4 abc 23.9  5.5 abc 24.6  2.3 ab

8.0  0 a 7.7  0.1 ab 7.9  0 ab 7.7  0.1 abc

0.1/0.4 0.1/0.7 0.1/1.8

6.2  1.3 def 7.3  1.8 bcde 7.9  1.2 abcd

20.3  2.5 bcd 17.9  2.3 de 20.3  1.8 bcd

7.6  0.1 abc 7.6  0.1 abc 7.1  0 cde

0.2/0.4 0.2/0.7 0.2/1.8

6.7  0.9 cde 7.9  1.3 abcd 8.9  2.1 ab

18.1  0.6 de 17.6  0 de 17.7  1.4 de

7.4  0.1 bcde 7.0  0.5 de 6.9  0.4 e

0.3/0.4 0.3/0.7 0.3/1.8

5.5  0.9 ef 7.3  1.1 bcde 9.8  0.8 a

15.3  0.5 e 18.1  1.3 de 19.5  0.7 cde

7.5  0.7 abcd 6.9  0.1 e 6.9  0 e

Viscozyme/Mayoquest 200 (%)a

a

Viscozyme (Novozymes) added as percentage of product as supplied. EDTA was supplied as Mayoquest 200 Callaway, with 38% EDTA. b Enzyme-retted straw passed through hand-card 2X and passed 1X through the Shirley Analyzer. c Average and standard deviation of 2 replicates of Shirley-cleaned fiber, with each replicate an average of 6 tests by Stelometer. d Average and standard deviation of 2 replicates of Shirley-cleaned fiber, with each replicate an average of 2 tests by airflow using approximately 5 g. e a–f values within columns followed by different letters differ at p  0.05. Source: Data adapted from Akin, D.E. et al., Text. Res. J., 72, 510–514, 2002. With permission.

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in finer fibers, as shown by airflow testing. Chelator level determined fine fiber yield, and within a single enzyme level a higher chelators amount tended to produce finer fibers. These results indicate that properties can be tailored with various formulations. The use of a commercial cleaning system integrated to the retting system is required, however, to fully compare costs and properties and to optimize enzyme retting for commercialization. With the above method as a framework for further evaluation, other commercial enzymes have been tested for retting efficiency based on fine fiber yield and fiber properties. Lyvelin (Lyven SA, Gagny, France), which contains a highly active endopolygalacturonase from A. niger, and BioPrep 3000L (Dexter Chemical LLC, Bronx NY), which is a commercial pectate lyase for bioscouring cotton and is active at high pH, were evaluated with the standard method using Viscozyme L and Mayoquest 200. Lyvelin produced fine fibers at relatively high yields at 0.05% and 0.1% of product as supplied. Fiber strength was lower than with Viscozyme L at these levels and further declined with increased enzyme levels. Fiber strength was extremely high with Bioprep, but fine fiber yield and fineness were inferior when compared with the other enzyme treatments. Other enzymes and chelators are available but our standard method with Viscozyme L and Mayoquest remains the procedure we compare to all others. While other enzyme formulations might be found to improve enzyme retting, other parameters should be tested for improved retting efficiency. In our standard test method, flax soaked with formulations is incubated for 24 h. Recent side-by-side laboratory tests showed that flax incubated with 0.05% Viscozyme L plus 18 mM EDTA (from Mayoquest 200) for 4 or 8 h resulted in similar fine fiber yields but coarser, stronger fibers than treatments for 24 h (Table 4.3). Variations in pH, temperature, and incubation time studied with various retting enzymes indicated that modifications in retting conditions may be used to advantage for costs and fiber yield and properties (Van Sumere, 1992). The choice of enzyme formulations, cost, and desired properties will dictate particular conditions for the most effective retting. Since the structure of the flax fibers, i.e., in bundles consisting of stiff brittle fibers, results in a continuous reduction in fiber fineness during processing, integration of retting and physical processing to clean or prepare flax is required to optimize the method. This integration should avoid the production of fiber dislocations (kink bands) and excessive attack on fiber nodes by cellulases that weaken the fibers. 4.1.3

Chemical/Mechanical Retting

Considerable research has been undertaken to find a replacement for dew retting. Chemical retting has been evaluated using a variety of methods, including ethylenediaminetetraacetic acid (EDTA) or other chemical chelators at high pH, detergents, strong alkali, and steam explosion.2,24,33,35 Sharma33 patented a chemical-retting method using chelating agents. Flash hydrolysis or steam explosion treatment, with or without impregnation before steam treatment, has been used to remove pectins and hemicelluloses Copyright © 2005 by Taylor & Francis

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TABLE 4.3 Yield and Properties of Water-Soaked and Unsoaked Flax Enzyme Retted for 4, 8, or 24 h Treatment a Unsoaked Unsoaked Unsoaked Unsoaked Soaked Soaked Soaked Soaked

Ret Time (h)

Shirley Yieldb (%)

Strengthc (g/tex)

Finenessd

7.4 16.3  1.0 a 15.1  0.6 a 16.7  1.3 a 16.3 19.6  1.1 a 20.1  0.1 a 17.5  2.7 a

37.9  6.8 40.9  7.3 a 34.5  0.1ab 28.8  1.8 bc 44.0  3.2 36.5  3.7 ab 32.2  1.1 bc 23.9  1.5 c

NDf, g 7.9  0.1 a 7.7  0.4 a 6.6  0.2 bc NDf 7.1  0.1 b 6.6  0.2 bc 6.3  0.2 c

0e 4 8 24 0e 4 8 24

a

Fifty-gram aliquots taken from water-soaked overnight or control, unsoaked crimped Ariane flax stems (SC 99) were retted in duplicate by soaking 2 min in 0.05% Viscozyme L plus 18.3 mM EDTA (from Mayoquest 200) in water at pH 5.0 and 40EC. b Shirley-cleaned fibers 1 as percentage of straw weight. For control, unsoaked samples, 50 g were used to calculate yield; for water-soaked samples, 58.1 g was used as starting weight, since 16.25% of material had been solubilized during water extraction. c Average  S.D. of two replications, each consisting of six tests by Stelometer. d Average  S.D. of two replications, each consisting of two tests by airflow (micronaire) method. e Nonretted control consisting of one replication. f Not determined. g abc, values within columns with different letters are significantly different at p  0.05. Source: Data adapted from Akin, D.E. et al., Text. Res. J., 72, 510–514, 2002. With Permission.

from decorticated flax;25 a fast decompression separates bundles to smaller bundles and ultimate fibers. Successful laboratory results have been reported with chemical-retting methods, but at times fiber properties are less satisfactory than those from other methods. Chemical treatment increases cost, but cost efficiencies are usually not reported. To help offset the cost, steam explosion could be used to overcome the wide natural variation to produce constant fiber qualities to specific fiber properties.28

4.2

Standards for Flax Fibers

No matter what the method of retting, being able to measure the fiber properties after retting is a major problem to widespread acceptance of natural Copyright © 2005 by Taylor & Francis

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fibers in the textile, composite, or even the paper industry. To eliminate this barrier, in the United States a group is working with ASTM International to develop standards for measuring fiber properties. The general business and trade practices related to cotton and standards serve as a useful example, especially for natural fibers. As far back as the early 1900s, the cotton industry recognized the need for standards to address problems related to marketing. A resolution was adopted in 1907 to establish uniform cotton standards “to eliminate price differences between markets, provide a means of settling disputes, and make the farmer more cognizant of the value of his product, and, therefore, put him in a better bargaining position, and in general be of great benefit to the cotton trade”.39 Over the next several years, laws were enacted authorizing the United States Department of Agriculture (USDA) to develop cotton grade standards. Considerable evolution related to grading methods has occurred over the years and work continues to modify and improve standards for cotton.31 The Agricultural Marketing Service, an agency of the USDA, uses the term cotton classification to refer to application of standardized procedures for measuring physical and aesthetic attributes of raw cotton that affect quality of products or manufacturing efficiency. Currently, properties of almost every bale of cotton produced in the United States are analyzed and classified within just a few seconds by HVI (High Volume Instrumentation). Properties included in this analysis are: strength, length and length distribution, fineness, color, and trash. It has long been recognized that marketing and applications in flax, in contrast to that with cotton, suffer from a lack of standards. Even though flax is considered the oldest textile fiber known, generally standards do not exist for flax fibers. There have been standards developed for bast fibers that address this issue. Examples are DIN 53 808, Bast fibers, Determination of length; DIN EN ISO 1073, Determination of fineness; and Din EN ISO 5079, Determination of strength and elongation. These documents have seen limited adoption as industry standards. Ross (1992) stated that flax is bought and sold by the subjective judgment of graders who appraise the look, feel, and strength. Various classification schemes include the following: source (e.g., Belgium, France, Russia, or China), processing history (e.g., water or dew retted), or application (e.g., warp or weft yarn). The various flax grading systems attempt to assess fineness, length and shape of fibers, strength, density, luster, color, handle, parallelism, cleanliness, and freedom from neps and knots.30 Extensive research has been carried out for objective measures of quality,28 particularly for strength, length, and fineness. Methods development is confounded by the nature of the bast fibers, and an objective system is still not available for grading flax fiber. Within companies, however, various grades of flax fibers are identified for marketing. Methods to derive objective values for various parameters, such as fiber strength, length, and fineness, are available10,36 and routinely used by research and industrial organizations for in-house testing of flax samples. Copyright © 2005 by Taylor & Francis

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The Stelometer, for example, provides data for fiber strength, but the method is time-consuming and no standards for flax fibers are available with this method as there are for cotton.36 Fineness is one of the most important properties for fibers. The International Standard (ISO) 2370 (1980), “Textiles—Determination of Fineness of Flax Fibers—Permeametric Methods to Determine Fineness,” is based on resistance to airflow for a known fiber mass in a known volume. The resistance is related to the surface area of the fibers. With constant mass and volume, fine fibers have more surface area than coarse fibers and, therefore, greater resistance to airflow. Proper assessment of airflow with calibrated gauges and appropriate equations, such as the Kozeny equation used in ISO 2370, permits comparisons of fiber fineness. ISO 2370 allows for analysis of both parallel (reference method) and random fibers. The Index of Fineness Standards (IFS), using reference samples and based on the tex system, “permits compensation for the fact that the fineness of flax fibers cannot be defined in an absolute manner” (International Standard (ISO) 2370, 1980). A series of sets of fibers, ranging from IFS values of 21.7 to 72.1, is available from the Institut Textile de France, Lille. The ASTM Standard D1448 (1999) for cotton fineness, measured in micronaire, was modified (flax fibers cut to 2.54 cm and loaded at 5 g) to test the IFS flax samples.8 Good agreement occurred between the two methods (R2 = 0.99). Variations in fineness of these fibers, as well as a series of other samples derived from various retting procedures, were verified by image analysis that gave average fiber widths and width distributions based on physical dimensions. Several instruments used to rapidly and objectively analyze cotton were evaluated using flax in cooperative trials between Zellweger Uster (Knoxville, Tennessee) and the IAF (Reutlingen, Germany) (Anja Schleth, personal communication). Hardware and software modifications allowed some success over time, but optimum performance of the equipment for flax analyses was not achieved. In order to measure flax fibers successfully with cotton equipment, a major redesign in the mechanics and software of instruments, such as the AFIS (Automated Fiber Information System) and HVI, was required. The amount of development required, along with predicted small market size and lack of standards, caused Zellweger Uster to discontinue work. Other groups, e.g., IAF and Applied Science Division, Department of Agriculture, Northern Ireland, continue to research rapid methods for flax fiber assessment. The Center for American Flax Fiber (CAFF), which is a not-for-profit organization dedicated to promoting flax fiber in North America, led in establishing a subcommittee to develop flax standards as part of ASTM D13. In 1999, an ASTM International Flax (Linen Content) Products Subcommittee, D13.17, was officially formed and began biannual meetings as part of “ASTM Textile Committee Week” conferences. Flax properties identified for standards development specifically included strength, length, fineness, color, and trash (nonfiber components). Task Groups within D13.17 Copyright © 2005 by Taylor & Francis

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were established to address each of these properties. Task Groups for Terminology and for Industry Liaison were added later, the latter group particularly for flax in composites for automobile parts. Terminology. Subcommittee discussions of flax properties showed the need for standard terms and language for clear communication. Development of a terminology standard is rather straightforward but does require some research to assure terms are not “reinvented” or, even worse, modified from already accepted definitions in ASTM Standard D 123 (“Terminology Related to Textiles”) or in Europe and other regions with a long history of flax. Patricia A. Annis, Department of Textiles, Merchandising & Interiors, University of Georgia, and long-time member of ASTM, agreed to lead in developing “Standard Terminology Relating to Flax and Linen.” This document, currently consisting of 19 definitions specific to flax properties and processing, has been published as ASTM D6798. Additional definitions will be added as D13.17 continues to be active.

4.2.1

Color

Color differences in flax retted by various means are obvious and wellknown to researchers.9 Water retting, which is brought about by anaerobic bacteria when flax is immersed in water, is lightly colored. In contrast, dew retting, which is due to colonization and partial degradation by fungal consortia, produces dark and nonuniform fibers. Enzyme retting, which is an objective of research in USDA, results in a light-colored flax that is similar but not identical to water-retted flax (Table 4.2). The color of flax, with lightness from black to white (L* value), green to red (a* value), and blue to yellow (b*), is influenced by retting, processing, cleaning, and cottonizing. The use of these established procedures based on CIELab measurements provides an efficient means for objective color determination.17 Problems related to color matching can be more objectively addressed and provide better use of flax from a broadened production system. Color determination is well-established, and a document entitled “Practice for Color Measurement of Flax Fiber” utilizing this method has been drafted by Helen H. Epps, Department of Textiles, Merchandising & Interiors, University of Georgia. This document, from fiber results of various sources, is currently being balloted by ASTM Committee D13.

4.2.2

Fineness

Information presented above on ISO 2370 indicates that airflow methods are appropriate for measuring the fineness of flax fibers. Earlier work8 was successful using a modification of the ASTM micronaire for the cotton grading system13 with IFS flax fibers. With the use of IFS samples for calibration, the long-line fibers were cut to 2.5 cm and used as a 5-g sample. Fineness readings and the relationship to physical measurements with image analysis Copyright © 2005 by Taylor & Francis

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for various fiber samples are given in Table 4.3. Fineness of fiber bundles of flax is influenced by many factors, especially retting and processing methods. For laboratory and small pilot plant tests, flax fibers can be cleaned and separated using a Shirley Analyzer (SDL America, Inc., Charlotte, North Carolina), which uses a rotating wire-wound drum and airflow to separate fine fibers from coarse fibers and shive. Fibers cleaned in this manner have been used to prepare test yarns from flax and cotton blends. Based on studies with a wide variety of samples, a document drafted by Jonn A. Foulk, ARS-USDA, Clemson, South Carolina, entitled “Standard Methods Practice for Assessing Clean Flax Fiber Fineness,” is being balloted by ASTM D13.17.

4.2.3

Trash (Nonfiber) Components

Bast fibers are formed in the cortex of the stem and are surrounded with a considerable amount of nonfiber tissues closely associated with fiber bundles. Stephens (1997) reported total fiber yields of about 20–30% for 44 cultivars grown in small plots, with the fiber varieties often producing nearly 30% fiber. Therefore, at least 70% of the cultivar can be nonfiber components. Chemical analyses indicate substantial differences in flax stem tissues that could be exploited in standards. Flax fiber is mostly cellulose, but considerable amounts of noncellulosic carbohydrates contribute to total fiber weight. Aromatics, as indicated with acid phloroglucinol, are sporadically located in cell corners between fibers in bundles.7,19 Nonfiber tissues in the bast region contain considerable amounts of pectin and matrix materials.37 The outer barrier (cuticularized epidermis), which protects the plant and is connected to fiber bundles before retting, contains waxes, cutins, aromatics, and carbohydrates.7,27 The shive component, comprised in large part by the woody core tissues located within the ring of fiber bundles and toward the stem center, contains high levels of aromatics, such as lignin, along with some waxes and carbohydrates.7 Shives as a major source of trash in fibers are the primary focus of the proposed ASTM trash standard. Developing a standard for trash has important implications for all industries interested in using flax fibers. A method to analyze trash toward the goal of developing a standard, using chemical analysis and near infrared spectroscopy, is being developed at the Russell Research Center by W. Herbert Morrison III and Franklin E. Barton II.

4.3

Conclusions

The use of glyphosate salts to achieve a more uniform degree of retting is a method that warrants more study. With a standing crop, the adverse effects of weather can lessen. In addition, the standing crop should be cleaner, with less dirt and trash having accumulated on the stems. With the Copyright © 2005 by Taylor & Francis

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fiber properties being equal to or better than dew retted, cost is the negative factor. With the large harvest losses associated with flax, any new method that would decrease these losses would be good. However, the impact on the second crop must not be ignored. If delaying the removal of flax crop moves the plant time of the second crop to a point where its income is lessened, the total system must be considered. This adds complexity that must be studied over different weather patterns and crop cycles. As reported, the standing crop tends to produce a higher-strength fiber, which may be an advantage in the emerging composite area. The SER method effectively rets flax from diverse sources of flax straw with a low liquid-to-fiber ratio. The inclusion of calcium-binding chelators with enzymes, and used at conditions optimal for enzymes, allows a reduction in amounts of enzymes for effective retting. Breaching the protective stem barrier, such as by crimping stems, improves penetration of the formulation. Cellulases, which often are present in commercial pectinase-rich mixtures, become problematic at higher enzyme levels or longer incubation times, apparently by attacking the nodes or dislocations in fibers and fiber bundles and reducing fiber strength. Various enzyme formulations can be used to tailor fiber properties. Enzyme retting must be integrated and optimized with physical processes for preparation and cleaning for a commercial method to be practical. The current status of retting can be summarized in the following statements: 1. Water retting was the practiced method of the past to make easier the processing of bast fiber plants with simple devices driven by hand. The quantities of processing are limited. This method cannot be expanded for cost and environmental reasons. 2. Dew retting is possible in humid zones only. It is impossible in arid zones. Dew retting is a natural process, which depends on air humidity and temperature. Both the air humidity and temperature cannot be controlled, which causes a large variation in the quality parameters of the fibers. Quality management is difficult and large quantities of fiber with consistent quality cannot be ensured. 3. The controlling of the retting by chemicals or enzymes is possible on principle. Such technologies are in the research and development stage and are not cost-effective. To date, practical applications of these technologies have not been achieved on a commercial scale. While chemical retting and physical retting have advantages, there has been very little progress in moving the technology forward in recent times. Complexity and costs of such operation remain major barriers. The ability to customize fiber properties to specific applications is a major advantage for these processes. For commercialization to occur, there must be methods by which the quality of the fibers can be determined. This is necessary to determine value, but Copyright © 2005 by Taylor & Francis

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it is also necessary to determine optimum processing. For example, by knowing the shiver content, cleaning can be limited to that necessary for the final application. This can preserve strength and length in the fibers. ASTM International has a well-established and proven method for developing standards that are recognized worldwide.

References 1. Adamsen, A.P., Akin, D.E., and Rigsby, L.L., Chelating agents and enzyme retting of flax, Textile Res. J., 72, 296–302, 2002. 2. Adamsen, A.P., Akin, D.E., and Rigsby, L.L., Chemical retting of flax straw under alkaline conditions, Text. Res. J., 72, 789–794, 2002. 3. Akin, D.E., Dodd, R.B., Perkins, W., Henriksson, G., and Eriksson, K.-E.L., Spray enzymatic retting: a new method for processing flax fibers, Text. Res. J., 70, 486–494, 2000. 4. Akin, D.E., Foulk, J.A., Dodd, R.B., and McAlister, III, D.D., Enzyme-retting of flax and characterization of processed fibers, J. Biotechnol. 89, 193–203, 2001. 5. Akin, D.E., Foulk, J.A., and Dodd, R.B., Influence on flax fibers of components in enzyme retting formulations, Text. Res. J., 72, 510–514, 2002. 6. Akin, D.E., Morrison, III, W.H., Rigsby, L.L., Evans, J.D., and Foulk, J.A., Influence of water presoak on enzyme-retting of flax, Ind. Crops Prod., [in press]. 7. Akin, D.E., Gamble, G.R., Morrison, III, W.H., and Rigsby, L.L., Chemical and structural analysis of fibre and core tissues from flax, J. Sci. Food Agric., 72, 155–165, 1996. 8. Akin, D.E., Rigsby, L.L., and Perkins, W., Quality properties of flax fibers retted with enzymes, Text. Res. J., 69, 747–753, 1999. 9. Akin, D.E., Epps, H.H., Archibald, D.D., and Sharma, H.S.S., Color measurement of flax retted by various means, Text. Res. J., 70, 852–858, 2000. 10. Archibald, L.B. 1992. Quality in flax fibre, in The Biology and Processing of Flax, Sharma, H.S.S. and Van Sumere, C.F., Eds., M Publications, Belfast, Northern Ireland, 1992, pp. 297–309. 11. ASTM D 1448-90, Standard Test Method for Micronaire Reading of Cotton Fibers, Annual Book of Standards, Section 7, Textiles, ASTM, West Conshohocken, PA, 1999. 12. ASTM D 1445-95, Standard Test Method for Breaking Strength and Elongation of Cotton Fibers (Flat Bundle Method), Annual Book of Standards, Section 7, Textiles, ASTM, West Conshohocken, PA, 1999. 13. ASTM D 1448-97, Standard test method for micronaire reading of cotton fibers. pp. 374–376, in Annual Book of ASTM Standards, Sec. 7, Textiles, ASTM, West Conshohocken, PA, 1999. 14. Brown, A.E., Epicoccum nigrum, a primary saprophyte involved in the retting of flax, Trans. Br. Mycolog. Soc., 83, 29–35, 1984. 15. Courtney, A.D. and Robinson, E., Glyphosate as a Preharvest Retting Agent in Flax, Proceedings of the British Crop Protection Conference–Weeds, 1982, pp. 231–236.

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16. Easson, D.L. and Cooper, K., A study of the use of trimesium slat of glyphosate to desiccate and ret flax and linseed and its effect on the yield of straw, seed and fiber, J. Agric. Sci., 183, 29–37, 2002. 17. Epps, H.H., Akin, D.E., Foulk, J.A., and Dodd, R.B., Color of enzyme-retted flax fibers affected by processing, cleaning, and cottonizing, Textile Res. J., 71, 916–921, 2001. 18. Foulk, J.A., Akin, D.E., and Dodd, R.B., Processing techniques for improving enzyme-retting of flax, Ind. Crops Prod., 13, 239–248, 2001. 19. Gorshkova, T.A., Salnikov, V.V., Pogodina, N.M., Chemikosova, S.B., Yablokova, E.V., Ulanov, A.V., Ageeva, M.V., Van Dam, J.E.G., and Lozovaya, V.V., Composition and distribution of cell wall phenolic compounds in flax (Linum usitatissimum L.) stem tissues. Ann. Bot., 85, 477–486, 2000. 20. Hamilton, I.T., Linen. Textiles, 15, 30–34, 1986. 21. Henriksson, G., Akin, D.E., Hanlin, R.T., Rodriguez, C., Archibald, D.D., Rigsby, L.L., and Eriksson, K.-E.L., Identification and retting efficiencies of fungi isolated from dew-retted flax in the United States and Europe. Appl. Environ. Microbiol., 63, 3950–3956, 1997. 22. Henriksson, G., Akin, D.E., Rigsby, L.L., Patel, N., and Eriksson, K.-E.L., Influence of chelating agents and mechanical pretreatment on enzymatic retting of flax. Text. Res. J., 67, 829–836, 1997. 23 Henriksson, G., Akin, D.E., Slomczynski, D., and Eriksson. K.-E.L., Production of highly efficient enzymes for flax retting by Rhizomucor pusillus. J. Biotechnol., 68, 115–123, 1999. 24. Henriksson, G., Eriksson, K.-E.L., Kimmel, L., and Akin, D.E., Chemical/physical retting of flax using detergent and oxalic acid at high pH, Text. Res. J., 68, 942–947, 1998. 25. Kessler, R.W., Becker, U., Kohler, R., and Goth, B., Steam explosion of flax – a superior technique for upgrading fibre value, Biomass Bioenergy, 14, 237–249, 1998. 26. Khalili, S., Akin, D.E., Pettersson, B., and Henriksson, G., Fibernodes in flax and other bast fibers, J. Appl. Botany [in press]. 27. Morrison, III, W.H. and Akin, D.E., Chemical composition of components comprising bast tissue in flax. J. Agric. Food Chem. 49, 2333–2338, 2001. 28. Nebel, K.M., Research and Development of Bast Fibers, in World Conference of the Textile Institute, Vol. 2, Niches in the World of Textiles, 1997, pp. 338–339. 29. Rognes, H., Gellerstedt, G., and Henriksson, G., Optimization of flax fiber separation by leaching, Cellulose Chem. Technol., 34, 331–340, 2000. 30. Ross, T., 1992. Preparation and spinning of flax fibre, pp. 275–296. in The Biology and Processing of Flax, Sharma, H.S.S. and Van Sumere, C.F., Eds., M Publications, Belfast, Northern Ireland. 31. Schneider, T., Heap, S.A., and Stevens, J.C., Proceedings of the 26th International Cotton Conference, Bremen, Faserinstitut, Bremen, Germany, 2002, 247 pp. 32. Sharma, H.S.S., An alternative method of flax retting during dry weather, Ann. Appl. Biol., 109, 605–611, 1986. 33. Sharma, H.S.S., Studies on chemical and enzyme retting of flax on a semi-industrial scale and analysis of the effluents for their physico-chemical components, Appl. Microbiol. Biotechnol., 26, 358–362, 1987. 34. Stephens, G.R., Connecticut fiber flax trials 1994–1995. Bulletin 946, The Connecticut Agric. Exp. Sta. New Haven, CT, p. 13, 1997.

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35. Tubach, M. and Kessler, R.W., Interdisciplinary Approach for New Flax Products: Examples of Applied Research at the IAF, in Proceedings of the Symposium on World Fibre Flax, Connecticut Agricultural Experiment Station, New Haven, CT, 1994, pp. 71–86. 36. van Langenhove, L. and Bruggeman, J.P., Methods of fibre analysis, in The Biology and Processing of Flax, Sharma, H.S.S. and Van Sumere, C.F., Eds., M Publications, Belfast, Northern Ireland, 1992, pp. 311–327. 37. Van Sumere, C.F., Retting of flax with special reference to enzyme-retting, in The Biology and Processing of Flax, Sharma, H.S.S. and Van Sumere, C.F., Eds., M. Publications, Belfast, Northern Ireland: 1992, pp. 157–198. 38. Van Sumere, C.F., and Sharma, H.S.S., Analyses of fine flax fibre produced by enzymatic retting, Aspects Appl. Biol. 28, 15–20, 1991. 39. United States Department of Agriculture, The classification of cotton, USDA, Washington, DC, 1995.

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5 Alternative Low-Cost Biomass for the Biocomposites Industry

Douglas D. Stokke

CONTENTS 5.1 Introduction 5.2 Wood Residues 5.2.1 Wood Processing Residue 5.2.2 Forest Residues 5.3 Biomass Alternatives 5.3.1 Recycled Materials 5.3.2 Municipal Solid Waste 5.3.3 Construction and Demolition Waste 5.3.4 Waste Paper 5.3.5 Other 5.3.6 Dedicated Biomass Crops 5.3.7 Agricultural Production Residues 5.3.8 Agricultural Processing Residues 5.4 Further Considerations 5.5 Conclusion Acknowledgments References

ABSTRACT Melt-blend processing of biocomposites is accomplished predominantly with wood fiber (flour) as the “bio” component. Wood fiber is derived from residues generated by primary or secondary wood conversion industries. Increased efficiency of wood processing manufacturers, competing uses of wood residue for other products or energy, and shifts to alternative, nonwood materials will likely keep wood residue supplies tight in the near future. As a result, biomass alternatives should be identified and evaluated. This chapter provides an overview of several categories of low-cost

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material that may have potential as wood fiber alternatives. These include recycled materials (municipal solid waste, construction and demolition waste, waste paper, and other recyclable materials), dedicated biomass crops, agricultural production residues, and agricultural processing residues. A wide range of these alternatives could be considered for use in melt-blend biocomposites. A number of studies and trials have been and continue to be carried out to evaluate the technical characteristics of these materials. However, it is the infrastructure needed to efficiently harvest or retrieve, process, and distribute biomass alternatives that may play the primary role in determining which materials are adopted. In the United States, infrastructure considerations suggest that the biocomposites industry should examine closely the potential for use of agricultural production and agricultural processing residues for use as suitable alternatives to wood fiber.

5.1

Introduction

The uses of plant-derived biomass are seemingly too great to enumerate. Similarly, the term biocomposite may be applied to a staggering range of materials derived wholly or in part from renewable biomass resources. The general limiting view of this subject, as described in this chapter, is to focus on short fiber-reinforced or particulate-filled composites in which the “bio” component of the composite material is the fibrous or particulate embedded in a matrix which may or may not be biobased itself. Thus, the scope of this chapter is limited to biomass alternatives that may be suitable as fillers or reinforcements of matrix composite materials. The general perspective herein is that of fiber/plastic materials formed via melt-blend processing. The review begins with wood residue, not because it is an alternative, but because it is the primary natural material on which the extant fiber/composite industry relies. A case may be made that shortages of this type of material, in the quantity and quality demanded by future industrial growth, may accrue. Thus, there arises the need to assess alternatives. The alternative materials reviewed herein include recycled materials (municipal solid waste, construction and demolition waste, waste paper, and other recyclable materials), dedicated biomass crops, agricultural production residues, and agricultural processing residues. The order in which these materials are addressed represents a taxonomy that builds a hierarchy of increasingly viable alternatives to wood fiber. This hierarchy depends not only on material properties, which in many cases are yet unproven for the applications envisioned, but also on economic and infrastructure considerations that would tend to favor (despite obstacles to adoption) the use of such alternatives. Ultimately, technology, economics, and practicality will dictate the materials that will be used in marketable products. The data examined in this chapter focus on the situation in the United States. Readers interested in worldwide data on

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plant-based biomass are referred to the recent review by White and Cook.27 Furthermore, this chapter is specific to plant-based biomass, although it is recognized that animal sources may prove viable as alternatives to wood fiber. For instance, recent research has shown potential for the use of chicken feather fibers in combination with wood fiber for medium-density fiberboard28 or turkey feather fibers for various textile applications.8 Such uses of poultry feathers are possible due to a process that has been developed to efficiently separate feather fiber from the quill.7

5.2

Wood Residues

5.2.1

Wood Processing Residue

The fiber/plastic composite industry currently relies primarily, or almost exclusively, on wood processing residue for the “biofiber” component of its products. Most manufacturers rely on a third party to acquire, process (grind, screen, dry), and supply the wood fiber, generally in grades consisting of various particle size distributions. These materials are more particulate than fibrous, that is, they have a low aspect ratio (length to width or diameter ratio), generally less than two, vs. ~100:1 for individual whole wood fibers. Estimates of primary processing residue, at over 82 million t (Table 5.1), reveal that this is the single largest category of residual wood fiber

TABLE 5.1 Estimated Biomass Availability in the United States, 1999 Biomass Type Forest residues (standing poletimber excluded) Primary mill residues (wood processing waste) Agricultural residues (corn stover and wheat straw only) Dedicated energy cropsa (switchgrass; hybrid willow and poplar) Urban wood waste (yard waste, pallets, packaging, construction, and demolition waste) Sum a

Dry t
106 (delivered at  $55/dry metric ton) 40.7185 82.0490 136.7011 170.6522 33.4346

463.5554

Biomass for dedicated energy crops is largely an estimate of the quantities that could be grown if dedicated biomass crops were cost-competitive with other crops. While there are some biomass energy plantations in the United States, the vast majority are small research plots. Source: Adapted from Walsh, M.E. et al., U.S. Dept. of Energy, Oak Ridge National Laboratory, 1999. With permission.

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supply. The quantity of “roundwood” (logs) vastly overshadows this number (roundwood data not shown), but it seems unlikely that it will become economic to directly convert roundwood into wood flour. Despite the large amount of primary mill residue (i.e., processing by-products of sawmills and veneer mills), very little typically remains after all uses are taken into account. That is, most primary mill residues have value and use for pulp, molded wood products, hardboard and medium-density fiberboard, particleboard, fuel, animal and pet bedding, and mulch, to name a few. Data from 1991 showed that of the 74.5 million t of primary mill residue (compare this figure to the 1999 value of 82 million t in Table 5.1), only 6% was not being used; the unused material was likely in remote locations which precluded economic use.11 Anecdotal information confirms that many users of wood residue often face local shortages of this raw material, particularly when energy prices are high. High energy costs tend to force primary mills to divert their residue streams for energy generation in their own facilities, thus significantly reducing supplies for their customers. Although the focus here is on primary mill residues, so-called “secondary” residues are also used. These are wastes (e.g., sawdust) generated in secondary processing operations such as furniture, cabinet, or window and door manufacturing. While these are viable and valuable resources (generally “clean,” dry, and relatively homogeneous), census information on this material is often difficult to obtain. At any rate, the fact remains that most mill residues, be they derived from primary or secondary sources, already have sound markets that result in generally tight supplies of these materials for the various industries competing for them. Shifts in wood manufacturing operations, increased efficiency, and use of alternative materials will likely continue to keep tight mill residue supplies into the foreseeable future. Once again, herein lies the problem for the biocomposites industry: if the staple material supply is short in the face of rapid overall growth of biocomposites manufacturing, it seems rather certain that wood fiber supplies will need to be supplemented with alternative biomass sources. 5.2.2

Forest Residues

Forest residues are comprised of the tree tops, branches, small or poor-quality logs, and other materials that are generally uneconomic to recover during logging operations. Substantial quantities of logging residues, amounting to a quantity equivalent to about half of the total primary wood mill residues, are potentially available. However, the poor quality (high proportion of bark, juvenile wood, knotty material, dirt and grit, etc.) coupled with access problems (residues are left in the woods due to technical, economic, and environmental considerations) combine to make this a generally undesirable source for biocomposites. When available, forest residues, along with urban wood waste, are probably best used for fuel, compost, or mulch (Figure 5.1). Further development of separation and cleaning technology could render modest quantities of these fiber sources amenable to use in biocomposites. Copyright © 2005 by Taylor & Francis

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FIGURE 5.1 Forest residues, construction and demolition wastes, and other urban wood wastes could be viable for the biocomposites industry, provided that efficient reduction, cleaning, and classification technology is available. (Photo by Monlin Kuo. With permission.)

5.3

Biomass Alternatives

Before we begin our examination of biomass alternatives for the biocomposites industry, a little perspective is in order. The plastics industry is estimated to currently use ~2.95 million t of fillers of all types (mainly inorganics), of which ~0.227 million t are natural fibers (mainly wood flour).2 Industry experts believe that the demand for biobased or natural fibers for fiber/plastic composites will grow by at least sixfold in the next five to seven years. Thus, U.S. industry could face a short-term need for upwards of 1.4 million t of biomass suitable for melt-blend processing. This number is quite small if compared to demands of other fiber-using industries (e.g., pulp and paper or wood composite panels) and is similarly small compared to various types of available biomass. However, sheer inventory numbers for alternative biomass are in some ways irrelevant. What does matter is whether materials will be available in sufficient quality and quantity for the intended application. Given that industries in general that rely on wood residues routinely face periodic fiber shortages, it seems reasonable to assume that similar shortages will face the fiber/plastics industry if growth projections for that industry hold true. Copyright © 2005 by Taylor & Francis

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Table 5.1 shows estimated availability of significant classes of potentially low-cost biomass, including forest residues, primary mill residues, agricultural residues, dedicated energy crops, and urban wood waste. The estimates represented here are for the amounts of materials that could be available at a given delivered price, here, up to $55/dry metric ton ($0.025/lb). This price level could be considered “low” cost, recognizing, however, that additional processing costs will be incurred to convert these materials into usable form for biocomposites. By comparison, wood flour typically ranges from ~$0.05 to $0.20 per pound ($110 to $440 per metric ton), depending upon grade (species, mesh size), with recent prices (2002 and 2003) ranging from $0.06 to $0.0925 per pound ($133.24 to $203.87 per metric ton), Free on Board (FOB) mill, for 40 or 60 mesh oak, maple, pine, or spruce wood flour.18 These prices represent a finished fiber product that is ready for blending with plastic matrices. The delivered price of $55/metric ton for biomass alternatives therefore leaves some room for processing cost and perhaps represents an estimated limit for competitive pricing of unprocessed alternatives to wood fiber. Even though the fiber/plastic industry may require “only” 1.4 million t of natural fiber within five years, this quantity would represent 4% of all urban wood waste or 3.3% of all forest residues, or 1.7% of all primary wood processing mill residues (based on data in Table 5.1). If we assume that all of the current biofiber used by fiber/plastic industries comes from primary mill residues, current consumption of 0.227 million t amounts to just 0.28% of such residues. It seems highly unlikely that the industry could acquire six times that amount within five years without substantial cost increases for the fiber. Admittedly, this is a crude analysis, but it does serve to illustrate the need to evaluate biomass alternatives.

5.3.1

Recycled Materials

Recycled materials represent the first category of materials that could become biomass alternatives for fiber/plastic composites. This category includes municipal solid waste, construction and demolition waste, recycled paper, and other materials.

5.3.2

Municipal Solid Waste

Municipal solid waste (MSW) forms the lowest tier of material supplies, for obvious reasons. Wastes are commingled, of extremely variable composition, and contaminated with food, liquids, and all manner of disposables. The resource component of MSW of most interest for biocomposites would be paper, comprising nearly 40% of MSW by weight in 1993.11 Significant quantities of this paper are currently recovered in a relatively uncontaminated form (such waste paper is discussed in a separate section below). Ince and McKeever11 estimated that an additional 28 million t (60% of a 47 million ton Copyright © 2005 by Taylor & Francis

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disposal burden left after clean paper was recovered for recycling) would represent the “maximum additional [paper] recovery potential” (1993 data). However, despite efforts to increase paper recovery (Figure 5.2), it is highly unlikely that paper or other cellulosic resources reclaimed by such operations would be of sufficient cleanliness and quality for biocomposites. Rather, these materials remain destined for use as refuse-derived fuels. Thus, much of the nearly 200 million t of MSW will remain an untapped resource for such uses in the foreseeable future unless suitable alternate technologies are either developed or introduced. 5.3.3

Construction and Demolition Waste

Construction and demolition (C&D) waste is a significant component of the “urban wood waste” in Table 5.1. New construction waste (from singlefamily residential, multifamily residential, residential remodeling and repair, and nonresidential building sites) was estimated as 6.1 million t of wood plus 0.2 million t of paper and paperboard, with perhaps 88% of the total recoverable (5.4 million t, 1993 data).11 Since (new) construction waste

FIGURE 5.2 Waste paper, the single largest component of municipal solid waste (~40% by weight), could be attractive to the biocomposites industry when collected as a clean, segregated resource. Facilities such as this separate household garbage into recyclable metal, glass, and combustibles (paper and plastic). The latter “refuse-derived fuel” is generally too contaminated for use other than as fuel or compost. This particular facility has served the Ames, Iowa (U.S.) community of about 50,000 residents since 1976. (Photo by Heath Gieselman. With permission.)

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is relatively clean, it might represent a viable biomass source in larger municipalities. Demolition waste (~23 million t, with perhaps one third of this recoverable for recycling) is likely too contaminated and variable for consideration. 5.3.4

Waste Paper

Waste paper (old newsprint, onp; old corrugated containers, occ; and mixed office paper) is one component of MSW of potential interest for biocomposites. Indeed, research has shown technical feasibility for melt-blend processing with waste paper.4 The challenges of using this resource are probably more economic in nature in that waste paper markets are notoriously volatile. In addition, the paper industry has slowly built manufacturing capacity, especially during the 1990s, to handle increasing volumes of recycled paper. Thus, any industry seeking to use the “clean” grades of waste paper (onp, occ, and even mixed office) must compete for raw material directly with the pulp and paper industry. In 1993, 32 million t of paper and paperboard were recovered for recycling, of which 26 million t went into domestic paper production, 5 million t for export, and 1 million t for miscellaneous uses.10 5.3.5

Other

Additional biomass sources include materials such as leather processing waste, denim processing wastes, or cotton processing residues. Some of these materials have been or are being used today in biocomposites. In the case of cotton residues (linters and motes), current competing uses (specialty papers and nonwovens) make their availability for biocomposites questionable.20 In recent years, many lignocellulosics have been investigated as fillers for plastics, including jute, sisal, corn stalks and cobs, waste paper, kenaf, oat hulls, rice hulls, peanut hulls, soybean hulls, oat and wheat straw, jojoba seeds, and oil palm empty fruit bunch fibers, to name only a few.3,4,15,16,19,21 This list is by no means comprehensive of the materials that have been evaluated, and not all are necessarily “recycled” (i.e., some are better classified as agricultural production or processing residues). Nevertheless, even if technical feasibility is shown for materials such as these, the problems of infrastructure (collection, processing, storage, and distribution) may preclude their introduction as viable biofiber alternatives for the biocomposites industry. This issue will be revisited when we turn our attention to agricultural processing residues. 5.3.6

Dedicated Biomass Crops

Data in Table 5.1 suggest that dedicated biomass crops could be the answer to many of our questions regarding fiber supply, not only for biocomposites Copyright © 2005 by Taylor & Francis

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of the variety considered here, but also for many other wood-based composite materials, pulp and paper, and energy. However, the 170.6522 million t of dedicated biomass shown in Table 5.1 is only an estimate of what might be IF such crops were cultivated on a wide scale. The U.S. Department of Energy has invested many years and dollars in research on dedicated biomass crops such as hybrid poplars, willow, sycamore, and other woody plant species, as well as switchgrass and other grasses. As a result, a solid research base has been built, and it would be technically possible to move forward with some of these plants as dedicated crops. Some are, in fact, in the implementation stage. For example, short rotation intensive culture (SRIC) tree plantation area is increasing due to projects designed to supply pulp and paper manufacturing. Table 5.2 shows recent trends in U.S. SRIC area. Though encouraging, the area of dedicated woody crop plantations (92.2 thousand ha) remains quite small. By way of comparison, the total area of forest land in the United States is 298 million ha, or about 33% of the U.S. land base.22 It should be noted, however, that we must be careful to distinguish between SRIC plantations (by definition, tree plantings in which the trees will be harvested at young stand ages; 4 to 12 years of age for poplars or less than 20 years of age for pines) and other plantation forests with longer “rotation ages” (tree age at harvest) designed to produce pulpwood, sawlogs, or veneer logs. For example, pine plantings in the southern United States totaled in excess of 12.154 million ha in 1999.1 Most of this plantation acreage would not be classified as SRIC due to the longer rotation ages for production of wood products. Similarly, grass biomass crops are not yet planted on a wide scale. Although the U.S. Department of Agriculture collects data on forage crop acreage and harvest (e.g., hay), Agricultural Census data does not yet include specific biomass crops such as switchgrass.24 Dedicated nonwood biomass crops in general face a particularly tough economic climb in that it is difficult to recover land rent values and costs of production to produce a relatively low-value biomass product. It makes little economic sense to cultivate such crops on lands that would be suitable for grain production. Marginal agricultural land is therefore targeted for biomass crops. TABLE 5.2 Short Rotation Woody Crops in the United States, 1995 and 2000. Values are in ha
103 Region

1995

2000 Projection

Pacific Northwest North Central Southeast Total

20.2 3.8 12.1 36.1

46.5 18.2 27.5 92.2

Source: Adapted from Land, S.B., Jr. et al., Proceedings of the 35th LSU Forestry Symposium, Louisiana State University, Baton Rouge, 1996. With permission.

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Production costs at this point have been generally too high to compete with materials the biomass would supplant, e.g., coal for energy or wood residue for composites manufacturing. As a result, it would seem that agricultural residues accruing from the production of economic crops (e.g., grains) would have the upper hand economically as an alternative biomass source. 5.3.7

Agricultural Production Residues

Of the potential alternatives examined thus far, it would seem that agricultural production residues would be most appealing in terms of cost (land rents and production costs are largely carried as a cost of producing the primary crop) and infrastructure (farmers already have technology, experience, and incentive to collect many agricultural production residues). Agricultural production residues include corn and soybean stover (stalks and leaves), wheat straw, oilseed flax straw, rice straw, and sugarcane bagasse. Bagasse, a fibrous residual of sugar processing, could actually be classified as an agricultural processing residue, but it will be included here with the agricultural production residues primarily due to the inclusion of bagasse in source data examined below. Gallagher et al.5 conducted a detailed analysis of agricultural crop residues that could be available for industrial processes such as ethanol production from lignocellulosics. Their analysis looked at agronomic production capacity, supply and utilization, environmental constraints (e.g., required soil conservation practices that include leaving crop residue on the soil), competing uses and value of residue (e.g., animal forage, value of crop as fertilizer), and other related factors. The analysis therefore provides supply functions for crop residues that include far more consideration than just raw acreage of crop multiplied by biomass production per acre. Figure 5.3 shows the regions evaluated by Gallagher et al.5,6 They identified five major U.S. crop-producing regions that generate the vast majority of crop residues. In addition, the crops examined have infrastructure not only for harvesting the primary crop (grain or sugar cane), but also infrastructure for collection and use of the crop residues (Figure 5.4). The regions examined include the Corn Belt (corn and soybeans are predominant crops; only corn stover included in biomass estimates), Great Plains (wheat), West Coast (wheat), Delta (rice), and Southeast (sugarcane). Table 5.3 presents agricultural production residues by region, including net residue supply and the resultant “industrial supply” remaining after subtracting feed use. Feed use supplies could be diverted to industrial use, but only at higher prices (Gallagher et al.5 estimate cattle feed value of biomass from $6.31/ton for sugarcane bagasse to $42.51/ton for sorghum stover, based on nutritional value). Corn stover feed value is reported as $41.90/ton. However, the market for stover at this price level is limited by the cattle census within reasonable stover transportation distance. For a corn stover lignocellulose-to-ethanol facility, supply curves show that stover prices in the range of $12 to $14.50/ton would produce a stover supply of Copyright © 2005 by Taylor & Francis

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FIGURE 5.3 Map of major crop biomass-producing regions in the United States. The Corn Belt supplies corn stover; Southeast, sugarcane bagasse; Delta, rice straw; Great Plains and West Coast, wheat straw. (Reproduced from Figure 5.2, Gallagher, P.W. et al., Environ. Resour. Econ., 24, 335–358, 2003. With kind permission of Kluwer Academic Publishers.)

about 81.67 million t, or about 90% of the Corn Belt supply. If this market were thus in place, the biocomposites industry would have to meet the same price level for stover ($0.006 to 0.0075/lb or $13.22 to $16.53/dry metric ton). Currently, the price would be somewhat higher than these estimates, in that custom stover baling typically costs Iowa farmers $12–14 per ton. It is interesting to note the comparison between the total biomass availability shown in Table 5.3 (142.083 million t) with the agricultural residue estimate (corn stover and wheat straw only) shown in Table 5.1 (136.7011 million t). Such estimates typically vary quite significantly, to a much greater degree than these do. For example, White and Cook27 cited estimates for both U.S. and worldwide tonnage of “fibrous raw materials from field crops” using data from Atchison,* the USDA,† and FAO.† † The approach used in these * Atchison, J.E., Present status and future prospects for use of nonwood plant fibers for paper grade pulps, Presentation at the American Forest and Paper Association 1994 Pulp and Fiber Fall Seminar, Tucson, AZ, 1994. † United States Department of Agriculture, New industrial uses, markets for U.S. crops: status of technology and commercial adoption, Cooperative State Research Service, Office of Agricultural Materials, Washington, DC, 1993. †† Food and Agriculture Organization of the United Nations, Statistical Reports on Production Data for Nonwood Fiber Crops, FAO, Rome, Italy, 1994.

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FIGURE 5.4 Midwestern U.S. farmers have substantial infrastructure for collection of crop production residues. Soybean stover bales such as this, weighing over 500 kg each, are collected for animal fodder or bedding. Collection and baling of soybean stover is not nearly as routine as that for corn stover. Large bales of corn stover provide the basis for the 90 million t of crop production biomass available in the U.S. Corn Belt. (Photo by D. Stokke. With permission.)

TABLE 5.3 Agricultural Production Residues (t
106) in the United States Region

Net Residue Productiona

Corn Belt Great Plains West Coast Delta Southeast Total

94.006 36.768 3.347 4.734 3.228 142.083

Industry Supplyb 89.762 32.232 2.180 4.195 3.228 131.597

a

Net residue production includes conservation adjustments to yield and erosion-based restrictions required for the harvested area. b Industry supply is net residue production less feed use; feed use indicates livestock demand minus hay supplies (data for feed use omitted from Table). Source: Adapted from Table 4 of Gallagher, P.W. et al., Environ. Resour. Econ., 24, 335–358, 2003. With permission.

estimates was to multiply crop area by biomass yield estimates for the various types of crops. As a result, these estimates are above-ground biomass, as contrasted with the data examined in this chapter in which the authors Copyright © 2005 by Taylor & Francis

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(Gallagher et al.5 and Walsh et al.26) sought to estimate available crop residues when considering delivered prices, soil conservation, residue demand for other uses, etc. For total field crop fiber biomass,* White and Cook27 show a U.S. total of 284 million t based on Atchison’s data (see footnote * on p. 169) or 455.2 million t based on USDA data (see footnote † on p. 169). This type of variation in biomass estimates is not uncommon. The relative congruence of the source data used in this chapter does not necessarily corroborate for accuracy, but it does suggest some agreement on biomass potentially available under the constraint of reasonable assumptions. Overall availability of agricultural production biomass, coupled with price and existing infrastructure lend some advantage to these materials relative to previously considered alternatives. However, the technical characteristics of agricultural residues may be more limiting. Crop residues contain high proportions of extractable chemical constituents and hemicelluloses that are heat-labile.23 This characteristic is particularly troublesome for melt-blend processing. Thus, technical challenges for process technology and biomass pretreatment to improve heat stability need to be addressed in order to effectively use the low-cost, high-volume agricultural processing residues for biocomposites. Nevertheless, substantial efforts are underway to commercialize crop biomass residue, such as wheat straw, for use by the fiber/plastic composites industry.12 5.3.8

Agricultural Processing Residues

A final category of alternative biomass for consideration here is that of agricultural processing residues. Whereas agricultural crop residues are those plant components left following harvest, agricultural processing residues are those materials remaining following industrial operations on the primary crop. These residues include corn, soybean, or rice hulls, defatted corn germ meal, distillers dried grains, or bagasse (the latter was included in the discussion of agricultural production residue). These materials enjoy some potentially significant advantages in terms of infrastructure in that they are collected, processed, and available at existing central processing locations, they are relatively “clean” and homogeneous, they are typically available year-round, and supplies are generally well in excess of market demand. Many of these materials are used in the same general market, that of animal feed. Most of these are low-quality feed, so they are blended with higherprotein feed. Excess supplies of these agricultural processing residues tend to keep them relatively low in cost at less than $0.05/lb ($110/metric ton). Of the potential agricultural processing residues, only three will be considered here, namely, corn hulls, soybean hulls, and rice hulls. Corn hulls * The fibrous raw materials from field crop residues included cereal straw (barley, oats, rice, rye, and wheat); corn stover; cotton lint, linters, stalks, and mote; oilseed flax straw; seed grass straw; sorghum stalks; and sugarcane bagasse (i.e., a wider range of materials than those included in the estimates shown in Tables 5.1 and 5.3 of the present chapter).

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consist of the “pericarp” or outer covering of a corn kernel. Hulls are generated in corn wet milling operations, yielding about 6 lb of hulls (also called “corn fiber”) per bushel of corn or ~10% of the corn input by weight. It is estimated that the U.S. corn wet milling industry produces ~3.63 million t of corn fiber per year.9 Soybean hull (reported as “soybean millfeed [hullmeal]”) production was ~2.74 million t in 1997.25 Market volumes depend on how much 44% protein feed meal is sold compared to 47.5% high-pro meal, as the hulls are used to dilute soy protein to the 44% meal specifications.* The technical characteristics of grain hulls for melt-blend processing are largely unknown, although some studies have been done.† Soybean hulls tend to have good “flow” characteristics; that is, they handle easily with few “bridging” problems during feeding in small pilot-scale extrusion experiments. Notably, both corn hulls and soybean hulls have high proportions of hemicelluloses, at 66.4 and 49.2% of extractive-free substance, respectively.† † Generally, materials with such high hemicellulose contents would not appear to be good candidates for melt-blend processing due to their heat sensitivity. Rice hulls represent another agricultural processing residue of interest for biocomposites. Approximately 1.81 million t are produced annually in the United States, with a market price of around $0.03 per pound ($66.12 per metric ton) for fine grades suitable for fiber/plastic composites.17 Rice hulls have high (20% by weight) silica content, making them somewhat unique among agricultural residues in this regard. They are currently used by some manufactures of natural fiber/plastic composites.

5.4

Further Considerations

The focus of this chapter has been from a U.S. perspective and one that has taken a view of biomass alternatives that have potential for wide availability. I have also suggested that agricultural crop residues or agricultural processing residues might be the better of the alternatives explored, at least from an infrastructure standpoint. This is not to imply, however, that this examination is neither exhaustive nor even correct. For example, the European automobile industry uses ~25,000 t of the 200,000 t of annual European natural fiber production.13 Flax, a “dedicated biomass crop” of sorts (although it is

* Smith, K., personal communication, Consultant, United Soybean Board, Mar. 24, 2003. † Julson, J.L., Subbarao, G., Stokke, D.D., Geiselman, H.H., and Muthukumarappan, K., Mechanical properties of biorenewable fiber/plastic composites, J. Appl. Polym. Sci., 93, 2484–2493, 2004. †† Stokke, D.D. and Ai, J., Unpublished Data, presented at the American Chemical Society Annual Meeting, March 25–27, 2003, New Orleans, LA.

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primarily cultivated for textiles), is a primary source of this material; industrial hemp is another. Kenny13 also reports the use of imported fiber (jute, sisal, coconut fiber) by European automakers. But, these examples are not exactly parallel to the perspective taken throughout this chapter (i.e., focus on short fiber/particulate melt-blended composites) vs. the types of thermoformed materials to be made from the long fibers of flax, hemp, jute, and the like. In addition, these fiber sources are not really “low-cost” alternatives, comparatively speaking. U.S. prices, FOB, were quoted in mid-2003 as from $0.23 to $0.47 per pound ($507 to $1036 per metric ton) for hemp, flax, kenaf, and sisal fibers of various grades.18

5.5

Conclusion

A wide range of biomass alternatives can be considered for use in melt-blend biocomposites. A number of studies and trials have been and continue to be carried out to evaluate the technical characteristics of these materials. However, it may prove that the infrastructure needed to efficiently harvest, process, and distribute biomass alternatives may play the primary role in determining which materials are adopted, as suggested by Riccio and Orchard.20 In the United States, infrastructure considerations suggest that the biocomposites industry should examine more closely the potential for use of agricultural production and agricultural processing residues.

Acknowledgments I would like to acknowledge Susanne Knutson, Graduate Student, for corroboration of some data sources. I also thank Heath Gieselman, Research Associate, for assistance with photos, figures, and general support. Susanne and Heath are both in the Department of Natural Resource Ecology and Management at Iowa State University. I would also like to thank Dr. Paul Gallagher, Department of Economics at Iowa State University, for background information concerning his analysis of crop biomass availability.

References 1. Conner, R.C. and Hartsell, A.J., Forest area and conditions, in Southern Forest Resource Assessment, Wear, D.N. and Greis, J.G., Eds., General Technical Report SRS-53, U.S. Department of Agriculture, Forest Service, Southern Research Station, Asheville, NC, 2002, Chap. 16, pp. 357–402.

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2. Eckert, C., Opportunities for Natural Fibers in Plastic Composites, Proceedings of the Conference on Progress in Woodfibre-Plastic Composites Conference, May 25–26, 2000, Toronto, Canada, University of Toronto, Toronto, Ontario, CA. 3. English, B., Chow, P., and Bajwa, D.S., Processing into composites, in Paper and Composites from Agro-Based Resources, Rowell, R.M., Young, R.A., and Rowell, J.K., Eds., CRC Lewis Publishers, Boca Raton, FL, 1997, pp. 269–299. 4. English, B., Clemons, C.M., Stark, N., and Schneider, J.P., Waste-wood-derived Fillers for Plastics, General Technical Report FPL-GTR-91, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1996. 5. Gallagher, P.W., Dikeman, M., Fritz, J., Wailes, E., Gauthier, W., and Shapouri, H., Supply and social cost estimates for biomass from crop residues in the United States, Environ. Resour. Econ., 24, 335–358, 2003. 6. Gallagher, P., Dikeman, M., Fritz, J., Wailes, E., Gauther, W., and Shapouri, H., Biomass from Crop Residues: Cost and Supply Estimates, Agricultural Economic Report No. 819, U.S. Department of Agriculture, Office of the Chief Economist, Office of Energy Policy and New Uses. Washington, DC, 2003, 26p. 7. Gassner III, G., Schmidt, W., Line, M.J., Thomas, C., and Waters, R.M., Fiber and fiber products produced from feathers, U.S. Patent 5,705,030, January 6, U.S. Government Patent Office, Washington, DC, 1998. 8. George, B.R., Evazynajad, A., Bockarie, A., and McBride, H., Production and characterization of yarns and fabrics utilizing turkey feather fibers, in Natural Fibers, Polymers and Composites, Wallenberger, F.T. and Weston, N.E., Eds., Kluwer Academic Publishers, Boston, MA, 2004, 370pp. 9. Hicks, K.B., Moreau, R.A., and Doner, L.W., Corn Fiber: An Old By-product with a Cornucopia of Future Uses, Program Proceedings, Corn Utilization Conference VI, June 4–6, 1996, St. Louis, MO, National Corn Growers Association and the National Corn Development Foundation. 10. Ince, P., Recycling of Wood and Paper Products in the United States, General Technical Report FPL-GTR-89, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 1996. 11. Ince, P. and McKeever, D.B., Estimates of Paper and Wood Recovery for Recycling and Potential for Additional Recovery in the United States, Proceedings of the 3rd International Conference on Woodfiber–Plastic Composites, No. 7293, Forest Products Society, Madison, WI, 1996, pp. 144–154. 12. Johnson, D.A., Jacobson, R., and Maclean, W.D., Wheat Straw as a Reinforcing Filler in Plastic Composites, Proceedings of the 4th International Conference on Woodfiber–Plastic Composites, May 12–14, 1997, No. 7277, Forest Products Society, Madison, WI, pp. 200–205. 13. Kenny, J.M., Natural Fiber Composites in the European Automotive Industry, Proceedings of the 6th International Conference on Woodfiber–Plastic Composites, No. 7251, Forest Products Society, Madison, WI, 2002, pp. 9–12. 14. Land, S.B., Jr., Exell, A.W., Schoenholtz, S.H., Tuskan, G.A., Tschaplinski, T.J., Stine, M., Bradshw, H.D., Kellison, R.C., and Portwood, J., Intensive Culture of Cottonwood and Hybrid Poplars, in Proceedings of the 35th LSU Forestry Symposium, Carter, M.C., Ed., Louisiana State University, Baton Rouge, LA, USA, 1996, pp. 167–189. 15. Manarpaac, G.A., Aaman, K., and Harun, J., Oil Palm Empty Fruit Bunch Fibers–polypropylene Composites: The Effects of Electron Beam Radiation and Reactive Addititves on Some Properties, Proceedings of the 6th International Conference on Woodfiber–Plastic Composites, No. 7251, Forest Products Society, Madison, WI, 2002, pp. 55–60. Copyright © 2005 by Taylor & Francis

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16. Pereira, N., Souza, M.L., Agnelli, J.A.M., and Mattoso, L.H.C., Effect of processing on the Properties of Polypropylene Reinforced with Short Sisal Fibers, Proceedings of the 4th International Conference on Woodfiber–Plastic Composites, May 12–14, 1997, Madison, WI, No. 7277, Forest Products Society, Madison, WI, pp. 206–214. 17. Principia Partners, Rice hulls: an ample supply, Nat. Wood Fiber Compos. Newslett., 2, 3, 2003. 18. Principia Partners, Wood flour prices rise; natural fiber prices holding steady, Nat. Wood Fiber Compos. Newslett., 2, 4, 2003. 19. Rana, A.K., Mandal, A., Mitra, B.C. Jacobson, R., Rowell, R., and Banerjee, A.N., Short jute fiber-reinforced polypropylene composites: effect of compatibilizer, J. Appl. Polym. Sci., 69, 329–338, 1998. 20. Riccio, F.A., Jr. and Orchard, L.P., Nonwood Fiber Resources: Availability, Infrastructure, and Feasiblity, Proceedings of the 5th International Conference on Woodfiber–Plastic Composites, No. 7263, Forest Products Society, Madison, WI, 1999, pp. 211–215. 21. Rowell, R.M., Sanadi, A.R., Caulfield, D.F., and Jacobson, R.E., Utilization of natural fibers in plastic composites: problems and opportunities, in LignocellulosicPlastic Composites, Leao, A.L., Carvalho, F.X., and Frollini, E., Eds., Universidade de Sao Paulo Press, Sao Paulo, Brazil, 1997, pp. 23–51. 22. Smith, W.B., Faulkner, J.L., and Powell, D.S., Forest Statistics of the United States, 1992, Metric Units, General Technical Report NC-168, U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station, 1994. 23. Stokke, D.D., Kuo, M.L., Curry, D.G., and Gieselman, H.H., Grassland Flour/Polyethylene Composites, Proceedings of the 6th International Conference on Woodfiber–Plastic Composites, No. 7251, Forest Products Society, Madison, WI USA, 2002, pp. 43–53. 24. United States Department of Agriculture, Crop Production 2002 Summary, National Agricultural Statistics Service, Report Cr Pr 2–1, 2003. 25. United States Department of Commerce, Soybean Processing, 1997 Economic Census of Manufacturing, Industry Series, EC97M-3112E, issued Oct., 1999. Economics and Statistics Administration, U.S. Census Bureau, 1997. 26. Walsh, M.E., Perlack, R.L., Turhollow, A., de la Torree Ugarte, D. Becker, D.A., Graham, R.L., Slinsky, S.E., and Ray, D.E., Biomass Feedstock Availability in the United States: 1999 State Level Analysis, U.S. Department of Energy, Oak Ridge National Laboratory, 1999, http:// bioenergy.ornl.gov/resourcedatat/ index.html. 27. White, G.A. and Cook, C.G., Inventory of agro-mass, in Paper and Composites from Agro-Based Resources, Rowell, R.M., Young, R.A., and Rowell, J.K., Eds., CRC Press, Boca Raton, FL, 1997, pp. 7–21. 28. Winandy, J.E., Muehl, J.H., Micales, J.A., Raina, A., and Schmidt, W., Potential of Chicken Feather Fibre in Wood MDF Composites, Proceedings Paper P.20 in EcoComp 2003, Queen Mary, University of London, 2003.

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6 Fiber-Matrix Adhesion in Natural Fiber Composites

Pedro J. Herrera Franco and Alex Valadez-González

CONTENTS 6.1 Introduction 6.2 Properties of Natural Fibers 6.2.1 Isolation of Natural Fibers 6.2.2 Fiber Geometry 6.2.3 Cellulose Crystallinity 6.2.4 Hydrophilicity of Cellulose 6.3 The Fiber-Matrix Interphase 6.4 Fiber Surface Modification Methods 6.4.1 Physical Methods of Modification 6.4.2 Alkali Swelling and Substitution Reactions 6.4.3 Impregnation of Fibers 6.4.4 Chemical Modification 6.4.5 Silane Treatments 6.5 Fiber-Matrix Adhesion 6.5.1 Testing of Adhesion in Fiber-Matrix Composites: Micromechanical Characterization 6.6 Experimental Procedure and Materials 6.6.1 Materials 6.6.2 Fiber Surface Treatments 6.6.3 Adsorption Isotherm 6.6.4 X-ray Photoelectron Spectroscopy (XPS) 6.6.5 Infrared Spectroscopy (FTIR) 6.6.6 Pullout Test Sample Preparation 6.6.7 The Single-Fiber Fragmentation Test Sample Preparation 6.6.8 Elaboration of the Composite 6.6.9 Scanning Electron Microscopy (SEM) 6.7 Results

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6.7.1 Surface Treatments 6.7.2 Adsorption Isotherms 6.7.3 X-ray Photoelectron Spectroscopy (XPS) 6.7.4 Infrared Spectroscopy (FTIR) 6.8 Fiber-Matrix Adhesion: Characterization of the Adhesion Level Using Micromechanical Techniques 6.9 Single-Fiber Fragmentation Test 6.10 Effect of Fiber-Matrix Adhesion on Mechanical Properties 6.10.1 Tensile Properties 6.10.2 Iosipescu Shear Strength 6.11 Conclusions References

ABSTRACT It is well known that natural fibers impart polymeric matrix composites high specific stiffness and strength, a desirable fiber-aspect ratio, and biodegradability. Also, cellulosic fibers are readily available from natural sources, and most importantly, they have a low cost per unit volume. One difficulty that has prevented a more extended utilization of natural fibers is the lack of good adhesion to most polymeric matrices. The hydrophilic nature of natural fibers adversely affects adhesion to a hydrophobic matrix, resulting in poor strength properties. In order to give an overview of the problems that have to be overcome when using a natural fiber, a review of their nature and configuration is made. Then, the different approaches to improve fiber-matrix adhesion are explored for a specific fiber-matrix system, namely, henequen fibers (Agave fourcroydes) and a thermoplastic matrix, high-density polyethylene (HDPE). Both the mechanical and the chemical aspects of fiber surface modification are presented. The interfacial shear strength (IFSS) between the natural fibers and the thermoplastic matrix was improved by morphological and chemical modifications of the fiber surface. The alkaline treatment had two effects on the fiber:1 it increased the surface roughness therefore resulting in better mechanical interlocking, and2 it incremented the amount of cellulose exposed on the fiber surface, thus increasing the number of possible reaction sites. The fiber preimpregnation allowed a better fiber wetting, which, in a normal fiber-polymer mixing procedure, would not have been possible because of the high polymer viscosity. Thus, the preimpregnation also enhanced the mechanical interlocking between fiber and matrix. Modification of the henequen fiber with a silane coupling agent was carried out and the changes in their surface physicochemical properties were measured. Adsorption isotherms showed that the efficiency of the silane treatment was higher for the alkaline-treated fiber than for the untreated one, e.g., there is a higher amount of silane deposited when the fibers was modified with an aqueous alkaline solution. This indicated that the partial removal of lignin and other alkali-soluble compounds from the fiber increased the number of reactive sites on the fiber surface. It was also found Copyright © 2005 by Taylor & Francis

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that the formation of polysiloxanes inhibited the adsorption of the silane onto the henequen fiber surface. This fact was particularly important since at highly concentrated silane solutions there was no additional silane adsorption onto the fibers. The XPS surface studies of the henequen fibers confirmed the presence of the silane group deposited on the surface of both silane-treated (FIBSIL) and the alkaline plus silane fibers (FIBNASIL). The analysis of these emission spectra showed evidence of a chemical reaction between the hydrolyzed silane and the henequen fibers, at least as observed from the results for FIBNASIL. The chemical reaction between the organosilane coupling agent and the henequen fibers was also confirmed by FTIR spectroscopic analysis. The fiber-surface silanization results in a better interfacial load transfer efficiency but does not seem to improve the wetting of the fiber. Fiber-matrix adhesion was studied using micromechanical techniques commonly used in the characterization of the IFSS of circular cross section, smooth fibers. The IFSS relationships developed for circular fibers were modified to incorporate the natural fiber perimeter instead of an equivalent fiber diameter. The use of the fiber critical aspect ratio (l /d) in the Weibull analysis resulted in higher values for IFSS, but the trends were the same as those obtained from the use of the fiber cross-section perimeter for both the fragmentation test and the pullout test. The first technique, however, resulted consistently in higher values of IFSS than the second one. Slight discrepancies were observed between the results obtained from the two micromechanical techniques for fibers that had been subjected to a preimpregnation process. From the pullout experiment, higher (IFSS) values were obtained than those obtained with the single-fiber fragmentation test. Such discrepancies were attributed to the differences in the experimental configurations and mechanical solicitation in both arrangements and to differences in the mechanical behavior of the fiber that had been impregnated with the matrix. The results obtained from the single-fiber fragmentation test seem to be in better agreement with the effective mechanical properties measured for the laminated material. The influence of the surface treatments on the effective mechanical properties of a HDPE-henequen fiber composite was also assessed. It was noted that modification of the fiber surface with only an aqueous solution of NaOH was not sufficient to improve the strength of the material. Additional treatment with a silane-coupling agent to promote the adhesion by chemical interactions or by enhancing the mechanical interlocking by a preimpregnation process clearly improved the tensile properties of the composite. The silane treatment of the fiber surface resulted in an increase of 28% of the tensile strength of the composite material whereas the preimpregnation process yielded only a 4.7% increase. The contact between fiber and matrix seemed to have a larger effect on shear strength. Fiber preimpregnation by itself had no great contribution to shear strength. The largest increase in shear strength was observed for the fiber treated with both the aqueous alkaline solution and the silane coupling agent. With such treatment shear strength increased by 25%. Copyright © 2005 by Taylor & Francis

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Introduction

Composites have encompassed almost all material domains (house furnishing, packaging, car, aerospace, transportation, sport, leisure, and so on). All synthetic polymers (thermoplastics, thermosets, and elastomers) can be used as matrices. As fillers, besides inorganic powders, extensive use has been made of inorganic man-made fibers such as glass, silicium carbide, or organic fibers like carbon and aramid, in the form of individual fibers (chopped or continuous), mats, or 2D–3D fabrics. Except for glass fibers, these reinforcing fibers are expensive. Various fibers like cellulose, wool, silk, and composites thereof are produced in nature. Among others, wood is a good example of a natural composite where cellulosic fibers are in close association with hemicelluloses to reinforce the lignin matrix.1 Cellulosic fibers are located in cell walls and are referred to as bast fibers in the stems and stalks (flax, jute, ramie, etc.) as leaf fibers (sisal, banana, etc.) and as seed fibers (cotton, kapok, etc.). Besides its renewability, cellulose is available worldwide and it is biodegradable. Raw materials are often used as furnished by nature, but cellulose and wood present poor dimensional stability due to swelling by water, in relation to relative humidity, and a protection against weathering conditions is needed (paints, varnishes, etc.). The return to natural materials is based on many advantages present in the materials themselves, as well as in combination with other materials in composites. As composites, large quantities of waste arising from woodworking (chips and clippings) are reused in chipboards, where they are compounded using a thermosetting resin: formaldehyde associated with phenol, melamine, or urea.2 Research efforts made over the past 15–20 years have centered in the incorporation of vegetal materials in synthetic matrices, with or without processing additives or compatibilizing agents. More recently, cellulosic fibers have been used as reinforcement for thermoplastic matrices. Their mechanical properties are excellent: in the case of flax, the ultimate fibril has a tensile modulus almost as high as aramid.3 Compared with glass fibers, composites made with cellulosic fibers have lower density, but the same mechanical performances are achieved at identical filler weight contents.3,4 However, during processing of the composites, the cellulosic material does not lead to abrasion of the processing machines. Moreover, cellulose can withstand the temperature needed for the processing of polyethylene and polypropylene (PP) as it withstands the curing temperature for thermoset and wood chips. Finally, the bulk composite made from polyolefins and cellulose may eventually be recycled or burned to recover heat, without production of residues or toxic by-products. Such usage justifies the production of agricultural, but nonedible products, especially in third world countries. Such advantages are interesting for the automotive industry which requires light materials, good performance/weight ratio, minimization of the number of materials, recycling possibilities, and no pollution. Recycling of both waste paper with waste polyolefins has been studied.5–7 J. George et al. has Copyright © 2005 by Taylor & Francis

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recently published a review on composites made of various vegetal materials and synthetic polymers.8 Association between various materials is always the source of different problems (good contact between the materials, adherence, resistance in wet conditions) that are common to many industrial sectors: paper industry, paper/PP laminates,9 fabrication of wood polymer composites10–14 with many patents available in this field, adherence of paints, varnishes on wood, adhesives on paper, cardboard and fabrics, printing on paper, and dye printing on cotton fabrics. Cellulosic fibers, like henequen, sisal, coconut fiber (coir), jute, palm, bamboo, and wood, in their natural conditions, and several waste cellulosic products such as shell flour, wood flour, and pulp have been used as reinforcement agents of different thermosetting and thermoplastic resins. In the case of henequen fibers, during leaf defibration and also during the transformation of the raw fibers into cordage, ~10% of waste fibers is produced. Waste fibers could be profitably used in the manufacture of short-fiber polymer-reinforced composites because they possess attractive physical and mechanical properties. Earlier reports15,16 on henequen show that these fibers contain ~60% cellulose, 25% hemicelluloses, 8% lignin, and 2% waxes and that they possess physical and mechanical properties that make them suitable to reinforce plastics.17 The cellulose pulps can be easily obtained and could also be used to reinforce polymeric materials. One difficulty that has prevented a more extended utilization of henequen fibers in composites is the lack of good adhesion to most polymeric matrices. The hydrophilic nature of natural fibers adversely affects adhesion to a hydrophobic matrix resulting in poor strength properties. To prevent this, the fiber surface has to be modified in order to promote adhesion. Due to their high cellulosic components content, henequen fibers are of a hydrophilic nature, and it is necessary to modify their surface properties prior to using them as a reinforcement on most of the commercially available thermoplastic resins that are of a hydrophobic nature. An important aspect with respect to optimal mechanical performance of fiber-reinforced composites in general and durability in particular, is the optimization of the interfacial bond between fiber and polymer matrix. The quality of the fiber-matrix interface is significant for the application of natural fibers as reinforcement for plastics. Several reports concerning the surface modification of natural fibers for their use as reinforcement of plastics can be found in the literature. Moharana et al.18 chemically modified jute fibers through graft copolymerization of methyl methacrylate on the fiber surface. Samal and Bhuyan19 grafted acrylonitrile onto pineapple leaf fibers and characterized its functionality changes. Karmaker20 showed that in a jute-filled PP composite, water absorption and Izod impact strength of the composite were improved when the surface properties of the jute fibers were modified with a maleic anhydride-grafted PP. Felix and Gatenholm21 modified the surface properties of α-cellulose (Nymolla AB) fibers using a PP-maleic anhydride copolymer. They used x-ray photoelectron spectroscopy (XPS) and Fourier-transformed infrared spectroscopy (FTIR) to identify the chemical structure and found Copyright © 2005 by Taylor & Francis

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that the coupling agent was localized at the surface of the fibers. They also reported that the mechanical properties of PP-cellulose fiber composites were improved when treated fibers were used. Zadorecki and Flodin22,23 used triazine derivatives containing double bonds for the modification of bleached softwood kraft paper cellulosic fibers. Using FTIR, they characterized the products of the reactions between the hydroxyl groups of cellulose and the reactive species of the coupling agent and XPS for the fiber surface characterization. They found that such coupling agents were able to react with the cellulosic fibers and that these coupling agents were concentrated on the surface rather than uniformly distributed throughout the fibers. They also pointed out that the use of triazine compounds as coupling agents with cellulosic fibers improved the environmental aging behavior of cellulose-polyester composites, since the triazines compound acted as a chemical bridge between the polyester matrix and the cellulosic reinforcement. Herrera-Franco et al.25 used a silane coupling agent to promote adhesion between henequen fibers and a high-density polyethylene matrix and have also shown that the adhesion between the natural hard fiber and the matrix played an important role on the final mechanical properties of the composite. Singh et al.26 also observed improved mechanical properties on treatment of a sisal-polyester composite. This chapter will first introduce the elements of fiber-matrix adhesion and the different approaches to achieve it with a natural fiber. In order to give an overview of the problems that have to be overcome when using a natural fiber, it is necessary to review their nature and configuration. Then, the different approaches that have been used to improve fiber-matrix adhesion will be explored for a specific fiber-matrix system, namely, henequen fibers (Agave fourcroydes) and a thermoplastic matrix, high-density polyethylene (HDPE). Both mechanical and chemical aspects of fiber surface modification will be presented and how such modification is characterized. The level of fiber-matrix adhesion has been studied both from the chemical and mechanical point of view; therefore, such approach will be presented. Then, the effect of fibermatrix adhesion on the effective mechanical properties will be discussed.

6.2

Properties of Natural Fibers

Depending on their origin, natural fibers can be grouped into bast (jute, flax, hemp, kenaf, mesta), leaf (pineapple, sisal, henequen, screw pine), and seed or fruit fibers (coir, cotton, oil palm). Cellulose is the main component of natural fibers, and the elementary unit of a cellulose macromolecule is anhydro-D-glucose, which contains three hydroxyl (OH) groups. These hydroxyl groups form hydrogen bonds inside the macromolecule itself (intramolecular) and between other cellulose macromolecules (intermolecular). A schematic drawing of cellulose molecules including the intramolecular Copyright © 2005 by Taylor & Francis

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O(3) OH 6

H

o 5

3 HO

2

o

1

o

H

OH

3

o

1

5 2

4 6

o

o

OR

FIGURE 6.1 Schematic drawing of cellulose molecules including the intramolecular and intermolecular hydrogen bonds.

and intermolecular hydrogen bonds is shown in Figure 6.1. Therefore all natural fibers are hydrophilic in nature. Unlike conventional fibers like glass, aramid, and carbon that can be produced with a definite range of properties, natural fibers will vary considerably. The cellulose of natural fibers contains different natural substances such as lignin and waxes. The fibers are made up of cellulose microfibrils bonded together by lignin. The physical properties of natural fibers are basically influenced by the chemical structure such as cellulose content, degree of polymerization, orientation, and crystallinity, which are affected by conditions during growth of plants as well as extraction methods used. The fiber properties vary considerably depending on where they are taken from a plant, the plant quality, and location. Different fibers have different lengths and cross-sectional areas and also different defects such as microcompressions or pits or cracks.

6.2.1

Isolation of Natural Fibers

The cellulosic fiber is generally associated with other organic compounds that have to be eliminated: pectic cements, tannins, hemicelluloses, and lignin. Various methods are used to isolate the fiber: traditional rotting for flax, hemp, sisal, etc., followed by washing with alkaline aqueous solutions, which eliminates mainly the pectic cements, lignin, and hemicelluloses [washing of raw sisal fibers, for instance, induces a loss of 24 wt%48], and steam explosion.28–30 Wood fibers are more frequently used since they are obtained in the preparation of pulps for the paper industry. The treatment consists in defibrillation by mechanical pulping with heat and water. Eventually alkali treatment eliminates the lignin and hemicelluloses. The socalled thermomechanical pulps (TMP) or chemothermomechanical pulps (CTMP) present well-defined and almost constant properties. All the treatments performed to separate and isolate the individual fibers are always detrimental to the mechanical properties of these fibers and a compromise must be found. Copyright © 2005 by Taylor & Francis

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Fiber Geometry

Man-made fibers emerging from a spinneret are cylindrical, with approximately constant diameter and specific area. This is not the case for cellulosic fibers that present many defects caused by twisting31 in the stacking of the cellulose chains. These defects are apparent as “knees” at the fiber surface and constitute points where the fiber may rupture more easily. In addition, an important parameter is the aspect ratio (length/diameter), which has an influence on the mechanical properties of the composite.32 This aspect ratio is highly modified by attrition during processing (extrusion, injection).33 Table 6.1 shows dimensions and chemical composition of some common natural fibers. It should be emphasized that cellulose swells when exposed to a polar media such as water, dimethylformamide, dimethylsulfoxyde, tetrahydrofuran, and pyridine. The hydroxyl groups are accessible for reaction, but some of these swelling solvents may be entrapped in the cellulosic structure.34 In contrast, nonswelling media (benzene, toluene, aliphatic hydrocarbons, etc.) force the hydroxyl groups to turn toward the inside of the cellulosic structure.35 It is also possible, by progressive replacement of a swelling medium by media of lower and lower polarity, to maintain in nonpolar medium the initially swelled structure.36 6.2.3

Cellulose Crystallinity

Cellulosic materials present amorphous and crystalline domains and a high degree of organization. The crystallinity rate depends on the origin of the TABLE 6.1 Dimensions and Chemical Composition of Some Common Natural Fibers Fiber Dimension (mm) Type of Fiber Cotton Seed flax Hemp Abaca Coniferous wood Sisal Bamboo Kenaf Jute Esparto Papyrus Sugar cane bagasse Cereal straw Corn straw Wheat straw Rice straw Deciduous wood Coir

Cellulose (%) Lignin (%) Mean length Mean width 85–90 43–47 57–77 56–63 40–45 47–62 26–43 44–57 45–63 33–38 38–44 32–37 31–45 32–35 33–39 28–36 38–49 35–62

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0.7–1.6 21–23 9–13 7–9 26–34 7–9 21–31 15–19 21–26 17–19 16–19 18–26 16–19 16–27 16–23 12–16 23–30 30–45

25.0 30.0 20.0 6.0 4.1 3.3 2.7 2.6 2.5 1.9 1.8 1.7 1.5 1.5 1.4 1.4 1.2 0.7

0.02 0.02 0.022 0.024 0.025 0.02 0.014 0.02 0.02 0.013 0.012 0.02 0.023 0.018 0.015 0.008 0.03 0.02

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material; bacterial cellulose has the highest degree of organization and is often used as a reference. Cotton, flax, and ramie have high degrees of crystallinity (65–70%), but the crystallinity of regenerated cellulose is only 35–40%. Progressive elimination (by dissolution with chemicals, attack by microorganisms) of the less organized parts leads to fibrils with ever increasing crystallinity, until almost 100% crystallinity whiskers. Crystallinity of cellulose results partly from hydrogen bonding between the cellulosic chains, but some hydrogen bonding also occurs in the amorphous phase,37 although its organization is low. Numerous structural forms of cellulose38 are recognized and are still the object of investigation.39–43 Table 6.1 presents the chemical composition of some common natural fibers. 6.2.4

Hydrophilicity of Cellulose

Because of its chemical structure, many hydroxyl groups are available for interaction with water molecules by hydrogen bonding.47 In contrast to glass fibers where water adsorption is important only at the surface, cellulosic fibers interact with water not only at the surface, but also in the bulk. The quantity of sorbed water depends on the relative humidity of the confined atmosphere with which the fiber is in equilibrium. The sorption isotherm of cellulosic material (water sorbed vs. water partial pressure) depends on the following: (1) The purity of cellulose: raw cellulosic material such as nonwashed sisal fibers absorb at least twice as much water as washed fibers.48 This is due to the presence of 24% of pectic cements. (2) The degree of crystallinity: all OH groups in the amorphous phase are accessible to water whereas only a small amount of water interacts with the surface OH groups of the crystalline phase. All these characteristics of the cellulosic fiber play an important role when the fiber is to be incorporated in a matrix: Strong adherence is needed to take advantage of the high modulus of the fiber, and this is controlled.

6.3

The Fiber-Matrix Interphase

A major disadvantage of cellulose fibers is their highly polar nature, which makes them incompatible with nonpolar polymers. Also, the poor resistance to moisture absorption makes the use of natural fibers less attractive for exterior applications. Several types of polymers have been used as matrices for natural fiber composites.49–57 The most commonly used are thermoset polymers such as polyester, epoxies, and phenolics. Thermoplastics like polyethylene (PE), polystyrene (PS), and polypropylene (PP) have also been used.49–52 These polymers have a different affinity toward the fiber owing to the difference in their chemical structure. It was reported that sisal/LDPE composites showed a better reinforcing effect because of high matrix ductility and a high strength/modulus ratio of sisal Copyright © 2005 by Taylor & Francis

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as compared to that of LDPE matrix.51 It was reported by Joseph et al.51 that among polyester, epoxy, and phenol formaldehyde composites of sisal fiber, the phenolic-type resin had better tensile and flexural properties matrix than epoxy and polyester resins owing to the high interfacial bonding in phenolic composites. He also found52 that the reinforcing ability of sisal fiber in PP matrix is lower. It was found that kudzu fiber (Pueraria lobata) extracted from kudzu vines and banana strands has potential economic value with polyester to make a commercial fabric woven on power looms.53,54 The flexural behavior of coir, straw, and jute fibers in polyester has been studied and analyzed by various workers.55–57 The potential of sunhemp/polyester composites in terms of tensile and impact properties has been investigated by Rohatgi and co-workers.58 Zadorecki and Flodin23 used cellulose fiber in the form of paper sheets as reinforcement in unsaturated polyester composites. The reinforcing effect of sisal, jute, and bamboo fiber in epoxy composites is also reported.59–62 Several studies have been carried out by Kokta and co-workers using chemithermomechanical pulp in different thermoplastics.63,64 Crystallization kinetic studies were carried out by Chen and Porter65 on composites made of polyethylene and kenaf fiber, which is extracted from the bast of the plant Hibiscus cannabinus. The effects of filler content and size on the mechanical properties of PP/oil palm wood flour composites were reported by Zainy et al.66 The stress transfer at the interface between two different phases is determined by the degree of adhesion. A strong adhesion at the interfaces is needed for an effective transfer of stress and load distribution throughout the interface. This situation calls for the development of strategies for the surface modification of cellulosic surfaces, thereby an effective control over the fiber–polymer interface. In order to improve the mechanical properties of composites, a coating is applied, which generally consists of coupling agents or compatibilizing agents that introduce chemical bonds between the fiber and matrix. Effects of coupling agents on cellulosic fiber-reinforced thermoplastic composites and their influence on mechanical properties have been reported.67–69 Singh et al.70 reported the effect of various chemical treatments such as organotitanate, zirconate, silane, and N-substituted methacrylamide on the properties of sisal fiber-reinforced polyester composites. Reinforcing fibers can be modified by physical and chemical methods.

6.4 6.4.1

Fiber Surface Modification Methods Physical Methods of Modification

Physical methods involve surface fibrillation, electric discharge71,72 (Corona, cold plasma), and so on. Physical treatments change structural and surface properties of the fiber and thereby influence the mechanical bonding with the matrix. Surface modification by discharge treatment such as low-temperature Copyright © 2005 by Taylor & Francis

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plasma, sputtering, and corona discharge is of great interest in relation to the improvement in functional properties of natural fibers. Low-temperature plasma treatment causes mainly chemical implantation, etching, polymerization, free radical formation, and crystallization, whereas sputter etching brings about chiefly physical changes such as surface roughness, and this leads to increase in adhesion.72 Low-temperature plasma is a useful technique to improve the surface characteristics of the fiber and polymeric materials by utilizing ingredients such as electrons, ions, radicals, and excited molecules produced by electrical discharge. Low-temperature plasma can be generated under atmospheric pressure in the presence of helium.72 The action of these plasmas involves abstraction of protons and creation of unstable radicals that convert functional groups such as alcohols, aldehydes, ketones, and carboxylic acids. Electrical discharge methods are used for cellulose fiber modification to decrease the melt viscosity of cellulose-polyethylene composites73 and to improve the mechanical properties of composites. Corona treatment is one of the most interesting techniques for surface oxidation activation. It changes the surface energy of the cellulosic fibers, which in turn affects the melt viscosity of composites.30 Mechanical and rheological properties of cellulose-PP composites subjected to corona treatment were reported by Bataille et al.74 and Dong and Sapieha.75 Corona treatment modifies the surface composition and therefore the surface properties of the composite components. 6.4.2

Alkali Swelling and Substitution Reactions

Mercerization is an old method of cellulose fiber modification which is an alkaline treatment of cellulose fibers. Its efficiency depends on the type and concentration of the alkaline solution, time of treatment, and temperature. It was reported that after immersion in alkali for 48 h, the globular pultrusions present in the untreated fiber disappeared, leading to the formation of a larger number of voids.76 Loss of cuticle by the rupture of alkali- sensitive bonds leads to a rough surface. Varma et al.77 studied the thermal stability of natural and chemically treated coir-fiber including mercerization in the nitrogen atmosphere. Effects of NaOH treatment and preirradiation on coconut fiber/PF composites were reported by Owolabi et al.78 In general, this treatment reduces the cementing material followed by the removal of volatile products with the rupture of bonds. Optimal conditions of mercerization ensure the improvement of tensile properties. George et al.79 reported reduction of tensile properties of PALF/LDPE composites at higher concentrations of NaOH. In substitution reactions, those that substitute OH groups of cellulose molecules in the presence of alkali can be utilized. By impregnating fiber with alkaline swelling agents and by reacting with some chemical agents that can be substituted for the hydroxyl groups on cellulose molecules in the presence of alkali, highly decrystallized fibers can be obtained.80 Effective substitution reactions include acetylation by acetic anhydride and cyanoethylation, and it was found that acetylation of wood increases the dimensional stability and reduces the susceptibility to decay.81 Copyright © 2005 by Taylor & Francis

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Increased strength, rot resistance, and hydrophobicity of jute fiber after acetylation are reported by Anderson and Tillman.82 In a review article, Bledzki and Gassan reported the surface modification of natural fibers, the processing and properties of natural fiber composites, and their technical applications.83

6.4.3

Impregnation of Fibers

A better interaction between fiber and polymer is achieved by impregnation of the first with polymer matrices compatible with the polymer. Fibers are impregnated in liquid monomer and then these monomers are polymerized in situ using a catalyst, heat, or radiation.84 A wide range of composite properties could be attained by varying the ratio between the monomer with a cross-linking agent.84 It is known that the elevated viscosity of the matrix during processing of the composite hinders the monomer; therefore, the fiber microcavities, the method and the polymerization process should be carefully selected. Improved dimensional stability of wood polymer (WPC) using a solvent exchange technique of impregnation was reported and complete impregnation of the fibers resulted in a low mechanical interaction with the matrix. Monomer solutions of low viscosity are used for the purpose.85 Herrera-Franco et al.25 reported the improved interfacial properties of henequen/HDPE composites by the impregnation method. The HDPE was deposited on the henequen fibers from a dilute solution of xylene at 115°C.

6.4.4

Chemical Modification

Strongly polarized cellulose fibers are not inherently compatible with hydrophobic polymers. The compatibility and dispersability of fiber and matrix can be improved by developing a hydrophobic coating of a compatible polymer on the surface of filler before being mixed with polymer matrix. Generally, coupling agents facilitate the optimum stress transfer at the interface between fiber and matrix. Coupling agents are molecules possessing two functions. The first is to react with OH groups of cellulose and the second is to react with functional groups of the matrix. The selection of a coupling agent that can combine both strength and toughness to a considerable degree is important for a composite material. The most common coupling agents are silanes,67,87–89 isocyanates,69,87,90–95 and titanate-based compounds: Their chemical composition allows them to react with the fiber surface, which forms a bridge of chemical bonds between the fiber and matrix. It is expected that the formation of a primary type of bond—covalent bonds between cellulose and isocyanate—and a secondary type of weak bond between thermoplastics and isocyanates improves the mechanical properties of wood fiber-filled thermoplastics. Pretreatment of fibers by encapsulated coating with silanes or isocyanates, grafting,96–105 and so on, provides better dispersion by reducing fiber–fiber interaction. Copyright © 2005 by Taylor & Francis

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Silane Treatments

Several theories have been proposed to explain the interfacial bonding mechanisms of silane coupling agents. Among these, the most widely accepted is chemical bonding theory. In the chemical bonding theory, the bifunctional silane molecules act as a link between the resin and the cellulose by forming a chemical bond with the surface of cellulose through a siloxane bridge while its organofunctional group bonds to the polymer resin. This coreactivity with both the cellulose and the polymer via covalent primary bonds gives molecular continuity across the interface region of the composite. A number of factors affect the microstructure of the coupling agent, which in turn controls the mechanical and physical properties of the composites.86 They are the silane structure, its organofunctionality, its acidity, the drying conditions and homogeneity, the topology, and the chemical composition of the fiber surface. The general chemical formula of silane is X3Si–R, a multifunctional molecule that reacts at one end with the cellulose fiber surface and at the other end with the polymer phase. R is a group, which can react with the resin, and X is a group, which can hydrolyze to form a silanol group in aqueous solution, and these react with the hydroxyl group of the cellulose surface. R-groups may be vinyl, aminopropyl, methacreloxypropyne, etc. The X-group may be chloro, methoxy, ethoxy, etc. The type of organofunctional group and pH of the solution dictates the composition of silane in dilute solution. It is essential that the R-group and the functional group be chosen so that they can react with the functional group in the resin under given curing conditions. Furthermore the X-group must be chosen so that it can hydrolyze to allow reactions to take place between the silane and the OH-group on the cellulose surface. When the treated fibers are dried, a reversible condensation takes place between the silanol and –OH groups on the cellulose fiber surface, forming a polysiloxane layer, which is bonded to the cellulose surface. When the silane-coated cellulose surface gets in contact with the resin, the R-groups on the fiber surface react with the functional groups present in the polymer resin, forming a stable covalent bond with the polymer. Once all these reactions occur, the silane coupling agents may function as a bridge to bond the cellulose fibers to the resin with a chain of primary strong bonds. Figure 6.2 illustrates the bonding between a silane coupling agent, cellulose, and a polymeric resin. Apart from the chemical structure of silane, dispersion aids such as solvent and initiator (different organic peroxides) provide various chemical reactions as well as physical interactions at the interface. In this way they play important roles in the mechanical properties of the composites.87 Treatment of fibers with silane coupling agents significantly improves the interfacial adhesion and therefore the mechanical properties of the composites.69,88,89 The performance of silane coupling agents in hardwood aspen fiber in different polymeric systems was reported.67,87 The coupling action of silanes is accelerated by the presence of solvents and initiators. Copyright © 2005 by Taylor & Francis

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Hydrolysis RSi(OR′)3 + 3 H2O

RSi(OH)3 + 3 ROH (Silanol) R

Condensation 3 RSi(OH′)3

HO

R

Si

O

R

Si

O

Si

OH OH (Siloxane)

Hydrogen bonding

R

Biofiber

Si

OH + HO

OH

R O

OH

OH + 2 H2O

R O

Si

Si

OH

OH

OH R

HO

R

Si

O

Si

R

Si

O

H

O

O

Si

O H

H

O

H

O

O H

H

O

H

Biofiber

R

R

R

R

Si

O

H

OH

Biofiber

HO HO

Si

Biofiber

Surface grafting R

O

O

O

O H

R

Si

O

Si

O

Si

OH

OH O

O

O

Biofiber

Biofiber

Biofiber

O H

H

O

H

+ 3 H 2O

Biofiber

Biofiber

Biofiber

FIGURE 6.2 Bonding between a silane coupling agent, cellulose, and a polymeric resin.

6.5

Fiber-Matrix Adhesion

Several theories describe the adhesion phenomenon and include mechanical interlocking, primary bonding, absorption, interdiffusion, and electronic theories.106 Fundamentally, adhesion is a thermodynamic event, with the Copyright © 2005 by Taylor & Francis

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enthalpy of adhesion determining the strength of adhesion at equilibrium.107 There is also a thermodynamic need to decrease the interfacial tension at interfaces.108 Good wetting is a requisite for these thermodynamic demands, and this can only occur if the surface energy of the wetting substance is lower than that of substrate.107 6.5.1

Testing of Adhesion in Fiber-Matrix Composites: Micromechanical Characterization

To measure the IFSS the pullout test was used. In this test, one end of the fiber is embedded in a block of a polymer matrix [Figure 6.3(a)], and a force is applied to the free end to pull it out of the matrix while the force is continuously monitored and recorded. The average IFSS can be calculated from the force at which debonding occurs using the following equation: F τ⫽ᎏ π dl

(6.1)

where F is the maximum load measured prior to debonding of the fiber, d is the fiber diameter, and l is the fiber embedded length. The single-fiber fragmentation test was also used to calculate the IFSS. A single fiber is embedded in a dogbone-shaped tensile coupon, which in turn is subjected to a tensile load [Figure 6.3(b)]. Tensile forces are transferred from the matrix to the fiber, and, depending on the level of fiber-matrix adhesion, tensile stresses build up in the fiber. The fiber tensile strength is reached at some point where the stress concentration is high enough for it to fracture. This loading process continues until the fiber fragment lengths are so small that tensile stresses induced in the fiber can no longer reach the fiber tensile strength and the fiber fragmentation process ceases. The final fiber fragment length is referred to as the fiber critical length, lc. The fiber critical length is a good indicator of the ability of the interphase to transmit loads between the two constituents. The ratio (lc /d) can be used as an indicator of the fiber-matrix bond strength. From this value of the critical length, the IFSS is calculated according to the equation developed by Kelly and Tyson:110 σf d τ⫽ᎏ ᎏ (6.2) 2 lc

冢冣

where d is the diameter of the fiber, lc is the critical length of the fiber, and σf is its maximum tensile strength. Drzal et al.111 recognized that the distribution of fiber fragments fits well a two-parameter Weibull distribution and rearranged the Kelly and Tyson equation, proposing the following modification to calculate the IFSS: σf 1 τ ⫽ ᎏ Γ 1⫺ ᎏ (6.3) α 2β

冢

冣

where α and β are the shape and scale parameters of the Weibull distribution, respectively, and Γ is the gamma function. Several authors have Copyright © 2005 by Taylor & Francis

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Load F Microvise

Fiber

Fiber Microdrop L

Load F

2r Matrix

2r

(a)

σ

σ

σ1

Length σ2

Length σ3

Length

lc

σ1 > σ2 > σ3 τ σ

σ + δσ

(b)

τ FIGURE 6.3 Schematic representation of the micromechanical techniques. (a) Pullout test. (b) Single-fiber fragmentation test. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

reported the physical and mechanical properties of natural hard fibers such as sisal (Agave sisalana) and henequen (Agave fourcroydes). CazaurangMartínez et al.,109 Mukherjee and Satyanarayana,112 and Barkakaty113 have shown that the cross section and apparent diameter of these fibers vary Copyright © 2005 by Taylor & Francis

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considerably along their length. From photomicrographs of the cross section of these natural hard fibers,109,112 it was shown that they are not circular. In order to measure the average area and perimeter of the henequen fibers, they were embedded in an epoxy resin, cured in an air convection oven, and then microtomed at several points along the fiber. The microtomed films were photographed and geometrical parameters, such as the perimeter and the cross-sectional area, were calculated using a standard image analysis software. The typical cross sections of the henequen fibers used in this study are shown in Figure 6.4(a). It was observed that the variability of the perimeter, equivalent diameter, and cross-section area of the henequen fibers, shown in Figure 6.4(b–d), fit well the two-parameter Weibull distribution, α

冦 冢 冣 冧 x⬎0

x F(x)⫽1⫺exp ⫺ ᎏ β

(6.4)

FIGURE 6.4 (a) Typical cross sections of henequen fibers at different points along their length. (b–d) Histograms of the geometrical parameters: (b) perimeter, (c) diameter, and (d) cross-sectional area. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

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35

30

30

α = 3.92, β = 0.96

20 Frequency

Frequency

25 20 15

5

5 0

15 10

10

(b)

α = 4.91, β = 0.2

25

0.5 0.6 0.7 0.9 1.0 1.1 1.2 Perimeter (mm)

0

1.5

1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 ×10−1 Diameter (mm)

(c)

30 α = 4.11, β = 0.03

25

Frequency

20 15 10 5 0

(d)

1.1 1.5 1.9 2.2 2.6 3.0 3.4 3.7 4.1 4.7 ×10−2 Area (mm2)

FIGURE 6.4 (Continued)

The maximum likelihood method was employed to estimate the parameters of the Weibull distribution. These parameters are solutions to

冱Xαi lnXi ⫺ ᎏ1 ⫺ ᎏ 冱lnXi ⫽ 0 ᎏᎏ α α n 冱X i

冦

1 ᎏ α

冧

1 α β ⫽ ᎏ 冱Xi n

(6.5)

where X is the parameter being studied and α and β have the same meaning as indicated above. It is observed from these figures that the perimeter of the fiber cross section shows a lower dispersion than that observed for the equivalent diameter or the cross-section area. This suggests that the perimeter could be a better parameter for calculation of the IFSS. Dong and Copyright © 2005 by Taylor & Francis

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Sapieha,114 in a study concerning the characterization of the interfacial adhesion between cellulose and a thermoplastic matrix composite, modified the Kelly and Tyson equation by using the ratio of the cross section to perimeter of the fibers instead of the apparent diameter to calculate the IFSS. However, they do not report about any of their geometrical parameters. Karlsson et al.,115 using the Wilhelmy plate method, measured the apparent perimeter of regenerated cellulose and used this parameter to calculate the IFSS. In order to take into account the inherent variability of the henequen fibers, Equations (6.2) and (6.3) were rearranged to use the perimeter Pf instead of the diameter to calculate the IFSS. Following this approach the IFSS was calculated for the pullout test with the expression: Ff τ⫽ᎏ Pf le

(6.6)

where Ff is the force of debonding, Pf is the measured perimeter of the henequen fiber, and le is the fiber-embedded length. In the case of the singlefiber fragmentation test the IFSS was calculated with 2Ff ᎏ τ ⫽ β Γ 1⫹ᎏ1ᎏ P α f

冢

冣

(6.7)

where Ff is the tensile load at break for the henequen fiber, Pf is the perimeter, α and β are the shape and scale parameters of the fragment length Weibull distribution, and Γ is the gamma function.

6.6 6.6.1

Experimental Procedure and Materials Materials

High-density polyethylene (HDPE-Petrothene) extrusion grade was supplied by Quantum Chemical, Inc., with a melt-flow index (MFI) of 0.33 g/10 min, a density of 0.96 g/cm3, and a melting point of 135°C. The MFI was determined following the ASTM standard D-1238-82116 at 190°C with a weight of 2160 g. The ASTM standard D-792-86,117 with benzene as the immersion liquid, was used to determine the density. The melting point was determined in a DSC-7 Perkin Elmer calorimeter. The natural hard fiber used was Henequen (A. fourcroydes) supplied by DESFIYUSA. The chemical composition and mechanical properties of the henequen fibers are shown in Tables 6.2 and 6.3. Sodium hydroxide and xylene, reactive grade from Técnica Química S.A., were used by the various surface treatments. As a Copyright © 2005 by Taylor & Francis

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TABLE 6.2 Chemical Composition of Henequen Fiber (Agave fourcroydes) Component

Percent (w/w)

TAPPI Standarda

Cellulose Hemicellulose Lignin Extractives Ashes

60 28 8 4 0.5

TAPPI T9 m-54 TAPPI T9 m-54 TAPPI T9 m-54 TAPPI T12 m-59 TAPPI T15 m-58

a

Technical Association of Pulp and Paper Industry. Source: Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.

TABLE 6.3 Mechanical Properties of Henequen Fibers (Agave fourcroydes) Tensile strength (MPa) Strain at break (%) Tensile modulus (GPa)

500 ⫾ 70 4.8 ⫾ 1.1 13.2 ⫾ 3.1

Source: Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.

coupling agent, the vinyltris (2-methoxy-ethoxy) silane (Silane A-172) from Union Carbide was used. 6.6.2

Fiber Surface Treatments

The fibers were subjected to one or several surface treatments (see Table 6.4) as follows: first, the fibers were treated with a NaOH aqueous solution (2% w/v) for 1 h at 25°C, then they were washed with distilled water until all the sodium hydroxide was eliminated. This treatment is referred to as FIBNA. Subsequently, the fibers were dried to 60°C for 24 h.109 For the second surface treatment, the fiber, 1% silane, and 0.5% dicumyl peroxide (Polyscience), weight percentage with respect to the fiber, were dissolved and hydrolized in a mixture of methanol-water (90/10 w/w). The pH of the solution was adjusted to 3.5 with acetic acid and stirred continuously for 10 min. Then, the fibers were immersed in the solution and left for 1 h under agitation. Finally, the fibers were dried at 60°C for 24 h (FIBNASIL). For the third surface treatment (FIBNASILPRE), henequen fibers were first impregnated with a 1.5% w/w HDPE:xylene solution for that purpose: HDPE in powder form was added to xylene at 110°C in a Pyrex beaker and stirred continuously with a magnetic bar to dissolve the polyolefin. The natural fibers were put in a stainless-steel basket and carefully immersed in the hot solution and stirred continuously for 5 min. Then, the basket was removed and the fiber lumps were transferred to a flat tray and kept in an oven at 60°C for 24 h to allow the solvent to evaporate. The lumps of impregnated fibers were dispersed before blending with the matrix. Copyright © 2005 by Taylor & Francis

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TABLE 6.4 Different Treatments for Fiber Surface Physical and Chemical Modification Applied to Henequen Fibers Keyword

Surface Treatment

FIB FIBNA FIBNASIL FIBNAPRE

No treatment Treated with an aqueous alkaline solution Treated first with alkaline solution and then with a silane coupling agent Treated with alkali and then impregnated with a dilute matrix solution

Source: Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.

6.6.3

Adsorption Isotherm

For the adsorption studies 2-mm-long henequen fibers were cut and thoroughly washed with distilled water for 48 h at 25°C and then dried at 60°C. In a typical adsorption experiment, 1.5 g of henequen fibers and 30g of a silane solution were weighted in a round bottom polypropylene tube. During the adsorption experiment, this tube was placed in an Orbital Shaker (New Brunswick Science) at room temperature (25°C). After a given time, the tube was taken from the shaker and the contents were centrifuged at 3000 rpm for 1 h in an IEC HNSII centrifuge (International Equipment Co.). The solution was then decanted and filtered using a cellulose nitrate membrane having a 0.45 µm pore size. All the laboratory material was washed with a silane solution and cured in a convection oven prior to performing the adsorption experiments in order to minimize the adsorption of the silane on them. For measuring the change of silane concentration in the solution, a blank sample with no silane was simultaneously studied as a reference sample. A Milton Roy refractomonitor IV differential refractive index detector, thermally equilibrated at 25°C ± 0.05°C, was used to measure the change in concentration. Before obtaining a calibration curve for the silane solution, the refractive index detector was previously calibrated with a sucrose solution. This procedure allowed the measurement of concentrations with a precision better than 1 ppm. 6.6.4

X-ray Photoelectron Spectroscopy (XPS)

The XPS spectra of the untreated and silane-treated henequen fibers were recorded on a PERKIN ELMER PHI model 550 ESCA/SAM spectrometer using MgKα1,2 at 600 W (10 kV-60 mA) at an angle of 50°. Binding energies were registered with an accuracy of 0.5 eV and the analysis of the spectra was performed using a commercial curve-fitting software. 6.6.5

Infrared Spectroscopy (FTIR)

The FTIR analysis was performed on a Nicolet model Protege 460 Magna IR spectrometer. Diffuse reflectance spectroscopy (DRIFTS), with a deuterated Copyright © 2005 by Taylor & Francis

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triglycine sulfate (DTGS-KBr) detector, was used. In order to obtain a good resolution spectra it was necessary to mill the henequen fibers to an average length of 0.2 mm. The milled fibers were then mixed with an analytical grade KBr and the spectra were recorded with a 2 cm⫺1 resolution and 400 scans. 6.6.6

Pullout Test Sample Preparation

The pullout specimens were made using the following procedure: The henequen fibers were cut in fragments 20 cm long and attached to a frame using high-temperature adhesive tape. Then they were placed between two sheets of HDPE and the mold was subjected to a 1 ton constant pressure, at 180ºC, in a Carver laboratory press for 10 min and then cooled to room temperature. The specimens were cut in rectangles of 3⫻1 cm in such a way that one fiber was embedded along the main axis of the rectangle. The nominal fiber-embedded length was produced by perforating the sample at a specified position. The actual embedded length and the apparent diameter of the fiber of each probe were recorded with a Specwell M820-SM optical microscope. The specimens were subjected to a tensile force using a universal testing machine equipped with a PC computer, and the load-displacement curve was recorded. A cross-head speed of 1.2 mm/min was used in all the experiments. 6.6.7

The Single-Fiber Fragmentation Test Sample Preparation

The coupons needed to carry out the single-fiber fragmentation experiments were prepared as follows: The henequen fibers were carefully aligned and attached to a frame. Then they were placed between two sheets of polyethylene and pressed at 180°C in a Carver laboratory press for 10 min and cooled to room temperature. ASTM D-638 standard118 type IV dogboneshaped tensile specimens were cut using a Ceast Co. pneumatic hollow die punch, according to the ASTM standard D-638,118 in such a way that one henequen fiber remained oriented along the central axis of the coupon. The coupons were subjected to a tensile load in an Instron 1125 universal testing machine at 25°C and a cross-head speed of 2 mm/ min. The application of load was stopped before yielding of the polyethylene matrix when the fragmentation process had ceased. The length of the formed fragments was recorded with an optical microscope coupled to a calibrated TV camera and monitor system. 6.6.8

Elaboration of the Composite

A composite with a 20% fiber volume fraction was elaborated to determine the effect of the henequen fiber surface treatments on the tensile properties of the material. Henequen fibers that were 6 mm long were incorporated to the polyethylene matrix using a BRABENDER intensive mixer at 180°C; the resulting material was laminated at a temperature of 180°C and a pressure Copyright © 2005 by Taylor & Francis

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of 1 ton using a Carver laboratory press. From these laminates, type IV tensile test specimens were obtained according to the D-638 ASTM standard.118 The tensile tests were carried out in an INSTRON universal testing machine, model 1125, using a cross-head speed of 5 mm/min. The Iosipescu Shear Test was carried out following the ASTM D-5379 standard119 using a Wyoming Shear Test Fixture adapted to the Instron machine, after conditioning at 25°C. The test specimens were cut from the laminates obtained previously, to the dimensions of 76 mm length, 19 mm width, and 2 mm thickness, and the distance between the two 90° notches was 12 mm. The cross-speed used was 0.5 mm/min.

6.6.9

Scanning Electron Microscopy (SEM)

Tensile fracture surfaces of the composite samples were coated with gold and then analyzed using a JEOL SEM Model LV 5400 scanning electron microscope operated at 20 keV.

6.7 6.7.1

Results Surface Treatments

The treatment of the henequen fibers with an aqueous alkaline solution promotes the partial removal of the hemicelluloses, waxes, and lignin present on the fiber surface and leads to some changes in morphology and chemical composition.113,120 The change in the surface morphology of the treated fiber was studied by scanning electron microscopy (SEM). Figure 6.5(a) and Figure 6.5(b) show a fiber without treatment (FIB) and one that has been treated with the NaOH aqueous solution (FIBNA), respectively; it can be seen that FIBNA possesses more imperfections on its surface than FIB. As a direct result, there are more crevices on the surface of FIBNA than on the surface of FIB. The change in the surface composition can be appreciated in the infrared spectra for FIB and FIBNA shown in Figure 6.6. It can be noted that there is a strong absorption in FIB at 1750 cm⫺1 which no longer exists in FIBNA. Several authors120–123 reported that hemicelluloses contain groups that absorb in the carbonyl region and that they are soluble in aqueous alkaline solutions. Furthermore, if these solutions are acidified and brought in contact with ethanol, the hemicelluloses precipitate.121 This procedure was carried out and it was observed that a precipitate flocculates. The precipitate was analyzed in a FTIR spectrometer, and a spectrum with a very intense band at 1750 cm⫺1 that corresponds to the carbonyl group was obtained. The alkaline treatment also removes other compounds like waxes and partially dissolves the lignin present in the henequen fiber. In addition, the removal of hemicelluloses with the alkaline treatment was corroborated Copyright © 2005 by Taylor & Francis

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FIGURE 6.5 SEM photomicrographs of henequen fibers for (a) FIB and (b) FIBNA showing their surface features. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

using a thermogravimetric analysis (TGA). Figure 6.7 shows the thermogram of FIB and FIBNA, and it can be appreciated that both curves suddenly decrease by ~10% at 100°C, corresponding to water evaporation, and also at 250°C where the decomposition of the fibers begins. However, there is an Copyright © 2005 by Taylor & Francis

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100 90 FIBNA

80

Transmittance (%)

70 60 50 40 FIB

30 20 10 4000

3500

3000

2500 2000 Wavenumber (cm−1)

1500

1000

500

FIGURE 6.6 Fourier transform infrared (FTIR) spectra for FIB and FIBNA. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

120

4

100 3

Residual mass (%)

80

2

60 1

40 20

0

d (residual mass)/dT

FIB FIBNA

0 −1

−20

0

100

200

300

400

500

600

−2

700

Temperature (°C) FIGURE 6.7 Termogravimetric analysis (TGA) decomposition thermograms for FIB and FIBNA. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

inflection in the FIB curve that is not present in FIBNA; this inflection corresponds to a weight loss of ~20%, which is similar to the hemicelluloses content in the henequen fibers, as can be seen in Table 6.2. Figure 6.8(a) shows Copyright © 2005 by Taylor & Francis

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FIGURE 6.8 EM photomicrographs of henequen fibers for (a) FIBNASIL and (b) FIBNAPRE showing their surface features. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

the SEM microphotographs corresponding to the silane-treated fiber. It can be seen that there is no dramatic change in the surface morphology of FIBNASIL fiber as compared to FIBNA. Copyright © 2005 by Taylor & Francis

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The change in the surface morphology of the preimpregnated fiber, FIBNAPRE, was followed by SEM and it is shown in Figure 6.8(b). It can be appreciated that there are lumps of the polyolefin deposited on the fiber. To determine the amount of resin deposited on the preimpregnated fibers a soxhlet extraction with xylene was done. This determination was carried out by triplicate and ~5% (w/w) of HDPE with respect to the fiber was found to be deposited on the fiber surface.

6.7.2

Adsorption Isotherms

The FTIR spectra for 5% and 0.1% w/w freshly mixed aqueous silane solutions are shown in Figure 6.9. Characteristic absorption peaks at 1115, 1061, and 1015 cm⫺1 of the polysiloxanes oligomers are higher for the concentrated silane solution than for the diluted one. It was also observed from experiments that the intensity of these peaks increased with the aging of the solutions; the peaks increased faster upon aging for the 5% w/w solution than for the diluted one. Several authors have reported the presence of homopolymer, e.g., polysiloxanes, in aqueous silane solutions and have reported that its formation rate depends on the initial silane concentration.124–126 Prior to

1061

1.4 1115 1.2

1015

1.0

Absorbance

890 922 0.8

5% w/w

1200

1370 0.6

0.4

0.1% w/w

0.2

0.0 1500

1400

1300

1200

1100

Wavenumber

1000

900

800

(cm−1)

FIGURE 6.9 FTIR spectra for 5% and 0.1% w/w, aqueous silane solution. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 321–331, 1999. With permission.)

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the determination of the adsorption isotherms for FIB and FIBNA, it was necessary to estimate the time required to reach the adsorption equilibrium. A plot of the amount of silane adsorbed by unit mass of fiber, as a function of adsorption time for FIB and FIBNA, is shown in Figure 6.10(a) for an initial 0.005% w/w silane solution. Some differences between these curves should be noted. In the case of FIB the adsorption started after ~12 h, and once this occurred, adsorption equilibrium was reached after 72 h. In the case of

Adsorption (silane/henequen fiber mg/g)

0.5 FIB FIBNA

0.4

0.3

0.2

0.1 Initial silane concentration: 0.005 % w/w 0.0 0

20

40

60

80

100

120

140

160

180

Adsorption time (h)

(a)

Adsorption (silane/henequen fiber mg/g)

0.6

0.4 0.3 0.2 0.1 0.0 0.00

(b)

FIB FIBNA

0.5

0.01

0.02

0.03

0.04

0.05

0.06

Silane solution initial concentration (% w/w)

FIGURE 6.10 (a) Amount of silane absorbed vs. time and (b) absorption isotherms vs. silane solution initial concentration for FIB and FIBNA at 25°C. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 321–331, 1999. With permission.)

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FIBNA no appreciable induction time was detected since the adsorption began practically as soon as the solution and the fiber were brought in contact. It took ~100 h to attain the adsorption equilibrium. The amount of silane adsorbed onto the alkali-treated fibers was higher than that absorbed by the nontreated. This difference in the amount of adsorbed silane could be attributed to the fact that the treated fibers (FIBNA) have a larger amount of exposed cellulose on the surface, and the cellulosic hydroxyl groups have a better capability to interact with the hydrolyzed silane. Some authors127,128 have pointed out that the main effect of washing a natural fiber with a mild alkaline solution is the removal of waxes, hemicelluloses, and a partial removal of the lignin present on their surface. The leaching of these compounds enhances the availability of sites for the cellulose–silane interaction. Furthermore, the surface of FIBNA is rougher than the surface of FIB, resulting in a larger effective contact area. Also, the surface of FIB is richer in lignin and waxes than the surface of FIBNA, so that the hydrolyzed silane has to diffuse first between the crevices of the fibers in order to be able to interact with the cellulosic hydroxyl groups. The adsorption rate curves show that the adsorption equilibrium is reached within an overall adsorption time of 100 h; therefore an adsorption period of 120 h was chosen for subsequent experiments. Figure 6.10(b) shows the adsorption isotherms for FIB and FIBNA at 25°C. It can be seen that in the case of FIB the maximum adsorption of silane occurs when the henequen fiber is treated with ~0.0075% by weight silane solution, that is, equivalent to 0.225% by weight of silane with respect to the fiber. It is also observed that there is a maximum point beyond which the silane adsorbed on the fibers decreases, first slightly and then dramatically as the silane solution concentration increases. In fact, at a concentration greater than 0.05% (1.15% by weight of silane with respect to the fiber), there is no increase of the adsorption at all. The decrease in the amount of silane adsorbed on the henequen fibers at higher concentrations could be due to the presence of condensation reactions that yield polysiloxanes. On one hand, these reactions diminish the concentration of the hydroxyl groups on the monomeric silane, and on the other, upon an increase of the size of the newly formed polymeric chains, they can hardly diffuse as monomeric molecules into the crevices of the fibers. As a result, they can no longer readily interact with the active groups of the cellulose due to steric hindrances. The adsorption isotherm for FIBNA is quite similar to that for FIB. However, it can be seen that the amount of silane adsorbed by the alkaline-treated fiber is always greater than that adsorbed by the untreated fiber. In the case of FIBNA the highest adsorption is reached with a 0.01% w/w silane solution concentration, which corresponds to a 0.3% by weight of the silane with respect to the henequen fiber. The silane adsorbed by FIBNA also shows a dramatic decrease as the silane solution concentration increases. This behavior reinforces the idea that the formation of polysiloxanes is responsible for such decrement in the amount of silane adsorbed on the henequen fibers, because the concentration of the dilute silane solution near the henequen fiber surface is high.126 Copyright © 2005 by Taylor & Francis

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X-ray Photoelectron Spectroscopy (XPS)

Figure 6.11 shows the x-ray photoelectron spectra of electron intensity as a function of the binding energy for FIB, FIBSIL, and FIBNASIL. For a lignocellulosic material it can be seen that the O1s and C1s are the predominant species and occur at 533 and 285 eV, respectively.129,130 Other species are present but only in very small concentrations. This is the case of Ca2p at 350 eV, Ca2s at 439 eV, Mg2s at 90 eV, and Cu2p at 954 eV. The presence of metallic atoms on the fibers is believed to arise from the use of metallic knives during the harvesting and subsequent decortication process of the leaves for fiber extraction. Also, the presence of copper atoms is attributed to the fact that a filament of such material was used to fix the fiber ends on the sample holder. In the case of the silane-treated fibers, the presence of silicon on their surface was detected from its characteristic emission peaks in the region between 150 and 155 eV, for the Si2s and 99 and 104 eV for the Si2p. In the insert of Figure 6.11, the 90–160 eV range emission peaks for FIBSIL and

FIB FIBSIL

N(E)/E (a.u.)

N(E)/E (a.u.)

FIBNASIL

80

100

120

140

160

Binding energy (eV)

800

600

400 Binding energy (eV)

200

0

FIGURE 6.11 XPS survey spectra for FIB, FIBSIL, and FIBNASIL fibers. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 321–331, 1999. With permission.)

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FIBNASIL are appreciated. In Figure 6.12(a) and (b), the carbon peaks for FIB and FIBNA are present. In the case of FIB, there are three peaks at 288, 285.7, and 285.5 eV, respectively. These peaks give an indication that there exists

N(E)/E (a.u.)

C1s

292

290

288 286 284 Binding energy (eV)

282

280

290

288

282

280

(a)

N(E)/E (a.u.)

C1s

292 (b)

286

284

Binding energy (eV)

FIGURE 6.12 XPS spectra of carbon peaks (C1s) for (a) FIB and (b) FIBNA. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 321–331, 1999. With permission.)

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more than one compound on the surface of the fiber without treatment. Noah and Prud’Homme,131 in a study of characterization of the surface of a tropical wood by XPS, also reported several emission peaks for the carbon atom. They attributed them to the different constituents such as, polysaccharides, lignin, and waxes that are present on the surface of wood, each containing carbon atoms in different chemical environments. Barry and Koran132 carried out a study to characterize the surface of the pulp of paper (cellulose) and also reported several emission peaks for the carbon atom. They also assigned these peaks to the different types of bonding on the paper pulp surface. As lignocellulosic fibers contain cellulose, lignin, and waxes, the appearance of multiple peaks of emission for the carbon atom is expected. The treatment with the aqueous alkaline solution eliminated waxes and part of the lignin from the surface of the fibers. In Figure 6.12(b), there is a slight shift of the peaks and the peak at 285.5 is no longer observed. The carbon peaks for FIBSIL and FIBNASIL are shown in Figure 6.13. From these plots it can be seen that the effect of the silane surface treatment is reflected in the emission of a carbon peak either for FIBSIL or for FIBNASIL. However, this effect is more pronounced for FIBNASIL than for FIBSIL, as indicated by the presence of a shoulder at ~288 eV. This peak is an indication of the presence of the C-O-Si bond on the fiber surface, and it means that a condensation reaction between the silane and the henequen fiber may have taken place. It is probable that this reaction also occurred for FIBSIL. However, due to the lower quantity of silane deposited on the fiber surface of FIB and a lower quantity of exposed cellulose compared to FIBNA, the efficiency of this condensation reaction may be poor, resulting in undetected C-O-Si bond in FIBSIL. Figure 6.14 shows the characteristic 2p and 2s emission peaks for silicon for FIBSIL and FIBNASIL. In FIBSIL [Figure 6.14(a)], it can be observed that there is a pair of peaks related to the 2p electrons, one of them at ~102 eV and the other close to 104 eV. Toth et al.130 and Morra et al.131 reported that the apparition of emission peaks at binding energies greater than 102 eV reflects the bonding of the silicon atom with more than two oxygen atoms. The coupling agent used in this study, has three ethoxymethoxy groups attached to the silicon atom, so the hydrolyzed silane could have three hydroxyl groups attached to the Si atom. If this is true, then a peak should be expected at a binding energy greater than 102 eV, as is the case for both FIBSIL and FIBNASIL. These findings seem to confirm the presence of the silane group deposited on the surface of both FIBSIL and FIBNASIL fibers and that there is a chemical reaction between the hydrolyzed silane and the henequen fibers, at least with FIBNASIL. 6.7.4

Infrared Spectroscopy (FTIR)

The chemical reaction between the silane coupling agent and the cellulose on the henequen fiber was confirmed using FTIR spectroscopic analysis in a diffuse reflectance mode (DRIFTS). The spectra in the 1500 cm⫺1 to 600 cm⫺1 range for the untreated and treated henequen fibers are shown in Copyright © 2005 by Taylor & Francis

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N(E)/E (a.u.)

C1s

292

290

288 286 284 Binding energy (eV)

282

280

290

288 286 284 Binding energy (eV)

282

280

(a)

N(E)/E (a.u.)

C1s

292 (b)

FIGURE 6.13 XPS spectra of carbon peaks (C1s) for (a) FIBSIL and (b) FIBNASIL. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 321–331, 1999. With permission.)

Figure 6.15. The difference spectra of FIBSIL and FIB, that is, the result of subtracting FIB to FIBSIL and the FIBNASIL-FIBNA difference spectra are shown in Figure 6.16(a) and (b), respectively. The presence of two new absorption bands around 765 and 700 cm⫺1 in the curve for FIBSIL can be Copyright © 2005 by Taylor & Francis

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Si2p

N(E)/E (a.u.)

Si2s

155

150

(a)

105 Binding energy ( eV)

100

95

Si2p

N(E)/E (a.u)

Si2s

160 (b)

155

150

145 110 Binding energy (eV)

105

100

95

FIGURE 6.14 XPS spectra of silicon peaks (Si2s and Si2p) for (a) FIBSIL and (b) FIBNASIL. (From ValadezGonzalez, A. et al., Composites: Part B, 30, 321–331, 1999. With permission.)

seen in Figure 6.15(a). The absorption band at 765 cm⫺1 corresponds to the -Si-C- symmetric stretching bond,134 and that at 700 cm⫺1, to the -Si-O-Sisymmetric stretching. Also, a slight increase in the broad peak around 950–1150 cm⫺1 in FIBSIL should be noted. The growth of this band could be attributed to the presence of the asymmetric stretching of -Si-O-Si- and/or to the -Si-O-C- bonds.135,136 The former bond is indicative of the existence of polysiloxanes deposited on the fiber and the latter would confirm the Copyright © 2005 by Taylor & Francis

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0.65 1105 1030

0.60

FI BSI L 700

Absorbance

0.55 965

765

0.50

0.45

FI B

0.40

0.35 1400

1200 1000 Wavenumber (cm−1)

(a)

800

600

1.4 1105 1030

Absorbance

1.3

FIBNASIL

1.2

700 1370 965 765

1200

1.1

FIBNA

1.0

0.9 1400 (b)

1200

1000

800

600

Wavenumber (cm−1)

FIGURE 6.15 FTIR survey spectra for (a) FIB and FIBSIL and (b) FIBNA and FIBNASIL. (From ValadezGonzalez, A. et al., Composites: Part B, 30, 321–331, 1999. With permission.)

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0.08

930 860

0.06

Absorbance

765 700

1187 965

0.04 1200 1145

1030

0.02

0.00

1400

(a)

1200 1000 Wavenumber (cm−1)

800

600

0.12

0.08

1105

1080

Absorbance

1030 0.04

1200

965

700 840 765

1370 1330 0.00

1400 (b)

1200 1000 Wavenumber (cm−1)

800

FIGURE 6.16 FTIR difference spectrum for (a) (FIBSIL-FIB) and (b) (FIBNASIL-FIBNA). (From ValadezGonzalez, A. et al., Composites: Part B, 30, 321–331, 1999. With permission.)

occurrence of a condensation reaction between the silane coupling agent and the henequen fiber. The FIBNA and FIBNASIL spectra are shown in Figure 6.15(b). An absorption band at 1200 cm⫺1 and the shoulders at 765 and 700 cm⫺1 should be noted in FIBNASIL. The well-defined absorption band at 1200 cm⫺1 is associated to the Si-O-C bond whereas the absorption Copyright © 2005 by Taylor & Francis

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bands at 700 and at 765 cm⫺1, as mentioned above, are characteristic of the Si-O-Si and the Si-C bonds, respectively. There is also an increase in the intensity of the 950–1150 cm⫺1 broad band. This behavior, like that of FIBSIL, is attributed to the presence of the -Si-O-Si- and/or to the -Si-O-Cbonds on the treated henequen fiber. The difference spectra FIBSIL-FIB is shown in Figure 6.16(a). The characteristic absorption bands of -Si-O-Si- at 700, 1030, 1145, and 1187 cm⫺1 confirms the presence of polysiloxanes126,135 and the shoulders at 965 and 1200 cm⫺1, characteristic of the -Si-O-Cbond,135 seems to confirm the reaction between the hydrolyzed silane and the henequen fiber. It is interesting to note that the absorption bands at 860 and 930 cm⫺1 corresponding to the -Si-OH bond126 reveals the presence of residual hydrolyzed silane in FIBSIL. Figure 6.16(a) shows the difference in the spectra for FIBNASIL and FIBNA. The presence of the small absorption bands at 700 and 1030 cm⫺1 shows the presence of polysiloxanes in FIBNASIL. On the other hand, the well-defined absorption bands at 965, 1200, and 1370 cm⫺1 confirm the presence of the -Si-O-C- bond in FIBNASIL. Hence, it confirms the condensation reaction between the hydrolyzed silane and the henequen fiber135 to a greater extent with respect to FIBSIL. These findings show that there is a chemical reaction between the A-172 silane coupling agent and the henequen fibers and that the efficiency of this reaction is higher for FIBNASIL than for FIBSIL.

6.8

Fiber-Matrix Adhesion: Characterization of the Adhesion Level Using Micromechanical Techniques

The typical load-displacement curves for the native (FIB), alkali-treated (FIBNA), silane-treated (FIBNASIL), and preimpregnated henequen (FIBNAPRE) fibers in the pullout test are shown in Figure 6.17. It can be noted that all the curves exhibit the nonlinear behavior characteristic of a ductile matrix.134–139 Also, the slope of the straight portion of each curve is not the same for all of the fiber surface conditions. This is an indication that the stiffness corresponding to each slope is not equal for all cases. However, it cannot be asserted from these results that this slope corresponds to that of the interphase because the failure process also includes matrix and fiber failure processes. It is also observed that once the load reached its maximum value there are significant differences in the way these curves drop. For the untreated henequen fiber, it can be seen that the load increases gradually. When it reaches a maximum value there is a smooth transition, and it begins to decrease in a linear fashion until the total embedded length of the fiber is pulled out. This behavior agrees well with the behavior of a poor interphase that results from the incompatibility between the hydrophilic fiber and the hydrophobic matrix. This behavior shows a slight change in the case of FIBNA because the higher the roughness of the alkali-treated fiber the better the Copyright © 2005 by Taylor & Francis

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1.0 FIB FIBNASIL FIBNAPRE FIBNA

Force (kg)

0.8

0.4

0.0 0.0

0.4

0.8 1.2 Displacement (mm)

1.6

FIGURE 6.17 Typical pullout test load-displacement curves for HDPE and henequen fibers subjected to different surface treatments. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

fiber-matrix mechanical interlocking, i.e., the fiber-matrix interaction is improved. For the silane-treated fiber, the load-displacement curve depicts a weakly bonded interphase; after the interphase has failed, the fiber can be extracted in a controlled way and friction was measured until the fiber was completely pulled out.137 The load displacement curve for FIBNAPRE resembles a very strongly bonded interphase: the interface fails immediately after fiber extraction, as reported by Désarmont and Favre.137 On the other hand, it can be seen in Figure 6.18 that the dependence of the force of debonding on the embedded length follows a linear behavior, with a different slope for all treatments. This seems to indicate some bond ductility138,139 and that the surface treatments increase the fiber-matrix interaction.137 The slope of these curves indicates that fiber-matrix interaction increases in the order FIB ⬍ FIBNA ⬍ FIBNASIL ⬍ FIBNAPRE. The IFSS calculated using the equivalent diameter (Equation 6.1) and the measured perimeter (Equation 6.4) of the henequen fiber for the pullout test are presented in Figure 6.19. It can be appreciated that the fiber surface treatments increased the IFSS in the order FIB ⬍ FIBNA ⬍ FIBNASIL ⬍ FIBNAPRE. As can be seen, the alkali treatment shows a slight increase in the fiber-matrix interaction mainly due to the higher fiber surface roughness that allows certain mechanical interlocking. However, the preimpregnated fiber (FIBNAPRE) gave clearly better results than FIBNA and even FIBNASIL. This seems to indicate that wetting of the fiber plays a key role in fiber-matrix adhesion because it increases the mechanical interlocking. Thus, the high viscosity of the matrix in the molten state does not allow a proper fiber wetting, and this could explain the small increase on the IFSS for FIBNA. Copyright © 2005 by Taylor & Francis

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1.5 FIB FIBNA FIBNASIL FIBNAPRE

Debonding force (kg)

1.2

0.9

0.6

0.3

0.0 0

2

4

6

8

10

Embedded length (mm) FIGURE 6.18 Debonding force vs. embedded length for nontreated and treated henequen fibers. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

10

Diameter (Eq. 6.1) Perimeter (E q. 6.6)

Interfacial shear strength (MPa)

8

6

4

2

0 FIB

FIBNA FIBNASIL FIBNAPRE Fiber treatment

FIGURE 6.19 Effect of the surface treatment on the interfacial shear strength measured using the pullout test. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

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The silanization of the fiber surface also enhances fiber-matrix interaction as can be observed from the increase of the IFSS, although such increase is somewhat lower than that observed for the preimpregnated fiber. Several authors126,140–142 have pointed out that the silanization of natural fibers modifies the surface properties and that it increases the fiber-matrix interaction. The silane used in this work has two functional groups: a hydrolyzable alkoxy group able to condense with the hydroxyls of the henequen surface fiber, and an organofunctional (vinyl) group capable of interacting with the thermoplastic matrix. The hydrolyzed alkoxy silanes can undergo condensation and bond formation stage under baseand acid-catalyzed mechanisms. Besides these reactions of the silanol and the fiber surface hydroxylic groups, the formation of polysiloxanes structures can also occur.143 Schematics of the various interphases formed for the nontreated and treated fibers are proposed in Figure 6.20. It can be seen in this figure that for FIBNAPRE the matrix penetrates into the fiber whereas in the case of FIBNASIL the silane groups are chemically attached to the fiber surface on one side and bonded with some chains of the matrix in the other side.

HO HO

HO Cellulose surface

Cellulose surface

HO HO

HO (a)

(b) Polymer

Polymer

HO C

Si

O

HO Cellulose surface

HO

HO Polymer C

Si

HO

O

(c)

Cellulose surface

(d)

FIGURE 6.20 Schematic representations of the interphases formed on the henequen fibers for (a) FIB and fibers subjected to different surface treatments: (b) FIBNA, (c) FIBNASIL, and (d) FIBNAPRE. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

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Single-Fiber Fragmentation Test

The calculated IFSS from the SFFT test results using the equivalent fiber diameter (Equation 6.3) and the measured perimeter (Equation 6.5) is shown in Figure 6.21. It can be seen that all treatments improve the fibermatrix interface strength, and it is observed in this case that the silanetreated fibers yielded the best results. It should be noted that Equation 6.5 always yields lower values than Equation 6.3 but the same trend is observed. These results can easily be explained because the treatment with NaOH removed some lignin and hemicelluloses from the surface of the fiber and thus increased the fiber surface area. Such fiber surface increase resulted in a larger contact area between the fiber and the matrix. Therefore, the hydroxyl groups on the cellulose fibers were able to better interact with the silane-coupling agent because of the availability of a larger number of possible reaction sites. It is interesting to note that the pullout test also yielded lower IFSS values than those of the fragmentation test; this phenomenon has been observed previously for high-performance fiberreinforced polymers.144–146

28

Interfacial shear strength (MPa)

24

20

Perimeter (Eq. 6.7) Diameter (Eq. 6.3)

16

12

8

4

0 FIB

FIBNA FIBNASIL FIBNAPRE Fiber treatment

FIGURE 6.21 Effect of the surface treatment on the interfacial shear strength measured using the single-fiber fragmentation test. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

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Effect of Fiber-Matrix Adhesion on Mechanical Properties Tensile Properties

The tensile strength of the composite HDPE-henequen fibers (80:20 v/v) is shown in Figure 6.22. It is noted that modification of the fiber surface with only an aqueous solution of NaOH is not sufficient to improve the strength of the material. Either further treatment with a silane-coupling agent to promote the adhesion by chemical interactions or by enhancing the mechanical interlocking by a preimpregnation process clearly improves the tensile properties of the composite. The silane treatment of the fiber surface increases the tensile strength of the composite material from 21 MPa to 27 MPa, whereas the preimpregnation process yields 23 MPa. Figure 6.23 and Figure 6.24 show the fracture photomicrographs of the specimens subjected to tensile loads. It can be seen in Figure 6.23(a) that for FIB fibers, extensive “pullout” occurred more than for FIBNASIL and FIBNAPRE shown in Figure 6.24. In the picture corresponding to FIBNA [Figure 6.23(b)], both fiber pullout and some tearing of the fibers occur as a consequence of possibly other adhesion mechanisms induced by the greater roughness of the fiber. The

30

Tensile strength (MPa)

25

20

15

10

5

0

HDPE

FIB

FIBNA

FIBNASIL FIBNAPRE

Fiber treatment FIGURE 6.22 Effect of the surface treatment on the tensile strength for HDPE-henequen fiber composites. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

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FIGURE 6.23 Fracture surface SEM photomicrographs of (a) FIB and (b) FIBNA. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

effect of the coupling agent is shown in Figure 6.24(a) where it can easily be seen that the fibers were broken and torn. There is no evidence of fiber pullout. In addition, it is interesting to note that the fibers are completely Copyright © 2005 by Taylor & Francis

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FIGURE 6.24 Fracture surface SEM photomicrographs of tensile test specimens of (a) FIBNASIL and (b) FIBNAPRE. (From Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.)

impregnated by the matrix. In the case of the preimpregnated fiber, Figure 6.24(b), more tearing of the fibers can be observed together with some cavities left by pulled out fibers. When the fiber is preimpregnated with the Copyright © 2005 by Taylor & Francis

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matrix, the resulting IFSS seems to be higher than that obtained for fibers that are treated only with a silane coupling agent, when it is measured with the fiber pullout test. However, when the single-fiber fragmentation test is used, the silanization results in higher IFSS values. It should be pointed out that there are differences between the mechanical solicitations on the specimens used for fiber pullout and fiber fragmentation tests. In the first case, the fiber is subjected to an axial force, and this results in a Poisson contraction in the radial direction. The normal radial stress on the fiber resulting from matrix shrinkage may be reduced, resulting in a decrease in the frictional component of the interfacial strength. This may facilitate the initiation and propagation of interfacial failure. In the fiber fragmentation test, the load is applied directly to the matrix, and the load is transferred from the matrix to the fiber through the interphase. In this case, the tensile forces also produce a reduction of the cross section in both the fiber and the matrix because of a Poisson effect. Such cross-sectional reduction induces a radial compressive stress component on the fiber, and the frictional component of the adhesion should increase, depending on the net cross-sectional reduction between fiber and matrix. In the case of preimpregnated fibers, such cross-sectional contraction due to a Poisson effect will not take place as easily as in an untreated fiber because the polymeric matrix that has penetrated into the fiber will hinder such contraction. When the fiber is subjected to a silanization there is no impediment for such contraction, and the adhesion between the fiber and the matrix is attributed to chemical bonds between the silane coupling agent present on the fiber surface and the matrix. Also, in the pullout experiment, the fiber is completely extracted off the matrix. In the case of fragmentation, the fiber is not extracted but broken inside the matrix. In order to completely extract the fiber off the matrix, it is necessary to apply a large force on the fiber because the frictional component of adhesion has to be broken. In doing so the polymer that has penetrated into the fiber fails either under shear or tensile forces for fiber pullout. When the fibers have been treated with the silane coupling agent, the resulting adhesion between fiber and matrix will be more efficient than for the preimpregnated fibers only if they do not suffer a considerable contraction upon load application. When both fiber surface modification processes are combined, namely, the preimpregnation and the silanization processes, it is expected that the IFSS will be higher because the load-transfer efficiency at such interphase will be further enhanced by a synergistic effect of both treatments. This is indicated by the results shown in Table 6.5 where both micromechanical tests result in higher IFSS values.

6.10.2

Iosipescu Shear Strength

The nondimensionalized shear strength of the HDPE-henequen fiber composites plotted as a function of surface treatment is shown in Figure 6.25. Similar observations made earlier for the tensile strength on the effect of fiber-matrix adhesion are evident here. However, the effect of increased fiber surface contact area with the matrix seems to result in a large contribution Copyright © 2005 by Taylor & Francis

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TABLE 6.5 A Comparison of the Interfacial Shear Strength (IFSS) Obtained by Pullout and Single-Fiber Fragmentation Test (SFFT) for Several Fiber Treatments Treatment

IFSS (MPa) Pullout

IFSS (MPa) SFFT

FIBNAPRE FIBNASIL FIBNASILPRE

8.0 5.0 9.0

9.0 16.0 20.0

Source: Valadez-Gonzalez, A. et al., Composites: Part B, 30, 309–320, 1999. With permission.

Relative shear strength

1.3

1.2

1.1

1.0

FIBNA

FIBPRE FIBNAPRE FIBSIL Type of fiber surface treatment

FIBNASIL

FIGURE 6.25 Effect of the surface treatment on the relative shear strength for HDPE-henequen fiber composites.

on the shear strength, unlike the fiber preimpregnation by itself. The largest increase in shear strength is observed for the fiber treated with both the aqueous alkaline solution and the silane coupling agent. The tensile properties for this fiber surface treatment combination increased ~20%. In the case of shear strength, such increase is of the order of 25%. Features on SEM analysis of the failure surface of Iosipescu shear test samples further clarify the importance of fiber adhesion on shear strength results. In Figure 6.26(a), the untreated fibers appear to be free of any matrix material adhering to them, thus indicating poor fiber-matrix adhesion. In Figure 6.26(b), the fibers treated with the aqueous NaOH solution show more matrix material still on the fiber surface because of a better impregnation. Copyright © 2005 by Taylor & Francis

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FIGURE 6.26 Fracture surface SEM photomicrographs of shear test specimens for (a) FIB, (b) FIBNA, (c) FIBNAPRE, and (d) FIBNASIL.

However, the fiber surface seems to be smooth, and only on areas of deep resin penetration into the crevices did the matrix fail, either by tearing or considerable flow. This is an indication of poor fiber-matrix adhesion. When the fibers are preimpregnated [Figure 6.26(c)], more tearing of the fibers could be observed, together with some cavities left by the pulled out fibers. Despite this fact, there is some fiber pullout but the fiber is coated with the matrix polymer. Another feature of this fiber treatment is on the matrix failure mode, because appreciable shear yielding rather than tearing is observed. It can be inferred that mechanical interlocking and friction are responsible for the observed composite strength increase. Figure 6.26(d) shows the failure surface for the composite with fibers treated with the silane coupling agent. It can be observed that the fibers are still coated with the matrix and that the matrix failed by shear yield flow and tearing from the fiber. If the matrix failure mode of the untreated fiber composite is compared to the pre-impregnated fiber composite, it can be seen that it changes from Copyright © 2005 by Taylor & Francis

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tearing mode to shear yielding. Hence there exists a higher force transfer capability at the fiber-matrix interface in the composites with fibers treated by preimpregnation and a silane coupling agent. It should be pointed out that the matrix properties are an upper bound for the composite strength, especiallly when fiber-matrix adhesion is strong. In this case, such upper bound is given by the yield strength of the matrix.

6.11

Conclusions

The IFSS between natural fibers and thermoplastic matrices has been improved by morphological and chemical modifications of the fiber surface. The alkaline treatment has two effects on the fiber: (1) it increased the surface roughness, therefore resulting in a good mechanical interlocking, and (2) it increased the amount of cellulose exposed on the fiber surface, thus increasing the number of possible reaction sites. The fiber preimpregnation allowed a better fiber wetting, which in a normal fiber-polymer mixing procedure would not have been possible because of the high polymer viscosity. Thus, preimpregnation enhances the mechanical interlocking between fiber and matrix. Modification of the henequen fiber with a silane coupling agent was carried out and the changes in their surface physicochemical properties were measured. The adsorption isotherms for both FIB and FIBNA show that the efficiency of the silane treatment was higher for the alkaline-treated fiber than for the untreated one, e.g., there is a higher amount of silane deposited on FIBNA compared to FIB. This indicates that the partial removal of lignin and other alkali-soluble compounds from the fiber increases the number of reactive sites on the fiber surface. It was also found that the formation of polysiloxanes inhibits the adsorption of silane onto the henequen fiber surface. This fact is particularly important since at highly concentrated silane solutions there is no additional silane adsorption onto the fibers. The XPS surface studies of the henequen fibers confirm the presence of the silane group deposited on the surface of both FIBSIL and FIBNASIL fibers. The analysis of these emission spectra show evidence of a chemical reaction between the hydrolyzed silane and the henequen fibers, at least for FIBNASIL. The chemical reaction between the organosilane coupling agent and the henequen fibers was also confirmed by FTIR spectroscopy. The level of fiber-matrix adhesion is further enhanced by the presence of a silane coupling agent. The fiber-surface silanization results in a better interfacial load transfer efficiency and also seems to improve the wetting of the fiber. The fiber-matrix adhesion was studied using micromechanical techniques commonly used in the characterization of the IFSS of circular, smooth fibers. The IFSS relationships developed for circular fibers were modified to incorporate the natural fiber perimeter instead of an equivalent fiber diameter. The use of the fiber critical aspect ratio (lc /d) in the Weibull analysis resulted in Copyright © 2005 by Taylor & Francis

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higher values for IFSS, but the trends were the same as those obtained from the use of the fiber cross-sectional perimeter for both the fragmentation test and the pullout test. The first technique, however, resulted consistently in higher values of IFSS than the second one. Slight discrepancies were observed between the results obtained from the two micromechanical techniques for fibers that had been subjected to a preimpregnation process. From the pullout experiment, higher IFSS values were obtained than those obtained with the single-fiber fragmentation test. Such discrepancies were attributed to the differences in the experimental configurations and mechanical solicitation in both arrangements and to differences in the mechanical behavior of the fiber that had been impregnated with the matrix. The results obtained from the single-fiber fragmentation test seem to be in better agreement with the effective mechanical properties measured for the laminated material. The influence of this surface treatment on the effective mechanical properties of an HDPE-henequen fiber composite was also assessed. It was noted that modification of the fiber surface with only an aqueous solution of NaOH improved the strength of the composite. Further treatment either with a silane coupling agent to promote the adhesion by chemical interactions or by enhancing the mechanical interlocking by a preimpregnation process clearly improved the tensile properties of the composite. The silane treatment of the fiber surface resulted in a 28% increase of the tensile strength of the composite whereas the preimpregnation yielded only a 4.7% increase. The increase of contact area between fiber and matrix seemed to have a large effect on the shear strength. Fiber preimpregnation by itself had no great contribution to the shear strength. The largest increase in shear strength was observed for the fiber treated with both the aqueous alkaline solution and the silane coupling agent. Such shear strength increase was on the order of 25%.

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131. Noah, J. and Prud’Homme, R., Spectroscopic analysis of Fraké treated with periodic acid. I. Electron spectroscopy for chemical analysis (ESCA), J. Appl. Polym. Sci., 53, 763–768, 1994. 132. Barry, A.O. and Koran, Z., Surface analysis by ESCA of sulfite post-treated CTMP, J. Appl. Polym. Sci., 39, 31–42, 1990. 133. Morra, M., Ochiello, E., and Garbassi, F., The effect of plasma-deposited siloxane coatings on the barrier properties of HDPE, J. Appl. Polym. Sci., 48, 1331–1340, 1993. 134. Ishida, H. and Koeing, J., Fourier Transform Infrared Study of the Glass Matrix Interface, Proceedings of the 31st Annual Technical Conference, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc. Section 6-C, 1976, pp. 1–7. 135. Britcher, L., Kehoe, D., Matisons, J., and Swincer, G., Siloxane coupling agents, Macromolecules, 28, 3110–3118, 1995. 136. The Aldrich Library of Infrared Spectra, Edition III, Charles J. Pouchert. 62,123,1535. 137. Désarmont, G. and Favre, J.P., Advances in pull-out testing and data analysis, Comp. Sci. Technol., 42, 151–187, 1991. 138. Pigott, M.R. and Dai, S.R., Fiber pull-out experiments with thermoplastics, Polym. Eng. Sci., 31, 1246–1249, 1991. 139. Jian-Xin Li., Analysis of the pull-out of single fiber from low density polyethylene, J. Appl. Polym. Sci., 53, 225–237, 1994. 140. Herrera-Franco, P., Valadez-González, A., and Cervantes-Uc, M., Development and characterization of a HDPE-sand-natural fiber composite, Composites Part B, 28B, 331–343, 1997. 141. Gonon, L., Momtaz, A., Hoyweghen, D.V., Chabert, B., Gérard, J.F., and Gaertner, R., Physico-chemical and micromechanical analysis of the interface in a poly(phenylene sulfide)/glass fiber composite.–a microbond study, Polym. Comp., 17, 265–274, 1996. 142. Singh, B., Gupta, M., and Verma, A., Influence of the fiber surface treatment on the properties of Sisal–polyester composites, Polym. Comp., 17, 910–918, 1996. 143. Gassan, J. and Bledzki, A.K., Effect of the moisture content on the properties of silanized jute-epoxy composites, Polym. Comp., 18, 179–184, 1997. 144. Herrera-Franco, P.J. and Drzal, L.T., Comparison of methods for the measurement of fiber/matrix adhesion in composites, Composites, 23, 2–27, 1992. 145. Rao, V., Herrera-Franco, P., Ozello, A.D., and Drzal, L.T.J., A direct comparison of the fragmentation test and the microbond pull-out test for determining the interfacial shear strength, J. Adhes., 34, 65–77, 1991. 146. Miller, B., Muri, P., and Rebenfeld, L., Compos. Sci. Technol., 28, 17, 1987. 147. Valadez-Gonzalez, A., Cervantes-Uc, J.M., Olayo, R., and Herrera-Franco, P.J., Effect of fiber surface treatment on the fiber-matrix bond strength of natural fiber reinforced composites, Composites: Part B, 30, 309–320, 1999. 148. Valadez-Gonzalez, A., Cervantes-Uc, J.M., Olayo, R., and Herrera-Franco, P.J., Chemical modification of henequen fibers with an organosilane coupling agent, Composites: Part B, 30, 321–331, 1999.

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7 Natural Fiber Composites in Automotive Applications

Brett C. Suddell and William J. Evans

CONTENTS 7.1 Introduction: History of Natural Fiber Applications within the Motor Industry 7.2 Materials 7.3 Wood Fiber Composites 7.4 Route to Market 7.5 Harvesting 7.6 Retting 7.7 Fiber Collection and Storage 7.8 Licensing 7.9 Markets 7.10 Manufacturing Methods 7.11 Matrices 7.12 Thermoplastics 7.13 Thermosets 7.14 The Use of Thermoplastics and Thermosets 7.15 Structure of the Automotive Components Market 7.16 Mechanical Characterization 7.17 Current Automotive Applications 7.18 Interior Components 7.19 Exterior Components 7.20 Benefits 7.21 Limitations 7.22 Driving Forces 7.23 Issues to Address 7.24 Future Outlook 7.25 Summary References

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ABSTRACT This chapter covers the results of a survey carried out recently into the current status of natural fiber composites within the automotive sector. It gives a broad overview of the route to market and of various applications in the automotive sector. The chapter discusses the advantages and disadvantages of natural fibers as well as the reasons why these novel materials are being used more frequently. The future that lies ahead will also be discussed.

7.1

Introduction: History of Natural Fiber Applications within the Motor Industry

The combination of natural fibers with other materials to form composites is not new. It was some 3000 years ago that the first “natural” composite systems were being used—in that case it was for housing applications—clay, reinforced with straw was used to build walls of dwellings in ancient Egypt.1 With the development of more durable materials such as metals, the interest in “natural” materials was lost.2 It wasn’t until the early 1900s when natural materials reemerged as possible future materials for use within automotive applications.3 In the early 1930s, Henry Ford walked into his company’s research laboratory with a bag of chicken bones, dumped them on the desk, and asked his technicians to see what they could make out of them. They responded by experimenting with a variety of natural materials including cantaloupes, carrots, cornstalks, cabbages, and onions in a search of materials to build an organic car body.4 Eventually, in 1940 Ford scientists discovered that soybean oil could be used to make high-quality paint enamel and could also be molded into a fiber-based plastic. The company claimed that the material had 10 times the shock-resistance of steel. Henry Ford delighted in demonstrating the strength of the material by pounding a soybean boot lid with an axe. If it was not for the fact that the material required a long cure time and was difficult to mold, we might well be driving cars made from this material today. In 1941, composites, particularly those based on natural fiber reinforcements, received increased attention.5 They were used for making seats, bearings, and fuselages in aircraft during World War II, which was due to the shortage of aluminum at that time, and for bearings in ships. One example was “Gordon-Aerolite,” a composite of unidirectional, unbleached flax yarn impregnated with phenolic resin and hot pressed. This was used in aircraft fuselages.6 Another example was a cotton-polymer composite which was reportedly the first fiber-reinforced plastic used by the military for aircraft radar.7,8 In 1942 Henry Ford developed the first prototype composite car made from hemp fibers. The car, unfortunately, did not make it into general production due to economic limitations at the time.

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In Europe, the body of the East German “Trabant” car, produced between 1950 and 1990, was one of the first to be built from materials containing natural fibers. In this case, cotton fibers were embedded within a polyester matrix. Daimler-Benz has been exploring the idea of replacing glass fibers with natural fibers in automotive components since 1991. A subsidiary of the company, Mercedes-Benz, pioneered this concept with the “Beleem project” based in São Paolo, Brazil. In this case, coconut fibers were used in their commercial vehicles over a 9-year period. Mercedes first used jute-based door panels in its E-class vehicles in 1996. Then in September, 2000, DaimlerChrysler began using natural fibers for their vehicle production based in East London, South Africa. The company was able to implement technology transfer from their German plants to South Africa for the entire process chain. Mat manufacturers and subsequently vehicle component suppliers, none of whom were involved in this type of activity prior to this application, used sisal grown by local farmers. History has seen numerous attempts to incorporate natural materials into automotive components. Some have been more successful than others; however, the environmental driving force has never been as important as it is in today’s automotive market.

7.2

Materials

Fibers can be classified into two main groups: man-made and natural. In general, natural fibers can also be subdivided as to their origin: plants, animals, or minerals, as shown in Figure 7.1. The vegetable/plant fibers, which are the main type of fibers discussed here, can be further subdivided into subgroups, such as bast fibers, leaf fibers, seed fibers, fruit fibers, and wood fibers, as shown in Figure 7.2. This figure also shows examples of each classification, for example, the bast fiber subgroup includes fibers of the flax, hemp, jute, and kenaf plants, to name but a few. Further commercially important fiber types along with their world production levels are presented in Table 7.1.

Fibers Man-made

Natural Vegetable

Animal

FIGURE 7.1 Classification of fibers.

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Mineral

Synthetics

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The range of different areas of application for bast fibers is shown in Chart 7.1. As can be seen, the opportunities for bast fibers are wide and varied, from textiles to paper and building materials to other industrial products.

Vegetable fibers − Cellulosic fibers Bast fibers − Flax − Hemp − Jute, kenaf

Leaf fibers − Sisal − Curaua − Banana

Seed fibers − Cotton − Capok

Fruit fibers − Coconut

Wood fibers − Pinewood

FIGURE 7.2 Vegetable fiber classification.

TABLE 7.1 Commercially Important Fiber Sources Fiber Wood Bamboo Cotton lint Jute Kenaf Flax Sisal Roselle Hemp Coir Ramie Abaca Source:

Species

World Production (103 t)

Origin

(⬎10,000 species) (⬎1250 species) Gossypium sp. Corchorus sp. Hibiscus cannabinus Linum usitatissimum Agave sisilana Hibiscus sabdariffa Cannabis sativa Cocos nucifera Boehmeria nivea Musa textiles

1,750,000 10,000 18,450 2,300 970 830 378 250 214 100 100 70

Stem Stem Fruit Stem Stem Stem Leaf Stem Stem Fruit Stem Leaf

Bolton, J., Outlook Agricult., 24, 85, 1998. With permission.

Bast fibers Textiles

Apparel Nappies Fabrics Handbags Clothes Denim Socks Shoes

Technical textiles

Twine Rope Nets Canvas bags Tarps Carpets Geotextiles

Paper

Printing Fine Speciality Technical filter paper Newsprint Cardboard Packaging

Building materials

Other industrial products

Fiberboard Insulation material Fiberglass substitute Cement blocks Mortar

CHART 7.1 Bast fiber applications. Source: IENICA (Ref. 1495) Summary Report—Fiber Crops, August, 2000.

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Agrofiber composites Compression molded points Brake/clutch linings

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The most important of the natural fibers in terms of automotive applications are the bast fiber group, so called because they come from the bast part of the plant. Plant bast is composed of an inner woody core surrounded by bundles of long hollow fibers and an outer protective skin. It is the bast that stabilizes the plant and provides plant fibers with good mechanical properties; for example, they exhibit the greatest strength so it is only logical that these fibers are used for the more demanding composite applications. This attribute in conjunction with a low density ensures that bast fibers have the potential to be outstanding reinforcements in lightweight composite parts. The advantages of using bast fibers in the automotive industry include: ● ● ●

● ●

Renewable and sustainable plant fiber resource Recyclable Weight savings of between 10 and 30% (which is of primary importance to the automotive industry) Cost savings Abundant supply which is accessible to car manufacturing plants in many regions of the world

Further advantages of natural fibers in general are discussed later in Section 7.20.

7.3

Wood Fiber Composites

New materials derived from wood and thermoplastic polymers have attracted considerable attention in recent years.9 Throughout history, the unique characteristics and comparative abundance of wood have made it a natural material for homes and other structures such as furniture, tools, vehicles, and decorative objects. The bulk of plant-based fiber composites are made using wood flour (waste from saw mills) or wood fiber (obtained from waste wood products such as pallets and furniture). These are used as inexpensive fillers in polypropylene (PP) and polyvinyl chloride (PVC). These composites are often referred to as “plastic lumber.” The wood fiber content in these products can range from 30 to 70% and are commonly used in outdoor applications, automotive panels, and furniture. As these products do not experience high stresses in service, their mechanical properties can be moderate to low. The market for such composites has grown strongly in the past few years and is expected to continue in a similar vein in the future.10 The North American market for extruded wood-plastic composites has emerged as one of the fastest growth industries.11 The well-developed areas include those such as the construction sector, which is the converse of the situation in Europe, still in its infancy. However, this situation is now changing within Europe, with wood-plastic window profiles being developed.12 Copyright © 2005 by Taylor & Francis

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The extruded natural fiber-thermoplastic composite market in North America is now in rapid growth, with a recent forecast suggesting that the annual growth rate of this area would be around 60% for the period 2000–2005.10 In particular, the growth areas which were highlighted included window and door profiles, fencing, and outdoor decking applications. The use of wood fiber-plastic composites is now standard technology in the production of decking materials.13 The demand for natural fibers in plastic composites within North America has been very small in comparison to the demand for wood fiber-flour during the period 1980–2000.10,14 The current sectors utilizing wood fiber and natural fiber composites in North America can be seen in Chart 7.2.14 The largest sectors for both materials are clearly the building products. However, the transportation sector for the natural fiber composites is larger (16%) than it is for the wood fiber composites (10%). It should also be noted that the total amount of wood fiber composites used in all four sectors amounts to 308,200 t whereas the use of natural fiber composites within the four sectors is only 31,000 t, a mere tenth of the wood fiber composite materials proportion.

7.4

Route to Market

We can examine the production route for hemp as being a representative example of natural fibers in general. Hemp (Figure 7.3) is grown in the U.K. by one particular company called Hemcore.15 It is an annual crop, planted in late spring when the soil temperatures are just starting to warm up, and has a remarkable growth rate, often reaching heights in excess of 3 m by the month of August, with a field density of some 150–200 plants/m2. Following the harvesting process, the crop is stored by the grower prior to delivery to the factory. The resultant hemp straw is processed and the outer fibers are separated from the woody inner core. The resultant fiber finds applications within the following markets (specific to Hemcore): paper, automotive, construction, and horticulture. The woody core, which is very absorbent, is also sold as horse bedding. A typical hemp plantation can be seen in Figure 7.4.

7.5

Harvesting

Hemp is cut once the flowering commences at the end of July. The growth of the hemp plant virtually ceases after flowering starts. Crops are cut by most makes of disc mower or by a reciprocating knife mower (Figure 7.5). The advantage to growing hemp is the close growing of the plants obviating the need for the application of herbicides in order to control the growth of Copyright © 2005 by Taylor & Francis

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Natural fiber composites (total for all sectors 31,000 tonnes) Industrial/consumer 6%

Transportation 16%

Infrastructure 4%

Building products 74%

Wood fiber composites (total for all sectors 308,200 tonnes) Industrial/consumer 6%

Transportation 10%

Infrastructure 18%

Building products 66% CHART 7.2 Total consumption of natural and wood fiber composites in North America.

weeds. In Figure 7.4 and Figure 7.5, it can be seen just how dense the growth of the hemp crop in a field is. An objective of the harvesting is to leave the crop as thinly spread across the field as possible to avoid a tightly packed swath prior to the retting process. Copyright © 2005 by Taylor & Francis

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FIGURE 7.3 Hemp leaf.

FIGURE 7.4 Hemp plantation.

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FIGURE 7.5 Hemp crop being harvested. (Courtesy of Hemcore.)

7.6

Retting

The definition of retting is as follows: (verb) “To moisten or soak (flax, for example) in order to soften and separate the fibers by partial rotting.”* Once cut, the crop remains on the ground for between 3 to 4 weeks in order for the leaf to senesce and fall from the stem and to allow retting of the crop to take place. This natural process of wetting and drying of the stems initiates the separation of the outer fibers from the inner pithy core and is a critical part of the harvesting operation. If the crop becomes over retted the fibers will lose part of their strength and their market potential becomes limited. It is during the retting process that the hemicellulose and lignin are removed from the fibers.16

7.7

Fiber Collection and Storage

In a process known as “rowing up,” under normal conditions the crop will ret when spread thinly across the field. Once cleared for baling, the crop should be rowed up using a single-wheel horizontal rotary rake (also known as a silage rake). For baling of the crop to take place, hemp in this instance must be baled with belt-type variable chamber round balers. Bales are generally 1.5 m in diameter. The bales should be immediately removed from the field and stored under cover in at least a Dutch barn environment. It should be noted that hemp bales do not shed rainwater. Hemp is an excellent break crop providing a good barrier to pests and diseases and its deep roots are very beneficial for soil structure. * The American Heritage Dictionary of the English Language, 4th Ed., Houghton Mifflin Company, 2000.

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Licensing

The term industrial hemp is applied to varieties of the Cannabis sativa family that have been specifically produced by plant breeders where the THC (tetrahydrocannabinnol) content is less than 0.2%. The THC component is the psychoactive drug found in Cannabis, which in the marijuana plant would be of the order 10–15%. A problem still exists, though, in that industrial hemp and cannabis plants are visually identical to the general public, thereby promoting the known theft of hemp plants from licensed crops. As the plant is a member of the cannabis family, the law in the U.K. requires growers to be licensed by the Home Office (a government department within the U.K.). There are certain restrictions in place for growers such as fields that offer easy access from busy public roads, fields in close proximity to housing estates, and fields close to schools or leisure areas. It should also be noted that there are international restrictions in place in relation to the growing of hemp. For example, in Canada the crop is grown under licence issued by the government but this is not the case for the United States where the growing of the crop is outlawed. The situation in Europe is slightly different in that the crop is allowed to be grown under licence similar to in the U.K. but the THC content of the plant must not exceed the stated amount of 0.2%.

7.9

Markets

The natural fiber-reinforced plastics market is already well established in Europe, which is ahead of North America in the development and adoption of “biocomposites.”17 The specific nature of the car industry (manufacturers, suppliers, legislation, etc.) means it is necessary to look at these novel materials from a European perspective. The automotive industry started using natural fiberpolymer composites for both commercial and technical reasons. The automotive market is attractive to the natural fiber producer, as a vehicle platform life is generally a minimum of 5 years but more usually 7–8 years, and then there is the continued time period for product support after vehicle production has concluded. This therefore ensures a sustained period of demand for natural fibers within this industry. Indeed, the use of natural fibers has risen dramatically in recent years, as seen from Table 7.2. The German automotive industry has increased its usage, from 4,000 t in 1996 to 15,500 t in 1999, with usage in the rest of Europe growing from 300 t to 6,900 t in the same period. Projections for 2005 and 2010 suggest that the total application of natural fibers in the European automotive sector could rise to between 50,000 and 70,000 t in 2005 and to more than 100,000 t by 2010, as shown in Figure 7.6.18 Copyright © 2005 by Taylor & Francis

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TABLE 7.2 European Use of Natural Fibers (t) 1996

1999

Fiber

Germany

Rest of Europe

Germany

Rest of Europe

Flax Jute Hemp Kenaf Sisal Totals

1800 1800 na 400 na 4000

na 300 na na na 300

11000 2000 1100 900 500 15500

4900 1400 600 na na 6900

Total consumption (tonnes)

100000 80000 60000 40000 20000 0 1996

1999

2000

2005

2010

Year

FIGURE 7.6 Total consumption (actual and forecast) for natural fibers within Western Europe.

This equates to a potential European market in excess of £70 million, excluding any fibers, which are exported. Hemp use, in particular, is increasing, making it the second most important natural fiber in the industry after flax. The forecast for natural fibers in automotive products in 2005 for North America alone is projected to be in excess of 45,500 t,19 a similar amount to Europe’s requirements considering that North America is 8 years behind Europe in its application of natural fibers within the automotive industry. A study by the Nova Institute in 2000 reviewed market potential for short hemp and flax fibers in Europe.20 In this study, a survey of German flax and hemp producers showed that 45% of hemp fiber production went into automotive composites in 1999. One of the attractions of hemp, as compared with flax, is the ability to grow the crop without pesticide application. The potential for fiber yield is also higher with hemp. In 1996, the consumption of natural fibers was 4,300 t per annum. This figure rose to 22,000 t in 1999. The difference between fibers being used in Germany and the rest of Europe can be seen in Figure 7.7. The growth outlook for natural fibers in automotive components is expected to increase by 50% per annum. Between 1982 and 2002 the European Union used in excess of $60 million in subsidies which was directed toward the development of new flax and hemp applications. However, in Germany alone in excess of Copyright © 2005 by Taylor & Francis

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Natural fibers (tonnes)

18000 16000

1996

14000

1999

12000 10000 8000 6000 4000 2000 0 Germany

Rest of Europe

FIGURE 7.7 Comparison between the use of natural fibers in Germany with the rest of Europe.

$104 million was invested in research and development in terms of new harvesting, fiber processing, and refining technologies for these fibers. A further investment of over $89 million is planned by Germany in the future in this area.20 The success of the German manufacturers is for two reasons. First, the German government allocates 10 times the amount of funding which is available within the U.K. (in 1999, these figures were $75.3 million for Germany and $7.53 million for the U.K.). Second, the German government has a long tradition of subsidy and benefits to promote the growth of industrial crops by the general public. This therefore led to the supply of fibers being dictated by price and was taken up rapidly by local farmers, which led to the expansion of the industrial crop market in Germany. North America has been slower to recognize the potential of these materials. However, the situation there also is changing rapidly to the extent that the market share is predicted to rise from $150 million in 2000 to $1.4 billion in 2005. It is estimated that the market in 2005 will require 45,450 t of natural fibers.

7.10

Manufacturing Methods

The fabrication methods for natural fiber composites are similar to those used for glass fibers.9

7.11

Matrices

There are two main production techniques for press molding natural fiber composites used in automobile components. The first technique involves blending natural fibers with polypropylene fibers or polyethylene to form Copyright © 2005 by Taylor & Francis

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mats that are then used in a number of different layers. Alternatively, nonwoven mats are used interdispersed with a layer of polypropylene fibers which are then hot pressed, causing the polypropylene to bind the layers of nonwoven materials together. This technique utilizes thermoplastic resins which soften when heated, harden when cooled and allow for reprocessing. The second production method involves using thermoset resins such as polyurethane or epoxy, which are sprayed or allowed to soak into natural fiber mats. These are then press molded to produce the final component shape. The composite is created as a result of polymerization and subsequent hardening.

7.12

Thermoplastics

The number of thermoplastics, which can be used successfully with natural fibers, is limited, which is due to the lower thermal stability of natural and wood fibers (confined to temperatures up to 230°C).21 Typical examples include polypropylene; high-, medium-, and low-density polyethylene (PE); polyvinyl chloride,22 and others consistent with the lower thermal stability of the natural fibers.21,45 Types of processes which can be used with natural fibers and thermoplastics include extrusion, compression molding, and injection molding. The most commonly used thermoplastic is polypropylene used in a 50:50 blend.23 For special applications where additional strength is required, the more expensive polymer polyester is sometimes used.24

7.13

Thermosets

Thermoset polymers are used as the matrix for filled plastics and fiberreinforced composites in automotive body panels, for example. Examples of thermosetting resins include epoxy and polyurethane.25 A number of manufacturing methods can be used with these polymers, including resin transfer molding (RTM), sheet moulding compound (SMC), and bulk molding compound (BMC).

7.14

The Use of Thermoplastics and Thermosets

A study conducted by the Nova Institute in 200020 reported that both the thermoplastic and thermoset resins were being used in the automotive industry in equal measure. However, there has now been a shift and the automotive industry is using more of the thermoplastics rather than the thermosets. This Copyright © 2005 by Taylor & Francis

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is being attributed to the fact that the thermoplastics exhibit a reduced environmental impact over the thermosets, which cannot be recycled, whereas thermoplastics can.26

7.15

Structure of the Automotive Components Market

The sectors within the automotive industry value chain are described as follows:24 ● ●

●

●

OEMs: Original equipment manufacturers, i.e., car manufacturers Tier One Suppliers: Suppliers of specialized interior systems for assembly by the OEMs Substrate Suppliers: The nonwoven producers in the textile industry and plastics producers in the case of new natural fiber granulate technology Natural Fiber Suppliers: Organizations that supply hemp, flax, and other fibers

To influence specification and usage of natural fibers, suppliers must work with all four sectors of the automotive industry as identified above. The actual customers for natural fibers are the substrate suppliers. The various steps with examples of the stages in the production of components can be seen in Figure 7.8.

7.16

Mechanical Characterization

The mechanical properties of selected natural and synthetic fibers are presented in Table 7.3. While these fibers may not be as strong as graphite or aramids, on a “per weight” basis the natural fibers such as flax, hemp, jute, and bamboo all have higher moduli (stiffness) than E-glass fibers.22

7.17

Current Automotive Applications

The automotive industry requires composite materials that meet performance criteria as determined in a wide range of tests. Typical market specification includes the following criteria: ● ●

Ultimate breaking force and elongation Flexural properties

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HEMCORE

Natural fiber suppliers

Substrate suppliers (nonwoven mat maker)

Tier one supplier

OEM FIGURE 7.8 Production stages from fiber to component. (Courtesy of Hemcore.)

● ● ● ● ● ● ● ● ●

Impact strength Fogging characteristics Flammability Acoustic absorption Suitability for processing: temperature and dwell time Odor Water absorption Dimensional stability Crash behavior

Most of the composites currently used by industry are designed with longterm durability in mind and are made using nondegradable polymeric resins, such as epoxies and polyurethane, along with high-strength, manmade fibers such as graphites, aramid, and glass. It should also be noted that Copyright © 2005 by Taylor & Francis

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TABLE 7.3 Mechanical Properties of Natural Fibers

Fiber Cotton Jute Flax Hemp Sisal Coir Bamboo Soft wood Pineapple Ramie E-glass S-glass Aramid (normal) Carbon (standard)

Diameter Range (mm)

Density (g/cm3)

Elongation at Break (%)

Tensile Strength (MPa)

Young’s Modulus (GPa)

1.5 1.3 1.5 1.4 1.5 1.2 0.8 1.5 — 1.5 2.5 2.5 1.4 1.4

7.0–8.0 1.5–1.8 2.7–3.2 1.6 2.0–2.5 30.0 — — 1.6 3.6–3.8 2.5 2.8 3.3–3.7 1.4–1.8

287–597 393–773 345–1035 690 511–635 175 391–1000 1000 413–1627 400–938 2000–3500 4570 3000–3150 4000

5.5–12.6 26.5 27.6 35 9.4–22.0 4–6 48–89 40 34.5–82.5 61.4–128.0 70 86 63–67 230–240

12–38 10–25 5–38 10–51 8–41

11–80 10 10 12 7–10

many of these polymers and fibers are derived from petroleum, which is a nonrenewable resource.22 The various applications where natural fibers have found use, depending upon fiber length, can be seen in Figure 7.9.

7.18

Interior Components

In recent years there has been increasing interest in the replacement of glass fibers in reinforced plastic composites by natural plant fibers such as flax, hemp, and sisal.27,28 Like glass, the natural fibers combine readily with a thermoplastic or thermosetting matrix to produce commodity goods.29 In the early stages of the application of these materials within automotive components those leading the development of natural fibers were predominantly German, with BMW, Audi, and DaimlerChrysler as leading examples. Now, virtually all manufacturers incorporate natural fiber composites in their vehicles. The German companies represent in most cases the premium end of the automotive sector. Virtually all of the major car manufacturers in Germany (i.e., DaimlerChrysler, Mercedes, Volkswagen Audi Group, BMW, Ford, and Opel) now use natural fiber composites in applications such as those listed in Table 7.4. Figure 7.10 illustrates the range of automotive components within the Mercedes “E” Class which incorporate natural fibers. The U.K. government department previously known as MAFF (Ministry for Agriculture, Fisheries and Food), which, following the foot and mouth epidemic in the U.K., became known as DEFRA (Department for Copyright © 2005 by Taylor & Francis

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Fiber applications

Yarns Textile processing Braiding rolling + Polymers e.g. UPES, epoxide PPS Leisure/sport industry (cycle frames, surf board poles, fishing rods)

Nonwovens Wet laid

Dry laid

Bonding

Pressing

+Binder + Fiber

+ Thermosets + Film stacking + Additives

BMC/RTM

Blends with thermoplastics

Pulping

Pulverisation

Paper (tea bags, special papers)

Filling agents

Extrusion Granulate

Insulation material

Molded articles

Filters

Au tomotive interior parts

Packaging material

Furniture

4−100 mm

4−100 mm

Friction lining (blended yarns from metal, aramid, cellulosic fibers)

Max. fiber length

Blends with thermosetting plastics

Molding Pultrusion

Molding

Molded articles

Molded parts

Cages

Foils

Automotive parts (bumpers, spoilers, body parts)

Profiles

5−25 mm

<5 mm

1−5 mm

-

concrete polymers food varnish paints

ca. 1 mm

Fiber length

FIGURE 7.9 Promising nontextile applications of bast fibers.

Environment, Food and Rural Affairs), commissioned a study in 1999.24 This study assessed natural fiber applications and the potential usage of natural fibers within the automotive industry. The study found that 50 million vehicles are produced on an annual basis, with up to 20 kg of natural fibers being required per vehicle. Each new model range requires between 1,000 and 3,000 t of natural fibers per annum. Currently, more than 15,000 t of natural fibers are used each year by the automotive industry in Europe alone. DaimlerChrysler, already a major user of natural fiber composites, aims to increase the proportion of natural fiber materials in their cars. Interior trim components such as dashboards and door panels using polypropylene and natural fibers are produced by Johnson Controls, Inc. for DaimlerChrysler.30 BMW currently uses from 8 kg to over 13 kg of natural fibers per vehicle while Ford uses from 5 to 13 kg (these weights include wool and cotton).31 Current well-established applications of natural fibers in automotive vehicles are presented in Table 7.5, with the typical weight of natural fibers being incorporated for the more established components. Components under development utilizing natural fibers (along with the weight of the fibers) are presented in Table 7.6. Copyright © 2005 by Taylor & Francis

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TABLE 7.4 Automotive Manufacturers, Car Models, and Components Utilizing Natural Fibers Automotive Manufacturer Audi BMW DaimlerChrysler Fiat Ford Peugeot Renault Rover Saab SEAT Vauxhall Volkswagen Volvo

Model and Application A2, A3, A4, A4 Avant, A6, A8, Roadster, Coupe: Seat backs, side and back door panel, boot lining, hat rack, spare tyre lining 3, 5 and 7 series and others: Door panels, headliner panel, boot lining, seat backs A, C, E and S-series: Door panels, windshield/dashboard, business table, pillar cover panel Punto, Brava, Marea, Alfa Romeo 146, 156 Mondeo CD 162, Focus: Door panels, B-pillar, boot liner New model 406 Clio Rover 2000 and others: Insulation, rear storage shelf/panel Door panels Door panels, seat backs Astra, Vectra, Zafira: Headliner panel, door panels, pillar cover panel, instrument panel Golf A4, Passat, Bora: Door panel, seat back, boot lid finish panel, boot liner C70, V70

FIGURE 7.10 Top of the range automotive vehicles incorporate many components made from natural fibers, as shown here for the Mercedes “E” class.

An example of one of the first interior door panels to be created from natural materials can be seen in Figure 7.11. This particular panel was manufactured by the Institute of Structural Mechanics based within the DLR of Copyright © 2005 by Taylor & Francis

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TABLE 7.5 Well-Established Interior Automotive Components Utilizing Natural Fibers Automotive Component Front door liners Rear door liners Boot liners Parcel shelves

Typical Weight of Fibers (kg) 1.2–1.8 0.8–1.5 1.5–2.5 ⬍2

TABLE 7.6 Automotive Components under Development Utilizing Natural Fibers Automotive Component Seat backs Sunroof interior shields Headrests

Typical Weight of Fibers (kg) 1.6–2.0 ⬍ 0.4 ~2.5

FIGURE 7.11 Biocomposite interior door panelling constructed by DLR in cooperation with Johnson Controls Interiors.

Germany in the early 1980s.Typical materials used in the American automotive market include those given in Table 7.7. The main difference from the European market is the use of wood and kenaf, which grows well in the United States. In 2000, Audi launched the A2 midrange car, which was the first mass-produced vehicle with an all-aluminum body. To supplement the weight reduction afforded by the all-aluminum body, door trim panels were made of polyurethane reinforced with a mixed flax/sisal mat. These panels have not Copyright © 2005 by Taylor & Francis

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TABLE 7.7 Typical Natural Fibers used in the American Automotive Market with a Polypropylene Matrix Application

Fiber

Fiber (%)

Door panel Inserts Rear parcel shelves

Kenaf/hemp, 50:50 Wood fiber Kenaf Flax Wood Flax Flax Wood Kenaf Flax

50 50 50 50 85 50 50 50 50 50

Seatbacks Spare tyre covers Other interior trim

Source: Suddell, B.C., Food and Agriculture Organisation of the United Nations, Joint meeting, July, 2003, Salvador, Brazil. With permission.

only an extremely low mass per unit volume but also very high-dimensional stability.

7.19

Exterior Components

Recently, DaimlerChrysler has increased its research and development investment in flax-reinforced polyester composite for exterior or semiexterior applications. In 2000 alone, DaimlerChrysler spent $1.5 billion on environmental protection, and from this $870 million was spent on environmentally friendly products and production processes.32 A truck with flax-based, rather than glass-based exterior skirting panels is now in production. Tests carried out by the DaimlerChrysler Research Centre in Ulm, Germany showed that these composite components stood up to impact without shattering into splinters. This is an important consideration in crash behavior tests. The composites were also dimensionally stable and weather-resistant. In Germany, car manufacturers are aiming to make every component of their vehicles either recyclable or biodegradable.33 Now the researchers at DaimlerChrysler are going one step further. They are using natural fibers to reinforce exterior components of a vehicle, whereas in the past the applications have been confined to interior components. The exterior components must be able to withstand extreme conditions compared to their interior cousins, such as exposure to wetness and chipping. The Mercedes-Benz Travego travel coach is equipped with a flax-reinforced engine/transmission cover. This is the first use of natural fibers for standard exterior components Copyright © 2005 by Taylor & Francis

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in a production vehicle and represents a milestone in the application of natural fibers. More importantly, though, from an automotive standpoint, are the benefits that natural fibers provide when compared with glass fibers. These include a weight reduction of the engine/transmission cover in this particular instance of 10% and a reduction in cost of 5%. While the overall usage of natural fibers is continuing to grow on an annual basis, it is the use of flax, hemp, and kenaf which is growing at the fastest rate. In contrast, the use of jute and sisal is actually in decline. Flax, hemp, and kenaf are favored due to their excellent combination of economic and functional properties. Short fiber flax has dominated the market since 1996 with a market share of 70%. This high share is achieved despite the fact that the short fiber price and quality is highly dependent on whether linen (long fiber flax) is fashionable in the clothing industry. Hemp and flax account for nearly 85% of the natural fibers used in composite materials.34

7.20

Benefits

When compared to glass fibers traditionally used within the automotive sector, natural fibers demonstrate considerable benefits. The important aspect here is that these benefits are achieved without a significant change in material functional performance such as degradation of their mechanical or acoustic properties. The most important benefits include: ●

●

●

●

A reduction in cost: Typically, natural fibers are between 25 and 50% cheaper than glass fiber composites. A reduction in weight: Four injection-molded ABS car door panels weigh 9 kg. The same panels based on natural fibers weigh only 5 kg for equivalent mechanical properties. Light weight (less than half the density of glass fibers): This is also an important environmental benefit as ~85% of energy usage is when a vehicle is being driven. Plant fibers have a maximum density of 1.5 g/cc (that of cellulose), which results in high specific strength and stiffness and hence low component weight. Safer: Safer crash behavior in accident tests, particularly in relation to high stability and an absence of splintering, is also observed.20

Additional benefits also include the following: ● ● ●

●

Good acoustic properties due to their hollow cellular structure. Good thermal insulation properties. Easier to cut and nonabrasive to processing equipment—glass fibers blunt tools, so by using natural fibers instead, the life of the equipment is prolonged. Equivalent strength values to glass fiber composites.

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Exhibit excellent formability—deep moldings can be produced. Natural fibers are superior to glass fibers from a health standpoint. The dermal irritation or respiratory problems associated with glass fibers for the operator do not occur. Good availability and abundant worldwide.

The main reason for the interest in natural fiber composites is the cost and weight. However, natural fibers also demonstrate considerable environmental benefits in relation to glass fibers,35 which include: ●

●

●

●

Ecological sustainability. The fibers are potentially sustainable. Being based on biomass they are widely available throughout the world. Low energy requirements for production. The energy for the production of natural plant fibers comes free from the sun. The only energy cost to society is that required for harvesting and subsequent transportation of the fibers, and for their separation from the remainder of the plant. For example, natural fibers such as kenaf require only one quarter of the energy necessary to produce the same weight of glass fiber, i.e., it takes 6,500 British Thermal Units (BTUs) to produce 2.2 kg of kenaf, whereas the energy requirements to produce the same weight of glass fibers is 23,500 BTUs.36 End of life disposal. They can be composted or combusted at the end of their useful life. This is complimented when the fibers are used with a biodegradable resin matrix. Carbon dioxide neutrality. Natural fibers are carbon dioxide neutral, i.e., they do not return excess carbon dioxide into the atmosphere when they are composted or combusted. This aspect is consistent with a sustainable development strategy.

From a technical point of view, natural fibers are generally inferior to glass, having lower inherent strengths and stiffness. It should also be noted that natural fibers show wide variations in their strength properties due to different regions in which they are grown, which is not observed with synthetic fibers. However, they have only half the density of glass. They are, therefore, very competitive with glass in weight-sensitive applications, such as the automotive industry. Indeed, one automotive producer has reported that molded ABS door panels incorporating natural fibers as reinforcement were only 60% the weight of similar panels having equivalent mechanical properties but made using glass fiber as the reinforcement. The attributes listed above such as low-energy requirements in production, end of life disposal, and their carbon dioxide neutrality are becoming increasingly important as life cycle costs, particularly costs in relation to their disposal, start to be taken into account at the initial manufacture stage. This is especially apparent in the European automotive industry where high landfill costs, coupled with strong environmental legislation, have made cost of disposal a major design issue.24 Copyright © 2005 by Taylor & Francis

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A further advantage of using natural fibers over the alternative wood fibers is that plants are renewed on an annual basis, whereas a typical tree requires 20–25 years to grow to maturity. However, wood fiber is available as a waste material from industry. The key point, though, in the comparison of the two types of fibers is the better strength and stiffness the plant fibers demonstrate in relation to wood.

7.21

Limitations

The high potential of plant fiber as a reinforcement is greatly diminished by its hydrophilic nature37 and the lack of sufficient adhesion between untreated fibers and the matrix materials being employed.38 As a result, considerable effort is currently being directed toward improving the mechanical performance of fiber-reinforced composites through optimization of the interfacial bond between the fibers and the polymer matrix.18 The variable quality of natural fibers, which is dependent on the growing conditions and the fiber-processing techniques, is an additional drawback when considering the use of these fibers in high-performance technical applications.39 The natural fibers also exhibit low impact strength, which is a result of a high concentration of fiber defects imparted into the material during the growing or processing stages.

7.22

Driving Forces

The automotive industry is the prime driver of “green” composites because the industry is faced with issues for which these types of materials offer a solution. Faced with pressures to produce fuel-efficient, low-polluting vehicles, the industry needs biofriendly fiber-reinforced plastic composites to make its products lighter. The two most important factors now driving the use of natural fibers by the European automotive industry are cost and weight. The benefits realized in the Travego coach include a weight reduction in the engine/transmission cover of 10% and a cost reduction of 5%. Fibers are typically purchased at present for about 0.5–0.6 Euros/kg. This compares with ~2 Euros/kg for glass fibers.13 Disposal of composites after their intended life is a major concern as well as being very expensive. As composites are made using two dissimilar materials, they cannot be easily recycled or reused. Most composites end up in landfills, while some are incinerated after use, with the possible release of dangerous chemicals. Only a small quantity is recycled. These disposal alternatives are Copyright © 2005 by Taylor & Francis

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expensive and wasteful and can also contribute to pollution in the world. Another important consideration is that the number of landfill sites is being reduced. For example, in the United States, the number of sites reduced from 8000 to 2314 over a 10-year period (1988 to 1998).22 There has also been an increase in environmental awareness among consumers, manufacturers, and governments. It is estimated that the world is consuming petroleum at an “unsustainable” rate: some 100,000 times faster than the rate at which it is created by nature.22 Therefore, alternative sources need to be found and utilized. This is why a number of governments have established laws to encourage the use of recycled and/or biobased green products. From the automotive sector standpoint, European legislation* coupled with consumer pressure is a major driver for the uptake of natural fiber composites. There is now an increasing trend for the use of natural fibers as a result of government legislation on environmental issues. This is particularly important in those countries where products from agricultural sources offer an attractive and cheap alternative in the development of degradable materials.40 The end of life vehicle (ELV) directive came into force in October, 2000. All member states of the European Union were required to transpose the directive into national law by April, 2002. The legislation states that by 2015 vehicles must be constructed of 95% recyclable materials, with 85% recoverable through reuse or mechanical recycling and 10% through energy recovery or thermal recycling. The directive aims to reduce the amount of waste from vehicles (which includes automobiles and vans) and also includes requirements for member states to introduce strict standards for the treatment of ELVs at authorized treatment facilities.41 An important consequence is that the use of natural fibers in composites is much higher in Europe than in North America.

7.23

Issues to Address

The potential use of natural fibers as a reinforcement is greatly reduced because of their hydrophilic nature, i.e., the ability to absorb water and moisture,37 and the lack of sufficient adhesion between untreated fibers and the polymer matrix.38 The lack of adhesion is specifically an issue with certain thermoplastics and not necessarily with polymers in general. If this area of interfacial adhesion could be addressed, then one could see an improvement in the mechanical performance of the natural fiber composite materials. However, this has not hindered the growth in applications within the automotive industry to date.

* EU directive Article 7 [2000/53/EC] on vehicle end of life disposal.

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For there to be an improvement in the impact properties of these materials, there will need to be a greater focus in a different area, this being the use of impact modifier additions. There are also quality problems that could influence the future development of natural fiber composites. For example, fiber quality assurance is a concern, with contamination of jute and sisal crops in particular being reported. Additionally, the variability of fibers from year to year, and within particular harvests, needs to be addressed. In essence, there is a real need for a quality assurance protocol for natural fibers to be established.

7.24

Future Outlook

Engineers who previously worked for large automotive companies but felt they could be doing more toward helping the environment have developed a concept car. It is called the EcoCar and is advertised as a sustainable vehicle for the future, running on biofuels. It is built from natural fiber composite panels that incorporate biodegradable resins as the matrix material.42 A recent forecast predicts that the most important technologies that incorporate natural fiber composite materials in 2005 will be natural fibers for injection-molded products (32%), followed by natural fibers with a bioplastics matrix (19%) and modified fibers for use in advanced applications (19%), Figure 7.12.43 Natural fibers for press molding 8%

Natural fibers for injection molding 32%

Fibers for advanced applications 11%

Others 11%

Modified fibers for advanced applications 19%

Natural fibers + bioplastics 19%

FIGURE 7.12 Forecast for natural fiber composite technologies which will gain in significance by 2005.

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The diverse range of products now being produced, utilizing natural fibers and biobased resins derived from soybeans, is giving life to a new generation of biobased composites for a number of applications. These include automotive vehicles (including trucking) and hurricane-resistant housing and structures, especially in the United States.44 Other areas where these novel materials could find a market include construction sectors and the leisure industry, to name but a few. In Germany, car manufacturers are aiming to make every component of their vehicles either recyclable or biodegradable.33

7.25

Summary

The use of natural fibers within composite applications is being pursued extensively throughout the world. As a result, many components for the automotive sector are now made from natural fiber composite materials. These materials are based largely on polypropylene or polyester matrices incorporating fibers such as flax, hemp, and jute. Until now the motivation for use of these fibers has been price and marketing rather than technical demands. Therefore, the resultant products are limited mainly to lowstrength interior automotive components. Important shortcomings of these materials are hydrophilicity and poor impact strength. However, work is being conducted in order to improve the mechanical properties of the materials so that they can be used in exterior components where the conditions are more severe and include requirements for impact strength and resistance to water and moisture. In a New Scientist article, the future for these materials was summed up quite nicely:33 The car of the future could be moulded from cashew nut oil and hemp. And that’s not all. In the boot could be a set of golf clubs built around jute fibers, nestling next to a tennis racket stiffened with coconut hair. The bicycle frames strapped to the flax-based roof rack may derive their strength from any one of the 2000 other suitable plants. The truth is, natural fibers are undergoing a high tech revolution that could see them replace synthetic materials.

References 1. Suddell, B.C., The Current Situation and Future Outlook for Natural Fibers within the Automotive Industry, Food and Agriculture Organisation (FAO) of the United Nations, Joint Meeting of the 32nd Session of the Intergovernmental Group on Hard Fibers and the 34th Session of the Intergovernmental Group on Jute, Kenaf and Allied Fibers, July 8–11, 2003, Salvador, Brazil.

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2. Suddell, B.C., A Survey into the Application of Natural Fiber Composites in the Automotive Industry, in Proceedings ISNaPol Natural Polymers and Composites IV, Mattoso, L.H.C., Leao, A., and Follini, E., Eds., Sao Paolo, Brazil, Sept. 1–4, 2002. 3. Suddell, B.C. and Evans, W.J., The Increasing Use and Application of Natural Fiber Composite Materials Within the Automotive Industry, Plenary Address at 7th International Conference on Woodfiber–Plastic Composites (and other Natural Fibers), May 19–20, 2003, Monona Terrace Community and Convention Centre, Madison, Wisconsin, USA. 4. Evans, W.J., Isaac, D.H., Suddell, B.C., and Crosky, A., Natural Fibers and their Composites: A Global Perspective, Keynote Paper, in Proceedings of the 23rd Riso International Symposium on Materials Science: Sustainable Natural and Polymeric Composites–Science and Technology, Lilholt, H. et al., Eds., Riso National Laboratory, Roskilde, Denmark, 2002. 5. Joseph, K. et al., Natural Fibre Reinforced Thermoplastic Composites, in Proceedings, Natural Polymers and Agrofibers Composites, Frollini, E., Leao, A.L., and Mattoso, L.H.C., Eds., Sao Carlos, Brazil, 2000, pp. 159–201. 6. Hughes, M., Sustainable Composites–Fact or Fiction?, Oral Presentation, Composites Processing 2002, Composites Processing Association, Holiday Inn, Birmingham airport, Jan. 24, 2002. 7. Lewin, L.M. and Pearce, E.M., Handbook of Fiber Science and Technology, Vol. IV, Fiber Chemistry, Marcel Decker, New York, 1985. 8. Piggot, M.R., Load Bearing Fiber Composites, Pergamon Press, Oxford, 1980. 9. Smit, H.H. et al., Agrofiber Reinforced Sheet Moulding Compound ACUN-2: Composites in the Transportation Industry, UNSW, Sydney, Australia, Feb. 14–18, 2000. 10. Kline & Company Inc., Opportunities for Natural Fibers in Plastic Composites, published by Kline & Co. Inc., 2000. 11. Song, W., Challenges in Extrusion of Woodfiber–plastic composite, Progress in Woodfiber–Plastic Composites Conference, May 23–24, 2002, Toronto, Canada. 12. Plackett, D., The Natural Fiber-Polymer Composite Industry in Europe –Technology and Markets, Proceeding, Progress in Woodfiber–Plastic Composites Conference, May 23–24, 2000, Toronto, Canada. 13. Wolcott, M.P. and Englund, K., A Technology Review for Wood–Plastic Composites, Proceedings of the 33rd International Symposium on Particleboards/ Composite Materials, Washington State University, Pullman, WA, 1999, pp. 103–111. 14. Principia Partners, Speciality Additives in Natural/Wood Composites, Progress in Woodfiber–Plastic Composites Conference, May 23–24, 2002, Toronto, Canada. 15. Information provided, Courtesy of Hemcore Ltd., Latchmore Bank, Little Hallingbury, Bishop’s Stortford, Hertfordshire, CM22 7PJ, UK, www.hemcore.com. 16. Hemcore Company Flyer, Hemcore Ltd., Latchmore Bank, Little Hallingbury, Bishop’s Strortford, Hertfordshire CM22 7PJ, UK. 17. Marsh, G., Next step for automotive materials, Mater. Today, Apr. 2003, pp. 36–43. 18. Thomas, S., Effect of Chemical Modification on the Properties of Natural Fiber Reinforced Composites, Proceedings of the 10th European Conference on Composite Materials (ECCM-10), 3–7 June, 2002, Brugge, Belgium. 19. Riedel, U., Nickel, J., and Herrmann, A., High Performance Applications of Plant Fibers in Aerospace and Related Industries, Natural Fibers Performance Forum, May 27–28, 1999, Denmark, Copenhagen.

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20. Karus, M., Kaup, M., and Lohmeyer, D., Study on Markets and Prices for Natural Fibres (Germany and EU), Nova Institute Report Project No. 99NR163, March 2000. 21. Bledzki, A.K., Sperber, V.E., and Faruk, O., Natural and Wood Fiber Reinforcement in Polymers, Rapra Review Report No. 152, Vol. 13, No. 8, 2002. 22. Netravali, A.N. and Chabba, S. Composites get greener, Mater. Today, Apr. 2003, pp. 22–29. 23. Ellison, J., The Use of Natural Fiber Composites in the European Automotive Industry, Proceedings of the Natural Fibers for the Automotive Industry Conference, published by Actin, UMIST, Manchester, UK, Nov. 28, 2000. 24. Ellison, G.C. and McNaught, R., The Use of Natural Fibres in Non-Woven Structures for Applications as Automotive Component Substrates, UK Government MAFF Report Reference NF0309, February 2000. 25. Flodin, P. and Zadorecki, P., Composite Systems from Natural and Synthetic Polymers, Elsevier, Amsterdam, 1986, p. 59. 26. Mohanty, A.K., Sustainable Green Composite Materials from Renewable Resources: The Present Status and Future Perspectives, Proceedings of the Global Outlook for Natural Fiber and Advanced Wood Composites 2001, Sheraton World Resort, Orlando, FL, 3–5 Dec., 2001. 27. Bledzki, A.K. and Gassan, J., Composites reinforced with cellulose based fibers, Prog. Polym. Sci., 24, 221–274, 1999. 28. Kessler, R.W. and Kohler, R., New strategies for exploiting flax and hemp, Chemtech, 34–42, Dec. 1996. 29. Brouwer, W.D., Natural fiber composites: where can flax compete with glass? SAMPE J., 36, 18–23, 2000. 30. ‘Green’ door-trim panels use PP and natural fibers, Plast. Technol., 46, 27, 2000. 31. Taylor, A., Case Study on Fibers in Composite Materials, e.g. Hemp in Automotive Applications, 4th Meeting of Government-Industry Forum on NonFood Uses of Crops, GIFNFC 4/4 Fibres in Composite Materials, Jan. 22, 2002, DTI Conference Centre, London. 32. Automotive Industries, DaimlerChrysler “Goes Natural” for Large Body Panel, Aug. 2000, p. 9. 33. Hill, S., Cars that grow on trees, New Scientist, Feb. 1997, pp. 36–39. 34. Department for Environment, Food and Rural Affairs, Annual Report of the Government-Industry Forum on Non-Food Uses of Crops, DEFRA publications, Aug. 2002. 35. Eichorn, S.J., Baillie, C., Zafeiropoulos, N., Mwaikambo, L.Y., Ansell, M.P., Dufresne, A., Entwistle, K.M., Herrera-Franco, P.J., Escamilla, G.C., Groom, L., Hughes, M., Hill, C., Rials, T.G., and Wild, P. M., Review–current international research into cellulosic fibers and composites, J. Mater. Sci. 36, 2107–2131, 2001. 36. Mohanty, A.K., Sustainable Green Composite Materials from Renewable Resources: The Present Status and Future Perspectives, Proceedings of the Global Outlook for Natural Fiber and Advanced Wood Composites 2001, Sheraton World Resort, Orlando, FL, 3–5 Dec., 2001. 37. Saheb, D.N. and Jog, J.P., Natural fibre polymer composites: a review, Adv. Polym. Technol., 18, 351–363, 1999. 38. Van de Weyenberg, I. and Verpoest, I., Polyester Composites with Flax Knit Reinforcement, Proceedings of the 10th European Conference on Composite Materials (ECCM-10), June 3–7, 2002, Brugge, Belgium.

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39. Rowell, R.M., The State of Art and Future Development of Bio-based Composite Science and Technology towards the 21st Century, Proceedings of the 4th Pacific Rim Bio-based Composites Symposium, Indonesia, 1998. 40. Bernal, C., Cabral, H., and Vazquez, A., Mechanical Properties of Jute-PP Composites, Proceedings of the 10th European Conference on Composite Materials (ECCM-10), June 3–7, 2002, Brugge, Belgium. 41. Peijs, T., Composites for recyclability, Mater. Today, Apr. 2003, pp. 30–35. 42. Kenning, D. (Splendid Engineering Ltd.), The Splendid EcoCar Project, Presentation at SusCompNet 4, Jan. 22, 2003, Queen Mary and Westfield College, London. 43. Brouwer, R., Natural Fiber Composites–State of the Art, Joint Meeting of the 32nd Session of the Intergovernmental Group on Hard Fibers and the 34th Session of the Intergovernmental Group on Jute, Kenaf and Allied Fibers, July 8–11, 2003, Salvador, Brazil. 44. Wool, R.P. et al., Composites from Soybeans, Proceedings of the 10th European Conference on Composite Materials (ECCM-10), June 3–7, 2002, Brugge, Belgium. 45. Rowell, R.M., Han, J.S., and Rowell, J.S., Characterization and Factors Effecting Fibre Properties, Natural Polymers and Agrofibres Composites, Frollini, E., Leao, A., and Mattoso, L.H., Eds., Sao Carlos, Brazil, 2000.

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8 Natural Fiber Composites for Building Applications

Brajeshwar Singh and Manorama Gupta

CONTENTS 8.1 Introduction 8.2 Natural Fiber Composites in Buildings—An Experience 8.3 Natural Fibers 8.4 Surface Modification of Natural Fibers 8.4.1 Fiber-Coupling Agent Interaction 8.4.2 Modified Fiber–Resin Interaction 8.4.3 Chemically Treated Fibers in Polymer Composites 8.5 Properties of Natural Fiber Composites 8.6 Weathering Studies 8.7 Natural Fiber-Based Building Materials 8.7.1 Laminates/Panels 8.7.2 Door Shutters 8.7.3 Jute Pultruded Door Frames 8.7.4 Roofing Sheets 8.7.5 Composite Shuttering Plates 8.7.6 Dough Molding Compounds 8.8 Concluding Remarks Acknowledgments References ABSTRACT Natural fiber composites are attracting more attention as alternate building materials, especially as wood substitutes in the developing countries. With the advent of surface modification, moisture-resistant natural fibers are utilized in low-pressure molding to overcome their well-defined problem of moisture absorption. As a result, dimensionally stable panel products, profiles, sheets, and molded items have been prepared alternatives to

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boards/plywood. In this chapter, the developmental work carried out on natural fiber-based building materials is presented. Chemical modification of natural fibers and their role in unsaturated polyester resin in terms of aspect ratio, volume fraction, number of fiber plies, and glass/natural fiber combination is detailed. Behavioral performance of these composites is also reported under various humidity, alternate wetting and drying cycles, and weathering conditions. A comparison of the test results on these natural fiber-based products with the requirements mentioned in the relevant standard specifications gives a clear guideline for users in large-scale adoption in practice.

8.1

Introduction

Diversification of natural fibers from their traditional markets to other potential sectors (building and furniture) has gained interest in developing countries to meet the pressing demands of conventional materials, especially wood substitutes.1–11 One of the promising applications of natural fibers is in polymer composites that can be molded into a variety of flat and complexshaped components by exploiting their attractive reinforcing potential. The incentive for producing these natural fiber-based components originates from their low cost, light weight, and oriented employment compared to synthetic fiber-based products. As a result, various items such as school buildings,3 food grain silos,3 prototypes of low-cost housing units,3,4 roofings,2 wood substitutes,4,5 and pipes17 have been made. Recently, research is being directed at producing low-cost composites by use of natural fiberthermoplastic blends with the objective of substituting mineral fillers with chopped fibers in plastics.12,13 This area also encompasses a new means of effective utilization of natural fibers in packaging to replace wooden crates used for this purpose. For ensuring proven building materials, work has been extended by focusing on the hydrophobic fiber-matrix interface, improved dimensional stability against moisture attack, and nonvulnerability toward other environmental agents.7,14–18 The physicomechanical properties of the resulting composites have been reported with mixed results, thus affecting their wider acceptability. This situation necessitates the generation of sufficient performance data, including weatherability, of natural fiber composites prior to use as building components.

8.2

Natural Fiber Composites in Buildings—An Experience

Various attempts have been made recently to rationally utilize abundantly available natural fibers such as jute, sisal, coir, palm, bagasse, and wood fibers in polymer matrices like polyester, epoxy, and phenolics, to be used as building materials for an assortment of applications. Notable contributions in this field Copyright © 2005 by Taylor & Francis

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are the construction of inexpensive primary school buildings using jute fiberreinforced polyester in Bangladesh (1972–1973) under the auspices of the Cooperative of American Relief Everywhere (CARE) and United Nations Industrial Development Organization (UNIDO).3 Subsequent efforts toward the development of a prototype for low-cost housing units and food grain silos in India (1976–1977) demonstrate the use of appropriate technology in developing countries. In the 1980s, building panels and roofing sheets made from bagasse/phenolics were installed in houses in Jamaica, Ghana, and the Philippines.19 In another program, developmental work on low-cost building materials based on henequen/palm/sisal fibers and unsaturated polyester resin has been undertaken as a cooperative research project between the Government of Mexico and UNIDO for appropriate utilization of natural resources.20 In the 1990s, United Nations Development Programme (UNDP), in association with the Government of India, supported a program to develop jute-based composites and molded products as wood substitutes in packaging and building sectors.21 Attempts to prepare wall panels and roofing sheets using jute/polyester/epoxy/polyurethane resin for temporary shelters, bunker houses, storage silos, post office boxes, helmets, and roofing sheets made from coir/polyester have drawn considerable attention.22 It is widely known that these composites failed in wet conditions either through surface roughening caused by fiber swelling or delamination, possibly owing to crack propagation between plies. This has resulted in an effort to develop water-resistant natural fiber composites for making dimensionally stable building products. Use of natural fibers as reinforcement in a cement matrix has also been practiced for making low-cost building materials such as panels, claddings, roofing sheets and tiles, slabs, and beams. Sisal and coir are two of the most studied fibers, but bamboo, jute, hemp, reeds, and grasses have also been studied for making fiber concrete and sheeting materials.23 The Swedish Cement and Concrete Institute has been involved with sisal products for many years, especially with respect to their durability. Field experience has also been gained by installing sisal fiber-based roof tiles in South Africa.24 Appropriate Technology International (ATI) and Save for Child Federation (SCF) have actively promoted fiber-cement roofing tiles since the 1970s. Thousands of tiles have been made in countries such as Kenya, Sri Lanka, and in Latin America using indigenous plant fibers as reinforcements. Based on their findings, both ATI and SCF have concluded that, as currently manufactured, such fiber-reinforced tiles are unsuitable for rural housing. In 1987, the International Labor Organization (ILO) supported some craft industries for the production of fiber-cement roofing tiles in Ivory Coast.25 Studies indicate that the quality of tiles produced by these manufacturers is generally poor because of inadequate quality control during production. Commonwealth Scientific & Industrial Research Organization (CSIRO), Australia adopted an autoclave process using wood fibers as a reinforcement for making asbestosfree cement products in the 1980s.26 A key concern with the use of these natural fibers in cement relates to their long-term durability due to weakening of the link between individual fiber cells because of alkali attack and moisture sensitivity, particularly under severe exposure conditions. Efforts are being Copyright © 2005 by Taylor & Francis

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made to solve these problems by pretreatments of fibers and also by using a low alkaline cement binder.

8.3

Natural Fibers

Natural fibers are generally lignocellulosic in nature, consisting of helically wound cellulose microfibrils in a matrix of lignin and hemicellulose. According to a Food and Agricultural Organization survey, Tanzania and Brazil produce the largest amount of sisal. Henequen is grown in Mexico. Abaca and hemp are grown in the Philippines. The largest producers of jute are India, China, and Bangladesh. Presently, the annual production of natural fibers in India is about 6 million t as compared to worldwide production of about 25 million t. The typical diameter of individual plant fibers is in the range of 15–35 µm, and their aspect ratio ranges from 100 to 200, except for hemp (550), ramie (4000), and cotton (2000). The strength of these fibers varies from fiber to fiber depending on their origin and physicochemical properties.23,27 The main parameters influencing the fiber properties are crystalline cellulose content, microfibrillar angle, cell number, and cell aspect ratio. It is reported that higher cellulosic content and lower microfibrillar angle are essential for high fiber strength. This fact is clearly supported microscopically as shown in Figure 8.1. The low strength of coir, bhabar, and munj is attributed mainly to the presence of cells in large numbers with low aspect ratio, whereas lesser cells with high aspect ratio and large lumen size, as exhibited by sisal and sun hemp, support their high strength. Besides cellulose content and microfibrillar angle, cell aspect ratio also can be considered as one of the parameters to establish structure-property relationships in natural fibers.28 One of the major problems associated with the use of natural fibers in composites is their high moisture sensitivity, leading to severe reduction of mechanical properties and delamination. The reduction in mechanical properties may be due to poor interfacial bonding between resin matrices and fibers. It is therefore necessary to modify the fiber surface to render it more hydrophobic and also more compatible with resin matrices.

8.4

Surface Modification of Natural Fibers

Interface interactions have been widely recognized for their important role in improving the hygrothermal stability and mechanical properties of natural fiber-reinforced polymer composites. Several methods15,17,20,29–43 have been reported to modify the surface of natural fibers, not only to reduce moisture absorption, but also to offer an opportunity to improve interfacial adhesion between the fiber and the matrix. Pretreatment of these natural Copyright © 2005 by Taylor & Francis

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FIGURE 8.1 SEM of natural fibers showing internal structures. (a) Coir. (b) Sunhemp. (c) Bhabar. (d) Munj. (e) Sisal. (From Bhal, N.S. and Singh, B., Proceedings of the 2nd International Conference on Composites in Infrastructure, Arizona, USA, Jan. 5–7, 1998, p. 661. With permission.)

fibers by vinyl polymer grafting/coating,17,20,29,30,32–34,36,42,43 alkali treatment,33,38,39,41 and coupling agents15,31,34,35,37,40 provide improved physicomechanical properties to the resulting composites. Mercerization of fibers prior to silane treatment is another effective means of achieving better properties retention of composites exposed to moisture.31 Choice of surface treatment depends on its specificity for a particular resin. The potential advantage of using the coupling agent method is the natural attraction of Copyright © 2005 by Taylor & Francis

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both the fiber and the resin matrix during curing. The alkoxy groups of coupling agents react with the surface functional groups of fibers to form covalent bonds (fiber–O–M; M
central metal atom), while the organofunctional portion interacts with the polymer matrices by a chemical reaction following the chemical bonding/interpenetrating networks theory.44 The effectiveness of these coupling agents in composites is known to be influenced by their physical state on the surface of fibers and fiber surface characteristics themselves. Undercured coupling agent layers may diffuse so far into the resin matrix that they are lost to the potential cross-linking site at the interface. Under wet conditions, the most acute problem is the moderate stability of coupling agent–fiber adhesion to hydrolysis. This situation justifies the necessity of developing hygrothermally stable bonds by properly adjusting the treating conditions and by selecting the specificity of a particular coupling agent.

8.4.1

Fiber-Coupling Agent Interaction

The efficacy evaluation of various coupling agents, such as organosilane (gamma-methacryloxypropyltrimethoxysilane), titanate (neopentyl (diallyl) oxy, tri (dioctyl) pyro-phosphatotitanate), zirconate (neopentyl (diallyl) oxi, triacrylzirconate), and N-substituted methacrylamide (methacrylamide functional amine adduct of pyro-phosphatotitanate) on the surface modification of jute and sisal fibers has been studied.45,46 It has been found that a high contact angle and reduced hydroxyl groups on titanate-treated fibers favor their improved hydrophobicity over other treatments. The extent of moisture/ water absorption has also been reduced significantly. The surface free energy of untreated and silane-treated fibers is almost similar (Table 8.1). On the other hand, lowering of the surface free energy has been observed when the fiber surface is modified with titanate, zirconate, and N-substituted methacrylamide. These modifications change the surface property of fibers from their initial inorganic to organic nature. The reason for enhanced interaction between sisal fiber and N-substituted methacrylamide is suggested by the

TABLE 8.1 Surface Energetic Properties of Untreated and Surface-Treated Sisal Fibers Treatment Untreated Silane-treated Zirconate-treated Titanate-treated N-substituted methacrylamide-treated

Surface-Free Energy (mJ m2)

Polar Component (mJ m2)

Dispersive Component (mJ m2)

48.30 48.60 39.50 35.10 40.90

25.90 25.30 14.30 13.80 10.50

22.40 23.30 25.20 21.30 30.40

Source: From Singh, B. et al., J. Appl. Polym. Sci., 70, 1847, 1998. With permission from John Wiley & Sons, Hoboken, NJ.

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formation of a hydrogen bond, besides extracting a surface-active proton from the fiber surface by its alkoxy group to form a covalent bond. An optimum treating condition, such as pH ~9, concentration ~1 wt% of fibers, treatment time 30 min, and drying time 1 h at 110°C, of sisal/jute fibers has been reported for an effective adsorptive interaction.45 The deposition of compound in the form of an aggregate onto fiber surface has also been observed due mainly to the formation of intramolecular hydrogen bonding between the alkoxy and amide group of the attached coupling agent.

8.4.2

Modified Fiber–Resin Interaction

The concept of improving fiber-matrix adhesion has been used for developing water-resistant natural fiber composites.47–50 The occurrence of fiber breakage and broken end sites on the fracture surfaces of treated fiber composites implies that the fiber-matrix interface is intact [Figure 8.2(a)]. On the other hand, the fracture surfaces of untreated fiber composites shown in Figure 8.2(b) have indicated a sign of interface failure with a darkened region extending down the side of the fiber. The enhanced adhesion in sisal/polyester composites is attributed to attachment of methacrylamide organofunctionality onto the fiber surface [Figure 8.3(a)].47 This seems to provide good wetting to the fibers during application. The presence of bulky organic groups on the nitrogen atom further contributes solution compatibility with the polyester resin. It is expected that the vinyl group of methacrylamide functionality attached to the fiber surface can coreact with styrene present in the polyester resin to form styrenated methacrylamidemodified fibers, as reported by Ishida and Koenig,51 for reaction between the styrene and vinyl groups of methacryloxypropyl trimethoxysilane. This fiber, in turn, reacts with polyester resin via the styrene attachment following the conventional route [Figure 8.3(b)]. Thus, a stable bond is formed, enabling enhanced fiber-matrix adhesion. Additionally, the possibility of bonding between methacryl functional attached fibers and unsaturation of polyester resin also exists via an additional reaction [Figure 8.3(c)].

8.4.3

Chemically Treated Fibers in Polymer Composites

The effect of adsorbed coupling agents on the mechanical properties of sisal/polyester composites is given in Table 8.2. An improvement of ~15–33% in tensile strength, ~45–79% in tensile modulus, and ~21–29% in both flexural strength and flexural modulus has been observed with respect to untreated sisal composites. About a 38% increase in area under the stress-strain curve for treated composites has also been observed over untreated ones, resulting from the absorption of energy through fiber breakage and matrix cracking.52 The effect of humidity on the tensile strength of sisal/polyester composites is shown in Figure 8.4.52 About 30–44% reduction in strength was observed for both untreated and surface-treated sisal composites. However, the strength Copyright © 2005 by Taylor & Francis

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FIGURE 8.2 SEM of fracture surfaces of sisal/polyester composite showing (a) fiber fracture/broken end sites, and (b) interfacial failure (debonding). (From Gupta, M. et al., Curr. Sci., 74, 526, 1998. With permission.)

retention in the case of treated composites was considerable due to the formation of hydrophobic interphase that prevents wicking action of water. The SEM image of a pulled out surface of untreated fibers embedded in a polyester matrix shows weak, swollen, and separated fibrils [Figure 8.5(a)]. On the other hand, pulled out surfaces of treated sisal in composites exhibit comparatively less swelling due to the tightly bonded coupling agent coating retained on the fibers [Figure 8.5(b)]. At 95% RH (relative humidity), all surface-treated samples except organosilane show a 40–62% improvement in wet tensile strength compared with laminates reinforced with untreated fibers. In contrast, Copyright © 2005 by Taylor & Francis

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O Fiber

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H O

Ti(OR1)3. nR22NR3N

O

CH3

C

C

CH2

H N-substituted methacrylamide modified sisal fiber

(a) O

H

Fiber

O

Ti(OR1)3. nR22NR3N

O

CH3

C

C

CH2 CH-C6H5

H

CH2 O

O

O

O

O O

O O

O

O

O

n

Interaction of methacryl functional attached fiber with polyester resin through styrene

(b)

O Fibre

H O

Ti(OR1)3. nR22NR3N

O

CH3

C

C

CH2

H O

O

O

O

O O

O O

O

O

n

O

Interaction of methacryl functional attached fiber with polyester resin

(c)

Polyester resin

Coupling agent controlled interphase

Sisal fiber

FIGURE 8.3 Schematic representation of sisal fiber-polyester resin interaction. (From Gupta, M. et al., Curr. Sci., 74, 526, 1998. With permission.)

at 95% RH/50°C, only a 35–37% improvement was observed over the control wet samples. This is probably because of bond weakness across the interface under the combined action of moisture and temperature.53 When immersed in water, the strength of the composites was ~13–31% lower than these composites exposed at 95% RH. It is noted that the silane coupling agent has a detrimental effect on the tensile strength (~11%) compared with an untreated sisal composite. Fibers treated with N-substituted methacrylamide displayed superior behavior under this condition, as previously mentioned. Copyright © 2005 by Taylor & Francis

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TABLE 8.2 Mechanical Properties of Untreated and N-Substituted MethacrylamideTreated Sisal Fiber-Polyester Compositesa Property Tensile strength (MPa) (a) Fresh (b) Exposed 95% RH 95% RH/50°C Immersed in water Tensile modulus (GPa) Energy to break (MJm )  10 2

5

Flexural strength (MPa) (a) Fresh (b) Exposed 95% RH 95% RH/50°C Immersed in water Flexural modulus (GPa)

Untreated Composites

Treated Composites

% Improvement

29.66

39.48

33.10

17.32 22.63 14.99

28.00 31.10 23.91

61.66 37.43 59.51

1.15

2.06

79.13

7.96

11.06

38.94

59.57

76.75

28.84

29.42 26.15 20.52 11.94

33.97 32.00 29.62 15.35

15.46 22.37 44.34 28.55

a

Exposure in wet conditions ~60 days. Source: From Gupta, M. et al., Curr. Sci., 74, 526, 1998. With permission.

Dry 95%RH

Zirconate treated

Titanate treated

10

Silane treated

20

Untreated

Tensile strength (Mpa)

30

N-substituted methacrylamide treated

95%RH / 50°C Immersion in water

40

0 FIGURE 8.4 Effect of wet environments on tensile strength of sisal/polyester composites (fiber content ~50 wt%) for 35 days. (From Singh, B. et al., Polym. Compos., 17, 910, 1996. With permission.)

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FIGURE 8.5 (a) SEM of tensile fracture surfaces of untreated sisal/polyester composite exposed at 95% RH showing swelling and separation of fibrils. (b) SEM of tensile fracture surfaces of N-substituted methacrylamide-treated sisal fiber composite showing intact fibrils after exposure at 95% RH. (From Singh, B. et al., Polym. Compos., 17, 910, 1996. With permission.)

8.5

Properties of Natural Fiber Composites

The mechanical behavior of fiber-reinforced composites results from a complex interplay of the properties of the constituent phases such as resin, fiber, Copyright © 2005 by Taylor & Francis

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and the interfacial region. The principal relevant parameters are the following: the volume fraction of fibers, the fiber aspect ratio, the modulus and strength of the fibers, the fiber-matrix adhesion, and the toughness of the matrix. Slight variation of some of these parameters yields composites with poor properties.1,35,54–56 Hybridization with glass fibers provides a method to improve the mechanical properties of natural fiber composites, and its effect in different modes of stress depends on the design and construction of the composite.7,8,15,54,57 Hybrid composites have been prepared with the help of 20–25% filler, 30–35% sisal fibers, 8–10% glass fibers, and unsaturated polyester resin. The physicomechanical properties of these composites are given in Table 8.3. It has been found that the tensile and flexural properties of sisal/glass hybrid and sisal/glass/filler particulate hybrid composites increased due to the inherent better properties of glass fibers as compared to parent sisal/polyester composites. Hybrid composites containing a small amount of glass fibers fail by rupture of the sisal fibers followed by failure of the glass fibers. An improvement of ~15% in flexural strength for particulate hybrid composites has been observed over that of sisal/polyester composites. It is noted that the properties of particulate hybrid composite are inferior to those of glass fiberreinforced plastics.54 Water absorption tests indicate that laminates containing a substantial number of voids absorb water more readily than those which do not. It is obvious that at high fiber loading, the water uptake will be significant. The dimensions (swelling in thickness) of the composites also changed during TABLE 8.3 Comparative Physicomechanical Properties of USP and Compositesa Reinforcement Content

Material USP RM/USP SRP S/RM/USP GRP (Wt%28–32) S/GRP (G.cont-15%) S/G/RM/USP (G.cont-8%)

Red Sisal Water Swelling in Tensile Mod. of Flexural mud fiber Density Absorption Thickness Strength Elasticity Strength (wt%) (wt%) (gcm3) 24 h (%) 24 h (%) (MPa) (GPa) (MPa) — 20 — 20 —

— — 50 38 —

1.12 1.31 1.05 1.09 1.60

0.13 0.20 3.50 4.71 1.03

Negligible Negligible 5.0 0.58 —

26.40 17.30 40.0 23.50 163.0

1.91 2.38 2.13 0.64 26.0

61.26 64.12 85.03 40.02 362.0

—

46

1.20

4.12

3.03

65.20

2.60

98.0

20

34

1.24

3.65

Negligible

45.20

3.25

98.10

USP
unsaturated polyester; RM/USP
red mud/polyester; SRP
sisal-reinforced polyester; S/RM/USP
sisal/red mud/polyester; GRP
glass-reinforced polyester; S/GRP
sisal/ glass-reinforced polyester; S/G/RM/USP
sisal/glass/red mud/polyester. Source: From Singh, B. et al., Constr. Build. Mater., 9, 39, 1995. With permission from Elsevier.

a

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immersion in water. Maximum swelling has been observed in laminates containing ~74 wt% fiber content. The tensile strengths of sisal/polyester, hybrid, and particulate hybrid composites were evaluated before and after 2 h in boiling water immersion. The reduction in strength (~11–38%) was observed due to the combined effect of water and temperature, and also the plastification of the resin. Partial separation of fiber plies from each other was noticed due to the hydrolysis of the polyester resin and also swelling of fibers. The reduction in mechanical properties under immersed water conditions could be avoided by the development of stable covalent bonds between fibers and resin matrices and also by the application of resin-rich coating on both sides of the samples.

8.6

Weathering Studies

One early effect of weathering noted is the appearance of a milky color on the surface of composites.18,58 The color fading continues as the exposure proceeds in both natural (Roorkee-India) and accelerated UV weathering. The deterioration of natural fiber composites is initiated by fiber ridging followed by resin film rupture through cracking, and then the fiber pops out (Figure 8.6). At the end of 2 yr of natural exposure, a few black spots at the edge were visually observed, showing a large number of long rounded hyphae-like structures. Under accelerated weathering, fiber blooming was pronounced on the front surface after 750 h of exposure, while the rear side of exposed samples had a large number of black spots as similarly observed under humid conditions. Fibrillation of the jute was also noticed, indicating degradation of lignin, which acts as an adhesive for holding the cellulose fibrils together. The tensile strength dropped by 50% in 2 yr of outdoor and ~47% in 750 h of accelerated weathering exposure. The reduction in flexural strength was more pronounced in samples subjected to accelerated exposure than those simply left outdoors. A comparison of the weather-induced degradation of jute composites and glass fiber composites has been made. The tensile and flexural strength of jute composites decreased by approximately half (~50%), although weight loss was limited to 2–3%. On the other hand, surface deterioration in weathered glass fiber-reinforced phenolic was insignificant. The loss of tensile and flexural strength was in the order of ~5–15%. The weight loss of weathered glass composites was comparatively small (0.32%). In order to prevent weather-induced degradation, a polyurethane (PU) coat was applied on the exterior surface of samples. It was observed that PU-coated samples had very little surface deterioration in terms of color changes and fiber blooming. Improved understanding about the development of hydrophobic matrix interphase and UV-stabilized resin-rich outer layers is needed prior to the use of natural fiber composites in outdoor environments. Copyright © 2005 by Taylor & Francis

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FIGURE 8.6 Weathering exposure of jute composites (resin content ~30 vol%). (a) Fresh. (b) Two years outdoors. (c) Rear side of exposed surface showing black spots. (From Singh, B. et al., Compos. Sci. Technol., 60, 581, 2000. With permission.)

8.7

Natural Fiber-Based Building Materials

The building industry is under tremendous pressure and unable to cope with the new demands from different construction subsectors. For example, wood is in very short supply and good-quality, seasoned timber is not readily available. India’s share in wood production is ~2% of world production (24 million m3), which implies a per capita output in India of 0.025 m3 as compared to 1.89 m3 in the United States. Anticipating the need to combat wood scarcity, substitute materials are continuously being looked into and developed. The existing wood substitute materials such as reconstituted panel products (particle/fiberboards), plywood, and other materials cannot meet the increasing demand for wood without new efforts. One strong Copyright © 2005 by Taylor & Francis

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option is to promote the development and utilization of composite materials based on natural fibers. This type of industrial composite can provide suitable alternatives, particularly in the face of universal concern for ecological degradation caused by continuing dependence on depleting forest resources. Various natural fiber-based composite products such as laminates, panels, partitions, doorframes and shutters, roofings, and concrete shutterings have been produced. Manufacturing of single-layered natural fiberbased panels, alternatives to plywood, has failed to possess desired properties as building materials due to their inadequate specific strength, stiffness, and dimensional stability. Attention is being given to standardize these products to acceptable levels that conform to existing specifications. The salient features of these products are outlined here. 8.7.1

Laminates/Panels

Composite laminates/panels can be prepared using nonwoven/woven sisal/jute/coir mats and unsaturated polyester/phenolic/polyurethane resin by a compression molding technique.59–62 The outer surface of laminates/ panels can be gel coated and reinforced with glass fiber surfacing mats to obtain smooth, glossy, and colorful products. The properties of these panels are given in Table 8.4. The panels meet the requirements in IS:12406-88 (Indian standard specification for medium-density fiberboard for general purpose use). In order to reduce overall weight and also have favorable costs, sandwich panels can be made from jute/sisal fiber laminates as the face and corrugated sheets as the core. These panels are manufactured by the wet layup molding technique in one step and also by bonding the face and core with adhesive in a second step. Lightweight panels can also be prepared from jute stick in cylindrical form as a honeycomb core and jute/glass lamination as face sheets. These panels are comparable with the alternatives available in the market in terms of cost and performance. In order to reduce the manufacturing cycle, the corrugated/honeycomb core was replaced by in situ foaming based on polyester-urethane hybrid resin in sandwich panels.63 The rigidity of foam was further improved by a particle strengthening process. TABLE 8.4 Physicomechanical Properties of Natural Fiber/Polyester Composite Laminates Property

Sisal/Polyester (Resin ~50 wt%)

Jute/Polyester (Resin ~70 wt%)

Density (g/cm3) Water absorption, 24 hr(%) Swelling thickness (%) Tensile strength (MPa) Elongation (%) Tensile modulus (GPa) Flexural strength (MPa)

1.05 3–4 5 40 4–6 2.13 77

1.22 1.09 — 66.01 2.31 4.42 93.80

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Coir/Polyester (Resin ~70 wt%) 1.4 3–4 5–6 20.40 0.4–1.0 1.2–2.0 41.54

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The panels are lightweight, and possess good bending stiffness and excellent thermal and sound insulation (Figure 8.7). They can be used as alternatives to single-walled pressed boards for door panels and walling. In semistructural

FIGURE 8.7 Laminates/panels in (a) laboratory scale, and (b) commercial scale.

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uses, hybrid composites have been prepared from glass fibers, sisal/jute fibers, and polyester resin. The tensile strength and elastic modulus of hybrid composites were ~56 MPa and ~2 Gpa, respectively. These plates can be used as an alternative to plywood for concrete shuttering work.

8.7.2

Door Shutters

Door shutters made from materials like wood-based panel products are required to have certain performance attributes. These include resistance to impact and slamming, shape stability against operational loads (latching and locking), torsional and diagonal deformation performance (misuse), wind pressure and suction forces, and durability. Currently, existing door shutters made from agrofibers and plantation timber do not have adequate moisture resistance and screw- and nail-holding properties. Their flexural strength is also limited except for plywood. Acceptability of these substitute door shutters requires new efforts to meet the required performance levels. Composite doors can be made by bonding the jute/sisal laminate face sheets with rigid foam cores.63 In this work, the core made from polyesterpolyurethane hybrid resin possesses adequate screw-holding and nailing properties comparable to wood. This helps to avoid additional wood insertion for edge lipping/framing. The density of rigid core foam was ~ 0.4 to 0.6 g/cm3 and its thermal conductivity (K value) was ~ 0.12 kcal/h °Cm for a 32 mm thick slab. During localized thermo-mechanical analysis, absence of well-defined obvious transitions is conclusive to a fully cured rigid foam. The compressive strength of core was ~ 14 MPa in the direction of foam rise and ~ 12 MPa across the rise. The internal bond strength of the core was ~ 4–6 MPa. Its screw-holding property was ~ 800–1200 N across the rise of the foam and ~ 500–900 N parallel to the rise of foam. The natural fiber composite sheet face can be sawn, nailed, and screwed and is moisture resistant. For door construction, an in situ core slab can be cast on one of the face panels containing the spacer of a desired thickness without inserting a wooden frame. Lipping and rebating can be done in the cellular core without any difficulty. The face laminates can be glued to both faces of the core under pressure at the time of stage B resin curing. Additional heat-resistant adhesive was omitted during the construction process. As mentioned, the prepared doors satisfy the requirements as mentioned in IS: 4020-1998 (Indian standard specification, door shutters, methods of tests) for slamming, shock resistance, impact indentation, edge loading, misuse, etc. (Table 8.5 and Figure 8.8). The thermal conductivity (U value) of the door panel was ~0.18 kcal/h °Cm2 and its screw withdrawal strength was ~1500 N, compared to the typically specified value of 1000 N for wood. For the end immersion test, the door was dimensionally stable and the bonding between the face sheets and the core was intact even on applying a knife for delamination purposes. The weight of the door was ~12–14 kg/m2. Fixtures such as handles, locks, and hinges Copyright © 2005 by Taylor & Francis

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TABLE 8.5 Properties of Jute/Sisal Composite Doors Property

Wooden Door (IS: 4020-98)a

Local planeness test Edge loading test Impact indentation test Shock resistance test Slamming test End immersion test Glue adhesion test Knife test Screw with drawl resistance test Misuse test

 0.50 mm Deflection  5 mm  0.2 mm (No abnormal defect) No visible damage No visible damage No delamination Pass the test No delamination  1000 N Pass the test

a

Composite Door 0.02–0.1 Negligible 0.05–0.1 (Pass) Pass Pass Pass Pass Pass 1500 N Pass

Indian standard specification, door shutters, methods of tests.

can be fixed in the finished door shutters. The cost of the composite door (32 mm thickness) was comparable to conventional ones. Jute composite doors can also be prepared using jute stick blocks as a core and jute/polyester laminates as a face sheet. The slices of chemically bonded jute sticks can be placed suitably and fixed with an adhesive. The lamination on these blocks can be carried out by two layers of polyester-impregnated jute fabrics. Glass surfacing mats with pigmented gel coat were used as the outermost layer. Wooden blocks were inserted in the locations for fixing hinges and handles. Extra care was taken to seal the edges to make it water resistant. The door panels tested under slamming, shock, and edge bending have shown encouraging results.64 Coir composites can also be used for the manufacture of flush door shutters using coir/phenolic, or resin-coated rubber timber, as core and a random jute/phenolic layer as facing material. The core blocks can be joined with corrugated nails in the shutter cavity. The frame assembly can be made with coir-laminated veneer lumber containing a rubber timber core. The performance of these door shutters was reported to be satisfactory.65

8.7.3

Jute Pultruded Door Frames

Composite profiles can be made from jute hessian cloth and phenolic resin by the pultrusion technique. Jute mat was impregnated with resin and passed through a hot die at a speed of 0.4–1 m/min. The cured profiles contain 50–60% jute fibers. The sections can be assembled by meter joints fortified with a reinforced adhesive pack to form doorframes of various sizes accommodating 30–35 mm thick door shutters. Three holdfasts can be fixed on each side of the frame, one at the center, and the other two at 20 cm from the top and bottom of the frame. The physicomechanical properties of profiles used in frame construction were evaluated under humidity levels, hydrothermal aging, and weathering Copyright © 2005 by Taylor & Francis

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FIGURE 8.8 Jute/sisal doors. (a) Prototype of door and panels. (b) Composite door shutters.

conditions (Roorkee-India). The density of the profiles was ~ 0.87 g/cm3 and its moisture content was ~4.4%. At higher humidity levels, the weight gain in the profiles was in the order of ~10–12%, whereas in an immersed water condition, the change in weight was ~20% after 60 days of exposure. Tensile and flexural strength decreased by ~13% and ~36.14% at 95% RH/50°C and Copyright © 2005 by Taylor & Francis

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~24.29% and ~52.68% in an immersed water condition, respectively. Under hydrothermal aging, the reduction in tensile strength was more pronounced in cyclic exposures, whereas reduced flexural strength was noticed in accelerated aging. The internal bond strength of profiles was also reduced in all aging conditions. The profiles exhibited an initial increase in tensile strength with respect to fresh samples after 30 days of natural exposure and 100 h of an accelerated UV environment. Extending the exposure period, flexural strength dropped in the order of ~22% in 6 months natural and ~19.40% in 500 h accelerated UV exposure, respectively. Exposed surfaces after 5 yr of weathering outdoors show resin erosion, fiber mat ridges, and cracking. Comparative evaluation of pultruded jute frames and wooden doorframes (IS 4021-95, Indian standard specification for timber door, window, and ventilator frames) was also made (Table 8.6). It has been observed that the moisture content of jute profiles was nearly half that of the wooden frame materials. The profiles do not require any kind of seasoning or preservative treatments, as do wooden frames, prior to assembling. These frames were installed in the brick works as similar to wooden doorframes to assess their actual behavior under normal use (Figure 8.9). The results have so far indicated no signs of dimensional instability in terms of warping and bulging after 3 yr.66

8.7.4

Roofing Sheets

Roofing is generally said to be the most important and difficult problem in housing. In many situations, roofing alone represents more than 25–30% of the total construction cost of a low-cost house. Because of the health hazards

TABLE 8.6 Properties of Pultruded JRP Doorframe Property Moisture content (%) Seasoning/treatment Dimensions/size (cm) Thickness (mm) Hold fasts Gluing of joints Installation Finish Weathering Dimensional stability a

Wooden Doorframe (IS: 4021–83)a 8–14 Prevented from warping and mold growth H-199–209, W-79–99 60 3 BWR adhesive Solid doorframe, no grouting is required to fill inside of frame Priming followed by varnish/paint Al priming required No warping/twisting

Pultruded JRP Doorframe 4.40 Not required H-214, W-92 60 3 Reinforced adhesive Frame cavity is filled by concrete/foams PU paint/varnish/melamine Resin rich layer Dimensionally stable

Indian standard specification for timber door, window, and ventilator frames.

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FIGURE 8.9 (a) Jute frames. (b) Installed jute frame.

associated with the handling of asbestos fibers, various R&D institutions have been engaged for the past two to three decades in finding a suitable substitute for roofing materials. The replacement of asbestos by natural Copyright © 2005 by Taylor & Francis

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fibers is seen as one of the options, and sisal-cement tiles are being produced in several developing countries. An appropriate technology for the production of coir-cement roofing sheets has been developed.67 In the manufacturing process, coir fibers are first soaked in mineralized water for at least 2 h. The free water is drained from the fiber, which is then mixed with dry cement in a 1:5 ratio by weight. The sheet can be made with a wet mix of cement-coated fibers either by a manual process or a semimechanized process. It is held under pressure for 4–8 h, then demolded and cured in the shade. A cement wash can be applied on both sides of dried sheets. It can be produced in the size of 2 m  1 m of 6–8 mm thickness. Experimental structures constructed in 1990 with coir-cement roofing sheets have shown satisfactory performance with little maintenance needed. Roofing sheets of 3–4 mm in thickness can be prepared from chopped sisal strand mats, fibrous filler, antiaging agent, and unsaturated polyester resin.68 Wooden blocks with corrugated face were used as molding male and female dies. The properties of roofing sheets are given in Table 8.7. It can be seen in the load-deflection curve that the sheet did not break under flexure when the peak load was reached (Figure 8.10). Test results indicate that sheets have an adequate strength and rigidity to merit application as roofing materials (Figure 8.11). In order to improve outdoor durability, a neopentyl glycolbased gel coat can be used on exterior sides of the sheet. Laboratory investigations on sisal/cashew nut shell liquid-based resin have shown that the roofing sheets made from these composites were light in weight, strong, resilient, easy to cut, and can be easily transported without any risk of breaking. The sheet was almost 50% cheaper than corrugated iron sheets.69

8.7.5

Composite Shuttering Plates

Timber used as shuttering and formwork is being replaced by plywood and mild steel. Though plywood has certain advantages, the problems of TABLE 8.7 Properties of the Corrugated Roofing Sheets Property

Sisal-Polyester Sheet

Pitch length (mm) Pitch depth (mm) Weight (kg/m2) Density (g/cm3) Water absorption, 24 hr (%) Thickness (mm) Bending strength (MPa) Deflection (mm) Thermal conductivity (kcal/m h °C)

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75 19.25 3–4 1.02 3–5 3.31 45–58 30–40 0.12–0.15

Coir-Cement Asbestos-Cement Sheet Sheet 146 48 12 1.10 8–10 7.0 9–11 — 0.125

146 48 13.50 2.0 25 6.0 25–30 — 0.24

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1.20

1.00

Load (KN)

0.80

0.60

0.40

0.20

0

0

10

30 20 Deflection (mm)

40

50

FIGURE 8.10 Load deflection curve of sisal/polyester roofing sheet.

preservation and dimensional instability affect its reuse value. On the other hand, pressed steel shuttering sheets have problems of corrosion, defective surface, and poor resistance against penetration of external heat and cold in extreme climates. Recently, pultruded glass fiber-reinforced composites are being considered as an alternative shuttering material for casting of concrete slabs. These plates can be assembled in standard sizes by nuts and suitably backed with mild steel angle stiffeners. They are lightweight, smooth, corrosion resistant, waterproof, easy to release, and can be used multiple times. Recently, a high-pressure compression-molded sisal/glass fiber-epoxy shuttering plate (sisal fiber content ~30 wt%) has been prepared as an alternative to plywood.63 The tensile strength and modulus of the plate were ~57 MPa and ~2.6 GPa, whereas the flexural strength and modulus were ~98 MPa and ~4.7 GPa, respectively. The water absorption of the plate was in the order of 4%. A modular shuttering panel (12.50 kg/m2) has been designed for casting of 120 mm thick horizontal/flat concrete slabs with supports at 0.90 m distances. The designed load includes self weight of panel, fresh weight of concrete, and live and impact loads. The modular composite plate (8 mm), with a steel frame as a stiffener, was subjected to 600 kg/m2 of static load for 7 days in the laboratory. The deflection in the plate was Copyright © 2005 by Taylor & Francis

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FIGURE 8.11 Sisal/polyester roofing sheet.

~1.95 mm upon initial loading and ~2.57 mm after 7 days. It has been observed that the deflection in the natural fiber sheet (8 mm) was under permissible limits (3 mm) and also comparable to the 12 mm plywood (Table 8.8). The modular panel was also be tested by casting a 130 mm thick concrete slab. It was found that the surface of the slab was smooth. Application of release agent on the plate surface before casting could be desirable to obtain an increased use of the plate. More research is needed on natural fiber-based shuttering plates for large-scale use.

8.7.6

Dough Molding Compounds

The tendency of using short natural fibers as replacement to glass fibers in dough molding compound has been reported as a lightweight and costeffective option.70–72 Wood flour, sisal, coir, and jute have been used to prepare natural fiber-based dough molding compounds for the automotive and construction industries. The performance of these moldings is questioned when high strength and severe humidity are to be experienced. Copyright © 2005 by Taylor & Francis

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TABLE 8.8 Properties of Natural Fiber Shuttering Plates Plywood-IS: 4990-1993a

Composite Shuttering

Materials

Rotary/slice cut veneers

Specific gravity Moisture content (%) Tensile strength (MPa) Adhesion of plies

0.65 5–15 55 Resistance to veneers separation No sign of separation of edge of veneers and no blisters or surface deterioration Defective surface

Epoxy with natural fiber and fibrous filler 1.1 2–3 57 No separation of plies

Property

Resistance to water

Cement poisoning Shuttering requirements Surface finish Treatment Oiling Storage Weight (kg/m2) Thickness (mm) Deflection (mm) a

Smooth and defect-free Required Required Free circulation air 3.9 for 3 mm 12 3

Edge swelling in absence of sealing; surface smoothness intact after casting Not observed Smooth and rigid Not required Not required Free circulation air 12.5 for 8 mm 8 1.60

Indian standard specification for plywood for concrete shuttering work.

It is expected that these shortcomings may be overcome by combining the moisture-resistant natural fibers with other reinforcements. The combination of chemically treated sisal fibers and wollastonite whiskers has recently been documented to retain the optimum physical strength of moldings in spite of higher glass fiber replacement.72,73 The physicomechanical properties of moldings were studied as a function of various constituents such as thickeners, monomers, surface modifiers, and sisal/wollastonite/glass fiber content. The reinforcing effect of wollastonite to a level of ~50 wt% in an unsaturated polyester resin produced positive results due of its favorable physicomechanical properties and microstructural features. It is noted that ~11 wt% sisal fibers in wollastonite/polyester and ~3–5 wt% glass fibers in sisal/wollastonite/polyester give optimum results. The high loss area observed at α and β transitions in loss modulus curves under dynamic mechanical analysis supported the elastic behavior of sisal/wollastonite/polyester molding compared to a corresponding wollastonite/polyester system. Based on these findings, a dough composition comprising titanate-treated sisal fibers, silanized wollastonite, glass fibers (optional), zinc oxide, calcium stearate, methyl methacrylate monomer, and catalyzed styrenated unsaturated polyester resin was proposed. The microstructure of molding indicates that the constituents are well dispersed in the doughy mixture and that reinforcements are arranged nearly perpendicular to the advancing crack front, thus requiring higher loads to mobilize cracks (Figure 8.12). Copyright © 2005 by Taylor & Francis

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FIGURE 8.12 SEM showing uniform dispersion of constituents in dough moulding compound (DMC) and reinforcements arrangement nearly perpendicular to the advancing crack front.

TABLE 8.9 Properties of Sisal/Wollastonite Moldings vis-à-vis Molding Compounds Developed by Other Workers

System Sisal fiber/wollastonite/ polyester molding Sisal fiber/wollastonite/glass fiber/polyester molding Coir/glass fiber/CaCO3/ polyester molding Wood fiber/polyester molding Glass fiber/polyester molding (10–16% glass) (low/medium quality)

Tensile Strength (MPa)

Elastic Modulus (GPa)

Flexural Strength (MPa)

Reference

25–27

5–6

45–48

72,73

42.28

7.8

—

72,73

27.80

1.4

18–22 26–41

— 12.40

25

70

38 —

71 71

Improved mix workability, reduced mold shrinkage, and extended shelf life can be achieved by adding a blend of methyl methacrylate and styrene monomers to the polyester resin. The physico-mechanical properties of these Copyright © 2005 by Taylor & Francis

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moldings are comparable to coir/polyester molding, wood fiber/polyester molding, and glass fiber/polyester molding (Table 8.9). These developments make natural fibers worthy of consideration as an alternative to traditional glass fiber reinforcement for producing molding products that are less expensive. The potential of sisal-dough molding compound in construction can be considered for developing substitute materials such as checkered floor plates, roof tiles, artificial marbles, and sanitary wares.

8.8

Concluding Remarks

Natural fiber composites offer immense opportunities for an increasing role as alternate materials, especially wood substitutes in the construction market. They have special relevance to developing countries in view of their low cost, savings in energy, and applications as substitute materials. Problems related to natural fibers, such as inconsistency in product performance due to fiber variability, nonavailability in the molding forms of reinforcements (roving, long fiber strands, prepeg, chopped strand mats, etc.), use of fiber in a partially prepared state, and poor fiber-matrix interface, need to be carefully tackled to achieve industrial exploitation. Application developments of natural fiber composites as alternate building materials must be thoroughly studied for their durability and cost-effectiveness in order to obtain consistent product performance under service conditions. A code of practice on product specification will be developed for wider acceptance.

Acknowledgments The authors thank Prof. V.K. Mathur, Director, Central Building Research Institute, Roorkee, and Dr. K.P. Aggarwal, National Coordinator, NATP, ICAR, NEW DELHI, for their continued support and encouragement. Some of the experimental work reported in this chapter forms part of the normal R&D program of the Institute.

References 1. Bhattacharyya, D.N., Chakraverty, I.B., and Sengupta, S.R., J. Sci. Ind. Res., 20D, 192, 1961. 2. Nagabhushan, T., Radhakrishnan, G., Joseph, K. P., and Santappa, M., Proceedings of the 2nd Symposium on New Fibers and Composites, India, Jan. 10–11, 1977, p. 3.1. 3. Winfield, A.G., Plast. Rubb. Int., 4, 23, 1979.

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4. Pal, P.K. and Ranganathan, S.R., Pop. Plast., 31, 22, 1986. 5. Davidson, R. and Hancox, N.L., Proceedings of the International Symposium on Biocomposites and Blends based on Jute and Allied Fibers, New Delhi, Dec. 1–2, 1994, p. 83. 6. Roe, P.J. and Ansell, M.P., J. Mater. Sci., 20, 4015, 1985. 7. Shah, A.N. and Lakkad, S.C., Fiber Sci. Technol., 15, 41, 1981. 8. Clark, R.A. and Ansell, M.P., J. Mater. Sci., 21, 269, 1986. 9. Oksman, K., Wallstrom, L., Berglund, L.A., and Filho, R.D.T., J. Appl. Polym. Sci., 84, 2358, 2002. 10. Li, Y., Mai, Y.W., and Ye, L., Compos. Sci. Technol., 60, 2037, 2000. 11. Paira, J.M.F. and Frollini, E., J. Appl. Polym. Sci., 3, 880, 2002. 12. Karmakar, A.C. and Hinrichsen, G., Polym. Plast. Technol. Eng., 30, 609, 1991. 13. Rana, A.K., Mandal, A., Mitra, B.C., Jacobson, R., Rowell, R., and Banerjee, A.N., J. Appl. Polym. Sci., 69, 329, 1998. 14. Mukherjee, R.N., Pal, S.K., and Sanyal, S.K., J. Appl. Polym. Sci., 28, 3029, 1983. 15. Varma, I.K., Anathakrishnan, S.R., and Krishnamoorty, S., Composites, 20, 383, 1989. 16. Rowell, R., Lange, S. et al., Proceedings of the International Seminar on Jute and Allied Fibers: Changing Global Scenairo, Calcutta, Feb. 5–6, 1998, p. 97. 17. Semsarzadeh, M.A., Polym. Comp., 7, 23, 1986. 18. Singh, S.M. and Jain, S.K., Plast. Rubb. Mater. Appl., 5, 65, 1980. 19. Salyer, I.O. and Usmani, A.M., Ind. Eng. Chem. Prod. Res. Dev., 21, 17, 1982. 20. Belmares, H., Barrera, A., Castillo, E., Verheugen, V.E., Monjaras, M., Patfort, G.A., and Buequoye, E.N., Ind. Eng. Chem. Prod. Res. Dev., 20, 555, 1981. 21. Saptharishi, L.V., United Nations Development Programme on Nonconventional Uses of Jute (IND/92/302), 1992–97 IJIRA, Calcutta, India. 22. Satyanarayana, K.G., Sukumaran, K., Ravikumar, K.K., Brahmakumar, M., Pillai, S.G.K., Pavithran, C., Mukherjee, S., and Pai, B.C., Possibility of Using Natural Fiber Polymer Composites as Building Materials in Low Cost Housing, CBRI, Roorkee, November 12–17, 1984, p. 177. 23. Swamy, R.N., Ed., Natural Fiber Reinforced Cement and Concrete, Vol. 1, Concrete Technology and Design, Blackle, London, 1988. 24. Gram, H.E., Durability of Natural Fibers in Concrete, Swedish Research and Concrete Research Institute, Stockholm, 1983. 25. Lola, C.R., Fiber Reinforced Concrete Roofing Technology Appraisal Report in RILEM FRC 86, Swamy, R.N., Wagstaffe, and Oakley, D.R., Eds., RILEM Technical Committee 49-TFR, paper 2.12. 26. Clouts, R.S.P., From Forest to Factory Fabrication Fiber Reinforced Cement and Concrete, Proceedings of the 4th RILEM International Symposium, Swamy, R.N., Ed., E & FN, SPON, Tokyo, July 20–23, 1992, p. 31. 27. Rohatgi, P.K., Composite Materials from Local Raw Material Resources: New and Advanced Materials, UNIDO, Vol. 3, 1996, p. 30. 28. Bhal, N.S. and Singh, B., Proceedings of the 2nd International Conference on Composites in Infrastructure, Department of Civil Engineering and Engineering Mechanics, Arizona, USA, Jan. 5–7, 1998, p. 661. 29. Barkakaty, B.C. and Robson, A.J., Appl. Polym. Sci., 24, 269, 1979. 30. Sridhar, M.K., Basavarajappa, G., Kasturi, S.G., and Balasubramanian, N., Ind. J. Text. Res., 7, 87, 1982. 31. Bisanda, E.T.N. and Ansell, M.P., Compos. Sci. Technol., 41, 165, 1991. 32. Mishra, S., Misra, M., Tripathy, S.S., Nayak, S.K., and Mohanty, A.K., Polym. Compos., 23, 164, 2002.

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33. Mohanty, A.K., Khan, M.A., and Hinrichsen, G., Compos. Sci. Technol., 60, 1115, 2000. 34. Ganan, P. and Mondragon, I., Polym. Compos., 23, 383, 2002. 35. Rong, M.Z., Zhang, M.Q., Liu, Y., Yan, H.M., Yang, G.C., and Zeng, H.M., Polym. Compos., 23, 182, 2002. 36. Escamilla, G.C., Cupul, J.I.C., Mendizabal, E., Puig, J.E., Torres, H.V., and Franco, P.J.H., Composites: Part A, 30, 349, 1999. 37. Gassan, J. and Bledzki, A.K., Polym. Compos., 20, 604, 1999. 38. Rong, M.Z., Zhang, M.Q., Liu, Y., Yan, H.M., Yang, G.C., and Zeng, H.M., Compos. Sci. Technol., 61, 1437, 2001. 39. Gassan, J. and Bledzki, A.K., Compos. Sci. Technol., 59, 1303, 1999. 40. Zadorecki, P. and Flodin, P., J. Appl. Polym. Sci., 30, 3971, 1985. 41. Barkakaty, B.C., J. Appl. Polym. Sci., 20, 2921, 1976. 42. Rout, J., Misra, M., Tripathy, S.S., Nayak, S.K., and Mohanty, A.K., Compos. Sci. Technol., 61, 1303, 2001. 43. Joseph, K., Thomas, S., and Pavithran, C., Comp. Sci. Technol., 53, 99, 1995. 44. Plueddmann, E.P., Silane Coupling Agents, Plenum Press, New York, 1982. 45. Singh, B., Gupta, M., and Verma, A., J. Appl. Polym. Sci., 70, 1847, 1998. 46. Singh, B., Gupta, M., Verma, A., and Tyagi, O.S., Polym. Int., 49, 1444, 2000. 47. Gupta, M., Verma, A., and Singh, B., Curr. Sci., 74, 526, 1998. 48. Zhou, L.M., Mai, Y.W., Ye, L., and Kim, J.K., Key Eng. Mater., 549, 104–107, 1995. 49. Kim, J.K., Zhou, L.M., and Mai, Y.W., J. Mater. Sci., 28, 3923, 1993. 50. Rials, T.G., Wolcott, M.P., and Nassar, J.M., J. Appl. Polym. Sci., 80, 546, 2001. 51. Ishida, H. and Koenig, J.L., J. Polym. Sci. Polym. Phys. Ed., 17, 615, 1979. 52. Singh, B., Gupta, M., and Verma, A., Polym. Compos. 17, 910, 1996. 53. Peters, P.W.M. and Springer, G.S., J. Comp. Mater., 21, 157, 1987. 54. Singh, B., Gupta, M., and Verma, A., Const. Build. Mater., 9, 39,1995. 55. Singh, B., Gupta, M., Verma, A., and Hajela, R.B., Res. Ind., 39, 38, 1994. 56. Tobias, B.C., Proceedings of the International Conference on Advanced Composite Materials, Chandra, T. and Dhingra, A.K., Eds., T M S Publication, Australia, 1993, p. 623. 57. Pavithran, C., Mukherjee, P.S., Brahmakumar, M., and Damodaran, A.D., J. Mater. Sci., 26, 455, 1991. 58. Singh, B., Gupta, M., and Verma, A., Compos. Sci. Technol., 60, 581, 2000. 59. Singh, B. and Gupta, M., Proceedings of the 3rd International Conference on Advances in Composites ADCOMP-2000, Bangalore, Aug. 24–26, 2000, p. 75. 60. Pillai, M.S., Proceedings of the Advances in Polymeric Building Materials, POLY BUILT- 2003, Roorkee, Mar. 6–7, 2003, p. 16. 61. Bolton, A.J., Jones, G.L., and Loxton, C., Proceedings of the International Symposium on Biocomposites and Blends Based on Jute and Allied Fibers, New Delhi, Dec. 1–2, 1994, p. 35. 62. Rana, A.K., Mandal, A., Jayachandran, K., and Bandyopadhyay, S., Proceedings of the International Conference on Composites, UNSW, Sydney, 2002, p. 363. 63. Singh, B. and Gupta, M., Proceedings of the Advances in Polymeric Building Materials, POLY BUILT-2003, Roorkee, Mar. 6–7, 2003, p. 5. 64. Rana, A.K. and Mandal, A., Proceedings of the Advances in Polymeric Building Materials, POLY BUILT-2003, Roorkee, Mar. 6–7, 2003, p. 144. 65. Bagchi, P.K., Proceedings of the 3rd International Conference on Advances in Composites, ADCOMP-2000, Bangalore, Aug. 24–26, 2000, p. 176.

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66. Singh, B., Gupta, M., and Verma, A., Report on Suitability Assessment of JRP Pultruded Profile as Door Frame Materials in Buildings (F(C) 0176), 1998. 67. Singh, S.M., Coir, 3, 1975. 68. Singh, B. and Gupta, M., Proceedings of the National Seminar on Production and Characterisation of Natural and Man-made Fibers, Mumbai, Jul. 3, 1999, p. 73. 69. Bisanda, E.T.N., J. Mater. Process. Technol., 38, 369, 1993. 70. Owolabi, O., Czvikovszky, T., and Kovacs, I., J. Appl. Polym. Sci., 30, 1827, 1985. 71. Freischmidt, G., Michell, A.J., Lewis, M.J., and Vanderhoek, N., Polym. Int., 24, 113, 1991. 72. Singh, B., Gupta, M., and Verma, A., Proceedings of the International Symposium on Polymers Beyond AD 2000-Polymer 99, New Delhi, Jan. 12–15, 1999, p. 576. 73. Singh, B., Gupta, M., and Verma, A., Composites: Part A, 34, 1035, 2003.

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9 Thermoset Biocomposites

Dipa Ray and Jogeswari Rout

CONTENTS 9.1 Introduction 9.2 Materials 9.2.1 Thermosetting Resins 9.2.2 Natural Fibers 9.3 Thermoset: Natural Fiber Bonding and the Effect of Moisture 9.4 Surface Modification of Natural Fibers 9.5 Thermoset Biocomposites 9.5.1 Fabrication Techniques 9.5.1.1 Hand Layup 9.5.1.2 Press Molding 9.5.1.3 Filament Winding 9.5.1.4 Pultrusion 9.5.2 Properties and Structure 9.5.2.1 Mechanical Properties 9.5.2.1.1 Jute–Thermoset Composites 9.5.2.1.2 Coir–Thermoset Composites 9.5.2.1.3 Sisal–Thermoset Composites 9.5.2.1.4 Pineapple Leaf Fiber–Thermoset Composites 9.5.2.1.5 Sunhemp–Thermoset Composites 9.5.2.1.6 Straw–Thermoset Composites 9.5.2.1.7 Banana Fiber–Thermoset Composites 9.5.2.1.8 Other Lignocellulosic Fiber–Thermoset Composites 9.5.2.2 Dynamic Mechanical Properties 9.5.2.3 Fatigue Properties 9.5.2.4 Fracture Characteristics 9.5.3 Effect of Hybridization 9.5.4 Application 9.5.5 Biodegradability

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9.6 Conclusion Acknowledgments References ABSTRACT This chapter reviews the field of natural fiber-reinforced thermosetting resin matrix composites. Natural fiber-reinforced thermoset composites are now finding extensive uses in various fields from household articles to automobiles. The primary advantages of natural fibers over synthetic fibers have been their low cost, light weight, high specific strength, renewability, and biodegradability. Both the physical and mechanical properties of the natural fibers vary widely depending on their species, area of growth, climate, age of the plant, and their processing method to break down into the fiber level. The physical properties of the natural fibers are determined by their chemical and physical composition, such as the structure of the fibers, cellulose content, microfibrillar angle, cross section, and the degree of polymerization. Absorption of moisture causing swelling of the fibers has been a major drawback for natural fibers, which leads to a weak bond at the fiber/resin interface in the composites. To improve the bond between the fiber and the resin, therefore, there is a need to restrict the water sorption into the reinforcing fibers before incorporating them in the composites. Several approaches on the surface treatments and modifications of the natural fibers have been attempted and were found to improve their properties. As this field of research activity is very wide and gaining popularity, an attempt has been made in this review to report the mechanical properties of natural fiber-reinforced composites, with special attention on the dynamic mechanical properties of the composites, which clearly depict the significance of the interfacial interactions in a composite. Fatigue, which very often is the cause of failure of the composites, is also discussed in respect to natural fiber composites. The failure modes of the fiber and the composites reflecting the strength of bonding at the interface, as observed by researchers, is also examined. The studies made on hybrid composites containing some synthetic fibers along with natural fiber reinforcement for highstrength application at a lower cost are also reported. For the lack of a standardized procedure to produce such composites and insufficient property data from the used components, acceptance of the thermoset biocomposites as a structural material has remained restricted. Very recently, biodegradability has drawn the attention of the scientific community. A brief account of future research and development on ecofriendly biocomposites from plant-derived fibers and crop-derived plastics (bioplastics) as a novel material for the 21st century to arrive at a solution to growing environmental pollution is also given.

9.1

Introduction

Composite materials have been in existence for centuries, but only within the past few decades the utilization of natural plant fiber-reinforced Copyright © 2005 by Taylor & Francis

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composites has attracted the attention of the scientific community. There is a growing interest in composites reinforced with natural fibers because of their low cost, light weight, high specific strength, renewability, and biodegradability. These are now finding extensive uses in packaging, furniture, building, and other common industries. Yet natural fibers, as a construction material for buildings, were known long before. For centuries, mixtures of straw and loam, dried in the sun, were employed as construction composite, e.g., in Egypt.1 Each fiber is a “natural composite” by itself, wherein the crystallites (mostly cellulosic) remain embedded in a matrix of hemicellulose and lignin in a given orientation. Cellulose is one of the stiffest and strongest organic constituents in a natural fiber. Nature provides a host of inexpensive cellulosic fibers like wood fiber, jute, coir, sisal, and PALF. Wood fibers are the most abundant fiber source for various applications such as timber, pulp, paper, and medium-density fiberboard. Wood fibers and wood fiber-based composites, themselves, constitute a vast area of study. However, in this chapter we will confine our discussion to natural plant fibers and natural plant fiber-reinforced thermoset composites and exclude wood fibers and wood fiber-based composites such as medium-density fiberboard, which have very low matrix (or mainly thermoset adhesive). Although these natural fibers have relatively poor mechanical properties compared to man-made synthetic fibers like carbon, glass, and kevlar, they form large renewable resources in developing countries such as India. Pipes, pultruded profiles, and panels with polyester matrices are quite popular. Large projects have been promoted where jute-reinforced polyester resins are used for buildings, e.g., the Madras House, 1978, and grain elevators. Today, a renaissance in the use of natural fibers as reinforcement in technical applications is taking place, mainly in the automobile and packaging industries (e.g., egg boxes). In the automotive industry, textile waste has been used for years to reinforce plastics used in cars, especially in the Trabant.1 Mercedes at present has planned to develop a K-car series, where the “K” stands for “kraut” and “compost.” Local European renewable fibers, such as flax and hemp, are used for these cars. Components were developed for door panels, molded wood (shaping and curving the wood with the help of various woodworking tools), natural fiber moldings, laminated panels, and car roofs. Such composites are made of natural fiber–fleecy flax with epoxy resins or polyurethane resins. The use of flax fibers in car disk brakes to replace asbestos fibers is another example of this type.1 While natural fiber-reinforced thermoplastic composites are currently being manufactured for various ever widening applications, thermoset biocomposites still constitute the major part of the natural fiber-reinforced composite market. Mainly polyester, epoxy, and vinylester resins are commonly used for preparing such composites as the matrix material. An attempt is made here to review the findings thus far on thermoset biocomposites, their processing techniques, and their resultant properties, which are correlated with their microstructures. Copyright © 2005 by Taylor & Francis

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Materials Thermosetting Resins

Polyester resin is the most widely used resin system, particularly in the marine industry. The majority of dinghies, yachts, and workboats are built with composites reinforced with glass fibers having this resin as the matrix material. Polyester resins such as these are of the “unsaturated” type. Unsaturated polyester resin is a thermoset capable of being cured from a liquid to a solid state when subjected to appropriate conditions. A whole range of polyesters is made from different acids, glycols, and monomers, all having varying properties. There are two principal types of polyester resins used as a standard laminating system in the composites industry. Orthophthalic polyester resin is the standard economic resin commonly used, and it yields highly rigid products with a low heat resistance property. Isophthalic polyester resin is now becoming the preferred material in the marine industry, where its superior water resistance is desirable (Table 9.1, Figure 9.1). Epoxy resins are a large family of resins that represent some of the highperformance resins available in the market. Epoxies generally outperform most other resin types in terms of mechanical properties and resistance to environmental degradation, which leads to their almost exclusive use in the aircraft industry. As a laminating resin their increased adhesive properties and resistance to water degradation make these resins ideal for use in applications such as boat building. Here epoxies are widely used as a primary construction material for high-performance boats or as a secondary application to sheath a hull or replace water-degraded polyester resins and gel coats (Table 9.1, Figure 9.2). Vinylester resins are relatively recent additions in the family of thermosetting resins used for preparing fiber-reinforced composites. They combine the excellent chemical resistance and the thermal and mechanical properties of epoxy resins with ease of processing and rapid curing characteristics of polyester resins. These resins show better moisture resistance than epoxy when both are cured at room temperature. They have opened up broad new applications for thermoset resins because of the following: (1) they can be cured with relatively nontoxic catalysts like polyester resins; (2) they have excellent wetting characteristics, like epoxy resin; (3) they retain high elongation at moderate and high heat distortion temperatures due to a controlled cross-linking structure; and (4) they have very good heat aging properties when novolac epoxy resins are a part of the basic structure. Vinylester resins are similar in their molecular structure to polyesters (Table 9.1, Figure 9.3), but differ primarily in the location of their reactive sites, these being positioned only at the ends of the molecular chains. As the whole length of the molecular chain is available to absorb shock loadings, this makes vinylester resins tougher and more resilient than polyesters. The Copyright © 2005 by Taylor & Francis

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TABLE 9.1 A Study of the Basic Characteristics of Commonly Used Thermosets (Polyester, Epoxy, and Vinylester) Common Manufacturing Ingredients

Chemical Structure An idealized chemical structure of a polyester is shown in Figure 9.1.

Epoxy

Produced by the reaction between epichlorohydrin and bisphenol-A.

An idealized chemical structure of a typical epoxy is shown in Figure 9.2.

Copyright © 2005 by Taylor & Francis Vinylester Produced by the reaction between epoxy resin and an ethylenically unsaturated carboxylic acid resulting terminal unsaturation. Commonly used acids are acrylic or methacrylic acid.

An idealized chemical structure of a vinylester resin is shown in Figure 9.3.

Catalyst for room-temperature curing is methyl ethyl ketone peroxide (MEKP) or cyclohexanone peroxide. For high-temperature curing benzoyl peroxide is used as the catalyst. Promotor used is cobalt compound like napthenate, octoate, etc. Accelerator: tertiary amines like N,N-dimethyl aniline. Amine hardeners like diethylene triamine, triethylene tetramine, etc., are commonly used to cure epoxy by an addition reaction where both the materials take part in the chemical reaction. Anhydride hardeners are sometimes used with the amines to speed up gel and cure time. Catalyst for room-temperature curing is methyl ethyl ketone peroxide (MEKP). For high-temperature curing benzoyl peroxide or t-butyl perbenzoate is used as the catalyst. Promotor used is cobalt compound like napthenate, octoate, etc. Accelerator: tertiary amines like N,N-dimethyl aniline.

Source: From Pritchard, G., Appl. Sci., 32, 1980; Kroschwitz, J., Encyclopedia Polym, Sci. Eng., 3, 128, 1985; Sarkar, B.K. et al., J. Inst. Eng. (India), 78, 31, 1997. With permission.

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Glycols used are propylene glycol, ethylene glycol, diethylene glycol, etc. Unsaturated acid used are maleic acid or anhydride. Saturated acid used is commonly pthalic anhydride.

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Curing Behavior

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O

O

C C O C C O C *

HO C C *

O

O

C O C C O C C *

C C O C C OH * n n = 3 to 6

* Denotes reactive sites Ester groups

FIGURE 9.1 Idealized chemical structure of a typical polyester resin.

CH3 CH2

CH

CH2

O

O

C

O

CH2

CH

CH2 O

CH3

FIGURE 9.2 Idealized chemical structure of a typical epoxy.

C

C

C

R

O

O

CH2 CH

CH2 O

C

O

CH2

CH OH

OH

CH2

O

C

C

O

R

C

n

FIGURE 9.3 Idealized chemical structure of a typical vinylester resin.

vinylester molecule also features fewer ester groups. These ester groups are susceptible to water degradation by hydrolysis, which means that vinylesters exhibit better resistance to water and many other chemicals than their polyester counterparts. The polyesters, vinylesters, and epoxies discussed here probably account for some 90% of all thermosetting resin systems used in structural composites. The properties and the main advantages and disadvantages of each of these resins are given in Tables 9.2 and 9.3, respectively.2−6 Also of importance to the composite designer and builder is the amount of shrinkage that occurs in a resin during and following its cure period. Shrinkage is due to the resin molecules rearranging and reorienting themselves in the liquid and semigelled phase. Polyesters and vinylesters require considerable molecular rearrangement to reach their cured state and can show shrinkage of 8% or more. The different nature of the epoxy reaction, however, leads to very little rearrangement and with no evolution of volatile by-products, typical shrinkage of an epoxy is reduced to less than 5%. The absence of built-in stresses due to shrinkage is, in part, responsible for the improved mechanical properties of epoxies over polyester resins, not withstanding the development of a “print-through” of the pattern Copyright © 2005 by Taylor & Francis

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TABLE 9.2 A Comparative Study of the Properties of Polyester, Epoxy, and Vinylester Resin Properties

Polyester Resin

Epoxy Resin

Vinylester Resin

Density (Mg3) Young’s modulus (GPa) Tensile strength (MPa) Compressive strength (MPa) Tensile elongation to break (%) Cure shrinkage (%) Water absorption 24 h at 20oC Fracture energy (kPa)

1.2–1.5 2–4.5 40–90 90–250 2 4–8 0.1–0.3 —

1.1–1.4 3–6 35–100 100–200 1–6 1–2 0.1–0.4 —

1.2–1.4 3.1–3.8 69–83 — 4–7 — — 2.5

Source: From Sarkar, B.K. et al., J. Inst. Eng. (India), 78, 31, 1997; Mukherjee, R.N. et al., J. Polym. Mat., 1, 69, 1984; Iijima, T. et al., J Appl. Polym. Sci., 43, 1685, 1991. With permission.

TABLE 9.3 A Comparative Study of the Advantages and Disadvantages of Thermosetting Resins Resin

Advantages

Polyester

Easy to use Lowest cost of resins available (£1–2/kg)

Vinylester

Very high chemical/environmental resistance Higher mechanical properties than polyesters High mechanical and thermal properties High water resistance Long working times available Temperature resistance can be up to 140°C wet/220°C dry Low cure shrinkage

Epoxy

Disadvantages Only moderate mechanical properties High styrene emissions in open molds High cure shrinkage Limited range of working times Postcure generally required for high properties High styrene content Higher cost than polyesters (£2–4/kg) High cure shrinkage More expensive than vinylesters (£3–15/kg) Critical mixing Corrosive handling

Sources: From Pritchard, G., Appl. Sci., 32, 1980; Kroschwitz, J., Encyclopedia Polym, Sci. Eng., 3, 128, 1985; Sarkar, B.K. et al., J. Inst. Eng. (India), 78, 31, 1997. With permission; Sarkar, B.K. et al., J. Inst. Eng. (India), 78, 31, 1997; Mukherjee, R.N. et al., J. Polym. Mat., 1, 69, 1984; Iijima, T. et al., J Appl. Polym Sci., 43, 1685, 1991. With permission.

of reinforcing fibers, a cosmetic defect that is difficult and expensive to eliminate. Besides polyesters, vinylesters, and epoxies, there are a number of other thermosetting resin systems that are used when their unique properties are required. Phenolics are primarily used where high fire resistance is required. Phenolics also retain their properties well at elevated temperatures. For room-temperature curing materials, corrosive acids are used, which leads to unpleasant handling. The condensation nature during curing tends to lead Copyright © 2005 by Taylor & Francis

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to the inclusion of many voids and surface defects, and the resins tend to be brittle, causing reduced mechanical properties. Although for synthetic fiber-reinforced composites, some other thermosetting resins like cyanate esters, polyurethanes, and bismaleimides are used for some specific highperformance requirements, for natural fiber composites these resins are not much in use. However, coir-reinforced polyurethane matrix composites have been reported as being used as a substitute for wood (see chapter 8).7

9.2.2

Natural Fibers

After decades of exhaustive high-tech development of costly artificial manmade fibers such as glass, carbon, and aramid fibers, natural fibers have recently attracted much attention from the scientific community for use as reinforcement in composite materials. A large abundance of natural fibers like jute, coir, sisal, pineapple leaf fiber, flax, hemp, kenaf, and banana has enhanced the interest of researchers in exploring new value-added applications. New environmental regulations and societal concern have triggered the search for new products and processes that are compatible with the environment. This has motivated researchers to utilize the renewable resources of natural plant fibers as reinforcement in composites and ensure good economic returns for cultivation of these natural fibers. Recent reports state that plant-based natural fibers can be used as reinforcement in polymer composites. The advantages of these natural fibers over synthetic fibers have been their low cost, light weight, high specific strength properties, carbon dioxide sequestration, renewability, and biodegradability. The natural fibers can be classified into the following categories, depending on the part from which they are extracted. These are (1) bast fiber (hemp, flax, kenaf, jute, banana, etc.), (2) leaf fiber (sisal, pineapple, etc.), (3) fruit fiber (coir), (4) seed fiber (cotton), and (5) stem (serial straw). From a structural point of view, natural fibers are multicellular in nature, consisting of a number of continuous, mostly cylindrical honeycomb cells (Figure 9.4) which have different sizes, shapes, and arrangements for different types of fibers.8 These cells are cemented together by an intercellular substance which is isotropic, noncellulosic, and ligneous in nature, with a cavity termed the lacuna, whose position and dimensions differ for each type of fiber. Individual cells in turn consist of several concentric layers, the primary wall, and the secondary wall, the layers of which differ in composition and orientation of cellulosic microfibrils (Figure 9.5 and Figure 9.6).1,9 There is a central cavity in each cell called the lumen, the dimensions of which are again dependent on the external dimensions of the fiber. The microfibrils in the central walls form a constant angle (microfibrillar or helical angle) for each type of fiber with the fiber axis (Figure 9.7), so that the crystallites are arranged in a spiral form, the pitch of which varies from one fiber to another.1 Thus, each fiber is a “natural composite” by itself, wherein crystallites (mostly cellulosic) remain rooted in a matrix in a given orientation. Hence the properties of the single fibers Copyright © 2005 by Taylor & Francis

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Lumen

Lacuna

Cementing material

Single cell

FIGURE 9.4 Schematic view of a lignocellulosic fiber showing the arrangement of the individual cells. (From Satyanarayana, K.G. et al., Handbook of Ceramics and Composites, 1, 341, 1990. Courtesy of Marcel Dekker, New York and Elsevier, London.)

S2

S3 S1

Lumen Secondary walls (fibrils of cellulose in a lignin/hemicellulose matrix)

Primary wall (fibrils of cellulose in a lignin/hemicellulose matrix) FIGURE 9.5 Representation of the constitution of a natural fiber cell. (From Bledzki, A.K. and Gussan, J., Prog. Polym. Sci., 24, 221, 1999. Courtesy of Elsevier, London.)

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Primary wall Secondary wall Lumen Middle lamella

FIGURE 9.6 Schematic presentation of the cross section of a jute fiber. (From Majumdar, S.K., Ph.D. thesis, Calcutta University 1956. With permission.)

Crystalline fibrils

Noncrystalline regions

(a)

(b)

FIGURE 9.7 The fibrillar arrangement of a single cell (a) 3-D view; (b) 2-D view. (From Bledzki, A.K. and Gussan, J., Prog. Polym. Sci., 24, 221, 1999. Courtesy of Elsevier, London.)

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depend on the crystallite content, their size, shape, orientation, L/d ratio of cells, thickness of cell walls, and finally, their defects such as lumen and lacuna. This structure is reflected in the stress−strain diagrams of the fibers, some of which are shown in Figure 9.8. The most important factor controlling the different types of natural fibers is their species, i.e., the properties of the natural fibers are different between different species. In addition, the properties of fibers within a species vary depending on area of growth, climate, and age of the plant. Lastly, the properties of natural fibers vary greatly depending on their processing method used to break down to the fiber level. With the exception of cotton, the chemical constituents of natural fibers are cellulose, hemicellulose, lignin, pectin, waxes, and some water-soluble substances. By virtue of their relative superior specific strength, stiffness, and low density, the natural fibers are in general found attractive to reinforce thermosetting as well as thermoplastic polymers. Hence their mechanical properties are an important factor for composite application. The work of fracture of the natural fibers lies in the range of 10−100 kJ/m2,8 which on a weight basis is almost comparable with that of the ductile metals, on a weight basis. It is also reported8 that this value was higher than that required for breaking the interatomic bonds (~2 J/m2) in timber and the energy absorbed by the fiber pullout mechanism observed in conventional

Pineapple

800

Stress (MPa)

600 Banana Sisal 400

Coir

Palmyrah 200

0 0.00

Talipot

0.01

0.02

0.03

0.04

0.05

0.36

0.39

Strain FIGURE 9.8 The stress-strain curves of some natural fibers. (From Satyanarayana, K.G. et al., Handbook of Ceramics and Composites, 1, 341, 1990. Courtesy of Marcel Dekker, New York and Elsevier, London.)

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synthetic fiber composites. The fracture of each fiber depends on the parameters mentioned earlier, such as its constituents, shape, and orientation. It was observed that the fibers with higher cellulosic content and lower microfibrillar angle when pulled in tension possess higher ultimate tensile strength (UTS) and initial modulus with intracellular fracture, while fiber with lower cellulose content and higher micro-fibrillar angle show a low UTS, initial modulus with an intercellular fracture.10 In other words, a higher microfibrillar angle and lower cellulose content impart ductility to the fibers, as in the case of a coir fiber (Figure 9.9). Pavithran et al.11 showed that the microfibrillar angle in natural fibers plays an important role in determining the impact behavior of natural fiber-reinforced composites. Other major characteristic of the fibers affecting the properties are their discontinuous length, nonuniform diameter, and frequency of building nodes as the joints. These have imposed serious problems in characterizing the fiber strength properties, not withstanding their source. Hence, large variations are found by different experimenters. Due to nonuniformity in the diameter of natural fibers, it is very difficult to measure the cross-sectional area of the single fibers accurately. To simplify matters, a dimension related to cross section is often used in the case of natural fibers, which is called the “linear density” of the fiber. It is expressed as “tex” in SI units, which is the weight in grams of 1 km of the fiber.12 The strength is commonly expressed as the maximum force per unit area of cross section which a material can sustain before rupture. In the case of natural fibers, it is customary to express the strength in terms of force per unit of linear density of the fiber. In this form, as tenacity, it represents the number of unit lengths of the fiber that are supported before the rupture. Tenacity of a single natural fiber is thus defined as the force per unit linear density at the break point, the units being N/tex. The modulus of a single natural fiber is thus obtained as the ratio of the breaking stress (tenacity) to the breaking strain.

9.3

Thermoset: Natural Fiber Bonding and the Effect of Moisture

As far as the bonding between the natural fibers and the resin is concerned, three factors affect the bonding: mechanical anchoring, molecular (physical) attractive forces (van der Waals force and hydrogen bond), and the chemical bonds between the natural fiber and the resin. Hydroxyl groups (—OH) in the main backbone chain of a resin provide sites for hydrogen bonding to the surface of the natural fibers, which contain many hydroxyl groups in their chemical structure. The probable bonding between a resin and a natural fiber is shown in Figure 9.10. Thus, the polyester resin, having no hydroxyl group in its backbone chain, generally has the weakest bonding, hence the lowest adhesive properties compared to epoxy and vinylester resins. Vinylester resins Copyright © 2005 by Taylor & Francis

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600 Sisal

500

Strength (MPa)

400 Banana

300 Jute Bark of petiole (coir) 200

Mesta Leaf sheath

Sunhemp 100

Coir

Spath Rachis of coir Ipomoea carnea

0

0

10

20

(a)

30

40

50

Spiral angle (°) 600 Sisal

500

Strength (MPa)

400

300 Jute Bark of petiole (coir)

200 Coir

Kusha

Leaf sheath

100

Ipomoea carnea Spath 0

(b)

Rachis of coir

40

50

60

70

Content of cellulose (wt %)

FIGURE 9.9 Dependence of fiber strength on (a) spiral angle, and (b) content of cellulose. (From Bledzki, A.K. and Gussan, J., Prog. Polym. Sci., 24, 221, 1999. Courtesy of Elsevier, London.)

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OH

OH α-cellulose + hemicellulose + lignin

+

OH

OH

OH

OH

Natural fiber Thermosetting resin with −OH groups in the backbone chain

H

O α-cellulose + hemicellulose + lignin

O

H H

O

O

H H

O H

O

Natural fiber Thermosetting resin with −OH groups in the backbone chain

FIGURE 9.10 Probable bonding between a natural fiber and a resin containing hydroxyl groups in its backbone chain.

show improved bonding over polyesters but epoxy systems offer a strong bond. As epoxies cure with low shrinkage, the various surface contacts set up between the liquid resin and the adherends are not disturbed during the cure. However, the bond strength of these natural fiber-reinforced composites are often lowered by the absorption of moisture. Jute fibers, being hydrophilic in nature, absorb moisture from the environment. The hydrogen bonds are formed between the –OH groups of the cellulose molecules and water, shown in Figure 9.11. The first water molecules are absorbed directly onto the hydrophilic groups of the fibers and after that, the other water molecules are attracted either to other hydrophilic groups or they may form further layers on top of the water molecules already absorbed.12 Thus, these natural fibers can absorb and desorb moisture from the atmosphere and equilibrate with the surrounding environment. Hence, the composites reinforced with such fibers undergo swelling and shrinkage in moist and dry Copyright © 2005 by Taylor & Francis

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environments, respectively, affecting their dimensional stability. This characteristic of the natural fibers leads to poor adhesion with the resin matrix, ultimately causing debonding. In wet conditions, therefore, such composites show very poor mechanical properties. Drying of the fibers before processing is an important factor because water on the fiber surface acts like a separating agent in the fiber-matrix interface (Figure 9.12). Further, because of evaporation of moisture during the reaction process, voids appear in the matrix. All these lead to a decrease in mechanical properties of the natural fiber-reinforced composites with time. For the jute−epoxy composites, the tensile strength of the maximally predried fibers rose by about 10% compared to the minimally dried fibers.1 HOH H

HOH

OH

CH2OH O

OH

O

H

H

H

HOH H

O

H

O

H HOH OH

H

O CH2OH

OH

H

HOH

HOH

FIGURE 9.11 Absorption of moisture by natural fibers. Fiber

Fiber

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H-bonding

Resin (a)

Dry

Resin (b)

Wet

FIGURE 9.12 (a) H-bonding between a fiber and the resin. (b) Ingress of moisture causing dissociation of the H-bonds at the interface.

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Surface Modification of Natural Fibers

It had been observed by many workers that the composites containing lignocellulosic fibers in the as-received condition showed inferior strength properties, and only small amount of fibers could be incorporated.13−42 A higher resin consumption and higher moisture absorption by the composites were also observed. To improve interfacial bonding and to reduce moisture absorption, surface modification of the lignocellulosic fibers has been performed, similar to the sizing of synthetic fibers. Such modifications were effected by both physical and chemical methods (Table 9.4). This renders effective stress transfer from TABLE 9.4 Types of Surface Treatments of Natural Fibers to Increase Their Suitability as Reinforcing Fibers Physical Methods

Chemical Methods

Physical methods can 1. Change of surface tension: Treatment with suitable change the structural and chemical reagents can change the surface tension of fibers the surface properties of the and improve their dispersion in the resin. fibers and thereby, influence 2. Impregnation of fibers: Impregnation of the reinforcing the mechanical bonding to fibers with suitable polymer matrices compatible to the the resins. binder resin may improve the interfacial bonding. Some common physical 3. Chemical coupling: methods are: stretching, (a) Graft copolymerization: Free radicals are created on the calendaring, thermotreatment, cellulose molecules by reaction with the selected ions corona treatment, cold and afterward, these free radical sites are treated with plasma treatment suitable solution, compatible to the polymer matrix. Grafting can be done with vinyl monomer, acrylonitrile, methylmethacrylate, etc. (b) Treatment with compounds containing methanol groups: Chemical compounds containing ⫺CH2OH groups like polyesteramidepolyol form stable covalent bonds with cellulose fibers. (c) Treatment with isocyanates: Isocyanate treatment is an effective method for improving mechanical properties of the composites. Poly(methylene) (polyphenyl isocyanate), or PMPPIC, hexamethylene diisocyanate, etc., are used. (d) Treatment with triazine coupling agents: Triazine derivatives form covalent bonds with cellulose fibers. (e) Treatment with organosilane coupling agents: Mainly used for glass fiber composites. Also attempted on natural fiber composites. 4. Alkali treatment: The most common chemical method and economically the most viable one. Alkai treatment is often follwed by acetylation, diazotization, cyanoethylation, dinitrophenylation, etc. Source: From Bledzki, A.K. and Gassan, J., Prog. Polym. Sci., 24, 221, 1999. Courtesy of Elsevier, London.

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the resin matrix to the reinforcing fibers; thus, a full utilization of the strength of the fibers could be achieved. It had been found that such surface modifications had been effective not only in improving the interfacial bonding, but also in reducing resin consumption, with the possibilities of incorporating higher amount of fibers. Most of the experimental work carried out on natural fiber-reinforced thermoset resin composites used chemically modified fibers to maximize the efficiency of the fibers as reinforcement. The alkali treatment has been the most simple and economically viable tool to modify natural fibers into a better reinforcement. Apart from alkali treatment, cyanoethylation, bleaching, and grafting (PMMA, PAN, etc.), treatment with polyol compounds, organosilane coupling agents, and triazine have also been reported by some authors. Various surface treatments of natural fibers have been discussed in detail by Bledzki and Gassan.1

9.5 9.5.1

Thermoset Biocomposites Fabrication Techniques

Fabrication techniques suitable for manufacturing natural fiber-reinforced thermoset composites include the hand layup technique for unidirectional fibers/mats/fabric, sheet molding (SMC)/bulk molding (BMC) for short and chopped fibers, filament winding, and pultrusion for continuous fibers. However, in natural fibers, there are no continuous fibers such as in manmade fibers. The length of some natural fibers (single) reaches up to 4 m, and if it is a bundle of fibers connected to the length direction, then the fiber length can be longer than 4 m. Prepregs are also used.8

9.5.1.1 Hand Layup In this method, the mold is first prepared with a sufficient degree of finish. A release agent is applied to the mold. A gel coat consisting of a resin with or without a pigment is then applied to the mold to get a resin-rich top surface. Then the resin is mixed with suitable proportions of accelerator, promotor, and catalyst. The resin-impregnated fiber/mat/fabric is laid on the prepared mold to a desired thickness. The material is pressed with a hand roller to eliminate any entrapped air bubbles and extra resin. After curing, the product is subjected to a postcuring treatment. However, not much fiber loading is possible using this technique. The amount of fiber loading depends largely on the processing method. This also depends on the anatomical features of the natural fibers, which have intrafiber voids called lumen. If the fiber has large lumens and small cell walls, the fiber loading can be increased by compression of such voids in the fibers. It was found that up to 10 wt% coir fiber/mat, banana/sisal, and 14 wt% of banana cotton fabric could be incorporated in the polyester resin by this technique. Natural fiber reinforcements Copyright © 2005 by Taylor & Francis

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should be dried properly in an oven prior to resin impregnation to avoid poor wetting and moisture entrapment in the composites.8

9.5.1.2 Press Molding Press molding includes sheet molding, bulk molding, and cold press and hot press methods. The procedure is similar to the hand layup technique, except that herein a set of matched dies are used, which are closed before cure takes place, by the application of pressure. By this method, nearly 70 wt% of fibers could be incorporated and thickness of the product can be 1−10 mm. For cold press and hot press molding, the curing temperature is normally 40o−50oC and 80o−100oC for 1−2 h, respectively. This technique has been successfully used for incorporating up to 40 wt% of coir fiber/mat, 70 wt% of sisal, 40−60 wt% of banana, pineapple, and jute, and 25−30 wt% of banana cotton fabric in the polyester resin.8 Here, a much larger volume of fiber loading is possible compared to the hand layup process, as here, the applied pressure compresses the intrafiber voids or lumens more effectively and allows a greater amount of fiber incorporation. 9.5.1.3 Filament Winding Here, the continuous fibers are impregnated in a resin mix (resin ⫹ accelerator ⫹ catalyst ⫹ promotor) bath and then wound on a rotating mandrel. Sisal− epoxy and jute–polyester composites have been fabricated successfully to prepare cylinders with longitudinal or helical and hoop reinforcements.8 9.5.1.4 Pultrusion Pultrusion is a useful tool in making continuous products for varied application. Ray et al.43 used the hand pultrusion method to fabricate their composite rod samples for experimentation. The hollow glass tubes were used as a mold. The predried fibers were mixed with the resin mix (resin mixed with accelerator, catalyst, and promotor) and pulled through the glass tubes by hand to produce composites in the form of rods. Continuous profiles of any dimension can be manufactured by this method. While this technique is very popular for manufacturing synthetic fiber-reinforced composites, there remains immense scope in utilizing this method for manufacturing biocomposites for widespread applications.

9.5.2

Properties and Structure

9.5.2.1 Mechanical Properties There have been various studies on the mechanical properties of the composites reinforced with natural fibers. A brief review follows of the published literature on untreated as well as chemically modified natural fiber-reinforced thermosetting resin composites. Copyright © 2005 by Taylor & Francis

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9.5.2.1.1 Jute−Thermoset Composites Jute fiber has been one of the most important natural fibers used as reinforcement in composites. The tensile strength, modulus, and work of fracture of the jute–polyester composites were examined as a function of fiber volume fraction by Roe and Ansell.42 They prepared the samples by the press molding technique. The derived fiber strength and Young’s modulus were calculated to be 442 MPa and 55.5 GPa, respectively. Composites were fabricated by the press molding technique allowing a maximum loading of 0.6 volume fraction of fiber. Polyester resin formed an intimate bond with the jute fibers up to a volume fraction of 0.6, above which the quantity of resin was insufficient to wet out the fibers completely. At this volume fraction, the composite tensile strength was 250 MPa, Young’s modulus was 35 GPa, the work of fracture by Charpy impact test was 22 kJ/m2, and the interlaminar shear strength was 24 MPa. Ray et al.43 reported that the treatment of jute fibers with 5% NaOH solution for four different time periods (2, 4, 6, and 8 h) resulted in an improvement in the flexural strength and modulus of the treated fiber-reinforced vinylester resin composites. However, a maximum of 35% loading could be achieved as the samples were prepared by the hand pultrusion method. The results are given in Table 9.5. Maximum improvement in the composite properties were observed with 4 h. alkali-treated fibers. Authors suggested that an optimum combination of increased fiber strength properties and improved interfacial bonding was responsible for this rise in properties. The increase in fiber strength properties was attributed to the reduction of the linear density of the fibers on alkali treatment and also to the closer packing of the cellulose chains and molecular reorientation on prolonged soaking in the alkali solution (4−8 h), as was seen for coir and flax.44 There have been many reports on alkali-treated jute fiber−thermoset composites prepared by the filament winding technique. Gassan and Bledzki45,46 reported an improvement of 120% and 150% in the tensile strength and modulus of the isometric jute yarns, respectively, when treated with a 25% NaOH solution for 20 min. They also reported a 60% improvement in the jute−epoxy composite properties reinforced with these treated yarns. The mechanical properties of the tossa jute fibers were optimized by using NaOH solution with different concentrations and shrinkages. They observed that the shrinkage of the fibers during treatment had a significant effect on fiber structure, hence on the mechanical properties and fracture behavior of the composites. An attempt was made47 to overcome the high resin consumption and absorption and desorption of moisture in jute−polyester composites by coating the fibers with 10 wt% lignin solution (10 wt% lignin dissolved in acetone) for 30 min, followed by a treatment with ethylene diamine (EDA) before incorporating the fibers into the polyester matrix. It was reported that the resin wastage with the treated fibers was considerably reduced and resin consumption was found to be half that required for untreated fibers during the fabrication of unidirectional composites. The EDA treatment has brought down the moisture absorption of the fibers in the composites without Copyright © 2005 by Taylor & Francis

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TABLE 9.5 Flexural Properties of the Vinylester Resin and the Untreated/Alkali-Treated Jute–Reinforced Composites

Jute Volume (%) 0 8

15

23

30

35

Source:

Modulus (GPa) Type of Fiber — Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h

Mean value 2.915 4.220 3.446 4.205 3.967 3.130 5.544 6.024 6.539 5.546 5.337 7.355 8.065 9.384 8.542 7.132 10.030 10.990 12.850 12.490 11.170 11.890 12.700 14.690 14.890 12.320

Flexural Strength (MPa)

Breaking Energy (J)

Toughness (kJ/m2)

SD

Mean value

SD

Mean value

SD

Mean value

SD

0.10 0.33 0.53 0.71 0.66 0.43 0.55 0.04 0.42 0.21 0.26 0.51 0.79 0.44 0.41 0.45 0.62 0.27 1.07 0.94 0.42 0.62 0.37 0.85 1.17 0.35

120.70 106.30 96.27 121.20 101.80 93.97 128.60 134.70 146.50 121.50 127.60 145.70 157.70 172.70 155.40 145.80 180.60 189.40 218.50 195.90 197.50 199.10 205.20 238.90 232.00 204.20

11.18 7.88 3.99 4.02 6.14 4.95 3.00 3.50 6.40 7.93 3.50 12.60 1.10 6.70 10.70 6.50 1.40 15.00 13.50 10.04 7.60 7.60 6.00 17.60 7.70 1.20

0.8227 0.2948 0.2497 0.3634 0.2270 0.2488 0.3399 0.3530 0.4016 0.2569 0.3351 0.3531 0.4048 0.4198 0.3553 0.3762 0.4799 0.4816 0.5061 0.4319 0.5042 0.5543 0.4570 0.5695 0.5678 0.5099

0.33 0.02 0.03 0.03 0.04 0.02 0.02 0.01 0.05 0.04 0.03 0.08 0.04 0.05 0.04 0.04 0.02 0.10 0.02 0.05 0.05 0.07 0.04 0.10 0.02 0.02

34.00 11.75 10.13 14.75 9.28 9.92 13.80 14.53 16.30 10.24 13.60 14.08 16.61 16.74 14.37 15.27 19.14 19.55 20.69 17.53 20.32 22.10 18.55 21.92 23.05 19.97

13.63 0.79 1.21 1.21 1.63 0.79 0.81 0.41 2.02 1.59 1.21 3.19 1.64 1.99 1.61 1.62 0.79 4.05 0.81 2.02 2.01 2.79 1.62 3.84 0.81 0.78

From Ray, D. et al., Composites, Part A, 32, 119, 2001. Courtesy of Elsevier, London.

significantly affecting the tensile strength and modulus of jute−polyester composites. Chawla and Bostos48 reported a linear increase in the tensile properties of the laminates with increase in volume fraction of jute fibers. The impact energies and tensile strengths of jute fiber-reinforced polyester composites were studied by Semsarzadeh et al.49 Jute−polyester composites showed a drop in mechanical properties when more than three layers of cloth were used, because of incomplete wetting of the fabric.49 In an attempt to improve fiber-matrix interactions in jute–polyester composites, poly(vinyl acetate), plasticized or in its commercial form, was used as an emulsion on the jute cloth in polyester resin, which increased the flexural strength and the mechanical properties of the jute-reinforced polyester composites.50 It was reported that the impact energies and tensile strength of Copyright © 2005 by Taylor & Francis

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these composites were greatly improved by calendering of the cloth prior to spraying with poly(vinyl acetate). The calendering of the jute cloth assisted orientation of the fibers and thus allowed for more efficient packing and fewer microvoids in the final composites, lower water absorption, lower soluble matter loss, and higher mechanical properties. However, intensive calendering led to fiber damage and breakage. Modifications of the jute fibers with phenol formaldehyde (PF) or resorcinol formaldehyde (RF) resins by different approaches were observed to have enhanced the reinforcing character of the jute fiber in jute−unsaturated polyester (USP) resin-based composites.51 In one set of modifications, the jute was treated with preformed PF or RF resoles to about 4 wt% resin add-on, with the objective of introducing PF/RF resin at the interface between the reinforcing jute fiber and polyester resin matrix. In another set of modifications, the jute/oxyjute was treated with P and F (or R and F) in successive steps (in situ resin formation in the presence of jute) with the intention of establishing chemical linkages between the phenolic resin formed in situ and the fiber surface, through the reducing –CHO groups present in the cellulosic and hemicellulosic constituents of the fibers, thus proving a durable compatibilizing phenolic resin layer (4−8% add-on) at the interface. Table 9.6 shows some of the mechanical properties of unmodified and modified fiber− polyester composites. Table 9.6 revealed that both sets of treatments brought substantial improvements in the mechanical properties of jute–polyester composites. The flexural strength values improved by 20−40% and the flexural modulus values by 40−60%. The best all-round improvement in strength of the composite was obtained on treatment of jute with the RF resin system. The authors also carried out a weathering study of their composites and reported improved environmental and chemical resistance of the composites based on the modified jute fiber. The authors suggested that the presence of layers of hydrophobic phenolic resin moieties at the interface of the jute fiber sharply hindered, if not eliminated, the approach of moisture or water, particularly to the fiber−resin interface. Ghosh and Ganguly51 have reported the effect of grafting on the performance of unsaturated polyester (USP) resin-based jute composites. Chemical modifications of jute were achieved through graft copolymerization with acrylonitrile (AN) or methyl methacrylate (MMA) using an aqueous Cu2⫹− IO4 combination as initiator.52 Grafting of either polyacrylonitrile (PAN) or polymethyl methacrylate (PMMA) onto the jute fibers brought about a notable improvement in the strength and environmental resistance of the jute/polyester composites. Table 9.7 shows the effect of grafting on the mechanical properties of jute−polyester composites. From the table, it is clear that PAN grafting showed higher improvement in the strength of the composites in comparison with PMMA grafting. Generally, vinyl grafting (10−25%) led to an improvement in flexural strength and flexural modulus by 10−25% and 10−30%, respectively. The highest improvement in flexural strength (FS) and flexural modulus (FM) was apparently realized with 15% grafting of PAN or PMMA on jute. Again both PAN grafting and PMMA Copyright © 2005 by Taylor & Francis

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TABLE 9.6 Effect of Modifications of Jute Fibers with Resins on the Properties of Jute–USP Composites Flexural Properties after Exposure to

Mechanical Propertiesc

Water Reinforcing Jute Fiber Useda Unmodified Oxidized Modified with preformed PF(4%) Modified in situ with PF (4%) Modified with preformed RF (4%) Modified in situ with RF (4%) Modified in situ with RF (4%)b Modified in situ with RF(8%)b

Moisture

FS FM TS TM ILSS FS FM FS FM (MPa) (GPa) (MPa) (GPa) (MPa) (MPa) (GPa) (MPa) (GPa) 89 67 107

9.00 7.48 12.6

59.2 49.2 64.6

6.48 5.38 7.92

8.01 6.4 8.99

60.6 — 81.5

4.24 — 6.84

67.2 — 84.6

5.0 — 8.6

109

12.5

66.0

7.99

9.10

79.0

6.64

85.0

8.2

133

14.05

82.0

8.27

101.0

7.92

107.3

9.97

120

14.30

79.0

8.21

9.98

92.0

7.90

97.2

9.93

111

13.4

66.6

7.98

9.35

82.5

7.12

89.4

8.8

112.5

13.7

68.0

7.94

9.4

85.0

7.09

90.8

9.2

10.0

Jute fiber content of the composites ⫽ 44 ⫾ 2% (w/w). Modified oxy–jute. c FS, Flexural strength; FM, flexural modulus; TS, tensile strength; TM, tensile modulus. Source: From Ghosh, P., and Ganguly, P.K., Plast. Rubb. Camposites Proc. Appl., 20, 171, 1983. With permission. a

b

TABLE 9.7 Effect of Grafting of Jute Fiber on the Properties of Jute–Polyester Composites Flexural Properties after Exposure to

Mechanical Propertiesb

Water Reinforcing Jute Fiber Useda Unmodified 10% PAN grafted 15% PAN grafted 25% PAN grafted 10% PMMA grafted 15% PMMA grafted 25% PMMA grafted

Moisture

FS FM TS TM ILSS FS FM FS FM (MPa) (GPa) (MPa) (GPa) (MPa) (MPa) (GPa) (MPa) (GPa) 89.0 98.5 110.0 109.0 99.0 99.0 95.0

9.00 9.96 11.80 11.20 9.93 10.06 9.7

59.2 62.8 66.1 67.6 60.98 61.5 59.9

6.48 7.20 7.85 7.91 6.98 6.98 6.71

8.01 8.69 8.94 8.98 8.26 8.30 8.99

60.6 78.0 — — 76.4 — —

4.24 6.96 — — 6.90 — —

67.2 82.6 92.7 91.3 81.3 82.7 79.8

5.0 7.25 8.65 8.50 7.45 7.68 7.47

Jute fiber content of the composites ⫽ 44 ⫾ 2% (w/w). FS, Flexural strength; FM, flexural modulus; TS, tensile strength; TM, tensile modulus. Source: From Ghosh, P. and Ganguly, P.K., Plast. Rubb. Composites Proc. Appl., 20, 171, 1983; Ganguly, P.K., Ph.D. thesis, Calcutta University, 1992. With permission. a

b

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grafting of the jute improved the environmental resistance of jute−polyester composites significantly. Both PAN and PMMA grafting appeared to be equally effective in this respect, yielding about 75−80% retention of FS and about 65−70% retention of FM of jute−polyester composites after an exposure to moisture (95% RH, 35°C, 14 weeks), water, and a dilute solution of chemicals like 3% NaCl, 5% CH3COOH, and 2% Na2CO3 (35°C, 72 h). The improved strength of the composites based on grafted jute fabrics could be attributed to the improved compatibilities between the polyester resin and jute fibers through vinyl polymer moieties deposited on the fiber surface during the grafting process. Recently, Dash et al.53 fabricated a jute−polyester composite by using solution impregnation and hot curing methods. They have used both untreated and bleached jute slivers with various percentages to prepare the composites. It was observed that the composites with 60 wt% of jute fiber yielded the best results. The various mechanical properties of the composites are listed in Table 9.8. From the table, it is clear that both the untreated jute− polyester and bleached jute–polyester composites have very high tensile properties but bleached jute–polyester composites have slightly lower tensile properties than untreated jute–polyester composites. Again the flexural strength, modulus, interlaminar shear strength (ILSS), and so on were better for the bleached jute composites. The better flexural properties of the bleached jute composites could be due to less stiffness and a more flexible character of the fibers after delignification. Again, the bending forces were better propagated along the longitudinal direction of the fibers in the case of the bleached jute–polyester composites. The authors also reported that the storage modulus of the bleached jute composite was higher than that of the untreated jute composite. The better flexural properties of the bleached jute composites were explained on the basis of fiber-matrix adhesion. The much TABLE 9.8 Mechanical Properties of Jute Sliver (60 wt%) – Polyester Composites Unweathered

Properties Ultimate stress (MPa) Ultimate strain (%) Tensile energy absorption (N/mm) Tensile modulus (GPa) Flexural yield strength (MPa) Flexural modulus (GPa) Inter laminar shear strength (MPa) Source:

Weathered

Untreated jute polyester

Bleached jute polyester

Untreated jute polyester

132.40 5.834 9.767

117.00 6.677 10.600

88.00 5.417 8.775

78.15 4.460 6.675

2.956 140.4 13.85 3.865

2.106 171.8 18.44 4.032

3.639 100.7 8.94 3.192

3.902 105.76 7.33 3.30

From Dash, B.N., Polym. Composites, 20, 62, 1999. With permission.

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Bleached jute polyester

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higher flexural properties of the bleached fiber composites compared to the untreated fiber composites were attributed to a greater interfacial adhesion in bleached fiber composites, as observed from their fractographs.54 The greater adhesion between the bleached jute fiber and the polyester matrix could be due to greater chemical and physical bonding between the free hydroxyl (produced due to removal of lignin during bleaching) and free carboxyl functions of the polyester matrix. The above prepared composite specimens were subjected to natural outdoor weathering for 45 days to assess their relative environmental performance per the ASTM-D-1435 standard.55 The various mechanical properties of the composites after weathering are listed in Table 9.8. The tensile strength of the untreated and the bleached fiber composites decreased while the tensile modulus increased after weathering. The flexural strength, modulus, and interlaminar shear strength (ILSS) of the weathered specimens were lower than those of the unweathered ones. The bleached fiber composites also showed lesser water absorption (8.48%) than the composites reinforced with the untreated fibers (12.25%). Philip56 and the Fabric Research Laboratories57 also reported the weathering of jute− polyester laminates exposed to water. They found that there was a loss of mechanical strength upon immersion in water and seawater. They suggested that this loss could be minimized by the use of a glass fiber surface layer, gel coat, and edge protection. An extensive review on the jute composites has been made by Mohanty and Mishra.58 Mukherjee et al.59,60 attempted a multifunctional resin polyester amide polyol (PEAP) as the interfacial agent in jute−epoxy and jute–polyester composites, and significant improvement in the mechanical properties of the composites were observed. In the case of unidirectional jute−epoxy composites with raw jute fibers, the flexural strength was 291 MPa whereas with the incorporation of PEAP, the strength was increased up to 386 MPa. Since PEAP linked the fiber and the matrix through hydrogen bonding, the hydroxyl value of PEAP played an important role. They optimized the hydroxyl value so that maximum strength could be obtained. They found that in the case of unidirectional jute− epoxy composites, a maximum flexural strength of 386 MPa could be attained when the hydroxyl value of PEAP was 66.71. Gassan and Bledzki61 prepared jute−epoxy composites using untreated/ silane treated jute woven as the reinforcement and investigated the effect of cyclic moisture absorption−desorption on the mechanical properties of the composites. Silane treatment of the jute fibers led to an increase in the tensile, flexural strength, and modulus of the composites up to 30%. Absorption− desorption cycles changed the fracture mechanisms of the fibers. Microscopic examination revealed that the absorption−desorption cycles of the composites led to debonding of the resin from the fibers, and cracks were formed in the adjacent resin, leading to a lowering of mechanical properties. Bledzki et al.62 prepared jute-flax woven fabric reinforced epoxy foam composites and investigated their impact properties by measuring their loss of energy and damping parameters. They observed that the void content had a significant effect on the impact resistance of the composites. They also Copyright © 2005 by Taylor & Francis

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reported that with an increase in void content and impact energy, the loss energy and damping index exhibited approximate linear increases under comparable test conditions. 9.5.2.1.2 Coir−Thermoset Composites Coir fibers, due to their hard-wearing quality, durability, and other properties, have been used for making a wide variety of floor furnishing materials, yarn, rope, etc. But these traditional coir products consume only a small percentage of the potential total world production of coconut hush. Hence, apart from the conventional uses of coir as mentioned above, research and development efforts have been under way to find several new areas of use, including utilization of the coir as a reinforcement in polymer composites. Attempts were made to incorporate up to 9 wt% of coir into polyester in the form of laminates.63 Mechanical properties, including UTS modulus, flexural and impact strengths, and selected electrical properties of these composites are listed in Table 9.9. Recently, Rout et al.64,65 developed composites from unidirectional nonwoven coir mat with the polyester resin. The fiber weight percentage in composites was varied from 0 to 33%. It was observed that the optimum mechanical properties of coir−polyester composites were obtained at 25 wt% fiber content. Both the tensile strength and flexural strength of composites at 25 wt% of fiber content were 26% higher than that of the neat polyester resin whereas both properties of composites at 25 wt% fiber were 71% higher than the composites containing 10 wt% of coir fiber. For the above behavior, the authors suggested that at lower fiber content a weak interface was created due to a poor wetting of the fibers in the matrix. Hence, at lower tensile stress, weak interface cracks could have formed leading easily to failure. However, at higher fiber loading (17 and 25 wt%), adequate fiber content in the composites led to a better wetting. The effect of fiber loading on the various mechanical properties of the composites is given in Table 9.10. TABLE 9.9 Properties of Coir–Polyester Composites with 9 wt% Coir Density (kg/m3) Strength (MPa) Tensile Flexural Modulus of elasticity (GPa) Impact resistance (kg/m2) Water absorption (24 h, room temperature) (%) Volume resistivity (Ω cm), 100 V DC Dielectric strength, 2.5 mm thick Dielectric constant at 1.5 MHz

1160 18.61 38.51 4.045 391.0 1.36 400 100 kV/min 3.5

Source: From Satyanarayana, K.G. et al., Proc. 1st Int. Conf. Composite Structures, 1981. With permission.

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To improve the interfacial adhesion between the coir fiber and the polyester matrix, the fibers were chemically modified through various methods like dewaxing, alkali treatment, cyanoethylation, bleaching, and vinyl polymer grafting. The mechanical properties such as tensile strength, flexural strength, and impact strength of various chemically modified coir fiber-reinforced polyester composites are listed in Table 9.11.64,65 As expected, all types of chemical TABLE 9.10 Effect of Fiber Loading on Mechanical Properties of Coir–Polyester Composite Fiber (wt%) 0 10 17 25 33

Tensile Strength (MPa) 20.60 15.22 21.30 26 24.55

SD

Flexural Strength (MPa)

SD

Izod Impact Strength (J/m)

0.48 2.65 1.9 1.28 1.55

48.50 35.84 51.49 61.34 21.38

0.85 4.51 2.34 4.02 3.84

14.78 277.08 433.53 967.56 568.0

Source: From Rout, J. et al., Polym. Composite, 22, 468, 2001; Rout, J. et al., Comp. Sci. Tech., 61, 1303, 2001. With permission.

TABLE 9.11 Mechanical Properties of Various Chemically Modified Coir (17 wt%)–Polyester Composites Sample

TS (MPa)

SD

Untreated Dewaxed Alkali treatment (%) 2 5 10 Cyanoethylation (at °C) 50 60 70 Bleaching (at °C) 65 75 85 PMMA grafting (%) 5 10 20 PAN grafting (%) 5 10 15

21.30 22.94

1.9 0.80

26.80 24.70 17.34

FS (MPa)

SD

IS (J/m)

51.49 55.75

2.34 1.39

433.53 496.89

1.50 1.88 2.12

59.01 60.45 49.3

1.37 0.98 1.92

521.84 634.59 494.68

22.58 26.30 24.27

3.68 4.43 1.94

53.86 54.13 53.09

3.17 4.09 3.36

729.49 758.54 731.33

20.32 19.27 17.91

2.50 1.26 2.65

61.65 57.80 55.32

6.02 3.50 5.07

458.31 425.28 389.73

25.01 21.58 14.81

2.78 1.69 3.83

63.68 57.26 37.81

3.68 5.58 6.20

723.17 251.84 66.23

22.51 24.36 18.73

1.9 2.76 1.73

56.35 59.13 48.93

3.01 3.56 3.72

477.01 596.33 189.63

Source: From Rout, J. et al., Polym. Composite, 22, 468, 2001; Rout, J. et al., Comp. Sci. Tech., 61, 1303, 2001. With Permission.

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modifications improved the mechanical properties of the composites. Moderate alkali treatment (2% and 5%), cyanoethylation, and low percentage (5−10%) of vinyl polymer (PMMA and PAN) grafting were found to be effective chemical modifications for the coir fiber to produce mechanically strong coir composites. In the case of cyanoethylated coir composites, the improved mechanical properties could be due to a strong polar interaction between the β-cyanoethyl group of the fiber with the polymer matrix, leading to greater fiber-matrix adhesion. For the vinyl polymer-grafted coir polyester composites, the authors suggested a better wetting and adhesion between the polymer matrix and coir fiber through the vinyl polymer moieties on the fiber surface owing to the fiber modification treatment. Although bleached fiber composites showed poor tensile and impact strengths, the flexural strengths of composites were much higher than untreated fiber composites. The authors examined fiber-matrix interface by taking scanning electron micrographs of a tensile fractured surface of the composites. The results corroborated with the mechanical properties of the composites. Alkali treatment of the coir fibers35 for 72 to 96 hours resulted in a 10−15% increase in tensile strength, a 40% increase in tensile modulus, and 90% increase in debonding strength from the polyester. The charpy-type impact strength of the composite containing 0.2 volume fraction of the alkali-treated fibers was 35% higher than that of the polyester. The flexural strength of the polyester was lowered by 15% with an incorporation of 0.3 volume fraction of alkali-treated fibers, although a 5−10% increase in flexural modulus was observed. The mechanical properties of the coir−polyester composites containing both untreated and alkali-treated coir fibers is given in Table 9.12. Sound attenuation values were reported to be slightly higher for the composites containing untreated fibers compared to the alkali-treated fibers.

TABLE 9.12 Mechanical Properties of Coir–Polyester Composites Containing Untreated and Alkali-Treated Coir Fibers Volume Fraction of Coir

Treatment Given to Coir

Porosity (%)

Flexural Strength (MPa)

Flexural Modulus (GPa)

0 (Plain polyester) 0.10

— Untreated 5S72 5SR 144 Untreated 5S72 5SR 144 Untreated 5S72 5SR 144

— 3.71 0.65 0.50 9.17 1.60 0.89 11.24 4.50 2.43

48.5 33.5 34.0 34.5 33.0 42.3 42.0 29.0 41.5 40.5

3.077 2.792 3.026 3.004 2.497 3.380 3.390 1.720 3.334 3.380

0.20

0.30

Charpy-Type Impact Strength (103 N mm⫺2) 8.33 — — — 7.44 11.33 — — — —

Source: From Prasad, S.V. et al., J. Mater. Sci., 18, 1443, 1983. With permission of Kluwer Academic.

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Owolabi et al.66 observed that the treatment of natural coconut fibers with NaOH enhanced the bonding between the fiber and the resin and improved its properties. 9.5.2.1.3 Sisal−Thermoset Composites Sisal fiber is yet another natural fiber found to be of interest. Chand and Rohatgi67 have reported poor bonding between untreated fiber and polyester resin, with some increase in tensile strength of the composites with an increased volume fraction of the fiber. To improve the bonding, they modified the surface of the sisal fiber and found that a 72-hours alkali treatment with 5% concentration improved tensile and impact strengths of the polyester composites. The pullout load of the alkalized sisal fibers increased from 10 N to 15.5 N and adhesion increased correspondingly up to an alkalizing time of 90 h.67 The effects of several chemical treatments on the sisal−polyester resin composites (∼50 vol%) by organotitanate, zirconate, silane, and N-substituted methacrylamide were investigated by Singh et al.68 (Table 9.13). From the table it is clear that the chemical modifications of sisal fiber caused an improvement of 15 to 33% in tensile strength, 45 to 79% in tensile modulus, and 21 to 29% in both flexural strength and flexural modulus. However, the silane-treated samples showed an improvement of ∼62% in flexural strength properties. The effect of humidity on the tensile and flexural strength of sisal−polyester composites was evaluated by Singh et al.68 A significant reduction in strength (∼30 to 44%) was obtained for both untreated and chemically treated sisal composites. It was observed that N-substituted methacrylamide-treated sisal composites exhibited superior properties under dry as well as wet conditions. Recently, Mishra et al.69 developed composites from sisal fiber with polyester resin. In their study, they used sisal fiber in the form of unidirectional TABLE 9.13 Effect of Chemical Treatments of Sisal Fiber on the Physicomechanical Properties of Sisal-Polyester Composite (Fiber Content ∼ 50 vol%)

Property

Untreated

N-Subsitituted Methacrylamide Treated

Density (g/cm3) Void content (%) Tensile strength (MPa) Elongation (%) Tensile modulus (GPa) Energy at break (MJ/m2) × 105 Flexural strength (MPa) Flexural modulus (GPa)

0.99 16.11 29.66 9.52 1.15 7.96 59.57 11.94

1.05 5.88 39.48 9.75 2.06 11.06 76.75 16.35

Silane Treated

Titanate Treated

Zirconate Treated

1.12 3.01 34.14 5.71 1.75 4.78 96.88 19.42

1.02 12.55 36.26 8.00 1.67 8.60 75.59 15.13

1.00 13.30 34.69 9.51 1.39 10.13 72.15 14.46

Source: From Singh, B. et al., Polym. Composites, 17, 910, 1996. Courtesy of the Society of Plastics Engineers, Inc., United States.

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non-woven sisal mat. The fiber content was varied from 0 to 40 wt% and the various mechanical properties such as tensile strength, flexural strength, and impact strength of resulting composites were evaluated. It was observed that with an addition of 30 wt% fiber, the tensile strength and the flexural strength of the polyester resin increased by 198% and 122%, respectively. The impact strength of the composites also showed an increasing trend up to 30 wt% fiber content, and then with higher fiber content (40 wt%) the values decreased. It was observed that optimum mechanical properties were obtained at 30 wt% of fiber content in the composites. The authors also investigated the effects of various chemical modifications such as dewaxing, alkali treatment (5% and 10%), bleaching, cyanoethylation, and vinyl monomer (AN and MMA) grafting of sisal fiber on the physicomechanical properties of the composites. The various mechanical properties of the composites are given in Table 9.14. It was observed that the composites with an alkali- (5%) treated fiber achieved better mechanical properties as compared to untreated, dewaxed, 10% alkali-treated and bleached sisal fiber-based composites. The tensile strength (TS), flexural strength (FS), and impact strength (IS) of 5% alkali-treated fibers were found to have increased by about 22, 43, and 21%, respectively, over those of untreated fiber composites. It was observed that the composites made with cyanoethylated sisal fibers showed improved mechanical properties as compared to untreated fiber composites. The composite of cyanoethylated sisal fiber (60°C) was found to achieve maximum TS (84.29 MPa), FS (149.09 MPa), and IS (192.46 J/m) as compared to the composites containing cyanoethylated sisal fibers at 50°C and 70°C. Table 9.14 showed the effects of AN and MMA grafted sisal fibers TABLE 9.14 Physicomechanical Properties of Sisal (30 wt%)–Polyester Composites Chemically Treated Sisal Untreated Defatted Alkali treated (5%) Alkali treated (10%) Bleached (75°C) Cyanoethylated (50°C) Cyanoethylated (60°C) Cyanoethylated (70°C) AN-grafted (5%) AN-grafted (10%) AN-grafted (20%) MMA-grafted (5%) MMA-grafted (10%) MMA-grafted (20%)

TS (MPa)

FS (MPa)

IS (J/m)

Water Absorptiona(%)

68.33 74.21 83.25 51.49 47.51 71.69 84.29 69.18 69.11 75.38 32.58 61.72 73.13 30.27

107.8 121.23 153.94 79.91 112.19 115.27 149.09 110.71 102.31 135.67 81.61 100.91 122.18 80.52

163.58 172.81 197.88 167.23 171.17 171.27 192.46 175.38 160.27 171.15 102.43 158.29 169.71 99.37

10.21 8.99 7.52 — 7.49 — 5.58 — — 5.34 — — 7.23 —

a

Immersion time 24 h at room temperature. Source: From Mishra, S. et al., J. Reinf. Plast. Composites, 20, 321, 2001. Courtesy of Sage, Thousand Oaks, CA.

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on the mechanical properties of the resultant composites. With a 5% AN grafting, tensile strength (TS) increased by 11%, whereas flexural strength (FS) and impact strength (IS) decreased by 5.1% and 2%, respectively. However, an increase in the percentage of grafting (to 10%) increased the FS and IS by about 25.8% and 4.6%, respectively. Further increasing the AN grafting to 20% decreased all the mechanical properties of the composites. At the lower percentage of MMA grafting (5%), mechanical strengths of the composites showed reduced values as compared to untreated fiber composites whereas the composites prepared from 10% MMA-grafted sisal fibers exhibited an improved TS, FS, and IS by about 7%, 13.3%, and 3.7%, respectively. However, with further increase of MMA-grafting (20%), the mechanical strength of the composites decreased drastically. Bisanda and Ansell70 modified the surface of the sisal fibers by mercerization. They observed that mercerization improved the wettability of the fibers with the resin and enhanced the interfacial bonding by giving rise to additional sites for mechanical interlocking, thus helping more resin penetration at the fiber interface. It resulted in a 21% rise in the compressive strength of the dry composites (compressive strength of the sisal−epoxy composites, with the untreated fibers having a volume fraction of 0.4 in the dry condition, was 148 MPa, whereas with mercerized fibers compressive strength was 183.1 MPa). They also observed that the treatment of the mercerized sisal fibers with aminosilanes70 before forming the sisal−epoxy composites markedly improved the moisture-repellant property of the composites. They used alcoholic solutions of 5% with a catalyst as well as pure silane, and the duration of the treatment was 24 h. They concluded that the water absorption of the sisal−epoxy composites could be remarkably reduced by the treatment of the fibers with a pure silane. However, the silane treatment did not increase the flexural strength and the flexural stiffness of the composites. In contrast, the compressive strength was clearly increased by the coupling agent. These examples showed that the theories used for the silane treatment of natural fibers were contradictory, and thus, require further studies. Rong et al.71 tried alkali treatment, acetylation, cyanoethylation, treatment with organosilane coupling agent, heat treatment, and mixed treatments of the sisal fibers and observed the effect of various chemical treatments on the strength properties of the treated sisal−epoxy composites. They reported that with the chemically treated fibers, the flexural strength of the epoxybased composites became higher than that of the untreated versions due to the intrinsically increased strength of the fibers themselves having better wetting characteristics. However, not all the chemical treatments showed similar improvement in tensile strength of the composites. 9.5.2.1.4 Pineapple Leaf Fiber–Thermoset Composites Mishra et al.69 developed composites from PALF and polyester resin. They studied the effect of fiber loading and chemical modification of fibers on

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their mechanical properties. The fiber content was varied from 10 to 40 wt%, and it was observed that all mechanical properties, such as tensile strength (TS), flexural strength (FS), and impact strength (IS), increased gradually with an increase of fiber loading from 10 to 30 wt%. Addition of 30 wt% of fiber increased the tensile strength (TS), flexural strength (FS), and impact strength (IS) by 89%, 76%, and 44% against that of neat polyester resin. However, further increase in fiber content to 40 wt% decreased the TS, FS, and IS of composites. The decreased mechanical strength with higher fiber content (40 wt%) was explained in terms of stress concentration and dispersion problems. The effects of various chemical modifications i.e., dewaxing, alkali treatment, cyanoethylation, and AN grafting of PALF, on mechanical properties and water absorption are given in Table 9.15. As seen from the table, all types of chemical modifications of the fiber improved the mechanical strength of the resultant composites as well as reduced the percentage of water absorption. By studying the fractured surfaces of the composites using SEM, they showed that the superior mechanical properties of the composites containing appropriate chemically modified fiber were obtained due to improved fiber-matrix adhesion. 9.5.2.1.5 Sunhemp–Thermoset Composites Various workers developed polyester composites from sunhemp fiber.39,72,73 Continuous unidirectionally aligned sunhemp polyester composites with varying fiber volume fraction were prepared using a modified hand layup technique for low-volume fractions and a pultrusion technique for highvolume fraction composites. The tensile strength and linear elastic modulus of the composites were found to increase linearly with volume fraction of the fibers (up to 0.4 volume fraction). The izod impact strength of the composites was 21 kJ/m2, which increased linearly with the increase in the volume fraction of fiber, indicating a high toughness property.

TABLE 9.15 Physicomechanical Properties of PALF (30 wt%)–Polyester Composites Chemically Treated PALF Untreated Defatted Alkali-treated Cyanoethylated AN-Grafted (5%) AN-Grafted (10%) AN-Grafted (20%)

TS (MPa)

FS (MPa)

IS (J/m)

Water Absorptiona (%)

43.38 37.71 44.77 43.75 40.16 48.36 21.97

85.81 98.78 104.51 121.65 75.65 97.9 66.27

80.29 81.37 91.45 102.04 76.98 83.31 57.97

11.40 9.65 8.62 9.49 10.50 5.82 5.58

Immersion time ⫽ 24 h. Source: From Mishra, S. et al., J. Reinf. Plast. Composites, 20, 321, 2001. Courtesy of Sage, Thousand Oaks, CA.

a

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9.5.2.1.6 Straw−Thermoset Composites Wheat straw–polyester composites were developed by White and Ansell15 for the manufacture of constructional boards. It was found that up to 50 wt% straw fiber could be incorporated within polyester resin. The processes involved were as follows: (1) combining both straw fiber and resin with a catalyst in an open mold and a close fitting plunger, (2) alternative layers of fiber and resin were built in the mold and subjected to a pressure of 5 MPa for about 18 h, and (3) postcuring for 2 h at 80°C and removing the mold. Higher fiber content (50 wt%) posed difficulty in removing composites from the mold. It was also reported that a pretreatment of the straw fiber was necessary to remove the waxy layer in the epidermis of the straw fibers in order to get a good bonding with the polyester resin. The mechanical properties of these composites are given in Table 9.16. The straw fibers improved the stiffness, strength, and toughness of the resin alone, and reduced its density. 9.5.2.1.7

Banana Fiber−Thermoset Composites

The mechanical, failure and aging characteristics of the short banana fiberreinforced polyester composites were studied by Pothan et al.74 They observed that the maximum tensile strength was achieved at 30 mm fiber lengths, while impact strength gave maximum value for 40 mm fiber lengths. Treatment of banana fibers with silane coupling agent improved the tensile strength of the composites by 28% and the flexural strength of the TABLE 9.16 Mean Flexural and Tensile Properties of Straw–Polyester Composites Flexural Weight Fiber (%)

Volume Fiber (%)

Density ρc (kNm⫺3)

0

0

11.28

10

21

20

37

30

50

40

61

40a

61

50

70

9.91 (⫾ 0.68) 8.83 (⫾ 0.68) 8.04 (⫾ 0.49) 7.36 (⫾ 0.39) 7.36 (⫾ 0.39) 6.77 (⫾ 0.29)

a

E (GPa)

σmax (MPa)

4.4 (⫾ 0.05) 2.7 (⫾ 0.10) 4.7 (⫾ 0.39) 4.9 (⫾ 0.40) 5.5 (⫾ 0.45) 7.3 (⫾ 0.40) 6.2 (⫾0.35)

32 (⫾ 0.05) 25 (⫾ 0.3) 40 (⫾ 5.6) 47 (⫾ 6.0) 53 (⫾ 5.2) 56 (⫾ 4.3) 34 (⫾ 2.5)

Tensile E (GPa) 3.3 (⫾ 0.30) 4.2 (⫾ 0.44) 4.7 (⫾ 0.42) 5.6 (⫾ 0.45) 8.2 (⫾ 0.35) 8.0 (⫾0.30) —

Contains 40 wt% of selected aligned fibers. The standard deviation is recorded in parentheses. Source: From White, N.M. and Ansell, M.P., J. Mater. Sci., 18, 1549, 1983. Courtesy of Kluwer Academic.

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composites by 43%. They showed a 6% decrease in the tensile strength of the samples after thermal aging and 9% decrease for the samples kept exposed to sun and rain for 6 months. However, the samples subjected to an accelerated water aging test suffered a decrease of 32% in their tensile strength. Composites were made with banana cotton fabric, with banana in the weft direction and cotton in the warp direction, in the polyester resin in various proportions from 9 wt% to 25 wt% fiber.14 It was found that up to 14 wt% fabric could be incorporated by hand layup without applying any load. Mechanical properties such as tensile strength, flexural strength, and Young’s modulus of these composites have been evaluated (Table 9.17). Confirmed by optical and SEM studies, the decrease in the UTS and flexural strength of the composite, compared with the polyester matrix, had been attributed to a poor bonding between the fabric and the matrix. No degradation was observed on exposing the laminates and consumer articles like castings for voltage stabilizers, 16 mm projector covers, mirror casings, and scooter boxes to indoor weathering conditions. 9.5.2.1.8

Other Lignocellulosic Fiber−Thermoset Composites

Zadorecki and Flodin75 tried a few coupling agents based on trichloros-triazine and introduced polymerizable double bonds on the surface of a bleached softwood kraft pulp fiber. During the curing of the polyester resin through free radical polymerization, the double bonds introduced on the fiber surfaces through the coupling agents also took part in the reaction, forming covalent bonds between the fiber and the matrix. The authors observed that all types of surface treatments decreased water absorption from 15% in the case of untreated fibers to 10−12% in the case of treated fibers in 45% cellulose−polyester composites. A reduction of the mechanical properties in a wet condition was also observed. Depending on the fiber, matrix, and type of surface treatment, the coupling agents induced a noticeable improvement in the characteristic values of the composites, as given in Table 9.18. A comparative study of the various natural fiber-reinforced polyester resin composites74 is shown in Figure 9.13. TABLE 9.17 Mechanical Properties of Banana Cotton–Polyester Composites Banana Cotton Fabric–Incorporated Polyester Property

Polyester Resin

(9 wt%)

(14 wt%)

(18 wt%)

Tensile strength (MPa) Flexural strength (MPa) Modulus of elasticity (GPa)

41.38 89.69 2.06

25.86 52.38 1.36

30.96 61.24 2.03

29.50 60.40 1.90

Source: From Satyanarayana, K.G. et al., Proc. 1st Int. Conf. Composite Structures, 1981. With permission.

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TABLE 9.18 Influence of Coupling Agents on the Mechanical Properties of Natural FiberReinforced Composites Increase in Properties (%) Fiber/matrix

Tensile strength

Young’s modulus

Acrylic acid Polyesteramidepolyol Silane Dimethanolmelamine

Constant 10

Isocyanate Stearinic acid Maleic anhydride– PP copolymer Silane Maleic anhydride

Coupling Agent

Compression strength

Impact energy

— 10

— —

100 —

25 Constant

— —

30 —

— 100

30 30 100

Constant 15 Constant

— — —

50 50 —

Constant 50

50 100

— —

— —

Thermosets Jute/EP Jute/UP and EP Sisal/EP Cellulose/UP Thermoplastics Cellulose/PS Cellulose/PP

Flax/PP

Source: From Bledzki, A.K. and Gassan, J., Prog. Polym. Sci., 24, 221, 1999. Courtesy of Elsevier, London.

9.5.2.2 Dynamic Mechanical Properties Dynamic mechanical analysis (DMA) has been a very important tool in interpreting the interphase region in the composites, which reflects the nature of interfacial bonding between the fiber and the matrix and has the most significant contribution to the performance of the composites. Many workers have interpreted the level of interactions at the fiber-matrix interface in synthetic fiber-reinforced composite materials using the data from DMA. In contrast to the synthetic fiber-reinforced composites, dynamic mechanical behavior of the natural fiber-reinforced composites is still poorly investigated. Ghosh et al.76 carried out dynamic mechanical analysis of FRP hybrid composites based on glass and jute fiber reinforcements and epoxy resin as the matrix material. A dynamic mechanical study of banana/glass fiber woven fabric-reinforced polyester composites was also reported by Pothan et al.77 They used two-, three-, and four-layered composites for the study. The loss modulus curves showed three peaks, and they concluded that the peaks were due to the contribution of the resin, glass, and banana fiber, respectively. Dynamic mechanical properties of jute–polyester composites were studied by Saha et al.78 They found that the storage modulus and the thermal transition temperatures of the composites shifted to higher values when cyanoethylated jute fiber was used as reinforcement. Ray et al.79 investigated the dynamic mechanical properties of Copyright © 2005 by Taylor & Francis

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100

100 Tensile strength (MPa) Flexural strength (MPa)

80

80

2 Impact strength (KJ/m )

60

60

40

40

20

20

0

0 Jute/poly

Straw/poly

Sisal/poly

PALF/poly Banana/poly

Type of composites

Coir/poly (9 wt %)

FIGURE 9.13 Comparative study of the properties of natural fiber–polyester composites prepared by hand layup technique (30%). (From Pothan, L.A. et al., J. Reinf. Plast. Composites, 16, 744, 1997. With permission.)

the untreated as well as of alkali-treated jute fiber-reinforced vinylester resin in the temperature range 30o−210oC. The storage modulus (E′) values increased with increase in fiber loading in the composites (Figure 9.14). The E′ values decreased with increases in temperature, and the drop was maximum in the temperature range 110o−170oC (Figure 9.14). The glass transition temperature of the vinylester resin (Tg) corresponding to the maximum of the loss modulus peak (E′′max) was 101.2oC and was attributed to the mobility of the resin molecules. The primary transition peak of the composites (Tg) shifted to a higher temperature with the increase in fiber loading (Figure 9.15). The immobilization of the resin molecules near the surface of the jute fibers due to various molecular interactions raised the Tg of the composites. In the composites with the alkali-treated fibers, the improved bonding in the interfacial zone increased the interphase region and reduced the free resin matrix beyond the interphase, affecting the slight diminishing trend of the Tg toward the vinylester resin. The 35% composites with untreated, 4 h, and 8 h alkali-treated fiber loading showed Tg values of 128.3oC, 127.7oC, and 125.1oC, respectively. A tiny hump was observed in the loss modulus (E′′) vs. temperature curves of all the composites at a temperature higher than Tg around 160oC due to the presence of the jute fibers in the composites. With the increase in fiber loading, this second transition Copyright © 2005 by Taylor & Francis

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18

Storage modulus (GPa)

15

5

12

4 6 3 2

9

6

3

1

0 0

50

100

150

200

250

Temperature (°C) FIGURE 9.14 The variation of the storage modulus of (1) vinylester resin, (2) 23% untreated juted fiberreinforced composite, (3) 30% untreated jute fiber-reinforced composite, (4) 35% untreated jute fiber-reinforced composite, (5) 35% 4 h treated jute fiber-reinforced composite, (6) 35% 8 h treated jute fiber-reinforced composite, as a function of temperature. (From Ray, D. et al., Comp. Sci. Tech., 62, 911, 2002. Courtesy of Elsevier, London.)

peak shifted to a higher temperature (157.2oC, 159.7oC, and 166.7oC in the 23%, 30%, and 35% composites, respectively). In the alkali-treated fiberreinforced composites at 35% fiber loading, this second transition peak showed a decreasing trend (166.7oC, 159.7oC, and 154.7oC in the untreated, 4 h, and 8 h treated composites, respectively). The tan δ value was very high for the resin (Figure 9.16). Incorporation of the fibers restricted the mobility of the resin molecules, raised the storage modulus values, and reduced the viscoelastic lag between the stress and the strain, lowering the tan δ values in the composites. Much of the work cited here on the dynamic mechanical behavior of natural fiber−thermoset composites is relatively inconclusive and requires more studies to be carried out to have a clear understanding about the molecular interactions between such fibers and the thermosets at the interface.

9.5.2.3 Fatigue Properties Considering the complexity of fatigue and the extensive work carried out on the fatigue behavior of synthetic fiber-reinforced polymer composites, very little work has been reported on the fatigue behavior of natural fiber-reinforced Copyright © 2005 by Taylor & Francis

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1200

Loss modulus (MPa)

1000

800

600

5 4

400 6 200

1

3

2

0 0

50

100 150 Temperature (°C)

200

250

FIGURE 9.15 Variation of the loss modulus of the composites as a funtion of temperature for (1) vinylester resin, (2) 23% untreated, (3) 30% untreated, (4) 35% untreated, (5) 35% 4 h treated, and (6) 35% 8 h treated. (From Ray, D. et al., Comp. Sci. Tech., 62, 911, 2002. Courtesy of Elsevier, London.)

10

tan δ

1

0.1

vinyl esterresin 23% untreated jute composite 30% untreated jute composite 35% untreated jute composite 35% 4 hrs treated jute composite 35% 8 hrs treated jute composite

0.01

1E-3 0

50

100

150

200

250

Temperature (°C) FIGURE 9.16 Variation of tan δ as a function of temperature. (From Ray, D. et al., Comp. Sci. Tech., 62, 911, 2002. Courtesy of Elsevier, London.)

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polymer composites. Fatigue behavior of wood, which, being a cellulosic material, resembles natural fiber composites to a large extent, was conducted by Tsai and Ansell80 under four-point bending. The fatigue properties of wood were also investigated81 under tension, compression, and shear. However, a study on the cyclic dynamic behavior of natural fiber−thermoset composite was reported by Bledzki and Gassan.1 The fatigue behavior of natural fiber-reinforced composites remains relatively unexplored and very little information is available on the nature of their fatigue failure. The study of S-N type behavior of jute fiber-reinforced vinylester resin composites under repeated impacts and its correlation with its microstructures and other properties was done by Ray et al.82 The composites reinforced with untreated, 4 h, and 8 h alkali-treated jute fibers were subjected to impact fatigue study in a repeated impact tester. A four-region fatigue curve was obtained for the composites reinforced with the untreated fibers. The endurance limit found was 32% of the single-impact value at region IV. In contrast, however, the composites reinforced with 4 h and 8 h alkali-treated fibers displayed only three region fatigue curves. The progressive decrease in the applied impact energies showed longer endurance (region II), finally ending in region III at their endurance limits over 104 cycles of 23.7% and 44.5% of the single-impact values for the composites reinforced with the 4 h and 8 h alkali-treated fibers, respectively. The slope of region II was steeper for the case of 4 h alkali-treated composites, indicating that the damage mechanisms like matrix cracking, fiber/matrix debonding, and fiber fracture were more rapid. A comparative study of the fatigue resistance property of the untreated-treated jute−vinylester composite is given in Table 9.19. Extensive studies on the fatigue behavior of biocomposites are still lacking, and need more attention from the scientific community for their improved performance under cyclic loading conditions during their varied service life. TABLE 9.19 Fatigue Property of Vinylester Composites Reinforced with Untreated and AlkaliTreated Jute Fibers Type of Sample (Jute/Vinylester Composite) 35% composite (untreated) 35% composite (4 h alkali treated) 35% composite (8 h alkali treated) Source:

Single Impact Energy (N mm)

Endurance Limit (N mm)

Toughness (kJ/m2)

Fatigue Ratio (Endurance Limit/Single Impact Energy) (EL/Ec)

438.75

137.24

17.80

0.312

342.124

79.36

13.88

0.232

315.98

140.77

13.27

0.430

From Ray, D. et al., Composites, Part A, 33, 233, 2002. Courtesy of Elsevier, London.

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Fracture Characteristics

Unlike the fracture of the synthetic fibers, the fracture behavior of natural fibers itself is a complex phenomenon. The structure and characteristics of the natural fibers vary widely from one fiber to another and their fracture behavior, therefore, varies accordingly in the composites and processing techniques. Natural fibers themselves should be considered as oriented short fiber-reinforced composites containing varying volume fractions of cellulosic microfibrils in a noncellulosic matrix containing hemicellulose and lignin, with variation in their cell wall microstructures. Microfibril orientation, chemical composition, and layer structures are different in different positions within the cell wall. Roe and Ansell42 reported that the failure mode of the tensile fracture of jute–polyester composites up to a volume fraction of 0.15 was essentially brittle. As the fiber content increased, the pullout lengths were increased and the fracture paths were macroscopically longer and more complex, involving delamination between the strands of jute sliver as well as brittle fiber and matrix fracture. They observed a similar trend under impact fracture as well. As the fiber content increased, the failure mode changed from tensile to tensile plus shear, increasing the brushiness until at 0.6 volume fraction, the failure was by shear alone. The morphology of the natural fibers varies greatly depending on their processing method into the fiber level. The surface characteristics of a jute fiber before and after alkali treatment are shown in Figure 9.17.44 The multicellular nature of the jute fiber was clearly visible and removal of the cementing materials on alkali treatment was evident. It was observed that longer treatment time produced thinner fibers and the fibrillar structure of the individual cells was also partly visible. The morphology of the natural fibers varies greatly depending on their processing method into the fiber level. The size, surface characteristics, and other physical properties of the fibers are greatly influenced by the processing method. Ray et al.83 also examined the fractured surfaces of the composites reinforced with untreated, 4 h, and 8 h alkali-treated jute fibers and attempted a comparative analysis of the nature of fracture under flexural bending and repeated impact fatigue testing. The two different types of loading (slow, uniform and continuous loading in the case of flexural test, and fast, sudden, and discontinuous loading in the case of impact fatigue test) were found to have considerable effect on the fracture behavior of the composites. The fracture surfaces of the composites under the three-point bend and impact fatigue test have been shown schematically in Figure 9.18 and the characteristic features of the different fracture surfaces are given in Table 9.20 and shown in Figures 9.19−9.25.82,83 The surface characteristics of an untreated coir fiber and a 5% alkalitreated coir fiber are shown in Figure 9.26. Rout et al.65 showed a comparison of the tensile-fractured surfaces of 10% acrylonitrile- (AN) grafted coir and 5% alkali-treated coir composites with untreated coir composites. The

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FIGURE 9.17 The surface characteristics of jute fibers (a) untreated, (b) 2 h alkali treated, (c) 4 h alkali treated. (From Ray, D. and Sarkar, B.K., J. Appl. Polym. Sci., 80, 1013, 2001. Courtesy of Wiley Interscience, Hoboken, NJ.)

fractographs showed the fractured surfaces of the untreated coir−polyester composite with the fiber pullouts, with many holes left after the fibers were pulled out of the matrix (Figure 9.27). The incompatibility of the untreated coir surface with the polyester matrix was also reflected whereas an improved bonding with a lesser fiber pullout and better fiber dispersion was Copyright © 2005 by Taylor & Francis

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Three point bend test

Impact fatigue test

Untreated jute composite

4 Hours alkali treated jute composite

8 Hours alkali treated jute composite FIGURE 9.18 Schematic diagram showing the fracture behavior of the composties under three-point bend test and impact fatigue test. (From Ray, D. et al., J. Appl. Polym. Sci., 85, 2588, 2002. Courtesy of Wiley Interscience, Hoboken, NJ.)

observed in the AN-grafted coir fiber composite (Figure 9.28). Pulled out fibers along with the existence of cracks at the broken fiber ends in 5% alkali-treated coir−polyester composites were also evident (Figure 9.29). The fractograph of the tensile fractured surface of 5% PMMA grafted coir− polyester composite is shown in Figure 9.30.84

9.5.3

Effect of Hybridization

Natural fibers are quite often used along with synthetic fibers as a reinforcement to produce hybrid composites. Moreover, the water absorption property of the natural fiber-reinforced composites is also lowered by hybridization. The hybrid composites showed significant improvement in flexural strength. The systems studied so far include jute−glass-resin, coir−glass-resin, and banana− glass-resin composites. Mechanical properties of a few jute−glass-reinforced hybrid composites are given in Table 9.21.8 Incorporation of glass fibers along with jute fibers has been found to improve the mechanical properties of the hybrid composites due to the inherent better properties of glass fibers themselves. It was found that 20% coir hybridization resulted in a 130% increase in UTS, 60−90% improvement in FS, 83% improvement in FM, and a 53% increase in impact strength.8 Hybrid composites were prepared from glass fiber mat (7 wt%) and chemically modified coir fiber mat (13 wt%)/polyester resin.65 A comparative study of the water absorption of the hybrid composites with nonhybrid composites is given in Table 9.22 where a lowering in water absorption

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TABLE 9.20 A Comparative Study of the Nature of Fracture of Jute–Vinylester Composites under Three-Point Bend and Impact Fatigue Test Fracture under Type of Composite

Three-point bend test

Impact fatigue test

Untreated fiber composite

Interfacial debonding with extensive fiber pullout

Figure 9.19

4 h alkalitreated fiber composite

Improved interfacial bonding with much less fiber pullout having shorter pullout lengths

Figure 9.20

8 h alkalitreated fiber composite

Strong interfacial bonding with a shear fracture of the composite

Figure 9.21

Extensive fiber pullout with long pullout lengths, having the fibers bent in the direction of applied stress 1. Interfacial debonding, followed by fiber breakage producing “sawlike” fracture surface 2. Corrugated fracture surface showing clearance at the interface Catastrophic fracture of the fibers at the crack plane showing some microfibrillar pullout

Figure 9.22 (a,b)

Figure 9.23

Figure 9.24 Figure 9.25

Source: From Ray, D. et al., Composites, Part A, 33, 233, 2002; Ray, D. et al., J. Appl. Polym. Sci., 85, 2588, 2002. Courtesy of Elsevier, London.

FIGURE 9.19 Fiber pullout and breakage in 23% untreated jute–vinylester composite (threepoint bend test). (From Ray, D. et al., Composites, Part A, 32, 119, 2001. Courtesy of Elsevier, London.)

was evident.65 However, detailed studies are required to optimize the hybridization and to get the desired level of properties for specific applications. 9.5.4

Application

The main criterion for justifying the natural fiber/thermoset composites in various fields of applications is their cost-effectiveness in satisfying the Copyright © 2005 by Taylor & Francis

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FIGURE 9.20 The fractured surface of 35% 4 h alkali-treated jute–vinylester composite showing improved bonding at the fiber-matrix interface (three-point bend test). (From Ray, D. et al., J. Appl. Polym. Sci., 85, 2588, 2002. Courtesy of Wiley Interscience, Hoboken, NJ.)

FIGURE 9.21 Fractured surface of 23% 8 h alakli-treated jute–vinylester composite showing shear frecture of the fibers (three-point bend test). (From Ray, D. et al., J. Appl. Polym. Sci., 85, 2588, 2002. Courtesy of Wiley Interscience, Hoboken, NJ.)

requirement of low strength, noncritical applications. The adequate raw material availability facilitates their utilization as a replacement for expensive synthetic fibers. Commercialization of such products is highly desired to enhance the growth of this industry. These products include furniture, housing panels, doors and windows, ceilings, flooring, household articles, containers, mechanical components, and corrugated boards. Currently, advanced research and development is going on in various parts of the Copyright © 2005 by Taylor & Francis

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FIGURE 9.22 Fiber pullout at the fracture surface of 35% composites reinforced with untreated jute fibers. (a) Impact energy, 447.29 N mm; endurance, 1 cycle. (From Ray, D. et al., Composites, Part A, 33, 233, 2002. Courtesy of Elsevier, London.) (b) Impact energy, 350.36 N mm; endurance, 93 cycles. (From Ray, D. et al., J. Appl. Polym. Sci., 85, 2588, 2002. Courtesy of Wiley Interscience, Hoboken, NJ.)

world to make natural fiber-reinforced composites suitable for use as building material for structural applications. In developing countries commonly used building materials are clayand soil, stone, wood (paneling, partitions, windows, etc.), concrete blocks, straw, and palm leaves. Hence, a substitute to the extent possible for these conventional materials with natural fiber− thermoset composites would prove profitable in the long run. Although the use of such composites is yet to be accepted for large-scale production, field trials of such composites have proved their potential for noncritical applications. In fact, a jute−polyester house model, coir− polyester roofing, and post boxes have withstood outdoor exposure for more Copyright © 2005 by Taylor & Francis

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FIGURE 9.23 The “saw-like” fractured surface of 4 h alkali-treated jute–vinylester composite (impact energy, 137.24 N mm; endurance, 2088 cycles). (From Ray, D. et al., Composites, Part A, 33, 233, 2002. Courtesy of Elsevier, London.)

FIGURE 9.24 The “corrugated” fracture surface of the fiber in 4 h treated fiber composites under repeated impact loading (impact energy, 204.73 N mm; endurance, 126 cycles). (From Ray, D. et al., J. Appl. Polym. Sci., 85, 2588, 2002. Courtesy of Wiley Interscience, Hoboken, NJ.)

FIGURE 9.25 Catastrophic fracture of 8 h treated fiber composites with some microfibrillar pullout under repeated impact loading (impact energy, 256.09 N mm; endurance, 128 cycles). (From Ray, D. et al., Composites, Part A, 33, 233, 2002. Courtesy of Elsevier, London.)

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FIGURE 9.26 Surface characteristics of coir fiber (a) untreated, and (b) alkali treated. (From Rout, J. et al., Comp. Sci. Tech., 61, 1303, 2001. Courtesy of Society of Plastics Engineers, Inc., United States.)

than 5 years without any deterioration in properties. Satyanarayana et al.8 give a good account of their uses for mundane and sophisticated purposes. Similarly, a 16 mm projector cover, mirror casing, stabilizer covers, paper weights, and laminates made of a banana-cotton fabric−polyester composite have withstood 7 years of indoor exposure. There have been other reports on the use of cotton−polyester composites for radar domes in aircraft, and cotton−phenolic composites in the roll necks of steel and nonferrous rolling mills, which saved up to 25% of energy. There have been successful field trials Copyright © 2005 by Taylor & Francis

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FIGURE 9.27 SEM of fractured surface of untreated coir–polyester composite. (From Rout, J. et al., Comp. Sci. Tech., 61, 1303, 2001. Courtesy of Society of Plastics Engineers, Inc., United States.)

FIGURE 9.28 SEM of fractured surface of 10% PAN grafted coir–polyester composite. (From Rout, J. et al., Comp. Sci. Tech., 61, 1303, 2001. Courtesy of Society of Plastics Engineers, Inc., United States.)

over a period of 1 year on banana−polyester composite laminate replacing a heavy and brittle glass fiber-reinforced composite (GFRP) in a Volkswagen car in Germany. A composite containing vegetable fibers in a polymer matrix

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FIGURE 9.29 SEM of fractured surface of 5% alkali-treated coir–polyester composite. (From Rout, J. et al., Comp. Sci. Tech., 61, 1303, 2001. Courtesy of Society of Plastics Engineers, Inc., United States.)

FIGURE 9.30 SEM of fractured surface of 5% PMMA grafted coir–polyester composite. (From Rout, J., Ph.D. thesis, Utkal University, India, 2000. With permission.)

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TABLE 9.21 Mechanical Properties of Hybrid Laminates Type of Reinforcement Thermoset

Jute (wt%)

E-glass (wt%)

Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Polyester Polyester Polyester Polyester Polyester Polyester

32.9 18.0 20.0 14.4 — — 21.8 10.1 — — Jute —

— 40.0 30.0 48.0 68.2 — — 38.5 69.14 — — E-glass

UTSa (MPa) 104.0 157.0 143.0 238.0 429.0 59.0 84.0 200.0 391.0 37.0 441.5 3270.0

YM (GPa)

Fracture Strain

UCS (MPa)

UFS (MPa)

FM (GPa)

0.69 0.62 0.63 0.78 1.04 1.62 0.69 1.10 1.01 0.91 1.80 4.80

95 158 137 204 320 115 123 — — 99 — —

150 445 418 624 938 127 125 229 816 69 — —

14.646 20.778 20.561 28.253 37.376 3.630 8.069 17.560 32.765 4.758 — —

15.042 25.408 22.661 30.607 41.300 3.649 12.213 18.149 38.769 4.096 25.506 68.67

a

UTS, Ultimate tensile strength; YM, Young’s modulus; UCS, ultimate compressive strength; UFS, ultimate flexural strength; FM, flexural modulus. Source: From Satyanarayana, K.G. et al., Handbook of Ceramics and Composites, 1, 341, 1990. Courtesy of Marcel Dekker, New York.

TABLE 9.22 Water Absorption of Hybrid and Nonhybrid Coir – Polyester Composites Water Absorption (%) Sample Untreated Alkali-treated (5%) PMMA grafted (5%) PAN grafted (10%) Cyanoethylated Bleached

Nonhybrid

Hybrid

8.53 4.994 3.98 4.119 3.6 5.8

5.186 3.147 2.663 2.997 3.138 3.718

Source: From Rout, J. et al., Comp. Sci. Tech., 61, 1303, 2001. With permission.

has been used successfully in automobiles in Australia and sunhemp− polyester composites were used as a chair seat replacing GFRP. The future product profiles of natural fiber−thermoset composites include multipurpose panels with special properties such as sound insulation, thermal insulation, abrasion and fire resistance, as well as other physical and mechanical properties.85 Thus, a good potential of using natural fiber−thermoset composites as an engineering material is clearly evident and we look forward to further development in the near future. Copyright © 2005 by Taylor & Francis

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Biodegradability

Although natural fibers are biodegradable in nature and synthetic thermosetting resins are not, the product as a whole is not biodegradable in nature. The ideal product for sustainable development are biodegradable composites where the matrix part is also biodegradable. Ecofriendly biocomposites from plant-derived fibers and crop-derived plastics (bioplastics) are novel materials of the 21st century and would be of great importance to the materials world not only as a solution to the growing environmental threat but also as a solution to the uncertainty of the petroleum supply.86 Biopolymers are now moving into mainstream use and may soon be competing with common synthetic polymers in the market. The best examples of biopolymers based on renewable resources are cellulose plastics, polylactides (PLA), starch plastics, soy-based plastics, and polyhydroxyalkanoate polymers (PLA).86,87 Combination of natural fibers, which are biodegradable, and renewable resource-based bioplastic can be designed to meet the demands of specific applications, and their unique balance of properties may open up a new market development opportunity for biocomposites in the 21st century green materials world.

9.6

Conclusion

Thermoset biocomposites have a potential future, yet their acceptability on a large scale has remained limited due to several reasons, notwithstanding their durability and reproducibility. Due to the moisture-absorbing property of natural fibers, these composites tend to lose their dimensional stability in moist environments and the durability of these products is adversely affected. Reproducibility is another most important criterion which controls the consistency of the final product. Hence, while using natural fibers as reinforcement in thermosetting matrices, some points are worth remembering. Due to the inherent variation in the properties of natural fibers, process control at all possible intermediate steps is required to ensure reproducible end-use properties. Hence, proper drying of the reinforcement prior to fabrication is very important. The bonding between the fiber and the matrix is a significant factor controlling the strength of the composites. Using a suitable interfacial agent to achieve a strong bond at the interface will help to produce sufficiently strong and useful composites. Natural fiber-reinforced composites have proved highly encouraging in the emerging field of composites. The development of well-characterized processing techniques and modification methods is not yet over and, it is hoped, many more improvements are yet to come.

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Acknowledgments The authors are grateful to the editors of this book, Dr. M. Misra, Dr. A.K. Mohanty, and Dr. L.T. Drzal, for inviting us to contribute this chapter. The authors take this opportunity to express their gratitude to Dr. M. Misra for giving them such an opportunity. One of the authors, D.R., conveys her deepest sense of gratitude and respectful regards to her Ph.D. guide and supervisor, Professor B.K. Sarkar, who introduced her to this emerging and exciting field of biocomposites and whose constant encouragement and guidance helped her to complete her research work smoothly and successfully. D.R. also would like to thank Dr. A.K. Rana (IJIRA, Kolkata, India) and Dr. N.R. Bose (CG&CRI, Kolkata, India) for their support during the course of her research work. D.R also takes this opportunity to thank CSIR, Government of India, for providing her the financial support during the course of her investigation on biocomposites. J. Rout takes this opportunity to thank DST and CSIR, Government of India, for providing her the financial support during the course of her investigation on biocomposites. The authors are also thankful to the publishers who granted permission to use tables and figures from their journals.

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10 Thermoplastic Wood Fiber Composites

Shankar Godavarti

CONTENTS 10.1 Introduction 10.2 Raw Materials 10.2.1 Wood 10.2.2 Structure, Composition, and Properties 10.2.3 Softwood vs. Hardwood 10.2.4 Matrix Polymers 10.2.5 Thermoplastics 10.2.6 Structure—Property 10.2.7 Coupling Agents 10.2.7.1 Types of Coupling Agents 10.2.7.2 Characteristics Imparted by Coupling Agents 10.2.8 Methods to Manufacture Composite 10.2.9 Modification of the Wood Fiber 10.2.10 Compatibilization 10.2.11 Coupling, in situ Reactions 10.3 Processing 10.3.1 Introduction 10.3.2 Compounding 10.3.3 Processing Technology 10.3.4 Process Characterization 10.3.5 Extrusion 10.3.6 Processing Technology 10.3.7 Process Characterization 10.3.8 Injection Molding 10.3.8.1 Processing Technology 10.3.8.2 Process Characterization 10.4 Other Process Platforms 10.5 Foam Technology 10.5.1 Materials

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10.5.2 Processing 10.6 Novel Technologies 10.7 Future References ABSTRACT This chapter provides a global perspective of the thermoplastic wood fiber composite material technology. This includes raw materials and their properties, associated processing technologies, and the future of wood fiber composites. In the case of raw materials, numerous tables and figures are used to illustrate the important differences and properties relevant from wood fiber composite perspective. The different species of wood and their compositional differences and the relative importance of these are discussed. In the case of polymers, the importance of molecular weights in relation to properties and processability is discussed. An overview of different coupling agents is also provided. In addition, the inter dependencies between the material properties and processing are also discussed. The processing of wood fiber composites is discussed at length and highly detailed information is provided. Process windows for different process technologies, their limiting issues, and methods of characterizing these processes are also discussed. Process technologies include extrusion, compounding, injection molding, and compression molding. Foaming of wood fiber composites is also discussed. This section provides both an overview and detailed discussion of different types of blowing agents and the trend towards environmentally benign physical blowing agents. The issues involved with foaming of wood fiber composites using physical blowing agents including volatiles generated are discussed. Finally, the future of these composites is presented in relation to biopolymers and processing, primarily stressing the need for applications development.

10.1

Introduction

Applications for thermoplastic wood fiber composites are gaining acceptance due to renewed interest in the environment. The trend toward “recycling,” “protection of natural resources,” and “biodegrability” is the driving force behind the increased use of wood fiber polymer composites. In the automotive industry, weight reduction, improved mechanical and acoustic properties, recycability, and cost are the dominant factors. Wood fiber has been blended with plastics to improve performance virtually since the invention of polymers. Beginning with bakelite in the early 1900s, engineers and scientists have continued to work to improve the various attributes of both thermosets and thermoplastics through the addition of wood fiber. Interest in this class of composites was rekindled in the United Copyright © 2005 by Taylor & Francis

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States during the mid-1980s due to a strong environmental concern and a desire to develop materials and products from renewable resources. The most successful results from this work can be seen in the weatherable Wood Fiber Plastic Composites (WPC) decking market. Manufactured from post-consumer waste, this product embodies many of the early environmental goals. This, and other product successes have fueled continued development in the field, resulting in new composite blends tailored to specific applications. The current generation of WPC comprises a very broad range of materials from those manufactured from 100% post-consumer waste to those comprised of pulped wood and engineering resins. Formulating for the desired outcome is always key and wood fiber-based composites often outperform more conventional filled or reinforced products in many applications. Resins used in manufacturing these composites include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyvinyledene fluoride (PVdF), polyphenylene oxide (PPO), polybutylene terephthalate (PBT), acrylonitrile-styrene-acrylate (ASA), acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), and others.1,2 Currently, large volumes of WPC are manufactured from recycled resins; however, virgin materials are used to achieve a higher level of consistent performance in certain applications.3 Cellulosic fibers originate from both hard and soft woods and from other cellulosic materials, as shown in Figure 10.1. Preprocessing of the fiber or filler ranges from simple sizing to pulping and chemical modification of the surface, depending on the final application. If the surface energies of the fiber and polymer of choice are significantly different due to their different polarities, some technique of compatibilization between the fiber and resin will be necessary to achieve an effective bond. Compatibilization methods employed in WPC include the use of commercially available packages4–6 to enhance wetting and steam explosion of the wood followed by acetylation or maleation used to chemically derivatize the fiber.7 WPC possess the properties of both the fiber and plastic and as such are true composites. Effective compounding of the fibers into the continuous polymer phase requires that adequate shear, pressure, and specific mechanical energy (SME) be delivered consistently throughout the melt. The

Cellulosic fibers

Wood fibers

Fruit fibers

Seed fibers

Leaf fibers

Bast fibers

Hardwood/softwood

Coconut

Cotton

Sisal, banana

Flax, hemp, jute kenaf, rami

FIGURE 10.1 Plant fiber classification.

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equipment and processes used vary according to the intended outcomes. When low levels of SME are required and the surface finish of the final product is not excessively demanding, single-screw compounding extruders can be employed and are often directly coupled to a simple die. These applications do not require subsequent calibration which means cost and lead times are at a minimum. To achieve the full potential of the raw materials usually requires a significantly higher SME. Optimum levels require the use of intermeshing twin-screw extruders, atmospheric and vacuum degassing systems, and often a secondary single- or twin-screw extruder dedicated to pelletizing. Regardless of the equipment being used, properly formulated and manufactured WPC completely encapsulate and bond the wood fibers to the continuous polymer phase and fill the majority of available lumens. Achieving this goal is often a greater challenge with wood than with mineral fillers. Wood fiber is hygroscopic and when not properly handled, will draw moisture from the air. The basic molecules holding the wood lumens together, lignin, are volatile at elevated temperatures and will ignite at temperatures in the 235°C range. To overcome these challenges for high-end products requires complete degassing of the moisture and volatile organic compounds present in the wood during compounding. As a result WPC are significantly more resistant to moisture, rot, and attack by fungus than treated lumber, when well formulated and processed. Products manufactured from WPC in the United States are predominantly extruded as either solid or hollow profiles. Planking used in decking, docks, furniture, and rails are typical applications of both solid and hollow profiles. Solid profiles benefit from simple dies and minimal downstream calibration. They are easy to shape with conventional woodworking tools; however, they suffer from weight and cooling disadvantages. Incomplete cooling often results in products wherein the integrity of the profile center is compromised. Hollow profiles require more sophisticated downstream processing equipment to achieve the tight tolerances (± 0.010 in.) and high surface finishes for which they are often designed. However, their lower weight, faster cooling, and higher line speeds, up to 25 ft/min (7.62 m/min), often make up for the disadvantages. Whether extruding solid or hollow profiles, conventional plastic extrusion equipment can be used as long as precautions are taken to mitigate corrosion when processing halogenated resin-based WPC. The use of WPC in products is diverse, ranging from automotive applications where WPC serve as door panels and gearshift knobs to building materials, where WPC have been successfully used as decking,8 and in the manufacture of window framing.9 The opportunities for products derived from this relatively new class of composites are very large. However, the WPC field is dominated by vertically integrated companies making materials only for internal applications. This results in the materials and engineering know-how in the form of design guides and training materials being unavailable to the large majority of plastic extruders and injection molders. This chapter is dedicated to the cause of conveying the practical application of knowledge of the state of the field of WPC to a wider group of practitioners. Copyright © 2005 by Taylor & Francis

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Raw Materials Wood

This section presents wood from a materials science perspective in brief. More detailed discussion can be found in the literature.10,12,13 Consider, for example, that the knowledge gained from spinning of cellulose helped in the synthesis of cellulose nitrate used to make celluloid plastic and cellulose acetate used in photographic film and rayon or synthetic silk when spun into fibers. Products ranging from tissues to honeycomb structures for aircraft are possible from the fiber, which forms the internal structure of wood, depending on how it is modified or treated. The unique properties of cellulose in combination with its wide availability, renewable nature, adaptability to chemical treatments, favorable viscoelastic behavior, and its remarkable anisotropic properties with orthotropic ratios of 50 are fast making it a material of choice, especially in technologies with significant environmental impacts.10 Its fibrous nature makes it an efficient load-carrying structure and the fiber is the smallest structural element used commercially. Compared to other inorganic fillers, its low cost per unit volume, lower abrasion effects on processing equipment, lack of health hazards, low density, ability for surface modifications, and its abundance in nature make it a suitable alternative in both filler and fiber forms. Sources include wood shavings, newsprint, paper, and waste wood, such as demolition wood, wooden pallets, shipping containers, and scrap from construction sites. However, its properties can vary between and within trees of the same species depending on source, species, and growth cycle.10 Its poor compatibility with hydrophobic thermoplastics, low thermal stability above 190°C, dimensional instability due to excessive moisture uptake, and tendency for phase separation at freezing temperatures has limited its usage in thermoplastics composites, especially in PP, PE, and PS. Most of these issues have been overcome only recently. This section provides a brief review of its composition, types, and reinforcing nature.

10.2.2

Structure, Composition, and Properties

The chemical makeup of wood is complex. Wood is made up of cellulose, hemicellulose, and lignin and is infiltrated with many other compounds. The type and amount of the other compounds are often distinctive with certain species of tree and can have a pronounced effect on properties. The four major constituents of wood include: 1. 2. 3. 4.

Cellulose Hemicellulose Lignin Extractives

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The ratio of these components differs between hardwood (deciduous) and softwood (coniferous) species. Table 10.1 lists the composition and major functions of these four constituents. Cellulose functions as the primary structural component within the wood fiber cell walls. It is a long-chain unbranched condensation polymer of β-Dglucose units, with a degree of polymerization as high as 30,000. Cellulose molecules are completely linear and have a strong tendency to form intra- and intermolecular hydrogen bonds. This bonding between adjacent cellulose forms tightly packed crystalline structures called microfibrils. Microfibrils build up fibrils and finally cellulose fibers. The fibrous structure along with strong hydrogen bonds makes cellulose insoluble in most solvents while rendering it high tensile properties, equivalent to steel.10 The cellulose present in wood fibers is different from those in industrial products such as cellophane. The differences are mainly physical in nature but in some cases are chemical. For this reason, the cellulose found in wood and plant tissues is termed native to distinguish it from industrially modified celluloses. Cellulose retains its properties to 190°C and loses about 10% of its strength when exposed for 10 min at 200°C.11 In thermoplastic wood fiber composites, cellulose is primarily used for reinforcement. The hydroxyl groups on the fiber surface are usually either blocked or modified to be more reactive with thermoplastics. Hemicellulose is a short chain with a degree of polymerization in the low hundreds and so is a low-molecular-weight (LMW) polymer. It primarily serves as connecting agent that links or bonds the microfibrils providing additional structural reinforcement to the wood fiber cell wall. It varies from relatively unbranched, alkali-resistant species to relatively nonlinear, soluble type. The nature and proportions of hemicellulose found in different wood varies but tends to follow broad consistent patterns. Lignin, an amorphous polymeric material, acts as cement in bonding the cellulose filaments providing stiffness to wood. Softwood lignin is different from hardwood lignin. Its degradation temperature is low (110°C) but its rate of degradation is very slow. Its precise structure is complex and is still unknown. The thermoplastic characteristics of lignin enable the manufacture of hardboard-type wood products. TABLE 10.1 Typical Wood Composition Component % Composition Cellulose

44–50

Hemicellulose

20–25

Lignin Extractives

20–30 0–10

Polymeric Nature

Water Affinity

Linear, Hydrophilic crystalline Branched, Hydrophilic amorphous Amorphous Polymeric

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Hydrophobic Hydrophobic

Degradation > 200°C 160°C

Role Strength

Bind microfibrils, structural reinforcement 110°C Stiffness 100°–200°C Encrusting

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Extractives are often low-molecular-weight oleophilic (organic loving) compounds. They can be removed by organic solvents or water. These components are formed from the accumulation and degradation of sugars within the tree (physiological) or to protect the tree against biological damage (pathological). Extractives are comprised of numerous types of compounds, some of them volatile. The volatiles can be divided into three main subgroups: aliphatic compounds, terpenes, and terpenoids, and phenolic compounds consisting of gums, fats, resins, sugars, oils, starches, alkaloids, and tannins. The composition varies greatly between and within species. Softwoods contain up to 10% extractives. Extractives experience the greatest weight loss between 100° and 200°C. Extractives can be removed from the structural (hemicellulose, cellulose, and lignin) portions of a tree. The influence of extractives is significant especially during foaming of thermoplastic wood fiber composites, as discussed in section 10.5. 10.2.3

Softwood vs. Hardwood

In WPC applications, softwoods are generally preferred since they have higher aspect ratios, in addition to providing a regular structure of lumens. Tables 10.2 and 10.3 list the primary cell types found in wood for both softwoods and hardwoods. Softwood anatomy is less complex than that of hardwood. The two main cell types, which constitute softwoods, are tracheids and parenchyma. Longitudinal trachieds represent about 90% of the volume of cells, with the transversely oriented parenchyma occupying the remaining 10% of volume. Tracheids have hollow centers called lumens, giving them the appearance of a straw with the ends pinched shut. These longitudinal trachieds provide all structural support and conduction necessary in the longitudinal direction. A schematic of lumen cross section is shown in Figure 10.2. Lumens are capillarytype structures present in wood. Filling of these lumens during compounding and processing of WPC is the key in optimizing composite properties and is presented in Section 10.3.3. These longitudinal cells are bonded together with TABLE 10.2 Comparison of Wood Species Property Aspect ratio Moisture absorption (times wt of fiber) Acidity Specific gravity Ash content (%) Bulk density (lb/ft3) Typical pH in solution Moisture content (max, %)

Softwood 4 5.5 4.7 0.4 0.5 7–17 3.4 8

Source: American Wood Fibers, Schofield, WI. With permission.

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Hardwood 3 4 5 0.2–0.4 0.7 11–21 3.3–3.7 3–7

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TABLE 10.3 Softwood and Hardwood Cell Types Softwood (S) or Hardwood (H) Hardwood Hardwood Hardwood

Cell Type or Name

Softwood

Fiber tracheid Vessel element Longitudinal parenchyma Ray or transverse parenchyma Longitudinal tracheids

Softwood

Ray parenchyma

Hardwood

Main Function Structural support Conduction Conduction and storage Transverse conduction and storage Structural support and conduction Transverse conduction and storage

Proportion of Xylem Volume Accounted for by Cell Type 15–60% 20–60% 0–24% 5–30% 90% 10%

FIGURE 10.2 A schematic of lumen cross section.

lignin. The ray parenchyma provides transverse conduction and storage in the transverse direction. When using sawdust or other particles of wood for filling or reinforcement, the exposed surface could be a random mixture of all the cells found in softwoods. The softwood cells have strategically placed holes within their sidewalls, which are directly in line with holes located in adjacent Copyright © 2005 by Taylor & Francis

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cells. These holes are called bordered pits, and are the primary means by which liquids and vapors are transported between cells through the cell structure in softwoods. This infrastructure of openings that link cells together becomes very important when considering drying wood or wood fiber and introducing polymers during compounding or extrusion operations. In contrast to softwoods, hardwoods are relatively more complex, with more cell types present. The roles of conduction and support are carried out by different cells in hardwoods. Conduction cells are both longitudinal and transverse. Both vessel elements and longitudinal parenchyma provide conduction. However, longitudinal parenchyma also provides a storage function. The transverse or ray parenchymas carry nutrients transversely through the stem of the hardwood tree and also provide the primary storage function. Vessel elements in hardwoods provide a vast majority of the infrastructure necessary to transport liquids and vapor. Similar to the longitudinal tracheids in softwoods, vessel elements have numerous openings in their sidewalls, which align with adjacent vessel elements. These openings provide the means for exchange of liquids and vapors from one vessel to another, and are crucial pathways for absorption and desorption of liquids and vapor. The longitudinal cells providing the structural support are referred to as fiber trachieds. Thus, most hardwood species contain four types of cells; vessel elements, fiber trachieds, ray, and longitudinal parenchyma. The ratios and volume for each of these cell types is quite variable between species and can be estimated using ranges presented in Table 10.3. The ratio of thickwalled fiber trachieds to the larger thin-walled vessel elements generally determines the hardwood species density. Thick-walled fibers in hardwoods also have bordered pits for transport; however, their role is primarily to provide structural support.

10.2.4

Matrix Polymers

Matrix polymers in WPC include both thermosets and thermoplastics. Thermosets have been in use as a binder in numerous applications. Phenolics, melamines, epoxies, and urea are some of the commonly used thermoset resins. Although thermoset polymers have lower water absorption while offering higher thermal stability at low costs compared to thermoplastics, the increased demand for recyclability combined with alternative processing techniques is driving the thermoplastic-based composites. Applications using thermoplastic polymers are more recent and the focus of this chapter. The type of matrix polymer has a significant influence on the properties of the WPC. Thermoplastics are characterized by their ability to soften and regain flow characteristics upon heating from solid state. They can be classified into four distinct categories based on their properties and cost as follows: 1. Commodity resins: The majority of polymers fall into this category. Examples include PP, PS, PE, and PVC. Copyright © 2005 by Taylor & Francis

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2. Engineering resins: These resins provide a combination of high strength, stiffness, toughness, and chemical, heat, and wear resistance. Examples include polyamide, polycarbonate, acetals, and polyphenylene oxide. 3. Thermoplastic elastomers: These materials exhibit properties of vulcanized rubber combined with the ease of processing thermoplastics and can also be recycled. 4. Alloys and blends: These materials are obtained by mixing combinations of the above polymer types. These extend the range of performance properties in a relatively easy and inexpensive manner.

10.2.5

Thermoplastics

The use of thermoplastics in wood fiber applications is primarily limited by the thermal degradation of wood fiber. Typically, wood degrades around 220°C and this serves as the upper limit for the melting point of thermoplastic matrix resin systems.1,2 The most widely used thermoplastics are commodity resins. The range of properties provided by these polymers generally suffice to cover the greater range of current applications, such as decking, fencing, window lineal, and outdoor furniture. Blends of these polymers are also possible and composites have been developed to further the range of properties to meet specific application requirements, higher tensile strength, higher impact strengths at low temperatures, and reduced costs.14 Further, the introduction of new catalyst technologies for polymerization has extended the range and balance of properties that can be achieved using polyolefins. The trend, especially in Europe, has been to replace PVC with polyolefins, primarily PP for “green” reasons, and also to improve product cost–value due to its lower density. However, PP or PE in general requires some special considerations. These include their flammability compared to PVC, which is inherently flame retardant. Also, polyolefins have low polarity thereby making painting or printing issues more involved and the need for coupling agents to bind to the wood fiber. In general, polyolefins have excellent melt processability. Further, based on application, the inherent UV stability of these matrix resins becomes critical. Therefore, it is common to utilize products with two distinct material compositions, namely, the core and shell. The core provides the products structural properties, forms the majority of the product, and may be used in multiple products.3 The shell forms the minority of the product and provides the appearance properties. Many different shell formulations are possible. The shell is also referred to as capstock material. The capstock is specially formulated using hindered amine light stabilizers (HALS), among other additives, including fillers, pigments/dyes, processing aids, matting agents, and compatibilizers to provide extended periods of protection to the substrate. The capstock material should also provide for excellent adhesion to

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the core composite in addition to providing all physical characteristics including color, stain resistance, surface crack, scratch and abrasion resistance, and weatherability combined with wide processing latitude. WPC cost is largely mitigated by the low-cost wood fiber. The weight fraction of the filler is normally in the range of 40–70% (40–60% by volume depending on matrix polymer). The upper limit is primarily dictated by the critical volume fraction. Further, based on intended final application, recycled (post-consumer waste stream) polymer can be used in an effort to lower costs. However, the increasing demand for recycled polymers has raised its price to nearly that of virgin polymers. Current range of applications primarily target replacement of pressuretreated wood. Flexural modulus, heat distortion temperature, and impact strength are the primary mechanical properties that are optimized. 10.2.6

Structure—Property

The molecular weight and polydispersity index of the matrix play an important role in processing and determining the final polymer properties. Figure 10.3 — depicts the relation between molecular weight M and temperature. It must be noted that at any given temperature, the polymer can behave either as a solid or a viscous liquid, depending on time scale.15 The thermal degradation point of the polymer, however, remains constant. This is generally referred to as the viscoelastic behavior and can be modeled as linear viscoelasticity at small deformations or as nonlinear viscoelasticity at large deformations as encountered during polymer processing operations. Figure 10.4 is a schematic

Thermal degradation

Vis

cou

s li

qui

d

Temperature (T)

Flow temperature Visc

ous

Elastic solid

liqu

id

Glass transition temperature region

Molecular weight (M) FIGURE 10.3 Relationship between molecular weight and temperature. (From Osswald, T.A. and Menges, G., Materials Science of Polymers for Engineers, 1995. With permission from Gardner Publications, Cincinnati, OH.)

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Impact strength

Mn

Resistance to melt fracture

Flowability

Speed of crystallization

Melt elasticity

Mw FIGURE 10.4 — — Effect of M w and M n on polymer properties. (From Osswald, T.A., and Menges, G., Materials Science of Polymers for Engineers, 1995. With permission from Gardner Publications, Cincinnati, OH.)

— representing the relationship between Mw (weight average molecular weight) — and Mn (number average molecular weight) on some of the critical properties for composite materials. Melt viscosity at processing temperatures is also critical in the selection of the processing parameters in addition to final composite properties. Melt flow index (MFI) in the case of polyolefins and k-values for PVC are commonly used to indicate ease of processability. For WPC, the combination of high-impact strength and ease of processability pose challenges and usually blends are used in such cases.

10.2.7

Coupling Agents

The development of quality WPC are constrained by two physical limits, the upper temperature at which wood can be processed, and the difference between the surface energy of the wood and the polymer matrix. Wood can generally be processed at temperatures up to 230°C. When this limit is exceeded the effects are obvious. However, as the difference in surface energies grows the probability of achieving strong interfacial adhesion dwindles in less obvious ways. Because interfacial adhesion between the wood fiber and the polymer matrix defines the final physical properties of a WPC, it is in most cases necessary to compatibilize or couple the blend. Compatibilization is any operation performed upon the polymer and/or the wood fiber, which increases the wetting within the blend. Coupling is a process when dissimilar polymers and/or filler are made into an alloy by use of external agents called coupling agents. These operations fall into four broad classes including Copyright © 2005 by Taylor & Francis

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modification of the polymer matrix, modification of the fiber, addition of surface-active agents, and high shear compounding. This issue is further discussed in this book. Only an overview of these is presented in this section.

10.2.7.1 Types of Coupling Agents Coupling agents can be segmented into surface-active agents and reactive chemistries, sometimes referred to as functional modifiers. Surface-active agents are materials that increase interfacial adhesion by acting as a solid surfactant. This class of compatibilizers does not form covalent bonds to either the polymer matrix or the wood fiber and is often presented as processing aids. Reactive chemistries are comprised of materials that form covalent bonds to either the wood fiber or polymer matrix. Many of these materials react in situ during compounding due to cost economics and reaction conditions. 10.2.7.2 Characteristics Imparted by Coupling Agents Coupling agents are added to the blend to enhance interfacial adhesion and to aid in fiber dispersion and processing. Increase in interfacial adhesion between the polymer and fiber results in an improvement in mechanical and rheological properties of the composite.14,20 Also, the weight fraction of the ingredients influences the final physical properties of the blend. In the case of WPC using polyolefins, maleic anhydride-grafted polymers (MA-g-PP, MAg-PE) are widely used. These are characterized by their molecular weight and percentage of maleic anhydride grafted. The molecular weight is critical from the perspective of molecular entanglement affecting the impact behavior of the composite. The percentage of MA grafted is important from the perspective of forming efficient linkages between fiber and matrix polymer for transferring stress and subsequently the composite tensile and flexural properties. Based on the application requirements, rigorous statistical methods can be used to identify and optimize the coupling agent characteristics for achieving the desired performance attributes at low costs.16 The optimal characteristics depend on the volume fraction of the ingredients, moisture level in the wood fiber, and the molecular weight (MFI in g/10 min) of the matrix resin. Alternatively, coupling agents based on aminosilane, titanate, zirconate, and aluminate chemistries can be used and details can be found in Reference 17. Degree of mixing to ensure excellent dispersion, the sequence of addition, the critical pigment volume concentration (CPVC), and finally the amount and the solubility of the coupling agents are critical in maximizing their benefits.

10.2.8

Methods to Manufacture Composite

Chemically, compatibilizers vary significantly from grafted and block copolymers, which behave as surfactants, to olefins with reactive maleic anhydride grafts. In recent years growth in the field of WPC has resulted in a large variety of these materials being commercially available, facilitating Copyright © 2005 by Taylor & Francis

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brisk materials development. In the event a company needs to develop and manufacture the optimal coupling agent internally, the fundamentals of reactive extrusion have been well documented and discussed in Reference 18. Wood fiber producers have as yet to “size” or functionalize their products as is common in glass fibers, talc, and other inorganic materials.

10.2.9

Modification of the Wood Fiber

Preparation of wood fiber for subsequent compounding into WPC ranges in complexity from drying, grinding, and sizing to pulping and derivitization. Grinding is an important first step that establishes the surface morphology of the wood and the result varies greatly based on the type of mill, process, and moisture of the material processed. WPC have been manufactured from pulped wood; however, the performance increase does not justify the cost at the current scale. Instead, work has been focused on recycled paper products such as old newsprint when the lignin content must be reduced. Morphological characteristics and density of the wood fiber were seen to have more influence on mechanical properties than fiber surface modification.19 However, fiber surface chemistry was found to be an important factor determining the wettability of the wood fibers by the matrix resin. Wood fiber possesses a reasonably reactive surface. The wood fiber surface contains many reactive hydroxyl groups. Competing hydroxyls present in wood fiber can also be found in the lignin and in the free and bound water present in most commercially available materials. Many chemistries have been reacted with wood such as aldehydes, epoxides, anhydrides, siloxanes, and isocyanates in an effort to compatabilize wood for different polymer matrixes. Acetylation with acetic anhydride has been studied in much detail and was used to reveal the reaction order of hydroxyls in wood as cell water bound and free lignin cell wall. Acetylation is carried out at temperatures between 120°C and 160°C with acetic anhydride as the reactant. These reactions can be performed in batch or more economically as continuous reactions as well as in conjunction with the compounding step. The moisture content of wood is critical when working with any of the aforementioned chemistries as reactions with the free and bound water present in wood do not contribute as effectively to a strong interface as do bonds formed directly to the cell wall. To achieve reasonable yields of homogeneous material requires good mixing, temperature control, vacuum degassing, and accurate metering of the reaction components.

10.2.10 Compatibilization Compatibilization chemistries, such as surfactants and block copolymers, are commercially available. Caution should be exercised when using a proprietary additive package as the mechanism. The resulting process control can be a black box in nature, leaving the manufacturer at the mercy of the supplier. Copyright © 2005 by Taylor & Francis

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10.2.11 Coupling, in situ Reactions Most commercially available coupling agents based on reactive groups are derivatives of maleic anhydride, or siloxanes. These chemistries or functionalities can be added to the compounder and reacted in situ. Development of new composites can be accelerated through the use of mixture experimental designs. Important control variables for the wood component include fiber size distribution, aspect ratio, morphology, and moisture content. For coupling agents, the backbone, molecular weight, moles of reactive group, and concentration should be considered. Control of moisture in the process is critical because free or bound water in the presence of anhydrides form carboxylic acids that often catalyze polymer.

10.3

Processing

Final product form, performance attributes, cost, and ease of manufacturing are the primary drivers for the selection of appropriate process technology of the thermoplastic wood fiber composite. In addition, the feed form of the raw ingredients will influence the final process selection. It is imperative to understand the interrelationships between materials, the processing technique chosen, and final product design. This interrelationship has a direct bearing on the final quality of the composite. The majority of the WPC applications are directed at replacement of wood and its products. This requirement has primarily driven the development of the processing technology. Extrusion, injection molding, and compression molding processing technologies represent these areas of development. 10.3.1

Introduction

Appropriate selection of processing technology or platform to manufacture the desired product is vital. In the case of thermoplastic wood fiber composites, the ability of the process to: 1. Effectively distribute the wood fiber within the thermoplastic matrix 2. Encapsulate the wood fiber 3. Fill the majority of the available lumens 4. Couple the wood fiber to the matrix resin 5. Minimize attrition of the wood fiber for reinforcement purposes 6. Impart the desired orientation effects 7. Shape the product is a primary influencing factor driving the selection of process technology. These factors directly affect the composite properties and the final product Copyright © 2005 by Taylor & Francis

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attributes.20 Other relevant factors include capital cost, ease of scale up, and adaptability (transfer to different geographic locations). The importance and approach to some of the above-mentioned factors are discussed below. 10.3.2

Compounding

Compounding processes are gaining wide acceptance due to the high degree of consistency feasible in the pellet. The central purpose of a compounding operation is to form a pellet-shaped product suitable for many subsequent processing and manufacturing operations. Compounding refers to the intimate mixing, dispersive followed by distributive, in addition to the standard extrusion unit operations as listed below. This is achieved using twin-screw extruders of several different configurations. Of these, corotating, selfwiping, intermeshing configurations are found most suitable for achieving the desired final attributes and in executing the sequence of unit operations. Other screw configurations could include counterrotating twin-screw extruders of either conical or parallel configurations and planetary screw extruders, including single-screw extruders. During compounding, independent of extruder configuration, several unit operations are accomplished. 1. Feeding: Introduction of raw material components into processing equipment 2. Heating: By shear heating within the material or by conduction from external electrical heaters 3. Dispersive mixing: Agglomerates are broken into individual components by elongational and planar shearing forces 4. Distributive mixing: Spatial homogenization of the filler/fiber within the matrix polymer using narrow kneading elements for high and efficient melt division rates 5. Devolatilization: Removal of moisture and volatiles in wood fiber and associated by-products 6. Pumping: Overcome the flow resistance offered by the presence of the die Development of a robust process requires efficient sequencing of the unit operations, characterized by their individual L/D (extruder length to diameter ratio).

10.3.3

Processing Technology

The parallel, self-wiping, intermeshing corotating twin-screw extruders (TSE) are modular in nature, allowing for efficient sequencing of unit operations. In the case of WPC, moisture control of the fibers is a must for the efficient design of compounding operation. Residence time under the Copyright © 2005 by Taylor & Francis

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vent, rate of surface renewal, and exposed area of the melt under the vent are the primary design factors influencing devolatilization efficiency. Several innovative processes have been developed and patented to overcome this issue.21,22 Moisture control along with the specific mechanical energy (SME) generated during processing play an important role in establishing the final composite properties. Moisture also determines the compounding process window of WPC. For example, the moisture content of wood sampled at different ports along the length of a ZSK-58 twin-screw compounder, with a 44 L/D, is shown in Figure 10.5. As shown, an efficient process yielding the removal of moisture in excess of 96% can be designed.20 This is especially critical in case of coupling agents and for filling of lumens. During processing, maintaining the critical aspect ratio of the wood fibers is essential in order to obtain the reinforcing characteristic of the fiber; otherwise, the fiber will behave as low-cost filler at aspect ratios less than the critical aspect ratio. Critical fiber length, lc (or critical aspect ratio), is the minimum fiber length for a given diameter (or aspect ratio) at which the fiber fails in tension rather than the interface failing in shear mode. The degree of fiber attrition is dependent on initial fiber length, fiber volume fraction, and several process variables including screw design, shear rates and melt viscosity. At the fiber ends, the tensile stress tends to be zero with maximum shear stress. This is shown in Figure 10.6. Figure 10.7 shows the relative change in fiber length distribution in a ZSK-58 TSE. Effects of fiber size on storage modulus, G’, and HDT are shown in Figure 10.8 and Figure 10.9, respectively.20 Further, the fiber-reinforcing efficiency is greatly influenced 5

% Moisture

4

3

2

1

0 0

12

24 Process L/D

FIGURE 10.5 Moisture content of WPC during compounding on a TSE.

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Pellet

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Stress

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Shear stress at interface

Tensile stress in fiber

Ic /2

Fiber

D I FIGURE 10.6 Schematic of critical fiber length. (From Proc. 5th Int. Conf. Woodfiber–Plastic Composites, 1999. With permission.)

70 Feed

60

Throat

% Particles

50

Pellet Profile

40 30 20 10 0 30

40

50

60 70 Mesh size

80

−80

FIGURE 10.7 Wood fiber particle size distribution in a compounding process.

by the interfacial strength as in the case of using coupling agents. Good interfacial strength is essential, especially in the case of designing structural applications so that load from the matrix is efficiently transferred to the fiber. A striking difference between wood fibers and conventional inorganic fibers is the filling of lumens during compounding. Figure 10.10 plots this phenomenon as a function of two extrusion process variables, die head pressure and Copyright © 2005 by Taylor & Francis

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6000

Storage modulus (MPa)

AR = 8.8 +

+

4000

+ + + +

AR = 6.8 + + + +

2000

+

AR = 4.2

0 − 40

−20

0

20 40 Temperature (°C)

60

80

100

FIGURE 10.8 Effect of aspect ratio (AR) on PVC (60%) wood fiber (40%) composite storage modulus, G⬘. 150

Small

Medium

Large

125

HDT (°C)

100 75 50 25 0 Blend of polyolefins

Alloy with LMW coupling agent

Alloy with HMW coupling agent

FIGURE 10.9 Effect of aspect ratio on composite HDT.

melt temperature. As shown, there is a narrow region wherein the filling of lumens can be optimized. Thermal degradation of the wood fiber occurs when its decomposition temperature (Td) is exceeded while low melt temperatures (Tm) essentially result in incomplete melting of the polymer. Also, Copyright © 2005 by Taylor & Francis

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excess die head pressure results in crushing of the lumens while insufficient pressure results in poor wetting and thereby poor mechanical properties. Figure 10.11 shows a completely filled lumen, an empty lumen, and a crushed lumen.14,20 The process parameters along with the initial number of available lumens and cell wall thickness are primary determinants of the

Crushed lumens

Pressure

Pmax Unmelt

Optimum filling region

Thermal degradation

Pmin Poor wetting

Tm

Td

Temperature

FIGURE 10.10 Process window map for filling of lumens.

FIGURE 10.11 SEM micrograph of filled, empty, and crushed lumens in PVC (60%) wood fiber (40%) composite.

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final percentage of filled lumens. Typically 5% to 50% of the available lumens are filled during compounding and extrusion operations and most of these tend to be thick walled.14,20 This range could be attributed to different residence time, differences in matrix and composite viscosity, degree of moisture removal, die design, and related die pressure buildup mechanisms. Figure 10.12 and Figure 10.13 show the effect of increasing percentage of filled lumens on storage modulus (G⬘) and moisture uptake in a PVC matrix using 40% wood fiber, respectively.14,20 Increasing the percentage of filled lumens from 6% to 24% by optimizing processing conditions tends to increase the G’ by nearly 30% at room temperatures. Similar improvements can be obtained in the case of percentage of moisture absorbed. These results suggest that increasing the percentage of filled lumens is important and is beneficial to both composite strength and for product applications. The final pelletization unit operation during a compounding operation can be accomplished by several means. A hot die face-pelletizing die can be directly mounted on the TSE or a melt-fed single screw in tandem with the TSE is fitted with the pelletization die. Alternatively, water ring pelletization techniques can also be used to cool the pellets but the hygroscopic nature of the composite limits this application. The melt-fed option is popular because it has traditionally yielded a higher production rate. This also aids in separating the compounding unit operations from the pumping stage, thus aiding in the development of more robust processes.

6000

Storage modulus (MPa)

22−28% Filled lumens

4000 12−18% Filled lumens + + + + 2−8% Filled lumens

+

+

+

+

+

+

2000

+

Unfilled polypropylene resin

0 − 40

−20

0

20 40 Temperature (°C)

60

80

FIGURE 10.12 Relationship between percent filled lumens and composite storage modulus, G⬘.

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5.0% 50% Fiber - 6% filled 4.0% 40% Fiber - 3.5% filled 3.0%

40% Fiber - 11% filled

2.0%

1.0%

0.0% 0

200

400

600

800

1000

1200

FIGURE 10.13 Effect of percent filled lumens on profile moisture absorption.

10.3.4

Process Characterization

A fully characterized process reveals considerable insight into the interactions existing between material and process variables. It can also provide useful economic information to drive business decisions. Rigorous use of statistical design of experiments (DOE) is made in identifying the critical process or material variables and their interactions affecting the desired outcomes. Figure 10.14 is a typical DOE schematic depicting design, fixed, and noise variables with the desired responses exiting the box on the right. Figure 10.15 is a process window map of a compounding process with different boundary conditions as indicated. The operational region is shown in the shaded area. Moisture in the wood fiber greatly influences the operational region. The process window aids in optimizing the efficiency of the compounding operation in order to lower manufacture costs. However, the desired attributes could be different at different points in the process window. Therefore, it is necessary to use graphical overlay techniques to identify optimum processing conditions for specific application or specification.

10.3.5

Extrusion

The majority of current applications target replacement of traditional lumber and therefore the bulk of current products are extruded. In addition to profiles, sheet extrusion for automotive applications represents the other major extrusion segment. Extruded profiles are available in numerous shapes and could be broadly classified as either hollow or solid profiles. The foremost difference

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Control factors, Xi

X1

X2

X3

X4

X5

X6 Responses, Yi

Fixed inputs In process responses

Extruder

Sub process 1

Screw design

Yi

Y1

Sub process 2

Mechanical properties

Y2 Cost

Die design

Yi

Y3

Feeders

Flow characteristics

Z1

Z2

Time

Shift

Z3 RM lots

Z4

Z5

Amb. temp

Etc.

Uncontrollable factors, Zi noise FIGURE 10.14 Schematic of a DOE.

600 Upper boundary limit (high melt temperature)

RPMs

500

400

Machine related boundary limit (> 100% torque) Operational space

300

Lower boundary limit (low fusion)

200 500

750

1000

1250 1500 1750 Production rate

2000

2250

2500

FIGURE 10.15 Process window map for a compounding process.

between these profile types lies in the ability to cool the profiles, especially given the low thermal conductivity of these composites. Heat dissipation directly affects the extrusion line speeds and therefore the profile design Copyright © 2005 by Taylor & Francis

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appreciably affects final product costs. In addition, for products intended for outdoor applications, a satellite extruder(s), usually a 24 L/D single-screw extruder, is used for coextruding a capstock or shell layer. The capstock provides protection against environmental weathering to the substrate. The capstock formulation typically includes UV stabilizers, impact modifiers, color pigments, and flame-retardant packages and is extruded at a 0.01 in. thickness. The complexity and high viscosity of the composites make it essential that the extruder be capable of consistently delivering thermally homogeneous material to the die at high pressures. Die design is critical to ensure uniform melt delivery across the die to minimize cross-channel velocity gradients and resulting physical property gradient effects. Counterrotating, twin-screw extruders either in parallel or conical configurations are also widely used. For solid profiles with less demanding application requirements, single-screw extruders are more common. Several unit operations similar to compounding are carried out during an extrusion process. These include: feeding, conveying, devolatilization, melting, and pumping. Feeding is generally in flood mode rather than starve mode. The angle of the hopper sidewall in relation to the angle of internal friction of the material is an important criterion to avoid bridging. Devolatilization capability is important since moisture in the melt not only results in surface defects leading to poor physical properties but also in an unsteady-state extrusion process. Thermoplastic wood fiber pellets require conditioning prior to feeding since they are hygroscopic. Also, unlike compounding extruders, pressure development capability is the main purpose of these extruders and therefore the low degree of mixing offered by these extruders is not a major issue. In addition, the screws are heated by various means, which enables higher production rates. 10.3.6

Processing Technology

Figure 10.16 presents a graphical representation of a typical extrusion line showing the unit operations. Extruders are characterized by L/D ratio and typically range from 24 to 36. Conical counterrotating TSE are suitable for higher output rates when using smaller machines while the parallel configurations are better when using larger machines. Parallel configurations are preferred for machine sizes over 70 mm. This is because the more robust gear designs in smaller machines are no longer a benefit in larger-sized machines and also processing becomes increasingly difficult in larger conical extruders. Postextrusion unit operations are critical in determining the final composite properties. These include die swell, die design, rate of cooling, and drawdown exerted by puller velocity. Of these, die design, die swell, and drawdown are interrelated, and both empirical and analytical models can be developed to optimize the process. Die design may be complex, especially in the case of hollow profiles with varied geometries, multiple materials of varying rheology, and shrinkage. The design of die entry and exit angles is key in affecting the mechanical properties of the composites. The nature of matrix resins affects shrinkage Copyright © 2005 by Taylor & Francis

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FIGURE 10.16 Schematic of profile extrusion line showing unit operations.

behavior, with semicrystalline materials tending to shrink more than amorphous materials. In case of thick sections, shrink voids are possible. Die swell is affected by the L/D of the die land region. Die swell decreases with increasing die land length or L/D at constant temperature. Drawdown is the ratio of the difference in puller velocity to die exit velocity expressed in percent. Some amount of drawdown is required to maintain tension in the profile. In addition, this also aids in aligning the polymer chains influencing mechanical properties anisotropically. In general, die swell can be compensated for by manipulating percentage of drawdown. The force exerted by puller induces stresses in the profile, and if nonuniform stresses are applied, warpage of the profile is likely. Also, the temperature at which these stresses are imposed is crucial in determining the final profile dimensions upon relaxation. Stresses induced at high temperatures for very short periods tend to relax more than those induced at lower temperatures. The effect of drawdown on moisture uptake is shown in Figure 10.17.20,23 A strong correlation between bulk density and drawdown was also noticed, suggesting the presence of voids with increasing percent drawdown.

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2.5

% Increase in weight

33 sec−1

52 sec−1

79 sec−1

2 1.5 1 0.5 0 0

14.3

25

0

6.7

17.8

28.9

0

5.6

16.7

30.6

% Drawdown FIGURE 10.17 Effect of percent drawdown on profile moisture absorption.

Cooling is primarily limited by the low thermal conductivity of the composite. The low melt strength at processing temperatures requires vacuum calibration. Vacuum calibrators are used to cool the profile rapidly and impart enough strength so that it can be hauled away by the puller. Most of the drawdown, therefore, occurs in the region between the die and calibrator, where the polymer is in molten state. Use of vacuum calibrators can be expensive and is commonly used in complex profiles requiring stringent dimensional tolerances. In the case of profiles with lower tolerances, support brackets or slide calibrators can be used with air cooling instead of water cooling. Material shrinkage must be compensated for in calibrator designs. The number of calibrators will depend on line speed, since the residence time is affected. Also, use of more calibrators increases the drag force and will necessitate higher puller forces or capacities. Ultimately, the cooling of profiles should result in uniform temperature gradients throughout the profile cross section, minimizing bow and warpage while ensuring good surface finish at optimized line speeds. 10.3.7

Process Characterization

In the case of extrusion, all sources of variation are critical. In order to optimize the complete extrusion line, it is essential that each unit operation be well characterized using DOEs. Subsequently, with the use of multivariate optimization techniques, the best combinations of all operations can be identified to minimize downtime and costs while maximizing productivity and composite performance characteristics. In certain instances, for example, cooling, dimensionless numbers can provide valuable insight. Several dimensionless numbers are used in extrusion including Reynolds number, Fourier number, Brinkman number, Peclet Copyright © 2005 by Taylor & Francis

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number, Graetz number, Prandtl number, Nahme number, Biot number, and Nusselt number. In the case of cooling, a transient heat transfer phenomena, Fourier number (Fo), is used. In general, a Fo greater than 1 is indicative of the profile acheving thermal equilibrium within the cooling length and time. And if Fo is very small, typically less than 1, it suggests that only the surface layer has cooled or alternatively there exists large thermal gradients in the profile.24 With the use of a combination of analytical and experimental approaches, curves as shown in Figure 10.18 can be developed.23 These curves can be used to estimate cooling times required for profile line speeds, average (Tavg) and maximum (Tmax) profile temperatures, length of the cooling tank required for a profile geometry, etc. As discussed in Section 10.3.3, filling of lumens is also process dependent and Figure 10.19 depicts this. The extrusion process contributes the most to filling of lumens. A schematic of the filling is shown in Figure 10.2. Extrusion characterization and optimization thus requires understanding of not only extrusion process but also materials science and design issues. These in combination with statistical process control (SPC) and other statistical tools such as EVOP (evolutionary operation) can be very beneficial in developing a robust profile extrusion line.

10.3.8

Injection Molding

Injection molding is a versatile process and accounts for nearly one third of all polymeric materials processed by weight. In 1872, the stuffing machine used by the Hyatt brothers to inject cellulose into molds was patented and the technology has evolved since then.25 The reciprocating screw is the current industry standard and standard compression ratio ranges from 1.8 to 3.5. The shearing action of the screw accounts for nearly 70–90% of the heat required for melting. 500

Temperature (°F)

400 300 200 2

Tmax R = 0.66 100 Tavg R 2 = 0.87 0 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Fo FIGURE 10.18 Relationship between Fo and final profile temperature in a profile extrusion.

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0.35

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30.0 Biscuit Pellet Profile

% Filled lumens

25.0 20.0 15.0 10.0 5.0

Profile

0.0 Sample 1

Sample 2

Pellet Sample 3

Sample 4

Sample 5

Biscuit Sample 6

Sample 7

Sample 8

FIGURE 10.19 Effect of process on percent filled lumens in PVC (60%) wood fiber (40%) composite.

The injection molding machines are classified using the following international convention: Manufacturer type T/P,where T refers to clamping force in metric tons and P is defined as product of maximum shot size in cm3 and maximum injection pressure in bars divided by 1000. Clamping force can vary from 1 metric ton to over 10,000 tons. The benefits of injection molding include excellent dimensional tolerances and short cycle times coupled with few, if any, postprocessing operations. These benefits are being explored with thermoplastic wood fiber composite products. Thermoplastic wood fiber pellets are routinely molded using ASTM family molds for preparing test specimens for determining mechanical properties. Currently several products are being explored for both structural and nonstructural purposes. Examples include railing posts for decking applications, coasters for tables, and promotional items. However, injection molding of thick parts is a challenge due to shrinkage and resulting voids. Wood fibers in the form of short fiber reinforcement are more common because of their higher bulk density and free-flowing nature in the case of injection molding. Given the varied applications and technological innovations in this field, this section will only discuss the major trends in processing technology.

10.3.8.1 Processing Technology The processing technology is primarily being driven by automotive applications. Characteristics of wood fibers such as acoustic damping properties, Copyright © 2005 by Taylor & Francis

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low-cost processing with little or no postprocessing, and cost and weight reduction in combination with recycability are some of the primary benefits. Some current applications include spare tire covers, door trim panels, package trays, seat back panels, instrument panels, cargo covers, door panels, and head and arm rests. The aim of conventional injection molding is the mass production of parts with complex geometries at low costs. The molded part is uniform with a finite thickness of 2–5 mm and incorporation of thickness variations is not easily feasible. Further, the range of materials is limited to a single thermoplastic or thermoset. In order to overcome these limitations and meet the ever-increasing range of applications and product features, a new set of special processes have been developed. Figure 10.20 provides a schematic of these special injection molding processes.26 The processes shown include coinjection, gas-assisted injection molding, injection-compression molding, in mold decoration/lamination, low-pressure injection molding, microinjection molding, and microcellular injection molding. In the case of automotive applications using natural fibers with high aspect ratios, reaction injection molding (RIM) and its different variations such as structural reaction injection molding (SRIM) and reinforced reaction injection molding (RRIM) processes are very popular. Nearly 50% of RIM products use fiber-based materials.27 RIM processes mainly use thermosets such as polyurethane (PU). The constituent groups in PU are primarily isocyanates, polyols, and low-molecularweight chain extenders. The isocyanates react with polyols to form PU in the presence of chain extenders. Cellulose and lignins in natural fibers are essentially polyols, and isocyanates have a natural affinity for the hydroxyl groups in cellulose-based fibers. Using plant fibers with high aspect ratios enables SRIM processes for automotive applications, including load-bearing

Micro injection molding Thin-wall injection molding Injection-compression molding

jec -in Co

tio

old nm

Structural foam molding (polymer melt + foaming agents)

ing

Powder injection molding (polymer melt + metal/ceramic powders)

In-mold decoration In-mold lamination Low-pressure injection molding Overmolding Insert/outsert molding Fusible core injection molding Rheomolding Push-pull injection molding Live-feed injection molding

Microcellular injection molding (polymer melt + super-critical CO2 or N2) Lamellar (microlayer) injection molding Gas-assisted injection molding Water-assisted injection molding Liquid gas-assisted injection molding

FIGURE 10.20 Figure of special injection molding processes. (From Turng, L.S., Injection Molding Technol., 5, 160, 2001. With permission.)

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applications. The low viscosity of the matrix resins ensures complete wetting of the fibers, and the short cycle time of these processes enables lowering manufacturing costs. However, the presence of fiber hydroxyl groups tends to change the chemical ratio and the balance in SRIM process at high fiber levels. The specific process parameters for applications vary depending on the specific requirements and part size.

10.3.8.2 Process Characterization Injection molding is a cyclic process for the mass production of products with thin wall geometries through a melt-inject-pack-cooling cycle. A typical injection molding cycle is shown in Figure 10.21. As can be seen, cooling accounts for nearly 80% of the process. A typical process window is shown in Figure 10.22 depicting the possibility of an infinite number of combinations for the production of acceptable parts. Similar to the compounding process window (Figure 10.15), the properties of the composites could vary within the window.

Cycle ends Cycle starts

n

ctio

i

1. M

/eje

ning

old c

ope

losin

old

g

7. M

d

r wa

un

n tio

ec

nj

I 2.

or tf

old

/m tion

g

fillin

jec

3. In

nit nu ctio 5. I

e ur

s es

pr

nje

ng

di

ol

H 4.

bac k

6. Plasticating/recovery

FIGURE 10.21 Typical injection molding cycle. (From Osswald, T.A. and Menges, G., Materials Science of Polymers for Engineers, 1995. With permission from Gardner Publications, Cincinnati, OH.)

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In fiber-reinforced composites, fiber attrition occurs and, as shown in Figure 10.23, most of the damage occurs in the transition section of the screw, with little damage occurring in the metering section and the mold.27 Even

Temperature Thermal degradiation

Short shot

Flash

Melting Pressure

FIGURE 10.22 Process window map for injection molding process.

Fiber length (mm)

3

2

1 Number average Weight average

0

Conveying zone

Transition zone

Metering zone

9D

3D

7D

Nozzle

Part

FIGURE 10.23 Fiber attrition in an injection molding process. (From Proc. 5th Int. Conf. Woodfiber–Plastic Composites, 1999. With permission.)

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though the extent of fiber attrition in the nozzle and mold is small, it is very significant because of the high stresses the fiber experiences, leading to reduction in their aspect ratios. Further, the distribution of fiber length in the molded part is not uniform. The skin region has shorter fibers than the core. In addition to reducing filler aspect ratio, mold and gate design are also critical in that they affect the total shrinkage. Distinction must be made between material shrinkage and part shrinkage. Material shrinkage is a fundamental property and should be accounted for during mold design. Part shrinkage can be overcome by altering the part design. Figure 10.24 shows a PVT diagram used to characterize the injection molding process. Five distinct stages can be traced: (1) isothermal injection (0–1): material being injected into the mold with increasing pressure, (2) isothermal packing (1–2): material being packed under pressure during the hold phase and is very short, (3) isobaric cooling (2–3): material cooling at hold pressure, (4) isochoric cooling (3–4): gate is frozen and no additional material can be injected, and (5) isobaric cooling (4–5): part is ejected and cools to room temperature at atmospheric pressure.

cm3 g

Polystyrene

1.05 1 bar 0 Specific volume (v)

200

1

1.00

600 2 3

4

1000

∆v 5

1600

0.95

0.90 0

50

100

150

200

Temperature (°C)

FIGURE 10.24 PVT diagram used to characterize an injection molding process. (From Osswald, T.A. and Menges, G., Materials Science of Polymers for Engineers, 1995. With permission from Gardner Publications, Cincinnati, OH.)

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The point at which isobaric cooling (stages 4–5) begins controls the amount of part shrinkage. Melt temperature and hold pressure are the two primary process variables that influence this point. Stages 2 and 3 are usually combined into one single packing stage. However, note that stage 2 is very short. Use of DOE with an in-depth understanding of the process physics enables development of robust and efficient processes. In the case of thermoplastic wood fiber composites, the “fountain flow” effects can be very significant. These result in gradient effects in the molded composite, especially at high filler contents. In fountain flow the low-viscosity matrix polymer migrates to the surface and freezes immediately while the high-viscosity filler accumulates in the center. Material gradients therefore exist in both directions, with gradients in flow direction being more significant. Figure 10.25 shows this for a highly filled polypropylene composite. These gradient effects lead to poor interpretation of the mechanical test data. In addition, thermally induced residual stresses can be significant. In order to overcome these limitations, an alternate quality control test has been developed.28 The alternate analytical test method is based on determining the acid number of the composite, which is shown to directly correlate and hence can be used to predict the mechanical properties of the composite.

10.4

Other Process Platforms

These platforms29 include Buss kneaders, thermoforming extruded sheet for automotive applications, resin transfer molding, and continuous mixing technology based on Banbury® batch mixers. The benefits of these alternate

> 50% Resin

Highly filled WPC

No gradient effect at low filler

FIGURE 10.25 Gradient effects in a highly filled injection-molded WPC.

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platforms include compounding at low shear and temperatures with minimal degradation of wood fiber and higher filler loading up to 70% with good devolatilization capability. Compression molding is undoubtedly another major platform to consider. It is primarily used to manufacture large parts in the automotive industry that are light, strong, thin, and large. Matrix polymers could include either thermosets or thermoplastics. The primary advantage of this process is low fiber attrition. Many variations have been developed that are suitable for processing plant fibers. In general, the difference lies in the manner in which the fiber and polymer are brought together.30 An alternate process is to combine injection molding with compression molding primarily to automate the operation. Also, this tends to reduce fiber attrition25 compared to injection molding.

10.5

Foam Technology

The advantages obtained from using wood as reinforcement in thermoplastics such as improved flexural strength, tensile yield strength, and creep resistance can be further advanced by foaming of these composites. Foamed composites offer additional advantages, including the following: 1. Improved mechanical properties a. Impact strength and toughness b. Increased resistance to shrinkage c. Improved COTE (coefficient of thermal expansion) d. Improved fastener retention e. Improved thermal conductivity 2. Improved economics a. Weight and cost reduction b. Easy installation and assembly into useful structures 3. Insulation properties with thermal and acoustic applications Most common applications are conventional industrial, commercial, and residential architecture. These applications make use of both structural and nonstructural components of concrete, stone, glass, wood, and metal. Wood deteriorates due to fungus and insect attack.21 Further, cost issues tend to be considerable related to availability of suitable wood and the substantial upkeep comprised of painting and staining. Metals used in similar applications are typically aluminum or steel. These suffer from rust or corrosion problems and require their own particular skill set.31,32 Foaming of thermoplastic wood fiber composites is achieved by dispersing or expanding a gaseous phase throughout a liquid polymer matrix and filler phase to create foam comprising a polymer component and included

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gas component. The preservation of the resultant foamed state is important to maintaining the desired structural properties. Foamed thermoplastic wood fiber composites can be manufactured using either chemical blowing agents (CBA) or physical blowing agents (PBA). Use of PBA in thermoplastic wood fiber composites is relatively new. Generally, CBA are mixed with thermoplastic pellets before introduction of the blended material into an extruder inlet while PBA can be metered into the polymer melt for intimate mixing prior to foaming in a lower pressure zone. The trend is to replace CBA with PBA, since PBA are more economical, environmentally safe, and allow for higher cell density and a large volume expansion ratio. However, application and processing of PBA tends to be more technically challenging. The foaming process involves three discrete steps. At first, small discontinuities or cells are created in a fluid or plastic phase. Second, these discontinuities grow to desired volume to produce a cell structure. Finally, the cell structure is then stabilized by physical cooling or chemical cross-linking means to form the resultant foamed or cellular polymer structure. Foams can be classified based on their cell density and cell size or on expansion ratio,33 as given in Tables 10.4 and 10.5, respectively. Low-density foams are used in insulation and packaging applications while automotive, building, and furniture applications utilize high-density foams. Microcellular foams characterized by high cell density and small cell size tend to improve impact strength by acting as crack arresters. They blunt the crack tips, thereby enhancing part toughness.34 They also permit higher value uses of mixed polymer streams.35 Getting fine-celled structure at high fiber loadings in TABLE 10.4

Foam Classification—Cell Density and Size Foam Type

Cell Size

Cell Density

Conventional Fine celled Microcellular

⬎ 300 µm 10–300 µm ⬍ 10 µm

⬍10 6 cells/cc 10 6–10 9 ⬎10 9

TABLE 10.5 Foam Classification—Expansion Ratio Foam Density High Medium Low Very low

Expansion ⬍ 4-fold 4- to 10-fold 10–40 ⬎ 40

Source: Courtesy of University of Toronto-MMOIndustry Wood-Fiber Composite Foaming Research Program, 2000.

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WPC has not been possible due to several constraints, primarily moisture and volatiles issues. 10.5.1

Materials

The type of foaming agent, matrix polymer, wood fiber, and coupling agent all play major roles in obtaining the desired cell structure and resultant density. All thermoplastic wood fiber foamed composites are blown using inert gas either by direct injection in the case of PBA or by decomposing CBA. CBA operates by decomposing at elevated temperatures to an inert gas. PBA operates by dissolving or dispersing in the polymer melt stream and flashing into the gaseous state at lower pressures, creating the growth of the cellular structure. Gases released during decomposition of CBA include CO2, N2, NH3, H2O, and combinations of these. In addition to commonly used CBA, low-boiling liquids, which can produce gas on vaporization in lower pressure zones, include aliphatic hydrocarbons such as propane, butane, pentane, and isomers thereof. The decomposition temperature of the CBA should be greater than the minimum processing temperature and lower than the maximum processing temperature. Various types of CBA are available including endothermic, exothermic, and a combination of the two. Differential scanning calorimetry (DSC) studies are used to identify the relevant thermal parameters. The solubility of the gases affects the final density. Both CO2 and N2 have low solubility in polyolefin matrixes. However, solubility of CO2 is 4-fold greater than the solubility of N2 as measured at 200°C and 4000 psi36 in several polyolefin matrixes. Solubility in grams of gas per grams of polymer, is commonly determined using the pressure decay method.37 Diffusivity of these gases is important in maintaining the cell structure and resulting density. Moisture foams at low die temperatures and has very high diffusivity and permeation rates, resulting in its immediate expansion upon exiting from the die and is therefore difficult to entrap. Also, the moisture can interfere with or change the constancy of the foamed product, affecting density, void formation, and surface aesthetics. Large organic molecules from extractives tend to have low diffusivity and permeation rates and take more time to diffuse through the composite matrix. Smaller molecules like CO2 diffuse out quickly from extrudate and contribute to density reduction.38 In addition, the volatiles present in wood fiber act as blowing agents and cannot be controlled. Their solubility and nucleation mechanism are unknown. The viscosity of the matrix resin is critical in determining the wetting of the wood fiber and in controlling the melt strength. Aspect ratio of the wood fiber is also important. Higher aspect ratio fibers yield composites with higher viscosity compared to composites with higher fines due to higher packing fraction. Dispersion of the wood fiber is aided by the use of coupling agents. Coupling agents40–81 also help in decreasing the density of the thermoplastic wood fiber composites by dramatically reducing the volume of the free cavity around the wood fiber. Copyright © 2005 by Taylor & Francis

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Processing

Extruders of several different configurations can be used to foam thermoplastic wood fiber composites. Twin-screw extruders, either parallel or conical configurations, are more common while using CBA for their efficient devolatilization, mixing, and dispersion capabilities. For PBA, cascading or tandem extrusion configuration or parallel twin screws with large L/D ratios and good pumping characteristics are preferred due to the additional cooling required to raise the low melt viscosity of the composite after gas addition, for efficient pumping and shaping. The material processed by the extruder can be comprised of any of the following combinations31 while using CBA:(1) thermoplastic, wood fiber, and CBA as three independent streams, (2) composite of fiber and thermoplastic and CBA, (3) preformed pellet consisting of thermoplastic, wood fiber, and blowing agent, and (4) thermoplastic wood fiber pellet dry blended with blowing agent. Each approach is unique and requires defining the process windows and cost economics. The most common approach is to separate the compounding operation from foaming operation, as in option 2. During processing, the major issues have been the weak interface between the thermoplastic and wood fiber, the high viscosity of the composite at high wood fiber contents, and the effects of moisture and volatiles present, which are difficult to predict and control. The weak interface is usually overcome by using optimal coupling agents, which aid in dispersing and wetting of the fiber. Control of water to a level of less than 8 wt% in the pellet has been determined to be critical,31,32 depending on the devolatilization efficiency of the extruder. Other major issues include variation in the water content of the wood fiber source and the sensitivity of extrudate to water content. Excess moisture can also lead to deterioration of the foam structure in terms of uniformity, cell shape, cell density, and surface quality in addition to causing equipment corrosion. Therefore, the moisture level and volatiles content should be minimized to ensure fine cell morphology in thermoplastic wood fiber composites. No matter how dry the wood fiber, volatiles are released at typical extrusion processing temperatures. This effect has been demonstrated by thermogravimetric analysis studies. Several strategies have been proposed for reducing these volatile emissions33,38 using a tandem extrusion process by adjusting the primary process temperature vis-à-vis secondary processing temperatures. A critical region for die temperature was identified wherein maximum density reduction can be obtained. Lower die temperatures with higher melt viscosities aided in entrapping the gases released, thereby maximizing density reduction. Depending on the melt strength of the composite, the cells could either coalescence or collapse as a result of gas escaping from the matrix. Celuka process and/or capping the foamed substrate using coextrusion techniques have been used to control the cell structure. In the case of PBA, concentration is critical. A schematic of gas dissolution process is shown in Figure 10.26. The three-phase

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Morphological Change of Gas-Plastic/WF System 2 Phase plastic/WF mixture

Intensive mixing

3 Phase gasplastic/WF mixture

gas Moisture

Homogeneous plastic/WF mixture Devolatization of moisture

Devolatilized plastic/WF mixture

Gas injection Diffusion

Cell growth

Cell nucleation

(P decreased)

Fine-celled structure

Dissolution

(T decreased)

Fine-celled structure

2 Phase gasplastic/WF mixture

3 Phase gasplastic/WF mixture

FIGURE 10.26 Schematic of gas dissolution process during foaming of WPCs. (Courtesy of University of Toronto-MMO-Industry Wood-Fiber Composite Foaming Research Program, 2000.)

system obtained after gas addition is gradually converted into a more homogeneous and stable two-phase system when the gas has completely dissolved. Higher concentration of PBA is good for desirable volume expansion while excess PBA would lead to cavities and could actually be detrimental to cell nucleation and volume expansion. Also, the processing pressures should be greater than the vapor pressure of the gas for good cell nucleation and expansion ratio. On exiting from the extrusion die, high- pressure drop per unit time is desirable for optimal cell nucleation and structure. Solubility data for different gases used in thermoplastic wood fiber composites is not available and would need to be developed. The primary barriers to efficient foaming processes and their characterization are the inherent moisture either in the pellet or the wood fiber, presence of volatiles from extractives, poor dispersion and irregular nucleation and bubble growth.

10.6

Novel Technologies

Microcellular injection molding processes using supercritical gases are new and can find applications in thermoplastic wood fiber composites. This Copyright © 2005 by Taylor & Francis

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process can produce parts with excellent dimensional stability using lower injection pressure, shorter cycle times, and less material. In this process, a supercritical gas is mixed with polymer melt in the machine barrel and a single-phase polymer gas solution is created. Size and density of the cells are controlled by varying process conditions. The supercritical gas acts as a lubricant by placing itself in the polymer molecules thereby reducing its viscosity and glass transition temperature (Tg) such that a lower pressure and temperature can be used to mold the part. As the gas diffuses, the material recovers its Tg. Sink marks are eliminated and also dimensional stability is improved due to the internal pressure arising from the foaming process. Detailed descriptions of this process can be found in Reference 39 and other cited references. This is an emerging technology and as such the processing know-how is yet to be established. The swirling pattern on part surfaces has a tendency to compromise its cosmetic attributes. Also, the process of gas diffusion needs to be stabilized before undertaking any further downstream operations.

10.7

Future

Use of lignocellulosic fibers will continue to rise. However, the type of fiber in a geographic location is dictated by the factors including, but not limited to, price, availability, yield, and mechanical properties. In North America, it is mainly wood fiber whereas in Europe, flax and hemp are dominant. In Asia, jute and fruit fibers are more common. The new fiber decortication technologies are spawning new approaches wherein plant fibers are grown specifically for their industrial application. Also, with the development of new catalyst technologies for polymerization of polyolefin resins, a wide range of properties for different applications is feasible. The poor flame retardancy of polyolefins will continue to demand research for developing low-cost flame retardants. It is also feasible to combine this feature with UV stability so as to offer a one-pack system for higher performance. The increasing range of coupling agents would help further lower the costs of these WPC. Durability of the WPC continues to be an issue, with an increasing range of applications. New technologies specifically targeting moisture uptake are being addressed. Chemicals or additives that preferentially absorb water, thereby limiting degradation of the WPC when used for outdoor applications, are on the rise, especially in North America. In processing, compression molding and resin transfer molding will continue to lead the applications, especially automotive. For more advanced processes such as microcellular injection molding, however, the process know-how will require substantial research. Factors fueling automotive market growth include the dramatically lower weight and good insulation properties of these composites. This translates Copyright © 2005 by Taylor & Francis

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into lower costs to build while allowing for higher fuel efficiency. The acoustic properties offered by these composites are known but remain to be characterized. Technology will increasingly shift in favor of composites when life-cycle costs are considered, but for now, applications need to be developed and demonstrated for a multitude of applications. In order to reap the benefits of WPC, technology development needs to proceed hand in hand with performance-based code development and continued education of design engineers. Finally, the concept of a completely “green” composite material will require the development of polymers that are environmentally benign.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19. 20. 21. 22. 23. 24.

U.S. Patent 5,981,067. U.S. Patent 5948524. Andersen Corporation, Bayport, Minnesota, USA. Eastman Chemical Company, Kingsport, Tennessee, Epolene Literature. Dupont – Fusabond Literature. Uniroyal Chemical – Polybond Polymer Modifiers Literature. Saheb, N.D. and Jog, J.P. Natural fiber polymer composites: a review, Adv.Polym. Technol. 8, 4, 351, 1999. TREX Company, Winchester, VA. Renewal by Andersen, Cottage Grove, MN, USA. Wnagaard, F.F., Wood: its Structure and Properties. University of Toronto-MMO-Industry Wood Fiber Composite Foaming Research Program – September 2002. Handbook of wood and wood based materials for Engineers, Architects and Builders, Forest Product Society, WI, USA. Wood Handbook, Forest Products Society, Madison, WI, USA. Aspen Research Corp. - Andersen Corporation, 2000-2001 project reports. Osswald, T.M. and Menges, G., Materials Science of Polymers for Engineers, Hanser Publishers, Cincinnati, USA. Godavarti, S., Optimization of Coupling Agent Characteristics for Maximizing Performance of Wood Fiber Thermoplastic Composites, ANTEC 2003—Proceedings of the 61st Annual Technical Conference and Exhibition, Vol. XLIX, Nashville, TN, May 4–8, 2003, Society of Plastics Engineers, p. 2047. KEN-REACT—Reference manual, Kenrich Petrochemicals Inc., Bayonne, NJ. Reactive extrusion—Xanthos. Kazayawoko, M. et al., Effects of wood fiber surface chemistry on the mechanical properties of wood fiber-polypropylene composites, Int. J. Polym. Mater., 37, 237, 1997. Aspen Research Corp–Andersen Corporation, 2001–2002 project reports. U.S. patent 6,280,667. Davis Standard Corp. Connecticut, USA—WoodtruderTM literature. Aspen Research Corp–Andersen Corporation, 1999–2000 project reports. Rauwendaal, C., Polymer Extrusion III Edition, Hanser Publishers, Cincinnati, 1994.

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25. Osswald, T.M., Polymer processing fundamentals, Hanser Publishers, Cincinnati, 1998. 26. Turng, L.-S., Special and Emerging Injection Molding Processes, J. Injection Molding Technol., 5, 160, 2001. 27. Proceedings of the 5th International Conference on Woodfiber–Plastic Composites, Madison, WI, May 26-27, 1999. 28. Godavarti. S. An On-Line Analytical Method for Quality Control of Bio-Fiber Reinforced Composites, ANTEC 2003—Proceedings of the 61st Annual Technical Conference and Exhibition, Vol. XLIX, Nashville, TN, May 4–8, 2003, Society of Plastics Engineers, p. 2044. 29. Proceedings of the 4th International Conference on Woodfiber– Plastic Composites, Madison, WI, May 12–14, 1997. 30. Michael Karus and Gero Leson,Opportunities for German Hemp Results of the ‘Hemp Product Line Project, http://www.commonlink.com/~olsen?HEMP/IHA/ jiha4110.html. 31. U.S. Patent 6,342,172. 32. U.S. Patent 6,054,207. 33. Progress review meeting IV—Univeristy of Toronto—MMO-Industry Wood fiber composite foaming research program, March 2000. 34. Winata, H. et al., Applications of Polyamide/Cellulose Fiber/Wollastonite Composites for Microcellular Injection Molding, ANTEC 2003—Proceedings of the 61st Annual Technical Conference and Exhibition, Vol. XLIX, Nashville, TN, May 4–8, Society of Plastics Engineers, p. 701. 35. Rachtanapun, P., Matuana, L.M., and Selke, S.E.M, Cell Morphology and Impact Strength of Microcellular HDPE/PP Blends, ANTEC 2003—Proceedings of the 61st Annual Technical Conference and Exhibition, Vol. XLIX, Nashville, TN, May 4–8, Society of Plastics Engineers, p. 1762. 36. Progress review meeting IV—University of Toronto—MMO-Industry Wood fiber composite foaming research program, October 1999. 37. Progress review meeting IV—University of Toronto—MMO-Industry Wood fiber composite foaming research program, February 2001. 38. Progress review meeting IV—University of Toronto—MMO-Industry Wood fiber composite foaming research program, March 2002. 39. Lih-Sheng Turng, Microcellular Injection Molding, ANTEC 2003—Proceedings of the 61st Annual Technical Conference and Exhibition, Vol. XLIX, Nashville, TN, May 4–8, Society of Plastics Engineers, p. 686. 40. Urreaga J.M. et al., Effects of coupling agents on the oxidation and darkening of cellulosic materials used as reinforcements for thermoplastic matrices in composites, Polym. Eng. Sci., 40, 407, 2000. 41. Bledzki, A.K. et al., Natural fiber reinforced polymers make a comeback, Int Polym. Sci. Technol., 27, 28, 2000. 42. Costa, H.S. et al., Statistical experimental design and modeling of polypropylene-wood fiber composites, Polym. Testing, 19, 419, 2000. 43. Balatinecz, J.J. and Sain, M.M., The influence of recycling on the properties of wood fiber-plastic composites, Macromole. Symp., 135, 167, 1998. 44. Maldas, D. and Kokta, B.V., Interfacial adhesion of lignocellulosic materials in polymer composites: an overview, Compos. Interf., 1, 87, 1993. 45. Marcovich, N.E. et al., Modified woodflour as thermoset fillers Part I: effect of the chemical modification and percentage of filler on the mechanical properties, Polymer, 42, 815, 2001.

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46. Maldas, D. and Kokta, B.V., Influence of organic peroxide on the performance of maleic anhydride coated cellulose fiber filled thermoplastic composites, Polym. J., 23, 1163, 1991. 47. Leaversuch, R.D., Wood fiber composites build promising role in extrusion, Mod. Plast., December 2000. 48. Karnani, R. et al., Biofiber-reinforced polypropylene composites, Polym. Eng. Sci., 37, 476, 1997. 49. Barbosa, S.E. and Kenny, J.M., Processing of short-fiber reinforced polypropylene. I. Influence of processing conditions on the morphology of extruded filaments, Polym. Eng. Sci., 40, 11, 2000. 50. Barbosa, S.E. and Kenny, J.M., Processing of short-fiber reinforced polypropylene. II. Statistical Study of the effects of processing conditions on the impact strength, Polym. Eng. Sci., 39, 1880, 1999. 51. Bigg, D.M. et al., High performance thermoplastic matrix composites, J. Thermoplas. Compos. Mater., 1, 146, 1998. 52. Jang, J. and Lee, E., Improvement of the flame retardancy of papersludge/polypropylene composite, Polym. Testing, 20, 7, 2001. 53. Kelleher, P.G., Advances in the technology of fiber reinforced thermoplastics during 1992, Advances in Polym. Technol., 13, 157, 1994. 54. Myers, G.E. et al., Wood flour/polypropylene composites: Influence of maleated polypropylene and process and composition variables on mechanical properties, Int. J. Polym. Mater., 15, 21, 1991. 55. Myers, G.E. et al., Wood flour/polypropylene or high density polyethylene composites: Influence of maleated polypropylene concentration and extrusion temperature on properties, Int. J. Polym. Mater., 15, 171,1991. 56. Chen, C.C. and White, J.L., Compatibilizing agents in polymer blends: Interfacial tension, phase morphology, and mechanical properties, Polym. Eng. Sci., 33, 923, 1993. 57. Kazayawoko, M. et al., Effect of ester linkages on the mechanical properties of wood fiber-polypropylene composites, J. Reinf. Plasto Compos., 16, 1383, 1997. 58. Park, B. and Balatinecz, J.J., Effect of impact modification on the mechanical properties of wood fiber thermoplastic composites with high impact polystyrene (HIPP), J. Thermoplasto Compos. Mater., 9, 342, 1996. 59. Maiti, S.N. and Chawla, C.P., Effect of wood flour on the mechanical properties of polypropylene, J. Polym. Mater., 4, 155, 1987. 60. Park, B. and Balatinecz, J.J., Mechanical properties of wood-fiber/toughened isotactic polypropylene composites, Polym. Compos., 18, 79, 1997. 61. Kokta, B.V. et al., Use of wood flour as filler in polypropylene: Studies on mechanical properties, Polym.-Plast. Technol. Eng., 28, 247, 1989. 62. Krzysik, A.M. and Yougnquist, J.A., Bonding of air-formed wood fiber/polypropyelene fiber composites, Int. J. Adhesion Adhesives, 11, 235, 1991. 63. Wang, Y. et al., Twin-screw compounding of PE-HD wood flour composites, Int. Polym. Process., XVI, 100, 2001. 64. Balatinecz, J.J. and Woodlands, R.T., Wood-plastic composites: doing more with less, J. For., 91, 22, 1993. 65. Kazayawoko, M. et al., Surface modification and adhesion mechanisms in woodfiber-polypropylene composites, J. Mater. Sci., 34, 6189, 1999. 66. Matuana, L.M. et al., The effect of low levels of plasticizer on the rheological and mechanical properties of polyvinyl chloride/newsprint-fiber composites, J. Vinyl Additive Technol., 3, 265, 1997.

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67. Matuana, L.M. et al., Influence of interfacial interactions on the properties of PVC/wood-fiber composites, Polym. Compos., 19, 446, 1998. 68. Matuana, L.M. et al., Effect of surface properties on the adhesion between PVC and wood veneer laminates, Polym. Eng. Sci., 38, 765, 1998. 69. Mengeloglu, F. et al., Effects of impact modifiers on the properties of rigid PVC/wood-fiber composites, J. Vinyl Additive Technol., 6, 153, 2000. 70. Woodhams, R.T. et al., Wood-fiber as reinforcing filler for polyolefins, Polym. Eng. Sci., 24, 1166, 1984. 71. Liao, B. et al., Influence of modified wood fibers on the mechanical properties of wood fiber-reinforced polyethylene, J. Appl. Polym. Sci., 66, 1561, 1997. 72. Lu, J.Z. et al., Chemical coupling in wood fiber and polymer composites: a review of coupling agents and treatments, Wood Fiber Sci., 32, 88, 1993. 73. Heck, R.L., A review of commercially used chemical foaming agents for thermoplastic foams, J. Vinyl Additive Technol., 4, 113, 1998; Kosin, J.A. and Tice, C.L., Novel endothermic chemical foaming agents and their applications, J. Cell. Plast., 26, 332, 1990. 74. Rachtanapun, P. et al., Microcellular Foam of Polymer Blends of HDPE/PP and their composites with wood fiber, J. Appl. Polym. Sci., 88, 2842, 2003. 75. Rizvi, G. et al., Foaming of PS/wood-fiber composites in extrusion using moisture as a blowing agent, Polym. Eng. Sci., 40, 2124, 2000; Balatinecz, J.J. and Park, B.D., The Effects of temperature and moisture exposure on the properties of wood-fiber thermoplastic composites, J. Thermoplast. Compos. Mater., 10, 476, 1997; Morris, P.I. and Cooper, P., Recycled plastic/wood composite lumber attacked by fungi, For. Prod. J., 48, 86, 1998. 76. Matuana, L.M. et al., Cell morphology and property relationships of microcellular foamed PVC/wood-fiber composites, Polym. Eng. Sci., 38, 1862, 1998; Mankowski, M. and Morrell J.J., Patterns of fungal attack in wood-plastic composites following exposure in a soil block tests, Wood Fiber Sci., 32, 340, 2000. 77. Matuana, L.M. et al., Processing and cell morphology relationships for microcellular foamed PVC/cellulosic-fiber composites, Polym. Eng. Sci., 37, 1137, 1997; Mengeloglu, F. and Matuana, L.M., Foaming of rigid PVC/wood-flour composites through a continuous extrusion process, J. Vinyl Additive Technol., 7, 142, 2001. 78. Matuana, L. M. and Mengeloglu, F., Microcellular foaming of impact-modified rigid PVC/wood-flour composites, J. Vinyl Additive Technol., 7, 67, 2001; Stark, N.M., Influence of moisture absorption on mechanical properties of wood flour-polypropylene composites, J. Thermoplast. Compos. Mater., 14, 421, 2001; Rangaraj, S.V. and Smith L.V., Effects of moisture on the durability of a wood/thermoplasticcomposite, J. Thermoplast. Compos. Mater., 13, 140, 2000; Matuana, L.M. and Mengeloglu, F., Microcellular foaming of impact-modified rigid PVC/wood-flour composites, J. Vinyl Additive Technol., 7, 67, 2001;Felix J. M. and Gatenholm, P., The nature of adhesion in composites of modified cellulose fibers and polypropylene, J. Appl. Polym. Sci., 42, 609, 1991. 79. Patterson, J., New opportunities with wood-flour-foamed PVC, J. Vinyl Additive Technol., 7, 138, 2000. 80. Takase, S. and Shiraishi, N., Studies of composites from wood and polypropylenes. II, J. Appl. Polym. Sci., 3, 645, 1989. 81. Clemons, C., Wood plastic composites in the United States: The interfacing of two industries, For. Prod. J., 52, 10, 2002.

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11 Bamboo-Based Ecocomposites and Their Potential Applications

Kazuo Kitagawa, Umaru S. Ishiaku, Machiko Mizoguchi, and Hiroyuki Hamada

CONTENTS 11.1 Introduction 11.1.1 Materials 11.1.2 Processing 11.2 Mechanical Properties 11.2.1 Bamboo Content and Surface Treatment 11.2.2 Flexural Properties 11.2.3 Impact Test 11.2.4 Polymer Blending 11.2.5 Soil Biodegradability 11.3 Conclusions References ABSTRACT Short bamboo fiber-reinforced biodegradable polymer composite was prepared by an injection molding process; bamboo fibers, treated with a 1,2-Ethanedial solution of various concentrations, aimed at improving fiber-matrix adhesion, were employed with alteration of fiber contents with respect to the matrix. Mechanical properties of the composites produced were measured. It was found that tensile modulus increased with increasing bamboo content, whereas tensile strength decreased with increasing the bamboo content. The surface treatment improved both tensile modulus and tensile strength, although different optimum amounts were indicated. Flexural modulus increased with increasing bamboo content. The surface treatment was also effective in improving the modulus. The addition of bamboo fibers decreased impact strength but the role of the compatibilizer was not clearly indicated. It was demonstrated that compounding order plays a significant

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role in filled blend systems. It seems that initially impregnating the fiber with one of the component polymers improves mechanical properties. It could also be deduced that initial impregnation with the lower viscosity polymer further improved mechanical properties. Soil burial tests indicated that the 1,2-Ethanedial surface treatment used inhibited biodegradability.

11.1

Introduction

Wastes from plastics have become a worldwide environmental problem, not only in developed countries but also in developing countries. Production of biodegradable polymers is now widely expected to contribute to the solution of the problem, since biodegradable polymers would enter the material cycles in the environment. Biobased polymers including synthetic polymers derived from renewable resources and biopolymers are quickly rising in their significance. However, practical products from biodegradable polymers are limited in scope for usage as film and sheet materials, since the mechanical properties of these polymers are similar to those of commodity polymers such as polyethylene and polystyrene. In recent years, the concept of “ecomaterials” has become of key importance due to the need to preserve our environment. The meaning of ecomaterial should include “safe” material systems for human and other life forms at all times. Longer-term use of materials should not result in the emission of toxic materials. Experiences in the past 50 years have shown that it is necessary to characterize materials and determine which ones are safe for both short- and long-term utilization. Particular attention needs to be paid to the fate of materials after expiration of their useful lives. However, the long-term characteristics of many kinds of materials are not well understood, especially their effects on our air, water, and soil. If a new material has negative consequences on these environments after 50 years, for example, it will create a bad heritage for our future generations. Considering these circumstances, the meaning of ecomaterial should include “human friendly” material among various kinds of material systems. This is a wider definition of ecomaterial. Selection of a material system that satisfies not only industrial requirements but also the wider definition of ecomaterials, as described above, is an urgent necessity. Here, the most appropriate concept for material selection is composite material. In order to satisfy a wide range of requirements it is not sufficient to use a sole material. Instead, composite materials have the potential to satisfy these different requirements. Historically, the origin of the concept of composite material is based on natural resources such as bamboo and wood. Our ancestors used this concept in building houses that consisted of straw and soil in building walls, for example. Fifty years ago fiber-reinforced plastic (FRP) emerged as the material that satisfies the requirements of being both lightweight and of high strength. Gradually, FRP has come to occupy a prominent position in the industrial Copyright © 2005 by Taylor & Francis

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material system. FRP is increasingly used in the construction of airplanes and bridges. However, according to our wider definition, FRP is definitely not ecomaterials. Is FRP therefore to be discarded or abandoned? To this question, the answer should be NO. In their present form, FRPs contribute to the realization of lightweight vehicles and airplanes and to materials in the construction industry. FRPs therefore reduce total energy consumption, which makes them part of the ecomaterial system, according to our initial definition. The life span of a material should be a key consideration. For example, the material in a car has a life span of less than 20 years; in the construction industry, it is longer than 50 years; in plastic film for food packaging the life span may be 1 week. The first two examples can be considered as sustainable materials while the third should be a recyclable material. The sustainable material should possess a 50- to 100-year life span and be guaranteed to be nontoxic. The recyclable material should be an ecomaterial because its life span is very short. After the material is thrown away, it immediately impacts the environment. Among the various combinations of composite materials, the most suitable pair to be considered an ecomaterial is the combination of natural fiber and/or particle and biodegradable polymer matrix. Biodegradable polymers have been developed in recent years and many kinds have appeared. Polymer matrix composites using natural fibers as reinforcement1–6 have been investigated for many applications due to their low impact on our environment. Natural fibers such as jute, flax, hemp, kenaf, sisal, and cotton also have excellent mechanical properties such as high specific strength as well as high specific modulus. Unfortunately, the strength and modulus of these polymers are not sufficient in terms of industrial requirements, and also their processability is not good. To enhance the mechanical properties of these polymers, composite material formation is an excellent idea and biomass fibers are most suitable. However, natural fibers exhibit a large scatter in mechanical properties. Thus, the application of natural fibers in filled biodegradable polymers is limited to low strength and low rigidity requirements. Note that this chapter does not refer to natural fiber“reinforced” biodegradable polymers. It describes instead natural fiber-“filled” biodegradable polymers. In the actual application of this material system, injection molding is the suitable processing method that enables the production of various kinds of shapes within short processing cycles. Many kinds of natural fibers exist in various countries around the world and bamboo is particularly notable in Japan. As will be described next, the Japanese love bamboo and hence have established a kind of bamboo culture. From ancient times, the Japanese have loved bamboo to the extent that an original bamboo culture emerged. Kyoto was the capital of Japan from the 8th to the 18th century, AD. There are many Buddhist temples and Shinto shrines around which bamboo forests exist. Many Buddhist monks meditated while walking through these bamboo forests. Hence the philosophy of meditation became the base of our Japanese mentality. Bamboo deeply pervaded other aspects of human life as well. In terms of food, bamboo shoot is a famous dish Copyright © 2005 by Taylor & Francis

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in Japan, particularly in spring. Even now, bamboo chopsticks are commonly used and sometimes provide a kind of peace because of their smell and feel. In the field of sports bamboo has been used as the sword in kendo competition. Kendo is an original Japanese sport similar to European fencing. Japanese bamboo also contributed to the invention of the electric lightbulb by Thomas Edison. When Edison was designing the electric lightbulb, the material for the filament was an important issue in terms of making the bulb long-lasting. Bamboo grown in various areas such as China, Southeast Asia, and Central America were tried but the bamboo grown in Japan, especially in the Yawata region in Kyoto, was found to be the best. Moreover, 10-yearold bamboo cut during the autumn and winter was considered most suitable. Later, Edison successfully made the filament from this bamboo and made better bulbs. Accordingly, it can be concluded that our modern life supported by electricity had started with Japanese bamboo. Bamboo fiber is also a good candidate for natural fibers in composite materials. This chapter focuses on the processing and mechanical properties of bamboo fiber-filled biodegradable polymer and particularly introduces injection molding. This is the first attempt to use bamboo-biodegradable polymer composites in injection molding. Soil degradability and mechanical properties were measured by using injection-molded specimens. The investigation also dealt with the importance of fiber surface treatment on bamboo fiber. Surface treatment affects not only the mechanical properties but also degradability. Fortunately, the Japanese government and a number of societies for polymer processing recognize the usefulness of biodegradable polymers and therefore have established large research groups. The research groups consist of universities, national institutions, and companies. The aim is to establish the application of bamboo filled biodegradable polymers in various applications requiring short material life. 11.1.1

Materials

The matrix resin used is polybutylene succinate (PBS) (Showa High Polymer, Co.), and the bamboo fiber shown in Figure 11.1 was used as filler. A scanning electron micrograph of bamboo fiber is shown in Figure 11.2. The bamboo filler was treated with 1,2-Ethanedial and trimethoxymethylmeramine whose chemical structures are indicated in Figure 11.3. They are commonly used for the fabrication of pulps such as paper. They are mixed with water and the concentrations are varied in order to observe the effect of the concentrations on mechanical properties. The bamboo filler can be seen in Figure 11.4 after the filler’s treatment with reagents. A thin layer was generated on the surface of the bamboo fiber. 11.1.2

Processing

A twin-screw extruder was used for compounding of bamboo and biodegradable polymer. The cylinder temperature was set at 135°C and the Copyright © 2005 by Taylor & Francis

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FIGURE 11.1 Picture of bamboo fibers.

FIGURE 11.2 Bamboo fibers observed by scanning electron microscopy. HN

N

CH2OCH3

N H

H H

(a)

C

C

O

O

N

H H3COH2C

N

N CH2OCH3

(b)

FIGURE 11.3 Chemical structures of agent used for surface treatments on bamboo fiber. (a) 1,2-Ethanedial. (b) Trimethoxymethylmelamine.

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FIGURE 11.4 SEM micrographs of bamboo fiber after surface treatments. (a) Surface treated with 10 wt% of 1,2-Ethanedial. (b) Surface treated with 10 wt% of trimethoxymethylmeramine.

revolution of the screw was 200 rpm. The bamboo fiber content investigated was 5% and 10%, while the pure polymer was used as a control. The compounds were produced in pellet form that was subsequently used to fabricate test specimens by injection molding. The specimens were fabricated by injection molding with an inline screw-type injection molding machine (Ti30, Toyo Machinery Co.). The screw temperature was set at 140°C. The holding pressure varied from 0 to 40 MPa. A tensile test was carried out by the INSTRON universal testing machine at room temperature. The length between the grips was 60 mm and the crosshead speed was set at 50 mm/min. Soil biodegradability was investigated by burying the samples in soil in the campus of Kyoto Institute of Technology for 1, 3, 6, 9, and 12 weeks. The change of appearance and weight of the samples were evaluated. The weight was measured after rinsing the samples in water and drying them in the homoiothermal room for 2 days.

11.2 11.2.1

Mechanical Properties Bamboo Content and Surface Treatment

Figure 11.5 shows the tensile properties of the specimen in which the bamboo content was varied. Tensile modulus increased with increasing bamboo content, whereas tensile strength decreased with increasing bamboo content.7,8 This is the reason why we did not refer to this material system as “bambooreinforced composites.” The strength depends on the interfacial condition, and poor adhesion at the interface becomes a defect and leads to poor strength. The number of defects increases as a function of the content and results in a decrement of the strength. Figure 11.6 presents SEM micrographs Copyright © 2005 by Taylor & Francis

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40

1.1

Tensile strength (MPa)

Tensile modulus (GPa)

1.2 1.0 0.9 0.8 0.7 0.6 0.5 0.4

0

5 Bamboo content (wt%)

10

35 30 25 20 15 0

5 Bamboo content (wt%)

10

FIGURE 11.5 Tensile properties of the specimen without surface treatments.

FIGURE 11.6 Fracture surface of the specimen used showing bamboo fiber without surface treatments after tensile test.

of the fracture surface. It was found that the resin was not uniformly distributed around bamboo, which leads to poor tensile strength. The effect of surface treatment is next discussed. Figure 11.7 shows the relationship between tensile properties and concentration of 1,2-Ethanedial and trimethoxymethylmelamine in the case of bamboo content of 10 wt%. The tensile moduli showed a peak at 1 wt% and were constant over 5 wt%. It is clear that surface treatment affected and improved tensile modulus. Optimum conditions are between 1 and 2 wt% for both reagents. Tensile strength increased with increasing concentration and reached the peak at 2 wt%. Tensile modulus increased with increasing bamboo content, whereas tensile strength decreased with increasing the bamboo content. Figure 11.8 shows SEM micrographs of the fracture surface for specimens treated with 2 wt% 1,2-Ethanedial [Figure 11.8(a)] and 2 wt% trimethoxymethylmeramine [Figure 11.8(b)]. It is indicated that the bamboo fillers are covered with a layer of resin and the gap in the case of nontreatment disappeared. This is evidence of improvement of the interphase condition which leads to improvement of tensile properties. Copyright © 2005 by Taylor & Francis

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35 Treated with 1,2-Ethanedial Treated with methylmeramine

Tensile strength (MPa)

Tensile modulus (GPa)

1.4

1.2

1.0

0.8

0.6

0

2

4 6 Concentration (%)

8

10

Treated with 1,2-Ethanedial Treated with methylmeramine

30

25

0

2

4 6 Concentration (%)

8

10

FIGURE 11.7 Relationship between tensile properties and concentration of the agent for surface treatments.

FIGURE 11.8 Fracture surface of the specimen with surface treatments after tensile testing. (a) 2 wt% of 1,2-Ethanedial-treated bamboo fiber. (b) 2 wt% trimethoxymethylmeramine-treated bamboo fiber.

11.2.2

Flexural Properties

Figure 11.9 presents the flexural modulus for 0, 5, and 10 wt% of bamboo with or without 2 wt% of 1,2-Ethanedial treatment. Flexural modulus increased with increasing bamboo content. The surface treatment was also effective in improving the modulus. The modulus of 10 wt% specimen with surface treatment was Copyright © 2005 by Taylor & Francis

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1100

45

1000 900 800 700 600

Nontreated

Treated Flexural strength (MPa)

Flexural modulus (MPa)

Nontreated

0

5 10 Bamboo fiber content (wt %)

Treated

42

39

36

0

5 10 Bamboo fiber content (wt %)

FIGURE 11.9 Flexural properties of the specimen used comparing bamboo fiber treated with 2 wt% of 1,2-Ethanedial and nontreated fiber.

considerably higher than the specimen without surface treatment. The flexural strength could be improved by increasing the bamboo content and also the surface treatment. The 10 wt% and treated specimen achieved 13% improvement in strength. This is an indication that surface treatment coupled with bamboo fiber content can lead to improved flexural strength. However, this observation should be treated with cautious optimism since the opposite trend was observed in the case of tensile strength. The authors do not have any explanation for this contradiction, and it will be a subject of further investigation. 11.2.3

Impact Test

The effect of bamboo content and surface treatment on impact property is presented in Figure 11.10. The impact property decreased with the addition of bamboo. However, slight improvement was obtained by increasing the bamboo content. Here the effect of surface treatment was not very clear since 5% and 20% showed decreases, although an increase was observed in the case of 10%. 11.2.4

Polymer Blending

The effect of holding pressure in injection moldings was investigated. Table 11.1 gives the density of injection moldings in which surface treatment was not applied. The density increased slightly with increasing holding pressure. However, it was found that the holding pressure did not affect mechanical properties. The important factor to improve the mechanical properties of bamboo particle-filled biodegradable composite is the improvement of interfacial adhesion between bamboo and polymer. The surface of the bamboo fiber is rather rough, so that polymer should adequately wet the fiber surface. Hence, the anchor effect would be expected by controlling compounding and injection molding conditions. The combination of conditions for compounding and/or injection molding also affects interfacial properties. This concept was illustrated by observing Copyright © 2005 by Taylor & Francis

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Izod impact strength (kJ/m2)

25 Nontreatment

Treatment

20 15 10 5 0 0

5 10 Bamboo fiber content (wt%)

20

FIGURE 11.10 Impact property of the specimen used comparing bamboo fiber treated with 2 wt% of 1,2-Ethanedial and nontreated fiber.

TABLE 11.1 Density of the Specimens Without Surface Treatments Molded in Different Holding Pressures Density 2

Holding Pressures (kgf/cm )

5 wt% of bamboo fiber

10 wt% of bamboo fiber

10 20 30 40

1.70 1.71 1.72 1.71

1.72 1.72 1.73 1.74

the effect of compounding order in a filled polymer blend system. Figure 11.11 shows the mechanical properties of the specimens fabricated with three different compounding conditions. Type A is the condition in which the poly(caprolactone) (PCL) and bamboo were compounded first and then PBS was added later. The contents of PCL, PBS, and bamboo are fixed. Type B is the opposite in which poly(butylenes succinate) (PBS) and bamboo were compounded initially and then PCL was added later. In Type C, the blend between PBS and PCL was melt mixed and then the filler was added later (Table 11.2). There are apparent differences between all types. Type A showed higher tensile strength. In all cases, it can be seen that Type C gave the least values. This demonstrates that compounding order plays a significant role in filled blend systems. From here, it seems that initially impregnating the fiber with one of the component polymers improves mechanical properties. It could also be deduced that initial impregnation with the lower viscosity polymer further improved mechanical properties. Figure 11.12 shows SEM micrographs of the fracture surface. While the surface of bamboo is covered with resin in Type A, there are gaps around Copyright © 2005 by Taylor & Francis

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Tensile Strength (MPa)

Tensile modulus (GPa)

1.0 0.8 0.6 0.4 0.2 0

Type A

Type B

Type C

24 22 20 18 16

Type A

Type B

Type C

FIGURE 11.11 Tensile properties of the specimen molded by different compounding condition using bamboo fiber, PCL, and PBS.

TABLE 11.2 The Combination of Conditions for Compounding with Nontreated Bamboo Fibera Type A Type B Type C a

(PCL ⫹ bamboo) ⫹ PBS (PBS ⫹ bamboo) ⫹ PCL (PCL ⫹ PBS) ⫹ bamboo

Bamboo fiber:PCL:PBS = 5:47.5:47.5 (wt%).

FIGURE 11.12 Fracture surfaces of the specimen molded by different compounding condition using bamboo fiber, PCL, and PBS.

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FIGURE 11.13 Photographs of the sheet specimens after soil biodegradation. (a) PBS resin. (b) 5 wt% of nontreated bamboo fiber/PBS composites. (c) 10 wt% of nontreated bamboo fiber/PBS.

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bamboo in Type B. Type A may be expected to have much higher interfacial adhesion between polymer and bamboo, leading to higher tensile strength. PCL has a higher melt index value and would easily fit the bamboo surface. Hence, this is also a key to improving properties. Type C displays properties that are similar to type B.

11.2.5

Soil Biodegradability

Figure 11.13 presents the weight changes of the specimen after being buried in the soil for specified periods. The whitening was observed in the 0 wt% specimen buried for 6 weeks, and the shape of the specimen did not change. Whitening was observed in 3 weeks for the 5 wt% specimen and in 1 week for the 10 wt% specimen. In particular, the 10 wt% specimen could not retain its shape within 9 weeks, and it is obvious that biodegradation progressed severely. Figure 11.14 presents the relationship between weight change and time of soil burial for bamboo fiber/PBS composites. For all the specimens, the weight more or less remained unchanged until 6 weeks, after which weight reduction occurred. A comparison of the 1,2-Ethanedial-treated fiber composites and the untreated one reveals that the weight reduction in the untreated one is more significant. This indicates that the interface between the fiber and resin affects biodegradation in the soil. Moreover, it is shown that fiber volume fraction and surface treatments account for biodegradability. Figure 11.15 shows SEM micrographs of composites containing 10 wt% bamboo fiber after 9 weeks of soil burial for 1,2-Ethanedial-treated and untreated fiber composites. In the case of the untreated fiber composite [Figure 11.15(a)], biodegradation of both fiber and matrix is indicated and

1.02 1.00

Weight retention

0.98 0.96 0.94 0.92 PBS 5 wt% of bamboo fiber (nontreated) 10 wt% (nontreated) 5 wt% (2 wt% 1,2-Ethanedial treated) 10 wt% (2 wt% 1,2-Ethanedial treated)

0.90 0.88 0.86 0.84

0

2

4

6

8

10

12

Weeks FIGURE 11.14 Changes of weight retention ratio of sheet specimens after soil biodegradation.

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FIGURE 11.15 SEM photographs of the surface of the specimens filled with 10 wt% of bamboo fiber after 9 weeks of soil biodegradation. (a) Nontreated fiber. (b) 2 wt% of 1,2-Ethanedial-treated fiber.

the gap around the fiber, i.e., the interfacial region, was degraded in the soil. The scenario is different for the 1,2-Ethanedial-treated fiber [Figure 11.15(b)], whereby the extent of degradation was much less. These micrographs correspond well with the extent of weight reduction. Weight reduction coupled with evidence of material loss from the SEMs can be used as indicators of biodegradation in this case. Therefore it may be inferred that surface treatment inhibits biodegradation while biodegradation is enhanced with increase in fiber weight content, as the 10 wt% specimen showed a greater decrease as compared to the 5 wt% specimen in Figure 11.14.

11.3 Conclusions The incorporation of bamboo fibers into the biodegradable PBS matrix increased tensile modulus, which was further enhanced with increase in bamboo fiber content. Alternatively, the opposite trend was shown in the Copyright © 2005 by Taylor & Francis

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case of tensile strength. 1,2-Ethanedial surface treatment improved both tensile modulus and tensile strength although different optimum amounts were indicated. Flexural modulus also increased with increasing bamboo content. The surface treatment was also effective in improving flexural modulus. The addition of bamboo fibers decreased impact strength although the role of the compatibilizer was not clearly indicated. It was demonstrated that compounding order plays a significant role in natural fiber filled blend systems. It was established that initially impregnating the fiber with one of the component polymers improves mechanical properties. This observation could be further refined to state that initially impregnating the fiber with the lower viscosity polymer enhanced mechanical properties to a greater extent. Soil burial test indicated that the 1,2-Ethanedial surface treatment tended to inhibited biodegradability.

References 1. Joshi, S.V., Drzal, L.T., Mohanty, A.K., and Arora, S., Are natural fiber composites environmentally superior to glass fiber composites? Compos. Part A: Appl. Sci. Manuf., 35, 371–376, 2004. 2. Vande Weyenberg, I., Berghmans, B., Vangrimde, B., Verpoest, I., Influence of processing and chemical treatment of flax fibers on their composites, Compos. Sci. Technol., 63, 1241–1246, 2003. 3. Osman, K., Skrifvars, M., Selin, J.-F., Compos. Sci. Technol., 63, 1317–1324, 2003. 4. Lodha, P. and Netravali, A.N., J. Mater. Sci., 37, 3657–3665, 2002. 5. Lee, S.H., Ohkita, T., and Kitagawa, K., Holzforschung, 58, 529–536, 2004. 6. Okubo, K., Fujii, T., and Yamamoto, Y., Development of bamboo-based polymer composites and their mechanical properties, Compos. Part A: Appl. Sci. Manuf., 35, 377–383, 2004. 7. Kitagawa, K., Tsutsui, K., Watanabe, D., Mizoguchi, M., and Hamada, H., Development of Injection Molded Bamboo Fiber Biodegradable Composites, Proceedings of the 1st International Conference on Eco-Composites, London, 2001. 8. Kimura, Y., Kori, Y., Hamada, H., Aiba, S., Kitagawa, K., Okumura, H., Yoshida, T., Shimodoi, Y., Nagai, T., and Kimura, A., Development of Natural Fiber/Biodegradable Polymer Composites with Designable Interphase, Proceedings of the 2nd International Conference Eco-Composites, London, 2003.

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12 Oil Palm Fiber–Thermoplastic Composites

Hj D. Rozman, Zainal A. Mohd Ishak, and Umaru S. Ishiaku

CONTENTS 12.1 Introduction 12.2 Oil Palm Fibers 12.2.1 Morphological Properties of Oil Palm Fiber in Comparison with Hardwood and Softwood 12.2.2 Availability of Oil Palm Biomass 12.2.3 Research and Development on Oil Palm Fibers– Thermoplastic Composites 12.2.3.1 Mechanical Properties of High-Density Polyethylene (HDPE) Composites Filled with OPF and EFB 12.2.3.2 Mechanical Properties of Various Thermoplastic Composites Filled with EFB 12.2.3.3 The Effect of Compounding Techniques 12.2.3.4 The Effect of Surface Chemical Treatments 12.2.3.5 The Effect of Oil Extraction 12.3 Conclusions References ABSTRACT Oil palm fibers have been extensively studied for the production of various composites, such as thermoplastic composites, particleboard, medium-density fiberboard, polymer-impregnated oil palm trunk, and thermoset composites. These fibers are either used fully or partly in combination with wood. For thermoplastic composites, empty fruit bunch (EFB) and oil palm frond (OPF) have been the focus of various research works. These are mainly due to availability, cost, and properties. The studies that have been carried out encompass various preliminary works to ascertain the suitability of these kinds of fibers to be used as filler in the thermoplastic composites. The results show that the composites produced are comparable with other

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lignocellulosic-filled thermoplastics. The optimization, especially in the enhancement of properties with respect to surface treatment, has also been looked into, i.e., incorporation of various commercial or common coupling agents and compatibilizers as well as new coupling agents. From the results, it is clear that these oil palm fibers respond well with the treatment, which is reflected in the enhancement of mechanical properties, such as tensile, flexural and impact, as well as physical properties, such as dimensional stability in water. Overall, EFB and OPF have shown a great deal of potential to be used as filler in thermoplastic composites.

12.1

Introduction

Interest in using lignocellulosic materials, which predominantly consist of cellulose, lignin, and hemicellulose, in the production of plastic composites has gained momentum in recent years. The push is attributed to the growing demand for lightweight, high-performance materials together with diminishing natural fiber resources, wood in particular, and escalating costs of raw materials and energy. Although lignocellulosic fibers are abundantly available, they have not been fully exploited, principally because of a lack of adequate technology development. The impetus to utilize such fibrous materials is further dictated by environmental pressure groups and stringent environmental laws. One of the materials of this category that is of great relevance to the world and Malaysia in particular is the large quantity of biomass generated by palm oil industries. At present, there are about 3.1 million ha of land under oil palm cultivation in Malaysia, producing a total of over 9 million t of crude palm oil annually.1–5 However, the oil production represents only 10% of the total biomass produced by the industry, of which the remaining 90% consists of mainly lignocellulosic material. Various studies, especially at the academic level, have been carried out to exploit the full potentials of these lignocellulosic materials.1–5 Oil palm or Elais guineensis was first introduced into Malaysia in 1870. Like the coconut palm, the oil palm is grown mainly for its oil-producing fruit. Owing to its commercial importance, the botanical and cultivation aspects of the oil palm have been extensively studied.6–8 Its two main products are palm oil and palm kernel oil. Traditionally, these products are used mainly in the manufacture of compound fat and soap, but now their usage has widened and varied considerably. Recently, much attention has been channeled toward finding suitable applications for oil palm industry byproducts. In the light of the scarcity of timber and different environmental issues, various types of by-products have been studied to see whether they can serve as replacements for timber or alleviate environmental problems. At the palm oil mills, the by-products consist of shell, empty fruit bunches Copyright © 2005 by Taylor & Francis

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Fruit bunch Oil palm frond (OPF)

Empty fruit bunch (EFB)

FIGURE 12.1 Photographs of oil palm tree.

(EFB), pressed fruit fibers (mesocarp fibers), and palm oil mill effluent (POME) (Figure 12.1). Oil palm fronds (OPF) from pruning are constantly generated in the plantations, and these are mainly used in interrow mulching. Oil palm stems or trunks are produced in large quantity during replanting. At present, palm kernel shells and pressed fruit fibers are used as inexpensive boiler fuel. EFB has been used in the production of potassium fertilizer as well as mulching (to recycle nutrient, conserve soil moisture, and reduce soil surface erosion). OPF is only used for mulching. POME has been used in producing animal feed (dried and mixed with other supplement) and biogas (from anaerobic digestion) for heat and electricity generation. Lignocellulosic materials, especially wood, have stimulated much interest in the manufacture of composites during the past decade, i.e., used as filler materials instead of conventional fillers such as mica, clay, and glass fibers. By looking at various factors, this is going to change considerably. In recent years, the search for appropriate utilization of lignocellulosic materials (other than wood) has been growing, either for replacing existing wood species in making conventional panel products4 or for producing plastics composites.1 The increasing trend in using these nonwood materials has been induced by the growing demand for lightweight, highperformance materials in an age of diminishing natural fiber resources (wood in particular) and escalating raw materials and energy.3–4 Thus, the Copyright © 2005 by Taylor & Francis

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prospect of using oil palm by-products in various products is increasingly bright in the light of the demand for lignocellulosic materials in vast areas of applications. In general, the utilization of lignocellulosic material in the production of plastic composites is becoming more attractive, particularly for low-cost/high-volume applications. There are several factors responsible for the observed trend. Lignocellulosic-derived fillers possess several advantages compared to inorganic fillers, i.e., lower density, greater deformability, less abrasiveness to equipment, and lower cost.3 More importantly, lignocellulosic-based fillers are derived from a renewable resource, available in relative abundance, of which the potential has not been really tapped. Lignocellulosic materials including wood and oil palm by-products such as EFB and OPF have significantly lower density than the common inorganic fillers mentioned earlier. Thus, specific mechanical properties (strength to weight ratio) of these lignocellulosic-plastic composites, in addition to those characteristics mentioned before, often exceed those of other filled plastics owing to this favorable density difference. However, there are some drawbacks posed by these materials, especially in plastic composites, such as poor compatibility with nonpolar thermoplastics, thermal instability at temperatures above 220oC, hygroscopicity contributed by the polar nature of the lignocellulosic surface, low bulk density, and difficulty in mixing in ordinary plastic mixing equipment. Nevertheless, through research and development, most of the processing problems have been overcome. These include mixing processes, the use of suitable types of polymer, compatibilizers and coupling agents, and suitable forms of lignocellulosic materials. With regard to the trend in the plastic-related industries, composites are expected to be one of the main areas of growth and development. Lignocellulosic-based plastic composites will constitute a bigger portion of the industry, since lignocellulosic-based plastic composites are a largely price-driven commodity and they have adequate properties, as mentioned before. Market potential for lignocellulosic-thermoplastic composites can be categorized into three main areas:9 1. Automotive industries: door trim panels, trunk liners, seat backs, package trays, and speaker covers 2. Recreational industry: lumber-size profiles for playground structures 3. Nonstructural applications: furniture, packaging, building components, fencing, and highway guardrail components. As more interest has been developing in the wood-filled industry and as a better understanding of wood–plastic interaction has been gained, the effort to find substitutes for wood has been stimulated, especially by the abundance of similar resources. Copyright © 2005 by Taylor & Francis

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Oil Palm Fibers

It is obvious that oil palm fibers have been the focus of various studies in Malaysia as well as throughout the world. It is no surprise as the fibers derived from various components of the oil palm tree have not so far found a solid economic value. Hence, the move to study and produce an oil palmbased lignocellulosic thermoplastic composite is based on two reasons: first, the incorporation of high-loading, low-cost lignocellulosic fillers into relatively expensive thermoplastics will be an effective strategy to reduce the cost of composite products. The oil palm materials used in making the composites are derived from EFB and OPF, components of an oil palm tree and by-products of the palm oil industry. EFB consists of a bunch of fibers in which palm fruit are embedded. It is readily available at a lower cost than OPF, at a token price of US$10.00/t. OPF is readily available at US$30.00/t.9 Second, the EFB and OPF waste generated by the palm oil industry in Malaysia is estimated to be about 8 and 13 million t per year, respectively.9 Thus, finding useful applications for these materials will surely alleviate environmental problems related to the disposal of oil palm wastes and produce materials that could offer a favorable balance of quality, performance, and cost. 12.2.1

Morphological Properties of Oil Palm Fiber in Comparison with Hardwood and Softwood

Fibers in oil palm biomass exist in the form of vascular bundles (Figure 12.2). Generally, fibers from oil palm biomass resemble those from hardwood such as eucalyptus (Table 12.1).10 The fiber length lies between that of hardwood and softwood except for EFB, which is shorter than hardwood. EFB shows high distribution of short fiber, i.e., less than 0.5 mm, with fiber thickness of about 2.3 µm compared to only 2 µm in hardwood. In addition, the chemical compositions of EFB and OPF are quite comparable with coir, but considerably lower in cellulose as compared to flax and jute (Table 12.2).11 12.2.2

Availability of Oil Palm Biomass

The annual amount of oil palm biomass produced is on the average of about 90 million t.12 At the moment, out of this amount, only oil and palm kernel cake (PKC) are considered the products of economic value. The others such as the shell and pressed fruit fibers (PFF) are used by the mill to generate steam and electricity. These biomass make up ~16% of the total biomass produced. The remaining biomass of about 77 million t consists of EFB, OPF, OPT, roots, and POME. Some of the major losses that cannot be accounted for are the OPF bases and stalk of the EFB that are still attached to the palm during harvesting and pruning processes.12 Copyright © 2005 by Taylor & Francis

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FIGURE 12.2 Scanning electron micrograph of EFB fiber bundles.

TABLE 12.1 Morphological Properties of Oil Palm Fiber in Comparison with Hardwood and Softwood Properties

EFB

OPF

Oil Palm Trunk

Hardwood

Softwood

Fiber length (mm) Width of fiber (µm) Width of lumen (µm) Runkel ratio Area of fiber (µm2)

0.67 12.50 7.90 0.59 75.60

1.03 15.10 8.20 0.84 126.20

1.37 20.50 17.60 0.26 86.70

0.83 14.70 10.70 0.37 79.00

2.39 26.80 19.80 0.35 256.10

Source:

From Basiron, Y. et al., Proc. 4th Semin. Util. Oil Palm Tree, 173, 1997. With permission.

12.2.3 Research and Development on Oil Palm Fibers–Thermoplastic Composites

12.2.3.1

Mechanical Properties of High-Density Polyethylene (HDPE) Composites Filled with OPF and EFB Two types of oil palm fibers–thermoplastic composites were produced, i.e., HDPE–OPF13 and HDPE–EFB14 composites. Both OPF and EFB fibers were Copyright © 2005 by Taylor & Francis

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TABLE 12.2 Chemical Composition of Oil Palm Components Plant Fiber EFB OPF Coir Flax Jute

Cellulose (%)

Hemicellulose (%)

Lignin (%)

Extractives (%)

47.9 42.2 47.3 73.8 69.7

17.1 26.4 15.2 13.7 12.6

24.9 22.3 31.4 24.9 13.8

3.7 3.3 4.2 1.7 2.2

Source: From Khalil, H.P.S. and Rozman, H.D., Polym.–Plast. Technol. Eng., 39, 757, 2000. With permission.

TABLE 12.3 Mechanical Properties of OPF–HDPE Composites Flexural Sample % of Filler Mesh 80–100 30% 40% 50% 60% Mesh 60–80 30% 40% 50% 60% Mesh 35–60 30% 40% 50% 60%

MOE (MPa)

Tensile

MOR Toughness (MPa) (kPa)

Modulus (MPa)

Strength (MPa)

Impact Strength (J/m)

306.2 371.0 397.9 391.8

15.3 15.8 15.0 14.6

24.1 38.3 22.3 20.3

427.6 559.8 647.2 613.8

11.9 10.7 8.4 8.3

148.6 102.3 83.0 61.0

470.6 580.1 645.8 643.9

17.0 13.7 12.8 8.7

30.9 28.9 27.8 10.2

488.9 474.8 478.6 360.7

8.4 9.0 8.0 6.3

80.2 71.7 61.4 44.3

248.1 444.1 475.0 462.5

5.9 8.7 7.2 5.9

8.8 12.5 14.1 11.9

393.6 574.5 497.28 487.6

12.5 10.2 8.9 7.35

51.4 53.7 44.8 32.9

Source: From Rozman, H.D. et al., Intern. J. Polymeric Mater., 39, 161, 1998. With permission.

ground to different mesh sizes before use. The materials were compounded with a single-screw extruder. The addition of both fillers of all sizes does not seem to produce any significant effect on the modulus of rupture (MOR) of the composites (Tables 12.3 and 12.4). However, it can be seen that samples with smaller sizes displayed greater MOR than the larger ones. This implies that the samples are capable of withstanding higher stress before failure than the ones with larger particle size. Both OPF and EFB composites with filler size of 80 mesh showed the least reduction in MOR as compared to those with larger size. As shown by studies on other lignocellulosic fillers,15 this can be attributed to the greater interaction and/or dispersion of the finer OPF and EFB particles in the PE matrix. Copyright © 2005 by Taylor & Francis

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TABLE 12.4 Mechanical Properties of EFB–HDPE Composites Flexural Sample % of Filler Mesh 80–100 30% 40% 50% 60% Mesh 60–80 30% 40% 50% 60% Mesh 35–60 30% 40% 50% 60%

Tensile Modulus (MPa)

Strength (MPa)

Impact Strength (J/m)

26.0 25.5 21.5 19.0

322.4 312.5 341.9 347.8

13.0 11.9 9.2 7.3

70.9 52.4 48.1 57.3

28.9 24.6 20.8 12.9

226.3 283.7 338.3 358.2

9.7 9.7 7.5 7.3

59.7 63.2 55.7 48.7

13.68 14.1 18.2 14.6

328.5 364.6 438.2 375.4

13.2 11.7 11.1 9.7

73.6 65.5 65.2 52.4

MOE (MPa)

MOR Toughness (MPa) (kPa)

370.3 383.2 431.3 439.0

15.5 15.3 14.9 14.0

318.5 409.2 470.2 451.0

11.2 10.9 9.9 7.7

299.8 360.1 412.7 453.0

9.89 9.9 9.8 7.0

Source: From Rozman, H.D. et al., Polym.–Plast. Technol. Eng., 37, 493, 1998. With Permission.

In the OPF and EFB–HDPE composites, poor interfacial interaction is expected owing to poor compatibility between polar OPF and EFB with nonpolar PE matrix, which forms weak interfacial regions. As lignocellulosic materials, both OPF and EFB surfaces are covered by polar hydroxyl groups contributed by cellulose, hemicellulose, and lignin.11 This wetting is further decreased as more filler is added. As more filler is incorporated in the composite, more incompatible interfacial regions between polar lignocellulosic and nonpolar polyethylene are created. The weak interfacial regions result in the reduction in the efficiency of stress transfer from the matrix to the reinforcement component. As highlighted by several workers,16–18 the quality of interfacial bonding is determined by several factors, such as the nature of lignocellulosic and thermoplastic materials as well as their compositions, the fiber aspect ratio, the types of incorporation procedures, processing conditions employed, and the treatment of the polymer or fiber with various chemicals, compatibilizers, coupling agents, etc. Rozman et al.13,14 found that the stiffness or modulus of elasticity (MOE) of the OPF and EFB–HDPE composites increased as the filler loading was increased. As shown by various studies, incorporation of fillers is able to impart greater stiffness in the composites.19 Generally, samples with smaller particle size filler showed higher modulus. The toughness of the samples decreased as the filler loading was increased. This is very much expected, since the lignocellulosic materials were added without any pretreatment with any coupling agents or compatibilizers to improve the compatibility of the Copyright © 2005 by Taylor & Francis

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polar nature of the lignocellulosic and the nonpolar nature of the hydrocarbon chain of PE. Various factors that influence physical and mechanical properties have been mentioned by several workers,20–22 which include the chemical treatment of the polymer or fiber with various chemicals or additives, method of incorporating the lignocellulosic into the resin, types of lignocellulosic and thermoplastic, and the aspect ratio of the fiber. The results showed that samples with smaller particle size displayed higher toughness than those with bigger particle size. As toughness is a measure of energy needed for failure, the results generally demonstrate that more energy is needed to break samples with smaller size particles. Tensile strength of the OPF and EFB composites decreased gradually with filler loading. Filler sizes did not show any significant influence on the strength of the composites. This is true because incorporation of filler into a thermoplastic matrix does not necessarily increase the tensile strength of a composite. Fibers with uniform circular cross section and a certain aspect ratio normally improve the strength.23 However, the capability of irregularly shaped fillers with low aspect ratio, as in OPF and EFB, to support stresses transferred from the polymer matrix is significantly reduced. Both EFB and OPF fibers have the tendency to exist in fiber bundles. This would mean that the fibers embedded in the matrix would have greater diameters as compared to other wood-based fillers or inorganic filler such as glass fiber. This in turn would reduce the aspect ratio of the fibers. Significant improvement in the tensile modulus was observed with increasing filler loading. Both OPF and EFB composites with smaller filler size displayed better modulus than those with larger filler sizes. Smaller or finer particles with larger specific area may impart greater interaction with the polymer matrix and can result in uniform filler dispersion in the composite. This indicates the ability of EFB fillers to impart greater stiffness to the HDPE composites. These results are in agreement with the trend observed in other lignocellulosic-filled thermoplastics.24–27 The impact strength of both OPF and EFB composites showed a decreasing trend as the filler loading was increased. This clearly indicates that the presence of OPF and EFB fibers has reduced the energy-absorbing capabilities of the composites. The poor adhesion or bonding between the fibers and polymer matrix creates weak interfacial regions, which will result in debonding and frictional pullout of fiber bundles and inhibit the ductile deformation and mobility of the matrix.28 This in turn lowers the ability of the composite system to absorb energy during fracture propagation. A similar trend has been reported by Myers et al.29 in the case of wood-flour filled HDPE. By comparing the performance of both OPF and EFB–HDPE composites, it can be seen that both types of composites show comparable properties, i.e., in flexural and tensile strength and flexural toughness. However, OPF–HDPE composites show superiority on both flexural and tensile modulus as compared to the EFB–HDPE composites. This may be due to the greater compressibility of the former (which is lower in density and hollow in the middle of the fiber bundle) than the latter. Copyright © 2005 by Taylor & Francis

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12.2.3.2

Mechanical Properties of Various Thermoplastic Composites Filled with EFB Rozman et al.,30 in one of their studies, attempted to produce EFB (mesh size of 35–60) composites with various commonly known thermoplastics such as polystyrene (PS), polyvinylchloride (PVC), PP, and HDPE. All types of composites displayed a decreasing trend in MOR as the filler loading was increased (Figure 12.3). As mentioned earlier, the incorporation of fillers may disrupt the continuity of polymer matrix, which may result in the creation of more stress concentration points. Tensile strength results showed that the strength of the EFB composites with different thermoplastic matrix depended on the type of matrix used (Figure 12.4). EFB composites with PVC matrix displayed the highest tensile strength and were followed by PP, HDPE and PS, respectively. The impact strength for various composites showed a decreasing trend as the filler loading was increased (Figure 12.5).29 The impact strength of EFB–HDPE composite was significantly higher than the others. This may be attributed to the ability of HDPE matrix to undergo plastic deformation in the form of crazing and shear yielding during the crack propagation process. 12.2.3.3 The Effect of Compounding Techniques Two compounding techniques have been attempted to produce EFB–PP composites, i.e., internal mixer (IM) and single-screw extruder (EX). The detailed description is given elsewhere.31 Composites produced by both techniques show a decreasing trend in tensile strength as the filler loading

50 PE

PP

PS

PVC

MOR (MPa)

40

30

20

10

0 0

10

20

30 % filler loading

40

50

60

FIGURE 12.3 MOR of various EFB–thermoplastic composites. (From Rozman, H.D. et al., Intern. J. Polymeric Mater., 44, 179, 1999. With permission.)

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40 PE

PP

PS

PVC

Tensile strength (MPa)

35 30 25 20 15 10 5 0

0

10

20

30 % filler loading

40

50

60

FIGURE 12.4 Tensile strength of EFB composites made from various thermoplastics. (From Rozman, H.D. et al., Intern. J. Polymeric Mater., 44, 179, 1999. With permission.) 600 PE

PP

PS

PVC

Impact strength (J/m)

500 400 300 200 100 0 0

10

20

30 % filler loading

40

50

60

FIGURE 12.5 Impact strength of EFB composites made from various thermoplastics. (From Rozman, H.D. et al., Intern. J. Polymeric Mater., 44, 179, 1999. With permission.)

increases (Figure 12.6). This is in agreement with the trend observed in other lignocellulosic-filled composites.32,33 As mentioned earlier, this may be caused by the irregular shape of EFB fillers (Figure 12.2), which affects their capability of supporting stress transmitted from the thermoplastic matrix. Thus, the strength enhancement in the filled composite is in general much Copyright © 2005 by Taylor & Francis

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30 IM

EX

Tensile strength (MPa)

25 20 15 10 5 0 0

10

20

30

40

50

60

% of filler loading FIGURE 12.6 The effect of mixing techniques on the tensile strength of EFB–PP composites. (From Rozman, H.D. et al., J. Appl. Polym. Sci., 70, 2647, 1998. With permission.)

lower than that of fiber-reinforced systems. The results reveal that IM composites yielded a relatively higher strength than EX composites. This indicates that the mode of mixing plays an important role in determining the strength of a composite. As the degree of compounding and distribution of fillers in the polymer matrix determines the extent of formation of the interfacial region between these two components, it is believed that there is a better transfer of stress in the IM composites than in the EX composites. The better efficiency of stress transfer is also believed to be responsible for the higher tensile modulus displayed by the IM composites (Figure 12.7). This behavior is consistent with the earlier study by Raj et al.33 on PP–wood flour composites. It is interesting to note that IM composites display a markedly higher modulus than EX composites. For composites with 60% filler loading, for instance, the modulus for IM composites reaches as high as ~900 MPa as compared to about 400 MPa for EX composites. Thus, it can be inferred that the inherent stiffness of the EFB fillers can be greatly exploited by using the internal mixer. Apparently, the internal mixer can distribute the filler more evenly throughout the polymer matrix than the extruder. It is common to observe that the improvement in the tensile strength is at the expense of the elongation at break (EB).33 The EB for both composites decreases as the filler loading is increased (Figure 12.8). This may be attributed to the reduction in deformability of a rigid interface between filler and the matrix component, which is also reflected in the increase of stiffness as shown earlier in the tensile modulus result. Similar observations have been reported by several workers for other lignocellulosic composites.15,18,23,24,34 Copyright © 2005 by Taylor & Francis

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1000

Tensile modulus (MPa)

IM EX 800

600

400

200

0 0

10

20

30 % of filler loading

40

50

60

FIGURE 12.7 The effect of mixing techniques on the tensile modulus of EFB–PP composites. (From Rozman, H.D. et al., J. Appl. Polym. Sci., 70, 2647, 1998. With permission.)

14 IM

EX

Elongation at break, EB (%)

12 10 8 6 4 2 0 0

10

20

30

40

50

60

% filler loading FIGURE 12.8 The effect of mixing techniques on the elongation at break of EFB–PP composites. (From Rozman, H.D. et al., J. Appl. Polym. Sci., 70, 2647, 1998. With permission.)

It should be noted that the degree of reduction in EB for EX composites is substantially higher than for IM composites. It can also be seen that IM composites display lower EB than EX composites. As shown by tensile modulus results (Figure 12.7), IM composites are stiffer than EX composites. This obviously influences the EB properties of the composites. As better filler Copyright © 2005 by Taylor & Francis

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200

Impact strength (J/m)

IM

EX

150

100

50

0 0

10

20

30

40

50

60

% of filler loading FIGURE 12.9 The effect of mixing techniques on the impact strength of EFB–PP composites. (From Rozman, H.D. et al., J. Appl. Polym. Sci., 70, 2647, 1998. With permission.)

distribution is achieved in the internal mixer, more interfacial regions are expected to be formed between the filler and the polymer matrix. This would render the region stiffer and subsequently reduce the deformability of the composite. The impact strength results clearly indicate that the presence of EFB has reduced the energy-absorbing capabilities of the composites (Figure 12.9). A similar trend has also been reported by Mohd. Ishak et al.35 in the case of EFB-filled HDPE. As mentioned earlier, the filler system used in this study consists of irregularly shaped filler with low aspect ratio. This influences the energy absorption capability of the composite as well as the capability of supporting stress transferred from the polymer matrix. IM composites exhibit higher impact strength than EX composites. This suggests that composites made by mixing in the internal mixer are of higher strength than those prepared through extrusion. This may be attributed to the mixing characteristics of the former. The mixing process in an internal mixer is done by counterrotating rotors that give a high shear rate; the mixture is enclosed and confined in a mixing chamber. The process is more effective in improving the distribution of filler in the thermoplastic matrix. Consequently, the wetting of the filler surface is enhanced significantly. Composites with higher degrees of homogeneity in wettability and filler dispersion can be expected to possess superior mechanical properties.

12.2.3.4 The Effect of Surface Chemical Treatments Several research works have been carried out to study the effect of various surface treatments. Mohd. Ishak et al.35 attempted to produce EFB–HDPE Copyright © 2005 by Taylor & Francis

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composites using various chemicals as coupling agent. They found that with 3-aminopropyltrimethoxysilane (APM) as coupling agent, the MOEs of the samples were significantly higher than those with 3-aminopropyltriethoxysilane (APE), especially at 1 and 3% of the coupling agent (Table 12.5). The difference shown may be attributed to a stiffer molecular structure of APM with one methyl group as compared to APE with an ethoxy group. Composites with APM showed a decreasing trend as the loading was increased, while APE composites did not show significant changes. Composites with APM also displayed a significantly higher MOR than those with APE, also at 1 and 3% filler loadings. Tensile modulus was significantly increased with the addition of 1% of both coupling agents. As noted by Riley et al.,36 there are three main factors affecting the modulus: (1) filler modulus, (2) filler loading, and (3) filler aspect ratio. Since these three factors have been kept relatively constant, it can be inferred that the presence of coupling agents has led to a significant improvement in the filler–matrix interfacial bonding. The improvement obviously results in an increase in the efficiency of stress transfer from the matrix to the filler, which subsequently gives rise to higher modulus. Unlike tensile modulus, the incorporation of both types of coupling agents did not produce any significant effect on tensile strength. A significant improvement in the impact strength was observed with 1% loading of coupling agent followed by a stable strength at higher coupling agent loading. The improvement in filler–matrix adhesion, which has produced a pronounced effect on tensile modulus, has resulted in a similar effect on the impact strength of the composites. Addition of poly(propylene-acrylic acid) (PPAA) as compatibilizer was found to improve the flexural stiffness (MOE) of the composites (Table 12.5).35 TABLE 12.5 Effects of Coupling Agents and Compatibilizers on EFB–HDPE Compositesa Flexural Sample APM 1% 3% 5% APE 1% 3% 5% PPAA 1% 3% 5%

Tensile Modulus (MPa)

Strength (MPa)

Impact Strength (J/m)

92.0 55.4 40.8

767 762 784

12.6 13.2 11.8

116.5 123.1 123.2

20.0 19.0 18.7

52.7 30.7 43.4

737 651 547

12.0 12.0 12.0

123.9 117.0 137.3

10.1 9.8 11.4

24.2 25.0 21.0

362 328 292

9.9 9.3 9.2

60.0 56.8 59.3

MOE (MPa)

MOR Toughness (MPa) (kPa)

629.6 509.0 288.8

24.4 22.8 14.8

405.8 389.3 438.24 427.2 397.8 575.0

a

Mesh 35–60, 40% EFB. Source: From Mohd. Ishak, Z.A., J. Appl. Polym. Sci., 68, 2189, 1998. With permission.

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However, it was clearly seen that incorporation of the compatibilizer did not result in flexural strength improvements as shown by MOR results. As shown by MOE results, modification of the filler–matrix interfacial bonding through the presence of the compatibilizer produced a significant effect on tensile modulus of composites. PPAA failed to impart any positive effects to the tensile and impact strength. This again supports earlier claims that the shape factors outweigh any improvement in the filler–matrix interaction even in the presence of coupling agent or compatibilizers. Several attempts were made to use new chemicals as coupling agents in the EFB–thermoplastic composites. One chemical in particular was isocyanatemodified Alcell lignin.37,38 Two types of isocyanate-modified Alcell lignin were employed as coupling agents, i.e., toluene diisocyanate (TDI) and hexamethylene diisocyanate (HMDI) modified lignin. The results showed that both types of lignin were able to impart greater compatibility between EFB and PP. These were reflected in higher tensile (Figure 12.10) and flexural strength (Figure 12.11) shown by the composites with both TDI and HMDImodified Alcell lignin (both with isocyanate chemical loading of 91% by weight) than those with the unmodified lignin as well as without lignin. Rozman et al.39,40 conducted studies on the chemical modification of EFB surface with maleic anhydride (MAH) and the effect of the modification on the mechanical and physical properties of the EFB–PP composites. EFB filler has been chemically modified with MAH. The modification involved the reaction of EFB and MAH at 90oC. The composites with MAH-treated EFB showed higher flexural strength (Figure 12.12), flexural modulus 25 Unmodified lignin

TDI-modified lignin

HMDI-modified lignin

Tensile strength (MPa)

20

15

10

5

0 0

2

4

6

8

10

% of coupling agent FIGURE 12.10 Tensile strength of EFB–PP composites with unmodified, TDI-modified, and HMDI-modified lignin. (From Rozman, H.D. et al., Intern. J. Polymeric Mater., 46, 195, 2000; Rozman, H.D. et al., Polym. Int., 50, 56, 2001. With permission.)

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50

Flexural strength (MPa)

40

30

20

10 Unmodified lignin

TDI-modified lignin

HMDI-modified lignin

0 0

2

4

6

8

10

% of coupling agent FIGURE 12.11 Flexural strength of EFB–PP composites with unmodified, TDI-modified, and HMDI-modified lignin. (From Rozman, H.D. et al., Intern. J. Polymeric Mater., 46, 195, 2000; Rozman, H.D. et al., Polym. Int., 50, 56, 2001. With permission.) 60 20%

Mesh 100 40%

60%

Flexural strength (MPa)

50

40

30

20

10 0

5

10

15

20

25

30

35

Chemical loading of MAH (%) FIGURE 12.12 The effect of chemical loading of MAH in the EFB fibers on the flexural strength of EFB–PP composites. (From Rozman, H.D. et al., Polym. Testing, 22, 335, 2003. With permission.)

(Figure 12.13), and impact strength (Figure 12.14) than those with untreated EFB. The improved properties were attributed to the enhanced adhesion between the MAH-treated EFB and PP matrix as shown in Figure 12.15(b) as compared with loosely embedded fiber in composites with untreated EFB Copyright © 2005 by Taylor & Francis

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Flexural modulus (GPa)

5

4

3

2 20%

Mesh 60 40%

60%

1 0

5

10 Chemical loading of MAH (%)

15

20

FIGURE 12.13 The effect of chemical loading of MAH in the EFB fibers on the flexural modulus of EFB–PP composites. (From Rozman, H.D. et al., Polym. Testing, 22, 335, 2003. With permission.)

Impact strength (J/m)

100

80

60

40 20%

Mesh 60 40%

60%

20 0

5

10

15

20

Chemical loading of MAH (%) FIGURE 12.14 The effect of chemical loading of MAH in the EFB fibers on the impact strength of EFB–PP composites. (From Rozman, H.D. et al., Polym. Testing, 22, 335, 2003. With permission.)

fiber [Figure 12.15(a)]. It was found that for samples with MAH-treated EFB, better adhesion was detected, i.e., in the form of a more cohesive interface between the matrix and EFB, and a nicely embedded MAH-treated EFB in the matrix. The MAH-treated PP composites also showed lower water absorption (Figure 12.16) and thickness swelling (Figure 12.17) than those Copyright © 2005 by Taylor & Francis

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FIGURE 12.15 (a) EFB–PP composite (with untreated EFB). (b) EFB–PP composite (with MAH-treated EFB). (From Rozman, H.D. et al., Polym. Testing, 22, 335, 2003. With permission.)

with untreated EFB. EFB being a lignocellulosic material possesses hygroscopic hydroxyl groups in the cell walls. These functional groups are contributed by cellulose, hemicellulose, and also lignin. The absorption of moisture or water could happen through the formation of hydrogen bondings between these groups with water. Treating EFB with MAH would result in the OH groups being converted to a more hydrophobic ester group (Figure 12.18). This might explain why the water absorption and thickness swelling of the composites with MAH-treated EFB were lower than those without treated EFB. Copyright © 2005 by Taylor & Francis

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35

Water absorption (%)

30 25 20 15 10 5 Mesh 60 (MAH loading 10.08%)

Mesh 60 (untreated)

0 0

10

20

30

40

Immersion time (days) FIGURE 12.16 Water absorption of EFB–PP composites with untreated EFB and MAH-treated EFB. (From Rozman, H.D. et al., J. Appl. Polym. Sci., 87, 827, 2003. With permission.) 10

Thickness swelling (%)

8

6

4

2 Mesh 60 (MAH loading 10.08%)

Mesh 60 (untreated)

0 0

10

20 30 Immersion time (days)

40

50

FIGURE 12.17 Thickness swelling of EFB–PP composites with untreated EFB and MAH-treated EFB. (From Rozman, H.D. et al., J. Appl. Polym. Sci., 87, 827, 2003. With permission.)

12.2.3.5 The Effect of Oil Extraction It is well known that compatibility between lignocellulosic material and polymer matrix plays a crucial role in determining the properties of a composite. Lignocellulosic materials, which have polar hydroxyl groups on the surface, contributed predominantly by cellulose, hemicellulose, and Copyright © 2005 by Taylor & Francis

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O

O

C C EFB-OH +

EFB

O

O

O C C C C OH

C C O

Dicumyl peroxide + PP

Maleic anhydride

O EFB

O

O C C C C OH C C C C C C

C

FIGURE 12.18 Reaction mechanisms between MAH, EFB and PP matrix. (From Rozman, H.D. et al., J. Appl. Polym. Sci., 87, 827, 2003. With permission.)

40 35

Flexural strength (MPa)

30 25 20 15 10 5 0 T

N

U

3

3

T

/ XT

T/

N

U

E

TP

T/

U

N

IC

IC

PM

M

E4

E4

EX

PP

PP

/T

T EX

PM

PM

T/

N

U

T/

EX

FIGURE 12.19 Flexural strength of extracted and unextracted composites. (From Rozman, H.D. et al., Polym.–Plast. Technol., 40, 103, 2001. With permission.)

lignin, have difficulty in forming a well-bonded interface with a nonpolar polymer matrix, such as PP and HDPE. The compatibility is made worse in the case of EFB where residues of oil are still present on the fiber. The ester compounds in the oil may adversely affect the coupling efficiency between the EFB and coupling agents. A study has been carried out to investigate the effect of the oil on the EFB surface on the mechanical properties of the EFB–PP composites as well as coupling efficiency of the EFB and coupling agents.41 From Figure 12.19, it is obvious that the flexural strength of the composites Copyright © 2005 by Taylor & Francis

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with oil-extracted EFB fibers (without coupling agents) (EXT) is significantly higher than those without extraction (UNT). A similar trend is shown by composites with coupling agents compared to their respective unextracted composites. Composites with extracted EFB fibers, with the incorporation of poly[methylene(polyphenyl isocyanate)] (PMPPIC) as coupling agent (EXT/PMPPIC), show the highest degree of enhancement of about 20% (compared to the unextracted counterpart, UNT/PMPPIC), followed by composites with 3-(trimethoxysilyl)propyl methacrylate (TPM) (15%) and Eastman maleated PP, E43 (12%). This indicates that the presence of oil on the EFB surface may be a critical factor in determining the strength of a composite. For PMPPIC, its effectiveness to function as a coupling agent depends on its isocyanate reactivity with hydroxyl groups of EFB. Thus, the results show that oil residue on the EFB surface may inhibit this kind of interaction. A similar trend is shown for the results of flexural toughness (Figure 12.20). Tensile strength and toughness of the composites are significantly improved as the result of oil extraction (Figure 12.21 and Figure 12.22, respectively). These results also indicate that the compatibility of the components in the composites is greatly enhanced as the result of the extraction process. The stress concentrations created by the incompatible layer of esters of the oil may be reduced as the latter is removed. This may be possible through the formation of a continuous phase at the interfacial region between the matrix and the fibers. This phenomenon is supported by the SEM results.

60

Flexural toughness (kPa)

50 40 30 20 10 0 T

N

U

3

3

T

/ XT

T/

N

U

M

E4

E4

EX

E

N

U

IC

IC

M

TP

TP

T/

PP

PP

/ XT

PM

T/

E

U

N

FIGURE 12.20 Flextural toughness of extracted and unextracted composites. (From Rozman, H.D. et al., Polym.–Plast. Technol., 40, 103, 2001. With permission.)

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M

/P

T EX

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Tensile strength (MPa)

20

15

10

5

0 3

T

T

3

E4

EX

N

U

M

E4

T/

N

TP

T/

PP

/ XT

N

E

U

M

TP

/ XT

T/

E

U

IC

PP

PM

IC

PM

T/

EX

N

U

FIGURE 12.21 Tensile strength of extracted and unextracted composites. (From Rozman, H.D. et al., Polym.–Plast. Technol., 40, 103, 2001. With permission.)

250

Tensile toughness (kPa)

200

150

100

50

0 T

N

U

3

T

3

E4

EX

N

U

M

E4

T/

E

/ XT

N

U

IC

M

TP

TP

T/

PM

T/

E

N

U

FIGURE 12.22 Tensile toughness of extracted and unextracted composites. (From Rozman, H.D. et al., Polym.–Plast. Technol., 40, 103, 2001. With permission.)

Copyright © 2005 by Taylor & Francis

IC

PP

/ XT

PP

M

/P

T EX

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Figure 12.23 shows a micrograph of tensile fracture surface of unextracted composites without coupling agent. It is clearly seen that the fiber is loosely embedded and detached in the matrix. Figure 12.24 clearly shows that the effect of oil extraction gives rise to the continuous region at the interface between EFB and PP matrix in the EXT–PMPPIC composites. This may be attributed to the increased interaction between the matrix and EFB fibers. The isocyanates groups from PMPPIC may effectively come into contact with hydroxyl groups of the EFB, thus producing an efficient

FIGURE 12.23 SEM micrograph of composites with unextracted EFB fiber. (From Rozman, H.D. et al., Polym.–Plast. Technol., 40, 103, 2001. With permission.)

FIGURE 12.24 SEM micrograph of composites with extracted EFB fiber and PMPPIC. (From Rozman, H.D. et al., Polym.–Plast. Technol., 40, 103, 2001. With permission.)

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FIGURE 12.25 SEM micrograph for composites with unextracted EFB fiber and PMPPIC. (From Rozman, H.D. et al., Polym.–Plast. Technol., 40, 103, 2001. With permission.)

bridging between the matrix and EFB fibers. The interaction is also made possible by the greater ductility of the matrix, which produces homogeneous and continuous matrix. In contrast, the ductility of the matrix is relatively lower in the UNT–PMPPIC composites (Figure 12.25). The evidence clearly supports the argument given for the improvements in the strength and toughness of the EXT–PMPPIC composites.

12.3

Conclusions

Overall, oil palm EFB and OPF possess a variety of appealing properties to be used in thermoplastic composites of at least moderate strength. With their low cost and consistency in availability, these by-products can serve as a new or alternative filler to wood. As shown by the mechanical properties results, it is obvious that the properties of the EFB and OPF composites are comparable with those of wood-filled thermoplastic composites. The mechanical properties of the composites could be improved with the incorporation of various suitable coupling agents. Moisture repellency and dimensional stability are improved by these treatments. Oil palm by-products with reference to EFB and OPF demonstrate a promising future in lignocellulosic– thermoplastic composites. However, further research and development need to be carried out so that these composites might become a potential substitute for synthetic fibers such as glass and carbon fibers. Copyright © 2005 by Taylor & Francis

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References 1. Zaini, M.J., Fuad, M.Y.A., Ismail, Z., Mansor, M.S., and Mustafah, J., Polym. Int., 40, 51,1996. 2. Ismail, H., Rozman, H.D., Jaffri, R.M., and Mohd. Ishak, Z.A., Eur. Polym. J., 33, 1627, 1997. 3. Kumar, R.N., Rozman, H.D., Abusamah, A., and Chin, T.H., Sheet Moulding Compounds Based on Palm Fruit Pressed Fibre, National Seminar on Utilization of Palm Tree and other Palms, Forest Research Institute of Malaysia (FRIM), 1994. 4. Chew, L.T. and Ong, C.L., Particleboard from Oil Palm Trunk, Proceedings of the National Symposium on Oil Palm By-Products from Agra-Based Industries, Kuala Lumpur, PORIM Bulletin 11, 1985, p. 99. 5. Wan Daud, W.R., Law, K.N., and Valade, J.L., Cell. Chem. Technol., 32, 1998. 6. Hartley, C.W.S., The Expansion of Oil Palm Planting. Advances in Oil Palm Cultivation, Kuala Lumpur, Incorp. Soc. Planters, 1972. 7. Hartley, C.W.S., The Oil Palm, 3rd ed., Longmans, London, 1977. 8. Corley, R.H.V., Hardon, J.J., and Wood, B.J., Development in Crop Sciences 1: Oil Palm Research, Netherlands, Elsevier Scientific, 1976. 9. Rozman, H.D. and Wan Daud, W.R., Developments of oil palm based lignocellulose polymer blends, in Polym. Blends, Shonaike G.O. and Simon, G.P., Eds., Marcel Dekker, NY 719, 2000. 10. Basiron, Y. and Husin, M., Availability, Extraction and Economics of Oil Palm Biomass Utilization, Proceedings of the 4th Seminar on Utilization of Oil Palm Tree, 173, 1997. 11. Khalil, H.P.S. and Rozman, H.D., Polym. –Plast. Technol. Eng., 39, 757, 2000. 12. Husin, M., Utilization of Oil Palm Biomass for Various Wood-Based and Other Products. Advances in Oil Palm Research, Vol. II, Malaysian Palm Oil Board, Ministry of Primary Industries, Malaysia, 2000. 13. Rozman, H.D., Ismail, H., Jaffri, R.M., Aminullah, A., and Mohd. Ishak, Z.A., Int. J. Polym. Mater., 39, 161, 1998. 14. Rozman, H.D., Ismail, H., Jaffri, R.M., Aminullah, A., and Mohd. Ishak, Z.A., Polym. Plast. Technol. Eng., 37, 493, 1998. 15. Raj, R.G., Kokta, B.V., and Daneault, C., Intern. J. Polym. Mater., 12, 239, 1989. 16. Raj, R.G. and Kokta, B.V., J. Applied Polym. Sci., 38, 1987, 1989. 17. Myers, G.E., Clemons, C.M., Balatinecz, J.J., and Woodhams, R.T., Effects of Composition and Polypropylene Melt Flow on Polypropylene-Waste Newspaper Composites, Proceedings of the 1992 Annual Conference Society, Plastics Engineers (SPE/ANTEC), vol. 1, 1992, p. 602. 18. Myers, G.E., Chahyadi, I.S., Coberly, C.A., and Ermer, D.S., Intern. J. Polym. Mater., 15, 21, 1991. 19. Maldas, D. and Kokta, B.V., Polym.–Plast. Technol. Eng., 29, 119, 1990. 20. Felix, J.M. and Gatenholm, P., J. Appl. Polym. Sci., 42, 609, 1991. 21. Xanthos, N., Plast. Rubber Process. Appl., 3, 223, 1983. 22. Maldas, D., Kokta, B.V., Raj, R.G., and Daneault, C., Polymer, 29, 1255, 1988. 23. Rozman, H.D., Seng, T.G., Kumar, R.N., Abubakar, A., Ismail, H., and Mohd. Ishak, Z.A., Polym.–Plast. Eng., 38, 997–1011, 1999. 24. Rozman, H.D., Kumar, R.N., Addli, M.R.M., Abusamah, A., and Mohd. Ishak, Z.A., J. Wood Chem. Technol., 18, 471, 1998.

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Yam, L., Gogoi, B.K., Lai, C.C., and Elke, S.E., Polym. Eng. Sci., 30, 693, 1990. Rietveld, J.X. and Simon, M.J., Intern. J. Polym. Mater., 18, 213, 1992. Joseph, K., Thomas, S., and Pavithran, C., Polymer, 37, 5139, 1996. Woodhams, R.T., Thomas, G., and Rodgers, D.K., Polym. Eng. Sci., 24, 1166, 1984. Myers, G.E., Chahyadi, I.S., Gonzales, C., Coberly, C.A., and Ermer, D.S., Intern. J. Polym. Mater., 15, 171, 1991. Rozman, H.D., Lim, P.P., Abusamah, A., Kumar, R.N., Ismail, H., and Mohd. Ishak, Z.A., Intern. J. Polym. Mater., 44, 179, 1999. Rozman, H.D., Peng, G.B., and Mohd. Ishak, Z.A., J. Appl. Polym. Sci., 70, 2647, 1998. Raj, R.G., Kokta, B.V., Groleau, G., and Daneault, C., Polym. Plast. Technol. Eng., 29, 339, 1990. Raj, R.G., Kokta, B.V., and Daneault, C., Intern. J. Polym. Mater., 14, 223, 1990. Raj, R.G., Kokta, B.V., Groleau, G., and Daneault, C., Plast. Rubber Process Appl., 11, 215, 1989. Mohd. Ishak, Z.A., Aminullah, A., Ismail, H., and Rozman, H.D., J. Appl. Polym. Sci., 68, 2189, 1998. Riley, A.M., Paynter, C.D., McGenity, P.M., and Adams, J.M., Plast. Rubber Process Appl., 14, 85, 1990. Rozman, H.D., Tan, K.W., Kumar, R.N., Tay, G.S., and Abubakar, A., Intern. J. Polym. Mater., 46, 195, 2000. Rozman, H.D., Tan, K.W., Kumar, R.N., and Abubakar, A., Polym. Int., 50, 561, 2001. Rozman, H.D., Saad, M.J., and Mohd. Ishak, Z.A., J. Appl. Polym. Sci., 87, 827, 2003. Rozman, H.D., Saad, M.J., and Mohd. Ishak, Z.A., Polym. Test., 22, 335, 2003. Rozman, H.D., Tay, G.S., Kumar, R.N., Abusamah, A., Ismail, H., and Mohd. Ishak, Z.A., Polym.–Plast. Technol., 40, 103, 2001.

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13 Natural Fiber−Rubber Composites and Their Applications

Seena Joseph, Maya Jacob, and Sabu Thomas

CONTENTS 13.1 Introduction: Advantages and Disadvantages of Using Natural Fibers in Composites 13.2 Natural Fibers 13.2.1 Bast Fibers 13.2.1.1 Bagasse 13.2.1.2 Flax 13.2.1.3 Hemp 13.2.1.4 Jute 13.2.1.5 Kenaf 13.2.1.6 Ramie 13.2.2 Leaf Fibers 13.2.2.1 Banana 13.2.2.2 Sisal 13.2.2.3 Pineapple 13.2.3 Seed Fibers 13.2.3.1 Coir 13.2.3.2 Oil Palm 13.3 Rubber Matrices 13.3.1 Natural Rubber 13.3.2 Styrene Butadiene Rubber 13.3.3 Butyl Rubber 13.3.4 Halogenated Butyl 13.3.5 Butadiene Rubber 13.3.6 Nitrile Rubber 13.3.7 Ethylene Propylene Diene Rubber 13.3.8 Silicone Rubber 13.3.9 Thermoplastic Elastomers

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13.4 Short Fiber-Reinforced Rubber Composites 13.4.1 Theory of Reinforcement 13.4.2 Factors Affecting Reinforcement 13.4.3 Mechanical Properties 13.4.4 Fiber–Rubber Adhesion 13.4.5 Dynamic Properties 13.4.6 Rheological and Aging Studies 13.4.7 Processing 13.4.7.1 Milling 13.4.7.2 Calendering 13.4.7.3 Injection Molding 13.4.7.4 Extrusion 13.4.8 Applications 13.5 Conclusion References ABSTRACT Lignocellulosic fibers have become the wonder materials of this era. This chapter deals with the different aspects of fiber-reinforced rubber composites. A detailed section on natural fibers and their classification is presented. The different types of rubber matrices and theories of reinforcement are discussed. A report of the various investigations of natural fiberreinforced rubber composites has been included. Studies ranging from viscoelastic properties to rheological properties are presented. Finally, the processing techniques and applications of rubber composites are discussed.

13.1

Introduction: Advantages and Disadvantages of Using Natural Fibers in Composites

Nowadays, ecological concerns have resulted in renewed interest in natural materials. Issues such as recyclability and environmental safety are becoming increasingly important to the introduction of materials and products. Natural fibers like flax, hemp, banana, sisal, oil palm, and jute have a number of technoeconomical and ecological advantages over synthetic fibers like glass fibers. The combination of interesting mechanical and physical properties together with their environmentally friendly character has tiggered interest by a number of industrial sectors, notably the automobile industry. The major advantages are the following: ●

●

●

Low specific weight, which results in a higher specific strength and stiffness when compared to glass-reinforced composites Renewable resource with production requiring little energy. CO2 is used while oxygen is given back to the environment Processing atmosphere is friendly with better working conditions and therefore there are reduced dermal and respiratory irritations

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High electrical resistance Thermal recycling is possible Good thermal and acoustic insulating properties Biodegradability

The disadvantages include: ● ● ● ●

Enormous variability Poor moisture resistance Poor fire resistance Lower durability

It is quite clear that the advantages outweigh the disadvantages and most of the shortcomings have remedial measures in the form of chemical treatments. The natural fibers that are generally used in rubber composites are given in Table 13.1. The lignocellulosic fibers have an advantage over synthetic ones since they buckle rather than break during processing and fabrication. In addition, cellulose possesses a flattened oval cross section that enhances stress transfer by presenting an effectively higher aspect ratio.

13.2

Natural Fibers

Natural fibers are subdivided, based on their origins, coming from plants, animals, or minerals. Generally plant or vegetable fibers are used to reinforce polymers. Over the past few years, a number of researchers have been TABLE 13.1 Typical Reinforcing Fibers Used for Natural Fiber–Rubber Composites Fiber

Diameter (µm)

Bagasse 20–40 Banana 80–250 Coir 100–460 Flax 8–31 Hemp 13–40 Jute — Kenaf 4–9 Oil palm 150–500 Pineapple 20–80 Ramie 17–64 Sisal 50–200

Density (g/cm3) 1.25 1.35 1.15 1.54 1.48 1.45 — 0.7–1.55 1.45 1.5 1.45

Cellulose Content (%) 40 65 43 71.2 70.2 71.5 36 65 81 76.2 73.1

Hemicellulose Content (%) 30 8 10–20 18.6 23 13.6 21.5 19 — 14.6 14.2

Lignin Content (%) 20 5 45 2.2 5.7 13.1 17.8 13.2 12 0.7 11

Source: From Harris, M., Handbook of Textile Fibers, Harris Research Lab., Washington, DC, 1954; and Zylinski, T., Fiber Science, Office of Technical Services, U.S. Dept. of Commerce, Washington, DC, 1964. With permission.

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involved in investigating the exploitation of natural fibers as load-bearing constituents in composite materials. The use of such materials in composites has increased due to their relative low cost, their ability to recycle, and for the fact that they can compete well in terms of strength per weight of material. Natural fibers can be considered as naturally occurring composites consisting mainly of cellulose fibrils embedded in a lignin matrix. The cellulose fibrils are aligned along the length of the fiber, which render maximum tensile and flexural strengths, in addition to providing rigidity. The reinforcing efficiency of a natural fiber is related to the nature of cellulose and its crystallinity. The main components of natural fibers are cellulose, hemicellulose, lignin, pectins, and waxes.1 Cellulose is a natural polymer containing thousands of β-D-glucose units linked by glucosidic linkages at the C1 and C4 positions 2 (Figure 13.1). Solid cellulose forms a microcrystalline structure with regions of high order, i.e., crystalline regions, and regions of low order, i.e., amorphous regions. Hemicellulose is not a form of cellulose and the name is a misnomer. It comprises a group of polysaccharides that remain associated with cellulose after lignin has been removed. Hemicellulose differs from cellulose in three aspects. First, it contains several different sugar units whereas cellulose contains only 1,4-β-D-glucopyranose units. Second, it exhibits a considerable degree of chain branching, whereas cellulose is a linear polymer. Third, the degree of polymerization of native cellulose is 10−100 times higher than that of hemicellulose. Lignins are complex hydrocarbon polymers with both aliphatic and aromatic constituents. They are totally insoluble in most solvents and cannot be broken down to monomeric units. Lignin is totally amorphous. Pectin is a collective name for heteropolysaccharides. Waxes make up the last part of fibers and consist of different types of alcohols. The general classification and properties of natural fibers are given in Table 13.1.3,4

13.2.1

Bast Fibers

Bast consists of a wood core surrounded by a stem. Within the stem, there are a number of fiber bundles, each containing individual fiber cells or fila-

1 HO 4

H

OH

OH H

H

H

O

2 H H C 1

O

CH2OH O H OH

H

FIGURE 13.1 Structure of cellulose.

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OH

OH H

H

1 HH 4

4

CH2OH

O C

H

O

2 H

OH 3

n−2

CH2OH

CHOH 1

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ments. Stronger composites are obtained if the bundles are pretreated in such a way that removing the lignin between the cells separates cells. Boiling in alkali is one of the ways to separate cells. Bast fibers are usually grown in warm climates, e.g., bagasse, flax, hemp, jute, kenaf, and ramie.

13.2.1.1 Bagasse Bagasse is a by-product of the sugarcane industry and it grows extensively in tropical countries like Brazil, India, Pakistan, Indonesia, Philippines, and Jamaica.5 Bagasse consists of the sheath and pith of the cane stalks. Bagasse fiber-reinforced composites have been used as structural materials; e.g., bagasse−rubber composite is a rigid, tough, red structure which can be molded into a corrugated roof. The product exhibits excellent durability. 13.2.1.2 Flax This fiber is extracted from the plant Linum usitatissimum L., grown chiefly in former USSR, Poland, France, Belgium, and Ireland. The plant is cultivated mostly for its oil-bearing seed (linseed), although it is also an important source of a vegetable fiber. Flax is an annual plant with a slender, grayish green stem growing to a height of 90−120 cm. The plants are pulled by hand or machine for highest yield and quality although mowing is practiced for some grades. After deseeding, the straw is retted, i.e., the fibers are liberated through enzymatic action on pectinous binding material in the stem. Dew or water retting is also employed. The fiber is then hackled for alignment and final cleaning. 13.2.1.3 Hemp The hemp fiber is extracted from the plant Cannabis sativa, which originated in Central Asia. The plant grows readily in temperate and tropical climates but its commercial production for fiber is in China and Eastern Europe. The hemp plant is grown for fiber from the stem, for oil from the seeds, or for drugs from the flowers or leaves. The mature hemp stalk is harvested at the proper time to ensure the highest quality and yield by hand cutting and spreading or by a harvester-spreader for dew retting. The retted and dried stem is further treated by either hand breaking and hackling to remove the woody stem portion or by mechanical breaking. The grading is by color, lustre, density, spinning quality, and strength. 13.2.1.4 Jute The jute fiber is obtained from two species of the annual herbaceous plant Corchorus capsularis of Indo-Burma origin and C. olitorius from Africa. The major jute production areas are India and Bangladesh. The two sources of jute are differentiated by their seeds and seed pods Corchorus capsularis is round-podded and is called white jute; C. olitorius is long-podded and is known as tossa or daisee. The plants are harvested by hand at an early seed Copyright © 2005 by Taylor & Francis

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stage using knives and the stems are left on the ground for several days to promote defoliation. The defoliated stems are bundled and taken for wet retting in canals, ditches, or ponds for periods of 10−20 days. The jute is then baled for shipment to domestic users.

13.2.1.5 Kenaf The kenaf fiber is extracted from the plant Hibiscus cannabinus, which belongs to the Malvaceae family. The plant is native to Egypt and Russia. The plant is a herbaceous annual growing in a single stem to heights of 1−4 m. Kenaf is harvested as flowering begins and it can be hand cut, mowed, or pulled. It is often stem-retted followed by hand or mechanical stripping and washing. Kenaf is graded into three grades based on color, uniformity, strength, and cleanliness. 13.2.1.6 Ramie This fiber is produced from the stems of Bochmeria nivea, a nettle native to central and western China but growing in regions varying from temperate to tropical including China, Brazil, Taiwan and Japan. Another name for the raw fiber is China grass. The plant grows 1−2 m high or higher with stems 8−16 mm in diameter. The ramie fiber is contained in the bark and is usually extracted by hand stripping and scraping. The raw ramie fibers contain 25−35% plant gums and small quantities of parenchyma cells that must be removed before the fibers can be spun. The plant is cut green and defoliated manually, and bast ribbons are stripped from the woody stem. Ramie fibers are graded according to length, color, and cleanliness.

13.2.2

Leaf Fibers

In general, leaf fibers such as sisal, abaca, banana, and henequen are coarser than bast fibers. Within the total production of leaf fibers, sisal is the most important and is obtained from the agave plant. Its stiffness is relatively high and it is often applied as binder twines. The abaca fiber from the banana plant is durable and resistant to seawater.

13.2.2.1 Banana Banana fiber is extracted from the plant Musa sapientum. Each of the stalks is 2.7−6.7 m tall with a trunk 10−20 cm wide at the base. The sheaths before expanding are 2–4 m long, 13−20 cm wide, and about 10 mm thick at the center. The fibers run lengthwise in the sheaths. The sheaths vary in length, width, and color. The mature stalks are cut off at the roots and at a point just below where they begin to expand. In some countries the stripping method is used while modern plantations have replaced this hand-pulling operation with power-driven pulling machines. The annual production of banana is Copyright © 2005 by Taylor & Francis

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200
103 tons. The main countries of origin are India, Indonesia, and Philippines.

13.2.2.2 Sisal This fiber is extracted from the leaves of the plant Agave sisalana, widely cultivated in the Western Hemisphere, Africa, and Asia. The agaves have rosettes of fleshy leaves, usually long and narrow, which grow out from a central bud. As the leaves mature, they gradually spread out horizontally and are 1−2 m long, 10−15 cm wide and ~6 mm thick at the center. The fibers are embedded longitudinally in the leaves and are most abundant near the leaf surfaces. The leaves contain ~90% moisture, but the fleshy pulp is very firm and the leaves are rigid. The fiber is removed when the leaves are cut because dry fibers adhere to the pulp. The fiber is removed by scraping away the pulpy material, generally by a mechanical decortication process. In the decortication process, the leaves are fed through sets of crushing rollers. The crushed leaves are held firmly at their centers and both ends are passed between pairs of metal drums on which blades are mounted to scrape away the pulp. The centers are scraped in the same way. The fiber strands are then washed and dried. 13.2.2.3 Pineapple Pineapple leaf fibers at present are a waste product of pineapple cultivation. The fibers are extracted from the leaves of the plant Ananus cosomus, belonging to the Bromeliaceae family. The fiber is extracted by hand scraping after beating the leaves to break up the pulpy tissue or after a retting process that partially ferments and softens the leaves. 13.2.3

Seed Fibers

Cotton is the most common seed fiber and is used for textile all over the world. Other seed fibers are applied in less demanding applications such as stuffing of upholstery. Coir is an exception to this and is used to make mats and ropes. Other examples are kapok and oil palm.

13.2.3.1 Coir The fiber is contained in the husk of the coconut tree (Cocos nucifera), which is positioned beneath the outer covering of the fruit and envelops the kernel or coconut. Sri Lanka and India are important producers of coir. The fruits are gathered just short of ripeness and the husks are broken by hand or by use of a bursting machine. The extraction of the fiber involves retting at the edges of rivers and also in pits or in modern operations in concrete tanks. The retted husks are beaten with sticks to remove extraneous matter. The dried fiber is suitable for spinning. The fibers can also be removed from the husk by a decorticating machine in connection with copra production. Copyright © 2005 by Taylor & Francis

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In the extraction of coir fiber from coconut husk, a large quantity of coir waste is obtained. The coir waste consists of coconut pith resulting from the retting process, broken fibers from coir spinning, and shear waste from mats and matting industries. Coir pith constitutes about 70% of coconut husk with very low density. About 500,000 tons/year of coconut pith are produced in India. Studies have indicated that coir pith and waste do not have a reinforcing action on the vulcanizates even though they can lessen the costs of the products considerably.

13.2.3.2 Oil Palm Oil palm6 is one of the most economical and very high potential oil-producing crops. It belongs to the species Elaeis guineensis under the family Palmaceae, and is commonly found in the tropical forests of West Africa. Major industrial cultivation is in Southeast Asian countries such as Malaysia and Indonesia. Large-scale cultivation has grown in Latin America. Oil palm cultivation in India has risen to the level of attaining possible self-sufficiency in oil production. Oil palm empty fruit bunch (OPEFB) fiber and oil palm mesocarp fiber are two important types of fibrous materials left in the palm oil mill. OPEFB is obtained after the removal of oil seeds from fruit bunch for oil extraction. The average yield of OPEFB fiber is about 400 g per bunch. The fibers are extracted by retting followed by cleaning and drying.

13.3

Rubber Matrices

The principal classes of rubber composites that have been used for short fiber composites are natural rubber (NR), styrene butadiene rubber (SBR), butyl rubber (IIR), butadiene rubber (BR), nitril rubber (NBR), cloroprene rubber (CR), ethylene propylene diene rubber (EPDM), polyurethane rubber, silicone rubber, and thermoplastic elastomers. The most widely used rubber matrix is natural rubber. 13.3.1

Natural Rubber

Natural rubber (NR) belongs to a class of compounds known as elastomers. The Mayans in the Western Hemisphere used NR for centuries before it was introduced into Europe by Columbus. The term rubber was coined, however, by Joseph Priestly. Natural rubber is indispensable in our daily lives. The main uses of NR are concentrated in four key areas: medical devices, industrial products, domestic and recreational goods, and automobile products. The current elastomer consumption in the world is 18 million t/yr. NR supplies about one-third of the world demand for elastomers. It is also used as an industrial raw material. NR is a naturally occurring elastomeric polymer of Copyright © 2005 by Taylor & Francis

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isoprene (2-methyl-1,3-butadiene). It can be extracted from latex of only one kind of tree, the Hevea braziliensis. Hevea rubber is produced in many tropical regions of Southeast Asia, Africa, and Central and South America. There is probably only one other potential source of NR, the guayule shrub (Parthenium argentatum). Polyisoprene exists naturally in the form of two stereoisomers, namely, cis-1,4-polyisoprene and trans-1,4-polyisoprene (Figure 13.2). Chemically, NR is cis-1,4-polyisoprene. A linear, long-chain polymer with repeating isoprenic units (C5H8), it has a density of 0.93 g/cm3 at 20°C. Natural trans-1,4-polyisoprene is a crystalline thermoplastic polymer, which is mainly used in golf ball covers, root canal fillings in dentistry, special adhesives, and, to a lesser extent, in wire and cable coverings. It is obtained as gutta percha from Palaquium oblongofolium in South East Asia. It is much harder and less soluble than Hevea rubber. Certain trees such as the Chicle (Achras sapota) produce latex, which is a physical mixture of cis- and transpolyisoprene. cis-1,4-polyisoprene can be produced synthetically by stereospecific polymerization reaction using Zieglar−Natta heterogeneous catalyst. The important components of NR are given in Table 13.2.

CH 3 C

CH CH 2

CH 2 CH 3 CH 2

C

n

Cis -1,4-polyisoprene CH

CH 2

Isoperene CH 2

CH 3 C

CH

CH 2 n Trans -1,4-polyisoprene

FIGURE 13.2 Structure of natural rubber.

TABLE 13.2 Typical Analysis of NR Components

Percentage

Rubber hydrocarbon Acetone extract Protein Moisture Ash

93.3 2.9 2.8 0.6 0.4

Source: From Barlow, F.W., Rubber Compounding, Principles, Materials and Techniques, Marcel Dekker, New York, 1993. With Permission.

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The most important factor governing the properties of polyisoprene is the stereoregularity of the polymer chain. The unique characteristic of NR is its ability to crystallize under strain, a phenomenon known as strain-induced crystallization.7,8 Stretching of vulcanizates of polyisoprene having at least 90% cis content leads to crystallization, which in turn leads to strengthening of rubber. Another interesting aspect of isoprene rubbers is their low hysteresis, yielding low heat buildup during flexing. The combination of high tensile properties and low hysteresis explains the requirement of NR as the primary rubber in heavy vehicle tires. In addition, NR has excellent tensile and tear properties, good green strength, and building tack. However, NR is not very resistant to oxidation, ozone weathering, and a wide range of chemicals and solvents, mainly due to its unsaturated chain structure and nonpolarity. One of the main modifications of NR is deprotenized rubber (DPNR). This is very useful when low water absorption is wanted, vulcanizates with low creep are needed, or more than ordinary reproducibility is required. Normally NR has between 0.25 and 0.5% nitrogen as protein; deproteinized rubber has only about 0.07%. A drawback is that since protein matter in the rubber accelerates cure, deproteinized rubber requires more acceleration. DPNR is made by treating NR latex with a bioenzyme, which hydrolyzes the proteins to watersoluble forms. A protease such as Bacillus subtilis is used at about 0.3 phr. When the enzymolysis is completed the latex is diluted to 3% total solids and coagulated by adding a mixture of phosphoric and sulfuric acid. The coagulated rubber is then pressed free of most of the water, crumbed, dried, and baled. Another modification is oil-extended NR (OENR). There are three ways to make this kind of rubber: (1) coagulation of latex with oil emulsion, (2) Banbury mixing of oil and rubber, and (3) allowing the rubber to absorb the oil in pans until almost all is absorbed, then milling to incorporate the remaining oil. Recently rubber and oil have been mixed using an extruder. A comparatively new modification of NR is epoxidized NR. The rubber molecule is partially epoxidized, with the epoxy groups randomly distributed along the molecular chain. The main advantages of ENR over NR are improved oil resistance and low gas permeability. Another new modification is thermoplastic natural rubbers (TPNR). These are physical blends of natural rubber and polypropylene, mixed in different proportions to yield rubbers with different stiffness properties. They are suitable for injection molding into products for automotive applications such as flexible sight shields and bumper components. Many other chemically modified forms of natural rubber were available in the past. These included cyclized rubber, chlorinated rubber, hydrochlorinated rubber, isomerized or an anticrystallizing rubber, and depolymerized rubber. 13.3.2

Styrene Butadiene Rubber

Styrene butadiene rubbers (SBR) are the most commonly used synthetic rubbers today. They are produced by copolymerization of butadiene and styrene. Copyright © 2005 by Taylor & Francis

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Other raw materials used in the manufacturing of SBR are extending oils, soaps, antioxidants, antiozonants, short-stopping agents, coagulating agents, molecular weight modifiers, catalysts, catalyst modifiers, water, and hydrocarbon solvents. The total nonrubber constituents are rarely more than 10% except in oil-extended rubbers. The monomer butadiene is usually manufactured from petroleum sources. In the Phillips process, a mixture of 70% butane and 30% 1-butene and 2-butene is preheated to 550°C, mixed with superheated steam, and then fed to a catalytic dehydrogenator. The product is separated and purified. In the Houndry process n-butane is dehydrogenated in a single-step process with a chromia alumina catalyst. For emulsion polymerization the butadiene containing less than 5000 ppm acetylenic impurities is enough, whereas for solution polymerization the tolerance limit is less than 50 ppm. The other monomer, styrene, is derived from ethylene and benzene in the presence of aluminum chloride to yield ethyl benzene, which is catalytically dehydrogenated at about 630°C to form styrene. Ethylbenzene is obtained by alkylating benzene with ethylene using Friedel−Crafts catalysts such as aluminum chloride. It is then mixed with superheated steam and dehydrogenated over chromia alumina or iron oxide catalyst using high temperature, low pressure, and a high ratio of steam to ethyl benzene. The copolymerization of butadiene and styrene is usually effected by either emulsion or solution polymerization. Polymerization at 5°−10°C is called cold process and the product is called cold SBR, as distinct from hot SBR, which is made at about 50°C. Blackmasterbatch SBR is also supplied by synthetic rubber products for fabrication of rubber goods in locations where mixing of carbon black is not convenient. The compounding of SBR is similar to NR and other unsaturated hydrocarbon rubbers. Emulsion SBR is the prototype of general-purpose rubber and is said to have cure compatibility and excellent processability. Besides its Tg, the good processability of SBR results from its favorable combination of molecular weight, molecular weight distribution, and considerable proportion of branches in its molecule. Compounding recipes with low sulfur or with organically bound sulfur, as in thiazoles, leads to vulcanizates with better aging but slower curing. Zinc stearate or zinc oxide plus strearic acid is the most common activator for SBR. The recipe may also contain plasticizers, softeners, tackifiers, and other ingredients that have given evidence of solving compounding problems. The preparation of SBR compounds is similar to that of rubber. 13.3.3

Butyl Rubber

Butyl rubber (IIR) is produced by cationically copolymerizing isobutylene (97%) with small amounts of isoprene (3%). The polymerization is done below –95°C, with aluminum chloride catalyst. The most widely used butyl rubbers are copolymers of isobutylene and isoprene. Grades are distinguished by molecular weight and mole percent of Copyright © 2005 by Taylor & Francis

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unsaturation. The mole percent of unsaturation is the number of moles of isoprene per 100 moles of isoprene. The molecular characteristics of low levels of unsaturation between long segments of polyisobutylene produce unique elastomeric qualities that find application in a wide variety of finished rubber articles. These special properties can be listed as follows: 1. 2. 3. 4. 5.

Low rates of gas permeability Thermal stability Ozone and weathering resistance Vibration damping and higher coefficient of friction Chemical and moisture resistance.

Butyl rubber is commercially vulcanized by three basic methods. These are accelerated sulfur vulcanization, cross-linking with dioximes, and polymethylol-phenol resin cure. In sulfur vulcanization, in contrast to the higher unsaturated varieties, adequate levels of sulfur vulcanization can be obtained with very active thiuram and dithiocarbamate accelerators. Other less active accelerators such as thiozols derivatives may be used as modifiers to improve safety of processing scorch. Thiurams provide the primary accelerating activity, which promotes the most efficient use of sulfur. Thiazole reduce scorch during processing. High levels of dithiocarbamates and low levels of sulfur favor the formation of more stable monosulfidic cross-links. In dry rubber processing, the dioxime cure is largely used in butyl rubber electrical insulation formulations to provide a maximum of ozone resistance and moisture impermeability to the vulcanizate.

13.3.4

Halogenated Butyl

The introduction of chlorine to the butyl molecule, in ~1/1 molar ratio of chloride to the double bond, achieved a broadening of vulcanization latitude and rate and enhanced covulcanization with general purpose, high-unsaturation elastomers while preserving many unique attributes of the basic butyl molecule. Bromination in the same molar ratio further enhanced cure properties and provided greater opportunity for increased covulcanization and adhesion. Thus, the halogenated butyls contributed the required level of covulcanizability and adhesion to the highly unsaturated tire elastomers to carry the advantages of butyl inner tubes forward through the development of the radial tubeless tire. For example, chlorobutyl has proved to be highly useful in many commercial rubber products. These products take advantage of desirable characteristics generic to butyl polymers such as resistance to environmental attack and low permeability to gases. In addition, chlorobutyl offers the superimposed advantages of cure versatility, highly heat-stable cross-links, and the ability to vulcanize in blends with highly unsaturated elastomers. Copyright © 2005 by Taylor & Francis

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Bromobutyl, because of its low permeability to gas and moisture vapor and its ability to adhere to high-unsaturation rubbers, is generally the prefered choice for 100% halobutyl inner liner formulations in tubeless tires.

13.3.5

Butadiene Rubber

Polybutadiene (BR) is a synthetic thermoplastic polymer made by polymerizing 1,3-butadiene with a stereospecific organometallic catalyst (butyl lithium), though other catalysts such as titanium tetrachloride and aluminum iodide may be used. The cis isomer, which is similar to natural rubber, is used in the tread due to its abrasion and crack resistance and low heat buildup. Large quantities are also used as blends in SBR. The configurations of polybutadiene are cis, trans, and vinyl. Since either or both of the double bonds in butadiene can be involved in the polymerization mechanism, the resulting polymer may have a variety of configurations. These result from the fact that the special arrangement of the methylene groups in the polybutadiene backbone allows for geometric isomerism to occur along the polymer chain. The processing characteristics of polybutadiene are influenced by the polymer microstructure, molecular weight, molecular distribution, and degree of branching. Most polybutadienes are highly resistant to breakdown and have poor mill-banding characteristics and a rough extrusion appearance compared to SBR. Certain chemical peptizers slightly increase breakdown and improve processing. A lower Mooney viscosity also improves processing but may lead to cold flow problems. A broad molecular weight distribution and branching both improve milling and extrusion behavior as compared to a linear polymer. Polybutadienes are cured using conventional sulfur recipes or peroxide systems. For curing, thiazols and sulfenamides are generally employed alone or in combination with other accelerators. Cure rates are close to SBR using similar loads of sulfur. SBR blended with cis-polybutadiene is a potential partial or full replacement for SBR and has only recently become commercially important to the rubber industry. Cured properties of cis-1,4-polyisoprene vulcanizates are in general similar to those for natural rubber. Both show high gum tensile strength, good mechanical strength in filled compounds, high resilience, and low heat buildup in related applications.

13.3.6

Nitrile Rubber

Nitrile rubber is prepared by random polymerization of acrylonitrile with butadiene by a free radical catalyst. The monomers are emulsified in water, a free radical- generating catalyst is added, and the mixture is agitated while a constant temperature is maintained. After the desired degree of polymerization is reached, a shortstop and stabilizers are added, residual monomers Copyright © 2005 by Taylor & Francis

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are removed, the latex is concentrated, coagulated, washed, dewatered, and the resulting crumbs are dried and compacted to form bales. Nitril rubbers are available in several forms such as sheets, crumb powders, and liquid. Oil resistance is the most important property of nitril rubber. Nitril rubber is available in several grades of oil resistance, based on the acrylonitril content of the polymer. Available grades range from 15% to 50% acrylonitril content. A modification of the nitril rubber molecule, i.e. the addition of carboxyl groups, results in a rubber with the usual oil-resistant characteristics of nitril rubber, but with much improved abrasion resistance. Blends of poly(vinyl chloride) and nitril rubber also are produced, which have much improved ozone resistance and improved flame-retardant properties. Nitril rubber can be compounded to obtain a broad range of properties. It is compounded in the same fashion as NR. Polymer selection is important to obtain the best balance of oil resistance and low-temperature flexibility. The higher acrylonitrile polymers have the highest oil and fuel resistance and low-temperature properties. The lower acrylonitrile content polymers have good low-temperature properties at some sacrifice of oil and fuel resistance. A good antioxidant is usually added in all nitrile compounds to improve stability. The choice of antioxidant depends on the requirements, such as high heat resistance, extraction resistance, or staining characteristics. Ozone protective waxes usually at a 1- to 2-part level are generally added to the compound in addition to ozone inhibitors to induce ozone resistance. To improve processing and low-temperature properties, plasticizers are also added to the compound. Sulfur, sulfur donors, or peroxide systems can achieve vulcanization. Because of oil and fuel-resistant characteristics, nitrile rubber finds application in the hose industry where oil, fuel, chemicals, and solutions are transported. Typical hose constructions requiring a nitrile rubber tube or liner are automotive, marine, aircraft fuel lines, bulk fuel transfer, oil tanker, chemical transfer, industrial air hoses, and dairy and creamery hoses.

13.3.7

Ethylene Propylene Diene Rubber

Ethylene propylene diene rubber (EPDM) is an elastomer based on stereospecific linear terpolymers of ethylene, propylene, and small amounts of nonconjugated dienes. The unsaturated part of the polymer molecule is pendant from the main chain, which is completely saturated, and can be vulcanized with sulfur. The dienes are so structured that only one of the double bonds will polymerize and the unreacted double bond acts as the site for sulfur crosslinking. The later unsaturation is designed so that it does not become part of the polymer backbone but becomes a side group. As a consequence, the terpolymer retains the excellent ozone resistance that copolymer possesses. Conventional rubber equipment is used for the processing of EPDM compounds. Selection of the polymer or blend of polymers is an essential consideration in order to obtain optimum processability. Copyright © 2005 by Taylor & Francis

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EPDM has found extensive use in a wide variety of applications. A significant new application now developing involves the production of various grafts on the polymer, opening the way for the modification of many other plastics such as polystyrene, poly(acrylonitril-costyrene), and nylon where improved aging and weathering are desired.

13.3.8

Silicone Rubber

Silicone rubbers differ from other rubbers in one important aspect: they have an organic backbone made of silicone and oxygen atoms and belong to a family of compounds known as silicones. The most important properties of these rubbers are their retention of flexibility (a measure of elongation), resilience (a measure of rebound), and tensile strength (a measure of toughness) to an amazing degree in a wide temperature range of 150°F to 600°F. All other rubbers get hardened and turn brittle at low temperatures and become soft and lose their elasticity at higher temperatures. Silicone rubber is the best available electrical insulator as it retains its dielectric strength, power factor, and insulation resistance over a wide temperature range when other insulations become useless. It has excellent resistance to ozone and its corona resistance approaches mica. Some other important properties of silicone rubbers include its release characteristics, chemical, fuel, and oil resistance, inertness, nontoxicity, high permeability to gases, and resistance to weathering.

13.3.9

Thermoplastic Elastomers

Thermoplastic elastomers are materials that combine the properties of thermoplasticity and rubber-like behavior. This is a two-phase system in which one of the phases is a hard polymer that does not flow at room temperature, but becomes fluid when heated. The other phase is a soft, rubbery polymer. Five types of commercially important thermoplastic elastomers are polystyrene elastomer block copolymers, polyester block copolymers, polyurethane block copolymers, polyurethane block polymers, polyamide block polymers, and polypropylene/EP copolymer blends. In the block copolymers, the two phases are formed from the segments of the same chain molecule. Polyurethane rubbers have higher tensile strength than any other rubber and have excellent tear and abrasion resistance. They tend to have a high hardness and low resilience. They also show resistance to ozone and oxygen and to aliphatic hydrocarbons. In the case of polystyrene/elastomer block polymers, three types of elastomer segments [polyisoprene, polybutadiene, and poly(ethylene-cobutylene)] are used commercially. The products are named S-IS, S-B-S, and S-EB-S, respectively. The polystyrene/elastomer block copolymers are prepared by block copolymerization. An alkyl lithium initiates the polymerization of a Copyright © 2005 by Taylor & Francis

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monomer (A). The resulting polymer, (A)n, can initiate the polymeriza-tion of a second monomer, (B), to give a block copolymer (A)n-(B)n, and so forth.

13.4

Short Fiber-Reinforced Rubber Composites

Short fiber rubber composite can be defined as a compounded rubber matrix containing discontinuous fibers that are distributed within the rubber to form a reinforcement phase. A tremendous amount of research has been carried out on natural and synthetic fiber-reinforced rubber composites. The reinforcement of an elastomer with fibers combines the elastic behavior of rubber with the strength and stiffness of the reinforcing fiber. Short fibers are also used to improve or modify certain thermodynamic properties of the rubber for specific applications or to reduce the cost of the fabricated articles.9 According to O’Connor,10 the limitations to date of short fibers in rubber compounding applications have been due to difficulties in achieving uniform dispersion of fibers, fiber breakage during processing, and difficulties in handling, incorporating, and bonding the short fibers into the rubber matrix. In order to use a fiber-reinforced elastomeric composite most effectively, a basic understanding must be obtained of how the various properties depend on the fiber properties, the matrix properties, and the processing methods.11 The presence of fibers in a rubber increases the modulus but other improvements are also sought with fiber addition. The possibility of much improved properties brought about by the alignment of fibers, to give a composite with some measure of anisotropy, is of great interest. In 1936, Guth et al.12 derived an equation for calculating the modulus of a fiber-reinforced matrix, applicable to fiber-reinforced rubbers, which has been much quoted. This equation is commonly referred to as the “Guth− Gold” equation and is expressed as: G  Go (1 0.67 fc  1.62 f 2c2)

(13.1)

where G  modulus of composite material, Go  modulus of matrix material, f  length to diameter ratio (aspect ratio) of fiber, and c  volume concentration of fiber. When the fiber aspect ratio is in the range 10−50, moduli ratios of 102−103 can be achieved if there is good adhesion between the fiber and the matrix. A later modification taking into account the fiber orientation is the Boustany−Coran equation: Ecomp  Er {1 Kf Vf [ 26  0.85 (l/d) ]}

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(13.2)

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where Ecomp  modulus of the composite, Er  modulus of rubber, K  constant, f  a function of fiber orientation, l/d  fiber aspect ratio, and Vf  fiber volume fraction. In 1975, a paper by Anthoine et al.13 dealt mainly with the use of natural short-length cellulose fiber as a rubber reinforcement. They claimed that this fiber, being naturally available in short lengths (from wood pulp), would circumvent the complicated route of making short fiber from synthetic polymer and thus would be more cost-effective. The authors reported that the oriented cellulose fibers gave an increase in tensile strength, but the most important changes were in increased modulus and reduced elongation at break. The modulus of the fiber-reinforced materials, as defined by Young’s modulus, showed a steady increase with increasing fiber concentration. Tensile strength, however, first fell with increasing fiber level and then increased beyond that of the matrix material at higher fiber loadings. This was explained by the fact that at low concentrations the matrix is not restrained by enough fibers and high strains occur in the matrix at low stresses, causing debonding. The matrix strength is thus reduced by the debonded fibers. When enough fibers are present, as concentration increases, the matrix is restrained. Eventually, at high concentrations, fiber orientation becomes difficult because the rubber matrix cannot easily flow around the fibers and a reduction in tensile strength occurs again.

13.4.1

Theory of Reinforcement

The most common assumption in the case of short fiber-reinforced elastomers is that either the stress state or the strain is uniform everywhere in the deformed composite material. Stiffness is usually considered in terms of the linear elastic Young’s modulus. Even though elastomers do not exhibit substantial linear elastic regions of deformation in comparison to their useful extensions, the analysis of elastomer-based composites can proceed on this basis because in the reinforced state linear elasticity is more closely followed. In the case of reinforcements with fibers of sufficiently high aspect ratio, it is found that as long as a substantial orientation is obtained in the direction of the applied stress, the assumption of constant strain is valid. In other words, the fibers of low extensibility relative to the elastomeric matrix are controlling the overall deformation state so that the matrix experiences the same low extension except for regions of stress concentration in the vicinity of the fiber ends. Thus, linear elasticity is generally applicable, but it must be realized that in actuality the matrix exists in a rather complex state of nonuniform tension and shear. The high elastic moduli of the fibers (14 GPa) differ widely from those of the elastomeric matrix (generally about 3 MPa), which might yield a 103 greater Young’s modulus ratio, Ef/Em, than for a hard plastic with the same fiber and have an effect on reducing the efficiency of reinforcement.

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The tensile stress generated in a fiber-reinforced elastomer will in general be distributed between the fiber (f) and matrix (m) phases,

σc  σf  σm

(13.3)

where c refers to the fiber−rubber composite. If each phase is assumed to be linearly elastic, this relationship can be further written as

σc  Efεfφf  Emεmφm

(13.4)

where E is Young’s modulus, ε the tensile elongation, and φ the volume fraction of each phase. Although the micromechanical stress state is highly complex in a nonuniform composite, the role of discontinuous fibers in providing reinforcement can be gained by considering them to be well aligned in the direction of applied stress. It is through the shear stresses applied through the matrix that a tensile stress is generated in the fiber. As these shearing stresses are larger at the fiber ends and diminish to zero in the midsection of the fiber, the tensile stress builds up from a small value given by the matrix-supported tensile stress at the fiber ends toward a constant elevated value in its midsection (Figure 13.3). The fiber tensile stress does not continue to increase in unbounded form with increasing distance from the fiber end but will rather reach a peak at the point where the

Stress

σf(1)

Length

Stress

σf(1)

t=Lc/2 Length FIGURE 13.3 Tensile and shear stress distributions along a fiber length. Top, actual. Bottom, idealized (plastic). (From Goettler, L.A., Handbook of Elastomers, Anil K. Bhowmick and Howard L. Stephens, Eds., Marcel Dekker, New York, 1998. With permission.)

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fiber strength is reached, or will plateau at some high level due to the eventual decay of the shear stress as the matrix shear deformation goes to zero in regions of fully developed stress state between fibers. When a substantial portion of the fibers lies at an off-axis angle in the range of 15°−60° to the stress direction, the composite is more likely to fail in a shear mode, since these stresses are then more readily loaded to the fracture point. The typical variation of Young’s modulus with fiber angle is given in Figure 13.4. In general, it has been found that low aspect ratio, nonuniform dispersion, and partial or improper fiber orientation limit the stress development in short-fiber composites.

13.4.2

Factors Affecting Reinforcement

The parameters affecting the performance of a fiber-reinforced rubber composite17 are given in Table 13.3. The degree and type of adhesion can be

Tensile modulus (MPa)

80

60

40

20

0

0

10

20

30

40

50

60

70

80

90

Fiber angle, θ FIGURE 13.4 Angle dependence of tensile modulus in short fiber rubber composites. (From Goettler, L.A., Handbook of Elastomers, Anil K. Bhowmick and Howard L. Stephens, Eds., Marcel Dekker, New York, 1998. With permission.)

TABLE 13.3 Factors Influencing Performance of Fiber-Reinforced Composites Fiber concentration Fiber aspect ratio Direction and extent of fiber orientation Degree of fiber dispersion Rubber matrix adhesion to the fiber Void content Source: From Goettler, L.A., Handbook of Elastomers, Anil K. Bhowmick and Howard L. Stephens, Eds., Marcel Dekker, New York, 1998. With permission.

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estimated quantitatively by micromechanical or spectroscopic methods. Aspect ratio has a considerable effect on composite properties, hence it is important to conserve fiber length as much as possible during rubber composite processing operations. Fiber aspect ratio must be in the range of 100− 200 for optimum effectiveness. Fiber orientation, however, has the largest effect on composite properties. During processing, the fibers tend to orient along the flow direction, causing mechanical properties to vary in different directions. Poor fiber dispersion results in a loose bundle, embracing an effectively lower aspect ratio with less reinforcing potential than a single fiber. In addition, the bundle itself may be low in strength due to poor adhesion. Both the above factors reduce the overall strength of the composite. The development of strength in a composite depends on the existence of a strong interface, which can be enhanced through the selection of an optimum bonding system. A suitable reaction or a high degree of physical compatibility with both the fiber and matrix is required. Though the presence of a covalent bond would increase the levels of adhesion tremendously, many of the bonding systems rely only upon physical forces. The extent of physical forces is quite large in the case of polar polymers and hydrophilic surfaces. The typical bonding systems tried out in the case of most cellulosic fibers are resorcinolbased adhesives such as a resorcinol−formaldehyde−latex (RFL) cord-dip formulation or a resorcinol−hexamethylene tetramine−hydrated silica (HRH) bonding system. Another version substitutes hexamethoxymethylmelamine as the methylene bridge donor. Isocyanate-based resins and organofunctional silanes or titanates act as good bonding systems for the more polar classes of fiber−rubber composites. 13.4.3

Mechanical Properties

The primary effects of short fiber reinforcement on the mechanical properties of natural rubber composites include increased modulus, increased strength with good bonding at high fiber concentrations, decreased elongation at failure, greatly improved creep resistance over particulate-filled rubber, increased hardness, and a substantial improvement in cut, tear, and puncture resistance. Hysteresis and fatigue strength are also improved.14 When fibers are aligned parallel to the stress direction, tensile strength develops a characteristic drop with increasing fiber volume content until a critical fiber level is reached. Higher reinforcement concentrations generally cause the strength to increase. The critical concentration level at which the unreinforced matrix strength is recovered varies directly with the critical fiber aspect ratio. The stiffness and modulus of short fiber composite increase with fiber concentration, though not necessarily linearly. Tear strength is highly dependent on fiber loading and is seen to increase with concentration. The mechanical properties of lignocellulosic fiber-reinforced NR composites have been extensively studied. It has been reported by Dzyura15 that the minimum amount of fibers to restrain the matrix is smaller if the matrix Copyright © 2005 by Taylor & Francis

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strength is higher. Natural rubber is a very strong matrix because of its strain-induced crystallization. Generally it has been seen that the tensile strength initially drops down to a certain amount of fiber and then increases. The minimum volume of fiber is known as the critical volume above which the fiber reinforces the matrix. The critical volume varies with the nature of fiber and matrix, fiber aspect ratio, and fiber-matrix interfacial adhesion. At low fiber concentrations, the fiber acts as a flaw in the rubber matrix and the matrix is not restrained by enough fibers, causing highly localized strains to occur in the matrix at low stress. This causes the bond between fiber and rubber to break, leaving the matrix diluted by nonreinforcing debonded fibers. As the fiber concentration increases, the stress is more evenly distributed and the strength of the composite increases. The incorporation of fiber into the rubber matrix increases the hardness of the composite, which is related to strength and toughness. The close packing of fibers in the compounds increases the density while resilience decreases. Geethamma et al.16 investigated the mechanical properties of coir fiber-reinforced natural rubber composites. It was seen that tensile strength decreases sharply with increase in fiber loading up to 30 phr, and then showed a slight increase for composites containing 40 and 60 phr fiber loading (Figure 13.5). This trend was observed in both longitudinal and transverse directions. The 20

Longitudinal Transverse

Tensile strength (MPa)

15

10

5

0

10

20

30

40

50

60

Fiber loading (phr) FIGURE 13.5 Variation of tensile strength with fiber loading. (From Geethamma, V.G. et al., Polymer, 39, 1483, 1998. With permission.)

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modulus values increased with fiber loading along with tear strength. The authors determined the extent of fiber orientation from green strength measurements. It was observed that orientation is lowest when fiber loading is small and increases with loading. Maximum orientation was observed at 30 phr loading. At extremely high loading (60 phr), the orientation became chaotic due to entanglement of fibers. The same trend of an initial decrease in tensile strength followed by an increase in the case of sized cellulose composites was also observed by O’Connor.10 The mechanical properties of sisal fiber-reinforced natural rubber composites were studied by Varghese et al.17 It was observed that the tensile strength decreased up to 17.5% volume loading and then increased. The tear strength and modulus values showed a consistent increase with loading. Table 13.4 shows the mechanical properties of sisal fiber-reinforced natural rubber composites at different fiber loading; fiber length was 10 mm. The effect of concentration on the mechanical properties of oil palmreinforced natural rubber composites was studied by Ismail et al.18 They observed the general trend of reduction in tensile and tear strength with increasing fiber concentration. The modulus and hardness of composites registered an increase with fiber loading. The reinforcement effect of bagasse in NR was examined by Nassar et al.19 A tensile strength retention of 97% was observed. Sabbagh et al.20 studied the physicomechanical properties of NR vulcanizates loaded with kenaf fibers. An increase in rheometric and mechanical properties was observed. The fiber-loaded composites also possessed good thermal stability and swelling resistance. Bhattacharya et al.21 analyzed the mechanical properties of pineapple leaf fiber-reinforced NR composite. It was observed that there was an increase in tensile strength at fiber loadings above 10 phr. Short silk fibers have been used as reinforcement for natural rubber by Setua and De.22 They found that critical loading was 7.5 phr, after which the fibers started reinforcing the matrix. TABLE 13.4 Mechanical Properties of Sisal Fiber-Reinforced Natural Rubber Composites J (gum)

Property

Orientation

Modulus for 10% elongation (MPa) Tensile strength (MPa) Elongation at break(%) Tear strength (kN/m)

L T

0.32 0.31

L T L T L T

17.3 17.41 1072 1060 37.6 37.4

Source:

K (10 phr)

L (20 phr)

M (30 phr)

N (40 phr)

1.13 0.73

3.38 1.19

3.74 1.49

6.82 2.04

12.2 11.6 792 796 37.7 37.05

8.7 6.50 565 586 42.37 41.85

7.15 5.01 445 466 47.05 44.05

8.07 3.96 120 101 59.23 44.83

From Varghese, S. et al., J. Adhesion Sci. Technol. 8, 235, 1994. With permission.

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Arumugam et al.23 also reported similar studies for jute−NR composites. An interesting study by Ismail et al.24 reported on the fatigue and hysteresis behavior of oil palm−wood flour filled natural rubber composites. It was observed that the composite with the highest loading was the most sensitive toward changes in strain energy and exhibited the highest hysteresis. Another interesting area is that of hybrid composites. The incorporation of two or more fibers within a single matrix is known as hybridization, and the resulting material is referred to as hybrid composite. The behavior of hybrid composites is a weighted sum of the individual components in which there is a more favorable balance between the inherent advantages and disadvantages. The study of mechanical properties of sisal/oil palm hybrid fiber-reinforced natural rubber composites was conducted by Jacob et al.25 Increasing the concentration of fibers resulted in reduction of tensile strength and tear strength, but increased modulus of the composites. The variation of tensile strength with fiber loading is given in Figure 13.6. Sisal and oil palm were taken in a 50/50 ratio. Tensile strength was found to decrease, reach a maximum at 50 phr, and then decrease. The treatment of both sisal and oil palm fibers resulted in an increase of tensile strength and modulus. The mechanical properties of lignocellulosic fiber-reinforced natural rubber composites have been well documented by Jacob et al.26−30

L0 P30 R45 S60 T90

8

Tensile strength (MPa)

7 6 5 4 3 2 1 0 0

10

20

30 40 Fiber loading

50

FIGURE 13.6 Variation of tensile strength with fiber loading of sisal–oil palm hybrid fiber-reinforced rubber composites. (From Jacob, M. et al., Comp. Sci. Technol., 64, 955–965, 2004. With permission.)

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−Rubber Adhesion Fiber−

The compatibility of hydrophobic polymer and hydrophilic cellulose fiber can be enhanced through the modification of polymer or fiber surface.31−33 The extent of adhesion is usually increased by the use of bonding agents and chemical modification of fibers. Chemical composition and cell structure of natural fibers are quite complicated. The microfibrils are helically wound along the fiber axis to form ultimately hollow cells. The uncoiling of these spirally oriented fibrils consumes a large amount of energy and is the predominant failure mode. As a result, pretreatment of the fibers would result in chemical and structural changes not only on the fiber surface but also in the distinct cells, which in turn influences the properties of the fibers and composites. Interfacial strength depends on the surface topology of the fiber. Although the fiber possesses hydroxyl groups on its surface, its brittle nature and low cellulose content makes it less effective reinforcement. Hence to improve adhesion between fiber and natural rubber, the fiber should be subjected to some type of pretreatment. Geethamma et al.16 have shown that the surface morphology of coir fiber is modified by sodium hydroxide treatment, as is clear from the scanning electron micrographs of untreated and NaOHtreated coir fibers (Figure 13.7). The globular protrusions present in the untreated fiber disappear, leading to formation of a large number of voids. As mentioned earlier, the common bonding systems employed are silica− phenol−formaldehyde and resorcinol−hexamethylenetetramine−silica. Silica is believed to act as the controller for resin formation and helps in developing adhesion between rubber and fiber. The importance of silica is still a matter of controversy. Studies by Murty and De34 have indicated that 5 phr silica is an essential component for enhancing the development of adhesion between fiber and rubber in the case of jute−NR composites. Varghese et al.35 indicated that bonding between sisal and NR can be enhanced by use of resorcinol and hexamethylenetetramine in the ratio 5:3.2. It was also found that adhesion imparted by this system was found to be better than that of the normal HRH system. Geethamma et al.36 also reported that silica was not essential in producing good adhesion between coir and NR. Figure 13.8 shows that silica is not needed in the bonding system for coir fiber-reinforced NR composites. Raw coir fibers are used as reinforcements in mixes L and M, but mix L exhibits higher tensile strength than mix M. This can be explained by the presence of an unnecessary component of silica in the bonding system in mix M. Similarly, the tensile strength of mix N is higher than that of mix O even though both of these mixes contain sodium carbonate-treated coir fibers. This is also due to the presence of silica in mix O. The tear strength of the mixes exhibits the same trend as the tensile strength and moduli. This is apparent in Figure 13.9. Thus, it is clear that the nature of both rubber matrix and reinforcing fiber determines whether silica is needed or not as one of the components. The adhesion of short oil palm fiber with NR was investigated37 using various bonding agents such as phenol-formaldehyde, resorcinol-formaldeCopyright © 2005 by Taylor & Francis

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FIGURE 13.7 Scanning electron micrographs of the surface of (a) raw coir fiber and (b) alkali-treated coir fiber. (From Geethamma, V.G. et al., J. Appl. Polym. Sci., 55, 583, 1995. with permission.)

hyde:silica:hexamethylenetetramine, and resorcinol formaldehyde:silica. It was observed that different bonding agents gave different cure characteristics and mechanical properties. Better mechanical properties were obtained for the bonding system RF:Sil:Hex (5:2:5). The effectiveness of bonding systems was found to follow the order: RF:Sil:Hexa (5:2:0)  RF:Sil (5:2)  PF (10 phr) Copyright © 2005 by Taylor & Francis

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9

8 7

Stress (MPa)

6 N 5 4

L

3

O M

2 1

0

152

303

455

606

758

Strain (%) FIGURE 13.8 Stress-strain curves of mixes L (untreated fiber  resorcinol  Hexa), M (untreated fiber  resorcinol  Hexa  7.5 parts silica), N (sodium corbonate-treated fiber  resorcinol  Hexa), and O (sodium carbonate-treated fiber  resorcinol  Hexa  7.5 parts silica). (From Geethamma, V.G. et al., Polymer, 39, 1483, 1998. With permission.)

The variation of cure time with different bonding agents was also analyzed. Table 13.5 shows that the maximum and minimum torque values increase with the presence of various bonding agents in composites. This is due to strong bonding at the fiber-matrix interface and consequently the composite becomes stronger, harder, and stiffer. It can be seen that compounds with treated fiber show higher torque value compared to compounds with untreated fiber due to better adhesion achieved between the fiber and the rubber matrix. The adhesion between pineapple leaf fiber and NR was found to improve considerably in the presence of hexamethylenetetramine−resorcinol− hydrated silica (HRH).21 A reduction in scorch time and increment in optimum torque values with increase in concentration of bonding agent was observed. The HRH bonding system was found to be effective for silk fiberreinforced NR composites as well.22 The modulus values of kenaf fiber-reinforced NR were found to increase in the presence of the HRH bonding system.20 O’Connor10 investigated the effect of bonding agents on the properties of cellulose fiber−NR composites. The bonding agents studied were an Copyright © 2005 by Taylor & Francis

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50

Tear strength (kNm−1)

Longitudinal

Transverse

40

30

20

10 l

J

K

Effect of chemical treatment on coir

M

L

O

N

Effect of silica in HRH system

FIGURE 13.9 Tear strength of the mixes I (untreated fiber  NR solution  TDI solution), J (sodium hydroxide-treated fiber  NR solution  TDI solution), K (sodium hydroxide-treated fiber  liquid NR solution  TDI solution), L (untreated fiber  resorcinol  Hexa), M (untreated fiber  resorcinol  Hexa  7.5 parts silica), N (sodium carbonate-treated fiber  resorcinol  Hexa), and O (sodium carbonate-treated fiber  resorcinol  Hexa  7.5 parts silica). (From Geethamma, V.G. et al., Polymer, 39, 1483, 1998. With permission.)

TABLE 13.5 Cure Characteristics of Oil Palm Natural Rubber Composites with Different Bonding Systemsa Cure Characteristics Maximum torque (Nm) Minimum torque (Nm) Cure time t90 (min) Scorch time t2 (min)

B (gum) 10 0.4 17.5 6.1

H (U)

I (T)

J (U)

K (T)

L (U)

M (T)

10.4 0.6 30.6 5

12.8 0.7 30.7 4.4

12.2 0.6 26.9 10.1

13.4 0.7 23.9 9.3

13.6 0.8 21.0 5.5

14.8 0.9 21.4 5.3

a

B, gum sample; H, phenol–formaldehyde untreated; I, phenol–formaldehyde treated; J, resorcinol–formaldehyde–silica (5:2) untreated; K, resorcinol—formaldehyde–silica (5:2) treated; L, resorcinol–formaldehyde–silica–hexa (5:2.5) untreated; M, resorcinol–formaldehyde–silica–hexa (5:2.5) treated. Source: From Ismail, H. et al., Polymer, 38, 4059, 1997. With permission.

HRH system, a resorcinol hydrated silica (RH) system that omitted silica, and a resin bonding agent. Better composite properties were obtained with the RH system (Table 13.6). Copyright © 2005 by Taylor & Francis

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TABLE 13.6 Effect of Bonding Agents on Properties of Short Cellulose Fiber-Reinforced Natural Rubber Compositesa

Fiber

Cellulose

Fiber Bonding (vol%) Agent 17

0

17.1

Res

17

RH

17.6

HRH

Young’s Modulus Orientation (MPa) L T L T L T L T

64.8 16.1 46 10.1 97.5 29.1 92.1 22.7

Tensile Strength Elongation at (MPa) Break (%) 7.5 6.7 9.1 6.9 19 10.1 17.2 8.5

230 277 233 207 13 73 13 50

Tear Strength (KN/m) 41.5 30.6 41.3 31.3 68.9 31.9 57.3 32.9

a

Res, resorcinol; RH, resorcinol–hydrated silica; HRH, Hexamethylenetetramine–resorcinol hydrated silica. Source: From O’ Connor, J.E., Rubber Chem. Technol., 50, 945, 1977. With permission.

Figure 13.10 shows enhancement in fiber-matrix adhesion of coir fiberreinforced natural rubber composites with bonding agent. In the unbonded composite there are a large number of holes that are produced as a result of the pullout of fibers from the rubber matrix whereas broken fiber ends can be seen instead of the pullout phenomenon. In the case of sisal−oil palm hybrid fiber-reinforced composites, Jacob et al.26 found that the presence of resorcinol and hexamethylene tetramine increased the mechanical properties of alkali-treated composites. Prasantha Kumar et al.38 showed the effect of fiber surface modifications, NaOH treatment, acetylation, and benzoylation on the interfacial adhesion of sisal fiber and SBR matrix, and are given in Scheme 13.1. The schematic models of the interfaces in acetylated and benzoylated sisal fiber-reinforced sisal fiber composites are given in Scheme 13.2.35

13.4.5

Dynamic Properties

Rubber products generally undergo dynamic stressing during service. Therefore their behavior in dynamic load application is highly important. In dynamic applications, bonding between the fiber and rubber plays an important role. The influence of fiber-matrix adhesion on the viscoelastic properties of short jute fiber-reinforced rubber composites has been studied by Murthy and De.34 Varghese et al.39 studied the effects of fiber treatment, fiber orientation, fiber loading, bonding agent, and temperature on dynamic mechanical properties of sisal fiber-reinforced rubber composites. Prasantha Kumar et al.38 analyzed the dynamic mechanical behavior of sisal fiber-reinforced styrene butadiene rubber. They found that the storage modulus of the composites increases with increasing fiber volume fraction, Copyright © 2005 by Taylor & Francis

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FIGURE 13.10 Scanning electron micrographs of coir fiber–natural rubber composites (a) without bonding agent, (b) with bonding agent. (From Geethamma, V.G. et al., J. Appl. Polym. Sci., 55, 583, 1995. With permission.)

fiber surface modifications, and bonding agent. Figure 13.11 shows the variation of storage modulus (E’) and loss modulus (E”) of sisal fiber-reinforced SBR composites in the presence of bonding agents and surface modifications. The increasing use of short fiber composites in static and dynamic applications led to the importance of stress relaxation measurements. The stress relaxation behavior of short jute fiber-reinforced nitrile rubber composites Copyright © 2005 by Taylor & Francis

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CH2OH O H H OH H

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O + NaOH H

OH

CH2 O Na+ O H H OH H

O

+ H2O

OH

(a)

H

CH2 O Na+ O H H OH H

O + (H3C-CO)2O

CH3COOH O

OH

H

H

OH H

H

O

OH

CH2 O C CH3 O

(b)

H

CH2 O Na+ O H H OH H

O

O + Cl C

OH

O

OH

H

H

OH H

H

O CH2 O C O

(c) SCHEME 13.1 Effect of surface modifications on interfacial adhesion of sisal fiber and SBR matrix. (a) NaOH treatment, (b) acetylation, (c) benzoylation.

was studied in detail by Bhagawan et al.40 Flink and Stenberg41 studied the stress relaxation behavior of short cellulose fiber-reinforced natural rubber composites. They reported that the stress relaxation measurements give an idea about the level of adhesion in fiber−rubber composites. The stress relaxation behavior of acetylated short sisal fiber-reinforced natural rubber composites with special reference to the effects of strain level, fiber volume fraction, bonding agent, and temperature was analyzed by Varghese.42 13.4.6

Rheological and Aging Studies

A number of rheological studies have been carried out on specific types of fiberreinforced rubber, alone or in conjunction with more general investigations of mechanical behavior.43−45 Rubber reinforced with short fibers generally shows a highly pseudoplastic viscosity, obeying the power law relationship over a wide range shear rates. The effect of fiber reinforcement is more exaggerated as Copyright © 2005 by Taylor & Francis

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Cellulose H O

CH3 CO

O H

CH3 CO

O

CH3 CO

(CH2-CH)-CH2-CH-CH-CH2)

H O

CH3 CO

O

CH3 CO

O

CH3 CO

(CH2-CH)-CH2-CH-CH-CH2)

(CH2-CH)-CH2-CH

CH-CH2)

(CH2-CH)-CH2-CH

CH-CH2)

(a)

Cellulose H O

O H CO-Cl

Cl-CC

(CH2-CH)-CH2-CH- CH-CH2)

H O Cl-CO

(CH2-CH)-CH2-CH-CH-CH2)

(b)

SCHEME 13.2 Schematic model of interface after (a) acetylation treatment (b) benzoylation treatment.

the shear rate is reduced. Die swell has been found to reduce to nil at high fiber volume fractions. The rheological and extrusion behaviors of short fiber-reinforced rubber compounds have been reported by Goettler et al.45 The melt flow behavior of natural rubber composites containing untreated, acetylated, and gamma ray irradiated coir fibers was studied by Geethamma et al.46 It was found that the pseudoplasticity of the melt increased with fiber loading. Incorporation of fibers decreased the extrudate deformation, and this improvement was more prominent at higher fiber loading (Figure 13.12). Copyright © 2005 by Taylor & Francis

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108 Frequency - 10 Hz Untreated 18% NaOH Benzoylated

10% NaCl Acetylated Bonding agt

Storage modulus E ′(Pa)

107.6

107.2

106.8

20

48

(a)

76

104

Temperature (°C)

Loss modulus, E ′ (Pa)

106.9 Untreated

106.7

Acetylation

Frequency -10Hz 10% NaCl Benzoylation

18% NaOH Bonding agt

106.5 106.3 106.1 105.9 105.7 105.5

0

5

(b)

10

15

Volume of fibers (%)

FIGURE 13.11 (a) Effect of temperature on E’ as a function on chemical treatment at 10 Hz for sisal fiber SBR composites. (b) Effect of temperature on E” as a function on chemical treatment at 10 Hz for sisal fiber SBR composites. (From Prasantha Kumar, R., Ph.D. thesis, Mahatma Gandhi University, Kottayam, Kerala, India, 1992. With permission.)

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6

0 5 10 20 30

FIGURE 13.12 Photomicrographs of extrudate fibers of different loading at two shear rates, 3.67 and 1244.66 sec–1 (From Geethamma, V.G. et al., Int. J. Polym. Mater., 32, 147, 1996. With permission.)

The die swell of composites containing coir fibers was lower than that of gum compound and was negligible for composites containing 20 and 30 phr of fibers. Similar studies were carried out by Varghese et al.47 in the case of sisal fiber-reinforced natural rubber composites. Prasantha et al.48 studied the rheological behavior of short fiber-reinforced SBR composites and found that the short fibers present in the matrix prevent the shape distortion of the extrudates, and the increase in fiber content decreased the shape distortion. Rubbers like NR, which retain their double bonds in the vulcanized structure, are sensitive to heat, light, and oxygen. Unless protected with antioxidants, NR ages by an autocatalytic process accompanied by an increase in oxygen content. The result of aging may be either softening or embrittlement, suggesting a competing process of degradation and cross-linking of polymer chains. NR is far more sensitive than other rubbers to oxygen and ozone attack. The resistance of natural rubber to aging and ozone is poor due to the presence of the reactive double bond in the main chain. This is due to the action of degrading agents such as oxygen, ozone, heat, and high-energy radiation. The effects of these degrading agents depend mainly upon the chemical structure of the polymer chain. The thermooxidative degradation of rubber has been extensively discussed in the literature.49 Mechanical properties such as tensile strength, elongation, modulus, and hardness are decreased as a result of chain scission, while cross-linking has the opposite effect on these properties. The action of different degrading agents on the properties of sisal−NR composites was investigated by Varghese et al.50 The authors reported that at low irradiation, the retention in tensile strength was almost constant beyond 6% volume loading of fiber. On prolonged irradiation, degradation of polymer chains was the main reaction taking place at low fiber loading, whereas at higher fiber loading retention of tensile properties was higher (Figure 13.13). Unsaturated elastomers, especially those containing activated double bonds in the main chain, are severely attacked by ozone, resulting in deep cracks in a direction perpendicular to the applied stress. Protection against ozone attack can be achieved by blending with any saturated elastomers or by mixing with antiozonants. In the case of composites, the fibers incorporated in the mixes prevent crack initiation and also hinder crack propagation. Figure 13.14 is an optical photograph of NR−sisal fiber composite without any bonding agent after exposure to ozonized air Copyright © 2005 by Taylor & Francis

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Percentage retention of tensile strength

130

110

90

70

50

30

0

6

12

18

24

30

Volume loading of fiber (%) FIGURE 13.13 Percentage retention of tensile strength in the transverse direction with fiber loading at different radiation doses. (From Varghese, S. et al., Polym. Degradation Stability, 44, 55–61, 1994. With permission.)

FIGURE 13.14 Photographs of natural rubber–sisal fiber composites with bonding agent after exposure to ozone for 40 h. J, gum NR; P (10 phr); Q (20 phr); and S (40 phr) NR–sisal composites. (From Varghese, S. et al., Polym. Degradation Stability, 44, 55–61, 1994. With permission.)

for 40 h. Even in the absence of bonding agent, all the composites are resistant to ozone attack except the gum sample. Since the fibers are chemically treated, there is sufficient interfacial bonding between fiber and rubber. Copyright © 2005 by Taylor & Francis

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Processing

13.4.7.1 Milling The first step in milling is to oven-dry the whole fiber to reduce moisture to below 0.1%. As discussed earlier, the fibers can also be modified by chemical treatments to make them more compatible with the rubber matrix.51 The second step is the mixing of the treated fiber into the rubber formulation during the rubber compounding operation in an intensive (Banbury) or two-roll mill. The product from this step is a homogenous rubber compound reinforced with fiber. The compound is heated on a mill roll into manageable sheets for handling. The final process step is the compression molding at elevated temperature and pressure to cure the rubber. During mixing in a two-roll mill, high shear forces develop, leading to fiber breakage. The breakage pattern can be studied by means of a fiber length distribution curve. It has been reported that the breakage is more common for synthetic fibers than for natural fibers. 13.4.7.2 Calendering Calendering usually converts the rubber mix into a uniform continuous sheet. The product of this step is a roll of thin flexible rubber that can be trimmed with scissors or knives to fit into any desired mold. The addition of fibers renders a rubber compound less elastomeric and extensible and therefore the technique of calendering is quite difficult. 13.4.7.3 Injection Molding The molding of short fiber-reinforced rubber composites follows closely the technology of plastic composites.52,53 The natural fiber-reinforced rubbers are less abrasive to machine and tool surfaces, causing less wear common for synthetic fiber-plastic resins. The highly automated injection molding requires fibers that are shorter and less concentrated than those in compression molding. An advantage of injection molding is its less labor-intensive operation under well-controlled sequencing for good reproducibility at low cost. The typical parts that are made from short fiber-reinforced rubber composites include diaphragms, gaskets, and certain flexible automotive parts. 13.4.7.4 Extrusion Rubber extrusion is assisted by the lower rigidity of the organic fiber reinforcement commonly used.54−56 In this area a number of processability criteria take on added importance with short fiber-reinforced rubber. These are the direction of fiber orientation, surface appearance, and flow balancing in the die to minimize tearing and for the control of downstream postdie sizing operations. Elastomeric matrices possess the advantage of green strength that is essential to allow a free-surface forming operation such as extrusion. In the case of fiber-reinforced plastics, extrusion is impossible Copyright © 2005 by Taylor & Francis

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due to the severe degradation of extrudate as it emerges from the die. A unique application of this technology for extruding short fiber-reinforced hose allows the hose to be curved into a shape as it emerges from the expanding die by mechanically adjusting the die geometry to effect local variations in the flow uniformity within the die itself. Another attractive feature is that the fiber-reinforced green hose retains its shape during handling and the bend areas are nearly as strong as the straight sections. At present hose reinforced with oriented natural short-length cellulose fiber is available commercially in Europe. Natural fiber rubber composites must be processed and vulcanized to reach property development in such a way that fiber orientation patterns yielding optimal mechanical anisotropy are generated and preserved. 13.4.8

Applications

Short fibers have the potential for reinforcing low-performance tires. In automotive and truck tires they find application in better abrasion resistance for the chafer strip and in improved cut resistance to treads, especially for trucks and OTR vehicles.52 As short fibers have higher green strength and cut, tear, and puncture resistance, they can be used for sheeting. The fibers also provide higher green strength and cut, tear, and puncture resistance. Some applications for such reinforced sheeting are in roofing membranes. Short fibers can be utilized as the sole reinforcement for a moderate-performance hose or as an auxiliary reinforcement with cord constructions. They can provide stiffening to soft inner tubes for the application of metal braids and can extend hose life by bridging the stresses across weakened filaments. Other uses are as belts, diaphragms, and gaskets. Some other applications are roofing, hose, dock and ship fenders, and general uses such as belts, tires, and other industrial articles. Short fibers can reinforce and stiffen rubber in fenders and other impact applications in accordance with simple design equations.

13.5

Conclusion

A brief outlook on the different kinds of lignocellulosic fibers is given. The theory and factors of reinforcement in natural fiber-reinforced rubber composites are discussed. The different kinds of rubber matrices and their properties are also discussed. Several studies concerning natural fiber-reinforced rubber composites are also looked into. The mechanical, rheological, viscoelastic and ageing characteristics of fiber-reinforced rubber composites are explained. The different processing techniques used for the fabrication of rubber composites are accounted for. Finally the applications of rubber composites are enumerated. Copyright © 2005 by Taylor & Francis

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References 1. Bledzki, A.K. and Gassan, J., Prog. Polym. Sci., 24, 221, 1999. 2. Nevell, T.P. and Zeronian, S.H., Cellulose Chemistry and its Applications, Wiley, New York, 1985. 3. Harris, M., Handbook of Textile Fibers, Harris Research Laboratories, Inc., Washington, DC, 1954. 4. Zylinski, T., Fiber Science, Office of Technical Services, U.S. Dept. of Commerce, Washington, DC, 1964. 5. Hamed, P. and Coran, A.Y., Additives Plast., 1, 29–50,1978. 6. Sreekala, M.S., Kumaran, M.G., and Thomas S.J., Appl. Polym. Sci., 66, 821–835, 1997. 7. Ver Strate, G. and Lohse, D.J., in Science and Technology of Rubber, Mark, J.E., Erman, B., and Eirich, F.R., Academic Press, San Diego, 1994, chap. 4. pp. 95–188. 8. Gent, A.N., in Science and Technology of Rubber Mark, J.E., Erman, B., and Eirich, F.R., Eds., Academic Press, San Diego, 1994, chap. 10. pp. 472–512. 9. Hamid, S.H., Maahhah, A.G., and Usmani, A.M., Polym. Plast. Technol. Eng., 21, 173, 1983. 10. O’ Connor, J.E., Rubber Chem. Technol., 50, 945, 1977. 11. Evans, C.W., Ed., Developments in Rubber and Rubber Composites: Part 2, Applied Science Publisher, NY, p.125, 1983. 12. Guth, E., Gimha, R., and Gold, O., Koll. Zh., 74, 266, 1936. 13. Anthoine, G., Arnold, R., Boustany, K., and Campbell, J., Eur. Rubber J., July, 28, 1975. 14. Goettler, L.A., Short Fiber Reinf. Rubber Compo. 15. Dzyura, E.A., Int. J. Polym. Mater., 8, 165, 1980. 16. Geethamma, V.G., Joseph, R., and Thomas, S., J. Appl. Polym. Sci. 55, 583, 1995. 17. Varghese, S., Kuriakose, B., Thomas, S., and Koshy, A.T., J. Adhesion Sci. Technol. 8, 235,248, 1994. 18. Ismail, H., Rosnah, N., and Ishiaku, U.S., Polym. Int., 43, 223,1997. 19. Nassar, M.M., Ashour, E.A., and Washid, S.S., J. Appl. Polym. Sci. 61, 885, 1996. 20. El Sabbagh, S.H., El Hariri, D.M., and Abd. El Ghaffar, M.A., Proceedings of the 3rd International Symposium on Natural Polymers and Composites: ISNa Pol, May 14–17, 2000, pp. 469–483. 21. Bhattacharya, T.B., Biswas, A.K., Chaterjee, J., and Pramanick, D., Plast. Rubber Process. Appl., 6, 119,1986. 22. Setua, D.K. and De, S.K., Rubber Chem. Technol., 56, 808, 1983. 23. Arumugam, N., Tamareselvy, K., Venkata Rao, K., and Rajalingam P., J. Appl. Polym. Sci., 37, 2645, 1989. 24. Ismail, H., Jaffri, R.M., and Rozman, H.D., Polym. Int., 49, 618, 2000. 25. Jacob, M., Thomas, S., and Varughese, K.T., Proceedings of the USM-JIRCAS Joint International Symposium on Lignocellulose – Material of the Millenium: Technology and Application, Mar., 20–22, 2001, USM, Malaysia. 26. Jacob, M., Thomas, S., and Varughese, K.T., Mechanical properties of sisal/oil palm hybrid fiber reinforced natural rubber composites, Compos. Sci. Technol., 64, 955–965 2004. 27. Jacob, M., Thomas, S., and Varughese, K.T., Sisal/oil palm hybrid fiber reinforced natural rubber composites: Tensile and swelling characteristics, J. Appl. Polymer Sci., 93, 5, 2305–2312, 2004.

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28. Jacob, M., Thomas, S., and Varughese, K.T., Dynamic mechanical analysis of sisal/oil palm hybrid fiber reinforced natural rubber composites, Composites Part A (in press). 29. Jacob, M., Thomas, S., and Varughese, K.T., Mechanical properties of hybrid biofiber reinforced natural rubber composites: A Statistical Approach, Compos. Sci. Technol. (in press). 30. Jacob, M., Seena Joseph, Laly A. Pothan, and Thomas, S., Advances in characterization of fiber surfaces and interfaces in lignocellulosic fiber reinforced composites: A Study, Composite Interfaces (In press). 31. Frollini, E., Leao, A.L., and Mattosi, L.H.C. Eds., Natural Polymers and Agrofibers Based Composites, 2000. 32. Prasantha Kumar, R. and Thomas, S. J., Adhesion Sci. and Technol., 15, 633, 2001. 33. Prasantha Kumar, R. and Thomas, S., Bull. Mater. Sci., 18, 1021,1995. 34. Murty, V.M. and De, S.K., J. Appl. Polym. Sci. 27, 4611, 1982. 35. Varghese, S. Kuriakose, B., Thomas, S., and Koshy, A.T., Indian J. Nat. Rubber Res., 4, 55–67, 1991. 36. Geethamma, V.G., Thomas Mathew, K., Lakshminarayanan, R., and Thomas, S., Polymer, 39, 6–7, 1998. 37. Ismail, H., Rosnah, N., and Rozman, H.D., Polymer, 38, 16, 1997. 38. Prasantha Kumar, R., Ph.D. thesis, Mahatma Gandhi University, Kottayam, Kerala, India, 1992. 39. Varghese, S., Kuriakose, B., Thomas, S., and Koshy, A.T., Ind. J. Nat. Rubber Res., 5, 1–2, 1992. 40. Bhagawan, S.S., Tripathy, D.K., and De, S.K., J. Appl. Polym. Sci., 33, 1623, 1987. 41. Flink, P. and Stenberg, B., Br. Polym. J., 22, 193, 1990. 42. Siby Varghese, Ph.D. thesis, Mahatma Gandhi University, Kottayam, Kerala, India, 1992. 43. Setua, D.K., Int. J. Polym. Mater., 11, 67, 1985. 44. Murty, V.M., Gupta, B.R., and De, S.K., Plast. Rubber Process. Appl., 5, 307, 1985. 45. Goettler, L.A., Leib R.I., and Lambright, A.J., Rubber Chem. Technol., 52, 838, 1979. 46. Geethammma V.G., Ramamurthy, Janardhan R., and Thomas S., Int. J. Polym. Mater., 32, 147, 1996. 47. Varghese S., Kuriakose B., Thomas S., Premalatha C.K., and Koshy A.T., PlastRubber Compos. Proc. Applic., 20, 93, 1993. 48. Prasantha Kumar, R., Manikandan Nair, K.C., Thomas, S., Schit, C., and Ramamurthy, K., Comp. Sci.Technol., 60, 1737, 2000. 49. Razumovskii, S.D. and Zaikov, G.E., in Developments in Polymer Stabilisation - 6 Scott, G., Ed., Applied Science Publishers, London, 1982. 50. Varghese, S., Kuriakose, B., and Thomas, S., Polym. Degradation and Stability, 44, 55–61, 1994. 51. Goettler, L.A., The 3rd Annual Meeting, of the Polymer Processing Society. Stuttgart, Germany, 1987. 52. Goettler, L.A., in The Role of the Polymeric Matrix in the Processing and Structural Properties of Composite Materials, Plenum, New York, p. 289. 1983. 53. Goettler, L.A., Polym. Compos., 5, 60, 1984. 54. Goettler, L.A., Polym. Compos., 4, 249, 1983. 55. Goettler, L.A., Sezna, J.A., and Di Mauro, P.J., Rubber World, 187, 1, 1982. 56. Walker L.A. and Harber J.B., Kautsch Gummi Kunstst., 38, 494, 1985.

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14 Straw-Based Biomass and Biocomposites

Xiaoqun Mo, Donghai Wang, Xiuzhi S. Sun

CONTENTS 14.1 Introduction 14.2 Structure and Chemical Composition of Straw 14.2.1 Anatomy and Morphology 14.2.2 Physical and Chemical Properties 14.3 Straw-Based Particleboards and Fiberboards 14.3.1 Types of Resins 14.3.1.1 Synthetic Resins 14.3.1.1.1 Methylene Diphenyl Diisocyanate and Polymeric Methylene Diphenyl Diisocyanate Resins 14.3.1.1.2 UF Resins and PF Resins 14.3.1.2 Protein-Based Resins 14.3.1.3 Other Resins 14.3.2 Pretreatment of Straw for Straw-Based Composites 14.3.2.1 Mechanical Treatments 14.3.2.2 Chemical and Enzyme Treatments 14.3.3 Straw–Plastic Composites 14.4 Current Status and Industrial Applications 14.5 Conclusion References

ABSTRACT The surplus of agricultural residues, as well as the limited resources of wood and petroleum, have driven the utilization of straw as an alternative or supplement for wood or plastic composites. Beside traditional uses, there is still an abundance of straw leftovers for industrial uses such as particleboard, fiberboard, and filler for thermoplastic composites. The objective of this chapter is to review the morphology, chemical composition, and availability of wheat straw and rice straw, discuss resins and technologies

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that have been explored in manufacturing straw panels, and evaluate straw as filler for plastic composites. Straw panels have been successfully manufactured using synthetic methylene diphenyl diisocyanate resin with comparable mechanical and water-resistant properties to wood-methylene diphenyl diisocyanate based panels. The low-cost urea formaldehyde resins had poor bondability with straw, but can be improved by pretreatment of straw or by incorporating a cross-linking agent into the UF resin. Modified soy protein-based resin has also shown great potential in manufacturing straw panels with comparable properties to UF resin.

14.1

Introduction

Large quantities of agricultural residues are produced annually throughout the world. For example, the estimated production of wheat straw and rice straw in 2002 was 1000 million tons and 510 million tons, respectively. The straw was traditionally used for low-value purposes, such as livestock feeding, mulch, and bedding materials for animals.1 Straw resources are underutilized in many countries. Current strategies adopted to handle surplus straw include burning or incorporation or removal from the field. Dumping of straw in landfills or burning of straw has created serious environmental pollution. Legislation has been enacted to prohibit straw burning in North America and Europe. There is an urgent need to find a profitable outlet for surplus underutilized straw. Global deforestion and the fact that petroleum resources are finite have driven the search for alternative resources for petrochemicals. Cereal straw represents a potentially valuable source of fiber and has the potential to alleviate the shortage of wood fiber and petroleum resources. It can be used as a direct substitute for wood composites, and also can be used to make plastics composites, which are currently made from petrochemicals. The successful production of straw-based composites would add value to straw and have economic and environmental benefits. More importantly, an increasing usage of straw in composites would conserve existing wood and petroleum resources. A composite is any combination of two or more resources held together by some type of mastic or matrix. The mastic or matrix can range from a simple physical entanglement of fibers to more complicated systems based on thermosetting or thermoplastic polymers. During the past 20 years, great progress has been made in exploring straw as a potential fiber for composites, such as particleboards and fiberboards,2–6 and as a filler for composites derived from polyester and polypropylene polymers.7 The straw-based composites take advantage of the unique properties of straw as well as other Copyright © 2005 by Taylor & Francis

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resources such as plastics, metals, and glass that can be combined to meet end-use specifications. The new applications and new markets for strawbased composites could be explored in such areas as packaging, furniture, housing, and automotive products.

14.2 14.2.1

Structure and Chemical Composition of Straw Anatomy and Morphology

The length of straw varies greatly between and within species. Wheat straw is commonly ~50–100 cm long, and rice straw varies widely in length, from 30 to 500 cm, depending on cultivation methods. A typical whole cereal straw consists of the true stem made up of a variable number of internodes, separated by the nodes, which are the points at which leaves are attached (Figure 14.1). In oats and rice, the top section of the straw is branched and

Rachis

Node Internode

Leaf blade

Leaf sheath Disintegrated leaf

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FIGURE 14.1 Anatomical fractions of wheat straw. (From Theander, O. and Åman, P., in Straw and Other Fibrous By-products as Feed, Sundstφl, F. and Owen, E., Eds., Elsevier, Amsterdam, 1984, chap. 4. With permission.)

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consists of the threshed-out remains of the grain-bearing panicle. In wheat, the top section consists of a rachis composed of a number of nodes with very short internodes to which some of the chaff or awn normally adheres.8 The structure of the internodes is essentially similar in appearance for wheat and rice straw (Figure 14.2). The predominant, tubular part of the straw is made up of an epidermis, an outer ring of numerous small vascular bundles set in mainly sclerenchymatous tissue, and an inner ring of large vascular bundles set in mainly parenchymatous tissue, which is a soft, cellular material composed mainly of cellulose. The central lumens are more or less filled with pith. The cellulose-rich epidermis, with a concentration of silica on the surface, forms a hard, rigid outer layer, which protects the living cells within and stiffens the stems in conjunction with the lignin material. The lignin material forms a thin layer around the parenchyma, and although it contains vascular bundles, its main function is to stiffen the stem structure. The vascular bundles transport water and nutrients along the stem.

14.2.2

Physical and Chemical Properties

Generally, the wheat straw fiber has a length of 1.5 mm and diameter of 15 µ m, producing an aspect ratio of 100, and the rice straw fiber has a length in the

Large vascular bundles with an outer ring of lignified tissue Small vascular bundles

Central lumen Pith (unlignified) Lignified parenchyma Densely lignified sclerenchyma FIGURE 14.2 Transverse sections of the internodes of wheat straw. (From Theander, O. and Åman, P., in Straw and Other Fibrous By-products as Feed, Sundstφl, F. and Owen, E., Eds., Elsevier, Amsterdam, 1984, chap. 4. With permission.)

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range of 0.65–3.48 mm and diameter of 5–14 µ m, with a long aspect ratio of 170.9 Wheat or rice straw contains cellulose, hemicellulose, lignin, silica, and proteins (Table 14.1).10,11 Rice straw usually contains higher silica content and lower lignin content than wheat straw. Cellulose and hemicellulose are the predominant polysaccharides. A small amount of polysaccharides that contain mannose, galactose, and probably pectin components are also present. Cellulose occurs naturally as a large crystalline form and is composed of partially aligned or oriented linear polymer chains, consisting of up to 10,000 β-1,4-linked glucopyranosyl units. Cellulose chains are compacted packed aggregates that are held together by hydrogen bonds forming a three-dimensional structure, which imparts mechanical strength to cellulose and contributes to its resistance toward biological degradation and acid hydrolysis. Milling, steaming, or treating with a swelling agent, such as alkaline, can make the cellulose less crystalline and/or less hindered by associated components, such as lignin or silica.12 Hemicellulose is mainly composed of β-1,4-linked D-xylopyranoyl units with side chains of various lengths containing L-arabinose, D-glucuronic acid, or its 4-O-methyl ether, D-galactose, and possibly D-glucose.13 Apart from the structural polysaccharides, lignin is a main component of straw. Lignin is a family of branched polymers, made up of phenylpropane units. Lignin is encrusted in the cell wall and seems to be particularly associated with hemicellulose polysaccharides, to which it also forms covalent bonds. This association could be destroyed by steaming or alkaline treatment. Hydrophobic layers are attached to the cell walls and act as diffusion barriers. Cuticle acts as a cover for aerial organs and consists of a polymer cutin that is embedded in waxes. Cutin is an amorphous polymer primarily built up of a C16 and C18 family of acid. The waxes associated with cutin are usually a complex mixture of several classes of aliphatic components, as hydrocarbons, esters, free alcohols, and acids.14 Straw is a cellulose fiber-reinforced composite material consisting of microfibrils in an amorphous matrix of hemicellulose and lignin. The microfibrils are very rigid and quite stable, imparting high tensile strength. The polymers lignin and hemicellulose are responsible for most of the physical and chemical properties such as biodegradability, flammability, sensitivity to moisture, and thermoplasticity. This composition is also responsible TABLE 14.1 Chemical Composition of Straw (Percent of Dry Matter) Straw

Cellulose

Hemicellulose

Lignin

Silica

Proteina

Wheat Rice

39 33

36 26

10 7

6 13

3.6 4.2

a

Joint United States–Canadian Tables of Feed Composition, National Research Council, Washington, D.C. (1964). Source: Jackson, M.G., Anim. Feed Sci. Technol., 2, 105, 1977. With permission.

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for good heat, sound and electrical insulating properties, a high coefficient of friction, and combustibility.15 Also, the high silica content tends to make them naturally fire resistant.

14.3

Straw-Based Particleboards and Fiberboards

Wood fibers and chips have been the traditional sources for lignocellulosic material. Using straw as raw materials for panel products has received considerable attention in recent years. This interest has been driven by a number of factors including a limited supply of wood fiber, environmental issues, and government policies.6 Agricultural lignocellulosic fibers such as rice and wheat straws could easily be crushed to chips or particles, which are similar to wood particle or fiber. Straw contains the same basic chemical components as wood, but varies in composition. Straw usually has lower cellulose content and higher hemicellulose and lignin content than coniferous wood fiber.16 The ash content of straw was found to be extremely high (6.4%) when compared to industrial wood particles (0.54%).17 The difference in chemical properties between straw and wood has posed a challenge to production of straw-based panel using existing manufacturing technology for wood panels. Great efforts have been made to develop straw-based panels and study the effects of various treatments on panel performance.

14.3.1

Types of Resins

14.3.1.1 14.3.1.1.1

Synthetic Resins Methylene Diphenyl Diisocyanate and Polymeric Methylene Diphenyl Diisocyanate Resins Methylene diphenyl diisocyanate (MDI) and polymeric methylene diphenyl diisocyanate (PMDI) resins have been used in straw panels.2–4,6,17,18 The quality of straw particleboards bonded with 3% MDI resin meets the standard set by the American National Standards Institute (ANSI).18 Grigoriou17 found that bending strength, water resistance, and dimensional stability of straw–wood composites bonded with PMDI increased with straw level (Table 14.2). The panels made of 50, 75, and 100% straw complied with the European standard (EN) 300 for heavy-duty load-bearing oriented strand boards (OSB) in humid conditions. The straw OSB have some advantages over traditional OSB panels, such as uniform bending strength in all plane directions and superior surface smoothness. The higher adhesion strength of MDI is probably due to its ability to penetrate more deeply into the wax and silicate film of straw.19 MDI can easily react with active hydrogen-containing compounds, such as plant materials, to form urethane bonds. Water also plays an important role in the gluing

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TABLE 14.2 Properties of Straw Panel as Affected by Binding Resins Properties Resins

a

629 666 656 650 659 667 647 698 693 689 400 600 — 700 700 700

MOR (MPa)

MOEa (MPa)

IBa (MPa)

24.4 22.1 29.3 40.4 18.9 16.9 13.8 26.4 28.5 31.0 0.86 4.8 0.24 9.34 8.04 9.60

3730 — — — — — — — — — — — — 1859 1851 1601

0.60 1.61 1.07 0.85 0.99 0.41 0.16 0.66 0.72 0.97 — — — 0.19 0.22 0.35

MOR: modulus of rupture; MOE: modulus of elasticity; IB: internal bond; TS: thickness swell.

TSa (%) 7.6 15 9.7 5.7 23.9 23.3 27.5 11.5 9.6 8.7 — — — 72 156 115

Reference Wasylciew, 199818 Grigoriou, 200017 Grigoriou, 200017 Grigoriou, 200017 Grigoriou, 200017 Grigoriou, 200017 Grigoriou, 200017 Grigoriou, 200017 Grigoriou, 200017 Grigoriou, 200017 Yang et al., 200321 Yang et al., 200321 Yang et al., 200321 Mo et al., 200333 Mo et al., 200333 Mo et al., 200333

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Wheat 3% MDI Wood 8% MDI Wood/straw (50/50) 8% PMDI Straw 8% PMDI Wood 8% UF Wood/straw (75/25) 10% UF Wood/straw (50/50) 10% UF Wood/straw (50/50) 10% UF:MDI (8:2) Wood/straw (50/50) 10% UF:MDI (7:3) Wood/straw (50/50) 10% UF:MDI (5:5) Wood/rice (70/30) 10% UF Wood/rice (70/30) 10% UF Insulation board — Taylor & Francis Copyright © 2005 by Bleached straw 8% UF Bleached straw 15% SF Bleached straw 10% SPI

Density (kg/m )

a

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3

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process by reacting with the isocyanate to form a urea derivative, which, after reacting with an additional isocyanate, may also form a cross-linked interpenetrating network that contributes to the panel’s mechanical strength and dimensional stability.20 Moisture content of 12–14% would be favorable for MDI reaction. MDI is an excellent binder that imparts superior properties to panels. Panels bonded with MDI are lightweight, biodegradable, free from formaldehyde emission, and have good water resistance.20 However, MDI is three to four times more expensive than phenol formaldehyde (PF) and urea formaldehyde (UF) resins, and the price of PMDI is about 10 times that of UF resin. This considerable increase in production costs has created a large price gap with wood particleboards, which use the less expensive UF resin.4,17 In addition to the economic disadvantages, conventional strawboard technology employing MDI binders has shown technical disadvantages during production. Specifically there are additional costs for release agents needed to prevent panels from sticking to press plates, and a lack of mat adhesiveness. 14.3.1.1.2 UF Resins and PF Resins Panel boards bonded with UF resins are mainly for interior applications, such as for furniture and industrial core boards, cabinets, wall paneling, and flooring. Panels for exterior applications, such as wall sheathing, siding, roof sheathing, and fence board, are commonly made with the more expensive PF resins. Agricultural fiber residues have potential for use as industrial composite panels. Several researchers have succeeded in developing straw panels using UF resins.21,22. Rice straw–wood particleboards with UF resins were manufactured as insulation boards using existing technology for wood-based panels.21 Composite boards with a density of 800 kg/cm3 containing up to 20% straw bonded with UF resins had similar bending strength to boards made from wood only. The composite boards with density of 400 and 600 kg/cm3 containing up to 30% rice straw showed significantly higher strength than the standard commercial insulation boards (Table 14.2). Those low- to mediumdensity composite boards showed higher sound absorption coefficients in the medium- to high-frequency range than commercial particleboards, fiberboards, and plywoods, and they had greater bending strength than insulation boards. The suggested applications for the composites were roof and wall sheathing, subflooring, interior surfaces for walls and ceiling, as bases for plaster, and as insulation strips for foundation walls and slab floors.21 These composite boards can partially or completely replace wood-based insulation materials. The potential of manufacturing insulation boards from the rice straw to completely substitute wood was examined by Sampathrajan et al.22 The rice strawboards had a noise reduction capacity of 0.54, which was comparable to commercial wood-based boards. The rice strawboards also showed a

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uniform transmission loss of 26 dB in the frequency range of 250–2000 Hz. The attenuation factor of the boards was about 3700 dB m
1. Those boards could serve as good echo-dissipating medium. Low-density (0.19 g/cm3) rice strawboards had relatively low mechanical properties; however, their low thermal conductivity satisfied the requirement for insulation boards. Therefore, they could be used to replace insulation boards made from wood.1 The technology to produce UF-straw panels has not been commercialized. Sauter reported that the internal bond (IB) and thickness swell (TS) of panel were reduced significantly with straw substitutions as low as 10% of the wood panel weight, and panels delaminated at substitutions higher than 50%.3 Grigoriou17 also indicated that the partial substitution of wood with wheat straw in panels using UF resulted in decreased mechanical properties, water resistance, and dimensional stability as straw level increased (Table 14.2). The poor bondability of wheat straw with UF resins is attributed to the high silica and wax content, the high pH and acid buffering capacity in comparison to wood, and the reduced thermal conductivity.3,6 The most critical factor was the wax coating on the straw surface. The inherent nonpolar, hydrophobic characteristics of the straw waxy surface inhibit the penetration of water-based UF to bond with the active hydroxyl groups of the cellulose, resulting in poor adhesion between the straw and UF resins, which are polar and hydrophilic in nature.23 In addition, curing of the pH-sensitive UF resins was likely inhibited by the extremely high acid buffering capacity of the straw.3,24 Although UF resins showed poor performance with wheat straw in panel production, they can be strengthened by incorporating cross-linking agents and other resin additives. PMDI resins were used in combination with UF resins in constructing straw panels to improve panel quality.17 As expected, panels made from a wood–straw mixture bonded with pure UF resins had low IB and water resistance, except for linear swelling. The internal bond strongly correlated to adhesive bond strength, indicating limited cross-linking between UF resins and straw particles. However, incorporation of PMDI with UF in wood-straw panels could improve bending strength, water resistance, and internal bond strength. The properties of panels with 50% straw bonded with UF:PMDI combinations were considerably improved as PMDI content increased (Table 14.2). The UF:PMDI glue formulations containing 2% PMDI or higher met the requirement of EN 300 standard for load-bearing OSB in dry conditions. Furthermore, when the PMDI amount in an adhesive system was 3% or more, the panels met the requirement of EN 312-5, the standard regarding thickness swell. Those results indicated that high-quality panels well exceeding the current commercial standard could be produced from straw or a wood–straw mixture using PMDI or UF–PMDI. Panels made of pure straw or a wood–straw mixture bonded with PMDI or the appropriate UF:PMDI combination can compete with wood particleboards, medium-density fiberboards (MDF), and OSB in certain applications, as they satisfy the requirement of the related European standard. Copyright © 2005 by Taylor & Francis

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Similar results were obtained by Liu and Hao.25 They found that UF alone could not produce a high-quality board, but the board bonded with a mixture of PMDI and UF resin at a ratio of 2:9 met the commercial standard. Silane coupling agent (SCA) has been added to UF resins to improve stability and strength.26 The IB and TS, and dimensional stability of straw particleboards were improved with addition of SCA to UF up to 5% of UF resin solid. A good correlation between the contact angle of wheat straw surface and the improved board properties was observed. The SCA could improve wetting, spreading, and surface tension of the straw, thus improving bond strength.27

14.3.1.2 Protein-Based Resins In recent years, soybean-based resins have been considered as a potential alternative to petroleum-based adhesives because they are renewable, biodegradable, and environmentally friendly.28 Soy proteins have been used to replace UF or PF and to reduce formaldehyde emission and to solve bonding problems in high-density particleboards.29–31 Low-density wheat straw particleboards have been fabricated with modified soy protein (SPI) adhesives.32 The mechanical properties of the particleboards were largely affected by protein modification/denaturation, initial straw moisture content, and chemical treatment of the straw. Treatment of straw with bleach gave the best results among all surface treatments. Particleboards with sodium hydroxide-modified soy proteins had the best mechanical properties among those bonded with soy proteins modified with sodium hydroxide, urea, or dodecylbenzene sulfonic acid. The bondability of modified soy proteins was also dependent on straw moisture content. About 30% to 40% initial straw moisture content was needed to obtain good bonding. The equilibrium moisture contents were similar for straw particleboards with sodium hydroxide-modified soy proteins and MDI adhesives at 30% or 60% relative humidity. Although the boards with soy protein-based adhesives had poorer mechanical properties than those with MDI adhesives, they are environmentally friendly and could be used in filters or as lightweight core materials where the requirement for mechanical strength is not as stringent. The adhesive performance of sodium hydroxide-modified SPI and soybean flour (SF) was further investigated in preparing medium-density wheat straw particleboards.33 Mechanical properties, water absorption, thickness swell, and equilibrium moisture content of the particleboards were evaluated. Boards with SPI resins showed better mechanical strength than those with UF resins for particleboards made from bleached straw. The SF, which contains about 50% protein compared to SPI with 90% protein, showed similar bonding properties to the UF resins (Table 14.2), indicating that soybeanbased adhesives could replace UF resins for indoor construction and furniture to avoid formaldehyde emission from UF. SPI resins also have been incorporated with MDI in the fabrication of lowdensity particleboards from wheat straw and corn pith.34 Among different Copyright © 2005 by Taylor & Francis

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formulations, the particleboards made from 70% wheat straw coated with 4% MDI, and 30% corn pith coated with sodium hydroxide-modified SPI had the highest mechanical strength. The board thickness change was less than 0.25% within the temperature ranges of 27° to 50°C and relative humidity range of 35% to 90%.

14.3.1.3 Other Resins Gomez-Bueso and others examined the properties of straw fiberboards as affected by three kinds of biocomponent thermoplastic resins (outlayer/core: A polyester/polypropylene, B polyethylene/polypropylene, and C polyester/polyester).35 Fiberboard strength was affected by the type and concentration of the thermoplastic resins. Fiberboard performance increased as thermoplastic resin concentration increased and thermoplastic resin B showed the best mechanical properties. The compatibility of rice straw with cement was investigated by analyzing the material ratio, addition of polyvinyl alcohol, and use of fast-curing agents on the properties of cement-bonded boards.36 It was found the optimum water–cement ratio was 1:8 for rice straw and optimum material ratio was 30:60:10 (plant:cement:filler). Addition of polyvinyl alcohol and fastcuring agents for cement could improve bending strength and significantly reduce water absorption, TS, and improve dimensional stability.

14.3.2

Pretreatment of Straw for Straw-Based Composites

Straw has a dense coating on its surface containing wax, silica, and protein, which serves to protect the epidermis against moisture loss. This layer inhibits bonding with resins, hydrogen bonding, and autoadhesion, and poses a problem in the production of straw-based particleboards and fiberboards.37 The pH of straw is neutral and its buffering capacity is 8.8 to 9.2. In comparison, spruce MDF fibers have a buffer capacity of 4.5.38 Buffering capacity greatly affects curing of UF resins. The low pH values of wood were considered to contribute to a better bonding strength between the wood and acid-setting UF adhesives.24 Recent studies showed that various pretreatment of straw using mechanical, chemical, and enzymatic methods could remove wax and extractives to improve its bondability with UF.39

14.3.2.1 Mechanical Treatments The bonding ability of straw can be improved if its surface structure is opened up, and thus made more accessible to wetting by aqueous adhesives such as conventional acid-curing UF resins.38,40 By applying high shear forces, the wax and silica layer can be destroyed, improving bonding with formaldehyde-based resins. The pressure-refining process is one of the methods explored. This method could reduce the extreme acid buffering capacity of wheat straw, remove the straw surface waxy layer, improve Copyright © 2005 by Taylor & Francis

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wettability, as shown by the reduced contact angle,41 and make it more suitable for bonding with the pH-sensitive UF resin. The IB, modulus of rupture (MOR), and modulus of elasticity (MOE) of MDF made from pressurerefined straw were comparable to the industrial MDF standard. However, the TS and linear expansion (LE) properties of the MDF panel still need to be improved.3 Three different straw preparation systems were examined to apply intensive shear forces to straw chips to overcome the wax and silica layer.38 Panels bonded with UF were produced with straw processed using a twin-screw extruder system or a pressurized refiner. The boards showed excellent appearance, surface smoothness, machinability, edge appearance, and good bonding properties. The IB and MOR values, except swelling properties, were comparable to particleboard standards (Table 14.3). With use of combined extruder-refiner technology, the UF bonded panels gave excellent visual characteristics, bending and IB strength values that exceeded MDF standards, and swelling attributes that were close to particleboard and MDF standards. Wasylciw compared three straw preparation systems for the performance of panels bonded with UF and MUF (melamine urea formaldehyde).42 The pressure-refined straw had significantly decreased buffer capacity and pH, followed by atmospherically refined straw and milled straw. A similar trend was observed for IB: the panels from pressure-refined straw had the highest values, well exceeding the ANSI MDF standard, and the panels from milled straw had the lowest IB. The combination of steaming, pressure, and retention time in the pressure-refined process promoted the formation of weak and strong acids and reduced the straw pH and buffer capacity. The lowered pH level favored curing of pH-sensitive UF resin. The interactions of straw fiber preparation, acid treatment, and UF/MUF on the panel performance were further investigated.42 The application of acid with either pre- or post-atmospheric refining by spray and in low add-on rates resulted in bonding of UF-based resins with straw well above the ANSI standard. Applying acid to milled straw showed no improvement in straw and MUF bond performance. An acetic acid treatment before refining could enhance UF resin performance, but not with MUF resin (Table 14.3). Acid treatment after refining proved detrimental to both resins. The acid acts as a chemical modifier, which drops the pH level and reduces the buffer capacity of the straw. Pressure refining has the same effect as adding acid to atmospherically refined straw. As acids are applied to previously pressure-refined acidified straw, the pH drops below the optimal level for UF resin. Therefore, it is necessary to determine the combination of resin, chemical addition, and refining conditions that will achieve the desired panel performance.

14.3.2.2 Chemical and Enzyme Treatments Both chemical and enzymatic methods could successfully improve bonding performance of straw by removal of the waxy layer from the straw surface, Copyright © 2005 by Taylor & Francis

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TABLE 14.3

Properties Treatments

Resins

Density (kg/m )

b

762 675 725 739 763 736 700 700 700 — — 900 900

a

MOR (MPa)

MOEa (MPa)

IBa (MPa)

TSa (%)

20.3 20 25.7 — — — 13 23 14 13.2 16.8 35 32

— 2800 3640 — — — 2500 3700 2900 — — 2800 2500

0.56 0.45 0.2 0.87 1.018 1.074 0.06 0.24 0.17 0.28 0.44 2.1 1.8

16.2 16 24.2 — — — 130 70 85 11.6 8.8 25.1 1.8

MOR: modulus of rupture; MOE: modulus of elasticity; IB: internal bond; TS: thickness swell. EB: ethanol benzene; SCA: silane coupling agent.

Reference Mantanis and Berns, 200138 Mantanis and Berns, 200138 Mantanis and Berns, 200138 Wasylciew, 200142 Wasylciew, 200142 Wasylciew, 200142 Han, et al., 200126 Han, et al., 200126 Han, et al., 200126 Zhang et al., 200343 Zhang et al., 200343 Gomez-Bueso et al., 200046 Gomez-Bueso et al., 200046

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Extruder 12% UF Refiner 13% UF Extruder + refiner 13% UF Refiner 10% UF Refiner 10% MUF Add acid before refiner 10% UF Non-EB, Non-SCA 12% UF EBb 12% UF SCAb 12% UF Nonenzyme UF CellulaseCopyright © 2005 byUF Taylor & Francis Nonacetylation 10% PF Acetylation 10% PF a

3

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Properties of Panels Made from Mechanical or Chemical Enzyme-Treated Wheat Straw

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reducing buffering capacity, improving surface wettability, and increasing the surface free-radical concentration. Han et al. studied effects of ethanolbenzene (EB) extraction of straw on the properties of wheat particleboard.26 The EB treatment could remove wax from the straw surface, facilitating the adherence of UF resins to the active hydroxyl site of cellulose. EB treatment improved both MOR and MOE by about 70% and IB of wheat strawboard by 300%. Improvements in dimensional stability included substantial reductions in TS and LE (Table 14.3). Zhang et al. studied the effects of enzyme treatment (lipase from Candida rugosa, cellulase from Trichoderma reseii, and cellulase from Aspergillus niger) on the properties of particleboards.43 The pH value of the straw was almost the same as that prior to treatment; however, the straw buffering capacity was reduced after treatment. All boards made from enzyme-treated straw showed improved mechanical properties and dimensional stability. The cellulase from A. niger was most efficient in reducing buffer capacity and increasing straw radical concentration, as well as showing the best panel performance. Schmidt et al. evaluated the effects of two pretreatment processes (alkaline wet oxidation and enzyme treatment with laccase) for applications in particleboards and fiberboards.37 After alkaline wet oxidation, fibers enriched in cellulose were obtained, and about 80% of the hemicellulose and a large portion of the lignin were solublized. Both treatments resulted in more hydroxyl groups, and the enzyme treatment gave a more reactive surface than alkaline wet oxidation. Laccase treatment attacks lignin and increases thermoplasticity of lignin. It looses the rigid structure of lignin, allowing improved flow during board consolidation and increased interfacial adhesion.44 Laccasetreated straw was more reactive than wheat straw prepared from thermal mechanical pulping (TMP). The TMP process can only break up the protective outer layer of the straw, but the resultant layer of condensed lignin that remains limits reactivity. On the other hand, the enzyme treatment greatly increased surface reactivity by disrupting the condensed lignin surface, which greatly increases the number of sites available for reaction. Lignocellulosic products have a well-documented problem with water sorption and lack of dimensional stability. Both properties can be largely improved in composites by chemical modification of the lignocellulosic material.45 Acetylated lignocellulosic material, which is chemically modified by means of acetic anhydride, reduces the thickness swelling of fiber composites by up to 90% relative to control.46 Acetylation also increases the surface free energy of the ligocelluosic fibers, which improves wetting and the spreading of the melting thermoplastic on the acetylated lignocellulosic fiber surface.47 Karr and Sun studied the effects of acetylation on water resistance and dimensional stability of strawboards bonded with MDI.48 Both water resistance and dimensional stability increased as the acetylation level increased. Maximum reductions of 80% in thickness swell and 50% in linear expansion were achieved at 24% acetylation weight gain. It was suggested that the acetylated straw was more hydrophobic and absorbed less moisture, and the acetyl Copyright © 2005 by Taylor & Francis

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group caused permanent expansion of the wheat straw cell wall, which resulted in limited swelling caused by water. However, the mechanical properties decreased as the acetylation level increased. The work by Gomez-Bueso et al. also showed that dimensional stability greatly improved for the fiberboards made from acetylated straw.46 The TS of a panel with acetylated straw was about 1.8% compared to 25.1% of a board with unmodified straw, and the residual internal bond obtained for acetylated fiberboards were 85% of the original strength after a water soaking-freezing-drying cyclic test. However, the fiberboards bonded with phenol-formaldehyde resin had a decreased MOR and MOE (Table 14.3). The acetylated strawboards were less compact and had reduced board density and more void space than the untreated strawboards, which may have caused the decrease in mechanical properties. 14.3.3

Straw–Plastic Composites

Currently, over 500 million pounds of fibrous material (primarily glass) is used by the plastics industry.49 Use of lignocellulosic materials, such as straw, as a reinforcing component in polymer composites has become more attractive. These materials could replace traditional fibers such as glass, carbon, and other inorganic fibers in composites, particularly in low price/ high volume applications. There are several advantages to using annual growth agrofibers as fillers and reinforcement over their inorganic counterparts in plastics. Agrofibers are less expensive, less dense, less abrasive to expensive molds and mixing equipment, have greater deformability, are biodegradable and renewable, and have better filling levels, possibly giving improved stiffness properties.15,50,51 Using straw as a fiber also has health benefits to workers because glass fiber in the form of airborne particles can cause respiratory problems.15,51 The primary drawback of the use of agrofibers is the lower processing temperatures required, which limits the type of thermoplastic resins mainly to polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polysterene (PS). However, these plastic resins account for 70% of the total thermoplastic consumed by the plastic industry, and the amount used as filler and reinforcement in these plastics is far more than that used in other types of plastics.50 The second drawback is the high moisture absorption of the fibers, which affects composite performance and dimensional stability. Chemical modification of the fiber and improved fiber-matrix bonding could dramatically reduce moisture absorption.50 A third drawback is the limited bonding ability with the plastic polymers. Cellulose fibers are very effective in forming intermolecular hydrogen bonds with thermoplastic polymers and producing low-density, high-modulus/high-strength composites.52 However, the straw contains large amounts of hemicellulose and lignin, which are nonfibrous amorphous polymers. Better performance of composites with straw could be achieved by fractionation of lignocellulosics into their components to obtain pure cellulose or cellulose associated with a small amount of hemicelluloses and lignin, or by changing the morphology and structure of the straw to enhance its ability to form interfacial bonds with thermoplastic polymers. Copyright © 2005 by Taylor & Francis

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The necessary intimacy of contact essential for good adhesion in composite fabrication could be achieved by good wetting of one component by the other. The compatibility of the two components would improve adhesion and strength of the composite.53 Maleic anhydride-grafted polypropylene (MAPP) was reported to improve adhesion for a lignocellulosic-PP system. The maleic anhydride in the MAPP can provide polar interactions and covalent links to the hydroxyl groups on the lignocellulosic fiber.50,54 Several studies demonstrated the feasibility of using wheat straw fiber as filler for plastics. Wheat–polypropylene composites with 30% wheat straw increased tensile strength by 16%, and flexural strength by 48% compared to virgin polypropylene. The MOE of the composites had a value twice that of polypropylene and the MOR value was 2.5 times greater. Impact properties of the composites decreased compared to polypropylene.49 In contrast, Hornsby et al. reported that incorporated straw doubled composite modulus and had similar tensile strength and Charpy impact strength as pure polypropylene.55 They also found that addition of 5% maleic anhydride-PP increased the tensile strength of PP–straw composites by promoting fiber-matrix interaction (Table 14.4). The conflicting results might be caused by the different processing conditions: the latter study compounded the blend in a high-speed mixer, while the former study used a twin-screw extruder. White and Ansell demonstrated that a composite material could be manufactured from straw and polyester resin.7 Incorporation of up to 60% rolled straw fiber in polyester resin showed improved strength, stiffness, and toughness, and reduced density. The flexural and tensile properties increased as TABLE 14.4 Properties of Straw–Plastic Composites Charpy Impact Strength (KJm
2)

Density (kg/m3)

Young’s Modulus (MPa)

Tensile Stress (MPa)

Flexural Modulus (MPa)

Polyester

1151

3300

—

4400

—

Polyester/straw (90/10) Polyester/straw (60/40) PP

1010

4200

—

2700

—

751

8000

—

7300

—

900

1180

33

1200

2.46

999

2630

30

2350

2.22

—

2390

32

2340

2.21

—

1400

19

—

—

—

1700

9

—

—

Types of Composites

PP/straw (75/25) PP/straw (75/25) with 5% MA-PP PHBV PHBV/straw (70/30)

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Reference White and Ansell, 19837 White and Ansell, 19837 White and Ansell, 19837 Hornsby et al., 1997 55 Hornsby et al., 199755 Hornsby et al., 199755 Avella et al., 200057 Avella et al., 200057

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straw content increased from 10 to 40%, while density decreased (Table 14.4). The composite with 40% straw had a flexural stiffness of 0.99 (m  106), which was 2.5 times greater than the pure resin. At 50% straw the composite had a work of fracture close to 10 kJm
2, which was almost in the category of tough engineering materials (10 kJm
2). During impact the straw fibers were capable of arresting and diverting the progress of propagating cracks from an edge notch and blunting the crack by delamination. It was suggested that the straw acted as a cost-reducing agent in the composites and can be regarded as a successful engineering material or as a lightweight building material. Straw can be used as an inexpensive filler with biodegradable thermoplastic PHB (polyhydroxybutyrate) and PHVB [poly(3-hydroxybutyrate-co3-hydroxyvalerate)] in the production of natural fiber-based composites. The performance of composites made from PHB using steam explosion-treated wheat straw as a reinforcement was examined by Avella et al.56 PHB is a polymer with relatively low impact resistance. The addition of 10% to 20% steam-exploded straw to PHB markedly increased the physicomechanical properties of the PHB because of the intermolecular interactions between the carbonyl group of PHB and the hydroxyl groups of the straw. The thermal, mechanical, and biodegradability of 10–30% wheat straw fiber-reinforced PHBV were also investigated.57 The addition of straw fibers increased the rate of PHBV crystallization. The glass transition temperature increased as straw content increased, indicating increased interactions between carbonyl groups of the PHBV and hydroxyl groups of the straw, which decreases the molecular flexibility of the polymer chain. The composites had higher Young’s moduli and lower tensile strength compared to the pure PHBV (Table 14.4). The presence of straw did not affect biodegradation rate evaluated in a liquid environment and in long-term soil burial tests. Both PHB–straw and PHVB–straw fiber composites represent a potential new class of materials that may have applications in agricultural mulching or transplantation, where biodegradable properties are needed.

14.4

Current Status and Industrial Applications

The volume of straw in housing, automotive, packaging, and other low-cost, high-volume applications is expected to increase. Recent interest in environment-friendly materials is leading to the development of new materials or composites. In the light of natural resource shortages and pressures to decrease dependence on wood and petroleum products, there is an increasing interest in maximizing the use of renewable materials. Europe is stimulating sustainable development to build an economy based on renewable resources.15 The use of agricultural materials, including straw, not only provides a renewable material source, but also generates a nonfood source of economic development for farming and rural areas. Copyright © 2005 by Taylor & Francis

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Cereal straw appears to have the most promise for panel (low-density insulation boards, medium-density fiberboards, hardboards, particleboards) and other building system components, such as walls and roofs. The binders could be synthetic thermosetting resins, modified naturally occurring resins like tannin or lignins, starches, proteins, themoplastics, inorganics, or no binder at all.38 A number of factories have been established to manufacture high-quality straw panels. The Daproma Company of Sweden built a wheat strawboard factory in North America in 1996. Annual capacity currently is 53,100 cubic meters and the panel is sold under the trademark brand name of WheatBoard. In 1998, the Isoboard Company of Canada built a straw panel factory with an annual production of 2,000,000 cubic meters.59,60 Wheat straw is also used to produce panels in the northwest region of the United States.61 Plants in several countries produce thick (5–15 cm) straw panels with kraft paper faces and a low density of 0.25 g/cm. The panels can be cut for prefabrication into housing and other structures.3,62 The panels are fairly resilient and test data showed that housing built with these panels are especially earthquake resistant. Pure straw panels are characterized by improved bending strength as a result of the slenderness and compaction ratio of the straw; they are particularly suitable for applications demanding high flexure properties. Straw is suitable for the production of good-quality surface layers for particleboards bonded with PMDI or a combination of UF and PMDI. The manufacture of surface-layer chips is more expensive than that of core-layer chips. Straw can be processed with less costly chipping and drying equipment. Straw has a uniform structure, light color, is thinner, and has higher length-to-thickness and compaction ratios. These properties endow it with certain advantages as a raw material for particleboard surface layers compared to wood.63 Three-layer particleboards were fabricated, with the middle layer consisting of industrial wood particles and bonded with UF. The surface layer was made from straw or straw–wood mixture and bonded with PMDI. The surface layer made from straw showed a lighter color and superior surface smoothness to those made from straw-wood mixture. Boards with surface layers containing more than 50% straw were comparable with MDF for load-bearing applications. A low-cost wheat straw insulation board was made using MDI resin.64 Good mechanical properties were obtained at resin content of 2%. The boards with a density of 128 and 160 kg/m3 had thermal resistivities of 24 to 26 m2K/W. The boards with a density of 160 kg/m3 and 2–4% resin content had an estimated materials cost of $1.22 per unit of thermal resistance per square meter of area, which was substantially less than the cost of expanded polystyrene or the cost of rigid board insulation. In the former USSR, rice straw hardboard was produced and found to have excellent properties, with superior bending strength and water resistance.65 Typically, the order of high, medium, and low process for furnishings is assumed to be MDF, strawboards, and particleboards, respectively. In the market, strawboards have the most advantageous features, including closed panel edges, relatively low density, and the potential for zero-level formaldehyde emissions. Strawboards are now competing with wood products such Copyright © 2005 by Taylor & Francis

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as particle and fiberboards in the markets for floor underlays, furniture, and cabinet construction. While strawboards are used primarily for interior walls, floors, and ceilings in commercial and residential buildings, they have a multitude of other uses: outside walls using a covering agent, storage sheds, freestanding garages, etc. Strawboards have a significant quality advantage because of their built-in insulation, fire retardance, and sound suppression.66 The amount of agricultural residues generated each year far exceeds the present and foreseeable composite panel requirement.58 A new material from straw needs to be developed to fully exploit the application of the fiber. Compared to the panel industry, applications of straw-based composite are limited in the automotive industry, which has mainly focused on natural fiber-reinforced synthetic polymers. Straw–plastic composites could be used for automotive nonwoven-polypropylene composite components, highstrength composites, and automotive interior products, such as ashtrays and cup holders. However, the principal fibers now being used are the bast fibers: flax, hemp, jute, etc. Research on straw-based fillers and fiber-filled plastics could lead to new value-added uses of agricultural fiber residues.

14.5

Conclusion

Current applications of straw-based composites suggest that they are still in the early stages of applications in lightly loaded structures. The composites seem to possess strong potential for medium to heavily loaded structures. The development of biobased resins compatible with straw is important for manufacturing environmentally friendly panels. Straw-based composites have the potential to become a serious competitor to inorganinc fiber composites because they are lighter, biodegradable, and cheaper than synthetic fibers. However, thermal stability, moisture absorption, and straw–matrix compatibility of straw-reinforced composites need to be further studied. Extensive efforts are also needed in developing and fully exploiting the potential of biodegradable composites. The future is dependent on whether they are capable of competing with the existing composites in the market in view of technology, economy, and ecology.

References 1. Sampathrajan, A., Vijayaraghavan, N.C., and Swaminathan, K.R., Mechanical and thermal properties of particleboards made from farm residues, Bioresour. Technol., 40, 249, 1992. 2. Zucaro, J. and Reen, R., The Second Forest: Filling the Wood Source Gap While Creating the Environmental Performance Board of the 21st Century.

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Developing Composites from Wheat Straw, Proceedings of the Washington State University International 29th, Particleboard/Composite Materials Symposium, 1995, pp. 225–231. Sauter, S.L., Developing Composites from Wheat Straw, Proceedings of the Washington State University Internationl 30th, Particleboard/Composite Materials Symposium, 1996, p. 197. Dalen, H. and Shoram, D.T., The Manufacture of Particleboard from Wheat Straw, Proceedings of the Washington State University International 30th, Particleboard/Composite Materials Symposium, 1996, p. 191. Russel, W.C., The Straw Resource: A New Fiber Basket?, Proceedings of the Washington State University International 30th Particleboard/Composite Materials Symposium, 1996, p. 183. Hague, J., McLauchlin, A., and Quinney, R., Agri-Materials for Panel Products: a Technical Assessment of their Viability, in Proceedings of the Washington State University International 32nd Particleboard/Composite Materials Symposium, 1998, p. 151. White, N.M. and Ansell, M.P., Straw-reinforced polyester composites. J. Mater. Sci., 18, 1549, 1983. Staniforth, A.R., The nature of cereal straw, in Cereal Straw, Clarendon Press, Oxford, 1979, p. 16. Atchison, J.E. and McGovern, J.N., History of paper and the importance of agrobased plant fibers, in Pulp and Paper Manufacture, Vol. 3, Secondary Fibers and Agrobased Pulping, Hamilton, F. and Leopold, B., Eds., TAPPI Press, Atlanta, 1983. Jackson, M.G., Review article: the alkaline treatment of straws, Anim. Feed Sci. Technol., 2, 105, 1977. Joint United States-Canadian Tables of Feed Composition, Committee on Animal Nutrition, Agricultural Board and National Academy of Science-Committee on Animal Nutrition, Publication 1232, National Research Council, Washington, DC, 1964, p. 167. Theander, D., Review of straw carbohydrate research, in New Approaches to Research on Cereal Carbohydrates, Hill, R.D. and Munck, L., Eds., Elsevier, Amsterdam, 1985, p. 217. Lawther, J.M., Sun, R.C., and Banks, W.B., Extraction, fractionation, and characterization of structural polysaccharides from wheat straw, J. Agric. Food Chem., 43, 667, 1995. Kolattukudy, P.E., Cutin, suberin and waxes, in The Biochemistry of Plants, Vol. 4, Lipids: Structure and Function, Stumpf, P.K. and Conn, E. E., Eds., Academic Press, New York, 1980, pp. 571–645. Kandachar, P. and Brouwer, R, Applications of bio-composites in industrial products, in Materials Research Society Symposium—Proceedings, Vol. 702, 101, 2002. Rowell, R.M., Opportunity for lignocellulosic materials and composites, in Emerging Technologies for Materials and Chemicals from Biomass, ACS Symp. Series, 476, American Chemical Society, Washington, DC, 1992, p. 12. Grigoriou, A.H., Straw-wood composites bonded with various adhesive systems, Wood Sci. Technol., 34, 355, 2000. Wasylciw, W., The straw panel manufacturing process and straw’s potential in flooring substrates, Plastic Laminates Symposium, 1998, p. 291. Marcinko, J.J. et al., The Nature of MDI/Wood Bond, Proceedings of the Washington State University International 28th Particleboard/Composite Materials Symposium, 1995, p. 175.

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20. Borda, J., Teslery, B., and Zsuga, M., Biologically degradable fiber-reinforced urethane composites from wheat straw, Polym. Compos., 20, 511, 1999. 21. Yang, H.S., Kin, D.J., and Kim, H.J., Rice straw-wood particle composite for sound absorbing wooden construction materials, Bioresour.Technol., 86, 117, 2003. 22. Sampathrajan, A., Vijayaraghavan, N.C., and Swaminathan, K.R., Acoustic aspects of farm-residue-based particle boards, Bioresour. Technol., 40, 67, 1991. 23. Osama, A.A., Daoud, S.D., and Shatha, M., Properties of particleboard from reedtypha mixture, J. Petrol. Res., 7, 197, 1988. 24. Johns, W.E., Effect of pH and buffering capacity of wood on gel time of UF resin, Wood Fiber, 30, 255, 1980. 25. Liu, Z.T. and Hao, B.Y., Technology of Rice Straw Particleboard Boned by UreaFormaldehyde Resin Modified by Isocyanate, in Pacific Rim Bio-based Composites Symposium, New England, 1992, p. 295. 26. Han G.P. et al., Effects of silane coupling agent level and extraction treatment on the properties of UF-bonded red and wheat straw particleboards, J. Wood Sci., 47, 18, 2001. 27. Bryant, B.S., Studies in wood adhesion-interaction of wood surface and adhesive variables, Forest Prod. J., 18, 57, 1968. 28. Wang, D.H. and Sun, X.S., Composites from agricultural-derived resins and fibers, Agro Food Industry Hi Tech., 7, 14, 2001. 29. Ignjatovic, N.L. et al., Determination of optimal conditions for modification of a urea-formaldehyde adhesive by an enzymically modified soybean adhesive, Hem. Ind., 52, 286, 1998. 30. Kuo, M., Adams, D., Myers, D., and Curry, D., Properties of wood/agricultural fiberboard bonded with soybean-based adhesives, Forest Prod. J., 48, 71, 1998. 31. Clay, J.D., Vijayendran, B., and Moon, J., Rheological study of soy protein-based prf wood adhesives, Annual Technical Conference, Society Plast. Eng., 57, 1298, 1999. 32. Mo, X.Q., Hu, J., Sun, X.S., and Ratto, Jo. A., Compression and tensile strength of low-density straw-protein particleboard, Ind. Crops Prod., 14, 1, 2001. 33. Mo, X.Q., Cheng, E.Z., Wang, D.H., and Sun, X.S., Physical properties of medium-density wheat-straw particleboard using different adhesives, Ind. Crops Prod., 68, 47, 2003. 34. Wang, D.H. and Sun, X.S., Low density particleboard from wheat straw and corn pith, Ind. Crops Prod., 15, 43, 2002. 35. Gomez-Bueso, J., et al., Composites made from acetylated lignocellulosic fibers of different origin, Part II. The effect of nonwoven fiber mat composition upon molding ability, Holz als-und Werkstoff, 57, 178, 1999. 36. Eusebio, D.A. and Suzuki, M., Production and properties of plant-materials cement-bonded composites, Bull. Exp. Forests, 27, 27, 1990. 37. Schmidt, A.S. et al., Comparison of the chemical properties of wheat straw and beech fibers following alkaline wet oxidation and laccase treatments, J. Wood Chem. Technol., 22, 39, 2002. 38. Mantanis, G. and Berns J., Strawboards Bonded with Urea Formaldehyde Resins, Proceedings of the Washington State University International 35th, Particleboard/Composite Materials Symposium, 2001, p. 137. 39. Loxton, C. and Hague, J., Utilization of Agricultural Crop Materials in Panel Products, Proceedings no. 7286: The Use of Recycled Wood and Paper in Building Applications, Forest Prod. Society, Madison, WI, 1996, p. 190.

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40. Markessini, E., Roffael, E., and Rigal, L, Panels form Annual Plant Fiber Bonded with Urea-Formaldehyde Resins, Proceedings of the Washington State University International 31st, Particleboard/Composite Materials Symposium, 1997. 41. Han, G.P., Development of high-performance reed and wheat straw composite panels, Wood Res., 88, 19, 2001. 42. Wasylciw,W., Straw-based composite panel-attributes, issues, and UF bonding technology, Proceedings of the Washington State University International 35th, Particleboard/Composite Materials Symposium, 2001, p. 145. 43. Zhang, et al., Wheat straw particleboard bonding improvement by enzyme treatment, Holz als Roh-und Werkstoff, 61, 49, 2003. 44. Barsberg, S., Thygesen, L.G., and Felby, C., Detection of Laccase Induced Modification of Beech- and Spruce TMP Fibres by Luminescence Spectroscopy, Proceedings of the International Conference on Biotechnology in the Pulp and Paper Industry, Vol. 3, Vancouver, Canada, 1998, p. 123. 45. Rowell, R.M., Chemical modification of wood, in Wood and Cellulosic Chemistry, Hon, D.N.S. and Shiraishi, N., Eds., Marcel Dekker, New York, 1991, p. 15. 46. Gomez-Bueso, J. et al., Composites made from acetylated lignocellulosic fibers of different origin, Part I. Properties of dry-formed fiberboards, Holz als-und Werkstoff, 58, 9, 2000. 47. Lui, F.P. et al., Characterization of the interface between cellulosic fibers and a thermoplastic matrix, Compos. Interf., 2, 419, 1995. 48. Karr, G.S., and Sun, X.S., Strawboard from vapor phase acetylation of wheat straw, Ind. Crops Prod., 11, 31, 2000. 49. Johnson, D.A., Jacobson, R., and Maclean, W.D., Wheat Straw as a Reinforcing Filler in Plastic Composites, Proceedings of the 4th International Conference on Woodfiber-Plastic Composites, Madison, WI, 1997, p. 2000. 50. Sanadi, A. R. et al., Agro-fiber thermoplastic composites, in Paper and Composites from Agro-bases Resources, Rowell, R. M., Young, R.A., and Rowell, J.K., Eds., CRC Press, Boca Raton, Fl., 1997, pp. 378–399. 51. Rozman H.D. et al., The effect of chemical modification of rice husk with glycidyl methacrylate on the mechanical and physical properties of rice husk-polystyrene composites, J. Wood Chem. Technol., 20, 93, 2000. 52. Maldas, D., Kokta, B.V., and Daneault, C., Thermoplastic composites of polystyrene: effect of different wood species on mechanical properties, J. Appl. Polym. Sci., 38, 413, 1989. 53. Westerlind, B.S. and Berg, J.C., Surface energy of untreated and surface-modified cellulose fibers, J. Appl. Polym. Sci., 36, 523, 1988. 54. Felix, J.M., Gatenholm, P., and Schreiber, H.P., Controlled interactions in cellulose-polymer composites. I. Effect on mechanical properties, Polym. Compos., 14, 449, 1993. 55. Hornsby, P.R., Hinrichsen, E., and Tarversi, K, Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibers, Part II Analysis of composite microstructure and mechanical properties, J. Mater. Sci., 32, 1009, 1997. 56. Avella, et al., Natural Fiber Composites, Proceedings of the 9th International Conference on Composite Materials, Madrid, Spain, 1993, p. 864. 57. Avella, et al., Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and wheat straw fiber composites: thermal, mechanical properties and biodegradation behavior, J. Mater. Sci., 35, 829, 2000.

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58. Youngquist, J.A. et al., Agricultural Fibers for Use in Building Components, Proceedings of the Conference on the Use of Recycled Wood and Paper in Building Applications, Madison, Wisconsin, 1996, p. 123. 59. David, P., Wood process adapted to straw particleboard, Wood Technol., 9, 20, 1997. 60. Donnell, R., BCT is Key participant in wheat straw panel mill surge in United States, Panel World, 5, 33, 1998. 61. Loken, S., Spurling, W., and Price, C., Guide to resource efficient building elements, Center for Resourceful Building Technology, Missoula, MT. 62. United Nations Industrial Development Organization, Review of agricultural residue utilization for production of panels, Food and Agricultural Organization of the United Nations, FO/WCWBP/75, Doc, 127, 1975. 63. Grigoriou, A.H., Straw as alternative raw material for the surface layers of particleboard, Holzforschung und Holzverwertung, 50, 32, 1998. 64. Norford, L.K. et al., Development of low-cost wheat-straw insulation board, ASHRAE Trans., 106, 339, 2000. 65. Kluge, Z.E., Tsekulina, L.V., and Savel’eva, T.G., Manufacture of hardboards from rice straw, Tekhnol. Modif. Drev., 55, 1978. 66. Parker, P., A summary report on building materials produced from wheat straw, Inorganic-Bonded Wood Fiber Compos. Mater., 5, 47, 1997.

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15 Sorona® Polymer: Present Status and Future Perspectives

Joseph V. Kurian

CONTENTS 15.1 Development of Sustainable Technologies 15.1.1 Introduction 15.1.2 Sustainable Growth 15.2 History and Development of Sorona 15.2.1 History 15.2.2 Uniqueness of Sorona 15.2.2.1 Molecular Structure and Effects on Mechanical Properties 15.2.2.2 How Do Molecular Shape and Crystalline Structure Translate into Beneficial Properties? 15.2.2.3 Other Properties 15.3 Starting Materials 15.3.1 1,3-Propanediol 15.3.2 PDO from Traditional (Chemical) Sources 15.3.3 Bio-PDO 15.3.4 DMT/TPA 15.3.4.1 Terephthalic Acid (TPA) 15.3.4.2 Dimethyl Terephthalate (DMT) 15.3.5 TPA vs. DMT 15.3.6 Other Materials 15.4 Chemistry and Technology of Polymerization 15.4.1 Polymerization Process 15.4.2 Terephthalic Acid Process 15.4.2.1 Esterification 15.4.2.2 Polycondensation 15.4.3 Dimethyl Terephthalate Process 15.4.3.1 Transesterification 15.4.3.2 Batch Process

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15.4.3.3 Continuous Process 15.4.3.4 Make 3GT at a 2GT Plant? 15.4.3.5 Remelt Process 15.5 Polymer Shaping 15.5.1 Fibers: Apparel and Carpets 15.5.2 Films 15.5.3 Nonwovens 15.5.4 Engineering Components 15.5.5 Monofilaments 15.6 Applications and End Uses 15.6.1 Apparel 15.6.2 Bicomponents 15.6.3 Fiber Blends 15.6.4 Floor Coverings 15.6.5 Nonwovens 15.6.6 Engineering Markets 15.6.7 Other Markets 15.7 Creating the Future 15.7.1 Direction and Vision 15.7.2 Realization 15.8 Conclusion Acknowledgments References Endnotes: Patent References ABSTRACT DuPont Company recently introduced a new polymer platform, Sorona®, based on 1,3-propanediol. Sorona polymer can be shaped into fibers and other articles to offer a unique combination of softness, comfort-stretch and recovery, dyeability, and stain resistance. Sorona imparts distinctive characteristics to fabrics and a vast array of other applications such as upholstery and specialty resins. The breakthrough technology behind Sorona polymer enables manufacturers of apparel, upholstery, specialty resins, and other materials to use their existing assets to make new, higher-value products to meet customer needs. The development and commercialization of Sorona polymer is an example of DuPont’s commitment to provide value-added sustainable products from biologically renewable resources. DuPont has recently announced plans for the commercialization of the biological process for the large-scale production of 1,3-propanediol. Currently, DuPont is testing the jointly developed bioprocess with Genencor at a pilot plant operated by Tate & Lyle. Because corn is a renewable resource and can be produced anywhere in the world, PDO becomes less expensive to manufacture. DuPont dedicated the world’s first continuous polymerization plant in September of 2000 for making Sorona polymer. This Kinston, North Carolina facility was built in response to textile fiber manufacturers’ uncompromising requirement for consistent, high-quality polymer. In contrast to batch manufacturing, continuous polymerization (CP) is a round-the-clock process that feeds precisely measured ingredients into the Copyright © 2005 by Taylor & Francis

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polymer-making process. The result is uniform, consistent polymer that meets the demanding standards of textile fiber producers. One of the competitive advantages for DuPont is the technology to retrofit existing polyester CPs to produce Sorona polymer. Retrofit costs are a fraction of the cost to build a new CP. Sorona is also finding applications in engineering thermoplastics, films, nonwovens, mono- and multicomponents, and several other markets. DuPont has licensed Sorona fiber technology to a number of fiber manufacturers for applications development. From corn to polymer to future, Sorona polymer offers a multitude of advantages. Learn all about the advantages by visiting http://www.dupont.com/sorona.

15.1

Development of Sustainable Technologies

15.1.1

Introduction

Several factors arising over the last quarter of the 20th century have led to increased global interest in bioderived materials and biotechnology. 1. Instability of price and availability of crude oil over the last two decades has prompted manufacturing companies to reduce their dependence on petroleum as a source of material for their products. 2. Beginning first on a large scale in Europe, and rapidly spreading globally, is the tendency of governments to look at products through their entire life cycle—manufacture, care and use, disposal—for regulatory purposes. The result has been an increased concern on the part of manufacturers for recyclability of their products. 3. The Kyoto Protocol, by establishing a marketplace for environmental credit, is providing economic stimulus to companies who are already “good at it” to go further in their self-clean-up than they would otherwise. These factors play an increasingly important role in determining how global manufacturing companies manage themselves for both the present and the future. An example is E. I. du Pont de Nemours and Company, just starting its third century. DuPont CEO Chad Holliday states: Like most companies in our industry during the past century, we measured success by how much stuff we produced and sold. Revenues and profits were directly tied to throughput of energy and raw materials and output of product. To increase one you had to increase the other. As we approached the 21st century, however, we recognized that such an approach could not lead a company toward sustainability. One reason was the obvious: global industry does not have unlimited supplies of non-renewable raw materials, and the earth’s ecosystems have limited capacity to absorb waste and emissions. But, more fundamentally, if we wanted our businesses to be sustainable, we had to find a way to create Copyright © 2005 by Taylor & Francis

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business growth as measured by revenues and earnings while minimizing the pounds of product necessary to do so…. One way to do that is to sell knowledge, and knowledge intensity became one of the three prongs of our corporate strategy…. Another prong of our strategy is productivity improvement—really basic eco-efficiency. In a general sense, most productivity improvements in manufacturing contribute to sustainability…. Our third strategic prong is integrated science. We have for over a century made products and profits by integrating chemistry and physics. Now we are focusing on integrating chemistry, biology, physics, and information to create products that can make a difference in people’s lives. One new product line, Sorona® [polymer], will soon use a biological process to obtain a key intermediate. Essentially, we will be fermenting corn sugar, a renewable resource, to create a key component of a recyclable polyester fiber. Previously, such ingredients would have been produced from petrochemicals…. We at DuPont have a long way to go in the journey to true sustainability, but we believe it can be done…. We are committed to sustainable growth. That is how we will get our bottom line and that is how we will create value for our shareholders and for society. Holliday, Charles O., CEO, DuPont Company, with Phil Watts, Chairman, Royal Dutch Shell, and Stephan Schmidheiny, Chairman, Anova. Walking the Talk: The Business Case for Sustainable Development, Greenleaf Publishing Limited, 2002, p. 24

15.1.2

Sustainable Growth

Formed in 1802 as a gunpowder manufacturer, DuPont has transformed itself when necessary to survive and grow in a world of changing economic, political, cultural, and social imperatives. In the early 20th century the company moved from explosives to chemicals, and at the turn of the 21st century into sustainable growth through knowledge intensity, productivity, and integrated science. These transformations are summarized graphically in Figure 15.1. To drive the move toward sustainability, DuPont has set the following goals for 2010: ●

● ●

●

To derive 25% of company revenues from businesses not requiring depleteable raw materials (up from 14% in 2002) To meet 10% of the company’s energy needs from renewable sources To reduce global carbon-equivalent greenhouse gas emissions by 65%, using 1990 as a base. (The company has already surpassed this goal with a 68% reduction.*) To hold energy use flat using 1990 as a base year.

* In January 2003, as part of its sustainability mission, DuPont became a founding member of the Chicago Climate Exchange, a voluntary cap-and-trade program for reducing and trading greenhouse gas emissions. Members of the Chicago Climate Exchange have made a commitment to reduce their emissions of greenhouse gases by 4% below the average of their 1998–2001 baseline by 2006.

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Maturity

Maturity Growth Growth

Maturity

Birth

Growth

Chemistry, biology... knowledgeintensive solutions

Birth Chemicals, energy

Birth Explosives

Biobased Materials 1802

1830

1850

1900

1925

1945

1990

2000

2050

2090

FIGURE 15.1 DuPont technology, history, and strategy.

The strategy to meet the first of these targets includes the use of biological processes to make products from renewable raw materials. In support of that strategy, DuPont formed the Biobased Materials business (BBM) unit in 2000. BBM’s first commercial product is the Sorona family of poly(trimethylene terephthalate) (PTT, or “3GT”) polymers. Other companies shared DuPont’s view of sustainable growth. Concurrently, Dow Chemical Company in a joint venture with Cargill has recently commercialized the NatureWorks family of polylactic acid (PLA) biodegradable polymers.

15.2 15.2.1

History and Development of Sorona History

Polyester is arguably the most widely used category of synthetic fiber in the world today, in apparel, home furnishings, and industrial applications. Since its invention more than 50 years ago, the dominant polyester has been polyethylene terephthalate, also called PET or 2GT. Other varieties of the polymer have been known since the beginning. For example, instead of reacting ethylene glycol with the terephthalic acid ingredient to make 2GT, one could use 1,4-butanediol to make 4GT or 1,3-propanediol for 3GT. Until recently, three factors have effectively kept 3GT polymer off the market: the relatively high cost of the 1,3-propanediol; an inability to make high-quality polymer for downstream end uses; and more difficult process requirements compared to 2GT. Copyright © 2005 by Taylor & Francis

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Since the 1990s, however, significant developments have taken place to improve the business prospects for 3GT. More economical methods have been established independently by DuPont and by Shell for producing 1,3propanediol (“PDO,” also called 3G) in commercial quantities from petrochemical sources. Production of PDO has been demonstrated to be feasible from renewable resources as well, via biological processes.iii,iv,v,vi Finally, continuous polymerization processes have been refined to enable commercial manufacture of 3GT polymer to “fiber-grade” standards of quality and uniformity. In the past decade, DuPont has generated over 100 patents related to the cost-effective production, processing, and applications of 3GT. A few of those patents are listed in the Endnotes section of this chapter.

15.2.2

Uniqueness of Sorona

15.2.2.1 Molecular Structure and Effects on Mechanical Properties Sorona 3GT polymer is one member of a family of polymers based on fibergrade 1,3-propanediol. It is a linear crystallizable polymer with a melting temperature of ~228°C and a glass transition temperature of about 50°C. The structure of 3GT polymer is shown in Figure 15.2. As produced, 3GT is a clear polymer, but it becomes opaque when crystallinity is developed. The introduction of titanium dioxide pigment provides a milky white appearance. The molten polymer can be spun into fibers, injection molded into articles, blow-molded into bottles, and cast into films. Crystallization and crystal structure play an important role in determining the properties of 3GT. Crystallization behavior may be observed with a differential scanning calorimeter (DSC). A representative DSC scan of 3GT is shown in Figure 15.3. A freshly polymerized sample of 3GT, when first heated from ambient conditions, shows a glass transition temperature of 45°−55°C. As the temperature is raised a little further, a cold crystallization temperature is observed around 71°C. A photomicrograph of spherulitic crystal formation of 3GT is shown in Figure 15.4.1 The melting endotherm, Tm, is observed between 228° and 232°C. During cooling, crystallization from melt, Tc, is observed between 160° and 185°C. With nucleating additives, Tc could be as high as 200°C. The structure of PTT has been studied extensively.2−4 The beneficial properties of Sorona 3GT polymer are derived from a unique, semicrystalline molecular structure featuring a pronounced “kink,” as shown in Figure 15.5.

(

O

O

O CH2 CH2 CH2 O C

C

FIGURE 15.2 Molecular structure of 3GT.

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)

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15

Heat flow (mw)

Crystallization 160 − 185°C

Cold crystallization 71°C

10 5

Cooling 0 −5

Heating

−10

Melting 228 − 232°C

Glass transition 45− 55°C

−15 30

80

130

180

230

Temperature (°C) FIGURE 15.3 DSC trace for 3GT.

FIGURE 15.4 Example of spherulitic crystal formation in 3GT.

15.2.2.2

How Do Molecular Shape and Crystalline Structure Translate into Beneficial Properties? 3GT polymer has a zigzag shape, a consequence of the convolutions of the bonds in the trimethylene constituent (see Figure 15.5). This zigzag shape means that tensile or compressive forces translate at the molecular level to Copyright © 2005 by Taylor & Francis

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bending and twisting of bonds, rather than simply stretching. This is analogous to the tensile behavior of a coil spring compared to a straight wire. When the polymer is cooled from a liquid state, it forms crystalline and amorphous regions. 3GT solidifies rapidly and at lower temperatures than other commercially important terephthalic polyesters. In the crystalline regions the polymer molecules are aligned together with the same orientation, like springs in a mattress. Molecules are essentially locked together and must move as one. Accordingly, the entire crystalline region acts like a spring. All of the crystalline region’s strain is recovered when forces are removed, as long as the integrity of the region is preserved. With 3GT, the modulus of the crystalline regions is significantly lower than that of other commercial polymers, e.g., PET (see Figure 15.6). The fiber can take an appreciable level of applied strain (up to 15−20%) and recover completely, e.g., no permanent set, when forces are removed (see Figure 15.7).

SoronaTM 3GT Polymer

2GT

FIGURE 15.5 Molecular shape and crystal structure of 3GT.

4GT

PET

5 Stress, g/denier

Nylon SoronaTM 3GT Fiber

4 3

PBT

2 1 0 0

10

20

30

Strain (%)

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40

50

FIGURE 15.6 Comparison of tensile characteristics.

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Table 15.1 compares properties of Sorona 3GT with other synthetic and natural fibers and with polylactic acid (PLA), another bioderived polymer. In addition to its unique stretch-recovery characteristics, Sorona 3GT provides all the advantages generally associated with polyesters, i.e., excellent physical and chemical properties, dimensional stability, low moisture absorption, easy care, good weather resistance, easy processability, and recyclability. Sorona polymer can be easily modified to achieve desirable functional properties as well.

Total recovery (%)

100 Sorona 3GT Fiber

90 80 70

PET

N66

PBT

60 50 0

5

10 15 20 Applied strain (%)

25

30

FIGURE 15.7 Comparison of stretch recovery characteristics.

TABLE 15.1 Polymer and Fiber Properties Synthetic Fibers Fiber Property Nylon 6 PET Specific gravity Tg (oC) Tm (oC) Tenacity (g/d) Moisture regain (%) Elastic recovery (5% strain) Refractive index a

b

1.14 1.39 40–60 70–80 215 255 5.5 4.1 89 1.52

Acrylics 1.18 320

(degrades)

6.0 0.2–0.4 65

4.0 1.0–2.0 50

1.54

1.50

Melt-Spinnable Natural Based Soronaa

Natural Fibers

PLAb

Rayon Cotton Silk Wool

1.33 1.25 45–55 55–65 228 130–175

1.52 1.52 1.34 1.31 — — — — None None None None

4–5 6.0 0.2–0.3 0.4–0.6 100 —

93

2.5 11 32

1.35–1.45 1.52

4.0 7.5 52 1.53

4.0 10

1.6 14–18

52

69

1.54

1.54

Sorona is currently produced using a chemical process; full-scale commercialization of bioderived polymer is expected by the middle of 2006. PLA data from Dow/Cargill Web site.

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15.2.2.3 Other Properties In addition to the stretch and recovery characteristics, Sorona offers several advantages over both conventional polyester (PET) and Nylon. It can be effectively disperse-dyed at 100°C, eliminating the need for carriers or pressurization in the dyeing process (see Figure 15.8). Once dyed, the fabric has deeper shades and superior washfastness over other products. Other advantages of Sorona become more evident in actual use conditions. The fiber is highly resistant to most stains without the need for surface treatment with additives or coatings. It resists UV degradation better than other fibers, and exhibits both low water absorption and low electrostatic charging.

15.3

Starting Materials

Sorona 3GT is made from 1,3-propanediol and dimethyl terephthalate (DMT) or terephthalic acid (TPA). There are alternatives both to the materials and to the way they are obtained.

15.3.1

1,3-Propanediol

This material is the “3G” in “3GT” and is what differentiates 3GT from other polyesters.1,3-Propanediol (PDO) is a clear, colorless, odorless liquid, completely miscible with water. It is stable at normal temperatures and pressures, nonreactive with most materials at room temperature, and is not considered

5

Depth of color

4 Sorona TM 3GT Fiber

3

PET

2

1 Water boiling point 0 60

70

80

90

100

110

Dyeing temperature (°C) FIGURE 15.8 Comparison of dyeing characteristics.

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130

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a hazardous material by the United States Department of Transportation, or the Environmental Protection Agency. The CAS number for PDO is 504-63-2. The basic properties of PDO are summarized in Table 15.2, and specific analytical properties for polymer manufacture are given in Table 15.3.

15.3.2

PDO from Traditional (Chemical) Sourcesi,ii

Development of Sorona took place within a business model that requires the commercial success of a product to be established independently of environmental or sustainability considerations. Consequently, early product development was with PDO from conventional sources. Nonetheless, it was a major challenge to obtain suitable polymer-grade PDO. DuPont’s initial objective was to develop 3GT polymer for fiber applications, requiring the highest purity, consistency, and uniformity standards. DuPont worked with an established chemical producer, Degussa AG, to design, build, and qualify a brand new facility in Wesseling, Germany (see Figure 15.9) to make PDO of “polymer and fiber-grade” purity. The Degussa process consists of the following steps: 1. Oxidation of propylene to acrolein [CH2⫽CHCH3 ⫹ O2 → CH2⫽CHCHO] 2. Selective hydration to 3-hydroxypropionaldehyde (3-HPA) [CH2⫽CHCHO ⫹ H2O → OCHCH2CH2OH] 3. Catalytic hydrogenation to PDO [OCHCH2CH2OH ⫹ H2 → HO(CH2)3OH] TABLE 15.2 Basic Properties of 1,3-Propanediol (PDO) Chemical structure: HOCH2CH2CH2OH Chemical formula C3H8O2 Molecular weight 76.1 Viscosity at room temperature 52 cp Specific gravity 1.05 Boiling point 214°C Freezing point ⫺32°C Refractive index 1.4386

TABLE 15.3 Specific Analytical Properties of 1,3-Propanediol (PDO) Used for Polymer Manufacture Purity (GC) 99.9% Color (APHA, ASTM D1209) ⬍ 5 Water contents 0.05% maximum Ash content ⬍ 0.001% by weight

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FIGURE 15.9 Degussa AG facility in Wesseling, Germany, for production of PDO.

Other processes to prepare PDO are known. For example, Shell Chemical uses a two-step process: 1. Hydroformylation of ethylene oxide to 3-HPA O [H2C

CH2 + CO + H2 → OCHCH2CH2OH]

2. Catalytic hydrogenation to PDO [OCHCH2CH2OH + H2 → HO(CH2)3OH] 15.3.3

Bio-PDOiii–vi

The need for development of a biological source and process for PDO grew out of several factors: ●

●

“Polymer and fiber-grade PDO” is not easy or inexpensive to produce, even by conventional chemical means. Recent advances in biological sciences and biotechnology.

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The demonstrated efficiency of early trials of bio-PDO processes showed the potential for them to be economically competitive with established processes. Besides efficiency, the bioprocesses are generating lower levels of greenhouse gases. DuPont had made a commitment to integrated science, which includes biological science and biotechnology. The company is ideally positioned to integrate this with long-established polymer and fiber technology competency.

Initial development of biologically manufactured PDO has been with corn sugar, well-known as a plentiful and inexpensive raw material. Conversion of glucose to PDO has been known to occur in nature in two stages: first by yeast to an intermediate product, glycerol, then by bacteria to PDO (Figure 15.10). DuPont and Genencor International developed a bacterium (“biocatalyst”) to do both steps in a single fermentation stage. DuPont and Tate & Lyle, a major corn-based products company with expertise in fermentation processes, partnered to develop the manufacturing process based on this biocatalyst. The process is represented in Figure 15.11, and the facility in

FIGURE 15.10 Conversion of glucose to PDO in nature occurs in two steps.

Corn or other renewable sugar source

Fermenting engineered biocatalyst

FIGURE 15.11 Pictorial representation of process: corn to PDO.

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Separating and refining

Fiber grade PDO to polymer making unit

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Figure 15.12. The trial product was confirmed to have attributes and quality equivalent or superior to chemically produced PDO.

15.3.4

DMT/TPA

The second major ingredient, which can be either terephthalic acid or its dimethyl ester, is also used to make PET and other aromatic polyesters. Currently these materials are made from petrochemical raw ingredients, although suppliers are looking for economically attractive sustainable alternative processes.

15.3.4.1 Terephthalic Acid (TPA) The process of making TPA consists of the following steps: 1. Generation or isolation of low molecular weight aromatics from crude oil constituents by any of the following methods: a. Aromatic extraction from heavy naphtha fraction straight-run distillate b. Dehydrogenation of cycloalkanes c. Dehydroisomerization of cyclopentanes d. Dehydrocyclization of alkanes 2. Extraction of p-xylene, from xylene mixture by fractional distillation and fractional crystallization

FIGURE 15.12 Pilot scale fermentation facility of Tate & Lyle, Decatur, IL, USA.

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3. Oxidation of p-xylene in the presence of cobalt and manganese salts of heavy metal bromides to produce crude terephthalic acid (see Figure 15.13) 4. Purification of TPA via the following sequence of steps: a. Removal of acetic acid and unreacted p-xylene by evaporation b. Washing in hot water to remove other impurities c. Hydrogenation of the major remaining impurity, p-formylbenzoic acid, to p-methoxybenzoic acid. The TPA can then be separated by fractional crystallization which yields “purified terephthalic acid,” or PTA, which is 99.9% pure.

15.3.4.2 Dimethyl Terephthalate (DMT) DMT can be made in either of two ways: ● ●

Esterification reaction of TPA with methanol (see Figure 15.14) Manufacture directly from p-xylene by reaction with methanol and oxygen in the presence of cobalt catalyst (see Figure 15.15). The DMT produced from this one-step reaction must undergo a fivecolumn distillation procedure to yield material pure enough to be used in polymerization reactions.

O CH3

C

OH

[O2]

C

CH3

O

C

O

O

OH

C

OH

FIGURE 15.13 p-xylene oxidation to terephthalic acid.

OCH3

CH3OH

C O

C

OH

O

O CH3

C

OCH3

FIGURE 15.14 TPA esterification with methanol to DMT.

OCH3

CH3OH/Co O2 CH3

C O

OCH3

FIGURE 15.15 Reaction of p-xylene with methanol and oxygen to DMT.

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TPA vs. DMT

In the 1970s, DMT was favored over TPA by 3 to 1 for the manufacture of PET because it was easier to achieve the necessary purity for fiber uses. However, development of purification techniques in the 1980s have reduced the price of purified TPA so that currently the two terephthalates are used in approximately equal amounts worldwide. Poly(trimethylene terephthalate) requires an even higher standard of quality and consistency than PET, and DuPont has a strict program of certification for DMT/TPA suppliers. 15.3.6

Other Materials

Other materials besides TPA/DMT and diol that go into the manufacture of 3GT include: ●

● ●

15.4 15.4.1

Organotitanium compounds as catalyst (e.g., tetraisopropyl titanate or Tyzor® TPT, and tetrabutoxy titanium or TBOT) are preferred over other esterification catalysts containing heavy metals (see DuPont patents). White pigment (TiO2) as a delustrant Other polymer and functional additives

Chemistry and Technology of Polymerization Polymerization Processvii–x

Preparation of 3GT involves a set of basic steps, similar to other polyesters. The most notable difference is in the catalyst. Since the catalyst remains with the product until the end of the life cycle process, it was important for DuPont to eliminate heavy metal catalysts and the consequent hazards associated with product disposal and recycling. Either terephthalic acid or dimethyl terephthalate may be used for 3GT polymerization. Both processes are described below. 15.4.2

Terephthalic Acid Process

15.4.2.1 Esterification PDO, TPA, catalyst, and additives are mixed to form a slurry, then raised to an elevated temperature (but lower than that for PET) slightly above atmospheric pressure. Under these conditions, PDO and TPA react to form oligomers, mostly bis(hydroxypropyl)terephthalate, and water. The slurry contains an excess of PDO to help drive the polymerization forward, as shown in Figure 15.16. The water from the esterification process is separated, Copyright © 2005 by Taylor & Francis

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O

O C OH + HOCH2CH2CH2OH

HO C TPA

PDO Esterification and polymerization vacuum

O ( O C

O C OCH2CH2CH2 ) PTT

FIGURE 15.16 Polymerization reaction of TPA and PDO to 3GT.

stripped of other by-products, and sent to a biological wastewater treatment facility for final clean-up. Degradation products from the esterification reaction include acrolein and allyl alcohol. These would normally have to be separated and disposed of or recycled; however, DuPont has developed process refinements to minimize the amounts of these by-products in the first place.xi

15.4.2.2 Polycondensation Additional titanium catalyst, and pigment such as TiO2 if desired, are added just prior to this stage.Temperature is raised further, and pressure is reduced significantly less than atmospheric. Under these conditions, and with the help of the titanium catalyst, the oligomers react to form high molecularweight polymer. Pressure is lowered further to help remove excess PDO for recycling in the process. Polymerization is continued until the desired average molecular weight is reached. Typical number average molecular weight (Mn) for 3GT polymer for fiber use is between 22,000 and 30,000 g/mol. Melt or solution viscosity measurements are used to monitor the progress of polymerization process. Intrinsic viscosity (IV) is useful as it correlates closely with polymer molecular weight. The IV is obtained by two or more conventional viscosity measurements at different polymer concentrations, which are then extrapolated to zero concentration. The IV is defined as the limiting value of the specific viscosity/concentration ratio at zero concentration. The polymer thus obtained from polymerization is a molten, high-viscosity liquid, and is converted to a usable form. It is extruded to form polymer strands a few millimeters in diameter, quenched with cold water to solidify, and cut into pellets (also known as chips or “flake”). Copyright © 2005 by Taylor & Francis

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Titanium is the preferred catalyst for both reactions. This eliminates the need to deactivate a catalyst after esterification, simplifies the process, and avoids the environmental consequences of heavy metals. (Other polyester processes often use antimony, which is less effective for 3GT and also poses serious disposal problems.xii)

15.4.3

Dimethyl Terephthalate Processxiii–xv

15.4.3.1 Transesterification PDO, DMT, and catalyst are mixed as in the TPA process. At elevated temperature, DMT and PDO reacted to form bis(hydroxypropyl)terephthalate and methanol. The transesterification reaction is much faster and easier to manage than the esterification reaction involving TPA. The methanol generated from the ester exchange reaction is removed and refined, and may be either sold or burned to recover its energy. By-products such as acrolein and allyl alcohol in methanol are quantified. DuPont has identified process conditions that will not generate acrolein/allyl alcohol in significant concentrations. Polycondensation is similar to the polycondensation stage following esterification of TPA/PDO. The excess PDO needs to be removed from the polymer to increase the molecular weight. Various characterization techniques are used to determine polymer properties, including polymer chain endgroup concentration. As with the TPA process, titanium catalyst is recommended for both the transesterification and polycondensation steps.

15.4.3.2 Batch Process Both processes described above have been carried out in a variety of commercial batch polymerization facilities ranging in size from 2 to 5 t. In the batch process, molten polymer is discharged from the autoclave before polymerization is complete. These intermediate molecular weight polymer strands are quenched and cut into pellets. Pellets are dried, and solid phase polymerized at elevated temperature (just below melt temperature) to allow further increase in molecular weight. Solid-phase polymerization is typically carried out in a tumble dryer or in a fluidized column reactor. After achieving the desired IV, pellets are cooled and dedusted for further use. Note that after polymerization is essentially complete, a certain quantity of cyclic oligomers remains within the pellets. Equilibrium concentration of cyclic oligomer in the melt phase is around 3% by weight, but is significantly lower in the solid-phase polymerized (SPP) pellets. SPP pellets oligomer concentration as shipped is typically 0.8% or lower; however, when the chips are remelted, oligomer concentration increases to above 2% within 7 min at 265°C.xvi With sufficient time in the melt phase, cyclic oligomer concentration will increase to equilibrium level. Copyright © 2005 by Taylor & Francis

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15.4.3.3 Continuous Processxvii The polymerization process described above has been carried out in continuous-polymerization (CP) facilities of up to 10,000 t/yr and is currently used to make commercial quantities of 3GT polymer at DuPont’s Kinston, NC site. 1. PDO, DMT, catalyst, and additives are fed into an ester exchanger. 2. Reaction by-product, methanol, is separated and removed from the transesterification process. 3. The reaction products are fed to a flasher where much of the excess PDO is removed and recovered. 4. The oligomer is next fed to a prepolymerizer, where the polycondensation reaction takes place, and more PDO is recovered. 5. Polymer is sent to the finisher under high vacuum, and a specially designed finisher-agitator creates sufficient surface area to enable the last of the PDO vapor to escape, thereby further increasing the molecular weight of the polymer. 6. Polymer is extruded, water-cooled and cut into pellets, and packaged for shipping. (Note: The concentration of cyclic oligomer in CP pellets is the same as the melt equilibrium concentration of around 3%.) Unlike the batch process, CP process provides high IV polymer of better quality and consistency.The CP process is graphically shown in Figure 15.17. A photo of commercial pellet output is shown in Figure 15.18. Although the Kinston process is currently using chemical PDO, the plant is capable of shifting to bio-PDO whenever needed. Typical properties of CP polymer, as reflected in the current DuPont specification, are shown in Table 15.4.

Vacuum jets MeOH

Excess PDO

PDO DMT Catalyst

Finisher Additives

Flasher Ester exchanger

Prepolymerizer Pelletizer

FIGURE 15.17 3GT continuous polymerization process schematic.

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Sorona®

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FIGURE 15.18 3GT polymer chips being produced at DuPont’s Kinston, NC plant.

TABLE 15.4 Properties of Continuously Polymerized 3GTa Limits Property Intrinsic viscosity Color, Hunter b Color, Hunter L COOH (µeq/g) Melting point (°C) TiO2 (wt%) Moisture, ppm (as packaged) Pellet size, wt. of 50 pellets (g) Appearance Package description

Aim

Min.

Max.

Test Method

1.02 1.01 1.03 TM-0590-91 — 5 8 GEN-07 — 80 — GEN-07 — — 15 TM-1145-91 228 224 232 SP-509 0.30 0.27 0.33 TM-1090-90 — — 500 SP 525 1.6 1.4 1.8 GEN-08 No foreign substances,dust, or gross Visual particles Lined Gaylord boxes, 1475 lb (670 kg) nominal per box

a

Product: Sorona 3GT (a trademark of E. I. du Pont de Nemours and Company). Type: Semi-dull Homopolymer. Process: Continuous polymerization (CP). Specification date: March 30, 2001.

15.4.3.4 Make 3GT at a 2GT Plant? The DuPont 3GT process chemistry and technology was developed for retrofitting to existing 2GT facilities: ●

In recent years there has been significant PET overcapacity in North America and the Asia/Pacific region, and consequently there are numerous PET CP facilities standing idle.

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Retrofitting an existing facility, especially if it’s in decent shape to begin with, has significant advantages vs. building from scratch, both in capital cost and in time-to-startup.

When considering such a modification, the following are a few of the factors that must be kept in mind: ●

●

●

Storage and handling conditions for PDO are more stringent than for ethylene glycol, the corresponding ingredient for PET. A nitrogen atmosphere is usually recommended for storage of PDO, and special care must be taken to prevent contamination by even small amounts of other chemicals, especially ethylene glycol. The 3GT process occurs at a somewhat lower temperature than PET, and changes to heat transfer fluid (Dowtherm®) and operating settings are probably required. Closer control of process variables is required to preserve quality and avoid thermal degradation of the polymer.

15.4.3.5 Remelt Process As with most polymers, 3GT is shipped from production facilities in the form of pellets and delivered to customer companies where they are remelted to molten polymer and cast, spun, or otherwise processed into intermediate and end use products. Because remelt and processing performance of 3GT polymer is more sensitive to moisture and impurities, special effort must be made to ensure 3GT is clean, dry, and free of other polymers. Pellets must be dried to ⬍ 40 ppm H2O before remelting. A nitrogen environment is recommended for drying.

15.5

Polymer Shaping

Sorona 3GT polymer can be converted into a variety of products by conventional methods, a few of which are described here. Regardless of the polymer shaping method used, Sorona polymer must be dried before remelting to prevent the loss in molecular weight (IV). An oxygen-free processing environment is preferred. 15.5.1

Fibers: Apparel and Carpets

Polymer pellets are remelted, pumped under elevated temperature and pressure to spinnerets and extruded as filaments which cool and solidify, and are wound up as filament yarn on tubes. A texturing step may follow, which gives bulk, loft, and crimp to the filaments. The yarn is processed into fabric by knitting or weaving; dyeing may be either to the yarn or to the Copyright © 2005 by Taylor & Francis

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knitted or woven fabric. Carpets are made by dyeing the fiber and then tufting it into a backing layer. The advantages of Sorona polymer are summarized in Table 15.5, and described below. Spinning: Lower melt temperature than PET/Nylon 6,6 gives a number of benefits. The most obvious is lower energy costs. Additionally, “spun-dyed” yarn may be made with a greater variety of pigments and colors afforded by the lower temperature. Moreover, since the melt temperature is similar to Nylon 6, 3GT fiber can be spun on “short stack” spinning machines originally built for Nylon 6 and polypropylene bulk continuous filament (BCF) yarns.xviii–xx Most fiber is spun with a round cross section. However, other cross sections (e.g., delta, trilobal, star) used with Nylon, polyester, or polypropylene can also be applied to 3GT for desirable attributes. Draw: Less draw is required for 3GT partially oriented yarns (POY).xxi Conventional units can be used. Windup: Higher stretch and recovery makes winder setup easier for 3GT. Standard windup units used in the industry can be used for 3GT fibers. Texturing: Similar settings to PET may be used except to allow for lower Tg and higher stretch.xxii Dyeing: Disperse dyeing is effective at atmospheric boil (100°C), which makes 3GT compatible with a wide range of disperse dyes and pigments, and with a variety of dye processes. With disperse dyes, 3GT fabrics exhibit bright, vibrant colors. Fabric formation (weaving, knitting, tufting): Equipment setup must accommodate lower fiber modulus and higher stretch recovery characteristic of Sorona relative to other polyesters. Textured yarns: Sorona 3GT textured yarns exhibit higher residual shrinkage and higher crimp shrinkage than PET textured yarns. Fabric formation with textured yarn must take these differences into account in both mechanical settings and heat-setting temperature.

TABLE 15.5 Sorona 3GT Advantages in Textile Processing Polymerization Fiber quality Modifications → Unique compositions

→

Spinning Productivity Fiber design Aesthetics Quality

→

Texturing Productivity Bulk → Quality

Environment Recyclable Renewable sources for ingredient Minimized waste

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Films

Due to its relatively low melting point, 3GT can be cast into films at settings comparable to polypropylene or Nylon 6. Process optimization is required to eliminate film brittleness of cast films. Biaxially oriented films can also be prepared using 3GT. Modified 3GT polymers (e.g., copolymers with isophthalic acid) or polymer blends of 3GT (e.g., other commercially available polymers such as 2GT and 4GT) can be successfully used in films to obtain desirable property advantages and increased value. 15.5.3

Nonwovens

The suitability of Sorona polymer for various nonwoven processes is summarized in Table 15.6 and described below. Spunbonded nonwovens of 3GT are softer, more flexible, more air-permeable, and have better elastic recovery than equivalent webs of PET or polypropylene. Meltblown nonwovens applications are in filter media, thermal insulators, battery separators, oil absorbents, and laminates. 3GT can be processed like polypropylene due to close melt temperatures. The two polymers can be coprocessed and shaped to produce curly (crimped or twisted) bicomponent microfibers with benefits in filtration performance. 3GT processes are similar to PET, with advantages of softer handfeel and drapability in staple nonwovens. Feasibility in flash spun nonwovens has been demonstrated with 3GT polymer and various halogenated hydrocarbon solvents.xxiii Various bonding methods for nonwovens include thermobonding, needlepunching, hydroentanglement, chemical treatment, and chemical binder. Hydroentangled nonwoven fabrics of 3GT exhibit the softness/drape of Nylon 6 and high strength of PET, with greater elastic recovery than either. Needlepunched fabric of 3GT also exhibited greater softness/drape than PET. TABLE 15.6 Nonwovens: Process Suitability of Sorona Polymer Nonwovens Forming methods Carded Airlay Spunbonded/ meltblown Flashspun Bonding methods Chemical bonding Thermal bonding Needlepunch Hydroentanglement or hydrolacing

Stretch/ Strength Resilience Chemical Recovery Compatibility ●

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Engineering Components

All conventional thermoplastic formation techniques can be used with 3GT: injection molding, pressure molding, blow molding, casting, rotary molding, etc., with attention paid to minimize melt time. Otherwise, molding processes are comparable to Nylon 6, PBT, PET, or polypropylene.xxiv 3GT molding conditions are similar to that of PBT. As with many other plastics, the addition of glass fiber reinforcement significantly increases tensile strength, modulus of elasticity, bending strength and modulus, impact strength, and heat deflection temperature in molded parts of 3GT. 3GT provides tensile strength and stiffness higher than that of PBT. A DuPont proprietary nucleated 3GT polymer composition is available for fast mold cycle times and enhanced process. In addition to glass fibers, other natural fibers can be incorporated in 3GT to take advantage of lower process temperature and to increase total bioderived materials content in the final product.

15.5.5

Monofilaments

Monofilaments are made by extrusion similar to filament textile yarns, although the single filament is many times heavier than a textile filament. 3GT may offer benefits in various end uses, including fishing lines, fishnets, and brush bristles (e.g., toothbrush).xxv

15.6

Applications and End Uses

Sorona polymer exhibits mechanical properties equal to or in some cases better than Nylons (e.g., Nylon 6, Nylon 6,6) in combination with chemical properties equal to or better than PET. This combination of properties gives 3GT numerous advantages in a wide variety of applications, a few of which are summarized in Table 15.7, and listed below.

15.6.1

Apparel

The most significant advantages of Sorona 3GT fiber for apparel are softness and natural hand, printability, and easy dyeability. These and other properties, and their suitability for various apparel applications, are summarized in Table 15.8.

15.6.2

Bicomponents

T-400 is DuPont Textile and Interior’s (DTI; this division is recently sold to Koch Industries and called INVISTA) bicomponent fiber made of 3GT and Copyright © 2005 by Taylor & Francis

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TABLE 15.7 Market Advantages of Sorona Polymer Market Apparel

Upholstery Resins

Others

Market Segments

Benefits

Ready-to-wear; performance garments; hosiery and intimate apparel, linings, denim Automotive; home furnishings Sealable closures; connectors; extrusion coatings; blister packs; liquid containers Medical garments; monofilaments, brushes

Stretch/recovery; soft touch; dye and print capability Same as Apparel, plus softness, stain resistance, and resiliency Toughness and flexibility; puncture resistance; barrier (O2 and H2O) Soft resilient garments; higher hot/ wet strength

TABLE 15.8 Sorona Advantages for Apparel Apparel Ready-to-wear Intimate Hosiery Lining Socks Activewear Swimwear Nonwovens Fleece Sweater

Stretch/ Recovery

Softness

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Chlorine Resistance

UV Resistance

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PET for which the Federal Trade Commission has recently granted a new generic name, elasterell-p.5 T-400 adds stretch and recovery performance to garments such as jeans, trousers, shirts, and white athletic socks previously possible only by adding elastane. When fabrics pass DTI certification tests, those fabrics can be branded with the Lycra® trademark. T-400 is a direct-use fiber that eliminates added steps such as air-jet texturing or core spinning that are necessary with elastane. It uses an environmentally friendly meltspinning process that does not require the solvents needed for elastane spinning. T-400 stretch fabrics can be made with mill processes similar to those of PET; for example, obtaining a variety of stretch denim fabrics is possible because the chemical stability of T-400 allows special treatments such as bleaching, stone washing, and enzyme treatments. Garment durability and easy-care properties are equivalent to PET. T-400’s stretch/recovery and high chlorine resistance, compared to that of elastane,xxvi,xxvii make it an ideal candidate for competitive swimsuits. Copyright © 2005 by Taylor & Francis

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Fiber Blends

3GT fibers can be blended with other natural (e.g., cotton, wool) or synthetic (e.g., PET, acrylics) fibers for enhanced softness, stretch recovery, and other functional attributes. 15.6.4

Floor Coverings

Sorona is an excellent polymer for carpeting applications. It can be made in a variety of colors and styles with good dye uniformity. Sorona offers superior bulk, resilience, texture retention, stain resistance, easy dry, and a softer feel. Pigmented yarns with deep vibrant colors can be used to create any styles customers wish to have. They are resistant to fading in the presence of UV or chlorine, and they have low electrostatic properties. 3GT can be easily prepared to manufacture rugs and other floor covering materials and are as easy to recycle as PET polyester. 15.6.5

Nonwovens

The advantages of Sorona 3GT for apparel nonwovens are the same as for fabrics: softness and natural hand, drapability, dyeability, stretch/recovery characteristics, easy printability, and easy care. Examples of advantages for other nonwoven applications are filtration efficiency, heat resistance, and air permeability. 15.6.6

Engineering Markets

The advantages of Sorona 3GT for films and engineering applications are summarized in Table 15.9. As an example, films for food packaging must often be built of multiple layers of different materials in order to provide all the properties required for that application. Typical required properties may include oxygen barrier, water vapor barrier, UV shield, puncture and tear resistance, printability, and heat sealability. Films of 3GT or modified 3GT have excellent barrier properties with respect to both oxygen and water vapor, and are printable TABLE 15.9 Sorona Advantages for Films and Engineering Products

Resins Packaging/ films Extruded/ molded parts

Barrier Tensile ToughProcess- Properties Reinforcement Puncture Strength ness Flexibility ability (O2, H2O) Capability Resistance

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and heat sealable. This means that in many food packaging applications a single 3GT or modified 3GT layer may take the place of several layers of presently manufactured laminated films.xxviii 15.6.7

Other Markets

There are several other markets where 3GT will have significant value. Readers are encouraged to look into the latest news releases from DuPont.

15.7

Creating the Future

15.7.1

Direction and Vision

The development and commercialization of biobased Sorona 3GT is consistent with DuPont’s direction: from explosives in its first century, to chemicals and energy in its second century, and to sustainable growth in the third century and beyond, through use of the following: ●

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15.7.2

Knowledge intensity: increased knowledge component of our products Productivity improvement: reduced capital, environmental footprint, energy/resource usage, waste, emissions, and social cost Integrated science: historically, chemistry and physics; in the future, chemistry, biology, physics, and information science. Realization

Carrying these principles forward, by DuPont and others, will lead to the following: ● ●

●

15.8

Greater use of renewable biological resources such as crop vegetation Further development of biological manufacturing processes, e.g., fermentation (“biorefinery”) for intermediates and end products Products with integrated information technology, such as “smart” garments that automatically sense a need for insulation vs. cooling/ventilation and respond accordingly.

Conclusion

Sorona polymer provides functionality and attributes different from that of other polymeric materials in the market today. With Sorona polymer, either Copyright © 2005 by Taylor & Francis

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Re

Bio

Functionality

ble

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cla

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Biodegradable

FIGURE 15.19 DuPont’s product strategy for sustainable development and environmental responsibility.

alone or in conjunction with other polymers, a variety of functionalities can be developed. From the 3G-based platform other similar polymers can also be developed to satisfy growing needs. Figure 15.19 represents the combination of attributes for sustainable growth and environmental responsibility.

Acknowledgments Assistance from the Sorona R&D and Business team is greatly appreciated. Also I like to thank Brian D. Hanson for helpful suggestions in preparing this chapter.

References 1. Courtesy of Dr. Kathy Singfield, St. Mary’s University, Halifax, NS. 2. Desborough, J., Hall, I.H., and Neisser, J.Z., The structure of poly(trimethylene terephthalate), Polymer, 20, 545–552, 1979. 3. Ward, M. and Wilding, M.A., The mechanical properties and structure of poly(mmethylene terephthalate) fibers, J. Polym. Sci., Polym. Physics ed., 14, 263–274, 1976. 4. Jakeways, R. et al., Crystal deformation in aromatic polyesters, J. Polym. Sci., Polymer Physics ed., 13, 799–813, 1975. 5. Stretch fibers and fabrics, Performance Apparel Markets, 2nd 2002, pp. 13–44.

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Endnotes: Patent References i. ii. iii. iv. v. vi. vii. viii. ix. x. xi. xii. xiii. xiv. xv. xvi. xvii. xviii. xix. xx. xxi. xxii. xxiii. xxiv. xxv. xxvi. xxvii. xxviii.

US Patent 6,140,546 Process for the production of 1,3-propanediol. US Patent 6,284,930 Process for the production of 3-hydroxypropanol. EP 1204755 Process for the biological production of 1,3-propanediol. EP 1076708 Method of production of 1,3-propanediol by recombinant organisms comprising genes for vitamin B-12 transport. WO 0112833 Process for biological production of 1,3-propanediol with high titer. WO 0111070 1,3-propanediol and polymer derivatives from a fermentable carbon source. US 6,331,264 Low emission polymer compositions. US 6,325,945 Process of making a polyester or polyol. US 6,281,325 Preparation of Poly(trimethylene terephthalate). WO 0158980 Continuous process for producing Poly(trimethylene terephthalate). US 6,277,289, also WO 0102305; EP 1200358 Treatment of aqueous aldehyde waste streams. US 6,255,442 Esterification Process. US 5,840,957, also EP 1064247 Transesterification process using Lanthanum compound catalyst. US 6,350,895 Transesterification process using Y and Sm compound catalysts. WO 0158982 Continuous process for producing bis(3-hydroxypropyl terephthalate). US 6,335,421 also EP 1206497 Preparation of PTT with low level of di(1,3propane glycol). US 6,353,062 also EP 1254188 Continuous process for production of Poly(trimethylene terephthalate). US 6,458,455 Poly(trimethylene terephthalate) tetrachannel cross-section staple fiber. US 6,383,632 Fine denier yarn from Poly(trimethylene terephthalate). EP 1183409 Poly(trimethylene terephthalate) yarns. US 6,287,688 Partially oriented Poly(trimethylene terephthalate) yarns. US 6,333,106 Draw textured Poly(trimethylene terephthalate) yarn. US 6,458,304 Flash spinning process and solutions of polyester. US 6,245,844 also EP 1114095 Nucleating agent for polyesters. WO 99 05936 (by Unilever) Monofilaments of PTT. WO 0153573 2GT/3GT bicomponent spinning. US App. 2002002543 A1, also EP 1248870 Method for high speed spinning of bicomponent fibers. US App. 20020012807 A1 (1/31/2002) Low temperature heat sealable polyester film and method for producing same.

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16 Polylactic Acid Technology

David E. Henton, Patrick Gruber, Jim Lunt, and Jed Randall

CONTENTS 16.1 Introduction 16.2 Lactic Acid 16.3 Life Cycle Analysis 16.4 Polymerization of Lactide 16.5 PLA Physical Properties 16.5.1 Linear Optical Copolymer Structures and Blends 16.5.1.1 Density 16.5.1.2 Glass Transition and the Amorphous Phase 16.5.2 Rheology 16.5.2.1 Melt Rheology of Linear PLA 16.5.2.2 Melt Stability 16.5.2.3 Solution Properties 16.5.2.4 Branching 16.5.2.5 Solid-State Viscoelastic Properties 16.5.3 Crystallinity 16.5.3.1 Morphology 16.5.3.2 Degree of Crystallinity 16.5.3.3 Crystallization Kinetics 16.5.3.4 Stereocomplex 16.5.3.5 Stress-Induced Crystallization 16.5.4 Degradation and Hydrolysis 16.5.4.1 Hydrolysis 16.5.4.2 Hydrolysis of Solid Samples Suspended in Aqueous Media 16.5.4.3 Solution Hydrolysis 16.5.4.4 Hydrolysis of Samples Exposed to Humidity 16.5.5 Enzymatic Degradation 16.6 Applications and Performance 16.7 Summary

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Acknowledgments Reference

ABSTRACT Poly(lactic acid) (PLA) is the first commodity polymer produced from annually renewable resources. Some of the environmental benefits of PLA, such as low energy to produce and reduced green house gas production, are presented and opportunities for the future are given. This chapter on PLA reviews the chemistry and material properties of the polymer and its copolymers. The current technology for the production of lactic acid, lactide, and polymer are presented. Applications for fabricated articles such as films and fibers are also discussed.

16.1

Introduction

Polylactic acid (PLA) is a rigid thermoplastic polymer that can be semicrystalline or totally amorphous, depending on the stereopurity of the polymer backbone. L(
)-lactic acid (2-hydroxy propionic acid) is the natural and most common form of the acid, but D()-lactic acid can also be produced by microorganisms or through racemization and this “impurity” acts much like comonomers in other polymers such as polyethylene terephthalate (PET) or polyethylene (PE). In PET, diethylene glycol or isophthalic acid is copolymerized into the backbone at low levels (1–10%) to control the rate of crystallization. In the same way, D-lactic acid units are incorporated into L-PLA to optimize the crystallization kinetics for specific fabrication processes and applications. PLA is a unique polymer that in many ways behaves like PET, but also performs a lot like polypropylene (PP), a polyolefin. Ultimately it may be the polymer with the broadest range of applications because of its ability to be stress crystallized, thermally crystallized, impact modified, filled, copolymerized, and processed in most polymer processing equipment. It can be formed into transparent films, fibers, or injection molded into blowmoldable preforms for bottles, like PET. PLA also has excellent organoleptic characteristics and is excellent for food contact and related packaging applications. In spite of this unique combination of characteristics, the commercial viability has historically been limited by high production costs (greater than $2/lb). Until now PLA has enjoyed little success in replacing petroleumbased plastics in commodity applications, with most initial uses limited to biomedical applications such as sutures.1 PLA is not new to the world of polymers. Carothers2 investigated the production of PLA from the cyclic dimer (lactide) of lactic acid as early as 1932. Even before that, low molecular weight dimers and oligomers were detected

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when water was removed from an aqueous solution of lactic acid.3 The announcement of the formation of a new company, Cargill Dow LLC, in 1997 brought two large companies together to focus on the production and marketing of PLA with the intention of significantly reducing the cost of production and making PLA a large-volume plastic.4 In today’s world of green chemistry and concern for the environment, PLA has additional drivers that make it unique in the marketplace. The starting material for the final polymer, lactic acid, is made by a fermentation process using 100% annually renewable resources. The polymer will also rapidly degrade in the environment and the by-products are of very low toxicity, eventually being converted to carbon dioxide and water. PLA can be prepared by both direct condensation of lactic acid and by the ring-opening polymerization of the cyclic lactide dimer, as shown in Figure 16.1. Because the direct condensation route is an equilibrium reaction, difficulties of removing trace amounts of water in the late stages of polymerization generally limit the ultimate molecular weight achievable by this approach. Most work has focused on the ring-opening polymerization of lactide, although other approaches, such as azeotropic distillation to drive the removal of water in the direct esterification process, have been evaluated.5 Cargill Dow LLC has developed a patented, low-cost continuous process for the production of lactic acid-based polymers.6 The process combines the substantial environmental and economic benefits of synthesizing both lactide and PLA in the melt rather than in solution and, for the first time, provides a commercially viable biodegradable commodity polymer made from renewable resources. The process starts with lactic acid produced by fermentation of dextrose, followed by a continuous condensation reaction of aqueous lactic acid to produce low molecular weight PLA prepolymer (Figure 16.2). Next, the low molecular weight oligomers are converted into a mixture of lactide stereoisomers using a catalyst to enhance the rate and selectivity of the

H2O

O

O O

HO

OH H

H

CH3

L-PLA

O H

L-Lactic acid

O H2O

CH3

CH3

O H O L-Lactide

FIGURE 16.1 Polymerization routes to polylactic acid.

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n CH3

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O OH

HO

O

HO

O Lactic acid

OH

O n

O

O High molecular weight PLA

Condensation −H2O

Ring-opening polymerization O O

O

HO

O n

O

O

OH

O

Depolymerization O

O

Prepolymer Mn ~ 5000 FIGURE 16.2 Schematic of PLA production via prepolymer and lactide.

Dextrose

Corn

Unconverted monomer PLA polymer

Lactide formation

Prepolymer

Meso lactide Polymerization Low D lactide

Distillation

Lactic acid

Distillation

Fermentation

FIGURE 16.3 Nonsolvent process to prepare polylactic acid.

intramolecular cyclization reaction. The molten lactide mixture is then purified by vacuum distillation. Finally, PLA high polymer is produced using an organo tin-catalyzed, ring-opening lactide polymerization in the melt, completely eliminating the use of costly and environmentally unfriendly solvents. After the polymerization is complete, any remaining monomer is removed under vacuum and recycled to the beginning of the process (Figure 16.3). Copyright © 2005 by Taylor & Francis

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This process is currently in operation in a recently constructed 300 million lb/yr commercial-scale PLA plant in Blair, Nebraska, USA. Cargill Dow LLC has also announced the construction of an additional plant in Europe in the near future.7 PLA has been extensively studied by many researchers and reviewed in detail in several recent publications.8 This chapter will not review all of the growing volume of information on PLA, but will focus on the key elements of technology that will change this polymer from a specialty material to a large-volume commodity plastic.

16.2

Lactic Acid

Lactic acid (2-hydroxypropionic acid) is the basic building block for PLA. It is a highly water-soluble, three-carbon chiral acid that is naturally occurring and is most commonly found in the L(
) form. It is used as an acidulant in foods, as a building block for biodegradable polymers (PLA), and is converted to esters and used as a green solvent for metal cleaning, paints, and coatings. PURAC has been the world’s largest producer of lactic acid, but the recent start-up of the Cargill Dow lactic acid plant, with a capacity of 400,000,000 lb, exceeds that of all producers combined. There are different grades of lactic acid, each varying in purity depending on their end use application. The lowest to highest purity is technical grade, edible grade, pharmaceutical grade, and analytical grade. The lower grades contain sulfates, metals, amino acids, and various carbohydrates. With the need for polymerization-grade lactic acid it has become clear that even the highest purity grade, analytical grade, is not pure enough. The slight traces of carbohydrates and amino acids remaining in analytical grade cause color in the process and even parts per million traces of cations such as Na lead to racemization of the acid, resulting in the less desired D() lactic acid. A good summary of lactic acid chemistry is found in the book by Holten,9 Lactic Acid, Properties and Chemistry of Lactic Acid and Derivatives. An older, but still informative review of lactic acid production via fermentation is found in a chapter on lactic acid by H. Schopmeyer.10 The major issues of fermentation, purification, and a history of lactic acid growth in the United States are reviewed. It also contains a good literature review up to that time on lactic acid. A more recent review11 (1976) of lactic acid processes by Cox and Macbean looked closely at the details of distillation, liquid extraction, esterification, salt processes, electrodialysis, thermal methods, and ion exchange approaches to providing lactic acid from fermentation of carbohydrates. There have been a large number of proposed routes and patents issued claiming to have improved processes to isolate lactic acid from fermentation broth, but most are combinations or improvements on these basic concepts. Copyright © 2005 by Taylor & Francis

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The main stages of lactic acid production consist of (1) fermentation, (2) cell mass and protein removal, (3) recovery and purification of lactic acid, (4) concentration of lactic acid, and (5) color removal. Research and improvements in lactic acid production have been centered around each of these steps, with continued improvement in the cost and efficiency of each. Better microorganisms for the fermentation that are more robust to temperature, pH, and the organic products they produce have evolved in recent years. Engineering improvements have increased the size and efficiency of the plants, while improved technology for water removal (multiple effect evaporators and membranes) has lowered the cost for production. By far the greatest developments and diversity of approaches have been in the area of separation and purification. Some of the separation and purification processes and their proposed key features are listed in Table 16.1. Most processes to produce lactic acid rely on bacterial fermentation of dextrose in aqueous slurry with a continual addition of a base, such as calcium hydroxide, to maintain a neutral pH. Bacteria are generally quite sensitive to pH and are only productive in a narrow pH range. This sensitivity to pH is demonstrated in Table 16.2 for the production of acetic acid by Clostridium thermoaceticum.12 Generally, deviation of pH by more than a couple of pH units totally inhibits bacterial fermentation. For the production of neutral species such as ethanol, this is not an issue. However, the production of organic acids such as acetic and lactic acid shifts the pH as the acids are produced. To maintain the optimum pH, bases are continuously added. At a pH of about 7 (neutral pH), essentially all of the lactic acid is ionized. Isolation of the lactic acid and removal of water requires that a strong mineral acid, such as sulfuric acid, be added to protonate the lactate salt, while at the same time producing a mole of salt for every two moles of acid produced. On a weight basis, about a pound of salt per pound of acid is produced. This significant amount of waste salt contributes to the cost of the lactic acid and is undesirable from a sustainability point of view. The process of the near future will contain an organism that is tolerant of temperature and acidic conditions and be combined with lactic acid removal technology that does not result in the generation of waste salts. This process will be cost- advantaged and beneficial to the environment.

16.3

Life Cycle Analysis

In recent years, products, processes, and technology have been judged for their overall sustainability. Those that use large quantities of energy, deplete natural resources, pollute the environment, or emit greenhouse gases are viewed as less sustainable. Green chemistry is the use of chemistry for pollution prevention or the production of materials that fulfill the definition of sustainability. More specifically, green chemistry is the design of chemical products and processes Copyright © 2005 by Taylor & Francis

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Feature Electrodialysis

Lactic acid is continuously removed from the fermentation or acidified broth by preferential partitioning into a solvent.

Ion exchange (acidification)

The lactate salt is acidified by a strong acid ion exchange resin.

Ion exchange (purification)

Lactic acid is removed from the aqueous solution by complexing with an amino containing resin.

Distillation Lactic acid is separated from less volatile Copyright © 2005 by Taylor & Francis components by vacuum steam distillation.

Insoluble salt processes

Esterification

The fermentation or purification process is run at a concentration that exceeds the solubility of the lactate salt (e.g., CaSO4), which is isolated and later acidified. Lactate esters are prepared and the volatile esters are distilled.

1. Distillation and separation of esters gives high-quality product. 2. Requires reconversion to acid.

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Liquid extraction

1. Does not require acidification of fermentation. 2. Energy cost and capital. 1. Higher productivity because one can maintain low acid level in fermenter. 2. Fouling of the membrane. 3. Requires acidic pH stable organism. 1. Suitable for continuous process and provides efficient removal from many non-acidic impurities. 2. High cost of capital. 3. Solvent loss costs. 1. Eliminates the need to add a strong acid to the fermentation. 2. Cost of resin and issues of resin regeneration. 1. This is the solid equivalent of t-amine extraction technology without the solvent loss issues. 2. Regeneration of the resin. 3. Cost and availability of the resin. 1. Lactic acid can be steam distilled. 2. Significant purification must be done prior to distillation. 3. Depending on conditions, some degradation and oligomerization can occur. 1. Simple process that utilizes low-cost capital. 2. The crystallization of CaSO4 occludes impurities and results in relatively impure acid.

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Reverse osmosis

Can be used to continuously remove lactic acid (lactate ions) through a membrane driven by electrical potential. Lactic acid is continuously removed through a membrane.

Advantage/Disadvantage

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TABLE 16.2 Fermentation Performance of Clostridium Thermoaceticum vs. pH in Acetic Acid Production pH Productivity (g/hr) Acetate –g/l (as acid) Yield (%)

6.8 0.8 45 85–90

5.5 0.6 16 77

that reduce or eliminate the use and generation of hazardous substances. In a production supply chain, green products and processes integrate engineering, chemistry, and environmental issues. The supply chain evaluation incorporates the energy, wastes, and emissions for operations during the production and transportation of all raw materials throughout the entire process, from production of all raw materials to disposal of the final articles. An excellent method to quantify the sustainability of a product or process is to do a life cycle analysis (LCA). LCA looks at all factors from raw materials, transportation, product manufacture, end use, to disposal. The energy, air and water pollution, as well as the hazards of the chemicals are assessed and judged against alternate products or solutions. Each step from cradle to grave is quantified and the total impact on the environment in each of the areas is thus known. Cargill Dow’s process embodies the well-known Principles of Green Chemistry by (1) preventing pollution at the source through the use of a natural fermentation process to produce lactic acid, (2) substituting annually renewable materials for petroleum-based feedstock, (3) eliminating the use of solvents or other hazardous materials, (4) completely recycling product and by-product streams, and (5) efficiently using catalysts to reduce energy consumption and improve yield. In addition, NatureWorksTM PLA products can be either recycled or composted after use. PLA, as produced by Cargill Dow LLC, uses from 20 to 50% less fossil fuel than conventional plastic resins (Figure 16.4), meaning that up to 2 to 5 times more PLA can be produced with a given amount of fossil fuel than petrochemical-derived plastics. The ability to produce the polymer on a favorable cost/performance basis is critical to realizing these human health and environmental benefits. This has been achieved through efficient use of raw materials, reduced energy utilization, and elimination of solvents. PLA is also a low-impact, greenhouse gas polymer because the CO2 generated during PLA biodegradation is balanced by an equal amount taken from the atmosphere during the growth of plant feedstocks. In contrast, petrochemical-based polymers contribute to volatile organic compound (VOC) emissions and CO2 generation when incinerated. LCA calculations indicate that PLA has a greenhouse gas emission rate of about 1600 kg CO2/metric ton, while polypropylene, polystyrene, PET, and nylon have greenhouse gas values of 1850, 2740, 4140, and 7150 kg CO2/metric ton, respectively.13 Figure 16.5 shows the relative comparison of greenhouse gas Copyright © 2005 by Taylor & Francis

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142

140

MJ/kg

120 91

92

100

87 77

80

76 57

60 34

40

5

20

te r

m

5 er ng Lo

PL

A

ye

ar ye

or

A PL

am

ar

1

. ph

PP T

C

PE

el

N

lo

G

PP

S

e TM ph

an

IP S H

yl

on

66

0

FIGURE 16.4 MJ/kg of energy required to produce various polymers (From Gruber, P., Keynote address, Massachusetts Green Chemistry Symposium, Amherst, MA, 2001. With permission.)

14000

13030

12000

10000

8000

6870 6000

4143 4000

2737

2561 1852

2000

1820

−81

0 Nylon 66

HIPS

CellophaneTM GPPS

PP

PETAM

−1500

PLA year 1 PLA year 5 PLA long term

−2000

FIGURE 16.5 Greenhouse gas emissions (kg/ton) for production of various polymers. Long-term emissions for PLA are based on utilization of biomass for the production of lactic acid.

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contribution to the environment for common plastic products. Longer term, as PLA is produced from field wastes or other biomass, PLA can become a CO2 sink and actually contribute to a net reduction in greenhouse gases. Although problematic in the past, recycling can offer an environmentally attractive disposal option for plastics. However, recycled plastics are typically contaminated with other incompatible materials and thus have reduced properties. Consequently, recycled plastics are often relegated to lowperformance/low-value applications. In contrast, PLA articles can be readily hydrolyzed with water to form lactic acid, which is then purified and polymerized to remake prime polymer. PLA can also be naturally composted into humus, carbon dioxide, and water, with complete degradation occurring within only a few weeks under typical compost conditions. However, polymer hydrolysis and monomer recovery offer the preferred method for PLA recycling.

Polymerization of Lactide

16.4

Many catalyst systems have been evaluated for the polymerization of lactide including complexes of aluminum, zinc, tin, and lanthanides. Even strong bases such as metal alkoxides have been used with some success. Depending on the catalyst system and reaction conditions, almost all conceivable mechanisms (cationic,14 anionic,15 coordination,16 etc.) have been proposed to explain the kinetics, side reactions, and nature of the end groups observed in lactide polymerization. Tin compounds, especially tin(II) bis-2-ethylhexanoic acid (tin octoate), are preferred for the bulk polymerization of lactide due to their solubility in molten lactide, high catalytic activity, and low rate of racemization of the polymer. Conversions of  90% and less than 1% racemization can be obtained while providing polymer with high molecular weight. The polymerization of lactide using tin octoate is generally thought to occur via a coordination-insertion mechanism16 with ring opening of the lactide to add two lactyl units (a single lactide unit) to the growing end of the polymer chain shown schematically in Figure 16.6. The tin catalyst facilitates the

O O

R-OH O O

O

O

O

O

O

O

O

Sn(Oct)2 O

H O Sn R (Oct)2

O O

O

O O Sn R

H

O

H RO Sn(Oct)2

O

O O

H

Sn(Oct)2

RO

(Oct)2

FIGURE 16.6 Generalized coordination-insertion chain growth mechanism of lactide to PLA: R, growing polymer chain.

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polymerization, but hydroxyl or other nucleophilic species are the actual initiators. There is generally several hundred parts per million of hydroxyl impurities in the lactide from water, lactic acid, and linear dimers and trimers. High molecular weight polymer, good reaction rate, and low levels of racemization are obtained with tin octoate-catalyzed polymerization of lactide. Typical conditions for polymerization are 180°–210°C, tin octoate concentrations of 100–1000 ppm, and 2–5 h to reach 95% conversion. The polymerization is first order in both catalyst and lactide. Frequently hydroxyl-containing initiators such as 1-octanol are used to both control molecular weight and accelerate the reaction. Copolymers of lactide with other cyclic monomers such as caprolactone17 can be prepared using similar reaction conditions (Figure 16.7). These monomers can be used to prepare random copolymers or block polymers because of the end growth polymerization mechanism. Cyclic carbonates, epoxides, and morpoline diones have also been copolymerized with lactide. In addition to the work done on catalysts and comonomers, a significant amount of work has been done designing the optimum polymerization process from a cost and versatility perspective. Most of this work has used lactide dimer as the starting point. Batch polymerization as well as continuous processes can be used. Continuous processes (Figure 16.3) have a cost and productivity advantage and thus are the focus of most work. Stirred tank or pipe reactors have been evaluated alone as well as in combination. Because of the low energy of the ring-opening polymerization and the potential to obtain high rates of polymerization, melt extruders have been extensively evaluated as reactors to produce PLA. A number of groups have reported or patented specific aspects of the use of extruders or combinations of several reactor concepts to produce PLA.18–31 DuPont19 recognized the importance of high rates of lactide polymerization and the economic viability of using extruders. They investigated the issues surrounding hydroxyl initiators, catalysts, and acidic impurities. DuPont’s patent19 (US 5,310,599) provides examples showing the dramatic (negative) effect of acidic impurities, such as the linear dimer (DP2) on the rate of lactide polymerization. The acid impurities coordinate with the tin catalyst and render it ineffective as a ring-opening polymerization site. As an example, 389,000 MW PLA can be made with a 5-min residence time (6000:1 O O

O O

O

O

Sn(Oct)2

O Lactide

ε-Caprolactone

FIGURE 16.7 Copolymerization of lactide and caprolactone.

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O

O

+

On

m

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monomer: Sn octoate) at 180oC in the absence of acidic impurities, and 98% conversion is obtained. The conversion in 5 min drops to 50% at an impurity:catalyst ratio of 2:1 and to 12% in 5 min at a 6:1 ratio. The rate of polymerization of lactide can be accelerated by the use of hydroxyl initiators such as butanol or by higher catalyst loading. However, increased initiator levels will reduce the molecular weight while more catalyst will reduce the thermal stability and increase the color of the PLA.

16.5

PLA Physical Properties

Several excellent reviews have been written which include details on the properties and characteristics of PLA.32–36 A discussion of the variety of structures and their thermal and mechanical behavior follows. The physical characteristics of high molecular weight PLA are to a great extent dependent on its transition temperatures for common qualities such as density, heat capacity, and mechanical and rheological properties. In the solid state, PLA can be either amorphous or semicrystalline, depending on the stereochemistry and thermal history. For amorphous PLAs, the glass transition (Tg) determines the upper use temperature for most commercial applications. For semicrystalline PLAs, both the Tg (~58°C) and melting point (Tm), 130°–230°C (depending on structure) are important for determining the use temperatures across various applications. Both of these transitions, Tg and Tm, are strongly affected by overall optical composition, primary structure, thermal history, and molecular weight. Above Tg amorphous PLAs transition from glassy to rubbery and will behave as a viscous fluid upon further heating. Below Tg, PLA behaves as a glass with the ability to creep until cooled to its β transition temperature of approximately
45°C. Below this temperature PLA will only behave as a brittle polymer.

16.5.1

Linear Optical Copolymer Structures and Blends

Having two optical arrangements for lactic acid (L-lactic acid, D-lactic acid), and three optical arrangements for lactide (L-lactide, D-lactide, meso-lactide), the variety of primary structures available for PLA is substantial. In addition, many other monomers have been copolymerized with lactic acid and/or lactide, the subject of which will not be covered in this chapter. Techniques for measuring PLA stereosequences by NMR have been published by Thakur et al.37 Containing the discussion to strictly PLA linear optical copolymers, there are several notable primary structures which lead to a variety of highly ordered crystalline structures. The simplest is that of the isotactic homopolymer Copyright © 2005 by Taylor & Francis

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poly(L-lactide) (PLLA) or its opposite enantiomer poly(D-lactide) (PDLA) where all of the monomers in the chain are of the same optical composition. A greater amount of research has been done on PLLA due to the fact that the commercial production of lactic acid has been predominantly L-lactic acid. The crystal structure of PLLA homopolymer exists in α and β forms. A more detailed description of the crystal structures and unit cell parameters follows. The most common commercial polymers of PLA are optical copolymers of predominantly L-lactide, with small amounts of D- and meso-lactides, made through bulk polymerization with tin octoate catalyst via ring-opening polymerization (ROP). While these copolymers are generally described as random, there is evidence of some degree of stereoselectivity. Selectivity of tin octoate is discussed by Thakur et al.38 Condensation polymerization of mostly L-lactic acid with small amounts of D-lactic acid, polymerized in solution, has also been used. The optical comonomers introduce “kinks” in PLA’s natural helical conformation and “defects” in the crystal arrangement, which results in depression of the melting point, reduction in the level of attainable crystallinity, and reduction in the rate of crystallization. Optical purity (OP) is a common nomenclature to describe polymers of this variety. As OP decreases, crystallization eventually becomes impossible and the polymer is amorphous (occurring about when OP  0.78). For the purposes of this chapter, the designation a-PLA will be used for PLAs made with insufficient optical purity to crystallize, and c-PLA will be used to designate PLAs that are crystallizable. Stereoselective catalysis has been used to develop optical copolymers having a highly tapered concentration of optical purity (OP) along the chain.39 Random copolymers made from meso-lactide result in an atactic primary structure referred to as poly(meso-lactide) and are amorphous. Random optical copolymers made from equimolar amounts of D-lactide and L-lactide are commonly referred to as PDLLA or poly(rac-lactide). PDLLA is also essentially atactic, but the primary structure is segregated into optical doublets of the lactyl group, and it is also amorphous. Ovitt and Coates40 have made two novel stereoregular primary structures using chiral catalysts. The first was syndiotactic PLA, made by ROP of mesolactide with a chiral aluminum alkoxide catalyst, which selectively opened the L-lactyl side of the meso-lactide molecule and resulted in alternating stereocenters along the growing chain. This can be visualized as an optical headto-tail addition and results in an –LDLDLD- arrangement of lactyl stereocenters along the chain. In this case, optical purity (OP) will refer to the fraction of stereoregular syndiotactic sequences. The material was shown to crystallize with a melting point of about 152°C with an OP of 0.96. The second structure, referred to as di-syndiotactic PLA or heterotactic PLA, was made by ROP of meso-lactide with a racemic catalyst that would add mesolactide. However, the reacted lactide stereocenter would add in an alternating fashion, in what can be visualized as an optical head-to-head addition resulting in sequences of –LLDDLLDD- along the chain. The heterotactic PLAs produced did not crystallize, and had a Tg of about 40°C. Ovitt also showed that Copyright © 2005 by Taylor & Francis

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racemic catalysts can be used to polymerize rac-lactide into stereoblock copolymers that have long sequences of poly(L-lactide) and poly(D-lactide) blocks, referred to as isotactic stereoblock PLA.41 Similar work had been performed earlier by Yui et al.42 With sufficiently long isotactic run lengths, these polymers are able to crystallize into the stereocomplex crystal arrangement. Blends of PLA optical copolymers have also offered a variety of new properties. The most notable is that of the stereocomplex PLA, which is made by solvent or melt blending PLLA and PDLA where the polymers undergo stereocomplexation or racemic crystallization. This phenomenon has been an area of intense study since first described by Ikada et al.43 A more detailed description of stereocomplex will follow. A second blend receiving attention is the blend of crystallizable PLA (c-PLA) with noncrystallizable PLA (a-PLA). Key melting point and glass transition data for PLA structures and blends are listed in Table 16.3.

16.5.1.1 Density Volumetric measurements show that PLA and lactide isomers in the liquid state have approximately the same density as a function of temperature, with a thermal coefficient of expansion α1  7.4  10
4°C
1. The melt density can be approximated using Equation (16.1) by Witzke:32

ρ150°C ρ (g/cm3)   1α1(T(°C)
150)

(16.1)

where ρ150  1.1452. The solid amorphous PLA density is ~1.25 g/cm3. The purely crystalline PLLA density32 is estimated to be 1.37–1.49 g/cm3.

16.5.1.2 Glass Transition and the Amorphous Phase The PLA glass transition (Tg) increases with Mn and OP. Orientation and physical aging also affect the Tg. The infinite molecular weight Tg for PLLA, poly(meso-lactide) and PDLLA were estimated to be 61°, 46°, and 53°C, respectively, by Witzke,32 who developed an equation describing the Tg of unoriented poly(L-lactide-co-meso-lactide), Equation (16.2) 180,000 Tg  45
  16wL-mer
7wmeso Mn

(16.2)

where wL-mer and wmeso are the starting mole fractions of L-lactide and mesolactide, respectively. Physical aging affects the glass transition of PLA. It can occur between about
45°C and Tg.32 The free volume reduction, characteristic of physical aging, is observed by DSC as an endothermic volume expansion immediately after Tg and is often referred to as the enthalpy of relaxation. Crystallization and orientation reduce the rate and extent of PLA aging. The rate of aging increases with temperature up to the Tg. Copyright © 2005 by Taylor & Francis

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TABLE 16.3 Melting Point and Glass Transition Data for PLA Structures and Blends Description LLLLLLLLL or DDDDDDDDDD

Random optical copolymers

Random level of meso or D-lactide in L-lactide or D-lactic acid in L-lactic acid

PLLA/PDLA LLLLLLLL mixed with stereocomplex DDDDDDDD PLLA/PDLA stereoblock LLLLLLLLLLLDDDDDDDD complexes Syndiotactic poly(meso-) DLDLDLDLDLDL PLA Al-centered R- chiral catalyst Copyright © 2005 by Taylor & Francis

170–190

130–170 220–230 Ikada et al. (1987) 205 Yui et al. (1990) 179 Ovitt and Coates (2000) 152 Ovitt and Coates (1999)

Tm° (°C)

Tg(°C)

205 Tsuji and Ikada (1995) 211 Runt (2001) 215 Kalb and Pennings (1980) 165–210 Runt (2001)

45–65

279 Tsuji and Ikada (1996)

65–72 Tsuji and Ikada (1999)

55–65

40 Ovitt and Coates (1999)

Heterotactic (disyndiotactic) poly(mesolactide) Atactic poly(meso-lactide)

LLDDLLDDLLDDLLDD Al-centered rac- chiral catalyst

40 Ovitt and Coates (1999)

No stereocontrol

Atactic PDLLA

No stereocontrol

46 Witzke (1997) 53 Witzke (1997)

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Isotactic poly(L-lactide) or poly(D-lactide)

Tm (°C)

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A significant reduction in the Tg is found for PLLA crystallized under partially constrained conditions, where the polymer is prevented from shrinking.44 It was proposed that the constrained crystallization process increases the net free volume in the amorphous phase. Tg depression increased with increasing crystallinity. In contrast, when samples are allowed to shrink or crystallize quiescently, Tg increases with crystallinity. It was shown that the Tg could vary by as much as 30°C between the two methods, while the melting point (Tm), heat of fusion (∆Hm), and overall degree of crystallinity (xc) were not influenced. It is possible that the reduction in Tg under partially strained conditions may be related to suppressing physical aging. Blends of PLAs of different optical compositions have been studied for miscibility and compatibility. The most notable blend is that of the PLA stereocomplex, which occurs when PLLA is mixed with PDLA and the homopolymers cocrystallize into a new arrangement with a markedly higher melting point. The interesting features of the stereocomplex crystal will be discussed later. Tsuji and Ikada45 reported that the Tg of stereocomplex PLA is 5°C higher than that of the nonblended components. This was attributed to the strong interaction between the L-lactyl and D-lactyl unit sequences in the amorphous region of the blends, resulting in a more dense chain packing. A second very interesting blend is to blend a crystallizable polymer (c-PLA) of high optical purity (OP) with a polymer of low OP (a-PLA). Urayama et al.46 investigated blends of PLLA with PDLLA by dynamic mechanical analysis (DMA). Tg behavior supported compatibility of both polymers. The modulus remained constant regardless of blend ratio. Modulus changes were consistent with homopolymers, where the E of the blends dropped from 2–3  109 Pa to a rubbery plateau of 1–3  106 Pa across the Tg for amorphous specimens. The rubbery plateau increased to a different level depending on the crystallinity of the blend. Blends of this type provide a route to modify flexibility of the polymer above the glass transition by controlling the overall level of crystallinity (xc). 16.5.2

Rheology

Rheological properties are useful in evaluating thermoplastics for their performance during processing operations. The literature reports a wide variety of processes and products using PLA, spanning from high orientation processes such as fiber spinning and oriented films to lower orientation processes such as thermoforming and foams. The time scales of these processes are very different. Considerations for material flow characteristics, orientation, and crystallization must be made to gauge and predict the resulting properties of the products made from these processes. Intrinsic factors affecting the flow characteristics of PLA are molecular weight distribution, degree and type of branching, optical composition, optical block length distributions, and melt stability.

16.5.2.1 Melt Rheology of Linear PLA PLA, like many thermoplastics, is a pseudoplastic, non-Newtonian fluid. Above the melting point, PLA behaves as a classic flexible-chain polymer Copyright © 2005 by Taylor & Francis

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across all optical compositions.47 PLA’s zero shear viscosity and shear thinning behavior have been measured by a variety of groups. However, complicated by the array of optical copolymers studied, the difficulty associated with measuring the absolute molecular weight of PLA, and the issues of melt stability, the interpretation of PLA’s rheology has not always been consistent. The most comprehensive studies of linear PLAs in the melt were performed by Janzen and Dorgan48–50 who showed that zero shear viscosity (ηo) scales very close to the usual 3.4 power with Mw, and that PLA obeys the Cox-Merz rule well into the shear thinning region when in the linear viscoelastic range. The plateau modulus for PLLA is ~1 MPa, which corresponds to a critical molecular weight of about 9000 g/mol. Previous reports by Cooper-White and MacKay51 measured the rheological properties of PLLA with molecular weights ranging from 2,000 to 360,000 and reported a dependence of zero shear viscosity (ηo) on chain length to the 4.0 power (Mw4.0). However, only a very small number of samples were used to build the 4.0 power relationship. Dorgan also showed that PLA exhibits strain hardening during extension in the melt, a characteristic necessary for fiber spinning. The complex viscosity for PLA with 213,000 Mw (relative to polystyrene) and 13.0% D was measured at many temperatures and shear rates. The shifted data shown in Figure 16.8 depict rheological behavior across six orders of magnitude in shear for 180°C.

1.0E + 09 1.0E + 08 1.0E + 07 1.0E + 06 1.0E + 05 1.0E + 04

G' (Pa)

1.0E + 03

G'' (Pa)

1.0E + 02

|eta*| (Pa*s)

1.0E + 01

fit

1.0E + 00 1.0E − 01 1.0E − 02 1.0E − 03 1.E − 02 1.E + 00

1.E + 02

1.E + 04

1.E + 06

1.E + 08

1.E + 10 1.E + 12

aT ω (rad/s) FIGURE 16.8 Melt rheology of 213.000 Mw (relative to polystyrene) 13.0%D PLA shifted to 180°C. *Measurements by Jay Janzen, Colorado School of Mines.

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16.5.2.2 Melt Stability Issues with PLA melt stability during processing and rheological testing have been cited by Witzke,32 Dorgan et al.,48 Ramkumar and Bhattacharya,52 and Feng and Hanna.53 Reactions leading to poor melt stability include chain scission due to hydrolysis, chain scission due to thermal instability, and lactide formation by the well-documented “back-biting” mechanism. Hydrolysis can be reduced or essentially eliminated with conventional drying operations where water is removed prior to melting. Recommended drying conditions are available for commercial PLAs. When PLA is exposed to a humid environment following drying, moisture is quickly absorbed into the polymer up to its equilibrium level of swelling. Kinetics for melt hydrolysis depend on water concentration, acid or base catalysis, and polymerization catalyst concentration. The proton in the CH group of the main chain of PLA is labile. It has been suggested that the proximity of this labile proton to the ester group affects the thermal sensitivity of the polymer.52 The proton may be abstracted resulting in the breakdown of the molecular chain. In practical extrusion processes, thermal stability becomes important at temperatures greater than 250°C. Using PLA polymerized with tin octoate catalyst, Cicero et al.54 showed that the melt stability of PLA can be improved by the addition of small amounts of tris(nonylphenyl)phosphite (TNPP), likely due to balancing chain extension with degradation reactions. Adding TNPP during rheological testing greatly stabilized the polymer and lengthened the time available for testing. The thermodynamic equilibrium between polylactide and lactide monomer varies with temperature and is about 4.3% lactide at 210°C.32 Since lactide imparts undesirable characteristics during processing, such as fuming, it is desirable to depress the lactide formation kinetics and stabilize the polymer. Gruber et al.55 showed that lactide formation could be substantially reduced under normal processing by complexing the catalyst with additives, resulting in a melt-stable PLA polymer. 16.5.2.3 Solution Properties The dilute-solution properties of PLA are critical for characterizing molecular weight, chain flexibility, and defining solvent quality. Factors reported to affect PLA’s intrinsic viscosity ([η]) are molecular weight and distribution, optical composition, and branching. A recent publication submission by Janzen et al.47 systematically measured the dilute-solution properties of PLAs of wide OP in three solvent systems. Mark-Houwink parameters were determined by comparing [η] to a broad range of molecular weights determined by light scattering in a mixed solvent system first used on PLA by Kang et al.56 Tables 16.4 and 16.5 are taken from Janzen’s publication submission. Surprisingly, no effect of optical composition was found to affect [η], within the error of the experiments. Earlier work by Schindler and Harper57 had shown a significant difference between the Mark-Houwink parameters of PLLA and PDLLA. Copyright © 2005 by Taylor & Francis

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TABLE 16.4 Dilute-Solution Constants for PLAs at 30°C Parameters Schulz-Blaschke concentration factor Mark-Houwink fit Stockmayer-Fixman fit Characteristic ratios Source:

kSB: K, mL/g: a: Ke, mL/g: 104b: C :

CHCl3

Solvent THF

CH3CN & CH2Cl2 Mix

0.302 0.005

0.289 0.005

0.373 0.016

0.0131 0.0048 0.777 0.031 0.112 0.017 5.82 0.69 6.74 0.67

0.0174 0.0046 0.736 0.023 0.101 0.014 4.76 0.65 6.29 0.57

0.0187 0.0075 0.697 0.034 0.096 0.011 2.30 0.36 6.08 0.46

Janzen, J. et al., submitted to Macromolecules, Feb. 11, 2003. With permission.

TABLE 16.5 Flexible Linear Polymer Melt Chain Dimension Comparisons Values Parameters

PLA (140°C)

PEO (140°C)

it-PP (190°C)

M1, relative molecular mass per backbone bond L2, Å2 ρ, g/cm3 GN0, MPa Me, entanglement molecular mass Me/M1, backbone bonds Mc, critical molecular mass Mc /M1, backbone bonds r20/M, Å2 C p, packing length, Å dt, tube diameter, Å dt/p (Ronca-Lin ratio)

24.02

14.68

21.04

Source:

2.05 1.152 1.0 3,959 165 9,211 383 0.574 6.7 2.51 47.7 19

2.11 1.034 1.8 1,973 134 5,330 363 0.805 5.6 1.99 39.9 20

2.37 0.759 0.43 6,797 323 13,635 648 0.694 6.2 3.15 68.7 21.8

PE (140°C) 14.027 2.42 0.792 2.6 1,046 75 3,164 226 1.25 7.3 1.68 36.2 21.6

Janzen, J. et al., submitted to Macromolecules, Feb. 11, 2003. With permission.

Calculated values for some key PLA parameters, including entanglement molecular weight, critical molecular weight, and reptation parameters (Reference 47) are shown in Table 16.5. The characteristic ratio (C ) for PLA is 6.74 0.67. In sharp contrast to this value for C , Kang et al.56 characterized PLA of varying OP by static and dynamic laser light scattering in conjunction with Raman spectroscopy, and calculated a C of 12.4 for PLLA. Some debate still remains as to the flexibility of PLAs of varying optical composition.

16.5.2.4 Branching With the introduction of branching, the rheological properties of PLA can be significantly modified. Since the linear polymer exhibits low melt strength, Copyright © 2005 by Taylor & Francis

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for certain applications it is desirable to increase the melt strength by introducing long-chain branching. Several routes have been used to obtain longchain branching. Figure 16.9 shows the effect of branching on the complex viscosity of PLA. 1. Multifunctional polymerization initiators have been used to make star- and comb-shaped molecules. A number of studies have examined reacting lactides with branched polyols since first proposed by Schindler et al.58 Dorgan et al.59 published rheological characterization of linear and star PLAs. Relaxation spectra show that the transition zones for the linear and branched materials are nearly indistinguishable, while the star polymers have greater contributions to the terminal regime and a much steeper dependence of ηo as a function of Mw. Zhao et al.60 made starburst structures by polymerizing lactide onto dendrimer macroinitiators. From 16 to 21 polylactide arms were attached to the surface of the initiator. Molecular weight is controlled by the ratio of monomer to initiator. 2. Hydroxycyclic ester and/or carbonate polymerization initiators are comprised of an initiation group (e.g., hydroxyl) and a ring capable of copolymerizing with a growing PLA chain. Drumright et al.61 reported polymerizing PLA with hydroxycarbonate initiators, resulting in long-chain branching and enhanced melt elasticity. Tasaka et al.62 made branched PLA with mevalonic lactone initiator.

Complex viscosity (poise)

105

104

Peroxide-modified PLA Linear PLA

1000 0.01

0.1

1 Frequency (rad/s)

10

FIGURE 16.9 Complex viscosity of branched PLA made by peroxide treatment.

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100

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3. Multicyclic ester, multicyclic carbonate and/or multicyclic epoxy comonomers are comprised of multiple rings (most commonly two) capable of copolymerizing with a growing PLA chain. Drumright et al.63 also reported on polymerizing PLA with bicyclic diesters and/or dicarbonates to control rheological properties. The bicyclic diesters and dicarbonates copolymerize easily with lactide to form copolymers that can have tailored levels of branching. The copolymers showed excellent rheological properties, including increased melt tensions and improved shear thinning, compared to the analogous linear polymers. A typical polymer was manufactured by polymerization of L-lactide at 130°–185°C with 0.1% α,α-2,5-dioxabicyclo[2.2.2]octane-3,6-dione. Since multicyclic lactones do not contain an initiator, the chemistry can theoretically be used to cross-link PLA to infinite molecular weight. A patent by Gruber et al.64 describes rheological modification of PLA by copolymerizing with multifunctional expoxidized oil. 4. Cross-linking via free radical has been accomplished in the melt by treating PLA with peroxides.65 Small amounts of peroxide result in long-chain branches. The polymer remains melt processable and is stable. Large improvements in melt elasticity have been measured using die swell, melt tension, and parallel plate rheometry. Blends of linear and branched PLAs showed good compatibility and predictable melt rheology by the mixing rule.66

16.5.2.5 Solid-State Viscoelastic Properties Solid-state rheological properties of amorphous PLA typically show a storage modulus (E’) of about 2–3  109 Pa below the glass transition temperature. As the sample is heated, a sharp drop in modulus occurs between 55° and 80°C, and E’ decreases to ~1–3  106 Pa as the glass transition (Tg) is exceeded.67 Semicrystalline samples show less of a drop in E’ across the Tg, and also a higher Tg with increasing crystallinity. Peak height of the tan δ curve will decrease, but the width of the tan δ will increase as the percentage of crystallinity increases. Highly crystalline samples will typically show a tan δ peak at 70°–80°C. Optical composition and crystallization conditions will strongly influence the onset of melting, and the corresponding drop in E’. The β transition of PLA is very small and is scarcely detectable by DMA. 16.5.3

Crystallinity

16.5.3.1 Morphology The morphology of PLA crystals is influenced by composition and thermal history. Abe et al.68 showed that PLLA crystals grown at different temperatures go through three regime changes associated with nucleation and crystal growth rates. A morphology change was observed between regimes I and II. For PLLA crystallized isothermally from the melt, spherulitic morphology Copyright © 2005 by Taylor & Francis

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is observed at crystallization temperatures below 145°C (regime II). During isothermal growth, the radius of the spherulites increases linearly with time, and the slope of the curve is the lineal crystal growth rate (LCR). At higher temperatures, a smaller number of spherulites are formed due to a decrease in the nucleation density. This leads to larger spherulites with increasing crystallization temperature. Crystallizing PLLA at temperatures greater than 150°C (regime I) results in hexagonal lamellar stacking crystal morphology. Spherulites have been shown to contain stacked lamellar morphology, with the long axis of the lamellar crystal running parallel to the spherulite radius. Within the crystal, PLA exists as a polymeric helix, with an orthorhombic unit cell with dimensions a  1.070 nm and b  0.614 nm. Amorphous polymer resides between the lamellae. The lamellar thickness of PLA varies with the temperature of crystallization (Tc), crystallization time (tc), optical purity (OP), and molecular weight. PLLA grown at 120°, 140°, and 160°C had lamellar periodicities measured at 14–16, 16–18, and 18–22 nm, respectively, measured by AFM.68 For L-rich polymers with small amounts of D- and meso-lactides, there is an observed decrease in the equilibrium melting point with decreasing OP. At least two possible explanations for this have been considered. In one extreme case it is assumed that the optically impure units are incorporated into the crystal structure as defects. This results in a decreasing melting point (Tm) due to the decrease in the enthalpy of fusion. In the other extreme case, it is assumed that the optically impure units are rejected from the crystal structure, causing a reduction in Tm due to entropy effects. Baratian et al.69 studied the degree of crystallinity, spherulitic growth rate, lamellar thickness, and melting temperature of a series of optical copolymers of this variety, comparing the effects of D-lactide and meso-lactide optical impurities. Smallangle x-ray scattering (SAXS) results showed that lamellar thickness decreases with decreasing OP at a given degree of supercooling. It was proposed that a critical sequence length that is free of stereo defects is necessary for crystallization and that the stereo defects are excluded from the crystalline regions. In a different study by Zell et al.,70 it was shown that, at least under long crystallization times, a certain fraction of optical impurity (in this case, a small percentage of labeled L-lactide copolymerized into a polymer of predominantly D-lactide) is incorporated into the crystal lamellae. Zell et al.70 went on to develop a model for determining the fraction of crystallizable PLA based upon a critical chain length necessary for crystallization that allows stereo defects to be included in the crystalline region if they are surrounded by a sufficient isotactic run length. The spherulite structure for L-lactide-rich PLA was studied by Pyda et al.71 Figure 16.10 shows the crystal imaging using AFM and polarized light microscopy. A graph showing the observed melting point by DSC after isothermal treatments at crystallization temperature Tc is shown in Figure 16.11 for a series of poly(L-lactide-co-D-lactide)s that are rich in L-lactide content.69 Polymers were made by ROP using tin octoate catalyst. Copyright © 2005 by Taylor & Francis

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FIGURE 16.10 Morphology of PLA by optical microscopy and AFM. (a) Molecular formula of PLA. (b) Picture of the central region of a spherulite by optical microscopy. (c) Copy of ab AFM picture of a section of the spherulite shown in (b). (d) Schematic drawing of the lamellae within a spherulite. (From Pyda et al., Proceedings of the 30th NATAS Annual Conference on Thermal Analysis and Applications, 463, 2002. With permission.)

Using the Hoffman-Weeks72 method for determining equilibrium melting temperature (Tm0 ) of a polymer [linear extrapolation of experimentally observed melting points (Tm) vs. crystallization temperature (Tc) Copyright © 2005 by Taylor & Francis

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200 190 180

PLLA PD0.015L0.985LA

Tm (°C)

170

PD0.03L0.97LA

160 150 PD0.06L0.94LA

140 130 120 110 100

110

120

130 140 Tc (°C)

150

160

170

FIGURE 16.11 Melting point as a function of crystallization temperature for a series of poly(L-lactide-co-Dlactides. (From Baratian, S. et al., Macromolecules, 34, 4857, 2001. With permission.)

intersecting with the line Tm  Tc), the equilibrium melting point for PLLA has been observed to be 211°–212°C.73 However, some have questioned the appropriateness of the Hoffman-Weeks approach for copolymers. Baratian et al.69 elected to use a Gibbs-Thompson approach for estimating Tm0 that requires direct measure of the lamellar thickness and uses the following relationship: 1–2σe Tm  Tm0  ∆Hm0(lc)

冤

冥

(16.3)

where Tm is the observed melting point for lamella of thickness lc, σe is the end (fold) surface free energy, and Hm0 is the enthalpy of fusion for 100% crystalline PLA. Tm0 is estimated by extrapolation of Tm vs. 1/lc data to 1/lc  0. The lamellar thickness for PLLA measured by Baratian et al. agrees reasonably well with that of Abe et al.68 Their combined data are shown in Figure 16.12. Experimental equilibrium melting points for a series of L-lactide-rich polymers are shown in Figure 16.13. During normal processing and crystallization procedures, polymers do not reach their equilibrium melting temperature. A practical equation describing melting point for PLAs of predominantly L-lactide with mesolactide is simply Tm (°C) ≈ 175°C-300 wmeso where wmeso is less than 0.18.32 The melting point is depressed about 3°C for every 1% initial meso-lactide.

16.5.3.2 Degree of Crystallinity Most commonly, an enthalpy of fusion of 93.1 J/g is used for a 100% crystalline PLLA or PDLA homopolymers (i.e. ∆Hmo 93.1 J/g)74 having infinite Copyright © 2005 by Taylor & Francis

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24

Lamellar thickness (nm)

22 20 18 PLLA (Abe) 16 14 PLLA (Runt) 12 10 110

120

130

140 Tc (°C)

150

160

170

FIGURE 16.12 PLLA lamella thickness as a function of crystallization temperature.

220

200

180

Tm

°

160

Runt - D-lactide Runt - meso-lactide

140

Tsuji - PLLA Runt - PLLA Linear fit to all

120

100 0

1

2

3 4 % D isomer

5

6

7

FIGURE 16.13 Equilibrium melting point for PLA L-lactide-rich homopolymers as a function of increasing percent D-isomer.

crystal thickness, and Equation (16.4) is used to calculate percent crystallinity from DSC scans: ∆Hm
∆Hc Crystallinity (%)  100 93.1 Copyright © 2005 by Taylor & Francis

(16.4)

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where ∆Hm is the measured heat of fusion and ∆Hc is the heat of crystallization. This value of 93.1 J/g is used throughout the PLA literature. However, since Tm0 decreases with decreasing OP, it is expected that using a constant value for ∆Hmo across all optical compositions will introduce error. No reference was found that systematically calculated ∆Hmo for homopolymers of reduced OP. It is well known that for PLAs of essentially random optical copolymers of predominantly L-lactide, with small amounts of D- and mesolactides, the attainable percentage of crystallinity (xc) decreases with decreasing optical purity (OP), and crystallization is essentially nonexistent about when OP  0.78.

16.5.3.3 Crystallization Kinetics The crystallization kinetics of PLA are strongly dependent on the optical copolymer composition. The degree of crystallinity, nucleation rate, and spherulite growth rate decrease substantially with decreasing optical purity (OP). Under quiescent crystallization conditions, Kolstad showed that the bulk crystallization half time increases by roughly 40% for every 1 wt% increase in the meso-lactide.75 PLA has the highest rate of crystallization between 100° and 130°C. Two phenomena contribute to the bulk crystallization rate, nucleation and crystal growth. For instantaneous nucleation and spherulitic growth (Avrami n  3), the crystallization rate constant is given by 4 k  πNG3 3

(16.5)

x 1
e
kt

(16.6)

For the rate equation, n

where x  ∆Hm(t)/∆Hm(∞), N  the density of primary nucleation sites to initiate spherulitic growth, and G is the lineal growth rate of a spherulite. The rate of PLA lineal crystal growth (G) of a spherulite is dependent on Tc, OP, and molecular weight. Nulceation density (N) depends on Tc, polymer characteristics, thermal history, and impurities. When PLA is isothermally crystallized from the melt or from a quenched state, the size of the spherulites changes dramatically. Pluta and Galeski76 showed that spherulites were larger in samples crystallized via cooling from the melt than those in samples crystallized via heating from the glassy amorphous state. On the lamellar level, the structures were similar for samples prepared by the two methods under similar time and temperature conditions. The thermal properties, Tm and Hm, were similar. Technology to increase the rate of crystallization of PLA has been investigated because even the fastest crystallizing PLAs are considered to be slow when compared to many conventional thermoplastics. Kolstad75 showed that talc is an extremely effective nucleating agent for PLA homopolymers. Copyright © 2005 by Taylor & Francis

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Schmidt and Hillmyer77 showed that a small level of stereocomplex crystal is effective for seeding the crystallization of PLA homopolymers. Liao et al.78 studied the crystallization characteristics of blends of PLLA and PDLLA. Crystallization behavior of PLLA in the blends was similar to that of the homopolymer when PDLLA  30%.

16.5.3.4 Stereocomplex Stereocomplexation has been reported in the literature for a number of polymers including polylactide. The PLA racemic crystal structure consists of alternating L- and D-PLA chains packed side by side with a 1:1 ratio of L:D monomer units. The chain conformation in the stereocrystal is a 31 helix vs. the 103 for the unstrained homopolymer crystal. The 31 helix winds a little tighter, going from 108° to 120º rotation per residue, resulting in a significant change in the molecular shape. The 31 configuration allows a chain-to-chain separation difference of 0.4 Å smaller.79 This tighter packing results in more favorable nonbonding interactions that contribute to the higher crystalline melting point. The equilibrium melting temperature of PLA stereocomplex has been reported by Tsuji and Ikada to be 279°C,80 a very marked improvement over homocrystals, with the minimum tacticity sequencing needed to produce stereocomplex for the L-rich and D-rich PLAs to be 15 lactyl (or 7.5 lactide) units.81 The heat of fusion for pure stereocomplex has been determined to be 142 J/g by Loomis and Murdoch.82 Melting points for stereocomplex are dictated by the optical purity (OP) of both the L-lactide-rich and D-lactide-rich polymers (in terms of optically pure run lengths) as well as molecular weight and blend ratio. Maximum regularity is attained with a blend of PLLA and PDLA. The greater the deviation in OP of the L-rich and/or D-rich chains, the greater the decrease in stereocomplex melting peak. This follows the equivalent behaviors of PLA homocrystals, but is shifted to a higher temperature. A practical upper attainable melting temperature that would be achievable for commercial resins approaches 230ºC. Melting point (Tm) and enthalpy of melting (∆Hm) of the stereocomplex is dependent on the ratio of L-lactide-rich polymer and D-lactide-rich polymer. For a constant optical purity of racemic polymers, the Tm varies only by a few degrees across the possible blend ratios, whereas the stereocomplex ∆Hm varies directly with the blend ratio with maximum at 1:1. Tsuji et al. published an excellent series of papers on the crystallization, mechanical behavior, and hydrolysis properties of PLA stereocomplexes.83–94 Kang et al.95 have examined the PLA stereocomplex by Raman spectra and determined that CO stretching bands are related to the packing formation of the polymer chain. By combining Raman spectroscopy and DSC techniques, they deduced the enthalpy of fusion for stereocomplex to be 126 J/g. During crystallization, packing order led the conformation order during early stages. The CO dipole interaction is believed to be the driving force for crystallization. Copyright © 2005 by Taylor & Francis

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16.5.3.5 Stress-Induced Crystallization While the slow quiescent crystallization kinetics of PLA may limit its speed of processing for some applications, opportunities utilizing stress-induced crystallization (SIC) with PLA have been enjoying success commercially because of the naturally wide processing window. It is possible to highly orient PLA in the rubbery or molten state during operations such as film stretching or fiber spinning. Fabrication processes such as this take full advantage of the semicrystalline nature of PLA to develop different properties. Kokturk et al.96 showed that PLA exhibited almost ideal stress-strain behavior in the temperature range 65°–80°C. As films are stretched, crystallization and strain hardening leads to uniform thickness at high deformation. This effect is referred to as self-leveling, and it is a highly desirable property for orientation processes. Crystallization of homopolymers, during orientation processes, results in β crystals in a 31 helix conformation. Puiggali et al.97 characterized PLA crystals made through SIC, using electron diffraction and conformational energy analysis. Chains were shown to be arranged as a frustrated packing of 31 helixes in a trigonal unit-cell of parameters a  b  1.052 nm, c  0.88 nm. The frustrated packing is of the type described as North-South-South (NSS). The effects of uniaxial drawing conditions on the orientation of various poly(L-lactide) (PLLA) samples have been investigated using Fourier transform IR dichroism and thermal analysis.98 Orientation in uniaxially stretched PLLA was influenced by the drawing rate, drawing temperature, and draw ratio. It was shown that spherulites could be deformed at high temperature under drawing conditions into new crystal morphologies. Kister et al.99 first examined the Raman and IR spectra of poly(L-lactic acid) in the amorphous and semicrystalline states from 3600 to 100 cm
1. Vibrational assignments were proposed, and Kister et al. indicated that Raman data supported the 103 helical conformation for PLA. Raman spectroscopy has been further developed for PLA more recently. Smith et al. and Kang et al. have reported on a technique for characterizing orientation in both the amorphous and crystalline phases for films in multiple axes.100–102 Correlations to level of crystallinity were also developed. Film shrinkage was correlated to crystallinity level and orientation of the amorphous phase. Randall et al.103 studied the melting points of PLAs made by SIC fabrication processes of very different time scales using DSC. At a given OP, PLA homopolymers tended to develop the same melting point during film orientation, bottle inflation, and fiber spinning. Cicero et al.104,105 characterized the crystal morphology of fibers made with linear and branched architectures. The level of crystallinity increased with draw ratio. The morphology was fibrillar, with microfibril diameters ranging from 20 to 30 nm for both architectures. Cicero et al. noted that the orientation of the crystal blocks was similar to nylon. Two-step drawn fibers undergo a morphology transition from class 2 to class 1 within the classification proposed by Keller et al.106 at higher draw conditions.

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Degradation and Hydrolysis

Under conditions of high temperature and high humidity, as in active compost, for example, PLA will degrade quickly and disintegrate within weeks to months. The primary mechanism of degradation is hydrolysis, followed by bacterial attack on the fragmented residues. The environmental degradation of PLA occurs by a two-step process.107 During the initial phases of degradation, the high molecular weight polyester chains hydrolyze to lower molecular weight oligomers. The rate of hydrolysis is accelerated by acids or bases and is dependent on moisture content and temperature. Article dimensions, crystallinity, and blends will affect the rate of degradation. PLA products rapidly degrade in both aerobic and anaerobic composting conditions:

Stage

Molecular Weight

Rate of MW Decrease

First stage

High

Second stage

Critical (Mn: 10,000 – 20,000) Low

Weight Loss

Slow (rate None determined) Onset Rapid

Rapid

Hydrolysis Reaction Nonenzymatic

Degradation Mechanism Bulk

Nonenzymatic Bulk and and enzymatic surface

Under typical use conditions, PLA is very stable and will retain its molecular weight and physical properties for years. This is typified by its growing use in clothing and durable applications. High molecular weight PLA is also naturally resistant to supporting bacterial and fungal growth, which allows it to be safely used for applications such as food packaging and sanitation. A recent review by Tsuji108 gives an excellent account of PLA degradation studies.

16.5.4.1 Hydrolysis Degradation of PLA is primarily due to hydrolysis of the ester linkages, which occurs more or less randomly along the backbone of the polymer. It requires the presence of water according to the following reaction: CH 3

CH 3 PLA

CH

C

O

CH

O

CH

OPLA

+ H2O

O

CH 3 PLA

C

CH 3 C

OH + HO

O

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CH

C O

OPLA

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The rate of hydrolysis is determined by its intrinsic rate constant, water concentration, acid or base catalyst, temperature, and morphology. Two major challenges to the stabilization of PLA with regard to hydrolysis are the fact that it is quite permeable to water and that the hydrolysis reaction is autocatalytic. Polyethylene terephthalate, for example, is also a polyester that hydrolyzes, but the inherent rate of hydrolysis is slower than PLA and it is not autocatalytic. The autocatalytic hydrolysis reaction is the following: R-COOH

ka

R-COO¯ + H + CH 3

CH 3 PLA

CH

C O

O

CH

C

OPLA + H 2 O + H +

kh

O CH 3 PLA

CH

CH 3 C

OH + HO

CH

O

C

OPLA + H +

O

An equation describing decrease in ester concentration [E] over time is the following: d(1/Mn) d[E]  k[
COOH][H2O]  dt dt

(16.7)

for random chain scission where [
COOH] ∝ 1/Mn and the product [H2O][E] is constant. Rearranging to Mnd(1/Mn)kdt

(16.8)

ln Mn,t  ln Mn,o
kt

(16.9)

the integrated form becomes

where Mn,t  number average molecular weight at time t, Mn,o  number average molecular weight at time zero, and k is the hydrolysis rate constant. These kinetics were derived by Pitt et al.,109 and were again supported by Tsuji.108 Siparsky et al.110 proposed a slightly different equation using an autocatalytic model: d[
COOH]  kh[
COOH][H2O][E] dt

(16.10)

They found that this equation did not fit their solution kinetics so they derived a second equation using quadratic autocatalysis kinetics. This model Copyright © 2005 by Taylor & Francis

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fit the data much better. It is of the following form: d[x]  kh[E][H2O][x] dt where x  Ka[
COOH]1/2, and Ka is the acid dissociation constant. This form accounts for the dissociation of the acid. Stated differently, d[E]  kh[H2O][E]([
COOH]ka)1/2 dt

(16.11)

The expression makes it possible to take into account all the acid species in the polymer. It also removes the water dependence of the rate constant expression. A comprehensive predictive hydrolysis model for PLA autocatalysis also requires temperature dependence for kh above and below Tg, and accurate prediction of water concentration. Combining this expression with mass transfer terms for [H2O] and [
COOH] as a function of position in an article may be useful for some applications. Strategies for stabilizing PLA to hydrolysis include reducing the level of residual monomer to as low a level as possible and lowering the water concentration in PLA and preventing autocatalysis. The equilibrium moisture content can be decreased by controlling morphology (highly crystalline, oriented, fibular structure). Reduction in the rate of autocatalysis has been accomplished by incorporation of basic, buffering salts such as CaCO3. Certain impurities and classes of additives increase the rate of PLA hydrolysis, including lactide and oligomers and some acidic and basic additives. A second approach to preventing autocatalysis was reported by Lee et al.111 by functionalizing the PLA end group chemistry. End groups studied included OH-, COOH-, Cl-, and NH2-. NH2- and Cl-terminated PLAs were more resistant to thermal and hydrolytic degradation. The thermal stability of the OH-terminated PLA was poor (likely due to lactide formation), and the hydrolytic stability of the COOH-terminated PLA was also poor. The hydrolysis of PLA has been studied using at least four experimental designs: melt hydrolysis (discussed in rheology section above) to simulate extrusion conditions, solution hydrolysis of PLA dissolved in various solvents, and two types of solid-state hydrolysis methods. The first type of solid-state hydrolysis experiment utilizes solid samples suspended in aqueous media. The second type utilizes exposure of solid samples to humidity. Since the product of the hydrolysis reaction is also a catalyst for further reaction, the hydrolysis of PLA is autocatalytic. Melt hydrolysis, solution hydrolysis, and solid exposure to humidity experimental designs generally give rise to homogeneous hydrolysis. In contrast, exposure of solid samples suspended in water or aqueous buffers gives rise to very inhomogeneous hydrolysis. This is due to differences in hydrolysis rate as a function of depth within the sample and it is caused by extraction of PLA oligomers from the surface of the sample into the aqueous medium. It is the chain end groups of the PLA oligomers that catalyze the rate of hydrolysis, and therefore the Copyright © 2005 by Taylor & Francis

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inside of the sample actually hydrolyzes faster than the surface for thick samples submersed in water. It is important that the data used for the determination of any type of stability analysis be consistent with the type of hydrolysis conditions of the intended application. In vivo degradation of PLA is outside the scope of this chapter. Four key features mark the hydrolytic degradation of PLA: 1. The pKa of PLA’s carboxylic acid end group and its oligomers is unusually low (~3) compared to most carboxylic acid groups (4.5 to 5). PLA’s carboxylic acid end groups catalyze the ester hydrolysis, which generally leads to a faster rate of degradation as the polymer degrades (autocatalysis). 2. Two reaction mechanisms apparently occur. The first is a random chain scission and the second a chain-end hydrolysis. The chainend scission is about 10 times faster than the random scission reaction at low pH values but is negligible at neutral and basic pH values. The number of ester groups along the chain is much greater than that of the chain ends and is statistically much more important. 3. The crystalline regions hydrolyze much more slowly than the amorphous regions. 4. The rate of hydrolysis (slope) is much greater above the glass transition temperature (Tg) than below. In the following, we summarize each of the four hydrolysis experimental design conditions independently. Melt hydrolysis is covered in the rheology section pertaining to melt stability.

16.5.4.2 Hydrolysis of Solid Samples Suspended in Aqueous Media Hydrolytic degradation of PLA is autocatalytic due to the production of carboxylic acid end groups that catalyze further hydrolysis.112 An effect of this is that for a thick sample immersed in a 7.4 pH buffer at 37°C, bulk hydrolysis occurs faster than surface hydrolysis.113,114 The pH of the surface remains that of the 7.4 pH buffer whereas the pH of the interior of the sample is reduced due to the accumulation of PLA acid end groups and the inability of the oligomers to quickly diffuse into the buffer medium. Vert et al.115 summarized the literature on the hydrolytic stability of solid, suspended PLA in 1997 and captured several key points including autocatalysis and leaching effects. In addition, morphology and structure were addressed. The rate of degradation is strongly influenced by the number average molecular weight.116 Lower Mn PLA degraded faster as did PLA with higher levels of oligomers. The presence of lactide also increased the rate of hydrolytic degradation.117 Effects of Mn, oligomers, and lactide can all be attributed to autocatalysis. PLA’s amorphous domains are more susceptible to hydrolysis than the crystalline domains. This is due to the ability of Copyright © 2005 by Taylor & Francis

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water to permeate within the amorphous phase but not the crystalline phase, and leads to differences in water concentration throughout the article on the microscale. Hydrolysis of the crystalline domain occurs mainly by a surface erosion mechanism. Grizzi et al. used the fact that surface hydrolysis is slow compared to bulk hydrolysis to stabilize PLA to hydrolysis simply by using small dimensions.113 By suspending PLA samples of varying size in water and determining the rates of hydrolysis, they showed that leaching could be used to slow down the rate of solid-state hydrolysis. They estimated that the skin, which was formed due to the leaching of catalytic oligomers, was about 200 to 300 µm in thickness. Samples smaller than this size were found to degrade much more slowly than large samples. The heterogeneity of the process makes it difficult to determine hydrolysis kinetics constants for solids suspended in aqueous media. Translation of data from these kinds of studies to other environments is suspect for this reason. In alkaline media, Iwata and Doi118 studied the change in morphology and crystalline state of solution-grown single crystals of PLLA. Samples were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), and gel permeation chromatography (GPC). As hydrolysis proceeded, the molecular weight of the degraded crystals corresponded to the value calculated from lamellar thickness measured by AFM. The explanation for this is that the PLA first degrades in the more loosely packed chain folding regions. In the later stages of hydrolysis, the more persistent crystalline region’s molecular weight approaches that of the lamellar thickness, and the degradation mechanism will change to that of surface erosion. Tsuji and Nakahara119 tested PLLA films at pH 2.0 (1. HCl solution, 2. D,L-lactic acid solution), 7.4 (phosphate buffered), and 12 (NaOH soln). While the alkaline media degraded faster than the neutral pH control, the samples in acid media degraded at about the same rate as the neutral pH control. Using microcapsules suspended in buffer solutions, Makino et al.120 showed that PLA degrades slowest at pH 5.0 and increases in acidic and alkaline solutions. The activation energy of hydrolysis at pH 7.4 and ionic strength 0.15 was 19.9 kcal/mol for PDLLA microcapsules and 20.0 kcal/mol for PLLA microcapsules when the initial Mw was about 100,000 for both polymers. The degradation characteristics of PLA in seawater are important for certain applications and potential disposal routes. Yuan et al.121 tested PLLA fibers in saline buffer at 37°C for molecular weight loss and mechanical properties. The results suggested that the tensile properties of the PLLA fibers were stable for 35 weeks, even though their molecular weights decreased remarkably and obvious spotty defects appeared on their surfaces. Tsuji and Suzuyoshi122 showed that PLA degraded faster in natural seawater than controlled static seawater, although the rate of molecular weight loss is slow. Makino et al.120 showed that AlCl3, CaCl2, and KCl had no or little effect on the degradation rate. At temperatures above the glass transition, PLA’s degradation rate increases substantially. Tsuji et al.123 tested PLLA films in buffers at 97°C and showed Copyright © 2005 by Taylor & Francis

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that the hydrolysis takes place predominantly and randomly at the chains in the amorphous region. Amorphous samples crystallized while degrading and higher initial crystallinity slightly slowed the rate of hydrolysis. In a very extreme case showing the effect of morphology, fibers (drawn to a draw ratio of about 8 using a solution spinning approach) were examined. These fibers possessed few defects (chain folds, microvoids) together with high crystallinity, low fractional free volume, and small cross-sectional dimension.112 The PLLA fiber was very stable over a 5-year period immersed in water at room temperature under a static load and lost 10% of its diameter and less than 25% of its tensile strength. The perfection of morphology was thought to reduce the penetration and diffusion of water into the fiber. Effects of processing conditions were also examined by Iannace et al.124 Samples of PLLA crystallized under isothermal and nonisothermal conditions were prepared. Initial morphology developed during the isothermal crystallization affected the rate of hydrolysis in buffer solutions. Larger crystals developed at higher Tc were more resistant to erosion compared to less perfect and smaller crystals developed at higher degrees of supercooling. Tsuji and Suzuki94 also studied stereocomplex degradation over the course of 30 months, in 7.4 pH buffer solution using 1:1 blends and nonblended films prepared from PLLA and PDLA. Properties and molecular weight were monitored with GPC, tensile tests, DSC, SEM, optical polarizing microscopy, x-ray diffractometry, and gravimetry. The rate of molecular weight loss, tensile strength, Young’s modulus, melting temperature, and mass remaining of the films in the course of hydrolysis was more stable for the stereocomplex film than for the nonblended films.

16.5.4.3 Solution Hydrolysis Solution hydrolysis is generally thought to occur by random scission of the ester linkages in PLA.125 It has been reported, however, that base and neutral hydrolysis occur by random chain scission but acid hydrolysis occurs by a mixture of two mechanisms, random and chain-end scission, with chain-end scission being the faster.126 This was determined in dioxane/water solution with 1H NMR and GPC using a statistical method where the level of lactic acid produced by hydrolysis is compared to the degree of degradation across the entire molecular weight distribution. The chain-end scission is sometimes referred to as “unzipping.” PLA solution hydrolysis was also studied in acetone/water by Zhang et al.127 and acetonitrile/water by Siparsky et al.110 The latter solvent had a much higher dielectric constant (ε  36) than acetone (ε  21) or dioxane (ε  2) so that the carboxylic acid groups were more ionized, as in water (ε  79). Autocatalysis was observed in acetonitrile whereas it was not in the lower dielectric solvents due to a lower extent of ionization of the carboxylic acid. The solution hydrolysis rate constant (in dioxane solution) did not depend on the optical purity (OP) of the polymer whereas it does in solid-state degradation (probably due to crystallinity differences).128 CaCO3 was Copyright © 2005 by Taylor & Francis

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observed to slow the rate of hydrolysis due to its buffering capacity and water insolubility.

16.5.4.4 Hydrolysis of Samples Exposed to Humidity Samples exposed to temperature and humidity best simulate the shelf-life environment for many PLA applications. The equilibrium moisture level as a function of humidity is known for PLA at room temperature. It has not been as thoroughly studied for elevated temperatures. Hydrophilic impurities such as lactide and carboxylic acid end groups, whose concentration increase as degradation proceeds, increase the level of moisture. Sharp et al.129 measured the water uptake of PDLLA using a quartz crystal microbalance in humid environments at various temperatures. Kinetics of water adsorption and equilibrium concentrations were measured, and the Flory Huggins interaction parameter X was found to be 3.5 at 20°C using binary mixing theory. The equilibrium swell level of water in PLA increases with decreasing molecular weight due to the strong affinity of acid and hydroxyl end groups for water. It was also shown that the Tg of high molecular weight PLA was depressed by 11°C at 7000 ppm water (the highest level of swelling studied) compared to zero ppm water. Witzke32 measured the swelling concentrations of water in PLA using Karl-Fisher coulometric titration and arrived at an empirical model fit for estimating swell concentration: Water (ppm)  40.8RH(%)6.610
7RH(%)5

(16.12)

where water is in parts per million by weight and RH is the relative humidity. A plot of this equation is shown in Figure 16.14.

Moisture content (ppm)

Low 16000 15000 14000 13000 12000 11000 10000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0

0

10

Typical

20

30

40 50 60 Humidity (%RH)

High

70

80

90

FIGURE 16.14 Equilibrium swelling concentration of water in high molecular weight PLA in humid environments.

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100

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Crystalline pellets reach an equilibrium level of moisture much more slowly (by a factor of 2.4) than amorphous pellets, and crystalline pellets dry 1.5 times slower than amorphous pellets130 due to differences in water diffusion.113 However, Siparsky et al. found that once PLA was equilibrated with moisture, crystallinity had little effect on the solubility or diffusion constant of water.131 They also showed that the diffusion of water in PLA is much faster than the rate of hydrolysis in the temperature range between 20° and 50°C. Li and McCarthy found that the water absorption of PLA increased markedly as the hydrolysis proceeded, presumably due to the production of the more hydrophilic acid and alcohol end groups.132 Others substantiated this point.133 Two distinct populations of absorbed water were observed for poly(D,L-lactide-co-glycolide), one bound and one mobile.134 The rate of hydrolysis of the copolymer correlated with the level of bound water. The rate of hydrolysis increases markedly above Tg. Compost studies on PLA articles show that at temperatures above the Tg, the rate of hydrolysis is fast enough to completely consume articles in a matter of months. Results of a Cargill Dow LLC internal study showing Mw loss over time at various aggressive storage conditions are shown in Figure 16.15. Lactide has a large effect on hydrolytic degradation in humid environments because it rapidly hydrolyzes to a relatively strong acid, which promotes hydrolysis. At high levels (2%) the presence of lactide has a dramatic effect on PLA molecular weight loss. Acidic or basic additives catalyze the rate of PLA hydrolysis. MgSO4 and SiO2 promoted degradation whereas CaCO3 inhibited hydrolytic degradation.135 Li and Vert showed that the presence of porous coral in a blend with PLA (60% coral) dramatically reduced the rate of PLA degradation by neutralizing the effect of autocatalysis.136 1000000

Mw by GPC

40°C, 50% RH 40°C, 80% RH

100000

10000

60°C, 80% RH

80°C, 80% RH 80°C, 20% RH 1000 0

20

40

60

80 100 120 Time (days)

140

160

180

200

FIGURE 16.15 Cargill Dow LLC internal study showing molecular weight loss for amorphous PLA stored at various aggressive temperatures and humidity.

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The crystalline regions of PLA were found to hydrolyze very slowly.112,125 The rate of molecular weight loss follows a two-step process, the first occurring very fast (interpreted as loss of the amorphous phase) and the second occurring much slowly (interpreted as loss of the crystalline phase). The apparent percentage of crystallinity increases during the amorphous phase hydrolysis, consistent with the interpretation. The molecular weight of the remaining PLA after the amorphous phase was degraded was about 5000 to 9000 (DP  70 to 125), roughly that of the fold length of the lamella.74,137 Crystalline injection-molded samples lost physical properties faster than amorphous samples, although the rate of Mw loss was similar between them.125 This was interpreted as the loss of amorphous tie chains between the crystals causing catastrophic failure of the system. Also, chain folds and tie chains connecting crystallites are thought to hydrolyze preferentially because they are stressed due to a “reeling in” force.138 These observations may be the result of chain end groups and catalysts concentrating in the amorphous regions. Morphology also affects water permeation in another way. Microvoids or porosity allow water to migrate more freely into the material and thereby enhance the rate of hydrolysis. This is probably a small effect for samples exposed to humidity because the rate of hydrolysis is slow compared to the water diffusion rate, as discussed above. Localized regions of high water absorption may cause mechanical stresses due to increased osmotic pressure.

16.5.5

Enzymatic Degradation

Several enzymes have been shown to catalyze PLA hydrolysis. Enzymatic hydrolysis of PLLA with varied crystal morphologies was studied using proteinase K by Tsuji and Miyauchi.139,140 It was concluded that the enzymatic degradation proceeded via mainly a surface erosion mechanism. Amorphous regions hydrolyzed more readily than crystalline regions. The enzymatic hydrolysis rate (REH) was modeled using the following equation: REH(µg/(mm2·h)0.36(1
xc)

(16.13)

where xc  the fraction of crystalline PLLA. Tie chains degraded more readily than folds or restricted amorphous chains.

16.6

Applications and Performance

Since PLA is an environmentally friendly polymer that can be designed to controllably biodegrade, it is ideally suited for many applications in the environment where recovery of the product is not practical, such as agricultural Copyright © 2005 by Taylor & Francis

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mulch films and bags. Composting of post-consumer PLA items is also a viable solution for many PLA products. However, the large growth seen for PLA in many applications does not depend upon the biodegradability of the material. PLA resins can be tailor-made for different fabrication processes, including injection molding, sheet extrusion, blow molding, thermoforming, film forming, or fiber spinning. The key is controlling certain molecular parameters in the process such as branching, D-isomer content, and molecular weight distribution. The ability to selectively incorporate L-, D-, or meso-lactide stereoisomers into the polymer backbone allows PLA to be tailored for specific applications. The ease of incorporation of various defects into PLA allows for control of both crystallization rate and ultimate crystallinity. Typical properties of NatureWorks PLA from Cargill Dow LLC for injection molding and extrusion applications are given in Table 16.6. The various grades of NatureWorks PLA differ in stereochemical purity, molecular weight, and additive packages. Each grade is optimized for both processing and end use performance in its intended application. TABLE 16.6 Properties of Extrusion/Thermoforming and Injection Molding Grades of NatureWorks PLA PLA Polymer 2000Da Physical Property Specific gravity (g/cc) Melt index, g/10 min (190°C/2.16 kg) Clarity Mechanical Properties Tensile strength at break, psi (MPa) Tensile yield strength, psi (MPa) Tensile modulus, kpsi (GPa) Tensile elongation (%) Notched izod impact, ft-lb/in (J/m) Flexural strength, psi (MPa) Flexural modulus, kpsi (GPa) a

b

1.25 4–8

ASTM Method D792 D1238

Transparent

PLA Polymer 3010Db 1.21 10–30

ASTM Method D792 D1238

Transparent

7,700 (53)

D882

8,700 (60)

D882

500 (3.5)

D882

6.0 0.24 (0.33)

D882 D256

7,000 (48)

D638

2.5 0.3 (0.16)

D638 D256

12,000 (83)

D790

555 (3.8)

D790

2000D is a product of Cargill Dow LLC designed as an extrusion/thermoforming grade; properties typical of extruded sheet. 3010D is a product of Cargill Dow LLC designed as an injection molding grade; properties typical of injection molded tensile bars.

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Injection molding of heat-resistant products requires rapid crystallization rates that can be achieved by PLA, typically containing less than 1% D-isomer and often with the addition of nucleating agents.75 These compositions allow high levels of crystallinity to develop during the fast cooling cycle in the mold. Extrusion-thermoforming is optimized at a D-isomer content that does not allow crystallization to occur during the melt processing steps, with 4–8% D-content being the effective range. Branching can be introduced by a variety of methods,62–64, 66 thus enhancing melt strength during fabrication and opening up new application opportunities in the areas of foams and extrusion coating. Examples of a number of articles fabricated from PLA are shown in Figure 16.16. In short, the rheological characteristics and physical properties of PLA can be tailored for use in a variety of processes and applications. Fiber is one of the largest potential application areas for PLA. PLA is readily melt spinnable, stress crystallizes upon drawing, and can be designed for

FIGURE 16.16 Articles produced from PLA. (a) Injection stretch-blow molded bottles. (b) Films. (c) Extrusion-thermoformed containers. (d) Fiberfill products. (e) Carpet and coverings.

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Properties of NatureWorks PLA Fabricated into Fibers Material Properties

Denier (dpf) Total denier/filament count Strength (kg) Tenacity (g/den)

1.25–1.28 1.08–1.12 55–60 (130–140) 145–155 (295–310) 5–15

1.25–1.28 1.08–1.12 60–65 (140–150) 160–170 (320–340) 15–30

Monofilament 512 2.9 5.75

Elongation (%) 26.2 Copyright © 2005 by Taylor & Francis

Boiling water shrinkage (%) a b c d

Cotton-Type Staple 1.5 staple/N.A. 5.7

ASTM Method D792 D1238 D3417 D3418 D1238A & B

POYc

FDYd

4.75 114/24

3.04 73/24

2.8

5.7

35

61

29

0

21

10

6700D is a product of Cargill Dow LLC designed for extrusion into mechanically drawn monofilament. 6200D is a product of Cargill Dow LLC designed for extrusion into mechanically drawn staple fibers or continuous filaments. Partially Oriented Yarn (POY). Fully Drawn Yarn (FDY).

D1907 D1907 D2256 D2256 multifilament, D3822 single filament D2256 multifilament, D3822 single filament D2102

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Fiber Properties

PLA Polymer 6200Db

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Specific gravity Melt density, 230°C (445°F) Tg, °C (°F) Tm, °C (°F) Melt flow rate, 210°C (410°F)

PLA Polymer 6700Da

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many fiber applications. Some of the current fiber uses include hollow fiberfill for pillows and comforters, bulk continuous filament (BCF) for carpet, filament yarns, and spun yarns for apparel, spunbond, and other nonwovens and bicomponent fibers for binders and self-crimping fibers. Most fiber applications require polymer with high optical purity (OP) to allow high levels of crystallinity to develop and to have adequate heat resistance in the application. Binder fibers are unique in that low crystallinity in the sheath layer is desired to allow ease of melting and adhesion to other fibers; thus, high (8–20%) D- or meso-lactide content is incorporated. PLA can be processed on standard thermoplastic fiber spinning equipment with the appropriate temperature profiles relative to its crystal melting point. Melt temperatures of 200°–240°C are typically used. As with all melt processing of PLA, care must be taken to ensure that the material is dry and does not pick up moisture, otherwise unacceptable molecular weight loss will occur. Typical drying conditions are 2–4 h in a hopper dryer with – 40oC dew point air with a resulting moisture content of less than 50 ppm. Some of the properties of PLA fibers are shown in Table 16.7 and compared to other natural and synthetic fibers in Table 16.8. PLA fiber can be combined with natural or regenerated fibers including cotton, wool, silk, viscose, lyocell, and others along with synthetic fibers made from PET, nylon, and other petroleum-based synthetics. PLA can be included as a minor component (5–15%) or as the major fiber, depending on the balance of properties and appearance desired. One of the fastest growing application areas is in nonwoven wipes containing 35% PLA and 65% viscose. PLA is replacing PET in these applications because of its superior performance and the fact that the disposable products can be produced from fibers that are from 100% renewable resources and are also 100% biodegradable. Some of the beneficial characteristics of PLA fiber products include its natural soft feel, ease of processing, and unique stain and soil resistance. PLA excels at resistance to stain in standard tests with coffee, cola, tea, catsup, lipstick, and mustard. PLA also burns with low smoke generation, has good ultraviolet resistance, is easily dyeable, and brings good wickability of moisture to applications.

TABLE 16.8 Properties of Synthetic and Natural Fibers Fiber Property

Nylon 6

PET

Acrylics

PLA

Rayon Cotton Silk

Specific gravity 1.14 1.39 1.18 1.25 1.52 Tenacity (g/d) 5.5 6.0 4.0 6.0 2.5 Moisture regain (%) 4.1 0.2–0.4 1.0–2.0 0.4–0.6 11 Elastic recovery 89 65 50 93 32 (5% strain) Flammability Medium High Medium Low Burns smoke smoke UV resistance Poor Fair Good Good Poor

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Wool

1.52 4.0 7.5 52

1.34 4.0 10 52

Burns

Burns Burns slowly Fair Fair

Fair

1.31 1.6 14–18 69

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TABLE 16.9 Properties of Biaxially Oriented NatureWorks PLA Film Properties Specific gravity (g/cc) Tensile strength (kpsi) MDb TD Tensile modulus (kpsi) MD TD Elongation at break (%) MD TD Elmendorf tear (g/mil) MD TD Spencer impact (J) Transmission rates O2, cc-mil/m2/24 h atm CO2, cc-mil/m2/24 h atm H2O, g-mil/m2/24 h atm Optical characteristics Haze (%) Gloss, 20° Thermal characteristics Tg, °C (°F) Tm, °C (°F) a

b

PLA Polymer 4040Da

ASTM Method

1.25

D1505

16 21

D882 D882

480 560

D882 D882

160 100

D882 D882

15 13 2.5

D1922 D1922

550 3000 325

D1434 D1434 E96

2.1 90

D1003 D1003

52 (126) 135 (275)

D3418 D1003

4040D is a product of Cargill Dow LLC suitable for preparation of biaxially oriented film. Properties on 1 mil film oriented 3.5 in MD and 5 in TD. MD refers to machine direction and TD refers to transverse direction.

Films are the second largest application area for PLA. Again, the ability to modify the crystallization kinetics and physical properties for a broad range of applications by D- or meso-comonomer incorporation, branching, and molecular weight change makes PLA extremely versatile. Films are transparent when stress crystallized and have acceptance by customers for food contact. PLA films can be prepared by the blown double bubble technology or preferably, cast-tentering. Cast-tentered films have very low haze, excellent gloss, and gas (O2, CO2, and H2O) transmission rates desirable for consumer food packaging. PLA films also have superior dead fold or twist retention for twist wrap packaging. Some of the properties of PLA films are shown in Table 16.9.

16.7

Summary

Polymers of polylactic acid have finally become a commercial reality with the construction of a world-scale plant. The cost-performance balance of PLA has Copyright © 2005 by Taylor & Francis

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resulted in its use in many applications, including packaging, paper coating, fibers, films, and a host of molded articles. PLA is not being used in these applications solely because of its degradability nor because it is made from renewable resources. PLA is in both durable and short-term applications. The use of PLA as a cost-effective alternative to commodity petrochemical-based plastics will increase demand for agricultural products such as corn and sugar beets, and is an advanced example of sustainable technology. One of the additional benefits of large commercial production of lactic acid for PLA is the increased availability and reduced cost of lactic acid or lactide for the production of lactic acid esters. These esters, especially ethyl lactate, are finding increased use as environmentally friendly solvents, cleaning agents, and diluents. They are replacing more hazardous solvents such as chlorinated organic and aromatic compounds. As we move into the 21st century, increased utilization of renewable resources will be one of the strong drivers for sustainable products. Reduced energy consumption, waste generation, and emission of greenhouse gases will take on greater emphasis. Polylactic acid is the first commodity plastic to incorporate these principles and Cargill Dow LLC was awarded the 2002 Presidential Green Chemistry Award for their process to produce NatureWorks PLA.141 Future developments will include blends of PLA, copolymers, and impact-modified products, which will further expand the applications where this unique polymer can be used.

Acknowledgments The authors would like to thank Kevin McCarthy, Patrick Smith, and Ray Drumright for their contributions to this manuscript, and Cargill Dow, LLC for permission to publish it.

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79. Spinu, M., Jackson, C., Keating, M.Y., and Gardner, K.H., Material design in poly(lactic acid) systems: block copolymers, star homo- and copolymers, and stereocomplexes, J. Macromol. Sci., Pure. Appl. Chem., A33, 1497, 1996. 80. Tsuji, H. and Ikada, Y., Crystallization from the melt of poly(lactide)s with different optical purities and their blends, Macromol. Chem. Phy., 197, 3483, 1996. 81. Tsuji, H. and Ikada, Y., Stereocomplex formation between enantiomeric poly(lactic acid)s. XI. Mechanical properties and morphology of solution-cast films, Polymer, 40, 6699, 1999. 82. Loomis, G.L. and Murdoch, J.R., Polylactide stereocomplexes, Polym. Prepr. 31, 55, 1990. 83. Okihara, T., Kawaguchi, A., Tsuji, H., Hyon, S.H., Ikada, Y., and Katayama, K., Lattice disorders in the stereocomplex of poly(L-lactide) and poly(D-lactide), Bull. Inst. Chem. Res., Kyoto Univ., 66, 271, 1988. 84. Tsuji, H., Horii, F., Hyon, S.H., and Ikada, Y., Stereocomplex formation between enantiomeric poly(lactic acid)s. 2. Stereocomplex formation in concentrated solutions, Macromolecules, 24, 2719, 1991. 85. Okihara, T., Tsuji, M., Kawaguchi, A., Katayama, K., Tsuji, H., Hyon, S.H., and Ikada, Y., Crystal structure of stereocomplex of poly(L-lactide) and poly(D-lactide), J. Macromol. Sci., Phy., 30, 119, 1991. 86. Tsuji, H., Hyon, S.H., and Ikada, Y., Stereocomplex formation between enantiomeric poly(lactic acid)s. 3. Calorimetric studies on blend films cast from dilute solution, Macromolecules, 24, 5651, 1991. 87. Tsuji, H., Hyon, S.H., and Ikada, Y., Stereocomplex formation between enantiomeric poly(lactic acid)s. 4. Differential scanning calorimetric studies on precipitates from mixed solutions of poly(D-lactic acid) and poly(L-lactic acid), Macromolecules, 24, 5657, 1991. 88. Tsuji, H., Hyon, S.H., and Ikada, Y., Stereocomplex formation between enantiomeric poly(lactic acids). 5. Calorimetric and morphological studies on the stereocomplex formed in acetonitrile solution, Macromolecules, 25, 2940, 1992. 89. Tsuji, H., Horii, F., Nakagawa, M., Ikada, Y., Odani, H., and Kitamaru, R., Stereocomplex formation between enantiomeric poly(lactic acid)s. 7. Phase structure of the stereocomplex crystallized from a dilute acetonitrile solution as studied by high-resolution solid-state carbon-13 NMR spectroscopy, Macromolecules, 25, 4114, 1992. 90. Tsuji, H. and Ikada, Y., Stereocomplex formation between enantiomeric poly(lactic acid)s. 6. Binary blends from copolymers, Macromolecules, 25, 5719, 1992. 91. Tsuji, H. and Ikada, Y., Stereocomplex formation between enantiomeric poly(lactic acids). 9. Stereocomplexation from the melt, Macromolecules, 26, 6918, 1993. 92. Tsuji, H., Ikada, Y., Hyon, S.H., Kimura, Y., and Kitao, T., Stereocomplex formation between enantiomeric poly(lactic acid). VIII. Complex fibers spun from mixed solution of poly (D-lactic acid) and poly (L-lactic acid), J. Appl. Polym. Sci., 51, 337, 1994. 93. Tsuji, H., In vitro hydrolysis of blends from enantiomeric poly(lactide)s part 1. Well-stereocomplexed blend and non-blended films, Polymer, 41, 3621, 2000.

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94. Tsuji, H. and Suzuki, M. In vitro hydrolysis of blends from enantiomeric poly(lactide)s. 2. Well-stereocomplexes fiber and film. Sen’i Gakkaishi, 57, 198, 2001. 95. Kang, S., Aou, K., Pekrul, R., and Hsu, S.L., Crystallization of poly(lactic acid) stereocomplexes, 224th ACS National Meeting, Boston, MA, US, Abstr., Aug. 18–22, 2002. 96. Kokturk, G., Serhatkulu, T.F., Cakmak, M., and Piskin, E., Evolution of phase behavior and orientation in uniaxially deformed polylactic acid films, Polym. Eng. Sci., 42, 1619, 2002. 97. Puiggali, J., Ikada, Y., Tsuji, H., Cartier, L., Okihara, T., and Lotz, B., The frustrated structure of poly(L-lactide), Polymer, 41, 8921, 2000. 98. Lee, J.K., Lee, K.H., and Jin, B.S., Structure development and biodegradability of uniaxially stretched poly(L-lactide), Eur. Polym. J., 37, 907, 2001. 99. Kister, G., Cassanas, G., Vert, M., Pauvert, B., and Terol, A., Vibrational analysis of poly(L-lactic acid), J. Raman Spectrosc., 26, 307, 1995. 100. Smith, P.B., Leugers, A., Kang, S., Yang, X., and Hsu, S.L., Raman characterization of orientation in poly(lactic acid) films, Macromol. Symp. Polymerization Processes Polym. Mater. II, 175, 81, 2001. 101. Smith, P.B., Leugers, A., Kang, S., Hsu, S.L., and Yang, X., An analysis of the correlation between structural anisotropy and dimensional stability for drawn poly(lactic acid) films, J. Appl. Polym. Sci., 82, 2497, 2001. 102. Kang, S., Hsu, S.L., Stidham, H.D., Smith, P.B., Leugers, A.M., and Yang, X., A Spectroscopic analysis of poly(lactic acid) structure, Macromolecules, 34, 4542, 2001. 103. Randall, J., McCarthy, K., Krueger, J., Smith, P., and Spruiell, J., Properties of Polylactic Acid Optical Copolymers Achieved Through Stress Induced Crystallization, 58th Annual Technical Conference - Society of Plastics Engineers, Vol. 3, 3728, 2000. 104. Cicero, J.A., Dorgan, J.R., Garrett, J., Runt, J., and Lin, J.S., Effects of molecular architecture on two-step, melt-spun poly(lactic acid) fibers, J. Appl. Polym. Sci., 86, 2839, 2002. 105. Cicero, J.A., Dorgan, J.R., Janzen, J., Garrett, J., Runt, J., and Lin, J.S., Supramolecular morphology of two-step, melt-spun poly(lactic acid) fibers, J. Appl. Polym. Sci., 86, 2828, 2002. 106. Keller, A., Barham, P., and Wills, H., J. Polym. Sci., 13, 197, 1975. 107. Lunt, J., Large-scale production, properties and commercial applications of polylactic acid polymers, Polym. Degrad. Stability, 59, 145, 1998. 108. Tsuji, H., Polylactides, in Biopolymers, Wiley-VCH, 2002, chap. 5. 109. Pitt, C.G., Jeffcoat, A.R., and Zweidinger, R.A., Sustained drug delivery system. I. The permeability of poly(caprolactone), poly (DL-lactic acid), and their copolymers, J. Biomed. Mater. Res., 13, 497, 1979. 110. Siparsky, G., Voorhees, K., and Miao, F., J. Environ. Polym. Degrad., 6, 31, 1998. 111. Lee, S., Kim, S.H., Han, Y., and Kim, Y., Synthesis and degradation of end-groupfunctionalized polylactide, J. Polym. Sci., Part A: Polym. Chem., 39, 973, 2001. 112. Joziasse, C.A.P., Grijpma, D.W., Bergsma, J.E., Cordewener, F.W., Bos, R.R.M., and Pennings, A.J., The influence of morphology on the hydrolytic degradation

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115.

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117. 118. 119. 120. 121. 122.

123.

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126. 127. 128. 129.

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of as-polymerized and hot-drawn poly(L-lactide), Colloid Polym. Sci., 276, 968, 1998. Grizzi, I., Garreau, H., Li, S., and Vert, M., Hydrolytic degradation of devices based on poly(DL-lactic acid) size-dependence, Biomaterials, 16, 305, 1995. Nakamura, T., Hitomi, S., Watanabe, S., Shimizu, Y., Jamshidi, K., Hyon, S.H., and Ikada, Y., Bioabsorption of polylactides with different molecular properties, J. Biomed. Mater. Res., 23, 1115, 1989. Vert, M., Li, S., Garreau, H., Mauduit, J., Boustta, M., Schwach, G., Engel, R., and Coudane, J., Complexity of the hydrolytic degradation of aliphatic polyesters, Ange. Makromol. Chem., 247, 239, 1997. Mauduit, J., Perouse, E., and Vert, M., Hydrolytic degradation of films prepared from blends of high and low molecular weight poly(DL-lactic acids), J. Biomed. Mater. Res., 30, 201, 1996. Hyon, S., Jamshidi, K., and Ikada, Y., Effects of residual monomer on the degradation of DL-lactide polymer, Polym. Int., 46, 196, 1998. Iwata, T. and Doi, Y., Alkaline hydrolysis of solution-grown poly(L-lactic acid) single crystals, Sen’i Gakkaishi, 57, 172, 2001. Tsuji, H. and Nakahara, K., Poly(l-lactide). IX. Hydrolysis in acid media, J. Appl. Polym. Sci., 86, 186, 2002. Makino, K. et al., Preparation and in vitro degradation properties of polylactide microcapsules, Chem. Pharm. Bull., 33, 1195, 1985. Yuan, X., Mak, A., and Yao, K., In vitro degradation of poly(L-lactic acid) fibers in phosphate buffered saline, J. Appl. Polym. Sci., 85, 936, 2002. Tsuji, H. and Suzuyoshi, K., Environmental degradation of biodegradable polyesters 2. Poly(epsiln.-caprolactone), poly[(R)-3-hydroxybutyrate], and poly(L-lactide) films in natural dynamic seawater, Polym. Degrad. Stability, 75, 357, 2002. Tsuji, H., Nakahara, K., and Ikarashi, K., Poly(L-lactide), 8a. High-temperature hydrolysis of poly(L-lactide) films with different crystallinities and crystalline thicknesses in phosphate-buffered solution, Macromol. Mater. Eng., 286, 398, 2001. Iannace, S., Maffezzoli, A., Leo, G., and Nicolais, L., Influence of crystal and amorphous phase morphology on hydrolytic degradation of PLLA subjected to different processing conditions, Polymer, 42, 3799, 2001. Amass, W., Amass, A., and Tighe, B., A review of biodegradable polymers: uses, current developments in the synthesis and characterization of biodegradable polyesters, blends of biodegradable polymers and recent advances in biodegradation studies, Polym. Int., 47, 89, 1998. Shih, C., A graphical method for the determination of the mode of hydrolysis of biodegradable polymers, Pharm. Res., 12, 2036, 1995. Zhang, X., Wyss, U., Pichora, D., and Goosen, M., Investigation of poly(lactic acid) degradation, J. Bioact. Compat. Polym., 9, 80, 1994. Li, S.M. and Vert, M., Degradable Polymers: Principles and Applications, Chapman & Hall, London, 1995. Sharp, J. S., Forrest, J.A., and Jones, R.A.L., Swelling of poly(DL-lactide) and polylactide-co-glycolide in humid environments. Macromolecules, 34, 8752, 2001.

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130. McCarthy, K., Cargill Dow LLC product literature, Minnetonka, Minnesota, 1998. 131. Siparsky, G.L., Voorhees, K.J., Dorgan, J.R., and Schilling, K., Water transport in polylactic acid (PLA), PLA/polycaprolactone copolymers, and PLA/polyethylene glycol blends, J. Environ. Polym. Degrad., 5, 125, 1997. 132. Li, S. and McCarthy, S., Further investigations on the hydrolytic degradation of poly (DL-lactide). Biomaterials, 20, 35, 1999. 133. Burg, K.J.L. and Shalaby, S.W., Water fugacity in absorbing polymers, J. Biomed. Mater. Res., 38, 337, 1997. 134. Schmitt, E.A., Flanagan, D.R., and Linhardt, R.J., Importance of distinct water environments in the hydrolysis of poly(DL-lactide-co-glycolide), Macromolecules, 27, 743, 1994. 135. Renstad, R., Karlsson, S., Sandgren, A., and Albertsson, A., Influence of processing additives on the degradation of melt-pressed films of poly(ε-caprolactone) and poly(lactic acid), J. Environ. Polym. Degrad., 6, 209, 1998. 136. Li, S. and Vert, M., Hydrolytic degradation of coral/poly(DL-lactic acid) bioresorbable material. J. Biomater. Sci., Polymer ed., 7, 817, 1996. 137. Fischer, E.W., Structure of partially crystalline polymer systems, Prog. Colloid Polym. Sci., 57, 149, 1975. 138. Nijenhuis, A.J., Grijpma, D.W., and Pennings, A.J., Highly crystalline as-polymerized poly(L-lactide), Polym. Bull., 26, 71, 1991. 139. Tsuji, H. and Miyauchi, S., Poly(L-lactide): VI effects of crystallinity on enzymatic hydrolysis of poly(L-lactide) without free amorphous region, Polym. Degrad. Stability, 71, 415, 2000. 140. Tsuji, H. and Miyauchi, S., Poly(L-lactide): 7. enzymatic hydrolysis of free and restricted amorphous regions in poly(L-lactide) films with different crystallinities and a fixed crystalline thickness, Polymer, 42, 4463, 2001. 141. United States Environmental Protection Agency Presidential Green Chemistry Challenge, Alternate Solvents/Reaction Conditions Award, June 2002.

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17 Polylactide-Based Biocomposites

David Plackett and Anders Södergård

CONTENTS 17.1 Introduction 17.1.1 Background 17.1.2 Lactic Acid-Based Polymers: Polylactides 17.2 Research on PLA Biocomposites 17.2.1 Introduction 17.2.2 Natural Fiber Composites 17.2.3 Natural Fiber Biocomposites 17.2.4 PLA Biocomposites 17.2.4.1 Previous and Current Research Activities in PLA–Natural Fiber Composites 17.2.4.2 Processing and Processability of PLA–Natural Fiber Composites 17.2.4.3 Mechanical Properties of PLA–Natural Fiber Composites 17.2.4.4 Environmental Stability of PLA–Natural Fiber Composites 17.2.4.5 Other Characteristics of PLA–Natural Fiber Composites 17.2.4.5.1 Thermal Properties 17.2.4.5.2 Flammability 17.2.4.5.3 Recycling by Reprocessing 17.3 Applications 17.4 Future Directions References ABSTRACT In the past decade numerous research groups have explored the production and properties of biocomposites in which the polymer matrix is

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derived from renewable resources. The increasing commercial availability of polylactide (PLA) and interest in the utilization of this polymer, based on mechanical properties and other attributes, has meant that PLA biocomposites have been a logical choice for research activity. PLA biocomposites containing inorganic fillers or reinforcements have received much attention because of their biocompatibility and are briefly discussed in this chapter; however, the main focus is on PLA reinforced with natural fibers. In addition to the wellknown advantages of using natural fibers in composites, such as low cost, high specific properties, and nonabrasive processing, their use in combination with biopolymers has the potential to provide materials with enhanced mechanical properties entirely derived from nature. Replacement of a certain percentage of biopolymer with fiber also reduces material costs, and such materials should remain fully biodegradable. The scientific literature to date contains a relatively limited number of reports on natural fiber−PLA biocomposites. The research topics that have been studied include processing methods and their influence on biocomposite mechanical properties, the addition of plasticizers, biodegradability, modifications of fiber or polymer to improve adhesion, the effect of water and exposure to the environment, thermal properties, flammability, and composite recycling. The chapter provides a summary of key findings from research on each of these topics. Practical applications for natural fiber−PLA composites are at a very early stage of development at present; however, the use of these materials as automotive components is attracting some interest spurred on by the existing well-established use of other natural fiber−polymer composites in vehicle interiors. Another area of development, especially in Japan, is in the use of biopolymers or reinforced biopolymers in the construction of notebook computers. Further applications can be anticipated based on the increasing need for environmentally sustainable products across a wide range of consumer goods. In addition to economic factors, a number of technical improvements in natural fiber-reinforced PLA composites are desirable. For some applications, improved toughness is necessary and therefore research is still needed on ways to achieve this by means of such methods as plasticizer addition or the use of copolymers or polymer blends. The development and use of naturally derived process additives is also required in keeping with the concept of biocomposites manufactured entirely from renewable resources. Beyond the current generation of materials there are exciting possibilities for composites in which even greater property enhancements might be achieved through the use of nanoscale fibers.

17.1 17.1.1

Introduction Background

The industrial use of plastics produced from non-renewable raw materials, especially in the fields of packaging and consumables including medical

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uses, is continuously increasing worldwide. Today, the United States is the largest producer and also one of the largest markets for thermoplastics. The growth in sales of the main commodity thermoplastics from 1997 to 2001 was 4.2% for packaging and 2.0% for consumer and institutional applications. The corresponding compound growth rate for the same thermoplastics across all applications was 3.0% over the same period. The development and use of biodegradable polymers from renewable resources has been strongly encouraged in recent years in response to concerns about disposal of plastics. Interest in these polymers has also been stimulated partly as a result of emerging regulations (e.g., the European Union norm on biodegradable packaging materials, environmental taxes, end-of-life vehicle regulations)1−3 and also because of the growing awareness of the unique properties of materials made by nature. The indications are that polymeric materials made from renewable resources and with controllable degradation rates will become increasingly attractive in the future. This probable trend provides a challenging opportunity for polymer scientists to develop new composite materials and production methods using biodegradable polymers for a variety of products and applications.

17.1.2

Lactic Acid-Based Polymers: Polylactides

Lactic acid-based polymers (polylactides) are polyesters made from lactic acid, a compound found in both plants and animals. Lactic acid can be produced by a large number of different types of bacteria and bacterial strains and the feedstock for lactic acid production can be agricultural products (e.g., sugar beet, corn) or even waste products (e.g., whey, cellulose waste). Lactic acid is already extensively used as an additive in the food and foodrelated industries. Lactic acid can be converted into esters for use as solvents or polymerized into macromolecules. Preparation of high molar mass lactic acid polymers (PLAs) can be conducted by (1) ring-opening polymerization (ROP) of the dehydrated ring-formed dimer or dilactide, (2) polycondensation and manipulation of the equilibrium between lactic acid and polylactide by removal of the reaction water using drying agents, or (3) polycondensation and linking of lactic acid prepolymers. The repeating units of polylactide can have two different configurations (D- or L-) and the relative amount and distribution of the two stereoforms determine many of the properties of the polymer.4 In general, a polymer prepared by ROP and built of L-stereoisomer elements is referred to as poly(L-lactide) (PLLA), and, accordingly, a polymer containing both D- and L-stereoisomer elements is referred to as poly(D-,Llactide) (PDLLA). The two different stereoforms provide a tool for varying the polymer properties to a certain extent without using additives or comonomers. The general term polylactide (PLA) is used when the type of isomer is not specified. For polylactides prepared by polycondensation the name poly(lactic acid) is preferred.

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PLA can be formed into products of different shapes by standard plastic processing and manufacturing techniques (e.g., extrusion, injection molding, and thermoforming). The manufacturing technology and properties of polylactides are described in more detail in another chapter of this book.

17.2 17.2.1

Research on PLA Biocomposites Introduction

Polymeric biocomposites can be defined as materials based on a biodegradable polymer matrix of synthetic or natural origin and a reinforcing phase in the form of spheres, powder, flakes, or fibers. In the case of PLA biocomposites two different categories can be distinguished, one in which products are mainly used in the field of medical applications and a second in which applications are in the field of structural plastics intended for other uses. The focus in this chapter is on the second of these fields and on products in which the reinforcing phase is a natural fiber. PLA biocomposites used in medicine have often been reinforced with bioactive mineral fillers like zirconia, magnesium oxide, β-tricalcium phosphate,5 or hydroxyapatite,6 and the use of carbon fibers has also been reported.7 The use of inorganic fillers in polylactides has been described in various reports and scientific papers.8−10 The main reason for the potential use of such composites is to mimic the mechanical properties of bone in surgical applications. Jones5 prepared composites of PLLA and 10 w/w% fillers (whiskers of hydroxyapatite, zirconia, magnesium oxide, and β-tricalcium phosphate) by injection molding. The most apparent change due to the incorporation of the fillers was a 30% decrease in tensile strength as well as a significant increase in stiffness. The degradation behavior of the prepared materials was also tested both in vitro and in vivo. In another study, the use of hydroxyapatite in polylactide was reported to improve the strength, stiffness, and hardness of the material.6 Studies involving inorganic fillers have also included reinforcement of PLLA with calcium metaphosphate fibers,8 talc,11 and clay.12 The Young’s modulus for the PLLA−clay composite increased with increasing clay addition levels at filler contents below 10 w/w%. The clay was also found to influence polymer nucleating behavior.12 The incorporation of flake-formed mineral filler (mica) into PLLA has been shown to affect the heat distortion temperature at filler contents between 10 and 20 w/w%.13 Lactic acid-based polyesterurethanes have been filled with several different inorganic fillers and their mechanical properties have been studied.14 The use of mineral fillers (e.g., hydroxyapatite and magnesium oxide) in polylactide slows down autocatalytic hydrolysis by the buffering effect of the minerals.15 Similar observations concerning hydrolysis have been made for an injection-molded bioresorbable composite of PDLLA and granules of natural coral (40/60 w/w%).16

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Natural Fiber Composites

Natural fiber-reinforced thermoplastics are attracting increasing interest for application in the automotive, building materials, and other industries.17,18 A number of obvious advantages are associated with the use of natural fibers, including low cost, availability from renewable resources, low density, high specific properties, nonabrasive processing, and lack of residues upon incineration. The most apparent disadvantages are moisture absorption leading to fiber swelling, low thermal resistance, local and seasonal quality variations, and flammability.19 In North America, wood fiber-reinforced polyethylene is now taking market share from preservative-treated wood or naturally durable wood products in the outdoor decking market.20 In Europe, where this industry is presently less developed than in North America, a number of companies are producing similar products for a variety of applications including cladding, highway or railway noise barriers, window joinery, and industrial paper roll cores. In both Europe and North America, the automotive industry now uses wood- or plant fiber-reinforced thermoplastics in car interiors because of their ease of processing, low cost, and low weight. In the context of the European automotive industry, such materials derived at least in part from renewable resources have marketing appeal and fit with European Union legislation on energy efficiency and end-of-life vehicle disposal. Structural composite materials have also been manufactured by using natural fibers of flax, hemp, jute, and sisal in combination with a matrix of polyolefins or thermosetting resins.21

17.2.3

Natural Fiber Biocomposites

As an extension to the considerable amount of research already undertaken on processing and properties of natural fiber composites, a number of researchers have explored the concept of natural fiber-reinforced biodegradable polymer composites. One of the earliest groups to investigate this topic was based at the Institute of Structural Mechanics of the German Aerospace Centre in Braunschweig.22 There are several reasons for interest in natural fiber biocomposites including mechanical property enhancement relative to the parent polymer, improved thermal stability, biodegradability, and net cost reduction resulting from the partial replacement of polymer by natural fibers. In cases where the biodegradable polymer is derived from renewable resources (i.e., a biopolymer), a further advantage arises because the composite is then totally derived from renewable raw materials. Herrmann et al.23 patented biocomposites containing natural fibers and biodegradable matrices for applications as building materials. In this patent the title materials contain natural fibers (e.g., flax, hemp, ramie, sisal, or jute) and a biodegradable matrix such as cellulose diacetate or a starch derivative. In a review article, Mohanty et al.24 discussed biofibers, biopolymers, and biocomposites and commented on the relative lack of research on biocomposites. In general, research on biodegradable polymers as composite matrices Copyright © 2005 by Taylor & Francis

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has probably been rather limited in comparison with research on commodity thermoplastics because of the relatively poor availability and high price of biodegradable polymers. However, as discussed by Mohanty et al.,24 a variety of biopolymers have in fact been investigated as matrices for production of natural fiber biocomposites. For example, Wollerdorfer and Bader25 prepared composites based on Biopol (Metabolix, Inc., Cambridge, MA, USA), a polyhydroxybutyrateco-valerate (PHBV), and a blend containing Mater-Bi (Novamont SpA, Novara, Italy), a thermoplastic starch, among other polymers. The research focused on the influence of natural fibers on the mechanical properties of biodegradable polymers but PLA was not included in these studies. Other examples in the literature include the processing of cellulose with polyhydroxybutyrate (PHB)26 and the investigation of the effects of different reinforcing fibers such as wheat straw, wood, or pineapple on PHB or PHBV.27−29 The review by Mohanty et al.24 contains no reference to research on PLA biocomposites. The relative lack of attention to PLA as a composite matrix is possibly explained by the fact that, unlike Biopol, for example, this biopolymer has only become available for sampling in recent years. 17.2.4

PLA Biocomposites

There are a number of reasons for potential interest in PLA as a matrix in natural fiber biocomposites. First, of all the biopolymers, PLA is arguably now one of the most advanced in terms of its commercialization. Second, PLA has good mechanical properties that are similar to those of polystyrene. Third, PLA can be melt-processed with standard processing equipment at temperatures below those at which natural fibers start to degrade.

Previous and Current Research Activities in PLA−Natural Fiber Composites There has been European Union support for research on biocomposites based on PLA in combination with natural fibers. An example is the project FAIR-CT-98-3919 (New functional biopolymer−natural fiber composites from agricultural resources) in which one of the key objectives was to manufacture demonstration parts on a pre-competitive level with the automotive industry as the main potential market. Within this project, Lanzillotta et al.30 prepared biocomposites with flax fibers and either PLA or starch as the biopolymer matrix. The research was focused on the idea of converting biocomposites into products for real automotive applications. Research on PLA−natural fiber composite materials was undertaken within an industry-university consortium project in Finland.31 The preparation of the composites of PLA and natural fibers was performed in a twostep procedure in which the polymer was first compounded with the fibers using a twin-screw extruder. In the second step the compounded material was injection molded into standard ISO 3167 multi-purpose test bars. The 17.2.4.1

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natural fibers were either flax or cotton and the fiber content was fixed at 30 w/w%. Composite mechanical properties were characterized and properties such as long-term stability under various conditions, hydrolytic stability, flammability, and the effect of recycling or reprocessing were also studied to some extent. Nishino et al.32 at Kobe University in Japan prepared composites from kenaf fiber and PLLA and evaluated composite mechanical properties. Shibata et al.33 studied the use of abaca fibers as reinforcement in a number of biodegradable polymers, including a polylactide with weight-average molecular weight (Mw) of 123,000. In this research, materials were agglomerated using a twin rotary mixer and then granulated and injection molded to obtain samples for mechanical property testing. Enzyme-retted flax fiber reinforcement of PLA has been investigated by Oksman et al.34 The fibers were dried to less than 1% moisture before processing and then used in the form of a hand-made roving fed into a twinscrew extruder with PLA. A naturally derived plasticizer, triacetin or glyceryl triacetate, was added. Injection-molded test specimens were examined using mechanical tests, dynamic mechanical thermal analysis (DMTA), gel permeation chromatography (GPC), and scanning electron microscopy (SEM). Plackett et al.35 used a film-stacking method, combining PLA films with jute fiber mats, to make composite panels (Figure 17.1). Andersen36 has previously described the rapid press consolidation technique used as part of this method. The resulting PLA composites were characterized in terms of mechanical properties. Tensile specimen fracture surfaces were examined using SEM, and degradation of PLA during processing was studied by means of GPC.

PLA matrix Jute mats PLA matrix

Pre-compression Heating under vacuum Cooling and consolidation

Composite

FIGURE 17.1 Schematic of the process for making PLA–jute composites using a film-stacking method.

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17.2.4.2 Processing and Processability of PLA–Natural Fiber Composites In the European Union FAIR project involving Fiat Research and other partners,30 injection-molded PLA composites containing flax fibers were prepared using temperatures of 155°C in the hopper and 165°−170°C in the nozzle combined with a mold temperature of 40°C. The flax fibers were pre-pelletized in order to allow successful feeding into the processing equipment and, as in normal PLA processing, the polymer granulate was predried before use. The impact of processing on flax fiber length was noted. Pure flax fiber with a starting length of 14 mm was reduced to 0.3−0.4 mm after injection molding. In an alternative process, Lanzillotta et al.30 made hybrid fleeces from flax and PLA fibers by a wet fleece technique with flax percentages of 30−65%. PLA fibers were dried thoroughly, first in a circulating air drier at 40°C for 3 days and, second, in a vacuum drier at 40°C for at least 8 hours. The dried fleeces were pre-heated for a couple of minutes in an oven at 195°−200°C and then hot-pressed for about 15 min. Compression-molded sheets with 50% flax content showed better tensile strength properties than those obtained by injection molding but the Young’s modulus was reduced, which the authors suggested was due to the presence of voids in the composite panels, as shown by SEM analysis. The production of demonstration parts using flaxreinforced PLA included an injection-molded car interior cover panel and a compression-molded fan blade for a truck engine (Figure 17.2). This research demonstrated that PLA biocomposites could be produced without processing difficulties; however, it was not possible to significantly increase the properties of the biopolymer matrix. The conclusion was that PLA biocomposites might still be of interest but in the context of flax as a low-cost plantderived natural filler for the relatively expensive PLA biopolymer. One of the FAIR project findings was that PLA had to be plasticized in order to obtain a match between the mechanical properties of the biofibers and the polymer matrix. As noted by other researchers, feeding of plant fibers into an extruder was a significant challenge. Tests on biodegradation

FIGURE 17.2 PLA–flax composites for application in the automotive industry. (From Lanzillotta, C. et al., Proc. Annu. Tech. Conf. Soc. Plast. Eng., 60, 2185, 2002. With permission.)

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of the composites showed complete biodegradability of PLA and the fiber reinforcement. Future actions proposed by the project organizers in their final report included steps to improve the toughness of PLA and larger scale production of automotive demonstration parts by injection molding or compression molding. The film-stacking process lends itself to use of compatibilizer films, and this approach, including its application to PLA, is contained in an international patent application.37 A key feature of the process is the use of a stage in which a polymer film/fiber mat lay-up is heated under vacuum. The application of vacuum is thought to provide a number of advantages including improved consolidation, reduced composite porosity, and a reduced tendency to thermal and hydrolytic degradation of polymers or fibers. A recent review of natural fiber modification for use in biocomposites discusses fiber treatments using compatibilizers to improve performance in combination with biopolymers such as Biopol and Sconacell A.38 The susceptibility of PLA to degradation during processing has been reported in a number of studies.39,40 Nurminen41 also found that the polymer degraded during preparation of PLLA−flax composites. Variations in processing and potential ways of modifying both the matrix and the fibers in injection-molded composites were examined in a Finnish study.31

17.2.4.3 Mechanical Properties of PLA−Natural Fiber Composites The mechanical properties of fiber-reinforced composites are affected by the degree of filling, fiber length, the aspect ratio of the fibers or fiber bundles, fiber morphology, and the orientation of the fibers or fiber bundles42 as well as the degree of cellulose polymerization within the fiber. The mechanical properties of PLA composites are also likely to depend on the properties of the specific polylactide (e.g., molar mass, residual lactide content) as well as on processing conditions (e.g., catalyst, predrying and processing temperatures). Lanzillotta et al.30 discovered that the tensile strength of injection-molded PLA was not improved when 20−40 w/w% flax was incorporated. The authors attributed this to poor adhesion between flax and PLA. The dramatic reduction in fiber length observed by these researchers may also partly explain the lack of improvement in composite tensile strength. However, the Young’s modulus of the biocomposites did increase linearly with increasing flax content. The authors also investigated chemical modification of fibers (e.g., etherification, esterification) as well as reactive extrusion to modify PLA during processing but concluded that the resulting composite property improvements did not justify the costs involved. Melt modification of composites was also investigated in a Finnish project.31 The tensile strength of PLLA more than doubled as a result of melt modification of the polymer using unsaturated anhydrides in combination with organic peroxides. The same concept was applied earlier to improve interfacial adhesion in blends of PLA and starch.43 Surprisingly, although the type of fiber influenced the composite tensile strength, this property seemed Copyright © 2005 by Taylor & Francis

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not to be affected in any significant way by the order of addition of the chemicals. The highest tensile strength measured was 78 MPa, which was obtained for a PLLA−flax composite modified with 0.25 w/w% maleic anhydride and 0.25 w/w% t-butyl peroxybenzoate. The tensile strength was also found to depend on the treatment of the flax fibers and the possibility of correlating composite properties to the oxygen/carbon ratio, as measured by xray photoelectron spectroscopy at the surface of the fibers, was suggested.44 Melt modification of PLLA−flax composites with peroxide and maleic anhydride caused an increase in the Charpy impact strength.45 The research group at the Institute of Structural Mechanics of the German Aerospace Centre produced and tested PLA biocomposites using natural fiber mats. The results of their research gave a composite tensile strength of about 90 MPa and Young’s modulus of about 10 GPa.46 In a later report, the same authors mentioned a bending modulus for PLA biocomposites that was comparable with that of glass fiber-reinforced polymers.22 The incorporation of fibers in a polymer matrix is known to affect the moduli of the material significantly, but this effect is dependent on the type and content of fiber in the matrix. The elasticity modulus measured for PLLA filled with 30% cotton linter or flax fibers has been shown to increase in the range of 1.5 to 2.5 times that of unreinforced PLA, depending on the type of fiber modification.31,41 The mechanical properties of the kenaf fiber and PLLA composites prepared by Nishino et al.32 were better than those of either kenaf fiber or PLLA alone. The kenaf sheet had anisotropic mechanical properties leading to a similar anisotropy in the composites. The Young’s modulus and tensile strength were said to be comparable with those of traditional composites and therefore kenaf fiber was proposed as a good candidate for reinforcement in high-performance biodegradable composites. Levit et al.47 prepared PLA biocomposites using extrusion and compression molding. Reinforcements included paper waste fibers, rayon non-woven fabric, and wood flour. The mechanical properties of PLA were retained when the polymer was filled with up to 32% waste paper fiber. Tensile strength was retained with wood flour up to 15 w/w% filler and improved with rayon fabric reinforcement. The authors also reported that paper coated with PLA showed an improved tensile strength under standard test conditions and had significantly improved tensile strength in comparison with uncoated paper after wetting. The mineralization rate of PLA in an aerobic soil test increased considerably with the addition of a cellulosic component. The results reported by Oksman et al.34 showed that PLA−flax composites had better mechanical properties than PP(polypropylene)−flax composites and that, although the Young’s modulus was significantly improved through flax reinforcement, the composite tensile strength was not significantly better than that of the parent polymer. These results are similar to those obtained by Lanzillotta et al.30 as discussed above. Addition of triacetin plasticizer at levels of 5−15% resulted in a decrease in both the tensile modulus and tensile strength of the PLA−flax composites. Triacetin also had no Copyright © 2005 by Taylor & Francis

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influence on the Charpy impact strength of PLA or PLA−flax composites. As noted by other researchers,48 triacetin plasticizer addition causes a significant decrease in the glass transition temperature (Tg) when added to PLA, but this was not observed when triacetin was added to PLA−flax combinations. Oksman et al.34 concluded from DMTA studies that there was no interaction effect between flax and PLA unless the plasticizer was added. GPC results indicated that there was no degradation of PLA after extrusion in the case of either PLA or PLA/flax processing. In the research by Shibata et al.,33 the flexural modulus of the PLA−abaca composites increased from ~ 3.5 GPa to 5.5 GPa as fiber content increased from 0 to 20 w/w%. However, flexural strength at about 110 MPa was not significantly changed by fiber addition. The authors also investigated several fiber treatments and found that a further increase of flexural modulus was possible when the surfaces of abaca fibers were esterified. The film-stacking technique used by Plackett et al.35 is a new method for preparing PLA−plant fiber composites. Typical mechanical properties for composites prepared using this method are included in the general PLA biocomposite property overview given in Table 17.1. As in the work reported by Oksman et al.,34 the authors used GPC to analyze the molecular weight of the PLA before and after processing. Results of the GPC analyses indicated a general trend of decreasing Mw with processing from granulate to film to composite and also a decrease in Mw with increasing processing temperature. However, given that differences in Mw of 10,000 or less were probably not significant using this method, the changes were not as important as they might otherwise seem, especially if, as is likely, a relatively low processing temperature (e.g., 190°C) was selected. The best tensile properties were obtained when processing at 210°C, which may suggest that polymer flow TABLE 17.1 Tensile and Impact Properties of PLA–Natural Fiber Compositesa Fiber Type and Content

PLA Type

Flax (30%)

PLLA

Flax (40%)

Polylactic acid Polylactic acid Polylactic acid PLLA

Flax (40%) Flax (50%) Cotton linter (30%) Jute (40%) a

PLLA

Tensile Strength (MPa)

Young’s Modulus (GPa)

Impact Strength (kJ/cm2)

Reference

Extrusion + melt modification Injection molding

70

8.4

17.8

31

68

7.2

N.S.

30

Injection molding

45

7.2

11.0

34

Compression molding Extrusion + melt modification Compression molding

99

6.0

N.S.

30

30

6.8

5.8

45

100

9.4

14.3

35

Manufacturing Method

N.S., not specified.

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properties were optimum at this temperature while the composite properties were not yet compromised by thermal degradation under these conditions. Images of tensile fracture surfaces obtained using an environmental scanning electron microscope or ESEM (Figure 17.3), show void spaces between fibers and matrix. This suggests that better PLA−natural fiber interaction should be possible and might lead to further improvement in some mechanical properties.

17.2.4.4 Environmental Stability of PLA−Natural Fiber Composites Hydrolytic degradation of unmodified PLA−flax composites has been studied under controlled degradation conditions.41 The degradation of the matrix proceeded in a similar way to the degradation of unfilled PLA but at a somewhat faster rate. This finding may be due to faster water diffusion into the composite through the fibers or at the fiber-matrix interface.

FIGURE 17.3 ESEM photomicrograph of a PLA-jute tensile fracture surface showing void spaces (see white arrows) between jute fiber bundles and the PLA matrix.

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Mäkinen45 investigated the environmental stability of PLA−flax composites and found that the mechanical properties did not change noticeably during 6 months at 23ºC and 50% relative humidity (RH). This is a clear indication that the incorporation of flax fibers into PLLA can improve the stability of the polymer. Modification of the matrix by peroxides showed a decreasing effect on polymer stability, as did the use of cotton linter instead of flax fibers. Weathering tests of PLLA−natural fiber composites have also shown that cotton linter-reinforced PLLA is more sensitive to changes in the environment than the corresponding flax composites.45

17.2.4.5 Other Characteristics of PLA−Natural Fiber Composites 17.2.4.5.1 Thermal Properties One clear advantage of PLA−natural fiber composites is that the temperature window for practical applications is significantly enlarged. In one study, the tensile strength at elevated temperature (50ºC) was compared with that measured at 23ºC. On average the tensile strength at 50ºC was 31.4 ± 4.5 MPa, which was about 60% of the tensile strength at 23ºC (50.1 ± 4.5 MPa).41 This change in mechanical properties in going from 23°C to 50°C was less than expected. The change in mechanical properties of PLA−natural fiber composites was also studied under different conditions (various temperatures and relative humidities). The effect of the environment on mechanical properties (i.e., tensile strength, modulus, and impact strength) was found to depend on the material selection (i.e., fibers), the treatment of the fibers, and modifications during compounding. The most harmful environment in terms of composite properties involved repeated exposure to temperature changes at high humidity (90−100% RH).45 17.2.4.5.2 Flammability The flammability of PLA−natural fiber composites has been studied using the test procedure UL 94 (Test for flammability of plastic materials for parts in devices and appliances).31 This UL standard is now harmonized with ISO standard 9772.49 None of the composites passed the requirements for this flammability test. An ammonium polyphosphate-based fire retardant was tested and the highest anti-flame class (V0) was reached when the additive was added either to the fibers or to both the fibers and the matrix. The second highest anti-flame protection (V1) was obtained when the additive was added only to the matrix. However, a clear decrease in the mechanical properties was noticed due to the addition of the fire retardant to the matrix.31 17.2.4.5.3 Recycling by Reprocessing A study conducted by researchers from Finland45 investigated the recycling of a PLA−flax composite by means of secondary processing. A decrease in the tensile strength (23%), impact strength (8%), and E-modulus (5%) was

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reported as well as a corresponding decrease in the PLA molar mass. The effect of a decrease in the fiber length was not studied but was considered to be one factor contributing to the observed changes in mechanical properties.

17.3

Applications

Applications of natural fiber biocomposites based on PLA as the matrix are relatively undeveloped at this time, but there are some interesting and potentially large-scale opportunities on the horizon. As outlined by Lanzillotta et al.,30 the automotive sector has already adopted natural fiber composites for car interior and exterior uses, and composites based on biopolymers are a logical next step. The reason for this interest is largely driven by the appeal of low-weight materials with attractive performance characteristics entirely derived from renewable resources. Automotive applications could include headliners, door panels, parcel racks, and trunkliners as well as other products such as the fan blade produced in the FAIR project.30 Automobile manufacturers are already studying possible applications. For example, the Toyota environmental report for 2002 indicates that this company is exploring the use of biodegradable polymers and has been conducting research on aspects such as heat resistance for eventual application of these materials in car interiors. Prototype parts (e.g., interior pillar garnishes) made from biodegradable polymers were included in the Toyota ES3 concept car unveiled at motor shows in 2002. These developments follow Toyota’s takeover of Shimadzu’s PLA business and technology in Japan. A logical next step from evaluation of biopolymers is the use of natural fiberreinforced biopolymers in automotive applications. Further information about natural fiber applications within the automotive sector can be found in Chapter 7 of this book. Japanese companies recently announced the development of notebook computers in which the housing is based on PLA. Fujitsu Ltd and Fujitsu Laboratories Ltd claimed the first such development in June 2002. Fujitsu’s biodegradable plastic is said to use an optimized formula to produce a polylactic acid polymer derived from the starch of corn, potatoes, and other plants, and with the same strength and rate of shrinkage as PC/ABS plastic (polycarbonate/acrylonitrile-butadiene-styrene copolymer). Fujitsu also claims that production of parts from its biodegradable plastic consumes only about half of the energy required to produce conventional plastic components. Japanese companies are now starting to use natural fiber−biodegradable polymer composites for notebook computer housings.50 NEC announced development of kenaf−PLA products in January 2003. Significantly, the NEC press release indicated that 20% kenaf fiber reinforcement resulted in an increase in thermal deformation temperature from 67°C to 120°C and an increase in bending modulus from 4.5 GPa to 7.6 GPa. These

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properties exceed those of ABS resin and fiberglass-reinforced ABS resin used in packaging. There are also products under development in Japan based on paper in combination with PLA.51 A wider range of applications for biocomposites, including those based on PLA, can be forecast as a result of the increasing need for environmentally sustainable products. Examples could include furniture, television housings, and other consumer goods. Nickel and Riedel22 described several projects in which biocomposites have been studied for use in designer office chairs, car door paneling, and extruded profiles. Another area of application may be in packaging where PLA film is already of considerable interest for some food products because of its barrier properties, transparency, and biodegradability. Schaible and Fritschi52 have filed a patent application for compostable molded articles, especially for use as disposable packaging articles, containing plant and other natural raw materials, including lactide polymers.

17.4

Future Directions

A number of challenges lie ahead if PLA biocomposites are to realize their full commercial potential. These challenges are both economic (i.e., reductions in the price of PLA for some applications) as well as technical. Among the technical challenges, better composite properties should be achievable through improvements in adhesion between PLA and natural fibers. For some uses improved composite toughness (i.e., reduction in brittle character) will be especially important and further developments can be anticipated from research on toughening of PLA by means of polymer blending, copolymers, and other approaches. A further challenge will be to identify and utilize process additives (e.g., compatibilizers, lubricants, coloring agents, fire retardants) that are derived from renewable resources and are therefore fully consistent with the concept of environmentally sustainable composites. For practical purposes, PLA−natural fiber biocomposites will have to meet performance requirements and also not degrade over the expected product service life. In this context, resistance to moisture absorption and assurance of long-term durability under specified conditions will be important (i.e., biodegradability when required and not before). As with other natural fiber composites, heat resistance and odor characteristics will have to meet tight specifications set by manufacturers if, for example, automotive interior applications are to be achieved. In terms of next-generation materials, the investigation of PLA−natural fiber nanocomposites is a promising area of research that has so far received relatively little attention. The use of nanoscale natural reinforcing fibers should, in principle, allow a greater percentage of the strength and stiffness of cellulose to be utilized while also opening up the possibility of other

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composite enhancements related to thermal properties, mechanical properties, and visual appearance. In conclusion, as with many developments in new materials, the best approach for the future direction of natural fiber biocomposites incorporating PLA and/or other biopolymers should not necessarily target competition with and replacement of existing composite products. Instead, a preferred approach will be to use the beneficial properties of these composites, including their availability from fully renewable raw materials, in applications where such properties are considered particularly advantageous.

References 1. European Standard EN 13432, Packaging Requirements for Packaging Recoverable Through Composting and Biodegradation—Test Scheme and Evaluation Criteria for the Final Acceptance of Packaging, 2000. 2. Loick, H., Biologisch abbaubare Verpackungen im non-food-Bereich, 7. Fachtagung Biologisch abbaubare Werkstoffe 2000, Würtzburg, Germany, Feb. 23–24, 2000. 3. Directive 2000/53/EC of the European Parliament and of the Council of 18 September 2000 on end-of life vehicles—Commission Statements, Off. J. L 269, 21/10/2000, pp. 0034–0043. 4. Södergård, A. and Stolt, M., Properties of lactic acid-based polymers and their correlation with composition, Prog. Polym. Sci., 27, 1123, 2002. 5. Jones, N.L., The Development of a Degradable Polymer Composite to be Used as a Clinical Device, Vols. I–II, Ph.D. thesis, University of Liverpool, UK, 1996. 6. Verheyen, C.C.P.M. et al., Evaluation of hydroxylapatite poly(L-lactide) composites—Mechanical behaviour, J. Biomed. Mater. Res., 26, 1277, 1992. 7. Wan, Y.Z. et al., In vitro degradation behavior of carbon fiber-reinforced PLA composites and influence of interfacial adhesion strength, J. Appl. Polym. Sci., 82, 150, 2001. 8. Kelley B.S., Dunn, R.L., and Casper, R.A., Totally resorbable high-strength composite material, Polym. Sci. Technol. (Plenum) 35 (Adv. Biomed. Polym.), 75, 1987. 9. Bleach, N.C. et al., Effect of filler content on mechanical and dynamic mechanical properties of particulate biphasic calcium phosphate–polylactide composites, Biomaterials, 23, 1579, 2002. 10. Roether, J.A. et al., Novel bioresorbable and bioactive composites based on bioactive glass and polylactide foams for bone tissue engineering, J. Mater. Sci.— Mater. Med., 13, 1207, 2002. 11. Södergård, A. et al., Processable polylactides, European Patent 0 737 219 B1, 1999. 12. Ogata, N. et al., Structure and thermal/mechanical properties of poly(L-lactide)clay blend, J. Polym. Sci., Part B: Polym. Phys., 35, 389, 1997. 13. Södergård, A., unpublished data, Åbo Akademi University, 1997. 14. Hiljainen-Vainio, M., Heino, M., and Seppälä, J., Reinforcement of biodegradable poly(esterurethane) with fillers, Polymer, 39, 865,1998.

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15. Van der Meer, S.A. et al., The Influence of Basic Filler Materials on the Degradation of an Amorphous Copolymer of D- and L-Lactide, Proceedings of the 12th European Conference on Biomaterials, Porto, Portugal, Sept. 10–13, 1995. 16. Li, S. and Vert, M., Hydrolytic degradation of coral/poly(DL-lactic acid) bioresorbable material, J. Biomat. Sci., Polym. ed., 7, 817, 1996. 17. Bledzki, A.K., Sperber, V.E., and Faruk, O., Natural and Wood Fibre Reinforcement in Polymers, RAPRA Review Report 13 (8), Report 152, 2002. 18. Evans, W.J. et al., Natural Fibres and Their Composites: A Global Perspective, Proceedings of the 23rd Risø International Symposium on Materials Science: Sustainable Natural and Polymeric Composites—Science and Technology, Lilholt, H. et al., Eds., Risø National Laboratory, Roskilde, Denmark, 2002, p. 1. 19. Stamboulis, A., Baillie, C.A., and Peijs, T., Effects of environmental conditions on mechanical and physical properties of flax fibers, Composites: Part A, 32, 1105, 2001. 20. Clemons, C., Wood-plastic composites in the United States—the interfacing of two industries, Forest Prod. J., 52, 10, 2002. 21. Bledzki, A.K. and Gassan, J., Composites reinforced with cellulose based fibres, Prog. Polym. Sci., 24, 221, 1999. 22. Nickel, J. and Riedel, U., Structural materials made of renewable resources (Biocomposites), in Biorelated Polymers: Sustainable Polymer Science and Technology, Chiellini, E. et al., Eds., Kluwer Academic/Plenum Publishers, New York, 2001. 23. Herrmann, A.S., Hanselka, H., and Niederstadt, G., European Patent Application EP 687,711 (Cl. C08L97/02), 1995. 24. Mohanty, A.K., Misra, M., and Hinrichsen, G., Biofibres, biodegradable polymers and biocomposites: an overview, Macromol. Mater. Eng., 276/277, 1, 2000. 25. Wollerdorfer, M. and Bader, H., Influence of natural fibres on the mechanical properties of biodegradable polymers, Ind. Crops Prod., 8, 105, 1998. 26. Gatenholm, P. and Mathiasson, A., Biodegradable natural composites. II. Synergistic effects of processing cellulose with PHB, J. Appl. Polym. Sci., 51, 1231, 1994. 27. Avella, M. et al., Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and wheat straw composites: thermal, mechanical properties and biodegradation behaviour, J. Mater. Sci., 35, 829, 2000. 28. Reinsch, V.E. and Kelley, S.S., Crystallization of poly(hydroxybutyrate-cohydroxyvalerate) in wood fiber-reinforced composites, J. Appl. Polym. Sci., 64 1785, 1997. 29. Luo, S. and Netravali, A.N., Interfacial and mechanical properties of environment-friendly green composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin, J. Mater. Sci., 34, 3709, 1999. 30. Lanzillotta, C., Pipino, A., and Lips, D., New Functional Biopolymer Natural Fiber Composites from Agricultural Resources, Proceedings of the Annual Technical Conference of the Society of Plastics Engineers, Vol. 60, 2002, p. 2185. 31. Haapanen, P. and Mäkinen, K., Biokomposiitit konstruktiivisina materiaaleina, Project Report, PRO6/P3014/03, VTT Processes, Tampere, Finland, 2003. 32. Nishino, T. et al., Kenaf reinforced biodegradable polymer composites, Comp. Sci. Tech., 63, 1281, 2003. 33. Shibata, M. et al., Biocomposites made from short abaca fiber and biodegradable polyesters, Macromol. Mater. Eng., 288, 35, 2003. 34. Oksman, K., Skrivfars, M., and Selin, J.-F., Natural fibres as reinforcements in polylactic acid composites, Comp. Sci. Tech., 63, 1317, 2003.

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35. Plackett, D.V. et al., Biodegradable composites based on L-polylactide and jute fibres, Comp. Sci. Tech., 63, 1287, 2003. 36. Andersen, T.L., Development of a Rapid Press Consolidation Technique for Continuous Fibre Reinforced Thermoplastic Composites, Proceedings of the 18th Risø International Symposium on Materials Science: Polymeric Composites – Expanding the Limits, Andersen, S.I. et al., Eds., Risø National Laboratory, Roskilde, Denmark, 1997, p. 237. 37. Andersen, T. L. and Plackett, D.V., Natural fiber-reinforced thermoplastic composites and their manufacture and use, PCT Int. Appl., 35pp., WO 0264670 A1 20020822, 2002. 38. Mohanty, A.K., Misra, M., and Drzal, L.T., Surface modifications of natural fibers and performance of the resulting biocomposites: an overview, Comp. Interf., 8, 313, 2001. 39. Södergård, A. and Näsman. J., Stabilization of poly (L-lactide) in the melt, Polym. Degrad. Stab., 46, 25, 1995. 40. Wachsen, O., Platkowski, K., and Reichert, K.-H., Thermal degradation of polyL-lactide—studies on kinetics, modelling and melt stabilisation, Polym. Degrad. Stab., 57, 87, 1997. 41. Nurminen, A., Pellavakuitulujitetun polylaktidin ruiskuvalu ja ominaisuudet, Master Thesis, Technical University of Tampere, Tampere, Finland, 2000. 42. Strong, A.B., Composite properties, in High Performance and Engineering Thermoplastic Composites, Strong, A.B., Ed., Technomic Publishing, Lancaster, PA, 1993, chap. 4. 43. Carlson, D. et al., Maleation of polylactide (PLA) by reactive extrusion, J. Appl. Polym. Sci., 72, 477, 1999. 44. Korsman, Å., Fysikalisk-kemisk karakterisering av naturfibrer använda som förstärkningmaterial i bionedbrytbara polymerkompositer, Master Thesis, Åbo Akademi University, Turku, Finland, 2002. 45. Mäkinen, K., Biokomposiittien pitkäaikaiskestävyys, Master Thesis, Technical University of Tampere, Tampere, Finland, 2002. 46. Riedel, U. and Nickel, J., Natural fibre-reinforced biopolymers as construction materials–new discoveries, Die Ange. Makromol. Chemie, 272, 34, 1999. 47. Levit, M.R. et al., Composites Based on Poly (lactic acid) and Cellulosic Fibrous Materials: Mechanical Properties and Biodegradability, Proceedings of the Annual Technical Conference of the Society of Plastics Engineers, Vol. 54, 1996, p. 1387. 48. Ljungberg, N. and Wesslén, B., The effect of plasticizers on the dynamic mechanical and thermal properties of poly (lactic acid), J. Appl. Polym. Sci., 86, 1227, 2002. 49. ISO Standard 9772, Cellular plastics—Determination of Horizontal Burning Characteristics of Small Specimens Subjected to a Small Flame, International Standards Organisation, 2001. 50. Nishino, T., personal communication, 2003. 51. Kawashima, N., personal communication, 2003. 52. Schaible, T. and Fritschi, H., Method for Manufacturing Compostable Moldings and Pellets for This Purpose, European Patent Application, EP 97-114959 19970829, 1998.

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18 Bacterial Polyester-Based Biocomposites: A Review

Alma Hodzic

CONTENTS 18.1 Introduction 18.1.1 Bacterial Polyesters 18.1.2 Natural Fibers 18.2 Chemical Modification of Natural Fibers 18.3 Effect of Processing Temperature 18.4 Effect of Silane Coupling Agent 18.5 Effect of Hydrogen Bonding Additives 18.6 Effect of Plasticizers 18.7 Physics of Transcrystalline Region 18.8 Conclusion and Future Directions References ABSTRACT This chapter is an overview of modern research attempts to optimize the thermomechanical properties of bacterial polyester-based natural fiber composites. The composite materials containing natural fibers and bacterial polyesters, either as individual biopolymers or biopolymer blends, suffer from low-grade fiber-matrix adhesion, pullout under low stress, and generally poor mechanical properties due to brittleness of bacterial polyesters. Furthermore, the nominal processing temperatures of 180°C and above have been found to thermally degrade the bacterial polyesters and rapidly reduce their mechanical properties. The art of improving the quality of natural fibers by chemical treatments, in conjunction with the appropriate choice of additives to enhance the fiber-matrix adhesion and to provide flexibility into the initially brittle composite, produced the biocomposites having enhanced stiffness up to 145% and reduced their processing temperature down to 168°C. The chapter discusses that the outstanding improvements of

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the mechanical properties of the biocomposites could be attributed to the formation of a transcrystalline region between the natural fibers and the biopolymers, which is further enhanced with the activity of molecular bonding promoted by the additives.

18.1 Introduction Since the beginning of this century, global economic activities have increased more than 50-fold.1 The incredible rate of growth has raised concerns about the consequences of accelerated production and consumption, and greater attention has been given to development of sustainable economic and industrial practices that rely on natural and renewable sources of energy. One of the most important developments emerged in the utilization of biologically derived polymers, suitable for blending and composite processing. The obvious choice of reinforcement for such biopolymers can be any type of natural fiber. The natural fiber-based biocomposites should be designed according to the following criteria: (1) reliable consolidation of natural fibers and biopolyesters, (2) improved mechanical properties and flexibility as compared to the pure biopolymer, (3) improved biological resistance during the life cycle of the composite, (4) processing and manufacturing methods should be economically justified, and, most importantly, (5) the novel biodegradable biocomposites should be competitive to easily produce thermoplastic composites. Currently, biocomposites are produced only in a few laboratories for investigative purposes, as the high cost of such composites is dominated by the extremely expensive biopolyesters of limited availability. The present challenge is to design a biocomposite with increased thermomechanical properties at lower costs, and thus economically justify further increase in production of biopolyesters, and generate further reduction in costs. Such a closed economical loop of biocomposites manufacturing will establish a whole new sustainable market. Scientists have approached the point where the first biodegradable biocomposites are being introduced into the automotive market. 18.1.1 Bacterial Polyesters Polyesters represent a primary choice among biodegradable plastics due to their hydrolysable ester bonds. The polyester family is made of two major groups: aliphatic (linear) polyesters and aromatic (aromatic rings) polyesters. Biodegradable aliphatic polyesters, which have been developed commercially, are listed in Table 18.1. The main representatives of poly(hydroxyalkanoates) (PHA) are poly(3hydroxybutyrate) (PHB) and poly(3-hydroxyvalerate) (PHV). PHAs are naturally produced by microbial process on sugar-based medium acting as Copyright © 2005 by Taylor & Francis

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carbon and energy storage material in bacteria. Homopolymers and copolymers of hydroxybutyric acid and hydroxyvaleric acid are synthesized by microbes where the polymer accumulates in the microbes’ cells during growth. Aliphatic polyesters are readily biodegradable, unlike aromatic polyesters, which exhibit excellent mechanical properties and resistance to microbial attack. All polyesters degrade eventually through the mechanism of hydrolysis, degradation induced by water. The rate of soil degradation of various biodegradable plastics has previously been measured by Hoshino et al.2 Copolymer of PHB and PHV (or PHBV) and a few other polymers were subjected to soil burial for 12 months, with 3-month inspection intervals and weight loss measurements. The rate of degradation of PBHV was found to be similar to that of poly(lactic) acid (PLA). Life cycle assessment for the bacterial biopolyesters is not yet available; however, a number of authors have estimated the energy requirements and CO2 or greenhouse gas emissions.3–5 It has been concluded that energy use and greenhouse emissions are often larger for PHB than for conventional polymers, considering the current technology. The results from the study are presented in Table 18.2. The present goal of PHA fermentation is thus to gain experience with this material and to offer potential solutions for initiating the sustainable market.6 Subsequently, the processing and manufacturing methods will undergo optimization in time and in energy dissipation. TABLE 18.1 Definition of Aliphatic Polyesters Aliphatic Polyesters Synthetic renewable

Synthetic nonrenewable

Naturally produced renewable

PLA: polylactic acid

PBS: poly(butylene succinate) PBSA: PBS adipate PCL: poly (caprolactone)

PHA: poly(hydroxyalkanoates) and their copolymers PHB: poly(hydroxybutyrate) PHB/PHV PHV: poly(hydroxyvalerate) PHB/PHH PHH: poly(hydroxyhexanoate)

TABLE 18.2 Energy Requirements for Plastic Production Cradle-to-Factory Gate Fossil Energy Requirements in GJ/t Plastic Substance

Process energy

Feedstock energy

Total

PHA grown in corn plants PHA by bacterial fermentation HDPE PET (bottle grade) PS (general purpose)

90 81 31 38 39

0 0 49 39 48

90 81 80 77 87

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PHB is a linear saturated polyester behaving as conventional thermoplastic material.7 The advantageous properties of PHB and its copolymers are biodegradability and biocompatibility, with the present drawbacks of poor thermostability and relatively low impact resistance. PHB undergoes a decrease in molecular weight above 170°C, which is proportional with the time, and the thermal degradation follows a random scission at the ester groups.8 Blending and composite manufacturing with PHB may result in a decrease of the melting temperature and improved manufacturing properties at lower processing temperatures. A recent overview9 of biodegradable polymers discussed potential use of biochemically synthesized PHB and its copolymers in industry. PHB has attracted much attention as a biodegradable thermoplastic polyester,10–13 and has high potential as an environmentally degradable plastic14 as it can be degraded to water and carbon dioxide under environmental conditions. Brittleness and slow crystallization of PHB inconvenience its processability.15 In order to improve these properties, PHB and PHV are biosynthesized into a copolymer (PHBV).16 PHBV copolymers were first manufactured in 1983 and were intended as biodegradable substitutes for various containers.17 The current and potential uses of PHB and its copolymers have been reviewed for motor oil containers, film formation, and paper-coating materials.18 Due to its biodegradability through natural media and easy processability, PHBV copolymers have been targeted for the replacement of nondegradable polymers in commodity application.19–21 The commercialization of PHBV has been prevented due to high cost, the small difference between thermal degradation and melting temperature, and low impact resistance due to its high crystallinity. The melting temperature has been reported at 180°C.22 However, it can be changed (decreased) with varying PHV content (see Table 18.323). Another disadvantage with PHB is its brittleness due to its high crystallinity. Many attempts to improve its properties include blending with polyalcohols such TABLE 18.3 Thermal and Mechanical Properties of Biopolyester Blends and Composites at 25°C Compared with Isotactic Polypropylene Material/Blend PHBV Copolymers 0% HV 11% HV 20% HV 28% HV 34% HV i-propylene

Melting Temperature (°C)

Glass Transition Temperature (°C)

175 157 114 102 97 174

9 2 ⫺5 ⫺8 ⫺9 ⫺17

Tensile Strength (MPa)

Elongation (%)

Elastic Modulus (GPa)

45 38 26 21 18 30

4 5 27 700 970 10

3.8 3.7 1.9 1.5 1.2 1.5

Source: From Mark, H.F. et al., Encyclopedia of Polymer Science and Technology, WileyInterscience, New York, 1964. With Permission.

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as poly(ethylene glycol),24 poly(propylene glycol), and castor oil.25 Plasticizers such as acylglycerols26 and dibutylphthalate27 have also been used with some success. Lower glass transition temperatures (Tg) as well as a decrease in crystallization temperatures (Tc) were obtained from incorporation of plasticizers. Copolyesters of PHB with PHV show increased flexibility.28 The two monomers in the copolymer have been shown to cocrystallize, a very unusual phenomenon for comonomers of differing structure in polymers. The cocrystals form slower than the crystals of homopolymer PHB, the crystallinity is decreased, and the melting temperature lowered in the copolymers. The decreased crystallization rate could be a problem for production of composites in the times experienced with typical processing equipment. In addition, the glass transition temperatures of the copolymers are decreased relative to the amount of comonomer included. This is beneficial in increasing the flexibility of the biopolyesters. The properties of polymers are determined by the polarity, the geometry, the stereochemistry, and segmental mobility of their chains. Polyesters are slightly polar polymers and their chains are interconnected via weak van der Waals atomic bonds. Little changes in chemical structure can strongly influence the chemical and physical properties of these polymers.7 18.1.2 Natural Fibers Recent research discusses a wide range of natural fibers that have been successfully used in many composites: jute, hemp, kenaf, ramie, sisal, and flax. These natural fibers are classified as bast natural fibers and provide structural strength to the plants of their origins. They possess low density, have high strength and stiffness relative to their density, low cost, good thermal and acoustic insulating properties, and are friendly to processing equipment (see Figure 18.1). In addition, the natural fibers are fully recyclable and combustible without any noxious gases. However, when used as reinforcement in polymers, a major problem is their incompatibility with hydrophobic polymers due to their hydrophilic nature. Insufficient wetting of the fibers by the polymer matrix is proven to lower the tensile strength29 and stiffness of a composite, as a weak interface yields under the stress being transferred from the polymer matrix to the fibers. Another problem with natural fibers is moisture absorption, as the inherent moisture is known to cause voids, reducing the strength of the composite.30 The moisture content in natural fibers is variable, depending on relative humidity or wetting of the composite. Moisture will interfere with melt dispersion and processing of the composites, since processing temperatures of the order of 180°C are necessary. In addition, moisture can adversely affect the curing process. The following sections cover the methods used in attempts to overcome the above limitations of biopolyesters and natural fibers to produce a competitive, well-consolidated strong biocomposite material with good fibermatrix adhesion. Copyright © 2005 by Taylor & Francis

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Young's modulus (max.) [GPa] Specific modulus [GPa cm3/g] Glass 80

60

70

40 Sisal 38

Flax

20 28

50

29 0 21 30

35.7

37 55

Jute

Hemp

FIGURE 18.1 Properties of natural fibers compared with those of glass fibers. (The specific modulus is the modulus divided by density.)

18.2 Chemical Modification of Natural Fibers Chemical modification of natural fibers is a necessary processing step in order to remove undesirable substances: waxes, lignin, and pectin material naturally inherent in cellulose fiber. These organic substances are useful as the natural matrix material during a plant’s life; however, the purity of the fiber’s surface will directly influence the mechanical properties of the composite. As a rule, a variety of surface treatments and grafting modifications are applied to all systems of natural fibers and biopolymers before other parameters are considered. The resulting mechanical properties are assessed in order to determine the optimal chemical procedure for a given system. The recently reported chemical treatment of jute fibers in jute–Biopol composites31 involved numerous steps of chemical treatment, which consisted of (1) washing in detergent solution (defatting), (2) washing with distilled water, (3) subjecting to a mixture of alcohol and benzene, (4) another cycle of distilled water, (5) alkali treatment with 5% NaOH solution, and, (6) the alkali-treated yarns were then subjected to graft copolymerization of Copyright © 2005 by Taylor & Francis

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acrylonitrile (AN) in a controlled manner. The chemical treatment is presented schematically in Figure 18.2. The mechanical properties used to compare the properties of pure and reinforced Biopol were established as the ratio between the properties of the composite and that of the pure polymer. For instance, the tensile strength factor was Tf, defined as the ratio of tensile strength of a composite to that of pure Biopol. The same principle was applied to bending strength factor (Bf), impact strength factor (If) and bending E-modulus factor (Bmf). All properties were measured along the yarn winding direction and perpendicular to the yarn winding direction (see Figure 18.3). Composites of alkali-treated jute yarn were found to reach better mechanical properties in comparison to composites with defatted yarn as well as NaO

Jute

OH

(Alkali-treated jute)

H C CN

H3C C COOCH3

30°C NaoH

CH2 H3C C COOCH3 Jute

H C CN

nCH2

CH2 O

CH2

CH3

O CH2

C COOCH3 (MMA) Ho +2 − Cu− IO 4

CH2

Jute

OH

CH CN (AN) +2

−

Cu− IO 4

Jute

O CH2

H C CN

H3C C COOCH3 CH2

60°C CH2

CH CN/Pyridine

CH2 H C CN

H3C C COOCH3 (MMA-g-Jute)

CH2 O

NC

CH2 CH2 O

Jute

O CH2 CH2 CN

(AN-g-Jute)

(Cynoethylated jute)

FIGURE 18.2 A schematic representation of the various surface modifications of jute. (From Mohanty, A.K. et al., J. Mater. Sci., 35, 2589, 2000. With permission.)

(a)

(b)

(c)

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FIGURE 18.3 Schematic representation of yarn winding and mechanical properties measurement (a) along the yarn winding direction, (b) perpendicular to the yarn winding direction, and (c) in either direction of cross-winding yarn-based composite. (From Mohanty, A.K. et al., J. Mater. Sci., 35, 2589, 2000. With permission.)

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AN-grafted yarn, at all percentages of jute in the composite. The alkalitreated yarns showed enhancement of 119% in tensile strength, 47% in bending strength, 145% in impact strength, and 39% in flexural modulus. The same properties for defatted yarn composites were 96, 26, 105, and 49%, respectively. Both composites exhibited better mechanical properties than that of the AN-grafted system. The superior mechanical properties of alkali-treated jute yarn composites were discussed in the light of improved fiber-matrix adhesion, the well-known crucial element in composite design. It was discussed that the alkali treatment improved the fiber surface adhesion by removing organic and inorganic impurities and producing a rough surface topography. This was also discussed elsewhere.32 More importantly, alkali treatment leads to fiber fibrillation and increasing the effective surface area for contact with the matrix, thus increasing the useful interface bonding region in the composite. In addition, it has been suggested that the alkali treatment results in improvement in interfacial bonding by increasing mechanical adhesion between the fiber and the matrix.33 AN grafting also contributed to better fiber-matrix adhesion and therefore the effective increase in the mechanical properties; however, to a lesser degree than the alkali treatment. It could be the case that the optimal degree of grafting was not assessed and analyzed at that stage, and that more attention was given to the effect of temperature and type of chemical treatment. It was reported in the later publication by the same group of authors34 that the increase in AN grafting amount reduced the mechanical properties of jute–Biopol composites. It therefore remains a possibility that grafting amounts of AN less than 10% might lead to optimization of the grafting layer and better mechanical properties. Scanning electron microscopy images of jute–Biopol composites under various treatments are presented in Figure 18.4(a–d), showing a significant decrease of pullout for alkali-treated and AN-grafted composites.

18.3 Effect of Processing Temperature Investigation of the processing temperature usually follows the optimization of the chemical treatment, or the two processes can be assessed simultaneously. The optimal processing temperature of natural fiber–biopolyester composites is determined as the temperature at which the natural fibers and biopolymer completely blend and form sound interface regions without subsequent thermal degradation of the biopolyester and/or cellulose fiber. The mechanical properties at the optimal processing temperature are usually notably higher than the surrounding values, even within a few degrees range. The cellulosic fibers tend to degrade at about 200°C and quickly become friable with loss of water after prolonged exposure to increased temperatures. This friability must be avoided if cellulosic fibers are to be used as reinforcement in composites.35 Bacterial polyesters have a low enough Copyright © 2005 by Taylor & Francis

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FIGURE 18.4 SEM of jute–Biopol composites: (a) detergent-washed jute–Biopal composites; (b) dewaxed jute–Biopal composites; (c) alkali-treated jute–Biopal composite; and (d) 10% AN-grafted jute–Biopal composite. (From Mohanty, A. K. et al., Composites: Part A, 31, 143, 2000. With permission.)

processing temperature (170°C) to allow molding without degradation of the fibers. However, good adhesion between the natural fibers and biopolyesters is usually achieved around 180°C, and thermal degradation of bacterial polyesters has been earlier reported to initiate around 175°C. In order to preserve the benefits of optimal processing temperature with minimum thermal degradation of the matrix, the processing time should be kept at a minimum to avoid loss in mechanical properties. During processing temperature investigation,7 the mechanical properties of jute–Biopol composites were measured along the direction of yarn winding and perpendicular to the direction of yarn winding. The mechanical properties of jute–Biopol composite along the yarn winding direction were found to increase up to 180°C and decrease thereafter [see Figure 18.5(a,b)]. The tensile strength factor along the yarn winding direction at 180°C was found to be 2.13 (113% increase relative to pure Biopol) and the bending strength factor was 1.52 (52%). The flexural (bending) properties were found to be below that of pure Biopol at 160°C, and the increase in the flexural properties at 180°C was significantly lower as compared to the increase in tensile Copyright © 2005 by Taylor & Francis

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2.5 Bmf

2.0

1.5

Bf

Bmf 1.0

0.5 (a)

Tf

160 165 170 175 180 185 190 Temperature (°C)

1.00 Mechanical properties factors

Mechanical properties factors

If

0.75

If

Tf 0.50 Bf

0.25 (b)

160 165 170 175 180 185 190 Temperature (°C)

FIGURE 18.5 Effect of processing temperatures on mechanical properties factors of jute–Biopal composites (a) along the yarn winding direction and (b) perpendicular to the yarn winding direction. (From Mohanty, A.K. et al., J. Mater. Sci., 35, 2589, 2000. With permission.)

properties. The indication should be sought in the results for the measurements perpendicular to yarn winding direction [Figure 18.5(b)]. The tensile, bending, and impact factors at all processing temperatures were below 1.00, therefore having lower mechanical properties than that of pure Biopol. The results indicate that the fiber-matrix interface is constituted of weak mechanical atomic bonds, thus representing a weak spot in the composite, particularly at temperatures below 180°C. This directly affects the flexural properties along the yarn winding direction, since flexural stress consists of components from both principal directions. Perhaps better properties could be obtained with more random orientation of fibers throughout the composite. At 190°C, the degradation of Biopol might have formed crotonic acid and volatile products which are responsible for decreasing trends of mechanical properties. Clearly, the optimal processing temperature of natural fiber–biopolyester composites remains in a narrow range, and the temperature represents the limiting factor in the production of biodegradable composites.

18.4 Effect of Silane Coupling Agent Silane coupling agents have been initially designed to provide adhesion between glass fibers and polymer composites. Today, there are many types of silane coupling agents that provide a chemical bond between organicinorganic, organic-organic, and inorganic-inorganic materials. Table 18.4 Copyright © 2005 by Taylor & Francis

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shows the summary of the results of biopolyester–flax composites with varying degrees of copolymerization and with addition of silane coupling agent (trimethoxymethacrylsilane).35 After silane coupling agent was applied on the flax fibers, sporadic crystalline regions appeared at the edges of the flax fibers, as shown in Figure 18.6(a). A similar pattern was observed for nontreated flax TABLE 18.4 Thermal and Mechanical Properties of Biopolyester Composites with and without Application of Silane Coupling Agent Material/ Composite PHB–flax PHBV 5%–flax PHBV 12%–flax S-PHB–flax S-PHBV 5%–flax S-PHBV 12%–flax

Melting Temperature (°C)

Glass Transition Temperature (°C)

Storage Modulus (GPa) at ⫺50°C

174 18.2 2.31 165.2 24 3.34 159 17.4 4.18 Silane coupling agent-treated flax fibers 172.7 24.5 4.76 170 23.3 4.03 163.5 26 4.67

Storage Modulus (GPa) at 10°C 2.01 3.23 3.96 4.2 3.5 4.45

FIGURE 18.6 OM images showing the process of nucleation during crystallisation after melt of S-PHBPHV5-F. (From Wong, S. et al., J. Appl. Polym. Sci., 91, 2114 –2121, 2004. With permission.)

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fibers and PHBV copolymer. With addition of silane coupling agent, nucleation was expected to increase. The four OM images [Figure 18.6(a–d)] show the process of rapidly growing spherulitic nucleation from the fiber edges toward the polymer, thus creating brittle borders and inconsistencies in the polymer structure. Since silane coupling agent did not affect crystallization of PHB to such a high degree, it was assumed that the phenomenon could be attributed to the better flow of the copolyester, since it had a lower melting temperature. The highest value of the storage modulus at ⫺50°C was for the composite of PHB with silane-treated fibers (4.76 GPa). This is greater than the modulus of isotactic polypropylene (~ 3–4 GPa). The modulus at ⫺50°C of the PHB fiber composites without silane treatment was 2.31 GPa, so that silane treatment was shown to provide a significant improvement to the mechanical properties of the biocomposites (106%). Though the values quoted were at ⫺50°C, the comparison between the treated and untreated fiber composites is similar at any other temperature over the range of temperatures studied (⫺50°–100°C). It was discussed that the improved modulus when the fibers were silane treated was through better interfacial bonding between PHB and the fibers, and an increase in transcrystallinity near the fiber interfaces.

18.5 Effect of Hydrogen Bonding Additives Some interesting results have been achieved by attempts to improve the interfacial adhesion between flax fibers and PHB by using dihydric phenols as hydrogen bonding additives. The authors36 hypothesized that if hydrogen bonding could exist between the hydroxyl groups of cellulose with the hydroxyl and carbonyl groups of the aliphatic polyesters, this would lead to a potential improvement in the mechanical properties. Thus, various percentages of 4,4’-thiodiphenol (TDP) were used to assess the optimal concentration in a PHB–flax composite. Upon addition of TDP, the mechanical properties of all modified composites showed higher values than the pure PHB–flax composite across the applied temperature range (⫺50°–100°C). The maximum stiffness was displayed at 5% of TDP out of the concentration range tested (see Figure 18.7). The observed improvement was ascribed to the hydrogen bonding that was formed between the neighboring PHB chains and between the fiber-matrix interface. Higher concentrations of TDP resulted in lower values than that of the composite with 5% TDP, possibly due to an excessive amount of additive migrating into the matrix. The thermal properties of PHB were also affected by TDP, hindering crystallization and subsequently resulting in a decrease in crystallinity and melting temperature.

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14.0 PHB-F + 2.5% TDP + 5% TDP + 7.5% TDP + 10% TDP

12.0

G' (GPa)

10.0

8.0

6.0

4.0

2.0

0.0 −50

−25

0

25

50

75

100

Temperature (°C) FIGURE 18.7 Storage moduli of PHB composites with varying TDP concentrations. (From Wong, S. et al., Compos. Sci. Technol., 64, 1321–1330, 2004. With permission.)

The most important result from this study is shown through the 135% increase in composite stiffness with 5% TDP being applied as a hydrogen bonding agent. Presence of the fiber encouraged formation of transcrystalline region, which was further strengthened with hydrogen bonding (TDP). The two mechanisms increased the amount of strong molecular bonds within the material by connecting the crystals to the fiber through strong hydrogen bonds, and increasing the number of molecular interactions among the adjacent crystals [see Figure 18.8(a–e)]. More information on physics of the transcrystalline region is provided and discussed below. The storage modulus ranged from ~13 GPa at ⫺50°C to ~ 4.5 GPa at 100°C, which exceeded the stiffness of commercial polypropylene by a great percentage, as well as the properties of flax−PP composites.37 TDP also improved processing thermal conditions by reducing the melting temperature from 176°C down to 168°C. The flax fibers suffer a degree of degradation when subjected to temperatures of 170°C onward, and processing below the temperature eventually results in a stronger composite able to withstand more recycling cycles. Thus, the application of TDP resulted in overall thermomechanical improvement of the biocomposite.

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FIGURE 18.8 Optical microscope images of (a) PHB composite with (b) 2.5% TDP, (c) 5% TDP, (d) 7.5% TDP, and (e) 10% TDP. (From Wong, S. et al., Compos. Sci. Technol., 64, 1321–1330, 2004. With permission.)

18.6

Effect of Plasticizers

The moisture content of cellulose fibers varies and depends on relative humidity, wetting of the composite, type of natural fiber, and presence of additives for improved bonding efficiency. Moisture interferes with melt dispersion and processing of the composites between 180° and 200°C. The high temperature of processing indicates that water evaporates through the Copyright © 2005 by Taylor & Francis

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fibers’ open ends and interferes with the process of consolidation. If the moisture is altogether removed from the natural fibers, the fibers become hollow and brittle and lose much of their effectiveness as reinforcement. Dried natural fibers can be modified either by monomers or by plasticizers, which permeate the volume previously occupied by water through capillary action and protect the cellulose structure from increased frailness. In a study reported by Wong et al.,38 three types of plasticizers were used to investigate the resulting mechanical properties of PHB−flax composites: glycerol triacetate (GTA), tributyl citrate (TBC), and a low molecular weight poly(ethylene glycol) (PEG). Plasticizers were impregnated into the flax by addition of a weighed amount of flax fibers into the neat plasticizer and the mixture heated at 120°C for 8 h in order to ensure that the air was expelled from the interstitial spaces and the plasticizer fully penetrated the flax fibers. The effect of plasticizers in PHB was investigated by comparing the thermomechanical properties of the biopolymer mixed with ~4% vol fraction of each plasticizer, and the same properties of PHB−flax composites with plasticizers. The results revealed an interesting scientific phenomenon. The addition of plasticizers into PHB slightly varied the glass transition temperature and crystallinity of the biopolymer. However, the storage modulus of the plasticized PHB (analogous to the elastic modulus of the material), measured by dynamic mechanical analysis, was reduced relative to pure PHB (PHB-GTA had improved storage modulus after the glass transition temperature). Thus, it was expected that PHB−flax composite would also reveal better mechanical properties than its plasticized modifications, and that the role of plasticizers would conclude with increased flexibility and better environmental resistance of the composite. However, inclusion of flax fibers contributed differently to the overall mechanical properties, and it also altered the formation of crystals and behavior of the plasticized material. The storage moduli obtained for the plasticized systems with flax differed from those without flax. For the pure PHB composite and the composite with TBC-treated flax, the values were similar to their counterparts without flax at ambient temperatures, which showed that flax fiber reinforcement was not effective in those cases. It was expected that through addition of fibers, mechanical properties would improve due to higher stiffness. However, no increase was observed. On the other hand, higher mechanical properties were obtained for the composites treated with GTA and PEG, suggesting a formation of the strong interfacial region between the matrix and the fibers. The optical microscope images presented in Figure 18.9(a,b) show the formation of transcrystalline regions between the flax fiber and PHB, and with impregnation of the fiber with GTA, respectively. Transcrystallinity is a type of morphology that results from dense heterogeneous nucleation of the matrix crystals at the fiber surface. The crystals are impelled to grow perpendicular to the fiber axis since parallel growth is inhibited by dense distribution of neighboring nuclei,39 thus growing the region, which has been reported to have a range of different mechanical properties, depending on the fiber-matrix system. The density of the transcrystalline layer for the Copyright © 2005 by Taylor & Francis

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FIGURE 18.9 OM micrographs of PHB–plasticizer–flax systems. (a) PHB–flax and (b) PHB–GTA–flax. (From Wong, S. et al., Macromol. Mater. Eng., 287, 641, 2002. With permission.)

plasticized systems was lower than that for the system with PHB and the untreated fiber.

18.7 Physics of Transcrystalline Region The fiber-matrix interphase in composite materials results from perturbations induced at fiber surfaces and expands from the fiber surface into the polymer matrix.40 Under certain conditions, a transcrystalline interphase layer, different from that of spherulites in polymer, develops in semicrystalline polymers during the cooling cycle. In a study reported by Zhang et al.,41 two systems using the same polymer matrix with and without transcrystalline interphase have been examined. The reported results showed that the fracture of the composite that lacked transcrystalline interphase revealed cavity at the fiber end, and the propagation of interfacial microcracks along the fiber sides due to shear stress also left traces on the fractured specimen. In contrast, the system containing transcrystalline interphase did not suffer debonding, and the fracture was initiated by the local ductile flow in the polymer matrix, leaving the (transcrystalline) interphase intact. In the majority of the works presented in this chapter, the biocomposites formed perfect transcrystalline regions between the fibers and the biopolymers, either due to copolymerization or by incorporation of various additives. Evidently, the transcrystalline region has a strong influence on mechanical properties and its formation needs to be investigated further. Without the presence of the fiber, the process of crystallization reaches its chain formation balance in forming perfectly aligned spherulites. Inclusion of fibers disrupts the balance among crystals by attracting them to the fibers’ surfaces and results in an attractive force between the polymer crystals and the fibers. The half-spherulites, aligned linearly along the fibers, are being balanced by the crystal-fiber interface, and a strong transcrystalline interphase is formed. Copyright © 2005 by Taylor & Francis

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An interesting work on simulation of interphase between crystals42 reported that the topology of the crystal interphase may be characterized in terms of chain populations. The molecular simulation of the intercrystalline phase of chain molecules was related to the boundary region between two crystals in a semicrystalline polymer. However, the proposed chain formations in crystals can be related to cellulose−PHBV composites. Topology was described as a distribution of chain length and their entry and exit points at the crystal boundaries. The kinetic process of crystallization results in large-scale morphologies which are not representative of the global minimum free energy state. Thus the interlamellar interphases can be assumed to be in a state of local equilibrium. It was reported that 43% of the chains in simulation terminated in chain ends (so-called tails), 54% of the chains looped back into the crystal, and only 2% of the chains formed bridges from one crystal to the next, which also decreased with increasing distance between crystals. Therefore, it can be concluded that the properties of a semicrystalline polymer, randomly filled with chains organized into spherulites, possess weak micromechanical points at the boundary between the crystals and the noncrystalline phases. Inclusion of the fiber, which induces formation of transcrystalline region, further strengthened with hydrogen/silane/plasticizer bonding and AN grafting, and increased the amount of strong molecular bonds within the material by connecting the crystals to the fiber by strong hydrogen bonds, thus increasing the number of “bridging” connections among the adjacent crystals and equilibrating a large number of crystals with a single fiber. These synergetic phenomena need to be investigated in more detail in order to quantify such enormous increases in mechanical properties, which seem to be mainly due to the formation of a transcrystalline interphase region in the material.

18.8 Conclusion and Future Directions The processing of bacterial-based biocomposites is in its infancy due to limited availability of bacterial polyesters and the high costs of their production. The modern science of biocomposites has moved forward by successfully producing natural fiber biocomposites having thermomechanical properties better than some conventional plastic materials. Improved molecular structure of biopolymers by copolymerization, deep chemical treatment of natural fibers, better fiber-matrix adhesion due to careful selection of bonding additives, and the presence of a transcrystalline region all contribute to an extraordinary enhancement in mechanical properties of these novel materials. The next step is to design a sustainable manufacturing process which will economically justify the utilization of biocomposites on the market and subsequently reduce the price of biopolymer processing. Copyright © 2005 by Taylor & Francis

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References 1. U.S. Congress, Office of Technology Assessment, Biopolymers: Making Materials Nature’s Way–Background Paper, OTA-BP-E-102, U.S. Government Printing Office, Washington, DC, September 1993. 2. Hoshino, A., Sawada, H., Yokota, M., Fukuda, K., and Kimura, M., Influence of weather conditions and soil properties on degradation of biodegradable plastics in soil, Soil Sci. Plant Nutr., 47, 35 – 43, 2001. 3. Gerngross, T.U., Can biotechnology move us toward a sustainable society?, Nat. Biotechnol., 17, 541, 1999. 4. Gerngross, T.U. and Slater S., How green are green plastics?, Sci. Am., 37, August 2000. 5. Kurdikar, D., Paster, M., Gruys, K.J., Fournet, L., Gerngross, T.U., Slater, S.C., and Coulon, R.,Greenhouse gas profile of a plastic derived from a genetically modified plant, J. Ind. Ecol., 4, 107, 2001. 6. Patel, M., Environmental life cycle comparisons of biodegradable plastics, in Biodegradable Plastics, Bastioli, C., Ed., RAPRA Technology Ltd., [in press]. 7. Avella, M., Martuscelli, E., and Raimo, M., Review Properties of blends and composites based on poly(3-hydroxy)butyrate (PHB) and poly(3-hydroxybutyratehydroxyvalerate) (PHBV) copolymers, J. Mater. Sci., 35, 523, 2000. 8. Kunioka, M. and Doi, Y., Thermal-degradation of microbial copolyesters poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly (3-hydroxybutyrate-co4-hydroxybutyrate), Macromolecules, 23, 1933 –1936, 1990. 9. Mohanty, A.K., Misra, M., and Hinrichsen, G., Biofibers, biodegradable polymers and biocomposites: an overview, Macromol. Mater. Eng., 276/277, 1–24, 2000. 10. Avella, M., Immirzi, B., Malinconico, M., Martuscelli, E., and Volpe, G.M., Reactive blending methodologies for biopol, Polym. Int., 39, 191–204, 1996. 11. Holmes, P.A., in Development of Crystalline Polymers, Vol. 2, Basset, D.C., Ed., Elsevier, London, 1988, p. 1. 12. Doi, Y., Microbial Polyesters, VCH Publishers, New York, 1990. 13. Zhang, L., Deng, X., Zhao, S., and Huang, Z., Biodegradable polymer blends of poly(3-hydroxybutyrate) and starch acetate, Polym. Int., 44, 104–110, 1997. 14. Holmes, P.A., Applications of PHB - a microbially produced biodegradable thermoplastic, Phys. Technol., 16, 32–36, 1985. 15. Barham, P.J. and Keller, A., The relationship between microstructure and mode of fracture in polyhydroxybutyrate, J. Polym. Sci., Polym. Phys. Ed., 24, 69–77, 1986. 16. Doi, Y., Macromol. Chem. Phys., Macromol. Symp., 98, 585, 1995. 17. Pool, R., In search of the plastic potato, Science, 245, 1187–1189, 1989. 18. Hocking, P.J. and Marchessault, R.H., Chemistry and Technology of Biodegradable Polymers, Blackie & Son, Glasgon,1994, p. 48. 19. Amass, W., Amass, A., and Tighe, B., A review of biodegradable polymers: uses, current developments in the synthesis and characterization of biodegradable polyesters, blends of biodegradable polymers and recent advances in biodegradation studies, Polym. Int., 47, 89–144, 1998. 20. King, P.P., Biotechnology – an industrial view, J. Chem. Technol. Biotechnol., 32, 2, 1982.

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21. Lehrle, R.S. and Williams, R.J., Thermal-degradation of bacterial poly (hydroxybutyric acid) — mechanisms from the dependence of pyrolysis yields on sample thickness, Macromolecules, 27, 3782–3789, 1994. 22. Howells, E.R., Single-cell protein and related technology, Chem. Ind., 7, 508–511, 1982. 23. Mark, H.F., Gaylord, N.G., and Bikales, N.M., Eds., Encyclopedia of Polymer Science and Technology, Interscience Publishers, Wiley, New York, London, Sydney, 1964. 24. He, Y., Asakaw, N., and Inoue, Y., Biodegradable blends of high molecular weight poly(ethylene oxide) with poly(3-hydroxypropionic acid) and poly(3hydroxybutyric acid): a miscibility study by DSC, DMTA and NMR spectroscopy, Polym. Int., 49, 609–617, 2000. 25. Cyras, V.P., Fernadez, N.G., and Vazquez, A., Biodegradable films from PHB –PHV copolymers and polyalcohols blends: crystallinity, dynamic mechanical analysis and tensile properties, Polym. Int., 48, 705 –712, 1999. 26. Abe, H., Doi, Y., Satkowski, M.M., and Noda, I., Proceedings of Biodegradable Plastics and Polymers, Japan, Nov. 9–11, 1993, p. 591. 27. Ceccorulli, G., Pizzoli, M., and Scandola, M., Plasticization of bacterial poly(3hydroxybutyrate), Macromolecules, 25, 3304–3306, 1992. 28. Zhang, L.L., Goh, S.H., Lee, S.Y., and Hee, G.R., Miscibility, melting and crystallization behavior of two bacterial polyester/poly(epichlorohydrin-co-ethylene oxide) blend systems, Polymer, 41, 1429–1439, 2000. 29. Tjong, S.C., Xu, Y., and Meng, Y.Z., Composites based on maleated polypropylene and methyl cellulosic fiber: Mechanical and thermal properties, J. Appl. Polym. Sci., 72, 1647–1653, 1999. 30. Saha, A.K., Das, S., Bhatta, D., and Mitra, B.C., Study of jute fiber reinforced polyester composites by dynamic mechanical analysis, J. Appl. Polym. Sci., 71, 1505 –1513, 1999. 31. Mohanty, A.K., Khan, M.A., Sahoo, S., and Hinrichsen, G., Effect of chemical modification on the performance of biodegradable jute yarn-Biopol composites, J. Mater. Sci., 35, 2589, 2000. 32. Bisanda, E.T.N. and Ansell, M.P., The effect of silane treatment on the mechanical and physical-properties of sisal-epoxy composites, Compos. Sci. Technol., 41, 165 –178, 1991. 33. Mohanty, A.K., Khan, M.A., and Hinrichsen, G., Composites Part A [in press]. 34. Mohaty, A.K., Khan, M.A., and Hinrichsen, G., Influence of chemical surface modification on the properties of biodegradable jute fabrics-polyester amide composites, Composites: Part A, 31, 143, 2000. 35. Shanks, R.A., Hodzic A., and Wong, S., Thermoplastic biopolyester natural fibre composites, J. Appl. Polym. Sci., 91, 2114–2121, 2004. 36. Wong, S., Shanks, R., and Hodzic, A., Interfacial improvements in poly(3 hydroxybutyrate)-flax fibre composites with hydrogen bonding additives, Compos. Sci. Technol., 64, 1321–1330, 2004. 37. Hodzic, A., Shanks, R.A., and Leorke, M., Polypropylene and aliphatic polyester flax fibre composites, Polym. Polym. Compos., 10, 281–290, 2002. 38. Wong, S., Shanks, R.A., and Hodzic, A., Properties of poly (3-hydroxybutyric acid) composites with flax fibres modified by plasticiser absorption, Macromol. Mater. Eng., 287, 641, 2002. 39. Dean, D.M., Marchione, A.A., Rebenfeld, L., and Register, R.A., Flexural properties of fiber-reinforced polypropylene composites with and without a transcrystalline layer, Polym. Adv. Technol., 10, 655 – 668, 1999.

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40. Pompe, G. and Mader, E., Experimental detection of a transcrystalline interphase in glass-fibre/polypropylene composites, Comp. Sci. Tech., 60, 2159–2167, 2000. 41. Zhang, M., Xu, J., Zhang, Z., Zeng, H., and Xiong, X., Effect of transcrystallinity on tensile behaviour of discontinuous carbon fibre reinforced semicrystalline thermoplastic composites, Polymer, 37, 23, 5151–5158, 1996. 42. Balijepalli, S. and Rutledge, G.C., Molecular Simulation of the inter-crystalline phase of chain molecules, J. Chem. Phys., 109, 6523–6526, 1998.

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19 Cellulose Fiber-Reinforced Cellulose Esters: Biocomposites for the Future

Guillermo Toriz, Paul Gatenholm, Brian D. Seiler, and Debra Tindall

CONTENTS 19.1 Introduction 19.2 Case Study: Automotive Interior Applications 19.2.1 Materials 19.2.2 Methods: Matrix Fiber Adhesion Studies 19.2.3 Composite Preparation 19.2.3.1 Low-Density Composites 19.2.3.2 High-Density Composites 19.2.3.3 Viscosity and Dynamic Mechanical Thermal Analysis 19.2.3.4 Mechanical Tests 19.2.3.5 Scanning Electron Microscopy 19.3 Results and Discussion 19.3.1 Interfacial Adhesion 19.3.2 Rheological Properties 19.3.3 Dynamic Mechanical Thermal Analysis 19.3.4 Mechanical Properties 19.3.5 Injection-Molded and Low-Density Prototypes 19.4 Conclusions 19.5 Appendix 19.5.1 Cellulose Esters 19.5.2 Cellulose Esters Manufacture and Characteristics References

ABSTRACT This chapter describes the advantages of using cellulose esters as matrix binders for cellulose fibers. Biocomposites with an attractive combination of mechanical and thermal properties, at low density, were obtained by

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compounding cellulose fibers from various sources (including sisal, hemp, flax, coir, cotton, jute kenaf, and wood) with cellulose esters. During evaluation of various processing techniques such as extrusion, compression molding, and injection molding, we found that the rheological properties of cellulose esters are of critical importance. These properties can be adjusted by selection of different compositions of cellulose esters (varying acetyl, butyryl, and propionyl content), molecular weight, and addition of plasticizer. We found good adhesion characteristics of the cellulose fiber−cellulose ester interface. These findings obviate the need of using expensive coupling agents.

19.1 Introduction Today society faces numerous challenges concerning the transitions toward sustainability. Technical development has resulted in increased energy consumption, overuse of nonrenewable resources, waste, and a rise in carbon dioxide emissions. The increased carbon dioxide emissions with collateral global warming and the future shortage of fossil resources require many fronts of research to alleviate their consequences. Even though strategies for management of solid wastes have been ongoing for several years, the amount of nondegradable substances is rising steadily, while landfill space is being depleted. These problems, coupled to the urgency for better and integrated utilization of agricultural and forest resources, demand prompt and efficient actions both from society as well as research and development organizations. The benefits obtained from biomass as a source of energy, chemicals, and materials are manifold. Since biomass requires carbon dioxide as a source for growth, it represents a harmonic response to achieve sound equilibrium between consumption and emission of carbon dioxide. The great availability of natural renewable resources offers, through its efficient management, a favorable balance in terms of price and performance of products obtained from them. Moreover, biomass and the products obtained from it are amenable to recycling and composting practices, thus exploiting inherently biodegradable features. The production of biomass is around 172 billion tons/year of which lignocellulosics in forests represent 82%.1 Lignocellulosics are materials composed of cellulose, hemicelluloses, lignin, and extractives. Wood is the most important component of lignocellulosics; however, many plants are grown for their fibrous material and used for commercial purposes. For example, leaf fibers (sisal, abaca, hemp) are used for cordage; flax, jute, and kenaf, known as bast fibers, are used for textiles; fruit fibers such as coir from coconut husks are used for cordage and floor coverings. Cotton, a seed fiber, is the most widely grown and used textile fiber in the world. On the other hand, agricultural residues and other plant substances such as grasses and Copyright © 2005 by Taylor & Francis

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water plants represent a very large biomass source that could be more extensively utilized.2 The potential of agrobased fibers to be incorporated into materials with engineered properties is enormous. Availability, price, and performance are some of the factors that have catalyzed the surge of using lignocellulosic fibers, first as fillers and more recently as reinforcements, into polymeric materials. However, the large diversity of lignocellulosic fibers results in high variability in their properties. Complex interactions between external factors (the most important being moisture) and structural parameters that differ from ångström to micron levels result in multifarious properties. Intrinsic factors that dictate the mechanical properties of lignocellulosic fibers are polymer molecular weight, orientation and organization of cellulose chains which result in crystalline regions, hemicellulose and lignin content, interaction between components, and defects on fibers at microscopic and molecular levels. For example, the combination of chemical composition, expressed as cellulose content and crystallinity of the cellulose portion, results in variation in mechanical properties, as seen in Table 19.1.3 The stiffness of the fibers, i.e., the elastic modulus, ranges from 17 to 108 GPa, which should be related to the fibers’ lignin content, fibril angle, and all the factors mentioned above. Despite the variability on the mechanical fibers, natural fibers offer many advantages when compounded with, for instance, thermoplastics. Low density and abrasiveness, high specific properties, and high filling levels make the incorporation of lignocellulosic fibers into thermoplastics highly desirable. Previous work on lignocellulosic fibers in thermoplastics has concentrated on wood-based flour or wood fibers and polyolefins. There is a wealth of information on the manufacture and properties of synthetic thermoplastics and lignocellulosic fiber composites.2,4−15 In spite of the advances for incorporating natural renewable fibers into synthetic matrices derived from nonrenewable resources, the composites

TABLE 19.1 Mechanical Properties of Selected Lignocellulosic Fibers Lignocellulosic Fiber

Cellulose (%)

Crystallinity (%)

Strengtha (MPa)

Elastic Modulus (GPa)

Elongationa (%)

Cotton Flax Ramie Jute Sisal Wood pulp Regenerated cellulose

95 71 76 72 73 40–47 99

73 67 64 59 — 60 35II monoclinic

200–800 820 900 580 520 — 200–400

20–40 78–108 48–69 26 17 20–80 15–30

6–12 1.8 2.3 1.5 2.8 — 9

a

These properties measured at break. Source: Adapted from Rials, T.G. and Wolcott, M.P., in Paper and Composites from Agro-based Resources, CRC Press, Boca Raton, FL, 1997. With permission.

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produced are only about halfway in satisfying environmental concerns. To be truly sustainable, the new generation of composites must be based entirely on renewable materials, including the continuous phase. Bacteriaproduced biopolymers that are genuinely biodegradable and with mechanical properties that equal or even exceed those of traditional thermoplastics, as well as starch or polylactic acid, are under intense investigation to produce biocomposites reinforced with lignocellulosic fibers.16−21 Cellulose derivatives have been overlooked as potentially cocomponents in composites with lignocellulosics; only a few studies have considered the use of cellulose esters as matrices in biocomposites.16,22−24 Cellulose esters are very well suited for use as matrix binders in natural fiber-based composites. The predominant raw material employed in the manufacture of cellulose esters originates from softwood forests, harvested under programs of sustainable yield. Liquid chemicals derived from coal are used in esterification reactors to commercially produce these important thermoplastic materials. Some grades of cellulose esters are readily biodegradable (environmentally induced) to gases, water, and biomass.25−27 Other cellulose esterbased structures can be “triggered” to biodegrade.28 Many cellulose ester formulations (e.g., acetates, propionates, butyrates) can be burned without generating toxic products or residues, thus allowing their use in the incineration of waste for recovery of heat value. However, not all cellulose derivatives degrade at appreciably fast rates to be considered “biodegradable.” In general, as the degree of substitution and the length of the side chain substituent decreases, the rate of biodegradation increases. Selective use of fillers, plasticizers, and other additives can also influence biodegradation rate. Cellulose esters can be injection molded, extruded, blow and rotationally molded into structural components, thermoformed from sheet, and extruded in films. Typical processing temperatures lie between 180° and 240°C. Typical mold shrinkage is about 0.4% in the flow direction and 0.7% in the transverse direction, and post-mold shrinkage is negligible. All of the major commercial grades of cellulose ester plastics are suitable for normal cutting and machining operations. Most grades are easy to process and fabricate by conventional techniques and provide excellent flow properties, giving fine detail reproduction. Generally, cellulose esters have a wide, forgiving processing “window.” Cellulose ester-based plastics are noted for their ability to readily accommodate secondary fabrications such as thermoforming, painting, printing, polishing (mechanical or solvent), and cutting.29 Solvent solutions of cellulose esters can be cast into films or sheets, spun into fibers, or coated as lacquers. A high gloss finish is possible with cellulose ester-based coatings, and dust pickup is usually minimal. However, cellulose ester-based plastics will begin to thermally degrade at temperatures in excess of 260°C, leading to discoloration and eventual loss of properties. In addition, cellulose ester resins are fairly expensive (relative to many plastic materials) and as such, they are frequently targeted for substitution. The mechanical properties of cellulose ester-based plastics can be varied by plasticizer level to suit a variety of needs. Lower plasticizer content yields a Copyright © 2005 by Taylor & Francis

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harder surface, higher heat resistance, greater rigidity, higher tensile strength, and better dimensional stability. Higher plasticizer content increases impact strength and toughness even at relatively low temperatures. Cellulose esterbased plastics are not very susceptible to stress cracking. It is important to point out that cellulose ester-based plastics generally tend to creep under load. Glass transition temperatures (Tg values) for the cellulose esters used in common plastics applications fall in a range of ~140°−190°C. Cellulose esters burn cleanly, yielding simply carbon dioxide and water. The low specific heat and thermal conductivity are responsible for the pleasant feel of handles and articles made of cellulose esters, with consequent applications for handles, pens, barrels, typewriter keys, etc. Pure cellulose esters will typically burn slowly, although they can be formulated with a variety of fireretardant plasticizers and additives to eliminate or reduce this tendency. In film and sheet applications cellulose esters offer unique combinations of toughness and transparency. An almost unlimited color range is available, and the materials are easily varied from transparent, to translucent, to opaque. For many years, cellulose esters have been employed extensively in photographic film base applications due to considerations including relative toughness, high clarity, and dimensional stability. Cellulose triacetate is the major component found in contemporary 35mm film base. Cellulose ester films are generally relatively easy to tear; they do not stretch or “neck” easily, and they are easily written upon. Because of these properties, extruded cellulose diacetate film makes an excellent base for transparent tapes. Solvent casting of cellulose triacetate into films for liquid crystal displays (protective films for the commonly used polarizing layers) is another important and growing commercial application,30 where the relatively high moisture vapor transmission rate is a valued property in combination with clarity and toughness. Typical refractive index values for cellulose esters fall in the 1.46−1.49 range. Cellulose acetate butyrate has also found some application in tinted polarizing films and sheets of the types used in sunglasses and the special privacy films employed in computer screen assemblies where very restricted viewing angles are desirable (e.g., displays used at automatic teller machine installations). On the other hand, the optical properties of cellulose ester films are somewhat inferior to acrylics. The scratch resistance of cellulose ester films and sheets is generally regarded as not particularly good, but this property is strongly influenced by substituent type, degree of substitution, plasticizer type, and loading level of plasticizer. Also, it should be noted that the tear strength of extruded films (freestanding) is relatively low. Cellulose esters exposure to moisture does not greatly affect their mechanical properties (hardness and stiffness drop slightly with moisture uptake). Cellulose acetate butyrate and cellulose acetate propionate have reasonably good dimensional stability (dependent upon level of plasticizer). Cellulose esters are generally resistant to attack by common household chemicals such as bleach, detergents, and vegetable or mineral oils. As a general rule, cellulose esters also display good visible and ultraviolet (UV) light stability. The common organic cellulose esters are appreciably distinguished from the Copyright © 2005 by Taylor & Francis

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inorganic ester nitrocellulose with regard to light/UV stability, particularly in coatings applications. Cellulose esters are susceptible to attack by strong acids and alkalis, which can be used advantageously for surface modification or triggered degradation. Many types of cellulose ester-based plastics and films are dissolved or swollen by alcohols, esters, ketones, and aromatic and chlorinated hydrocarbons. This can be used advantageously for solvent polishing the surface. However, chemical resistance is highly dependent on composition. Cellulose esters tend to have higher permeabilities to both water and gases than most polymers, and permeability can be controlled to a certain extent. This can be used advantageously in membrane and other types of separation applications. The commercially important types of cellulose esters are inherently nontoxic and can be combined with nontoxic plasticizers approved by the U.S. Food and Drug Administration (USFDA, or similar international regulatory approval agencies) for food contact uses. Some grades are produced under “current Good Manufacturing Practices” (cGMP, regulations promulgated by the USFDA) for use in controlled-release coatings and other pharmaceutical applications,31 including cellulose acetate phthalate (C-A-P), which is used in enteric coatings.32 “NF” grades of cellulose acetate have also been developed for sales into the pharmaceutical excipients market. (NF indicates registry in the National Formulary published by the U.S. Pharmacopoeia.) Cellulose diand triacetates have found widespread use in hemodialysis membrane applications, i.e., hollow-fiber membrane structures for “artificial kidneys,” because of exceptional biocompatibility and somewhat unique permeability features.33 Cellulose ester-based materials offer relatively high dielectric constants (e.g., 3.2−6.2 @ 106 Hz), good dielectric strengths (e.g., 14.5−16.6 kV/mm), and relatively high volume resistivities (e.g., 1013−1015 ohm-cm).34 In coatings applications, a valued property provided by cellulosic materials is resistance to electrostatic dust attraction. However, cellulose ester-based materials have a high electrical dissipation (power) factor (e.g., 0.2−0.5 @ 106 Hz). Many cellulose esters can be blended with other synthetic and natural polymers.26,35,36 These blends can extend the physical properties of both components. The blending of cellulose esters with certain polymers, such as polyesters/copolyesters, can eliminate the need for plasticizing cellulose esters in thermoplastic applications. Cellulose esters can tolerate very high levels of fillers, allowing the construction of unusual composites. The Appendix (Section 19.5) provided for this chapter gives detailed information on the basic history and production of cellulose esters.

19.2 Case Study: Automotive Interior Applications Biocomposites are of great importance for the automotive industry because they offer a combination of good mechanical properties, low weight, and Copyright © 2005 by Taylor & Francis

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environmental benefits during the life end. We have used different lignocellulosic fibers as reinforcement for cellulose esters matrices. Prototypes of automotive components have been manufactured and evaluated. 19.2.1 Materials Cellulose esters were provided by Eastman Chemical Company (Kingsport, TN, USA). The physicochemical characteristics are listed in Table 19.2. Flax fibers (Linum usitatissimum) were supplied by Flexprop Production AB (Halmstad, Sweden). Flax fibers were retted and nonwoven needled (grammage 1750 g/m2) with a single fiber diameter of 23 µm. Sisal (Agavacea sisalana) fibers from United Rope (Rotterdam, Holland) and ramie fibers (Boehemeria tenacissima; 23 µm diameter) supplied by SCA Mölnlycke (Mölndal, Sweden) were tested as well. Triethylcitrate was purchased from Aldrich (Milwaukee, WI). Air-laid and thermobonded prepreg materials were produced at The Royal Veterinary and Agricultural University in Denmark. The prepreg compositions consisted of 45% CAB or CAP, 5% of a bicomponent fiber (polyethylene/polypropylene sheath/core fiber, to facilitate thermobonding), and different amounts of flax, sisal, and wood fibers. CAB-based prepregs consisted of 20% flax, 25% sisal, and 15% wood fibers; another prototype was made with 10% sisal, 10% flax, and 30% wood fibers. CAP prepregs had 20% flax, 15% sisal, and 15% wood fiber. 19.2.2 Methods: Matrix Fiber Adhesion Studies Single fiber fragmentation testing was carried out with CAB and CAP as matrices combined with single fibers of flax and ramie. Single fibers were placed between CAB or CAP films. The films were melted in a hot press at 210ºC. Total press time was 3 min, holding no pressure the first minute and applying 2.25 ⫻ 106 Pa for the rest of the time. Dogbone-shaped specimens were punched out from the films and mounted in a Minimat miniature tensile tester (Polymer Labs, Thermal Sciences, Inc.). The tensile tester was TABLE 19.2 Physicochemical Characteristics of Commercial Cellulose Esters Acetyla Butyryla CAB 381-20 CAP 482-20

13.5 2.5

Propionyla

Hydroxyla

Mp (°C)

Tg (°C)

Density (g/cm3)

MWn (g/mol)

37.0

—

1.8

195–205

141

1.20

70000

—

46.0

1.8

188–210

147

1.22

75000

a

Percentage in weight. Source: Eastman Publication E-325, Eastman Chemical Company, Kingsport, TN, 2003.

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placed under an optical microscope (Olympus BH2-UMA) equipped with cross polarizers, which made it possible to observe birefringent stress patterns in the thin matrix. The cross-section speed was set at 5 mm/min, and the sample was elongated until no further breaking of the fiber occurred (~10% elongation). The evaluation of the interfacial shear strength is based upon measurements of 50 fragments for each experiment.

19.2.3 Composite Preparation

19.2.3.1 Low-Density Composites Mats of flax of a specific grammage were pressed with appropriate CAB amounts according to the protocol followed for the single fiber composite. 19.2.3.2 High-Density Composites Composite granulates were manufactured using a twin-screw extruder (Coperion Werner & Pfleiderer ZSK 25 WLE). The matrix material was fed into the main k-tron feeder; wood fibers were fed into the secondary k-tron feeder, and sisal fibers were fed into the side extruder. The fiber content in the composite was calculated according to kilograms of matrix per hour, kilograms of wood fiber per hour, and feeding speed and the weight of the roving per meter. Articles were injection molded at 200°C, with an injection time of 1.1 sec. A back pressure of 3.0 ⫻ 106 Pa was applied. The holding pressure was maintained at 65.0 ⫻ 106 Pa for 10 sec. A cooling time of 22 sec was selected. 19.2.3.3 Viscosity and Dynamic Mechanical Thermal Analysis Viscosity measurements were carried out in a Rheometrics dynamic analyzer RDAII (Piscataway, NJ) in parallel-plate geometry; frequency sweeps at constant strain were performed from 0.1 to 80 Hz. Dynamic mechanical thermal tests were done in the Rheometrics apparatus with temperature sweeps at 2ºC/min rate in the range of 25°−200ºC at 1 Hz. 19.2.3.4 Mechanical Tests Composites were conditioned at 23ºC and 50% relative humidity for at least 40 h and tested according to ASMT tests (D 790-97/D 638-97 flexural/tensile properties of unreinforced and reinforced plastics and electrical insulating materials; D 6110-97 charpy impact resistance of notched plastic specimens). 19.2.3.5 Scanning Electron Microscopy The gold-sputtered composite fracture surfaces were examined with a Zeiss scanning electron microscope (model DSM 940 A; Oberkochen, Germany) at 15 kV working energy. Copyright © 2005 by Taylor & Francis

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19.3 Results and Discussion 19.3.1 Interfacial Adhesion Interfacial adhesion is one of the crucial properties in short fiber composites. The property of the interface controls the shear stress transfer between the matrix and the fibers. The interfacial shear strength is a function of the fiber strength, diameter, and critical fiber length. It is inversely proportional to the strength and diameter of the fiber and directly proportional to the critical fiber length.37 Figure 19.1 shows an optical micrograph of a fragmented flax fiber in a CAB matrix. The fragment lengths are clearly seen and were measured. Table 19.3 shows the results obtained. It can be seen that flax fibers have a much better adhesion to CAB than ramie fibers. Owing to the greater strength at the interface, the critical fiber length is shorter, and the tensile strength is almost four times larger than that of CAB−ramie fibers. It can also be seen in Table 19.3 that flax fibers have a comparable adhesion in a CAP matrix, which will be of interest for further discussion in automotive applications. However, due to the better interfacial properties and thermal stability, most of the results shown here will focus on CAB−flax composites.

FIGURE 19.1 Single flax fiber fragmented in CAB.

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TABLE 19.3 Results from Single Fiber Fragmentation Test (Flax and Ramie Fibers, CAP and CAB Matrices) Fiber Matrix

Ramie CAB

Flax CAB

Flax CAP

Fiber diameter (µm) Single fiber tensile strength (MPa) Average fiber fragment length (mm) Standard deviation (mm) Critical fiber length, lc (mm) Interfacial shear strength, τ (MPa)

25 180 0.74 0.27 (36%) 0.99 2.32

23.4 580 0.51 0.16 (31%) 0.68 9.98

23.4 580 0.59 0.27 (46%) 0.79 8.59

FIGURE 19.2 Fracture surface of CAB−flax composite.

19.3.2 Rheological Properties Melt flow is an important process parameter in compression molding. Appropriate melting of the matrix will ensure wetting of the fibers. CAB−flax composites were prepared by hot pressing at 170ºC and 2.5 MPa at 50 wt% fiber content. Figure 19.2 shows the fracture surface of the composite. Copyright © 2005 by Taylor & Francis

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It can clearly be seen that the poor wetting of the matrix resulted in fiber pullout. Lack of contact between the fiber and matrix is due to insufficient matrix melt flow, which has implications for the mechanical properties, as will be discussed in the next section. Rheological measurements of CAB were carried out to investigate the viscosity as function of shear rate and temperature. It is important to point out that in automotive interiors, a large number of low-density composites are required. These composites are achieved by preparing the fiber mats at a required grammage and then subjecting them to hot press molding with the matrix of interest. The low density of the composites is achieved by heating at low pressure. This sets requirements for low viscosity at rather low shear rates. Figure 19.3 shows the melt viscosity of neat CAB. It can be seen that the viscosity decreased as a function of both temperature and shear rate. Appropriate low viscosities are then achieved either at high shear rates and/or high temperatures. Shear generated during hot pressing is low, and therefore the only other variable to be manipulated is temperature. Lignocellulosic fibers, however, are not amenable to be processed at temperatures higher than 200ºC because they undergo thermal decomposition. Hence the poor wetting of the fibers was due to low melt flow of the matrix, as seen in Figure 19.2.

105 170 (°C) 180 (°C) 200 (°C) 210 (°C) 220 (°C) 230 (°C)

η' (

) (Pa-s)

104

103

102 10−1

100

101 ω (Hz)

FIGURE 19.3 Melt viscosity of neat CAB as a function of shear rate and temperature.

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Several approaches have been applied in order to reduce the melt viscosity of CAB. The use of furfuryl alcohol (FA) as a solvent for CAB matrices has been previously reported.16 FA addition reduced the melt viscosity of the mixtures to values lower than 200 Pa ⫻ s at 170ºC, which are appropriate for compression molding. Then FA was polymerized in the presence of CAB and flax fibers. It was shown that the storage modulus of the composites could be tuned through the addition of appropriate amounts of FA. Impact strength, however, was decreased. Another approach to reduce the melt viscosity of CAB is through the use of plasticizers. Citrates are plasticizers from renewable resources, manufactured by enzymatic reaction from readily available sugars. Citrates show a useful balance of performance characteristics, since they are not toxic; acetyl triethyl citrate is commercially important and is used in PVC, PVdC, and PVA. Figure 19.4 shows the melt viscosity of CAB−triethyl citrate (TEC) as a function of plasticizer content at 170ºC. It can be seen that the viscosity achieved at 30% TEC loading is sufficiently low for compression molding. It should be noted that in increasing the temperature 10°−15ºC, the viscosity of the matrix is further reduced to values around 100 Pa ⫻ s.

105 Plasticizer content I 0% L 10% J 20% K 30%

η' (I) (Pa-s)

104

103

102

101 10−1

100

ω (Hz)

101

FIGURE 19.4 Melt viscosity of CAB as a function of shear rate and triethylcitrate content at 170ºC.

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19.3.3 Dynamic Mechanical Thermal Analysis The shear storage moduli of neat and flax-reinforced CAB are shown in Figure 19.5. The effect of the reinforcing fiber in promoting stiffness of the composite can clearly be seen over the whole range of temperatures studied. TEC-plasticized CAB storage moduli curves are shown in Figure 19.6. At 10% plasticizer loading, the storage modulus is decreased at temperatures higher than 60ºC; higher plasticizer content significantly decreased the stiffness of the matrix. Reinforced plasticized matrices (Figure 19.7) showed moderate stiffness as compared to unplasticized composites (Figure 19.6).

19.3.4 Mechanical Properties The flexural properties of CAB−flax hot-pressed composites are given in Table 19.4. The reinforcing effect of the fibers is clearly seen. These results show, however, that the elongation is reduced. Sisal fibers can be considered as good candidates for reinforcing purposes. Tensile properties of high-density prototypes that include CAP as matrix are given in Table 19.5. These prototypes were compounded in an extruder and prepared by injection molding. The composites were prepared at 30 wt% fiber content with 15% sisal fibers and 15% wood fibers. It can be seen that the tensile properties are greatly improved in the CAP composites. Impact strength, as shown in Table 19.6,

1.E+10 Flax-reinforced CAB Unreinforced CAB

1.E+09

G' (Pa)

1.E+08

1.E+07

1.E+06

1.E+05 20

40

60

80 100 Temperature (°C)

120

140

FIGURE 19.5 Shear storage moduli of neat CAB and CAB−flax unplasticized composites.

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109

G' (I) (Pa)

108

107 Plasticizer content (%) I 0% J 10% K 20% L 30%

106

105

20

40

60

80 100 Temperature (°C)

120

140

160

FIGURE 19.6 Shear storage moduli of triethyl citrate-plasticized CAB.

1010

Plasticizer content I 10% J 20% K 30%

G' (I) (Pa)

109

108

107

106

20

50

80

110 140 Temperature (°C)

FIGURE 19.7 Shear storage moduli of flax-reinforced plasticized CAB.

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200

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TABLE 19.4 Flexural Properties of CAB−Sisal/Flax Fiber Biocomposites Prepared by Hot Pressing Technique at 50% Fiber Content Breaking Strength

Elongation

Flexural Modulus

CAB/Fiber

MPa

Std

%

Std

GPa

Std (MPa)

Neat Flax Sisal

70.3 82.3 87.6

12.5 16.5 27.9

4.9 3.6 2.5

0.1 0.3 0.7

1.35 5.71 7.85

50 1613 1553

TABLE 19.5 Tensile Properties of Injection-Molded CAB− or CAP−Sisal/Wood Fibers at 30% Fiber Content

CAB CAB/15/15a CAP CAP/15/15a a

Tensile Modulus (GPa)

Stddev ⫾) (⫾

Stress at Break (MPa)

Stddev ⫾) (⫾

Max. Strain (%)

Stddev ⫾) (⫾

1.3 3.4 1.5 4.1

0.2 0.06 0.1 0.06

25.2 26.0 26.7 41.5

0.3 0.6 0.4 0.4

— 2.6 — 2.1

— 0.3 — 0.2

Composites prepared with 15 wt% sisal/15 wt% wood fibers.

TABLE 19.6 Impact Strength of CAB− or CAP−Sisal/Wood Fibers at 30% Fiber Content Composite

Impact Strength (kJ/m2)

⫾) Stddev (⫾

CAB CAB/15/15a CAP CAP/15/15a

80.2 11.4 83.5 10.4

9.2 1.5 6.2 1.8

a

Composites prepared with 15 wt% in wood fibers and 15 wt% sisal fibers.

was dramatically reduced. Further studies are being undertaken to improve the impact strength of that kind of composite. 19.3.5 Injection-Molded and Low-Density Prototypes Figure 19.8 and Figure 19.9 show injection-molded and hot-pressed items, respectively, to point out the possibility of using these materials for specific applications. Copyright © 2005 by Taylor & Francis

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FIGURE 19.8 Coffee mug made of 70% CAB (left) and 70% CAP (right), 15% sisal, and 15% wood fibers.

FIGURE 19.9 Cover lid made of 70% CAB, 15% sisal, and 15% wood fibers.

19.4 Conclusions Cellulose esters represent a unique class of derivatized polymers from renewable resources that are very well suited for use as matrix binders in natural fiber-based composites. These materials are largely decoupled from petroleum-based feedstocks. In the course of our research, we have learned how to effectively combine cellulose esters with cellulose fibers sourced from renewable crops (including sisal, hemp, flax, coir, cotton, jute, kenaf, ramie, and wood) for the construction of novel and useful composite

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structures. Several different conversion approaches involving extrusion, compression molding, and injection molding have been evaluated. Physicalmechanical data promise a bright future for biocomposites based on cellulose esters reinforced with cellulose fibers. Many of the challenging fitness-for-use requirements for articles like interior automotive parts can be met, but more work is needed to optimize the materials combinations and methods of fabrication.

19.5 Appendix 19.5.1 Cellulose Esters The ester derivatives produced from the highly abundant biopolymer cellulose have a long and storied history. The basic types of cellulose esters used today were first synthesized in the 19th century, although (perhaps with the exception of nitrocellulose) very limited commercial value was extracted from them during this century of discovery. However, throughout the 20th century and particularly after about 1920, an incredible amount of research and development activity was focused on cellulosic materials, with high emphasis on the ester and ether derivatives of cellulose. By the late 1950s, these materials were firmly entrenched in many coating and film applications and had emerged as the most dominant materials found in plastic applications.38 The reach of these semisynthetic (as they are often described) biopolymer derivatives into numerous diverse commercial markets was and is quite remarkable, and to this day cellulose ester and ether-based materials persist in many applications originally developed during the first half of the past century. In certain market spaces, product life cycles have been atypically long, and the photographic film base application for cellulose triacetate (developed in the late 1940s) provides a good example of this. With the current emphasis on the development of plastics, coatings, fibers, and complex structures based on renewable resource-based feedstocks, the cellulose biopolymer derivatives are well-positioned to be “reengineered” into products for the 21st century. Products envisioned include composite structures where materials such as cellulose esters can be conveniently and effectively combined with natural fibers derived from annual crops or even tree forests under programs of sustainable yield. Hence, the terminology “derived from renewable resource-based feedstocks” certainly applies to the major classes of commercially available cellulosic materials.

19.5.2 Cellulose Esters Manufacture and Characteristics Unlike purely synthetic polymers prepared via common step-growth (condensation) or addition reactions, cellulose esters generally acquire some of

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their fundamental characteristics (upper molecular weight limit, polydispersity, etc.) from the “base” biopolymer cellulose, which in commercial practice is almost exclusively isolated from different plant species and predominantly different tree species. It is this distinction that has led to the categorization of cellulose esters as semisynthetic materials by many. Consequently, any detailed discussion of cellulose esters must logically begin with some background information related to the cellulose feedstocks from which these materials are produced. Figure 19.10 shows a commonly used depiction of the base biopolymer cellulose. The structure can be summarized most succinctly as a linear polysaccharide comprised of cellobiose repeat units, linked β -1→ 4.39 A few generalizations can be made concerning cellulose: celluloses extracted from natural sources are high molecular weight linear polysaccharides (i.e., no evidence has been found for branching), and conformational considerations explain a great tendency to form intermolecular hydrogen bonds. Additionally, the β -1→ 4 glycosidic linkages produce a structure that has quite low water solubility. Secondary and tertiary structural effects (promoted by extensive hydrogen bonding) make discrete cellulose fibers very insoluble and relatively impermeable to liquids.40 A somewhat simplified and commonly seen representation of cellulose focuses on the repetitive utilization of the anhydroglucose unit, C6H10O5, as the building block for the polymeric structure. Careful structure elucidation work indicates that every other anhydroglucose unit is actually flipped ~180 degrees,41 hence the cellobiose repeat unit viewpoint shown in Figure 19.10 is technically more accurate. However, the anhydroglucose repeat unit viewpoint provides a convenient basis for nomenclature, and accordingly degree of polymerization (DP) is commonly defined by the average number of anhydroglucose repeat units per molecule. DP values vary greatly according to the type of cellulose, and it should be noted that plant-derived celluloses feature high degrees of polydispersity. In native forms, average DP estimates (by the anhydroglucose repeat unit definition) falling in the range of 4,000−20,000 have been cited for celluloses found in different species of plants.40 The corresponding molecular weight range would be roughly 650−3240 kDa, but the reader is reminded of the issues encountered in describing populations of polymer chains, where number-, weight-, and Z-average molecular

CH2OH

OH

O H O

OH

OH O

FIGURE 19.10 Structural representation of cellulose.

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CH2OH O

O OH

OH

O

CH2OH

OH

O OH

O

CH2OH n

OH n = 2000−10000

OH

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weight treatments are appropriately chosen for describing polydisperse materials.42 In the refining of cellulose, particularly into high-purity dissolving grades of the type used in ester manufacture, DP loss (or chain length degradation) will inevitably occur. And further, in the actual production of cellulose esters, considerable loss of DP commonly takes place. DP values falling in the range of 800−1500 are typical for many of the commercially important esters of cellulose. For certain application areas (such as coatings additives), polymer chain length reduction is needed to provide desirable properties, and considerable DP loss may actually be promoted in the manufacturing processes. The structural representation involving anhydroglucose repeat units also forms the basis for the degree of substitution (DS) terminology associated with cellulose esters. Each six-carbon anhydroglucose unit presents three hydroxyl groups that can theoretically be esterified (at the 2, 3, and 6 positions, by IUPAC naming convention). Hence by common terminology, the maximum degree of ester substitution is three. Figure 19.11 provides a generic cellulose ester depiction in which the common “R” group (acyl) substituents are acetyl, propionyl, or butyryl. It should be noted that in commercial manufacturing significant populations of hydroxyl groups (R = H) are retained, dependent on intended application. Higher molecular weight acyl groups can also be esterified onto the polymer backbone, but those explicitly listed in Figure 19.11 represent the organic cellulose esters that have found the most widespread commercial utility. Cellulose acetate is sold primarily in di- and triacetate forms (nominal DSacetyl of ~ 2.5 and ~ 2.9, respectively, or ~ 39 and ~ 44 wt% acetyl). Common abbreviations are CDA for cellulose diacetate (although CA is often used to designate cellulose diacetate as well) and CTA for cellulose triacetate. The “pure” propionate or butyrate esters have never achieved much commercial importance (due to manufacturing issues), but the so-called mixed esters of acetate-propionate and acetate-butyrate (abbreviated CAP and CAB) are quite important in commerce. Figure 19.12 shows a generic reaction scheme

CH2OR

OR

O H O

OR

OR

OR

CH2OR

O

O OR

OR

O

O

CH2OR

O OR

OH

O

CH2OR n

OR

n = 400−750 R = H, or Acetyl

O

Propionyl

C CH3

FIGURE 19.11 Structural representation of cellulose esters.

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O C CH2CH3

Butyryl

O C CH2CH2CH3

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O

O O

HO OH

O

+ R

O

R

H2SO4

n Anhydride

Cellulose

CH3 (Acetyl) R = CH2CH3 (Propionyl) CH2CH2CH3 (Butyryl)

Cellulose acetate Cellulose acetate propionate Cellulose acetate butyrate

OR′

O O

R′O OR′

n

Cellulose ester R′ = COR or H

FIGURE 19.12 Reaction scheme for cellulose esters.

for producing cellulose esters based on the simplified depiction of cellulose as a polymer of repeating anhydroglucose units. In the presence of a strong acid catalyst (e.g., sulfuric acid), organic anhydrides can be employed to effect esterification. The basis for the “maximum DS of three” is clearly seen by this simplistic depiction. A point to be made here is that esterification is typically performed on relatively high molecular weight cellulose, and that the art and science of manufacturing cellulose esters lie in careful derivatization, with much attention paid to controlling the inevitable molecular weight breakdown (DP loss) that occurs with industrial scale processing. Gedon and Fengl43 prepared an excellent review article (with extensive bibliography) covering organic cellulose esters, and this article includes a large amount of manufacturing and processing information (lab through commercial scale). Discussion of basic structure−property relationships is also provided, as is application coverage up until about 1992. Similar though somewhat complementary manufacturing and application coverage was provided in an encyclopedia article published earlier.44 This coverage will not be repeated here. A number of the basic properties of cellulose esters can have a profound impact on observed properties in common applications. These include molecular weight, molecular weight distribution (polydispersity concept), type of substitution (acyl), extent of substitution (DS), and the substitution pattern (e.g., “blocky” vs. random). Other factors such as extent of crystallinity can certainly come into play, but those listed above represent the main properties that are either controlled or closely monitored in the commercial manufacture of cellulose esters. The exception is substitution pattern. To date, attempts to deliberately effect substitution pattern (along the polymer backbone) have been largely unsuccessful, and essentially random substitution patterns are found for all of the cellulose esters that have Copyright © 2005 by Taylor & Francis

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achieved commercial importance. That is not to say that large-scale production of cellulose esters with regioselective substitution is not worth pursuing, for it could represent breakthrough technology of the type seen with stereoregular polyolefins. Rather, this is a situation where no one has found an economically viable route to these types of materials.

References 1. Hon, D.N.-S., Embrapa instrumentação agropecuária, in Natural Polymers and Agrofibers Composites, Frollini, E., Leão, A. L., and Mattoso, Eds., São Carlos, Brazil, 2000, p. 292. 2. Rowell, R.,M., Young, R. A., and Rowell, K. J., Paper and Composites from Agrobased Resources, CRC Lewis Publisher, Boca Raton, FL,1997. 3. Rials, T.G. and Wolcott, M.P., in Paper and Composites from Agro-based Resources, Rowell, R.M., Young, R.A., and Rowell, K.J., Eds., CRC Lewis Publishers, Boca Raton, FL, 1997, p. 446. 4. Felix, J., Gatenholm, P., and Schreiber, H. P., J. Appl. Polym. Sci., 51, 285, 1994. 5. Felix, J.M., Carlsson, C.M.G., and Gatenholm, P., J. Adhesion Sci. Technol., 8, 163, 1994. 6. Felix, J.M. and Gatenholm, P., J. Appl. Polym. Sci., 42, 609, 1991. 7. Felix, J.M. and Gatenholm, P., J. Appl. Polym. Sci., 50, 699, 1993. 8. Felix, J.M. and Gatenholm, P., J. Mater. Sci., 29, 3042, 1994. 9. Felix, J.M., Gatenholm, P., and Schreiber, H.P., Polym. Compos., 14, 449, 1993. 10. Gatenholm, P., Bertilsson, H., and Mathiasson, A., J. Appl. Polym. Sci., 49, 197, 1997. 11. Hedenberg, P. and Gatenholm, P., J. Appl. Polym. Sci., 56, 641, 1995. 12. Hedenberg, P. and Gatenholm, P., J. Appl. Polym. Sci., 60, 2377, 1996. 13. Karlsson, J.O. et al., Polym. Compos., 17, 300, 1996. 14. Zini, E., Scandola, M., and Gatenholm, P., Biomacromolecules, 4, 821, 2003. 15. Frollini, E., Leão, A.L., and Mattoso, L.H.C., Natural Polymers and Agrofibers based Composites, Embrapa, San Carlos, S.P. Brazil, 2000. 16. Toriz, G. et al., J. Appl. Polym. Sci., 88, 337, 2003. 17. Gatenholm, P. and Mathiasson, A., J. Appl. Polym. Sci., 51, 1231, 1994. 18. Gatenholm, P., Kubat, J., and Mathiasson, A., J. Appl. Polym. Sci., 45, 1667, 1992. 19. Riedel, U. and Nickel, J., Ange. Makromol. Chem., 272, 34, 1999. 20. Dubief, D., Samain, E., and Dufresne, A., Macromolecules 32, 5765, 1999. 21. Dufresne, A., Kellerhals, M.B., and Withold, B., Macromolecules, 32, 7396, 1999. 22. Matsumura, H., Sugiyama, J., and Glasser, W.G., J. Appl. Polym. Sci., 78, 2242, 2000. 23. Seavey, K.C. et al., Cellulose, 8, 149, 2001. 24. Franko, A. et al., Cellulose, 8, 171, 2001. 25. Buchanan, C.M. et al., J. Macromol. Sci.–Pure. Appl. Chem., A32, 683, 1995. 26. Buchanan, C.M., Gardner, R.M., and Komarek, R.J., J. Appl. Polym. Sci., 47, 1709, 1993. 27. Komarek, R.J. et al., J. Appl. Polym. Sci., 50, 1739, 1993. 28. Offerman, R.J. U.S. Patent 6,462,120, USA, 2002.

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Eastman Publication PP-100, Eastman Chemical Company, Kingsport TN, 1999. Tremblay, J.-F., Chem. Eng. News, 21, 7, 1999. Eastman Publication EFC-223, Eastman Chemical Company, Kingsport TN, 1997. Eastman Publication EFC-205, Eastman Chemical Company Kingsport TN, 1998. Edgar, K.J. et al., Prog. Polym. Sci., 26, 1605, 2001. Licari, J.J., Plastic Coatings for Electronics, McGraw-Hill, New York, 1970. White, A.W. et al., J. Appl. Polym. Sci., 52, 525, 1994. Mayer, J.M. et al., J. Macromol. Sci.-Pure Appl. Chem., A32, 775, 1995. Netravali, A.N. et al., Polym. Compos., 10, 226, 1989. Paist, W.D., Cellulosics, Reinhold, USA, 1958. Gilbert, R.D. and Kadla, J.F., in Biopolymers from Renewable Resources, Kaplan, D. L., Ed., Springer, NY, 1998, chap. 3. Robyt, J.F., Essentials of Carbohydrate Chemistry, Springer, NY, 1998. Brown, R.M.J., Saxena, I.M., and Kudlicka. Trends Plant Sci., 1, 149, 1996. Campbell, I.M., Introduction to Synthetic Polymers, Oxford University Press, New York, 2000. Gedon, S. and Fengl, R., in Kirk-Othmer Encyclopedia of Chemical Technology, Wiley & Sons, New York, 1993, p. 503. Eicher, T., Wandel, M., and Astheimer, H.-J., in Ullmann’s Encyclopedia of Industrial Chemicals, VCH, Weinheim, Germany, 1986, p. 438.

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20 Starch Polymers: Chemistry, Engineering, and Novel Products

Bor-Sen Chiou, Gregory M. Glenn, Syed H. Imam, Maria K. Inglesby, Delilah F. Wood, and William J. Orts

CONTENTS 20.1 Introduction 20.2 Starch Blends and Biodegradability 20.3 Baked Starch-Based Foams 20.4 Starch-Based Foams: Moisture Protection 20.5 Compression/Explosion Process 20.6 Microcellular Foams: Preparation and Characterization 20.7 Microcellular Foams: Encapsulation of Volatile Compounds 20.8 Frozen-Food Trays 20.9 Lightweight Concrete from Aquagels 20.10 Starch-Based Wood Adhesive 20.11 Starch Modifications for Tailored Properties 20.12 Summary References ABSTRACT With the global interest in producing environmentally friendly polymers as an alternative to petroleum-based synthetic polymers, efforts are under way to develop and produce starch-containing ecoefficient plastic products for consumer applications. These efforts are in part driven by consumers’ heightened environmental awareness, international treaties, and numerous environmental legislations. At the U.S. Department of Agriculture (USDA) laboratories and elsewhere, research on starch polymers has focused on several areas, including utilizing commodity starches in foams, composites, building materials, adhesives, and blends. Particularly, in light of recent advances made in developing novel processing techniques, optimization of engineering parameters, and better understanding of starch

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polymer interfacial properties, USDA scientists have successfully engineered numerous products, including starch-based composites for disposable containers, microcellular foams for encapsulation of volatile compounds, lightweight concrete, mulch films, and wood adhesives. These starch-based products are not only completely biodegradable, but have physical and mechanical properties comparable to products made from synthetic polymers. The results suggest that developing consumer products from renewable starch polymers or starch polymer blends is a viable alternative to petrochemicals in the near future.

20.1

Introduction

To fully understand and appreciate the versatility of starch polymers and their applications in bioplastics, and to serve the interest of the readership, some background information on the physical and chemical properties of starch is reviewed here. Starches used in industrial applications are usually extracted from cereal seeds (corn, wheat, and rice), tubers (potato), and roots (tapioca). Starches from various sources are chemically similar, yet their granules are heterogeneous with respect to their size, shape, and molecular constituents.1,2 Probably their most critical descriptor is their proportion of the polysaccharides amylose and amylopectin.1,2 The chemical structures of amylose and amylopectin are shown in Figure 20.1. Naturally occurring types of cereal endosperm starches contain 72−82% by weight of amylopectin and 18−33% by weight of amylose. However, barley (Hordeum vulgare) and rice (Oryza sativa) may contain up to 70% amylose. Modified genotypes such as “waxy” starches may contain less than 1% amylose, whereas high-amylose genotypes may contain up to 70% amylose.1,3,4 Most amylose molecules (molecular weight ~105−106 Da) are linear and consist of (1→4) linked α-D-glucopyranosyl units. A few molecules are branched to some extent by (1→6) α-linkages.3,4 Amylose molecules can vary in their molecular weight distribution and in their degree of polymerization (DP). These fundamental molecular characteristics affect their solution viscosity, which is essential for processing, and their retrogradation/recrystallization behavior, which is critical for product performance.1 Amylopectin is the highly branched polysaccharide component of starch that consists of hundreds of short chains formed of α-D-glucopyranosyl residues with (1→4) linkages. These are interlinked by (1→6)-α-linkages, from 5 to 6% of which occur at the branch points. Despite the amylopectin’s high molecular weight (107−109 Da), its intrinsic viscosity is very low (120−190 ml/g) because of its extensively branched molecular structure.1,3,4 In addition to amylose and amylopectin, native starches contain three categories of minor, nonstarch components: particulate matter, extractable surface components, such as proteins, and internal components. Of these, the internal Copyright © 2005 by Taylor & Francis

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6 CH2OH 5 O

CH2OH O

H

H OH

H

H

H

H 4 OH

O

HO H

3 H

OH

CH2OH H

O

H

H OH

H 1

OH

O

2 OH

H

H

H

OH

N Amylose

G G G G G

G G G G G G G G G G G G G G G

G

G

G

G

G

G

G

G

G

G

G

G G G G G G G G G G G G G G G G G G G G G G G G G G G G G G G G G G G

O

C

G

O H2 H

H H O

O

H

G G G G G G G

G G G

H

G

G

Amylopectin

H H

H O

O H

O H OH

CH2OH

CH2

CH2OH H

H

H

O

O H OH

OH

H

H

O H OH

H

H

OH

H

O

O H

H

H

OH

Branch point

FIGURE 20.1 Chemical structures of amylose and amylopectin. G is an anhydroglucose unit.

components, consisting largely of lipids, can affect starch-plastics processing and product performance. Integral starch lipids are found within the intact granules of cereal starches. Integral lipids in barley and wheat consist entirely of lysophospholipids, whereas 30−60% of the integral lipids in rice, oats, and maize are free fatty acids.1 On the other hand, tuber starches contain mineral fractions in which the cereal starches have negligible concentrations. Root and tuber starches contain phosphate monoesters, with a high level found in potato starch (0.089%). For the starches containing phosphates, their viscosity during processing may be affected by the presence of cations.1,3,4 Copyright © 2005 by Taylor & Francis

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Starch granules are usually processed by heating in aqueous media, which results in their gelatinization. Gelatinization occurs from irreversible swelling of the granules, leading to starch solubilization as amylose leaches from the granules and amylopectin becomes fully hydrated. This process results in the loss of molecular order and the melting of native starch crystals. The gelatinization onset temperature is critical from a processing standpoint and varies considerably with starch type.1,4,5 Upon cooling, with release of water and other solvent molecules, the starch tends to retrograde or recrystallize. The carbohydrate chains associate into a thermodynamically more stable form through hydrogen bonding. The rate and degree of retrogradation depend on the proportion of amylose to amylopectin, the amylose DP, and the presence of lipids. The mainly linear amylose recrystallizes irreversibly and rapidly, whereas the highly branched amylopectin recrystallizes reversibly and much more slowly.1,6 Also, low molecular weight amylose molecules reassociate more rapidly than the high molecular weight ones.1,7,8 For example, Ellis et al.1 reported low retrogradation rates for potato and tapioca starches with DPs of 3000 and high retrogradation rates for maize and wheat starches with DPs of 800. Waxy starches, with only small concentrations of amylose, typically exhibit very low recrystallization rates. In addition, the presence of fatty acids has been reported to retard retrogradation by forming amylose−lipid inclusion complexes.1,7, 8 Ozcan and Jackson7 reported that increasing the chain lengths of the fatty acids decreased the percentage of recrystallization. This recrystallization or retrogradation with time translates into “brittleness” of the final starch product. To improve performance, further investigations are required to elucidate the phenomenon at the molecular level. The gelatinization process does not occur without a plasticizer, since both the glass transition temperature (Tg) and the melting temperature (Tm) of pure, dry starch are greater than the decomposition temperature.9,10 Water has been the most commonly used plasticizer in starch processing; however, with respect to starch plastics, its sole use has serious drawbacks. Water can rapidly escape the product, causing the product to become brittle over time.9−11 Consequently, other plasticizers, such as glycols, sugars, and amides can be added to the sample to lower the Tg and produce a more rubber-like starch product.6 Efficient plasticizers generally have low molar mass, a high boiling point, and exhibit low viscosities and low temperature coefficients of viscosity. It is critical that the plasticizer be homogeneously blended in the polymer.6 In addition to differences in chemical composition, native starch granules exhibit two main types of x-ray diffraction patterns, the A structure for cereal starches and the B structure for tuber and amylose-rich starches. The diffraction diagram of legume starches (smooth pea and various beans), originally designated as C type, has been shown to be a mixture between the A and B types.3,4 The most recent models of these structures are based on 6-fold left-handed double helices; the A-structure is packed with the space group B2 in a monoclinic unit cell, whereas the B structure is packed with the space Copyright © 2005 by Taylor & Francis

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group P61 in a hexagonal unit cell.3 Upon gelatinization, another pattern, called the V structure (Verkleisterung), forms as starch precipitates from solution or complexes with various organic molecules, such as lipids in cereal starches.4 In contrast to the double helical nature of native A and B types, V-type structures consist of single helices with ordered regions having significantly larger dimensions.12 Understanding the crystal structure and degree of crystallinity of starch samples can provide information about the retrogradation process. Structural changes may be ongoing in the final starch products, depending on the type of starch used, the starting compounds (product formulation), and the processing conditions. In addition, correlations between crystallite dimensions and mechanical properties, such as tensile strength and modulus, may be established. This chapter highlights recent studies aimed at generating innovative processing techniques and products from commodity starches, focusing mostly on research done by the USDA Agricultural Research Service’s scientists at the Western Regional Research Center, located in Albany, CA.

20.2

Starch Blends and Biodegradability

Although starch can be plasticized, starch-based products are moisture sensitive and are more brittle, especially after aging, when compared to typical synthetic plastics. For example, upon gelatinization, starch can be used to produce a clear film in the absence of any additives, but such films have relatively poor physical properties and are sensitive to water. To obtain commercially acceptable products, starch must be modified or blended with other materials to improve its physical properties and to minimize its water sensitivity. Researchers at the USDA have either chemically cross-linked or blended starch with other biopolymers or totally biodegradable synthetic polymers to improve their properties. Some of these polymers include chitin/chitosan,13 pectin and cellulose,14 bacterially derived polyester poly(β -hydroxybutyrate-co-β -hydroxyvalerate) (PHBV),15−18 poly(lactic acid) (PLA), and poly(vinyl alcohol) (PVOH).19−21 The blends have been cast into films as well as extruded and injection molded into various products. Both PLA and PHBV have received considerable attention as biodegradable alternatives to conventional plastics. Plastic products produced from these polymers have reasonable mechanical properties, leading to some commercialization. However, the primary deterrents to widespread use of these polymers include their high costs and relative thermal immiscibility with starch. Regardless of their potential benefits to the environment, the cost of the final product must be in the same range as alternative synthetic polymers used in that application. Scientists at the USDA have incorporated up to 30−50% low-cost starch as a filler into PHBV-based composites and successfully produced low-cost, biodegradable extruded and injection-molded Copyright © 2005 by Taylor & Francis

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articles that have excellent mechanical properties.15−18 Micrographs from scanning electron microscopy (SEM) showed homogenously distributed starch granules embedded in a PHBV matrix, typical of a filler material. As starch in these films degraded, granules were rapidly removed or depleted from the PHBV matrix (Figure 20.2). These composites have been evaluated with respect to their performance and biodegradability in a variety of environments, including compost, soil, municipal activated sludge, and an active marine environment. Injection-molded blends of PHBV with 30−50% native cornstarch exhibited tensile strength and elongation at break values ranging from 15 to 24 MPa and 21 to 38%, respectively. Under most environmental

FIGURE 20.2 SEM micrographs of PHBV blend with 50% starch (a) before, and (b) after exposure to compost for 125 days. (Reproduced from Imam, S.H. et al., J. Environ. Polym. Degrad., 6, 91, 1998. With permission.)

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conditions, both starch and PHBV polymers in a blend were degraded, and the weight loss was accompanied by a rapid deterioration in tensile strength and percentage of elongation (Figure 20.3). The removal or disappearance of polymer in a PHBV blend due to degradation could be quantified using FTIR spectroscopy. The neat cornstarch and PHBV exhibit distinct absorption peak spectrums, as shown in Figure 20.4(a,b), respectively. The starch spectrum has a broad OH peak centered around 40 100% PHBV 30% starch, 70% PHBV 50% starch, 50% PHBV 30% PEO coated starch, 70% PHBV

Tensile strength (MPa)

30

50% PEO coated starch, 50% PHBV

20

10

(a)

0 50

% Elongation

40

30

20

10

0

0

25

(b)

50

75 Days

100

125

FIGURE 20.3 The (a) tensile strength and (b) percentage of elongation at break of starch PHBV blends exposed to a natural compost for 125 days. (Reproduced from Imam, S.H. et al., J. Environ. Polym. Degrad., 6, 91, 1998. With permission.)

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1.0

Absorbance

0.8 0.6 0.4 0.2 0.0

(a)

(b)

1.0

Absorbance

0.8 0.6 0.4 0.2 0.0 3800 3400 3000 2600 2200 1800 1400 1000 800

(c)

Wavenumber (cm−1)

3800 3400 3000 2600 2200 1800 1400 1000 800

(d)

Wavenumber (cm−1)

FIGURE 20.4 FTIR spectra of (a) neat cornstarch, (b) neat PHBV, and a PHBV blend with (c) 30% starch and (d) 50% starch exposed for 125 days in compost. (Reproduced from Imam, S.H. et al., J. Environ. Polym. Degrad., 6, 91, 1998. With permission.)

3400 cm⫺1 and a set of strong C-O bands between 960 cm⫺1 and 1190 cm⫺1 in the fingerprint region. In contrast, the PHBV has a characteristic carbonyl peak at 1725 cm⫺1. These features are easily distinguishable in the PHBV blends containing 30% [Figure 20.4(c)] and 50% starch [Figure 20.4(d)]. Degradation of starch and PHBV was accompanied by substantial changes in the molecular weight distribution. In the PHBV blends containing 30% and 50% starch, the number average molecular weight (Mn) of PHBV decreased by an average of 44% after 125 days of exposure in a compost environment.16 Interestingly, comparison of the 1H-NMR signals generated from terminal methyl groups revealed that the rate and extent of degradation of PHB (indicated by H-4′) and PHV (indicated by H-5′) moieties within the copolymer were similar, regardless of the presence of starch (Figure 20.5). Most importantly, starch in the PHBV blend not only degraded at a much faster rate, its incorporation as filler also accelerated the PHBV degradation process. A predictive mathematical model for loss of individual polymers from a 30% starch−70% PHBV blend in marine environments indicated that PHBV degradation was delayed 50 days until more than 80% of the starch was consumed. The predicted starch and PHBV half-lives in the blend were 19 and 158 days, respectively (Figure 20.6). The slow degradation of PHBV in seawater was attributed to nitrogen deficiency and low oxygen demand typical of this environment. Addition of urea as a nitrogen source in Copyright © 2005 by Taylor & Francis

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O 1

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2

3

O

O

1′

PHB

H-3

4′

5′ CH3

H-4 H-2 + H-2′

3′ O

2′ PHV

H-5′

H-4′

H-3′ (a)

(b) 5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5 PPM

FIGURE 20.5 1 H proton NMR spectra of (a) starch−PHBV control sample and (b) a similar sample exposed to compost for 125 days. (Reproduced from Imam, S.H. et al., J. Environ. Polym. Degrad., 6, 91, 1998. With permission.)

100 90 80 Blend Weight loss (%)

70 60 50

PHBV

40 30 Starch 20 10 0 0

50

100

150

250 200 Time (days)

300

350

400

FIGURE 20.6 Computed polymer weight loss profiles (curves) for a 30% starch−70% PHBV blend exposed in the marine environment. Symbols represent experimental data, whereas lines represent model predictions. (Reproduced from Imam, S.H. et al., J. Environ. Polym. Degrad., 6, 91, 1998. With permission.)

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subsequent formulations showed increased PHBV degradation, thus validating the predictive model. Efforts to overcome mechanical deficiencies of starch have also included blending with PVOH and the family of polymers related to PVOH. PVOH, a hydrolysis product of poly(vinyl acetate), is a highly polar and hydrophilic synthetic polymer. Consequently, it is thermally miscible with starch. PVOH is also a biodegradable material. Starch−PVOH blends represent one of the few commercial successes in the industry, having been marketed domestically as water-soluble laundry bags and agricultural mulch films as well as in Europe as replacements for many commodity plastics. Although starch−PVOH extruded films initially exhibited desirable properties, their quality deteriorated with time. Scientists at the USDA have investigated starch−PVOH blends extensively, including pioneering studies that yielded starch−PVOH films by gelatinizing starch with PVOH in the presence of a plasticizer, glycerol, and a cross-linking agent, such as formaldehyde. Starch−PVOH films had also been coated with hydrophobic polymers, such as poly(vinyl chloride), to produce agricultural mulch films.22,23 Examination of various polyols, including glycerol, as plasticizers for starch−PVOH cast films indicated a loss of glycerol from aged films as well as a loss of plasticizer effectiveness in films with higher starch loads.24 Addition of 3% polyethylene-co-acrylic acid in a starch−glycerol−PVOH formulation had been shown to improve mechanical properties of starch− PVOH films.25 Significant increase in the elongation of starch−PVOH blown films had also been reported when 10% glycerol was added as a synergistic plasticizer along with 5% talc.26 Cross-linking the starch and PVOH with hexamethoxymethylmelamine in starch−PVOH films resulted in much improved tensile properties and water resistance.20 Also, increased PVOH content improved elongation of films, particularly at relative humidities of 80% or higher. Interestingly, cross-linking or increased PVOH content in films did not significantly affect film biodegradation in the compost. In addition, increasing the glycerol concentration from 0 to 35% in the starch−glycerol−PVOH formulation affected the material properties. Data from Dynamic Mechanical Analysis (DMA), shown in Table 20.1, reveal that both tan δ and storage modulus decreased proportionally with increased glycerol content. As expected, increasing the glycerol content lowered the glass transition temperatures (Tg).21 Blending starch with poly(ethylene-co-vinyl alcohol) (EVOH), a compostable polymer, offered several practical advantages over PVOH.27 The hydroxyl groups within the vinyl alcohol promote good compatibility with starch, and the ethylene co-monomers improve the sample’s moisture resistance and ductility.28−30 Thus, starch−EVOH blends are biodegradable, are thermally miscible for the complete range of starch compositions, and can have their water-resistant qualities controlled by the degree of ethylene comonomer replacement.31 The physical and mechanical properties of starch−EVOH blends depended heavily on the moisture content of the starch feed. Samples made Copyright © 2005 by Taylor & Francis

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TABLE 20.1 Dynamic Mechanical Analyses (DMA) and Estimated Tg Values of Starch–Glycerol–PVOH Formulations Starch–Glycerol–PVOH Ratio

T 1a (°C)

T 2b (°C)

55 38 61 48 15 8 –4

75 60 70 70 57 35 6

0–0–100 80–20–0 100–0–10 85–15–10 80–20–10 70–30–10 65–35–10

Tg (estimated from E⬘⬘ and tan δ ) 65 49 66 59 36 22 5

a

Temperature at the onset of drop of storage modulus (E⬘). Temperature at the peak of strain/stress phase difference tan δ. Source: Reproduced from Mao, L. et al., J. Polym. Environ., 8, 205, 2000. With permission. b

from feeds with higher moisture contents exhibited lower melting temperatures, tensile strength, tensile modulus, and strain to break values. The optimal blend formulation, based on miscibility, strength, aging characteristics, and capability to replace PS foam was 65−70% wheat starch, 20−25% EVOH, and 5−10% plasticizer.31 Various amino acids have also been found to successfully plasticize starch32,33 as well as starch−EVOH blends.30 Although the plasticizing effects of starch−EVOH blends changed over time, their aging properties depended more strongly on the relative humidity in which the samples were stored than on the introduction of any particular plasticizer. This is shown in Figure 20.7 for the various blends under different storage conditions. When the samples were stored under dry conditions [Figure 20.7(a)], the elongation to break decreased rapidly in value and only started to level off after several weeks. Under these conditions, the samples containing glycerol and, perhaps, β -alanine maintained the greatest flexibility, indicating glycerol was generally one of the better plasticizers. When the samples were stored under 52% relative humidity conditions [Figure 20.7(b)], the elongation to break did not decrease nearly as much, due, presumably, to the absorption of water, which acted as an additional plasticizer. Under these conditions, the samples containing β -alanine, sarcosine, and L-proline were significantly better than glycerol in maintaining flexibility. Samples stored under high relative humidity conditions (82%) exhibited little change in elongation to break and showed little difference in plasticizer effectiveness. Differences in aging characteristics can be attributed to the relative degree of crystallinity, as determined by x-ray wide-angle analysis. Crystallinity values for the glycerol samples after 24 weeks at relative humidities of 0, 52, and 82%, were 46, 44, and 26%, respectively. The high relative humidity retards recrystallization, significantly affecting starch−EVOH blend properties. Copyright © 2005 by Taylor & Francis

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140

140 D,L Pipecolinic acid 1-Piperidine propionic acid L-Proline 4-Aminobutyric acid Sarcosine Beta-alanine Glycerol

100 80 60 40

100 80 60 40 20

20

0

0 0

(a)

D,L Pipecolinic acid 1-Piperidine propionic acid L-Proline 4-Aminobutyric acid Sarcosine Beta-alanine Glycerol

120

Elongation to break (%)

Elongation to break (%)

120

2

4

6

8

0

10

2

(b)

Time (weeks)

4

6

8

10

Time (weeks)

140

Elongation to break (%)

120 100 80 60 D,L Pipecolinic acid 1-Piperidine propionic acid L-Proline 4-Aminobutyric acid Sarcosine Beta-alanine Glycerol

40 20 0 0

(c)

2

4

6

8

10

Time (weeks)

FIGURE 20.7 Elongation to break (%) of starch−EVOH−plasticizer blends as a function of time for samples stored under (a) dry conditions, (b) 52% relative humidity, and (c) 75% relative humidity.

20.3

Baked Starch-Based Foams

Single-use disposable packaging is typically made of either expanded polystyrene (EPS) foam or paper-based products. EPS has desirable mechanical and thermal insulating properties but suffers from perceived environmental disadvantages. Most EPS disposable packaging is not recycled, but sent directly to landfills. Since EPS has not been designed to break down or degrade, it lasts for decades. Another environmental concern with EPS is that it is made from a nonrenewable resource. In recent years, there has been a shift away from EPS disposable packaging and toward paper-based products. Paper-based products have a more favorable environmental perception but have worse mechanical properties than polystyrene. Even the perceived environmental advantages of paper-based Copyright © 2005 by Taylor & Francis

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products may be erroneous because most paper-based products are coated to improve water resistance and do not always degrade under most municipal landfill conditions. Consequently, there is increasing interest toward developing viable, “environmentally friendly” alternatives to polystyrene foams derived from renewable resources. For that purpose, the USDA has focused on developing starch-based foam composites as an alternative to EPS. A technique for making starch-based foams involves baking starch dough in heated closed molds for several minutes.34 This is similar to the wafer baking process for making wafer cookies. The foam’s shape and thickness can be controlled by selecting the geometry of the mold. In this way, various shapes, such as clamshells, cups, and trays, can be fashioned from the foams. In addition, a smooth surface can be achieved with this technique. Several studies have examined the effects of using different types and concentrations of starch on the foam’s physical properties. One study involved baking foam panels from wheat, corn, tapioca, and potato.35 The panels were baked at 180°C for 3 min to produce 0.2 cm thick samples. The baked foams were then conditioned at various relative humidities to produce samples with moisture contents ranging from 3.4 to 29%. The potato and tapioca foams had densities ~30% lower than those of corn and wheat foams at 11% moisture content. Another study36 found similar results for their baked starch foams. In addition, results from both studies indicated that foams made from feeds with higher solids content had higher densities. The reason may be that more solids led to higher viscosities, which effectively reduced the steam bubbles’ expansion rate. The amylose content in the starch feed also had a significant effect on the physical properties of the foams. Several studies36,37 have shown that foams made with high-amylose cornstarch had a higher density than that of native cornstarch. Consequently, more high-amylose starch was required to completely fill a mold during baking.37 One reason for this behavior may be that high-amylose samples gelatinize at higher temperatures. In addition, Shogren et al.36 claimed that the high-amylose starch may contain more amylose−lipid complexes, which act as physical cross-links. Both factors led to an increase in the batter’s viscosity over the time period of the baking process and resulted in less steam bubble expansion. The baked foams, made from wheat, corn, tapioca, and potato starch, had, in some respects, comparable mechanical properties to those of the commercial containers made from EPS and paperboard.35 In particular, the starch foams had flexural strength and modulus values that lie within those obtained from the commercial samples. One major difference between most of the starch samples and the polystyrene and paperboard samples involved the flexural and tensile percentages of elongations at break. The percentages of elongations for the polystyrene and paperboard samples were greater than 5%, whereas the percentages of elongations for most of the starch samples were less than 5%. Only a potato starch sample had a flexural percentage of elongation at break of 5%. Another study36 also showed that the potato foams had higher deformation to break values than those of the other Copyright © 2005 by Taylor & Francis

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starch foams. At an elevated moisture content, elongation at break improved considerably.35 Another way to modify the physical and mechanical properties of foams, without varying the type and concentration of starch, involved adding fillers. Incorporating fillers, such as wood fiber and CaCO3, into the foams had been shown to change the densities of the samples.35 In fact, samples with the wood fibers actually had slightly lower densities. However, these density values were still at least two times greater than those of the expanded polystyrene foams. When CaCO3 was added to the foams, the densities for all the samples increased markedly. For example, the density of the wheat foam almost doubled in value. The fillers also had a large effect on the mechanical properties of the foams.35 Adding from 3.6 to 6.9% wood fibers to the starch foams improved their mechanical properties. The composites had higher flexural strength and modulus values as well as higher tensile strength and modulus values than those of the neat foams. In addition, the flexural strains to break slightly improved with the addition of fibers. Moreover, the tensile strains to break almost doubled in value. Another study38 also showed that when aspen fiber was added to cornstarch foams, the maximum force to break increased in value. The addition of from 6.7 to 22.2% CaCO3 to the various starch foams produced mixed results. The CaCO3 caused the flexural and tensile strengths to decrease in value, the tensile modulus to remain the same, and the flexural modulus to increase in value. Also, for these composites, the flexural and tensile strains at break decreased in value. Adding both wood fiber and CaCO3 to the starch foams produced stronger, but less flexible samples than foams without any fillers. These samples had higher flexural and tensile modulus values, but slightly lower flexural and tensile strains at break. In fact, adding both types of fillers negated the improved flexibility found from just adding the wood fiber. The mechanical properties of baked foams can also be modified by adding PVOH38,39 and cross-linkers39 to them. Corn and potato foams showed improved strength, with an increase in the maximum force at break, after blending them with up to 30% PVOH.39 In addition, the blends had improved flexibility, as indicated by the increase in the maximum displacement at break. The foams’ flexural modulus did not seem to be affected by the addition of PVOH. In another study,38 waxy corn and waxy potato foams also showed improved strength and flexibility after adding PVOH. However, the foams became stiffer and less flexible when zirconium acetate and Ca(OH)2 cross-linkers were added to them.39

20.4

Starch-Based Foams: Moisture Protection

One major disadvantage for some applications of starch-based foams is their susceptibility to water absorption. The basic building block of starch, which Copyright © 2005 by Taylor & Francis

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is an α-D-glucopyranosyl unit, has multiple hydroxyl groups on it, making starch hydrophilic. The absorption of water causes detrimental effects on the sample’s physical and mechanical properties. For instance, it changes the foam’s properties over time. In addition, samples with higher moisture content have lower tensile strength and modulus values. Moisture resistance in starch-based foams can be improved by utilizing several different techniques. One technique involved laminating the starch foam in a one-step baking and lamination process.40 This represented a clear improvement over a two-step method in which a protective coating was applied after the baking process.41 The additional coating step increased costs and led to longer production times during manufacturing. In the one-step technique, the dough was sandwiched between two layers of laminate in the mold. The mold and its contents were then baked at 160°C for 5 min. The dough consisted of wheat starch, wood fiber, and perlite or wood flour. Five types of laminates were used: cellulose fiber sheets (tissue paper), foil paper, PVOH film, poly(vinyl chloride) (PVC) film, and weighing paper. The lamination process produced 0.2 cm thick foams with moisture contents ranging from 6 to 8%. The different laminated foams had varying microstructures and surface qualities. This is shown in Figure 20.8, which presents SEM micrographs of the nonlaminated and laminated samples’ surface and cross-sectional views. For the most part, all the laminated samples had a dense outer skin (laminate material) and a porous starch core due to the presence of large cells. The surface properties of the laminated samples varied. The tissue paper laminate had an opaque, white surface, whereas the foil laminate had an opaque finish with some defects that appeared as small pores. The PVOH laminate produced a perforated surface. However, reducing the baking temperature from 160° to 130°C led to a glossy and transparent surface. The PVC laminate also had a glossy, transparent surface, whereas the weighing paper laminate produced a semiglossy, transparent appearance. All the laminates improved moisture resistance to some degree. The foil, PVOH (baked at 130°C), and PVC laminates achieved the largest reduction in water permeance values, up to more than several orders of magnitude. However, the PVOH laminate processed at 160°C only achieved a 60% reduction because of its perforated surface. In addition, the tissue and weighing paper samples had 37% and 70% reductions in permeance, respectively, due to their somewhat porous structures. Other techniques have been used to improve water resistance in starchbased foams. One common method involved blending less hydrophilic polymers or additives with starch. For example, adding highly hydrolyzed (~98%) PVOH and cross-linkers to corn and potato starch foams had been shown to improve their moisture barrier properties.39 The PVOH reduced the water uptake because this grade of PVOH is highly crystalline and coldwater insoluble. Adding zirconium acetate and Ca(OH)2 cross-linkers further improved their water adsorption properties. Also, monostearyl citrate was found to reduce water absorption in several different starches.38 In addition, modifying starch to produce starch acetate resulted in greater Copyright © 2005 by Taylor & Francis

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FIGURE 20.8 SEM micrographs of (a) cross-sectional view of nonlaminated foam, (b) cross-sectional view of PVOH laminate processed at 160°C, (c) surface view of tissue paper laminate, (d) surface view of weighing paper laminate, (e) cross-sectional view of foil laminate, and (f) cross-sectional view of poly(vinyl chloride) laminate. Scale bars: (a) 1 mm, (b) 500 µm, (c) 200 µm, (d) 500 µm, (e) 1 mm, (f) 1 mm. (Reproduced from Glenn, G.M. et al., Ind. Crops Prod., 14, 125, 2001. With permission.)

hydrophobicity of the samples.42 In the study, the starch acetates with the greater degrees of substitution were better at preventing water absorption.

20.5

Compression/Explosion Process

One major disadvantage of baking starch foams in a closed mold is the relatively long time required to produce a sample. The dough is usually held Copyright © 2005 by Taylor & Francis

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inside the heated mold for more than a minute. To reduce this baking time, a novel compression/explosion process had been developed.43 This technique required only 10 sec to complete, thus reducing energy costs and increasing product throughput. First, the mold was filled with starch feed and then placed in a hydraulic press between two temperature-controlled platens. The mold was subsequently clamped together and heated at 230°C for 10 sec. After this, the hydraulic pressure was released and the mold was allowed to open. This produced an expanded foam that had a thickness greater than 1 cm and contained ~2% moisture. The type of starch used in the feed and its moisture content affected the physical and mechanical properties of the samples produced from the compression/explosion process. Three different starches were used: wheat, corn, and potato. The starch feed was conditioned to moisture contents ranging from 8 to 20% on a dry weight basis before being placed in the mold. The feeds with higher moisture contents produced foams with lower densities. This may be due to better foaming at higher moisture levels. In addition, feeds containing 14% moisture content produced corn and wheat foams with maximum compressive strength and compressive modulus values. In contrast, feeds containing only 8% moisture content were required to produce potato starch foams with maximum compressive strength and modulus values. Moreover, the feed’s moisture content had a large effect on the foams’ microstructure. SEM micrographs showed that the samples made from lower moisture feeds had closed-cell structures and thick cell walls. On the other hand, the samples made from higher moisture feeds had nonuniform cell sizes and thinner cell walls. Some of the foams produced from the compression/explosion process had comparable physical properties to those of polystyrene foams, but proved to be stiffer. The corn and wheat foams made from feeds containing 17% or greater moisture had comparable densities to polystyrene foams. In addition, potato foams made from feeds containing 14% or greater moisture also had comparable densities. However, these samples had compressive strength and modulus values approximately one order of magnitude larger than those of the polystyrene foams.

20.6

Microcellular Foams: Preparation and Characterization

Microcellular foams usually have cells under 10 µm in diameter and can be produced from various polymers. One method of preparing starch-based microcellular foams involved first producing aquagels and then drying them by utilizing various techniques.44 Aquagels are semirigid materials containing a low concentration of starch (~8% by weight) that is swollen by water. Once dried, they form a highly porous solid. Aquagels were prepared by first mixing water with starch to form a starch solution. This solution was then heated until the starch gelatinized and its viscosity reached a maximum value. The solution was then poured into a mold and allowed to chill Copyright © 2005 by Taylor & Francis

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overnight at 5°C to promote retrogradation. This produced a firm gel-like material. Subsequently, in some samples, water was displaced by ethanol to produce alcogels. This process reduced surface tension effects that might disrupt the gel’s microstructure while drying. Various drying techniques were then utilized to remove the ethanol or water from the alcogels or aquagels. Different types of starches, including wheat, corn, and highamylose (~70%) corn, were used to produce the gels. Four different drying methods were used to dry the alcogels and aquagels. One method involved drying the alcogels with a stream of dry air. Another approach was to critical point dry them using CO2. This involved placing the samples in an autoclave and first filling it up with ethanol. The ethanol was subsequently replaced by adding liquid CO2. After several more washings with CO2, the autoclave temperature and pressure were adjusted above the critical temperature and pressure of CO2 to dry the samples. A third drying method involved liquid CO2 extraction of alcogels by washing them with liquid CO2 in the autoclave. The samples were eventually dried by depressurizing the autoclave without ever reaching the critical point of the CO2. The fourth drying method involved freeze-drying the aquagels. The starch type and drying method can have a large effect on the foams’ physical and mechanical properties. To ensure proper comparison between foams, all the foams were conditioned at 50% humidity for several days to reach a final moisture content of 11.5%. Freeze drying corn and wheat foams resulted in samples with a lower density than that obtained from the other drying methods. This was due to the formation of large cavities. This, in turn, resulted in lower compressive strength and modulus values for the freeze-dried samples. Figure 20.9 shows several SEM micrographs of freeze-dried wheat foam as well as polystyrene foam for comparison. In contrast to freeze-drying, the critical point dry and liquid CO2 extraction methods produced samples with comparable density and mechanical properties. Of all the drying methods evaluated, the ethanol solvent exchange in conjunction with air drying was the most successful and economical. Freeze-drying was not recommended as it had adverse effects on the microstructure and mechanical properties and was a costly process. Foams produced in the autoclave had excellent properties but were the most costly to produce. Overall, compared to other starches, the cornstarch foams had the highest density, compressive strength, and compressive modulus. The wheat and corn foams also had comparable thermal conductivities, which were higher than that of the high-amylose corn foams. Micrographs from SEM showed that some starch granules remained in all the foams, although to a larger extent in the corn and wheat samples.

20.7

Microcellular Foams: Encapsulation of Volatile Compounds

A potential application of microcellular foams involved encapsulation and controlled release of volatile compounds.45,46 These can include flavor Copyright © 2005 by Taylor & Francis

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FIGURE 20.9 SEM micrographs of (a,c) polystyrene foam and (b,d) freeze-dried wheat foam. Scale bars: (a,b) 500 µm, (c,d) 100 µm. (Reproduced from Glenn, G.M. and Irving, D.W., Cereal Chem., 72, 155, 1995. With permission.)

compounds as well as insect pheromones, which are used to control agricultural pests. These foams had a large percentage of 0.25- to 0.5-µm pores, with some pores as small as 0.5−1.4 nm. Some advantages of using starch foams in these applications included their biodegradability and their processing versatility. The types and physical states of the starches had a large effect on their ability to sorb volatile compounds. Wheat and high-amylose cornstarch in foam and powder form were used in the sorption experiments. Sorption was determined by first placing the volatile compound and the starch inside a container. After the sample had equilibrated, the partial pressure above the starch was measured. Table 20.2 presents the sorption results for the various starch foams and powders. The results indicated that the foams were better than the powders at sorbing all of the volatile compounds, since the foams caused greater partial pressure reductions. This may be due to the foams having a greater surface area in the pores for the compounds to sorb. Also, the foams seemed to be better at sorbing the more polar compounds than the powders. For instance, the foams showed a greater ability to sorb several pyridines and pyrazines. In contrast, the foams were only slightly better than the powders at reducing the partial pressure of decane, a nonpolar compound. Generally, the high-amylose corn foam can better sorb the volatile compounds than the Copyright © 2005 by Taylor & Francis

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TABLE 20.2 Ratio of Partial Pressures of Volatile Compounds with Microcellular Foams (p) to That with Starch Powder (p0) p/p0 Compound Dimethyl sulfide Decane (S)-(−)- Limonene Benzaldehyde Isobutyl cyanide 3-Pentanone Acetophenone Hexanal Hexyl acetate Octanol (Z)-3-Hexanal 2-Octanone 3,5-Dimethylpyridine 2,5-Dimethylpyrazine 2-Ethyl-3-methylpyrazine 3-Methyl-2-cyclohexen1-one (E)-2,4-Decadienoate (E)-5-Decen-1-yl acetate 3,5-Lutidine 9-Decen-1-ol

Wheat starch

High-amylose cornstarch

0.91 0.71 0.34 0.11 0.11 0.07 0.05 0.03 0.03 0.02 0.02 0.01 0.01 0.008 0.007 0.37 0.36 0.31 0.20 0.16

— — — — — — — — — — — — — — — 0.42 0.06 0.13 0.12 0.01

wheat foam. This may be due to the higher amylose concentration, since amylose has been shown to complex with a wide variety of molecules.2,3 Another reason may be that the high-amylose corn foam had a higher porosity and larger surface area for sorption. The wheat foams can be made to release their sorbed compounds and can have their activity restored through heating.45 The various sorbed compounds can be partly released by adding water to the wheat foams. For instance, the partial pressure of 2-octanone was largely restored (⬎ 50%) within 30 min of adding water. The foams also lost their sorption activity after several months. This activity can be largely restored after heating the foams for several hours at 100°−110°C. The starch-based foams can be compressed under different pressures to control their permeability to the various volatile compounds.46 The foam’s permeability was characterized by measuring the vapor transmission rates of the compounds as they diffused through the sample. To control the vapor transmission rates, the foams were compressed at pressures of 1.4, 6.9, and 69 MPa. The compressed foams showed a 28, 62, and 91% average reduction, respectively, in the vapor transmission rates compared to those of the uncompressed foam. These results indicated that the more highly compressed samples with lower porosity and higher density were better at reducing the vapor transmission rates. Copyright © 2005 by Taylor & Francis

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Frozen-Food Trays

Another possible application for microcellular foams involved compressing them into sheets for use as frozen food trays in oven and microwave baking.47 The sheets can be made simply by compression at ambient temperatures. Compression pressures ranging from 34.5 to 138 MPa were applied to wheat foams to produce white and opaque trays. The starch-based trays had comparable mechanical properties to polypropylene in some areas, but fell short in others. Polypropylene is a commonly used material for making frozen-food trays. The foam’s tensile strength increased from 12.9 to 19.6 MPa for compression pressures of from 34.5 to 69 MPa. However, the tensile strength leveled off at higher pressures. Similarly, the tensile modulus increased in value to about 1.2 GPa until the compression pressure exceeded 103 MPa. After this, the modulus did not increase substantially. These tensile strength and modulus values were comparable to those of polypropylene. However, the starch-based trays had percentages of elongation to break values of ~2.5% for the entire compression pressure range, well below the 50% value for the polypropylene sample. To examine the starch foam’s usefulness as frozen-food trays, three types of frozen foods, high moisture (spinach), low moisture (pancakes), and oily (macaroni and beef with tomatoes), were initially placed on the trays. The foam’s mechanical properties were then measured after aging at ⫺20°C for 2 months and after baking in convection and microwave ovens. The starch foams exhibited some deficiencies when used as frozen-food trays. After aging, the samples had a much lower tensile and flexural strength as well as a lower tensile modulus than those of the fresh samples. This may be due to retrogradation, which caused the samples to become more brittle. The mechanical properties also degraded after the samples were baked in an oven. For the most part, the baked samples had comparable tensile strengths and comparable or greater flexural strengths than those of the aged samples. In addition, the baked trays had comparable or lower tensile modulus values than those of the aged samples. Overall, the oily food had the greatest detrimental effect on the mechanical properties. Moreover, all the trays baked in both convection and microwave ovens exhibited some type of blistering on their surfaces. This was due to the vaporization of trapped water within the starch matrix.

20.9

Lightweight Concrete from Aquagels

Lightweight concrete is used commercially for nonstructural applications such as floor fills and roof decks as well as insulation around fireplaces. Lightweight concrete can be made by incorporating lightweight aggregates Copyright © 2005 by Taylor & Francis

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in the concrete mixture or by incorporating air using specialized mixers. Each of these two methods has its limitations and drawbacks. A third method of making lightweight concrete is by adding aquagels, durable gellike aggregates made of water and a gelling agent. One advantage of using aquagels that utilize starch as the gelling agent is that they are comparable in cost to other commercial lightweight aggregate fillers, such as expanded perlite (perlite is a generic term for naturally occurring siliceous rock). Lightweight concrete with starch-based aquagels was made by first dispersing the aquagels into a concrete mixture, which then set and encased the aquagels. Over time, the aquagels shrank as they lost moisture, which created voids in the concrete matrix and resulted in a lightweight, less dense material. The incorporation of aquagels into concrete had a large effect on its physical and mechanical properties.48 Different types of aquagels were added to the cement mixture, including wheat starch, high-amylose (~70%) cornstarch, agar, and algin. In addition, perlite was added to concrete as a comparison. The aquagel-to-cement ratio was varied from 0.19 to 0.83, whereas the perlite-to-cement ratio was varied from 0.02 to 0.35. The incorporation of aquagels led to a decrease in the concrete density. For example, the oven-dried density of the samples decreased from 1.95 g/cm3 for the samples without any aquagel to 0.91 g/cm3 for the samples containing the highest aquagel concentration. Cross sections of the concrete samples revealed that all the aquagels were evenly dispersed in the concrete matrix. This is shown in Figure 20.10 for the various concrete samples. In addition, a linear relationship existed between density and thermal conductivity of the samples, with an increase in density resulting in an increase in thermal conductivity. The concrete samples containing high-amylose cornstarch, agar, and perlite had comparable compressive strengths over the entire concrete density range. These compressive strengths were always greater than those of the wheat and algin samples. The effects of aquagel shape and size on the concrete’s physical and mechanical properties had also been examined.49 The aquagel shapes included spherical beads, angular particles, and cylinders. These are shown in Figure 20.11, along with regular starch granules. The beads were produced by quenching the gelatinized starch into a bath of chilled vegetable cooking oil, whereas the angular particles were produced by milling dried starch gels. The starch cylinders were produced by extrusion. The angular aquagels and beads were separated into three sizes: large (1.40−1.98 mm), medium (0.85− 1.39 mm), and small (0.15−0.84 mm). In addition, three types of starches were used to produce the aquagels, since they are cheaper than the algin and agar used previously. The starches included wheat, corn, and high-amylose (~70%) corn. The shape and size of the aquagels had a greater effect than the starch type on the physical and mechanical properties of concrete. Concrete samples made with the different types of starch all had comparable plastic, cured, and oven-dried densities. The plastic density denotes the wet density of the concrete. In addition, all samples had comparable compressive strengths and percentage shrinkages. Adding different-shaped aquagels did Copyright © 2005 by Taylor & Francis

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FIGURE 20.10 Optical micrographs of cross-sectional views of samples containing (a) neat concrete, (b) a 0.03 wheat starch/concrete ratio, (c) a 0.07 wheat starch/concrete ratio, (d) a 0.13 wheat starch/concrete ratio, (e) a 0.21 wheat starch/concrete ratio, (f) a 0.35 perlite/concrete ratio, (g) a 0.21 wheat starch/concrete ratio, (h) a 0.067 high-amylose cornstarch/concrete ratio, (i) a 0.0084 algin/concrete ratio, and (j) a 0.017 agar/concrete ratio. Scale bar: 25 mm. (Reproduced from Glenn, G.M. et al., Ind. Crops Prod., 8, 123, 1998. With permission.)

produce concrete samples with some different physical and mechanical properties. All the samples containing the different aquagel shapes had comparable plastic, cured, and oven-dried densities. However, the samples containing spherical beads had greater compressive strength than that of the samples containing angular aquagels. The aquagel size had the greatest effect on the concrete densities. The concrete samples containing medium and large aquagels had lower densities than those of the samples containing small aquagels. In addition, the large aquagel samples had lower thermal conductivities and compressive strengths than those of the other samples. One problem with adding starch to cement is that starch degrades under the alkaline conditions present in cement mixtures. Starch degrades to form saccharinic acids, aldonic acids, and other products.50−52 These acids, in turn, Copyright © 2005 by Taylor & Francis

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FIGURE 20.11 SEM micrographs of (a) wheat starch granules, (b) angular particle, (c) spherical beads, and (d) extruded cylinder. Scale bars: (a) 25 µm, (b−d) 500 µm. (Reproduced from Glenn, G.M. et al., Ind. Crops Prod., 9, 133, 1999. With permission.)

may act as retarding agents and increase the setting times of cement mixtures. In fact, all the aquagel samples had longer setting times than that of the perlite sample.48 In addition, the setting times depended on how long the samples were mixed, with longer mixing resulting in longer setting times.53 If an aquagel sample was mixed for 20 min or more, the cement did not set. Several approaches have been used to reduce aquagel degradation in cement.53 One approach involved treating the aquagel with NaBH4 to block the starch chain’s reducing end groups, which might be susceptible to attack. Another approach was to cross-link the starch using epichlorohydrin. A third approach involved blending the aquagels with CaSO4 and CaCO3. The effectiveness of these techniques can be determined by monitoring starch degradation using UV absorbance. This was accomplished by measuring enediol formation at an absorbance of 282 nm. The results indicated that the NaBH4 and epichlorohydrin treatments as well as incorporating 20% CaSO4 in the aquagel reduced enediol formation. However, the setting times for these samples remained comparable to those of the samples containing untreated aquagels. This suggested that other mechanisms might be responsible for the retarding effect on setting time. Another approach to produce stable starch gels under alkaline conditions involved extruding cornstarch with various polymers.53 These polymers Copyright © 2005 by Taylor & Francis

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included a copolyester (Ecoflex FBX7011) from BASF, an epoxy (Blox 110) from Dow, EVOH from EVALCA, and PVOH from Sigma-Aldrich. These samples were placed in a 1 N NaOH solution to test their solubility. All the blends except for the starch/PVOH sample were soluble in the NaOH solution. In addition, adding the starch/PVOH samples to cement only slightly increased the setting time. This suggested that blending starch with PVOH might produce a useful filler for making lightweight concrete.

20.10 Starch-Based Wood Adhesive Formaldehyde-based thermosetting resins, such as urea-formaldehyde, phenol-formaldehyde, resorcinol-formaldehyde, and melamine-formaldehyde, are currently used for wood-to-wood bonding because of their durability, low cost, and bonding strength. The United States alone consumes over one billion pounds of formaldehyde-based resins every year in the manufacture of particleboard, oriented strength board, and plywood. However, formaldehyde is a known carcinogen, and starch-based adhesives may offer a viable alternative. Starch has been used as an adhesive in a wide range of products, including binders, sizing materials, glues, and pastes, but not as a wood adhesive, mainly due to its poor performance under extreme conditions. Recently, USDA scientists developed a starch-based adhesive in which starch was chemically cross-linked with PVOH using hexamethoxymethylmelamine (Cymel 303 and Cymel 323) as a cross-linking agent.54 Cross-linking occurred through a transetherification reaction between methoxy groups in Cymel and hydroxyl groups in starch, PVOH, and wood. The reaction is shown in Figure 20.12. This produced an efficient network of inter- and intracross-links between wood, starch, and PVOH, resulting in high bond strength. For instance, two hardwood pieces glued together with this adhesive could not be separated even after applying over 4000 lb of pressure. In fact, the wood usually failed instead of the adhesive joints. Addition of about 5−7% natural latex in the formulation provided excellent moisture barrier properties to the adhesive. The viscosity, curing temperature, moisture resistance, and bonding strength of this cross-linked starch adhesive makes it very well suited for commercial production of engineered wood products.55

20.11 Starch Modifications for Tailored Properties The many hydroxyl groups on starch allow it to be easily altered through derivatization. Some commercially available starch derivatives include Copyright © 2005 by Taylor & Francis

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N

N

N

H 2C N

CH2 OCH3

H O

Wood

CH2 OCH3

+ H O

Starch

H O

PVOH

N N

CH2

CH2

OCH3 OCH3 Catalyst

PVOH Wood

O H2C O

N

N

N

H 2C

CH2 O CH2 O

N

Starch PVOH

N N

Starch

CH2

C H2

O

O

Wood

FIGURE 20.12 Schematic representation of chemical cross-linking reaction for starch-based wood adhesives. (Reproduced from Imam, S.H. et al., Polym. Degrad. Stab., 73, 529, 2001. With permission.)

carboxymethyl and hydroxypropyl ethers as well as acetate esters. In fact, starch esters and ethers have been tested as thermoplastic competitors to cellulose esters and ethers to produce clear, tough plastic materials.56 Researchers at the USDA have successfully derivatized cornstarch to obtain specialty starches with specific properties. Particularly, chemical modifications of starch, as shown in Figure 20.13, allowed the tailoring of biodegradable materials with variable surface morphology, susceptibility to microbial adhesion, and controlled biodegradation.57 For example, replacing the hydroxyl hydrogen at C-2 of starch molecule with an n-butyl group enhanced the binding of starch-degrading microbes. This starch derivative tended to swell in cold water, producing an increased surface area for binding. The presence of n-butyl groups also disrupts intramolecular hydrogen bonding between contiguous anhydroglucose units, exposing more cell-binding sites on starch molecules for bacterial interaction. Such modified starches are particularly useful in single-use consumer products requiring quick degradation after their use. On the other hand, palmitate substitution of starch inhibited cell attachment and/or microbial interaction. The reason was the hydrophobic palmitate chain interfered sterically with binding sites on the surface of the starch granule. Such modified starches could be useful in products where slower or delayed starch degradation is highly desirable, such as biodegradable suitors or as slow-release encapsulating matrices. Copyright © 2005 by Taylor & Francis

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CH2OH

CH2OH

O OH

+

O

I

O OH Starch

O

−OH

OH

Na2SO4 O 2-propanol

n-Butyl oidide

O O 2-O-Butyl starch

CH2OH

CH2OH O OH O

[1]

O

O

N

O + C Cl

OH

(CH2)14CH3

O C

OH

[2]

O

O

(CH2)14CH3

O

Starch

Palmitoyl chloride

Starch palmitate O

HO

O

O (−

+

−)

HO

O

Polyethylene-co-acrylic acid (EAA)

H3C CO

O

O O

O

O O

Acetic anhydride EAA anhydride +

O CH2OCCH3

CH2OH

NaH

O OH

O

DMSO

O

O O

[3]

OH O

O OH

O

(−

−)

Starch

Starch EAA ester

FIGURE 20.13 Schematic of chemical derivatizations of cornstarch. Reactions 1, 2, and 3 show end products as 2-O-butyl-substituted, palmitate-substituted, and EAA-substituted starches, respectively. (Reproduced from Imam, S.H. and Harry-O⬘Kuru, R.E., Appl. Environ. Microbiol., 57, 1128, 1991. With permission.)

20.12 Summary In light of the need for material engineers to optimize their use of cost-effective feedstocks and with the relative abundance of starch, research continues on Copyright © 2005 by Taylor & Francis

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the development of starch-based composites for applications such as disposable containers, microcellular foams for encapsulation of volatile compounds, lightweight concrete, mulch films, and wood adhesives. Starch-based products are not only completely biodegradable, but have physical and mechanical properties comparable to products made from synthetic polymers. This suggests that developing consumer products from renewable starch polymers or starch polymer blends is a viable alternative to petrochemicals. The ultimate goal is to produce drop-in replacements for synthetic polymers, such as EPS foams, whereby environmentally friendly starch-derived plastics and other agriculturally derived composite materials are formed into commercial products using the same polymer processing equipment as for commodity plastics.

References 1. Ellis, R.P. et al., Starch production and industrial use, J. Sci. Food Agric., 77, 289, 1998. 2. Zobel, H.F., Molecules to granules: a comprehensive starch review, Starch, 40, 44, 1988. 3. Buleon, A. et al., Starch granules: structure and biosynthesis. mini review, Int. J. Biol. Macromol., 23, 85, 1998. 4. Whistler, R.L. and Daniel, J.R., Molecular structure of starch, in Starch, Chemistry and Technology, 2nd ed., Whistler, R.L., BeMiller, J.N., Paschall, E.F., Eds., Academic Press, Inc., Orlando, FL, 1984, chap. 6. 5. Jenkins, P.J. and Donald, A.M., Gelatinization of starch: a combined SAXS/WAXS/DSC and SANS study, Carbohydr. Res., 308, 133, 1998. 6. de Graaf, R.A., Karman, A.P., and Janssen, L.P., Material properties and glass transition temperatures of different thermoplastic starches after extrusion processing, Starch, 55, 80, 2003. 7. Ozcan, S. and Jackson, D.S., The impact of thermal events on amylose–fatty acid complexes, Starch, 54, 593, 2002. 8. Singh, J., Singh, N., and Saxena, S.K., Effect of fatty acids on the rheological properties of corn and potato starch, J. Food Eng., 52, 9, 2002. 9. Shogren, R.L., Effects of moisture and various plasticizers on the mechanical properties of extruded starch, in Biodegradable Polymers and Packaging, Ching, C., Kaplan, D.L., and Thomas, E.L., Eds., Technomic Publishing Co. Inc., Lancaster, 1993, p.141. 10. Nashed, G., Rutgers, R.P.G., and Sopade, P.A., The plastisation effect of glycerol and water on the gelatinisation of wheat starch, Starch, 55, 131, 2003. 11. Della Valle, G. et al., Relationship between structure and viscoelastic behavior of plasticised starch, J. Rheol., 42, 507, 1998. 12. Walenta, E. et al., Structure-property relationships of extruded starch 2: extrusion products from native starch. Macromol. Mater. Eng., 286, 462, 2001. 13. Nin Eo, K.A. et al., Extruded plastics containing starch and chitin: physical properties and evaluation of biodegradability, in Biopolymers: Utilizing Nature’s

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Advanced Materials, Imam, S.H., Greene, R.V., and Zaidi, B.R., Eds., ACS Symp. Ser. 723, 1999, chap. 12. Chiellini, E. et al., Composite films based on biorelated agro-industrial waste and poly (vinyl alcohol): preparation and mechanical properties characterization, Biomacromolecules, 2, 1029, 2001. Shogren, R.L., Poly(ethylene oxide)-coated granular starch-poly(hydroxybutyrate-co-hydroxyvalerate) composite materials, J. Environ. Polym. Degrad., 3, 75, 1995. Imam, S.H. et al., Biodegradation of starch-poly(β-hydroxybutyrate-co-valerate) composites in municipal activated sludge, J. Environ. Polym. Degrad., 3, 205, 1995. Imam, S.H. et al., Biodegradation of injection molded starch-poly (3-hydroxybutyrate-co-3-hydroxyvalerate) blends in a natural compost environment, J. Environ. Polym. Degrad., 6, 91, 1998. Imam, S.H. et al., Degradation of starch-poly(β-hydroxybutyrate-co-β-hydroxyvalerate) bioplastic in tropical coastal waters, Appl. Environ. Microbiol. 65, 431, 1999. Bastioli, C. et al., Biodegradable Articles Based on Starch and Process for Producing them, U.S. Patent 5,262,458, 1993. Liang, C. et al., Starch-polyvinyl alcohol crosslinked films: performance and biodegradation, J. Environ. Polym. Degrad., 5, 111, 1997. Mao, L. et al., Extruded cornstarch-glycerol-polyvinyl alcohol blends: mechanical properties, morphology and biodegradability, J. Polym. Environ., 8, 205, 2000. Otey, F.H. et al., Starch-based film for degradable agricultural mulch, Ind. Eng. Chem. Pro. Res. Dev., 13, 90, 1974. Otey, F.H., and Mark, A.M., Degradable Starch-based Agricultural Mulch Film, U.S. Patent 3,949,145, 1976. Westhoff, R.P., Kwolek, W.F., and Otey, F.H., Starch-polyvinyl alcohol filmseffects of various plasticizers, Starch, 31, 163, 1979. Lawton, J.W. and Fanta, G.F., Glycerol-plasticized films prepared from starchpoly(vinyl alcohol) mixtures: effect of poly(ethylene-co-acrylic acid), Carbohydr. Polym., 23, 275, 1994. Wang, X., Ph.D. dissertation, University of Massachusetts, Lowell, MA, 1994. Bastioli, C. et al., Method of Producing Plasticized Polyvinyl Alcohol and its Use for the Preparation of Starch-based, Biodegradable Thermoplastic Compositions, U.S. Patent 5,462,981, 1995. Villar, M.A., Thomas, E.L., and Armstrong, R.C., Rheological properties of thermoplastic starch and starch/poly(ethylene-co-vinyl alcohol) blends, Polymer, 36, 1869, 1995. Simmons, S. and Thomas, E.L., The use of transmission electron microscopy to study the blend morphology of starch/poly(ethylene-co-vinyl alcohol) thermoplastics, Polymer, 39, 5587, 1998. Orts, W.J. et al., Amino acids as plasticizers in blends of starch with ethylene vinyl alcohol copolymers. [to be submitted, 2004]. Nobes, G.A.R. et al., Blends of Starch with Ethylene Vinyl Alcohol Copolymers for use in Foam Containers, in ANTEC’01, Proceedings on the Society of Plastics Engineers, 2001, p. 1865. Stein, T.M. and Greene, R.V., Amino acids as plasticizers for starch-based plastics, Starch, 49, 245, 1997.

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33. Stein, T.M., Gordon, S.H., and Greene, R.V., Amino acids as plasticizers II. use of quantitative structure-property relationships to predict the behavior of monoammoniummonocarboxylate plasticizers in starch-glycerol blends, Carbohydr. Polym., 39, 7, 1999. 34. Tiefenbacher, K., Haas, F., and Haas, J., Process of Producing Rottable ThinWalled Shaped Bodies Made of Starch, U.S. Patent 5,376,320, 1994. 35. Glenn, G.M., Orts, W.J., and Nobes, G.A.R., Starch, fiber and CaCO3 effects on the physical properties of foams made by a baking process, Ind. Crops Prod., 14, 201, 2001. 36. Shogren, R.L. et al., Structure and morphology of baked starch foams, Polymer, 39, 6649, 1998. 37. Lawton, J.W., Shogren, R.L., and Tiefenbacher, K.F., Effect of batter solids and starch type on the structure of baked starch foams, Cereal Chem., 76, 682, 1999. 38. Shogren, R.L., Lawton, J.W., and Tiefenbacher, K.F., Baked starch foams: starch modifications and additives improve process parameters, structure and properties, Ind. Crops Prod., 16, 69, 2002. 39. Shogren, R.L. et al., Starch-poly(vinyl alcohol) foamed articles prepared by a baking process, J. Appl. Polym. Sci., 68, 2129, 1998. 40. Glenn, G.M. et al., In situ laminating process for baked starch-based foams, Ind. Crops Prod., 14, 125, 2001. 41. Haas, F., Haas, J. and Tiefenbacher, K., Process of Manufacturing Rottable ThinWalled Starch-Based Shaped Elements, U.S. Patent 5,576,049, 1996. 42. Shogren, R.L., Preparation, thermal properties, and extrusion of high-amylose starch acetates, Carbohydr. Polym., 29, 57, 1996. 43. Glenn, G.M. and Orts, W.J., Properties of starch-based foam formed by compression/explosion processing, Ind. Crops Prod., 13, 135, 2001. 44. Glenn, G.M. and Irving, D.W., Starch-based microcellular foams, Cereal Chem., 72, 155, 1995. 45. Buttery, R.G., Glenn, G.M., and Stern, D.J., Sorption of volatile flavor compounds by microcellular cereal starch, J. Agric. Food Chem., 47, 5206, 1999. 46. Glenn, G.M. et al., Sorption and vapor transmission properties of uncompressed and compressed microcellular foams, J. Agric. Food Chem., 50, 7100, 2002. 47. Glenn, G.M. and Hsu, J., Compression-formed starch-based plastic, Ind. Crops Prod., 7, 37, 1997. 48. Glenn, G.M., Miller, R.M., and Orts, W.J., Moderate strength lightweight concrete from organic aquagel mixtures, Ind. Crops Prod., 8, 123, 1998. 49. Glenn, G.M. et al., Starch-based lightweight concrete: effect of starch source, processing method, and aggregate geometry, Ind. Crops Prod., 9, 133, 1999. 50. Barrett, L., Sowell, S., and Sowell, E.A. Production and use of hypocholorite-oxidized starches, in Starch Chemistry and Technology, Vol. II, Whistler, R.L. and Paschall, E.F., Eds., Academic Press, New York, 1967, p. 237. 51. Greenwood, C.T., Aspects of the physical chemistry of starch, in Advances in Carbohydrate Chemistry, Wolfrom, M.L., Ed., Academic Press, New York, 1956, p. 335. 52. BeMiller, J.N., Alkaline degradation of starch, in Starch Chemistry and Technology, Vol. I, Whistler, R.L. and Paschall, E.F., Eds., Academic Press, New York, 1965, p. 521. 53. Glenn, G.M. et al., Novel Building Materials Containing Native Starches and Starch Blends. Proceedings of the Third International Starch Technology Conference, 2003, pp. 100–110.

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54. Imam, S.H. et al., Wood adhesive from crosslinked poly(vinyl alcohol) and partially gelatinized starch: preparation and properties, Starch, 51, 225, 1999. 55. Imam, S.H. et al., Environmentally friendly wood adhesive from a renewable plant polymer: characteristics and optimization, Polym. Degrad. Stab., 73, 529, 2001. 56. Seiberlich, J., Films and plastics from starch, Mod. Plastics, 18, 64, 1941. 57. Imam, S.H. and Harry-O’kuru, R.E., Adhesion of Lactobacillus amylovorus to insoluble and derivatized cornstarch granules, Appl. Environ. Microbiol., 57, 1128, 1991.

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21 Lignin-Based Polymer Blends and Biocomposite Materials

Satoshi Kubo, Richard D. Gilbert, and John F. Kadla

CONTENTS 21.1 Introduction 21.2 Thermal and Chemical Properties of Lignin 21.3 Thermal Treatment of Lignin 21.4 Change in Chemical Structure of Lignin during Thermal Spinning 21.5 Lignin–Synthetic Polymer Blends 21.5.1 Lignin–Polyolefin Blend Fibers 21.5.2 Lignin–Polyester Blend Fibers 21.5.3 Lignin–Hydrophilic Polymer Blend Fiber 21.5.4 Lignin–Amphiphilic Polymer Blend Fibers 21.6 Application of Lignin–Synthetic Polymer Blend Fibers as Precursors for Carbon Fibers 21.6.1 Processability 21.6.2 Carbon Fiber 21.7 Conclusion References ABSTRACT Lignin is a widely available natural polymer well suited for composite fiber applications. Depending on the wood species, and process conditions utilized, technical lignins can have a variety of thermal and chemical properties. Three technical lignins, a softwood kraft lignin (SKL), a hardwood kraft lignin (HKL), and a hardwood organosolv lignin (Alcell) were studied, and the suitability as precursor materials for composite fibers was determined. The hardwood lignins were easily extruded into fiber form, whereas the softwood kraft lignin displayed poor thermal processability. This observation was not unexpected, as the thermal moldability of lignins is very much dependent on its molecular structure. Chemical analysis, GPC, FT-IR,

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and HMQC NMR spectroscopy revealed distinct differences between the three technical lignins. The corresponding lignin fibers from the three technical lignins were brittle and difficult to handle. To improve processability and the mechanical properties of the lignin fibers, the technical lignins were blended with various synthetic polymers. Three classes of synthetic polymers, hydrophobic, hydrophilic, and amphiphilic, were thermally blended with the lignins. The spinnability or processability of the polyblends was dependent on the nature of the synthetic polymer incorporated. In the spinning of lignin and hydrophobic polymers, polyethylene or polypropylene, the spinnability was strongly dependent on the thermal fluidity of the precursor polymer; low thermal viscosity polymers showed significant phase separation from lignin whereas the high viscosity polymers could not be extruded into fibers with lignin at all. The fibers produced from these blends had a unique core and shell fiber morphology. Polyblends of lignin and poly(ethylene terephthalate) produced lignin-based fibers with good physical properties. These ligninbased fibers were readily transformed into carbon fibers with properties suitable for general performance-grade applications. By contrast, the thermal spinning of lignin blends with hydrophilic polymers, such as poly(vinyl alcohol), was very difficult. Lignin−poly(vinyl alcohol) blends showed a binary phase morphology with discrete polymer domains on the fiber surface visible using polarized microscopy. The processability and flexibility of the lignin fibers were dramatically improved by blending with poly(ethylene oxide) (PEO); in fact, SKL-based fibers could be thermally extruded upon blending with PEO. The molten viscosity behavior of this blend was different from that of any other polyblend systems. The blending of PEO with SKL decreased the temperature required to produce fibers to lower than that used for either of the homopolymers. This was not observed in any of the other systems and is indicative of the special intermolecular interactions present between the two components. On the basis of this study, the physical properties of lignin-based polyblends were found to be dependent not only on the source of technical lignin and the properties of the synthetic polymer being incorporated, but also on the extent and type of intermolecular interactions between components.

21.1

Introduction

Forestry and agriculture are among the largest industries in the United States and Canada. Natural by-products of these industries are biomass residues in the form of wood chips, sawdust, straw, cotton, cotton gin trash, and the like. The use of biologically derived polymers (biopolymers) is emerging as an important component for economic development. By transforming forest and agricultural feedstocks, a new class of renewable, biodegradable, and biocompatible materials (biomaterials) is being introduced. Emerging applications for biopolymers range from packaging to Copyright © 2005 by Taylor & Francis

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industrial chemicals, medical implant devices, and drug delivery systems. In addition to producing green materials with unique physical and functional properties, the processes used to create biobased materials lead to new manufacturing opportunities that minimize energy consumption and waste generation. As the list of environmental problems grows, the advantages of using renewable biopolymers to meet the materials requirements of an expanding economy has to receive increasing attention. The utilization of biopolymers is not new. The advantages of cellulosics in fibers, films, and membranes as well as proteins and other carbohydrates have been present for over 100 years. Unfortunately, many of the applications of natural polymers could not compete with synthetic polymers in which properties can be tailored to needs. Petroleum-based products normally have consistent properties, unlike those associated with crops, whose component properties can vary significantly with growth site, agronomic factors including weather, and other complicated considerations. Moreover, molecular weight, degree of branching, and chemical functionality in side and main chain sites can be more readily tailored and controlled for synthetic polymers than for most natural polymers and materials. However, in some cases, chemical modifications of natural polymers can lead to materials of particularly interesting properties; e.g., hydrolyzed starch−polyacrylonitrile graft copolymers, the first superabsorbent.1 In general, however, the use of natural polymers are comparatively unattractive at the commercial level compared to those derived from petrochemicals. In fact, more than 1/2 of our clothes are derived from oil, and plastics are replacing paper containers. A large shift in the markets from natural to synthetic materials has occurred. Recent socioeconomical changes are making natural polymers once again worth of consideration for many applications. Oil embargoes and higher oil prices and concern over sustainability of fossil fuels have forced alternative, and especially renewable natural sources of materials, to be reconsidered. Environment concerns are also driving the consideration of biodegradable natural polymer-based materials and chemicals. Biobased materials have low toxicity and low disposal costs. For certain applications, such as fillers in composite materials, natural polymers are extremely attractive owing to the high price of most synthetic matrixes and the low prices of natural polymers such as starch, cellulose and lignin for example. The utilization of chemicals derived from plants and trees as monomers for biobased polymers, or for pharmaceutical applications is on the rise. Numerous non-toxic, biodegradable polymers and biocompatible chemicals are being developed from natural sources. Poly(hydroxy alkanoates) such as poly(lactic acid), poly(glycolic acid), their copolymers and blends are being used in everything from food packaging to tissue engineering. Likewise, ethyl lactate, derived from corn and other natural materials, is rapidly replacing many petroleum-based hydrocarbon solvents. As the world addresses a growing list of environmental problems, the possibility of using renewable biopolymers to meet the materials requirements of an expanding economy is receiving increasing attention. Copyright © 2005 by Taylor & Francis

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Surprisingly, there is a lack of solid information on the molecular properties of many natural polymers. This has been attributed to difficulties in isolating and characterizing these molecules.2 Biopolymers typically have large molecular weights, which are often so great and so entangled or agglomerated through physical interactions that processing has a significant effect on their inherent biopolymer properties. Similarly, biopolymers often possess limited solubility in common organic solvents; the solubilities and phase separation characteristics of natural polymers are not well understood. The importance of such information has been emphasized for protein systems,3 but is still lacking for wood-based biopolymers. Lignin, second only to cellulose in natural abundance, is readily available, relatively inexpensive, and structurally appealing (Figure 21.1). It is a high molecular weight aromatic biopolymer with a reported total worldwide production of ~100 million tons/yr.4 The product of wood-free papermaking, technical or commercial lignins are predominately used as a fuel in the chemical recovery process. However, as a fuel source it is very inefficient, producing less than 1/4 as much energy as middle distillate (diesel, jet, and boiler) fuels. Nonetheless, lignin combustion plays a critical role in the papermaking chemical recovery process and is vital to that industry. However, an ever-increasing number of paper mills have become chemical recovery limited; if paper production is to be maximized, the by-product lignin can no longer be used in its traditional role as a fuel. Commercial lignins, such as those isolated from alkaline, acidic, or organic solvent-based processes, have undergone extensive fragmentation and degradation. As a result, a wide variety of lignins, in terms of chemical properties and structures, can be obtained. Specifically, depending on the type and length of chemical processing, the lignins will vary in molecular weight, functional groups present, degree of condensation, types of intermonomeric linkages, and types and ratios of monomeric units. Due to this inherent chemical and molecular weight inhomogeneity, lignin and lignin derivatives have limited utility in applications demanding a constant well-defined feedstock. In fact, less than 2% of the total available lignin is reportedly being used in higher-value products.5 In 1996, total sales in lignin-based specialty products, such as animal bypass protein, agrochemicals, dispersants, adhesives, and surfactants were reported at $600 million. One of the most important applications of lignin and its derivatives is in the area of plastic products. The utilization of lignin in plastic materials has been around since the 1930s.6 As a result, a considerable amount of information exists regarding the modification of lignins toward the engineering of plastics. Unfortunately, in most cases, the incorporation of various monomers or polymers into the lignin structure results in properties unsuitable for structural materials.7 Glasser and co-workers8,9 have shown that through the manipulation of the network structure and substituents in lignin, the physical properties of lignin-based materials can be manipulated. They found that the noted brittleness of lignin, caused by the globular structure of lignin fragments, could be abolished by incorporating a variety of polyether components Copyright © 2005 by Taylor & Francis

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OH OH

HO

OH

LigninO

HO

HO

OCH 3

HO

O

OH

OH

HO

H 3 CO

O HO

OCH 3 OH H 3CO OH

O HO

OCH 3

OH O

O

O

OH

O

OH

H 3CO

OCH 3

OR

OCH 3

OH

O O OCH 3

OCH 3 O

OH OH HO

RO

OCH 3

O

H 3CO

OH

OH OH

O

OH

O OCH 3 H 3CO

O

OCH 3

O

OH

R = H, Alkyl, Aryl

O H OCH 3

HO

H 3 CO

O

OH

OCH 3

HO

H 3 CO

HO

OH O

OCH 3

H 3 CO

HO OH

O

HO

CH 3 O

OH H 3 CO

O O

H 3 CO

O

O

OH

OCH 3

O

O

HO OH

OCH3 O

OH OCH 3 OH

O

H 3CO

OCH 3

HO

O

OCH 3

O

HO

OH

HO

O

OH

OCH 3

RO

OH

H 3CO

H 3 CO

OH

H 3CO

O CH 3

HO HO

O Lignin

OH

OCH 3

OH OCH 3

O OR

O OCH 3

OH OH H 3CO

OH

OCH 3

FIGURE 21.1 Proposed model structure for softwood lignin (top) and hardwood lignin (bottom).

in the network structure. That is, a decrease in glass transition temperatures and brittleness could be achieved through the introduction of “soft” molecular segments capable of a plastic response to mechanical deformation. Lignin-based polyblends or polymer alloys have also been developed to enhance the thermoplastic behavior of lignin.10 However, the amount of Copyright © 2005 by Taylor & Francis

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lignin incorporation into the polymeric materials is generally limited, due to the inherent brittleness of the lignin phase or phase separation due to immiscibility. Recently, Li and co-workers11 have produced homogeneous blends containing 85% (w/w) underivatized industrial kraft lignin with poly(vinyl acetate) and two plasticizers. The series of lignin-based thermoplastics exhibited promising mechanical properties, with the tensile behavior of these polymeric materials directly dependent upon the degree of association between the intrinsic kraft lignin components. One area of lignin utilization that is receiving increasing attention is in the development of advanced composite fibers.12 Carbon fibers have been manufactured from lignin.13 Lignin-based carbon fibers, Kayacarbon, were first developed and made commercially available by Nippon Kayaku Co. on a pilot scale. The process involved carbonization of dry-spun fibers from lignin dissolved in an alkali solution with poly(vinyl alcohol) added as a plasticizer.14,15 Similarly, Sudo and co-workers showed that lignin could be converted into a molten viscous material with suitable properties for thermal spinning by hydrocracking, phenolation, or hydrogenolysis followed by heat treatment in vacuum.16–18 These modification methods seem to induce flow by eliminating hydroxyl and hydroxy methyl functional groups from the lignin; however, detailed microstructural analyses were not made. The resulting carbon fibers showed superior properties to those of the Kayacarbon. Sano et al. have produced carbon fibers with properties suitable for midrange markets from Organosolv lignin obtained by aqueous acetic acid pulping.19–21 Unfortunately, in each system problems concerning production costs exist. Either plasticization or lignin modification was required or noncommercially available lignins were used. Recently, the first carbon fibers produced from a commercially available kraft lignin were reported, without any chemical modification.22 The process involved thermal spinning followed by carbonization. A fusible lignin with excellent spinnability to form a fine filament was produced with a thermal pretreatment under vacuum. Blending the lignin with poly(ethylene oxide) further facilitated fiber spinning, but at PEO levels greater than 5%, the blends could not be stabilized without the individual fibers fusing together. Carbon fibers produced had an overall yield of 45%. The tensile strength and modulus increased with decreasing fiber diameter and are comparable to those of much smaller diameter carbon fibers produced from phenolated exploded lignins. In view of the mechanical properties, tensile strength 400−550 MPa and modulus 30−60 GPa, kraft lignin should be further investigated as a precursor for general grade carbon fibers. Lignin is a renewable, nontoxic, commercially available, and low-cost natural resource. Lignin has enormous potential as a raw material for the polymer and composites industries. In spite of many years of development effort, the full potential of lignin has not been fully utilized. One area of potential utilization is in polyblend fibers for composite applications. In this chapter we discuss the thermal and chemical properties of several commercial lignin preparations and polyblends and the resulting thermal processability. Copyright © 2005 by Taylor & Francis

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Thermal and Chemical Properties of Lignin

There are several reports in the literature describing the effect of chemical structure on the observed thermal properties of lignin.5,7,23–25 In the thermal processing of lignin for structural applications, the relationship between chemical structure and thermal properties is of extreme importance. Differential scanning calorimetric (DSC) analysis of a softwood kraft lignin (SKL), hardwood kraft lignin (HKL), and organosolve lignin (Alcell) is shown in Figure 21.2. The glass transition temperature (Tg) of the HKL and Alcell appear at 93ºC and 70ºC, respectively. These temperatures are much lower than that of the SKL, 119ºC. The observed difference in thermal transition temperatures between the softwood and hardwood lignin has been seen in other lignins, e.g., periodate lignin and acetic acid lignin.26 This difference in Tg between hardwood and softwood lignins can be explained based on differences in chemical structure between the two lignins. Hardwood lignins are made up of both guaiacyl (G) and syringyl (S) structures, whereas softwood lignins are primarily composed of guaiacyl units (Figure 21.1). As a result, softwood lignins contain more condensed interunit linkages, such as 5-5’ and β -5. These cross-links or branch-points produce a more complex network-like structure and hinder the rotational and translational motions within the lignin macromolecule. As a result, the thermal mobility of the lignin decreases, increasing the Tg. Although a minor component of native lignins, accounting for ~ 20% and 10% of the interunit linkages in softwood and hardwood lignins, respectively, these condensed units are less reactive toward chemical processing. Thus, the relative amount of these condensed linkages increases, becoming more significant in residual and technical lignins.27

Alcell

70°C

HKL

End.

93°C

SKL

119°C

0

50

100 Temperature (°C)

FIGURE 21.2 DSC curves of isolated lignin.

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150

200

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Noncovalent interactions can also affect the thermal properties of the lignin macromolecule. Intra- and intermolecular hydrogen bonding can reduce the segmental motion of lignin and thereby the behavior to thermal processing. Both aliphatic and phenolic (G and S) hydroxyl groups participate in hydrogen bonding, with phenolic hydroxyl groups preferentially forming intramolecular hydrogen bonds. Again, the conditions of chemical treatment and wood species will affect the type and amount of hydroxyl groups present in the lignins. Table 21.1 lists the results of functional group analysis for three technical lignins. As can be seen from the results listed in Table 21.1, the content of phenolic hydroxyl groups in SKL, 3.8 mmol g⫺1, is lower than that in HKL, 4.3 mmol g⫺1. This is partially a result of the difference in delignification rate between G and S structural units under alkaline conditions.28 By contrast, the aliphatic hydroxyl content in SKL, 5.6 mmol g⫺1, is higher than that of HKL, 4.1 mmol g⫺1. As well, the molecular weight of SKL is higher than that of hardwood lignin. Therefore, with an increased propensity to form intramolecular hydrogen bonding ([aliphatic hydroxyl] ⬎ [phenolic hydroxyl]) combined with the increased condensed interunit linkages, the higher Tg of SKL relative to HKL is not unexpected. In addition to wood species, processing conditions can affect the thermal properties of lignin. Functional group and molecular weight analysis of Alcell are listed in Table 21.1. The molecular weight of Alcell is slightly lower than that of HKL, as is the content of aliphatic hydroxyl groups, 3.6 mmol g⫺1 (cf. 4.1 mmol g⫺1). The S to G ratio, estimated by nitrobenzene oxidation, is almost two times greater for Alcell than for HKL, 2.3 mol/mol vs. 1.2 mol/mol, respectively. These data suggest Alcell lignin has a greater condensed structure with the propensity to form fewer intermolecular interactions than HKL; the result is a lower Tg for Alcell as compared to HKL. However, the methoxyl content of Alcell lignin, 5.6 mmol g⫺1, is lower than that of HKL, 5.9 mmol g⫺1. This implies that Alcell contains more condensed guaiacyl units than HKL; biphenyl moieties (5-5⬘) are not detected as vanillin from nitrobenzene oxidation. As well, it is well established that under the acidic conditions of the Alcell process, several carbonyl compounds (Hibbert’s ketones) are produced from the acidic cleavage of β -O-4 bonds.29 Infrared (IR) analysis of Alcell reveals a substantially higher carbonyl content TABLE 21.1 Chemical Properties of Lignins Functional Groups (mmol g⫺1) Hydroxyl Sample SKL HKL Alcell

Molecular Mass

Aliphatic

Phenolic

Methoxyl

5.6 4.1 3.6

3.8 4.3 4.3

4.2 5.9 5.6

Copyright © 2005 by Taylor & Francis

SG

⫺1

— 1.2 2.3

MW

Dispersity

2800 2400 2200

2.0 1.8 1.7

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Intensity

Alcell

HKL

SKL

1800

1700

1600

1500

FIGURE 21.3 FT-IR spectra of isolated lignin at carbonyl region.

as compared to both of the kraft lignins (Figure 21.3). The higher biphenyl content and oxidized structure will reduce the thermal motion of Alcell and contradicts the observed relatively low Tg (Table 21.1). To better understand the differences in thermal properties between the various lignins, further structural investigations were performed using nuclear magnetic resonance (NMR) spectroscopy. Figure 21.4 shows the oxygenated aliphatic region of the HMQC NMR spectra for each of the lignins. Included in the spectra are the assignments for the pinoresinol (β -β), phenylcoumaran (β -5), and β -O-4 structure along with α- and γ -hydroxyl group assignments. As can clearly be seen, β -O-4 structures survive chemical processing and remain in the industrial lignins. Comparison of the HMQC spectra of HKL and Alcell reveal distinct differences between the two lignins; denoted A−D in the Alcell spectra (discussed below). The breakdown of lignin under alkali conditions (kraft lignin) and acidic conditions (Alcell) relies on the cleavage of the many aryl-ether bonds found in native lignin. Under acidic conditions, this is particularly true for α-ethers, but β -ethers also play a role. In alkaline systems, the cleavage of β -ethers is more important.30 During alkaline processing, side reactions, such as reverse aldol condensation reactions, occur wherein the Cγ group is released as formaldehyde. In fact, kraft pulping of 14C-labeled lignin woodmeal revealed 45% of the Cγ was eliminated during the 3-h reaction.31 By contrast, the Cγ position is relatively stable toward acidolysis. In the presence of acid, hydrolysis of benzyl ether linkages occurs, producing a resonancestabilized benzyl carbocation. This reactive intermediate can then undergo Copyright © 2005 by Taylor & Francis

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45

 in -

 in -5

SKL

50 55

Methoxyl

HO

60 65

 in -O-4

H
/C
correlations

3

 O 4

 1

6

OCH3

2

70 75

 in -   in -5


R 5

HO

-O-4 structure

80 85

 in -O-4 (guaiacyl)

 in - 

6

90 95 45

HKL

HO


4   O 1 6 2

50 55 60 65

5

-5 structure (phenylcoumaran)

70 75

2

80

 in -O-4 (syringyl)

85




90

 6

1  O 2

95 45 Alcell

D

50

6

60 65

HO

70 75

A

1


’

- structure (resinol)

55

C

O 

B

80

O


R 5  O 4

 1

6

2

3 OCH3

85 90

 -ethoxy--aryl ether

95 6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

FIGURE 21.4 Oxygenated aliphatic region of the HMQC NMR spectra of the SKL, HKL, and Alcell.

nucleophilic addition by the solvent (ethanol) to produce structure 3, or other solubilized lignin moieties (condensation reactions) to yield structure 4, as shown in Figure 21.5. Copyright © 2005 by Taylor & Francis

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HO

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R

R1

HO

 

HO H

O




R

R1

O

OCH3

OCH3

H H3CO

R R1

+

H3CO

(1)

R

O

R1

(2)

O

R1

OH OCH3 O O HO


 

O

R1

R

R R1

HO


 

O OCH3

H3CO

H3CO

(3)

O

R

O

R1

OCH3

H3CO

R R1

R1

(4)

O

FIGURE 21.5 Proposed reaction scheme of α position in β-O-4 type lignin moieties during Alcell pulping.

Based on the delignification mechanism of the Alcell process, further assignment of the Alcell HMQC spectra (Figure 21.4) can be made. The β -position in the β -O-4 lignin moieties (δH ~ 4.0−4.8/δC ~ 80−86) is much more complex in Alcell than in HKL. Ethoxylation of the benzylic position (structure 3 in Figure 21.5) shifts the β proton to lower magnetic field and the β carbon to higher magnetic field. Accordingly, the chemical shift for the α-ethoxy-β -aryl ether moieties are assigned A and B for the G- and S-units, respectively (Figure 21.4). Similarly, the γ position of Alcell will not only be more intact, but due to the large variety of oxidation states of the β carbon, new correlation peaks will appear around δH/δC ⫽ 3.2⫺4.5/58⫺66 (assigned C in Figure 21.4).32 It has been shown that alkylation and/or acylation can affect the thermoplasticity of lignin.33 Therefore, etherification of the benzylic position in the Alcell lignin will reduce Tg. Similarly, condensation reactions (structure 4 in Figure 21.5), which would affect the chemical shift of the β -position in the β -O-4 lignin to a similar extent, may also enhance the thermal mobility of the lignin provided the molecular size of condensed unit is low, i.e., monomeric or dimeric.18 Finally, the average length or amount of the intact proponoid side Copyright © 2005 by Taylor & Francis

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chain is also greater for Alcell as compared to HKL (more intact γ -positions) and may also contribute to the lower Tg.

21.3

Thermal Treatment of Lignin

Isolated lignins contain low molecular fractions that hinder thermal processing.21 To improve processability, the lignins were thermally treated under reduced pressure prior to thermal spinning to remove volatile materials. The resulting thermally pretreated lignins, HKL, and Alcell exhibited good thermal processability, and continuous fiber spinning was achieved. However, SKL was very difficult to thermally process, showing poor thermal fluidity. During thermal processing, all lignins showed die swelling (Barus effect) indicative of the viscoelastic properties of lignin. Table 21.2 lists the spinning temperatures used for HKL and Alcell. The spinning temperature of Alcell (145°−165°C) is much lower than that of HKL (195°−228°C), both well below the thermal decomposition temperature of either lignin (5% weight loss ~270°C). The lower thermal flow temperature and thermal viscosity of Alcell relative to HKL is in agreement with the results from the thermal and structural analysis previously described. The mechanical properties of Alcell and HKL lignin fibers are included in Table 21.2. The tensile strength of HKL fiber is slightly greater than that of Alcell. Molecular mass is an important factor affecting mechanical properties of plastic materials. Therefore, the observed difference in tensile properties between the two lignins may be due to the difference in molecular mass between the lignins (Table 21.1). The lignin fibers produced from both HKL and Alcell are very brittle and show low elongation.

21.4

Change in Chemical Structure of Lignin during Thermal Spinning

In the thermal spinning of acetic acid lignin, it was reported that polycondensation reactions occurred through dehydration of hydroxyl groups.21 As TABLE 21.2 Spinning Temperature and Tensile Properties of Lignin Tensile Properties Sample HKL Alcell

Spinning Temperature (°C)

Strength (MPa)

Modules (GPa)

Elongation (%)

195–228 145–165

23.1⫾3.3 18.7⫾4.8

3.85⫾0.27 4.57⫾0.36

0.60⫾0.08 0.41⫾0.11

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a result, the average relative molecular mass of the lignin increased. For both HKL and Alcell, the relative average molecular weight increased with each thermal processing step (Figure 21.6). The increase in relative average molecular weight of the HKL fiber was larger than that of the Alcell fiber. The rate and extent of thermal polycondensation is likely related to the reaction temperature, where the reaction rate is higher for higher reaction temperatures. Therefore, the difference in the change in relative average molecular weight between HKL and Alcell is likely the result of the difference in spinning temperatures between the two lignins. The change in chemical structure of lignin during fiber spinning was analyzed by FT-IR spectroscopy. Figure 21.7 shows the FT-IR spectra of the original HKL and the HKL fiber. There appears to be no significant difference between the two samples in the fingerprint region. However, some band deformation is evident in the hydroxyl stretching region. The original HKL shows two broad bands centered at ~ 3350 and ~ 3500 cm⫺1, the higher wavenumber band assigned to the intramolecular hydrogen bonding in the phenolic groups. After thermal spinning, this band remains unchanged, but the lower wavenumber band (~ 3350 cm⫺1), assigned to intermolecular hydrogen bonding appears to have decreased, and a higher wavenumber band (~ 3422 cm⫺1) increased. These results indicate that some deformation in the hydrogen bonding system has occurred as a result of thermal spinning. Hydrogen bonding properties will be affected by the type, e.g., phenolic or aliphatic, and number of hydroxyl groups. Acetylation followed by FT-IR

4500

3.0

2.5 3000

2.0

2000

HKL fiber

HKL pellet

Heat-treated HKL

HKL

Alcell fiber

1.5 Alcell pellet

Alcell

1000

Heat-treated Alcell

Molecular mass (Da)

4000

Polydispersity

M n; M w; M w/M n ;

FIGURE 21.6 Change in relative average molecular weight of HKL and Alcell during thermal processing.

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3422

3518

~3350

Intensity

fiber

3700

3500

3300

3100

original

4000

3000

2000

1500

1000

400

−1

Wavenumber (cm ) FIGURE 21.7 FT-IR spectra of HKL and HKL fiber.

analysis is a convenient method to determine the relative amount of phenolic and aliphatic hydroxyl groups in lignin.34 After acetylation, the acetoxyl bands of the phenolic and aliphatic acetyl groups appear at 1766 cm⫺1 and 1744 cm⫺1, respectively. Spectral subtraction of the unacetylated lignin from the acetylated lignin provides information about the relative concentration of the phenolic and aliphatic hydroxyl groups. To enable the accurate determination of the change in hydroxyl content during thermal processing, the respective FT-IR spectra must be normalized prior to spectral subtraction. This was accomplished using the aromatic skeleton vibration band, ~1514 cm⫺1. This band was chosen based on the fact that less than 5% weight loss was recorded over the course of the spinning process, and that the aromatic nuclei are not expected to be degraded under these treatment conditions. The FT-IR difference spectra for HKL are shown in Figure 21.8. The band intensity of the phenolic acetoxyl groups (1766 cm⫺1) in HKL does not change during initial thermal processing, heat pretreatment, and pellet formation. However, the intensity of this band increased slightly upon fiber formation. As previously discussed, HKL contains ether linkages such as β-O-4 structures. The bond energy of these ether linkages is relatively low; DoCO is estimated to be ~65 kcal mol⫺1.35 Some new phenolic hydroxyl groups may be created via the aryl ether bond cleavage upon heating and/or shear during the spinning process. However, the increase in phenolic Copyright © 2005 by Taylor & Francis

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phenolic aliphatic

fiber

original

Absorbance

Heattreated pellet fiber

1850

1800

1750

1700

original

4000

3000

2000

1500

1000

400

−1

Wave number (cm ) FIGURE 21.8 FT-IR difference spectrum of HKL.

hydroxyl content is not significant as the change in peak intensity is not very large. By contrast, a clear change is observed in the aliphatic acetoxyl region. The band intensity of this group gradually decreases through thermal treatment steps. Therefore, as the phenolic hydroxyl groups possess strong intramolecular hydrogen bonds and the FT-IR difference spectra show a decrease in aliphatic hydroxyl groups, the observed change in the hydroxyl stretching region of the FT-IR spectra (Figure 21.7) is due to dehydroxylation and polycondensation reactions.

21.5

−Synthetic Polymer Blends Lignin−

Although both Alcell and HKL show good spinnability, the resulting lignin fibers are brittle and difficult to handle. In the past, lignin-based plastic materials were either chemically modified, introducing a flexible segment such as a long alkyl chain, and/or blended with another thermoplastic polymer. Polymer blending is a convenient method to develop products with desirable properties. The chemical and physical properties of the polymer blends are dependent on monomer type(s), molecular weight, and distribution and Copyright © 2005 by Taylor & Francis

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composition of the respective polymers. For instance, thermal-resistant engineered plastics used in automotive and electronic industries are produced from blends of atactic polystyrene and poly(phenylene ether). Most polymers, however, are immiscible due to the low entropy of mixing.36 Only through specific intermolecular interactions can favorable polymer blending occur and composite materials with desirable properties be produced. Lignin is generally categorized as a hydrophobic polymer due to its phenyl propanoid structure. This implies that lignin should have compatibility with other hydrophobic polymers. However, lignin contains various polar functional groups, such as hydroxyl and carbonyl groups. These polar functional groups enable lignin to form strong interactions with various hydrophilic polymers through electrostatic and/or acid-base interactions. This amphipathic nature of lignin enables a wide variety of polymer blends to be produced.37 In this section, the effect of polymer blending on the thermal processability, i.e., fiber spinning, is discussed. 21.5.1

Lignin–Polyolefin Blend Fibers

Two of the most commonly produced polyolefins are polyethylene (PE) and polypropylene (PP). PE and PP are widely used in a variety of industrial applications such as films, bags, and fibers. In addition, PE is used as an extrusion aid and release agent in some polymer systems as well as an additive in hot-melt adhesives. Both PE and PP possess good mechanical properties with easy processability, low cost, and recyclability. Lignin-based polyolefin blends were prepared from both PE and PP. Low molecular weight PE, Mn⬍5500, was first studied to improve processability during lignin fiber spinning. Of the various lignin preparations examined, only the Alcell−PE blend system could be extruded and transformed into fiber. The thermal spinning of HKL−PE was very difficult. The incorporation of PE decreased the brittleness of Alcell-based fibers. The spinning temperatures required to produce satisfactory fibers from Alcell−PE blends (169°⫺186°C) were higher than that for Alcell alone (138°⫺165°C). In addition, the spinning properties were relatively poor. It was observed that during the thermal processing of Alcell−PE blends, the PE phase separated from the Alcell, reducing the moldability. The solubility parameter of lignin is reported to be ~22.5 MPa1/2 38 as compared to 16.4 MPa1/2 for PE.39 Therefore, it is difficult to disperse the PE molecules into Alcell. However, this phase separation resulted in a unique fiber morphology wherein a heterogeneous core−shell structure was observed, the Alcell sheath covering the PE core. Unlike PE, lignin-based PP fibers were produced from both Alcell and HKL. Lignin−PP blends were prepared from syndiotactic PP (127,000 Da) and three types of isotactic PP (580,000, 190,000, and 12,000 Da). These PP are abbreviated S-PP, H-PP, M-PP, and L-PP, respectively. The solubility parameter of PP is 19.0 MPa1/2, slightly higher than that of PE.39 As a result, the spinning properties of the lignin−S-PP blends were similar to that of the Alcell−PE blend. Again a core/shell-type phase separation was observed. Copyright © 2005 by Taylor & Francis

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20 µm

5 µm

FIGURE 21.9 SEM micrographs of HKL–S-PP blend fiber.

SEM analysis of the HKL−S-PP fibers revealed a unique morphology, shown in Figure 21.9. It can be seen that there are many PP nanofibers inside of the lignin−S-PP blend fiber. The molecular weight of the isotactic PP had a dramatic effect on the spinnability and fiber morphology of the blended fibers. Fibers produced from the HKL−L-PP blends were dificult to spin and did not show the same included PP nanofiber morphology as that for the S-PP. The low molten viscosity L-PP seems to completely phase separate from the higher viscosity HKL. By contrast, the spinning properties of HKL−M-PP blends were dramatically improved. Continuous fiber spinning at the maximum takeup rate of the equipment was achieved over the entire range of blend compositions, even though core/shell-type phase separation was also observed. The spinning temperatures of the HKL−M-PP blend fibers are listed in Table 21.3. No relation between spinning temperature and blend composition was observed. This result may indicate that there is no interaction between the HKL and M-PP. In general, the molten viscosity of a synthetic polymer Copyright © 2005 by Taylor & Francis

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TABLE 21.3 Spinning Temperatures of Lignin/Polyolefin Blends

Lignin HKL

Alcell a

b c

Spinning Temperature (°C)

Blending Ratio (lignin/polymer)

PE

S-PPa

L-PP

M-PP

H-PP

100/0 95/5 90/10 85/15 80/20 75/25 62.5/37.5 50/50 0/100 100/0 75/25

195–228 — — — — ⫻ — — ⫻ 145–165 169–186

195–228 204–213 208–226 b — — 208–220 — ⫻ ⫻ 145–165 165–180

195–228 208–210 209–212 208–210 209–210 ⫻ ⫻ ⫻ ⫻ 145–165 —

195–228 208–210 208–210 209–210 209–212 209–211 209–210 214–216 208–210 145–165 —

195–228 166–170 180–191 ⫻c ⫻ ⫻ ⫻ ⫻ 229–230 145–165 —

S-PP, Syndiotactic PP; L-PP, 12,000 Da isotactic PP; M-PP, 190,000 Da isotactic PP; H-PP, 580,000 Da isotactic PP. Blending ratio is HKL/S-PP=87.5/12.5. X, poor spinning ability.

decreases with increase temperature. As the molten viscosity is one of the most important factors for the predictability of thermal spinning, if the molten viscosity of the HKL−M-PP changed with the change in blend composition, the spinning temperature will also be expected to change. This observation indicates that the molten viscosity of HKL is close to that of MPP under spinning conditions. By contrast, fiber spinning of the HKL−H-PP blends was difficult. The spinnability of blends that incorporated more than 10% H-PP was very poor. The molten viscosity of H-PP was much higher than that of HKL. Therefore, good fiber spinnability can be achieved by selecting lignin−polymer blends with similar molten viscosities, regardless of whether the two polymers are miscible.

21.5.2

Lignin–Polyester Blend Fibers

Poly(ethylene terephthalate), PET, is the most common polyester. The solubility parameter of PET is 21.9 MPa1/2,39 slightly lower than that of lignin. However, PET has various functional groups that can interact with lignin. PET contains polar oxygen atoms that may form hydrogen bonds with the hydroxyl groups in lignin. As well, PET is an aromatic polyester, thereby enabling π -type interactions. Spinning temperatures for lignin−PET blends are higher than those used for the other lignin−synthetic polymer blend systems (Table 21.4). This may be due to the high spinning temperature of the PET component. This temperature is almost the same or slightly higher than the lignin thermal decomposition temperature. As a result of the high spinning temperatures the processability of the lignin−PET blends is decreased Copyright © 2005 by Taylor & Francis

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TABLE 21.4 Spinning Temperatures of Lignin–PET Blends

Lignin HKL

Alcell a

Spinning Temperature (°C)

Blending Ratio (lignin/polymer)

Lab-grade

IV ⫽ 0.63

IV ⫽ 0.93–0.96

100/0 95/5 85/15 80/20 75/25 50/50 25/75 15/85 0/100 100/0 75/25

195–228 — 211–227 219–231 — — 278–281 241–271 — 145–165 169–186

195–228 208–228 213–225 — 218–242 ⫻a 231–263 245–265 246–273 145–165 165–180

195–228 — — — 245–248 — — — 248–272 145–165 221–267

X, poor spinning ability.

because of the evolving decomposition products. Moreover, the molten viscosity of the PET blend system decreases abruptly with increasing fiber spinning temperature, making temperature control difficult. This viscosity change is not observed during the thermal spinning of either homopolymer: lignin or PET. This increase in blend viscosity may be an indication of strong specific intermolecular interactions between lignin and PET. If the specific intermolecular interactions are suddenly disrupted through enhanced thermal molecular motion, a dramatic change in viscosity might occur. This may explain why the molten viscosity of the blend decreases abruptly during the spinning of lignin−PET fibers. However, further experimentation is required to confirm this presumption, such as spectroscopic analysis under high temperature conditions. The spinnability of the industrial PET blends was better than that of research-grade PET blends. The intrinsic viscosity of industrial PETs is higher than that of research-grade PET. Thus, the change in molten viscosity of the research-grade PET may be larger than that of the industrial PET during fiber spinning. Although the fiber spinning temperature of Alcell is lower than that of HKL (Table 21.4), the spinning temperatures of the Alcell−PET and HKL−PET blends are almost the same. This result may indicate that the interaction between lignin and PET is more significant in affecting the viscous behavior of the blends than the thermal nature of the lignin itself.

21.5.3

Lignin–Hydrophilic Polymer Blend Fiber

Poly(vinyl alcohol), PVA, was blended with lignin to observe the processability of lignin−hydrophilic polymer composites. Commercial PVA is derived from poly(vinyl acetate). The solubility parameter of PVA is 25.8 MPa1/2.39 The physical and chemical properties of PVA are affected by Copyright © 2005 by Taylor & Francis

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molecular weight and degree of hydrolysis. In our study, two different molecular weight PVA samples (98−100% hydrolyzed) were used. PVA, a hydrophilic polymer, adheres to hydrophilic surfaces, such as cellulose through hydrogen bonding and/or electrostatic interactions. Therefore, the hydroxyl groups of PVA may have a good affinity for the polar functional groups within lignin. The thermal spinning of the high molecular weight PVA (ca. 100,000 Da) was very difficult. In fact, incorporation of as little as 5% PVA into the lignin dramatically affected fiber spinning. It has been reported that the thermal moldability of PVA is very poor because the melting temperature of PVA is very close to its thermal degradation temperature.40 By contrast, low molecular weight PVA (13,000−23,000 Da) showed good spinnability, with the spinning temperatures (230°−231°C) almost the same as that of HKL. The blending of PVA with HKL did not significantly change the spinnability, and the spinning temperatures of HKL−PVA blends were ~220°−230°C over the entire blend ratio (Table 21.5). These results are similar to those observed for the HKL−M-PP system. However, core/shelltype phase separation was not observed for HKL−PVA fibers, although under polarized light microscopy, it was observed that small PVA particles were dispersed on the fiber surfaces.41 21.5.4 Lignin–Amphiphilic Polymer Blend Fibers Due to the nature of the lignin macromolecule (see above), amphiphilic polymers may show good affinity toward lignin. The effects of polymer blending PEO having two different molecular weights (100K and 600K Da) with lignin were assessed (Table 21.6). The PEO samples are abbreviated PEO100K and PEO600K. The solubility parameter of PEO is 21.2 MPa1/2, slightly lower than lignin (~22.5 MPa1/2).39 Over the entire blend ratio, better fiber spinning was achieved with PEO than any of the other lignin−synthetic polymer blend systems. The spinning temperatures of lignin−PEO100K blends were lower than that of the respective homopolymers, lignin and PEO100K. This tendency was not observed for any of the other lignin−polymer blends. In TABLE 21.5 Spinning Temperatures of Lignin–Low MW PVA Blends Lignin

Blending Ratio (lignin/polymer)

Spinning Temperature (°C)

100/0 95/5 87.5/12.5 75/25 50/50 25/75 0/100

229–230 220–230 219–232 227–230 229–232 227–230 230–231

HKL

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TABLE 21.6 Spinning Temperatures of Lignin–PEO Blends

Lignin HKL

Alcell

SKL

a

Spinning Temperature (°C)

Blending Ratio (lignin/polymer)

PEO100K

PEO600K

100/0 95/5 87.5/12.5 75/25 62.5/37.5 50/50 40/60 35/65 25/75 10/90 100/0 100/0 95/5 87.5/12.5 75/25 62.5/37.5 50/50 40/60 35/65 25/75 10/90 100/0 87.5/12.5 75/25 62.5/37.5 50/50 37.5/62.5 25/75 12.5/87.5

195–228 205 187–190 168–170 162–179 165 164–177 185–190 210 220–232 216–222 145–165 178–180 165–170 159–160 148–155 149–150 154–169 173–180 195–197 148–155 ⫻ ⫻ ⫻ 219–224 238–240 224–225 234–235 234–236

195–228 200–212 209–231 190–236 — 204–250 — 229–258 218–248 270 ⫻a 145–165 191–203 195–212 189–204 189–240 — 224–254 — 192–254 268–270 — — — — — — — —

X, poor spinning ability.

the lignin−PEO100K blend compositions, as the PEO content increased, the fibers fused together on the takeup spools. This is likely due to the decrease in Tg of the polymer-blended fibers with increasing PEO content. This phenomenon was observed over a wider blend composition for Alcell−PEO blends than in HKL−PEO blends, which is likely due to the lower Tg of Alcell as compared to HKL (Figure 21.2). The spinning of the high molecular weight PEO blends and PEO600K, required higher spinning temperatures than that of PEO100K, especially for PEO-rich blends. As mentioned, the thermal fluidity of SKL is much lower than that of HKL and Alcell, preventing SKL fibers from being formed. The blending of PEO100K with SKL improved thermal processing, but fiber spinning was still problematic. Incorporation of 50% or more of PEO100K was required to enable continuous fiber spinning. Copyright © 2005 by Taylor & Francis

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−Synthetic Polymer Blend Fibers as Application of Lignin− Precursors for Carbon Fibers

Carbon fibers are one of the most important engineering materials in advanced composites. They are lightweight, fatigue-resistant materials that possess high strength and high stiffness. These unique properties result from their flawless structure and the development of highly anisotropic graphic crystallites oriented along the fiber axis during the production process.42 Carbon fibers are manufactured by thermally treating fibers at 1000°−2000oC in an inert atmosphere while maintaining the fibrous structure. This is aided by a stabilization stage in which the precursor fibers are heated under tension at 200°−300oC in the presence of air. This causes cross-linking on the fiber surfaces, among other reactions, and prevents shrinking, melting, and fusing. Carbon fibers can be classified into two groups based on mechanical properties: general purpose (GP) and high performance (HP). Properties of the carbon fibers are mainly designed by the selection of the precursor material. There are primarily two types of precursor materials of commercial significance: pitch (petroleum or coal) and polyacrylonitrile (PAN).43 Of these, PAN is the most important for high-performance structural applications. It is an excellent precursor material that has been widely researched and is in wide commercial production. Almost 80% of commercially available carbon fibers are derived from PAN. However, current PAN technology is expensive, thereby limiting its utilization in lower cost general performance applications, and research continues toward decreasing PAN precursor costs, e.g., Amlon (BP Chemicals).44 Carbon fibers have been manufactured from lignin.12 However, based on the physical properties of the lignin fiber and subsequently the carbon fibers, lignin-based carbon fibers are suited for GP-grade applications. GP-grade carbon is utilized as a precursor for activated carbon fiber, ACF, as well as composite material with concrete, CFRC (carbon fiber reinforced concrete).

21.6.1 Processability The general production process for carbon fibers consists of three main steps: (1) the selection of a precursor material and fiber spinning, (2) thermostabilization, and (3) carbonization and graphitization. The thermostabilization process is indispensable in the manufacturing of carbon fibers, required to ensure that the “green” fibers maintain their fiber form during the high-temperature conditions of the carbonization process. In general, the thermoplastic precursor fiber is transformed into a thermoset material through oxidative reactions induced by heating under an air atmosphere. Reactions occurring during thermostabilization increase Tg. As the temperature increases at the slow heating rate, Tg can increase faster than the heating temperature, maintaining the material in the glassy state (Tg⬎T): Copyright © 2005 by Taylor & Francis

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nontacky. At higher heating rates, thermostabilization reactions are not able to maintain Tg⬎T, and the material devitrifies, entering a liquid/rubbery state, tacky, and thus fuses together. Gillham and co-workers have described such phenomena in continuous-heating transformation (CHT) diagrams.45,46 During the thermal stabilization stage, the temperature was carefully increased at varying rates and the effect on fiber stability was observed (Table 21.7). Fiber stability was dependent on thermostabilization conditions and polymer blend content. Due to the range of softening points for lignin and the lignin−synthetic polymer blends, the heating rate of the thermostabilization stage varied. Slow heating rates were required to successfully convert the lignin fibers into thermostabilized fibers. As can be seen from Table 21.7, higher heating rates could be used in the thermostabilization process of lignin−PET blend fibers. The melting temperature of PET, ca. 270°C, is higher than the softening point of lignin, even though the glass transition temperatures of both polymers are approximately the same. The melting temperature of PET was slightly decreased upon blending with lignin; still higher than the oxidation temperature. This observation suggests the crystalline phase of PET may support the blend to maintain a fibrous form at high temperature, an advantage of this lignin−synthetic polymer blend fiber. By contrast, high heating rates could not be applied to the thermostabilization of lignin−PP or lignin−PEO blends. 21.6.2 Carbon Fiber The total carbon fiber yield decreased with the increased blending of synthetic polymers (Table 21.8). However, these yields are still higher than those reported for petroleum pitch.18 Moreover, if all the synthetic polymer was pyrolyzed during carbonization, the ideal yield should be ~34.3% (based on 100% lignin yield and amount of synthetic polymer incorporated). Therefore, some portion of the synthetic polymer (13% and 16% for PP and PET, respectively) was carbonized by the blending with lignin. Mechanical properties of the various carbon fibers are listed in Table 21.9. The average tensile strength and Young’s modulus of the lignin−PET carbon TABLE 21.7 Thermostabilization of HKL–Synthetic Polymer (75/25 w/w) Blend Fibersa Heating Rate (ºC h⫺1 ) Sample

12

30

60

120

180

300

HKL HKL-PEO -PET -PP


⫻


 ⫻


 ⫻


 ⫻
⫻

⫻ ⫻  ⫻

⫻ ⫻  ⫻

a

X, fibers fused together; , fibers stick/do not fuse;
, fibers remain intact.

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TABLE 21.8 Carbon Fiber Yield (%) of HKL–Synthetic Polymer Blends HKL

HKL–PEO (95–5)

Lignin–PET (75–25)

Lignin–PP (75–25)

45.7

41.6

38.4

37.6

TABLE 21.9 Mechanical Properties of HKL–Synthetic Polymer-Based Carbon Fibers Sample Tensile strength (MPa) Young’s modulus (GPa)

HKL

HKL–PET (75–25)

HKL–PP (75–25)

422 39.6

511 66.3

113 22.8

fibers are 1.21 and 1.67 times higher than those for the lignin carbon fiber, respectively. The tensile properties of the lignin−PET blend carbon fibers are comparable to that of GP-grade pitch-based carbon fibers. SEM analysis of the lignin−PP carbon fibers revealed a porous structure on the surface and in the cross section of the fibers.47 A number of these pores were evident even after thermostabilization. It appears that some of the PP is melted out from the lignin−PP fibers prior to pyrolysis. The porous fibers possessed much lower tensile strength as compared to the other lignin-based carbon fibers. However, the mechanical properties, particularly Young’s modulus, of these fibers are comparable to commercial-activated carbon fibers.48 Interestingly, no pores or voids were observed in the lignin−PET carbon fibers, although more than 80% of the PET was pyrolyzed during the carbonization process. In this miscible polymer blend fiber, both polymers are intimately mixed. If pores are formed through pyrolysis of the PET during the first stage of the carbonization process, those pores are eliminated by the molecular organization associated with carbonization. The same smooth fibers were observed for the miscible lignin−PEO carbon fibers.

21.7

Conclusion

General performance–grade Carbon fibers can be produced from technical lignins. Hardwood kraft and organosolv lignin were readily extruded into fiber form, however, the fibers were brittle and difficult to handle. Polymer blending improved fiber properties. The physical properties of lignin-based polyblends were found to be dependent not only on the source of technical Copyright © 2005 by Taylor & Francis

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lignin and the properties of the synthetic polymer being incorporated, but also on the extent and type of intermolecular interactions between components. Lignin blending with poly(ethylene oxide) showed improved processability and fiber flexibility as a result of enhanced intermolecular interactions. In fact, SKL-based fibers could be thermally extruded upon blending with PEO and displayed a unique molten viscosity behavior; blending PEO with SKL decreased the temperature required to produce fibers to lower than that used for either of the homopolymers. Polyblends of lignin and poly(ethylene terephthalate) produced lignin-based fibers with good physical properties and subsequent carbon fibers suitable for general performance-grade applications. By contrast, incorporation of hydrophobic polymers such as polyethylene or polypropylene resulted in phase separation and produced with unique core and shell fiber morphology. The resulting carbon fibers were porous, and depending on PP incorporation levels exhibited a hollow core structure.

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22 Soy Protein-Based Plastics, Blends, and Composites

Amar K. Mohanty, Wanjun Liu, Praveen Tummala, Lawrence T. Drzal, Manjusri Misra , and Ramani Narayan

CONTENTS 22.1 Introduction 22.2 Classification of Soy Protein 22.3 Protein Structure 22.4 Composition of Soy Proteins 22.5 Proteins Denaturation 22.6 Plasticization Thermodynamics 22.7 Soy Protein Plastics 22.8 Soy Protein Plastics Blends 22.9 Natural Fiber-Reinforced Soy-Based Biocomposites 22.10 Future Research Directions of Soy Protein Materials Acknowledgments References ABSTRACT The significant environmental stresses caused from the manufacture and consumption of petroleum-based plastics has spurred much research into the development of non-petroleum-based polymer feedstock. Biopolymers produced from various natural botanical resources such as starch, protein, and cellulose are regarded as an attractive alternative since they are abundant, renewable, inexpensive, environmentally friendly, and biodegradable. Soy protein, in particular, has tremendous potential to replace traditional plastics and has now become an important resource for bioplastics to be used in packaging applications. This chapter offers a summary of the current research and development in soy protein-based plastics, including the plasticization of soy protein and the modification of soy protein plastics. Additionally, strategies to tune the structure and properties of

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soy protein plastic blends to improve compatibility, interfacial properties, and interaction between different components will be discussed. Likewise, the influence of fiber properties, content, length, and surface treatment on the performance of natural fiber-reinforced soy-based biocomposites will be summarized. Finally, future research directions of soy-based materials are given based on the fundamental structure and properties of soy protein and the difference between soy protein and plastic.

22.1

Introduction

Since their inception nearly 40 years ago, plastics have become one of the most important materials in our daily life. However, environmental pollution from their consumption has become a serious issue, particularly from packaging materials, disposable containers, and agriculture mulch films. With tighter environmental regulations and increasing waste disposal costs, it has become necessary to develop alternative materials to replace traditional plastics. Thus, there has been an increased interest in the production and use of fully biodegradable polymers to replace nonbiodegradable plastics. Commercially available biodegradable synthetic plastics include polycaprolactone, polyhydroxyalkanotes, poly(3-hydroxybutyrate), poly(butylene succinate-co-adipate), poly(tetramethylene adipate-co-terephthalate), and polyester amide. Though these materials exhibit good biodegradation properties and can be used for the production of blown film and injection-molded materials, their high cost inhibits widespread use.1 Biopolymers derived from natural resources such as starch, protein, and cellulose are also an attractive research area because they are the most abundant, renewable, and inexpensive natural biopolymers from various botanical sources. Using biomass or natural polymers will reduce our dependence on petroleum-based product and save energy. Soy protein has been considered as a viable alternative to the petroleum polymers in the manufacture of adhesives, plastics, packaging materials, and various binders. About 5000 years ago, Chinese farmers grew soybeans which were mechanically crushed for the oil to be used for lubricants, lamp oil, coatings, cosmetics, and waterproofing. In 1804, a Yankee clipper ship brought soybeans to the United States. In 1829, U.S. farmers first grew soybeans for soy sauce. During the Civil War, soldiers used soybeans as “coffee berries” to brew “coffee” when real coffee was scarce. In the late 1800s, many farmers began to grow soybeans as forage for cattle. In 1904, at the Tuskegee Institute in Tuskegee, AL, George Washington Carver began studying the soybean for food uses.2 His discoveries changed the way people thought about the soybean; no longer was it just a forage crop. Currently, valuable products such as soy protein and soy oil are extracted from the soybean. Figure 22.1 shows the major countries contributing to world soybean production for the year 2003. Copyright © 2005 by Taylor & Francis

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Other 7%

Brazil 28%

US 34%

Argentina 18%

India 4% China 9%

FIGURE 22.1 Year 2003 world soybean production (Adapted from the American Soybean Association (http://www.soystats.com/2004/ page_30.htm). With permission.)

Soy protein is the major coproduct of soybean oil and is readily available, renewable, and has functional properties desired by industry. The soybean crop is the world’s foremost provider of protein and oil. Through the research, development, and commercialization of new industrial uses for soybeans supported by the United Soybean Board (USB) New Uses Committee (NUC), soybeans now have been used to produce adhesives, coatings, and printing inks, lubricants, plastics, and other specialty products. Likewise, soy protein plastic can be used for automobile parts and packaging applications. Alternative industrial products have seen a heightened demand due to increasing government regulations. Improving the quality of the environment, as well as working conditions is one of the primary objectives for industries to turn to green products as alternatives to petroleum-based chemicals. Soy-based products bring more benefit to environmental and worker safety while offering high-performance characteristics. This chapter is a review of the current research and development in soy protein plastics, blends, and natural fiber-reinforced soy-based biocomposites as well as of the fundamental structure and properties of soy protein for better understanding soy protein materials and broadening such work in this area.

22.2

Classification of Soy Protein

Dehulled soybeans are used to make full fat soy flours. The dehulled soybean is very similar in composition to whole soybean as the hull comprises only about 10% of the total bean. The soybean lipid is removed either by mechanical means or by solvent extraction. Removal of the lipid yields defatted soy flour. On a dry weight basis, the protein content of the defatted soy flour must be at least 50%.3 The defatted soy flour is the starting material for several other soy products. The defatted soy flour is further purified to yield soy protein concentrates and soy protein isolates. Purification increases the dry weight percentage of protein content by removing the Copyright © 2005 by Taylor & Francis

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TABLE 22.1 Composition of Different Types of Soy Protein 16 Component (%) Moisture Protein Fat Fiber Ash PERa a

Full Fat Soy Flour 3.4 41.0 22.5 1.7 5.1 2.1

Defatted Soy Flour 6.5 53.0 5.1 3.0 6.0 2.3

Soy Concentrate 9.0 65.3 0.3 2.9 4.7 2.3

Soy Isolate 4.8 92.0 — 0.25 4.0 1.1–2.6

Note: Protein Efficiency Ratio (PER) is a measurement of the efficient utilization of the proteins in the body.

soluble carbohydrate material in the case of soy protein concentrates and removing the soluble proteins along with the soluble carbohydrates in the case of soy protein isolates.4 Soy protein concentrates have a dry weight protein content of at least 70% by definition. The carbohydrate primarily consists of sugars such as sucrose, raffinose, and stachyose, which are typically removed using one of three procedures. The first procedure involves washing the defatted soy flour with dilute acid at the protein’s isoelectric point (around pH 4.5). Provided that the salt content is not too high, the soy protein is insoluble at this pH. The second procedure is to soak the flour in a mixture of alcohol and water that contains enough alcohol (50–70%) to prevent the proteins from becoming soluble but enough water to remove the carbohydrate.5 In the third procedure, the soy proteins are denatured with steam to make them insoluble and the soluble carbohydrate is then removed with water. Soy protein isolates have a dry weight protein content of at least 90% by definition. Defatted flakes are utilized as the starting material. The soluble protein is removed by extraction with dilute alkali (pH 7–9) at moderate temperatures. The insoluble carbohydrate (fiber) and some proteins are left behind. The proteins are then recovered from the soluble carbohydrates and ash by isoelectric precipitation at pH 4.5.6 The compositions of various types of soy protein are given in Table 22.1.

22.3

Protein Structure

Proteins are biopolymers made up of 19 different α-amino acids and one imino acid linked via amide bonds, known as peptide bonds. The constituent amino acids differ only in the chemical nature of the side-chain group at the α -carbon atom. The physicochemical properties, such as charge, solubility, and chemical reactivity, are dependent on the chemical nature of the sidechain group. Generally, the amino acid side chain may be conveniently Copyright © 2005 by Taylor & Francis

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classified as aliphatic, aromatic, acidic, basic, polar uncharged, and sulfur containing.7 The protein structure can be classified into four levels, namely, primary, secondary, tertiary, and quaternary structures. The primary structure of a protein denotes the linear sequence in which the constituent amino acids are linked via peptide bonds, namely, its chemical structure. Chemical structures of protein are significant and will affect the direction of modified protein through enzyme and chemicals. The secondary structure relates to the periodic structure in polypeptides and protein. The most common secondary structure in protein is helical with a pitch of 5.4 Å involving 3.6 amino acid residues. Tertiary structure refers to the preferred three-dimensional arrangement of the folded polypeptide chain with secondary structure segments aligned into a compact three-dimensional folded form. The tertiary structure of several single polypeptide proteins contains features known as domains with sizes of 100−150 amino acid residues. The quaternary structure refers to the spatial arrangement of a protein containing several polypeptide chains. This structure is referred to oligomeric structure with several subunits.8 Knowledge of the above physical structure is important because tertiary and quaternary structure, as well as hydrophobic interactions and hydrogen bonding will be changed when soy proteins are combined with modifiers or plasticizers. The interaction between protein molecules, including hydrophobic interaction, hydrogen bonds, electrostatic interaction, and van der Waals interaction are important as they will stabilize the protein tertiary structure and will be changed after soy protein is chemically modified. Hydrophobic interactions reflect a summation of the van der Waals attractive forces between nonpolar groups in the protein interior.9 The covalent disulfide bonds play a significant role in stabilizing the tertiary structure as well as quaternary structure of protein and hence maintain its stability.10 Proteins containing SS bonds tend to be quite heat stable and show higher enthalpies of denaturation. Most SS bonds are buried inside the protein molecules and turn outward during denaturation.

22.4

Composition of Soy Proteins

Soy protein is a complex biomacromolecule, which contains different amino acid components. Studies of soy proteins by analytical ultracentrifugation in a phosphate buffer of pH 7.6 with an ionic strength of 0.5 containing 0.01 M mercaptoethanol have revealed the presence of four Schlieren peaks. These peaks have approximate Svedberg coefficients of 2S, 7S, 11S, and 15S.11 The fractions 7S and 11S make up about 70% of total protein; the remainder of the protein exists as 2S (20%) and 15S (10%) fractions. These fractions are mixtures of proteins. The 2S fractions consist of low molecular weight polypeptides (in the range of from 8,000 to 20,000 Da) and are comprised of the soybean trypsin inhibitors. The 7S fractions are highly heterogeneous. Copyright © 2005 by Taylor & Francis

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Their principal component is γβ-conglycinin, a sugar containing globulin with a molecular weight on the order of 150K Da.12 The fraction is also comprised of enzymes (β-amylase and lipoxygenase) and hemagglutinins. The 11S fractions consist of glycinin, the principal protein of soybeans. Glycinin has a molecular weight of 320K−350K Da and is built from 12 subunits associated through hydrogen bonding and disulfide bonds.13 The ability of soy proteins to undergo association-dissociation reactions under known conditions is related to their functional properties and particularly to their texturization. The 15S proteins are probably dimers of glycinin and have molecular weights around 600K Da. Conglycinin and glycinin are storage proteins and they are found in the protein bodies within the cells of the cotyledons.

22.5

Proteins Denaturation

The term denaturation has been used to refer to any change from the biologically active, native state, including poorly understood irreversible alteration such as covalent modification and aggregation. Commonly, protein denaturation is defined as any noncovalent change in the structure of a protein.14 This change may alter the secondary, tertiary, or quaternary structure of the molecules. The main mechanism of protein denaturation is the unfolding of the protein molecule. Protein denaturation will occur because of changes of various external parameters, e.g., increasing temperature, pressure, denaturant concentration, and changing the pH.15 When proteins are exposed to increasing temperature, and depending upon the protein and the severity of the heating, the changes may be irreversible.16 As the temperature is increased, a number of bonds in the protein molecule are weakened. First affected are the long-range interactions that are necessary for the presence of tertiary structure. As these bonds are first weakened and broken, the protein obtains a more flexible structure and the groups are exposed to the plasticizer. If heating ceases at this stage, the protein should be able to readily refold to its native structure. As heating continues, some of the cooperative hydrogen bonds that stabilize the helical structure will begin to break. As the helical structure is broken, hydrophobic groups are exposed to the plasticizer. The protein attempts to minimize its free energy by burying as many hydrophobic groups as possible, while exposing as many polar groups as possible to the plasticizer. While this is analogous to what occurred when the protein folded originally, it is happening at a much higher temperature. This greatly weakens the short-range interactions that initially direct protein folding and the resulting conformations will often be vastly different from the native protein. Upon cooling, the structures obtained by the aggregated proteins may not be those of lowest possible free energy, but kinetic barriers will prevent them Copyright © 2005 by Taylor & Francis

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from returning to the native form. Any attempt to obtain the native structure would first require the breaking of the hydrophobic bonds that caused the aggregation. This would be energetically unfavorable and highly unlikely. Only when all the intermolecular hydrophobic bonds are broken would the protein begin to refold as directed by the energy of short-range interactions. The exposure of this large number of hydrophobic groups to the plasticizer, however, presents a large energy barrier that makes such a refolding kinetically unlikely. Thus, exposure of most proteins to high temperatures results in irreversible denaturation. Heat-induced denaturation is called thermal denaturation. Heating may cause the protein to undergo thermal degradation or decomposition. From this point, the nature of protein thermal denaturation is the unfolding of molecular chain, accompanying the thermal decomposition of some of the protein molecule. The denaturation of soy protein is influenced by many factors including molecular weight and composition of the soy protein, heating rate, moisture content, as well as the nature and content of the plasticizer. Generally, in the presence of a plasticizer, the denaturation temperature of the soy protein will decrease due to the interaction between the plasticizer and the soy protein macromolecule.

22.6

Plasticization Thermodynamics

Plasticization introduces pliability and distensibility into a plastic composition. There are three theories that account for the phenomenon of plasticization. These are the lubricity, gel, and free volume theories.17−19 According to the lubricity theory, the plasticizer acts as a lubricant to facilitate the movement of the resin macromolecules over each other, thus reducing the internal resistance to deformation. The gel theory considers the rigidity of an unplasticized resinous mass to be caused by an internal three-dimensional honeycomb structure or gel formed by loose attachments between resin macromolecules, and occurs at intervals along the molecular chains. The mechanism of plasticization, according to the gel theory, is accounted for by an application of the mechanistic theory of the plasticizer action. The gel theory extends the lubricity theory by presuming that the plasticizer breaks the resin–resin attachments and, by masking these centers of attachment from each other, prevents their reformation. Finally, there is the free volume theory whereby a plasticizer may depress the glass transition temperature by increasing the polymer free volume. The fundamental concept underlying all of these theories is that a plasticizer can interpose itself between the polymer chains and decreases the forces holding the chains together. Apart from these theories, thermodynamics supplies two criteria of solvent (plasticizer) ability, namely, the Hildebrand solubility parameter, δ, and the Flory interaction parameter, χ.20−22 The Hildebrand solubility parameter Copyright © 2005 by Taylor & Francis

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gives a measure of compatibility between the plasticizer and a resin based on the solvating power of the plasticizer for the resin under investigation. The thermodynamic criteria of solubility are based on the free energy of mixing, ∆GM. Two substances are mutually soluble if ∆GM is negative. By definition, ∆GM ⫽ ∆HM ⫺ T∆SM

(22.1)

where ∆HM is the enthalpy change of mixing and ∆SM is the entropy change of mixing. As ∆SM is generally positive, there is a certain limiting positive value of ∆HM below which dissolution is possible. According to Hildebrand, the enthalpy of mixing can be calculated by ∆Hm ⫽ φ1φ2 (δ1 ⫺ δ2)2

(22.2)

where φ1 and φ2 are the volume fractions of components 1 and 2 and δ1 and δ2 are the solubility parameters of components 1 and 2. From the Hildebrand equation, ∆Hm ⫽ 0 if δ1 ⫽ δ2, so that two substances with equal solubility parameters should be mutually soluble due to the negative entropy factor. This is in accordance with the general rule that chemical and structural similarity favors solubility. As the difference between δ1 and δ2 increases, the tendency toward miscibility decreases. In this way, it is reasonable to conclude that as a requirement for the plasticizer to be effective, the solubility parameters of the resin and the plasticizer must be of similar values. The solubility parameter of soy flour, calculated experimentally by measuring the surface tension of soy flour, was 20.25 (J/cm3)1/2.23 The solubility parameter of sorbitol and glycerol (Figure 22.2), calculated using the method of Hoftyzer and Van Krevlen,24 are 20.78 and 19.88 (J/cm3)1/2, respectively. Thus, sorbitol and glycerol are appropriate plasticizers for the soy flour. The plasticization mechanism of soy protein, as discussed above, is the interaction between the plasticizer and the protein macromolecule, such as polar interactions (with hydroxyl groups), thus increasing the free volume for the protein molecule. Therefore, through plasticization soy protein can be made into soy protein plastic using various processing methods.

Sorbitol

Glycerol

CH2OH H

C

OH

HO

C

H

H

C

OH

H

C

OH

CH2OH OH

C

H

CH2OH

CH2OH M.W. = 182

M.W. = 92

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FIGURE 22.2 Molecular structures of D-sorbitol and glycerol.

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Soy Protein Plastics

Through plasticization by plasticizers such as glycerol, ethylene glycerol, propylene glycerol, 1,2-butanediol, 1,3-butanediol, polyethylene glycol, sorghum wax, and sorbitol, the mechanical properties and the water resistance of the soy protein plastics can be adjusted to a given extent.15−28 The addition of a cross-linking agent such as zinic sulfate or formaldehyde, acetic anhydride, glyoxal, or a polyphosphate filler not only improves the mechanical properties of soy protein plastics but also leads to a significant enhancement to the water resistance of the soy protein plastics.29−33 Additionally, the mechanical properties of the soy protein plastic can be controlled and optimized by adjusting processing parameters including the molding temperature, pressure, and the initial moisture content.24−38 Zhong and Sun39,40 investigated the thermal, mechanical, and the water absorption properties of sodium dodecyl sulfate (SDS) and guanidine hydrochloride (GuHCL) modified soy protein 11S. Results indicated that both SDS and GuHCL not only acted as denaturants, but also as plasticizers for the soy protein 11S, decreasing its glass transition temperature as their concentration increased. The interactions of SDS and GuHCL with the 11S protein also affected its structure and this led to an improvement of the tensile strength, elongation, as well as the water resistance of the 11S proteinbased plastics. In another study, Mo and Sun41,42 used SDS and urea to modify soy protein isolate to form soy protein plastic. They found that SDS increased the degree of denaturation of soy protein, and the glass transition temperature of soy plastic decreased with increasing SDS concentration. A 1% SDSmodified soy plastic showed improved tensile strength and elongation. The urea-modified soy plastic also showed increased tensile strength and modulus with increasing the urea concentration, and reached their maximum values at a urea concentration of 8 M. Additionally, the water resistance of the soy protein plastics modified with urea also improved. The researchers attributed the improvement in properties to the interaction between urea and the soy protein. At lower urea concentration, urea functioned as a plasticizer. At higher urea concentration, urea functioned as a plasticizer, crosslinking agent, and filler and hence increased the entanglement, number of cross-links, and molecular weight of soy plastic. Thus, mechanical strength and modulus of the urea-modified soy plastic improved. They also observed that urea lowered the denaturation temperature of soy protein as well as the glass transition temperature of the soy plastic, compared with unmodified soy plastic. Therefore, urea functioned as a plasticizer, a denaturant, as well as a filler for the soy protein. Chemical modification of soy protein with monomers or low molecular weight polymers (oligomer) with functional groups that can react with hydroxyl or amino groups in protein has been studied. Wu et al.43,44 modified protein using low molecular weight polycaprolactone (PCL)/hexamethylene Copyright © 2005 by Taylor & Francis

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diisocyanate (HDI) prepolymer. They found that amino acids in the protein reacted with the HDI-modified PCL to form urea-urethane linkage. After modification, the resulting plastic showed excellent water resistance. In addition, the elongation was 15 times higher than that of the unmodified plastic. However, they noted that the tensile strength was reduced by 50% compared with unmodified protein plastic. Ly et al.45 grafted soy protein with vinyl monomers such as methyl methacrylate, ethyl acrylate, butyl acrylate, and hexyl acrylate and, using free radical methods, they polymerized vinyl polymer-modified soy protein plastics. They found that the water resistance of the modified soy protein plastics increased compared with the unmodified one. However, the tensile strength, elongation, tensile modulus, and yield strength of the modified soy protein plastic decreased. Ly and co-workers suggested that the grafting of the vinyl monomer to the soy protein generated branches that disrupted strong inter- and intramolecular forces associated with hydrogen bonding, electrostatic interaction, and disulfide bridges, resulting in products with poor mechanical properties. Chen et al.46 investigated the thermal properties, mechanical properties, and water resistance of polyurethane prepolymer (PUP) modified soy protein plastics. Because of grafting and cross-linking reaction between the amide groups in PUP and the hydroxyl and/or various amino functional groups in soy protein, the compatibility between PUP and soy protein was good and the resulting products showed enhanced toughness and good water resistance. By increasing the PUP content, the rigid soy-based plastics become elastomer-like materials. Work has been done in soy protein modification using enzymes with the aim of increasing protein functionality and altering its properties. Kumar et al.47 studied the influence of enzyme modification on the mechanical properties of soy plastics. Because modification involves enzyme hydrolysis of selected peptide bonds leading to a lower molecular weight, the tensile strength decreased significantly but elongation increased.

22.8

Soy Protein Plastics Blends

The studies cited above have demonstrated that soy protein can be made into a bioplastic through plasticization. By varying the amount of a plasticizer or by adding a cross-linking agent to soy protein, bioplastic with varying strength and modulus can be obtained. However, the relatively low strength and higher moisture absorption of the resulting plastics limit their applications. In order to improve the properties of soy protein plastic, the simplest and most effective approach is to blend soy protein plastic with natural polymer or other biodegradable polymer to form soy protein plastic blends, which is also called soy protein-based biodegradable plastics, Copyright © 2005 by Taylor & Francis

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namely, soy-based bioplastic. This is a preferred method for developing new polymer alloys for different applications. Starch and cellulose have been used as filler materials for soy protein plastics because of their ability to interact with soy protein through hydrogen bonding and other polar interactions.48−53 Otaigbe and Jane48−51 investigated the use of starch to reinforce soy protein plastics by using cross-linking agents. They found that starch as a filler could increase the mechanical properties and reduce the water absorption of the product. Paetau et al.52 and Huang et al.53 used cellulose as filler for soy plastics. They found that cellulose increased the mechanical properties, including modulus and strength, as well as the water resistance of soy protein plastics. Lignin is an amorphous natural polymer that is based on phenyl propane derivatives. It has been evaluated as a potential raw material for the chemical industry because it is nontoxic, commercially available, and inexpensive.54 Huang et al.55,56 studied the structure and thermal and mechanical properties of soy protein plastics blended with lignosulfonate and alkaline lignin. They found that lignosulfonate (LS) and alkaline lignin were able to cross-link with soy protein, thus enhancing tensile strength and elongation, and also improving water resistance of soy protein plastics. Huang and co-workers also proposed a theoretical model for the physical cross-linking interactions between soy plastic LS. They proposed that during the heat processing of the blends, the SPI molecules undergo a conformational change from a compact coil to an expanded chain and interact with the LS center through hydrogen bonding, dipole–dipole, charge–charge, and hydrophobic interactions to form a complete cross-linked network. These reactions resulted in blends with improved mechanical properties and water resistance. Starch, cellulose, and lignin are only used as fillers in blends with soy protein plastics because they cannot be melted during heat processing. If synthetic biodegradable polymers are to be used to produce blends with soy protein plastic, they need to be adequately melted during the processing. Due to the structural difference between soy protein plastic and biodegradable polymers, a compatibilizer is required to facilitate their mixing and assure good performance of the soy plastic blends. Zhong and Sun57,58 used methylene diphenyl diisocyanate (MDI) as a compatibilizer to produce blends of soy protein with polycaprolactone or poly(ethylene-co-ethyl acrylate-co-maleic anhydride) (PEEAMA). They found that MDI compatibilized soy protein and PCL because it could react with both soy protein and PCL, and as a result the mechanical properties and water resistance of soy protein−PCL blends were enhanced. However, the researchers observed that MDI partially compatibilized soy protein− PEEAMA blends, and therefore their mechanical properties and thermal properties did not improve. Only the water resistance improved after the addition of MDI. Mungara et al.59 used a poly(vinyl lactam) (PVL) as a compatibilizer to blend soy protein and polyesters, including polycaprolactone (PCL) and Biomax (a commercial biodegradable polyester). Because the amide side Copyright © 2005 by Taylor & Francis

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groups in PVL contribute to its compatibility with soy protein and the hydrophobic main chain interacts with the polyesters, PVL is an excellent compatibilizer for soy protein and biodegradable polyesters. Thus, the processed soy protein-based blends showed high tensile strength and modulus as well as low water absorption. John and Bhattacharya60 used maleic anhydride grafted polyesters as compatibilizers to enhance the compatibility of soy protein and biodegradable polyesters. The improved compatibility of the blends was attributed to the chemical reactions or hydrogen bonding between hydroxyl groups, amino or carboxyl groups in soy protein, and the anhydride groups in the compatibilizer. Additionally, the physical entanglement interaction between the compatibilizer and the polyester improved the compatibility of the blends. As a result, the blended products show enhanced mechanical properties as well as water and oil resistance of the blends. Using reactive extrusion methods, Liu and co-workers61 prepared functional monomer glycidyl methacrylate grafted polyester amide (PEA-g-GMA) and used it to prepare blends of soy protein plastic and polyester amide. They found that PEA-g-GMA was an excellent compatibilizer for soy protein plastic and polyester amide because the epoxy group of PEA-g-GMA may react with hydroxyl or amino groups of soy protein (Figure 22.3). Additionally, the polyester amide and PEA-g-GMA interacted through physical interpenetration between chain segments. Thus, PEA-g-GMA enhanced interfacial adhesion between soy protein and polyester amide and hence improved tensile properties [Figure 22.4(a)] without decreasing impact strength [Figure 22.4(b)]. However, polyethylene-grafted maleic anhydride (PE-g-MA, EPOLENE C16, a commercial product from Eastman) was not a good interfacial agent due to the difference in polarity between polyester amide and polyethylene. Waikul62 studied the effect of processing parameters on the mechanical properties of soy protein−copolyester blends. She found that low processing temperatures led to nonuniform dispersion of the ingredients, and the blends showed diminished mechanical properties. On the other hand, higher processing temperatures led to better dispersion and reduced particle

CH3 PEA

CH2

CH

O

+

CH2

CH3 PEA

CH2

H

O

N

C

C

H

R

O

CH

O

CH2

CH OH

CH2

O

OH H O

C

N

C

R

H

FIGURE 22.3 Scheme of soy protein reacted with polyester amide-grafted glycidyl methcrylate (PEAg-GMA) during extrusion processing at higher temperature.

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Tensile strength (MPa)

18 15

0.5 Strength

Modulus 0.4

12 0.3 9 0.2 6 0.1

3 0

(a)

Tensile modulus (GPa)

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0 A

B

C

D

A

B

C

D

Impact strength (J/m)

50

(b)

40 30 20 10 0

FIGURE 22.4 Tensile properties (a) and impact strength (b) of compatibilized soy-based bioplastic. (A) Soybased bioplastic (50% soy plastic/50% polyester amide); (B) soy-based bioplastic with 10% (PEA-g-MA); (C) soy-based bioplastic with 10% (PEA-g-GMA); and (D) soy-based bioplastic with 10% (PEA-g-MA).

size. However, the products showed signs of increased thermal degradation. Waikul also found that the screw configuration affected the mechanical properties of soy-based bioplastic blends. Moderate and high shear kneading blocks led to better mixing, resulting in blended products with better mechanical properties, including tensile strength and elongation. During preparation of soy protein plastic blends, the processing window of the synthetic polymer should be matched with that of soy protein plastic to prevent thermal degradation of protein that can adversely affect the mechanical properties of the final product. In order to keep the biodegradability and environmental friendliness of soy protein plastics, most of the polymers being used to blend with soy protein plastic are biodegradable polyesters. They include polyester amide, Ecoflex, PCL, PLA, Biomax, and Eastar bio, whose processing windows match well with that of soy protein. In a recent study, a polyester amide was blended with soy protein plastic to form soy protein-based bioplastic with improved mechanical properties and water resistance. The polyester amide was chosen because amide Copyright © 2005 by Taylor & Francis

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groups contributed to its compatibility with soy protein. To investigate the influence of different plasticizers on morphology and thermal and mechanical properties of soy-based bioplastic, plasticizers, including glycerol and D-sorbitol, and blends of the two were used.63 It was found that D-sorbitolplasticized soy flour plastics were more rigid, with a higher tensile modulus and tensile strength than those plasticized with glycerol [Figure 22.5(a)]. The soy flour plastic formulated using a combination of the two plasticizers resulted in a midrange of tensile modulus and strength. As expected, glycerol-plasticized bioplastic had the highest impact strength [Figure 22.5(b)], whereas that plasticized with sorbitol had the highest thermal stability among the three plastics studied. Also, there was a noticeable difference between the fracture surfaces of the sorbitol and the glycerol-plasticized soy flour-based plastic (Figure 22.6). Sorbitol-plasticized soy flour-based plastic had brittle features, whereas those plasticized with glycerol had local ductile features. These results demonstrate the effects of different plasticizers on the properties of soy-based plastics. This also indicates that using a combination of plasticizers can adjust and control the balance between strength, modulus,

1.0

15 Modulus

12

0.8

9

0.6

6

0.4

3

0.2 0

0 (a)

Tensile modulus (GPa)

Tensile strength (MPa)

Strength

A

B

C

A

B

C

Impact strength (J/m)

40 30 20 10 0 (b)

FIGURE 22.5 Comparison of tensile properties (a) and flexural properties (b). (A) Glycerol-plasticized soy flour bioplastic; (B) D-sorbitol-plasticized soy flour bioplastic; (C) glycerol and D-sorbitolplasticized soy flour bioplastic.

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FIGURE 22.6 Tensile fracture surface of the soy flour bioplastics of (a), glycerol-plasticized soy flour bioplastic; (b), D-sorbitol-plasticized soy flour bioplastic; (c), glycerol and D-sorbitolplasticized soy flour bioplastic.

and toughness of soy protein plastic as well as that of the soy protein-based bioplastic.

22.9

Natural Fiber-Reinforced Soy-Based Biocomposites

Although soy-based bioplastics with fairly good properties can be manufactured, the modulus and strength are not high enough for load-bearing applications. Fibers, on the other hand, possess high strength and are sufficiently rigid. However, they cannot be used on their own for load-bearing applications Copyright © 2005 by Taylor & Francis

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because of their fibrous nature. Therefore, the fibers are used for the reinforcement of soy−plastic-based composites to provide strength and stiffness. In the past decade, natural fibers such as kenaf, flax, jute, hemp, sisal, henequen, pineapple leaf fiber, grass, corn stalk, rice straw, and wheat straw have been used as fiber to produce reinforced composites. These fibers are attractive for industrial applications because they have many advantages including low cost, low density, acceptable mechanical properties, and ease of separation. They are also ecofriendly, sustainable, biodegradable, and their use is good for carbon dioxide sequestration.64,65 An additional advantage is their excellent insulation against heat and noise. The density, modulus, and strength of commonly used natural fibers and glass fiber are given in Table 22.2. The strength and modulus of fiber-reinforced composites are theoretically determined by the volume fraction. Therefore, the density or the specific strength and modulus of natural fiber are the most important properties to consider during the preparation of natural fiber-reinforced composites. Natural fiber-reinforced soy-based biocomposites are generally referred to as green composites because of their environmentally friendly nature. The composites are totally biodegradable, and 80% of the composite comes from renewable resources. Much research on natural fibers and their use in producing reinforced soy-based biocomposites has been performed.71−79 Hemp fiber has good mechanical properties and is also a commercially available fiber. It can be used to reinforce soy-based bioplastics to form biocomposites.71−75 Hemp fiber-reinforced soy-based biocomposites have shown considerable enhancement in tensile, flexural, and impact properties compared to soy-based bioplastic. For example, the tensile strength, flexural strength, tensile modulus, and flexural modulus values of this composite, including 30 wt% hemp fiber, increased by as much as 125%, 300%, 850%, and 1050%, respectively, in comparison to soy flour-based bioplastics (see Table 22.3). An improvement in impact properties of the biocomposite was also observed. Thermal property studies with 30 wt% hemp fiber show an increase in heat deflection temperature by ~35°C due to an increase in modulus of the biocomposites. The high fiber loading significantly improved the TABLE 22.2 Comparison of Natural Fibers to Glass Fiber Fiber Indian grass Hemp Kenaf Henequen Pineapple leaf fiber (PALF) Jute Flax E-Glass

Density (g/cm3) Strength (MPa) Modulus (GPa) References 1.25 1.29 1.4 1.57 1.44 1.3–1.45 1.5 2.5

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264 695 284–800 372 413–1627 393–773 345–1100 2000–3500

28 42–70 21–60 10 35–83 13–27 28–80 70

72 64,72 64,68,70–72 66,67 64,69 64 64, 65 64

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TABLE 22.3 Hemp Fiber-Reinforced Soy-Based Biocomposites

Samplea

Tensile Strength (MPa)

Tensile Modulus (GPa)

Flexural Strength (MPa)

Flexural Modulus (GPa)

Impact Strength (J/m)

Heat Deflection Temperature (°C)

8.5 17.8 19.8

0.2 1.5 2.1

8.3 26.9 32.4

0.2 1.6 2.5

32.9 36.7 44.7

35 61 71

A B C

A, Soy-based bioplastic; B, 15% hemp fiber-reinforced soy-based biocomposites; C, 30% hemp fiber-reinforced soy-based biocomposites.

3.5

25 Modulus

3 2.5

15

2 1.5

10

1 5

0.5 0

0 A (a)

A+15% Henequen

A+15% Flax

A+15wt% Hemp

A+15wt% Kenaf

40

4 Strength

35 Flexural strength (MPa)

Tensile modulus (Gpa)

Tensile strength (Mpa)

Strength 20

Modulus

30

3

25 20

2

15 1

10

Flexural modulus (GPa)

a

5 0

0 A

(b)

A+15% Henequen

A+15% Flax

A+15wt% Hemp

A+15wt% Kenaf

FIGURE 22.7 The comparison of tensile properties (a) and flexural properties (b) of different natural fiberreinforced soy-based biocomposites.

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physicomechanical performance of the resulting green composites. Henequen, flax, and kenaf fiber were also used to reinforce soy-based bioplastic76 and the biocomposites were fabricated by extrusion and injection molding. Those biocomposites had similar mechanical properties to the hemp fiber-reinforced biocomposites (Figure 22.7). Most natural fibers are lignin-cellulose fibers. Because lignin is on the surface of fiber, the interaction between cellulose and soy−plastic matrix is hindered, leading to poor adhesion. In order to improve the adhesion between fiber and matrix, it is necessary to treat the fiber surface to remove the lignin. After removal of lignin from the fiber surface through alkali treatment, the resulting fibers showed improvement in thermal properties and a decrease

FIGURE 22.8 ESEM micrographs (550X) (scale bar ⫽ 100 µm) of raw and alkali-treated Indian grass fibers for (a) raw fiber, (b) grass fiber treated with 5% alkali solution for 2 h, (c) grass fiber treated with 10% alkali solution for 2 h, and (d) grass fiber treated with 10% alkali solution for 4 h.

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in the cementing force between interfibrils (Figure 22.8).77 As a result, the dispersion of fiber in soy-based bioplastic matrix improved (Figure 22.9) and the aspect ratio of fiber increased. Therefore, the contacting area between fiber and matrix increased. After alkali treatment of the fiber, the content and activity of hydroxyl groups in the fiber surface increased and the interaction between fiber and soy matrix increased so as to improve the interfacial adhesion between fiber and soy−plastic matrix. The above factors led to increases in mechanical properties (Figure 22.10) including tensile strength and modulus, bending

FIGURE 22.9 ESEM micrographs (260X) of fracture surface in liquid nitrogen of Indian grass fiber-reinforced soy-based composites, (a) raw fiber (scale bar ⫽ 200 µm), (b) grass fiber treated with 5% alkali solution for 2 h (scale bar ⫽ 200 µm), (c) grass fiber treated with 10% alkali solution for 2 h (scale bar ⫽ 200 µm), and (d) grass fiber treated with 10% alkali solution for 4 h (scale bar ⫽ 150 µm).

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3.0

16 Tensile strength (MPa)

Strength

Modulus 2.5

12 2.0 8

1.5 1.0

4 0.5 0

0 A

(a)

B

C

D

E

3

30 Flexural strength (MPa)

Strength

Modulus

25 2

20 15

1

10 5 0

(b)

Tensile modulus (GPa)

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0 A

B

C

D

E

FIGURE 22.10 Tensile properties (a) and impact strength (b) of (A) soy based bio-plastic and (B) raw fiber-reinforced soy plastic composites, (C) 5% alkali solution-treated (2 h) fiber-reinforced composites, (D) 10% alkali solution-treated (2 h) fiber-reinforced composites, and (E) 10% alkali solution-treated (4 h) fiber-reinforced composites.

strength and modulus, impact strength, as well as heat deflection temperature of natural fiber-reinforced composites.78 It is necessary to point out that the alkali-treated fiber behaved like single glass fiber because the cementing force between fibrils was eliminated and soy-based biocomposites reinforced with alkali-treated fibers were similar to glass fiber-reinforced composites. Lodha and Netravali79 investigated the interfacial and mechanical properties of ramie fiber-reinforced soy protein isolate green composites. Their results indicated that the fracture stress increased with an increase in fiber length and content. Fibers that were 5 mm long (10 wt%) did not have a reinforcement effect. Instead, the short fibers acted as filler and led to a reduction in tensile properties because of fiber aggregation. By further increasing the fiber length to 15 mm and fiber content to 30 wt%, a significant increase in Young’s modulus and tensile stress occurred, although elongation decreased. They also used Zweben et al.’s model80 to predict and fit their Copyright © 2005 by Taylor & Francis

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experimental data. They found good agreement between the data with increasing fiber length and fiber content. The properties of soy-based biocomposites reinforced with natural fiber did not significantly change even after one month of aging at room temperature. Tensile strength and modulus increased but impact strength slightly decreased (Figure 22.11). This is attributed to the thermodynamic change from the nonequilibrium state to an equilibrium state, leading to relaxation of the molecular chain. As a result, there were only slight changes in the microstructure and mechanical properties of soy composites.81 In contrast, soy protein plastic lost its strength and toughness and became stiff and brittle over time because the excess enthalpy of the soy protein plastic relaxed with time.82 Another possible reason was the leaching of plasticizer from soy

20

4 Strength of aging samples Modulus of aging samples

16

3

12 2 8 1

4 0

0 A

(a)

Tensile modulus (GPa)

Tensile strength (MPa)

Strength Modulus

B

C

D

80 Impact strength (J/m)

Impact strength

Impact strength of aging sample

60

40

20

0 (b)

A

B

C

D

FIGURE 22.11 Tensile properties (a) and impact strength (b) of natural fiber-reinforced soy-based biocomposites after one month of aging at room temperature for (A) raw fiber-reinforced soy plastic composites, (B) 5% alkali solution-treated (2 h) fiber-reinforced composites, (C) 10% alkali solution-treated (2 h) fiber-reinforced composites, and (D) 10% alkali solution-treated (4 h) fiber-reinforced composites.

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10

Tensile strength (MPa)

40 Strength Elongation

30

8 6

20 4 10

2

0

Tensile elongation (%)

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0 A

B

C

D

FIGURE 22.12 Strength and elongation of unmodified and trithiol-modified wheat gluten-based bioplastic for (A) unmodified wheat gluten-based bioplastic, (B) trithiol-modified wheat gluten-based bioplastic, (C) unmodified wheat gluten-based bioplastic after 4 months of aging, and (D) trithiol-modified wheat gluten-based bioplastic after 4 months of aging. (Adapted from Woerdeman, D.L. et al., Biomacromolecules, 5, 1262, 2004.)

protein plastic, further reducing the interactions between plasticizer and soy protein. A similar phenomenon (see Figure 22.12) was found in aging wheat protein-based bioplastics (both an unmodified and a trithiol-modified wheat gluten-based bioplastic).83 Woerdeman et al. believe that a favorable change in the freeze-dried powder’s storage conditions during the summer months was responsible for the improvement in mechanical properties (both strength and elongation with no loss in stiffness). In light of above research, further studies need to be performed to determine how the properties of these various bioplastics can be controlled. In addition, determination of mechanical property with aging is also an effective method to measure the stability of the bioplastic and biocomposites.

22.10 Future Research Directions of Soy Protein Materials Soy protein plastics have been demonstrated to have such a large potential for application in many areas that research is expected to continue in the foreseeable future. Simultaneously, under the optimum condition of unchanged biodegradability of soy protein, other approaches including biotechnology methods or chemical methods should be added to this research area. Thus, soy protein can be converted to a thermoplastic. Similarly, in situ modification of soy protein using enzymes or chemical catalysts during processing, e.g., reactive extrusion, is another research direction. The chemical modification or surface activation of soy protein in solid state is a valuable method, worthy of consideration. Additionally, close Copyright © 2005 by Taylor & Francis

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attention should be paid to the relationship between the structure and the properties of soy protein, such as denaturation, crystallization, and processing, which would be helpful in finding new methods to solve the problems of soy protein plastic and hence create a novel research area for applications of soy protein.

Acknowledgments The authors are thankful to USDA-NRI Award # 2001-35504-10734, GREEEN (Generating Research and Extension to meet Economic and Environmental Needs) 2002 Award # GR02-066, and NSF 2002 Award # DMR-0216865, under “Instrumentation for Materials Research (IMR) Program,” for providing financial support.

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57. Zhong, Z. and Sun, X.S., Properties of soy protein isolate/polycaprolactone blends compatibilized by methylene diphenyl diisocyanate, Polymers, 42, 6961, 2001. 58. Zhong, Z. and Sun, X., Properties of soy protein isolate/poly(ethylene-co-ethyl acrylate-co-maleic anhydride) blends, J. Appl. Polym. Sci., 88, 407, 2003. 59. Mungara, P. et al., Processing and physical properties of plastics made from soy protein polyester blends, J. Polym. Environ., 10, 31, 2002. 60. John, J. and Bhattacharya, M., Properties of reactively blended soy protein and modified polyesters, Polym. Int., 48, 1165, 1999. 61. Liu, W. et al., Preparation of glycidyl metharylate modified polyester amide and its compatibilization effect on blends of plasticized soy flour/polyester amide, unpublished data, 2003. 62. Waikul, L.H., Processing Parameter-Property Relations of Soy ProteinCopolyester Blends, MS. thesis, Michigan State University, East Lansing, MI, 1998. 63. Tummula, P. et al., Influence of plasticizer on thermal, mechanical properties and morphology of soy flour based bioplastic, unpublished data, 2002. 64. Mohanty, A.K., Misra, M., and Hinrichsen, G., Biofibres, biodegradable polymers and biocomposites. An overview, Macromol. Mater, Eng., 276–277, 1, 2000. 65. Mohanty, A.K., Misra, M., and Drzal, L.T., Sustainable Bio-Composites from Renewable Resources: Opportunities and Challenges in the Green Materials World, J. Polym. Environ., 10, 19, 2002. 66. Cazaurang-Martinez, M.N., Herrera-Franco, P.J., Gonzalez-Chi, P.I., and AguilarVega, M., Physical and mechanical properties of henequen fibers, J. Appl. Polym. Sci., 43, 749, 1991. 67. Aguilar-Vega, M. and Cruz-Ramos, C.A., Properties of henequen cellulosic fibers, J. Appl. Polym. Sci., 56, 1245, 1995. 68. Parikh, D.V. et al., Use of Kenaf Fibers in Automotive Nonwovens Part I: Improved Chemical Retting of Kenaf Fibers, www.nal.usda.gov/ttic/ tektran/data/000012/58/0000125878.html 69. Lee, S.M., International Encyclopedia of Composite—Vol. 4, VCH, New York, 1991, p. 9. 70. Chen, H.-L. and Porter, R.S., Composite of polyethylene and kenaf, a natural cellulose fiber, J. Appl. Polym. Sci., 54, 1781, 1994. 71. Drzal, L.T., Mohanty, A.K., and Misra, M., unpublished data, 2001. 72. Composite Materials and Structures Center, Michigan State University, Mechanical properties of natural fibers, unpublished data, 2003. 73. Tummala, P. et al., Green Composites From Biofiber and Soy Protein Based Bioplastic, Proceedings of the 9th Annual Global Plastics Environmental Conference (GPEC), Detroit, MI. Feb. 26-27, 2003, 183. 74. Drzal, L.T. et al., Environmentally friendly bio-composites from soy-based bioplastic and natural fiber, Polym. Mater. Sci. Eng., 87, 117, 2002. 75. Hokens, D., Design and Engineering of Eco-Friendly Biocomposites From Hemp Fiber and Low Cost Novel Soy Protein-Based Bioplastic, MS. thesis, Michigan State University, East Lansing, MI, 2002. 76. Tummala, P. et al., Green composites from soy based bioplastic and natural fibers, unpublished data, 2003. 77. Liu, W. et al., Effects of alkali treatment on the structure, morphology and thermal properties of native grass fibers as reinforcements for polymer matrix composites, J. Mater. Sci., 39, 1051, 2004.

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78. Liu, W. et al., Injection Molded Green Composites From Native Grass And Soy Based Bioplastic, Proceedings of the 18th Annual Technical Conference American Society for Composites, Oct. 19–22, Paper 189, 2003, Gainesville, FL. 79. Lodha, P. and Netravali, A.N., Characterization of interfacial and mechanical properties of “green” composites with soy protein isolate and ramie fiber, J. Mater. Sci., 37, 3657, 2002. 80. Zweben, C., Hahn, H.T., and Chou, T.W., in Mechanical Behavior and Properties of Composite Materials, Delaware Composites Design Encyclopedia, Vol. 1, Carlsson, L.A., and Gillespie, J. W., Eds., Technomic Publishing, Lancaster, PA, 1989, p. 34. 81. Liu, W. et al., Influence of aging on mechanical properties of natural fiber reinforced soy-based biocomposites, unpublished data, 2003. 82. Mo, X. and Sun, X., Effect of storage time on properties of soybean protein based plastic, J. Polym. Environ., 11, 15, 2003. 83. Woerdeman, D.L. et al., Designing new materials from wheat protein, Biomacromolecules, 5, 1262, 2004.

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23 Synthesis, Properties, and Potential Applications of Novel Thermosetting Biopolymers from Soybean and Other Natural Oils

Fengkui Li and Richard C. Larock

CONTENTS 23.1 Introduction 23.2 Biopolymers from the Cationic Polymerization of Soybean Oil 23.2.1 Soybean Oils as Cationic Monomers 23.2.2 Polymer Synthesis and Structural Characteristics 23.2.3 Thermophysical and Mechanical Properties 23.2.4 Good Damping Properties 23.2.5 Shape Memory Properties 23.3 Biopolymers from the Cationic Polymerization of Other Natural Oils 23.3.1 Vegetable Oil-Based Polymers 23.3.2 Tung Oil-Based Polymers 23.3.3 Fish Oil-Based Polymers 23.4 Biopolymers from the Thermal Polymerization of Natural Oils 23.5 Biopolymers from the Free Radical Polymerization of Natural Oils 23.6 Biopolymers from the Free Radical Polymerization of Natural Oil Derivatives 23.7 Potential Applications 23.8 Conclusions Acknowledgments References ABSTRACT This chapter summarizes the preparations, characterizations, and potential applications of novel thermosetting biopolymers from soybean and other natural oils. Three different approaches, i.e. cationic, thermal, and

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free radical polymerizations, have been employed and the characteristics of the resulting polymeric materials have been reviewed. (1) Cationic copolymerization of soybean oil, styrene, and divinylbenzene affords a wide variety of dark brown-colored, opaque polymeric materials, ranging from elastomers to tough and rigid plastics. These new polymers not only possess industrially viable thermophysical and mechanical properties, but also unique good damping and shape memory properties. Cationic copolymerization of other natural oils with alkenes produces new polymeric materials with similar characteristics to soybean oil polymers. Apparently, their mechanical properties are intimately related to the degrees of unsaturation of the vegetable oils employed. (2) Thermal copolymerization of tung oil, styrene, and divinylbenzene results in light yellow-colored, transparent rubbery materials and plastics. Semitransparent and white opaque polymeric materials are obtained when other biological natural oils are employed. (3) Free radical polymerization has been successfully employed to copolymerize conjugated vegetable oils and acrylonitrile. The resulting plastics appear to be transparent or opaque, depending on reaction conditions. Vegetable oil derivatives containing more reactive C⫽C bonds than those of the original oils are able to readily copolymerize free radically with styrene to afford hard and rigid plastics. All of the natural oil-based polymeric materials prepared by different polymerization methods possess a wide range of industrially promising mechanical properties and may find many potential applications.

23.1

Introduction

Biopolymers from renewable natural resources have become increasingly interesting because of their low cost, ready availability, and possible biodegradability. A wide range of naturally occurring biopolymers, cellulose, and starch are already available for material applications.1 With rapid advances in our understanding of fundamental biosynthetic pathways and methods to modulate or tailor these pathways through genetic engineering, a variety of biopolymers have been derived from polysaccharides, proteins, lipids, and polyphenols, and specialty polymers have been produced by bacteria, fungi, plants, and animals.2−4 Meanwhile, new biopolymers may be obtained from low molecular weight natural substances containing various functionalities by the synthetic polymerization methods widely used for petroleum-based monomers. Natural oils (vegetable oils and marine fish oils) possess a triglyceride structure with highly unsaturated fatty acid side chains.5,6 The multiple C⫽C bonds make these biological oils ideal monomers of natural origin for the preparation of biopolymers. Of the available vegetable oils, soybean oil is the most abundant, accounting for ~30% of the world’s vegetable oil supply.7 Typically, the majority (~80%) of soybean oil produced each year is used for Copyright © 2005 by Taylor & Francis

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human food. A further 6% is used for animal feed, whereas the remainder (14%) finds nonfood uses (soap, lubricants, coatings, paints, etc.). The nonfood uses have mainly involved reactions and the polymerization of soybean oil derivatives such as fatty acids and epoxidized oils.8 Recently, a variety of new polymeric materials have been prepared from soybean oil and its derivatives, which possess industrially viable thermophysical and mechanical properties and thus may find structural applications. One of the major efforts in this field has taken advantage of either the original C⫽C bonds of soybean oil,9−18 tung oil,19,20 corn oil,21 and fish oils,22−26 or the C⫽C bonds of soybean oil derivatives.27−29 The other major process has involved the conversion of vegetable oils into polyols by epoxidizing the C⫽C bonds of the triglyceride oil, followed by oxirane ring opening of the epoxidized oil. The vegetable oil-derived polyols are then further reacted with diisocyanates to produce polyurethanes.30−34 The purpose of this chapter is to review the synthesis and characterization of viable polymeric materials from the direct polymerization of the C⫽C bonds of soybean oils and their derivatives.9−18,27−29 This chapter also covers other vegetable oils,19−26 including corn (COR), peanut (PNT), sunflower (SUN), safflower (SAF), walnut (WNT), linseed (LIN), and tung (TUN) oils, as well as marine fish oils. Various polymerization techniques have been employed, including cationic, thermal, and free radical polymerization. The biobased polyurethanes prepared through condensation polymerization of vegetable oil polyols and diisocyanates are described in a separate chapter in this book.

23.2

23.2.1

Biopolymers from the Cationic Polymerization of Soybean Oil Soybean Oils as Cationic Monomers

Table 23.1 lists three different soybean oils, which have been successfully employed in the preparation of biopolymers. The triglyceride structure of the soybean oils is illustrated in Scheme 23.1. The three fatty acid side chains are composed primarily of oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:3). Naturally occurring, regular soybean oil (SOY) has ~4.5 nonconjugated C⫽C bonds per triglyceride. Low-saturation soybean oil (LSS) is a genetically modified triglyceride oil with considerably more linoleic acid (C18:2) than the SOY, possessing ~5.1 nonconjugated C⫽C bonds per triglyceride. Conjugated LSS (CLS) is derived from LSS using a rhodium catalyst developed in the Larock group at Iowa State University.35 Essentially all of the C⫽C bonds that can be conjugated in the LSS are conjugated in the CLS. The C⫽C bonds of the SOY and LSS are slightly more nucleophilic than those of ethylene and propylene and are susceptible to cationic polymerization. The conjugated C⫽C bonds in the CLS are even more reactive toward Copyright © 2005 by Taylor & Francis

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TABLE 23.1 Compositions of Various Soybean Oilsa C⫽C Soybean Oil SOY LSS CLS a b c

Percent Fatty Acidsb

Type

Numberc

C16:0

C18:0

C18:1

C18:2

C18:3

Nonconjugated Nonconjugated Conjugated

4.5 5.1 5.1

10.5 4.5 4.5

3.2 3.0 3.0

22.3 20.0 20.0

54.4 63.5 63.5

8.3 9.0 9.0

The price of soybean oil was about 15–27 cents/lb in 2001–2003. For example, C18:2 represents the fatty acid (ester) with 18 carbons and 2 C⫽C bonds. The average number of C⫽C bonds per triglyceride is calculated by 1H NMR spectral analysis.

CH3 ( CH2)7 CH= CH (CH2)7 CO2CH2 CH3 (CH2)4 CH=CHCH2 CH =CH (CH2)7 CO2CH CH3CH2CH = CH CH2CH = CH CH2 CH=CH ( CH2)7 CO2CH2 SCHEME 23.1

cationic polymerization. Thus, thermodynamically, the SOY, LSS, and CLS oils, just like ethylene and propylene, are cationically polymerizable monomers or “cationic monomers.” 36 However, unlike ethylene and propylene that usually yield only oligomers due to the relatively low stability of intermediate carbocations (which favor secondary reactions competing with chain propagation),36 the cationic polymerization of soybean oil is more likely to afford high molecular weight polymers with cross-linked polymer networks because (1) soybean oils themselves possess relatively high molecular weights, (2) each of the side chains of the soybean oil triglyceride is likely to participate in the cationic polymerization, resulting in cross-linking, and (3) the cationic intermediates produced from the conjugated dienes in CLS are considerably more stable. 23.2.2

Polymer Synthesis and Structural Characteristics

Protonic acids, Lewis acids, and combinations of these materials are the most efficient carbocationic initiating systems.36 A variety of cationic initiators have been shown to effectively polymerize soybean oil under mild conditions. However, boron trifluoride diethyl etherate (BFE) has proved to be the most efficient initiator.9 Typically, the simple homopolymerization of the SOY, LSS, or CLS results in viscous oils with increased molecular weights or solid rubbery but very weak materials containing ~40−60 wt% unreacted soybean oils.18 However, the SOY, LSS, and CLS can readily copolymerize with certain cationic alkene monomers such as divinylbenzene (DVB) or mixtures of styrene (ST) plus DVB. In the copolymerization, the BFE initiator generally Copyright © 2005 by Taylor & Francis

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has to be modified by a fatty acid ester, e.g., Norway fish oil ethyl ester (NFO), which is miscible with both the soybean oil triglyceride and the alkene comonomers. Scheme 23.2 illustrates the cationic copolymerization of a soybean oil triglyceride, ST and DVB. Note that in addition to DVB, the soybean oil triglyceride also contributes directly to cross-linking. The gel time of the reactants varies from a few minutes to several hours, depending on stoichiometry and curing temperatures. The activation energies for the gelation process are 95−122 kJ/mol, which are slightly higher than those of some epoxy resins (70−90 kJ/mol).16 Generally, 20−50 wt% of the soybean oil reactants are converted into cross-linked polymers at gelation and subsequent vitrification. Fully cured thermosets are obtained after post-curing processes at elevated temperatures.16 The resulting soybean oil-based bulk polymers are typically, opaque materials and have a glossy dark brown color and a slight odor, and range from elastomers to tough and rigid plastics. The bulk polymers are composed primarily of soybean oil−ST−DVB cross-linked polymers, plus a small amount of unreacted soybean oil. It has been determined that the ST and DVB completely participate in the copolymerization to form cross-linked polymers. The more reactive the soybean oil is, the less unreacted oil that remains after polymerization. The maximum amount of soybean oil is incorporated into the cross-linked polymer when the soybean oil constitutes ~45−50 wt% of

CH

CH

CO2

CH

CH

CO2

CH

CH

CO2

+

+ DVB

ST

Triglyceride cat.BF3.OEt2 (BFE)

25°C/12 h 60°C/12 h 110°C/24 h O

O

CH2

CH

m

CH

CH

CH

O

O C

C

O CH n

SCHEME 23.2

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C CH

O CH n CH2

CH

m

CH2

CH

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the reactants. Under appropriate conditions, almost all of the CLS reactants are converted into cross-linked polymer networks.16 The cross-link densities of the soybean oil-based polymers range from 1 ⫻ 102 to 4 ⫻ 104 mol/m3. The elastomeric rubbery materials have low cross-link densities, whereas the rigid hard plastics possess rather high cross-link densities. As expected, the more reactive CLS oil affords polymers with noticeably higher cross-link densities than the SOY and LSS oils with the same stoichiometries. 23.2.3

Thermophysical and Mechanical Properties

The bulk polymers derived from soybean oils by cationic polymerization are thermally stable at temperatures below 200°C under either an air or a nitrogen atmosphere, and only begin to lose significant weight above 300°C. Typically, the materials lose 10% of their weight at temperatures of 310°−350°C, depending primarily on the content of the unreacted oil present in the bulk polymers.10,11 Some soybean oil−DVB polymers exhibit two glass transition temperatures (Tg’s) during dynamic mechanical analysis. The glass transition α1 at about 80°C corresponds to the glass transition of the cross-linked polymer, and glass transition α2 at 0°–10°C corresponds to the glass transition of the unreacted oil.10 On the other hand, soybean oil− ST−DVB polymers possess a single glass transition, indicating that the small amounts of unreacted oil mix well with the polymer main chains.12 The broad temperature regions of the glass transitions are ascribed to the segmental heterogeneities upon cross-linking. Typically, polymers with a greater amount of DVB possess higher cross-link densities, resulting in higher Tg’s. The more reactive CLS oil affords polymers that tend to have higher Tg’s than the corresponding SOY and LSS oils. The Tg’s of these soybean oil polymers have been determined to vary from 0° to 105°C, indicating that both rubbery materials and plastics are obtained. The tensile mechanical testing of the bulk polymers also exhibits characteristics typical of materials ranging from elastomeric rubbers to tough and rigid plastics. Figure 23.1 shows the tensile stress-strain curves for CLS polymers prepared by varying the DVB concentration while the total ST plus DVB concentration remains constant.13 As the DVB content increases, the resulting polymers evolve from an elastomer with ~300% elongation at break to tough plastics with yielding behavior, to a rigid plastic with breakage occurring in the linear stress-strain region. The fracture surfaces of the rubbery materials appear to be featureless, due, as expected, to the elastic recovery behavior of the stretched polymer chains. The ductile plastics clearly show evidence of a plastic deformation, even though the fracture surfaces are still relatively smooth. The rigid plastics possess very rough fracture surfaces, one of which is shown in Figure 23.2. Typically, the fracture is initiated by a flaw, which is surrounded by a slowgrowth mirror region, an area with a smooth, glossy appearance. Then a rapid-growth mist region covers the remainder of the surface. The stick-slip process in the ridges of the mist region presumably contributes in a major Copyright © 2005 by Taylor & Francis

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20

CLS45-ST47-DVB00-(NFO5-BFE3) CLS45-ST42-DVB05-(NFO5-BFE3) CLS45-ST37-DVB10-(NFO5-BFE3) CLS45-ST32-DVB15-(NFO5-BFE3) CLS45-ST27-DVB20-(NFO5-BFE3) CLS45-ST00-DVB47-(NFO5-BFE3)

Stress (MPa)

15

10 .

5

0 0

50

100

150 Strain (%)

200

250

300

FIGURE 23.1 Tensile stress-strain behavior of soybean oil polymers. (Reproduced from Li, F. and Larock, R.C., J. Polym. Sci., Part B: Polym. Phys., 39, 60, 2001. With permission of John Wiley & Sons Inc.)

way to the high tensile strength (21 MPa), the highest among all of the soybean oil-based plastics thus prepared.13 Table 23.2 summarizes the mechanical properties of the soybean oil polymers prepared by cationic copolymerization. Generally, Young’s moduli of these soybean oil polymers vary from 0.03 to 0.60 GPa. The ultimate tensile strengths vary from 0.3 to 21.0 MPa, and the elongations at break vary from 2% to 300%. The soybean oil-based plastics also possess industrially viable compressive strengths (10.5−46.5 MPa), flexural strengths (9.2−23.5 MPa), and Izod impact strengths (9.6−18.9 J/m). The CLS polymers possess higher mechanical properties than the SOY and LSS polymers with the same stoichiometries. When the amount of soybean oil is equivalent to the amount of comonomers, the resulting polymers possess the highest toughness.

23.2.4

Good Damping Properties

Damping materials find numerous applications in the aircraft, automobile, and machinery industries for both the reduction of unwanted noise and the prevention of vibration fatigue failure.37,38 Excellent viscoelastic properties of polymers make them ideally suited for use as damping materials, because of their ability to dissipate mechanical energy in the transition region from the glassy to rubbery state during dynamic mechanical analysis (vibration damping).39,40 Typically, the temperature range for efficient damping (tan δ ⬎ 0.3) of known homopolymers is 20°−30°C. Good damping materials Copyright © 2005 by Taylor & Francis

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FIGURE 23.2 SEM photographs of the tensile fracture surface of the rigid plastic CLS35–ST39−DVB18− (NFO5-BFE3). (Reproduced from Li, F. and Larock, R.C., J. Polym. Sci., Part B: Polym. Phys., 39, 60, 2001. With permission of John Wiley & Sons Inc.)

should exhibit a high loss factor (tan δ ⬎ 0.3) over a temperature range of at least 60°−80°C.41 Soybean oil−ST−DVB polymers with appropriate compositions have been shown to possess good damping properties.14 Figure 23.3 shows the loss Copyright © 2005 by Taylor & Francis

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TABLE 23.2 Summary of Soybean Oil-Based Polymers Prepared by Cationic Polymerization Property Tensile strength (MPa) Elongation at break (%) Modulus of elasticity in tension (GPa) Compressive strength (MPa)a Flexural strength (MPa)a Izod impact strength (J/m)a Coefficient of thermal expansion (10⫺5 per oC) a

0.3–21.0 2–300 0.03–0.60 10.5–46.5 9.2–23.5 9.6–18.9 9–11

Data of rubbery materials are not included.

4

SOY45-ST42-DVB05-(NFO5-BFE3) SOY45-ST37-DVB10-(NFO5-BFE3) SOY45-ST32-DVB15-(NFO5-BFE3) SOY45-ST27-DVB20-(NFO5-BFE3)

3

SOY45-ST22-DVB25-(NFO5-BFE3) SOY45-ST17-DVB30-(NFO5-BFE3)

Tan δ

SOY45-ST12-DVB35-(NFO5-BFE3) SOY45-ST07-DVB40-(NFO5-BFE3) SOY45-ST00-DVB47-(NFO5-BFE3)

2

1

0 −40

−20

0

20

40 60 80 Temperature (°C)

100

120

140

160

FIGURE 23.3 Temperature dependence of the loss tangent for the SOY polymers SOY45−(ST+DVB)47− (NFO5-BFE3) prepared by varying the DVB concentration. (Reproduced from Li, F. and Larock, R.C., Polym. Adv. Tech., 13, 436, 2002. With permission of John Wiley & Sons Inc.)

factor as a function of temperature for a series of SOY-based polymers. As the DVB content increases, the damping profiles evolve from a narrow, extremely intense damping peak to less intense, but significantly broadened, damping peaks. This is because DVB increases the degree of cross-linking, thus reducing the damping intensity to some extent and significantly broadening the damping region of the bulk materials. When the DVB constitutes 5−15 wt%, the temperature regions for efficient damping vary from 80° to 110°C, indicating the very good damping properties of the resulting materials. Copyright © 2005 by Taylor & Francis

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The tan δ area (TA), the area under the loss factor-temperature curve, represents the overall damping capacity of the material. The dependence of the TA values on the cross-link density of the various soybean oils is shown in Figure 23.4. With an increase in cross-link density, the TA value of the soybean oil polymers decreases and the damping temperature region significantly broadens simultaneously. The CLS polymers tend to possess higher TA values than the LSS and SOY polymers with the same cross-link densities. This is because the CLS oil is more reactive and a greater number of CLS fatty acid side chains are covalently bonded to the cross-linked polymer backbone, contributing more to the damping intensity, as suggested by the literature.37 Therefore, one can tailor good damping soy oil-based materials by deliberately designing their structures. The incorporation of a large number of fatty acid ester groups into the polymer chains results in high damping intensities. However, as expected, the fatty acid esters alone afford strong damping peaks over only a very narrow temperature range. Fortunately, cross-linking yields segmental inhomogeneities and thus effectively broadens the damping region. Cross-linking also simultaneously reduces the damping intensities by restricting segmental motions. Thus, the good damping properties of the soybean oil polymers result from optimal combinations of the fatty acid ester content and the degree of cross-linking. Overall, the most promising damping 140 SOY45-(ST+DVB)47-(NFO5-BFE3) LSS45-(ST+DVB)47-(NFO5-BFE3) CLS45-(ST+DVB)47-(NFO5-BFE3)

120

100

TA

80

60

40

20

0 10

100

1000

10000

100000

υe (mol/m3) FIGURE 23.4 Dependence of the TA values on the cross-linking densities of different soybean oil polymers. (Reproduced from Li, F. and Larock, R.C., Polym. Adv. Tech., 13, 436, 2002. With permission of John Wiley & Sons Inc.)

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soy-based materials show loss factor maxima (tan δ ) max of 0.8−4.3, TA values of 50−124 after correcting for the background, and temperature ranges for efficient damping of ∆T of 80°−110°C.14 23.2.5

Shape Memory Properties

In addition to good damping properties, soybean oil−ST−DVB polymers can be tailored to show typical shape memory properties.15 Shape memory refers to the ability of some materials to remember a specific shape on demand even after very severe deformations. The soybean oil polymers are composed of long polymer chains inherently interconnected by chemical crosslinks. A material with a high Tg appears to be a hard plastic at room temperature. However, it becomes an elastomer and can be readily deformed by an external force above its Tg. The majority of the deformation is retained upon being cooled back down to room temperature due to the frozen micro-Brownian motion. This temporary shape may remain “permanent” as the bulk material is in its glassy state at room temperature. However, the material returns to its original shape upon being reheated, a process driven by the strong relaxations of the oriented polymer chains between cross-links. The shape memory properties of the soybean oil polymers have been studied using a bending test. Their ability to deform as an elastomer at T ⬎ Tg, the ratio of fixed deformation to the original deformation at room temperature, and their final shape recovery upon being reheated have been determined.15 Typically, by varying the compositions of the soybean oil−ST−DVB polymers, a higher deformability at the elastomeric state (T ⬎ Tg) is always accompanied subsequently by a lower ratio of fixed deformation to the original deformation at room temperature, and vice versa. The deformability at the elastomeric state is inherently related to the cross-link density, and a relatively low degree of cross-linking is favorable for the deformation. The more rigid the polymer backbone is, the easier it is to fix the mechanical deformation at room temperature. Apparently, it is impossible to increase the rigidity of the soybean oil−ST−DVB polymer backbones and simultaneously decrease the degree of cross-linking. However, by introducing other comonomers, such as norbornadiene (NBD) and dicyclopentadiene (DCP), which are more rigid but less effective in cross-linking than DVB, one can easily achieve the appropriate combination of chain rigidity and cross-link density. The resulting polymers possess excellent processibility in the elastomeric state, being able to fix over 97% of their deformations at room temperature, and yet they completely recover their original shape upon being reheated.15 In other words, a good shape memory soy polymer should have a Tg well above room temperature and an appropriate degree of cross-linking. The shape recovery process is essentially determined by the glass transition characteristics of the shape memory polymers. In addition, the soy oil polymers are also found to possess good reusability, with a constant shape memory effect over at least seven processing cycles. Copyright © 2005 by Taylor & Francis

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Biopolymers from the Cationic Polymerization of Other Natural Oils Vegetable Oil-Based Polymers

Table 23.3 shows the compositions of other vegetable oils. These vegetable oils have a triglyceride structure very similar to soybean oil, with various fatty acid side chains predominantly composed of oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:3). The degree of unsaturation increases from peanut (PNT) to sesame (SES) to corn (COR) to sunflower (SUN) to safflower (SAF) to walnut (WNT) to linseed (LIN) oils, due primarily to differing amounts of the above mentioned three major fatty acids. The cationic copolymerization of these vegetable oils with DVB or ST plus DVB results in hard plastics similar to soybean oil polymers.42 Table 23.4 shows that vegetable oil−DVB polymers typically possess two glass transitions, i.e., α1 in a high temperature region and α2 in a low temperature region. Obviously, the polymers derived from the more unsaturated vegetable oils possess two Tg’s closer to each other. On the other hand, vegetable oil−ST− DVB polymers possess a single glass transition. Due to the broad transition region, it is difficult to identify the effect of various oils on the Tg’s. However, the cross-link densities and thermal properties of the resulting polymers are obviously dependent upon the vegetable oils employed. As the degree of unsaturation increases from PNT to LIN, the cross-link densities of the vegetable oil−DVB polymers increase from 9.8 ⫻ 10⫺5 to 2.2 ⫻ 10⫺4, and those of the vegetable oil−ST−DVB polymers increase from 1.6 ⫻ 10⫺4 to 5.1 ⫻ 10⫺4. This is expected as the more highly unsaturated vegetable oils contribute more to cross-linking. As a result, the highly unsaturated oil-based polymers TABLE 23.3 Compositions of Various Vegetable Oils

Oil PNT SES COR SUN SAF WNT LIN a b

C⫽C Numbera 3.2 3.7 4.3 4.5 4.9 5.5 5.8

Fatty Acidsb C14:0

0.1 0.01

C16:0

C18:0

C18:1

C18:2

11.0 7.3 11.5 7.0 6.7 4.4 5.5

2.3 4.4 2.2 3.3 2.7 0.9 3.5

51.0 46.0 26.6 14.3 12.9 16.9 19.1

30.9 35.2 58.7 75.4 77.5 69.7 15.3

C18:3

C20:0

0.8

0.7 0.4 0.2

3.1 56.6

0.5 0.01

C20:1

0.5

The average number of C⫽C bonds are calculated from the 1H NMR spectra. The fatty acid concentrations have been obtained from Jamieson, G.S., Vegetable Fats and Oils, Chemical Catalog Co., New York, 1932, and Salunkhe, D.K. et al., World Oilseeds: Chemistry, Technology and Utilization, Van Nostrand Reinhold, New York, 1991.

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TABLE 23.4 Characteristics of Various Vegetable Oil-Based Polymers Prepared by Cationic Polymerizationa νe (mol/m3)

Tg (°C) DVB Polymers α1 PNT SES COR SUN SAF WNT LIN a b

90 68 74 68 80 80 80

α2 ⫺10 ⫺5 ⫺4 6 —b 20 ~30b

⫹DVB ST⫹

DVB

81 73 63 64 53 52 57

9.8 ⫻ 10⫺5 9.9 ⫻ 10⫺5 1.3 ⫻ 10⫺4 1.3 ⫻ 10⫺4 1.4 ⫻ 10⫺4 2.1 ⫻ 10⫺4 2.2 ⫻ 10⫺4

T at 10% Wt Loss (°C)

Tensile Mechanical Property (ST+DVB)

Tensile Modulus strength ⫹DVB DVB ST⫹ ⫹DVB E(MPa) σb (MPa) ST⫹ 1.6 ⫻ 10⫺4 1.3 ⫻ 10⫺4 4.1 ⫻ 10⫺4 3.9 ⫻ 10⫺4 3.8 ⫻ 10⫺4 4.2 ⫻ 10⫺4 5.1 ⫻ 10⫺4

272 276 296 308 335 354 358

273 298 303 322 325 334 326

32 ⫾ 3 48 ⫾ 6 48 ⫾ 5 49 ⫾ 6 76 ⫾ 6 85 ⫾ 2 75 ⫾ 6

2.5 ⫾ 0.2 3.8 ⫾ 0.3 4.0 ⫾ 0.4 5.1 ⫾ 0.3 4.5 ⫾ 0.3 6.1 ⫾ 0.2 7.0 ⫾ 0.5

DVB or ST⫹DVB in the table represents the comonomers employed. The glass transition temperature, α2, is difficult to distinguish from a shoulder overlapping the glass transition temperature α1.

are more thermally stable, as evidenced by the temperature at 10% weight loss in the thermogravimetric analysis data shown in Table 23.4. The vegetable oil−DVB polymers are typically rigid, but relatively brittle plastics, which are fragile when making specimens for mechanical property testing. On the other hand, the vegetable oil−ST−DVB polymers appear to be hard and tough plastics, which possess good machining qualities. As expected, Young’s moduli and ultimate tensile strengths increase in a uniform manner with an increase in the unsaturation of the vegetable oil. It follows that the soybean oil cationic polymerization method is equally applicable to other vegetable oils. The characteristics of the resulting materials are predominantly determined by the degree of unsaturation of the vegetable oil. 23.3.2

Tung Oil-Based Polymers

Tung oil is readily available as a major product from the seeds of the tung tree.43 It is practically colorless in its natural state, but the commercial product is generally a light yellow color and possesses an earthy odor. Tung oil also has a triglyceride structure with three fatty acid side chains. However, unlike the vegetable oils whose fatty acid side chains are composed chiefly of oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:3), the principal acid constituent (~84%) of tung oil is α -elaeostearic acid, i.e., cis-9, trans-11, trans-13-octadecatrienoic acid, a conjugated triene.43,44 The remaining acids include 8% oleic acid, 4% linoleic acid, and 4% of a saturated acid. Proton NMR spectroscopy indicates that tung oil possesses ~7.5 C⫽C bonds per triglyceride, most of which are conjugated. Consistent with this, tung oil Copyright © 2005 by Taylor & Francis

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is substantially more reactive toward cationic polymerization than other previously mentioned vegetable oils. Indeed, viscous tung oil undergoes a very fast BFE-initiated homopolymerization at room temperature, which is very difficult to control experimentally. The tung oil forms a dark brown, irregularly-shaped solid instantly upon adding 5% BFE at room temperature. Upon cooling, tung oil becomes too viscous to readily undergo any reaction. The addition of DVB comonomer dilutes the tung oil and facilitates the cationic copolymerization at relatively low temperatures, i.e., 0°C, and thus the gel time is reduced to some extent.19 The gel time is found to reach a maximum of ~15 sec when DVB constitutes ~35− 40 wt% of the reactants. In order to better control the polymerization, the gel time is further reduced to 55 sec by decreasing the amount of BFE to 1%. After postcuring at elevated temperatures, the resulting polymers are hard and rigid plastics with a dark brown color, which appear to be similar to the previously mentioned vegetable oil-based polymers. These bulk plastics are composed of 90−95% cross-linked polymers. Proton NMR spectroscopy indicates that all of the TUN and DVB react and the rest of the materials consist of initiator fragments. The tung oil−DVB polymers possess a single Tg at around 100°C, and storage moduli of ~2.0 GPa at room temperature. These plastics are thermally stable below 200°C with 10% weight loss in air at around 430°C. The cationic copolymerization of tung oil, ST, and DVB is less vigorous than the tung oil and DVB systems. The gel time ranges from 30 sec to a few minutes, depending on the compositions employed. A variety of polymeric materials have been obtained ranging from flexible and ductile materials to hard and rigid plastics. The Tg’s of the polymers vary from 45° to 130°C, and the elastic moduli vary from 0.1 to 1.9 GPa. Compared to other vegetable oil-based polymers, tung oil polymers appear to be substantially superior in their properties, due presumably to the high reactivity of the tung oil itself.

23.3.3

Fish Oil-Based Polymers

Typically, natural fish oils also have a triglyceride structure with a substantially higher degree of unsaturation. Norway triglyceride fish oil (TFO) possesses ~10 nonconjugated C⫽C bonds per triglyceride. Unlike the vegetable oils mentioned above, the major fatty acid components of TFO are docosa-4, 7,10,13,16,19-hexaenoic acid (DHA) and eicosa-5,8,11,14,17-pentaenoic acid (EPA). Commercial Norway fish oil ethyl ester (NFO), which is derived from TFO, has ~ 3.6 nonconjugated C⫽C bonds per chain.45 Approximately 90% of the C⫽C bonds in NFO that can be conjugated have been conjugated in conjugated NFO (CFO) by a process catalyzed by Wilkinson’s catalyst.46 A unique feature of the cationic copolymerization of NFO and CFO with DVB is that the BFE initiator can be used directly with no modification.22−24 The resulting densely cross-linked materials appear to be rigid plastics with Tg’s ranging from 50° to 150°C. By partially replacing the DVB with the Copyright © 2005 by Taylor & Francis

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comonomer ST, a variety of industrially viable NFO- and CFO-based polymeric materials ranging from elastomeric rubbery materials to tough and rigid plastics are obtained.25 The Tg’s of the resulting materials range from below ambient temperature to 105°C. Typically, CFO is more reactive and results in polymers with higher mechanical properties than NFO. TFO has a triglyceride structure that contributes to cross-linking. Thus, TFO polymers possess higher mechanical properties than NFO and CFO polymers with the same compositions. Figure 23.5 shows the fracture mechanisms of a rigid NFO plastic. Basically, the fracture characteristics of the fish oil polymer are similar to the corresponding soybean oil thermoset. However, the tensile strength of the fish oil polymer is almost twice that of the soybean oil polymer. The high tensile strength of the fish oil thermoset is presumably due to the apparent plastic deformation, as evidenced by the thin layer of broken polymer fibrils in the glossy mirror region. Overall, Young’s moduli of the fish oil polymers vary from 0.02 to 0.90 GPa, the tensile strengths vary from 0.4 to 42.6 MPa, and elongations at break vary from 2% to 160%. In addition to competitive mechanical properties, the fish oil polymers also possess unique good damping and shape memory properties, just like the aforementioned soybean oil and other vegetable oil polymers. The best

FIGURE 23.5 SEM photograph of the tensile fracture surface of the rigid plastic NFO30−ST46−DVB21−BFE3. (Reproduced from Li, F., Perounoud, A., and Larock, R.C., Polymer, 42, 10133, 2001. With permission of Elsevier Science Ltd.)

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damping fish oil polymer, NFO60−ST25−DVB12−BFE3, is capable of efficient damping over a temperature range of 86°C from ⫺8° to 78°C. The best fish oil shape memory polymer, NFO49−ST38−DVB10−BFE3, exhibits an excellent ability to undergo mechanical deformation above its Tg and subsequently fix its deformation below its Tg. The polymer completely returns to its original shape upon being reheated.

23.4

Biopolymers from the Thermal Polymerization of Natural Oils

A thermal polymerization process was first used to polymerize tung oil at 200°−300°C in the beginning of the last century.47 Chin in 1950 determined that the thermal polymerization of tung oil was chiefly caused by the dimerization of elaeostearic acid.48 Subsequently, the formation of monocyclic dimeric fatty acid groups was also found in the thermal polymerization of tung oil at 200°C, and intramolecular reactions of the glycerides were determined to be of very little importance.49 Typically, the thermal polymerization of the tung oil had to be carried out at relatively high temperatures (200°− 300°C), and the resulting products have been viscous oils or weak rubbery films with little utility as structural materials. Tung oil readily copolymerizes thermally with aromatic alkenes, such as ST and DVB, to form viable polymeric materials potentially useful for structural applications. Since ST and DVB undergo thermal polymerization at temperatures as low as 100°C, the thermal copolymerization of tung oil, ST, and DVB can be carried out at relatively low temperatures. A rigid polymer was obtained in 1940 by simply heating tung oil and styrene at 125°C for 72 hrs. However, tung oil actually constituted only 0.1−2.0% of the formulation.50 Recently, a variety of new polymeric materials ranging from elastomeric rubbery materials to tough and rigid plastics have been prepared by the thermal copolymerization of tung oil, ST, and DVB with tung oil present in substantial quantities (30−70%).20 Gelation of the reactants typically occurs at temperatures higher than 140°C, and metallic catalysts have been found to accelerate the thermal polymerization. Fully cured thermosets have been obtained after postcuring at 160°C. Basically, 90−100% of the reactants are converted into cross-linked polymers. The resulting new thermosetting polymers shown in Figure 23.6 appear to be light yellow and transparent, with glossy surfaces. Table 23.5 lists the characteristics of the tung oil polymers prepared by varying the stoichiometry and metallic catalysts. These polymers possess Tg’s of ⫺2° to 116°C, cross-link densities of 1.0 × 103 to 2.5 ⫻ 104 mol/m3, coefficients of linear thermal expansion of 2.3 ⫻ 10⫺4 to 4.4 ⫻ 10⫺4 per degree centigrade, compressive moduli of 0.02 to 1.12 GPa, and compressive strengths of 8 to 144 MPa. The materials are thermally stable below 300°C and exhibit a major thermal degradation with a maximum degradation rate at 493°−506°C. Copyright © 2005 by Taylor & Francis

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FIGURE 23.6 Photograph of the tung oil bulk polymers prepared by thermal polymerization.

TABLE 23.5 Characteristics of Tung Oil Polymers Prepared by Thermal Polymerization TGA Results, oC (wt% loss)b

Compressive Test

νe (mol/m3)

CLTEa per oC

T1

TUN30-ST28-DVB42 116 TUN40-ST24-DVB36 93 TUN50-ST20-DVB30 82 TUN60-ST16-DVB24 17 and 65 TUN70-ST12-DVB18 ⫺2

2.5 ⫻104 1.2 ⫻ 104 6.1 ⫻ 103 3.0 ⫻ 103 1.0⫻ 103

2.9 ⫻ 10⫺4 2.9 ⫻ 10⫺4 3.3 ⫻ 10⫺4 3.2 ⫻10⫺4 3.9 ⫻10⫺4

502 (90.0) 499 (93.1) 500 (93.1) 496 (94.7) 489 (95.4)

632 (10.0) 619 (6.9) 608 (6.9) 617 (5.3) 610 (4.6)

1.12 0.86 0.39 0.04 0.02

144 138 89 31 8

TUN50-ST30-DVB20 TUN50-ST20-DVB30 TUN50-ST10-DVB40 TUN50-ST00-DVB50

77 82 71 65

3.5 ⫻ 103 6.1 ⫻ 103 1.7 ⫻ 104 1.5 ⫻ 104

2.2 ⫻ 10⫺4 3.3 ⫻ 10⫺4 2.7 ⫻ 10⫺4 2.9 ⫻ 10⫺4

483 (93.5) 500 (93.1) 503 (90.9) 506 (90.9)

608 (6.5) 608 (6.9) 625 (9.1) 629 (9.1)

0.17 0.39 0.46 0.54

44 89 110 132

TUN50-ST20-DVB30 TUN50-ST20-DVB30 Co TUN50-ST20-DVB30 Co⫹Ca TUN50-ST20-DVB30 Co⫹Zr TUN50-ST20-DVB30 Co⫹Ca⫹Zr

82 72

6.1 ⫻ 103 8.9 ⫻ 103

3.3 ⫻10⫺4 500 (93.1) 608 (6.9) 3.5 ⫻ 10⫺4 496 (100) —

0.39 0.43

89 88

90

8.4 ⫻ 103

2.3 ⫻ 10⫺4 496 (100)

—

0.48

86

77

8.8 ⫻ 103

4.4 ⫻ 10⫺4 497 (100)

—

0.40

71

82

1.0 ⫻ 104

4.4 ⫻ 10⫺4 499 (100)

—

0.44

77

Polymer

Tg (oC)

T2

E (GPa)

σb (MPa)

a

Coefficient of linear thermal expansion. The results outside the parentheses represent the temperatures at which the fastest weight loss rate occurs during the corresponding decomposition stage, and the results inside the parentheses represent the total weight loss during the corresponding decomposition stage. Source: Reproduced from Li, F. and Larock, R.C., Biomacromolecules, 4, 1018, 2003. With the permission of American Chemical Society.

b

Other natural oils, including linseed oil and soybean oil, can be thermally copolymerized with ST and DVB. Compared with the transparent tung oil polymers, the other vegetable oil-based polymers are white and Copyright © 2005 by Taylor & Francis

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opaque, possessing more unreacted oil and relatively low mechanical properties.42

23.5

Biopolymers from the Free Radical Polymerization of Natural Oils

Regular vegetable oils with nonconjugated C⫽C bonds do not undergo free radical polymerization initiated by common free radical initiators, such as benzoyl peroxide (BPO) and tert-butyl hydroperoxide (TBHP), to produce viable materials. However, conjugated C⫽C bonds are relatively easily attacked by free radicals in conjugated vegetable oils.52 The CLS oil has been found to undergo free radical polymerization initiated by either BPO or TBHP or combinations of these materials, and the resulting polymerized oil becomes more viscous than the original oil. Subsequently, a variety of comonomers, including ST, DVB, acrylonitrile (AN), isoprene (ISP), and methyl metha-crylate (MMA), have been copolymerized with CLS. AN affords the best looking materials to date, but there are major problems with the samples cracking. Best results have been obtained when BPO and TBHP are used in combination. Typically, upon being fully cured, the resulting polymers experience substantial shrinkage. The CLS−AN bulk polymers are yellow transparent hard plastics, which become darker at high curing temperatures. These materials are essentially completely cross-linked random copolymers. No unreacted starting materials or oligomers have been identified by Soxhlet extraction using methylene chloride as the refluxing solvent. These materials are relatively thermally stable below 300°C and lose 5% of their weight at around 350°−420°C. The Tg’s of the materials vary from 60° to 125°C, and the room temperature storage moduli vary from 0.7 to 3.7 GPa.

23.6

Biopolymers from the Free Radical Polymerization of Natural Oil Derivatives

Due to the low reactivity of nonconjugated C⫽C bonds, a number of different approaches have been employed to modify the C⫽C bonds of natural oils in order to produce materials that will more readily undergo free radical polymerization. One approach discussed earlier has been to move the C⫽C bonds into conjugation. Subsequent polymerization generates more stable allylic free radicals. This section deals with the replacement of vegetable oil C⫽C bonds with other more reactive C⫽C bonds through chemical modifications.27−29 Scheme 23.3 shows one approach used to prepare more reactive Copyright © 2005 by Taylor & Francis

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CH

CH

CO2

HO

CH

CH

CO2

+ HO

CH

CH

CO2

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CH

CO2

230−240°C Catalyst

HO

H

O C C O C C

< 100°C

HO

+

HO

H O Maleic anhydride

Monoglyceride

Glycerol

Triglyceride

CH

O CH

CH

CO2

HO2 C

CH

CH

CO2

HO2 C

CH

CH

O

CO2

Monoglyceride bis-maleate half ester

C CH 2 CH

m

CH

CH

CH

O

O

Initiator

C

O CH n CO2H

C CH

O CH n CH2 CO2H

CH

m

SCHEME 23.3 (Reproduced from Can, E., Kusefoglu, S., and Wool, R.P., J. Appl. Polym. Sci., 81, 69, 2001. With permission of John Wiley & Sons Inc.)

vegetable oil derivatives for subsequent free radical polymerization.28 The monoglycerides prepared by the transesterification of natural oil triglycerides are esterified with maleic anhydride to form half esters, and the resulting polyfunctional monoglyceride bis-maleate half ester is free radically copolymerized with a reactive diluent, such as styrene. A network polymer with a high degree of connectivity is formed. Soybean oil monoglyceride bis-maleate half esters (SOMG maleates) have been prepared and subsequently copolymerized with styrene (ST).27−29 SOMG maleates themselves are viscous, but soluble in ST. A mixture of SOMG maleates with 30% ST affords reactant mixtures with viscosities around 475 cps, a value acceptable to manufacturers. The reactant mixtures can be polymerized at room temperature or at higher temperatures using the well-established catalyst system MEKP/Conaphthenate/BPO. The cured product is a straw-colored, clear, rigid solid. Solvent extraction and swelling experiments have proven the cross-linked nature of the product. Chloroform and dimethylsulfoxide solvents swell and fragment the product, but do not extract any unreacted monomers or oligomers. Preliminary tests show very useful properties for these products. The Tg determined by DSC can be as high as 135°C. The dynamic flexural modulus as determined by DMA is 0.9 GPa at 35°C and the surface hardness is 72 Shore (D). The polymer possesses a tensile strength of 29.4 MPa and Young’s modulus of 0.84 GPa.28 When other more rigid diols including neopentyl glycol (NPG) and bisphenol A (BPA) are added to the maleinization mixture, a more rigid polymer backbone is obtained.29 The polymer based on SOMG/NPG maleates and ST possesses a Tg of 145°C, a tensile strength of 15.6 MPa, and an increased Young’s modulus of 1.5 GPa, and the polymer based on SOMG/BPA maleates and ST possesses a Tg of 131°C and a Young’s modulus of 1.3 GPa. Copyright © 2005 by Taylor & Francis

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Potential Applications

Recent work on soy oil-based polymers reveals that in most cases the specific mechanical properties of the biopolymers are comparable to widely used plastics, including polyethylene, polystyrene, and unsaturated polyesters. Various complex structures, such as tubes, panels, and automotive parts, that are currently made from petroleum-based plastics might be replaced by these soy oil plastics. The soy resins can also be fabricated into biocomposites with high strength fillers.27 These biocomposites possess much higher mechanical properties and thus may find applications where engineering plastics are currently being widely used. The good damping properties of some of the soy oil biopolymers may greatly broaden their applications. Damping materials, which reduce unwanted noise and prevent vibration fatigue failure, have found numerous applications in the aircraft, automobile, and machinery industries.37 There are many conventional polymers presently being used commercially for damping, including asphaltics, polyurethanes, polyvinyl acetate, acrylics, natural rubber, SBR, and silicone rubber. Multicomponent polymeric materials, such as interpenetrating polymer networks (IPNs), constitute a new class of advanced damping materials.37 These IPNs are capable of efficient damping over wide temperature and frequency ranges. The good damping soy oil polymers show damping characteristics similar to the IPNs.14,17 Shape memory polymers, such as heat-shrinkable tubing and films, have been widely used in cable and wire as well as the medical and packaging industries.53 Due to their low cost, easy processibility, and large variations in starting materials, shape memory polymers also possess great potential as materials for civil engineering, machinery manufacturing, electronics and communications, household materials, etc. Shape memory soy oil plastics are a new class of shape memory materials. More importantly, these materials are the first shape memory polymers prepared from renewable natural resources. Thus, the new soy oil plastics have certain advantages over petroleum-based shape memory polymers, and have definitely broadened the variety and applications of these functional materials.

23.8

Conclusions

Soybean oil and other natural oil triglycerides have been employed for a long time as a feedstock for paints, coatings, lubricants, etc. In recent years, important advances in the synthesis of structural polymeric materials from these natural oils have been made. Recent advances in cationic, thermal, and free radical polymerization have made it possible to readily polymerize a wide variety of natural oil triglycerides and their fatty acids under mild conditions. To date, Copyright © 2005 by Taylor & Francis

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only a very limited number of comonomers have been employed in these copolymerizations. There are a large number of petroleum-derived monomers available as potential comonomers for the copolymerization of natural oils. The addition of such comonomers reduces the renewable resource content of the final resins, but can result in higher thermal and mechanical properties and thus significantly broaden the variety and applications of the products. However, some synthetic problems remain. For example, shrinkage during free radical polymerization and the replacement of expensive comonomers are a couple of current problems which require solutions. Meanwhile, the commercialization of these new technologies challenges chemists and engineers, since it not only involves technical problems, but also involves economic and political issues. Fortunately, some natural oilbased polymers possess conventional, industrially useful characteristics as well as unique properties, such as good damping and shape memory properties. As this technology becomes more economical and industrially feasible and the more unique aspects of these natural oil-based polymers become clear, the use of renewable raw materials as starting materials is likely to become a major thrust of the polymer industry in the years to come.

Acknowledgments The research reported in this chapter were in part supported by the Iowa Soybean Promotion Board, the Iowa Energy Center, and the USDA.

References 1. Kaplan, D.L., Introduction to biopolymers from renewable resources, in Biopolymers from Renewable Resources, Kaplan, D.L., Ed., Springer, New York, 1998, chap. 1. 2. Bogdansky, S., Chasin, M., and Langer, R., Biodegradable Polymers as Drug Delivery Systems, Marcel Dekker, New York, 1990. 3. Byrom, D., Biomaterials—Novel Materials from Biological Sources, Stockton Press, New York, 1992, p. 1. 4. Kaplan, D.L. et al., Biosynthetic polysaccharides, in Biomedical Polymers: Designed to Degrade Systems, Shalaby, S., Ed., Carl Hanser, New York, 1994, chap. 8. 5. Jamieson, G.S., Vegetable Fats and Oils, The Chemical Catalog Company, Inc., New York, 1932. 6. Salunkhe, D.K. et al., World Oilseeds: Chemistry, Technology and Utilization, Van Nostrand Reinhold, New York, 1991. 7. Fehr, W.R., Soybean, in Oil Crops of the World: Their Breeding and Utilization, Robbelen, G., Downey, R.K., and Ashri, A., Eds., McGraw-Hill, New York, 1989, chap. 13, p. 283.

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8. Gunstone, F.D., Industrial Uses of Soybean Oil for Tomorrow, Special Report ‘96, Iowa State University and the Iowa Soybean Promotion Board, 1995. 9. Larock, R.C. and Hanson, M.W., Lewis Acid–Catalyzed Polymerization of Biological Oils and Resulting Polymeric Materials, U.S. Patent 6,211,315, issued 4/03/01. 10. Li, F., Hanson, M.W., and Larock, R.C., Soybean oil–divinylbenzene thermosetting polymers: synthesis, structure, properties and their relationships, Polymer, 42, 1567, 2001. 11. Li, F. and Larock, R.C., New soybean oil–styrene–divinylbenzene thermosetting copolymers. I: synthesis and characterization, J. Appl. Polym. Sci.,80, 658, 2001. 12. Li, F. and Larock, R.C., New soybean oil–styrene–divinylbenzene thermosetting copolymers. II: dynamic mechanical properties, J. Polym. Sci., Part B: Polym. Phys., 38, 2721, 2000. 13. Li, F. and Larock, R.C., New soybean oil–styrene–divinylbenzene thermosetting copolymers. III: tensile stress–strain behavior, J. Polym. Sci., Part B: Polym. Phys., 39, 60, 2001. 14. Li, F. and Larock, R.C., New soybean oil–styrene–divinylbenzene thermosetting polymers. IV: good damping properties, Polym. Adv. Tech., 13, 436, 2002. 15. Li, F. and Larock, R.C., New soybean oil–styrene–divinylbenzene thermosetting polymers. V: shape memory effect, J. Appl. Polym. Sci., 84, 1533, 2002. 16. Li, F. and Larock, R.C., New soybean oil–styrene–divinylbenzene thermosetting copolymers. VI: time–temperature–transformation cure diagram and the effect of curing conditions on the thermosets properties, Polym. Int., 52, 126, 2002. 17. Li, F. and Larock, R.C., Novel polymeric materials from biological oils, Polym. Mater. Sci. Eng., 86, 379, 2002. 18. Li, F. and Larock, R.C., Novel polymeric materials from biological oils, J., Polym. Environ., 10, 59, 2002. 19. Li, F. and Larock, R.C., Thermosetting polymers from the cationic copolymerization of tung oil: synthesis and characterization, J. Appl. Polym. Sci., 78, 1044, 2000. 20. Li, F. and Larock, R.C., Synthesis, structure, thermophysical and mechanical properties of new tung oil–styrene–divinylbenzene copolymers prepared by thermal polymerization, Biomacromolecules, 4, 1018, 2003. 21. Li, F., Hasjim, J., and Larock, R.C., Synthesis, structure, thermophysical and mechanical properties of new polymers by the cationic copolymerization of corn oil, styrene and divinylbenzene, J. Appl. Polym. Sci., 90, 1830, 2003. 22. Li, F., Marks, D., Larock, R.C., and Otaigbe, J.U., Fish oil polymeric system: synthesis, structure, properties and their relationships, SPE ANTEC Techn. Pap., 3, 3821, 1999. 23. Li, F., Marks, D., Larock, R.C., and Otaigbe, J.U., Fish oil thermosetting polymer: synthesis, structure, properties and their relationships, Polymer, 41, 7925, 2000. 24. Li, F., Larock, R.C., and Otaigbe, J.U., Fish oil thermosetting polymers: creep and recovery behavior, Polymer, 41, 4849, 2000. 25. Li, F., Perounoud, A., and Larock, R.C., Thermophysical and mechanical properties of novel thermosets prepared from lewis acid–initiated cationic copolymerization of fish oils, styrene and divinylbenzene, Polymer, 42, 10133, 2001. 26. Marks, D., Li, F., and Larock, R.C., Synthesis of thermoset plastics by lewis acid initiated polymerization of fish oil ethyl ester and alkenes, J. Appl. Polym. Sci., 81, 2001, 2001.

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27. Khot, S.N. et al., Development and application of triglyceride–based polymers and composites, J. Appl. Polym. Sci., 82, 703, 2001. 28. Can, E., Kusefoglu, S., and Wool, R.P., Rigid thermosetting liquid molding resins from renewable resources. I: synthesis and polymerization of soybean oil monoglyceride maleates, J. Appl. Polym. Sci., 81, 69, 2001. 29. Can, E., Kusefoglu, S., and Wool, R.P., Rigid thermosetting liquid molding resins from renewable resources. II: copolymers of soybean oil monoglyceride maleates with neopentyl glycol and bisphenol a maleates, J. Appl. Polym. Sci., 83, 972, 2002. 30. Petrovic, Z.S., Guo, A., and Zhang, W., Structure and properties of polyurethanes based on halogenated and nonhalogenated soy–polyols, J. Polym. Sci., Part A: Polym. Chem., 38, 4062, 2000. 31. Janvi, I. et al., Thermal stability of polyurethanes based on vegetable oils, J. Appl. Polym. Sci., 77, 1723, 2000. 32. Guo, A., Janvi, I., and Petrovic, Z.S., Rigid polyurethane foams based on soybean oil, J. Appl. Polym. Sci., 77, 467, 2000. 33. Guo. A. et al., Polyols and polyurethanes from hydroformylation of soybean oil, J. Polym. Environ., 10, 49, 2002. 34. Petrovic, Z.S et al., Effect of OH/NCO molar ratio on properties of soy–based polyurethane networks, J. Polym. Environ., 10, 5, 2002. 35. Larock, R.C. et al., Preparation of conjugated soybean oil and other natural oils and fatty acids by homogeneous transition metal catalysts, J. Am. Oil Chem. Soc., 78, 447, 2001. 36. Kennedy, J.P. and Marechal, E., Carbocationic Polymerization, Wiley, New York, 1982, pp. 31–55. 37. Sperling, L.H., Sound and vibration damping with polymers: basic viscoelastic definitions and concepts, in Sound and Vibration Damping with Polymers, Corsaro, R.D. and Sperling, L.H., Eds., ACS Symposium Series No. 424, American Chemical Society, Washington, DC, 1990, chap. 1. 38. Sperling, L.H., in Interpenetrating Polymer Networks, Klempner, D., Sperling, L.H., Utracki, L.A., Eds., ACS Symp. Ser. No. 239, American Chemical Society, Washington, DC, 1994, p. 1. 39. Aklonis, J.J. and MacKnight, W.J., Introduction to Polymer Viscoelasticity, 2nd Ed., Wiley–Interscience, New York, 1983. 40. Murayama, T., Dynamic Mechanical Analysis of Polymeric Materials, Elsevier, Amsterdam, 1978. 41. Yao, S., Means to widen the temperature range of high damping behavior by IPN formation, in Advances in Interpenetrating Polymer Networks, Vol. IV, Klempner, D. and Frisch, K.C., Eds., Technomic Publishing Company, Lancaster, PA, 1994. 42. Li, F. and Larock, R.C., unpublished data, 2003. 43. Wood, E.C., Tung Oil: A New American Industry, U.S. Government Printing Office, Washington, DC, 1949. 44. Kinabrew, R.G., Tung Oil in Mississippi: The Competitive Position of the Industry, University of Mississippi, MS, 1952. 45. Technology International Exchange, Inc., Certificate of Analysis for Pronova Fish Oil, Pronova Biocare, 1996, p. 6. 46. Marks, D.W., MS thesis, Iowa State University, 1998. 47. Mazume, T. and Nagao, S., Heat polymerization of tung oil and ethyl ester of elaeostearic acid, J. Chem. Soc. Jpn., Ind. Chem. Sect., 31, 473, 1928.

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48. Chin, C., Tung Oil. II. Polymerization of tung oil, J. Chem. Soc. Jpn., Ind. Chem. Sect., 53, 281, 1950. 49. Boelhouwer, C., Klaassen, W.A., and Waterman, H.I., Thermal polymerization of tung oil, Res. Correspondence, Suppl. to Research (London), 7, S62, 1954. 50. Stoesser, S.M. and Gabel, A.R., Styrene–Tung Oil Copolymer, U.S. Patent 2,190,906, 1940. 51. Zhang, L., MS thesis, Iowa State University, 1999. 52. Lendlein, A. and Kelch, S., Shape–memory polymers, Angew. Chem., Int. Ed. Engl., 41, 2034, 2002.

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24 Houses Using Soy Oil and Natural Fibers Biocomposites

Mahmoud A. Dweib, Annmarie O’Donnell, Richard P. Wool, Bo Hu, and Harry W. Shenton III

CONTENTS 24.1 Introduction and Background 24.2 Materials 24.2.1 Soy Oil-Based Resin 24.2.2 Natural Fiber Mats 24.3 Foams 24.4 Composite Processing and Manufacturing: Natural Composite Panels 24.4.1 Manufacturing Using VARTM 24.4.2 Permeability Measurements 24.4.3 Dynamic Mechanical Analysis (DMA) 24.4.4 Effect of Fiber Content 24.5 Applications: Housing Construction Material 24.5.1 Introduction 24.5.2 Beam Design 24.5.3 Structural Composite Manufacturing Using VARTM 24.5.4 Four-Point Bending Test and Results 24.6 Other Potential Applications 24.6.1 Stay-in-Place Bridge Decking Form 24.6.2 Furniture Application 24.7 Conclusions References ABSTRACT Room temperature curing of an acrylated epoxidized soybean oil (AESO) resin gave a flexural modulus of 1.1 GPa. Natural fiber reinforcement of 20 to 50 wt% fiber increased the flexural modulus to 2–5 GPa. The same resin reinforced with woven E-glass gave a flexural modulus of about 17 GPa.

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Applications such as monolithic hurricane-resistant house roofs, stay-in-place bridge decking forms, and furniture were successfully tested. As part of developing an all-natural composite roof for housing applications, structural panels and unit beams were manufactured with soybean oil-based resin and natural fibers (flax, cellulose, pulp, recycled paper, chicken feathers) using vacuumassisted resin transfer molding (VARTM) technology. Physical and chemical investigations and mechanical testing of the beams yielded good results in line with the desired structural performance. Foam core composites were infused in three dimensions and cured at room temperature. Recycled paper from corrugated cardboard boxes was successfully used and superseded other natural fibers in strength and stiffness. Results show that the recycled paper beam with chicken feathers and the recycled paper beam with corrugated cardboard have flexural rigidities comparable to a cedar 2-by-4 wood sample. The beam with some E-glass is as stiff as all three of the wood members. On the other hand, all the composite beams are stronger than the weakest wood members, and in most cases, the composite beams have strengths nearly equal to or exceeding that of the strongest wood members. The comparison shows that the composite beams made from recycled paper have strengths and stiffness that make them suitable for use in structural applications where wood members would normally be used. Porous fiber mats such as chicken feathers, flax, and a corrugated forms of the recycled paper were used to provide flow channels for the resin, since the recycled paper is very compact and flow through the beam web was very slow. Woven E-glass was also used in a very small quantity along with the recycled paper to provide resin flow channels and additional ductility that keeps the beam together after failure. Using this type of composite in building construction introduces many advantages such as high strength and stiffness to weight, survivability in severe weather conditions, desired ductility, fatigue resistance, and design flexibility (three-dimensional forms, molded in place, easy to install, and structure replacement). A biobased (natural and biodegradable) matrix reinforced by natural fibers also provides an important environmental advantage, as renewable resources are used instead of petroleum-based materials.

24.1

Introduction and Background

A new global awareness of the depletion of natural resources has led legislators and scientists to work collectively toward developing environmentally sound alternatives to traditional structural materials. The extensive use and disposal of large quantities of these conventional structural materials typically composed of phenolic, epoxy, or polyester resins reinforced with glass, carbon, or aramid fibers present quite a challenge. Recent advances in the area of biobased materials and natural fiber composites provide significant evidence for the use and replacement of these conventional materials in everyday products. These Copyright © 2005 by Taylor & Francis

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composites are typically composed of natural or synthetic resins reinforced with natural fibers. The ACRES (Affordable Composites from Renewable Sources) group at the University of Delaware has been actively involved in developing biobased thermosetting resins from plant oils, such as soybean, as comparably strong alternatives to the conventional petroleum-based resins.1–4 In addition, when low-density natural fibers are imbedded into the natural oilbased polymeric matrices, the specific properties of the composite improve significantly.5–8 Most natural fibers are significantly lighter in weight and cheaper than synthetic fibers in terms of availability and energy consumed in the fibers’ manufacturing process. They are also easy to process, are rapidly renewable, typically on a semiannual basis, and reduce our dependency on foreign and domestic petroleum oil. Natural oils and fibers are indigenous to many countries, and this green engineering technology can be readily applied universally to a broad spectrum of materials applications in developing nations. Despite the environmental and cost advantages of these new green materials, relatively few high-volume applications of these environmentally conscious materials have been developed. Therefore, these materials have yet to compete economically with the traditional plastics market. Recent advances in the use of natural fibers (e.g., flax, cellulose, jute, hemp, straw, switch grass, kenaf, coir, and bamboo) in composites have been reviewed by several authors.9–28 Bledzki and Gassan9 reported that natural fibers were used as early as 1908 in the fabrication of large quantities of sheet material, where paper or cotton was used to reinforce sheets made of phenol- or melamineformaldehyde resins. Hemp fibers were the world’s largest agricultural crop in the early nineteenth century, but the demand for the material has declined with advances in the field of synthetic fibers. Recently, natural fibers have been used to reinforce traditional thermoplastic polymers in automotive applications.9 Polypropylene has often been used as the matrix material. The influence of surface treatments of natural fibers on interfacial characteristics was also studied by several authors22–26 and was reviewed by Mohanty et al.25 Global environmental issues have led to renewed interest in biobased materials with the focus on renewable raw materials that can be made recyclable or biodegradable at a reasonable cost. In this current work by the ACRES group at the University of Delaware, new natural fiber mats were used to reinforce natural resin from plant oil for large volume applications such as building construction materials, roofs, house walls, stay-in-place forms for bridge decks, and furniture.

24.2 24.2.1

Materials Soy Oil-Based Resin

Extensive research has been devoted to the development of polymers from triglyceride oils as a natural alternative to petroleum-based polymers.1–4 Copyright © 2005 by Taylor & Francis

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Triglyceride oils are found in abundance all over the world and are derived from both plant and animal sources. Soybean oil is mainly composed of triglyceride molecules. Each triglyceride contains three fatty acid chains joined by a glycerol center, as shown in Figure 24.1. The fatty acid chains have 0-3 double bonds and vary in length from 16 to 22 carbon atoms. In order to produce a rigid cross-linked thermoset, the triglycerides must be chemically functionalized.1–4 The preferred degree of functionalization for composite resins is from 4 to 6 groups per triglyceride. Several types of functionalization can be obtained at various active sites within the triglyceride structure: the double bond, the ester group, the allylic carbons, and the carbons alpha to the ester group. Various chemical pathways for functionalization of these triglycerides have been studied. The selection of plant oil (soy, corn, linseed, sunflower, genetically engineered high oleic, etc.), the optimization of the chemical functionalization, and its effect on the mechanical and thermal properties of the resin have been recently analyzed by LaScala and Wool.29 The viscoelastic properties of resin samples made of acrylated epoxidized soybean oil (AESO) and cured at room temperature with varying amounts of styrene were studied. Styrene is a minor component of the soy oil-based resin. Since it is derived from petroleum and has considerable volatile emissions, it was necessary to optimize the amount of styrene blended. It was found that both the storage modulus E⬘ and glass transition temperature Tg increased

Glycerol O

O

Fatty acid chain

O

O O

O

0-6 Double bonds

12-22 Carbons long

1. Formic acid H2O2 2. Acrylic acid catalyst (AMC2)

Acrylate

O

O O

O O

O

OH

O

O O

Acrylated epoxidized soybean oil

FIGURE 24.1 Chemical structure and reaction leading from triglyceride oil to AESO.

Copyright © 2005 by Taylor & Francis

O OH

O

O

O

Epoxy

OH

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with increasing styrene content in the resin system. The styrene content at 33.3 wt%, or a 2:1 AESO/styrene ratio, was considered optimal, giving maximum properties at minimal styrene content. At 33.3 wt% styrene content, the storage modulus was 1.1 GPa with a loss modulus E” of 193 MPa and a glass transition temperature of 66ºC. The neat resin properties can be further improved with higher levels of acrylation and the addition of maleic anhydride to the hydroxyl groups on the triglycerides.1–5 The reaction kinetics of this optimized resin system were studied using Fourier transform infrared spectroscopy (FTIR). From this study, the autocatalytic model provided a reasonable fit of the kinetic data. Typically, the ultimate conversion of styrene in the system was determined to be 0.72 while the ultimate conversion of acrylates was 0.89. 24.2.2

Natural Fiber Mats

Natural fibers serve as the low-cost reinforcement of the resin in biocomposites, improving both the strength and stiffness of the resulting composites. Natural fibers are typically grouped into four different types: leaf, bast, fruit, and seed, depending on their source. The leaf and bast fibers are generally used in composite processing. Examples of leaf fiber include sisal, henequen, and pineapple leaf fiber (PALF). Bast fiber examples are flax, hemp, ramie, cellulose, and jute. One of the major difficulties of natural fibers is that their properties are intrinsically dependent on where they are grown (locality), what part of the plant they are harvested from (leaf or stem), the maturity of the plant (age), and how the fibers are harvested and preconditioned in the form of mats or chopped fibers, woven or unwoven. These factors result in significant variation in properties compared to their synthetic fiber counterparts (glass, aramid, and carbon). Figure 24.2 shows images of some natural fibers and Table 24.1 presents all the different natural fibers used in this study, including descriptions of the fiber mats in terms of contents and processing conditions.

24.3

Foams

Elfoam, T300, manufactured by Elliott Company, Indianapolis, IN, was used as the foam core in manufacturing the structural composites in this study. The foam is a closed-cell polyisocyanurate that is chemically similar to, but higher performing than polyurethane foam. Outstanding thermal and moisture resistance make this foam a highly effective insulating material. The foam is widely used in composite sandwich construction due to its light weight. T300 foam has a density of 44.85 kg/m3 (2.8 lb/ft3) and operating temperature range from ⫺297° to ⫹ 300°C.

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FIGURE 24.2 (a,b) Flax mat; (c) cellulose mat; (d) jute; (e) hemp; (f) chicken feathers mat.

24.4

24.4.1

Composite Processing and Manufacturing: Natural Composite Panels Manufacturing Using VARTM

The VARTM (vacuum-assisted resin transfer molding) process is a variant of vacuum-infusion RTM in which one of the solid tool faces is replaced by a flexible polymeric film (Figure 24.3). This process, or a modified version, has Copyright © 2005 by Taylor & Francis

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TABLE 24.1 Natural Fiber Reinforcements used in AESO Composites Chart Reference Flax/PET 40/40 Flax mat 60/40 Flax mat 85/15 20 oz Cellulose (200 g/m2) CTMP pulp Fluff pulp Chemically-treated pulp Caustic-treated pulp Cellulose (150 g/m2) Flaxcraft hemp Flaxtech flax Newspaper Recycled paper Chicken feathers mat

Description 40% Flax, 40% PET, 20% starch binder, supplied by Cargill Ltd. 60% Flax, 40% binder, supplied by Cargill Ltd. 85% Flax, 15% binder 20 oz., supplied by Cargill Ltd. Air laid 200 g/m2, 84% cellulose, 16% binder, supplied by Concert Fabrication, Canada Chemical thermal mechanical pulp, supplied by M&J Fibretech a/s, Denmark Wet laid fluff pulp low-density mat, 100% cellulose 640 g/m2, supplied by Rayonier Chemically treated pulp contains a hydrophobic debonder, supplied by Rayonier Caustic-treated pulp, 100% cellulose, high-porosity mat used for filtration products, supplied by Rayonier Air laid 150 g/m2, 82% cellulose, 18% binder, supplied by Concert Fabrication, Canada 550 g/m2 nonwoven hemp, supplied by Flaxcraft Flax distribution of flax fibers with varying lengths and low binder content, supplied by Flaxtech Newspaper 110 g/m2 recycled paper from cardboard boxes, supplied by Interstate Resources, PA, USA 97% chicken feathers ground to about 6 µm diameter and 8 mm long fibers with 3% low molecular weight polymeric binder, solid feather density is 0.8 g/cm3 40

also been called SCRIMP.30 The VARTM process is a very clean and economical manufacturing method. The process draws resin into the dry reinforcement on a vacuum bagged tool using only the partial vacuum to drive the resin (Figure 24.3). The process increases the component mechanical properties and fiber content by reducing void percentage when compared to other large-part manufacturing processes such as hand layup.31 For higher performance composites, VARTM offers the potential for reduced tooling costs where matched tooling is being used, such as that used in RTM or compression molding. As one of the tool faces is flexible, the molded laminate thickness depends in part on the compressibility of the fiber–resin composite before curing and the vacuum’s negative pressure. Recent advances in room-temperature VARTM processing, large-scale process automation, and flow modeling behavior have been reported at this laboratory.32–34 Composite panels were manufactured from AESO and various natural fiber reinforcements using the VARTM process. The composite panel specimens were manufactured with the dimensions 30.5 ⫻ 30.5 ⫻ 0.635 cm (12 ⫻ 12 ⫻ 0.25 in.). The preform is vacuum bagged on a one-sided mold, as shown in Figure 24.3, and the resin is drawn into the preform under the Copyright © 2005 by Taylor & Francis

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FIGURE 24.3 Schematic showing vacuum-assisted resin transfer molding process and resin flow into the fiberbed.

negative pressure created by the vacuum. The resin is cured at room temperature and gels after ~3–5 h. However, the panel specimens were left under vacuum overnight to improve consolidation and then demolded. Full vacuum, if reached, is equivalent to atmospheric pressure (14.7 psi or 101.3 kPa). This level of pressure may seem to be low, but it is very significant when parts with a large surface area (such as a house roof) are molded compared with the same pressure to be achieved mechanically using a body force. Bagging a large part like a structural panel and applying vacuum at one side gives a uniform negative pressure over the whole part equivalent to about 100 kPa; exerting this pressure over a 7 ⫻10 m roof panel would require a body force of about 700,000 kg. 24.4.2

Permeability Measurements

In order to manufacture large composite structures using VARTM, resin permeability through the fiber mats becomes a very important parameter that dictates the processing and quality of the final parts. Permeability is defined as the volume of a fluid of unit viscosity passing through a unit cross section of the medium in unit time under the action of a unit pressure gradient. It is a constant determined by the structure of the medium. The natural fiber mats used in this work can have random or oriented fibers, with or without Copyright © 2005 by Taylor & Francis

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binders; they can be processed using an air-laid or wet-laid process and the fiber length can be varied. Mat permeability plays a key role in determining fiber content of the resulting composite. Permeability is used in characterizing the flow behavior of a resin as it impregnates a preform or fiber bed. Darcy’s law is commonly used to model the fluid flow through a porous media and to predict permeability parameters.35 Darcy’s law in 1-D describes the average velocity, u 苶 (m/s), and can be written as K dP u⫽ ᎏ * ᎏ 苶 µ dx

(24.1)

where K is the permeability parameter, m2, µ is the viscosity of the injected fluid (resin), Pa.s, P is pressure, Pa, and x is flow direction, m. At constant flow rate, Q (m3/s), the unsaturated permeability in a fiber bed with known dimensions and fiber content, can be written as 1 Q 2 µ Kuns⫽ ᎏ * ᎏ * ᎏ A (1⫺Vf) dP冫dt

冢 冣

(24.2)

where Kuns is the unsaturated permeability parameter, m2, A is the crosssectional area of the mold cavity, m2, Vf is the fiber volume fraction in the cavity, µ is viscosity of the injected fluid (resin), Pa.s, and dP/dt is the pressure change rate, Pa/s. This law describes the macroscopic relationship between the flow rate, Q, and the pressure change rate, dP/dt, rather than using the momentum conservation equations that require more detailed information concerning the geometry of every channel present in a fiber bed. Therefore, Darcy’s law accounts for the average ease of flow through these channels via the permeability parameter, K, which characterizes the flow through the fiber porous medium. Table 24.2 shows permeability readings for some of the natural fibers used in this study. 24.4.3

Dynamic Mechanical Analysis (DMA)

Composite panels were made out of the 13 different fiber mats listed in Table 24.1. The storage modulus, E⬘, the loss modulus, E⬙, and the glass transition temperature, Tg, were measured at a temperature range of 35.0°–150.0°C for the various room temperature cured AESO natural fiber TABLE 24.2 Permeability Measurements Fiber Mat

Fiber Volume Fraction (%)

Unsaturated Permeability (m2)

42.3 31.0 18.3 29.2

3.60 ⫻ 10⫺13 1.02 ⫻ 10⫺10 6.00 ⫻ 10⫺10 3.12 ⫻ 10⫺10

Recycled paper Flax mat 85/15 20 oz Cellulose 200 Chemically treated pulp

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composites. The glass transition temperature was obtained from the maximum point of the tan δ curve. The storage modulus E⬘ of the neat resin was 1.1 Gpa, and with natural fiber reinforcements, E⬘ increased to more than 5 GPa at ~ 50 wt% fiber. The highest E⬘ values were obtained for cellulose derived from newspaper or recycled paper. Significantly, the recycled paper was the cheapest of all the natural fibers examined in this work and is therefore an excellent candidate for use in high-volume large structures, such as houses. Values of the loss modulus were observed at two different temperatures corresponding to a temperature of 37°C and also at the temperature at which the loss modulus achieves its maximum value. Similar behavior was observed for the loss modulus; the neat resin had a loss modulus of ~190 MPa, and with fiber reinforcement it increased to ~ 430 MPa. The structural or material damping of a composite material may also be analyzed using DMA testing. Tan δ is the ratio of the loss modulus to storage modulus, or the ratio of the energy lost to the energy retained during a loading cycle. Tan δ values were measured at 37°C and also at its maximum value (the glass transition temperature). The most significant result was obtained from the cellulose composites with a maximum tan δ of ~0.3. This result indicates that natural fiber- (cellulose-based) reinforced composites have good structural damping properties that could also be used in the automotive industry. Since the main objective of this study is utilizing natural and low-cost products to make composite materials, recycled paper from corrugated cardboard boxes was considered a cheap source for cellulose fiber. Recycled newspaper was initially considered and tested, and despite the flow problem associated with making large parts using newspaper, the resulting composites exhibited very good mechanical properties at the lab scale. The positive experience with recycled newspaper directed our attention to a more porous recycled (cardboard) paper, which showed no flow problems, and the resin effectively infiltrated and bonded into the paper bed. Mechanical testing showed that these natural composite materials are suitable for housing applications such as roofs, walls, and construction lumber. Due to their reduced weight, environmental survivability, and noise suppression, several automotive applications can also be considered. Lastly, the glass transition temperature of the room temperature cured AESO was 66°C and was relatively unaffected by the fiber reinforcement. This value can be further improved by changing the curing conditions of the resin (i.e., high-temperature curing or postcuring), by altering the chemical structure of the resin, or by improving the adhesion between the fiber and the matrix. LaScala and Wool29 found that Tg is proportional to the level of chemical functionalization of the oil, and this has the largest impact on Tg modification. Future work will also focus on studying the interface between the resin and the natural fiber and studying possible sizing agents that might help improve interface properties. Table 24.3 summarizes the DMA results at 37°C. Copyright © 2005 by Taylor & Francis

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TABLE 24.3 Summary of Dynamic Mechanical Analysis Data According to Fiber Mat wt% for Various Room Temperature Cured AESO Composites Composite Reinforcement/AESO Pure AESO resin Cellulose 200 Cellulose 150 Flax mat 85/15 20 oz Flax mat CCF200C FlaxTech flax Flax mat 60/40 Rayfloc XJE Rayfloc JLDE CTMP pulp

Recycled paper

Porosanier JHP Flaxcraft hemp News journal E-Glass fiber

Description

Fiber Mat (wt%)

Neat resin Cellulose, 200 g, by Concert, Canada Cellulose 150 g, by Concert, Canada Flax mat 85/15 20 oz, by Cargill, Durafiber Flax/PET 40/40, by Cargill, Durafiber Flax supplied FlaxTech Flax mat 60/40, by Cargill, Durafiber Chemically treated pulp, by Rayonier Fluff pulp, by Rayonier Chemical thermal mechanical pulp, by M&J Fibretech a/s, Denmark Recycled corrugated cardboard, by Interstate Resources, PA, USA Caustic treated pulp, by Rayonier Hemp mat, by Flaxcraft, Inc. Newspaper Woven E-Glass Fiber

Tg (ºC) E’ (Mpa) E” (Mpa)

0.0% 19.1%

66.0 73.5

1108 2176

68 201

20.0%

57.0

1989

246

23.0%

57.5

2090

260

23.5%

68.0

1404

171

29.5% 30.0%

67.0 70.0

1493 2072

199 200

45.9%

71.3

3673

341

47.8% 53.4%

73.0 73.5

4550 3776

320 285

55.2%

62.5

5242

361

57.7%

73.0

4556

287

X X 75.2%

65.5 68.0 61.5

2160 4874 17,947

271 375 916

X, unknown value.

24.4.4

Effect of Fiber Content

Table 24.3 shows that the storage modulus increased with increasing fiber weight fraction in the different composite panels. Because of the complexity and irregularity of natural fiber mats, the fiber modulus was back calculated using the rule of mixtures for every composite panel, depending on the experimental composite modulus, the pure resin modulus, and the fiber volume fraction. The fiber volume fraction is also a complex parameter as natural fibers are very porous materials and the resin may partially fill the voids in the natural fibers when composites are made. This makes the actual density of the fiber in the composite a very difficult parameter to define. However, in this work the solid density of cellulose was used (1.5 g/cm3) for the purpose of calculating fiber volume fraction. Natural fiber mats usually contain binder and sometimes other polymeric products to keep the mat together. In this case, the binder and PET, where applicable, in the mats were Copyright © 2005 by Taylor & Francis

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considered part of the matrix. The fiber volume fraction was based on the amount of flax or pure cellulose content in the mats. Calculation of the fiber modulus for every natural fiber composite panel yielded an average value of Ef ⫽ 8.9 GPa. This value was then used to model the storage modulus of the composite E, following the rule of mixtures (Equations 24.3 and 24.4) and the Halpin–Tsai (Equation 24.5) micromechanical models.36 The pure resin modulus was experimentally measured as Em ⫽ 1.1 GPa. The rule of mixtures is as follows: E ⫽ EfVf ⫹ EmVm

(upper bound)

(24.3)

1/E ⫽Vf/Ef ⫹ Vm /Em (lower bound)

(24.4)

The Halpin–Tsai model is 1⫹ξχVf E ⫽ Em ᎏ 1⫺χVf

冢

冣

(24.5)

where Ef⫺Em χ⫽ ᎏ Ef⫹ξEm E is the modulus of composite, MPa, Ef and Em are the moduli of fiber and matrix, respectively, MPa, Vf and Vm are the volume fractions of fiber and matrix, respectively, and ξ is the Halpin–Tsai parameter. ξ is a parameter that serves to adjust the modulus value between the upper and lower bound solutions. When ξ ⫽ ∞, the Halpin–Tsai model gives the upper bound rule of mixtures (Equation 24.3) in which the fiber stiffer modulus dominates, and when ξ ⫽ 0, the Halpin–Tsai model gives the lower bound rule of mixtures (Equation 24.4), where the less stiff resin modulus dominates. The ξ value has often been related to the aspect ratio of the fibers such that high aspect ratios approach the upper bound while lower aspect ratios approach the lower bound, as expected. Figure 24.4 shows that the upper bound rule of mixtures and the Halpin–Tsai analysis, with ξ ⫽100 or over, provide the best fit for these experimental data. Discrepancy in the experimental data is due to the nature of the reinforcement fiber mats, as previously discussed, and to the fact that each point came from different experiments and different natural fiber mats.

24.5 24.5.1

Applications: Housing Construction Material Introduction

New efforts in the application of natural composite materials in the building products sector show significant potential. An analysis completed by Kline Copyright © 2005 by Taylor & Francis

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6000 Experimental data

5000

Rule of mixture

Modulus (MPa)

Halpin−Tsai, xi = 100 Halpin−Tsai, xi = 10

4000

Halpin−Tsai, xi = 1

3000

2000

1000

0 0

10

20

30

40

50

60

Cellulose fiber volume fraction (%) FIGURE 24.4 Modeling of the storage modulus as a function of fiber volume fraction using the composite rule of mixtures and the Halpin–Tsai model, changing xi between 1 and 100.

& Company, Inc., in 2001 forecasted the building market for natural fibers for the years 2000–2005 to grow 60% per year. More specifically, by 2005, the market in building products was projected to require 3 billion pounds of natural fibers for applications, led by decking and followed by new applications in siding and shingles.37 One area of specific interest to this project is the use of these natural fiber-reinforced composites for structural applications, particularly roofing applications. Over the past 15 years, the United States has been hit with three major hurricanes, Hugo (1989), Andrew (1992), and Iniki (1992), causing over $27.5 billion cumulatively in damage to insured property.38 In 2004, the damage from the four hurricanes which impacted Florida (Charley, Frances, Ivan, and Jeanne) is expected to reach at least $30 billion. Typically, when houses are exposed to hurricane forces, the roofs are most susceptible to damage, followed by walls and openings, and then foundations. The most common roof damage is the loss of cladding or sheathing (tiles, shingles, etc.) caused by very high suction pressures that develop at the roof–wall interface. Once the sheathing is lost, the roof no longer acts as a diaphragm, and the lateral load carrying of the structure is compromised. In addition, the loss of sheathing can result in costly water damage. Researchers at the Insurance Institute for Property Loss Reduction and the Insurance Research Council write the following:39 A 40-year period of relatively benign weather left southern Florida with a false sense of security regarding its ability to withstand hurricanes. This led to complacency about hurricane risk, leading to “helter-skelter” development, lackluster code enforcement, building code amendments,

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shortcuts in building practices, and violations that seriously undermined the integrity of the [building] code and the quality of the building stock. Conservative estimates from claim studies reveal that approximately 25 percent of Andrew caused insurance losses (about $4 billion) were attributable to construction that failed to meet the code due to poor enforcement, as well as shoddy workmanship. At the same time, concentrations of population and property exposed to hurricane winds in southern Florida grew many-fold.

The inability of the traditional roof structure to maintain its structural integrity during a hurricane creates an opportunity to completely overhaul the structure by developing a monolithic, all-natural composite roof system. The so-called hurricane resistance of this roof will be obtained by making the structure monolithic (one unit), without cladding (see Figure 24.5). In doing so, the composite will not rely on numerous small sheathing panels to form the diaphragm, thereby becoming “hurricane resistant.” By using VARTM processing to make the composite structures, complicated three-dimensional architectures of the roof can be obtained. Furthermore, the use of natural fibers and soybean oil resins results in a cost-efficient composite material. These advantages of the all-natural alternative roof will create significant consumer demand. Moreover, mass production of this novel roof structure will require significant quantities of natural fiber composite raw materials so that these composites can compete with conventional synthetic composite materials. 24.5.2

Beam Design

To study the possibility of manufacturing the roof presented in Figure 24.5, beams of the proposed material were first manufactured and tested for strength requirements. Presented in Figure 24.6 is a schematic of the prototype

FIGURE 24.5 A schematic showing a monolithic house roof made of natural composites as a hurricaneresistant roof. (From Newsweek, October 27, 2003. With permission.)

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8.0 5.75 0.250

0.875

3.5' 0.875

0.250 0.250

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3.0 3.5

0.250

Section

Side view

FIGURE 24.6 A schematic showing the dimensions of the test beams. Dimensions are shown in inches.

beam studied in the investigation. The beam is of a sandwich construction with a top horizontal face sheet, bottom face sheet, and two vertical webs. The overall dimensions of the beam are 1067 ⫻ 89 ⫻ 203 mm (42 ⫻ 3.5 ⫻ 8 in.); the face sheets as well as the webs have a nominal thickness of 6.4 mm (0.25 in.). The foam core is required for the manufacture of the beam and is integral to it, but while it contributes significantly to thermal and sound insulation, it is not expected to contribute significantly to the strength and stiffness of the member. The beam was designed as a flexural member to carry loads transverse to the longitudinal axis of the beam. It should be noted that for this preliminary investigation the beams were not designed to satisfy any specific structural design criteria. Instead, the specimen was designed to be of a size and shape that would facilitate testing and expose any limitations or difficulties in the fabrication of the member. However, data presented in Section 24.5.4 show a comparison of the composite beams with some typical structural sections made from conventional construction materials. 24.5.3

Structural Composite Manufacturing Using VARTM

VARTM process, as discussed in Section 24.4.1, was also used to manufacture structural composites using foam cores. As stated, recycled paper produced good composites when flat composite sheets were made, but the first attempt at making a three-dimensional structure was not successful due to resin flow problems through the web. To overcome the flow problem, other porous fibers were used in small quantities along with the main reinforcement (recycled paper) to provide flow channels for the resin, especially through the beam web. Figure 24.7 is a schematic showing how porous fiber mats Copyright © 2005 by Taylor & Francis

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Fluffy fiber (high porosity)

Recycled paper

Structural foam FIGURE 24.7 A schematic showing the porous fiber in the preform.

were used in combination with recycled paper to provide better flow and yield the properly infused beam, as shown in Figure 24.8. Three different types of porous fiber mats or fibers with an open channel structure were used for this purpose: chicken feather mats, a corrugated form of the same recycled paper, and woven E-glass fiber. The materials were selected to improve processing and strength or to reduce cost. The concept of using chicken feathers as a polymer composite reinforcement originated in 2001 in discussions between the ACRES group and Tyson, Ltd., a major producer of chicken products in the region. Chicken feathers are a waste product and are generated in large quantities. Tyson Ltd. agreed to make mats of ~97% chicken feathers and 3% low molecular weight polymer, an example of which is shown in Figure 24.2. The chicken feather mats were then used by the ACRES group to manufacture composites.40 They improved the resin modulus of the neat resin by a factor of two, and decreased the density and dielectric constant while increasing thermal and sound resistance. In this application, however, the mats, which are very porous, were used to provide flow channels for the resin to better distribute the resin to the dense and compacted recycled paper. Chicken feathers were used primarily because they were an inexpensive, available waste product and were proven to be good for providing the channels needed for the resin flow while also increasing the resin strength. As a second alternative in the design, a single ply of corrugated cardboard, made from the same recycled paper, was used to provide the channels needed to facilitate flow of the resin through the preform web. The advantage of using the corrugated paper is that all of the reinforcement material for the beam is from one source rather than two or more sources. Using all recycled paper also eliminates the interface and possible compatibility issues between chicken feather mats and paper mats. As the final modification to the design, woven E-glass was used to provide the needed channeling for the resin. The E-glass was added to study the effect of a small amount of glass on the strength, stiffness, and ductility of the beam. Copyright © 2005 by Taylor & Francis

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FIGURE 24.8 Fracture surface showing failure modes. (a) Brittle failure of flax beam. (b) Ductile failure of recycled paper with one-ply of woven E-glass fiber.

24.5.4

Four-Point Bending Test and Results

Four-point bending tests were done on each of the structural beams, which were loaded to failure. This testing gives load vs. deflection results and strain measurements. All data were obtained using a data acquisition system. The specimen was first loaded reversibly three times in the elastic region and then was taken to failure; Figure 24.8(a) shows a broken beam Copyright © 2005 by Taylor & Francis

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TABLE 24.4 Comparison of Composite Beam Properties vs. Typical Wood Section Properties Flexural Rigidity–EI (kN-m2)

Beam Recycled paper/chicken feathers Recycled paper/corrugated paper Recycled paper/E-glass fiber Douglas fir Spruce Cedar

Composite beams 12.4 14.8 19.9 Wood beams 18.0–30.3 16.0–25.0 10.0–26.4

Strength (kN) 24.2 25.8 25.6 15.4–29.7 10.7–24.5 9.5–28.8

and the fracture surface after testing. This type of failure was noticed for flax beam, recycled paper with chicken feathers, and recycled paper with corrugated recycled paper. The different beams were tested to failure and Table 24.4 compares three beams made of recycled paper from old cardboard boxes and three different interlaminar or integral distribution media in the form of chicken feather mat, one-ply corrugated paper, or one-ply woven E-glass fiber. Table 24.4 also presents a comparison between the beams and the three most common wood structures used in building construction and shows that the newly developed material properties matched or surpassed that of the wood structures. Using woven E-glass fiber ply as an interlaminar integral distribution media provided ductility and prevented undesired brittle failure. Figure 24.8(b) shows the failure mode of the beam made of recycled paper and one-ply woven E-glass fiber. Additional improvements to the hurricane-resistant roof design have been developed by Wool et al.41

24.6

Other Potential Applications

Natural composites developed in this research program are potentially cheaper than petroleum-based resin products, are available in large quantities, and are mostly dependent on annual crops like soybean, flax, jute, hemp, and other cellulose-rich plants. This has an advantage over using wooden structures which need fully grown trees that take tens of years to reach that stage. In addition to economic advantages, there are also environmental and energy advantages to using natural composites. Natural composites are not only good as housing structural material, as presented earlier, but also can be used as infrastructural material, such as bridge decking forms, and in furniture manufacturing to replace traditional woods and make it easier to mold furniture components, saving intensive labor usually invested in woodworking. Copyright © 2005 by Taylor & Francis

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Stay-in-Place Bridge Decking Form

Stay-in-place (SIP) bridge forms are corrugated sheets of material that span the distance between bridge girders. SIP forms are formwork for the concrete bridge deck and are designed to carry the dead load of the deck while the concrete cures. Before SIP forms were invented, wooden formwork was used for the same purpose. However, wooden forms are labor-intensive, requiring scaffolding to be built from the ground up and then requiring removal after the concrete has cured. The first revolution in form design occurred more than 16 years ago when forms were made of corrugated steel to replace wooden formwork. This was the birth of SIP forms. SIP forms are widely used today. SIP forms are made of light-gauge steel and are ~2 ft (0.610 m) wide by 4 (1.22 m) to 10 (3.05 m) ft long. They are screwed into angles that are welded onto bridge stringers. The benefits of SIP forms include speed and ease of installation, and decreased labor costs because the forms do not need to be removed after deck curing. However, there are several disadvantages of conventional SIP forms. The first is that the steel pans can collect moisture and keep it trapped between the steel and the bottom of the concrete deck. This can cause corrosion of the concrete rebar, leading to possible failure of the deck. Another problem is that the form itself prevents bridge inspectors from inspecting the bottom of the deck. The SIP form hides cracking or corrosion that can occur at the bottom of the deck. There are several advantages of a natural composite SIP form over a conventional steel form. The first is that the form is able to “breathe,” or allow water to pass through and away from the concrete deck, thereby reducing the risk of corrosion. Second, the form is biodegradable and would break down naturally, thereby allowing bridge inspectors to examine the bottom of the deck. Finally, the forms are lightweight compared to their steel counterparts and would allow for faster installation and lower labor costs. The first SIP form manufactured at the University of Delaware was made of soy oil-based resin and woven E-glass fiber. The form was successfully manufactured using the VARTM process and then was tested by three-point bending and in situ by poring concrete, as shown in Figure 24.9 and Figure 24.10.

FIGURE 24.9 Stay-in-place testing in 3-point bending and wooden frame for poring concrete.

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FIGURE 24.10 Concrete experiment setup and the final 8-in.-thick concrete slab after curing.

FIGURE 24.11 All-natural composite chairs made of soybean oil-based resin and flax fiber mats.

24.6.2

Furniture Application

Natural resin and flax fiber mats were used to manufacture a chair at the CCM laboratory using the VARTM process. A mold was made out of solid material to assemble the chair shape and the fiber was laid up and then bagged. Figure 24.11 shows a chair after it was cleaned and attached to a metal frame.

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Conclusions

Thermosetting resin suitable for liquid molding was successfully developed and manufactured from plant oil (soybean), then was successfully processed in manufacturing composite materials with natural fibers such as flax, jute, cellulose, recycled paper, and chicken feathers. The biobased composite materials were successfully tested and met established standards in applications as construction material, infrastructural material, and furniture material. A weather protection layer or coating is being developed and will be used where the application is for a permanent or long-term structure. This will act as a weather and fire protection skin, although fire-retarding additives will be used in the composite itself during manufacturing. This newly developed material can be optionally biodegradable as needed, serving the purpose of catering to short-term and temporary installations or structures such as decking and matting materials.

References 1. Wool, R.P., Kusefoglu, S.H., Zhao, R., Palmese, G., and Khot, S., U.S. Patent 6,121,398, Date N/A. 2. Can, E., Kusefoglu, S., and Wool, R.P., Rigid thermosetting liquid molding resins from renewable resources. II. Copolymers of soybean oil monoglyceride maleates with neopentyl glycol and bisphenol A maleates, J. Appl. Polym. Sci., 83, 972–980, 2002. 3. LaScala, J.J. and Wool, R.P., Effect of FA composition on epoxidation kinetics of TAG, J. Am. Oil Chem. Soc., 79, 373–378, 2002. 4. Khot, S.N., LaScala, J.J., Can, E., Morye, S.S., Williams, G.I., Palmese, G.R., Kusefoglu, S.H., and Wool, R.P., Development and application of triglyceridebased polymers and composites, J. Appl. Polym. Sci., 82, 703, 2001. 5. Williams, G.I. and Wool, R.P., Composites from natural fibers and soy oil resins, Appl. Compos. Mater., 7, 421, 2000. 6. Dweib, M.A., Hu, B., O’Donnell, A., Shenton, H.W., and Wool, R.P., All-natural composite sandwich beams for structural applications, Compos. Struct., 63, 147–157, 2004. 7. O’Donnell, A., Dweib, M.A., and Wool, R.P., Plant oil-based resins and natural fiber composites, Compos. Sci. Technol., 2003. [in press] CSTE 2566. 8. Dweib, M.A., O’Donnell, A., Hu, B., Shenton, H.W., and Wool, R.P., Natural Composites Materials for Structural and Automotive Applications, ICCM-14, San Diego, CA, July, 2003, pp. 14–18. 9. Bledzki, A.K. and Gassan, J., Composites reinforced with cellulose based fibres, Prog. Polym. Sci., 24, 221–274, 1999. 10. Mwaikambo, L.Y. and Ansell, M.P., Chemical modification of hemp, sisal, jute, and kapok fibers by alkalization, J. Appl. Polym. Sci., 84, 2222–2234, 2002.

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11. Gassan, J., A study of fibre and interface parameters affecting the fatigue behaviour of natural fibre composites, Compos. Part A: Appl. Sci. Manuf., 33, 369–374, 2002. 12. Ruys, D., Crosky, A., and Evans, W.J., Natural bast fibre structure, Int. J. Mater. Prod. Technol., 17, 2–10, 2002. 13. Mishra, S., Tripathy, S.S., Misra, M., Mohanty, A.K., and Nayak, S.K., Novel ecofriendly biocomposites: biofiber reinforced biodegradable polyester amide composites—Fabrication and properties evaluation, J. Reinforced Plast. Compos., 21, 55–70, 2002. 14. Prabhu, K. and Rik, B., Applications of Bio-Composites in Industrial Products, Materials Research Society Symposium–Proceedings, Vol. 702, 2002, pp. 101–112. 15. Anon, The Competitiveness of Natural Fibers Based Composites in the Automotive Sector: the Sisal Agribusiness in Brazil, Mater. Res. Soc. Symp.–Proceedings, 702, 113–139, 2002. 16. Santulli, C., Post-impact damage characterisation on natural fibre reinforced composites using acoustic emission, NDT & E Int., 34, 531–536, 2001. 17. Van de Velde, K. and Kiekens, P., Thermoplastic pultrusion of natural fibre reinforced composites, Compos. Struct., 54, 355–360, 2001. 18. Gassan, J., Chate, A., and Bledzki, A.K., Calculation of elastic properties of natural fibers, J. Mater. Sci., 36, 3715–3720, 2001. 19. Eichhorn, S.J., Baillie, C.A., Zafeiropoulos, N., Mwaikambo, L.Y., Ansell, M.P., Dufresne, A., Entwistle, K.M., Herrera-Franco, P.J., Escamilla, G.C., Groom et al., Current international research into cellulosic fibres and composites, J. Mater. Sci., 36, 2107–2131, 2001. 20. Salvatore, I., Raed, A., and Luigi, N., Effect of processing conditions on dimensions of sisal fibers in thermoplastic biodegradable composites, J. Appl. Polym. Sci., 79, 1084–1091, 2001. 21. Braun, D. and Braun, A., Natural thermosets, Kunststoffe Plast. Eur., 91, 36–38; 83–86, 2001. 22. Corbie`re-Nicollier, T., Laban, B.G., Lundquist, L., Leterrier, Y., Manson, J.-A.E., and Jolliet, O., Life cycle assessment of biofibres replacing glass fibres as reinforcement in plastics, Resour., Conservation Recycling, 33, 267–287, 2001. 23. Hepworth, D.G., Hobson, R.N., Bruce, D.M., and Farrent, J.W., The use of unretted hemp fibre in composite manufacture, Composites: Part A, 31, 1279–1283, 2000. 24. Zafeiropoulos, N.E., Williams, D.R., Baillie, C.A., and Matthews, F.L., Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part I. Development and investigation of surface treatments, Composites: Part A, 33, 1083–1093, 2002. 25. Mohanty, A.K., Misra, M., and Drzal, L.T., Surface modifications of natural fibers and performance of the resulting biocomposites: an overview, Compos. Interf., 8, 313–343, 2001. 26. Mohanty, A.K., Drzal, L.T., and Misra, M., Engineered natural fiber reinforced polypropylene composites: Influence of surface modifications and novel powder impregnation processing, J. Adhesion Sci. Technol., 16, 999–1015, 2002. 27. Silva, J.L.G. and Al-Qureshi, H.A., Mechanics of wetting systems of natural Æbres with polymeric resin, J. Mater. Process. Technol., 92–93, 124–128, 1999. 28. Cichocki Jr., F.R. and Thomason, J.L., Thermoelastic anisotropy of a natural fiber, Compos. Sci. Technol., 62, 669–678, 2002. 29. LaScala, J.J. and Wool, R.P., Property Analysis of Triglyceride–based Thermosets [submitted].

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30. Seemann, W.H., Plastic Transfer Molding Techniques for the Production of Fiber Reinforced Plastic Structures, U.S. Patent No. 4902215, filed 30 March 1989. 31. Williams, C.D., Grove, S.M., and Summerscales, J., The compression response of fibre-reinforced plastic plates during manufacture by resin infusion under flexible tooling methods, Composites: Part A, 29, 111–114, 1998. 32. Mathur, R., Heider, D., Hoffmann, C., Gillespie Jr. J.W., Advani, S.G., and Fink, B.K., Flow Front Measurements and Model Validation in the Vacuum Assisted Resin Transfer Molding Process, Polym. Compos., 22, 477–490, 2001. 33. Heider, D., Hofmann, C., and Gillespie Jr. J.W., Automation and Control of LargeScale Composite Parts by VARTM Processing, Proceedings of the 45th International SAMPE Symposium/Exhibition, “Bridging the Centuries with SAMPE’s Materials and Processes Technology,” Long Beach, CA, May 21–25, 2000. 34. Hsiao, K.-T., Mathur, R., Advani, S.G., Gillespie, J.W., and Fink, B.K., A closed form solution for flow during the vacuum assisted resin transfer molding process, J. Manuf. Sci. Eng., 122, 463–475, 2000. 35. Advani, S.G. and Sozer, E.M., Process Modeling in Composites Manufacturing, Marcel Dekker, New York, 2003. 36. Whitney, J.M. and McCullough, R.L., Micromechanical Materials Modeling, Delaware Composites Design Encyclopedia, Vol. 2, Technomic Publishing Co., PA, USA, 1990. 37. Kline & Company, Proceedings of the 6th International Conference on Wood FiberPlastic Composites, May 14, 2001. 38. Ayscue, Jon K., Natural Hazards Research Working Paper #94. Retrieved from http://www.colorado.edu/hazards/wp/wp94/wp94.html, November, 1996. 39. Insurance Institute for Property Loss Reduction (IIPLR) and Insurance Research Council, Coastal Eposure and Community Protection: Hurrican Andrew’s Legacy, IIPLR, Boston, MA, 1995. 40. Wool, R.P. and Hong, C.K., Low Dielectric Constant Materials from Plant Oils and Chicken Feathers, Patent application No. 60/396319, 2002. 41. Wool, R.P., Dweib, M.A., Shenton III, H.A., and Chapas, R., A Monolithic Hurricane Resistant Roof Made from Low Density Composites, U.S. Patent Application, 2003.

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25 Biobased Polyurethanes and Their Composites: Present Status and Future Perspective

Jean-Pierre Latere Dwan’Isa, Amar K. Mohanty, Manjusri Misra, and Lawrence T. Drzal CONTENTS 25.1 Introduction 25.2 Polyols from Petroleum 25.2.1 Polyether Polyols 25.2.2 Polyester Polyols 25.2.3 Polycarbonate Polyols 25.3 Isocyanates 25.4 Biobased Polyurethanes 25.4.1 Polyols from Plant Oils 25.4.1.1 Castor Oil and Lesquerella Oil 25.4.1.2 Chemical Modification of Vegetable Oils 25.4.2 Polyols from Wood 25.4.3 Polyols from Carbohydrates 25.4.4 Polyols from Lignin 25.4.5 Polyols from Cashew 25.4.6 Polyols from Cork 25.5 Biobased Polyurethane Composites 25.5.1 Biobased Polyurethane Composites from Merginate Polyols 25.5.2 Biobased Polyurethane Composites from Soybean Phosphate Ester Polyol 25.5.2.1 Experimental Section 25.5.2.2 Results and Discussion 25.6 Future Perspectives 25.7 Conclusions Acknowledgments References

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ABSTRACT A review of the preparation of polyurethanes using natural resources is provided. Basically, such polyurethanes are produced by combination of petroleum-basedisocyanate reactants with polyols derived from resources such as plant oils, carbohydrates, wood, lignin, cashew, and cork. Depending on the procedure used to derive them from natural resources, polyols with tunable hydroxyl content can be produced. Properties of biobased polyurethanes range from flexible to rigid materials and are thus able to compete with commercially available polyurethanes made from petroleum-based products. Despite a dearth of research on biobased polyurethane composites, this class of reinforced materials is certainly of interest, since addition of fibers such as glass or natural lignocellulosic (i.e., hemp, jute, flax) may result in improved thermomechanical properties, as shown by data collected in our laboratory. When using natural lignocellulosic fibers, formation of covalent bonds between the fiber and the isocyanate component is possible, with improved interaction between the matrix and the fiber.

25.1

Introduction

Polyurethanes are a class of polymers produced by combining a large number of simpler molecules called monomers. Their versatility has allowed their use in the automotive industry, architecture, aerospace, office machinery, agriculture, the marine industry, mining, packaging, footwear, and medical devices. Typically, the monomers used to form polyurethanes are isocyanates and polyols (Figure 25.1). Polyurethanes are used in various applications1,2 as bulk plastics, elastomers, fibers, foams (rigid to flexible), surface coatings, adhesives, and sealants (Figure 25.2).1 The key to polyurethane versatility lies in the nature and the molecular characteristics of the reactants, i.e., polyol and isocyanate. Indeed, their chemical and morphological structure and final properties depend on the reactants structure, molecular weight, and functionality. For example, increasing the functionality of both polyol and isocyanate results in an insoluble and highly cross-linked polymer.

25.2

Polyols from Petroleum

The polyol used to make polyurethane is crucial. Today, different types of polyols are commercially available. As a general observation, most of them are made from chemicals derived from petroleum. They can be classified into three groups, which are discussed below. Copyright © 2005 by Taylor & Francis

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HO

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OCN

NCO

Polyol

+

HO

OH Polyol

Diisocyanate

O

O

O

O

OCN

NCO

OCN

NCO

H

H

H

H Polyurethane

FIGURE 25.1 Polyurethane synthesis from polyol and diisocyanate.

Shoes 5%

Others 27%

Insulation 29%

Automotive 12%

Furniture and mattresses 27%

FIGURE 25.2 World polyurethane consumption 2004 by applications (total: 10 ⫻ 106 t/a). (Adapted from Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA (Electronic Encyclopedia), 7th Ed., 2005. With permission from Wiley & Sons, Inc.)

25.2.1

Polyether Polyols

These polyols result from the oligomerization or polymerization of cyclic ether such as ethylene oxide, propylene oxide, their mixtures, or tetrahydrofuran initiated by hydroxyl-terminated compounds such as ethylene glycol, propylene glycol, glycerin, pentaeruthritol, sucrose, and sorbitol. Amine-terminated polyether polyols are also available. They are formed by substitution of hydroxyl groups by primary or secondary amino functions. Polyether polyols are by far the most used polyol in preparation of polyurethanes, representing more than 90% of the commercially available polyols. 25.2.2

Polyester Polyols

Polyester polyols are formed by condensation of a di- or trihydroxyterminated compound such as glycol, glycerine, or trimethylolpropane with a diester or a diacid. Usually, adipic, glutaric, and azeloic acids are used. The Copyright © 2005 by Taylor & Francis

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condensation reaction is controlled in order to produce polyols bearing hydroxyl end groups. These polyols represent about 10% of available polyols for preparation of polyurethanes. Polyester polyols based on the ring-opening polymerization of ε-caprolactone initiated by a glycol or trimethylolpropane are available on the market.

25.2.3

Polycarbonate Polyols

Polycarbonate polyols are also available on the market (~1%). They are produced from nonvicinal diols by condensation with diarylcarbonate, dialkylcarbonate, dioxolanones, phoshene, or bischlorocarbonic acid esters of urea.

25.3

Isocyanates

Commercially available isocyanates can be divided into three categories: (1) aromatic, (2) aliphatic and cycloaliphatic, and (3) specialty isocyanates. A 2004 world report of raw materials (%) usage in polyurethane production (according to Reference 1) is presented in Figure 25.3. The aromatic isocyanates, i.e., toluene diisocyanate and either crude or polymeric diphenylmethane diisocyanate (MDI), are the most used in polyurethane synthesis (Figure 25.4). For a review on their characteristics and their synthesis see Reference 2.

25.4

Biobased Polyurethanes

In recent years, there has been a move toward the replacement of petroleumbased products by renewable resource-based materials.3 Indeed, limited MDI 31% Polyol 52%

TDI 17% FIGURE 25.3 World 2004 report of raw materials (%) usage in polyurethane production. (Adapted from Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA (Electronic Encyclopedia), 7th Ed., 2005. With permission from Wiley & Sons, Inc.)

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OCN

H3C

Toluene 2,4-diisocyanate

NCO

OCN

H3C

Toluene 2,6-diisocyanate OCN

OCN

CH2

NCO

OCN

CH2

CH2

4,4′ -methylene diphenyl diisocyanate (MDI)

NCO

NCO

n

Polymeric methylene diphenyl diisocyanate (pMDI)

FIGURE 25.4 Chemical structures of aromatic isocyanates most used in polyurethane synthesis.

availability of petroleum and its erratic higher cost combined with environmental problems have prompted governments and researchers to encourage and develop alternative ways of production of today’s materials from renewable resources. The advantages of utilizing renewable resources are multifold, e.g., respect for the environment, reduction of petroleum consumption, low cost, and abundance. A biobased product can be defined as a product made from renewable agricultural and forestry feedstock, including crops and crop by-products, animal by-products, and wood and its residues. In this chapter, some of the approaches described in the scientific literature for preparation of renewable resource-based polyurethanes, i.e., biobased polyurethanes will be reviewed. Note that the terminology biobased polyurethanes refers principally to polyurethanes in which the polyol is produced from renewable resources. Indeed, attempts to produce isocyanates from renewable resources have not been conclusive as yet in terms of physico-chemical properties and cost. Therefore, the terminology used refers to polyurethanes containing “biobased polyols.” In the forthcoming paragraphs, a list of nonexhaustive examples of renewable resources employed for production of polyurethanes is presented. Copyright © 2005 by Taylor & Francis

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Polyols from Plant Oils

Naturally occurring plant oils are mixtures consisting primarily of triglycerides (Figure 25.5) containing at least two different fatty acid groups, not statistically distributed.4 Free fatty acids, phospholipids, waxes (esters of fatty alcohols and fatty acids), and unsaponifiable matter consisting of longchain mono- or dihydric alcohols, sterols, aliphatic hydrocarbons, and terpenoids are also found in plant oil. Because of their abundance, plant oils represent a very interesting source in the preparation of renewable resourcebased monomers and thus polymers (Table 25.1). Fatty acids are mainly even-numbered, straight-chain, aliphatic (saturated or unsaturated) monocarboxylic acids with chains ranging from C4 to C24. Plant oil properties (chemical, physical, and biological) are determined by the fatty acid groups and their distribution. As an illustration of the influence of fatty acids, they are liquid at ambient temperature because of their high content of unsaturated fatty acids. According to their unsaturation, plant oils have been classified into three groups based on their iodine number. They are drying (iodine number above 130), semidrying (iodine value 90−130), and nondrying oils (iodine value below 90).5

CO O

CH2

CO O

CH

CO O

CH2

FIGURE 25.5 General structure of triglyceride.

TABLE 25.1 World Production of Oils (million tons/yr) Oils Soybean Palm Rapeseed Sunflower Groundnut Cottonseed Coconut Palm kernel Olive Corn Sesame Linseed Castor

2001

2002a

27,592 23,453 13,743 8,340 4,932 3,997 3,571 2,897 2,694 2,017 737 639 524

29,495 23,750 13,570 7,177 5,579 4,419 3,469 2,951 2,550 2,190 755 645 475

a Forecast. Source: Adapted from Oil World (December 14, 2001). With permission.

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25.4.1.1 Castor Oil and Lesquerella Oil Although most triglycerides contain unsaturation, a few oils naturally contain other groups. Triglycerides of castor and lesquerella oils are characterized by the presence of ricinoleic and lesquerolic fatty acids, respectively (Table 25.2). These acids bear hydroxyl groups on their backbone (Figure 25.6). Castor oil is extracted from the Ricinus communis L. bean.6,7 It is grown commercially worldwide, especially in India, Brazil, and China. Lesquerella oil is obtained from the seed of Lesquerella fendleri, an annual native plant of the southwestern United States (Arizona, New Mexico, Oklahoma, and Texas). Both oils are interesting sources of polyols for preparation of biobased polyurethanes. However, castor oil presents an obvious advantage over lesquerella oil since the hydroxyl content is intrinsically higher because ricinoleic acid represents ~90% of the fatty acids while lesquerolic acid content is ~50% in lesquerella oil. The hydroxyl groups of these oils can react with isocyanates to form branched polyurethanes. Castor oil is frequently utilized in the synthesis of cross-linked polyurethanes and interpenetrating networks (IPN).8−11 Castor oil-based polyurethanes have been used in the telecommunications and

TABLE 25.2 Composition of Fatty Acids in Castor and Lesquerella Oils Content (%) Fatty Acid

Castor oil

Ricinoleic Lesquerolic Oleic Linoleic Linolenic Stearic Palmitic

90 — 3 4 ~0 2 1

Lesquerella oil — 51 18 14 13 2 1

OH OH O

Lesquerolic acid OH

OH O Ricinoleic acid FIGURE 25.6 Chemical structures of lesquerolic and ricinoleic fatty acids.

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electrical industries because of their insulation properties as well as in coatings, adhesives, and sealants applications.12 They have also served in biomedical applications because of their biocompatibility.13 Flexible, semiflexible, and rigid polyurethanes can be produced with castor oil. Foams based on this triglyceride polyol are resistant to moisture and have good low-temperature flexibility. Because of the similarity of the chemical structure of lesquerolic and ricinoleic acids, lesquerella oil can also be used as a source of poylol for synthesis of polyurethanes. The lower hydroxyl content of this oil, when compared to castor oil, results in lower cross-linking densities and thus in flexible polyurethanes.14 Addition of cross-linking agents increases results in semiflexible polyurethanes. So far, little information on lesquerella oil-based polyurethanes can be found in the scientific literature due to its lower hydroxyl content as well as its to lower production when compared to castor oil. Indeed, no commercial data are yet available on current production and yields of lesquerella oil. However, trials are currently being carried out in a number of countries across Europe (Belgium, Italy, Netherlands, and the United Kingdom) and in the United States.

25.4.1.2 Chemical Modification of Vegetable Oils Besides naturally occurring plant oils containing hydroxyl groups, other oils may be used. Indeed, the major drawback of oils like castor and lesquerella is their limited production and thus their higher price compared to other oils such as soybean oil or palm oil. Researchers have taken up the challenge of converting naturally occurring oils into polyols by chemical modification. Two approaches have been and are still currently being investigated for the synthesis of plant oil-based polyols from the most abundant oils, i.e., soybean, palm, and rapeseed. The first approach is conversion of double bonds into hydroxyl groups. Most oils contain unsaturation, specifically double bonds. Therefore, conversion of these double bonds by chemical reaction is a way to prepare polyols. Two distinctive technologies have been successfully developed to prepare polyols from plant oils. The first technology is hydroformylation/hydrogenation. In this technology, the double bonds of oil are first converted to aldehydes through hydroformylation using a metal catalyst (usually cobalt or rhodium) with a mixture of H2 and CO gases. The aldehydes are subsequently hydrogenated, still in the presence of a catalyst, to hydroxyl groups (Figure 25.7). The hydroxyl content of the generated triglyceride polyols depends on the efficiency of the catalysts used. Depending on the degree of conversion, flexible to rigid polyurethanes can be obtained.15−18 Results indicate that the catalyst used influences the mechanical properties of polyurethanes because it controls the conversion of unsaturation into polyols and thus the hydroxyl content. Recent results showed that polyurethanes behaving like hard rubber (Young’s modulus of 13 MPa and elongation at break of 93%) were obtained with a soybean-based Copyright © 2005 by Taylor & Francis

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O Hydroformylation

CH

Hydrogenation OH CH

FIGURE 25.7 Hydroformylation of double bond followed by hydrogenation of the generated aldehyde.

Epoxidation

O

Hydrolysis

OH

OH FIGURE 25.8 Epoxidation of a double bond followed by hydrolysis of the epoxy function.

polyol prepared with cobalt as catalyst while rigid plastics (Young’s modulus⬎300 MPa and elongation at break of 13%) were synthesized from polyols made with a rhodium-based catalyst.19 The rigidity of these materials is significantly improved upon addition of glycerine, which acts as a low molecular weight cross-linker. The second technology is epoxidation/hydrolysis. In this technology, the double bonds of oil are epoxidized followed by hydrolysis of the epoxy groups to produce hydroxyl groups (Figure 25.8). The industrial process currently used for epoxidation of plant oils consists in reacting performic acid generated in situ from hydrogen peroxide and formic acid. However, other reactants have been used for this epoxidation step, which provide high selectivity.20,21 Ring opening of the epoxy groups is acid-catalyzed. Usually fairly harsh conditions (50% phosphoric acid, 100°C, and vigorous stirring) result in the direct synthesis of vicinal hydroxyl groups on the triglyceride polyol backbone. The reaction can also be performed in organic solvent with much lower reaction temperature and acid concentration. Other methods of ring opening of epoxy groups have been developed but conditions are either too harsh or too costly to implement.22,23 Copyright © 2005 by Taylor & Francis

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O R C O CH2

HO CH2

O R C O CH

+

HO CH

O R C O CH2

HO CH2

O

O

R C O CH2

HO CH2

HO CH2

R C O CH2 O

O HO CH

+

R C O CH

+

HO CH O

HO CH2 1-monoglyceride

+

R C O CH O

HO CH2

R C O CH2

R C O CH2

2-monoglyceride

1,1-diglyceride

1,2-diglyceride

FIGURE 25.9 Glycerolysis of triglyceride of plant oil to form a mixture of mono- and diglycerides.

Examples of biobased polyurethanes produced from plant oil-based polyols prepared by chemical transformation of unsaturations include elastomers manufactured from material with low hydroxyl content and rigid foam and rigid plastics from high hydroxyl content polyols using oils like soybean, rapeseed, palm, and linseed.23−29 In both technologies, the key parameter is the extent of unsaturation conversion to hydroxyl groups. Since the hydroxyl content controls the degree of cross-linking, and to some extent the stiffness, a broad spectra of polyols with various characteristics can be produced, depending on the conversion. The stiffest samples prepared with soybean-based polyols showed Young’s modulus as high as 1.1 GPa and elongation at break lower than 10% for glass transition temperature (Tg) of about 75°C27 without addition of any cross-linking agent. The second approach for the synthesis of plant oil-based polyols is alcholysis of triglyceride ethers. Ester groups of triglycerides are another reaction site for preparation of plant oil-based polyols. The synthesis consists in reaction of the oil with an alcohol, such as glycerol, with triglycerides ester to produce a mixture of mono- and diglycerides (Figure 25.9).30 The clear advantage of this technology over the one based on conversion of unsaturation is its applicability to all kinds of triglycerides. The polyols produced in this way, also known as oil alkyd resin, can react with both aromatic and aliphatic isocyanates to form biobased Copyright © 2005 by Taylor & Francis

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polyurethanes.31,32 Many plant oil-based polyols produced by alcoholysis of plant oils are commercially available. The most commonly used oils are soybean, palm, castor, coconut, linseed, and olive. Polyurethanes can be prepared by addition of an isocyanate. These kinds of biobased polyurethanes are generally used in paint technology and coating applications because of their hardness and abrasion resistance.33 Paint films are highly waterproof and show improved resistance to chemicals. They are also used in pigmented paints to produce high-gloss and matt decorative coatings. Another application is as varnish binders for the interior and exterior coating of wood.34,35

25.4.2

Polyols from Wood

Wood is a mixture of lignin and carbohydrates, such as cellulose and hemicellulose, which forms the lignocellulosic structure of the wood. However, other extractives (gums, fats, resins, oils, alkaloids, starches, tannins) and inorganics (recovered after combustion of wood as ash) are found in wood. The content of each constituent varies with variables such as soil composition, geographic location, weather, and location within a given tree. The average composition based on dry weight for wood is cellulose 40−45% (about the same in softwoods and hardwoods); lignin 25−35% in softwoods and 17−25% in hardwoods; hemicellulose 20% in softwoods and 15−35% in hardwoods; and 25−35% lignin while extractives vary from 1 to more than 10%. In order to be of use as a reactant for polyurethane synthesis, wood must be liquefied (Scheme 25.1). Liquefaction of wood and its conversion into WOOD

Liquefaction

Liquefied WOOD polyol

1. Isocyanate

+ Isocyanate

2. Processing (e.g., solution casting, compression molding)

+ Surfactant

Polyurethane film or sheet SCHEME 25.1 Polyurethane materials from wood.

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+ Foaming agent

Polyurethane foam

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liquid materials either by chemical or thermochemical treatments has been reported. Various techniques have been developed.36−39 The first typical liquefaction experiments were conducted under drastic conditions, i.e., high pressure and high temperature in the presence of catalyst and/or reducing gases (CO, H2, and CO/H2). In recent applications, organic solvents (e.g., dioxane, DMSO, DMF, and acetone) have been used. The use of phenol results in products rich in phenol units, which can be used to prepare phenolic resins.40−42 In the presence of alcohols, such as ethylene glycol, liquefied wood with hydroxyl content suitable to react with isocyanate can be obtained. Kurimoto et al. prepared different polyurethanes using liquefied wood as polyol and showed that polyurethane foams with substantial strength can be obtained.43 These authors also prepared polyurethane films from liquefied wood with polymeric MDI.44,45 Films were made by solution casting and their mechanical properties were measured as a function of the amount of liquefied wood in the polyurethane. Results showed that increase in the amount of the liquefied wood significantly enhanced Young’s modulus and reduced the ductility of the films. Films containing ~30 wt% of liquefied wood had a Young’s modulus of ~0.6 GPa and a tensile strength of 32 MPa. The authors also showed that cross-linking densities and glass transition temperature increase significantly with the isocyanate to hydroxyl ratio. Madas and Shiraishi also used liquefied wood as polyol component for synthesis of polyurethane foams by reaction with polymeric MDI.46 Foams with densities of ~0.03 g/cm3, strength of 0.01 MPa, and Young’s modulus of 3.8 MPa were prepared. In a separate work, polyols resulting from liquefaction of a mixture of wood and starch were used to prepare foams.47 They had densities of ~0.03 g/cm3, compressive strengths of 80 to 150 kPa, and Young’s moduli (3 to 10 MPa) comparable to conventional rigid polyurethane foams. The composition of the polyol had significant influence on the properties of the foams. Results indicated that starch helps to increase the compressive strength and Young’s modulus as well as the brittleness. 25.4.3

Polyols from Carbohydrates

Carbohydrates are carbon compounds that contain large amounts of hydroxyl groups. Therefore they are potential reactants in the preparation of biobased polyurethanes. They can be divided into three categories: (1) monosaccharides, e.g., glucose, galactose, and fructose; (2) oligosaccharides (combination of from two to ten monosaccharides, such as disaccharides and trisaccharides); (3) polysaccharides (long chains of monosaccharide units, such as cellulose, starch, glycogen, and hemicellulose). Commonly occurring monosaccharides are glucose and fructose (found in fruits, honey, and processed food), and galactose found in milk (Figure 25.10). The most abundant oligosaccharide in nature is sucrose (disaccharide of glucose and fructose). It comes from sugarcane or sugar beets. Maltose is also a disaccharide of glucose and galactose. The majority of carbohydrates found in nature are present as polysaccharides. The two most abundant Copyright © 2005 by Taylor & Francis

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CH2OH

CH2OH O

H H OH

OH

OH

H

O H OH

H

OH H

H

OH

Glucose

H

H

H OH

H

OH

Galactose

O

CH2OH

H

H

OH OH

OH

H

Fructose

FIGURE 25.10 Structures of some common monosaccharides.

polysaccharides are starch and cellulose. Starch occurs preferentially in cereal grains, leguminous grains, and tuberized roots. The principal sources of starch are wheat, maize, potato, manioc, and banana. Cellulose is the single most abundant organic molecule but is also the most wasted natural polymer. It is found mainly in plants. Wood is largely cellulose while cotton and paper are almost pure cellulose. Like starch, it contains glucose as monomer. However, cellulose differs profoundly from starch in its properties. Starch is a branched polymer while cellulose is a long, rigid molecule. Because of their abundance, easy recovery from nature, and low cost, carbohydrates represent an interesting source for production of renewable resources-based products. Unfortunately, these carbohydrates cannot be used as found in nature for the synthesis of biobased polyurethanes. They must be liquefied, just like wood, in order to react with isocyanates. Liquefaction in the presence of alcohols has been used for starch and cellulose. Liquefied carbohydrates, particularly the abundant starch and cellulose, can be used as polyol for preparation of biobased polyurethanes. Shiraishi et al. have studied the preparation of polyurethanes from starch.46,48−51 The starch-based polyols were obtained by liquefaction of starch in the presence of polyethylene glycol-dominant reaction reagents by using sulfuric acid as a catalyst under either a refluxing condition or a reduced-pressure condition (Scheme 25.2). The starch content of such polyols varied from 50 to 70 wt%, polyethylene glycol (PEG) varied from 20 to 50 wt%, while glycerine represented less than 10 wt% of the total mixture. The authors have been able to prepare polyols with hydroxyl content ranging from 200 to 400 mg KOH/g. It was found that these liquefied starch polyols have suitable characteristics for making polyurethanes with comparable properties to conventional “petrobased” polyurethanes.47,51 They were reacted with MDI. In a recent variation of this process, the authors liquefied in the same conditions corn branch and prepared polyurethanes from the obtained polyols.52 Liquefied cellulose polyol has also been used to make biobased polyurethanes. Paper being almost pure cellulose, it has been used to make liquefied cellulose polyols by Shiraishi et al. They used wastepaper, which is produced in significant amounts every year and is recycled but still incinerated or discarded into the environment. The liquefaction process described Copyright © 2005 by Taylor & Francis

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Starch

Liquefaction solvent (PEG/glycerine/H2SO4)

Solution of starch

Neutralization of H2SO4 with NaOH

Liquefied starch polyol

SCHEME 25.2 Preparation of liquefied starch polyol.

in Scheme 25.1 was used and polyols were obtained from newspaper, box paper, and business paper.53 Depending on the reaction conditions and the kind of paper used, polyols with hydroxyl content ranging from 300 to 400 mg KOH / g were prepared. By reacting these polyols with MDI, polyurethane foams with satisfactory densities and mechanical properties (as good as those of foams synthesized from liquefied wood and starch polyols) were obtained. These results suggest that cotton, can be liquefied in the same way and cotton-based polyurethanes can possibly be prepared. It is important to note that some carbohydrates have been used in the synthesis of petroleum-based polyether polyols (e.g., sucrose and starch). They serve as initiator for oligomerization of polymerization of the cyclic oxide. In this chapter, we do not consider polyurethanes resulting from these polyether polyols as biobased products since the total content of carbohydrates represents only a few percentage points.

25.4.4

Polyols from Lignin

Lignin, the second most abundant organic substance on Earth after cellulose, is a renewable and complex amorphous biopolymer whose structure can vary from plant to plant. It contains phenylpropane units, methoxy groups, phenolic hydroxyl groups, and some terminal aldehyde groups in the side chains. Few of the phenolic hydroxyl groups are free and most of them are occupied through linkages to neighboring phenylpropane (Figure 25.11).54,55 As mentioned earlier, lignin is the chief noncarbohydrate constituent of wood; it binds to cellulose fibers to harden and strengthen cell walls of plants. The chemical pulping of wood, one of the ten largest industrial activities in North America, involves the removal of lignin from wood. Utilization of this by- (or co-) product of the pulp and papermaking process has resulted Copyright © 2005 by Taylor & Francis

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HO

OCH3

HO HO OH O OH

OCH3 CH3 O

HO O HO OCH3

HO

O

HO

HO

CH3 O

HO

O

HO

OH

O

OH CH3 O

O

OCH3 HO

OH

O

O

OH

HO CH3 O HO CH3 O

O OH

OCH3

OH

OH

OCH3

O HO

O O CH3 O

HO

CH3 O

FIGURE 25.11 Representative structure of lignin.

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OH

CH3O

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in lignin being utilized in many diverse products ranging from road dust binder to a polymer component in printed circuit boards. More than one million tons of lignin is now sold annually worldwide, thus making this byproduct an industrial commodity.56 Different types of lignin materials are produced as by-products of industrial processes that break down woody materials into fibrous materials and/or chemicals. All these lignin products contain various amounts of nonlignin chemicals. There are five important type of lignin industrially produced: ●

●

●

●

●

Hydrolysis lignin is produced by strong hydrolysis of woody materials to produce sugars for fermentation to alcohol. Kraft lignin is obtained through kraft pulping, a process that involves the cooking of wood chips in a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S) at temperatures of 155°–175°C. Lignosulfonates are lignin products recovered from woody materials after chemical treatments. Different lignosulfonates can be produced depending on the reactants used. Examples include lignosulfonates from acid sulfide pulping, bisulfide pulping, neutral sulfide semichemical processing, and alkaline sulfide (antraquinone) pulping. However, commercially available lignosulfonates come mainly from acid sulfide pulping and secondarily from hydrolysis and alkaline sulfite (anthraquinone) pulping. Organosolv lignins are lignins produced from different organic solvent-based systems. They are not marketed because of the high cost associated with the production of pulp using these systems. Steam explosion lignin is obtained by separation into fibers of woody material through high-temperature/high-pressure treatment with steam.

Some of these lignin-based materials have been used to prepare polyurethanes. Saraf and Glasser have synthesized polyurethane films using kraft lignin and steam explosion lignin as the only polyol component in reactions with hexamethylene diisocyanate (HDI) and tolylene diisocyanate (TDI).57 The polyurethanes obtained were generally stiff materials showing high values of Young’s modulus (1−2 GPa) and low ultimate strain upon tensile testing (5–15%). The addition of up to 18% poly(ethylene glycol) to this system, in order to introduce soft segments to the polymer, produced somewhat more flexible polyurethanes with HDI, while the use of TDI still resulted in stiff polyurethanes having a high Young’s modulus and low ultimate strain.58 Yoshida et al. also used kraft lignin as the polyol component together with a polyether triol and polymeric MDI to make polyurethanes.59,60 Polyurethanes having mechanical properties ranging from soft to hard were synthesized by varying the kraft lignin content. High content of kraft lignin (⬎30−35%) results in rigid and brittle plastics while lower amounts provide soft polyurethanes. Copyright © 2005 by Taylor & Francis

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Hatakeyama et al. also synthesized lignin-based polyurethanes.61,62 The authors used kraft lignin from different plants, as well as organosolv lignins that they dissolved in poly(ethylene glycol) or poly(propylene glycol). They reacted the mixtures with MDI to prepare polyurethane sheets or foams and observed that increasing the amount of lignin-based polyol yields stiffer polyurethanes, suggesting that lignins act as hard segments. Glass transition temperature (Tg) of ~90°C was measured while Young’s moduli as high as 60 MPa and elongation at break of ~15% were recorded for polyurethane containing ~50 wt% of organosolv lignin polyol.

25.4.5

Polyols from Cashew

Among the renewable resources studied to make biobased polyurethanes, cashew nut shell has been used. Cashew, Anacardium L., is a small genus of trees, shrubs, and subshrubs indigenous to the neotropics. It is ranked second only to the almond among the nine tree nuts of importance in the world.63 From the fruit to the shell, cashew is basically edible and useful. However, of the various natural products of the cashew, it is the caustic oil, the cashew nut shell liquid (CNSL), removed by a roasting process, which has been found to be of most interest, as it can be used in the plastics and varnish industries.64 Cashew nut shell liquid is a mixture of phenolics (i.e., cardanol, anacardic acid, cardol, and 2-methyl cardol) extracted from the shells and is a good alternative to petrochemically derived phenol. The interesting structure of cardanol (Figure 25.12) gives it attractive properties such as quick drying after baking, good electrical insulation, and good thermal stability. Typical applications that exploit its properties include brake linings, paints, varnishes, epoxy resins, and laminates. Bhunia et al. described a procedure to prepare polyurethane from cashew nut shell.65 Cardanol was recovered from cashew nut shell liquid by double vacuum distillation, and converted into a difunctional polyol which is reacted with a diisocyanate. A semicrystalline thermoplastic polyurethane was thus obtained with higher thermal stability than the petropolyetherbased polyurethanes. In other syntheses of polyurethanes, cardanol was reacted with glycol in stoechiometric amounts in the presence of phosphoric acid as catalyst. The resulting mixture was purified and reacted with aromatic isocyanate. Flexible OH

C15H(31-n) n = 0, 2, 4, 6

FIGURE 25.12 Chemical structure of cardanol.

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polyurethane films were obtained.66 Rigid polyurethanes were prepared from mixtures of cardanol, formaldehyde, and diethanolamine blended first with polyethylene glycol and then with polymeric MDI to obtain foams.67 25.4.6

Polyols from Cork

Cork is a natural material constituting the outer bark of Querbus suber L. It is constituted of suberin (45%), a natural aliphatic polyester, cross-linked to components of the plant cell wall matrix, i.e., polysaccharides (12%), lignin (27%), and other lignin-like structures (10%). The cork oak grows in the sunny south of Portugal, Spain, and North Africa. Portugal accounts for more than 30% of the world’s total cork production. It is the climate and soil types of this region that keeps the trees growing. Although cork trees are also grown in some Asian countries, only cork harvested from the Mediterranean region is of a quality good enough to be commercially viable. Cork is used in many applications because of its insulation properties (efficient for acoustics, heat, and mechanical vibration), light weight, biodegradability, recyclability, impermeability, compressibility, and high resistance to wear and tear. Therefore, the cork industry generates substantial wastes, which are usually burnt. Nowadays, researchers are working on making better use of these wastes. In a recent paper, Evtiouguina et al. have described the conversion of solid cork into polyols through a treatment with propylene oxide under pressure and high temperature in the presence of potassium hydroxide.68 The prepared viscous liquid polyols have been used to prepare polyurethane foams showing densities of ~0.03 g/cm3 and 40% of open cells for a cork-based polyol content of 30 wt%.69 In another approach, these authors extracted suberin from cork and used it to prepare polyurethanes. The process adopted consisted of sequential extraction of cork powder with dichloromethane, ethanol, and water, followed by hydrolysis of the residue by methanolic NaOH solution. Suberin was then recovered by extraction with chloroform and removal of this solvent by vacuum distillation.70 Polyurethanes were synthesized using polymeric MDI and TDI. No thermomechanical data on these products were given by the authors.71,72 The reason might lie in the fact that this process is costly because of the solvents used, and thus only limited yields are achieved, thereby limiting the amount of polyol available for polyurethane preparation.

25.5

Biobased Polyurethane Composites

Today, polyurethanes are finding a growing interest in applications such as composites due to the increasing demand for lightweight, durable, and costeffective compounds for sectors such as the automotive market. Owing to Copyright © 2005 by Taylor & Francis

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the versatility of polyurethane chemistry, a broad range of properties and applications is possible for reinforced composites, such as for seat pans, sun shades, door panels, package trays, and truck box panels.73 So far, biobased polyurethanes are not produced commercially on a large scale. Some polyols made from renewable resources are now marketed, such as Merginate® polyols, commercialized by Hobum Fettchemie in Germany, and SoyOil® in the United States. Both types of polyols are made from vegetable oils. Another polyol under development by Atofina Chemical is the soybean phosphate ester polyol. Production of biobased polyurethane composites is not yet widespread. However, some studies have been conducted by various research groups.

25.5.1

Biobased Polyurethane Composites from Merginate Polyols

Dahlke et al. studied biobased polyurethanes from Merginate polyols, vegetable oil-based products, and polymeric MDI, reinforced with natural fibers like flax, sisal, and jute.74 The Composites contained up to 80% of renewable materials, i.e., plant polyol and natural fibers. These composites were evaluated for automotive applications as interior car trims. The authors claimed that properties of pieces made of these biobased polyurethane composites were comparable to glass fiber-reinforced parts and superior to parts made from fiber-reinforced thermoplastics because of good compatibility of Merginate polyols with aromatic isocyanate, and also due to good adhesion on natural fibers.

25.5.2

Biobased Polyurethane Composites from Soybean Phosphate Ester Polyol

The rest of this chapter will focus on the use of biobased polyurethanes made from renewable resources and the dynamic mechanical properties of polyurethane made by reaction of soybean ester phosphate polyol (SOPEP) with polymeric MDI reinforced with fibers, natural or synthetic (i.e., hemp and E-glass fibers).

25.5.2.1 Experimental Section Materials. Soybean phosphate ester polyol was received from ATOFINA. Baydur 410 IMR aromatic isocyanate (pMDI) was a gift from Bayer. Pure hemp mat was received from Flaxcraft (NJ), and chopped E-glass fiber (1.5 in.) mat from Kemlite (IL). Composite Fabrication. Composites were prepared by first mixing SOPEP with pMDI. The isocyanate index used was 130 (30 mol% excess of isocyanate groups compared to hydroxyl groups). The mixture was stirred for 2 min and laid up on both sides of the fiber mat (5.5 in.⫻7 in.), which was previously dried overnight at 80°C in a vacuum oven. The wetted mats were Copyright © 2005 by Taylor & Francis

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maintained under vacuum at room temperature to remove trapped gas (~5 min) and then compression molded for 10 min at 110°C in a Carver® laboratory press to a thickness of 0.1 in. SOPEP-based polyurethane and its composites containing 20 wt% of fiber were obtained and postcured at 130°C for 2 h in an air oven. Fiber Treatments. During alkali treatment, hemp mat was immersed in 5 wt% NaOH solution for either 1 or 5 h at room temperature, followed by washing with conductivity water until pH ~6 was achieved. For the graft copolymerization of acrylonitrile, the required amount of fiber mats was vacuum dried prior to treatment. The dried fiber mat was soaked in a solution containing 3 wt% acrylonitrile, 0.5 wt% dicumyl peroxide, and 96.5 wt% ethanol for 15 min. The mat was dried overnight in hood after draining out the excess solution. The mats were vacuum dried overnight before using. Characterization. Specimen densities were determined by the mass to volume equation. Dynamic mechanical analysis (DMA) was performed with a DMA 2980 TA Instrument. Samples were tested in a three-point bending mode at fixed frequency (1 Hz) with a heating rate of 5°C/min. Thermogravimetric analysis (TGA) was carried out under nitrogen atmosphere with a Hi-Res TGA 2950 Thermogravimetric Analyzer at a heating rate of 20°C/min from 30° to 600°C.

25.5.2.2 Results and Discussion Densities of neat polyurethane and its composites were determined by the classical equation, i.e., the ratio of mass to volume (Figure 25.13). Upon hemp fiber loading and chemical treatment, the average density did not significantly change. It ranged from 1.11 to 1.13 g/cm3. However, in the case of reinforcement with E-glass, the average density was clearly higher, i.e., 1.24 g/cm3, due to the higher density of glass compared to hemp (2.6 vs. 1.3 g/cm3). Dynamic mechanical analysis (DMA) of these soybean-based polyurethanes was performed in a three-point bending mode in order to determine thermomechanical properties. The temperature dependence of the storage modulus (G⬘) for each specimen was recorded (Figure 25.14). In the case of neat polyurethane, the value of G⬘ was almost constant at low temperature (glassy state) before dropping in the region between 40° and 90°C (glass transition region). As the temperature further increased, G⬘ leveled off in the rubbery state. The presence of a region where G⬘ remained relatively constant indicated that a stable cross-linked network existed. The patterns of the curves of temperature dependence for the composite specimens were similar in nature to neat polyurethane. However, over the temperature range studied, G⬘ was substantially increased upon fiber loading (either for hemp or E-glass) due to a greater stress transfer at the matrix− fiber interface, thereby increasing the stiffness of the overall material. However, the storage modulus was systematically lower over all ranges of temperature studied for the composites containing sized hemp, i.e., alkali treated or acrylonitrile grafted, compared to raw hemp-reinforced polyurethane. Copyright © 2005 by Taylor & Francis

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1.5 1.4 1.3

Density (g/cm3)

1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 A

B

C

D

E

F

FIGURE 25.13 Temperature dependence of storage modulus (G⬘) of SOPEP-based polyurethane and its composites. (A) Neat polyurethane; (B) 20 wt% hemp; (C) 20 wt% alkali-treated 1 h hemp; (D) alkali treated 5 h; (E) acrylonitrile-grafted hemp; (F) E-glass.

4000 B

G' (MPa)

3000

F

2000 D C 1000 E A 0 30

50

70

90 110 Temperature (°C)

130

150

FIGURE 25.14 Storage modulus (G⬘) and specific modulus (G⬘/density) (at 30°C) of SOPEP-based polyurethane and its composites. A) Neat polyurethane; B) 20 w% hemp; C) 20 wt% alkali treated 1h hemp; D) alkali treated 5h; E) acrylonitrile grafted hemp; F) E-glass.

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A comparison of the average G⬘ values measured at 30°C is shown in Figure 25.15 together with the specific G⬘ (G⬘/density). At this temperature, the specific G⬘ increased from an average of 1199 MPa (SOPEP-based polyurethane) to 2937 MPa in the 20 wt% hemp-reinforced polyurethane, and 2848 MPa when 20 wt% of E-glass was added. It bears mentioning that the error bars overlap, thus indicating that the specific G⬘ was the same in both composites. These values represent respective improvements of 145% and 138% compared to neat SOPEP-based polyurethane. Alkali treatment and acrylonitrile grafting of hemp did not improve G⬘ but led to lower values, although higher than those of neat polyurethane. The average specific G⬘ values were 2178 and 2077 MPa for the 1 and 5 h alkali-treated fibers, respectively, and 2314 MPa for the acrylonitrile-grafted one. The improvement is thus 82%, 73%, and 93% in each case. However, error bars overlap, suggesting no statistically difference between those values. The glass transition temperatures (Tg) were determined from the peak of the tan delta (ratio of loss modulus, G⬙, to storage modulus, G⬘) curves (Figure 25.16). Only one Tg (95.6°C) was observed for the polyurethane, suggesting a single-phase system. Upon fiber loading the Tg of this biobased polyurethane shifted to higher values and was determined to be 110.6°C for the hemp-reinforced composite, 113.1°C for the 1 h alkali-treated, 108.6°C for the hemp mat-treated for 5 h, and finally 111.3°C in the acrylonitrile-grafted sample. In the case of E-glass, no change in the value of Tg (95.6°C) was 4500 4000

G' G ' (specific)

3500

G ' (MPa)

3000 2500 2000 1500 1000 500 0 A

B

C

D

E

F

FIGURE 25.15 Storage modulus (G⬘) and specific modulus (G⬘/density) (at 30°C) of SOPEP-based polyurethane and its composites: (A) Neat polyurethane; (B) 20 wt% hemp; (C) 20 wt% alkali-treated 1 h hemp; (D) alkali treated 5 h; (E) acrylonitrile-grafted hemp; (F) E-glass.

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120 115 110

Tg (°C)

105 100 95 90 85 80 A

B

C

D

E

F

FIGURE 25.16 Glass transition temperature (Tg) of SOPEP-based polyurethane and its composites. (A) Neat polyurethane; (B) 20 wt% hemp; (C) 20 wt% alkali-treated 1 h hemp; (D) alkali treated 5 h; (E) acrylonitrile-grafted hemp; (F) E-glass.

noted. The increase in Tg for the hemp−polyurethane composites suggests an increase in the degree of cross-linking and thus a restricted chain mobility. Good dispersion, efficient wetting, and good adhesion at the fiber-matrix interface are requirements for composites with improved mechanical properties.75 The mechanism responsible for adhesion between the fibers and the polyurethane matrix is not clearly determined. The enhancement of stiffness and Tg in hemp-reinforced polyurethane is attributed to good fiber dispersion, efficient wetting, and good fiber-matrix adhesion. Adhesion through chemical bonding can be favored since covalent bonds may be formed by reaction of free hydroxyl groups on the surface of the fiber with the isocyanate, as suggested by the literature.76 Therefore, chemical modification of the surface (i.e., alkali treatment and acrylonitrile grafting) might have a direct influence on wetting, fiber distribution, and fiber-matrix adhesion. In the case of E-glass composite, a study carried out in our laboratory on the nature of interfacial interactions between polyurethane and glass showed that the contribution of chemical bonding, covalent or ionic, is not important. Formation of an interphase region in which hydrogen bonding plays a key role is more likely to occur in glass-reinforced polyurethanes.77,78 However, further investigations need to be carried out.

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Thermal stability was studied by means of thermogravimetric analysis (TGA) under nitrogen. In the case of neat polyurethane, thermal degradation starts at ~320°C, and two maximum peaks corresponding to the maximum of thermal degradation are observed at 389° and 483°C, suggesting that at least two mechanisms of degradation are involved. The pattern of curves corresponding to the temperature dependence of the weight loss for composites is presented in Figure 25.17. Residual weight at 600°C is ~13% for neat polyurethane and a little higher for its composites with hemp (raw or sized). In the case of E-glass composite the value reached is ~34% (Table 25.3). This result is not surprising since glass does not degrade while hemp fiber begins to degrade at ~300°C. Two temperatures of maximum thermal degradation were also recorded for each sample of hemp (raw or sized) and E-glass-reinforced composites (Table 25.3). According to data given in the table, alkali-treated hemp composite is the less stable among all specimens while the maxima of decomposition are slightly shifted to higher values for composites that contain raw hemp and acrylonitrile. These results reveal that polyurethane composites reinforced with 20 wt% of either hemp or glass fibers that have improved mechanical properties can be obtained with these soybean-based polyols despite the differences in surface chemical composition and topology of the two fibers. It is worth noting that the natural lignocellulosic fiber used, i.e., hemp, proved to be a better

100

Weight (%)

80

60

D

F B

40

C 20

A

0 0

100

200

300 400 Temperature (°C)

500

600

FIGURE 25.17 Temperature dependence of weight loss for of SOPEP-based polyurethane and its composites. (A) Neat polyurethane; (B) 20 wt% hemp; (C) 20 wt% alkali-treated 1 h hemp; (D) alkali treated 5 h; (E) acrylonitrile-grafted hemp; (F) E-glass.

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TABLE 25.3 Temperature of Maximum Decomposition (Td) of SOPEP-Based Polyurethane and Its Composites Entry

Polyurethane Sample

Td1 (°C)

Td2 (°C)

1 2 3 4 5

Neat Raw hemp Alkali-treated hemp Acrylonitrile-g-hemp Glass

389 391 384 394 391

483 497 477 504 502

Residual Weight (%) 13.2 15.1 16.7 18.3 34.2

reinforcement than E-glass when one considers the higher Tg (by 15°C) and the lower density of the composites, as well as the price of this fiber (one third that of glass). Surface modification of hemp, such as alkali treatment and acrylonitrile grafting, decreases stiffness while the glass transition temperature is almost unchanged compared to nontreated hemp mat. Thermogravimetric analysis shows that the presence of hemp slightly enhances thermal degradation for the composites containing raw hemp and alkali-treated hemp. In the case of acrylonitrile-grafted hemp and E-glassreinforced composites, thermal degradation is retarded under nitrogen.

25.6

Future Perspectives

The polyurethane market is clearly on the move, as shown by world-wide consumption of this polymer (more than 8.4 million tons in 2000), with a forecasted annual growth rate higher than 5% for the next 5 years.79 The estimated growths for different parts of the world are: Asia-Pacific (7.5%), Africa/Middle East (6%), Latin America (4.5%), Europe (4%), and North America (4%). Those values are well above average for polymeric materials. This strong forecasted demand is explained by the versatility of polyurethanes, which helps them to be globally used in fast-growing and crucial sectors such as transportation, construction, and furniture. As mentioned, polyurethanes can be manufactured as flexible, semiflexible, and rigid material whose properties can be further controlled by foaming agent, surfactants, and fillers. The increasing demand of lightweight, durable, and cost-effective compounds for markets such as the automotive industry is also a key element in the future of polyurethanes. In 2000, more than 900,000 tons of polyurethanes were used by the automotive industry.73 Developments in reinforced reaction injection molding (RRIM) and structural injection molding (SRIM) of glass-reinforced polymers are expected to increase the use of polyurethanes in the automotive industry. Despite these optimistic prospects for polyurethanes, which so far are essentially made from petroleum-based raw materials, one important concern Copyright © 2005 by Taylor & Francis

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arises: the dependence on petroleum, a product in depletion with highly erratic cost. Therefore for polyurethanes such as petroleum-dependent polymers, researchers need to find new production alternatives and substitutes. The use of biobased-products as starting materials is certainly a valuable alternative to petrobased products. Indeed, they are renewable, ecofriendly, and, depending on their manner of production, can be inexpensive. Studies have proved that biobased polyurethanes in which the polyol part is made from natural resources show similar properties to their petrobased counterparts. Moreover, reinforcements of such materials with fibers (synthetic or natural) lead to materials with improved thermomechanical properties that may be used for structural applications. Still, there is a challenge that needs to be overcome: the replacement of the current widely used petrobased isocyanates with petroleum-free products. There is little doubt that in the next 20 years, biobased polyurethanes and their composites will find increasing interest and applications. As an illustration, Dow Chemical has begun commercialization of a biobased polyurethane foam for use in the carpet industry and marketed it under the trade name of Biobalance®. Biobalance polymers are manufactured using SoyOyl®, a soybean-based polyol. The current drive toward cheap, lightweight, and green products in industries such as automotive is also a factor that will result in commercialization of biobased polyurethane composites in the future.

25.7

Conclusions

Polyurethanes have been made from various renewable resources ranging from plant oils to wood. Different technologies have been successfully implemented. So far, the polyol component is easily extracted or synthesized from natural resources such as plants, some of their oils, and wood. However, synthesis of the isocyanate component is more challenging and thus satisfactory results have not yet been reached. Therefore, 100% biobased polyurethanes with acceptable properties are not available. Still, researchers have developed materials containing more than 50% biobased component and showing thermomechanical properties comparable to petrobased polyurethanes, thus opening the way to partial replacement of such petrobased products. Polyurethanes ranging from soft to rigid materials have been prepared. Control of the hydroxyl number and other additives such as a cross-linking agent or a chain extender enables modification of flexibility, rigidity, toughness, and brittleness. However, because of the higher functionality of biobased polyols used, highly cross-linked systems are obtained, i.e., thermosets. Therefore, plastics or foams behaving like thermoplastics are more difficult to make. The cost of these biobased polyurethanes varies according to the cost of the extraction and/or production of polyols from renewable resources. Polyols obtained from liquefaction of wood, cellulose, starch, or abundant plant oils (e.g., soybean or palm oil) can be competitive when compared to petrobased Copyright © 2005 by Taylor & Francis

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products. As an illustration, soybean phosphate ester polyol is forecasted in the range of from 60 to 70 cents/lb, which is comparable to most petrobased polyether polyols (50 to 70 cents/lb). According to Urethane Soy Systems Company, SoyOyl polyol costs less than petrobased polyols. The key parameter is the technology used in the manufacturing of the polyol component since, to a large extent, it controls the price of the produced polyol. Biobased polyurethanes have, however, some intrinsic limitations. Indeed, usually the chemical structure, molecular weight, and molecular weight distribution of the biobased polyol components are not totally controlled. Besides, problems such as color and odor are associated with some of these biobased products. Another very important determining element is the hydrophilicity of the products prepared from renewable resources such as carbohydrate-based materials. Such materials tend to absorb water with time; therefore their thermomechanical and morphological properties may change with time. Some results presented in this chapter indicate that biobased polyurethanes reinforced with either natural or glass fibers can be easily prepared. Composites reinforced with hemp fibers show improved mechanical properties and can compete with glass-reinforced composites. The net advantage lies in the higher glass transition temperature and the lower cost (20−30 cents/lb for raw hemp vs. 90 cents/lb for E-glass) of the hemp composite while comparable stiffness is achieved in both cases. Increasing the amount of fibers should result in higher mechanical properties and a lower cost of composites. All these facts suggest that there is a future for biobased polyurethanes and their composites. The first generation of such products, in which biobased polyols are incorporated, is now making its way to the market. We expect to see their number and applications increase in the coming years. However, more efforts are expected from researchers in improving the performances of these materials but also making a second generation of biobased polyurethanes that are exempt from petroleum-based products.

Acknowledgments Authors are grateful to GREEEN (Generating Research and Extension to meet Economic and Environmental Needs) 2001 Award No. GR01-037 for financial support.

References 1. Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA (Electronic Encyclopedia), 7th Ed., 2005. 2. Szycher, M., in M. Szycher’s Handbook of Polyurethanes, CRC Press. Boca Raton, 1999. 3. Mohanty, A.K., Misra, M., and Drzal, L.T., J. Polym. Environ., 10, 19–26, 2002. Copyright © 2005 by Taylor & Francis

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4. Gunstone, F.D., An Introduction to the Chemistry and Biochemistry of Fatty Acids and their Glycerides, Chapman & Hall, London, 1967. 5. Kirschenbauer, H.G., Fats and oils. An outline of their chemistry and technology, Reinhold Publishing Corporation, 1944. 6. Naughton, F.C., Duneczky, F., Swenson, C., Kroplinkski, C.R., and Cooperman, M.A., Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 5, Wiley, New York, 1979. 7. Johnson, R.W. and Fritz, E., Fatty Acids in Industry–Process, Properties, Derivatives, Applications, Marcel Dekker, New York, 1983, p. 13. 8. Ghatge, D. and Phadke, V.B., Elastomers from castor oil, J. Appl. Polym. Sci., 11, 629–638, 1967. 9. Petrovic, Z.S. and Fajnik, D., Preparation and properties of castor oil-based polyurethanes, J. Appl. Polym. Sci., 29(4), 1031–1040, 1984. 10. Sperling, L.H., Carraher, C.E., Qureshi, S.P., Manson, J.A., and Barrett, L.W., in Biotechnology and Polymers, Gebelain, C.G., Ed., Plenum Press, New York, 1992. 11. Kim, S.C. and Sperling, L.H., IPN’s Around the World: Science and Engineering, Wiley, New York, 1997. 12. Oertel, G., Polyurethane Handbook, Chemistry–Raw Materials–Processing– Application–Properties, Hanser Publisher, 1985. 13. CasChem Technical Bulletin 100, Biothane Systems. 14. Wong, S.W., Wang, S.B., Graves, J.F., and Odocha, I., Applications of polyols derived from renewable resources in polyurethanes, Adv. Urethane Sci. Technol., 13, 73–111, 1996. 15. Frankel, E.N. and Thomas, F.L., Selective hydroformylation of polyunsaturated fats with a rhodium-triphenylphosphine catalyst, J. Am. Chem. Soc., 49, 10–14, 1972. 16. Frankel, E.N., U.S. Patent 3,787,459. 17. Frankel, E.N. and Pryde, E.H., Catalytic hydroformylation and hydrocarboxylation of unsaturated fatty compounds, J. Am. Oil Chem. Soc., 54, A873–A881, 1977. 18. Lyon, C.K., Garrett, V.H., and Frankel, E.N., Rigid urethane foams from hydroxymethylated castor oil, safflower oil, oleic safflower oil, and polyol esters of castor acids, J. Am. Oil Chem. Soc., 51, 331, 1974. 19. Guo, A., Demydov, D., Zhang, W., and Petrovic, Z.S., Polyols and polyurethanes from hydroformylation of soybean oil, J. Polym. Environ., 10, 49–52, 2002. 20. Biermann, U., Friedt, W., Lang, S., Luhs, W., Machmuller, G., Metzer, J.O., Rusch, M., Schafer, H.J., and Schneider, M.P., New syntheses with oils and fats as renewable raw materials for the chemical industry, Angew. Chem. Int. ed., 39, 2206–2224, 2000. 21. Sherringham, J.A., Clark, A.J., and Keene, B.R.T., New chemical feedstocks from unsaturated oils, Lipid Technol., 12, 129–132, 2000. 22. Dahlke, B., Hellbardt, S., Paetow, M., and Zech, W.H., Polyhydroxy fatty acids and their derivatives from plant oils, J. Am. Oil Chem. Soc., 72, 349–353, 1995. 23. Petrovic, Z., Guo, A., and Javni, I., U.S. Patent 6,107,433. 24. Guo, A., Javni, I., and Petrovic, Z., Rigid polyurethane foams based on soybean oil, J. Appl. Polym. Sci., 77, 467–473, 2000. 25. Javni, I., Petrovic, Z.S., and Guo, A., Fuller, R., Thermal stability of polyurethanes based on vegetable oils, J. Appl. Polym. Sci., 77, 1723–1734, 2000. 26. Guo, A., Cho, Y.-J., and Petrovic, Z.S., Structure and properties of halogenated and nonhalogenated soy-based polyols, J. Polym. Sci., 38, 3900–3910, 2000. 27. Petrovic, Z.S., Guo, A., and Zhang, W., Structure and properties of polyurethanes based on halogenated and nonhalogenated soy-polyols, J. Polym. Sci., 38, 4062–4069, 2000.

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28. Chian, K.S. and Gan, L.H., Development of a rigid polyurethane foam from palm oil, J. Appl. Polym. Sci., 68, 509–515, 1998. 29. Badri, K., Ahmad, S.H., and Zakaria, S., Production of a high-functionality RBD palm kernel oil-based polyester polyol, J. Appl. Polym. Sci., 81, 384–389, 2001. 30. Sonntag, N.O.V., Glycerolysis of fats and methyl esters–status, review and critique, J. Am. Chem. Soc., 59, 795, 1982. 31. Kurth, T.M., Kurth, R.A., Turner, R.B., and Les P. Kreifels, U.S. Patent 0,119,321. 32. Harris, R.R. and Pollack, W.J., U.S. Patent 4, 116,902. 33. Bentley, J., in The Use of Oils and Fats in Paints and Surface Coatings in Lipid Technologies and Applications, Marcel Dekker, New York, 1997. 34. Lin, K.F., in Paints, Varnishes, and Related Products in Bailey’s Industrial Oil and Fat Products, John Wiley, New York, Vol. 5, 1996. 35. Stoye, D., Funke, W., Hoppe, L., Hasselkus, J., Curtis, L.G., Hoehne, K., Zech, H.-J., Heiling P. et al., Ullmann’s Enclycopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH, Weinhem, Germany, 2002. 36. Chornet, E. and Overend, R.P., Proceedings of the International Conference on Thermomechanical Biomass Conversion, Estes Park, Colorado, Oct. 18–22, 1982, p. 87. 37. Bestue-Labazuy, C., Soyer, N., Bruneau, C., and Brault, A., Wood liquefaction with hydrogen or helium in the presence of iron additives, Can. J. Chem. Eng., 63, 634–638, 1985. 38. Figueroa, C., Schaleger, L.L., and Davis, H.G., Proceedings of the 6th Annual Conference on Energy from Biomass Wastes, Lake Buena Vista, FL, 1982, pp. 1097–1112. 39. Bouvier, J.M., Gelus, M., and Maugendre, S., Wood liquefaction–an overview, Appl. Energ., 30, 85–98, 1988. 40. Kresta, J.E., Polymer Science and Technology, Vol. 18, Plenum Press, New York, 1982. 41. Hesse, W. and Jung, A., Eur. Pat. Appl. EP43097, 1980. 42. Ono, H., Yamada, T., Hatano, Y., and Motohashi, K., Adhesives from waste paper by means of phenolation, J. Adhes., 59, 135–145, 1996. 43. Kurimoto, Y., Shirakawa, K., Yoshioka, M., and Shirishi, N., Proc. Symp. Chem. Modifi. of Lignocell., New Zealand, Bull., 176, 192, 1992. 44. Kurimoto, Y., Takeda, M., Koizumi, A., Yamauchi, S., Doi, S., and Tamura, Y., Mechanical properties of polyurethane films prepared from liquefied wood with polymeric MDI, Bioresour. Technol., 74, 151–157, 2000. 45. Kurimoto, Y., Takeda, M., Doi, S., Tamura, Y., and Ono, H., Network structures and thermal properties of polyurethane films prepared from liquefied wood, Bioresour. Technol., 77, 33–40, 2001. 46. Madas, D. and Shiraishi, N., Liquefaction of wood in the presence of polyol using NaOH as a catalyst and its application to polyurethane foams, Int. J. Polym. Mater., 33, 61–71, 1996. 47. Yao, Y., Yoshioka, M., and Shiraishi, N., Rigid polyurethane foams from combined liquefaction mixtures of wood and starch, Mokuzai Gakkaishi, 41, 659–668, 1995. 48. Uehori, Y., Yoshioka, M., and Shiraishi, N., Abstracts of 42th Annual Meeting of Japan Wood Research Society, Nagoya, 1992, p. 295. 49. Kurimoto, Y., Shirakawa, K., Yoshioka, M., and Shiraishi, N., Liquefaction of untreated wood with polyhydric alcohols and its application to polyurethane foams, FRI (NZ) Bull., 176, 163, 1992.

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50. Lin, L., Yoshioka, M., Yao, Y., and Shiraishi, N., Physical properties of moldings from liquefied wood resins, J. Appl. Polym. Sci., 55, 1563, 1995. 51. Yao, Y., Yoshioka, M., and Shiraishi, N., Water-absorbing polyurethane foams from liquefied starch, J. Appl. Polym. Sci., 60, 1939–1949, 1996. 52. Lee, S-H., Yoshioka, M., and Shiraishi, N., Liquefaction of corn bran (CB) in the presence of alcohols and preparation of polyurethane foam from its liquefied polyol, J. Appl. Polym. Sci., 78, 319–325, 2000. 53. Lee, S-H., Teramoto, Y., and Shiraishi, N., Acid-catalyzed liquefaction of waste paper in the presence of phenol and its application to novolak-type phenolic resin, J. Appl. Polym. Sci., 83, 1482–1489, 2002. 54. Adler, E., Lignin chemistry–past, present and future, Wood Sci. Technol., 11, 169, 1977. 55. Glasser, W.G. and Kelley, S.S., Encyclopedia of Polymer Science and Engineering, 2nd ed., Vol. 8, Wiley, New york, 1987, p. 795. 56. American Forest and Paper Association, US. Pulp and Paper Industry’s Energy Use, Washington, DC, 1984. 57. Saraf, V.P. and Glasser, W.G., Engineering plastics from lignin. III. Structure property relationships in solution cast polyurethane films, J. Appl. Polym. Sci., 29, 1831–1841, 1984. 58. Saraf, V.P., Glasser, W.G., Wilkes, G.L., and McGraph, J.E., Engineering plastics from lignin. VI. Structure–property relationships of PEG-containing polyurethane networks, J. Appl. Polym. Sci., 30, 2207–2224, 1985. 59. Yoshida, H., Morck, R., Kringstad, K.P., and Hatakeyama, H., Kraft lignin in polyurethanes. I. Mechanical properties of polyurethanes from a kraft ligninpolyether triol-polymeric MDI system, J. Appl. Polym. Sci., 34, 1187, 1987. 60. Yoshida, H., Morck, R., Kringstad, K.P., and Hatakeyama, H., Kraft lignin in polyurethanes. II. Effects of the molecular weight of kraft lignin on the properties of polyurethanes from a kraft lignin-polyether triol-polymeric MDI system, J. Appl. Polym. Sci., 40, 1819–1832, 1990. 61. Hatakeyama, H., Sirose, S., Nakamura, K., and Hatakeyama,T., Cellulosics: Chemical, Biomedical and Material Aspects, Kennedy, J.F., Phillips, G.O., and Williams, P.A., Eds., Ellis Horwood, New York, 1993, pp. 523–536. 62. Hatakeyama, H., in Chemical Modification, Properties, and Usage of Lignin, Thomas Q. Hu, Ed., Kluwer Academic/Plenum Publishers, Dordrecht, 2002. 63. Blazdell, P., Inter. Science Rev., 25, 220–226, 2000. 64. Anand, L.C., CNSL based polymers and their applications, Paint India, 28, 13–33, 1978. 65. Bhunia, H.P., Jana, R.N., Basak, A., Lenka, S., and Nando, G.B., Synthesis of polyurethane from cashew nut shell liquid (CNSL), a renewable resource, J. Polym. Sci. Chem., 36, 391–400, 1998. 66. Minh Tan, T.T., Cardanol-glycols and cardanol-glycol-based polyurethane films, J. Appl. Polym. Sci., 65, 507–510, 1997. 67. Ghatge, N.D., Gujar, K.B., and Mahajan, S.S., Synthesis of polyurethane rigid foams from cardanol, Ind. J. Technol., 23, 195–197, 1985. 68. Evtiouguina, M., Barros, A.M., Cruz-Pinto, J.J., Neto, C.P., Belgacem, N., Pavier, C., and Gandini, A., The propoxylation of cork residues: preliminary results, Bioresour. Technol., 73, 187–189, 2000. 69. Evtiouguina, M., Barros, A.M., Cruz-Pinto, J.J., Neto, C.P., Belgacem, N., and Gandini, A., Oxypropylation of cork and the use of the ensuing polyols in polyurethane formulations, Biomacromolecules, 3, 57–62, 2002.

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70. Cordeiro, N., Belgacem, M.N., Silvestre, A.J.D., Neto, C.P., and Gandini, A., Cork suberin as a new source of chemicals. 1. Isolation and chemical characterization of its composition, Int. J. Biol. Macromol., 22, 71–78, 1998. 71. Cordeiro, N., Belgacem, M.N., Gandini, A., and Pascoal Neto, C., Urethanes and polyurethanes from suberin: 1. Kinetic study, Ind. Crops Prod., 6, 163–167, 1997. 72. Cordeiro, N., Belgacem, M.N., Gandini, A., and Pascoal Neto, C., Urethanes and polyurethanes from suberin 2. Synthesis and characterization, Ind. Crops Prod., 10, 1–10, 1999. 73. www. huntsman.com 74. Dahlke, B., Poltrock, R., Larbig, H., and Scherzer, H.D., Natural fiber reinforced foams based on renewable resources for automotive interior applications, J. Cell. Plast., 34, 361–379, 1998. 75. Hull, D. and Clyne, T.W., in An Introduction to Composite Materials, 2nd ed., Cambridge University Press, London, 1996. 76. Rials, T.G. and Wolcott, M.P., Morphology-property relationships in wood-fiberbased polyurethanes, J. Mater. Sci. Lett., 17, 317–319, 1998. 77. Agrawal, R.K. and Drzal, L.T., Adhesion mechanisms of polyurethanes to glass surfaces. I. Structure-property relationships in polyurethanes and their effects on adhesion to glass, J. Adhes., 54, 79–102, 1995. 78. Agrawal, R.K. and Drzal, L.T., Adhesion mechanisms of polyurethanes to glass surfaces. III. Investigation of possible physicochemical interactions at the interphase, J. Adhes., 55, 221–243, 1996. 79. www.pu.bayer.com

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26 Cellulose-Based Nanocomposites

Lars Berglund

CONTENTS 26.1 Introduction 26.2 Cellulose Structure and Properties 26.3 Tunicin-Based Nanocomposites 26.4 Microfibrillated Cellulose (MFC) Nanocomposites 26.4.1 MFC from Wood Pulp 26.4.2 MFC from Parenchyma Cell Wall Pulp 26.4.3 MFC from Algae 26.4.4 Disintegration of MFC 26.5 Bacterial Cellulose Nanocomposites 26.6 Nanoscale Modified Plant Fiber Structures 26.7 Concluding Remarks Acknowledgments References ABSTRACT Nature is using cellulose in the form of nanoscale fibrils as the load-bearing constituent in plants. It is of interest to use cellulose in this form for synthetic nanocomposites. Tunicin-based nanocomposites serve as inspiration for such new materials, and have been studied extensively. Microfibrillated cellulose (MFC) is a more likely candidate for commercial materials. Different plant sources are discussed as well as the disintegration procedure. Bacterial cellulose is another interesting form of cellulose for advanced materials. Technical challenges to more widespread development of cellulose nanocomposites are identified. Regarding the advantages, cellulose nanofibrils have fine diameter, large aspect ratio, biocompatibility, high strength and modulus as well as other favorable physical properties associated with the highly crystalline extended chain conformation. Advanced materials based on this constituent are therefore likely to be developed in the near future.

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Introduction

The concept of nanostructured materials design is often attributed to Richard Feynman, Nobel Prize winner in physics. He gave a dinner speech at the annual meeting of the American Physical Society in 1959.1 The speech is famous for posing the question “Why cannot we write the entire 24 volumes of the Encyclopedia Brittanica on the head of a pin?” But Feynman also discussed the potential of new materials organized at a very fine scale under the headings “Rearranging the Atoms” and “Atoms in a Small World.” In 1990, researchers at Toyota in Japan published a much cited study on polamide 6 nanocomposites where montmorrilonite clay was used.2 Caprolactam was polymerized together with the clay mineral and converted into thermoplastic composite pellets. The pellets were injection molded and the resulting nanocomposite showed remarkable properties. The Young’s modulus, tensile strength, and heat distortion temperature were dramatically improved at very low clay content. In retrospect, we now know that the improvement was caused by a combination of fine dispersion of individual clay silicate layers of high aspect ratio, preferred orientation of silicate layers and polymer molecules, as well as silicate layer effects on molecular mobility and morphology of the matrix. The strong reinforcement effects at low volume fraction resulted in a tremendous interest from the scientific community as well as from industry. With this as an inspiration, it is therefore interesting to consider the potential of nanoscale cellulose structures as reinforcement in novel composite materials. Cellulose is already used extensively, for instance in paper products and in the processing of cellulose derivatives. Cellulose is a fibrous, semicrystalline polymer of high molar mass and the most common biopolymer on Earth. It is the main constituent of plant structures, where its extended chain conformation and microfibrillar morphology contributes significant load-carrying capability. The mechanical properties of cellulose are utilized in applications of such materials as wood, paper, hemp, flax, sisal, and cotton in buildings, furniture, rope, textiles, packaging, etc. However, cellulose microfibrils are not specifically exploited in synthetic structural materials. The most important application of related interest is based on microcrystalline cellulose (MCC). This is a type of pure cellulose produced by acid degradation of celluloses, for instance, by disintegration of the cellulose in wood pulp.3 However, mechanical properties are of limited interest in current applications. MCC, for example (a well-known commercial name is Avicel), is used as a pharmaceutical tablet binder and as a rheology control agent in foods. The axial Young’s modulus of cellulose has been measured to be 137 GPa.4 This is similar to aramid fibers (trade name Kevlar). A typical diameter of a wood-based microfibril is 30 nm.5 The length will depend on the disintegration procedure. The concept of cellulose nanocomposites for load-bearing applications is fairly new. Compared with lignocellulosic microcomposites, we expect property enhancements due to higher Young’s modulus of a pure Copyright © 2005 by Taylor & Francis

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cellulose reinforcement and more finely distributed reinforcing microfibrils. In addition, improved understanding of basic nanoscale effects in cellulosic materials is likely to inspire efforts on new materials and processing routes. Because of the high Young’s modulus of cellulose, one may wonder why cellulose microfibrils have not already been disintegrated from plant structures and commercially used in structural materials. The first major problem is disintegration of cellulose from the plant cell wall at reasonable cost and without severe degradation. The second major problem is dispersion of cellulose microfibrils in a polymer matrix. The high density of hydroxyl groups at the microfibril surface induces strong interaction between the microfibrils, and they tend to agglomerate. The present objective is to provide a review of the literature on cellulose nanocomposites. Materials from different cellulose sources and prepared by different processing methods are presented. Cellulose structure and topics such as bacterial cellulose are discussed in an introductory manner for completeness, although other sources are much better for these subjects. Instead, emphasis is on the preparation procedures, resulting composites structures, and mechanical properties of cellulose nanocomposites. With the exception of extensive studies by Dufresne, Cavaille, and colleagues at CERMAVCNRS in Grenoble, France, only a limited amount of work is available in the literature. The subject appears to generate substantial interest, but is still in its infancy. There are still significant scientific and technical challenges to address before we can turn an interesting possibility into technical reality.

26.2

Cellulose Structure and Properties

Cellulose has important mechanical functions in plants, bacteria, fungi, algae, and in some animals. It is organized in stiff microfibrils where the constituent cellulose is estimated to have a Young’s modulus of at least 130 GPa.6 Crystallographic and ultrastructural studies are often performed on green algae (Valonia spp. and Cladophora spp.) of high crystallinity, on cellulose synthesized by the bacterium Acetobacter xylinum, on cellulose from tunicin (a sea animal containing large cellulose whiskers), and on cellulose in the secondary cell wall of wood. Cellulose from higher plants may be present in the primary, secondary, and tertiary cell wall, although the content is very low in the tertiary wall. In the secondary cell wall of wood tracheids, the microfibrils are predominantly organized in a unidirectional manner. The average microfibril angle is typically 10−30° with respect to the axial direction of the tracheid. In the primary cell wall, the cellulose is in the form of a woven mesh of microfibrils. Also, the microfibril itself may have a nanocomposite structure, often containing hemicelluloses such as xylan. Potato tuber tissue is an example of a source for primary wall cellulose. Here, the cellulose serves to withstand the turgor pressure from water contained in the parenchyma cell. Copyright © 2005 by Taylor & Francis

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≈ 30 nm

We may view the plant cell wall as a composite material consisting of stiff cellulose microfibrils in a polymer matrix.7 For simplicity, we may assume that Young’s modulus of the cellulose microfibrils is independent of plant species. The microfibril orientation is in the plane of loading. The Young’s modulus of the cell wall will then depend on the volume fraction of cellulose and the average microfibril angle (MFA) with respect to the loading direction. Such a model clarifies the effect of MFA on wood modulus.8 The effect of MFA on modulus is also clear from data on sisal fiber specimens and modulus differences between different bast fibers.6 A simple model of the cellulose microfibril structure is sufficient for the purpose of the present chapter. Fengel and Wegener5 present a microfibril structure in the secondary wall of wood as in Figure 26.1. The idea of making cellulose nanocomposites is usually to utilize these microfibrils of 10−50 nm in width as reinforcement in a polymer matrix. Depending on the source of cellulose and the cellulose disintegration procedure, the microfibril dimension will vary. In Figure 26.1, so-called elementary fibrils of around 3 nm in width are combined in parallel arrangement, forming microfibrils of ~30 nm in width. The width of elementary fibrils depends on the source of cellulose, with Valonia spp. having large fibrils, 20 nm. There is some controversy regarding the existence of elementary fibrils, as discussed by O’Sullivan.9 At the molecular scale, cellulose is a linear polysaccharide composed of D-glucose molecules linked by (1−4) glycosidic bonds. The stiff pyranose rings are in the chair configuration 4C1 and in nature, this fibrous biopolymer is present as semicrystalline extended chains. During biosynthesis of cellulose, enzyme complexes are adding glucose residues to the growing chain.9 The biopolymerization process takes place with almost instantaneous

3 nm Cellulose elementary fibril Hemicelluloses Lignin

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FIGURE 26.1 Model of the wood microfibril, consisting of elementary fibrils. (After Fengel, D. and Wegener, G., Wood Chemistry, Ultrastructure, Reactions, Walter deGruyter, Berlin, 1984. With permission.)

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crystallization as hydrogen bonds are formed between neighboring chains. In nature, cellulose exists in the crystalline form of cellulose I and the molar mass is high. In the secondary cell wall of wood, for instance, cellulose has ~10,000 glucopyranose repeating units.10 The structure of the crystalline cellulose I unit cell is discussed by O’Sullivan.9 The high modulus of the cellulose microfibril is due to molecular alignment and dense packing in the unit cell. Covalent bonds then dominate the loading response along the predominant direction of the molecules. As with polyethylene or aramide fibers, the polymer is in an extended chain conformation. The commonly used estimate of a Young’s modulus of 137 GPa for cellulose is supported by direct experimental data for ramie.4 Another argument favoring this value is that experimental data for wood modulus as a function of MFA can be predicted using well-established composite mechanics models and the quoted value for cellulose modulus.8 The crystalline structure of cellulose shows considerable complexity. Differences between species may be due to the order of the stacking of two kinds of crystalline sheet, up and down.9 In addition, cellulose I has two different polymorphs, Iα and Iβ. The reactivity of cellulose I is influenced by the relative proportion of the two, since Iα is metastable and has higher reactivity. The content of Iα varies between different species. Valonia spp. has 64%, ramie and cotton 20%, and tunicin 0%. Another aspect of the complexity is that cellulose present at microfibril surfaces is less ordered and sometimes termed amorphous. In Valonia spp., only 6−7% is amorphous, whereas in wood, around 30% of the cellulose may be classified as amorphous. However, the term amorphous is somewhat misleading, since the molecules are still extended in the microfibril direction although their packing density is reduced. Although many studies provide detailed knowledge regarding the morphology and crystallography of different types of cellulose, the Young’s modulus of microfibrils from different sources and subjected to different types of hydrolysis and mechanical treatment is seldom discussed. In the context of mechanical properties, this is important. Microfibrils of lower crystalline order will be less stiff since the cross-sectional density of covalent bonds is lowered. The extent of stabilizing interchain hydrogen bonds will also influence mechanical properties. In addition, the organization of microfibrils is important. Composites based on dispersed discrete whiskers are likely to behave differently as compared with those based on microfibrils in the form of a loose web of connected microfibrils.

26.3

Tunicin-Based Nanocomposites

In 1995, Favier, Chanzy, and Cavaille, at CERMAV-CNRS in Grenoble, published an excellent paper where they presented data on composites based on tunicin cellulose whiskers in a copolymer acrylate latex film.11 This paper

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and numerous subsequent studies using other matrices have renewed the interest in composites based on cellulose microfibrils. Although cellulose whiskers from tunicin are of little commercial interest, this work provides an inspiring demonstration of the reinforcing potential of high aspect ratio cellulose microfibrils. The paper presents a model nanocomposite based on unusually well-defined cellulose whiskers. The widths are 10−20 nm and the average aspect ratio is typically 70−100. A micrograph of the cellulose whiskers is presented in Figure 26.2, where the high aspect ratio is apparent. The whiskers are disintegrated from an edible tunicate, a sea animal of around 5−10 cm in diameter. The animals contain a 1-cm-thick cellulose tunic. This is cut in smaller pieces and is deproteinized by bleaching treatments. The bleached mantles are disintegrated in dilute water suspension in a blender and then in many passes through a laboratory homogenizer. The aqueous tunicin suspension is then hydrolyzed by sulfuric acid, sonicated, neutralized, and washed. The presence of surface sulfate groups is critical, since agglomeration or sedimentation is avoided. In the 1995 paper,11 the cellulose suspension is mixed with a styrene (65 w/w), butyl acrylate (35 w/w) latex containing small amounts of acrylic acid. Films 2 mm in thickness are solution cast, with an evaporation time of 1 month. In the following, this matrix is often termed acrylate latex. Many nanocomposite property studies from the group at CERMAV-CNRS tend to focus on data from dynamic mechanical thermal analysis (DMTA).

FIGURE 26.2 Electron micrograph of cellulose microfibrils from tunicin. Scale bar 0.5 µm. (From Favier, V. et al., Macromolecules, 28, 6365, 1995. With permission.)

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The effects of cellulose whisker addition to the acrylate latex are summarized in Figure 26.3. Significant improvements in modulus are observed both below and above Tg. The studies largely focus on the reinforcement effects above Tg, most likely because the relative improvement in matrix modulus is stronger above Tg. In References 12 and 13, glassy state micromechanics predictions for the modulus of cellulose whisker−acrylate latex composites with random-in-the-plane fiber orientation distribution (quasiisotropic properties) agree very well with experimental data. It thus appears as if nanofibers provide no unique modulus reinforcement effect in the glassy state. In several CERMAV-CNRS studies, the high reinforcement efficiency above Tg is discussed in great detail. The strong effects of small amounts of tunicin whiskers on nanocomposite modulus (see Figure 26.3) are explained by a percolation phenomenon. At a critical minimum volume fraction, estimated to be around 1% in one case,14 the cellulose whiskers become connected and form a continuous path. At this percolation threshold, individual whiskers are connected by hydrogen bonding, and form a rigid whisker network. Tunicin whisker network formation is concluded for composites based on the acrylate-containing latex,11,14 bacterial PHO polyesters,15 and epoxy,16,17 whereas network formation is not mentioned in the case of tunicin/PVC.18,19 Although no direct experimental morphology evidence is presented, three main arguments are provided in support of the hypothesis of percolating whisker network formation. One is that the rubbery plateau shows an

10

9 14% 6%

Log (G )

8

3% 7 1% 6 0%

5 4 200

300

250

350

Temp (K) FIGURE 26.3 Storage shear modulus, G’, vs. temperature for acrylate latex nanocomposites reinforced with tunicin cellulose. (After Favier, V. et al., Polym. Adv. Technol., 6, 351, 1995. With permission.)

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unusual stability so that it is unaffected by temperature over a wide interval. At low or no cellulose concentration where no whisker network is expected, the rubbery plateau modulus decreases with temperature for the acrylate composite (see Figure 26.3). The second argument is that a disproportionate increase in modulus occurs above the percolation threshold. This argument is clearly illustrated by data for amorphous PHO15 and for chitin whisker composites.20 The third argument is essentially that the observed increase in modulus is much higher than possible for a composite reinforced by discrete fibers. Support for this is provided from theoretical predictions of composite modulus in the rubbery state using what is termed the Halpin−Kardos equations. Beyond an assumption of random fiber orientation in the plane, the real predictive basis is the Halpin−Tsai (H−T) equations. The argument is that since the H−T equations underestimate the experimental rubbery state modulus, some mechanism must operate which is not accounted for. This argument is not correct, since the semiempirical H−T model often is not valid for large moduli differences between matrix and fiber. The discrepancy between H−T predictions and predictions from exact solutions has been shown to be large for the case of a stiff fiber in an elastomeric matrix. This is discussed in great detail in Reference 21. As a consequence, the H−T predictions presented in several publications11−14,16,22,23 for cellulose microfibrils in a rubbery matrix will underestimate experimental data even without a cellulose network. For acrylate composites11,14 and amorphous PHO,15 the whisker network interpretation is supported by a flat rubbery plateau at higher whisker concentrations and by a very strong reinforcement effect, in particular at higher whisker contents. However, whisker network formation is not a general phenomenon. It is not mentioned for PVC,18,19 and careful reading reveals that it is not the case for semicrystalline PHO either.15 Semicrystalline PHO does not show as dramatic reinforcement effect as the acrylates and the amorphous PHO. In the studies on thermoset epoxy composites,16,17 the authors also conclude the existence of a cellulose whisker network. H−T predictions are an important argument in Reference 16. Since the neat epoxy shows a flat rubbery plateau, this argument is not valid as a whisker network indication. Furthermore, the reinforcement effect in the rubbery state is much stronger in the previously discussed acrylate and amorphous PHO composites. For this reason, we predicted the rubbery state modulus using an exact solution discussed in Reference 21. Predictions were much closer to experimental data than H−T results, but still underestimated the rubbery state modulus significantly. It thus seems as if whisker network formation does indeed take place, provided a favorable latex particle coalescence process takes place. Possibly, network formation and whisker−whisker bonding is facilitated by the concentration of whiskers to the latex particle−particle interfaces. The percolating network is a phenomenon of considerable scientific interest. It also has technical implications, for instance in the case of thermal or electrical conductivity. An interesting question is how important the Copyright © 2005 by Taylor & Francis

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network is in practice for mechanical performance. References 12 and 13 demonstrate that fiber networks have no specific effects on the modulus of composites based on glassy polymers. Local fiber strain is probably much more important than cooperative fiber network deformation for this case. In the rubbery state, we have already discussed the dramatic modulus increase in the small strain DMTA test when fiber networks have formed. Hajji et al. report the percolating network effect to be important also in quasi-static tensile tests on cellulose whisker−acrylate composites.12 For 6 wt% composite films produced by water evaporation casting, the maximum stress seems to be reached at ~25% strain. Above ~50%, the stress starts to decrease until final failure at ~550 to 600%. The authors interpret the maximum stress location as a point associated with network degradation. A consequence is then that in this case the network appears to show significant integrity and reinforcement effect at strains as high as 25%. It is also interesting to consider the cases when no whisker network is formed, since this raises questions regarding network formation mechanisms and the distribution of fibers in the films. Network formation involves fiber−fiber bonding. Apparently this is prevented in the PVC system. In the semicrystalline PHO, Dufresne speculates15 that a transcrystalline layer is nucleated at the fiber surface and prevents fiber−fiber bonding. Networks seem to predominantly form when solvent casting of latex polymers is used. In Reference 12, strong effects on mechanical properties were observed for different processing routes. The highest rubbery state modulus was observed for film casting by water evaporation. Freeze drying and hot pressing of the same latex gave a much lower modulus (5 MPa as compared with 30 MPa at 6% by weight of cellulose). In Reference 23, significant throughthe-thickness inhomogeneity is reported in composites based on wheat straw microfibrils. The fiber content is highest in the bottom layer due to sedimentation of microfibrils during casting. In related tunicin studies, sedimentation seems to be hindered by the whisker network formation. The importance of percolating network formation is a main theme in the CERMAV-CNRS studies. The reports on processing effects12,23 show that strong cellulose network formation is intimately associated with the solution casting method. For this reason, freeze-dried and hot-pressed wheat straw cellulose−acrylate latex composites show much lower increases with cellulose content than the cast films.23 An important scientific issue is if cellulose whiskers cause reduced molecular mobility of the matrix close to the fiber. Tsagaropoulus and Eisenberg demonstrate such effects in nanocomposites based on very fine silica particles.24 They also observe some flattening of the rubbery plateaus, although they assign this phenomenon to a second glass transition corresponding to Tg of a loosely bound polymer phase. Chazeau et al.25 claim the presence of an immobilized PVC layer next to the cellulose whisker. Their analysis of viscoelastic DMTA data is based on a fitting procedure using the H−T equations and a three-phase assumption in order to describe changes in the hightemperature peak for the composite shear loss modulus, G⬙. The slight Copyright © 2005 by Taylor & Francis

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decrease in the tan δ peak height and peak area at Tg with cellulose addition indicate a small immobilization effect. In contrast to this weak effect, Dufresne15 found strong immobilization effects in a cellulose whisker composite based on amorphous bacterial polyester, polyhydroxyoctanoate (PHO). The effect is also evidenced by a sharp decrease in tan δ peak area and height at Tg as the cellulose content increases. This particular effect is stronger than effects on peak height reported in Reference 24. The effect is also strong in the tunicin−acrylate latex composites.14 The phenomenon indicates that the fraction of polymer matrix molecules participating in the glass transition decreases as the cellulose content increases. It confirms the presence of immobilized or partially immobilized polymer phases due to the cellulose whiskers. This is a scale effect caused by the large specific microfibril surface area. Dufresne’s paper is very important because it demonstrates that there are unique nanocomposite reinforcement effects also in composites based on cellulose microfibrils.15 The reduced molecular mobility phenomenon is discussed in Reference 24 for silica nanocomposites. The phenomenon reduces diffusion coefficients and influences other physical properties sensitive to matrix molecular mobility. Complex modulus changes with temperature above Tg are usually influenced as well.24 In the case of a semicrystalline PHO composite, no immobilization effects were observed, and Dufresne speculates that transcrystallization may cause the lack of molecular mobility changes.15 From Dufresne’s work on amorphous and semicrystalline PHO, it is clear that the immobilization phenomenon may be very significant for cellulose whisker composites and is sensitive to the nature of the polymer matrix. The cellulose surface may also cause morphological effects. Noishiki et al. investigated composite films based on tunicin whiskers and silk fibroin.26 The silk fibroin was obtained from silk cocoon and dissolved in LiSCN. Composite films were prepared by solution casting. The Young’s modulus of the films increased almost linearly with cellulose content. Pure silk had a modulus of ~3 GPa and the film of 100% cellulose showed a modulus of ~10 GPa. The tensile strength of the tunicin−silk composites showed a maximum of ≈155 MPa at ~70−80% cellulose. It was convincingly demonstrated by FTIR and XRD that the silk crystallized as silk II, which is in the form of an antiparallel β -sheet. From the variation in silk morphology with cellulose content, it seems that silk II formation is induced at the fiber-matrix interface by the highly ordered tunicin whisker surface. In Reference 15, corresponding tunicin film modulus data from the CERMAV-CNRS group are reported to be 15 GPa. A film from tunicin cellulose whiskers is probably a fiber network with some porosity. The mechanical properties are then sensitive to several parameters, for instance, density, fiber orientation distribution, fiber−fiber bonding, and moisture content. Cellulose nanocomposites based on plasticized starch are of great interest since the material is an all-biopolymer system. Such materials have complex morphology though, since the system contains not only starch, which is Copyright © 2005 by Taylor & Francis

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complex in itself, but also glycerol, water, and cellulose. The mechanical behavior is also complex. In particular, the reinforcing effect in tunicin whisker composites based on plasticized starch is disappointing.27 Glycerol and water are interpreted to destroy interfacial adhesion and therefore the effect of the cellulose reinforcement. However, at 35% relative humidity and 25% by weight of tunicin cellulose, the stress-strain behavior in uniaxial tension improved considerably as compared with lower cellulose content. This is interpreted to be due to the formation of a “sparse cellulose whiskers percolating network”.27 DMTA data indicate increased modulus already at 16.7% by weight of cellulose, so the effect of the network is apparently sensitive to the type of mechanical test performed.

26.4 Microfibrillated Cellulose (MFC) Nanocomposites 26.4.1

MFC from Wood Pulp

The terminology used in the literature for different types of cellulose microfibrils is somewhat confusing. Microcrystalline cellulose, MCC, is an established term for a commercially available material used as a rheology modifier and as a tablet binder in the pharmaceutical industry. It is produced by chemical hydrolysis, often in combination with mechanical milling of some kind. It is available as a powder. Although MCC consists of cellulose microfibrils, the purpose is neither to preserve cellulose molar mass during extraction nor to utilize MCC as a reinforcement phase in composites. The term microfibrillated cellulose (MFC) was introduced by Turbak and others.28,29 Today there are few, if any, commercial applications. It was originally produced without any chemical degradation step.28 In many of the more recent scientific studies, the objective is clearly to obtain disintegrated microfibrils as a reinforcement phase, although both hydrolysis and mechanical homogenization is used. It still seems adequate to predominantly use the term MFC in the context of composite materials, since the term is descriptive. Regardless of extraction/disintegration route, MFC seems like a suitable term when the desired final cellulose for reinforcement is microfibrillar in nature. A well-known trade name for microcrystalline cellulose is Avicel™. The material is 100% cellulose and very pure due to the typical applications. MCC is described in great detail in a monograph by Battista.3 As compared with the production of other types of cellulose containing microfibrils, MCC emphasis is on chemical hydrolysis of plant pulp. MCC is often produced by disintegration of wood pulp by acid hydrolysis, sometimes combined with severe shearing or milling of a dilute water suspension. Obviously, other sources of cellulose, for instance, cotton, may also be used. During hydrolysis, pectins and hemicelluloses are removed Copyright © 2005 by Taylor & Francis

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and covalent bonds of the cellulose molecules are broken at sites of molecular disorder. MCC forms a stable gel in water, and this is a major reason for its use. Preserving the molar mass of the native cellulose is therefore not a primary goal in commercial production of MCC. A typical degree of polymerization, DP, is 145.30 Since DP correlates strongly with microfibril aspect ratio,3 the aspect ratio of MCC is lower than what can be obtained from milder hydrolysis or by mechanical disintegration of wood pulp. It is of some interest to study composites based on MCC, since even short microfibrils are expected to contribute favorably to the mechanical behavior of most polymer composites. MCC may be viewed as readily available baseline reinforcement. Its average microfibril aspect ratio and fibrillar structure controls the reinforcement effects. A problem is that once the MCC is dried, the microfibrils are very difficult to disintegrate since hydrogen bonds cause strong adhesion between the individual microfibrils. Just water is not sufficient for dissolution. In Reference 31, dimethyl formamide, DMF, and lithium chloride were used to dissolve individual microfibrils in MCC. A polyurethane elastomer was then polymerized in the solution and the DMF was evaporated during a nanocomposite film casting procedure. Since the solvent can dissolve cellulose at high concentrations and suitable conditions, the preservation of the cellulose microfibrils needed confirmation. XRD and TEM confirmed the presence of crystalline cellulose and microfibrils. The property enhancement was significant; in particular, tensile strength increased and the strain to failure increased from 390% to more than 800% as 5% by weight of MCC was added. The rubber plateaus in the DMTA data show only a weak reinforcement effect, which indicates that there is no cellulose network and the average MCC microfibril aspect ratio is not very large. However, FTIR spectroscopy showed changes in the chemical structure of the polyurethane network, and also chemical bonds between cellulose and the network. The increased strain to failure must therefore be related to these effects and the small scale of the reinforcing microfibrils as well as their reorientation at high strains. Boldizar et al. subjected wood pulps to different hydrolysis procedures so that celluloses of different DPs were produced.30 Thermoplastic composites were then compounded, injection molded, and the mechanical properties determined. The reinforcement effect was significantly better as compared with thermoplastics mixed with conventional pulp fibers. For instance, PP composites based on ~37% by volume of cellulose showed a Young’s modulus of ~5.2 GPa for conventional bleached pulp and 6.8 GPa for hydrolyzed cellulose. This was interpreted to be a result of the increased reinforcement aspect ratio (fiber length divided by diameter). PP composites based on Avicel MCC showed a weaker reinforcement effect as compared with hydrolyzed cellulose of higher DP. For PS at ~26% by volume of cellulose, E ≈ 4.8 GPa for conventional bleached pulp and around 7.3 GPa for hydrolyzed cellulose. An interesting and neglected part of the study was the preparation of PVAc films containing large amounts of hydrolyzed and homogenized cellulose. Most likely, this is the first report on cellulose nanocomposites. Copyright © 2005 by Taylor & Francis

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The Young’s modulus was improved from 63 MPa to 1.6–2.9 GPa at a cellulose content of 40 wt%. The best results were obtained with cellulose at DP 200–400. Lower DP gave weaker reinforcement effects. The aspect ratio was estimated to be ⬎100, although this seems too high. For the injectionmolded composites, an aspect ratio of 50 used in combination with a semiempirical composite micromechanics equation was able to correctly predict experimental data for Young’s modulus as a function of cellulose content. Attempts to injection mold composites from homogenized cellulose latex composites gave disappointing results. This was interpreted as being due to agglomeration of the cellulose. It was also mentioned that polyamide (PA6, 66, 11, and 12) cellulose composites produced by compounding and injection molding did not show strong reinforcement effects. In 1983, Herrick et al.29 and Turbak et al.28 described a process for preparation of MFC. In this case, wood pulp fibers were cut in the dry state to a fiber length of 0.7 mm and suspended in water. The suspensions (1–2%) were subjected to repeated high-pressure (55 MPa) homogenizing action at 70°–90°C in lab homogenizers. A drawing of the homogenizer is presented in Figure 26.4. Disintegration of cellulose into microfibrils occurs due to the high shear forces generated in the narrow slit apparent in the figure. In the mentioned studies, almost no reduction in degree of polymerization took place, although a highly microfibrillated material resulted. Microfibrillation was facilitated with neverdried pulp fibers. Cellulose DP was ~1000, which is similar as in the wood pulp.29 In Reference 28, DP was as high as 2200 after microfibrillation as compared with 2500 previously. MFC cannot normally be dried since it then loses its microfibril character. During drying, strong hydrogen bonds form between microfibrils and dissolution becomes extremely difficult. The major use of the original MFC was as a suspending medium for other solids, as an emulsifying base and with many applications as a food additive, in paints, cosmetics, as a binder in nonwoven textiles, etc. Due to processing requirements, MFC was available as a dilute aqueous suspension of ~2%.

Microfibrillated cellulose

Valve seat Pulp fibers

Spring

Flow

Homogenizer valve

Impact ring FIGURE 26.4 Homogenization machine.

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Films have also been produced on the basis of MFC.32 The tensile strength of films based on wood pulp MFC was 2.5 times that of commercial grade print paper. Films based on tunicin whiskers showed similar properties. Nakagaito and Yano have used kraft fiber-based MFC with a phenol− formaldehyde resin matrix to produce high-modulus composites33. Although the same Young’s modulus E was obtained with microscale pulp fiber composites, the flexural strength of the nanocomposites was substantially higher. The reason is the fine structure of the microfibrils. Microscale cracks are most likely delayed to higher strain in a material with welldispersed nanoscale reinforcement. Yano and Nakahara produced MFC materials of 16 GPa modulus using MFC only.34 This is similar to the bacterial cellulose films reported in Reference 35. As Yano and Nakahara added 2% starch to their MFC films, strength increased although modulus decreased significantly (from 16 to 12.5 GPa). MFC in the form of rods have been observed to form ordered structures in water suspensions. Orts and others analyzed MFC suspensions by small angle neutron scattering.36 The microfibrils were prepared by sulfuric acid hydrolysis of pulp from wood and cotton. The presence of sulfate groups induces repulsion between the microfibrils in suspension. The distance between microfibrils decreases with increased microfibril concentration. The behavior of aqueous cellulose suspensions was typical of chiral nematic liquid crystals. Their order could be improved by application of a magnetic field. The dimensions of one fraction of the microfibrils were determined to be 280 nm, with an average aspect ratio of 47. Interestingly, the data indicate that the rodlike microfibrils are twisted. The relaxation rate after shearing the suspension was slower for higher aspect ratio MFC. It would be interesting to investigate if microfibril ordering can take place during solution casting of cellulose nanocomposites. Araki et al.37 also reported on aqueous cellulose suspensions, which were isotropic at low concentration (⬍ 1%). Different surface charges of microfibrils were introduced by different sulfation procedures. Surface charge influences the phase transitions in cellulose suspensions. One sulfonated suspension separated into an isotropic and an anisotropic phase at 5−11% concentration. One specimen formed a gel at 7% concentration. These observations point to the complexity of cellulose microfibril organization in aqueous suspensions. The gel observation also supports the CERMAV-CNRS studies on microfibril networkformation in their latex composites.

26.4.2

MFC from Parenchyma Cell Wall Pulp

MFC based on parenchyma cells (i.e., sugar beet, potato tuber) tends to show small microfibril diameter, and the fibrillar structures resemble swirled mats of connected microfibrils rather than discrete rods. Dufresne et al. used potato pulp as a source of cellulose microfibrils and prepared interesting composites based on starch matrices.38 Potato pulp is available as cattle feed Copyright © 2005 by Taylor & Francis

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and consists of 40% cellulose. It is a residual from the production of starch, which has largely been removed in the cattle feed product. In addition to cellulose, the cattle feed potato pulp contains residual starch, pectins, hemicelluloses, and lignin. The potato pulp is first hydrated and ground. This is followed by sodium hydroxide extraction and bleaching in sodium chlorite. The cellulose is disintegrated in a blender and then passed 15 times through a slit homogenizer at elevated temperature. After this final step, cellulose microfibrils are obtained. One may note that the fibril form is different as compared with MFC from wood pulp sources. The microfibrils have smaller diameter and seem to be intertwined rather than present as individual rods (see Figure 26.5). The yield is 20% as compared with the starting material. The homogenized potato pulp MFC is an interesting constituent for novel nanocomposites. Starch−cellulose composite films were therefore produced. The starch was also modified by glycerol, which serves as a plasticizer. Up to 50% cellulose content was added to the starch. This led to an increase in glassy state modulus to 6.9 GPa as compared with 2.1 GPa for unmodified starch in the dry state. Interestingly, MFC addition significantly lowered the water absorption. In some cases, the cellulose constituent seemed not to absorb any moisture at all. In 1998, Dufresne and Vignon produced starch−cellulose composites which showed extreme reinforcement effects.39 At 3.4% by weight potato pulp MFC, the rubbery state modulus was increased several orders of

FIGURE 26.5 Electron micrograph of cellulose microfibrils from potato cell wall. (After Dufresne, A. et al., J. Appl. Polym. Sci., 76, 2080, 2000. With permission.)

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magnitude, and it was nearly showing glassy state behavior. One may note that the starch matrix system was complex and contained both glycerol and starch (complex in itself). Surprisingly, doubling the amount of cellulose hardly produced any changes in modulus above Tg. Still, the data indicate the formation of a microfibril network providing unique reinforcement to a rubbery state matrix. In fact, one may speculate that the effect is biomimetic, since the cellulose content in plant parenchyma cell walls is not very high despite the considerable turgor pressure it must sustain. The later research on the same material38 showed a slightly different picture. Room temperature tensile tests were performed at different glycol and moisture contents. The reinforcement effect in a starch matrix plasticized by both moisture and glycerol was weaker than observed in Reference 39 for dry rubbery modulus. The moisture sensitivity of highly glycerol plasticized starch is probably due to strongly reduced adhesion between the MFC and the matrix. Since the strains involved most likely are higher as compared with the DMTA tests, this may also contribute to a weakening of the reinforcement effect. Starch−cellulose composites are interesting materials in an environmental perspective. No chemical reaction is involved, the constituents are native, and processing can be carried out in water solution at room temperature. It was demonstrated that at high cellulose content, the moisture content is lowered significantly as compared with the neat starch material.38 An interesting detail is that glycerol addition to the starch matrix significantly lowers the diffusion coefficient, but not the maximum moisture content. MFC was also prepared from sugar beet pulp.40 This is the residue from sugar production and is also traditionally used as cattle feed. The study used different cellulose extraction routes so that the purity of the cellulose varied. The pectin content was of prime concern. The MFC was converted to films and film properties were measured. The presence of pectin improved mechanical properties of films. The pectin is probably able to bind microfibrils together and stiffen the network. As previously, MFC was extracted by a combination of chemical and mechanical treatment. The film properties were improved as both chemical and mechanical treatment was carried out. This is probably because a larger degree of disintegration improves the microfibril aspect ratio and the microfibril network. However, the extent of fiber−fiber bonding may also increase with disintegration. The lower modulus as compared with conventional paper was discussed and higher water content was one reason. One may speculate that out-of-plane orientation of microfibrils and lower density could be other factors. The idea to keep pectin as a matrix is interesting. It is highly compatible and the procedure to just leave pectin in the material (to function as a matrix or binder) is elegant. The analogy for wood microfibrils would be to leave some hemicellulose. The disadvantage with both pectin and hemicelluloses is moisture sensitivity. Hepworth and Bruce41 prepared composites based on swede root parenchyma cells and poly(vinyl alcohol) (PVOH). The preparation procedure Copyright © 2005 by Taylor & Francis

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of the cellulose was less cumbersome than the homogenization procedure discussed previously, although true MFC was not produced. A food processor was used to disintegrate tissue into a fine paste. A detergent was used to destroy lipid membranes. The pulp was collected, washed, and then subjected to HCl hydrolysis at 50°C for 2 h to remove pectins. After washing and grinding, 90% of the water was squeezed out. This paste was mixed with a 10% PVOH solution and water was again squeezed out in a muslin bag placed between metal plates. The paste was compressed to sheet form and subjected to 60°C for 5 h. The cellulose microfibrils were not completely disintegrated in this study since optical micrographs showed that cell walls were only partially broken down by the treatment. The cellulose volume fraction was 22% and the Young’s modulus was 5.4 GPa with a tensile strength of 70 MPa. Water absorption is very large in this material. A composite placed in water increased in weight by 4.4 times during 30 min. 26.4.3

MFC from Algae

Algae are potential sources of cellulose for use in composite materials. A substantial part of the literature on algae cellulose focuses on the structure of the green alga Valonia, as in Reference 42. Valonia ventricosa can be harvested outside Florida. This species has become a standard for native cellulose I since it has large microfibrils of very high crystallinity. Each microfibril consists of 1200−1400 extended cellulose chains in parallel organization. The microfibril is essentially a single crystal with a width of up to 100 nm. The proportion between Iα and Iβ structure in Valonia is 65/35. Variation in proportions between these two polymorphs is an important reason for the differences between cellulose structures in nature. Ek et al.43 studied the morphology of cellulose powder from Caldophora spatza algae from the Baltic Sea. As compared with the Avicel plant MCC powder, the surface area was more than 50 times larger. The microfibrils in the algae cellulose powder are therefore more accessible and dissolution or chemical modification is facilitated. The degree of crystallinity is close to 100% of Cladophora sp. whereas it is 74% of plant MCC. The microfibril width is around 38 nm. Hydrolyzed MFC from Cladophora sp. collected in the Baltic Sea has also been produced.44 It was solution cast with ethylene-vinylacetate (EVA) from THF. Avicel MCC was used as a reference material. In order to disperse the MFC in the fairly nonpolar EVA, silane modification was necessary. This was carried out by attaching 17.3% by weight of organosilane to the algae cellulose. For the MCC, the corresponding amount was only 8.7% by weight. The MCC composite showed lower reinforcement effects due to agglomeration. The large surface area of algae MFC facilitates surface modification, which helps dispersion. Obviously, the MFC cost is increased by surface modification. The Young’s modulus in the rubbery state was increased 10 times at 5% by weight of algae MFC. This effect is weaker than for cast acrylate latex−wheat straw cellulose composites. However, the effect is stronger than for freeze-dried Copyright © 2005 by Taylor & Francis

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and hot-pressed films of the same material, where no cellulose network is expected. The effective stiffness of the algae MFC may then be interpreted as higher than for the wheat straw cellulose. The effective stiffness refers to the combined effect of microfibril modulus and its aspect ratio. We may conclude that algae microfibrils at least are of scientific interest as composite material reinforcement. A mechanical homogenization procedure28 after hydrolysis could probably further disintegrate the algae cellulose, improve the MFC aspect ratio, and improve dispersion in the polymer matrix. 26.4.4

Disintegration of MFC

Extraction and disintegration of cellulose from plants and animals is not straightforward. Biochemical or chemical processes such as enzymatic hydrolysis, acid hydrolysis, or bleaching are used in the first step. Polymers such as hemicelluloses, lignin, pectin, proteins, and starch are extracted from the plant or the animal structure, whereas the cellulose remains. The term disintegration is often used to describe the process where microfibrillar cellulose is loosened from its microfibrillar neighbors or surrounding material. The chemical and biochemical processes for extraction therefore often lead to disintegration. So far, chemical hydrolysis and severe mechanical homogenization procedures have been repeatedly mentioned in connection with MFC preparation. However, chemical hydrolysis often degrades the molar mass and therefore the aspect ratio of cellulose microfibrils. Repeated mechanical homogenization of dilute water suspensions of swollen cellulose-rich pulp is currently the best method to produce MFC. However, the method is energy demanding and large amounts of water need to be removed during subsequent materials processing. Cutting or milling procedures for the pulp may help by reducing the number of runs required through the homogenizer. Steam explosion processing is a method to consider in this context.45 Today, the decompression phase can be carefully controlled and nonaqueous environments can be used. This allows better tailoring of the morphological and supermolecular structure of the resulting material. The steam ionization and acetic acid formation during steam explosion lead to degradation of lignin and hemicelluloses whereas the degradation of cellulose is claimed to be limited. The cellulose becomes more reactive and accessible. Rapid expansion of water present in pores of the cell wall increases the accessibility and surface area of the material. For instance, enzymatic hydrolysis is facilitated and the tendency of cellulose to dissolve in certain solvent systems is increased. One reason, in addition to larger surface area, may be that the intermolecular hydrogen bonds are modified. Enzymatic hydrolysis of cellulose in plants and animals is an interesting alternative to chemical hydrolysis since it takes place at low temperature without any need for strong acids. As an example, the action of crude cellulase enzyme complexes on Valonia has been studied.46 Each microfibril was fibrillated into longitudinal subelements of widths between 2 nm and the Copyright © 2005 by Taylor & Francis

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initial microfibril width. The enzymes of the enzyme complex are of considerable size, with diameters in the range of 2−7 nm. For this reason, mechanical or physicochemical pretreatment of the cellulose substrate can increase the cellulolytic action dramatically, since surface area is increased, making the cellulose more accessible. Delaminated Valonia cell walls show microfibrils of ~20 nm in width.46 The length seems to be several micrometers. After 48 h digestion by a cellulase complex, subfibrils of 2−20 nm in widths are apparent. It appears as if a more advanced state of enzymatic hydrolysis also leads to degradation of microfibril length. Also, the molecular order in the crystallites decreases dramatically with microfibril degradation. Possibly the state of order is paracrystalline rather than crystalline. As microcrystalline, chemically hydrolyzed Valonia cellulose is subjected to the cellulase enzyme complex, the resulting microfibril length is in the range 500 nm to several micrometers.46 With bacterial cellulose, 50−80 microfibrils compose one ribbon. During that case of enzymatic degradation, it is likely that individual microfibrils are separated from each other. With Valonia, Chanzy and Henrissat46 demonstrate that enzymes can release not only loose hydrogen bonds holding microfibrils together but also stronger hydrogen bonds linking chains within crystallites.

26.5

Bacterial Cellulose Nanocomposites

Cellulose is produced from some bacteria, and A. xylinum, vinegar, or acetic acid bacteria are the ones most widely studied. Bacterial (or microbial47) cellulose (BC) is an extracellular product which is excreted into the culture medium. Acetic acid bacteria are not photosynthetic but can convert glucose, sugar, glycerol, or other substances to pure cellulose. A typical single cell can convert up to 108 glucose molecules per hour into cellulose. The activities in a single cell are numerous. Each cell acts as a spinneret and produces a bundle of submicroscopic fibrils fetching polymer aggregates through a row of pores.35 The bacteria produce a membrane of pure cellulose, which is very strong in its never dried state. It also has an extremely large absorbance of water. BC is frequently studied in order to clarify the mechanism for biosynthesis of cellulose.48 Details of mechanisms for biopolymerization and simultaneous crystallization in long parallel rows of exended chains are understood fairly well. As an example of the nature of such studies, effects of pectin and xylan medium on the crystallite morphology have been demonstrated.48 The properties of BC films are interesting. Iguchi et al.35 report on studies where the in-plane modulus of BC films approached 30 GPa. The films are produced by a heat-press method where a gel BC sheet is sandwiched between stainless steel or similar meshes to allow escape of water. The density of interfibrillar hydrogen bonds is expected to be much higher than in conventional paper. Copyright © 2005 by Taylor & Francis

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Grunert and Winter49 produced cellulose nanocomposites based on BC and cellulose acetate butyrate (CAB). Solution casting of films was carried out using acetone. The BC was hydrolyzed using sulfuric acid. The dimension of the cellulose crystals was 50 by 8 nm in cross section and a few hundred to several thousand nanometers in length. The BC is commercially available under the trade name Primacel™. The BC was silylated. The unmodified BC showed an increase in modulus at 81°C from 900 MPa to 1750 MPa at 10% by weight of cellulose. The authors state that native cellulose shows slightly better reinforcement effect, although they also explain that 18% by weight of silylated BC is due to the silyl groups. This difference in volume fraction of BC must be taken into account, and it is then clear that native and silylated BC show similar modulus reinforcement efficiency, as expected from composite micromechanics. The study illustrates that surface modification of nanoscale microfibrils requires a substantial amount of modifying compound. An interesting detail is the tan δ data by Grunert and Winter. At Tg, the peak height decreases significantly with increasing BC content, indicating significant interphase effects. Thus, the molecular mobility of the matrix is strongly influenced by the cellulose microfibrils. This indicates that the fraction of CAB chains participating in the glass transition decreases with increasing cellulose content. The effect of silylation on tan δ changes is small and almost negligible. Considerable interphase and matrix morphology effects are then indicated for tunicin whisker composites15,26 and BC composites.49 Perhaps large microfibrils of high surface perfection are more suited to induce interphasial effects than smaller microfibrils based on wood pulp or parenchyma cell walls. BC composites have considerable scientific interest. However, widespread industrial use of BC composites would probably require the development of efficient large-scale fermentation technology. Only then can BC be available at a competitive cost. In a review from year 2000, the dry-base cost at that time was estimated at $30 US/kg.35

26.6

Nanoscale Modified Plant Fiber Structures

The nanocomposites discussed so far are all based on the concept of mixing cellulose with a polymer and then processing the mixture into a composite material. The disadvantage is that the control of microfibril orientation distribution in the new composite material is very difficult. The application of magnetic fields during processing can orient MFC in aqueous suspension and has been mentioned as a potential route to cellulose nanocomposites of controlled MFC orientation.50 In wood tracheids, the microfibrils are predominantly organized parallel to each other and at a fairly small microfibril angle (MFA) with respect to the axial direction of the cell. For this reason, wood pulp fibers or wood veneer are of interest as precursors for material synthesis. Copyright © 2005 by Taylor & Francis

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Matsumura, Sugiyama, and Glasser51 prepared interesting nanostructured materials from pulp fibers. Wood pulp fibers are in the first step reacted in a solvent medium that does not fully penetrate the fibers. In the following step, the partially modified pulp fibers are hot-pressed at elevated temperature. A semitransparent sheet of thermoplastic character is obtained from the procedures. During the first step, a thermoplastic cellulose ester layer is formed at the surface of the microfibrils. The material is thermoformable but still biodegradable. The Young’s modulus of the material is ~1.3 GPa and its tensile strength 25 MPa. The starting material is dissolving grade pulp fibers, commonly used in industries producing cellulose derivatives such as cellulose textile fibers. The cellulose ester formation takes place in a mixed p-toluene sulfonic/hexanoic anhydride system in a cyclohexane-based nonswelling medium. The derivatization causes no apparent changes to the shape of the pulp fibers. Fiber swelling reduces significantly by the modification. XRD data proves that ordered cellulose I crystallites are present in the composite. At least part of the native cellulose morphology is therefore preserved. The material may be viewed as composite of cellulose microfibrils surrounded by a thermoplastic cellulose ester matrix (cellulose hexanoate). The fairly low value of Young’s modulus may indicate that the volume fraction of remaining cellulose I is not so high. The processing concept for this material is very interesting and seems to rely on esterification proceeding from the microfibril surface and inward. The effect of degree of substitution (DS) on Young’s modulus E was investigated and a maximum was found at DS ⫽0.5–0.6. In a second publication,52 the morphology of the material is characterized by atomic force microscopy (AFM). The scale of the distributed phase in compression-molded pulp fibers is several 10s of nanometers. This is what we would expect for cellulose microfibrils. Etching with enzymes produces protruding entities of the same scale and periodicity. Regenerated Lyocell cellulose fibers are used as model materials in Reference 52 and also esterified to a DS of 0.6. The AFM data demonstrate a 1-µm-thick surface layer in the 12 µm diameter fiber, whereas the core appears unsubstituted. Thermal analysis showed a Tg of the surface layer in agreement with the Tg of cellulose hexanoate. Yano et al. preserve the microfibril organization of wood veneer in their composite work.53 Pine veneer is the starting material. The basic idea is to increase the volume fraction of cellulose microfibrils in the material. This is carried out by the combination of two routes. One is to partially remove the lignin and hemicellulose wood polymers. The other route is to compress the veneer so that the porosity of the cell lumen is reduced. Phenol formaldehyde resin (PF) is used to preserve the compressed microstructure of the material and to bond the microfibrils. Lignin is removed by chlorite delignification. Remaining lignin and hemicelluloses are then removed by a combination of chlorite and sodium hydroxide treatment under mild conditions. The challenge is to remove as Copyright © 2005 by Taylor & Francis

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much as possible of the matrix wood polymers without degradation of cellulose and with preservation of the wood cell wall structure. After repeated treatment with chlorite and sodium hydroxide, rinsing and drying, the samples were soaked in aqueous PF-resin solution. The soaking was carried out for 3 days. The materials were then dried and the final weight gain was 21–25%. Veneers were then stacked and laminated under pressure at elevated temperature. The cellular structure was compressed and the PF resin polymerized so that the compressed microstructure was preserved and the microfibrils were bonded together. The final specific gravity of the material was ~1.4. Compared with the control (no removal of wood polymers), the Young’s modulus was improved from 26.1 GPa in the axial direction to 38.7 GPa. The reason is that the volume fraction of cellulose microfibrils increased substantially, the weight loss after matrix removal was 29.8%, although the integrity of the cellular structure was still preserved. It is interesting to note that the flexural strength was as high as 454 MPa (control 317 MPa). Because of the nature of flexural fracture in brittle materials, we expect the tensile strength to be much lower. Hepworth et al.54 carried out a solvent exchange methodology on flax fibers in order to introduce epoxy into the cell wall. The flax was swollen in an aqueous urea solution. The urea was washed out and the water replaced by ethyl alcohol. As the cell wall was in this state, epoxy was able to penetrate the cell wall and the lumen. The mean perimeter of the fiber cells increased from 0.064 (initial dry state) to 0.083 mm (epoxy impregnated after solvent exchange). The Young’s modulus of composites based on the same weight of unidirectional fibers increased from 11.5 GPa to 15 GPa whereas the tensile strength was unchanged. It is not clear what the volume fraction of fibers was in the material. The difference between the two composites was in the distribution of epoxy. The stiffer composite had part of the epoxy matrix in the cell wall forming a nanostructured cellulose−epoxy cell wall. The constant fiber weight in both composites meant that the volume fraction occupied by the epoxy-impregnated cell wall phase was higher than for the cell wall in the composite based on dry untreated flax fibers.

26.7

Concluding Remarks

Nanocomposites based on microfibrillated cellulose (MFC) represent a new way of utilizing the inherently high modulus and strength of cellulose. The numerous studies undertaken at CERMAV-CNRS in Grenoble are likely to inspire an expansion of this research area. In order for these materials to make the transition into industrial technology, a favorable cost/performance ratio is required. In this context, wood pulp and waste products from the food industry in the form of vegetable and fruit pulp (or cattle feed pellets based on those pulps) are of interest as sources of cellulose. Extraction and disintegration of Copyright © 2005 by Taylor & Francis

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cellulose are typically carried out by chemical hydrolysis and bleaching combined with severe mechanical treatment in a slit homogenizer. Wood pulp offers the advantage that MFC can be obtained by homogenization only. The majority of the MFC composites described in the literature are produced by solvent casting. Other processing concepts need to be developed or investigated more thoroughly, such as extrusion, injection molding, and compression molding. In particular, environmentally friendly processing routes at low temperatures and pressures are of interest. Processing of water suspensions needs to be considered, since MFC is obtained in that form after homogenization. In connection with processing, the issue of MFC dispersion in the polymer matrix is critical. Tendencies for MFC agglomeration are strong in many cases, since the polymer matrix is often nonpolar in nature. Mild sulfuric acid hydrolysis seems to improve MFC dispersion in aqueous suspension due to introduction of sulfate groups. Surface modification of MFC by silanes or other compounds can in principle be used. However, due to the large surface area of MFC, up to 20% by weight of silane or similar compound may be required. Many studies focus on low MFC content composites in the rubbery state. Unique nanocomposite effects have been demonstrated, for instance, due to decreased molecular mobility of the polymer matrix. This finding is very important, since it will favorably influence many properties, depending on molecular mobility of the matrix close to the fibril surface, for instance, diffusion coefficients. Although this effect is well known for inorganic nanofillers, it is not self-evident in the case of organic entities such as cellulose microfibrils. Under certain conditions, in particular after solvent casting, rubber nanocomposites may also contain microfibril networks, which provide unique reinforcement effects such as dramatically increased modulus. In the glassy state, there appear to be no unique nanocomposite modulus reinforcement mechanisms. This means that composite modulus primarily will be controlled by MFC content, MFC aspect ratio, MFC modulus, and MFC orientation distribution. High MFC content composites and MFC films show high mechanical performance. Increased attention needs to be paid to the desired nanostructures of this class of MFC nanocomposites and how to obtain them. Nanoscale-modified plant structures such as paper or bast fibers and veneer represent a different approach to preparation of nanocomposite structures. Paper fibers may be derivatized so that the cellulose surface becomes thermoplastic whereas the interior of the microfibrils is preserved. Solvent exchange techniques may be used to impregnate the secondary plant fiber cell wall, and this has been shown to improve some mechanical composite properties. As existing plant structures are used, one advantage is that the cellulose microfibrils are already preferably oriented in directions nearly parallel to the cell axis. In contrast, with composites based on aqueous MFC suspensions, it is difficult to control MFC orientation distribution, although the application of strong magnetic fields has been demonstrated to have an effect. Copyright © 2005 by Taylor & Francis

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There are many unanswered scientific questions with regard to MFC and their composites. It is perhaps typical for studies on a new type of material that focus tends to be on the preparation, structure, and properties of a specific material rather than on more general mechanisms. For instance, it is not always clear how different cellulose sources and disintegration processes influence the shape (final aspect ratio) and modulus of the MFC. Mechanisms in operation during extraction, disintegration, composites processing, dispersion, and so on are not all well understood. In addition, the difference in reinforcement efficiency between different forms of MFC (rodlike MFC, weblike primary wall cellulose, etc.) in composites is not clear. Specific questions regarding composite material structure include mechanisms for MFC effects on interface, interphase, matrix morphology, Tg, and molecular mobility. Strong matrix effects from MFC have already been demonstrated in certain systems based on tunicin or bacterial cellulose.

Acknowledgments Financial support from BiMaC, the Biofiber Materials Center at the Royal Institute of Technology in Stockholm, is gratefully acknowledged. The encouragement and discussions with Professor Tom Lindström on the subject of microfibrillated cellulose have been a major source of inspiration.

References 1. Regis, E., Nano!, Little, Brown, Boston, 1995 and www.zyvex.com/nanotech/feynman.html 2. Okada, A., Kawasumi, M., Usuki, A., Kojima, Y., Kurauchi, T., and Kamigaito, O., Mater. Res. Symp. Proc., 171, 45, 1990. 3. Battista, O.A., Microcrystal Polymer Science, McGraw Hill, New York, 1975. 4. Sakurada, I., Nukushina, Y., and Ito, I., J. Polym. Sci., 57, 651, 1962. 5. Fengel, D. and Wegener, G., Wood Chemistry, Ultrastructure, Reactions, Walter de Gruyter, Berlin, 1984. 6. Wainwright, S.A., Biggs, W.D., Currey, J.D., and Gosline, J.M., Mechanical Design in Organisms, Princeton University Press, Princeton, 1982. 7. Mark, R.E., Cell Wall Mechanics of Tracheids, Yale University Press, 1967. 8. Cave, I.D., Wood Sci. Technol., 3, 40, 1969. 9. O’Sullivan, A.C., Cellulose, 4, 173, 1997. 10. Sjöström, E., Wood Chemistry Fundamentals and Applications, Academic Press, New York, 1981. 11. Favier, V., Chanzy, H., and Cavaille, J.Y., Macromolecules, 28, 6365, 1995. 12. Hajji, P., Cavaille, J.Y., Favier, V., Gauthier, C., and Vigier, G., Polym. Comp., 17, 612, 1996.

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13. Dubief, D., Samain, E., Dufresne, A., Macromolecules, 32, 5765, 1999. 14. Favier, V., Canova, G.R., Cavaille, J.Y., Chanzy, H., Dufresne A., and Gauthier, C., Polym. Adv. Technol., 6, 351, 1995. 15. Dufresne, A., Comp. Interf., 7, 53, 2000. 16. Ruiz, M.M., Cavaille, J.Y., Dufresne, A., Gerard, J.F., and Graillat, C., Compos. Interf., 7, 117, 2000. 17. Ruiz, M.M., Cavaille, J.Y, Dufresne, A., Graillat, C., and Gerard, J.F., Macromol. Symp., 169, 211, 2001. 18. Chazeau, L., Cavaille, J.Y., Canova, G., Dendievel, R., and Boutherin, B., J. Appl. Polym. Sci., 71, 1797, 1999. 19. Chazeau, L., Cavaille, J.Y., and Terech, P., Polymer, 40, 5333, 1999. 20. Paillet, M. and Dufresne, A., Macromolecules, 34, 6527, 2001. 21. Hui, C.Y. and Shia, D., Polym. Eng. Sci., 38, 774, 1998. 22. Helbert, W., Cavaille, J.Y., and Dufresne, A., Polym. Compos., 17, 604, 1996. 23. Dufresne, A., Cavaille, J.Y., and Helbert, W., Polym. Compos., 18, 198, 1997. 24. Tsagaropoulus, G. and Eisenberg, A., Macromolecules, 28, 6067, 1995. 25. Chazeau, L., Paillet, M., and Cavaille, J.Y., J. Polym. Sci., Part B: Polym. Phys., 37, 2151, 1999. 26. Noishiki, Y., Nishiyama, Y., Wada, M., Kuga, S., and Magoshi, J., J. Appl. Polym. Sci., 86, 3425, 2002. 27. Neus Angles, M. and Dufresne, A., Macromolecules, 34, 2921, 2001. 28. Turbak, A.F., Snyder, F.W., and Sandberg, K.R., J. Appl. Polym. Sci. Appl. Polym. Symp., 37, 813, 1983. 29. Herrick, F.W., Casebier, R.L., Hamilton, J.K., and Sandberg, K.R., J. Appl. Polym. Sci.: Appl. Polym. Symp., 37, 797, 1983. 30. Boldizar, A., Klason, C., Kubat, J., Näslund, P., and Saha, P., Int. J. Polym. Mater., 11, 229, 1987. 31. Wu, Q., Liu, X., and Berglund, L.A., in Sustainable Natural and Polymeric Composites-Science and Technology, Lilholt, H., Madsen, B., Toftegaard, H.L., Cendre, E., Megnis, M., Mikkelsen, L.P., and Sørensen, B.F., Eds., Risø National Lab, Roskilde, Denmark, 2002, p. 107. 32. Taniguchi, T. and Okamura, K., Polym. Int., 47, 291, 1998. 33. Nakagaito, A.N. and Yano, H., Appl. Phy. A., 78, 547, 2004. 34. Yano, H. and Nakahara, S., Proceedings of the 6th Pacific RIM Bio-based Composite Symposium, Portland, USA, 2002, p. 171. 35. Iguchi, M., Yamanaka, S., and Budhiono, A., J. Mater. Sci., 35, 261, 2000. 36. Orts, W.J., Godbout, L., Marchessault, R.H., and Revol, J.F., Macromolecules, 31, 5717, 1998. 37. Araki, J., Wada, M., Kuga, S., and Okano,T., Langmuir, 16, 2413, 2000. 38. Dufresne, A., Dupeyre, D., and Vignon, M.R., J. Appl. Polym. Sci., 76, 2080, 2000. 39. Dufresne, A. and Vignon, M.R., Macromolecules, 31, 2693, 1998. 40. Dufresne, A., Cavaille, J.Y., and Vignon, M.R., J. Appl. Polym. Sci., 64, 1185, 1997. 41. Hepworth, D.G. and Bruce, D.M., Composites Part A, 31, 283, 2000. 42. Baker, A.A., Helbert, W., Sugiyama, J., and Miles, M.J., J. Struct. Biol., 119, 129, 1997. 43. Ek, R., Gustafsson, C., Nutt, A., Iversen, T., and Nyström, C., J. Mol. Recog., 11, 263, 1998. 44. Wu, Q., Liu, X., and Berglund, L.A. [in preparation]. 45. Focher, B., Marzetti, A., Beltrame, P.L., and Alvella, M., Biomass Bioenergy, 14, 187, 1998.

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46. Chanzy, H. and Henrissat, B., Carbohydr. Polym., 3, 161, 1983. 47. White, D.G. and Brown, R.M., in Cellulose and Wood—Chemistry and Technology, Schuerch, C., Ed., Wiley, New York, NY, 1989, 573 and www.botany. utexas.edu/facstaff/facpages/mbrown/position1.htm 48. Tokoh, C., Takabe, K., Sugiyama, J., and Fujita, M., Cellulose, 9, 65, 2002. 49. Grunert, M. and Winter, W.T., J. Polym. Environ., 10, 27, 2002. 50. Sugiyama, J., Chanzy, H., and Maret, G., Macromolecules, 25, 4232, 1992. 51. Matsumura, H., Sugiyama, J., and Glasser, W.G., J. Appl. Polym. Sci., 78, 2242, 2000. 52. Matsumura, H. and Glasser, W.G., J. Appl. Polym. Sci., 78, 2254, 2000. 53. Yano, H., Hirose, A., Collins, P.J., and Yazaki, Y., J. Mater. Sci. Lett., 20, 1125, 2001. 54. Hepworth, D.G., Vincent, J.F.V., Jeronimidis, G., and Bruce, D.M., Composites Part A, 31, 599, 2000.

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27 How Sustainable Are Biopolymers and Biobased Products? The Hope, the Doubts, and the Reality

Martin Patel and Ramani Narayan

CONTENTS 27.1 Introduction 27.2 PHA vs. Petrochemical Polymers: Energy and GHG Argument 27.3 There Is More than PHA: More Results for Further Biobased Polymers 27.4 Environmental Argument 27.5 Discussion and Conclusions References

ABSTRACT Biobased polymers are considered promising candidates for sustainable development because, among other reasons, they contribute to the substitution of renewable resources for depletable petrochemical feedstocks, help to decrease greenhouse gas (GHG) emissions, and, if used in the form of biodegradable polymers, allow the closing of the loop of organic carbon and nutrients by means of composting. Life Cycle Assessment (LCA) is an elaborate and acknowledged methodology for assessing environmental sustainability. In LCAs, usually a large number of environmental indicators are analyzed to evaluate and compare process chains. In the past few years, some articles were published which, using the LCA concept, questioned the utility of biodegradable/biobased polymers. In this chapter we review and analyze this body of work, thereby attempting to correct misperceptions and misconceptions. Polyhydroxyalkanoates (PHA) have been discussed controversially in terms of their environmental benefits. Having reviewed the body of environmental assessments on PHA, we conclude that energy use and CO2 emissions are nowadays often larger for PHB than for conventional polymers, but that there is also potential to avoid this disadvantage if the entire

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system covering all stages of the life cycle is carefully optimized. A review of twenty studies for several other biobased polymers and natural fibers shows that savings of ~20% for energy and CO2 are typically achieved. Substantially higher savings of up to 50% and beyond are considered feasible for certain starch polymers, printed wiring boards, and certain natural fiber composites. Further improvements can be expected because most biobased polymers are still in their infancy, while substantial optimization potential has remained untapped. In summary, available LCA studies and environmental assessments strongly support further development of biodegradable and biobased polymers. Careful monitoring of various environmental impacts continues to be necessary both for decision makers in companies and in policy. If combined with good practice targets, such monitoring may accelerate and focus ongoing product and process innovation.

27.1

Introduction

There are numerous definitions of “sustainable development,” but the most frequently quoted one was stated by the Brundtland Commission. It defines sustainable development as “development that meets the needs of present without compromising the needs of future generations to meet their own needs”.1 Based on the work done by the Brundtland Commission and others, there is currently a consensus that sustainable development must embrace (at least) the environmental, the economic, and the social dimension. Biobased polymers are considered as very promising candidates for sustainable development for a variety of reasons, with the most important being the following: ●

●

●

●

●

Substitution of renewable resources for depletable petrochemical feedstocks The possibility of lower greenhouse gas (GHG) emissions by sequestering CO2 from the atmosphere in polymers and other organic chemicals, thereby closing the biogeneous carbon loop instead of net addition of fossil carbon to the atmosphere in the case of petrochemicals Closure of the loop of organic carbon and nutrients which, in the case of biodegradable polymers, can be returned to the soil by composting (biocycling) Possibly lower production cost (or lower system cost) due to cheaper raw materials, tailored raw materials, and shorter process chains Opportunities for growth and employment in agriculture and the chemical industry.

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dimensions being in a rather early stage of development and acceptance (e.g., external cost2 and other integrated approaches).3 On the other hand, with Life Cycle Assessment (LCA), there is an elaborate methodology for the assessment of environmental sustainability.4−7 In LCAs, usually a large number of environmental indicators are analyzed to evaluate and compare process chains. Common environmental indicators are resource depletion (biotic and abiotic), global warming, acidification, euthrophication, ozone depletion, human toxicity, and ecotoxicity; however, these are often not studied simultaneously in one study. Before LCA became common, comparable studies were conducted which focused on (primary) energy as the only indicator. These studies are referred to as energy analyses8 and the indicator used is called GER (Gross Energy Requirements) or Cumulative Energy Demand (CED).8,9 Currently, energy analyses, sometimes only extended by one or two other environmental indicators (such as global warming), are referred to as LCA. This is especially true for new products where real data measured on plants are scarce, as is the case for biobased polymers. In the past few years, some articles were published which questioned the utility of biodegradable/biobased polymers on the basis of energy and environmental analyses.10,11 Serious doubts were expressed about whether they contribute to the reduction of fossil energy use and GHG. The outcome of assessments for polyhydroxyalkanoates (PHA) has been used as a sweeping indictment of all biodegradable and biobased polymers, vis-à-vis their energy and environmental benefits. Meanwhile, further LCA studies on biobased products have been released and comparative reviews published by us and by others. In this chapter we review and analyze this body of work, thereby attempting to correct misperceptions and misconceptions. Due to its importance to the discussion, we will begin with PHA and then continue with other biobased polymers.

27.2

PHA vs. Petrochemical Polymers: Energy and GHG Argument

Table 27.1 compares Gerngross and Slater’s11 cradle-to-factory gate energy data with respective values for petrochemical polymers according to the Association of Plastics Manufacturers in Europe (APME).12 The table shows that the total cradle-to-factory gate fossil energy requirements of PHA can compete with high-density polyethylene (HDPE), depending on the type of PHA production process. The minimum total energy consumed (energy input into the system) for PHA production (fermentation) is slightly higher than for polyethylene terephthalate (PET) production (81 vs. 77 GJ/ton plastic), comparable to HDPE (81 vs. 80 GJ/ton plastic), but lower than polystyrene (PS) (81 vs. 87 GJ/ton plastic). However, if one were to consider only process energy requirements, then the PHA values are two to three times Copyright © 2005 by Taylor & Francis

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TABLE 27.1 Cradle to Factory Gate Fossil Energy Requirements for Plastics Productiona Process Energy PHA grown in corn plants PHA by bacterial fermentation HDPE PET (bottle grade) PS (general purpose)

Feedstock Energy

Total

90 81

0 0

90 81

31 38 39

49 39 48

80 77 87

a In GJ/tonne plastic. Source: Data for green plastics from Gerngross, T.U. and Slater, S., Sci. Am., 37, August 2000; data for petrochemical polymers from APME, in Eco-profiles of Plastics and Related Intermediates, Brussels, Belgium, 1999.

higher than for petrochemical polymers (Table 27.1). Gerngross and Slater base their arguments on these process energy data, totally excluding the feedstock energy values which are significant (0 for biobased polymers vs. 39−49 for petrochemical-based plastics (GJ/ton plastic); see Table 27.1). It is therefore not surprising that, on the basis of process energy values, Gerngross and Slater conclude that PHA does not offer any opportunities for emission reduction10,11 or energy savings. This finding may be valid for certain system boundaries, e.g., for the system cradle-to-factory gate, the output of which are plastic pellets.* The conclusion may also be correct if all plastic waste is deposited in landfills. In contrast, the finding is not correct if other types of waste management processes are assumed within the cradle-to-grave concept. As the last column of Table 27.1 shows, total fossil energy requirements are practically identical for PE and PHA manufactured by bacterial fermentation. Hence, if combusted in a waste incinerator without energy recovery, both plastics result in comparable CO2 emissions throughout the life cycle. If, on the other hand, energy is recovered by incineration in a waste-to-energy facility, then the consumption of nonrenewable energy used elsewhere for the production of power and/or heat is avoided. Energy is recovered independent of whether the polymer is biobased or petrochemical. The amount of energy recovered may differ considerably between a biobased polymer with a relatively low calorific value [such as polyhydroxybutyrate (PHB), lower heating value: 22.0 GJ/t] and a petrochemical polymer with a high calorific value (e.g., polyethylene, lower heating value: 43.3 GJ/t). In practice, however, the difference is only large if the energy recovery rate of waste-to-energy facilities is high which, on average, is still not the case in most industrialized countries.

* In the system “cradle-to-factory gate,” the feedstock energy remains embodied in the material and is hence not released. The same is true for landfills where petrochemical polymers hardly get degraded under anaerobic conditions.

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It must also be taken into consideration that PHA production both by bacterial fermentation and by production in the plants is in an early stage of development while the manufacture of petrochemical polymers has been optimized for decades and that PHA is currently produced for a high-value but low-volume market (e.g., for medical applications) where efficiency improvements from scale-up of production and process optimization remain largely unexploited. It is therefore not valid to draw final conclusions. Heyde13 compared the energy requirements of PHB production by bacterial fermentation using various feedstocks and processes to those of HDPE and PS. The PHB options studied included substrate supply from sugar beet, starch, fossil methane, and fossil-based methanol. Moreover, in the processing stage, the options of enzymatic treatment and solvent extraction were considered. As Figure 27.1 shows, the energy requirements for biotechnological PHB production can substantially exceed the requirements for conventional plastics, but there is also scope to outpace fossil-based polymers in terms of energy requirements (PHB best case). The latter is confirmed by LCA results from Japan, discussed later. It can be concluded that energy use and CO2 emissions are currently often larger for PHB than for conventional polymers but that there is scope to avoid this disadvantage if the entire system covering all stages of the life cycle is carefully optimized. Moreover, since most biobased polymers are still in their infancy, substantial improvements can be made in a relatively short time, as indicated in Figure 27.2. Since Monsanto did not consider the target yields for the production of PHA in genetically modified plants to be feasible within a few years, the company stopped all R&D activities related to PHA in 1999. Also, R&D on fermentative production, which was seen as an interim step, was abandoned at that time.

700 573.2

Nonrenewable energy resources (MJ/Kg)

600 500 400 300 200 100

66.1

73.8

91.7

PHB best case

HDPE

PS

0 PHB worst case

FIGURE 27.1 Cradle-to-factory gate requirements of nonrenewable energy for the production of various polymers. (From Heyde, M., Polym. Degrad. Stability, 59, 3, 1998.)

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450

Energy (finite) in GJ/t PHA

400 350 300 250 200 150 100 50 0 1991

1992

1993

1994

1995

1996

1997

1998

1999

FIGURE 27.2 Development of cradle-to-factory gate energy use for Monsanto’s production of polyhydroxyalkanoates by fermentation. (From personal communication with Monsanto, 1998; dotted line represents projected data.)

27.3

There Is More than PHA: More Results for Further Biobased Polymers

While the overall discussion about environmental benefits of biobased plastics has been overshadowed, especially in the United States, by Gerngross’ publications on PHA, the total picture proves to be much brighter if evidence from other countries and for other polymers is considered. Japanese researchers have conducted LCA studies under the sponsorship of the Ministry of Trade and Industry (MITI), looking at the very same question, and arrived at very different answers. Figure 27.3 shows how various plastics stack up with regard to energy consumption vs. CO2 emissions. Polyhydroxybutyrate (PHBB) shows lower cumulative energy and CO2 emissions than PET, PS, or the biodegradable petrochemical polyester polybutylene succinate (PBS). The biodegradable starch-based plastic (SPCL) shows cumulative energy requirements equivalent to PE but with much reduced CO2 emissions. Note that this study did not take into account disposal via composting and the positive credits in CO2 emissions and energy savings resulting from compost application. The impact categories resource use, CO2 emission, solid waste generation, energy consumption, and price of resin were plotted in Figure 27.4.14 It can be readily seen that biobased biodegradable plastics (PHBB, SPCL) show clear advantages in all the impact categories except in price. Polybutylene succinate, a biodegradable plastic (from petrochemical feedstocks) is comparable Copyright © 2005 by Taylor & Francis

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Cumulative energy consumption MJ/t

90000

PET PBSC PHBB

60000

PS

SPCL

PE

30000

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Cumulative CO2 emission kg/ton FIGURE 27.3 Cradle-to-factory gate requirements for the production of biobased and petrochemical polymers. PE: polyethylene; PET: polyethylene terephthalate; PHBB: polyhydroxybutyrate; PBS: polybutylene succinate; PS: polystyrene; SPCL: starch-based plastic. (From Ministry of Trade and Industry, Tokyo, Japan, 2001.)

to nonbiodegradable polyesters in CO2 emissions, but has clear advantages in the resource and solid waste impact categories. A review of twenty studies,15 mainly of Swiss and German origin, also results in a completely different picture compared to Gerngross’ analysis.10,11 The rest of this section will present the review results in an aggregated manner; for more detailed information, the reader is referred to Reference 15. The materials covered in the review are starch polymers (both thermoplastic starch, i.e.,TPS, and blends with petrochemical copolymers), PHA, polylactides (PLA), lignin-epoxy resins, epoxidized linseed oil, and composites reinforced with natural fibers such as flax, hemp, and china reed (miscanthus). The first three materials are biodegradable, while the remaining materials studied are not. The types of end products covered are primary plastic materials (mainly pellets, i.e., granules; not to be confused with transportation pallets), loose-fill packaging material (packaging chips), films, bags, mulch films, printed wiring boards (for electronics), thickener for lacquer, two different panels for passenger cars (Mercedes A Class Copyright © 2005 by Taylor & Francis

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PS PE

CO2 emission

PET PHBB SPCL PBSC

2 1 0 Price of resin

−1

Energy consumption

−2

Resources

Solid waste

FIGURE 27.4 Environmental and economic indicators for the production of biobased and petrochemical polymers. (From Ministry of Trade and Industry, Tokyo, Japan, 2001.)

and Audi A3), and transport pallets. These products are compared with equivalent products made from petrochemical polymersin many cases polyethylene, polypropylene, or polystyrene. An overview of life cycle inventory results for various types of primary plastics (pellets) and end products is given in Tables 27.2 and 27.3. Only those impact categories are listed for which data were available from several sources (nonrenewable energy use,* GHG emissions, ozone precursors, acidification, eutrophication). All materials listed in Table 27.2 are commercially available and they are produced according to the current state of the art. In contrast, future options, e.g., the production of PHA in plants and the medium- and long-term production processes for PLA, are not listed. In Table 27.3, inventory data are presented for end products. All products listed in this table except for the printed wiring boards, the underfloor panel for the Mercedes A Class, and the transport pallets are currently on the market. In the case of printed wiring boards, the lack of low-priced, pure lignin represents an obstacle to industrial implementation while, in the case of the underfloor panel, the management decision about introduction in series production was pending when this chapter was written. For transportation pallets reinforced with china reed, technical problems still need to be resolved (material stiffness). * The term nonrenewable energy use is used here to account for the fact that electricity is partly produced by nuclear plants in some countries. Nonrenewable energy use is hence interpreted as the total of fossil and nuclear energy and can be seen as a synonym for "finite energy." The data in Table 27.1 are based on the assumption that no power from nuclear plants is used. The energy values of Tables 27.1, 27.2, and 27.3 may therefore be compared (equivalence of fossil energy and nonrenewable energy).

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TABLE 27.2 Summary of Key Indicators from LCA Studies Reviewed (state-of-the-art technologies only)

b c

80 72.3 80.6 91.7 120 77 87 84 88 87

Incineration Incineration Incineration 80% incin.⫹20% landfilling Incineration Incineration Incineration Incineration None (cradle-to-factory gate) None (cradle-to-factory gate)

4.84 4.54 5.04 5.20 7.64 4.93 5.98 5.88 2.80 2.72

n/a n/a n/a 13.0 n/a n/a n/a n/a 43.0 1.2

n/a n/a n/a 0.70 n/a n/a n/a n/a

25.4 25.5 25.4 18.9

Incineration 80% incin.+20% compost. 100% composting None (cradle-to-factory gate)

1.14 1.20 1.14 1.10

n/a 4.7 5.0 0.2

n/a 0.20 0.20

24.9 48.3 52.3 18.5 32.4 36.5 53.5 57 81 66–573

Incineration Incineration Incineration None (cradle-to-factory gate) Composting Waste water treatment plant Composting Incineration n/a n/a

1.73 3.36 3.60 1.08 0.89 1.43 1.21 3.84 n/a n/a

n/a n/a n/a 0.23–0.64 5.5 5.8 5.3 n/a n/a n/a

n/a n/a n/a

n/a n/a n/a

Total of process energy and feedstock energy. Nonrenewable energy only, i.e., total of fossil and nuclear energy. In the “cradle-to factory gate” concept the downstream system boundary coincides with the output of the polymer or the end product. Hence, no credits are ascribed to valuable by-products from waste management (stream, electricity, secondary materials). Only CO2 Embodied carbon: 3.14 kg CO2 /kg PE, 2.34 kg CO2/kg nylon 6, 2.29 kg CO2/tPET, 3.38Kg CO2/tPS, 2.32 kg CO2/tPCL, 2.00 kg CO2/tPVCH. No credit for carbon update by plants.

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a

GHG Emissions Ozone [kgCO2eq/functional Precursors Human Toxicity b unit] [g ethylene eq.] [a m]c

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Petrochemical polymers HDPE 1 LLDPE 1 LDPE 1 LDPE 1 Nylon 6 1 PET (bottle grade) 1 PS (general purpose) 1 EPS 1 EPS 1 EPS (PS+2%SBR⫹Pentan⫹Butan) 1 Biobased plastics (pellets) TPS 1 TPS 1 TPS 1 TPS (maize starch⫹ 5.4% 1 maize grit⫹12.7% PVCH) TPS⫹15% PVCH 1 TPS⫹Copyright 52.5% PCL 1 © 2005 by Taylor & Francis TPS⫹ 60% PCL 1 Starch 1 Mater-Bi foam grade 1 Mater-Bi foam grade 1 Mater-Bi film grade 1 PLA 1 PHA by fermentation 1 PHB, various processes 1

Type of Waste Treatment Assumed for Calculation of Emissions

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Type of Plastic

Functional Unit (kg)

Cradle-to-Gate Nonrenewable Energy Use [MJ/ functional unit]a

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TABLE 27.3

Foam (loose fills) Starch foam

1 m3 (10 kg)

492

Starch foam

1 m3 (12 kg)

277

PS foam PS foam

1 m3 (4.5 kg) 1 m3 (4 kg)

680 453

Waste water treatment plant 30% incin., 70% landfilling Incineration 30% incin., 70% landfilling 30% incin., 70% landfilling

PS foam (by 1 m3 (4 kg) recycling of PS waste) Mater-Bi starch 1 kg loose fills FloPak starch © 20051by kgTaylor & Francis Copyright loose fill EPS loose fill 1 kg FloPak EPS loose 1 kg fill EPS loose fill (by 1 kg recycling of PS waste) Films and bagsc TPS film 100 m2, 150 µm

GHG Emissions Ozone [kg CO2 eq./ Precursors functional [g ethylene unit] eq.]

Human Toxicity [a m3]d

Acidification [g SO2 eq.]

Eutrophication [g PO4 eq.]

21.0

115

276

39.0

33.5

10

83

9.9

56.0 22.5

1200 57

325 85

42.0 8.0

18.6

55

107

9.9

49

2

12

0

28

4

23

3

1

0

7

1

151 113

12 6

267 14

0 0

72 21

9 2

90

5

14

0

27

2

239

103.0

361

649

80% incin. ⫹ 20% landfilling

25.30

100

4.33

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Type of Waste Treatment Assumed for Calculation of Emissions

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Type of Plastic

Functional Unit

Cradle-to-Gate Nonrenewable Energy Use [MJ/ functional unit]b

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Summary of LCA Key Indicators for End Products (some of the products listed are commercialized, others not)a

100 m2, 150 µm

1340

80% incin. ⫹ 20% landfilling

1 kg

~17 e

1 kg

~12 e

Disposal as MSW Disposal as MSW Disposal as MSW

~10 e

2.98

14.0

66.70

180

9.70

26.5

2.8

238

15.0

100% ⱕ–67% ⱕ–67%

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Organosolve 1 kg lignin/epoxy Under-floor panel for passenger car Glass-fiber reinforced polypropylene 1 panel Flax reinforced 1 panel polypropylene Interior side panel for passenger car ABS copolymer 1 panel

Composting

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133

155 134

132

Hemp fiber/epoxy 1 panel 73 composite ABS copolymer 1 kg 117 HempCopyright fiber/epoxy kgTaylor & Francis 89 © 20051by composite Transport pallet Glass-fiber 1 pallet 1400 reinforced polypropylene (GF) China reed 1 pallet 717 reinforced polypropylene (CF)

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Printed wiring boards Conventional epoxyd Kraft lignin/epoxy

100 m2, 20 µm

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No waste m’mt (cr.-to-fact. gate) No waste m’mt (cr.-to-fact. gate)

5.4 4.7 5 6

No waste m’mt (cr.-to-fact. gate)

75.3

208

653

68.2

No waste m’mt (cr.-to-fact. gate)

40.4

133

432

62.8

(Continued)

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TABLE 27.3 (Continued)

a b

c

d e

f

93

61

GHG emissions [kg CO2 eq./ functional unit]

Ozone Precursors [g ethylene eq.]

Human Toxicity [a m3]d

Acidification [g SO2 eq.]

Eutrophication [g PO4 eq.]

No waste m’mt (cr.-to-fact. gate)

5.0

14

0

44

4.5

No waste m’mt (cr.-tofact. gate)

3.4

11

0

37

5.3

For details, see Reference 15. Total of process energy and feedstock energy. Nonrenewable energy only, i.e., total of fossil and nuclear energy. In the “cradle-to factory gate” concept the downstream system boundary coincides with the output of the polymer or the end product. An important explanation for the large difference between the values reported in the following columns is that Carbotech assumes a film thickness of 150 µCopyright m while it©is2005 onlyby 20Taylor µm in & the case of Composto. Francis Epoxy resin (FR4) cured with dicyandiamide (DICY). The publication (IBM, 2001) provides only energy data for two cradle-to-grave systems. These are first, 100% incineration for metal reclamation and, secondly, a combination of 50% disposal as municipal solid waste and 50% incineration for metal reclamation. Based on the results for these two cases we have roughly estimated the cradle-to-gate energy use. The data in these two rows have been calculated from the preceding two rows by using the pallet weight (1 GF pallet = 15 kg; 1 CF pallet = 11.8 kg).

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Glass-fiber 1 kg pallet reinforced polypropylene (GF)f China reed 1 kg pallet reinforced polypropylene (CF) f

Type of Waste Treatment Assumed for Calculation of Emissions

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Functional Unit

Cradle-to-Gate nonrenewable energy use [MJ/ functional unit]b

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In the case of starch polymer pellets, energy requirements are mostly 25−75% below those for PE and greenhouse gas emissions are 20−80% lower. These ranges originate from the comparison of different starch−copolymer blends, different waste treatment, and different polyolefin materials used as reference. Regarding the latter, APME12 data for LLDPE (72.3 MJ/kg) and LDPE (80.6 MJ/kg) were assumed, which are lower than the value according to Carbotech16 (91.7 MJ/kg; see Table 27.2). The lower APME values serve also as reference for comparison with the other biobased polymers (see later). Starch polymers (both TPS and copolymers) also score better than PE for all other indicators listed in Table 27.2, with eutrophication being the sole exception. The lower the share of petrochemical copolymers, the smaller the environmental impact of starch polymers. However, the application areas for pure starch polymers and blends with small amounts of copolymers are limited due to inferior material properties. Hence, increased blending can extend the applicability of starch polymers and thus lower the overall environmental impact at the macroeconomic level. The cradle-to-factory gate energy requirements for PLA are 20−30% below those for PE, while GHG emissions are ~15−25% lower. The results for PHA vary greatly (only energy data are available). Cradle-to-factory gate energy requirements in the best case (66.1 GJ/t) are 10−20% below those for PE. For more energy-intensive production processes PHA does not compare well with petrochemical polymers. Since all data in Table 27.2 refer to the current state of the art, technological progress, improved process integration, and various other possibilities for optimization are likely to result in more favorable results for biobased polymers in the future. The results for starch polymer loose fills (Table 27.3) differ decisively depending on the source. Much of these differences can be explained by different assumptions regarding the bulk density of the loose fills (see second column in Table 27.3) and different approaches for the quantification of the ozone depletion potential (inclusion vs. exclusion of NOx). It therefore seems more useful to compare the results of each study separately. One can conclude from both the Composto and the BIFA/IFEU/Flo-Pak study (2002)17 that starch polymer loose fills generally score better than their equivalents made of virgin EPS. GHG emissions represent an exception where the release of CH4 emissions from biodegradable compounds in landfills results in a disadvantage for starch polymers (only according to BIFA/IFEU/Flo-Pak).17 The other sources reviewed may not have taken this emission source into account. By analogy to loose fills, the range of results for starch polymer films and bags is to a large extent understandable from the differences in film thickness. Taking this factor into account, the environmental impacts of the starch films and bags are lower with regard to energy, GHG emissions, and ozone precursors. The situation is less clear for acidification. For eutrophication, PE films tend to score better. For printed wiring boards (PWBs), ~30−40% energy can be saved, and the GHG mitigation potential is estimated to lie in a similar range. The replacement of fiberglass by natural fibers in composites is reported to save between 14% Copyright © 2005 by Taylor & Francis

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TABLE 27.4 Energy and GHG Savings by Biobased Polymers Relative to Their Petrochemical Counterparts Energy Savings (M J/kg biobased polymer)a

CHG Savings (kg CO2 eq/kg biobased polymer)a

51 52 28 24 42 23 19 5 195 28 33

3.7 3.1 1.4 1.2 3.6 3.6 1.0 n/a 8.3 –0.9 1.6

Biobased plastics (pellets) TPS TPS+15% PVOH TPS+52.5% PCL TPS+60% PCL Mater-bi foam grade Mater-bi film grade PLA Printed wiring boards Lacquer Interior side panel for passenger car Transport pallet a

Max. ⫹/⫺ 15%, depending on whether LDPE or LLDPE according to APME is chosen as reference (cf. Table 27.1).

(Mercedes’ underfloor panel) and 45−50% energy (Audi’s interior side panel; transport pallet). Leaving PHA aside as the only exception, it can be summarized that biobased polymers and natural fibers typically enable savings of ~20% (energy and CO2). Substantially higher savings up to 50% and beyond are considered feasible for certain starch polymers, PWBs, and certain natural fiber composites. Table 27.4 summarizes the energy and GHG savings per kilogram of biobased polymer. These values have been determined by deducting the values for biobased polymers from the respective values of their petrochemical counterparts. It is obvious also from this table that biobased polymers and natural fiber composites offer very attractive potentials for energy saving and GHG emission reduction.

27.4

Environmental Argument

The final disposal and waste system has an important role in the overall ecobalance of the materials. This particularly may be the case for biodegradable materials. If a biobased material is recycled through composting and the compost applied to land, then significant emission and energy credits can accrue because of the value of the compost to sustainable agriculture (see discussion later). On the other hand, a biobased material dumped in a landfill could produce negative effects by uncontrolled emissions of methane, as pointed out by Kurdikar et al.25 and BIFA/IFEU/Flo-Pak.17 Figure 27.5 shows the composition of the U.S. municipal solid waste stream. As can be seen, 67% of the waste generated (paper, yard, food, wood) is biodegradable organic waste. It does not make sense to “sequester” these Copyright © 2005 by Taylor & Francis

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Glass 6.2% 12.8 mil tons

Wood 7.1% 14.9 mil tons

Metals 7.6% 15.8 mil tons

Other 6.3% 20.2 mil tons

Paper 39.2% 81.5 mil tons

Total generation: 208 mil tons

Plastics 9.1% 19.0 mil tons

Yard waste 14.3% 29.8 mil tons

FIGURE 27.5 Total waste generated (before recycling) in the United States in 1995. (From Environmental Protection Agency, Characterization of MSW in the U.S., Washington, D.C., 1996.)

valuable biowastes in our sanitary landfills and prevent it from entering the natural biological carbon cycle. However, composting is an environmentally sound approach to transfer biodegradable waste, including the new biodegradable plastics, into useful soil amendment products. Biodegradation of such natural materials will produce valuable carbon-rich compost as the major product, along with water and carbon dioxide. The CO2 produced does not contribute to an increase in greenhouse gases because the next season crop plantation will fix the CO2 and reinitiate the biological carbon cycle (biocycling). Compost-amended soil has beneficial effects by increasing soil organic carbon, increasing water and nutrient retention, reducing chemical inputs, and suppressing plant disease. Composting is increasingly a critical element for maintaining the sustainability of our agriculture system.26,27 One may argue that the contribution of biodegradable polymers to the buildup of compost is negligible since the bulk of the compost material originated from green waste such as garden and vegetable waste. However, biodegradable polymers are an important facilitator without which undegradable polymers may remain in the compost or tedious and costly additional separation costs might be required in the composting facilities. Negative public perception and economic disadvantages may ultimately hamper the implementation, while biodegradable collection bags can change the picture as the experience from several municipalities and regions has already shown (e.g., Ann Arbor in the United States and numerous cities in northern Italy). An LCA study by the International Nutrition and Agricultural Consultancy group29 on fertilization with compost as opposed to NPK chemical fertilizer in agricultural production systems clearly shows the environmental benefits of compost fertilization in a number of areas. Thus, in the potato system the compost showed a clear advantage in all effect categories over the chemical NPK fertilizer. The compost application showed an environmental relief potential between 26% (fossil energy carriers) and 91% (photooxidants) compared to that of the chemical fertilizer application (Figure 27.6). Similarly, in the winter wheat system, compost fertilization showed clear advantage in all the effect categories, with values ranging between 76% and 90% (Figure 27.7). Copyright © 2005 by Taylor & Francis

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60 48.57

Potatoes-NPK

50

Potatoes-eco and compost

36.61

40 30 20

14.93

10

9.81

10.25

8.13 3.15

4.31

10.61

0.4

0 Fossil energy carriers (very high)

Greenhouse effect (low to medium)

Photoxidants (low)

Acidification (very high)

Nutrient input (very high)

FIGURE 27.6 Comparison of conventional fertilization and compost fertilization for potatoes (calculated, 1.00 E-3). (From Meier-Ploeger, A. et al., in Eco-balance Compost vs. NPK Fertilizers, Int. Nutrition Agric. Consultancy, April, 1996. With permission.)

131.32

140 120 100

126.42

Winter wheat-MPK1 Winter wheat-eco + compost

80 60 40

33.93 33.17

37.33

33.58

34.95

17.11

20

10.31

0 Fossil energy carriers (very high)

Greenhouse effect (low to medium)

1.39 Photoxidants (low)

Acidification (very high)

Nutrient input (very high)

FIGURE 27.7 Comparison of conventional fertilization and compost fertilization for wheat. (From MeierPloeger, A. et al., in Eco-balance Compost vs. NPK Fertilizers, Int. Nutrition Agric. Consultancy, April, 1996. With permission.)

Apart from the waste management phase, environmental benefits may also accrue from the use phase. For plastics and composites, these savings typically become available by weight reduction in transportation (e.g., a lightweight component of a car or a crate that is being transported). According to the studies available, savings of this type, sometimes referred to as “secondary savings,” are as high or even clearly higher than the “primary savings” (typically related to cradle-to-factory gate systems). For example, the secondary savings of a china reed-reinforced pallet and a hemp fiber-based interior side panel for a car were determined to be up to three times as high as the primary savings. Exceptionally high secondary savings are reported for tires containing starch-based fillers where secondary energy savings exceed primary savings by a factor of more than 20.15 Copyright © 2005 by Taylor & Francis

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Discussion and Conclusions

The results presented herewith clearly document that composting of biowastes along with biodegradable plastics offer significant environmental advantages in all effect categories. Thus, biodegradable materials, instead of having negative environmental impact, can make significant positive contributions to the environment and contribute to sustainability. Currently, most products are designed with limited consideration toward their ecological footprint, especially as it relates to their ultimate disposability. Of particular concern are plastics used in single-use disposable packaging and in consumer goods. Designing these materials to be biodegradable and biobased and ensuring that they end up in an appropriate disposal system such as composting along with other biowastes is environmentally and ecologically sound. The most important uncertainties in published LCA studies relate to the waste management phase, especially regarding methane emissions from landfills, energy recovery yields in waste-to-energy facilities, and carbon sequestration due to composting.15 The uncertainties need to be addressed partly by further research and analysis and partly by following the good practice guidelines for biobased and biodegradable materials in addition to the ISO standards for LCAs.4−7 In spite of these uncertainties and the information gaps already mentioned, the body of work analyzed overwhelmingly indicates that biodegradable and biobased polymers and natural fiber composites offer important environmental benefits today and for the future. Of all materials studied, starch polymers are considered to perform best for the environment under the current state of the art, with some differences among the various types of starch polymers. Compared to starch polymers, PLA yields less environmental benefits (LCA results only available for energy and CO2). For PHA, the achievable environmental advantage currently seems to be very small compared to conventional polymers (LCA results only available for energy use). For both PLA and PHA, the production method, the scale of production, and the type of waste management treatment can decisively influence the ultimate conclusion about overall environmental balance. For natural fibers, the extent to which these can replace fiberglass (which is heavy and energy intensive to produce) mainly determines the net environmental benefits in the cases studied. The advantages according to cradleto-factory gate analyses were rather limited in one case (–14% for underfloor panel) and very attractive in two other cases (– 45% to – 50% for interior side panel and transport pallet). The LCA studies addressing savings in the use phase (secondary savings) show that these can be as high or even higher than the primary savings (typically related to cradle-to-factory gate systems). Secondary savings could hence be an important driver for substitution. The ECCP Working Group on Renewable Raw Materials30 concluded that total secondary savings may Copyright © 2005 by Taylor & Francis

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exceed primary savings by about one order of magnitude. This expectation is clearly confirmed by the case study for tires, while it seems somewhat too optimistic if compared to the results for natural fibers. Starch polymers are currently the only type of biobased polymer for which several comprehensive LCA studies are available. According to these assessments starch polymers do not perform better than their fossil fuelbased counterparts in all environmental categories, including biodiversity and soil quality, which are generally outside the scope of LCAs. However, most studies come to the conclusion that starch polymers (pellets and end products) are more beneficial in environmental terms than their petrochemical counterparts; this conclusion is drawn by comparing the number of scores by impact category (without weighting and in most cases without significance thresholds). The preferences among the environmental targets determine whether biobased polymers are considered to be environmentally attractive. Full-sized LCA studies for further biobased materials are indispensable for deriving conclusions and recommendations that have better underpinning and are more focused. For the time being, it is not possible to make a concluding general judgment whether biobased plastics should be preferred to petrochemical polymers from an environmental point of view. This has partly to do with the limited availability of comprehensive LCAs. But even if more LCA studies were available, one would be left with considerable uncertainties because it is theoretically impossible to cover all possible products and all possible impact categories (cf. Finnveden).31 In spite of these limitations, one can conclude that the results for the use of fossil energy resources and GHG emissions are already more favorable for most biobased polymers in use today. As an exception, landfilling of biodegradable polymers can result in methane emissions (unless landfill gas is captured), which may make the system unattractive in terms of reducing greenhouse gas emissions. As a potential source of N2O emissions, fertilizers also require special attention. To maximize the environmental benefits from biobased and biodegradable polymers, further R&D will be necessary in order to optimize production by increasing efficiencies of the various unit processes involved (e.g., separation processes) and by process integration. Substantial scope for improvement can be expected here from upscaling and further technological progress (especially for biobased and biodegradable materials which are still in their infancy while manufacture of petrochemical polymers has been optimized for decades). Some of the LCA studies previously discussed were already outdated when these conclusions were drawn, since substantial progress has been made in manufacturing and processing biobased polymers (e.g., for films). This means that the real environmental impacts caused by biobased and biodegradable polymers tend to be lower than established in the LCA studies reviewed. As a guide for future R&D, good practice targets for green products could be very useful. Based on the outcome presented in Table 27.4, a first attempt is made here to specify such targets. It is recommended that, relative to their conventional counterparts, green polymers and natural fiber composites should Copyright © 2005 by Taylor & Francis

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Save at least 20 MJ (nonrenewable) energy per kilogram of polymer Avoid at least 1 kg CO2 per kilogram of polymer Reduce most other environmental impacts by at least 20%.

In view of the results shown in Table 27.4, these criteria are rather conservative but complying with them would ensure continuous innovation and reduction of environmental impacts. Note that this set of criteria ●

●

●

is dynamic, since it accounts for reductions in energy use and environmental impacts achieved in petrochemical plastics (in other words, it is a moving target) is only indicative, since it refers to one mass unit of polymers, while a more accurate study would be at the level of end products (due to the myriad of end products, a selection would need to be made, and for the same reason, the simpler approach of using 1 kg of polymer as a functional unit has been chosen here) can be applied without any discrimination to advanced petrochemical materials as well, which outpace the standard products.

From this point of view, green plastics and composites can be defined in a broad and target-oriented manner. It can be summarized that the available LCA studies and environmental assessments strongly support the further development of biodegradable and biobased polymers. Careful monitoring of various environmental impacts continues to be necessary both for decision makers in companies and in policy. If combined with good practice targets, such monitoring may accelerate and focus ongoing product and process innovation. For some materials the environmental benefits achieved are substantial even today. In many other cases, the potentials are very promising and need to be exploited.

References 1. U.N. Brundtland Commission/World Commission on Environment and Development, Our Common Future, New York, 1987. 2. European Commission, ExternE–Externalities of Energy, Vol.7: Methodology–1998 Update (EUR 19083), Office for Official Publications of the European Communities, Luxembourg, 1999, Results are also available at http://ExternE.jrc.es/publica.html. 3. Hofstetter, P., Perspectives in Life Cycle Impact Assessment–A Structured Approach to Combine Models of the Technosphere, Ecosphere and Valuesphere, Kluwers Academic Publishers, Dordrecht, 1998. 4. International Organisation of Standardization, ISO, Environmental Management–Life Cycle Assessment–Principles and Framework (ISO 14040), 1997.

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5. International Organisation of Standardization, ISO, Environmental Management– Life Cycle Assessment–Goal and scope definition and inventory analysis (ISO 14041), 1998. 6. International Organisation of Standardization, ISO, Environmental Management –Life Cycle Assessment–Life cycle impact assessment (ISO 14042), 2000a. 7. International Organisation of Standardization, ISO, Environmental Management –Life Cycle Assessment–Life cycle interpretation (ISO 14043), 2000b. 8. Boustead, I., Hancock, G.F., Handbook of Industrial Energy Analysis, The Open University, Milton Keynes, Ellis Horwood Ltd., New york, 1979. 9. Smith, H.H., The Cumulative Energy Requirements of Some Final Products of Chemical Industry, Transactions of the World Energy Conference, Moscow, 1968 (year of publication: 1969), pp. 71–89. 10. Gerngross, T.U., Can biotechnology move us toward a sustainable society? Nat. Biotechnol., 17, 541–544, 1999. 11. Gerngross, T.U. and Slater, S., How Green are Green Plastics? Sci. Am., 37–41, August, 2000, and www.sciam.com/2000/0800issue/0800gerngross.html 12. APME (Association of Plastics Manufacturers in Europe, 1999/2000) Ecoprofiles of plastics and related intermediates (about 55 products). Prepared by Ian Boustead for the APME. Downloadable from the internet (http://www.apme.org), Brussels, Belgium. 13. Heyde, M., Ecological considerations on the use and production of biosynthetic and synthetic biodegradable polymers, Polym. Degrad. Stab., 59, 3–6, 1998. 14. MITI Ministry Trade and Industry (MITI) - Japan, Tokyo, Japan, 2001. 15. Patel, M., Bastioli, C., Marini, L., and Würdinger, E., Life-cycle assessment of biobased polymers and natural fibers, in Biopolymers, Vol. 10, Wiley-VCH, 2003. 16. Carbotech Ökobilanz stärkehaltiger Kunststoffe (Nr. 271), 2 volumes. Study prepared by Dinkel, F., Pohl, C., Ros, M., Waldeck, B., Carbotech, Basel, for the Bundesamt für Umwelt und Landschaft (BUWAL), Bern, Switzerland, 1996. 17. BIFA/IFEU/Flo-Pak Kunststoffe aus nachwachsenden Rohstoffen - Vergleichende Ökobilanz für Loose-fill-Packmittel aus Stärke bzw. aus Polystyrol (final report, DBUAz. 04763). Study prepared by Würdinger, E., Roth, U., Wegener, A., Borken, J., Detzel, A., Fehrenbach, H., Giegrich, J., Möhler, S., Patyk, A., Reinhardt, G.A., Vogt, R., Mühlberger, D., and Wante, J., (2002) Bayrisches Institut für Angewandte Umweltforschung und –technik, Augsburg (BIFA; project leader), Institut für Energie- und Umweltforschung Heidelberg (IFEU), Flo-Pak GmbH, Germany, 2002. 18. Fraunhofer ISI C-STREAMS - Estimation of Material, Energy and CO2 Flows for Model Systems in the Context of Non-Energy Use, From a Life Cycle Perspective (Vol. I) (in German; English abstract). Report by Patel, M., Jochem, E., MarscheiderWeidemann, F., Radgen, P., and von Thienen, N., Fraunhofer ISI Karlsruhe, Germany, 1999. 19. Composto, Life cycle assessment of Mater-Bi bags for the collection of compostable waste, Study prepared Estermann R., Schwarzwälder, B. by Composto, for Novamont, Novara, Italy. Olten, Uerikon, Switzerland, 1998. 20. Composto, Life cycle assessment of Mater-Bi and EPS loose fills, Study prepared by Estermann, R., Schwarzwälder, B., Gysin, B., Composto for Novamont, Novara, Italy. Olten, Switzerland, 2000. 21. Cargill Dow, NatureWorks–A New Generation of Biopolymers. Presentation by E. Vink, Cargill Dow on 29 March 2001, Birmingham, United Kingdom. 2001.

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22. IBM, Introducing biobased materials into the electronics industry. Paper by Kosbar, L.L., Gelorme, J.D., Japp, R. M., and Fotorny, W.T., J. Ind. Ecol., 4, 93–105, 2001. 23. Daimler-Chrysler Ökologischer Vergleich von NMT- und GMT-Bauteilen, Paper by Diener, J. and Siehler, U., Angew. Makromol. Chem. 272, 1–4, 1999. 24. Braunschw. Univ. & Seeber, Life cycle studies on hemp fibre reinforced components and ABS for automotive parts, Paper by Wötzel, K., Wirth, R., and Flake, R., Angew. Makromol. Chem., 272, 121–127, 1999. 25. Kurdikar, D., Paster, M., Gruys, K.J., Fournet, L., Gerngross, T.U., Slater, S.C., and Coulon, R., Greenhouse gas profile of a plastic derived from a genetically modified plant, J. Ind. Ecol., 4, 107–122, 2001. 26. Narayan, R., Biodegradation of polymeric materials (anthropogenic macromolecules) during composting, in Science and Engineering of Composting: Design, Environmental, Microbiological and Utilization Aspects; Hoitink, H.A.J. and Keener, H.M., Eds., Renaissance Publications, Ohio, 1993, p. 339. 27. Narayan, R., Impact of governmental policies, regulations, and standards activities on an emerging biodegradable plastics industry, in Biodegradable Plastics and Polymers, Doi, Y. and Fukuda, K., Eds., Elsevier, New York, 1994, p. 261. 28. EPA Characterisation of MSW in the U.S, U.S. Environmental Protection Agency (EPA), Washington, DC, 1996. 29. Meier-Ploeger, A., Vogtmann, H., and Zehr, A., Eco Balance Compost vs. NPK Fertilizers, prepared by the International Nutrition and Agricultural Consultancy group, April 1996. 30. ECCP, European Commission ECCP (European Climate Change Programme) – Long report. http://europa.eu.int/comm/environment/climat/eccp_longreport_0106.pdf. Brussels, Belgium, 2001. 31. Finnveden, G., On the limitations of life cycle assessment and environmental system analysis tools in general, Int. J. Life Cycle Assessment, 5, 229–238, 2001.

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