Chemical Modification of Lignocellulosic Materials

Chemical Modification of Lignocellulosic Materials

edited by

David N.-S. Han Clemson University Clemson, South Carolina

Marcel Dekker, Inc.

New York - Basel- Hong Kong

Library of Congress Cataloging-in-Publication Data

Chemical modification of lignocellulosic materials I edited by David N.-S. Hon. p. cm. Includes index. ISBN 0-8247-9472-9 (alk. paper) 1. Wood. 2. Lignocellulose. 3. Fibrous composites. 4. WoodChemistry. I. Hon, David N.-S. TA419.C496 1996 620.1 '2-dc20 95-34365 CIP

The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the address below. This book is printed on acid-free paper. Copyright © 1996 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any infonnation storage and retrieval system, without pennission in writing from the publisher. Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 Current printing (last digit): 10 9 8 7 6 5 4 3

2

PRINTED IN THE UNITED STATES OF AMERICA

Preface

Lignocellulosic materials are important natural renewable resources . Wood, the predominant source, is among the most extensively used engineering materials. As a consequence, it occupies a position of great importance in the global raw materials picture. Because of the ever-increasing demand for wood as well as new restrictions on wood production, the need for wood products with enhanced engineering properties and performance is greater than at any time in the past. Wood is the oldest composite material. It and other lignocellulosic materials consist of flexible cellulose fibers assembled in an amorphous matrix of lignin with a hemicellulosic polymer. These three principal constituents make up cell walls and are responsible for most of the physical and chemical properties of wood. Wood has been used as an engineering material because it is low in cost, renewable, and strong, and it requires low processing energy. For instance, while the production of most plastics used today consumes 30-90 million Btu/ton, most solid-wood products use only 5-10 million Btu/ton. Unfortunately, wood products have undesirable properties such as dimensional instability caused by moisture sorption with varying moisture contents, biodegradability, flammability, and degradability by ultraviolet light, acids, and bases. Consequently, if wood composite products are used under adverse environmental conditions, their quality will deteriorate. They will have a limited service life. Because these changes are chemical, it is possible to eliminate or avoid them, or to

iii

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Preface

decrease their rate by modifying the basic chemistry of the lignocellulosic polymers. Moreover, wood has limited thermoplasticity. Although it can be bent under steam and chemical treatment, wood normally burns before it melts or becomes sufficiently plastic for heat molding or extrusion. These two techniques are important ways of shaping materials in high-speed composite production and are therefore keys to the cost-efficient penetration of lignocellulosic materials into the composites market. Chemical modification of wood offers a means of improving its thermoplasticity. In addition to wood, other lignocellulosic materials of commercial value, such as bamboo, kenaf, and rice straw, also lend themselves to chemical modification for use in composites. It is clear that processes for chemical modification of wood and other lignocellulosic materials offer opportunities to produce a new generation of high-performance, high-quality products that can compete with thermoplastics as well as fiber composite materials. Interest in chemical modification of lignocellulosic materials has been evident in the scientific literature for the past 15 years, and researchers around the world are increasingly recognizing the potential of chemical modification and its application to products derived from biobased materials. The interest in wood modification in the Pacific Rim countries was evident at the Pacifichem international meeting in Honolulu in December 1989 at which an international symposium on the chemical modification of wood was successfully held. A similar symposium was also held in Kyoto, Japan, in May 1991. The chemical modification of lignocellulosic materials conference was so well received that international symposia with the same theme were held in Rotorua, New Zealand, in November 1992 and in Vancouver, Canada, in November 1994. The next Pacifichem international meeting is scheduled to be held in Honolulu in December 1995. A symposium on chemical modification of lignocellulosic materials will also be organized for that meeting. These active international meetings have drawn global attention to improving utilization and performance of lignocellulosic materials. Although scientific information is actively exchanged during these meetings, most of the research documents are either scattered in the literature or unpublished. The need for a reference book has been expressed by many scientists, including those who attended the meetings. After numerous discussions with colleagues throughout the world, I decided to organize this book. The principal objective was to underscore the importance of chemical modification as a major branch of wood chemistry as well as to provide academic

Preface

v

and industrial research scientists and technologists with a broad background in current principles and practices. Based on the suggestions of many colleagues in academia and industry, the important topics that should be included on chemical modification of lignocellulosic materials were enumerated. Each chapter was written by recognized experts in the field. The format of an edited book seems particularly useful for a book on chemical modification because the subject has grown too large for anyone person to be an expert on every aspect, and outstanding work has been done by many investigators. This book combines the best work in various areas of chemical modifications to produce a comprehensive and high-quality reference source. The book is aimed at practicing wood chemists, polymer chemists, material engineers, and scientists who work in the wood, cellulose, and agrofiber industries. It is designed to provide both solid fundamental information and an up-to-date review of recent innovations applicable to real-life wood utilization problems. I believe that the chemical modification of lignocellulosic materials will continue to receive great attention in the years ahead and that this book is a forerunner of things to come.

David N.-S. Hon

Contents

Preface III Contributors

ix

1.

Functional Natural Polymers: A New Dimensional Creativity in Lignocellulosic Chemistry 1 David N.-S. Ron

2.

Chemical Structures of Cellulose, Hemicelluloses, and Lignin Gyosuke Meshitsuka and Akira Isogai

3.

Reactivity and Accessibility of Cellulose, Hemicelluloses, and Lignins 35 Yuan-Zong Lai

4.

Chemical Modification of Cellulose David N.-S. Ron

5.

Chemical Modification of Lignin John J. Meister

6.

Chemical Modification of Solid Wood Rideaki Matsuda

11

97

129

159

vii

viii

Contents

7.

Liquefaction of Wood 185 Mariko Yoshioka, Yaoguang Yao, and Nobuo Shiraishi

8.

Surface Modification and Activation of Wood Makoto Kiguchi

9.

Chemical Modification of Nonwood Lignocellulosics Roger M. Rowell

197

229

10.

Characterization of Chemically Modified Wood Takato Nakano

II.

Weathering of Chemically Modified Wood 277 David V. Plackett, Elizabeth A. Dunningham, and Adya P. Singh

12.

Physical and Mechanical Properties of Chemically Modified Wood 295 Roger M. Rowell

13.

Viscoelastic Properties of Chemically Modified Wood Misato Norimoto

14.

Biological Properties of Chemically Modified Wood Munezoh Takahashi

Index

363

247

311

331

Contributors

Elizabeth A. Dunningham Wood Products Division, New Zealand Forest Research Institute Limited, Rotorua, New Zealand David N.-S. Hon Department of Forest Resources, Clemson University, Clemson, South Carolina Akira Isogai Department of Forest Products, University of Tokyo, Bunkyoku, Tokyo, Japan Makoto Kiguchi Ibaraki, Japan

Forestry and Forest Products Research Institute, Tsukuba,

Yuan-Zong Lai Faculty of Paper Science and Engineering, SUNY College of Environmental Science and Forestry, Syracuse, New York Hideaki Matsuda Research Laboratory, Okura Industrial Co., Ltd., Marugame, Kagawa-ken, Japan John J. Meister Department of Chemistry, University of Detroit Mercy, Detroit, Michigan Gyosuke Meshitsuka Department of Forest Products, University of Tokyo, Bunkyo-ku, Tokyo, Japan Takato Nakano Timber Engineering Division, Hokkaido Forest Products Research Institute, Asahikawa, Hokkaido, Japan

ix

Contributors

x Misato Norimoto Japan

Wood Research Institute, Kyoto University, Uji, Kyoto,

David V. Plackett Composites and Treated Wood Products Department, Forintek Canada Corporation, Vancouver, British Columbia, Canada Roger M. Rowell Forest Products Laboratory, USDA Forest Service~ and Department of Forestry, University of Wisconsin, Madison, Wisconsin Nobuo Shiraishi Department of Wood Science and Technology, Kyoto University, Sakyo-ku, Kyoto, Japan Adya P. Singh Wood Products Division, New Zealand Forest Research Institute Limited, Rotorua, New Zealand Munezoh Takahashi oto, Japan

Wood Research Institute, Kyoto University, Uji, Ky-

Yaoguang Yao Department of Wood Science and Technology, Kyoto University, Sakyo-ku, Kyoto, Japan Mariko Yoshioka Department of Wood Science and Technology, Kyoto University, Sakyo-ku, Kyoto, Japan

1 Functional Natural Polymers: A New Dimensional Creativity in Lignocellulosic Chemistry David N.-S. Hon

Clemson University Clemson, South Carolina

I.

INTRODUCTION

Mankind has always recognized the value of lignocellulosic materials, especially wood. Throughout recorded history wood has proven to be one of man's most valuable natural resources. It has always been readily available and is adaptable for use in a wide variety of applications. Since the dawn of civilization, human beings have acquired and applied technical knowledge and skills to harness lignocellulosic materials such as wood for making the weapons, domestic utensils, tools, building materials, and fuel they required. At the most basic level these items satisfied their needs or wants in shelter, clothing, defense, transport, and leisure. This fact has been exemplified in such statements as "man has no older or deeper debt than that which he owes to trees and wood" [1] and "a culture is no better than its woods" [2]. Even in today's rapidly changing technological world, wood products and other lignocellulosic materials continue to serve mankind in thousands of ways. Without these renewable resources, not only would our cultural and social life suffer but the economy of the nation would be altered considerably. Thus, global forest resources have recently attracted unprecedented attention. They give rise to more widespread concern than at any time in history.

1

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II.

Han

CHANGING WORLD

In spite of the eminent role played by wood produClli, replacement of them with products derived from petrochemicals took effect dramatically from the late 1950s. The emergence of high carbon and stainless steel, structural aluminum alloys, organometalics, ceramics, plastic composites, and other engineering materials has also reduced significantly the market share of lignocellulosic-derived products. Not until the energy and raw material crises of 1973 was the intrinsic renewable value of lignocellulosic materials (or biomass) recognized and the complete and effective utilization concepts revived. Lignocellulosic materials give rise to more widespread concern than at any time in history, even though they are no longer a vital part of the power baliC of major countries as they were before the days of coal and oil. With this indelible experience, the forest products industry acquiesced that in order for it to hold its place it must compete aggressively with the low-cost, nonrenewable, but fashionable oil-derived products. General consensus confinns that the forest products industry can no longer stand pat on the statement "wood is good" and remain aloof from the real competitive forces of the marketplace. Technical, marketing, and managerial innovations have occurred in the forest products industry throughout its history. The overriding challenge facing the industry today is to accelerate the current pace of innovation. Unless more emphasis is placed on innovation, the forest products industry will continue to lose economic ground to more aggressive competitors. In order to strengthen the competitive position of wood and allied lignocellulosic products, scientislli recogni:t..c the need to improve properties and pcrfonnance of these naturally produced products by chemical modification of their polymeric chemical components [3-6]. Many innovative chemical modifications have already enhanced the performance and extended market opportunities for lignocellulosic materials. In addition to this undertaking, another critical challenge will be to accommodate new production technologies within increasingly strict and rigid environmental codes and constraints. The wood products industry will have to operate with close attention to delicate ecological balances. The challenges are formidable but not insunnountable. Fortunately, since lignocellulosic materials are renewable, they are received in a better light than those from nonrenewable sources.

III.

SOURCES AND PROPERTIES OF LIGNOCELLULOSIC MATERIALS

The primary production of biomass, which is narrowly defined as materials of terrestrial plant origin (7), is about 172 billion tons/year on land of which

Functional Natural Polymers

3

about 82% is the existing lignocellulosic materials in forests. Wood, therefore, is the most important component. Other lignocellulosic materials include agricultural residues, water plants, grasses, and other plant substances. These materials are unique in their chemical composition as well as their chemical, physical, and mechanical properties. They consist mainly of cellulose, hemicelluloses, lignins, and a small amount of extractives. Several important sources of lignocellulosic materials are listed in Table 1. As a natural product of biological origin, lignocellulosic materials are characterized by a high degree of diversity and variability in their properties. They are available in various forms, give a feeling of "warmth" to the touch, and have a pleasant appearance, none of which are offered by other nonwoody engineering materials. The fact that wood is very strong and easy to machine with low energy consumption, yet light in weight, makes it an ideal building material. The attractive colors and grain patterns of wood as well as its ease of cutting and fabrication are responsible for its unsurpassed beauty and wide use in fine furniture. In addition to wood, other lignocellulosic fibrous

Table 1 Chemical Composition of Lignocellulosic Materials (%) Lignocellulosic source

Cellulose

Hemicellulose

Lignin

Extract

Hardwood Softwood Abaca Bagasse Coir Corn cobs Com stalks Cotton Flax (retted) Flax (unretted) Hemp Henequen Istle Jute Kenaf Ramie Sisal Sunn Wheat straw

43-47 40-44 63.72 40 32-43 45 35 95 71.2 62.8 70.2 77.6 73.48 71.5 36.0 76.2 73.1 80.4 30

25-35 25-29 5-10 30 10-20 35 25 2 20.6 12.3 22.4 4-8 4-8 13.6 21.5 16.7 14.2 10.2 50

16-24 25-31 21.83 20 43-49 15 35 0.9 2.2 2.8 5.7 13.1 17.37 13.1 17.8 0.7 11.0 6.4 15

2-8 1-5 1.6 10 4.5 5 5 0.4 6.0 13.1 1.7 3.6 1.9 1.8 2.2 6.4 1.7 3.0 5

Han

4

materials are also insulating to heat and electricity, exhibit little thermal contraction and expansion, and have good acoustic properties. The strengthto-weight ratio is very high for lignocellulosic fibers when compared to almost every other fiber. Given these properties, lignocellulosics compare favorably to other products. Wood and its reconstituted products, such as plywood, particleboard, ftakeboard, and strandboard, can be durable when used or maintained under proper conditions. Many wooden temple buildings that were built in Japan 1300 years ago are still in good condition, and in the harsh climate of the mountains of Norway all-wood stave churches can be found that were built about 800 years ago. In spite of their many indisputably excellent characteristics, lignocellulosic materials do have some problems. Most of the lignocellulosic materials are relatively hygroscopic. They will take up and give off moisture depending on the temperature and relative humidity of the surrounding atmosphere. Wood shrinks as it loses moisture below the fiber saturation point; conversely, it swells upon gaining moisture. This dimensional instability certainly is a counterbalancing disadvantage in the utilization of wood for many products. Flammability of wood also imposes a disadvantage. Wood and wood-based products are also sensitive to weathering in which sunlight, moisture, and air pollutants may trigger discoloration, checking, and surface deterioration. Biological agents readily attack wood to cause staining, softening, and decay. Such deterioration may result from fungi, marine borers, a variety of wood-boring beetles, and even bacteria that cause deterioration in water-stored logs. Products derived from lignocellulosic materials other than wood also exhibit more or less identical properties. With regard to utilization of these renewable resources in the performance-driven competitive world, improvement of these unattractive properties is imperative and has been addressed to some extent.

IV.

CHEMICAL CHARACTERISTICS AND REACTIVITIES

Lignocellulosic materials contain cellulose, hemicelluloses, and lignins. These polymers possess many active functional groups susceptible to reaction (3,8]. Professors Meshitsuka and Isogai discuss the chemical structures of these polymers in detail in Chapter 2. They also include in their discussion the solid state structures of cellulose, degree of polymerization, and chemical linkages. In short, the potential sites for chemical reactions in lignocellulosic materials, cellulose, and lignin are plainly illustrated in Figs. I and 2. These

Functional Natural Polymers

5

reaction sites or functional groups are primary and secondary hydroxyls, carbonyls, carboxyls (ester), carbon-carbon, ether, and acetal linkages. Ethylenic and sulfur-containing groups may also be found in lignins. Virtually every type of reagent capable of reacting with these functional groups can be applied to wood, and the literature is full of examples [8-12]. Hence, based on the variety of functional groups, etherification, esterification, alkylation, hydroxyalkylation, graft copolymerization, crosslinking, and oxidation have been conducted to produce a series of products with many applications. Depending on product properties, the magnitude of the reaction may vary. Although wood species, density, and thickness play an important role in chemical treatments, the chemical reaction also depends heavily on the distribution of reactive functional groups of wood and their accessibility and reactivity (Fig. 3). The reaction system and media should be considered for a specific purpose. Whether the system should be limited to the surface reaction or bulk, to the amorphous or crystalline region, and whether the reaction should be conducted in a homogeneous or a heterogeneous system has to be defined (Fig. 3). These variations influence the uniformity of the reaction products in the modified wood. In Chapter 3, Professor Lai summarizes the major factors affecting the reactivity and accessibility of cellulose, hemicelluloses, and lignins under both acidic and alkaline modification conditions. During the performance of chemical modification, often chemical degradation takes place simultaneously. Professor Lai includes this factor in his discussion. Many products with industrial value are manufactured from wood and lignocellulosic materials. The optimum conditions for the reactions have been established by scientists from academia or industry. Hence, to utilize this basic chemistry to redesign wood products or lignocellulosic materials with superior properties will be the main issue of the chemical modification of

o

I -C=C

I

OR \

O--C\ / C / / .... C

/

/ ........C

/

"

Figure 1 A simplified illustration of functional groups in lignocellulosic materials.

Cellulose 'X~_ I -cI

~/

I -eI -c- . - - ---.-e-+-- ---.

;~CH3 H3CO"~~CH' OH ~ t

t Lignin

Figure 2

Potential sites for chemical reactions in cellulose and lignin.

OH

Functional groups: • Distribution • Accessibility •Reactivity Conditions: • Surface vs.Bulk • Crystalline vs. Amorphous • Homogeneous vs. Heterogeneous Figure 3

6

Reaction products are governed by the modification parameters.

Functional Natural Polymers

7

wood for the twenty-first century. Along with the careful consideration of the chemical modification conditions, it is just as important to consider product development with regard to the intrinsic properties of the starting materials (aesthetic value; natural warmth; mechanical, physical, and chemical properties), processing properties of new products (chemical and mechanical), and new product application properties (performance, aesthetic, and maintenance properties). With reference to product properties, one should consider the following parameters for evaluation: tensile and compressive strength; elastic, tensile, and compressive moduli; density; thermal stability; electrical conductivity; biodegradability; weatherability; and dimensional stability. The criteria for selection of endproducts should include visual appearance, compatibility, ease of fabrication, weight, cost, quality, maintainability, performance, and availability of process facility and materials.

v.

CHEMICALLY MODIFIED PRODUCTS

As discussed earlier, most lignocellulosic materials are relatively hygroscopic. This exerts dimensional instability. In the attempts to solve this problem, many techniques have been developed, especially in the past 10 years. These activities are summarized by Dr. Matsuda in Chapter 6 and by Dr. Kiguchi in Chapter 8. In Dr. Matsuda's chapter, various etherification and esterification procedures are discussed in detail and the utilization of these products is emphasized. Dr. Kiguchi's chapter includes a discussion of surface activation and thermoplasticization of wood surfaces by etherification. Self-bonding properties and other applications of surface-modified woods are reviewed. Wood and wood-based products are also sensitive to weathering. Chemical modification of the wood surface appears to provide the weather resistance properties, which is the subject of Chapter II by Plackett. Since lignocellulosic materials are biomaterials, they are biodegradable. In many areas of application, microbiologically resistant products are required. Through unique chemical modification, biodegradability of wood can be improved. In Chapter 14, Professor Takahashi summarizes the work in this area. Products derived from lignocellulosic materials other than wood also exhibit more or less identical properties. These nonwood lignocellulosic materials can also undergo chemical modification to improve their physical and mechanical properties. Dr. Rowell discusses the methods of modification and the characteristics of the agro-based products in Chapters 9 and 12. In addition, Dr. Nakano discusses various methods of physical characterization of chemically modified wood in Chapter 10. His discussion emphasizes the molecular mobility and

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8

relaxation properties of wood components in chemically modified woods. The viscoelastic properties of endproducts were also included in Chapter 13 by Dr. Norimoto. It is recognized that wood, cellulose, and lignin encompass a number of diverse properties that enable them to perform a particular task. It is also recognized that these materials may not encompass the required properties and hence it becomes necessary to transform them into new products by carrying out chemical reactions on them. The reactions were mainly carried out with the objectives of improving or modifying their chemical or structural properties and of making them suitable for specific purposes. Dr. Shiraishi and his group continue their effort in transforming wood and lignocellulosic materials into innovative products. They have used various dissolution methods to create liquefied woods. These activities are summarized in Chapter 7. New products from cellulose and lignin are also summarized in Chapters 4 (Hon) and 5 (Meister). In Chapter 5, Dr. Meister discusses modification in terms of thermal or chemical decomposition of lignin to produce various new products. He also includes a discussion on the preparation and utilization of potential graft copolymers of lignin. Based on the materials highlighted by the authors of this book, it is clear that scientists throughout the world have worked diligently to improve the properties, performance, and utility of wood and other lignocellulosic materials. High-level chemical modification research activities have been developed and continuing. Chemical modification specifically tailored for macromolecular structures of lignocellulosic materials is a fascinating scientific endeavor in its own right as well as a useful art for the creation of specialty polymeric materials for technological applications. By utilizing suitable chemical reactions, new products with hybrid properties of nature and synthesis will take an important position with regard to utilization in the competitive world.

VI.

CONCLUDING REMARKS AND FUTURE PROSPECTS

Lignocellulosic research today is poised on the threshold of a new era of research breakthroughs. It has enabled the use of a wide variety of lignocellulosic materials, low-quality wood species and sawdust, and low-value lignin products. Lignocellulosic and cellulosic research efforts are under way to produce novel products for construction, transportation, plastics, fiber, packaging, and medical applications. Some of the major activities in chemical modifications of wood, cellulose, and lignins are the main features of this book.

Functional Natural Polymers

9

Chemical modification of lignocellulosic materials represents a magnificent range of achievements in both academic research and industrial innovation. Although extraordinary progress has been made in terms of understanding structures and chemistries of the raw materials as well as modification parameters, challenges also remain. The chemically modified lignocellulosic products must compete with other engineering materials for their share of an often highly specialized market. Hence, the future uses of lignocellulosic materials and the degree of efficiency in lignocellulose utilization largely depend on continued research and development. During the past 15 years, unprecedented progress has been made in the development of new engineering materials from lignocellulosic residues. These materials, which include wood-polymer composites, thermoplasticized woods, and adhesives from liquefied woods, open up new engineering possibilities. Although scientists have achieved great success in producing modified lignocellulosics with unique properties, it is by no means certain that the products will be successfully commercialized. These chemical technologies are still in their infancy, and cost-effective use of modified engineering products and environmentally friendly fabrication processes are yet to be demonstrated in large-scale commercial applications. Hence the future holds many opportunities for chemists, biochemists, engineers, and materials scientists in the rapidly expanding science of renewable resource materials. They will be challenged to develop and improve the production of economically and environmentally viable products. This gives mankind a future that is not locked in the ever-dwindling supplies of fossil products (oil, coal, natural gas) and geological products (iron, aluminum, rock, sand, etc.).

REFERENCES I. 2. 3. 4.

5. 6. 7.

M. Bramwell (ed.), The International Book of Wood. Simon and Schuster, New York, 1976. W. H. Auden, Woods. Listener, Dec. II (1952). D. N.-S. Hon, Polym. News 17:102 (1992). Proceedings of the International Symposium on Chemical Modification of Wood, Kyoto, Japan, May 17-18, 1991. Proceedings of Chemical Modification of Lignocellulosics, Pacific Rim BioBased Composites Symposium, Rotorua, New Zealand, November 7-8, 1992. Proceedings of Second Pacific Rim Bio-Based Composites Symposium, Vancouver, Canada, November 6-9, 1994. Productivity of World Ecosystems, 5th Symp. Proc. Gen. Assembly Spec. Comm. Int. BioI. Prog., International Biological Program, National Research Council, National Academy of Science, Washington, D.C., 1975.

10

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8.

D. N.-S. Hon and N. Shiraishi (eds.), Wood and Cellulosic Chemistry, Marcel Dekker, New York, 1991, p. 1020. D. Fengel and G. Wegener, Wood: Chemistry, Ultrastructure and Reactions, Walter de Gruyter, Berlin, 1984, p. 613. K. V. Sarkanen and C. H. Ludwig (cds.), Lignin, Wiley-Interscience, New York, 1971, p. 916. N. M. Bikales and L. Segal (cds.), Cellulose and Cellulose Derivatives, WileyInstescience, New York, 1971, Parts IV and V. D. N.-S. Hon (ed.), Graft Copolymerization of Lignocellulosic Materials (ACS Symposium Series 187), American Chemical Society, Washington, D.C., 1981.

9. 10. 11. 12.

2 Chemical Structures of Cellulose, Hemicelluloses, and Lignin Gyosuke Meshitsuka and Akira Isogai

University of Tokyo Bunkyo-ku, Tokyo, Japan

I.

CHEMICAL STRUCTURE OF CELLULOSE

Higher plants including wood, some algaes, tunicates (sea animal), and some bacteria produce cellulose, which is a homopolysaccharide consisting of [3-o-glucopyranose residues linked by glucoside bond at their C 1 and C4 hydroxyl groups (Fig. 1). Cellulose has three hydroxyl groups per anhydroglucose residue and thus some functional groups are introducible into cellulose by esterification, etherification, deoxyhalogenation, and other reactions. However, such chemical modifications of cellulosic materials are sometimes difficult to achieve freely at the hydroxyl groups. Solid state structures of cellulose, i.e., intra- and intermolecular hydrogen bonds, crystallinity, crystal size, crystal structures, interactions with water, molecular mass and molecular mass distributions, presence of lignin or hemicelluloses, shape and size of cellulosic materials and others, greatly influence reactivity of the cellulosic materials, efficiency of the reactions, and finally properties of the chemically modified cellulosic materials.

A.

Solid State Structures of Cellulose

1.

Native Cellulose

Although cellulose has the simple chemical structure of homopolysaccharide as shown in Fig. 1, it can take various rotation angles at C1-04 (
11

Meshitsuka and Isogai

12 06

Figure 1

Chemical structure of cellulose.

(\fJ), and C5-C6 (K) for the heavy atoms as well as the three hydroxyl groups

(rotation angles of C2-02, C3-03, and C6-06) in solid state; one anhydroglucose residue alone can take various conformations. Cellulose chains easily form the intramolecular hydrogen bond between the proton of C3-OH and the C5 oxygen atom of the adjacent glucose ring, and thereby the linearity of cellulose chains may appear not only in solid states but also in some solution ones (I]. The driving force of this chain linearity may also be explained by sterically stable conformations at the glucoside bond (i.e., the rotation angles and \fJ with the lowest repulsion between two protons, C' I-H and C4-H). The presence of intra- and intermolecular hydrogen bonds in solid cellulose leads in some cases to heterogeneous distribution of substituents. When cellulose is etherified through heterogeneous alkali cellulose, relative reactivity of hydroxyl groups in cellulose is in the order of C2-0H ~ C6-0H » C3-OH for methylation, carboxymethylation, and hydroxyalkylation [2]. Cellulosic materials usually form crystal structures in part, and water cannot penetrate the inside of crystalline domains at room temperature. Native celluloses form crystalline microfibrils or bundles of cellulose chains 2-5 nm in width for higher plant cell uloses and 15-30 nm for algal celluloses, which are observable by electron microscope. Almost all native cell uloses have Xray diffraction patterns of cellulose I with crystallinity indexes (CI) [3] of about 40-95 %. Recently, solid state 13C-NMR analysis of native celluloses has revealed the presence of two different crystal structures in native cellulose I, celluloses Ia and 113 , on the basis of the resonance patterns of C I; singlet for Ia and

Chemical Structures

13

doublet for If3 (Fig. 2) [4]. Higher plants and tunicate cell uloses mostly have cellulose If3' whereas bacteria and algal ones have a mixture of celluloses Ia and 113 with ratios of about 6:4. Electron diffraction analysis of algal and tunicate celluloses showed that cellulose Ia has a triclinic unit cell consisting of one cellulose chain with the space groups of PI and that cellulose 113 has a monoclinic unit cell consisting of two chains with P2 1 [5]. Furthermore, it seems that cellulose Ia and 113 domains are separately distributed along one cellulose microfibril [5]. Cellulose la can be irreversibly converted to cellulose 113 without any changes in shape and sizes of the microfibrils by steam treatments at about 230-280°C (Fig. 3) [6]. This result suggests that cellulose chains are packed as the parallel mode in one microfibril for both celluloses la and 1f3.

2.

Conversions to Other Crystal Structures

As to cellulose allomorphs, cell uloses I, II, Ill, and IV have been reported so far based on their X-ray diffraction patterns (Fig. 4). Swelling treatments of native celluloses with some solvents, which can penetrate into the crystalline domains, bring about conversions to other crystal structures and/or partial decrystallization. When native celluloses are soaking in aq. NaOH solutions with higher than about 15% (wt/wt) followed by washing with water to remove the alkali and drying, the X-ray diffraction pattern of cellulose II appears. Treatments of native celluloses with liquid ammonia or an organic amine such

I

100

80

I

60 PPM

Figure 2 13C-NMR spectra of cclluloses la and 16 , (Reproduced from Ref. 4. Copyright 1984 American Chemical Society.)

Meshitsuka and Isogai

14 Cl

C2,J,5 C4

C6

I ".1",,1

120

I , 100

.1.

I ••

I 80

I

"1,,,,1

I

I.

.1

60

ppm from TMS

Figure 3 Conversion from cellulose lu-rich structure to 16 by steam treatments. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)

Chemical Structures

5

15

10

15

Diffraction angle deg 26 Figure 4 Typical X-ray diffraction patterns of celluloscs I, II, III, and IV and amorphous cellulose.

as ethylene diamine followed by washing with alcohol results in conversion to cellulose III with partial decrystallization. These conversions to other crystal structures are influenced by lignin content, shape and size of cellulosic materials used, time and temperature of the soaking treatments. When wood chips or wood meals are used, the conversions are difficult to achieve and only partial decrystallization occurs. On the other hand, the crystal conversions clearly take place in mechanical pulps, where lignin and hemicellulose contents are almost equal to those of wood. These swelling treatments of cellulosic materials generally enhance their reactivity to chemical modifications resulting from an increase in accessibility to reagents with partial decrystallization.

Meshitsuka and Isogai

16

3.

Noncrystalline Domain and Its Distribution in Cellulose

Chemical modifications of cellulosic materials initially proceed at their regions accessible to the reagents, and thus characterizations of the noncrystalline domains as well as those of the initial reactions between reagents and the accessible regions are sometimes necessary for understanding the behavior of heterogeneous reactions of cellulosic materials. Ball milling of native celluloses under dry conditions decreases their CI together with severe depolymerization and is sometimes used for preparing amorphous cellulose samples [7}. Noncrystalline celluloses with virtually no depolymerization and with a stable noncrystalline structure under aqueous conditions can be prepared from various native cell uloses by regeneration from their nonaqueous cellulose solutions in water [8]. The distributed state of the noncrystalline domains in cellulose affects the properties of chemically treated higher plant celluloses. It is well known that degrees of polymerization (DP) of higher plant celluloses are drastically reduced to about 200-250 at the initial stage of the heterogeneous acid hydrolysis at high temperature. These DP values are maintained throughout the acid hydrolysis, although the weight loss of the recovered celluloses proceeds gradually with an increase in the hydrolysis time (Fig. 5). This leveling off of DP is observed only on cell uloses of higher plants and may be explained

c o

",p

IS

"~

E o

~

a.

'0

.....

r

.................... ..................

~

o

Hydrolysis time

Figure 5 Changes in degrees of polymerization and yields of higher plant cellulose during acid hydrolysis at 80-95°C.

Chemical Structures

17

in terms of the distributed states of noncrystalline or acid-accessible regions in higher plant cell uloses. These regions may possibly be located with intervals of about 200-250 DP. If the noncrystalline domains of higher plant celluloses are present in this manner, it may be a fatal shortcoming for chemical treatments of cellulose with less depolymerization under acid and high-temperature conditions such as autohydrolysis, chemical modification, pulping, and bleaching. However, no such periodic units along microfibrils have been observed by electron microscope or other methods. Thus, systematic explanation about the behavior of the leveling off DP in terms of the solid state structures of higher plant cellulose is necessary.

B.

Degrees of Polymerization and DP Distribution of Cellulose

Approximate molecular weights of cellulose are conveniently obtained by viscosity measurements of cellulose solutions in cuppriethylenediamine or cadoxene and are expressed as degrees of polymerization (DPs) or viscosities (cps) [9]. On the other hand, size exclusion chromatography (SEC) is used for obtaining both DPs and DP distributions. In this case, a derivatization of cellulose with less depolymerization and with complete substitution at all hydroxyl groups is required for solubilizing in tetrahydrofuran or other solvents for SEC analysis using columns of styrene-divinylbenzene copolymer gel. Nitration and phenylisocyanation of cellulose have commonly been used for this SEC analysis in order to obtain, for example, information about changes in DP and DP distributions of cellulosic materials during chemical treatments [10,11]. Recently, LiCI-N,N-dimethylacetamide (DMAc), one of the new cellulose solvent systems, was applied to the solvent of cellulose and chitin without derivatizations for viscosity measurement and SEC analysis

(12-14].

C.

Chemical Linkages with Hemicelluloses and Lignin

The conventional procedure for purifying higher plant cell uloses is (1) repetition of bleaching with an aq. sodium chlorite solution under acid conditions for removing lignin to prepare holocelluloses, followed by (2) extraction of the holocelluloses with an aq. alkali solution for removing hemicelluloses to prepare a-cellulose. Although almost pure cellulose can be obtained as acellulose by this method, small amounts of mannose and xylose residues are still detected by the sugar composition analysis [15]. As described in the

Meshitsuka and Isaga;

18

following section, the presence of chemical linkages (ethers, esters, and/or glycoside bonds) between lignin and hemicelluloses in wood cell wall is well accepted, and these linkages may influence in part the pulping and bleaching reactions. Wood cellulose and pulps soluble in S02-diethylamine-dimethylsulfoxide, one of the nonaqueous cellulose solvent systems, can be pennethylated in one step by using CH 3 I-powdered NaOH in the homogeneous system 116]. The experimental results of this permethylation followed by gas chromatographic (GC) analysis of the hydrolyzates of pennethylated wood celluloses suggested the presence of small amounts of chemical linkages between cellulose, hemicelluloses, and Iignins (15). Not only these chemical interactions but also the physical ones between hemicellulose, lignin, and especially noncrystalline domains of cellulose in wood cell wall are unsolved questions, hopefully to be clarified in tenns of biosynthetic processing of the three components in situ.

II.

STRUCTURE OF HEMICELLULOSE

Hemicellulose is also one of the main components of lignified cell wall. Monosaccharide residues constituting hemicelluloses are arabinose and xylose (pentoses), glucose, galactose, and mannose (hexoses), rhamnose and fucose (6-deoxyhexoses), and galacturonic, glucuronic, and 4-0-methylglucuronic acids (uronic acids). The dominating hemicellulose in hardwood is an 0acetyl(4-0-methylglucurono)xylan (so-called hardwood xylan) accompanied by a small amount of glucomannan. On the other hand, softwood contains an O-acetylgalactoglucomannan (so-called softwood glucomannan) as the major hemicellulose with a small amount of an arabino(4-0-methylglucurono)xylan (so-called softwood xylan) 117-19]. These hemicelluloses are known to be biosynthesized by a corresponding synthetase, xylan synthetase, which is responsible for the fonnation of xylan from UDP-D-xylose, which is produced by the decarboxylation of UDP-Dglucuronic acid 120]. Glucomannan, on the other hand, is biosynthesized from GDP-D-mannose and GDP-D-glucose. It is important to note that the activities of hemicellulose synthetases are controlled depending on the stage of cell wall fonnation.

A.

Structure of Hardwood Hemicelluloses

Hardwood xylan is a ~-(1-4)-linked polymer of D-xylopyranose residue. Average DP of hardwood xylan is 150-200, and every tenth of xylose resi-

Chemical Structures

19

dues, on average, is substituted at C2 by a 4-0-methylglucuronic acid residue. Hardwood xylan contains 9-14% acetyl group which corresponds to DS = 0.36-0.54. Acetyl groups are mainly introduced to C2 and some are to C3 and C2 and C3 of a xylose residue. It is interesting to note that small amounts of galacturonic acid and rhamnose residues are contained as the integral parts of hardwood xylan. Based on the characterization of the enzymatic hydrolyzate of xylan, every reducing end of xylan was proved to have a structure of -~-o-Xylp-(l-4)-~-o-Xylp­ (l-3)-a-L-Rhap-(l-2)-a-o-GaIAp-(l-4)-o-Xyl [2I,22). Glucomannan is a minor fraction of hardwood hemicelluloses (about 3-5%), and is a ~-(l-4)-linked linear copolymer of glucose and mannose at the ratio of 1: 1-2.

B.

Structure of Softwood Hemlcelluloses

Softwood glucomannan is a linear copolymer (DP = 35-140) of o-glucose and o-mannose at the ratio of I :3, containing low and high amounts of 0galactose. The former has the ratio of o-Gal:o-Glu:o-Man = 0.1: 1:3 and is simply called glucomannan. On the other hand, the latter has the ratio of o-Gal:o-Glu:o-Man = I: 1:3 and is called galactoglucomannan. Softwood glucomannan has 4.3-8.8% of acetyl groups (DS = 0.17-0.36) at C2 and C3 of o-mannose residue. A 6-0-acetyl group was also recently confirmed by Tanaka and coworkers [23]. Pectic polysaccharides are other important cell wall components characteristic of the primary wall and middle lamella. Three pectic polysaccharides have been structurally characterized. Those are rhamnogalacturonan I, rhamnogalacturonan II, homogalacturonan, arabinan, arabinogalactan, and galactan. Rhamnogalacturonan I has a backbone of 4-linked a-o-galacturonic acid and 2-linked a-L-rhamnose residues and has a 4-linked side chain of arabinan, arabinogalactan, or galactan [24]. Rhamnogalacturonan II is a complex polysaccharide having a backbone of 4-linked a-o-galacturonic acid residues with 2- and 3-linked side chains [25]. Arabinan was found in maritime pine wood and has a backbone of 5-linked a-L-arabinose residues with substitutions of some sugar residues at C2 and C3 [26). Arabinogalactan was found in the cambium layer of aspen wood and has a 3-linked backbone of ~-o-galactose residues with 6-lined side chains (27). The molecular structure of this polysaccharide is very similar to that found in larch heartwood, which will be mentioned below.

20

C.

Meshitsuka and Isogai

Conclusion

Hemicelluloses in reaction woods are quite different from those in the normal woods, namely, galactan and ~-( 1-3)-glucan in compression wood and galactan in tension wood. It is also well known that a remarkable amount of a water-soluble polysaccharide, arabinogalactan, is contained in the heartwood of larch. Since this polysaccharide occurs mainly in the lumen of tracheids and is not a cell wall component, it may not be included in hemicelluloses. Although structures and distributions of hemicelluloses have been comprehensively studied in the last 20 years, their physiologic meanings in a cell wall are not known yet. This must be the most important point for the future study of hemicelluloses.

III.

STRUCTURE OF LIGNIN

Lignin is a polymeric material of phenyl propane structural units and is found in every kind of vascular plant, including the herbaceous, as one of its main components. Lignin was first confirmed by Anselme Payen in 1838 when he treated wood with concentrated nitric acid. The residual white fibrous material turned out to be cellulose. This is the reason he has been called the discoverer of cellulose. The material removed by this treatment was found to have a higher carbon content than cellulose and to be responsible for the stiffness (hardness) of wood. The material was named lignin from a Latin word for wood, Lignun. Since historical developments of lignin chemistry were comprehensively reviewed in Lignins by Sarkanen and Ludwig and some others textbooks [28-311, the recent advances in structural understanding of lignin will be emphasized here.

A.

Determination of Lignin

It is important for lignin chemistry to have an exact and reliable method for the determination of lignin. There are basically two types of methods: (l) determination of solid lignin after removal of the carbohydrates as soluble degradation products and (2) determination of soluble lignin by spectroscopic methods. Klason lignin determination is typical of the former type and has been widely used as an indispensable step for the characterization of lignocellulosic materials. Since a method standardized by Tappi (Tappi Test Method T222 om-83) takes 2 h for immersion in 72% H 2S0 4 and 4 h for acid hydrolysis in boiling 3% H 2S04 , altogether 6 h for one sample, development of a modified method of essentially 2 h immersion in 72% H 2S04 and 0.5 h autoclave

Chemical Structures

21

treatment at 121°C was very useful [32]. A part of lignin soluble in a diluted sulfuric acid solution should be determined spectroscopically and added to Klason lignin content for the evaluation of total lignin. Klason lignin is fairly pure although a negligible amount of carbohydrate is incorporated [33] and is used for elementary analysis. Of the spectroscopic detenninations of lignin, an acetyl bromide method [34] has been most commonly accepted. This method is extremely useful for small-scale samples (= 10 mg) but was known to be disturbed by phenolic extractives. The modified method proposed by Iiyama and Wallis [35,36] is essentially the addition of a small amount of perchloric acid as an acetylation catalyst and is said to be applicable to a wood sample containing various phenolic extractives. The standard procedure of the modified method is as follows: A dry lignocellulosic sample containing 0.6-21.3 mg lignin is sealed in a glass bottle with 5 ml of 25% acetyl bromide in acetic acid containing 0.2 ml of 70% perchloric acid. After a heat treatment at 70°C for 30 min, the reaction mixture is diluted with acetic acid in a lOO-ml volumetric flask containing 10 ml of 2 M sodium hydroxide. The sample scale for the modified method may be even smaller, if it is necessary. The amount of lignin dissolved in the solution is determined based on the absorbance at 280 nm using absorptivity 20.091 g - I cm - I for both softwood and hardwood lignins.

B.

Isolation of Lignin

Isolation of lignin from wood or other lignocellulosic materials without any chemical change has long been one of the most important but difficult tasks for lignin chemists. This is because some undesirable structural changes, namely, condensation and oxidation, may occur during the isolation process. Milled wood lignin (MWL), originally developed by Bjorkman [37,38], has been the best preparation for the structural study of lignin. When thoroughly dried wood meal is ground in a vibratory ball mill in the dry state or in nonswelling solvents, such as toluene, the cell wall structure is destroyed and lignin is obtained by the subsequent extraction of the milled wood meals with a dioxane-water mixture. MWL suffers no serious structural modifications in the preparation process. However, it has at least two drawbacks. The first one is some carbohydrate contamination and the second is its limited yield. Concerning the first drawback, MWL may contain 2-8% of carbohydrate even after the ordinary purification. Harkin [39] recommended the preextraction of milled wood with cold ethyl acetate to remove low molecular weight fragments. MWL obtained by

22

Meshitsuka and Isogai

subsequent extraction with dioxane-water (9: I) was purified by the gradual addition of 0.08 part of benzene together with a small amount of neutral alumina. By this method, carbohydrate content in the purified MWL was claimed to be less than 0.2%. Another purification method was proposed by Lundquist [40]. A crude MWL dissolved in pyridine-acetic acid-water (9: 1:4 v/v/v) mixture is extracted with chlorofonn by a liquid-liquid extraction apparatus to give a pure MWL preparation. Carbohydrate content in the purified MWL was claimed to be 0.2-0.3% for spruce lignin and 3.7% for birch lignin. Concerning the second drawback of MWL, the yield of MWL is usually not more than 50%, sometimes 15-20% based on Klason lignin content of wood meals. In others words, even though it is sufficiently pure, MWL may not be representative of total lignin in wood meals. This drawback may be more serious when lignin is found to be not structurally homogeneous. Lee et al. [41] successively extracted MWL from birch wood meals at different milling times and found that MWL extracted at an early stage of milling was relatively rich in guaiacyl units and the contribution of guaiacyl units decreased with the increase of milling time (Table I). Based on these results, they concluded that at least MWL extracted at the early stage of milling should be originated from compound middle lamella. On the other hand, Whiting and Goring [42] concluded that the secondary wall of tracheids should be the origin of spruce MWL based on the behaviors of lignin-rich fraction obtained by milling and centrifugal treatment. These inconsistent results may indicate that it seems too early to conclude the origin of MWL in the cell wall. Lignin in wood may also be isolated not only by purified cellulase but by cellulase and hemicellulase mixture treatments as well. In these cases, the reactions are essentially the enzymatic hydrolysis of carbohydrates without structural changes of lignin [43,44]. Lignin preparation obtained by these treatments is named cellulolytic enzyme lignin (CEL). Although this method is successfully applied for the isolation of lignin from pulp [45], it is important to note that an appreciable amount of carbohydrate remains in this lignin preparation. Periodate lignin may be another type of lignin preparation representative of whole lignin in wood [46]. Wood meal suspending in water with a small amount of acetic acid is treated with sodium metaperiodate at room temperature, by which vic-glycol structures in carbohydrates are cleaved to give aldehydes. After the subsequent reduction of aldehydes to alcohols, carbohydrates are readily hydrolyzed even at room temperature in a mild acid condi-

9


3

[

Table 1

I ( I)

Nitrobenzene Oxidation Products of MWLs and LCC

MWL fraction (milling time)

MWL-I (24 h)

MWL-II (48 h)

MWL-II1 (96 h)

MWL-IV (144 h)

MWL-V (216 h)

12.23 15.05 27.25 1.04 0.15 5.96

10.68 15.85 26.53 1.24 0.29 3.28

9.96 16.04 26.00 1.35 0.52 1.60

9.24 16.44 25.68 1.49 0.59 1.53

9.71 18.20 27.91 1.57 0.70 1.23

---

Aldehyde: Vanillin Syringald. Total SIva molar ratio S'IG molar ratio Condo G/noncond. G

LCC

Residue

Wood meal

1.18 4.22 5.40 3.00

1.13 3.85 4.98 2.85

1.81 5.63 7.44 2.59

·S. Syringaldehydc; V. vanillin; S'. Syringyl unit (230); G. guaiacyI unit (200). 31 x (I + X) +- 31 x 2 x SIV . . Note: 200 x (l +- X) + 230 x SIV x 100 = methoxyI content (%). X: condensed type gualacyI umt.

~

24

Me5hit5uka and 150gai

tion. Although sodium metaperiodate was known to oxidize a free phenolic guaiacyl structure to an v-quinone structure 147], these undesirable structural changes of lignin may be avoidable if phenolic hydroxyl groups are previously etherified. Light yellow periodate lignin quantitatively prepared from wood meals seems to be one of the recommendable lignin preparations for the structural study. Of the lignin fractions remaining in milled wood after the extraction of MWL, a considerable portion is dissolved in dimethylsulfoxide or dimethylformam ide as lignin-carbohydrate complexes (LCCs). LCC has been extensively characterized to determine whether or not chemical linkages are present between lignin and carbohydrates. At present, some types of chemical linkages are almost accepted and certain evidence for the direct linkage between lignin and cellulose has been proposed [481, although direct evidence even for the linkages of lignin and hemicelluloses have not yet been obtained. Of the procedures proposed for the preparation of LCC, it is interesting to note that Koshijima et al. [49,501 developed two procedures to isolate LCC from milled wood meals. One is the isolation of LCC from the water-soluble fraction of dioxane-water extract and the other is the hot water extraction after the extraction of MWL. LCCs prepared by these procedures were claimed to have practically the same characteristics as Bjorkman LCC in terms of chemical composition and molecular weight. Concerning types of linkages, ether and ester bonds through the benzyl position of lignin side chain seem to be most likely. Glucuronic acid in xylan, presumably in the secondary wall, and galacturonic acid in pectic substance in the compound middle lamella may be the carbohydrate parts of ester-type LCC linkages. In this context, by the treatment with a purified pectinase, endopectinlyase, about 90% of neutral sugars, especially 99% of xylose residues, which are predominantly sugars in the original LCC, was removed (Table 2) [51]. This strongly indicates that the hemicellulose fraction in a hardwood LCC is connected to lignin through pectic substance. Another interesting behavior of neutral sugars at the pectinase treatment is the stability of galactose residue. Practically the same content of galactose before and after pectinase treatment indicates the different nature of galactose from other sugar residues in LCC in terms of the association with lignin. Azuma and Koshijima [52-55] found that benzyl ether bonds in nonphenolic structural units are effectively and selectively cleaved by (DDQ) dichlorodicyanoquinone oxidation and proposed this treatment for the analysis of benzyl ether-type LCC bonding structure.

9 ~

[

C/)

2 o 2' ~ (/)

Table 2

Sugar Composition of LCC Before and After Pectin Lyase Treatment

Sample

Lignin

Neutral sugar

LCC-I PL-Treated LCC-I

19.7% 71.0%3

68.9% 29.09P

Acidic sugar

11.4%

Relative composition of neutral sugars Rha.

Ara.

Xyl.

Man.

Gal.

Glu.

0.8% 6.8% (0.7)

2.3% 15.3% (1.5)

69.2% 4.2%

12.9% 12.5% (1.2)

3.1% 47.7% (4.8)

11.7% 13.5% ( 1.3)

(0.4)

'Detennined from Sepharosc CL-6B gel filtration curves. ( ) Based on total sugar content in LCC.

~

26

c.

Meshitsuka and Isogai

Linkages Between Lignin Structural Units

It is well known that lignin structural units are linked to each other with various types of linkages, namely ~-0-4, a-0-4, 4-0-5 (diaryl), ~-5, ~-~, 5-5 (biphenyl), and ~-1. The three types are ether bonds and the rest are carbon-carbon bonds. Table 3 shows the approximate frequencies of these linkages based on the data of Adler [56], of which ~-0-4 is the major one and ~-5 is of secondary importance in softwood lignin. The frequencies of ~-~, ~-l, and 5-5 are not clear and may be much lower than the estimated values. In the case of hardwood lignin, the contribution of ~-0-4 is higher than that of softwood lignin. Although the relative importance of ~-0-4 linkage may be certain, the quantitative determination was not fully established. An electron-withdrawing p-toluenesulfinyl group introduced in the 'Y-hydroxyl group promotes the cleavage of ~-0-4 linkage by ~-alkoxy elimination reaction. Contribution of ~-0-4 linkage was estimated to be 0.46/C9 based on the number of newly formed phenolic hydroxyl groups [57] after this treatment. Concerning the studies on stereostructures of lignin side chains, Matsumoto et al. [58] found, and later Sarkanen and Anderson [59] confirmed, that ozonolysis is quite useful. Two diastereoisomers of side chains are determined as shown in Fig. 6 as the corresponding tetronic acids, erythronic and threonic acids. Although there are some earlier trials, this is the first case of the application of ozonolysis for the structural study of lignin. KMn0 4 may be used as the oxidizing agent instead of ozone but is less effective than ozone [59]. Erythro and threo forms of ~-0-4 structures have been determined based on the benzyl proton signals of side chain in IH-NMR, if the sample

Table 3

Percentage of Different Types of Linkages in Spruce and Birch MWLs Percentage

Type of linkage Arylgycerol-~-aryl ether Phenylcoumaran Noncyclic benzyl aryl ether Biphenyl Diaryl ether Glyccraldehyde-2-aryl ether 1,2-Diarylpropane ~,J3-Linked structures

Spruce MWL

46 9-12

Birch MWL

60

6

6-8

6-8

9.5-11 3.5-4 2 7 2

4.5 6.5 2 7 3

Chemical Structures

27

CH20H

CH20H

H-i-OjQ> H-C-OH

O 3 CH

I

H-C-OH

OJ

OW

~OCH3

I

H-C-OH

I

COOH

OH

Erythronic acid

Erythro form

CH20H

CH20H

H-i-o -? H~OC::H3

I

H-C-OH

OJ

ow

I

HO-C-H

I

COOH

OH

Threo form Figure 6

Threonic acid

Formation of tetronic acids from B-ether type of structures by ozonation.

is soluble in NMR solvents. On the other hand, the ozonolysis method may be applicable to any kind of solid samples. Matsumoto further extended the ozonolysis method for the quantitative analysis of 13-5 and other minor bonding structures [60-62]. The 13-1 structure is formed by the liberation of glyceraldehyde-2-aryl ether structure at the stage of lignification and is presumably distributed in secondary wall lignin. Therefore, the 13-1 structure may be estimated by the determination of the glyceraldehyde-2-aryl ether structure. By the sequential administration of NaBH 4 reduction, ozonolysis, and alkali treatment, this will be determined as glycerol (60). In this case, it is important to note that glycerol is not formed from carbohydrate and that the glycerol yield varies depending on the number of annual rings.

D.

Structural Heterogeneity of Lignin

Lignins are classified into several types according to their composition of structural units. Lignin in softwoods is largely composed of guaiacyl units and is called "guaiacyl lignin" or "G lignin." On the other hand, lignin in hardwood is the copolymer of guaiacyl and syringyl units in different ratio and is called "guaiacyl syringyllignin" or "G-S lignin." Even though soft-

28

Meshitsuka and Isogai

wood lignin is mainly composed of guaiacyl units, it also contains some amount of p-hydroxyphenyl units and very small amounts of syringyl units. Structural heterogeneity of lignin in various topochemical regions of wood must be considered for total understanding of lignin. UV microscopic characterization of lignin in each topochemical region of wood was first introduced by Lange [63) and later developed by Goring and his coworkers [64-68). EDXA analysis developed by Saka et al. [69-71] had a higher sensitivity and was very useful for the evaluation of the topochemistry of delignification during pulping and bleaching. When an electron beam impinges on the specimen, a characteristic X-ray for each element is emitted. In this case, X-ray characteristic to bromine atoms which are introduced in lignin is detected and used for the estimation of lignin content in each region. Since the intensity of the X-ray is dependent on the number of bromine atoms, the reactivity of each type of lignin was carefully examined by the use of lignin model compounds [71). It seems uncertain, however, as to whether or not lignin in the cell wall, especially in the middle lamella, is fully brominated as are the lignin model compounds. For the detailed structural study of lignin in a certain morphologic region of wood, isolation of a lignin fraction from that particular region is very important. In this context, gravitational and centrifugal fractionations were reported by several authors [72-76]. Nakano et al. [72,731. Hardell et al. [74], and Obst [75) tried the fractionation of mechanically disintegrated highyield pulp suspended in water. Depending on the different density of wood components, the cellulose-rich fiber fraction starts to precipitate first but the lignin-rich compound middle lamella fraction remains suspended in water. Therefore, it is possible to obtain a lignin-rich fraction if the suspending particles are carefully collected. A film-like fraction obtained by Nakano et al. [73) from a birch wood neutral high-yield sulfite pulp contained 43.8% of lignin, which is about double the lignin concentration determined for secondary wall by UV microscopy. Although this is still lower than the expected lignin concentration for middle lamella, it is certain that this fraction is rich in middle lamella. Centrifugal treatment is another method (76) by which disintegrated wood particles suspended in organic solvent system were forced to separate according to their density. Density of the solvent system (carbon tetrachloride-l A-dioxane) was adjusted to be in between those of cellulose (1.5 glml) and lignin (1.4 g/ml). In these cases it is important to note that although gravitational and centrifugal fractionation methods gave us fairly lignin-rich fractions, the yields of those fractions, namely, 0.4% for

29

Chemical Structures

the latter case, may be too low as a representative sample of middle lamella lignin. Meshitsuka and coworkers [77-82] proposed to collect the differentiating xylem, so-called soft xylem, as the representative sample of the compound middle lamella. This is because differentiating xylem is at the beginning of secondary wall formation and consists almost exclusively of compound middle lamella lignin. The lignin fraction thus obtained from birch wood was characterized by degradation and spectroscopic methods. It was found that this lignin fraction is highly condensed and rich in guaiacyl units. Total aldehyde yield by alkaline nitrobenzene oxidation was only 4.0% (instead of 42.3% for total lignin) [77J. A small amount of p-hydroxybenzaldehyde was also confirmed in the oxidation products of this lignin fraction. Concerning the contribution of each aromatic structure, namely, syringyl (S), guaiacyl (G); and p-hydroxyphenyl (H), methoxyl group content per C9 unit of sulfuric acid lignin (Klason lignin) was evaluated (72). In this case, it is important to note that Klason lignin is practically pure and demethylation of methoxyl groups during its preparation is negligible. The number of methoxyl groups thus obtained are 0.56/C9 and 1.39/C9 for the differentiating xylem lignin and total lignin, respectively. The relative contribution of each aromatic structure was calculated to be S IG I H = 0.05:0.46:0.49 for the differentiating xylem lignin. This is in clear contrast with that for total lignin, S G H = 0.33:0.67:0.00. H units are obviously one of the main components of the compound middle lamella lignin, which is deposited at the early stage of cell wall formation, although most of them are incorporated in the polymer lignin by carbon-carbon bondings. In this context, Goring et al. also claimed the important contribution of H units for softwood lignin [831, although some possibility of the contamination of compression wood lignin was pointed out [84]. Concerning the biosynthetic background of the structural heterogeneity of lignin, Terashima and his coworkers [85-89] accomplished extensive results by the administration of radioisotope-labeled lignin precursors to a living tree. Results obtained by this method clearly support the structural heterogeneity of lignin discussed before. In other words, lignin structurally specific to the type of a cell and to the stage of cell maturation is deposited in the cell wall regardless of the structural types of the precursors adminstered. Contribution of H units for the compound middle lamella lignin was confirmed [87,89J. For the further study of the structural heterogeneity of lignin, mild and selective cleavage of bonds, pyrolysis gas chromatography-malis spectrometry (GC-MS), and solid state nuclear magnetic resonance are particularly important.

I I

Meshitsuka and Isogai

30

Selective cleavage of ether bonds is useful to detennine the contribution of carbon-carbon bonds for polymer lignin. Pivaloyl iodide [90J is known to cleave a-ether bonds selectively and trimethylsilyl iodide [91,92J can cleave a- and ~­ ether bonds quite effectively under the proper reaction conditions. Because of the very small amount of sample required, pyrolysis GC-MS may be applied for the analysis of a specific morphologi region of a cell wall.

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2. 3. 4. 5. 6. 7. 8. 9. 10. II.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

D. Gagnaire, J. St.-Gennain, and M. Vincendon, J. Appl. Polym. Sci., Appl. Polym. Symp. 37, 261 (1983). A. Isogai, A. Ishizu, and J. Nakano, Sen'i Gakkaishi 40, T504 (1984). L. Segal, J. J. Creely, A. E. Martin, Jr., and C. M. Conrad, Text. Res. J. 29, 786 (1959). D. L. VanderHart and R. H. Atalla, Macromolecules 17, 1465 (1984). J. Sugiyama, R. Vuong, and H. Chanzy, Macromolecules 24,4168 (1991). F. Horii, H. Yamamoto, R. Kitamaru, M. Tanahashi, and T. Higuchi, Macromolecules 20, 2946 (1987). P. H. Hennans and A. Weidinger, J. Am. Chem. Soc. 78,2457 (1946). A. Isogai and R. H. Atalla, J. Polym. Sci., Polym. Chem. 29, 113 (1991). Tappi Test Method, T230 om-82 (1991). L. R. Schroeder and F. C. Haigh, Tappi J., 62, 103 (1979). S. Kuga, N. Muto, A. Isogai, M. Usuda, and M. Brown, in Cellulose, Structural and Functional Aspects (B. Phillips et aI., eds.), Ellis Horwood, New York, 1989, p. 81. C. L. MacConnick, P. A. Callais, and B. H. Hutchinson, Jr., Macromolecules 18, 2394 (1985). A. L. Kvemheim and E. Lystad, Acta Chem. Scand. 43, 209 (1989). M. Hasegawa, A. Isogai, and F. Onabe, J. Chromatogr. 635, 334-337 (1993). A. Isogai, A. Ishizu, and J. Nakano, Holzforschung 43,333 (1989). A. Isogai, A. Ishizu, J. Nakano, S. Eda, and K. Kato, Carbohydr. Res. 138, 99 (1985). T. E. Timell, Adv. Carbohydr. Chem. 19,247 (1964). T. E. Timell, Adv. Carbohydr. Chem. 20, 409 (1965). K. Shimizu, in Wood Cellulosic Chemistry (D. Hon and N. Shiraishi, eds.), Marcel Dekker, New York, 1991, p. 177. G. P. Bolwell, G. Dalessandro, and D. H. Northcote, Phytochemistry 24,699 ( 1985). K. Shimizu, M. Ishihara, and T. Ishihara, Mokuzai Gakkaishi 22,618 (1976). M. Ishihara, K. Shimizu, and T. Ishihara, Mokuzai Gakkaishi 24, 108 (1978). R. Tanaka, F. Yaku, J. Iyoda, T. Koshijima, and A. Azuma, Mokuzai Gakkaishi 31,859 (1985).

Chemical Structures 24. 25. 26. 27. 28. 29. 30. 31. 32.

31

M. McNeil, A. G. Darvill, and P. Albersheim, Plant Physiol. 66,1128 (1980). A. G. Darvill, M. McNeil, and P. Albersheim, Plant Physiol. 62,418 (1978). A. J. Roudier and L. Eberhard, Bull. Soc. Chirn. France (1965), 460. B. W. Simson and T. E. Timell, Cellulose Chem. Technol. 12,63 (1978). K. V. Sarkanen and C. H. Ludwig, Lignins, Wiley Intcrscience, New York, 1971. I. R. Pearl, The Chemistry of Lignin, Marcel Dekker, New York, 1967. E. Sjostrom, Wood Chemistry, 2nd ed., Academic Press, San Diego, 1993. J. Nakano (ed.), Lignin no Kagaku, 2nd ed., Yuni, Tokyo, 1990. K. Yoshihara, T. Kobayashi, T. Fujii, and I. Akamatsu, J. Japan Tappi 38(4), 86 (1984). 33. Y. Matsumoto, A. Ishizu, J. Nakano, and K. Terasawa, J. Wood Chem. Technol., 4,312 (1984). 34. D. B. Johnson, W. E. Moore, and L. C. Zank, Tappi, 44,793 (1961). 35. K. liyama and A. F. A. Wallis, Wood Sci. Techno!., 22, 271 (1988). 36. K. liyama and A. F. A. Wallis, Appita 41, 442 (1988). 37. A. Bjorkman, Svensk Papperstidn. 59, 477 (1956). 38. A. Bjorkman and B. Person, Svensk Papperstidn. 60, 158 (1957). 39. K. V. Sarkanen and C. H. Ludwig, Lignins, Wiley-Interscience, New York, 1971, p. 168. 40. K. Lundquist, B. Ohlsson, and R. Simonson, Svensk Papperstidn. 80, 143 (1977). 41. Z. Z. Lee, G. Meshitsuka, N. S. Cho, and J. Nakano, Mokuzai Gakkaishi 27, 671 (1981). 42. P. Whiting and D. A. I. Goring, Svensk Papperstidn. 84, RI20 (1981). 43. H.-M. Chang, E. B. Cowling, W. Brown, E. Adler, and G. Miksche, Holzforschung 29, 153 (1975). 44. B. Bezuch and J. Pokin, Cellulose Chem. Technol. 12, 473 (1978). 45. T. Yamasaki, S. Hosoya, c.-L. Chen, J. S. Gratzl and H.-M. Chang, Proc. Int. Symp. Wood Pulping Chemistry, Stockholm, Vo!. 3, 1981, p. 34. 46. P. F. Ritchie and C. B. Purves, Pulp Paper Mag. Can., 48(12), 74 (1947). 47. E. Adler and S. Hemestam, Acta Chem. Scand. 9, 319 (1955). 48. A. Isogai, A. Ishizu, and J. Nakano, J. Wood Chern. Technol. 7, 463 (1987). 49. T. Watanabe, J. Azuma, and J. Koshijima, Mokuzai Gakkaishi 33,798 (1987). 50. J. Azuma, N. Takahashi, and T. Koshijima, Carbohydr. Res. 93,91 (1981). 51. G. Meshitsuka, Z. Z. Lee, J. Nakano, and E. Eda, J. Wood Chem. Technol. 2,251 (1982). 52. T. Koshijima, T. Watanabe, and J. Azuma, Chern. Lerr. 1984, 1737. 53. T. Watanabe, S. Kaizu, and T. Koshijima, Chern. Lerr. 1986, 1871. 54. T. Koshijima, T. Watanabe, and J. Azuma, Chem. Lerr. /984, 1737. 55. T. Watanabe and T. Koshijima, Mokuzai Gakkaishi 35, 130 (1989). 56. E. Adler, Wood Sci. Technol. JJ, 169 (1977).

32

Meshitsuka and Isogai

57.

Y. Matsumoto, A. Ishizu, K. Iiyama, and J. Nakano, Mokuzai Gakkaishi 28, 249 (1982). Y. Matsumoto, A. Ishizu, and J. Nakano, Holzforschung 40, Suppl., 81 (1986). Y. Tsutsumi, A. Islam, C. D. Anderson, and K. V. Sarkanen, Holzforschung 44, 59 (1990). Y. Matsumoto, A. Ishizu, and J. Nakano, Mokuzai Gakkaishi 30,74 (1984). N. Habu, Y. Matsumoto, A. Ishizu, and J. Nakano, Holzforschung 44, 67 (1990). Y. Matsumoto, N. Habu, K. Minami, A. Ishizu, and J. Nakano, Proc. Fifth Int. Syrnp. Wood Pulp Chern., Raleigh, North Carolina, 1989, p. 365. P. W. Lange, Svensk Papperstidn. 57, 525 (1954). B. J. Fergus and D. A. I. Goring, Holzforschung 24, 113, 118 (1970). B. J. Fergus, A. R. Procter, J. A. N. Scott, and D. A. I. Goring, Wood Sci. Technol. 3, 117 (1969). Y. Musha and D. A. I. Goring, Wood Sci. Technol. 9, 45 (1975). J. M. Yang and D. A. I. Goring, Holzforschung 32, 185 (1978). J. M. Yang and D. A. I. Goring, Can. J. Chern. 58, 2411 (1980). S. Saka, R. J. Thomas, and J. S. Gratzl, Wood Fiber II, 99 (1979). S. Saka and R. J. Thomas, Wood Sci. Technol. 16, 1 (1982). S. Saka, S. Hosoya, F. G. T. St-Germain, and D. A. I. Goring, Holzforschung 42, 79 (1988). T. Iwamida, Y. Sumi, and J. Nakano, J. Japan Tappi 29,324 (1975). N. S. Cho, S. Lee, G. Mcshitsuka, and J. Nakano, Mokuzai Gakkaishi 26,527 (1980). H. L. Hardell, G. J. Leary, M. Stoll, and U. Westermark, Svensk Papperstidn. 83,44,71 (1980). J. R. Obst, Holzforschung 36, 143 (1982). P. Whiting, B. D. Favis, F. G. T. St.-Germain, and D. A. I. Goring, J. Wood Chern. Technol. 1, 29 (1981). G. Meshitsuka and J. Nakano, J. Wood Chern. Technol. 5,391 (1985). T. J. Eom, G. Meshitsuka, and J. Nakano, Mokuzai Gakkaishi 33,576 (1987). T. J. Eom, G. Mcshitsuka, A. Ishizu, and J. Nakano, Mokuzai Gakkaishi 33, 716 (1987). T. J. Eom, G. Meshitsuka, A. Ishizu, and J. Nakano, Cellulose Chern. Technol. 22, 211 (1988). T. J. Eom, G. Mcshitsuka, and A. Ishizu, Mokuzai Gakkaishi 35, 820 (1989). Y. S. Kim, G. Mcshitsuka, and A. Ishizu, Mokuzai Gakkaishi 40,407 (1994). D. A. I. Goring, Wood Sci. Technol. 16, 261 (1982). U. Westermark, Wood Sci. Technol. 19, 223 (1985). N. Terac;hima, K. Fukushima, and K. Takabe, Holzforschung 40, Suppl., 101 (1986).

58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85.

Chemical Structures 86. 87. 88. 89. 90. 91. 92. 93.

33

N. Terashima, K. Fukushima, and K. Takabe, Holzforschung 42, 347 (1988). N. Terashima, K. Fukushima, and K. Takabe, Wood Sci. Technol. 22, 259 (1988). K. Fukushima and N. Terashima, J. Wood Chern. Technol. 10,413 (1990). K. Fukushima and N. Terashima, Holzforschung 49, 87 (1991). N. Fukagawa, G. Meshitsuka, and A. Ishizu, J. Wood Chern. Technol. /2,425 (1992). G. Meshitsuka, T. Kondo, and J. Nakano, J. Wood Chern. Technol. 7, 161 (1987). S. Makino, G. Meshitsuka, and A. Ishizu, Mokuzai Gakkaishi 36,460 (1990). K. Fujino, G. Meshitsuka, and A. Ishizu, Mokuzai Gakkaishi 38, 956 (1992).

3 Reactivity and Accessibility of Cellulose, Hemicelluloses, and Lignins Yuan-Zong Lai

SUNY College of Environmental Science and Forestry Syracuse, New York

I.

INTRODUCTION

Chemical modification reactions continue to playa dominant role in improving the overall utilization of lignocellulosic materials [1,2]. The nature of modification may vary from mild pretreatment of wood with alkali or sulfite as used in the production of mechanical pulp fibers [3] to a variety of etherification, esterification, or copolymerization processes applied in the preparation of wood- [4], cellulose- [5] or lignin- [6] based materials. Since the modification of wood polymers is generally conducted in a heterogeneous system, the apparent reactivity would be influenced by both the chemical and the physical nature of the substrate as well as of the reactant molecules involved. The significance of the accessibility factor in affecting the reactivity of cellulose has been well recognized [7-13] and is generally related to crystallinity of the substrate. However, the reactivity of cellulose [14-17] cannot be explained entirely by its apparent crystallinity. It is also profoundly influenced by its supramolecular structure. For example, the reactivity of native cellulose toward acetylation or nitration reaction decreased significantly with mercerization [14], which in fact would reduce the cellulose crystallinity. Also, regenerated cellulose samples did not dissolve in certain organic solvents [15,16], such as S02-diethylamine-dimethylsulfoxide (DMSO), which easily dissolved the native cellulose samples. The variation of native and regenerated

35

Lai

36

cellulose in hydrogen bonding patterns was suggested as being a contributing factor to this unique behavior [14-17]. The accessibility factor also plays a role in certain reactions of the hemicellulose and lignin components, although these polymers appear to be amorphous when present in the plant tissues [18,19]. Sepall and Mason 120], based on the tritium exchange reaction, observed an accessibility of only 52% for a birch xylan whereas higher values were obtained for wood cellulose (58%) and amylose (99%). The observed low accessibility of an amorphous xylan could be partially attributed to the presence of hydrogen bonding between acetyl groups and the neighboring hydroxyl groups [211. The reactivity of wood components in situ is further complicated by their highly heterogeneous nature across the cell wall in terms of both a qualitative and a quantitative variation [19,22-25]. Also, the presence of lignin-carbohydrate complex (LCC) or possible linkages 122,26-28] has been suggested as playing a significant role in the degradation and dissolution of the lignin component 129-34]. In addition to these chemical factors, the pore structure of the cell wall matrix would influence both penetration and diffusion processes. Thus the accessibility factor is anticipated to have an even more profound impact on the reaction of wood polymers in situ. This chapter discusses major factors affecting the reactivity of cellulose, hemicelluloses, and lignin under both acidic and alkaline modifications.

II.

CELLULOSE

Cellulose molecules, because of their unique structure, have a strong tendency to form extensive hydrogen bonds [13,35-46]. Two major intramolecular types were proposed 145]. One is between the C3 hydroxyl (OH) groups and the pyranose ring oxygen of an adjacent glucose residue (03-H· . ·05'). The second one is between the C2 OH and the C6 oxygen of a neighboring glucose residue (02-H· . ·06'). The major intermolecular hydrogen bond occurs between the C6 OH and C3 oxygen (06-H· . ·03') along the b axis. As a result of these hydrogen bonding and van der Waals forces, cellulose molecules could align together in a highly ordered fashion to form a crystalline region, whereas the less ordered molecules being noncrystalline are usually termed amorphous materials. It is generally accepted [44] that cellulose chains are aggregated to form microfibrils, in which highly ordered regions alternate with the less ordered regions. Microfibrils are further aggregated to form fibrils and then fibers. The proportion of ordered and disordered regions (crystallinity) of cellulose varies considerably with its origin and the extent of

Reactivity and Accessibility

37

physical or chemical treatment encountered [7,351. The crystallinity of cotton and wood cell uloses [7} generally decreases in the order of cotton (73%), wood pulp (60%), mercerized cotton (51 %), and regenerated cellulose (35%).

A.

Accessibility

The accessibility of cellulose has been a subject of extensive discussions by many authors [7-13,17,20,211. Jeffries et al. [71 summarized the approximate percentage of disordered component in native, mercerized, and regenerated cellulose. Their data reproduced in Table 1 display significant variations among various cellulose samples obtained by different methods, but several conclusions are evident. The noncrystalline or amorphous content identified indirectly from the X-ray diffraction or density measurement is not entirely equivalent to accessibility measured by chemical reactions. The relative content of disordered materials among various substrates, however, is consistent and increases in the order of decreasing crystallinity. Values from the density, deuteration, and moisture regain methods were comparable and slightly higher than that of the X-ray diffraction measurement. Significantly lower values were obtained by the chemical techniques. This is reasonable because only

Table 1

Approximate Percentage of Disordered Components in Various Celluloses as Summarized by Jeffries et al. Disordered components (%)

Technique Noncrystalline: X-ray diffraction Density Accessible: Deuteration Moisture regain Nonfreezing water Iodine sopriotn Acid hydrolysis Periodate oxidation Formylation Source: Ref. 7.

Cotton

Wood pulp

Mercerized cotton

Regenerated cotton

27 36

40 50

49 64

65 65

42 42 16 13

55 49

10

8 21

27 14 8 31

59 62 23 32 20 10

35

72 77 48 52 28 20 63

Lai

38

a portion of functional groups in the disordered region are capable of interacting with the specific reactant involved. Thus accessibility is a relative parameter specific to a given environment.

1.

Crystallites

The surface of crystallites also represents a portion of the cellulose component readily accessible to chemical agents. Thus, the nature of crystallite surface along with crystallinity influences the apparent accessibility of cellulose, especially when measured by chemical reactions. Among major polymorphs, cellulose I and II are most important in cellulose reactions. Cellulose I refers to native cellulose and the chain polarity is parallel or oriented in the same direction [13,37,41]. Cellulose II is produced by a mercerization process [34,37,47,48] or when native cellulose is first dissolved and then precipitated or regenerated. It has an antiparallel chain structure [46,49,53] and a more complex hydrogen-bonding network [36,49,52]. An additionaI02-H· . ·02' intennolecular bond was proposed as occurring along the long ab diagonal. This intermolecular bond was suggested as being a factor contributing to a high stability of the cellulose II structure and certainly would affect the accessibility of the C2 OH group.

2.

Pore Structures

Cellulosic substrates contain pores or voids of different sizes [10,12,13]. Pore structure detennines the internal accessible surface area of a cellulose substrate and thus affects its accessibility or reactivity.

Volume and Surface Area.

The void volume of native cellulose estimated by density measurements was approximately 3-4% [54]. Lin et al. [55], based on small-angle X-ray scattering technique, reported the value of cotton samples being in the 0.7-3.4% range with a considerably higher value (17%) for Valonia cellulose. Both alkali mercerization and NH 3 treatments reduced the void fraction. Similarly, the specific inner surfaces of hydrocellulose and Valonia samples were 15.3 and 45.2 m 2/cm 3 , respectively. These values are 5-10 times higher than that of cotton cellulose by the nitrogen sorption method (0.3-1. 9 m2/g). The X-ray method was suggested as being able to detect the inaccessible pore surface. Relatively large values (in m 2/g) were also obtained by using the vapors of ethanol 7.3; acetic acid 18.3; and water 137 156]. Thus, pore structures could be opened up by swelling agents but only a small portion of the pore structures opened up by water (15%) can be retained even after solvent exchange [57,58]. Drying significantly reduces the internal active surface areas

Reactivity and Accessibility

39

[13] and this is probably a major contributing factor to the so-called hornification processes [13,59]. The dehydration process likely induces the fonnation of new hydrogen bonding, especially for those molecules located on the less ordered and highly swollen surface areas of the fibrils. Pore Size. The pore size of cellulosic fibers depends considerably on the processing conditions. In general, native cellulose has more of the larger pores while voids of colloidal sizes are dominant in precipitated or regenerated cellulose [ 13]. The pore size distribution of fibers in wet state is often measured by the solute exclusion technique [60-64], based on the partitioning of macromolecule across the phase boundary of a swollen gel. Although the solute exclusion technique could provide infonnation on the distribution of pore sizes, the data interpretation, as Lindstrom [65] pointed out, is not as straightforward as the simple argument suggests. Stone et al. [631 reported that a never-dried cellophane sample was more swollen (2.5 mllg) than a rayon yarn (1.2 mllg) or super tire cord (0.76 mll g) cellulose. The maximum pore sizes decreased in the order cellophane (200 A) > rayon (100 A) > tire cord (50 A). Drying was shown to significantly reduce fiber swelling and pore size, especially for cellophane samples.

3.

Nature of Amorphous Components

The amorphous or disordered component plays an enonnous role in the physicochemical properties of cellulose. Its nature is still poorly defined [66-69] and like crystallites is influenced by physical and chemical treatments as reflected in solubility and reactivity characteristics [14-17]. The alkali solubility of cotton cellulose cannot be correlated entirely with its apparent amorphous content [16]. A regenerated sample prepared from a cuprammonium solution having a 94% amorphous content was totally soluble in 10% NaOH while a powdered cellulose obtained by ball milling to a similar amorphous content (92%) had an alkali solubility of only 58%. Different solubility behaviors were observed in organic solvent systems. Regenerated cellulose did not dissolve in the S02-amine-DMSO system which, however, readily dissolved the native and mercerized samples [15-171. Similarly, rayon and mercerized cellulose, unlike native cellulose, were insoluble in the dimethylformamide (DMS)-chloral-pyridine solvent system [ 14]. Thus, the morphology of the amorphous component in addition to its content plays a significant role in the overall reactivity of cellulose. It has been suggested [14,16,17,70,71] that the hydrogen bonding patterns of the amorphous component in cellulose I differs significantly from that of cellulose II. Regarding the intramolecular hydrogen bonds [105], an extra

Lai

40

06-H· . ·02' type was proposed for the cellulose II structures. The alkali solubility of cellulose was shown to have a higher correlation with the reduction in intramolecular hydrogen bonding than with the apparent amorphous content [16,71].

4.

Reaction Environments

Major reaction parameters affecting the apparent accessibility include the nature of media and reactant molecules. Swelling increases pore size and surface area, and thus accelerates the diffusion of reagents into the cellulose structure 14,72]. Water being able to disrupt the hydrogen bonding network is an effective swelling agent, and its effectiveness in promoting the chemical reactions of cellulose has been well documented [8,13,72-74J. Also, the diffusion or penetration of large reactants into the fiber matrix may be limited by the size of pore structures. As Rowland and Bertoniere pointed out [10], the percent accessibility of hydroxyl groups in native cotton decreased considerably with increasing molecular weight (MW) of the reactant: water (37% with 18 MW) > N,N-diethylaziridinium chloride (20% with 135 MW) > diphenyl fast red 5BL (3% with 676 MW). In addition, the relative reactivities of different hydroxyl groups in cellulose toward etherification reactions were significantly influenced by the alkali concentration [75-78J or solvent used 179J. Figure 1 illustrates the reaction of methyl ~-D-glucopyranaside with N,N-diethylaziridinium chloride (DAC) agent [80]. The reactivity of the 2-0H, 3-0H, and 4-0H groups decreased sharply with increasing alkali concentration whereas only a very small reduction was observed for the 6-0H group. The latter became the most reactive at alkali concentration above the 4 M level. Thus, reaction environment affects not only the physical aspects of accessibility but also the chemical reactivity of functional groups (Fig. 1).

B.

Reactivity of Reducing Endgroup

1.

Nature

The reducing endgroup of cellulose is hemiacetal in nature and is partially converted to the open-chain aldehyde function in solution. The anomeric hydroxyl groups being the most acidic in cellulose [82,83] can be selectively etherified [84]. They also behave practically like an aldehyde and can be reduced or oxidized to a glucitol or gluconic acid moiety. Reduction with sodium borohydride is often used for a quantitative estimation of the reducing endgroup content [69J. Oxidation may be achieved by a variety of agents [85], but the yield is generally

Reactivity and Accessibility

41

0.06

~

0.05

~ 0.04

~

~

~ 0.03

o 25

0.02

o o

0.01

~

CI')

2

3

4

5

6

7

NORMAUTY OF NaOH

Figure 1 Influence of alkalinity on the fonnation of monosubstituted diethylaminoethyl (DEAE) derivatives from the reaction of methyl B-o-glucopyranoside with N,Ndiethylaziridinium chloride. (From Ref. 80.)

not quantitative. In alkaline solutions, oxygen [86-90} and anthraquinone (AQ) [91-93} degrade reducing endgroups to a mixture of aldonic acid endgroups including gluconic, mannonic, arabinonic, and erythronic acids.

2.

Accessibility

Gentile et al. [94} studied a fibrous hydrocellulose and an amorphous sample prepared by regenerating a cellulose solution in DMSO-parafonnaldehyde (PF) solvent [69}. The accessible reducing endgroups were detennined by reduction with tritiated sodium borohydride under normal conditions. A total reducing endgroup was estimated similarly after regenerating the cellulose from a DMSO-PF solution. They observed that approximately 12% of the reducing endgroups in fibrous cellulose were inaccessible whereas the amorphous sample was totally accessible. Interestingly, a large difference in hydroxyl accessibility between these two samples was indicated by the deuteration method (51 % vs. 99%) [95). Thus the concentration of this functional group is probably significantly higher in the amorphous component than in the crystallites.

42

3.

Lsi

Endwise Degradation (Peeling)

Reducing endgroups playa significant role in the chemical process of cellulose conducted in alkaline media by being able to undergo the so-called peeling or endwise depolymeriztion reaction. Mechanism. The peeling reaction as indicated in Fig. 2 is initiated by the enolization of reducing endgroups to fonn several enediol intennediates [81,95]. The intennediate (3) undergoes the ~-elimination process resulting in detachment of endgroups from the cellulose chain (A). The peeled end unit may proceed by either benzylic acid rearrangement to yield isosaccharinic acid (5) or ring cleavage leading to the formation of mainly lactic acid (11)

0~OH

H

RO

0-

OH

(1)

H HJ:~H (5:==::-

RO~ OH

H~~~H,OH

RO~-O·

(2)

(3) B

1_ f~OH

lc 0H OH

fr

CHO

J;Z2

H

RO

-

O.

(6)

I

_A....o.-_ _

RO

OH

OH (8)

H OH

kj

(10)

J

1

(9)

(11)

C~H

RO

OH (7)

Figure 2

The peeling process of cellulose.

Reactivity and Accessibility

43

[34,81]. The average acidic products produced per glucose unit degraded is fairly constant, being approximately 1.5 mol regardless of the reaction temperature or alkalinity [34,96]. The remaining cellulose chain containing a new reducing endgroup can repeat the same process until a stable endgroup is formed [97]. Major types were metasacchaerinic (7) (71 %) and C-2 methylglyceric (9) (23%), which are formed from l3-elimination of the C3 OH (C) and from ring cleavage of the C4-C5 linkage (B), respectively. Accessibility. Many reports [98-103] indicated that when hydrocellulose was treated with an alkali the stable residues still contained noticeable amounts of reducing endgroups [94,101-103]. This phenomenon was ascribed to a physical stopping process [100] when a degrading end reached a crystalline region inaccessible to the alkali. The number of peeled-off glucose units for each reducing endgroup was nearly independent of reaction temperature or alkalinity below the mercerizing strength «2 M NaOH) [100,104-106]. Average values of 68 and 40 were observed for native and mercerized cellulose, respectively. As Lai and Sarkanen suggested [106], the observed constancy of degradable chain lengths must be attributed to the submicroscopic structure of microfibrils or more specifically to the average length of accessible segments of the cellulose molecules. A lower degradable chain length for mercerized cellulose may be ascribed to a shorter accessible segment. It appears that the fringe area of ordered regions of the cellulose II type is inaccessible to peeling reaction but conversion to stable acidic endgroups may occur, whereas in corresponding areas of the native cellulose both reactions are impeded. Gentile et al. [94] showed that both alkaline peeling and chemical stopping occurred more rapidly in an amorphous hydrocellulose than in the disordered regions of a fibrous sample. Chemical Factors. The peeling reaction of cellulose, as summarized by Lai [27], is significantly influenced by the chemical environment, notably the nature and concentration of the alkali. Alkali concentration affects both physical accessibility of the substrates and relative rates of the peeling and chemical stopping reactions. Lai and Ontto [108] observed that the extent of peeling of hydrocellulose at 120°C increased with alkali concentration up to approximately 6 M and decreased sharply thereafter (Fig. 3). The enhanced degradation initially was ascribed to increased accessibility of the cellulose while reduced degradation at the high-alkalinity region was caused by an increased formation of stable acidic endgroups.

La;

44

0.5..,......----------------,

"3 '"0

0.4

th

0 ~

0.3

0

:; ~

u

U

e0

0.2

c: 0 .::: 0

f

u.. 0.1

0-+-----r----~---r_--__1

o

5

10

15

20

[OH-l, mole/L

Figure 3 Effect of alkali concentration on the extent of hydrocellulosc degradation at 120°C for I h. (From Ref. 108.)

c.

Reactivity of Glycosidic Linkages

Figure 4 illustrates one of the generally accepted mechanisms for the cleavage of glycosidic linkages catalyzed by acid [109-112 J or alkali [109,112-117]. Both reactions display common features. An initial rapid equilibrium-controlled process involves acidic protonation of the glycosidic oxygen or alkali ionization of the C2 OH. In the slow rate-determining step, the conjugate acid (13) decomposes in an unimolecular heterolysis to form a carbonium ion intermediate (15), whereas the C2 hydroxyl anion (17) undergoes an intramolecular displacement process to yield a I ,2-anhydride intermediate (18). Both acidic and alkaline hydrolysis reactions are significantly influenced by the type of glycosides as illustrated in Table 2 for a variety of methyl pyranosides pertinent to wood polysaccharides [117-120].

1.

Acidic Degradation

Overall Process. The heterogeneous degradation of cellulose is characterized by two distinct phases [121-1261 and may be monitored either by weight loss equivalent to the formation of soluble materials (Fig. 5) [1261 or by DP

Reactivity and Accessibility

]i)oRI H

45

J)H RJ) - RHPRI -o

ka

+ H+

I

ka -OR1

OH

OH (12)

OH

(13)

(14)

I

15-

Degradation products •

H

OH

R

(15) OH

£;ORI H

k

+ -OH ~

R

£;ORI H

R

t

~

ka -OR 1

R

0

OH (16)

(17)

0

(18)

Figure 4 Generally accepted mechanisms of acidic and alkaline hydrolysis of glycosidic linkages.

Table 2

Relative Hydrolysis Rate of Methyl Pyranosides in 0.5 M HCl at 75°C [118] and in 2.5 M NaOH at 170°C [119) Relative rate Acid

Methyl pyranosides of: D-Glucose D-Mannosc o-Galactosc o-Xylose L-Arabinose L-Rhamnose o-Glucuronic acid b

ex Anomer

I 2.4 5.2

Alkali ~

Anomer

1

2.8

1.1

9.2

I

5.7

1.2

5.8

9.1

9.0 19.0 0.62

rrhc relative rate of methyl L-arabinofuranosidc is 32. bFrom Ref. 120.

Anomer

1.9

4.5

0.47

~

5.7

13.1

8.3

ex Anomer

lOa

280

2.5

1

Lai

46 30.....-------------------,

25

20

15

10

5

0fl----r------r---~---r__-----1

o

10

20

30

40

50

TIME,H

Figure 5

Rate of weight loss in acidic degradation of a-cellulose in 2 M and 4 M (From Ref. 126.)

Her at 9Q c. 0

changes in the cellulose residue (Fig. 6) [122]. The initial phase associated with a small dissolution of the material (7%) and a sharp DP reduction has been attributed to hydrolysis in the easily accessible region. This concept was often used to indicate the percent amorphous component of cellulose (Table 1). The results, however, varied significantly with hydrolytic conditions, e. g. , acid concentrations and temperature. The slow degradation stage was characterized by relatively few DP changes, especially after reaching the so-called leveling-off DP (LODP)

Reactivity and Accessibility

47

2500,---------------...,

2000

~

1500 > ~

o

1000 ~

500

ilu-----__

--n

O-i----...,.----r--.---__ .-----l o

100

200

300

400

Reaction Time, H Figure 6 Rate of DP reduction in acidic degradation of cotton cellulose in I M Hel at 50°C. (From Ref. 122.)

[ 123-125]. Both the initial and slow degradation processes are in confonnity with a pseudo-first-order kinetics. The hydrolysis rate of amorphous components was generally one to two orders of magnitude higher than that of crystallites [123,126].

Influence ofPhysical Structure.

The hydrolytic behavior of cellulose is much influenced by its physical structure and lateral order [121-132]. Wood cellulose was hydrolyzed twice as fast as cotton [125]. Hydrolysis rate was significantly increased by physical or chemical pretreatment, with the effect depending on the source of cellulose. Hill and coworkers [127,128] reported that mercerization increased the hydrolysis rate of cotton (by 40%) and of ramie (7%), whereas the opposite effect was observed for linen and a-cellulose samples showing an approximately 30% reduction. Based on kinetic analysis, they concluded that the end-attach model proposed by Sharples [121] can only be applied to the cellulose II structure and not to the cellulose I crystallite. Thus, the confonnation of cellulose is also a significant factor affecting its reactivity and possibly the hydrolytic mechanism as well.

Reaction Conditions.

The hydrolysis rate of cellulose generally increases with increasing acid concentration (Fig. 5). The conversion of cellulose in

La;

48

dilute acid at elevated temperatures results in glucose yields not exceeding

60% [133]. A complete cellulose hydrolysis for quantitative analysis requires an initial treatment with strong acid (72% H 2S0 4 ) to effect dissolution followed by boiling in dilute acid (3-4%) [134]. When using concentrated acids, e.g., 51% H 2S04 [135], 12% HCI [136,1371. or 70% H 3P04 [138], the hydrolysis process approaches a homogeneous reaction. Also, the addition of ethanol, propanol, or methyl ethyl ketone accelerates the degradation whereas DMSO has a negative effect [131]. The nature of solvent effects has been explained in terms of affecting the hydronium ion reactivity (1391 or the relaxation of structural stress in the sample [131]. The nonswelling media may help to preserve the structural stress of cellulose and thus enhances the hydrolytic degradation. In addition, cellulose can be effectively depolymerized in ethylene glycol at high temperatures (200-240°C) and the depolymerized residues have been shown to have a high accessibility toward cellulose hydrolysis [140].

2.

Alkaline Degradation

Overall Process. Figure 7 illustrates the alkaline degradation of cellulose at elevated temperatures [102,106]. The existing reducing endgroups will

G -G -G -G -G -G -G -G -G -G,

!

k, (Rapid)

G -G -G -G -G -G-G -G -G -Gs

!

G -G -G -

G -G,

+

k. (Slow)

! +

G -G -G -G -Gs

k2 (Rapid)

-G -G -Gs

Figure 7 106.)

+

Alkaline degradation of cellulose at elevated temperature. (From Ref.

Reactivity and Accessibility

49

rapidly initiate endwise degradation (k l ). The alkaline hydrolysis of glycosidic linkages (kh) generates new reducing endgroups which give rise to a similar peeling process (k 2 ). Both the peeling and glycosidic cleavage reactions of cellulose, like homogeneous reactions of simple glycosides [115] and amylose [96], have been shown to confonn with pseudo-first-order kinetics [100,104-107]. According to Lai and Sarkanen's estimation [106], the peeling process is relatively rapid in 150-190°C range and roughly by a factor of 107 times faster than the alkaline hydrolysis reaction.

Physical Structure. The hydrolysis rate of mercerized cellulose was approximately 70% higher than that of native cellulose [106]. This suggests that the number of accessible glycosidic linkages in mercerized cellulose is higher than in native cellulose by a factor of 1.7. Gentile et al. {94] detected alkaline hydrolysis reaction for an amorphous hydrocellulose under relatively mild conditions (I M NaOH, 60-80°C). They suggested that the discorded regions associated with the cellulose I and II polymorphs had different degrees of structural order and reactivity. Molecular confonnation, along with molecular mobility and accessibility, appears to influence the alkaline susceptibility of glycosidic linkages.

D.

Reactivity of Hydroxyl Groups

Among the interunits of cellulose the 2-0H group is generally accepted as being the most acidic one 180,84,85,141-143] and this has been attributed to the activating effect of the anomeric center. The acidity of hydroxyl groups was also influenced by other factors including the hydrogen bonding system. Hearne et al. [144] observed that methyl ~-D-ribopyranoside was more acidic than methyl ~-D-xylopyranoside and that they differ only at the C3 confonnation. The hydroxyl reactivities in a heterogeneous system are further affected by the accessibility factor. Inaccessibility may arise from either regions being inaccessible to a reagent or functional group being hydrogen-bonded to other units. Reported data on the reactivity of cellulose varied considerably with the type and the condition of modification. Often significant variation was observed for the same reaction by different authors.

1.

Esterification

Cellulose can be esterified by reacting with acids, acid chloride, anhydrides, or unsaturated agents [5,73] such as CS 2, phenyl isocyanate [128,145] and urea.

Model Glucosides. The esterification of simple glycoside in homogeneous system generally indicated [84,141] that the 6-OH group is more reactive

50

La;

than the secondary OH groups. The relative reactivity of the 2-0H and 3-0H groups depends appreciably on the reagent used [146] and on the nature of the glycosides [147-149]. Acetylation with acetic chloride favored reaction at the 2-0H site whereas it occurred preferentially at the 3-0H with acetic anhydride. The reaction of methyl a-D-glucopyranoside with I M equivalent of acetic anhydride in pyridine [147] indicated that the DS was essentially identical for the C2, C3, and C4 positions (0.2) and higher (0.4) for the C6. In benzolylation of a series of methyl a-D-glycopyranosides of glucose, mannose, and galactose [148], the 6-0H group was the most reactive and the 4-0H group the least reactive. Both the glucoside and galactoside showed a higher reactivity for the 2-0H group than the 3-0H group, whereas a reverse order was found for the mannoside indicating the equatorial hydroxyl group being preferentially benzolylated. In the acetylation of benzyl 4-0-methyl ~­ D-xylopyranoside [149], the ratio of the 2- and 3-acetates varied approximately from 2: I with acetic anhydride-sodium acetate to 1:3 with an acetic anhydride-perchloric acid system. Influence of Morphology. Although the morphology of cellulose is known to have a profound influence on the overall esterification process, the detailed mechanism involved is still not entirely clear [8]. A heterogeneous reaction is generally controlled by the diffusion of reagents into the fibers [81. The reaction was initially very rapid and proceeded at a decreasing rate as illustrated in Fig. 8 for formylation of a rayon and sulfite pulp [150]. Based on the assumption that the rapid phase was attributed to esterification in the readily accessible region, formylation has been used to estimate the content of the disordered region in the cellulose (Table I) [151]. Similarly, initial nitration of the cellulose occurred mainly on the fiber surface [152]. The primary wall was extensively nitrated and formed a fluffy coating which may restrict swelling of the cell wall. Under similar acetylation conditions [153], the acetyl content of cotton (4.3%) was more than twice that of ramie (2.1%) or linen (1.3%) cellulose. Mercerization [8,154, ISS] greatly enhances the reactivity (or accessibility) of cellulose if it is maintained in a never-dried state. Drying of mercerized cellulose considerably reduced it~ reactivity to even less than half that of the unmercerized samples [8]. However, its impact, can be alleviated by dehydmtion through a solvent exchange process. The reactivity of mercerized samples expressed by acetyl content increased from 1.7% to 17.7% if waterwashed sample was solvent-exchanged with pyridine and to 29% if washing directly with absolute ethanol and a subsequent pyridine exchange.

Reactivity and Accessibility

51

25-r-----------------..., 20 -0

'0

<0

'§ &

15

"8

.5

.D

e

8

10

5

On-----r-----...-----T----~ 15 5 10 20

o

Time,H

Figure 8

Rate of fonnylation of sulfite pulp and textile rayon at 55°C. (From Ref.

150.)

The influence of drying on reactivity or homification was significantly affected by the NaOH concentration of alkali treatments [14]. As indicated in Fig. 9, the reduction in acetylation became noticeable at approximately 7% NaOH and reached a maximum level at about 12% NaOH for a ramie and kraft pulp, or at 14% for a cotton linter sample. Thus, homification was more pronounced in the cellulose II than in the cellulose I lattice. The reactivity of dry-mercerized samples, however, can be largely restored by a wetting process prior to acetylation or nitration. This pretreatment did not affect the apparent crystallinity and probably resulted in a loosening of the hydrogen bonding networks in the amorphous region. In this connection, presoaking cellulose in water followed by solvent exchange to pyridine was found to improve the carbanilation process [74]. Hydroxyl Reactivity. The reactivities of hydroxyl groups, measured on partially esterified samples, generally reflected the reaction of glucose residues

La;

52

70 60 ~ :I:

g

50

$

(b)

(a)

10

(e)

0+-----......-----,......-----,.----.....,

o

5

10

15

20

Cone of NaOH Soln Used for Mercerization (%)

Figure 9 Influence of hydroxide concentration in alkaline treatment on the acetylation of mercerized linter (a), pulp (b), and ramie (c). (From Ref. 14.)

located either in the amorphous region or on the surface of crystallites. Table 3 summarizes the reported relative reactivities of cellulose hydroxy Is in response to tosylation, formylation, nitration, acetylation, and xanthation. Although the primary hydroxyl group was generally shown to be most reactive, its reactivity relative to 2-0H and 3-0H varied considerably among reported data. Several trends are indicated. Nuclear magnetic resonance (NMR) analysis [160] generally showed smaller variations among different hydroxyl groups as compared to chemical analysis. For example, in heterogeneous tosylation of a regenerated sample [ 158], the relative rates between the 6-0H and the 2-0H plus 3-0H was about 21 by chemical analysis, whereas 13C-NMR analysis indicated that reactivity of the 6-0H was only slightly higher than that of the 2-0H (1.6 vs. 1.1). However, it is difficult to reconcile these huge variations. The relative reactivity of hydroxyl groups depends considerably on reaction

Table 3

Relative Reactivities of Hydroxyl Groups in Tosylation (TS), formylation (FO), Nitration (NT), Acetylation (AC), and Xanthation (XA) of Cellulose Relative reactivity Reaction/substrate

DS

C2

C2&C3

C3

~ Q) (')

:::!:

C6

s.

-<

Ref.

Q) ~

TS:

Q.

Ethyl. cell. Cell aceta. Regen. cell.

-0.23 1.18

33 19.6 .-

-

1

214 213 21 10 5.8 1.8

-

I

1.7

Takahashi (I60J

1.8 1.2

-

1.06

-

1 1 1

5.8 1.4 1.12

Wu Clark Clark

(161) [162J (162)

Maim Maim Miyamoto Miyamoto Miyamoto

(166) [166J (165) (165) [165)

Adamek Adamek Doane Doane Takahashi

[168J (168) [169J 1169) 1160J

Regen. cell. Regen. cell.

-

-

1.4

1.1

Regen. cell.

2.4

1.3

1.9-2.3 2.37 2.83

1 1

.

1 1 1

-.

Mahoney Gardner Honeyman Honeyman Heuser Takahashi

1156) 1157J [158J (158) [159J [160)

:b (') (') (b CI) CI)

0:

s:

-<

FO: NI: Cellulose Cotton AC: Cell. Cell. Cell.

-

1 1

-

0.87 1.21 1.87

2.5 1.7 1.4

-

1.1 0.6 0.12 0.33 0.4

-

62 42

44

-

-

--

-

10

-

1 1 I

2.5 5.2 3.4 1.9

XA: Starch Starch Cell

4.5

-

0 0 0 I 0.35

38 58 56 11.2 -

~

54

Lai

conditions and the degree of substitution [162]. Patterson et al. [163] reported that initial nitration occurred mainly at the 6-0H group of molecules in the amorphous region. Nitration of the 2-0H and 3-0H groups began later at about 0.28 and 1.83 OS, respectively. Maim et al. [166] reported the relative reactivity between the primary and secondary hydroxys. The acetylation of a hydrolyzed cellulose acetate sample under uncatalyzed conditions was in the 10-16 range conducted at room temperature and was lower (4-7) at 1()()OC. An even lower ratio of 2.5 was obtained using catalysts, e.g., sulfuric acid or pyridine. Acetylation in a benzene suspension as compared to a pyridine solution was more selective for the 6-0H group. Miyamoto et al. [164,165] conducted homogeneous acetylation of cellulose in a 10% LiCI-dimethylacetamide solvent. The 6-0H group was about twice as reactive as the 2-0H group for OS up to 1.2, and the factor was reduced to 1.4 for higher OS samples. One of the major factors contributing to reported variations in xanthation is probably the stability of xanthate groups [170-172]. The primary xanthate at C6 was generally shown to be more stable than those at C2 and C3 positions and the latter groups could be hydrolyzed 15-20 times faster under certain conditions. Thus the extent of hydrolysis and the redistribution of xanthate groups varied with the duration of xanthate solution (viscose) in storage (ripening).

Influence of Solvents. Esterification is significantly influenced by the nature of the reaction medium depending on whether it is a swelling agent, or a solvent for cellulose or esterified products. The function of solvents [173] is often involved in the formation of the cellulose complex or chemical derivatives, which could affect the reactivity of specific hydroxyl groups. Miyamoto et al. [1651 observed a more uniform acetylation among different hydroxyl groups in LiCI dimethylacetamide (OMAC) as compared to heterogeneous reactions (Table 3). Cellulose dissolved in OMSO-PF is known to form methylol derivatives, especially for the 6-0H group. Acetylation of cellulose in this system [174-176] was shown to occur preferentially at the methylol hydroxyl group generated in situ. Philipp et al. [177,1781 studied the esterification of cellulose dissolved in N 20 4-OMF solution, which is known to form nitrite derivatives preferentially at the C6 position. Sulfation was shown to occur mainly at the 6-0H group presumably as a result of a transesterification reaction. Such a process, however, did not occur in phosphorylation, which appeared to take place directly on the 2-0H and 3-0H groups.

Reactivity and Accessibility

2.

55

Etherification

Etherification, in general, is obtained by alkali-catalyzed substitution or addition reactions using three types of reagents. The first type includes a variety of alkyl or aryl halides or sulfates to yield alkyl ethers. The second type is alkene oxide to yield hydroxyalkyl ethers with the new hydroxyl group generated also being active toward further etherification. The third type is unsaturated agents including diazomethane, acrylonitrile, and acrylamide leading to the fonnation of methylcyanoethyl and carbamoethyl derivatives, respectively. The etherification of cellulose is also considerably influenced by its morphologic structure and reaction environments.

Model Glucosides.

Reported data on simple glycosides [84,179] indicate that the etherification with bulky groups, e.g., trityl chloride, occurs preferentially at the 6-0H group. The relative reactivity between the primary and secondary hydroxyl groups for most other reactions, however, cannot be simply generalized as indicated in Table 4 for methylation or reaction with N,N-diethylaziridinum chloride (DAC). The latter chloride, generated in situ from a solution of 2-chloroethyldiethylamine, reacts with hydroxyl groups to fonn diethylaminoethyl (DEAE) derivatives [Eq. (I)].

H

RO

~

1:) 0

H~~.~H2.N-(C2H')2 +...I CH2

+ (C2 HS)2 1, , \ I

CI

c~

OH

-

---

RO~

(I)

OH

(C2HS)2NCH2CH~1

Several trends are apparent. Among the three hydroxyl groups pertinent to cellulose and glucomannan, the 3-0H is certainly the least reactive. The relative reactivity of the 2-0H and 6-0H groups seems to be largely determined by the alkalinity used. Reactions conducted in dilute alkali generally resulted in a higher reactivity for the 2-0H while a high-alkali reaction gave an apparently higher reactivity for the 6-0H group. For example, Lenz [142] observed in the methylation of monosodium salt of methyl a:-o-glucopyranoside with methyl iodide that the 2-0H group was twice as reactive as the 6-0H. Rowland et al. [75) observed only a 30% higher reactivity for the 2-0H group on partial reaction of the same compound with DAC conducted in 6 M NaOH solution. On the other hand, Norman [181] observed reactivity of the 6-0H group to be approximately

~

Table 4 Relative Reactivities of Hydroxyl Groups in Methyl a- and J3-D-Glucopyranosides (MG), Methyl-a-DMannopyranoside (MM), and Benzyl-4-0-Methyl J3-o-Xylopyranoside (BX) Toward Methylation or Reaction with N.N-diethylaziridinium Chloride (DAC) Reactive reactivity Glycoside MGI-a

MG-J3

MM-a

BX-J3

Reagent CH 31 DAC (CH 3hS04 CI1 31h (CH 3hS04 (CH 3hS04 (CH 3hS04 DAC DAC DAC DAC DAC CH 31b CH 3lc (CH 3hS04 (C11 3 hS04

NaOH

6M 19%

19% 19% 19% 0.5M 2M 6M 6M BAH d

19%

"May include some 4-0 isomer. bplus Ag}O. 'The glucoside was a 4-0 benzyl derivative. dBenzyltrimethylammonium hydroxide (40%). (Plus Nail and dimethylsulfoxide.

DS 1.2 0.34 0.1-0.2

0.1-0.2

0.9 c 0.17 0.13 0.10 0.4 0.2

0.4-1

C2

C3

5 2.2 7.8 2.2 7.3 4 2 1.3 1.4 1.3 1.5 1.6 1.1 1.6 1.3 2.3

1a 1 I I I I 1 1 I I I I I I I I

C4

C6

Ref.

-

2.5 1.7 13 1.6 12 4 3 0.9 1.5 2.2 2.1 1.4 0.8 1.0 1.6

[142] [75] (181] [184] (181] [180] 1180] 175] 175] (75] [75] (75] [182] 1182] [182] [183]

4.3 0.7 3.1 0.5 -

-

0.8 1.0 0.8 ....

-

~

Reactivity and Accessibility

57

70% higher than that of the 2-0H group with dimethyl sulfate in 19% NaOH. Accessibility. The extent of etherification is often detennined by the accessibility of a substrate under a given environment. As Segal discussed [8], the extent of methylation with ethereal diazomethane (indicated by methoxyl contents) increased in the order hydrocellulose (5.9%) < cotton (7%) < mercerized cotton (14.9%) < cuprammonium rayon (18.6%) < ball mill-ground cotton (20.6%). Crystallinity of the cellulose was considered to be a significant factor. In a series of successive methylations of cotton cellulose with dimethyl sulfate-DMSO [185] the process displayed two distinct stages (Fig. 10). The rapid methylation corresponded to a DS of 0.7, which is equivalent to approximately 25% substitution of the hydroxyl groups in cellulose. These 1.4,..--------------------,

1.2

CI)

0.8

Cl

~ 0.6 o

i

~ 0.4

0.2

10

20

30

Number of Methylation Treatment Figure 10 Influence of repeated methylation of cotton (presoaked in 2 M NaOH) with dimethyl sulfate in DMSO on the degree of substitution. (From Ref. 185.)

58

Lai

methyl groups, however, were found being associated with 44% of the glucosyl units. The extent of this rapid methylation was lower with diazomethane [186] and was enhanced by alkali swelling. Hydroxyl Reactivity. Tables 5 and 6 summarize the relative reactivities of hydroxyl groups observed in partial etherification of cellulose. Besides a distinctly high selectivity of the C6 OH group in tritylation, the relative reactivity of the 2-0H and 6-0H groups was considerably affected by the alkylation conditions, such as fiber swelling and the nature of etherifying conditions. In successive methylation with dimethyl sulfate [7,185], the 2-0H group as compared to the 6-0H was much more reactive initially and became quite comparable when DS exceeded the 0.7 level. Interestingly, the 6-0H group was shown to be slightly more reactive than the 2-0H group, when the methylation was either with diazomethane [7, 185,189] or with dimethyl sulfate in an S02-diethylamine (DEA)-DMSO solution [79]. Also, the 3-0H group was most reactive when using a methyl iodide in an S02-DEA-DMSO solution [79]. The discrepancies among reported data, besides possibly being caused by different analytic techniques employed, may be partly attributed to variations in the alkali concentration used as shown in Fig. I. Rammas and Samuelson [197] also demonstrated that the reactivity of the 2-0H and 6-0H with ethylene oxide was quite comparable in dilute alkali and that the C6 hydroxyethylation was preferentially promoted by an increase in the alkali concentration. In etherification of cotton cellulose with sodium 2-aminoethyl sulfate [192], sodium allyl sulfate [193], or acrylamide [194], the 6-0H group was generally found to be more reactive than the 2-0H group. The carboxymethylation of cellulose with sodium chloroacetate in an aqueous system generally showed that the 2-0H group was more reactive than the 6-0H group, whereas the 3-0H was the least reactive. A low water content medium resulted in a relatively more unifonn reactivity than a high water content reaction [202]. Similarly, carboxymethylation in a nonaqueous system (S02-DEA-DMSO) [79] resulted in a more unifonn reaction and a higher reactivity of the 6-0H group as compared to an aqueous reaction. Interestingly, carboxymethylation with sodium iodoacetate substantially enhanced the reactivity of the 3-0H, which was found to be most reactive under this condition. Rowland et al. [10,208-210] conducted extensive studies on the reaction of cellulose with 2-DAC [Eq. (I)] as summarized in Table 7. The data were consistent in showing the 2-0H as being the most reactive whereas the

Table 5 Relative Reactivities of Hydroxyl Groups in Tritylation, Alkylations, 2-Aminocthylation, Allylation, and Carbamoethylation of Cellulose

~

Q) (')

=t

Relative reactivity Agent

Reaction Tritylation Reg. cell. Methylation Mercer. cotton Cotton Milled cotton Cotton

TrCl

Ethylation Mercer. cotton 2-Aminoethylation Allylation Carbamocthylation

DS

C2

C3

Pyridine

0.3 1.2

Ia Ia

-

0.8 0.6 0.2 0.2 0.6 1.6 0.1 0.4

5 3.5 1.2 9

C6

~

Ref.

Q)

::J Q. ~

58

to

Honeyman [158J Honeyman [158]

(') (')

a>

(I) (I)

5= CH 3CI (CH J hS04 CH 2N2 (CII J hS04 CH 3CI (CH J hS04 CH 31

Avicel

NaOH

s.

C 2HsCI C 2HsCI AES d ALS e ACA f

18.9% 18.9% -

2M

b c

16.4% -

24% 20.7% 25%

0.8

0.14 0.07

.-

1.8 1.1 0.8

I I I I I I I I

2 2 1.5 3 8 1.7 1.3 0.6

4.5 4.3 4.6 3.5 9

I I I I I

2 4.5 7.1 5 19

to

Croon [187J Croon [188] Croon [I89J IIaworth [185] Haworth [185 J Isogai [79J Isogai [79] Isogai [79J Croon Reuben Roberts Hoiness Tonzinski

s:

~

[190J [191] [192] [193] [194J

'Also included C3. t>conducted in SOrdiethylamine (DEA)-DMSO containing 4% NaOH of cellulose. 'As in b excepted using a 2% NaOH charge. d2-Aminocthyl sulfate. rAllyl sulfate. fAcrylamidc.

0, <0

Table 6 Relative Reactivities of Hydroxyl Groups in Hydroxyethylation (HE), Hydroxypropylation (HP), and Carboxymethylation (CM) of Cellulose

~

Relative reactivity Reaction

NaOH

DS (MS)

C2

C3

18.9%

0.6 (1.9-3.7) 0.56 0.23 (3.5-4.5) (0.8- 1.2) 1.1 (2.6) 2.5 (4.2)

3 3 1 4.7 3 3 6 1.6

I

2 2.5 4.6 2 2.1 2.5 1.0 0.7 2.8 2.1

C6

Ce

Ref.

HE Cotton Comrner. a Rayon Cotton Cell.

-

3% 18% 1M 1.2M -

-

HP CM Commer. Wood cell.

-

0.8-1.0

b

-

Commer.

c

0.5--1.8 0.6-2.2 1.4 1.2 0.6 0.7 0.7

Commer. Avicel Cell.

7%d 9%e

I

3 1 I 1 I 1 I I I I I I I

I I I

.-' "Commercial samples. b-cConductcd in reactions containing 7 and 14 mol water per mol cellulose. respectively. <Jconducted in S02 - DEA- DMSO solution and alkali charge was percent cellulose. (Conducted in SOJ-DEA-DMSO using sodium iodoacetate.

10 lO I X.5 I 3 II 1.6

2.5 1.8 3.6 1.5 1.6 1.7 1.6 0.5 2.5 1.5

20 10 1.5 12 2 2 35 2.7

-

-

Croon Wirick Rammas Rammas Glass Glass Reuben Lee

[195] [I96J [1971 [197] [198] [198] [199] [2001

Croon [201] Buyt. [202] Buyt. [202] Ho [203] Reuben [2041 Isogai [79] Isogai [791 Gronski [206] A.-Malik [207]

~

Reactivity and Accessibility

61

Table 7 Relative Distribution of 2-0iethylaminoethyl (OEAE) Groups from Treating Methyl ~-()-Glucopyranoside and Five Cellulose Samples with 2-0iethylaminoethyl Chloride in 0.5 M NaOH at 25°C for 45 min OEAE distribution Sample Methyl ~-D-glucoside Decrystallized cell. Mercerized cotton Hydrocellulose I Native cotton Hydrocellulose II

OS

C2

0.105

2.1 2.6 3.3 9.3 10.2 11.7

0.016 0.015 0.02

C3

C6

Ref.

0.6 1.2 1.4 2.4 4.0 4.2

[2081 [76] [78] [76] (75) [77]

Source: Ref. 209.

3-0H was the least reactive. The substitution patterns of decrystallized and mercerized cellulose were quite similar and distinctly different from that of hydrocellulose and native cellulose. Both the reactivity of 2-0H and 6-0H groups (relative to 3-0H) increased in the order of increasing crystallinity: model glucoside, decrystallized, mercerized, hydrocellulose I, native cellulose, and hydrocellulose II. These trends are certainly related to the variations of cellulose structures in hydrogen bonding systems.

Influence of Solvents. Etherification is also considerably affected by the solvent system used [211-2171. The preparation of cellulose trimethylsilyl ether derivative was considerably enhanced by conducting it in a DMSO-PF solution 12121. In a heterogeneous dihydroxypropylation of cellulose with glycidol, the DS cannot exceed the I. 5 level [2151. As noted earlier [791, the methylation and carboxymethylation of cellulose in S02-DEA-DMSO solution resulted in a more uniform reactivity among the three hydroxyl groups. The methylation or carboxymethylation of cellulose in DMSO-PF solution [215] resulted in preferential substitution at the 2-0H and 3-0H groups. This was attributed to the presence of methylol groups formed mainly at the C6 position during the cellulose dissolution process.

3.

Oxidation

The oxidation of cellulose, besides endgroups, may be discussed in three major types as illustrated in Fig. II. The first type (A) is a glycol cleavage oxidation of <x,~-diol units with periodate giving the dialdehyde derivative

Lai

62

A

J~\OA' CH

CH

8 8

))0A, ):;0A, (20)

B

H

_

H

A

(21) OH

A£lA'

OH

~ o

~2J

OA,

C

Further Degradation

H

OH

(23)

(19)

l

J;;oA'~ ~OAI H

(24) D

COi'i

(26)

t

AX/A, o

OH

(25) E

~

HOi'i

A

OH

+~

(27)

Figure 11

H

OH

OH

(28)

Major types of oxidation of cellulose.

(20)[218]. The second type is hydroxyl oxidation which can occur at the C6 (B), C2 (C), or C3 (D) position. The third type is a direct attack on the anomeric carbon (CI) resulting in glycosidic cleavage or oxidation of the Cl position (E). The tendency of these oxidations depends substantially on the nature of the oxidants [222-231] and the hydroxyl configuration [84,219-221].

Reactivity and Accessibility

63

Model Glycosides. The 6-0H group of methyl a-and ~-D-glucopyranosides was selectively oxidized by platinum black to the corresponding uronic acid derivatives being obtained with 87% and 68% yields, respectively [202]. A slightly lower preference was observed on oxidation with nitrogen dioxide which also gave the 2-,3-, and 4-keto derivatives [203]. Reactions with chromic acid [224] or Fention's reagent (Fe 2 + plus H 20 2) [2251 result in rather nonspecific oxidation. On the other hand, the initial oxidation of methyl ~-D-glucoside in oxygenalkali [226-229] or peroxide-alkali [227) solutions occurred mainly at the 2-0H or 3-0H group. The resulting ketoglucoside could be further oxidized to the 2,3-dicarbonyl intermediate (24) leading to the formation of ethyl 2-carboxY-~-D-pentofuranoside derivative (26). The reactivity of methyl glycosides increased in the order ~-xyloside (12%) < a-glucoside (18%) < ~-glucoside (20%) < a-mannoside (28%). Glycosides containing an axial hydroxyl group seem to promote the oxidative degradation process. Also, the oxidation of glycosides with aqueous bromine [230] was practically confined to the secondary hydroxyl groups. The reactivity may be related to the hydroxyl conformation as indicated in Table 8. Methyl a-D-glucopyranoside yielded mainly the 2- and 4-keto derivatives whereas oxidation of the corresponding ~-glucoside was less specific and resulted in additional formation of the 3-keto compound. The a-anomer of manno- and galactopyranoside, containing axial hydroxyl groups at the C 2 and C 4 position, respectively, gave mainly the corresponding 2-keto and 4-keto products.

Table 8 Formation of Mono-Keto Glycosides from Bromine Oxidation of Methyl Pyranosides at pH 7

Type Glucoside Mannoside Galactoside

Unreacted material

Yield of keto (%)

Methyl glycoside Anomer

C2

C3

C4

%

26 17

35 24

ex

17

~

II

9

29 24

6

ex ~ ex ~

44

10

31 19

43 34 36

Note: The reaction was conducted at 30°C until a complete consumption of the bromine (after 4-6 h), which was charged twice the amount of the glycoside. Source: Ref. 230.

La;

64

Cellulose Glycol cleavage. The initial pcriodate oxidation of cellulose, like other chemical reactions, was largely limited to the readily accessible component and has also been used to indicate the accessibility of cellulose substrates [151] (Table 1). Rowland and Cousins \232], based on the influence of periodate oxidation in the crystallinity of cotton, observed about 40% of the component being noncrystalline. Since the cis-diol unit is generally more reactive than the trans-diol, the cleavage of the man nose residues would proceed faster than that of the glucose or xylose residues. Secondary hydroxyls. Snyder et al. 1237] observed in the reaction of cotton with DMSO-acetic anhydride (AC 20) that 47-62% of the oxidation occurred at C6 producing aldehyde group, while the remaining 38-53% oxidation was at the C2 or C3 keto groups. However, oxidation of cellulose with DMSO-AC 20 in the DMSO-PF solvent system gave a selective oxidation at the 3-0H group [238]. This was suggested as being attributable to a reversible formation of hydroxy methyl and poly(oxymethylene)ol groups at the C 2 and C 6 positions. In the case of oxygen-alkali systems 1239,2401 the initial oxidation appears to occur preferentially at the 2-0H or 3-0H group. Recently, the oxygen-alkali- and cobalt-hydrogen peroxide-induced oxidation of cellulose was shown to be significantly influenced by its morphology 1241]. Amorphous cellulose was degraded more rapidly than kraft pulp or highly crystalline cellulose samples. Thus, accessibility plays a dominant role in the oxidative degration of cellulose.

III.

HEMICELLULOSES

Hemicelluloses contain, in addition to glucose, a variety of other sugar units [18, 19,242-2441. These nonglucose units, because of their different ring structures or hydroxyl configurations, often display distinctly different reactivities from the glucose residue. They are generally more reactive than cellulose and can be selectively removed from cellulosic substrates. Most studies on the reaction of wood hemicelluloses, except for structural analysis, were largely associated with the delignification or biomass component separation process. Relatively few hemicellulose derivatives have been reported.

A.

Accessibility

Although hemicelluloses occurring in plant tissues are likely to be amorphous [18, 19], they, like cellulose, are capable of forming strong hydrogen bonds

Reactivity and Accessibility

65

and have a tendcncy to crystallize after certain degradation resulting in removal of side chain units. As noted in the introduction section, Sepall and Mason 120] observed a relatively low accessibility of 52% for birch xylan toward tritium exchange reactions as compared to wood cellulose (58%) and amylose (99%). The presence of strong hydrogen bonding betwecn acetyl and the neighboring hydroxyl groups were thought be be a significant factor in reducing the accessibility of xylan [211. Xylan in solution has a strong tendency to deposit on the surface of cellulose fibers [245-247] as a rcsult of strong hydrogen bonding. In alkaline pulping [34], a significant portion of the residual xylan was very resistant to alkaline extraction or heterogeneous acid hydrolysis 1245, 246]. It was suspected that the adsorbed xylan, being low in uronic acid content, might have cocrystallized on the cellulose surface [248]. Marchessault and Liang [248] suggested that the 0-3H· . ·05' intramolecular hydrogen bond occurring in cellulose was probably also present in the xylan. The presence of galactose side chains in galactoglucomannans certainly would contribute to their water solubility and probably prevent them from aligning together to form strong hydrogen bonds. Partially degraded glucomannans containing few side chains are capable of crystallizing [249-251]. The true mannan is known to consist of two crystalline forms, I and II [252-254]. Both are stabilized by intra- and intermolecular hydrogen bonds. A similar 0-3H· . ·05' intramolecular hydrogen bond was observed in both mannan and cellulose.

B.

Chemical Degradation

Thc chemical degradation of amorphous hemicelluloses is similar to that of cellulose (Sect. II. C.), but it procceds much more rcadily and extensively because of their relatively high accessibilities.

1.

Acidic Process

Consistent with the behavior of simple glycosidcs (Table 2), the homogeneous hydrolysis rate of B-(l-4)-linked polysaccharides, as BeMiller summarized [255,256], increased in the order cellulose (I) < mannan (2-2.5) < xylan (60-80) < galactan (300). This further demonstrates the significant role of acccssibility in acidic degradation reactions. The prescnce of uronic acid groups significantly reduces the hydrolysis rate of glycosidic linkages (Tablc 2). Thus, high yields of aldobiuronic acid dimers were obtained on partial hydrolysis of xylans [18, 19, 242]. The hydrolysis rate of different hardwood xylans was closely related to their uronic acid content 1257]. A higher stability of softwood xylan as compared to

La;

66

hardwood xylan in sulfite pulping may be partly attributed to its higher uronic acid group content [18,19]. The acetyl groups in hardwood xylans have been found to exhibit a remarkable stability under the relatively drastic conditions of acid sulfite cooking [258]. In steam treatments of birch wood, the acetyl group was more stable than the 4-0-methyl glucuronosyl unit [259]. In softwood xylans, the arabinofuranoxyl linkage is very labile and can be selectively hydrolyzed under mildly acidic conditions (0.05 M H 2S0 4 at 97°C for 3 h) [260, 261]. Similarly, the a-D-(l-6)-galactosidic linkage present in galactoglucomannans is very labile under acidic conditions [262]. Its high reactivity, however, cannot be satisfactorily explained in terms of the behavior of simple glycosides (Table 2). Alkali-induced deacetylation of glucomannan increased its resistance toward acid hydrolysis as evidenced in acid sulfite cooking [263,264].

2.

Alkaline Process

Alkali-induced deacetylation and hydrolysis of the uronic acid group of xylans proceed readily under alkaline pulping conditions and contribute significantly to xylan redeposition onto the fibers. The galactose side chain in galactoglucomannans is fairly resistant to alkaline hydrolysis (Table 2). Wood xylans are much more stable than glucomannans under alkaline conditions [34]. The peeling of xylans is basically similar to that of cellulose (Fig. 2). Extended alkali treatment of a rye flour arabinoxylan at room temperature resulted in only a 29% reduction in molecular weight [265]. As anticipated, the polysaccharide, after reduction with sodium borohydride, was completely stable to alkali. The moderate stability of xylans in alkali can be partly attributed to their unique endgroup arrangements and 4-0-methylglucuronic acid groups. Johansson and Samuelson [266,2671 showed that the peeling of xylan molecules was retarded when reaching at either a galactouronic or a xylose unit being substituted at the C2 position. Softwood xylans also contain arabinose units substituted at the C3 position of xylose residues, which would induce the chemical stopping reaction. Glucomannans, in contrast to xylans, undergo extensive endwise depolymerization [268,269], partly because of the lack of substitutents at the C2 or C3 position. The extent of glucomannan degradation at 100°C [269] was higher for pine galactoglucomannans (57%) than for spruce glucomannans (47%).

Reactivity and Accessibility

c.

67

Reactivity of Hydroxyl Groups

Few derivatives of wood hemicellulloses have been subjected to detailed studies. Most of the modification reactions were related to structural studies of these polysaccarides [18,19].

1.

Esterification

Organic esters of xylans were generally prepared by first dispersing or dissolving them in anhydrous formam ide [271] followed by the addition of acylating agents. The acetate prepared from white birch xylans displayed molecular association in chloroform [270]. The nitrates of undegraded xylans were insoluble in common solvents and decomposed readily at room temperature [18]. Philipp et al. [178] studied the sulfation of xylans dissolved in N 20 4OMF solution and observed little difference between the 2-0H and 3-0H groups in reactivity.

2.

Etherification

Croon and Timell [272,273] reported the methylation of a 4-0-methyl glucuronoxylan with dimethyl sulfate, finding that the 2-0H group was more reactive than the 3-0H group. A series of carboxymethyl (CM) ethers with OS varying from 0.13 to 0.92 was prepared by Schmorak and Adams 1274] from beechwood xylan. No xylose residue carried more than one CM group. Sjostrom [275] observed in the carboxymethylation of hardwood pulps that xylan was carboxymethylated to a higher OS than cellulose. Also, the 2-0H group was much more reactive than the 3-0H group by a factor of 2.4 and 3.3 for the xylan and cellulose components, respectively. Manzi and Cerezo [276] concluded from the methylation of galactomannans in organic media that the extent of reaction was significantly influenced by the orderly structures of the polymer.

IV.

LIGNIN

Lignin usually plays a negative role in the chemical utilization of lignocellulosic materials and must be modified, partially degraded, or completely removed depending on the end uses of the final products. These lignin modification or delignification reactions constitute the commercial pulping and bleaching operations in producing paper-making fibers [34]. Although the approximate contents of major lignin linkages are now reasonably well understood [23,277], the chemical structure of lignin, unlike cellulose or hemicellulose, still cannot be precisely defined. Since carbon-carbon

La;

68

linkages are very resistant to chemical attack, the degradation or fragmentation of lignin is largely limited to cleavages of ether units at the ex and ~ positions. These hydrolytic reactions are often accompanied by condensations that would have a significant impact on reactivity of the resulting lignin. Thus, the nature of technical lignin varies considerably with the type of delignification process, pulping conditions, and raw materials.

A.

Accessibility

Lignin appears to be amorphous, occurring in plant tissues or in isolation forms, and like cellulose and hemicelluloses has a high tendency to form hydrogen bonds. Michell [2781 concluded from infrared analysis of MWL (milled wood lignin) samples and related lignin model compounds that all detectable hydroxyl groups in lignin were involved in hydrogen bond formation. Both the ex- and phenolic hydroxyl groups appeared to be preferentially involved in intramolecular hydrogen bonds. Similarly, crystal ~-0-4 lignin model dimers were shown to contain a variety of intra- and intermolecular hydrogen bonds [279]. The morphology or fine structures of lignin, unlike wood polysaccharides, still has not been subjected to extensive investigation [22-26,280].

1.

Sorption of Vapors and Alkali

Periodate lignin was shown to adsorb as much water as cellulose [281,282], even though lignin had a considerably lower hydroxyl content. Drying also had a pronounced influence on the extent of benzene sorption by periodate lignin [283]. A water-swollen lignin, when solvent-exchanged to benzene and then freeze-dried, adsorbed 50 times as much benzene as air-dried lignin from water. The reduced adsorption by drying from water, similar to homification in cellulose, is likely caused by the formation of strong hydrogen bonding networks. In this connection, colloidal lignin precipitated from an organic solvent with water was shown to have a high enzymic degradability [284]. Thus, the reactivity of lignin, like cellulose, is influenced by its physical state. The sorption of alkali metal ions by periodate lignin increased rapidly with increasing alkali concentration and leveled off to an approximate value of 1.5 meq g lignin at about 1% alkali 1285]. Interestingly, both alkali sorption and swelling of the lignin were found to decrease at alkali concentrations above the 9% level.

2.

Solubility

Schuerch [286J demonstrated that the ability of a solvent to dissolve isolated lignins was a function of both the cohesive energy density (CEO) and hydrogen

Reactivity and Accessibility

69

bonding capacity of a solvent, and is often expressed by the Hildebrand's solubility parameter O. The 0 is the square root of CEO. Good lignin solvents like dioxane, acetone, methyl cellosolve, pyrindine, and OMSO were shown to have 0 values between 10 and 11 (callml) 1/2. Lindberg [287] showed that the tendency of lignin to precipitate from dioxane solution with chlorobenzene increased considerably with decreases in the excess hydrogen bonding capacity of the molecule. Kudo and Kondo [288] concluded that major factors affecting the solubility of lignin include content of hydrophilic groups, molecular weight, and degree of condensation.

3.

Association

The association of lignin in solution [289-296) has often been postulated to explain the variation of molecular weight distribution observed in different solvent systems. The nature of this noncovalent interaction or association, however, is still far from clear. Benko 1289} reported a gradual decrease in molecular weight of a spruce kraft lignin solution with increasing pH from 7 to 11.5. Yaropolov and Tishehenko 12971 observed a higher association of alkali lignins when the pH was lowered. Yean and Goring [298] showed that a high molecular weight lignosulfonate gave constant molecular weight in different solvents ranging from 0.1 M aqueous sodium chloride to OMSO. Lindstrom [290] concluded that carboxylic acids playa significant role in the association of kraft lignin in aqueous solution, and they may engage in hydrogen bonding with ether oxygens.

B.

Reactivity of Ether Linkages

Thy hydrolyzable ether units in the lignin are ether linkages.

~-aryl,

a-aryl, and a-alkyl

1. Acidic Conditions Fundamental Aspects. Figure 12 illustrates a general scheme for acidic degradation of a lignin model trimer containing both a- and ~-ether units 132,277,2991. The reaction is initiated by protonation of the benzyl oxygen followed by a-ether elimination to give a benzylic carbonium ion intennediate (31), which may undergo three competing processes. Reaction A forms C 6C 3 enol ether (32), which undergoes ~-ether hydrolysis to yield ketol (33), and then Hibbert's ketones (34). Reaction B involves a carbon-carbon bond cleavage between the ~ and "y positions to give fonnaldehyde and C 6C 2 enol ether (35), which may be degraded slowly to yield homovanillin (36). Reaction C involves intermolecular condensations to give mainly the a-6 diphenylmethane unit (38) plus some a-5 condensed structure.

Lai

70

Hibberts ketones G,-CH-C-CH 3 H2V OH

9CHJ

Ry-~%

f=O

~

~-

I~ .&

OR

OH

(29)

11

t

H

H2V OH

HC- 1

h-

Gt: ~+

I

.&

8

&

9CHJ

Hy-~~

%

A-

-R,OH

-slow

~

OR

~

G 1-CH 2 -C-CH 3

(34)

~OH

9CHJ

1_~~

8 bH G,-CH-C-CH 3

(33)

a:R,=H b:R,=Aryl

II

OH 0 G, -CH-C-CH3

I

-OR'

+

I

H 2 COH

OR

9HJ

¢l~ OH

(38)

(30)

~J7CH2~

8

OH

I

OH

(39)

HCHO - - - - - - - - - - - - - - ,

H~-~%

oOR

0CH:3

Acidic degradation of

OR

0CH:3

(36)

(35)

Figure 12

~ +~

(X-

OH

(37)

and B-ethcr units.

Additionally, fonnaldehyde may condense with two aromatic units to fonn another diphenylmethane-type condensed structure (39). These condensation reactions have been well established in lignin model studies 132,300,301]. Table 9 shows the relative hydrolysis rate of lignin models reported by Johansson and Miksche [302]. Both a- and ~-aryl ether hydrolysis were enhanced by the presence of a phenolic hydroxyl groups. It is evident that a-aryl ether is much more reactive than the ~-ether by a factor of 25 and 65

Reactivity and Accessibility

71

Table 9 Relative Hydrolysis Rates of Lignin a- and HCI of Aqueous Dioxane at 50°C

~-Ether

Models in 0.2 M

Linkages Type

Structure (Fig. 12)

Relative rate

~-Aryl

ether: Nonphenolic Phenolic a-Aryl ether: Nonphenolic Phenolic a-Alkyl: Phenolic 8

(29a), R = CH 3 (29a), R= H

12

(29b), R = CH 3 (29b), R = H

65 305

I

0.8

IEstimated from the data in Ref. 303; G = guaiacyl. Source: Ref. 302.

for phenolic and etherified units, respectively. Leary and Sawtell [303] showed that a p-hydroxybenzyl aryl ether was about 400 times more reactive than a p-hydroxybenzyl alkyl ether. Thus, LCC of the a-ether type, if present in lignin, can only be hydrolyzed slowly [304]. a-Aryl ether. Meshgini and Sarkanen [305] observed that the acid hydrolysis of etherified benzyl aryl ether dimers was significantly affected by the nature of the solvent, the benzyl, and the a-ether group. Benzyl units (ring A) of the syringyl type, as compared to that of guaiacyl or p-methoxybenzyl moiety, reduced the hydrolysis rate. On the other hand, a syringyl moiety on the a-ether unit (R 1 group) had a positive effect. The overall a-aryl ether hydrolysis was shown to increase with increasing the solvent polarity. ~-Aryl ether. Solvents were shown to significantly affect the hydrolysis rate of ~-aryl ether dimers (29), being higher in aqueous dioxane than in water or aqueous ethanol media [306]. Both dioxane and ethanol favor the C 6C 2 enol ether (35) formation, especially at high temperatures [307]. Carbon-carbon linkages. Acidic cleavages of carbon-carbon linkages in lignin is mainly limited to the bond between the ~-and ')'-carbon atoms, as indicated in reaction B (Fig. 12) for a ~-0-4 dimer. Similar reactions also occur with the ~-l or ~-5 type units [299,308]. Under acidolysis conditions [308], the formaldehyde yield from lignin model dimers decreased in the order ~-l (15%) > ~-5 (9%) > ~-0-4 (3%) [309,310].

72

Lai

Condensation reactions. The nature of phenyl units and reaction conditions has been shown to influence lignin condensation reaction [311-313]. Syringyl nuclei condensed more readily than guaiacyl nuclei with vanillyl alcohol [311] . Yasuda et aI. (312] observed the formation of benzyl chloride on treatments of ~-0-4 dimers in hydrochloric acid; this would reduce condensation at the benzyl position. They also observed an intramolecular condensation product of a phenylcoumaran type (313]. This intramolecular condensation was shown to be dominant in an 85% formic acid solution [314,315] and was insignificant in 50% aqueous ethanol containing 0.2 M HCI [306].

Lignin.

On acidic treatments of lignin in aqueous media, lignin condensation reactions dominate and lead to the formation of acid-insoluble residues. This principle serves as the basis for quantitative determination of lignin content in plant materials (261. Lignin condensation reactions, however, can be minimized by using mild conditions, organic solvents, or nucleophiles. Aryl ether cleavages. Lai and Guo (316,317] determined the acid-catalyzed hydrolysis of aryl ether linkages in wood lignin. As indicated in Fig. 13, temperature had a significant influence on the aryl ether hydrolysis reaction. Noncyclic a-aryl ether units were selectively hydrolyzed at temperatures below 65°C and determined to be 4% and 6% of C 9 units for spruce and aspen lignin, respectively (317]. It is evident that aspen lignin contained a high proportion of ~-aryl ether units with hydrolysis rates substantially higher than those of spruce. Mild hydrolysis. Nimz (318,319] subjected wood to water percolation at 100°C for several weeks. Approximately 20% and 40% of the lignin from spruce and beech wood, respectively, became soluble. Sakakibara [3201 obtained similar results using a 50% aqueous dioxane at 180°C. These soluble products were assumed to come mainly from the cleavages of a-aryl ether units. Solvent systems. Under typical acidolysis conditions (with 0.2 M hydrogen chloride in a 9: I dioxane-water mixture at 100°C for 4 h) (277,299], lignin is depolymerized through a- and ~-aryl ether cleavages. The yield of monomeric and dimeric products was substantially higher from birch (30%) than from spruce (17%) lignin. Based on solvent-assisted delignification, several acid-catalyzed organosol v processes have been reported [321- 3241. The Alcell process, using aqueous ethanol under autocatalyzed conditions, has been in plot-scale operations (325].

Reactivity and Accessibility

73

IOO·C

E

100

~"'C

8.s U

~

C

.2'

~..J

is :r~

vO

'§

~ 80

r

45"C

l. o

2

4

6

8

10

12

Reaction nme. h

Aspen

Figure 13 Influence of temperature on phenolic hydroxyl group formation in treatments of Norway spruce and aspen wood lignin with 0.1 M Hel. (From Ref. 316.)

La;

74

2.

Alkaline Conditions

The alkaline cleavage of a- and ~-aryl ether linkages as well as condensation reactions have been extensively studied using lignin model compounds [22,24,27,32,277,326-343], but it is still not entirely clarified in the degradation of wood lignin. Fundamental. Figure 14 illustrates a general scheme for the alkaline degradation of hydrolyzable ether units. The reaction of phenolic units (40) is initiated by the phenoxide ion (41) to yield a quinonemethide intermediate (42) with elimination of the a-ether unit R I . This elimination reaction occurs quite readily for an a-aryl ether unit (in I M NaOH at 25°C) [326], whereas the a-ether linkage of a lignin-carbohydrate model was shown to be stable under the same conditions [304]. The quinonemethide intermediate (42) may participate in several reactions depending on the alkali environment. In kraft liquor it reacts readily with hydrosulfide ion and the resulting adduct (43) undergoes intramolecular displacement leading to the ~-ether cleavage (B). In a soda cook, the quinonemethide intermediate undergoes mainly cleavage of the ~-'Y linkage to form formaldehyde and C 6C 2 enol ether (46). Thus, soda cooking of phenolic ~-aryl ether units resulted in only limited ether cleavages (A). On the other hand, alkaline cleavage of the etherified ~-aryl ether unit (47) proceeds by an intramolecular displacement mechanism (D). Under kraft cooking conditions, the ~-aryl ether cleavage of phenolic type could be 12-50 times faster than that of the etherified type depending on the hydroxide and sulfide ion concentrations [331 J. Lignin condensation reactions include the formation of diphenylmethanes (51) and (52) and a-carbohydrate ether linkage (53) that may be derived from the quinonemethide (42) 132,277,336-340] or the epoxide (49) [342,343] intermediate. Coniferyl alcohol may be involved in the formation of ~­ 'Y-Iinked condensed unit (54) [338,339]. The overall degradation of ether units, as revealed from lignin model reactions, is profoundly influenced by both chemical structure and reaction conditions. Phenolic units. In soda liquor, although the phenolic ~-aryl ether of the guaiacyl type (40) gave mainly the C6C 2 enol ether (46) (70%), this enol ether formation was shown to be minor (5%) for the corresponding syringyl ~-ether dimers [334,335]. Besides sodium sulfide, the alkaline cleavage of phenolic ~-aryl ether units was enhanced by AQ 1344,345], reducing sugars [346,347], and sodium sulfite 1348,349].

Reactivity and Accessibility

75

G

HCHO

H~-'i2-D "

~ I~ A

CH:J

OH

(46)

I Condensation Reaction

~OH _

H~~,Ei? · ~CH:J OH

(51)

~OH 9CHJ

Hy-~

,(to-carb

+

~CH:J Q-i (54)

(53)

~OH

~OH

HC,

o

°

I ~

HC-O H

~-

~CH:J (48)

Figure 14

OCH:J (49)

1-O H

Q: I~ A

CH:J

OCH:J (50)

General scheme for the alkaline degradation of

(l-

and B-ether units.

Lai

76

Etherified units. Consistent with the generally accepted mechanism (0, Fig. 14), the erythro dimer of etherified ~-aryl ether was about four times more reactive than the threo isomer [350]. The ether cleavage, besides being enhanced by increasing alkalinity, was facilitated in the presence of monoethanolamine [351] or in a OMSO-potassium-tertiary butoxide solution [330J.

Lignin. The alkaline degradation of lignin was practically all related to the delignification of biomass 1352]. Gierer et al. [353] determined the alkaline cleavage of aryl ether linkages in MWL and its diazomethane-methylated samples. Soda cook was shown to release fewer phenolic hydroxyl groups than kraft cook. The significance of aryl ether cleavages in the alkaline delignification process is well established as reflected by a considerable increase of the phenolic hydroxyl group content in both dissolved lignin [354,355] and residual pulp lignin [354,3561. The nature of lignin condensation reactions, however, has yet to be clarified 13571. Kraft lignin, as compared to MWL, is characterized by a higher content of phenolic hydroxyl group, a-carbonyl, and stilbene structure, but a lower aliphatic hydroxyl group content.

c.

Reactivity of Hydroxyl Groups

Among the three major types of hydroxyl groups in lignin, the phenolic hydroxyl unit (being most acidic) plays a dominant role in alkali-catalyzed reactions (Fig. 14). The a-hydroxyl group, under acidic conditions, is readily transformed into active benzylic carbon cations that may undergo a variety of addition or transformation reactions (Fig. 12). The 'Y-carbinol group, when present in phenolic units, is often released as formaldehyde under either acidic (B, Fig. 12) or alkaline (C, Fig. 14) conditions.

1.

Etherification

Alkylation. Figure 15 illustrates that individual hydroxyl groups of a phenolic ~-O-4 dimer may be selectively methylated with proper agents. Methylation with diazomethane is largely limited to the phenolic hydroxyl group plus nonenolizable carbonyl units [332,358]. The benzylic hydroxyl group can be specifically methylated with methanolic hydrochloric acid 1359]. All the phenolic and aliphatic hydroxyl groups can be methylated with dimethyl sulfate in alkali [332]. Treatments of spruce MWL in methanol-dioxane solution containing ptoluenesulfonic acid (at 30°C) resulted in hydrolysis of the a-aryl ether units and methylation of the benzylic hydroxyl groups 1360,3611. A complete

Reactivity and Accessibility

77

H

~ H

~~

-OU

~I ~

H

H 3

(57)

(58)

Figure 15

Methylation of a lignin B-aryl ether dimcr.

methylation required 4-6 days, and resulted in the introduction of methoxyl (0.5) and phenolic hydroxyl (0.05) groups per C9 unit. Thus acid-catalyzed alkylation provides a means to estimate the content of both ex-aryl ether and benzylic hydroxyl groups.

Hydroxyalkylation. The reaction of lignin model compounds with alkene oxides in alkali generally showed that the phenolic hydroxyl groups were more reactive than aliphatic hydroxyl groups [362-364J. Guaiacol reacted about 35 times faster than glycerol with propylene oxide [362]. The reactivity of phenolic hydroxyl groups toward ethylene oxide [364] was considerably

La;

78

suppressed by the presence of an a-carbonyl group. The hydroxyl function generated from the reaction with alkene oxide may initiate homopolymerization [363J. The reaction of lignin with alkene oxide resulted in the formation of lignin polyol derivatives [363-368J. The hydroxylation of lignosulfonates with ethylene oxide in 5% NaOH occurred mainly at the phenolic hydroxyl groups [364J. WU and Glasser [3631 showed that the reaction of kraft lignin with propylene oxide in alkali displayed two distinct phases: an initial slow copolymerization followed by a rapid homopolymerization. Also the reaction conducted in a toluene suspension [363,365J was reported to facilitate homopolymerization, whereas an aqueous reaction was largely confined to the hydroxypropylation of phenolic hydroxyl groups.

Carboxymethylation. Few studies were reported on carboxymethylation of lignin preparations [370-3731. Lange and Schweers [371] observed in the reaction of kraft and ethanol organosolv lignin with bromoacetic acid that approximately 60-70% of the total hydroxyl groups were carboxymethylated. The bulk of carboxymethylation was probably involved in the phenolic hydroxyl groups.

Miscellaneous.

In the reaction of lignosulfonate model compounds with cyanuric chloride in alkali [374], the reactivity of phenolic hydroxyl groups was considerably higher than that of aliphatic hydroxyl groups. For example, guaiacol reacted readily with cyanuric acid at pH 10.5 whereas n-propanol was totally unreactive. Condensation of lignosulfonate with cyanuric chloride resulted in a significant resin formation, which was facilitated by increasing the reagent charge or the consistency of the reaction media.

2.

Esterification

M[msson [3751 showed that deacetylation of aromatic units in pyrrolidine (aminolysis) proceeded much faster than that of aliphatic groups. This principle served as the basis of an aminolysis method for the determination of phenolic hydroxyl group content in lignin. Thus, the acetylation of phenolic hydroxyl groups is expected to proceed faster than that of aliphatic hydroxyl groups. Similarly, the phenolic hydroxyl groups were generally shown to be more reactive than aliphatic hydroxyl groups toward acid chloride [376J. The reaction of lignosulfonate with terephthaloyl chloride [376J gave a poor resin formation. Thus, the crosslinking reaction may have involved mainly an intramolecular condensation process. On the other hand, the benzylic hydroxyl groups were shown to be more

Reactivity and Accessibility

79

reactive than the phenolic hydroxyl groups toward diisocyanate [377-379]. The reaction appeared to be affected by both electronic and steric factors. The reaction of technical lignin with diisocyanate agents has been emphasized in the preparation of polyurethane derivatives [379-384]. The suitability for making polyurethane network films increased in the order of lignin preparations: acid hydrolysis < MWL < kraft < organosolv < steam explosion lignin. Solubility of the lignin was suggested as being a key parameter affecting its reactivity. The uniformity of lignin reactions with diisocynates [380-382] could be improved by blocking phenolic hydroxyl groups of the lignin with alkene oxide such as propylene oxide. Also, the perfonnance of lignin-polyurethane products may be improved by the addition of polyethylene glycols during the copolymerization process [381,383].

3.

Oxidation

The phenolic hydroxyl and etherified benzyl alcohol group can be selectively oxidized by periodate [385] and 2,3-dichloro-5,6-dicyano-1 A-benzoquinone (DDQ) [386], respectively. The periodate oxidation of guaiacyl and syringyl nuclei yields O-quinones plus methanol and has been used to estimate the phenolic hydroxyl group content of lignin. DDQ oxidation has been applied to estimate the content of lignin-carbohydrate linkages [28].

4.

Condensation

Lignin can be modified by condensation with phenol, formaldehyde, amino acids [373], or diazonium salts [387,388].

Phenolation. Acid-catalyzed phenolation of lignin occurs mainly at the ex position [391]. The reactivity of the lignin is considerably influenced by intennolecular condensations encountered during its preparation and was higher for steam explosion lignin [3921 than for kraft lignin [389,390]. Acid lignin prepared by hydrochloric acid was substantially more reactive than sulfuric acid lignin [393,394]. Formaldehyde Addition Acidic condition. Acidic hydroxymethylation occurs mainly at the C2 or C6 position of aromatic nuclei which may be phenolic or etherified units. It is used in the preparation of wood adhesives. The influence of lignin structure on reactivity and perfonnance of the resulting resin product is still not entirely understood, as discussed by Nimz [3951. Van der Klashorst [396-398] reported the acidic condensation of alkali lignin and lignin model compounds with fonnaldehyde. Initial hydroxymethylation was faster with syringyl units

La;

80

than with guaiacyl units and was promoted by the presence of phenolic hydroxyl groups. The subsequent crosslinking reaction was facilitated by an increase in acid concentration and reaction temperature. Alkaline condition. Alkali-induced hydroxymethylation of lignin [399] occurs mainly at the C5 position of uncondensed phenolic units via Lederer-Manasse reaction (52 in Fig. 14) and also at the ~ position of a-carbonyl groups via Tollens reaction. Thus, the hydroxymethylation of lignin [399-403] depends considerably on its contents of phenolic units and carbonyl groups. The extent of hydroxy methylation of kraft lignin [399,403], based on percent C9 units, was 30-33% on the C5 position and 3-7% on the side chains. Softwood kraft lignin was more reactive than hardwood steam explosion lignin [400]. The activation energy of hydroxy methylation was considerably lower for lignin than for simple phenol reaction (15 vs. 24 kcall mol). This suggests that accessibility is a significant factor in the hydroxymethylation of lignin. Nitrogen-Containing Agents. Psotta and Forbes [387,388] conducted the reaction of lignosulfonate and related model compounds with diazonium salts in mild alkali. The number of azo groups introduced into the lignin was in the 0.3-0.5 range per C9 units by reacting with bcnzenediazonium bisulfate. The condensation reaction appeared to occur mainly at the C5 position of phenolic units. Brezny et al. [373] reported the Mannich reaction of kraft or organosol v lignin with glycine and iminodiacetic acid in acetic acid. The range of substitution per C9 units observed for different lignin was 0.25-0.38 and 0.44-0.49 for the glycine and iminodiacetic derivatives, respectively.

5.

Sulfonation

Nakano et al. [404] observed that approximately 90% of an acid hydrolysis lignin residue from white birch became soluble after treatment with 3% sodium sulfite at 200°C. Reactivity of the hydrolysis lignin was improved by milling, which was assumed to increase accessibility, and may also induce some chemical degradation. Lignin sulfonation is the dominant reaction during sulfite-based treatments or delignification of wood [32,348]. Since the sulfonate content of chemimechanical pulps is a major factor in determining their strength properties [405-407], there has been strong interest in finding means to maximize sulfonation reactions without causing excessive yield loss. The extent of fiber sulfonation under preferred neutral or slightly alkaline conditions is generally

Reactivity and Accessibility

81

100 • Softwood o Hardwood

80

c

c

C

01

2! 'c

uo:=iOl 60 Q,JO

02 c,

2 0 40 -E

~E

r= 0.977 slope"", I. 065

20

20

40

GO

80

100

Phenolic Hydroxyl Content mmol/IOOg Lignin

Figure 16 Relationship between the sulfonate and phenolic hydroxyl group content of sulfonated wood meal samples. (From Ref. 408.)

thought of as being limited to the phenolic units of lignin. This contention was recently demonstrated by Lai and Guo 14081 as shown in Fig. 16. An excellent relationship between lignin sulfonation and phenolic hydroxyl group content was observed for a variety of wood species. Since many lignin reactions proceed through quinonemethide intermediates resulting from the ionization of phenolic hydroxyl groups, the capacity to form such intermediates may serve as a relative measure of the reactivity of lignin. Consequently, it appears that the response to neutral sulfite treatments may be suitable for measuring reactivity of the lignin.

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Reactivity and Accessibility 197. 198. 199. 200. 201. 202. 203. 204. 205. 206. 207.

208. 209. 210.

211. 212. 213. 214. 215. 216. 217. 218. 219. 220. 221. 222. 223. 224.

89

O. Rammas and O. Samuelson, Svensk Papperstidn., 71:829 (1968). J. E. Glass, A. M. Buettner, R. G. Lowther, C. S. Young, and L. A. Cosby, Carbohyd. Res. 84:245 (1980). J. Reuben and T. E. Casti, Carbohydr. Res. 163:91 (1987). D.-S. Lee and A. S. Perlin, Carbohydr. Res. 106:1 (1982). I. Croon and C. B., Purves, Svensk Papperstidn. 62:876 (1959). F. A. Buytenhuys and R. Bonn, Papier 31:525 (1977). F. F.-L. Ho and D. W. Klosiewicz, Anal. Chem. 52:913 (1980). J. Reuben and H. T. Conner, Carbohydr. Res. JJ5:1 (1983). L. T. Bach Tuyet, K. Iiyama, and J. Nakano, Mokuzai Gakkaishi 31:8 (1985). W. Gronski and G. Hellman, Papire 41:668 (1987). M. M. Abdel-Malik and M. Yalpani, in Cellulose: Structural and Functional Aspects (1. F. Kennedy, G. O. Phillips, and P. A. Williams, eds.), Ellis Horwood, Chichester, 1989, p. 263. E. J. Roberts, C. P. Wade, and S. P. Rowland, Carbohydr. Res. 17:393 (1971). S. P. Rowland, in Modified Cellulosics (R. M. Rowell and R. A. Young, eds.), Academic Press, New York, 1978, p. 147. S. P. Rowland, in Encyclopedia Polymer Science and Technology (N. M. Bikales, ed.) John Wiley and Sons, New York, Suppl. No. I, 1976, pp. 146-175. A. Isogai, A. Ishizu, and J. Nakano, J. Appl. Polym. Sci. 29:3873 (1984). N. Shiraishi, Y. Miyagi, S. Yamashita, T. Yokota, and Y. Hayashi, Sen-I Gakkaishi 35:T466 (1979). T. Sato, M. Minoda, and T. Miyamoto, in Ref. I, p. 403. I. Hagiwara, N. Shiraishi, T. Yokota, M. Norimoto, and Y. Hayashi, J. Wood Chem. Technol. 1(1):93 (1981). M. D. Nicholson and D. C. Johnson, Cell. Chem. Technol. JJ :349 (1977). T. Morooka, M. Norimoto, and T. Yamada, J. Appl. Polym. Sci. 32:3575 (1986). T. P. Nevell, in Cellulose Chemistry and Its Applications (T. P. Nevell and S. H. Zeronian, eds.), Ellis Horwood, Chichester, 1985, p. 243. A. S. Perlin, in The Carbohydrates, Vol. IB (W. Pigman and D. Horton, eds.), Academic Press, New York, 1980, pp. 1167-1215. O. Theander, Adv. Carbohyd. Chem. 17:223 (1962). O. Theander, in The Carbohydrates, Vol. IB (W. Pigman and D. Horton, eds.), Academic Press, New York, 1980, pp. 1013-1099. O. Theander, Tappi 48(2):105 (1965). S. A. Barker, E. J. Bourne, and M. Stacey, Chem. Ind. (London):970 (1951). A. Assarsson and O. Theander, Acta. Chem, Scand. 18:553 (1964). A. Assarsson and O. Theander, Acta. Chem. Scand. 18:727 (1964).

90 225. 226. 227. 228. 229. 230. 231. 232. 233. 234. 235. 236. 237. 238. 239. 240. 241. 242. 243. 244. 245. 246. 247. 248. 249. 250. 251. 252. 253. 254. 255. 256. 257. 258. 259.

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Reactivity and Accessibility 260. 261. 262. 263. 264. 265. 266. 267. 268. 269. 270. 271. 272. 273. 274. 275.

276. 277. 278. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. 291 . 292. 293.

91

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294. 295. 296. 297. 298. 299. 300. 301. 302. 303. 304. 305. 306. 307. 308. 309. 310. 311. 312. 313. 314. 315. 316. 317. 318. 319. 320. 321. 322. 323. 324. 325. 326. 327. 328.

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Reactivity and Accessibility 329. 330. 331. 332. 333. 334. 335. 336. 337. 338. 339. 340. 341. 342. 343. 344. 345. 346. 347. 348. 349. 350. 351. 352. 353. 354. 355. 356. 357. 358. 359. 360. 361. 362.

93

G. E. Miksche, Acta Chem. Scand. 27:1355 (1973). T. J. Fullerton, Svensk Papperstidn. 78(6):224 (1975). S. Ljunggren, Svensk Papperstidn. 83(13):363 (1980). J. Gierer and I. Noren, Holzjorschung 34(6):197 (1980). J. R. Obst, Holzjorschung 37(1):23 (1983). R. Kondo, Y. Tsutsumi, and H. Imamura, Holzjorschung 41:83 (1987). Y. Tsutsumi, R. Kondo, and H. Imamura, J. Wood Chem. Technol. 13 (1):25 (1993). H. Taneda, J. Nakano, S. Hosoya, and H.-M. Chang.,J. WoodChem. Technol. 7(4):485 (1987). B. Johansson and G. E. Miksche, Acta Chem. Scand, 23:924 (1969). G. Brunow and G. E. Miksche, Appl. Polym. Symp. 28:1155 (1976). J. Gierer, F. Imsgard, and I. Pettersson, Appl. Polym. Symp. 28: 1195 (1976). S. Yasuda, B.-H. Yoon, and N. Terashima, Mokuzai Gakkaishi 26(6):421 (1980). D. R. Dimmel and L. F. Bovee, J. Wood Chem. Technol. 13:583 (1993). J. Gierer and S. Wannstrom, Holzjorschung 40:347 (1986). J. Gierer, I. Noren, and S. Wannstr6m, Holzjorschung 41:79 (1987). J. Gierer, O. Lindeberg, and I. Noren, Holzjorschung 33(6):213 (1979). L. L. Landucci, Tappi 63(7):95 (1980). T. J. Fullerton and L. J. Wright, Tappi 67(3):78 (1984). T. J. Fullerton and A. L. Wilkins, J. Wood Chem. Technol. 5(2): 189 (1985). G. Gellerstedt, Svensk Papperstidn. 79:537 (1976). H.-T. Chen, M. Funaoka, and Y.-Z. Lai, Holzjorschung 48 (Suppl.):14O (1994). G. E. Miksche, Acta Chem. Scand. 26:3275 (1972). J. R. Obst and N. Sanyer, Tappi 63(7):111 (1980). J. Marton, in Lignins (K. V. Sarkanen and C. H. Ludwig, eds.), WileyInterscience, New York, 1971, p. 639. J. Gierer, B. Lenz, and N. H. Wallin, Tappi 48:402 (1%5). G. Gellerstedt and E.-L. Lindfors, in Proc. Int. Pulp Bleaching Conf., Stockholm, Vol. 2, 1991, p. 73. E. Evstgneyer, H. Maiyorova, and A. Platonov, Tappi J. 76:177 (1992). Y.-Z. Lai, S.-P. Mun, S.-G. Luo, H.-T. Chen, M. Ghazy, H. Xu, and J. E. Jiang, Holzjorschung, in press. V. L. Chiang and M. Funaoka, Holzjorschung 42 :385 (1988). J. Gierer and N.-H. Wallin, Acta. Chem. Scand. 20:2059 (1%6). E. Adler and J. Gierer, Acta Chem. Scand. 9:84 (1955). E. Adler, G. Brunow, and K. Lundquist, Holzjorschung 41 :199 (1987). G. Brunow and K. Lundquist, Holzjorschung 45: 37 (1991). L. N. Mozheiko, M. F. Gromova, L. A. Bakalo, and V. N. Sergeeva, Polym. Sci. USSR 23(1): 141 (1981).

363. 364. 365. 366. 367. 368. 369. 370. 371. 372. 373. 374. 375. 376. 377. 378. 379. 380. 381. 382. 383. 384. 385. 386. 387. 388. 389. 390. 391. 392. 393. 394. 395.

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Reactivity and Accessibility

3%. 397. 398. 399. 400. 401. 402. 403. 404.

405. 406. 407. 408.

95

G. H. van der Klashorst, in Ref. 6, p. 346. G. H. van der Klashorst and H. F. Strauss, J. Polyrn. Sci. A Polyrn. Chern. 24:2143 (1986). G. H. van der Klashorst, J. Wood Chern. Technol. 8(2):209 (1988). J. Marton, T. Marton, S. I. Falkehag, and E. Adler, Adv. Chern. Ser. 59:125 (1966). D. J. GardnerandG. D. McGinnis,J. Wood Chern. Technol. 8(2):261 (1988). D. J. Gardner, G. D. McGinnis, and L. W. Amos, J. Wood Chern. Technol. 9(2):219 (1989). W. Peng, A. O. Barry, and B. Riedl, J. Wood Chern. Technol. /2(3):299 (1992). L.-W. Zhao, B. F. Griggs, c.-L. Chen, J. S. Gratzl, and c.-Y. Hse, J. Wood Chern. Technol. /4(1): 127 (1994). J. Nakano, K. Sasaki, C. Takatsuka, and N. Migita, J. Jpn. Wood Res. Soc. 9: 107 (1963). C. Heitner and T. Hauula, J. Pulp Paper Sci. /4:J6 (1988). R. P. Beatson, C. Heitner, M. Rivest, and D. Atack, Pap. Puu 67:702 (1985). Y.-Z. Lai and W. Situ, J. Wood Chern. Technol. 12:149 (1992). Y.-Z. Lai and X.-P. Guo, Holzforschung 46:477 (1992).

4 Chemical Modification of Cellulose David N.-S.

HOD

Clemson University Clemson, South Carolina

I.

INTRODUCTION

One of the major components in wood is cellulose. In tree or in wood, cellulose plays an important role in providing basic structural element and strength. Cellulose is also the major chemical component in other lignocellulosic materials such as bamboo, bagasse, cotton, flax, hemp, jute, kenaf, and ramie (Table 1). The purpose of isolating cellulose from wood or lignocellulosic materials through mechanical, chemimechanical, and chemical means is to acquire it as a polymeric material for the making of paper and textile materials; or as a chemical source for the making of alcohol and other chemicals [1,2]. Although cellulose in its polymeric form is an excellent material for the manufacturing of many useful products, often it does present limitation for many areas of applications. In order for cellulose to be used in these areas and to improve its intrinsic value, modification of cellulose structure is mandatory. In order to improve its competitive position as a functional material as well as the development in emerging technology for raw materials, the restless and probing minds of chemists have continually sought to prepare novel cellulose derivatives having unique and useful properties [3,4]. Many managers and scientists with an interest in cellulose technology are aware of the great economic investment involved in the industrial production of cellulose

97

Han

98 Table 1 Cellulose Content in Lignocellulosic Materials Cellulose content Source

(%)

Bagasse Bamboo Cotton Flax Hemp Jute

35-45 40-55 90-99 70-75 75-80

Kapok

70-75 70-75 40-50 40-50

Ramie Straw Wood

60-65

derivatives for inclusion in a wide variety of commodities, such as textile fiber, foodstuffs, phannaceutical aids, paints, plastics, and explosives.

II.

REACTIVITY AND ACCESSIBILITY

Cellulose is a polydisperse linear syndiotactic natural polymer. The basic monomeric unit of cellulose is D-glucose, which links successively through a glycosidic bond in the ~ configuration between carbon 1 and carbon 4 of adjacent units to fonn long-chain I ,4-~-glucans (see Fig. I). As shown in the figure, cellulose possesses one primary and two secondary hydroxyl groups per glucose unit. Like any hydroxyl-containing compound, these hydroxyl groups can undergo addition, substitution, and oxidation reactions. Due to the inductive effects of neighboring substituents the acidity and the tendency for dissociation is increased in the order H06 < H03 < H02. Hence, depending on the reaction media, whether it is acid or alkaline, the reactivity of these hydroxyl groups will be different. For example, etherification of hydroxyl group is conducted in an alkaline condition, hence, H02 is most readily etherified among the three hydroxyl groups. On the contrary, for esterification, the primary hydroxyl group (H06) is the most active. Although these hydroxyl groups are active, they may not be accessible for reaction due to the morphologic characteristics of cellulose. The hydroxyl groups govern the morphologic characteristics of cellulose. They frequently fonn intra- and intennolecular hydrogen bondings within and

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99

H.OH

(a)

(b) Figure 1 Structure of cellulose illustrates in Haworth formula (a) and chiar conformation (b).

among molecules. These bondings, combined with other secondary forces such as Van der Waals attraction, aggregate portions of the molecular chains into various degrees of lateral order ranging from perfect geometric packing of the crystal lattice (so-called crystalline region) to random conditions (amorphous region). This topochemistry actually controls the chemical reactivity of cellulose. Essentially, the hydroxyl groups located in the amorphous regions, being in a highly accessible environment, react readily in many chemical reactions. However, in the crystalline regions, where there is a close packing and strong interchain bonding, these groups are not readily accessible to reactant molecules and are occasionally completely inaccessible to some. In order to enable a significant portion of cellulose molecules to participate in a reaction, crystalline regions must be made accessible to reactants. Accessibility of the cellulose molecules in the fiber is frequently detennined by fiber reactivity, which obviously also depends on the nature of the reactants as well as on such conditions as time, temperature, pressure, and solvents. Due to the strong hydrogen bondings, cellulose is not readily dissolved in common organic solvents. Thus, most of the reactions on cellulose are heterogeneous in nature. As with other semicrystalline polymers, cellulose undergoes two-phase crystalline-amorphous reaction, in which four reaction patterns are possible (5]: (l) surface, (2) macroheterogenous, (3) microheterogenous, and (4) pennutoid. Surface reactions involve only the cellulose mole-

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cules at the microscopic surface or, more precisely, their segments located at the surface. The macroheterogeneous reaction starts at the surface but proceeds through the fiber from layer to layer as the reacted cellulose dissolves or swells in the surrounding solvent. The microheterogeneous reaction occurs when the cellulose is swollen by the reaction medium, but the crystalline, ordered, or intrafibrillar regions are not accessible to the reagent. Pennutoid reactions (or so-called intrafibrillar or intracrystalline reactions) extend to the highly ordered and crystalline regions without dissolving them and usually lead to the transfonnation of the lattice. Microheterogeneous and pennutoid reactions are also often distinguished as intennicellar and intramicellar reactions, respectively. While photochemical and weathering reactions of cellulose generally are considered a surface reaction, the nitration of cellulose is considered to be pennutoid. Reactions 3 and 4 are detennined by the solvent power. The acetylation of cellulose in a solvent for cellulose acetate that does not swell cellulose is a macroheterogeneous reaction. If a partial acetylation of cellulose is carried out by applying the acetic anhydride from a swelling agent for cellulose, then it is a microheterogeneous reaction. Many activation treatments, such as swelling, solvent exchange, inclusion of structure-loosening additives, or mechanical action, can be used to increase reactivity. Once the original hydrogen bonds have been broken and intramicellar swelling achieved, the cellulose hydroxyIs are capable of reacting like an ordinary aliphatic hydroxyl group. Recently, many cosolvents have been developed for cellulose which consequently improve cellulose accessibility and reactivity. These cosolvents are dimethylsulfoxide/parafonnaldehyde, dimethylacetamide (DMAC)/parafonnaldehyde, lithium chloride/dimethylacetamide, dinitrogen tetroxide/dimethylfonnamide, and sulfur dioxide/nitrosyl chloride [6,7]. With these cosolvents, cellulose modification can be conducted in a homogeneous system. For example, a high degree of substitution, organosol trimethylsilylcellulose has been prepared in DMAC/LiCI system (8). Several cellulose sulfonates have been prepared in the DMF/chloral solvent system [9,10]. The number of hydroxyl groups available for reactions can vary from as few as 10-15% in highly crystalline cellulose materials to as much as 85-95% in decrystallized cellulose [11]. Even higher accessibility (i.e., 98-100%) can be achieved from a regenerated noncrystalline cellulose material [12].

III.

SERENDIPITY DISCOVERIES

As with so many significant discoveries, serendipity smiled on Christan Schonbern when he accidentally synthesized cellulose trinitrate by using his

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wife's apron to clean up the spill of nitric and sulfuric acids. The history of cellulose modification began around 1830 when Brocannot first described the nitration of cellulose [13]. Regeneration of cellulose fiber from cellulose nitrate through denitration, dissolution, spinning, and regeneration was detailed by Count Hilaire de Chardonnett in 1885 [14]. After 1890, additional methods were found to solubilize cellulose by acetylation, xanthation, and cuproxyammoniation, to spin the resulting solutions by coagulating them in the fonn of a filament or fiber. This period was considered to be the beginning of cellulose chemical modification. With this series of esterifications, a number of cellulose ethers of technical importance have emerged. These products include ethylcellulose, methylcellulose, and carboxymethylcellulose. The properties of the cellulose ethers and esters depend heavily on the type, distribution, and unifonnity of the substitution groups. Reactions at the hydroxyl groups can occur either on a one-to-one basis or with the fonnation of side chains depending on the choice of reagent employed to modify the cellulose. In the fonner case, the tenn degree of substitution (DS) is used to identify the average number of sites reacted per ring. The maximum value is 3, corresponding to the number of hydroxy Is available for reaction. Moreover, ethers or ether esters with hydroxyalkyl groups attached are characterized by the degree of reaction (DR), also frequently named molar substitution (MS), i.e., the average number of molecules of reagent reacted with each anhydroglucose unit. Its value can exceed 3. Thus, the ratio MS/DS expresses the average length of the pendant chain. In some cases, the extent of substitution and of reaction are expressed on a weight percentage basis and the substitution index (SI), i.e., the percentage of substituted anhydroglucose units, has been used [15,16]. Several cellulose esters and ethers with commercial values are summarized in Table 2. Many cellulose esters, such as cellulose nitrate, cellulose acetate, and mixed esters of cellulose acetate butyrate, have found popularity in commercial scale production. Many new esters continue to appear in the market. Traditionally, esterification is conducted on a heterogeneous system (topochemical reaction); however, homogeneous systems employing mixed organic solvents have recently been developed. For example, Ikeda et al. [17] demonstrated that homogeneous esterification and acetalization of cellulose in LiCU DMAC can be achieved. Cellulose ethers also have gained their positions on the market due to their multifunctional effects. They are soluble in both water and organic solvents, functioning as thickeners, flow control agents, suspending aids, protective colloids, water binders, liquid crystals, film fonners, or thennoplastics. Because of their properties, they are used in such diverse industries as food,

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Table 2

Commercially Available Important Cellulose Esters and Ethers

Cellulose derivative

Symbol

Cellulose nitrate Cellulose acetate Cellulose triacetate Cellulose acetate butyrate Cellulose acetate priopionate Cellulose acetate phthalate Cellulose acetate trimellilate Methylcellulose Ethylcellulose Hydroxyethylcellulose hydroxypropylcellulose Ethylhydroxyethylcellulose Hydroxybutylmethylmethylcellulose Hydroxyethylmethylmethylcellulose Carboxymethylcellulose Sodium carboxymethylcellulose Calcium carboxymethylcellulose

CN CA CTA CAB CAPr CAP CAT MC EC HEC HPC EHEC HBMC HEMC CMC NaCMC CaCMC

paint, oil recovery, paper, cosmetics, pharmaceutical, adhesives, printing, agriculture, ceramics, textiles, and building materials (18). The properties of cellulose can be improved by crosslinking reactions. The crosslinking agents in common used are generally water soluble, di- or trifunctional agents capable of reaction with cellulose under relatively mild acidic conditions. Covalent and ionic crosslinking agents have been used for improving textile and paper properties. Covalent bonding is usually achieved by the fonnation of ester linkages by reaction of cellulose with a polycarboxylie acid and by fonnation of imine linkages by reaction of polyamines with oxidized cellulose. Urea-fonnaldehyde, melamine-fonnaldehyde, and the polyamide-amine polymers have been used to fonn water-resistant bonds between fi bers. Cellulose can also be modified by introducing long-chain polymer(s) onto its main chain. The preparation of a graft copolymer requires the fonnation of a reactive site on cellulose in the presence of a polymerizable monomer. The principal techniques frequently used are (I) grafting initiated by free radical polymerization, (2) grafting initiated by ionic polymerization; (3)

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grafting by condensation and ring opening. The first of this reaction was reported in 1943 [164]. The same year, Ushakov also reported the copolymerization of unsaturated cellulose derivatives. Several excellent monographs [70,165] and review articles [166,167) are available. Production of cellulose derivatives require considerable characterization and analysis. Excellent review papers are available for this subject [19,20]. Applications and properties of cellulose derivatives for various life sciences were summarized by Doelker [21] and Hon [3].

IV.

REVISIT OF UNUSUAL CELLULOSE DERIVATIVES

As discussed earlier, many cellulose derivatives are available commercially. Many review articles are also available on this subject. To avoid repetition of previous works, in what follows only unusual cellulose derivatives will be discussed.

A.

Oxycellulose and Polyalcohol from Cellulose

Cellulose can be oxidized to products with acidic properties. It is oxidized selectively at C6 to yield 6-carboxycellulose by oxidation with nitrogen dioxide in a nonpolar solvent such as tetrachloromethane [22]. The most selective process of cellulose modification is, however, the oxidation of cellulose by periodic acid and its salts to form a dialdehyde cellulose, which can be further oxidized to dicarboxylcellulose, tricarboxylcellulose, or reduced into an acyclic, stereoregular polymer of [(2r,4s,5r)-2,4,5-tris(hydroxymethyl)-1 ,3-dioxopentamethylene] [23], as shown in Fig. 2. A recent review article on this subject is available [24]. The 2,3-dialdehyde cellulose was found to be a bioabsorbable material with a requisite mechanical strength and antimicrobial activity [25]. It has been noted that under suitable conditions periodate oxidation of cellulose can yield products containing high levels of carboxyl or acidic enediol function [26] and methyl ester derivatives [27]. The high level 2,3-dicarboxycellulose produced by oxidation with HCI0 2 was completely soluble in water and took up various metallic ions other than alkali metals to form precipitates [28]. Homogeneous periodate oxidation of cellulose was attempted by using methylol cellulose, in which a uniform cleavage of C2C3 bonds by the periodate ion was achieved [29]. Examination of the thermal deformation and tensile properties revealed that no notable cellulose degradation occurred during the reaction. A new synthetic polycarboxylic of oxidized

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cellulose, in which the p-aminobenzoic acid was linked via anesthesin (paminoethylbenzoate) to carboxyl groups of chlorite-oxidized cellulose, both as spacer and to increase the hydrophobicity of the polymer through the phenyl group, is presented by Uglea et al. [30]. The structure of modified cellulose was characterized by thin-layer chromatography (TLC) and by infrared (IR) and nuclear magnetic resonance (NMR) spectrometry. The impregnation of crotonized crotonaldehyde cellulose with e-aminocaproic acid and CaCI 2, (CH 3C0 2hCu, CuS04 , or AgN0 3 leads to the formation of bioactive gauzes with hemostatic and antimicrobic action [31]. Moreover, the oxidation of unprotected (regenerated) cellulose with DMSO-Ac20 or DCC/DMSO/pyridine/triftuoroacetic acid affords a mixture of2-oxy-, 3-oxy-, and 2,3-dioxycelluloses [32]. Recently, it was observed that oxidation of unprotected cellulose with DMSO-Ac 20 in the DMSO/paraforrnaldehyde

Figure 2

Synthesis of oxycelluloses.

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105

solvent system affords exclusively 3-oxycellulose, due to the reversible formation of 0-6 and 0-2 hydroxymethyl and poly (oxymethylene)ol side chains [33]. The methyl ester of oxycellulose, produced by alkaline permanganate oxidation of cuprammonium cellulose followed by treatment with diazomethane, has been reacted with protein by the azide method [34]. Acidic oxycelluloses are also able to react with alcohols and amines, including proteins, to form esters or amide derivatives.

B.

Esterification

Because it is a polyhydroxyl alcohol, cellulose can be esterified in strong acid mediums, anhydrides, and catalysts, and requires the absence of water for completion because it is a reversible reaction. Normally, the reaction proceeds rapidly and is permitted to continue until the three hydroxyl groups on each anhydroglucose unit have been replaced with the acyl group of the organic acid or mixture of acids.

1.

Cellulose Sulfates

Cellulose can be sulfated by sulfating agents such as sulfuric acid in organic solvents, chlorosulfonic acid in the presence of amines, and liquid or gaseous sulfur trioxide [35]. When prepared to the appropriate degree of sulfate ester substitution, cellulose sulfate esters are water-soluble and of interest as detergents; antistatic coatings for photographic film; viscosity modifiers for enhanced oil recovery; thickening agents for foods, cosmetics, and pharmaceuticals; and low-calorie food additives [36-41]. Although cellulose sulfates have been known since 1819, new processes for making this inorganic cellulose ester continue to appear. High molecular weight cellulose sulfate esters with a high degree of sulfate ester substitution and an excellent thermal stability have been synthesized [42,43]. This method uses preformed dialkylamide sulfur trioxide complexes as a sulfating reagent in the corresponding dialkylarnide solvent. The reaction is heterogeneous and the cellulose remains fibrous throughout the sulfation. Completely watersoluble, highly viscous sodium cellulose sulfate semiesters are obtained in homogeneous systems by the reaction of cellulose nitrite [44]. The intermediate, cellulose nitrite, that is formed and dissolved is obtained in the N 20 41 DMF system and is at the same time transesterified by the S03-DMF complex [44]. Such transesterified products can be crosslinked by metal ions to form highly effective thickening agents in aqueous media [45]. This process has also been developed to produce cellulose sulfate ester with interesting rheo-

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logic and gel fonning properties [46]. Mixed esters such as cellulose acetate sulfates, cellulose acetate butyrate sulfates, cellulose acetate propionate sulfate, and ethyl cellulose sulfates are described in several patents [160-163]. Being ionic compounds, cellulose sulfates have ion exchange properties. They have been recommended for use as cation exchangers [42,47,48]. Sodium cellulose sulfates are also known to have blood anticoagulant activity. The correlation of molecular characteristics of these derivatives with their anticoagulant activity has been investigated [49]. However, it has been reported that cellulose sulfate exhibits a certain toxicity [50].

2.

Cellulose Carbamates

Cellulose reacts with isocyanates in anhydrous pyridine or with urea and substituted ureas at relatively high temperature to yield carbamates. The optimum carbamation reaction of microcrystalline cellulose with urea in a dry solid mixture has been studied [51]. In addition, a preferentially C6-modified cellulose carbamate derivative has been obtained [52]. Heating of cellulose with thiourea at 180°C yielded cellulose thiocarbamate [53]. Heat treatment of cellulose isocyanate products has been utilized for the production of urethanes [54]. When cellulose was treated with phenylisocyanate at 100°C in DMF in the presence of dibutyltin dilaurate and triethylenediamine, cellulose bisphenylcarbamate was fonned [55]. Treatment of cellulose with urea at temperatures at or above the latter's melting point (where urea decomposes into isocyanic acid and ammonia) has been employed for the production of cellulose carbamates fibers [56]. The advantages and disadvantages of using urea as an intennediate for production of fiber have been discussed [57]. Metal chelating amino acid derivatives of cellulose were recently obtained via modification of cellulose with 2,4-toluenediisocyanate, followed by treatment with amino acid ester derivatives [58,59]. Diisocyanates are able to crosslink cellulose chains and/or to yield reactive cellulose isocyanate, depending on the reaction conditions. Sato and his coworkers [60] examined the optimum conditions for the reaction between cellulose and 2,4-toluenediisocyanate and succeeded in introducing 0.30 mol of free isocyanate group per glucose unit. Cellulose isocyanate was further converted into isothiocyanate [61]. This derivative has also been synthesized by condensation of cellulose with 2,4-diisocyanototoluene, followed by hydrolysis and thiophosgene treatment [61]. Cellulose carbamate and its derivatives are able to immobilize enzymes easily with the help of dialdehydes such as dialdehyde starch, glutaraldehyde, and glyoxal [62]. Since cellulose triphenylcarbamate or tricarbanilate prepared

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without degradation showed good solubility in many organic solvents, it was used for detennining the DP and DP distribution of cellulose by gel penneation chromatographic analysis [63,64].

3.

Cellulose Acetate

Cellulose acetate is the most important organic ester because of its broad application in fibers and plastics. The historical perspective, supply and demand, properties, and manufacture of this group of cellulose derivatives can be found in a recent review [65]. Cellulose acetate can also be further modified to improve its properties. For example, cellulose diacetate can be perfluoroacylated with straight-chain perfluoroalkanoyl chlorides and with oligohexafluoropropene oxides in the presence of amines to generate mixed esters having oil and water repellency properties [66]. Pure cellulose propionate and cellulose butyrates are difficult to produce [67]. However, some mixed cellulose esters, such as cellulose acetate propionate and cellulose acetate butyrate, can be prepared in one step from cellulose with the corresponding acid and acetic anhydride in the presence of sulfuric acid. These mixed cellulose esters find applications as lacquers, plastics, and hot-melt coatings [68]. Commercially available cellulose acetate phthalate is produced by reacting secondary acetate with phthalic anhydride in acetic acid with a sodium acetate catalyst [69]. Graft copolymerization of cellulose acetate to improve its properties has gained popularity recently [70]. Details are discussed in a subsequent section.

4.

Cellulose Phosphate

Cellulose phosphate esters are of considerable interest because of their inherent flame resistance and ion exchange capability. Cellulose phosphates with a low phosphorus content are obtained by reacting cellulose or linters with phosphoric acid in an urea melt [71]. Higher phosphorus contents and a lower degradation rate of the cellulose may be obtained with excess urea at higher temperatures (l30-150°C) for 15 min. The Ban-Flame process [72], one of the first commercially feasible flameproofing procedures for cotton fabric, was based on this method. Water-soluble cellulose phosphate with a high degree of substitution may be obtained from a mixture of phosphoric acid and phosphorous pentoxide in an alcoholic medium. Phosphorylated cellulose fibers show increased swelling after partial hydrolytic degradation and transfer into the alkali salt fonn and were suggested for use as adsorbents [73]. All cellulose esters containing phosphorus have fire retardant properties [74] and have attracted some interest due to their ion exchange characteristics [75,76].

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5.

Others

Cellulose carbonate has been prepared by the reaction of cellulose in dimethylsulfoxide with ethyl chlorofonnate in the presence of triethylamine [159]. It is unstable in water, yielding acyclic carbonates by the ring opening reaction. p-Toluenesulfonic esters of cellulose can be obtained by treating rayon or alkali cellulose in pyridine with a pyridine solution of p-toluenesulfonyl chloride [77]. Methanesulfonic esters can be made by first swelling the cellulose in about 20% sodium hydroxide. The base is washed out and the cellulose either solvent-exchanged through methanol into pyridine or just washed with pyridine to remove excess water. Methanesulfonation is accomplished by treating the swelled cellulose with a solution of methanesulfonyl chloride in pyridine. Bromoacetylcellulose has been prepared by treating cellulose with bromoacetic acid and bromoacetyl chloride in dioxane [78]. The p-Aminobenzoyl ester of cellulose has been made by reaction of cellulose with p-nitrobenzoyl chloride followed by reaction with Ti(lII) or V(II) [79,80]. A polycarboxylic acid esterified with cellulose through the fonnation of an acid anhydride intennediate was identified by Fourier transfonn infrared spectrometry [81]. Long-chain fatty acid cellulose esters were synthesized by the acid-chloride-pyridine reaction with different degrees of substitution. Hydrolyzed soybean oil was used as an unsaturated fatty acid [82]. The preparation of cellulose esters with substituted benzoic acids in the presence of p-toluenesulfonyl chloride in pyridine was investigated [83]. The substituents included N02 , CI, Me, and MeO. The substituent and its position (ortho, meta, or para) did not influence the reaction significantly. A relatively novel class of derivatives is obtained by the covalent incorporation of organometallic moieties into cellulose. For example, cellulose ferrocenyl derivatives have been prepared by esterification of cellulose with an intennediate derived from ferrocene carboxylic acid and triphenyl phosphite in the presence of pyridine [84]. An enzymatically cleavable cellulose ester has been developed [85], and prodrugs have been coupled to the hydroxyl or carboxyl functions of C-tenninal aromatic amino acids of cellulose peptide derivatives for controlled release applications [86].

c.

Etherification

In contrast to esterification, etherification is carried out in an alkaline medium and the etherifying agents are alkyl halides. The general reaction is tenned aliphatic nucleophilic substitution and, employed under nonnal conditions, is of the bimolecular type.

Chemical Modification of cellulose

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Classical cellulose ethers, such as carboxymethylcellulose (CMC), alkylcellulose, and hydroxyalkylcellulose will not be discussed here. Readers should refer to classical publications [6,18,67,88]. The reactivity of cellulose toward tri(p-toluenesulfonyl)methane chloride was recently examined [89]. The tosyl reagent is more reactive than trityl chloride, and the primary hydroxyl position exhibited 43 times more reactivity than the secondary hydroxyl groups. The products were used as intennediates in the synthesis of selectively modified cellulose derivatives [89]. As mentioned earlier, a high OS, organosol trimethylsilylcellulose has been prepared in OMAc/LiCI (10]. The condensation of polysaccharides with triphenylmethyl (trityl) chloride proceeds generally with preference for the primary hydroxyl positions. The tritylation of cellulose occurs initially 58 times faster at the hydroxyl group at C6 than at either C2 or C3 [90]. Cellulose can be modified with organostannane chlorides, such as dibutyl or triphenyl derivatives [91,92], or with organotin halides in the presence of bisethylenediamine copper(II) hydroxide [93]. Epoxy-activated cellulose was prepared by reacting cellulose acetate fibers with sodium methoxide, followed by reacting it with epichlorohydrin in OMSO. This epoxy-activated cellulose has proved to be a useful intennediate to react with substances containing active hydrogen, such as amine, amino acid, or carboxylic acids [94], as shown in Fig. 3. Epoxidized cellulose has also been converted to a thiol derivative via reduction of a thiosulfate intermediate [95], and sulfoethylcellulose has been obtained from sodium chloroethanesulfonate [96]. Cellulose ROC~-~:C~-NH-R'

ROCH2-C HCH2-NH-R'

6H

~N-R:

~-(NHCH2CH,)n

ROC H2-.....\-/-.,

HOOC(CH,), C0

7

ROCH2-~:CH2-OCO(CH2hCOOH

Figure 3

o ~CH2COOH ROCH2-~:H2-NHC~COOH

Reaction of epoxy-activated cellulose. R denotes cellulose.

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monoresorcinol ether can be made by refluxing a mixture of powdered cellulose and benzene, with phosphorous tribromide followed by refluxing of the cellulose bromide in alcohol with an excess of sodium m-hydroxyphenoxide (monosodium salt of resorcinol) [97,98].

1.

Modified Cellulose Ethers

Isogai and coworkers [99] recently prepared a series of tri-O-alkylcellulose ethers using a technique that was originally developed for permethylations and involves the use of alkyl halides, powdered sodium hydroxide, and nonaqueous solvents. Water-soluble phosphonomethylcellulose products have been produced by modification of cellulose ethers with chloromethanephosphonic acid derivatives [87,100]. Low levels of hydrocarbon residues can be incorporated into cellulose ethers, such as hydroxyethylcellulose, to yield high-viscosity, water-soluble products that display non-Newtonian behavior at low shear rates [101,102]. Small amounts of2-(N,N-diethylamino)ethylcellulose can be produced by the Williamson reaction of alkali cellulose with the hydrochloride of 2-chloroethyldiethylamine [103]. Chemical modification of hydroxyethylcellulose or hydroxypropylcellulose with long-chain hydrocarbon alkylating reagents, such as C8-C24 epoxides or halides, has been reported to yield novel water-soluble compositions exhibiting enhanced low-shear-rate solution viscosities and polymeric surfactant properties [104,105]. Patents have also been issued for water-soluble phosphonomethylcellulose and phosphonomethylhydroxyethylcellulose [106,107]. The preparation of predominantly 06-substituted carboxymethylcellulose can be achieved in a homogeneous solution, using the N-methylmorpholineN-oxide/DMSO solvent system (108). A suitable novel route for the preparation of CMC via 6-0-triphenylmethyl cellulose substituted only at the C2 and C3 positions was reported [109]. Acetylation of highly substituted carboxymethylcellulose has also been reported [110]. It has reported that the presence of additives such as formam ide and H 3P04 affected the aggregate structure and morphology of cyanoethylcellulose acetate in acetone solution by forming hydrogen bond and/or nucleophilic interaction [111]. The incorporation of mercury into cellulose has been accomplished via treatment of cellulose aniline ether derivatives with mercuric acetate [112]. Arsenic-containing cellulose derivatives have been obtained from sodium arsenate and diazotized cellulose precursors [113]. Platinum-containing polysaccharide derivatives have also been reported [114]. By an in situ method, cellulosics can be used to produce ferrites in the nanoscale size range, producing a "cellulosic nanocomposite" to be used as superparamagnetic materials

Chemical Modification of Cellulose

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[115]. Magnetic bead cellulose can also be prepared from viscose and ferrite powder by employing the suspension procedure using the thennal sol-gel transition [116]. The particles thus obtained can be stirred if acted on by an external rotary magnetic field of a common pennanent magnet. However, magnetic bead cellulose prepared from supennagnetic powder of the SmCoFe alloy shows a better behavior in the magnetic field than ferrite materials. Cellulose can be reacted to cellulose titanates in a heterogeneous reaction system by reacting it with titanium tetrachloride in DMF or with chlorinated anhydrides, chlorinated ester anhydrides, and esters of the hypothetical orthotitanic acid [117]. Cellulose derivatives with electrical properties have also been prepared [118]. In this case, cellulose was treated with 2,3-epoxy-lpropanol to give 0-2,3-dihydroxypropylcellulose which was then cyanoethylated to fonn derivatives having high dipole moments. Cyanoethylated 02,3-dihydroxypropylcellulose having a degree of substitution of 0.6-1.6 and a molar substitution of 0.7-8.5 showed dielectric constant 18-31 at I kHz and room temperature. Treatment of cyanoethylcellulose with borane-dimethylsulfide or boranetetrahydrofuran complexes in tetrahydrofuran has resulted in the quantitative conversion to 3-aminopropylcellulose. Such aminopropylcellulose derivatives have also been employed as intennediates for acetamido or aryluredo products, and in grafting reactions [119]. m-Aminobenzyloxymethyl ether of cellulose can be made by wetting the cellulose with an acetate buffered solution of N-m-nitrobenzyloxymethylpyridinium chloride, drying the cellulose at 6O-80°C, and heating to 125°C for 40 min. The nitro groups are then reduced with an aqueous solution of sodium dithionite for 30 min [80]. The general reaction scheme is shown in Fig. 4. p-Aminobenzylcellulose can be made by heating, for 4 h at 95°C, a mixture of p-nitrobenzyl chloride and cellulose powder in 40% sodium hydroxide. Extensive washing with water, ethanol, and acetone is required. The nitro group is then reduced to an amino group by suspending the p-nitrobenzylcellulose in boiling ethanol and adding a water solution of sodium dithionite [120]. The general reaction scheme is shown in Fig. 5. The synthesis of aminoalkylcarbamoylcellulose derivatives from either CMC or cyanoethylcellulose was reported recently. This family of derivatives exhibits properties suitable for applications as biodegradable carriers for phannacons and agricultural chemicals and for production of high molecular weight polymeric quaternary salts [121]. Carboxymethylcellulose can be further modified by basic amine, amino acid, or protein. 4-p-Aminophenylanilide can be made by allowing benzidine

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20H

-6 H H

H

H

H

0+

[CNCH'OCH'-< SO'

fcr

OH

1

NO,

H,OCH,OCH,-<

5

0-

1

NA,S,O.

CH,OCH,OCH,-<

NH,

5

0-

Figure 4

Preparation of m-aminobenzyloxymethyl ether of cellulose.

to react with carboxymethylcellulose in water or methanol, in the presence of dicyclohexylcarbodiimide or other diimides to fonn the half amide, one amino group remaining free, as shown in Fig. 6a [122]. Carboxymethylcellulose amide of tyramine has been made by reacting tyramine to carboxymethylcellulose in dimethylfonnamide using dicyclohexylcarbodiimide as the reacting agent (Fig. 6b) [123]. The reaction of carboxymethylcellulose in water with I-hydroxyl-5-naphthylamide, in the presence of dicyclohexylcarbodiimide, to make I-hydroxy-5-naphthylamide of carboxymethylcellulose was made successfully (Fig. 6c) [124]. Acid chloride of CMC reacted with p-phenylazoaniline yielded p-phenylazoanilide of CMC. The product was a clear colored cellulose, yellow in acid, red in base [125].

Chemical Modification of Cellulose

113

I;I~"O-

~ H OH

+

CIH,C --< }-NO'

!NaoH CH,OCH,-{ }-NO' o

0-

H

18,0; CH,OCH,-{ }-NH' o

0-

H

Figure 5

Preparation of p-aminobenzylcellulose.

A simple method of chemical modification of cellulose was proposed by Zhdanov and coworkers [126]. This method was based on the' 'hydrophobic mercerization" of cellulose in a superbasic medium such as the dimethylsulfoxide-solid sodium hydroxide mixture, followed by etherification or esterification. Methyl sulfate, benzyl chloride, acetic anhydride, methyl brornoacetate, triethyleneglycol ditosylate, and p-toluenesulfonyl chloride were used as the modification agents. This method simplified appreciably the preparation of acetylcellulose and methoxycarbonylmethylcellulose. Cellulose ethers can be prepared with high etherification yield, uniform substitution, and good oxidation resistance by alkalinization and alkylation of aqueous cellulose in the presence of water-miscible cyclic ethers as suspending agents [127].

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CARBODlMIDE

•

NHR -

-NH-O-O-~

(a)

(b)

-NH-Q

(c)

~H

Figure 6 Modification of carboxymethylcellulose by using the carbodiimide reaction.

D.

Deoxycellulose

Due to the practical importance of deoxycellulose, the halogenation of cellulose has been the subject of many studies which were reviewed by Gal 'braikh and Rogovin [24], Vigo [128], and Ishizu 14]. Deoxycelluloses denote anhydroglucose units in which the three hydroxyl groups are partially or completely replaced by other functional groups which do not contain an oxygen atom attached to the ring carbon atom. The possibility of synthesizing deoxycellulose was first demonstrated by Shorygin and Makarov-Zemlyanskaya during an investigation of the cleavage of cellulose ethers by solutions of metallic sodium in liquid ammonia [22]. Today many reactions have been used to prepare deoxycelluloses. Of these, halogenation is the most popular one.

1.

Halodeoxycellulose

Chlorodeoxycellulose has been the most widely studied halodeoxycellulose. This derivative has useful properties such as resistance to flame and rotting. Slight fluorination increases oil resistance and lowers the soiling potential of cellulose fibers. It has also been employed as an intennediate in the preparation

Chemical Modification of Cellulose

NHy'EtOH I-hNCH2R/n-BuOH NaNHNH,lNH,NH, HS(CH)nCOOH H2N(CH2hNH2H2S3 NaSCNIDMF KCNJDMF NaNYTh\.fF KIIDMF NaIIHD t· Bti>K/DMSO

Figure 7

115

---.

CeD----C H2NHz Cen-cH2NHCHzR Cen-cI-hNH2~

CeU--CHS(CH2)nCOOH CeO-CH2(S) nCH2-CeD Cen-c~SCN

Cen-cHzCN Cen-cI-hN 3 ---. CeU-cHO Cen-cHzI S06 •

Cellubsene

Reactions of chlorodeoxycellulose. Cell denotes cellulose.

of many cellulose derivatives, as shown in Fig. 7. The most widely used method for preparing halodeoxycellulose is the nucleophilic displacement of good leaving groups by halides in various solvents. The most frequently employed leaving groups are tosylate and mesylate. Other leaving groups, such as nitrate, and N ,N-dimethylfonnamide from a fonniminium salt intennediate, have also been used. Recently, a sulfuryl chloride (S02CI2)-pyridine system was also used [129]. Chlorination with S02Cl2 and pyridine proceeded in parallel at C6 and C3 in a heterogeneous system. A homogeneous LiCIdimethylacetamide system was also attempted by Ishizu [4]. Porous cellulose gel beads were halogenated under heterogeneous conditions with N-halosuccinimide and triphenylphosphine in aprotic organic solvents by Furuhata et al. [130]. They found that the degree of substitution of OH groups by halogen was higher for chlorination than for bromination. The halodeoxycellulose gel beads were resistant against hydrolysis by cellulases. Carboxyl-substituted aminodeoxycellulose (Cell-CH 2NH-R-C0 2H) prepared by the reaction of amino acids with chlorodeoxycellulose adsorbed various heavy metal ions with high efficiency [131]. Cellulose isocyanate reacted with amino acids or their esters in DMSO at low temperature to yield cellulose derivatives containing amino acid residues [60]. These derivatives adsorbed various kinds of metal ions in their free acid state with relatively high

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adsorption values for Cu(II) and Fe(III) [132,133]. The derivative containing cysteine was especially effective for the adsorption of Hg(II) ions [133].

2.

lododeoxycellulose

Since iodide ion is a strong nucleophile, the action of iodides on cellulose tosylate, mesylate, and nitrate in ketones, and chlorodeoxycellulose in DMF or 2,5-hexanedione can generate iododeoxycellulose. In use of chlorodeoxycellulose, the chlorine substituents were almost completely replaced by iodide in 2,5-hexanedione. The iododeoxycellulose thus prepared was 6-dexoy-6iodocellulose. lododeoxycelluloses having iodo substituents at C2 and C3 were also prepared by treating various sulfonates having a DS higher than 1.0 or 6-0-trityl-2(3)-p-nitrobenzenesulfonate of cellulose with sodium iodide in DMF. Ishii [134,135] succeeded in the almost quantitative preparation of 5,6-cellulosene acetate by the treatment of acetylated 6-deoxy-6-iodocellulose (DS 0.8) with 1,8-diazobicyclo [5,4,0]undec-7-ene in DMF. Novel cellulose derivatives containing viologen were prepared from a halogenated cellulose or tosylated cellulose. These products are expected to be a good polymeric electron carrier because they fonn a stable hydrophilic membrane as compared with other polymers [136]. Deoxythiocyanate cellulose fabrics exhibited moderate antibacterial activity [137]. The hydroxyl group of cellulose can be specifically substituted with fluorescent probe groups, and their fluorescence behavior can be studied with the change in the concentration of the tetrahydrofuran solution [138].

E.

Block and Graft Copolymers

Whereas block copolymers are linear chains fonned by introducing active sites in the tenninal units, graft copolymers are their branched equivalents whereby the active site is included on an internal monomer unit.

1.

Block Copolymers

Two block copolymers of trimethylcellulose-b-polyoxyethylene have been reported (139). The trimethylcellulose blocks containing one a-chloro ether end were treated with silver hexafluoroantimonate, AgSbF6 , in THF solution between - 10°C and + 23°C to facilitate a living cationic THF polymerization of polyoxytetrarnethylene blocks, as shown in Fig. 8. Monofunctional I-hydroxycellulose triesters, such as tributyrate and propionate acetate derivatives, have been coupled to bis(4-isocyanotophenyl)disulfide to obtain macroinitiators for the radical syntheses of three block copolymers of the type cellulose-initiator-initiator-cellulose [140].

Chemical Modification of Cellulose

~

117

OR

o

CI

/RO

OR

Figure 8 Synthesis of trimethylcellulose-b-polyoxyethylene block copolymer starting with an et-chloroether end of the trimethylcellulose.

2.

Graft Copolymers

Graft Copolymers of Cellulose. A large number of graft copolymers of cellulose have been prepared, and although there has been little commercial exploitation, there is considerable interest from the perspective of modification of cellulose. Three methods have been commonly used for graft copolymerization, namely, radical polymerization, ionic polymerization, and condensation and ring-opening copolymerization. Of these, radical modification is the most popular one. However, problems in using this method frequently occur. For examples, elimination of homopolymer formation, better control of molecular weights and molecular weight distribution of the grafted side chains, better reproducibility of the grafting yields are some of the problems of great concern. There are some formidable challenges facing those engaged in the synthetic aspects of cellulose grafting if viable commercial processes are to be developed as pointed out by Stannett [141] and Narayan [142]. In spite of these problems, many graft copolymers of cellulose continued to appear via the radical initiation method. Recently,

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grafted products of cellulose exhibiting thennoplastic properties have also been reported [143]. Graft polymerization of 2-methyl-5-vinyltetrazole onto cellulose fiber decreased the temperature for initial oxidative thennal degradation and increased the activation energy of degradation compared with that for the initial cellulose [144]. It was reported that the oxidative thennal degradation of the grafted fiber depended on the amount of grafted polymer and on the structure of the cellulose substrate (cotton cellulose, rayon, etc.). UV- or -y-radiation-induced grafting of cellulose films with styrene in methanol solution was significantly enhanced in the presence of H2S04 , lithium salts, urea, and other organic compounds and polyfunctional monomers [145]. In the graft polymerization of methacryloyllupinine onto cotton cellulose in the presence of Ce(NH 4h (N0 3)6' the induction period of methacryloyllupinine polymerization increased with decreasing temperature and was higher in air than in a vacuum. Vinylacetylene copolymers, derived from grafting of dimethylvinylethylcarbinol onto cellulose, afford metal-containing polymer derivatives on treatment with copper or silver salts. The preparation of cation exchange cellulose was examined by grafting of acrylic acid [146] and itaconic acid [147]. Wood cellulose grafted with polyacrylic acid or 2-acylamido-2-methylpropanesulfonic acid was reacted with hexadecyltrimethylammonium bromide to improve its hydrophobicity in order to utilize the graft copolymers as an oil absorbaent. This system was developed by Fanta and coworkers [148]. Scoured cotton was chemically modified to dialdehyde cellulose and hydrazinated dialdehyde cellulose by treating cellulose successively with aq. NaI0 4 , N 2 H4 , and H 20. Vinyl-grafted cellulose was subsequently prepared by graft copolymerization of methyl methacrylate on hydrazinated dialdehyde cellulose using K 2S 20 3 as the catalyst in a limited aqueous system. SEM studies of cellulose, dialdehyde cellulose and the vinyl-grafted fibers were carried out to get an idea of the changes brought about in the surface morphology of cellulose by different types and degrees of chemical modification. The orientational patterns of the fibrils of cellulose get somewhat blurred on oxidation and the fiber surface suffers some damage on hydrazination. Vinyl grafting causes deposition of vinyl polymer on the fiber surface thus leading to further damage and masking of the fibrils and their orientation pattern [149]. Graft Copolymers ofCellulose Derivatives. Modification of cellulose derivatives via graft copolymerization reaction has gradually gained popularity. An

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excellent source covering grafting on chemically modified cellulose up to 1980 was reviewed by Hebeish and Guthrie [70]. Several new graft copolymers of cellulose derivatives have been developed since then. It was reported that grafting levels of acrylonitrile on allylic-modified bleached cellulosic material were higher than graft polymer of acrylonitrile on the cellulosic material (ISO]. Polyglutamate and polyglutamine have been grafted onto cellulose or its acetate to afford blood-compatible polymers. Single amino acid residues have been grafted onto cellulose acetate. Vinyl acetate have been grafted onto cellulose acetate-and ethylene oxide-modified wood cellulose [70]. Dextran has been grafted onto carboxymethylcellulose [151]. Ultraviolet graft polymerization of arylamide onto cellulose acetate reverse osmosis membranes yielded grafted membranes with higher salt retention and lower water flux compared with pristine cellulose acetate [152]. Acid-catalyzed grafting of styrene on cellulose acetate reverse-osmosis membranes imparted a higher salt rejection rate (92.4%) to the membrane than those of ungrafted membranes (80.8%) and heat-shrunk membranes (90.2%) [153]. Cellulose acetate butyrate and hydrolyzable silanes were reacted at 80°C for 10 h to give a silyl group-modified CAB for coating application [154]. Kinetic data for the graft polymerization of acrylamide with hydroxyethylcellulose in the presence of a Co(III)-cyclohexanol redox initiating system are available [155]. Several graft copolymers were prepared based on the anionic polymerization method to overcome some of the major problems encountered in radicalinitiated grafting. This method is used to prepare a living synthetic polymer with mono- or dicarbanions to react with modified cellulosic substrates under homogeneous conditions. For example, polyacrylonitrile carbanion was prepared to react with cellulose acetate to generate a cellulose acetate-polyacrylonitrile graft polymer [142]. Well-defined tailored cellulose-styrene graft copolymers were also prepared by anionic polymerization. Preliminary bonding studies showed that these graft copolymers could function effectively as compatibilizers or interfacial agents to bond hydrophobic polystyrene to wood, evolving into a new class of composites [156]. Likewise, polystyrene was grafted onto cellulose with precise control over molecular weights and narrow molecular weight distribution. Crosslinked cellulose-grafted copolymers grafted with exactly defined polymer chain segments between crosslink points have been synthesized (142].

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Another unconventional technique of grafting with success is to graft a polymer with tenninating functional groups which are capable of reacting with hydroxyl groups in cellulose. For example, reaction of anhydride-tenninated polyisobutylene with sodium cellulosate gives polyisobutylene-grafted cellulose products in high graft yields (110%) and grafting efficiency [157). Likewise, cellulose is reacted with polybutadiene bearing succinic anhydride groups in dimethylfonnamide and N,N-dimethylbenzylamine between 90°C and 150°C through esterification. The grafted copolymer possesses high tack, which is suitable for surgical adhesive and skin barrier applications [158).

v.

FUTURE TRENDS

For the past 90 years, efforts were made to produce industrial products of cellulose with specific properties by means of chemical modifications. Today this activity continues to receive worldwide attention as evidenced by the many papers presented at various conferences, the voluminous patents, and the extensive journal literature. Although progress has been made, challenges remain. The advent of energy crisis, shortage of raw materials and the concern of environment have certainly drawn more attention to cellulose research and development. On the other hand, with the chemical industries coming under federal and state regulations on discharges of air, water, and solid wastes, much attention has to be directed to improving the effectiveness and efficiency of existing technologies and to produce cellulose derivatives that are environmentally acceptable. They must be low in cost and, to a greater extent, reusable and recyclable. It is suspected that more cellulose derivatives will be produced in solution or homogeneous systems. The success of this system has to depend heavily on nonpolluting, solvent-recyclable processes. An area of future expansion of research and development should be in the area of biomedical applications of cellulose derivatives. The future of cellulose looks good. The years ahead offer great challenges but also great rewards for chemists and material scientists with the vision to take advantage of cellulose's greatest asset, i.e., renewability, versatility, and adaptability. This gives mankind a future that is not locked into the everdwindling supplies of oil-based products.

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C. Feger and H.-J. Cantow, Polym. Bull. 3:407 (1980). T. Mezger and H.-J. Cantow, Angew. Makromol. Chern. l/6:13 (1983). V. Stannett, in Graft Copolymerization of Lignocellulosic Materials (D. N.S. Hon, ed.), ACS Symposium Series, 187:1-20 (1982). R. Narayan, in Cellulose and Wood: Chemistry and Technology ( C. Schuerch, ed.), John Wiley & Sons, New York, 1988, p. 945. N. Shiraishi, Chemtech 366 (1983). E. P. Vysotskaya, J. S. Gal'braikh, I. N. Andreeva, S. Kuznetsova, S. Yu, and E. V. Fronchek, Khim. Drev. 3:24 (1989). P. A. Dworjanyn, B. Fields, and J. L. Garnett, ACS Symp. Ser. 381:112 (1989). C. I. Simionescu, S. Brandisteanu, and D. G. Rumega-Stoianovici, Cellulose Chern. Technol. 17:687 (1983). K. Dimov, D. Dimitrov, E. Terlemezian, M. Semkova, and M. Bandova, Cellulose Chern. Technol. 14:665 (1980). G. F. Fanta, R. C. Burr, and W. M. Doane, in Renewable-Resource Materials: New Polymer Sources (C. E. Carraher, Jr. and L. H. Sperling, eds.), Plenum Press, New York, 1986, p. 107. P. Ghosh and J. C. Dalal, Indian J. Technol. 27:189 (1989). F. O. Agbonlahor, E. N. Ejike, A. Gbinije, F. E. Okieimen, and J. U. Otaigbe, Acta Polym. 40:723 (1989). D. J. Sikkhema, J. Appl. Polym. Sci. 30:3523 (1985). W. Yan, P. Yang, and Y. Wang, Shuichuli Jishu 12:213 (1988). W. Van, P. Yang, X. Zhang, and H. Zhang, Mo Xexue Yu Jishu 7:38 (1987). N. Yamakado, S. Kawakami, and H. Hata, Japan Kokai Tokkyo Koho JP 63, 254, 101. I. A. Nemchinov, V. A. Molotkov, Kurlyankina, V. I. Vysokomol, Soedin, Ser A. 31:123 (1989). R. Narayan, C. J. Biermann, M. O. Hunt, and D. P. Horn, ACS Symp. Ser. 385:337 (1989). S. Coleman-Kammula and H. Hulskers, in Cellulose and Cellulosics (J. F. Kennedy, G. O. Phillips, and P. A. Williams, eds.), Ellis Horwood, Chichester, England, 1987, pp. 195-202. D. N.-S. Hon and L.-M. Xing, Cellulose-Polybutadiene Copolymer: Properties and Applications Paper presented at the 200th ACS National Meeting, Cellulose, Paper and Textile Division, Washington, D.C., 1990. (Abstracts CELL15). S. A. Barker, H. C. Tun, S. H. Doss, C. J. Gray, and J. F. Kennedy, Carbohyudr. Rse. 17:471 (1971). Eastman Kodak, US Patent 1963, 3,075,962. Eastman Kodak, US Patent 1963, 3,075,963. Eastman Kodak, US Patent 1963, 3,075,964.

142. 143. 144. 145. 146. 147. 148.

149. 150. 151. 152. 153. 154. 155. 156. 157.

158.

159. 160. 161. 162.

Chemical Modification of Cellulose 163. 164. 165.

166.

167.

127

Eastman Kodak, US Patent 1963, 3,078,007. S. N. Ushakov, Fiz. -Mat. Nauk (USSR), 1:53 (1943). D. N.-S. Hon (ed.), Graft Copolymerization ofLignocellulosic Materials. ACS Symposium Series 187, American Chemical Society, Washington, D. C., 1982, pp. 381. D. N.-S. Hon, in Manmade fiber: Their Origin and Development (R. B. Seymour and R. S. Porter, eds.), Elsevier Applied Science, London, 1993, pp. 91-106. J. M. Cardamone, in Manmade fiber: Their Origin and Development (R. B. Seymour and R. S. Porter, eds.), Elsevier Applied Science, London, 1993, pp. 107-141.

5 Chemical Modification of Lignin John J. Meister University of Detroit Mercy Detroit, Michigan

I.

INTRODUCTION

Lignin (8068-00-6) is a natural product produced by all woody plants. It is second only to cellulose in mass of the natural polymer formed per annum [1]. Lignin constitutes 15-40% of the dry weight of wood with variation in lignin content being caused by species type, growing conditions, parts of the plant tested, and numerous other factors [2]. The data of Table 1 show the variation of lignin content by species type. Plants use lignin to (1) add strength and structure to their cellular composites; (2) control fluid flow; (3) protect against attack by microorganisms; (4) act as an antioxidant, a UV absorber, and possibly a flame retardant; and (5) store energy [3]. When considering the present and future use of this biopolymer, it is important to realize that any archeological age, such as the iron age, starts and ends before the participants realize it. We are currently at the end of the age of oil. The slow decline in available oil reserves during the early twenty-first century will make lignin a more important source of chemicals for our future society. When fundamental technology within a society changes, decades of work preceding the change must have occurred to develop new technologies to replace those that are obsolete. As the age of oil changes to the age of biomass, some of the chemical modifications described below will become important industrial processes for producing the chemicals and materials that society needs.

129

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130

Table 1 Chemical Composition of U.S. Woods as Detennined at U.S. Forest Products Laboratory from 1927 to 1968 Scientific name/common name A. Hardwoods Acer macrophyllum Purshlbigleaf maple Betula alleghaniensis Britton/yellow birch Carya cordiformus (Wangenh.) K. Koch/bitternut hickory Populus tremoides Michx.lquaking aspen Quercus falcata Michx.lsouthem red oak Quercus rubra L.lnorthern red oak Fagus grandifolia Ehrh.lAmerican beech Gleditsia triacanthos L.lhoney locust Liriodendron tulipifera L.lyellow poplar Populus delelOides Bartr. ex Marsh.lEastem cottonwood Salix nigra Marsh./black willow B. Softwoods Abies balsamea (L.) Mill./Balsam fir Larix occidentalis Nutt./Westem larch Picea glauca (Moench) Voss.lWhite spruce Pinus banksiana Lamb.lJack pine Pinus elliott;; Engelm.lSlash pine Pinus strobus L./Eastem white pine Sequoia sempervirens (D. Don) Endl./redwood Old growth Second growth Tsuga canadensis (L.) Carr.lEastem hemlock

Klason lignin

25 21 (2)25 19 (22) 25

24 22 (2) 21

20 23 (3) 21 (2) 29 27 29 27 27 27

(16)

(3) (8)

(27) (15)

(5)

33 33

33 (7)

IN umbers in parenthesis are number of independent detenninations for the component. In some cases, the trees are from different locations. Values are wt % contained in moisture-free wood. Data are from R. C. Petterson, ChemicaJ composition of wood, in The Chemistry of Solid Wood (R. Rowell, 00.), Advances in Chemistry Series, Vol. 207, Amer. Chern. Soc., 1984, p. 76, Table 3.

A.

Recovery of Lignin

For lignin to be used as the class of chemicals it is, it must be removed from the plant. As shown in Chapter 2 and emphasized above, the lignin produced by a plant is a species- and plant part-specific compound that has different composition and structure even within the same plant. Added to this diversity of repeat units and bonding patterns is the chemical alteration introduced by

Chemical Modification of Ugnin

131

each means of removing lignin from wood. Lignin recovery processes that extract lignin from wood change the chemical and functional group composition of lignin and make this material extremely heterogeneous. Methods for recovering lignin are the alkali process, the sulfite process, ball milling, enzymatic release, hydrochloric acid digestion, and organic solvent extraction. Alkali lignins are produced by the kraft and soda methods for wood pulping. They have low sulfur content « 1.6 wt %), contain sulfur contamination present as thioether linkages, and are water-insoluble, nonionic polymers of low (2000-15,000) molecular weight. Approximately 20 million Mg of kraft lignin is produced in the United States each year [4]. The sulfite process for separating lignin from plant biomass produces a class of lignin derivatives called lignosulfonates. Lignosulfonates contain approximately 6.5 wt % sulfur present as ionic sulfonate groups. These materials have molecular weights up to 150,000 and are very water-soluble. Less than 1 million Mg of lignosulfonates is produced in the United States each year and production is declining from year to year. Environmental restrictions are putting the sulfite pulp mills that produce Iignosulfonates out of business. Because they are a slowly disappearing commodity, lignosulfonates will receive little attention in the following discussion of chemical modification. Milled wood lignin (MWL) is produced by grinding wood in a rotary or vibratory ball mill. Lignin can be extracted from the resulting powder using solvents such as methylbenzene or 1,4-dioxacyclohexane [5]. Milling only releases 60 wt % or less of the lignin in wood, disrupts the morphology of lignin in wood, and may cause the fonnation of some functional groups on the produced lignin [6]. Despite these limitations, milling appears to be an effective way of recovering lignin from plants with only slight alteration. Enzymes that hydrolyze polysaccharides can be used to digest plant fibers and release lignin. After digestion, the lignin is solubilized in ethanol [7]. Extensive analytic studies support the idea that enzymatically produced lignin has undergone no major modification in removal from plant material [8-12]. Acid hydrolysis of the polysaccharide portion of wood releases lignin but also causes major condensation reactions (13] that remove many ether bonds in the lignin and replace them with carbon-carbon bonds. These reactions can be minimized by using 41 wt % hydrochloric acid in place of other mineral acids but some condensation reactions still occur (14]. This is not an effective method by which to obtain unaltered lignin. On the other hand, lignin can be solvent-extracted from wood at temperatures of 175°C using solvent mixtures such as 50:50 (by volume) water-l,4-dioxacyclohexane (15]. Changes in lignin under these conditions appear to be minor.

132

B.

Meister

Uses of Extracted Lignin

Outside of the plant, lignin is useful as a component of the diet of ruminant mammals; a soil property improver in the process of natural decay; and a source of peat, lignite, and coal. As a commodity forest product, however, it has a long history as a waste product for which functional uses are sought. This means that when a woody plant is rendered for its chemical content, about 25% of the dry weight of the plant has little or no economic value. For this reason, the most common use of lignin from pulping operations or ethanol from biomass processes is as a fuel. The lignin produced is burned for its 26.5 kJlg of energy, 40% of the solar energy [4] stored by the plant.

II.

MODIFICATION OF LIGNIN

Other uses for lignin can be broken into three large groups, two of which require chemical modification of the biopolymer. These groups are (I) breaking lignin down into component aromatics or repeat units, (2) using the biopolymer as extracted from the wood, and (3) adding to the lignin biopolymer, treating it as a starting material to be built on to make useful materials.

A.

Decomposition of Lignin

The decomposition of lignin into aromatic repeat units is a long practiced art that reached its zenith around 1800 A.D. Production of chemicals by wood pyrolysis was extensively practiced until, between 1750 and 1850 A.D., coal slowly displaced wood as the major chemical source available to man. Wood is usually pyrolyzed at 260-410°C and lignin at 300-440°C to produce 50 wt % charcoal, 10-15% tar, and lesser amounts of 2-propanone, ethanoic acid, and methanol [16-18]. The tar is often called wood creosote and is a complex mixture of substituted phenols and aromatics. It contains phenol, 2- and 4-methylphenol, 2,4-dimethylphenol, 2-methoxyphenol, 4-methyl-2methoxyphenol, and 4-ethyl-2-methoxyphenol [19). This technology has lead to lignin-based surfactants. A group at Texaco has shown that after retorting the lignin, the phenols can be ethoxylated to fonn nonionic surfactants that are both inexpensive and highly useful in industrial processes such as oil recovery [20-22). Alternatively, pyrolysis in a reducing atmosphere of hydrogen can be used to make cresylic acid in yields of 35 [23] to 40 [24] wt % of the lignin charged to the reactor. Cresylic acid is a mixture of alkyl phenols that boils between 180°C and 240°C. This process may be a future source of aromatic alcohols.

Chem~alModfflcauonofUgnm

133

This decomposition technology for lignin is emphasized in the literature as a major means of lignin utilization but it makes little thennodynamic or economic sense. With two fifths of the plants absorbed energy being used to make the one quarter of its dry mass that is lignin, lignin represents a large investment of biochemical effort by the plant. Reducing this macromolecule to CO2 or aromatic fragments destroys much of that investment. Keeping the molecule as extracted from the wood or adding to it are thennodynamically preferred approaches to lignin utilization, but these approaches face significant, practical barriers. Lignin is a deep brown, fluffy powder that can be thennofonned into hard, brittle solids when heated above its glass transition temperature. This transition from a brittle amorphous solid to a ductile thennoplastic occurs when lignin is heated above 90°C when it contains 13 wt % moisture or up to 195°C when it contains 0 wt % moisture. Lignin thus changes its properties sharply when relative humidity changes and is a brittle glass at common application temperatures, 20-25°C. Further more, the deep brown color is a product of free radicals in the lignin that if bleached away will slowly refonn and react with atmospheric oxygen [25]. This behavior can be a major drawback for applications of lignin to consumer products. Added to these difficulties are the variations in lignin produced by different sources and extraction processes and the chemical complexity already described above and in Chapter 2. Despite these difficulties, the enonnous amount of lignin available at low cost has driven numerous efforts to utilize it.

B.

Using Lignin as Extracted from the Plant

Using lignin in the fonn obtained when it is extracted from the plant does not mean that the lignin exists in the application exactly as it did when withdrawn from the plant. It means that the lignin enters the application process as a reagent and is often reacted with other components of the product as product is produced. This is definitely the case in the largest current application for unaltered lignin, its use as a replacement for phenol in phenolfonnaldehyde (methanal) adhesives.

1.

Lignin in Phenol-Methanal Adhesives

Phenol-methanal adhesives are currently used in about one tenth of all plywood and particle board. The binding technology represented by these "Bakelite" resins would be more widely applied if the reagents, particularly the phenol, were cheaper. Using lignin in place of phenol will sharply reduce the cost of the binder [26]. Unfortunately, lignin is not structurally equivalent to phenol.

134

Meister

Phenol has five free sites on the aromatic ring and no ortho or para substituents around the hydroxyl group. Lignin has only 35 phenolic hydroxyl groups per 100 C9 repeat units and 5 benzylic hydroxyl groups per 100 C9 repeat units. For virtually all lignin phenolic hydroxyl groups, the aromatic ring is parasubstituted by the propyl chain of the I-propylphen-4-01 (coumaryl) structural unit. In softwoods, the hydroxyl group is often next to a methoxyl group in the number 3 position on the ring, whereas in hardwoods it is completely ortho-substituted by methoxyl groups. This leaves only the meta position open for reactions on the aromatic ring of a lignin phenol. The implications of this structure on lignin reactivity in phenol/methanal crosslinking polymerizations can be seen from the mechanism of the phenol-methanal reaction, shown in Fig. I. In crosslinking with methanal, an aromatic hydroxyl group ionizes to fonn ortho (2,6) and para (4) anionic sites through which to react with a positively charged methylene group. Lignin has most sites ortho and para to its aromatic hydroxyl groups blocked by organic, functional groups. This is why lignin reacts more slowly with methanal than does phenol and why lignin can only be used to replace between 40 and 70 wt % of the phenol in an adhesive fonnulation. Lignin simply has too few highly reactive sites to create a high density of crosslinks without at least 30 wt % phenol being present. The rates of reactions detennined by Dr. Douglas Gardner are compared between hardwood, steam-exploded lignin; softwood, kraft lignin; and phenol in Table 2A [27]. The rate of the hardwood lignin/methanal reaction is, as would be expected from the dimethoxyl substitution on the ring, only 46% as fast as phenol at 30°C and only 12% as fast at 60°C. Softwood lignin has, under the same reaction condition.s, a rate that is 68% as fast as phenol at 30°C and 14% as fast at 60°C. The open ortho positions on softwood lignin obviously allow the softwood lignin to react more readily with methanal and should lead to more extensive crosslinking of the softwood lignin as compared to the hardwood lignin. This is confinned by the data of Table 2B. Here the number of methanal groups added to each C9 repeat unit of the two lignins is detennined by three different methods. The data show that hardwood lignin only reacts with 0.23 methanal units per C9 whereas softwood lignin reacts with 0.40 methanal units. With adjustments in composition to compensate for the chemical features of each aromatic hydroxyl source, a wood binder fonnulated with any of the materials discussed above [28] will be deemed highly effective if it can be (I) fonnulated at lower cost, (2) applied with conventional equipment, (3) reacted under the same process conditions, and (4) so strong an adhesive that

Chemical Modification of Ugnin

135

or

o

Na+

,1

HO

#

+

0 I

· - O~'--- 6 o

II

at. 0'

Na+ 0

II

I

01.08 +

Na+

~

~

6 I

01.08 +

~

Figure 1 Mechanism of the phenol-methanal reaction.

wood parts fonned with it fail in the wood phase most of the time and not in the adhesive phase. Adams and Schoenherr [29] achieved most of these benchmarks by fonnulating an adhesive consisting of a 40 wt % solids solution of kraft lignin in phenol-methanal-sodium hydroxide. This fluid had a viscosity of 10 Pals and thus was a very thick and energy-consuming adhesive to spread. However, when this binder was used in the manufacture of three ply panels of Douglas

Meister

136

Table 2 Kinetic Parameters for the Phenol-Methanal or Lignin-Methanal Reactions A: Component Rate constant k( 10 - 2 M - I min - I)

Temp.(OBBC)

Phenol Kraft lignin Steam-exploded lignin

30

40

50

60

2.17 1.44 0.98

6.65 1.83 1.60

24.9 5.50 4.37

79.4 11.3 9.44

Preexponential factor A (min-I)

Activation energy E.(kcal/mol)

5.8 x 101~ 3.25 x lOS

24.2 14.5

x 109

15.5

1.32

B: Degree of Methanal (HCHO) Substitution per C9 Unit of Lignin by Various Methods' Degree of methanal substitution Method

Kraft lignin

Steam-exploded lignin

HCHO uptake IH-NMR b I3C-NMR

0.39 0.38 (0.35) 0.42

0.25 0.18 (0.15-0.20) 0.27

'Data from Ref. 24. Fonnula for the rate of reaction, k, is k = A "Values in parentheses from Ref. 27.

* e-(E/R1).

fir, destructive testing of the plywood showed wood failure 92% of the time. A more easily applied adhesive can be prepared by blending 37 wt % lignin in phenol-methanal-sodium hydroxide [30] and only partially crosslinking the mixture. This blend has a viscosity of 0.46 Pals but sets into an adhesive layer under 1.2 MPa pressure for 6 min at 140°C that breaks in the wood phase 94% of the time. These data show that despite its chemical deficiencies lignin is a functional replacement for much of the phenol in Bakelite adhesives. Appropriately blended, lignin-containing adhesives will, under common treatment conditions for binding plywood or particle board, set into an adhesive that is stronger than the wood [31] and therefore capable of producing bonds that will be the last part of the structure to fail. As of 1991, lignin constitutes 17% of the resin solids in phenol-methanal adhesives used to make exterior grade plywood [32]. This technology is providing a stable market for the lignin fraction of wood. Growth areas of wood composites, oriented strand-

Chemical Modification of Ugnin

137

board, oriented waferboard, medium density fiberboard, and laminated veneer lumber will provide a growing market for lignin.

2.

Lignin Photostabilizers

DePaoli and Furlan studied the use of sugar cane bagasse as a photostabilizer for butadiene rubber [33]. The logic for this application is that the phenolcontaining repeat units of lignin have structures proximate to compounds currently known to act as photostabilizers in rubber. Hindered alkylphenols with long chains para to the hydroxyl group, structure (1) in Fig. 2, are known to inhibit photo-induced bond cleavage in rubbers by forming stable phenoxyl radicals [34]. This stable, hindered radical prohibits the formation of a peroxide radical on the rubber backbone, thereby preserving the structural integrity of the elastomer. Bagasse lignin contains approximately 2 wt % of structures (2) and (3) with the frequency ratio between them being 4 of structure (2) to 1 of structure (3). These structures are not only similar to those of common photostabilizers; these repeat units appear in a lignin chain. Polymer-bound, hindered phenols are more effective than free molecular phenols because the polymer chain restricts migration and dimerization of the formed radicals [35]. Bagasse lignin was tested as a mixture of 90 wt % lignin and 10 wt % N' ,N-bis(lethyl-3-methylpentyl)p-phenylenediamine in butadiene rubber. Diamines are commonly used in conjunction with hindered phenols to inhibit photodegradation in rubber. Rubber samples containing the lignin blend and commercial

~ VO

OH

CH3

OH

(1)

(2)

OH (3)

Figure 2 Base structure of photoinhibitor (1) and common bagasse lignin repeat units (2, 3).

Meister

138

stabilizers were irradiated at 350 ± 20 nm in air and rates of photodegradation were measured. The data showed that 0.37 wt % diamine could be replaced by 2.25 wt % lignin without affecting photostability of the blend. The lignin stabilized the rubber by both capturing radicals and absorbing the ultraviolet light directly. The effect on physical properties of compounding butadiene rubber with over 2 wt % lignin was not investigated. While these data are positive, they fail to verify that the photostabilized rubber possesses all of the application properties that the rubber must have to be used as a commercial product. Lignin has long been known to be an excellent reinforcing agent for rubber if the low molecular weight lignin and nonlignin constituents of industrial byproduct lignin are removed [36]. If contaminants are not removed from industrial lignin, they promote clustering of the lignin particles [37], lowering of the softening temperature of the rubber, and lowering of the reinforcing ability of the lignin. Unfortunately, the bagasse lignin used in the photostability studies was only 93 wt % lignin and was not fractionated to remove low molecular weight portions of the blend. Because of these deficiencies, the photostability data are of limited significance.

3.

Lignin Electrodes

Lignin pyrolyzed at 700°C under nitrogen forms a cohesive, conducting solid that can act as an electrode in a storage battery. This modification to dehydrogenate and deoxygenate the lignin forms a charcoal with the capacity to absorb or donate 6 mmol of electrons per gram [38]. Batteries have been formed from these electrodes and the cells have produced 45 W-hlkg per electrode at 70% efficiency (charge recovered/charge put in). While the W-h/kg rating of these electrodes is about two thirds of the value of a lead oxide plate, the lignin-based electrodes polarize rapidly and suffer a rapid drop in discharge voltage [39]. These two performance properties work against effective use of the electrodes. Furthermore, the redox capacity of carbon structures is quite limited when compared to that of metals, so that the utility of this modification has yet to be verified. However, since the internal structure and composition of these electrodes are unknown and, under current technology, controlled solely by pyrolysis conditions, there is extensive room for improvement of these biomass electrodes.

c.

Adding to the Lignin Biopolymer

1.

Halogenation

One of the simplest addition reactions to lignin is the addition of a halide to the alkylaromatic backbone of lignin. The reaction is shown in Figure 3.

Chemical Modification of Ugnin

139

2 ci

ct"

-

+

HCI

OCH3 OH

OH

+

cf

--

Figure 3 Fonnation of Halolignin.

The reaction is run by bubbling chlorine into spent pulping liquor and following that addition with additions of bromine and chlorine [40]. The wt % halide in the product is raised to between 20% and 40%. Since the halogenated alkyl aromatic is hydrophobic, it precipitates. Previously, the halogenated lignin has been recovered as a fire retardant for use in building materials and consumer goods. By 1994 standards, however, this halogenated organic presents very significant environmental problems for virtually any application. As a result, it is very improbable that this chemistry will be used today or in the future.

2.

Grafting by Free Radical Polymerization

Once lignin is separated from other plant products it can be grafted. Extensive studies on the modification of lignin by graft copolymerization have been made [41] because of the enonnous mass of kraft lignin produced each year by the pulp and paper industry. Graft copolymerization sharply changes the properties of lignin and allows useful products to be made from this underutilized portion of biomass [42]. Lignin has been grafted with ethenylbenzene [43,44] (styrene), 4-methyl2-oxy-3-oxopent-4-ene [45,46] (methylmethacrylate) , 2-propenamide (acrylamide), 2-propene nitrile [47] (acrylonitrile), cationic monomers, anionic monomers, and propenoic acid ethoxylates. An index of compounds listing structure, product name, and trivial name is given in Table 3. Two types of

Meister

140 Table 3

Trivial Names for Compounds and Copolymers

Name

Structure

Trivial name

Poly-I-phenyl ethylene

Polystyrene .( eR

Ethenylbenzene

Styrene

o

D'

CH3

I

Poly-I-( l-oxo-2-oxy propyl)-lmethylethylene

Poly methyl methacrylate

-CHr C -

I C=O

I O- CH 3

m 2-0xy-3-ox0-4methylpent-4-ene

Methyl methacrylate

I O-CH 3

I

C=O

Polyacrylamide

I NH 2

2-Propenamide

m

Acrylamide

eH [Poly-I-cyano ethylene

Polyacrylonitrile

H

2-

I

- ]

C=N m

2-Propenenitrile

I

C=O

CH 2 - CH Poly-l-amido ethylene

CH 2=c-CH 3

Acrylonitrile

Chemical Modification of Lignin

141

methods, radiation and chemical, have been used to attach side chains of these repeat units to lignin. The radiation methods have used both electromagnetic and particle radiation to produce grafting. Low-energy, electromagnetic irradiation based on visible or ultraviolet light relies on exciting or decomposing a particular bond either in lignin or in an initiator present in the reaction mixture. This method, photoinitiation, has not been used to graft lignin. High-energy radiation grafting using either electromagnetic or particle beams proceeds by ionization and excitation reactions that produce anionic, cationic, and free radical sites. Radiation-initiated, ionic grafting reactions have not been conducted on lignin and therefore only the free radical polymerization is known to contribute to grafts. Lignin is quite stable to ionizing radiation having a GR value of 0.6-0.718. This stability makes lignin a poor candidate for radiation grafting because, in the presence of neat monomer or in solution, initiation will occur far more readily to fonn homopolymer than it will to fonn graft copolymer. Homopolymer contains only polymerized monomer. Because of these deficiencies radiation grafting of lignin will not be discussed further. If additional infonnation on radiation grafting is needed, references are provided above to radiation grafting studies. Those are the references cited in the list of monomers grafted to lignin. The major deficiency of radiation grafting is production of homopolymer instead of graft copolymer. Some reduction in the amount of homopolymer produced can be achieved by initiating the grafting reaction by chemical methods. Indeed, Stannett showed that chemical initiation of grafting is over eight times more effective in converting monomer to graft copolymer than is radiation when both are applied to hydrochloric acid lignin. He notes [44] that "chemically initiated grafting at 60°C was more effective than radiationinduced grafting at room temperature." Chemical initiation can be applied in two ways. First, a reagent that attacks functional groups on the lignin backbone to produce a grafting site can be used as a grafting initiator. Alternatively, a reagent that reacts with lignin to fonn a reactable functional group is used to fonn a derivative of lignin. The added groups are structures such as peroxide or ethene bonds, which are then treated or reacted to initiate grafting. The chemical method can be used to initiate all polymerization reactions that do not require a solid supported catalyst [48], but only step and free radical chain reactions have been conducted on lignin. Unfortunately, for most of the products reported in the literature, neither of these methods has been used to make the material claimed to be a graft copolymer. If the polymerization reaction does not start off of the material to be grafted, then almost all of the monomer is polymerized into homopoly-

142

Meister

mer with no lignin in the chain. This is a "polymerization in the presence of' and it wastes the monomer that was supposed to be used in a grafting reaction. Thus, a key and often overlooked point in conducting grafting reactions is to ensure that the initiation of the polymerization occurs on the backbone to be grafted. This requires a special chemistry to initiate the polymerization. Polymerization methods without such chemistries merely make homopolymer. We have achieved the grafting of lignin-containing materials by developing an initiation system that preferentially attacks repeat units in lignin to create a site for polymer chain growth. The reaction appears general and works on almost all ethene monomers. By this reaction, we convert lignin to process polymers for industrial use or thermoplastics for use in consumer items. Figure 4 is a general diagram showing the materials reacted with lignin and the applications for the resulting products. A typical grafting reaction mixes monomer in nitrogen-saturated, organic or aqueous/organic solvent containing lignin, calcium chloride, and a hydroperoxide [49]. What appears to happen in this reaction is that the hydroperoxide and chloride ion react to form a chlorine atom. The chlorine then abstracts hydrogen from lignin to form the free radical site on the natural backbone and initiate polymerization. Chloride ion could act as a catalyst in this mechanism if the hydrochloric acid formed in the hydrogen abstraction later dissociates. My research group has developed this reaction chemistry over the past 10 years and shown that it almost quantitatively converts lignin to graft copolymer. We first synthesized, characterized, and tested a spectrum of watersoluble lignin copolymers that were nonionic, anionic, or cationic[50] and then showed that the water-soluble polymers were effective dispersing, flocculating, and surface-active agents. The nonionic polymers and their hydrolysis products are effective thinners and suspending agents for drilling mud formulations [50], as shown by the data in Table 4. These test samples compare polylignin-g-( I-amidoethylene)-r-( l-carboxylatoethylene), poly-( 1amidoethylene)-r-(l-carboxylatoethylene), and chrome lignosulfonate as aqueous drilling mud dispersants. The copolymer performs as well as the homopolymer and is more thermally stable than the lignosulfonate. The anionic polymer products are thickening agents for fluid flow control [51], as shown by the data of Table 5. The high limiting viscosity numbers of these copolymers cause rapid viscosity increase in water as a function of copolymer concentration. The cationic polymer products are dewatering aids for sewage treatment [52], as shown by the data of Table 6.

Chemical Modification of Ugnin

143 C~=C-CH:1

C~=CH

I

I

Thickeners Drag Reduction How Control Flocculation

2 - Met h y I Plastics C=Opropenolc: Composites Acid Methyl Mulch Elter C~=CH

C=o

I

I

I

~CH.J

NH 2 2-Propenamlde

Plastics Composites

<EN I-Cyanoethene

TC~=CH

h c

I

C~=CH

6

C=O

k

I

en

OH

e

I-Pbenyletbene Plastics C~=CH Composites

r 2·Propenolc:

CH2 =CH

I

I

c=o

NH- C=O

I

I

CH:1- C - C 1\-S O:J H

I

C~

(OCI\-C~)p-OH

C=O

I

+

p-Ethoxylate Ester or 2-Propenolc Add 2 N -( 2-Propena mA do )-2-MethylPlastics Propaae Sulfonic Add Cf or S<\C~· Urethanes Thickeners Foams Tr I met h y 1(" -ox a-3 -oxy he x -S-e ny I) Brine Tolerance Ammonium Chloride or Methylsulfate Flocculation Sewage Treatment Dewatering O-(Cl\h-N-(ClIJh

Figure 4 Modification of lignin.

The ethoxylate esters of propenoic acid are useful as prepolymers for urethane formation but these materials are not as effective as those prepared by Wolfgang Glasser's research group. Therefore, only Dr. Glasser's products will be discussed. In reactions with ethenylbenzene, lignin was used to make thermoplastic materials. Data for a spectrum of reactions run to optimize yield and create samples of different molecular weights and composition [53] are given in Table 7. Samples PE-I to PE-I 0 were made with ethenylbenzene as monomer. These products have been shown to be polylignin-g-(l-phenylethylene)-containing materials by a series of solubility and extraction tests and are formed

Meister

144 Table 4

Properties of Test Muds Before and After Hot Rolling·

Base mud Property

Before

After

Graft copolymer fraction Before

After

Viscosity in centipoise at a shear rate of: 1020 S-I 74 69 42 52 510 S-I 47 49 24 36 340 S-I 29 40 30 17 170 S-I 28 27 21 10 Gel strength in Ib/l00 ft 2 mud has set for: 12 5 3 6 10 sec 35 20 3 10 min. 25 Apparent viscosity (cp) 26 37 21 33 Plastic viscosity 22 (cp) 16 25 18 Yield point 25 24 (lb/l00 ft 2) 20 6 API filtrate volume (ml) 12.0 13.8 7.8 8.0 9.1 8.6 pH 9.0 8.0 High-pressure, high-temperature filtrate (ml) 62.8 52.4

Homo polymer fraction

Chrome lignosulfonate

Before

After

Before

After

74 49 40 27

43 24 17 10

29 16 12 7

50 33 26 18

5 20

2 3

2 9

11 24

37

22

15

25

25

19

13

17

24

5

3

16

7.8 9.0

8.4 8.0

11.6 9.5

46.0

14.0 8.2 64.0

·Hot rolling is agitating the mud at 121°C for 16 h.

with 90% or more grafting efficiency for lignin. These materials have been shown to be thermoplastic [53], coupling agents for wood and plastic [54], and biodegradable plastics [55]. The thermoplasticity of the graft copolymers can be verified by measurements of the glass transition temperature of the new solids. The glass transition temperature is the temperature at which an amorphous solid becomes ductile and is a characteristic of thermoplastic materials. Samples of 5-10 mg of reaction product were heated at 10°C/min in a differential scanning calorimeter to monitor heat capacity as a function of temperature. The temperature of each transition produced by each copolymer is shown in Table 8. These products also occupy surfaces on wood and act to alter the wetting

ChemicalModfflcationofUgnm Table 5

145

Synthesis Data and Physical Characteristics of Graft TerpolymerA Reactants

Sample 2-propen amide (g) Number I

2 3 4 5 6 7 8 9 10

1.60 1.60 1.60 1.60 1.60 1.60 1.60 2.58 2.56 21.98

A (g) 4.66 4.66 4.66 4.66 5.16 5.16 4.66 1.87 1.86 15.99

Dimethyl sulfoxide CaCl z B (ml) (g) (g) 20 50 50 40 30 30 30 30 30 219

0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.53 4.35

0.15 0.15 0.25 0.40 0.15 0.15 0.15 0.39 0.39 3.35

Yield (wt %)

[ "h ] (dllg)

70.12 86.98 78.40 69.82 78.79 77.27 87.28 67.89 79.49 91.02

10.52 11.40 7.40 9.30 12.59 6.81 10.46 .953 2.46 1.97

IAII reactions, save #10, contained 0.50 g of lignin and 0.15 ml of cerium (+4) ion. Reaction # 10 contained 4.39 g of lignin and 1.28 ml of cerium ion. A = 2,2-dirnethyl-3imin0-4-oxohex-5-ene-I-sulfonic acid. B = Hydroperoxide. Samples 1-7: Values are weight of 1,4-dioxa-2-hydroperoxycyclohexane in g. Samples 8-22: Values are amount of aqueous solution of 1,2-dioxy-3,3-dimethylbutane in ml. EquivalenVml = 7.23 x 10- 3 .

properties of the plant material. The lignin and copolymer solutions gave smooth, adherent surface coatings on the wood with contact angles against water of 90-1100 whereas plastics and plastic-lignin mixtures did not give adherent coatings. These data show that copolymers of lignin are surfaceactive, preferentially orienting the lignin portion of the product toward wood while the plastic sidechain is oriented outward to create a new surface with different wetting properties. Thus, these copolymers are surface-active coupling agents that can bind wood to hydrophobic phases such as plastic. This coupling process works best when the wetting agent has been synthesized so that the side chain attached to the lignin during the preparation of the macromolecular surface-active agent is chemically identical to the plastic hydrophobic phase that is to be bound or connected to the wood. Thus, to bind poly-l-phenylethylene (trivial name = polystyrene) to wood, coat the wood with polylignin-g-( I-phenylethylene), and to bind poly-l-cyanoethylene (trivial name = polyacrylonitrile or orion) to wood, coat the wood with polylignin-g-( l-cyanoethylene). The coupling of wood by a grafted plant part to a plastic that has the same

Meister

146

Table 6 Synthesis and Application Data for Cationic Graft Copolymer A: Synthesis Data Reactant weight (g) Sample number ! 2 3 4

E

Yield

Lignin

CaCl 2

A

C

DMSO

(m!)

%

0.50 0.50 0.50 0.50

0.93 0.99 1.02 1.07

1.92 1.29 0.64

5.15 8.36 10.21 12.77

30.72 29.28 31.40 30.71

0.50 0.50 0.50 0.50

80.18 70.44 91.46 70.91

8: Dewatering Data of Cationic Lignin Graft Copolymer with Methylsulfate Anion Filtrate volume after being on the filter for the given number of minutes Sample number Blank 1a Ia

3 4 Comm Pol.

Concentration in sludge (ppm) 00.0 300 450 150 150 150

35 30

2

3

4

45 45

55 55

60 65

5

10

<10 70 70 28 52 54

10

46 74

nb

acloudy filtrate. bperformance data for the commercial polymer currently in use at the Detroit Water and Sewerage Works. A = 2-propenamide. C = Cationic monomer. 2-methyl-N7 ,N7-trimethyl-7-ammonium-3-ox0-4-oxyoct-I-ene chloride or 2-methyl-N7 .N7-trimethyl-7-ammonium-3-ox0-4-oxyoct-I-ene methyl sulfate. E = 30% hydrogen peroxide (equivalent weight: 8.383 meq/ml).

or similar composition as the side chain on the plant part increases the binding strength of the plastic to the wood. This was proven by performing lap shear tensile strength tests on birch strips onto which were injection-molded blocks of plastic. The samples with a grafted product coating with the same repeat units in the side chain as in the plastic gave 20-50% higher tensile strength. The coupling experiments were performed as follows. Birch tongue depressors, a medical product of 1.75 mm thickness, were cut into suitable

Chem~aIModmca~onofUgnm

Table 7

147

Copolymerization Reactions of Lignin and Ethene Monomers Reactants (g)

Sample number PE I PE 2 PE 3 PE 4 PE 5 PE 6 PE 7 PE 8 PE 9 PE 10 CY-AM I CY-AM 2 CY-AM 3 CY-AM 4 CY-AM 5 CY I CY 2 CY 3 CY 4 CY 5 MBuD I MBuD I MBuD I MPrPe I MPrPe 2 MPrPe 3 MPrPe 4 MPrPe 5 MPrPe 6 MPrPe 7 MPrPe 8 MPrPe 10

Lignin

Monomer

CaCI 2

H 20 2 (ml)

Solvent

Yield (glwt %)

2.00 2.01 3.03 2.00 2.01 2.01 2.01 2.02 8.00 8.04 4.10 0.50 0.50 0.50 0.50 4.10 3.95 4.05 4.02 4.00 4.11 4.01 3.98 4.00 4.03 3.97 4.00 4.05 8.00 8.1 7.99 2.00

18.76 18.78 18.78 18.76 4.69 9.39 14.07 23.45 28.15 18.76 9.35/0.0a 0.35/4.17 0.78/3.74 1.19/3.33 0.79/3.75 9.35/0.0 6.08/0.0 3.13/0.0 6.15/0.0 6.16/0.0 9.32 3.14 6.27 17.17 11.57 6.37 11.78 11.53 6.1 2.41 17.52 18.07

2.02 2.02 2.00 1.01 2.04 2.02 2.03 2.04 8.00 8.00 3.05 0.5 0.5 0.5 0.5 3.05 3.07 3.02 3.05 2.51 3.02 3.05 3.07 3.03 3.09 3.06 2.98 2.57 3.03 3.08 3.11 2.06

1.0 5.0 2.0 2.0 2.0 2.0 2.0 2.0 8.0 8.0 4.00 0.53 0.53 0.53 0.53 4.0 4.0 4.0 4.0 5.0 4.0 4.0 4.0 4.48 4.48 4.48 4.48 5.60 4.70 4.48 8.96 4.48

20.04 20.02 20.00 20.10 20.01 20.00 20.10 20.07 40.02 40.03 20.04 28.91 28.91 28.91 28.91 20.04 20.07 20.57 25.02 20.01 20.29 20.32 21.36 20.04 20.11 20.27 20.06 20.35 30.42 30.21 30.23 20.11

17.80/85.7 18.53/89.1 19.14/87.8 18.84/90.8 5.68/84.8 10.42 / 91.4 14.95/92.8 23.76/93.3 33.16/91.7 24.14/90.1 13.86 / 103.0 4.82/96.4 4.91 / 97.8 3.50/69.7 3.28 / 65.1 13.86 / 103.0 9.83/98.0 6.34/88.3 10.14/99.7 9.29/94.3 4.21/31.3 4.39/61.4 3.71/36.2 19.54/92.30 15.29 / 98.01 9.69/93.71 16.2 / 102.67 15.69/ 100.71 13.53 / 95.96 9.86 / 93.82 24.22 / 94.94 20.63 / 102.79

·First number = weight of 2-propenenitrile added and second number added.

= weight of 2-propenamide

Meister

148 Table 8

Differential Scanning Calorimetry Data for Lignin, Poly- I-phenylethylene, and Graft Copolymer

Sample number Amoco RIPO (pure poly-lphenylethylene) Kraft pine lignin (pure lignin) Copolymer I Copolymer 2 Copolymer 3 Copolymer 4 Copolymer 5 Copolymer 6 Copolymer 7

Lig. % in Reaction

0

Lig. % in A

Peak(sOC)

Ramp OC/min.

102.6

10

116.17-130.09

10

0

100.0

100.0

9.6 22.0 30.0 30.0 30.0 46.0 46.0

10.3 27.3 32.2 34.5 32.3 50.5 51.8

94.82 98.43 98.23 102.35 95.73 94.11 101.63

(114.62)8 (133.97)8 (l24.IO)b (I 44.48)b 133.25 125.12 143.27

10 10 10 20 10 10 20

A = copolymerization reaction product.

8Yery small peale. bSmall peak.

sizes to match an injection mold. Kraft pine lignin was reacted into a graft copolymer as previously described. The homopoly-I-phenylethylene used in coupling tests is a recovered fraction of the reaction product of mechanical pulp and ethenylbenzene. Coatings of copolymer or any comparison mixtures or "control" blanks were prepared as a 10 wt % solution in dimethylfonnamide and spread on the wood. The plastic phase was Amoco RIPO (Amoco Chemical Company). Injection molding was done on a Milberry Model 50 Mini-lector. Experimental conditions were as follows: cylinder temperature, 288°C; nozzle temperature, 172-176°C; pressure, 3450 kPa; pressure holding time, 12 s; and chilling time, 1-2 min. Lap shear strength of the pieces of wood with plastic injection molded to them was tested on a Instron Model 4200 universal testing instrument. Experimental conditions were as follows: room temperature, 23°C; room relative humidity: 50%; crosshead speed, 2.54 mm/min; with the sample in hand-fastened grips and an aluminum specimen holder. The lap shear strengths of the wood-plastic samples are summarized in Table 9. The copolymer samples were fractionated by benzene extraction. The reaction product was labeled product A. The benzene extract of the product became "ben. ex." Whereas the benzene-insoluble portion of the product was labeled product B.

Chemical Modification of Ugnin Table 9

149

Summarized Adhesion Strength Results

Coating material 3O-151-2A (10.45% lignin) 30-151-2B 30-151-2 ben. ex. 10% Lig. + 90% PS 35-120-1A (24.23% lignin) 35-120-1B 35-120-1 ben. ex. 24% Lig. + 76% PS 35-11O-3A (32.17% lignin) 35-11O-3B 35-110-3 ben. ex. 32% Lig. + 68% PS 35-115-3A (51.70% lignin) 35-115-3B 35-115-3 ben. ex. 50% Lig. + 50% PS Poly(l-phenylethylene) (PS) 10% Lig. + 90% PS 24% Lig. + 76% PS 32% Lig. + 68% PS 50% Lig. + 50% PS Lignin Blank (treated with DMF) Blank (treated with nothing)

Adhesion strength (kPa) 2422.1 2313.9 2126.4 2209.1 2313.9 2278.7 2094.0 2027.1 1930.5 1911.2 2670.4 1949.2 2838.6 2723.4 2707.6 1843.0 2040.9 2209.1 2027.1 1949.2 1843.0 2123.6 2022.2 1825.7

± 219.3 (3)' ± 488.2 (3) ± 44.1 (3) ± 251.0 (5) ± 81.4 (3) ± 294.4 (2) ± 213.7 (3) ± 185.5 (4) ± 304.1 (3) ± 184.8 (3) ± 207.5 (3) ± 265.4 (5) ± 60.0 (3) ± 328.9 (3) ± 70.3 (3) ± 91.0 (5) ± 206.8 (5) ± 251.0 (5) ± 185.5 (4) ± 265.4 (5) ± 91.0 (5) ± 164.1 (5) ± 120.7 (2) ± 165.5 (4)

'Number of valid repetitions of the tensile strength test is in parentheses.

In almost all cases, coating the wood with any of the three fractions of the graft copolymer of lignin and ethenylbenzene (product A, product B, and product ben. ex.) provides stronger adhesion between wood and commercial poly-l-phenylethylene than coating the wood with mechanical mixtures, pure poly-l-phenylethylene, pure lignin, or nothing (blank). The data show the copolymers to be effective coupling agents. White rot basidiomycetes were able to biodegrade graft copolymers of lignin and ethenylbenzene containing different proportions of lignin and polyl-phenylethylene. The biodegradation tests were run on ligninlethenylbenzene copolymerization products that contained 10.3, 32.2, and 50.4 wt % lignin.

Meister

150

The polymer samples were incubated with white rot, lignin-degrading organisms Pleurotus ostreatus, Phanerochaete chrysosporium, Trametes versicolor, and brown rot, cellulose-degrading organism Gleophyllum trabeum. Over a 68-day period, white rot fungi degraded the plastic samples at a rate that increased with increasing lignin content in the copolymer sample. Both poly-I-phenylethylene and lignin components of the copolymer were readily degraded. Pure poly-I-phenylethylene pellets were not degradable in these tests. Observation by scanning electron microscopy of incubated copolymers showed a deterioration of the plastic surface. Brown rot fungus did not affect any of these plastics. White rot fungi produced and secreted oxidative enzymes associated with lignin degradation in liquid media during incubation with lignin-poly-I-phenylethylene copolymer. All of these applications represent significant markets for modified lignin. In examples Cy-Am 1-5 of Table 7 the monomers used were 2-propenenitrile (107-13-1) and 2-propenamide (79-06-1). In examples Cy 1-5 of Table 7, the monomer used was 2-propenenitrile (107-13-1). The compounds in the first group are water-absorbing agents whereas those in the second group are thermoplastics and biodegradable plastics [56]. In examples MBuD 1-3 of Table 7, the monomer used was 2-methyl-I,3-butadiene (78-79-5). These materials are uncrosslinked elastomers and potential rubber additives. In examples MPrPe 1-9 of Table 7, the monomer used was 2-methyl-2-oxy-3oxopent-4-ene (80-62-6). These materials are thermoplastics and biodegradable plastics [57].

3.

Grafting by Anionic Chain Polymerization

Lignin can be grafted with alkane epoxides to form polyether adducts and this reaction has been extensively studied by the research group of Dr. Wolfgang Glasser at Virginia State University and Polytechnic Institute. The reaction is initiated by base attack on the phenolic hydroxyl groups to form phenoxide groups. The phenoxide groups then attack the polar epoxide ring. A significant side reaction in this ring opening polymerization is a nucleophilic attack on the ring by hydroxyl anion. These reactions are shown in Fig. 5. Alkoxylation is critically important as a precursor reaction that changes both the physical properties of the lignin and its chemistry. The most important physical property changes is the glass transition temperature of the lignin. As the weight fraction of ethylene or propylene oxide in the product increases, the glass transition temperature of the product falls. Since a reduction in Tg is synonymous with a lowering of viscosity at any temperature and a lowering of the temperature at which flow starts, this change makes lignin a flowable

Chemical Modification of Ugnin

A1.

Q+ Oa

151

~+

OH"

b'~

Ott

A2.

~

°

B. Figure 5

0

26

CtiJ +

H2 O

6

/0......... .CH2-CH2 •

6

OCH 3

·C~· 0·

Ott-

Alkoxylation of a lignin repeat unit.

liquid at temperatures lOO-200°C below the usual temperatures for lignin viscous flow. Simultaneously, Fig. 5 shows that alkoxylation leaves lignin "capped" with chains ending in primary hydroxyl groups. These alcohols are more reactive in some chemistries than the phenolic hydroxyl groups that they have capped. A particularly important example of one such chemistry is the reaction of the alkoxylated lignin with an organic isocyanate or diisocyanate. The reaction of the alkoxylated lignin with an isocyanate-tenninated polymer allows Glasser et al. to add polymeric side chains of cellulose [58] or polycaprolactam [59] to the lignin graft copolymer. Reactions of alkoxylated lignin with diisocyanates produce thennoset materials because the lignin polyol is always polyfunctional with a functionality greater than 2. The isocyanate-alcohol reaction produces a urethane linkage that when repeated creates a crosslinked, nonrefonnable polyurethane. This is shown in Fig. 6. A broad spectrum of lignin-based urethanes have been made and tested. The data show that these materials match if not exceed the properties of synthetic polyurethanes made without lignin [60]. The alkoxlyated lignin requires an isocyanate to hydroxyl group ratio of 1.5 to fonn effective networks. This is a significantly higher ratio of expensive isocyanate to hydroxyl units than is used in commercial practice. The work

Meister

152 (8)

- Lignin - CH2 - CH2 - OH +

OCN - R - NCO

~

o - Lignin - CH 2 - CH 2 - 0 -

(b)

~ - NH -R - NCO

Lignin -[ (CH2 - CH2 - O)n H ]m +

m • - Lignin - CH 2 - CH2 - 0 -

0

I~ - NH -R - NCO

Crosslinked Network Figure 6

Reaction of ethoxylated lignin with a diisocyanate.

of Glasser et al. shows that the T g of a alkoxlyated lignin-diisocyanate network depends on the weight percent of lignin in the network, the chemical structure of the alkoxide, and the chemical structure of the diisocyanate [60]. As lignin content increases, the T g of the network increases. Networks made with oxyprop-l ,2-ylene repeat units, -OCH 2CH(CH 3)-, have lower Tg values than networks made with oxyeth-I ,2-ylene repeat units when the diisocyanate used in the network is 1,6-diisocyanaohexane, an alaphatic diisocyanate. When the diisocyanate is aromatic, however, there is no difference between the Tg values of the materials made with oxypropyl or oxyethyl repeat units. Networks made with aliphatic diisocyanate generally have lower Tg values than networks made with aromatic diisocyanate. The mechanical properties of these networks, modulus of elasticity and ultimate strength, are dependent on the method of preparing the network (60]. While the chemical composition and repeat units of the network are important factors that influence the properties of the crosslinked product, the most important influence on physical properties of the network is the way the polymeric alkoxide chain is introduced into the network. Products made by the addition of a pure polyether that has not been grafted onto lignin have higher modulus of elasticity and ultimate strength than products made with a1koxylated lignin copolymer. This difference in physical properties probably reflects the steric hindrance and reduced diffusion produced by grafting a

Chemical Modification of Ugnin

153

polyether onto lignin. The copolymer produces a less uniform distribution of crosslinks and a greater segregation of network parts than does the blend of pure polyether with the other reagents. A key feature in forming this material that is not widely recognized outside of the polyurethane industry is that the reactants must be fluids at 60°C, the common temperature for conducting this reaction. This is why the reduction in glass Tg with increasing degree of alkoxylation in lignin is so important. By decreasing the temperature at which modified lignin will flow, the alkoxylation reaction allows lignin to not only be a uniformly reactive, poly primary alcohol but also a fluid polyol that can be processed by procedures such as reaction injection molding into thermoset solids. The prospects for application of this chemistry to foams, networks, and consumer goods is great because the materials match the properties of existing products, the area of application is high-profit-margin specialty chemicals, and lignin sharply reduces cost by replacing more expensive polyol.

III.

CONCLUSIONS

Utilization of lignin will increase in the next quarter century as the demand for aromatic carbon exceeds the supply available from a decreasing inventory of oil. Lignin is a more homogeneous material than petroleum that can provide a relatively uniform supply of alkyloxyaromatics for decomposition, utilization as extracted from the plant, and use as a backbone to create larger, altered polymers. The current markets for lignin, as a moisture retention agent in cement grouts, a dust suppressor for road treatments, or a thickening agent in inks, are very small but stable. The market that not only has the highest rate of growth in 1994 but also promises the largest increases in utilization of lignin is that which makes use of lignin as an adhesive in wood composites. Lignin constitutes 17 wt % of the solids in most exterior grade plywood and will become a progressively larger fraction of the binder in laminates and fiber, strand, or wafer board. Lignin has a potential to become a functional photostabilizer and free radical trap because of its high molar absorptivity at UV wavelengths below 300 nm and its ability to trap and maintain free radicals. However, these applications will require careful formulation of a product that will include a chemically altered lignin in place of the product that can be extracted from the plant. Thermal or chemical decomposition of lignin to produce oxyaromatics will grow when the depletion of low-cost petroleum becomes pronounced between 2015 and 2020 A.D. Examples of this technology would be nonionic surfac-

154

Meister

tants containing retorted, ethoxylated lignin or cresylic acid produced by pyrolysis. Although this is a market for a small mass of lignin, it is a highprofit-margin market with specialty chemical applications stretching from adhesives to zeolite manufacture. Modified lignin may be used in the commodity chemical market if it is graft-copolymerized. Copolymerization of lignin with polar monomers to create process polymers will pennit the use of lignin in water treatment, sewage dewatering, thickening, and dispersion. These nonionic, anionic, and cationic water-soluble polymers will be industrial process polymers that allow the production of consumer and manufactured products. Copolymerization of lignin with nonpolar monomers to create thennoplastics will pennit the use of lignin in the commodity plastics market. These materials are biodegradable thennoplastics, thennoplastic composites, and coupling agents to incorporate wood and plastic into a single phase. The alkoxylation of lignin will pennit its use in processes requiring a fluid reagent for further modification. The largest application for this new material will be the production of urethane solids and foams from the reaction of the alkoxylignin with a diisocyanate. This high-value material will be used in engineering plastic or as insulation. Alkoxylated lignin is also useful as an intennediate to be grafted into new materials by reaction with step reaction polymers tenninated with a group that creates covalent bonds with hydroxyl groups. While carboxylic acid-tcnninated step polymers can be reacted with alkoxylignin to fonn alkoxylignin esters, thennodynamic and equilibrium forces dictate that the best products from this chemistry will come from step reaction polymers capped with isocyanate groups. New products of alkoxylated lignin covalently bonded to cellulose acetate or caprolactam are known and are available for utilization.

ACKNOWLEDGMENT Portions of the testing of the graft copolymers described here were supported by the U.S. Department of Agriculture under Grant No. 89-34158-4230, Agreement No. 71-2242B and Grant No. 90-34158-5004, Agreement No. 61-4053A. Additional support of property testing of the grafted products was provided by the Center of Excellence in Environmental Engineering and Science of the University of Detroit Mercy.

Chemical Modification of Ugnin

155

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21.

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32. 33. 34. 35. 36. 37. 38. 39.

40. 41. 42.

43. 44. 45. 46. 47. 48. 49. 50. 51.

Meister H. H. Nims, in Lignin-Based Wood Adhesives (A. Pizzi, ed.), Marcel Dekker, New York, 1983. D. J. Gardner and G. D. Mcginnis, J. Wood Chem. Tech., 8 (2):261-288 (1988). P. C. Muller and W. G. Glasser, J. Adhesion 17:157 (1984). J. W. Adams and M. W. Schoenherr, U.S. Patent 4,306,999 (1981). J. W. Hollis and M. W. Schoenherr, U.S. Patent 4,303,562 (1981). D. J. Gardner, Selected Investigations into the Chemistry and Utilization of Biomass Lignins, Ph.D. thesis, Forest Products Laboratory, Mississippi State University, Columbus, MS, DA8609771 T. Sellers, Jr., Panel World 31(5):26-29 + 44, 1991. M. A. DePaoli and L. T. Furlan, Polym. Degrad. Stabil 11:327-337 (1985). M. A. DePaoli, G. W. Schultz and L. T. Furlan, J. Appl. Polym. Sci. 29: 2493 (1984). K. W. S. Kularatne and G. Scott, Eur. Polym. J. 14:835 (1978). J. J. Keilen, W. K. Dougherty, and W. R. Cook, Ind. Eng. Chem. 44:163 (1952). Y. Lin, Progress in Biomass Conversion, Vol. 4 (D. A. Tillman and E. C. Jahn, eds.), 00.31-78, Academic Press, 1983. A. K. Roy, Indian Pulp Paper (India) 28(2):10-13 (1983). F. Shalizadeh, K. V. Sarkanen, and D. A. Tillman (eds.), Thermal Uses and Properties of Carbohydrate and Lignins, Academic Press, New York, 1976 p. 217. E. Zeigerson and M. R. Block, U. S. Patent 3,962,208 (1976). David N. Hon (00.), Graft Copolymerization of Lignocellulosic Fibers. ACS Symposium Series #187, Am. Chern. Soc., 1982, p. 187. Chem. Eng. News 62 (#39): 19-20 (1984). T. Koshijima and E. Muraki, Zairy O. 16 (169):834-838 (1967). R. B. Phillips, W. Brown, and V. T. Stannett,J. Appl. Poly. Sci. 15: 2929-2940 (1971). T. Koshjima and E. Muraki, Nihon Mokuzai Gakkaishi 10(3): 110-115 (1964). Tetsuo Koshijima and Einosuke Muraki, Nihon Mokuzai Gakkaishi 10(3):1l6-119 (1964). Cr. Simionescu, A. Ceratescu-Asandei, Stoleru, Cellul. Chem. Tech. 9(4):363-380, (1975). Robert W. Lenz, Organic Chemistry of Synthetic High Polymers, Interscience, New York, 1967, pp. 161-172,718. J. J. Meister and D. R. Patil, Macromolecules /985:1559-1564. J. J. Meister, D. R. Patil, and H. Channell, Ind. Eng. Chem. Prod. Res. Dev. 24(2): 306-313. (1985). J. J. Meister, D. R. Patil, and C. Augustin, J. Z. Lai, in Lignin: Properties and Uses (Simo Sarkanen and Wolfgang Glasser, eds.), ACS Symp. Series #397, Washington, D.C., 1989, pp. 294-311.

ChemicalModfflcaffonofUgnm 52.

53. 54.

55. 56.

57.

58. 59. 60.

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J. J. Meister, C. T. Li, K. K. Tewari, and S. Simoliunas, Spring National Meeting of the American Chemical Society, April 22-27, 1990 at the Symposium on the Chemical Modification of Biopolymers, Boston, MA. ACS Abst. 199: 418 (1990). J. J. Meister and M. J. Chen, Macromolecules 24(26):6843-6848 (1991). D. W. Gunnells, D. J. Gardner, M. J. Chen, and J. J. Meister, Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering, 67, 227, (1992). ACS Abstracts: 204, August, 1992, pp 125. Presented at the Fall Meeting, 1992, Washington, D.C. O. Milstein, R. Gersonde, A. Hutterrnann, M. J. Chen, and J. J. Meister, App. Environ. Microbiol. 58(10):3225-3232(1992). J. J. Meister, A. Aranha, and A. Wang, Proceedings ofthe American Chemical Society, Division of Polymeric Materials: Science and Engineering, submitted 5/15/93, Chicago, August 23-26, 1993. J. J. Meister, M. J. Chen" Y. Lou, A. Aranha, and Z. Zhao, Bio/Environmentally Degradable Materials Society, Second International Meeting, Chicago, August 19-20, 1993, submitted 4/30/93. V. Demaret and W. G. Glasser, Polymer 30 (3):570-5(1989). W. DeOliveira and W. Glasser, Polym. Prepr. 31 (1):655, (1990). S. S. Kelley, Incorporation of lignin copolymers into polyurethane materials, Ph.D. thesis, Virginia Polytech. Inst. State Univ., 1987, Univ. Microfilms Int., #DA8814587.

6 Chemical Modification of Solid Wood Hideaki Matsuda

Okura Industrial Co., Ltd. Marugame, Kagawa-ken, Japan

I.

INTRODUCTION

Most of the research in the area of chemical modification of wood was conducted for improving either its dimensional stability or its biological resistance. Wood is made up primarily of cellulose, hemicellulose, and lignin. Originally, chemical modification of wood was a chemical reaction between some reactive part of a wood component and a simple chemical reagent to form a covalent bond between the two [1]. The most abundant reactive sites in wood are the hydroxyl groups. The major important types of covalent bonds formed by chemical modification of wood are ethers and esters. The treated wood must still possess the desirable properties of wood. Past research work on the chemical modification of wood was reviewed extensively in 1975 [1] and 1984 [2] by Rowell. On the other hand, as in one recent notable chemical processing of wood, there is a method in which wood as a whole is chemically modified, with a view to providing properties not observed in the original wood. In this method, it was found that wood can be made thermally meltable by appropriate esterification or etherification. Further, by efficiently introducing active functional groups into wood and utilizing the introduced functional groups, plastic-like crosslinked woods having properties not observed in the original wood could be obtained. Thus, advantageous methods for thermoplasticization of wood

159

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for composites were found. Shiraishi et al. [3-7] reviewed extensively the thermoplasticization of wood. This chapter deals with the chemical modification of wood by such methods as etherification, esterification, and thermoplasticization of wood, with emphasis on recent and new research in these fields.

II.

ETHERIFICATION

Etherification of wood can be conducted by reacting wood with alkyl halides, acrylonitrile (AN), epoxides, J3-propiolactone (acid conditions), aldehydes, and dimethyl sulfate [2]. Preparation of the following etherified woods has recently been studied actively.

A.

Benzylation

Benzylation of wood has been carried out with wood meal in order to convert wood to thermoplastic materials [4,8]. Hon and Ou [8] studied various parameters for obtaining benzylated woods with different degrees of substitution. Experimental data showed that pretreatment of the wood with NaOH as a swelling agent and water as a solvating agent, as well as varying reaction temperatures, had critical effects on the benzylation reaction. The reaction proceeds by the following mechanism [Reactions (I and (2)].

+ NaOH -------.. Wood-O - Na+ + H20

(I)

+CICHf@---+-Wood-O-CH2-@fHaCI

(2)

Wood-OH

Wood-ONa

Kiguchi recently reported benzylation of surfaces of wood blocks [9, 10] and wood chips [10, II] for obtaining hot-melted surfaces of wood, keeping the wood structure intact. The hot-melted surfaces were prepared by hotpressing of the benzylated wood blocks at 160°C. The surfaces had high levels of glossiness and water resistance. As a result of electron spectroscopy for chemical analysis (ESCA), chemical bonds of carbon on the benzylated surface were found to be almost all of the hydrocarbon type. Hot-melted particle-

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boards prepared from the benzylated wood chips had great flexural strength, as did commercial high-density particleboards, and superior water resistance and dimensional stability [10]. In addition, the benzylated particleboards with weight gains (WGs) above 38% showed that their dimensional stability and decay resistances were superior to those of the conventional particleboards made by using phenol-fonnaldehyde resin [II]. More recently, Kiguchi reported benzylation of wood particles with the solvent dilution and vapor phase methods for reducing the amount of benzyl chloride in the etherification agent [12]. In the vapor phase benzylation below the boiling point of benzyl chloride, greatly thennoplasticized particles could be produced after 2-4 h at 140°C. However, in the solvent dilution method, higher reaction temperatures and longer reaction times than those in the ordinary liquid phase method were found to be necessary to obtain the thennoplasticized particles.

B.

Allylatlon

The early work on allylation was done by Kenaga and Sproull [13] using allyl chloride in pyridine. In the allyl chloride-pyridine case, antishrink efficiency (AS E) [2,14] is due not to the fonnation of allyl ethers in cellulose or lignin but rather to the bulking caused by the fonnation of water-soluble allyl pyridinium chloride polymers [15]. Recently, Shiraishi and Goda [16] studied allylation of wood meal with allyl chloride or allyl bromide [Reaction (3)].

In this case, the wood meal was pretreated with a NaOH aqueous solution. It was found that allyl bromide gave better results than allyl chloride. That is, by using allyl bromide, WGs of -22-28% were obtained under mild conditions of 6O-80°C for 2 h, whereas higher temperatures (above 90°C) were needed to obtain the same WGs in the case of allyl chloride. Similarly, surfaces of wood blocks [9,10] and wood chips [10,17], pretreated with NaOH aqueous solution, have been allylated with allyl bromide. The allylated surfaces could be self-bonded by hot-pressing without using any adhesives.

162

C.

Matsuda

Cyanoethylatlon

By reacting wood with AN, cyanoethylation occurs [Reaction (4)]. The early work was conducted for improving dimensional stability and decay resistance [18,19]. In this case, before reaction, wood was pretreated with NaOH aqueous solution and the degree of reaction was generally low.

Wood-OH + CH 2 =CH-CN

(4)

In 1986, Morita and Sakata [20] reported cyanoethylation of wood meal for imparting thennoplasticity to wood. They carried out the reaction after pretreating wood meal with an NaOH solution approximately saturated with a neutral swelling agent such as NaSCN or NaI. The swelling agents in conjunction with dilute alkali increased the accessibility of wood substrate and resulted in a high N content (9-10%) of cyanoethylated wood. Further, treatment of the cyanoethylated wood with chlorine solution was found to result in an improvement of solubility in organic solvents such as N,N-dimethylfonnamide (DMF) and dimethylsulfoxide (DMSO) [21]. Films prepared by casting DMF solution of the cyanoethylated wood onto a glass surface were subjected to a dynamic mechanical measurement [22]. It was found that the segmental motions of the main chains were restrained by lignin, but these restraints were released with chlorination of cyanoethylated wood. Meanwhile, Hon and San Luis [23] recently studied cyanoethylation of wood meal to convert wood to thennoplastic material as a means of utilizing low-quality wood species as well as wood waste materials. With control of the reaction conditions, they obtained a cyanoethylated wood with an N content up to 13%. More recently, surfaces of wood blocks [9,10] and fibers [24] have been cyanoethylated. Hot-melted surfaces, prepared by hot-pressing the cyanoethylated wood blocks at 160°C, had high levels of glossiness and hardness, but water resistance was very poor [9]. This might be attributed to a hygroscopic subproduct such as amides originating in hot-melting. When the cyanoethylated fibers were hot-pressed at 240°C, a board could be obtained by autoadhesion of the thennally melted fibers [24]. Because cyanoethylated wood contains reactive nitrile groups, various functional derivatives are obtained by utilizing the nitrile groups [25]. For example, carbamoylethylated wood (water-soluble, nonionic, liquid crystal-

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163

line), carboxyethylated wood (water-soluble, anionic), aminoethylated wood (water-soluble, cationic), and so forth are obtained.

D.

Acetalation

When formaldehyde (FA) is reacted with wood, formalization occurs as follows (Reactions (5 and (6)]:

Wood-OH + HCHO --...... Wood-O-CH20H

Wood-O-CH 2 0H + Wood-OH

• Wood-O-CH 2-O-Wood + H20

(5) (6)

This treatment was first reported by Tarkow and Stamm (26] in 1953. The reaction is catalyzed by strong acids, such as HCI (26,27], HN0 3 [26], etc., and zinc chloride [28]. However, formalization results in degradation of wood, due to the strong acids. FA-treated wood shows high ASE values at low WG, e.g., an ASE of 90% is achieved at a WG of 7% [28]. Meanwhile, mechanical properties are reduced by the FA treatment [26,28,29]. Furthermore, the treatment causes an embrittlement of wood, probably attributable to the short, inflexible, crosslinking unit of the -O-C-O- linkage. Recently, Minato and Yano [30] reported that wood was treated with FA vapor in the presence of S02 catalyst. S02 was found to be a good catalyst for formalization of wood with a small decrease in strength. Wood has been treated with trioxane (cyclic trimer of FA) [31] and tetraoxane (cyclic tetramer of FA) (32] in the presence of S02' The S02-catalyzed formalization with tetraoxane was the most effective among the various formalization methods for improvement of dimensional stability and retention of strength [32]. The decay resistance of FA-treated wood to a white-rot fungus (Coriolus versicolor) is remarkable, but not to a brown-rot fungus (Tyromyces palustris) [33]. Meanwhile, the termite resistance is enhanced regardless of wood species and the method of treatment [33]. Yano et al. [32,34,35] reported that FA treatment improves the acoustic properties of wood. Further, the S02-catalyzed formalization with tetraoxane is the most effective for the improvement of the acoustic properties [32]. More recently, vaporous FA treatment has been applied to the improvement of dimensional stability of medium-density fiberboard (MDF) [36]. Among

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the S02-catalyzed fonnalizations with various reagents, tetraoxane gave the highest ASE (>60%), which is sufficiently durable for the practical uses of MDF. It was suggested that the dimensional stabilization of MDF is attributable mainly to the interfiber bondings. Furthennore, wood has been treated with non-FA crosslinking reagents such as glyoxal, glutaraldehyde, and dimethyloldihydroxyethyleneurea [37). Using the glyoxal and glutaraldehyde treatments, ASE reached around 70%, which is comparable to the values attained by the FA treatment, although the accompanying WG was much larger than the latter. Meanwhile, dimethyldihydroxyethyleneurea did not give sufficiently high ASE.

E. Treatment with Epoxlde When wood is treated with epoxides, etherification occurs as follows [Reaction (7»):

Wood-OH + A-CH-CH z ---+. Wood-O-CH CH-A \,../ zI \J OH

(7)

Usually, basic catalysts have been used in the reaction. The early work was done by Zimakov and Pokrovskil [38], McMillin [39], Barnes et al. [40], etc. For a typical example, ethylene oxide (EO), catalyzed with triethylamine, was used as a vapor phase treatment [39]. Rowell and Ellis [41] reported detailed studies on the reaction of epoxides with wood in 1984. They examined many epoxides including propylene oxide (PO), butylene oxide (BO), epichlorohydrin (EPC), and so forth, using triethylamine as catalyst. Low molecular weight epoxides penetrate wood cell walls and react with the cell wall polymer hydroxyl group fonning stable chemical bonds. Bulking of the wood, caused by the bonded chemical, results in 50-70% dimensional stability at a chemical WG of 20-30%. Above this level of treatment, the increase in added chemical causes the cell wall structure to break down and dimensional stability is lost. Good resistance to biological attack and marine borer attack is observed in the modified wood. The resistance to biological degradation is thought to be due to changes in chemical confonnation of the potential substrate, mainly in the carbohydrate fraction, that result from the chemical modification [41]. The effect of alkylene oxides, such as PO and BO, on dimensional stability has been studied also by Guevara and Moslemi [42]. Results indicated that

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165

PO and BO enhanced with the crosslinking agent trimethylolpropanetrimethacrylate applied to ovendry wood were the most efficient chemical treatments in controlling hygroscopic dimensional changes. Dynamic mechanical properties of PO- or BO-treated wood were recently investigated in relation to their structure at both the cellular and the molecular level [43]. The chemical treatment reduced the specific dynamic Young's modulus E'/-y. Wood flakes have been reacted with BO/triethylamine [44]. Flakes modified to 20% WG gave a flakeboard that absorbed 25% less water and had reduced thickness swelling up to 50% as compared to an untreated flakeboard. Similarly, wood particles were treated with PO prior to board manufacture [45]. PO-treated boards showed excellent decay resistance. More recently, Norimoto et al. [46] reported that etherification with PO and BO, in which the hydrophilic nature of the bulking agent is not counterbalanced by crosslinking, yielded high ASE values but increased mechanosorptive creep induced by moisture changes under load. On the other hand, Matsuda et al. [47] recently conducted systematic studies on catalysts for etherification reaction of wood with phenylglycidyl ether (PGE) [47]. It was found that, among various basic catalysts, the K salts of fatty acids accelerate the reaction remarkably, and a high WG indicating a high degree of polyetherification could be obtained. As shown in Table 1,

Table 1 Effect of Carbon Number of Fatty Acid on Yield and Weight Gain of Etherified Wood Meal for Etherification Reaction of Wood Meal (3 g) with PGE (30 g) at 140°C for 3 h in the Presence of Various K Salts of Fatty Acids as Catalyst (2 mol % Based on PGE) Fatty acid K salt of fatty acid HCooK CH 3COOK CH 3CH 2COOK CH3(CH2)2COOK CH3(CH2)3COOK CH3(CH2)4COOK CH3(CH2)6CooK CH 3(CH 2)gCOOK

C number I 2 3 4 5 6 8 10

Etherified wood meal Yield (g)

Weight gain

pKa 3.77 4.76 4.88 4.82 4.81 4.85 4.85 4.82

15.28 15.67 13.82 12.63 8.44 7.38 7.09 6.55

409.3 422.3 360.7 321.0 181.3 146.0 136.3 118.3

(%)

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166

CH 3COOK gives the highest WG of 422%. With increase in the C number of fatty acid, the WG decreases markedly. This tendency seems to be attributable to an increase in steric hindrance of fatty acid with increase in the C number because the pK a values of the fatty acids are almost equal except for fonnic acid. However, even above the C number of 6, the WG values are still 118-146%. It is inferred that epoxy groups adduct to OH groups of wood to fonn new OH groups, to which PGE molecules adduct by a polyetherification reaction, leading to high WG. Thermal stability of the etherified wood has a tendency to increase as the WG increases.

III.

ESTERIFICATION

Preparation of the following esterified woods was recently studied actively.

A.

Acetylation

In the reaction of wood with acetic anhydride, acetylation occurs as follows [Reaction (8)]:

o

II

Wood-OH

0 II

0

I I

+ CH 3-C-O-C-CH J ---+~ Wood-O-C-CH 3 + CHJCOOH

(8)

The reaction is acid- or base-catalyzed, and various catalysts have been studied [1,2]. Acetylation has been the most studied of all the chemical modifications of wood. The early studies done by Tarkow et a1. [48] and Goldstein et a1. [49] are notable. Vast amounts of data are available on acetylation of wood. Rowell [1,2] reviewed past research work in this field. Norimoto [50] reviewed recent trends in this field. The reaction with acetic anhydride can be conducted in a liquid or vapor phase. However, the vapor phase treatment results in a product with poorer properties than the liquid phase [51]. Therefore, the treatment has been carried out usually in liquid phase. Goldstein et a1. [49] reported that uncatalyzed acetylation in xylene at l00-130°C is the best condition. It is well known that acetylation results in an increase of density as well as an improvement of dimensional stability and decay resistance [1,2,50]. Also, tennite resistance [48,50], electric insulation [50,52], and acoustic properties [50,53] are improved. ASE effectiveness caused by acetylation is due mainly to chemical blocking of the hydroxyl groups [2].

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Yano et al. [53] studied acoustic properties of acetylated Sitka spruce by specific dynamic Young's modulus and by logarithmic decrement. For ovendried specimens, both the modulus and the decrement have been found to increase. Meanwhile, mechanical properties are generally unchanged and adhesive strength is reduced by acetylation [2]. Furthermore, creep deformation of wood under humidity change is remarkably reduced by acetylation [54]. Recently, particleboards have been prepared from mixtures of acetylated and untreated wood chips [55]. Thickness swelling and water absorption after soaking in water for 24 h decrease as the number of acetylated chips increases. The specimens containing 100% of acetylated chips show no sign of decay. Further, particleboards from acetylated wood particles have been reported to have higher mechanical properties than those from POtreated particles [56]. Rowell et al. [44] prepared ftakeboards from acetylated wood flakes. Their water absorption was 50% less and thickness swelling was reduced 85%. There was an 85% reduction in thickness swelling when the boards were subjected to 90% relative humidity over a period of 20 days. It is of interest that veneer-faced low-density particleboards made from acetylated veneers and acetylated core particles showed excellent dimensional stability in both liquid water and humidity tests and were resistant to attack by fungi in an 8-week soil block test [57]. Furthermore, during the 150-day bending-creep test, the totally acetylated boards showed no strength or weight loss during exposure to T. palustris. More recently, fiberboards have been made from acetylated bagasse fiber [58] and acetylated aspen fiber [59,60]. The fiberboards swelled at a much slower rate and to a lesser extent than control fiberboards [58], and their internal bond strength [58] and biological resistance [59] were greater. The preparation of acetylated wood fiber was recently studied on a commercial scale [61]. It seems that the process has come close to commercialization. Acetylated wood can be prepared also by reacting with ketene gas [Reaction (9)] [1,2]:

o

II

Wood-OH + CH2=C=O -------.. Wood-O-C-CH3

(9)

However, the reaction with ketene generally results in a much lower WG.

168

B.

Matsuda

Treatment with Higher Aliphatic Acids, Acid Anhydrides, or Acid Chlorides

When wood is reacted with aliphatic acids, acid anhydrides, or acid chlorides, aliphatic acid esters of wood are obtained as follows [Reactions (10)-(12)]:

o

II

Wood-OH

+ RCOOH - - , Wood-O-C-R + H2 0

000 Wood-OH + R-C-O-C-R - - - - . + lfood-O-C-R II

II

II

o

(II)

0

II

Wood-OH + R-C-C I

+ RCOOH

(10)

II

--~,

Wood-O-C-R + He I

(12)

The early work was done with acetyl chloride catalyzed with lead acetate [2], propionic and butyric anhydrides in xylene without catalyst [49], and several unsaturated carboxylic acids in the presence of trifluoroacetic anhydride (TFAA) [62,63]. It is notable that Shiraishi et al. [64] prepared higher aliphatic acid esters of wood in an N20 4-DMF cellulose solvent medium. They studied esterification of wood meal with a series of aliphatic acid anhydrides and chlorides in the nonaqueous N20 4-DMF cellulose solvent. The N20 4-DMF-pyridine solution used as the reaction medium plays a role in destroying the molecular order of the cellulose within the wood, enabling the cellulose to be unifonnly substituted by acyl groups along its chain. These unifonnly distributed blocking groups result in the pennanent decrystallization of wood. Thus, a series of esters, from propionate to stearate, have been prepared. Also, they have prepared these esters by acylating wood in a TFAA-aliphatic acid system (TFAA method) and an aliphatic acid chloride-pyridine-DMF system (chloride method) [3,5]. Nakano and Nakamura [65-69] studied viscoelasticity of wood specimens prepared with TFAA and aliphatic acids such as propionic, n-valeric, ncaproric, n-capric, lauric, and palmitic acids. It was found that, with increase in the ester content, the relaxation rigidity decreases markedly and its temperature dependence becomes high [65]. The real part of complex rigidity decreases rapidly with increasing number ofe atoms of the introduced acyl group [68]. The rate of decrease increases with increasing temperature. Further, the

Chemical Modification of Solid Wood

169

effect of molar volume of the introduced acyl groups on the viscoelastic properties has been investigated [69].

c.

Treatment with Dicarboxyllc Acid Anhydrides

When dicarboxylic acid anhydrides are reacted with wood, esterified woods bearing carboxyl groups are obtained as follows [Reaction (13)]:

):0-,.

Wood-OH + ~o;o

-------.. Wood-OOC-R-COOH

(13)

The early work was done with phthalic anhydride (PA) [70,71]. The mechanism of ASE effectiveness by phthalylation is mainly by mechanical bulking of the submicroscopic pores in the wood cell wall [72]. Recently, Mastuda et aI. [73-75] efficiently obtained a series of carboxyl group-bearing esterified woods by the addition reaction [Reaction (13)] of wood meal with maleic anhydride (MA), PA, and succinic anhydride (SA). The reaction proceeds at room temperature in DMF or DMSO [73]. It is noteworthy that, even in the absence of solvent, the esterification reaction proceeds easily at high temperatures (60-200°C) [74]. The esterification reactions without solvent, as compared with those with the solvent, are industrially advantageous and give esterified woods with a wide range of monoester contents [74]. The hygroscopicity and initial weight loss temperature of the esterified wood meals decrease with increase in the monoester content [76]. Also, wood in the fonn of blocks has been esterified with MA and PA [77,78]. Since MA has a melting point of 52.6°C, the reaction with MA proceeds smoothly at 80-120°C even in the absence of solvent, to give esterified wood blocks. Meanwhile, it is desirable to use a solvent such as DMF in the reaction with PA because PA has a melting point of 131 ° C. The esterified wood blocks show some degree of improved dimensional stability against moisture and water [77]. For example, the MA-treated wood block with a WG of 25.7% shows an ASE of 48%. However, the ASE value decreases with repeated wet-dry tests due to hydrolysis of the monoester. It was recently reported [79,80] that grafting through esterification between refiner-ground pulp and MA-modified polypropylene occurred when they were kneaded at 180°C for 10 min. This resulted in an enhanced adhesion between the polyolefin matrix and the pulp fibers, giving high levels of physical properties to the composites [79].

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More recently, aspen fiber was esterified with MA and PA [60]. Esterified fiber in fiberboards had reduced moisture sorption and rate of swelling in liquid water.

D.

Ollgoesterificatlon

The carboxyl groups introduced into wood by Reaction (13) are reactive with epoxy groups. Mastuda et al. [81,82] obtained epoxide-adducted esterified woods by the addition reaction of the carboxyl group-bearing esterified wood meal with epoxides as follows [Reaction (14)]:

Wood-OOC-R-COOH + A-CH-rH 7

'0

2

,

Wood-OOC-R-COOCH2~H-A

(14)

OH

In this case, PGE, allylglycidyl ether (AGE), and glycidyl methacrylate (GMA) were used as the epoxide. Also, EPC has been adducted to the carboxyl group-bearing esterified wood blocks, resulting in markedly improved dimensional stability [77]. This effect is attributed to the blocking effect caused by replacing the carboxyl group by a more hydrophobic ester linkage as well as to the bulking effect of the introduced EPC residue. When the epoxide-adducted esterified wood meals were further allowed to react with the anhydride and the epoxide at high temperatures, alternately adding esterification reactions occurred, to produce oligoesterified woods [81]. Polymerizable oligoester chains have been introduced into wood by the oligoesterification reaction of wood meal with the anhydrides and the epoxides such as AGE or GMA as follows [Reaction (15)] [83-85]:

R'

CH 2=C-rr Wood-f-OOC-R-COOCH 2 -CHl;OH R: -CH:CH-, ~ :R': H-,CH 3-;R": -CH 2OCH2-' -COOCH2-

(15)

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171

This reaction without the process of isolating the intennediate esterified woods is advantageous from the industrial standpoint. Products of the reaction consist of an acetone-insoluble part (predominantly) and a soluble part. The insoluble parts are oligoesterified woods bearing polymerizable oligoester chains. The soluble parts that are viscous liquids consist mainly of polymerizable free oligoesters not linked with the wood matrix. The fonnation of the free oligoesters is considered to be caused by active hydrogen-containing impurities in the reaction mixture. The oligoesterified woods thus obtained are of interest in that they will be crosslinked at high temperatures and under high pressures, accompanying thennal flow of the wood components, to give plastic-like crosslinked woods. Similarly, oligoesterified wood blocks have been prepared by a one-step chemical treatment with a reactant solution of the anhydride and EPC as follows [Reaction (16)] [86-88]:

(16)

Among the preparation methods investigated, the heating-suction method [88] is advantageous. In this method, wood blocks impregnated with the reactant solution are heated and then subjected to suction under reduced pressure and heating to remove unreacted reactant solution. The oligoesterified wood blocks thus obtained contain small amounts (-4-8%) of free oligoesters not linked with the wood matrix. Their dimensional stability against moisture and water becomes greater with an increase in the apparent total WG due to the oligoester chains and the free oligoesters [86-89]. In this case, the free oligoesters have some degree of contribution of bulking effect to ASE. The oligoesterified wood blocks based on PA and EPC exhibit higher compressive and flexural strengths as well as greater chemical resistance and electric insulation than untreated wood blocks [90]. Furthennore, excellent resistance is observed against weathering, biodeterioration [91], decay, and tennite invasion [92]. More recently, crosslinked oligoesterified woods have been prepared by simultaneous oligoesterification and vinyl polymerization, as follows [Reaction (17)] [93]:

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172

-f-CH2-~(CH3~

(17)

COO~H2

Wood-f-OOC-R-COOCH 2-CHtaoH In this case, wood blocks impregnated with a reactant solution of MA, PA, or SA and GMA were subjected to one-step chemical treatment. The crosslinked oligoesterified wood blocks obtained had hard and smooth surfaces as well as improved dimensional stability.

E.

Treatment with Maleic Acid and Glycerol Mixtures

Recently, Fujimoto et al. [94] treated wood flakes with 60% aqueous solutions of maleic acid and glycerol mixtures (MG) to obtain various WGs, and then made particleboards from the MG-treated flakes by hot-pressing at 210°C for 15 min. Water absorption and thickness swelling were greatly reduced with increase in MG weight. The strength properties of these particleboards were superior to those of untreated particleboards. These effects were ascribed to the formation of three-dimensional networks by chemical reaction (esterification) between the carboxyl groups of maleic acid and the hydroxyl groups of glycerol and wood components. Furthermore, they reported that for the production of dimensionally stable MG particleboards, chemical costs can be reduced when hot-pressing is done at high temperatures [95]. Also, the MG treatment was more recently applied to wood blocks [96,97]. In this case, wood blocks were treated with 30% methanolic MG solution [96] or 10-30% aqueous/methanolic MG solution [97], followed by heating at 160-180°C for 3-5 h. The MG-treated wood blocks showed an ASE of 80%. Even after an outdoor exposure test, the smoothness of the surface was retained in the MG-treated specimens [97]. This means that the MG treatment effectively prevents wood components from photodegradation. Furthermore, few changes were observed in the deformed parts that had been bent by a microwave irradiation method.

IV.

THERMOPLASTICIZATION

It is notable that Shiraishi et al. [3,5-7,98,99] first reported that wood becomes thermally meltable by esterification such as lauroylation al)d stearoylation,

Chemical Modification of Solid Wood

173

and also by etherification such as benzylation. Generally, lignin is believed to be a three-dimensional, phenolic molecule of complex structure and ultrahigh molecular weight. However, from the fact that the chemically modified woods are thermally meltable, Shiraishi [100] postulates that lignin might also be regarded as a highly branched, linear high polymer with large branches. Thermoplasticization of the following chemically modified woods has been studied actively.

A.

Etherified Woods

Shiraishi [4] found that benzylation of wood meal to WG above 40% resulted in benzylated woods that have good thermal fluidity and can give slightly yellowish films by hot-pressing. The benzylated wood films have tensile strength of 27.7-40.6 MPa and elongation (at break) of 1.61-9.49%. Similarly, Hon and Ou [8] recently reported that benzylated woods exhibited good melting properties and a wide range of Tg from 66°C to 280°C was achieved. These were largely dependent on the WG. Molded and extruded products obtained from the benzylated wood meal exhibited acceptable mechanical strength for structural engineering applications. Kiguchi and Yamamoto [11] prepared hot-melt, self-bonded particleboards from benzylated wood particles. Their internal bonding strengths were about twice those of conventional particleboards. The interparticle bondings of the benzylated particleboards were plane adhesion. More recently, it was reported that the properties of benzylated wood were improved by the addition of plasticizers such as epoxydized soybean oil, polycaprolactone, and acetyltriethyl citrate [101]. Although tensile strength decreased, elongation increased and flow temperature decreased with an increase in the plasticizer content. The moldability of the benzylated woods is remarkably improved with the treatment. Shiraishi and Goda [16] reported that allylated wood meals were given thermoplastic properties by blending with appropriate synthetic polymers or low molecular weight plasticizers such as dimethylphthalate or resorcinol. Mere allylation did not render wood thermally meltable. Films from the allylated wood-polyethylene and allylated wood-polypropylene (1 :2) blends exhibit tensile strengths of 92.2 and 159.0 MPa and elongations of 14.6 and 3.8% respectively, [16]. Kiguchi [10] reported that hot-melted wood surfaces treated by benzylation and allylation lost surface glossiness by weathering because of the benzyl or allyl groups induced by the etherification of the surfaces. These groups can absorb ultraviolet rays and degrade the surfaces. As a result of ESCA, the

174

Matsuda

benzylated and allylated surfaces had increased proportions of 0 atoms from photooxidation of ultraviolet rays. Ohkoshi [I 02J allylated wood surfaces and then copolymerized the surfaces with styrene by hot-pressing to attain crosslinking of wood. The shear strengths of bonding obtained were dependent on both the degrees of the thennal softening of the allylated wood and the grafting of styrene. The bonding strengths were somewhat less compared with those of specimens bonded without copolymerization of the styrene. This was attributed to external plasticization of the allylated wood by styrene during hot-pressing. More recently, Ohkoshi et al. [103J studied the mechanism of thennoplasticization of wood by allylation. They considered that decrystallization of cellulose within wood during allylation permits the wood to soften thennally and the allylated lignin within wood increases the softening through acting as a plasticizer. Morita and Sakata [20J reported that cyanoethylated wood meal showed thennal flow at around 250°C. The temperature at which the wood could flow decreased with increasing cyanoethyl content of the wood. Treatment of the wood meal before cyanoethylation with sodium periodate or sodium chlorite lowered the flow temperature. The flow temperature was also decreased by blending the cyanoethylated wood with appropriate synthetic polymers or plasticizers. Treatment of the cyanoethYlated wood with chlorine solution was found to be the most effective method of lowering the flow temperature. The lowering may be interpreted in tenns of the plasticizing effect of chlorination on the lignin moiety in the wood. Recently, Hon and San Luis [23] studied the thennal properties of cyanoethylated wood by DSC and dynamic mechanical thermal analyzer (DMTA). Depending upon the N content, the cyanoethylated wood exhibited a softening temperature ranging from 162°C to 177°C and melting temperature ranging from 240°C to 270°C. The DMTA measurements suggest that wood materials are susceptible to degradation upon cyanoethylation. Kiguchi [IOJ conducted weathering tests of hot-melted wood surfaces. It was found that the hot-melted wood surfaces treated by cyanoethylation were effective in ultraviolet ray resistance. However, one year of outdoor exposure showed that the weathering ability of the surfaces was poor because of the lack of water resistance on the cyanoethylated surfaces. Further, Yamawaki et al. [24J hot-pressed surface cyanoethylated wood fibers at 240°C to make a board by autoadhesion of the thennally melted fibers. When the cyanoethylated fibers were chlorinated, the pressing temperature was reduced to 130°C. The thennal flow temperature was decreased

Chemical Modification of Solid Wood

175

further by adding a small amount of a metal halide such as ferric chloride or cupric chloride. Matsuda et al. [47] reported that PGE-treated etherified woods with high WG could be molded into sheets by hot-pressing.

B.

Esterified Woods

While untreated wood meal shows a thennal softening point of . . . . 260°C , esterified wood meals prepared in the N20 4 -DMF-pyridine solution have a softening point of around 100°C or less and a thennal flow temperature of 220-250°C [98]. The flow temperature shows a tendency to decrease with increase in the C number of acyl group. This was first found by Shiraishi et al. [98] that wood meal is converted as a whole to thennoplastic material. In this case, a very high degree of acylation is not always required to provide wood with the thennally meltable property. The products become thennoplasticized materials when almost one-third of the hydroxyl groups in wood are acylated [99]. Also, Shiraishi et al. [4,5] found that the acylated wood meals prepared by the TFAA and chloride methods are thennally meltable. In addition, the acylated wood samples prepared by the TFAA method show somewhat lower flow temperatures than those prepared by the chloride method. It is noteworthy that acetylated wood meals prepared by the TFAA method melted clearly at 320°C under a pressure of 0.29 MPa [4,5]. Other methods of acetylation resulted in products that did not undergo complete flow. However, thennal properties of the acetylated wood were enhanced by mixed esterification with other acyl groups. That is, esterified woods containing either propionyl or butyryl groups in addition to acetyl revealed meltable properties [4,5]. A film prepared from the acetylated-butyrylated wood meal has a tensile strength of 41.0 MPa and an elongation of 12.5% [4]. Further detailed thennal and other properties of the above aliphatic acid esters of wood have been described in the reviews by Shiraishi et al. [3-7]. Matsuda and Veda [104] found that carboxyl group-bearing esterified wood meals prepared by Reaction (13) can be molded into yellowish or reddish brown sheets by hot-pressing. In this case, an increase in the monoester content in the esterified wood results in higher moldability. However, as the diester content increases, the moldability decreases, probably due to a decrease in the thennal fluidity by crosslinking. Similarly, epoxide-adducted esterified wood meals and oligoesterified wood meals described above can be molded by hot-pressing [105]. More recently, Clemons et al. [60] reported that esterification of aspen

Matsuda

176

fibers with MA or SA imparted thennoplasticity to the fibers, whereas acetylation did not affect thennal properties. Fiber modified with SA appeared to exhibit the greatest thennoplasticity. Further, Matsuda and Veda [106] have investigated the crosslinking reaction of the carboxyl group-bearing esterified wood meals with bisphenol A diglycidyl ether (BADG). It was found that in the reaction at high temperatures the epoxy groups in BADG adduct to the carboxyl groups in the esterified wood to produce ester linkages, resulting in a fonnation of crosslinks between the esterified woods via BADG. TIle crosslinked structure may be written as follows:

-E-B-E-(wood matrix)-E-B-E-

f

I

~

B I

E

E

-E-B-E-(wood'I matrix)-E-B-E-(wood ~atrix)-E-B-EI

~

f I

I

E

-B-E-(wood ~atrix)-E-B-EE : -OOC-R-COO-: B: BAnG

resid~e

Both at high temperatures (l50-190°C) and under high pressure (18.0-27.0 MPa), the wood components thennally flow to give reddish, yellowish, or blackish brown, crosslinked wood boards whose surfaces are smooth, glossy, and plastic-like. In this case, it is advantageous that BADG works as a plasticizer for the wood components. Generally, these boards exhibit properties that are much superior to those of usual woody boards such as fiberboards and particleboards (106]. The boards based on MA exhibit the highest flexural strength of 86.8 MPa. Meanwhile, compressive strength, hardness, and heat distortion temperature (HDT) are the highest in the boards based on PA, and are 198.7 MPa, 116, and 116°C, respectively. It is characteristic of these plastic-like crosslinked wood boards that they show very high compressive strength. In addition, the PA-based board has higher water resistance than the MA- and SA-based ones. This is considered to be attributable to the high hydrophobicity of the phenyl ring.

Chemical Modification of Solid Wood

c.

177

Oligoesterified Woods

As described above, the products of oligoesterification [Reaction (15)] consist of oligoesterified woods and viscous liquids consisting mainly of polymerizable free oligoesters [83-85]. The oligoesterified woods do not show good thermoplastic properties. Similarly, in the case of the wood-MA-AGE series [83], when the products, the oligoesterified wood-containing mixtures, are subjected to hot-pressing, a great portion of the liquid part exudes from the system and sufficient thennal flow of the wood components is not observed. Meanwhile, in the case of the mixtures into which a catalytic amount of dicumyI peroxide has been added, both at high temperatures and under high pressures, the wood components thennally flow to give reddish or yellowish brown, crosslinked wood boards whose surfaces are smooth, glossy, and plastic-like. In this case, the exudation of the liquid part is not observed, indicating that the free oligoesters are combined by crosslinking with the oligoesterified woods, resulting in the fonnation of a network structure. The free oligoesters that are hardening work as a plasticizer for the wood components. The crosslinking is due largely to polymerization of the allylic double bonds. On the other hand, in the case of the products of the wood-PA-GMA [84] and wood-MA-GMA [85] series, the wood components thennally flow under hot-pressing to give plastic-like crosslinked wood boards even in the absence of a radical initiator. Table 2 shows the physical and other properties of the plastic-like crosslinked wood boards obtained from the products of the wood-MA-GMA series [85]. HOT is largely influenced by wood content and exhibits values of above 183°C, and at wood contents of 45-50%, the HOT values are above 210°C. These values are much higher than those of the plastic-like crosslinked wood boards from the carboxyl group-bearing esterified woods and BAOG [106], probably due to higher crosslinking density of the fonner boards. Tensile strength shows high values of - 59-68 MPa over a wide range of wood content. flexural strength and Rockwell hardness are not greatly affected by the wood content. Impact strength increases slightly with increase in the wood content. Compressive strength exhibits values of -220 MPa at wood contents of 60-80% and below -60% shows a tendency to increase with decrease in the wood content. As for water resistance, linear swelling was not greatly influenced by the wood content, showing values in the range of 0.06-0.28%. However, water absorption and thickness swelling showed a tendency to increase with an

..... ~

Table 2

Physical and Other Properties of Plastic-like Crosslinked Wood Boards from Products of Wood-MA-GMA Series

Physical and other properties

Wood content of plastic-like crosslinked wood boardl 45%

50%

55%

60%

65%

70%

75%

80%

HDT (OC) TS (MPa) FS (MPa) IS (JIm) RH (M scale) CS (MPa) WA (%) THSW (%) LSW (%)

210< 62.1 56.6 14.5 119 253.1 0.61 0.69 0.07

210< 59.7 66.4 15.8 120 242.7 0.77 1.08 0.06

196 62.7 57.9 13.5 121 223.1 1.09 1.28 0.07

204 62.9 60.7 13.1 122 220.8 1.21 1.36 0.13

188 58.5 60.7 15.3 121 225.9 1.91 1.96 0.11

187 62.9 56.5 15.9 122 223.3 2.45 2.78 0.23

184 68.3 56.2 16.2 122 221.7 3.18 3.80 0.23

183 67.1 56.3 16.8 122 219.9 3.26 3.70 0.28

IPrepared by hot-pressing at 180°C, 43.15 MPa, 30 min. HDT, heat distortion temperature; TS, tensile strength; FS. flexural strength; IS, impact strength; RH. Rockwell hardness; CSt compressive strength; WA. water absorption; THSW, thickness swelling; LSW. linear swelling.

~

Cit

c:

~

Chemical Modification of Solid Wood

179

increase in the wood content due to the increase in the remaining hydrophilic hydroxyl groups in the wood matrix. The molded boards referred to above show excellent properties in HDT, tensile and compressive strengths, which are superior to those of the other series. The molded boards of the wood-MA-AGE series [83] exhibit HDT values of above 165°C and compressive strength of -160-230 MPa. The molded boards of the wood-PA-GMA series [84] exhibit outstanding properties in tensile strength (-69 MPa), flexural strength (-88-100 MPa), and Rockwell hardness (-120). These products of oligoesterification are able to give, by compression molding or injection molding, various types of plastic-like crosslinked wood samples as shown in Fig. 1.

Figure 1

Plastic-like crosslinked wood samples.

REFERENCES I.

2. 3.

R. M. Rowell, Am. Wood Preserv. Assoc. Proc. 7/:41 (1975). R. M. Rowell, in The Chemistry of Solid Wood (R. M. Rowell, ed.), ACS, Washington, D.C., 1984, pp. 175-210. N. Shiraishi, Kobunshi Kalw 3/:500 (1982).

180

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4. 5.

N. Shiraishi, Sen-i Ga!
6. 7. 8. 9. 10. II. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Chemical Modification of Solid Wood 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65.

181

P. V. Zimakov and V. A. Pokrovskil. Zh. Prikl. Khim. 27:346 (1954). C. W. McMillin, For. Prod. J. 13:56 (1963). H. M. Barnes, E. T. Choong, and R. L. McIlhenny, For. Prod. J. 19:35 (1969). R. M. Rowell and W. D. Ellis, U.S. For. Servo For. Prod. Lab., Res. Pap. EPL 451, 1984. R. Guevara and A. A. Moslemi, Wood Sci. Technol. 18:225 (1984). H. Akitsu, M. Norimoto, and T. Morooka, Mokuzai Gakkaishi 37:590 (1991). R. M. Rowell, A. -M. Tillman, and L. Zhengtian, Wood. Sci. Technol. 20:83 (1986). V. C. Mallari, K. Fukuda, N. Morohoshi, and T. Haraguchi, Mokuzai Gakkaishi 36: 139 (1990). M. Norimoto, J. Gril, and R. M. Rowell, Wood Fiber Sci. 24:25 (1992). H. Matsuda, H. Dohi, and M. Ueda, Mokuzai Gakkaishi 33:884 (1987). H. Tarkow, A. Atamm, and E. C. O. Erickson, U.S. For. Servo For. Pest Leaft. Rep. No. 1593 (1950). I. S. Goldstein, E. B. Jeroski, A. E. Lund, J. F. Nielson, and J. F. Weater, For. Prod. J. 1/:363 (1961). M. Norimoto, Mokuzai Kenkyu Shiryo 24:13 (1988). R. M. Rowell, A. -M. Tillman, and R. Simonson, J. Wood Chem. Technol. 6:293 (1986). G. -J. Zhao, M. Norimoto, F. Tanaka, T. Yamada, and R. M. Rowell, Mokuzai Gakkaishi 33:136 (1987). H. Yano, M. Norimoto, and T. Yamada, Mokuzai Gakkaishi 32:990 (1986). M. Norimoto, J. Gril, K. Minato, K. Okumura, J. Mukudai, and R. M. Rowell, Mokuzai Kogyo 42:14 (1987). K. Nishimoto and Y. Imamura, Mokuzai Kogyo 40:14 (1985). V. C. Mallari, K. Fukuda, N. Morohoshi, and T. Haraguchi, Mokuzai Gakkaishi 35:832 (1989). R. M. Rowell, Y. Imamura, S. Kawai, and M. Norimoto, Wood Fiber Sci. 2/:67 (1989). R. M. Rowell and F. M. Keany, Wood Fiber Sci. 23:15 (1991). W. C. Feist, R. M. Rowell, and J. A. Youngquist, Wood Fiber Sci. 23:260 (1991). C. Clemons, R. A. Young, and R. M. Rowell, Wood Fiber Sci. 24:353 (1992). A. D. Sheen, Proc. Pacific Rim Bio-Based Composites Symposium, Rotorua, 1992, FRI Bull. 176, p. 1. T. Nakagami, H. Arimoto, and T. Yokota, Bull. Kyoto Univ. For. 46:217 (1974). T. Nakagami and T. Yokota, Bull. Kyoto Univ. For. 47:178 (1975). N. Shiraishi, T. Matsunaga, T. Yokota, and Y. Hayashi, J. Appl. Polym. Sci. 24:2347 (1979). T. Nakano and H. Nakamura, Mokuzai Gakkaishi 32:176 (1986).

182 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99.

Matsuda T. T. T. T.

Nakano and H. Nakamura, Mokuzai Gakkaishi 32:337 (1986). Nakano and H. Nakamura, Mokuzai Gakkaishi 32:820 (1986). Nakano and H. Nakamura, Mokuzai Gakkaishi 33:472 (1987). Nakano, Mokuzai Gakkaishi 34:516 (1988). J. Risi and D. F. Arseneau, For. Prod. J. 8:252 (1958). R. Popper, Bull. Schweizer. Arbeitsgemeinsch. Holzjorsch. J:3 (1973). R. Popper and M. Bariska, Holz Roh-Werkst. 33:415 (1975). H. Matsuda, M. Veda, and M. Hara, Mokuzai Gakkaishi 30:735 (1984). H. Matsuda, M. Veda, and K. Murakami, Mokuzai Gakkaishi. 30: 1003 (1984). H. Matsuda, Wood Sci. Technol. 2J:75 (1987). H. Matsuda and M. Veda, Mokuzai Gakkaishi 3J:103 (1985). H. Matsuda, M. Veda, and K. Murakami, Mokuzai Gakkaishi 34:140 (1988). H. Matsuda, Wood Sci. Technol. 27:23 (1993). H. Kishi, M. Yoshioka, A. Yamanoi, and N. Shiraishi, Mokuzai Gakkaishi 34:133 (1988). S. Takase and N. Shiraishi, J. Appl. Polym. Sci. 37:645 (1989). H. Matsuda and M. Veda, Mokuzai Gakkaishi 3J:267 (1985). H. Matsuda and M. Veda, Mokuzai Gakkaishi 3J:468 (1985). H. Matsuda, M. Veda, and H. Mori, Wood Sci. Technol. 22:21 (1988). H. Matsuda, M. Veda, and H. Mori, Wood Sci. Technol. 22:335 (1988). H. Matsuda, M. Veda, and H. Mori, unpublished data. H. Matsuda, M. Veda, and K. Murakami, Mokuzai Gakkaishi 34:597 (1988). H. Matsuda, K. Murakami, and M. Veda, Mokuzai Gakkaishi 34:844 (1988). H. Matsuda, K. Murakami, and M. Veda, Mokuzai Gakkaishi 34:1004 (1988). K. Murakami and H. Matsuda, Mokuzai Gakkaishi 35:924 (1989). K. Murakami, H. Matsuda, and M. Veda, Mokuzai Gakkaishi 35:328 ( 1989). K. Murakami and H. Matsuda, Mokuzai Gakkaishi 36:538 (1990). K. Murakami, H. Matsuda, M. Veda, J. Y. Ryu, Y. Imamura, and M. Takahashi, Mokuzai Gakkaishi 39:436 (1993). M. Veda, H. Matsuda, and K. Matsumoto, Mokuzai Gakkaishi 38:458 (1992). H. Fujimoto, T. Anazawa, and K. Yamagishi, Mokuzai Gakkaishi 34:904 (1988). H. Fujimoto, T. Anazawa, Y. Ohmiya, and K. Yamagishi, Mokuzai Gakkaishi 37:456 (1991). H. Fujimoto and K. Yamagishi, Proc. Int. Symp. Chemical Modification oj Wood, Kyoto, 1991, p. 83. H. Fujimoto, Proc. Pasific Rim Bio-Based Composites Symposium, Rotorua, 1992, FRI Bull. J76, p. 87. N. Shiraishi, T. Matsunaga, and T. Yokota, J. Appl. Polym. Sci. 24:2361 (1979). H. Funakoshi, N. Shiraishi, M. Norimoto, T. Aoki, S. Hayashi, and T. Yokota, Holzjorschung 33:159 (1979).

Chemical Modification of Solid Wood 100. 101. 102. 103. 104. 105. 106.

183

N. Shiraishi, Mokuzai Kogyo 35:200 (1980). S. Honma, K. Okumura, M. Yoshioka, and N. Shiraishi, Proc. Pacific Rim Bio-Based Composites Symposium, Rotorua, 1992, FR/ Bu/l/76, p. 140. M. Ohkoshi, Mokuzai Gakkaishi 37:917 (1991). M. Ohkoshi, N. Hayashi, and M. Ishihara, Mokuzai Gakkaishi 38:854 (1992). H. Matsuda and M. Veda, Mokuzai Gakkaishi 3/:215 (1985). H. Matsuda and M. Veda, Mokuzai Gakkaishi 31:579 (1985). H. Matsuda and M. Veda, Mokuzai Gakkaishi 31:903 (1985).

7 Liquefaction of Wood Mariko Yoshioka, Yaoguang Yao, and Nobuo Shiraishi Kyoto University Sakyo-ku , Kyoto, Japan

I.

INTRODUCTION

The term liquefaction of lignocellulosics has hitherto chiefly referred to those procedures for producing oil from biomass under very severe conditions for conversion [1,2]. For example, Appel et al. [1] converted cellulosics to oil by using homogeneous Na2C0 3 catalyst in water and a high-boiling-point solvent mixture (anthracene oil, cresol, etc.) at pressures of 140-240 atm with synthetic gas, CO/H2 • Treatments for 1 h at 300-350°C resulted in 40-60% yield of benzene solubles (oil) and a 95-99% conversion of the starting materials. This type of liquefaction can be more precisely called the oilijication of lignocellulosics. This chapter presents recent progress on lignocellulosic liquefaction under milder treating conditions, Le., at a temperature of 240-270°C without catalyst, or at a temperature of 80-150°C with an acidic catalyst. One special group of chemically modified woods can be dissolved in cresols even at room temperature as will be shown later. The liquefaction or dissolution of chemically modified wood has been developed [3-6], and the liquefaction of untreated wood has also been found to be possible [5,7,8]. The latter can also be compared with the organosolve pulping of wood. While it can be pointed out that the conditions of the liquefaction are more severe than those for the organosolve pulping, the degree of difference is not so large compared with that between the liquefaction and the oilification.

185

Yoshioka et a/.

186

After the discovery of the phenomena of these wood liquefactions, various trials have been done in order to (I) increase the biomass concentration in the liquefaction mixture, (2) obtain the true liquefaction degree in relation to the soluble properties of liquified biomass in organic solvents, and (3) understand the mechanism of the liquefaction. Applications of these wood liquefactions have also been developed in the preparations of adhesives, moldings, foams, and so forth.

II.

LIQUEFACTION OR DISSOLUTION OF WOOD

A.

Chemically Modified Wood

Chemically modified woods have been found to liquify and dissolve in neutral aqueous solvents, organic solvents, or organic solutions, depending on the characteristics of the modified wood [9]. So far, three methods have been found for chemically modified wood liquefaction. The first trial of the liquefaction of wood was accomplished by using very severe dissolving conditions [10]. One example used wood samples esterified with a series of aliphatic acids, which could be liquified in benzyl ether, styrene oxide, phenol, resorcinol, benzaldehyde, aqueous phenols, a chloroform-dioxane mixture, or a benzene-acetone mixture after treating at 200-270°C for 20-150 min. Carboxymethylated wood, allylated wood, and hydroxyethylated wood have been liquified in phenol, resorcinol, or their aqueous solutions and formalin after standing or stirring at 170°C for 30-60 min [3]. The second method for liquefaction makes use of solvolysis during the process [8, II]. By using conditions which allow phenolysis of part of the lignin, especially in the presence of an appropriate catalyst, the liquefaction of chemically modified wood into phenols could be accomplished under milder conditions (at 80°C for 30-150 min). Allylated wood, methylated wood, ethylated wood, hydroxyethylated wood, acetylated wood, and others have been found to dissolve in polyhydric alcohols such as 1,6-hexanediol, 1,4butanediol, 1,2-ethanediol, 1,2,3-propanetriol (glycerin), and bisphenol A using the liquefaction conditions just described. Each of them caused partial alcoholysis of lignin macromolecules [4]. The liquefaction processes can produce paste-like solutions with a considerably high concentration of wood solute (70%). Such solutions are meaningful for the utilization of biomass including wood wastes. The third method of liquefaction or dissolution involves postchlorination.

Uquefaction of Wood

187

When chemically modified woods are chlorinated, their solubility in solvents is tremendously enhanced. For example, at room temperature cyanoethylated wood can dissolve in a-cresol by only 9.25%. However, once chlorinated, it can dissolve almost completely in the same solvent at room temperature. The chlorinated-cyanoethylated wood can also dissolve in resorcinol, phenol, and an LiCI-dimethylacetamide solution under heating.

B.

Untreated Wood

The liquefaction and dissolution of chemically modified wood has so far been reviewed. More recently, untreated wood has also been found to liquify in several organic solvents [8]. This phenomenon was found during an investigation evaluating the effect of the degree of chemical modification of wood on liquefaction. For example, after treating at above 250°C for 15-180 min, wood chips and wood meals were liquified in phenols, bisphenols, alcohols such as benzyl alcohol, polyhydric alcohols such as 1,6-hexanediol and 1,4butanediol, oxyethers such as methyl cellosolve, ethyl cellosolve, diethylene glycol, triethylene glycol, polyethylene glycol, l,4-dioxane, cyclohexanone, diethyl ketone, and ethyl n-propyl ketone. Liquefaction of untreated wood can also be achieved at a lower temperature of 150°C and at atmospheric pressure in the presence of a catalyst [12]. Phenolsulfonic acid, sulfuric acid, hydrochloric acid, and phosphoric acid were used as catalysts. In this acid catalyst method, phenols and polyhydric alcohols can also be used for the coexisting organic solvents. Phenol, cresol, bisphenol A and F, and so forth are successfully adoptable as the phenols. Polyethylene glycols, polyether polyols (epoxide additionally reacted polyether polyol, polyethylene terephthalate polyol) have been found to liquify wood resulting in polyol solutions [13]. Liquefaction of wood in the presence of E-caprolactone, glycerin, and sulfuric acid has also been accomplished. It was confirmed in this case that liquefaction and polymerization, the latter of which produces polycaprolactone, take place in the reaction system at the same time [14]. Besides the wood material, it has become apparent that trunk and coconut parts of palm, barks, bagasse, coffee bean wastes, and used OA papers can also be liquified [15]. In the cases of the noncatalyst method, it is possible to obtain paste-like solutions with a high concentration of wood solute of up to 70%. The wood concentration of 70% is an extreme, obtained for the liquefaction in the presence of phenol, and usually the concentration will be lower. The values became even lower when polyhydric alcohol was used. In order to obtain the

Yoshioka et al.

188

liquified solution with enriched biomass content, a combined liquefaction of wood and starch in a polyethylene glycoVglycerin blend system was successfully studied [16]. As observed in the explosion and the autohydrolysis processes for wood, recondensation of degraded wood components also occurred more easily as the ratio of liquid to wood (liquid ratio) became smaller. This phenomenon makes it difficult to obtain a concentrated wood solution. After having found that starch can be Iiquified easily at a low liquid ratio without recondensation, a so-called stepwise liquefaction method was proposed for the preparation of large-biomass-content liquefaction mixture. That is, after the wood was Iiquified to a satisfactory degree, meaningful amounts of starch were added and liquified, resulting in a highly concentrated solution. Table I lists the results obtained by this method. It can be seen that a liquefaction mixture containing as large as 62.5% biomass can be obtained with a liquefaction rate of 90.7% [161. Because the liquified solutions thus obtained are paste-like, it is necessary to dilute them with some suitable solvents in order to measure the liquefaction rate. In that sense, the soluble properties of the liquified wood and starch (Iiquified biomass) were investigated using a series of diluent solvents [17]. It was found that the liquefaction mixture consists of complicated aggregates of the degraded and modified wood components. In most cases, any single solvent could not dissolve all of the liquified components completely, as shown in Table 2. It was found that the binary systems composed of different-

Table 1

Results of the Stepwise Liquefaction Procedure for Wood and Starch Liquefaction timeb

Reaction compositions Wood (parts)

1.0 1.5 1.5 2.0 2.0 2.0

Starch (parts)

Solvent 8 (parts)

2.0 1.5 3.0 1.0 2.0 3.0

3 3 3 3 3

Total biomass content

3

·Liquefaction solvent composition: bReaction temperature 150°C.

Combined liquefaction time (min)

Liquefaction rate

(%)

Preliquefaction time of wood (min)

50.0 50.0 60.0 50.0 57.0 62.5

90 90 90 80 80 80

20 20 20 15 15 15

98.2 93.8 96.0 84.5 88.3 90.7

PEG-400/glycerinJH~04=

80:20:3 (by weight).

(%)

Uquefaction of Wood Table 2

189

Soluble Behaviors of Liquefied Wood- in Various Single Solvents

Solvents PEG/glycerin b Water Ethylene glycol Methanol Acetone Dioxane Tetrahydrofuran

Hydrogen bonding capacities (A~D(cm -

390.0 206.0 187.0 97.0 97.0 90.0

I»

Solubility parameters (8)

Solvent-insoluble residue (%)

23.4 15.7 14.7 9.8 9.8 9.3

12.8 64.0 14.2 45.8 48.0 35.5 49.0

-Liquefaction compositions and conditions: woodJPEG-400/glycerinlH 2S04 = 50:80:20:3, reaction temperature 150aC, time 90 min. bpEG-400/glycerin = 80:20.

polar solvent, one-electron-donor moderately polar solvent, such as dioxane, tetrahydrofuran, acetone and so forth, and the other hydroxyl-containing electron donor-acceptor greatly polar solvent, such as methanol, ethylene glycol, water, and so forth, were good diluent solvents for liquified biomasses prepared in various liquefaction solvents. Figure I shows the solubility of a Iiquified wood in two of this kind of binary solvents. It can be seen that the insoluble residues are reduced to minimum values when diluting solvents are composed of suitable ratios. The minimum values are correspondent to the complete dissolution of the liquified fraction of the wood. Table 3 shows the solubilities of the liquified woods prepared from different liquefaction solvents in one of the typical binary solvents, dioxane/water 8:2. It can be found that the insoluble residues measured in the binary solvent are actually the same as those measured in the liquefaction solvents themselves. These phenomena can be illustrated by consulting with physicochemical properties of the binary solvent systems, i.e., positive deviation of the activities of each constituent from its ideal behavior. After liquefaction, the wood components were found to be degraded and became reactive. The obtained wood solute can be used to prepare adhesives, moldings, and other products, opening a practical new field for utilizing wood materials. In order to further elucidate the mechanism of phenol liquefaction of wood, several experiments have been continued. From these negligible conversion of wood into gaseous substances during the liquefaction treatment, lowering

Yoshioka et al.

190

100

...-..

--

~ 0

80

Solvents: o:Dioxane/waler A :Dioxane/methanol

=' "0

'Vi G)

.... :0

60

<3 Vl

40

,0""",,0

0

='

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0 100

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20

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20

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80

0 100

Mixing ratio of solvents Figure 1 Solvent-insoluble residue of liquefied wood in different binary systems as functions of the mixing ratios of the two solvents. Note: liquefaction solvent composition: PEG-400/glycerinlH 2S04 80:20:3, liquid ratio 2, reaction time 90 min.

Table 3 Percent Residues of Various Liquefied Wood Mixtures Measured in Different Diluents Percent residues (%) Liquefaction solvents

Liquefaction solvents

PEG-400 PEG-400/glycerin (8:2) PEG-400/l,I,I-tris(hydroxymethyl)propane (8:2) Polycaprolactone 303 e-Caprolactone/glycerin (75:25) Phenol Note: liquefaction conditions are the same as noted in Table 2.

21.4 12.8 18.9 14.0

Dioxane/water

21.0 13.7 19.3 14.3 8.5 17.1

Uquefaction of Wood

191

of the molecular weight, cleavage of bonds in the wood components through phenolysis, and significant phenolation of the degraded fragments of the wood components were shown [18].

III.

APPLICATIONS OF THE LIQUEFACTION OF WOOD

A.

Chemically Modified Wood

There are many potential applications for the liquefaction and dissolution of chemically modified wood. Examples include the fractionation of modified wood components, the preparation of solvent-sensitive and/or reaction-sensitive wood-based adhesives [4,9,11,19], the preparation of resinified woodbased moldings such as the foam type [4], and the preparation of wood-based fibers and their conversion to carbon fibers [20]. To fractionate modified wood components, the dissolution-precipitation technique has been successfully used. Quantitative precipitation of the wood components can be made by pouring the cresol solutions of chlorinatedcyanoethylated wood into an excess of ethanol, ethyl ether, or other solvents. Since the recovered amounts of the precipitates were found to change with the type of nonsolvent, the fractionation of modified wood components was thought to be possible [21]. In the preparation of adhesives from chemically modified wood, phenols, bisphenols, and polyhydric alcohols have been used as solvents and the obtained resins contained a significant amount of modified wood [4,9,11,19,22,23]. The combined use of these reactive solvents with reactive agents, e.g., a crosslinking agent and/or hardener, has given rise to phenolfonnaldehyde resins (such as resol resin), polyurethane resins, epoxy resins, etc., all of which contained a significant amount of chemically modified wood and possessed outstanding gluability [9,11,19,24, 25]. Molded materials such as foams or pressed shapes can also be obtained from chemically modified wood solutions [4]. Foams, for example, can be prepared by adding an adequate amount of water as a foaming agent and a polyisocyanate compound as a hardener to a 1,6-hexanediol solution of allylated wood, all of which are then mixed well and heated. When heated at 100°C, foaming and resinification are initiated within 2 min and completed within several minutes. If a promoter such as triethylamine is added, rapid reactions occur even at room temperature and foams can be obtained within several minutes. The foam has a low apparent density of 0.04 glcm 3 , a

Yoshioka at al.

192

substantial strength, and significant restoring force to compressive defonnation [4]. In order to elucidate the role of the chemically modified wood within the foams, comparative experiments in preparing the foams without the chemically modified wood have been conducted. It was found that foaming actually occurs during the resinifying process, but immediately after that a contraction occurs resulting in an apparent density around 0.2 glcm 3 with little foamed-cell structure remaining. This result reveals that the chemically modified wood plays a positive role in maintaining the shape of the foams during their fonnation. One other application of modified wood solutions is the fonnation of filaments or fibers [20]. After making a phenol solution of acetylated wood, hexamethylene tetramine was added and the mixture was heated at 120°C to promote the addition-condensation reaction for a resinified solution with high spinnability. From this solution, filaments were spun and hardened in an oven at a certain heating rate. The maximum temperature for hardening was 250°C, and continuous filaments could easily be obtained by this method. These filaments can then be carbonized to produce carbon filaments. Carbonization was carried out in an electrically heated furnace at a maximum temperature of 9OQ°C with a heating rate of 5.5°C/min. The strength of these carbon filaments has been measured according to the Japan Industrial Standard (1IS R7601) and a tensile strength up to I GPa has so far been obtained. This strength is comparable to that of the pitch carbon fibers of general-purpose grade. Greater strength can be expected by improving the methods for spinning and carbonization.

B.

Untreated Wood

From Iiquified solutions of untreated wood, almost the same products have been prepared as those from chemically modified wood [7,8,23,26]. For example, resol-type phenol resin adhesives prepared from five parts of wood chips and two parts of phenol, liquified at 250°C without catalyst, did not require severe adhesion conditions and were comparable to the corresponding commercial adhesives in their gluability. Acceptable waterproof adhesion was attained from the adhesives after gluing wood veneers at 120-130°C with a hot-pressing time of 0.5 min to I mm-thick plywood [27]. Resol-type phenol resin adhesives were also prepared from wood-phenol solutions Iiquified at 150°C with phenolsulfonic acid catalyst and their gluabilities were examined [12]. The results revealed that when these adhesives were used, it was easily possible to realize completely satisfactory waterproof

Liquefaction of Wood

193

adhesion even under hot-press conditions at 120°C, with a hot-pressing duration of 0.5 min applied to I-mm-thick plywood. This adhesion temperature of 120°C is at least 15°C lower than that ordinarily used for resol resin adhesives. As the second example, foams can be prepared from untreated wood-polyethylene glycol solutions [23]. Both soft and hard types of foams can be produced according to the preparation conditions. The prepared foams had a density of around 0.04 g/cmJ, substantial strength, and strong restoring force against deformation. Bio- and photodegradable properties were also found for these foams. These results imply that the wood components were not merely blended with the foam bubbles but also played an important role in maintaining the dimensional stability of the foams. Foams with enhanced properties have been developed with the liquified wood in the presence of polyether polyols and polyester polyols [28]. An example is shown in Fig. 2. The third application example is Novolak resin-type moldings prepared from untreated wood-phenol solutions [29,30]. After one part of wood meal had been liquified in two parts of phenol, the unreacted phenols were distilled

Figure 2

Foam prepared from liquefied wood in the presence of polycaprolactone.

194

Yoshioka st al.

off under reduced pressure. The obtained reactive powders from the liquified wood-phenol solution can be cured directly, after the wood meal fillers and hexamethylene tetramine are added, and hot-pressed at 150-190°C. The flexural strength of the moldings was comparable to those made from commercial Novolak. Plant scale production of this molding has started and an example of the products is shown in Fig. 3. After the untreated phenol was distiIled off, the liquefied wood in phenol showed a curing ability comparable with that for the commercial Novolak resin. This fact was observed through measurements with a differential scanning calorimeter (DSC). a measuring apparatus for dynamic viscoelasticity. and a laboplastmiIl [18). Almost the same DSC curve could be obtained for the phenol-liquefied wood powder as that for the commercial Novolak, revealing that the curing reactivity of the constituting components of the former is concentrated at the same level as the latter. However, it has also been shown that the curing ability increases with an increase in the amounts of combined phenol [31). The amounts of combined phenol are those that react with the liquefied and degraded wood components. The amount of combined phenol increases with an increase in liquefaction

Figure 3

Sushi trays made from liquefied wood in the presence of phenol.

Uquefaction of Wood

195

temperature, liquefaction time, catalyst content, or liquid ratio. By removing the free phenol, the resulting liquefied woods become Novolak-Iike resins as described above. The measurements of flow properties of these liquefied woods reveal that the melts of liquefied woods behave as pseudoplastics and their flows obey the Ostwald de Waele power law equation. The amount of combined phenol within the liquefied woods and the presence of filler in the liquefied woods have great influence on their flow properties. The flowing temperature, activation energy, and zero shear viscosity of the liquefied woods show a tendency to increase with an increase in combined phenol (31). The carbon fibers already described could also be prepared from an untreated wood solution. Tensile strength up to 1.2 GPa has been obtained so far. Even better physical properties can be expected with further development. The present state of studies on wood liquefaction and dissolution has been briefly reviewed. This new field in the chemical processing of wood has great potential, but more studies are needed.

REFERENCES I.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14.

H. R. Appel, Y. C. Fu, E. G. Illig, F. W. Steffgen, and R. D. Miller, in Conversion o/Cellulosics Wastes to Oil, U.S. Bureau of Mines, 1975, RI 8013, 27. C. Vanasse, E. Chornet, and R. P. Overend, Can. J. Chern. Eng. 66:112 (1988). N. Shiraishi and K. Goda, Mokuzai Kogyo 39:329 (1988). N. Shiraishi, S. Onodera, M. Ohotani, and T. Masumoto, Mokuzai Gakkaishi 31:418 (1985). N. Shiraishi, Tappi Proc, International Dissolving Pulps Conference, 24-27 March, Geneva, 1987. N. Shiraishi; Jap. Patent 1456560 (1988). N. Shiraishi, Mokuzai Kogyo 42:42 (1986). N. Shiraishi, N. Tsujimoto, and S. Pu, Japan Patent Publication Unexamined, 261358 (1986) (Submitted on May 14, 1985). N. Shiraishi, Mokuzai Gakkaishi 32:755 (1986). N. Shiraishi; Jap. Patent 1475080 (1989). N. Shiraishi and H. Kishi, J. Appl. Polym. Sci. 32:3189 (1986). S. Pu, M. Yoshioka, Y. Tanihara, and N. Shiraishi, presented by M. Yoshioka, Adhesives and Bonded Wood Products Symposium, 19-20 November, Seattle, Washington, 1991. N. Shiraishi, K. Shirakawa, K. Kurimoto, Jap. Patent Publication, Unexamined, 106128 (1992). Y. Uehori, M. Yoshioka, and N. Shiraishi, presented by Y. Uehori, Japan Wood Research Soc., 42th Annual Meeting, Nagoya, 1992.

196 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

27. 28. 29. 30. 31.

Yoshioka et al. N. Shiraishi, M. Yoshioka, and K. Itoh, presented by K. Itoh, Japan Wood Research Soc., 40th Annual Meeting, Nagoya, 1992. Y. Yao, M. Yoshioka, and N. Shiraishi, Mokuzai Gakkaishi 39:930 (1993). Y. Yao, M. Yoshioka, and N. Shiraishi, Mokuzai Gakkaishi 40:176 (1994). A. Kurozu, M. Yoshioka, and N. Shiraishi, presented by M. Yoshioka, Japan Wood Research Soc., 40th Annual Meeting, Nagoya, 1992. H. Kishi and N. Shiraishi, Mokuzai Gakkaishi 32:520 (1986). N. Tsujimoto, Japan Wood Research Soc., 14th Symposium on Chemical Proceeding of Wood, Kyoto, 1984. M. Morioka, K. Ahigematsu, and I. Sakata, presented by K. Shigematsu, Japan Wood Research Soc., 35th Annual Meeting, Tokyo, 1985. N. Shiraishi, H. Itoh, and S. V. Lonikar, J. Wood Chem. Technol. 7:405 (1987). N. Shiraishi, in Cellulosic Utilization: Research and Rewards in Cellulosics (H. Inagi and G. O. Phillips, eds.), Elsevier, London, 1989, pp. 97-109. N. Shiraishi, Y. Tamura, and N. Tsujimoto, Mokuzai Kogyo 42:492 (1987). N. Shiraishi, Y. Tamura, and N. Tsujimoto, Mokuzai Kogyo 43:2 (1988). N. Shiraishi, in Lignin Properties and Materials (W. G. GlasserandS. Sarkanen, eds.), ACS Symposium Series 397, American Chemical Soc., Washington, D.C., 1989, pp.488-495. S. Pu, M. Yoshioka, and N. Shiraishi, Adhesive Technology for Tropical Woods Symposium, 25-28 May, Taipei, 1992. K. Kurimoto, K. Shirakawa, M. Yoshioka, and N. Shiraishi, presented by K. Kurimoto, Japan Wood Research Soc., 42nd Annual Meeting, Nagoya, 1992. K. Kato, M. Yoshioka, and N. Shiraishi, presented by K. Kato, Japan Wood Research Soc., 40th Annual Meeting, Tsukuba, 1990. N. Shiraishi, and K. Kato, Jap. Patent Publication Unexamined, 43442 (1991). L. Lin, M. Yoshioka, Y. Yao, and N. Shiraishi, J. Appl. Polym. Sci., 55:1563-1571 (1995).

8 Surface Modification and Activation of Wood Makoto Kiguchi Forestry and Forest Products Research Institute Tsukuba, Ibaraki, Japan

I.

INTRODUCTION

Wood has many properties that make it superior to other materials in everyday use. For example, it has beautiful grain and color, comfortable touch, moisture absorptive and desorptive properties, high specific mechanical strength, and so on. These characteristic properties result in part from its cellular structure. Ideally, chemical modification of wood should improve properties in which wood is deficient without compromising those that give it its appeal. There are many properties demanded of wood surfaces depending on how and where they are used. These include weathering resistance, hardness, water resistance, water repellency, wettability for adhesives or coatings, and so on (Fig. 1). Most of these properties may be improved by means of surface modification. Bulk chemical modification of wood is a very effective means of improving wood properties such as dimensional stability or biological resistance. In general, such modification is achieved under heterogeneous conditions, involving solid-liquid phase reactions. These reactions require an excess of chemical reagents, so that reaction efficiencies are comparatively low. Treatments also usually require high temperatures and special costly corrosionresistant reactors. Furthermore, the recovery of unreacted reagents is an important consideration in commercial scale applications. For these reasons the

197

004

Properties Demanded of Wood Surfaces

tt later Resistance

~

~

Surface Modifications

2·

I

Surface Thermoplasticization I

•

I

Surface Activation I

•

I

Introduction of Functional Groups

•

I

Compression Set

) Hot-Welted and Self-Bonded Board

tt later Repellency tt leather Resistance

) Self Bonding, Improvement of Adhesive

tt Light Stability tt Flatness and Glossiness

I

>

Surface Graft Polymerization

tt Milde,proof tt Antifouling Property tt Improvement of lettability tt Hardness

~

Figure 1 Surface modifications of wood.

". I

IPC

i

Surface Modification and Activation of Wood

199

production cost of chemically modified wood is very high, which is why its commercial application has been limited. Surface modification of wood is a noteworthy new technique of wood improvement in which only surfaces are treated. The inner parts of the wood are unmodified and as a result retain their inherent properties. Of course, less chemical is needed to modify only the surfaces of wood compared to bulk modification. Vapor phase reactions are particularly suitable for surface treatments. This could reduce the amount of reagents required for modification and make the removal of unreacted reagent easier, thereby reducing treatment costs. This chapter describes the surface activation and surface thennoplasticization of wood. The production of hot-melted and self-bonded boards through surface thennoplasticization of wood particles is also described.

II.

SURFACE ACTIVATION OF WOOD

A.

Characterization of Wood Surfaces

Analysis by ESCA (electron spectroscopy for chemical analysis) of chemical bonding of carbon atoms (CIs) on wood surfaces reveals hydrophobic chemical bonding, i.e., carbon-to-carbon bonding (C-C) or carbon-to-hydrogen bonding (C-H) [1,2]. Figure 2 shows ESCA (Cis) spectra of untreated and alcoholbenzene extracted Sugi (Japanese cedar: Cryptomeriajaponica D. Don) heartwood surfaces [3]. An untreated wood surface was rich in hydrophobic chemical bonds at around 285 eV of binding energy. Generally, timber is subject to cutting and drying processes before it is used, and during such processing water in wood migrates centrifugally, depositing hydrophobic fatty acids and resins at wood surfaces [4]. However, when wood is extracted with an ethanol and benzene mixture these extractives are removed and the surface shows hydrophilic properties. The ESCA spectrum of such surfaces shows a large increase in carbon-to-oxygen bonding at around 287 eV (Fig. 2). Generally, it is necessary to remove extractives from wood surfaces to improve their wettability and adhesive properties.

B.

Surface Oxidation of Wood

Nitric acid, sulfuric acid, and hydrogen peroxide have often been used as surface oxidation reagents for wood, with the aim of introducing carbonyl or carboxyl groups to wood surfaces [5-7]. Surface oxidation treatments have generally been used as pretreatments to facilitate self-bonding of wood sur-

Kiguchi

200

%--------------

100

50

290 100

280

Extracted wood

50

294

290

280

Binding Energy (eV) Figure 2 ESCA (CIs) spectra of untreated and extracted wood surfaces. PI, Peak of C-e or C-H bonding~ P2, peak of C- 0 bonding~ P3, peak of C = 0 or o-e = 0 bonding. (From Ref. 3.)

faces. Many studies have used such treatments for the production of particleboards containing no adhesive. The ability of surface oxidation to induce self-bonding depends on the wood species and the character of the wood surfaces. Moreover, bonding strength is often unstable, and the "self-bonding" treatments invariably require gap fillers such as lignosulfonate and furfuryl alcohol mixtures to im-

Surface Modification and Activation of Wood

201

prove water resistance [7,8]. The bonding mechanisms of surface oxidation treatments have not been fully clarified, but it is likely that crosslinking or condensation reactions between activated particles play an impol:ant role. Surface oxidation processes have also been used as pretreatments for improving the bonding strength of adhesives. Brink et al. [9] reported that the wet bonding strength of plywoods or particleboards manufactured using phenol fonnaldehyde increased after pretreatment of wood with nitric acid. Mari et al. [10] also reported that nitric acid oxidation reduced the amount of isocyanate resin adhesive required to manufacture particleboard and improved the mechanical properties and biological resistance of boards.

c.

Introduction of Functional Groups to Wood Surfaces

Introduction of carbonyl or carboxyl groups is one of the most effective simple activation treatments for wood surfaces. Carbonyl or carboxyl groups are effective functional groups for improving the wettability and the bond strength of adhesives or coatings on wood surfaces. Graft polymerization of vinyl monomers on wood, which may also improve surface properties, is more complex and requires the prior introduction of vinyl groups to wood surfaces. Williams reported the graft polymerization of a functional UV absorber (hydroxyepoxypropoxybenzophenone) to wood surfaces to improve their weather resistance [11], but the grafting efficiency was low. However, Hon et al. reported that the weather resistance of wood was improved by treatment of wood surfaces with polymerized UV absorbers [12], and it is possible that the introduction of functional groups at wood surfaces could facilitate subsequent graft polymerization of UV absorbers.

1.

Introduction of Vinyl Groups

Ohkoshi introduced vinyl groups to wood surfaces by allylation after pretreatment with sodium hydroxide and this improved the water resistance of hotmelt and self-bonded surfaces [13]. A grafting ratio in excess of 200% was achieved by graft polymerization of styrene of methyl methacrylate (MMA) onto allylated wood surfaces (Figs. 3 and 4). After such treatment graftpolymerized, hot-pressed, and self-bonded wood specimens showed much higher wet shear strength than ungrafted allylated wood (Fig. 5). Graft polymerization of many kinds of vinyl-containing monomers or oligomers on wood surfaces can be achieved by prior introduction of vinyl groups.

Kiguchi

202

)/0

/

/0

.g !!

E 1; ~

100

/0 o o

'

,

,

10

20

30

, 40

\Ye9lt-98lcent gain (./.)

Figure 3 Relationships between the weight percent gains due to allylation and the grafting ratios of styrene on wood meal. Note: copolymerized at 100°C for 2 h using BPO 0.041 mollL as an initiator. (From Ref. 13.)

2.

Introduction of Carboxyl Groups

Matsuda introduced carboxyl groups to wood meal using dicarboxylic anhydrides. Maleic (MA), succinic (SA), and phthalic anhydride (PA) were used as acid anhydrides, N,N-dimethylbenzylamine was used as a catalyst, and dimethylsulfoxide (DMSO) or dimethylformamide (DMF) was used as solvent [14]. The carboxyl groups in the esterified wood were then further reacted with epichlorohydrin to produce oligoesterified woods. A detailed description of these reactions is given in Chapter 6. Matsuda's study outlined one of a number of techniques that can be used to produce high-quality wood materials from wood meal, but can also be used to introduce carboxyl groups to wood surfaces thereby greatly increasing the reactivity of the modified surfaces with epoxy groups. This allows techniques like the introduction of vinyl or vinyl groups to be used as a pretreatment to facilitate grafting of functional monomers or polymers to wood surfaces.

3.

Plasma Treatment

Introduction of functional groups to wood is mainly achieved in the liquid phase using strong acids, allylation, or esterification. Because wood is a porous and

Surface Modification and Activation of Wood

203

a)

b)

WPG: 10.1% Graftilg ratio : sari. WPG: 2481. Graftilg ratio : 129.0'1.

WPG : 36.4'1. Gtaftilg ratio : 192.0'1.

e)

Graftilg ratio ; tO%

,

~

,

,

,

,

§ §

I

I

,

I

,

§

,

,

,

•

I

~

, ,

, , ,

§

Wavecunb«s (em",

Figure 4 IR spectra of allylated wood copolymerized with styrene. WPG, Weight percent gain due to allylation. Note: (a) allylated wood; (b-d) allylated woods copolymerized with styrene; (e) alkalinepretreated woods copolymerized with styrene. (From Ref. 13.)

bulky material these treatments need an excess of reaction reagents. Furthennore, after treatment, removal of unreacted reagents from wood is difficult and this increases treatment costs. Treated woods are also damaged by the high temperatures, strong acids, or swelling agents used in the reactions and this results in decreased mechanical strength. For these reasons, as mentioned above, vapor phase treatments are preferable for the surface modification of wood. Plasma treatment is a very interesting and unique technique for wood surface modification. The plasma is in an ionized gaseous state and this is

Kiguchi

204 (i'l water

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i

5

9

~ c

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ti)

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0

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non-

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Figure 5

non-

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Wet shearing strength of styrene-grafted allylated wood. (From Ref. 13.)

created by subjecting a gaseous monomer to heating, radiowave bombardment, or discharging under reduced pressure. Gaseous monomers are blown into the vacuum chamber and are polymerized onto the surfaces of the materials to be treated. For example, oxygen atoms may be introduced onto wood surfaces using oxygen gas plasma and the surfaces then become hydrophilic. Fluorine atoms can also be introduced by CF4 plasma treatment [15] resulting in the treated surfaces becoming very hydrophobic and showing good antifouling properties. ESCA (CIs) spectra of wood surfaces modified by these plasma treatments are shown in Fig. 6. Amino groups may be introduced onto wood surfaces using nitrogen, ammonia, or pyridine plasma treatments. The degree of exhaustion on wood surfaces containing amino groups induced by ammonia plasma was about three times higher than on untreated wood surfaces [15]. Plasma treatments using various gases can introduce different functional groups to wood surfaces. They may also be used to improve the adhesive properties of plastic coatings. For example, plasma treatments are used industrially as a method of coating automobile bumpers. However, there are some problems in applying plasma treatments to wood. First, the equipment required for plasma treatments is expensive. Second, it is difficult to treat porous woods with plasma under reduced pressures. The latter problem has led to the investigation of normal pressure plasma treatments of wood.

Surface Modification and Activation of Wood

205

~treated wood

o1=::Z:::::Ir::I::I~""'''''~~---&'~

296 294 292 290 288 286 284 282 280 278

4000

Oxygen plasma

2000

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296 294 292 290 288 286 284 282 280 278

4000

CF 4 plasma

2000

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292

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288

284

280

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Figure 6 ESCA (Cis) spectra of plasma-treated wood surfaces. (From Ref. 15.)

III.

THERMOPLASTICIZATION OF WOOD SURFACES

A.

Thermoplasticization of Wood Surfaces

1. Thermoplasticization of Wood In the late 1970s, Shiraishi and his group reported that wood could be converted to thennoplastic materials by esterification with higher fatty acids [16].

Kiguchi

206

The thermoplasticization of wood was subsequently reviewed in detail by Shiraishi [17]. Cellulose occupies about half of wood's chemical components and approximately 70% of cellulose is crystalline, forming strong hydrogen bonds. In general, wood is carbonized by heating and does not show melting properties. However, wood can be converted to a thermoplastic material by chemical modification. Thermoplasticity of wood is achieved by swelling the crystalline cellulose and derivatizing hydroxyl groups to organic functional groups with higher molecular weight and lower polarity. At the same time, the thermoplasticity of amorphous lignin and hemicelluloses in wood, which show higher thermoplasticity than cellulose [18], drastically increases. Therefore these treatments act as "internal plasticizers," and whole wood may "melt" during hot-pressing. Figure 7 shows a simple mechanism for the thermoplasticization of benzylated wood. Thermoplasticity of chemically modified wood may be increased further by chlorination, using chlorine water as a posttreatment. Chlorination depolymerizes lignin and increases its thermoplasticity [19]. However, this treatment generates organic chlorine compounds, which may cause environmental problems. When wood meals are kneaded with synthetic thermoplastic resins, the mixture shows hot-melting behavior as these plastics act as "external plasticizers" [20]. However, such wood plastic composites lack a fibrous construction and do not show the excellent properties of solid wood such as beautiful grain and color, tactility, or insulation. Surface thermoplasticization of solid wood or particles is one of the more useful chemical modification techniques because most of the inherent properties of wood are retained. External plasticizers can only be applied to wood Benzylated Wood

Wood

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'--

sweIng

Figure 7 Illustration of the thermoplasticization of benzylated wood.

....

""••••

Surface Modification and Activation of Wood

207

meals; therefore surface thennoplasticization of solid wood is limited to internal plasticization treatments such as esterification or etherification. The thennoplasticization of wood can be subdivided into two categories (Fig. 8): (I) the total thennoplasticization of wood meals to produce products that can be used as wood based adhesives or as moulded substitutes for synthetic plastics [21-23] and (2) partial thennoplasticization of wood. The development of hot-melted and self-bonded wood materials and methods of improving the surface properties of natural wood by partial thennoplasticization of wood are reviewed below.

2.

Thermoplasticization of Wood Surfaces by Etherification

Etherification of Wood.

Etherification of wood involves the derivatization of hydroxyl groups to other functional groups. In general, etherification of wood produces more stable bonding and generates lower heat of reaction than esterification. Swelling of wood and fonnation of sodium salt are commonly applied as a pretreatment prior to etherification. Benzylation, cyanoethylation, allylation, and/or hydroxyethylation are commonly used in the etherification of wood. Some methods of etherification of cellulose may also be applied to wood. It is possible to introduce various functional groups in wood, from hydrophilic to hydrophobic and from low molecular weight to high molecular weight, by etherification. Many reports have been published on the characterization of etherified wood. For example, allylated wood showed higher dimensional stability than acetylated wood [24] and cyanoethylated wood showed high decay resistance [25]. In Japan, composite boards are manufactured industrially using wood that has been etherified with hydrophobic alkyl halides. The boards show good water resistance, dimensional stability, and biological resistance [26], although they do not possess thennoplasticity.

Surface Thermoplasticization ofWood by Etherification.

Aoki and Norimoto [27] used lauroylation (esterification) and benzylation (etherification) to produce surface-thennoplasticized wood and also examined some properties of the modified wood. Lauroyl chloride was used for the lauroylation and benzyl chloride for the benzylation. Although thennoplasticization was observed using both treatments, separation of melting layers was observed at the surface of lauroylated wood. Benzylated Buna (Japanese beech: Fagas crenata BL.) had high plasticity and moisture absorptive and desorptive abilities without showing large dimensional changes in the radial direction (Fig. 9). In addition, defonnation of

:£

None (Iood Weals)

j

)

~

Wolding Materials (alternative saterials of general plastics) Liquefaction

~

Wolding Waterials

~ Resins

(Adhesives. Coatings). Films (Biodegradational. Photodegradational)

Flatness lood Fiber Organization

food Surfaces

~

Surface Processing

i

later Resistance Dimensional Stability Biological Resistance Existence (Surface Wodification

Hardness leather Resistance Particles

) Self-Bonding

~

Hot-Welted Board

~

Biodegradation

~ Photodegradation Figure 8

Two major directions of studies of the thennoplasticization of wood.

~ c: ~

Surface Modification and Activation of Wood

209

o o QJ

-

~ 15

o


6

10

8

12

(%)

Figure 9 Relationships between dimensional changes in the radical direction (J3R) and water absorption (4)) of surface-benzylated wood. J3R, Dimensional changes of radical direction; cD, water absorption; 0, untreated wood; 6, partially thennoplasticized wood; 0, thennoplasticized wood. (From Ref. 27.)

surfaces was repressed by melting materials, and the dielectric constant (E) of modified surfaces was constant at 2.14 regardless of moisture absorption. This was lower than that of untreated wood surfaces (E = 2.72) indicating that the electrical insulation of benzylated thermoplasticized wood surfaces was high. The surface thennoplasticization of Sugi using etherification was also examined [3]. Acrylonitrile, allyl bromide, and benzyl chloride were used in the cyanoethylation, allylation, and benzylation of wood surfaces (Fig. 10). Cyanoethylation and benzylation produced thermoplasticized surfaces with high glossiness, but allylated surfaces did not exhibit this property. Cyanoethylated wood surfaces were a reddish brown color and showed high glossiness, Brinell hardness, and good ultraviolet light resistance, but they did not show any water resistance (Fig. II). During cyanoethylation many complicated competing reactions [28] take place and hydrophilic subproducts may be produced. This may account for their poor water resistance. Furthennore, acrylonitrile, which is used in the cyanoethylation treatment, is very poisonous and any residual unreacted acrylonitrile remaining in the wood after treatment is dangerous to human health. For this reason, cyanoethylation of wood surfaces with acrylonitrile is unlikely to be commercialized. Benzylated wood surfaces were light yellowish in color and had high plasticity, glossiness, and water resistance. Infrared (IR) spectra of benzylated wood indicated that the melting materials on wood surfaces were benzylated

Kiguchi

210

NaOH

Wood-OH - - -......-

Wood-ONa

+

1'\0

1) CYANOETHYLATION Wood-ONa

+

CH 2

:

CHCN

- - 1... -

(Acrylonitrile)

Wood-o-CHzCHzCN (Cyanoethylated Wood)

2) BENZYLATION Wood-ONa. CICl\-@----WOod-O-CHz-@+NaCI (Benzyl Chloride)

(Benzylated Wood)

3) ALLYLATION Wood-ONa

+

BrCHzCH

=CH z -

(Alyl Bromide)

Wood-O-CHzCH

=CH z • NaBr

(AHylated Wood)

Flgure 10 Etherification treatments for surface thennoplasticization of wood.

wood components (Fig. 12). The benzyl content of wood increased with increasing weight percent gain (WPG), and absorption of the hydroxyl vibration at about 3400 cm-· diminished with increasing WPGs. New doublet peaks at 735 and 695 cm-· appeared due to benzylation and their intensities increased with increasing WPG. These absorptions, which are characteristic of monosubstituted benzene rings of the benzyl group, indicate that the hydrogen of hydroxyl groups in wood were replaced by benzyl groups. However, when reacted for longer times, the WPG of benzylated wood tended to decrease as highly benzylated components were eluted from the wood. The eluted material (BE-EM) showed almost the same Fourier transfonn IR spectrum as the benzylated wood. Benzylated thennoplasticized wood surfaces showed good water resistance, but because of the aromatic construction of the benzyl group (Fig. 13), they showed high absorption of ultraviolet (UV) light. As a result of this, benzylated wood surfaces did not show good UV and weather resistance. Benzylated

Surface Modification and Activation of Wood

211

80,.-----------------70

4

9

Wet and Dry Cycles (times)

Figure 11 Contact angle of a water drop on etherified wood surfaces after wet and dry cycle testing. BE, Benzylated wood; AL; allylated wood; CE, cyanoethylated wood. Note: wet and dry cycle test was soaking in water and drying at 60°C for 24 h. (From Ref. 3.)

wood coated with oil-based, highly durable transparent coatings containing acrylic, silicon, or fluorocarbon resins may, however, perform well irrespective of UV resistance because of the high water resistance, dimensional stability, and increased wettability of benzylated wood (Fig. 14) [29]. Since the hardness of the melting materials on benzylated wood surfaces was not high, hardness of surfaces was not improved by benzylation [29]. The development of UV-degradable films from benzylated lignocellulosic materials is now underway [30].

B.

Hot-Melted and Self-Bonded Boards by Surface Benzylating Wood Particles

1.

Bonding Wood by Thermoplasticizing Surfaces

Hot-Melt and Self-Bonding of Wood. Ohkoshi examined the self-bonding of allylated Makamba (Japanese red birch: Betula maximowicziana Regel) wood [31]. Makamba blocks were allylated on both sides with sodium hydroxide and allyl bromide, and then hot-pressed at 160°C and I MPa to produce shear specimens. After modification, maximum shear strength of specimens

Kiguchi

212

UT

Wavenumbers (em")

Figure 12 IR spectra of benzylated wood meals. UT, Untreated wood meal; BE 27%, 31%, 37%, benzylated wood meal with 27 WPG, 31 WPG, and 37 WPG, respectively; BE-EM, eluted material during benzylation.

in compression was about 20 MPa. In general, allylated wood does not melt because of the low molecular weight of the introduced allyl group [17]. On the basis of microscopic observations, Ohkoshi suggested that thennoplasticization occurred between the allylated wood surfaces due to the high temperature and pressure during pressing. Shear strength in compression was influenced by the concentration of sodium hydroxide, pretreatment reaction times and temperature, and the pressing temperature. The self-bonding of benzylated wood surfaces after hot-pressing without

Surface Modification and Activation of Wood

213

UV irradiated

BE

294

290

280

Binding Energy (eV)

Figure 13 ESCA (Cis) spectra of UV-irradiated benzylated wood surface. BE, Benzylated wood. Note: benzylated wood was irradiated by UV for 96 h using a highpressure mercury lamp.

the addition of binders has been examined by the author [29]. Maximum shear strength in compression of modified specimens was about 15 MPa, and this is almost equal to the strength of blocks glued with a commercial polyvinyl acetate hot-melt adhesive. The shear strength of benzylated wood was also influenced by concentrations of sodium hydroxide solution used as a pretreatment, pretreatment reaction times and temperature, and pressing temperature. The glossiness of benzylated surfaces also influenced shear strength. Some properties of surface-therrnoplasticized wood produced by etherification are shown in Table 1. Thermoplasticity of Benzylated Wood. Benzylation is the most favorable treatment for the partial therrnoplasticization of wood surfaces. Benzylation of wood requires a pretreatment with sodium hydroxide to increase the swelling of wood and its reactivity with benzyl chloride. The molecular weight of

Kiguchi

214 100,-----------------,

----

... ..... "

". <.0

III

CIl

.~

".

50

...----------

III

o

a

0 20 UJ ~

15

CIl

()

C CIl

£0 10 0

<5

0

5

24

48

72

96

UV Irradiation Time (h)

Figure 14 Changes in the glossiness and color difference of etherified Sugi surfaces during UV irradiation. 0, Cyanoethylated wood; 6, allylated wood; 0, benzylated wood; e, untreated wood; A, acrylic urethane resin-coated wood; _, acrylic silicon resin-coated wood. Note: UV lamps were high-pressure mercury lamps with 80 WI em. Coatings were transparent. (From Ref. 30.)

the benzyl group is about two times higher than that of the acetyl group, and it shows lower polarity and hydrophobicity. As mentioned above, thermoplasticity of benzylated wood depends on the concentration of sodium hydroxide solution used as a pretreatment. Melting was not observed when the concentration of sodium hydroxide was less than 20% (Fig. 15) and in general a 40% sodium hydroxide solution was used for pretreatment prior to benzylation. This is disadvantageous as it results in high

Table 1 Some Properties of Etherified Surface Thennoplasticized Wood Etherification Untreated Sugi sap wood Urethane resin-coated wood Cyanoethylated wood

UV Thennal softening Thenno- Glossiness, Shearing strength 8 Water Gr(60) point (OC) (kgf/cm2) plasticity repellancy resistance

Brinel hardness (kgf/mm2)

Color

-

None

5-15

-

Poor

Poor

0.20 (SG = 0.27)

-

None

70-80

-

Good

Medium

1.80

76.9

Good

53-73

63

Poor

Good

4.10

Reddish brown

Good Poor

32-75 14-33

146 49

Good Good

Medium Medium

0.95 0.30

Light yellow Light white

(Subproducts) Benzylated wood Allylated wood

58.6 138.1

'Surface-etherified Makamba (Betula maximowicziana Regal) specimens were used for shear strength tests. Other tests used Sugi (Cryptomeria japonica D. Don) sap wood. SG, air-dry specific gravity.

Kiguchi

216

80,...--------

10

20

30

40

Concentration of NaOH (010)

Figure 15

Effect of sodium hydroxide concentrations as a pretreatment for benzylation on the thennoplasticity of Sugi surfaces. Note: pretreatment time was I h. Benzylation conditions were at 120°C for 1 h. Sugi (Cryptomeria japonica D. Don). Angle of glossiness was 60°.

production costs for benzylated wood. Highly concentrated sodium hydroxide solutions are t however t viscous t penetrating only the surface of wood t and therefore they are particularly suitable as a pretreatment for the thennoplasticization of wood surfaces. Figure 16 shows thennal softening curves of benzylated Akamatsu (Japanese red pine: Pinus densifrola Sieb. et Zucc) particles. The curves were produced using the thennomechanical analysis (TMA) penetration method. Thennoplasticity of particles increased with increasing WPGs. The thennal degradation point of the benzylated wood was almost the same as for untreated wood, and major weight losses due to heating started at around 250°C (Fig. 17). The softening point of benzylated wood was about 80°C and benzylated wood with more than 37% WPG showed thennal flow before its thennal degradation point [32]. However, more highly benzylated wood was soluble and materials were eluted from it during heating. The degree ofthennoplasticity of benzylated wood was not in proportion to its WPG. t

Surface Modification and Activation of Wood

217

0 20

C 40

~

l!

l

60

0..

80 tOO 25

100

50

150

250

200

300

Temperature ('I.)

Figure 16 Thennal softening curves of benzylated wood meals. BE27%, 31%, 37%, Benzylated wood meals with 27 WPG, 31 WPG, and 37 WPG, respectively. UT, Untreated wood meal. Note: measuring conditions were 5°Clmin in air. Load was 25 g. (From Ref. 32.)

0

C

20

.9

40

en en

i cf

--- -----",'~\: , , I

I

(J

I

,

60 ......................

.....,..,

80

~ ...... _._----

100 30

500

100 Temperature (Oe)

Figure 17 Thennal gravimetric curves of benzylated wood meal. BE, Benzylated wood meal with a 37 WPG; EX, ethanol benzene-extracted wood meal. Note: measuring conditions were 3°Clmin in air. (From Ref. 32.)

Kiguchi

218

Preparation of Hot-Melt and Self-Bonded Boards. Reaction conditions for benzylation are dependent on wood species and specimen size. For Akamatsu particles, pretreatments and reaction conditions are as follows: wood is first oven-dried at 105°C for 24 h and then immersed in 40% sodium hydroxide solution for 1-2 h at room temperature. These particles are then squeezed to remove excess sodium hydroxide and reacted with benzyl chloride at 120°C for 1-2 h. Benzylated particles are then washed in water and any unreacted reagent is removed by squeezing. Particles are then washed further in a mixture of water-methanol (1:2 v/v) and air-dried for 48 h at 20°C. In general, benzylation of softwood species is more difficult than for hardwoods, and the reactivities of wood from fast-growing trees, i.e., willow or monocotyledons, i.e., bamboo, with some reagents are higher than for softwoods. Accordingly, surface thennoplasticization of these fast-growing species can be undertaken using milder benzylation reaction conditions.

2.

Some Properties of Hot-Melted and Self-Bonded Boards

Since the bonding strength of benzylated wood surfaces was similar to that achieved using commercial hot-melt adhesives, the development of selfbonded wood materials using surface benzylated particles was possible [32]. Suzuki and Iwakiri [33] reported that self-bonded fiberboard produced from benzylated asplund-processed fibers had high thennoplasticity, dimensional stability, and water resistance. Morita and Sakata also produced hot-melt cyanoethylated fiberboards, but they showed poor water resistance [34]. Both benzylated and cyanoethylated fiberboards lacked many of the inherent "good" properties of solid wood because they did not retain the cellular construction of wood. Morphologic Properties. SEM photographs of hot-melted boards prepared by benzylating Akamatsu particles (Figs. 18-20) revealed that board surfaces were very smooth and covered with hydrophobic melting materials. Fibers were not observed on board surfaces (Fig. 18), but some small cavities were present on the surface and these contained fibers (Fig. 19). Cross-sections through benzylated boards (Fig. 20) revealed many tracheid walls and lumens. Photographs of the interfaces between particles revealed that internal board adhesion occurred via surface hot-melting. These photographs (Figs. 18-20) show that hot-melt and self-bonded boards have very smooth and hydrophobic surfaces, and because they retain a fibrous structure they also have good insulating properties.

Surface Modification and Activation of Wood

Figure 18 Ref. 32.)

219

Micrograph of the surface of benzylatcd board with 51 WPG. (From

Mechanical Properties. The bending strength of benzylated and hot-melted boards increased with increasing thennoplasticity of particles and specific gravity of boards (Fig. 21) but was slightly lower when compared to conventional particleboard bonded with a phenol fonnaldehyde resin binder. The amount of wood substance in benzylated particleboards decreases with increasing WPG of particles. Wood particles may also be damaged during benzylation as a result of pretreatment with sodium hydroxide and reaction with benzyl chloride at high temperatures. Both of these factors may explain the inferior bending strength of benzylated boards. On the other hand, the internal bond strength of benzylated boards manufactured using particles with 51 WPG was about two times higher than that of a conventional board (Fig. 22). The bonding between particles in conventional particleboard is via point adhesion, whereas in hot-melt and self-bonded boards it occurs by plain adhesion. Accordingly, the internal bond strength

220

Figure 19

Kiguchi

Micrograph of hot-melted fibers (lracheids) in a cavity on the board.

between particles in hot-melt boards is higher than for conventional boards. This is one of the interesting properties of self-bonded and hot-melted boards. Dimensional Stability. Hot-melted boards were immersed in water for 24 h at 25°C. Thickness swelling of the boards decreased with increasing WPG of the particles and board-specific gravity (Fig. 23). The thickness swelling of hot-melted boards manufactured using benzylated particles with a WPG of 51 % and a specific gravity of 1.0 was less than I %. Since substituted benzyl groups are hydrophobic and board surfaces were also covered with hydrophobic melting materials, hot-melted and self-bonded boards showed high water resistance. Furthermore, strong adhesion between the particles via plain adhesion also enhanced board dimensional stability. Decay and Weather Resistance. The decay resistance of hot-melted and self-bonded boards was evaluated using pure cultures of the white-rot fungus (Coriolus versicolor L. ex Fr.) and the brown-rot fungus (Tyromyces palustris

Surface Modification and Activation of Wood

221

Figure 20 Micrograph of the transverse surface of benzy1ated board with a 51 WPG. (From Ref. 32.)

Berk. and Curt.). Soft rot testing involving a soil test was also undertaken. Weight percent loss of boards in the decay tests decreased with increasing WPG of particles (Table 2). A benzylated board with 51 WPG showed excellent decay resistance. This board was covered with mycelia during incubation, but the mycelia were easily removed from the board after the test, and the weight and surface glossiness of the board did not change during the test period. This suggests that benzylated boards might be resistant to the extracellular metabolites produced by decay fungi. Thus decay resistance is created by the change in the chemical composition of the wood, not by the toxicity of the introduced benzyl groups. In general, fungi are not able to attack chemically modified wood because of their substrate specificity. For this reason, the decay resistance of hot-melted benzylated boards is achieved in

Kiguchi

222 50,..-.-----------:-------"

40

30

20

10

°0.... .5-~0 .....6--0..... 7-~0'-:'.8-~O~.9-~tO Specific Gravity

Figure 21 Modulus of rupture in bending of surface-benzylated and self-bonded boards. BE21 % (0): benzylated board with a 21 WPG~ BE38% (£\): benzylated board with a 38 WPG~ BE51 % (0): benzylated board with a 51 WPG. PF (e), conventional particleboard bonds using phenol-fonnaldehyde resin binder acted as a control. (From Ref. 32.)

a more benign manner than that of conventional particleboards and therefore presents fewer problems for the environment. The weather resistance of hot-melted benzylated boards was not high because benzyl groups on board surfaces absorbed UV light and were degraded. Benzylated boards exposed outdoors showed surface color changes and the development of many small cracks. However, as benzylated boards show low polarity due to the presence of benzyl groups, dimensional stability, water resistance, and adhesive properties of coatings are improved. There was no peeling of transparent acrylic, silicon, or fluorocarbon resin coatings on hotmelted benzylated boards after 3 years of outdoor exposure [35]. Vapor Phase Benzylation. It has generally been concluded that vapor phase

Surface Modification and Activation of Wood

223

4,.-----------------,

3

O--------''------'-----''----L-_....J 0.5

0.6

0.8

0.9

to

Specific Gravity

Figure 22

Internal bond strength of surface-benzylated and self-bonded boards. Abbreviations are the same as in Fig. 21. (From Ref. 32.)

benzylation is not a suitable procedure for wood. The boiling point of benzyl chloride is about 190°C and wood particles may be degraded at such high temperatures. However, a mist of benzyl chloride may be generated at around 140°C and under such conditions the vapor phase benzylation of wood particles may be feasible [36], although at this temperature longer reaction times would be needed. Figure 24 shows the preparation of surface-thennoplasticized particles by vapor phase benzylation at 140°C for 2 h. Such treatment (Fig. 24) reduces the amount of benzyl chloride required for modification and hence lowers the cost of the treatment when compared to liquid phase reaction. Combining Chemically Modified Particles. Hot-melted and self-bonded boards show slightly lower mechanical properties, except for internal bond strength, compared to untreated particleboard. A composite board containing benzylated particles at the surface and acetylated particles in the core showed

Kiguchi

224 30~------------""

Figure 23 Thickness swelling of surface-benzylated and self-bonded boards. Abbreviations are the same as in Fig. 21. Note: samples were immersed in water for 24 h at 25°C. (From Ref. 32.)

Vapor 140"C

0 20

~

~

40

~

60

e G)

0..

80 100 Akamatsu Particle

25

50

100

150

200

250

Temperature ("C)

Figure 24 Thermal softening curves of vapor phase bcnzylated particles. h, Vapor benzylation time (hour) at 140°C.

Surface Modification and Activation of Wood

225

similar surface properties and dimensional stability compared to a benzylated board, and its mechanical properties were slightly superior (Figs. 25 and 26). Future Potential of Hot-Melted and Self-Bonded Boards. Surface thennoplasticization of wood enhances many properties: dimensional stability, durability, and water resistance can be improved without loss of any of the inherent properties of wood. In addition, hot-melt bonding can be achieved without the use of costly binders. However, there are some difficulties associated with the commercial production of hot-melt benzylated boards. Although the price of benzyl chloride in Japan is almost the same as that of acetic anhydride, it requires careful handling as it produces strong skin and eye irritation and is corrosive to metals. After benzylation, removal of the unreacted reagent and byproducts is difficult and costly because they are composed of hydrophilic sodium hydroxide and hydrophobic benzyl chloride. The softening point of benzylated particles is about 80°C, and this also limits the potential commercial viability of hot-melt benzylated boards. Hot-melted and self-bonded boards show curve moldability during secondary processing, and some of the properties of the boards may be altered by modifying the shape of the wood particles or by changing the reaction conditions. Such boards show the properties of both wood and plastics. Thus a 70 .·X

.::-

Cil 60 a..

!.

50

'" :; 0.. 40 :3 a: (;

30

CIl

:3

"3

20

"0

0

:::lE

10 0

IU) (SG 0.85)

(A) (SG 0.85)

(8)

(SG tOO)

IB+ U) (SG 0.95)

IB+ Al (SG 0.95)

Figure 25 Modulus of rupture of particleboards combined with chemically modified particles. U, Untreated particles; A, acetylated particles; B, benzylated particles; SG, specific gravity of board. Note: isocyanate resin was used as a binder for particles U and A. Benzylated particles were used on the surfaces of the boards. Willow (Salix Arakiana koidz.) particles were used to manufacture boards.

Kiguchi

226 50

-r--------------------,

40

..

30 20

Thickness swelrng Q (Uj

Volume swelr.ng

Waler abSOfPbon

E!3

on

lID

I!II

(A)

IB)

(B .U)

(B.A)

Figure 26 Water resistance of particleboards containing combination of chemically modified particles. Abbreviations are the same as in Fig. 25. Note: samples were immersed in water for 24 h at 25°C.

new and unique woody material may be produced by surface thermoplasticization of wood particles. This technique is an important processing technique with great future potential that is not limited by the quality of wood used in manufacture.

REFERENCES 1.

2. 3. 4. 5. 6. 7. 8. 9.

G. M. Dorris and D. G. Gray, Cell. Chern. Technol. 12:9 (1978). R. A. Young, R. M. Ramman, S. S. Kelley, and R. H. Gillespie. Wood Sci. /4: 110 (1982). M. Kiguchi. Mokuzail Gakkaishi 36:651 (1990). G. E. Troughton and S. -Z. Chow, Wood Sci. 3:129 (1971). J. Stofko and E. Zavarin, U.S. Patent 4,007 ,312 to the Regents of the University of California (1977). W. E. Johns and T. Nguyen, For. Prod. J. 27(1):17 (1977). R. A. Young, M. Fujita, and B. H. River, Wood Sci. Technol. 19:363 (1985). J. L. Philippou, E. Zavarin, W. E. Johns, and T. Nguyen, For. Prod. J. 32(5):55 ( 1982). D. L. Brink, B. M. Collett, A. A. Pohlman, A. F. Wong, and J. Philippou, ACS Syrnp. Ser. 43:169 (1977).

Surface Modification and Activation of Wood 10. II. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

227

E. L. Mari, T. Sakuno, I. Furukawa, and J. Kishimoto, Mokuzai Gakkaishi 35:669 (1989). R. S. Williams, J. Appl. Polym. Sci. 28:2093 (1983). D. N. -So Hon, S. -T. Chang, and W. C. Feist, J. Appl. Polym. Sci. 30:1429 (1985). M. Ohkoshi, Mokuzai Gakkaishi 37:917 (1991). H. Matsuda, M. Ueda, and K. Murakami, Mokuzai Gakkaishi 34:140 (1988). K. Setoyama, Mokuzai Kogyo 44:653 (1989). H. Funakoshi, N. Shiraishi, M. Norimoto, T. Aoki, H. Hayashi, and T. Yokota, Holzjorschung 33: 159 (1979). N. Shiraishi, in Wood and Cellulosic Chemistry (D. N. -So Hon and N. Shiraishi, eds.), Marcel Dekker, New York, 1991, pp. 861-906. D. A. I. Goring, Pulp Paper Mag. Can. 64:517 (1963). M. Morita and I. Sakata, Cell Chem. Technol. 3/:831 (1986). N. Shiraishi, Kobunshi Kako 3/:500 (1982). N. Shiraishi and K. Goda, Mokutai Kogyo 39:329 (1984). D. N. -So Hon and N. -H. Ou, J. Polym. Sci. A 27:2457 (1989). D. N. -So Hon and J. M. S. Luis, J. Polym. Sci. A 27:4143 (1989). D. L. Kenaga, R. C. Sproull, and J. Esslinger, South. Lumberman /80:45 (1950). R. H. Baechler, For. Prod. J. 9(5):166 (1959). K. Onodera and K. Kitano, Mokuzai Kogyo 44:644 (1989). T. Aoki and M. Norimoto, Wood Res. 22:66 (1986). J. H. MacGregor, J. Soc:. Dyers Col. 67:66 (1951). M. Kiguchi, Mokuzai Gakkaishi 36:867 (1990). N. Shiraishi, Kogyo Zairyo 39(8):40 (1991). M. Ohkoshi, Mokuzai Gakkaishi 36:57 (1990). M. kiguchi and K. Yamamoto, Mokuzai Gakkaishi 38:150 (1992). M. Suzuki and S. Iwakiri, Bull. Exp. For., Tokyo Univ. Agric. Technol., 22:25 (1986). M. Morita and I. Sakata, Proc. 19th Symp. Chern. Processing of Wood (Japan), 1989, p. 19. M. Kiguchi, FRI Bull. /76:77 (1992). M. Kiguchi, Mokuzai Gakkaishi 39:80 (1993).

9 Chemical Modification of

Nonwood Lignocellulosics Roger M. Rowell USDA Forest Service and University of Wisconsin Madison, Wisconsin

I.

INTRODUCTION

When considering lignocellulosics as possible engineering materials, either in solid form (such as wood) or in fiber form (such as paper and fiberboard), there are several very basic concepts to be considered. First, lignocellulosics are hygroscopic resources that were designed to perform in nature in a wet environment. Second, nature is programmed to recycle lignocellulosic resources in a timely way through biological, thermal, aqueous, radiation, chemical, and mechanical degradations. In simple terms, nature builds a lignocellulosic from carbon dioxide and water and has all the tools to recycle it back to the starting chemicals. We harvest a green lignocellulosic (e. g., a tree) and convert it to dry products, and nature, with its arsenal of degrading reactions; starts to reclaim it at the first opportunity (Fig. 1). In order to produce lignocellulosic-based engineering materials with a long service life, it is necessary to interfere with the natural degradation processes for as long as possible. This can be done in several ways. Traditional methods for decay resistance and fire retardancy, for example, are based on treating

The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This chapter was written and prepared by U.S. government employees on official time, and it is therefore in the public domain and not subject to copyright.

229

Rowell

230 Biological degradation-fungi, bacteria, insects, termites Enzymatic reactions-oxidation, hydrolysis, reduction Chemical reactions-oxidation, hydrolysis, reduction Mechanical-chewing Fire degradation-lightning, sun, man Pyrolysis reactions-dehydration, hydrolysis, oxidation Water degradation-rain, sea, ice Wood-water Interactions-swelling, shrinking, freezing, cracking Weathering degradation-sun, water, heat, wind Chemical reactions-oxidation, hydrolysis Mechanical-erosion

Chemical degradation-acids, bases, salts Chemical reactions-oxidation, reduction, dehydration, hydrolysis, acid rain Mechanical degradation-storms (wind, rain, hail, snow), loads Mechanical-stress, cracks, fracture, abrasion

Figure 1 Degradation reactions that occur when lignocellulosics are exposed to nature.

the product with toxic or corrosive chemicals that are effective in providing decay and fire resistance but can result in environmental concerns. There is another approach based on the premise that the properties of any resource are a result of the chemistry of components of that resource. In the case of Iignocellulosics, cell wall polymers, extractives, and inorganics are the components that, if modified, would change the properties of the resource. In order to make property changes, you must first understand the chemistry of the components and the contributions of each in the properties of the resource. Following this understanding, you must then devise a way to modify what needs to be changed to get the desired change in property. Properties of lignocellulosics, such as dimensional instability, flammability, biodegradability, and degradation caused by acids, bases, and ultraviolet radiation, are all a result of chemical degradation reactions that can be prevented or, at least, slowed down if the cell wall chemistry is altered [1-7].

II.

FEATURES OF LIGNOCELLULOSICS

Lignocellulosics are three-dimensional, polymeric composites made up primarily of cellulose, hemicelluloses, and lignin. Table 1 shows the chemical composition of several different types of natural fibers. It is interesting to

Chemical Modification of Nonwood Lignocellulosics

231

Table 1 Chemical Composition of Some Common Fibers Chemical Component (%) Type of fiber Stalk fiber Straw Rice Wheat Barley Oat Rye Cane Sugar Bamboo Grass Esparto Sabai Reed Phragmites communis Bast fiber Seed flax Kenaf Jute Leaf fiber Abaca (Manila) Sisal (agave) Seed hull fiber Cotton linter Wood Coniferous Deciduous

Cellulose

Lignin

Pentosan

28-36 29-35 31-34 31-37 33-35

12-16 16-21 14-15 16-19 16-19

23-28 26-32 24-29 27-38 27-30

15-20 4.5-9 5-7 6-8 2-5

9-14 3-7 3-6 4-6.5 0.5-4

32-44 26-43

19-24 21-31

27-32 15-26

1.5-5 1.7-5

0.7-3.5 0.7

33-38

17-19 22.0

27-32 23.9

6-8 6.0

44.75

22.8

20.0

2.9

47 31-39 45-53

23 15-19 21-26

25 22-23 18-21

5 2-5 0.5-2

60.8 43-56

8.8 7-9

17.3 21-24

1.1 0.6-1

80-85 40-45 38-49

Ash

Silica

2.0

0.8-2 26-34 23-30

7-14 19-26


note that while there are many different kinds of lignocellulosics, there are only minor differences in composition. This might lead to a simple conclusion that reactions involving these three polymers and the changes in properties resulting from those modifications might be the same for all types of lignocellulosics. That is to say, that one does not have to devise a different reaction scheme for wood fiber than was used for jute or kenaf or, within limits, any other fi ber.

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Table 2 shows the fiber length of different types of lignocellulosic fibers. Fibers do come in different individual fiber length and in different aggregate fiber lengths. Without bias to any given industry (i.e., wood, kenaf, jute, bamboo, etc.), fiber selection for a given product could be made just on the basis of chemical composition and/or fiber length. Fiber selection is also based on availability, ease of handling, cost, and other factors. The object of this discussion is to point out that researchers should review all literature on the chemical modification and property improvement of all types of agrofibers and not restrict infonnation to just one type of fiber. There is a vast amount of infonnation, e.g., on the chemical modification of cotton, jute, and other agricultural fibers that is not considered relevant to the people doing research in wood chemistry. Another shortcoming in literature searching is the use of computerized searches, which in most cases do not go very far back in time into the scientific literature so that many of the old but yet excellent references are overlooked.

III.

DEGRADATION OF LIGNOCELLULOSICS

Figure 2 shows the cell wall polymers involved in each composite property as we understand it today [8]. Lignocellulosics change dimensions with changing moisture content because the cell wall polymers contain hydroxyl and other oxygen-containing groups that attract moisture through hydrogen bonding [9,10]. The hemicelluloses are mainly responsible for moisture sorption, but

Table 2

Dimensions of Some Common Lignocellulosic Fibers Fiber dimension (mm) Length

Average length

Width

Cotton

10-60

Flax

5-60 5-55 2.5-12 1.5-4 0.5-2 1-3.4 1.5-5 1-1.8 3.5-5

18 25-30 20

0.02 0.012-0.027 0.025-0.050 0.025-0.040 0.025-0.040 0.013 0.023 0.02 0.03 0.025

Type of fiber

Hemp Manila hemp Bamboo Esparto Cereal straw Jute Deciduous wood Coniferous wood

6

2.5 1.5 1.5 2

.Chemical Modification of Nonwood Ugnocellulosics

233

Biological degradation Hemicelluloses > > > accessible cellulose > noncrystalline cellulose > > > > crystalline cellulose > > > > > lignin Moisture Sorption Hemicelluloses > > accessible cellulose> > > noncrystalline cellulose > lignin > > > crystalline cellulose Ultraviolet degradation Lignin > > > > > hemicelluloses > accessible cellulose > noncrystalline cellulose > > > crystalline cellulose Thermal degradation Hemicelluloses > cellulose > > > > > lignin Strength Crystalline cellulose > > noncrystalline cellulose + hemicelluloses + lignin > lignin Figure 2 Cell wall polymers responsible for the properties of lignocellulosics.

the accessible cellulose, noncrystalline cellulose, lignin, and surface of crystalline cellulose also play major roles. Moisture swells the cell wall, and the fiber expands until the cell wall is saturated with water. Beyond this saturation point, moisture exists as free water in the void structure and does not contribute to further expansion. This process is reversible, and the fiber shrinks as it loses moisture. Lignocellulosics are degraded biologically because organisms recognize the carbohydrate polymers (mainly the hemicelluloses) in the cell wall and have very specific enzyme systems capable of hydrolyzing these polymers into digestible units. Biodegradation of the high molecular weight cellulose weakens the fiber cell wall because crystalline cellulose is primarily responsible for the strength of the cell wall [5]. Strength is lost as the cellulose polymer undergoes degradation through oxidation, hydrolysis, and dehydration reactions. The same types of reactions take place in the presence of acids and bases. Lignocellulosics exposed outdoors undergo photochemical degradation caused by ultraviolet light. This degradation takes place primarily in the lignin component, which is responsible for the characteristic color changes [II]. The lignin acts as an adhesive in the cell walls, holding the cellulose fibers together. The surface becomes richer in cellulose content as the lignin degrades. In comparison to lignin, cellulose is much less susceptible to ultraviolet light degradation. After the lignin has been degraded, the poorly bonded

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carbohydrate-rich fibers erode easily from the surface, which exposes new lignin to further degradative reactions. In time, this "weathering" process causes the surface of the composite to become rough and can account for a significant loss in surface fibers. Lignocellulosics burn because the cell wall polymers undergo pyrolysis reactions with increasing temperature to give off volatile, flammable gases. The hemicellulose and cellulose polymers are degraded by heat much before the lignin is [11]. The lignin component contributes to char formation and the charred layer helps insulate the composite from further thermal degradation.

IV.

CHEMICAL MODIFICATION SYSTEMS

There are several approaches to chemically modifying the lignocellulosic cell wall polymers. The most abundant single site for reactivity in these polymers is the hydroxyl group and most reaction schemes have been based on the reaction of hydroxyl groups. Sites of unsaturation in the lignin structure can also be used as a point of reactivity as well as free radical additions and grafting. However, the most studied class of chemical reactions are those involving hydroxyl substitutions. In modifying a lignocellulosic for property improvement, there are several basic principles that must be considered in selecting a reagent and a reaction system [I]. Of the thousands of chemicals available, either commercially or by synthetic means, most can be eliminated because they fail to meet the requirements or properties listed below. If hydroxyl reactivity is selected as the preferred modification site, the chemical must contain functional groups that will react with the hydroxyl groups of the lignocellulosic components. This may seem obvious but there are several failed reaction systems in the literature using a chemical that could not react with a hydroxyl group. The overall toxicity of the chemicals must be carefully considered. The chemicals should not be toxic or carcinogenic to humans in the finished product and should be as nontoxic as possible in the treating stage. The chemical should be as noncorrosive as possible to eliminate the need for special stainless steel or glass-lined treatment equipment. In considering the ease with which excess reagents can be removed after treatment, a liquid treatment chemical with a low boiling point is advantageous. A gas system can be used to great advantage in a fiber system but there may be problems in pressurized gas handling in a continuous reactor. Likewise if the boiling point of a liquid reagent is too high, it will be very

Chemical Modification of Nonwood Lignocellulosics

235

difficult to remove the chemical after treatment. It is generally true that the lowest member of a homologous series is the most reactive and will have the lowest boiling point. The boiling point range for liquids to be considered is

30-150°C. Accessibility of the reagent to the reactive chemical sites is a major consideration. The increase accessibility to the reaction site, the chemical must swell the lignocellulosic structure. If the reagents do not swell the structure, then another chemical or cosolvent can be added to meet this requirement. Accessibility to the reactive site is a major consideration in a gas system unless there is a condensation step in the procedure. Almost all chemical reactions require a catalyst. With lignocellulosic as the reacting substrate, strong acid catalysts cannot be used as they cause extensive degradation. The most favorable catalyst from the standpoint of lignocellulosic degradation is a weakly alkaline one. The alkaline medium is also favored as in many cases these chemicals swell the cell wall matrix structure and give better penetration. The properties of the catalyst parallel those of reagents, i.e., low boiling point liquid, nontoxic, effective at low temperatures, etc. In most cases, the organic tertiary amines or weak acids are best suited. The experimental reaction conditions that must be met in order for a given reaction to take place is another important consideration. The temperature required for complete reaction must be low enough that there is little or no fiber degradation, i.e., less than 150°C. The reaction must also have a relatively fast rate of reaction with the cell wall components. It is important to get as fast a reaction as possible at the lowest temperature without lignocellulosic degradation. The moisture present in the lignocellulosic is another consideration in the reaction conditions. It is costly to dry lignocellulosics to less than I % moisture, but it must be remembered that the OH group in water is more reactive than the OH group available in the lignocellulosic components, i.e., hydrolysis is faster than substitution. The most favorable condition is a reaction that requires a trace of moisture and the rate of hydrolysis is relatively slow. Another consideration in this area is to keep the reaction simple. Avoid the multicomponent systems that will require complex separation after reaction for chemical recovery. The optimum here would be that the reacting chemical swells the lignocellulosic structure and is the solvent as well. If possible, avoid byproducts during the reaction that have to be removed. If there is not a 100% reagent skeleton add-on, then the chemical cost is higher and may require recovery of the byproduct for economic reasons.

236

Rowell

The chemical bond fonned between the reagent and the lignocellulosic components is of major importance. For pennanence, this bond should have great stability to withstand weathering. In the order of stability, the types of covalent chemical bonds that may be fonned are ethers> acetals > esters. The properties of these bonds have been described before and it is obvious that the ether bond is the most desirable covalent carbon-oxygen bond that can be fonned. These bonds are more stable than the glycosidic bonds between sugar units in the wood polysaccharides, so that the polymers would degrade before the grafted ether. It may be desired, however, to have the bonded chemical released by hydrolysis or enzyme action in the final product so that an unstable bond results from the modification. The hydrophobic nature of the reagent needs to be considered. The chemical added to the lignocellulosic should not increase the hydrophilic nature of the lignocellulosic components unless that is a desired property. If the hydrophilicity is increased, the susceptibility to microorganism attack increases. The more hydrophobic the component can be made, the better the substituted lignocellulosic will withstand dimensional changes in the presence of moisture. Single-site substitution vs. polymer fonnation is another consideration. The greater the degree of chemical substitution of lignocellulosic components, the better it is for biological resistance. So, for the most part, a single reagent molecule that reacts with a single hydroxyl group is the most desirable. Crosslinking can occur when the reagent contains more than one reactive group or results in a group that can further react with a hydroxyl group. Crosslinking can cause the lignocellulosic to become more brittle, so reagents in this class must be chosen carefully. Polymer fonnation within the cell wall after initial reaction with the hydroxyl groups of the lignocellulosic components gives good biological resistance, and the bulking action of the polymer gives the added property of dimensional stabilization. The disadvantage of polymer fonnation is that a higher level of chemical add-on is required for the biological resistance than is required in the single-site reactions. The treated lignocellulosic must still possess the desirable properties of lignocellulosics. That is, no reduction in fiber strength, no change in color, retention of electrical insulation properties, safety of the final product, and gluability unless one or more of these properties is the object of change in the product. A final consideration is, of course, the cost of chemicals and processing. In laboratory scale experimental reactions, the high cost of chemicals is not

Chemical Modification of Nonwood Lignocellulosics

237

a major factor. For commercialization of a process, however, the chemical and processing costs are very important factors. Laboratory scale research is generally done using small batch processing; however, rapid, continuous processes should always be studied for scale-up. Economy of scale can make an expensive laboratory process economical. In summary, the chemicals to be laboratory-tested must be capable of reacting with lignocellulosic hydroxyIs under neutral or mildly alkaline or acidic conditions at temperatures below 150°C. The chemical system should be simple and capable of swelling the structure to facilitate penetration. The complete molecule should react quickly with lignocellulosic components yielding stable chemical bonds, and the treated lignocellulosic must still possess the desirable properties of untreated lignocellulosics.

v.

CHEMICAL MODIFICATION FOR PROPERTY ENHANCEMENT

As was stated before, because the properties of lignocellulosics result from the chemistry of the cell wall components, the basic properties of a fiber can be changed by modifying the basic chemistry of the cell wall polymers. Many chemical reaction systems have been published for the modification of agrofiber and these were recently reviewed [12]. By far the most research has been done on the reaction of acetic anhydride with cell wall hydroxyl groups to give an acetylated fiber. Many different types of lignocellulosic fibers have been acetylated using a variety of procedures including wood [3,13], bamboo [14], bagasse [15], jute [16-20], kenaf [21,22], pennywort, and water hyacinth [23]. Without a strong catalyst, acetylation using acetic anhydride alone levels off at approximately 20 weight percent gain (WPG) for the softwoods, hardwoods, grasses, and water plants. By replacing some of the hydroxyl groups on the cell wall polymers with acetyl groups, the hygroscopicity of the lignocellulosic material is reduced. Two very interesting observations that we recently made in the acetylation of many different types of agro-based fibers were that the rate of acetylation of many different types of agrofibers was essentially the same (Fig. 3) and that the relationship between the reductions in equilibrium moisture content (EMC) at 65% RH of these different types of acetylated fibers referenced to unacetylated fiber as a function of the bonded acetyl content results in a straight line plot (Fig. 4) [23]. The data represented in Fig. 4 indicate that the rate of acetylation is essentially the same for all types of lignocellulosic fibers and that even though

Rowell

238

2S

20

~ 41

()

c:r

lS

0~

0

---------~

y

2

3

§

10

/0

S

0

/~

0

0

4

Reaction time (h)

Figure 3

Rate of reaction of Iignocellulosics with acetic anhydride.

the points shown in Fig. 4 come from many different lignocellulosic fibers, they all fit a common line. A maximum reduction in EMC is achieved at about 20% bonded acetyl. Extrapolation of the plot to 100% reduction in EMC would occur at about 30% bonded acetyl. Because the acetate group is larger than the water molecule, not all hygroscopic hydrogen bonding sites are covered. These findings would indicate that it does not matter which type of lignocellulosic resource is used as a substrate for acetylation. Wood as well as agricultural residues and other sources of agromass can be incorporated into acetylated composites. The fact that EMC reduction as a function of acetyl content is the same for many different lignocellulosic materials indicates that reducing moisture sorption and, therefore, achieving cell wall stability is controlled by a common factor. The lignin, hemicellulose, and cellulose contents of all the materials plotted in Fig. 4 are different. Earlier results showed that the bonded acetate was mainly in the lignin and hemicelluloses [24] and that isolated wood cellulose does not react with uncatalyzed acetic anhydride [25]. Many other chemicals have been used to modify natural fibers including

Chemical Modification of Nonwood Lignocellulosics

239

100

II 31°/. I I

90

I

/ I

I

80

I

/ I

70

/

G

/

~

~ ::I:

60

0:

;: Ln

lD

'0 u

50

:r

UJ

!:: c:

40

~

v

~

30

'0 G

£r

C 20 41 V

Gi

Cl.

10 0

0

2S

30

% Acetyl

Figure 4 Reduction in equilibrium moisture content (EMC) as a function of bonded acetyl content for various acetylated lignocellulosic materials. 0, southern pine; O. aspen; 6, bamboo; 0, bagasse; X. jute; +. penny wort; V', water hyacinth.

ketene, phthalic, SUCCiniC, maleic, propionic, and butyric anhydride, acid chlorides, carboxylic acids, many different types of isocyanates, formaldehyde, acetaldehyde, difunctional aldehydes, chloral, phthaldehydic acid, dimethyl sulfate, alkyl chlorides, ~-propiolactone, acrylonitrile, ethylene, propylene, and butylene oxide, and difunctional epoxides [3,12].

240

VI.

Rowell

CHEMICAL MODIFICATION FOR COMPATIBILIZATION

Before 1980, the words blend and alloy were essentially unknown in the plastics industry. Today there are more than 1000 patents relating to plastic blends and alloys and it is estimated that one out of every 5 kg of plastic sold is an alloy or blend [26]. In the plastic industry, the word blend is defined as a mechanical mixture of two or more plastics and an alloy is an actual molecular bonding of the chemical elements within the plastics. Blends and alloys have revolutionized the plastics industry as they offer new materials with properties never before available and materials that can be tailored for specific end uses. The agro-based composites industry has an opportunity to follow this trend and greatly expand markets for new materials based on blends and alloys with other materials. Most of the research going on today is focused on agrofiber/plastic-compatibilized blends in an attempt to produce materials with consistent, uniform, continuous, predictable, and reproducible properties. There is research underway to produce compatibilized blends of kenaf and jute with polypropylene. This research is directed at developing the technologies needed to combine dissimilar resources for improved bonding, impact resistance, moldability, and to decrease creep. The two materials remain as separate phases, but if delamination and/or void formation can be avoided, properties can be improved over those of either separate phase. To take advantage of the high tensile strength and low weight of the kenaf or jute fiber as a true reinforcing phase in the composite, it is necessary to modify the kenaf fiber and/or the plastic matrix to optimize stress transfer, minimize stress concentrations, and maximize final material properties. Interfacial quality is likely to be enhanced through the introduction of chemical bonds across the interface or through increased secondary interactions [27]. Although various modification options have been extensively studied, the emphasis has been on reaction chemistry and on gross composite mechanical properties [28,29]. The bonding of a hydrophilic agro-based fiber to hydrophobic thermoplastic in such a way as to achieve a synergistic effect (i.e., the best of the properties of each material translated into the composite material) requires compatibilization of the two phases. It is therefore extremely important to understand the properties of the fiber-matrix interphase and interface. The compatibilizers that are presently under investigation for kenaf or jute!

Chemical Modification of Nonwood Ugnocellulosics

241

polypropylene composites are based on a maleic anhydride-grafted polypropylene (PP) or an acrylic acid-grafted pp. The effectiveness of the compatibilizer depends on the number of reactive end groups and the molecular weight of the copolymer. These compatibilizers have the potential of reacting with the hydroxyl groups on the cell wall polymers to fonn a graft point and the polyolefin end can then mix with the thennoplastic phase. Table 3 shows the properties of a kenaf-pp-compatibilized blend using a maleated (1.5%) that was prepared in a thennokinetic mixer, granulated, dried, and then injected-molded [28J. There are significant improvements in tensile and flexural properties of the kenaf-filled PP as compared to those of unfilled PP. The tensile modulus of the kenaf fiber system is about equivalent to that of a 40% mica-PP or a 30% glass-PP composite (approximated from data available on 20% and 40% glass-filled PP), but significantly higher than a 40% CaCoJ or a 40% talcfilled PP. The flexural modulus of the kenaf system is in the same range as that of the stiffest system, which is the mica-filled PP. The tensile and flexural strengths of the kenaf system are lower than that of the glass-filled PP but higher than all of the other systems. The notched Izod impact strength of the kenaf system is much lower than that of the glass fiber-filled PP but about the same as all other fillers and mica systems. Short fiber lengths present in the kenaf system due to the compounding system used and molding are probably responsible for the poor impact strengths [30J.

Table 3

Comparison of Properties of Kenaf-Filled Polypropylene with Other Commercially Filled Polypropylenes Filler/reinforcement Property

None

Kenaf

Talc

CaC0 3

Glass

Mica

% Filler by weight % Filler by volume Tensile modulus (GPa) Elongation at break (%) Flex. strength (MPa) Flex. modulus (GPa) Notched Izod impact (J/m) Specific gravity Water sorption (%, 24 h)

0 0 1.7 to 41 1.4 24 0.9 0.02

50

40

40

18 4

18 3.5

63 4.3 32 1.27 0.02

48 3.1 32 1.25 0.02

20 8 9 2.5 97 3.8 98 1.05 0.02

40

40

7.3 2.2 91 7.1 32 1.07 0.95

18 7.6 2.3 62 6.9 27 1.26 0.03

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242

The water sorption of the kenaf system is higher than that of any other system but this could be solved by chemically modifying the kenaf fiber before processing to decrease moisture sorption in the fiber. Creep due to thermal deformation restricts thermoplastic-based composites from structural uses. However, creep in thermoplastic-based composites may be controlled through crosslinking chemical reactions or surface plasma modifications.

VII.

CHEMICAL MODIFICATION FOR THERMOPLASTICIZATION

All agro-based fibers are composed of a crystalline, thermoset polymer (cellulose) in a thermoplastic matrix (lignin and the hemicelluloses). The melting point of the thermoplastic matrix is too high to allow this phase to flow at temperatures that do not degrade the fiber. If the glass transition temperature of the thermoplastic matrix is reduced through chemical modification, it is possible to plasticize the fiber allowing it to become more thermoformable through thermopressing, extrusion, or injection. There are several critical issues to consider in this research area. The kenaf fiber must not be degraded by the chemical modification procedures. To maintain the strength of the kenaf fiber, depolymerization or degradation of the cellulose must be avoided. It is possible to thermoplasticize only the lignin and hemicellulose polymers of the cell wall using succinic anhydride [31]. If a nondecrystallizing reaction condition is used, it is possible to chemically modify the lignin and, possibly, the hemicelluloses but not the cellulose. This selective reactivity has been shown to occur if uncatalyzed anhydrides are reacted with wood fiber [25]. Reaction of kenaf with succinic anhydride were done in xylene at 120°C and WPGs up to 80% were achieved [32]. Dynamic mechanical analysis was done on acetone-extracted esterified fibers in the WPG range of 30-80. The data showed that there was a reduced transition temperature from about 170°C down to about 135°C and that there was no change in this first transition temperature as the WPG increases. The data showed that complete modification of that melting species had taken place at a WPG of i m35. This thermal behavior is similar to reported trends observed for water-plasticized lignin in wood. Figure 5 shows scanning electron micrographs of hot-pressed control and esterified kenaf fiber. The control fiber (a) shows little tendency to thermally flow under the pressure of the hot press at 190°C whereas the esterified fiber

Chemical Modification of Nonwood Lignocellulosics

243

(a)

(b)

Figure 5 Scanning electron micrographs (x 50) of pressed kenaf fiber; control (a) and esterified (80 WPG, b).

Rowell

244

(b) shows thennal flow at this temperature, indicating that fiber thennoplasticization had taken place.

VIII.

CONCLUSIONS

High-perfonnance composite materials with unifonn densities, durability in adverse environments, and high strength can be produced by using agrobased fiber, high-perfonnance adhesives, and fiber modification to overcome dimensional instability, biodegradability, flammability, and degradation caused by ultraviolet light, acids, and bases. Products with complex shapes can also be produced using flexible fiber mats, which can be made by nonwoven needling or thennoplastic fiber melt matrix technologies. Taking advantage of fiber cell wall modification chemistry and combining bast fiber with other materials provides a strategy for producing advanced composites and materials that take advantage of the enhanced properties of all types of materials, and it allows the scientist to design materials based on end-use requirements within the framework of cost, availability, renewability, recyclability, sustainability, energy use, and environmental considerations.

REFERENCES 1. 2.

3. 4. 5.

6. 7.

8.

R. M. Rowell, Proc. Am. Wood Preservers' Assoc. 1-10 (1975). R. M. Rowell and R. L. Youngs, USDA Forest Service Res. Note FPL-0243, Forest Products Laboratory, Madison, WI, 1981. R. M. Rowell, Commonwealth Forestry Bureau, Oxford, England, Vol. 6(12), 1983, pp. 363-382. R. M. Rowell and P. Konkol, USDA Forest Service, Forest Products Laboratory Gen. Tech. Rep. FPL-GTR-55, Madison, WI, 1987. R. M. Rowell, G. R. Esenther, J. A. Youngquist, D. D. Nicholas, T. Nilsson, Y. Imamura, W. Kerner-Gang, L. Trong, and G. Deon, in Proceedings: IUFRO Wood Protection Subject Group, Honey Harbor, Ontario, Canada, Canadian Forestry Service, (1988), pp. 238-266. D. N. -So Hon, Polym. News 17: 102-107 1992. R. M. Rowell, in Composites Applications: The Role of Matrix, Fiber, and Interface T. L. Vigo and B. J. Kinzig, eds.), VCH, New York, 1992, pp. 365-382. R. M. Rowell, in Proc. Composite Products Symposium, Rotorua, New Zealand, November 1988, FRI Bull 153:57-67 (1990).

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A. J. Stamm, Wood and Cellulose Science; Ronald Press, New York, 1964. R. M. Rowell and W. B. Banks, USDA Forest Service Gen. Tech. Rep. FPL 50, Forest Products Laboratory, Madison, WI, 1985. II. R. M. Rowell, Chemistry of Solid Wood, Advances in Chemistry Series No. 207, American Chemical Society, Washington, DC, 1984. 12. R. M. Rowell, in Handbook on Wood and Cellulosic Materials (D.N.-S. Hon and N. Shiraishi, eds.), Marcel Dekker, New York, 1991, pp. 703-756. 13. R. M. Rowell, A. -M. Tillman, and R. Simonson, J. Wood Chem. Technol., 6(3): 427-448 (1986). 14. R. M. Rowell and M. Norimoto, J. Jpn. Wood Res. Soc. 33(11):907-910 (1987). 15. R. M. Rowell and F. Keany, Wood Fiber Sci. 23 (1):15-22 (1991). 16. H. J. Callow, J. Textile Inst, T423-32 (1952). 17. A. B. Sen Gupta and S. K. Deb, Sci. Culture 29(7):362-363 (1963) 18. M. Mosihuzzaman, J. C. Roy, and M. Z. Ali, Dacca Univ. Stud., Part B, 28(1) 47-52 (1980). 19. M. Andersson and A. -M. Tillman, J. Appli. Polym. Sci. 37:3437-3447 (1989). 20. R. M. Rowell, R. Simonson, and A. -M. Tillman, European Patent 0,213,252 (1991) 21. R. M. Rowell, Food and Agricultural Organization of the United Nations, ESC:JU/IC 93115, 1-12 (1993). 22. R. M. Rowell and S. E. Harrison, in Proc. Fifth Annual International Kenaf Conference (M.S. Bhangoo, ed.), California State University Press, Fresno, 1993, pp. 129-136. 23. R. M. Rowell and J. S. Rowell, in Cellulose and Wood (C. Schuerch, ed.), John Wiley and Sons, New York, 1989 pp.343-356. 24. R. M. Rowell, Wood Sci. 15(2):172-182 (1982). 25. R. M. Rowell, R. Simonson, S. Hess, D. V. Plackett, D. Cronshaw, and E. Dunningham, Wood Fiber Sci. 26(1):11-18 (1994). 26. V. Wigotsky, Plastics Eng. Nov., 25 (1988). 27. A. R. Sanadi, R. M. Rowell, and R. A. Young, J. Mat. Sci. 28:6347-6352 (1993). 28. A. R. Sanadi, D. F. Caulfield, and R. M. Rowell, Plastic Eng. (in press). 29. A. R. Sanadi, R. A. Young, C. Clemons, and R. M. Rowell, J. Reinforced Plastics Composites, 13:54-67 (1994). 30. A. R. Sanadi, K. Walz, L. WieIoch, D. F. Caulfield, and R. M. Rowell, in Proc., Sixth Annual International Kenaf Conference, 1994. 31. R. M. Rowell and C. M. Clemons, in Proc.lnt. Particleboard/Composite Materials Symposium (T. M. Maloney, ed.), Pullman, WA, 1992, p.251. 32. R. M. Rowell, D. F. Caulfield, A. Sanadi, J. O'Dell, and T. G. Rials, in Proc., Sixth Annual International Kenaf Conference, 1994. 9. 10.

10 Characterization of Chemically Modified Wood Takato Nakano Hokkaido Forest Products Research Institute Asahikawa, Hokkaido, Japan

I.

INTRODUCTION

Various chemical modifications of wood have been tried to improve native properties such as swelling and shrinking and to apply new properties such as thermoplasticity, solubility, and so on [1-5]. These treatments will increase the use of wood as an important renewable resource. Several kinds of treatments have been applied to cellulose alone, whose properties have been studied by many scientists [6-9], e.g., esterification, etherification, and cyanoethylation. As for wood, the introduced side chains with these reactions are combined with OH groups of its several components, especially those of cellulose chains. The treatments give new properties due to the introduced side chains and to the change of the conditions around the side chains, so that the interaction between wood components' molecular main chains is remarkably varied. Thus, new physical and chemical properties are applied to wood with such chemical modifications. In the present chapter, characterization of chemically modified wood, especially physical characterization, will be discussed. The molecular mobility of wood components is examined in terms of various properties that were improved by treatment based on relaxational properties that depend on mobility of molecules and the interaction between molecules. Moreover, the effects of various side chain factors on the mobility of wood component molecules and their mechanisms are discussed.

247

248

II.

Nakano

THERMOPLASTICITY AND THE MECHANISM

Wood can be converted to thennoplastic materials by chemical modification (1-3], though native wood is not thennoplastic. Shiraishi and coworkers measured the softening points and melting points of chemically modified woods by the use of the apparatus devised by Goring and also took scanning electron microscope photographs of the melting of chemically modified wood to visually confinn melting of the treated wood [1,10]. They speculated that facility in the application of thennoplasticity was related to the volume and number of introduced side chains. With respect to the factors of thennoplasticity, Nakano and Nakamura studied effects of the number and length of introduced side chains on the viscoelastic properties of esterified block wood (11-15]. Moreover, Nakano qualitatively examined the relationship between these factors and the free volume, and then found the mechanism of thennoplasticity of esterified wood on the basic of results of the viscoelastic measurements (16]. In this section, the mechanism of thennoplasticity will be mentioned in tenns of the concept of free volume.

A.

Factors for Mobility of Molecules

The variation of wood with chemical modification should be caused by the number and nature of introduced side chains because the treatment is an introduction of new side chains into wood substance. Thus, on the basis of viscoelastic properties, the effects of both factors on the mobility of wood components molecules are examined. The specimens were treated according to the method of Nakagami and coworkers (17,18]. Japanese linden (Tilia japonica Smik.) was treated with trifluoroacetic acid anhydride and the fatty acids (TFAA method), which included acetic acid, propionic acid, valeric acid, hexanoic acid, decanoic acid, lauric acid, and palmitic acid. Dynamic measurements were made with a torsion pendulum apparatus under a vacuum. An increasing temperature rate was 2°C/min. The amount of introduced side chain per gram of wood is about 4-6 mmollg [16]. The chemical structure of the treated wood is presented by the fonnula: Wood -

CO -

(CH 2)n-2

-

CH 3

For esterified wood, the change in dynamic loss (Gil) as a function of temperature with the change in the number of carbons of introduced side chains (n) is shown in Fig. 1. Five dispersions are found for n ~ 3. They are attributed to the main chain motion of lignin (a ' ), the restricted main

249

Characterization of Chemically Modified Wood

5 3

2

8

Y

/3

~IttxI:i

cr

cr

ammmmsmmxJIJXDIiO

untreated

o

-100

Temperature

100

ee)

Figure 1 The effect of the length of side chains on dynamic loss (G"). n, Number of carbon atoms in introduced side chains. (From Ref. 16.)

chain motion of other wood components (a), the micro-Brownian motion of the modified cellulose main chains in the amorphous region (~), the local mode of wood components related to water ("I), and the introduced side chain motion (&) [13]. Both a and a dispersions have been speculated but not yet confirmed. Their dispersions systematically change with an increase in n. The a', a, and ~ dispersions shift to lower temperatures with an increase in n. The a ' dispersion overlaps the a dispersions and then apparently disappears. Moreover, this a dispersion overlaps the ~ dispersion too. Accordingly, we find only two dispersions: ~ and &. On the other hand, for the & dispersion, the position for n < 5 shifts to lower temperatures with an increase in n, I

250

Nakano

while that for n > 6 shifts to higher temperatures. The ~ dispersion disappears gradually. Figure 2 shows the effect of the amount of the introduced side chains (M) on dynamic loss for decyanoylated wood. We find similarity between the effects of M j and n when Fig. 2 is compared with Fig. I. That is, the (x' and (X dispersions overlap to the ~ dispersion and then disappear. There are only two dispersions (the ~ and 8 dispersions) for M j = 5.19 mmoll g. The change of the 8 dispersion, however, is different from that of Fig. I. The 8 dispersion in Fig. 2 shifts only to lower temperatures with an increase in Mi' There is no reversion such as that shown in Fig. I. From the above discussion, we can conclude that the contribution of elonga-

Mi=5.19 m mol/g

0.24

~!1

am

llt$~

untreated

-100

o Temperature

100

ee)

Figure 2 The effect of the amount of introduced side chains (M j ) on dynamic loss (Gil) for decanoylated wood. (From Ref. 16.)

Characterization of Chemically Modified Wood

251

tion of introduced side chains to the peak shift for the ex', a, and (3 dispersions is equivalent to that of an enlargement of M j without the 0 dispersion.

B.

Swelling and Free Volume

We expect that the modification creates the free volume (Vr) in wood substance from the similarity of the effect of M i and n on viscoelasticity. The discussion for wood, however, is impossible on the basis of a concept of the free volume, although the flexibility of molecular motion for synthetic amorphous polymers is discussed. Unfortunately, we can not directly know the created free volume because the time-temperature superposition principle is not valid for wood [19]. The principle is related to WLF equation by which the free volume is calculated. The free volume, however, relates to volumetric swelling as follows. The chemical modification should make space between molecular chains because even in the case of close-packed filling the introduced side chains are not tightly packed without space between molecular chains. For this reason, the swelling with the treatment (V) is proportional to the sum of the occupied volume of the introduced side chains (Vi) and the empty space volume created by the introduction of them (Vr): Vr is the free volume. Thus V is represented by

k

=

constant

(I)

On the other hand, a relationship between Vi and V is derived from the experiment as shown in Fig. 3. The Vi can be calculated by using the volume for various atomic groups, which Slonimskii and coworkers 1201 calculated to obtain the packing coefficient for various pOlymers. The relationship is represented by k'

=

constant

(2)

From both equations, we obtain the following equation:

v=

/('Vr.

/(' =

constant

(3)

Equation (3) represents the relationship between V and Vr. Thus, we can discuss the results of viscoelastic measurement on V instead of Vr.

c.

Mechanism of Thermoplasticity

Figures 4 and 5 show relationships between the peak temperature of the ex, (3, and 0 dispersions and V. The peak temperature for the ex and (3 dispersions

Nakano

252 100

/

,~

;I. 01

>

o

IP

o Figure 3

~

f..C1_Q

VI

(cm 3 jg)

Relationship between the volume of introduced side chains (Vi) and the swelling of wood with esterification (Y). 6, n = 2; n 3; n = 5; Q, n = 6; Q, n = 10;,0, n = 12; -0, n = 16. (From Ref. 16.)

a,

cr,

does not depend on the nature of the introduced side chains but on Valone (Fig. 4). On the other hand, that of the & dispersion depends on the nature of the side chain (Fig. 5). Additionally, the relationship between the peak temperature and V is linear for each side chain and the position of the straight line depends on the nature of the side chain. The position shifts to lower temperatures for n ~ 5 and to higher temperatures for n ~ 5. We can explain the results shown in Figs. 4 and 5, taking into consideration that the volumetric swelling V is proportional to the free volume V f • For the a and (3 dispersions, since the interaction between side chains is very low in this temperature region, the peak temperature does not depend on the nature of the introduced side chain, so that the flexibility of the main chains relates only to free volume created with the modification. Thus, when the free volume increases with an increase in the number or length of the introduced side chains, the flexibility of main chains increases, so that the peak temperature shifts to lower temperatures with an increase in V as shown in Fig. 4. The lengthy side chain, however, causes entanglement between side chains [21,22]. In this work, when the side chain hali the length of n = 5, the interaction appears to enlarge and the peak shifts to higher temperatures. For this reason, the peak temperature of the &dispersion shows dependence of a

Characterization of Chemically Modified Wood

253

6

~~C1~6 QC1

e

100

a

Q_<>~

f-

o

100

V (%)

Figure 4 Relationship between the peak temperatures of the alpha and beta dispersions and the swelling of wood with esterification (V). Note: symbols are shown in Fig. 3. (From Ref. 16.)

-'0--

_ '0-'0-

QQ Q.)

6

:;

~

-100

2 -Q~1 2 9-

-6~

10

Q.)

0-

3

E

..-

Q.)

-200

L...-

o

"""--

V

(%)

-'

100

Figure 5 Relationship between the peak temperatures of the & dispersion and the swelling of wood with esterification (V). n, Number of carbon atoms in introduced side chains. (From Ref. 16.)

Nakano

254

side chain length on the peak position and the shift direction for the straight line is reversed at n = 5. From the above discussion, we obtain the conclusion that thennoplasticity of chemically modified wood is due to the bulky free volume, since the creation of the free volume enables flexible molecular motion such as softening or melting under appropriate conditions. Thus, the treated wood should soften and melt in heat if the ester content of the modified wood is higher than that in the present report. The creation of bulky free volume, however, does require the breaking down of intramolecular bonds of lignin, since the lignin network inhibits the swelling of wood substance with the treatment. In this regard, Shiraishi and coworkers have reported the decrease of the molecular weight of lignin during the acetylation after pretreatment with trifluoroacetic acid (TFA) [2]. Furthennore, the results of stress relation of wood treated with TFA supported the theory by Shiraishi and coworkers [11]. Consequently, the thennoplasticity is due to creation of the bulky free volume by the introduction of side chains into wood substance with partial destruction of the lignin network.

III.

IONIZATION OF SIDE CHAINS AND INTERACTION BETWEEN MOLECULES

Mechanical properties of the chemically modified wood depend on the nature of the introduced side chain. If the introduced side chain is ionized, the interaction between wood component molecules is influenced by not only the free volume but also by electrostatic repulsion of ionized side chains. For example, the confonnation of polyelectrolyte depends on the degree of ionization [23-28]. The structure is transformed from random coil to rod-like conformation with an increase in the degree of ionization. A screening effect also influences the interaction because electrostatic action is inhibited by electrolytes such as NaCI. The effect of pH and the content of salt on relaxation properties will be examined for succinylated wood, which has a carboxyl group at the end of the side chain: pH is related to the degree of ionization and the content of salt by the screening effect.

A.

Ionization and Swelling

The relationship between pH and relaxation rigidity (G r ) at 1 and 1000 is shown in Fig. 6 for both untreated and succinylated wood [29]. Specimens prepared from Japanese ash (Fraxinus mandshurica Repr. var. japonica

Characterization of Chemically Modified Wood X 107

255

10 1s

"1>-_

o

5

10

pH Figure 6 Relationship between relaxation rigidity (G r ) at I sand 1000 s and pH. Untreated wood; 0, succinylated wood. (From Ref. 29.)

e,

Maxim.) were succinylated by the procedure of Matsuda and coworkers [5]. Stress relaxation measurements were made in various pH solutions at 30°C using a torsional relaxation apparatus. For untreated wood, pH showed little influence on Gr whereas for succinylated wood it does (Fig. 6). The difference in G r between untreated and succinylated wood increases with an increase in pH. In explanation of the above results, we assume that there are two factors with respect to the dependence of G r on pH: one is deesterification in succinylated wood and the other is the ionization of carboxyl groups at the end of side chains. Figure 7 shows infrared (lR) spectra for specimens after stress relaxation measurements. The absorption at 1735 cm - I due to carbonyl groups (CO<) increases with succinylation [30]. Since the absorption after stress relaxation measurements does not depend on pH in the region of pH 3.03-10.13, succinylated wood is not deesterified during the stress relaxation measurement. Thus, we can expect that the dependence of Gr on pH is related to the ionization of the introduced side chains into wood substance. Confonnation of polyelectrolyte in aqueous solution changes with variation of pH. Crosslinked polyelectrolyte swells with an increase in the degree of ionization [31]. Though the confonnation of wood components has not been exactly clarified, the effect of the ionization on confonnation appears to be

Nakano

256

untreated

succinylated

Q)

o c ra t:::

pH

'E CIl

c

3.03

ra

.=

6.99

1500 Wave number

1000 (cm- 1)

Figure 7 Infrared spectra of succinylated woods after stress relaxation measurement in solution at various pH values. (From Ref. 29.)

similar to that of crosslinked polyelectrolyte. Thus we expect that the dependence of pH on swelling is the same as that of G r if the results in Fig. 6 are due to ionization of side chains. Figure 8 shows relationships between pH and swelling for untreated and succinylated wood. Swelling of untreated wood depends little on pH, whereas obviously pH affects succinylated wood. The change in swelling for succinylated wood is observed at pH 5-8 and leveled off above pH 8. The result shown in Fig. 8 suggests that swelling occurs by ionization of carboxyl groups at the end of the introduced side chains in succinylated wood. Swelling for crosslinked polymethacrylate gel by divinylbenzene increases with an increase in ionization, and there is a tendency for leveling off with an increase in crosslinked density [31]. The reason for this

257

Characterization of Chemically Modified Wood

8

•

•

•

•

·o~ /

o

0

0

__ 0

--0-0

4

5

10 pH

Figure 8 Relationship between swelling and pH for succinylated wood impregnated with solution at various pH values. AT, Dimensional change in tangential direction; untreated wood; 0, succinylated wood. (From Ref. 29.)

e,

is that the swelling pressure is balanced by an elastic restoring force due to crosslinkings. From an analogy with crosslinked polyelectrolyte, we consider the mechanism of dependence of Gr on pH for succinylated wood as follows. Wood components swell by electrostatic repulsion with an increase in the degree of ionization; this swelling increases with an increase in pH. However, the swelling pressure levels off as a result of balancing by elastic repulsive force due to the ultrastructure of wood. The mobility of wood components increases under such a condition; then Gr decreases and the stress relaxes remarkably.

B.

Effect of Ionization Change

If the above discussion is valid, Gr should decrease rapidly when we replace low-pH solution with high-pH solution during the stress relaxation measurement. Stress relaxation was measured in solutions of varying pH from 2.97 to 10.10 at log t = 3. The results of untreated and succinylated wood are shown in Fig. 9 and 10. As would be expected, untreated wood changes slightly by the replacement, whereas succinylated wood does remarkably. This is because the numbers of ionizative side chains differ from one another. A slight decrease for the untreated wood appears due to the ionization of

Nakano

258

carboxyl groups that native wood components have. On the other hand, succinylated wood has many ionizative side chains introduced with esterification. Thus, the interaction between main chains is strong in pH 2.97 solution in comparison with that in pH 10.10 because their side chains are ionized little. By the replacement, the side chains begin to ionize, so that main chains repulse one another and then the stress relaxes rapidly. Such a change occurs only slightly for untreated wood. Furthennore, we should also expect a screening effect from an electrolyte such as NaCI, if electrostatic repulsion of side chains is a factor for decrease of Gr. Figure 11 shows the effects of NaCI on G r for untreated and succinylated wood. Though samples of succinylated wood vary over the range of 0 to 3 mollL, G r of succinylated wood has an obvious tendency to approach that of untreated wood with an increase in the concentration of NaCI. This result suggests that the addition of NaCI causes neutralization of ionized side chains. From this discussion, the mechanism of dependence of pH on relaxation properties is as follows. Introduced side chains are ionized with an increase in pH, so that wood swells increasingly because of electrostatic repulsion of main chains of wood components. The swelling pressure increases until the balancing by elastic repulsive force due to the ultrastructure of wood is reached 8

pH a: 2.97

7

b:2.97-10.10

o log t

2 (5)

3

4

Figure 9 Effect of pH solution exchange during measurement on stress relaxation of succinylated wood. Note: time of the exchange is log t = 3. (From Ref. 29.)

259

Characterization of Chemically Modified Wood 8

<0

~

b pH

....

(!) Ol

o

a: 2.97

7 "'-4-b:2.97-10.10

o

3

2

log t

4

(s)

Figure 10 Effect of pH solution exchange during measurement on stress relaxation of untreated wood. Note: time of the exchange is log t = 3. (From Ref. 29.)

8 X 10

...------------o ~--=-_

_5

N

E o ......

e-o~

~o

e_---=-

_

0

o

c >-

~

o o

1 NaCI

2

3

(mol/I)

Figure 11 Relationship between G r at 1000 s and sodium chloride (NaOH) concentration. e, Untreated wood; 0, succinylated wood. (From Ref. 29.)

Nakano

260

and then levels off. The interaction between main chains decreases under this condition. Consequently, the mobility of main chains increases and the stress relaxes remarkably during constant strain.

IV.

BINDING OF METALS TO MODIFIED WOOD

Modified wood with introduced ionizative side chains should be able to bind to various metals, since it is expected that the side chains such as carboxymethyl and succinyl groups bind easily to produce metal salts. Bonding of metals with wood is an interesting subject from practical or fundamental viewpoints. The properties, especially mobility of wood component molecules, are considered to depend on bonding fonnation between the side chains and metals. Binding of metals and the confinnation for carboxymethylated and succinylated wood will be mentioned in the following discussion.

A.

Variation of Infrared Spectrum

Figure 12 shows variations of IR spectra of carboxymethylated wood binding various metals. Nakano's procedure [32] for carboxymethylation and Matsuda's procedure [5] for succinylation were adopted as the chemical modifications. The introduction of metals was carried out by the use of metal salt solution [33]. With the introduction of metals, the densities of the absorption band at 1595 cm - I due to -COO - increase whereas those at 1735 cm - I due to >CO decrease. IR spectra of succinylated wood show a similar result. This result shows the variation from WOOD-COOH to WOOD-COO-. Modified wood containing metals is hereafter referred to, for example, as CMW-Na for carboxymethylated wood containing Na + • Figure 13 shows a relationship between absorbance ratios 1595 cm - 1 to 1505 cm - I due to aromatic ring stretching vibration and the amounts of introduced side chains for both carboxymethylated and succinylated wood. We find a linear relationship and no difference between the two. This fact suggests that metals bind to carboxyl groups at the end of introduced side chains.

B.

Amount of Binding Metals

From the result of IR spectra, it should be expected that the amount of binding metals relates to the number of introduced side chains. When the concentration of salt solution is sufficient for the treatment, the quantity of binding metals levels off, i.e., binding sites are saturated [33]. Figure 14 shows relationships between the amount of introduced side chains and that of binding Zn for

Characterization of Chemically Modified Wood

261

Q)

u c

~

E (/) c

«l

.=

1900

1500 Wave number

(cm- 1)

Figure 12 Infrared spectra of carboxymethylated wood with various introduced metals. (From Ref. 33.)

carboxymethylated and succinylated woods that have been treated with a metal salt solution of sufficient concentration. We find that the amounts correlate with each other. The amount of Zn increases with an increase in the number of introduced side chains. Moreover, there is no difference between carboxymethylated wood and succinylated wood. Figure 15 shows relationships between the number of introduced side chains and that of various metals for carboxymethylated wood. Each curve is of a similar shape to a curve shown in Fig. 14. The position of the curve shifts to the lower amount of metals with an increase in valency. In other words, the amount of binding metals is monovalency < divalency < trivalency when the number of intro-

Nakano

262 2.0 , . . . - - - - - - - - - - - - . . . ,

L{)

oL{)

o

•

",1.5

/'

L{) 0) L{)

o

1.0 '--

o am

--.l'

--.JI

1

2

(m mol/g)

Figure 13 Relationship between absorbance ratios D(S9S/D(50S and the amounts of introduced side chains (~m). 0, Carboxymethylated wood; e, succinylated wood. (From Ref. 33.)

1.0

Oi ......... ""6 E

/

.s 0.5

/0

•

~.-

•

/0

/

<]

•

/

•

0

0

/

0 0

1 6m

2

(m mol/g)

Figure 14 Relationship between the amount of binding metals (61) and that of introduced side chains (~m). 0, Carboxymethylated wood; e, succinylated wood. (From Ref. 33.)

Characterization of Chemically Modified Wood 1.0

263

r--------------,

,rn

c

/'

/;~

(5

E

~ 0.5

13~

<]

.-.--.-.. i!Y

if~

0 0

1

6. m

2

(m mOl/g)

Figure 15 Effect of the amount of introduced side chains (~m) for carboxymethylated wood on the amount of binding metal (~m). D, Na; O. Zn; e, Ca; (), AI; -.. Mg; 6, Fe(IlI); A, Fe(II). (From Ref. 33.)

duced side chains is equal. For carboxymethylated and succinylated woods, the amount of binding metals is shown in Table I where both the amounts of metals per gram of modified wood (C m ) and per side chain per gram of modified wood (Cs )' Both modified woods have a tendency for the amount of binding metals to decrease with an increase in valency. Some metals are impossible to bind to succinylated wood. This fact suggests the selectivity of binding metals. The relationship between the amount of binding metals and a valency is qualitatively explained as follows. Bonding formation between the modified wood and metals should be either ionic bonding or coordinate bonding. The latter produces metal complexes. If the formation is ionic bonding, the number of binding side chains is one per monovalent metal, two per divalent, and three per trivalent, so that the amount of binding metal decreases with an increase in valency. Moreover, in this case the amount of binding metal per side chain (Cs in Table 1) should be equal for all valencies. As shown in Table 1, C s is approximately equal for both modified woods except for a few metals. The decreased values of C s are about 2 for carboxymethylated wood (the introduced side chains = 1.07 mmol/g) and about 2.5 for succinylated wood (that of 2.00 mmol/g). Thus the bonding between these metals and

Nakano

264

Table 1 Metal Contents (C m) per Gram of Modified Wood and Amount of Binding Side Chains per Mol of Metal (Cs) for Modified Woods Containing Various Metals

Carboxymethylated wood a

Succinylated wooda

Cm

Cs

Cm

Cs

Metal

(mmol/g)

(mol-I)

(mmol/g)

(mol-I)

Na Mg Ca Fe(I1) Co Zn Pb Al Cr Fe(lII)

0.662 0.457 0.481 0.100

1.62 2.34 2.22 10.70

0.970 0.720 0.760

2.06 2.78 2.63

0.528

2.03

0.465

2.30

0.359

2.98

0.646 0.806 0.801 0 0.156 0

3.10 2.45 2.50 0 12.80 0

rrhe amounts of introduced side chains are 1.07 mmol/g for carboxy methylated wood and 2.00 mmollg for succinylated wood. Source: Ref. 33.

modified wood appears to be ionic. However, we expect that C s should be higher if the fonnation is coordinate bonding, since such a metal binds to much more side chain. In Table I, for bonding between Fe(II) and carboxymethylated wood and bonding between Cr and succinylated wood, values of C s are more than 10. Consequently, there is the possibility that these binding fonnations are coordinate bonding.

v.

INTERACTION BETWEEN METALS AND MOLECULES

As mentioned in the previous section, metals bind to the modified wood which has carboxyl groups at the end of the introduced side chains because their groups ionize in an aqueous solution. Thus, metals bound to more than two side chains produce crosslinkings, so that the mobility of wood components should be influenced by the bonding. The flexibility is considered to depend on the nature of the metal as well as the amount. The interaction between metals and the modified wood will be examined and the relation between factors related to the flexibility will be discussed on the basis of the interaction.

Characterization of Chemically Modified Wood

265

On the basis of the discussion in the previous section, we hereafter represent binding metals by metal ions because bonding between metals and modified wood is ionic in most cases.

A.

Binding of Metals and Mobility of Molecules

Figure 16 shows relationships between the number of introduced side chains and relaxation rigidity (G r ) at 900 s for carboxymethylated wood binding various metal ions [34J. Wood specimens were prepared from Japanese linden (Tilia japonica Smik.). Carboxymethylation and the introduction of metal ions was the same procedure as mentioned in the previous section [32,33J. Stress relaxation measurements were carried out in an aqueous solution at 30°C. The relaxational property of carboxymethylated wood without metal ions is first discussed. For carboxymethylated wood (a broken line in Fig. 16), G r (900) decreases with an increase in the number of introduced side chain. This rapid decrease appears to be caused by two factors. One is the effect of sodium hydroxide (NaOH). Young's modulus of wood treated with an aqueous solution of NaOH decreases remarkably under wet conditions, especially at concentrations above 10% NaOH [35J. The other factor is the electrostatic repulsion of ionized carboxymethyl groups in carboxymethylated wood, as mentioned in the above section [29J. For example, confonnation of polypeptide is influenced by the ionization of the side chains, and the structural change of the helix-coil transition has been interpreted as a reversible transfonnation. Theoretical treatment of the transfonnation has been reported to explain the mechanism [23-25, 36-43]. The confonnation of component molecules in wood, however, cannot change markedly by ionization in comparison with soluble polyelectrolytes in water, because carboxymethylated wood is not dissolved in water. Only space among the main chains is expanded by the electrostatic repulsion due to negatively charged side chains. For these reasons, G r (900) of carboxymethylated wood decreases with an increase in the number of introduced side chains. A 13+ and Fe(III)3 + bound to carboxymethylated wood reduces the decrease of G r (900), especially at a concentration of more than 0.5 mmollg, whereas Na"', Mg2 "', Ca2 +, and Zn 2 + do not give such an effect (Fig. 16). There is slight difference of relaxation magnitude between Na + and the other divalent metal ions. The difference appears to be due to a screening effect by Na + as mentioned before. Relationships between G r (900) and the amount of binding metal ions are shown in Fig. 17. For AI 3+ and Fe(III)3 + , Gr (900) obviously increases with an increase in the amount of the metal ions. In the previous

Nakano

266

2

I I I \ I I

, \ \

\ \ \

\\

rJIl\",,--(/ 0 ............. -."

c

/

0

--~----

0 1 2 Carboxymethyl group

(m mol/9)

Figure 16 Relationship between relaxation rigidity at 900 s G r (900) and content of carboxymethyl group in carboxylated wood (CMW). 0, CMW-Na; to., CMW-Mg; to., CMW-Ca; A, CMW-Zn; 0, CMW-AI; _, CMW-Fe(II1), carboxymethylated wood without metal ions. (From Ref. 34.)

section, we found that divalent and trivalent metal ions produce crosslinkings between ionizative side chains in main chains of wood component molecules. The results shown in Figs. 16 and 17, however, indicate that such crosslinkings are broken in water for some metal ions. In this connection, it should be noted that the degree of ionization is related to ion species. The degree is Na+ > Mg 2 + > Ca2+ > Zn 2 + from the solubility of cellulose carboxymethylates [44]. A1 3 - and Fe(lII)3..- ionize little. Thus, the results shown in Figs. 16 and 17 are explained in terms of difference in ionization: crosslinkings are broken down for monovalent and divalent metal ions but remain intact for trivalent metal ions. We noted that the crosslinking between main chains remains in water for carboxymethylated wood binding AI 3 - or Fe(III)3 +. Figure 18 shows the

Characterization of Chemically Modified Wood

267

8

x 10 r----------------.., 2

o o ~m

0.5 (m mol/g)

1.0

Figure 17 Relationship between relaxation rigidity at 900 s Gr (900) and metal ion content (Am). 0, CMW-Na; 6, CMW-Mg; &, CMW-Ca; A, CMW-Zn; D, CMWAI; _, CMW-Fe(III). (From Ref. 34.)

experimental results in order to confirm the guess. The figure shows change of relaxation curves of carboxymethylated wood binding AI3+ and untreated wood when water is replaced with acid solution at log t = 2.78 s (10 min) after the measurement. If the crosslinkings remain in water, G r will decrease rapidly by replacement of water with acid solution because the ionic bonding between metal ions and carboxyl groups at the end of the introduced side chains should be broken down. We find that G r decreases little by the replacement for untreated wood, whereas G r decreases rapidly for carboxymethylated wood binding AI 3+. In the case of Fe(lII)3 + , the same results are obtained. The slight decrease in untreated wood appears due to native ionized side chains. Moreover, the relaxation magnitude after replacement depends on metal ion contents and increases with increasing content. The above results support the theory that crosslinkings produced by Al 3+ or Fe(lII)3 + remain in water. From the above discussion, we obtain the following conclusion for the interaction in water between metal ions and main chains of carboxymethylated

Nakano

268

9

E 0

~

.......

u:

c >-

~8

(; Ol

~

7

3

2 log t

4

(s)

Figure 18 Stress relaxation curves of carboxylated woods containing aluminum ion when water was replaced with 5% acetic acid solution at 10 min (log t = 2.78 s). -, Untreated wood; ---, untreated under replaced condition; 0, CMW-AI in water; e, CMW-AI under replaced condition. (From Ref. 34.)

wood. The binding of carboxymethylated wood to monovalent or divalent metal ions is influenced by two factors; electrostatic repulsion of ionized side chains and screening effect. For Na + , the effect of electrostatic repulsion is reduced by the screening effect since ionization of Na + is the greatest among the metal ions examined. Thus, the interaction of carboxymethylated wood binding Na + is stronger than the other binding divalent metal ion interactions. On the other hand, for carboxymethylated wood binding A1 3 + or Fe(III)3+ , the interaction is the strongest in comparison with the others, since the crosslinkings are not broken down in water.

B.

Factors for Flexibility of Molecular Motion

Molecular motion of chemically modified wood is influenced by binding metal ions. One factors is valency, as mentioned previously. In general, however, there is not only valency but also other factors related to mobility [45-47]. Eisenberg reported that the mobility of polymers binding metal ions relates to three factors: charge, amount of metal ion, and metal ion radii [48]. The mobility of molecules of carboxymethylated wood, especially that of

269

Characterization of Chemically Modified Wood

carboxymethylcellulose main chains, will be examined on the basis of results of dynamic viscoelastic measurement. Moreover, the factors will be discussed according to Eisenberg [49]. Figure 19 shows dynamic shear modules (G') and loss tangent (tan 8) as a functions of temperature for carboxymethylated wood binding various metal ions [49]. The content of carboxymethyl groups in treated wood is about 1.07 mmollg for each specimen. Dynamic viscoelastic measurements were carried out under vacuum. There are three dispersions in the range below 100°C: the ~ dispersion near 50°C for the carboxymethylcellulose main chain motion in the modified wood, the 'Y diversion near O°C for local mode of wood components related to water, and the 8 dispersion near - 60°C for the side chain

Eo

~ 9.0

~

C) 0>

o

...J

8.5

0 :CMW-Na

/:}.: CMW-Mg

0.05

o : CMW-AI -:CMW

0.01

L-_...L-

-100

-'-

o Temperature

.l.-

100

ee)

-..L_---l

200

Figure 19 Temperature dependence of G' and tan 0 for carboxymethylated woods (CMWs) and for CMWs containing metal ions. (From Ref. 49.)

270

Nakano

motion. The figure shows the effect of metal ions on the dispersions, especially marked on the ~ dispersion. The position of the ~ dispersion (T~) shifts to higher temperatures and the height to lower tan 0 with an increase in valency. Since the temperature and the height relate to the mobility of molecules for the corresponding relaxation process, the results shown in Fig. 19 suggest that the mobility of molecules binding divalent and trivalent metal ions decreases. Such metal ions fonn crosslinkings between molecules, as mentioned before. In what follows, for more divalent and trivalent metal ions, the effect and the factors on the ~ dispersion are discussed. Variation of the ~ dispersion by the binding of six metal ions are shown in Fig. 20. The effect on the ~ dispersion is systematic. The T~ shifts to higher temperature and the height decreases to lower tan 0 with increasing valencies. The ionic radius appears to relate to the variation of both: the radii are Mg 2 + < Ca2 '" < Zn 2 + for

0.04

r----------------, o :Na /). : Mg

it : Ca A : Zn 0.03

o

0.02

• 0

-50

0

50

Temperature

Figure 20

Effects of metal ions on the

~

: Fe(ill) : AI

100

ee)

dispersions of CMWs. (From Ref. 49.)

Characterization of Chemically Modified Wood

271

divalent metal ions and Al 3+ < Fe(III)3 -to for trivalent metal ions. Figure 21 shows relationships between the T~ and the amount for the ~ dispersion (~m). The figure suggests that the mobility is inhibited with an increase in amount of metal ion, since the T~ increases with increasing metal ion. From these results, factors relating to the mobility of carboxymethylcellulose in carboxymethylated wood are presumably the valency of a metal ion, which are proportional to electric charge, amount, and radius. For synthetic polymers with polar side chains, the glass transition temperature (Tg) increases with the introduction of metal ions [45-47]. In this respect, Eisenberg reported that the Tg of polyphosphoric acid depends on metal ion specimens and the amount. In the report, the bonding energy (E) between the side chains and metal ions is represented by

E = I'QaQc

(4)

I' = const.

a

where Qa' Qc' and a are the anion charge, the cation charge, and the internuclear distance, respectively: a is taken as the sum of the ionic radii. For a cation-anion bonding, E is an electrostatic attractive energy between cations and anions. Moreover, the following relationship was experimentally found.

a

0.5 ~m

1.0

(m mol /9)

Figure 21 Relationship between the positions for J3 dispersions. (From Ref. 49.)

(T~)

and amounts of metal ion (~m)

Nakano

272 Tg = L"Qc!a

L" = const.

(5)

This equation suggests that the bonding energy between the side chains and the metal ions is reasonably explained by Eq. (4). Accordingly, the mobility of molecules is related to Qc!a. Next, we examine our case: the factors related to the mobility are the electric charge, the amount, and the radius from our results. From analogy with Eisenberg's discussion, T~ should also be represented by a similar equation, if the bonding between metal ions and side chains is ionic. Thus T~ is represented by

TC3 = m' Qc!r

m' = const.

(6)

where r is adopted for a to simplify our discussion, since r = a r': r' is the radius of an ionized side chain. Furthermore, from the results in Fig. 21, Tf3 must also be proportional to the amount of metal ion (~). Therefore, T13 is expected to be represented by

=

T~

(m"Qc!r) ~

m"

=

const.

(7)

The relationship between Tf3 and Qc!r . ~m is shown in Fig. 22, where dQ is Qc!r . ~m. The plots show a linear relationhip, represented by

+

TC3 = 40.5

50

~

. ..Y:

~

..

6.

3.5(Qc!r .

~m)

(8)

/

~o 40

0

2

4

AQ

Figure 22 Relationship between Tfl and ~Q ( = Qr!r . ~m) for the ~ dispersions. Qc' the charge of the metal ion; !1m, amount of metal ion; r, radius of metal ion. (From Ref. 49.)

Characterization of Chemically Modified Wood

273

This equation suggests that the above discussion is reasonable. That is, application of Eisenberg's theory is valid for carboxymethylated wood containing metal ions. Accordingly, the mobility of carboxymethylate cellulose in carboxymethylated wood appears to depend on three factors: electric charge, amount, and radius. The relationship is represented by Eq. (8). This result shows that Eisenberg's theory is valid for chemically modified wood containing metal ions.

REFERENCES l.

N. Shiraishi, T. Matsuda, and T. Yokota, Thennal softening and melting of esterified wood prepared in an N20 4-DMF cellulose solvent medium. J. Appl. Polym. Sci. 24:2361-2368 (1979). 2. N. Shiraishi and M. Yoshioka, Plasticization of wood by acetylation with triftuoroacetic acid pretreatment. Sen-i Gakkaishi 42:T346-T355 (1986). 3. N. Shiraishi, Cellulose Utilization (H. Inagaki and G. O. Phillips, eds.), Elsevier, London, 1970, pp.97-109. 4. M. Morita and I. Sakata, An improvement of the thennoplasticity and the solventsolubility of cyanoethylated wood by halogen treatment. Cellulose Chem. Technolo 21:255-265 (1987). 5. H. Matsuda, Preparation and utilization of esterified woods bearing carboxyl groups. Wood Sci. Technol. 21:75-88 (1989). 6. D. N.-S. Hon and N. H. Ou, Thennoplasticization of wood. I. J. Polym. Sci. 27:2457-2463 (1989). 7. D. N.-S. Hon and J. San Luis, Thennoplasticization of wood. II. J. Polym. Sci. 27:4143-4149 (1989). 8. R. M. Rowell and R. A. Young, Modified Cellulosics, Academic Press, New York, 1978. 9. E. Otto, H. M. Spurlin, and M. W. Grafftin, Cellulose and Cellulose Derivatives, Interscience, New York, 1955. 10. H. Funakoshi, N. Shiraishi, M. Norimoto, T. Aoki, H. Hayashi, and T. Yokota, Studies on the thennoplasticization of wood. Holzjorschung 33: 159-166 (1979). 11. T. Nakano and H. Nakamura, Viscoelasticity of esterified wood specimens. I. Mokuzai Gakkaishi 32: 176-183 (1986). 12. T. Nakano and H. Nakamura, Viscoelasticity of esterified wood specimens. II. Mokuzai Gakkaishi 32:337-343 (1986). 13. T. Nakano and H. Nakamura, Viscoelasticity of esterified wood specimens. III. Mokuzai Gakkaishi 32:820-826 (1986). 14. T. Nakano and H. Nakamura, Viscoelasticity of esterified wood specimens. IV. Mokuzai Gakkaishi 33:472-477 (1987).

274 15.

Nakano T. Nakano, Viscoelasticity of esterified wood specimens. V. Mokuzai Gakkaishi

34:516-521 (1988). 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

32.

T. Nakano, Mechanism of thermoplasticity of chemically modified wood. Holzforschung 48:318-324 (1994). T. Nakagami, H. Amimoto, and T. Yokota, Esterification of wood with unsaturated carboxylic acids. I. Bull. Kyoto Univ. Forest 46:217-224 (1974). T. Nakagami, M. Ota, and T. Yokota, Esterification of wood with unsaturated carboxylic acids. II. Bull. Kyoto Univ. Forest 47:178-205 (1975). T. Nakano, Time-temperature superposition principle on relaxational behavior of wood as a multi-phase material. Holzals Roh-und WerkstofJ 53:39-42 (1995). G. L. Sionimskii, A. A. Askadskii, and A. I. Kitaigorodskii, About packing of macromolecules in polymers. Vysokomol Soedin AJ2:494-512 (1970). C. J. Maim, J. W. Mench, D. L. Kendall, and G. D. Hiatt, Aliphatic acid esters of cellulose. Ind. Eng. Chem. 43:688-691 (1951). C. E. Rehberg and C. H. Fisher, Properties of monomeric and polymeric alkyl acrylates and methacrylates. Ind. Eng. Chern. 43:1429-1433 (1948). L. Peller, On a model for the helix-random coil transition in polypeptide. I. J. Phys. Chem. 63:1194-1199 (1959). L. Peller, On a model for the helix-random coil transition in polypeptide. II. J. Phys. Chem. 63:1199-1206 (1959). A. Wada, Helix-coil transformation and titration curve of poly-L-glutamic acid. Molec. Phys. 3:409-416 (1960). U. P. Strauss and N. L. Gershfeld, The transition from typical polyelectrolyte to polysoap. I. J. Phys. Chem. 58:747-753 (1954). N. S. Schneider and P. Doty, Macro-ions. IV. J. Phys. Chem. 58:762-769( 1954). F. E. Harris and S. A. Rice, A chain model for polyelectrolytes. I. J. Phys. Chem. 58:725-732 (1956). T. Nakano, Dependence of relaxation property of succinylated wood on sidechain ionization. Holzforschung 47:278-282 (1993). T. Nakano and S. Honma, Physical properties of chemically-m-odified wood containing metals. I. Mokuzai Gakkaishi 36:1063-1068 (1990). P. J. Flory, Principles of Polymer Chemistry (in Japanese). Maruzen, Tokyo, 1957. T. Nakano, S. Honma, S. Ehata, and A. Matsumoto, Carboxymethylation of wood by ethanol-water reaction medium. Mokuzai Gakkaishi 36: 193-199 (1990).

33.

34.

T. Nakano, S. Honma, S. Ehata, and A. Matsumoto, Introduction of metals to chemically-modified woods. Mokuzai Gakkaishi 36:644-650 (1990). T. Nakano, Relaxation properties of carboxymethylated wood plus metallic salts in aqueous solution. HoJzforschung 47:202-206 (1993).

Characterization of Chemically Modified Wood 35.

275

T. Nakano, Plasticization of wood by alkali treatment. Mokuzai Gakkaishi

35:431-437 (1989). 36. 37. 38. 39. 40.

B. H. Zimm, Theory of melting of the helical form in double chains of the DNA type. l. Chem. Phys. 33:1349-1356 (1960). B. H. Zimm and J. K. Brag, Theory of the phase transition between helix and random coil in polypeptide chains. l. Chem. Phys. 3/:526-535 (1959). B. H. Zimm and S. A. Rice, The helix-coil transition in charged Macromolecules. Molec. Phys. 3:391-407 (1960). J. H. Gibbs and E. A. Dimarzio, Statistical mechanics of helix-coil transition in biological macromolecules. l. Chem. Phys. 30:271-282 (1959). S. Lifson, Theory of the helix-coil transition in DNA considered as a copolymer.

Biopolymers /:25-32 (1963). 41.

S. Lifson and A. Roig, On the theory of helix-coil transition in polypeptides.

l. Chem. Phys. 34:1963-1974 (1961). 42. 43. 44.

S. Lifson and B. H. Zimm, Simplified theory of the helixcoil transition in DNA based on a grand partition function. Biopolymers 1: 15-23 (1 %3). M. Ozaki, M. Tanaka, and E. Teramoto, Dependence of the transition temperatures of DNA Molecules upon their base. l. Phys. Soc. lpn. 18:551-557 (1%3). Y. Tanaka, Serogen Monogarari (in Japanese), Daiichikogyoseiyaku, Kyoto,

1968. 45. 46.

47. 48. 49.

E. T. Otocka and T. K. Kwei, Macromolecules 1:401-405 (1968). W. T. Macknight, L. W. Mckenna, and B. E. Read, l. Appl. Phys.

38:4208-4212 (1967). W. E. Fitzgerald and L. E. Nielsen, Proc. Roy. Soc. A282:137-146 (1964). A. Eisenberg, Adv. Polym. Sci. 5:59-112 (1967). T. Nakano, Physical properties of chemically-modified wood containing metal.

II. Mokuzai Gakkaishi 37:924-929 (1991).

11 Weathering of Chemically Modified Wood David V. Plackett Forintek Canada Corporation Vancouver, British Columbia, Canada

Elizabeth A. Dunningham and Adya P. Singh New Zealand Forest Research Institute Limited Rotorua, New Zealand

I.

WEATHERING OF WOOD SURFACES

The weathering of wood in outdoor environments occurs as a result of a complex combination of processes involving the effects of light, water, fungi, and airborne dirt and pollutants. As would be expected, climatic factors such as hours of sunshine and amount of rainfall significantly influence the rate at which wood weathers. The visible effects of weathering depend on the wood species and the particular circumstances, including such factors as orientation with regards to north and south, i.e., whether the exposed wood is inclined vertically as in a wall, horizontally as in decking, or at an angle as may occur with roofing materials, and the presence or absence of nearby trees or shrubs that may provide shelter from wind and sun. In general, wood surfaces fully exposed to the weather gradually take on a grey color and may support molds or fungal staining organisms over a period of time. The effect of wetting and drying cycles will also often cause splits or checks in wood surfaces and the extent to which this may happen will depend in part on the manner in which the timber has been sawn from the log. There is a considerable amount of literature dealing with the phenomenon of wood weathering and methods that have been used to gain a better understanding of weathering processes. It is useful to consider some of this literature

277

278

Plackett et al.

before discussing methods that might be used to interfere with wood weathering. Hon [I] pointed out that air pollutants such as sulfur dioxide and nitrogen dioxide have become more significant in the atmosphere over the last 40 years. He established experiments to determine the impact of radiation on southern yellow pine (Pinus spp.) when exposed to these gases. Fourier transform infrared (FfIR) spectroscopy and ultra violet (UV) spectroscopy were used to monitor surface oxidation of the cellulose, hemicellulose, and lignin components in wood as well as the color changes that occurred as a function of wood species and length of exposure. FfIR confirmed that carbonyl groups were formed during photoirradiation and a decrease in an absorption band at 1743 cm - I was cited as evidence of lignin degradation. It was concluded that there was some evidence for both lignin and carbohydrate degradation in these experiments and that there was an increase in degradation products in the presence of the pollutant gases. Electron spin resonance (ESR) spectroscopy was used to provide evidence for reactions between photoinduced free radicals on the wood surface and oxygen, sulfur dioxide, or nitrogen dioxide. ESR has also been used in other studies to demonstrate the involvement of singlet oxygen in the photodegradation of wood surfaces [2]. In these experiments, ESR showed that free radicals were formed at the wood surface during photoirradiation and that reaction of these radicals with oxygen led to formation of peroxide radicals. The concentration of hydroperoxide in photoirradiated wood increased when singlet oxygen generators were present and decreased when wood was irradiated in the presence of singlet oxygen quenchers. The participation of singlet oxygen in the photodegradation of wood surfaces was therefore strongly indicated. Evans et al. [3] studied the rate of surface delignification of radiata pine (Pinus radiata D. Don) using thin veneers exposed to natural weathering and demonstrated substantial loss of lignin after 3 days of exposure. Sample weight loss results were compared with rainfall records and were consistent with rainwater leaching of lignin breakdown products. The work of Evans et al. also involved microscopic examination of degradation in the lignin-rich middle lamella and the use of tensile strength measurements under controlled conditions to monitor the effects of weathering. Using FfIR, lignin degradation in the first 1-2 JJ.m of the wood surface was monitored by following changes in the 1505 cm- I band. This band corresponds to the aromatic C=C bond stretching in lignin. Indications that chemical changes in lignin can occur so rapidly on weather exposure certainly has implications for the performance of surface finishes on wood. Potential problems in performance of

Weathering of Chemically Modified Wood

279

finishes when applied over preweathered wood have been recognized by a number of researchers [4-7]. Kuo and Hu [8] studied ultrastructural changes in red pine (Pinus resinosa Ail.) sapwood when exposed to UV light. Their rationale for the research was the need to obtain a better understanding of the wood weathering process so that systems to interfere with this process can potentially be found. They referred to earlier work by Norrstrom [9] in which it was suggested that 80-95% of wood degradation caused by UV occurred in lignin, 5-20% in carbohydrates, and only 2% in the extractives. Kuo and Hu used scanning (SEM) and transmission (TEM) electron microscopy to show that lignin in cell wall comers and in the middle lamella was preferentially degraded during the early stages of UV irradiation in laboratory experiments. Massive cell wall degradation did not occur until wood surfaces were exposed to UV light for more than 10 days. Hon and Chang [10] showed that the lignin content in the surface of southern yellow pine dropped from 28% to 14.5% on exposure to UV light. There was also the suggestion that UV light absorption by lignin could result in energy transfer and could contribute to the breakdown of cellulose. Hemicelluloses have UV absorption characteristics almost identical to those of cellulose [11] and may produce more water-soluble degradation products than cellulose at the same degree of polymerization. Research by Kalnins [12] indicated that extractives in Douglas fir were antioxidants and therefore these extractives might provide a protective effect against wood photodegradation. Miniutti [13] used SEM to show that the main changes occurring in UVirradiated wood were the destruction of pit borders in radial cell walls and the fonnation of microchecks along the fibril angle in tangential cell walls. In this same study, SEM was also used to illustrate the preferential degradation of lignin in cell comers and in compound middle lamellae during the early stages of UV irradiation. Groves and Banana [14] used SEM to study natural weathering of radiata pine and found that deterioration of the wood microstructure was observable after 4 months exposure in Canberra, Australia. Complete surface degradation and some erosion of the wood surface was observed after 6 months of exposure. Feist [15] summarized significant aspects of the weathering of wood in structural applications, pointing out the aesthetic effects that occur such as changes in color, roughness, surface checking, dirt pickup, and mildew growth. He noted that these changes can be quite rapid but then there is often little further noticeable change for years in the absence of decay. Hon [16] reviewed weathering reactions and the protection of wood surfaces and stated

Plackett et aJ.

280

that most of the weather-induced discoloration occurred in wood between 3 and 4 months after initial exposure (Fig. 1). Studies in New Zealand involving exposure of radiata pine and western red cedar (Thuja pUcara) gave similar results (Figure 2). Kalnins and Feist [17] investigated the increased wettability of wood as a function of weathering time and suggested that photo-oxidation and leaching of extractives contributed to a gradual loss in water repellency of the wood surface. They indicated that further research on the role of extractives in the weathering process might be justified. Hon and Feist (18] used diffuse reflectance FfIR (DRIFf) to look at the weathering behavior of southern yellow pine and red oak (Quercus spp. Erythrobalanus) under various light conditions. Evidence wa~ .
80

W S g

70

Q)

60

e

50

GI

:::: '0

+

+

+

0 40

0

0

30

0

20

•+

10

6 Douglas-fir

Westem red cedar Redwood Southern yellow pine

0 0

60

120

180

240

300

360

-420

480

Weathering time (days)

Figure 1 Changes in color of outdoor weathered western red cedar, redwood, southern yellow pine, and Douglac; fir in the USA. (From Ref. 16.)

Weathering of Chemically Modified Wood

281

20 - . - - - - - - - - - - - - - - - - - - - - - - - - - - ,

10 iiJ S

8c ~

CD

It:

0

'5

0

'0

(.)

-10

• Radiata Pine o Western Red Cedar

-20 - t - - - - - - . - - - - - - . - - - - - - - r - - - - - . . . , - - - - - - ;

o

20

40

60

80

100

Weathering time (days)

Figure 2 Changes in color of outdoor weathered radiata pine and western red cedar in New Zealand.

Faix and Nemeth [21] also reported the use of DRIFf to study photodegradation in robinia (Robinia pseudoacacia) and poplar (Populus tremula). They summarized the spectral changes that were observed when both unextracted and water-extracted samples were studied using DRIFf and concluded that this was a powerful and convenient analytic technique for monitoring photodegradation of wood and its components. Andersen et al. [22] obtained the infrared spectra in diffuse reflectance mode for western red cedar, southern yellow pine, and Douglas fir (Pseudotsuga menziesii) after samples had been subjected to a variety of artificial weathering conditions. Initially, the spectra of the surfaces were different but the spectra became essentially identical after 2400 h of artificial weathering and then corresponded closely with those of the cellulosic polymer components of wood. A photochemically induced pathway for the weathering process was proposed. Hon [23] used electron spectroscopy for chemical analysis (ESCA) to study weathering of yellow poplar. The ESCA spectra showed increases in the intensities of peaks attributed to carbon-oxygen bonds and oxygen-carbon-oxygen bonds and increases in the oxygen-to-carbon ratio, as well as decreases in peaks attributed

282

Plackett ef aI.

to carbon-carbon and carbon-hydrogen bonds on weathered and UV-irradiated wood surfaces. The surface nature of wood weathering was confinned when it was shown that oxidation processes were extremely slow 100 IJ.m below the exposed wood surface. Spectral changes were consistent with a surface rich in cellulose but poor in lignin. Kabir et al. [24] reported on laboratory methods to predict the weathering characteristics of wood. The methods involved detennination of antiswelling efficiency (ASE) and water-repellent effectiveness (WRE). Different degrees of water repellency and dimensional stability were introduced into southern yellow pine sapwood by various chemical treatments, and ASE and WRE values were assessed after sample exposure to natural weathering. WRE values and moisture content changes in boards were considered to be the best predictors of weathering perfonnance. In this study, treatments imparting high WRE values gave material with the best perfonnance in exterior weathering. None of the treatments gave sufficient improvement in wood dimensional stability to allow full evaluation of ASE as a factor in predicting exterior weathering perfonnance. In summary, research has shown that weathering is a process confined to the very surface of the wood and involving photo-induced breakdown of lignin to water-soluble reaction products. These breakdown products are leached away from the wood surface to leave behind a surface rich in cellulosic components. Evidence for the breakdown of lignin in particular comes from FfIR and other analytic methods and there is some suggestion that degradation of lignin may also be associated with the breakdown of carbohydrates in the weathering process. ESR provides evidence for the role of free radicals in oxidative processes occurring during weathering. The key factors causing weathering of wood appear to be UV light and water, although there is evidence for involvement of visible light as well. Research also indicates that certain air pollutants such as sulfur dioxide and nitrogen dioxide may exacerbate the wood weathering process. The fact that wood weathers and therefore requires some fonn of surface protection such as a paint or stain finish in many applications, and that the finish itself requires care and maintenance, is an influencing factor in some wood products markets and has probably contributed to loss of market share to other materials in some areas. If wood is to regain market share for highvalue exterior products in the future, the development of "weather-resistant" wood or wood products that will give much longer service lives for applied finishes is a desirable goal. Having discussed the nature of the weathering process, the remaining two sections of this chapter address ways in which

Weathering of Chemically Modified Wood

283

wood may be treated to improve weather-related properties. The next section deals with simple chemical pretreatments that have been studied in the past whereas the third section discusses the weathering properties of chemically modified wood in which chemical moieties have been covalently bound to the wood cell wall to provide a new material with improved properties.

II.

PROTECTION OF WOOD SURFACES AGAINST WEATHERING: CHEMICAL PRETREATMENTS

Wood is susceptible to degradation in a number of ways, including rot or decay, insect attack, fire, and weathering. Each of these fonns of degradation is essentially chemical in nature and can therefore potentially be inhibited by chemical means. Weathering of wood also has physical and biological aspects; therefore any methods developed to interfere with weathering at the wood surface should ideally provide greater physical stability and biological durability. The ultimate fonn of wood protection involves applying several coats of an opaque paint. If properly applied and well maintained, paint protects wood from the damaging effects of the sun's rays, slows the ingress of water into the wood material, and provides a clean, aesthetically appealing appearance. In exterior situations, it is often desirable to see the grain and texture of the wood and this calls for transparent or semitransparent finishes. Such finishes generally have a shorter service life than paints because they offer less protection from UV or visible light exposure or from water entry into the product. The need to develop simple pretreatments that will inhibit wood weathering has therefore been driven by an interest in improving wood as a substrate for finishing as well as for other reasons. This section deals with such simple pretreatments. By some definitions these treatments could fall under the category of chemical modification of wood but we shall deal with this type of treatment, defined as covalent binding of chemical groups to the wood substance, in the third section. Perhaps the most extensive area of research in simple pretreatment systems to enhance the weathering properties of wood has involved application of inorganic salt solutions. Much of the early work in this field was undertaken at the U.S. Forest Products Laboratory in Madison, Wisconsin and was aimed at improving the perfonnance of transparent finishes. Black [25] described an experimental chromate-based wood finish and Black and Mraz [26] discovered that both acid and ammoniacal-copper chromate treatments significantly improved the perfonnance of clear finishes on western red cedar, redwood (Sequoia sempervirens) , and Douglas fir plywood. Later research [27] demon-

284

Plackett et al.

strated that a solution of chromium trioxide in water was the most effective system for improving finish performance. There is also evidence that such chromium-based treatments can be used alone to protect wood without a surface finish under some circumstances. The treatments are thought to stabilize the wood surface against UV light degradation and to provide a degree of water repellency and protection from staining fungi. Researchers in other countries have also investigated the chromium treatment concept and demonstrated its effectiveness under a range of weathering conditions and with a range of wood species 128-30]. However, despite the apparent effectiveness, chromium-based treatments to improve wood weathering and finishing properties have received very little commercial attention because of chemical toxicity and handling issues. There has been a considerable amount of research undertaken to establish the mechanism by which chromium treatments photostabilize wood. The rationale for this research has usually been that a better understanding of the exact wood protection mechanism might allow new, less toxic treatments to be developed. Research by Pizzi and others [31,32] suggests that chromium(VI) pretreatments cause oxidation of the wood surface and the fonnation of a chromium-based network involving lignin. The fonnation of this network provides the observed water-repellent effect. However, it is apparent that the detailed mechanism by which chromium interacts with lignin, cellulose, and other wood components at the molecular level has yet to be clarified. Studies on metal ion-lignin interactions were undertaken by Hon and Chang [33] using model compounds and they suggested that photoprotection could involve energy transfer from wood-to-wood complexes and subsequent release of the energy hannlessly from wood surfaces. It was also considered possible that wood-ion complexes might decompose peroxide impurities fonned at wood surfaces and that this could interfere with photodegradation chain reactions. The beneficial effect of chromium in interfering with wood weathering and improving the perfonnance of finishes has also been demonstrated in copperchromium-arsenic (CCA)-treated wood [34]where surface quantities of chromium are typically much less than when 5% chromium trioxide pretreatments are applied to wood surfaces. There is also evidence that chromium(lII) (e.g., chromium nitrate) has some wood-photostabilizing activity [35] and there would be benefits in using chromium(III) chemicals in tenns of reduced toxicity when compared with chromium(VI); however, heat is required to fix chromium(III) pretreatments on wood. Schmalzl et al. [36] evaluated a series of novel chemical treatments to protect radiata pine against weathering using chromium trioxide as a control treatment. Their experiments involved the use of weight loss measurements and a zero-span tensile strength test on veneers,

Weathering of Chemically Modified Wood

285

described elsewhere by Evans [37], to monitor wood weathering. FfIR internal reflectance spectroscopy was also used to follow chemical changes occurring on the wood surface. The novel treatments generally retarded the loss of tensile strength and the authors took this as evidence that complexation with cellulose was occurring. FfIR showed that the chromium trioxide treatment resulted in the appearance of a new band at 1509 cm - 1 and the reduction in intensity of a band at 1506 cm - 1. The spectra of untreated veneers pointed to the complete loss of lignin during weathering whereas the chromiummodified lignin polymer on wood surfaces appeared to be very resistant to degradation. The authors concluded that the novel treatments would have potential benefits when used as wood priming agents. Other simple chemical treatments to enhance wood weathering characteristics have been studied. Hon et al. [38] prepared a clear acrylic coating containing an added benzophenone UV absorber and compared the effectiveness of this coating with several penetrating chemical agents when southern yellow pine was exposed to UV irradiation. Theoretically, addition of UV absorbers, light stabilizers, or free radical quenchers to coating films and/or wood surfaces should have the potential to interfere with the photochemical processes that occur during UV irradiation and therefore could possibly disrupt the wood weathering process. The work of Hon et al. [38] showed that wood surface application of commercial glycols such as PEG-400 and triol-G400 certainly inhibited wood discoloration and it was suggested that this might occur through deactivation of photoexcited chromophoric groups via an energy transfer process. Since this work was reported there has been considerable development in the types of UV absorbers and light stabilizers typically added to transparent coatings or plastics [39], and further possibilities for combining wood surface treatment with coating stabilization can be envisaged [40]. Feist and Sell [41] investigated heat treatment of spruce and beech under nitrogen at 175-195°C. Natural and artificial weathering studies showed that beech had significantly reduced hygroscopicity, improved dimensional stability, and greater resistance to weathering after such treatment. Unfortunately, heat treatment of spruce resulted in diminished weathering resistance; however, the improvement in the properties of beech was considered sufficient to justify further examination under practical conditions.

III.

PROTECTION OF WOOD SURFACES AGAINST WEATHERING: CHEMICAL MODIFICATION

This section of the chapter addresses the performance of chemically modified wood against weathering. Although, as indicated earlier, some form of chemi-

286

Plackett et al.

cal modification in a broader sense can be considered to have occurred when wood has been treated with reagents such as chromium trioxide, for the purposes of this chapter chemical modification will be defined as the covalent binding of chemical moieties to the wood cell wall to obtain improved properties. Chemical modification of wood is a research topic that dates back some four decades. The earliest work in the field involved acetylation of solid wood using acetic anhydride [42] and acetylation methods and characteristics of acetylated wood have remained of interest to the present day. The driving force for research in wood modification over the years has been an interest in improving the durability and dimensional stability of wood products so that they can more effectively compete with nonwood materials in high-value applications. Rowell [43] summarized the state of wood modification research in 1983 and more recently reviewed the application of chemical modification to lignocellulosics and composites [44]. In recent years the emphasis in acetylation research has turned away from basic properties such as durability and dimensional stability toward other properties that have included gluing characteristics, color stabilization, and weathering properties among others. There has also been much activity directed to alternative methods of wood modification that might offer the benefits of acetylation without the disadvantages associated with handling reagents such as acetic anhydride and removing the acetic acid byproduct from the acetylated wood product. In addition, building on the earlier knowledge base, there is now much interest in treatment of wood surfaces to enhance compatibility with synthetic polymers so that a new generation of woodJnonwood (e.g., plastic) composites can be manufactured. As in other areas much of the early research on weathering characteristics of chemically modified wood was undertaken at the U.S. Forest Products Laboratory. Rowell et al. [45] showed that the earlywood of southern yellow pine, chemically modified with butylene oxide or methyl isocyanate for enhancement of durability, was not adequately protected against the effects of weathering. Increasing the dimensional stability of the wood and blocking lignin phenolic hydroxyl groups was apparently not enough to stop weathering in this case. Lumen filling with polymethyl methacrylate (PMMA) did, however, reduce surface erosion by about 50% and this was attributed to reduced water uptake and a retardation of the rate of leaching of weathering breakdown products. A combined treatment of butylene oxide followed by a methyl methacrylate (MMA) lumen filling treatment was found to be effective in protecting wood from accelerated weathering over a period of 90 days. In later work [46], Feist and Rowell studied the effect of UV light and a combination of

Weathering of Chemically Modified Wood

287

UV light and water on physical and chemical changes occurring in modified wood. The extent of weathering on exposed sections of test samples was detennined microscopically. As a result of their research, Feist and Rowell concluded that southern yellow pine modified with butyl isocyanate or butylene oxide was not resistant to the effects of UV light and water. Weight loss studies indicated that chemical modification using these reactants did not reduce the rate of wood surface degradation. Although both treatments increased the dimensional stability of the wood and were thought to block lignin phenolic hydroxyl groups, these factors were not sufficient to stop the effects of weathering. The effectiveness of combined lumen filling and cell wall modification treatments was attributed to the ability of the cured PMMA to hold surface wood fibers in place during weathering. As in earlier research, weight losses for samples receiving combined treatments were at least 50% less than those of the samples modified with butyl isocyanate or butylene oxide. Surface erosion rates were also correspondingly lower. This research was not repeated using an acetylated softwood at that time; however, Feist and Rowell [47] later examined the weathering perfonnance of acetylated aspen (Populus tremuloides) and acetylated aspen that had received a secondary MMA lumen filling treatment. Aspen was selected because of an interest in applying chemical modification to composite products that frequently incorporate this species. Acetylation to 18 weight percent gain (WPG) was found to reduce surface erosion by about 50% when compared with unmodified material. MMA treatment alone reduced erosion by 40%, whereas acetylation to 12 or 18 WPG followed by MMA treatment reduced surface erosion by 74% and 85%, respectively. Wood surfaces were analyzed for acetyl content before and during the weathering process by scraping away the outer 0.5 mm of wood material with a razor blade. Acetyl contents were detennined by a gas chromatography (Ge) method following deacetylation of ground samples with sodium hydroxide [48]. Results showed an average reduction in acetyl content of 48% in the surface 0.5 mm after 700 h of accelerated weathering. Thus, although some weathering protection was obtained, the analytic results were consistent with earlier studies on acetylated or methylated lignocellulosic materials in which UV light-induced deacetylation or demethylation was demonstrated [49]. Analyses of wood components indicated that chemical changes in lignin and cellulose in acetylated aspen were similar to those changes occurring in unmodified aspen. There was, however, less change in the reduction of xylans and mannans. This was interpreted to mean that a reduction in weathering effects in acetylated aspen may be a result of modification of both lignin and hemicelluloses.

288

Plackett et al.

In subsequent research, Feist et al. [50] acetylated aspen fiber obtained from a commercial hardboard manufacturing plant and produced fiberboard in the laboratory using a commercial phenol-formaldehyde resin. Finished and unfinished samples were then subjected to accelerated or natural weathering. The study showed that acetylation greatly reduced the rate of swelling of fiberboards, reduced the rate of surface roughening, and decreased the amount of mildew growth occurring on boards exposed to the outdoors. Imamura [51] examined the resistance of spruce (Picea jezoencis Carr.) or sugi (Cryptomeria japonica D. Don), acetylated to various weight gains, against degradation during exposure to sunlight only or to natural weathering. Color measurements were used to monitor the effects of sunlight only over an initial 2 and 12 weeks of exposure. Almost no change in absolute color difference was obtained in the case of wood acetylated to 24 WPG. Even after 1 year of exposure, wood acetylated above 20 WPG was said to maintain nearly its original color. SEM was used to show that cell wall degradation was relatively restricted in acetylated samples. Plackett and Dunningham [52] used untreated or acetylated rotary-peeled radiata pine sapwood veneer as the test substrate and glued this material to solid wood support blocks for the purposes of weather exposure. Checking and delamination from the support blocks was much reduced in the acetylated samples after a year of exposure. However, in general, acetylated and untreated samples had the same appearance in terms of color and apparent fungal growth after about 8 weeks of exposure at 45° facing equatorially. In a parallel accelerated weathering study [52], acetylated samples showed greater dimensional stability and less checking after 3000 h of exposure than untreated samples or material that had been modified with maleic anhydride to an average of 8.0 WPG. Wakeling et al. [53] studied the susceptibility of acetylated and untreated radiata pine sapwood to growth of surface molds when exposed to a laboratory test procedure. The results of a 3-week test showed a significantly slower rate of colonization on 20.0 WPG acetylated samples than on untreated wood. It was speculated that this might be due to an absence of readily assimilated sugars and starches in acetylated wood. Later work [54] based on exterior exposure trials showed that there was little difference in appearance between acetylated and untreated samples after 8 weeks of exposure. Kiguchi [55] used ESCA to monitor surface changes in chemically modified sugi during and after exposure to UV light from 20-W UV fluorescent lamps. Chemical modification treatments included (1) methylation and reduction and (2) etherification with butylene oxide. In addition, wood samples that had been benzylated and then thermoplasticized by hot pressing were coated with

Weathering of Chemically Modified Wood

289

an acrylic silicone varnish and exposed outdoors at 45° for 1 year. ESCA C( 1s) spectra of methylated and reduced samples showed that the area due to carbonyl and carboxyl groups increased as did the oxygen-to-carbon ratio, indicating that this type of treatment did not protect wood from photooxidation. Fujimoto [56] examined the weathering behavior of Japanese bass wood (Tiliajaponica) that had been chemically modified with a maleic acid-glycerol (MG) mixture. In a vertical outdoor exposure test, the surface smoothness of MG-treated samples was maintained over an I8-month period and the results suggested that this type of treatment might provide wood products with good exterior performance properties. Rowell et al. [57] examined the stability of acetylated southern yellow pine and aspen to environmental changes involving such factors as pH, temperature, and humidity. This research was undertaken in order to address concerns about the long-term stability of the acetyl group in modified wood. Results suggested that acetylated wood should be stable for a very long time under ambient conditions of temperature and humidity. This research did not address the outdoor weathering environment where, as indicated earlier [47], evidence suggests that UV light-induced deacetylation may occur over time. Dunningham et al. [58] investigated the accelerated weathering of chemically modified radiata pine using a slightly modified standard method [59]. Exposure conditions included water spray only, water spray and UV light, and UV light only for 1275 h. Extent of weathering was assessed by measuring depth of erosion on the tangential/longitudinal face of samples using a light microscope with a calibrated focal scale. One third of each sample was covered during weathering and this unexposed area was used as a baseline from which to measure erosion at 10 places over the 15-mm width of the test samples. Depth of erosion data for acetylated, succinylated, formaldehyde crosslinked, and CCA-treated samples are shown in Table 1. As expected, mean erosion was higher where samples had been exposed to both UV and water; however, there was no statistically significant difference between UV/water and UVonly exposures for untreated and CCA-treated material. Interestingly, only CCA-treated samples showed significantly less erosion than untreated samples after 1275 h of accelerated weathering. FfIR spectra obtained during the weathering experiment confirmed that lignin was undergoing photodegradation on the surface of both untreated and chemically modified wood samples. Although acetylation does not appear to confer resistance to the effects of weathering at the wood surface, improved dimensional stability is obtained as shown, for example, in thickness swell measurements for phenol-formalde-

Plackett et al.

290

Table 1 Erosion of Radiata Pine Samples Exposed to UV Only (UV), Water Only (WAT), or UV and Water (UVW) Under Accelerated Weathering Conditions for 1275 h Average depth of erosion (J-Lm) Treatment

Unexposed

UV

Untreated Acetylated 00 WPG) Acetylated (20 WPG) Succinylated 00 WPG) Succinylated (20 WPG) Succinylated (30 WPG) Fonnaldehyde crosslinked (4.3 WPG) CCA-treated

20.6 efghi 16.4 efghij

56.6 be 17.2 fghijk

26.5 e 10.6 hijkl

101.4 ab 117.4 a

13.4 ghijkl

21.4 efgh

9.0 jkl

127.8 a

12.0 jkl

31.1e

21.4 efghi

124.6 a

9.4 kl

WAT

8.6 kl

UVW

99.4 ab

12.8 ijkl

60.4 bc

18.6 efghi

14.2 fghijk

26.8 ef

22.5 efg

93.6 ab

14.4 fghijkl

31.2 de

10.41

52.6 ed

126.0 a

Note: Values with the same letter are not statistically different at 95% confidence limits, calculated on the log transformation of the erosion data. Source: Ref. 58.

hyde-bonded radiata pine particleboard exposed to natural weathering (Fig.

3). Cronshaw [60] studied accelerated weathering of unmodified, acetylated, and chloroacetylated radiata pine. Scanning electron microscopy with energydispersive X-ray analysis (SEM-EDXA) was used to localize chlorine in the cell wall of chloroacetylated samples and an increase in chlorine content in the middle lamella with increasing WPG was indicated. FfIR spectroscopy was used to show that the lignin band at 1510 cm - I decreased in intensity at about the same rate for unmodified, acetylated, and chloroacetylated samples during exposure to accelerated weathering.

IV.

CONCLUDING REMARKS

The weathering of wood surfaces under natural or artificial conditions has been widely studied in recent years and the application of modem instrumental

Weathering of Chemically Modified Wood

291

2..5.,-------------=:::a:::::::::::-----------,

E

2.0

.§.

I

1.5

~

1.0

~

0.5

0.0

-f-~---_r_----~----~----r__---____l

o

5

I- Untreated

10 15 Tme exposed (weeks) A Acetylated 10 WPG

20

+ Acetylated 20 WPG

25

I

Figure 3 Thickness swell of particleboard made from untreated, lO-WPG acetylated, or 20-WPG acetylated radiata pine furnish on exposure to natural weathering in New Zealand. (From Ref. 58.)

techniques such as FfIR and ESR spectroscopy, as well as microscopy, has provided us with an improved understanding of the weathering process. In particular, most of the chemical and physical changes during weathering of wood are now thought to occur within 1()() J.Lm of the surface and degradation of lignin to water-soluble products of lower molecular weight is recognized as a key factor. The treatment of wood to reduce or eliminate the effects of weathering could potentially open up new market opportunities for wood as a material and research has therefore targeted chemical pretreatments that might provide such benefits. Chromium-based pretreatments are particularly effective in this regard but, presumably for environmental reasons, such systems have generally not been adopted by industry and the search continues for effective, lowtoxicity photostabilizing agents for wood. Chemical modification of wood, in which chemical moieties are covalently bound to the wood, offers tremendous potential as a general way of upgrading important wood properties such as biological durability and dimensional stability, and may also provide a route to "weather-resistant" wood products in the future. Research to date has shown that some types of chemically modified wood appear to at least partially resist the surface effects of weathering. The challenge for the future will be to better understand the factors contributing

292

Plackett et al.

to weather resistance and to thereby develop cost-competitive modified wood products with superior outdoor perfonnance.

REFERENCES 1.

D. N.-S. Hon, in Cellulosics: Pulp, Fibre and Environmental Aspects (1. F. Kennedy, G. O. Phillips, and P. A. Williams, eds.), Ellis Horwood, New York, 1993, pp. 329-334. 2. D. N.-S. Hon, S.-T. Chang, and W. C. Feist, Wood Sci. Technol. /6:193 (1982). 3. P. D. Evans, A. J. Michell, and K. J. Schmalzl, Wood Sci. Technol. 26:151 (1992). 4. K. Kleive, J. Coat. Technol. 58(740):39 (1986). 5. R. S. Williams, P. L. Plantinga, and W. C. Feist, Forest Prod. J. 40(1):45 (1990). 6. M. Arnold, W. C. Feist, and R. S. Williams, Forest Prod. J. 42(3):10 (1992). 7. R. S. Williams and W. C. Feist, Forest Prod. J. 43(1):8 (1993). 8. M.-L. Kuo and N. Hu, Holzforschung. 45(5):347 (1991). 9. H. Norrstrom, Svensk. Paperstidn. 72:25 (1969). 10. D. N.-S. Hon and S.-T. Chang, J. Poly. Sci. Polym. Chem. 22:2227 (1984). 11. D. N.-S. Hon, in Developments in Polymer Degradation (N. Grassie, ed.), Applied Science Publishers, London, 1981, pp. 229-281. 12. M. A. Kalnins, in Surface Characteristics of Wood as They Affect Durability of Finishes, US Forest Servo Res. Pap. FPL 57 (1966). 13. V. P. Miniutti, Forest Prod. J. /4(12):571 (1964). 14. K. W. Groves and A. Y. Banana, J. /nst. Wood Sci. 10(5):210 (1986). 15. W. C. Feist, in Structural Use of Wood in Adverse Environments (R. W. Meyer and R. M. Kellogg, eds.), Van Nostrand Reinhold, New York, 1982, pp.156-178. 16. D. N.-S. Hon, J. Appl. Polym. Sci. Appl. Polym. Symp. 37:845 (1983). 17. M. A. Kalnins and W. C. Feist, Forest Prod. J. 43(2):55 (1993). 18. D. N.-S. Hon and W. C. Feist, Wood Fiber Sci. 24(4):448 (1992). 19. D. N.-S. Hon and W. C. Feist, Wood Sci. Technol. 20:169 (1986). 20. D. N.-S Hon, G. Ifju, and W. C. Feist, Wood Fiber Sci. /2(2):121 (1980). 21. O. Faix and K. Nemeth, Holz als Roh Werkstoff. 46:112 (1988). 22. E. L. Andersen, Z. Pawlak, N. L. Owen, and W. C. Feist, Appl. Spectrosc. 45(4):641 (1991). 23. D. N.-S. Hon, J. Appl. Polym. Sci. 29:2777 (1984). 24. F. R. A. Kabir, D. D. Nicholas, R. C. Vasishth, and H. M. Barnes, Holzforschung. 46(5):395 (1992). 25. J. M. Black, USDA Forest Servo Res. Note FPL-0134 (1973). 26. J. M. Black and E. A. Mraz, USDA Forest Servo Res. Paper FPL 232 (1974). 27. W. C. Feist, USDA Forest Servo Res. Paper FPL 339 (1979).

Weathering of Chemically Modified Wood 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

45. 46.

47. 48. 49. 50. 51. 52.

53.

54. 55.

293

A. Pizzi, Hohforschung Holverwertung. 31(6):128 (1979). A. F. Preston and C. M. Chittenden, J. Coat. Technol. 50(637):59 (1978). K. Ohtani, Chromium Rev. 8:4 (1987). A. Pizzi, J. Appl. Polym. Sci. 25: 2547 (1980). R. S. Williams and W. C. Feist, Wood Fiber Sci. 17(1):184 (1985). D. N.-S. Hon and S.-T. Chang, Wood Fiber Sci. 17,(1):92 (1985). D. V. Plackett and D. R. Cronshaw, Surface Coat. Aust. 29,(11):11 (1992). R. S. Williams and W. C. Feist, Forest Prod. J. 38,(11112):32 (1988). K. J. Schmalzl, A. J. Michell, L. J. Vickers, and P. D. Evans, presented by K. J. Schmalzl, Forest Products Conference, Melbourne, Australia, 1990. P. D. Evans, Wood Fiber Sci. 20:487 (1988). D. N.-S. Hon, S.-T. Chang and W. C. Feist, J. Appl. Polym. Sci. 30:1429 (1985). H. Bohnke and E. Hess, Eur. Coat. J. 5:222 (1990). D. V. Plackett, Surface Coat. Aust. 30, (4): 14 (1993). W. C. Feist and J. Sell, Wood Fiber Sci. 19(2):183 (1987). H. Tarkow and A. J. Stamm, J. Forest Prod. Res. Soc. 3:33 (1953). R. M. Rowell, Commonwealth Forestry Bureau Rev. 6,(12):363 (1983). R. M. Rowell, in Emerging Technologies for Materials and Chemicals from Biomass (R. M. Rowell, T. P. Schultz, and R. Narayan, eds.), American Chemical Society, Washington, D. c., 1992, pp. 12-27. R. M. Rowell, W. C. Feist, and W. D. Ellis, Wood Sci. 13, (4):202 (1981). W. C. Feist and R. M. Rowell, in Graft Copolymerization of Lignocellulosic Fibres (D. N.-S. Hon, ed.), American Chemical Society, Washington, D. c., 1982, pp. 349-370. W. C. Feist and R. M. Rowell, Wood Fiber Sci. 23,(1):128 (1991). R. M. Rowell, A.-M. Tillman, and R. Simonson, J. Wood Chem. Technol. 6,(3):427 (1986). G. J. Leary, Tappi 51, (6):257 (1968). W. C. Feist, R. M. Rowell, and J. A. Youngquist, Wood Fiber Sci. 23,(2):260 (1991). Y. Imamura, Wood Res. 79:54 (1993). D. V. Plackett and E. A. Dunningham, in Proceedings of the International Symposium on Chemical Modification of Wood, Kyoto, Japan, 1991, pp. 136-141. R. N. Wakeling, D. V. Plackett, and D. R. Cronshaw, in Proceedings of the International Symposium on Chemical Modification of Wood, Kyoto, Japan, 1991, pp. 142-147. R. N. Wakeling, D. V. Plackett, and D. R. Cronshaw, International Research Group on Wood Preservation, Document No. IRG/WP/1548-92 (1992). M. Kiguchi, in Chemical Modification of Lignocellulosics (D. V. Plackett and E. A. Dunningham, compilers), FRI Bulletin No. 176, New Zealand Forest Research Institute, Rotorua, 1992, pp. 77-86.

294 56.

57. 58. 59. 60.

Plackett et a/. H. Fujimoto, in Chemical Modification of Lignocellulosics (D. V. Plackett and E. A. Dunningham, compilers), FRI Bulletin No. 176, New Zealand Forest Research Institute, Rotorua, 1992, pp. 87-96. R. M. Rowell, R. S. Lichtenberg, and P. Larsson, Wood Fiber Sci. 25, (4):359 (1993). E. A. Dunningham, J.-Q. Zhu, and A. P. Singh (personal communication). ASTM Standard G26-84 (1984). D. R. Cronshaw, M.Sc. (Tech) thesis, Department of Chemistry, University of Waikato, Hamilton, New Zealand (1992).

12 Physical and Mechanical Properties of Chemically Modified Wood Roger M. Rowell USDA Forest Service and University of Wisconsin Madison, Wisconsin

I. INTRODUCTION Chemical modification will be defined for this chapter as any chemical reaction between some reactive part of a wood cell wall component and a simple single chemical reagent, with or without catalyst, that fonns a covalent bond between the two components. This excludes in situ polymerizations of monomers in the lumen structure of the wood and those reactions that result in cell waH-penetrating polymer systems that do not result in any cell wall attachment. It is well known that lumen-filling polymer treatment results in large improvements in mechanical properties, but these are mainly a result of the properties of the new polymer introduced [I]. To the extent possible, this chapter will deal with the changes in physical properties of wood as a direct result of chemical modification of the cell wall and not as a result of the reaction conditions used to modify the wood. For example, strong acid or base catalysts used in some chemical modification reaction systems will result in a reduction in physical properties due to hydrolysis or decrystallization of cellulose and these changes are not due to the

The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This chapter was written and prepared by U.S. government employees on official time. It is in the public domain and not subject to copyright.

295

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296

chemical added to the cell wall. The actual change in physical properties that resulted from the chemical modification may be small but masked by the massive changes due to catalyst degradation. In many cases, it is very hard to detennine what changes are due to the reaction conditions and those resulting from a change in cell wall chemistry. This chapter deals mainly with changes in the mechanical properties resulting from the chemical modification of solid wood. There has been a lot of research done on modification of wood chips/particles/fiber that is then used to make wood composite. It is difficult to detennine the mechanical changes in a composite made of chemically modified furnish because the properties of the adhesive and glue line influence the mechanical properties more than or as much as changes resulting from the chemical modification. Changes in gluability of chemically modified wood will be discussed as well as some changes in mechanical properties of wood composites.

II.

CELL WALL CHEMISTRY AND MECHANICAL PROPERTIES

Any change in the chemistry of the wood cell wall polymers results in a change in physical and mechanical properties of the wood. These properties can vary from simple color change in the wood to major changes in modulus properties. work to maximum load. brittleness, hardness. wet strength, wet stiffness, impact strength, compressive strength, gas permeability, density, and moisture sorption. Improvements in dimensional stability and biological resistance and decreases in moisture sorption have been the motivation for much of the research done over the years in the chemical modification of wood [2-4]. Changes in cell wall moisture content resulting from chemical modification have a very large influence on mechanical properties.

A.

Relationship Between Cell Wall Moisture Content and Mechanical Properties

Changes in the moisture content of the wood cell wall have a major effect on the mechanical properties of wood [5]. At moisture contents from ovendry (00) to the fiber saturation point (FSP). water accumulates in the wood cell wall (bound water). Above the FSP. water accumulates in the wood cell cavity (free water) and there is no tangible strength effect associated with a change in free water content. However, at moisture contents between 00 and the FSP. water does affect strength. Increased amounts of bound water interfere with and reduce hydrogen bonding between the polymers of the cell

Physical and Mechanical Properties

297

wall, which decreases the strength of wood. The approximate relationship between cell wall moisture content and strength is shown in Tables I and 2 [6]. It can be seen from the data in these tables that the moisture content of the cell wall has a great influence on all strength properties of wood. Table I shows that fiber stress at proportional limit, work to proportional limit, and maximum crushing strength are the mechanical properties most affected by changing moisture content by only + I % below the FSP. The change in mechanical properties from green to ovendry are shown in Table 2.

B.

Effect of Chemical Modification on Cell Wall Moisture Content

Table 3 shows the changes in cell wall equilibrium moisture content (EMC) for several types of chemically modified wood. It can be seen that all bonded chemicals lower the EMC of the wood by about half that of nonreacted wood Table 1

Approximate Change in the Mechanical Properties of Clear Wood When Subjected to Change in Moisture Content Changes per I % change in moisture content Property Static bending Fiber stress at proportional limit Modulus of rupture Modulus of elasticity Work to proportional limit Work to maximum load Impact bending Height of drop causing complete failure Compression parallel to grain Fiber stress at proportional limit Maximum crushing strength Compression perpendicular to grain Fiber stress at proportional limit Shear parallel to grain Maximum shearing strength Hardness End Side

(%)

5 4

2 8

0.5 0.5 5 6 5.5 3 4

2.5

Rowell

298 Table 2 Relationship Between Some Mechanical Properties of Wood and Moisture Content Moisture content Green

Property· Modulus of rupture Compression parallel to grain Modulus of elasticity Modulus of rupture Compression parallel to grain Modulus of elasticity

Douglas fir 62 52 80 Aspen 61 50 73

19%

12%

8%

Oven-dry

76 68 88

100 100 100

117 124 108

161 192 125

75 67 83

100 100 100

118 126 III

165 199 137

'All values are expressed as a percentage of property at 12% moisture content.

at each RH tested except propylene oxide. EMC is not reduced as much at equal weight gains with propylene oxide as it is with acetylation or crosslinking with fonnaldehyde. This may be due to the fact that a new hydroxyl group is fonned at the bonding site during the addition of the propyl group. Table 4 shows the FSP of control and acetylated aspen flakes. The FSP for acetylated aspen is reduced over 65% at about 17 weight percent gain (WPG). The data given in Tables 4 and 5 demonstrate the great decreasing effect of chemical

Table 3

Equilibrium Moisture Content of Control and Chemically Modified Pine

Chemical Control Acetic anhydride Fonnaldehyde Propylene oxide Butylene oxide

Weight percent gain

Equilibrium moisture content at 27°C 0% RH

35% RH

60% RH

85% RH

0 20.4

1.0 0.7

5.0 2.4

8.5 4.3

16.4 8.4

3.9 21.9

0.4 1.4

3.0 3.9

4.2 6.1

6.2 13.1

18.7

1.3

3.5

5.7

10.7

Physical and Mechanical Properties

299

Table 4 Fiber Saturation Point of Control and Acetylated Aspen Flakes Weight percent gain

Fiber saturation point

(%)

(%)

o

46

8.7 13.0 17.6

29 20 15

Table 5 Changes in Volume of Southern Pine upon Drying and Modification with Propylene Oxide or Acetic Anhydride B

A

B - A

Oven-dry volume (cm 3)

V (%)

57.0 59.0 59.0

53.1 53.1 55.1

-6.9 -10.0 -6.7

23.3 23.7

21.1 21.3

-9.4 -10.1

Green volume (cm 3)

Weight percent gain (%)

C

C - B

C-A

Volume after modification (cm 3)

V (%)

V (%)

Propylene oxide

15.9 21.1 41.0

56.0 59.0 61.0

+5.3 + 10.0 +9.7

-1.7 0 +3.3

23.0 23.7

+8.3 + 10.1

-1.3 0

Acetic anhydride

3.9 22.8

modification on the moisture content of the modified cell wall, which translates to major effects on strength properties.

C.

Effect of Chemical Modification on Wood Volume

Table 5 shows the increases in cell wall volume resulting from reaction of southern pine with either propylene oxide or acetic anhydride. The oven-dry wood volume after modification with either of these chemicals at approximately 20 WPG is equal to the original wood green volume. This means that chemically modified wood has fewer fibers per centimeter than nonmodified wood. This means that if equal cross-sections of control and modified wood are used for mechanical tests there will be fewer fibers to test in the modified

Rowell

300

wood as compared to the control. Since the cross-section of equal-sized control specimen contains about 10% more fibers than the modified specimen, the mechanical properties of the control should be higher than that of modified wood. Because of differences in volume and fibers per cross-section, it is difficult to compare properties between control and modified wood and make conclusions unless the difference in properties are very large.

III.

CHANGES IN MECHANICAL PROPERTIES OF SOLID WOOD RESULTING FROM CHEMICAL MODIFICATION

There are always some changes in physical properties of wood when it is subjected to a chemical treatment. Vibrational properties change depending on the type of modification [7] and the moisture content of the modified cell wall [8] as do acoustical properties [9]. There is usually a color change associated with modification that may vary from almost no change to major, usually darkening, changes. The most dramatic mechanical property improvement in chemically modified wood over nonmodified wood is in wet strength and wet stiffness properties. Creep properties due to moisture sorption are also greatly reduced in chemically modified wood [10]. Most mechanical properties reported in the United States are done according to standards described by the American Society of Testing and Materials (ASTM). The standard method for testing small, clear specimens of timber (ASTM 0143) calls for the test to be conducted at 65% RH. Control specimens have a moisture content of approximately 9% (Table 3), whereas specimens reacted with acetic anhydride or formaldehyde have a moisture content of only about 4% and specimens reacted with butylene oxide 6%. This means that the standard test is run on specimens of different moisture contents. Because of the great effect of moisture content on mechanical properties (Table 1), it is misleading or even invalid, to compare strength properties of control and modified wood since they were tested at different moisture levels. Despite the problems associated with differences in fibers per cross-section and cell wall moisture content, the literature is full of comparisons of strength properties of control vs. modified wood. In many cases, the test procedure was not given or the RH used in the test was not reported, so that it is even harder to compare results from one author to another. The following is a summary of the literature on mechanical properties of chemically modified wood. However, as was stated earlier, it is hard to make comparisons or draw

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conclusions due to differences in testing procedures, levels of modification, relative humidities of the tests, and the use of various hard and softwoods.

A.

Changes Due to Esterification

Reacting wood with acetic anhydride to about a 15-20 WPG gives an increase in density from about 5% to 20% [11-13]. The color of very light woods (such as pine, maple, oak) usually darkens and the color of dark woods (such as walnut, cherry, teak) usually lightens due to acetylation. Penneability of gases decreases in acetylated wood as compared to control [14]. Shear strength parallel to the grain decreased from 12% to 24% [13]. In static bending tests, modulus of elasticity (MOE) varied from - 6% to + 2% [13,15], modulus of rupture (MaR) varied from - 8% to + 17 depending on the wood tested (13,16,17], fiber stress at proportional limit increased from +7% to 20% [13], and work to proportional limit increased from 25% to 42% [13]. Ball hardness increased from 22% to 31 % [13] and Brinell hardness increased tangentially 25% and radially 20% [17]. Impact strength varied from - 13% to + 16 [12,18]. Compressive strength perpendicular to the grain increased by 22-31 % [13], and compressive strength parallel to the grain increased by 10% [16] . Wet compression strength at proportional limit increased by 93-144% [12,18]. Toughness varied from -7% to + 17% [11,13]. Work to failure decreased by 5-12% and tensile strength by decreased 1-4% [4]. Elongation at break in tension varied from - 17% to + 42% [11]. All of these values are comparing acetylated wood to controls. Other anhydrides, such as propionic, butyric, and phthalic, have been reacted with wood [18,19] but no data have been published on mechanical properties of these modified woods. Isocyanates have also been reacted with wood to give high levels of dimensional stability, but no mechanical tests have been conducted [20].

B.

Changes Due to Crosslinking

Wood reacted with fonnaldehyde with an acid catalyst results in crosslinking between two hydroxyl groups in the cell wall polymers. The mechanical properties of fonnaldehyde-treated wood are all reduced from those of untreated controls. Toughness and abrasion resistance are greatly reduced [21-23], crushing strength and bending strength are reduced by about 20% and impact bending strength are reduced up to 50% [24] in specimens reacted to 4-7 WPG. The loss in toughness properties is directly proportional to the

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gain in dimensional stability as measured in antishrink efficiency (ASE). For example, a specimen with a 60% ASE is equal to a 60% loss in toughness [21 ]. A definite embrittlement is observed in fonnaldehyde-treated wood that may result from the short crosslinking unit (O----C-o) fonned between fonnaldehyde and fibrils in the cell wall. If the inner carbon unit were longer, there would be more flexibility in this unit and the embrittlement should be reduced. Part of the loss in strength properties of formaldehyde may come from hydrolysis of cellulose by the strong acid catalyst.

C.

Changes Due to Etherification

Wood has been reacted with propylene oxide and mechanical properties determined [25]. Maple specimens were reacted to 20-22 WPG and subjected to standard ASTM tests. The following is a summary of the results comparing propylene oxide-modified specimens to controls. MOE was decreased by 14%, MOR decreased by 17%, fiber stress at proportional limit reduced by 9%, maximum crushing strength decreased by 10%, radial hardness increased by 5%, tangential and longitudinal hardness remained unchanged, and the diffusion coefficient increased by 29%.

D.

Changes Resulting in Cellulose Decrystallization and/or Matrix Modification

Because cellulose is the major component in strength properties of wood [6,23], if this polymer is depolymerized due to severe reaction conditions, or decrystallized, major changes in the mechanical properties of wood will result. The cellulose matrix of hemicelluloses and lignin can also be modified resulting in plasticization, which also causes major changes in mechanical properties. In some cases these changes result from the use of a strong oxidizing acid catalyst and the loss of mechanical properties is not desired. In other cases, the matrix is modified or the cellulose decrystallized in an attempt to make films and thennoplastics out of whole wood. In this case, the lowering of mechanical properties is not considered a loss because the wood structure is partially or totally lost in the new product. Shiraishi and coworkers have had a 10-year program on wood molding at Kyoto University in Japan [26]. Their approach renders the entire wood structure thennoplastic through chemical modification of wood meal, which means that lignin and hemicelluloses are modified and the cellulose is decry-

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stallized and modified. Their work has emphasized esterification of wood using triftuoroacetic acid (TFAA). Thennoplasticity of esterified wood was found to depend on the acyl group, the method of preparation, and the degree of substitution. They found that as the size of the aliphatic group is increased the melting temperature of the modified wood at 3 kg/cm 2 is decreased. Matsuda and Veda [27] also extensively investigated the esterification of wood in order to make a totally thennoplastic material. They esterified wood with a solvent by simply heating wood meal with succinic anhydride for 3 h at temperatures greater than 60°C. The wood meal was readily molded at 180°C under a pressure of 570 kg/cm 2 for 10 min. The moldability of various esterified woods decreased in the following anhydride order: succinic anhydride > maleic anhydride> phthalic anhydride. Hon and Ou [28] also produced a thennomoldable product by benzylation of wood powder. The degree of substitution was varied by changing the reaction alkalinity, temperature, and time. Sodium hydroxide concentrations greater than 25% were necessary to obtain a high weight gain, presumably because of the need to swell the lignocellulosic substrate. Different species showed variation in reaction rates. The thennoplasticized woods exhibited good melting properties and were readily moldable into bulk materials or extruded into films and sheets. A wide range of glass transition temperatures, from 66°C to 280°C, was observed for the benzylated wood and was largely dependent on weight gain. The molded and extruded products exhibited acceptable mechanical strength for structural engineering applications. The lignin and hemicellulose matrix can also be thennoplasticized using various anhydrides such as maleic or succinic anhydrides [29]. If a nondecrystallizing reaction condition is used, it is possible to chemically modify the lignin and hemicellulose but not the cellulose. This selective reactivity has been shown to occur if uncatalyzed anhydrides are reacted with wood fiber [30]. This research is intended to produce wood veneers that are thennofonnable.

IV.

CHANGES IN MECHANICAL PROPERTIES OF WOOD COMPOSITES RESULTING FROM CHEMICAL MODIFICATION

It is very hard to compare data from chemically modified wood to data from composites made from chemically modified wood. There are so many variables in composite manufacturing alone that introducing a new chemical composition variable makes analysis even more complicated and tenuous.

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Size, geometry, and orientation of particles in the composite has a great influence on mechanical properties as does density, type and level of adhesive used, solubility of resin in carrier liquid, ability of resin to penetrate into wood, moisture content, defects, wood species used, and many other variables.

A.

Adhesion and Adhesives

The mechanism of adhesion is also an important factor in failure analysis in composites [31]. Some adhesi ves work due to a physical entanglement of the resin into the wood structure whereas others require a free hydroxyl group on one of the cell wall polymers to participate in a chemical reaction with the resin. Substitution of hydroxyl groups was shown to decrease adhesion between chemically modified veneers due to the loss of hydroxyl functionality [32]. Resins that are water-soluble and depend on a hydrophilic substrate for penetration will be less efficient in chemically modified wood due to the decreased hydrophilic nature of the cell wall resulting from modification [33]. Many different types of adhesives have been studied in the gluing of chemically modified wood, especially acetylated wood [34]. The adhesion of 18 thermoplastic and thermosetting adhesives was reduced by the level of acetylation, some to a minor degree and others to a severe degree. Many adhesives were capable of strong and durable bonds at a low level of acetylation (8 WPG) but not at higher levels (14-20 WPG). Most adhesives contained polar polymers, and all but four were aqueous systems, so that their adhesion was diminished in proportion to the presence of the nonpolar and hydrophobic acetate groups in acetylated wood. Thermosetting adhesives were capable of high shear strengths in the dry condition. With the exception of an acidcatalyzed phenol-formaldehyde adhesive, thermosetting adhesives that were hot-pressed became mobile and tended to overpenetrate the wood because of the limited capacity of the acetylated wood to sorb water from the curing bond line. The abundance of hydroxyl groups in a highly reactive resorcinol adhesive permitted excellent adhesion at room temperature despite the limited availability of hydroxyl groups in acetylated wood. An emulsion polymer-isocyanate adhesive, a crosslinked polyvinyl acetate adhesive, a resorcinol-formaldehyde adhesive, a phenol-resorcinol-formaldehyde adhesive, and an acid-catalyzed phenolic-formaldehyde adhesive developed bonds of high shear strength and wood failure at all levels of acetylation in the dry condition. A neoprene contact bond adhesive and a moisture-curing polyurethane hot-melt adhesive performed as well on acetylated wood as untreated wood in tests of dry strength. Only a cold-setting resordnol-formal-

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dehyde adhesive and a phenol-resorcinol-formaldehyde adhesive, along with a hot-setting acid-catalyzed phenolic adhesive, developed bonds of high strength and maximum wood failure at all levels of acetylation when tested in a water-saturated condition [34].

B.

Lumber Laminates

Vick and others [35] studied the dry and wet shear strength of acetylated spruce and pine lumber laminates. Using four different types of adhesives, they found that a resorcinol-fonnaldehyde adhesive on acetylated spruce showed a decreased (15%) dry shear strength but an increased (33%) wet shear strength as compared to controls. Similar results were found for an emulsion polymerisocyanate resin. A phenol-resorcinol-fonnaldehyde adhesive showed an increased (18%) dry shear strength and a greatly increased (52%) wet shear strength comparing acetylated laminates to controls. A polyvinyl acetate adhesive gave about the same increase in dry (20%) shear strength as in wet (28%) conditions comparing acetylated laminates to controls. Similar trends were found for pine laminates. Acetylation greatly reduced the swelling that occurs during the water soaking tests, so that there was essentially no stress fracturing in the acetylated wood. Nonacetylated wood developed many water-related stress fractures during the water soaking tests. A very high level of wood failure occurred in both acetylated and nonacetylated wood showing that the mechanism of failure was not due to the failure of the glue line.

c.

Veneer Composites

The first work to compare mechanical properties of chemically modified wood to a composite made from chemically modified veneers was done in 1950 by Tarkow and Stamm [11]. They made a nine-ply parallel-laminated composite of 16-mm rotary-cut Sitka spruce veneers that had been acetylated to 28 WPG and compared properties to a control laminate and control and acetylated single veneers. A hot-press phenolic resin was used as an adhesive in the laminated composite. While acetylated single veneers had a MOE, in tension parallel to the grain, of + 10% greater than control veneers, the acetylated laminate MOE was lower than controls by 21 %. Ultimate strength, in tension, of acetylated single veneers was 10% higher than controls but was 14% lower in the acetylated laminate than controls. These results show that mechanical properties were lowered due to the adhesion between veneers. In the control laminates, 97% of the failure of the glue line occurred in the wood whereas only 89% of the failure occurred in the wood for acetylated veneers.

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In general, other properties of the acetylated laminates showed, improved properties over control laminates. For example, acetylated laminates in compression tests parallel to grain showed a 3% increase in stress at proportional limit, 8% increase in ultimate strength, 6% decrease in MOE, and 21 % increase in deformation at failure as compared to controls. In compression tests perpendicular to the grain, acetylated laminates showed a 30% increase in stress at proportional limit, 47% increase in ultimate strength, 21 % increase in MOE, and 10% increase in deformation at failure as compared to controls.

D.

Flakeboards

In flakeboards made from acetylated aspen flakes, internal bond (- 36%), MaR (- 34%), and MOE (- 11 %) were decreased as compared to boards made from nonacetylated flakes [33,36]. The adhesive used was a watersoluble phenol-formaldehyde at a solids content of 6%. Electron micrographs showed that fragmentation, intercell, intrawall, and transwall separations increased as the level of acetylation increased. These defects were found only in the outermost layer of surface flakes and were not considered to influence the mechanism of failure of the glue line. In these studies, 75-90% of the failures during test occurred in the glue line of the acetylated wood and not in the wood, showing that adhesive failure was the main mechanism of failure [33]. In unacetylated flakeboards, more than 95% wood failure occurred. The reduced moisture content of the acetylated flake made them less compressible and required higher press pressures to make the flakeboards [36]. Using an isocyanate resin, at 3% solids content, on the same aspen flakes as described above, dry internal bond strength decreased by only 2%, MOR decreased by 23%, and MOE decreased by 15% on boards made from acetylated flakes as compared to boards made from nonacetylated flakes [37]. This is just one more example of how the type and level of the resin used influenced composite board mechanical properties.

E.

Flberboards

There have been several studies on the changes in mechanical properties of fiberboards made from chemically modified wood fiber. Hardboards made from control and acetylated hemlock fiber using 7% phenyl-formaldehyde adhesive were tested. In static bending, MOR was reduced by 23% and MOE reduced by 16% in acetylated boards as compared to control boards [38]. Tensile strength parallel to the surface was reduced by 5% but there was no

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change in the tensile strength perpendicular to the surface in acetylated boards compared to controls. Fiberboards made from control and acetylated aspen fiber were made using 8% phenol-formaldehyde resin and tested in static bending. MOR increased by 15% and MOE increased by 40% in acetylated fiberboards compared to controls [39]. The acetylated boards were reported to have a more uniform density and a more consolidated surface as compared to controls. Finally, changes in mechanical properties have been conducted on control and acetylated mixed wood (MW) fiber and recycled old newspaper (ONP) fiber. The boards were made using a phenol-formaldehyde resin at several different levels of resin solids. Comparing data generated in static bending tests on fiberboards made using 7% resin, MOR was reduced by 4% and MOE increased by 3% for boards made from acetylated ONP compared to controls, and MOR was reduced by 35% and MOE reduced by 21 % for boards made from acetylated MW compared to controls [40]. It is interesting to note that in the same tests MOR was reduced by 55% and MOE reduced by 63% comparing control WF to ONP fiberboards whereas MOR was reduced by only 12% and MOE by 34% comparing acetylated WF to ONP.

V.

CONCLUSIONS

If there are no major disruptions, depolymerizations, or decrystallization of cell wall polymers during the reaction chemistry to modify solid wood, there are no statistical differences in mechanical properties of chemically modified wood as compared to nonmodified wood. There are so many differences in moisture levels, specific gravities, and fibers per unit cross-section in control vs. modified woods that no definitive conclusions can be made. If the reaction chemistry used to modify solid wood does result in major disruptions, depolymerizations, or decrystallization of cell wall polymers, then there are major statistical differences between control and chemically modified solid wood. Losses in mechanical properties can vary from large decreases in all properties to complete loss of cell wall structure and wood is converted to a thermoplastic film. In both solid wood and wood composites, the major improvement in mechanical properties resulting from chemical modification is in wet strength and wet stiffness. There are so many variables to consider in wood composites that is it also difficult to draw conclusions on the effect of chemical modification on mechanical properties of composites. The size, geometry, and orientation of

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flakes, particles, fibers, and so forth in the composite has a great influence on mechanical properties as do variations in specific gravity throughout the boards, stratification of fines in boards, type and level of adhesive used, ability of the resin system to penetrate into wood, moisture content during testing, defects/gaps arising in the board forming process, hydrophilic/hydrophobic nature of the cell wall, availability of reactive hydroxyl groups in the modified accessible cell wall polymers, and wood species used. As was concluded for solid wood, there does not seem to be any statistical difference in mechanical properties between wood composites made from chemically modified and nonmodified furnish except in wet strength properties.

REFERENCES I. 2.

3. 4. 5. 6.

7. 8. 9. 10. II. 12. 13. 14.

R. M. Rowell and P. Konkol, USDA, Forest Service, Forest Products Laboratory Gen. Technical Report FPL-GTR-55, Madison, WI, 12 pp (1987). R. M. Rowell, in The Chemistry of Solid Wood (R. M. Rowell, ed.), American Chemical Society Advances in Chemistry Series No. 207, Washington, D.C., 1984, pp. 135-210. R. M. Rowell, Proceedings of the Composite Products Symposium, Rotorua, New Zealand, November 1988, FRI Bull. 153:57-67 (1990). R. M. Rowell and W. B. Banks, USDA, Forest Service General Technical Report FPL 50, Forest Products Laboratory, Madison, WI, 1985. U.S. Department of Agriculture, Forest Service, Wood Handbook, Agric. Handbook 72, rev.; Washington, D.C., 1987. J. E. Winandy and R. M. Rowell, in Chemistry of Solid Wood (R. M. Rowell, ed.), American Chemical Society Advances in Chemistry Series No. 207. Wac;hington, D.C., pp. 211-255. M. Norimoto, J. Grill, and R. M. Rowell, Wood and Fiber Sci. 24(1):25-35 (1992). H. Akitsu, M. Norimoto, T. Morooka, and R. M. Rowell, Wood Fiber Sci. 25(3) 250-260 (1993). H. Yano, M. Norimoto, and R. M. Rowell, Wood Fiber Sci. 25(4) 395-403 (1993). M. Norimoto, J. Grill, K. Minato, K. Okamura, J. Mukudai, and R. M. Rowell, Wood Industry (Japan) 42(18):14-18 (1987). H. Tarkow and A. J. Stamm, USDA Forest Service, Forest Products Lab Report No. 1593, 1950. Koppers Company, New Materials Res. Paper E-I06, Pittsburgh, 1961. W. A. Dreher, I. S. Goldstein, and G. R. Cramer, Forest Prod. J. 14(2):66-68 (1964). S. Kumar, S. P. Singh, and M. Sharma, J. Timber Dev. Assoc. India 25(3):5-9 (1979).

Physical and Mechanical Properties 15. 16. 17.

18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

30. 31.

32. 33. 34. 35. 36. 37. 38.

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D. Narayanamurti and B. K. Handa, Papier 7:87-92 (1953). H. Militz, Int. Res. Group on Wood Preservation Proceedings, Document No. WP 3645, 22nd Annual Meeting, Kyoto, Japan, 1991. P. Larsson, Licentiate thesis, Department of Forest Products and Chemical Engineering, Chalmers University of Technology, Gothenburg, Sweden, 1993, pp. I-II. I. S. Goldstein, E. B. Jeroski, A. E. Lund, J. F. Nielson, and J. M. Weater, Forest Prod. J. lJ(8):363-370 (1961). J. Risi and D. F. Arseneau, Forest Prod. J. 8(9):252-255 (1958). R. Rowell, Commonwealth Forestry Bureau, Oxford, England, Vol. 6(12) 363-382 (1983). H. Tarkow and A. J. Stamm, J. Forest Prod. Res. Soc. 3:33-37 (1953). A. J. Stamm, Tappi 42:39-44 (1959). A. J. Stamm, Wood and Cellulose Science, Ronald Press, New York, 1964. A. Bunnester, Holzforschung 21(1):13-20 (1967). R. M. Rowell and W. D. Ellis, USDA Forest Service Research Paper FPL 451, Forest Products Laboratory, Madison, WI, 1984. N. Shiraishi. Wood and Cellulose Chemistry (D. N. -So Hon, and N. Shiraishi, eds.), Marcel Dekker, New York, pp 861-906, 1991. H. Matsuda and M. Ueda, Mokuzai Gakkaishi 31(3):215 (1985). D. N.-S. Hon and N.-H. Ou, J. App. Polym. Sci. A Polym. Chem. 27:2457 (1989). R. M. Rowell and C. M. Clemons, Proceedings of the International Particleboard/Composite Materials Symposium (T. M. Maloney, ed.), Pullman, WA, 1992, pp. 251-258. R. M. Rowell, R. Simonson, S. Hess, D. V. Plackett, D. Cronshaw, and E. Dunningham, Wood Fiber Sci. 26(1):11-18 (1994). R. V. Subramanian, in Chemistry ofSolid Wood (R. M. Rowell, ed.), American Chemical Society Advances in Chemistry Series No. 207, Washington, D.C., 1984, pp. 323-348. A. W. Rudkin, Aust J. App. Sci. I: 270-283 (1950). R. M. Rowell, J. A. Youngquist, and I. B. Sachs, Int. J. Adhesion Adhesives 7(4): 183-188 (1987). C. B. Vick and R. M. Rowell, Int. J. Adhesion Adhesives 10(4):263-272 (1990). C. B. Vick, P. Ch. Larsson, R. L. Mahlberg, R. Simonson, and R. M. Rowell, Int. J. Adhesion Adhesives 13(3):139-149 (1993). J. A. Youngquist, R. M. Rowell, and A. Krzysik, Holz als Roh- Werkstoff 44( 12):453-457 (1986). J. A. Youngquist and R. M. Rowell, Int. J. Adhesion Adhesives 10(4):273-276 (1990). J. A. Youngquist, R. M. Rowell, N. Ross, A. M. Krzysik, and P. Chow,

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Rowen Proceedings: 1990 Joint International Forest Products Conference, Taipei, Republic of China, 1990, pp. 159-162. R. M. Rowell, J. A. Youngquist, J. S. Rowell, and J. A. Hyatt, Wood Fiber Sci. 23(4):558-566 (1991). A. M. Krzysik, J. A. Youngquist, R. M. Rowell, J. H. Muehl, and P. Chow, in Materials Interactions Relevant to Recycling ofWood-Based Materials (R. M. Rowell, T. L. Laufenberg, and J. K. Rowell, eds.), Materials Research Society, Pittsburg, 1992, pp. 73-79.

13 Viscoelastic Properties of Chemically Modified Wood Misato Norimoto Kyoto University Uji, Kyoto, Japan

I. INTRODUCTION Wood has many excellent mechanical properties that result from its exceptional combination of microstructural, ultrastructural, and molecular features. The main drawback is its dimensional instability in the presence of moisture. One main reason for modifying wood chemically is to reduce this instability. Other chemical treatments of wood reach the crystalline region of cellulosic microfibrils, destroying the crystalline structure, thus eliminating most of the composite structure of wood. The resulting material does not have any of the characteristic properties of wood but it may be provided with thermoplasticity. A chemical treatment of wood as defined here excludes such radical modifications. We refer to a chemical treatment that may reduce some defects relative to wood utilization and enhance its properties while keeping the bulk of the superior mechanical properties of wood. In many situations where wood is used as a structural member it undergoes other types of dimensional changes such as abnormal creep deformations when subjected to humidity changes. The viscoelastic properties such as the dynamic specific modulus and loss tangent may also be significantly dependent on humidity. This is one of the main factors causing tonal instability in wooden musical instruments. Any type of instability of wood caused by humidity changes originates in its hygroscopicity. In order to characterize the ability of a chemical treatment to stabilize viscoelastic properties of wood, it

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is important to have a clear understanding of the relationship between the structural modifications and changes in properties. In this chapter, a classification of chemically modified woods is proposed and their viscoelastic properties including creep and dynamic mechanical properties along the grain are presented and analyzed in relation to the type of chemical modification involved.

II.

CLASSIFICATION OF CHEMICALLY MODIFIED WOODS

A.

Model of Chemical Modification of Wood

We classify chemical modifications of wood with respect to their actions at both the cellular and molecular levels. Six types of modification of a single cell are represented in Fig. IA [1,2]. The cell wall remains untreated in AI to A-3 and the cell wall is modified in A-4 to A-6. A resin or any other product is deposited on the internal faces of the lumen in A-2 and A-5, whereas the lumen in A-3 and A-6 is filled partially or totally. In Fig. I B, nine patterns, including the untreated one (B-1), of the possible modifications of the cell wall materials at the molecular level are shown. The chain (a) in B-1 refers to the crystalline core of a cellulose microfibril or a part of the lignocellulosic material of the cell wall that is not affected by water. It has a water-reactive zone at its boundary, illustrated by a hydroxyl group (b) represented by the open circle, which is available for hydrogen bonding with water. Neighboring chains are linked to each other by a set of water-reactive zones, shown here by the hydrogen bond (c). Water sorption on (c) has a double effect. First, the change of matrix volume forces lateral displacement of the two chains (a) in the direction indicated by (d). Second, it weakens the connection between the chains and facilitates slippage in the direction indicated by (e) resulting from local shear stresses. At the macroscopic level, the first effect results in swelling or shrinkage and the second in creep or stress relaxation. The small closed circles indicate that the hydroxyl group has been substituted with chemical bonding and is not available any more. The large closed circles indicate the bulking effect caused by the introduction of large molecules between the constituents, which replace water molecules. Hydrogen bonding is represented by two hydroxyl groups facing each other and strong molecular bonding is shown by a line. The crosslinking effect is thus pictured by an unbroken sequence of lines and closed circles, either small or large, linking

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3

4

(8)

5

6

e

f

a

d-~:1-d ~ ~ 1 a

H

~~1

Figure 1 Model for the chemical treatment of wood. (A) Cellular level. (1 )-(3) Untreated ceIl waIl, (4)-(6) treated ceIl wall; (I) untreated; (4) no chemical deposits in lumen; (2) and (5) deposits on cell wall surface; (3) and (6) filling of lumen. (B) Modification of lignoceIlulosic material at molecular level. (c) Hydroxyl group available for hydrogen bonding; (e) substitution of hydroxyl group; (e) bulking agent.

the two chains. In patterns B-2 and B-3, crosslinking occurs without the bulking effect by using molecules of low molecular weight that link the two reaction sites with hydroxyl groups. In pattern B-2, the reaction is done in a dry state with a short linkage, preventing both lateral expansion (d) and molecular movement (e). In pattern B-3, the reaction is done in a swollen

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state so that after drying the structure develops considerable looseness. Both a crosslinking and a bulking effect occur in patterns B-4 and B-5. In pattern B-4 the reactant is hydrophobic, and in pattern B-5 it is hydrophilic as shown by the small open circles attached to the big closed circle. In the remaining four patterns there is a bulking effect but no crosslinking. In patterns B-6 and B-7, the reactant establishes a stable bond on one side. It is hydrophobic in pattern B-6 and hydrophilic in pattern B-7. In pattern B-7 the reactant suppresses hydrogen bonding on one side but simultaneously creates a new sorption site at the other. In the last two patterns no stable bond is established between the reactant and the constituents. In pattern B-8, the bulking agent is extremely hydrophilic and establishes extensive hydrogen bonding with the constituents, and in pattern B-9 it is hydrophobic and does not interact with the constituents or with water. Any type of chemical modification can be characterized by a combination of the two criteria A and B of Fig. 1.

B.

Chemical Modification Treatments

Specimens of Sitka spruce (Picea sirchensis) and Glehn spruce (Picea glehnii) were modified by eight kinds of chemical treatment. Weight percent gain (WPG) resulting from each chemical treatment was detennined based on the over-dry weight of the specimen before and after treatment. Fonnalization is a reaction involving the formation of oxymethylene bridges between the hydroxyl groups of the chemical constituents of wood by fonnaldehyde. The reaction in vapor phase (F s) was done in a closed vessel with 0.0215 moUL fonnaldehyde and 0.004 mollL sulfur dioxide as a catalyst for 24 h at 120°C. The reaction in liquid phase (F w) was done by immersing the wood in a water solution containing 3.6% fonnaldehyde, 3.7% HCI, and 75% acetic acid for 6 days at 24°C. In the vapor phase formalization, a small molecular bridge is made while the chains are close to each other; thus the structure of the resulting products is pattern (A-4) + (B-2). On the other hand, in the liquid phase fonnalization the reaction is made in a swollen state, so that after drying the structure develops considerable potential looseness; thus the pattern (A-4) + (B-3). Acetylation of wood (A) was done in neat acetic anhydride at 120°C for 10 h, followed by leaching 10 h in boiling water and oven-drying overnight. Acetylation leads to the structure of pattern (A-4) + (B-6), hydrophilic hydroxyl groups being substituted with hydrophobic and bulky acetyl groups. Etherification of wood (PO) was done using propylene oxide and 5%

Viscoelastic Properties of Wood

315

triethylamine as a catalyst in an autoclave at 120°C for 0.5-6 h. To avoid loss of the reactant during the process, the specimens were not leached. The addition of epoxides provides wood with the structure of pattern (A-4) + (B-7). Treatment of wood with maleic acid and glycerol (MG) was done by impregnating the specimen with a 20% aqueous solution of maleic acid and glycerol in a ratio of 2: I (v/v), reacting at 180°C for 3 h, leaching for 10 h in boiling water, and oven-drying overnight. The resulting structure belongs to patterns (A-5) + (B-5) and (B-7). Impregnation with phenol-formaldehyde resin, with high molecular weight (PFH) or low molecular weight (PFL), was done by soaking the specimens in a 20% aqueous solution until complete saturation of resin was achieved. The resin-impregnated specimens were cured for 30 min at 130°C. When an aqueous solution of phenol-formaldehyde resin with a low molecular weight of around 300 penetrates the cell wall, patterns (A-5) + (B-9), (B-6) and (8-4) may be expected. However, that with a high molecular weight cannot penetrate the cell wall, so that the structure belongs to patterns (A-2) and (A-3) + (B-1). Impregnation with polyethylene glycol (PEG-I 0 4 , PEG-I 0 3 , PEG-600) was done by saturating the wood specimens first in water and then in a 25% aqueous solution of polyethylene glycol for 12 h at room temperature, followed by oven-drying overnight. The molecular weight should be lower than 1000 to allow penetration into the cell wall and result in pattern (A-5) + (8-8). For a molecular weight of about 10,000, there is no penetration at all and pattern (A-2) and (A-3) + (B-1) results. Wood-plastic composite specimens (WPC) were prepared by impregnating the oven-dry specimens with a solution of methyl methacrylate containing 1% a,a' -azobisisobutyronitrile as a catalyst, wrapping the specimens in aluminum foil, and curing the polymer at 80°C for 3 h. The structure of pattern (A-5) and (A-6) + (B-6) and (8-9) can be obtained under experimental conditions, but pattern (A-2) and (A-3) + (B-1) can be expected in industrial practice. Wood-inorganic material composite specimens (WIC) were prepared by immersing the wood specimens in an aqueous solution containing 32.0% BaCI 2 and 7.2% boric acid at 50°C for 24 h, followed by a second immersion in an aqueous solution of 37.4% (NH 4 hHP04 and 16.6% boric acid for 8 h at 50°C. All specimens were boiled in water for 12 h and then oven-dried overnight. This modification provides a material with fireproof qualities that can be used for fire prevention. This modification yields the pattern (A-5) + (B-8) because the molecule involved is a salt that is extremely water-reactive.

316

Norimoto

IU.

CREEP OF CHEMICALLY MODIFIED WOODS

A.

Creep During Humidity Changes

Creep of chemically modified woods along the grain was measured in threepoint bending tests at 30°C using humidity cycles between 29% RH and 86%RH. The initial load was adjusted to specimen dimensions after chemical modification to obtain a stress level of 10 MPa in the dry state. The following procedure was imposed on all specimens: loaded at 29% RH, kept 10 min, moved to 86% RH for 24 h, returned to 29% RH for 24 h, unloaded at 29% RH, kept 10 min, moved to 89% RH for 24 h, and, finally, returned to 29% RH for 24 h. A small fraction of the initial load remained on the specimen after unloading. To characterize the ability of the modification to reduce creep, the following anticreep efficiency factor ACE was defined [1]. ACE

=

dJ u - dJ x 100 dJ u

where w is the width of the specimen in the dry state, t is the thickness of the specimen in the dry state, l is the span, F is the applied load, II is the deflection after loading, 12 is the deflection at the end of the experiment, dJ is the increase of compliance induced by a complete cycle of humidity, and dJ u is the value obtained from an untreated specimen (U). A specimen with 100% ACE would have no creep at all and negative ACE indicates instability. On the other hand, the dimensional stability provided by treatment is evaluated by use of an antiswelling efficiency factor (AS E) given by the following expression: ASE

= Su- S x

100

Su where Vd is the dry volume, V w is the wet volume, S is the volumetric swelling of chemically modified specimen between a dry state and a wet state, and Su is that for untreated wood.

B.

Antlcreep Efficiency Factor and Antiswelllng Efficiency Factor

Figure 2 shows the relative deflection plotted against time for each chemical modification [1]. Solid and dotted lines show the modified and control data (U), respectively. The initial 10 min period of creep at 29% RH is hardly visible on the graph. The change in deflection would be minimal, even after

Viscoelastic Properties of Wood 2

317

r ,----,

~

:~~ ° 2

,

,----,

,,_-_.1

g

3

3

2

:

2

1

~ --- __U A 't0L.------''----L---'-----L.. 2 3 ,

°

C

.~ '0

:

3

~

2

a:: 2

FtG#

'~--------U oL----lL.----L---'----'-

°

3

2

,

,---

... --~

,--'---------U I

'---....L_--'-_-'-_-----.L.

, °°

oL----lL.---'----'---L-

2

°

2

3

2

3

4

2

'Me I

_____ ---- U M3

°°

2

~·--------U oL.-----l~____L

°

3' Time (day)

_

2

__'__

3

_J._

4

Figure 2 Creep-recovery tests of chemically treated woods. U, untreated wood; Fs, vapor phase formalization; F w, liquid phase formalization; A, acetylation; PO, etherification with propylene oxide; MG, treatment with maleic acid and glycerol; PFL , impregnation with low molecular weight phenol-formaldehyde resin; PEG-I
a long time, if the specimen were kept at 29% RH. Increasing the humidity induces a marked increase in deflection, and the following reduction in humidity induces another increase in deflection. After the specimen is unloaded at 29% RH, only a slight recovery will occur a long period as long as the specimen remains at this RH level. On the other hand, a high RH level induces

318

Norimoto

a marked recovery even though the recovery is slightly masked by the final equilibrium at 29% RH. (Fs), WPG: 2.1 %, yielded a high ASE for a low WPG. It was also the most efficient treatment regarding ACE. (Fw), WPG: 0.5%, is usually perfonned on paper so that cohesion is retained even in water. Such a treatment is of no practical interest in the case of wood, which has a natural cohesion in the green state. As a matter of fact, the treatment leads to only small improvement in ASE and ACE according to all criteria. The comparison between (A) and (PO) is of special interest. These treatments have high ASE. However, the most striking difference between these treatments is the opposite effect on creep. (A), WPG: 23%, yielded positive ACE values, but (PO), WPG: 26%, yielded extremely negative values. This means that epoxide treatments stabilize stress-free wood but destabilize loaded wood. Although the reactant saturates hydroxyl groups, it produces new ones during the reaction. Furthennore, as a bulking agent, the reactant increases the accessibility of water molecules to the water-reactive regions. This results in more water sorption and more creep. Both ASE and ACE have increased for (MG), WPG: 21 %. Heat treatment for 3 h at 180a C was compared to (MG), which involves similar conditions. Heat treatment alone yielded only small ASE and ACE values. The improvement with (MG) must be attributed mostly to the effects of bulking and crosslinking. (PEG-l0 3), WPG: 110%, yielded results similar to those resulting from (PO) and resulted in an excellent ASE value, but the ACE value was negative. (WIC), WPG: 105%, has similar drawbacks to those of (PEG-IcY), although the drawbacks are not as pronounced. Moreover, the treatment did not yield a high ASE value. The hydrophobic nature of the bulking agent of (PFd, WPG: 13%, resulted in a good ACE value for a modest ASE value. For (WPC), WPG: 105%, the low ASE value shows that only a small proportion of the methyl methacrylate penetrated into the cell wall. Therefore, this treatment is closer to the pattern (A-2) + (B-1) than to the pattern (A6) + (8-9). Neither ASE nor ACE can be affected unless the cell wall is modified at the molecular level. The ACE and ASE values obtained by the various modifications are compared in Fig. 3. The effect of chemical modifications on creep of wood has been characterized by ACE, analogous to ASE that quantifies the ability of the treatment to reduce load-free moisture expansion. Modification of the cell wall at the molecular level is required to affect either moisture expansion or creep. Depending on the type of treatment, this can be obtained through

Viscoelastic Properties of Wood

319

100

PFl

• MG•

o t.Fw --:lI;;----+------j WPC

50

100

ASE (0/0)

w

U


-100

PO -200

•

FtG.rj

•

Figure 3 Anticreep efficiency factor (ACE) compared to antiswelling efficiency factor (ASE). See legend to Fig. 2 for treatment abbreviations.

molecular crosslinking, bulking, or a combination of these. In the case of treatments that induce crosslinking, a good correlation can be expected between ASE and ACE. The bulking effect alone may induce high ASE values but negative ACE values. This occurs when the bulking agent is hydrophilic.

IV.

DYNAMIC MECHANICAL PROPERTIES OF CHEMICALLY MODIFIED WOODS

A.

Dynamic Specific Modulus and Loss Tangent

Dynamic specific modulus £'/"1, the ratio of dynamic Young's modulus to specific gravity, and loss tangent tan 0 can be used to study the viscoelastic nature of wood. £'/"1 is related to sound velocity and tan 0 to sound absorption or damping within the wood. A large £'/"1 and small tan 0 characterize the acoustic quality of soundboard wood [3]. £' /"1 and tan 0 were measured using a free-free beam method. The specimens were suspended horizontally with an iron piece at both ends by two fine threads at the nodal points. They were excited by an electromagnetic driver at one end, while the vibration was detected by an electromagnetic

320

Norimoto

transducer at the other end. E' /-y was calculated from the resonant frequency and tan 0 from the logarithmic decrease of the signal following a period of forced vibrations at the resonant frequency. There is a good linear correlation between logarithms of tan 0 and E' /-y for untreated wood [4]. This relationship was found regardless of wood species [5,6], measuring direction [7], or moisture content [8]. The regression line obtained for untreated wood using all of the values measured before the treatments at 20°C and 60% RH was log tan 0 = - 0.66 log E' /-y - 1.21. Some comment should be made concerning this regression line. In the longitudinal direction, E' /-y characterizes the average rigidity of the cell wall [9], whereas tan 0 represents the relative amount of viscous strain to elastic strain and the participation of the matrix in the deformation process. Both quantities depend mainly on the mean microfibrillar angle of the cell wall [4] and very little on the wood density. As the variations of both E' /-y and tan 0 originate from the same ultrastructural factor, they should not be independent of each other. In practice, it has been observed for a number of untreated specimens from different wood species that the higher the E' /-y, the smaller the tan 0 [5,6]. When different viscous behavior is apparent between a treated specimen and the untreated reference, we do not know a priori whether the difference is due to the treatment or originates from bad matching of the two specimens. However, we did confirm that it is not the latter case by observing that the point representing the modified wood data not only differs from that of untreated wood but also lies far from the dotted line. Figures 4 and 5 show the results of the individual measurements for the treatments [10, II]. The tan 0 vs. the E' /-y (in GPa) is drawn on a log-log scale. The regression line for untreated wood is drawn with the dotted line on each graph. In the case of (A), the range of initial values was especially wide, indicating a large range of mean microfibrillar angles. Although the initial plots are scattered on either side of the correlation line, the treatment resulted in a systematic decrease of log E' /-y and log tan O. In the case of (WPC), the initial plots were grouped on the graph and the final plots were grouped ali well, indicating few variations of the treatment effect. This holds also for the other treatments. In Fig. 6, all of the experimental results were grouped on a single graph using appropriate average values for each treatment.

B.

Uniaxial Viscoelastic Modeling

The model used for explaining the actions of chemical treatments along the grain is shown in Fig. 7 [11,12]. Wood is supposed to be equivalent to a

Viscoelastic Properties of Wood

321

-1.8

-1.6

-2.0

.... ....

-0

c -2.2

E

~, ....

-0

........

A.t

8'

O.J

-1.8

QJ

2- 2.0

.... ....

A. .... ....

.... ,

0"1

~-2.2

- -2.4 -2·6

0

'fi\

PO

-2.6

U3

1.6

\4

1.2

1.2

log E'/y

1.6

- t6 A.

....

4A. A.

-1.8

O.J

-1.8

a- 2.O

1.4

log E'/y

-1.6

O.J

-0

-8' 'it.,k° '6

c-20

l:1:1-

--2.2

-2.4

:1-

l:1 "

-2·4

Fs

-2·8

-0

~A. ' .... ~...

A. A.

£ ~-2.2

....

-2.4

A k lO

-2·6 1.2

1.4

log E'/y

1.6

FEG-600

k

-2.6 1.2

14

lO

1.6

log E'/y

Figure 4 Effects of chemical treatments on relationships between logarithm of E' / 'Y and logarithm of tan B. Dotted lines represent experimental correlations for untreated specimens. Various values of relative increase of matrix rigidity (k) and mobility factor (J.1.) were simulated. (6,.) Experimental values of untreated and treated specimens. (0, e) Theoretical plots for zero mean microfibrillar angle and theoretical plots assuming swelling effect only. See legend to Fig. 2 for treatment abbreviations.

parallel arrangement of three components, cellulosic fibrils (t), amorphous matrix (m), and lumens containing air only (a). In the case of (WPC), a fourth component, the polymer (p), has replaced a part of the air. The four components occupy the volumes V f , V m' Vp , and Va' The model is based on a simplified picture of both the cellular and macromolecular structures with elastic microfibrils disposed parallel to the fiber axis and embedded in a

Norimoto

322 -1.8

-2·0

~

" ,,

(a)

~ -2.2

-E:

"...A

(b)

a

(c)

-2-' Ep tmllp 4.5 0.00 (b) 0.1 I (a)

-2·6

(c) 0.1

0.001

(d) 10 (e) 10

0.001

I

(f)I34 0.001

-2-8 1.2

1.4

1-6

1.8

log E'/y Figure 5 Effects of wood-polymer composite treatment on relationship between logarithm of E' /-y and logarithm of tan O. (.6, A) Experimental values of untreated and treated specimens. Dotted line represents experimental correlation line for untreated specimens. Various values of polymer rigidity (Ep) and loss tangent (tan op) were simulated (curves a to f).

viscoelastic matrix, and the cell walls are organized as a perfectly bidimensional honeycomb. The microfibrils are presented by a spring of rigidity E 1 and the viscoelastic matrix by a Maxwell model made of a spring of rigidity E 2 and a dashpot of viscosity T2E2' The polymer is represented by another Maxwell element with rigidity E 3 and viscosity T3E3' In the case of a sinusoidal loading of pulsation w, assuming that the frequency is big enough to ensure (WT2)2 > > I and (WT3)2 > > I, so that the glassy transition is exceeded, the E' /"'1 and tan 8 can be expressed by the following equations: E' "'I

= E1 +

E2 "'I

+

E 3 = VrEr Vr"'lr

tan 8 = E l l + E2

+

E3

+ VnEm + + Vm"'lm +

(~ + .E.L) WT2 WT3

VpEp Vp"'lp

Viscoelastic Properties of Wood

323

-1.6

F£G-«:o -1.8

0()

c

-2.()

£ 01

o

-2.2

-2.~

-2.6

1.2

1-4

16

log E'(y Figure 6 Effects of chemical treatments on relationships between logarithm of log E'/-y and logarithm of log tan B. DOlled line represents experimental correlation for untreated specimens. Arrows represent average experimental values of Figs. 4 and 5. See legend to Fig. 2 for treatment abbreviations.

Figure 7 Rheologic model of chemical modifications. f, Cellulosic fibril; m, amorphous matrix; a, air; p, polymer; E I • E2 • E3 • elastic moduli; T" T2' T), characteristic times.

324

Norimoto I

VrEr + VnPm + VpEp

+

"Y = Vr"Yr

(VnPm

V m"Y m V

WT m

+

VpEp)

+

WT p

Vp"yp

where E r, Em' and E p are Young's modulus of microfibrils, matrix, and polymer, respectively, and T m' and T p the characteristic times of the viscous processes involved in dynamic experiments for the matrix and the polymer, respectively, and "Yr, "Ym' and "Yp specific gravity of microfibrils, matrix, and polymer, respectively. V = Vr + V m + Vp + Va is the total volume. Let's consider the case of a wood with an air-dried density "Y = 0.45, corresponding to a cell wall density of 1.45, to a void volume fraction of 0.69. In Table I, the physical quantities used for estimating the values of the parameters used in the model are shown. The volumes indicated are normalized for untreated wood, so that Vu = Vr + VUm + VUa = I. E r = 134 GPa and EUm = 2 GPa are reasonable estimates of the cellulose and matrix Young's modulus, respectively. The calculated value of E'/"y was 46.9 GPa, which was a value compatible with measurements at various mean microfibrillar angles, prolonged to 0° [4J. The value of TUm was adjusted to yield the estimate of tan & = 0.0046 deduced from measurements [4J. These two values correspond to a point shown by an open circle placed at an extreme position down and right on the regression line in the log E' /"y - log tan 8 graph. For simplicity, we assume that no filling of the cell wall is provoked by the treatments, so that Vp = 0, as well as no change of lumen sizes and specific gravities or Young's moduli of microfibrils. Only the matrix volume, specific gravity, and mechanical properties are modified by the treatment.

Vm = VUm + ~Vm' "Ym

=

_ Vr"Yr "Y -

"YUm

+

+ ~"Ym' Em = kEum , "Y = (VUm

I

+ ~Vm)("Y\,;m + + ~Vm

ST Um '

V

= I +

~Vm'

~"Ym)

The parameter values used in the simulations are shown in Table I. Typical values of V and "Y measured after treatment were used to estimate ~ V m and ~ "Ym' respectively. As a (PEG-600) treatment involves a partial filling of the

325

Viscoelastic Properties of Wood Table 1

Parameters Used in Simulation

(A) Untreated Woods

Microfibrils (f) Matrix (m) Air (a)

Volume fraction

Specific gravity

Components

'Yr 'YUm

= =

1.55 1.35

0

Vr VUm VUa

= 0.155 = 0.155 = 0.690

Elastic modulus (GPa)

Er E um

= =

134 2 0

Characteristic time (s)

TUm

=

2.035

X

10- 3

(B) Treatment Actions Treatment

'Y ~Vm

~'Ym ~

k

U

F

A

PO

PEG-600

WPC

0.45 0 0 0 1

0.45 0 0 -0.3 0

0.49 0.1 -0.18

0.49 0.1 -0.18

0.59 0.12 0.18 0.3 10

0.45-1.28 0 0 0 0

0 1

cell wall, a nonzero value of Vp should have been used, but for the sake of simplicity the calculation of A'Ym was done as if all of the mass increase was due only to the penetration of polyethylene glycol molecules into the matrix, which explains the abnormal increase of A'Ym in Table 1. This assumption is equivalent to neglecting the contribution of the free polyethylene glycol molecules to the rigidity. The results of the simulations are shown on the right side of the graphs in Fig. 4. The closed circle corresponds to the theoretical case where the matrix mechanical properties have remained unchanged (k = s = 1). Therefore, the shift from the open circle to the closed circle is due to matrix swelling only. For the given A V m and A'Ym' a univocal relationship between E' /'Y and k is obtained. On the other hand, tan 5 depends on both k and s. For convenience, the nondimensional quantity j..L = log (kls) will be named here the matrix mobility factor. For each treatment except (WPC), k was varied between approximately 0 to 10 for several fixed values of j..L from - 0.3 to 0.3. Although the experimental data shown correspond to nonzero mean microfibrillar angle, the tendency remains valid when extrapolating to zero mean microfibrillar angle, as far as the shift in the log-log plane is concerned. Confrontation of the experimental shift with the theoretical one may yield indications on the appropriate k and j..L values for each treatment. In (Fs )' WPG: 3.9%, the circles and squares are confounded because

326

Norimoto

zero swelling of the matrix was assumed. The simulation shows that the experimental shift to a slightly greater E' /-y and much smaller tan 8 implies a greater matrix modulus (k > I) and smaller mobility factor (~ < 0), which is what the crosslinking would be expected to produce. (PO), WPG: 22%, is similar to (A), WPG: 20%, in tenns of conventional stabilization but tends to increase the molecular mobility instead of reducing it because of the hydrophilic nature of the bulking agent. As the two treatments induced the same amount of volume and weight increases, the position of the filled circle as well as the results of the simulations are the same in both cases. While in (PO) the shift from the open circle to the closed circle corresponds to the experimental shift, indicating a more or less unmodified matrix mobility (J.L = 0), in (A) the observed drop of tan 8 implies a strong reduction of matrix mobility (~ < 0). Contrary to (Fs), (A) does not markedly increase matrix rigidity. (PEG-600), WPG: 8.9%, is similar to (PO) in tenns of chemical modification of the cell wall, except that the hydrophilic nature of the molecule is even more pronounced. Although the experimental shift in (PEG-600) was considerably more than that in (PO), the simulation shows that most of the difference is explained by the greater volume increase, with probably, in addition to this an increase of matrix mobility (~ > 0). These simulations evidence the importance of ~ as characterizing the molecular mobility. It should be noted that kJs is proportional to the square of matrix Young's modulus divided by viscosity, so that an alternative definition could have been ~ = log k2/r, where r = TJdTJum is the relative increase of matrix viscosity. On the other hand, in the treatments involving no cell wall modifications, the modification of mechanical properties is due only to the nonzero value of Vp and E p • For the simulation, rheological properties of polymethyl methacrylate(Ep = 4.5GPa,tan8p = l/wTp = 0.065atwl21T = 250 Hz [13]) were used. Line (a) in Fig. 6 shows the effect of increasing Vp values. The closed circle on line (a) corresponds to the average experimental Vp value (= 0.5 for a WPG = 126 % and -Yp = 1.2). This theoretical prediction shows good agreement with the experimental shift shown on the same graph. Various theoretical cases of polymer properties were considered in Fig. 6. The values of E p or Tp used in the simulation are not necessarily those of existing polymers. They were tried in order to illustrate the limits of properties modified by treatments involving no modifications of the cell walls. The case (0, for instance, equivalent to filling the lumens with pure cellulose, is the only one inducing an increase of E' /-y and a decrease of tan 8 at the same time.

Viscoelastic Properties of Wood

C.

327

Stabilizing Action of Chemical Modifications

Figure 8 shows the relationship between average values of log E' /-y and log tan & for chemically modified woods at four RH levels, together with the corresponding reference lines [14]. In the case of (Fs)' WPG: 3.9%, crosslinking occurred in an unswollen state of the cell wall and prevented swelling. This decreased the mobility of the molecular chains, thus reducing tan & at an humidity levels. In the case of (Fw), WPG: - 1.2%, crosslinking occurred in a swollen state. The tan &value at 0% RH was lower than that for untreated wood, though the value was much larger than those for (Fs). This lowering of tan & may be ascribed to rearrangement of molecular chains during reaction. At 60% RH, tan &for modified wood became greater than that for untreated wood. However, because the data points for (F w) at 85% RH appeared on the reference line, we suggest that the molecular motion in the highly swollen state is restricted by crosslinking. E' /-y for (A), WPG: 20%, was slightly lower than for untreated specimens

1.6

0()

"',

1.8

1.6

' ....

c

B

~

. " WPCe

Fm«lO

""

2.0

\~-ol .... ....

.

!~~

2.2

O·'.RH

0()

1.8

~

2·0

0\

1.6

1.4 1.2 tog E"y

1.8

WR: ........... "

C

B 0\

2.0

..... .....

.2

..

.....

2.2

fw

35·,.RH

M3

2.4 0·8

1.0

1.2

tog E"y

1.0

0·8

..

.....

1.8

.2 0\

,.1,

1.2

'·6

log Fly ... "'~G~oJ ...... ro

. 600

... ...

C

t-<;· ............fW

2.0

.2

A

...

~.Fs

2.2

' .... ~rw

1.4

. . fS......

~l

2-'

0()

....

.. ...........roe

twG.\.. . . .

1.6

'A."..

ePffi-lO'

60·'.RH

1-6

• •·PEG-lO'

. .... e

2.2

PECT600

0()

",

...... ...

efR.

1.0

~

e

yYfC

.2

2.4

0.8

"""",

",

......

..

8S·'.RH 2·4

1.6

0·8

1.0

'·2

1.4

1.6

log E'/y

Figure 8 Relationships between logarithm of £' /-y and logarithm of tan &for chemically treated woods at various relative humidities. Dotted lines represent experimental correlation for untreated specimens. See legend to Fig. 2 for abbreviations.

328

Norimoto

as a result of the slight increase in -y. Hydrophobic acetyl groups were subjected to hydrogen atoms in the cell wall. This reduced tan & because of the steric hindrance of the chain imposed by the bulky acetyl group. This effect was seen at all humidity levels. However, humidity-induced changes of E' /-y and tan &values were greater than those for (Fs ). Because not all available hydroxyl groups were acetylated, water still was able to hydrogen-bond with cell wall polymers and act as a plasticizer. (PO), WPG: 22%, resulted in bonded cell wall bulking as in the case of (A), except the introduced group was hydrophilic. This made a big difference as the RH increased. At 0% RH, (PO) had slightly lower E' /-y because of the increase in specific gravity and tan &was lower than that for untreated wood. At 35% RH, tan & was almost the same as that for untreated wood, but at 60% RH and 85% RH, tan &was much larger. This was due to the flexibility introduced into the cell wall because the hydrophilic ether allowed water to act as a plasticizer. (MG), WPG: 23%, resulted in a similar effect on E' /'Y and tan & as with (PO) where the group introduced into the cell wall is hydrophilic. Unlike (PO), the polymer formed with (MG) is rigid in nature and some crosslinking may occur with the cell wall hydroxyl groups. This results in less molecular motion in the cell wall, resulting in lower tan & than that of untreated wood. However, because of the hydrophilic nature of the polymer formed, tan & increases at 85% RH. Treatment with low molecular weight phenol-formaldehyde resin introduced a bulky, hydrophobic group in the cell wall. A small amount of the resin possibly was bonded to the cell wall hydroxyl groups. There was a large increase in specific gravity and E' /-y was lower for (PFd, WPG: 45%, compared to that for untreated wood. Because of the presence of a rigid benzene ring in the resin backbone as well as reduced moisture sorption, molecular mobility was reduced, which lowered tan &for all humidity levels tested. (PEG-I03), WPG: 39% and (PEG-600), WPG: 49%, polymers are low molecular weight and hydrophilic polymers that enter the cell wall. These very flexible hydrophilic polymers swell and plasticize the cell wall even at 0% RH, resulting in very large tan & in the dry condition. The hydrophilic nature of the cell wall does not change as the RH increases because it does not matter whether polyethylene glycol or water is acting as a plasticizer. (WPC), WPG: 138%, indicates that a small amount of this polymer may enter the cell wall, but in general this treatment only fills the lumen space. The increase in specific gravity decreases E' /-y. Water can still enter the cell wall and act as a plasticizer, so that tan & increases as the RH increases. All these data can be used to derive a general relationship for sensitivity

329

Viscoelastic Properties of Wood

or stability to changes in humidity for chemically modified wood with respect to E'/-y and tan &. E'/-y stability SE and tan & stability Sl are defined by the following expressions [14]:

6.(E'/-Yh (E'/-Yh

6.(E'/-y)u

(E'/-y)v

6.(E'/-y)u

x 100

(E'/-y)u ~(tan &h

6.(tan &)u

=

S I

(tan &)v (tan &h 6.(tan &)u (tan &)u

x 100

where U is untreated, T is treated, and ~(E' l-y) and ~(tan &) are changes of E'/-y and tan & between 35% RH and 85% RH, respectively. Figure 9 shows the results of these calculations. The SE and Sl measure how E'/-y and tan &changed with changing RH conditions. (Fs)' (PFd, (A), (WPC), (PFH), WPG: 23%, (PEG-Ht), WPG: 37%, and (Fw) have positive

e 100 PEG-ro>

•

WPC

eFfie • e.Fs

PEG·lOJ PEG-w:>'

-100

St(Ofo)

Pm

~

o •

W1C. MG

A

SEr/.) 100

Figure 9 Relationship between tan & stability Sl and E' /-y stability SE for chemically treated wood. See legend to Fig. 2 for abbreviations.

Norimoto

330

SE and St. (PEG-I0 3) and (PEG-600) have negative SE and positive St. (WIC), WPG: 47%, and (MG) have positive SE and negative SI' (PO) has negative SE and St·

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

M. Norimoto, J. Gril, and R. M. Rowell, Wood Fiber Sci. 24:25 (1992). M. Norimoto, in Current lapanese Materials Research, Vol 11 (N. Shiraishi, H. Kajita, and M. Norimoto, eds.), Elsevier, London, 1993, pp. 135-154. M. Norimoto, T. Ono, and Y. Watanabe, l. Soc. Rheol. lpn. 12:115 (1984). M. Norimoto, F. Tanaka, T. Ohogama, and R. Ikimune, Wood Res. Technol. Notes 22:53 (1986). T. Ono and M. Norimoto, lpn. l. Appl. Phys. 22:611 (1983). T. Ono and M. Norimoto, Rheol. Act. 23:652 (1984). T. Ono and M. Norimoto, lpn. l. Appl. Phys. 24:969 (1985). T. Sasaki, M. Norimoto, T. Yamada, and R. M. Rowell, Mokuzai Gakkaishi. 34:794 (1988). M. Norimoto, T. Ohogama, T. Dno, and F. Tanaka, l. Soc. Rheol. lpn. 9: 169 (1981). H. Akitsu, M. Norimoto, and T. Morooka, Mokuzai Gakkaishi 37:590 (1991). H. Akitsu, J. Gril, and M. Norimoto, Mokuzai Gakkaishi 39:258 (1993). H. Akitsu, J. Gril, T. Morooka, and M. Norimoto, FR/ Bull (New Zealand) 176:130 (1992). G. W. Becker, Kolloid-Zeitschrift. /40:1 (1955). H. Akitsu, M. Norimoto, T. Morooka, and R. M. Rowell, Wood Fiber Sci. 25:250 (1993).

14 Biological Properties of Chemically Modified Wood Munezoh Takahashi Kyoto University Uji, Kyoto, Japan

I.

INTRODUCTION

Although both toxic and nontoxic chemical treatments have been extensively studied for protecting wood, the fonner is still the dominant method for preserving wood. Toxic preservatives in wide commercial use have been subjected to public criticism, so that there is a strong demand for the development of low-toxicity alternatives or safer nontoxic treatments. Nontoxic treatments such as chemical modification will be more and more important for wood preservation because they yield no risk of hazard to health or the environment in the use of endproducts. Chemical modification of wood originally involved any chemical reaction between the hydroxyl groups of principal wood components and a single chemical reagent. In recent years, chemical modification has also included several chemical systems that affect the cell wall and fill the void spaces in wood. Some of these modifications of wood that have been studied to improve hygroscopic, mechanical, viscoelastic, and fire-retardant properties are interesting for wood preservation techniques too. The effectiveness of chemical modification in enhancing biological resistance has been assumed to be mostly due to crosslinking, bulking, or a combination of both for dimensional stabilization. Hydroxyl groups in the cell wall polymers are not only the water adsorption sites but also the biological enzymatic reaction sites. Wood-rotting fungi and tennites each have a very specific enzyme system capable of degrad-

331

Takahashi

332

ing wood polymers into digestible units. Therefore, if the substrate for these systems is chemically changed, this enzymatic action cannot take place. However, as described later, the wood preservation effect of all modifications is not fully explained by this mechanism, and relevant fundamental research should be made as should research in applied technology. In this chapter, biological resistance of chemically modified wood and its potential applications are discussed, based mostly on our recent work to develop high-perfonnance wood products.

II.

ENHANCEMENT OF BIOLOGICAL RESISTANCE OF SOLID WOOD BY CHEMICAL MODIFICATION

As is well known, the extent of chemical modification is detennined by weight percent gain (WPG) in treated wood blocks. The degree of dimensional stability is commonly evaluated as antiswelling efficiency or antishrink efficiency (ASE), or as relative efficiency (RE). RE is the quotient of ASElWPG. Wood preservation effectiveness of modified wood is generally shown by WPG or ASE, which gives no virtual attack of modified wood by wooddeteriorating organisms.

A.

Esterification

Although there are many reports of reactants to achieve dimensional stabilization by ester bonding with hydroxyl groups of the wood components, an additional increase in biological resistance has been studied only for acetic anhydride [1-12] and several isocyanates [13-15] such as methyl isocyanate and allyl isocyanate.

1.

Acetylation

Among the chemical modifications of wood, decay resistance of acetylated wood has been most frequently reported. However, reports are somewhat limited as to the resistance of treated softwood against brown-rot fungi. The different effects among decay types or between softwood and hardwood have been studied to a lesser degree. The author's work revealed that brown-rot fungi were more resistant to acetylation than were white- and soft-rot fungi (Fig. 1). About 20% of WPG was required to eliminate decay by Tyromyces palustris (brown rot) in any wood species (Fig. IA). On the other hand, Coriolus versicolor (white rot) failed to attack acetylated Japanese cedar even at 6% of WPG, although 20% WPG was necessary to suppress its decay of

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Figure 1 Effect of acetylation on the decay resistance of wood against Tyromyces palustris (A). Serpula lacrymans (B), Coriolus versicolor (C), and soil burial (D). (0) Japanese cedar (Crypromeria japonica); (e) Japanese red pine (Pinus densijiora); (6) albizzia (Albizia !alcata); (A) Japanese beech (Fagus crenata). Relative weight loss = W21W 1 X 100, WI: weight loss of control wood; W2: weight loss of acetylated wood.

Japanese beech and albizzia (tropical fast-growing hardwood) (Fig. Ie). The well-known brown-rot fungus Serpula lacryman was also less deterred by acetylation than was C. versicolor but T. paluslris was still the most aggressive of the fungi tested against acetylation (Fig. 1B). Soft-rot fungi might be ranked between brown- and white-rot fungi, based on the result of a soil burial test with acetylated wood (Fig. ID). According to Rowell [7}, the degree of substitution in lignin does not have an effect on the decay resistance at low levels of chemical modification, even though lignin is always more substituted than holocellulose and the difference

Takahashi

334

of degree of substitution between these components is larger at the low level of modification. However, our results suggested that modification of lignin did prevent white-rot decay of softwood even at the low level of acetylation. Different resistances between softwood and hardwood against C. versicolor are possibly involved with different chemical reactivities between the two types of lignin, namely, the guaiacyl (softwood) lignin and the syringyl (hardwood) lignin [16], and/or possibly different cellular distributions of introduced acetyl bonds between softwood and hardwood. S. lacrymans has been characterized by its ligninolytic ability as has Gloeophyllum trabeum. Such ability is lacking in most brown-rot fungi including T. palustris [17-19]. Different resistances to acetylation between S. lacrymans and T. paLustris might be due to their different physiologic properties. Based on reports by many researchers, virtual elimination of decay is achieved in any wood species at 20% WPG, which gives 60-70% ASE. The tennite resistance of acetylated wood has been studied to a lesser degree. Elimination of attack was gained under laboratory conditions at 20% WPG for Reticulitermes jlavipes [2] and R. speratus [20]. Although a reduction of attack was seen because of the acetylation, satisfactorily high resistance was not achieved against a destructive Coptotermes formosanus at that WPG [10,11,20]. However, higher mortality of workers, comparable to starving, was evidenced in acetylated softwood (Table 1). As in most forced-feeding tests with modified wood, the mortality of soldiers was far higher than that of workers in acetylated wood. This is primarily due to the decline of food supply from donating workers, which must be attributable to the digestion deterrence through feeding on acetylated wood. This was supported by the observation of symbiotic protozoan fauna in the hindgut of workers fed acetylated wood [20]. In C. formosanus, three species of protozoa exist as symbiotants. They decreased quickly during the first week. Among them, the largest, Pseudotrichonympha grassii, disappeared quickly. It has the role of breaking down high molecular weight celluloses [21-24]. Such a defaunation of protozoa indicates the prevention of their special cellulolytic enzyme systems, which gives rise to a slower death like starvation rather than toxic effects by tenniticides.

2.

Isocyanate Modification

In the reaction of wood hydroxyls with isocyanates, a nitrogen-containing ester is fonned. A limited reduction of fungal attack was reported earlier in beechwood modified with diisocyanate [13]. Considerably more decay protection than acetylation has been shown in the methyl and allyl isocyanate modifications. Little decay was evident in methyl isocyanate-modified southern pine at WPGs between 14% and 19% in laboratory assay using G. trabeum,

Biological Properties of Wood

335

Table 1 Mortality of Worker Termites During Forced-Feeding Test on Acetylated Spruce Blocks with 20% Weight Gain Mortality (%) in week Coptotermes formosanus

Treatment Control Starved Acetylated

0 6 8

Reticulitermes speratus

2

3

4

5

6

I 15 23

3 27

5 56 74

7 92 96

8 96 98

40

I

13 60

2

3

2 80 90

5 100 100

Source: Ref. 20.

Lentinus lepideus, and C. versicolor [14,15]. Southern pine modified with allyl isocyanate also showed greatly improved decay resistance to these fungi when WPG exceeded 11 % [15]. To compare the decay resistance of various types of modified wood on an equal basis, degree of substitution (DS) is considered more rational than WPG because few large molecular weight molecules could give higher WPG than large numbers of small molecules attached to wood with much greater frequency. At the calculated DS between 0.1 and 0.2, modification with these isocyanates gave wood more pronounced protective effect than did acetylation. In the case of isocyanate modification, an additional effect of fungi-toxic amines that were fonned from wood carbamate derivatives was suggested [15]. Allyl isothiocyanate treatment also gave a high decay resistance of wood comparable to methyl and allyl isocyanates, but the main reaction appaeared to be polymerization, which left isothiocyanate groups intact [15].

B.

Etherification

Among the various chemicals to fonn ether bonds with hydroxyl groups in wood, a high decay resistance of wood treated with f3-propiolactone [25] and acrylonitrile [26,27], both at 25% WPG, has been reported earlier. However, an undesirable carcinogenic effect in f3-propiolactone as well as loss of impact strength and low ASE in acrylonitrile-treated wood discouraged further works on these treatments [7]. Detailed resistance tests against decay fungi and tennites have been conducted on epoxide-treated wood. Brown-rot fungi, L. lepideus and G. trabeum, yielded 44.2% and 62.9% weight loss (WL) of control southern pine, respectively, after 12-week soil block tests.

Takahashi

336

At about 20% WPG, butylene oxide-treated southern pine blocks resisted (2% or less WL) attack by these fungi, whereas propylene oxide-treated ones were not very resistant, gaining 4.8% and 32.8% WLs even at higher WPG [28,29]. Control of attack by soft-rot fungi and tunneling bacteria was evidenced in butylene oxide-modified ponderosa pine with a 23.7% WPG after 6-week soil burial tests [30]. Virtual elimination of attack by the subterranean tennite R. jlavipes was also gained in epoxidc-treated southern pine at 34% WPG for propylene oxide and 27% WPG for butylene oxide under laboratory conditions [31]. Weight loss due to decay was also reduced in cpichlorohydrintreated wood [32]. As shown in Tables 2 and 3, the modified blocks were not very resistant to decay, but they suffered only minor attack by the destructive tennite C. formosanus in a laboratory forced-feeding test (Table 4). No tennite attack was shown for epichlorohydrin-treated pine with WPG above 15% after 6-month exposure to a laboratory-reared colony and a 3-year field test [32].

C.

Crosslinking

1.

Formalization

Reaction of fonnaldehyde with wood hydroxyls has been characterized as yielding high ASE with low WPG. As is well known, the effectiveness of

Table 2 Laboratory Tests of Epichlorohydrin-Treated Wood Inoculated with the Brown-Rot Fungus Tyromyces pa!ustris Wood

Cedar

Pine

Beech

Source: Ref. 32.

Weight percent gain

Percent weight loss after 12 weeks

Control 11.8 26.7 40.7 Control 11.5 19.9 24.0 Control 14.8 21.5 26.5

53.1 10.2 6.3 4.5 44.4 3.4 2.3 2.2 68.7 7.7 6.8 5.5

BwlogicalPropert~sofWood

337

Table 3

Laboratory Tests of Epichlorohydrin-Treated Beech Inoculated with the White-Rot Fungus CorioIus versicolor or the Soft-Rot Fungus Chaetomium globosum Weight percent gain

Percent weight Joss after 12 weeks

Coriolus versicolor

Control 12.7 19.5

Chaetomium globosum

Control 14.7

58.8 9.1 6.9 5.8 38.7 7.7 6.4

Fungi

25.4

25.8 Source: Ref. 32.

Table 4

Laboratory Tests of Epichlorohydrin-Treated Wood Exposed to the Subterranean Termites Coptotermes formosanus Wood

Weight percent gain

Percent weight loss after 3 weeks

Cedar

Control

28.3

11.8

4.7

26.7

3.7 38.0 3.5 2.0

Pine

Control 11.6

22.6 Beech

Control 15.2

23.5

12.3 2.1 1.8

Source: Ref. 32.

fonnalization in dimensional stabilization is caused by crosslinking cell wall polymers. A 60% ASE was reported by Stevens et al. [33] at 5% or less WPG in several softwoods and hardwoods. They also evidenced very low WLs due to decay in fonnalized pine and poplar inoculated with brownrot fungi, Coniophora puteana and Poria monticola, and white-rot fungus, Coriolus versicolor. No decay by G. trabeum occurred for fonnalized spruce with 2.5% or less WPG [1]. Virtual decay elimination by fonnalization was also reported by other workers [34-36]. Detailed study on the relationship between decay resistance and the extent

Takahashi

338

of formalization was made recently by Minato et al. [37]. In this study, the effectiveness of vapor and liquid phase treatments was evaluated. The vapor phase formalization was conducted for 2-24 h at 120°C under S02 catalysis, using tetraoxane for the vapor source of formaldehyde. The liquid phase formalization was made by soaking in the mixing solution of formaldehyde (3.6%), hydrogen chloride (3.7%), acetic acid (75.0%), and water (17.7%) for 1-4 days at room temperature, according to Pierce and Frick [38]. The latter treatment at room temperature was considered milder than the former in higher temperature. Results shown in Table 5 show the following: (I) Very low WPG, less than 2.3%, was enough to eliminate decay of cedar by C. versicolor and by soil microorganisms for both of vapor and liquid phase treatments. (2) Formalization was far more effective for cedar (softwood) than for beech (hardwood) in achieving dimensional stabilization and decay resistance. (3) T. palustris (brown rot) was more resistant to formalization than were C. versicolor (white rot) and soft-rot fungi. The large negative ASE values in the liquid phase treatment mean that the sample swelled to a larger extent in the Table 5 Weight Percent Gain (WPG) and Antiswelling Efficiency (ASE) of Fonnalized Cedar and Beech Blocks, and Their Weight Loss (WL) Due to Decay After 12 Weeks Laboratory Tests with Tyromyces palustris and CorioIus versicolor, and After 9 Months Soil Burial Test WL (%) Wood Cedar

Reaction phase Control Vapor

Liquid Beech

Control Vapor

Liquid Soura: Ref. 37.

Time (h)

WPG

ASE

(%)

(%)

2 5 10 24 24 96

-1.0 -0.6 0.7 1.6 2.3 1.9

-4.3 26.7 48.7 58.7 -21.6 -24.4

2 5 10 24 24 96

-0.1 -0.1 0.8 1.0 1.0 -1.2

4.4 11.5 33.6 44.2 -15.4 -32.6

Tyromyces palustris

35.2 25.1 8.7 0.2 0.2 29.4 39.0 27.8 23.2 21.0 19.2 13.3 26.7 27.6

Coriolus versicolor

Soil burial

44.5 40.0 0 0 0 0 0 66.4 62.9 49.1 18.7 4.4 32.1 29.7

25.3 16.8 0 0 0 0 0 22.5 22.7 21.7 10.2 1.9 8.8 5.6

Biological Properties of Wood

339

reaction solution than in water, but the new oven-dry volume was close to or smaller than the volume of the oven-dry sample before treatment. Stamm and Baechler ( I] explained the decay elimination at a low level of crosslinking on the basis that it must be tying the structural units of the wood together so effectively that the enzymes of decay fungi are prevented from entering the structure. However, the higher decay-inhibiting effect in fonnalized cedar could not be explained, unless some relevant microscopic or chemical difference is evidenced between cedar and beech. The crosslinking is known to occur on lignin as well as on holocellulose [39], but the preferential reactivity of lignin, as found in wood reacted with acetic anhydride and methyl isocyanate [7], is not evidenced in formalization. Therefore, a different decayinhibiting effect between brown- and white-rot fungi should be explained on another basis. The possible causes are that (I) wood-degrading enzymes of white-rot fungi act in the immediate vicinity of the hyphe and their attacks progress from the lumen outward, (2) the action of brown-rot systems is diffused over the entire cell wall and at a considerable distance from the hyphae [40], and (3) the reaction of fonnaldehyde with wood hydroxy Is takes place densely at the lumen surface [41]. Table 6 shows that a virtual elimination of attack by R. speratus was gained in fonnalized cedar and beech at very low WPG for both vapor and liquid phase treatments. WLs due to attack by C. formosanus did not reduce to a satisfactory low level for fonnalized cedar but did for treated beech. As shown in Figs. 2 and 3 on the mortality of worker tennites of C. formosanus, feeding on vapor phase fonnalized blocks caused 100% mortality even when they suffered large WLs due to attack. When comparing a rise of mortality curves between vapor phase-treated cedar and beech, it was significantly slower for cedar than for beech, but all mortality curves went over the mortality curve of starvation. This is different from the case of acetylation, in which both mortality curves passed in a similar manner. In the case of feeding with liquid phase treated wood, mortality curves were somewhat similar to starving, particularly for treated beech. The effectiveness of fonnalized wood in resisting attack by tennites or killing them is still unknown, but substrate alteration, unpalatability of treated wood, and/or slow-acting toxicity of the bonded chemical may play an important role.

2.

Modification with Nonformaldehyde Agents

As stated above, improved biological resistance by fonnalization has been of most interest until recently. However, vapor of fonnaldehyde is toxic to the human body; therefore, special caution should be paid in the treatment process of fonnalization. For the chemical processing of cotton fabrics for pennanent

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Table 6 Weight Loss (WL) of Formalized Cedar and Beech Blocks Due to Tennitc Attack After Forced-Feeding Tests with Coptotermes formosanus for 9 Weeks and with Ret;culitermes speratus for 2 or 4 Weeks WL (%) Wood Cedar

Reaction phase Control Vapor

Liquid Beech

Control Vapor

Liquid

Time a (h) 2 5 10 24 24 96 2 5 10 24 24 96

Coptotermes formosanus

Ret;culitermes speratus

38.6 31.8 13.8 10.8 9.5 21.2 21.3 25.4 25.1 22.0 4.3 1.6 2.2 4.2

3.8 b 2.6 0 0 0 0.4 0.3

3.5 e 2.6 2.1 0.4 0.1 1.3 0.2

aFor resultant WPG and ASE. see Table 5. bAfter 2 weeks. and Cafter 4 weeks. Test ceased when mortality reached lOOCk on treated wood. Source: Ref. 37.

press and wrinkle resistance, non formaldehyde crosslinking agents such as glyoxal, glutaraldehyde, and dimethyldihydroxyethyleneurea (DMDHEU) have been utilized instead of formaldehyde [42]. These dialdehydes and ethyleneurea compounds are known to form crosslinking with cellulosic materials. Biological resistance of wood treated with these agents was recently investigated by Yusuf et al. [43]. Sapwood blocks of cedar and beech were vacuumimpregnated with solutions of these agents at room temperature. Blocks were kept in the solution for I week to gain the optimum swelling until they were sunk to the bottom. air-dried for I week, and cured at 120°C for 24 h under S02 catalysis. After treatment they were thoroughly rinsed in running water for several days to leach out the unreacted agent. Glutaraldehyde and DMDHEU modifications gave cedar blocks ASE of 40-50% at WPG of 5-10%, but the latter could not yield 70% of ASE at 25%. Glyoxal treatment gave a lower WPG and ASE than the former two

Bro/og~aIPropert~sofWood

341

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60

0

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2 TEST

6 PERIOD

8

(WEEKS)

Figure 2 Mortality of worker tennites of Coptotermesformosanus fed on fonnalized Japanese cedar. (----0---) Control, ( - - 6 - - ) vapor phase for 2 h, ( - - A - - ) vapor phase for 5 h, ( - - 0 - - ) vapor phase for 10 h, ( - - B - - - vapor phase for • ) liquid phase for 4 days, 24 h, ( - 8 - - ) liquid phase for 1 day, ( (---e---) starvation.

treatments. The order of ASE at the same WPG was not the same in cedar and beech and not followed in the order of molecular weight. These results may be related to different reactivities of agents between cedar and beech. As for the dimensional stabilizing effect, these three agents could not yield high ASE with low WPG as recognized in formalization. Therefore, crosslinking was possible for these compounds but the ASE achieved was mainly caused by bulking. The decay resistance of wood modified with the three agents is shown in Tables 7 and 8. Among them, glutaraldehyde was most effective in eliminating the attack on cedar and beech by decay fungi. Decay was almost nil at less than 10% of WPG, which gave about 50% and 30% of ASEs for cedar and beech, respectively. The glutaraldehyde treatment is considered superior to formalization because the decay resistance of formalized beech

Takahashi

342 100

l :>

80

I-

:J <:

IeI:

0

60

~

W I-

:E eI:

40

W I-

20

o

2

4

6

8

TEST PERIOD (WEEKS)

Figure 3 Mortality of worker termites of Coptotermesformosanus fed on fonnalized Japanese beech. Symbols are same as those in Fig. 2.

was not so improved by its modification [371. DMDHEU was ranked after glutaraldehyde mainly due to its unsatisfactory effect in preventing decay of beech by T. palustris. Glyoxal treatment exhibited a generally poor effect in preventing decay, although 45% of ASE was gained in cedar blocks at 15% of WPG. Glutaraldehyde treatment was also most effective in preventing attack by the destructive termite C. jormosanus (Tables 9 and 10). The effect was somewhat higher than that of formalization. WL decreased to less than 5% in cedar at 14% WPG and in beech at 4%. WL fell below 5% also in DMDHEU-treated beech, but a higher WPG of 8% was required. Less than 5% WL was not achieved in cedar treated with this agent. Mortality of worker termites fed on cedar and beech blocks reached 100% even at low-level treatments with glutaraldehyde and DMDHEU. As in the decay resistance test, glyoxal-treated wood had a poor resistance to C. jormosanus when it was evaluated as to the WL reduction of treated wood. However, mortality reached 100% when wood was treated with a higher concentration of the solution. The effect of these treatments was different between the two subterranean termites tested. In the case of R. speratus, glutaraldehyde was ranked

Biological Properties of Wood

343

Table 7 Weight Percent Gain (WPG) and Antiswelling Efficiency (ASE) of Cedar Blocks Treated with Three Crosslinking Agents, and Their Weight Loss (WL) Due to Decay After 12 Weeks Laboratory Tests with Tyromyces paluslris and CorioIus versicolor

WL (%) Agent Control Glutaraldehyde

DMDHEU

Glyoxal

Cone.

WPG

ASE

(%)

(%)

(%)

5 to 15 25 5 to 15 25 5 to 20

9.1 13.7 26.3 32.7 9.4 23.5 37.6 54.3 -3.6 5.7 14.5

43.6 62.1 73.5 78.6 50.0 59.4 57.9 55.6 6.2 28.4 44.8

Tyromyces paluslris

44.0 0.5 0.2 0.1 0.3 4.8 6.8 5.8 12.5 46.4 35.1 35.9

CorioIus versicolor

53.4 0.1 0.4 0.2 0.5 1.7 1.8 2.7 4.9 17.3 14.8 22.9

Source: Ref. 43.

below other chemicals. The mechanism of effectiveness of these agent-treated woods in resisting to decay fungi and termites is unknown. Since all treated blocks were thoroughly rinsed in running water for several days, it is highly improbable that any of the biological resistance is due to residual bioactive agents.

D.

Resin-Based Treatments

1.

Phenolic Resin Impregnation

The impregnation of PF (phenol-formaldehyde) resin into wood was studied in the early part of the twentieth century. PF resin wood composites have been commercialized as "Impreg" and "Compreg" for their high mechanical strength and dimensional stability. Although the enhancement of decay resistance by PF resin treatment was also found earlier [ 1], it has not been applied to wood preservation techniques for the prevention of biodeterioration. Ryu et al. [44] compared the wood preservation effects of PF resin treatment with those of acetylation. When using low molecular weight PF resin

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344

Table 8 Weight Percent Gain (WPG) and Antiswelling Efficiency (ASE) of Beech Blocks Treated with Three Crosslinking Agents, and Their Weight Loss (WL) Due to Decay After 12 Weeks Laboratory Tests with Tyromyces palustris and Coriolus versicolor WL (%) Agent Control Glutaraldehyde

DMDHEU

Glyoxal

Cone.

WPG

ASE

(%)

(%)

(%)

5 10 15 25 5 10 15 25 5 10 20

1.3 3.8 6.9 8.5 -1.3 5.8 8.5 23.2 -8.1 -7.1 0.7

10.4 25.0 40.8 40.0 28.6 42.8 48.9 60.1 -11.4 8.7 32.3

Tyromyces palustris

Coriolus versicolor

58.3 58.3 1.2 0.7 1.2 47.2 34.0 32.8 19.7 34.5 31.0 29.8

62.6 19.8 22.6 2.1 5.0 13.7 9.0 4.5 3.9 73.3 46.5 33.6

Source: Ref. 43.

such as 170 of Mn (number-averaged molecular weight), treated softwood perfonned well at 10% of WPG against T. palustris, which was still resistant to acetylation at similar treatment level. They also reported a better protecting effect of PF resin treatment to tennite attack than acetylation. Also, Morishita et al. [45] reported that PF resin-treated stakes with 16% WPG were free from tennite attack after 2Y2 years of field test. The biological resistance of PF resin-treated wood in relation to the molecular weight and some related properties of resins was investigated recently by Ryu et al. [46]. Seven resins-three by one manufacturer (Mn: 369, 621, and 1143) and four by another (Mn: 385, 545, 791, and 991)-were used for the treatments. Sapwood blocks of Japanese cedar were impregnated with several concentrations of PF resin solutions, heat-cured, and submerged in water for 2 weeks at room temperature to remove the unpolymerized portions of impregnated PF resins. Treated blocks were exposed to decay fungi and subterranean tennites. In both resin groups, the smaller Mn resins gave higher

B~/cg~aIProperlwsofWood

345

Table 9 Weight loss (WL) of Cedar Blocks Treated with Three Crosslinking Agents Due to Termite Attack, and Mortality (MT) of Worker Termites After Forced-Feeding Tests with Coptotermes formosanus for 9 Weeks and with Reticulitermes speratus for 3 Weeks.

Cone. Agent Control Glutaraldehyde

DMDHEU

Glyoxal

Coptotermes formosan us

Reticulitermes speratus

(%)8

WL (%)

MT (%)b

WL (%)

MT (%)b

5 10 15 25 5 10 15 25 5 10 20

35.3 2.5 4.0 2.6 3.3 12.7 8.5 6.1 13.0 25.0 22.3 13.9

22 1004 I ()()4 1004 I ()()4 1005 I ()()~

6.0 0.7 1.7 0.8 1.2 1.5 2.0 1.1 2.4 0 0 1.8

22 1002 1002 1002 lQ02 100 100 100 1002 100 100 100

IW 1005 58 100 1005

aFor resultant WPG and ASE, see Table 7. bSupcrscript numbers are the week when mortality reached 100%. Source: Ref. 43.

ASE values and greater biological resistance than the larger ones at the same WPGs (Tables 11 and 12). Scanning electron microscope (SEM) observation of PF resin-treated wood evidenced that the low molecular weight resins were extensively deposited in wood cell walls (47). Thus, they are effective in reducing the swelling of wood blocks in water immersion, mainly by the fonnation of hydrogen bonds between the hydroxyl groups of wood and the phenol alcohols, and/or a physical blocking of the hydroxyl groups. The smaller ASE value in wood blocks impregnated with PF resin of Mn 383, which was compared to those with Mn 369 in another resin group, might be due to the different constitutions of these resins. Mn 369 resin consisted exclusively of phenolic monomers in which 2,4,6-trimethyrolphenol was the major component, whereas Mn 383 resin was a mixture of various monomeric phenols with one to three alcohol groups and dimeric phenols [48]. The smaller molecular weight monomer

Takahashi

346

Table 10 Weight Loss (WL) of Beech Blocks Treated with Three Crosslinking Agents Due to Termite Attack, and Mortality (MT) of Worker Termites After Forced-Feeding Tests with Coptotermes formosanus for 9 Weeks and with Reticulitermes speratus for 3 Weeks

Cone. Agent Control Glutaraldehyde

DMDHEU

Glyoxal

Coptotermes formosanus

Reticulitermes speratus

(%)a

WL (%)

MT (%)b

WL (%)

MT (%)b

5 10 15 25 5 10 15 25 5 10 20

18.9 16.1 4.6 3.9 2.7 9.5 4.5 3.0 2.7 26.6 24.7 7.8

19 70 1006 1005 1005 1008 laos

5.7 5.5 3.1 4.1 1.4 1.6 1.6 0.8 1.0 0 1.6 0.5

32 67 95 80 97 100 1002 1002 1002 100 100 100

lW IW 14 17 1008

aFor resultant WPG and ASE. see Table 8. hSuperscript numbers are the week when mortality reached 100%.

Source: Ref. 43.

could penetrate better into cell walls than the larger molecular weight dimer, and phenol trialcohol reacts greatly with the hydroxyl groups of wood to fonn hydrogen bonds. Thus, wood blocks impregnated with Mn 369 resin were made more dimensionally stable. As shown in Table II, impregnations with Mn 369 and Mn 621 resins (group A) achieved the virtual elimination of decay in monoculture and soil burial tests at 10% of WPG. Mn 1143 resin also gave high resistance against T. palustris and C. versicolor, but was not effective in reducing fungal attacks in soil burial tests. Mn 791 and Mn 991 resins (group B) yielded poor decay resistance to C. versicolor and soil burial (Table 12). The soil burial test conducted in a humid "fungus cellar" 1461 has been characterized as accelerating the microbial deterioration of test samples by their exposures to diverse soil microflora containing a wide range of microfungi, many of which are known as causal agents of soft rot. Other organisms may be involved in the

Brorog~alProperlmsofWood

347

Table 11 Weight Loss (WL) of PF Resin-Impregnated Cedar Blocks After 12 Weeks Decay Tests, After 9 Months Soil Burial Test, and After 8 Weeks Termite Test (Group A Resins)

Mn l Control 369

ASE

(%)

(%)

TYpb

cove

S-Bd

COpe

2 5

12 33 42 54 58 8 19 20 22 24 0 0 0 0 0

27.7 3.8 1.8 0.9 0 0 1.0 0.5 0 0 0 9.5 3.2 1.4 0 0

24.8 2.7 0.7 0 0 0 1.6 0.5 0 0 0 11.1 1.4 0.5 0 0

36.4 5.3 0.8 0 0 0 8.6 1.1 0 0 0 35.3 25.6 16.4 6.9 5.3

24.0 14.4 8.3 8.1 6.6 3.9 15.8 13.6 9.2 5.6 1.4 14.7 14.4 13.9 9.8 8.9

10

621

20 30 2 5 10

1143

WL (%)

WPG

20 30 2 5 10

20 30

"Number-averaged molecular weight, t>ryromyces pa/ustris, 'Coriolus versicolor, dsoil burial, CCoptotermes formosanus. Source: Ref. 46.

alteration of the introduced chemicals or substrates allowing soft rot to occur [49]. According to the performance requirement prescribed in Japanese standards, a good wood preservative should have less than 3% of mean weight loss in treated blocks. Minimum WPGs to meet the requirement in all fungal exposures, including soil burial, were 5% for Mn 369 and Mn 621 resins, and 20% for Mn 383 resin. Treatments with any other PF resin were unsuccessful in meeting the requirement even at a 30% of WPG. The overall superiority of group A resins over group B resins in resisting decay can be explained by the different molecular weight distributions stated before. Although Mn 621 and Mn 545 resins exhibited similar ASE values at every WPG, the former showed a clear advantage over the latter by having greater decay resistance. Both resins contained monomeric and dimeric phenols, and Mn 545 resin

Takahashi

348

Table 12 Weight Loss (WL) of PF Resin-Impregnated Cedar Blocks After 12 Weeks Decay Tests, After 9 Months Soil Burial Test, and After 8 Weeks Tennite Test (Group B Resins) WL (%)

WPG

ASE

Mn a

(%)

(%)

TYpb

cove

S-B d

COP

383

2 5 10 20 30 2 5 10 20 30 2 5 10 20 30 2 5 10 20 30

II 27

1.1 0.6 0.3 0 0 2.5 2.4 1.4 0.4 0 3.3 2.8 1.9 0.6 0 6.1 4.7 1.7 1.1 0

6.1 1.3 1.1 1.7 1.2 22.2 7.2 2.8 2.2 3.6 31.4 30.0 8.9 5.0 4.2 33.9 26.7 25.0 10.6 6.7

14.7 11.6 6.3 2.3 2.8 21.5 17.5 9.8 3.3 3.5 31.6 27.8 21.8 15.2 10.1 34.9 31.1 19.7 9.6 7.6

12.9 6.3 6.3 .5.6 3.5 13.4 14.2 12.7 11.9 10.9 18.2 28.4 27.6 25.3 19.2 29.6 25.8 25.1 21.8 12.2

545

791

991

31 33 33 16 19 23 25 25 II 12 13 13 13 2 4 6 7 7

-Number-averaged molecular weight, lYryromyces palusrris, cCoriolus versicolor, dsoil burial, tCoplOlermes formosanus. For WL of control blocks, see Table II. Source: Ref. 46.

also had small amounts of trimeric phenols [48]. However, in this case, the better decay-inhibiting effect of Mn 621 resin might be related to its lower pH value (10.0) compared to that of Mn 545 resin (12.0). A reduction of modulus of elasticity (MOE) was evidenced in wood blocks treated with highalkaline PF resin, and both a detrimental swelling of crystalline cellulose and a weakening of intennolecular bonding of cellulose were suggested [48]. Thus, the protective effect of resin deposits in the cell walls and the blocking of hydroxyl groups might be discounted by the deterioration of the cell wall

Brorog~alProperlwsofWood

349

structure during the highly alkaline treatment. The poor decay resistances of Mn 791 and Mn 991 resins-treated wood also can perhaps be explained by larger pH values (12.5 and 12.8) of these resins than that of Mn 1143 resin (11.5). Although Stamm and Baechler [I] reported that 70% of ASE was required to eliminate decay in PF resin treatment, our results showed that a far smaller ASE (20-35%) was enough to eliminate decay when treating with Mn 369 and Mn 621 resins at only 5% of WPG. This suggests that the elimination of decay by the impregnation of low molecular weight PF resin can be involved with some additional factor such as the fonnation of bioactive polymers in the wood structure. The molecular weight of PF resin affected tennite resistance less clearly than decay resistance, but resins with up to 621 Mn gave better tennite resistance than larger molecular weight resins. Although total elimination of tennite attack was not achieved by any PF resin treatment, activities oftennites fed on the treated wood declined at a greater rate than that of starved tennites. At 10% of WPG, all treated blocks caused a 100% mortality of tennites after 6 or 7 weeks feeding.

2.

Furfuryl Alcohol Impregnation

Furfuryl alcohol (FFA) is produced by a partial reduction of furfural, which is derived from pentose-containing plant materials such as com cob, straw, and hardwood residues. Dimensional stabilization has been achieved by the impregnation and polymerization of FFA into wood with or without a catalyst [50]. Acid polymerization of FFA is known to involve intennolecular dehydration of alcoholic groups under fonnation of a polymer composed of furan nuclei linked by methylene bridges [51]. Heating at 100°C gave a high yield of furan resin into the wood impregnated with FFA solution [52]. FFA has been studied as a nonconventional bonding agent to fonn coavalent bonds with the wood surface [51] but has not been studied as a method of wood preservation. Biological resistance of FFA-treated wood was first studied by Ryu et al. [53]. The treatment was more effective for cedar and pine than for beech in enhancing decay resistance (Table 13). Unexpectedly, the highest ASE was less than 50% at 30% of WPG. As generally recognized in chemical modification, the treatment was more effective against white-rot fungus, C. versicolor, than brown-rot, T. palustris. FFA-treated wood was less attacked by the destructive tennite C. formosanus and caused their high mortality with an increase of WPG. However, the effectiveness of FFA treatment in resisting

Takahashi

350

biological attacks was ranked below any other treatment described in this chapter.

E.

Wood-Inorganic Material Composites

Wood-inorganic material composites (WIC) have been developed in Japan [54]. A double-diffusion treatment of wood by two aqueous solutions of inorganic chemicals leads to the formation of water-insoluble deposits within the cell walls and voids. Water-impregnated wood was introduced into solutions (I and II) successively at 50°C for a desired period of time. Saturated aqueous solutions were prepared from (I) barium chloride plus a small amount of boric acid and (II) ammonium phosphate plus a small amount of boric

Table 13 Weight Loss (WL) of FFA-Impregnated Wood Blocks After 12 Weeks Decay Tests, After 9 Months Soil Burial Test, and After 8 Weeks Tennites Test

Wood

WPG

ASE

(%)

(%)

Control

Cedar 5 10 20 25 30

16 28 44 46 47 Control

Pine

10 18 29 31 31

5 10 20 25 30 Control

Beech 5 10 15 20 30

15 25 32 34 35

WL (%) TYpa

COyb

S-B c

COP!

35.5 22.1 17.2 6.9 4.5 0.7 45.5 19.9 16.4 10.1 3.5 NT 36.7 53.5 34.6 24.5 17.1 NT

46.9 17.2 12.4 2.8 2.1 0 44.1 16.8 5.2 3.8 1.4 NT 51.7 49.7 40.6 24.4 20.3 NT

47.6 16.3 10.8 6.0 4.8 4.8 N'fC NT NT NT NT NT 99.4 91.0 56.6 44.0 NT NT

18.1 16.1 8.9 5.2 4.1 1.9 25.6 12.6 9.4 6.1 4.1 2.4 15.6 5.2 3.7 2.0 NT NT

'Tyromyces paluslris, bCoriolus versicolor, 'soil burial, dCoplOlermes formosanus, <not tested. Source: Ref. 53.

Biological Properties of Wood

351

acid. After treatment, wood was exposed to running water to wash out the excess chemicals, dried under ambient condition for a suitable period, and oven-dried. The WPG calculated on an oven-dried weight basis indicates the level of treatment. The main research target of this treatment has been an improvement in the fire resistance of wood. Plywood made from a treated veneer of Douglas fir was tested for its combustibility and the results indicated that it met the performance requirements for a "very slow-burning material" designated in Notification No. 1231 of the Ministry of Construction of Japan [54]. Whereas a higher WPG, approximately 100%, is required to improve the fire resistance, the reduction of biological attacks was achieved by a relatively low WPG. Research has been extended to examine other combinations of inorganic chemicals and the role of added boric acid in wood preservation performance 155-57]. Sapwood blocks of Japanese cedar served as substrate for the doublediffusion treatment with inorganic chemicals. Treatment was conducted by consecutive soaking at 80°C for 24 h in each different aqueous solution and followed by washing in nonrunning water at room temperatures for 85 h (Table 14). Water-insoluble deposits, such as barium hydrogen phosphate, barium sulfate, calcium hydrogen phosphate, and calcium sulfate, were formed during the treatment. Treatments 1-4 and 11, in which boric acid was incorporated into both cation and anion solutions or into the second anion solution and barium hydrogen phosphate was expected to be precipitated within wood, well protected treated wood from decay fungi (Table 15). In contrast, treatments 9 and 13, in which an addition of boric acid to anion solution was not made, failed to control decay by both test fungi. As for treatments 5-8, 10, 12, and 14 to form barium sulfate, a similar tendency was noticed. When no boric acid was added to anion solution (10 and 14), WL of treated wood caused by C. versicolor exceeded 3% even if WPG by water-insoluble deposits was as high as 30%. However, these treatments were effective in resisting T. palustris. Both calcium hydrogen phosphate and calcium sulfate (15 and 16) were satisfactorily effective against the two test fungi at 30% of WPG. Treatments 1-4, 9, II, and 13 were highly effective against termite C. formosanus (Table 15). When a small amount of boric acid was added to anion solution, 100% mortality was achieved in all cases regardless of WPG values. Barium hydrogen phosphate appeared to perform better than any of the other deposits. Barium sulfate also succeeded in controlling termite attack with the aid of boric acid in both treatment solutions or in the second solution.

352

Takahashi

Table 14 Double-Diffusion Treatment for Preparing Wood-Inorganic Material Composites and Target Weight Gain (WPG) Chemicals treating cone. (mollL water) Treatment

Solution I

(cation)

Solution II

(anion)

Target WPG (%)

1 2 3 4

BaCI 22H 2O + HJBO J

0.17,0.17 0.33, 0.33 0.53, 0.53 0.92,0.92

(NH 4hHP0 4 + HJBO J

3.5,4.0 3.5,4.0 3.5,4.0 3.5,4.0

10.0 20.0 30.0 50.0

5 6 7 8

BaC1 22H 2O + H3BO J

0.18,0.18 0.33,0.33 0.47, 0.47 0.75,0.75

(NH 4hS04 + H3B0 3

3.5,4.0 3.5,4.0 3.5,4.0 3.5,4.0

10.0 20.0 30.0 50.0

9

BaC1 22H 2O

0.53,0.53

(NH 4hHP0 4

3.5

30.0

10

+ H3 B0 3

0.47, 0.47

(NH 4hS04

3.5

30.0

II

BaC1 22H 2O

0.53

(NH 4hHP04 +HJBO J

3.5,4.0

30.0

12

BaCI 22H 2O

0.47

(NH 4hS04 + H3 B0 3

3.5,4.0

30.0

13

BaC1 22H 2O

0.53

(NH 4hHP04

3.5

30.0

14

BaC1 22H 2O

0.47

(NH 4hS04

3.5

30.0

15

0.91,0.91

(NH 4hHP0 4 + H3B0 3

3.5,4.0

30.0

0.80,0.80

(NH 4hS04 +H 3B0 3

3.5,4.0

30.0

CaC1 22H 2O 16

+ H3B0 3

Source: Ref. 56.

This tendency was quite similar to the case of decay-inhibiting effectiveness. Calcium compounds produced in treatments 15 and 16 exhibited a good tenniticidal efficacy at 30% WPG. The resull'i clearly demonstrated that barium hydrogen phosphate perfonned best in protecting wood from biological attacks. The small amount of boric acid added to anion solution thoroughly resulted in the increased efficacy in controlling

Biological Properties of Wood

353

Table 15 Weight Loss (WL) of Wood-Inorganic Material Composites After 90 Days Decay Test and 21 Days Termite Test WPG Treatmenta

(%)

TYpb

coye

COP

13.2 21.2 31.2 52.2 13.0 24.8 34.6 50.0 33.0 30.1 33.0 30.0 30.7 30.2 30.0 26.8

BHpe BHP BHP BHP BSF f BSF BSF BSF BHP BSF BHP BSF BHP BSF CHPg CSP

46.5 0 4 0.1 1.7 0 0.2 0.3 1.0 4.3 0.6 2.7 0.4 5.5 0.9 0 0.2

29.6 0.7 0.1 0.9 1.9 0.6 0.1 0.7 0.8 8.3 3.9 1.5 0.7 5.9 3.2 1.1 0.3

24.1 0.2 0 0 0.4 1.2 1.2 2.2 1.7 0 4.1 0 2.6 0 6.1 0.7 0.5

Control I

2 3 4 5 6 7 8 9 10 II

12 13 14 15 16

WL (%)

Deposits formed

·See Table 14. t>ryromyces palustris. cCoriolus versicolor. dCoptotermes formosanus. tI>arium hydrogen phosphate. fbarium sulfate. 8calcium hydrogen phosphate. hcalcium sulfate. Source: Ref. 56.

the attack by decay fungi and subterranean tennites. An ion chromatographic analysis of WIC exposed to leaching in nonrunning water revealed that a very small amount of the boric acid added remained even after 132 h leaching, at which no boric acid was detected from the leaching water [57]. The amount was far less than 1.0 kg/m 3 wood which has been recognized as the threshold retention to yield a good biological resistance. Furthennore, X-ray diffractometric analysis evidenced that boric acid remained as an inorganic borate within the cell walls. Its identification has not yet been made but it seemed to bind with the barium hydrogen phosphate. Therefore, a higher biological resistance of WIC incorporating boric acid might be attributed to a cooperative action of the deposited barium salt and borate.

354

III.

Takahashi

APPLICATIONS TO RECONSTITUTED WOOD PRODUCTS

As described in the preceding pages, virtual elimination of decay and termite attack has been achieved in several chemically modified solid woods. However, commercial application of thick modified solid wood has not been realized. Rowell [121 pointed out the two major concerns in the cost ineffectiveness of its application. The first was the need to dry the wood to less than 3% moisture for minimizing reagent hydrolysis, and the second was the poor penetration of the reacting chemical into the deeper region of thick solid wood. Developed technology and world demand for saving forest resources have increased the production and utilization of reconstituted wood products. Chemical modification is most effective for thin or small-size wood because of its good chemical penetration. The first commercialization of chemically modified reconstituted products was an acetylated laminated veneer lumber (LVL) in which 3-mm-thick veneer was treated with an acetic anhydride under sodium acetate catalysis prior to hot-pressing. In Japan, acetylated LVL has been applied to endproducts that are exposed to continued dampness or outdoor weathering conditions such as bathroom elements, external sign boards, and roof shingles on traditional wooden shrine buildings [58]. Property enhancement by acetylation has been frequently reported over the years in other reconstituted wood products such as ftakeboards, particleboards, and fiberboards [8,9,11,12,59-64]. Table 16 shows the laboratory decay test of low-density acetylated particleboards made from perishable albizzia wood. They were resistant to attack by Tyromyces palustris (brown rot), CorioIus versicolor (white rot), and Chaetomium globosum (soft rot) above 12% WPG. These acetylated boards with 20% WPG also exhibited an improved resistance to attack by the destructive Formosan termite, Coptotermes formosanus, in the laboratory. However, their performance was unsatisfactory in the wet tropics with a higher hazard of termite attack. High resistance to fungal and bacterial attack in acetylated southern pine and aspen ftakeboards was evidenced in laboratory and fungus cellar tests r12]. Greatly improved resistance to biological attack has been achieved also in particleboards made from PF resin-impregnated particles of Japanese cedar [65,66]. Particles had average dimensions of 0.43 mm in thickness, 2.83 mm in width, and 29.6 mm in length. They were oven-dried until moisture content reached about 3%. A water-soluble PF resin of Mn 389 was used for the treatment of particles. Two methods were employed for incorporating the resin into the particleboards: method I-pretreatment by dipping of particles

Biological Properties of Wood

355

Table 16 Weight Loss (WL) of Acetylated Albizzia Particleboard After 8 Weeks Decay Tests WL (%)

WPG Adhesive PP

(%)

TYpe

COyd

CHG~

26.8 24.1 4.3

21.7 18.4 1.1 0 24.3 21.6 0

10.0 7.7 0.7 0 15.0 0.7 0

0

0

0

0 5

IC b

12 20 0 5 12 20

IPhenol fonnaldehyde resin, bisocyanate resin. CTyromyces palusrris. dCoriolus versicolor. cChaeromium globosum. Source: Ref. II.

into aqueous solutions of PF resin for 30 min prior to board manufacture; method II-mixing a compatible PF resin with liquid bonding PF resin (Mn 962) and spraying the two simultaneously. For method I, particles saturated with PF resin solution were air-dried and then oven-dried at 60°C. After that, bonding resin was sprayed on these particles to produce board materials. Particleboards were made with target specific gravity (SG) of 0.5, 0.6, and 0.7, with a solid content of 8% for bonding resin. Pressing lasted for 10 min at 170°C. After pressing, board was conditioned in an oven at 180°C for 1 h to cure the incorporated resins completely. Test blocks prepared from these boards were soaked in water to avoid the toxic effect of free phenols from the phenolic binder and exposed to biological attack. Results are shown in Table 17. Required WPGs to eliminate decay of board were different between the test fungi. For C. versicolor, 5% WPG was enough to eliminate decay in the dipping treatment (method I) and 10% WPG in the spraying treatment (method II), while 15% WPG was required for T. palustris in both treatments. This higher tolerance of the latter fungus has often been recognized in chemically treated wood, as described previously in this chapter. Method I was more effective than method II (spraying) in yielding a higher decay resistance of treated boards, particularly against C. versicolor. Method I also gave a lower TS (thickness swelling) of boards than those by method II. These results can be explained by deeper and more even penetration of PF resin into particles

Takahashi

356 Table 17 Weight Loss (WL) PF Resin-Treated Cedar Particleboards After 12 Weeks Decay Tests, After 9 Months Soil Burial Test, and After 6 Weeks Termite Test

Treatment Control

Dipping8

Specific gravity 0.5 0.6 0.7 0.5

0.6

0.7

Sprayingb

0.5

0.6

0.7

WL (%)

WPG (%)

TYpe

COyd

S-BC

COP

5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15

40.0 28.9 31.1 3.0 7.2 0 5.1 3.0 0 10.2 2.6 0 24.7 6.0 11.5 14.9 16.2 0 13.8 11.1 0

34.0 25.5 22.1 0 0 0 0 0 0 0 0 0 14.5 0 0 1.7 0 0 2.1 0 0

16.8 13.0 11.4 10.3 6.5 1.7 8.3 3.5 1.0 10.5 4.6 1.0 7.3 1.7 4.3 5.6 5.9 3.0 6.7 5.4 1.4

16.2 13.3 11.0 8.3 10.0 3.0 8.7 3.8 1.3 7.9 4.8 3.0 4.6 3.8 2.9 7.8 8.1 7.6 8.6 3.7 0.6

'Particles were dipped into PF-resin solution for 30 min prior to board manufacture. bpF resin solution was sprayed on particles together with bonding PF resin. CTyromyces pa/uslris, dCoriolus versicolor, csoil burial. fCoplolermes formosanus. Source: Ref. 65.

in the dipping treatment. Less than 2% WL in boards treated by dipping was gained at 15% WPG in 9 months of soil burial. In control boards, TSs were measured at 9-17% but were less than I% in treated boards with 15% WPG 166]. For enhancing resistance to termite attack, the effect of treatment method was not evident. The spraying treatment employed is a one-step treatment that is considered preferable for time saving and/or cost efficiency in board

357

Biological Properties of Wood

production. However, the results showed the overall superiority of the dipping treatment to the spraying, especially for yielding higher decay resistance. Medium-density fiberboard (MDF) has been of much interest as the most promising reconstituted product. MDF can be utilized not only for interior materials and furniture but for construction materials. One of the inferior properties of MDF is the dimensional instability when exposed to humid condition. It is also quite often invaded by wood-degrading organisms. Vapor phase formalization was applied to improve several properties of MDF including biological resistance [67,68]. Both 9-mm-thick softwood MDF (S-MDF) and hardwood MDF (H-MDF) were used for biological tests. Results are shown in Table 18. Although WLs due to biological attack decreased in treated MDF blocks, the results were unsatisfactory when compared with those of solid wood (Tables 5 and 6). Excessive reaction time (24 h) caused a rather detrimental effect on dimensional stability and on biological resistance of treated boards. This can possibly be explained by a thermal degradation of urea-formaldehyde resin as an adhesive.

Table 18 Antiswelling Efficiency (ASE) of Fonnalized Medium-Density Fiberboards (MDF), and Their Weight Loss (WL) After 12 Weeks Decay Tests, After 9 Months Soil Burial Test, and After 9 Weeks Tennite Test

Board S-MDP

H-MDP

Reaction time (h) Control 2 5 12 24 Control 2 5 12 24

WL (%) WPG

ASE

(%)

(%)

0.7 2.7 4.8 4.8

2.3 49.8 66.0 58.8

1.2 2.5 4.8 5.3

4.0 27.1 60.5 46.5

TYpe

COV d

S-Be

COP

8.0 6.7 7.7 13.5 11.0 3.3 3.1 4.4 5.3 7.0

18.8 5.2 4.3 7.2 10.5 68.5 47.9 8.3 4.4 8.1

29.9 24.3 11.1 12.8 14.1 22.0 23.7 13.2 8.8 11.0

38.1 35.5 14.8 11.7 9.1 9.9 7.9 6.0 4.3 4.9

'Softwood MDF. ~ardwood MDF. CTyromyces palustris, dCoriolus versicolor, csoil burial. (Coptotermes formosanus. Source: Ref. 68.

358

IV.

Takahashi

CONCLUSION

Biological resistance of perishable solid wood can be greatly improved by several methods of chemical modification. Most of these methods are derived from those for improvement of dimensional stability, such as by bulking and crosslinking of the cell wall polymers or by impregnation with water-soluble resin to form water-insoluble polymers in void spaces in wood. Therefore, the resistance can be explained on the bases that insufficient water can be taken up to support decay and that hydroxyl groups are blocked, so that enzyme systems of wood-degrading organisms cannot work. However, different mechanisms might be involved in the resistance of wood-inorganic material composite (WIC), and some additional factor might be related in phenolic resin-impregnated wood, because their resistance was achieved at low levels of dimensional stability. Acetylation and phenolic resin impregnation are found to be applicable to reconstituted wood products in yielding good biological resistance. Treatments with crosslinking agents and WIC can be applied to these end products but further studies are required to improve production technology. Even if several other properties and cost effectiveness could be improved, chemically modified wood products cannot completely replace preservativetreated products. Nontoxic chemical treatments are principally for prevention, not for remedial and extermination treatments. However, current high dependence on preservatives for wood protection can be reduced by proper use of chemically modified wood products.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

A. J. Stamm and R. H. Baechler, Forest Prod. J. 10:22 (1960). I. S. Goldstein, E. B. Jeroski, A. E. Lund, J. F. Nielson, and M. Weater, Forest Prod. J. 11:363 (1961). H. Tarkow, A. J. Stamm, and E. C. O. Erickson, USDA Forest Service, Forest Pest. Leaft. Rep. No. 1953 (1961). Koopers' Acctylated Wood, New Materials Technical Information (RDW-400) E-I06 (1961). M. D. Peterson and R. J. Thoma." Wood Fiber 10(3):149 (1978). L. R. Gjovik and H. L. Davidson, USDA Forest Service, FRLRP-02 (1979). R. M. Rowell, in The Chemistry of Solid Wood (R. M. Rowell, cd.), ACS Symposium Series 207, Washington, D.C., 1984, pp. 175-210. Y. Imamura, K. Nishimoto, Y. Yoshida, S. Kawai, T. Sato, and M. Nakaji, Wood Res. 73:35 (1986).

Bromg~alProperlmsofWood

9. 10.

It. 12.

13. 14. 15. 16. 17.

18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

359

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39:190 (1993).

Index

Accelerated weathering (see Weathering, artificial) Accessibility, 15, 35, 36, 40, 43, 49, 57,235 cellulose, 5, 37, 38, 43, 64, 99, 100 cotton, 40 lignin, 81 measurement, 37 wood, 5, 162 xylan, 36, 65 ACE (see Anticreep efficiency) Acetal, 5, 236 Acetalization, 10 I, 163 Acetyl bromide, 21 spectroscopic determination of lignin, 21 Acetyl content, 50, 237, 287 Acetyl group, 214, 237, 289, 328, 332 glucomannan, 19 4-0-methyl glucuronosyl unit, 66 xylan, 19, 66 Acetylation, 52, 166, 237, 238, 286, 301,314 acetic anhydride, 100, 237 acetic chloride, 50 bagasse, 167

[Acetylation] board, 167, 306 bromoacetylcellulose, 108 carboxymethylcellulose, 110 cellulose, 35, 54, 100, 101 fiber, 167, 237, 307 hydrolyzed cellulose acetate, 54 laminate, 305, 306 4-0-methyl I3-D-xylopyranoside, 50 perchloric acid, 21 phenolic hydroxyl groups, 79 veneers, 167, 305 wood, 166, 167, 175, 186,286 Acid hydrolysis, 16, 20, 65, 66, 79, 80, 131 Adhesion, 173, 175, 192, 193, 218, 219, 220, 304, 305 Adhesive, 79 cellulose ethers, 102 graft copolymer, 120 hot-melt, 213, 218 isocyanate, 201, 304 lignin, 133, 135, 136, 153, 233 liquefied wood, 9, 186 phenolic, 305 phenol-formaldehyde, 304, 306

363

Index

364 [Adhesive] phenol-methanal, 133, 136,219 phenol-resorcinol-formaldehyde, 304, 305 polyurethane, 304, 305 polyvinyl acetate, 304 resol, 192 resorcinol, 304, 305 thermoplastic, 304 thermoset, 304 urea-fonnaldehyde, 357 wood-based, 207 Agrofiber, 232, 237 Air pollutant, 4, 278, 282 Alkylation,S, 12,58,76,77,113 Alkaline hydrolysis, 44, 49, 66 Allylated wood, 174, 186, 201, 203, 204,207,210,211,212,214 American Society of Testing and Materials (ASTM), 300, 382 Amorphous,S, 36, 37, 39, 51, 54, 133, 144, 249 cellulose, 16, 39, 46, 64 hemicelluloses, 65 hydrocellulose, 41, 43, 49 lignin, 68, 206 plant tissue, 64 xylan, 36 Anticrcep efficiency, 316, 318 Antioxidant, 129, 279 Antishrink efficiency (ASE), 161, 282, 302,316,332 acetylation, 166 acrylonitrile, 335 cellulose, 161 dimcthyldihydroxyethylcneurea, 164, 340 formaldehyde, 336 glutaraldehyde, 164, 340 glyoxal, 164, 340 lignin, 161 maleic anhydride, 169 oligoesters, 172 phenol fonnaldehyde, 349

[Antishrink efficiency] phthalylation, 169 propylene oxide, 318 tetraoxane, 164 wood, 163 Arabinose, 18, 19, 66 ASE (see Antishrink efficiency) ASTM (see American Society of Testing and Materials) Bagasse, 97, 137, 167, 187,237 Bamboo, 97, 218, 232, 237 Benzylation, 160, 174,207,218,225 benzyl chloride, 207 thermoplasticization, 173, 207, 209, 213, 303 vapor phase, 161, 223 wood, 160, 161, 173, 209 Biodegradability, 7, 230, 243 Biodegradation, 149 cellulose, 233 test, 149 Biological resistance, 167, 197,201, 207, 236, 331, 332, 339, 340, 343, 344, 345, 349, 353, 357, 358 Biomass, 2, 64, 129, 131, 132, 138, 185, 186, 188 delignification, 77 liquified, 188, 189 utilization, 186 Bleaching, 17, 18,28,67 Brown rot, 150 Coniophora puteana, 337 G. lrabeum, 335 Gleophyllum trabeum, 150 L. lepideus, 335 Poria monticola, 337 Serpula lacrymans, 333 Tyromyces palustris, 163, 220, 332, 333, 338, 354 Building materials, 1,3,4, 102, 139, 354 shrine, 354 temple, 4

Index Carbonyl, 5, 77, 81, 199, 201, 255, 280,289 a-carbonyl, 77, 79, 81 dicarbonyl, 63 Carboxycellulose, 103, 105 6-carboxycellulose, 103 2,3-dicarboxycellulose, 103, 104 Carboxyl, 5, 103, 104, 108, 169, 170, 172,176,177,199,201,202, 254, 255, 256, 258, 260, 264, 267,289 carboxylated wood, 266, 268 Carboxylic acid, 69, 109, 155, 168, 239,280 dicarboxylic acid, 169 Carboxymethylated wood, 186, 260, 261,263,265,266,267,268, 269,271 Carboxymethylation, 12, 58, 61, 67, 78,260,265 Cellulolytic enzyme lignin, 22, 334 Cellulose I (see Crystal structure) Cellulose II (see Crystal structure) Cellulose III (see Crystal structure) Cellulose IV (see Crystal structure) Cellulose acetate, 54,100,101,106, 107, 109, 110, 119, 154 Cellulose carbamate, 106 Cellulose phosphate, 107 Cellulose sulfate, 105, 106 Chromium, 284, 285, 286, 291 Color, 3, 133, 151, 197,206,209, 222, 233, 236, 277, 278, 279, 288, 296, 300 (see also Discoloration) Color measurement, 288 Color stabilization, 286 Condensation diisocyanotoluene, 106 grafting, 103, I 17 lignin, 21, 68, 69, 70, 72, 74, 76, 79, 80, 131 polysaccharides, 109 wood,201

365 Copolymerization, 35, 79, 139, 155, 174,201 block, 116, 117 condensation, 117 graft, 5, 102, 103, 107, 116, 117, 118, 119, 139, 141, 142, 144, 149, 154, 170, 174 ionic, 117, 150 radical, 117, 139 Crosslinking, 5, 79, 80, 102, 134, 136, 164, 174, 176, 177, 191,201, 236, 242, 264, 267, 298, 301, 312, 326, 328, 339 Crosslinking density, 177, 256 Crosslinking unit, 300 Crystal structure, II, 12, 13 cellulose, II, 12, 13 cellulose I, 12,38,39,47,49 cellulose la, 13 cellulose Ib, 13 cellulose II, 13, 15, 38, 40, 43, 47, 49,51,61 Crystallinity, II, 12,35,37,61 cellulose, 35, 37, 57 cotton, 37, 64 density measurement, 37, 38 Crystallite, 38, 39,41,47,52 Cyanocthylation, 162, 174, 175,207, 209 Decay, 4, 132 Decay resistance, 161, 162, 172,220, 221, 283, 339 acetylation, 166, 207, 332, 334 acrylonitrile, 335 aldehyde, 341 allyl isocyanate, 334 bcnzylated board, 221 cyanoethylation, 162 epichlorohydrin, 336 formalization, 337, 341 glyoxal, 342 isothiocyanate, 335 methyl isocyanate, 334

366

Index

(Decay resistance I propiolactone, 335 treated with tetraoxane, 163 treated with trioxane, 163 treated with propylene oxide, 165 Decrystallization, 151 cellulose 15, 295, 302 cell wall polymer, 307 wood, 168, 174 Degree of polymerization (DP). 4, 279 cellulose, 16, 17,44, 107 glucomannan, 19 level-off degree of polymerization (LODP). 46 Degree of substitution, 54, 100, 101,

303, 333, 335 Delignification, 28, 64, 67, 68, 72, 76,

80,278 Dcoxycellulose, 114 aminodeoxycellulose, 115 halodeoxycellulose, 114, 115, 116 iododeoxycellulose, 116 Dialdehyde, 61, 103, 106, 118, 340 Differential scanning calorimeter (DSC),

170, 194 Dimensional stability, 7,159,161,197,

220, 282, 296, 358 acetylation, 166 benzylated boards, 222, 225 boards, 207 cyanocthylation, 162 esterified wood, 170 fiberboard, 163 formalization, 163 wood products, 286, 287 Dimethylacetamide (DMAC), 54, 100,

101,109 Discoloration, 4, 280, 285 (see also Color) Dispersion, 248, 249, 250, 252, 269,

270,271 ex dispersion, 249, 250, 251 (3 dispersion, 249, 250, 251, 252,

269,270

DMAC (see Dimethylacetamide) DMTA (see Dynamic mechanical thermal analyzer) DP (see Degree of polymerization) DS (see Degree of substitution) DSC (see Differential scanning calorimeter) Dynamic loss, 248, 249, 250, 252 Dynamic mechanical thermal analyzer (DMTA), 169, 174, 175 Dynamic shear modulus, 269 Dynamic specific modulus, 311, 319 Dynamic Young's modulus, 167, 265,

319, 324, 326 Electron spectroscopy for chemical analysis (ESCA), 160, 174, 199,

204,213,281,288,289 Electron spin resonance (ESR), 278,

280, 282, 291 Engineering material, 2, 3, 9, 229 Enzymes, 106, 131, 150,233,236,

331, 334, 339, 358 ESCA (see Electron spectroscopy for chemical analysis) Ester, 5, 67, 101, 105, 106, 107, 168,

170, 236, 332 alkoxylignin esters, 155 cellulose aminobenzoyl ester, 108 cellulose ester, 101, 105, 107, 108 ccllulose methanesulfonic esters, 108 cellulose phosphate esters, 107 cellulose sulfate ester, 105 cellulose toluenesulfonic esters, 108 linkage, 17, 24, 102 mixcd esters, 10 I, 106, 107 oligocster, 170, 172, 177 oligocsterified wood, 171, 172, 175,

177,202 wood esters, 168, 202, 248 Esterification, 5, 7, II, 35, 49, 50, 54,

79, 98, 101, 105, 166, 169, 207,247,301,332 cellulose, 54, 108

Index

IEsteri fication) wood, 170 Ether,S, 55, 131 alkyl ether, 72 allyl ethers, 164 allylglycidyl ether, 170 aminobenzyloxymethyl ether, III aryl ether, 70, 72, 73, 75, 78 carboxymethyl ether, 67 cellulose aniline ether, 110 cellulose ethers, 101, 109, 110, 113 cellulose trimethyl ether, 61 linkage, 17,24,26,29,69,75,77, 236 Etherification,5,7, 11,35,55,57,61, 67,77,98, 108, 160,207,247, 314,335 cellulose, 55, 58, 207 cotton, 58 surface, 174, 207 wood, 165, 207, 314 Fiberboard (see Forest products) Flakeboard (see Forest products) Foam, 153, 154, 186, 191, 192, 193 Forest, I, 2, 3 Forest products, 2, 132 fiberboard, 137, 163, 167, 170, 177, 218,229,288,306,307,354, 357 (see also Wood products) flakeboard,4, 165, 167, 306, 354 particleboard,4,161,167,172,173, 177,201,219,222,223,225, 290, 354 plywood,4, 133, 136, 153, 192, 201,283,351 strandboard , 4 Formalization, 163, 164,314,317, 336, 337, 338, 339, 341, 342, 357 Formylation, 50, 51, 52 Free radical, 282 chain reaction, 141 ESR (see Electron spin resonance)

367 [Free radical) grafting, 139, 234 lignin, 133, 141, 142, 153 quenchers, 285 radiation, 141 wood, 278, 280 Free volume, 248, 251, 252, 254 FTIR (see Fourier transform infrared spectroscopy) Functional groups, 4, 5, II, 38, 40, 49, 114, 120, 131, 134, 141, 159, 201,202,206,207,234 Fungi 4, 221, 277, 284, 331, 339 (see also Brown rot, Soft rot, White rot) Gas chromatography, 18, 287 Gas chromatography-mass spectrometry, 29,30 GC (see Gas chromatography) GC-MS (see Gas chromatography-mass spectrometry ) Gel permeation chromatographic analysis, 107 Glucosidic bond, II, 12 Glycosidic bond, 98, 236 Hardness, 20, 162, 177, 179, 197,209, 211, 296, 30 I, 302 Hemiacetal, 40 Hydrocellulose, 38, 41, 43, 49,57, 61 Hydrogen bond, 36, 39, 51, 65, 68, 99, 206,312,345 intermolecular, II, 12, 68, 98 intramolecular, 11, 12,40, 68 Hydroxyalk ylation, 5, 12, 77 Hygroscopicity, 169,237,285,311 Infrared spectroscopy, 104,209,255, 260 Fourier transform infrared spectroscopy (FTIR), 278, 280, 282, 285, 289, 290

368 Ion chromatographic analysis, 353 IR (see Infrared spectroscopy) Jute, 97, 231, 232, 237, 240 Kenaf, 97, 231, 232, 237, 240, 241, 242 Klason, 20, 21, 22, 29 LCC (see Lignin-carbohydrate complex) Lignin alkali lignin, 69, 79, 131 kraft lignin, 69,76,78,79,80, 131, 134, 135, 139 Lignin-carbohydrate complex, 24, 36, 71 Lignin structure guaiacyl, 22, 24, 27, 28, 29, 71, 72, 74, 79, 80, 334 heterogeneity, 27, 28, 29 syringyl, 27, 28, 29, 71, 107, 117, 118, 139, 149, 154, 174 LOD? (see Degree of polymerization) Loss tangent, 269, 311, 319, 322 Maleic anhydride, 169, 241, 288, 303 Mannan, 18, 19, 55, 65, 66, 287 Mannose, 17, 18, 19,50,64 Mechanical, properties 3, 152, 163, 201,219,254,295,297,302, 311 acetylation, 167 benzylated wood, 225 butylene oxide treated wood, 165 hot-melt, 223 moisture content, 296, 300 propylene oxide treated wood, 165, 167,302 Mercerized cellulose, 39, 43, 49, 61 Microfibril, 12, 13, 17,36,43,311, 322 specific gravity, 324 Young's modulus, 324

Index Middle lamella, 19, 22, 24, 28, 29, 278, 279, 290 Milled wood lignin, 21 preparation, 131 Model compound, 28, 68, 75, 78, 79, 80, 81, 284 Molecular weight, 24, 69, 206, 254 cellulose, 17, 233 cotton, 40 lignin, 69, 138 lignosulfonate, 69,131 phenol-formaldehyde, 315, 317, 328, 345, 349 polyethylene glycol, 315 MWL (see Milled wood lignin) Nitration, 17,35,50,51,52,54, 100, 101 Nuclear magnetic resonance (NMR), 27,52, 104 13C, 12, 52 tH,26 Oxidation, 5, 21, 24, 40, 61, 98,118 cellulose, 61, 62, 64 cyanorluinone, 24 glycoside, 63 HCI0 2 , 103 nitric acid, 20 1 nitrobenzene, 29 nitrogen dioxide, 63, 103 periodate, 61, 64, 79, 80, 103 periodic acid, 103 photo, 280 products, 29 surface, 199, 201, 278 wood,284 Oxycellulose, 103 3-oxycellulose, 105 Ozonolysis, 26, 27 Particleboard (see Forest products) Peeling, 42, 43, 49, 66, 222 PEG, 315, 285

Index Periodate lignin, 22, 24, 68 Photochemical reactions, 100 bond cleavage, 137 initiation, 141 lignocellulosics, 233 Photodegradation, 137, 138, 173, 279, 284, 289 lignocellulosics, 233 photodegradable, 193 robinia, 281 poplar, 281 wood,278 Photoirradiation, 278 Photooxidation, 174, 278, 280 Photostabilizer, 137, 153 Phthalic anhydride, 107, 169, 202, 303 Plastics, 8,98, 146, 151,207,225, 285 biodegradable, 145, 151 cellulose acetate, 107 cellulose esters, 107 industry, 240 market, 155 Plasticizers, 173, 174, 176, 177, 206, 328 Plywood (see Forest products) Preservation, 331, 332, 343, 349 Pyrolysis, 132, 230 biomass, 138 cell wall, 234 cresylic acid, 154 ethoxylated lignin, 154 wood, 132 Pyrolysis gas chromatography-mass spectrometry, 29, 30 Rayon,39,50,57,108,118 Reducing endgroup, 40, 41, 42, 43, 48, 49 Reduction, 230 aldehyde, 22 sodium borohydride, 27, 40. 41, 66 thiosulfate, 109 Renewable, I, 2, resources, 4, 9, 247

369 Scanning electron microscope (SEM), 118,218,279,288. 290, 345 SEC (see Size exclusion chromatography) SEM (see Scanning electron microscopy) Size exclusion chromatography, 17 Softr~, 221, 346, 347 Chaetomium g/obosum, 354 Solubility, 39, 247 alkali, 39 cellulose, 40 chlorinated wood, 187 cyanoethylated wood, 162 galactoglucomannans, 65 Hildebra's solubility parameter, 69 lignin, 68, 69 liquified wood, 189 Stiffness, 20, 296, 300, 307 Strandboard (see Forest products) Succinic anhydride, 120, 169, 242, 303 Sulfonation, 80, 81, 108 Sunlight, 4, 230, 277, 283, 288

Tan & (see Loss tangent) Termite, 172, 230, 331, 336 Coptotermes formosan, 354 Coptotermes formosanus, 336, 339, 342 R. jiavipes, 336 Termite resistance, 163 acetylation, 166, 334 barium sulfate, 351 calcium compounds, 352 phenol formaldehyde, 344, 349 Termiticides, 334, 335 Thermoplasticity, 145, 206, 247 benzylation, 213 esterified wood, 303 graft copolymers, 145 fiber, 176,218 lignin, 206 wood,206

370 Thermoplasticization bcnzylated wood, 206, 209, 210, 214,216 etherification, 207, 209 wood, 160, 162, 173, 199,207 wood surface, 7, 205, 207, 226 Thermoplasticized wood, 9, 159, 288, 303 Thermoplastics, 101, 142, 143, 145, 151, 175,302 biodegradable, 155 lignin, 155 properties, 118, 174 Trifluoroacetic acid, 104, 248, 254, 303 Uronic acid, 18, 19,24,63,65,66 UV absorber, 129, 201, 285 deacetylation, 289 degradable film, 211 degradation, 279, 284 irradiation, 214, 279, 285 light, 210, 222, 279, 280, 282, 283, 286, 287, 289 microscopy, 28 radiation, 118 resistance, 210, 211 spectroscopy, 278 wavelength, 153 Viscoelasticity, 251 dynamic, 194 measurement, 194,248,251,269 wood, 168 Viscoelastic properties, 8, 169, 248, 311, 312 (see also Dynamic specific modulus, Loss Tangent) esterified wood, 248 Water absorption, 167, 172,209 Water repellency, 107, 197,280,282, 284

Index Weathering, 4, 172, 174, 230, 234, 236, 277, 282, 283 artificial, 281, 285, 286, 287, 288, 289,290 cellulose, 100 hardwood, 280 natural, 278, 282, 288, 290 pine, 279, 280 poplar, 281 process, 277, 279, 280, 281, 282, 285 protection, 279, 287 reaction, 279 resistance, 197, 285 tests, 175 wood, 7, 277, 279, 284 Weight percent gain, 203, 210, 212, 217,237,287,314,332 Wettability, 197, 199,201,211,280 White rot, 150 Basidiomycetes, 149 Coriolus versicolor, 163, 220, 332, 337, 338, 354 Pleurotus ostreatus, 150 Phanerochaete chrysosporiurn, 150 Trametes versicolor, 150 Wood products, 1, 2, 5, 282, 286, 289, 332. 354, (see also Forest products) industry, 2, market, 282 reconstituted, 354, 358 weather-resistant, 291 WPG (see Weight percent gain) Xanthation, 52, 54, 101 X-ray diffraction, 12, 13, 37, 38, 353 cellulose I, 12 cellulose II, 13 cellulose III, 13 cellulose IV, 13 Xylan, 18, 19,24,36,65,66,67,287 Xylose, 17, 18, 19,24,64,66,67