Biodegradable polymers for industrial applications

Biodegradable polymers for industrial applications

Related titles from Woodhead's materials engineering list: Green composites ± polymer composites and the environment (ISBN 1 85573 739 6) Life cycle assessment is of paramount importance at every stage of a product's life, from initial synthesis through to final disposal, and a sustainable society needs environmentally safe materials and processing methods. With an internationally recognised team of authors, Green composites examines polymer composite production and explains how environmental footprints can be diminished at every stage of the life cycle. Green composites is an essential guide for agricultural crop producers, government agricultural departments, automotive companies, composites producers and material scientists all dedicated to the promotion and practice of eco-friendly materials and production methods. Recent advances in environmentally compatible polymers (ISBN 1 85573 545 8) Based on the proceedings of the eleventh international Cellucon conference held in Tsukuba, Japan, this book offers a comprehensive overview of recent research undertaken into all aspects of environmentally compatible polymers. It deals with natural and synthetic polymer materials such as gels, fibres, pulp and paper, films, foams, blends and composites and shows how environmental compatibility such as biodegradability and recyclability can de developed by utilising natural polymers such as polysaccharides and polyphenols. Environmental impact of textiles (ISBN 1 85573 541 5) This comprehensive book examines the effects that textile production and use have on the environment. It looks at the physical environment affected by these processes, including resource depletion, pollution and energy use and the biological environment, by considering what happens as a result of manufacture. It also considers the degradation suffered by textile materials within the environment whether by air pollution, wind, water and other agents. The most recent solutions adopted by the industry are considered and an analysis offered of the likely effectiveness of these strategies. It is an essential reference for anyone concerned with the environmental footprint of the global textile industry. Bast and other plant fibres (ISBN 1 85573 684 5) Environmental concerns have regenerated interest in the use of natural fibres for a much wider variety of products, including high-tech applications such as geotextiles, and in composite materials for automotive and light industry use. This new study covers: the chemical and physical structure of these natural fibres; fibre, yarn and fabric production; dyeing; handle and wear characteristics; economics; environmental and health and safety issues. Details of these books and a complete list of Woodhead's materials engineering and textile technology titles can be obtained by: · visiting our web site at www.woodheadpublishing.com · contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 893694; tel.: +44 (0) 1223 891358 ext. 30; address: Woodhead Publishing Limited, Abington Hall, Abington, Cambridge CB1 6AH, England) If you would like to receive information on forthcoming titles in this area, please send your address details to: Francis Dodds (address, tel. and fax as above; e-mail: [email protected]). Please confirm which subject areas you are interested in.

Biodegradable polymers for industrial applications Edited by Ray Smith

Published by Woodhead Publishing Limited Abington Hall, Abington Cambridge CB1 6AH England www.woodheadpublishing.com Published in North America by CRC Press LLC 2000 Corporate Blvd, NW Boca Raton FL 33431 USA First published 2005, Woodhead Publishing Limited and CRC Press LLC ß 2005, Woodhead Publishing Limited The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. 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 information storage or retrieval system, without permission in writing from the publishers. The consent of Woodhead Publishing Limited and CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited or CRC Press LLC for such copying. Trademark notice: product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress: Woodhead Publishing Limited ISBN 1 85573 934 8 CRC Press ISBN 0-8493-3466-7 CRC Press order number: WP3466 The publishers' policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elementary chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Project managed by Macfarlane Production Services, Markyate, Hertfordshire ([email protected]) Typeset by Godiva Publishing Services Ltd, Coventry, West Midlands Printed by TJ International Limited, Padstow, Cornwall, England

Contents

Contributor contact details

xiii

Part I Classification and development 1 1.1 1.2 1.3 1.4 1.5

2 2.1 2.2 2.3 2.4 2.5 2.6

3 3.1 3.2 3.3 3.4 3.5

Classification of biodegradable polymers

A - M C L A R I N V A L and J H A L L E U X , CRIF, Belgium

3

Introduction Biopolymers from natural origins Biopolymers from mineral origins Conclusions References

3 4 21 29 29

Polyhydroxyalkanoates

32

G G - Q C H E N , Tsinghua University, China

Introduction 32 Mechanical and thermal properties of PHA 37 Process development and scale up for microbial PHA production 42 Applications of PHA 48 Future developments 50 References 50

Oxo-biodegradable polyolefins

57

Introduction Polyolefin peroxidation Control of polyolefin lifetimes Oxidative degradation after use Aerobic biodegradation

57 58 62 63 66

D M W I L E S , Plastichem Consulting, Canada

vi

Contents

3.6 3.7 3.8 3.9

Applications of oxo-biodegradable polyolefins Environmental impact Future developments References

66 69 73 74

New developments in the synthesis of aliphatic polyesters by ring-opening polymerisation

77

4

R J E R O M E and P L E C O M T E , University of LieÁge, Belgium

4.1 4.2 4.3 4.4 4.5 4.6 4.7

5

5.1 5.2 5.3 5.4 5.5 5.6

6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

Introduction Synthesis of aliphatic polyesters by ring-opening polymerisation Reactive extrusion Supercritical carbon dioxide as a medium for the ring-opening polymerisation of lactones and lactides and a processing aid for aliphatic polyesters Future developments Acknowledgements Bibliography

91 101 102 102

Biodegradable polyesteramides

107

Introduction Poly(ester amide)s synthesis Polydepsipeptides Concluding comments Further information References

107 107 124 132 132 132

Thermoplastic starch biodegradable polymers

140

Introduction Properties of starch Thermoplastic starch and their blends Modified thermoplastic starch polymers Commercial applications and products for thermoplastic starch polymers Thermoplastic starch polymers ± looking beyond traditional polymer applications Future developments Further information

140 141 149 153

P A M L I P S and P J D I J K S T R A , University of Twente, The Netherlands

P J H A L L E Y , The University of Queensland, Australia

77 77 87

155 156 157 158

Contents 6.9 6.10

Acknowledgements References

vii 159 159

Part II Materials for production of biodegradable polymers 7 7.1 7.2 7.3 7.4 7.5 7.6

8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11

9

Biodegradable polymers from sugars

165

Introduction Biodegradable polymers obtained from monosaccharides and disaccharides Biodegradable polymers obtained from synthetic polysaccharides Biodegradable polymers obtained from natural polysaccharides Future developments ± `biodegradable' polymers obtained from hemicelluloses References

165

Biodegradable polymer composites from natural fibres

189

Introduction Natural fibres as polymer reinforcement Natural fibre-polyhydroxyalkanoate (PHA) composites Natural fibre-polylactide (PLA) composites Natural fibre-starch composites Natural fibre-soy resin composites Natural fibres in combination with synthetic biodegradable polymers Commercial developments Conclusion Further information References

189 190 191 198 203 208 210 211 213 213 214

Biodegradable polymers from renewable forest resources

219

A J V A R M A , National Chemical Laboratory, India

D P L A C K E T T , Risù National Laboratory, Denmark

T M K E E N A N , S W T A N E N B A U M and J P N A K A S , College of Environmental Science and Forestry Syracuse, USA

9.1 9.2

Lignocellulosic biomass as a renewable and value-added feedstock for biodegradable polymer production Cellulose: as a platform substrate for degradable polymer synthesis

166 173 178 180 184

219 223

viii

Contents

9.3 9.4 9.5 9.6

Hemicellulose and its application as a feedstock for biodegradable polymers Sources of further information Conclusions and future developments References

226 244 246 246

10

Poly(lactic acid)-based bioplastics

251

10.1 10.2 10.3 10.4 10.5 10.6 10.7

Introduction Properties of PLA Blends of PLA Plasticization of PLA-based bioplastics Aging and biodegradation Applications of PLA based bioplastics References

251 252 261 270 275 280 281

11

Biodegradable protein-nanoparticle composites

289

Introduction Delaminating clay using ultrasonics Processing protein-nanoparticle composites using extrusion Microstructure and mechanical properties of proteinnanoparticle composites Conclusion References

289 293 298

11.1 11.2 11.3 11.4 11.5 11.6

J - F Z H A N G and X S U N , Kansas State University, USA

K D E A N and L Y U , CSIRO ± Manufacturing and Infrastructure Technology, Australia

298 306 307

Part III Properties and mechanisms of degradation 12

Standards for environmentally biodegradable plastics

313

12.1 12.2 12.3 12.4 12.5 12.6

Why standards are necessary Bio-based polymers The post-use treatment of plastics for the recovery of value Mechanisms of polymer biodegradation Laboratory studies The development of national and international standards for biodegradable plastics Lessons from the past and future developments

313 316 317 319 322

12.7

G S C O T T , Aston University, UK

323 329

Contents 12.8 12.9

ix

Acknowledgements References

331 332

Material properties of biodegradable polymers

336

13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.10 13.11 13.12 13.13 13.14 13.15 13.16 13.17 13.18

Introduction Biodegradation Natural polymers Microbial polyesters Synthetic polyesters Poly-lactic acid Poly(glycolic) acid Polycaprolactone Poly(alkene succinate) Aliphatic-aromatic copolyesters Poly(orthoesters) Polyanhydrides Polycarbonates/polyiminocarbonates Blends Water-soluble polymers Future developments Acknowledgements References

336 337 340 341 343 343 345 345 345 346 346 347 347 347 348 349 352 352

14

Mechanism of biodegradation

357

Introduction Biodegradation Biodegradation Biodegradation Biodegradation Biodegradation Biodegradation Biodegradation Biodegradation Future trends Bibliography References

357 359 362 365 372 376 382 384 389 393 394 395

13

14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10 14.11 14.12

M B H A T T A C H A R Y A , University of Minnesota, USA, R L R E I S , V C O R R E L O and L B O E S E L , University of Minho, Portugal

S M A T S U M U R A , Keio University, Japan mechanism: overview mechanism of naturally occurring polymers mechanism of polyesters mechanism of polycarbonates and polyethers mechanism of poly(vinyl alcohol) mechanism of polyurethanes mechanism of poly(amino acid) mechanism of miscellaneous polymers

x

Contents

15

Enzymatic degradation of polymers

411

15.1 15.2 15.3 15.4 15.5 15.6

Introduction Vinyl polymers Hydrolyzable polymers Natural biodegradable polymers Conclusion References

411 414 419 423 427 428

G M A D R A S , Indian Institute of Science, India

Part IV Industrial applications 16

Oxo-biodegradable polyolefins in packaging

437

16.1 16.2 16.3 16.4 16.5 16.6 16.7

Introduction Characteristics of packaging plastics Oxo-biodegradable polyolefins Disposal Recovery Environmental impact References

437 439 440 444 447 448 449

17

Biodegradable plastics in agriculture

451

17.1 17.2 17.3 17.4 17.5 17.6

Plasticulture Oxo-biodegradation of polyolefins in the environment The impact of degradable plastics on the environment Future developments Acknowledgements References

451 464 466 470 471 471

18

Generation of biodegradable polycaprolactone foams in supercritical carbon dioxide

474

Introduction Generation of polycaprolactone foams Effect of processing conditions on the foaming cell Crystallinity of foamed polycaprolactone Conclusion References

474 477 480 488 490 491

D M W I L E S , Plastichem Consulting, Canada

G S C O T T , Aston University, UK

L Y U and K D E A N , CSIRO ± Manufacturing and Infrastructure Technology, Australia and Q X U , Zhengshou University, China 18.1 18.2 18.3 18.4 18.5 18.6

Contents

19

19.1 19.2 19.3 19.4 19.5 19.6 19.7

xi

Biodegradable polymers in agricultural applications

494

Introduction Materials applied in agriculture Evaluating properties of biodegradable materials in agriculture Market issues Conclusion Further information References

494 495 501 510 513 514 515

Index

517

S G U I L B E R T , ENSA.M, INRA, France, P F E U I L L O L E Y , CEMAGREF, France, H B E W A , ADEME, France and V B E L L O N - M A U R E L , CEMAGREF, France

Contributor contact details

Introduction Dr Ray Smith Department of Materials Queen Mary and Westfield College University of London Mile End Road London E1 4NS UK Tel: +44 (0) 207 882 5164 Fax: +44 (0) 208 981 9804 Email: [email protected] Chapter 1 Ir Anne-Marie Clarinval CRIF Charleroi Avenue Georges LemaõÃtre, 22 6041 Gosselies Belgium Tel: 32 71 250 363 Fax: 32 71 250 398 Email: [email protected] Jacques Halleux CRIF LieÁge Science Park Rue du Bois Saint-Jean, 12 B4102 Seraing Belgium

Tel: 32 4 3618700 Fax: 32 4 3618702 Email: [email protected] Chapter 2 Dr George Guo-Qiang Chen Professor of Microbiology Department of Biological Sciences and Biotechnology Tsinghua university Beijing 100084 China Email: [email protected] Chapters 3 and 16 Dr David M. Wiles Plastichem Consulting Victoria, British Columbia Canada V8N 5W9 Email: [email protected] Chapter 4 Professor R. Jerome and Dr P. Lecomte Center for Education and Research on Macromolecules (CERM)

xiv

Contributor contact details

University of LieÁge, Sart-Tilman, B6a 4000 LieÁge Belgium Email: [email protected] [email protected] Chapter 5 Dr Pieter J. Dijkstra and Dr Priscilla A. M. Lips University of Twente Faculty of Science and Technology Department of Polymer Chemistry and Biomaterials and Institute for Biomedical Technology P.O. Box 217 7500 AE Enschede The Netherlands Tel: 31-53-4893004 Email: [email protected] [email protected] Chapter 6 Dr Peter Halley Director ± Centre High Performance Polymers Division of Chemical Engineering School of Engineering The University of Queensland QLD 4072 Australia Tel: +61-7-3365-4158 Fax: +61-7-3365-4199 Email: [email protected]

Chapter 7 Dr A. J. Varma Deputy Director Chemical Engineering Division National Chemical Laboratory Pune-411008 India Tel: +91-20-25893300 Email: [email protected] Chapter 8 Dr David Plackett Senior Scientist Danish Polymer Centre Risù National Laboratory 4000 Roskilde Denmark Tel: +45-4677-5487 Fax: +45-4677-4791 Email: [email protected] Chapter 9 T. M. Keenan, S. W. Tanenbaum and J. P. Nakas State University of New York, College of Environmental Science and Forestry Dept of Environmental and Forest Biology 1 Forestry Drive Syracuse, New York 13210 USA Tel: 315-470-6769 Email: [email protected]

Contributor contact details Chapter 10 Dr Xiuzhi Susan Sun Professor Department of Grain Science and Industry Kansas State University Manhattan, KS 66506 USA

Chapter 13 Professor Mrinal Bhattacharya Department of Biosystems and Agricultural Engineering University of Minnesota 1390 Eckles Avenue St. Paul, MN 55108 USA

Tel: 785-532-4077 Email: [email protected]

Email: [email protected]

Dr Jian-Feng Zhang Research Associate Department of Grain Science and Industry Kansas State University Manhattan, KS 66506 USA Tel: 785-532-4810 Email: [email protected] Chapters 11 and 18 Dr Long Yu FRACI Chem. CSIRO, CMIT Melbourne, Vic. 3168 Australia Email: [email protected] Chapters 12 and 17 Professor Gerald Scott Professor Emeritus in Chemistry and Polymer Science Aston University Aston Triangle Birmingham B4 7ET UK Fax: +44 (0) 121 359 4094 Email: [email protected]

xv

Rui L. Reis Associate Professor 3B's Research Group Dept. of Polymer Engineering University of Minho Campus de Gualtar, 4710-057 Braga Portugal Tel: +351-253-604498 (Direct) or +351-253-604782/1 (Personal Assistant) Email: [email protected] Luciano F. Boesel 3B's Research Group Dept. of Polymer Engineering Univ. of Minho Campus de Azurem 4800-058 Guimaraes Portugal Tel: + 351-253510395 Email: [email protected] Chapter 14 Professor Shuichi Matsumura Department of Applied Chemistry Faculty of Science and Technology Keio University 3-14-1, Hiyoshi, Kohoku-ku Yokohama-shi, 223-8522 Japan Email: [email protected]

xvi

Contributor contact details

Chapter 15 Dr Giridhar Madras Department of Chemical Engineering Indian Institute of Science Bangalore 560 012 India

H. Bewa ADEME (Biocombustibles, biomateÂriaux) 2 Square La Fayette ± BP 90406 F-49004 Angers Cedex 1 France

Tel: (91-80) 22932321 Fax: (91-80) 23600683 Email: [email protected]

Email: [email protected]

Chapter 19 Professor S. Guilbert Directeur de L'Unite Mixte de Recherche IngeÂnierie des AgropolymeÂres et Technologies Emergentes ENSA.M, INRA, UM II, CIRAD 2 Place P. Viala F 34060 Montpellier France Email: [email protected] P. Feuilloley Responsable Equipe BiodeÂgradabilite des MateÂriaux CEMAGREF 361 Rue Jean-FrancËois Breton ± BP 5095 F-34033 Montpellier Cedex 1 France Email: Pierre.feuilloley@ montpellier.cemagref.fr

V. Bellon-Maurel Responsable de l'Unite Mixte de Recherche `Information et Technologies pour les AgroproceÂdeÂs' CEMAGREF, ENSA.M, CIRAD 361 Rue Jean-FrancËois Breton ± BP 5095 F-34033 Montpellier Cedex 1 France Email: [email protected]

Part I

Classification and development

1

Classification of biodegradable polymers

A - M C L A R I N V A L and J H A L L E U X , CRIF, France

1.1

Introduction

It is not easy to decide how to classify biodegradable polymers. They can be sorted according to their chemical composition, synthesis method, processing method, economic importance, application, etc. Each of these classifications provides different and useful information. In the present overview, we have chosen to classify biodegradable polymers (hereafter called biopolymers) according to their origin into two groups: natural polymers, polymers coming from natural resources and synthetic polymers, polymers synthesised from crude oil. Biopolymers from natural origins include, from a chemical point of view, six sub-groups: 1. 2. 3. 4. 5. 6.

polysaccharides (e.g., starch, cellulose, lignin, chitin) proteins (e.g., gelatine, casein, wheat gluten, silk and wool) lipids (e.g., plant oils including castor oil and animal fats) polyesters produced by micro-organism or by plants (e.g., polyhydroxyalcanoates, poly-3-hydroxybutyrate) polyesters synthesised from bio-derived monomers (polylactic acid) a final group of miscellaneous polymers (natural rubbers, composites). Biopolymers from mineral origins include four sub-groups:

1. 2. 3. 4.

aliphatic polyesters (e.g., polyglycolic acid, polybutylene succinate, polycaprolactone) aromatic polyesters or blends of the two types (e.g., polybutylene succinate terephthalate) polyvinylalcohols modified polyolefins (polyethylene or polypropylene with specific agents sensitive to temperature or light).

We have chosen to classify polylactic acid in the category of biopolymers from natural origins because its monomer (lactic acid) is today largely produced

4

Biodegradable polymers for industrial applications

by fermentation. It is also mentioned in the category of the synthetic aliphatic polyesters because it can be synthesised from oil. There is finally a group of commercial biodegradable polymers, the blends of polymers from different origins. They have been formulated so that they offer interesting properties, limiting the amount of costly materials in their composition. In this family fall blends of starch with aliphatic polyesters, polylactic acid, polycaprolactone, polyvinylalcohol or cellulose acetate. The properties of starch (low cost material) are improved by the controlled addition of other more costly biopolymers. They will be described in section 1.2.1.

1.2

Biopolymers from natural origins

1.2.1 Polysaccharides Starch polymers Starch is a polymer of D-glucose organised in two major constituents of huge molecular weights: amylose and amylopectin. Amylose contains amorphous and crystalline regions. It forms a linear structure constituted by repeating units of 1-4-glucose (Fig. 1.1). Amylopectin is branched on amylose in starch (Fig. 1.2) (Moore and Saunders, 1997; Flieger et al., 2003). The natural crystalline structure of starch must be dismantled in order to produce a thermoplastic material. It is achieved by the application of heat, pressure, mechanical work or by addition of plasticisers such as glycerine, polyols or water.

1.1 Structure of amylose.

1.2 Structure of amylopectin.

Classification of biodegradable polymers

5

First generation of starch polymers Historically, this category was one of the first generation of biopolymers. To improve their resistance to shock and moisture, polyolefins were added in small quantities (about 10±15%) or in large proportions up to 85±95% to starch (Flieger et al., 2003). Those polymer mixtures disappeared during the biodegradation process leaving small fragments whose degradation time was a function of their carbon chain length. This type of product gave a very poor image to the first `biodegradable' polymers. Today most of them are not produced any longer but they gave birth to a new generation of blended plastics used for soil environment applications. They are composed of starch and polyolefin polymers including a catalyst. The catalyst improves the photo and thermo-oxidative degradation of the polyolefin phase (Bastioli, 2002; Arnaud et al., 1994; Scott, 1971). The first step of starch microbial degradation initiates further polyolefin degradation by increasing the porosity, the void formation and the loss of the plastic skeleton integrity. Currently, plastic films used in agricultural mulch are made with low-density polyethylene (LDPE) containing transition metal compounds soluble in the thermoplastics matrix (the catalyst) and about 6±15% of starch. However, the degradation duration is still high and can reach a few years for some of these products that do not respond to certain norms of biodegradability. Sometimes, a pre-treatment of the starch with a silane coupling agent is required in order to improve compatibility with the hydrophobic phase of the thermoplastic. This technology can also be applied to other matrixes such as PVC or polyester derivatives. Second generation of starch based polymers This second generation of polymers includes two kinds of products. The first one is produced from flour (flour biopolymer) and the second one is produced by plasticisation of starch with another biopolymer (Lourdin et al., 1999). The starch appears here more as filler. The flour biopolymers are made from rye, wheat or corn. They are generally cheaper than the second category and are suitable for use in catering (cutlery, forks, dishes, etc.). In this category, we can also classify the biopolymers produced from the whole plant (including starch, cellulose fibres, hemicellulose, lipids). Some commercial biopolymers in this category are listed below. Supol (Supol, Germany) Potato flour is submitted to a thermal treatment under pressure. Pellets can be injected to produce single-use dishes which are microwavable and which are compostable or can be added to animal food.

6

Biodegradable polymers for industrial applications

Evercorn (Cornstarch, Japan) Plasticised maize starch can be injected in order to make small parts for catering or for horticultural applications. This product is compatible with other biopolymers such as PHBV, PLA, PCL polyesters. VeÂgeÂmat (VeÂgeÂmat, France) This material, made with the complete corn plant, is relatively cheap (1 ¨/kg) (Forest, 2000). Grades have been developed for injection moulding and the maximum wall thickness is one millimetre. Without treatment this material is sensitive to humidity and is completely biodegraded after eight weeks. Paragon (Paragon Products BV, The Netherlands) This is a thermoplastic starch made from potatoes, wheat, maize or tapioca. The applications are found in food packaging, dog toys and veterinary accessories, and for injection moulding of complex parts. Clean Green Packing (Starchtech Inc., USA) This is soluble in water and is compostable. To compensate for the inconvenience of plastics made with pure starch, it can be chemically treated to improve its resistance to humidity. One treatment consists of the acetylation or of the esterification of the free hydroxide groups present in the chain by an anhydrous propionic acid. Another possibility is to add to the formulation of the polymer hydrophobic substances such as natural wax or biodegradable plastics which are not sensitive to humidity, however, all these treatments increase the production costs. Novon (Novon International, USA) was originally developed by Warner Lambert for the fabrication of pharmaceutical capsules, includes up to 80±90% of starch. Some grades are edible; others have been developed for thermoforming, sheet and film extrusion or for injection moulding applications. The second way to improve the mechanical properties of this cheap material (i.e., starch) is to blend it with a more expensive one that has better properties (e.g., polyesters, PVA, cellulose acetate). Some commercial blends are described below. Starch + PVA This blending category is commercialised mainly by Mater-bi (Novamont), Envirofil (Enpac), Greenfill (Green Light Products Ltd).

Classification of biodegradable polymers

7

Mater-Bi (Novamont, Italy) Mater-Bi is one of the main biopolymers commercialised in Europe (Bastioli, 2001). It is a copolymer of thermoplastic starch with natural plasticisers. Following grading, it can also contain cellulose derivatives or polyesters such as -caprolactone or ethylene vinyl alcohol. This family of materials is compostable. The main applications are for the production of mulch films, shopping bags, food packaging (yogurts), nappies and personal hygiene products (Facco and Bastioli, 2000). The film production capacity of Mater-Bi is about 20,000 tons/year (2003). In Europe, hundreds of cities use Mater-Bi bags for the collection of organic waste (Bastioli, 2002). Starch + aliphatic polyester Blends of biodegradable synthetic aliphatic polyesters and starch are used to produce sheets and films for packaging by film extrusion or blown film methods. Up to 50% of the synthetic polyester can be replaced with starch. A polyester synthesised from the poly-condensation of 1,4-butanediol and a mixture of adipic and succinic acids has been blended with wheat starch by Lim (1999) (Nolan-ITU Pty Ltd, 2002). The blends were found to have melting points near that of the polyester alone. Plasticisers were also added to the starch to improve flexibility and processability of the blend. The modified blends were found to retain a high tensile strength and elongation, even at high starch concentrations. Starch + PCL Blending starch with degradable synthetic aliphatic polyesters such as PCL has been studied. Biodegradable plastics can be prepared by blending up to 45% starch with degradable PCL. Due to a low melting point of 60 ëC and poor mechanical properties, the applications for starch±PCL blends are limited (Nolan-ITU Pty Ltd, 2002). Bioplast (Biotec, Germany) Bioplast grades are formulated for injection, blowing injection and flat extrusion. These grades are blends of starch and polycaprolactone. They are moisture sensitive. These blends have been developed for biodegradable film applications like lawn and leaf collection compost bags, agricultural mulch film, etc. The technology involves the following steps: · plasticisation of the starch using glycerol as plasticiser · polymerisation with -caprolactone directly in an extruder · compounding of the new, branched polymer by reactive blending with thermoplastic starch during the extrusion polymerisation operation

8

Biodegradable polymers for industrial applications

· preparation of compatibilised poly(-caprolactone)-thermoplastic starch blends. This new starch-PCL resin is being marketed under the name ENVAR for film applications like compost, trash and retail carrier bags. Properties are comparable to LDPE films and better than pure polycaprolactone film. Two other companies, Novamont (Italy), and Milleta (Biotech Division, Germany), manufacture and sell starch-PCL blends for film applications (e.g. compost bags, trash bags) (SINAS, undated). Starch + Poly(lactic acid) PLA (Ecostar (Novon)) Blends of PLA and 10±20% starch have been commercialised by Novon as additives for traditional thermoplastics in order to make them biodegradables (Flieger et al., 2003). Starch + PBS + PBSA (Bionolle (Showa)) To improve the interfacial adhesion between starch/biodegradable phases, the performance of two compatibilisers has been studied by Gormal (2002) to create a mechanically improved blend for food packaging film applications. Starch + cellulose acetate (Bioflex and some Bioplast (Biotec, Germany)) Bioflex is a blend of starch and cellulose acetate which is rapidly degraded by composting. It is mainly intended for the production of trash bags and films. The material is resistant to oils and greases and can be printed by flexography or by offset (without corona surface pre-treatment). BIOPLASTÕ GF 105/30 is a plasticiser-free thermoplastic material suitable for injection moulding as well as sheet film extrusion. Applications are short-life products, film coating for foamed starch and fibre trays and as a substitute for food wrapping paper, packaging, etc. (Biotec, undated).

Cellulose and cellulose derivative For industrial applications, cellulose comes mainly from wood and in small proportions from stalks of sugar cane bagasse (dry pulp after juice extraction in sugar cane). Raw cellulose is a cheap material costing 0.5±1 ¨/kg. The main uses of cellulose are for paper, membranes, dietary fibres, explosives and textiles. Figure 1.3 represents schematically the structure of cellulose. The strong glucosidic bonds ensure the stability of the cellulose in various media. Cellulose is generally insoluble and highly crystalline. Chemical reactions such as etherification and esterification are conducted on the free hydroxyl groups to

Classification of biodegradable polymers

9

1.3 Structure of cellulose.

improve its thermoplastic behaviour. Numerous derivatives are commercialised such as cellulose acetate, ethyl cellulose, hydroxyl ethyl cellulose, hydroxyl propyl cellulose, hydroxyl alkyl cellulose, carboxy methyl cellulose, fatty acid esters of cellulose (Chiellini et al., 2002). Bio-Compo (Mitsufuku, Japan) This material is made from cellulose powder and is suitable for thermoforming. The main applications are found in horticulture. Cellophane Cellophane films are obtained by dissolution of cellulose in a sodium hydroxide and carbon disulphide solution (Xanthation) and than by recasting in a sulphuric acid bath. The aspect is brilliant and transparent. Degradation takes place after six weeks of composting. Cellophane films are mainly used in food packaging where they are appreciated for their barrier properties against micro-organisms, gases and smells. The other main properties are resistance to infra-red light, oil, heat and transparency to the microwave. Labels are easy to stick on cellophane which is also printable. The cellulose (di/tri) acetates Cellulose acetate contains COCH3 radicals in place of free OH groups on sugar (Fig. 1.4) (Flieger et al., 2003). Cellulose acetate is mainly used in the synthesis of membranes for reverse osmosis. Bioceta (Mazzucchelli, Italy) Bioceta (developed by RhoÃne Poulenc) is a cellulose diacetate. It is produced from cotton linter or from wood pulp. The modified cellulose is mixed with a colourant, a stabiliser, a natural plasticiser catalysing the biodegradation. This product is transparent and can be injected, extruded or blown depending on the grade type. It can also be recycled or incinerated. The applications are

10

Biodegradable polymers for industrial applications

1.4 Structure of cellulose acetate.

packaging, flower pots, small objects (tooth brushes, etc.). Biocellat comes from the same family of material. EnviroPlastic Z (Planet Polymer Technologies, USA) This is made from modified cellulose acetate by using a high temperature process which improves the biodegradability of the material. The composting duration is low (about one or two years). This product can be injected or film extruded for packaging applications. Celgreen (Daicel Chemical Industries, Japan) Daicel commercialises various biopolymers using this label. The grade P-CA is produced with cellulose acetate. Lignin and wood powder blends Lignin is one of the main constituents of wood. It is a very stable and complex product, insoluble in water and resistant to a number of physical and chemical treatments. The composition of lignin slightly changes from one plant species to another and is a function of the growing conditions but it is always a threedimensional biopolymer composed of three different units of the phenyl propane family: p-hydroxy phenyl, Guaiacyl and syringic aldehydes (Fig. 1.5). These units are linked by aliphatic and aromatic carbon bonds and ether bonds. In wood, the lignin is closely associated with cellulose and bound to plant polysaccharides in order to form hemicellulose. This complex chemistry and polymer architecture is the reason why it is really difficult to isolate and to plasticise lignin by a cheap process (Chiellini et al., 2002). The usual source of commercial lignin is waste liquor from the wood pulp industry. It contains sodium ligninates or lignin sulfonates. Previously, liquefaction of lignocellulosic products was achieved using several hard treatments. One consisted of treatment at 320±400 ëC in aqueous or organic solvents (Widsten et al., 2002). A second treatment used an acidic catalyst

Classification of biodegradable polymers

11

1.5 Structure of main subunits of lignin.

solution at a temperature between 80±150 ëC. Today phenols can be used for the liquefaction of wood and lead to the production of thermosetting materials. Sulphuric, oxalic or phosphoric acids also enhance the liquefaction of wood. The derivative product is then a kind of novolac based resin which can be used in adhesives, mouldings or fibres. Sugar cane waste is another raw material that can be treated in a hot solution of concentrated acetic acid in hydrochloric acid solution. After re-concentration, the lignin is then precipitated in warm water and finally recovered by dissolution in acetone. Due to all this complex chemistry, the major commercialised `wood polymers' are blends. These plastics contain wood powder, starch or lignin. The presence of lignin as a filler in other polymers improves the quality of the biodegradation. Some of those products are reinforced with flax or hemp. Arboform (Tecnaro, Germany) Arboform is a thermally treated mixture of lignin, flax and hemp. This product can be injected and presents a good dimensional stability. Applications are found in car dashboard panels, computer or television frames, GSM housings. Fasal (IFA, Austria) Fasal products are made with wood waste, corn floor, natural resins and small quantities of a plasticiser, lubricants and a colourant. It can be processed by injection or extrusion without previous drying. The products look slightly like wood and can be milled, painted, or varnished in the same way as wood. Treeplast Treeplast is a product of the same kind developed thanks to a European CRAFT project and is still not commercialised (Eilbracht, 2001).

12

Biodegradable polymers for industrial applications

Lignopol (Borregaard LignoTech, Germany) Lignopol is a natural biodegradable composite blended with natural proteins, wood, lignin, and natural resins. It is in the form of pellets, which can be processed by extrusion or injection. Products look like wood and can be milled. Ecoplast (Groen Granulaat, Holland) This product is composed of wood powder, starch and a binder. It can be injected or thermoformed. Objects made from this material are composted in six weeks. Napac (Napac, Switzerland) Napac results from the transformation of Chinese reeds with a natural binder (starch and pine tree resin). These raw materials can be mixed with a colourant and extruded in pellets. The fibre concentration is around 70±75%. Pellets are then moulded by hot compression. This material is perfectly stable outdoors and is formulated to resist exposure to UV light. The applications are flower pots, CD boxes, interior car parts and non-food packaging. Finally, to complete the picture of natural biopolymers, one can mention the existence of materials produced by lignin/styrene copolymerisation and by lignin/methyl methacrylate copolymerisation. In both cases, the increase of lignin improves the biodegradation of the product by fungi. New research is being conducted into the idea of modifying lignin polymer using enzymes like peroxydase or laccase. The latter enzyme has now been commercialised by a Danish company, Novo Nordisk, and will certainly promote the commercial appearance of new lignin products. Chitin and chitosan Chitin is one of the most widespread polysaccharides in nature and is particularly abundant in the cell walls of insect cuticles, of many fungal species and of shellfish or mollusc exoskeletons. The chemical composition of chitin is based on the repetition of the unit (1-4) 2 acetamide-2-desoxy-D-glucose (or Nacetylglucosamine) (Flieger et al., 2003). Chitin is composed of a linear chain of acetylglucosamine groups (Fig. 1.6) (Lim and Hudson, 2003). Most chitins and derivatives are extracted from crab shells, lobsters and shrimps or from the waste of fungi fermentation (e.g., Aspergillus sp.) in concentrated NaOH solution. The swelling involves a modification of its natural crystal structure ( or ). After washing in water, the recovered  structure is chemically resistant due to the hydrogen bonds between the chains. The -chitin

Classification of biodegradable polymers

13

1.6 Structure of chitin.

is a better candidate to promote the production of derivative products like benzyl chitin or carboxymethyl chitin. It is also better adapted to be transformed by reactions such as acetylation, tosylation, tritylation and acetolysis. Chitosan is produced by the complete or partial elimination of acetyl groups (CH3-CO ± deacetylation) which are replaced by an amino-group (Fig. 1.7) (Rathke and Hudson, 1994). The properties of chitosan depend strongly on the molecular characteristics (molecular weight and degree of acetylation). Chitosan is soluble in water and in some organic solvents. The difference between chitin and chitosan is defined by their solubility in a dilute solution of weak acids. Chitosan dissolves in dilute acetic acid. It presents a unique combination of properties, brought about by its polysaccharide structure, large molecular weight, and a cationic character. Chitin and chitosan are biocompatible and present antithrombogenic and hemostatic properties. These polymers can be extruded to make films for packaging applications. They are edible and can be used in the agricultural (crop protection) and food sectors, and also in wastewater treatment, textiles or cosmetics and toiletries. They are also used for biomedical applications (biomedical devices, and drug delivery systems). Chitosan and its derivatives form air permeable films. This property facilitates cell regeneration when the films are used to protect tissues against microbiological attack. For this reason chitin and chitosan are also good candidates for artificial skin, and biodegradable sutures. Producers of chitine and chitosan will not be presented here because there are 63 main companies; 30 are located in Asia, 14 in the USA, 12 in Europe, 6 in Canada, and one in Russia.

1.7 Structure of chitosan.

14

Biodegradable polymers for industrial applications

1.2.2 Proteins For thousands of years people have been using natural proteins such as wool, silk and hair (-keratin) for clothes, adornment or to display their wealth. Proteins are natural chains of -amino acids joined by amide linkages. They are degraded by enzymes (proteases). The first industrial applications of protein as polymer were in the early 1930s and 1940s with casein and with soy protein. Even though protein biopolymers did not develop as quickly as starch derivates, they remained present in some niche markets such as encapsulates (pharmaceutical), coatings (food industry), adhesives or surfactants (Guilbert, 2002). They can be classified with animal proteins (casein, whey, keratin, collagen and gelatine) and in plant proteins (wheat, corn, soy, pea and potato proteins) (Chiellini et al., 2002). Collagen and gelatine Collagen and gelatine represent the most well-known animal polymers. Collagen is a relatively non-extensible protein presenting good stiffness. Gelatine derives from the physical and chemical denaturising of collagen. The good quality of gelatine depends on its high solubility in hot water, its polyampholite character and its intrinsic ability to form thermally reversible gels. Gelatine grades are also available in a wide range of viscosities. The classical applications are for the manufacturing of pharmaceutical products (drug caps), for X-rays, photographic film development and food processing. As a biocompatible material, gelatine displays several advantages. It does not show antigenity and is resorbable in vivo. Its physico-chemical properties can be suitably modulated. Gelatine can be plasticised thanks to the addition of water or of glycerol. There is, however, a limit to the use of this interesting material because there is a risk of viral animal contamination. Finally blends of polyvinyl alcohol and gelatine are the object of studies and researches. Casein Casein is a natural polymer extracted from skim milk proteins. It represents a small but important percentage of all the natural polymers used for the manufacturing of water-based adhesives. The casein formulations are highly soluble in alkaline solutions and in water. Casein polymers (modified or not) are mainly used in the manufacture of adhesives and the packaging industry for breweries, wineries and refrigerated products. Casein is also a binder for paints and an additive for adhesives formulations. It can also be used as a plasticiser for concrete. Beyer Richard (2002) demonstrated the feasibility of preparing casein polymer to make edible films and for food products containing this polymer.

Classification of biodegradable polymers

15

Wheat and corn gluten Polymers made from gluten are flexible, resistant, transparent, and completely biodegradable. They are thermoplastic and present a yellow or slightly brown look. They are relatively impermeable to oxygen and to CO2 but are sensitive to humidity and do not give protection against desiccation. Potential applications are the production of soluble pockets for the controlled release of a chemical product (e.g., toilet detergent). The world-wide production of wheat gluten is about 400,000 tons per year. Moreover, as an edible material, gluten is a good candidate for food packaging or single units of coffee or other food. Soy proteins Soy beans contain about 18% oil, 38% protein, 30% saccharides (15% soluble saccharides and 15% starch) and 14% moisture and ash. In 1940, Henry Ford presented a car body made from soybean-based materials. Soy proteins allow the development of various biodegradable materials. They are mainly formaldehyde-based thermoset composites. Water resistance can be improved by adding polyphosphate fillers (Otaigbe and Adams, 1997). Many applications have been developed thanks to its very high Young's modulus. A grade has also been formulated for medical applications. The plasticiser is the glycerol and aminopropyltriethoxi silane is used as coupling agent. In India, many studies have been undertaken into the production of coextruded films of soy proteins with an aliphatic polyester. The research goal is to decrease the brittle character of the material. The main commercialisation of soy protein plastics is done in the USA (Heartland, Resource Technologies, Iowa, Urethane Soy System Company, Illinois; and Dow Chemical with SoyOilÕ and BioBalanceÕ) (Flieger et al., 2003). Polypeptides of aspartic acid and lysine The wetting level of these polypeptide polymers in water is very high. They are now commercialised by Mitsui Chemical for horticultural applications.

1.2.3 Lipids Plant oils and animal fat are mostly unsatured fatty acids. Some of these oily products are already well known by the public from their use in paint (e.g., flax (linseed); tung oils are drying oils used in paints, varnishes and enamels) or in soaps, detergents, cosmetics and lubricant applications. Plant oils increasingly become a source of raw material to produce thermoset resins that can be mixed with natural fibres in order to achieve light and resistant composite materials. The combination of bio-based resins with natural fibres (plant and poultry) or

16

Biodegradable polymers for industrial applications

1.8 Structure of triglyceride in castor oil.

lignin, produce new low-cost composites that are economical in many highvolume applications (Beckwith, 2003). These composites are used in agricultural equipment, automotive sheet-moulding compounds (SMCs), civil and rail infrastructures, marine applications, housing and the construction industry (Wool, 2003). Plant oils represent about 80% of the worldwide lipid production. Soybean and palm oils are the most important ones but European oils (rapeseed, sunflower and linseed) contain more than 90% of unsatured fatty acid. The best candidates are triglycerides presenting a high level of unsaturation, and comprising active sites such as double bonds, allylic carbons, ester groups and carbons alpha to the ester group. By using the same synthetic techniques that have been applied in the synthesis of petrochemical-based polymers, these active sites can be used to introduce polymerisable groups on the triglyceride (Fig. 1.8). Castor oil contains ricinoleic acid presenting a hydroxyl group that allows polymer formation. This OH group participates in the formation of polyurethane and polyesters. Chemical functionalities such as aromatic or cyclic structures are introduced in the chemical structure of the triglyceride to improve stiffness in the polymer network. The material produced with this kind of resin and reinforced with fibres shows very high mechanical properties (e.g., tensile strength moduli of a resin reinforced with glass fibres can range up to 1±2 GPa) (Kuesefoglu et al., 2000). The chemistry of thermoset resins made from plant oils could be addressed in a separate chapter. In this overview we will simply mention that from plant triglycerides, it is possible to produce polyolefins, polyurethane, polyesters, polyethers or polyamide resins.

1.2.4 Polyesters directly produced by micro-organisms Some polyesters are synthesised by certain bacteria such as metabolites which in this way ensure energy stock. Certain bacteria can accumulate up to 80% of their dry weight before auto-hydrolysis (Braunegg, 2002). In this category are found polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), poly(hydroxybutyrate-hydroxyvalerate) (PHB/HV) and poly--caprolactones (PCL). These

Classification of biodegradable polymers

17

Table 1.1 Categories of polyesters Polymer/grade (company)

Radical

PHA PHB PHV PHBHx (Kaneda) PHBO (Nodax) PHBOd (Nodax)

ö CH3 ±CH2±CH3 ±CH2±CH2±CH3 CH3 or/and ±(CH2)4±CH3 CH3 or/and ±(CH2)14±CH3

polymers are polyesters. Table 1.1 shows the specified radical of their chemical structure (Moore and Saunders, 1997). Generally, a decrease of the length of the aliphatic chain causes a decrease of the melting and glass transition temperatures. These products are easier to process and are more flexible. Most of these polymers are biocompatible and bioresorbable. This is why numerous applications, generally patented, are in the medical or veterinary sector (implants, sutures). Nevertheless, certain companies have developed more usual products. Polyhydroxyalkanoate (PHA) Polyhydroxyalkanoate is a polyester identified in 1925 by the microbiologist Maurice Lemoigne. It can be synthesised by various bacteria (Alcaligenes Eutrophus, cyanobacteria). Lower concentrations of carbon, nitrogen and phosphorus sources increase the yield and the quality of the polymers produced (SteinbuÈchel, 2002). There are numerous potential applications for PHA (cosmetics containers, disposable articles, medical implants, paper coatings). Moreover, PHA can be formulated in many grades, from elastic products to crystalline ones, it is a good candidate for blends and easy to process with traditional equipment (Whitehouse, 2000). Polyhydroxybutyrate (PHB) PHB has a melting temperature (Tm) of 180 ëC, a glass transition temperature (Tg) of 5 ëC and a high molecular weight. It is naturally not crystalline, and is converted in a more crystalline form during the extraction process. Research has been undertaken to avoid this transformation step that causes a decrease in the mechanical properties. The properties of PHB are similar to those of polypropylene, except for its biodegradability. It is also more rigid, more brittle and denser than PP. It resists oxidation but presents low chemical resistance. PHB is insoluble in water and relatively resistant to hydrolysis, the opposite of most biopolymers.

18

Biodegradable polymers for industrial applications

Biomer (Biomer, Germany) This polyhydroxybutyrate is produced by Alcaligenes Latus. Pellets are commercialised for classical plastic transformation processes. The low viscosity of the melted polymers allows the injection of objects with thin walls. This product is very hard and can be used from ÿ30±120 ëC. Composting duration is about two months. Nodax (Kaneka, Japan) Previously, Nodax was a Procter & Gamble product. The originality of the grades is the variation in the nature of the radical of the ester. This company also produces PHA products. Genetically modified plants are also studied for producing PHB. Potential plants are watercress, colza (Arabidopsis thaliana), corn and tobacco but yields are very low, just a few percent of the total weight of the vegetable. Copolymers and blends of PHB/V/A The biodegradablility of blends of poly(3-hydroxybutyrate) with a copolymer of poly(3-hydroxybutyrate) and poly(3-hydroxyvalerate) is better than that of each component taken separately. This is an advantage found in many other blends of biodegradable plastics. The same fact is observed for blends of poly(3-hydroxybutyrate) and polyethylene glycol. This is due to the hydrophilic character of polyethylene glycol. Biopol (Metabolix, USA) The owner of this plastic material has changed many times, developed by ICI, bought by Zeneca, then by Monsanto. It is now the property of Metabolix. It is a copolymer of hydroxybutyrate and hydroxyvalerate. This thermoplastic is adapted for injection and blow moulding, fibre and film production. Adaptations of the product for foaming, laminating and thermoforming processes are in development. The antistatic properties of Biopol make it a good candidate for electric and electronic packaging applications. Despite its high degree of crystallinity, Biopol is sensitive to hydrolysis. Companies (e.g., Fluka and Toray Industries) are involved in the study of medical applications. Metabolix PHA (Metabolix, USA) This product is a blend of PHA, PHB and PHO (polyhydroxyoctanoate). This last material is an elastomere. Metabolix has transferred the coding genes in Escherichia coli K12 (agreed strain by the FDA) for the production of food

Classification of biodegradable polymers

19

additives. These cells can produce PHA in only 24 hours and can accumulate a polymer quantity equivalent to 90% of their own weight.

1.2.5 Polyesters synthesised from bio-derived monomers Polylactic acids or polylactide (PLA) Polylactic acids or Polylactide are terms used to indicate the same biodegradable aliphatic polyester (Moore and Saunders, 1997). The difference in terminology indicates simply the synthesis method chosen to produce the polymer from lactic acid. The interest in this material started in the 1930s with the work of Carothers but the molecular weight and the mechanical properties were weak. In 1954, DuPont patented a PLA presenting higher molecular weight and in 1972, the first co-polymers allowed the production of medical resorbable sutures. PLA comes from the esterification of lactic acid produced by fermentation. The micro-organism can be Lactobacilli, Pediococci or certain fungi such as Rhizopus Oryzae for example. Nowadays, the PLA cost decrease is due mainly to the improvement of the bacterial yield (DarteÂe, 2002). From lactic acid, there are two pathways to produce PLA. The first one has been developed by Mitsui Toatsu. First of all, the aqueous lactic acid solution is purified and concentrated. Then, the direct condensation and cyclisation reactions are performed at elevated temperatures in the presence of a catalyst. The condensate is removed by distillation. This process produces a polylactic acid of high molecular weight presenting a broad distribution. The second process is indirect. First, a lactide is produced from two lactic acid molecules by cyclisation and dimerisation. Lactide oligomers can then be polymerised in polylactide. The dimerisation step is a critical and more expensive pathway. The Cargill Dow process consists in producing a low molecular weight polylactic acid by the first process. Then, the PLA is depolymerised and converted in lactide which is transformed in PLA with a higher and more homogeneous molecular weight distribution. The properties of PLA change from one producer to another but general properties are resistance to fat, food oil, humidity, solvent and smells. Biodegradation occurs by composting (in 3±4 weeks). Some grades are really bright and transparent but are also more brittle. PLA can be processed by extrusion, thermoforming, injection, blow moulding, fibre spinning or stretching. It is printable and heat sealable. The actual or potential applications are found in the crop and food sectors (films, food packaging, soft drinks) and for non-woven materials in hygienic products. The properties of biocompatibility and of bioresorption of PLA permits the development of suture threads and clips, orthopaedic fixations (screws, pins) and of resorbable implants (Clarinval, 2002). Some of the main products are given below.

20

Biodegradable polymers for industrial applications

Lacea (Mitsui Toatsu Chemical, Japan) Different pellets exist for injection, moulding of foam products, blow moulding, thermoforming, extrusion and fibre spinning. Eco-Pla ± NatureWorks (Cargill Dow Polymers, USA) Materials are mainly dedicated to packaging applications ± films, thermoformed and injected products ± paper coatings. The degradation duration is about 4±6 weeks. Cargill has also developed fibres for clothes, hygiene products or carpets. With a capacity up to 140,000 tons/year (2003), Cargill Dow LLC is the biggest producer of PLA resin in the world (DarteÂe, 2002). Due to the closed properties and similar applications of the products from Cargill Dow LLC and Mitsui Chemicals, a research and commercial collaboration has been decided between both these giants of PLA production. Lacty (Shimadzu Corporation, Japan) Lacty produces PLA pellets for film and fibre extrusion. Lactron (Kanebo Goshen, Japan) Lactron are fibres dedicated to the production of nets used in agriculture or for fishing. It exists also as a non-woven product used in hygiene products and this is a medical grade. Solanyl (Rodenburg Biopolymers, Holland) The production capacity of Rodenburg Polymer is about 8,000 tonnes per year (2003) and 40,000 tonnes per year has been announced (2005). PLA is made here from potatoes. Grades for injection are commercially available and allow the injection of objects presenting thin walls (0.5 mm). Another application is for the release of fertiliser rods. Galactic (Belgium) Pellets are commercialised for the production of films and fibres. Galactic is involved in new application developments.

1.2.6 Miscellaneous natural polymers Natural rubber Since the 19th century, natural rubber latex has been well-known for its ageing behaviour. Chemists have mainly worked to decrease its mechanical alteration

Classification of biodegradable polymers

21

Table 1.2 Properties of natural fibres (Drzal et al., 2003) Fibre

Cotton Jute Flax Hemp Ramie/China grass Sisal Coir Abaca

Density (g/cm3)

Textile strength (MPa)

Young's modulus

Elongation at break (%)

1.5±1.6 1.3±1.45 1.40±1.50 1.48 1.50 1.33±1.45 1.25 1.50

290±700 395±773 345±1100 550±900 400±900 468±700 230 980

5.5±12.6 13±27 28±60 70 61.4±125.0 9.4±32.0 6.0 ö

3.0±10.0 1.16±1.80 2.7±3.2 1.6 1.2±3.8 2.0±7.0 15±25 ö

due to oxidation and eventual bioassimilation in soil or in a tropical environment. The most studied natural polymer is cis-polyisoprene produced by the rubber tree Hevea Braziliensis. Nowadays, this molecule is also synthesised by adding polymerisation from isoprene. The main research trends have been directed at inhibiting its degradation by adding, for example, aromatic amines, antioxidants or some other constituents during the vulcanisation process. Composite Blends of two or more biopolymers are not presented here, they are included in the paragraph describing the main constituent of their matrix. Starch or lignin are often added as a filler. Depending on their concentration, these blends are described in the section dedicated to starch/lignin or to their main matrix component. The main natural fibres that can be added to biopolymers are cotton, jute, flax, hemp, ramie/china grass, sisal, kenaf, kapok and abaca (Beckwith, 2003). Table 1.2 summarises the major properties of these fibres (Drzal et al., 2002)

1.3

Biopolymers from mineral origins

The polymers are divided in this section into four groups: aliphatic polyesters; aromatic polyesters; polyvinylalcohols and modified polyolefins. Polyesters represent a large family of polymers having in their structure the potentially hydrolysable ester bond (Fig. 1.9). The polyesters can be classified following the composition of their main chain. There are aliphatic and aromatic polyesters

1.9 Structure of ester bond.

22

Biodegradable polymers for industrial applications

1.10 Biodegradable polyesters.

(Fig. 1.10). In the family of aliphatic polyesters are found polymers of natural origin (PHA, PHB, PHV, PHH), mineral origin (PBS, PBSA, PCL) or those which originate from both (PLA and PGA). In the family of aromatic polyesters, those coming from PET or from PBT (PBST, PBAT, PTMAT) and copolymers are separately classified.

1.3.1 Aliphatic polyesters Aliphatic polyesters are generally sensitive to hydrolysis and are biodegradable (Gross and Bhanu, 2002). They are formed by the polycondensation reaction of an aliphatic glycol with an aliphatic dicarboxylic acid. Among the aliphatic polyesters there is a family of polymers, the poly(-hydroxy acids) such as polyglycolic acid (PGA), polylactic acid (PLA), and some of their copolymers, which have been used in a number of clinical applications; sutures, plates and fixtures for fracture fixation devices and scaffolds for cell transplantation. Commercially available aliphatic polyesters The structure of polyglycolic acid (PGA) is shown in Fig. 1.11.

1.11 Structure of polyglycolic acid.

Classification of biodegradable polymers

23

Dexon (American Cyanamide Corp) PGA is a rigid thermoplastic material with high crystallinity (46±50%) produced by ring opening of glycolide, a diester of glycolic acid. The glass transition is 36 ëC and the melting temperature is 225 ëC. PGA is not soluble in most organic solvents but has a high sensitivity to hydrolysis. It can be processed by extrusion, injection and compression moulding. The attractiveness of PGA as a biopolymer in medical applications is the fact that its degradation product (glycolic acid) is a natural metabolite. Polylactic acid (PLA) (Fig. 1.12) is produced by ring opening of the lactide, a diester of lactic acid that can be obtained from oil even though the natural origin is now largely known. PLA exists in three isomeric forms d(-), l(+) and racimic (d,l). Poly(l)LA and poly(d)LA are semi-crystalline solids having hydrolytic degradation rates similar to PGA. PLA is more hydrophobic and more resistant to hydrolysis than PGA. The (l) isomer of lactic acid (LA) is preferentially chosen because it is better metabolised in the body.

1.12 Structure of polylactic acid.

Copolymers of PGA and PLA PL(l)LA, poly(lactic-glycolic acid) (PLGA) copolymers and PGA have a FDA approval for human clinical use. For example, Vicryl (Ethicon Inc), is composed of 8% (l)LA and 92% GA. The main application of (d,l-LA/GA) copolymer has been in the field of controlled drug release. The structure of polycaprolactone (PCL) is shown in Fig. 1.13 (Gunatillake et al., 2003). PCL is a semicrystalline polymer with a glass transition temperature of about ÿ60 ëC and a melting temperature of 59±64 ëC. PCL degrades at a much lower rate that PLA and is used as a base polymer for developing long-term, implantable drug delivery systems. PCL is prepared by ring-opening polymerisation of -caprolactone with catalysts such as stannous octoate and initiators such as low molecular weight alcohols to control the molecular weight of the polymer.

1.13 Structure of polycaprolactone.

24

Biodegradable polymers for industrial applications

Commercial products of PCL CAPA (Solvay) There are several grades of PCL produced by Solvay. CAPA 650 can be processed by injection and extrusion. CAPA 680 can also be processed by extrusion blowing. Both polymers have a Tm of 60±62 ëC and a Tg of ÿ60 ëC. Tone polymers (Dow) Polybutylene succinate (PBS) and polybutylene succinate adipate (PBSA) (Nolan-ITU Pty Ltd, 2002) PBS has properties similar to PET. It has a crystallinity of 35±45%, a glass transition temperature of ÿ32 ëC and a melt temperature of 114±115 ëC. PBS is generally blended with other compounds, such as starch and adipate copolymers (to form PBSA). PBSA has a crystallinity of 20±35%, a Tg of ÿ45 ëC and a Tm of 93±95 ëC. Its properties are close to those of LDPE (linear low density polyethylene). These polymers can be processed via conventional melt processing techniques (blow moulding, extrusion, injection) and applications include mulch film, packaging film, bags and `flushable' hygiene products. Bionolle's (Showa Denko) are a family of aliphatic polyesters synthesised by polycondensation of glycols and dicarboxylic acids. There are two series: the 1000 series with PBS obtained from 1,4 butane diol and succinic acid and the 3000 series consisting of PBSA copolymers from 1,4 butane diol and a mix of succinic acid and adipic acid (Fig. 1.14). The structure of PBA is linear or branched (Showa High Polymer Ltd, 1998; Kettle Belinda, 1998). Modified aliphatic polyesters Polyester amides (PEA) are obtained by polycondensation of butanediol with adipic acid and caprolactame (Fig. 1.15). They can be classified in the polyester urethane group. BAK 1095 (BASF) Monomers such as -caprolactame, adipic acid and butane-1, 4-diol are combined by polycondensation. Properties depend on the process. The polymer is easily degraded (Horn et al., 2002).

1.14 Structure of polybutylene succinate.

Classification of biodegradable polymers

25

1.15 Synthesis of polyester amides.

1.3.2 Aromatic polyesters Aromatic polyesters are formed by the polycondensation of aliphatic diols and aromatic dicarboxylic acids. The aromatic ring gives the polymer an excellent resistance to hydrolysis and to chemical agents. They are difficult to hydrolyse and therefore not biodegradable. For example, PET (polyethylene terephthalate) and PBT (polybutylene terephthalate) are well-known polyesters obtained by polycondensation of aliphatic glycols and terephthalic acid. They can be modified by the addition of hydrolysis sensitive monomers (ether, amide or aliphatic groups) giving a family of biodegradable polyesters. Modified aromatic polyesters Aliphatic-aromatic polyesters are formed by the polycondensation of aliphatic diols and a mix of aliphatic and aromatic dicarboxylic acids. Commercially available polyesters obtained by modification of PBT (polybutylene terephthalate) with aliphatic dicarboxylic acids are listed below. Polybutylene succinate terephthalate (PBST): butanediol + succinic and terephthalic acid Biomax (DuPont) are a family of aromatic polyesters issued from the polymerisation of polyethylene terephthalate PET with different aliphatic monomers like dimethylglutarate and diethylene glycol. The monomers create weak spots sensitive to hydrolysis. Biomax can be processed by thermoforming, blow or injection moulding on standard equipment. Applications include household wipes, yard waste bags, components in disposable nappies, disposable eating utensils, agricultural films, plant pots, etc. Biomax's properties can be adapted to meet customer requirements. Melting points are in the range of 200 ëC and mechanical strength can be tuned to be as low as the properties of LDPE or as high as those of strong aromatic polyester films (DuPont, 1998). Polybutylene adipate terephthalate (PBAT) is formed by the reaction of butanediol with adipic and terephthalic acids (Fig. 1.16)

26

Biodegradable polymers for industrial applications

1.16 Structure of polybutylene adipate terephthalate.

Ecoflex (BASF) Ecoflex is a thermoplastic material similar to LDPE but with better mechanical properties. It is extruded to make tear-resistant and flexible films for packaging applications. It is resistant to water and is used to make breathable films because of its moderate water vapour permeability. The glass transition temperature of Ecoflex is ÿ30 ëC and the melting temperature is 110±115 ëC (Yamamoto et al., 2003). Polytetramethylene adipate terephthalate (PTMAT) is made by reaction of tetramethylene glycol with adipic and terephthalic acids. Eastar Bio (Eastman Chemical) Eastar Bio is designed for blown and cast film extrusion, extrusion coating, and fibre or non-woven applications. Eastar Bio can be processed on conventional polyethylene extrusion equipment. The general purpose grade resin Eastar Bio GP has a glass transition temperature of ÿ30 ëC and a melting temperature of 108 ëC. The high viscosity grade Eastar Bio Ultra copolyester, designed for blown film processing, has a Tg of ÿ33 ëC and a Tm of 102±115 ëC (Eastman, Eastar Bio GP, 2001 and 2002).

1.3.3 Polyvinylalcohols (PVA) The structure of PVA is shown in Fig. 1.17. Hydrolene (Hydroplast) Hydrolene can be processed to make films for applications in the medical sector, the packaging sector, agriculture and the car industry. It can be injection moulded to make small parts (Idroplast, undated).

1.17 Structure of polyvinylalcohol.

Classification of biodegradable polymers

27

PVA Erkol (Erkol) PVA Erkol is produced by polymerisation of vinylacetate to polyvinylacetate PVAC, followed by the hydrolysis of PVAC in PVA. The degree of polymerisation determines the molecular weight and viscosity of PVA Erkol in solution. The degree of hydrolysis (saponification) signifies the extent of conversion of the PVAC to PVA. Partially hydrolysed PVA has a Tg of 58 ëC and a Tm of 180 ëC. Totally hydrolysed PVA has a Tg of 85 ëC and a Tm of 230 ëC. PVA can be used in the production of paper, clothes, glues, paints, pharmaceutical products, building materials, ceramics, etc. (Erkol, undated). Solplax (Millenium Plastic Corp)

1.3.4 Modified polyolefins Polyolefins like polypropylene (PP) and polyethylene (PE) are very resistant to hydrolysis and are totally non-biodegradable. It is possible to alter their structure by the addition of an agent that will, by an oxidative radicalair mechanism, degrade the carbon chain of the polymer. Heat or light can initiate the mechanism. The agent-containing transition metal ions transform the polymer into low molecular mass carboxyl acids and alcohols. Bacteria, fungi and enzymes of the milieu then degrade the residues into biomass and CO2. Used photosensitisers include diketones, ferrocene derivatives (aminoalkyferrocene) and carbonyl-containing species (Nolan-ITU Pty Ltd, 2002). Some commercially available products are mentioned below. TDPA (totally degradable plastic additives) and DCP (degradable and compostable plastics) from EPI environmental technologies TDPA is an additive that causes the degradation of polyolefins to lower and lower molecular weights. They become brittle, disintegrate and are ultimately digested by micro-organisms. TDPA can control the degradation rates of plastics in various degrees, from as short as a few weeks to months or years, at a competitive cost. DCP are additives used to make compostable bags and bin liners with polyethylene. These additives represent a significant improvement to the properties of earlier biodegradable films based on starch-filled polyethylene. The starch-based bags had significantly inferior physical and mechanical properties compared with the new modified polyethylene that has very similar mechanical properties to polyethylene. These products are commercialised under the brand Envirocare (Ciba Specialty Chemicals) for agricultural uses. One can also mention the commercial PVA Addiflex from Add-X Biotech AB.

Table 1.3 Processing possibilities of typical commercial biodegradable polymers

Starch Cellulose PHB PHB-PHV PLA PBS PCL PBST PBAT PTMAT PVA PP,PE + agents Starch + PVA Starch + cellulose acetate

Injection moulding

Extrusion

Extrusion blowing

Extrusion casting

X X X X X X X X

X X X X X

X

X

X X

X X

X X X X X X X X

X

X X X X

X X X X

X X X X X X

Blowing

Fibre spinning

Thermo forming

X X X X

X X

X X X

X

X

X X X

X X X X

X X X X X

Classification of biodegradable polymers

29

Table 1.4 Comparison of typical biodegradable polymer properties with polyolefins

(ëC)

Tensile strength (MPa)

Tensile modulus (MPa)

Elongation at break (%)

ÿ100 ÿ60 ö ÿ30 ÿ30

98±115 59±64 110±115 110±115 108±110

8±20 4±28 35±80 34±40 22

300±500 390±470 600±850 ö 100

100±1000 700±1000 580±820 500±800 700

70±115 ö 40±70 0 ÿ30±10 0±30 58±85 ö

100 ö 130±180 140±180 70±170 100±190 180±230 115

34±50 55±120 48±53 25±40 18±24 25±30 28±46 10

2300±3300 3000±5000 3500 3500 700±1800 600±1000 380±530 460

1.2±2.5 18±55 30±240 5±8 3±25 7±15 ö 13±15

PET PGA PEA

73±80 35±40 ÿ20

245±265 225±230 125±190

48±72 890 25

2800±4100 7000±8400 180±220

30±300 30 400

1.4

Conclusions

LDPE PCL Starch PBAT PTMAT PS Cellulose PLA PHB PHA PHB-PHV PVA Cellulose acetate

Tg

Tm

(ëC)

There are now a variety of biopolymers, having different chemical structures and different properties. They offer a large field of applications and after use, composting is a sustainable option. They can be processed by the traditional methods of thermoplastic processing. Table 1.3 summarises the possibilities offered by some of the commercial biopolymers. Their mechanical properties are comparable with those of conventional polymers (Clarinval, 2002). There are flexible materials with medium service temperature like polyethylene (PE), relatively stiff materials with medium service temperature like polystyrene (PS) and stiff materials with a high service temperature like polyethylene terephthalate (PET). Table 1.4 gives a comparison of the typical properties of biopolymers with those of polyolefins. The main applications are in the fields of industrial and domestic packaging, bags, fibres and textiles, agricultural films, catering and fast-food, toys, leisure, medicine, hygiene and cosmetics (Weber, 2000; KaÈb, 2002; Kolybaba et al., 2003).

1.5

References

Arnaud, R., Dabin, P. and Lemaire, J. (1994) `Photooxydation and biodegradation of commercial photodegradable polyethylenes', Polymer Degradation and Stability, 46, 211±224.

30

Biodegradable polymers for industrial applications

Bastioli, C. (2002) `Starch-polymer composites' in Geralds Scott, Degradable Polymers Principles and Applications 2nd edn, Kluwer Academic Publishers, 133±161. Bastioli, C. and Facco, S. (2001) Biodegradable Plastics, 2001 Conference, Frankfurt, Germany, November 26±27. Beckwith, S.W. (2003) `Natural Fiber ± Reinforcement Materials, lower cost for composite applications', Composite Fabrication (November/December), 1±6. Beyer Richard (2002) Patent: WO9809537. Biotec (undated) BIOPLASTÕ GF 105/30, technical information. Braunegg, G. (2002) `Sustainable Poly(hydroxyalcanoate) (PHA) production' in Geralds Scott, Degradable Polymers Principles and Applications 2nd edn, Kluwer Academic Publishers, 235±293. Chiellini, E. Chiellini, F. and Cinelli, P. (2002) `Polymers from renewable ressources' in Geralds Scott, Degradable Polymers Principles and Applications 2nd edn, Kluwer Academic Publishers, 163±233. Clarinval, A.-M. (2002) `Classification and comparison of thermal and mechanical properties of commercialized polymers' International Congress & Trade Show, The Industrial Applications of Bioplastics, 3rd, 4th and 5th February. DarteÂe, M. (2002) `Quality achievements in PLA based plastics' International Congress & Trade Show The Industrial Applications of Bioplastics, 3rd, 4th and 5th February. Drzal, L., Mohanty, A. and Misra, M. (2002) `Biocomposites: Opportunities for Valueadded Biobased Materials', Proceedings of Creating Value for Biobased Resources ± Moving beyond Petroleum, Kansas City, MI, USA. DuPont, Biomax, press release, Wilmington, DE, USA 1998. Eastman, Eastar Bio GP and Ultra Copolyester, product data sheet, July, 2001 and May, 2002. Eilbracht P. (2001) Treeplast news, PE design and engineering nv, Delft, The Netherlands. Erkol (undated), PVA Erkol, technical data sheet. Facco, S. and Bastioli, C. (2000) `Mater-bi-starch based polymers' Biodegradable Plastics Conference, Frankfurt, Germany, 6th and 7th June. Flieger M., Kantorova M., Prell A., Rezanka T. and Votruba J. (2003) `Biodegradable plastics from renewable sources', Folia Microbiology, 48 (1), 27±44. Forest, J.-P. (2000) `PolymeÁre biodegradable: une premieÁre francËaise', Caoutchoucs and Plastiques, 791 (deÂc.), 3. Gormal Stewart (2002), Compatibilisation of biodegradable starch/Bionolletm blends, Thesis, Department of Chemical Engineering, University of Queensland, Australia, May. Gross Richard A. and Kalra Bhanu (2002) `Biodegradable polymers for the environment', Science, 297, 803±807, August 2. Guilbert, S. (2002) `Protein-based Bio-Plastics: formulation, thermoplastic processing and main applications' International Congress & Trade Show The Industrial Applications of Bioplastics, 3rd, 4th and 5th February Gunatillake P.A. and Adhikari R. (2003) `Biodegradable synthetic polymers for tissue engineering', European Cells and Materials, 5, 1±16. Horn Stefan, Bader Hans Joachim and Buchholz Klaus (2002) `Plastics from renewable raw materials and biologically degradable plastics from fossil raw materials', Green Chemistry, Royal Society of Chemistry, December. Idroplast (undated), Hydrolene, technical data sheet.

Classification of biodegradable polymers

31

KaÈb Harald (2002) `Back to nature, Trends in development and market of biodegradable materials', Kunststoffe, 92, 9, 24±40. Kettle Belinda (1998) `Biodegradable Polymer Blends ± Engineering Thesis, Bachelor of Engineering (Honours), Department of Chemical Engineering, University of Queensland, 16 October. Kolybaba M., Tabil L.G., Panigrahi S., Crerar W.J., Powell T. and Wang B. (2003) `Biodegradable Polymers: Past, Present, and Future', 2003 CSAE/ASAE Annual Intersectional Meeting, Fargo, North Dakota, USA, October 3±4, Department of Agricultural and Bioresource Engineering, University of Saskatchewan. Kuesefoglu et al., (2000) Patent US6121398. Lim, S.-H. (1999) `Structure and properties of biodegradable gluten/aliphatic polyester blends', European Polymer Journal, 35, 1875±1881. Lim, S.-H. and Hudson, S. (2003), `Review of Chitosan and its derivatives as antimicrobial agents and their uses as textile chemicals', J. Macromolecular Science, Part C, Polymer Reviews Vol. C43. No. 2, 223±269. Lourdin, D., Della Valle, G., Colonna, P. and Poussin, D. (1999) `Mise en úuvre et proprieÂteÂs de l'amidon', Caoutchoucs et Plastiques, 780, octobre, 39±42. Moore, G.F. and Saunders, S.M. (1997) Advances in Biodegradable Polymers, rapra Technology Ltd, Volume 9 (2), 17±31. Nolan-ITU Pty Ltd, Biodegradable Plastics ± Developments and Environmental Impacts, Australia, October, 2002, prepared in association with ExcelPlas. Otaigbe J.U. and Adams D.O. (1997) `Bioabsorbable soy protein plastic composites: effect of polyphosphate fillers on water absorption and mechanical properties', J. Environ. Polym. Degrad., 5, 199±208. Rathke, T. and Hudson, S. (1994) `Review of Chitin and Chitosan as fiber and film formers', J.M.S. Rev. Macromol. Chem.. Phys., C34(3), 375±437. SteinbuÈchel, A. (2002) `Achievements in biotechnological production of PHAs' International Congress & Trade Show The Industrial Applications of Bioplastics, 3rd, 4th and 5th February. Scott, G. (1971) UK Patent. 1,356,107. Showa High Polymer Ltd, technical data sheet, October 20, 1998. SINAS (undated), Starch Institute for Non-Traditional Applications of Starch, Center for Plant Products and Technology, Michigan State University, MI, USA. Weber J. Claus (2000) Biobased packaging materials for the food industry, Status and perspectives, KVL, The Royal Veterinary and Agricultural University, November. Whitehouse, R. (2000) `The potential for plant-based biodegradable plastics based on polyhydroxy-alkanoates (PHAs)', Biodegradable Plastics Conference, Frankfurt, Germany, 6th and 7th June. Widsten, P., Laine, J., Qvintus-Leino, P. and Tuominen, S. (2002) `Effect of hightemperature defibration on the chemical structure of hardwood', Holzforschung, 56 (1), 51±59. Wool, R. (2003), `Affordable bio-based materials from renewable resources', Business Briefing, CPI Technology, 32±36. Yamamoto, M., Witt, U., Skupin, G., Dieter, B.D. and MuÈller, R.-J. (2003) `Biodegradable Aliphatic-Aromatic Polyesters: EcoflexÕ', Biopolymers, Institute of Microbiology, University of MuÈnster, Alexander SteinbuÈchel.

2

Polyhydroxyalkanoates G G - Q C H E N , Tsinghua University, China

2.1

Introduction

Polyhydroxyalkanoates (PHA) are a family of intracellular biopolymers synthesized by many bacteria as intracellular carbon and energy storage granules (Fig. 2.1). It is generally believed that PHA synthesis is promoted by unbalanced growth.1±4 The plastic-like properties and biodegradability of PHA offer an attraction as a potential replacement for non-degradable polyethylene and polypropylene. Many efforts have been made to produce PHA as environmentally degradable thermoplastics.3±10

2.1.1 Structure variations of PHA Among the PHA family, poly-3-hydroxybutyrate (PHB) is the most common member, it belongs to the short chain length PHA (scl PHA) with its monomers containing 4±5 carbon atoms. Other PHA containing monomers consisting of 6± 16 carbon atoms have been designated as medium chain length PHA (mcl PHA), poly(hydroxyoctanoate-co-hydroxydecanoate) or P(HO-co-HD) is a typical mcl PHA.11±13 Copolyesters of PHA containing scl monomers such as 3hydroxybutyrate (HB) and mcl monomers such as 3-hydroxyhexanoate (HHx) have been found to have a dramatic improvement in their mechanical properties compared with PHB.14 However, only a few wild type microorganisms were reported to be able to produce HB and mcl HA polymers.4,14±17

2.1 General molecular structure of polyhydroxyalkanoates (PHA). m ˆ 1, 2, 3, yet m ˆ 1 is most common, n can range from 100 to several thousands. R is variable. When m ˆ 1, R ˆ CH3 , the monomer structure is 3-hydroxybutyrate, while m ˆ 1 and R ˆ C3 H7, it is a 3-hydroxyhexanoate monomer.

Polyhydroxyalkanoates

33

Many structure variations of PHA have also been synthesized. Due to the small number of these unconventional PHA, little physical characterization and application research has been carried out so far.18±22 By reviewing the PHA research carried out to date, it is clear that many works have been directed towards the design, biosynthesis, and properties of biodegradable and biocompatible materials, these materials can be explored for bioengineering new optical and other `smart' chiral materials.20 Additionally, the functional groups of these unconventional PHA provide a lot of opportunities for further chemical modifications. Since current production technology is still unable to produce any PHA that is competitive with conventional plastics such as polyethylene, polypropylene or polystyrene which are manufactured on a large scale, the application of PHA as environmentally friendly packaging materials is still unrealistic. Therefore, increasing research is focused on the biosynthesis of PHA with unconventional structures that may bring new properties and new applications for PHA (Figs 2.2±2.6).19±29 As well as the change of the PHA site chain, the synthesis of PHA

2.2 PHA monomer structures derived from vernolic acid identified in euphorbia oils (on the left) and from ricinolic acid identified in castor oil (on the right). Both were used for growing Pseudomonas aeruginosa 44T1.20

2.3 Monomer structures of PHA synthesized by Pseudomonas putida grown in a mixture of octanoate and para-cyanophenoxyhexanoate.21

34

Biodegradable polymers for industrial applications

2.4 A monomer structure of PHA synthesized by Pseudomonas stutzeri 1317 grown in soybean oil.22

2.5 Monomer structures of PHA synthesized by Pseudomonas putida grown on phenoxyalkanoates (a), a mixture of nonanoic acid and 10-undecynoic acid (b) and a mixture of nonanoic acid and fluorinated acid cosubstrates (c), respectively.25±28

2.6 Biosynthesis of sulfur-containing polymers with thioester linkage by Ralstonia eutropha.29

Polyhydroxyalkanoates

35

with a non-carbon main-chain is also possible (Fig. 2.6).29 This has opened a completely new area for PHA study.

2.1.2 Screening of PHA producing bacteria FT-IR rapid screening of PHA producing bacteria Fourier-transform infra-red spectroscopy (FT-IR) was demonstrated to be a powerful tool for studying microorganisms and their cell components in intact form.30±32 It has been reported that PHB was observable in FT-IR spectra in intact bacteria.33±35 In our study we extended the observation to find that not only PHB but also mcl PHA can be detected rapidly by the FT-IR technique in intact cells.36 Purified PHB, mcl PHA and P(HB + mcl HA) showed their strongest band at 1728 cmÿ1 , 1740 cmÿ1 and 1732 cmÿ1, respectively, in FT-IR spectra (Fig. 2.7a).36 The methylene C-H vibration near 2928 cmÿ1 had the strongest band in spectra of mcl PHA, the weakest one in PHB. Other characteristic bands for PHB and mcl PHA were visible near 1282 cmÿ1 and 1165 cmÿ1 respectively. In comparison, intact cells of PHB-producing Azotobacter vinelandii UWD, mcl PHA-producing Pseudomonas mendocina AS 1.2329 and P(HB + mcl HA)producing Pseudomonas pseudoalkaligenes AS 1.2328 demonstrated their ester carbonyl bands at 1732 cmÿ1, 1744 cmÿ1 and 1739 cmÿ1 respectively (Fig. 2.7b). Other characteristic bands visible in their pure forms were all observable in the spectra of the intact cells, albeit at a different position and with weaker bands. No band was observable near 1744 cmÿ1 when Pseudomonas mendocina AS 1.2329 accumulated no PHA (Fig. 8B), further demonstrating that the band between 1728 cmÿ1 and 1744 cmÿ1 is characteristic of PHA.36 Several methods have been developed for qualitative analysis of PHA, including GC, nucleic magnetic resonance and pyrolysis.37±38 These methods often require extensive and complicated sample preparation, like hydrolysis, extraction, purification or methylation, etc. The FT-IR method does not require extensive sample preparation and it is thus very convenient and useful. A broad screening process using the FT-IR technique was carried out.39 Samples were collected from various geological locations around China. The FT-IR method proved very effective. It was found that PHA compositions depend very much on the geological locations.39 In some sugar rich locations, bacteria mainly synthesized short-chain-length PHA, in oil contaminated locations, medium-chain-length PHA (mcl PHA) was accumulated by inhabiting bacteria. Additionally, the synthesis of blend polymers consisting of PHB, shortchain-length PHA and mcl PHA is a common phenomenon among the bacteria studied. Of the 371 strains cultivated on six substrates 40% were able to synthesize PHA, with many of them making blends of PHB and mcl PHA,39 particularly for those inhabiting oil-polluted soils. This result will help polymer researchers to identify sources of PHA synthesizing bacteria.

2.7 Fourier-transform infra-red (FT-IR) spectra of pure polyhydroxyalkanoates (PHA) extracted from various cells. (A): (a) polyhydroxybutyrate (PHB) from Azotobacter vinelandii UWD; (b) medium-chain-length (mcl) PHA from Pseudomonas mendocina AS 1.2329; (c) PHA containing hydroxybutyrate (HB) and mcl hydroxyacetate (HA) monomers from Pseudomonas pseudoalkaligenes AS 1.2328. (B): FT-IR spectra of (a) PHB-producing cells of strain Azotobacter vinelandii UWD; (b) cells of Pseudomonas mendocina AS 1.2329 not containing PHA; (c) mcl PHA-producing cells of Pseudomonas mendocina AS 1.2329; (d) cells of Pseudomonas pseudoalkaligenes AS 1.2328 producing PHA containing HB and HA monomers.36 (Courtesy of SpringerVerlag.)

Polyhydroxyalkanoates

37

2.1.3 Biodegradation of PHA One of the most important properties of PHA is their biodegradability. To assess the polyhydroxyalkanoate (PHA)-biodegrading capacity of soil, numbers of aerobic PHBV-degrading microorganisms (degraders) were estimated by Song et al.40 The numbers of PHBV degraders were estimated to be 4.3  105 per gram of dry garden soil, 5.06  105 per gram of dry paddy-field soil, and 3.87  105 per gram of dry river-bank soil. It was found that the PHBV-biodegrading capacity of the soil increased as the number of PHBV degraders in the soil increased. The weight loss after one week in garden soil suspension supplemented with 20 mM of glucose was 2.60%, which was lower than that in garden soil suspension (GSS) (7.14%). After five weeks, the weight loss had increased to 24.97% in the presence of glucose but only to 18.26% in the absence of glucose. The results showed that glucose played important roles in the inhibition and acceleration of different biodegrading phases and finally accelerated the PHBV biodegradation in soil suspension.40 PHB with different initial properties were degraded at virtually equal rates. PHBV copolymers with a lower crystallinity and different microstructure in comparison to PHB were degraded at a 20±30% higher rate on average.41 The ambient temperature significantly influenced the rate of PHA biodegradation. To investigate the biodegradability of novel thermoplastics, PHBHHx were subjected to degradation in activated sludge and compared with PHB and Ecoflex, a biodegradable product of BASF. After 18 days degradation, 40% of PHBHHx and 20% of PHB were degraded, while Ecoflex lost only 5% of its weight. Scanning electron microscopy (SEM) revealed that the surface of Ecoflex was much smoother than that of PHBHHx and PHB. At the same time, PHBHHx degradation in a simplified system containing 0.1 g/l lipase in phosphate buffer saline was found to be affected by its HHx content. It was found that P(HB-co-12%HHx) was degraded faster compared with PHB, P(HBco-5%-HHx) and P(HB-co-20%-HHx). SEM results revealed that P(HB-co12%-HHx) had the most porous surface after degradation. All this indicates that surface morphology played an important role in degradation of PHBHHx. P(HBco-12%-HHx) combining the advantage of low crystallinity and rough surface was degraded the fastest.42

2.2

Mechanical and thermal properties of PHA

The most typical PHA family member is PHB which is a scl PHA; scl PHA includes also poly(hydroxybutyrate-co-hydroxyvalerate) or P(HB-co-HV) or PHBV. Scl PHA is normally brittle (Fig. 2.8). The most typical mcl PHA is P(HO-co-HD), which is elastic in property (Fig. 2.8). Copolyester of hydroxybutyrate and hydroxyhexanoate or abbreviated as P(HB-co-HHx) or PHBHHx, combines the properties of scl- and mcl PHA and becomes a real

2.8 Molecular structures of scl PHB, PHBV, PHBHHx and mcl P(HO-co-HD). Mechanical property of the above PHA changes from brittle to flexible to elastic from left to right, while crystallinity decreased from left to right.

Polyhydroxyalkanoates

39

Table 2.1 Properties of various PHA and conventional plastics29 Samples PHB P(HB-co-10% HV) P(HB-co-20% HV) P(HB-co-10% HHx) P(HB-co-17% HHx) Polypropylene Polystyrene PET HDPE

Tm (ëC)

Tg (ëC)

Tensile strength (MPa)

Elongation at break (%)

177 150 135 127 120 170 110 262 135

4 ö ö ÿ1 ÿ2 ö ö ö ö

43 25 20 21 20 34 50 56 29

5 20 100 400 850 400 ö 7300 ö

HV: 3-hydroxyvalerate; HHx: 3-hydroxyhexanoate; PET: poly(ethylene teraphthalate); HDPE: high density polyethylene

thermoplastic (Table 2.1). PHB, PHBV, PHBHHx and P(HO-co-HD) were the biopolyesters that were produced on a large scale,4±6,11 and therefore most studies have been focused on these biopolyesters.

2.2.1 Improvement on PHA mechanical properties As can be observed from Table 2.1, PHB has the poorest mechanical properties compared with PHBV and PHBHHx. However, PHB has been the most cost competitive biopolyester so far. Efforts have been made to improve the mechanical properties of PHB, Iwata et al. prepared uniaxially oriented films of PHB, with sufficient strength and flexibility by cold-drawing from an amorphous preform at a temperature below, but near to, the glass transition temperature.43 Melt-crystallized and solvent-cast films of PHB are usually quite brittle, and the orientation is critical and difficult to reproduce consistently. Melt-quenched films with rubber state were stretched easily and reproducibly to a draw ratio of more than 1000% and, when annealed under tension, acceptable mechanical properties were generated by their research. The tensile strength, elongation to break, and Young's modulus were 237 MPa, 112% and 1.5 GPa, respectively. They found that when the two-step drawing procedure was applied, the mechanical properties were further improved (287 MPa, 53% and 1.8 GPa). In X-ray fibre diagrams of highly oriented films, shape reflections assigned to the beta-form (zigzag conformation) together with those derived from the normal orthorhombic crystal system (alpha-form, 2(1) helix conformation) could be observed. The improvement of mechanical properties is due not only to the orientation of molecular chains but also the generation of zigzag conformation and network structure formed by fibril and lamellar crystals (shishkebab structure). The mechanical properties of uniaxially oriented films remained almost unchanged for four months at room temperature, suggesting that the high

40

Biodegradable polymers for industrial applications

orientation and crystallinity avoid secondary crystallization. Their work demonstrated that it is possible to improve the mechanical properties of PHB through the process conditions.43 Fischer et al. processed the uniaxially oriented films with high mechanical properties from ultra-high-molecular-weight PHB by using a two-step drawing procedure.44 When the annealing procedure was applied to one-step hot-drawn films at a temperature (100 ëC) showing the maximum growth rate of spherulites, the tensile strength reached up to 277 MPa with 84% elongation to break. The group concluded that the increase of mechanical properties is due to the orientation of molecular chains and the finger-joint structure of lamellar crystals in the shishkebab structure. To increase further the mechanical properties, twostep drawing was performed at room temperature against hot-drawn films before annealing. Tensile strength of two-step drawn films increased with an increase in the two-step draw ratio at room temperature. The tensile strength of the two-step drawn film reached nearly 400 MPa, indicating that the two-step drawing procedure is quite useful to obtain high strength films of PHB homopolymer. These mechanical properties of PHB remained unchanged for two months, suggesting that the development of a planar zigzag conformation and the highly ordered structure with finger-joint structure of lamellar crystals avoid deterioration by secondary crystallization.44 Similar mechanical property improvement was observed with films of PHBHHx.45

2.2.2 Two-dimensional Fourier transform infra-red correlation spectroscopy (2D FT-IR) for studying PHA Generalized two-dimensional (2D) Fourier transform infra-red correlation spectroscopy was used to investigate the effect of the comonomer compositions on the crystallization behavior of two types of biosynthesized random copolymers, PHBHHx and PHBV. The carbonyl absorption band around 1730 cmÿ1 was sensitive to the degree of crystallinity. 2D correlation analysis demonstrated that the 3-hydroxyhexanoate units preferred to remain in the amorphous phase of the semicrystalline PHBHHx copolymer, resulting in decreases in the degree of crystallinity and the rate of the crystallization process. The PHBV copolymer maintained a high degree of crystallinity when the 3hydroxyvalerate fraction was increased from 0 to 25 mol% because of isodimorphism. The crystalline and amorphous absorption bands for the carbonyl bond for this copolymer, therefore, changed simultaneously.46 The molecular level pre-melting process of purified biosynthesized polyesters PHBV (20.4% HV) and PHBHHx was investigated by 2D FT-IR. Intensity variations and band shifts in the characteristic spectral regions for C=O groups (1710±1770 cmÿ1), C-H groups (2910±3010 cmÿ1), and C-O-C groups (1220± 1310 cmÿ1) were selected for a detailed study of the thermally induced phase transition of the copolymers.47±49 The 2D correlation approach successfully

Polyhydroxyalkanoates

41

demonstrated that a fully amorphous liquid-like structure was formed following the disappearance of the crystals in the sample, by going through an intermediate state related to the lost conformation of the highly ordered and helical polyester main chain. Similarly, 2D correlation analysis on changes of the crystalline and amorphous conformation of PHB indicated that the appearance of a fully amorphous component did not occur simultaneously with the disappearance of the crystalline component, suggesting that there is an intermediate state between the ordered crystalline and amorphous states in PHB, which may probably be responsible for the band near 1730 cmÿ1.36 Also the intermediate state was observed in the crystallization process of melted amorphous PHB.47 This intermediate state may be very interesting for the processing of this unique material. As well as the change of melting processes, DSC measurements showed that the incorporation of HV and HHx decreased the melting temperature relative to that of PHB by 70 ëC,50 which is convenient for melt processing.

2.2.3 PHA melting and crystallinity On PHB banded spherulites, concentric rings were observed between cross polarizers or without polarizers. The rings used to be considered as cracks. Atomic force microscopy (AFM) confirmed that these concentric rings with varying spacing were growth terraces rather than cracks. The height of the terraces reached up to several hundred nanometers. It was observed that the external terraces were higher than the internal terraces, which, it was proposed, resulted from layer-by-layer growth. 51 Real-time AFM observation demonstrated that the terrace forms at the front of the growing spherulites just before or exactly when two spherulites impinge on each other. Terraces were observed on the spherulites crystallized from melt confined between glass or polyimide slides rather than poly(ethylene terephthalate) slides. The formation of the terraces may have resulted from instability of the moving boundary of the melt film confined between the spherulite surface and cover slide. Wettability of the substrate played an important role in the formation of the terraces.52 Melting and crystallization of PHB and PHBHHx were studied via in situ Fourier transform infra-red spectroscopy (FT-IR).53 The absorbance variances of the crystalline and the amorphous bands revealed that melting of PHB occurred within a narrow temperature range, while melting of PHBHHx copolymer occurred within a wide temperature range, and it was shown that the latter had a much wider distribution of lamellar thickness. These results were consistent with those from DSC. The alkyl groups reached close packing prior to crystallization in PHB and followed the crystallization in PHBHHx due to the longer branch propyl group. The FT-IR band at 1230 cmÿ1 was first assigned to the conformational band of the helical segments, and the absorbance ratio of the band at 1230 cmÿ1 to the reference band at 1453 cmÿ1 was used to determine quantitatively the crystallinity of PHA after adjustment with DSC.53

42

Biodegradable polymers for industrial applications

In another study, banded spherulites of PHBV random copolymer were prepared by isothermal crystallization at 90 ëC for ten hours.54 Using tapping-mode AFM, the concentric periodic ridges and valleys on the surface of the banded spherulites of this polymer were found to consist of edge-on and flat-on lamellae, respectively. The periodic concentric ridges and valleys observed by AFM corresponded to the periodic extinction rings observed by polarized optical microscopy. AFM measurements showed that the interaction between the AFM probe and the sample surface can be significantly influenced by lamellar orientation.54 Real-time AFM observation was carried out during crystallization on thin films of chiral PHBHHx copolymer. The lamellae exhibited complicated growth behaviors: twisting, bending, backward growth, and branching. The lamellae continuously twist to show alternating edge-on and flat-on views along the radii of the spherulites. Giant screw dislocations give birth to new lamellae. Interaction between the leading and trailing lamellae contributes to cooperative stacking of the twisting crystals. The lamellae twist before screw dislocations appear, demonstrating that screw dislocations are not causal of twisting. All the observed twisting occurs in the right-handed sense, and is likely to result from the chirality of the crystal structure. Increased crystallization temperature resulted in decreased magnitude of lamellar twisting and bending.54

2.3

Process development and scale up for microbial PHA production

Although many PHA have been found, only three of them were produced on a large scale for commercial exploitation, these are PHB, PHBV and PHBHHx. Small-scale production of mcl PHA was also conducted. There is still a lot to improve for the production of these unique polyesters.

2.3.1 Microbial PHB and PHBV production Extensive work was conducted for microbial production of PHB and PHBV.3,5±9 Accumulated experience has led to the industrial production of PHB by Chemie Linz AG/Austria in the 1980s, using the bacterial strain Alcaligenes latus.5 However, the poor mechanical properties and weak processibility limited development of PHB as an environmentally friendly packaging material. PHBV was subsequently developed by Zeneca/UK (then ICI or Imperial Chemical Industries Co. Ltd), using Wautersia eutropha as a production strain. PHBV had improved properties over PHB (Table 2.1) and ICI developed PHBV with the Trade name of BIOPOL. PHB production by Bacillus spp. Bacillus spp. were among the very first to be reported as PHB producers.55 However, we were surprised to learn that no PHB production research in terms

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of process development was conducted with this organism although Bacillus spp. have long been known to grow rapidly. They are also capable of using various cheap carbon sources for growth, in addition, they are very resistant to contamination by other bacteria. Chen et al. reported the production of PHB from 11 Bacillus spp. randomly selected from German Culture Collection (DSM), their PHB production never exceeded 50% of cell dry weight when grown in shake flasks.8 To investigate the possibility for PHB production using Bacillus spp., a Bacillus strain isolated from molasses-contaminated soil was used as a model.56 It appeared that PHB formation was growth associated, factors that normally promote PHB production including high ratios of carbon to nitrogen, carbon to phosphorus and low oxygen supply, did not lead to high PHB production. Instead, these factors resulted in sporulation, which further leads to reduced PHB contents and cell dry weight. Molecular weights of PHB produced by this Bacillus sp. were all low. The competition of PHB synthesis and sporulation seemed to be the reason for low PHB production. Therefore, Bacillus spp. may not be a suitable PHB industrial production strain. Furthermore, the thick Gram positive cell wall will make the breakage of cells and PHB extraction difficult.56 PHB production by Alcaligenes latus Alcaligenes latus is one of the strains that satisfy the requirements for industrial PHB production.5 The strain grows rapidly in sucrose, glucose and molasses. PHB accumulation can be as high as over 90% of the cell dry weight.57 There was even an attempt to produce PHB from waste materials using A. latus.58 Chemie Linz AG/Austria (later btf Austria) produced PHB in a quantity of 1000 kg/week in a 15 m3 fermentor using Alcaligenes latus DSM 1124.59 The cells were grown in a mineral medium containing sucrose as a carbon source. The PHB produced by Alcaligenes latus was used to make sample cups, bottles and syringes for application trials. The PHB production and processing technology are now owned by Biomer in Germany. Different products including combs, pens and bullets have been made from PHB produced by Alcaligenes latus. PHB production by Wautersia eutropha Wautersia eutropha (formerly known as Ralstonia eutropha) was used to conduct PHB production research in a 1 m3 fermentor under the joint auspices of the Institute of Microbiology affiliated to the Chinese Academy of Sciences and Tianjin Northern Food Inc./China. Growth was carried out for 48 h in a glucose mineral medium. At the end of the cell growth, cell density reached 160 g/l. The cells produced 80% PHB in their dry weight. Most surprisingly, the strain grown to such a high density did not require oxygen-enriched air. This was perhaps the

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highest cell density for PHB production achieved in a pilot scale production (unpublished results). Further details can be obtained from China Ningbo TianAn Biomaterial Co. Ltd. PHB production by Azotobacter vinelandii Azotobacter vinelandii strain UWD was demonstrated to grow rapidly in a molasses medium.10 The strain has a large size, ranging from 1±8 m. It can produce PHB up to 90% of cell dry weight. At the same time, the strain produces PHB with a molecular weight ranging from 1±4 million Dalton,9 this is rarely seen with any wild-type microorganism. PHB production could be promoted by lower aeration, therefore, PHB production can be separated into two-stages, one for cell growth under high aeration and another for PHB accumulation under lower aeration.10 In a small-scale lab top fermentor, 36 g/l PHB were produced from molasses after 48 h of growth. A collaboration between the Microbiology Lab at Tsinghua University and Guangdong Jiangmen Center for Biotech Development/China for pilot PHB production by A. vinelandii UWD was carried out on molasses medium. The pilot study was done in a 4 m3 fermentor without automatic oxygen supply control. After 48 h of growth, the cells reached a density of 75±80 g/l. The PHB content in the cells was as high as 72% of the cell dry weight. The cell size was at least 6 lm in diameter. Due to the high PHB accumulation efficiency and the large cell size, separation of biomass from the fermentation broth using continuous disk centrifuges was convenient. At the same time, the cells were easily broken by a 0.2% SDS solution at 60 ëC for 2 h, making the downstream processing relatively easy. The major problem with this strain has been the difficulty in growing the cells to a high density, as this strain requires a high dissolved oxygen concentration for high-density growth (unpublished result). The supply of oxygen enriched air for industrial fermentation is impossible due to its explosive nature and high cost of pure oxygen supply. PHB produced by the strain is now the subject of study by the Institute of Polymer Sciences and Engineering at Tsinghua University. Major efforts have been focused on improving the mechanical strength and the exploitation of tissue engineering applications for this polyester. PHBV production by Wautersia eutropha The strain is able to grow on glucose and produce the copolymer PHBV to a density as high as 70±80 g/l after over 70 h of growth. Shampoo bottles were produced from PHBV (trademarked as BIOPOL) and were available in supermarkets in Europe. However, due to economic reasons, the Biopol products did not succeed and the PHBV patents were sold to Monsanto and further to Metabolix.

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NingBo TianAn Co. Ltd/China, in collaboration with the Institute of Microbiology affiliated to the Chinese Academy of Sciences, has developed a model process that can produce PHBV with great efficiency. Without a supply of pure oxygen, R. eutropha grew to a density of 160 g/l cell dry weight within 48 h in a 1000 l fermentor. The cells accumulated 80% of PHBV with a production efficiency of 2.5 g/h/l. The HV content in the copolymer ranged from 8±10%. This process can significantly reduce the production costs for PHBV. Only by achieving the high growth rate, high PHBV production efficiency and high cell and PHBV densities can the polymers become economically competitive. We assume that PHBV or other PHA can become cost effective after extensive improvement in fermentation and downstream processes.

2.3.2 Microbial PHBHHx production Recently, Tsinghua University in Beijing/China, in collaboration with Guangdong Jiangmen Center for Biotech development/China, KAIST/Korea and Procter & Gamble in the USA succeeded in producing PHBHHx by Aeromonas hydrophila grown in a 20 m3 fermentor.4 The PHBHHx production was carried out on glucose and lauric acid for about 60 h. Cell dry weight reached 50 g/l, only 50% of PHBHHx was produced in the cell dry weight. The extraction of PHBHHx was a very complicated process involving the use of ethyl acetate and hexane, which increased the polymer production cost dramatically. PHBHHx produced by Jiangmen/China has now been exploited for application in areas of flushables, nonwovens, binders, films, flexible packaging, thermoformed articles, coated paper, synthesis paper, coating systems and medical devices (www.nodax.com). Copolymers consisting of HB and medium-chain-length HA have been trademarked by P&G as NODAX. Current production cost for PHBHHx is still too high for real commercial application. However, many efforts have been made to improve the production process for PHBHHx including the downstream process technology. Most efforts have been focused on increasing cell density and simplifying the downstream process. A better production strain able to utilize glucose will be one of the most important issues of reducing PHBHHx production costs. Akiyama et al. simulated large-scale fermentative production of PHBHHx (P(3HB-co-5mol% 3HHx)) from soybean oil as sole carbon source using a recombinant strain of Wautersia eutropha harboring a polyhydroxyalkanoate (PHA) synthase gene from Aeromonas caviae.60 Annual production of 5000 tons of P(3HB-co-5mol% 3HHx) is estimated to cost from 3.5±4.5 US$/kg, depending on presumed production performances. Similar scale production of PHB from glucose is estimated to cost 3.8±4.2 US$/kg. In contrast to the comparable production costs between P(3HB-co-5mol% 3HHx) and PHB, life cycle inventories of energy consumption and carbon dioxide emissions favor the former product over the latter, reflecting smaller inventories and higher

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production yields of soybean oil compared to glucose. The life cycle inventories of energy consumption and carbon dioxide emissions of bio-based polymers are markedly lower than those of typical petrochemical polymers.60 Metabolic engineering for PHBHHx production Aeromonas hydrophila was a microorganism that had never been used for genetic engineering. However, A. hydrophila was found to be able to produce PHBHHx and an industrial process for production of PHBHHx was developed using strain 4AK4.4 The strain produced PHBHHx with a stable HHx content ranging from 10±15% regardless of growth conditions. Aeromonas hydrophila coded as CGMCC 0911 isolated from lake water was found to be able to synthesize PHBHHx consisting of 3-hydroxybutyrate (HB) and 4±6 mol% 3-hydroxyhexanoate (HHx). The wild-type bacterium accumulated 49% PHBHHx containing 6 mol% HHx in terms of cell dry weight (CDW) when grown on lauric acid for 48 h. When A. hydrophila CGMCC 0911 expressed the Acyl-CoA dehydrogenase gene (yafH) of Escherichia coli, the recombinant strain could accumulate 47% PHBHHx, while the HHx content reached 17.4 mol%.61 It was also found that the presence of changing glucose concentration in the culture changed the HHx content both in wild type and recombinant A. hydrophila CGMCC 0911. When 5 g lÿ1 glucose was added to a culture containing 5 g lÿ1 lauric acid as co-substrate, 45% PHBHHx/CDW consisting of 8.8 mol% HHx was produced by wild-type A. hydrophila CGMCC 0911 compared with only 5% in the absence of glucose. When the recombinant A. hydrophila CGMCC 0911 was grown on a mixed substrate containing lauric acid and 8±10 g lÿ1 glucose, the HHx content could be further increased to 35.6 mol%. When the glucose concentration exceeded 10 g lÿ1, cell growth, PHA content and mole percentages of HHx in PHBHHx were significantly reduced.61 Therefore, we could manipulate the PHBHHx contents by changing the strain's metabolic pathways or by changing the growth conditions. Attempts have also been made to manipulate E. coli PHBHHx production.62 Acyl-CoA dehydrogenase gene (yafH) of E. coli was expressed together with polyhydroxyalkanoate synthase gene (phaCAc) and (R)-enoyl-CoA hydratase gene (phaJAc) from Aeromonas caviae. The expression plasmids were introduced into E. coli JM109, DH5alpha and XL1-blue, respectively. Compared with the strains harboring only phaCAc and phaJAc, all recombinant E. coli strains harboring yafH, phaCAc and phaJAc accumulated at least four times more PHBHHx. Cell dry weights produced by all recombinants containing yafH were also considerably higher than those without yafH. It appeared that the overexpression of acyl-CoA dehydrogenase gene (yafH) enhanced the supply of enoyl-CoA which is the substrate of (R)-enoyl-CoA hydratase. With the enhanced precursor supply, the recombinants accumulated more PHBHHx.62

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To regulate the unit fraction in PHBHHx, phbA and phbB genes encoding beta-ketothiolase and acetoacetyl-CoA reductase in Wautersia eutropha, were introduced into A. hydrophila 4AK4.63 When gluconate was used as a cosubstrate of dodecanoate, the recombinant produced PHBHHx containing 3± 12 mol% 3HHx, depending on the gluconate concentration in the media. Vitreoscilla hemoglobin gene, vgb, was also introduced into the above recombinant, resulting in improved PHBHHx content from 38 to 48 wt% in shake flask study. Fermentor studies also showed that increased gluconate concentration in a medium containing dodecanoate promoted the recombinant strain harboring phbA and phbB genes to incorporate more 3HB units into 1 PHBHHx, resulting in a reduced 3HHx fraction. Recombinant A. hydrophila harboring phbA, phbB and vgb genes demonstrated better PHBHHx productivity and higher conversion efficiency from dodecanoate to PHBHHx than those of the recombinant without vgb in fermentation study. Combined with the robust growth property and simple growth requirement, A. hydrophila 4AK4 appeared to be a useful organism for metabolic engineering. As well as the production of PHBHHx using metabolic engineering, some research has succeeded in producing monomers of PHA using metabolic engineering,64±70 these chiral monomers will also be very useful for the production of other high-value-added chemicals.

2.3.3 Microbial production of medium-chain-length PHA (mcl PHA) Medium-chain-length (mcl) PHA can be produced by Pseudomonas oleovorans and Pseudomonas stutzeri as well as other Pseudomonas spp. Research demonstrated that many bacteria isolated from oil-contaminated sources were able to synthesize mcl PHA.71±78 Pseudomonas oleovorans forms medium-chain-length poly(3-hydroxyalkanoate) (PHA) most effectively at growth rates below the maximum specific growth rate. Under adequate conditions, PHA accumulates rates in inclusion bodies in cells up to levels higher than half of the cell mass, which is a timeconsuming process.79 For PHA production, Jung et al. developed a two-stage continuous cultivation system with two fermenters connected in series as a potentially useful system.79 It offers production of cells at a specific growth rate in a first compartment at conditions that lead cells to generate PHA at higher rates in a second compartment, with a relatively long residence time. Transientstate experiments allowed investigation of Dilution-1 and Dilution-2 over a wide dilution rate range at high resolution in time-saving experiments. Furthermore, the influence of temperature, pH, nutrient limitation, and carbon source on PHA productivity was investigated by this group. Results similar to optimum conditions in single-stage chemostat cultivations of P. oleovorans were found. With all culture parameters optimized, a volumetric PHA productivity of

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1.06 g l-1 hÿ1 was determined. Under these conditions, P. oleovorans contained 63% (dry weight) PHA in the effluent of the second fermenter. This is the highest PHA productivity and PHA content reported thus far for P. oleovorans cultures grown on alkanes. Strain P. stutzeri 1317 isolated from oil-contaminated soil was found to grow on a variety of carbon sources including glucose and soybean oil.22,75 The strain produced over 63% mcl PHA when grown on soybean oil, while on glucose, 51% mcl PHA was synthesized by this organism. The strain is currently under intensive investigation into the possibility of increasing the mcl PHA production level.

2.4

Applications of PHA

Current applications of PHA research focus on biodegradable and environmental packaging, as well as implant biomaterials

2.4.1 Biodegradable packaging materials NodaxTM Polymers are a series of PHA copolyesters consisting of short-chainlength HB and mcl HA currently in development by Procter & Gamble (P&G, USA), with efforts focused on achieving low cost fermentation production and targeted polymer specifications. The development is being conducted on a global basis, combining P&G's resources with the support of several companies and research institutes. The global combination of resources provides the best available technology, scientists and engineers, as well as continuous localized feedback (www.nodax.com). P&G is working with converters and end users. This aspect of the value chain includes conversion of the formulated resins into initial forms, like films, fibers or molded articles, as well as secondary conversion into nonwovens, laminated packages and papers. End users then use these to make or package consumer or industrial products, like diapers, hamburgers or computers. Finally, the products are used and then eventually disposed of in one of several ways (composting, landfill, digestion, incineration) where the biocycle is completed.

2.4.2 Biomaterials for implant purposes The biocompatibility of PHBHHx were evaluated in vitro.80 The mouse fibroblast cell line L929 was inoculated on films made of PHB, PHBHHx and their blends, polylactic acid (PLA) as control. It was found that the growth of the cells L929 was poor on PHB and PLA films. The viable cell number ranged from 8.8  102 to 1.8 x 104/cm2 only. Cell growth on the films made by blending PHB and PHBHHx showed a dramatic improvement. The viable cell number observed increased from 9.7  102 to 1.9  105 on a series of PHB/

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PHBHHx blended film in ratios of 0.9/0.1 to 0/1, respectively, indicating a much better biocompatibility in the blends contributed by PHBHHx. Biocompatibility was also strongly improved when these polymers were treated with lipases and NaOH, respectively. However, the effects of treatment were weakened when PHBHHx content increased in the blends. It was found that lipase treatment had more increased biocompatibility than NaOH. After the treatment biocompatibility of PHB was approximately the same as PLA, while PHBHHx and its dominant blends showed improved biocompatibility compared to PLA.80 Scanning electron microscopy showed that PHB films changed their surface from multi-porous to rough non-porous after the lipase or NaOH treatment, while PHBHHx films showed little change after these treatments. The results seemed to show that the polyester surface morphology played an important role in affecting cell attachment and growth on these materials.81 Blended PHB and PHBHHx was turned into films and scaffolds.81 The films made from blending polyesters showed that the elongation to break of the blending PHBHHx/PHB film increased from 15% to 106% when PHBHHx content in the blend increased from 40% to 60%. Scaffolds made of PHBHHx/ PHB consisting of 60 wt% PHBHHx showed strong growth and proliferation of chondrocytes on the blending materials. Energy dispersive X-ray analysis of the extra cellular matrix on the scaffolds demonstrated a high level of calcium and phosphorus elements in a molar ratio of Ca/P at 1.66, this is approximately equal to that of natural material hydroxyapatite which has a Ca/P ratio of 1.67. This suggested that the chondrocyte cells grown on PHBHHx/PHB scaffolds presented effective physiological functions for the generation of cartilage. 81±84 Polymer scaffold systems consisting of PHBHHx/PHB were investigated for possible application as a matrix for the three-dimensional growth of chondrocyte culture. Blend polymers of PHBHHx/PHB were fabricated into threedimensional porous scaffolds by the salt-leaching method. Chondrocytes isolated from rabbit articular cartilage (RAC) were seeded on the scaffolds and incubated over 28 days, with a change of the culture medium every four days. A PHB scaffold was taken as a control. Results showed that chondrocytes proliferated better on the PHBHHx/PHB scaffolds than on the PHB one. As for the blend polymers, cells grew better on scaffolds consisting of PHBHHx/PHB in ratios of 2:1 and 1:2 than they did on PHBHHx/PHB of 1:1. In addition, chondrocytes proliferated on the scaffold and preserved their phenotype for up to 28 days.82 This result was also proven by semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) used to assay collagen 11 mRNA for evaluation of the ability of the blend scaffolds to induce collagen 11 production.83 After bone marrow stromal cells were seeded and cultured on PHBHHx, their proliferation was investigated by MTT. Differentiation of the cells was assessed by measuring alkaline phosphatase activity and by histochemical assay. The wettability and thermal properties of PHBHHx films were also studied. The

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results showed that bone marrow stromal cells can attach, proliferate and differentiate into osteoblasts on PHBHHx films. These results suggest that PHBHHx has good affinity with bone marrow stromal cells and may have potential applications in bone tissue engineering.85,86 The above studies strongly suggested that PHA, especially PHBHHx has good potential to be developed into biocompatible implant materials.

2.5

Future developments

Although many possibilities have been explored to lower the production cost of PHA, they are still not in a position to challenge the conventional plastics such as polypropylene and polystyrene that cost only $1/kg. The success of transgenic plants that produce large quantities of PHA could eventually lower the cost of PHA to a level comparable to conventional plastics. To achieve this goal, much research work is still needed to improve PHA genetic expression levels in economically interesting plants, such as oilseeds and potatoes. The synthesis of PHA with novel monomer structures will open up another interesting research field. More and more bacterial strains which have shown some unusual ability to synthesize various PHA have been isolated from various locations. Some strains have high PHA productivity and can produce unusual PHA structures from simple substrates such as glucose and sucrose. By using unconventional precursors with specific functional groups, PHA with such functional groups that render the PHA with desirable properties have been produced. PHA with enhanced piezoelectricity, nonlinear optical activity, biodegradability and biocompatibility will provide PHA with the potential to challenge other functional polymers produced by conventional chemical synthesis. In addition to the application as plastics, PHA can also be a potential source of chiral hydroxy acid feedstock for the fine chemical industry. In contrast to the introduction of new polymers, the PHA hydroxy acids and the related derivatives can be readily integrated into existing fine chemical markets. The long-term development for PHAs will be a promising subject for research which has the potential to be profitable to many industries. Collaboration between microbiologists, molecular biologists, polymer scientists, material scientists and industry is the key for PHA exploration. Many new developments should be expected in the new century.

2.6

References

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38. Morikawa H and Marchessault RH, Pyrolysis of bacterial polyhydroxyalkanoates. Can J Chem., 1981, 59: 2306±2313. 39. Wu Q, Sun SQ, Yu PHF, Chen AXZ and Chen GQ, Environmental Dependence of Microbial Synthesis of Polyhydroxyalkanoates. Acta Polymerica Sinica, 2000, 6: 751±756. 40. Song CJ, Wang SF, Ono S, Zhang BH, Shimasaki C and Inoue M, The biodegradation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB/V) and PHB/Vdegrading microorganisms in soil. Polymer Adv Technol., 2003, 14: 184±188. 41. Volova TG, Belyaeva OG, Plotnikov VF and Puzyr AP, Studies of biodegradation of microbial polyhydroxyalkanoates. Appl Biochem Microbiol., 1998, 34: 488±492. 42. Wang YW, Mo WK, Yao HL, Wu Q and Chen GQ, Biodegradation studies of Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). Polym Degrad Stability, 2004, 85: 815±821. 43. Iwata T, Tsunoda K, Aoyagi Y, Kusaka S, Yonezawa N and Doi Y, Mechanical properties of uniaxially cold-drawn films of poly([R]-3-hydroxybutyrate). Polymer Degradation and Stability, 2003, 79: 217±224. 44. Aoyagi Y, Doi Y, Iwata T. Mechanical properties and highly ordered structure of ultra-high-molecular-weight poly[(R)-3-hydroxybutyrate] films: Effects of annealing and two-step drawing. Polymer Degradation and Stability, 2003, 79: 209±216. 45. Fischer JJ, Aoyagi Y, Enoki M, Doi Y and Iwata T, Mechanical properties and enzymatic degradation of poly([R]-3-hydroxybutyrate-co-[R]-3-hydroxy hexanoate) uniaxially cold-drawn films. Polymer Degradation and Stability, 2004, 83: 453±460. 46. Tian G, Wu Q, Sun SQ, Noda I and Chen GQ, Two-Dimensional Fourier-Transform Infrared Spectroscopy Study of Biosynthesized Poly(hydroxybutyrate-co-hydroxyhexanoate) and Poly(hydroxybutyrate-co-hydroxyvalerate). J of Polymer Sci., 2002, Part B 40 (7): 649±656. 47. Tian G, Wu Q, Sun SQ, Noda I and Chen GQ, Study of Thermal Melting Behavior of Microbial Polyhydroxyalkanoates Using Two-Dimensional Fourier-Transform Infrared FT-IR Correlation Spectroscopy. Appl. Spectroscopy, 2001, 55 (7): 888±894. 48. Tian G, Wu Q, Sun SQ, Noda I and Chen GQ, Study of Pre-melting and Crystallization Process of Biosynthesized Poly(3-hydroxybutyrate) Using Twodimensional Fourier-Transform Infrared Spectroscopy. Chem J Chinese Universities, 2002, 8: 1627±1631. 49. Wu Q, Tian G, Wu Q, Sun SQ, Noda I and Chen GQ, Study of Microbial Poly(hydroxybutyrate-co-hydroxyhexanoate) Using Two-Dimensional FourierTransform Infrared Correlation Spectroscopy. J Appl Polym Sci., 2001, 82: 934±940. 50. He JD, Cheung MK, Yu PHF and Chen GQ, Thermal Analyses of Poly(3hydroxybutyrate), Poly(3-hydroxy-butyrate-co-hydroxylvalerate) and Poly(3hydroxybutyrate-co-hydroxyl -hexanoate). J. Appl. Polym. Sci., 2001, 82 (1): 90±98. 51. Xu J, Guo BH, Zhang ZM, Chen GQ and Wang XF, Topography of polyhydroxybutyrate banded spherulites. Chem J Chinese Univ., 2002, 23: 1216±1218. 52. Xu J, Guo BH, Chen GQ and Zhang ZM, Terraces on banded spherulites of polyhydroxyalkanoates. J Polym Sci Pol Phys., 2003, 41 (18): 2128±2134. 53. Xu J, Guo BH, Yang R, Wu Q, Chen GQ and Zhang ZM, In situ FT-IR study on melting and crystallization of polyhydroxyalkanoates. Polymers, 2002, 43 (25): 6893±6899. 54. Jiang Y, Zhou JJ, Li L, Xu J, Guo BH, Zhang ZM, Wu Q, Chen GQ, Weng LT, Cheung ZL and Chan CM, Surface properties of poly(3-hydroxybutyrate-co-3-

54

55.

56. 57. 58.

59. 60. 61. 62. 63. 64.

65. 66. 67. 68. 69.

Biodegradable polymers for industrial applications hydroxyvalerate) banded spherulites studied by atomic force microscopy and timeof-flight secondary ion mass spectrometry. Langmuir, 2003, 19 (18): 7417±7422. Jun X, Guo BH, Zhang ZM, Zhou JJ, Jiang Y, Yan S, Li L, Wu Q, Chen GQ and Schultz JM, Direct AFM observation of crystal twisting and organization in banded spherulites of chiral poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). Macromolecules, 2004, 37: 4118±4123. Macrae RM and Wilkinson JF, Poly-beta-hydroxybutyrate metabolism in washed suspensions of bacillus cereus and Bacillus megaterium. J. Gen. Microbiol. 1958, 19: 210±220. Wu Q, Huang HH, Hu GH, Chen JC, Ho KP and Chen GQ, Constitutive Production of Poly-3-hydroxybutyrate by strain of Bacillus aureus JMa5 Cultivated in Molasses Media. Antonie van Leeuwenhoek, 2001, 80 (2): 111±118. Chen GQ, Produktion von Poly-D(-)-3-HydroxybuttersaÈure und Poly-D(-)-3HydroxyvaleriansaÈure mit Einzel- und Mischpopulation von Alcaligenes latus DSM 1122, 1123 bzw. 1124. Ph.D. Thesis, Graz University of Technology, Graz/ Austria (1989). Yu PH, Chua H, Huang AL, Lo W and Chen GQ, Conversion of Food industrial Wastes into Bio-plastics. Appl. Biochem. and Biotechnol., 1998, 70: 603±614. Hrabak O, Industrial production of poly- -hydroxybutyrate. FEMS Microbiol. Rev. 1992, 103: 251±256. Akiyama M, Tsuge T and Doi Y, Environmental life cycle comparison of polyhydroxyalkanoates produced from renewable carbon resources by bacterial fermentation. Polym Degradation and Stability, 2003, 80: 183±194. Lu XY, Wu Q and Chen GQ, Production of Poly(3-hydroxybutyrate-co-3- hydroxyhexanoate) with Flexible 3-hydroxyhexanoate Content in Aeromonas hydrophila CGMCC 0911. Appl Microbiol Biotechnol., 2004, 64: 41±45. Lu XY, Wu Q and Chen GQ, Enhanced production of poly(3-hydroxybutyrate-co- 3hydroxyhexanoate) via manipulating the fatty acid oxidation pathway in E. coli. FEMS Microbiol Lett., 2003, 221: 97±101. Qiu YZ, Ouyang SP, Wu Q and Chen GQ, Metabolic Engineering for Production of Copolyesters Consisting of 3-Hydroxybutyrate and 3-Hydroxyhexanoate by Recombinant Aeromonas hydrophila Harboring phbA and phbB Genes. Macromolecular Biosci., 2004, 4: 255±261. Gao HJ, Wu Q and Chen GQ, A Novel Genetically Engineered Pathway for Production of D-(-)-3-hydroxybutyric acid by recombinant Escherichia coli. FEMS Microbiol Lett., 2002, 213: 59±65. Zhao K, Tian G, Zheng Z, Chen JC and Chen GQ, Effect of acrylic acid on Production of D-(-)-3-hydroxybutyric acid and D-(-)-3-hydroxyalkanoic acid by recombinant Escherichia coli. FEMS Microbiol Lett., 2003, 218: 59±64. Wu Q, Zheng Z, Xi JZ, Gao HJ and Chen GQ, Production of 3-(R)-Hydroxybutyric acid by Recombinant Escherichia coli HB101 Harboring Genes of phbA and phbB. Chem Eng J Japan, 2003, 36: 1170±1173. Zheng Z, Zhang MJ, Zhang G and Chen GQ. Production of 3-hydroxydecanoic acid by recombinant Escherichia coli HB101 harboring phaG gene. Antonie van Leeuwenhoek, 2004, 85: 93±101. Zheng Z, Gong Q, Liu T, Deng Y, Chen JC and Chen GQ, Thioesterase II of Escherichia coli plays an important role in 3-hydroxydecanoic acid production. Appl Environ Microbiol., 2004 70: 3807±3813.

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70. Lee SY, Lee Y and Wang FL, Chiral compounds from bacterial polyesters: Sugars to plastics to fine chemicals. Biotechnol Bioeng., 1999, 65: 363±368. 71. He WN, Zhang ZM, Hu P and Chen GQ, Microbial synthesis and characterization of polyhydroxyalkanoates by strain DG17 from glucose. Acta Polymerica Sinica, 1999, 6: 709±714. 72. Xi JZ, Wu Q, Yan YB, Zhang ZM, Yu HP, MK Cheung, Zhang RQ and Chen GQ, Hyperproduction of polyesters consisting of medium-chain-length hydroxyalkanoate monomers by strain Pseudomonas stutzeri 1317. Antonie van Leeuwenhoek, 2000, 78: 43±49. 73. Hong K, Chen GQ, Yu PH, Zhang G and Liu Y, Effect of C/N ratio on monomer composition of polyhydroxyalkanoate (PHA) produced by Pseudomonas mendocina 0806 and Pseudomonas pseudoalkaligenus YS1, Appl. Biochem. and Biotechnol., 2000, 84±86: 971±980. 74. Tian WD, Hong K, Chen GQ, Wu Q, Zhang RQ and Huang WY, Production of polyesters consisting of medium chain length 3-hydroalkanoic acids by Pseudomonas mendocina 0806 from various carbon sources. Antonie van Leewenhoek, 2000, 77: 31±36. 75. Chen GQ, Xu J, Wu Q, Zhang ZM and Ho KP, Synthesis of copolyesters consisting of medium-chain-length-hydroxyalkanoates by Pseudomonas stutzeri 1317. Reactive & Functional Polymers, 2001, 48: 107±112. 76. Zhang G, Hang XM, Green P, Ho KP and Chen GQ, PCR Cloning of Type II Polyhydroxyalkanoate Biosynthesis Genes from two Pseudomonads strains. FEMS Microbiology Lett., 2001, 198 (2): 165±170. 77. Hang XM, Zhang G, Wang GL, Zhao XH and Chen GQ, PCR cloning of polyhydroxyalkanoate biosynthesis genes from Burkholderia caryophylli and their functional expression in recombinant Escherichia coli. FEMS Microbiol Lett., 2002, 210: 49±54. 78. Hang XM, Lin ZX and Chen GQ, Polyhydroxyalkanoate Biosynthesis by Pseudomonas pseudoalcaligenes YS1. FEMS Microbiol Lett., 2002, 212 (1): 71±75. 79. Jung K, Hazenberg W, Prieto M and Witholt B, Two-stage continuous process development for the production of medium-chain-length poly(3-hydroxyalkanoates). Biotechnol Bioeng., 2001, 72: 19±24. 80. Yang XS, Zhao K and Chen GQ, Effect of Surface Treatment on the Biocompatibility of Microbial Polyhydroxyalkanoates. Biomaterials, 2002, 23 (5): 1391±1397. 81. Zhao K, Yang XS, Chen JC and Chen GQ, Effect of Lipase Treatment on the Biocompatibility of Microbial Polyhydroxyalkanoates. J Mater Sci. Mater in Med., 2002, 13: 849±854. 82. Zhao K, Deng Y, Chen CJ and Chen GQ, Polyhydroxyalkanoate (PHA) Scaffolds with Good Mechanical Properties and Biocompatibility. Biomaterials, 2003, 24 (6): 1041±1054. 83. Deng Y, Zhao K, Zhang XF, Hu P and Chen GQ, Study on the three-dimensional proliferation of rabbit articular cartilage derived chondrocytes on polyhydroxyalkanoate scaffolds. Biomaterials, 2002, 23 (20): 4049±4056. 84. Zheng Z, Deng Y, Lin XS, Zhang LX and Chen GQ, Induced Production of Rabbit Articular Cartilage-Derived Chondrocytes Collagen II on Polyhydroxyalkanoates Blend. J Biomater Sci. Polymer Edn, 2003, 14: 615±624. 85. Deng Y, Lin XS, Zheng Z, Deng JG, Chen JC, Ma H and Chen GQ,

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Poly(hydroxybutyrate-co-hydroxyhexanoate) Promoted Production of Extracellular Matrix of Articular Cartilage Chondrocytes in vitro. Biomaterials, 2003, 24: 4273± 4281. 86. Yang M, Zhu SS, Chen Y, Chang ZJ, Chen GQ, Gong YD, Zhao NM and Zhang XF, Studies on bone marrow stromal cells affinity of poly(3-hydroxybutyrate-co-3hydroxyhexanoate). Biomaterials, 2004, 25: 1365±1373.

3

Oxo-biodegradable polyolefins D M W I L E S , Plastichem Consulting, Canada

3.1

Introduction

Most scientists, even some polymer chemists, will not be familiar with this type of thermoplastic, so a description is called for. The term oxo-biodegradable plastic is commonly restricted to those plastics that, owing to particular compositional features or by means of additive chemistry, undergo oxidative degradation much more rapidly than would otherwise occur in a variety of environments. Although the plastic initially is bioinert, the oxidation products are biodegradable.

3.1.1 Characteristics This recent designation for a specific category of thermoplastics covers those hydrocarbon polymers that oxidize in the environment following which the oxidation products are assimilated by naturally occurring microorganisms. In principle, of course, all polyolefins will eventually undergo oxidative degradation in the environment, at rates that depend on conditions. Factors such as temperature, UV radiation (i.e., exposure to sunlight) and mechanical stress (e.g., wind, rain) control the rates at which polyolefins oxidize. Oxygen must, of course, be present. Even in the absence of antioxidants, the time required for unmodified polyolefins to degrade to brittleness by oxidation in various environments can vary from months to decades. Oxo-biodegradable polyolefins, on the other hand, can become brittle and disintegrate in the environment in a matter of months or even weeks, depending on the conditions. Polyolefins are generally understood to be relatively bioinert. Indeed, polyethylene is specified as a negative control in the ASTM Standard Test Method (D5338) for Determining Aerobic Biodegradation of Plastic Materials. It is now well established, however, that the oxidation products of polyethylene are readily biodegradable (see section 3.5). In order to achieve rates of oxobiodegradation for polyolefins that are compatible with established practices of plastics use and disposal, it is necessary to add one or more of an assortment of

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Biodegradable polymers for industrial applications

activating/sensitizing additives or structural units to conventional polyolefin formulations (see section 3.3.2). It is these `activated' polyolefins that are referred to as oxo-biodegradable polyolefins. It is worth remembering that oxo-biodegradation is of major significance in nature as a principal pathway by which the chemical elements in organic materials, e.g., wood and other lignin-containing materials, natural rubber, are returned to the biocycle.

3.1.2 Definitions The definition of biodegradable plastic used by the ASTM (D6002 or D6400) is `a degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi and algae.' This is fine for that part of the degradation of plastics that is biotically driven but it ignores the abiotic molar mass reduction that occurs by hydrolysis in the case of linear polyesters, for example, prior to the bioassimilation of the hydrolysis products. The ASTM definition also ignores the oxidative degradation of polyolefins that precedes the bioassimilation of the oxidation products. Clearly, a better definition is required. In this context, it is useful to consider for universal acceptance the following definition for oxo-biodegradable plastics: plastics that undergo degradation resulting from peroxidative and cell-mediated phenomena, either simultaneously or successively. This is based on the definition of oxo-biodegradation written into the proposed new BSi standard by Professor Gerald Scott (see Ch. 12). Another approach is that which emphasizes the parallelism between the two major classes of biodegradable plastics. Hydrobiodegradation is defined as `biodegradation in which polymer chain cleavage is primarily due to hydrolysis which may be mediated by abiotic chemistry, microorganisms or a combination of both.' Oxo-biodegradation is defined as `biodegradation in which polymer chain cleavage is primarily due to oxidation which may be mediated by abiotic chemistry, microorganisms or a combination of both.' These definitions are being considered for incorporation in a terminology document (N88) by TC249/ WG9 of the CEN (see Ch. 12).

3.2

Polyolefin peroxidation

The oxidative degradation of hydrocarbon (or carbon chain) polymers such as polyolefins is a branching chain reaction involving, among other things, the cyclical formation and reaction of hydroperoxide groups attached to carbon atoms in the polymer backbone. It is a series of individual, simple processes that collectively are referred to as oxidation, or oxidative degradation, or peroxidation, which terms are used interchangeably in this chapter.

Oxo-biodegradable polyolefins

59

3.2.1 Basic chemistry It is generally recognized that there will be some oxidation of polyolefins (usually trace amounts) during their melt processing, notwithstanding the routine use of processing antioxidants and the exclusion of air from the processing equipment. The net result of this will be a very low concentration of hydroperoxide groups that are attached to carbon atoms in the polymer chains. It is commonly the case that the level of hydroperoxides is too low to be detected by FTIR spectroscopy but these chromophores are believed to be significant in the photosusceptibility of polyolefins which, if pure, would be transparent to terrestrial sunlight and therefore unaffected by it. It is not necessary or helpful to discuss in detail here all the potential chromophoric impurities in commercial polyolefins, and it is not important in this chapter to do so. Since the peroxidation chemistry of polyolefins is a cyclic chain reaction, it is a matter of convenience to summarize the important processes starting with the hydroperoxide group, as follows: RH (heat, O2 , stress) ! ROOH ROOH (heat and/or UV light) ! RO  ‡  OH OH ‡ RH ! H2 O ‡ R R  ‡O2 ! RO2  RO2  ‡RH ! ROOH ‡ R

3:1 3:2 3:3 3:4 3:5

where RH represents a polyolefin molecule This is interesting chemistry that has been investigated by many scientists in many laboratories1±12 and there is no doubt about the universal applicability of it to many thermoplastic products. In the absence of stabilizing additives, the oxidation is auto-accelerating for a time, owing to the fact that it is a branching chain reaction. The well-defined termination processes are not shown here, for the sake of brevity. In the context of oxo-biodegradable plastics, however, it is necessary to consider those associated chemical processes that result in polymer chain cleavage, and the loss of mechanical properties that is the inevitable result. These latter processes must also be involved in the conversion of stable, inert polyolefins into biodegradable intermediates (see section 3.5). RO  ‡RH (very rapidly) ! alcohols, acids

3:6

esters, ketones The alkoxy radical (RO) is exceedingly unstable. It will decompose spontaneously to form ketones or aldehydes (depending on the structure of the `parent' hydroperoxide group) and these carbonyl compounds will undergo further oxidation to produce acids esters and the like that are similar to naturally occurring materials. Professor Albertsson and her colleagues13,14 have

60

Biodegradable polymers for industrial applications

investigated the products of the thermal and photochemical degradation of low density polyethylenes, some of which contained prodegradant additives. These products are formed in specific processes that follow on from reactions of the alkoxy radical, such as O | ~CH2±C±CH2~ | R0 O | ~CH2±C±CH2~ | R0

!

O || ~CH2±C±R0 ‡ CH2~

3.7

!

O || ~CH2±C±CH2~ ‡ R0

3.8

where R0 can be an alkyl group, in the case of a tertiary alkoxy radical, or H in the case of a secondary alkoxy. Unless the parent hydroperoxide group is close to the end of the polymer molecule to which it is attached, the spontaneous reaction (referred to as the scission reaction) of the RO group will lead to significant molar mass reduction two times out of three. The other type of reaction of the alkoxy radical is abstraction of a hydrogen atom from a nearby polymer chain segment (reaction 3.9). O | ~CH2±C±CH2~ | R0

RH ÿ!

OH | ~CH2±C±CH2~ ‡ R | R0

3.9

This will not result directly in polymer chain scission, i.e., molar mass reduction, but it does result in the formation of another reactive alkyl radical (R) on a polymer chain that can initiate another oxidation sequence, e.g., reaction 3.4. The individual processes shown above (reactions 3.1 to 3.9) do not by any means represent all the relevant chemistry that occurs upon the exposure of unstabilized polyolefins to heat and/or UV light in the environment. What these equations do emphasize, however, is the involvement of the hydroperoxide groups in the molar mass reduction phenomenon. It must also be emphasized that the formation of these groups is rate-determining in the heat or UV-light initiated oxidative degradation. The only major difference between these two types of initiation is that the intermediate ketones that are formed are UV-

Oxo-biodegradable polyolefins

61

sensitive but stable to heat. Otherwise the oxidation chemistry is very similar. Additional details can be found elsewhere.1±3

3.2.2 Additive chemistry The addition of small amounts of non-polymeric materials to rubbers and plastics is well established as a practical and cost-effective method for obtaining and preserving specific properties. Indeed, many commercial products contain several kinds of additive, each performing a specific task. Antiozonants, antistatic agents, antioxidants, flame retardants, photostabilizers and of course dyes and pigments have been used to good effect for many years. It is doubtful that any products made primarily from rubbers and plastics, except perhaps those employed in vivo, are produced or used without any additives. It must be remembered that there is a limit to the amount of additive that can be used with rubbers and plastics. Not only are the additives usually more expensive than the polymers, they do not contribute to those useful properties of the product that derive from the macromolecular nature of the base material. Moreover there is a limit to how much additive can be incorporated and retained (e.g., by dissolution) in the macromolecular matrix2 and excessive amounts will exude to the surface. For many years, a primary focus for polymer scientists and technologists was the development of materials and methods for prolonging the service life of plastics. As the details of the peroxidation of hydrocarbon polymers were identified, emphasis was placed on the hydroperoxide groups referred to above.4±7 Long-term stabilization of several types of thermoplastics involved the use of two kinds of stabilizing additives, those that could reduce the rate of formation of hydroperoxides during processing and use, and those that could bring about the decomposition of hydroperoxides that did form, in harmless, i.e., non-radical reactions. Stabilizing additives were also developed specifically to protect susceptible hydrocarbon polymers from the degradative effects of terrestrial sunlight. UV stabilizers are commonly added to polyolefins when such materials are expected to have a long service life outdoors. It should be noted that portfolios of highly effective stabilizers are routinely used today, having specific structural and chemical features that provide for processing antioxidant, long-term antioxidant and UV stabilization characteristics. It is additives in each of these categories that collectively have contributed in a major way to the ubiquity of plastics in virtually all facets of life. While most plastics are produced, fabricated and used today with the help of additives of various types, the emphasis here is on those used to enhance the usefulness of polyolefins. Thousands of publications in the past 35 years deal with various facets of polyolefin degradation and stabilization. The interested reader is urged to consult monographs and reviews in articles1±12 to obtain a `feel' for this complex and fascinating area of endeavour. It

62

Biodegradable polymers for industrial applications

continues to be an intellectual challenge as well as a subject of immense commercial significance.

3.3

Control of polyolefin lifetimes

3.3.1 Longer As was mentioned in the previous section, a great deal of effort has been expended in developing stabilizing additives that prolong the useful life of polyolefin plastic products. This work, performed in several countries over many years, has resulted in highly durable materials for extended use in transportation, construction and other consumer product areas. The damaging factors in the use environments are primarily heat, the UV component of sunlight, and mechanical stress, although combinations of these factors are usually involved. One of the most stressful and chemically damaging things that can be done to a polyolefin (or any other thermoplastic for that matter) is to heat it well above its melting point and force it through a small orifice. And that is precisely what must be done, commonly at least twice, in order to fabricate products made from polyolefins. The additives that protect the polymers during processing, thermal antioxidants, are remarkably effective in `deactivating' radicals that, if ignored, will inevitably cause the formation of hydroperoxide groups. In general, however, processing antioxidants are not particularly effective in protecting polyolefins from thermally initiated oxidation over a prolonged period of use. They are also not effective in providing protection against oxidative degradation initiated by near-UV radiation (unfiltered sunlight). The use of photostabilizers is required and these are available. Control of product lifetime necessitates the use of the right amount of the correct stabilizer(s). Invariably, reduction in the rate of formation and decomposition of polymer-bound hydroperoxides must occur in order to obtain long service life. References to earlier work, and relevant discussion are found in Ch. 17.

3.3.2 Shorter In many cases, products made from polyolefins are needed for only a short time and are then disposed of in one of several ways. Packaging in general and food packaging in particular are uses for which only a relatively short service life is required (see Ch. 16). Typically carrier bags or shopping bags made of polyethylene are used a few times only. Including storage life and use life, the total time during which they are required to maintain their useful properties is likely to be a year or so, perhaps even less. In some places, shopping bags are collected after use for recycling, but in many instances used bags are disposed of in landfills or, unfortunately, sometimes end up as litter. Commonly, used shopping bags are utilized for the collection of kitchen and other household

Oxo-biodegradable polyolefins

63

waste before being dumped in landfills. In such environments the polyethylene bags can persist for decades and can also retard the biodegradation of food waste, paper and the like.12 Similar situations and conditions exist for used, discarded trash bags, food wrappings, containers for prepared and `fast' foods, and analogous products. What would seem to be required here are polyolefin products that have the normal storage and service lives during which the normal very useful characteristics of polyolefins persist. Following use and disposal, however, these properties must be lost relatively quickly so that valuable landfill space is not occupied for decades by recalcitrant plastics, and so that the normally biodegradable waste organics that persistent plastics will otherwise `protect' will biodegrade aerobically. Polyolefins having this rapid degradation-after-disposal characteristic cannot be produced simply by leaving out processing antioxidants and adding an oxidizing agent. Such materials would not survive processing and could not have a predictable storage and use life; they would be likely to fall apart prematurely. Useful oxo-biodegradable polyolefins must and do have controlled lifetimes.

3.4

Oxidative degradation after use

In the sequence of chemical and microbiological events that are required to convert polyolefins to carbon dioxide and water, it is oxidative degradation that is required initially to reduce molar mass values by an order of magnitude and concomitantly convert hydrophobic plastics into water wettable materials. In contrast to polyolefins in the as-produced and as-used condition, oxidized polyolefins are biodegradable, as explained in section 3.5. The challenge is to ensure that significant oxidative degradation of oxo-biodegradable plastics does not occur during fabrication, storage or use. After these plastics are discarded, however, the normal peroxidation processes must occur much more rapidly than with normal polyolefins, at rates which are commensurate with the disposal environment. This can be accomplished by the incorporation in the resin of a small amount of an additive that catalyzes the decomposition of hydroperoxide groups that are fastened to carbon atoms in the polymer chains. Transition metal salts are commonly used as prodegradant additives, and the chemistry3,16 is shown here using iron as an example. Fe2+ ‡ ROOH ! Fe3+ + RO ‡ OHÿ Fe3+ ‡ ROOH ! Fe2+ ‡ ROO ‡ H‡

3.10 3.11

It has been known for some time that these redox reactions amount to a lowering of the activation energy of bimolecular peroxide decomposition.3 This decomposition by a redox couple (a transition metal in two oxidation states) will be a factor in both heat and light-induced oxidation.

64

Biodegradable polymers for industrial applications

It must be emphasized that metal salt additives of this type are not, by themselves, initiators of the oxidation of oxo-biodegradable plastics. They do not affect the normal processing or use of conventional polyolefins. The usual fabricated products look and act like conventional polyolefin products during storage and use. The products can be recycled in the normal ways. In order for transition metal-based prodegradants to demonstrate their benefits, initiation of oxidation of the polyolefins must occur as a result of one or more factors in the disposal environment. These factors, the initiators of the conventional oxidation processes, are heat, UV light (unfiltered sunlight) and mechanical stress. Usually there is a combination of these factors operating in the environment. Let us see how this works.

3.4.1 Initiation Details concerning the thermal and photochemical oxidation have been published in a large number of papers and numerous books, e.g., references 1±12 and references therein. The summary in section 3.2 of this chapter refers to the formation of polymer-bound oxidation products and to molar mass reduction. After use, polyolefin plastic products that are not recycled will, in many cases, be discarded in landfills. Conventional polyolefins will not begin to oxidize until all residual stabilizers are consumed and, even then, will not oxidize rapidly after `burial' in the mass of waste material. They have been shown to remain intact for decades. 15 For some applications, such as agricultural mulch films, components in the additive formulations of oxo-biodegradable polyolefins provide for stabilities of several months, up to a year or more Oxobiodegradable polyolefins designed for a short use-life, in contrast, will begin to oxidize immediately after being disposed of, because there is no stabilizer present and the rate of peroxidation is greatly enhanced by the presence of the prodegradant additive. The microbially generated heat in a landfill is significant, enough to raise temperatures well above ambient air temperatures. In this environment, polyolefins containing pro-oxidant additives oxidize and disintegrate readily under conditions that leave conventional polyethylene virtually unchanged. EPI Environmental Products Inc. has developed a range of prodegradant formulations called Totally Degradable Plastic AdditivesÕ (TDPA). When added to conventional polyolefin resins, TDPA results in controlled product lifetimes (before the onset of oxidative degradation) followed by enhanced degradation and fragmentation. Results of the exposure to heat and light of polyethylene and polypropylene films containing EPI's TDPA are illustrated in Table 3.1 for laboratory and outdoor testing. The characteristics of a TDPA-polyethylene film product in a landfill environment are illustrated in Table 3.2. The temperatures in commercial composting are much higher than in a landfill, commonly above 70 ëC for some days and in excess of 60 ëC for weeks.

Oxo-biodegradable polyolefins

65

Table 3.1 Accelerated degradation of TDPAÕ-containing polyolefins Sample

MIa (g/10 min.)

b (MPa)

c (%)

0.15 ö ö 17.1

41.7 20.6 16.8 16.2

548 18.1 9 3

HDPE carrier bag/TDPA : no exposure QUV exposure (hours): 96 144 216

HDPE carrier bag/TDPA: outdoor exposure: carbonyl index = 0.31 after 59 days (with no TDPA, no embrittlement after outdoor exposure) PP packaging bag/TDPA : no exposure QUV exposure (hours): 48 72 96

14.6 ö ö ö

47.3 23.9 17.8 10.3

904 407 139 1

PP packaging film/TDPA: outdoor exposure: MI = 40 after 90 days (with no TDPA, no embrittlement after outdoor exposure) PP packaging film/TDPA: lab oven at 71 ëC: film fragmented after 36 days (with no TDPA, no embrittlement after heat ageing) a: melt index; b: tensile breaking strength; c: elongation at break

The oxidative degradation of oxo-biodegradable polyolefins in such environments is very rapid indeed. The temperatures that develop in landfills or even in composting are not nearly as high as processing temperatures, and so the question arises as to how one can process oxo-biodegradable polyolefins above the melt and still retain a built-in sensitivity to heat-induced oxidation that will operate after use and disposal. The answer, of course, is the presence of a small amount of processing antioxidant in the resin prior to film/bag/container fabrication. This can be in the form of residue from a previous blending/pelletizing procedure or it could be added intentionally prior to fabrication. Very little of the processing antioxidant needs to persist after the fabrication of the consumer products in order for the articles to have a practical shelf life/use life combination. Table 3.2 Effects of landfill burial (10 months) on LDPE film with and without TDPAÕ prodegradant Sample Control (unburied) Control (recovered) TDPA-PE (unburied) TDPA-PE (recovered)

MIa (g/10 min.)

A (C=O)b

Mwc

0.75 1.11 0.76 13.3

0 0.15 0 2.31

114,000 107,000 115,000 4,250

a: melt index (ASTM D1238); b: IR absorbance at 1715 cmÿ1; c: weight average molecular weight, by GPC

66

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3.5

Aerobic biodegradation

It is widely understood that conventional polyolefins are bioinert,17±20 meaning that there is negligible conversion of the carbon in these plastics to carbon dioxide as a result of the activities of microorganisms. This is a major advantage for polyolefins as materials for food packaging, for example, and in a great many applications where long-term durability is called for. Transportation, construction and consumer products applications are ubiquitous because the microbial conversion of polyolefins is not a factor. In the case of packaging plastics, however, and other plastics for which the use life is measured in months, persistence after use and disposal is a problem and bioinertness is a distinct disadvantage. Applied microbiology is a complex subject but it is a reasonable simplification to consider that the bioinertness of polyolefins is the result of their being hydrophobic, having relatively high molar mass values, and containing none of the functional groups that are readily attacked by microorganisms. For example, extra-cellular microbes operate in aqueous media but they cannot wet the polyolefin surface. On the other hand, polyethylene that has undergone significant oxidative degradation can support microbial growth.17±24 This is because the formation of oxidation products on and near the surface of the plastic have rendered it water-wettable, and the reduction in molar mass has resulted in the formation of many more polymer chain ends, the point at which many extra-cellular enzymes commonly react with a substrate. The products of the aerobic biodegradation of polyethylene are carbon dioxide, water, biomass (dead microbial cells) and humic material. This is presumed to be the situation with conventional polyethylene, although in the usual disposal environments, stabilized polyethylene will take a very long time to oxidize. Oxo-biodegradable polyethylene (or activated, or photosensitized polyethylene) in contrast oxidizes very much more rapidly after disposal and, as a consequence, the biodegradation occurs very much sooner. It is the oxidation in oxo-biodegradation that is the rate-determining process. Perhaps owing to some misunderstandings created by laboratory work that was published more than 30 years ago, there was a commonly held conviction that only hydrocarbon molecules below a molar mass of 500 would support microbial growth. The results of that work were valid, but they have no relevance to the biosusceptibility of polyethylene oxidation products in various environments. The actual molar mass values at which biodegradation of these molecules occurs is very much higher, probably in the tens of thousands.19

3.6

Applications of oxo-biodegradable polyolefins

It was noted17 more than 30 years ago that unwanted plastic waste, in particular discarded polyethylene, could be disposed of by oxidizing it prior to exposure to

Oxo-biodegradable polyolefins

67

thermophilic fungi. Preoccupation with littered plastics at that time tended to focus attention on photooxidative degradation by unfiltered sunlight as a means of changing bioinert polyethylene to biodegradable oxidation products. This led in turn to the synthesis of photosensitive copolymers containing polymer-bound ketone groups. The requirement was seen to be for polyolefin copolymers that would, after being discarded, photodegrade, become brittle, and fragment to the point of disappearing, as a result of outdoor weathering. Subsequently, research has focused on commodity plastics that will be sensitive to heat as well as to near UV light so that oxidation during or after use does not depend solely on exposure to sunlight. There are many applications for polyolefin products that are characterized by a limited use period (months) following which degradation should occur relatively rapidly (also months) leading to bioconversion (up to two years, or even longer). They all depend on having a short but controlled service life and their final disposal may not include exposure to sunlight. This type of oxo-biodegradable polyolefins rely on the addition of a small amount of prodegradant blended with conventional polyolefin resin. The chemistry involved has been described in sections 3.2, 3.3 and 3.4. Let us look at these two different types of technology in turn.

3.6.1 Ketone copolymers It has been known for many years that carbonyl groups fastened to hydrocarbon polymer molecules would sensitize those molecules to the absorption of near UV wavelengths in sunlight. Quantum considerations apply to the absorption of photons so that in the absence of specific chromophores, polyolefins would be transparent to sunlight. If ketone groups, for example, are introduced into polyolefin molecules, they will absorb incident radiation having near UV wavelengths. A review of the science of ethylene-carbon monoxide copolymers and their use in plastics litter control has been published.25 The science is based on the research of Hartley and Guillet at the University of Toronto, and the widespread use of the concept was first implemented by Hi-ConeTM as a photodegradable version of their loop carriers for beverage can six-packs. These carriers have been suitably developed so as to meet the legislative requirements in many of the US states, and the requirements of the US Environmental Protection Agency. The carriers are considered to be highly litter-prone but the photodegradable versions have been singularly successful in reducing both litter accumulation and wildlife entanglement problems that used to occur. Professor Guillet and his colleagues discovered that the quantum efficiency for photodegradation was far higher if the ketone group was alpha to the main polymer chain rather than part of it. He went on to develop the EcolyteTM process26 by which vinyl ketones are copolymerized with ethylene, propylene, styrene, etc., in addition to polymerization. Analogous procedures can be used to produce readily photodegradable condensation polymers. Such plastics can be

68

Biodegradable polymers for industrial applications

synthesized to have very little sensitivity to indoor lighting but to photooxidize quite rapidly in sunlight or even skylight. It should be noted that lifetime control is achieved by polymer design; increasing the ketone content will increase the rate of photodegradation. You need a different polymer for each different application. It was reported from Union Carbide25 that the photooxidation products of ethylene-carbon monoxide copolymers do not reach low enough molar mass values to support bioassimilation. This conclusion may have been based, however, on a misinterpretation of the earlier work with unoxidized hydrocarbons and a very limited number of microbial cultures. Guillet, in contrast, demonstrated26 that photodegraded Ecolyte polyethylene and polypropylene are bioassimilated by common soil bacteria, albeit rather slowly.

3.6.2 Scott/Gilead technology The first specific evidence that conventional polyethylene film could be oxidized rapidly using a metal salt prodegradant additive to produce biodegradable products was published by Scott, Lemaire and coworkers in 1994.19 They showed that abiotic iron-catalyzed oxidation (photo- or thermal oxidation) of commercial polyethylene was followed by biodegradation of the oxidation products from the surface. Among the other significant results was the observation that relatively high molar mass carboxylic acid oxidation products, with Mw as high as 40,000, were bioassimilated before water was able to leach them away. Of particular importance is the controlled performance of polyethylene agricultural products based on Scott/Gilead technology,27±31 and on the proof that there are no toxic or harmful residues (see Ch.17). It should be noted that the ferrous/ferric redox couple was used as an example in section 3.4 of prodegradant catalytic activity. The Scott/Gilead technology uses iron salts that are stabilizers initially but which are converted photochemically to prodegradants.

3.6.3 EPI technology More than ten years ago EPI Environmental Products Inc. developed additive technology that has since been commercialized in a wide variety of oxobiodegradable materials.31±33 TDPAÕ (Totally Degradable Plastic Additives) concentrates are blended with conventional polyolefin resins by EPI's customers to fabricate controlled-lifetime products such as carrier bags, compost bags, food wrapping, trash bags, packaging and cutlery for fast-food outlets, and daily landfill covers. Ciba Specialty Chemicals, in collaboration with EPI, has developed a number of agricultural products that require a controlled life of several months followed by relatively rapid oxo-biodegradation, e.g., mulch films. These products, marketed under the trade name EnvirocareTM also involve TDPA

Oxo-biodegradable polyolefins

69

technology. Initiation of oxidation at the end of the service life is principally the result of exposure to the near-UV component of terrestrial sunlight; biodegradation of the oxidation products is the result of soil microorganisms. The requirements for one-way commercial compost bags are an ideal `fit' with the properties of those made using EPI concentrate and conventional polyethylene resins. These properties are strength, stretchiness, toughness, flexibility, light weight, high wet strength, and low cost. In addition, during the composting process, the bags must undergo significant molar mass reduction and become brittle as a result of thermal oxidation, and they must then disintegrate as a result of mechanical handling. The compost bags must not interfere with the composting of the normal organic waste material, and must themselves begin to biodegrade, producing top quality compost and leaving no harmful residue. Conventional polyethylene bags cannot meet the degradation requirements. A major trial of compost bags that had been fabricated using EPI's TDPA technology was carried out at the Vienna Neustadt facility in Austria. All the requirements were met.33 Many types of post-consumer plastic packaging that are not recovered by recycling are disposed of in landfills. There are many environmental advantages (see section 3.7) to be achieved by using oxo-biodegradable polyolefins in these applications but safety considerations are equally important. Keller & Heckman (the US/International specialist law firm) conducted the certification and validation work for TDPA formulations required by the US Food and Drug Administration. Most TDPA formulations have been cleared for use in any type of food packaging, for any type of food. Likewise, TDPA formulations are considered safe by the SCF (Scientific Committee for Food) of the European Union.

3.7

Environmental impact

It has been known for a number of years that there are environmental advantages in using plastics instead of other materials as packaging, as containers and in numerous other applications.26,34 Specifically, as an example, the advantages of polyethylene shopping bags versus unbleached kraft paper bags include lower gaseous and waste-water burdens and lower energy requirements for production, and high wet strength, lower weight and volume for the product ± all in favour of the polyethylene material. Oxo-biodegradable polyolefins retain all these advantages, of course, but have the additional advantage of controlled lifetime and relatively rapid oxo-biodegradation after being discarded, without precluding the potential for recycling.

3.7.1 In landfills There will be a period of time (months, possibly a year or more) after the dumping in a landfill of each container of waste when there is enough access of

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Biodegradable polymers for industrial applications

Table 3.3 Degradation of EPI's EnviroÕCover in landfill trials Sample location

MIa (g/10 min.)

b (MPa)

c (%)

A(C=O)d

0.44 0.55 0.72

24.5 14.0 8.0

550 450 130

0.18 0.26 0.42

ö ö ö

24.0 22.6 10.1

480 450 40

0.24 0.26 0.59

Landfill in Canadae Control (unburied) 1 m below surface 2 m below surface Landfill in Chinaf Control (unburied) 20 cm below surface 2 m below surface

a: melt index (ASTM D1238); b: tensile breaking strength; c: elongation at break; d: IR absorbance at 1715 cmÿ1; e: Chilliwack landfill site, British Columbia, Canada, from Dec. 95 to Mar. 96; f: Shenzhen Xiapin landfill site, China, from Oct. to Dec. 98

air and water to the organic waste (including the oxidized plastic) to allow vigorous aerobic biodegradation. This will reduce the volume of waste relatively rapidly and thereby prolong the useful life of the landfill. Much more of the carbon in the organic waste will be converted to carbon dioxide than to methane (the major product from anaerobic biodegradation of organic waste) provided that plastic bags and other discarded plastics do not remain intact and restrict the flow of gases and liquids through the waste mass. Eventually, compaction of the waste at the lower levels will preclude aerobic processes. Discussion of the environmental advantages of this situation is to be found in Ch. 16 and elsewhere.15,32 Proper management of landfills requires the application of a daily cover for aesthetic and hygienic reasons.15 This cover has commonly been 15 cm of soil which, although effective, is also usually expensive and consumes quite a bit of space. A polyethylene film applied daily to the active face could do the job but, because the film would persist, this would exacerbate the problem of restricted flow of gases and liquids through the bulk of the waste. It is this problem, caused by persistent plastics, that retards the aerobic biodegradation of organic material in landfills and thereby contributes to the premature filling of the sites. Daily landfill cover that is made from EPI's TDPA-polyethylene does the required covering and protecting job but subsequently oxidizes and disintegrates in the warmth of the landfill over a period of months (see Table 3.3). In this way the aerobic biodegradation of organic waste is enhanced.

3.7.2 Composting In commercial composting, it is essential that good quality material is produced. It must look good, and this means that no unsightly pieces of intact plastic may

Oxo-biodegradable polyolefins

71

persist after the composting operations are completed. The compost bags, based on EPI's TDPA technology, that were evaluated in the Vienna Neustadt facility, disintegrated to the extent that this criterion was met.33 Premium quality compost must meet national and international requirements for very low levels of residual metals. EPI bags met this requirement as well. It is essential that there be no harmful or toxic residues from the oxo-biodegradation of polyolefins. For this reason, the product from the composting of 10,000 of the TDPA-PE bags at the Austrian facility was evaluated for ecotoxicity at the BVA laboratory in Linz, Austria and at the OWS laboratory in Ghent, Belgium. The results from Austria showed no negative effects in the plant tolerance test, and no seeds were detected. The level of metals in the compost was very much below the allowable levels. In addition, no negative effects were observed in the work at the Belgian laboratory in the following tests: cress, summer barley plant growth, daphnia, earthworm (Table 3.4). Finally, there should be as much `unconverted' biomass and humic material as possible in the compost because this is what imparts the nutritive value in horticultural and agricultural applications of compost. If all the carbon in the compost bags had been converted to carbon dioxide during composting, then there is no `recovery' and a resource will have been wasted. This is a problem for some of those hydrobiodegradable plastics that biodegrade as rapidly as is required by ASTM D6400 and EN13432 (see Ch. 12) but is not a problem for bags made using EPI's compostable TDPA formulation. The ASTM recognized some years ago that the bioassimilation of the biomass in compost may usefully continue for some years after it has been applied to arable land for soil improvement.

3.7.3 Litter In spite of decades of consumer `education' and anti-litter regulations, there are still people who discard things carelessly. It would be unwise not to assume that such carelessness and indifference will continue. Owing to the persistence in the environment of discarded plastics, it must be conceded that advantages in litter reduction will accrue from the widespread use of oxo-biodegradable plastics in packaging. It has been demonstrated,26 using a simple mathematical model, that limiting the outdoor lifetime of discarded plastics is an effective method of preventing litter accumulation. The factors that initiate oxidative degradation in weathering outdoors include combinations of UV light, heat (especially in warm climates) and mechanical stress in the form of wind and precipitation. There is no suggestion here that littering should be condoned, let alone encouraged. It is worth noting, however, that there are oxo-biodegradable plastics available commercially right now that automatically prevent litter accumulation in addition to performing as they are designed to in composting, agricutural applications, and degradable packaging followed by landfill disposal.

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Biodegradable polymers for industrial applications

Table 3.4 Effect of TDPA-PE after composting (AC) A: On germination and plant yields Mediuma Compost ÿ blank Compost ‡ TDPA-PE ÿ AC Compost ÿ blank Compost ‡ TDPA-PE ÿ AC

Species

Germination (%)b

Plant yield (g)

cress cress barley barley

32.3 33.3 92 94

1.42 1.68 14.0 14.2

a: compost/soil ˆ 1/3; b: average germination rate as a % of total seeds added

B: On survival of daphnia Medium Compost ÿ blank Compost ‡ TDPA-PE ÿ AC

Dilution factorc

Survival (%)

10.2 6.4 4.0 10.2 6.4 4.0

100 60 12 97 83 40

c: dilution with fresh water

C: On survival and growth of earthworms Medium (wt%) (artificial soil/compost)

Survival (%)

Live weight (g/worm)

100/0 35/65 20/80 0/100

Compost contains no composted TDPA-PE 100 88 10 0

0.56 0.36 0.26 0

35/65 20/80 0/100

Compost contains TDPA-PE-AC 100 68 20

0.43 0.39 0.27

Polyethylene and polypropylene films, bags, containers and other articles made using EPI's TDPA technology meet all the essential requirements.31,32,35 Discarded plastics are a serious problem in freshwater as well as marine environments. The difficulties in addressing this situation include the complexity of identifying the origins of this pollution and the variety of plastics involved. Partial alleviation of these problems can be envisaged for those short use-life items that are or could be made from polyolefins. If these products are made from oxo-biodegradable polyolefins, then the combination of heat, sunlight and

Oxo-biodegradable polyolefins

73

mechanical stress in the environment will result in the oxidation of the material and bioconversion of the products. It has already been demonstrated36 that oxobiodegradable polyethylene film incorporating TDPA will, following oxidative degradation, undergo biodegradation in laboratory experiments involving water containing bacteria. Much of the pollution by discarded plastics in rivers, lakes and oceans has probably originated as land-based discards, and has subsequently been washed or blown into the water. Thus, the widespread use of oxobiodegradable polyolefins in packaging, for example, could result in a significant reduction of marine litter that started out on land.

3.8

Future developments

A great many consumer products are available at present that are made from oxo-biodegradable polyolefins, and the list continues to lengthen. Whenever recycling is not possible or not economically viable, the use of these newer materials would seem to be justifiable. Each new application requires more trials and testing but the science is sound and the technology is in place or being developed. It is to be hoped that the recent practice of taxing or banning the use of plastic carrier bags in a few jurisdictions does not spread. Those who advocate such actions either never knew or have known and subsequently forgotten the significant environmental and energy advantages of plastic containers over those made from any other materials.26,34 The preferred way to deal with unwanted persistence of used plastics is to reduce their lifetime after disposal.

3.8.1 Polypropylene and polystyrene The science and technology of oxo-biodegradable polyolefins is expected to be valid for any hydrocarbon polymer for which the peroxidation mechanism involves the formation/reaction of hydroperoxide groups as the rate-determining process. Obviously polypropylene is a prime candidate, and EPI technology involving TDPA is used in numerous polypropylene products that have a short use-life followed by disposal in landfills. It is well known that unstabilized polypropylene is readily oxidized by heat and UV light, and the use of TDPA prodegradants engenders controlled oxidative degradation after use and disposal.37 Thus, polypropylene packaging that incorporates TDPA has the same environmental advantages in landfill disposal as the TDPA-PE products described above. TDPA-PP has been shown37 to be effective in a wide variety of applications, e.g., straws, food containers, BOPP films. Recent studies38 have indicated that a number of polyolefins, including commercial isotactic polypropylene, are biodegradable after a photooxidative pre-treatment with near-UV radiation. Evidence of biodegradation was obtained using a variety of physical/chemical measurements, following incubation in a

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Biodegradable polymers for industrial applications

laboratory compost vessel, or a fungal culture test with A. niger. It remains to be determined how rapidly carbon dioxide will be evolved from oxidized polypropylene in biometric measurements. The oxidative degradation of polystyrene also involves the formation and degradation of hydroperoxide groups, primarily at the tertiary backbone carbon position in each repeat unit. This means that the incorporation of redox-type peroxide decomposers as prodegradants should produce controlled-lifetime polystyrene materials that undergo much more rapid oxidative degradation than is observed with conventional polystyrene. A collaborative project involving the US National Industries for the Blind and EPI demonstrated that injectionmoulded polystyrene cutlery containing TDPA could be fabricated in the normal way, using conventional equipment, to produce cutlery having the usual characteristics. It was much more sensitive to thermal oxidation, however.36 Such cutlery will degrade relatively rapidly in a landfill but it remains to be seen how rapidly it can biodegrade following oxidation.

3.8.2 Anaerobic environments Most of the disposal environments in which used, oxo-biodegradable polyolefins are likely to occur are characterized by enough oxygen and water to ensure both timely oxidative degradation and subsequent biodegradation of the oxidation products. Commercial composting and agricultural applications come to mind. Landfill disposal is not as straightforward, however, since conditions change as the landfill is used, and `ages' with time. Aerobic conditions obtain at the active face, of course, and for some time (not well defined) after, as the waste burden increases. As the site is filled up, however, air and water (from precipitation) will gradually be eliminated, and aerobic conditions will dominate in the lower levels. The principal target for oxo-biodegradable plastics is peroxidation to brittleness and fragmentation, within a year or so after disposal in a landfill. This permits the free movement of gases and liquids in the bulk of the waste so that as much of the organic component as possible biodegrades aerobically. This maximizes the conversion of carbon in the waste materials to carbon dioxide, and this is environmentally preferable to its conversion to methane under anaerobic conditions because methane is 24.5 times more potent a greenhouse gas than is carbon dioxide. But what happens to the oxo-biodegradable materials in the landfill that have oxidized but not yet bioassimilated? The answer to that question awaits the development of a suitable test method.

3.9

References

1. Scott G, Atmospheric Oxidation, Amsterdam, Elsevier, 1965. 2. Billingham N C and Calvert P D in Allen N S, Degradation and Stabilization of Polyolefins, London, Applied Science, 1±28, 1983.

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3. Carlsson D J and Wiles D M, Encyclopedia of Polymer Science and Engineering, 2nd edn, New York, Wiley, 4, 631±696, 1986. 4. Wiles D M, `The photodegradation of fiber-forming polymers' in Geuskens G, Degradation and Stabilization of Polymers, London, Applied Science, 137±155, 1975. 5. Wiles D M, `Photostabilization of macromolecules by excited state quenching', Pure Appl Chem, 1978, 50, 291±297. 6. Garton A, Carlsson, D J and Wiles D M, `Photooxidation mechanisms in commercial polyolefins' in Allen N S, Developments in Polymer Photochemistry ± 1, London, Applied Science, 93±123, 1980. 7a. Al-Malaika S and Scott G, `Thermal stabilization of polyolefins' in Allen N S, Degradation and Stabilization of Polyolefins, London, Applied Science, 247±281, 1983. 7b. Al-Malaika S and Scott G, `Photostabilization of polyolefins' in Allen N S, Degradation and Stabilization of Polyolefins, London, Applied Science, 283±333, 1983. 8. Hawkins W L, `The thermal oxidation of polyolefins ± mechanisms of degradation and stabilization' in Geuskens G, Degradation and Stabilization of Polymers, London, Applied Science, 77±94, 1975. 9. Chien J C W, `Hydroperoxides in degradation and stabilization of polymers' in Geuskens G, Degradation and Stabilization of Polymers, London, Applied Science, 95±112, 1975. 10. Grassie N and Scott G, Polymer Degradation and Stabilization, Cambridge, Cambridge University Press, 1985. 11. Scott G, `Autoxidation and antioxidants: historical perspective' in Scott G, Atmospheric Oxidation and Antioxidants I, London, Elsevier, 1±44, 1993. 12. Scott G, `Photodegradation and photostabilization of polymers' in Scott G, Atmospheric Oxidation and Antioxidants II, London, Elsevier, 385±489, 1993. 13. Karlsson S, Hakkarainen M and Albertsson A-C, `Dicarboxylic acids and Ketoacids formed in degradable polyethylenes by zip depolymerization through a cyclic transition state', Macromolecules 30, 7721±7728, 1997. 14. Khabbaz F, Albertsson A-C and Karlsson S, `Chemical and morphological changes of environmentally degradable polyethylene films exposed to thermo-oxidation', Polym Deg Stab 63, 127±138, 1999. 15. Swift G and Wiles D M, `Biodegradable and degradable polymers and plastics in landfill sites' in Kroschwitz J I, Encyclopedia of Polymer Science and Technology, Hoboken, John Wiley & Sons, in press. 16. Osawa Z, `Metal catalyzed oxidation and its inhibition' in Scott G, Atmospheric Oxidation and Antioxidants II, London, Elsevier, 327±362, 1993. 17. Eggins H O W, Mills J, Holt A and Scott G, `Biodeterioration and biodegradation of synthetic polymers' in Sykes G and Skinner F A, Microbial Aspects of Pollution, London, Academic Press, 267±277, 1971. 18. Albertsson A-C and Karlsson S, `The 3 stages in degradation of polymers ± polyethylene as a model substance,' J Appl Polym Sci, 35 (5) 1289±1302, 1988. 19. Arnaud R, Dabin P, Lemaire J, Al-Malaika S, Choban S, Coker M, Scott G, Fauve A and Maarooufi A, `Photooxidation and biodegradation of commercial photodegradable polyethylenes', Polym Deg Stab 46, 211±224, 1994. 20. Weiland M, Daro A and David C, `Biodegradation of thermally oxidized

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polyethylene', Polym Deg Stab 48, 275±289, 1995. 21. Karlsson S and Albertsson A-C, `Techniques and mechanisms of polymer degradation' in Scott G and Gilead D, Degradable Polymers: Principles and Applications, London, Chapman & Hall, 29±42, 1995. 22. Albertsson A-C, Anderson S O and Karlsson S, `The mechanism of biodegradation of polyethylene', Polym Deg Stab 18, 73±87, 1987. 23. Chiellini E, Corti A and Swift G, `Biodegradation of thermally oxidized, fragmented low-density polyethylenes', Polym Deg Stab 81, 341±351, 2003. 24. Jakubowicz I, `Evaluation of biodegradable polyethylene', Polym Deg Stab 80, 39± 43, 2003. 25. Harlan G and Kmiec C, `Ethylene-carbon monoxide copolymers' in Scott G and Gilead D, Degradable Polymers: Principles and Applications, London, Chapman & Hall, 153±168, 1995. 26. Guillet J E, `Plastics and the environment' in Scott G and Gilead D, Degradable Polymers: Principles and Applications, London, Chapman & Hall, 216±246, 1995. 27. Bonhomme S, Cuer A, Delort A-M, Lemaire J, Sanceline M and Scott G, `Environmental biodegradation of polyethylene', Polym Deg Stab 81, 441±452, 2003. 28. Scott G, `Photo-biodegradable plastics' in Scott G and Gilead D, Degradable Polymers: Principles and Applications, London, Chapman & Hall, 169±185, 1995. 29. Gilead D, `Photodegradable plastics in agriculture' in Scott G and Gilead D, Degradable Polymers: Principles and Applications, London, Chapman & Hall, 186± 199, 1995. 30. Fabbri A, `The role of degradable polymers in agricultural systems' in Scott G and Gilead D, Degradable Polymers: Principles and Applications, London, Chapman & Hall, 200±215, 1995. 31. Scott G and Wiles D M, `Programmed-life plastics from polyolefins: a new look at sustainability', Biomacromolecules 2, 615±622, 2001. 32. Scott G and Wiles D M, `Degradable hydrocarbon polymers in waste and litter control' in Scott G, Degradable Polymers: Principles and Applications, 2nd edn, Dordrecht, Kluwer Academic Publishers, 449±479, 2002. 33. Raninger B, Steiner G, Wiles D M and Hare C WJ, `Tests on composting of degradable polyethylene in respect to the quality of the end-product compost' in Insam H, Klammer, S and Riddich N, Microbiology of Composting, Berlin, Springer-Verlag, 299±308, 2002. 34. Scott G, Polymers and the Environment, Cambridge, Royal Society of Chemistry, 19±37, 1999. 35. Billingham N C, Wiles D M, Cermak B E, Gho J G, Hare C W J and Tung J-F, `Controlled-lifetime environmentally degradable plastics based on conventional polymers', Addcon World, Basel 2000, RAPRA Publishing, p. 6, 2000. 36. Gho J G, chairman and CEO, EPI Environmental Products Inc., Vancouver, BC, personal communication. 37. Tung J-F, Wiles D M, Cermak, B E, Gho J G and Hare C W J, `Totally degradable polyolefin products', Addcon World, Prague, RAPRA Publishing, p. 17, 1999. 38. Pandey J K and Singh R P, `UV-irradiated biodegradability of ethylene-propylene copolymers, LDPE and I-PP in composting and culture environments', Biomacromolecules 2, 880±885, 2001.

4

New developments in the synthesis of aliphatic polyesters by ring-opening polymerisation à M E and P L E C O M T E , University of LieÁge, Belgium R J E R O

4.1

Introduction

Over the last few years, steadily increasing attention has been paid to the production of biodegradable and biocompatible aliphatic polyesters. In the first section of this chapter, the key role of tin and aluminium alkoxides as initiators for the ring-opening polymerisation of lactones, lactides and glycolide is emphasised. This polymerisation process has been up-graded to the industrial production of poly(lactide) and poly(-caprolactone). It is worth noting that poly(lactide) can be made available from agricultural renewable resources. Contamination of the aliphatic polyesters by potentially toxic metallic residues is a concern for many applications, particularly for biomedical applications. In order to alleviate this problem, organometallic initiators have been successfully replaced by lipases and full organic systems. The second section is dedicated to ring-opening polymerisation in a twin-screw extruder with the twofold advantage of getting rid of any organic solvent and replacing the current batch technology by a continuous process. The challenge is that the ring-opening polymerisation must be close to completion within the very short residence time in the extruder. Section 4.4 deals with ring-opening polymerisation in supercritical carbon dioxide, more environmentally friendly than the usual organic solvents. Either enzymes or metal alkoxides are used as initiators Additional advantages in using supercritical carbon dioxide can be found in the polymer purification, foaming and processing as particles and nanocomposites. Finally, the perspectives for aliphatic polyesters in the new century will be discussed.

4.2

Synthesis of aliphatic polyesters by ring-opening polymerisation

4.2.1 Aliphatic polyesters The broad range of biodegradable and biocompatible aliphatic polyesters shown in Fig. 4.1 have been prepared over the last forty years. Among them, poly(-

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Biodegradable polymers for industrial applications

4.1 Usual aliphatic polyesters.

caprolactone) (PCL) and poly(lactide) (PLA) are produced on an industrial scale (Gross, 2002). PCL, a semi-crystalline polyester commercialised under the tradenames CelgreenTM, TONETM and CAPATM respectively by Daicel, Union Carbide and Solvay, has a low glass transition temperature of ÿ60 ëC and a melting temperature of 60 ëC. It has good adhesive properties, and is highly miscible with many pigments, fillers and polymers, including poly(vinyl chloride). PLA is produced by a joint venture between Dow and Cargill in a plant recently built in North America with a capacity of 1.4 million tonnes per year. It is sold under the trade name NatureworksTM (Vink et al., 2003). A much lower capacity of PLA production can be found at Boehringer, Galactic and Shimadzu. PLA contains chiral carbons, such that the properties are highly dependent upon the chain microstructure. As a rule, poly(D,L-lactide) (PDLLA) is amorphous, whereas, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) are semicrystalline and as stiff and brittle as polystyrene. Changing the content and distribution of the D, L and D,L units by copolymerisation is a tool to modify the PLA properties (Vert, 2000). For a long time, PLA was restricted to biomedical applications because of a high cost ranging from 450 to 800 euros per kilo. Nowadays, the price is down to two euros per kilo, which paves the way to a broader range of applications, for instance in packaging, textiles or agriculture (Bogaert et al., 2000). The development of new applications requires the fine tailoring of the properties of the aliphatic polyesters, such as biodegradation rate, crystallinity, bioadherence, hydrophilicity and mechanical properties. For this purpose, a variety of strategies are currently considered, that involve copolymerisation, polymer blending, loading with fillers and/or plasticisers, and chemical transformation. This aspect is however beyond the scope of this paper.

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79

4.2.2 Preparation of aliphatic polyesters by step-growth polymerisation Aliphatic polyesters can be prepared by two distinct processes, i.e., step-growth polymerisation and ring-opening polymerisation. The step-growth polymerisation process relies on condensation of hydroxy-acids or mixtures of diacids and diols. The major drawbacks of polyadditions have been known for a long time. Any departure from the reaction stoichiometry has a deleterious effect on the chainlength. Condensation at high temperature and usually for a long reaction time is favourable to side reactions. Finally, the reaction is equilibrated, and water has to be removed from the polymerisation medium for increasing conversion and molecular weight. Mitsui in Japan produces high molecular weight PLA under the tradename LACEATM, by polycondensation of lactic acid in the presence of a suitable catalyst, with removal of water by azeotropic distillation of high boiling solvents (Enomoto et al., 1994) (Fig. 4.2) However, the use of an organic solvent is hardly compatible with rapidly increasing environmental concerns.

4.2 Mitsui process for the production of polylactide by polycondensation of lactic acid.

4.2.3 Preparation of aliphatic polyesters by ring-opening polymerisation Introduction The second process, ring-opening polymerisation of lactones, lactides and glycolide (Fig. 4.3), is free from these limitations. High molecular weight polyesters can be easily prepared under mild conditions from lactones of different ring size, substituted or not by functional groups (Lou et al., 2003). A broad range of anionic, cationic and coordinative initiators/catalysts have been reported in the scientific literature. As a rule, ionic initiators are much reactive and responsible for detrimental intra- and intermolecular transesterification reactions with formation of low molecular weight chains and broad molecular weight distribution (Fig. 4.4). Many organometallics derivatives of metals with d-orbitals of favourable energy such as Al, Sn, Y, Nd, Yb, Sm, La, Fe, Zn, Zr, Ca Ti and Mg, turned out to impart control to the polymerisation in contrast to their anionic counterparts, as recently reviewed (Lecomte et al., 2004). In the more favourable cases, the

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4.3 Ring-opening polymerisation of lactones, lactides and glycolide.

ring-opening polymerisation of lactones and lactides leads to polyesters of narrow molecular weight distribution, with a molecular weight predetermined by the monomer-to-initiator molar ratio. Two major polymerisation mechanisms have been identified (Lecomte, 2004). First of all, some organometallics are catalysts, which activate the monomer by complexation with the carbonyl group (Fig. 4.5). Polymerisation is then initiated by any nucleophile, e.g., water and alcohol, present in the polymerisation medium, as either adventitious impurity or compound added on purpose. In an alternative mechanism, the organometallic acts as an initiator and the polymerisation proceeds through an `insertioncoordination' mechanism (Fig. 4.6). Metal alkoxides are typical initiators, which first coordinates the carbonyl of the monomer, followed by the cleavage of the acyl-oxygen bond of the monomer and simultaneous insertion into the metalalkoxide bond. Other mechanistic proposals can be found in the scientific literature, which are however not general enough to be discussed in this chapter. For the time being, tin octaoate and aluminium and tin alkoxides are the most widely used organometallic mediators for the ring-opening polymerisation under consideration. Ring-opening polymerisation initiated by aluminium alkoxides The high selectivity of aluminium alkoxides is the major reason for their success. For instance, propagation is 100 times faster than bimolecular transesterification in the ring-opening polymerisation of LLA in THF at 80 ëC, (Baran et al., 1997) and molecular weight is very well controlled. In reference to the `insertioncoordination' mechanism shown in Fig. 4.6, the chain end-groups are welldefined and predictable. Indeed, the polyester chains are -end-capped by an ester group RO-C(=O), where RO is the alkoxy group of the initiator. Because a huge variety of aluminium alkoxides can be prepared by reaction of

4.4 Intra- and intermolecular transesterification reactions during ring-opening polymerisation initiated by anionic species.

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4.5 Ring-opening polymerisation catalysed by organometallic species [M] in the presence of nucleophiles Nu-H.

triethylaluminium with an alcohol ROH, polymeric or not, functionalised or not, the choice of the -end-group is very flexible. Moreover, hydrolysis of the propagating chains systematically results in a !-hydroxyl end-group. Other nucleophiles can also be used to terminate the chains and impose accordingly the structure of the !-end-group. The very good control imparted to ring-opening polymerisation by aluminium alkoxides is a unique platform for the macromolecular engineering of aliphatic polyesters, for instance, making comb-like, star-shaped, graft and hyperbranched (co)polyesters available (Mecerreyes et al., 1999). Tin (IV) alkoxides are potential substitutes for aluminium alkoxides, although the polydispersity of the chains is then higher (~1.5). Ring-opening polymerisation initiated by tin octoate Whenever biomedical applications are concerned, contamination of the aliphatic polyesters by toxic metallic residues, difficult to extract, is a severe drawback, which explains the success of tin octoate, Sn(O(O)CCH(C2H5)C4H9)2 or Sn(Oct)2, which is accepted as a food additive by the US Federal and Drug Agency (FDA). The intimate polymerisation mechanism has been a matter of controversy for a long time (Kowalski et al., 2000). However, Penczek and colleagues have reported recently that Sn(Oct)2 is converted into tin alkoxide, the actual initiator, by reaction with alcohols (Fig. 4.7) or other protic impurities, at least in the investigated conditions (THF, 80 ëC) (Kowalski et al., 1998). As a consequence of this new insight into the mechanism, the deliberate addition of a predetermined amount of alcohol to the polymerisation medium is an effective way to control the molecular weight by the monomer-to-alcohol molar ratio.

4.6 Ring-opening polymerisation initiated by metal alkoxides according to the `insertion-coordination' mechanism.

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4.7 In situ formation of tin alkoxides by reaction of Sn(Oct)2 with an alcohol ROH.

Industrial production of PCL and PLAs Being more tolerant to impurities than aluminium alkoxides, Sn(Oct)2 is widely used for the industrial production of PCL and PLAs mainly in bulk, within batch reactors. Any discussion on the industrial production of polymers has to integrate not only the polymerisation process, but also the monomer production. CL is prepared by the Baeyer-Villiger oxidation of cyclohexanone (Renz et al., 1999; Rocca et al., 2003), which is produced by the catalytic oxidation of cyclohexane, itself resulting from the catalytic reduction of benzene, made available from oil, a non-renewable resource (Fig. 4.8). PLA is produced by Cargill from corn, a renewable agricultural resource, according to the strategy schematised in Fig. 4.9 (Vink et al., 2003). Corn is milled, starch is separated from the raw material and processed into unrefined dextrose, which is transformed into lactic acid by a fermentation process similar to that used by beer and wine producers. Alternatively, lactic acid is also produced by other companies from beet and wheat. In contrast to synthetic lactic acid, which is a racemic mixture of L and D isomers, lactic acid produced by fermentation contains 95.5% of L isomer. Lactic acid is then converted into lactide according to a two-step process. Lactic acid is first oligomerised by water extraction, followed by catalytic depolymerisation of the oligomers at high temperature and reduced pressure. Ring-opening polymerisation of lactide is then carried out by a solvent-free melt process. It is interesting to note that copolymerisation of LLA with DLA or meso-LA improves the poor impact resistance of PLLA. The impact of the complete Cargill process on the environment has been assessed, taking into account corn production, monomer production and polymerisation. The energy consumption amounts to 54 MJ/kg PLA, and produces 1.8 kg CO2-equivalents/kg PLA (Vink, 2003). The Dow Cargill process consumes 20±50% less fossil fuel compared to the production of comparable petroleum-based products. Finally, poly(lactide) is degraded into

4.8 Synthesis of -caprolactone.

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4.9 Production of lactide from corn according to the Dow-Cargill process.

water and carbon dioxide and therefore the basic chemicals used by vegetable photosynthesis. Enzymatic ring-opening polymerisation As already stressed, the availability of aliphatic polyesters uncontaminated by possibly toxic metallic residues is of the utmost importance for biomedical applications. Potential of non-metallic catalysts/initiators in ring-opening polymerisation of lactones and lactides has thus been investigated. Enzymatic catalysis is a typical example of an environmentally friendly approach of the ring-opening polymerisation of lactones by lipases (Gross et al., 2001; Kobayashi et al., 2001). First reports on the ring-opening polymerisation of CL by lipases were published in 1993 (Uyama et al., 1993; Knani et al., 1993). Although this polymerisation is out of control, it is worth noting that lipases are active in the polymerisation of large-size lactones, which is a difficult task when any traditional catalysts/initiators are used. The generally accepted polymerisation mechanism is an `activation-monomer' mechanism as shown in Fig. 4.10. The rate-determining step would be the formation of a lactonelipase complex. The reactivity of lactones is not controlled by their ring strain but by the ease of the lactone recognition by the lipase (Duda et al., 2002). Moreover, lipases are optically active, which is promising for stereoselective ring-opening polymerisation with the formation of aliphatic polyesters of tailored tacticity (Kobayashi et al., 2001). All organic mediators for ring-opening polymerisation Metal-free polyesters have been prepared by using `all organic' initiators. Shibasaki et al. showed that the ring-opening polymerisation of CL and VL could be initiated by an alcohol in the presence of HCl.Et 2O (Shibasaki et al.,

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4.10 Mechanism of lipase-catalysed ring-opening polymerisation.

4.11 `Monomer activation' mechanism of ring-opening polymerisation catalysed by nucleophiles.

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87

2000). Nevertheless, molecular weight does not exceed 15,000, except for poly(-valerolactone) that has been prepared with molecular weight up to 50,000. The use of nucleophilic catalysts is promising. Indeed, ring-opening polymerisation of LA initiated by alcohols and catalysed by nucleophiles, such as tertiary amines (e.g., 4-dimethylaminopyridine, 4-pyrrolidinopyridine) (Nederberg et al., 2001) and N-imidazolium carbenes (Connor et al., 2002) is well controlled. A `monomer activation' mechanism (Fig. 4.11), similar to the one reported for biocatalysis, has been proposed.

4.3

Reactive extrusion

The replacement of the usual batch reactors by twin-screw extruders is highly desirable in order to make the process continuous and economically viable (Jacobsen et al., 1999). Moreover, after reactive extrusion, aliphatic polyesters can be further extruded into films, fibres, bottles and various shaped articles in a continuous process. The interest of industries in reactive extrusion aimed at producing aliphatic polyesters is testified by several patents dealing with ringopening polymerisation of CL (Wautier, 1995; Narayan et al., 1998) and lactides (Fritz et al., 1998). The polymerisation kinetics must be fast enough for reaching high conversion within the very short residence time in the extruder. Moreover, the high processing temperature may be favourable to the occurrence of undesired transesterification reactions. Therefore, the initiating and propagating species must be carefully selected for making polyesters of high molecular weight without residual monomer available, thus with high mechanical properties. In this respect, aluminium alkoxides (Wautier, 1995; Narayan, 1998) and titanium alkoxides (Gimenez, 1999) have been used to polymerise CL in an extruder within less than 30 minutes. According to Wautier, aluminium alkoxides that contain one oxygen atom in the alkoxy chain deserve interest because they are liquid and miscible with CL which prevents any solvent from being used prior to polymerisation (Wautier et al., 1997). Sn(Oct)2 is not appropriate to the CL polymerisation in an extruder because an exceedingly long time is required to reach an acceptable monomer conversion (Reichert et al., 1989; Wautier, 1995). Although tin octoate is known to catalyse the bulk polymerisation of LLA at 180 ëC, undesirable transesterification and degradation reactions take place during polymerisation and during further melt processing. JeÂroÃme et al. have shown that the addition of an equimolar amount of triphenylphosphine (PPh3) to tin octoate significantly enhances the bulk polymerisation rate of LLA (DegeÂe et al., 1999). The experimental results collected in Table 4.1 unambiguously show that the time required to reach 90% and 100% is decreased by triphenylphosphine. Moreover, the polymerisation control is improved as testified by higher molecular weight and narrower molecular weight distribution.

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Table 4.1 Bulk ring-opening polymerisation of LLA in a glass reactor at 180 ëC [LA]0/[n(Oct)2.PPh3]

Cocatalyst t90 (min.)

5,000 5,000 10,000 10,000

None PPh3 None PPh3

27 20 48 36

t100 (min.)

Mn

Mw/Mn

60 45 150 120

102,000 153,000 128,000 259,000

2.0 1.6 1.8 1.5

The faster kinetics is accounted for the coordination of the Lewis base onto the metal, which polarises the metal alkoxide bond and makes the monomer insertion easier (Fig. 4.12). An excess of triphenylphosphine is however not beneficial to polymerisation. Worse, this excess can compete with the monomer for coordination to aluminium, which is detrimental to the kinetics. Thermal stability of the as-polymerised PLLAs (at a [LLA] 0 / [Sn(Oct)2.PPh3]0 ratio of 5000), thus contaminated by the residual tin-based catalyst, has been analysed by Thermal Gravimetric Analysis. Table 4.2 shows that the degradation rate is decreased by PPh3 when the monomer-to catalyst molar ratio is higher than 5000.

4.12 Coordination of PPh3 to Sn(Oct)2.

Table 4.2 Effect of PPh3 on the thermal stability of PLLA prepared by bulk ringopening polymerisation promoted by Sn(Oct)2 Degradation rate (% minÿ1  103) [Monomer]0/[Catalyst]0

Sn(Oct)2

Sn(Oct)2.PPh3

1,000 5,000 10,000

260 160 135

1100 200 50

4.13 Twin-screw extruder for ring-opening polymerisation of LA by Sn(Oct)2.PPh3.

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Table 4.3 Comparison of ring-opening poltmerisation of LA in glass ampoule and extruder Process Glass ampoule Reactive extrusion

Mn

Mw/Mn

Conv (%)

Time (min.)

246.0 91.1

1.9 1.8

98.5 99.0

40 7

It thus appears that the addition of an equimolar amount of PPh3 onto Sn(Oct)2 has a twofold beneficial effect. It increases the polymerisation rate and it delays noxious transesterification reactions. An acceptable balance between propagation and side reactions is then reached, and polymerisation is fast enough to be conducted in an extruder. LLA bulk ring-opening polymerisation by Sn(Oct)2.PPh 3 has been implemented on a larger scale in a typical low capacity closely intermeshed co-rotating twin-screw extruder (Jacobsen et al., 2000a) shown in Fig. 4.13. LLA is mixed with 5 wt% of Ultranox prior to polymerisation in order to increase the stability of PLLA. Table 4.3 compares the ring-opening polymerisation of LLA at 180 ëC, performed in a glass ampoule and in the twin-screw extruder (Jacobsen et al., 2000a). Although a comparison of the two processes is questionable because of the inherently different experimental conditions, the superiority of reactive extrusion has to be found in a much faster kinetics with, however, a lower molecular weight at complete conversion. Nevertheless, Mn of approximately 105 is quite acceptable for most applications and might be explained by moisture contamination of lactide during the transfer from the gravimetric feeder to the extruder. A series of experimental parameters (mass flow rate, screw speed, extruder head pressure) have been optimised as reported by Jacobsen et al. (2000b). This process has been extended to the synthesis of di- or tri-block copolymers by using either Sn(Oct)2 (Stevels et al., 1996) or Sn(Oct)2.PPh3 (Jacobsen et al., 2000b) as a catalyst in the presence of an - or ,!-hydroxyl polymer, e.g., PCL and polyethylene glycol (PEG). In future, many copolyesters of diversified architectures already prepared in organic solvents could be produced by reactive extrusion. In an alternative process, lactones and lactides have been polymerised in an extruder fed by a nonreactive preformed polymer, such as poly(propylene) and poly(ethylene). Gimenez et al. have accordingly prepared blends of PCL with these polyolefins (Gimenez, 1999). Further research effort is, however, required in order to optimise this approach. The in situ formation of a compatibiliser by reactive blending might be a possible strategy.

Synthesis of aliphatic polyesters by ring-opening polymerisation

4.4

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Supercritical carbon dioxide as a medium for the ring-opening polymerisation of lactones and lactides and a processing aid for aliphatic polyesters

Substitution of supercritical carbon dioxide for organic solvents in the macromolecular engineering of aliphatic polyesters is also worth being considered for meeting the environmental concerns raised by industrial activities. Implementation of organic synthesis (Oakes, 2001) and polymerisation in its medium (Wells et al., 2001) has received increasing attention. Carbon dioxide is environmentally friendly, non-flammable, non-toxic and very cheap with easily accessible critical parameters (Tc ˆ 31 ëC; pc ˆ 73.8 bar). Supercritical fluids have remarkable solvent properties of high diffusivity and low viscosity similarly to gases, and their density is close to that of liquids. Moreover, density and solvent power can be tuned by changing temperature and pressure. Supercritical carbon dioxide is a very efficient agent for the extraction of impurities and preparation of materials with high purity. All these advantages may explain why many industrial processes rely on a supercritical carbon dioxide technology, e.g., extraction of caffeine, fluorination of polymers and hydrogenation processes (Beckman, 2004). The low solubility of many polymers, including the aliphatic polyesters, in supercritical carbon dioxide may be a drawback. For instance, most of the polymerisations conducted in this medium are precipitation polymerisations, which is not desirable at any time. Nevertheless, advantage can be taken of this low solubility, because supercritical carbon dioxide can be used either as an antisolvent for the preparation of nano- and microparticles or as a blowing agent in foaming processes. Plasticisation of polymers by supercritical carbon dioxide, may be beneficial to their processing and offer the possibility to incorporate guest molecules under mild conditions. The contribution of supercritical carbon dioxide to the ring-opening polymerisation of lactones and lactides and the processing of aliphatic polyesters has been thoroughly investigated over the last few years.

4.4.1 Ring-opening polymerisation of lactones and lactides in supercritical carbon dioxide Ring-opening polymerisation in supercritical carbon dioxide has been investigated in the presence of both organometallics and enzymes, as mentioned in section 4.2. In 2001, Kobayashi and co-workers reported on ring-opening polymerisation of CL, undecanolide and dodecanolide promoted by Candida Antarctica lipase in supercritical carbon dioxide (Takamoto, 2001). Control of ring-opening polymerisation is rather poor as testified by high polydispersity (>3) and low molecular weight (1,000). More recently, Howdle and co-workers

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supported Candida Antarctica onto macroporous beads (Novozym 435) and prepared PCL of higher molecular weight (up to 35,000) and lower polydispersity (1.5 and 2.0) in supercritical carbon dioxide (Loeker et al., 2004). This achievement is comparable to the one reported for the same polymerisation carried out in toluene under similar conditions (Kumar et al., 2000). Not only the polymer can be collected easily by venting the autoclave, but impurities can also be extracted by carbon dioxide. GPC and NMR analysis of PCL samples picked out before and after purification confirm the extraction of residual monomer and oligomers of molecular weight lower than 1,500. In order to facilitate the separation of the enzyme and the polyester after polymerisation, Loeker and co-workers placed the enzyme beads in a small wire filter pot at the bottom of the autoclave (Loeker et al., 2004). After removal of PCL, the filter pot was transferred to a second 60 ml autoclave and attached to the shaft of the motor driven stirrer blade. The autoclave was then pressurised at 4,000 psi at 35 ëC and the pot was spun at 4,500 rpm for 1 h in order to remove any residual polyester from the filter pot and to reuse it. The yield of the second polymerisation run can, however, decrease after cleaning due to partial leaching of enzyme. This phenomenon is not observed after the first polymerisation/ cleaning cycle, the enzymatic activity remaining basically constant for the following cycles. Remarkably, enough lipases are also very efficient in degrading polyesters into oligomers and monomers. Kobayashi and co-workers showed indeed that Candida Antarctica lipase is able to degrade PCL into a mixture of cyclic and linear oligomers, which may be further polymerised (Takamoto and Kobayashi, 2001). It is, however, necessary to add a small amount of a good organic solvent for PCL, e.g., acetone, for degrading PCL into oligomers with a molecular weight of ~500. Kondo and co-workers reported comparable results in the absence of acetone, provided that degradation was carried out in the presence of water (Kondo et al., 2002). Moreover, the cyclic caprolactone dimer can be selectively formed within high yield (more than 90%) under compressed carbon dioxide (18 Mpa) and in the presence of small amounts of water and lipase. This dimer can be further polymerised by the same enzyme in supercritical carbon dioxide in the absence of water. For the time being, metal alkoxides remain superior to enzymes in terms of control of the ring-opening polymerisation of low and medium-sized lactones and lactides. Their activity and capacity for controlling molecular weight have been tested in supercritical carbon dioxide. Hile and Pishko showed that Sn(Oct)2 mediates copolymerisation of lactide and glycolide (Hile et al., 2001). Molecular weight was quite low (3,500) and the mechanism and control of the copolymerisation were not considered. Stassin et al. observed that tin(IV) alkoxides are very efficient initiators for the ring-opening polymerisation of CL in supercritical CO2 (Stassin et al., 2001). The reactor used for this study is shown in Fig. 4.14. 1H NMR analysis of the chain ends confirmed that ring-

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4.14 Scheme of the reactor used for the ring-opening polymerisation of CL initiated by dibutyltin dimethoxide in supercritical CO2.

opening polymerisation proceeds through the usual coordination-insertion mechanism. The experimental molecular weight increases regularly with conversion and is predetermined by the monomer-to-initiator molar ratio, at least until 20,000 g/mol (Fig. 4.15), on the assumption that the two alkoxides of the initiator are active. For the sake of comparison, the apparent rate constants for ring-opening polymerisation of CL have been measured in different media: kapp is 56  10ÿ3 minÿ1 in toluene, 130  10ÿ3 minÿ1 in bulk, 15  10ÿ3 minÿ1 in CFC-113 and 3.95 x 10ÿ3 minÿ1 in supercritical CO2. Thus ring-opening polymerisation at 40 ëC is ca. 14 times faster in toluene and 33 times faster in bulk than in supercritical CO2. This very slow kinetics is consistent with an equilibrium between propagating species and dormant species. The reversible insertion of CO2 into the Sn-O bond leads to a carbonated tin compound, as shown in Fig. 4.16. This mechanism has been substantiated by spectroscopy and determination of the activation parameters (Stassin et al., 2003). According to Bergeot et al., Al(OiPr)3, Y(OiPr3), La(OiPr)3 are also deactivated by reversible carbonatation. The more ionic the initiator is, the higher is the reactivity towards carbon

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4.15 Dependence of Mn (SEC) on the monomer conversion and on theoretical M n for the CL ring-opening polymerisation initiated by Bu2Sn(OMe)2 in supercritical CO 2 . [CL] 0 = 1.39M, [CL] 0 /[Sn] 0 = 364 (square), 254 (diamond), 167 (triangle), and 88 (circle).

dioxide (Bergeot et al., 2004). Because ring-opening polymerisation initiated by these alkoxides in supercritical carbon dioxide is carried out at high temperature (100 ëC), transesterification reactions are favoured, which is detrimental to the control of the chain growth, as supported by high polydispersity (>1.5).

4.16 Reversible carbonatation of propagating tin (IV) alkoxide by CO2.

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4.17 Kinetic profile of the supercritical carbon dioxide extraction of CL from PCL containing 15 wt% of monomer (f=[CL]extracted/[CL]0).

For biomedical applications, ultra-pure aliphatic polyesters are needed. In this respect, supercritical carbon dioxide is a valuable vehicle for extracting monomer and catalyst residues. Figure 4.17 shows the first-order kinetic profile for the supercritical fluid extraction (SFE) of CL from a PCL sample containing 15 wt% of monomer (Lecomte et al., 2004). Based on the extraction constant, 95% of CL is extracted after c. 110 minutes, while 99% extraction would require ca. 175 minutes. Whenever ring-opening polymerisation is initiated by dibutyltin dimethoxide, the extraction of tin from PCL is a more difficult task because PCL-bound tin alkoxide has first to be derivatised into species soluble in supercritical CO2. A possible strategy relies on the reaction of the PCL-alkoxytin end-group with acetic acid and the release of dibutyltin diacetate which is extractable by supercritical CO2 (Lecomte et al., 2004). The kinetic profile is quasi-linear (Fig. 4.18) and the slope of the straight line allows the extraction constant to be determined. Precipitation of PCL as soon as it is formed in supercritical carbon dioxide can be avoided by substitution of chlorodifluoromethane (HCFC-22) for carbon dioxide. HCFC-22 has a shorter half life than CFCs, which results in a lower ozone depletion and thus in less detrimental impact on the environment. The higher solubility of aliphatic polyesters, e.g., PLA, PDLA, poly(DLLA-coglycolide), in HCFC-22 compared to carbon dioxide is accounted for by hydrogen bonding between the polymer ester groups and this solvent. A LCST behaviour is observed as reported by Lee et al. (2000). Pack et al. (2003b) prepared high molecular weight copolymers by homogeneous ring-opening polymerisation in supercritical HCFC-22 by using Sn(Oct)2 as catalyst in the presence or not of 1-dodecanol.

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4.18 Kinetic profile of extraction of tin residues after ring-opening polymerisation in supercritical carbon dioxide initiated by dibutyltin dimethoxide (f=[Sn]extracted/[Sn]0).

4.4.2 Processing of aliphatic polyesters in supercritical carbon dioxide The use of supercritical carbon dioxide has the unique advantage of combining synthesis of aliphatic polyesters and pocessing. For instance, the polyester formed in the high-pressure reactor can be in situ loaded by guest molecules, or collected as microparticles and porous material. Synthesis of micro- and nanoparticles Microparticles can be produced by a simple technique that consists of spraying a polymer, e.g., PLLA, solution in dichloromethane (or dimethylsulfoxide), through a nozzle into a reactor filled with supercritical carbon dioxide (Reverchon et al., 2000). This process is known as `supercritical antisolvent precipitation' (SAS). The experimental parameters have a limited influence on the particle size (1±4 m). A modified version of the process, known as the SAS-EM process, allows nanoparticles of a controlled size (30±50 nm) to be produced (Chattopadhay et al., 2002). In order to restrict the use of an organic solvent, Pack and co-workers fed the SAS reactor with a solution of PLLA prepared by homogeneous ring-opening polymerisation in supercritical HCFC22 (Pack et al., 2003a). Nevertheless, for the time being the strategy more widely implemented for the production of polyester particles relies on ring-opening polymerisation in

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supercritical carbon dioxide in the presence of a block or graft copolymer made of a CO2-philic block responsible for steric stabilisation and a CO2-phobic block (PCL) acting as the anchoring block. Because polymers are insoluble in supercritical carbon dioxide, the choice of the CO2-philic component is very limited, and usually restricted to silicones and fluorinated polymers. For instance, Hile et al. (2001) used a poly(1,1-dihydroperfluorooctyl acrylate) containing dispersing agent. Poly(CL-b-FPE-b-CL), where FPE stands for perfluoroether, was tested by Bratton et al. (2003). Ring-opening polymerisation of CL in supercritical CO2 has also been carried out in the presence of poly(CL-b-tetrahydroperfluorodecylacrylate) diblock copolymers (Lecomte and JeÂroÃme, 2004). Microspheres have accordingly been prepared with PCL (5K)-b-poly(tetrahydroperf;uorodecylacrylate) (24K) under the following conditions: 10 vol% CL, Mn,th = 20 K; 5 wt% surfactant, 40 ëC, 300 bar, 400 rpm, 15 h. They are shown in Fig. 4.19. Bratton et al. reported on the use of a non-fluorinated surfactant, poly(propylene glycol)-b-poly(ethylene glycol)-bpoly(propylene glycol) in order to prepare poly(glycolide) microparticles (10 m < size < 100 m), poly(propylene glycol) being the CO2-philic block (Bratton et al., 2004). Aliphatic polyester particles prepared in supercritical carbon dioxide, have potential in drug encapsulation and delivery. Encapsulation of proteins is an

4.19 Preparation of microspheres by dispersion ring-opening polymerisation of CL in supercritical CO2.

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99

example (Mishima et al., 2000). Magnetically responsive particles have also been prepared by entrapment of magnetite by the SAS-process (Chattopadhay et al., 2002). Preparation of aliphatic polyesters-clays nanocomposites Dispersion of lamellar nanoclays into aliphatic polyesters is a valuable strategy to improve a series of properties, such as thermal stability, mechanical strength, permeability to gases and moisture and flame resistance (Ray et al., 2003) even at clay contents of 3±5 wt%, thus much lower than the polymer microcomposites that contain more than 20 wt% of filler. PCL/clay nanocomposites are of special interest not only because of biocompatibility and biodegradability of PCL but also because of the miscibility of PCL with other polymers, including PVC. Melt blending of PCL and clay, i.e., natural montmorillonite (MMT) or MMT modified by quaternary ammonium salts, is a first method to prepare PCL/clay nanocomposites (Lepoittevin et al., 2002a; Pantoustier et al., 2001, 2002). They have also been prepared by the `in situ intercalative polymerisation' process (Pantoustier, 2002). Bulk CL polymerisation has been promoted by tin octoate (Kubies et al., 2002) and dibutyltin dimethoxide (Lepoittevin et al., 2002b) in the presence of MMT either native or modified by dimethyl 2-ethylhexyl and methyl bis(2-hydroxyethyl) containing ammonium cations (MMT-C8H17 and MMT-(CH2CH2OH)2). The targeted content of filler ranged from 1 up to 10 wt%. In the presence of native MMT, intercalated structures are observed in contrast to exfoliated structures, which are formed when the surface of MMT is modified by cations bearing hydroxyl groups. In this case, the PCL chains grow from the surface hydroxyl groups and they are accordingly grafted to the clay. The molecular weight can be controlled by the amount of monomer and dispersed clay, the polydispersity being, however, rather high (~2). Lower polydispersity (1.2±1.5) has been obtained by initiating the polymerisation of CL (Lepoittevin et al., 2002c) and LLA (Paul et al., 2003) by aluminium alkoxide produced by reaction of surface hydroxyl groups with triethylaluminium. The intercalation process has been extended to PCL nanocomposites with a high clay content (typically 25 wt% and higher). However, the conversion must be limited for the system to be processable, and the unreacted monomer must be eliminated in vacuo (Lepoittevin et al., 2003). These nanocomposites are thus nothing but master batches which can be redispersed in polycaprolactone or in any miscible polymers, particularly PVC (Lepoittevin et al., 2003). Supercritical carbon dioxide turns out to be a very efficient solvent for the exfoliation of lamellar nanoclays, particularly in the case of in situ polymerisation (JeÂroÃme et al., 2001). JeÂroÃme et al. initiated the CL polymerisation in supercritical carbon dioxide by dibutyltin dimethoxide in the presence of montmorillonite organomodified by the exchange of the Na + counterions by

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dimethyl 2-ethylhexyl and methyl bis(2-hydroxyethyl) containing ammonium cations respectively (Lecomte et al., 2004). Nanocomposites are collected as a powder, which is advantageous compared to traditional bulk polymerisation whenever the recovery of the material is a problem because of an exceedingly high melt viscosity. Because of the conjunction of beneficial effects, i.e., diffusivity of supercritical carbon dioxide in the nanoclays, plasticisation of PCL by supercritical carbon dioxide and no deleterious effect of melt viscosity, high clay loadings are achieved very easily at least up to 50 wt%. Foams Supercritical carbon dioxide is a very attractive blowing agent for the preparation of microcellular polymer foams in contrast to traditional processes, which require either large amounts of organic solvents (Wells et al., 2001) or possibly toxic foaming agents. Therefore, biocompatible polymeric materials with a well-defined porosity can be made available to bone and nerve reconstruction. In order to prepare foams, two processes can be considered. First, pellets of polyester are saturated with carbon dioxide followed by heating at a temperature above the glass transition temperature of the polymer. This technique produces foams with average cell size from 10 to 50 microns (Goel et al., 1994). This technique is however limited to materials which are not overly sensitive to temperature. It is a milder method to saturate the polymer at a higher pressure in the supercritical region, followed by a rapid pressure quench. As a rule, the rapid decrease of pressure results in the increase of the glass transition temperature above the temperature of the reactor. Nucleation is induced by the supersaturation caused by the sudden pressure drop, and the nuclei grow until the polymer vitrifies. The morphology of the foam is highly dependent on the processing conditions, e.g., pressure and temperature (Sparacio et al., 1997), magnitude of the pressure drop and rate of depressurisation (Goel, 1994; Mooney et al., 1996). Slow pressure drop is favourable to micropores (50 and 100 nm), whereas fast pressure drop produces mainly macropores (500 nm to 5 m). Moreover, the plasticisation of aliphatic polyesters under supercritical conditions is a tool to decrease the viscosity and to efficiently incorporate insoluble guest molecules, e.g., bioactive chemicals. For instance, in view of bone regeneration, Howdle et al. prepared porous poly(lactide-co-glycolide) and porous PLA containing 40 wt% of CO2-insoluble calcium hydroxyapatite and 10 wt% of ribonuclease A, respectively (Howdle et al., 2001). Remarkably, the enzymatic activity is retained under these mild conditions. Prospects In the future, more research work needs to be devoted to the preparation of biomedical-grade aliphatic polyesters by using supercritical carbon dioxide

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technology, particularly for biomedical applications. Strategies already implemented for non-biodegradable polymers have not been extended yet to aliphatic polyesters. For instance, it is known that polymers swell in supercritical fluids, which facilitates their chemical transformation by many chemical reactions. Chemical transformation of aliphatic polyesters is thus a promising approach in order to prepare a broader range of aliphatic polyesters, and to increase their potential as biomaterials.

4.5

Future developments

Because of steadily heavier ecological pressure, the next century should witness a tremendous development of green biodegradable aliphatic polyesters. Industrial production of aliphatic polyesters from renewable resources is a way to decrease our dependence on oil-based products. PLA, obtained from agriculturally renewable resources, should occupy a key position. Although PCL is produced from oil, it will remain very attractive in the future for a broad range of applications due to the remarkable properties that are rarely met in other polymer families. The price of aliphatic polyester has to be as low as possible for the conquest of new markets. Under the auspices of Cargill, huge progress has been made over the last few years and poly(lactide) is no longer only confined to the biomedical field. Nevertheless, the use of PLA as a commodity thermoplastic is still in its infancy. Current efforts aiming at decreasing production costs have to be pursued. In this respect, continuous processes by using reactive extrusion have to be considered in the future. As is usually the case, any increase in the demand will decrease the price of PLA. The industrial processes have to be optimised in order to decrease as much as possible impact on the environment. Cargill's 5±8 year objective is to decrease fossil energy use from 54 MJ/kg PLA down to approximately 7 MJ/kg PLA. In terms of greenhouse gases, the target is a reduction of 3.5 CO2-equivalents/kg PLA (Vink et al., 2003). These data take into account the agricultural production of corn, the preparation of the monomer and polymerisation. Cargill proposes to use crop residues such as stems, straws, husks and leaves from corn and other crops. Two fractions will be separated. The lignin-rich fraction will be used to produce steam and thermal energy, whereas cellulose and hemicellulose will be converted into fermentation sugars. The energy efficiency of the lactide preparation and polymerisation will be further optimised. Electricity can also be replaced by wind power. These improvements aim at reducing the consumption of fuel and raw material and at lowering air emissions, water emissions and solid waste production. In order to overcome the drawbacks related to metal contamination of polyesters prepared in the presence of tin and aluminium alkoxides, supercritical carbon dioxide is a promising polymerisation medium in order to prepare biomedical grade aliphatic polyesters, due to the possibility to extract the

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metallic residues. Another approach relies on the use of metal-free catalysts/ initiators. In this respect, lipases are particularly environmentally friendly catalysts but, despite some recent progress, enzymatic green polymerisation remains less efficient and controlled than chemical ring-opening polymerisation. The development of a new all-organic system to replace aluminium and tin alkoxides while maintaining polymerisation control should be a field of intense research. For many applications, properties of PLAs and PCL are not satisfactory. For instance, the brittleness of PLLA is a severe limitation for many applications. Another example can be found in the low melting temperature of PCL (60 ëC), which prevents it from being used in packaging applications in warm countries. Obviously, much research has to be dedicated in the future to the production at low cost of new aliphatic polyesters with improved properties for a broad range of applications.

4.6

Acknowledgements

The authors are much indebted to the `Belgian Science Policy' for support in the frame of the Interuniversity Attraction Poles Programme, PAI-5/03Supramolecular Chemistry and Supramolecular Catalysis. PL is Associate Researcher for the Fonds National de la Recherche Scientifique (FNRS).

4.7

Bibliography

Due to the ever-increasing number of publications dealing with ring-opening polymersation of lactones, lactides, and related materials, there is a need continuously to review and up-date the available information. Several review articles are now available and have been mentioned within the manuscript and may be found in the following list of publications. During the last few years, several research groups have actively contributed to progress in the field and the reader is advised to trace their scientific and technical contributions in the future. Websites of the companies selling PLAs and PCL are a very practical and rapid source of information about these materials (www.cargilldow.com, www.dow.com/tone, www.solvaycaprolactones.com). Baran J, Duda A, Kowalski A, Szymanski R, Penczek S (1997), `Quantitative Comparison of selectivities in the polymerization of cyclic esters', Macromol. Symp., 123, 93±101. Beckman E J (2004), `Supercritical and near-critical CO2 in green chemical synthesis and processing', J. of Supercritical Fluids, 28, 121±191. Bergeot V, Tassaing Th, Besnard, M, Cansell F, Mingotaud A-F (2004), `Anionic ringopening polymerisation of e-caprolactone in supercritical carbon dioxide: parameters influencing the reactivity', J. of Supercritical Fluids, 28, 249±261. Bogaert J C , Coszach Ph (2000), `Poly(lactic acids): A potential solution to plastic waste

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dilemma', Macromol. Symp. 153, 287±303. Bratton D, Brown M, Howdle S M (2003), `Suspension polymerisation of L-lactide in supercritical carbon dioxide in the presence of a triblock copolymer stabilizer', Macromolecules, 36, 5908±5911. Bratton D, Brown M, Howdle S M (2004), `Synthesis of poly(glycolide) in supercritical carbon dioxide in the presence of a hydrocarbon stabilizer', J. Chem. Soc., Chem. Commun., 808±809. Chattopadhay P, Gupta R B (2002), `Supercritical CO2 based production of magnetically responsive micro- and nanoparticles for drug targeting', Ind. Eng. Chem. Res., 41, 6049±6058. Connor E F, Nyce G W, Myers M, MoÈck A, Hedrick J L (2002), `First example of Nheterocyclic carbenes as catalysts for living polymerisation organocatalytic ringopening polymerisation of cyclic esters', J. Am. Chem. Soc. 124, 914±915. DegeÂe Ph, Dubois Ph, Jacobsen S, Fritz, H-G, JeÂroÃme R (1999), `Beneficial effect of triphenylphosphine on the bulk polymerization of L,L-lactide promoted by 2ethylhexanoic acid tin (II) salt', J. Polym. Sci, Polym. Chem. 37, 2413±2420. Duda A, Kowalski A, Penczek S, Uyama H, Kobayashi S (2002), `Kinectics of the ringopening polymerisation of 6-, 7-, 9-, 12-, 13-, 16-, and 17-membered lactones. Comparison of chemical and enzymatic polymerisations', Macromolecules, 35, 4266±4270. Enomoto K, Ajioka M, Yamaguchi A (1994), `Polyhydroxycarboxylic acid and preparation process thereof', Patent US 5310865. Fritz H G, Jacobsen S, JeÂroÃme R, DegeÂe Ph, Dubois Ph, (1998), `Aliphatischer polyester und/oder dopolyester und verfahren zu seiner herstellung', Patent DE 19628472. Gimenez J (1999), `Polymerization de l'e-caprolactone en extrudeuse: eÂtudes cineÂtiques et rheÂologiques en vue du controÃle du proceÂdeÂ', Thesis (PhD), University Claude Bernard-Lyon I. Goel S K, Beckman E J (1994), `Generation of microcellular polymeric foams using supercritical carbon dioxide. I: Effect of pressure and temperature on nucleation', Polym. Eng. Sci., 34, 1137±1147. Gross R A, Kumar A, Kalra B (2001), `Polymer synthesis by in vitro enzyme catalysis', Chem. Rev., 101, 2097±2124. Gross R A (2002), `Biodegradable Polymers for the environment', Science, 297, 803± 807. Hile D, Pishko M V (2001), `Emulsion copolymerisation of D,L-lactide and glycolide in supercritical carbon dioxide', J. Polym. Sci., Polym. Chem., 39, 562±570. Howdle S M, Watson M S, Whitaker M J, Popov V K, Davies M C, Mandel, F S, Wang J D, Shakeshell K M (2001), `Supercritical fluid mixing: preparation of thermally sensitive polymer composites containing bioactive materials', J. Chem. Soc, Chem. Commun., 109±110. Jacobsen S, DegeÂe Ph, Fritz H G, Dubois Ph, JeÂroÃme R (1999), `Polylactide (PLA) ± A new way or production', Polym. Eng. Sci., 39, 1311±1319. Jacobsen S, Fritz, H-G, DegeÂe Ph, Dubois Ph, JeÂroÃme R (2000a), `Single-step reactive extrusion of PLLA in a corotating twin-screw extruder promoted by 2-ethylhexanoic acid tin(II) salt and triphenylphosphine', Polymer, 41, 3395±3403. Jacobsen S, Fritz, H-G, DegeÂe Ph, Dubois Ph, JeÂroÃme R (2000b), `New developments on the ring opening polymerisation of polylactide', Industrial Crops and Products, 11, 265±275.

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JeÂroÃme R, Calberg C, Stassin F, Halleux O, Dubois Ph, Pantoustier N, Alexandre M, Lepoittevin B (2001), `Nanocomposite polyester preparation method', Eur. Pat. EP1247 829 A1. Knani D, Gutman A L, Kohn D H (1993), `Enzymatic polyesterification in organic media. Enzyme-catalyzed synthesis of linear polyesters. I. Condensation polymerisation of linear hydroxyesters. II. Ring-Opening polymerisation of e-caprolactone', J. Polym. Sci. Polym. Chem., 31, 1221±1232. Kobayashi S, Uyama H, Kimura S (2001), `Enzymatic polymerisation', Chem. Rev., 101, 3793±3818. Kondo R, Toshima K, Matsumara S (2002), `Lipase-catalyzed selective transformation of polycaprolactone into cyclic dicaprolactone and its repolymerisation in supercritical carbon dioxide', Macromol. Biosci., 2, 267±271. Kowalski A, Duda A, Penczek S (1998), `Kinetics and mechanism of cyclic esters polymerization initiated with tin(II) octoate, 1. Polymerization of e-caprollactone', Macromol. Rapid Commun., 19, 567±572. Kowalski A, Duda A, Penczek (2000), `Kinetics and mechanism of cyclic esters polymerization initiated with tin(II) octoate, 3. Polymerization of L,L-dilactide', Macromolecules, 33, 7359±7370. Kubies D, Pantoustier N, Dubois Ph, Rulmont A, JeÂroÃme R (2002), `Controlled ringopening polymerisation of e-caprolactone in the presence of layered silicates and formation of nanocomposites', Macromolecules, 35, 3318±3320. Kumar A, Gross R A (2000), `Candadida lipase B catalyse polycaprolactone synthesis: effects of organic media and temperature', Biomacromolecules, 1, 133±138. Lecomte Ph, JeÂroÃme R (2004), `Recent developments in controlled/living ring opening polymerization', Encyclopedia of Polymer Science and Technology, Hoboken, Wiley, http://www.mrw.interscience.wiley.com/epst/articles/pst497/abstractsfs.html. Lecomte Ph, Stassin F, JeÂroÃme R (2004), `Recent developments in the ring-opening polymerisation of e-caprolactone and derivatives initiated by tin(IV) alkoxides', Macromol. Symp., 15, 325±338. Lee J M, Lee B-C, Lee S-H (2000), `Cloud points of biodegradable polymers in compressed liquid and supercritical chlorodifluoromethane', J. Chem. Eng. Data, 45, 851±856. Lepoittevin B, Devalckenaere M, Pantoustier N, Alexandre M, Kubies D, Calberg C, JeÂroÃme R, Dubois Ph (2002a), `Poly(e-caprolactone)/clay nanocomposites prepared by melt intercalation: mechanical, thermal and rheological properties', Polymer, 43, 4017±4023. Lepoittevin B, Pantoustier N, Devalckenaere M, Alexandre M, Kubies D, Calberg C, JeÂroÃme R, Dubois Ph, JeÂroÃme R (2002b), `Poly(e-caprolactone)/clay nanocomposites by in-situ intercalative polymerisation catalysed by dibutyltin dimethoxide', Macromolecules, 35, 8385±8390. Lepoittevin B, Pantoustier N, Alexandre M, Calberg C, JeÂroÃme R, Dubois Ph (2002c), `Polyester layered silicate nanohybrids by controlled grafting polymerisation', J. Mater. Chem., 12, 3528±3532. Lepoittevin B, Pantoustier N, Devalckenaere M, Alexandre M, Calberg C, JeÂroÃme R, Henrist C, Rulmont A, Dubois Ph (2003), `Polymer/layered silicate nanocoomposites by combined intercalative polymerisation and melt intercalation: a masterbatch process', Polymer, 44, 2033±2040.

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Loeker F C, Duxbury C J, Kumar R, Gao W, Gross R A, Howdle S M (2004), `EnzymeCatalyzed Ring-Opening Polymerization of e-caprolactone in supercritical carbon dioxide', Macromolecules, 37, 2450±2453. Lou X, Detrembleur Ch, JeÂroÃme R (2003), `Novel aliphatic polyesters based on functional cyclic (di)esters', Macromol. Rapid Commun., 24, 161±172. Mecerreyes D, JeÂroÃme R, Dubois Ph (2000), `Novel macromolecular architectures based on aliphatic polyesters: relevance of the ``coordination-insertion ring-opening polymerization'' ', Adv. Polym. Sci, 147, 1±59. Michima K, Matsuyama K, Tanabe D, Yamauchi S, Young T J, Johnston K P (2000), `Microencapsulation of proteins by rapid expansion of supercritical solution with a nonsolvent', AIChe, 46, 857±865. Mooney D J, Baldwin D F, Suh N P, Vacanti J P, Langer R (1996), `Novel approach to fabricate porous sponges of poly(D,L-lactic-co-glycolic acids) without the use of organic solvents', Biomaterials, 17, 1417±1422. Narayan R, Krishnan M, Snook, J B, Gupta A, Dubois Ph (1998), `Bulk reactive extrusion polymerisation process producing aliphatic ester polymer compositions', Patent US 5801224. Nederberg F, Connor E F, MoÈller M, Glauser Th, Hedrick J L (2001), `New paradigm for organic catalysts: the first organocatalytic living polymerization', Ang. Chem. Int. Ed., 40, 2712±2715. Oakes R S (2001), `The Use of Supercritical fluids in synthetic organic chemistry', J. Chem. Soc., Perkin 1, 917±941. Pack J W, Kim S H, Park S Y, Lee Y-W, Kim Y H (2003a), `High molecular weight poly(L-lactide) and its microsphere synthesized in supercritical chlorodifluoromethane, Macromolecules, 36, 7884±7886. Pack J W, Kim S H, Park S Y, Lee Y-W, Kim Y H (2003b), `Kinetic and Mechanistic studies of L-lactide polymerisation in supercritical chlorodifluoromethane', Macromolecules, 36, 8923±8930. Pantoustier N, Alexandre M, DegeÂe Ph, Calberg C, JeÂroÃme R, Henrist C, Cloots R, Rulmont A, Dubois Ph (2001), `Poly(e-caprolactone) layered silicate nanocomposites: effect of clay surface modifiers on the melt intercalation process', ePolymers, no. 9. Pantoustier N, Lepoittevin, B, Alexandre M, Kubies D, Calberg C, JeÂroÃme R, Dubois Ph (2002), `Biodegradable polyester layered silicate nanocomposites based on poly(ecaprolactone)', Polym. Eng. Sci., 42, 1928±1937. Paul M A, Alexandre M, DegeÂe Ph, Calberg C, JeÂroÃme R, Dubois Ph (2003), Exfoliated polylactide/clays nanocomposites by in situ coordination insertion polymerisation, 24, 561±566. Ray S S, Okamoto M (2003), `Polymer/layered silicate: a review from preparation to processing', Prog. Polym. Sci., 28, 1539±1641 Reichert D, Klinger F, Schwall H, Christmann A, Buchholz B (1989), `Continuous process for manufacturing resorbable polyesters and their use', Patent WO9005157. Renz M, Meunier B (1999), `100 years of Baeyer Villiger Oxidations', Eur. J. Org. Chem. 1999, 737±750. Reverchon E, Della Porta G, De Rosa, I, Subra P, Letourneur, `Supercritical antisolvent micronization of some biopolymers', J. of Supercritical Fluids, 18, 239±245. Rocca M C, Carr G, Lambert A B, MacQuarrie D J, Clark J H (2003), `Process of the oxidation of cyclohexanone to e-caprolactone', Patent US 6531615.

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5

Biodegradable polyesteramides

P A M L I P S and P J D I J K S T R A , University of Twente, The Netherlands

5.1

Introduction

Degradable polymers have to meet many demands depending on the desired application. It has been recognized that especially the combination of adequate polymer material properties, thermal processing, low price and biodegradability are difficult to fulfil and are an ongoing challenge for polymer scientists. Strategies that may be followed include chemical modification of polymers and the synthesis of newly designed polymers. Aliphatic polyesters are biodegradable but often lack good mechanical and physical properties whereas aliphatic polyamides have good mechanical properties but are not biodegradable. Achieving successful combination of the favourable properties of both classes of polymers has been the reason for the development of poly(ester amide)s. The introduction of structural features like hydrogen bonding in poly(ester amide)s influences the material properties and degradability. In this respect, different strategies have been followed by placing the units in a random way in the polymer chain or using structurally well defined blocks. In section 5.2 these materials are reviewed for their structural characteristics and physical and mechanical properties and biodegradability. In section 5.3 the synthesis and properties of poly(ester amide)s prepared through ring-opening polymerization of cyclic depsipeptides are discussed. These materials, comprising hydroxy- and amino acid moieties, have been developed especially for biomedical applications like tissue engineering and drug delivery devices.

5.2

Poly(ester amide)s synthesis

Poly(ester amide)s are conveniently synthesized by polycondensation techniques as applied in the synthesis of polyesters and polyamides. One can choose either to use methods where preformed monomers are condensed, use ring-opening polymerization or combine both methods. The preformed monomers may contain either ester or amide functional groups and all the possible combinations lead to architectural variations of poly(ester amide)s. The placement of amide and ester

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groups along the polymer chain can thus be regulated and alternating, segmented (block) and random polymers have been prepared depending on the starting compounds and procedures applied. The structures of preformed symmetrical monomers are depicted in Fig. 5.1(a±d) and the polymerizations are summarized in Figs 5.2±5.10. The monomers used are described as diester-diamines, diamidediols, diamide-diesters or oligoesters. From a synthetic point of view alternating and (block) copoly(ester amide)s are mostly prepared from preformed monomers like diester-diamines, diamide-diols and diamide-diesters. Where an activated diacid is used this monomer never contains amide groups because intramolecular cyclization reactions are very common for these type of compounds.

5.2.1 Monomers Diester-diamines have been prepared by reacting an amino acid with an aliphatic diol (Fig. 5.1(a)) or alternatively an amino alcohol with succinic acid or tartaric acid (Villuendas et al., 1999, 2001) (Fig. 5.1(b)). A variety of amino acids and aliphatic diols, PEG or cyclic diols like dianhydrosorbitol or dianhydromannitol have been used (Aharoni, 1988; Nagata, 1999; Montane et al., 2002; Paredes et al., 1998a,b, 1999, 2000, 2001; Asin et al., 2001; Botines et al., 2002b; Arabuli et al., 1994; Katsarava et al., 1999; Han et al., 2003; Armelin et al., 2001; Gomurashvili et al., 2000; Okada et al., 2001). Diamide-diols (Fig. 5.1(c)) are generally prepared by ring opening reaction of -butyrolactone, -valerolactone or -caprolactone with a linear aliphatic diamine (x ˆ 2±16) or an amino alcohol in the melt or in solution (Stapert et al., 1998, 1999; Katayama et al., 1971; Katayama and Murakami, 1976; Brandt and Latawiec, 1989; Bera and Jedlinski, 1992, 1993; Sudha, 1996, 2000; Barrows, 1980). A side reaction that occurs is the ring-opening of a lactone by the generated hydroxyl endgroups resulting in oligomerization (Stapert et al., 1998; Barrows, 1988; Bera and Jedlinski, 1993; Katayama et al., 1971). This side reaction can partly be suppressed by performing the reaction in isopropanol at 5 ëC (Brandt and Murakami, 1989; Bera and Jedlinski, 1993b) and is not observed in the reaction of diamines with glycolic acid or lactic acid (Barrows, 1980; Stapert et al., 1998). Diamide-diesters have been prepared from a glycine ester or -aminocaproic ester and a diacid chloride (y ˆ 2±10) by a Schotten Bauman reaction in the presence of triethyl amine (Fig. 5.1(d)(i)) (de Candia and Maglio, 1982; Castaldo et al., 1982; Montane et al., 2002; Botines et al., 2002b). A di-amide di-ester with inverted amide groups has been prepared from 1,4-butanediamine and dimethyl adipate in the melt in the presence of titanium butoxide (Fig. 5.1(d)(ii)) (Stapert et al., 2000).

5.2.2 Polymers In this section the synthesis and properties of successively alternating, segmented (block) and random copoly(ester amide)s will be reviewed. To

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5.1 Diester-diamine A and B, diamide-diol C and diamide-diester D monomers used in the synthesis of poly(ester-amide)s. (a): R1 ˆ H, CH3 (L,D, DL), CH 2 C 6 H 5 , CH(CH 3 ) 2, CH 2CH(CH 3 )2 , CH(CH 3 )CH 2 CH 3, (CH 2 ) 3 CH 3 , (CH2)2S CH3; Y ˆ (CH2)2-12, PEG, dianhydrosorbitol or dianhydromannitol; (b): R2 ˆ H, OCH3; x ˆ 2-6; (c): y ˆ 1-5; x ˆ 2-16; (d(i)): y ˆ 2-10; z ˆ 1; X ˆ Cl; (d(ii)): y ˆ 4; X ˆ OCH3.

synthesize alternating poly(ester amide)s the monomers depicted in Fig. 5.1(a± d) can be condensed with an active (pentachlorophenyl, nitrophenyl) di-ester, diacid chloride or diol. In general diester-diamines are reacted with di-acids either in solution (Fan et al., 2001, 2002; Okada et al., 2001; Arabuli et al., 1994; Katsarava et al., 1999; Gomurashvili et al., 2000) or by interfacial polymerization (Rodriguez et al., 2000, 2003; Castaldo et al., 1982, 1992; Villuendas et al., 1999, 2001; Nagata, 1999; Montane et al., 2002; Paredes et al., 1998a,b, 1999, 2000, 2001; Asin et al., 2001) to give alternating copolymers. Diester-diamines have also been reacted with diacid-chlorides (y ˆ 2±10) in the presence of triethyl amine as an acid acceptor (Fig. 5.2(a)). In a similar way diester-diamines and an activated succinic or di-O-methyl-L-tartaric acid ester yield poly(ester amide)s in interfacial polymerization (Fig. 5.2(b)). Alternating

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5.2 Alternating poly(ester-amide) synthesis from monomers comprising amino acid or tartaric acid moieties. (a): R1 ˆ H, CH3 (L,D, DL), CH2C6H5, CH(CH3)2, CH2CH(CH3)2, CH(CH3)CH2 CH3, (CH2)3 CH3, (CH2)2S CH3 ; Y ˆ (CH2)212 , PEG, dianhydrosorbitol or dianhydromannitol; y ˆ 2-10; X ˆ Cl or pC6H4NO2; (b): x ˆ 2-6; R2 ˆ H, OCH3; R3 ˆ H, OCH3; X ˆ pentachlorophenyl.

poly(ester amide)s based on glycine or alanine, dianhydrosorbitol as the diol and activated dicarboxylic acid esters with methylene chain lengths of 4±10 (Fig. 5.2(a)) are examples of the large variations in the microstructural architecture (Gomurashvili et al., 2000; Katsarava et al., 1999; Okada et al., 2001). Most of these poly(ester amide)s are amorphous, except those prepared from sebacic acid and glycine or glycylglycine units, which are semicrystalline (Okada et al., 2001). Especially alanine and glycine based alternating poly(ester amide)s have been prepared and their properties studied in recent years (Rodriguez-Galan et al., 2000; Montane et al., 2002; Paredes et al., 1998a,b, 1999, 2000, 2001). Alternating poly(ester amide)s prepared from 1,12-dodecanediol and -alanine or glycine and sebacic acid chloride (Fig. 5.2(a)) have been studied with TEM and X-ray diffraction. The polymer derived from -alanine crystallizes like the  and forms of nylon with intermolecular hydrogen bonds along a single Ê ). Similar direction that runs parallel to the  crystallographic axis (4.80 A

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poly(ester amide)s based on 1,6-hexanediol and glycine and varying di-acid chlorides (y ˆ 2±8) showed strong hydrogen bonds (Fig. 5.2(a)). The lamellar crystals have a fairly constant thickness, which approximately corresponds to two chemical repeat units. Molecules are folded within the lamella along the Hbonded sheets (Paredes et al., 1998b, 1999, 2000, 2001). Instead of using an amino acid, 4-amino-butyric acid has also been used in monomer synthesis (analogous to Fig. 5.1(a)). Such poly(ester amide)s based on 4-amino butyric acid (Fig. 5.2(a)) have been compared with glycine based polymers. FT-IR spectra revealed that amide-amide H bonds and amide-carbonyl ester H-bonds are present in both polymers, whereas the poly(ester amide)s derived from 4amino butyric acid also contain amide groups and carbonyl ester groups in the free state (Han et al., 2003). Alternating stereoregular poly(ester amide)s based on 6-aminohexanol and di-O-methyl-L-tartaric acid (Fig. 5.2(b)) are optically active and are semi-crystalline (Villuendas et al., 1999, 2001). To prepare alternating poly(ester amide)s derived from diamide-diols, these monomers were reacted with di-esters or di-acid chlorides (Fig. 5.3) using solution polymerization (Barrows, 1980; Katayama and Murakami, 1976; Sudha et al., 1996; Sudha, 2000; Aharoni, 1988) or melt polymerization (Sudha et al., 1996, Sudha, 2000; Katayama and Murakami, 1976; Stapert et al., 1998, 1999). Melt polymerized polymers showed a lower melting temperature when compared to solution polymerized polymers possibly due to side reactions. When an asymmetric diol is used the melting temperature of the polymers is lower than those of comparable symmetrical polymers (Katayama and Murakami, 1976). The expected fibre forming properties of poly(ester amide)s makes them suitable as bioresorbable medical sutures. Thus poly(ester amide)s based on a diamine and natural metabolites like glycolic- or lactic acid (Fig. 5.3) have been investigated. The properties of the material could be optimized by the use of water soluble diamide-diols to minimize polymer resorption time. To optimize the polymer melting temperature and in-vivo fibre strength retention succinic acid was used as the comonomer (Barrows, 1980). Alternating poly(ester

5.3 Poly(ester-amide)s synthesized from diamide-diols and di-esters or diacid chlorides. x ˆ 2-16; y ˆ 2-5; z ˆ 2-14; R1 ˆ H, CH3; X ˆ OCH3, Cl.

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5.4 Synthesis of poly(ester-amide)s by polycondensation of diamide-diesters and diols. (a): y ˆ 2-10; p ˆ 2-12; z ˆ 1.5.

amide)s, prepared by melt polymerisation starting from diamide-diols and dimethyl adipate (Fig. 5.3), were obtained only as low molecular weight material, most likely due to the difficulty of retaining a 1:1 stoichiometry during the condensation reaction (Stapert et al., 1998, 1999). The problems encountered when starting from diamide-diols can be circumvented when starting from diamide-diesters as in the synthesis of polyesters (Fig. 5.4) (de Candia and Maglio, 1982; Montane et al., 2002; Stapert et al., 2000; Asin et al., 2001; Botines et al., 2002b; Ferre et al., 2003; Castaldo et al., 1982). Alternating semicrystalline poly(ester amide)s thus can be conveniently synthesized from diamide-diesters and diols (Fig. 5.4(a)). Within each series, for each polymer one single melting temperature was found which regularly decreases with increasing number of methylene groups in the diol (de Candia and Maglio, 1982). The material properties are highly dependent on the regularity of the units in the polymer chain. This is illustrated by the higher crystallinity of the alternating polymers as depicted in Fig. 5.4(a) compared to the random copoly(ester amide)s as depicted in Fig. 5.9(c), which comprises the same units in the polymer chain (Botines et al., 2002). The well known odd-even effect for aliphatic polyamides and poly(ester amide)s becomes visible in the higher melting temperature of adipic- compared to glutaric acid based poly(ester amide)s (Ferre et al., 2003). Block or segmented polymers have been prepared by replacing one of the components in a polyesterification by an amide containing monomer. Thus diamide-diols and diamide-diesters are the key elements (Bera and Jedlinski, 1992, 1993b; Stapert et al., 1998, 1999; Kaczmarczyk, 1998; Kaczmarczyk and Sek, 1995). High molecular weight segmented block poly(ester amide)s prepared by condensation of diamide-diesters, 1,4-butanediol and dimethyl adipate are easily prepared by melt polymerization (Fig. 5.5(a)). The uniform amide blocks are randomly distributed in the polymer chain and no cyclization reactions or ester-amide interchange occurs during the polycondensation reaction (Stapert et al., 2000). Segmented block poly(ester amide)s can also

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5.5 Pathways to segmented poly(ester-amide)s by melt polymerization of preformed diamide-diesters or diamide-diols. (b): X ˆ OCH3, OH.

be prepared starting from diamide-diols, 1,4-butanediol and dimethyl adipate (Fig. 5.5(b)). Polymers with molecular weights between 20,000 and 50,000 are affected by melt polymerization in such a way that the molecular weight decreases with increasing amide content. As no or little ester-amide interchange occurs between segments of the polymer chain, the symmetrical and uniform structure of the amide segments is retained in the polymer. Melting and glass transition temperatures increase with increasing amide content. Polarization microscopy revealed a biphasic birefringent melt (Stapert et al., 1998, 1999). When a hydroxyl end-capped oligoester and a low molecular weight oligo(ester amide) (Fig. 5.3) are reacted in the presence of a catalyst (Sb2O3) a high molecular weight segmented poly(ester-amide) is obtained. DSC studies revealed two glass transition temperatures, showing the presence of two phase separated domains, and one melting temperature. The stress-strain behaviour at different weight fractions of hard oligo(ester-amide) content in the copolymer shows a decrease in tensile strength and an increase in elongation at break as the hard oligomer content decreases (Bera and Jedlinski, 1992, 1993). Hydrogen bonding has been studied with temperature dependent IR for the low molecular weight oligo(ester amide) (Fig. 5.3) and two segmented poly(ester amide)s with different amide content. Amide-amide and amide-ester H-bonds are formed in

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5.6 Block and random poly(ester-amide)s by interfacial polycondensation. y ˆ 2-4; x ˆ 2-10; r ˆ 2-6.

all polymers. The largest amount of amide-ester H-bonds is present in the oligo(ester amide), in which the content of ester groups is considerably lower than in the segmented poly(ester amide). The amide-ester H-bonds appear stronger than the amide-amide H-bonds, the amide-ester bond is stable up to 210 ëC whereas the amide-amide H-bond disappears at 170 ëC (Kaczmarczyk 1998; Kaczmarczyk and Sek, 1995). When an oligoester, end-capped with carboxylic acid chloride terminal groups, is reacted with a diacid chloride and a diamine by interfacial polymerization, a block copolymer (Fig. 5.6) is obtained (Pivsa-Art et al., 2002; Castaldo et al., 1992). Interestingly such polymers have two crystalline phases, with the characteristic melting temperatures of linear polyesters and polyamides. Repetitive heat treatments cause randomization of the poly(ester amide) (Castaldo et al., 1982). A wide range of random copolymers has been prepared starting from monomers 5.1a, 5.1d and oligo-esters with different molecular weights, by condensing them with aliphatic di-amine and di-acid derivatives (Armelin et al., 2001; Andini, 1988; de Simone et al., 1992; Castaldo et al., 1982; Qian et al., 2003a,b; Alla et al., 1997; Perez-Rodriguez, 2000; Lee et al., 2002; Kawasaki et al., 1998; Gonsalves et al., 1992). When a mixture of a diester-diamine and 1,12-dodecanediamine is reacted with sebacoyl chloride, random poly(ester amide)s are obtained. IR spectra indicate that both amide units form H-bonds in the poly(ester amide). The melting temperature of these polymers increases with decreasing diamide-diester content as the amide/ester ratio increases (Armelin et al., 2001). In the development of new biodegradable polymers, poly(lactic acid) oligomers have been used to prepare random poly(ester amide)s. The poly(lactic

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acid) oligomer is reacted with an excess of sebacoyl chloride to form an oligomer with acid chloride end-groups which is then reacted with a diamine. Random poly(ester amide)s with polyester contents of 23±53 wt% could be prepared starting from poly(lactic acid) oligomers with molecular weights of 600, 1,000 and 1,500. The high melting temperatures found are attributed to the melting of the polyamide segments which was confirmed by WAXS measurements (Andini, 1988; de Simone et al., 1992). Castaldo synthesized slightly different polymers with high melting temperatures by using a diamidediamine instead of a diamine (Castaldo et al., 1992). Poly(ester amide)s based on 1,6-hexanediamine, adipoyl chloride and an oligoester (Fig. 5.6) reveal one melting endotherm in the range of melting temperatures of corresponding linear aliphatic polyamides. Poly(ester amide)s with higher ester content have an additional melting endotherm in the melting region of the corresponding linear aliphatic polyesters. The glass transition decreases drastically with an increase in ester content which suggests a homogeneous amorphous phase (Castaldo et al., 1982). Similar poly(ester amide)s with high amide contents (75 and 60%) show only one melting temperature, which is attributed to a polyamide rich phase, whereas poly(ester amide)s with lower amide content show two melting temperatures, assigned to both polyester and polyamide rich phases. Moreover, the mechanical properties decrease with increasing ester content (Gonsalves et al.,1992). It is interesting to note that these poly(ester amide)s possess a distinct characteristic polyamide melt transition while the melt transitions of random poly(ester amide)s prepared by ring opening (Fig. 5.9(a)) and alternating poly(ester amide)s (Fig. 5.3) are in between those of the corresponding polyamide and polyester. Incorporation of tartaric acid moieties in the polymer backbone (Fig. 5.7) affords polymers with high melting and glass transition temperatures, which decrease with increasing ester content. The mechanical properties as well as water uptake of these polymers decrease with increasing ester content (Alla et al., 1997). Perez synthesized similar polymers (Fig. 5.7)

5.7 Interfacial polymerization of diester-diamines, amines and activated diacids affording random poly(ester-amide)s. x ˆ 6; y ˆ 2; r ˆ 6; R2 ˆ H, OCH3; X ˆ pentachlorophenyl.

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5.8 Random poly(ester-amide)s from diamide-diacids/diacid mixtures and diols.

with 0, 3 and 10 mol% of succinate ester groups. The polymers were exposed at high humidity or incubated in phosphate buffered water at pH 7.4 and 37 ëC, and their thermal and mechanical properties were evaluated as a function of water absorption time. The plasticizing effect of water is clearly manifested in the decrease of both yield stress and elastic modulus and in the increase of strain at break (Perez-Rodriguez, 2000). Random poly(ester amide)s, prepared from diamide-diacid, 1,4-butanediol and sebacic acid (Fig. 5.8), with amide contents ranging from 10±30 mol% revealed two melting endotherms. The higher melting endotherm is a result of the introduction of amide segments. The glass transitions shifted to higher temperatures as the amide content increased, indicating a homogeneous amorphous phase. IR spectra showed intramolecular and intermolecular H-bonds between amide and ester units as well as between amide units. When the amide content is higher than 10%, H-bonds are mainly observed between amide groups (Lee et al., 2002). Random poly(ester amide)s based on 11-amino undecanoic acid and caprolactone or lactic acid with amide contents of 40±75 mol% were prepared by solution polymerisation (Fig. 5.9(a)). XRD measurements showed a diffraction pattern very similar to the a form crystal of the nylon 11 homopolymer. A decrease of the crystallinity with increasing ester content is observed and which is most likely due to the insertion of -caprolactoyl or lactyl units into the nylon 11 lattices. Such poly(ester amide)s with an ester/amide ratio of 40/60 have a typical microphase separated structure, including an ester rich phase, a middle phase composed of ester and amide segments and an amide rich phase (Qian et al., 2003). Ring-opening polymerization of lactones and lactams is an attractive alternative route to prepare random poly(ester amide)s. Several groups have focused

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5.9 Combined ring-opening and polycondensation of monomers to give poly(ester-amide)s. (a): x ˆ 5; y ˆ 5-10; (b): x ˆ 2-4; p ˆ 4, 6.

on this methodology aiming at a possible commercialization of this class of polymers. Random poly(ester amide)s based on -caprolactone and -caprolactam, with ester/amide composition varying from 75/25 to 10/90, are conveniently prepared by anionic ring opening polymerization (Fig. 5.9(a)). These poly(ester amide)s possess a random microstructure, a single melting temperature with a eutectic minimum at an amide content at 45%. The mechanical properties decrease with increasing ester content (Gonsalves et al., 1992). In recent years many random poly(ester amide)s have been prepared according to this same methodology (Fig. 5.9(b±d)) (Timmermann et al., 1995, 1997, 1998; Wiegand et al., 1999; Ferre et al., 2003). An example given here is poly(ester amide)s based on -caprolactam, adipic acid and 1,4-butanediol (Fig.

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5.9 (continued)

5.9(b)) with amide/ester ratios of 70/30 and 50/50. As the amide content decreases, the melting and glass transition temperature as well as the elastic modulus decrease. IR studies revealed the presence of amide-ester H bonds as well as amide-amide H-bonds (Ferre et al., 2003). BAK 1095 is a commercial semi-crystalline transparent poly(ester-amide) (Fig. 5.9(b)) and the polymer having an ester/amide ratio of 40/60 is easily processed and is suitable for a wide range of applications (Timmermann et al., 1995, 1997, 1998). Starting from !lauractam and -caprolactone anionic ring opening polymerization affords diblock poly(ester amide)s (Fig. 5.10(a)). Only the amide component in this polymer crystallizes, showing the form of nylon 12. At low amide group content (20 mol%) the polycaprolactone is crystallizing. IR spectra of these polymers indicate the co-existence of amide-ester and amide-amide H-bonds (Goodman and Valevanidis, 1984). Goodman also prepared diblock poly(ester amide)s based on -caprolactam and -caprolactone (Fig. 5.10(a)) which are crystalline over the entire range of compositions. Depending on the amide/ester ratio and the thermal history of the samples, the crystalline phases are either polyamide type or composed of coexisting and mutually incompatible polyamide and polyester entities. The mechanical properties of the polymers change discontinuously with

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5.10 Random poly(ester-amide)s by ring-opening copolymerization of lactams and lactones or from diols, diacids and diamide-diamine salts. (a): y ˆ 5,11.

composition, showing minima in the values of initial modulus, yield stress and break stress at 25±40% of amide content where dual crystallinity exists (Goodman 1984a,b,c). Similar polymers based on 12-hydroxydodecanoic acid and -caprolactam (Fig. 5.10(a)) have been prepared with varying amide/ester ratios. Up to 20 mol% of amide units, the polymers are semicrystalline and essentially modified polyesters (Goodman and Rodriguez, 1996). Random poly(ester amide)s based on 1,6-hexanediol, adipic acid and diamide-diamine salt (Fig. 5.10(b)) revealed a decreasing melting temperature, glass transition temperature, crystallinity and elastic modulus with decreasing amide content (Goodman and Sheahan, 1990a,b).

5.2.3 Degradation The increasing interest in poly(ester amide)s as biodegradable materials for environmental and biomedical applications also prompted researchers to study

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the biodegradation process, (bio)compatibility of the polymers and its degradation products, changes in physical and mechanical properties during degradation and cellular interactions. The hydrolytic degradation of alternating, amino acid containing poly(ester amide)s, in buffers at different pH values has been investigated. Comparative studies between polymers with a different architecture remain scarce but are becoming available and examples will be discussed. In general the hydrolysis of ester bonds takes place at a low rate (Lee et al., 2002; RodrigueÂz-GalaÂn et al., 2000; Villuendas et al., 1999; Paredes et al., 1998a,b; Botines et al., 2002; Bera and Jedlinski, 1993a; Armelin et al., 2001; Andini, 1988; Alla et al., 1997, 2000; Perez-Rodriguez, 2000; Gonsalves et al., 1992; Qian et al., 2003a). At elevated temperatures the degradation rate increases and bulk degradation with even the occurrence of surface to centre segregation has been observed (Villuendas et al., 2001; Ferre et al., 2003). The rate of the degradation process increases at low or high pH values. Similarly alternating polymers (Fig. 5.4) and BAK1095 (Fig. 5.9(b)) are hydrolytically degradable (Botines et al., 2002b). An important conclusion was that sequential poly(ester amide)s are more stable than random poly(ester amide)s to hydrolytic as well as enzymatic hydrolysis. Random poly(ester amide)s degrade slowly as shown by a decrease in intrinsic viscosity and weight loss. Increasing the amide content generally decreases the degradation rate (Armelin et al., 2001). However, Lee et al. (2002) showed that the hydrolytic degradation of random copolymers (Fig. 5.8) performed in a buffer solution of pH 11 at 35 ëC increased with increasing amide content due to the enhanced hydrophilicity. For all types of poly(ester amide)s the crystallinity and molecular weights are important to take into consideration because the hydrolysis of ester bonds occurs almost entirely in the amorphous region. If dissolution of oligomers takes place the crystallinity of the residual polymer may increase and this will lead to changes in the amount of the amideester H-bonds. Random copoly(ester amide)s based on 11-amino undecanoic acid and lactic acid show a similar degradation behaviour up to a period of one year (Qian 2003a,b). The hydrolytic degradation, under physiological conditions, of poly(ester amide)s (Fig. 5.2(b)) with different placement of amide and ester groups along the polymer chain (regicity) has been examined by Villuendas et al. (2001). Degradability increases with the ester group content and although effects started to be noticeable for ester contents as low as 10 mol%, the degradability of the polymer became clearly apparent when their concentration reached 20 mol%. Surprisingly, both sample weight and molecular weight of the syndioregic poly(ester amide)s were found essentially unaltered after eight months. On the contrary, the molecular weight of the isoregic poly(ester amide)s decreased to less than one-third of its initial value after two months. The aregic poly(ester amide) degraded faster than the isoregic variant, despite the lower amount of ester linkages. This effect is probably caused by a larger amount of amorphous

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phase. The chain scission in both isoregic and aregic poly(ester amide)s is said to take place by intramolecular amidolysis. This mechanism is unable to operate if the polymer chain has an entirely syndioregic microstructure (PeÂrezRodrõÂguez et al., 2000). The hydrolytic degradation of poly(ester amide)s based on tartaric acid (Fig. 5.7) immersed in pH 7.4 buffer at 37 ëC was followed by changes in molecular weight in time (8 h to 70 days) (PeÂrez-Rodriguez, 2000). In time a noticeable decrease in molecular weight and tensile properties was observed. Variations in glass and melting temperatures appeared to be slight, whereas crystallinity increased with incubation time. In another study poly(ester amide)s based on tartaric and succinic acid (Fig. 5.7) were investigated for their hydrolytic degradation over a period of 0±25 weeks (Alla et al., 2000). The molecular weight decreased rapidly during the first two months of incubation and ends in complete erosion of the polymer sample. The weight loss rate increases with the ester content. The degradation takes place essentially through cleavage of the ester bonds, and is accompanied by formation of cyclic succinimide units due to an intramolecular imidation. The enzymatic degradation of alternating poly(ester amide)s has been systematically investigated by RodrigueÂz-GalaÂn and Puiggali. Proteinase K, lipase and papain are typical enzymes applied (Ferre et al., 2003; Botines et al., 2002b; Armelin et al., 2001; Montane et al., 2002; RodrigueÂz-GalaÂn et al., 1999, 2000; Paredes et al., 1998, 2001). The degradation of these polymers has been compared to that of the BAK1095 polymers. The poly(ester amide)s that do hydrolytically degrade at a slow rate through ester hydrolysis reveal increased degradation rates in the presence of enzymes. Rates increase at higher temperatures and decrease with increasing crystallinity as expected. The degradation rate is lower for poly(ester amide) comprising D-amino acids instead of L-amino acids (Fan et al., 2002; Nagata, 1999). Alternating poly(ester amide)s (Fig. 5.2(a)) incubated with papain at 37 ëC show that the glycine derived poly(ester amide)s demonstrated a significantly improved degradability compared to poly(ester amide) films derived from 4-aminobutyric acid. The higher degradation rate has been explained by the presence of free C=O ester bonds as measured with IR for the glycine derived poly(ester amide) (Han et al., 2003). In a recent paper Botines et al. compared the enzymatic degradation of sequential and random poly(ester amide)s. The regular sequence as found in sequential poly(ester amide)s leads to an increase in crystallinity and consequently the degradation rate decreases (Botines et al., 2002a,b). Similar poly(ester amide)s have been described by Katsarava and subjected to enzymatic hydrolysis using -chymotrypsin (Katsarava et al., 1999; Arabuli et al., 1994). A wide range of polymers has been studied and it was concluded that an increase in the hydrophobic chain length of diol or diacid moieties increased the rate of degradation of these polymers. Enzymatic degradation tests of block type poly(ester amide)s similar to those presented in Fig. 5.6 with varying amide/ester ratios were performed by placing the

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polymers in pH 7.0 buffer solutions at 37 ëC for 24 h in the presence of enzymes (Pivsa-Art et al., 2002). Enzymes used in this study are lipases from Rhizopus arrhizus, Rhizopus delemar and Candida cylindracea and an esterase from trypsin and -chymotrypsin. It was concluded that both hydrophilicity and rigidity of the poly(ester amide)s are the main factors controlling the rate of degradation. The enzymatic degradation of some -caprolactam and -caprolactone based random copoly(ester amide)s (Fig. 5.10) was studied by incubation at pH 7.4 and 37 ëC of polymer films (amide content 19±46 wt%) with solutions of protease, collagenase, -chymotrypsin and pancreatin (Goodman and Rodriguez, 1999). The first three enzymes mentioned had no visible effect upon any of the polymer films, and there was no significant evidence, with any enzyme or substrate, of the formation of amino acids as a product of amide bond splitting. In contrast, incubation with pancreatin caused surface erosion of these poly(ester amide)s, with depletion of the ester group content in the surface layer and the development of an amide rich striated surface morphology. Both hydrolytic degradation and degradation by fungi has been investigated in a comparative study towards two types of poly(ester amide)s (Fig. 5.9(d)) by Gonsalves et al. (1992). Poly(ester amide)s prepared by polycondensation of 1,6hexanediamine, adipoyl chloride and 1,6-hexanediol slowly degraded in aqueous media whereas the poly(ester amide)s prepared by ring-opening polymerization of -caprolactone and -caprolactam degraded much faster. In their studies films were subjected to biodegradation in solid agar media in which either Fusarium moniliforme or Aspergillus niger were used as microorganisms (incubated at 25 ëC). The hydrolytic degradation at pH 7.4 and 37 ëC is much slower than the degradation by Fusarium moniliforme. The degraded samples were analysed with IR to discover structural changes. Upon degradation with Fusarium moniliformea a significant decrease in ester absorption intensity is observed but the amide content remained virtually unchanged (no significant decrease in intensities of both amide I and II bands). Random poly(ester amide)s have also been subjected to microbial degradation in basal mineral broth, under the attack of a yeast, Cryptococcus laurentii, at 20 ëC (Chen et al., 1993). Cryptococcus laurentii has proved to be effective in the degradation of aliphatic polyesters. The poly(ester amide)s films showed a significant weight loss in 30 days, while the films exposed to a buffer solution (hydrolytically) show no weight loss. The mechanical properties decreased dramatically upon degradation. In their studies towards new biomedical materials with fibre forming properties Barrows studied the in-vivo absorption of poly(ester amide)s (Fig. 5.3). The principal mechanism of in-vivo degradation appears to be hydrolysis of the ester bonds. This produces the diamide-diol monomer as a major metabolite which is non-toxic as determined by LD50 tests in rats. In separate experiments the diamide-diol monomer and the corresponding polymer showed no evidence of cytotoxicity (Barrows et al., 1986, Barrows and Horton, 1988).

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In vivo studies on high molecular weight segmented poly(ester amide)s indicate that these polymers are biocompatible and degrade to some extent. A comparison with alternating poly(ester amide) oligomers showed that these oligomers were completely fragmented after 30 days whereas segmented poly(ester amide)s formed a capsule initially and both were eventually resorbed after three months (Bera and Jedlinski, 1993a). Only a few studies of poly(ester amide)s on cell material interaction are available up to now. The increasing interest in tissue engineering also increases the search for new materials that can be applied as scaffold materials to grow tissues (Han et al., 2003). The cellular interaction of the poly(ester amide)s (Fig. 5.2(a)) was studied by measuring the proliferation of human dermal fibroblast (after seven days at 37 ëC) on polymer films. The cells proliferated faster on polymer films derived from 4-aminobutyric acid than on poly(ester amide)s derived from glycine. The higher hydrophilicity (lower contact angle) of the 4aminobutyric acid derived polymer films seems to favour the growth of human dermal fibroblasts. The results suggest that the poly(ester amide)s prepared in this study may serve as a potential cell-compatible biomedical material. The biocompatibility and cytotoxicity of alternating poly(ester amide)s (Fig. 5.2(a)) has been tested. Cellular proliferation is observed on the poly(ester amide)s material surface when seeded with mouse fibroblasts (Paredes et al., 1998a). No cytotoxic responses were detected, in either assay, after a 24 and 48 h incubation period with the cells, but the cytotoxic response increased after 72 h of incubation. A few studies have been directed towards the environmental degradation of poly(ester amide)s (Kawasaki et al., 1998; Okada et al., 2001; Pivsa-Art et al., 2002). Films have been buried in soil in a dessicator at 27 ëC, in which the relative humidity was adjusted to 70±80%. The enzymatic degradation tests with porcine pancreas lipase and papain took place in a pH 7.0 buffer solution at 37 ëC for 24 h. The soil burial degradation tests, biological oxygen demand (BOD) measurements in an activated sludge, and enzymatic degradation tests indicated that these poly(ester amide)s are biodegradable, and that their biodegradability markedly depends on the molecular structure. The poly(ester amide)s were, in general, degraded more slowly than corresponding polyesters. In the enzymatic degradation (porcine pancreas lipase) some poly(ester amide)s containing dicarboxylic acid components with shorter methylene chain lengths were degraded more readily than the corresponding polyesters, whereas most of the poly(ester amide)s were degraded more rapidly than the corresponding polyesters with papain. The effect of methylene chain length of poly(ester amide)s, and polyesters, on degradation has also been discussed (Okada et al., 2001). The rate of enzymatic degradation depends on the methylene chain length and poly(ester amide)s with an odd numbered methylene chain length were less readily degraded by papain.

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5.3

Polydepsipeptides

Biodegradable polymers have already found widespread applications and are being increasingly investigated for possible use in a wide variety of temporary medical applications. A wide variety of materials and composites are currently applied as biomaterials. The most well known classes of biodegradable polymers are the polyanhydrides, polyorthoesters, polyamino acids and aliphatic polyesters. Degradation of most of these polymers takes place by the hydrolysis of (enzymatically) hydrolytically unstable bonds (e.g., ester, amide and anhydride bonds) present in these materials. It is essential that the degradation products of such materials are non-toxic. The design of biodegradable polymers is therefore frequently based on building blocks derived from natural metabolites. Examples of biodegradable polymers developed on the basis of this approach are poly(-hydroxy acid)s and poly(-amino acid)s. Just by replacing a lactyl moiety in lactide or a glycolyl moiety in glycolide by an -amino acid a large variety of new materials with special properties was envisaged. About 20 years ago the synthesis of poly(ester amide)s through ring-opening polymerization of cyclic depsipeptides was initiated. Up to now many papers have been published on the synthesis and properties of these polymers. In this section approaches are summarized that have been used in the synthesis of polydepsipeptides by ringopening polymerization of cyclic monomers built from amino- and hydroxy acids, the morpholine-2,5-diones.

5.3.1 Monomer synthesis Because a variety of -amino acid residues can be used to synthesize morpholine-2,5-dione derivatives, ring-opening polymerization of these monomers provides a method to prepare a wide variation of alternating polydepsipeptides. Different strategies have been investigated for the synthesis of morpholine-2,5-dione derivatives. The monomers have been prepared by reaction of -amino acids with -halo acid halides, followed by cyclization of the intermediate N-(2-halogenacyl)-amino acids (route A1, Fig. 5.11) with the formation of an ester bond (Helder et al., 1985, 1986b; Dijkstra and Feijen, 2000). The other route uses the synthesis of an intermediate aminoacyl hydroxy acid via route B1. Cyclization via route A1 can be performed with triethyl amine in dimethyl formamide or by dry heating of the corresponding sodium salt. The dry heating method yields oligomeric and polymeric alternating polydepsipeptides as important side products, which in the case of R1 ˆ alkyl substituent and R2 ˆ H can be depolymerized to the corresponding morpholine-2,5-dione by heating with Sb2O3. Optically pure 3-alkyl substituted morpholine-2,5-diones can be prepared via route A1. Optically pure 3- and 6-alkyl substituted morpholine-2,5-diones cannot be prepared via route A1, because in the cyclization reaction racemization occurs at the chiral centre of the hydroxy

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5.11 Synthetic pathways for the synthesis of cyclic depsipeptides and polydepsipeptides.

acid residue. Optically pure 3- and 6- substituted morpholine-2,5-diones can be synthesized via route B1. Because in some cases the yields of cyclic products are low, in recent years the synthesis of cyclic depsipeptides has been re-investigated in detail by HoÈcker and co-workers (JoÈrres et al., 1998). They have prepared several substituted morpholinediones by cyclization of N-(-hydroxy acyl)--amino acid ethyl esters in moderate to good yields. However, when a lactic acid moiety is introduced, optically pure monomers remain difficult to obtain and in some cases racemization during cyclization has been observed at the lactyl methine carbon atom. A large range of (protected) substituted 2,5-morpholinediones has become available using the methods described above. Cyclo(ester amide)s with larger ring sizes are well known naturally occurring molecules with biological activity. One example is valinomycin that can act as a potassium ion transporter in biological membranes. Robertz et al. have studied the ring-expansion of, e.g., caprolactam through acylation and ring closure to give an 11-membered ring (Fig. 5.12) (Robertz et al., 1999a). Such a cyclic depsipeptide has been polymerized to give a semicrystalline polydepsipeptide. When a -alkyl substituted acylating agent was used, the obtained alkyl substituted 11-membered ring did give a ring expansion reaction rather than ring-opening polymerization (Robertz et al., 1999b). Recently Fey prepared a series of cyclic ester amides by ring-closing depolymerization of corresponding polymers based on adipic acid and ,!-amino alcohols (Fey et al., 2003b). In a similar way the depsipeptide based on glycolic acid and -alanine can be prepared by a depolymerization reaction (RodrigueÂz-GalaÂn et al., 2003). Another challenge has been the synthesis of functionalized polydepsipeptides using monomers comprising a functional -amino acid like lysine (in't Veld et

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5.12 Ring-opening copolymerization of functionalized morpholine-2,5diones with lactide or glycolide. R 1 ˆ H, CH 3 ; R 2 ˆ H, CH 3 , CH 2 CH 3 , CH(CH3)2; R3 ˆ H, CH3. If R2 is a functional amino acid the intermediate substituent is (CH2)4NHZ, CH2COOBz, (CH2)2COOBz, CH2SMBz, CH2OBz. Deprotection gives (CH2)4NH2, CH2COOH, (CH2)2COOH, CH2SH, CH2OH.

al., 1992; Ouchi et al., 1996; Barrera et al., 1995; JoÈrres et al., 1998), aspartic acid (in't Veld et al., 1992; Ouchi et al., 1993; Wang and Feng, 1997; Feng et al., 2002b) or glutamic acid (Ouchi et al., 1996), cysteine (in't Veld et al., 1992; Ouchi et al., 1998) or serine (Tasaka, 2001; John, 1996). The same strategies as described above have been used for the synthesis of monomers in racemic or optically pure form. Intermediate protection and deprotection reactions are envisaged in the eventual preparation of polymers having functional groups along the polymeric chain.

5.3.2 Polymer synthesis Ring-opening polymerization is an excellent alternative to polycondensation in the synthesis of aliphatic polyesters like, e.g., polylactides, polyglycolide, poly(-caprolactone) and their copolymers. High molecular weight materials are easily prepared and the development and use of coordination catalyst/initiator systems nowadays allows an accurate tailoring of different macromolecular structures (Stevels et al., 1997). Similar to the ring-opening polymerization of cyclic (di)esters, the ring-opening polymerization of cyclic depsipeptides (morpholine-2,5-dione derivatives) has been suggested as an attractive alternative to obtain (alternating) poly(ester amide)s (in this case polydepsipeptides) in a more facile way than by multistep synthetic routes (Dijkstra and Feijen, 2000). A large variety of polydepsipeptides has been synthesized in the years thereafter by ring-opening polymerization and their properties have been determined. Catalyst or catalyst-initiator systems, which have been successful in the ring-opening polymerization of glycolide and lactide, have been applied. The ring-opening takes place at the acyl oxygen bond. The first

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polydepsipeptides prepared through ring-opening polymerization mainly comprised a lactic acid or glycolic acid moiety and a glycine, alanine or valine amino acid moiety. In the synthesis of the monomer L-amino acids are used and the stereochemistry of the lactic acid moiety precursor was not taken in account. As a result the polymers prepared from 6-methyl substituted morpholine-2,5diones are amorphous. Polymers with the highest molecular weight were obtained when the polymerization was carried out mainly using stannous octoate as a catalyst at reaction temperatures close to the melting temperature of the monomer. The molecular weights of the polymers (Mw 2  104) were generally considerably lower than the molecular weights of polymers obtained in the stannous octoate initiated bulk polymerization of L-lactide (Mw 105±106) using similar reaction conditions. The ring-opening polymerization of 3- and/or 6-alkyl substituted morpholine2,5-diones have been reported by several researchers (Yonozawa et al., 1985; Samyn and Beylen, 1988; Fung, 1989). It always appeared difficult to obtain high molecular weight materials because of side reactions occurring at the elevated temperatures necessary for the polymerization (JoÈrres et al., 1998). In the ring-opening polymerization of 3S,6S-3-isopropyl-6-methyl-morpholine2,5-dione to give Poly(L-valine-alt-L-lactic acid) the polymer molecular weight increases with conversion but at high conversions molecular weight decreases drastically due to chain fragmentation reactions. Another example is the synthesis of the parent compound composed of a glycine and a glycolic acid moiety that is possible only just below the melting temperature of the monomer (200 ëC) (in't Veld et al., 1994). To prepare semi-crystalline polymers with a distinct stereochemistry was even more challenging. The synthesis of the precursor chiral monomers is straightforward when the depsipeptide contains a glycolyl moiety. When the 6position is substituted with a methyl group (lactyl moiety) the preparation of one of the enantiomers was possible only by a ring closing reaction with the formation of an amide bond. Several protecting and deprotecting steps are necessary to prepare those chiral monomers and the overall yield is generally low. The polymerization of (3S)-methylmorpholine-2,5-dione (Fig. 5.13, Rl ˆ CH3, R2 ˆ H) yielded a semi-crystalline poly(L-alanine-alt-glycolic acid) with a melting temperature of 232 ëC, whereas the polymerization of the achiral (3RS)methylmorpholine-2,5-dione gives a completely amorphous polymer. Recently JoÈrres et al. synthesized several alkyl-substituted morpholine-2,5-diones and studied the ring-opening polymerization using stannous octoate as a catalyst or stannous acetylacetonate as an initiator. It was shown that racemization takes place in the hydroxy acid residue and not in the amino acid residue (JoÈrres et al., 1998). The results of these ring-opening polymerizations can be summarized as follows: (i) no racemization is observed in the amino acid residue; (ii) polymers comprising a glycolic acid moiety and a chiral amino acid moiety (like L-

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5.13 Ring expansion of N-acyl-lactams and subsequent ring-opening polymerization of the cyclo-amide-ester.

alanine) give stereoregular polymers and are semi-crystalline; (iii) when a lactic acid residue is introduced (partial) racemization of the chiral lactyl carbon atom occurs; (iv) alternating and stereoregular polydepsipeptides are semi-crystalline but in a pure form have been prepared only by polycondensation of activated linear tetradepsipeptides (didepsipeptides show a high tendency to cyclize and are thus not useful). The successful synthesis of alternating polydepsipeptides by ring-opening polymerization of morpholine-2,5-dione derivatives having different alkyl substituents leads the way to synthesize polydepsipeptides with functional groups. The use of functional -amino acids like, e.g., L-aspartic acid, Lglutamic acid, L-lysine, L-serine and L-cysteine in the synthesis of morpholine2,5-dione derivatives offers a synthetic route to biodegradable polymers with pendant carboxylic acid, amine, hydroxyl and thiol groups, respectively. The ring-opening (co)polymerization of morpholine-2,5-dione derivatives with pendant functional groups has been investigated by several researchers (in't Veld et al., 1992; Barrera et al., 1995; JoÈrres et al., 1998; Ouchi et al., 1993, 1996, 1998; Tasaka et al., 1999, 2001; Wang and Feng, 1997; Feng et al., 2002b; John et al., 1997). Thus, -amino acid residues with protected side chain functional groups were incorporated into morpholine-2,5-dione derivatives, which subsequently could be (co)polymerized (Fig. 5.13), followed by deprotection of the pendant functional groups. Polymers with functional groups have been used for the preparation of combpolymers, the preparation of charged microspheres and scaffolds for tissue engineering, materials to be applied in the controlled release and/or targeting of drugs, and preparation of networks (Fig. 5.14) (John and Morita, 1999; Ouchi et al., 2002a,b,c).

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5.14 Synthetic routes to networks and comb type polymers.

Morpholine-2,5-dione derivatives having substituents like benzyl protected carboxylic acid, benzyloxycarbonyl protected amine and p-methoxybenzyl protected thiol groups, respectively, were prepared by reaction of N-[(2RS)bromopropionyl]amino acids with triethyl amine in DMF. The ring-opening homopolymerization of morpholine-2,5-dione derivatives with protected functional substituents and a methyl substituent at the 6-position failed, due to the low reactivity of the monomers. However, these derivatives could be copolymerized with -caprolactone and DL-lactide, to give poly(ester amide)s with pendant protected functional groups (in't Veld et al., 1992). The selective removal of the benzyl and benzyloxycarbonyl protective groups by catalytic hydrogenation yielded copolymers with pendant carboxylic acid and amine groups, respectively. Successful homopolymerization of a morpholine-2,5-dione constructed from a glycolic acid and a protected aspartic acid residue was first reported by Ouchi et al. (1993, 1996). The poly(aspartic acid-alt-glycolic acid) was synthesized using stannous octoate as a catalyst and was obtained as a polymer with moderate molecular weight. Similarly the poly(L-glutamic acid-alt-glycolic acid) and poly(L-lysine-alt-glycolic acid) were prepared. The homopolymerization of the poly(L-aspartic acid-alt-glycolic acid) was also described by Wang

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and Feng (1997). Morita described the ring-opening homopolymerization of a O-benzyl protected L-serine glycolic acid morpholine-2,5-dione and its deprotection (John et al., 1997) and the copolymerization with lactide or caprolactone. The deprotection reaction by catalytic hydrogenation is almost complete for the copolymers prepared but the homopolymer could not be completely converted. Copolymerization of morpholine-2,5-dione derivatives with other lactones provides a possibility to prepare various biodegradable polydepsipeptides with a wide range of properties which depend on the composition of the copolymers. Comonomers used are p-dioxanone, lactide, caprolactone and glycolide (Samyn and Van Beylin, 1988; in't Veld et al., 1990; Ouchi et al., 1996, 1997; Barrera et al., 1995). Copolymerization of morpholine-2,5-dione with -caprolactone or lactides generally affords random copolymers. Molecular weights are higher than found in the homopolymerization of the cyclic depsipeptides but do decrease with increasing morpholine-2,5-dione in the feed. In many cases the polymers will be amorphous or in case of copolymerization with L-lactide the melting temperature and crystallinity rapidly decrease with increasing depsipeptide units incorporated. Racemization is depending on the catalyst/initiator used and can be observed even at the amino acid residue when a catalyst like calcium hydride was used (Feng et al., 2002a). Copolymerization of lactones and amino acid carboxyanhydrides using stannous octoate as a catalyst yields random polydepsipeptides and may be an attractive way to prepare polymers with pendant functional groups (RypaÂcÏek et al., 1998). Recently the ring-opening polymerization of amino acid N-carboxyanhydrides and lactic acid anhydro sulphite has been reported (Deng et al., 2002; Liu et al., 2003). The structure of the polymer chain may be random or block like depending on the catalyst/ initiator system used. Macroinitiators like hydroxy terminated poly(ethylene oxide)s have been applied to prepare block copolymers (Feng et al., 2001). Enzymatic ring-opening polymerization of lactones is a new method to prepare aliphatic polyesters. Feng et al. have used the enzyme porcine pancreatic lipase to polymerize for the first time morpholine-2,5-diones carrying alkyl groups at the 3 and 6-position. During polymerization racemization of the amino acid residue as well as the hydroxy acid residue takes place. The polymerization possibly proceeds through ring-opening at the ester bond (Feng et al., 2000). Recently research focuses also on the synthesis and ring-opening polymerization of larger rings comprising an amide and ester bond. Alternating polymers show the odd-even effect in their melting temperatures and may be an excellent extension to the existing poly(ester amide)s so far known (Fey et al., 2003a,b). Polydepsipeptides with N-alkyl substituted -amino acid residues, such as poly(N-methylglycine-alt-DL-lactic acid) and poly(N-isopropylglycine-alt-DLlactic acid) could not be obtained by ring-opening polymerization of the corresponding N-alkyl substituted morpholine-2,5-diones (Helder, 1986).

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5.3.3 Degradation Non-enzymatic and enzymatic degradation in-vitro of polydepsipeptides and poly(ester amide)s containing depsipeptide units has been reported. Most of the materials studied are copolymers prepared by ring-opening polymerization of morpholine-2,5-diones with either p-dioxanone, lactide or -caprolactone. Helder et al. investigated the hydrolytic degradation in phosphate buffer at pH 7.4 of poly(DL-lactic acid-glycine) with a glycine content varying from 5 to 25 mol% (Helder et al., 1990). The degradation takes place by bulk hydrolysis of ester bonds autocatalyzed by the carboxylic acid groups generated. Although the initial degradation rates are comparable to that of poly(DL-lactic acid), weight loss takes place at an earlier stage due to the increased hydrophylicity of the oligomers generated. The degradation behaviour of these copolymers after being subcutaneously implanted in the backs of rats correlated with the in-vitro degradation of these copolymers (Schakenraad et al., 1989). Similarly incorporation of depsipeptide units in poly(p-dioxanone) accelerated the in-vitro and in-vivo resorption of melt spun monofilaments (Shakaby and Koelmel, 1983). Incorporation of depsipeptide units in poly(-caprolactone) or poly(L-lactide) also shows an increase in degradation rate compared to the homopolymer. Particularly copolymers comprising -caprolactoyl units and only 2 mol% of aspartic acid units were most rapidly degraded. However, during the degradation the depsipeptide content decreases while at the same time the crystallinity of the residual polymer increases to give highly crystalline poly(caprolactone). This may eventually decrease the degradation rates (in't Veld et al., 1993; Shirahama et al., 2002). Ouchi et al. studied the degradation rates of several alternating polydepsipeptides composed of glycolyl and lysyl, aspartyl or glutamyl or cystyl residues in the presence or absence of enzymes (Ouchi et al., 1993, 1996, 1997, 2002a). Degradation rates increase in the presence of specific enzymes that cleave ester bonds in the main chain of these alternating polydepsipeptides. These findings were extrapolated to the in-vitro degradation of poly(ester amide)s based on poly(L-lactide) with a low content of these depsipeptide units. The increase in degradation rates of these polymers in the presence of specific enzymes is considered to be a result of the substrate specificity of the enzyme but also has been related to the decreased crystallinity of the poly(ester amide)s. In a recent paper Ouchi et al. also showed that branched copolymers prepared through the incorporation of serine amino acid residues in polylactides and using the free hydroxyl groups as initiators for lactide ring-opening polymerization degrade much faster due to an increased number of end groups (Tasaka et al., 2001).

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5.4

Conclusions and remarks

Having summarized the synthesis and properties of poly(ester amide)s it can be concluded that there remains a need to synthesize and investigate (co)polymers that combine good mechanical performance and physical properties, low price, safe manufacturing and processing methods and complete (controlled) biodegradability, all in relation to the intended applications. Strategies that may be followed include (chemical) modification of polymers and the synthesis of (newly) designed polymers. Aliphatic poly(ester amide)s show the characteristics of structural organization through hydrogen bonding and by design these materials may fulfil the requirements with respect to polymer properties in this developing field. Poly(ester amide)s distinguish themselves from aliphatic biodegradable polyesters, which are biodegradable but often lack good mechanical and physical properties. Moreover, aliphatic polyamides have good mechanical properties but are not biodegradable. The combination of ester and amide groups in poly(ester amide)s affords polymers with adequate properties and the required biodegradability. The ring-opening polymerization of cyclic depsipeptides yields materials that that can be used in biomedical applications like tissue engineering and drug delivery devices. The possibility of using a variety of (functional) amino acids incorporated in the polymer backbone allows the design of novel materials with specific properties. Controlled polymerization using non-toxic catalyst/initiator systems remains an important feature to be investigated. The ring-opening polymerization of larger rings containing ester and amide groups has already shown a way to make poly(ester amide)s with a defined structure and allows further tailoring of polymers. In addition, there is also a need for a better understanding of the relationships between polymer structure, polymer morphology and biodegradation properties by carefully investigating the complete biodegradation process using well characterized polymer materials. Comparative studies using well controlled and standardized conditions are of utmost importance.

5.5

Further information

A general review of the chemical synthesis of biodegradable polymers incorporating poly(ester-amide)s has been published recently by Okada (2002). Further information on degradable polymers can be found in the book of Domb et al. (1997).

5.6

References

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acid based copolymers', J. Pol. Sci., 32 (6), 1063±1069. John G, Morita M (1999), `Synthesis and characterization of photo-cross-linked networks based on L-lactide/serine copolymers', Macromolecules, 32 (6), 1853±1858. John G, Tsuda S, Morita M (1997), `Synthesis and modification of new biodegradable copolymers: Serine/glycolic acid based copolymers', J. Pol. Sci. Part A: Polymer Chemistry 35 (10), 1901±1907. JoÈrres V, Keul H, HoÈcker H (1998), `Aminolysis of -hydroxy acid esters with -amino acid salts; first step in the synthesis of optically active 2,5-morpholinediones', Macromol. Chem. Phys., 199 (5), 825±833. Kaczmarczyk B (1998), `FTIR study of hydrogen bonds in aliphatic polyesteramides', Polymer, 39 (23), 5853±5860. Kaczmarczyk B, Sek D (1995), `Hydrogen bonds in poly(ester amide)s and their model compounds', Polymer, 36 (26), 5019±5025. Katayama S, Murakami T (1976), `Synthesis of alternating polyamide esters by melt and solution polycondensation of N,N'-di(6-hydroxycaproyl)diamines and N-6hydroxycaproyl aminoalcohol with terephthalic and adipic dimethyl esters and dichlorides', J. Appl. Pol. Sci., 20 (4), 975±994. Katayama S, Horikawa H, Ito Y, Gomyo N, Obuchi Y (1971), `Synthesis of alternating polyamideurethanes by reacting diisocyanates with N,N'-di-(6-hydroxy-caproyl)alkylenediamines and N-hydroxy-alkyl-6-hydroxycaproamide', J. Appl. Pol. Sci., 15 (4), 775±796. Katsarava R, Beridze V, Arabuli N, Kharadze D, Chu C C, Won C Y (1999), `Amino acid based bioanalogous polymers: Synthesis and study of regular poly(ester amide)s based on bis(amino acid) alkylene diesters and aliphatic dicarboxylic acids', J. Pol. Sci. Part A: Polymer Chemistry,37 (4), 391±407. Kawasaki N, Nakayama A, Maeda Y, Hayashi K, Yamamoto N, Aiba S (1998), `Synthesis of a new biodegradable copoly(ester)amide: poly(L-lactic acid-coepsilon-caprolactam)', Macromol. Chem. Phys., 199 (11), 2445±2451. Lee S Y, Park J W, Yoo YY T, Im S S (2002), `Hydrolytic degradation behaviour and microstructural changes of poly(ester-co-amide)s', Pol. Deg. Stab., 78 (1), 63±71. Liu Y, Yuan M, Deng X (2003), `Study of biodegradable polymers: synthesis and characterization of poly(DL-lactic acid-co-L-lysine) random copolymer', Eur. Pol. J., 39 (5), 977±983. Montane J, Armelin E, AsõÂn L, RodrigueÂz-GalaÂn A, Puiggalõ J (2002), `Comparative degradation data of polyesters and related polyesteramides derived from 1,4 butanediol, sebacic acid and amino acids', J. Appl. Pol. Sci., 85 (9), 1815±1824. Nagata M (1999), `Synthesis and enzymatic degradation of poly(ester amide) stereocopolymers derived from alanine', Macromol. Chem. Phys., 200 (9), 2059±2064. Okada M (2002), `Chemical synthesis of biodegradable polymers', Prog. Polym. Sci., 27, 87±133. Okada M, Yamada M, Yokoe M, Aio K (2001), `Biodegradable polymers based on renewable resources V: Synthesis and biodegradation behavior of poly(ester amide)s composed of 1,4:3,6-dianhydro-D-glucitol, amino acid and aliphatic dicarboxylic acid units', J. Appl. Pol. Sci., 81 (11), 2721±2734. Ouchi T, Shiratani M, Jinno M, Hirao M, Ohya Y (1993), `Synthesis and enzymatic hydrolysis of polydepsipeptides with functionalized pendant groups', Macromol. Rapid Commun., 14 (12), 825±831. Ouchi T, Nozaki T, Okamoto Y, Shiratani M, Ohya Y (1996), `Synthesis of poly-

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6

Themoplastic starch biodegradable polymers P J H A L L E Y , The University of Queensland, Australia

6.1

Introduction

Biodegradable polymers are an exciting novel range of polymer materials. Biodegradable polymers are polymers that under action of a biological enzyme break down to biomass, CO2 and water in a given time period (as defined by a biodegradation standard) and in a given environment (i.e., marine, compost, anaerobic sludge). Looking more broadly it could be argued that biodegradable polymers are a subset of sustainable polymers. Sustainable polymers are defined as polymers that are produced in a sustainable way. This may involve using sustainable or renewable resources for ingredients or may involve polymers that are processed via a processing method that reduces environmental impact of the process and product. Strategies for increasing sustainability of polymers are: · Reuse (which aims to reuse primary scrap in polymer processes) · Reduce (which aims to reduce the amount of polymer in a product in the first place) · Recycle (which examines secondary uses for plastic products) · Recover (which looks at recovering the chemicals and/or energy from plastic waste) · Renewable/non-biodegradable (which uses renewable feedstocks to produce non-biodegradable polymers ex. polyolefins from biofuel sources) · Renewable/biodegradable (which is the use of biodegradable polymers from renewable resources). These sustainable strategies are a slight extension on the `four R's of recyclability' widely used in the polymer industry, but regardless, are very important strategies used by plastic industries today to reduce their environmental impact and maximise long-term sustainability of their business. Biodegradable polymers can be further broken down into two main areas: renewable and non-renewable biodegradable polymers. Essentially renewable biodegradable polymers utilise a renewable resource (i.e., a plant by-product) in the development of the polymer, rather than a non-renewable (i.e., petroleum-

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based) resource. Obviously, the long-term research and development focus is on renewable/biodegradable polymers, but initial R&D work on petroleum based biodegradable polymers has been very instructive for many of the initial biodegradable products. Biodegradable polymers traditionally have also been categorised by base polymers. Table 6.1 summarises various biodegradable polymers, along with their generic advantages, disadvantages, potential applications and some current suppliers. As shown in Table 6.1 there is a wide variety of biodegradable polymer platforms being developed. This is particularly good for the development of biodegradable materials development as not only are there many individual polymers, there are also many opportunities to synergise strengths of various polymer systems through the use of polymer alloys, polymer blends and reactive compatibilisation strategies. For example, in broad terms there always has been a trade off between cost and performance in biodegradable plastics development. For example, renewable starch-based systems traditionally are low cost but suffer from poor processibility and final product properties. Whereas non-renewable synthetic polymers traditionally are easier to process and have excellent properties, but are typically too cost prohibitive to sell. Thus synergising the advantages of starch-based and synthetic biodegradable polymers via new alloying and blending technologies this author believes represents one key strategy for creating more applications and larger markets for biodegradable plastics. This chapter will focus on the development of these lowcost base thermoplastic starch-based polymers, however, first it is instructive to introduce some background on starch itself.

6.2

Properties of starch

Starch is the major polysaccharide reserve material of photosynthetic tissues and of many types of plant storage organs such as seeds and swollen stems. Starch occurs in nature as water insoluble granules. The starch granule is essentially composed of two main polysaccharides, amylose and amylopectin with some minor components such as lipids and proteins. Starches from different botanical origin have different biosynthesis mechanisms and hence may exhibit distinct molecular structure and characteristics as well as diversity in shape, size, composition and other macroscale constituents of the starch granules. Thus the ultimate processing and properties of the starch are linked to starch genetics as well as various structure levels from granule structure, macromolecular structure and crystalline macrostructures.

6.2.1 Starch genetics Different varieties of starch species generally have different granule sizes, granule size distributions, granule structures, starch compositions, molecular

Table 6.1 Types of biodegradable polymers Base polymer

Source type

Advantages

Disadvantages

Potential applications

Suppliers

Starch

Renewable

Low cost Fast biodegradation

Poor mechanical properties Hydrophillicity

Foams Films and bags Moulded items

Novamont (Materbi) Plantic Technologies (Plantic) Rodenberg (Solanyl) Biotec (Bioplast) National Starch (ECOFOAM)

Polyhydroxyalkanoates (PHA)

Renewable

Rapid biodegradation High cost Water stable

Moulded items

Biomer (Biomer) Metabolix P&G

Cellulose and cellulose acetates

Renewable

High strength Water stable

Difficult to process Very low biodegradability

Composites Fibre-board

UCB (Natureflex) Mazzucchelli (Bioceta)

Fatty acid (triglyceride oil based) polymers

Renewable

High strength

Brittle Low biodegradability

Composites Adhesives Compatibilisers

Dow UDelaware (research)

Lignin polymers

Renewable

High strength

Brittle Low biodegradability

Collagen/Gelatine polymers

Renewable

High strength

Non-reproducible properties Films

Polyactic acid (PLA) Renewable and non-renewable

High strength

Brittle

Injection moulding Fibres

Cargill-Dow (Natureworks) Boehringer

Polyglycolic acid (PGA)

Non-renewable

High strength

Brittle Soluble in water

Fibres Sutures

Davis and Greck Ethicon

PCL

Non-renewable

Water stable Hydrolysable

Low melting point

Compost bags Cold packaging

Solvay (Capa)

PVOH

Non-renewable

Good barrier properties

Low biodegradability Solubility in water

Synthetic polyesters

Non-renewable

High strength Good processing

Relatively high cost

Films Moulded items

BASF (Ecoflex) Showa (Bionolle) DuPont (Biomax) IRE Polymers Eastman (Eastar Bio)

Composites Adhesives Compatibilisers

Borregard (Lignopol)

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6.1 General scheme for starch biosynthesis in cereals (Rahman et al., 2000).

sizes and degree of branching of amylose and amylopectin. Recent advances in genetic engineering have developed an understanding of metabolic pathways in starch synthesis. Figure 6.1 for starch synthesis in wheat endosperm was proposed by Rahman et al. (2000). ADPG is responsible for the synthesis of amylose and amylopectin through addition of the glucose moiety of ADPG to the non-reducing end of a preexisting starch molecule. The extended starch polymer is then branched through the action of starch branching enzymes and these debranching enzymes have also been known to be responsible in forming the final structure of amylopectin. The amylose/amylopectin ratio is controlled by the (granule bound starch synthase) GBSS protein. Mutations in genes encoding (branching enzymes) BEI, BE II, (starch synthase enzymes) SSI, SSII, SSIII and (debranching enzyme) DBEs will affect the structure of amylopectins. By understanding the genes involved in the starch biosynthesis pathway, prediction of starch structure and molecular constituents from specific plants or biotechnologically modified starches can be made.

6.2.2 Starch granule diversity The diversity in starch granules from various botanical origins is illustrated in Table 6.2. Clearly there is a wide variety of sizes and content of constituent macromolecules (amyose and amylopectin, see below for structures) molecules

Table 6.2 Shape, size, composition and some properties of the macromolecular constituents of some starch granules (Blanchard, 1987) Amylose Shape Cereals Oats Wheat Maize Amylomaize Waxy maize Millet Barley Rice Rye Sorghum Pulses Horse bean Smooth pea Wrinkled pea

Amylopectin

Diameter (m)

Content

Degree of polymerisation

Iodine binding constant

Degree of polymerisation

Lenticular, polyhedric Polyhedric

5±15 2±38 5±25

1300 2100 940 1300

0.211

Polyhedric Polyhedric, spherical Lenticular Polyhedric Lenticular Polyhedric, spherical

4±12 2±5 3±8 12±40 4±24

27 26±31 28 52±80 0±1

20 19±20 25±26 23 20±22

22±29 14±32 28 21±34

1850

Spherical, ovoid Reniform (simple) Reniform (compd.)

17±31 5±10 30±40

32±34 33±35 63±75

1800 1300 1100

1.03 1.66 0.91

23 26 27

5±35 15±100

17 23

3200

1.06 0.58

24

Roots and tubers Manihot Semi-spherical, spherical Ellipsoidal Potato

0.905 0.11

0.591

26 26

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within each type of granule. This is obviously relevant to the final macromolecular structure and morphology of starch-based plastics derived from these various starch sources.

6.2.3 Macromolecular structure The major macromolecular components of starch are amylose and amylopectin. Amylose is essentially a linear molecule of (1!4)-linked -D-glucopyranosyl units with some slight branches by (1!6)--linkages (Fig. 6.2). Typically, amylose molecules have molecular weights ranging from 105 to 106 g molÿ1 (Buleon, 1998; Roger, 1996). Amylopectin is a highly branched molecule composed of chains of -Dglucopyranosyl residues linked together mainly by (1!4)-linkages but with (1!6) linkages at the branch points. Amylopectin consist of hundreds of short chains of (1!4)-linked -D-glucopyranosyl interlinked by (1!6)--linkages (Fig. 6.3). It is an extremely large and highly branched molecule (molecular weights ranging from 106 to 108 g mol-1) and thus it is unique compared to synthetic polymer (Thompson, 2000). Moreover, it has been established that the

6.2 The structure of amylose.

6.3 The structure of amylopectin.

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6.4 Types of chains in amylopectin (Peat et al., 1952).

branching within amylopectin is not random (Parker and Ring, 1998; Thompson, 2000). The basic organisation of amylopectin chains is usually explained in terms of A, B and C chains, which is based on how side chains are linked to the backbone of the molecule. The concept of A, B and C types of chains in amylopectin was first introduced by Peat (1952) and is shown schematically in Fig. 6.4. The outer chains (A) are glycosidically linked at their potential reducing group through C6 of a glucose residue to an inner chain (B). B chains are defined as chains bearing other chains as branches. The single C chain per molecule likewise carries other chains as branches but contains the sole reducing terminal residue (Buleon et al., 1998).

6.2.4 Crystallinity There are three types of crystallinity in starch as observed in the X-ray diffraction pattern (Fig. 6.5). They are the `A' type mainly cereal starches such as maize, wheat, and rice; `B' type such as tuber starches (potato, sago); and finally the `C' type crystallinity which is the intermediate between A and B type crystallinity, normally found in bean and other root starches (Blanshard, 1987; French, 1984). Another type of crystallinity is the Vh-type, which is the characteristic of amylose complexed with fatty acids and monoglycerides.

6.5 X-ray diffraction pattern of A-, B- and Vh-type starch.

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Biodegradable polymers for industrial applications

6.2.5 Combining structure levels in starch The structure and the organisation inside starch granule have been studied considerably by examining the granule after acid and enzymatic hydrolysis (Parker and Ring, 1998; Buleon et al., 1998). The most widely accepted model of the molecular organisation within the starch granule is shown in Fig. 6.6. From these studies, it was observed that starch granules contain alternating 120±400 nm amorphous and semi-crystalline layers or growth rings (Blanshard, 1987; Buleon et al., 1998). The semi-crystalline growth rings are composed of alternating amorphous and crystalline lamellae. The sum of one amorphous and one crystalline lamella is around 9±10 nm in size (Jenkins and Donald, 1995; Donald et al., 2001). Amylopectin is often presumed to support the framework of the semicrystalline layers in the starch granule. The short chains with polymerisation degrees ranging between 15 and 18 form a double helical conformation (Buleon et al., 1998) and associating into clusters (Jenkins and Donald, 1995). These clusters pack together to produce a structure of alternating crystalline and amorphous lamellar composition (Fig. 6.6). The side chains clusters which are predominantly linear and form double helices are responsible for the crystalline lamellae while the branching regions of the amylopectin molecule are

6.6 Schematic representation of starch granule structure (Jenkins and Donald, 1995): (a) a single granule, comprising growth rings of alternating amorphous and semi-crystalline composition; (b) expanded view of the internal structure; (c) the currently accepted cluster structure for amylopectin within the semicrystalline growth ring.

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responsible for the amorphous lamellae. The conformation of chain segments between branching points is not known, however, it has been shown that the branch point does not induce extensive defects in the double helical structure (Buleon et al., 1998). The exact conformation of free amylose inside the starch granule and the precise structural role played by amylose is still unclear. It has been suggested that a large portion of amylose could be found within the amorphous growth ring (Jenkins and Donald, 1995). However, amylose may also be partly involved in double helices with amylopectin short chains in the crystalline lamella and amylose-lipid complexes (Buleon et al., 1998). Jenkins and Donald (1995) has performed small angle X-ray scattering to investigate the changes in structure within the three regions (amorphous lamellae, crystalline lamellae and amorphous growth ring) with variation in amylose content (Jenkins and Donald, 1995). Although the role of amylose in starch granule structure was not fully resolved, however, they found that an increase in amylose content had the effect of increasing the size of the crystalline lamellae. The combined repeat distance of crystalline and amorphous lamellae however remained constant. It is clear that starches, unlike most commodity synthetic polymers, have complex interrelated molecular and macroscopic levels of organisation of structure and these must be understood before being able to use these materials as biodegradable polymer precursors.

6.3

Thermoplastic starch and their blends

Plasticised starch is essentially starch that has been modified by the addition of plasticisers (or similar) to enable processing. Thermoplastic starch is plasticised starch that has been processed (typically using heat and pressure) to completely destroy the crystalline structure of starch to form an amorphous thermoplastic starch. Thermoplastic starch processing typically involves an irreversible orderdisorder transition termed gelatinisation. Starch gelatinisation is the disruption of molecular organisation within the starch granules and this process is affected by starch-water interactions. Due to its relevance in starch processing both in the food and non-food industry, starch gelatinisation has been extensively studied in the past decades. Most starch processing involves heating in the presence of water and some other additives (for instance, sugar and salt to control gelatinisation in the food industry, or glycerol as a plasticiser for biodegradable plastics applications). Understanding the mechanism of starch gelatinisation and how the starch characteristics dictate the gelatinisation behaviour is thus necessary for better and more effective control of the structure development during processing and to allow the design of optimum processing conditions of starch polymer blends. From many studies, it is clear that starch gelatinisation involves: (i) the loss of crystallinity of the granule as measured by the loss of birefringence and its X-ray

150

Biodegradable polymers for industrial applications

6.7 Starch gelatinisation process: (a) raw starch granules made up of amylose (linear) and amylopectin (branched); (b) addition of water breaks up crystallinity and disrupts helices; (c)) addition of heat and more water causes swelling, amylose diffuses out of the granule; (d) granules, mostly containing amylopectin, are collapsed and held in a matrix of amylose (adapted from Lai and Kokini, 1991).

diffraction pattern, (ii) an uptake of heat as the conformation of the starch is altered, (iii) hydration of the starch as accompanied by swelling of the granules, (iv) a decrease in the relaxation time of the water molecules, (v) loss of molecular (double helical) order and (vi) leaching of the linear molecules (amylose) from ruptured granules (Donavon, 1979). Figure 6.7 highlights the gelatinisation process diagrammatically (Lai and Kokini, 1991). This figure shows raw starch granules made up of amylose (linear) and amylopectin (branched) molecules (step (a)). Then the addition of water breaks up crystallinity and disrupts helices (step (b)). Addition of heat and more water causes granules to swell and amylose diffuses out of the granule (step (c)). Granules, mostly containing amylopectin are collapsed and held in a matrix of amylose (step (d)). More complex models of gelatinisation have been described but usually follow three themes: (i) water mediated melting of starch crystallites (Donovan, 1979; Liu et al., 2002; Maaruf et al., 2001); (ii) melting process in semicrystalline polymers (Slade and Levine, 1988; Billiaderis, 1992; Blanshard, 1987; Roos, 1995); (iii) the breakdown of starch structure as side chain liquid crystalline polymer (SCLCP) (Donald et al., 2001; Jenkins et al., 1993; Waigh et al., 1998, 2000). Recent work in our laboratories (Tan et al., 2004) using modulated DSC to separate glass transition and gelatinisation events has further supported an extension of the SCLCP model and hypothesised gelatinisation occurs via (i) breakage of starch-starch±OH bonds, (ii) formation of starch-

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solvent ±OH bonds and (iii) an unwinding helix-coil transition. These insights into gelatinisation will help us understand and control the final structure and properties of the thermoplastic starch polymer. Thermoplastic starch polymers have been widely investigated since the 1970s and are attractive in that they start from a low cost base (are therefore on a par with competing synthetic non-biodegradable polymers) and are able to be modified or blended with other polymers in order to `engineer-up' their processing and properties. (This is opposite to the development of highperformance biodegradable polymers where the objective is to lower the cost base of the material). A review of initial research on processing, rheology and properties of thermoplastic starch is provided by Lai and Kokini (1991) and concentrates on effects of starch constituents and moisture on gelatinisation, rheological properties and fragmentation during extrusion of thermoplasticised starch. Effects of temperature, moisture content and additives on the rheological properties of thermoplastic starch have also been examined by Willett (1995a,b). They generally find a power-law behaviour of the viscosity-shear rate profile and a reduction in viscosity with increasing moisture, temperature and plasticisers, with the exception of glycerol monostearate (GMS) which increases viscosity (they propose, due to unmelted helical complexes wof starch-GMS). More detailed studies of effects of structural changes and rheological properties (Della Valle et al., 1998; Dintzis et al., 1995) highlighted the importance of the semi-crystalline gel like structure of starch on the rheological properties and the ability of strong shear conditions to disrupt this structure. Jane et al. (1993) also examine the effects of the addition of various salts on the breakdown of starch structure where the salts interact with free water to affect plasticisation. They find that there are both water structure effects (that depend on salt charge density as to whether salt increases (high charge density; structure maker with water, ex NaCl) or decreases (low charge density; structure breaker with water, ex NaCl) gelatinisation temperature) and electrostatic effects (where since starch is electronegative (OH-ion abundance) so cations destabilise starch granules and anions stabilise starch granules. Onteniente et al. (1998) examines the extrusion blending of starch plastics with epoxidised linseed oil and found some improvement in water resistance afforded by the oils. It is known that lipids (where lipids content varies between various starch types from potato (0.05%), corn (0.6%), wheat (0.8%), tapioca (0.1%) to waxy maize starch (0.15%)) reduce solubilisation, decrease thickening power and increase cloudiness by forming amylose-lipid complexes (Swinkels, 1985). However, although these studies are instructive in understanding thermoplastic starch processing it should be noted that thermoplastic starch polymers based solely on starch are extremely water sensitive, can suffer from significant molecular weight change in extrusion (Davidson et al., 1984; Sagar and Merrill, 1995a; Gomez and Aguilera, 1983) and are thus of limited practical

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value. Therefore most of the commercial research on thermoplastic starches has involved modified starches and/or blends with additives and other polymers. Early work on thermoplastic starch blends includes the study by Otey (1974) who investigated castable degradable mulch film derived from starchpolyvinylacetate (PVAc) blends with Polyvinyl chloride (PVC) coatings. Additionally Otey et al. (1980. 1987) investigated the development of blown starch based agricultural mulch films based on gelatinised corn starch, polyethylene (PE) and polyethylene-acrylic-acid (PEAA) polymers and various additives. Good film blowing performance was achieved with increased ammonia and urea (to improve starch-EAA interactions). Bastioli et al. (1994a,b) and Shogren et al. (1993) also investigated starch-ethylene-vinyl alcohol (EVOH) blends and Novamont MaterBi (starch-EVA) materials looking at the ratio of amylose and amylopectin and moisture content in the starch. However, note that these studies are symptomatic of early biodegradable plastics work, where the material was not entirely biodegradable and did not meet biodegradability standards (De Kesel et al., 1997; Breslin, 1993). But, of course, it should be noted that much of this earlier work has led to the science behind modern biodegradable blends. Mao et al. (2000) examined the extrusion of thermoplastic cornstarchglycerol-polyvinyl alcohol (PVOH) blends and noted the effect of PVOH to improve mechanical properties and slow biodegradation. Much debate on PVOH degradability has been well reviewed by Chiellini et al. (2003) which summarises that molecular weight and type of PVOH may affect its biodegradability, which although much slower than starch, appears to show degradability with specific enzymes. Doane (1992) has reviewed thermoplastic starch/ biodegradable polymer blends research and highlighted work at USDA focusing on developing either starch/biodegradable polymer blends (starch/polylactic acid(PLA), starch/polyhydroxybutrate(PHB), starch/polycaprolactam(PCL), and starch/polysaccharide/protein blends) or starch with grafted thermoplastic sidechains (free radical initiated starch-g-polymethacrylate (PMA), starch-gpolystyrene (PSty)) for blending with thermoplastics. All systems were generally plasticised with urea and other polyols. Doane (1992) and Doane et al. (1992) highlighted the development of fully degradable systems including starch/polylactic acid (PLA), starch-polycaprolactam(PCL) and reactively modified starch copolymers. However, as described by Jopski (1993), prices for some synthetic biodegradable polymers were still cost prohibitive. Recent research work has shown promise for reducing base costs of synthetic biodegradable polymers, where they examine development of polylactic acid (PLA) polymers via various novel conversion methods from starch (Warth et al., 1997; Gruber et al., 1996). Obviously the low cost conversion methods of Cargill-Dow's PLA production process established in 2003 (Natureworks TM) where PLA is developed from corn starch fermentation and polymerisation during extrusion is based on this earlier work.

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Ratto et al. (1999) has examined poly butylenes succinate adipate (PBSA) terpolymer/granular starch composites for blown films and showed they could produce good film tensile properties and control biodegradation with granular starch addition. Thermoplastic starch/cellulose fibre extrudates and injection moulded products were examined by Funke et al. (1998) and they showed a reduction in water adsorption with increasing fibre content. Work by Halley et al. (2001) examined the use of thermoplastic starch-polyester blends for use in mulch film applications noting excellent field performance and biodegradability for these materials. Modified processing techniques have also been useful for thermoplastic starch polymers. Recent work by Martin et al. (2001) and Martin and Avernous (2002) has examined the use of coextruded sheet processing to produce polyester/thermoplastic wheat starch/polyester multilayer films. It was found that adhesion strength between the layers and stability of the interface were crucial properties in controlling the final performance properties of the films. Work by Sousa et al. (2000) has also examined use of the novel shear controlled orientation injection moulding (SCORIM) process to control morphologies and provide tensile property increases of thermoplastic starch/synthetic blends. In summary this section has demonstrated that thermoplastic starch polymers and their blends provide an exciting foundation for developing low-cost biodegradable polymers. The next section will examine an extension of this research into modified thermoplastic starch polymers.

6.4

Modified thermoplastic starch polymers

In terms of modification of starch, many laboratory approaches have been taken from acetylation/esterification of starch to starch acetates, carbonilation of starch with phenyl isocyanates, addition of inorganic esters to starch to produce phosphate or nitrate starch esters, production of starch ethers, and hydroxypropylation of starches via propylene oxide modification (Gilliard, 1984). Generally all these modifications involve hydroxyl group substitution on the starch (increasing the degree of substitution (DOS) of OH groups) that will lower gelatinisation temperatures, reduce retrogradation (recrystallisation over time) and improve flexibility of final products. Takagi et al. (1994) examined corn starch acetate/polycaprolactone (PCL) blends and showed that the blends are able to maintain biodegradability (after acetylation) and have stable viscosities with increasing acetylation. This is due to the increasing stability and thermoplasticity afforded by the acetylation. Increasing acetylation also reduces retrogradation. Tomasik et al. (1995) examined the acetylation of starch via extrusion with succinic, maleic and phthalic anhydrides and found a decrease in water binding capacity of the extrudates. However they also noted the reactive extrusion process was difficult to control. Fringant et al. (1996) examined the acetylation of starch via the

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pyriding-acetic anhydride procedure and produced modified starches with DOS of 1.7 that were easily processible but had deteriorated in some mechanical properties. Of course acetylation levels can reduce biodegradation of the starch polymer (Rivard et al., 1995), so a balance between property water resistance and biodegradation must always be maintained. Fatty acid ester modification of starches was also examined by Sagar and Merrill (1995b), however, although thermoplastic processible starch-ester polymers could be produced, it was stated that the additional costs of these modifications probably would limit industrial use of these materials. Hydroxypropylation of normal and high amylase starch was conducted by Bae and Lim (1998) and showed an increase in ease of extrusion, decrease in water absorption and increase in flexibility and strength. Additionally, crosslinked starch may be induced by the addition of organic esters (i.e., succinic anhydride), inorganic esters (i.e., trisodium trimetaphosphate (TSTMP)), hydroxydiethers (i.e., epichlorohydrin) and irradiation. Kulicke et al. (1989, 1990) examined solution phase crosslinking of starch with epichlorohydrin and TSTMP, and although some of this work was extended to cast starch/PVOH films (Chen et al., 1997), no reported work has examined crosslinking thermoplastic starch in the melt during extrusion processing. Jane et al. (1993) examined the crosslinking of starch/zein cast films for improving water resistance. Reactive blending of thermoplastic starch/polymer blends has been examined recently and aims to increase properties and performance via control of blend morphologies. Mani et al. (1998, 1999) examined different techniques for compatibilising starch-polyester blends. They examined development of maleic anhydride grafted polyester/starch blends and starch-g-polycaprolactone compatibilised starch/polycaprolactone blends, and found significant increases in tensile properties of the compatibilised blends, over the uncompatibilised blends. Recent work extending this original work by Mani and Bhattacharya (2001) has shown improved properties for injection moulded starch/ biodegradable polyester materials via the addition of anhydride-modifed polyester compatibilisers. Dubois and Narayan (2003) also examines the reactive compatibilisation of starch/PLA and starch/PCL polymers via MA-gPLA and PCL-g-dextran compatibilisers. Interestingly, they find no difference between compatibilisation when using starch granules or thermoplastic starch, which may indicate the granules are quickly gelatinised during processing. In terms of nanocomposite reinforcement of thermoplastic starch polymers there has been much exciting new developments. Dufresne and Cavaille (1998) and Angles and Dufresne (2000) highlight work on the use of microcrystalline whiskers of starch and cellulose as reinforcement in thermoplastic starch polymer and synthetic polymer nanocomposites. They find excellent enhancement of properties, probably due to transcrystallisation processes at the matrix/fibre interface. McGlashan and Halley (2003) examines the use of nanoscale montmorillonite into thermoplastic starch/polyester blends and finds excellent improvements in film blowability and tensile properties (Table 6.3).

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Table 6.3 Improvement in tensile properties of thermoplastic starch based films blown with modified montmorillonite (McGlashan and Halley, 2003) Sample

Tensile strength (MPa)

Young's Modulus (MPa)

Strain at break (%)

Thermoplastic starch/ polyester film

13

17

1086

Thermoplastic starch/ polyester/nanocomposite film (low level nanoclay)

18

58

1500+

Thermoplastic starch/ polyester/nanocomposite film (high level nanoclay)

17

64

1500+

Perhaps surprisingly McGlashan (2003) also found an improvement in the clarity of the thermoplastic starch based blown films with nanocomposite addition which was attributed to disruption of large crystals. It is clear that research into modifications to thermoplastic starch based polymers is burgeoning and that property and processing improvements derived from this research will help thermoplastic starch polymers widen their application products and markets.

6.5

Commercial applications and products for thermoplastic starch polymers

Some commercial thermoplastic starch polymer based products were highlighted in Table 6.1, and some of them can be examined in more detail in this section. Probably one of the first starch based products developed was the National Starch expanded starch foam packaging material ECO-FOAMTM. ECO-FOAMTM materials are derived from maize or tapioca starch and include modified starches. This relatively short-term, protected-environment packaging use is ideal for thermoplastic starch polymers. National starch now has additional thermoplastic starch materials, blends and speciality hydrophobic thermoplastic starches for a range of applications including injection moulded toys, extruded sheet and blown film applications. [http://www.eco-foam.com/ loosefill.asp]. Novamont has been developing thermoplastic starch based polymers since 1990. Mater-BiTM polymers are based on thermoplastic starch-blend technologies and product applications include biodegradable mulch films and bags, thermoformed packaging products, injection moulded items, personal hygiene items and packaging foam. [http://www.novamont.com/] Rodenberg Biopolymers produce SolanylTM a thermoplastic starch based biopolymer which is focused on injection moulding applications. SolanylTM is

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derived from potato peels, and is able to be produced in grades with water solublility and decreased water sensitivity. [http://www.biopolymers.nl/] Biotec GmbH produces BioplastTM themoplastic starch based polymers for a range of applications including accessories for flower arrangements, bags, boxes, cups, cutlery, edge protectors, golf tees, horticultural films, mantling for candles, nets, packaging, packaging films, packaging material for mailing, planters, planting pots, sacks, shopping bags, straws, strings, tableware, tapes, technical films, trays and wrap film. [http://www.biotec.de/engl/ index_engl.htm] Earthshell are also developing thermoplastic starch based materials for thermoformed tray applications. Earthshell materials are derived from starch (from sources such as potatoes, corn, wheat, rice and tapioca) along with fibre, other processing agents, air, water and micro-thin biodegradable coatings for protection. Earthshell thermoformed foamed materials are processed in a patented cooking process, and not via conventional plastics processing equipment which can be seen as an advantage (new material structures are enabled) or a disadvantage (new process equipment is needed). Applications include foamed trays, plates and packaging films and laminate. [http:// www.earthshell.com/foam.html] Very recently Plantic Technologies Ltd produced soluble PlanticTM thermoformed trays for confectionery packaging [http://www.plantic.com.au]. PlanticTM material is based on thermoplastic corn starch polymer and is processible on standard polymer processing equipment.

6.6

Thermoplastic starch polymers ± looking beyond traditional polymer applications

The above is focused on thermoplastic starch polymers for traditional polymer markets, and thus much research and development has focused on engineering water resistance and better mechanical properties into the starch based materials. However, there is an existing and growing interest in soluble biodegradable polymer applications ± markets starch based polymers should already be well placed to enter. An excellent review of environmentally biodegradable water soluble polymers is given by Swift (2002). Swift first characterises the breakdown of soluble biodegradable polymers as either single stage via an aquatic environment or two stage via water treatment and then subsequent treatment such as composting or digestion. Then he describes various soluble polymer systems such as polyvinyl alcohol (PVOH) used in films, polycarboxylates such as polyacryllic acid (PAA) used in detergents, polyaspartic acid polymers used as dispersents and thickeners, polyethylene oxides (PEO) used in shampoos and detergents, and modified natural polymers such as starch grafted with soluble polymers (for detergent additives) and modified cellulose polymers such as hydroxypropyl cellulose and carboxy-

Themoplastic starch biodegradable polymers

157

methylcellulose (used in cleaning products). Paik et al. (1995) sees future applications for starch based materials as surfactants (alkyl- and alkenylchemically modified polyglycosides), sequestrants (oxidised starches) and builders (to replace carboxylic acids with carboxy-modified starches) in detergents. They discuss the potential for builders to replace carboxylic acid as by far the most highly researched area due to the potential of starches modified with carboxylic acid grafting, esterification and etherification reactions. There is clearly an opportunity here for soluble starch, modifed starch and starch-blend polymers to be used in these applications. Starch polymers have also been used as controlled drug delivery matrices. For example polyacrylate modifed starch microparticles have been used as protein and drug carriers, and starch/albumin microparticles have been designed for controlled protein release (Piskin, 2002). Research into starch based controlled release tissue scaffolds for growth factor release is also noted by Piskin (2002). In tissue engineering applications obviously control of the breakdown of the starch matrix is crucial to maintain control over drug or macromolecule release. Altpeter et al. (2003) also examined the use of novel shear controlled orientation injection moulding (SCORIM) to develop starchpolylactide and starch-poly(ethylene-co-vinyl alcohol) blends for replacing biomedical implants for temporary applications. However, although initial properties were improved by the novel processing, they found too high degradation rates for their applications. However, this work and earlier work (Marques et al., 2002) showed that starch-based polymers exhibit a cytocompatibility that was promising for their use as biomaterials. Thus opportunities for modified starch polymers and their blends in biomedical applications is also great.

6.7

Future developments

Clearly thermoplastic starch based polymers offer a very attractive low cost base for new biodegradable polymers due to their low material cost and ability to be processed on conventional plastic processing equipment. The engineering of more advanced properties into these low cost base materials will continue to be the main technological drive into the future. This development will most probably be in the form of integrating research already being developed in parallel from thermoplastic starch polymers (like blending, nanocomsposites and reactive modification) and novel research from conventional thermoplastic polymer (such as nano- and microstructure control using novel additives and novel polymer processing). Additionally the development of new biodegradable polymers with lower base costs (such as PLA from corn starch (Cargill Dow) and PHB from wheatgrass (under development)) will also widen the types and applications of biodegradable polymers from niche applications to wider markets in the future.

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6.8

Further information

Further excellent articles and references on starch and thermoplastic starch polymers can be found in the following list. · Petersen K, Nielsen PV, Bertelsen G, Lawther M, Olsen MB, Nilsson NH, Mortensen G, `Potential of biobased materials for food packaging', Trends In Food Science & Technology, v10 (2): 52±68 (1999). (An excellent review of food packaging applications for biodegradable polymers.) · Bastioli C, `Biodegradable materials ± Present situation and future perspectives', Macromolecular Symposia, v135: 193±204 (1998). (An excellent summary of thermoplastic starch polymers from one of Novamont's leading scientists.) · Fritz HG, Aichholzer W, Seidenstucker T, Widmann B, `Biodegradable polymer systems based on renewable raw materials ± Chances and limitations', Starch-Starke, 47 (12): 475±491 (1995). (Excellent review of potential of renewable biodegradable polymers.) · Shogren RL, Fanta GF, Doane WM, `Development of Starch-Based Plastics ± A Reexamination of Selected Polymer Systems In Historical-Perspective', Starch-Starke, v45 (8): 276±280 (1993). (Excellent review of original studies in starch based biodegradable polymers.) · Galliard T, Starch: Properties and potential, John Wiley & Sons, Great Britain (1987). (Excellent text on starch.) · Doane WM, `Starch ± Renewable Raw-Material For Chemical-Industry', Journal of Coatings Technology, v50 (636): 88±98 (1978). (One of the pioneering articles on starch based plastics.) Some societies and other websites focusing on thermoplastic starch and biodegradable polymers include: · Bio-, Environmental and Degradable Polymer Society (BEDPS); http:// www.bedps.org/ · Biodegradable plastics society (and certification society); http:// www.bpsweb.net/02_english/ · Society of Plastics Engineers ± Environmental division; http://www.4spe.org/ sectionsdivisions/divisions/d40.htm · International Biodegradable Polymers Association & Working Groups (IBAW) is an international coalition of companies and institutions for promoting the innovation of Biodegradable Polymers; http://www.ibaw.org. This society ran the Kassel project (where biodegradable plastics were used by an entire community) and this is reviewed in detail from their home page or directly at http://www.modellprojekt-kassel.de · An excellent website for biodegradable polymers being researched and on the market is www.biopolymer.net

Themoplastic starch biodegradable polymers

6.9

159

Acknowledgements

I would like to acknowledge my colleagues and students (the 35 researchers who know who you are) in biodegradable polymers. Specific thanks to Stewart McGlashan, Ihwa Tan, Peter Sopade and Steve Coombs for their discussions and input to this chapter.

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starch/EVOH blend aimed as an alternative biomaterial for temporary applications', Journal of Applied Polymer Science, v77 n6: 1303±1315. Swift G, (2002) `Chapter 11 Environmentally biodegradable water soluble polymers', in G Scott (ed.), Degradable Polymers, 2nd edn, 379±412 Kluwer Acad. Pub., The Netherlands. Swinkels JJM, (1985) `Composition and properties of commercial native starches', Starch, v37: 1. Takagi S, Koyama M, Kameyama H, Tokiwa Y, (1994) `Development of PCL/ Gelatinised starch blends and their enzymatic degradation', in Biodegradable plastics and polymers, Y Doi, K Fukuda (eds), 437. Tan I, Wee CC, Sopade PA, Halley PJ, (2004) `Investigating the starch gelatinisation phenomena in glycerol-water systems: application of modulated temperature differential scanning', submitted to Starch/Starke, January (2004). Thompson DB, (2000) `On the non-random nature of amylopectin branching'. Carbohydrate Polymers, v43: 223±239. Tomasik P, Wang YJ, Jane JL, (1995) `Facile route to anionic starches', Starch/starke, v47: 96±99. Waigh TA, Perry AP, Riekel C, Gidley MJ, Donald AM, (1998) `Chiral side-chain liquidcrystalline polymeric properties of starch', Macromolecules, v31 n22: 7980±7984. Waigh TA, Kato KL, Donald AM, Gidley MJ, Clarke CJ, Riekel C, (2000) `Side-chain liquid-crystalline model for starch', Starch/Starke, v52 n12: 450±460. Warth H, Muelhaupt R, Schaetzle J, (1997) `New thermoplastic carbohydrate derivatives', PPS-13 International Meeting, Polymer Processing Society, NJ, USA. Willett JL, Jasberg BK, Swanson CL, (1995a) `Rheology of thermoplastic starch ± effects of temperature moisture content and additives on melt viscosity', Poly. Eng. & Sci., v35, n2: 202±210. Willett J, Jasberg BK, Swanson CL, (1995b) `Rheology of thermoplastic starch', Poly. Eng. & Sci., v35: 202.

Part II

Materials for production of biodegradable polymers

7

Biodegradable polymers from sugars A J V A R M A , National Chemical Laboratory, India

7.1

Introduction

Nature has blessed man with a variety of renewable polymers (Kaplan, 1998) and renewable monomers (Carraher et al., 1983) from which to synthesize polymeric materials. From the days of prehistoric man until a few centuries ago, man depended almost solely on natural materials like wood, clay and some metals for his structural materials, and leaves and straw as packaging material. Wood, as we know, is one of the finest examples of a natural composite consisting of reinforcing sugar polymer fibres (cellulose) in a crosslinked lignin matrix. Cellulose, a polymer of glucose, was the first polymer to be structurally identified as a polymer, and cellulose acetate was the first plastic patented and then developed for commercial use, less than a century ago (Edgar et al., 2001). Till today, cellulose and its various derivatives constitute the largest group of polymeric molecules used by modern industry (Gilbert and Kadla, 1998). Indeed, with newer and `greener' process technologies for treating biomass from the plant kingdom (such as steam explosion techniques for pulping) in various stages of development, the potential for a further increase in their market share is but logical. Similarly, starch (Shogren, 1998), hemicelluloses (xylans, glucomannans) (Kirk Othmer's Encyclopedia of Chemical Technology, 1995), lignins (Glasser and Sarkanen, 1989; Argyropoulos and Menachem, 1998), and many other simple and complex polysaccharides are increasingly being viewed as invaluable polymeric organic raw materials for the future requirements of a variety of products for man. Sucrose, or table sugar, is one of the purest and cheapest multi-functional chemicals available for development as a polymer, or for incorporation into a polymer molecule. A variety of other simple carbohydrate molecules (xylose, glucose, lactose, degraded oligomers from polysaccharide processing, etc.) can be polymerized or incorporated into polymers, leading to a wide array of polymeric materials with different properties and for different specialty applications. Many of these polymers, by virtue of their sugar content, are expected to biodegrade when disposed of in the soil or sewage.

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Therefore, all the above mentioned are attractive materials for sustained current research and development, especially in view of the scarcity of synthetic chemicals, and the environmental issues that are increasingly important to modern society. The current importance of this subject can be gauged by the large number of important reviews appearing in recent literature. The question as to why we need to use renewable resources (Pacitti, 2003; Varma, 2003; Bozell, 2001, Narayan, 1997); their importance not only in plastics but also in the daily down-the-drain products like detergents (Gross and Kalra, 2002) and in large volume cementconcrete (Griffen and Gibbs, 2003); why we need biodegradable polymers (Scott and Wiles, 2002a; Doi and Shiotani, 1994); and, finally, what problems we encounter after use of plastics (Vert et al., 2002; Scott and Wiles, 2002b) are all detailed in the references cited. This chapter will look at polymers with sugar units (monosaccharides and disaccharides) anchored onto synthetic polymers, polymers prepared by the polymerization and copolymerization of chemically functionalized sugar molecules (synthetic polysaccharides), natural polysaccharides like cellulose, starch, and their derivatives, plant fibers (lignocellulosics), and hemicellulose based polymers (pentose sugar polymers) as a new generation of biodegradable polymer materials.

7.2

Biodegradable polymers obtained from monosaccharides and disaccharides

There has been a worldwide realization that nature-derived monosaccharides, disaccharides, oligosaccharides and polysaccharides can provide us with the raw materials needed for the production of numerous industrial consumer goods (Kunz, 1993; Varma, 2003; Pacitti, 2003). This section will deal with the role of sugar molecules anchored like pendants onto a synthetic polymer, reminiscent of the `crown ether' type molecules pendant on polystyrene and other synthetic chains which created a whole new area of research with far-reaching outcomes (Gokel and Durst, 1976; Varma, 1979; Varma et al., 1979; Smid et al., 1979a, Shah and Smid, 1978; Smid et al., 1979b; Varma and Smid, 1977). While the crown ether pendants created metal binding capacity to synthetic polymers, pendants of sugar molecules are expected to make the synthetic polymer biodegradable and/or biocompatible. In particular, the effect of anchoring only small quantities of sugar molecules like glucose or sucrose onto polymers to make them biodegradable plastics (thermoplastics) will be discussed in as much detail as possible, as this is an emerging field (Galgali et al., 2002, 2004). The use of sucrose in rigid polyurethane foams is only too well known (Hickson and Gould, 1977; Kobayashi et al., 2001; Kino et al., 2002). However, in such cases the sucrose content of the crosslinked thermoset polymer is substantial. The other major application of sugars grafted onto synthetic polymers is that of biocompatible hydrogels. Thus, disposable diapers

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containing acrylic acid-alkylsulfonic acid-glucose graft copolymer and hydroxyethylmethacrylate-(n-hydroxyethyl-n-vinylamine)-glucose graft copolymer were synthesized for their deoderizing properties (Onishi and Hisanaka, 2003). Work on microgels of styrene-divinylbenzene conjugated with glucose and maltohexose has also been of interest (Narumi et al., 2003). Specialized applications of glucose-responsive hydrogels useful as glucose sensors are also of great current interest (Lesko and Sheppard, 1994; Han et al., 2002). Indeed, grafting of sugar molecules via polymer analogous reactions has been practiced for decades but has achieved limited success. The earlier work of Pfannemuller (Andresz et al., 1978) was based on grafting monosaccharide segments onto natural polymers like amylose to obtain comb like polymers. Later on he diversified this strategy to include grafting of sugars onto synthetic polymers. The grafting of glucose and maltoligomers onto linear polymers like poly(ethylene glycol) having carboxymethyl end groups, poly(acrylic acid) etc., via hydrazone linkages was reported (Andresz et al., 1978). Similarly, mono-, di- and oligosaccharides were also linked via amide bonds to synthetic and natural polymers having carboxyl and amino functions, e.g., poly(acrylic acid) and poly(vinyl amine), as well as polysaccharide derivatives like chitosan. The number and the length of the saccharide branches were varied to obtain polymers exhibiting polyelectrolyte behaviour (Emmerling and Pfannemueller, 1983). Galactose was covalently linked to 2hydroxyethylmethacrylate-ethylene methacrylate copolymer and the resulting polymer was used as a stationary phase material for column chromatography of proteins (Karel et al., 1980; Jiri et al., 1978). Synthesis of synthetic polysaccharides by polymer analogous reactions was reported by other groups as well (Bahulekar et al., 1998a,b). Glucosamine hydrochloride and galactosamine hydrochloride were reacted with poly(acryloyl chloride) to obtain linear polyacrylamides with pendant sugar residues. Sucrose was grafted onto butadiene-acrylic acid copolymers and poly(butadiene carboxylate) (Alvarez et al., 1988). More recently, polymer surfaces were modified with carbohydrate derivatives by polymer analogous methods (Gruber and Knaus, 2000). The surface of poly(vinyl chloride) has been modified by polymer analogous reactions (Rios and Bertorello, 1997). The polymer film, suspended in acetone containing the initiators viz benzophenone and 2,20 azoisobutyronitrile and sucrose acrylate, was subjected to UV radiation to initiate the grafting reaction. The modification improved the interfacial phenomenon between the microorganism and the PVC surface. The base catalyzed adsorption of poly(vinyl alcohol) in DMSO with reducing carbohydrates to obtain pseudopolysaccharides through a chemically and enzymatically inert ether linkage has been reported (Kraska and Mester, 1978), They are useful in the solid phase syntheses of glycosides, as potential carriers of drugs and they also serve as a useful probe in the study of protein-

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carbohydrate interactions. The 6-O-epoxy propyl derivative of D-galactose-6-Oallyl ether was used for this study. Hemocompatibility of polymer surfaces has been shown to improve by grafting monomers like aÂ-amino acids, peptides and amino sugars like D-glucosamine onto polymers like poly(ether urethanes), poly(ethylene glycol) , poly(tetrahydrofuran), poly (vinyl alcohol) and dextran (Bamford et al., 1990). Maltamine, which is a mixture of -D-glucopyranosyl(1,6)-2-amino-2-deoxy-D-sorbitol and -D-glycopyroanosyl(1,6)-2-amino-2deoxy-D-mannitol was bound to chloroethylated poly(g-Me L-glutamate) and the resulting membrane was used to resolve optically active substances (Nakagawa et al., 1994). Sugars were bound to polymer supports via thiosemicarbazones and were used for immobilization of enzymes (Tweeddale et al., 1994). Glucose and N-acetyl glucosamine hydrazones were reacted with isothiocyanate substituted polystyrene. 3-azido styrene and N-p-vinyl benzyl(O-B-D-galactopyransoyl (l!4) ± D- gluconamide were polymerized and the resulting polymer was applied on a PVC dish and irradiated using ultraviolet radiation to obtain PVC fixed with sugar, which prevented adhesion of blood platelets on the plastic dish (Yura et al., 1997). A novel solvent evaporation technique was also used to prepare nano particles with carbohydrate chains on their surface (Maruyama et al., 1994). From the above review, it is clear that incorporation of sugar molecules onto synthetic polymers has been carried out with a view to developing new materials for low-volume niche applications. Indeed, no large-scale application of such studies has come forth, though the information generated has created a wealth of scientific information of great importance for structure-property evaluation of such polymer systems as well as their role in mimicking or acting as models of natural polymers. In spite of these advantages, compositional analysis of these polymer systems will continue to remain problematic (Klein, 1987) and will require state-of-the-art characterization tools. However, it cannot be denied that these studies have opened the doors for further research into the structural and functional mimicry of bio-systems, which promises to be an important field for the future. However, the potential for developing an application for commodity materials based on inexpensive sugar molecules like sucrose, glucose, etc., anchored onto commodity polymers (plastics like polystyrene and polyethylene) in order to obtain biodegradable commodity thermoplastics have only now been reported in published literature (Galgali et al., 2002, 2004). The methodology of grafting unprotected multifunctional sugars onto synthetic polymer backbones on a large scale has not been investigated intensively due to the crosslinking reactions which are unavoidable. However, this author is of the opinion that this methodology has much to offer in terms of tailored polymer properties, especially biodegradability. Recent researches from this group showed that sugars with protected as well as unprotected groups can be grafted onto a synthetic commodity polymer (Galgali et al., 2002, 2004). Mild reaction condi-

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tions were chosen in order to avoid formation of cross-linked products. Experimental conditions were carefully chosen to control the number of sugars being grafted as well as their random occurrence on the polymer chain, especially when low degrees of grafting are required. Dramatic enhancement in the rates of biodegradation of these polymers were reported by these workers, and has caught the attention of researchers and technology writers worldwide (Ball, 2002; Pacitti, 2003).

7.2.1 Biodegradable commodity plastics Due to excellent processing properties and inexpensiveness, polystyrenes and polyolefins have occupied a special status as commodity plastics. The significant annual growth of these plastics when used as packaging materials has imposed a colossal waste disposal problem for municipal sewage treatment plants, not to speak of the unseemly littering of public places by these materials. The major drawback of such packaging plastics is that they are nonbiodegradable and hence pose the severe problems (mentioned above) of their safe disposal after their useful life is completed. Several alternative and innovative methods have been investigated, but each one has drawbacks, and so the search continues for a `green' solution. Natural polymers are still some way from being developed as viable alternatives to petroleum derived polymers. This realization has led to voluminous researches on additives for polyolefins that can cause degradation of these polymers (Scott and Gilead, 1995). For example, if additives are added which can catalyze the degradation of the plastics, there is also the danger that the additives, often toxic, would leach out in landfills and contaminate groundwater and affect the soil microorganisms. With the aim of developing biodegradable polymers based on polyolefins, blending of starch with polyolefins (particularly with polyethylene) has been much explored and also put in practice in a limited way. The intention of such blending procedures was that after disposal, degradation of starch in the blend would create voids and weaken the integrity of the polyethylene and result in its degradation. However the main drawback of this methodology is that attainment of such properties demands larger volumes of starch (in the range of 30% or higher) due to which the physical properties of the polyethylene have to be compromised (Johnson et al., 1993; Lee et al., 1991). A new approach to synthesizing biodegradable plastics based on functionalized polystyrene, by chemically linking carbohydrate molecules onto the polymer, by polymer analogous reactions, and then testing their biodegradation rates using pure bacterial and fungal cultures has been reported (Galgali et al., 2002, 2004). Polystyrene, functionalized with maleic anhydride (14% by weight, obtained from Aldrich), was used as the base polymer onto which minute quantities of various monomeric sugars like glucose, lactose and sucrose were anchored via ester linkages. Instead of using a mixture of several

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7.1 Growth pattern of soil bacteria on sugar-linked poly(styrene maleic anhydride) polymers. Weight losses after 28 days for samples 3 and 4 were 10%. Key 1: Control 1 (minimal medium inoculated by Pseudomonas sp.). 2: Control 2 (glucose as the sole source of carbon degraded by Pseudomonas sp.). 3: Lactose-linked PSMAH degraded by Pseudomonas sp. 4: Lactose-linked PSMAH degraded by Serratia sp. 5: Unmodified PSMAH degraded by Pseudomonas sp. (Results similar to and adapted from P. Galgali, A.J. Varma, U.S. Puntambekar, and D.V. Gokhale, JCS Chemical Communications, 2884, (2002).)

microorganisms, three pure soil bacterial cultures (serratia marcescens, pseudomonas sp., and bacillus sp.) and three fungal cultures Aspergillus niger Trichoderma sp. and Pullularia pullulans) were chosen for studying their individual growth patterns on these new polymers in comparison to their growth in glucose solution or onto the unmodified polymer. The advantages of using pure cultures in biodegradation studies of polymers helps to identify the types of soil bacteria that preferentially attack a particular type of sugar polymer. This could be useful in designing biodegradation culture media. Figure 7.1 shows, as an example, the growth pattern of soil bacteria psuedomonas and serratia marcens on lactose-linked poly(styrene maleic anhydride) (PSMAH). About 1.2% lactose (by weight) linked to the PSMAH greatly increased the rates of biodegradtion over the unmodified polymer and other controls used, as seen from the increased optical density curves after about four weeks. The weight losses in this experiment were of the order of 10%. Table 7.1 shows weight loss data for fungal degradation of glucose, lactose, and sucrose-linked poly(styrene maleic anhydride) polymers. Here again, it is clear that the weight loss differences between the unmodified PSMAH (no

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Table 7.1 Weight loss data for fungal degradation of sugar-linked poly(styrene maleic anhydride) polymers. (Part of this data obtained from P Galgali, U S Puntambekar, D V Gokhale and A J Varma, Carbohydr. Polym., 55, 393±399 (2004).) Sample

Lactose-linked PSMAH (1.2 wt% lactose) Glucose-linked PSMAH (wt% of glucose < 0.5) Glucose-linked PSMAH (wt% of glucose < 0.35) Sucrose-linked PSMAH (0.9 wt% of sucrose) Sucrose-linked PSMAH (2.6 wt% of sucrose) Unmodified PSMAH

Aspergillus niger

% weight loss after 2 months P. ochro- P. pullulans Trichoderma chloron sp.

2.8

12

9.2

6.4

1.2

5.2

9.2

3.6

20.4

9.6

5.2

2.8

0.8

0.0

9.6

2.0

0.0 0.0

10.0 ö

14.0 0.0

19.6 0.0

weight loss recorded) is increased to about 20% weight loss for the glucose and sucrose linked PSMAH after two months. While these are preliminary experiments to prove a hypothesis, it is clear that small quantities of sugar molecules attached to PSMAH (used as a model compound for functionalized polystyrene) can induce dramatic changes in their rates of biodegradation. This opens up an area of research that can be pursued for widening the scope of the development, as well as for probing the possible mechanism of interaction of the bacteria with the sugar-linked polymer. A schematic view of the possible mechanism of biodegradation of sugarlinked PSMAH is shown in Fig. 7.2. A beginning has been made on which to build; much more work is needed to bring the weight losses up to at least 80%, identify the byproducts of degradation, and get a deeper understanding of the mechanism, so that tailor-made sugar-linked polymer systems can be synthesized for various applications and various degradation rates. It must be pointed out, as earlier done by Klein (1987), that chemical, structural, morphological and molecular weight characterizations of such polymers (before and after biodegradation) having very minute quantities of anchored sugar molecules (about 1% by weight) will entail difficulties due to problems of solubility, crosslinking (especially after biodegradation has set in), etc. Overlapping high resolution spectral studies is a simple way to see chemical changes occurring in the material; for example, in the FTIR spectra of poly(styrene maleic anhydride) grafted with sucrose and its biodegraded products, the biodegraded products showed significant reduction in the intensity of the bands at 3200 cm ÿ1 (sugarÐOH), 1780 cmÿ1 (anhydride carbonyl), 1716 cmÿ1 (ester carbonyl) and 1600 cmÿ1 (polystyrene phenyl ring). Thus, not

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7.2 Schematic view of synthesis of sugar-linked polystyrene and their attack by microorganisms (adapted from National Chemical Laboratory, Pune (India) website www.ncl-india.org).

only the sugar component but surprisingly, also the polystyrene component of the polymer can apparently be degraded by the microorganisms. This could be a general strategy that can be applied to all polyolefins, and ongoing detailed research in our laboratory in this area will be published in due course (Galgali et al., unpublished). The dramatic changes in the growth pattern of individual bacteria on the unmodified polymer as well as on the saccharide modified polymers can have wide-ranging ramifications in the design of sugar based biodegradable polymers, the biodegradation media, and the mechanism of biodegradation of sugar-laced polymers by these bacteria (Galgali et al., 2002, 2004). It may also be mentioned at this point that there have been several reports of chemically modifying polystyrenes with sugar molecules. Polystyrene derivatives with maltose, lactose and maltotriose substituents on each phenyl ring were synthesized by coupling the corresponding oligosaccharide lactones with p-vinylbenzyl amines followed by radical polymerization (Kobayashi and Sumitomo, 1983, 1985; Kobayashi et al., 1997, 1998). These polymers were water soluble, and are potential biomedical materials wherein the oligosaccharide moieties are used as recognition signals. However, there is no scope to develop these into bulk `commodity' plastics useful as packaging materials, in contrast to the use of polymer analogous reactions of sugars with PSMAH (previous paragraphs) wherein it has been shown that otherwise nonbiodegradable synthetic polymers can be incorporated with structural features (sugars molecules) which can induce biodegradability.

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Thus, chemically linking small amounts of carbohydrate molecules with synthetic polymers like polystyrene-maleic anhydride leads to new polymers that are (at least) partially degraded by bacterial as well as fungal cultures, more than could be expected from the very low sugar content of the polymer. The role of the carbohydrate molecule seems to be the key to the further biodegradation of the synthetic polymer. The differences in the physico-chemical properties and thermal stabilities of the different types of carbohydrate-linked polymers, as well as the weight losses in the different microbial cultures used will throw useful new light on the design of new biodegradable polymer systems. More work is needed to convert the modified synthetic polymer into an acceptable biodegradable polymer that can be processed for packaging applications, and work in this direction is proceeding in our laboratory.

7.3

Biodegradable polymers obtained from synthetic polysaccharides

In the previous section, we saw how small molecules of sugars, when anchored to polymer chains, can have dramatically different biodegradability properties, and have potential applications in commodity packaging materials. On the other hand, having sugar molecules pendant on each repeating unit of a synthetic polymer would alter the synthetic polymer to such a large extent that its original properties would be completely changed. The new polymers formed would be termed either as poly(vinylsaccharides) or as synthetic polysaccharides. In these cases the large amount of the sugar component on the polymer can make the new polymer molecule highly hydrophilic and biocompatible, with several niche applications (but not suited to packaging materials, where moisture resistance is a key property). Many of the niche applications will also be enumerated in this section. We will devote this section to an overview of the study of synthetic polysaccharides. Due to multi-step synthesis of such polymers, they are uneconomical for use in bulk applications, hence their use is generally limited to biomedical and other highly specialized low-volume high-value fields (Fraser and Grubbs, 1995; Kallin et al., 1989; Caneiro, et al., 2001; Kobayashi et al., 1985; Nishimura et al., 1990, 1991). However, with the renewed emphasis on research with carbohydrate materials, it is safe to predict new developments in applications. Structurally, the poly(vinylsaccharide)s have a synthetic carbon-carbon backbone with pendant carbohydrate molecules. Since sugars are a good nutrient source for micro-organisms, many poly(vinylsaccharide)s have the potential to be utilized as biodegradable polymers. The following are the general methods of preparing synthetic polysaccharides: (i) polymerization of vinyl sugars to give (polyvinylsaccharide)s, (ii) polymerization of anhydro-sugars to give (polyanhydrosugar)s, (iii) enzyme-mediated synthesis of carbohydrate

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polymers to give synthetic polysaccharaides, (iv) olefin metathesis reactions have also been employed for synthesis of poly(vinylsaccharide)s, and (v) grafting of sugars onto functionalized synthetic polymers by polymer analogous reactions (already discussed in the previous section). The more common methods of synthesis of poly(vinylsaccharide)s are either by homopolymerization of the vinyl sugars or by copolymerization of the vinyl sugar with other polymerizable vinyl monomers. Vinyl sugar synthesis can be carried out in any one of the following ways: incorporation of acrylic ester onto a sugar moiety and homopolymerizing or copolymerizing it with an acrylate using a radical catalyst (Patil et al., 1991a); converting the sugar into a sugar oxime and homopolymerizing it without protecting the hydroxyl groups (Zhou et al., 1997); condensation of an alkyl isocyanate with a sugar amine followed by its free radical polymerization to obtain a poly(vinylsaccharide) with a urea linkage (Zhou et al., 1999); oxidation of sugars to their corresponding lactones, which in turn are reacted with p-vinyl benzyl amine, followed by polymerization of the adducts by free radical polymerization (Kobayashi et al., 1985); and by converting sugars to their corresponding glycosyl amines, then to N-acryloyl derivatives, followed by radical polymerization (Kallin et al., 1989). The chemistry of anhydro sugar polymerizations dates back mainly to the mid-1960s. Anhydro-sugar synthesis and their polymerizations was pioneered by Schuerch, who established the most renowned school of research in this area. His group was the first to successfully synthesize a regular polysaccharide by anhydro-sugar polymerizations (Ruckel and Schuerch, 1966a,b). Many other important papers followed this work (Ruckel and Schuerch 1967; Zachoval and Schuerch, 1969; Uryu and Schuerch, 1971; Lin and Schuerch, 1972; Schuerch, 1981; Varma and Schuerch, 1981; Sharkey et al., 1981). Cationic polymerizations, initiated by carbonium ions, have been the most common methodology for the ring-opening polymerization of anhydro-sugars, since they lead to highly stereoregular polymers with high molecular weights. The complex monomer synthesis followed by the requirements of extreme purity of the monomers and solvents for effecting the polymerizations has precluded the use of such polymers for bulk applications. However, this method is very useful for obtaining polysaccharides with high stereoregularity, needed for biochemical studies (Uryu et al., 1981). Schuerch was also the pioneer for the synthesis of D-galactan (Uryu and Schuerch, 1971; Lin and Schuerch, 1972; Uryu et al., 1970), D-mannan (Lin and Schuerch, 1972; Frechet and Schuerch, 1969; Tkacz et al., 1972), and glucomannan synthesis (Kobayashi et al., 1977). The strategy of ring-opening polymerization of anhydro sugars was extended to the synthesis of glycoconjugates, wherein disaccharides were linked to various proteins (Eby and Schuerch, 1982). The anhydro sugars which could be synthesized and polymerized are 1,2-, 1,3-, 1,4-, and 1,6-anhydropyranoses and 1,2-, 1,3-, 1,5and 1,6-anhydrofuranoses. Most of such polymers have been investigated for

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biomedical applications. More recently arabinofuranan and xylofuranan were prepared from the corresponding 1,4-anhydro sugar polymerizations, and then sulfonated to various extents (Yoshida et al., 2000). The highly sulfonated derivatives (having degrees of sulfonation of 1.4±1.9) showed potent anti HIV activities and also exhibited higher blood anticoagulant activities. Another major and emerging method of producing highly stereoselective polysaccharides is via enzyme catalyzed reactions (Dordick et al., 1991; Kobayashi and Kamiya, 1996), wherein no protection of the hydroxyl groups of the sugar was required. Sucrose contains eight hydroxyl groups, all of which are capable of undergoing esterification reactions. However polycondensation of sucrose with diacids using enzyme catalysts yielded linear polymers, wherein only two hydroxyls of sucrose were functional (Patil et al., 1991b). The advantages associated with enzymatic reactions are that they can be carried out both in aqueous and non-aqueous media and, being highly selective, protection and deprotection of the sugars are avoided. There are also certain limitations involving enzyme-catalyzed reactions. Most known enzymes catalyze only selective reactions to produce specific sugar derivatives, therefore currently only a limited variety of vinyl sugar derivatives can be synthesized by this method. Examples of vinyl sugars that can be polymerized via enzyme mediation are sucrose1-acrylate (Patil et al., 1991a,b), methyl 6-acryloyl- -galactoside (Martin et al., 1992). The other limitations of enzyme catalyzed reactions are their slow reaction rates. This problem can be taken care of by using chemo-enzymatic methods of synthesis wherein the vinyl sugar is synthesized in a single step without protection of the sugar hydroxyls using enzymes and then polymerized by chemical means (Patil et al., 1991a; Nishimura et al., 1990; Chen et al., 1995). The chemo-enzymatic method capitalizes on both the enhanced regio-selectivity over chemical methods and on the speed of conventional chemical methods of polymerizations (Patil et al., 1991a). Sucrose acrylate was synthesized by enzymatic catalysis using an enzyme proleather (a protease from Bacillus Sp.) (Patil et al., 1991a). The sucrose acrylate was polymerized using potassium persulfate/hydrogen peroxide to obtain poly(sucrose acrylate). Tokiwa et al. (2000) reported esterification of glucose with adipic acid enzymatically and later on effected its polymerization by conventional methods to obtain biodegradable polymers. Similarly, -Dgalactose was acryloylated with vinyl acrylate enzymatically and later polymerized chemically. Martin et al. (1992) reported synthesis of a variety of monosaccharides with vinyl acrylate in pyridine to obtain 6-acryloyl esters and later on polymerized them in DMF solvent with AIBN as the initiator to give poly(acrylate) products. Enzymatic reactions have been used to synthesize linear polymers with sugar as part of the main chain (Patil et al., 1991b). More recently, several new lipase grafting reactions on polysaccharides have been carried out. Polysaccharides are

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generally modified by hydrophobic groups by chemical means. Enzymes, especially lipases were used to carry out this hydrophobic modification. Thus, hydroxyethyl cellulose was reacted with vinyl stearates using lipase-catalyzed transesterification reaction under much milder reaction conditions than the chemical reaction (Gu, 2003). This reaction was also successfully applied to an industrial thickening product, cationic guar (galactomannan substituted with 2hydroxypropyltrimethylammonium chloride) for reactions of vinyl stearate and vinyl acrylate. In a different type of application, polyvinylsaccharides have been used for stabilizing enzymes. Thus, horseradish peroxidase was stabilized by the addition of polyvinylsaccharides (Kuhlmeyer and Klein, 2003). It is clear from such a variety of studies on enzyme systems that enzyme catalyzed reactions will be utilized to an increasung degree in modifying polysaccharides. Olefin metathesis reaction, although not a general method of synthesis of poly(vinylsaccharide)s, has also been employed in some cases to obtain poly(vinylsaccharide)s (Mortell et al., 1996; Fraser and Grubbs, 1995).

7.3.1 Applications of synthetic polysaccharides As mentioned earlier, most of the applications of synthetic polysaccharides are in the field of biomedical applications. Some of the most recent developments are presented here. Synthetic polysaccharides having the same immunomodulating effect as some bacterial polysaccharides have been reported (Kournikakis et al., 2002). The synthetic polysaccharides were found to enhance the general or cell-mediated immunity of animals to various diseases. Synthetic dextran derivatives substituted with carboxymethyl, benzylamide, sulfonate, and sulfate groups in a random manner exhibit heparin-like properties such as anti-coagulant properties, have stimulatory effect on endothelial cells, possess antiproliferative capacity on smooth muscle cells of rats, etc., making synthetic polysaccharides of great interest for vascular therapy (Chaubet et al., 1999). A symposium on the synthesis of octadecylated amphiphilic, fluorinated, comb shaped branched, and graft copolymer polysaccharides was prepared using phosphorylase, and were seen to be useful tools to elucidate structure-function parameters in addition to exhibiting novel bio-functional properties (Kobayashi et al., 1985; 1994a; 1994b). Schuerch published an excellent review on use of synthetic polysaccharides having stereoregular structures as model compounds in basic research studies in the areas of allergy, immunity, the actions of enzymes and lectins, and for structure proof of naturally occurring polysaccharides of biomedical significance (Schuerch, 1992). Their use as cell surface mimics has also been noted (Fraser and Grubbs, 1995) while their use in lecithin or antibody-binding assays, wherein the recognition of pendant sugars on the liposomal surfaces by lecithin or enzyme was studied (Kitano and Ohno, 1994; Kitano et al., 1995). Copolymers of N-acryloyl-4-O-( -D-galactopyranosyl)- -D-glucopyranosyl-

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amine and acrylamide exhibited strong specific binding to antibodies against antigens in ELISA assays so they can be used as substitutes for glycolipid and glycoprotein antigens in immunological assays (Kallin et al., 1989). Artificial antigens were synthesized based upon a polyacrylamide copolymer containing 3-deoxy-D-manno-2-octulopyranosylonic acid residues (Kosma and Gass, 1987). Chromatographic supports for affinity chromatography and for the isolation of proteins with specificity for different sugar residues is an important application from the industrial point of view (Roy and Tropper, 1988; Kobayashi et al., 1985). Similarly, chiral templates derived from sugars could be useful for asymmetric synthesis and optical resolution of organic molecules (Kobayashi and Sumitomo, 1980). Molecularly imprinted polymers prepared from sugar acrylates have been used as chiral stationary phases for the resolution of the Dand L-isomers of CBz- Asp in polar organic eluents (Liu and Dordick, 1999). The hydrophilicity of sugar was applied to the design of a reverse osmosis membrane and a selectively permeable membrane (Kobayashi and Sumitomo, 1980). The industrial applications of such developments can be very significant. Other potential industrial applications are in the areas of synthetic rubbers, modification of the surfaces of hydrocarbon polymers, and so on. Silcone rubbers (polysiloxanes) possess highly hydrophobic surfaces, which is a drawback for biomedical applications such as surgical implants of contact lenses. Polysiloxanes containing glucose, sucrose and other carbohydrate derivatives have been reported to give better wettability and biocompability (Mossl et al., 1993; Gerd and Stadler, 1991; Volker and Stadler, 1998). Poly(pvinyl phenol) with grafted -bromo-3,4,6-tri-O-acetyl -D-glucosamine bactericide was useful in the treatment of steel (Keisuke et al., 1985a,b). They have potential as polyelectroytes in absence of salt (Emmerling and Pfannemueller, 1983). Polymers of unsaturated sugars and their copolymerization with comonomers like unsaturated carboxylic acids, esters, acrylic compounds, vinyl heterocycles, styrenes or maleic acid compounds are useful as thickeners which are also biocompatible (Buchholz et al., 1995). Also of industrial significance as biodegradable materials are the copolymers of glucose or sucrose with acrylic acid, sodium methallyl sulfonate, sodium 2-methacryloyl oxyethyl sulfate or vinyl phosphonic acid and are useful as sequestering agents for Ca, Fe and other ions, as additives in textile desizing, bleaching, dyeing or printing, as dispersing agents for pigments in paper coating compositions and as additives in leather manufacture for improving chrome tanning, softness, brightness, etc. (Krause and Klimmek, 1984). Complexation of heavy metals by sucrose containing gels is also quite promising (Alvarez et al., 1991). They are produced by crosslinking sucrose diacids with diepoxide crosslinking agents (Faulkner, 1977) or by transesterification processes (Carraher et al., 1981) or by reaction with organo-

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stannane dihalides. As seen above, there are a number of potential applications for synthetic polysaccharides, not only in the field of biomedical science, but also in industrial applications. There is no doubt that as processing techniques develop, the use of synthetic polysaccharides will also increase, and the property of biodegradability and biocompatibilty will play a crucial role in their acceptance.

7.4

Biodegradable polymers obtained from natural polysaccharides

The use of natural polysaccharides like starch and cellulose as well as the natural lignocellulose polymers (plant fibers like jute as well as forest residues and agricultural wastes) are becoming ever important, and are likely to capture more and newer markets in the future. One of the reasons is the perception of these materials to be environmentally friendly, and meeting the cradle-to-life life cycle analysis criterion. While there is scepticism and growing public resentment about the use of long-lasting petroleum polymers for short-lived packaging and disposable products applications, the use of natural biomass polymers has received a very favorable public response. Indeed, as shown in Fig. 7.3, only renewable resource based materials have a `closed carbon producing cradle-to-grave cycle', whereas the petroleum-based materials can have no such closed loop.

7.3 Completion of the carbon cycle for renewable resource based materials, but not for petroleum based polymers (adapted from E. Chiellini, Proceedings of UNIDO-ICS International Workshop on Environmentally Degradable Polymers, Nov. 10±15, (1997), p. 32).

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Many lignocellulose fibers are known for their tenacity, and can be used as reinforcing material for composites with polymers, similar to glass-reinforced plastics. Surface treatment of the cellulosic fiber, and adequate processing techniques, can result in novel composites which can often substitute glass fiber composites (Mohanty et al., 2001). If the polymer in question is also a naturederived polymer such as cellulose acetate, the composite can truly be called a `biocomposite' or a `green biodegradable composite' (Mohanty et al., 2001). Similarly, composites of biodegradable l-polylactides with jute fibers are an improvement over the non-reinforced l-polylactide, with no signs of fiber pullout (Plackett et al., 2003). Important reviews on biocomposites explain the various parameters, opportunities, and the challenges (Kandachar and Brouwer, 2002; Mohanty et al., 2001; Mueller, 2001; Mohanty et al., 2002). Three-dimensional moldings from plant fibers (up to 80%) and polyolefins have been developed for applications as containers, shock absorbers, and heat insulators (Takasaki and Naito, 2002). Instead of the plant fiber, lignins have also been used as fillers with biodegradable polymers like poly(l-lactic acid), and the resulting blend was found to be promising due to the good material properties and economics (Li et al., 2003). A very novel study that could be of importance for the future is that of functionalized cellulose nanofibers and nanocrystals blended with biodegradable polyesters and acrylic acid polymers (Winter and Bhattacharya, 2003). The nanocrystals were found to be markedly superior reinforcing agents than wood flour, and their behavior was similar to the exfoliated clays in terms of reinforcing properties (Winter and Bhattacharya, 2003). These continued new developments bode well for the future role of plant fiber based composite materials. Starch-based plastics are another area of biodegradable plastics that has the potential for large-scale deployment. The developments during the period up to the mid-1990s have been well covered by Narayan, who has contributed significantly to starch based biodegradable materials (Narayan, 1997; Bloembergen and Narayan, 1995). The pioneering starch based commercial biodegradable plastics products and technologies from Novamont in Italy are only too well known. This section reviews developments in the last three years. Most starch derivatives available commercially have a low degree of substitution (DS). A group from Germany has recently synthesized high DS (0.5±2.9) starches, incorporating carboxymethylation, sulfonylation, acylation, introducing cationic groups, etc. under both homogeneous and heterogeneous conditions, and this wide variety of properties generated thereby are under evaluation (Heinze, 2003). Starch esters with varying chain lengths and DS are being evaluated by a group in Britain (Fang et al., 2004). Nanoengineered blends of thermoplastic starch and malleated polyethylene have been synthesized, containing organically layered silicates to improve the barrier and mechanical properties (Ganguly and Dean, 2003). Native and modified industrial starches, and the advantages of specific industrial starches focusing on application in biodegradable packaging, have

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been reviewed (Nobes et al., 2001). Biodegradable mulch films based on starches is another large-volume application that is being vigorously explored. Modified starches blended with aliphatic polyesters have produced highperformance biodegradable products. Aspects of material formulation, film blowing, processing and scale-up are brought out, and insights into future developments in these materials are given (Halley et al., 2001). A recent patent describes synthesizing highly attenuated fibers containing thermoplastic polymer microfibrils formed within the starch matrix, which can be used for making non-woven webs and disposable articles (Bond et al., 2003a). Use of a blend that is cospun to give a biocomponent fiber minimizes the thermal degradation of the starch that occurs when the starch is heated above 180 ëC (Bond et al., 2003b). Mulch films based on basic starch blended with other biodegradable polymers and plant pots produced from such blends by injection molding have been investigated in Australia (Salt, 2002). With innovative process technologies, appropriate use of nanomaterials and nanotechnology, and a vast body of experimental data available, the time for a larger role for plant fibers and starch in the development of biodegradable polymer composites and packaging materials has now arrived.

7.5

Future developments ± `biodegradable' polymers obtained from hemicelluloses

Plant biomass consists of three main polymeric components; cellulose, hemicellulose and lignin. These three polymers constitute an intricate natural composite material. In softwoods, hardwoods, and the abundantly available agricultural residues such as wheat, rice and other cereal straws, sugarcane bagasse, corn stalks, corncobs, jute and cotton stalks, cellulose is the chief constituent (over 40% by weight) followed by hemicellulose (~30%) and lignin (~20%). Table 7.2 shows the constituents of some plant biomass. Thus, while cellulose is nature's most abundant polymer, hemicellulose and lignin are the second and third most abundant polymeric substances produced by nature. While cellulose is a well established commercial polymer (Kennedy et al., 1985; Hon and Shiraishi, 1991; Hon, 1996), and lignin is also well researched and used commercially though on a much smaller scale (Sarkanen and Ludwig, 1971; Glasser and Sarkanen, 1989; Lora and Glasser, 2002), hemicellulose, as yet, has had few takers. With such a large fraction of plant biomass (lignin and hemicellulose) finding its way into waste streams, particularly during paper pulping operations, and with `green technologies' and `green materials' being the order of the day, these two polymers (hemicellulose and lignin) are being looked at seriously as wasted resources that should be utilized and valorized. Hemicellulose, in particular, is industrially the least isolated and utilized component of plant material. Hemicelluloses from different types of plants have complex and widely different structures. There is significant disagreement and

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Table 7.2 Cellulose, hemicellulose, and lignin contents of some major plant biomass Source Hardwood*,y Softwood*,y Indian sugar cane bagassez Wheat straw*,# Corn cobs* Corn stalks*

Cellulose

Hemicellulose

Lignin

43±47 40±44 45 30 45 35

25±35 22±59 30 35±38 35 25

16±24 25±29 19±21 15 15 35

* Chemical Modification of Lignocellulosic Materials, D N S Hon, ed. Marcel Dekker, New York (1996), p. 3. y T E Timell, Adv Carbohydr Chem 19, 247±302 (1964). z Results from our laboratory (2003). # R C Sun, and J Tomkinson, Carbohydr Polym 50(3), 263±271 (2002).

uncertainty on the degree of polymerization (DP), chemical structure, bonding with lignin, and so on for hemicelluloses obtained from various sources. These aspects are beyond the scope of this chapter and will not be dealt with and readers are directed to other reviews on the subject (Kirk Othmer's Encyclopedia of Chemical Technology, John Wiley and Sons, 1995). The classical applications of hemicelluloses are shown in Fig. 7.4, which shows its use in specialty chemicals, solvents as well as biodegradable polymers (the latter being a recent addition). This section seeks to highlight the applications

7.4 Some chemicals obtained from hemicellulose (xylan).

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of isolated hemicelluloses, regardless of their source. The application can be as a biodegradable polymer, either by itself, or by chemical modification, or by complete hydrolysis followed by fermentation to monomers (like lactic acid) which can then be polymerized. Several fermentation processes have been developed to synthesize lactic acid (Rajgarhia, et al., 2003; Minami and Kozaki, 2003; Hause et al., 2003; Yun et al., 2003). Polylactic acids are well known biodegradable polymers, and are increasingly being viewed as viable alternatives to some currently used commodity plastics for various applications (Oome et al., 2003). It must be stated that the general assumption is that natural polymers (such as hemicellulose and lignin) are environmentally friendly materials, and are inherently biodegradable (Kaplan, 1998). Moreover, the very fact that they are being diverted from pollution streams to industrial applications makes the technology of utilizing these materials into a `green technology'. In the past, hemicelluloses were degraded into complex byproducts during the extraction of lignin by alkali in the pulping process, and were rarely recovered (Rydholm, 1965). In recent years, the use of techniques such as the `steam explosion process' (Shimizu et al., 1998; Glasser and Wright, 1998; Varma, 2004) and hydrothermal techniques (Sasaki et al., 2003; Varma, 2004) makes the recovery of hemicellulose facile. These non-chemical using `green technologies' appear to be the technologies to look out for in the coming years, as they are not only environmentally friendly but also economically viable. This makes possible the availability of vast quantities of hemicelluloses in the near future as potential raw material for industrial exploitation. Even though the hemicellulose extracted from softwood is quite heterogeneous in nature, that obtained from hardwoods (as well as from sugarcane bagasse) is more homogeneous consisting mainly of polymers of xylose. Indeed, it is a goal of this author's laboratory to develop technologies to obtain hemicellulose from sugarcane bagasse, available abundantly in India, in a pure and polymeric/oligomeric/monomeric form for further applications development. Laboratory scale work has been largely successful. Similarly wheat straw, another annually replenishable biomass resource available in many countries, contains about 35±38% hemicellulose (Sun and Tomkinson, 2002). Extraction of this hemicellulose, containing about 70% xylose, 13% arabinose and 14% gluclose by an ultrasound irradiation technique, improved the molecular weight as well as the thermal stability of the hemicellulose, and is a promising technique if used in conjunction with other techniques such as `steam explosion'. This will enable the extraction of hemicellulose in such a form that the chemical structure will be as little changed as possible from the native state. Publications pertaining to applications of hemicellulose, such as the xylans, have recently started to appear in the open literature. After starch production from corn, the corn hulls can be used to produce a highly branched heteroxylan, which has applications as a new food gum (Hromadkova and Ebringerova, 1995; Saulnier et al., 1998). The possibility of using xylan (in levels of 0±40% wt/wt)

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for the production of biodegradable composite films in combination with wheat gluten has also been investigated (Kayserilioglu et al., 2003). While studying the mechanical properties, solubility, and water vapor transmission rate, it was found that the presence of xylan did not adversely affect the film forming quality or the water vapor transmission rate, though the mechanical and solubility properties depended on the quantity of xylan in the wheat gluten. New biodegradable hydrogels based on using oligomeric hemicellulose from spruce chips as a component have been reported (Lindblad et al., 2001). In this study, hemicellulose was chemically modified with 2-(1-imidazolyl) formyloxyl) ethyl methacrylate and then polymerized with 2-hydroxyethyl methacrylate using a redox initiator. The resulting hydrogels were transparent, homogeneous and elastic materials. Biodegradable sorbents based on seed meal (containing hemicellulose) and other carbohydrate materials to produce environmentally friendly materials useful for absorbing spilled oil or hazardous chemicals, for releasing insecticides or larvicides into an environment, or alternatively useful as animal feed, have recently been patented (Barresi et al., 2002). In another significant study by Gabrielii et al., the investigators were able to extract polymeric hemicellulose of high molar mass (Gabriellii et al., 2000) in order to conduct a systematic study of hydrogels with chitosan admixtures. Thus, Aspen hemicellulose (xylan) having a weight average molar mass of 73,100 g/mol and a number average molar mass of 48,000 (corresponding to a polydispersity of 1.5), extracted by an alkali extraction process, and mixed with chitosan (10% and above) yielded continuous films. Films with a chitosan content less than 20% chitosan swelled in water and formed hydrogels; the presence of more than 20% led to gradual dissolution of the hydrogel film. The authors hypothesized that xylan and chitosan were able to cocrystallize. The crystalline arrangement of the polymers as well as the expected electrostatic interaction between the acidic groups of the xylan and amine groups of the chitosan (Gabrielii and Gatenholm, 1998) impart the necessary cohesive force to the hydrogels. These materials can be investigated for biodegradable hydrogel applications. Other applications of hemicellulose are in the area of agriculture, food and plant growth promotion (Ishihara, 2001). Applications in the role of drugs have also been developed (Ebringova and Heinze, 2000), but these developments are outside the scope of this chapter. Suffice to say, hemicelluloses are finally gaining recognition as important raw materials for a variety of diverse applications, including biodegradable polymeric materials, and their importance is expected to grow rapidly as more facile methods are developed to isolate these materials in substantially unaltered or undegraded form. The older applications in furfural solvent synthesis on a large scale, or the highly value-added xylitol are other applications that make the use of hemicellulose very attractive. Separation of other rare sugar components such as arabinose will further lead to the importance of hemicellulose as one of the most important raw materials of the future.

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7.6

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MD, USA, Sept 5±7, 652±669. Nakagawa T, Higushi A, Nin S, Taniguchi W, Hara T, Nakajima Y, (1994), Jpn. Kokai Tokkyo Koho JP 06145074. Narayan R, (1997), in Paradigm for successful utilization of renewable resources, Sessa D J and Willett J L (eds), Chap. 6, AOCS Press, Champaign, Illinois, USA. Presented at the 88th AOCS Annual Meeting & Expo, May, 1997. Narumi A, Kaga H, Satoh T, Kakuchi K, (2003), Polym Preprints (ACS), 44(2), 869±870. Nishimura SI-, Matsuoka K, Kurita K, (1990), Macromolecules, 23, 4182. Nishimura SI-, Matsuoka K, Furuike T, Ishii S, Kurita K, Nishimura K M, (1991), Macromolecules, 24, 4236±4241. Nobes G A R, Orts W J, Glen G M, Gray G M, Harper M V, (2001), 222nd ACS National Meeting, Chicago, IL, USA, Aug 26±30. Onishi K, Hisanaka, T, (29 July 2003), JP 2003210520 A2. Oome H, Kumasawa S, Kumaki J, (2003), JP 2003342459 A2. Pacitti S, (2003), Plastics in Packaging, 24, 14±16. Patil D R, Dordick J S, Rethwisch D G, (1991a), Macromolecules, 24, 2462±2463. Patil D.R., Rethwisch D.G., Dordick J.S., (1991b), Biotechnol. Bioeng., 1991, 37, 639. Plackett D, Logsturp Anderson T, Batsberg-Pedersen W, Nielsen L (2003), Composites Science & Technology 63(9), 1287±1296. Rajgarhia V, Casleson S, Ulson P, Suominen H, Pirkko B, Hause B, (2003), PCT Int Appl NO 2003102201 A2. Rios P, Bertorello H, (1997), J. Appl. Poly. Sci., 64, 1195±1201. Rydholm S A, (1965), Pulping processes, John Wiley and Sons, NY. Roy R, Tropper F D, (1988), J. Chem. Soc., Chem Commun., 1058. Ruckel E R, Schuerch C, (1966a), J. Org. Chem., 31, 2233. Ruckel E R, Schuerch C, (1966b), J. Am. Chem. Soc., 88, 2605. Ruckel E R, Schuerch C, (1997), Biopolymers, 5, 515. Salt D, (2002), Chemistry in Australia 69(5), 15±17. Sarkanen K V, Ludwig C H (eds), (1971), Lignins: Occurrence, formation, structure and reactions, John Wiley & Sons, NY. Sasaki M, Adschiri T, Ariai K, (2003), Bioresources Technol, 86(3), 301±304. Saulnier L, Chanliaud E, Thibault, J F, Despre, D, Messager A, (1998), Carbohydr as Org Raw Mat, IV, Wein, and WUV, 132±138. Schuerch C, (1981), Adv. Carbohydr. Chem. Biochem., 39, 157. Schuerch C, Fornes R, Gilbert R D, eds, (1992), in Polym. Fiber Sci.: Recent Adv, VCH, NY, 9±16. Scott G, Gilead D. eds, (1995), Degradable Polymers: Principles and Applications. Chapman & Hall: London, 416 pp. Scott G, Wiles D M (2002a), Degradable Polymers, pp. 1±15, Kluwer Acad., Dordrecht. Scott G, Wiles D M (2002b), Degradable Polymers, pp. 449±479, Kluwer Acad., Dordrecht. Shah S C, Smid J, (1978), J Amer Chem Soc, 100, 1426. Shimizu K, Sudo K, Uno H, Ishihara M, Fujii T, Hishiyama S, (1998), Biomass and Bioenergy, 14(3), 195±203. Shogren R L, (1998), in Biopolymers from renewable resources, Kaplan, D L, ed., Springer, Berlin Chap 2, 30±45. Sharkey P F, Eby R, Schuerch C, (1981), Carbohydr. Res., 96(2), 223±9. Smid J, Shah S, Sinta R, Varma A J, Wong L (1979a), Pure & Appl. Chem., 51, 111±122.

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8

Biodegradable polymer composites from natural fibres D P L A C K E T T , Risù National Laboratory, Denmark

8.1

Introduction

Composite materials consisting of a polymer matrix reinforced with fibres are becoming increasingly important for structural applications where a combination of high strength and stiffness, durability and relatively low weight are key requirements (Starr, 1999). Composites are now widely used in materials for construction, aerospace and defence, marine and offshore, transport and specialty products such as wind turbine blades. The polymer matrices vary according to the particular product but include unsaturated polyesters, epoxy resins, polyurethanes and thermoplastics like polypropylene. Composite reinforcements are typically such high strength fibres as glass, aramid and carbon. In practice, most of the polymers and fibres used in current commercial composites are derived from non-renewable petroleum resources; however, natural wood- or plant fibre-reinforced composites have been developing rapidly and are now used successfully as building materials and automotive interior components (Evans et al., 2002). In response to the accelerating use of non-renewable resources and products derived from these resources, many countries have sought to stimulate `green chemistry' and the production and use of `green products' derived from nature. This has led to a search for new products that are environmentally sustainable and compatible with the environment. Composite materials are no exception to this new paradigm and there is therefore considerable interest by manufacturers in developing new `green' composites. This chapter covers research and development in the field of biodegradable polymer composites in which natural fibres are used as reinforcement. This type of composite material is still in its infancy with most of the related process or product research initiated since the late 1980s and to date there are only a few fully commercial applications of the technology. Some of the earliest of the recent research on `green' plant fibre biocomposites has come from the Institute of Structural Mechanics at DLR (Deutsches Zentrum fuÈr Luft- und Raumfahrt) in Braunschweig, Germany, where a programme on biocomposites was started

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in 1989 (Hermann et al., 1998; Riedel and Nickel, 1999; Nickel and Riedel, 2001). In general, research on biodegradable composites has been largely driven by the availability of the raw materials from sustainable natural resources. Although varying from country to country, the increased use of composites in general raises questions about their eventual disposal in situations where incineration is not possible. Options exist for recycling and re-use of composites although these are not always easy to implement given that composites typically consist of two quite dissimilar materials. As an alternative solution, the use of biodegradable polymers in combination with biofibres (wood or plant fibre) potentially provides a route to fully biodegradable composites based on renewable resources and some recent review articles have highlighted this potential (Mohanty et al., 2000b, 2002).

8.2

Natural fibres as polymer reinforcement

Wood or plant fibres are of interest in polymer reinforcement for a number of reasons, especially their low cost, low weight and non-abrasiveness to processing equipment. In addition, natural fibres are CO2-neutral when burned, have attractive acoustic and thermal insulation properties and have good specific mechanical properties. The research literature contains many examples of studies in which the reinforcement of non-degradable thermoplastics like polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC) and polyesters with wood or plant fibres has been investigated (Bledzki and Gassan, 1999; Kandachar, 2002). Reinforcement of thermosets such as epoxies and polyurethanes with natural fibres has also been explored. A summary of the mechanical properties of selected plant fibres derived from various literature sources is shown in Table 8.1 (Lilholt and Lawther, 2000; Kandachar, 2002; Bledzki and Gassan, 1999; Wambua et al., 2003). Table 8.1 Density and mechanical properties of fibres as summarised from the literature Fibre type E-glass Hemp Flax Jute Sisal Coir Ramie

Density (%)

Tensile strength (MPa)

Young's modulus (GPa)

Elongation (%)

2.5 1.5 1.5 1.45 1.33 1.25 1.5

2000±3500 550±900 345±1500 200±800 100±850 130±220 400±938

70 30±70 28±80 10±55 9±38 4±6 44±128

2.5±3.0 1.6 1.2±3.2 1.2±1.8 2.0±3.7 15±40 1.2±3.8

Sources: Lilholt and Lawther (2000), Kandachar (2002), Bledzki and Gassan (1999) Wambua et al. (2003)

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From a commercial viewpoint, one of the most significant developments in the field of natural fibre composites has been in wood-plastics, typically based on either PP or PE. These products are used extensively in outdoor decking in North America and continue to show rapid market growth. Other uses for wood-plastics include railway ties, house cladding, window and door frames and mouldings. Fibre contents are typically in the 30±70% range. In the USA a large percentage of wood-plastic decking is based on recycled PE as the matrix. Processing technology is essentially similar to existing extrusion or injectionmoulding processes with some adjustments to accommodate the use of low bulk density fibres and process additives. Another well-developed application in the automobile sector involves the use of moulded fibre composite panels in which wood or plant fibres are combined with thermoplastics or thermoset resins. Virtually all vehicle manufacturers are now using these composites for interior components such as door panels, headliners, parcel racks and spare tyre covers.

8.3

Natural fibre-polyhydroxyalkanoate (PHA) composites

Polyhydroxyalkanoates (PHAs) are biopolymers that are obtained directly from certain bacteria in which they are produced as energy reserves. Of this polymer type, most attention has been paid to poly(3-hydroxybutyrate) or PHB (Fig. 8.1) which was first isolated and characterised by Lemoigne at the Pasteur Institute (Lemoigne, 1926). In the intervening years, PHB and other PHAs generated by bacteria have been widely studied and it is now known that bacteria can produce a large variety of polymers and copolymers of this type (SteinbuÈchel, 1995). One disadvantage of PHB is that it is a rather brittle polymer; however, the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) or PHBV is more flexible and has also attracted commercial interest (Asrar and Gruys, 2002). As a result of past research on PHAs and the commercial development of PHB and PHBV, it is perhaps not surprising that these polymers have also been examined for use in biodegradable composites. Examples of reinforcing fibres used with PHAs have included wood (Reinsch and Kelley, 1997; Peterson et al., 2002), wheat straw (Avella et al., 2000a, 2000b), pineapple leaf fibres (Luo and Netravali, 1999) and jute (Khan et al., 1999) amongst others.

8.1 Polyhydroxybutyrate (PHB).

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8.3.1 Mechanical properties of natural fibre-PHA composites There have been a number of studies in which the investigation of natural fibrePHA composite mechanical properties has been a key part. This section provides a review of key findings from these investigations. Pineapple leaf fibre-PHBV composites were prepared using a film-stacking technique in which layers of fibres were sandwiched between layers of PHBV films and then subjected to hot pressing (Luo and Netravali, 1999). Tensile and flexural tests were performed on specimens cut from the resulting composite panels. At 30% w/w fibre content the maximum tensile stress and maximum flexural stress were about 56 MPa and 86 MPa respectively. These values represent increases of 100% in tensile strength and 60% in flexural strength relative to unreinforced PHBV. The strain to failure in tension and the flexural strain at yield both decreased. When the fibre content was 20% w/w the corresponding increases in tensile and flexural strengths were 32% and 75% respectively. The authors used three fibre layers arranged in 0ë/90ë/0ë directions with 25% of the fibre weight in the top and bottom layers and the remaining 50% fibre weight in the middle layer. Microscopic examination of tensile fracture surfaces of 30% w/w pineapple fibre-PHBV composites showed that although some fibres broke at the fracture surface, a number of other fibres were pulled out of the polymer matrix with no polymer adhering to the fibres. This observation suggests weak fibre/matrix bonding and indicates that surface treatment to enhance the interfacial shear strength could improve the composite strength properties. The advantage of surface modifying plant fibres in order to improve the properties of biocomposites has been reviewed (Mohanty et al., 2001). The idea of treating jute fibres with different additives to enhance the properties of hot-pressed BiopolTM composites was examined by Khan et al. (1999). As in a number of other research studies, BiopolTM, a PHBV originally developed by ICI but now owned by Metabolix Inc of the US, was the chosen polymer matrix. Jute was used in the form of a fabric combined with sheets of polymer film. Fibre treatments included 2-ethyl hexyl acrylate (3%), methacryloxypropyltrimethoxysilane (2%) and trimethoxyvinylsilane (0.5%). These treatments were effective in improving composite mechanical properties by up to 80% over and above those properties obtained when using untreated fibre. Mohanty et al. (2000a) used various chemically modified jute yarns to prepare composites based on BiopolTM and found that tensile strength, bending strength, impact strength and bending modulus could be increased by up to 194, 79, 166 and 162% respectively over and above the corresponding property values for pure BiopolTM. When 10% acrylonitrile was grafted on jute yarn the tensile strength was enhanced by 102% whereas with 25% acrylonitrile grafted on the yarn the corresponding strength increase was 84%. Alkali treatment of fibres was shown to give better composite mechanical properties than yarns that had been chemically grafted or dewaxed.

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193

8.2 Schematic of the process for making natural fibre (e.g., jute)-PLA composites using a film-stacking method (source: Plackett et al., 2003).

Plackett and Andersen (2002) prepared jute-PHB composites by a filmstacking procedure involving layers of non-woven jute mat sandwiched between PHB films. The technique was based on the use of a heating stage in which the lay-up was subjected to vacuum while heating (Andersen, 1997). The potential advantages of this approach are: (i) avoidance of the need to dry the fibre mats, (ii) reduced porosity (i.e., void spaces) in the composite, (iii) enhanced consolidation of the lay-up, and (iv) reduced likelihood of thermooxidative or hydrolytic degradation of fibre or polymer. A schematic of the process is shown in Fig. 8.2. Composites containing 40% w/w fibre were prepared using temperatures in the range 190±220 ëC. The composite tensile strength was about 70 MPa, which compares with a value of ~20 MPa for the unreinforced polymer and the tensile modulus showed a corresponding increase from roughly 2 GPa to about 9 GPa. A summary of the tensile test results is shown in Fig. 8.3 indicating also the expected decrease in elongation and the reduction in strength occurring at the highest process temperature, possibly resulting from thermal degradation of the PHB matrix. Researchers have reinforced PHB with steam-exploded wheat straw and hemp fibres (Avella et al., 2000a, 2000b). Steam-exploded fibres were combined with PHB granulate in a Brabender mixer operated at 180 ëC for five minutes. The authors discussed the critical strain release rate and the critical stress intensity calculated from linear elastic fracture mechanics (LEFM) and concluded that the steam-exploded fibres played an important role in reinforcement of PHB because of good adhesion between fibre and the polymer matrix. The steam explosion process is believed to enhance the availability of hydroxyl groups on the fibre surface, which results in increased hydrogen bonding between fibre and matrix. The authors suggested that fibre-PHB composites could find applications in agricultural mulching and transplantation where product biodegradability would be advantageous.

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8.3 Mechanical properties of PHB and jute-PHB composites processed at various temperatures (source: Plackett and Andersen, 2002).

The mechanical properties and biodegradability of wood fibre-PHB composites based on BiopolTM have been investigated (Peterson et al., 2002). Prepregs (mat, fabric, non-woven material or roving pre-impregnated with resin and ready for molding) were prepared by interleaving dried, non-woven wood fibre mats with a layer of BiopolTM granules and consolidating under heat and pressure. Fibre mass fractions of 0, 15, 20 and 25% were used. Statistical analysis of composite tensile results showed which conditions of pressure, temperature, heating time and time of pressing gave the best strength and modulus results. Process temperature had the greatest influence on composite tensile strength with values varying from 23 MPa at 210 ëC to 17 MPa at 240 ëC. The tensile modulus varied from 2.9 GPa to 2.15 GPa over the same temperature range. Gatenholm et al. (1992) used dissolving cellulose pulp fibres to reinforce a PHB homopolymer. The samples were brittle but impact strength and elongation at break were improved when copolymers containing hydroxyvalerate were used as the matrix. Wollerdorfer and Bader (1998) processed a number of different plant fibre and biopolymer combinations using extrusion compounding followed by injection moulding. The extruder was fed manually because of the low bulk density of the fibres. Tensile testing of composites consisting of BiopolTM with either 25% cellulose fibre, 25% jute or 25% jute pre-treated with a surfacemodifying resin gave strength values in the range from 30 to 35 MPa, indicating that no effective reinforcement had been found. One proposed explanation was that a drastic shortening of the fibres had occurred during processing; however, in contrast, very significant gains in mechanical properties, attributed to better fibre-matrix interfacial interaction, were found in fibre-thermoplastic starch combinations. Flax-PHB composites have been prepared by mixing flax fibres with a chloroform solution of PHB (Shanks et al., 2004). The solvent was allowed to

Table 8.2 Mechanical properties of natural fibre-PHA composites Fibre type and content

Polymer

Manufacturing method

Jute (25%)

BiopolTM D 300G PHBV (Zeneca) PHB, (Biomer) BiopolTM D400GN

Pineapple (30%) Jute (40%) Wood (18%)

Note: N.S. = not specified

Tensile strength (MPa)

Tensile modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

Extrusion/hot-pressing

33.6

N.S.

N.S.

N.S.

Film stacking/ hot pressing Film stacking/ heating under vacuum Hot pressing mats and granulate

55.8

2.25

86.0

2.45

68

8.5

N.S.

N.S.

23

2.9

N.S.

N.S.

References

Wollerdorfer and Bader (1998) Luo and Netravali (1999) Plackett and Andersen (2002) Peterson et al. (2002)

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evaporate and the resulting mat was then cut into small pieces, which were hotpressed to obtain a composite sheet. The authors concluded that flax-PHB or flax-PHBV composites had mechanical properties that were comparable to or better than those of commercial thermoplastic polymers. As an alternative to using a PHBV copolymer, Wong et al. (2002) investigated the use of plasticisers in flax-PHB composites. A weighed amount of flax fibres was added to neat plasticiser followed by heating to 120 ëC for eight hours. After heating, excess plasticiser was removed by washing the fibres with acetone and the residual solvent was removed under vacuum. Flax-PHB composites with a 1:1 fibre/ polymer volume ratio were then made by dissolving PHB in chloroform and mixing plasticiser-modified fibres into the solution. The solvent was allowed to evaporate and the resulting mat was chopped into pieces before hot-pressing to obtain composite panels. Sample bars were annealed at 70 ëC for three hours to allow PHB to crystallise and interact with the fibres. As a control material, PHB containing 4% v/v plasticiser without added flax was prepared and subjected to the same treatment as the PHB-flax composites. Dynamic mechanical analysis (DMA) was used to assess composite properties in bending mode. Incorporation of glycerol triacetate and polyethylene glycol as plasticisers resulted in an increase in the loss modulus (G}), a measure of the energy dissipated into heat during deformation of a material. This finding was attributed to better binding between the fibres and the matrix caused by a change in fibre surface energy resulting from plasticiser addition. A summary of literature values for the mechanical properties of natural fibre-PHA composites is shown in Table 8.2.

8.3.2 Biodegradability of natural fibre-PHA composites From a practical perspective, biodegradability might be considered one of the most important properties of plant fibre biocomposites and it is therefore surprising that there has not been more research on this topic. A recent literature search suggests that there have been only a limited number of studies in which the biodegradability of plant fibre-PHA composites in particular has been investigated. Peterson et al. (2002) used high-temperature mechanical pulp (Pinus radiata) to prepare hot-pressed composite sheets based on BiopolTM. The biodegradation procedure employed optimum parameters determined from previous studies. Composites of varying fibre mass fraction were incubated in activated sludge soil for five weeks at 40 ëC and biodegradability was assessed by weight changes in conditioned samples over the test period. The wood fibre-BiopolTM composites were shown to degrade faster than pure BiopolTM. For example, composites with 25% w/w fibre exhibited a mean weight loss of about 55%, which compares with a corresponding weight loss of only about 15% for the unreinforced polymer over the same period. A mass fraction of 15% wood fibres

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197

was optimum for maximising degradation. The authors concluded that the fibres were acting as conduits for bacterial attack, allowing easier access to the material and therefore faster biodegradation. Mohanty et al. (2000c) also studied the biodegradability of BiopolTM composites and found that 34% weight loss occurred in unreinforced BiopolTM while dewaxed, alkali-treated and acrylonitrile-grafted fibre composites exhibited weight losses of 56%, 42% and 37% respectively under the same conditions. In contrast to the studies mentioned above, Avella et al. (2000a) discovered that steam-exploded wheat straw-PHBV composites containing 30% w/w fibre degraded at the same rate as unreinforced PHBV in a long-term burial test. In a composting simulation test, the composites showed a slower rate of degradation than unreinforced PHBV. These results emphasise the importance of comparing test results based on specific fibres and biodegradation procedures.

8.3.3 Processing of natural fibre-PHA composites Researchers have reported successful laboratory processing of natural fibre-PHA composites by a number of techniques (Table 8.2) and from these studies it appears that there are at least two important considerations; first, the impact of processing on the fibre and second, the impact of processing on polymer degradation. Considering the first point, the work of Wollerdorfer and Bader (1998) shows that fibre breakdown during extrusion can be a concern and can overwhelm any improvements in composite mechanical properties that would otherwise occur. Commercially practicable methods are therefore needed that will allow plant fibres to be fed into extruders without causing such complications. Regarding the second point, it is well known that PHB, for example, can thermally degrade at temperatures not far above the melting point. In studies on the processing of cellulose with PHB it has been shown that thermal breakdown of the polymer produces volatile organic acids that may degrade the fibres with a possible reduction in composite properties (Gatenholm et al., 1992; Gatenholm and Mathiasson, 1994). As shown in Fig. 8.3, Plackett and Andersen (2002) found a reduction in composite tensile strength when processing jute with PHB at 220 ëC, but in this case it is probable that degradation and a reduction in composite strength would have occurred at lower temperatures if a vacuum had not been applied during the heating stage of the process.

8.3.4 Other properties of natural fibre-PHA composites Several researchers have examined the impact of natural fibres on polymer crystallisation as a part of their research on PHA biocomposites. Reinsch and Kelley (1997) used modulated differential scanning calorimetry (MDSC) and hot-stage microscopy to examine crystallinity and crystallisation in PHB or

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PHBV combined with various cellulosic fibres. In general, the addition of fibres increased the polymer crystallisation rate from the glass and the melt; however, the ultimate crystallinity as determined from the heat of crystallisation in MDSC experiments was the same in reinforced and unreinforced materials. Straw fibre addition was found to have a similar effect on PHBV crystallisation (Avella et al., 2000a). Optical microscopy has also been used to show the increased nucleation occurring in PHB or PHBV as a result of fibre addition and the use of a silane coupling agent (Shanks et al., 2004).

8.4

Natural fibre-polylactide (PLA) composites

Polylactide (PLA) has received recent attention because of the development of a large-scale commercial plant in the US by Cargill-Dow (Drumright et al., 2000). In fact, this biopolymer has been used for some years in high-value medical applications but the Cargill-Dow development represents the first time that production for larger volume products using PLA films or fibres (e.g., packaging, apparel) has been targeted. In the Cargill-Dow process, PLA is derived from corn starch as a starting material, which is then converted to simple sugars and subsequently to lactic acid. A continuous condensation reaction involving aqueous lactic acid is then used to produce a low molecular weight PLA pre-polymer. Next, the pre-polymer is converted into a mixture of lactide stereoisomers that are purified and converted to a high molecular weight PLA by a tin-catalysed ring-opening reaction. Figure 8.4 illustrates the synthesis of PLA by either condensation or ring-opening polymerisation. PLA is attractive commercially because it is biodegradable, can be produced from renewable resources and resembles polystyrene in some of its properties.

8.4 Polymerisation of L-lactic acid to L-PLA by direct condensation or by ringopening via the L-lactide.

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199

As with other biopolymers, PLA remains relatively costly when compared with commodity thermoplastics such as polyethylene and polypropylene but a cost level competitive with polyethylene terephthalate (PET) is considered feasible. Possibly because the polymer has only recently been available in bulk to any great degree, there has so far been relatively little research on PLA as a matrix in natural fibre biocomposites.

8.4.1 Mechanical properties of natural fibre-PLA composites The findings from international research in which PLA has been combined with natural fibres to make composites for mechanical testing are summarised in this section of the chapter. Jute-PLA composites have been made using a film stacking procedure and a combination of non-woven jute mats and extruded PLA films (Plackett et al., 2003). The fibre content was 40% w/w and the process involved use of a vacuum in the heating stage. The results of composite tensile tests showed that an approximate doubling of tensile strength and almost tripling of tensile modulus could be achieved (Fig. 8.5). However, the addition of jute fibres had little effect on impact strength and elongation at maximum stress was reduced slightly when compared with the unreinforced PLA. Oksman et al. (2002) fed a hand-made flax roving through the side feeder of a twin-screw extruder in order to make flax-PLA composites. The fibre content was calculated based on the feeding speed and the weight of the roving per metre. The extrudate was then compression molded in order to obtain specimens for tensile testing. Interestingly, it was found that the addition of flax fibres did not significantly improve the tensile strength, which the authors attributed to

8.5 Tensile strength of compression-moulded jute-PLA composites showing the enhanced strength and brittleness of the composites processed at 190 ëC or 210 ëC as compared with the unreinforced polymer processed at 190 ëC (source: Plackett et al., 2003).

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Biodegradable polymers for industrial applications

poor adhesion between flax and PLA. In contrast, the addition of fibres increased the tensile modulus but this subsequently decreased when the fibre addition level went up from 30% to 40%. The authors indicated that the compression moulding process might have influenced fibre orientation with variations from one sample to another leading to the observed changes in tensile modulus. Further trials involved the use of triacetin as a plasticiser. Impact strength was improved at 5% triacetin content but did not improve further at higher plasticiser levels. Furthermore, triacetin addition caused a significant decrease in tensile modulus when plasticiser addition levels of 10 and 15% were employed. The authors suggested that triacetin changed the fibre structure making the fibres more brittle. Plasticiser-modified flax fibres have also been studied as PLA reinforcement (Wong et al., 2003). In this case, the plasticisers were triethyl citrate, tributyl citrate and glycerol triacetate and DMA was used as a composite characterisation technique. The plasticisers caused a marked increase in the storage modulus (G0 ), a measure of the elastic modulus or the energy stored by a material through deformation, possibly explained by improved composite morphology or by a smoother surface coverage of the fibres by the matrix. Injection-moulded flax-PLA composites were made as part of an EU FAIR project entitled `New functional biopolymer-natural fibre composites from agricultural resources' that examined the development and potential use of PLA composites in automotive applications (SchoÈnweitz, 2001). Tensile strength was not increased when 20±40% w/w flax was incorporated and this was explained by a lack of good fibre-PLA adhesion (Lanzilotta et al., 2002). However, fibres were significantly shortened during processing and this could also partly explain the results. The Young's modulus of the composites increased linearly with flax content. Chemical treatment of the fibres and modification of the polymer by reactive extrusion were both studied as a means of enhancing composite properties but it was decided that the results did not justify the additional costs involved. A commercial paper-like sheet of kenaf fibre was converted to a composite material by impregnation with a solution of a commercial L-PLA in dioxane solution (Nishino et al., 2002). Following this procedure, a composite material with 70% v/v fibre content was obtained. Tensile tests showed that a maximum tensile stress of about 60 MPa was found for the best composite material, comparing with a value of about 20 MPa for the unreinforced polymer processed in the same way. Similarly, a tensile modulus of about 6 GPa for the kenaf-PLA composite compared with a value of just over 1 GPa for the unreinforced polymer. The authors concluded that good stress transfer from the resin to the matrix had been obtained. In research by Shibata et al. (2003), a flexural modulus of 5.5 GPa was achieved at 20% w/w abaca fibre content in a PLA composite. This result compares with a flexural modulus of 3.5 GPa for the unreinforced polymer. A further increase in flexural modulus was obtained when

Biodegradable polymer composites from natural fibres

201

using chemically modified (e.g., esterified) fibres. Flexural strength, at about 110 MPa, was not significantly improved by fibre addition. A summary of the literature on mechanical properties of natural fibre-PLA composites is shown in Table 8.3.

8.4.2 Biodegradability of natural fibre-PLA composites The biodegradation of PLA is thought to proceed through initial abiotic hydrolysis of the polymer followed by enzymatic breakdown of lower molecular weight fragments. The degradation rate has been shown to increase in the compost environment in the presence of an active microbial community compared to the abiotic hydrolysis (Tuominen et al., 2002). Given this situation, it seems reasonable to expect that natural fibre-PLA composites will biodegrade at an acceptable rate in compost once the minimum conditions of temperature and moisture are present and, for example, Nurminen (2000) found that flaxPLA composites degraded faster than unreinforced reference samples under controlled degradation conditions. However, under conditions equivalent to an automotive interior, flax-PLA composites show no unwanted biodegradability (SchoÈnweitz, 2001).

8.4.3 Processing of natural fibre-PLA composites As in the case of PHA composites, researchers have demonstrated various techniques for preparing PLA biocomposites containing natural fibres (Table 8.3). Extrusion processing of PLA requires care to ensure good temperature control and pre-drying of the polymer granulate. Plackett et al. (2003) found that jute-PLA could be processed by film stacking at temperatures as high as 220 ëC without deterioration in composite tensile properties when a vacuum was applied at the heating stage; however, this type of batch process does not easily transfer to an industrial scale. Demonstration flax-PLA components with acceptable appearance for automotive applications have been produced via industrial-scale injection moulding or compression moulding, although not all material properties were fully tested and the cost for a specific car part remained much higher than for a conventional mineral-filled polymer (SchoÈnweitz, 2001).

8.4.4 Other properties of natural fibre-PLA composites The flammability of natural fibre-PLA composites has been studied using the UL 94 standard test procedure (Test for flammability of plastic materials for parts in devices and appliances) (Haapanen and MaÈkinen, 2003). The composites did not pass this test; however, the test was passed when an ammonium polyphophate-based fire retardant was included. Another Finnish study (Nurminen, 2000) showed that the tensile strength of a natural fibre-PLA

Table 8.3 Mechanical properties of natural fibre-PLA composites Fibre type and content

Polymer

Manufacturing method

Flax (30%)

PLLA

Flax (40%) Flax (40%)

Polylactic acid Polylactic acid (Pollait, Fortum) Polylactic acid Compression moulding Extrusion + melt PLLA modification PLLA (Biomer) Compression moulding Polylactic acid N.S. PLLA (Lacea, Solution impregnation Mitsui Chemicals) PLA (Lacty, Injection moulding Shimadzu)

Flax (50%) Cotton linter (30%) Jute (40%) Kenaf (20%) Kenaf sheets (70% v/v) Abaca (20%)

Note: N.S. = not specified

Extrusion + melt modification Injection moulding Injection moulding

Tensile strength (MPa)

Tensile modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

70

8.4

N.S.

N.S.

68 45

7.2 7.2

N.S. N.S.

N.S. N.S.

Haapanen and MÌkinen (2003) Lanzilotta et al. (2002) Oksman et al. (2002)

99 30

6.0 6.8

N.S. N.S.

N.S. N.S.

Lanzilotta et al. (2002) MÌkinen (2002)

100 N.S. 60

9.4 7.6 6.5

N.S. N.S. N.S.

N.S. N.S. N.S.

Plackett et al. (2003) NEC (2003) Nishino et al. (2002)

N.S.

N.S.

110

6.0

Shibata et al. (2003)

References

Biodegradable polymer composites from natural fibres

203

composite at 50 ëC was about 60% of the value at 23 ëC. This decrease is less than might be anticipated and suggests a wider than expected available temperature range for practical applications. In a further project, the effect of the environment on composite mechanical properties was evaluated and, although dependent on the particular fibres, treatment and modifications during compounding and processing, the most harmful environment was generally found to involve repeated exposures to temperature change at high humidity (MaÈkinen, 2002). In the same investigation, composite mechanical properties did not change significantly during six months at 23 ëC and 50% relative humidity.

8.5

Natural fibre-starch composites

Starch is a high molecular-weight polymer of anhydroglucose units linked by D-glycosidic bonds. The two main constituents of starch are amylose and amylopectin (Fig. 8.6). Amylose is a linear molecule with an extended helical twist and generally has a molecular weight of 1.0 to 1.5 million. Amylopectin is

8.6 Structure of the polysaccharide components of starch, amylose (a) and amylopectin (b).

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Biodegradable polymers for industrial applications

a branched molecule with a much higher molecular weight in the range of 50 to 500 million. Pure starch is a brittle polymer and therefore has to be plasticised for ease of processing. Plasticisation can be achieved through addition of plasticisers or by mechanical means. As explained by Jansson and Thuvander (2002), starch is commercially available on an industrial scale and is a candidate matrix material for biocomposites in which natural fibres are used as reinforcement.

8.5.1 Mechanical properties of natural fibre-starch composites The mechanical properties and water absorption behaviour of composites made from a starch-based thermoplastic matrix in combination with alkali-treated sisal fibres were studied by Alvarez et al. (2003). The variables in their experiments were fibre percentage, temperature and process time. The selected matrix was Mater-BiÕ-Y (Novamont), a blend based on cellulose derivatives, starch and additives (Cyras et al., 2001). Composites were prepared by blending MaterBiÕ-Y with fibres in a high-intensity mixer and then compression moulding the compounded material into panels. Mechanical tests showed that the tensile modulus more than doubled from 0.95 GPa to 2.2 GPa when the fibre content was increased from 0 to 15%. Fibre treatment with sodium hydroxide solution had no statistically significant effect on tensile modulus. Similarly, the flexural modulus increased from roughly 1.4 GPa to 2.8 GPa when Mater-BiÕ-Y was reinforced with 15% w/w sisal fibre but was not significantly increased when alkali-treated fibres were used. Wollerdorfer and Bader (1998) used a thermoplastic starch derived from wheat starch plasticised with sorbitol and glycerol, BioplastÕ (Biotec GmbH), a blend consisting of potato starch, modified cellulose and synthetic polymers, and Mater-BiÕ type Z101U, a blend consisting of corn starch and a biodegradable polyester, in their research on biocomposites. These authors found that the tensile strength of the thermoplastic starch could be increased by a factor of between two and four depending upon the particular fibre and the fibre percentage. The best improvement was obtained when the thermoplastic starch was reinforced with 15% flax fibre that had been previously treated with a maleinate-modified colophony resin (rosin derived as an exudate from certain species of pine tree). The tensile modulus displayed similar increases as a result of fibre reinforcement. Quite different results were obtained when the commercial starch blends (i.e., BioplastÕ or Mater-BiÕ Z101U) were mixed with fibres. For example, no significant enhancement of tensile strength was found when BioplastÕ was mixed with up to 35% flax fibre and the strength increase for Mater-BiÕ was fairly modest with virtually no increase above 15% fibre content. The authors postulated that the starch-containing matrices would have good fibre-matrix adhesion but this could be offset by a reduction in fibre length as a result of processing.

Biodegradable polymer composites from natural fibres

205

Hermann et al. (1998) reported the use of various fibres to reinforce a selection of biodegradable polymers. Mater-BiÕ and Sconacell AÕ (Buna SOW Leuna) were the modified starches that were included in their investigations. The tensile moduli of ramie-Sconacell AÕ and flax-Sconacell AÕ compounds were found to be about 50% and the tensile strengths about 60% of the corresponding properties of E-glass epoxy composites. When hemp was used as reinforcement the modulus was 143% of the value of E-glass epoxy composites while the tensile strength was about 60% compared with that of glass composites. The authors suggested that these results supported the case for biocomposites as an alternative to glass fibre-reinforced plastics in some structural applications. The effect of processing conditions on the mechanical properties of starchbased biocomposites has been studied (Ali et al., 2003). Mater-BiÕ-Z and Mater-BiÕ-Y were used as matrices and sisal fibres were used either as received or after alkaline treatment. Mixtures were extrusion compounded at various temperatures, screw rotation speeds and mixing times. After mixing, composites were prepared either by hot pressing or on a calendering machine. The results showed that composite tensile behaviour was very dependent on the polymer type and on the processing conditions including temperature and mixing time. For example, composites based on Mater-BiÕ-Z were reinforced in terms of both tensile strength and tensile modulus by use of sisal fibres; however, composites based on Mater-BiÕ-Y (plasticised starch and cellulose) had higher elastic moduli but lower tensile strength. Creep properties were improved by the use of sisal fibre reinforcement and this improvement was a function of fibre aspect ratio. The tensile fracture and failure behaviour of thermoplastic starch reinforced with unidirectional and cross-ply flax fibre has been explored (RomahaÂny et al., 2003). Composite production involved film stacking using Mater-BiÕ films in combination with flax fibres. The fibres were arranged either unidirectionally or in a cross-ply lay-up. The unidirectional lay-up was achieved by combing the fibres and fixing the fibre ends outside of the hot-pressing area using adhesive tape. Flax contents were 20%, 40% or 60% by weight and composites were prepared by hot pressing at a pressure of 3 MPa and at 140 ëC. Unidirectional composites gave average tensile strength values of 48, 73 and 78 MPa at 20, 40 and 60% fibre content respectively while the corresponding values for cross-ply composites were 30, 53 and 55 MPa. These results may be partly explained by a decrease in fibre wetting at fibre contents over 40%. In contrast, the tensile modulus increased almost linearly over the same fibre content range for both composite lay-ups, although the increase was less notable for the cross-ply composites. Funke et al.. (1998) used extrusion processing to examine the effect of different starch types, fibres, plasticisers and other compounding additives on thermoplastic processing. Raw starch materials from corn were plasticised by

206

Biodegradable polymers for industrial applications

extrusion and then converted into test products by injection moulding. Two types of commercial cellulose fibre were added at 2, 7 or 15% by weight. Significant improvement in tensile properties was obtained through addition of the cellulose fibres although there was a decrease in tensile strength for one of the fibre types when fibre content was increased from 7 to 15%. As expected, there was a parallel decrease in elongation during tensile testing as fibre content was increased from 7 to 15%. The impact properties of injection-moulded Mater-BiÕ (R) were increased by 30% when the matrix was reinforced with miscanthus fibres (Johnson et al., 2003). Key factors in determining composite properties were the temperature of the extruder barrel and the extruder screw speed. A summary of plant fibrethermoplastic starch composite mechanical properties is presented in Table 8.4.

8.5.2 Biodegradability of natural fibre-starch composites The biodegradability of thermoplastic starch has been established through past research; however, there are very few studies in which the biodegradability of natural fibre-starch composites has been investigated. In an EU FAIR project (SchoÈnweitz, 2001), two different Mater-Bi starch/synthetic polymer blends (Mater-BiÕ Y101U and Mater-BiÕ A105H) with flax fibres were injection moulded into small demonstration parts and then subjected to controlled composting conditions. Results after 126 days showed that the parts containing Mater-BiÕ A105H were only degrading slowly and were significantly less degraded than flax-PLA composite parts. The same materials showed no unwanted biodegradability under conditions equivalent to those of an automotive interior.

8.5.3 Processing of natural fibre-starch composites Thermal degradation during thermoplasticisation and mixing of starch with cellulose fibres was examined by Carvalho et al. (2003). The compounds and composites were prepared in an intensive batch mixer at 150±160 ëC with glycerol as plasticiser and with fibre contents of 5±15%. The findings were that an increased glycerol content reduced starch chain degradation while an increase in fibre content appeared to increase starch chain degradation. The high molecular weight fraction of the starch (i.e., amylopectin) was more susceptible to degradation during processing than the amylose fraction.

8.5.4 Other properties of natural fibre-starch composites The water absorption characteristics of natural fibre-starch composites have been investigated by Funke et al. (1998) and Alvarez et al. (2003). In the former study, it was found that the water uptake on exposure to 45% relative humidity

Table 8.4 Mechanical properties of natural fibre-starch composites Fibre type and content

Polymer

Sisal (15%)

Mater-BiÕ-Y Compression moulding (Novamont) Wheat starch/ Extrusion sorbitol Mater-BiÕ Z101U Extrusion (Novamont) Mater-BiÕ Z101U Extrusion (Novamont) Mater-BiÕ Film stacking with (Novamont) unidirectional fibres Film stacking with Mater-BiÕ (Novamont) cross-ply fibres Commercial Injection moulding starch

Flax (15%) Ramie (15%) Flax (15%) Flax (60%) Flax (60%) Cellulose (7%)

Note: N.S. = not specified

Manufacturing method

Tensile strength (MPa)

Tensile modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

16.8

2.2

N.S.

2.8

Alvarez et al. (2003)

37

N.S.

N.S.

N.S.

25.1

N.S.

N.S.

N.S.

20.8

N.S.

N.S.

N.S.

78

9.3

N.S.

N.S.

55

5.9

N.S.

N.S.

Wollerdorfer and Bader (1998) Wollerdorfer and Bader (1998) Wollerdorfer and Bader (1998) Wollerdorfer and Bader (1998) Romaha¨ny et al. (2003)

7

N.S.

N.S.

N.S.

Funke et al. (1998)

References

208

Biodegradable polymers for industrial applications

and 20 ëC was reduced by adding a few percent of cellulose fibre to starch. In the work by Alvarez et al. (2003), the addition of sisal fibres to a starch-based matrix (Mater-BiÕ-Y) produced a similar result when either 10 or 15% fibre was used. The authors suggested that a fibre network could impair diffusion of moisture through the matrix. Alkali-treated fibres, which should be more hydrophilic than untreated fibres, showed higher water gains at equilibrium in composites.

8.6

Natural fibre-soy resin composites

There has been an extensive programme over a number of years at the University of Delaware focused on the development of new composites from natural fibres and resins derived from renewable resources. The ACRES (Affordable Composites from Renewable Resources) group at the university has, for example, developed new chemistries to synthesise rigid polymers from plant oils (Williams and Wool, 2000). The resins prepared by the group fall largely into the thermoset category and involve use of plant oils as starting materials. These oils consist mainly of triglyceride molecules with long hydrocarbon chains and a few double bonds per molecule. The oils are activated by functionalisation of the hydrocarbon chain double bonds and the resulting active sites can then be used to initiate the synthesis of new monomers and new composite resins. The ACRES group has developed a number of resins from plant triglycerides that can be used in liquid moulding processes. Composites have been made by the ACRES group by combining either flax or hemp fibres with a modified acrylated epoxidised soy oil (Williams and Wool, 2000; Khot et al., 2001). A resin injection-moulding process was adopted using a chemically modified oil combined with styrene and divinylbenzene in the ratio 100 : 45: 5. Mechanical testing of composites containing 34% flax gave tensile strength and modulus values of 30 MPa and 5 GPa respectively and flexural strength and modulus values of 64 MPa and 4.2 GPa respectively. The ACRES group has collaborated with the John Deere company to produce fibrereinforced composites with excellent properties for potential use as hay baler doors (Wool et al., 2002). Since soybeans contain about 20% oil, efforts are now under way to use genetic modification to increase the oil content and thus provide a less expensive source of resins. The ACRES group has also developed a series of maleinised hydroxylated triglycerides derived from a number of plant oils. Although most of these resins have been found to be nondegradable, it may be possible through chemical modification to make them biodegradable. The same group has investigated a wide range of other composite materials including the concept that lignin might be used as an additive (Thielemans et al., 2002). Netravali (2002) prepared composites using cellulose fabrics and soy protein resin with the objective to better understand the fibre/soy protein resin interface. Random short cellulose fibre composites had moderate mechanical properties

Table 8.5 Mechanical properties of natural fibre-soy resin composites Fibre type and content

Polymer

Manufacturing method

Flax (35%)

Acrylated epoxidised soy oil resin Acrylated epoxidised soy oil resin Soy protein concentrate Soy protein concentrate

Hemp (20%) Ramie (65%) Ramie (65%)

Note N.S. = not specified

Tensile strength (MPa)

Tensile modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

Resin transfer moulding

30

4

65

4.3

Williams and Wool (2000)

Resin transfer moulding

35

4.4

35.7

2.7

Williams and Wool (2000)

Infiltration of unidirectional fibre lay-up Infiltration of transverse fibre lay-up

271

4.9

N.S.

N.S.

Netravali (2002)

7.4

0.9

N.S.

N.S.

Netravali (2002)

References

210

Biodegradable polymers for industrial applications

and were considered suitable for non-structural applications. As expected, superior tensile properties were obtained when unidirectional fibre lay-ups were used. Henequen fibre composites based on soy protein concentrate were also prepared and found suitable for packaging, non-structural consumer goods and automotive parts such as door trimmings. A summary of the mechanical properties of natural fibre-soy resin composites is provided in Table 8.5.

8.7

Natural fibres in combination with synthetic biodegradable polymers

There are a number of reports in the literature concerning research on reinforcement of synthetic biodegradable polymers. These polymers include polyester amides, poly(butylene succinate-co-butylene adipate) (PBSA) and polycaprolactone (PCL). For example, Tserki et al. (2003) examined the performance of composites in which cotton fibre wastes were combined with a commercial polyester of the PBSA type. A maleated derivative of the polyester was produced by mixing and heating together appropriate amounts of the polyester, maleic anhydride and an initiator. Hot-pressed composites prepared from cotton waste fibre in combination with the polyester were subsequently tested for a range of different properties. The use of cotton fibres resulted in a decrease in tensile yield stress, an increase in modulus and a significant decrease in both elongation at break and impact strength. The decrease in yield stress was not considered surprising because other studies have shown that the introduction of filler into a thermoplastic does not necessarily increase the composite strength. However, introduction of the maleated polyester as a compatibiliser led to a significant increase in composite mechanical properties. Sisal or pineapple leaf fibres have been used as reinforcement in a commercial polyester amide matrix (Mishra et al., 2002). Water absorption and poor fibre wettability between untreated fibre and matrix led to debonding over time; however, various chemical treatments were applied to the sisal fibres and this produced better mechanical properties. Tensile and flexural properties were optimum at a fibre loading of 50% w/w. Alkali treatment and acetylation appeared to give promising improvements in composite properties. Alkalitreated sisal gave about a 20% increase in tensile strength and acetylated fibres produced about a 14% increase in flexural strength when compared with reference composites made from untreated fibres. The fabrication and properties of polyester amide composites based on coir fibres has also been studied (Rout et al., 2001). The coir fibres were used in the form of non-woven mats that were sandwiched between polyester amide films and then hot pressed to give composite samples. Fibre contents were varied from 30% w/w to 60% w/w but even at the highest fibre loading the tensile strength increased only very slightly from 25 MPa for the pure polyester amide to about 29 MPa for the composite material. More significant increases were seen in the flexural strength although

Biodegradable polymer composites from natural fibres

211

this decreased when the fibre content was raised from 50 to 60%. The authors concluded that this finding was a result of poor fibre wetting. All of the chemical treatments that were investigated led to improved composite mechanical properties. Polycaprolactone (PCL), a biodegradable polyester, has also been explored as a component of plant fibre composites. Plackett and Andersen (2002) prepared jute-PCL composites using a rapid press consolidation technique with vacuum applied at the heating stage and found very significant improvements in tensile properties. However, although the low melting point of PCL allowed the composites to be fabricated at 80 ëC, for practical purposes this property could also seriously limit the number of applications for which fibre-PCL composites might be suitable. A summary of natural fibre-synthetic biodegradable polymer composite mechanical properties is presented in Table 8.6.

8.8

Commercial developments

In February 2003, NEC Corporation announced the development of a highstrength heat-resistant bioplastic based on polylactide reinforced with kenaf fibres. The material is said to have superior heat resistance and strength and to be a candidate for use in electronic devices. Specifically, the thermal deformation temperature was raised from 67 ëC to 120 ëC and the bending modulus was improved from 4.5 GPa to 7.6 GPa as a result of fibre reinforcement. These properties are said to exceed those of conventional oilbased resins used for packaging such as ABS and fibreglass-reinforced ABS. The development was achieved by cooperation between NEC and Nature Trust, Inc. of Japan, a company that is successfully growing kenaf in bulk in Australia. In a further development announced by press release on 26 January 2004, NEC indicated that a flame-resistant polylactic acid has been developed through use of a safe inorganic additive and that this should also assist the introduction of polylactic acid-based components in electronic products. In addition to these developments, there is considerable general interest in Japan in PLA-based parts and housings for electrical products and, as well as NEC, Fujitsu, Sony and Toyota have announced plans to study or start using PLA in products. Developments in paper-PLA composites are also said to be under way in Japan. In Europe, one wood-plastic extrusion company (Fasalex GmbH) based in Austria has produced wood-reinforced biodegradable polymer products for various interior applications (e.g., interior profiles, mouldings). The FasalexÕ line includes products based on natural fibres in combination with maize starch and various percentages of non-degradable polymers, such as PP or PVC, as well as one material that is almost entirely made from renewable resources. FasalexÕ products for exterior use are said to be under development. The American farm equipment manufacturer John Deere introduced new soybased polymer panel components (HarvestformÕ) for combine harvesters in

Table 8.6 Mechanical properties of natural fibre-synthetic biodegradable polymer composites Fibre type and content

Polymer

Manufacturing method

Cotton (50%)

PBSA (Bionolle, Showa) Polyester amide (BAK 1095, Bayer) Polyester amide (BAK 1095, Bayer) Polyester amide (BAK 1095, Bayer) Polycaprolactone (CAPA 680, Solvay)

Sisal (50%) Pineapple leaf (50%) Coir (50%) Jute (40%)

Note: N.S. = not specified

References

Tensile strength (MPa)

Tensile modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

Moulding

26

2.4

N.S.

N.S.

Tserki et al. (2003)

Film stacking

60

N.S.

78

N.S.

Mishra et al. (2002)

Film stacking

30

N.S.

40

N.S.

Mishra et al. (2002)

Film stacking

30

N.S.

55

N.S.

Rout et al. (2001)

Film stacking with heating under vacuum

65

7

N.S.

N.S.

Plackett and Andersen (2002)

Biodegradable polymer composites from natural fibres

213

2002. The panels are based on either soy urethanes or other soy- or corn-based resins. Future developments are anticipated in the use of flexible urethane foam components, such as seating, and the use of natural fibres, such as flax, in polymer composite panels.

8.9

Conclusion

As outlined in this chapter, international research has demonstrated the technical feasibility of manufacturing eco-friendly composites based on biodegradable polymers, either natural or synthetic, in combination with local wood or plant fibre resources. Although the polymers remain expensive relative to commodity plastics, perhaps presently three to five times or more the cost of resins such as PP, LDPE, HDPE and PVC, the incorporation of natural fibres provides a potential means of reducing total material costs while at the same time improving the material mechanical properties. As in most cases, one might expect that as products gain acceptance and production increases, costs should come down. Less expensive polymers and improved processing methods will clearly help with this trend. In terms of future development and aside from issues concerning polymer cost and processing, there are also challenges with use of plant fibres that have to be considered. For example, fibre properties depend on a number of factors such as plant source, plant age, processing techniques, geographic origin and climate. Moisture sorption and desorption over time can also lead to reduced strength and debonding from a polymer matrix. As discussed in this chapter, the use of fibre treatments can improve composite mechanical properties and can also enhance resistance to water uptake. In addition, most fibres cannot withstand temperatures much above 175 ëC for long periods, limiting their use in terms of processing with certain polymers. Practical processes for handling plant fibres in extrusion and injection moulding processes, without losing the advantages provided by these fibres, are also needed if the full potential of plant fibre biocomposites is to be realised in the future. To date, there have been relatively few research projects that have taken a comprehensive approach to natural fibre biocomposite property evaluation while at the same time employing industrially practicable production methods. Furthermore, from a fundamental research perspective there is still much to be learned about the morphology and fibre-matrix interactions in natural fibre biocomposites that should ultimately assist with their overall development.

8.10 Further information As a result of the relatively early state of development of natural fibre-reinforced biopolymer composites, there are few current documents outside of those cited as references in this chapter. One possible starting point for further related

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Biodegradable polymers for industrial applications

information is the web site for the UK-based Sustainable Composites network (SusComp). This network was established in 2001 to advance the development and commercialisation of sustainable composite materials. In this sense, sustainable composites are considered to be materials that have minimal impact upon the environment and include those that are readily recyclable. The Sustainable Composites network web site provides links to the National NonFood Crops Centre (NFCC), the Biomimetics Network for Industrial Sustainability (BIONIS), the Centre for Advanced and Renewable Resources (CARM) and the European Renewable Resources and Materials Association (ERRMA). In North America, the USDA Bio-based Products programme supports the production and use of materials from renewable resources and can be a link to information of relevance to natural fibre biocomposites. The final report on EU FAIR project CT98-3919 (SchoÈnweitz, 2001) provides useful detailed reading on the issues involved in development of fibrebiopolymer composites for automotive applications. A review article (Netravali and Chabba, 2003) discusses alternative approaches to `green' composites, developments in fully biodegradable composites and future prospects for these products. A recently published book entitled Natural fibres, plastics and composites (Wallenberger and Weston, 2004) has several chapters that provide further reading for those interested in natural fibre composites based on biodegradable polymers. Chapter topics of particular interest include those that discuss plastics and composites from polylactic acid, plastics and composites from soybean oil, plastics and composites from lignophenols, natural fibrereinforced automotive parts and nanoparticle-reinforced natural plastics.

8.11 References Ali R, Iannace S and Nicolais L (2003), `Effect of processing conditions on mechanical and viscoelastic properties of biocomposites', J Appl Polym Sci, 88, 1637±1642. Alvarez V A, Ruscekaite R A and VaÂzquez A (2003), `Mechanical properties and water absorption behaviour of composites made from a biodegradable matrix and alkalinetreated sisal fibres', J Comp Mat, 37(17), 1575±1588. Andersen T L (1997), `Development of a rapid press consolidation technique for continuous fibre reinforced thermoplastic composites', Andersen S I et al., Polymeric Composites ± Expanding the Limits, Proceedings of the 18th Risù International Symposium on Materials Science, Risù, Denmark, 1±5 September 1997, pp. 237±244. Asrar J and Gruys K (2002), `Biodegradable polymer (BiopolÕ)', in Doi Y and SteinbuÈchel A, Biopolymers Volume 4, Weinheim, Wiley-VCH Verlag, pp. 53±90. Avella M, La Rota G, Martuscelli E, Raimo M, Sadocco P, Elegir G and Riva R (2000a), `Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and wheat straw fibre composites: thermal, mechanical properties and biodegradation behaviour', J Mat Sci, 35 (4), 829±836. Avella M, Martuscelli E and Raimo M (2000b), `Review: Properties of blends and composites based on poly(3-hydroxy)butyrate (PHB) and poly(3-hydroxybutyrate-

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acid', Bull Soc Chem Biol, 8, 770±782. Lilholt H and Lawther J M (2000), `Natural Organic Fibers', in Kelly A and Zweben C, Comprehensive Composite Materials: Volume 1, Amsterdam, Elsevier Science, pp. 303±325. Luo S and Netravali A N (1999), `Interfacial and mechanical properties of environmentfriendly ``green'' composites made from pineapple fibres and poly(hydroxybutyrateco-valerate)', J Mat Sci, 34 (15), 3709±3719. MaÈkinen K (2002), `Biokomposiittien pitkaÈaikaiskestaÈvyys', Master thesis, Technical University of Tampere, Tampere, Finland. Mishra S, Tripathy S S, Misra M, Mohanty A K and Nayak S K (2002), `Novel ecofriendly biocomposites: Biofiber reinforced biodegradable polyester amide composites ± Fabrication and properties evaluation, J Reinf Plas Comp, 21 (1), 55±70. Mohanty A K, Khan M A, Sahoo S S and Hinrichsen G (2000a), `Effect of chemical modification on the performance of biodegradable jute yarn-Biopol composites', J Mat Sci, 35 (10), 2589±2595. Mohanty A K, Misra M and Hinrichsen G (2000b), `Biofibres, biodegradable polymers and biocomposites: An overview', Macromol Mater Eng, 276/277, 1±24. Mohanty A K, Khan M A and Hinrichsen G (2000c), `Surface modification of jute and its influence on performance of biodegradable jute-fabric/BiopolÕ composites', Comp Sci Technol, 60 (7), 1115±1124. Mohanty A K, Misra M and Drzal L T (2001), `Surface modifications of natural fibres and performance of the resulting biocomposites', Comp Interfaces, 8 (5), 313±343. Mohanty A K, Misra M and Drzal L T (2002), `Sustainable bio-composites from renewable resources: Opportunities and challenges in the green materials world', J Polym Environ, 10 (1±2), 19±26. NEC (2003), `NEC announces development of high-strength highly heat-resistant bioplastic', Press release, 10 February. Netravali A N (2002), ` ``Green'' composites from cellulose fabrics and soy protein resin', Proceedings of the 2nd International Workshop on Green Composites, Tokushima, Japan, November 19±20, 2002. Netravali A N and Chabba S (2003), `Composites get greener', Materials Today, 6 (4), 22±29. Nickel J and Riedel U (2001), `Structural Materials made of Renewable Resources', in Chiellini E, et al., Biorelated Polymers: Sustainable Polymer Science and Technology, Dordrecht, Kluwer Academic/Plenum Publishers, pp. 27±40. Nishino T, Hirao K, Kotera M, Nakamae, K and Inagaki H (2002), `Kenaf reinforced biodegradable composites', Comp Sci Technol, 63 (9), 1281±1286. Nurminen A (2000), `Pellavakuitulujitetun polylaktidin ruiskuvalu ja ominaisuudet', Master thesis, Technical University of Tampere, Tampere, Finland. Oksman K, Skrifvars M and Selin J-F (2002), `Natural fibres as reinforcement in polylactic acid (PLA) composites', Comp Sci Technol, 63 (9), 1317±1324. Peterson S, Jayaraman K and Bhattacharyya D. (2002), `Forming performance and biodegradability of wood fibre-BiopolÕ composites', Comps Part A: Appl Sci, 33 (8), 1123±1134. Plackett D V and Andersen T L (2002), `Biocomposites from natural fibres and biodegradable polymers: processing, properties and future prospects', Lilholt H et al., Sustainable Natural and Polymeric Composites ± Science and Technology,

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9

Biodegradable polymers from renewable forest resources T M K E E N A N , S W T A N E N B A U M and J P N A K A S , College of Environmental Science and Forestry at Syracuse, USA

9.1

Lignocellulosic biomass as a renewable and value-added feedstock for biodegradable polymer production

This chapter describes the major components of forest biomass and relates the associated value-added and biodegradable polymeric products that can be generated from each component. The chapter begins with a description of the major lignocellulosic resources available in forest ecosystems, with a focus on the two largest fractions, namely cellulose and hemicellulose, and how these renewable resources will be refined and processed in the near future. The history and rationale behind development of the `green' generation of biodegradable polymers is briefly outlined, as it pertains to the use of renewable forest resources as petroleum-displacing feedstocks. Following a brief description of the composition and magnitude of the woody biomass resource, the chapter then details biodegradable polymers derived from the glycans and heteroglycans present in cellulose and hemicellulose, respectively. Descriptions of these alternative polymers are warranted, as these novel materials represent an inevitable advancement in materials science during the `age of plastics' and an intensely investigated area of research in several major industrial, academic, and governmental laboratories (Aggarwal, 1999). First, structural considerations, applications, and the variety of factors influencing biodegradability are outlined for a class of degradable cellulose-based polymers, namely the cellulose esters. Next, the magnitude of the hemicellulosic component of woody biomass is underscored, as the composition of this feedstock and characteristics of the associated biopolymers become the primary focus of the remaining chapter. The bulk of this section of the chapter focuses on microbial polyhydroxyalkanoate (PHA) polymers, which can be derived from the carbohydrates present in the underutilized hemicellulosic component of forest biomass. Several studies regarding the biodegradation of PHA polymers are presented to illustrate the kinetics of degradation in a variety of environments. The carbohydrate composition of hardwood and softwood hemicellulose is described,

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as well as how this natural heteropolysaccharide has been applied to the fermentative production of biodegradable PHA polymers. Economic analyses regarding such forest-based PHAs, including manufacturing costs, potential applications, and future market viability are discussed using conventional, petroleum-based resins as a reference. The remainder of the hemicellulose section describes in detail, the production and physical-chemical characterization of a specific PHA copolymer, P(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co3HV)), produced by bacterial fermentation of wood-based substrates. The chapter concludes with a summary describing the evolution of the green generation of forest biomass-derived polymers and current perspectives on future developments and applications for these alternative polymeric materials.

9.1.1 Forest biomass: a vastly underutilized renewable resource Forest biomass represents an enormous reservoir of renewable carbon-rich material, which has the potential to be utilized as a feedstock for the production of a wide variety of industrial and commodity products, ranging from paper, lumber, and platform chemicals to a variety of fuels and advanced materials, including biodegradable polymers. The magnitude of this renewable resource is so great that the majority of currently operating paper/pulp mills will be reengineered in the foreseeable future to become complex biorefineries, where the array of renewable resources present in woody biomass will be processed and converted to value-added products. The total amount of plant biomass accumulated globally amounts to approximately 182 trillion kg, while only 5 trillion kg are utilized (primarily for cereal/oil, sugar, and wood) (Simon et al., 1998). Lignocellulosic biomass comprises approximately 50% of the global biomass (Galbe and Zacchi, 2002) and is by far the most abundant renewable organic resource on earth. This woody material is comprised of 30±50% cellulose, 20±50% hemicellulose, and 15±35% lignin, dependent upon the tree species and environmental (growing) conditions. While the bulk of the cellulosic fiber component is efficiently exploited by the paper industry, most of the hemicellulosic and lignin fractions are currently underutilized. Due to the enormous quantities of the major lignocellulosic streams, wood-based biorefineries will inevitably evolve, in order to refine and process all three of these resources. Such biorefineries will develop biotechnological and cultivation strategies for rapid, year-round woody biomass production, as well as efficient pre-treatment and separation techniques to generate and utilize the process streams described above. From these renewable forest-based feedstocks, biorefineries will have the potential for replacing the wide variety of platform chemicals and polymeric products derived from dwindling petroleum reserves with a `green' generation of materials. Optimally, these novel biorefineries will evolve waste-recycling and self-sustaining

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9.1 Flow chart depicting the NREL-patented `Clean Fractionation Process', by which lignocellulosic biomass is separated into its three major components. These fractions can then be processed through environmentally benign methods to purified feedstocks, which can be used to produce a variety of industrial products (Kulesa, 1999).

technologies, to harvest all of the potential value and energy present in woody biomass. The `Clean Fractionation Process', developed and patented by the National Renewable Energy Laboratory (Golden, CO, USA), is one example of an organic solvent-based system used to separate and purify the three major feedstocks present in lignocellulosic biomass (Fig. 9.1). Lignin and hemicellulose are disrupted and solubilized in the solvent mixture composed of water, methyl-isobutyl-ketone (MIBK), ethanol, and sulfuric acid (H2SO4), following a steam explosion treatment catalyzed by the acidic conditions created within the reactor due to the added sulfuric acid and endogenous acetic acid released during the hydrolysis. This environmentally benign process selectively separates cellulose, hemicellulose, and lignin with a high degree of purity, substantial energy savings, and lessened production cost (Kulesa, 1999). Due to the year-round availability and relatively low management requirements associated with the supply of renewable feedstocks present in properly managed forest ecosystems, the forest products industry must evolve to capitalize on all components of this renewable resource.

9.1.2 Replacement of petroleum-based polymers with `green' alternatives The rapid growth of synthetic plastics production has been a relatively recent, twentieth century phenomenon, attributable to the extraordinary versatility and relatively low cost of conventional petroleum-based feedstocks. Built for the long haul, these environmentally recalcitrant plastics are being produced at alarming rates to meet the ever-expanding needs of an `Information Age'

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society. Annual rates of production have been estimated at over 91 billion kg in 2000, with over 36 billion kg being produced in the United States (Stevens, 2002). The packaging industry represents one of the largest areas of plastics growth, utilizing the four major commodity resins, polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC) in a variety of applications including films, flexible bags, and rigid containers (Mohanty et al., 2000). These conventional plastic materials have the advantages of reasonable cost, strength, and durability, qualities that have contributed to their accumulation and associated ecological and environmental concerns. Although recycling efforts have evolved in response to the introduction of an enormous variety of reusable plastic products, daunting quantities of single-use, non-recyclable plastic materials continue to fill landfills and incinerators throughout the United States. Each year, over 10 million tons of plastics are discarded as waste in the United States and Europe (Mohanty et al., 2000). Ignoring the obvious waste disposal-associated problems, incineration of plastic waste brings about secondary environmental pollution with the production of potentially harmful gases and financial expenditures associated with incinerator wall corrosion, due to the enormous heat required for the process (Kim et al., 2000). Growing environmental awareness and emerging global concerns over limited fossil fuel reserves have prompted the search for novel polymeric materials and production processes that draw from sustainable, renewable feedstocks and enhance the environmental quality of the associated products. Over the last decade, replacement of conventional, environmentally persistent, petroleum-based plastics with the `green generation' of biodegradable plastics has become an important research priority. Recent heightened interest in the arena of biodegradable polymers is highlighted with the increase in worldwide consumption of biodegradable polymers from 14 million kg in 1996 to 68 million kg in 2001 (Gross and Kalra, 2002). However, when compared to over 70 years of research and development devoted to improving the production and performance of synthetic petroplastics, scientific progress regarding biobased and biodegradable polymers is clearly in its early stages with current production costs and a lagging consumer acceptance for the more expensive `green' generation of products hindering advancement. Other important factors that discouraged commercialization of degradable polymers during their initial appearance in the late 1980s and early 1990s, were the confusion and misunderstandings associated with the lack of clear, well-defined standards for industries and governmental agencies to evaluate and confirm degradability claims. The establishment of unambiguous, scientifically credible degradability standards and relatively recent advances in polymer chemistry and biotechnology, have helped to establish a growing degradable plastics industry. Several large national and international companies have advanced laboratory research and novel production processes to commercialize renewable feedstockbased, degradable polymers on a global scale (Narayan and Pettigrew, 1999).

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9.2

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Cellulose: as a platform substrate for degradable polymer synthesis

9.2.1 Structural considerations and derivatized forms of cellulose Cellulose, a polysaccharide composed of -D-glucopyranosyl subunits, represents the most significant component of forest biomass, and by mass, is the most abundant natural biopolymer in the world. The most significant cellulosic applications are in the paper, wood product, textile, film, and fiber industries. Many useful properties stem from unique functional characteristics related to the chemical structure of cellulose. These structural properties include an extended, planar chain conformation and oriented, parallel-chain packing in the crystalline state. The absence of branches in this 100% linear polymer contributes to efficient chain packing in the native crystalline state, resulting in stiff, dimensionally stable fibers. Cellulose fibers thus exhibit a high degree of crystallinity (upwards of 70%) when isolated and purified. However, cellulose fibers present in native woody biomass exhibit approximately 35% crystallinity, due to the presence of other lignocellulosic components. The free hydroxyl groups provided by the -D-glucopyranose subunits allow for inter- and intrachain hydrogen bonding, as well as for derivatization of the parent cellulose molecule. Derivatization reactions include selective carboxymethylation, acetylation, and esterification of the hydroxyl moieties. These modified forms of cellulose can be tailored to exhibit particular physical and chemical properties by varying the pattern and degrees of substitution within the cellulose backbone. The natural cellulosic carbon skeleton can be utilized in two major applications on an industrial scale. The first is as regenerated or mercerized cellulose (cellulose II, Rayon), which is not moldable and is used only for film and fiber production. The second represents a broader class of applications, which employs chemically modified celluloses, principally the cellulose esters (e.g. cellulose acetate (CA) and cellulose acetate butyrate (CAB)). Esterification of the cellulose backbone provides structural changes that allow for a greatly expanded range of applications, not available to the parent polysaccharide. Commercially available forms of cellulose acetate have degrees of substitution between 1.7 and 3.0 and are generally plasticized when used in thermoplastic applications (Mohanty et al., 2000). Plasticizing (to reduce the melting temperature, Tm) is critical, as the melt processing temperature often exceeds the decomposition temperature of the cellulose esters because of the thermal lability of the parent polysaccharide backbone. Recently, citric esters (e.g. triethyl citrate and acetyl triethyl citrate) have been introduced as biodegradable plasticizers that form miscible blends with cellulose acetates (Amass et al., 1998). These plasticized forms of cellulose acetate are reported to have reduced tensile moduli, increased elongation at break, and increased rates of biodegradation.

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Cellulose acetates are currently used in a variety of film and coating applications (e.g. low solids solvent-borne coatings for metal and automobile industries), but with tensile strengths similar to polystyrene, the resins are also injection-moldable. Plastics applications are numerous and widespread for cellulose esters owing to rigidity, moisture vapor permeability, grease resistance, clarity, and appearance (Edgar et al., 2001). Physical and mechanical properties displayed by the cellulose esters make these resins conducive to extrusion and molding, provided that the narrow thermal processing window is considered (or typically broadened by the addition of a plasticizer during melt-processing). Consumer items including clear adhesive tape, eyeglass frames, and textiles have been produced from esterified cellulose (Mohanty et al., 2000). Previous and expanding pharmaceutical applications of biodegradable cellulose esters include use as supports for controlled release of drugs, as hydrophobic matrices for sustained release of active compounds, and as enteric coatings for oral administration (Wu et al., 1997; Edgar et al., 2001). Cellulose esters have also been applied agriculturally for the controlled release of herbicides and pesticides and continue to find applications in composites and laminates due to the adhesiveness of these materials to other natural fillers (Edgar et al., 2001).

9.2.2 Biodegradability considerations: cellulose-based polymers Biodegradation or compostability of cellulose esters depends upon the chemical structure of the polymer, as well as physical and microbial characteristics of the environment in which the end product material is placed for ultimate degradation. The chemically modified cellulose esters are degradable only under certain circumstances, as more recalcitrant, hydrophobic ester groups replace the native glucopyranosyl hydroxyls (to varying degrees) in the esterification procedure. Structurally, the degrees of substitution and C-2 hydroxyl substitution patterns are important criteria in predicting biodegradation patterns for these polymers (Amass et al., 1998). Biodegradation rates of cellulose esters generally increase with decreasing degrees of acetate substitution. Buchanan et al. (1993) postulated that this effect is explained by changes in the balance of hydrophobicity to hydrophilicity, with some additional influence by the site of substitution. More recent studies of ether-substituted celluloses (Seneker and Glass, 1996) also support the importance of substitution patterns on biodegradation rates, demonstrating that a five or six-segment run of unsubstituted C-2 sites on the glucopyranosyl subunits is associated with an increased rate of degradation. Although biodegradation can be promoted by designing polymers with degradable chemical substituents, structures, and additives, this `green' plastic design strategy must also include the appropriate disposal infrastructure. The composting and bioconversion infrastructure for transforming and reprocessing

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all forms of biodegradable polymers must expand as the market for this `green' generation of materials continues to expand. There are several important aspects of the waste-disposal/degradation environment that must be considered when evaluating biodegradability of polymeric products. Plastics often degrade by several mechanisms (e.g. oxidative, hydrolytic, and photo-induced degradation) consecutively or simultaneously, and the following discussion focuses on microbially mediated degradation. The environmental conditions at the disposal site must be suitable for microbial metabolism and also for proliferation since the active degrading biomass of organisms in nature is often initially present in low numbers. The degradation environment must therefore contain sufficient nutrients, water, oxygen, trace elements, sub-threshold levels of toxins, and appropriate physical conditions (temperature, pH, etc.) to allow for microbial enzymatic activity and/ or growth. The materials to be degraded must also be bioavailable and accessible to microbes or associated extracellular degradative enzymes (and not sequestered or adsorbed to sites inaccessible to bacteria and fungi). Obviously, the above discussion assumes that there exists an active organism/enzyme that will catabolically transform the polymer molecules, but this may not be the case in inappropriate disposal environments. In some instances, degradative enzymes are inducible enzymes that are up-regulated following exposure to a particular polymeric compound (Alexander, 1999). For these reasons, it is important to dispose of the `biodegradable' plastic article in a microbially rich environment that will facilitate breakdown over a reasonable, defined period of time. Ensuring that appropriate and efficient disposal infrastructures are established to accommodate a growing market of biodegradable plastic products is an important scientific, industrial, and legislative priority. Composting is one of the most environmentally sound waste disposal approaches that recycles biodegradable waste into CO2, H2O, and nutrient rich humus. Composting biodegradable polymers along with yard, food, and agricultural wastes, thus has the potential of producing large volumes of high quality soil amendment products that can contribute to completing a closed loop of use, disposal, and re-use of annually renewable resources. The composting infrastructure is growing in the United States, with 1999 estimates showing close to 3,000 facilities composting yard waste, 150 composting sludge, 30 composting food and processing waste, and 20 composting mixed waste (Narayan and Pettigrew, 1999). Disposal environments range from microbially rich compost, soil, and sewage sludge sites to freshwater and marine environments, dry/wet landfills, or anaerobic digestion systems. The kinetics of biodegradation will differ significantly in these environments, which illustrates the importance of defining standard tests and assays, specific to the degradation environment that can be used to evaluate, compare, and confirm the degradability of an ever-expanding variety of new biodegradable products. For almost a century, cellulose esters have been used commercially in a variety of polymeric applications because of their useful physical and

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mechanical properties and production based on the natural, renewable cellulose polysaccharide backbone. Production costs of cellulose esters are relatively high because these polymers are constructed from highly purified cellulose and the manufacturing steps require processing of high molecular weight intermediates in solution, which mandates larger equipment and higher associated capital costs (Edgar et al., 2001). Despite these technical factors and the somewhat narrow thermal processing window, cellulose esters and other cellulose-based derivatives should continue to find new applications, as research and development of bio-based and biodegradable alternative polymeric feedstocks are becoming a global priority.

9.3

Hemicellulose and its application as a feedstock for biodegradable polymers

9.3.1 Composition of the hemicellulosic fraction of woody biomass In contrast to the homogeneous cellulosic polymer backbone composed of identical and repeating -1,4 glucopyranosyl subunits, hemicelluloses are generally heteropolymeric and composed of more diverse saccharide monomers and compositional arrangements, including numerous side chains on an otherwise linear backbone. The array of monosaccharide constituents found in terrestrial hemicellulose includes pentoses (e.g. D-xylose and L-arabinose), as well as hexoses (e.g. D-glucose, D-galactose, and D-mannose). Hardwood and softwood hemicelluloses often include an additional variety of uronic acids including 4-O-methyl-D-glucuronic, D-glucuronic, and D-galacturonic acids, depending upon the tree species. The monomeric sugars and uronic acids present in hemicellulose are polymerized (generally 150±200 sugar residues/molecular backbone) into an array of glycans including glucans, xylans, mannans, galactans, and glucuronides, with molecular masses that are generally lower than those of the extended cellulose chains (Coughlan and Hazelwood, 1993). The chemical composition of woody biomass and the individual lignocellulosic components differ among the softwood and hardwood species. In temperate locations, softwoods tend to accumulate more lignin (25±35% lignin in softwoods; 18±25% lignin in hardwoods) and mannose than hardwoods, with lower amounts of xylose (Coughlan and Hazelwood, 1993). The predominant form of hemicellulose in softwood tree species is a partly acetylated galactoglucomannan (20%), with a smaller fraction of xylan (10%) (Coughlan and Hazelwood, 1993). Hardwood species are distinguished by a relatively high content of a partly acetylated, acidic xylan (i.e. O-acetyl-4-Omethylglucuronoxylan comprises 20±35% of the hardwood biomass) and a small quantity of the glucomannan type of hemicellulose (Coughlan and Hazelwood, 1993) (Fig. 9.2). Hardwood hemicellulose is rather highly acetylated, with an

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9.2 Molecular structures of the predominant forms of hemicellulose in hardwood and softwood tree species.

average of 7 acetyl moieties per 10 xylose residues, as compared to 2±3 acetyl groups per ten-residue glucomannan molecule in softwood hemicellulose (Coughlan and Hazelwood, 1993). This high degree of acetylation in hardwood hemicellulose must be accounted for in acid hydrolysis and detoxification/prefermentative processing procedures, as it contributes a high degree of acidity and an inhibitory acetate load to subsequent fermentation processes.

9.3.2 Pretreatment of hemicellulose for fermentative application Most biotechnological applications of hemicellulose utilize acid and/or solventbased hydrolysates, in which the native xylan is cleaved to xylose and xylose oligomers. This saccharification can be achieved through such chemical means, but also by physical and/or enzymatic methods. Forest biomass is often treated with dilute acid or enzymatic hydrolysis as a pretreatment for fermentative application of the hemicellulosic fraction. Dilute acid hydrolysis generally uses low concentrations of mineral acids (H2SO4, HCl, 2±5%) at high temperatures (e.g. 150±170 ëC) and pressures (10 atm) (Sun and Cheng, 2002). Concentrated

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acid hydrolyses allow for lower operating temperatures and generally high yields, although carry the disadvantages of costly acid consumption, acid recovery processes, and equipment corrosion (Galbe and Zacchi, 2002; Jones and Semrau, 1984). Thus, dilute acid hydrolyses are more often used to convert xylan in wood chips to xylose, although the high temperature and low pH conditions required in these processes generally result in the generation of toxic sugar decomposition products. These products include the furan toxins, furfural and 5-OH-methylfurfural (5-HMF), produced by decomposition of pentoses and hexoses, respectively. These furfuraldehyde compounds have been found to be toxic and inhibitory to many fermenting microorganisms (Jeffries, 1983) and must therefore be removed or reduced in concentration via detoxification procedures prior to fermentation. Steam explosion of wood chips is also an effective pre-treatment method where conditions of high temperature and pressure, followed by sudden pressure release, lead to disruption of the lignocellulosic structure. Following the steam explosion process, cellulose fibers are rendered more accessible to physical removal or cellulolytic enzymes, while the hemicellulosic fraction is hydrolyzed to varying degrees based on the severity of treatment conditions. This pretreatment process has been favored for the associated environmental advantages, including relatively low energy requirements and minimal use of chemicals (Heitz et al., 1991; Vlasenko et al., 1997). Often these steam-exploded hydrolysates contain incompletely hydrolyzed xylose oligomers, which require further saccharification by additional chemical or enzymatic methods. There are a variety of xylanase enzymes produced by bacteria and fungi that can be applied in hemicellulose pre-treatment processes to hydrolyze the glycosidic bonds in xylan and xylose oligomers. The three major types of xylanase enzymes are -xylosidases (hydrolyze xylobiose and small xylose oligomers), endoxylanases (hydrolyse internal glycosidic bonds, ultimately leaving a mixture of xylose, xylobiose, and xylotriose), and exoxylanases (cleave external xylose residues with product inversion) (Reilly, 1981). These xylanolytic enzymes can be used to increase the xylose monosaccharide content of hydrolysates intended for fermentative bioconversions, as well as for selective removal of xylan from cellulose fibers. Even following chemical and enzymatic pretreatment of hemicellulose, resistant linkages (e.g. 4-O-methyl-D-glucuronic acid in glucuronoxylan) can sometimes persist in the xylan product, which decrease the bioavailability of the associated xylose or xylose oligomers to the fermentative microorganism. In these instances, treatment with additional enzymes (e.g. glucuronidases) may be necessary to increase the fraction of free xylose available for microbial fermentation (Timell, 1962). Detoxification procedures are often required for acidic and steam-exploded lignocellulosic hydrolysates, as variable concentrations of toxic furans (furfural and 5-HMF), lignin degradation compounds (aromatics and phenolics including vanillin, catechol, guaiacol, ferulic acid, syringaldehyde, and 4-hydroxy-

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benzaldehyde), and inhibitory organic acids (formic, acetic, and levulinic acids) are produced during the hydrolysis (Larsson et al., 1999). Thermal degradation of 5-HMF produces levulinic acid and formic acid is generated by degradation of both furfuraldehydes (Mussatto and Roberto, 2004b). Toxicity of these compounds usually depends on the concentration of inhibitory substances, tolerance of the fermenting microorganism to the substance(s), and general culture conditions (temperature, pH, dO2, etc.). Cantarella et al. (2004) determined the effects of several chemicals in the three main classes of toxins mentioned above, on subsequent enzymatic hydrolysis and simultaneous saccharification and fermentation (SSF) for the production of ethanol by Saccharomyces cerevisiae. Vanillin (0.5 g/l) was found to be the most potent inhibitor in SSF when compared to similar concentrations of 5-HMF and acetic acid. Longer lag phases and the most pronounced reduction in fermentation productivity were found when higher concentrations of acetic acid (i.e. 2 g/l) were used, while levulinic and formic acids at 1 g/l were found to reduce ethanol production by 38% and 48%, respectively. Efficient and selective removal or substantial dilution of vanillin and the organic acid inhibitors found in steam-exploded hydrolysates were concluded to be the most important pretreatment considerations for improving process productivity. Although the preceding study artificially amended hydrolysates with known toxins, actual concentrations of these fermentation inhibitors in the final steamexploded and acid-catalyzed hydrolysates depend on the severity of pretreatment conditions and the type of biomass being processed. Generally, removal or conversion of these inhibitory compounds is the most efficient pretreatment procedure for subsequent fermentation and bioconversion of the hydrolysate. There are a variety of physical, chemical, and biological detoxification procedures (Mussatto and Roberto, 2004b) that effectively reduce the concentration of toxic compounds or convert these substances to nontoxic derivatives. Biological pretreatments include the use of specific microorganisms or oxidative enzymes that remove or transform toxic substances in the hydrolysates to innocuous forms (Mussatto and Roberto, 2004b). Jonsson et al. (1998) demonstrated efficient detoxification of willow hydrolysates using laccase (a phenol oxidase) and lignin peroxidase enzymes isolated from the white rot fungus, Trametes versicolor. The mechanism of toxin removal was postulated to involve oxidative polymerization of monoaromatic phenolic compounds, which were identified as potent inhibitors of hemicellulosic fermentation. Physical methods of detoxification include roto-evaporation, which has been demonstrated to reduce the concentrations of volatile inhibitory compounds including the furfuraldehydes, acetic acid, and a variety of the lignin-derived aromatic compounds, which can impede microbial growth, bioconversion, and metabolic activity of the fermenting organism (Converti et al., 2000; Mussatto and Roberto, 2004b). This evaporative procedure reduces the volume of

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hydrolysate and can thus concentrate the contained sugars and non-volatile inhibitors including lignin degradation products and extractives (Mussatto and Roberto, 2004b). Hydrolysates may also be detoxified by chemical means including acid/base treatments (NaOH, Ca(OH)2, H2SO4, etc.) for pH-dependent ionization of certain inhibitors (Mussatto and Roberto, 2004b), which may be the basis for the detoxification effect associated with overliming hemicellulosic hydrolysates (adding Ca(OH)2 to a pH of 9±10). Some substances may be selectively precipitated and removed from the hydrolysates by filtration. Activated charcoal, diatomaceous earth, and ion exchange resins have been used to adsorb certain toxic compounds (Mussatto and Roberto, 2004b), with activated charcoal proving to be the most efficient and cost-effective method in several studies analyzing ethanol and xylitol production from detoxified hemicellulosic hydrolysates (Gong et al. 1993, Dominguez et al., 1996, Ribeiro et al., 2001). Combinations of the above-described treatments have been applied to detoxify hydrolysates. For example, Converti et al. (1999) used the combination of initial overliming (to pH 10), followed by H2SO4 (to pH 5.5), and activated charcoal to efficiently detoxify and ferment xylan hydrolysates to xylitol. The conditions associated with activated charcoal treatment have a significant bearing on the efficiency of adsorption and associated detoxification. Mussatto et al. (2004a) determined that optimal conditions for activated charcoal detoxification include 60 minutes of contact time at a pH of 2.0, 150 rpm, and 45 ëC, which produced significant removal of color and lignin degradation products for subsequent xylitol production. Following thorough hydrolysis and detoxification of the hemicellulosic fraction of woody biomass, this paper/pulping byproduct stream can be utilized in microbial fermentations for the production of a variety of value-added products. Detoxified hemicellulosic hydrolysates have been used as xylose-rich feedstocks in a variety of biotechnological applications including the microbial production of ethanol, xylitol, and biodegradable PHA polymers. Production of PHAs based on renewable, bio-based substrates could make PHA-derived thermoplastic products more economically competitive with petroleum-based plastics, as the major costs in PHA production are the carbon source and the separation process (Byrom, 1987). The next sections describe the history of research and development underlying this family of degradable microbial polyesters and discussion regarding the P(3HB-co-3HV) copolymer and its production from forest-based feedstocks.

9.3.3 History of microbial polyhydroxyalkanoates as biodegradable polymers Poly- -hydroxyalkanoates (PHAs) represent an intracellular carbon and energy storage reserve synthesized by a variety of microorganisms when carbon sources

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are provided in excess and growth is impaired by the lack of at least one other nutrient (Anderson and Dawes, 1990; Steinbuchel and Fuchtenbusch, 1998). Recently, PHAs have received increased attention because of their thermoplastic or elastomeric properties that resemble those of petroleum-based plastics, yet are completely biodegradable in the environment (Holmes, 1988). Thus, not only are the PHAs synthesized biologically, these alternative polymeric materials are also capable of being converted to the harmless degradation products of CO2 and H2O through natural microbiological degradation (Imam et al., 1999). In addition to compostability, PHA polyesters are also recyclable, similar to the petrochemical-derived thermoplastics (Madison and Huisman, 1999). The identification of poly-3-hydroxybutyrate (P(3HB)) as an intracellular reserve material in Bacillus megaterium by Lemoigne in 1927 marks the origin of PHA research. Scientific advancements outside of detection and cell-content estimation methods were limited over the next thirty years, until the late 1950s when interest in the microbial P(3HB) polymer began to increase markedly (Braunegg et al., 1998). The next fifteen years allowed for important research discoveries to evolve in the field of PHAs and by the early 1970s, the knowledge base was well established but directed primarily towards the polymer role as a physiological storage polymer for microorganisms. The usefulness of PHAs as biodegradable alternatives to petroleum-based commodity materials was not well-recognized and investigated until the threats of rising and unstable oil prices created a negative outlook for the petroleum-based polymer industry and created a niche for plastics derived from alternative feedstocks (Braunegg et al., 1998). Prompted by the oil crisis of the 1970s, ICI developed a large-scale fermentation process to produce P(3HB), the most common and well-characterized microbial PHA. Employing a two-stage, batch fermentation process, using a sugar-based feedstock and the bacterium Ralstonia eutropha (formerly Alcaligenes eutrophus), P(3HB) was produced despite a relatively high cost of extraction and poor processability (compared to conventional commodity plastics following oil price stabilization) (Holmes, 1988). Homopolymers of P(3HB) are highly crystalline and brittle, resulting in a rather limited range of applications (Doi, 1990; Holmes, 1988). Because of these limitations, investigations subsequently focused on the synthesis of a copolymer consisting of 3HB and 3-hydroxyvalerate to create poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)), a polyester with increased strength and more flexible mechanical properties conferred by its monomeric composition (Holmes, 1988). The chemical structure of the P(3HB-co-3HV) copolymer is depicted in Fig. 9.3. Pilot production of packaging films, fibers, and containers was accomplished using extruding and molding processes, although at a high process cost (partly due to isothermal degradation of P(3HB) from 170±200 ëC and the very slow crystallization rate of PHAs) (Amass et al., 1998). ICI subsequently improved the fermentation and downstream processing in efforts to begin large-scale,

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9.3 Chemical structure of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer (P(3HB-co-3HV)), where x and y refer to the number of 3-HB and 3-HV comonomer subunits in the polymer chain, respectively.

commercial production of the P(3HB-co-3HV) copolymer, as a biodegradable substitute for petroleum-based polyolefins in films, containers, and bottles (Holmes, 1985). In 1983, prompted by the biodegradability and renewable feedstock-based production associated with P(3HB-co-3HV), ICI began the first commercial production of the copolymer (containing 0±24 mol% 3HV) under the trade name BiopolÕ (Mohanty et al., 2000). Prohibitively high fermentation costs, production expenses, and the relatively low prices of commodity petroplastic resins made commodity-scale production of P(3HB-co-3HV) impossible. Subsequently, Monsanto began production of BiopolÕ (until 1998) and has also achieved experimental production of PHA polymers in transgenic plant cells. Wella AG, Darmstadt (Germany) began blow-molded production of shampoo bottles in 1990, successfully marketing these products due to environmentally conscious legislation and consumer acceptance of the relatively high product cost (Mohanty et al., 2000). Metabolix acquired the BiopolÕ rights from Monsanto in 1998, adding this copolymer to their existing family of PHAs, and recently obtained a Department of Energy grant to develop a transgenic PHA production system in switchgrass. Several other companies are developing novel PHAs and production processes, including Procter & Gamble with their recent NodaxTM addition to the broad class of PHAs (Narasimhan and Green, 2004). NodaxTM represents a family of PHAs consisting of copolymers containing 3HB and low levels of 3-hydroxyhexanoate and 3HB with low levels of longer side chain PHA monomers (i.e. greater than six carbons). These copolymers are useful alone or in combination with other polymers and have been used to produce films, fibers, and injection-molded articles (Bond, 2004)

9.3.4 Physical and chemical properties of PHA polymers Physical and chemical properties of the P(3HB-co-3HV) copolymer can be controlled by varying the mol percentage of 3HV, with greater elasticity conferred by an increased HV content. The industrially important monomeric composition of PHA copolyester can be regulated through a judicious choice of

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microorganism, substrate to cosubstrate ratios, and general fermentation conditions. The physical and mechanical properties of P(3HB), similar to those of polypropylene, are those of a thermoplastic, although P(3HB) is somewhat more crystalline and thus more stiff and brittle (Doi, 1990). The incorporation of 3HV into the P(3HB-co-3HV) copolymer chains improves the associated physical properties and decreases the crystallinity, making the plastic more flexible, with greater extension to break percentages (Doi, 1990). The lower melting temperatures achieved with higher mol % 3HV compositions broadens the thermal processing margin of safety, as this phenomenon allows for meltprocessing temperatures that can be sufficiently lower than the corresponding thermal decomposition temperatures (i.e. Tdecomp., which represents the temperature at which random chain scission and loss of polymer molecular mass occurs). Table 9.1 displays the melting temperature (Tm), glass transition temperature (Tg), and degree of crystallinity for the P(3HB) homopolymer and the related copolymer containing 20 mol % 3-hydroxyvalerate (P(3HB-co-20 mol % 3HV)), with the petroleum-based polymers, polypropylene and lowdensity polyethylene (LDPE), as references. The process, culture conditions, and substrate to cosubstrate ratio control afforded by the conventional microbial fermentation used to produce PHA, allows for the industrially important ability to control physical characteristics of the PHA polymers, especially the Tm and Tg. Mechanical characteristics of the P(3HB-co-20 mol% 3HV) copolymer are also improved relative to the P(3HB) homopolymer, as Young's modulus (stiffness) is decreased from 3.5 GPa to 0.8 GPa and elongation at break is increased from 5% to 50%, respectively (Sudesh et al., 2000). Compositional control (and the influence on physical properties) is important for copolymers, as some polymer applications may require more flexible and elastic polymers, while others may require more inelastic and rigid materials. Due to the lower melting/processing temperatures, improved thermal stability, and enhanced mechanical properties, P(3HB-co-3HV) copolymers are conducive to an expanded range of commercial applications. The broad class Table 9.1 Physical characteristics of polyhydroxyalkanoate polymers and petroleum-based plastics, including melting temperature (Tm), glass transition temperature (Tg), and degree of crystallinity (% crystall.) Polymer P(3HB)1 P(3HB-co-20%3HV)2 Polypropylene3 Polyethylene (LDPE)4 1,2 3,4

Tm (ëC)

Tg (ëC)

% crystall.

177 145 176 130

4 ÿ1 ÿ10 ÿ36

60 56 50±70 20±50

Data for P(3HB) and P(3HB-co-20%3HV) obtained from Madison and Huisman (1999). Data for polypropylene and low density polyethylene obtained fromTsuge (2002).

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of PHA polymers display physical and mechanical properties ranging from hard crystalline plastics to elastic rubbers, with melting temperature profiles that allow for commercial extrusion, injection molding, and fiber spinning to create a variety of value-added products (Sudesh et al., 2000). Other PHAs with longer side chains, referred to as medium side-chain length-PHAs, have also been produced for other applications due to much lower levels of crystallinity and more elastic mechanical properties (Gross et al., 1989). The lower degrees of crystallinity in this class of PHA polymers and copolymers is attributable to the longer side-chain functionalities of monomers comprising the backbone, which lead to less efficient chain packing in the crystalline lattice.

9.3.5 Biodegradation of PHA polymers As mentioned earlier regarding cellulose esters, the composition and structure of the polymer, as well as the nature of the degradation environment, will dictate the mechanisms and kinetics of decomposition. Under the current waste disposal infrastructure, the most likely environments for terminal degradation of PHA products would be municipal landfills and compost piles. Yue et al. (1996) studied the compostability of P(3HB) and P(3HB-co-20 mol % 3HV) in simulated municipal solid waste bioreactors, maintained at a constant temperature and moisture content of 55 ëC and 54%, respectively. Kinetics of biodegradation varied dramatically over the 24-day experiment, (maximum rates of weight loss, 50±55 g/mm2, observed between days 10 and 15) possibly due to variations in the numbers and populations of PHA-degrading microorganisms and/or increased polymer surface area due to fragmentation and pitting. Additionally, it was determined that the P(3HB-co-20 mol% 3HV) copolymer displayed a much higher rate of degradation than the P(3HB) homopolymer. These findings were consistent with those documented in another study showing greater rates of weight loss for P(3HB-co-4HB) copolymer films relative to P(3HB) homopolymers, by extracellular P(3HB) depolymerase isolated from Alcaligenes faecalis (Doi et al., 1990). This depolymerase enzyme was found to catalyze polymer degradation primarily by surface dissolution. Accelerated degradation of copolymer samples may relate to the structural imperfections caused by comonomer side chains, which contribute to lower degrees of crystallinity and potentially more facile access by microbially derived extracellular PHA depolymerase enzymes. Using both aerobic and anaerobic waste landfill bioreactors, Ishigaki et al. (2004) compared the degradation of several types of commercial biodegradable polymers including P(3HB-co-8 mol% 3HV) film samples. Anaerobic burial of the PHA samples resulted in no significant weight loss or film breakage over the 120-day study period. Forced aeration at 100 ml/min. caused a dramatic improvement in degradation rate, with initial sample weight reduced by almost 100% and significant film fragmentation was observed during the 120-day

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experiment. The populations of PHA-degrading microorganisms were determined to be much higher in the aerobic reactors and Ishigaki et al. (2000), in a previous study, suggest that the inherent biodegradation activity of the aerobic microbes was generally much higher than that of the anaerobic population. The physical characteristics of the disposal environment can have significant bearing on the kinetics of PHA polymer degradation by influencing the nature and activity of the associated microbial populations. Kim et al. (2000) characterized the degradation profiles of three commercially available biodegradable polymers including P(3HB), by incubating film samples in four different soil types (forest, sandy, activated sludge, and farm soils) inoculated with seven P(3HB)-degrading fungi at 28 ëC, 37 ëC, and 60 ëC. Biodegradation of P(3HB) (recorded as % weight loss) was greatest in activated sludge at 37 ëC, with four of five films almost completely removed after 25 days of incubation. Incubation in forest and sandy soils resulted in <10% weight loss over this period, most likely due to reduced microbial populations and restricted diversity in less favorable soil conditions.

9.3.6 PHA production from forest-based feedstocks: history and economics Generally, PHA production costs have not been economically competitive with conventional, petroleum-based commodity resins. This is the major factor hindering widespread commercialization of these biodegradable polymers. Initial commercial production cost of Biopol (P(3HB-co-3HV)) using the natural producer, Ralstonia eutropha with glucose and propionic acid as substrates, was estimated at US$16/kg PHA (Lee, 1996; Reddy et al., 2003). Lee (1996) determined the cost and specific yields of various substrates on production cost of P(3HB). Based on glucose or sucrose, with an approximately 40% yield of P(3HB)/g substrate, these substrates were calculated to cost US$1.35/kg of P(3HB) produced. Therefore, the use of glucose as the substrate leads to associated PHA production costs that are high compared to commodity petroplastics such as polypropylene (PP), for which the cost of carbon feedstocks (polypropylene substrate monomers) and market price are approximately US$0.185/ kg and ~US$1/kg, respectively. Thus, further reductions in overall PHA production cost (Biopol Õ was 16±18 fold more expensive than polypropylene) are mandated to approach those of non-degradable petroleum-based resins and broaden the market for PHA polymers. Although it is not reasonable to expect an instantaneous and complete replacement of petroleum-based polymeric materials, the unique features of PHAs including biodegradability, biocompatibility, and production from renewable feedstocks, create a variety of specialty applications, which have become the current market target (Luzier, 1992; Braunegg et al., 1998). The application potential and environmental

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benefits associated with PHAs as alternative polymers warrant research directed towards furthering strategies for more cost-efficient production. When determining total PHA production cost, the equation must include the expenses of substrates, fermentation, product recovery/isolation, and processing procedures. The efficiency of product recovery is strongly influenced by the degree of intracellular PHA accumulation relative to the biomass produced, with higher polymer contents leading to lower recovery costs. For example, Choi and Lee (1997) calculated a recovery cost of US$4.80/kg for a microbial system accumulating P(3HB) to a level comprising 50% (w/w) of the biomass and compared this to a much lower cost of US$0.92/kg for a similar process producing an 88% (w/w) accumulation of polymer. Recovery costs are dependent on the efficiency of polymer accumulation primarily because of the solvents and procedures required to extract polymer from cellular biomass and debris. Prior to 1999, the highest concentration of P(3HB-co-3HV) copolymer obtained was 117 g/l, with a 3HV content of 4.3 mol% (Kim et al., 1994). However, Choi and Lee (1999) recently reported a copolymer yield of 158.8 g/l using a strain of recombinant Escherichia coli harboring cloned PHA biosynthetic genes from Alcaligenes latus and a fed-batch fermentation using conventional P(3HB-co-3HV) substrates, glucose and propionic acid. These data translate into a production cost, based on a 100,000 metric tons per year scale of operation, of US$5.05/kg versus US$9.75/kg based on previous yields. Using an improved extraction procedure, based on NaOH digestion, cost estimates were further lowered to US$3.95/kg (Choi and Lee, 1999). This economic projection represents a cost comparable to that of other biodegradable plastic materials including polylactic acid (PLA) (Reddy et al., 2003). The most significant factor in the overall production cost calculation is the expenditure for substrates (Yamane, 1992). Since substrate costs have been reported to account for up to 50% of the total manufacturing cost (Choi and Lee, 1997), a great deal of research has been devoted to identifying inexpensive and renewable feedstocks for biosynthesis of PHA polymers. For these reasons and the numerous disadvantages associated with petroleum-based polymer production, there are a significant number of universities and industries that are actively investigating PHA production from inexpensive renewable resources such as agricultural, forestry, and food wastes. Substrates such as cane molasses and cheese whey have been estimated to potentially reduce the P(3HB) substrate costs to US$0.52/kg and US$0.22/kg, respectively (Lee, 1996). Based on the magnitude and renewable nature of the largely underutilized hemicellulosic component of forest biomass, PHA production based on this xylan-rich feedstock has also evolved to become an important research objective at several institutions. Utilizing hemicellulosic hydrolysates as the principal carbon source has been reported to significantly reduce the final PHA product cost. The cost comparison study by Lee (1996) reported that although substrate specific yields are

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relatively low (i.e. ~20% P(3HB) yield/g substrate), the use of hemicellulosic hydrolysate (approximate price: US$0.07/kg) as the principal carbon source reduces the final substrate cost to US$0.34/kg P(3HB), a cost roughly 2±4 fold lower than that of glucose. Young et al. (1994) and Ramsay et al. (1995) demonstrated the ability of B. cepacia to convert xylose and hemicellulosic hydrolysate to the P(3HB) homopolymer. The xylose-specific yield was calculated to reduce substrate cost by more than half compared to glucose (Ramsay et al., 1995). Deriving fermentable pentosic sugars from the xylan-rich component of such hemicellulosic hydrolysates was projected to reduce substrate cost to US$76/metric ton of PHB, a cost predicted to result in competitive economics if the xylose-specific yield could be tripled (Lynd, 1989; Ramsay et al., 1995). Currently there are several companies developing more cost-effective and novel PHA polymers, including Procter and Gamble's recent development of NodaxTM, a unique type of PHA copolymer described earlier. Current projections for NodaxTM marketing include 90±900 metric ton scale production in 2004 and delivery plans for 2005/2006 that would make PHA at production costs of approximately US$2.20/kg (Narasimhan and Green, 2004). Improving the technology involved in PHA polymer production has potential to make significant contributions to the `green generation' of plastic products because of the facile biodegradability of PHA through nontoxic intermediates and the potential for more cost-effective, large-scale production by utilizing renewable feedstocks, including waste streams.

9.3.7 Forest-based microbial PHAs Further reductions in overall production costs for PHAs must be attained to compete with those of commodity petroplastics such as polypropylene and polyethylene. Since reduction of substrate cost has the greatest potential for production cost improvement, current research (Keenan et al., 2004) has focused on novel processes by which to create PHA copolymers using totally renewable and inexpensive substrates and cosubstrates. The P(3HB-co-3HV) polymer has been a primary focus, because compared to P(3HB), the copolymer has physical and mechanical properties more conducive to melt-processing and subsequent commercial application. Investigators (Keenan et al., 2005; Nakas et al., 2004) have demonstrated the conversion of steam-exploded (aqueous) and organic solvent-extracted (`organosolv') hemicellulosic hydrolysates (National Renewable Energy Laboratory, NREL, Golden, CO) to P(3HB-co-3HV) copolymers using levulinic acid as a cosubstrate (see Table 9.3). Levulinic acid has been identified as a potentially forest-based, P(3HB-co3HV) cosubstrate (Bozell et al., 2000), which can be combined with xylose to produce P(3HB-co-3HV) (Keenan et al., 2004). Jang and Rogers (1996)

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demonstrated the use of levulinic acid as an organic acid cosubstrate for the production of P(3HB-co-3HV) by Alcaligenes sp. SH-69 from glucose as a principal carbon source. In their experiments, the use of levulinic acid as a cosubstrate for production of P(3HB-co-3HV) was found to enhance growth and PHA accumulation at concentrations from 0.05 to 0.1% (w/v) in shake-flask cultures. Levulinic acid (4-keto-valeric acid) can be produced cost-effectively from a vast array of carbohydrate-containing renewable biomaterials, including cellulose-containing forest and agricultural waste residues, paper mill sludge, and cellulose fines from paper production processes (Bozell et al., 2000; Cha and Hanna, 2002). The Biofine corporation (South Glens Falls, NY, USA) has developed a two stage/reactor process that begins with a dilute mineral acid hydrolysis of hexose sugars as the key step in levulinic acid production and can produce yields upwards of 60%, with minimal byproduct formation (Bozell et al., 2000). Based on the Biofine process, Bozell et al. (2000) estimated the production cost of levulinic acid to potentially fall as low as US$0.09±0.22/kg, and 2004 selling prices for refined levulinic acid in chemical applications have been projected to range from US$0.99±1.21/kg based on a potential 4.5 million kg scale of operation (Fitzpatrick, 2004). The apparent biomass enhancing effects of levulinic acid and subsequent increase in intracellular PHA accumulation, combined with utilization of xylose, also obtained from renewable carbon feedstocks, have been hypothesized (Keenan et al., 2004) to offer potential for significant reduction in the polymer production cost. As a preliminary investigation for the use of forest-biomass as a renewable feedstock for PHA production, the authors have demonstrated the ability of Burkholderia cepacia to produce P(3HB-co-3HV) from xylose and levulinic acid (Keenan et al., 2004). Results showed that the use of levulinic acid as a cosubstrate with xylose in shake-flask cultures of B. cepacia resulted in growth and PHA-accumulation enhancing effects over cosubstrate concentrations increasing to 0.52% (w/v), as can be observed by the associated dry biomass and P(3HB-co-3HV) yield maxima of 9.5 g/l and 4.2 g/l, respectively (Fig. 9.4). The P(3HB-co-3HV) samples produced by these shake-flask cultures were found to contain an average of 43 mol% 3HV, as determined by 300 MHz 1H NMR analyses. Concentrations of levulinic acid exceeding this level increased the 3HV content of the copolymer to 61 mol%, but this effect was accompanied by dramatic declines in both cell and PHA yields. The P(3HB-co-3HV) copolymer is unique among the PHA family in that the structural/size similarity of the 3HB and 3HV constituents and associated crystalline lattices allow for co-crystallization of the monomers in either the HB or HV crystalline lattices. This phenomenon, termed isodimorphism, is rare among polymers and manifests as the pseudoeutectic pattern displayed by a graph of Tm plotted as a function of increasing mol % 3HV composition in a

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9.4 Biomass and P(3HB-co-3HV) yields for shake-flask cultures of Burkholderia cepacia grown on 2.2% (w/v) xylose and various concentrations of levulinic acid. Composition of the copolymers, expressed as the mol% 3HV, is also plotted as a function of the levulinic acid cosubstrate concentration (Keenan et al., 2004).

series of P(3HB-co-3HV) copolymer samples. The physical basis for this `V'shaped Tm response is the HB to HV crystalline lattice transition, generally occurring in the range of 30 mol % 3HV (Bluhm et al., 1986; Bloembergen et al., 1989). Xylose and levulinic acid-based P(3HB-co-3HV) samples characterized by differential scanning calorimetry (DSC) exhibit such a pseudoeutectic pattern, as shown in Table 9.2 (Keenan et al., 2004). Thermogravimetric analysis (TGA) of P(3HB-co-3HV) samples revealed the onset temperature for thermal degradation (Tdecomp.) of the polymer chains to occur 54±100 ëC above the respective Tm, with an average differential of 92 ëC. This relatively wide temperature differential between Tm values and the corresponding onset temperatures for molecular degradation would theoretically allow for a margin of safety in polymer melt-related procedures, thus preserving the molecular mass and associated physical/mechanical properties of the PHA copolymeric products. Molecular mass and associated polydispersity are important characteristics of polymers that determine physical and mechanical properties, and thus dictate the associated range of applications for the ultimate products. Madison and Huisman

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(1999) reported that the molecular mass of PHAs varies by host microorganism between 50,000±1,000,000 Da. This range of polymer mass is sufficient for the associated material characteristics of the PHA to resemble those of conventional commodity resins (Madison and Huisman, 1999). From intrinsic viscosity ([]) measurements, viscosity average molecular weights (Mv) were approximated for P(3HB-co-3HV) samples derived from xylose and levulinic acid (Keenan et al., 2004), using calculations based on the corresponding intrinsic viscosity value (dl/g) and the Mark-Houwink constants reported for the PHB homopolymer, K ˆ 1.18  10ÿ4 dl/g and  = 0.78 (Akita et al., 1976) (Table 9.2). The viscosityderived molecular masses ranged between 469±919 kDa (average Mv ˆ 687 kDa) for the P(3HB-co-3HV) copolymers produced by B. cepacia in shake-flask cultures, and is relatively high compared to other microbially derived PHAs. Thus, these B. cepacia-based P(3HB-co-3HV) samples possess desirable molecular masses for commercial exploitation and the copolymers can also be produced with controllable composition, spanning a wide range of mol% 3HV (Table 9.2). Composition of the P(3HB-co-3HV) copolymer has been predictably controlled by regulating the ratio of levulinic acid to xylose in the fermentation medium, adding cosubstrate doses from 0.07% to 0.67% (w/v) (Keenan et al., 2004). The progressive increase in mol % 3HV is displayed by the expanded and Table 9.2 Physical-chemical characteristics of twelve P(3HB-co-3HV) copolymers containing 0.8±61 mol% 3HV, including melting temperature (Tm), glass transition temperature (Tg), thermal decomposition onset temperature (Tdecomp.), intrinsic viscosity ([]), and viscosity average molecular weight (Mv) Mol% 3HV 0.8 1.4 2.6 15 17 19 20 22 25 28 56 61 1

Tm (ëC)1

Tg (ëC)

174.3 173.7 172.8 166.1 161.4 160.9 157.2 157.9 154.2 154.5 159.5 171.8

2.1 1.6 2.3 0.8 0.5 ÿ0.5 ÿ0.6 ÿ0.8 ÿ0.9 ÿ1.1 ÿ10.6 ÿ11.9

Tdecomp. (ëC)2 [] (dl/g) 273.4 270.4 254.8 254.7 253.7 238.1 249.2 258.8 257.3 259.3 251.2 225.5

3.4 5.4 4.1 4.0 4.7 5.3 4.3 3.2 4.2 4.4 4.5 4.0

Mv (KDa)3 511 919 647 627 779 897 683 469 671 700 721 622

Tm determined to be the peak of the second, higher temperature endotherm in DSC cycles with multiple melting peaks. 2 Tdecomp. represents the onset temperature for thermogravimetric decomposition, arbitrarily defined as the loss of 0.032% of the original sample weight via analysis of the first derivative weight (%/ëC) curve. 3 Mv calculations based on the corresponding [] values and Mark-Houwink constants (K, ) published for the PHB homopolymer and thus represent approximations.

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9.5 Compositionally relevant chemical shift expansions of 300 Hz 1H NMR spectra obtained from P(3HB-co-3HV) samples, illustrating a progressive increase in the mol% 3HV fraction of copolymers produced by Burkholderia cepacia through regulation of the ratio of levulinic acid (cosubstrate) to xylose (substrate) added to the fermentation medium. Chemical structure schematic of P(3HB-co-3HV) displays the methyl side-chain residues of the corresponding monomers, represented quantitatively by the integrated areas of the 3HB doublet (1.27 ppm) and the 3HV triplet (0.88 ppm) (Keenan et al., 2004).

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9.6 Chemical structure and 75.5 MHz 13C NMR chemical shift assignments for the main- and side-chain carbon atoms in a P(3HB-co-20 mol% 3HV) sample produced by B. cepacia from xylose and levulinic acid. Expanded inset displays three distinct carbonyl chemical shifts and integrated peak areas, corresponding to the relative intensities of the different diad sequences (3HB3HB, 3HB-3HV, and 3HV-3HV, from right to left) in this statistically random copolymer (Keenan et al., 2004).

normalized 300 MHz 1H NMR spectra (Fig. 9.5). These peaks correspond to proton resonance of the methyl side-chain functionalities of these monomers and are conventionally used for quantification purposes. The 75.5 MHz 13C NMR chemical shifts and relative intensities of the expanded carbonyl resonance peaks corresponding to the 3HB and 3HV diad sequences (Fig. 9.6) are similar to those reported by Shang et al. (2004) and Bloembergen et al. (1989) for bacterial P(3HB-co-3HV), indicating that the PHA samples characterized in Table 9.2 are statistically random copolymers of these -hydroxyalkanoate monomers (Keenan et al., 2004). Investigators in the authors' laboratory have used the xylan-rich, hemicellulosic waste portion of aspen biomass as the primary carbon source for microbial conversion to PHA by B. cepacia. Specifically, several batches of

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Table 9.3 Physical-chemical characteristics of P(3HB-co-3HV) copolymers produced by shake-flask cultures of Burkholderia cepacia from hemicellulosic hydrolysate and levulinic acid as renewable, forest-based substrates and cosubstrates, respectively Mol% 3HV 16 25 50 52

Tm (ëC)1

Tg (ëC)

Tdecomp. (ëC)2

[]

Mv (KDa)3

156.1 155.7 97.4 103.3

ÿ0.8 ÿ0.6 ÿ7.3 ÿ7.1

224.1 210.9 245.7 254.7

3.73 3.67 3.22 3.68

587 575 569 578

1

Tm determined to be the peak of the second, higher temperature endotherm in DSC cycles with multiple melting peaks. Tdecomp. represents the onset temperature for thermogravimetric decomposition, arbitrarily defined as the loss of 0.032% of the original sample weight via analysis of the first derivative weight (%/ëC) curve. 3 Mv calculations based on the corresponding [] values and Mark-Houwink constants (K, ) published for the PHB homopolymer and thus represent approximations. 2

C5 hemicellulosic hydrolysates, produced by the NREL Clean FractionationTM procedure, have been detoxified by methods related to the TVA detoxification procedure (Strickland and Beck, 1984). With the appropriate addition of an organic acid cosubstrate, levulinic acid, also produced from renewable feedstocks, we have produced P(3HB-co-3HV) copolymers possessing physical and chemical properties that are potentially useful for application as industrial bioplastics. Thermal characteristics and viscosity-derived molecular masses of a set of P(3HB-co-3HV) samples produced microbially from detoxified hemicellulosic hydrolysate and levulinic acid, are presented in Table 9.3. Intrinsic viscosities and Mv calculations (average Mv ˆ 577) for these hemicellulose-based copolymers (Table 9.3) are similar to those of the commercial grade xylose-derived and levulinic acid-derived samples (Table 9.2) and well within the range of molecular masses required for melt-related application. The 16 and 25 mol% 3HV hydrolysate-based samples also exhibited Tm, Tg, and Tdecomp. profiles consistent with the copolymers reported previously (Table 9.2). The higher mol% 3HV samples (50 and 52 mol% 3HV) show comparable Tg and Tdecomp. values, although have much lower melting temperatures. The 50 and 52 mol% 3HV samples were produced and solventcast six to eight months earlier than the 16 and 25 mol % 3HV samples, allowing for a longer period of progressive, secondary crystallization (Bluhm et al., 1986). For these samples, the melting transitions were obtained from the initial heating cycle in DSC analyses, so that higher degrees of crystallinity and variations in crystallite size likely contribute to the lower Tm values, which are most sensitive to these physical variations. The 97 ëC and 103 ëC melting temperatures are consistent with results reported in other studies (Bluhm et al., 1986; Bloembergen et al., 1989) and must be considered as being highly dependent on the crystalline composition of the dynamic PHA copolymer.

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9.4

Sources of further information

9.4.1 Relevant articles and review papers (not included in references) Bertrand, J.L., B.A. Ramsay, J.A. Ramsay, C. Chavarie. 1990. `Biosynthesis of poly- hydroxyalkanoates from pentoses by Pseudomonas pseudoflava'. Appl. Environ. Microbiol. 56: 3133±3138. Lee, S.Y. 1998. `Poly(3-hydroxybutyrate) production from xylose by recombinant Escherichia coli'. Bioproc. Engin. 18: 397±399. Luengo, J.M., B. Garcia, A. Sandoval, G. Naharro, E.R. Olivera. 2003. `Bioplastics from microorganisms'. Curr. Opin. Microbiol. 6: 251±260. Ramsay, B. A., K. Lomaliza, C. Chavarie, B. Dube, P. Bataille, J. A. Ramsay. 1990. `Production of poly-( -hydroxybutyric-co- -hydroxyvaleric) acids'. Appl. Environ. Microbiol. 56: 2093±2098. Ramsay, B.A., J.A. Ramsay, D.G. Cooper. 1989. `Production of poly- -hydroxyalkanoic acid by Pseudomonas cepacia'. Appl. Environ. Microbiol. 55: 584±589. Salehizadeh, H., M.C.M. Van Loosdrecht. 2004. `Production of polyhydroxyalkanoates by mixed culture: recent trends and biotechnological importance'. Biotechnol. Advanc. 22: 261±279. Scott, G. 2000. ` ``Green'' polymers'. Polym. Degrad. Stabil. 68: 1±7. Trotsenko, Y.A., L.L. Belova. 2000. `Biosynthesis of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and its regulation in bacteria'. Microbiology 69: 635±645.

9.4.2 Websites and links to information regarding PHAs and forest biomass http://www.esf.edu/resorg/rooseveltwildlife/Research/biodegplastic/ biodegplastic.htm `Channelling New York State's forest-derived renewable resources towards the microbial production of polyhydroxyalkanoates (PHAs), as biodegradable polymers, etc.' http://www.nodax.com `Procter and Gamble combines the performance of plastics with environmental sustainability, etc.' http://www.metabolix.com `Metabolix applies the cutting edge tools of biotechnology to create a new generation of highly versatile, sustainable, environmentally-friendly plastics, etc.' http://www.unifr.ch/nepswiss/pages/research.html `Metabolic engineering for the synthesis of polyhydroxyalkanoates in plants, etc.'

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www.forestresearch.co.nz/internallink.asp?topic=Added%2520Value%2520 Biomaterials `The waste treatment group at Forest Research investigates bacteria that produce polymers such as polyhydroxyalkanoates (PHA) in the cell, etc.'

9.4.3 Additional considerations for PHA production based on renewable substrates Certain PHA production strategies have been estimated to have higher associated energy input cost and byproduct generation than some of the conventional petrochemical-based processes. Much of the energy input cost relates to the choice of substrate, which can require considerable effort in terms of harvest, delivery, and preparation for subsequent fermentation. For instance, an analysis was presented by Gerngross (2000) that showed some methods of `green manufacturing' may be more energy consuming and polluting than traditional methods. The economic analysis focused on the process by which corn is microbiologically converted to PHAs (e.g. similar to the conventional glucose/ propionic acid recipe for Biopol PHA). In this process, the corn must be grown, harvested, and delivered to a processor for glucose extraction and subsequent fermentation to intracellular PHA. The cells must then be washed, spun down by centrifugation, washed and spun down again, concentrated, and dried to a powder. This process was estimated to consume 19-fold more electricity, 22% more steam, and 7-fold more water than the chemical-based method of manufacturing polystyrene. These estimates included the indirect/latent use of fossil fuels for power generation in manufacturing, net energy use for the production of fertilizers, insecticides, and pesticides used in growing corn, and the energy required for crop harvesting/processing. Gerngross calculated that the production of 0.46 kg of PHA is equivalent to the consumption of 1.09 kg of fossil fuel resources, whereas the production of 0.46 kg of polystyrene through chemical manufacturing requires 1.03 kg of oil. In addition, the polluting effects of the `green approach' are greater because the fermentation process would burn the entire 1.09 kg of oil, in contrast to 0.57 kg burned in the chemical process. However, as Gerngross concluded, `it would be most unfortunate if this study were viewed as a general indictment of biological processing'. It was stated that we now have the tools for the evaluation of environmental impact associated with biological processes and we must recognize that while some strategies may fail, other (innovative) approaches may demonstrate superior performance. Hemicellulosic hydrolysates, when used as the principal carbon source in PHA fermentations, are not a potential food crop but rather represent a substantial waste stream produced by the pulp and paper industry. This eminently fermentable carbon source is at best burned for heat in the plant, but sometimes discarded as an environmental pollutant. By utilizing this C5 stream, derived

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from forest biomass as the PHA production substrate, the process dramatically reduces the energy/labor input associated with the growth and harvest of fermentable sugars. Creating such value-added PHA products thus not only reduces the environmental impact associated with petrochemical-based plastic products, but also minimizes the necessity for hemicellulosic waste discharge. The environmental impact of methods for the biosynthesis of PHA, based on renewable forest resources is minimized at both the level of production and in the associated waste stream reductions.

9.5

Conclusions and future developments

Synthetic polymers were initially designed for durability and resistance to degradation, and have therefore tended to accumulate in landfills. With growing concerns for the environment and limited fossil reserves, investigations regarding biodegradable polymers have become a global priority. Research and development in the field of biodegradable plastics is making significant advancements towards creating compounds that represent an economically viable and environmentally friendly alternative to traditional petroleum-based plastics. Benefits of conversion to this `green generation' of plastics are numerous and warrant continued research into synthesizing novel degradable polymers, developing more efficient production systems, and utilizing biobased, renewable feedstocks for sustainable production systems. Identifying inexpensive, renewable substrates and cosubstrates for PHA production, as well as novel feedstocks/monomers (e.g. tall oils produced as oleic and linoleic acid-rich byproducts from the wood pulping processes) to enhance the economics and physical properties of PHAs will help to advance PHA polymers to greater and more widespread applications. Utilization of renewable resources, such as those derived from forest biomass could contribute to substantial reductions in PHA production cost and creation of novel polymers and production processes. The commercialization of biodegradable polymers should continue to increase, especially in markets where plastic products have very short, single-use applications. The general attitude in society has come to value environmentally `green' products and consumers are slowly becoming to accept paying relatively higher prices for these novel products. This eco-conscious, biodegradable plastic market should only continue to expand, as bio-based resin prices are made more competitive with conventional commodity plastics and consumer misconceptions regarding the instability of `degradable plastics' are clarified.

9.6

References

Aggarwal, P. 1999. Degradation of a starch-based polymer studied using thermal analysis. Thermochimica Acta 340±341: 195±203.

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Alexander, M. 1999. Biodegradation and Bioremediation. Academic Press, New York, USA. Akita, S., Y. Einaga, Y. Miyaki, H. Fujita. 1976. Solution properties of poly(D-ûhydroxybutyrate). 1. biosynthesis and characterization. Macromolecules 9: 774±780. Amass, W., A. Amass, B. Tighe. 1998. A review of biodegradable polymers: Uses, current developments in the synthesis and characterization of biodegradable polyesters, blends of biodegradable polymers, and recent advances in biodegradation studies. Polymer International 47: 89±144. Anderson, A.J., E.A. Dawes. 1990. Occurrence, metabolism, metabolic role, and industrial uses of bacterial polyhydroxyalkanoates. Microbiol. Rev. 54: 450±472. Bloembergen, S., D.A. Holden, T.L. Bluhm, G.K. Hamer, R.H. Marchessault. 1989. Isodimorphism in synthetic poly(û-hydroxybutyrate-co-û-hydroxyvalerate): stereoregular copolyesters from racemic û-lactones. Macromolecules 22: 1663± 1669. Bluhm, T.L., G.K. Hamer, R.H. Marchessault, C.A. Fyfe, R.P. Veregin. 1986. Isodimorphism in bacterial poly(û-hydroxybutyrate-co-û-hydroxyvalerate). Macromolecules 19: 2871±2876. Bond, E.B., Noda, I., Satkowski, M.M. 2004. Nodax PHA: biotechnology at its best. Amer. Chem. Soc. Meeting, Abstract, Anaheim, CA, USA. Bozell, J.J., L. Moens, D.C. Elliott, Y. Wang, G.G. Neuenscwander, S.W. Fitzpatrick, R.J. Bilski, J.L. Jarnefeld. 2000. Production of levulinic acid and use as a platform chemical for derived products. Resour., Conserv. and Recycl. 28: 227±239. Braunegg, G., G. Lefebvre, K.F. Genser. 1998. Polyhydroxyalkanoates, biopolyesters from renewable resources: physiological and engineering aspects. J. Biotechnol. 65: 127±161. Buchanan, C.M., R.M. Gardner, R.J. Komarek, S.C. Gedin, A.W. White. 1993. In Biodegradable Polymers and Packaging. Kaplan, Thomas, Ching (eds). Technomic Publishing Co, Lancaster, PA, USA. Byrom, D. 1987. Polymer synthesis by microorganisms: technology and economics. Trends Biotechnol. 5: 246±250. Cantarella, M., L. Cantarella, A. Gallifuoco, A. Spera, F. Alfani. 2004. Effect of wood inhibitors released during steam explosion treatment of poplar wood on subsequent enzymatic hydrolysis and SSF. Biotechnol. Prog. 20: 200±206. Cha, J.Y., M.A. Hanna. 2002. Levulinic acid production based on extrusion and pressurized batch reaction. Ind. Crops and Products 16: 109±118. Choi, J., and S.Y. Lee. 1997. Process analysis and economic evaluation for poly(3hydroxybutyrate) production by fermentation. Bioprocess Eng. 17: 335±342. Choi, J., S.Y. Lee. 1999. High-level production of poly(3-hydroxybuyrate-co-3hydroxyvalerate) by fed-batch culture of recombinant Esherichia coli. Appl. Environ. Microbiol. 65: 4363±4368. Converti, A., P. Perego, J.M. Dominguez. 1999. Xylitol production from hardwood hemicellulose hydrolysates by Pachysolen tannophilus, Debaryomyces hansenii, and Candida guilliermondii. Appl. Biochem. Biotechnol. 82: 141±151. Converti, A., J.M. Dominguez, P. Perego, S.S. Silva, M. Zilli. 2000. Wood hydrolysis and hydrolysate detoxification for subsequent xylitol production. Chem. Eng. Technol. 23: 1013±1020. Coughlan, M.P., G.P. Hazelwood. 1993. Hemicellulose and hemicellulases. Portland Press, London, UK.

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Doi, Y. 1990. Microbial polyesters. VCH Publishers Inc., Yokohama, Japan. Doi, Y., Y. Kanesawa, M. Kunioka. 1990. Biodegradation of microbial copolyesters: poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybutyrate-co-4hydroxybutyrate). Macromolecules 23: 26±31. Dominguez, J.M., C.S. Gong, G.T. Tsao. 1996. Pretreatment of sugarcane bagase hemicellulose hydrolysate for xylitol production by yeast. Appl. Biochem. Biotechnol. 57/58: 49±56. Edgar, K.J., C.M. Buchanan, J.S. Debenham, P.A. Rundquist, B.D. Seiler, M.C. Shelton, D. Tindall. 2001. Advances in cellulose ester performance and application. Prog. Polym. Sci. 26: 1605±1688. Fitzpatrick, S. 2004. Personal communication regarding potential levulinic acid production costs based on Biofine technology. Biofine Corporation, South Glens Falls, NY, USA. Galbe, M., G. Zacchi. 2002. A review of the production of ethanol from softwood. Appl. Microbiol. Biotechnol. 59: 618±628. Gerngross, T.U. 2000. How green are green plastics. Amer. Chem. Soc. Meeting, Abstract, New Orleans, LA, USA. Gong, C.S., C.S. Chen, L.F. Chen. 1993. Pretreatment of sugarcane bagasse hemicellulose hydrolysate for ethanol production by yeast. Appl. Biochem. Biotechnol. 39±40, 83±88. Gross, R.A., C. DeMello, R.W. Lenz, H. Brandl, R.C. Fuller. 1989. Biosynthesis and characterization of poly( -hydroxyalkanoates) produced by Pseudomonas oleovorans. Macromolecules 22: 1106±1115. Gross, R.A., B. Kalra. 2002. Biodegradable polymers for the environment. Science 297: 803±807. Heitz, M., E. Capek-Menard, P.G. Koeberle, J. Gagne, E. Chornet. 1991. Fractionation of Populus tremuloides at the pilot plant scale: optimization of steam pretreatment conditions using Stake II technology. Bioresour. Technol. 35: 23±32. Holmes, P.A. 1985. Applications of PHB ± a microbially produced biodegradable thermoplastic. Phys. Technol. 16: 32±36. Holmes, P.A. 1988. Biologically produced PHA polymers and copolymers, pp. 1±65. In D.C. Bassett (ed.), Developments in Crystalline Polymers, vol. 2. Elsevier, London, UK. Imam, S. H., S. H. Gordon, R. L. Shogren, T.R. Tosteson, N.S. Govind, R.V. Greene. 1999. Degradation of starch-poly( -hydroxybuyrate-co- -hydroxyvalerate) bioplastic in tropical coastal waters. Appl. Environ. Microbiol. 65: 431±437. Ishigaki, T., T. Kawagoshi, M. Ike, M. Fujita. 2000. Abundance of polymer degrading microorganisms in sea-based solid waste landfill site. J. Basic Microbiol. 90: 400± 405. Ishigaki, T., W. Sugano, A. Nakanishi, M. Tateda, M. Ike, M. Fujita. 2004. The degradability of biodegradable plastics in aerobic and anaerobic waste landfill model reactors. Chemosphere 54: 225±233. Jang, J.H., P.L. Rogers. 1996. Effect of levulinic acid on cell growth and poly- hydroxyalkanoate production by Alcaligenes sp. SH-69. Biotechnol. Lett. 18: 219±224. Jeffries, T.W. 1983. Utilization of xylose by bacteria, yeast, and fungi. pp. 1±32. In A. Fiechler (ed.) Advances in biochemical engineering/biotechnology, vol. 27, Pentoses and Lignin. Jones, J.L., K.T. Semrau. 1984. Wood hydrolysis for ethanol production-previous experience and the economics of selected processes. Biomass 5: 109±135.

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Jonsson, L.J., E. Palmqvist, N.O. Nilvebrant, B. Hahn-Hagerdal. 1998. Detoxification of wood hydrolysates with laccase and peroxidase from the white-rot fungus Trametes versicolor. Appl. Microbiol. Biotechnol. 49: 691±697. Keenan, T.M., A.J. Stipanovic, S.W. Tanenbaum, J.P. Nakas. 2004. Production and characterization of poly- -hydroxyalkanoate copolymers from Burkholderia cepacia utilizing xylose and levulinic acid. Biotechnol. Prog. 20: 1697±1704. Keenan, T.M., S.W. Tanenbaum, J.P. Nakas. 2005. Microbial formation of polyhydroxyalkanoates from forestry-based substrates. ACS Symposium Series, American Chemical Society, in press. Kim, B.S., S.C. Lee, S.Y. Lee, H.N. Chang, Y.K. Chang, S.I. Woo. 1994. Production of poly(3-hydroxybutyric-co-3-hydroxyvaleric acid) by fed-batch culture of Alcaligenes eutrophus with substrate control using on-line glucose analyzer. Enzyme Microb. Technol. 16: 556±561. Kim, M.N., A.R. Lee, J.S. Yoon, I.J. Chin. 2000. Biodegradation of poly(3hydroxybutyrate), Sky-Green, and Mater-BiÕ by fungi isolated from soils. Europ. Polym. J. 36: 1677±1685. Kulesa, G. 1999. Clean fractionation-inexpensive cellulose for plastics production. http:// www.oit.doe.gov/chemicals/factsheets/ch_cellulose.pdf Larsson, S., A. Reimann, N. Nilvebrant, L. Jonsson. 1999. Comparison of different methods for the detoxification of lignocellulose hydrolyzates of spruce. Appl. Biochem. Biotechnol. 77/79: 91±103. Lee, S.Y. 1996. Plastic bacteria? Progress and prospects for polyhydroxyalkanoate production in bacteria. Trends Biotechnol. 14: 431±438. Luzier, W.D. 1992. Materials derived from biomass/biodegradable materials. Proc. Natl. Acad. Sci. USA 89: 839±842. Lynd, L.R. 1989. Production of ethanol from lignocellulosic materials using thermophilic bacteria: critical evaluation of potential and review. Adv. Biochem. Eng. Biotechnol. 38: 1±52. Madison, L., G.W. Huisman. 1999. Metabolic engineering of poly(3-hydroxyalkanoates): from DNA to plastic. Microbiol. Molec. Biol. Rev. 63: 21±53. Mohanty, A.K., M. Misra, G. Hinrichsen. 2000. Biofibres, biodegradable polymers, and biocompostites: an overview. Macromol. Mater. Eng. 276: 1±24. Mussatto, S.I., I.C. Roberto. 2004a. Optimal experimental condition for hemicellulosic hydrolysate treatment with activated charcoal for xylitol production. Biotechnol. Prog. 20: 134±139. Mussatto, S.I., I.C. Roberto. 2004b. Alternatives for detoxification of dilute-acid lignocellulosic hydrolysates for use in fermentative processes: a review. Bioresource Technol. 93: 1±10. Nakas, J.P., S.W. Tanenbaum, T.M. Keenan. 2004. Bioconversion of xylan and levulinic acid to biodegradable thermoplastics. Patent Pending. U.S. Patent and Trademark Office, USA. Narasimhan, K., P.R. Green. 2004. Nodax low cost delivery plans. Amer. Chem. Soc. Meeting, Abstract Anaheim, CA, USA. Narayan, R., C.A. Pettigrew. 1999. ASTM Standards: help and a new industry. ASTM J.S.N. Feb. 1999: 1±7. Ramsay, J., M. Hassan, B. Ramsay. 1995. Hemicellulose as a potential substrate for production of poly ( -hydroxyalkanoates). Can. J. Microbiol. 41: 262±266. Reddy, C.S.K., R. Ghai, Rashmi, V.C. Kalia. 2003. Polyhydroxyalkanoates: an overview.

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10

Poly(lactic acid)-based bioplastics J - F Z H A N G a n d X S U N , Kansas State University, USA

10.1 Introduction Petrochemical-based synthetic polymers have brought extensive benefits to mankind in many respects, but the ecosystem and environment are, therefore, disturbed and polluted as a result of the accumulation of petroleum-based disposable waste. Biodegradable materials would ease disputes on environment pollution and reduce reliance on fossil resources. Poly(lactic acid) (PLA) is a biocompatible and biodegradable semi-crystalline polyester and is commercially available. High-molecular-weight PLA is generally prepared by ring-opening polymerization of lactide, a cyclic dimer prepared by the controlled depolymerization of lactic acid, which, in turn, is obtained by the microbial fermentation of annual renewable sugar-based materials such as starch or cellulose (Berl and Scharngal, 1988; Nijenhuis et al., 1992; www.cdpoly.com). PLA has comparable mechanical performance to those petroleum-based polyesters, especially high elasticity modulus and high stiffness, thermoplastic behavior, biocompatibility, and good shaping and molding capability. PLA is classified as a water-sensitive polymer because it degrades slowly compared with water-soluble or water-swollen polymers (Gajria et al., 1996). Its thermal and mechanical properties are superior to those of the other biodegradable aliphatic polyesters, such as poly(butylenes succinate) (PBS), poly(3hydroxybutyrate) (PHB), and poly(-caprolactone) (PCL) (Kulkarni et al., 1971). With these characteristics, PLA has been utilized successfully in surgicalimplant materials and drug-delivery systems. The stereochemistry of PLAs is complex because of the chiral nature of lactic acid monomers, as shown in Fig. 10.1 (Palade et al., 2001). The stereo-isomeric L/D ratio of the lactate unit association influenced PLA properties (Tsuji and Ikata, 1992). There are three types of PLAs because there are two stereoisomeric forms of lactic acid, poly (levo-lactic acid) and poly (dextro-lactic acid), which are both semi-crystalline and have identical chemical and physical properties. Poly (D,L-lactic acid) or poly (meso-lactic acid), a racemic polymer obtained from an equimolar mixture of D- and L- lactic acid, is amorphous, with weak mechanical properties. The

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10.1 Chemical structure of poly(lactic acid) (PLA) and its constituent monomers (Palade et al., 2001).

involvement of D- and L-units in the sequences of PLLA and PDLA, has a profound effect on their thermal and mechanical properties. Generally, an increased stereo-isomeric ratio decreases crystallinity and, accordingly, a lower melting temperature (Urayama et al., 2001, 2002). Thus, control of the ratio of L to D monomer content is an important molecular feature of PLAs.

10.2 Properties of PLA 10.2.1 Rheology of PLA Melt rheological behavior is of particular interest for PLA to become a viable commercial material for engineering processing. Typical rheology properties are summarized in Table 10.1. By measuring the rheological properties at one temperature and clearing the temperature dependence (according to flowactivation energies or shift factor) and molecular-weight dependence (in terms of parameters K and  for the terminal viscosity, 0 ˆ K…Mw † (Fox and Loshaek, 1955)), the rheological properties at any other temperatures and molecular weights can be determined for the entire PLA system. Once the relation of temperature and molecular weight is known it can be applied in modeling applications. For instance, in screw extrusion and general flow simulations.

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Table 10.1 Melt rheology properties of poly(lactic acid) (Dorgan et al., 1999) Property

Value

M1, g/mol (molecular weight per skeletal bond) , g/cm3 GN0, MPa Me ˆ RT/GN0, (kg/mol) Mc ˆ (p*/p)Me, kg/mol /M, Ð2
mol/g Characteristic Ratio MKuhn, g/mol

24.02 1.117 1.0 4.21 ˆ 175M1 9.60 ˆ 400M1 0.57 6.7  7% 237 ˆ 9.85M1

The scaling of the zero-shear viscosity with molecular weight is presented in Fig. 10.2 for a wide variety of optical compositions. There is no systematic trend of the melt viscosity with changing composition. Dynamic and steady rheological tests found that the molecular weight between entanglement in PLA melt is close to 10  103 . This value corresponds to a characteristic ratio of C1 (a fundamental chain property defined as the ratio of the chain dimensions under  conditions to the size of a random walk) ˆ 12, implying that PLA chains

10.2 Zero shear viscosities versus weight averaged molecular weights for PLAs of varying optical composition and resulting scaling law. Numbers labeling symbols correspond to L/%D (Dorgan, Curtsey).

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are rather stiff. Amorphous polymers with such high values of C1 fracture in a brittle fashion, with crazing as the dominant deformation mechanism, as amorphous PLA (Grijpma et al., 1994). Chain architectures influence PLA rheology. For a linear architecture, the Cox-Merz rule (Dealy and Wissbrun, 1990) is valid for a large range if shear rates and frequencies are accessible. Both zero-shear viscosity and elasticity (measured by the recoverable shear compliance) increase with increasing branched content. For the linear chain, the compliance is independent of temperature, but this behavior is apparently lost for the branched PLA (Lehermeier and Dorgan, 2001). Chain branching and molecular weight distribution have significant effects on the rheological behaviors of PLA melts (Wang et al., 1997). For example, a consistent viscosity reduced the variation of complex viscosity with temperature at a molecular weight of Mw ˆ 1,000,000 by approximately 2/3 of the previous value for every 10 ëC increase (Lehermeier and Dorgan, 2001). The PLA melts follow an Arrhenius-type behavior. The horizontal activation energy increases with increasing molecular weight of PLA, up to 85 kJ/mol for the highest molecular weight PLA, as listed in Table 10.2. The vertical activation energy varies little from unity, and the calculated values can be considered as zero. Hence, only horizontal activation energy or the horizontal shift factor T need to be considered to describe the temperature dependence of PLA melts (CooperWhite and Mackay, 1999). The quantification of these dependencies eliminates the need to perform rheological measurements at multiple temperatures and molecular weights. The effects of molecular weight on zero-shear viscosity and elasticity coefficient for PLLA at 200 ëC are shown in Fig. 10.3. The exponent value of molecular weight, with regard to terminal viscosity, is slightly higher than the generally accepted value of 3.4. The elasticity coefficient for PLA melts shows a greater dependence on molecular weight, at a value of 8.0, than that for monodisperse polystyrene melts (Cooper-White and Mackay, 1999). The dynamic viscoelastic behavior of PLA, with molecular weights ranging from 2,000 to 360,000, was studied over a broad range of reduced frequencies (approximately 1  10ÿ3 to 1  103 sÿ1 ) by using the time-temperature superTable 10.2 Activation energies for flow of poly(lactic acid) melts (CooperWhite and Mackay, 1999) Polymer Horizontal activation energy EH (kJ/mol
K) Vertical activation energy EV (kJ/mol
K)

L206*

L210*

L214*

76.0

79.0

85.0

4.0

4.0

4.0

* Commercial names of PLLA, from Boehringer Ingelheim, Ingelheim, Germany.

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10.3 Effect of molecular weight on zero-shear viscosity and elasticity coefficient for poly( L-lactic acid) (PLLA) at 200 ëC (Cooper-White and Mackay, 1999).

position principle. The results indicated that PLA melts have a critical molecular weight for entanglement, Mc , of approximately 16,000 g/mol, and an entanglement density of 0.16 mmol/cm3 at 25 ëC. For PLA made from a 98:2 ratio of the L to D enantiomeric monomers, the entanglement molecular weight is approximately 9,000 per mole whereas the molecular weight for branch entanglement is inferred to be nearly 35,000 g per mole. There is little difference between activation energies as a function of chain architecture. This suggests that the flow activation energy is affected only by small-scale local motions of the polymer chain rather than by larger-scale selfdiffusive motions (Dorgan et al., 1999). PLA is noted to require substantially larger molecular weights to display similar melt viscoelastic behavior, at a given temperature, as that for conventional non-biodegradable polymers such as polystyrene. The reason for the deviation is suspected to be steric hindrance, resulting from excessive coil expansion or other tertiary chain interactions. Lowmolecular-weight PLA (~40,000) shows Newtonian-like behavior at shear rates typical of those achieved during film extrusion (Cooper-White and Mackay, 1999).

10.2.2 Thermal characteristics Because PLA is a semi-crystalline polymer, thermal history affects the physical properties of PLA, inducing changes in the crystalline/amorphous ratio, as well as large physical aging effects on the glassy amorphous phase (Gelli and

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Scandola, 1992). Crystallinity induced by thermal treatment of PLA depends on the initial molecular weight. Crystallization phenomena that occur in the material during thermal treatment are clearly documented by DMTA and DSC (Migliaresi et al., 1990, 1991). Crystallization behavior of PLA has been well investigated (Nijenhuis et al., 1991; Kalb and Pennings, 1980; Cohn et al., 1987; Migliaresi et al., 1991), including crystallization kinetics (Kolstad, 1996; Urbanovici et al., 1996), isothermal melting mechanism (Kishore et al., 1984), and the influence of undercooling and molecular weight of PLA on morphology and crystal growth (Vasanthakumari and Pennings, 1983). The equilibrium melting point and the glass transition temperature are about 215 ëC and 55 ëC, respectively (Kalb and Pennings, 1980). Below 114 ëC, it is not feasible to measure spherulitic radius growth rate because of high nucleation density. Isothermal melt crystallization kinetics of PLA are described adequately by the Avrami kinetic model (Urbanovici et al., 1996). The self-seeding technique to induce nucleation at very high temperatures is a valid method for measuring growth rate. Crystallization rate increases first, reaching a maximum peak, and then falls as the crystallization temperature increases. The growth rate increases with decrease of molecular weight. At high undercooling, well-defined spherulites form. As undercooling is reduced, the spherulites become of irregular shape and coarse-grained structure (Vasanthakumari and Pennings, 1983). Isothermal annealing of PLA indicates an increase in melting temperature and heat fusion (shown in Table 10.3) with annealing time, suggesting an increase in lamellar thickness and perfection of lamellae caused by the enrichment of the amorphous and crystalline phase in shorter and longer chains, respectively. Melting and thickening occur simultaneously during annealing. The apparent activation energy for melting Table 10.3 Melting point of the isothermally treated poly(lactic acid) at 80 ëC (Gupta and Deshmukh, 1982b) and at 145 ëC (Kishore et al., 1984) Time (hour) 0 0.5 1 1.5 2 2.5 5 8

Melting point (ëC)*

Melting peak temperature (ëC)**


Hf (J/g)

148 124 118 121 123 125

185.0

63.1

186.5

70.3

190.5 192.5

81.3 89.97

* from Gupta and Deshmukh (1982b). ** from Kishore et al. (1984). PLA crystallized at145 ëC for1h and then kept at175 ëC for different times.

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10.4 CRHR map of the appearance of the double-melting peaks: () doublemelting peaks and (o) single-melting peaks (He et al., 2001).

is 828 kJ/mol (Kishore et al., 1984). Yasuniwa et al. (2004) studied the melting and crystallization behavior of PLA (Mw ˆ 3  105 ) by DSC through various cooling rates (CRs) from the melt (210 ëC). The peak crystallization temperature and the exothermic heat of crystallization determined from the DSC curve decreased almost linearly with increasing log(CR). Double endothermic peaks, a high-temperature peak (H) and a low-temperature peak (L), appeared in the DSC curves at slow heating rates (HRs) for the samples prepared with a slow CR. Peak L increased with increasing HR, whereas peak H decreased (Fig. 10.4). The peak melting temperatures of L and H [Tm(L) and Tm(H)] decreased linearly with log(HR). Peak L decreased with increasing CR, whereas peak H increased. Both Tm(L) and Tm(H) decreased almost linearly with log(CR). The melting temperatures of PLA are significantly affected by isothermal treatment (Table 10.3). Initially, at one hour, there is a decrease in PLA melting point, suggesting that thermal degradation occurred during heating and that the initial decrease in melting point is caused by the superimposed depression in melting point. After one hour, the melting point of the PLA increases with

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heating time. This increase may be caused by the stiffening of chains, resulting from the decreasing number of ester linkages in the chain, and also caused by the change in crystallinity of PLA (Gupta and Deshmukh, 1982b). At temperatures in excess of 190 ëC and, in some instances, even at lower temperatures, chain scission reactions, along with thermohydrolysis, depolymerization and cyclic oligomerization, and intermolecular and intramolecular transesterifications, cause significant reductions in molecular weight (Jamshidi et al., 1988; Kopinke et al., 1996). Precautions must be taken when defining the material processing and annealing conditions to avoid PLA degradation by thermal cleavage. Concomitantly, PLA degrades due to thermal cleavage of the chains during processes such as melt molding and spinning. Thermal degradation kinetics of PLA follow the Avrimi-Erofeev equation as determined by using TG as: ‰ÿln…1 ÿ †Š1= ˆ kt

10:1

where  is the fractional weight loss,  is an exponent, and k is the rate constant. The equation suggests that the decomposition of PLA is due to the growth and nucleation of decomposition sites in the solid. In the presence of air, PLA undergoes thermal oxidative degradation, a single-stage process, and lactide is the decomposition product (Gupta and Deshmukh, 1982b). The first-order random decomposition of PLA occurs under isothermal conditions, and the activation energy for thermal oxidation range from 105 to 126 kJ molÿ1 (Gupta and Deshmukh, 1982a). The activation energy for thermal degradation of PDLLA and PLLA is 119 and between 72 and 103 kJ molÿ1, respectively (Babanalbandi et al., 1999; McNeil and Leiper, 1985). The typical thermal properties of a PLA are listed in Table 10.4 (Lu and Mikos, 1999). Terminal groups play an important role in decreasing the molecular weight, forming a low-molecular-weight cyclic monomer and oligomers in thermal degradation (Jamshidi et al., 1988; Babanalbandi et al., 1999; Lee et al., 2001). Although acetylation of the hydroxyl terminal group and removal of the nonpolymeric contents improves the thermal stability of PLA (Jamshidi et al., 1988), some additives as catalyst deactivators are successfully used to stabilize PLAs in the melt (Tsuji and Fukui, 2003). Also, the thermal stability of PLA can be improved by the addition of its enantioner PDLA and vice versa. The
Etd value of the L/D film was in the range of 205 to 297 kJ molÿ1, which was higher by 82 to 110 kJ molÿ1 than the average
Etd value of the L- and D- films (Tsuji and Fukui, 2003). The characteristics of the crystallization and double-melting behavior were explained by the slow rates of crystallization and recrystallization, respectively. PLLA could crystallize from the melt and form spherulites in the presence of PDLLA whose Mv ranged from 6:5  103 to 3:0  105 . PDLLA is miscible with PLLA, irrespective of the molecular weight of PDLLA, and increased entanglement between PLLA and PDLLA with increasing molecular weight

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Table 10.4 Typical thermal and physical properties of poly(lactic acid) (adapted from Lu and Mikos, 1999) Properties

Units

Degree of crystallinity Xc % Density,  Heat of fusion,
Hf

Heat capacity, Cp

gcmÿ3 kJ molÿ1

J Kÿ1 gÿ1

Glass transition temperature Tg

K

Melting temperature Tm

K

Decomposition temperature Td

K

Conditions

Value

D-PLA L-PLA D,L-PLA P(L-co-DL)LA

Semicrystalline 0-37 Amorphous

Amorphous Single crystal L-PLA complete crystalline L-PLA fiber As extruded After hot-drawing L-PLA of Mv ˆ 5,300 Mv ˆ (0.2-6.91)  105 L-PLA of various molecular weights D, L-PLA of various molecular weights D-PLA injection-molded, Mv ˆ 21,000 L-PLA of various molecular weights 5 L-PLA of Mw ˆ (0.5±3)  10 D,L-PLA of Mw ˆ (0.21±5.5)  105

1.248 1.290 146 2.5 6.4 0.60 0.54 326±337 323±330 444.4 418±459 508±528 528

reduces the growth rate and regularity of PLLA spherulites and PLLA crystallinity (Tsuji and Ikada, 1996b). To prevent shrinking during crystallization, partly constrained conditions were applied to a PLA melt; a reduction in Tg of PLA increased with increasing degree of crystallization, %X, or crystallization temperature, Tc. Unconstrained PLA exhibited the conventional trend of increasing Tg with %X or Tc, however, which is the result of an increase in the net free volume in the amorphous phase, because the increasing crystal volume fraction has a greater density than the amorphous fraction, which is not able to contract to maintain its nominal density (Fitz et al., 2002).

10.2.3 Mechanical properties Physical and mechanical properties of PLA largely depend on the L/D ratios, molecular weight, crystallinity, orientation, and preparation methods. Typical thermal and mechanical properties of PLA by various preparation methods are listed in Table 10.5 (Grijpma et al., 1994). The obvious drawback for PLA used as a plastic is its low flexibility upon bearing load. PLA is rigid, brittle, and likely to deform at temperatures in excess of its glass transition temperature; it is desirable

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Table 10.5 Thermal and mechanical properties of as-polymerized polylactides prepared in the monomer melt at 110 ëC with different catalysts and compression molded materials (C. M.) (Grijpma et al., 1994) L/D lac ratio

Catalyst or C. M.

 (MPa)

 (%)

Impact strength (kJ/m2)

Tm (ëC)


H (J/g)

100/0 100/0 100/0 85/15 100/0 100/0 85/15

Sn(Oct)2 Sn(Acac)2 Zn(DMH)2 Sn(Oct)2 C. M. C. M. quenched C. M.

59.5 34.5 25.3 62.3 72.3 67.8 64.5

12.2 3.3 3.0 7.5 8.1 18.1 7.6

13.5 2.7 2.4 4.0 13.1 6.4 4.3

192.1 198.0 201.3 ö 184.4 193.0 ö

76.0 95.5 100.7 0 47.9 12.7 0

to improve the mechanical and thermal properties of PLA, particularly for engineering purposes, to facilitate various applications (Urayama et al., 2003). One approach improves the mechanical properties of PLA by controlling the L/D ratio or polymerizing PLA by using specified catalysts. Low entangled and crosslinked PLA can be polymerized in bulk at temperatures below its melting temperatures, and these PLAs show very strong tensile strength of up to 805 MPa in the longitudinal direction upon drawing. Molecular weight between entanglements (Me) is one of the main parameters in determining the mechanical properties in PLA. In the melt, the value of this characteristic chain length is determined by the intrinsic stiffness of the PLA chain. To have an appreciable strength, the PLA should have a molecular weight that is at least a multiple of Me. Greater tensile strength is achieved by using higher-molecular-weight PLA (Grijpma et al., 1994). Orientation of PLA chains after melt processing by strengthening at various ratios improves mechanical strength. Amorphous non-crystallizable oriented PLA networks have a tensile strength of around 460 MPa (Grijpma et al., 2002). Strength of PLA is also closely correlated to its shape of product. Extrusion through a rectangular die by means of solid-state extrusion improved the flexural strength and flexural modulus up to the maxima of 202 MPa and 9.7 GPa, respectively (Lim et al., 2003). Self-reinforcing (TormaÈlaÈ et al., 1990) and hot strengthening (Eling et al., 1982; Leenslag et al., 1987; Fambri et al., 1997) also improve the mechanical properties of PLA. These can be achieved by aligning the polymer molecules to have a high degree of chain orientation, mostly accompanying the transformation of spherulitic to fibrillar structure. Typical mechanical properties of a PLA are listed in Table 10.6 (Lu et al., 1999). Mechanical properties of PLA are superior to poly(D, L-lactide), and its behavior can be significantly improved by crystallization. PLA slowly crystallized from the melt is much more impact-resistant, showing that the presence of crystalline domains also has a large positive effect on the ductility of

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Table 10.6 Typical mechanical properties of poly(lactic acid) (adapted from Lu and Mikos, 1999) Properties

Units

Conditions

Value

Tensile strength

MPa

L-PLA film or disk,

28±50

Tensile modulus

Flexural storage modulus Elongation at yield

Elongation at break

Shear strength Shear modulus Bending strength Bending modulus

MPa

MPa

%

%

MPa MPa MPa MPa

Mw ˆ (0.5±3)  105 L-PLA melt-spun fiber D, L-PLA film or disk, Mw ˆ (1.07±5.5)  105 L-PLA film or disk, Mw ˆ (0.5±3)  105 L-PLA melt-spun fiber D, L-PLA film or disk, Mw ˆ (1.07±5.5)  105 L-PLA film or disk, Mw ˆ (0.5±3)  105 D, L-PLA film or disk, Mw ˆ (1.07±5.5)  105 L-PLA film or disk, Mw ˆ (0.5±3)  105 D, L-PLA film or disk, Mw ˆ (1.07±5.5)  105 L-PLA film or disk, Mw ˆ (0.5±3)  105 L-PLA fiber spun from toluene L-PLA melt-spun fiber, Mw ˆ 1.8  105 D, L-PLA film or disk, Mw ˆ (1.07±5.5)  105 L-PLA spin L-PLA melt-spun monofilament L-PLA pin L-PLA pin

Up to 870 29±35 1200±3000 Up to 9200 1900±2400 1400±3250 1950±2350 3.7±1.8 4.0±2.5 6.0±2.0 12±26 25 6.0±5.0 54.5 1210±1430 132 2800

the specimen. Thermally annealed specimens possess higher values of tensional and flexural modulus of elasticity, Izod impact strength, and heat resistance. The plateau region of flexural strength as a function of molecular weights appears around Mw ˆ 3:5  104 for PDLLA and amorphous PLA and at higher molecular weight, around Mw ˆ 5:5  104 , for crystalline PLA. The temperature effect shows that only crystalline PLA still exhibits useful mechanical properties at 56 ëC (Perego et al., 1996).

10.3 Blends of PLA 10.3.1 Miscibility of PLA with polymers The application of PLA in the field of commodity plastics requires a dramatic reduction in the costs of this polymer, as well as reliable and controllable

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technical processes (Rafler et al., 2001). Blending PLA with other polymers or fillers is a common approach. PLA was reported to be miscible with other stereoisomers such as poly(D,L-lactic acid), and the blends had different properties according to the mixing ratio (Tsuji et al., 1996b; Perego et al., 1996). It is also known that PLA is able to form miscible blends with various other polymers, such as poly(ethylene oxide) (PEO) (Nijenhuis et al., 1996), poly(vinyl acetate) (PVA) (Gajria et al., 1996), poly(ethylene glycol) (PEG) (Sheth et al., 1997; Tsuji et al., 2001a), PBS (Park and Im, 2002), and poly(vinyl acetate-co-vinyl alcohol) (PVAc-co-VA) copolymers (Park and Im, 2003). There is a significant crystallization-induced phase separation for PLA containing more than 40% poly(butylenes succinate) (Park and Im, 2002). PLA/(PVAc-co-VA) blends are miscible systems for the entire composition region, but for blends with 10% hydrolyzed PVAc-co-VA copolymer, the phase separation and double-glass transition are generated. With increasing PVAc-coVA content, the fusion heat decreases and the melting peaks, measured by DSC, shift to lower temperature in PLA/(PVAc-co-VA) blends. The interaction parameters exhibit negative values for 10% hydrolyzed PVAc-co-VA copolymer, but the values increase to positive ones with increasing the degree of hydrolysis (Park and Im, 2003). The blend of PLA/PEG consists of two semimiscible crystalline phases dispersed in an amorphous PLA matrix. For more extreme compositions, only the major component is able to crystallize; the noncrystalline matrix that develops consists of the minor constituent and the amorphous phase of the major component of the blend. PLA and PEG are miscible with each other when the PLA fraction is below 0.5 (Tsuji et al., 2001a). The molecular weight of PEG affects the morphology generated because of the enhanced crystallizability of the longer PEG chains. Crystallization phenomena are viewed as a fundamental driving force for microphase segregation (Younes et al., 1988). The PLA has partial miscibility with poly(vinyl alcohol), which can be expected from the large and small differences, respectively, between the experimental solubility () values of the constituent polymers and those between their calculated  values shown in Table 10.7 (Tsuji et al., 2001a). PLA/poly(p-vinylphenol) (PVPh) blends are partly miscible, as characterized by shifts in the Tgs of the two component polymers. The Tg of the PLA-rich phase increases with increasing PVPh content, whereas that of the PVPh-rich phase decreases with increasing PLA content. The apparent melting temperature of PLA is significantly depressed with increasing PVPh content. Weak hydrogen-bonding interaction exists between the carbonyl groups of PLA and the hydroxyl groups of PVPh, as evidenced by FTIR spectra (Zhang et al., 1998). In the amorphous state, the hydrogen-bonded hydroxyl band of PVPh shifts to a higher frequency upon blending with PLA, whereas the carbonyl stretching band of PLA shifts to a lower frequency with the addition of PVPh. The crystallinity of PLA in the blends displays a marked decrease with increasing PVPh content. When the PVPh content is in excess of 40 wt%,

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Table 10.7 Solubility parameter () values for poly(L-lactic acid) (PLLA), poly(DL-lactide-co-glycolide) (P(DLLA-GA)), poly(vinyl alcohol) (PVA), and poly(ethylene oxide) (PEO) (Tsuji and Muramatsu, 2001) Polymer PLLA P(DLLA-GA) (50/50) PVA PEO

 (J0.5 cmÿ1.5) 22.7 19.0±20.5 24.4 25.1 21.6 25.8±29.1 17.8 20.2  2

Method Calcd (Fedors parameters) Experimental (swelling) Calcd (fedors parameters) Calcd (Hoy parameters) Calcd Experimental Calcd Experimental (inverse phase gas chromatography)

crystallization of PLA does not occur under isothermal conditions (Zhang et al., 1998). Atactic PHB (a-PHB) was selected as the second blend component in PLA in efforts to improve the flexibility and impact resistance. The a-PHB/PLA blends are miscible over the whole range of composition. The elastic modulus, stress at yield, and stress at break decrease, whereas the elongation at break increases, with increasing a-PHB content (Focarete et al., 2002). Immiscible binary blends of PLA and PCL exhibit immiscibility of the components among various ratios (Dell'Erba et al., 2001). The presence of amorphous PDLLA did not disturb crystallization of PCL over the PDLLA content [XPDLLA ˆ PDLLA/(PCL ‡ PDLLA)] from 0.1 to 0.9, and allowed PCL to form spherulites over XPDLLA ranging from 0.1 to 0.6. The spherulite radius is larger for the blends than for the nonblended PCL. Phase separation occurred for the blends with XPDLLA between 0.1 and 0.9. Tm of PCL remains unchanged in the XPDLLA range up to 0.6 but decreased at XPDLLA in excess of 0.6, whereas the crystallinity of PCL is constant, around 60%, irrespective of XPDLLA. The tensile strength, the yield stress, the Young's modulus, and the storage modulus of the blends increased monotonously with increase in XPDLLA. Elongation-atbreak of PDLLA increased dramatically, whereas elongation of PCL decreased remarkably, when a small amount of the other component was incorporated (Tsuji et al., 1996b).

10.3.2 PLA with fillers Of particular interest is the recently developed nano-composites technology, consisting of a polymer and organically modified layered silicate, because they often exhibit remarkably improved mechanical and various other performances compared with virgin polymers (Ray et al., 2003a,b,c). Organically modified layered silicate (Nam et al., 2003; Ray et al., 2002; 2003a,b,c; Ogata et al., 1997) and synthetic mica (Chang et al., 2003) are used for PLA-based

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nanocomposites preparation. All of the nanocomposites exhibit superior improvement in properties, such as mechanical modules, thermal stability, crystallization behavior, gas barrier property, and biodegradability, because of the interaction between the clay particles and the PLA matrix. The possibility of preparing biodegradable nanocellular polymeric foams via nanocomposites technology based on PLA and layered silicate was first reported by Fujimoto et al. (2003) who used supercritical carbon dioxide as a foaming agent, with silicate acting as nucleating sites for cell formation. Carbon-fiber reinforced PLA-matrix composites (Wan et al., 2001a,b) and glass fiber-PLA composites improved mechanical properties by surface treatment of fibers to generate a chemical reaction between the PLA matrix and the fibers to enhance interfacial adhesion. Cellulosic fibrous materials such as paper-waste fibers, rayon nonwoven fabric, and wood flour were blended with PLA by extrusion and compression molding. The mechanical properties were retained for PLA filled with as much as 32 wt.% of paper-waste fiber. The tensile strength was improved for PLA filled with rayon nonwoven fabric, compared with pure PLA. The tensile strength of PLA filled with wood flour was retained up to 15 wt.% content of the filler. Paper coated with PLA showed an improved tensile strength at standard conditions and showed significantly improved tensile strength after a wetting treatment, in comparison with uncoated paper (Levit et al., 1996). One of the simplest approaches to strengthening PLA was by incorporating inorganic or organic fillers into PLA, such as talc (Kolstad, 1996), other inorganic materials (Urayama et al., 2003), and starch (Ke and Sun, 2000). Because PLA is a crystalline polymer, its fiber and particle composites could increase heat stability. In particular, with crystallization of the PLA matrix, the heat deformation temperature should reach the melting temperature of PLA. If the PLA matrix remains amorphous, however, the reinforcement effect should be restricted to its glass transition temperature. For incorporating particle- and whisker-type fillers, the tensile moduli are 3.1±3.7 and 3.7±4.5 GPa, respectively, and the flexural moduli are from 4.1±4.8 and from 4.8±6.1 GPa. The greatest reinforcing effect is obtained with whiskers of potassium titanate and aluminum borate with a high aspect ratio. Their mechanical properties are also found to increase with increasing volume fraction of filler (Urayama et al., 2003). Low-molecular-weight compounds with hydroxyl groups can be important to improvement of PLA properties. The FTIR spectra of PLA/4,40 -thiodiphenol (TDP) blends suggest that there are interassociated hydrogen bonds between PLA chains and TDP molecules (He et al., 2001). The thermal and dynamic mechanical properties of PLA are greatly modified through blending with TDP, and PLA/TDP blends possess eutectic phase behavior. In addition to TDP, PLA blends with other low-molecular-weight hydroxyl compounds, including aromatic and aliphatic compounds, are also characterized (He et al., 2001; Zhang et al., 2004b). Incorporating dendritic hyperbranched polymer (DHP)

Poly(lactic acid)-based bioplastics

265

significantly improved mechanical properties of PLA. The blend with 2% DHP has substantially greater tensile strength, about 70 MPa, than that of pure PLA, at about 60 MPa. Elongation of the blends with DHP concentration from 2% to 8% is also increased compared with that of PLA alone. Addition of DHP also increased the recrystallization rate of PLA between glass-transition and melting temperatures. The improved mechanical properties are attributed to strong hydrogen bonds between DHP and PLA at a proper DHP concentration and to a certain crystallinity produced by nucleation of DHP (Zhang et al., 2004b).

10.3.3 Blends with starch In disposable applications, PLA competes with cheap mass polymers like polyethylene, polypropylene, or polystyrene. Therefore, reducing the price of PLA without sacrificing its excellent biodegradability while maintaining certain mechanical and thermal properties is an effective approach. Starch, a degradable carbohydrate biopolymer from natural resources, consists of polysaccharides amylose and amylopectin. Native starch usually exists in granular form, in which the polymer molecules are hydrogen bonded and aligned radically (Thomas et al., 1997). Starch can be physically blended with PLA, but remains in a separate conglomerate form in PLA matrix. Size of the conglomerates is determined by volume fraction, molecular weight, viscosity ratio, interfacial energy between starch and PLA, and shear history during process (Graaf et al., 2001). Starch loading level plays a key role in determining the mechanical properties of PLA/starch blends. Tensile strength and elongation almost linearly decreased as starch content increased. Thus, starch is typically characterized as a solid filler (Ke and Sun, 2000). The tensile strength of the blends can be described by eqn 10.2 (Nicolais et al., 1971), on the assumption that the adhesion between starch granules and PLA matrix does not exist. c ˆ 0 …1 ÿ 1:212=3 †

10:2

where 0 is the tensile strength of the only matrix and the constant of 1.21 is the 2/3th power of , the volume fraction of a sphere-shaped filler when the fillers are filled perfectly with a hexagon or face-centered cubic arrangement. The stress decrease rate with a starch fraction is slower than that predicted by eqn 10.2, which means certain interfacial adhesion occurred between PLA and starch because of physical absorption and hydrogen bonds (Ke and Sun, 2000; Park et al., 1999). The blends showed two distinct Tgs, but, the PLA phase changed toward the Tg of starch with the change in blend composition, also indicating some degree of interaction. Microscopic observations revealed non-uniformly dispersed PLA inclusions in the starch matrix, indicating that phase separation can occur (Martin et al., 2001a). The Young's modulus and dynamic storage modulus increased as starch concentration increased up to 70%. For example, a PLA blend with 50%

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Biodegradable polymers for industrial applications

Table 10.8 Mechanical properties of tensile bars from starch and poly(lactic acid) (PLA) with various blending ratiosa (Ke and Sun, 2000) Blend 0:100 Corn starch and PLA 20:80 40:60 50:50 60:40 70:30 80:20 Wheat starch and PLA 20:80 40:60 50:50 60:40 70:30 80:20 a

Tensile strength (MPa)

Elongation at break (%)

Modulus (GPa)

61.0 (1.07)

6.33 (0.33)

1.204

(0.132)

42.4 33.0 32.4 23.9 18.9 13.2

(2.7) (1.04) (3.4) (1.5) (0.43) (2.4)

2.90 2.44 2.15 1.52 1.10 0.84

(0.24) (0.18) (0.21) (0.09) (0.09) (0.16)

1.613 (0.054) 1.496 (0.153) 1.633 (0.088) 1.734 (0.142) 1.832 (0.110) 1.739 (0.059)

44.7 41.2 35.9 29.7 22.3 16.4

(3.8) (2.1) (1.8) (1.2) (1.5) (1.3)

3.96 2.96 2.39 1.94 1.19 1.12

(0.2) (0.17) (0.11) (0.08) (0.13) (0.15)

1.327 (0.152) 1.551 (0.158) 1.669 (0.092) 1.661 (0.105) 1.954 (0.192) 1.594 (0.140)

Standard deviations in parentheses.

cornstarch gave 1.6 GPa elastic modulus (Table 10.8) (Ke and Sun, 2000). At an equal starch loading level, high-amylase cornstarch gave a blend with greater tensile strength than did normal cornstarch having 25% amylase (Kim et al., 1998). Wheat starch and cornstarch had similar blending properties because of their similar chemical composition and granule size (Ke and Sun, 2000). Mechanical properties of immiscible PLA/starch blends should be greatly influenced by the particle size of the dispersed phase, independent of starch type, according to a report from analysis of low-density polyethylene/starch blends. Starch with smaller granule size imposed better mechanical properties than did starch with larger granule size (Sailaja et al., 2001).The PLA becomes very soft above Tg around 50±70 ëC within the rubber plateau, losing modulus. The modulus of starch is higher than 1 GPa; blending starch significantly improves the stiffness of PLA above the Tg. Melting temperature of PLA is evidently not affected by incorporation of starch, but crystallinity of PLA is enhanced from 47.7% for pure PLA to 51.8% for a PLA/starch blend at a 55/45 ratio (Wang et al., 2001). Cold crystallization between approximately 100 and 120 ëC is more visible by the addition of starch, and the phenomenon is more obvious at low starch concentrations. The cold crystallization and melting behavior are even more evident at 2% starch level (Zhang et al., 2004b) but thermal transition temperatures remain the same. In this instance, starch acts as a nucleation agent. The nucleation function of starch is also conducted by isothermal crystallization (Ke and Sun, 2003; Zhang et al., 2004b). Starch effectively increases PLA crystallization rate by forming

Poly(lactic acid)-based bioplastics

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heterogeneous nuclei, but this effect is less than that caused by talc (Ke and Sun, 2003). Crystallization rate decreases as starch concentration increases at a constant temperature, and also decreases with increased isothermal temperature (Zhang et al., 2004b). The PLA spherulites size and shape are influenced by the addition of starch; as a result, spherulites become smaller and less regular as starch content increases (Ke and Sun, 2001a; Park et al., 2000). The nucleation effect of starch for a rapid crystallization rate is suited for injection molding of heat-resistant PLA products (Drumright et al., 2000); the stiffness of PLA is improved as well. Starch is a hydrophilic polymer and is sensitive to moisture, and PLA is a hydrophobic polymer with high water resistance. Water absorption increased slowly at a starch content of less than 60%, but increased rapidly when starch content exceeded 70% (Ke and Sun, 2003a). At low starch content, PLA formed a very good continuous phase that covered the starch. As starch content increased to 60% or more, the PLA phase became discontinous, and starch granules were not completely covered by the PLA matrix, resulting in large water uptake. The water absorption of the blend in boiling water was much greater than in water at room temperature. Free starch also leaches from the blend into the boiling water, which may be caused by starch solubility at elevated temperatures. PLA and starch blends allow the combination of desirable properties of the individual components. PLA forms a continuous visco-elastic matrix with good mechanical properties, whereas starch functions as a second component bringing much stiffness, reducing costs and increasing biodegradability of the whole system. Hydrophobic PLA, with hydroxyl and carboxyl end groups, and hydrophilic starch, with plenty of hydroxyl groups, lack reactive functional groups, which leads to poor interfacial adhesion, making the blends rather weak and brittle. The calculated starch/PLA interfacial energy and work of adhesion at 221 ëC are 0.61 and 85.8 dyn/cm, respectively (Biresaw et al., 2001). Tensile strength and elongation decrease due to immiscibility of starch and PLA (Ke and Sun, 2000). Non-uniformly dispersed PLA inclusions in the starch matrix were observed by Martin et al. (2001a), confirming that phase separation occurred. Interfacial modification of PLA and starch improve interfacial adhesion and, consequently, the blends' mechanical properties. Compatibility of two polymers can be improved by adding a third component with functional groups compatible with PLA or starch and capable of reducing interfacial tension, strengthening the interaction between polymer phases, or chemically reacting with PLA or starch. A stable and strong chemical bond is formed so as to transfer the internal stresses from the filler to the matrix consequently enhancing the strength of the blend. Examination of the end groups of starch and PLA suggested that the possible coupling agent may be one containing carboxylic, amino, anhydride, or isocyanate groups. Most polymeric blends are commercially compounded by extrusion. The functional groups should react to form the required concentration of graft or block

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copolymers in the short residence time of extrusion. Anhydride groups may react more quickly than carboxylic groups because of their higher reactivity. Reaction of anhydride with hydroxyl to form an ester is not an equilibrium reaction. The functionality of the anhydride group can be incorporated into a polymer by copolymerization or grafting of anhydride (e.g., maleic anhydride (MA)) (Zhang et al., 2004b), generating amounts of interfacial agents to compatibilize the system in situ at normal processing temperatures (John et al., 1997; Liu et al., 1992, 2003; Vaidya et al., 1994, 1995; Yang et al., 1996). Temperature, residual time, monomer, and initiator concentrations are all important variables affecting the graft ratio. Desirable graft content, with minimal degradation, can be achieved by controlling these factors (Mani et al., 1999; Mani and Bhattacharya, 2001). Including MA with the initiator improves the interfacial adhesion between starch and PLA during reactive extrusion (Carlson et al., 1999; Zhang et al., 2004a). At 1% MA with 0.1% initiator, the PLA/starch (55/45) blend had a tensile strength of 52.4 MPa, which is much higher than the original PLA/starch blends (55/45). A

10.5 Proposed chemical reactions among poly(lactic acid) (PLA), starch, MA, and initiator L101 (Zhang and Sun, 2004b).

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10.6 Schematic presentation of proposed chemical reaction among poly (lactic acid) (PLA), starch, and methylenediphenyl diisocyanate.

proposed reaction between PLA and starch with addition of MA is illustrated in Fig. 10.5 (Zhang et al., 2004b). Diisocyanates with high reactivity, such as methylenediphenyl diisocyanate (MDI), toluene diisocyanate (TDI), isoporonediisocyanate, and 1,6diisocyanatehexane (DIH), easily react with both hydroxyl and carboxyl groups. The proposed reaction of PLA and starch with MDI is schematically presented in Fig. 10.6. PLA and starch are strongly bridged by urethane linkages. MDI is one of the most effective crosslinking agents for PLA/starch blends (Jun, 2000; Wang et al., 2001, 2002), and the shortest, whereas DIH is the longest and most flexible crosslinking agent. The PLA/starch (50/50) blend exhibits different responses among three crosslinking agents in tensile strength, in the order MDI < TDI < DIH, according to efficacy. The tensile strength of a PLA/starch blend with 0.5% MDI is about 66.7 MPa, which is even higher than that of pure PLA, and almost double that of as-prepared PLA/starch (55/45), as shown in Table 10.9 (Wang et al., 2001). With 0.5% MDI, a blend with 45% starch gave the blend the highest tensile strength value among various starch contents (Wang et Table 10.9 Mechanical properties of raw poly(lactic acid) (PLA) and PLA/starch blends at 55/45 weight ratio, with various methylenediphenyl diisocyanate (MDI) levels (weight) (Wang et al., 2001). Samples Raw PLA PLA/starch without MDI PLA/starch with 0.25% MDI PLA/starch with 0.5% MDI PLA/starch with 1% MDI PLA/starch with 2% MDI

Tensile strength (MPa)

Elongation (%)

Young's modulus (GPa)

62.1 bc 36.0 d 62.3 c 66.7 a 64.9 abc 65.3 ab

5.69 a 2.58 c 4.37 d 4.40 b 4.77 b 4.50 b

1.41 a 1.73 c 1.89 b 1.94 b 1.94 b 1.92 b

Levels of MDI are weight percentages based on blend.Values in the same column followed by the same letter are not significantly different (p < 0.05).

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al., 2002). The storage modulus (G0 ) of PLA/starch blends, even above Tg, is improved with MDI (Vasanthakumari et al., 1983). The stiffness of blends is also improved by blending with MDI (Wang et al., 2001). A two-step process, in which the reactive agent is added toward the end of compounding all other ingredients, is more effective than the one-step process. The elongation of the two-step process is three or four times greater than that of the one-step process (Jun, 2000).

10.4 Plasticization of PLA-based bioplastics The brittleness and stiffness of PLA are major drawbacks in many fields of application. Plasticizers are widely used in the plastics industry to improve processability, flexibility, and ductility of glassy polymers (Sears et al., 1982). In semicrystalline polymers like PLA, an efficient plasticizer is expected to reduce the glass transition of the amorphous domains; if the Tg is near or lower than ambient temperature, flexibility is achieved. Meanwhile, it depresses the melting point of the crystalline phase so as to reduce processing temperature and avoid thermal decomposition at high temperature. Plasticizers are mainly dispersed in the amorphous phase under the pressure of spherulites (Mark, 1990). Any factor influencing crystallinity or crystalline behaviors of PLA could disturb the distribution and compatibility of plasticizers with PLA and induce phase separation and segregation or migration of plasticizers. As a result, the mechanical properties and morphology of the materials can be degraded (Ljungberg et al., 2003). Lactide monomer is an effective plasticizer for PLA, but it tends to migrate to the material surface, causing the surface to become sludgy. As a consequence of plasticizer loss in response to heating or long storage time, the PLA gradually stiffens. Nontoxic citrate plasticizers derived from natural citric acid are compatible with PLA. A significant improvement in the flexibility of PLA blends is accomplished at the expense of tensile strength by incorporation of triethyl citrate (TC), tributhyl citrate, acetyl triethyl citrate (ATC), acetyl tributyl citrate (Labrecque et al., 1997; Ljungberg et al., 2003; Zhang et al., 2004d), or triacetine (Ljungberg et al., 2002, 2003) or dioctyl maleate (Zhang et al., 2004c). These plasticizers have better plasticizing efficiency than the intermediate-molecular-weight plasticizers; meanwhile, the lower-molecularweight citrates increase the enzymatic degradation rate of PLA. Their efficiency is evaluated in terms of Tg shift and mechanical properties improvement (Martin et al., 2001a). The Tg of PLA decreases linearly as plasticizer content increases. These plasticizers are miscible with PLA to an extent of ~25 wt%. At this point, the PLA seems to be saturated with plasticizer and the blends tend to phaseseparate when more plasticizer is incorporated. There are signs of phase separation occurring in samples heated at 35, 50, and 80 ëC, presumably attributed to the increase in crystallinity during heat treatment. Increasing

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plasticizer amounts can increase the crystallinity of PLA by enhancing molecular mobility, whereas the melting temperature of the blends decreases by 10 ëC, regardless of plasticizer concentration (Ljungberg et al., 2002). TC improves elongation of PLA/starch blends compatibilized by MDI, but the presence of TC also depresses compatibilizer efficiency (Ke and Sun, 2003b). Low-molecular-weight PEG (Jacobsen et al., 1996, 1999a,b; Kim et al., 1998), poly (propylene glycol) (Ke and Sun, 2001b), and partial fatty acid (Krishnan and Narayan, U.S. Patent) are also compatible with PLA. Molecular weight of the incorporated PEG does not have a significant influence on its plasticization effect on PLA (Jacobsen et al., 1996). The regular reduction of Tg with addition of plasticizer up to the solubility limit is satisfactorily described by two well-known equations. One was proposed by Fox (eqn 10.3) for the Tg/composition dependence of polymer/diluent mixtures and miscible polymer blends: 1 w2 w1 ˆ ‡ Tg Tg2 Tg1

10:3

and the other was described as the Gordon-Taylor equation (Gordon et al., 1952): Tgb ˆ …w1 Tg1 ‡ kw2 Tg2 †=…w1 ‡ kw2 †

10:4

The analysis of dependence of melting temperature on plasticizer content in the range in which PLA and acetyl tri-n-butyl citrate (ATBC) are miscible gives indications of polymer-plasticizer interactions (Baiardo et al., 2003). The Flory (1953) equation fits well for miscible polymer-diluent mixtures: 1 1 R Vu ÿ 0ˆ

…'1 ÿ 1
'21 † Tm Tm
Hu V1

10:5

where Tm0 and Tm are melting temperatures of the pure polymer and of the polymer in a mixture with diluent volume fraction '1 , respectively;
Hu and Vu are the heat of fusion and the molar volume per polymer repeating unit; V1 is the diluent molar volume; and 1 is the polymer-diluent interaction parameter. Figure 10.7 shows the reciprocal melting temperature of PLA/ATBC blends as a function of ATBC weight fraction (w1 ). Plasticizers typically reflect polarity of the polymers with which they are blended. Hydrophobic plasticizers are used extensively with petroleum-based polymers. In contrast, plasticizers used in plastics containing starch must be polar, allowing better compatibility with the hydrophilic polysaccharide polymer. These plasticizers usually are water, glycerol, sorbitol, sucrose, urea, PEG, etc. (Lawton et al., 1994; Stein et al., 1997). Being a very effective plasticizer, water brings starch special features, but it evaporates easily, leaving an extremely brittle, glassy material. The starch granule becomes swollen and disperses in the presence of water, and it gelatinizes when heated. Gelatinized

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Biodegradable polymers for industrial applications

10.7 Reciprocal melting temperature of poly(lactic acid) (PLA)/(acetyl tri-nbutyl citrate (ATBC)) blends as function of ATBC weight fraction (Baiardo et al., 2003).

starch disintegrated granules and overcame strong crystalline intramolecular forces before mixing with other polymers (Coffin et al., 1995). Poorly gelatinized starch with 10% water gives blends with PLA a 26 MPa tensile strength, whereas the well-gelatinized starch with 20% water gives the blend 36 MPa tensile strength (Ke and Sun, 2001a), with the result that the gelatinized starch improves interfacial adhesion between starch and PLA (Park et al., 2000). Starch granule structure at different degrees of gelatinization in PLA/starch blends are shown in Fig. 10.8 (Ke and Sun, 2001a). But the increased water absorption is the obvious disadvantage of gelatinized starch. It is necessary to remove the trapped water in gelatinized starch thoroughly before blending with easily hydrolyzed PLA but during the removal of water, gelatinized starch tends to reform a strong crystalline structure because of intramolecular hydrogen bonding. Glycerol is also an effective plasticizer much less volatile than water, used to prevent re-formation of a strong crystalline structure in starch (Park et al., 2000), which provided a workable PLA/starch blend (at 50% RH, tensile strength 7.5 MPa, and elongation 4.3%) (Stein et al., 1997). PLA blended with various plasticizers exhibited various thermal and mechanical properties, as listed in Table 10.10 (Martin et al., 2001a). The low-molecular-weight PEG (Mw, 6000) acts as plasticizer giving blends relative higher elongation, compared with PLA/ starch (50/50) blends, although tensile strength is rather low (Ke and Sun, 2003c). PLA and high-molecular-weight PEG miscibilized in the amorphous

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10.8 Photographs of starches extracted from the starch and poly(lactic acid) (PLA) blends at a 60:40 ratio, with (a) 0, (b) 11.9, (c) 20, (d) 40% moisture content (Ke and Sun, 2001a).

phase. At 20 wt%, the PEG acting as a plasticizer gave PLA 500% elongation at break (Nijenhuis et al., 1996). Typical stress-strain curves of PLA/starch (60/40) blends with 15% of various plasticizers are shown in Fig. 10.9 (Ke and Sun, 2001b). The degree to which tensile strength decreased is found to be dependent upon the species of solid plasticizer used, such that the following trend is observed (weakest to strongest formulation) for PLA systems containing starch (Stein et al., 1997): Ammonium chloride < urea < proline < lysine hydrochloride < glycine < sucrose < isoleucine. Plasticization of starch alone does not seem to be an effective way to significantly improve flexibility of PLA/starch blends, because PLA composes the major component, although a better dispersion of plasticized starch is achieved (Sailaja et al., 2001). Plasticization of PLA and starch seems the effective way. Pluronic F-108 (molecular weight 14,600; polyethylenepolypropylene glycol copolymer) is well known as a good PLA plasticizer, because of its hydrophilic-hydrophobic structure. Adding 10% of such a plasticizer gives a PLA/starch (75/25) blend 870% elongation (Jun, 2000). The application of plasticizers causes some adverse effects. One of them is that, for processing-improvement purposes, it probably has an adverse effect on the adhesion strength between the layers of plasticized starch-polyester

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Biodegradable polymers for industrial applications

Table 10.10 Thermal and mechanical properties of plasticized poly(lactic acid) (standard deviations are given in parentheses) (Martin and Ave¨ros, 2001a). Material Pure PLA Glycerol 10% 20% CITRO 10% 20% M-PEG 10% 20% PEG1500 10% 20% PEG400 10% 20% OLA 10% 20%

Tg (ëC)

Tc (ëC)

Tm (ëC)

Crystallinity E modulus (%) (MPa)

EB (%)

58

no

152

1

2050 (44)

9(2)

54 53

114 110

142 141

24.3 25.4

ö ö

ö ö

51 46

no no

144 142

12 20

ö ö

ö ö

34 21

94 75

148 146

22 24

1571 (51) 1124 (33)

18 (2) 142 (19)

41 30

105 85

152 148

17 25

ö ö

ö ö

30 12

82 67

147 143

26 29

1488 (39) 976 (31)

26(5) 160 (12)

37 18

108 76

144 132

21 24

1256 (38) 744 (22)

32 (4) 200 (24)

10.9 Typical tensile curves of extruded and molded poly(lactic acid)/starch (60:40) blends containing 15% of various plasticizers as representatives (Ke and Sun, 2001b).

Poly(lactic acid)-based bioplastics

275

structures (Martin et al., 2001b). Plasticizer thermal loss during processing or reheating, even in storage (Ljungberg et al., 2003), is another adverse effect. The loss amount is directly proportional to the temperature and concentration gradient of the plasticizer, and is inversely proportional to the molecular weight (Labrecque et al., 1997). The migration of plasticizers toward the surface of polymers is a procedure of the thermal loss, which reduces the layer adhesion in multilayer films (Martin et al., 2001b). The preferred plasticizers are those with a boiling point sufficiently higher than the processing temperature to overcome thermal loss (Krishnan and Narayan, U.S. Patent). Leaching of plasticizer in liquid media is a distinctive drawback for plasticized polymeric materials, which results in loss of plasticizer from the blends. Thus, the thermal, mechanical, and other properties are distorted (Jacobsen et al., 1999a). The solubility of plasticizer in water or other contacted media and molecular weight are major factors that favor leaching (Zhang et al., 2004d). Polymeric plasticizers such as nontoxic maleate avoid thermal loss and migration and are good candidates for application in biodegradable blends (Zhang et al., 2004c).

10.5 Aging and biodegradation Physical aging, a general phenomenon, is characteristic of the glassy state of all materials. This process is thermoreversible, which means that, although a material has been physically aged, this aging can be removed by simply heating the material in excess of its Tg. The aging normally occurs in the glassy state as a consequence of room-temperature storage, develops at a faster rate as the aging temperature Ta approaches Tg, and can be attributed to relaxation of the molecules toward equilibrium. Physical aging has a dramatic influence on polymer properties, such as reducing impact strength, because it increases the relaxation times of the polymer, and also results in the plasticizers' migration and loss (Soest et al., 1994; Shogren, 1992b). Aging experiments carried out at the same undercooling (
T ˆ Tg ÿ Ta ) on PLA samples of different molecular weight (Mv ˆ 5,300, 20,000, 691,000) show that a decrease in molecular weight increases the magnitude of the enthalpy relaxation at the glass transition (Gelli et al., 1992). Storage of plasticized PLA film at room temperature near the Tg resulted in an increase in crystallinity (Ljungberg et al., 2003). After being physically aged at room temperature, the thermal transition behavior of PLA/ starch/MDI blends is more evident with aging time increase as shown in Fig. 10.10 (Wang et al., 2003), which is the result of free volume relaxation during sub-Tg aging. Mechanical properties are distorted, while the tensile strength decreases from 63.6 MPa to 51.3 MPa after one year of aging, and the elastic modulus decreases from 1.75 GPa to 1.56 GPa (Wang et al., 2003). The biological degradation of PLA is less than other aliphatic polyesters such as PBS and PCL (Hirotsu et al., 2000). PLA degrades in moisture and at elevated temperatures. Reformation monomer, hydrolysis, and thermolysis are

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Biodegradable polymers for industrial applications

10.10 DSC curves of physical aged poly(lactic acid) (PLA)/starch/ methylenediphenyl diisocyanate (MDI) (55/45/0.1) blends (Wang et al., 2003).

all possible, depending on the conditions encountered. The environmental degradation of PLA occurs by a two-step process. During the initial phases, the high-molecular weight chains hydrolyze to lower-molecular-weight oligomers and this rate is slow (Mason et al., 1981; Ogama et al., 1988a,b). The process can be accelerated by acids or bases and is affected by temperature and moisture levels. This leads to a significant change in the chemical structure of the material. The change in the weight-loss and molecular-weight distribution of the PLA films during hydrolysis in acid media proceeds homogeneously along the film cross-section by mainly a bulk erosion mechanism, and the durability of PLA films in acid media is very similar to that in a neutral medium but is higher than that in an alkaline medium (Tsuji et al., 2002). More detailed hydrolysis mechanisms in various media are listed in Table 10.11. Embrittlement of PLA occurs during this step at a point at which the Mn decreases to less than about 40,000. At about this same Mn, microorganisms in the environment continue the degradation process by converting these lower-molecular-weight components to carbon dioxide, water (in the presence of oxygen), or methane (oxygen absent) (Drumright et al., 2000; Narayan, 1993). A typical degradation of PLA under composting conditions is shown in Fig. 10.11 (Baiardo et al., 2003). For a product of at least 1 mm thickness, degradation is faster inside than at the surface because of diffusion-reaction

Poly(lactic acid)-based bioplastics

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Table 10.11 Hydrolysis mechanism of poly(L-lactic acid) films under different conditions (Tsuji and Nakahara, 2002) pH

T (ëC)

Enzyme

2.0 7.4 12.0 7.4

37

No

37

Proteinase K

7.4

97

No

Hydrolysis mechanism Chain cleavage

Material erosion

Random in amorphous region Random in amorphous region Random in amorphous region Predominant cleavage at chains with free ends and tie chains in amorphous region Random in amorphous region

Bulk Bulk Surface Surface Bulk

10.11 Elongation at break (b ), tensile strength (b ), and elastic modulus (E) as a function of the glass-transition temperature of plasticized poly(lactic acid) (PLA). Plasticizer: (o) poly(ethylene glycol) (PEG) 400; (m) PEG 1.5K; (s) PEG 10K; (©) acetyl tri-n-butyl citrate (ATBC) (Baiardo et al., 2003).

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Biodegradable polymers for industrial applications

10.12 Proposed degradation mechanism of poly(lactic acid) (PLA) in the bulk state (Torres et al., 1996).

phenomena. When immersed in an aqueous medium, the PLA-based products absorb water, and autocatalytic cleavage of ester bonds starts. Later on, degradation at the surface becomes slower than inside because of the release of water-soluble acidic oligomers and lactic acid, whereas those located well inside remain entrapped. At the end, the whole inside is composed of water-soluble oligomers, and a hollow structure is formed, because of their release to the medium, while surface degradation continues slowly (Li et al., 1995; Torres et al., 1996). A proposed degradation mechanism of PLA in the bulk state is shown in Fig. 10.12 (Torres et al., 1996). PLA degradation is enzyme sensitive, a pure PLA can degrade by 20% of its total weight after 48 h of enzymatic exposure (Sheth et al., 1997), and the weight loss under the control degradation conditions (i.e., degradation without enzyme) is 1.6%. In addition to the environmental factors, the structure of PLA also affects the degradation. The dominant factor leading to the degradation of PLA over the stereochemical range study (MacDonald et al., 1996) is the degree of crystalline order (Gajria et al., 1996). Crystal structure prevents degradation because the amorphous portion receives hydrolysis preferentially. The hydrolytic degradation rate decreases with increasing crystallinity (Tsuji et al., 1996a). A

Poly(lactic acid)-based bioplastics

279

highly oriented PLA with a high crystallinity degrades more slowly and is able to keep the shape and mechanical properties for more than a year (Doi, 1995). But the radius of spherulites had practically no significant effect on the hydrolysis of PLA films, regardless of the hydrolysis medium (Tsuji et al., 2001b). The enzymatic degradation rate decreases as a function of physical aging (Cai et al., 1996). PLA with relatively more L-lactate units degrades slowly compared with PLA having fewer L-lactate units, because the local helices are formed in higher L-lactate materials so that the main chain esterlinkages are enveloped in the array of surrounding hydrophobic methyl groups. In this conformation, the attack of water moleculars is inhibited to retard the main chain hydrolysis (Urayama et al., 2002). The presence of low-molecular-weight lactic acid derivatives and PDLLA oligomers clearly accelerates degradability of PLA in a biotic medium, and the mechanism of degradation greatly depends on the content in the oligomers (Hakkarainen et al., 2000; Mauduit et al., 1996). Both hydrolytic and enzymatic degradability of plasticized PLA by citrate-based plasticizers is influenced by plasticizer solubility, Tg, and crystallinity in a combined manner (Labrecque et al., 1997). Weight-loss rate of PLA increases in response to the addition of a small amount of water-soluble PEO (Sheth et al., 1997). The blend of PLA and P(VAc-co-VA) is expected to have enhanced mechanical properties as well as a good degradation rate if the blend forms a miscible phase at the specific vinyl alcohol content (Park and Im, 2003). The biodegradability of pure PLA significantly increases after nanocomposites preparation by layered silicates (Ray et al., 2003a). The trend in degradation pattern after different treatments of PLA with 5% PVA films is as follows: deaged-free > aged-extruded > agedpolished > deaged-fixed > annealed. More detailed information is listed in Table 10.12 (Gajria et al., 1996). Surface tension influences biodegradation, because the adherence of bioorganisms or enzymes to the polymers should be the initial step. Plasma Table 10.12 Normalized weight loss of poly(lactic acid)/poly(vinyl acetate) (95/5) melt extruded films after different treatment (Gajria et al., 1996) Time (h) 8 16 24 32 40 48 56 64

Normalized weight loss (g mmÿ2) Aged-extruded 0.448 1.55 2.93 3.46 3.5 6.2 6.14 8.11

Deaged-fixed Aged-polished 1.2 1.19 1.19 2.03 2.7 2.93 3.0 5.62

1.79 2.99 2.38 2.87 4.21 6.41 5.21 6.69

Annealed

Deaged-free

0.8 0.88 0.96 1.31 1.5 1.7 1.76 1.96

2.42 3.1 3.1 5.53 7.83 8.07 9.79 10.6

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Biodegradable polymers for industrial applications

exposure is effective for surface-specific etching (i.e., removal the skin layer and charging the surface to hydrophilic). The hydrophilic changes increase the biological compatibility on the surface. The surfaces of PLA sheets are etched to form characteristic morphology, and the patterns are different, depending on the type of plasma discharge, either with O2-, He- or N2-. Polar groups composed of -COOH and -OH are incorporated by plasma treatment, and the surface become wettable. Surface modification becomes effective after a short treatment period (e.g., 30s). The surface properties are closely related to the increase in surface energy of the polar contribution (Hirotsu et al., 2002). Biodegradation of the PLA sheets is not enhanced practically, however, even though the surface becomes hydrophilic after plasma treatment. Therefore biological degradation is dominated by the bulk reactions of polymers through the cleavages of ester groups, and the surface effects are negligible in PLA. The interfacial adhesion strength has an obvious influence on the degradation characteristics of carbon fiber-reinforced PLA composites. Those having better adhesion require a relatively longer time to degrade than those with low adhesion (Wan et al., 2001b). Biodegradation also depends on the medium used. Molded PLA bars could degrade very little after one year buried in soil in the midwestern United States. Addition of cornstarch to the PLA bars facilitates degradation of PLA, but starch degrades much faster by strains of either bacteria or fungi (Shogren et al., 2002). In general, increasing the amount of starch in the blend increases fungus growth rate, and the rate depends on the ease with which starch can be accessed by the microorganism. Higher starch content, higher degree of gelatinization/ degradation of starch, and higher water permeability are factors that favor the rapid growth of fungus (Vaidya et al., 1994). Mineralization percentages of the materials' carbon content of PLA with coextruded starch evaluated in a liquid, in an inert solid, and in composting media are 65%, 59%, and 63%, respectively. These values are higher than the standard minimum requirement of at least 60% for degradation. A blend of PDLLA with PLA subjected to increasing hydrolysis in phosphate-buffered solution (pH 7.4 and 37 ëC) revealed that degradation takes place preferentially in the amorphous region, rather than in the crystalline region (Tsuji et al., 1997). According to the increasing degree of degradation by the media, the order was: liquid medium > composting medium > inert solid medium (Gattin et al., 2003).

10.6 Applications of PLA based bioplastics The medical field provides early applications of PLA, for use as suture and in drug delivery. Usage as bioplastics is also attracting more and more attention. Biaxially oriented film for food packaging is a major application of PLA because of its excellent barrier for flavor constituents and heat sealability (Smith et al., 2001). Also, PLA/starch materials are ideally suited for environmentally friendly usage

Poly(lactic acid)-based bioplastics

281

for applications in which recovery of the products is not practical. Such applications would be diverse, such as, mulch films for agriculture, trash bags, fast-food utensils, package containers, and single- or short-term usage items (Shogren et al., 1992a; Jacobsen et al., 1999b; Graaf et al., 2001; Petersen et al., 2001; Leaversuch, 2002). Extruded starch/PLA foams are also available as replacements for expanded poly(styrene) in loose-fill packaging applications, because of their lower densities and great radial expansion ratio (Willett et al., 2002). Greatest expansions and lowest densities are generally achieved through the use of modified high-amylose starches. Incorporation of PLA into starch increased radial expansion and decreased water solubility (Biby et al., 2001; Fang et al., 2000, 2001). Sandwiched PLA/starch/PLA laminate sheets by co-extrusion, with high peel strength, solved the rapid water absorption problem, which rendered its possible applications for food packaging, controlled release of drugs, pesticides, insect diets, etc. (Wang et al., 2000). Porous biodegradable PLA films can be also prepared by extraction of PEO from their blends (Tsuji et al., 2000).

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11

Biodegradable protein-nanoparticle composites

K D E A N a n d L Y U , CSIRO ± Manufacturing and Infrastructure Technology, Australia

11.1 Introduction 11.1.1 Natural biodegradable polymers Natural macromolecules such as protein, cellulose and starch are generally degraded quite rapidly by hydrolysis and the action of micro-organisms. As such they present an alternative to more traditional petroleum based plastics which can be relatively inert and take a significant amount of time to degrade (Paetau et al., 1994). Natural polymers may help to solve many solid-waste environmental issues, particularly in commercial applications such as packaging of single-use items. A significant amount of research has been undertaken to develop a thorough understanding of biodegradable polymers and their potential applications (Iwanami and Uemura, 1993; Chandra and Rustgi, 1998; Yu et al., 1998, 1999; Otaigbe et al., 1999; Salmoral et al., 2000; Mohanty et al., 2000; Yu and Christie, 2001; Jana et al., 2001; Gross and Kalra, 2002; Singh and Singh, 2003; Vaz et al., 2002, 2003). The focus of this chapter is plant protein. Chickpea and soy protein isolates (Salmoral et al., 2000) are two of the more common plant proteins that are used to produce biodegradable plastic film materials. However, there are many other proteins used in film production, including wheat (Bejosano and Corke, 1999), pistachio (Ma et al., 2002), sunflower (Orliac et al., 2003) and peas (Gueguen et al., 1998). Proteins are formed by the condensation polymerisation of various combinations of amino acid repeat units and different plant sources produce proteins with different amino acid combinations and thus exhibit different properties (McGrath and Kaplan, 1997). Their general structure is illustrated in Fig. 11.1. Commercially available soy products include soy isolate, soy concentrate and soy flours (Kumar et al., 2002; Paetau et al., 1994). Soy protein isolates are generally used in the preparation of soy-based plastics due to their high protein content (approximately 90%) (Paetau et al., 1994) and are generally prepared from an alkaline suspension of defatted soy flour which is acidified to the isotonic point at which the proteins precipitate. The precipitated proteins are

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11.1 Generalised protein structure.

generally washed and centrifuged (Paetau et al., 1994; Salmoral et al., 2000; Achouri et al., 1999). A number of authors have reported information on the processing and properties of plastics formed from soy protein (Zhang et al., 2001; Cho and Rhee, 2002). Extruded soy protein is generally plasticised with glycerol and water, and the resulting properties (mechanical and thermal) are quite dependent on the amount of plasticiser that is contained in the extruded soy protein (Zhang et al., 2001). Irradiation of proteins to promote crosslinking has been reported by a number of authors including Salmoral et al. (2000) and Mezgheni et al.,(Mezgheni et al., 2000). These authors utilised gamma-irradiation to produce crosslinked films and solid samples and this work showed that irradiated films had better mechanical properties and were quite resistant to water.

11.1.2 Nanocomposites Polymer nanocomposites generally consist of a nano-scale layered clay dispersed within a polymer matrix. There are two types of nanocomposite structures which may be formed, these are termed intercalated and exfoliated nanocomposites (see Fig. 11.2). In an intercalated nanocomposite often a single polymer chain will be driven between the clay silicate layers, but the system still remains quite well ordered in a stacked type of arrangement. In an exfoliated nanocomposite the silicate layers are completely delaminated from each other and are well dispersed. It is this second type ± the exfoliated nanocomposite, which has been shown to exhibit the most significant improvements in physical properties (Usuki et al., 1993; Kojima et al., 1993; Vaia and Giannelis, 1997; Giannelis, 1998). The degree of intercalation and exfoliation of layered silicates in polymer nanocomposites can be quantified using wide-angle X-ray diffraction (WAXS) and transmission electron microscopy (TEM). Generally, the intercalated layered silicate has a defined interlayer spacing basal reflection corresponding to the d001 spacing in a WAXS diffractogram, conversely there is no coherent X-ray diffraction from the exfoliated silicates. TEM is a complementary technique to WAXS where an image of the dispersion of the silicate within a polymer matrix can be quantified and analysed (Sinha Ray and Okamoto, 2003; LeBaron et al., 1999). Montmorillonite, hectorite and saponite are frequently used pristine layered silicates which are combined with polymeric materials to form nanocomposites

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11.2 Schematic of exfoliated and intercalated nanocomposite formation (Giannelis, 1998).

(Sinha Ray and Okamoto, 2003). These clays usually contain hydrated sodium or potassium ions (Giannelis, 1998) and in this state these silicates are miscible only with hydrophilic polymers such as poly(ethylene oxide) (PEO), poly(vinyl alcohol) PVA and natural polymers such as starches and proteins (Sinha Ray and Okamoto, 2003). Cationic surfactants (e.g. alkyl ammonium ions) may be ion exchanged with these hydrated ions to enable the intercalation of numerous engineering polymers. The alkyl ammonium cations in the layered silicates improve the wetting characteristics with the polymer and can provide functional groups that can react with the polymer or initiate polymerisation of monomers to improve the strength of the interface between the inorganic component and the polymer (Vaia and Giannelis, 1997; Giannelis, 1998; Alexandre and Dubois, 2000). In-situ polymerisation, solvent intercalation/exfoliation and melt intercalation/exfoliation are the three major pathways for the formation of nanocomposites. In-situ polymerisation involves the combination of clay and monomer, followed by the polymerization of the monomer, which ideally locks the exfoliated clay particles in the resulting polymer matrix. In solvent intercalation the clay is first swollen in a solvent and the polymer (intercalant) is dissolved in the solvent. Both solutions are then combined and the polymer chains intercalate and displace the solvent within the interlayer of the clay (Shen et al., 2002). In melt intercalation the clay and polymer are added together above the melting temperature of the polymer; they may be held at this temperature for a period of time, put under shear, or other conditions to encourage intercalation and exfoliation of the clay. The focus of this chapter is melt intercalation

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although due to the nature of some of these systems some crossover with solvent intercalation may be observed (Alexandre and Dubois, 2000).

11.1.3 Biodegradable nanocomposites The literature available for natural biodegradable nanocomposite materials is quite limited. Park and co-workers (2002, 2003) reported on the preparation and properties of starch/montmorillonite clay nanocomposites. In this work the naturally occurring Na+ montmorillonite (Cloisite Na+) and three alkyl ammonium modified clays (Cloisite 30B, 10A and 6A) were used to form the nanocomposites. Initially the starches were gelatinised with glycerol and water, and then allowed to sit for one hour prior to processing on a Haake mixer at 110 ëC, after which the gelatinised starch was cooled and cut into small pieces. The gelatinised starch pieces and the various clays were then dry mixed in a roller mixer for 20 minutes. WAXS showed intercalation in starch/Cloisite Na+. Cloisite 30B and 10A starch nanocomposites showed some broadening of the d-001 peak indicative of partial exfoliation but the peaks essentially remained in the same position indicating minimal further intercalation. For the Cloisite 6A nanocomposite the clay peak remained in a similar position to the neat Cloisite 6A but the intensity had decreased, perhaps due to dilution as the authors gave no indication of internal referencing. The TEM images of these four systems showed an intercalated structure for the starch/ Cloisite Na+, Cloisite 30B remained at a similar level of displacement between particles whether as the neat clay or when it was blended with the starch. The Cloisite 6A and 10A appeared as large particle agglomerates indicating lack of compatibility between clay and starch as may be expected for these two modifications (Park et al., 2002, 2003). Mechanical testing showed that the modulus of the starch/Cloisite Na+ nanocomposite exhibited the greatest increase of all the clay types used. Park and co-workers (2003) also investigated the effect of clay content on the barrier properties of the nanocomposites and found that increasing clay content led to an improvement in barrier properties. Wilhelm et al. (2003) investigated the formation of starch/clay nanocomposites, however in their case the clay used is a Ca2+ hectorite. In this study the starch was not melt blended with the clay, but rather, the clay was dispersed in distilled water prior to addition to an aqueous dispersion of starch. This blend was then degassed and heated to boiling point for 30 minutes to gelatinise the starch; after this glycerol was added to the hot solution, then the material was poured onto polypropylene dishes and the solvent allowed to evaporate. The first basal spacing (corresponding to the interplanar distance) for Ê , this was shifted below the resolution of the WAXS for the pure clay was 14.4 A 90:10 starch:clay mixture indicating almost total exfoliation in the starch matrix. Although this was a good result, starch requires some kind of plasticiser to reduce its brittleness. When glycerol was added by itself to the clay the

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Ê , however a peak at 9.3 A Ê also appeared interplanar distance increased to 18.5 A corresponding to the d-001 spacing for the dehydrated clay. Fischer and Fischer (2001; Fischer, 2003) also investigated starch/clay nanocomposites. In this work, a number of experimental pathways were investigated, including the dispersion of Na+ montmorillonite clay in water, followed by blending in an extruder at a temperature of 85±105 ëC with a premixed powder of potato starch, glycerol and water. The resulting material appeared to be fully exfoliated and exhibited a reduction in hydrophilicity, and improved stiffness, strength and toughness.

11.2 Delaminating clay using ultrasonics Sonifiers and ultrasonic baths have been used to improve the level of intercalation and exfoliation particularly for in-situ polymerised nanocomposites (Artzi et al., 2002; Liao et al., 2001; Okamoto et al., 2000). If a sound wave is of sufficient energy, cavitation bubbles are created at sites of rarefaction as the liquid fractures or tears because of the negative pressure of the sound wave in the liquid. As the wave fronts pass, the cavitation bubbles can eventually grow to be unstable under the influence of positive pressure. The high-speed collapse of the cavitation bubbles results in implosions, which cause shock waves to be radiated from the sites of the collapse. High temperatures and pressures can be generated at the implosion sites of cavitation bubbles. In this study a Branson sonifier (model 250 W cell disruptor) with a maximum mechanical vibration frequency of 20 kHz was used to aid intercalation and exfoliation of the clay and plasticisers. A standard half-inch diameter flat horn tip was used at approximately 40% output. In this study, separate dispersions of water/clay and glycerol/clay were sonified for varying times to observe the effects of the sonification treatment. For the extrusion scaleup water and glycerol (1:1 wt:wt) were initially blended together for two minutes in a larger beaker using a mechanical stirring device, the Cloisite Na+ was subsequently added continuing with a further two minutes of stirring. The beaker of clay/glycerol/water mixture was then sonified for one hour; the whole system was immersed in an ice bath to reduce the amount of evaporation of the water and glycerol due to the heat generated at the ultrasonic tip. The mixture was weighed after the ultrasonic treatment and was found to be unchanged within error (1%), indicating minimal evaporation from the system. The WAXS measurements were performed using a BruÈker D8 Diffractometer operating at 40 kV, 40 mA, Cu K radiation monochromatised with a graphite sample monochromator. A diffractogram was recorded between 2 angles of 1ë and 25ë. Monitoring the d-001 spacing corresponding to the interlayer spacing of Ê for Cloisite Na+) enabled intercalation and exfoliation to be the clay (at 10 A observed. The d-100 at 20ë2 was used as an internal standard in many cases to standardise diffractograms in relation to the percentage of clay in each system.

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11.3 WAXS diffractograms of Cloisite Na+:H2O (ratio 1:4) sonified for various periods of time.

The neat Cloisite Na+ (as received from the manufacturer) had a d-001 spacing Ê , when dried under vacuum at 100 ëC for 12 hours this was reduced to of 12.3 A Ê . The emulsions of Cloisite Na+ and water or glycerol were placed in liquid 10.0 A WAXS sample holders and scanned immediately to avoid the effects of evaporation. With a low ratio of water to clay (4:1 wt:wt) some broadening of the Ê (see Fig. 11.3 and Table 11.1), WAXS diffractogram was observed around 35 A although the extent of interaction was minimised due to the high viscosity of the pastes making complete sonification difficult. With the further addition of water (5:1 wt:wt water:clay) a greater amount of intercalation was observed (see Fig. Ê ) to 11.4 and Table 11.1), with a broadening of the WAXS peak (originally at 10 A Ê after 15 minutes of sonification. This broadening of the WAXS peak 35±40 A gave some indication of the range of different states in which the silicate existed. As expected, further increasing the water to clay ratio (10:1 wt:wt) increased the dispersion of the clay and enabled the sonification treatment to be more efficient Ê corresponded to (see Fig. 11.5 and Table 11.1). The original shift to 15.6 A approximately two layers of water molecules in the clay interlayers. After 15 Ê were also observed, so minutes of treatment higher order peaks up to 62 A Ê still existed at 25 minutes of treatment, not all clay although the peak at 15.6 A layers were separated by this distance and in fact there was most likely a range of Ê in separation (see Fig. 11.5). clay configurations from 15.6 to 62 A The WAXS diffractograms for Cloisite Na+:glycerol (ratio 1:10 wt:wt) sonified for varying times are illustrated in Fig. 11.6. Although the intercalation

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Table 11.1 Cloisite Na+, H2O (distilled) and glycerol d-001 spacing (Ð) for varying ratios at different sonification times Sonification time (minutes) 0 5 10 15 20 25 30

1:4 (clay: water)

1:5 (clay: water)

1:10 (clay: water)

1:10 (clay: glycerol)

1:10:10 (clay:glycerol: water)

15.7 15.45 15.65 and 32±35 Ð 15.7 and 32±35 Ð 15.7

15.6 15.609 32.585 15.712

15.5 15.538 33 15.49

18.3 18.3 18.3

18.3 54.4 63.9

32.585 15.62

33 15.568

38.5

Very small variations ranging from 55±65 Ð

41.1

Very small variations ranging from 55±65 Ð

41.5

None visible

62

None visible

32 45±33 broad 15.5129 peak 15.7 15.5 32 46 15.58 15.32 Broadening ö 62.16 up to 60 Ð 15.408 15.6

11.4 WAXS diffractograms of Cloisite Na+:H2O (ratio 1:5) sonified for various periods of time.

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11.5 WAXS diffractograms of Cloisite Na+:H2O (ratio 1:10) sonified for varying periods of time.

11.6 WAXS diffractograms of Cloisite Na+:glycerol (ratio 1:10) sonified for varying periods of time.

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Ê , there was a of glycerol increases the separation of the clay layers to 18.3 A minimal effect observed by the action of sonification. However, after 15 minutes of sonification treatment some small peaks were observed ranging from 38 to Ê (see Table 11.1). 62 A The WAXS diffractograms for Cloisite Na+:glycerol:H20 (ratio 1:10:10 Ê wt:wt:wt) are illustrated in Fig. 11.7. After initial mixing a broad peak at 18.3 A was observed, corresponding to the d001 spacing or interlayer clay spacing. This was similar to that observed in the ClositeNa+:glycerol system (see Fig. 11.6) where a relatively sharp peak was observed in the same position. The broadness of this particular peak may give an indication of the different states with which Ê , but the clay exists in this system. Some of the layers may be separated by 18.3 A Ê (corresponding to the outer edges may range from separations of 15.6 to 19.9 A of this WAXS peak). After five minutes of ultrasonic treatment this broad peak Ê had disappeared and a peak at around 64 A Ê could be seen (see Fig. 11.7 at 18.3 A Ê and Table 11.1). This higher peak at around 64 A was still visible after ultrasonic treatment for 20 minutes, but in the diffractograms of this system after 25 and 30 minutes of the treatment no peaks were observed, indicating good dispersion or exfoliation of clay particles in the system. The main mechanism for the expansion of the clay layers (as observed in the WAXS diffractograms) was by an interaction between the sodium ion of the sodium montmorillonite and the dipole of the water/glycerol (see Fig. 11.8). This kind of mechanism has been implied by other authors (Wilhelm et al., 2003; Kozak and Domka, 2004).

11.7 WAXS diffractograms of Cloisite Na + :glycerol:H 2 O (ratio 1:10:10 wt:wt:wt) sonified for varying periods of time.

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11.8 Ion-dipole interactions between the metal ion located between the clay layers and either water or glycerol.

11.3 Processing protein-nanoparticle composites using extrusion The materials used in this study included a water-soluble protein (Profam 974, supplied by Archer Daniels Midland), glycerol (supplied by Aldrich), sodium montmorillonite (Cloisite Na+, supplied by Southern Clay Products) and distilled water. For the neat protein formulation, 400 g of water and 400 g of glycerol were initially blended for two minutes in a large beaker using a mechanical stirring device. This mixture was then added drip-wise to 1200 g of the soy protein using a ten-litre laboratory scale high-speed mixer (HSM-10) for five minutes then extruded using a twin extruder (Theysohn 30) at the highest setting up temperature, 140 ëC. A die width of 300 mm was used. The sheets were pulled out of the die using a three-roller system with positive tension. The thickness of the sheets varied from 0.15±0.35 mm depending on drawing speed. For the protein nanocomposite formulation, 400 g of water and 400 g of glycerol were also blended in a large beaker with a mechanical stirring device for two minutes, prior to the addition of 60 g of Cloisite Na+. For the untreated system the mixture was then combined drip-wise with 1200 g of the soy protein using a ten-litre laboratory scale high-speed mixer (HSM-10) for five minutes prior to extrusion similar to the neat protein film. For the ultrasonically treated system the mixture was treated with a point source ultrasonic device (Branson sonifier model 250 W cell disruptor) for one hour in an ice-cooled environment (as outlined in section 11.2). This mixture was then combined drip-wise to 1200 g of the soy protein using a ten-litre laboratory scale high-speed mixer (HSM-10) for five minutes prior to extrusion similar to the neat protein film.

11.4 Microstructure and mechanical properties of protein-nanoparticle composites The main tools used for assessing the dispersion of clays in these nanocomposites were WAXS and transmission electron microscopy (TEM). The WAXS measurements were performed using a BruÈker D8 Diffractometer as described in section 11.2. Monitoring the d-001 spacing corresponding to the

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11.9 WAXS diffractograms of the neat protein/glycerol, neat Cloisite Na+ and the resulting nanocomposite films.

Ê for Cloisite Na+) enabled intercalation interlayer spacing of the clay (at 10 A and exfoliation to be observed. The d-100 at 20ë2 was used as an internal standard in many cases to standardise diffractograms in relation to the percentage of clay in each system. Used in conjunction with WAXS, transmission electron microscopy was a valuable tool used in the study of nanocomposites. TEM can give an indication of the levels of dispersion of clay particles, whether they exist as single particles or tactoids, however intercalated and exfoliated they are. The WAXS diffractogram for the untreated protein/glycerol/water/Cloisite + Na sample (Fig. 11.9) indicates a range of clay configurations, observed as a Ê to broad series of peaks corresponding to interlayer clay spacings from 10 A Ê 60 A. The diffractogram for the ultrasonically treated protein/glycerol/water/ Cloisite Na+ sample (Fig. 11.9) shows significantly different behaviour ± the broad series of peaks had disappeared, indicating the nanocomposite formed was exfoliated. The samples for TEM were crosslinked with Osmium Tetroxide (OsO4) prior to embedding in epoxy resin. The samples were agitated in uncured epoxy resin for 24 hours to encourage impregnation of the resin into the samples, followed by curing at 60 ëC for 24 hours. 70±90 nm sections of the samples were microtomed at room temperature using an Ultracut E microtome at a cutting speed of 0.05 mm/s. The sections were cut perpendicular to the flow direction of

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11.10 TEM image of untreated protein/glycerol/water/Cloisite Na+ sample.

the extruded sheet. A Jeol 100S TEM was used at 100 keV using magnifications from 25,000 to 100,000 times to study dispersions of clay particles. The TEM image of the untreated protein/glycerol/water/Cloisite Na+ sample (Fig. 11.10) confirms the WAXS data, which indicated that a range of clay configurations were present. In Fig. 11.10, a large agglomerate approximately 500 nm in width and 1,000 nm in length is clearly visible, along with other smaller tactoids of silicates (5±20 particles) and some single silicates. The TEM image of the ultrasonically treated protein/glycerol/water/Cloisite Na+ sample (Fig. 11.11) showed significantly different behaviour. There were no large agglomerates visible, there were some smaller tactoids (3±5 particles), but the dominant structure observed consisted of single exfoliated silicates. This dominant exfoliated structure was in agreement with the WAXS, where no clear peaks corresponding to particular interlayer silicate spacings were observed (see Fig. 11.9). The dynamic mechanical behaviour of the protein films was measured with a Perkin-Elmer Pyris Diamond DMA (sinusoidal oscillation of rectangular specimens in tension). The sheet samples were cut into rectangular bars (20 mm by 8 mm) with the longitudinal direction of the bar being parallel to the flow direction from the extruded sheet. The samples were thinly coated in silicone oil to minimise the evaporation of the plasticisers. The protein samples were scanned from ÿ100 to 130 ëC at 2 ëC/min. The glass transition temperature was determined by the maximum in tan at 1 Hz in the dynamic mechanical

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11.11 TEM image of treated protein/glycerol/water/Cloisite Na+ sample.

thermal analysis spectrum. Secondary transitions at lower temperatures were determined by smaller maxima in the tan at trace. Experiments were repeated to ensure consistency in the results. The DMA tan at traces for the neat protein, the untreated nanocomposite and the treated nanocomposite are illustrated in Fig. 11.12. The neat protein/glycerol/water sample shows a single Tg at 108.5 ëC. The secondary (or ) relaxation in the neat protein/glycerol/water sample was situated at ÿ52.0 ëC and may have been due to the motion of the hydrated soy protein side groups, or the crankshaft motion of the main chain as described previously (Zhang et al., 2001; Baschek et al., 1999). The protein/glycerol/water/Cloisite Na+ (untreated) sample also showed a single Tg at 112.5 ëC, however this Tg is at a slightly elevated temperature compared to the neat protein, due to the restriction in molecular motion of the protein by the dispersion of the nano-clay. The secondary (or ) relaxation in the neat protein/glycerol/water/Cloisite Na+ (untreated) sample was situated at ÿ55 ëC (3 ëC lower than in the neat protein). This lowering of the relaxation may have been due to the disruption of the protein side groups due to the large agglomerates of clay particles in the system (Zhang et al., 2001). The protein/glycerol/water/Cloisite Na+ (treated) sample also showed a single Tg at 118.0 ëC, 5.5 ëC higher than observed in the untreated nanocomposite, due to the further restriction in molecular motion of the protein by the predominantly exfoliated dispersion of the nano-clay. The secondary (or ) relaxation in the neat protein/glycerol/water/Cloisite Na+ (untreated) sample was situated at

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11.12 The DMA tan  trace for the neat protein, the untreated nanocomposites and the treated nanocomposite.

ÿ51 ëC, in a similar position to the neat protein/glycerol/water sample, indicating that the finely dispersed clay particles did not significantly affect the motion of the protein side groups or crankshaft motion of the main chain as described previously (Zhang et al., 2001) in protein systems. In all three systems (see Fig. 11.12), a broad transition due to ice melting (Zhang et al., 2001) was observed at around 0 ëC, however, due to the overlapping of the other stronger transitions of the protein, this peak cannot be resolved. The variation in storage modulus (E0 ) for the neat protein, the untreated nanocomposite and the treated nanocomposite over the temperature range from ÿ100 to 130 ëC (see Fig. 11.13), clearly showed that the ultrasonically treated protein/glycerol/water/Cloisite Na+ system exhibited a higher storage modulus throughout the whole range. At 25 ëC, the neat protein/glycerol/water exhibited a storage modulus close to 500 MPa, similar to that obtained by Sue et al. (1997) for a plasticised soy plastic. The untreated protein/glycerol/water/Cloisite Na+, showed a similar value of storage modulus at 25 ëC (ca. 500 MPa), however the ultrasonically treated protein/glycerol/water/Cloisite Na+ exhibited a storage modulus value of 1,200 MPa, reconfirming the positive structure/property relationship due to exfoliation of the clay particles. The glass transition temperatures (Tgs) were also measured using differential scanning calorimetry (Perkin-Elmer DSC-7) operated in scanning mode (20 ëC/ min) under a N2 atmosphere. The temperature and enthalpy were calibrated using high purity indium (transition point ˆ 156.61 ëC, transition energy ˆ 28.45 J/g) and high purity zinc (transition point ˆ 419.47 ëC) standards. A

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11.13 The storage modulus (E0 ) versus temperature in a scanning DMA trace for the neat protein, the untreated nanocomposite and the treated nanocomposite.

baseline was recorded using two empty aluminium pans that were scanned over the same temperature range of the experiment then subsequently subtracted from the scan of the sample. Approximately 10 mg of the protein samples were sealed in aluminium pans and scanned from ÿ35 to 120 ëC. The Tgs of the samples were determined using the mid-point method. The Tgs obtained from the DSC (using the midpoint method) showed similar trends to the maximum in tan  from the DMA (see Fig. 11.14). The neat protein/glycerol/water sample had a Tg of 51 ëC, the untreated protein/glycerol/water/Cloisite Na+ had a slightly higher Tg of 52.5 ëC and the ultrasonically treated protein/glycerol/water/Cloisite Na+ sample had a Tg of 58 ëC. Despite similarities in trends, the values of Tg from the DSC were significantly lower than found with DMA. This is due, in part, to the faster scanning (20ë/minute compared to 2ë/minute for the DMA) which had to be used to capture the Tgs, as all samples exhibited very broad glass transitions. Also, the DMA samples in tension may have lost moisture (despite their silicone oil coating), hence reducing the mobility of the protein chains, which would be observed as an increase in the Tg that was actually measured. To clarify this point further, DMA samples were also tested in compression (enclosed between parallel plates). Similar results were found, indicating that the moisture loss in tension was similar to that in compression. From this it may be concluded that the coating of silicone oil on the surface of the samples in tension was quite effective in reducing the amount of glycerol/water loss, but despite this some

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11.14 DSC thermograms of the neat protein, the untreated nanocomposite and the treated nanocomposite (scanning rate 20ë/minute).

glycerol and water loss occurred with any DMA scanning experiment. This was also confirmed by the brittle nature of the samples taken from the DMA on completion of the temperature scans. The effects of nanocomposite formation are reflected in the mechanical property improvements, where the most significant improvement in modulus is observed in the ultrasonically treated sample. The mechanical properties were measured as outlined in ASTM 638 using a United Testing Rig STM10 with a 1 kN load cell. The test speed used was 10 mm/min with an average of seven samples tested for each different system. The changes in mechanical properties as compared to the neat protein/glycerol/water sample are shown in Fig. 11.15. The most significant improvement observed in the mechanical properties was the increased modulus, in particular for the ultrasonically treated sample, which exhibited an increase of 84% compared with the neat protein/glycerol/water sample. The experimental values for the mechanical properties (including standard deviations) are listed in Table 11.2. An improvement in tensile strength was also observed for both nanocomposite protein materials, with the untreated protein/glycerol/water/Cloisite Na+ and ultrasonically treated protein/glycerol/ water/Cloisite Na+ showing a 23% and 47% increase in tensile strength respectively. As is expected from the inclusion of a hard silicate into a polymeric material, the break elongation for both nanocomposites is reduced moderately. The thermal behaviour of the protein samples was measured using a Perkin Elmer Thermal Gravimetric Analyser (TGA), with a scanning rate of 10 ëC/min to give a general thermal profile (see Fig. 11.16) and more specifically to estimate the variation in water and glycerol content due to extrusion and ageing for two weeks at ambient temperature. As illustrated in Fig. 11.16, there was

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11.15 Summary of the percentage change in mechanical properties (modulus, tensile strength and elongation at break) for the neat protein, the untreated nanocomposite and the treated nanocomposites (tested at 10 mm/min average seven samples).

Table 11.2 Experimental average values of modulus, tensile strength and break elongation for the neat protein/glycerol/water, the untreated protein/glycerol/ water/Cloisite Na+ and the ultrasonically treated protein/glycerol/water/Cloisite Na+

Neat protein/glycerol/water Protein/glycerol/water/ Cloisite Na+ untreated Protein/glycerol/water Cloisite Na+ ultrasonically treated

Modulus (MPa)

Tensile strength (MPa)

Break elongation (%)

531 SD 49 774.5 SD 42 979 SD 60

12.5 SD 0.87 15.34 SD 1.13 18.38 SD 0.94

29.87 SD 4.23 27.4 SD 4.5 24.86 SD 3.1

SD (standard deviation) and average calculated from seven samples.

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11.16 TGA results for the neat protein, the untreated nanocomposite and the treated nanocomposites.

little variation in weight between samples over the temperature range scanned. One may conclude that the untreated nanocomposite material was slightly lower in water content and/or lost slightly more water than the other samples, however a variation of 2% could quite easily be due to error. TGA experiments were performed to verify the accuracy of the mechanical property results, as variations in water and glycerol content could have had a significant effect on the mechanical properties of these samples.

11.5 Conclusion In this research protein-based nanocomposites have been successfully produced via a novel melt blending technique. In the blends, the unmodified sodium montmorillonite clay was initially treated with a high-powered sonifier in a solution of glycerol and distilled water. This solution was then added to a soy protein isolate and processed through an extruder. The ultrasonically treated nanocomposite material that was produced exhibited an exfoliated type structure (as observed by WAXS and TEM) and an improvement in modulus and tensile strength of 84% and 47% respectively (compare the neat protein/glycerol/water sample). The glass transition temperature (from the maximum of tan  in the DMA trace) for the ultrasonically treated protein-nanocomposite was 118.5 ëC, an increase of 9.5 ëC over the neat protein/glycerol/water, and the nanocomposite films produced were uniform and relatively transparent.

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This study has illustrated that clean ultrasonic energy promotes the intercalation and exfoliation of unmodified montmorillonite clays, and when melt blended in particular combinations with protein and plasticisers produces a biodegradable polymer film that shows significant improvement in thermal and mechanical properties. The complete exfoliation of clay silicates without the use of chemical modification is significant both in terms of cost and biodegradability. Complicated modifications of the silicate surfaces or grafting within the polymer matrix can be costly. Furthermore, the addition of chemical groups (either to the silicates or in the polymer matrix) may have a significant effect on the ability of these materials to degrade completely and safely. This clean technology is important in the development of future application for biodegradable protein nanocomposites. This may include biomedical or more general industrial applications such as environmentally friendly packaging. It has been shown that the thermal and mechanical properties of protein nanocomposites can be significantly manipulated using different treatments and additions of nano-clays. Future research in this area may include an investigation on the effects of nano-clay dispersions on biodegradation rates of natural polymers. Understanding and control of degradation rates is of particular significance in biodegradable packaging where synchronising the degradation of products' use-by dates and the onset of degradation of the packaging would be an advantage.

11.6 References Achouri, A., Zhang, W. and Shiying, X. (1999) Enzymatic hydrolysis of soy protein isolate and effect of succinylation on the functional properties of resulting protein hydrolysates. Food Research International 31 (9): 617±623. Alexandre, M. and Dubois, P. (2000) Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Materials Science and Engineering 28 (1±2): 1±63. Artzi, N., Nir, Y., Narkis, M. and Siegmann, A. (2002) Melt blending of ethylene-vinyl alcohol copolymer/clay nanocomposites: Effect of the clay type and processing conditions. Journal of Polymer Science Part B: Polymer Physics 40 (16): 1741± 1753. Baschek, G., Hartwig, G. and Zahradnik, F. (1999) Effect of water absorption in polymers at low and high temperatures. Polymer 40 (12): 3433±3441. Bejosano, F.P. and Corke, H. (1999) Properties of protein concentrates and hydrolysates from Amaranthus and Buckwheat. Industrial Crops and Products 10 (3): 175±183. Chandra, R. and Rustgi, R. (1998) Biodegradable polymers. Progress in Polymer Science 23 (7): 1273±1335. Cho, S.Y. and Rhee, C. (2002) Sorption Characteristics of Soy Protein Films and their Relation to Mechanical Properties. Lebensmittel-Wissenschaft und -Technologie 35 (2): 151±157. Fischer, H. (2003) Polymer nanocomposites: from fundamental research to specific applications. Materials Science and Engineering: C 23 (6±8): 763±772.

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Fischer, H. and Fischer, S. (2001) Biodegradable thermoplastic material. WO 01/68762 A1. Giannelis, E.P. (1998) Polymer-layered silicate nanocomposites: synthesis, properties and applications. Applied Organometallic Chemistry 12 (10±11): 675±680. Gross, R.A. and Kalra, B. (2002) Biodegradable Polymers for the Environment. Science 297 (5582): 803±807. Gueguen, J., Viroben, G., Noireaux, P. and Subirade, M. (1998) Influence of plasticizers and treatments on the properties of films from pea proteins. Industrial Crops and Products 7 (2-3): 149±157. Iwanami, T. and Uemura, T. (1993) Properties and applications of starch based biodegradable polymer `mater-bi'. Kobunshi Ronbunshu/Japanese Journal of Polymer Science and Technology 50 (10): 767±774. Jana, T., Roy, B.C. and Maiti, S. (2001) Biodegradable Film 6. Modification of the film for controlled release of insecticides. European Polymer Journal 37: 861±864. Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Fukushima, Y., Kurauchi, T. and Kamigaito, O. (1993) Mechanical properties of nylon 6-clay hybrid. Journal of Materials Research 8 (5): 1185±1189. Kozak, M. and Domka, L. (2004) Adsorption of the quaternary ammonium salts on montmorillonite. Journal of Physics and Chemistry of Solids 65 (2-3): 441±445. Kumar, R., Choudhary, V., Mishra, S., Varma, I.K. and Mattiason, B. (2002) Adhesives and plastics based on soy protein products. Industrial Crops and Products 16 (3): 155±172. LeBaron, P.C., Wang, Z. and Pinnavaia, T.J. (1999) Polymer-layered silicate nanocomposites: an overview. Applied clay science 15 (1-2): 11±29. Liao, Y., Wang, Q. and Xia, H. (2001) Preparation of poly(butyl methacrylate)/gammaAl2O3 nanocomposites via ultrasonic irradiation. Polymer International 50 (2): 207±212. Ma, Z., Morgan, D.P., Felts, D. and Michailides, T.J. (2002) Sensitivity of Botryosphaeria dothidea from California pistachio to tebuconazole. Crop Protection 21 (9): 829± 835. McGrath, K. and Kaplan, D. (1997) Protein-Based Materials. In: Sadat-Aalaee, D., (ed.) Chemical Synthesis of Peptides and Polypeptides, pp. 3±37. Boston: Birkhauser. Mezgheni, E., Vachon, C. and Lacroix, M. (2000) Bacterial use of biofilms cross-linked by gamma irradiation. Radiation Physics and Chemistry 58 (2): 203±205. Mohanty, A.K., Misra, M. and Hinrichsen, G. (2000) Biofibres, biodegradable polymers and biocomposites: An overview. Macromolecular Materials and Engineering 276± 277 (1): 1±24. Okamoto, M., Morita, S., Taguchi, H., Kim, Y.H., Kotaka, T. and Tateyama, H. (2000) Synthesis and structure of smectic clay/poly(methyl methacrylate) and clay/ polystyrene nanocomposites via in situ intercalative polymerization. Polymer 41 (10): 3887±3890. Orliac, O., Rouilly, A., Silvestre, F. and Rigal, L. (2003) Effects of various plasticizers on the mechanical properties, water resistance and aging of thermo-moulded films made from sunflower proteins. Industrial Crops and Products 18 (2): 91±100. Otaigbe, J.U., Goel, H., Babcock, T. and Jane, J. (1999) Processability and properties of biodegradable plastics made from agricultural biopolymers. Journal of Elastomers and Plastics 31 (1): 56±71. Paetau, I., Chen, C.-Z. and Jane, J. (1994) Biodegradable plastic made from soybean

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products. 1. Effect of preparation and processing on mechanical properties and water absorption. Industrial & Engineering Chemistry Research 33 (7): 1821±1827. Park, H.-M., Li, X., Jin, C.-Z., Park, C.-Y., Cho, W.-J. and Ha, C.-S. (2002) Preparation and Properties of Biodegradable Thermoplastic Starch/Clay Hybrids. Macromolecular Materials and Engineering 287 (8): 553±558. Park, H.-M., Lee, W.-K., Park, C.-Y., Cho, W.-J. and Ha, C.-S. (2003) Environmentally friendly polymer hybrids Part I Mechanical, thermal, and barrier properties of thermoplastic starch/clay nanocomposites. Journal of Materials Science 38 (5): 909± 915. Salmoral, E.M., Gonzalez, M.E., Mariscal, M.P. and Medina, L.F. (2000a) Comparison of chickpea and soy protein isolate and whole flour as Biodegradable Plastics. Industrial Crops and Products 11: 227±236. Shen, Z., Simon, G.P. and Cheng, Y.-B. (2002) Comparison of solution intercalation and melt intercalation of polymer-clay nanocomposites. Polymer 43 (15): 4251±4260. Singh, J. and Singh, N. (2003) Studies on the morphological and rheological properties of granular cold water soluble corn and potato starches. Food Hydrocolloids 17 (1): 63±72. Sinha Ray, S. and Okamoto, M. (2003) Polymer/layered silicate nanocomposites: a review from preparation to processing. Progress in Polymer Science 28 (11): 1539±1641. Sue, H.-J., Wang, S. and Jane, J.-L. (1997) Morphology and mechanical behaviour of engineering soy plastics. Polymer 38 (20): 5035±5040. Usuki, A., Kojima, Y., Kawasumi, M., Okada, A., Fukushima, Y., Kurauchi, T. and Kamigaito, O. (1993) Synthesis of nylon 6-clay hybrid. Journal of Materials Research 8 (5): 1179±1184. Vaia, R.A. and Giannelis, E.P. (1997) Polymer Melt Intercalation in OrganicallyModified Layered Silicates: Model Predictions and Experiment. Macromolecules 30 (25): 8000±8009. Vaz, C.M., Mano, J.F., Fossen, M., Van Tuil, R.F., De Graaf, L.A., Reis, R.L. and Cunha, A.M. (2002) Mechanical, dynamic-mechanical, and thermal properties of soy protein based thermoplastics with potential biomedical applications. Journal of Macromolecular Science ± Physics 41 (1): 33±46. Vaz, C.M., De Graaf, L.A., Reis, R.L. and Cunha, A.M. (2003) Effect of crosslinking, thermal treatment and UV irradiation on the mechanical properties and in vitro degradation behavior of several natural proteins aimed to be used in the biomedical field. Journal of Materials Science: Materials in Medicine 14 (9): 789±796. Wilhelm, H.-M., Sierakowski, M.-R., Souza, G.P. and Wypych, F. (2003) Starch films reinforced with mineral clay. Carbohydrate Polymers 52 (2): 101±110. Yu, L. and Christie, G. (2001) Measurement of starch thermal transitions using differential scanning calorimetry. Carbohydrate Polymers 46 (2): 179±184. Yu, L., Christov, V., Christie, G., Beh, H., Smyth, R., Gray, J., Dutt, U., Harvey, T., Do, M., Halley, P. and Lonergan, G. (1998) Mechanical Properties and Microstructures of PLA/Thermoplastic Starch Blends. Proceedings of the 37th IUPAC International Symposium on Macromolecules 420 (Abstract). Yu, L., Christie, G., Beh, H. and McAuley, J. (1999) Processing and Mechanical Properties of Thermoplastic Starch. Proceedings of the 6th Polymer Conference, Guangzhou 476 (Abstract). Zhang, J., Mungara, P. and Jane, J. (2001) Mechanical and thermal properties of extruded soy protein sheets. Polymer 42 (6): 2569-2578.

Part III

Properties and mechanisms of degradation

12

Standards for environmentally biodegradable plastics G S C O T T , Aston University, UK

12.1 Why standards are necessary When the public became alerted to the problems of the proliferation of plastic litter in the early 1970s, a number of universities initiated research programmes directed toward more environmentally acceptable packaging. For technical reasons at that time, the favoured materials for packaging were the synthetic polymers, notably the polyolefins, polystyrene and to a lesser extent, polyvinyl chloride. These commodity materials had considerable advantages over paperbased products because of their resistance to water and hence microorganisms. Their manufacture was relatively non-polluting1-3 and their properties were well understood.4 They were consequently very cost effective in food packaging on a weight basis. The first approach to solving the problem of plastic packaging litter was to modify the commodity plastics. Three types of modified polyolefin plastic emerged.

12.1.1 Photochemically unstable (`photodegradable') polymers Photochemists had shown that ketone groups, formed in polyolefins under environmental conditions, photolyse with scission of the C-CO-C bond5 and industrial chemists found that ketone-modified polyolefins could be readily produced, for example, by copolymerisation of ethylene and carbon monoxide.6 J. E. Guillet of Toronto University, who studied this chemistry in some detail, developed a versatile range of carbonyl-modified polymers by copolymerisation of the common commercial monomers, particularly polyethylene and polystyrene with vinyl ketones. These polymers were shown by Guillet to undergo chain-scission at the carbonyl group to give low molar mass products (Fig. 12.1) and were commercialised in the 1970s under the trade name `Ecolyte'.5 He subsequently demonstrated that photo-degraded `Ecolyte' Polystyrene, unlike commercial polystyrene mineralised readily in biometric tests.1

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12.1 Photodegradable polymers.

12.1.2 Polymers with enhanced oxidisability During a study of the environmental degradation of polyolefins, Scott et al.7,8 found that the outdoor stability of the polyolefins could be very markedly reduced by accelerated peroxidation in the presence of transition metal ions (Fig. 12.2). Metal ion catalysed peroxidation occurs uncontrollably during processing and on exposure to the environment and consequently it is of little practical value unless the formation of hydroperoxides is controlled by appropriate antioxidants.8 A variety of transition metal ions accelerate the oxidative degradation of the carbon-chain polymers by catalysing both the formation and the decomposition of hydroperoxides.8,9 Typically, cobalt-catalysed oxidation of hydrocarbons is used in the manufacture of terephthalic acid from p-xylene.10 These prooxidant reactions also accelerate the breakdown of polymer molecules to smaller fragments (see Fig. 12.2) but are effectively inhibited by metal deactivators.8,11 All antioxidants have some retarding effect, but the most effective are the peroxide decomposers (PD) that remove hydroperoxides as they are formed by ionic (non-free radical) reactions.9,12±14 Deactivated transition metal ions (e.g.

12.2 Formation of hydroperoxides during transition metal ion peroxidation of PE.9

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iron complexes) in which the ligand is also a peroxide decomposer is the basis of the Scott-Gilead (SG) system for agricultural films discussed in more detail in Chapter 17. It was also used successfully in the commercial development of environmentally degradable carrier bags by Amerplast in Finland in 1975 and later by Plastopil as Plastor SG in agricultural mulching films in Israel,15 by Enichem in Italy16 and by Plastigone in the USA in packaging and agricultural applications. Field trials carried out on (SG) mulching films by Plastopil, an Israeli pioneer in the field of agricultural films, demonstrated that oxidised particulate residues were produced in less than six months, even in northern climates and biodegradation studies showed that the oxidised products biodegraded readily in soil.7 This process has now been in commercial use in a variety of agricultural applications, but particularly mulching films, for over 20 years and will be discussed further in Chapter 17. The rate of biodegradation is controlled by abiotic processes in commodity plastics. The prooxidant-modified polymers behave like regular plastics during manufacture and use but subsequently fragment rapidly just before cropping17,18 and subsequent studies have confirmed that the peroxidised particles biodegrade rapidly in biotic environments (see section 12.5).

12.1.3 Starch filled polyethylene composites Starch-filled polyethylenes show some technological advantages over unmodified PE. Following a study of these materials at Brunel University, J. G. L. Griffin19 licensed this process to a wallpaper manufacturer, Coloroll. This product attracted the attention of North American starch manufacturers who saw the opportunity to profitably dispose of surplus cornstarch. They went on to claim that the presence of a small proportion of starch would induce biodegradability in PE-starch blends. This was subsequently shown not to be the case and Griffin subsequently modified the materials by incorporating transition metal ions that gave products with an acceptable rate of bioassimilation in the environment.

12.1.4 Legislation It was recognised in the 1980s that the concept of blending a naturally produced biodegradable polymer with polyethylene to induce biodegradability was simplistic. This well publicised failure to induce the biodegradability of hydrocarbon polymers by the addition of biodegradable materials caused an unfortunate backlash from the `green' movement. In particular, Greenpeace adopted an equally simplistic view that only biopolymers can be truly biodegradable. The following is a typical statement made on behalf of Greenpeace;20 `Because petrochemical products are not the outcome of

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biological evolution, living things lack enzymes that can break them down into components that can be assimilated into the biological cycles'. The authors concluded from this premise that `every pound of plastic that has been produced, if it has not been burned, is still with us'. This reasoning ignores the fact that abiotic as well as biotic processes are involved in the bioassimilation of all polymers, natural as well as synthetic. Thus the hydrophobic hydrocarbon polymers, cis-polyisoprene (cis-PI) and cis-polybutadiene (cis-PB) begin to biodegrade only after exposure to the environment, when peroxidation leads to low molar mass products that support microbial growth.9 The relevance of this fact to polyolefin bidegradation will be discussed further in Section 12.4. Representations from consumers concerning unsupported claims about `environmentally friendly' packaging by companies manufacturing starch-filled polyolefins led to an investigation by a working group of the Attorney General of the USA in the late 1980s. This resulted in the publication of the Green Report,21 which subsequently provided the basis of standards for degradable polymers. The salient conclusions of this report are as follows:22 1. 2. 3. 4.

To advertise polymers as degradable is deceptive unless the conditions are clearly defined. Degradable plastics must be compatible with existing waste management systems. Meaningful research should be carried out into the effects of degradable plastics in the environment. Testing procedures and protocols for degradability should be established.

12.2 Bio-based polymers It has been known for many years that bacteria may produce poly(hydroxy butyric acid) (PHB)) as a food store23 This biosynthetic process was `rediscovered' by ICI in 1983,24 and it was found that PHB could be modified to give acceptable technological performance (BiopolÕ). Although Biopol was quite uneconomic as a commercial commodity plastic, this biosynthetic polymer stimulated a new surge of interest in improving both the economics of the biological manufacturing procedures25 and in modifying the properties of the polymer.26 The commercial shortcomings of Biopol as a potential replacement for commodity plastics, led to an interest in synthetic aliphatic polyesters based on petrochemicals that began to emerge as compromise `green' candidates. These do not strictly qualify as `green' as envisaged by Greenpeace because their commercial manufacture involves synthetic intermediates; however, they were more biodegradable than other petrochemical-based polymers. The modification of gelatinised starch by the incorporation of biodegradable synthetic polymers and plasticisers27 is a similar compromise to the production of environmentally

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acceptable plastics, although it remains to be demonstrated whether they are more ecologically acceptable than biodegradable hydrocarbon polymers also manufactured from petrochemical sources.2,28

12.3 The post-use treatment of plastics for the recovery of value In modern waste management, the primary objective is not just to dispose of waste plastics harmlessly but also to recover value from these wastes. The European Waste Framework Directive (March 1991) defines recycling (recovery) as follows:29 `Recycling/reclamation of organic substances . . . use as fuel to generate energy and spreading on land resulting in benefit to agriculture and ecological improvement, including composting and other biological processes.'

12.3.1 Recovery options Plastics packaging may end up in a variety of environments.22 Some items can in principle be re-used and this is encouraged by local governments, which otherwise have the responsibility to collect and dispose of the waste but it is not always clear that the energy involved in collecting and cleansing used plastics is ecologically acceptable.3,29±31 High in popular esteem is mechanical recycling, since this word carries an aura of ecological sustainability. In practice reprocessing of plastics is only sustainable if it leads to a reduction in use of fossil resources. This is possible in the case of clean, homogeneous wastes ± generally from industrial sources29±31 but it is not a viable procedure for contaminated mixed waste plastics, particularly when it is collected from widely dispersed rural sites. Furthermore, the reprocessing operation is itself energy intensive and there are generally no clear ecological or technical benefits in reprocessing mixed wastes.28 On the other hand, the hydrocarbon portion of mixed plastics wastes has a high fuel value. For example polyethylene has the same calorific value as the oil from which it was manufactured32 and thus the fossil resource used in the molecular composition of polyolefins is given a second use as a fuel in combined heat and power incinerators.28,29

12.3.2 Recovery in biologically active environments Biodegradable plastics may end up in one of four biologically active environments:22 1. 2. 3. 4.

inland water courses, sewage systems and the oceans compost litter on or in the soil landfill.

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12.3 Time-scale for polymer biodegradation in different environments (reproduced with permission from G. Scott, Polymers and the Environment, Royal Society of Chemistry, 1999, p. 121).

Each of these processes requires a different time-scale for biodegradation22 (Fig. 12.3). Plastics that end up in sewage systems, in rivers or in the sea are required to disintegrate and biodegrade rapidly ± generally within weeks ± to ensure that there is no accumulation of plastics debris to cause damage to man, animals or fish. By contrast, plastics that are deliberately treated in industrial compost in order to recover fertiliser should fragment to particulates that are indistinguishable from normal fertiliser biomass during the composting process but, in order to avoid the rapid formation of `greenhouse' CO2 and retain the benefit of carbonaceous biomass, bioassimilation should take place over a year or more. Plastics that end up on land either deliberately as part of their function, as in the case of polyethylene agricultural films, (see Chapter 17), or accidentally, as in the case of packaging litter, are required to fragment and peroxidise rapidly in sunlight. However, as is the case of nature's lignocellulosic waste, provided the residues can be shown to be ultimately bioassimilable either by mass loss to cell biomass or by mineralisation in laboratory tests, the time scale is not critically important provided that, like Nature's slow biodegrading wastes, they do not accumulate in the soil.22,28 Finally, although landfill should in principle be the last resort for biodegradable materials,29 much household waste is still sent to landfill even in the developed countries and will eventually biodegrade. This process can be accelerated by the use of plastics packaging that disintegrates and by the use of landfill covers that fragment after burial.28 The rate of ultimate biodegradation is of little practical significance. It is important to recognise that domestic and industrial plastics packaging can end up in more than one and in some cases all of the above disposal systems. From which it follows that not only must they be capable of being manufactured in conventional processing equipment, but they should also be mechanically reprocessable and compostable if they are collected for recycling. If they

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intentionally or inadvertently end up as litter, they should be photobiodegradable. Very few classes of polymer satisfy all these requirements. The bioplastics, unlike the commercial synthetic plastics are generally thermally unstable and difficult to reprocess due to the tendency to scorch (starch, cellulose) or depolymerise (aliphatic polyesters, etc.) at the high reprocessing temperatures used in modern polymer technology.26 The synthetic polyolefins, on the other hand, are relatively resistant to reprocessing due to their carbonchain structure and the antioxidants they contain.14 They can also be pyrolysed to fuel and in some cases monomers.29 Due to their higher carbon content, they provide more energy than bioplastics when incinerated with energy recovery.

12.4 Mechanisms of polymer biodegradation Many biopolymers do not biodegrade rapidly. For example, natural rubber, hydrocarbon waxes and resins and lignocellulose are converted relatively slowly to carbon dioxide. In spite of this, it is often erroneously concluded that any polymer, be it natural or synthetic, that is not mineralised within the same timescale as cellulose in an aqueous mineralisation test, for example the Sturm test,33 is not biodegradable. On the basis of ambient biometric tests (see section 12.6), much of Nature's polymer wastes cannot be classified as `biodegradable'. Wood, twigs and straw are primarily composed of lignocellulose and this compound of lignin and cellulose biodegrades relatively slowly.9 In practice, pure cellulose is rarely found in Nature. It is normally chemically bonded to lignin, which markedly slows the bidegradation rate. The rate of mineralisation of lignocellulose is not linear due to the build-up of lignin in the system.9 It has been shown34 that straw is mineralised over a period of about ten years. Some species of wood when felled may last for hundreds of years in a biotic environment.22 These naturally abundant materials would thus be considered to be non-biodegradable on the basis of short-term biometric tests.

12.4.1 Hydro-biodegradation Naturally occurring polymers containing polysaccharides and many synthetic condensation polymers initially undergo molar mass reduction by hydrolytic processes, catalysed by enzymes. In the case of the polysaccharides, the products are sugars that can be readily bioassimilated by microoganisms indigenous to the environment with the formation of cell biomass and carbon dioxide. The former acts as a seedbed for new plant growth and the latter ultimately recycles through photosynthesis in the environment to new biological growth. In the case of the polyesters, (e.g. poly(hydroxy alkanoates (PHA) and poly(caprolactone) (PCL) hydrolysis with molar mass reduction must precede the bioassimilation of the dicarboxylic acids, diols and hydroxy carboxylic acids so formed.35,36

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12.4.2 Oxo-biodegradation of carbon-chain polymers Unlike hydro-biodegradable polymers, hydrocarbon polymers are resistant to water and do not hydrolyse or hydro-biodegrade but they do undergo systematic molar mass reduction by peroxidation at composting temperatures or in light at ambient temperatures. They thus give rise to small bioassimilable molecules similar or in some cases identical to those that are formed from the hydrobiodegradable polymers.9,37 Oxo-biodegradation is controlled less by the structure of the polymer than by the antioxidants added during manufacture to give them durability during use.14 Very significantly the rate of biodegradation of oxo-biodegradable polymers is directly related to the rate of abiotic peroxidation. Thus the poly(diene) rubbers are the most biodegradable and the halogenated polymers (e.g. PVC) the least biodegradable of the common carbon-chain polymers. The polyolefins lie between the two extremes, i.e., Cl CH3 CH3 | | | ±(CHCH2)n < ±(CH2CH2)n± < ±(CHCH2)n± < ±(CH2CH=CHCH2)n± < ±(CH2C=CHCH2)n PVC

PE

PP

cis-PB

cis-PI

Ikram and co-workers38 have shown that in normal soils at 25 ëC, natural rubber (NR) gloves showed 54% loss of thickness after four weeks and 94% mass loss after 48 weeks. On the other hand, nitrile and neoprene rubbers showed insignificant loss in this time and plasticised PVC showed a smaller mass loss (11.6%) due entirely to biodegradation of the plasticiser. Ikram went on to show (see Table 12.1) that the rate of mass loss is strongly dependent upon the nutritional quality of the soil.39 After 24 weeks NR in the high N (100 mg/l), P (150 mg/l) system had lost 61.5% of its mass whereas in the low N (10 mg/l), P (15 mg/l) system, only 23.6% mass was lost. Control (unfertilised) soil produced least mass loss (17.3%). Microbial growth measured on the rubber pieces were in decreasing Table 12.1 Effect of added soil nutrients on the mass loss of rubber and plastic films (%) after 40 weeks in soil (adapted from Ikram et al.39 with permission) Polymer NR Neoprene Nitrile Plasticised PVC

High* ÿ82.4 ‡0.3 ÿ4.3 ÿ26.1

Nutrient treatment Low* Control* ÿ38.5 ÿ13.0 ÿ3.2 ÿ13.4

ÿ29.7 ÿ1.1 ÿ3.5 ÿ11.1

* Nutrients added: High 100 mg/l N and 150 mg/l P; Low 10 mg/l N. 15 mg/l P; Control nil

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order as expected. Bacterial populations on the NR gloves (12,317/mg) were higher than for fungi (441.47/mg), which were in turn significantly higher than actinomycetes (297.02/mg). Nevertheless, Heisey and Papadatos40 isolated ten actinomycetes (seven strains of Streptomycetes, two strains of Amycolatopsis and one strain of Nocardia) from soil that reduced the mass of NR gloves from 10±18% in six weeks. The most biodegradable of the commodity polyolefins is polypropylene (PP). Pandey and Singh have recently shown that polypropylene (PP), after removal of antioxidants by solvent extraction, biodegrades much more rapidly than polyethylene by mass loss in compost.41,42 PP lost over 60% mass in six months whereas low density polyethylene (LDPE) lost about 10% in the same time. Ethylene-propylene (EP) co-polymers biodegraded at rates intermediate between PP and PE. As expected, prior UV irradiation (photo-oxidation) increased both the rate and extent of the bioassimilation. This is fully in accord with the rates of environmental peroxidation of these molecules43 and it has been shown that PP acts as a sensitiser for the peroxidation of LDPE.31 The rate of molar mass (Mw) reduction of biodegradable plastics at ambient temperatures, can be assessed by the Arrhenius equation (see Fig. 12.4).44 However, commercial polyolefins show no significant reduction in molar mass in typical ambient biometric tests over many months or even years. The resistance of hydrocarbon polymers to biodegradation during use is one of their major advantages over bio-based plastics in agricultural application. The useful life of oxo-biodegradable polyolefins is controlled commercially by appropriate antioxidants to give substantial induction periods (IP) to biodegradation to meet the demands of the user (see Ch. 17). The abiotic stage preceding oxo-

12.4 Arrhenius plots for molar mass change (reproduced from Jakubowicz I, Polym. Deg. Stab., 80, 42 (2003) with permission).

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biodegradation generally involves antioxidants that are environmentally sensitive. For example, the SG photo-biodegradable polyolefins for agriculture referred to in section 12.1.1 incorporate photo-sensitive light stabilisers that are destroyed in a controlled way by light.17 Consequently, after the decay of antioxidant activity, the rate of molar mass reduction proceeds at a rate similar to that of the same polymer without antioxidants or light stabilisers.44 The rate controlling step in the biodegradation of oxo-biodegradable polymers is thus not attack by micro-organisms, but abiotic oxidation of the polymer. The latter process is powerfully influenced by the presence of transition metal ion prodegradants and the antioxidants and stabilisers that control the abiotic and hence the biotic processes.45 The balance between durability during use in the outdoor environment and the rate of biodegradation is achieved by combinations of antioxidants and transition metal compounds by adjusting the antioxidantprooxidant ratio (Ch. 17). Biodegradable polyolefins are also now used commercially in compostable garden waste bags. Thus, EPI's TDPAÕ compost bags oxidise and fragment during normal composting procedures ± normally at ~60±70 ëC.28 They are also used as waste sacks for biodegradable household and garden wastes. When added to compost or landfill the sacks disintegrate rapidly releasing the contents to the biotic environment. In the case of landfill, the bulky covering of soil may be replaced by a thin biodegradable film, which again disintegrates within weeks to allow air access. The combination of these innovations leads to a much more rapid reduction in landfill volume.28

12.5 Laboratory studies Many environmentalists still believe that synthetic polymers cannot biodegrade in the environment. It is certainly true that some plastics may not degrade for a very long time even in sunlight. For example, properly stabilised PVC is widely used in out-door applications, such as window frames which do not biodegrade for many decades, possibly centuries. At the other extreme, the poly(dienes) ± for example cis-poly(isoprene) in the form of latex rubber ± biodegrades in a few months in soil (see section 12.2). The polyolefins lie somewhere between PVC and natural rubber.9 Biodegradable polyethylene (e.g. EPI TDPAÕ) shows a substantial induction period (IP) at 20 ëC, whereas at 60 ëC, the IP is very short. In a commercial weatherometer, the IP is even shorter.46,47 Epifluorescence spectroscopy, which follows biotic changes in the surface of plastic films, shows46 that even a mild heat treatment, such as compression moulding, changes the polymer surface making it favourable to colonisation by microorganisms.46,48 This technique can also be used to monitor the rate of complete colonisation of the polymer surface. This process, in the case of preaged polyethylene is followed rapidly by the bioerosion of the surface of the polymer.9,46

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Figure 12.4 shows the time scale for molar mass reduction at different temperatures during the peroxidation of PE. It is known that up to Mw 40,000, PE oxidation products are preferentially removed from the polymer.48 It can then be predicted by means of the Arrhenius plots that ultimately the polymer would disappear by bio-erosion. Photo-acoustic FTIR studies of the surface of biodegrading PE 46 shows the surface development of species identified as polysaccharides and protein, both attributable to the growth of microorganisms. Mass loss after removal of this biomass has also been measured during biodegradation by monitoring changes in thickness of a sample.48 At present, however, mineralisation is considered to be the most convenient method of demonstrating complete biodegradability and this will be discussed in the next section.

12.6 The development of national and international standards for biodegradable plastics The development of biodegradable polymers has been beset by misinterpretation of the way in which Nature deals with its waste products. In particular, the importance of abiotic processes has not been given sufficient emphasis in the process of bioassimilation.41 Consequently existing international standards for biodegradable polymers tend to be based on folklore rather than scientific evidence since they ignore completely the environmental role of abiotic chemistry.30,41 The earliest test methods for the biodegradation of plastics followed from the use of biological oxygen demand (BOD) that had originally been developed to evaluate the environmental persistence of synthetic detergents that were the cause of pollution in inland waterways during the 1950s. More recent standard test methods for water quality have been comprehensively reviewed together with their subsequent application to the biodegradation of plastics.33 The main critical parameters proposed by the International Standards Organisation (ISO) are either oxygen absorption49 or carbon dioxide evolution50 in the presence of microorganisms. The latter procedure was taken over directly by the European Standards Organisation (CEN) in EN 13432.51 However, low molar mass chemicals are quite different from plastics. The first evidence of physical deterioration of plastics is generally due to abiotic chemical processes, which commence at an early stage during exposure to the environment. Consequently the measurement of mechanical properties is an important indicator of degradation, although not necessarily of biodegradation. It is argued with some justification that these changes alone do not guarantee that the residues are eventually harmless to the environment. In principle, complete mineralisation of biodegradable plastics by biometric means is the most convenient way of demonstrating the extent of biodegradation. In practice the end of the induction period is the point at which biodegradation begins and this in turn is determined by the post treatment the

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polymer receives during or after its service life, for example, during composting for packaging plastics or during outdoor weathering for agricultural products. The durability of degradable plastics before the end of their useful life is of great importance to manufacturers of packaging and agricultural products such as mulching films (Ch. 17). Plastics technologists use forced air oven ageing or UV weatherometer tests to predict this in practice and a considerable amount of work has been done to establish correlations between these standard tests and exposure during service.47 Provided the same standard procedures are used to simulate the effects of the environment on degradable plastics in the prebiodegradation (user) stage, they have no direct relevance to the rate of bioassimilation. Consequently, concerns over the fate of particulate materials or low molar mass degradation products begin only at this stage. Hydro-biodegradable polymers may begin to biodegrade in a biometric test before they have ever been used as packaging, this is not normally so with carbon-chain polymers. In the latter case polymers do not begin to biodegrade until peroxidation of the polymer has produced a hydrophilic polymer surface that supports microbial colonisation.48 This inevitably involves collateral loss of mechanical properties, typically elongation at break (Eb). Technologists normally consider a sample to be brittle at 90% loss of Eb, (see Ch. 17). The key CEN Standard for composting of packaging plastics is EN 13432 `Packaging Requirements for packaging recoverable through composting and biodegradation Test scheme and evaluation criteria for final acceptance of packaging' 51 which is paralleled in ASTM D 6002-96, ASTM d 6400-99e1 and ISO CD 15986.33 In this standard, compostability is assessed by the following criteria, all of which must be satisfied. 1. 2. 3. 4. 5.

Identification of packaging constituents, dry solid content, ignition residues, and hazardous metal residues. Biodegradability: 90% of the total theoretical CO2 evolution in compost or simulated compost in six months. Disintegration: not more than 10% shall fail to pass through a <2 mm sieve. Compost quality: no negative effects on density, total dry solids, volatile solids, salt content, pH, total nitrogen, ammonium nitrogen, phosphorus, magnesium and potassium eco-toxicity effects on two crop plants. Recognisability: `must be recognisable as compostable or biodegradable by the end user by appropriate means'.

These criteria were arrived at pragmatically in association with the composting industries in order to cause minimal interference with the commercial operations of the industry.52 Criteria 2, 3 and 5 are subjective assessments associated with compost `quality' and have little to do with ecological behaviour. For example, criterion 2 is concerned with biological `burning' of the plastic to carbon dioxide in order to improve the visual appearance of the compost. This has nothing to do with the agronomic

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performance of the compost. Similarly, no basis has been experimentally demonstrated for the particle size limit in criterion 3. What agronomic evidence there is, suggests7 that particulate plastics have a beneficial effect in improving soil drainage provided they do not accumulate with time. `Recognisability' (criterion 5) refers not to the demonstrated quality of the compost at all but to the logo used to describe the product for commercial sale of the product. Thus scientific considerations appear to have played very little part in arriving at the above criteria.52 Detailed procedures for measuring disintegration and CO2 evolution are given Standards EN 14045 53 and EN 1404654 respectively. EN 13432 and EN 14046 require that the plastics as produced must mineralise within six months at 58  2 ëC. As observed above, oxobiodegradable plastics as normally manufactured do not undergo mineralisation to any significant extent under these conditions. In fact this requirement has little to do with composting, since it uses as a reference standard crystalline cellulose that is 68% mineralised under the chosen conditions in 38 days.52 This polymer is rarely found in pure form in Nature's wastes, since it is almost universally chemically bound to lignin as lignocellulose, which reduces its biodegradation rate (see section 12.4). A practical consequence of this is that polymers that mineralise at a similar rate to pure cellulose are rapidly ejected to the environment as CO2 and make only a minor contribution to the nutritional value of compost. The European Waste Framework Directive, however, requires that organic waste is reclaimed (see section 12.3) so that the product can be `spread on land resulting in benefit to agriculture and ecological improvement'. Consequently, a much better reference standard would be lignocellulose (e.g. straw), which remains in the soil, releasing the carbon nutrients slowly to growing plants. A new standard test method is clearly required that will accommodate oxobiodegradable plastics and satisfy environmentalists that these materials are fully biocompatible. The British Standards Institution (BSi PKW/0 `Packaging and the Environment') and ASTM (D 20.96) are currently developing alternative mineralisation processes for oxo-biodegradable plastics that incorporate controlled pre-ageing processes before the mineralisation test. The protocol shown in Fig. 12.5 outlines the BSi `twin-track' route to mineralisation.28 Key features of the test method are as follows. 1.

2. 3.

The plastic is subjected to a laboratory ageing procedure to the point at which it is oxidised to fragments. This involves heat ageing to simulate environmental exposure at temperatures up to 60 ëC (e.g. compost environment). For plastics residues that are also exposed to sunlight, for example, after spreading on soil, an appropriate commercial weathering process. Appropriate mineralisation procedures in a laboratory composting test using matured compost or soil as inocula.44,55

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12.5 Alternative testing procedures for mineralisation of biodegradable plastics.

4.

Soil toxicity tests on polymer fragments that may contain classified `heavy metals' to ensure that any degradation products released from the plastic in the compost have no long-term deleterious effects on plants or on animals that may imbibe them.

Procedures that may be used to mimic heat ageing during composting and outdoor weathering include conventional forced-air ovens and xenon arc weatherometers. Polyolefins treated in this way have been found to generate CO2 to over 60% of theoretical carbon dioxide formation in 18 months to two years in the presence of mature compost.44 Similarly, inoculation with soil gives a similar result after two years55 (see section 12.6.3).

12.6.2 Mineralisation test procedures Figure 12.6 shows the proposed laboratory test procedure for mineralisation of plastics at ambient temperature in a low-carbon soil inoculated with mature (2±4 months) compost. The plastic is introduced as produced. The maximum temperature permitted in EN 13432 is 58  2 ëC which is designed to maximise

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12.6 Biometric test for the mineralisation of degradable plastics in compost.54

the mineralisation of hydro-biodegradable polymers. However, modern continuous-flow composting plants operate at temperatures of up to 80 ëC. 70 ëC would thus be a satisfactory compromise. The BSi proposal is similar to this and is based on a method developed by Jakubowicz44 that provides a more effective percolation of air through the medium (Fig. 12.7). The ageing procedure that precedes the mineralisation test must in this case be matched to the environmental exposure conditions.

12.6.3 Degradable plastics in soil As already indicated, degradable polyolefins that end up on the surface of soil as litter have been used in agriculture for many years. This will be discussed further in Ch. 17. No accumulation of these materials has been observed and it will be

12.7 Biometer vessel for oxo-biodegradable plastics (unpublished work by I. Jakubowicz reproduced by kind permission).

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evident from the evidence already cited that they do indeed biodegrade in fertile soils and compost. It is now accepted by CEN (TC 249/WG9) that, in the case of oxobiodegradable polymers, biometric tests should be preceded by weathering tests (UV exposure in a weatherometer) before mineralisation. This together with the eco-tests, shown in simplified form in Fig. 12.5, provides a basis for ensuring that not only is the carbon of the plastic ultimately converted to useful fertiliser but also that no non-biodegradable toxic products are produced in the process. Chiellini et al.55 have shown that thermally aged oxo-biodegradable plastics mineralise in forest soils to over 60% in 18 months and CEN TC 249/WG9 has concluded that two years at temperatures up to 28 ëC in the bio-active stage of degradation to achieve 60% mineralisation is acceptable for polymers that require an ageing period before biodegradation commences. The evidence from studies of Jakubowicz44 and Chiellini et al.55 have shown that biodegradable polyethylenes evolve CO2 in an auto-accelerating mode, which continues progressively after this time. However, in the event that some polymers may not reach 60% mineralisation after two years at 28 ëC it is proposed to allow biometric mineralisation at temperatures up to 50 ëC. This is fully justified in view of the fact that the surface of the soil may exceed this temperature during the summer season17 so that the rate of peroxidation may be quadrupled.

12.6.4 Simulated weathering procedures Since temperatures may exceed 50 ëC under a black plastic film due to the synergistic influence of heat and light the rate of abiotic peroxidation is known to be stimulated. Several commercial xenon arc weatherometers provide similar temperatures and provided the UV shorter wavelength cut-off is not less than that experienced in sunshine (~290 nm),46-48 these are realistic practical conditions for plastics that end up on the soil (e.g. mulching films). International standards describe the use of typical commercial laboratory techniques.56±63 Weatherometers based on xenon arc simulate environmental exposure resulting in physical fragmentation of plastics (see Ch. 17). Since these procedures have been used by the manufacturer and user of plastics artefacts for many years to determine their durability during use,47 it is not considered that the details of the weathering procedure should be part of the standard itself, provided that full reference is made to the one chosen and why it is the most appropriate. ASTM D 5510-9462 describes a convenient heat-ageing oven for degradable plastics.

12.6.5 Mineralisation of oxidised plastics in soil Since abiotic peroxidation and bio-oxidation occur together during the bioassimilation of oxo-biodegradable plastics (see Fig. 12.8),9,41,46 it is of crucial importance that the mineralisation chamber is perfused with air during

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12.8 Model for the bioassimilation of oxo-biodegradable plastics in soil (adapted from Scott, G., Polymers and the Environment, Royal Society of Chemistry, 1999, p. 118 with permission).

the whole of the mineralisation procedure. Figure 12.6 shows the introduction of air at the bottom of a perforated plate, which ensures that the air is evenly distributed through the biologically active soil.

12.7 Lessons from the past and future developments The principle of a `level playing field for business' outlined in the `Green Report'21 is also enshrined in the EU Packaging and Packaging Waste Directive,64 which states Objective 1 to be as follows: `. . . to harmonise national measures . . . to ensure the functioning of the internal market and to avoid obstacles to trade and distortion of competition within the community'. This is a worthy aim that requires that Standards really are objective and based on the best scientific evidence available. As the earlier discussion makes clear, these ideals have not always been realised in International Standards for biodegradable polymers developed for packaging. This was recognised by the EU Environment Directorate, which issued the following conclusions on EN 13432.65 1. 2. 3.

`ISO 14851 (Oxygen consumption) and ISO 148 (Sturm test) do not simulate composting conditions. What is really needed is to know what the fate is of material under composting conditions and what happens once it is released to the soil. If the packaging material does not completely biodegrade during the composting process, it should be demonstrated that it eventually degrades in the soil.'

As currently formulated, EN 13432 discriminates against manufacturers of oxo-biodegradable polyolefins since certification of biodegradable plastics as compostable requires compliance with this published Standard.33 This clearly

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obviates the declared EU Directive, quoted above. In spite of this, TC 261/SC4/ WG2 the Working Group responsible for setting standards for compostable packaging has resisted pressure from polyolefin product manufacturers to modify EN 13432 in spite of the scientific evidence now available in the published technical literature and reviewed in this chapter. `Green' fundamentalism has been a serious hindrance to progress. Scientifically, it is not possible to distinguish the mechanism and rate of biodegradation of polymers such as PHB from synthetic polyesters nor natural rubber from synthetic cis-poly-(isoprene). Biodegradation depends only on the chemical composition of the polymer and not on its origin.66 (see section 12.4). Furthermore, the rate of biodegradation of commercial polyolefins is much less dependent on their chemical structure than on the additives used to provide durability during use and hence on their ultimate fate in the environment. Abiotic chemistry as well as biological processes are thus involved in the bioassimilation of both biological and synthetic waste products.9 It may be argued that man-made polymers have been over-stabilised for short-term applications such as one-trip commodity packaging and this had led to a fundamental re-thinking of the processes involved in bioassimilation of packaging and agricultural plastics in the environment. Time-controlled biodegradable polyolefins require a reproducible user life in the outdoor environment, followed by a rapid fragmentation of the films due primarily to photooxidation.17 This technology, which is already widely applied in agriculture, will be discussed in more detail in Chapter 17. It has become evident during the last ten years that there is a place both for bioplastics and synthetic plastics in packaging. Bio-based hydro-biodegradable plastics have an obvious advantage in products that are likely to end up in watercourses or in sewage systems due to the requirement that they must be substantially biodegraded during the waste treatment process. However, oxobiodegradable plastics are much more useful in applications that require assured and reproducible durability to random attack by microorganisms. The rate of hydro-biodegradation of natural polymers can be retarded by chemical modification. However, this process also retards biodegradation after discard, so that the balance between usefulness and biodegradability is rather precarious (Fig. 12.9).3,30 Oxo-biodegradable plastics by contrast degrade under the influence of specific aspects of the outdoor environment ± notably by light, heat, and mechanical action, whereas hydro-biodegradable plastics based on cellulose or starch are relatively resistant to these influences. Other factors also need to be assessed in comparing the ecological impact and sustainability of natural and man-made products, these have been discussed elsewhere.3,18,22,28,30,41,67 Since life cycle assessment (LCA) has not yet played a significant role in the thinking of Standards committees, they will not be discussed in detail here. It can be concluded, however that natural polymers have not so far demonstrated superiority over synthetic polymers in sustainability.30

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12.9 Effect of polymer modification on biodegradability and technological usefulness.

The reason for this is that frequently all the fossil resources used during the manufacture of bioplastics are not taken into account, particularly when chemical modification of the biological raw material is involved during manufacture. Nor has the question of alternative use of agricultural land for polymer intermediates been accounted for when it competes with food production.29,41 The calorific value of hydrocarbon polymers when burned in an appropriate waste-to-energy incinerator9,32 is similar to the oil from which they were manufactured, whereas bio-based polymers are generally less useful as fuels. Caution must therefore be observed before concluding that polymers based on fossil resources are less sustainable than bio-based polymers, particularly as they have not yet been shown to have better technological behaviour than the commodity synthetic polymers. Indeed, some properties are noticeably inferior. Moreover, if renewable energy is available cheaply in the future, many synthetic polymer feedstocks could be made from natural products. For example, ethene can be manufactured from ethanol, which may in turn be manufactured from carbohydrates. In the short term, polymer feedstocks from natural and fossil resources will co-exist and the primary determinant of the proportion of each utilised will depend on the relative ecological benefits and economics of each. Over the next decade the standards organisations will need to come to terms with the reality that end-of-life disposal is just one of the factors to be weighed in the ecological balance.

12.8 Acknowledgements I am grateful to Dr. Ignacy Jakubowicz for helpful discussions and for permission to reproduce Figure 18.4 from previously unpublished work. I also thank Dr. David Wiles, Professor Jacques Lemaire, Dr. Anne-Marie Delort and their co-workers for their helpful collaboration. I am also grateful to Professor Emo Chiellini, Dr. Graham Swift and their colleagues for helpful discussions.

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12.9 References 1. Guillet J (1995) `Plastics and the environment' in Scott G and Gilead D, eds, Degradable Polymers: Principles and Application, 1st edn, Chapman & Hall (Kluwer Acad. Pub.), Chapter 12. 2. Guillet J (2002) `Plastics and the environment' in Scott G, ed., Degradable Polymers: Principles and Application, 2nd edn, Kluwer Acad. Pub., Chapter 12. 3. Scott G (2002) `Why biodegradable polymers' in Scott G, ed., Degradable Polymers: Principles and Application, 2nd edn, Kluwer Acad. Pub., Amsterdam Chapter 1. 4. Scott G (1999) `Polymers in modern life', Polymers and the Environment, Royal Society of Chemistry Paperbacks, Chapter 1. 5. Guillet, J E (1973) Polymers and Ecological Problems, Plenum Press, New York. 6. Harlan G and Kmiec C (1995) `Ethylene-carbon monoxide copolymers' in Scott G and Gilead D, eds, Degradable Polymers: Principles and Application, 1st edn, Chapman & Hall (Kluwer), Chapter 8. 7. Eggins H O W, Mills J, Holt A and Scott G (1971) `Biodeterioration and biodegradation of synthetic polymers' in Sykes G and Skinner F A, Microbial Aspects of Pollution, Academic Press, London and New York, pp. 267±277. 8. Amin M U and Scott G (1978), Europ. Polym. J., 1019±1028. 9. Scott G (2002) `Degradation and Stabilisation of Carbon-Chain Polymers' in Scott G, ed., Degradable Polymers: Principles and Application, 2nd edn, Kluwer Acad. Pub., Amsterdam, Chapter 3. 10. Scott G (1965) `Metal ion induced decomposition of hydroperoxides', Atmospheric Oxidation and Antioxidants, Elsevier, pp. 41±51. 11. Osawa Z (1993) `Metal catalysed oxidation and its inhibition' in Scott G, ed., Atmospheric Oxidation and Antioxidants, 2nd edn, Vol. II, Chapter 6. 12. Scott G (1965) `Peroxide decomposers', Scott G, ed., Atmospheric Oxidation and Antioxidants, Elsevier, pp. 188±203. 13. Al-Malaika S (1993) `Antioxidants preventive mechanisms' in Scott G, ed. Atmospheric Oxidation and Antioxidants, 2nd edn, Elsevier, Amsterdam, Chapter 5. 14. Scott G (1999) `Evironmental Stability of Polymers', Polymers and the Environment, Royal Society of Chemistry Paperbacks, Chapter 3. 15. Gilead D (1995) `Photodegradable Plastics in Agriculture' in Scott G and Gilead D, eds, Degradable Polymers: Principles and Application, 1st edn, Chapman & Hall (Kluwer Acad. Pub.), Chapter 10. 16. Fabbri A (1995) `The role of Degradable Polymers in Agricultural Systems' in Scott G and Gilead D, eds, Degradable Polymers: Principles and Application, 1st edn, Chapman & Hall (Kluwer Acad. Pub.), Chapter 11. 17. Gilead D and Scott G (1982) `Time Controlled Stabilisation of Polymers' in Scott G, ed., Developments in Polymer Stabilisation-5, Applied Science Publishers, Barking, Chapter 4. 18. Scott G and Wiles D M (2002) `Degradable hydrocarbon polymers in waste and litter control' in Scott G ed., Degradable Polymers: Principles and Applications, 2nd edn, Kluwer Acad. Pub., Chapter 13, pp. 449±479. 19. Griffin G J L (1994) `Particulate starch-based products' in Griffin G J L, ed., Chemistry and Technology of Biodegradable Polymers, Blackie Academic and Professional, London. 20. Sadun A G, Webster, T F and Commoner B (1990) Breaking down the degradable

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plastics scam, for Greenpeace, Washington D.C. 21. `Green Report' (1990) Report of a task force set up by the Attorneys General of the USA to investigate `Green marketing'. 22. Scott G (1999) `Biodegradable Polymers', Polymers and the Environment, Royal Society of Chemistry Paperbacks, Chapter 5. 23. Lemoigne M (1925 Ann. Int. Plast., 39, 144. 24. Hammond T and Liggatt J J (1995) `Properties and applications of bacterially derived polyhydroxyalkanoates' in Scott G and Gilead D, eds, Degradable Polymers: Principles and Applications, 1st edn, Chapman & Hall (Kluwer Acad. Pub.), Chapter 5. 25. Braunegg G (2002) `Sustainable poly(hydroxyalkanoate) (PHA) production' in Scott G, ed., Degradable Polymers: Principles and Applications, 2nd edn, Kluwer Acad. Pub., Chapter 8. 26. Chodak I (2002) `Polyhydroxyalkanoates: `Properties and modification for high volume applications' in Scott G, ed., Degradable Polymers: Principles and Applications, 2nd edn, Kluwer Acad. Pub., Chapter 9. 27. Bastioli C (2002) `Starch-polymer composites' in Scott G, ed., Degradable Polymers: Principles and Applications, 2nd edn, (Kluwer Acad. Pub.), Chapter 6. 28. Scott G and Wiles D M (2002) `Degradable hydrocarbon polymers in waste and litter control' in Degradable Polymers: Principles and Applications, 2nd edn, Scott G, ed., Kluwer Acad. Pub., Chapter 13, pp. 455±460. 29. Scott G (1999) `Management of Polymer Wastes', Polymers and the Environment, Royal Society of Chemistry Paperbacks, Chapter 4. 30. Scott G and Wiles D M (2001) `Programmed-Life Plastics from Polyolefins: A New look at Sustainability', Biomacromolecules, 2, 615±622. 31. Sadrmohaghegh C, Scott G and Setudeh E (1985) `Recycling of mixed plastics', Polym. Plast. Tech. Eng., 24, 149±185. 32. Bousted J and Hancock G F (1981) Energy and Packaging, Ellis Horwood Publishers. 33. MuÃller R-J (2003) `Biodegradability of polymers: Regulations and methods for testing' in SteinbuÈchel A, Biopolymers Vol. 10, Wiley VCH, pp. 365±392. 34. Jansson S L (1963) `Nitrogen transformation in soil organic matter' in The use of Isotopes in Soil Organic Matter Studies, Report of the FAO/IAEA Technical Meeting, September 9±14, Pergamon Press, Oxford. 35. Li S and Vert M (1995) `Biodegradation of aliphatic polyesters' in Scott G and Gilead D, eds, Degradable Polymers: Principles and Application, 1st edn, Chapman & Hall (Kluwer Acad. Pub.), Chapter 4. 36. Li S and Vert M (2002) `Biodegradation of aliphatic polyesters' in Scott G Degradable Polymers: Principles and Applications, 2nd edn, (Kluwer Acad. Pub.), Chapter 5. 37. Karlsson S, Hakkarainen M and Albertsson A-C (1997) `Dicarboxylic acids and ketones formed in degradable polyethylenes by zip-depolymerisation through a cyclic transition state', Macromolecules, 30, 7721±7728. 38. Ikram A, Alias O and Napi D (2000) `Biodegradability of NR gloves in soil', J. Rubb. Res., 3, 104±114. 39. Ikram A, Alias O, Bahri A R S, Fauzi M S and Napi D (2001), `Effects of added nitrogen and phosphorus on the biodegradation of NR gloves in soil', J. Rubb. Res., 4, 102±117.

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40. Heisey R M and Papadatos S (1995) `Isolation of microorganisms able to metabolise purified natural rubber' App. Environ. Microbiol, 61, 3092±3097. 41. Scott G (2002) `Science and Standards' in Chiellini E and Solaro R, eds, Biodegradable Polymers and Plastics, Kluwer Academic/Plenum Pub., 3±32. 42. Pandey J K and Singh R P (2001) `UV-irradiated biodegradability of ethylenepropylene copolymers, LDPE and I-PP in composting and culture environments', Biomacromolecules, 2, 880±885. 43. Scott G (1993) `Photodegradation and photostabilisation of polymers' in Scott G, ed., Atmospheric Oxidation and Antioxidants, Vol. II, Chapter 8. 44. Jakubowicz I (2003) `Evaluation of biodegradable polyethylene (PE)', Polym. Deg. Stab., 80, 39±43. 45. Scott G (1999) `Antioxidant Control of Polymer Biodegradation' in Degradability, Renewability and Recycling; 5th International Scientific Workshop on Biodegradable Plastics and Polymers, Macromolecular Symposia, eds, Albertsson A-C, Chiellini E, Feijen J, Scott G and Vert M, Wiley-VCH, Weinheim, 1999, 113± 125. 46. Bonhomme S, Cuer A, Delort A-M, Lemaire J, Sancelme M and Scott G, (2003) `Environmental biodegradation of polyethylene', Polym. Deg. Stab., 81, 441±452. 47. Davis A and Sims D, (1983) Weathering of Polymers, App. Sci. Pub., Barking, Chapters 3, 4, 8. 48. Arnaud R, Dabin P, Lemaire J, Al-Malaika S, Chohan S, Coker M, Scott G, Fauve A and Maarooufi A (1994) `Photooxidation and Biodegradation of Commercial Photodegradable Polyethylenes' Polym. Deg. Stab., 46, 211±224. 49. ISO (1999a) `Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium Method by measuring the oxygen demand in a closed respirometer' ISO 14851. 50. ISO (1999b) (International Standards Organisation) `Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium ± Method of analysis of evolved carbon dioxide' ISO 14852. 51. CEN (2000) `Packaging ± Requirements for packaging recoverable through composting and biodegradation ± Test scheme and evaluation criteria for the final acceptance of packaging'. EN 13432. 52. Innocenti F D (2002) `Biodegradability and compostability' in Chiellini E and Solaro R, eds, BiodegradablePolymers and Plastics, Kluwer Academic/Plenum Pub., Chapter 2, 33±45. 53. CEN (2003) `Packaging ± Evaluation of the disintegration of packaging materials in practical oriented tests under defined composting conditions', EN 14045. 54. CEN (2003) `Packaging ± Evaluation of the ultimate aerobic biodegradability and disintegration of packaging materials under controlled composting conditions ± Method by analysis of released carbon dioxide, EN 14046. 55. Chiellini, E, Corti A and Swift G (2003) `Biodegradation of thermally oxidised, fragmented low-density polyethylenes', Polym. Deg. Stab., 81, 341±351. 56. ISO 4892-1 Plastics ± Methods of exposure to light sources ± Part 1: General guidance. 57. ISO 4892-2 Plastics ± Methods of exposure to laboratory light sources ± Part 2: Xenon-arc sources. 58. ISO 4892-3 Plastics ± Methods of exposure to laboratory light sources ± Part 3: Fluorescent UV lamps.

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59. ISO 4892-4 Plastics ± Methods of exposure to laboratory light sources ± Part 4: Carbon arc sources. 60. ASTM D 5071-99 ± Standard practice for operating xenon-arc type exposure apparatus with water for exposure of photodegradable plastics. 61. ASTM D 5208-01 ± Standard practice for operating fluorescent Ultraviolet (UV) and condensation apparatus for exposure of photodegradable plastics. 62. ASTM D 5510-94 ± Standard practice for heat aging of oxidatively degradable plastics. 63. ASTM D5272-92 ± Standard practice for outdoor exposure of photo-degradable plastics. 64. European Union (1994) `Packaging and Packaging Waste Directive'. 65. European Commission Directorate E, Industry and the Environment (2000) `Comment on prEN 13432; Organic recovery'. 66. Linos A and SteinbuÈchel A. (1998) `Microbial degradation of natural and synthetic rubbers by novel bacteria belonging to the genus Gordon, Kauchuk Gummi Kunststoffe, 51, 496±499. 67. Patel M (2003) `Do Biopolymers fulfil our expectations concerning environmental benefits' in Biodegradable Polymers and Plastics, eds Chiellini E and Solaro R, Kluwer Acad. Pub., Chapter 7.

13

Material properties of biodegradable polymers M B H A T T A C H A R Y A , University of Minnesota, USA, R L R E I S , V C O R R E L O and L B O E S E L , University of Minho, Portugal

13.1 Introduction Plastics are one of the fastest growing segments of the waste stream. Designed for their durability and special performance characteristics, plastics usage has doubled every four to five years since 1970. About 30% of all plastics are for one-time use, such as disposable packaging, service-ware items, and disposable non-wovens. Single-use items account for eight million tons of plastics annually in the US. Because of low consumer acceptance and poor appearance, little if any is recycled (<2%). Many packaging materials do not lend themselves to recycling because of contamination, and the cleaning necessary prior to recycling can be very expensive. It is estimated1 that plastic occupies approximately 14±28% of the volume of loose trash and 9±12% by volume of all municipal solid waste disposed of in landfills. It is becoming increasingly evident that the use of these long-lasting polymers for short-duration application is difficult to justify. Furthermore, plastics are manufactured with little consideration on the impact of the resources used in making them. Since most plastics are derived from petrochemicals (non-renewable feedstocks), there is some concern regarding sustainability. This, coupled with the recalcitrance of plastics, has led to serious concern in an environmentally conscious era, leading to legal requirements and mandates being imposed on a growing number of municipalities particularly in Western European countries and Japan, where landfill space is scarce. A possible and environmentally conscious alternative is to design/synthesize polymers that are biodegradable. Biodegradable plastics provide opportunities for reducing municipal solid waste through biological recycling to the ecosystem and can replace conventional synthetic plastic products. In addition, it is desirable that these biodegradable polymers come primarily from agricultural or other renewable resources for a sustainable environment. A second area of application for biodegradable polymers is in the field of medicine. It is desirable that an implant not require a second surgical procedure for removal. A bone fracture is often treated by fixating the bone with a rigid

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stainless implant. Since the stress is borne by the stainless steel, the bone does not carry sufficient load during the healing process (referred to as stress shielding). The bone is, thus, prone to refracture when the implant is removed. An implant that is biodegradable will gradually transfer load to the healing bone as it degrades. Drug delivery using biodegradable polymers has also been widely investigated. Degradable polymers are also being investigated for intraluminal grafts, temporary vascular grafts, and high-strength orthopedic implants, such as bone nails and screws.

13.2 Biodegradation There are several ways a polymer may degrade in the environment. These include biodegradation, photodegradation, oxidation, and hydrolysis. These processes are often interpreted by the general public as one and the same though they lead to very different end results. It is often conceived that the breakdown of a plastic into small, invisible (to the naked eye) fragments is biodegradation, when in reality these fragments may remain in the environment over a significant period of time. Biodegradable polymers when placed in bioactive environments, such as compost, will break down to carbon dioxide and water under the action of bacteria and fungi. There are two major steps in the biodegradation process. The first involves the depolymerization or chain cleavage of the polymer to oligomers, and the second is the resulting mineralization of these oligomers. The depolymerization step normally occurs outside the micro-organism and involves both endo- and exo-enzymes. Endo-enzymes cause random scission on the main chain, while exo-enzymes causes sequential cleavage of the terminal monomer in the polymer main chain. Once depolymerized, sufficiently small-sized oligomeric fragments are formed. These fragments are transported into the cell where they are mineralized. Mineralization is defined as the conversion of the polymer into biomass, minerals, water, CO 2, CH4, and N2. The mineralization step usually occurs intracellularly. Standard test procedures are available to evaluate the biodegradability of plastics. The American Society for Testing and Materials (ASTM), the Ministry of International Trade and Industry (MITI, Japan),2 and the Organization for Economic Cooperation and Development (OECD)3 have developed standard test procedures to evaluate the biodegradability of plastics. Most of these test methods measure the percent conversion of the carbon from the designed biodegradable plastic to CO2 and CH4 (plus some CO2) in aerobic and anaerobic environments, respectively. The absence of polymer and residue in the environment indicates complete biodegradation, whereas incomplete biodegradation may leave polymer and/or residue as a result of polymer fragmentation or metabolism in the biodegradation process. Failure in one test does not necessarily exclude biodegradation, it merely indicates that under the

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Table 13.1 Standard methods for estimating biodegradation of plastic materials Test

Environment

Property measured

ASTM D 5209-92 ASTM D 5210-92 ASTM D 5247-92 ASTM D 5271-93 ASTM D 5338-92 ASTM D 5509-94 ASTM D 5511-94 ASTM D 5512-94

Aerobic sewage sludge Anaerobic sewage sludge Aerobic specific micro-organism Activated sewage sludge Controlled composting Simulated compost High solids anaerobic digestion Simulated compost using external heated reactor Simulated landfill Accelerated landfill Mixed Microbial

Carbon-dioxide Carbon-dioxide/methane Molecular weight Oxygen/carbon-dioxide Carbon-dioxide Physical properties Carbon-dioxide/Methane Physical properties

ASTM D 5525-94 ASTM D 5526-94 MITI Test

Physical properties Carbon-dioxide/Methane Oxygen

environmental conditions and/or the timeframe where the experiment was conducted, no (or incomplete) biodegradation occurred. Usually, more than one test method is needed to fully assess the biodegradability of a given polymer.4 Table 13.1 summarizes the various tests under the ASTM standards that may be used to determine biodegradability of a material. The primary requirement for initiating the biodegradation process is that the polymer chain must contain chemical bonds that are susceptible to enzymatic hydrolysis or oxidation. The most common chemical functional group with these characteristics are esters. Peptide bonds in proteins can also be hydrolyzed enzymatically. The other factors which affect the rate of degradation are branching, hydrophilicity/hydrophobicity, molecular weight, crystallinity, stereochemistry, chain flexibility, and morphology. Polysaccharides and proteins are good substrates for enzymatic attack due to their hydrophilic nature. The lack of branching and lower crystallinity also enhances biodegradability. The next requirement for biodegradation is the existence of appropriate micro-organisms to synthesize the specific enzymes required to depolymerize and mineralize the targeted polymer. These two steps in the biodegradation process may not involve the same micro-organism. Naturally occurring polymers, such as polysaccharides, proteins, and cellulose, are easily biodegraded since many micro-organisms that produce the enzymes required to metabolize these compounds are readily available in nature. The final requirement for the biodegradation process is a well-tuned environment where the desired micro-organisms can thrive. Factors that affect the growth of micro-organisms include appropriate temperature range, moisture level, salt (type and level), oxygen (aerobic to anaerobic), trace metals, pH, redox potential, environmental stability or flux, and pressure. If any of these factors are not in the suitable range, the rate of the biodegradation process may be reduced or halted until the proper conditions are re-established.4 Sewage,

Material properties of biodegradable polymers

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marine water, landfills, compost, and soil, all furnish very different environments, and therefore the rate of degradation may well be different for different environments. It is possible that some polymers that are degradable in one environment may or may not degrade in another environment. Since most landfills are biologically inert, the most appropriate disposal venue for biodegradable polymers is a composting site. For example, newspapers have been cited as not being biodegradable in a landfill, even though cellulose itself is completely biodegradable in compost when enough moisture is provided. Composting is an accelerated biological decay process that has the potential to alleviate the solid-waste management crisis existing in many parts of the world. Compostable is defined as `capable of undergoing biological decomposition in a compost site as part of an available program, such that the material is not visually distinguishable and breaks down to carbon dioxide, water, inorganics and biomass at a rate consistent with known compostable materials'.5 To meet the compostability requirement under ASTM D5338-93 standard, 60% of a homopolymer and 90% of blends or copolymers must mineralize within six months. After land application of the compost, the remaining material should be converted to carbon dioxide by micro-organisms. Any remaining materials (additives) should not adversely affect the quality of the compost, i.e., the compost should not be toxic and should not deter plant growth. Most biomedical polymers degrade by hydrolysis of the polyester chain to produce low molecular weight water-soluble fragments that are then attacked by enzymes, to produce metabolites. In the case of polylactides or polyglycolides or their copolymers, they break down to their monomers (lactic acid or glycolic acid), enter the Kreb's cycle, and are further broken down to carbon dioxide and water and excreted through the normal processes. L-Lactic acid occurs in the metabolism of all animals and micro-organisms. Degradation of poly(glycolic acid) leads to the formation of glycine which enters the tricarboxylic acid cycle and metabolizes to carbon dioxide and water. Poly(orthoesters), a family of synthetic degradable polymers, degrades by erosion at the surface. Hence, polymer hydrolysis in the surface layers must occur at rates that are significantly higher than that in the bulk material. These devices degrade at the surface and become thinner with time. Since orthoester linkages are acid sensitive and also stable in base, erosion rates in these polymers can be accelerated by the addition of acidic excipients. Biodegradable polymers can be either natural or synthetic. Similarly, biodegradable polymers can be synthesized from renewable or petrochemical resources. It should be noted that the origin of the material and biodegradability are not related. Natural polymers like polysaccharides (starch and cellulose), proteins, and polyesters, such as polyhydroxyalkanoates are biodegradable. Synthetic polymers, such as linear aliphatic polyester, can also be considered biodegradable as the degradation products of hydrolytic or enzymatic chain cleavage, such as diacid and diol, can enter the metabolic cycle of microorganisms.

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Biodegradable polymers for industrial applications

13.3 Natural polymers The building blocks of carbohydrates are the -D and -D glucose that contain six carbon atoms and form a pyranose ring. Glucose molecules bond through enzymatic condensation that occurs predominantly at carbon 1 and 4 but occasionally between carbon 1 and 6. When only  1-4 linkage occurs a linear polymer (amylose) is formed. Amylopectin is a -1,6- branched -1,4 glucan polymer. Starch consists of both amylose and amylopectin whose relative ratio varies depending on the source and type of starch. Most common starches have amylose contents approximating 25%, though higher amylose-containing starches (up to 80%) are also available. Starch is known to be readily biodegradable. The -1,4 linkages in both the amylose and amylopectin are hydrolyzed by amylases while the -1,6 links at branch points in amylopectin are attacked by glucosidases. Unmodified starch polymers have poor processability and mechanical properties when compared to conventional synthetic polymers. They have high glass transition temperatures (Tg) and melting temperatures (Tm) that are close to their decomposition temperature. Plasticizers, such as water, can be added to aid in processing. Alternatively, modified starches, such as esters of starch, can behave as thermoplastics. However, care should be taken in the modification of natural polymer, as physical or chemical modification of a natural biopolymer may result in a loss of its biodegradability. For example, starch esters with degrees of substitution greater than 2.8 have drastically reduced biodegradation. Cellulose is one of the most abundant naturally occurring polymers in nature. Cellulose is water-insoluble and fully biodegradable. It is composed of Dglycopyranoside units linked by -(1-4) bonds, unlike starch where the Dglycopyranoside units are linked by -(1-4) bonds. This difference in structure influences the biodegradation rate and properties of the polymers. Pulp and viscose (cellophane) are processed celluloses with reduced molecular weight that makes them generally more readily biodegradable. Cellophane is one of the common forms of cellulose used for packaging and laminated wrappers for a variety of foods because of its barrier properties to oils and transparency. Cellulose is mineralized by micro-organisms due to the activity of the cellulose enzyme complex, which results in the formation of cellobiose and glucose leading to mineralization in cellular biochemical cycles.4 Several enzymes act synergistically in the breakdown of cellulose in a series of hydrolysis reactions, and an enzyme which can biodegrade via oxidative pathways has also been reported.6 Cellulose cannot be thermally processed as it undergoes thermal decomposition before melting due to the extensive hydrogen bonding. However, modified cellulose, such as cellulose acetate, is thermoplastic and is used in toothbrush handles. Like its other natural counterpart, starch, cellulose loses degradability as the degree of substitution increases. Cellulose acetate with a degree of substitution below 2.5 is biodegradable.

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Proteins are natural polymers that are fully biodegradable like starch and cellulose. In proteins, the amino acid chains are extended through the formation of amide linkages, which are readily degraded by proteases enzyme. Collagen, like other proteins, is susceptible to depolymerization due to the action of a variety of protease enzymes. Commercial collagen films are used as edible casings and wraps for meat products. The other proteins, which have been studied for film formation, include corn zein, wheat gluten, soy protein isolate, whey protein, and casein.7±9 Generally, films from these proteins are prepared from either ethanolic or aqueous solutions. Most of the proteins may exhibit thermoplastic behavior under the right conditions and have fairly large water vapor permeabilities. Many other protein-based polymers, such as casein, albumin, fibrinogen, silks, and elastins, have been considered for materials applications because of their inherent biodegradability. Most of these polymers have been primarily used as microspheres for encapsulation or slow release of pharmaceuticals. Proteins have not yet found widespread use as plastic material since they are difficult to process and do not melt without decomposition. They are also difficult to blend with most polymers because of their incompatibility. Their decomposition temperature is considerably lower than other natural polymers and hence find limited usage in blends. Proteins are also more expensive than most polysaccharides, which makes it difficult to justify their use. Natural rubber, the other natural hydrocarbon polymer, consisting mainly of cis-1,4-polyisoprene is relatively resistant to microbial attack in comparison to other natural polymers. A number of micro-organisms have been reported to degrade natural rubber.10±12 An enzyme which degrades the rubber was isolated from the extracellular culture medium of Xanthomonas sp., and the crude fractions which are capable of depolymerizing natural rubber in the latex state have been reported.13 The same authors14 reported on a Nocardia strain that used natural rubber as its sole carbon source.

13.4 Microbial polyesters Polyhydroxyalkoanates (PHAs) are intercellular storage materials that are synthesized by a wide variety of bacteria including Clostridium, Syntrophomonas, Pseudomonas, and Alcaligenes species. PHAs are synthesized in bacteria as a carbon and energy reserve. They can be accumulated to high levels (as high as 95% of cellular dry weight) in these bacteria. The bacterial PHA are aliphatic homo- or copolyesters of -hydroxyalkanoic acids grown under physiologically stressed conditions. Limiting nutrients, such as nitrogen, phosphorus, and sulfur or reducing oxygen concentration stimulate PHA formation.15 Similarly, the limitation of iron, manganese, potassium, and sodium can also lead to PHA accumulation.16 There are two major forms of PHAs, poly-beta-hydroxybutyric acid (PHB) and poly-beta-hydroxyvaleric acid (PHV).

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Biodegradable polymers for industrial applications

There are two groups of bacteria synthesizing PHAs, one group produces short chain length PHAs with monomers ranging from 3±5 carbons in length, and the other group produces monomers from 6±16 carbons in length. There are over 150 hydroxyacids in PHAs.17 The majority of PHAs are composed of R3 hydroxyacids ranging from 3 to 16 carbons in length. Some PHAs contain monomers of 4-hydroxy and 5-hydroxy acids. Functional groups (unsaturated, phenoxy, cyano, halogenated, etc.) have also been found on the side chain of monomers forming PHAs. Imperial Chemical Industries (ICI) began investigating the polymer-forming properties of bacteria in the mid 1970s. PHB is a highly crystalline material with properties similar to polypropylene. It has a melting point of 180 ëC and a glass transition temperature, Tg, of 5 ëC and is 100% optically pure and isotactic. This combination of high crystallinity and high Tg makes films and plastics produced from PHB very brittle. It also degrades at temperatures above its melting point making processing very difficult. Molecular weight of the polymer depends on the strain of micro-organism used, growth conditions and sample purity. PHB has properties comparable to that of polypropylene,18 except for elongation but has poor solvent resistance. PHB also has a very slow crystallization rate. Plasticization is necessary for processing as well as improving the properties of PHB. By introducing hydroxyvalerate (HV) groups on the PHB polymer backbone (about 0±30%) these thermoplastic polymers have properties that vary from soft elastomers to rigid brittle plastics. Statistical copolymers of PHBV with increasing HV composition have been synthesized.19,20 As the HV content is increased, a decrease in Young's modulus and increased impact properties are observed. These polyesters with a longer side-chain of about 3±6 carbons are also produced by a variety of bacteria.21 The side-chains in the copolymers reduce the crystallinity, melting point, and glass transition temperatures. Polyesters containing longer side-chains are elastomeric and have excellent toughness and strength. In addition, these polymers are inherently biodegradable, though the biodegradation rate is greatly reduced with increased chain length in the polymer indicating that hydrophilic/hydrophobic balance of polymer is important even for biodegradation of naturally occurring polymers.21 Saito et al.22 synthesized copolymers of [R]-3 hydroxybutyrate (3HB) and 4hydroxybutyrate (4HB). The composition of P3HB-co-4HB varied from 0 to 100 mol% depending on micro-organism and carbon source mixture. As the 4HB fraction increased, glass transition temperature and melting temperature decreased while elongation increased. The tensile strength of 100 mol% 4HB exceeded 100 MPa. Doi et al.23 reported on the biodegradation of PHAs. The mechanism of biodegradation of these polymers has been studied and involves the enzyme, bacterial esterases, which depolymerizes the polymer to monomers, dimers, and trimers.24 Budwill et al.25 reported the rapid mineralization of PHB and PHBV in anaerobic sewage sludge, and more than 90% metabolized to methane and

Material properties of biodegradable polymers

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carbon dioxide within 16 days. PHB and PHBV are also prone to thermal degradation during processing.26 At 190 ëC, PHB films were found to experience a 50% drop in molecular weight after one hour.18 Ramkumar and Bhattacharya27 reported that as the HV content increased, the rate of degradation decreased. PHAs ranging from PHB to PHBV and medium chain length PHA have been successfully synthesized in plants. PHB homopolymer was synthesized in Arabidopsis thaliana and was first reported by Poirier et al.28 Low levels of PHB synthesized in cytoplasm of tobacco29 and cotton.30 The transgenic plants had stunted growth, although seeds could be produced. The major problem was low yield and negative correlation between PHB production and plant health. Plants experienced decreased fertility, growth, and other metabolic changes. PHB can be produced in rapeseed at 8% dry weight without deleterious effect on plant growth and germination.31 Slater et al.32 produced PHBV in plants with 4± 17 mol% HV content in the polymer. However, the levels of PHBV production was lower than that of PHB. In the 1990s, Monsanto started a program to produce PHB in plants. However, the program met with minimal success and was discontinued a few years later. According to Poirier,33 PHAs in excess of 15% dry weight without altering the yield of other components (oils, proteins, and carbohydrates) are needed to be economically viable. PHB and PHBV is also receiving attention for potential biomedical applications such as controlled drug release and sutures. In vivo, PHB degrades to D-3-hydroxybutyric acid which is a normal constituent of human blood. Its application in non-medical use is doubtful given its high costs. PHAs remain the most characterized of all the biodegradable polyesters. Interested readers are directed to the reviews by Doi, 34 Gross35 and Steinbuchel.36

13.5 Synthetic polyesters Polyesters are macromolecules characterized by the presence of carboxylate ester groups in the repeating units of their main chain. These are synthesized by polycondensation reactions between diacids and diols or by ring-opening polymerization of lactone/dilactone or anhydrocarboxylate. The drawback of polycondensation reaction is that the water produced must be removed continuously, requiring lengthy reaction times and producing products of varying chain length. Hence, ring opening polymerization is the preferred method of synthesis.

13.6 Poly-lactic acid Originally developed by Carothers in 1932, polylactic acid is making a comeback on the commercial scale. Poly(lactic acid) (PLA) is a linear aliphatic polyester based on lactic acid which can be produced via biological (fermentation of starch) or chemical methods. Lactic acid has both hydroxyl and carboxyl

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Biodegradable polymers for industrial applications

groups. However, conventional polycondensation reactions do not yield high molecular weight PLA unless the water produced is removed using azeotropic distillation. PLA is a thermoplastic that is insoluble in water and biodegradable in compost in the presence of micro-organisms. Used primarily in medical applications due to its high cost, it is now finding use in many packaging and nonwoven applications. This is primarily due to significant process improvements achieved and economies of scale that have drastically reduced the cost. Poly(lactic acid) can be found in two forms as poly (L-lactic acid) and poly (DL-lactic acid) synthesized from L-lactic acid (naturally occurring) and Dlactic acid. Poly (L-lactic acid) is a crystalline polymer while poly(DL-lactic acid) is an amorphous polymer. The preferred route for producing high molecular weight PLA is the catalytic ring-opening bulk polymerization of lactide (dilactone of lactic acid). This is a two-step process. The first step involves the polymerization L-lactic acid, D-lactic acid or their mixtures to poly(lactic acid) oligomer. This lower molecular weight PLA is catalytically depolymerized through transesterification to form lactide. The resulting lactide (L, D, DL, or meso) is purified and converted to high molecular weight PLA by catalytic ring-opening polymerization. The ring-opening polymerization can be performed in solution or bulk. Poly (L-lactic acid), which is derived from pure L-lactide, has a high melting temperature, poor processability, and crazes easily because of its high crystallinity. The D, L-PLA is an amorphous polymer with Tg of 60 ëC, which is low for many packaging uses. Properties, such as melting point, mechanical strength, and crystallinity, are determined by the polymer architecture (determined by different proportions of L, D) and the molecular mass. Similarly, the time for degradation is also affected by the crystallinity and molecular mass of the polymer; higher values require longer time for degradation. Several reports have shown that PLA is a thermally unstable polymer above its melting temperatures.27,37±39 The presence of moisture has a particularly severe effect at higher temperatures as it induces hydrolysis. Thus, care has to be taken to ensure that PLA does not undergo degradation during processing. Drying samples before use or processing under nitrogen atmosphere is recommended. Another drawback is its low heat deflection temperature that makes application for hot foods impractical. Polylactic acid is a popular material in various biomedical applications, such as drug delivery, sutures and orthopedic devices. In the body, PLA is hydrolyzed to its monomeric form, lactic acid, which is then eliminated by incorporation into the Kreb's cycle. Cargill-Dow LLC is producing PLA to the tune of 140kt annually in Blair, Nebraska, USA. Their primary application is the production of fibers through melt-spinning for clothing. Other applications include food packaging such as thermoformed containers and pop bottles. The monomer, lactic acid, has the potential to become a new bio-based material from which other chemicals (acrylic acid, propylene oxide, ethyl lactate) can be synthesized.

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13.7 Poly(glycolic acid) Poly(glycolic acid) (PGA) is a highly crystalline, hydrophilic, linear aliphatic polyester used primarily for adsorbable sutures. In general, it is not possible to obtain high molecular weight polymers of PGA by direct condensation of glycolic acids. This is due to the reversibility of the condensation reactions and backbiting reactions. Therefore, like PLA, thermoplastic PGA is synthesized by ring-opening polymerization of the glycolide (cyclic diester dimers).19 PGA is a highly crystalline polymer (about 35±75%) relative to other biodegradable synthetic polymers and has a melting temperature of 225 ëC and a glass transition temperature of 35 ëC. It has excellent mechanical properties (strength and modulus). It is insoluble in most organic solvents. Poly(glycolic acid) biodegrades by hydrolysis of the readily accessible and hydrolytically unstable aliphatic-ester linkages. One drawback of PGA is its fast degradation time (~6 months). Glycolide and lactides are often copolymerized to extend the range of homopolymer properties. PGA and its copolymers are mainly used for biomedical applications. In the body, PGA is broken down either by hydrolysis or by non-specific esterases or carbosypeptidases. The glycolic acid monomer is either excreted through the urine or enters the Kreb's cycle.

13.8 Polycaprolactone Polycaprolactone (PCL) is a semi-crystalline linear aliphatic polyester synthesized by ring-opening polymerization of epsilon-caprolactone. The initial commercial grades have been of comparatively low molecular weight (15,000± 40,000) and the main lines of interest have been as precursors for polyurethanes and as additives in other polymers.40 Huang et al.41,42 and Woodward et al.43 have studied the biodegradation of PCL by fungi. PCL has been shown to degrade by random hydrolytic scission of its ester groups and under certain circumstances, by enzymatic degradation.44 The drawback when using PCL is that as it exhibits melting points of about 60 ëC, these PCL-based materials are not usable at elevated temperatures such as applications in hot beverages and applications requiring exposure to sunlight.

13.9 Poly(alkene succinate) A series of biodegradable polyesters using 1,4 butane diol and succinic acid has been produced using polycondensation reaction. The method yields prepolymers of molecular mass of approximately 10,000, which is then coupled to increase the molecular mass by chain extenders, such as di-isocyanate or diamines. Chain extenders are bifunctional low molecular weight chemicals that react with end groups in the polymers. In polyesters, the end groups are hydroxyl and carboxyl. Polybutylene succinate, known under the trade name Bionolle

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Biodegradable polymers for industrial applications

1000 (Showa Highpolymer), is available in melt flow indexes ranging from 1±50 and can be used for a wide range of applications. A corresponding co-polyester of succinic and adipic acid is also available (Bionolle 3000). Bionolle 1000 has a melt temperature of 115 ëC while Bionolle 3000 has a melt temperature of 85 ëC. The 1000 series is more crystalline and takes longer to degrade. Bionolle plastics have been found to degrade in compost, soil, fresh water, and sea water though the time taken for degradation varies. Both the 1000 and 3000 series are relatively stable during processing.

13.10 Aliphatic-aromatic copolyesters These copolyesters combine the biodegradability of aliphatic polyesters with the excellent properties imparted by aromatic polyesters. While aliphatic polyesters are easily biodegradable, they lack thermal stability and mechanical properties needed for many applications. Aromatic polyesters, on the other hand, provide excellent use properties but are resistant to microbial attack under environmental conditions.45 Both BASF and Eastman Chemicals produce aliphatic-aromatic copolyesters from terephthalic acid, adipic acid, and 1,4 butane diol. Witt et al. reported on a new group of copolyesters, which combine both biodegradability and excellent properties.46 These copolyesters are synthesized by conventional bulk condensation techniques from various aliphatic diols with a defined mixture of different aliphatic dicarboxylic acids and terephthalic acid. The key to biodegradability is the blocklength of the aromatic unit, which should preferably be no more than a trimer.

13.11 Poly(orthoesters) These polymers appear to be particularly useful for controlled drug release. Initially poly(orthoesters) were prepared by condensation of 2,2diethoxytetrahydrofuran and a dialcohol.47 Because the tetrahydrofuran component of the polymers does not provide the necessary rigidity (Tg varies from body temperature or lower), this material had limited utility as a solid implant designed for drug delivery. An alternative route uses 3,9bis(methylene)-2,4,8,10-tetraoxaspirol [5,5] undecane and a dialcohol.48 Mechanical properties of the polymer can be controlled by the choice of diols used in the condensation reactions. Use of rigid diols (trans-cyclohexane dimethanol) produces a rigid polymer whereas the use of 1,6-hexanediol produces a flexible material. Polymers with molecular weights in excess of 100,000 can be synthesized. Poly(orthoesters) are hydrophobic materials and are stable during storage without careful exclusion of moisture.

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13.12 Polyanhydrides Polyanhydrides are a class of hydrolytically unstable polymers that are usually either aliphatic, aromatic, or a combination of the two. They are prepared by reacting a purified diacid monomer with excess acetic anhydride to form an acetic acid mixed anhydride oligomer which is then polymerized under vacuum to yield a high molecular weight polymer. The polymerization reaction can be catalyzed to produce high molecular weight polymers in the range of 300,000. A number of polyanhydrides have been synthesized and mainly used in biomedical applications as implants and for controlled release. These polymers tend to be crystalline, and the mechanical properties of polyanhydrides are generally poor, tending to be brittle. Polyanhydrides degrade by surface erosion.

13.13 Polycarbonates/polyiminocarbonates Monomers of poly (trimethylene carbonate), an aliphatic polycarbonate, have been used in copolymerizations with glycolides and lactides for drug delivery application. These aliphatic polycarbonates become soft at 40±60 ëC and have been evaluated primarily as biomaterials in biomedical applications. Poly(bisphenol carbonate) is another polycarbonate derived from bisphenol A. It is non-biodegradable but has excellent mechanical properties.49 By replacing the carbonyl oxygen of poly (bisphenol A-carbonate) with an imino group, the resulting polyiminocarbonate copolymer becomes hydrolytically unstable while retaining its mechanical properties.50 Pulapura and coworkers51 replaced bisphenol A with derivatives of tyrosine dipeptide resulting in a iminocarbonate-amide copolymer. The iminocarbonate bond was formed between the phenolic hydroxyl group at the tyrosine side chains. These iminocarbonate-amide copolymers can be regarded as pseudopoly(amino acids)52 and are being evaluated in biomedical applications.

13.14 Blends The blending of different polymers is an attractive way of imparting new properties to a product. By controlling the blend morphology during processing, it is possible to impart unique properties to the mixture. Polymer compositions containing starch have been developed for different applications and are the subject of several patents.53±55 The first important commercial application of starch plastics has been the blending of polyethylene with starch as a filler. It was assumed that starch would accelerate the degradation of polyethylene. Another aspect of adding starch is that it reduces the cost of the blend, particularly with biodegradable aliphatic polyester. Starch has been used as a filler in degradable materials, in blends such as starch-urethanes, and starch-polyethylene with ethylene-acrylic acid copolymer,

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in graft copolymers with vinyl or acrylic monomers using a free radical process, in cross-linking reactions such as starch-xanthate for elastomeric materials, and in a variety of other technologies.56 Although the biodegradation of low molecular weight polyethylenes was studied by Potts57 and Albertsson,58 it is well recognized that high molecular weight polyethylene is virtually nonbiodegradable. An auto-oxidant, which can react with metal salts in soils or other environments to give peroxide radicals, has been incorporated into starchpolyethylene blends to degrade the polyethylene into oligomers which is reported to be susceptible to mineralization.59 Alternatively, starch can be blended with biodegradable polyesters for suitable applications. In general, physical blends of natural and synthetic polymers lead to poor properties. This is primarily due to the poor compatibility between the two major components of the blend. Good interfacial adhesion and compatibility between the blend components is important for producing materials with good mechanical properties. Reactive blending is an economical and commercially viable approach to enhance the interfacial compatibility and other properties in systems using high levels of starch. In reactive blending, the graft or block copolymers are formed in situ during the blend preparation by using polymers containing reactive functional groups, such as carboxylic acid, anhydride, epoxy, urethane, or oxazoline. Natural polymers, such as proteins (amine and carboxylic) and polysaccharides (hydroxyl), can be considered to be functionalized polymers. Such techniques have shown that acceptable properties are obtained at 70% by weight of starch or protein in the blend. This helps to reduce the cost of the resulting polymers significantly. Biodegradable polyesters have been blended and evaluated for their properties. In general, polymer blends can be miscible (single Tg) or immiscible. Jacob et al.60 evaluated the properties of several of binary and ternary blends of aliphatic copolyesters. All blends in their studies were immiscible. Blends containing PHA have been reviewed by Verhoogt et al.61 PHB/PHV blends prepared in the melt state are phase separated.62 Scandola et al.63 reported that solvent cast films of atactic PHB and isotactic PHBV were miscible. The authors also reported that P(3HB) and cellulose esters formed miscible blends when melt compounded. It should be noted that blending biodegradable polymers with nondegradable polymer serves no useful purpose and is not reviewed here.

13.15 Water-soluble polymers These polymers are used in a variety of household goods (detergents, toothpaste, shampoo, skin lotions, and conditioners) and industrial goods (flocculants, thickeners, and emulsifiers). These polymers are largely anionic or nonionic in character and assist in reducing slurry viscosity, and improve dispersion and anti-redeposition. There is a widespread misconception that because these polymers are water soluble they do not contribute to pollution. Neither are all

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water-soluble polymers biodegradable. In fact, water-soluble polymers persist in the environment just as plastics. The public at large is simply unaware of the pollution created since these polymers are invisible, but they pollute our lakes, rivers, and oceans just the same. These polymers normally enter the waste-water treatment plant. Hence, for water-soluble polymers, infrastructure for biodegradation is readily available (as opposed to developing compost for biodegradable plastic products). Wastewater treatment plants were unable to remove phosphate from detergents. Since phosphate salts promote plant growth in rivers, the potential for eutrophication can lead to environmental imbalance. In the 1980s, this led to the use of zeolites and citric acid. Most water-soluble polymers are synthesized from acrylic acid, methacrylic acid, anhydrides, and their combinations. Copolymers of acrylic and maleic acids are not biodegradable, and acid accumulation will result from their use. While these acids are non-toxic, their long-term environmental impact is uncertain. Several studies64 have shown that low molecular weight acrylic copolymers are biodegradable. Isotatic poly (acrylic acid) and poly (methacrylic acid) show biodegradability indicating stereochemical preferences by the enzymes involved in the degradation process. Poly (aspartic acid), a biodegradable polymer, is prepared by the thermal condensation of aspartic acid followed by the hydrolysis of the intermediate poly(succinimide).65,66 Poly (aspartic acid) is produced in two isomeric forms,  and . As the mole percent of increases, the properties become comparable to poly (acrylic acid). Poly (vinyl alcohol), which is used as a sizing agent in textiles, is another water-soluble biodegradable polymer and is used in textiles, paper coating, and adhesives. Nonionic surfactants based on hydrophobic polyethers can biodegrade, depending on the structure. Linear primary alkyl ethoxylates readily biodegrade64 while branching in the hydrophobic portion of the primary alkyl reduces the biodegradation rate. Poly(malic acid) is a biodegradable polyester that may have some applications in detergents. Starch and cellulose can be modified to produce water-soluble biodegradable polymers. Carboxymethyl cellulose and hydroxyethyl cellulose are examples of water-soluble biodegradable polymers. Xanthan is another widely used microbial polysaccharide. Polyacrylamide grafted starches have also been suggested as flocculants,67 though their biodegradability has not been evaluated.

13.16 Future developments From the data in Table 13.2, it is quite obvious that biodegradable polymers with a varying range of properties are available. Yet, with few exceptions, sales of these polymers remain anemic. A study by Business Communications Co, Inc, (BCC) estimates that the North American market for biodegradable polymers in 2000 reached 11.3 million kg. BCC (www.bccresearch.com) predicts that this

Table 13.2 Properties of biodegradable and common polymers Material

Property Glass Melting transition temperature temperature (ëC) (ëC)

Poly hydroxy butyrate Poly hydroxy butyrate-co-valerate Poly lactic acid Poly--caprolactone Poly glycolic acid Poly butylene succinate Poly butylene succinate-co-adipate Poly butylene terephthalate-co-adipate Low density polyethylene Linear low density polyethylene High density polyethylene Polypropylene Polystyrene Polyorthoesters Polyanhydrides Polyiminiocarbonates

4 ÿ4 to ÿ7 55±60 ÿ60 35 ÿ32 ÿ45 ÿ33

100 35±95 55±69

178 130±160 175 60 225 114 95 110

46±49 135

Density (g/cc)

Stress at break (MPa)

Elongation at break (%)

Tensile modulus (GPa)

1.25 1.25 1.21 1.15 1.25 1.2 1.2 1.25 0.93 0.92 0.94 0.9 1.05 N/A N/A N/A

43 20±40 45±60 30

5 8±25 6±15 600±700

40±60 35±45 17±22 4±16 13±28 22±27 30±40 30±60 19±27 4 40±50

170±250 350 600 650 900 1200 600 3±5 7±220 85 4±7

4 1±2 2.7±3.0 0.38 7.0 0.5 0.35 0.08 0.07±0.28 0.04±0.08 0.4±1.3 1±2 3.2±5.0 0.82±1.15 0.045 1.6±2.2

Material properties of biodegradable polymers

351

will increase to 15.9 million kg by 2005. The worldwide consumption of biodegradable polymer in 2001 was estimated to be in the neighborhood of 68 million kg.68 Loose-fill packing peanuts has the largest share of the market followed by compost bags. Other applications include agricultural films, hygiene products, paper coatings and disposable tableware. The primary reason for low sales is the high cost of biodegradable polymers when compared to commodity plastics. But with increased volume of production and process optimization, the cost of biodegradable polymers continues to decrease. For example, Cargill-Dow predicts that their PLA will cost approximately $2/kg once their 140 kt/year plant becomes fully operational. Similarly, the costs of other biodegradable polyesters (Binolle, Eastar) continue to decline as volume picks up, though they are still far away from the prices of commodity plastics. A second drawback is the lack of proper infrastructure for disposal. Biodegradable plastics do not degrade in landfills. Hence, the full potential benefit is realized only if these polymers actually biodegrade. In the United States, composting infrastructure is available in a limited number of suburbs for disposing of food and yard waste (in most cases it actually ends up in the landfill). These facilities do not accept biodegradable plastics. There are no agencies to certify products as biodegradable. Nor is there the infrastructure to collect and separate biodegradable products from the waste stream. Thus, it is more than likely that biodegradable products will end up in the landfill. In the long run, a more active role by the general public will be needed to solve the waste disposal problem. Awareness is often lacking on the significance of both disposal and the environmental costs that need to be added to the final cost of the product. A key criterion should use the Life Cycle Analysis (LCA) to estimate the true cost of a product. Life cycle analysis seeks to identify the true environmental impact of a product by considering its environmental effect at every stage of its life cycle or `cradle to grave'. This includes the impact of extracting the raw materials, processing them into a product, transporting that product, using it, and then disposing and/or recycling it. It attempts to identify and quantify all of the material and energy inputs and all of the outputs of a product or process. In practice, however, reliable procedures for generating LCA analysis are lacking as many of the costs are impossible to define precisely.69 The final disposal system has an important role in the overall ecobalance of the materials, and hence, its cost.70 If a biodegradable material is composted, and the compost is applied to the land, then significant emission and energy credits will accrue on the account of sustainability. According to Gross and Kalra,68 if 3.6 billion kg of fossil fuel-based polymers are replaced annually by PLA produced from renewable resources like starch, then 192 trillion BTUs of fossil-derived fuel will be saved per year, resulting in reduced carbon dioxide emissions estimated at 10 million tons. Patel69 estimates that starch polymer pellets require 25±75% of the energy required for polyethylene, while the

352

Biodegradable polymers for industrial applications

greenhouse gas emissions are 20±80% lower. The cradle-to-factory gate energy requirements for PLA are 20±30% below those for polyethylene, while greenhouse gas emissions are 20±30% lower. In the future, the need to derive more carbon from renewable resources for chemical processes will also drive the debate on biodegradable polymers. Many of the diacids and diols needed to synthesize aliphatic polyesters can be produced from renewable feedstocks. Sustainability will also be an important part of this debate. The chief reasons for this are utilizing the potential of photosynthesis for energy savings, curbing the greenhouse effect, developing eco-compatible processes and products, diversifying agriculture out of food production and, possibly, generating employment. Future design of new products will have to take into account emissions/waste during processing, source of raw materials, waste management, and life-cycle analysis. In this respect biodegradable polymers are well positioned for future growth.

13.17 Acknowledgements Mrinal Bhattacharya would like to thank FLAD for its generous support of this work. L. F. Boesel acknowledges FundacËaÄo CoordenacËaÄo de AperfeicËoamento de Pessoal do Ensino Superior (CAPES ± Brasilia, Brazil) for the PhD grant.

13.18 References 1. Municipal Solid Wastes in the United States ± 2000 Facts and Figures. United States Environmental Protection Agency, Washington DC, USA. 2. Ministry of International Trade and Industry, Japan, The Biodegradability and Bioaccumulation of New and Existing Chemical Substances, 1983. 3. OECD Guide lines for Testing Chemicals (Organization for Economic Cooperation and Development), Paris, 1981. 4. Kaplan, D. L., Mayer, J. M., Ball, D., McCassie, J., Allen A. L. and Stenhouse, P. In Biodegradable Polymers and Packaging, C. Ching, D. Kaplan and E. Thomas, eds, Technomic Publishing Co., Inc., Lancaster, PA, 1993, pp. 1±42. 5. American Society of Testing of Materials. 6. Shimada, M. and Higuchi, T. In: Wood and Cellulosic Chemistry, edited by D. N. S. Hon and N. Shiraishi, Marcel Dekker, Inc., New York, 1992, pp. 557±569. 7. T. H. McHugh, J. F. Aujard and J. M. Krochta, Plasticized whey protein edible films: water vapor permeability properties. J. Food Sci., 59(2), 416±419, 423 (1994). 8. Kim, S. and J. M. Krochta. 1998. Polymer chain immobilization factors for whey protein-sorbitol/beeswax edible emulsion films. In Paradigm for Successful Utilization of Renewable Resources, D. J. Sessa and J. L. Willett (eds), AOCS Press, Champaign, IL, pp. 198±213. 9. Krochta, J. M. 1998. Whey protein interactions: effects on edible film properties. In Functional Properties of Protein and Lipids, J. R. Whitaker, F. Shahidi, A. Lopez Munguia, R. Y. Yada and G. Fuller (eds), ACS Symposium Series 708. ACS, Washington, D.C., pp. 158±167.

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10. Borel, M., Kergomard, A. and Renard, M. F. Degradation of natural rubber by fungi imperfecti. Agric. Biol. Chem. 46: 877±881 (1982). 11. Cundell. A. M. and Mulcock, A. P. The biodegradation of vulcanized rubber. Dev. Ind. Microbiology. 16: 88±96 (1975). 12. Hutchinson, M., Ridgway, J. W. and Cross, T. Biodeterioration of rubber in contact with water, sewage and soil. In Microbial aspects of deterioration of materials. R.J. Gilbert and D. W. Lovelock eds, pp. 187±202 (1975). 13. Tsuchii, A. and Takeda, K. Rubber-degrading enzyme from a bacterial culture. Appl. Environ. Microbiol. 56: 269±274 (1990). 14. Tsuchii, A., Takeda, K. and Tokiwa, Y. Colonization and degradation of rubber pieces by Nocardia sp. Biodegradation: 7: 41±48 (1996). 15. Lafferty, R. M., Korsatko, B. and Korsatko, W. Microbial production of poly ± hydroxybutyric acid. In Biotechnology. Rehm, H. J. and Reed, G. (eds). VCH Publishers, New York 135±176 (1988). 16. Sasikala, C and Ramana, C.V. Biodegradable Polyesters. In Advances in Applied Microbiology. Volume 42, Niedelman, A. I. and Lashkin, A. I. (eds), Academic Press, pp. 97±218 (1996). 17. Steinbuchel, A. and Valentin, H. E. Diversity of bacterial polyhydroxyalkanoic acids. FEMS Microbial Letters. 128: 219-228 (1995). 18. Hocking, P. J. and Marchessault, R. H. Chemistry and Technology of Biodegradable Polymers, Blackie, Glasgow, Chapter 4. p. 48 (1994). 19. Dawes, E. Novel Biodegradable Microbial Polymers, NATO ASI Series, Series E, Applied Sciences, Vol 186. Kluwer Academic Publisher Boston. (1990). 20. Bloembergen, S., Holden, D. A., Hamer, G. K., Bluhm, T. L. and Marchessault, R. H. Studies of composition and crystallinity of poly( -hydroxybutyrate-co- hydroxyvalerate). Macromolecules 19: 2865±2871 (1986). 21. Byrom, D. Biopolymers: Novel Materials from Biological Sources. Stockton Press, New York (1991). 22. Siato, Y., Nakamura, S., Hiramitsu, M. and Doi, Y. Microbial synthesis and properties of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) Poly. Int. 39: 169±174 (1996). 23. Doi, Y., Kanesawa, Y., Kunioka, M. and Saito, T. Biodegradation of microbial copolyesters: poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate). Macromolecules. 23: 26±31 (1990). 24. Kawaguchi, Y. and Doi, Y. Kinetics and mechanism of synthesis and degradation of poly(3-hydroxybutyrate) in Alcaligenes eutrophus. Macromolecules. 25: 2324±2329 (1992). 25. Budwill, K., Fedorak, P. M. and Page, W. J. Methanogenic Degradation of Poly(3Hydroxyalkanoates). Applied Environmental Microbiology. 58: 1398±1401 (1992). 26. Amass, W., Amass, A. and Tighe, B. A Review of Biodegradable Polymers: Uses, Current Developments in the Synthesis and Characterizations of Biodegradable Polyesters, Blends of Biodegradable Polymers and Recent Advances in Biodegradation Studies. Polymer International. 47: 89±144 (1998). 27. Ramkumar, D. H. R. and Bhattacharya, M. Steady shear and dynamic properties of biodegradable polyesters. Polymer Engineering and Science. 38: 1426±1435 (1998). 28. Poirier, Y., Dennis, D. E., Klomparens, K. and Somerville, C. Polyhydroxybutyrate, a biodegradable thermoplastic, produced in transgenic plants. Science 256: 520±523 (1992).

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29. Nakashita, H., Arai, Y., Yoshioka, K., Fukui, T., Doi, Y., Usami, R., Horikoshi, K. and Yamaguchi, I. Production of Biodegradable Polyester by Transgenic Tobbaco. Bioscience Biotechnology Biochem. 63: 870. 30. John, M. E. and Keller, G. Metabolic Pathway Engineering in Cotton: Biosysnthesis of Polyhydroxybutyrate in Fiber Cells. Proc. Natl. Academy of Sci. USA 93: 12768± 12773. 31. Houmiel, K. L., Slater, S., Broyles, D., Casagrande, L., Colburn, S., Gonzales, K., Mitsky, T. A., Reiser, S. E., Shah, D., Taylor, N. B., Tran, M., Valentin, H. E., Gruys, K. J. Poly ( -hyroxybutyrate) production in oilseed leucoplasts of Brassica napus. Planta 209: 547±550 (1999). 32. Slater, S., Mitsky, T. A., Houmiel, K. L., Hao, M., Reiser, S. E., Taylor, N. B., Tran, M., Valentin, H. E., Rodriguez, D. J., Stone, D. A., Padgette, S. R., Kishore, G. and Gruys, K. J. Metabolic Engineering of Arabidopis and Brasica for poly (3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer production. Nature Biotechnology 17: 1011±1016 (1999). 33. Moire, L., Rezzonico, E. and Poirier, Y. Synthesis of novel biomaterials in plants. J. Plant Physiology. 160: 831±839 (2003). 34. Doi, Y. Microbial Polyesters. VCH Publishers (1990). 35. Gross, R. A. Bacterial Polyesters: Structural Variability in Microbial Synthesis. In Biomedical Polymers. S. W. Salaby ed. Hanser Publisher, pp. 172±188 (1994). 36. Steinbuchel, A. Polyhydroxyalkanoic acids. In Biomaterials: Novel Materials from Biological Sources. D. Byrom ed. Stockton Press, New York, pp. 123±213 (1991). 37. McNeil, I. C. and Leiper, H. A. Degradation studies of some polyesters and polycarbonates I. Polylactide ± general features of degradation under programmed heating conditions. Polym. Degrad. Stab., 11, 267±285, 309 (1985). 38. McNeil, I. C. and Leiper, H. A. Degradation studies of some polyesters and polycarbonates II. Polylactide ± degradation under isothermal conditions, thermaldegradation mechanisms and photolysis of the polymer. Polym. Degrad. Stab., 11, 309±326 (1985). 39. Jamshidi, K., Hyon, S. H. and Ikada, Y. Thermal characterization of polylactides. Polymer. 29: 2229±2234 (1988). 40. Brydson, J. A. Plastics Materials, 6th edn, p. 718. Butterworth-Heinemann Ltd, UK (1995). 41. Benedict, C. V., Cameron, J. A. and Huang, S. J. Polycaprolactone degradation by mixed and pure cultures of bacteria and yeast. J. Appl. Polym. Sci., 28, 335±342 (1983). 42. Benedict, C. V., Cook, W. J., Jarrett, P., Cameron, J. A., Huang, S. J. and Bell, J. P. Fungal degradation of polycaprolactones. J. Appl. Polym. Sci., 28, 327±334 (1983). 43. Woodward, S. C., Brewer, P. S., Moatamed, F., Schindler A. and Pitt, C. G. The intracellular degradation of Poly(-caprolactone). J. Biomed. Mater. Res., 19, 437± 444 (1985). 44. Pitt, C. G. In Biodegradable Polymers as Drug Delivery Systems. M. Chasin and R. Langer eds p. 17. Marcel Dekker, NY (1990). 45. Witt, U., Muller, R. J., Augusta, J., Widdecke, H. and Decker, W. D. Synthesis properties and biodegradability of polyesters based on 1,3-propanediol. Macromol Chem. Phys., 195: 793±802 (1995).

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46. Witt, U., Muller, R. J. and Decker, W. D. New biodegradable polyester-copolymers from commodity chemicals with favorable use properties. J. Environ. Poly. Degrad. 3: 215±223 (1995). 47. Heller, J. and Daniels, A. U. Poly (ortho esters). In Biomedical Polymers. S. W. Salaby ed. Hanser Publisher, pp. 35±67 (1994). 48. Heller, J., Penhale, D. W. H. and Helwing, R. F. Preparation of poly(ortho esters) by reaction of dikene acetals and polyols. J. Polym. Sci. (Polym. Lett. edn) 18: 619±624 (1980). 49. Goodman, I. and Rhys, J. A., eds, Polycarbonates in Polyester. Elsevier Publishing, New York, NY, pp. 141±153 (1965). 50. Kohn, J. and Langer, R. Poly(iminocarbonates) as potential biomaterials. Biomaterials 7: 176±182 (1988). 51. Pulapura, S., Li. C. and Kohn, J. Structure-property relationships for the design of polyiminocarbonates. Biomaterials, 11: 666±678 (1990). 52. Nathan, A. and Kohn, J. Amino acid derived polymers. In Biomedical Polymers. S. W. Salaby ed., Hanser Publisher, pp. 117±151 (1994). 53. Bastioli, C., Bellotti, V., Del Giudice, L., Del Tredici, G., Lombi R. and Rallis, R. PCT Int. Patent Appl. WO 90/10671 (1990). 54. Lay, G., Rehm, J., Stepto, R. F., Thoma, M., Sachetto, D., Lentz J. and Silbiger, J. U.S. Pat. 5,095,054 (1992). 55. Vaidya, U. R. and Bhattacharya, M. Compositions of Biodegradable Natural and Synthetic Polymers. US Pat. 5,321,064 (1994). 56. Narayan, R. In Assessment of Biobased Materials, H. L. Chum, ed., Report SERI/TR 234-3610, Solar Energy Research Institute, Colorado, pp. 7.1±7.25 (1989). 57. Potts, J. E. In Aspects of Degradation and Stabilization of Polymers, H. H. J. Jellinek, ed., Elsevier, Amsterdam, p. 617 (1978). 58. Albertsson, A. C. Biodegradable Polymers. J. Macromol. Sci., Pure and Appl. Chem., A30(9), 757±760 (1993). 59. Griffin, G. J. L. In Proceeding of Symposium on Degradable Plastics, The Society of Plastics Industry, Inc., Washington D. C., pp. 47±49 (1987). 60. Jacob, J., Mani, R. and Bhattacharya, M. Compatibility and Properties of Biodegradable Polyester Blends. J. Polymer Science: Part A: Polymer Chemistry 40(12): 2003±2014 (2002). 61. Verhoogt, H., Ramsay, B. A. and Favis, B. D. Polymer blends containing poly(3hydroxyalkoanates). Polymer 35: 5155±5169 (1994). 62. Pearce, R. P. and Marchessault, R. H. Melting and Crystallization in Bacterial Poly( -hydroxyvalerate), PHV, and Blends with Poly( -hydroxybutyrate-cohydroxyvalerate). Macromolecules. 27: 3869±3874 (1994). 63. Scandola, M., Ceccoruli, G. and Pizzoli, M. Miscibility of bacterial poly(3hydroxybutyrate) with cellulose esters. Macromolecules. 25: 6441±6446 (1992). 64. Swift, G. Water-soluble polymers. Polymer Degradation and Stability 45: 215±231 (1994). 65. Wolk, S. K., Swift, G., Paik, Y. H., Yocom, K. M., Smith, R. L. and Simon, E. S. One- and two-dimensional NMR characterization of poly(aspartic acid) prepared by thermal polymerization of L-aspartic acid. Macromolecules. 27: 7613±7620 (1994). 66. Roweton, S., Huang, S. J. and Swift, G. Poly(aspartic acid) ± synthesis, biodegradation, and current applications. Journal of Environmental Polymer Degradation. 5: 175±181 (1997).

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14

Mechanism of biodegradation S M A T S U M U R A , Keio University, Japan

14.1 Introduction The production of biodegradable polymers is now rapidly increasing, and new biodegradable polymeric materials have been developed based on various factors, such as polymer structure, chemical/enzymatic modification, blending and mechanical treatments. These factors are closely correlated with their biodegradation behaviors in addition to their physico-chemical and mechanical properties. In order to design and develop novel biodegradable polymers, understanding and consideration of the biodegradation mechanisms are essential. Furthermore, the environmental/enzymatic degradation mechanism can be applicable for the transformation of naturally abundant and waste polymers into useful materials by chemical recycling and new chemical feedstocks. Figure 14.1 summarizes the various modes of environmental polymer degradation. Polymeric materials were subjected to degradation by biological, chemical and/or physical (mechanical) actions in the environment. Polymeric materials generally undergo these factors concurrently in the environment. Generally, biodegradation involves successive chemical reactions, such as hydrolysis,

14.1 Environmental polymer degradation.

Table 14.1 Structure of main chain polymer and the specific enzyme Main chain structure

Polymer

Corresponding enzyme responsible to the primary degradation Oxidoreductase peroxidase oxidase lignin peroxidase dehydrogenase

C-C linkage

polyethylene natural rubber polyisoprene lignin

±C±C±C± | | OH OH

ÿHydroxy group

poly(vinyl alcohol)

±C±O±C±

Ether linkage

poly(ethylene glycol) poly(propylene glycol)

±C±O± || O

Ester linkage

polyesters PHA, PCL PLLA

±O±C±O± || O

Carbonate linkage

polycarbonate

lipase esterase

±C±N± || H O

Amide linkage

protein polylysine poly(glutamic acid) poly(aspartic acid)

protease

±N±C±O± H || O

Urethane linkage

polyurethane

±C±C± ±C± ±C±

dehydrogenase oxidase peroxidase

Hydrolase PHA depolymerase lipase, esterase protease

Mechanism of biodegradation

359

oxidation/reduction with/without the aid of enzymes in living organisms depending on the environmental conditions. Biological action is represented by enzymatic reactions. Typical examples related to biodegradation are biological hydrolysis by hydrolase enzymes and oxidation by oxidoreductase enzymes. The reaction rate by the former route is generally faster than that by the latter route. The hydrolase enzyme is responsible for the hydrolysis of ester, carbonate, amide and glycosidic linkages of the hydrolyzable polymers producing the corresponding low-molecular weight oligomers. The oxidoreductase enzyme is responsible for the oxidation and reduction of ethylenic, carbonate, amide, urethane, etc. Hydrocarbons such as polyethylene, natural and polyisoprene rubbers, lignin and coal are first subjected to biological oxidation by oxidoreductases, such as oxygenases, hyroxylases, monooxygenases, peroxidases and oxidases in the biodegradation process. Table 14.1 shows the structure of the main chain polymer and the specific example of the related enzyme. However, it should be remembered that degradation proceeds both by the abiotic and biotic actions in the environment. This chapter summarizes the general biodegradation mechanism and some specific biodegradation mechanisms for important as well as potentially important polymers.

14.2 Biodegradation mechanism: overview Biodegradable polymers are generally degraded through two steps of primary degradation and ultimate biodegradation (Fig. 14.2). Primary degradation is the main chain cleavage forming low-molecular weight fragments (oligomers) that can be assimilated by the microbes. Molecular weight reduction is mainly caused by hydrolysis or oxidative chain scission. Hydrolysis occurs using environmental water with the aid of an enzyme or under non-enzymatic

14.2 Biodegradation process of biodegradable polymers.

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Biodegradable polymers for industrial applications

conditions (abiotic). In this case, autocatalysis, heat, or catalytic metals are often responsible for the hydrolysis. Oxidative scission occurs mainly by oxygen, a catalytic metal, UV light or an enzyme. It should be noted that the polymer chain can also be cleaved by mechanical strain such as bending, pressing or elongation. The low-molecular weight fragments produced were incorporated into microbial cells for further assimilation to produce carbon dioxide, water and microbial cells/metabolic products under aerobic conditions. Under anaerobic conditions, methane is mainly produced in place of carbon dioxide and water. In this section, some characteristic degradation phenomena are reviewed with respect to the polymer chain scission and degradation pattern, and the main degradation mechanism by enzymes.

14.2.1 Polymer chain scission Polymer chain scission occurs in two ways, depolymerization (exogeneous scission) and random (endogeneous) scission. In the former the polymer chain is cleaved from the terminal of the chain. In this case, a water-soluble monomer/ oligomer is generally liberated into the reaction media and the rate of the molecular weight reduction of the residual polymer is small (curve I in Fig. 14.3). The biodegradation of polyethylene glycol is a typical example. In the latter way the polymer chain is randomly cleaved. In this case, the molecular weight of the remaining polymer quickly decreased (curve II in Fig. 14.3) (Sawada, 1986). At the same time the mechanical properties of the remaining polymer are also quickly decreased. The addition to these two types of polymer chain scissions, degradation at the weak link, is presented. That is, the polymer chain is cleaved at the relatively weak bond by the various physico-chemical

14.3 Degradation vs remaining molecular weight (adapted from Sawada, 1986).

Mechanism of biodegradation

361

actions. The thermal degradation of polystyrene and polymethacrylate are typical examples.

14.2.2 Degradation site of plastic materials In general, an amorphous region is more susceptible to degradation both by enzymatic and non-enzymatic hydrolysis. This is ascribed to the ease of water penetration into the amorphous region. There are two types of degradation of plastic materials, surface degradation (erosion) and bulk degradation, depending on the main degradation site. Surface degradation may occur when catalytic molecules such as an alkaline catalyst or enzyme, exclusively act on the surface of the plastics or water molecules cannot diffuse into the bulk layer. During surface degradation, spherulite formation may be observed and the sample weight may decrease soon after the beginning of degradation. Changes in the mechanical properties of the sample are dependent on the shape of the sample, thickness and crystallinity.

14.2.3 Degradation mechanism via hydrolysis Polyesters, polyanhydrides, polycarbonates and polyamides are mainly degraded by hydrolysis into low-molecular-weight oligomers at the primary degradation with subsequent microbial assimilation in the biodegradation process. Figure 14.4 shows the classification of the hydrolytic degradation of the hydrolyzable polymers. Hydrolytic degradation is divided into two types, catalytic hydrolysis and non-catalytic hydrolysis. Furthermore, catalytic degradation is divided into two types, external catalytic degradation and internal catalytic degradation. The former includes enzymatic degradation by hydrolase enzymes, such as depolymerase, lipase, esterase, glycohydrolase, etc., and non-enzymatic catalysts, such as alkaline metal and solid acids in the environment. Internal catalytic degradation involves autocatalysis by the terminal carboxyl groups of the polyester chain. In general, enzymatic hydrolysis accompanies nonenzymatic degradation.

14.4 Classification of hydrolytic degradation of hydrolyzable polymer.

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Biodegradable polymers for industrial applications

14.2.4 Miscellaneous degradation mechanisms Some other degradation mechanisms other than simple hydrolysis are presented for biodegradable polymers. Such mechanisms include oxidative cleavage by a radical mechanism. Oxidative degradation is the main mechanism for non-hydrolyzable polymers, such as polyolefins, natural rubber, lignins, and polyurethanes. For many polymers, hydrolysis and oxidation occur simultaneously in the environment.

14.3 Biodegradation mechanism of naturally occurring polymers Proteins and carbohydrates are the most important and renewable biological polymerics which are frequently used in the industrial and medicinal fields. Most enzymes catalyzing the primary degradation of protein and carbohydrates are hydrolase enzymes. Apart from these polymers, a relatively lower amount of natural polymers such as natural rubber, nucleic acids and lignin, are available. In this section, the biodegradation mechanisms of the more abundant and wellknown natural polymers, such as proteins, cellulose, starch, chitin, chitosan, nucleic acids, and their derivatives, are omitted because many excellent reviews and books are available.

14.3.1 Natural rubbers and poly(cis-1,4-isoprene) The rubber family can be divided into natural rubber and vulcanized natural rubber having a cross-linked network with an infinite molecular weight and synthetic polyolefin-type rubber. Among them, only natural rubber with/without vulcanization and synthetic poly(cis-1,4-isoprene) rubber are biodegradable. Natural rubber widely occurs in many grasses and plant species in addition to the rubber plant. Therefore, natural rubber degrading microbes are widely distributed in the environment. Natural rubber is degraded by various microbes. Degradation is initiated by oxidation at the double bond of the polymer chain. The residual rubber showed a decrease in the double bonds, and the formation of carbonyl, peroxide or epoxide groups was observed. (Cundell and Mulcock, 1975). Tsuchii et al. isolated two kinds of rubber-degrading microbial strains, actinomycete Nocardia sp. strain 835A and Xanthomonas sp. strain 35Y, from the soil. The growth of the organism essentially occurs on the insoluble rubber substrate and the cells are tightly bound to the rubber pieces during the initial stage of growth (Tsuchii et al., 1985; Tsuchii and Tokiwa, 1999a,b). Strain 35Y grew slowly on natural rubber latex and secreted an extracellular rubberdegrading enzyme (Tsuchii and Takeda, 1990). Natural rubber was degraded by the enzyme and produced two main fragments consisting of 2 and 113 isoprene units. Each fraction consisted of aldehyde and ketone end groups. The product specificity was very high, mainly forming 12-oxo-4,8-dimethyltrideca-4,8-

Mechanism of biodegradation

363

14.5 Proposed degradation mechanism of natural rubber and final product (adapted from Tsuchii and Takeda, 1990; Lions and SteinbÏchel, 2001).

diene-1-al as shown in Fig. 14.5. The enzymatic chain scission of natural rubber occurs at the double bond of the cis-1,4-polyisoprene moiety by oxygenase as shown in Fig. 14.5 (Tsuchii et al., 1985; Lions and SteinbuÈchel, 2001). Partially purified enzyme with a molecular weight of about 5  104 Da was obtained by the same authors. (Tsuchii and Tokiwa, 1999a,b). Biodegradation of natural rubbers was further studied by the same authors with respect to rubber waste treatment (Tsuchii and Tokiwa, 1999b). The molecular weight reduction of natural rubber from 640,000 to 25,000 implied an endo-cleavage mechanism in the biodegradation process as reported by Jendrossek et al. (1997). Based on these results, primary degradation takes place due to the extracellular enzyme secreted by the rubber-degrading microbes producing the lower-molecular weight fractions which are further assimilated by the microbes. (Linos et al., 2000). Among the synthetic rubbers, only poly(cis-1,4-isoprene) rubber has been reported to be biodegradable. It is also known that poly(cis-1,4-isoprene) is quite susceptible to oxidative degradation at the double bond. Thus, carbonyl ends are produced by the double bond scission as shown in Fig. 14.5 (Harayama et al., 1992). As shown in this figure, double bond cleaving dioxygenases require a transition metal as a cofactor for their interaction with dioxygen.

364

Biodegradable polymers for industrial applications

14.3.2 Lignins Lignin makes up as much as 30 percent of the carbon found in the organic matter of plants, and lignin is available from biomass refining as an abundant renewable carbon resource. However, its highly heterogeneous structure has limited its effective use as a chemical feedstock. In order to use it as a chemical feedstock, developing a selective transformation method is required. Clarification of the biodegradation mechanism of lignin will allow one to design a biotransformation method which may be the most feasible for such materials. The only organism capable of efficient lignin degradation is basidiomycetous white-rot fungus and related litter-decaying fungi (Kirk and Cullen, 1998). As the fungi extracellular lignin-modifying enzymes, laccase, lignin peroxidases and manganese peroxidases are the best known and their specific reaction mechanisms are presented. Because of the highly polydisperse materials of lignin, in this section, details of the degradation mechanism of lignin are not further described. Many excellent reviews have been published (Fritsche et al., 1999; Hatakka, 2001).

14.3.3 Coals Coal is one of the most important fossil resources because coal deposits are worldwide and may be more abundant than petroleum resources. Thus coal may again become an important and versatile raw material for the production of various chemicals as the next generation carbon resource. For the biotransformation of coal into lower molecular weight compounds, a bioprocess may become an important and environmentally benign process as well as saving energy for the next-generation chemical industry using coals. Coals are complex and heterogeneous polymer networks with aliphatic and ether bridges between aromatic moieties. There are two different bioconversion

14.6 The so-called ABCDE-mechanism of biological conversion of brown coal (adapted from Hofrichter and Fakoussa, 2001).

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routes of coals, solubilization and depolymerization. Various microbes capable of solubilizing coal to black liquids are reported, such as aerobic bacteria, actinomyceles, and micromycetes. The solubilization process mainly involves non-enzymatic process but also involves chelators and hydrolytic enzymes. The solubilization process was called the ABC-system by Fakoussa et al. (1997). This was later extended to the ABCDE-system to describe all possible mechanisms which are involved in brown coal bioconversion as shown in Fig. 14.6 (Fakoussa and Hofrichter, 1999; Hofrichter and Fakoussa, 2001). 1.

2.

3. 4. 5.

Alkaline substances: alkaline solubilization is attributed to the high content of carboxylic groups in coal humic acids, resulting in the formation of black water-soluble salts (Quigley et al., 1989). Alkaline conditions of the medium can be created by the secretion of alkaline metabolites, e.g., ammonium ions or biogenic amines. Free bases are liberated by the microbial utilization of organic acids used as growth substances containing mono and dibasic metals (Na, K, Ca) (Holker et al., 1995). Biocatalysts (oxidative enzymes): this enzyme system consists of peroxidases, such as manganese peroxidase, lignin peroxidase and other peroxidases, phenol oxidases, such as laccases, and various low-molecularweight agents, such as oxalate and malate. Chelators: chelator molecules can remove complexing metal ions forming bridges in the coal polymer networks. Detergents: chemo- and biosurfactants may solubilize coal or extract specific compounds from the coal, most likely interacting with aliphatic coal moieties. Esterases: the solubilization of brown coal is attributed to hydrolytic enzymes, such as esterases, which cleave ester bonds and other hydrolyzable bonds of the coal molecule (Fig. 14.6).

Primary degradation of coal occurs by the cleavage of the carbon-carbon covalent bond and ether bond by the extracellular peroxidases (Fritsche et al., 1999). Lignin peroxidase and manganese peroxidase from ligninolytic fungi depolymerize the coal liberating fulvic acid. This degradation proceeds by the cometabolic pathway.

14.4 Biodegradation mechanism of polyesters Biodegradable polyesters are grouped according to their synthetic methods, chemical synthesis and biological synthesis. Chemically synthesized polyesters include the polycondensates of diols with dibasic acids and of hydroxy acids. Microbially produced biopolyesters are exclusively based on hydroxyalkanoic acid polycondensates. These polyesters are equally biodegraded by hydrolysis at the ester bonds to produce low-molecular-weight intermediates during the initial degradation stage.

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14.4.1 Biodegradation of chemically synthesized aliphatic polyesters The biodegradability of the chemically synthesized polyesters is significantly influenced by the chemical structure and physicochemical properties, such as melting temperature and crystallinity of the polymer. (Tokiwa et al., 1988; Tokiwa and Jarerat, 2004; Albertsson and Ljungqvist, 1988a,b; Mathisen and Albertsson, 1990). The rate of degradation decreases with the increasing melting temperature. (Tokiwa and Suzuki, 1981). Also, orientation of the polymer molecules reduces the rate of degradation. The discovery of polyester degradation by lipase promoted the study of plastic degradation (Tokiwa and Suzuki, 1977a). A lipase randomly cleaves the ester bond along the main chain of the polymer molecules (Tokiwa and Suzuki, 1977b; Nakayama et al., 1997, 1998). Biodegradation of polyesters occurs by the chain scission by both simple non-enzymatic hydrolysis and enzymatic hydrolysis (Kasuya et al., 1998; Holland et al., 1986; Reeve et al., 1994). Poly(-caprolactone) Poly(-caprolactone) (PCL) is prepared by the ring-opening polymerization of caprolactone. High-molecular-weight PCL is an almost crystalline polymer with a moderately low Tm (60 ëC), and is a tough and semi-rigid material at room temperature. The biodegradation of PCL was demonstrated in the 1970s (Fields et al., 1974; Tokiwa and Suzuki, 1977a,b). In several biotic environments, PCL is degradable (Benedict et al., 1983; Albertsson and Edlund, 1998). However, PCL has been found to biodegrade more slowly than biopolyesters and starch in most environments. Microorganisms that degrade PCL are widely distributed in nature (Benedict et al., 1983; Nishida and Tokiwa, 1993). The proposed degradation mechanism is started by the hydrolysis of the polymer chain to 6-hydroxyhexanoic acid, an intermediate of !-oxidation, and then -oxidation to acetyl-CoA, which can then undergo further degradation in the TCA cycle as illustrated in Fig. 14.7 (Pitt et al., 1981). The molecular weight readily decreases during biodegradation, and is accompanied by a broadening of the molecular weight distribution (Benedict et al., 1983). The degree of crystallinity of PCL increases with the degradation, indicating preferential degradation in amorphous regions (Albertsson et al., 1998; Albertsson and Varma, 2002). PCL was also confirmed to be an analogue of cutin (the structural polyester of plant cuticle) from the fact that it is degraded by cutinase of phytopathogenic fungi (Nishida and Tokiwa, 1994; Murphy et al., 1996). Poly(alkylene alkanoate) Poly(alkylene alkanoate)s, such as poly(butylene succinate) (PBS), poly(butylene adipate) (PBA) and poly(ethylene adipate) (PEA), are synthetic

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14.7 PCL degradation mechanism (adapted from Pitt et al., 1981).

biodegradable plastics which are now commercially available. They readily undergo degradation in any environment (Fujimaki, 1998; Ishioka et al., 2002). Microbes are thought to degrade the main polymer chain into small fragments via enzymatic hydrolysis (Nishioka et al., 1994; Ando et al., 1998). PEAdegrading microbes are quite widely distributed; however, their distribution is smaller than that of PCL (Tokiwa and Suzuki, 1977a,b). The biodegradation of PBS is also demonstrated, and the PBS-degrading microbes, such as Amycolatopsis sp. HT-6 and Bacillus strain TT96, have been isolated (Pranamuda et al., 1995, Tansengco and Tokiwa, 1998a,b). Water-soluble 1,3butanediol, 4-hydroxy n-butyrate and succinic acid were temporarily accumulated during degradation of PBS. The biodegradable polyesters thus far developed do not always sufficiently exhibit mechanical and thermal properties. These properties are sometimes improved by incorporating aromatic units into the main chain of the aliphatic polyesters in ratios at which the biodegradability of the resulting copolymers may not be decreased (Honda et al., 2003). Poly(butylene adipate-coterephthalate) (trade name Ecoflex) is degraded into various aliphatic and aromatic oligomers by the action of the microorganism Thermomonospora fusca DSM43793 and at the end of this biodegradation, adipic acid, terephthalic acid and 1,4-butanediol are detected (Witt et al., 2001). Further degradation mechanisms with respect to the intermediate compounds during biodegradation were studied using lipase which is commonly present in the environment. The proposed enzymatic degradation mechanism of poly(butylene succinate-co-terephthalate) (PBST) is summarized in Fig. 14.8 (Honda et al., 2003). As the degradation product, a hexamer containing one terephthalate unit was the largest fragment, while a pentamer was the next largest one consisting only of succinate units. A variety of oligomeric fragments were also involved in the secondary hydrolysis into 4-hydroxybutyl succinate (BS) and 4-hydroxybutyl terephthalate (BT). The trimeric fragments, SBS and

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14.8 A plausible mechanism of enzymatic hydrolysis of PBST with lipase PS. The compounds shown in bold are the main products (adapted from Honda et al., 2003).

SBT, were slowly hydrolyzed by a nonspecific mechanism. The PBST degradation was regarded as an endo hydrolysis (Honda et al., 2003).

14.4.2 Poly(lactic acid) and Poly(glycolic acid) Poly(-hydroxy acid)s, such as poly(glycolic acid) (PGA) and poly(lactic acid) (PLA), are crystalline polymers with relatively high melting points. Lactic acid is produced by a fermentative process, and PLA will replace various conventional plastics produced from petrochemicals. PLA has a high degree of stability under normal conditions of use and storage, and can be rapidly degraded after use, such as in a composter (Kawashima et al., 2002). The rate of nonenzymatic hydrolysis of PLA in a buffer solution is much slower than that of PGA because of the hydrophobic nature of PLA having methyl groups. The hydrolytic degradability of PLA in a buffer solution has been extensively studied, especially for biomedical applications (Leenslag et al., 1987; Tsuji and Nakahara, 2002). On the other hand, the biodegradation of PLA was also confirmed. The biodegradability of PGA has not been clarified because of its rapid hydrolyzability. Microbial degradation of PLA PLA-degraders have a limited distribution and are rather scarce in the environment compared with those that degrade PHB, PCL and PBS (Nishida and Tokiwa, 1993; Pranamuda et al., 1995, 1997; Tokiwa and Pranamuda, 2001). In natural environments, the main PLA degradation proceeded in a twostep reaction. During the primary degradation step, PLA undergoes nonenzymatic hydrolysis, which is both temperature and humidity dependent. During the secondary degradation step where the Mn decreases to 10,000±

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20,000, microorganisms present in the soil begin to digest the lower-molecularweight oligomer and lactic acid, producing carbon dioxide and water (Torres et al., 1996; Pranamuda et al., 1997; Pranamuda and Tokiwa, 1999; Ho and Pometto, 1999; Meinander et al., 1997; Jarerat and Tokiwa, 2001; Jarerat et al., 2002, 2003; Lunt, 1998; Hakkarainen et al., 2000). During active fermentation in an actual compost, it showed the same degree of biodegradability as organic waste; 60% of the PLA was converted into inorganic substances in about 10 days, and >90% was degraded in about 20 days (Kawashima et al., 2002). Apart from this two-step mechanism, PLA was degraded by direct bacterial attack on the polymer in a single-step process (Mochizuki, 2002). The first report of microbial degradation of PLA using the Amycolatopsis strain HT-32 (Pranamuda et al., 1997) has promoted further studies on the microbial degradation of PLA. PLA-degraders are widely distributed within this genus (Pranamuda and Tokiwa, 1999; Ikura and Kudo, 1999; Tokiwa et al., 1999a; Nakamura et al., 2001). It was found that most strains of these actinomycetes exhibit a silk fibroin-degrading ability in addition to a PLAdegrading ability. This is ascribed to the stereochemical similarities between Lalanine and L-lactic acid. Silk fibroin powder is an effective culture substrate for inducing enzyme production by the Amycolatopsis strain 41 (Pranamuda et al., 2001). PLA-degrading Bacillus strains have been isolated from compost (Tomita et al., 1999; Sakai et al., 2001). Enzymatic degradation of PLA PLA was enzymatically degraded by proteinase K, a fungal serine protease of Tritirachium album. Most biodegradation studies of PLA were carried out using proteinase K as a convenient method (MacDonald et al., 1996; Moon et al., 2003; Matsumura et al., 1999a; Tsuji and Ishizaka, 2001; Cai et al., 1996; Iwata and Doi, 1998; Tsuji and Miyauchi, 2001). The PLA-degrading enzyme was purified from Amycolatopsis strain 41 by Pranamuda et al. (2001). The enzyme could degrade casein, silk fibroin and PLA, but not PCL nor PHB. Similar results were reported such that the purified PLA depolymerase isolated from Amycolatopsis strain K104-1 exhibited a degrading activity on PLA, casein and fibroin, but not PCL and PHB (Nakamura et al., 2001). PLA-degrading enzymes selectively cleave the -ester bond of the L-isomer as the L-alanine unit of silk fibroin (protein) (Tokiwa and Jarerat, 2004). Also, L-lactic acid units are preferentially degraded as compared to the D-lactic acid units (Reeve et al., 1994; MacDonald et al., 1996). Enzymes, such as tissue esterases, pronase, and bromelain, are also able to affect PLA degradation. The PLA-degrading enzyme seems to be a protease-type enzyme which recognizes the repeated L-lactic acid unit of PLA (Vert, 2002).

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14.4.3 Biodegradation of biopolyesters Many aerobic and anaerobic bacterial species, under nutrient-limiting conditions with a sufficient supply of carbon resources, accumulate polyesters composed of poly(R-3-hydroxyalkanoate)s (PHAs). The most common of these is poly(R-3-hydroxybutyrate) (PHB). PHB can be present in bacterial membranes and also in a wide variety of plant and animal tissues (Hocking and Marchessault, 1994). PHAs can be completely degraded to carbon dioxide and water through an aerobic bacterial action (Jendrossek and Handrick, 2002; Tokiwa and Calabia, 2004). The biodegradation of PHAs is demonstrated in natural environments such as soil (Mergaert et al., 1993), sea water (Doi et al., 1992a) and river water (Doi et al., 1996). The biodegradation processes are divided into two categories: intracellular and extracellular. The rate of biodegradation of PHA is dependent on environmental conditions such as temperature, moisture, pH, nutrient supply, and those related to the PHA materials themselves, such as monomer composition, crystallinity, additives and surface area (Abe and Doi, 2002). The intracellular biodegradation pathway of PHB is illustrated and combined with the biosynthetic pathway in Fig. 14.9 (MuÈller and Seebach, 1993; Senior and Dawes, 1971, 1973; Doi et al., 1992b; Haywood et al., 1988). PHB is first depolymerized by PHB depolymerase to produce the R-3-hydroxybutyric acid (R-3-HB) or R-3HB oligomers. The latter is further depolymerized by oligomer hydrolase to R-3HB monomer. R-3HB is dehydrogenated with NAD+ into acetoacetic acid which follows esterification with CoA-SH to produce acetoacetyl-CoA by the action of acetoacetyl-CoA synthase with the aid of ATP. The acetoacetyl-CoA is degraded into acetyl-CoA by -ketothiolase. This compound then enters the tricarboxylic acid (TCA) cycle to transform carbon dioxide and water under aerobic conditions. The extracellular enzyme is important for PHB degradation in the environment. By excreting depolymerase in the environment, some bacteria can grow on extracellular PHA as the carbon source (Nakayama et al., 1985; Shirakura et al., 1986; Fukui et al., 1988; Saito et al., 1989). Many extracellular PHB depolymerases have been purified and characterized (Tanio et al., 1982; Mukai et al., 1993; Yamada et al., 1993; Jendrossek et al., 1993; Klingbeil et al., 1996; Uefuji et al., 1997). The PHB depolymerase enzymes are comprised of a catalytic domain, a putative substrate-binding domain, and a linker region connecting the two domains. The substrate-binding domain of PHB depolymerase acts as an absorbent onto the water-insoluble polymer chain mainly by hydrophobic interactions. The catalytic domain is responsible for the hydrolyzing of ester bonds of PHA. The extracellular depolymerases degrade the PHB polymer into oligomers, mainly the dimer, and a small amount of 3HB monomer. The extracellular dimer hydrolase is excreted to further degrade oligomers into the R-3HB monomer (Tanaka et al., 1981). A depolymerase

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14.9 Intracellular PHB synthesis and degradation mechanisms.

enzyme isolated and purified from Alcaligenes faecalis T1 cleaves only the second ester linkage from the hydroxyl terminus of the trimer and tetramer and acts as an endo-type enzyme toward the pentamer and the higher oligomers in addition to the exo-hydrolase activity. (Marchessault et al., 1990; Shirakura et al., 1986). The water-soluble products of random copolymers generated by the PHB depolymerase showed a mixture of monomers and oligomers of R-3HB and hydroxyalkanoates units. A schematic model for the enzymatic hydrolysis of ester linkages in a polymer (PHA) chain by PHB depolymerase from Alcaligenes faecalis has been proposed by Abe et al. (1995), as illustrated in Fig. 14.10. The active site of the catalytic domain in PHB depolymerase recognizes at least three monomeric units as the substrate. The catalytic site may bind the substrate by interacting with the carbonyl group in the hydroxy-terminated R1 unit in order to assist the hydrolysis reaction of the second ester bond with the active site of the catalytic domain (Abe and Doi, 2002).

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14.10 A schematic model for the enzymatic hydrolysis of an ester bond in various sequences by PHB depolymerase (adapted from Abe, et al., 1995).

14.5 Biodegradation mechanism of polycarbonates and polyethers 14.5.1 Polycarbonates Bisphenol A-derived aromatic polycarbonates are widely used as long-term commodity plastics in many fields; however, they are highly resistant to biodegradation. On the other hand, aliphatic polycarbonates can be expected to be biodegradable and biocompatible plastics, especially for medical applications. The biodegradation of aliphatic polycarbonates has been studied and many microbial strains capable of biodegrading aliphatic polycarbonates were isolated (Pranamuda et al., 1999). Also, Roseateles depolymerans strain 61A was isolated as a poly(hexamethylene carbonate) degrader (Suyama et al., 1999). Poly(tetramethylene carbonate) was degraded by lipoprotein lipase from Pseudomonas sp. first forming tetramethylene carbonate oligomers. The oligomers were further degraded into small units consisting of di(4hydroxybutyl)carbonate and 1,4-butanediol as shown in Fig. 14.11a (Suyama and Tokiwa, 1997). With the microbial degradation of polycarbonate, the evolution of carbon dioxide and the oxidation of diol to the diacid occurred simultaneously. That is, Roseateles depolymerans strain 61A was shown to form di(6-hydroxyhexyl)carbonate and adipic acid from poly(6-hydroxyhexyl carbonate) and di(4-hydroxybutyl)carbonate and succinic acid from poly(4hydroxybutyl carbonate). Transiently accumulated intermediates will be further assimilated by the environmental microbes to produce carbon dioxide and microbial cells as the ultimate biodegradation (Fig. 14.11b) (Tokiwa, 2003a). The biodegradabilities of various copolyester-carbonates have been reported. Above all, copolycarbonates containing the lactide moiety have been extensively

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14.11 Enzymatic degradation of polycarbonate.

studied with respect to biodegradation and enzymatic degradation. (Matsumura et al., 1999a; Liu et al., 2003; Tsutsumi et al., 2002, 2003a,b; Pego et al., 2003a,b; Kim and Lee, 2002a,b). Copolycarbonates containing the urethane moiety have also been studied with respect to biodegradation and enzymatic degradation (Tang et al., 2002, 2003a,b; Matheson et al., 2002). Furthermore, biodegradable triblock copolycarbonates have been reported. (Zhou et al., 2004). The in vivo degradation of polycarbonates and copolymers has been reported. The in vivo behavior of poly(1,3-trimethylene carbonate) and the copolymers of 1,3-trimethylene carbonate with DL-lactide or -caprolactone were reported by Pego et al. (2003a,b), Christenson et al. (2004a,b) and Matheson et al. (2002).

14.5.2 Poly(ethylene glycol) (PEG) Poly(alkylene glycol)s are widely used as raw materials for the synthesis of detergents and polyurethanes. They are either water-soluble or oily liquids and generally they cannot be recycled or recovered. Therefore, a ready biodegradability will be essential. Among the poly(alkylene glycol)s, poly(ethylene glycol) (PEG) is manufactured in large quantities as a commodity chemical in various fields. The biodegradation of polyethers has been investigated since 1962, especially PEG (Fincher and Payne, 1962). They isolated some aerobic PEG-utilizing bacteria which assimilated PEGs with a variety of molecular weights up to 400. Since then, many reports have been published on aerobic PEG-assimilating bacteria (Ogata et al., 1975; Hosoya et al., 1978; Kawai, 1995, 2002). Various commonly occurring bacteria assimilated low-molecular weight PEGs up to molecular weights of 4,000, while higher molecular weight PEGs from 4,000 to 20,000 were assimilated by a limited number of species: Pseudomonas aeruginosa (up to 20,000); (Haines and Alexander, 1975); Pseudomonas

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14.12 Mechanism of symbiotic degradation of PEG (adapted from Kawai and Yamanaka, 1986).

stutzeri (up to 13,500); (Obradors and Aguilar, 1991) and Spingomonas species (up to 2,000); (Takeuchi et al., 1993; Kawai and Takeuchi, 1996). For the PEG degradation, a symbiotic culture played an important role. The mechanism for the symbiotic biodegradation of PEG by Sphingomonas terrae and Rhizobium sp. was elucidated as being due to the removal of a toxic metabolite, glyoxylic acid being formed during the biodegradation process as shown in Fig. 14.12 (Kawai and Yamanaka, 1986). The PEG degradation pathway under aerobic conditions described by Kawai is described as follows: the first stage is a dehydrogenation (PEG dehydrogenase); the second stage is an oxidation (an aldehyde dehydrogenase/ oxidase); and the third stage is an oxidation followed by hydrolysis to liberate a two-carbon fragment as glyoxylic acid. That is, PEG is successively oxidized to an aldehyde and a monocarboxylic acid, and this is followed by the cleavage of the ether bond, resulting in PEG molecules that are shortened by one glycol unit. Simultaneous oxidation of the two-terminal hydroxyl group of PEG is also possible. The glyoxylic acid produced may be metabolized by known pathways, such as the oxidative dicarboxylic acid cycle, TCA cycle and the glycerate pathway. The biodegradation exogeneously proceeds from a terminal hydroxy group of PEG and thus strictly depends on the terminal groups of PEG (Kawai, 1992, 1993). The monoalkyl PEG was biodegraded, but not the dialkyl PEG. Anaerobic biodegradation of PEG was also demonstrated (Schink and Stieb, 1983). Dwyer and Tiedje (1983) obtained methanogenic consortia from sewage sludge, which can degrade PEG with a molecular weight up to 20,000.

Mechanism of biodegradation

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14.13 Predicted pathways of PEO scission by oxygen insertion or hydrogen abstraction (adapted from Kerem et al., 1998).

Apart from these dehydrogenation mechanisms, a radical oxidative cleavage at the ether linkages was reported. Biodegradation of PEG by brown-rot fungi, Gloeophyllum trabeum occurred by depolymerization due to the oxidative C-C bond cleavage at random locations. The predicted pathways of the PEG scission of PEG after oxygen insertion or hydrogen abstraction are shown in Fig. 14.13 (Kerem et al., 1998).

14.5.3 Poly(propylene glycol) The susceptibility of polypropylene glycol to biological degradation has only a limited characterization. The strain Stenotrophomonas maltophilia grew on various PPGs with a number-average molecular weight of up to 4,000 but did not assimilate the PEGs (Tachibana et al., 2002). The degradation mechanism of PPG is possibly similar to that of PEG; oxidation of the terminal OH group must

14.14 Proposed mechanisms of PPG degradation (adapted from Kawai, 1993).

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Biodegradable polymers for industrial applications

precede the cleavage of an ether bond. In this case, the metabolic products of C3 might be pyruvaldehyde or pyruvate as shown in Fig. 14.14 (Kawai, 1993, 2002, 2003).

14.6 Biodegradation mechanism of poly(vinyl alcohol) Poly(vinyl alcohol) (PVA) is the largest, synthetic, water-soluble polymer produced in the world based on volume. The prominent properties of PVA may include its biodegradability in the environment, because the polyvinyl-type polymer consisting of a carbon-carbon main chain is hardly biodegradable (Chandra and Rustigi, 1998; Kawai, 1997; Watanabe, 1981; Tsuji, 2000; Matsumura, 2003). It suggested the occurrence of two PVA degradation mechanisms, a random-type attack and a terminal unzipping depolymerization process of the polymer chains (Solaro et al., 2000).

14.6.1 Microbial degradation of PVA The biodegradation of PVA has a long history of over 65 years since its first degradation by soil was reported by Nord in 1936. The first report on the PVAdegrading microbe, Pseudomonas boreopolis (Pseudomonas O-3), was published by Suzuki et al. (1973a,b). Similar PVA-degrading microbes were also reported (Pseudomonas vesicularis PD) (Watanabe et al., 1975, 1976), Pseudomonas vesicularis var. povalolyticus PH (Hashimoto and Fujita, 1985; Kawagoshi and Fujita, 1997, 1998), Bacillus megaterium (Mori et al., 1996a), Geotrichum sp. WF9101 (Mori et al., 1996b,c), Phanerochaete chrysosporium (Betty et al., 1999; Mejia et al., 1999), Alcaligenes faecalis KK314 (Matsumura et al., 1994, 1999b) and Pseudomonas sp. 113P3 (Hatanaka et al., 1995a,b). Apart from the aqueous PVA solution, the biodegradation of the PVA film and fibers was also demonstrated but the rate of degradation was relatively slow compared to that of the aqueous solution of PVA. (Chiellini et al., 1999a,b, 2001, 2003; Haschke et al., 1998a,b; Ermilova et al., 1979). In addition to aerobic biodegradation, the anaerobic biodegradation of PVA was confirmed using the anaerobic microbes. A longer time was needed to biodegrade PVA under anaerobic conditions compared to the aerobic conditions (Matsumura et al., 1993a; Li et al., 2001).

14.6.2 PVA biodegradation mechanisms and related enzymes The generally accepted biodegradation mechanisms occur via a two-step reaction by oxidation (dehydrogenation) of the hydroxyl group followed by hydrolysis. However, details of each degrading enzyme are varied according to the origin of the enzyme as shown in Figs 14.15 and 14.16. In the first step of the

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14.15 PVA degradation mechanism by SAO/PVADH and BDH.

degradation, there are two enzymes that have been reported, oxidase and dehydrogenase. The former requires molecular oxygen and the latter requires pyrroloquinoline quinone (PQQ). In the second step, two types of enzymes, hydrolase and aldolase, are reported. PVA biodegradation by oxidation (dehydrogenation) enzyme and -diketone hydrolase (Fig. 14.15) Secondary alcohol oxidase (SAO) SAO, or PVA oxidase, catalyzes the transformation of the hydroxyl group of PVA into the carbonyl group. SAO was first isolated from the culture medium of Pseudomonas O-3 and the degradation mechanism examined (Suzuki, 1976, 1978; Suzuki and Tsuchii, 1983). The following two successive reactions, the first one catalyzed by the SAO and the second one by -diketone hydrolase (BDH), were presented as the mechanism for PVA degradation (Morita and Watanabe, 1977; Morita et al., 1979; Sakai et al., 1985b; Kawagoshi and Fujita, 1997). Acidic SAO was also purified from the P. vesicularis PD (Sakai et al., 1983, 1985a,b; Kawagoshi and Fujita, 1997).

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PVA dehydrogenase (PVADH) PVADH catalyzes the transformation of the hydroxyl group of PVA into the carbonyl group. A PQQ-dependent PVADH was purified from the membrane fraction as a PVA-degrading symbiont, Pseudomonas sp. strain VM15C (Shimao et al., 1986). A PQQ-dependent PVADH was also isolated from Pseudomonas sp. 113P3 by Hatanaka et al. (1995a,b, 1996). -Diketone hydrolase (BDH) A hydrolase, named BDH, was purified from a culture broth of Pseudomonas vesicularis PD. The -diketone moiety was hydrolytically cleaved to produce methyl ketone and carboxylic acid terminals as shown in Fig. 14.15 (Sakai et al., 1981, 1984, 1985b,c, 1986). Enzymatic and biodegradation of PVA by a symbiotic mixed culture PVA was symbiotically utilized by two bacterial members. From a mixed culture, Pseudomonas putida VM15A (strain type II) and Pseudomonas sp.VM15C (strain type I) were isolated as essential members for PVA utilization (Sakazawa et al., 1981; Shimao et al., 1982, 1983a,b, 1985b, 1986). VM15C secreted two enzymes, PVADH and PVA oxidase (SAO), which were responsible for PVA degradation (Shimao et al., 1986, 1989). On the other hand, VM15A provided the essential growth factor of PQQ to VM15C (Salisbury et al., 1979; Duine et al., 1979; Van der Meer and Duine, 1986). The biodegradation mechanism of PVA by the two symbiotic bacterial strains was proposed. The 1,3-dihydroxyl moiety of PVA was first dehydrogenated by PVADH and PQQ; the former was secreted from strain I and the latter was provided from strain II to produce the diketone (oxi-PVA). This moiety was further hydrolytically cleaved by oxi-PVA hydrolase (BDH) secreted from strain I. PQQ produced by VM15A was effective in not only causing growth of the PVA-degrading bacterium on PVA but also in enhancing the growth rate (Shimao et al., 1984, 1985a, 1996).

14.6.3 Enzymatic degradation by PVADH and aldolase Another PVA degradation pathway was confirmed such that the hydroxyl group of PVA was first dehydrogenated by PVADH into the corresponding carbonyl group to form the -hydroxy ketone which was followed by the aldolase-type cleavage to produce the methyl ketone and an aldehyde by the aldolase (Fig. 14.16). Though the two enzymes having the same molecular weight of 67 kDa were isolated from the PVA-assimilating strain, Alcaligenes faecalis KK314, they were the identical proteins having both functions (Matsumura et al., 1999b, 2001). That is, the enzyme protein is the apoenzyme of PVADH which requires

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14.16 PVA degradation mechanism by PVADH and aldolase.

PQQ. More exactly, the apoenzyme of PVADH acted as an aldolase (Matsumura et al., 1999b).

14.6.4 Degradation of PVA via radical cation intermediates The fungal metabolization of PVA has been reported (Ines Mejia et al., 1999). The ligninolitic enzyme, lignin peroxidase, from Phanerochaete chrysoporium promoted the degradation of PVA chains through the formation of carbonyl groups as well as double bonds, thus increasing the unsaturation. The proposed degradation mechanism first involves the conversion to epoxides by the initial free radical reaction with the subsequent elimination of water and the formation of double bonds. As a low-molecular-weight end product, benzaldehyde was produced.

14.6.5 Polymer structure and biodegradation The polymer structures of PVA, such as the saponification degree, polymerization degree, tacticity, and 1,2-glycol content, are responsible for its biodegradability (Hatanaka et al., 1995a,b; Matsumura et al., 1994). It is reported that the 1,3-diols of PVA were dehydrogenated at a greater rate than the 1,2-diols which is produced by the head-to-head reaction of the monomer (Hatanaka et al., 1995a,b). The unreacted acetyl moiety of PVA was hydrolyzed by esterase, which was isolated from the PVA-assimilating microbe,

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Biodegradable polymers for industrial applications

Pseudomonas vesicularis PD, to produce the corresponding hydroxyl group (Sakai et al., 1998). The biodegradation of PVA is influenced by the stereochemical configuration of the hydroxyl groups of PVA. The isotactic moieties of PVA were preferentially degraded (Matsumura et al., 1994; Fukae et al., 1994, 2000).

14.6.6 PVA copolymers and derivatives (Fig. 14.17) PVA copolymers of vinyl alcohol and ethylene, poly[(vinyl alcohol)-coethylene] (EVOH), is industrially produced in significant quantities as an engineering plastic known as EVAL. A thermophilic EVOH-assimilating strain, Bacillus stearothermophilus, was isolated from the soil at 60 ëC by Tomita et al. (1997). The apparent Vmax/Km value of the PVA copolymer with 10 mol% ethylene was over 10-fold greater than that of PVA (Hatanaka et al., 1995b). The biodegradation mechanism of the PVA having side chain groups attached to the hydroxyl groups of PVA has not been extensively studied. However, these types of polymer have the side groups first deleted thus liberating PVA which will be further assimilated. Poly(sodium vinyloxyacetate) (PVOA), which is produced by the introduction of a carboxymethyl group into the hydroxy group of PVA, was biodegraded by activated sludge as well as isolated microbes, such as Bacillus cereus. The biodegradation of PVOA may occur by the following pathways: (i) hydrolytic splitting of the pendant oxyacetic acid group in PVOA

14.17 Proposed biodegradation mechanism of PVA derivatives.

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and (ii) oxidation or dehydrogenation of the resulting PVA by the enzyme, then the enzymatic hydrolytic cleavage as shown in Fig. 14.17 (Matsumura et al., 1988b,c). A novel biodegradable and water-soluble functional PVA consisting of three parts, the PVA main chain, the adipic ester moiety as a spacer arm, and glucose and thymidine moieties, was prepared and the complete biodegradation of the sugar-branched PVA was reported (Kitagawa and Tokiwa, 1997, 1998; Kitagawa et al., 1998, 1999, 2000a,b; Shibatani et al., 1997; Tokiwa et al., 1999b, 2000).

14.6.7 Design of biodegradable polymer using vinyl alcohol block Because the short chain vinyl alcohol block can be incorporated into a functional polymer chain by copolymerization of functional vinyl monomers with vinyl acetate and subsequent hydrolysis, the vinyl alcohol oligomer has attracted attention as a biodegradable segment in a polymer chain for the design of biodegradable functional polymers. However, vinyl alcohol blocks must be incorporated into a functional polymer chain in such a manner that they are accepted as a substrate by the PVA-degrading enzyme occurring in the environmental microbes (Matsumura et al., 1999a). High-molecular weight poly(sodium carboxylate) shows excellent builder performances for detergents; however, it is generally highly resistant to biodegradation. On the other hand, the sodium carboxylate oligomer with a molecular weight less than 500 generally exhibits a biodegradability, but not a builder performance. A vinyl alcohol block with the oligomerization degree greater than 3 shows biodegradability. Based on these facts, a biodegradable polycarboxylate can be prepared by combining the short chain sodium carboxylate blocks and vinyl alcohol blocks by the copolymerization of the corresponding monomer with vinyl acetate (Matsumura et al., 1993b, 1995; Matsumura and Tanaka, 1993, 1994; Matsumura and Shigeno, 1993). A significant increase in the PVADH activity was observed for poly[(disodium fumarate)-co-(vinyl alcohol)] [poly(DSF-co-VA)] which contained a vinyl alcohol block length of more than about five. Thus, the minimum block length (Ln), which acts as an enzymatically degradable segment in the copolymer chain, was estimated to be about five vinyl alcohol monomeric units for poly(DSF-co-VA). This tendency was in good agreement with the results obtained from BOD biodegradability using activated sludge. In a similar way, it was estimated that the minimum vinyl alcohol block length of disodium methylenemalonate and sodium acrylate copolymers was about 3 and 5±6, respectively (Matsumura and Tanaka, 1993, 1994; Matsumura et al., 1993b, 1994, 1997). The difference in the minimum vinyl alcohol block length may be attributable to the steric and electrostatic hindrance caused by the neighboring

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carboxylate groups and the entire polymer chain. Thus a longer vinyl alcohol block length in the polycarboxylate chain was needed to be accepted as a substrate for PVADH.

14.7 Biodegradation mechanism of polyurethanes Polyurethane is widely used in various fields, such as the manufacture of plastic foams, cushions, rubber goods, synthetic leathers, adhesives, paints, and fibers. Conventional polyurethane is produced using toxic diisocyanate, which is derived from the even more toxic phosgene, and it is generally resistant to biodegradation (Tokiwa, 2003b; Howard, 2002). This may be ascribed both to the complexity of the molecular structures and lack of enzymatically cleavable linkages in the polymer chain.

14.7.1 Microbial degradation of polyurethane Simple and high-molecular weight polyurethanes without hydrolyzable linkages, such as ester or carbonate linkages, are generally resistant to biodegradation. However, it is well known that low-molecular-weight urethane oligomers can be hydrolyzed by some microorganisms and that the hydrolysis is catalyzed by an esterase (Owen et al., 1996; Ohshiro et al., 1997). However, it is still unclear whether urethane bonds of high-molecular-weight polyurethanes are directly hydrolyzed. Only possible mechanism other than hydrolysis may be oxidation degradation. The biodegradation of polyurethane containing some hydrolyzable groups has been reported (Darby and Kaplan, 1968). The microbial degradation of poly(ester-urethane) is thought to be mainly due to the hydrolysis of ester bonds by esterases or lipases. The polyester segment of the polyurethane is the first cleaving site of the polymer. The oligomeric urethane will then be further degraded (Nakajima-Kambe et al., 1999; Dupret et al., 1999). Also, the in vitro degradation of poly(ether-urethane urea) was evaluated by exposure to enzymatic and aqueous environments. However, in this case, the mechanical properties deteriorated, but a significant molecular degradation was not detected (Zhao et al., 1987; Marchant et al., 1987)

14.7.2 Enzymatic degradation of polyurethane It is not clear whether urethane bonds in high-molecular weight polyurethane are hydrolyzed by an enzyme. Owen et al. (1996) reported that the fungus Exophila jeanselmei REN-11A was able to metabolize the low molecular-weight Ntolylcarbamate model compounds, having structures closely resembling the urethane linkages found in polyurethane based tolylene-2,4-diisocyanate (TDI), to produce toluene diamine. Figure 14.18 shows the proposed pathway for the biodegradation of toluene-2,4-dicarbamic acid diethyl ester (model compound)

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14.18 Proposed degradation scheme of urethane model compound (adapted from Owen et al., 1996).

by Exophiala jeanselmei REN-11A. It is at least demonstrated that only the lowmolecular weight urethane model compound could be hydrolyzed by the microbes. The Comamonas acidovorans strain TB-35, isolated by NakajimaKambe et al., is a gram-negative bacterium that has been reported to be capable of utilizing solid poly(ester-urethane). They isolated a solid poly(esterurethane)-degrading enzyme, which had unique characteristics. That is, this enzyme has a hydrophobic polyurethane-surface-binding domain and a catalytic domain, and the surface-binding domain was considered as being essential for polyurethane degradation. The enzyme structure is similar to PHB depolymerase; however, there was no significant homology between the amino acid sequence of the polyurethane esterase and that of the PHB depolymerase (Nakajima-Kambe et al., 1995, 1997; Nomura et al., 1998). It is easy to understand that poly(ester-urethane) is susceptible to hydrolysis by an enzyme, such as lipase and esterase. The poly(ester-urethane) was hydrolyzed by Rhizopus delemar lipase at the polyester moiety of poly(esterurethane) (Tokiwa et al., 1988). Santerre et al. (1994) and Wang et al. (1997) reported that cholesterol esterase degraded poly(ester-urethane), synthesized from TDI, polycaprolactonediol and ethylene diamine, and released the hardsegment components. One of the reasons why homopolyurethane is resistant to biodegradation may be due to the complexity of the urethane moieties. Conventional polyurethanes are produced using very reactive diisocyanates, such as TDI. Therefore, the molecularly pure and biodegradable diurethane moiety as a hard segment was combined by an enzymatically hydrolyzable carbonate or ester linkages to produce a novel biodegradable poly(carbonate/ester urethane). This synthetic scheme is shown in Fig. 14.19. The urethanediol produced by the reaction of 1,6-hexamethylenediamine with trimethylene carbonate or ethylene carbonate was readily biodegradable by activated sludge. The polycondensation of biodegradable diurethanediol with diethyl carbonate and diethyl alkanedioates using lipase afforded the corresponding poly(carbonate urethane) and poly(ester

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14.19 Enzymatic synthesis and chemical recycling of biodegradable poly(ester/carbonate urethane) (adapted from Soeda et al., 2004, 2005).

urethane), respectively. Also, the polyurethanes were readily transformed into a repolymerizable cyclic monomer/oligomer by lipase in an organic solvent. Thus, the produced poly(carbonate urethane) and poly(ester urethane) are chemically recyclable by lipase in addition to being biodegradable (Soeda et al., 2004, 2005). Apart from the hydrolytic degradation mechanism, an oxidative degradation mechanism was reported. Poly(carbonate urethane) underwent a slight degradation via a common oxidation mechanism for the biodegradation of these polymers. The observed in vitro degradation was inhibited by adding an antioxidant to the polyurethane film (Christenson et al., 2004a,b).

14.8 Biodegradation mechanism of poly(amino acid) Poly(amino acid)s (Fig. 14.20) are usually distinguished from proteins as they are polymers consisting of only one or two kinds of amino acids with molecular weight distributions and having amide linkages other than -linkages, such as the -, - and -amino acid linkages (Obst and SteinbuÈchel, 2004). As a naturally occurring poly(amino acid)s, poly( -glutamic acid) and poly(-Llysine) are well known. In addition to these, cyanophycin is reported and has recently attracted attention as a raw material of natural-type poly(aspartic acid). Poly(, -D,L-aspartic acid) has been extensively studied and its industrial production has started. The biodegradation of naturally occurring

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14.20 Structures of poly(amino acid)s.

homopolyamides, such as poly( -glutamic acid) and poly(-L-lysine), are studied using poly(amino acid)-degrading microorganisms (Oppermann et al., 1998; Obst and SteinbuÈchel, 2004).

14.8.1 Nylon Nylons are a typical example of synthetic poly(amino acids) (Fig. 14.20). Only the linear -aminohexanoic acid oligomer (nylon oligomer)-degrading microbe and the oligomer-hydrolyzing enzyme were obtained. The nylon oligomer hydrolyzing enzymes (hydrolalse) of Flavobacterium sp. KI72 has been characterized in detail (Kinoshita et al., 1977, 1981). The enzyme showed hydrolyzing activities towards dimeric to 20-mer of -aminohexanoic acid, but no activities against 100-mer of the amino acid (Kinoshita et al., 1981). Apart from the hydrolyzing enzyme, manganese peroxidase from a ligninolytic culture of the white rot fungus strain was isolated as a nylon degrading enzyme. Four end groups, CHO, NHCHO, CH3, and CONH2, were formed in the oxidatively biodegraded nylon-66 membranes. (Deguchi et al., 1997, 1998). Figure 14.21 shows the proposed mechanisms by Nomura et al. for nylon degradation by fungus peroxidase. The methylene group adjacent to the nitrogen atom in the polymer chain was attacked by the enzyme, and subsequently the reaction proceeded auto-oxidatively (Nomura et al., 2001). Different enzymatic degradation mechanisms were presented for the microbial degradation of nylon indicating that the extensive degradation of polyamide is feasible.

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14.21 Proposed mechanisms of nylon degradation by peroxidase (adapted from Nomura et al., 2001).

14.8.2 Poly(-L-lysine) Poly(-L-lysine)(-PL) is a homopolymer consisting of L-lysine with -linkages different from natural proteins with -linkages (Fig. 14.20). -PL is microbially synthesized by Streptomyces sp. (Shima and Sakai, 1981) and is now industrially available. -PL is used as a food preservative (Yoshida and Nagasawa, 2003). The -PL degrading microbial strains have been isolated which can grow using -PL as the sole carbon source (Kito et al., 2002a). -PL hydrolyzing enzyme was isolated as an extracellular enzyme of Chryseobacterium sp. OJ7 producing L-lysine oligomers via endo-wise cleavage (Kito et al., 2002b). Also, the intracellular -PL degrading enzyme was isolated, which can hydrolyze -PL to the lysine monomer by an exo-type degradation (Kunioka, 1995).

14.8.3 Poly( -glutamic acid) Poly( -glutamic acid) ( -PGA) is a homopolymer consisting of glutamic acid with -linkages (Fig. 14.20). -PGA is microbially synthesized by various Bacillus strains (Ivanovics and Erdos, 1937) and is now industrially available. The microbial -PGA is generally a copolymer of two enantiomers of L- and Dglutamic acids, but pure -L-PGA and -D-PGA are also found. Relatively few examples have been reported for the biosynthesis of -linked -PGA.

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The biodegradation of -PGA has been established. The extracellular -PGA hydrolase enzyme was isolated from a filamentous fungus, and -PGA consisting of L- and D-glutamic acids was endogeneously hydrolyzed between the L- and L-glutamic acid linkages by the enzyme to produce the L- and Dglutamic acid oligomers (Tanaka et al., 1993a,b). A novel degradation mechanism was presented using the purified extracellular enzyme such that a

-L-PGA homopolymer with a molecular weight of 490,000 and -D/L-PGA heteropolymer with a molecular weight of 11,000 were specifically liberated from the -D/L-PGA with a molecular weight of 1,500,000 and having a block copolymer-like structure (Suzuki and Tahara, 2003).

14.8.4 Cyanophycin Cyanophycin (CGP) is found in all groups of cyanobacteria and its molecular structure is related to that of poly(-aspartic acid) (Fig. 14.20). CGP is a comblike polymer with an -linked L-aspartic acid backbone and L-arginine residues bound to the -carboxylic groups of the aspartic acids (Fig. 14.20). CGP is highly resistant to hydrolytic cleavage by protease such as trypsin. The enzymatic degradation of CGP was extensively studied with respect to both intracellular and extracellular degradation (Richter et al., 1999; Obst et al., 2002). The extracelluar CGPase of Bacillus megaterium was isolated and characterized. The -Asp-Arg dipeptides and (Asp-Arg)2 tetrapeptides were produced as the final degradation products of CGP by the CGPase (Obst et al., 2002, 2004). The -Asp-Arg dipeptides were also exogeneously cleaved off by the reaction of CGP and extracellular CGPase from Pseudomonas anguilliseptica (Hejazi et al., 2002). CGP is attractive as a bio-based poly(aspartic acid), because it may be obtained by the enzymatic cleavage of the L-arginate side chain of CGP.

14.8.5 Poly(aspartic acid) Poly(aspartic acid) (PAsp) has been extensively studied as one of the candidates for the replacement of polycarboxylate compounds, because PAsp has some beneficial characteristics, such as calcium sequestration capacities and dispersing capacities. PAsp is industrially produced only by a chemical method. The produced PAsp is composed of the L- and D-enantiomers of aspartic acid and also having - and -linkages (Fig. 14.20). Poly(-L-aspartate) is prepared by the enzyme-catalyzed polymerization of L-aspartate using protease (Soeda et al., 2003). Poly(-L-aspartate) is readily biodegradable by the activated sludge from sewage treatment plants. On the other hand, thermally polymerized PAsp can be considered as biodegradable although the branching sites and succinic imide moieties as well as D-units of aspartic acid and the amide bond structure of the - or -linkages of PAsp may cause negative effects for rapid biodegradation. Due to these concerns, extensive studies on the enzymatic

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14.22 A proposed mechanism on the enzymatic hydrolysis of PAsp by PAsp hydrolase-1 and PAsp hydrolase-2 in the cell of Sphingomonas sp. KT-1 (adapted from Hiraishi et al., 2004).

degradation of thermally polymerized PAsp have been carried out and the biodegradability demonstrated. High molecular weight and thermally synthesized PAsp were degraded to low molecular weight oligomers by the combination of the two bacterial strains, Pedobacter sp. KP-2 and Sphingomonas sp. KT-1, isolated from river water (Tabata et al., 2000). Pedobacter sp. KP-2 hydrolyzed high molecular weight PAsp into aspartic acid oligomers, and the oligomers with molecular weights less than 5,000 were completely degraded by Sphingomonas sp. KT-1. Two PAsp-hydrolyzing enzymes, PAsp hydrolase-1 and PAsp hydrolase-2, were purified from Sphingomonas sp. KT-1 (Tabata et al., 2001). The proposed degradation mechanism by the combination of the two enzymes is as follows: first, PAsp hydrolase-1 cleaves only the specific amide bonds between the - aspartate linkages of PAsp via an endo-wise cleavage to yield aspartate oligomers (Mw 200±1,000) via an endo-type degradation with subsequent hydrolysis of both the - and -oligo(L-aspartic acid)s to aspartic acid monomers via exo-type degradation by PAsp hydrolase-2 (Fig. 14.22) (Hiraishi et al., 2003a,b, 2004). The Km values of the Asp hydrolase-2 are almost independent of the oligomerization degree, while the Vmax values are dependent on the oligomerization degree and show a maximum value at 5-mer.

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14.9 Biodegradation mechanism of miscellaneous polymers 14.9.1 Polyolefins The C-C polyvinyl type polymer containing side groups, such as short alkyl groups, and phenolic groups, are generally resistant to biodegradation. As an exceptional example, PVA is readily biodegradable by environmentally occurring microbes. Other than PVA, only polyethylene (PE) has been studied with respect to microbial degradation. PE with a molecular weight of less than 1,000 is biodegradable. (Albertsson and Banhidi, 1980; Cornell et al., 1984). Biodegradation of the low-molecular weight PE involves exogeneous elimination (!-oxidation) by the action of oxidoreductases, such as oxygenase, dehydrogenase, and oxydase, forming a fatty acid with subsequent -oxidation. The mechanism shows similarities with the typical -oxidation of fatty acids and n-alkanes. For PE degradation, an initial abiotic oxidation of the polymer chain is also a necessary step; once hydroperoxides have been introduced, a gradual increase in the keto groups of the polymer is followed by a decrease in the keto groups when short chain carboxylic acids are released as degradation products. The combined effect of an abiotic oxidative step with consequent biotic action will be a slow but definite and progressive mineralization (Albertsson et al., 1987; Albertsson and Karlsson, 1990). Figure 14.23 shows the proposed biodegradation mechanism of polyethylene and n-alkane.

14.9.2 Polyacrylate and polycyanoacrylate Poly(sodium acrylate)s are widely used as dispersing agents, chelating agents, scale inhibitors, flocculating agents, and detergent builders. It is generally believed that their biodegradation rate is very slow in the environment. However, sodium acrylate oligomers with molecular weights lower than approximately 500 to 700 are subject to biodegradation in the environment (Matsumura et al., 1988a; Larson et al., 1997). Hayashi et al. isolated the acrylate oligomer-degrading strain, Arthrobacter sp. NO-18, and the acrylate heptamer was the upper limit for rapid biodegradation by this strain (Hayashi et al., 1993). Furthermore, they first obtained a consortium of several bacterial species capable of relatively higher molecular weight acrylate polymers with a molecular weight of 4,000 (Hayashi et al., 1994). These results may indicate that the relatively low-molecular weight poly(sodium acrylate)s are slowly biodegradable. The biodegradation mechanism for poly(sodium acrylate) (average molecular weight, 2,100) by the bacterial consortium no. L7-98 was presented (Iwahashi et al., 2003). The proposed degradation pathway of poly(sodium acrylate) involves (i) the oxidation of a methylene group to a carbonyl group, (ii) decarboxylation to form an aldehyde group and dehydrogenation to form a double bond, and (iii) oxidation of the aldehyde group to a carboxyl group

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14.23 Proposed degradation mechanism of polyethylene and n-alkane (1) oxygenase, (2) dehydrogenase/oxidase, (3) dehydrogenase/oxidase, (4)acyl-CoA synthetase, (5) acyl-CoA dehydrogenase, (8) acetyl-CoA acyltransferase.

followed by elimination of an acetic acid. These are summarized in Fig. 14.24 (Iwahashi et al., 2003). Poly(alkyl cyanoacrylate)s have attracted attention as biomaterials, because they are biocompatible and also readily biodegradable both in vivo and in vitro (Hegyell, 1973). The complete degradation of poly(methyl or ethyl cyanoacrylate) occurs by releasing formaldehyde. However, higher alkyl esters, such as butyl and t-butyl, had decreased degradability (MuÈller et al., 1990). The proposed degradation mechanism of poly(alkyl cyanoacrylate) involves hydrolysis of alkyl ester groups which is accelerated in the presence of hydrolytic enzymes, and backbone degradation by eliminating formaldehyde (Liebmann-Vinson and Timmins, 2003).

14.9.3 Polydioxanone Poly(p-dioxanone) is an aliphatic poly(ether-ester) with bioabsorptibility, biocompatibility and biodegradability (Fig. 14.25) (Ray et al., 1981; Nishida et al., 2000a,b; Ping, 2002). Poly(p-dioxanone) exhibits excellent mechanical

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14.24 Proposed degradation pathway of poly(acrylic acid) (adapted from Iwahashi et al., 2003).

properties and a low toxicity as well as hydrolyzability for biomedical applications as has been anticipated (Yoon et al., 2003; Yang et al., 2002). Poly(p-dioxanone) is prepared by the ring-opening polymerization of pdioxanone which is derived by the intramolecular cyclodehydrogenation of diethylene glycol. The enzymatic and microbial biodegradability of poly(pdioxanone) has been demonstrated. Many types of poly(p-dioxanone)-degrading microorganisms are widely distributed in natural environments (Nishida et al., 2000a,b). The biodegradation mechanism of poly(p-dioxanone) first involves the hydrolytic cleavage to soluble oligomeric acids, which is similar to the biodegradation intermediates of polyethylene glycol. The hydrolyzates are further assimilated by the environmental microbes.

14.9.4 Polyorthoesters Polyorthoesters have been used as biomaterials, for example, in drug delivery systems (Suggs and Mikos, 1996). As a typical example, Heller et al. synthesized a poly(orthoester) from 3,9-bis(ethylidene-2,4,8,10-tetraokisaspiro[5,5]undecane) and glycols (Fig. 14.25). They are highly susceptible to hydrolysis therefore, in the environment, they undergo hydrolytic degradation with subsequent assimilation by environmental microbes.

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14.25 Chemical structures of synthetic biodegradable polymers.

14.9.5 Polyacetals Water-soluble poly(sodium glyoxylate) is synthesized by the polymerization of methyl glyoxylate followed by saponification (Fig. 14.25). The degradation of poly(sodium glyoxylate) occurs by abiotic hydrolysis producing monomeric glyoxylate with subsequent assimilation by environmental microbes (Gledhill and Saeger, 1987).

14.9.6 Polyanhydrides Polyanhydrides have been used for biomaterials, such as drug delivery systems (Burkoth and Anseth, 2000). They are synthesized by the melt polycondensation of diacids. Because they are highly susceptible to hydrolysis, their industrial application is limited. On the other hand, they showed excellent biocompatibilities in vivo (Laurencin et al., 1990). The degradation products of the polyanhydrides are nonmutagenic and noncytotoxic (Domb and Nudelman, 1995). Therefore, in the environment, they first undergo a hydrolytic degradation to produce low-molecular weight fragments which are further assimilated by the environmental microbes. The typical structure of the polyanhydrides is shown in Fig. 14.25.

14.9.7 Polyphosphazenes Polyphosphazenes are linear polymers having an N=P backbone structure. Polyphosphazenes have been used as biomaterials (Allcock and Kwon, 1989). Their main chain is hydrolyzed to produce phosphate and ammonium salts and liberating the side chain groups. Therefore, in the environment, they undergo degradation by hydrolysis. A typical structure of the polyphosphazene is shown in Fig. 14.25.

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14.10 Future trends An understanding of the degradation mechanisms of both natural and synthetic polymers by microorganisms and enzymes will open new prospects in the field of biodegradable plastics. The biodegradation mechanisms of the polymeric materials will contribute to further developments of the next generation materials having a high environmental acceptability and recyclability. Polymer degradation and transformation technologies are essential for polymer production and recycling. As an example, coal and lignin are widely distributed and they will become abundant resources for such polymers and fine chemical raw materials when environmentally acceptable transformation techniques are established. Bio-based polymers may become as important as the next generation of plastics. Poly(lactic acid), biopolyesters and microbial poly(amino acid)s may become the most promising commodity bio-based plastics because they can be produced from renewable resources. They can be recycled by various routes, such as bio-recycling, chemical recycling and material recycling. Also, the biorefining of such bio-based polymers by a bioprocess may contribute to establishing a sustainable chemical industry. In order to develop technologies for the design of biodegradable and bio-recyclable polymers, an understanding of the biodegradation mechanism for each polymer will become a powerful tool. Apart from these considerations, the degradation mechanism can be applicable for novel enzyme-catalyzed polymer syntheses and chemical recycling. Polyesters, polycarbonates and poly(amino acids)s can be synthesized and chemically recycled by novel enzyme-catalyzed polymerization and degradation methods, and these technologies will be realized as a sustainable chemical industry in the near future (Fig. 14.26). Based on these considerations, extensive studies of the biodegradation mechanisms should be further carried out in detail.

14.26 Sustainable polymer production and recycling.

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14.11 Bibliography Books Albertsson A-C, Huang S J (1995), Degradable polymers, recycling, and plastics waste management, Marcel Dekker, Inc, New York. Albertsson A-C, Karlsson S (1998), `Degradable Polymers', Macromolecular symposia, 130, Weinheim, Wiley-VCH. Albertsson A-C, Chiellini E, Feijen J, Scott G, Vert M (1999), `Degradability, renewability and recycling ± Key functions for future materials', Macromolecular Symposia, 144, Weinheim, Wiley-VCH. Chiellini E, Solaro R (2003), Biodegradable Polymers and Plastics, Kluwer Academic/ Plenum Publishers, Woodbury, New York. Doi Y, SteinbuÈchel A (2001), Biopolymers 3b, Polyesters II, Weinheim, Wiley-VCH. Doi Y, SteinbuÈchel A (2001), Biopolymers 4, Polyesters II, Weinheim, Wiley-VCH. Fahnestock S R, SteinbuÈchel A (2003), Biopolymers 7, Polyamides and complex proteinaceous materials I, Weinheim, Wiley-VCH. Griffin G J L (1994), Chemistry and technology of biodegradable polymers, Blackie Academic & Professional, London. Hofrichter M, SteinbuÈchel A (2001), Biopolymers 1, Lignin, humic substances and coal, Weinheim, Wiley-VCH. Huang S J (2002), `An overview of biodegradable polymers and biodegradation of polymers.' Scott G ed., Degradable Polymers (2nd edn), Kluwer Academic Publishers, Dordrecht, Netherlands, 17±26. Koyama T, SteinbuÈchel A (2001), Biopolymers 2, Polyisoprenoids, Weinheim, Wiley-VCH. Liebmann-Vinson A, Timmins M (2003), Biodegradable polymers: Degradation mechanisms, part 2, in PBM Series, vol. 2, Citus Books, 329±372. Matsumura S, SteinbuÈchel A (2003), Biopolymers 9, Miscellaneous biopolymers and biodegradation of polymers, Weinheim, Wiley-VCH. Sawada H (1986), `Depolymerization', in Mark et al., Encyclopedia of Polymer Science and Engineering, 2nd edn, vol. 4, New York, Wiley, 719±745. SteinbuÈchel A (1999), Biochemical Principles and Mechanisms of Biosynthesis and Biodegradation of Polymers, Wiley-VCH.

Reviews Briassoulis, D. (2004), `An overview on the mechanical behaviour of biodegradable agricultural films', J Polym Environ, 12(2), 65±81. Chiellini E, Corti A, D'Antone S, Solaro R (2003), `Biodegradation of poly(vinyl alcohol) based materials', Prog. Polym Sci. 28, 963±1014. Howard GT (2002), `Biodegradation of polyurethane: a review', Int Biodet Biodegr 49, 245±252 Kawai F (1995), `Breakdown of plastics and polymers by microorganisms', Adv Biochem Eng/biotech, 52, 152±194. Obst M, SteinbuÈchel A (2004), `Microbial degradation of poly(amino acid)s, Biomacromolecules, 5, 1166±1176. Tokiwa Y, Jarerat (2004), `Biodegradation of poly(L-lactide)', Biotechnol Lett, 26, 1181±1189. Tokiwa Y, Calabia B P (2004), `Degradation of microbial polyesters', Biotechnol Lett, 26, 771±777.

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14.12 References Abe H, Doi Y (2002), `Molecular and material design of biodegradable poly(hydroxyalkanoate)s', in Doi Y and SteinbuÈchel A, Biopolymers 3b, Polyesters II, Weinheim, Wiley-VCH, 105±132. Abe H, Doi Y, Aoki H, Akehata T, Hori Y, Yamaguchi A (1995), `Physical properties and enzymatic degradability of copolymers of (R)-3-hydroxybutyric and 6hydroxyhexanoic acids', Macromolecules, 28, 7630±7637. Albertsson A-C and Banhidi Z G (1980), `Microbial and oxidative effects in degradation of polyethene', J Appl Polym Sci, 25, 1655±1671. Albertsson A-C, Edlund U (1998), `Novel release systems from biodegradable polymers', Polym Preprints (Am Chem Soc, Div Polym Chem), 39, 186±187. Albertsson A-C, Karlsson S (1990), `The influence of biotic and abiotic environments on the degradation of polyethylene', Prog Polym Sci, 15, 177±192. Albertsson A-C, Ljungquist O (1988a), `Degradable polymers. IV. Degradation of aliphatic thermoplastic block copolymers', J Macromol Sci Chem, A25, 467±498. Albertsson A-C, Ljungquist O (1988b), `Degradable polyesters as biomaterials', Acta Polym, 39, 95±104. Albertsson A-C, Varma I K (2002), `Aliphatic polyesters', in Doi Y and SteinbuÈchel A, Biopolymers 4, Polyesters III, Application and commercial products, Weinheim, Wiley-VCH, 25±52. Albertsson A-C, Andersson S O, Karlsson S (1987), `The mechanism of biodegradation of polyethylene', Polym Degrad Stab, 18, 73±87. Albertsson A-C, Renstad R, Erlandsson B, Eldsater C, Karlsson S (1998), `Effect of processing additives on (bio)degradability of film-blown poly(-caprolactone)', J Appl Polym Sci, 70, 61±74. Allcock H R, Kwon S (1989), `An ionically cross-linkable polyphosphazene: Poly[bis(carboxylatophenoxy)phosphazene] and its hydrogels and membranes', Macromolecules, 22, 75±79. Ando Y, Yoshikawa K, Yoshikawa T, Nishioka M, Ishioka R, Yakabe Y (1998), `Biodegradability of poly(tetramethylene succinate-co-tetramethylene adipate): I. Enzymatic hydrolysis', Polym Degrad Stab, 61, 129±137. Benedict C V, Cameron J A, Huang S J (1983), `Polycaprolactone degradation by mixed and pure cultures of bacteria and a yeast', J Appl Polym Sci, 28, 335±342. Betty L L O, Amanda I M G, Ligia S G (1999), `Biodegradability of poly(vinyl alcohol)', Polym Eng Sci, 39, 1346±1352. Burkoth A K, Anseth K S (2000), `A review of photocrosslinked polyanhydrides: In situ forming degradable networks', Biomaterials, 21, 2395±2404. Cai H, Dave V, Gross R A, McCarthy S P (1996), `Effects of physical aging, crystallinity, and orientation on the enzymatic degradation of poly(lactic acid)', J Polym Sci Part B: Polym Phys, 34, 2701±2708. Chandra R, Rustigi R (1998), `Biodegradable polymers', Prog Polym Sci, 23, 1273±1335. Chiellini E, Corti A, Solaro R (1999a), `Biodegradation of poly(vinyl alcohol) based blown films under different environmental conditions', Polym Degrad Stab, 64, 305±312. Chiellini E, Corti A, D'Antone S, Solaro R (1999b), `Biodegradation of PVA-based formulations', Macromol Symp, 144, 127±139. Chiellini E, Cinelli P, Corti A, Kenawy E (2001), `Composite films based on waste gelatin: thermal-mechanical properties and biodegradation testing', Polym Degrad

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G M A D R A S , Indian Institute of Science, India

15.1 Introduction ISO 472: 1988 defines a biodegradable plastic as follows: A plastic designed to undergo a significant change in its chemical structure under specific environmental conditions resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastics and application in a period of time that determines its classification. The change in chemical structure results from the action of naturally occurring microorganisms.

The ASTM definition, updated in 1994 (ASTM Standard D-5488-84d), has led to the establishment of labeling terminology for packaging materials wherein biodegradable is defined as capable of undergoing decomposition into carbon dioxide, methane, water, inorganic compounds, or biomass in which the predominant mechanism is the enzymatic action of microorganisms, that can be measured by standardized tests, in a specified period of time, reflecting available disposal condition.

Biodegradation is a process whereby bacteria, fungi, yeasts and their enzymes consume the polymer leading to significant changes in the material's chemical structure. In essence, biodegradable plastics should break down cleanly, in a defined time period, to simple molecules found in the environment such as carbon dioxide and water. Biodegradation for limited periods is a reasonable target for the complete assimilation and disappearance of an article leaving no toxic or environmentally harmful residue (Chandra and Rustgi, 1998). It is important to note that the degradation of polymer can rarely be complete because a small portion of the polymer will be incorporated into microbial biomass, humus and other natural products (Alexander, 1977; Atlas and Bartha, 1997; Narayan, 1993). The versatility of carbon-carbon bonds and carbon to non-carbon bonds and other substituent groups with several configurations, orientation and stereo-

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chemistry has led to a wide variety of polymers. Synthetic polymers have widespread use in several industrial, domestic and biomedical applications. These polymers are typically highly stable and resistant to degradation in a normal environment. This is especially applicable to polymers that have only carbon linkages in the polymer backbone. Because of this, the majority of commercial polymers like polyethylene, polypropylene or polystyrene are virtually non-biodegradable. However, other polymers with heteroatoms in the main chain are potentially susceptible to hydrolytic cleavage of the ester bonds or amide bonds. It is generally accepted that if the polymeric structure is similar to that of a natural molecule, it is easily biodegradable. However, it should be noted that very small variations in the chemical structures might result in large differences in terms of biodegradability. There are several factors that influence biodegradation of polymers. Other than the right microbe, degradation depends on the structure, morphology and molecular weight of the polymer. Natural macromolecules, e.g., protein, cellulose, and starch are generally degraded in biological systems by hydrolysis followed by oxidation. Most of the synthetic biodegradable polymers contain hydrolyzable linkages (ester, urethane) along the polymer chain. Introduction of substituents like hydroxy, carboxy, methyl, and phenyl groups also increase biodegradability (Huang et al., 1978). Because the polymer chain must be flexible to fit into the active site of the enzyme, flexible aliphatic polyesters are much more biodegradable compared to rigid aromatic polyesters (Potts et al., 1984). Another reason for the ease of biodegradability of proteins when compared to that of synthetic polymers is because of the crystallinity of the synthetic polymers. Polyamides, for example, have short and regular repeating units with higher symmetries and strong interchain hydrogen bonding resulting in highly ordered crystalline morphologies, limiting the accessibility to enzyme attack. Generally, the degradation rates decrease with an increase in molecular weight. Thus, monomers and oligomers degrade at faster rates. Though polyethylene and polypropylene are resistant to microbial attack, microbes can degrade low molecular weight hydrocarbons. In an extracellular environment, polymer molecules are too large to enter the cell. In the case of natural polymers with high molecular weights, however, the enzymatic reactions occurring outside the microbial cell convert the polymer to lower molecular weight compounds, which are subsequently degraded within the cell. Enzymes are essentially biological catalysts, which enhance the reaction rates by reducing the activation energies of the reaction. The vast majority of enzymes are proteins with a complex three-dimensional structure and their activity is closely related to conformational structure. The three-dimensional structure of enzymes with folds and pockets creates certain regions on the surface with characteristic primary structures that constitutes an active site, wherein interaction between the enzyme and substrate occurs leading to a chemical

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reaction. Different enzymes have different actions, some enzymes change the substrate through a free radical mechanism while others follow alternative chemical routes. Typical examples are biological oxidation and biological hydrolysis. Many enzymes have an optimal temperature around 40 ëC and work best at a pH 6±8. However, there have been several investigations focused on enzymes in non-aqueous solvents. This is primarily because esterification reactions carried out in water will be limited by thermodynamic equilibrium. Several enzymes can react directly with oxygen such that they can be directly incorporated into the substrate. The enzyme can be hydroxylases or oxygenases. Hydroxylases are sometimes called monooxynases and catalyze the insertion of a single atom of oxygen in the substrate. Oxygenases catalyze the insertion wherein the oxygen atoms are incorporated as a part of a carbonyl (ÐCO) or a carboxyl (ÐCOOÐ) grouping. Oxidases are enzymes that function as a hydrogen acceptor that is capable of splitting the aromatic structures to produce (ÐCˆO) groups. Several different hydrolysis reactions occur in biological organisms. Proteolytic enzymes (proteases) catalyze the hydrolysis of peptide bonds and also the related hydrolysis of an ester bond. Two categories of enzymes, extracellular and intracellular depolymerases, are involved in biological degradation of polymers (Doi, 1990; Gu et al., 2000). During degradation, the polymers are broken down to smaller/short chains (oligomers) by exoenzymes. These are small enough to pass through the semipermeable outer bacterial membranes, and then to be utilized as carbon and energy sources. When the product evolved is primarily the monomer, the process is called depolymerization. When the products evolved are inorganic species like carbon dioxide, methane or water, the process is called mineralization. Degradation of polymers by enzymes is affected by various parameters (Marten et al., 2003) like the melting point (Tokiwa et al., 1990a,b), polymer crystallinity (Mochizuki et al., 1995) or tacticity (Kumagai and Doi, 1992; Hocking et al., 1995) of the polymers. When the degradation is carried out in solution, the degradation is influenced by the viscosity and surface tension of the solvent used (Sivalingam et al., 2003a,b). Among the various biological, chemical and physical phenomena involved in the microbial degradation of polymers the biocatalytic reaction at the solid surface of the usually hydrophobic material is assumed to play an important role in the overall process. Factors such as the chemical environment of the cleaved bonds, rigidity of the polymer chain, the molar mass of the polymer, adsorption and surface activation of the enzyme, removal and dissolution of scission products from the surface are often used to control the degradation process (Chandra and Rustgi, 1998; Huang et al., 1994). The purpose of this chapter is to summarize the research findings on the degradation of various polymers and contribute to a better understanding of the biodegradation mechanism of polymers that may possibly lead to novel engineered polymers with tailor-made properties and biodegradation

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characteristics. In addition to the introduction and summary, the chapter has three main sections. The first section deals with vinyl polymers, which are commercially popular but practically non-biodegradable. The second section deals with the degradation of polymers with hydrolysable linkages in the polymer backbone. The third section deals with naturally occurring degradable polymers.

15.2 Vinyl polymers Vinyl polymers, with few exceptions, are generally not susceptible to hydrolysis. Polymers with main chains having only carbon-carbon bonds (with few exceptions) are not suspectible to enzyme-catalyzed reactions. Their biodegradation requires an oxidation process, and most of the biodegradable vinyl polymers contain an easily oxidizable functional group. There have been recent approaches to insert functional groups like ester groups or carbonyl groups into the main chain. The introduction of the carbonyl group into the main chain makes the polymer suspectible to cleavage by photochemical reaction. The introduction of the ester groups allows cleavage by chemical hydrolysis. Copolymers of ethylene and 2-methylene-1,3-dioxepane were prepared by a high-pressure free radical solution polymerization (Bailey and Gapud, 1985) and this introduced an ester group into the backbone of the copolymer, resulting in biodegradation of the polymer. Similarly, main chain ester groups were inserted into polystyrene (Bailey et al., 1990).

15.2.1 Polyethylene High and low density polyethylenes are used in a wide variety of applications, ranging from short-term uses as packaging material to long-term uses in commercial products. The wide applicability of this material is primarily because of its properties, which result in slow degradation rates in a natural environment. Unfortunately, though polyethylene and polypropylene are the most widely used polymers, their enzymatic degradation has been not extensively studied. Various factors including the presence of antioxidants, plasticizers and coloring material can significantly alter the biodegradability of polyethylene. No microorganism or bacterium has been found so far that could degrade PE without additives (Karlsson et al., 1988). Many of the studies on the biodegradation of polyethylene (PE) has been centered on polyethylene-starch blends (Albertsson, 1980a,b; Breslin, 1993; Breslin and Swanson, 1993; Imam and Gould, 1990). Although starch has been used as a filler in plastics for several years, degradable composites of starch and polymer with good mechanical properties came into existence only after the work of Griffin (1976). However, the degradation of these composites is still of serious concern because the molecular weight of PE decreases only after a very long period (Yoon et al., 1996; Albertsson and Ranby, 1976).

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Extra cellular Streptomyces species cultures were found to degrade starch blended PE. Phanerochaete chrysosporium was also found to degrade starch blended LDPE in soil. A decrease of more than 50% reduction in elongation was observed when the soil is inoculated with the organism as against a decrease of only 12% with an uninoculated soil system (Orhan et al., 2000). High molecular weight polyethylene is also degraded by lignin-degrading fungi under nitrogenlimited or carbon-limited conditions, and by manganese peroxidase. Fungi like Mucor rouxii NRRL 1835 and Aspergillius jlavus and several strains of Streptomyces are capable of degrading polyethylene containing 6% starch. Degradation was monitored from the changes in the mechanical properties like tensile strength and elongation (El-Shafei et al., 1998). The biodegradability rate of blends of LDPE and rice or potato starch was enhanced when the starch content exceeded 10%. Various methods such as photodegradation coupled with biodegradation have been investigated. For example, the addition of the gelatin-coated ferric salt in PE extended the induction period of degradation and accelerated photodegradation after the removal of coating material by biodegradation. Many of these studies suggest that the degradation rate of PE could be controlled if more powerful photoactivators and/or coating material are developed (Yoon et al., 1996). The degradation of polyethylene in polyethylene-starch blends is questionable because microbial metabolites may contaminate the surface and may be erroneously interpreted as polyethylene degradation. Abiotic degradation of PE is evident by the appearance of carbonyl functional groups. However, an increase of double bonds was observed when polymers showed weight loss resulting from biodegradation (Albertsson et al., 1994). When a sufficient amount of the starch present in the blend is degraded and removed in this way, the sample should lose its strength and disintegrate but this effect occurs only for samples containing fairly large amounts of starch, of the order of 30% by volume, and polyethylene plastics and films containing so much granular starch have substantially decreased tensile, tear and impact strengths. This indicates that the effective connectivity and accessibility of the starch granules, which is required for extensive enzymatic hydrolysis and removal, is achieved only at relatively high starch contents. At lower starch contents, very little effect on mechanical properties results from the biodegradation of the accessible starch component (Peanasky et al., 1991; Gohen and Wool, 1991). Albertsson (1980a,b) studied the microbial and oxidative effects in the degradation of PE. Colin et al. (1981) studied the biodegradation of polyolefins and polyamide films under soil burial conditions extending for up to about three years. Based on the results of changes in elongation at break, the films have been ranked in the order of increasing sensitivity to degradation as: polyester=polypropylene
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of a nylon-degrading enzyme purified from a culture supernatant of white rot fungal strain IZU-154 were identical to those of manganese peroxidase. Albertsson and Karlsson (1988) and Karlsson et al. (1988) studied the degradation of polymers taking polyethylene as a model substance. The degradation is characterized by three stages: (i) a constant degradation rate, (ii) a parabolic decline in the rate of degradation, and (iii) a subsequent final increase in the rate of degradation. Microbial degradation of polyethylene is generally regarded as a two-step process involving an initial abiotic photooxidation, followed by a cleavage of the polymer carbon backbone. The mechanism of the second step is still under question (Gu, 2003).

15.2.2 Polypropylene The mechanism for the degradation of polypropylene may involve the formation of hydroperoxides which destabilize the polymeric carbon chain to form a carbonyl group (Cacciari et al., 1993; Severini et al., 1988). Plastics composed of biodegradable polycaprolactone (PCL) and conventional plastics like PE and PP have been investigated. However, the blends of PCL and LDPE or PP retained high biodegradability of PCL but did not influence the degradation of PE or PP. In general, it appears that the higher the miscibility of PCL and conventional plastics, the harder the degradation of PCL on their blends by the lipase (Iwamoto and Tokiwa, 1994).

15.2.3 Poly(vinyl alcohol) and poly(vinyl acetate) Among vinyl polymers, poly(vinyl alcohol) (PVA) is the easiest to degrade. PVA has also been used as a polymer carrier for pesticides and herbicides (Harris and Post, 1975a,b). The degradation of PVA has been studied in wastewater-activated sludges (Casey and Manly, 1976), using microbes and enzymes like secondary alcohol peroxidases isolated from Pseudomonas (Suzuki, 1979; Morita and Watanabe, 1977). It was concluded that the initial biodegradation step involves the enzymatic oxidation of the secondary alcohol groups in PVA to ketone groups. Hydrolysis of the ketone groups results in chain cleavage. Other bacterial strains, such as Acinetiobacter and Flavobacterium (Watanabe et al., 1976), have also been found to be effective in degrading PVA. Another potential route to degrade PVA, the controlled chemical oxidation of PVA (Fukunaga et al., 1976) was carried out to yield poly(enol-ketone) (PEK), which is more susceptible to hydrolysis and biodegradation than PVA (Huang et al., 1982, 1983). By using dyes as models it was found that blends of PEK and PCL blends are excellent materials for controlled release. Here, the water-soluble PEK acts as an excipient, whereas the hydrophobic and water-insoluble PCL acts as a barrier, keeping the device dimensions intact during the release period. Poly(vinyl alcohol) is completely degraded by a bacterial strain, pseudomonas

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O-3, as a source of carbon and energy. However, PVA degrading microorganisms are not ubiquitous within the environment and almost all the degrading strains belong to the genus pseudomonas. Poly(vinyl acetate) (PVAC) undergoes biodegradation slower when compared to that of PVA (Potts et al., 1984; Rosato, 1968). Because PVA is obtained from the hydrolysis of PVAC, a controlled hydrolysis of PVAC followed by controlled oxidation should be a better technique to handle this polymer. The effect of lipases on the side chain hydrolysis of poly (vinyl acetate) (PVAC) was investigated in toluene by various lipases (Chattopadhyay et al., 2003).

15.2.4 Polyacrylates Poly(alkyl acrylate)s and polycyanoacrylates are generally resistant to biodegradation. (Potts, 1984). Poly(methyl-2-cyanoacrylate) is the most degradable among the alkyl esters and the degradability of the polymer decreases as the size of the alkyl group increases. Poly(2-hydroxyethyl methacrylate), a widely used biomedical polymer, hydrolyzes slowly (Chandra and Rustgi, 1998).

15.2.5 Polyethers The main polyethers include polyethylene glycols (PEGs), polypropylene glycols (PPGs) and polytetramethylene glycol (PTMGs). They are used in pharmaceuticals, cosmetics, lubricants, inks, and surfactants. Poly(ethylene glycol) (PEG) is a water-soluble, waxy solid that is used extensively in the cosmetic and toiletry industry. It has been widely used to form various block copolymers with suitable hydrophilic characters for many biomedical and biotechnological applications, such as poly(ethylene glycol)polyester block copolymers (Li et al., 2002; Yuan et al., 2000; Bogdanov et al., 1998). The applicability of the polymer arises due to its hydrophilicity, high solubility in water and organic solvents, lack of toxicity (Herold et al., 1989), and absence of antigenicity and immunogenicity (Richter and Akerblom, 1983). The degradation of these polymers has been investigated in both oxic and anoxic environment (Kawai, 1987, 2002; Kawai and Moriya, 1991; Kawai, 2001; Kawai and Yamanaka, 1986; Dwyer and Tiedje, 1983; Frings et al., 1992; Schink and Stieb, 1983). The central pathway of PEG degradation is cleavage of an aliphatic ether linkage. PEG is completely mineralized in the presence of aerobic Flavobacterium sp. and Pseudomonas sp., though each bacterium alone cannot degrade PEG. The degradation proceeds through dehydrogenation to form an aldehyde and a further dehydrogenation to a carboxylic acid derivative (Kawai, 1987). Three enzymes are effectively involved in the degradation of PEG. They

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are PEG dehydrogenase, PEG-aldehyde dehydrogenase, and PEG-carboxylate dehydrogenase. Though all these enzymes are present in Flavobacterium sp., this bacterium alone cannot be used to degrade PEG. This indicates that though Pseudomonas sp. is not directly involved in the degradation, it utilizes the toxic metabolite that inhibits the activity of the Flavobacterium sp. PEG can also be degraded under anaerobic conditions (Dwyer and Tiedje, 1983) in the presence of Pelobacter venetianus (Schink and Stieb, 1983). Mono-, di-, tri-, and tetraethylene glycols and polyethylene glycols (PEG) were degraded by soil microorganisms using a strain of Pseudomonas aeruginosa (Haines and Alexander, 1975). The enzymatic degradation of copolymers of polycaprolactone (PCL) with monohydroxyl or dihydroxyl poly(ethylene glycol) were investigated in phosphate buffer solution with Pseudomonas lipase. Weight loss data showed that the introduction of PEG segments to the PCL main chain did not alter the enzymatic degradation of PCL significantly (He et al., 2003).

15.2.6 Polyamides Though the same amide linkage is found in polyamides and in polypeptides, the degradation of polyamides is extremely slow. This is primarily because of the high crystallinity of the polyamides. Though high molecular weight polymers are practically non-biodegradable, the low molecular weight oligomers can be degraded by enzymes and microbes (Negoro et al., 1983; Kinoshita et al., 1975, 1977, 1981). The introduction of substituents such as benzyl, hydroxy and methyl greatly improve biodegradation. Copolymers with both amide and ester groups are generally found to be readily degraded (Huang et al., 1978; Tikiwa et al., 1979; Katakai and Goodman, 1982). Naturally occurring homopolyamides are linear poly(amino acids) consisting either of glutamic acid (poly( -glutamic acid), -PGA), lysine (poly(-lysine)) or arginylaspartic acid (poly( arginylaspartic acid), cyanophycin) and the degradation of these polymers has been studied (Oppermanna et al., 1998). Poly(amide-enamine)s are synthesized because they degrade by surface erosion, unlike the degradation of other polymers like PGA which degrade homogeneously. These polymers are susceptible to hydrolysis and biodegradation by fungi and enzymes (Huang et al., 1981).

15.2.7 Polyurethanes and polyureas Polyurethanes can be considered to have both the structural characteristics of polyesters and polyamides, whereas polyureas are similar to that of poly(diamide)s. Their susceptibility to biodegradation can be expected to be similar to that of polyesters and polyamides. Polyurethanes derived from aliphatic diisocyanates are degraded faster than those derived from aromatic

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diisocyanates (Haung et al., 1981). The biodegradability of polyurethanes depends on whether the prepolymer is a polyester or a polyether (Carby and Kaplan, 1968). The polyether-based polyurethanes are resistant to biodegradation whereas the polyester polyurethanes are readily attacked. Many microorganisms (Aspergillus niger, Aspergillus funeigatus, Fusarium solanii, Cryplococcus lacirentii, etc.) and enzymes (papain, subtilisin, etc.) are effective in degrading polyurethanes. Two proteolytic enzymes, papain and urease, degraded medical polyester polyurethanes (Phua et al., 1987) while bacteria, Corynebacterium sp., Pseudomonas aeruginosa, could degrade polyurethanes in the presence of basal media (Howard, 2002). Several fungi can grow on the surfaces of polyurethanes and Curvularia senegalensis degrades polyurethane.

15.2.8 Polyanhydrides Polyanhydrides are fiber-forming polymers that are suspectible to hydrolysis (Leong et al., 1984) and are used in drug delivery as slow release formulations (Langer et al., 1986). An increase in the aliphatic chain length between the acid groups also notably improves their hydrolytic stability (Albertsson and Lundmark, 1990). While aliphatic polyanhyrides degrade rapidly, aromatic polyanhydrides are almost non-biodegradable.

15.3 Hydrolyzable polymers Polymers with hydrolyzable backbones have been found to be susceptible to biodegradation. Aliphatic polyesters, and especially poly(hydroxy acids) derived from lactic and glycolic acids, constitute one of the most widely used classes of degradable biomaterials due to their excellent biocompatibility and variable degradability. Almost the only high molecular weight compounds shown to be biodegradable are the aliphatic polyesters. The reason for this is the extremely hydrolyzable backbone found in these polyesters. In the past two decades, aliphatic polyesters such as poly(-caprolactone) (PCL), poly((R)-3hydroxybutyrate) (R-PHB), and poly(L-lactide), i.e. poly(L-lactic acid) (PLLA) have been intensively studied because of their hydrolyzability in the human body as well as in natural circumstances (Doi and Fukuda, 1994; Scott and Gilead, 1995; Hollinger, 1995; Hartmann, 1998; SteinbuÈchel and Doi, 2002; Tsuji and Ikada, 1999). Among these aliphatic polyesters, R-PHB and PLLA are producable from renewable resources using various methods. The environmental degradation of PCL (Doi et al., 1996; Kasuya et al., 1998; Tsuji et al., 1998), R-PHB (Doi et al., 1996, Kasuya et al., 1998; Doi et al., 1992), and PLLA (Doi et al., 1996; Gallet et al., 2001) has been studied in seawater (Kasuya et al., 1998; Doi et al., 1992), river water (Doi et al., 1996; Kasuya et al., 1998;), lake water (Kasuya et al., 1998;), and soil (Tsuji et al., 1998; Torres et al., 1996; Gallet et al., 2001). It was found that the

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biodegradability in the natural water media was higher for PCL than for R-PHB. The change in weight loss, tensile strength, and Young's modulus revealed that the biodegradabilities of the aliphatic polyesters in the controlled seawater decreased in the order: PCL>R-PHB>PLLA. The results of gravimetry, GPC, and DSC showed that the biodegradation of PCL and R-PHB films proceeds via surface erosion mechanisms. Polyesters derived from diacids of medium sized monomers are more readily degraded by fungi (Aspergillus niger and Aspergillus flavus), than those derived from longer or shorter monomers (Bitritto et al., 1979). In order for a synthetic polymer to be biodegradable by enzyme catalysts, the polymer chain must be able to fit into the enzyme's active site. Thus flexible aliphatic polyesters are degradable while rigid aromatic polyesters are usually non-biodegradable (Potts et al., 1973; Tokiwa and Suzuki, 1981).

15.3.1 Poly(glycolic acid) Poly(glycolic acid) and its copolymers with polylactic acid are used as degradable and absorbable sutures (Fiazza and Schmitt, 1971). They can be degraded by simple hydrolysis of the ester backbone in aqueous environments like body fluids. Homo- and copolymers of poly(lactide)s (PLA) and poly(glycolide)s (PGA) was studied in detail (Hakkarainen et al., 1996). The relative importance of the rate of hydrolysis reaction and the rate of water diffusion in the degradation of polyglycolide (PGA) has been investigated and a four-stage reaction±erosion front model of degradation was proposed (Hurrell et al., 2003).

15.3.2 Polylactic acid Polylactic acid (PLA) is a degradable thermoplastic polymer with excellent mechanical properties. Corn is fermented to lactic acid, which is subsequently polymerized. Higher molecular weight PLA is synthesized by ring opening polymerization of cyclic lactides. Because of the chirality of the lactyl unit, lactide exists in three diastereoisomeric forms, i.e., L-lactide, D-lactide and meso-lactide. Most studies of the degradation of PLA have focused on abiotic hydrolysis due to the long standing use of PLA as biomedical implants (Hakkarainen, 2002). The degradation occurs in two distinct stages, with diffusion of water into the material followed by hydrolysis of ester bonds. The effect of temperature and humidity on the rate of hydrolysis has been investigated (Hakkarainen, 2002; Ho et al., 1999). While PLA degraded in a mixture of microbes (Hakkarainen et al., 2000), the film did not undergo degradation in the abiotic medium. However, degradation is thought to occur by abiotic hydrolysis followed by biotic breakdown of the products (Hakkarainen, 2002; Agarwal et al., 1998). The mineralization of PLA in compost has been reported (Lunt et al., 1998; Itavaara et al., 2002).

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Several groups have investigated the enzymatic degradation of PLA polymers. While it has been long known that a specific enzyme, proteinase K, accelerates PLA degradation (Fukuzaki et al., 1989), it was observed that the enzyme (Reeve et al., 1994) preferentially degraded L-lactyl units as opposed to D-lactyl units with PLA0 being reported as nearly undegradable. The maximum degradation rate was found for PLA92 and was attributed to a disruption of crystallinity. The degradation rate of PLA96 decreased with increase in crystallinity (Cai et al., 1996) and this result was confirmed by studying the degradation of PLA50 and PLA100 (Li and McCarthy, 1999). The enzymatic degradation of three stereocopolymers was investigated in a buffer solution with proteinase K. The degradation of PLA50-mes was found to be much faster than that of PLA50-rac, PLA62.5 degrading at an intermediate rate (Li et al., 2000).

15.3.3 Polyhydroxybutyrate Natural polyesters have recently been receiving increased attention for applications as biodegradable polymers. The polymers, with structural formula of (OCRH±CH2CO)n, can be rigid or flexible depending on the composition of the polymer and the pendant group, Rˆ±(CH2)x±CH3. The most significant polymer among this class of polymers, is polyhydroxybutyrate (PHB) where xˆ0. The polymer is highly crystalline and very brittle and, therefore, it is copolymerized with other polymers with an x of 3±5. The mechanical properties of PHB are poor and are usually improved by blending or copolymerization. The main advantage of P(3HB) and its copolymers over common synthetic polyesters is their biodegradability by various extracellular depolymerases secreted by microorganisms. The biodegradation of PHB and its copolymers has been studied in soil, activated sludge and seawater (Kunioka et al., 1989a; Doi, 1990). High molecular weight PHB can be completely degraded to carbon dioxide, water and biomass in river water (Kusaka et al., 1999). Copolymers of 3HB and 4HB (9%) have been degraded in soil while the copolymer is completely decomposed in activated sludge (Kunioka et al., 1989b). The degradation of copolymers of PHB with hydroxy valerate (PHV) has also been investigated. Blends of PHB with polystyrene and polyethylene have also been investigated (Bhalakia et al., 1990; Dave et al., 1990). Poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate) (P(3HB-co-3HV)) has been produced commercially under the trade name of Biopol. Due to isodimorphism (Fischera et al., 2004), the mechanical properties of this copolymer are similar to that of PHB. Therefore, copolymers of 3HB and 3hydroxyhexanoate (3HH) (Doi et al., 1995) have been produced which show high elongation at break, but low tensile strength. To increase the strength, colddrawing and copolymerization has been proposed (Fischera et al., 2004) and the degradation has been examined (Cao et al., 1999).

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The hydrolytic degradation of microbial polyesters occurs in two stages, random chain scission wherein the molecular weight reduces followed by a second step where weight loss occurs (Doi et al., 1989). The degradation mechanism is a combination of both endo- and exo-cleavage (Hocking et al., 1996), though the predominant mechanism is exo-cleavage (Iwata et al., 1997a). The degradation of high molecular weight polyester to water soluble monomer and oligomers by extracellular PHB depolymerases like from Ralstonia pickettii T1 has been extensively investigated (Zhang et al., 1992) and kinetic models are available (Iwata et al., 1997b; Kasuya et al., 1995). Similar to other polyesters, hydrolysis occurs preferentially in the amorphous region of the polymer and thus the degradation rates of the polymer decrease with increasing crystallinity of the polymer (Kumagai et al., 1992; Cao et al., 1999).

15.3.4 Polycaprolactone The aliphatic polyester currently most important for commercial biodegradable plastics is poly(-caprolactone) (PCL). Poly(-caprolactone) (PCL) has been thoroughly studied as a substrate for biodegradation (Potts, 1984; Fields et al., 1974) and as a matrix in controlled-release systems for drugs (Pitt et al., 1979; 1980). The degradation of PCL by fungi has been investigated (Tokiwa and Suzuki, 1977). It has been proven, by using SEM/XRD, that the degradation of partially crystalline polycaprolactone film by fungi proceeds with the degradation of amorphous regions followed by the degradation of the crystalline region. The rate is influenced by the size, shape and number of the crystallites (Benedict et al., 1983a). Poly(-caprolactone) (PCL) is an aliphatic polyester that is degradable in several biotic environments, including river and lake waters, sewage sludge, farm soil, paddy soil, creek sediment, roadside sediment, pond sediment and compost (Benedict et al., 1983a; Nishida and Tokiwa, 1994; Pettigrew et al., 1995; Albertsson et al., 1998). While molecular weight and crystallinity plays an important role in the biodegradability of PCL (Benedict et al., 1983), temperature plays a key role in the degradation in anaerobic sludge (Albertsson et al., 1998). Contrary to the degradation of PHB, the molar mass decreases during biodegradation and low molar mass peaks appear in the molecular weight distribution (Tilstra and Johnsonbaugh, 1993; Benedict et al., 1983b). While studying the abiotic and biotic degradation of PCL (Albertsson et al., 1998), parallel grooves appeared in the polymer surface when degraded in a biotic environment. The degradation of PCL in various solvents and enzymes has been recently investigated (Sivalingam et al., 2003b). It was found both that the enzymes and the viscosity of the solvent play an important role in determining the degradation rate of the polymer. The biodegradability of PCL in the form of blend sheets is much reduced because the packed form of PCL protects it from enzymatic digestion (Hirotsu et al., 2000).

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15.3.5 Aromatic-aliphatic polyesters While aromatic polyesters like poly(ethylene terephthalate) (PET) or poly(butylene terephthalate) (PBT) provide excellent material properties and are commercially widely used, they are non-biodegradable. PCL, an aliphatic polyester, however, provides excellent biodegradable properties but are mostly excluded from commercial applications because of poor mechanical properties and low melting temperature. Pure aromatic polyesters like PET or PBT are quite insensitive to any hydrolytic degradation. The rate of hydrolysis based on accelerated degradation experiments (Edge et al., 1991; Allen et al., 1994) predicted the lifetime of PET to range from 16 to 48 years. No significant direct microbial or enzymatic attack of pure aromatic polyesters (e.g. PET, PBT or poly(ethylene naphthalate)) has been observed (Levefre et al., 1999). However, microbial degradation of polyesters from terephthalate, phthalate or isophthalate polycondensed with poly(ethylene glycol) (PEG) (Kawai, 1996). Attempts have been made to enhance the hydrolysis of aromatic polyesters by introducing aliphatic components into the aromatic polyester chains (Kint and Munoz-Guerra, 1999). Aliphatic-aromatic copolyesters combine the material properties of aromatic polyesters like PET or PBT with the biodegradability of aliphatic polyesters. Earlier studies (Tokiwa and Suzuki, 1977, 1981) indicated that these polymers degrade significantly with only low fractions of aromatic polyesters. However, later work by Witt et al. (1994, 1995) showed significant biological attack of a block-copolyester poly(trimethylene decanedoate)-block(trimethylene terephthalate) with 50 mol% terephthalic acid and copolyesters of PET, poly(propylene terephthalate) (PPT) and PBT with adipic acid and sebacic acid. These studies indicated a significant decrease of the average molar masses and a significant chemical hydrolysis. Recent work has examined the degradation of PBT with adipic acid as the aliphatic component in compost (Witt et al., 1997), degradation of PET with caprolactone (Jun et al., 1994a,b), adipic acid, sebacic acid and ethylene glycol (Nagata et al., 1997) using lipase in a phosphate buffer.

15.4 Natural biodegradable polymers 15.4.1 Polysaccharides For materials applications, the principal polysaccharides of interest are cellulose and starch, but increasing attention is being given to the more complex carbohydrate polymers produced by bacteria and fungi. Both cellulose and starch are composed of hundreds or thousands of D-glucopyranoside repeating units. However, the glucopyranoside ring is present in the -form and -form in starch and cellulose, respectively. Because of this difference, enzymes that catalyze acetal hydrolysis reactions during the biodegradation of each of these two polysaccharides are different and are not interchangeable. Polymers like cellulose, chitin, pullusan, and PHB are all biologically synthesized and can be

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completely and rapidly biodegraded by heterotrophic microorganisms in a wide range of natural environments (Berenger et al., 1985; Byrom, 1991; Chahal et al., 1992; Frazer, 1994; Gamerith et al., 1992; MacDonald et al., 1985; Yoshizako et al., 1992). Lignocellulose, the major component of biomass, makes up about half of the matter produced by photosynthesis. It consists of three types of polymers ± cellulose, hemicellulose, and lignin ± that are strongly intermeshed and chemically bonded by non-covalent forces and by covalent cross-linkages. Cellulose and hemicellulose are macromolecules from different sugars, whereas lignin is an aromatic polymer synthesized from phenylpropanoid precursors. Fungi are the best-known microorganisms capable of degrading these three polymers. Because the substrates are insoluble, both bacterial and fungal degradation have to occur exocellularly, either in association with the outer cell envelope layer or extracellularly.

15.4.2 Cellulose Cellulose makes up about 45% of the dry weight of wood. This linear polymer is composed of D-glucose subunits linked by -1,4 glycosidic bonds forming cellobiose molecules. Cellulose can appear in crystalline forms as well as in amorphous forms, with the latter being more susceptible to enzymatic degradation (BeÂguin and Aubert, 1994). Microorganisms capable of degrading cellulose produce a battery of enzymes with different specificities. Cellulases hydrolyze the -1,4-glycosidic linkages of cellulose. Traditionally, they are divided into two classes referred to as endoglucanases and cellobiohydrolases. Endoglucanases (endo-1,4±glucanases, EGs) can hydrolyze internal bonds (preferably in cellulose amorphous regions) releasing new terminal ends. Cellobiohydrolases (exo-1,4±glucanases, CBHs) act on the existing or endoglucanase-generated chain ends. Both enzymes can degrade amorphous cellulose but, with some exceptions, CBHs are the only enzymes that efficiently degrade crystalline cellulose. CBHs act synergistically with EGs to solubilize high-molecular-weight cellulose molecules. They are also glycosylated and present an optimum activity at acidic pH. P. chrysosporium has several glucosidases, whereas only one isoenzyme has been described in T. reesei. Cellulose esters represent a class of polymers that have the potential to participate in the carbon cycle via microbiologically catalyzed de-esterification and decomposition of the resulting cellulose and organic acids. Cellulose acetate is currently used in high-volume applications ranging from fibers, to films, to injection molding thermoplastics. The biodegradation of cellulose ethers has been studied extensively (Reese, 1957; Cantor and Mechalas, 1969; Gu et al., 1992; Buchanan et al., 1993a,b) and it is known that cellulose ethers with a DS of less than 1 will degrade due to attack of microorganisms at the unsubstituted residues of the polymers.

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15.4.3 Chitin Chitin is a macromolecule found in the shells of crabs, lobsters, shrimps and insects and can be degraded by chitinase (Muzzarelli, 1986). Chitin has been widely used for making artificial skin, absorbable sutures (Chandy and Sharma, 1990; Tashibana et al., 1988) cosmetics, wound treatment (Muzzarelli, 1990) and drug carriers (Tokura et al., 1990). Most of the enzymatic processes for degradation of chitin have resulted only in incomplete conversion of chitin into its monomer (Pichyangkura et al., 2002). The enzymes that have been investigated in details are strains of Serratia marcescens (Watanabe et al., 1997). However, they do not convert chitin to N-acetyl-D-glucosamine but to monomers and oligomers (Aloise et al., 1996). Chitinolytic enzymes from fungi (Trichoderma) have also been investigated (Lorito, 1998), which degrade chitin to N-acetyl-D-glucosamine almost exclusively. Recently, enzymatic degradation of three forms of chitin by Serratia/Trichoderma and Streptomyces/Trichoderma blends was studied (Giuliano et al., 2003).

15.4.4 Starch Starch, for example, is a physical combination of branched and linear polymers (amylopectin and amylose, respectively), but it contains only a single type of carbohydrate, glucose. Amylose is crystalline, constitutes nearly 20% of starch and is soluble in hot water. Amylopectin is insoluble in boiling water, but in their use in foods, both fractions are readily hydrolyzed at the acetal link by enzymes. In its application in biodegradable plastics, starch is either physically mixed in with its native granules, kept intact, or melted and blended on a molecular level with the appropriate polymer. In either form, the fraction of starch in the mixture that is accessible to enzymes can be degraded by either, or both, amylases and glucosidases. The starch molecule has two important functional groups, the ±OH group that is susceptible to substitution reactions and the C±O±C bond that is susceptible to chain breakage. The hydroxyl group of glucose has a nucleophilic character. By reaction of its ±OH group, modification of various properties can be obtained. One example is the reaction with silane to improve its dispersion in polyethylene (Huang et al., 1990). Crosslinking or bridging of the ±OH groups changes the structure into a network while increasing the viscosity, reducing water retention and increasing its resistance to thermomechanical shear. Starch was added as filler to various resin systems to make films that were impermeable to water but permeable to water vapour. Starch as a biodegradable filler in LDPE was reported (Griffin, 1973; Griffin and Turner, 1978) and disposable polyvinyl chloride (Westhoff et al., 1974). There have been contrasting reports on the mechanical properties on starch/PLA composites (Kim et al., 1998; Willett, 1998). However, a recent report confirms that the

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mechanical and thermal properties of the composite are enhanced with improvement in adhesion and lower pressures needed for injection molding (Garlotta et al., 2003).

15.4.5 Hemicellulose Hemicellulose is a complex carbohydrate polymer and makes up 25±30% of total wood dry weight. It is a polysaccharide with a lower molecular weight than cellulose. Hemicelluloses are biodegraded to monomeric sugars and acetic acid. Hemicelluloses are frequently classified according to their action on distinct substrates. Xylan is the main carbohydrate found in hemicellulose. Its complete degradation requires the cooperative action of a variety of hydrolytic enzymes. An important distinction should be made between endo-1,4±xylanase and xylan 1,4±xylosidase. The former generates oligosaccharides from the cleavage of xylan; the latter works on xylan oligosaccharides, producing xylose (Jeffries, 1994). In addition, hemicellulose biodegradation needs accessory enzymes such as xylan esterases, ferulic and p-coumaric esterases, -l-arabinofuranosidases, and -4-O-methyl glucuronosidases acting synergistically to efficiently hydrolyze wood xylans (Kirk and Cullen, 1998).

15.4.6 Lignin Lignin (along with cellulose) is the most abundant polymer in nature. It is present in the cellular cell wall, conferring structural support, impermeability, and resistance against microbial attack and oxidative stress. Structurally, lignin is an amorphous heteropolymer, non-water soluble and optically inactive; it consists of phenylpropane units joined together by different types of linkages. The structural complexity of lignin, its high molecular weight and its insolubility make its degradation very difficult. Extracellular, oxidative, and unspecific enzymes that can liberate highly unstable products, which further undergo many different oxidative reactions, catalyze the initial steps of lignin depolymerization. White-rot fungi are the microorganisms that most efficiently degrade lignin from wood. Of these, Phanerochaete chrysosporium is the most extensively used. For recent reviews on lignin biodegradation by white-rot fungi and advances in the molecular genetic of ligninolytic fungi, see Cullen (1997). Two major families of enzymes are involved in ligninolysis by white-rot fungi: peroxidases and laccases. Two groups of peroxidases, lignin peroxidases (LiPs) and manganese-dependent peroxidases (MnPs), have been well characterized. LiP has been isolated from several white-rot fungi. So far, it is the most effective peroxidase and can oxidize phenolic and non-phenolic compounds, amines, aromatic ethers, and polycyclic aromatics with appropriate ionization potential (Kirk and Cullen, 1998). Degradation of lignin and lignindegrading enzymes has also been reported for actinobacteria from the

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Streptomyces genus (Berrocal et al., 1997). Even though lignin biodegradation is accepted as an aerobic process, some authors have reported that anaerobic microorganisms in the rumen may alter, if not partially degrade, portions of lignified plant cells (Akin, 1980).

15.4.7 Polypeptides A polymer composed of multiple amino acid units linked by peptide bonds is known as a polypeptide. Fibrous proteins like wool, silk and collagen are not soluble and used in their natural form. Gelatin is a water-soluble, biodegradable polymer with extensive pharmaceutical and biomedical applications with uses in coating drugs (Tabata et al., 1993; Tabata and Ikada, 1993) and for preparing biodegradable hydrogels (Ziegler, 1991; Shinde et al., 1992). Gelatin, an animal protein, consists of 19 amino acids joined by peptide linkages and can be hydrolyzed by a variety of the proteolytic enzymes to yield its constituent amino acids or peptide components (Eastoe and Leach, 1977). Kumar et al. (1981) studied three copolymers of gelatin-grafted-poly(ethyl acrylate) with different grafting efficiencies and tested for their microbial susceptibility in a synthetic medium employing a mixed inoculum of Bacillus subtilis, Pseudomonas aeruginosa, and Serratia marcescens. The weight losses were found to be around 5±10% after six weeks of incubation, depending on the efficiency of grafting.

15.5 Conclusion The current worldwide consumption of biodegradable polymers has increased nearly eight times from the production of 14 million kg in 1996. The development of biodegradable polymers has to be viewed in the context of developing cleaner chemical processes and avoiding disturbance to the ecosystem. Though certain biodegradable polymers can be used in long-term applications, the commercialization of these polymers will continue to increase in markets for products that have a short lifetime. Biodegradable polymers are beneficial only when they can actually biodegrade in the environment. Many current technologies allow for the disposal of water-soluble biodegradable polymers but not for polymers that are insoluble in water. As described above, several methods exist for converting biodegradable polymers to valuable compost, chemical intermediates, and energy through aerobic and anaerobic processes. However, only limited knowledge about the detailed mechanisms of the enzymatic attack are available. For the design of new and improved materials and the evaluation of the degradation behavior under other environmental conditions, research needs to elucidate the degradation mechanism of biodegradable polymers and develop the genetic engineering of pathways yielding microbes that can convert feedstocks to building polymers for

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biodegradable polymers. Intensive research is required before biodegradable polymers can compete economically with the current synthetic polymers.

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Part IV

Industrial applications

16

Oxo-biodegradable polyolefins in packaging D M W I L E S , Plastichem Consulting, Canada

16.1 Introduction It is commonly suggested that about a third of all plastics production goes into packaging applications. More than a billion pounds of plastics are produced annually in the USA alone; the volume of plastics exceeds the volume of steel. This means that really huge amounts of plastics packaging are used each year worldwide. The following estimates have been made, however, concerning increases in waste if plastic packaging were eliminated and replaced with more traditional materials: the volume of packaging waste would increase by more than 250%, the tonnage would increase by more than 400%, the energy consumption would increase by 200%, and packaging costs would increase by more than 200%. In a direct comparison with unbleached kraft paper and paper combinations, it has been shown1 that the production of polyethylene carrier bags consumes much less energy, and causes much less air pollution and very much less water pollution (see Table 16.1). It would be logical to conclude that the ubiquitous use of plastics in general, and for packaging purposes in particular, is regarded universally as an unqualified success story. But this is not the case. Objections to the widespread use of plastic shopping bags have arisen recently, with the criticism of them focused on what may be called environmental grounds. Much attention has been and is being focused on plastic shopping or carrier bags. Their ubiquity and the numbers that one finds are certainly large. It is estimated, for example, that 10 to 20 billion bags are used and disposed of in the UK each year. Statistics are not readily available about how many plastic shopping bags are used in Canada but the city of Ottawa, for instance, collects more than 540 tonnes of them annually. There are other sources of plastic litter, however, including discards from agricultural uses such as silage wrap for the outdoor storage of hay.2 Used plastic shopping bags and the disposal of them have become issues of major concern to regulators and legislators in several parts of the world. Statements have been made to the effect that plastics comprise 25% of

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Table 16.1 Air and water pollution associated with the production of 50,000 carrier bagsa Environmental burden

Polyethylene

Unbleached kraft paper

Paper combinationsb

29

67

69

Air pollution (kg) SO2 NOX CH3 CO Dust

9.9 6.8 3.8 1.0 0.5

19.4 10.2 1.2 3.0 3.2

28.1 10.8 1.5 6.4 3.8

Waste water burden (kg) COD BOD

0.5 0.02

16.4 9.2

107.8 43.1

Energy (GJ) for production process

a

Data produced by the West German Federal Office of the Environment, 1988. The formulation used for most paper carrier bags approved for use in Germany. Reprinted from ref. 1 with kind permission of Kluwer Academic Publishers and Professor James Guillet.

b

municipal solid waste (MSW) and are a major component of litter. Actual measurements indicate that all plastics are about 7% by weight of MSW.3 Statements have been made that millions of plastic bags blow around the streets although evidence suggests that plastic bags account for less than 1% of all litter. These statements and the attitudes behind them, however illusory, have resulted in a tax on shopping bags in Ireland and proposals to ban the use of them or to tax them in Taiwan, South Africa, the UK and California. Use, re-use, recycling and appropriate disposal methods for plastic packaging will be discussed later in this chapter. Taxation should be considered to be yet another insidious way of extracting more money from the consumer rather than an intelligent way to address authentic environmental issues.

16.1.1 The use of polyolefin plastics in packaging applications is commonplace because these materials are inexpensive, easy to fabricate, have good barrier properties, are hydrophobic, and are available with a wide range of physical and mechanical properties. The various polyethylenes, polypropylene, blends and copolymers of them plus polystyrene collectively account for more production than all the other packaging plastics combined because of this versatility and on the basis of cost/benefit analysis. Carrier bags made of HDPE, trash bags made of LDPE, clear films of LDPE for food wrapping, and clear clothing bags made

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of oriented polypropylene are a few examples of many that could be cited as examples of packaging versatility. In common with other commodity plastics, the polyolefins require less labour input and lower energies during production and fabrication than is the case with more traditional packaging materials.1,2 Furthermore, although there is a chicken-and-egg element to it, efficient equipment for the extrusion and injection molding of polyolefins is available all over the world. Recycling of factory `waste' is commonplace, and postconsumer waste is recyclable as well. Like all thermoplastics, polyolefins and polystyrene `contain' large amounts of energy (in the form of heat content) that can be usefully recovered as a result of incineration. This is another advantage, compared to metals, glass and ceramics. Finally, polyolefins and polystyrene consist only of the elements carbon and hydrogen, and this simplifies the safe incineration of them.

16.2 Characteristics of packaging plastics Making a list ab initio of the characteristics that packaging materials should have is quite a challenge, and the list is rather long. Even with the limitation that it be nominally limited-use packaging, the list includes the following properties: inexpensive, light, strong, tough, flexible, impact resistant, easy to fabricate, inert, at least somewhat re-usable, recyclable, high wet-strength, safe during use and after disposal. More specific characteristics arise, of course, from specific uses such as safe for food contact, or durable outdoors for up to a year or more. Plastic packaging may be required to be transparent in order to display the goods contained in it or opaque, for instance, in the case of garbage bags; nobody wants to see discarded trash. In many types of packaging, the contents must be protected from mechanical damage and the use of plastics has significantly reduced waste through breakage. The widespread use of plastic food containers and wrappings is due in large part to the good barrier properties against permeation by water and water-borne microbes. Food spoilage and therefore illness have been greatly reduced as a result of the use of plastics food packaging. It can be argued4 that commercial processing of many foods is feasible and widely practised in developed countries owing to the common usage of plastic packaging. Indeed, this has resulted in less trash to be accommodated in landfills as well as higher standards for hygiene and food safety. It is safe to say that plastics offer a broader spectrum of properties for a larger number of packaging requirements than all other materials combined. What necessary characteristics are not found in individual commodity plastics can usually be obtained with blends or other combinations of them. Well, perhaps not quite all the characteristics are there. Single- or limited-use plastics have lifetimes that exceed their use-lives by a considerable margin. They can be considered to be excessively, unnecessarily durable. For the most part, packaging plastics are required to maintain their useful properties

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for a limited time, and they are then discarded. Frequently, used plastic films, bags and containers are collected and dumped in a landfill, where they persist for too long. Regrettably, some of the time these used plastics end up as litter. Some land-based litter is blown into the waterways and oceans where litter accumulation is a serious problem.

16.2.1 Adding controlled lifetimes It seemed, more than ten years ago, that it was time to add one more characteristic to the list of desirable properties for plastics that usually have a service life of 18 months or less. The concept is that of controlled-lifetime plastics that have a designed shelf-life/use life combination but that degrade chemically and mechanically far more rapidly than normal in whatever disposal environment they are placed, intentionally or inadvertently. When implemented correctly, this concept can alleviate the problems of the accumulation of discarded plastics in landfills as well as plastics litter accumulation. In the case of polyolefin packaging plastics, the concept is manifested in what have come to be called oxo-biodegradable polyolefins. As defined, described and discussed in Chapter 3, oxo-biodegradable polyolefins consist of conventional resins to which have been added small amounts of pre-compatabilized prodegradant. After the depletion of any stabilizing additives in the plastic, the prodegradant catalyzes the normal processes of oxidation chemistry so that molar mass reduction and material fragmentation occur orders of magnitude faster than would otherwise happen. It must be noted here, and emphasized, that this is not simply a matter of leaving out stabilizing additives when compounding the resin. To rely on this ploy would mean a reduced but uncontrolled lifetime and a persistence after discarding that would still be excessive. The value of oxobiodegradable polyolefins derives from the ability to control their lifetimes, and to control the much-reduced degradation times for them in a variety of configurations for numerous applications in a number of different climates.

16.3 Oxo-biodegradable polyolefins The materials of interest here are LDPE, LLDPE, HDPE, PP and to a certain extent PS. This is a list of hydrocarbon polymers whose normal processes of deterioration are radical chain reactions involving the formation and reaction of hydroperoxide groups, as rate-determining steps. Although a large number of individual reactions can be and usually are involved, the Arrhenius relationship can be applied to the overall process. Molar mass reduction and concomitant polar-group formation occur as a consequence of oxidative degradation. Mechanical consequences include the loss of tensile properties, embrittlement and fragmentation. The use of pro-degradant additives speeds up these processes dramatically.

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It is well known that as-produced, commercial polyolefins are bioinert. It was demonstrated unequivocally during the 1990s, however, that the oxidation products of polyolefins are biodegradable (see Ch. 3, section 3.5). The characteristics of specific products made from oxo-biodegradable polyolefins are the focus of the remainder of this chapter.

16.3.1 Performance It is important that oxo-biodegradability can be introduced to packaging materials in such a way that the performance of the resin in fabrication and the function of the products in service be maintained. The oxo-biodegradable resins compounded using the TDPAÕ (Totally Degradable Plastic Additives) developed by EPI Environmental Products Inc. are handled in the same way on the same equipment as the conventional resins of which they largely consist.5 The fabrication of carrier bags from TDPA-PE resin by blown film extrusion on standard film extrusion lines produces a product with properties that are indistinguishable from those of ordinary carrier bags. There is no need to modify the equipment or the way in which it is operated; there is no reduction in output or in output quality. The carrier bags can be `programmed' to have the same storage life/use life combination as is common for conventional bags, although this controlled lifetime can be changed to suit specific markets. The difference between ordinary carrier bags and those made using TDPA-PE becomes evident only after use and disposal. The oxo-biodegradable product undergoes normal oxidative degradation orders of magnitude faster than the ordinary product, even if it is made from the same resin. As has been explained in Chapter 3, the TDPA pro-degradant catalyzes the oxidation which follows initiation in the disposal medium by heat, or the near-UV component of terrestrial sunlight, or mechanical stress, or usually some combination of these factors. Details of the oxidation chemistry of polyolefins are found elsewhere6±11 and have been summarized in chapter 3. Although the degradation and disintegration of plastic films and other products is manifested in decreasing tensile and other mechanical properties (see Ch. 3) it is convenient to use infra-red spectroscopy to monitor the formation and build-up of oxidation products. In spite of the relatively low values for molar extinction coefficients at infra-red frequencies (compared to those in the UV, for example), the specificity of structure/frequency relations observed in infra-red spectroscopy make this the technique of choice. Carbonylcontaining oxidation products accumulate in polyolefins that are undergoing oxidation to produce a characteristic pattern of overlapping absorption bands, including those from ketones, aldehydes, esters, acids and lactones. Figure 16.1 shows typical results,12 obtained using FTIR spectroscopy, for polyethylene film samples heated in a laboratory oven for the same number of days. The lower trace shows negligible absorption in the unsaturation region, typical for a conventional film that has not undergone significant oxidation. The

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16.1 FTIR spectra of thin polyethylene films after heating in a laboratory oven: upper trace ± film containing TDPA; lower trace ± film contains no additive.

upper trace shows that considerable oxidation has occurred in the film containing EPI's TDPA pro-degradant. The oxidation processes of polyolefins initiated by heat or by near-UV light are not identical primarily because the ketone group is photo-labile but stable to heat, but they are very similar. The build-up of photooxidation products in polyethylene films containing a prodegradant invented by Professor Gerald Scott (henceforth referred to as Scott/ Gilead technology) shows13 a very similar pattern to the products of thermal oxidation in Fig. 16.1. Commercial polyolefins have a distribution of molar mass values, and the molecular sizes of their oxidation products also cover a wide range. This means that biodegradation of the smaller oxidation product molecules will begin soon after oxidation is under way, since susceptibility in microbially active environments has been observed13 at molar mass values as high as 40,000. It seems likely that biodegradation will proceed more rapidly for smaller molecules, and it certainly is essential that hydrophobic polyolefins be converted to waterwettable materials as a result of oxidation in order for microbial assimilation to occur. Invariably, oxidative degradation is the rate-determining part of the twostage oxo-biodegradation process.

16.3.2 Cost The cost of EPI's TDPA-based oxo-biodegradable polyolefins is only incrementally higher than that of the ordinary polyolefins that they replace.

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This is because most of the material is a conventional polyolefin resin and because processing is done on conventional equipment at the usual speeds. The amount of active pro-degradant required depends on the application and the disposal environment, but it is invariably small. The most rapid oxidation is required for materials that will be used in commercial composting but this is less important in materials that will be disposed of in landfills. Heat is the most prominent initiator of oxidation in landfill disposal and composting. Sunlightinduced initiation is obviously significant in agricultural applications and in the alleviation of litter accumulation. Mechanical stress is likely to be a factor in all of these situations. A major advantage of oxo-biodegradable technology is that the use-life and time-to-degrade periods can be controlled by means of adjustments to the additive formulation but using the same resin. There is no need for a different resin for each application and this represents a significant saving over other technologies. There are circumstances in which oxo-biodegradable carrier bags can be made available for considerably less than one cent more than an ordinary bag having equivalent performance during storage and use. In any case oxobiodegradable polyolefins are up to an order of magnitude less expensive than hydro-biodegradable plastics such as linear polyesters.

16.3.3 Safety There are two elements to safety that must be considered for packaging plastics safety during use, and safety after disposal. Packaging materials such as food wrapping, food and beverage containers (including straws) and carrier bags come in contact with prepared foods, fresh produce and the like purchased in supermarkets, and ready-to-eat foods and beverages are consumed in large quantities from fast-food outlets. Polyolefins are frequently used in most of these applications and it is acknowledged that the plastics themselves are entirely safe. Attention is, and should be, focused on any and all small molecules that are part of the packaging material. It must be demonstrated that no unacceptable contamination of food occurs as a result of additive migration from the plastic. EPI's TDPA formulations have been evaluated and certified according to USA and European standards (see Ch. 3, section 3.6.3). The residues from TDPA-polyethylene compost bags have been evaluated in laboratories in Austria and Belgium using what are called ecotoxicity tests.14 The results of standard plant tolerance tests, seed germination tests, daphnia and earthworm tests all showed no negative results. This is not unexpected since only the elements carbon and hydrogen are present in polyolefin plastics, and the oxidation products from them are assimilated and bioconverted by naturally occurring micro-organisms. The products of oxo-biodegradation from polyethylene agricultural mulch films, based on Scott/Gilead technology, have been shown to be harmless and

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Biodegradable polymers for industrial applications

non-toxic in extensive testing and trials. Fabbri15 showed that the crop-growth and yield benefits of these films are large but the disposal problems that are common with conventional polyethylene are avoided by the use of the S/G films. Equally important was his demonstration that there are no effects from the residue of the pro-degradant, even from continuous use of these films in the same area for many years. He used films containing a nickel-based prodegradant and, in some experiments he added nickel sulfate to the soil, equivalent to 180 years of mulching, before growing and testing vegetable crops. No effects on the constituents of the plants were observed, and this result has been confirmed by others. It is to be expected that the residues from oxobiodegradable plastics containing pro-degradants based on other transition metals would be equally innocuous. Ciba Specialty Chemicals are developing and marketing products for agricultural applications,16 based on EPI's TDPA technology, using the trade name EnvirocareTM. Extensive laboratory testing and field trials have demonstrated the effectiveness of these products. Several international standard tests have been used to demonstrate that both undegraded and degraded Envirocare-based agricultural plastics are non-ecotoxic.

16.4 Disposal There are programs in operation in many countries for the collection and recycling of post-consumer plastics. In Western Canada, for example, virtually every supermarket has receptacles in which shoppers deposit used carrier bags, which are subsequently re-fabricated into a variety of products. Some stores offer a three-cent reduction in the customer's bill for each bag the customer brings in as a substitute for a new bag. In other establishments, customers bring in their own used bags to avoid being charged for new ones. Molded plastic, single-use containers are collected by municipalities for re-use in other products, in what is called, locally, the `blue box' program, the boxes having been fabricated from post-consumer waste plastics. But this is not the end of the trail, just the beginning.

16.4.1 Landfill Following their use and re-use, polyolefin carrier bags are by no means invariably collected for recycling, nor are they always acceptable for recycling. Many are commonly used by householders to collect food and other household waste to become part of the MSW which is hauled to the local landfill. It is the case that much plastic packaging, including but not limited to carrier bags, is finally disposed of in a landfill. These repositories are considered morally indefensible by some and environmental disasters by others. Indeed, they can be both, but they need not be either. A properly managed landfill should not permit

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water pollution from liquid effluent or odor or disease vectors to become problems. The methane production from a landfill after anaerobic conditions obtain can be an asset or a problem, depending on circumstances, but the waste that is entombed in a landfill instead of being converted to something useful is discarded likely because its recovery and re-use is not economically feasible. Presumably there is widespread agreement that to prolong the useful life of existing landfills, so as to postpone if not avoid the creation of more of them, is a good thing. The use of oxo-biodegradable plastics in packaging can help in this regard,17 as we shall see. There is significant microbial activity in landfills owing to the high carbon content3 on the material in MSW and the ubiquity of micro-organisms and moisture. It is not uncommon for the temperature two metres below the surface to exceed 30 ëC even when the air temperature is below freezing. At the surface, and for several metres below there will be enough oxygen and water that aerobic biodegradation of the organic matter will occur. A vast array of fungal species and aerobic bacteria will be converting carbon to carbon dioxide. A limit on this activity will be the impervious plastic bags, sheets and films that prevent the free movement of gases and liquids in the mass of organic waste that is contained in or `protected' by this plastic. After a time (months, years) the MSW well below the surface or active face will no longer have an adequate supply of oxygen and water to support the aerobes. Then, whatever anaerobic bacteria are present will convert (much more slowly) the carbon in the remaining organic material largely to methane, which is 24.5 times2 more potent as a greenhouse gas than is carbon dioxide. It follows that there are advantages, both environmental and commercial, to encouraging rapid aerobic biodegradation while there is enough oxygen and water in the upper levels of MSW in the landfill. A simple and inexpensive way to do this is to use oxobiodegradable polyolefins in packaging. Faster bioconversion of the organic matter reduces the volume faster, thus postponing the need for a new landfill, and the conversion of carbon to carbon dioxide instead of to methane reduces the effect of greenhouse gas build up. Under normal conditions, degradation of EPI's TDPA-based polyethylene film will begin approximately thirty days after disposal in a landfill.3 This time to the onset of degradation could be as short as two weeks under ideal conditions or it could be as long as several months under cold, wet conditions. Fragmentation of the films, bags and other containers will follow after abiotic oxidation, aided by the inevitable mechanical stresses (e.g., from compacting equipment) in the landfill environment. This will allow the free circulation of air and water through the upper levels of the waste mass for some time ± months, perhaps a year or more ± and the readily biodegradable organic materials will be bioassimilated by aerobic micro-organisms. This relatively rapid conversion will reduce the volume of waste relatively rapidly, and this will prolong the useful life of the landfill. Obviously, with so much more material bioassimilated during

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the aerobic phase, there will be much less left to undergo slower anaerobic degradation to produce more damaging methane. Furthermore, the site, after filling and capping, will be available that much sooner for other purposes such as recreational fields.

16.4.2 Litter It seems to be an unfortunate fact of life that a small segment of the population is prone to thoughtlessly discarding unwanted materials outdoors. Publicized, if not enforced, penalties for littering do not appear to be effective in avoiding this problem. Since much of the littered materials are single-use packaging, a significant amount is plastics, and a large proportion of this is polyolefins. It has been shown1 that it is far more effective to reduce the outdoor lifetime of the littered material than to rely solely on penalties or `education' for the alleviation of the litter accumulation problem. The requirement in much of the United States that the polyolefin six-pack holder for beverage cans be photodegradable has resulted in a major reduction in the litter (and wildlife) problems caused by these devices when they are discarded outdoors. On a more general note, many different used plastic products are discarded carelessly and most of them can be considered packaging of one sort or another. The outdoor environment entails exposure to heat and mechanical stress, e.g., wind, precipitation, as well as to sunlight. Since the initiation of the abiotic deterioration of oxo-biodegradable polyolefins results from all three of these factors, the widespread use of these degradable plastics would be expected to ameliorate the problems that arise from plastics litter (see Ch. 3, section 3.7.3). Since much of this litter should have ended up in a collection/landfill system anyway, litter reduction can be considered as an extra advantage resulting from the use of oxo-biodegradable polyolefins in packaging. No one is suggesting that a special class of packaging be labeled `suitable for littering' or that the social stigma of being a litterer should be relaxed in any way.

16.4.3 Composting Plastics that are permitted to enter commercial composting plants must have no deleterious effects on the composting operation, must undergo biodegradation themselves, and must not leave a harmful or toxic residue at the end of the composting procedure (see Ch. 3, section 3.7.2). Premium quality compost must have no visible pieces of plastic, must meet metal-content restrictions, and must show no ecotoxicity effects in standard tests. It has been demonstrated14 that EPI's TDPA-polyethylene compost bags can meet all these requirements. An added advantage with the TDPA-based product is that much of the carbon from the plastic remains in the form of oxidation products, humic materials, after the formal composting process is finished. This maximizes the nutritive value of the

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compost since the complete conversion to carbon dioxide takes place only after the compost has been added to the soil.

16.5 Recovery It is commonly the case that traditional packaging materials such as steel, aluminum and glass can be re-used after suitable collection and cleaning. This is just as well since the energy requirements to produce beverage containers, for example, from these materials are considerably higher than to produce plastic beverage containers.1 Production of aluminum cans is particularly energy intensive, and yet the recovery of aluminum beverage cans in North America is not much better than 50%. The transportation of heavy glass bottles, and the cleaning of them before re-use requires a major expenditure of energy. Oxobiodegradable polyolefins have superior characteristics when it comes to recovery protocols.

16.5.1 Recycling The simplest scenario for the recycling of post-consumer plastic packaging is the return to central locations of relatively clean items made from a single plastic type. Thus the bins in supermarkets into which shoppers deposit used carrier bags are a successful supply for recycling. The inevitable inclusion in such supplies of some already degraded polyolefins is normally not a problem because of `dilution'. The addition of extra processing antioxidant to the mix can normally take care of higher than normal melt index values, and is certainly warranted if the products to be produced from the recycled material are to be used for more than a few months, in a warm environment or outdoors. In certain parts of the world, injection molded food and beverage containers are collected, and usually recycled into lower value products, such as `plastic lumber.' It can be a problem to balance the supply of post-consumer plastic with demand for it but, where this is achieved, the recycling of used plastics is successful. It is reasonable to ask if post-consumer oxo-biodegradable polyolefins can be recycled, and if such materials when mixed with ordinary polyolefins will interfere with existing recycling procedures. The answers at the present time are a cautious yes and no, respectively. The in-plant recycling of scrap is done routinely but there is not necessarily enough post-consumer oxo-biodegradable polyolefin material in the general marketplace for all conceivable problems to have arisen. Nevertheless, there is no reason to anticipate any problems, for the following reason. In the case of EPI's TDPA technology, the prodegradant does not initiate the oxidative degradation of polyolefins. It simply speeds up conventional oxidation initiated by heat or by exposure to sunlight or by mechanical stress. Residual antioxidant in virgin resin and/or additional antioxidant added for the recycling process itself will minimize any resin

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oxidation that might otherwise occur during re-extrusion. The dilution effect will prevent the products from being oxo-biodegradable, unless the products are supposed to degrade rapidly after use, in which case more prodegradant will need to be added.

16.5.2 Incineration The attractiveness of incineration of waste organic materials with heat recovery seems like a preferred disposal option as compared to a landfill. Used plastics in particular represent a valuable fuel source comparable in calorific value to fuel oil,2 and this is another major advantage of plastics compared to traditional materials such as metals and glass. In some countries such as Japan and Denmark, the recovery of energy from the incineration of combustible waste is carried out extensively. There can be severe air pollution problems, however, and there are concerns about the stack gases carrying the likes of chlorinated hydrocarbons and transition metal contaminants to downwind locations. There is public mistrust on incineration in much of Europe, Canada and the United States, for example. This, coupled with the cost of building and operating an efficient plant suggests that incineration in unlikely to become a major factor in the near future. The question needs to be answered, however, whether or not oxobiodegradable polyolefins can be incinerated safely. The answer is yes. These plastics still represent excellent fuel, no halogens are involved, and the residue from partial or complete oxidation is not toxic or harmful (see section 16.3.3).

16.6 Environmental impact A number of items could be discussed under this heading. For example, this could be the place for a discussion and summation of the evidence that no harm is done to the environment by the continued use of oxo-biodegradable plastics. This was already covered, however, in section 16.3.3, and there is no need to reiterate here the harmlessness of the plastics and the residues from them. Rather, let us be reminded of the benefits to the environment that accrue from using oxo-biodegradable polyolefins.

16.6.1 Scott/Gilead products The efficacy of using plastics as aids in several aspects of agriculture have been summarized 2,18 together with the major economic and environmental advantages that derive from the use of oxo-biodegradable polyolefins, especially as photodegradable films. The major advantages to the farmer of using polyethylene mulch films are retained but the awkward environmental problem of the disposal of used films is obviated when those films automatically become a soil improving material after the growing season (see Ch. 17). In other words,

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there is a significant environmental impact resulting from the use of oxobiodegradable polyolefin films in agriculture, and this impact is beneficial.

16.6.2 EPI products There are significant impacts arising also from the use of oxo-biodegradable polyolefins based on EPI's TDPA technology, and these are beneficial as well. It has been shown (section 16.4) how their use in carrier bags could prolong the use of landfills, and obviate the accumulation of litter. The availability of inexpensive, one-way compost bags made from TDPA-PE can encourage more composting of organic waste and improve the yield of premium quality compost product by not undergoing mineralization as rapidly as is required by ASTM D6400.

16.7 References 1. Guillet J E in Scott G, Degradable Polymers: Principles and Applications, 2nd edn, Dordrecht, Kluwer Academic Publishers, chapter 12, 2002. 2. Scott G, Polymers and the Environment, Cambridge, Royal Society of Chemistry, 1999. 3. Swift G and Wiles D M, `Biodegradable and degradable polymers and plastics in landfill sites' in Kroschwitz J I, Encyclopedia of Polymer Science and Technology, Hoboken, John Wiley & Sons, 2004. 4. Benjamin D K, `The good news about your garbage' in Consumers' Research, 86, 11, 28±30, 2003. 5. Tung J-F, Wiles D M, Cermak B E, Gho J G and Hare C W J, `Totally degradable polyolefin products', Proceedings of the Fifth International Plastics Additives and Modifiers Conference, paper #17, Prague, 1999. 6. Wiles D M, `The photodegradation of fiber-forming polymers' in Geuskens G, Degradation and Stabilization of Polymers, London, Applied Science, 137±155, 1975. 7. Hawkins W L, `The thermal oxidation of polyolefins ± mechanisms of degradation and stabilization' in Geuskens G, Degradation and Stabilization of Polymers, London, Applied Science, 77±94, 1975. 8. Chien L C W, `Hydroperoxides in degradation and stabilization of polymers' in Geuskens G, Degradation and Stabilization of Polymers, London, Applied Science, 95±112, 1975. 9. Garton A, Carlsson D J and Wiles D M, `Photooxidation mechanisms in commercial polyolefins' in Allen N S, Developments in Polymer Photochemistry ± 1, London, Applied Science, 93±123, 1980. 10. Al-Malaika S and Scott G, `Thermal stabilization of polyolefins' in Allen N S, Degradation and Stabilization of Polyolefins, London, Applied Science, 247±281, 1983. 11. Al-Malaika S and Scott G, `Photostabilization of polyolefins' in Allen N S, Degradation and Stabilization of Polyolefins, London, Applied Science, 283±333, 1983.

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12. Billingham N C, Wiles D M, Cermak B E, Gho J G, Hare C W J and Tung J-F, `Controlled-lifetime environmentally degradable plastics based on conventional polymers', Addcon World, Basel 2000, RAPRA publishing, p.6, 2000. 13. Arnaud R, Dabin P, Lemaire J, Al-Malaika S, Choban S, Coker M, Scott G, Fauve, A and Maarooufi A, `Photooxidation and biodegradation of commercial photodegradable polyethylenes', Polym Deg Stab, 46, 211±224, 1994. 14. Raninger B, Steiner G, Wiles D M and Hare C W J, `Tests on composting of degradable polyethylene in respect to the quality of the end-product compost' in Insam H, Klammer S and Riddich N, Microbiology of Composting, Berlin, SpringerVerlag, 299±308, 2002. 15. Fabbri A, `The role of degradable polymers in agricultural systems' in Scott G and Gilead D, Degradable Polymers: Principles and Applications, London, Chapman & Hall, 200±215, 1995. 16. Billingham N C, Bonora M and De Corte D, `Environmentally degradable plastics based on oxo-biodegradation of conventional polyolefins' in Proceedings of the 7th World Conference on Biodegradable Polymers and Plastics, Pisa, June 4±8, 2002. 17. Scott G and Wiles D M, `Degradable hydrocarbon polymers in waste and litter control' in Scott G, Degradable Polymers: Principles and Applications, 2nd edn, Dordrecht, Kluwer Academic Publishers, 454±457, 2002. 18. Gilead D, `Photodegradable plastics in agriculture' in Scott G and Gilead D, Degradable Polymers: Principles and Applications, London, Chapman & Hall, 186± 199, 1995.

17

Biodegradable plastics in agriculture G S C O T T , Aston University, UK

17.1 Plasticulture Plastics have achieved a dominant position in agriculture during the past 30 years. This is a direct consequence of their transparency, lightness in weight, impermeability to water and their resistance to microbial attack.1 The term `plasticulture' is now generally used to describe the use of plastics in agriculture, which since 1970 also includes the use of degradable plastics and rubbers.2 In the following sections, the main areas of application of degradable polymers in plasticulture and horticulture will be described.

17.1.1 Protective films A major limitation of normal (non-degradable) plastics in mulching films and temporary films is the residual materials that remain in the growing environment. The use of degradable plastics (e.g. polyethylene, PE) films in tunnels and mulching films for the growing of soft fruits and vegetables has thus become an important economic tool in commercial agriculture and horticulture. The earliest use of degradable polymers in mulching films was in Israel in the late 1970s, based on research at Aston University (see section 17.1.2).3 Initial applications were in strawberry growing in which the films with controlled photo-degradability very effectively replaced straw as the water retention medium and led to much cleaner fruit and increased yields. This was followed by tomato growing and when the technology was taken up in the USA and Italy in the early 1980s, it was applied equally successfully to melons.2,4 Mulching films also made possible the growth of crops such as chilli, and sweet potatoes in more northerly climates that had until then been grown only in tropical climates. Degradable PE mulching films were then quickly applied to the growing of vegetables and cereals, such as sweetcorn and forage maize ± even in northern Europe.5 The use of polypropylene in degradable hay and straw baler twines and of HDPE in protective bags to facilitate the ripening of bananas are both now well-established applications of degradable polyolefins.2 There are also

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Biodegradable polymers for industrial applications Table 17.1 Ratio of increased income to cost of mulching film1,2 Crop Melons Vegetables Peanuts Sugar cane Cotton Maize

Increased income: cost 13.0 5.0 3.9 3.6 3.0 2.5

important potential applications in forestry and in environmental improvement, for example growing of shrubs and trees for land stabilisation on discarded coal tips and on motorway embankments. By far the most important benefit of photobiodegradable plastics as opposed to regular commercial plastics is that they can be left on the soil and ploughed in after use without interfering with crops in the following season. For a detailed discussion of the biodegradability of photobiodegradable plastics, see Chapter 12. One of the most important markets for mulching films is in the Far East, particularly mainland China and Taiwan. China uses five times the quantity of plastics in agriculture than the USA.2 It will be evident from the above that mulching films are used in a variety of different environments and when used to their maximum potential, they greatly increase the value of commercial crops as a result of earlier and heavier cropping (Table 17.1). This is particularly advantageous in the case of the more exotic fruits and vegetables. In Taiwan, the cost of removing partially degraded mulching films from the field before the next growing season is $250/hectare, irrespective of the value of the crops.6,7 Scott-Gilead (SG) mulching films have been used successfully in the cultivation of a variety of crops throughout the world. Tables 17.2±17.5 show the typical increase in yields compared with bare ground cultivation.2 In 2001 the area of crops cultivated over mulching film in Taiwan was 31,220 hetares/annum, involving the removal of about 5,500 tonnes of plastic film annually.6,7 Table 17.2 indicates the importance of mulching film technology to the economy of the country. Since the crops listed are all expensive soft fruits, it Table 17.2 Planted area of mulching films in Taiwan4 Crop Watermelon Cantaloupe Musk melon Strawberry Pineapple Papaya

Planted area (hectares) 17,436 6,607 2,646 439 10,273 3,541

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Table 17.3 Yields of cotton fibre with and without mulching films in Israel2 Treatment Bare ground Mulched

Yield (kg/1,000 m2)

Increase in fibre (%)

292 ( 25) 422 ( 30)

49

Table 17.4 Yields of musk melons with and without mulching films in Canada2 Treatment

Yield (kg/7 plants)

Increase in yield (%)

62.7 92.6

47

Bare ground Mulched

Table 17.5 Yields of sweet corn with and without mulching film in USA6 Treatment

Yield (12/acre) 1986 1987 1988

Bare ground Mulched

1095 1382

1238 1306

1457 1623

Average

Increase (%)

1263 1437

14

is therefore not surprising that a great deal of research into all classes of degradable plastics has been conducted at the Taiwan Agricultural Improvement Station (see below). Very detailed studies of agronomic practice using SG degradable plastics has been carried out by A. Fabbri for Enichem in Italy.4 In particular, the concerns of the `green' movement about `heavy metals' in the environment, resulting from the use of prooxidant transitional metal ions, were addressed in collaboration with G.A. Casalicchio and Bertoluzza at the University of Bologna and were demonstrated to be unfounded.8,9 Similar studies were carried out and similar conclusions were reached by Taber and Ennis using Plastigone (SG) in the USA.10 The results of these investigations are discussed further in section 17.3.2. Some typical results obtained using the Scott-Gilead (SG) agricultural film systems2,3,11,12 in a variety of conditions as shown in Tables 17.3±17.5 and the main agronomic factors governing the economics of the use of degradable mulching films are discussed below. Soil temperature Plastics films spread on the ground act as a local `greenhouse'. The sun's energy is absorbed by the soil and is partly converted to heat. Typical soil temperatures

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Biodegradable polymers for industrial applications Table 17.6 Soil temperature (ëC) in soil in Israel (August±September) with and without mulching films3 Depth (cm) 5.0 15.0 20.0

Mulched Maximum Minimum 52 42 38

25 30 35

Unmulched Maximum Minimum 42 34 33

25 31 27

at various depths under plastic mulch in the Jordan Valley in September are shown in Table 17.6.3 The primary effect of this is to increase root growth remarkably, thus bringing forward the maturation time and increasing the yield of the crop. A second important effect is to accelerate the oxo-biodegradation of any plastics residues that may remain from earlier crops (see Ch. 12). It is significant that these elevated temperatures are within the thermophilic regime of bacterial action and that abiotic peroxidation and bio-oxidation then operate synergistically to increase the rate of bioassimilation of the residual plastic. Table 17.7 lists the products currently being examined in agricultural applications in Taiwan.13 Plastor SG and EPI TDPA are oxo-biodegradable polyolefins (see Chs 3 and 12) and have been shown to oxo-biodegrade after initiation by light and heat.14 PE-starch blends may contain transition metal ions Table 17.7 Commercial degradable plastics evaluated in mulch in Taiwan Class

Mechanism

Commercial products

Photodegradable

Photolysis

ECO-3 Ecolene (USIFE) Polymer-M Polygrade

Photo-biodegradable

Oxo-biodegradation

Plastor SG EPI TDPA Ecolyte

PE-Starch blends

Hydro-biodegradationinduced disintegration

Ecolene Green choice kk (China) Oligostarch Ecostar (Japan) Polystarch Ecostar plus (USA)

Biodegradable

Hydro-biodegradation

Bioflex Bioplastics Mater-bi Novon (Japan) Greenpol

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Table 17.8 Composition of Ecolene plastics Trade name

Composition

DG101 DG102 DG201 DG301 DG401

Photodegradable polyethylene Photodegradable PVC Biodegradable PE Bio/photodegradable PE Biodegradable PVA

(e.g. Mn) that are added to accelerate photo-biooxidation, but this is not normally disclosed and a single trade name may cover several mechanisms. Table 17.8 illustrates the difficulties associated with the use of degradable plastics in agriculture. Some of them are of very doubtful value. In particular the use of organo-chlorine compounds must be open to question unless the nature of the breakdown products are known and their toxicity is assessed. Furthermore, the names do not give a clear indication of the classes in Table 17.7 into which the materials fall. For example, Ecolene PE-starch formulations manufactured by USI in the Far East13 contain LDPE 20±60%, starch 10±60% and compatibiliser 10±20%. This is designated, `biodegradable' in spite of the fact that it has been shown that starch does not induce the biodegradation of polyethylene. The formulations containing 5%, 10%, 15% and 20% of starch in PE are called `bio/ photodegradable', although there is no photodegradant present. Photodegradable polyethylene is even more obscure, consisting of a `carbonyl group polymer' containing either benzophenone or `a tert-hydrogen polymer' as promoters. None of these formulations contain transition metal ion catalysts, which have been found to be essential to provide a satisfactory rate of subsequent oxo-biodegradation, even in the case of ethylene-carbon monoxide (E-CO) copolymers. Ecolyte polymers, on the other hand, have been shown to biodegrade.15 There is some doubt then about the ability of some of these plastics to be bioassimilated and the possibility of accumulation on the soil in the time scale considered. It will be seen later that although PE-starch blends do break down to particulate PE this does not occur rapidly enough in horticultural containers and pots (see section 17.3.1). The indiscriminate use of the `green' prefix `Eco' illustrates the need to demonstrate in each case ultimate bioassimilation by the application of standard tests for `biodegradable plastics' in order to advise the user in their use and to avoid long-term pollution of the environment. This has already been discussed in more detail in Chapter 12. Reduction in labour costs In modern automated agriculture, mulching films are laid and the plants inserted through pre-formed holes in the film in a single operation. The films must be

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strong and tough since the edges of the rows are turned under to avoid wind damage.2 Mulching films and tunnels made from conventional plastics are well stabilised against the processing operation and consequently in the environment during use. Plastics residues from regular polyolefins, when used as mulch films or low tunnels, remain on the soil for some years as large pieces and are not only an impediment to plant growth but also a potential hazard to animals if the land is subsequently put down to grass. They must be manually removed from the fields before the next planting season or they will interfere with root growth and reduce crop yields.2,5,7 Yang has estimated the use of Ecolene degradable agricultural films provides an overall 16% saving compared with the use of regular polyethylene due to the self-destruct properties of the mulch.13 Furthermore, many crops are harvested automatically so that the tough plastic residues remaining from conventional films clog the cropping machinery.2 This makes the use of conventional plastics practically impossible in automated agriculture since not only is it time-consuming but, with modern thin film technology (8±10 m), it is also virtually impossible to remove all the films from the soil.4 Plastics that are initially tough and strong but which degrade rapidly just before cropping result in considerable saving in cost (see above). Water and fertiliser conservation A further economic advantage of plastics mulch is economy in water and fertiliser usage. This is a major issue for the 21st century, since water will not be the plentiful commodity it has been in the past and conservation will be necessary to produce crops at all in many arid parts of the world. It has been found that water usage during irrigation can be reduced by 80% when it is released into the ground below a plastic cover.3 In modern commercial agriculture, fertilisers are applied to the plants through the irrigation system and, due to the reduced water usage, the use of fertilisers can also be reduced by 30% compared with their usage on bare ground.4 Consequently the `run-off' of fertilisers into rivers and lakes is considerably reduced with a beneficial effect on the environment. Reduction in the use of nitrogenous fertilisers also results in the conservation of carbon in the soil, rather than ejecting it to the environment as carbon dioxide.16 Weed control The growth of weeds reduces the soil nutrients available for growing plants and these are normally controlled by herbicides. The use of herbicide-resistant strains of food crops by genetic modification has so far proved very controversial and recently weeds have been found to build up their resistance to the same herbicides. A simpler and less controversial way of achieving the same result is to use dark coloured mulching films.2 These do not need to be

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black and `smoked' films manufactured from SG photo-biodegradable PE containing special grades of carbon black successfully achieve this without sacrificing the required rate of photodegradation. In hot climates, the temperature under black films may rise too high and may actually damage the roots of the plants. Under these conditions bi-layer films are used successfully. The outside layer contains white or `silver' pigments to reflect the heat and the inside layer contains carbon black or other dark pigments, preventing the growth of weeds.2,7,13 Root development An unexpected benefit of protective films on the surface of soil is that the lateral development of the roots of plants is favoured. This is generally beneficial to the plant in temperate climates because the soil temperature is higher in the surface layers due to the `greenhouse effect' (see Table 17.2). The increased temperatures also activate beneficial micoorganisms that increase the availability of nutrients in the soil.2 A further benefit is that mulching films condense evaporating water on the underside of the film and by drip feeding the plants with pure water they avoid the normal concentration of unwanted salts in the surface of the soil. This is a way of `sweetening' brackish soils and makes possible cultivation in areas where this would not normally be possible.2

17.1.2 Time-control of degradable plastics in protective films It will be evident that to optimise the benefits of this technology, the protective films must remain intact until just before harvesting. This is particularly important in the case of melons, bell peppers and sensitive vegetables that are normally irrigated with aqueous nutrients. Mulching films create a microenvironment at the roots of the plant, which take up only the water and nutrients that they need.4 Furthermore, excess water from heavy rainfall can be just as damaging to sensitive crops as too little water. If the films degrade prematurely, much of the benefit of the protective mulch may be lost with consequent loss of income to the farmer. It is important that mulching films have high tear strength when laying and during use but it is equally important that the films have little tear strength at the beginning of cropping since in automated agriculture, the cropping machinery may become clogged and incapacitated by the plastic. Elongation at break, Eb (Fig. 17.1) must remain high and constant until the end of the induction period (IP) and decline rapidly when the plastic fragments. This can not be achieved by the use of regular commercial plastics, since antioxidants and UV stabilisers are added to polymers not only to increase IP but also to slow down the rate of post-IP peroxidation and loss of mechanical properties.

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17.1 Ideal performance of biodegradable plastics in agriculture.1,7,8 Eb ˆ elongation at break, IPa,b ˆ time-controlled induction periods during which mechanical and chemical properties do not change.

It will be evident that the post-IP rate of polymer degradation must be as rapid as possible, consistent with the basic kinetics of polymer peroxidation. Short induction periods (up to two months) can normally be achieved by the use of conventional processing stabilisers but for longer maturing crops, most commercial antioxidants and stabilisers do not provide the inversion of mechanism depicted in Fig. 17.1 and special light stabilisers have been developed in the optimal technology. The principles underlying the molecular design of appropriate stabilisers will be discussed below but in practice five concentrate formulations are sufficient to provide the induction periods required for agriculture in any part of the world.2,5 Table 17.9 indicates the embrittlement time for each formulation when tested in a temperate environment. Blends of these may be used for intermediate time scales if required. Although there is a shift in fragmentation (embrittlement) time between the hot, sunny climes and temperate climes, it can normally be predicted with reasonable accuracy based on incident energy (mJ/m2) and temperature. Table 17.9 Standard SG polyethylene additive grades5 SG additive grade #221 #131 #19 #12 #112

Time to embrittlement* 6 weeks 3 months 4 months 6 months 12 months

* Average times for mid-Europe or mid-USA for spring planting.

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However, in making substantial changes in climate, small-scale trials are normally carried out first. SG time-controlled degradable plastics are widely used in many countries throughout the world under the brand name of the film manufacturer. The commercial success of the additive concentrate technology is based on the fact that it does not require a separate and relatively small-scale manufacture for each grade of degradable polymer. In practice the additive package simply replaces the additives used in commercial plastics. The conventional way of using plastics mulching films is in a single cropping regime in which the film disintegrates just before cropping. However, programmed-life biodegradable mulching films are now being developed for fast-growing vegetables to produce two crops in quick succession. 5,13 A second crop is planted in the same film immediately after the first is harvested. Both crops are grown over a single film and in this case the film is timed to degrade as the second crop is being harvested. For this purpose, the film may have to survive intact for one year before fragmenting.5,13 This system is discussed in more detail in reference 5. In irrigated systems this second procedure offers considerable advantages since quite apart from the lower costs, the irrigation tubes are not disturbed, ensuring a fast changeover. In temperate climates such as Taiwan, a late crop planted in September can be harvested in December and this may be followed by a second crop early in the next season by planting on the same film in December or January.13 Yang has compared the lifetimes of degradable plastics on soil in Taiwan (Table 17.10). Another procedure that is gaining in importance, is to sow seed directly into the soil under a complete plastic cover. This has the advantage of avoiding the `shock' of transplanting with consequent earlier maturity of the crop. `Mid-bed trenching', as this process is called,2,4 involves sowing the seed in a trench and laying the plastic above the growing plants (Fig. 17.2). The increase in yield achieved by this technique can be quite remarkable and is not possible apart from using time-controlled degradable films. A typical example is shown in Table 17.11. Table 17.10 Relative stability of commercial degradable agricultural films in the period November 30 to December 30 in Taiwan9 Trade name

Grade

Cracking (%)

Plastor SG Plastor SG Plastor SG Green choice Ecolene Mater-bi Regular PE

#221 #131 #12

61.0 0.49 1.36 25.3 0.72 49.0 0

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Biodegradable polymers for industrial applications

17.2 Mid-bed trenching using SG photo-biodegradable polyethylene.

Table 17.11 Yields of bell pepper with and without mid-trench cover in Texas, USA2 Treatment Bare ground Mid-bed cover

Yield (kg/hectare)

Increase (%)

1356 5880

433

The SG degradable films are timed to break under slight pressure (i.e. Eb < 10%) when the leaves of the plant contact the cover. If the film breaks too early, the greenhouse effect will be lost and if it breaks too late the plant will be misshapen. Longer-term films have also been evaluated as solar sterilisation films in tropical climates. The principle involved is that photodegradable films are laid on the soil for a period of time before the crops are planted. This results in the destruction of pathogenic bacteria, which tend to accumulate in intensively cultivated land. This obviates the use of the bacteriocide, methylene dichloride, which has been found to be a cause of ozone destruction in the upper atmosphere. The high temperatures achieved in the soil under transparent films destroy pathogenic bacteria but leave untouched beneficial microorganisms.2 Normally, the films are allowed to fragment before the crop is grown on a new photodegradable film but if the initial film is still strong the plants may be grown through holes punched in the original film.

17.1.3 Auxiliary products for farming and horticulture Biodegradable plastics are being increasingly used for auxiliary products that frequently end up in the environment as litter. In these applications, the time scale for disintegration and biodegradation is not so critical as it is in the case of crop protection film. They include irrigation tubing, clips for tying plants to retaining posts, seed pots, plug-trays and growbags. They are also used for protection bags for fruits such as bananas and guava and in seed release tapes and other controlled release devices. A recent application has been developed for in situ composting

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461

bags for domestic and garden waste, which although not directly an agricultural procedure, gives high quality fertiliser for agricultural and horticultural use. Plant pots Traditionally, bio-based materials such as paper, plant fibres, peat and wood have been used for biodegradable pots and trays used in horticulture. In 1994 the number used was estimated to be 70m. items.18 The main advantages of these natural materials is that they all biodegrade, although at quite different rates. Paper is rather fast for many containers and wood composites are rather too slow. The advantages of biodegradable containers for plants are obvious. They do not need to be disposed of and, if of appropriate design, they break apart as the roots develop. The soil available for root growth at any time in the plant growth appears to be the major limitation of the use of pots as compared with planting out without pots after germination.11 A further factor affecting the use of biogenic materials such as paper or starch is the amount of water used which markedly increases cost and reduces eco-efficiency due to rapid evaporation. It has been calculated that the additional fuel oil required for the provision of water is up to 670 litres per 1,000 m2 of nursery cultivation area.19 Coating starch-based pots using rubber latex or starch esters was found to decrease water usage without reduction of biodegradability. Other biodegradable materials are currently under investigation but have not so far achieved a significant share of the market. Oxo-biodegradable plastics are effective in reducing water evaporation through the walls of the containers. However, unlike the hydro-biodegradable plastics, they do not biodegrade when buried unless they have been photo-degraded by exposure to the environment after which they biodegrade more slowly when buried. On the other hand, cost favours the polyolefins since they are cheaper than bioplastics. Furthermore, this is an acceptable application for plastics recovered from the waste stream, which are normally contaminated and hence unsuitable for `high-quality' recovery processes (see below).5 Fritz has compared the germination of tomatoes in biodegradable pots with conventional non-biodegradable polystyrene pots in greenhouses19 (Table 17.12). It is not entirely clear why some of the bio-based materials appear to inhibit germination. This could be the result of more rapid depletion of nitrogen in the soil, which has been shown to be associated with some natural wastes but it could also result from more rapid evaporation of water through the more permeable containers. These are clearly factors that have to be considered in the choice of the most acceptable materials for seed and plant pots. Packaging materials The use of degradable plastics as a primary agronomic requirement, for example in protective films, seed pots, etc., brings a substantial cost-benefit to the user and

462

Biodegradable polymers for industrial applications Table 17.12 Performance of biodegradable plant pots Material ÚKOPUR FASAL MaterBi AI 05H Biomer P101 RT Polystyrene

Germination (%)

Biomass (mg/plant)

Relative biomass (% of PS)

100 83 93 100 100

89 145 26 124 1690

5.3 8.6 1.5 7.3 100

ÚKOPUR is a composite material made mainly from sugar beet residues (plant-root residues after sugar extraction), starch and plant resins. FASAL is similar but the sugar beet residues are replaced by sawdust and other ingredients are added to make it thermoplastic. Biomer P101 RT is a ready-to-use PHB material (granulate) provided by Biomer, Germany.

these applications were the first to make use of oxo-biodegradable polyethylene. There are much less obvious commercial benefits in using biodegradable plastics where the primary purpose is to produce a cleaner environment. Some auxiliary products such as animal feed bags can be recovered in a relatively clean form for recycling but the subsequent performance of recycled agricultural sacks is generally inferior to that of virgin materials.1 Some farmyard plastics detritus is too heavily contaminated and degraded during use to be worth recycling at all.20,21 Agricultural packaging and wrapping bags normally end up in landfill.5 However, fertiliser and animal feed sacks frequently escape collection and become highly visual litter in the countryside. The cost of recovering windblown plastics is prohibitive and as already noted they are unsuitable for recycling because of contamination and partial degradation. For this application accelerated photooxidation using longer-term degradants discussed in the last section are very suitable. The same applies to banana protection bags that hasten the ripening process and protect against fruit bats.2 SG polyethylene is used commercially for this purpose. Silage-wrap, hay-wrap and growbags A very persistent and visible environmental pollutant is silage stretch-wrap film. This has to remain tough and strong during the maturation period but after the silage has been fed to animals, the residual plastic becomes an environmental nuisance.1 This material is again highly contaminated with agricultural waste and soil and can only be used for sub-standard recycled products. Past attempts to reprocess to good quality have been largely unsuccessful20 primarily because well-meaning recyclers have little understanding of the lack of durability of recycled contaminated plastics. It has been pointed out that mechanical recycling may actually consume more energy in the long term than making the same product from virgin polymers.1,5

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In some countries, including the UK, discarded plastics stretch-wrap continuously accumulates in the farmyards and blows into the surrounding countryside, collecting on hedges and along riverbanks.1 In a volunteer collection in the Yorkshire Dales National Park in England 40 m3 of compressed polyethylene film was collected from one 13 km stretch of public footpaths along the river Swale.22 It was estimated that about 500 tonnes of film are used annually in just one of the British National Parks alone (estimated at 65,000 tonnes for all farmland in the UK) and this accumulates from year to year since very little is routinely collected for disposal. The cost of landfill disposal is increasing year by year. SG technology is available to make these materials photo-biodegradable with a time delay of one year or more if required (Table 17.6, e.g. #112) and manufacturers of silage stretch-pack film are now beginning to show an interest in the use of biodegradable materials as a result of public demand. Growbags are widely used by amateur gardeners for vegetables and flowers but the actual volume of such products is not of great significance and there is not at present a strong incentive to make them biodegradable. Some commercial crops by contrast utilise grow bags on a much larger scale. In particular growing of mushrooms involves hundreds of millions of individual growbags. In Taiwan, for example, 215m. paper growbags are used annually in the production of the three main varieties of mushroom.13 Starch-filled polypropylene does not degrade rapidly enough in this application and the starch appears to reduce the mushroom yields13 relative to normal PP. It has been shown that the photodegradation of polypropylene can be induced by the addition of photodegradable ethylene-carbon monoxide copolymers (E-CO) (see Table 17.13) and that this type of formulation is a satisfactory replacement for paper mushroom bags.23 However, longer-term bioassimilation studies are required before this material can be considered to be ecologically acceptable after discard. In the USA photo-biodegradable polypropylene baler twines based on SG oxo-biodegradable PP have been used for 15 years and substantially replaced conventional PP slit films in hay wrapping and crop-protection netting.2 These are given a useful life of one year and this is followed by rapid disintegration and bioassimilation.

Table 17.13 Loss of mechanical properties of E-CO polypropylene used in the cultivation of ganoderma mushrooms14 Polymer ECO-3 Regular PP

Elongation at break (MPa) Before harvesting After harvesting 624±686 710±788

0 588±794

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Biodegradable polymers for industrial applications

Controlled release systems for fertilisers, pesticides and seed sowing An important development in Japan is the use of biodegradable polyolefins in fertiliser encapsulation using SG polyethylene.24 This results in controlled release in leaching environments over an extended period of time compared with direct application. This in turn effectively reduces the pollution of streams and the eutrophication of lakes and watercourses. The empty fertiliser capsules remaining after the fertiliser has been leached out biodegrade in the soil.26 Controlled release of pesticides by encapsulation also has considerable potential by matching the application time to the life cycle of the pest.25 A recent application that has been evaluated in Taiwan is in `seeder' tapes, where the seed is encapsulated at precise intervals along the tape, leading to economies in the sowing of expensive crops.23 It is clear that each of the above developments requires different rates of physical disintegration and of bioassimilation of the material and that it is difficult, if not impossible, to embrace them all in a single international standard.

17.2 Oxo-biodegradation of polyolefins in the environment 17.2.1 Peroxidation and its control Hydrocarbon polymers in air degrade by a free radical chain reaction involving oxygen from the environment. This process, which has been discussed in some detail in Chapters 3 and 12, is accelerated predictably by heat and light. The primary products are hydroperoxides (see Ch. 12, Scheme 1) and the latter either thermolyse (
) or photolyse (h) with chain scission and produce by further oxidation low molar mass biodegradable products such as carboxylic acids, alcohols and ketones.27,28

17.2.2 Antioxidants and stabilisers It has been known since the development of natural rubber as an industrial product in the 19th century that antioxidants inhibit the chain reaction that leads to the formation of hydroperoxides and the subsequent physical degradation of hydrocarbon polymers.28,29 It was subsequently shown that antioxidants also inhibit the biodegradation of most carbon-chain polymers.30,31 However, it is not a viable technological solution to simply omit antioxidants and stabilisers during the manufacture of commercial products since they are added to protect the polymer against mechano-oxidation during the conversion process32 and to provide the required service life.33 Some processing stabilisers, such as the hindered phenols or phosphite esters, extend the life of the polyolefins in the outdoor environment32 even though they are not normally considered to be light stabilisers.

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Transition metal ions are effective accelerators for the peroxidation of hydrocarbon polymers and antioxidants and stabilisers retard this process (see Chapters 3 and 12). Phenolic (chain-breaking) antioxidants control the metal catalysed melt degradation of polyolefins during processing but are relatively ineffective in controlling service life, particularly out-of-doors.34,35 Photodegradable polyolefins based on transition metal compounds and conventional antioxidants and processing stabilisers are thus very effective in short-term mulching films or in packaging that ends up in aerobic compost. Polyolefin films that must remain intact in the out-door environment for three months or more in sunny climates require a different solution. Transition metal ions catalyse hydroperoxide formation (eqns 17.1±17.4). POOH + Mn ! PO + OHÿ + Mn+1 POOH + M

n+1



+

! POO + H + M

PO + PH ! POH + P ! POOH + P 

17.1

n

 O2 ‡PH

POO + PH ! POOH + P

17.2  O2 ‡PH

ÿ! POOH



ÿ! P + POOH

17.3 17.4

PH = Hydrocarbon polymer, POOH = Polymer hydroperoxide Reactions 17.1±17.4 lead to the rapid accumulation of hydroperoxides and the equally rapid formation of low molar mass oxidation products (see Ch. 12). This must therefore be controlled during the service life of the product by peroxidolytic antioxidants that catalytically destroy hydroperoxides as they are formed in the polymer in a process not involving free radical formation.3,11,12,25,30,33±35 Consequently, they inhibit peroxidation (and hence biodegradation) until the antioxidant has been depleted by the action of light or heat. Some peroxidolytic antioxidants such as nickel dialkyldithiocarbamates (NiDRC ) are also very effective light stabilisers for polymers because they are stable to UV light. Other peroxidolytic antioxidants, for example the zinc dialkyldithiocarbamates (R2NCSS)2M, are thermal antioxidants but not light stabilisers, whereas the iron complexes are processing stabilisers but subsequently photo-prooxidants after photolysis of the ligand.3 (R2NCSS)2M Dialkyl dithiocarbamates M ˆ Fe, thermo-antioxidant, photo-prooxidant M ˆ Zn, thermo-antioxidant, weak photo-antioxidant M ˆ Ni, thermo-antioxidant, photo-antioxidant Many dithiocarbamate complexes (MDRCs) are very sensitive to sunlight and so long as the sulphur ligand remains intact, they are photo-antioxidants but they invert to photo- and thermo-prooxidants after photolysis.1,11,12,17,31,33,35,36 The ultimate end-point of this process is the conversion of the original polymer to biodegradable low molar mass products.

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17.3 Mechanisms of biodegradation of synthetic polymers in the environment.17

17.2.3 Biodegradation Two main mechanisms have been recognised in the process of polymer bioassimilation.11,14,17 The first is hydro-biodegradation, the dominant process leading to the bioassimilation of cellulose, starch, polyesters and polyamides (hetero-chain polymers). Hydrolysis produces low molar mass biodegradable chemicals. An analogous abiotic process, oxo-biodegradation, occurs in the case of the carbon-chain polymers (including natural rubber and polyolefins). Abiotic peroxidation results in the formation of low molar mass chemical species similar to those formed in hydro-biodegradation (i.e. carboxylic acids, alcohols and esters) and these are rapidly bioassimilated by microorganisms in exactly the same way35,36,37 (Fig. 17.3). The reader is referred to Chapters 3 and 12 for further details of these processes. Both abiotic peroxidation by molecular oxygen and biodegradation involving oxygenase enzymes play an integral and synergistic role in the bioassimilation of carbon-chain polymer wastes. This is discussed in detail in Chapter 12. However, peroxidation, unlike hydrolysis, is very readily controlled by antioxidants.3,11,12,17,25,31,35,36 Soil organisms that biodegrade lignin have been found to also bioassimilate polyethylene (Ch. 12).

17.3 The impact of degradable plastics on the environment 17.3.1 The popular view Most non-specialists do not believe that man-made plastics ever biodegrade in the environment. Farmers who are by nature sceptical of new technology but

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who use degradable polyethylene mulching films in agriculture do not share this belief, since experience has taught them that degradable polyethylene not only disintegrates to small particles at the end of its useful life but also has no impact on subsequent crops (i.e. they show no eco-toxicity) in the following seasons. The misapprehensions of the public arise from well-publicised statements by the `green' organisations and the following is a typical example published in 1990 by `Greenpeace'38: From 1 kg of biodegradable polyethylene-based plastics spread on soil today, it will still be possible to detect 500 g of material in the year 2033.

This misunderstanding arises from the fact that the non-biodegradability of the commercial polyolefins in the environment has little to do with the inherent biodegradability of the polymer structure but depends almost entirely on the antioxidants and stabilisers added to give them durability. However, this is not obvious to non-specialists and many environmentalists have accepted the opinions of Greenpeace without question (see Ch. 12). Commoner,38 who was responsible for the statement quoted above, ignored the contribution of abiotic chemistry to the biodegradation of all materials in the natural environment (Chs 3 and 12). There is no question that commercial polyethylene can resist microbial attack in the environment for many years. However, aged or weathered degraded fragments of the same polymer containing iron, manganese or cobalt as typical prooxidants, support microbial growth in the absence of any other source of carbon (see Ch. 12). 14,37 Furthermore, there is also evidence that, if the antioxidant is removed from the polymer after the conversion process, then microbiological attack on the polymer can and does occur by a combination of biotic and abiotic chemistry and the overall rate of bioassimilation is controlled by the presence of antioxidants and stabilisers in the polymer (see Section 17.2.2).

17.3.2 The scientific evidence The term oxo-biodegradable25 is intended to describe the above process. This has now been accepted by CEN and has been defined by CEN TC 249/WG9 `Characterisation of degradability'39 as `Degradation resulting from peroxidative and cell-mediated phenomena, either simultaneously or successively'. This definition characterises the synergistic effects of abiotic peroxidation ± accelerated by heat and light ± in combination with cell mediated oxidation processes in the degradation of polyolefins. Standards for biodegradable plastics emphasise the need to be able to show the substantial mineralisation of degradable polymers (Ch. 12). After exposure of polyolefins to the environment, they are no longer `pure' hydrocarbons but contain considerable concentrations of hydrophilic oxidation products (see Chs 3 and 12). Hydrocarbon polymers, then, by their very nature cannot biodegrade unless they are first modified by peroxidation.

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Degradable polyethylene mulching films have been used for over fifteen years in the same fields without any evidence of accumulation of plastic. The fundamental characteristic and most positive value of degradable polyolefins in soil is their conditioning effect and the fertiliser value of the ultimate cell biomass produced. Therefore rapid mineralisation is not ideal for polymers in the agricultural environment where the carbon in the original plastic should be converted over a longer period of time to humus and only slowly to carbon dioxide. The oxo-biodegradable polymers are ideal for this purpose since controlled peroxidation is the rate-determining step in the overall process (see Ch. 12).30,40

17.3.3 Eco-toxicity of biodegradable polyolefins Transition metal ions are required to initiate thermo-and photo-oxidation of the polyolefins and concern has been expressed about the effects of `heavy metals' on plants and on humans who eat the plants. However, it is normally overlooked that many agricultural soils contain substantial concentrations of these same metal ions.4 Table 17.14 shows the concentrations of nickel and cobalt in typical rocks. Humans require all the above metal ions for normal metabolism and most of these are obtained from food plants, which in turn obtain them from the soil. Table 17.15 shows data from an Expert Group Report on the concentrations of these trace elements in some common fruits and cereals. The report also points out that nickel, which is popularly believed to be carcinogenic in man is in fact not so. The so-called toxicity of nickel came from early studies of workers in mines, where the dust caused mesothelioma in the same way as asbestos.41 In spite of the large quantities of transition metal ions in common soils, the evidence suggests that edible plants take up only the very small amounts they require. Furthermore, Gilead calculated, based on known parameters,2 that if the same field was to be covered every year with Ni-containing degradable plastic mulch it would take 500 years to increase the content of the topsoil by 1 ppm. Other workers have demonstrated experimentally that doping the topsoil with massive amounts of water-soluble nickel, in order to simulate the accumulation of this metal Table 17.14 Nickel and cobalt content (ppm) of some common rocks (as oxides)4 Rock type Gabbro Gabbro (medium grains) Sandstone Limestone Crystalline (with quartz)

Ni

Co

750 30 90 10±20 64

100 50

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Table 17.15 Trace elements in food and water (mg/kg)33 Cobalt

Fish: 0.01, nuts: 0.09, cereals: 0.01 Average daily intake: 0.12 mg/day, fresh water: up to 0.01 mg/l

Magnanese

Green vegetables: 2, nuts: 15, bread: 8, other cereals: 6.8, tea: 2.7, water: up to 0.01 mg/l

Nickel

Oats: 0.18, nuts: 1.8, water: variable Average daily intake: 0.016 mg/day (food and water)

Table 17.16 Concentration of nickel in melons grown on soils doped with nickel sulphate4 Control Leaves Stems Flesh Skin

17.3 5.0 2.7 3.0

60 years* 120 years* 180 years* 15.2 4.5 2.0 3.5

13.5 5.2 3.0 3.2

13.7 5.0 3.2 3.0

* The soil was sprayed with NiSO4 to give nickel concentrations in the topsoil equivalent to the accumulation from Plastor SG mulching films used for the number of years indicated.

Table 17.17 Yields of lettuce grown in soil containing added plastic mulch debris9 (with kind permission from S-R. Yang and G-H. Wu, Tainan District Agricultural Improvement Station) Planting date January 1992 January 1994 February 1995 December 1995 October 1996 October 1997

Yields (kg/15.6 m2) Without debris With debris 37.8 35.2 32.4 52.0 32.5 40.1

39.3 38.2 34.5 55.4 38.7 40.5

in the soil from mulch film, did not significantly increase the concentration in some common crops. The effect on melons is shown in Table 17.16. Clearly then, plants take up only the metal ion concentrations that they require from the soil. Finally the growth of vegetables in fields to which particulate degraded polyethylene containing transition metal compounds has been added has been studied in Taiwan for five years.13 Table 17.17 follows the yields of lettuce with and without plastic debris over this period. Again, no increase in the concentration of heavy metals was observed in the plants.

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17.4 Future developments The provision of adequate supplies of fresh water for irrigation will become an increasing problem in the 21st century and reduction of water usage will be a major challenge in arid parts of the world. Plastics mulching films create a microclimate over the roots of the plants, which conserves irrigation water and fertilisers in addition to increasing the soil temperature, thus advancing the harvest and increasing yields with consequent economic benefits to the farmer. Commensurately, there is an increasing awareness of man-made products that pollute the environment. Conventional polyethylenes used in mulching films are too durable and unless removed from the soil after cropping, they interfere with the growth of subsequent crops. Manual removal of essentially non-degraded plastics litter is expensive but this can be obviated by the use of programmed-life biodegradable plastics that are stable during manufacture and in service but which are destroyed by sunlight due to UV exposure. This technology is already in use in plasticulture throughout the world but legislation will in the future limit the use of materials that accumulate in soil or lead to the generation of long-term toxic products in the soil. Consequently there must be a balance between the economic benefits of biodegradable plastics and their long-term environmental effects. The use of programmed-life photo-biodegradable polypropylene has successfully solved the problem of twine litter. There are also a number of other potentially important uses of degradable polyolefins that are in an earlier stage of development, for example, in disposable silage-wrap and hay-wrap film and bird netting for expensive soft fruits. The encapsulation of fertilisers in biodegradable plastics that allow controlled release over a longer period are already making a significant contribution to the reduction of the pollution of rivers and lakes by agricultural chemicals. The development of biodegradable pesticides has had an unfortunate effect on the control of insects such as the mosquito which have a relatively long `action time'. Controlled release by encapsulation in biodegradable polymers offers considerable promise and could become a life-saving technology in this decade. In the above applications, photoxidation is the primary process that leads to bioassimilation in the soil. Some horticultural products require a different solution. For example, biodegradable pots used for propagation of plants from seed are required to fragment and biodegrade rapidly after immersion in the soil in order to permit unrestricted root growth. It is difficult to expose photodegradable pots with sufficient UV energy to guarantee fragmentation before immersion in the soil and subsequent bioassimilation will be very slow. By contrast, bioplastics such as cellulose and starch-based plastics, which normally undergo bioassimilation rapidly when buried, could provide a practical solution when made waterproof by an impermeable biodegradable coating.

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Experience with polyolefin mulching films has more recently led to the application of programmed-life polymers in other auxiliary products for agriculture. A particularly important target is waste polyolefin stretch-wrap films for hay and silage, which are now very visible pollutants in many parts of the world. There is little economic incentive to the plastics manufacturer to make these materials biodegradable, particularly if increased cost is involved. It may be necessary for governments to legislate against the use of non-degradable stretch-wrap in farming. Fortunately, solutions are already available to deal with this problem.

17.5 Acknowledgements I am indebted to Dr Shaw-rong Yang for helpful discussions and the provision of data not previously published in readily accessible literature. I am also grateful to Dr Johann Fritz for helpful discussions and for his contribution to section 17.1.3 on auxiliary products.

17.6 References 1. Scott G (1999) `Environmental impact of polymers', Polymers and the Environment, Royal Society of Chemistry Paperback, Cambridge, Chapters 2 and 4. 2. Gilead D (1995) in Scott G and Gilead D `Photodegradable plastics in agriculture'. Scott G and Gilead D, eds, Degradable Polymers: Principles and Applications, 1st edn, Chapman & Hall (Kluwer Acad. Pub.), Chapter 10. 3. Gilead D and Scott G (1982) `Time controlled stabilisation of polymers' in Scott G, ed., Developments in Polymer Stabilisation ± 5, App. Sci. Pub. Barking, Chapter 4. 4. Fabbri A (1995) `The role of degradable polymers in agricultural systems' in Scott G and Gilead D, eds, Degradable Polymers: Principles and Applications, Chapman and Hall (Kluwer Acad. Pub.), Chapter 11. 5. Scott G and Wiles D M (2002) `Degradable carbon polymers in waste and litter control in Scott G ed. Degradable Polymers: Principles and Applications, 2nd edn, Kluwer Acad. Pub., Chapter 13. 6. Agricultural Statistics Yearbook (2001) 7. Yang S-R and Wu C-H (1999) `Degradable plastic films for agricultural applications in Taiwan' Degradability, Renewability and Recycling, 5th International Scientific Workshop on biodegradable Plastics and Polymers, Macromolecular Symposia, eds Albertsson A-C, Chiellini E, Feijen J, Scott G and Vert M, Wiley-VCH, pp. 101± 112. 8. Casaliccio G H, Bertoluzzo A and Fabbri A (1990) `Photodegradable film research ± Initial research into the possible toxic effect of photodegradability inductors on sweet corn and melons' Plasticulture 86 (2), 21±28. 9. Casaliccio G H, Bertoluzzo A and Fabbri A (1990) `Photodegradable film research ± Further research into the possible toxic effect of photodegradability inductors on potatoes and canning tomatoes', Plasticulture 87 (3), 47±53. 10. Taber H G and Ennis R (1989) `Plant uptake of heavy metals from decomposition of PlastigoneTM photodegradable plastic mulch' Proc. N.A.P.A. Conference, Orlando,

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1989, 47. 11. Scott G (1997) `Abiotic control of polymer biodegradation', Trends in Polymer Science, 5, 361±368. 12. Scott G (1995) `Photo-biodegradable plastics' in Scott G and Gilead D, eds, Degradable Polymers: Principles and Applications, 1st edn, Chapman & Hall (Kluwer Acad. Pub.), Chapter 9. 13. Yang S-R (2000) Personal communication. 14. Arnaud R, Dabin P, Lemaire J, Al-Malaika S, Chohan S, Coker M, Scott G, Fauve A and Maaroufi A (1994) `Photooxidation and biodegradation of commercial photodegradable polyethylenes'. Polym. Deg. Stab., 46, 211. 15. Guillet J (2002) `Plastics and the Environment' in Scott G, ed., Degradable Polymers: Principles and Applications, Kluwer Acad. Pub., Chapter 12. 16. Soil Association (2001) Response from the Soil Association to the Policy Commission on Farming and Agriculture, p. 10. 17. Scott G (1999) `Biodegradable Polymers', Polymers and the Environment, Royal Society of Chemistry Paperbacks, Chapter 5. 18. Groot L (1996) `Biodegradable cultivation pots', Deutscher Gartenbau, 46. 19. Fritz J (2003) Personal communication. 20. Anon (1999) Wastes Management, December. 21. Scott G (1999, May) `The role of environmentally biodegradable polymers in waste management' Wastes Management. 22. Lovel S (1999) Yorkshire Dales National Park Authority, Personal communication. 23. Chien S Y and Yang S-R, Private communication. 24. Kawai F, Shibata M, Yokoyama S, Maeda S, Tada K and Hayashi S (1999) `Biodegradability of Scott-Gilead photodegradable polyethylene and polyethylene wax by microorganisms', Degradability, Renewability and Recycling, 5th International Scientific Workshop on biodegradable Plastics and Polymers, Macromolecular Symposia, eds, Albertsson A-C, Chiellini E, Feijen J, Scott G and Vert M, Wiley-VCH, pp. 73±84. 25. Scott G (2000) `Green Polymers', Polym. Deg. Stab., 68, 1±7. 26. Kawai F, personal communication. 27. Karlsson S, Haakarainan M and Albertsson A-C (1997) `Dicarboxylic acids and ketoacids formed in degradable polyethylenes by zip depolymerisation through a cyclic transition state', Macromolecules, 30, 7721±7728. 28. Scott G (1965), Atmospheric Oxidation and Antioxidants, Elsevier, Chapters 4, 8 and 9. 29. Scott G (1993) `Autoxidation and antioxidants: historical perspective' in Scott G, ed. Atmospheric Oxidation and Antioxidants, Vol. I, Elsevier, Chapter 1. 30. Grassie N and Scott G (1985) Polymer Degradation and Stabilisation, Cambridge University Press, Chapters 4 and 5. 31. Scott G (1999) `Antioxidant control of polymer biodegradation' in Degradability, Renewability and Recycling, 5th International ScientificWorkshop on biodegradable Plastics and Polymers, Macromolecular Symposia, eds. Albertsson A-C, Chiellini E, Feijen J, Scott G and Vert M, Wiley-VCH, pp. 113±125. 32. Scott G (1993) `Oxidation and stabilisation of polymers during processing' in Scott G, ed., Atmospheric Oxidation and Antioxidants, Vol. II, Elsevier, Chapter 3. 33. Scott G (1993) `Photodegradation and photostabilisation of polymers' in Scott G, ed., Atmospheric Oxidation and Antioxidants, Vol. II, Elsevier, Chapter 8.

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34. Scott G (1997) Antioxidants in science, technology, medicine and nutrition, Albion Publishing, Chichester, Chapter 4. 35. Scott G (2002) `Degradation and stabilisation of carbon-chain polymers' in Scott G, ed., Degradable Polymers: Principles and Application, 2nd edn, Kluwer Acad. Pub., Chapter 3. 36. Scott G (1999) `Biodegradable Polymers', Polymers and the Environment, Royal Society of Chemistry Paperbacks, Chapter 5. 37. Bonhomme S, Cuer A, Delort A-M, Lemaire J, Sancelme M and Scott G (2003) `Environmental biodegradation of polyethylene', Polym. Deg. Stab., 81, 441±452. 38. Sadun A G, Webster T F and Commoner, B (1990) Breaking down the degradable plastics scam, Report for Greenpeace, Washington, DC. 39. CEN TC 249/WG9, N 95 (2003) `Terminology in the field of degradable and biodegradable polymers and plastics'. 40. Pandey J K and Singh R P (2001) `UV-irradiated biodegradability of polyethylenepropylenecopolymers, LDPE and iPP in composting and culture environments' Biomacromolecules 2, 880±885. 41. Expert Group on Vitamins and Minerals, (2003) `Part 3 Trace Elements, Risk Assessment', UK Food Standards Agency.

18

Generation of biodegradable polycaprolactone foams in supercritical carbon dioxide L Y U and K D E A N , CSIRO ± Manufacturing and Infrastructure Technology, Australia and Q X U , Zhenghou University, China

18.1 Introduction The importance of developing biodegradable polymers has been well documented in the current literature (journals, books and government discussion papers), including this book, thus it has not been repeated in this chapter. However, it should be noted today that the major barrier for growth in applications for biodegradable polymers is price. Hence, developing some high value applications (for example in the medical field) and reducing price are the key drivers in the research and development of biodegradable polymers. Foaming using supercritical CO2 offers a potential avenue to achieve these two objectives. Foamed polymeric materials are produced in a wide range of bulk densities that mainly determine their mechanical properties. A high-density foam that has an improved tensile strength and modulus can be used for load-bearing applications, such as structural parts, while a low-density foam can be used in thermal insulation and packaging applications. In addition to the foam density, the size and distribution of cells also affects the final properties of the foam. Conventional plastic foams have relatively poor mechanical properties because the cell size is typically larger than 100 m and the cell size distribution is very non-uniform.1,2 In general, foams with very fine cell size exhibit better mechanical properties. Supercritical fluids, above their critical temperature and pressure conditions, exhibit unique behavior by combining the properties of conventional liquids and gases.3 In particular, they exhibit liquid-like densities allowing for solvent power of orders of magnitude higher than gases, while gas-like viscosities lead to high rates of diffusion. A novel technique for the use of supercritical fluids as blowing agents for the generation of microcellular foams has recently been developed. It has been shown that this method can produce microcellular foam with a cell density up to 1010 per cm3 and an average cell size varying from less than 10 microns to 50 microns depending on the conditions. Several authors4±14 have reported the method using CO2 and N2 as blowing agents. Initially, the

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polymeric pellets are saturated with gas at moderate pressure (5±6 MPa) followed by heating to a temperature above the glass transition temperature of the polymer. Beckman et al.9,10 reported an analogous, yet different scheme by which to generate microcellular foams in polymers using CO2 as blowing agent; a constant-temperature variable-pressure method based on saturating the polymer with CO2 at much higher pressure (25±30 MPa), in the supercritical region of CO2, followed by a rapid pressure quench. This method takes advantage of the depression of the transition temperature of the polymers by the presence of CO2. The growth of cells is allowed by the suppression of glass transition temperature resulting from the diluent effect rather than heating the polymer to a temperature above its normal glass transition temperature. Microcellular foams are characterized by a cell size between 10 to 50 m, a cell population density greater than 109 cells/cm3, and very narrow cell size distribution. Because of these unique structures, microcellular foamed plastics offer superior mechanical properties, such as impact strength, toughness, and fatigue life when compared to an un-foamed polymer. The enhanced properties of microcellular polymers make them highly competitive in many applications such as packaging, automotive parts, aircraft parts, sporting equipment, insulation, etc., when a cost-effective, continuous manufacturing process for these materials is developed. In packaging applications, where cost is a critical issue, lowering density and improving mechanical properties are two of the best ways to reduce the price of a product. This is an important consideration for the synthetic biodegradable polymers which are currently marketed for packaging, as they are still quite expensive in comparison to more traditional polyolefins. Figure 18.1 shows a schematic representation of a supercritical fluid phase diagram. In this pressure-temperature diagram there are three lines describing the sublimation, melting and boiling process. These lines also define the regions corresponding to the gas, liquid and solid states. Points along the lines (between the phases) define the equilibrium of two of the phases. The vapor pressure (boiling) starts at the triple point and ends at the critical point. The critical region has its origin at the critical point. At this point a supercritical fluid can be defined as substance that is above its critical temperature (Tc) and critical pressure (Pc). The critical temperature is therefore the highest temperature at which a gas can be converted to a liquid by an increase in pressure. The critical pressure is the highest pressure at which a liquid can be converted to a standard gas by an increase in the liquid temperature. In the so-called critical region, there is only one phase and it possesses some of the properties of both gas and liquid. Carbon dioxide, a nontoxic fluid with a relatively low critical point (Tc ˆ 31 ëC, Pc ˆ 7.376 MPa) is the most widely used in the supercritical fluid field. Supercritical liquid CO2 is found in the triangular region formed by the melting curve, the boiling curve and the line that defines the critical pressure.15 Carbon dioxide is known to swell and significantly plasticize many amorphous polymers, such as poly(methyl methacrylate), polystyrene, polycarbonate and

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18.1 Schematic representation of a supercritical fluid phase diagram.

poly(ethylene terephthalate).2±10 The driving force for growth of bubbles using supercritical CO2 is the temperature difference, which can be controlled by manipulating glass transition temperature via changes in the CO2 pressure, instead of changing temperature by directly heating the polymer. Nucleation is induced via supersaturation caused by a sudden pressure drop from the equilibrium solution state, and the nuclei grow until the polymer vitrifies at a lower pressure. The classical homogeneous nucleation theory is not able to fully describe the nucleation activities in the temperature variation processing.6,9,16,17 Poly(-caprolactone) (PCL) is one of the representative examples for ringopening polymerization of lactones to produce synthetic biodegradable polyesters. Because of its unique combination of biocompatibility, permeability and biodegradability, PCL and some of its copolymers with lactides and glycolides have been widely applied in medicine as artificial skin, artificial bone and containers for sustained drug release.18±20 An example of microcellular foamed PCL in the medical field is guided tissue regeneration and cell transplantation. As far as guided tissue regeneration is concerned, porous implants are used as size selective membranes to promote the growth of a special tissue in a healing site. Ideally, the implant should be inherently biocompatible, have well-defined cell size and be resorbable with appropriate biodegradation rates.21 PCL is a material that meets these demands well. PCL is a biocompatible and biodegradable aliphatic polyester that is bioresorbable and non-toxic for living organisms. Packaging is one of the largest potential markets for all biodegradable polymers, but in this market it is generally price that has been the limiting factor thus far. The use of supercritical CO2 foamed PCL as a biodegradable packaging material has the potential to reduce the cost (by using less material for each packaging unit) and also to expand the number of packaging applications for PCL. Since the glass transition temperature of PCL is very low (Tg ˆ ÿ60 ëC)

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which is far below the ice point, the experimental temperature used in this work is much higher than the Tg, and it is this temperature difference that distinguishes this work from previous studies reported by other researchers. As it has been outlined, PCL foaming is of importance both commercially and scientifically. In this chapter, microcellular foaming of low-Tg biodegradable and biocompatible polycaprolactone (PCL) in supercritical CO2 will be described. The effects of a series of variable factors, such as saturation temperature, saturation pressure, saturation time and depressurization time on the foam structures and density were studied through measurement of density and SEM observation. The experimental results show that higher saturation temperatures lead to a reduction in bulk densities; and that different saturation pressures result in different nucleation processes. In addition, saturation time has a profound effect on the structure of the product. Both X-ray diffraction (XRD) and differential scanning calorimetry (DSC) results show that the foaming treatment with supercritical CO2 increased the crystallinity of PCL.

18.2 Generation of polycaprolactone foams The polycaprolactone (PCL) described in this chapter is CAPA 640 from Solvay in the form of ivory-white granules (Mn ˆ 85000, Tg ˆ ÿ60 ëC, Tm ˆ 60 ëC). Carbon dioxide with purity of 99.9% was used as blowing agent. Reactions were run in a 50.0 ml high-pressure variable-volume stainless steel reactor with two glass-viewing windows. A high-pressure syringe pump (Beijing Satellite Manufacturing Factory, DB-80) was used to charge CO2 into the reaction vessel and attached to the reactor via a coupling and high-pressure tubing. A pressure gauge consisting of a transducer (IC Sensors Co., Model 93) and an indicator (Beijing Tianchen Automatic Instrument Factory, XS/A-1) with the accuracy of 0.01 MPa was also connected to the reactor to observe in situ the pressure change of the system. Schematic representation of the set-up is shown in Fig. 18.2. In the experiments, the reactor was placed in a constant-temperature circulator, which consists of a temperature control module (Thermo Haake, C10) and a bath vessel (Thermo Haake, P5). The temperature of the bath was controlled to 0.1 ëC. Foams were prepared in a glass tube (15 mm  50 mm) inside the reactor to facilitate removal of the foamed samples. PCL was placed into the tube and the PCL and tube were then placed in the reactor together. The closed reactor was preheated in a bath to a set temperature, and flushed for a few minutes with CO2. The cell was then filled to the desired pressure. At this pressure the resin was exposed to supercritical CO2 for a prescribed period of time. Finally, the valve of the reactor was opened and the pressure was released to the atmosphere. The depressurization time was recorded in order to quantify its influence on the final product. The system was maintained at zero pressure for approximately half an hour so that the bubbles could mature completely.

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18.2 Apparatus description diagram. 1. Gas cylinder. 2. Valve. 3. Syringe pump. 4. Vent. 5. Variable-volume reactor with view windows. 6. Sample. 7. Temperature circulator. 8. Pressure gauge.

The foams were characterized to determine their densities, cell sizes and cell shapes. Density was measured using a gravity bottle with a capillary tube in its lid. The weight of the bottle filled with distilled water was measured with an analytical balance (accuracy 0.001 g) at a preset temperature. Following this, the sample was put into the bottle, water of the same volume as the sample overflowed along the capillary tube. The bottle containing both water and sample was re-weighed. The density of the sample was calculated using the following equation: w1  0 ˆ 18:1 w1 ‡ w2 ÿ w3 where  ˆ density of sample;0 ˆ density of water; w1 ˆ weight of the sample; w2 ˆ weight of bottle filled with water; w3 ˆ weight of bottle containing both water and the sample. The cell structures of foamed samples were also studied using an AMRAY1000B scanning electron microscope (SEM). The samples were prepared by freezing in liquid nitrogen, fracturing the surface, mounting the fracture on stubs with carbon paint and sputter coated with gold, forming a layer of approximately Ê in thickness. The cell size for each sample was calculated through eight 100 A diameter measurements and the average value was regarded as cell diameter. The desorption kinetics of CO2 in PCL under a particular processing condition (40 ëC, 10 MPa) was studied using Berens' method.22±24 Through these measurements, average mass gain was calculated, and the results were plotted versus the square root of desorption time. In Fig. 18.3 the non-linear relationship between mass gain and the square root of desorption time illustrates the inability of Fickian's theory to explain the desorption data. When this desorption data was plotted versus the natural logarithm of desorption time (see Fig. 18.4), a better linear dependence of mass change on the logarithm of time was obtained.

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18.3 Mass uptake of CO 2 versus the square root of desorption time. (Temperature ˆ 40 ëC, pressure ˆ 10 MPa, saturation time ˆ3 h.)

Since the growth of the cells is allowed by the suppression of glass transition (Tg) resulting from the diluent effect rather than heating the polymer to a temperature T above its normal glass transition temperature, the driving force for growth of bubbles is still temperature difference `T ÿ Tg', which is controlled by manipulating Tg through changing the CO2 pressure. Nucleation is induced by supersaturation caused by a sudden pressure drop from the equilibrium solution state and nuclei growth until the polymer vitrifies at a lower pressure.

18.4 Mass uptake of CO2 versus the natural logarithm of desorption time (seconds). (Temperature ˆ 40 ëC, pressure ˆ 10 MPa, saturation time ˆ3 h.)

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18.5 SEM photographs of cross- and longitudinal sections of PCL foams. (Temperature ˆ 40 ëC, saturation time ˆ3 h, magnification ˆ 150.) (a) crosssection, pressure ˆ 16 MPa; (b) longitudinal section, pressure ˆ 16 MPa; (c) cross-section, pressure ˆ 15 MPa; (d) longitudinal section, pressure ˆ 15 MPa.

Observation via SEM shows that the PCL foam has the typical skin-core structure. The size of the average cell is larger than 30 m. It may be caused by the fact that the barrier force encountered during the nucleation process is quite low, since PCL is in the rubbery state at these experimental conditions. Another possible explanation is that since the PCL samples are placed into a glass tube, the bubbles are prevented from escaping by the aspect ratio of the glass tube. Figure 18.5 shows the SEM micrograph of the longitudinal section and cross-section of the foamed PCL. We can easily infer that during the nucleation process bubbles cannot grow up freely in the diameter direction because of the space limitation of the tube, in contrast to the complete growth of the cells in the length direction.

18.3 Effect of processing conditions on the foaming cell 18.3.1 Effect of temperature on foam structure In addition to advantages of the supercritical fluid (SCF), such as liquid-like densities which allow its solubility to be orders of magnitude higher than gases,

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18.6 Bulk foam density as a function of saturation temperature. (Pressure ˆ 10 MPa, saturation time ˆ2 h.)

and gas-like viscosities which lead to high rates of diffusion, the plasticization effect of CO2 causing depression of the substrate's Tg is also important to the polymeric materials in the previous studies. In this work, however, due to the low Tg of the PCL (ÿ60 ëC), plasticization appears not to be a key factor. At the lower temperature of 35 ëC, foaming materials can also be obtained after long periods of time (up to 24 h), however, the overall shape change between raw and foamed PCL is not significant. When the temperature is higher (above 40 ëC) with a constant saturation pressure of 10 MPa, samples melt when saturated with CO2 and a larger expansion of the foamed material is obtained. Figure 18.6 shows the effect of temperature on the bulk density under 10 MPa pressure. It is observed that the bulk density is increased with increasing temperature in the region between 40±50 ëC. The mechanism of this phenomenon can be explained by solubility and viscosity of PLC at different temperatures. With the increase of temperature, the solubility of CO2 will be decreased according to Henry's Law, however the viscosity of the polymer will be reduced simultaneously. The intramolecular and intermolecular forces will be decreased as the material softens, so the sample can be saturated at 40 ëC and a lower pressure of 8 MPa for a short time. If the experimental temperature is lowered to 35 ëC a higher pressure of 16 MPa is required for a longer period of time. If the system is heated to a temperature of 45 ëC or higher, the diameter of the bubbles can grow up to one centimeter in size during depressurization, however the cells will collapse due to the weight of their walls. Figure 18.7 shows the effect of temperature on the size of the cells. It is seen that the cell size increases with increasing saturation temperature in the region of 40 to 50 ëC. When the temperature is higher than 55 ëC, the cell size starts to decrease with increasing temperature.

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18.7 Effect of temperature on cell size (10 MPa pressure; 3 h).

This unique property gives the supercritical CO2-assisted foaming a promising direction for new applications: when the exterior or interior sizes of the product are closely restricted (such as artificial skins or bones, etc.), rough casts can be shaped first and then foamed at low temperatures; when the product has no strict restriction on geometrical sizes (such as drug-release systems, drug containers, etc.), the supercritical CO2-assisted foaming can be carried out at relatively high temperatures.

18.3.2 Effect of pressure on foam structure The effect of saturation pressure on the foaming structure at 40 ëC was studied in detail and the experimental results are shown in Figs 18.8 and 18.9. Figure 18.8 shows the SEM photographs of cross- and longitudinal sections of PCL foams at different pressures. Equivalently, the foam density is plotted in Fig. 18.9. It is seen that the cell size first increases with saturation pressure from 8 MPa until 14 MPa, then decreases sharply from 14 MPa to 16 MPa. As shown by the homogeneous nucleation theory, when the magnitude of the pressure drop increases, the energy barrier to nucleation decreases, leading to more cells nucleated in a given volume. Therefore the average cell size decreases with the increase of saturation pressure from 14 to 16 MPa. This homogeneous nucleation theory was also verified by Beckman.9 In the lower pressure range, the pressure release gradient is less than that of high saturation pressure range (14~16 MPa), and the effect of this release gradient is minimal when compared with the energy barrier to nucleation. At the same time, fluid density as well as the amount of CO2 in the system is crucial, larger CO2 bubbles were formed in the saturated PCL melt at higher pressure and not at the lower pressure.

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18.8 SEM photographs of cross-sections of PCL foams formed under different saturated pressure. (Temperature ˆ 40 ëC, saturation time ˆ3 h, magnification ˆ 150/50.) Top: 14 MPa; center: 15 MPa; bottom: 16 MPa.

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18.9 Average cell size as a function of saturation pressure. (Temperature ˆ 40 ëC, saturation time ˆ3 h.)

18.3.3 Effect of saturation time on foam structure The effect of saturation time on foaming structure was studied at a fixed pressure (10 MPa) and temperature (40 ëC). The experimental results are shown in Fig. 18.10, showing that the average cell size changes slightly at the beginning from 1.5 h to 3 h. Cell size then increases sharply from 3 h to 4 h, and decreases sharply from 4 h to 8 h. It should be noted that the time of saturation represents the time that the PCL sample is exposed to the high-pressure CO2 prior to the pressure quench, and has no relation to the rate of pressure quench, which controls the time period for nucleation and growth. Although pressure quench is a key factor in the resulting foam structure, our study indicates that saturation time also has a contribution to the foam structure. A similar phenomenon was also reported by Beckman9 in which longer exposure to the high-pressure CO2 caused greater absorption of the CO2 by the polymer. In the work presented here, the average cell sizes (see Fig. 18.10), measured from the SEM micrographs were taken in the central region of the samples, i.e., the influence of concentration gradient across the sample thickness was excluded. So only the influence of saturation time on the foam structure was considered in cell size calculation. It should be noted that there is a peak of cell size for both saturation pressure (see Fig. 18.9) and saturation time (see Fig. 18.10) at a particular temperature. When the pressure is lower or saturation time is shorter, increasing pressure or time will increase the amount of CO2 diffused into PCL, which results in larger

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18.10 Density of the bulk foams as a function of depressurization time. (Temperature ˆ 40 ëC, pressure ˆ 10 MPa, saturation time ˆ2 h.)

cell sizes. However, since there is a limitation to the melt strength of all materials, there is a maximum bubble size that materials can produce. Above this maximum size the bubble will collapse. The maximum size depends upon many factors, such as temperature, pressure and the physical and chemical variations in the materials during processing (crosslinking or decomposition, etc.). The experimental results indicate that there exists an optimum temperature to achieve a maximum expansion ratio for each temperature of polymer melt. Since high pressure and longer saturation time both decrease the Tg, which has a similar effect to controlling temperature, a study of the effect of temperature on the expansion and cell structure is important. Figure 18.11 is a schematic representation of the effect of temperature on the bulk density and cell size. There is a maximum point for the expansion. When the temperature is very low (below 35 ëC), the diffusion rate of the CO2 into the polymer is also very low so the polymer has little chance of foaming. Even under long saturation times when there is enough absorbed CO2, the foamed products are uniform because of melt fracture. The melt fracture phenomenon has also been observed during extrusion foaming.25 When the temperature is too high (above 55 ëC), the bubbles start to escape through the hot skin layer of the foam during expansion. The bulk density and cell size have a close relationship even though they do not correspond directly. There is a maximum point of bulk density and cell size. The volume expansion was strong as a function of both pressure and temperature at certain saturation times. When the temperature was lower than 30 ëC the cell size was small and the bulk density was higher, thus it may be concluded that the cell size

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18.11 Schematic representation of foaming at various temperatures.

will increase and bulk density will decrease with increasing temperature and saturation time.

18.3.4 Effect of depressurization rate on foam structure A series of experiments were carried out at 40 ëC and 10 MPa using a number of different depressurization rates. The experimental results are given in Fig. 18.12 and show that the bulk foam density increases with increasing depressurization time. It may be deduced that prolonging the depressurization time ensured the cells more time to contract at fixed sites, thus decreasing the bulk volume. Hence the bulk density increases with depressurization time.

18.12 Average cell size as a function of saturation time. (Temperature ˆ 40 ëC, pressure ˆ 10 MPa.)

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Since microstructure influences the performance of materials it is necessary to control the nucleation step that determines the cell size and size distribution of the foam. The models used for studying nucleation are classified as homogeneous nucleation and heterogeneous nucleation. The homogeneous nucleation models17,26,27 were built around the chemical nucleation theory. In short, the theory looks at the relative rate of obtaining a non-stable cluster of foaming phases over an activation barrier that is defined by the phase equilibrium and surface tension. The homogeneous nucleation models were used to interpret the negligible nucleation rate up to a gas saturation pressure of about 5,000 psi (34.5 MPa). However, true homogeneous nucleation is a difficult phenomenon to effect even in a laboratory utilizing temperature variation processing methods. Furthermore, homogeneous nucleation is an inherently random process and it may not be considered a foaming mechanism. Kumar and Suh28 have concluded that the cell nucleation phenomenon in polystyrene is not well described by the homogeneous chemical nucleation theory. The inadequacies of simple nucleation theory have been addressed by Kweeder et al.,8 by hypothesizing the existence of a population of preformed microvoids in the system around which nucleation takes place. Some heterogeneous nucleation methods8,9,29 have been developed to obtain a controlled, predictable nucleation mechanism resulting in the desired microstructure. However, the use and modeling of this principle relies on a good understanding of the heterogeneities introduced. Agreement with experimental data depends largely on how the physical system agrees with the model's assumptions. Campbell et al.8,29 developed a model to predict the heterogeneous nucleation through studying the effect of thermal and pressure history on the microdamage in polycarbonate and polystyrene. It was shown that samples cooled quickly under low pressure exhibited markedly more microscopic cracks. Under a constant-temperature variable-pressure processing, Beckman et al.9 have shown that the classical nucleation theory can be used to describe the nucleation under higher pressure conditions. In this constant-temperature variable-pressure processing, the polymer samples were swollen by supercritical CO2 over a sufficiently long period of time to ensure that the amount of liquid absorbed by the polymer was at equilibrium. The amount of supercritical CO2 absorbed is sufficient to reduce the Tg of the polymer to below the ambient temperature, generating a liquid, albeit concentrated, polymer solution. Quick reduction of the pressures at constant temperature generates both the pores and drives the system towards vitrification, freezing ± in the microstructure. As processing is carried out at a CO2 pressure much higher than needed to plasticize the polymer at the operation temperature, the system is in a homogeneous liquid state. Therefore, it has been shown that the classical nucleation theory can be used to describe the nucleation activity under these conditions. The homogeneous nucleation theory has also been used to study the nucleation of PMMA.

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18.4 Crystallinity of foamed polycaprolactone 18.4.1 Crystallinity of the foamed PCL One of the main purposes for developing the microcellular foams is their potential for applications in medicine, for example the containers for sustained drug delivery and as the raw material for artificial skin and bones. Apart from its biocompatibility with the body, its stability and biodegradability during the period of application are also important. Previous studies have shown that molecular weight and crystallinity are the dominant factors affecting the biodegradability of PCL30,31 and that the amorphous part of PCL degrades prior to the crystalline part in a biotic environment.32,33 XRD and DSC can be used to study the crystallinity of virgin and microcellular PCL. The X-ray diffraction (XRD) measurements were performed using a Bruker D8 Diffractometer operating at 40 kV, 40 mA, Cu K radiation monochromatized with a graphite sample monochromator. A diffractogram was recorded between 2 angles of 12ë and 45ë. The amorphous region of the sample was approximated using a Gaussian fit as illustrated in Fig. 18.13. The area of the amorphous region was subtracted from the total area of the diffractogram to give percentage crystallinity in the materials that were investigated. The XRD results for the virgin and foamed PCL are shown in Fig. 18.14 and summarized in Table 18.1. The XRD results indicate a significant increase in crystallinity (compared to the virgin PCL) for both the foamed systems, with an increase of 42.00% and 42.86% for the foamed PCL-1 (treated under 10 MPa for 2 h) and foamed PCL-2 (treated under 15 MPa for 3 h), respectively.

18.13 XRD diffractogram of foamed PCL and the Gaussian fit used to approximate the amount of the amorphous region in the sample.

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18.14 XRD results of: A, original PCL; B, original PCL melted and quenched rapidly; C, foamed PCL (pressure ˆ 10 MPa); D, foamed PCL (pressure ˆ 15 MPa); E, foamed PCL (pressure ˆ 15 MPa), re-melted and quenched.

The crystallinity of these materials, as measured by XRD, is in good correspondence to the data obtained via DSC. A Perkin-Elmer Pyris-1 DSC with internal coolant (Intracooler 1P) and nitrogen purge gas was used. Melting point and enthalpies of indium and zinc were used for temperature and heat capacity calibration. Samples were heated from 20 ëC to 100 ëC at 10 ëC/min to measure the heat capacity used for evaluation of crystallinity. The foamed samples were cooled down from 100 ëC then heated again under the same conditions to confirm the effect of foaming on crystallinity. The variation of relative crystallinity was calculated by: X ˆ …HX =Hv †  %

18:2

in which HX is the heat capacity of formed samples and Hv is the heat capacity of virgin PCL materials. Table 18.1 XRD results showing the effect of foaming on the crystallinity in the various PCL systems Sample Virgin PCL (as received) Virgin PCL (remelted and quenched) Foamed PCL-1 Foamed PCL-2 Re-melting-2

Crystallinity (%)

Variation of crystallinity (%)

42.0 35.0 49.7 50.0 30.7

ö ö ‡42.00 ‡42.86 ö

The PCL was foamed at 40 ëC. Foamed PCL-1 was treated for 2 h at 10 MPa and foamed PCL-2 was treated for 3 h at 15 MPa.

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18.15 DSC curves of virgin and foamed PCL. (Temperature ˆ 40 ëC, pressure ˆ 10 MPa, saturation time ˆ2 h.)

Figure 18.15 shows DSC curves of virgin and a foamed PCL (foamed PCL-1, treated at 40 ëC and 10 MPa for 2 h). It is seen that both onset temperature and melting are increased after supercritical CO2 foaming. This gives a strong indication that the crystal structure has been improved or that the thickness of crystalline lamellae has been increased. Some detailed results of crystallinity are listed in Table 18.2. The data in this table shows clearly that relative crystallinity is increased significantly after foaming. This increase of crystallinity can be explained by the orientation of polymer chains during foaming.34,35

18.5 Conclusion PCL foam can be produced in supercritical CO2. Various factors affecting the foaming structure have been studied in detail. The experimental results indicate that there exists an optimum temperature to achieve a maximum expansion ratio of each polymer melt. Since high pressure and longer saturation time both decrease the Tg, and in effect are similar to controlling the temperature, quantification of Table 18.2 Effect of SC CO2 treatment on thermal behaviors of PCL Sample

Virgin PCL Foamed PCL-1 Re-melting-1 Foamed PCL-2 Re-melting-2

Onset temperature (ëC)

Melting temperature (ëC)

Heat capacity (J/g)

Variation of crystallinity (%)

52.19 54.36 52.36 55.43 52.23

54.28 57.80 55.10 59.30 54.60

41.68 60.80 43.65 61.60 43.73

ö ‡45.87% ‡47.98%

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the effect of temperature on the expansion and cell structure is important. A maximum point of bulk density and cell size was found. The volume expansion was a function of both pressure and temperature under certain saturation times. When the temperature was lower than 30 ëC the cell size was small and bulk density was higher, thus generally the cell size could be increased and bulk density decreased by increasing both the temperature and saturation time. A peak of cell size was also found for both saturation pressure and saturation time at particular temperatures. When the pressure was lower or the saturation time was shorter, increasing pressure or time increased the amount of diffused CO2 into PCL, which resulted in larger cell sizes. However, since there is a limitation to the melt strength of all materials, there is a maximum bubble size that materials can produce. Above this maximum size the bubble will collapse. Under constant-temperature, variable-pressure processing it has been shown that the classical nucleation theory can be used to describe the nucleation process under higher pressure conditions. In this constant-temperature, variablepressure processing, PCL was swollen by supercritical CO2 over a sufficiently long period of time to ensure that an equilibrium amount of liquid was absorbed by the polymer. The amount of supercritical CO2 absorbed was sufficient to reduce the Tg of the polymer to below ambient temperature, generating a concentrated liquid polymer solution. Quick reduction of the pressures at constant temperature generates both the pores and also drives the system towards vitrification, freezing ± in the microstructure. Since the processing is undertaken at a CO2 pressure much higher than that needed to plasticize the polymer at the operating temperature, the system is in a homogeneous liquid state. Therefore, it has been shown that classical nucleation theory can be used to describe the nucleation activity under these conditions. The crystallinity of PCL was increased significantly after foaming as shown by both XRD and DSC. This was promising for improving biodegradability, as previous studies have shown that crystallinity was one of the dominant factors affecting the degradation of PCL.30,31 Furthermore, understanding the complex nature of the structure/property relationships of microcellular PCL foams produced using supercritical CO2, is essential in controlling their formation and unlocking their potential for applications in both medicine and other high performance areas.

18.6 References 1. Klempner D and Frisch K C, Handbook of Polymeric Foams and foam Technology, Hanser, New York, 1991. 2. Throne J, Science and Technology of Polymer Process, Suh N P and Sung N eds, pp. 77±131, MIT Press, Cambridge, Mass, 1979. 3. McHugh M A and Krrukonis V J, Supercritical Fluid Extraction, Butterworths, Stoneham, Mass. 1986.

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4. Aubert J H and Clough R L, `Low density microcellular polystyrene foams', Polymer, 1985, 26, 2047. 5. Young A T, `Polymer-Solvetn Phase Separation as a Route to Low Density, Microcellular Plastic Foams', J. Cell. Plast., 1987, 23, 55. 6. Kumar V and Weller J E, `Microcellular polycarbonate ± Part I: experiments on bubble nucleation and growth', SPE ANTEC Tech. Papers 1991, 37, 1401. 7. Kumar V and Van Der Wel M M, `Microcellular polycarbonate ± Part I: characterization of tensile modulus', SPE ANTEC Tech. Papers 1991, 37, 1406. 8. Kweeder J A, Ramesh N S, Campbell G A and Rasmussen D H, `The Nucleation of microcellular polystyrene foam', SPE ANTEC Tech. Papers 1991, 37, 1398. 9. Goel S K and Beckman E J, `Generation of Microcellular Polymeric Foams Using Supercritical Carbon Dioxide. I: Effect of Pressure and Temperature on Nucleation', Polym. Eng. Sci., 1994, 34, 1137. 10. Goel S K and Beckman E J, `Generation of Microcellular Polymeric Foams Using Supercritical Carbon Dioxide. II: Cell Growth and Skin Formation', Polym. Eng. Sci., 1994, 34, 1148. 11. Baldwin D F, Park C B and Suh N P, `A Microcellular Processing Study of Poly(ethylene Terephthalate) in the Amorphous and Semi-Crystalline States: Part I Microcell Nucleation', Polym Eng. Sci., 1996, 36, 1437. 12. Baldwin D F, Park C B and Suh N P, `A Microcellular Processing Study of Poly(ethylene Terephthalate) in the Amorphous and Semi-Crystalline States: Part II Cell Growth and Process Design', Polym Eng. Sci. 1996 36, 1446. 13. Arora K A, Lesser A J and McCarthy T J, `Compressive Behaviour of Microcellular Polystyrene Foams Processed in Supercritical Carbon Dioxide', Polym. Eng. Sci., 1998, 38, 2055). 14. Taylor L T, Supercritical Fluid Extraction, John Wiley & Sons, New York, 1995. 15. Ramesh N S, Rasmussen D H and Campbell G A, `Nucleation and Experimental Studies of Bubble Growth during the Microcellular Foaming Process', Polym. Eng. Sci., 1991, 31, 1657. 16. Colton J S and Suh N P, `The nucleation of Microcellular Thermoplastic Foam with Additives: Part I: Theoretical Considerations', Polym. Eng. Sci., 1987, 27, 485. 17. Pitt C G, Marks T A and Schindler A, `Biodegradable Drug Delivery Systems Based upon Aliphatic Polyesters: Application to Contraceptives and Narcotic Antagonists', in Controlled Release of Bioactive Materials, R Baker (ed.) Academic Press, New York, 1980. 18. Vert M., `Biomedical polymers from chiral lactides and functional lactones: properties and applications', Makromol.Chem., Macromol. Symp., 1986, 6, 109. 19. Gogolewski S and Pennings A., "Biodegradable Materials of Polyactides, Porous Biomedical Materials Based on Mixtures of Polyactides and Polyurethanes", Macromol. Chem., Rapid. Commun., 1982, 3, 839. 20. Popove V K, Mandel F S and Howdle S M, `Supercritical fluid assisted production of synthetic bone composites', in Proceedings of the 5th meeting on supercritical fluids, Nice, France, 1998. 21. Sparacio D and Beckman E J, `Generation of microcellular biodegradable polymers in supercritical carbon dioxide' Polym. Preprint, 1997, 2, 422. 22. Berens A R, Huvard G S, Korsmeyer E W and Kunig F W, `Application of compressed carbon dioxide in the incorporation of additives in to polymers', J. App. Polym. Sci., 1992, 46, 231±242.

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23. Liang M T and Wang C M, `Production of Engineering Plastics Foams by Supercritical CO2', Ind. Eng. Chem. Res., 2000, 39, 4622. 24. Liang M T and Wang C M, `Production of very low density microcellular polypropylene by supercritical carbon dioxide', in Proceedings of the 6th meeting on supercritical fluids, Nottingham, United Kingdom, 1999. 25. Park C B, Behravest A H and Venter R D, `Low density microcellular foam processing in extrusion using CO2' Polym. Eng. Sci., 1989, 38 (11), 1812±1823. 26. Martini J E, Waldman F and Suh N P, `The Production and Analysis of Mirocellular Thermoplastic Foams', Processing of SPE ANTEC-82, 1982, 28, 674. 27. Youn J R and Suh N P, `Processing of Microcellular Polyester Composites', Polymer Composites, 1985, 6, 175. 28. Kumar V and Suh N P, Proceedings of ANTEC-88, 1988, 715-718. 29. Adams M E, Campbell G A and Cohen A, `Thermal-stress induced damage in thermoplastic matrix materials for advanced composites', Polym. Eng. Sci., 1991, 31 (18): 1337±1343. 30. Berens A R, Huvard G S, Korsmeyer R W and Kunig F W, `Application of compressed carbon dioxide in the incorporation of additives into polymers', J. Appl. Polym. Sci., 1992, 46, 231. 31. Berens A R and Huvard G S, `Supercritical Fluid Science and Technology', Johnston K P, Penniger J M L (eds). ACS Symposium Series 406, American Chemical Society, Washington, DC (1989). 32. Benedict C V, Cameron J A and Huang S J, `Polycaprolactone degradation by mixed and pure cultures of bacteria and a yeast' J. Appl. Polym. Sci. 1983, 28, 335. 33. Field R D, Rodriguez F and Finn R K, `Microbial degradation of polyesters: Polycaprolactone degraded by P. pullulans' J. Appl. Polym. Sci. 1974, 18, 3571. 34. Nadella H P, Spruiell J E and White J L, J. Appl. Polym. Sci., 1975, 18, 2539. 35. Yu L, Shanks R A and Starchurski Z H, `Kinetics of polymer crystallization;, Polym. in Progress, 1995, 20, 651±702.

19

Biodegradable polymers in agricultural applications S G U I L B E R T , ENSA.M, INRA, France, P F E U I L L O L E Y , CEMAGREF, France, H B E W A , ADEME, France and V B E L L O N - M A U R E L , CEMAGREF, France

19.1 Introduction Since the 1950s, the use of plastics for agriculture has been booming. Between 1990 and 1999, the worldwide consumption increased by 62% and by 185% since 1985, to reach a volume estimated at 2,850,000 tons (Jouet, 2003) in 2003. This consumption only represents 4% of the global plastic consumption. The above volume generates a similar but often higher quantity of wastes that are difficult to dispose of by farmers. They have to comply with several obligations: · Legal obligations: several texts are published in the EU to regulate waste disposal. The most important is the Directive 75/442 (1975) modified by the Directive 91/156 (1991), both texts regulate waste elimination (recycling or incineration with energy recovery). In France, burning in the open and burying are banned. Only composting is allowed. · Technical obligations: plastics as mulching films or low tunnels films are dirty (70% of earth on films). Consequently, collecting, transportation and washing are costly for the farmer or for the industrial company. · Environmental obligations: farmers are now conscious of the environmental impact of plastics. Plastics create visual pollution, physical pollution (clogging of machines) and mortal risks for the fauna in the case of ingestion. Farmers do not want to be responsible for this pollution. This situation leads to a deadlock for agricultural plastic disposal. One way of solving this difficulty is to use biodegradable plastics. Such materials exist in nature (starch, cellulose, and proteins, etc.) or can be made using biotechnologies (fermentation) or are synthesised from petroleum sources. In addition to their biodegradability, these materials have the following advantages: · They are recyclable, incinerable and compostable. · They are often made up from agricultural sources and certainly offer a diversification and a new market for farmers.

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· Their energetic balance is positive: their life cycle needs three or four times less energy than polyolefin. Therefore, they limit CO2 emissions. · They have a favourable impact on the public who are motivated for the environment respect.

In agriculture, mulching films, low tunnel films and accessories (clips, twines, etc.) are difficult to eliminate. It is possible to replace them by biodegradable materials that can be composted at the farm or left on site to be biodegraded in the soil. In these cases the farmer does not need to collect them, transport them or pay for elimination. In agriculture, as well as in other sectors, it is estimated that 15% of the plastic market is substitutable by biodegradable materials. Technologies, uses and the market for biodegradable materials are rapidly moving forward. Biotechnologies and genetic engineering will revolutionise the design and production of biodegradable materials to be competitive with or even at lower cost than polyolefin ones. Materials based on natural fibres (flax, hemp, paper, etc.) have also a promising future.

19.2 Materials applied in agriculture 19.2.1 Principal applications in agriculture Due to their high cost, actual applications of biodegradable plastics are commercialised in special niches with environmental considerations. Loose fill packaging and compost bags are the two major end uses, constituting nearly 60% of demand in 2003 but applications in the field of agriculture are expected to be dominant in the near future. Applications of bioplastics can be classified in three categories (Guilbert, 1999): 1. 2. 3.

Plastics to be composted or recycled (fields where reuse or fine recovery are difficult). Plastics used in natural environment (fields where recovery is not economically or practically feasible). Specialty plastics (fields with specific features where bioplastics possess preferential properties).

The two last categories are relevant for materials for agriculture. The principal applications in agriculture are listed in Table 19.1. Among the agricultural applications listed above, mulching is the main application for biodegradable materials (Feuilloley, 1999). Mulching is a current operation that consists in covering the soil with a film in order to improve cultural conditions, namely: · Protection against weeds and cost reduction for weed killing · Limitation of moisture evaporation · Protection against plant disease

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Table 19.1 Applications of bioplastics in agriculture (Guilbert, 1999) Plastics used in natural environment (no recovery) Biodegradable/soluble/ controlled release materials for agriculture and fishery Agricultural engineering

Mulching plastic, films for banana culture, twines, nursery pots, etc. Materials for controlled release fertilisers or agrochemicals High water retention materials for planting Soluble sachets, biodegradable containers for fertilisers or agrochemicals Fishing lines and nets, etc. Retaining walls or bags for mountain areas or sea, protective sheets and nets for tree planting, etc.

Speciality ingredients or materials Edible films/coatings Matrix for controlled release systems Super-absorbents

· · · ·

Films, non woven tissues, etc. Barrier internal layers, surface coatings, `active' superficial layers Soluble sachets for instant dry/soft agrochemicals or additives formulations, etc. Slow release of fertilisers, agrochemicals, additives, etc. Material for plant planting in desert, etc.

Protection of fruits and vegetables against dirtiness Protection of soil against rain, erosion, splashing, washing, compaction, etc. Soil heating for earlier production (out of season crops) Improvement of luminosity with white mulches (for strawberries, vineyards, etc.)

Another application, under development, is the low tunnel technique. These low tunnels, the diameter of which is about 1m, are set over mulch in order to increase the mulch effect by a greenhouse effect. They are made of transparent plastic and fixed on the soil using small iron arches. The function of these low tunnels is similar to a greenhouse. Typically, melons are cultivated under these structures. In 2003 (Jouet, 2003), the above applications accounted for 12,860,000 ha (670,000 t of plastic) for mulching and 900,000 ha (170,000 t of plastic) respectively for low tunnels in the world. A third application, which is similar to mulching, is the use of direct covers. These films cover directly the canopy in order to protect it against meteorological hazards (storms, hail, and frost, etc.) but also against insect attack. Tests of direct covers made of biodegradable materials are ongoing. This application accounts for 105,000 ha (48,000 t of plastic) in the world. A last application involves accessories such as (i) twines for baling hay and straw, (ii) twines and clips to tie in plants (tomatoes, cucumbers, etc.) cultivated

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under greenhouses, (iii) nets, bags, plant pots, etc. Presently the world consumption for plastic twines is 195,000 t. For the accessories used under greenhouses the consumption is estimated at 600 kg/ha per year. Biodegradable twines and plant pots are still commercially available. Other future applications could be related to buried micro irrigation systems and to coating materials (seeds, fertilisers, etc.). In order to fulfil the requirements for agricultural applications described above, two main groups of biodegradable materials are available. The first group consists of fibrous materials such as paper and non-woven materials. These fibrous materials are abundant on the market and they present a large grade of functionality. The principal limitation of their usage is due to their relatively high price. Biodegradable thermoplastic materials or `bioplastic' represents the second group. These materials come from various sources as described below.

19.2.2 Origin of existing materials Various polymers, either synthetic or those obtained from products or byproducts from agricultural origin, were proposed for the formulation of thermoplastic biodegradable materials, films and coatings. These polymers (polysaccharides, proteins and lipids or polyesters) can be used alone or as a blend or mixture (see Fig. 19.1). They can be used in various forms (coating, simple or multi-layer film, 3-D items, simple materials, mixtures, blends and composites). The materials obtained from agro-polymers (i.e. from agricultural raw material) are fully renewable and biodegradable (apart from when some very severe chemical modifications are applied). They are generally non-ecotoxic for the soil and the environment. Synthetic bioplastics are not issued from renewable raw materials and, for some of them, problems of eco-toxicity are reported. Conventional `additived' plastics such as polyethylene additived with abiotic degradation catalysts are not considered in this inventory since they are `fragmentable' and not biodegradable according to the most recognised definitions and standard tests of biodegradation. It must be outlined that agricultural products can be a major source for bioplastics, using three different techniques (Guilbert, 1999): 1.

2. 3.

Agricultural polymers (polysaccharides or proteins) can be extracted and eventually purified. They can be used alone or in a mixture with a synthetic biodegradable polymer such as polycaprolactone or other synthetic biodegradable polyesters (e.g. blends of thermoplastic starch and polyesters). Agricultural products can be used as fermentation substrates to produce microbial polymers (e.g. polyhydroxyalcanoates). Agricultural products (or by products) can be used as fermentation substrates to produce mono or oligomers which will be polymerised by

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19.1 Main polymers proposed for the formulation of biodegradable thermoplastic materials.

conventional chemical processes (e.g. polylactic acid obtained by polymerisation of natural lactic acid produced by fermentation of corn).

19.2.3 The formulation The formulation of a bioplastic implies the use of at least one component able to form a matrix having sufficient cohesion and continuity. These polymers have the property to form crystalline or amorphous continuous structure under the conditions of preparation used. Only the polyesters, polysaccharides or the proteins are usable for making of `materials'. In order to improve the properties of the material, the support can be covered with a coating. The matrix is made of various types of materials Starch is the most commonly used agricultural raw material. Starch is inexpensive, widely available and relatively easy to handle. `All-starch' bioplastics are made from thermoplastic starch and formed with standard techniques for synthetic polymer films such as extrusion or injection moulding. The use of

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thermoplastics proteins was also investigated (Gontard and Guilbert, 1994, Guilbert and Cuq, 2005) but commercial applications are still being developed. Among proteins, milk proteins (casein, whey proteins), soya proteins and cereal proteins (wheat gluten, zein, etc.) have been more extensively studied (Gennadios et al., 1994, Guilbert et al., 2002, Redl et al., 2001). This type of material, based on hydrocolloids is generally not very resistant to water and their moisture barriers properties are poor. In some cases, water solubility or sensitivity to water is a functional advantage, e.g. for the formulation of soluble sachets to carry chemicals such as fertilisers or pesticides. For the majority of uses, the improvement of water resistance and water barrier properties is of first importance. Chemical modification of biopolymers and development of specific additives (cross linking agents or plasticisers) adapted to the polymer structure are then proposed. Regarding these developments, protein rich materials which have a `non monotonous' complex structure with very large potential functional properties are promising (Cuq et al., 1998). Commercial water-resistant starch based bioplastics are produced by using fine molecular blends of biodegradable synthetic polymers and starch. These materials are made with gelatinised starch (up to 60±85%) and hydrophilic synthetic polymers (e.g. ethylene vinyl alcohol copolymer) or hydrophobic synthetic polymers (e.g. polycaprolactone or Ecoflex Õ) and compatibility agents (Fritz et al., 1994). The most important starch based material on the market is proposed by Novamont as Mater-BiÕ. Microbial polymers (e.g. poly(3)-hydroxybutyrate-hydroxyvalerate) are excreted or stored by micro organisms cultivated on starch hydrolysates or lipidic mediums. Isolation and purification costs could be high for those products that are obtained from complex mixtures. Monsanto stopped the commercialisation of its product BiopolÕ in 1999. Since then, production has been low but some new producers are entering the market (e.g. Coopeazucar in Brazil which has built new facilities for a pilot plant production of these polyhydroxyalkanoates). Polylactic (and polyglycolic) acids are mainly produced by chemical polymerisation of lactic acid (and glycolic) acid obtained by Lactobacillus fermentation. Commercial applications of polylactic acid materials are growing up very rapidly under the trade marks of EcoplaÕ from Cargill/Dow Chemical or LaceaÕ from Mitsui. Synthetic biodegradable polyesters are produced by the major chemical companies such as Basf (EcoflexÕ), Eastman (EcostarÕ), Showa Denco (BionolleÕ) and Solvay. Thermoplastic biodegradable materials are sometimes formulated with paper, fibres or fibrous materials to form composites with optimised properties. The coating can be applied using other types of materials Bio-plastics, which have been proposed to formulate coatings, are numerous (Guilbert and Cuq, 1998, Cuq et al., 1995). Polysaccharide, protein or lipid

500

Biodegradable polymers for industrial applications

materials being used in various forms (simple or composite material, singlelayer or multilayer film). The polysaccharides used for material formulation are generally the same ones as those used as stabilising, thickening and gelling agents. These polysaccharides are of various origins: plant polysaccharides such as cellulose and derivatives, starches and derivatives, pectin or arabinoxylanes, algae gums such as alginates or carraghenanns and microbial gums such as pullulan, xanthan and gellan. Many plant and animal proteins have also been studied as raw material for films and coatings that are generally characterised by highly interesting functional properties (Guilbert and Cuq, 2005). Lipids and derivatives are used for their good water barrier properties and their low cost. The use of a polysaccharide or protein based matrix or support is generally advised. A few examples of applications to improve product appearance or conservation include wax, oil or fat coating for seeds. The application of a coating is an easy way to structurally strengthen certain products, to reduce particle clustering, to improve visual and tactile features on product surface and finally to improve shelf life and biological activity of seeds. Polymers with substantial gas and moisture barrier properties are required for many applications: to control gas exchange for plant leaves, to reduce moisture exchange with external atmosphere, etc. Retention of specific additives (agrochemicals, fertilisers, pesticides, etc.) in coatings can lead to a functional response confined to the product surface and to modification and control of `surface conditions'. This concept can be applied to the formulation of contact fertiliser or pesticide. Applied to the bulk product the use of a bioplastic to form a matrix for the controlled release of fertilisers is also very promising. In both cases the control of chemical retention and release is a problem of control of mass transport inside a biopolymer matrix combined with its biodegradation. Then the design of the material structure (molecular mass, cross-linking or entanglement degree, biodegradation speed, etc.) is of first importance.

19.2.4 Process used for building up biodegradable plastics Two general process pathways for bio-plastic material are distinguished: 1. 2.

The `dry process' such as thermoplastic extrusion which is based on thermoplastic properties of biopolymers when plasticised and heated above their glass transition temperature under low water content conditions. The `solvent process' or casting which is based on the drying of a filmforming solution or dispersion.

The casting process is used to form cast preformed films or applied to coatings directly on products (Guilbert and Cuq, 1998). This process is generally adapted for coating seeds or preparing fertiliser solutions (see below). Heat processing of agro-polymer based materials by techniques usually applied for synthetic

Biodegradable polymers in agricultural applications

501

thermoplastic polymers (extrusion, injection, moulding, etc.) is more costeffective. This process is often applied for making flexible films (e.g. mulching films for agricultural applications, packaging films, and cardboard coatings) or objects (e.g. biodegradable materials) that are sometimes reinforced with fibres (composite bioplastics). The material characteristics (polysaccharide, protein, polyester, plasticised or not, chemically modified or not, used alone or in combination) and the fabrication procedures (casting of a film forming solution, thermoforming) must be adapted to each specific agricultural product and usage condition (irrigation, temperature, cultivation duration, etc.). To conclude, commercial applications of these materials are growing up very rapidly. But because they are complex to develop and sometimes to produce, the total production of `modern thermoplastic bioplastics' is actually low with a total world consumption in 2004 of only of 70,000 to 90,000 t. Large-scale application is not therefore possible. This value is still inferior to the production capacity of `bioplastics' which is around 300,000 t.

19.3 Evaluating properties of biodegradable materials in agriculture 19.3.1 Properties related to biodegradability The biological degradation of polymers can be characterised by several laboratory degradation tests (Grima, 2002, Domenek et al., 2004). These tests are generally standardised (DeÂcriaud et al. 1998) but tests in real conditions can also be performed and consist of burying specimens in agricultural soils. A lot of data now exist concerning the biodegradation of polymers according to the above measurement methods (DeÂcriaud, 1998). For instance as a part of an international project (Labelling biodegradable products, 2002), ring tests have been performed on 20 materials proposed on the market and several methods have been used and tested, namely: · Laboratory methods The Sturm test (respirometric) Other respirometric tests (BODIS and head space tests) The anaerobic test (respirometric) The enzyme test (non respirometric) The controlled composting test (respirometric) The result is a coefficient of biodegradation (% of mineralisation of C). · Other testing methods under real life conditions Composting experiments in 140 l composter bin. Certain parameters such as temperature, pH and moisture content were regularly monitored Soil burial tests in soil containers of 1 l Soil burial tests in real agricultural soil (in situ)

502

Biodegradable polymers for industrial applications

The result is a coefficient of degradation (in general a percentage of mass losses). A short description of these methods is given in Table 19.2. Figure 19.2 gives a synthetic view of biodegradation rates for three materials using the above methods (see Table 19.2). Material A is made of polycaprolactone+starch, Material B is made of aromatic-aliphatic polyester, and Material C is made of polyethylene additivated by pro-oxidants. Biodegradation rate varies from zero to 100%, whatever the method (except in real soil). Material A degrades better than B, and B degrades better than C. For Materials A and B, the anaerobic test gives a significantly lower degradation rate than the aerobic tests do. This is not surprising, as the metabolism of anaerobic micro-organisms is very different. Anaerobic tests can obviously not be used to predict degradation in aerobic degradation environments such as composting plants and agricultural applications. The almost complete disintegration of Material C in an agricultural soil burial study could be explained by the fact that this film is made of polyethylene with additives that make this material disintegrable and not biodegradable. Negative values are linked to inhibition effects. Discrepancies in the test results could also be attributed to the duration of the test. The polyester-based material (Material B) degrades slowly and reaches higher degradation percentages in tests lasting longer. Another conclusion is that some materials are better designed to be degraded in compost (Material B) than in another medium, and conversely. This Table 19.2 Description of the methods used for the tests reported in Fig. 19.1 Type of test

Description and standards

StÏrm BODIS

StÏrm test (OCDE 301B, ISO 14852) Test of oxygen demand in solid medium (ISO 14851) Composting test at the laboratory scale (ISO/DIS 20200, EN 261085, ISO 14855) Anaerobic test (EN 13432, ASTM D 5210) Closed bottle test at 25 ëC (OCDE 301D, ASTM D5988-96 modified) Closed bottle test at 50 ëC (OCDE 301D, ASTM D5988-96 modified) Composting test at the pilot scale (EN 14045)

56±180 117

Soil test in laboratory (DIN 53739) Test in a real agricultural soil

28±84 330

Compost Anaerobic Headsp 25 ëC Headsp 50 ëC Compost pilot scale Soil lab test Agric soil (11 months) Enzymatic test

Enzymatic test

Average duration of tests (days)

50 58 48 48 84

ö

Biodegradable polymers in agricultural applications

503

19.2 Comparison of biodegradability methods for three materials.

is why the farmer should choose materials according not only to the biodegradation performances, but also according to the medium of disposal (soil or compost). Tests in a real agricultural soil have been performed. For agriculture, these in situ tests are needed in order to validate the laboratory tests and to provide relevant information for the future agricultural practices. Putting them between a plastic net sealed on PVC frames assesses the degradability of the samples in the field conditions. These frames are then buried in an agricultural soil and the degradation of the samples analysed at determined time intervals. The degradation is evaluated by mass losses or by image analysis (disappearance of areas). Similar tests are also performed in compost using this frame method. Figure 19.2 shows the results for this agricultural soil test. Materials C and A disintegrate at a very high level after 11 months of burial. However, this specific test shows only the disintegration behaviour of the materials and not the biodegradation performance. Recently, tests performed on similar materials led to analogous results (Fritz, 2003) and showed that the pre treatment of the polyethylene (Material C) by exposure to high temperature did not significantly improve its biodegradability that remained very weak (lower than 10% in soil tests, and lower than 20% in compost tests). The interpretation of the data strongly depends on how the results are expressed. With regard to the mass loss expressed as a percentage, a strong influence of the sample dimensions (e.g. film thickness) is noted. In this case,

504

Biodegradable polymers for industrial applications

the comparison of the degradability or the degradation rate of samples with different dimensions would lead to wrong conclusions. The results obtained indicate that thickness is an important factor on the `speed' of degradation. However, for similar thickness, biodegradation in soil and in the laboratory are correlated. There are significant differences between degradation under soil conditions compared to degradation in compost. As expected, the weight loss under composting conditions is, in most cases, higher than in soil. The higher temperature, the hydrolysis and the higher microbiological activity under composting conditions could explain this difference. For polyesters it has been shown that an increase in polymer chain mobility was the main factor in increasing degradation rates with increasing temperature. This polymer-related effect dominates changes in biological activity. In addition, the different microbial populations present in soil and compost may explain the lower disintegration rate of the cellulose-based samples in the compost. Other tests performed in situ and presently on going, show that 99% of fragments of biodegradable plastics disappeared after two years of cultivation (Charrier, 2003; Agrice, 2004). To conclude and generally speaking, it must be remarked that the measurement of weight loss cannot be used as a substantiation of biodegradability. The weight loss of the test material can be the consequence of migration of watersoluble additives such as plasticisers, the dissolution of water-soluble polymers, melting or embrittlement. Any conclusion of biodegradability based on weight loss measurement should be verified by other studies (e.g. respirometric tests).

19.3.2 Material behaviour during cultivation, impacts on plant yields and crop quality As stated above, the main objectives of mulching is to avoid weed growth and to warm the soil. These objectives are easily achieved using polyethylene films. However, with biodegradable material difficulties occur after a few weeks or months because the film gets biodegraded and does no longer fulfil its role. We are faced with contradictory needs and the challenge for biodegradable films is to solve this problem. Thanks to technological progress, the on field duration has been improved and when the degradation starts, the film has achieved its purpose and is no longer necessary for the plant. The mulching film is laid on the soil surface and buried on both lateral sides to hold in place. This operation is done using specific machines. Generally, the machine works over the seedbed or width of plantation and unrolls the plastic. The traditional system (Fig. 19.3) consists of opening the soil with a ploughshare in both sides of the cultivation area. The film ± stretched by tyres against the furrows ± is laid on the soil surface. Another ploughshare closes the furrow and covers the border of film ± 20 cm each side ± with earth to anchor it. The roll of film is loaded on a mandrel. After laying, the width of film exposed to sun is

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19.3 Traditional system for mulch films implementation.

around 100 cm. A newly developed process avoids the need for soil ploughing (Fig. 19.4). With this method, a coulter drills the soil to make a thin drain on both sides of the cultivation area. The film is inserted into this drain with a disc. The roll of film is laid on the soil by unrolling the film underneath the tyres of the machine as it goes along. This system increases around 10% the width of film laid on the soil, 115 cm instead 100 cm in the traditional system. We have noticed that 25 to 40 cm of the 140 cm width of film is buried in the soil (20 to 30%) and this starts biodegradation earlier than the film which is laid on the soil. The part, which is on the soil, is exposed to the changing climate conditions (sun and rain) and is susceptible to ageing. For usual cultivation in the open (e.g. melon, see Fig. 19.5) this exposure is for a short duration (one month generally) because the canopy totally recovers the film after one month of growth. Then, the film is no longer exposed to the sun and to the heat and ageing stops. Experiment and measurement show that in the south of France the amount of UV energy (in the range of 280±380 nm) received by the film in April (period of mulching) is about 10 MJ/m2 (Candela, 2003). Other tests conducted on the 55 days between 13 February and 9 April 2001 gave an amount of 19.5 MJ/m2 (Grima, 2002). Under greenhouse conditions (e.g. for salad, tomatoes, etc.) this UV radiation is very limited, because the greenhouse cover (plastic or glass) is a total filter to UV radiation. In addition, in greenhouses the heat exposure is lower than in the open because (i) the cover is also a filter to the long IR and (ii) the farmer manages his greenhouse to avoid heat shocks by aeration and by painting the cover with white paints (chalk mixture). Consequently, the ageing under greenhouses is reduced to zero. This ageing on soil consists in lesions that appear one or two weeks after laying. These lesions are cracks, holes, and tears that become more important as the time goes. However, the film is never totally degraded ± long pieces remain on the soil (see Fig. 19.6). At this point, the mulch has fulfilled its task, the plant is adult and does not need mulching any longer. After harvesting, the film is mixed into the soil with the green wastes using tilling machines (roto tilling).

19.4 New process for mulch film implementation.

506

Biodegradable polymers for industrial applications

19.5 Mulching films in test just after deposit (in March 2003) (photo by P. Feuilloley).

19.6 A biodegradable film after four months and after harvest (in June 2003) (photo by P. Feuilloley).

Biodegradable polymers in agricultural applications

507

Table 19.3 Soil temperatures under mulching in 2002 Soil temperature From 4 April to 15 May Sum of values Average PE M1 M2 M3

224 049 218 202 217 088 215 092

18.9 ëC 18.5 ëC 18.4 ëC 18.2 ëC

Another objective of mulching is to reheat the soil for an earlier cultivation (CEHM, 2003). Table 19.3 shows results of soil temperature measurements. There are no significant soil temperature differences between soils covered with a polyethylene mulch and soils covered with bioplastics (M1, M2 and M3). An impartial criterion used to assess the plant yield is the commercial yield, i.e. the net amount of harvest really put on the market, expressed generally in kg/ ha or in kg/m2. Products that are not commercial are destroyed. The commercial yields cultivated under bioplastic films have to be assessed to check if the behaviour of the bioplastic has affected yields compared to the yields obtained under polyethylene films. One explanation is that the use of bioplastic does not induce more plant disease compared to plants grown under polyethylene. This is because, in some cases (e.g. melon), there is a reduction of humidity under the mulch and consequently the microclimate around the fruit is dryer than under polyethylene. Concerning the commercial yields and other costs linked to them (manpower, consumables, etc.) specific data are now available and allow comparison with traditional mulching. The trends, in general, are as follows: · the consumption of bioplastic, in kg/ha, is lower than the consumption of polyethylene. The reduction is about 20% due to the use of films thinner than polyethylene · another positive result in favour of these films is that the commercial yield under bioplastic is higher than the commercial yield under polyethylene (‡5%). This may be due to the better sanitary status of the plants cultivated with these films. However, the cost of bioplastic is higher than the cost of polyethylene (+50%/ kg) and the yield increase does not balance out the higher cost, except for high value added cultivation (e.g. strawberries).

19.3.3 Environmental impact The biodegradable materials used by farmers are in contact with three media: soil, compost and water. According to the various existing definitions of

508

Biodegradable polymers for industrial applications

biodegradation, it is necessary that the materials should not release any toxic residues at the end of the process. Selecting a method to test the toxic effect of biodegradable materials is the most important step to obtain reliable data. A performant toxicity test should measure the relevant parameters and respond to the environmental characteristics of the site. The test should be fast, simple and reproducible (Labelling biodegradable products, 2002). The majority of ecotoxicity tests have been developed for the risk assessment of water pollution whereas the soil ecotoxicity tests have been developed for the risk assessment of contaminated soils. With regard to composted materials, there are currently few agreed methods for studying their ecotoxicity. Most of the methods are still under development within the framework of the standardisation organisation research and performed with compost applications of biodegradable materials. It should be stressed that the properties of compost differ from those of soil, in particular as the compost contains a smaller amount of mineral material. Therefore methods developed for soil are not appropriate for compost. Several tests are carried out to assess the environmental impact According to EN 13432 standard, ecotoxic tests should be performed on compost samples three-months old, obtained in the pilot scale composting process (this test is equivalent to the ISO/FDIS 16929 standard). The material is mixed at very high concentration to the compost (10%) and composted for three months. At this point, the modified plant growth test (OECD 208 guidelines) is a suitable method to test compost toxicity. The same test is also available to test soil toxicity. Similar tests are described in ISO/DIS 11269 standard. Tests on earthworms (OECD 207 guidelines) are also possible to assess toxic effects on animals. For water media, tests on algae (OECD 201 guidelines, ISO 8692 and NFT 90-375 standards) or on daphnia can be performed. Tests on micro organisms are frequently used. These tests are simple and fast (OECD 216 and 217 guidelines). Among these tests, the bioluminescent test with Vibrio fischeri is very effective. This test is based on the luminescence measurement of a luminescent bacteria (Vibrio fischeri). The luminescence decreases with the increase of toxicity. The light emitted by the microbe is measured with photo-sensors in a specific device (EN ISO 11348-3 standard). This luminescence bacteria test is widely used for measuring the acute toxicity of water-soluble compounds. It should be noticed that a biodegradation test (respirometric) is also an ecotoxic test by itself; if the material is toxic, the biodegradation rate leads to negative values (inhibition effects). New tests, not yet standardised, have been set up to detect the presence of micro fragments in soils or in compost. They can be used to show the difference between biodegradation and disintegration. Using the same three materials (A, B and C, made of poly-caprolactone, polyester alipha/arom, and polyethylene + pro oxidant), no toxic effect was

Biodegradable polymers in agricultural applications

509

detected using the plant growth test and the bioluminescent tests. A new test method with Vibrio fischeri bioluminescent bacteria was found suitable for the assessment of the toxicity of compost. Comparison of the results obtained with the method described in the harmonised EN 13432 standard and the direct contact test with bioluminescent bacteria Vibrio fischeri indicated a similar response. The suitability of the bioluminescent test was verified by studying biodegradation of cellulose acetate during a controlled composting test. However, milled cellulose acetate decreased the activity of the compost during the test and gave a toxic response in the toxicity test, leading to an immature compost. It is important to understand that there are restrictions on the application of toxicity tests. Special attention should be paid to several physical and chemical properties of the tested compost samples. For example, usually, the conductivity, the amount of phytotoxic substances due to the immaturity of the compost and its pH variations affect the validity of the terrestrial plant growth test. Other negative effects have been detected. The most important one is the formation of toxic by-products released during the early stages of biodegradation phases (Fritz, 2003) and the high level of oxygen demand. These phenomena occur when large quantities of non toxic biopolymers (e.g. starch, cellulose) are mixed with the medium (soil or compost). These phenomena are present in agriculture when straw is incorporated into soil by ploughing (depressing straw effect). To avoid this effect, the farmer must increase the nitrogen fertilisation; however, these effects are temporary and will end as soon as the biodegradation process is completed. Recently, ecotoxic tests performed on several materials showed that biomaterials (starch, cellulose, polylactic acid, etc.) in soil conditions had a toxic effect at the beginning of the process, and became non toxic after 60 days. On the contrary, polyethylene materials with pro oxidant additives showed a toxic response after 60 days (Fritz, 2004). Similarly, inhibition effects occur during Sturm tests with this material. One explanation could be the release in the environment of poisonous substances contained in the pro-oxidant additive (di thio carbamates) or in other unknown additives. These harmful substances accumulate in soils. For the above reasons and for agricultural requirements (no accumulation of toxic substances in soils), ecotoxic tests should be performed after a lag phase of three or six months. Another impact on the environment is the possible production of micro fragments during the biodegradation process. To assess this micro fragment production, experiments conducted in laboratory conditions and real conditions have been set up (Charrier, 2003). Three films were tested: (i) polyester aliph/ arom + starch, (ii) polyester aliph/arom + natural fibres and (iii) polyethylene + pro oxidant. The results are shown in Table 19.4. In test A, samples of films were incorporated into pots containing a natural soil. These pots were put in an oven at 30 ëC for six months, and were regularly watered to ensure a constant soil humidity. A similar experiment, test B, was performed in compost

510

Biodegradable polymers for industrial applications

Table 19.4 Micro fragments of plastic in soil and in compost, after 6 months Type of material

Test A Percentage Test B Percentage (on soil) of remaining (on compost) of remaining fragments fragments

Polyester aliph/arom + starch Soil test Polyester aliph/arom + natural fibres Soil test Polyethylene + pro oxidant Soil test

6.5

Compost test

<1%

5.9 100

Compost test Compost test

<1% 100

conditions at the real scale. After six months, the micro fragments of the films were sorted (either from the pots or from the compost) using sieves of several meshes (10 mm, 2 mm and 250 m). The comparison between the initial mass sample and the final total mass of micro fragments gives the degradation rate. The results in Table 19.4 show clearly that the polyethylene + pro-oxidant additive material does not degrade either in soil or in compost. In addition, this material is not biodegradable as shown in section 19.3.1. The fates of these fragments as well as of the toxic additives are still unknown and a risk of accumulation of all these substances is possible in soil or in the food chain (Feuilloley, 2004).

19.4 Market issues 19.4.1 Market and costs The market for biodegradable polymer is in its infancy, compared to the market of traditional plastics that started 20 years ago and has been booming for two or three years (Advanced Bioplastics, 2003). The world production passed from the pilot scale (during the 1990s) to the industrial scale. However, this amount remains lower than the market of plastics (about 150 million t in the world). For example the production increased from 450 t in 1990 to 254,000 t in 2002 (0.17% of the plastic market), distributed between biodegradable polymers from fossil origin (33,000 t) and from renewable resources (221,000 t, i.e. 87% of the biodegradable polymers production). Important manufacturers, like Cargill-Dow whose production capacity is 60%, expects to increase its capacity to 95%. This company has been planning the construction of a new plant able to produce 140,000 t/year of polylactic acid in USA and the construction of a similar plant in Europe for two years. If capacity continues at this rate, the global production capacity of biodegradable polymers should reach 1,000,000 t in 2010. Presently, the market is under the rule of huge producers such as Cargill-Dow (USA), Novamont (Italy) and BASF (Germany). Others such as Dupont (USA), Procter & Gamble (USA), Eastman (Holland) and UCB (UK) are ready to invest the market.

Biodegradable polymers in agricultural applications

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Table 19.5 Evolution of the production capacities of biodegradable polymers (t) Nature

1990

1995

Year 2000

2002

2005±2007

Polymers from fossil origin Polymers from renewable resources Total

100

5,000

23,000

33,000

75,000

350 450

13,200 18,200

26,000 44,000

221,000 254,000

420,000 495,000

Source: IBAW (2002).

In 2001, the consumption in the EU was 30,000 t, strongly supported by Novamont, the European leader. World consumption could be in the region of 70,000 t. In general, a two-year shift between the consumption and the production capacities has been noticed. Table 19.5 shows the evolution of the production capacities of biodegradable polymers. Several and various products are proposed for the market and are difficult to quantify. For example, we find materials made of renewable resources (starch and mixtures of starch with polylactic acid, polyhydroxyalcanoates, etc.), of fossil resources (polyesters, etc.) and of a mixture of both materials (composites). However, for environmental reasons and to promote sustainable development, the trend is to use renewable resources rather than fossil resources to make these materials. Table 19.6 shows examples of materials and producers. Table 19.6 Examples of materials and producers Polymers

Materials

Producers

Starch and mixtures of starch

Mater-Bi Solanyl Bioplast

Novamont (Italy) Rodenburg (NL) Biotec (UK)

Biomer

Biomer (Germany)

NatureWorks Lacea

Cargill Dow (USA) Mitsui Chemicals (Japan)

Polyhydroxyalcanoates (PHA) Polylactic acid (PLA) Cellulose

Polyester

Tenit

Bioceta

Fasal

Natureflex

Ecoflex

Eastar Bio

Biomax

Bionolle Capa

Eastman (USA) IFA (Austria)

Mazzucchelli (Italy) UCB (Germany)

BASF (Germany) DuPont (USA)

Eastman (USA) Showa Denko (Japan) Solvay (Belgium)

512

Biodegradable polymers for industrial applications

These materials have different features that match different applications. For instance, the Mater-Bi material of Novamont is more suitable for agricultural films or bags than the polylactic acid of Cargill that is used for packaging and non-woven materials. Manufacturers are presently complementary with a double function: buyer and provider. In other words, producers are not yet competitive on the same applications, they blend materials from one producer to another to obtain specific or expected film properties. However, these materials have been successful in `niche' markets, but less so in mass markets. The main sector concerns objects used once, or those with a short- or medium-life duration. Presently, the case for biodegradable mulch is almost completed. This is true for agricultural mulching films and other supplies for agriculture, horticulture and forestry. These products are generally mixtures of starch and biodegradable copolyesters. For example, Novamont (Italy) and Ulice (France) make a product that is a mixture of corn starch and a co-polyester from BASF (EcoflexÕ) or Eastman (Eastar Bio). Other mixtures are based on vegetal fibres and biodegradable polymers (composite materials). Horticultural and forestry pots for transplanting and mini greenhouses for plant protection, made of biodegradable polymers, are proposed for the market. Some final improvements are needed to better adapt them to the external conditions and to the various types of cultivation. New products such as low tunnels, twines, clips, etc., are being tested. The cost of biodegradable materials remain two or three times higher than the cost of conventional plastics. For instance, the present cost of mulch based on Ecoflex or Mater-Bi materials is 0.15 ¨/m2 with a thickness of 17 m versus 0.06 ¨/m for a polyethylene film with a thickness of 30 m. A recent study (Ernst & Young, 2003) shows that, including recycling, the cost of polyethylene is lower than the cost of biodegradable plastics. However, the same study shows that biodegradable plastic costs remain competitive if the support allocated for recycling is taken into account. Therefore, the advantage of biodegradable plastics would be the manpower saving dedicated to the film collecting (time saving) and would improve the environmental image of agriculture.

19.4.2 Competitive materials There are other materials on the market that are competition for biodegradable plastics. These materials are generally made of natural polymers, i.e. renewable materials such as paper and non-woven fibres. Paper is one of the oldest means of mulching and was used by farmers during the 1930s in the US. Paper is made of cellulose fibres, has a high level of biodegradability (higher than 90%) and is biodegraded quickly in soils (one or two months). This is an advantage for cultivation where development is short, such as salads (Colloquium, 2003). Another application for paper is the embedding of rice seeds in rice-swamps. Paper is durable and resists one month

Biodegradable polymers in agricultural applications

513

in water. A new generation of long-life duration papers are becoming available that should withstand exposure on soil for six months up to one year, or possibly longer. These papers are reinforced with biodegradable fibres and should be used for green area implementation. These materials are made of natural fibres from various origins: coconut fibres, flax, hemp, cotton, etc. The fibres are usually non-woven but are sometimes woven into a continuous material that can be rolled or cut into sheets. A large choice of thickness is available, from 16 g/m2 up until 400 g/m2 (for special applications, materials of 1100 g/m2 can be made). The thinner material is used as mulch or as direct cover for vegetable crops and can be used in place of biodegradable films. The thicker material is used for horticulture applications such as the support of young trees or other ornamentals. It is also used in civil engineering for the stabilisation of embankments near roads or highways to facilitate lawn seeding (the seeds could even be included in the mulch) or for the stabilisation of river banks (COBIO, 2000). In either case, the main role of these very thick materials is to avoid erosion and to help the growth of a new vegetation (Spot Cemagref, 2001). The duration of these materials is about four or five years (Le Figaro, 2001).

19.5 Conclusion After a long period of latency, biodegradable plastics are now credible. Major polymer manufacturers are entering the market and performances and processability are significantly improving. Furthermore, as their demand is potentially important, significant niche markets are opening up. New legislation in Western Europe and Japan, who are in favour of biodegradable plastic users, help to spur the demand but the difficulty of developing international standards for biodegradable polymers is still an obstacle. In addition to biodegradability, the absence of eco-toxicity is now required. Bioplastics are still unknown to the processing industry and their performances (e.g. water sensitivity of most agricultural based plastics) and processability often remain a problem. The industrial future of biodegradable plastics is still a matter of discussion due to numerous problems. One of the main obstacles to widespread use of bioplastics is the cost (actually similar to specialty plastics, i.e. 1.5 to 10 euro/kg) of commercially available products. However, due to existing production capacity and the new plants which are under consideration, costs are expected to start falling. Among the different categories of biodegradable plastics obtained from agropolymers, both starch/polyesters blends and `microbial' biodegradable plastics satisfied the majority of requirements asked by the plastic packaging industries (material qualities, processability, performances, etc.). Other bioplastics based on natural polysaccharides or proteins are mainly interesting for their low cost but their non-reproductive quality and lower performances are still a handicap.

514

Biodegradable polymers for industrial applications

Coatings provide a supplementary and sometimes essential means of preserving seeds and of controlling the release of agrochemicals, fertilisers and pesticides (e.g. modifying and controlling local conditions and level of functional agents and controlling the release of agrochemicals for soil and/or plant leaves). The concept of `active layers and matrices' for films and coatings which contribute themselves to seed preservation, agrochemical release in the soil or on the leaf surface can be introduced. This can be obtained by designing the bio-plastic matrix in order to control either gas or solute transfers or/and its biodegradation. The formulation of coatings and/or biodegradable bio-materials formed with several compounds (composite or complex materials) have been developed to take advantage of complementary functional properties of the different constitutive materials and to overcome their respective drawbacks. Most composite films studied to date combine one or several polyester (or lipidic) compounds with a hydrocolloid-based one. The future of biodegradable materials is therefore probably dependent on the development of applications where some of their preferential properties (e.g., controlled biodegradability combined with slow release) are enhanced or on the development of composite materials.

19.6 Further information Details of international and European standards cited in this chapter can be obtained from the standardization bodies below. EN 13432, NFT 90-375 Association FrancËaise de Normalisation (AFNOR) 11 Avenue Francis de Pressence F-93571 Saint Denis La Plaine France OECD 201, OECD 207, OECD 208, OECD 216, OECD 217 Organisation for Economic Co-operation and Development (OECD) 2 rue Andre Pascal F-75775 Paris Cedex 16 France ISO/FDIS 16929, ISO/DIS 11269, ISO 8692, EN ISO 11348-3 International Organization for Standardization (ISO) 1 rue de Varembe Case postale 56 CH-1211 Geneva 20 Switzerland

Biodegradable polymers in agricultural applications

515

19.7 References Advanced Bioplastics (2003) Applications ± Markets ± Sustainable Benefits. International Symposium. 12±13 February. Nuremberg (Germany). IBAW Publication, Marienstrasse 19/20, D-10117 Berlin. Agrice (2004) Contract N02 01 021, Unpublished Report. ADEME, 2 square Lafayette, F-49004 Angers cedex 1. Candela J. (2003) Traitement de donneÂes de rayonnement solaire dans le cadre de l'eÂtude de vieillissement de plastiques. MeÂmoire de DUT, IUT ± Universite de Montpellier, Juin. CEHM (2003) Bioplastics project, Unpublished Report, May. Charrier J. (2003) Recherche de microfragments de plastique biodeÂgradable dans les sols, IUP ± Universite de Picardie, Rapport de maitrise. COBIO (2000) Newsletter, 15, July±September. Colloquium (2003) `MateÂriaux biodeÂgradables et environnement', Rouen, May. ReÂgion Haute Normandie, HoÃtel de reÂgion, 25 Bd Gambetta, F-76174 Rouen cedex 1. Cuq, B., Gontard, N. and Guilbert, S. (1995) Edible films and coatings as active layers. In Active Food Packagings, M.L. Rooney (ed.), p. 111±142, Blackie Academic & Professional, Glasgow. Cuq, B., Gontard, N. and Guilbert, S. (1998) Proteins as agro-polymer for packaging production, Cereal Chemistry, 75 (1): 1±9. DeÂcriaud A. (1998) Evaluation objective de la biodeÂgradabilite des mateÂriaux: mise au point d'une meÂthode et d'un dispositif expeÂrimental. TheÁse aÁ l'INP ± Toulouse, 09 Juillet 1998 DeÂcriaud A., Bellon-Maurel V. and Silvestre F. (1998) Standard methods for testing the aerobic biodegradation of polymeric materials. Review and perspectives. Advances in Polymer Science, 135. Domenek S., Feuilloley P., Gratraud J., Morel M. H. and Guilbert S. (2004) Biodegradability of wheat gluten based bioplastics, Chemosphere, 54, 551±559. Ernst & Young (2003) Etude du marche des mateÂriaux biodeÂgradables. Rapport final Ernst & Young. May. ADEME, 2 square Lafayette, F-49004 Angers cedex 1. EU Directive 75/442 (1975) Official Journal L 194, 25/07/1975 P. 0039-0041. EU Directive 91/156 (1991) Official Journal L 078, 26/03/1991 P. 0032-0037. Feuilloley P. (1999) Plastiques biodeÂgradables: le deÂfi du XXIeÁme sieÁcle. ReÂussir Fruits et LeÂgumes, 174, May, 39±41. Feuilloley P. (2004) Ce plastique faussement biodeÂgradable, La Recherche, 374, April, 52±54. Fritz J. (2003) Strategies for detecting ecotoxicologic effect of biodegradable polymers in agricultural application. Macromol. Symp. 197, 397±409. Fritz J. (2004) Degradation of natural and synthetic polymers in compost and soil environment. CEN TC 249 WG9, N 109. Fritz, H.G., SeidenstuÈcker, T., BoÈlz, U., Juza, M., Schroeter, J., and Endres, H.J. (1994) Study on Production of Thermoplastics and Fibers Based Mainly on Biological Materials, Science Research Development, European Commission, EUR 16102 EN. Gennadios, A., High, T.H., Weller, C.L. and Krochta, J.M. (1994) Edible coatings and films based on proteins. In Edible Coating and Films to Improve Food Quality, J. M. Krochta, E. A. Baldwin and M. Nisperos-Carriedo (eds), Technomic Publishing Co. Inc. Lancaster, USA. Gontard, N. and Guilbert, S. (1994) Bio-packaging: technology and properties of edible

516

Biodegradable polymers for industrial applications

and/or biodegradable material of agricultural origin. In Food Packaging and Preservation, M. Mathlouthi (ed.), pp. 159±181, Blackie Academic & Professional, Glasgow. Grima S. (2002) BiodeÂgradation de mateÂriaux polymeÁres aÁ usage agricole: eÂtude et mise au point d'une nouvelle meÂthode de test, analyse des produits de deÂgradation et impact environnemental. TheÁse aÁ l'INP ± Toulouse, 16 DeÂcembre. Guilbert S. (1999) Proteins as bio-polymers for food packaging: A review, Bulletin of the Research Institute for Food Science, Kyoto University, 56, 38±62. Guilbert, S. and Cuq, B. (1998) Les films et enrobages comestibles. In L'emballage des DenreÂes Alimentaires de Grande Consommation, pp. 471±530, Technique et Documentation, Lavoisier, Apria, Paris. Guilbert S. and Cuq B. (2005) Protein as raw material for biodegradable products In Handbook of Biodegradable Polymers, C. Bastioli (ed.), Rapra tech. Ldt London. Guilbert S., Gontard. N., Morel M. H., Chalier P., Micard V. and Redl A. (2002) Formation and properties of wheat gluten films and coatings. In Protein-based Films and Coatings. A. Gennadios (ed.), CRC Press, Boca Raton, pp. 69±122. IBAW (2002) Biodegradable Polymers. IBAW Publication. Marienstrasse 19/20, D10117 Berlin. November. Jouet J. P. (2003) Situation de la plasticulture dans le monde, CIPA, 27 rue de Prony, Paris, DeÂcembre. Labelling biodegradable products (2002) EU Contract SMT 4 CT97-2167. Le Figaro (2001) 5 November Redl A., Guilbert S. and Morel M. H. (2001) Method for preparing composite materials containing natural binders. International Patent PCT/EP02/00665. Spot Cemagref (2001) 118, September.

Index

A, B and C chains 147 abaca 200, 202 ABCDE system 364, 365 acetyl tri-n-butyl citrate (ATBC) 271, 272 acetylation 153±4 acrylic copolymers 349 activation energy 254, 255 activation-monomer mechanism 85, 86, 87 additives 61±2, 458±9, 464±6 adipic acid 423 ADPG 144 aerobic biodegradation 66, 445±6 Aeromonas hydrophila 45, 46, 47 ageing 275±80 agricultural biodegradable polymers 451±73, 494±516 auxiliary products for agriculture and horticulture 460±4 biodegradability 501±4 competitive materials 512±13 environmental impact 466±9, 507±10 evaluating properties of biodegradable materials 501±7 formulation of bioplastics 498±500 future developments 470±1 market and costs 510±12 material behaviour during cultivation, impacts on yields and crop quality 504±7 materials applied in agriculture 495±501 origin of existing materials 497±8 oxo-biodegradation of polyolefins 464±6 plasticulture 451±64, 470±1 principal applications 495±7 process pathways 500±1 protective films 448±9, 451±60

time-controlled degradable plastics 457±60, 470 air permeable films 13 alanine 110 albumin 341 Alcaligenes latus 42, 43 alcohol 82, 85±7 aldolase 379 aliphatic-aromatic copolyesters 346, 423 aliphatic polyamides 107, 132 aliphatic polyesters 3, 21±5, 77±8, 107, 132, 419±20 biodegradation mechanism 365±9 ring-opening polymerisation see ringopening polymerisation starch blends with 7 step-growth polymerisation 79 alkaline solubilisation 365 n-alkane 389, 390 alkoxy radical 59±60 alkyl ammonium modified clays 292 all organic mediators 85±7, 102 alternating polyester amides 109±12, 120, 121, 123 aluminium 447 aluminium alkoxides 80±2, 87 American Society for Testing and Materials (ASTM) 324, 325, 328, 337±8 amino acids 14 amino butyric acid 111 amylopectin 4, 141, 144, 145, 340, 425 structure 4, 146±7, 203±4 structure of starch 148±9 amylose 4, 141, 144, 145, 340, 425 structure 4, 146, 203 structure of starch 149 anaerobic environments 74 anaerobic tests 502

518

Index

anhydride groups 268 anhydro-sugar polymerisation 174±5 antioxidants 65, 464±6 Arboform 11 aromatic-aliphatic copolyesters 346, 423 aromatic polyesters 3, 21±2, 25±6 Arrhenius plots 321, 323 aspartic acid 129±30 polypeptides of 15 atomic force microscopy (AFM) 41, 42 Avrimi-Erofeev equation 258 Azotobacter vinelandii 44 Bacillus spp. 42±3 bacteria see microbial degradation; microbial production bacterial esterases 342 BAK 1095 24, 118, 120 -diketone hydrolase (BDH) 378±9 bi-layer mulching films 457 Bioceta 9±10 Bio-Compo 9 biodegradability agricultural polymers 501±4 cellulose-based polymers 224±6 natural fibre composites 196±7, 201, 206 biodegradable polymers 140±1 classification see classification of biodegradable polymers types of 142±3 biodegradation 337±9, 411±12 mechanisms see biodegradation mechanisms PHA polymers 37, 234±5 PLA 275±80 polydepsipeptides 131 polyester amides 119±23 biodegradation mechanisms 319±22, 357±410 degradation site 361 future trends 393 hydro-biodegradation 319, 466 hydrolysis 361 miscellaneous mechanisms 362 naturally occurring polymers 362±6 overview 359±62 oxo-biodegradation 320±2, 466 PEG 373±5 polyacrylate and polycyanoacrylate 389±90 polyamino acid 384±8 polyanhydrides 392 polycarbonates 372±3

polydioxanone 390±1 polyesters 365±72 polyglyoxylate 392 polymer chain scission 360±1 polyolefins 389 polyorthoesters 391, 392 polyphosphazenes 392 polyurethanes 382±4 PPG 375±6 PVA 376±82 Bioflex 8 bioinertness 66 biological oxygen demand (BOD) 323 biologically active environments 317±19 bioluminescence test 508±9 Biomax 25 Biomer 18 biometric tests 319, 326±9 Bionolle 8, 345±6, 499 Bioplast 7±8, 156, 204 Biopol 18, 42, 44, 235, 316, 421, 499 composites with natural fibres 192, 194, 196±7 production 232 biorefineries 220±1 blends see composites block polymers 112±14, 121±2, 123 blowing 28 blowing agents 474±5 bone marrow stromal cells 49±50 branching enzymes 144 break elongation 304, 305, 457±8 British Standards Institute (BSi) 325±6 bulk density 477, 481, 486, 491 Burkholderia cepacia 238±43 Candida Antarctica lipase 91±2 CAPA 24, 78 carbon-chain polymers 320±2 carbon dioxide 74, 323, 445 supercritical see supercritical carbon dioxide Cargill 101 Cargill-Dow process 19, 84±5, 198 carrier bags 62±3, 69, 437±8, 447 casein 14, 341 casting process 500±1 castor oil 16 catalytic hydrolysis 361 cavitation bubbles 293 Celgreen 10, 78 cell growth 48±9 cell size 480, 481, 482, 484±5, 491 cellobiohydrolases (CBHs) 424

Index cellophane 9, 340 cellular interaction 123 cellulose 8±10, 142, 165, 178, 223±6, 319, 325, 340, 424 biodegradability 224±6 composites with starch 205±6, 207 derivatives 8±10, 223±4 materials and producers 511 plant biomass 180, 181, 219, 220±1, 223±6 structural considerations 223±4 cellulose acetate (CA) 9±10, 142, 165, 223±4, 340, 424 starch blends with 8 chain extenders 345 chelators 365 chemical detoxification treatments 229, 230 chemo-enzymatic synthesis 175 China 452 chiral monomers 127 chiral templates 177 chitin 12±13, 425 chitosan 12±13, 183 chlorodifluoromethane (HCFC±22) 96 chondrocytes 49 citrate plasticisers 270±1 classification of biodegradable polymers 3±31 aliphatic polyesters 3, 21±5 aromatic polyesters 3, 21±2, 25±6 lipids 3, 15±16 microbial polymers 3, 16±19 miscellaneous natural polymers 3, 20±1 modified polyolefins 3, 27±9 natural polymers 3, 4±21 polyesters synthesised from bio± derived monomers 3, 19±20 polysaccharides 3, 4±13 polyvinyl alcohols 3, 26±7 proteins 3, 14±15 synthetic polymers 3, 21±9 clays 99±100, 290±8 delaminating using ultrasonics 293±8 see also nanocomposites Clean Fractionation Process 221 Clean Green Packing 6 Cloisite Na+ 292, 293, 293±306 coal 364±6, 393 coatings 499±500, 514 cobalt 468, 469 coextruded sheet processing 153 coir 190, 210±11, 212 collagen 14, 143, 341

519

comb polymers 128±9 commercial yields 452±3, 507 composites 514 material properties 347±8 microbial polyesters 18 natural fibres see natural fibres PLA see polylactic acid/polylactide starch see starch see also nanocomposites compost bags 69, 70±1, 322, 460±1 composting 225, 234, 317±18, 339 infrastructure for 351 oxo-biodegradable polyolefins 70±1, 72, 446±7, 449 PLA 276±8 standards for compostability 324±5 tests for biodegradability 503±4 constant-temperature variable-pressure processing 487, 491 controlled-lifetime plastics 62±3, 67±8, 330, 440, 457±60, 470 controlled release systems 157, 337, 464, 470, 500, 514 copolycarbonates 373 copolyesters 346, 423 copolymers 345 acrylic 349 microbial polyesters 18, 342 PVA 380±1 corn 84±5, 101 corn gluten 15 corn zein 341 costs 351, 512, 513 oxo-biodegradable polyolefins 442±3 production costs for PHAs 235±7 reduced labour costs 455±6 cotton 210, 212 Cox-Merz rule 254 critical point 475, 476 critical pressure 475 critical temperature 475 crop quality 504±7 crosslinked starch 154 crown ether pendants 166 crystallinity 412 and degradation of PLA 278±9 foamed PCL 488±90, 491 natural fibre±PHA composites 197±8 PHA 40±2, 233 PLA 255±6, 258±9 PLA blends with starch 266±7 starch 147, 148±9 cultivation 504±7 cyanophycin (CGP) 385, 387

520

Index

daphnia 71, 72 DCP (degradable and compostable plastics) 27 degree of substitution (DS) 179 dendritic hyperbranched polymer (DHP) 264±5 depolymerase enzymes (depolymerases) 370±1, 413 depolymerisation 337, 360, 413 depressurisation rate 486±7 desorption kinetics 478, 479 detergents 365 detoxification treatments 228±30 Dexon 23 diamide-diesters 108, 109 polyester amide synthesis 109±19 diamide-diols 108, 109 polyester amide synthesis 109±19 dibutyltin dimethoxide 92±3, 94, 96, 97 diester-diamines 108, 109 polyester amide synthesis 109±19 differential scanning calorimetry (DSC) foamed PCL 489±90 protein-nanoparticle composites 302±4 diisocyanatehexane (DIH) 269 dilute acid hydrolysis 227±8 direct covers 496 disaccharides, polymers from 166±73 discarded plastics see litter disposal 351 agricultural plastics 494 oxo-biodegradable polyolefin packaging 444±7 DMA tan  trace 300±2 drug delivery 157, 337 Earthshell materials 156 earthworms 71, 72 Eastar Bio 26, 511, 512 Ecoflex 26, 37, 499, 511, 512 ECO-FOAM 155 Ecolene 454, 455, 456 Ecolyte 313, 454, 455 Ecolyte process 67±8 Ecopla 20, 499 Ecoplast 12 Ecostar 8, 499 ecotoxicity 468±9 ecotoxicity tests 326, 328 agricultural biodegradable polymers 507±10 oxo-biodegradable polyolefins in packaging 443±4 elasticity coefficient 254±5

elastins 341 elongation at break 304, 305, 457±8 embrittlement time 458 encapsulation 464, 470 endogeneous scission 360 endoglucanases (EGs) 424 endo-1,4-xylanase 426 energy requirements 351±2 enhanced oxidisability 314±15 entanglement molecular weight 254±5 ENVAR 8 Envirocare 68±9 environment for biodegradation 338±9 environmental impact agricultural biodegradable polymers 466±9, 507±10 ecotoxicity of biodegradable polyolefins 468±9 oxo-biodegradable poleolefins 69±73, 443±4, 448±9 popular view 466±7 scientific evidence 467±8 EnviroPlastic Z 10 enzymatic degradation 411±33 aromatic-aliphatic polyesters 423 cellulose 424 chitin 425 hemicellulose 426 hydrolysable polymers 419±23 lignin 426±7 natural polymers 423±7 polyacrylates 417 polyamides 418 polyanhydrides 419 polycaprolactone 422 polydepsipeptides 131 polyester amides 121±2, 123 polyethers 417±18 polyethylene 414±16 polyglycolic acid 420 polyhydoxybutyrate 421±2 polylactic acid 278, 279, 369, 420±1 polypeptides 427 polypropylene 416 polysaccharides 423±4 polyureas 418±19 polyurethanes 382±4, 418±19 polyvinyl alcohol 379, 416±17 polyvinyl acetate 416±17 starch 425±6 vinyl polymers 414±19 enzymatic hydrolysis 228 enzymatic ring-opening polymerisation 85, 86, 102, 130

Index enzyme-mediated polymerisation 175±6 enzymes 14, 338 main chain polymers and related enzymes 358, 359 PVA degradation mechanisms and related enzymes 376±9 EPI (Environmental Products Inc.) TDPA technology see TDPA (totally degradable plastic additives) technology epifluorescence spectroscopy (EP) 322 Escherichia coli (E. coli) 46 esterases 365 ethylene-carbon monoxide (E-CO) polypropylene 463 ethylene-propylene (EP) 321 European Standards Organisation (CEN) 323 standards 324±5, 329±30 European Union (EU) 494 Environment Directorate 329 Packaging and Packaging Waste Directive 329, 330 Waste Framework Directive 317, 325 Evercorn 6 exfoliated nanocomposites 290, 291 exogeneous scission 360 external catalytic degradation 361 extracellular depolymerases 413 extracellular PHB biodegradation 370±1 extraction constant 96, 97 extrusion 28 natural fibre composites 197, 201 protein-nanoparticle composites 298 reactive 87±90 extrusion blowing 28 extrusion casting 28 Fasal 11 Fasalex 211 fatty acid ester modification of starch 154 fatty acid polymers 16, 142 fertilisers conservation in plasticulture 456 controlled release 464 fibre-reinforced PLA-matrix composites 264 see also natural fibres fibre spinning 28 fibrinogen 341 film stacking 193, 201 flammability 201 flax 190 PHA composites 194±6

521

PLA composites 199±200, 201, 202 soy resin composites 208, 209 starch composites 204, 205, 207 Flory equation 271 flour biopolymers 5±6 foams 100 polycaprolactone see polycaprolactone foams forced air oven ageing 324, 326 forest biomass 219±50 cellulose 219, 220±1, 223±6 hemicellulose and its application 226±43 replacement of petroleum-based polymers with `green' alternatives 221±2 underutilized renewable resource 220±1 fossil resources 331 Fourier-transform infra-red (FTIR) spectroscopy PHA 35, 36, 40±1 polyethylene films 441±2 Fox equation 271 functional groups PLA blends with starch 267±70 polydepsipeptides 128±9 reactive blending 348 vinyl polymers 414 Galactic 20 gelatin(e) 14, 143, 427 gelatinisation 149±51, 271±2, 273 germination 461, 462 glass 447 glass transition temperature PHA 233, 240, 243 PLA 270±1 protein-nanoparticle composites 300±4 gluten 15, 341 glycerol 272 protein-nanoparticle composites 293±306 glycine 110±11, 339 glycolic acid 129±30 Gordon-Taylor equation 271 `green' generation of biodegradable polymers 221±2 Green Report 316, 329 greenhouse gases 74, 351±2, 445 Greenpeace 315±16, 467 growbags 463 growth rings 148 growth terraces 41 guaiacyl 10, 11 guided tissue regeneration 476

522

Index

hardwoods 226±7 Harvestform 211±13 hay-wrap 463, 471 heat 64±5 heat processing 500±1 heavy metals agriculture 468±9 complexation by sucrose containing gels 177±8 hectorite 290±1, 292±3 hemicellulose 165, 219, 220±1, 226±43, 424 biodegradable polymers from 180±3 biodegradation of PHA polymers 234±5 composition of hemicellulosic fraction of woody biomass 226±7 enzymatic degradation 426 forest-based microbial PHAs 237±43 history of PHAs as biodegradable polymers 230±2 PHA production from forest±based feedstocks 235±7 pretreatment for fermentative application 227±30 properties of PHA polymers 232±4 hemicellulosic hydrolysates 236±7, 243, 245±6 hemp 190, 208, 209 heterogeneous nucleation 487 high density polyethylene 350 homogeneous nucleation 487 horticulture 460±4, 470 humidity resistance 6 hydro-biodegradable plastics 330, 331 hydro-biodegradation 58, 319, 466 see also hydrolysis hydrogels 166±7, 183 hydrolase enzymes (hydrolases) 358, 359 PAsp 388 Hydrolene 26 hydrolysable polymers 419±23 hydrolysis/hydrolytic degradation 58, 319, 337, 338, 339, 357±9, 466 biodegradation mechanism 359±60, 361 dilute acid hydrolysis 227±8 PLA 276, 277, 278±9 polyester amides 120±1, 122 hydroperoxide groups 59±60, 61 hydrothermal techniques 182 hydroxylases 413 p-hydroxyphenyl 10, 11

immuno-modulation 176 implants 48±50, 336±7, 476 in situ polymerisation 291 in situ tests 325±9, 503±4 in vivo degradation studies 122±3 incineration 448 increased income/cost ratio 452 injection moulding 28 insertion-coordination mechanism 80, 83 intercalated nanocomposites 99, 290, 291 internal catalytic degradation 361 International Standards Organisation (ISO) 323 intracellular depolymerases 413 intracellular PHB biodegradation pathway 370, 371 intrinsic viscosity 240, 243 ionic initiators 79, 81 isocyanate groups 269±70 isodimorphism 238±9 Israel 453, 454 jute 190 PHA composites 192±3, 194, 195 PLA composites 199, 202 synthetic polymer composites 211, 212 kenaf 200, 202, 211 ketone copolymers 67±8 lab degradation tests 325±9, 501±4 labour costs reduction 455±6 laccase 12, 426 Lacea 20, 79, 499 lactic acid 19, 343±4 L-lactic acid and D-lactic acid 251±2 lactide monomer 251, 270 lactones 130 Lactron 20 Lacty 20 lamellae 42 lamellar nanoclays 99±100 landfill 317±18, 336, 351, 440 degradation of PHA 234±5 oxo-biodegradable polyolefins 69±70, 74, 444±6 levulinic acid 237±8, 240±2, 243 life cycle analysis (LCA) 351±2 light 64±5 lignin 165, 180, 181, 220±1, 393, 424 biodegradation mechanism 364 enzymatic degradation 426±7 polymers 10±12, 143 lignin peroxidases (LiPs) 426

Index lignocellulose 424 fibres see natural fibres slow biodegradation 319, 325 lignocellulosic biomass 219±22 see also cellulose; forest biomass; hemicellulose; lignin Lignopol 12 lipases biodegradable polymers from sugars 175±6 biodegradation of aliphatic polyesters 366 catalysis of ring-opening polymerisation 85, 86, 91±2, 102 lipids 3, 15±16, 499±500 litter 438, 440 agricultural plastics 462, 463 oxo-biodegradable polyolefins 71±3, 446 low density polyethylene (LDPE) 321, 350, 415 low tunnel technique 496 lysine 15 main chain structure, and enzymes responsible for degradation 358, 359 maleic anhydride 268±9 manganese 469 market 510±12 Mater-Bi 6, 7, 155, 499, 511, 512 mechanical properties 204, 205, 206 material properties 29, 336±56 aliphatic-aromatic copolyesters 346 biodegradation 337±9 blends 347±8 future developments 349±52 microbial polyesters 341±3 natural polymers 340±1 PLA 343±4, 350 polyalkene succinate 345±6 polyanhydrides 347, 350 polycaprolactone 345, 350 polycarbonates/polyiminocarbonates 347, 350 polyglycolic acid 345, 350 polyorthoesters 346, 350 synthetic polyesters 343 water-soluble polymers 348±9 mechanical properties nanocomposite reinforced thermoplastic starch polymers 154±5 natural fibre composites PHA 192±6

523

PLA 199±201, 202 soy resin 208±10 starch 204±6, 207 synthetic polymers 210±11, 212 natural fibres 190 PHA 37±9 improvement in 39±40 PLA 259±61 blends with starch 265±6 protein-nanoparticle composites 298±306 medical applications 48±50, 336±7, 476 medium chain length PHA (mcl PHA) 32 microbial production 47±8 medium side-chain length PHAs 234 melt intercalation 291±2 melt polymerisation 112±13 melting behaviour of PHA 41±2 melting temperature PHA 233, 238±9, 240, 243 PLA 256±9 mercerised (regenerated) cellulose 223 metabolic engineering 46±7 Metabolix PHA 18±19 metal salt additives 63±4, 68 methane 74, 445 methyl methacrylate 12 methylenediphenyl diisocyanate (MDI) 269±70 micro fragments 509±10 microbial degradation 338±9 biodegradable polymers from sugars 170±2 cellulose-based polymers 225±6 PLA 368±9 polyester amides 122 polyurethane 382 PVA 376 microbial polyesters 3, 16±19, 341±3, 499 see also polyhydroxyalkanoates (PHAs) microbial production 3, 16±19 mcl PHA 47±8 PHB and PHBV 42±5 PHBHHx 45±7 screening of PHA producing bacteria 35, 36 microparticles 97±9 mid-bed trenching 459±60 mineralisation 325±6, 337, 413 of oxidised plastics in soil 328±9 test procedures 326±7 Ministry of International Trade and Industry (MITI) (Japan) 337

524

Index

miscellaneous polymers 3, 20±1 see also composites; natural rubber model compounds 176±7 modified polyolefins 3, 27±9, 313±15 see also TDPA (totally degradable plastic additives) technology modified thermoplastic starch polymers 153±5 molar mass reduction 321±2, 323 molecular mass 239±40 monomer activation mechanism 85, 86, 87 monomers for synthesis of polyester amides 108, 109 synthesis for synthesising polydepsipeptides 124±6 monosaccharides, polymers from 166±73 montmorillonite 99, 290±1 Cloisite Na+ 292, 293±306 modified thermoplastic starch polymers 154±5 morpholine-2,5-dione derivatives 124±6 ring-opening polymerisation 126±30 mulching 495±6 mulching films 451±60, 470, 504±7 time-controlled plastics 457±60, 470 municipal solid waste (MSW) 336, 437±8 mushroom growbags 463 nanocomposites 289±309 biodegradable 292±3 delaminating clay using ultrasonics 293±8 extrusion processing of proteinnanoparticle composites 298 microstructure and mechanical properties 298±306 PLA with fillers 263±5 preparation of aliphatic polyester-clay nanocomposites 99±100 reinforcement of thermoplastic starchbased polymers 154±5 nanocrystals 179 nanoparticles 97±9 Napac 12 natural fibres 21, 189±218 agricultural applications 513 biodegradability of composites 196±7, 201, 206 biodegradable polymers from sugars 178, 179 commercial developments 211±13 composites with PHA 191±8

composites with PLA 198±203, 211, 264 composites with soy resin 208±10 composites with starch 203±8 composites with synthetic polymers 210±11, 212 density and mechanical properties 190 mechanical properties of composites 192±6, 199±201, 202, 204±6, 207, 208±11, 212 other properties of composites 197±8, 201±3, 206±8 as polymer reinforcement 190±1 processing of composites 195, 197, 201, 202, 206, 207, 209, 212 natural polymers 3, 4±21, 289±90 agricultural applications 512±13 biodegradation mechanism 362±6 enzymatic degradation 423±7 lipids 3, 15±16 material properties 339, 340±1 microbial polyesters 3, 16±19 miscellaneous 3, 20±1 polyesters synthesised from bio-derived monomers 3, 19±20 polysaccharides 3, 4±13, 178±80 proteins 3, 14±15 sustainability 330±1 see also under individual types natural rubber (NR) 20±1, 320±1, 341, 362±3 Natureworks 78 networks 128±9 nickel 468±9 Nodax 18, 48, 232, 237 non-catalytic hydrolysis 361 non-renewable biodegradable polymers 140±1 Novon 6 nuclear magnetic resonance (NMR) spectra 240±2 nucleation 476, 479, 487, 491 nucleophilic catalysts 86, 87 nylon 385±6 oceans/seas 317±18 olefin metathesis reactions 176 organically modified layered silicate 263±4 Organization for Economic Cooperation and Development (OECD) 337 organometallic catalysts 80, 82 oriented PLA chains 260 oxidases 377, 378±9, 413

Index oxidation 337, 338, 357±9 degradation mechanisms 359±61, 362 polyolefins 58±62, 464, 466 oxidative scission 359±60, 360±1 oxidisability, enhanced 314±15 oxidoreductases 358, 359 oxo-biodegradability 467 oxo-biodegradable plastics 330 mineralisation in soil 328±9 oxo-biodegradable polyolefins 57±76 aerobic biodegradation 66 agricultural applications 454, 467±8 applications of 66±9 characteristics 57±8 control of polyolefin lifetimes 62±3 cost 442±3 definitions 58 environmental impact 69±73, 443±4, 448±9 future developments 73±4 oxidative degradation after use 63±5 initation 64±5 packaging 437±50 performance 441±2 polyolefin peroxidation 58±62, 464, 466 additive chemistry 61±2 basic chemistry 59±61 safety 443±4 oxo-biodegradation 58 carbon-chain polymers 320±2 polyolefins 464±6 oxygen absorption 323 oxygenases 413 packaging 222, 336, 437±50 agricultural 461±2 characteristics of packaging plastics 439±40 disposal 444±7 environmental impact 448±9 foamed PCL 476 oxo-biodegradable polyolefins 440±4 PHA applications 48 recovery 447±8 palm oil 16 paper agricultural applications 512±13 paper combinations in packaging 437, 438 Paragon 6 performance 441±2 peroxidases 365, 366, 385, 386, 426 peroxidation see oxidation

525

peroxide decomposers (PD) 314±15 peroxidolytic antioxidants 464±6 pesticides 464 phosphate 349 photo-acoustic FTIR studies 323 photochemically unstable (photodegradable) polymers 313±14 photodegradation 337 photosensitive copolymers 67±8 physical detoxification treatments 229±30 pineapple leaf fibre 210, 212 plant biomass 180, 181 see also forest biomass plant fibres see natural fibres plant growth test 508±9 plant oils 15±16 plant pots 461, 462, 497 plant protein see protein-nanoparticle composites plant yields 452±3, 507 Plantic 156 plants, synthesis of PHAs in 343 plasticised starch 149 plasticisers 149 for cellulose acetate 223 natural fibre composites 196, 200 PLA-based bioplastics 270±5, 279 plasticulture 451±64, 470±1 auxiliary products 460±4 protective films 451±60, 470 time-controlled plastics in protective films 457±60, 470 Plastor SG 454 pluronic F-108 273 polar plasticisers 271±3 polyacrylates 389±90, 391, 417 polyalkene succinate 345±6 polyalkylene alkanoates 367±8 polyamide resins 16 polyamides 418 polyamino acids 384±8 polyanhydrides 347, 350, 392, 419 polyaspartic acid (PAsp) 349, 385, 387±8 polybutylene adipate (PBA) 366 polybutylene adipate terephthalate (PBAT) 25±6, 350, 367 polybutylene succinate (PBS) 24, 345±6, 350, 366 starch with PBS and PBSA 8 polybutylene succinate adipate (PBSA) 24, 210, 346, 350 starch with PBS and PBSA 8

526

Index

polybutylene succinate terephthalate (PBST) 25, 367, 367±8 polybutylene terephthalate (PBT) 25, 423 polycaprolactone (PCL) 23, 77±8, 101, 143 biodegradation mechanism 366 blends 416 natural fibre 211 starch 7±8 commercial products of 24 copolymers 418 enzymatic degradation 419±20, 422 industrial production 84±5 material properties 345, 350 nanocomposites 99±100 ring-opening polymerisation in supercritical carbon dioxide 91±7 polycaprolactone foams 474±93 crystallinity 488±90 effect of processing conditions on the foaming cell 480±7 depressurisation rate 486±7 pressure 482±4 saturation time 484±6 temperature 480±2, 485±6 generation of 477±80 polycarbonates 347, 350, 372±3 polycondensation reaction 343 polycyanoacrylate 389±90, 417 polydepsipeptides 107, 124±31, 132 degradation 131 monomer synthesis 124±6 polymer synthesis 126±30 polydioxanone 390±1, 392 polyenol-ketone (PEK) 416 polyester amides (PEA) 24±5, 107±39 biodegradation 119±23 monomers 108, 109 natural fibre composites 210±11 polydepsipeptides 107, 124±31, 132 polymer synthesis 108±19 synthesis 107±23 polyesters agricultural applications 511, 512 aliphatic see aliphatic polyesters aromatic 3, 21±2, 25±6 biodegradation mechanism 365±72 blends 348 microbial 3, 16±19, 341±3, 499 natural 3, 16±20 synthesised from bio-derived monomers 3, 19±20 synthetic 3, 21±6, 343 polyethers 16, 373±6, 417±18

polyethylene (PE) 57, 222, 321, 350 biodegradation mechanism 389, 390 blends with starch 315, 347±8, 414±15, 454±5 enzymatic degradation 414±16 polyethylene adipate (PEA) 367 polyethylene glycol (PEG) 262 biodegradation mechanism 373±5 enzymatic degradation 417±18 as plasticiser 272±3 polyethylene terephthalate (PET) 25, 423 polyglutamic acid (PGA) 385, 386±7 polyglycolic acid (PGA) 22±3, 143, 339, 499 biodegradation mechanism 368±9 enzymatic degradation 420 material properties 345, 350 polyglycolides 339 polyglyoxylate 392 polyhydroxyalkanoates (PHAs) 16±19, 32±56, 142, 230±43, 511 applications 48±50 biodegradation 37, 234±5 mechanism 370±2 blends 348 natural fibre composites 191±8 forest-based microbial PHAs 237±43 future developments 50 history of microbial PHAs as biodegradable polymers 230±2 material properties 341±3 mechanical properties 37±9 improvement in 39±40 physical and chemical properties 232±4 process development and scale up for microbial PHA production 42±8 production from forest±based feedstocks 235±7 production based on renewable substrates 245±6 screening of PHA producing bacteria 35, 36 structure variations 32±5 thermal properties 40±2 polyhydroxybutyrate (PHB) 16±19, 32, 316, 341 applications 48±9 biodegradation 37 mechanism 370±2 crystallinity 41 enzymatic degradation 419±20, 421±2 from forest biomass 231, 233, 234, 235 material properties 342±3, 350 mechanical properties 39

Index improvement of 39±40 microbial production 42±4 natural fibre composites 191, 193±7 PH3B-co-4HB 342 PHBHHx 37±9, 40±1, 42 implants 48±50 microbial production 45±7 polyhydroxybutyrate-co-hydroxyvalerate (PHBV) 39, 40±1, 42, 421 biodegradation 37 from forest biomass 231±3, 234, 235, 237±43 material properties 342±3, 350 microbial production 44±5 natural fibre composites 191, 192 polyhydroxyvalerate (PHV) 16±19, 341 polyiminocarbonates 347, 350 poly(cis)-1,4-isoprene 363 polylactic acid/polylactide (PLA) 3±4, 19±20, 78, 101, 143, 182, 251±88, 339 ageing 275±80 agricultural applications 499, 511, 512 aliphatic polyester 22, 23 applications of PLA-based bioplastics 280±1 biodegradation 275±80 mechanism 368±9 blends 261±70 with fillers 263±5 miscibility of PLA with polymers 261±3 with natural fibres 198±203, 211, 264 with starch 8, 265±70, 271±3, 274, 280±1 copolymers of PGA and PLA 23 enzymatic degradation 278, 279, 369, 420±1 industrial production 84±5 Mitsui process 79 PDLLA (polyDL-lactic acid) 260±1, 263, 344 plasticisation of PLA-based bioplastics 270±5 PLLA (polyL±lactic acid) 78, 84, 88, 102, 344, 419±20 properties 252±61 material properties 343±4, 350 mechanical properties 259±61 rheology 252±5 thermal characteristics 88, 255±9 ring-opening polymerisation in supercritical carbon dioxide 91±7

527

thermoplastic starch polymers 152 polylactic acid oligomers 114±15 poly--lysine (-PL) 385, 386 polymer analogous reactions 166±73 polymer chain scission 359±61 polymer structure and biodegradation 379±80 main chain structure and enzymes responsible for degradation 358, 359 polyolefins 5, 16, 169, 313 biodegradation mechanism 389 modified 3, 27±9 types of 313±15 oxidation 58±62, 464, 466 oxo-biodegradable see oxobiodegradable polyolefins oxo-biodegradation 464±6 polyorthoesters 339, 346, 350, 391, 392 polypeptides 427 of aspartic acid and lysine 15 polyphosphazenes 392 polypropylene (PP) 73±4, 222, 321, 350 E-CO polypropylene 463 enzymatic degradation 416 polypropylene glycol (PPG) 375±6, 417 polysaccharides 499±500 enzymatic degradation 423±4 natural 3, 4±13, 178±80 synthetic 173±8 see also cellulose; lignin; starch polysiloxanes (silicone rubber) 177 polysodium vinyloxyacetate (PVOA) 380±1 polystyrene (PS) 73±4, 222, 313, 350, 438±9 biodegradable polymers from sugars 169±73 polystyrene maleic anhydride (PSMAH) 170±2 polytetramethylene adipate terephthalate (PTMAT) 26 polytetramethylene glycol (PTMG) 417 polyureas 418±19 polyurethanes 16 biodegradation mechanism 382±4 enzymatic degradation 382±4, 418±19 polyvinyl acetate (PVAC) 417 polyvinyl acetate-co-vinyl alcohol (PVAC-co-VA) 262 polyvinyl alcohols (PVA or PVOH) 3, 26±7, 143 biodegradation mechanism 376±82 blends with starch 6±7

528

Index

blends with thermoplastic starch 152 enzymatic degradation 379, 416±17 polyvinyl alcohol-co-ethylene (EVOH) 380 polyvinyl chloride (PVC) 222, 313 polyvinylphenol (PVPL) 262±3 polyvinylsaccharides (synthetic polysaccharides) 173±8 pressure 482±4 primary biodegradation 359 processing conditions and foam structure 480±7 natural fibre composites 195, 197, 201, 202, 206, 207, 209, 212 pathways 500±1 possibilities 28, 29 processing antioxidants 62, 65 product lifetime control 62±3, 67±8, 330, 440, 457±60, 470 production capacities 510±11 production costs 235±7 programmed-life biodegradable plastics 457±60, 470 prooxidant-modified polymers 314±15 proteases (proteolytic enzymes) 413 protective films 451±60, 470 time-control of degradable plastics in 457±60, 470 proteinase K 369, 421 protein-nanoparticle composites 289±309 delaminating clay using ultrasonics 293±8 microstructure and mechanical properties 298±306 processing using extrusion 298 proteins 3, 14±15, 341, 499, 499±500 Pseudomonas oleovorans 47±8 Pseudomonas stutzeri 47, 48 PVA dehydrogenase (PVADH) 378±9 PVA Erkol 27 radical cation intermediates 379 ramie 190, 205, 207, 209 random polyester amides 114±19, 120, 121, 122 random scission 360 reactive blending 348 reactive extrusion 87±90 recovery 140 in biologically active environments 317±19 options 317 oxo-biodegradable polyolefins 447±8 recovery costs 236

recycling 140, 317, 393, 447±8 reduction strategy 140 regenerated (mercerised) cellulose 223 regicity 120±1 renewable/biodegradable polymers 140±1 renewable forest resources see forest biomass renewable/non-biodegradable polymers 140 renewable substrates 245±6 re-use 140, 317, 444 reversible carbonatation 93±4, 95 ring-opening polymerisation 77±106, 343 future developments 101±2 lactones and lactides in supercritical carbon dioxide 91±7 polydepsipeptides 126±30, 132 preparation of polyester amides 116±19 processing of aliphatic polyesters in supercritical carbon dioxide 97±101 reactive extrusion 87±90 synthesis of aliphatic polyesters 77±87 all organic mediators 85±7 enzymatic 85, 86, 102, 130 initiated by aluminium alkoxides 80±2 initiated by tin octoate 82±4 rivers 317±18 root development 457 rubber natural 20±1, 320±1, 341, 362±3 silicone 177 synthetic 363 safety 443±4 saponite 290±1 saturation time 484±6 scaffolds 49 scanning electron microscopy (SEM) 480, 482, 483 Scott/Gilead technology 68, 314±15 agricultural films 442, 443±4, 448±9, 452±3, 459 sebacic acid 423 secondary alcohol oxidase (SAO) 377, 378±9 seed sowing 464 segmented polyester amides 112±14, 121±2, 123 sewage 317±18 shake-flask cultures 238, 239

Index shear controlled orientation injection moulding (SCORIM) process 153, 157 shopping (carrier) bags 62±3, 69, 437±8, 447 side chain liquid crystalline polymer (SCLCP) 150±1 silage wrap 462±3, 471 silicone rubbers (polysiloxanes) 177 silks 341 sisal 190, 205, 207, 210, 212 sodium montmorillonite (Cloisite Na+) 292, 293±306 softwoods 226±7 soil 317±18 degradable plastics in 327±8 mineralisation of oxo-biodegradable plastics 328±9 soil bacteria 170±1 soil burial tests 501, 503±4 soil temperature 453±5, 457, 507 Solanyl 20, 155±6 solar sterilisation films 460 Solplax 27 solubilisation, alkaline 365 soluble biodegradable polymers 156±7, 348±9 Solvay 499 solvent intercalation 291±2 sonification 293±8 soy proteins 15, 289±90, 341 soy resin-natural fibre composites 208±10 soybean oil 16 stabilisers 61±2, 458±9, 464±6 standard test procedures 325±9, 337±8 standards 313±35 bio-based polymers 316±17 degradable plastics in soil 327±8 degradation mechanisms 319±22 development of national and international standards 323±9 laboratory studies 322±3 legislation 315±16 lessons from the past and future developments 329±31 mineralisation of oxidised plastics in soil 328±9 mineralisation test procedures 326±7 necessity of 313±16 post-use treatment of plastics for the recovery of value 317±19 simulated weathering procedures 328 starch 4±8, 141±9, 165, 178, 179±80 agricultural applications 498, 511, 512

529

blends commercial blends 6±8 material properties of blends 347±8 natural fibre composites 203±8 with PLA 8, 265±70, 271±3, 274, 280±1 with polyethylene 315, 347±8, 414±15, 454±5 with polyolefins 169 with synthetic polymers 499 crystallinity 147, 148±9 enzymatic degradation 425±6 first generation of starch polymers 5 genetics 141±4 granule diversity 144±6 granule structure 148±9 macromolecular structure 146±7 material properties 340 nanocomposites 292±3 second generation of starch polymers 5±8 synthesis in wheat endosperm 144 thermoplastic polymers see thermoplastic starch-based polymers steam explosion process 182, 193, 228 steel 447 step-growth polymerisation 79 storage modulus 302, 303, 304, 305 stress shielding 336±7 stress-strain curves 273, 274 stretch-wrap films 462±3, 471 styrene 12 substrate costs 236±7 succinic acid 115±16, 121 sugarcane bargasse 182 sugar-based biodegradable polymers 165±88 applications of synthetic polysaccharides 176±8 commodity plastics 169±73 from hemicelluloses 180±3 from monosaccharides and disaccharides 166±73 from natural polysaccharides 178±80 from synthetic polysaccharides 173±8 supercritical antisolvent precipitation (SAS) 97±9 supercritical carbon dioxide 91±102 PCL foams see polycaprolactone foams processing of aliphatic polyesters in 97±101 ring-opening polymerisation of lactones and lactides in 91±7

530

Index

supercritical fluids 474±6 phase diagram 475, 476 Supol 5 surface degradation 361 surface tension 279±80 sustainability 351±2 polymer production and recycling 393 strategies 140 symbiotic mixed culture 378±9 synthetic polyesters 3, 21±6, 143, 343, 499 aliphatic see aliphatic polyesters aromatic 3, 21±2, 25±6 synthetic polymers 3, 21±9, 339 blends with starch 499 modified polyolefins 3, 27±9, 313±15 natural fibre composites 210±11, 212 polyvinyl alcohols see polyvinyl alcohols synthetic polysaccharides applications 176±8 biodegradable polymers from 173±8 synthetic rubber 363 syringy 10, 11 Taiwan 452±3, 454±5, 459 tartaric acid 115±16, 121 taxation 438 TDPA (totally degradable plastic additives) technology 27 applications 68±9 compost bags 322, 446±7 degradation of polyolefins 64±5 environmental impact 70 oxo-biodegradable polyolefins in packaging 441±4, 445, 446±7, 449 use in polypropylene and polystyrene 73±4 temperature commercial composting and landfill 64±5 and growth of bubbles in PCL foams 479 and PCL foam structure 480±2, 485±6 range for natural fibre-PLA composites 201±3 tensile strength 304, 305 terraces, growth 41 thermal degradation onset temperature 239, 240, 243 thermal gravimetric analyser (TGA) 304±6 thermal loss due to plasticisers 275

thermoforming 28 thermoplastic starch-based polymers 140±62 applications 156±7 blends with other polymers 152±3, 154 commercial applications and products 155±6 future developments 157 modified 153±5 thermoplastic starch and their blends 149±53 thiodiphenol (TDP) 264 time-controlled biodegradable plastics 62±3, 67±8, 330, 440, 457±60, 470 tin(IV) alkoxide 92±4, 95 tin octoate 82±4, 87±90 toluene diisocyanate (TDI) 269 TONE 78 tone polymers 24 transition metal salt additives 63±4, 68 transition metals 465, 468±9 transmission electron microscopy (TEM) 290, 299±300, 301 Treeplast 11 triacetin 200 triglyceride oil-based polymers 16, 142 triphenylphosphine 87±90 triple point 475, 476 twin-screw extruders 87±90 twines 496±7 two-crop mulching films 459 two-dimensional FTIR (2D FTIR) correlation spectroscopy 40±1 ultimate biodegradation 359 ultrasonic delamination of clay 293±8 ultrasound irradiation 182 unbleached kraft paper 437, 438 UV stabilisers 61, 62 UV weatherometer tests 324, 326, 328 valinomycin 125 VeÂgeÂmat 6 Vibrio fischeri bioluminescence test 508±9 Vicryl 23 vinyl alcohol blocks 381±2 vinyl polymers enzymatic degradation 414±19 see also under individual types vinyl sugar synthesis 174 viscosity average molecular weight 240, 243

Index wastewater treatment plants 349 water conservation in plasticulture 456 as plasticiser 271±2 protein-nanoparticle composites 293±306 water absorption natural fibre-starch composites 206±8 PLA-starch blends 267 water resistance 499 water-soluble polymers 156±7, 348±9 Wautersia eutropha 42, 43±5 weathering tests 324, 326, 328 weed control 456±7 wheat gluten 15, 341 wheat straw 182 whey protein 341

531

wide-angle X-ray diffraction (WAXS) 290, 293±7, 298±9 wood 165 composites with wood fibre 191, 194, 195, 196±7 wood powder blends 10±12 X-ray diffraction (XRD) 488±9 wide-angle 290, 293±7, 298±9 xenon arc weatherometers 324, 326, 328 xylan 182±3, 426 xylan±1,4±xylosidase 426 xylose 237±8, 239, 240±2 yields, commercial 452±3, 507 zero-shear viscosity 253±5