Biodegradable and sustainable fibres (Woodhead Publishing Series in Textiles)

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Biodegradable and sustainable fibres

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Related titles from Woodhead’s textile technology list: Regenerated cellulose fibres (ISBN-13: 978-1-85573-459-3; ISBN-10: 1-85573-459-1) Bast and other plant fibres (ISBN-13: 978-1-85573-684-9; ISBN-10: 1-85573-684-5) Green composites (ISBN-13: 978-1-85573-739-6; ISBN-10: 1-85573-739-6) Environmental impact of textiles (ISBN-13: 978-1-85573-541-5; ISBN-10: 1-85573-541-5) Handbook of nonwovens (ISBN-13: 978-1-85573-603-1; ISBN-10: 1-85573-603-9)

Details of these books and a complete list of Woodhead’s textile technology titles can be obtained by: ∑ visiting our website 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)

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Biodegradable and sustainable fibres Edited by R. S. Blackburn

CRC Press Boca Raton Boston New York Washington, DC

WOODHEAD

PUBLISHING LIMITED Cambridge England iii

Published by Woodhead Publishing Limited in association with The Textile Institute Abington Hall, Abington, Cambridge CB1 6AH, England www.woodheadpublishing.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2005, Woodhead Publishing Limited and CRC Press LLC © 2005, Woodhead Publishing Limited; 2005, Chapter 13 © Mary M. Brooks The authors have asserted their moral rights. Every effort has been made to trace and acknowledge ownership of copyright. The publishers will be glad to hear from the copyright holders whom it has not been possible to contact concerning the following: Figs 10.1–10.14, 13.1, 13.4, 13.6, 13.8, 13.9, 13.11 and 13.12. 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 Woodhead Publishing Limited. The consent of Woodhead Publishing Limited 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 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-13: Woodhead Publishing Limited ISBN-10: Woodhead Publishing Limited ISBN-13: Woodhead Publishing Limited ISBN-10: CRC Press ISBN 0-8493-3484-5 CRC Press order number: WP3484

978-1-85573-916-1 (book) 1-85573-916-X (book) 978-1-84569-099-1 (e-book) 1-84569-099-0 (e-book)

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, Dunstable, Bedfordshire (email: [email protected]) Typeset by Replika Press Pvt Ltd, India Printed by T J International Limited, Padstow, Cornwall, England

iv

Contents

Contributor contact details

xi

Introduction

xv

R S BLACKBURN, University of Leeds, UK

1

Microbial processes in the degradation of fibers

1

P M FEDORAK, University of Alberta, Canada

1.1 1.2 1.3

1.7 1.8 1.9

Introduction Background and terminology Incubation conditions used for studying biodegradation of fibers and films Sources of microorganisms and enzymes for laboratory incubations Analytical methods used to assess biodegradation of fibers and films Examples of types of bonds that are susceptible to enzymatic attack Future trends Acknowledgements References

2

Bast fibres (flax, hemp, jute, ramie, kenaf, abaca)

1.4 1.5 1.6

1 1 8 12 17 24 29 31 31 36

R KOZLOWSKI, P BARANIECKI and J BARRIGA-BEDOYA, Institute of Natural Fibres, Poland

2.1 2.2 2.3 2.4 2.5 2.6 2.7

Introduction Flax Hemp Jute Ramie Kenaf Abaca

36 37 51 60 70 78 81 v

vi

Contents

2.8 2.9

Comparison of fibre properties References

86 86

3

Alginate fibers

89

J M MURI and P J BROWN, Clemson University, USA

3.1 3.2 3.3 3.4 3.5 3.6 3.7

Introduction The chemical nature of alginate materials Physical properties of alginate-based materials Industrial applications of alginates Fabrication of alginates as useful flexible substrates in medical textile-based products Alginates in bioengineering References

89 92 96 100 101 105 107

4

Cellulosic fibres and fabric processing

111

D CIECHAÑSKA, Institute of Chemical Fibres, Poland and P NOUSIAINEN, Tampere University of Technology, Finland

4.1 4.2 4.3

4.6 4.7 4.8 4.9

Introduction Life cycle assessment (LCA) The mechanisms of enzymatic reactions on wood and cellulose Biodegradability of cellulose fibres in textile blends Biotechnology for manufacture and modification of cellulosic fibres Enzyme applications in fabric and dyestuff processing Hygienic and medical fibres Future trends References

133 140 144 150 151

5

Lyocell fibres

157

4.4 4.5

111 112 120 131

®

P WHITE, M HAYHURST, J TAYLOR and A SLATER, Lenzing Fibers Ltd, Derby, UK

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

Introduction Process description Lyocell sustainability Lyocell fibre properties Lyocell in textiles Lyocell – a versatile, high performance fibre for nonwovens Marketing Future trends Sources of further information

157 159 165 171 172 181 187 188 188

Contents

6

Poly(lactic acid) fibers

vii

191

D W FARRINGTON, Consultant, UK, J LUNT, S DAVIES, NatureWorks LLC, USA and R S BLACKBURN, University of Leeds, UK

6.1 6.2 6.3 6.4 6.5 6.6 6.7

Introduction Chemistry and manufacture of PLA polymer resin PLA fiber properties Applications Environmental sustainability Future trends References

191 192 197 200 211 218 219

7

Poly(hydroxyalkanoates) and poly(caprolactone)

221

I CHODÁK, Polymer Institute of the Slovak Academy of Sciences, Slovakia, and R S BLACKBURN, University of Leeds, UK

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10

Introduction PHA-based oriented structures Poly(caprolactone)-based fibres Structure of drawn fibres Thermal properties Enzymatic and hydrolytic degradation Other biodegradable and sustainable polyesters Application of polyester-based biodegradable fibres Future trends and concluding remarks References

221 222 232 235 236 237 238 239 241 242

8

The route to synthetic silks

245

F VOLLRATH and A SPONNER, University of Oxford, UK

8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

Introduction Silk structures Development of fibre: the feedstock Development of fibre: spinning Performance characteristics Applications Future trends Acknowledgements References and sources of further information

245 245 248 255 256 262 262 264 264

9

Biodegradable natural fiber composites

271

A N NETRAVALI, Cornell University, USA

9.1 9.2 9.3

Introduction Biodegradable fibers Biodegradable resins

271 274 279

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Contents

9.4 9.5 9.6 9.7

Soy protein-based green composites Conclusions and future trends Acknowledgements References

295 304 304 305

10

Biodegradable nonwovens

310

G BHAT, University of Tennessee, USA and H RONG, Johnson Controls Inc., USA

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 11

Introduction Nonwoven fabrics Fiber consumption in nonwovens Web formation methods Web bonding techniques Technology and relative production rate Recent research on biodegradable nonwovens Applications of biodegradable nonwovens Flushable nonwovens Leading producers of nonwovens Sources of further information and advice References

310 311 314 315 319 321 322 336 337 338 338 340

Natural geotextiles

343

C LAWRENCE, University of Leeds, UK and B COLLIER, University of Tennessee, USA

11.1 11.2 11.3 11.4 11.5 11.6 11.7

Introduction Fundamental aspects of geotextiles Fibres used for natural geotextile products Fibre extraction and preparation Production of natural geotextile products Measurement of the properties of natural geotextiles References

343 344 345 351 355 362 365

12

Conversion of cellulose, chitin and chitosan to filaments with simple salt solutions

367

H S WHANG, N AMINUDDIN, Fiber and Polymer Science Program, USA, M FREY, Cornell University, USA, S M HUDSON and J A CUCULO, Fiber and Polymer Science Program, USA

12.1 12.2 12.3 12.4 12.5

Introduction Cellulose in liquid ammonia/ammonium thiocyanate solutions Fibers from chitin and chitosan Future trends Sources of further information

367 368 380 393 394

Contents

ix

12.6

References

395

13

Soya bean protein fibres – past, present and future

398

M M BROOKS, University of Southampton, UK

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 Index

Introduction The soya bean plant Naming regenerated protein fibres The need for new fibre sources Generalised method for producing soya bean fibre in the mid-twentieth century Contemporary research into alternative protein fibre sources Contemporary methods for producing fibres from soya bean protein Fibre characteristics Identifying soya bean protein fibres Degradation behaviour A truly biodegradable and ecological fibre? Conclusion Acknowledgements References

398 398 400 401 413 420 422 425 428 431 434 434 435 435 441

x


(* = main contact)

Introduction

Chapter 2

Dr Richard S. Blackburn Green Chemistry Group Centre for Technical Textiles University of Leeds Leeds LS2 9JT UK

Dr Ryszard Kozlowski*, Przemyslaw Baraniecki and Jorge Barriga-Bedoya Institute of Natural Fibres ul. Wojska Polskiego 71 B 60630 Poznan¢ Poland

Tel: +44 (0)113 343 3757 Fax: +44 (0)113 343 3704 E-mail: [email protected]

E-mail: [email protected]

Chapter 3 Chapter 1 Dr Phillip M. Fedorak Biological Sciences Building University of Alberta 114 St–89 Ave Edmonton Alberta Canada T6G 2M7 Tel: (780) 492-3670 Fax: (780) 492-9234 E-mail: [email protected]

Dr J. M. Muri and Dr Philip J. Brown* Clemson University School of Materials Science & Engineering Clemson University 265 Sirrine Hall Clemson, SC 29634-0971 USA E-mail: [email protected]

xi

xii


Chapter 4 Dr Danuta Ciechañska* Institute of Chemical Fibres ¢ 19/27 ul. M. SkSodowskiej-Curie ¢ Lódz 90-570 Poland E-mail: [email protected] Professor Pertti Nousiainen* Kuitumateriaalitekniikka Institute Tampere University of Technology PO Box 527 33101 Tampere Finland E-mail: [email protected]

Chapter 5 Patrick White MBE, Dr Malcolm Hayhurst, Jim Taylor* and Andrew Slater Lenzing Fibers Limited 1 Holme Lane Spondon Derby DE21 7BP UK

Dr Richard S. Blackburn* Green Chemistry Group Centre for Technical Textiles University of Leeds Leeds LS2 9JT UK Dr James Lunt and Mr Steve Davies NatureWorks LLC 15305 Minnetonka Boulevard Minnetonka MN 55345 USA

Chapter 7 Dr Ivan Chodak* Slovak Academy of Sciences Centre of Excellence CEDEBIPO 872 36 Bratislavia Slovakia E-mail: [email protected]

Tel: 01332 682359 E-mail: [email protected]

Dr Richard S. Blackburn* Green Chemistry Group Centre for Technical Textiles University of Leeds Leeds LS2 9JT UK

Chapter 6

Chapter 8

Mr David W. Farrington* Beech Edge 7 The Common Quarndon Derby DE22 5JY UK

Professor Fritz Vollrath* and Professor Alexander Sponner Department of Zoology University of Oxford South Parks Road Oxford OX1 3PS UK




Chapter 9 Dr Anil N. Netravali Cornell University 9 St Joseph Lane Ithaca NY, 14850 USA

xiii

Professor Carl Lawrence* Centre for Technical Textiles University of Leeds Leeds LS2 9JT UK E-mail: [email protected]


Chapter 12 Chapter 10 Professor Gajanan S. Bhat* Department of Material Science and Engineering University of Tennessee Knoxville TN 37996 USA E-mail: [email protected] Dr Haoming Rong Materials Process Engineer Johnson Controls, Inc., Holland MI 49423 USA

Chapter 11 Dr Billie J. Collier* Associate Vice Chancellor for Research Compliance Director, Textiles and Nonwovens Development Center 1534 White Avenue University of Tennessee Knoxville TN 37996 USA Tel: 865-974-2474 Fax: 865-974-7400 E-mail: [email protected]

Dr Hyun Suk Whang, Dr Norman Aminuddin, Dr Samuel M. Hudson* and Dr John A. Cuculo Fiber and Polymer Science Program North Carolina State University College of Textiles 2401 Research Drive Box 8301 Raleigh NC 27695 USA E-mail: [email protected] Dr Margaret Frey Cornell Center for Materials Research 627 Clark Hall of Science Cornell University Ithaca NY 14853 USA

Chapter 13 Mary M. Brooks Textile Conservation Centre University of Southampton Park Avenue Winchester Hampshire SO23 8DL UK Tel: 02380 597100 E-mail: [email protected]

xiv

Introduction R S B L A C K B U R N, University of Leeds, UK

At the start of the third millennium the world population was approximately six billion, which is expected to rise to ten billion by the middle of the twentyfirst century. The exponential increase in population increases the demand on food, energy, water, resources and chemicals, and effects a corresponding increase in environmental pollution and a depletion of finite resources (e.g. fossil fuels). Since the 1930s, research and development into synthetic chemical products has afforded a significant improvement in the quality of life and availability of products for consumption. Not least being synthetic polymers, specifically fibres, for apparel and furnishing applications. Wallace Carothers and DuPont developed the first synthetic polyamide, nylon, in 1935; Whinfield, Dickson, Birtwhistle and Ritchie advanced the early research of Carothers, creating the first polyester fibre called Terylene (based on polyethylene terephthalate) in 1941 manufactured by Imperial Chemical Industries; DuPont followed this up with the invention of Dacron in 1946. Other synthetic fibres were also developed, including polyurethane (Bayer, 1937), acrylic (DuPont, 1944), polypropylene and high density polyethylene (both Banks and Hogan, 1951). The main problems with synthetic polymers are that they are non-degradable and non-renewable. Since their invention, the use of these synthetic fibres has increased oil consumption significantly, and this continues today; arguably, polyester now is the most used of all fibres, taking over from cotton. Oil and petroleum are non-renewable (non-sustainable) resources and at the current rate of consumption, these fossil fuels are only expected to last for another 50– 60 years; the current petroleum consumption rate is estimated to be 100,000 times the natural generation rate.1 The Energy Information Administration projects that world conventional oil production will peak somewhere between 2021 and 2112, depending on the annual production growth rate (0–3%) and resource estimates (2248–3896 billion barrels). A maximum production growth rate (3% per year) combined with a low resource estimate (2248 billion barrels) gives a peak production year of 2021. For the expected (mean–resource) USGS case (3003 billion barrels) the peak will be somewhere between 2030 and 2075. This means that the raw material for fibres will change. xv

xvi

Introduction

An even more important problem with the use of fossil energy is the huge translocation of carbon from the ground into the atmosphere accompanied by emissions of sulphur and nitrogen oxides as well as all kinds of hydrocarbons and heavy metals. Fossil fuels are also the dominant global source of anthropogenic greenhouse gases (GHG), rising concentrations of which are widely understood to drive global warming;2 a growing majority of the scientific community believes this will lead to an unstable and unpredictable climate. Global warming can lead to more frequent and more extreme weather events such as floods, droughts, heatwaves, wind-storms, ice-storms, hurricanes and cyclones. Other negative effects are an increase in air pollution; increase in water- and food-borne diseases; the arrival of diseases like malaria, dengue fever and yellow fever; an increased number of wildfires; the loss of land by sea level rising; the forced migrations of people, plants and animals that can result in a serious reduction in the number of species; drop in prosperity and even starvation. Even climate change sceptics have expressed support for increased efforts to better understand the issues. More cautious business leaders increasingly view fossil-fuel-related emissions and global climate change as a key risk parameter, with strong potential to adversely impact long-range business planning goals and objectives. Of even more concern is the ability of polymeric fibres to remain unchanged in the environment as such polymers do not degrade very readily, which has exacerbated the already existing ecological and environmental problems of waste building; the volume in waste disposal and landfill is very high. Landfills are decreasing in number, making less space available to discard waste. In the last few years the Republic of Ireland declared that they no longer had any space for landfill, imposing large taxes on the use and disposal of polymers. Landfill space in the UK is decreasing and in the US alone, the number of landfills dropped from 8000 to 2314 between 1988 and 1998. 1 Many governments, in response, have established laws to encourage recycling;3 some governments have enforced stricter ‘take-back’ rules requiring manufacturers to take back packaging and products at the end of their life.

Biodegradable fibres A material is defined as ‘biodegradable’ if it is able to be broken down into simpler substances (elements and compounds) by naturally occurring decomposers – essentially, anything that can be ingested by an organism without causing that organism harm. It is also defined that it must be non-toxic and able to be decomposed in a relatively short period even on a human time scale.4 Albertsson and Karlsson5 defined the biodegradation of a polymeric material as ‘an event which takes place through the action of enzymes and/or chemical decomposition associated with living organisms (bacteria, fungi, etc.) and their secretion products’. Biodegradable polymers can be classified6 into three main categories:

Introduction

xvii

1. Natural polysaccharides and biopolymers; ∑ e.g. cellulose (Chapters 2, 4, 5 and 12), alginates (Chapter 3), wool, silk (Chapter 8), chitin (Chapter 12), soya bean protein (Chapter 13). 2. Synthetic polymers, particularly aliphatic polyesters; ∑ e.g. poly(lactic acid) (Chapter 6), poly(e-caprolactone) (Chapter 7). 3. Polyesters produced by microorganisms; ∑ e.g. poly(hydroxyalkanoate)s (Chapter 7). Biodegradable polymers and the fibres that can be produced from them are very attractive in offering a possible solution to waste-disposal problems, but these polymers tend to have a high price associated with them (Table I.1), hence applications of these polymers need to be found and taken on by manufacturers in order to consume sufficiently large quantities of these materials and drive the price down so that they can compete economically in the market. Table I.1 Cost comparison of traditional and biodegradable polymers7 Material

Average cost $/kg–1

Traditional polymers

poly(propylene) high density poly(ethylene) poly(ethylene terephthalate)

0.73 0.82 1.15

Biodegradable polymers

poly(lactic acid) poly(hydroxyalkanoate)s

3.30–6.60 8.80–13.90

One of the most important factors in developing new biodegradable fibres that can compete economically is the public perception of what a biodegradable polymer is (or should be); demand for such products can be driven by the public and the media. ‘Biodegradable’ chemistry is generally perceived by the public to be good for the environment (although that statement alone could be seen as a paradox, the term ‘chemistry’ often being associated with ‘dirty’ processes). Some industries use this to their advantage, but what is purported to be ‘green’ is often not so in reality. The paper industry claimed that paper packaging should be used as an alternative to plastic because paper was ‘biodegradable’ and plastic was not, without having any scientific evidence to support these claims; in actual fact, in a well-engineered landfill environment neither paper nor plastic is biodegradable. Polymer producers developed the first generation of ‘degradable’ polymers in the 1980s, which consisted of polyolefin polymers with starch additives that would cause fragmentation of the composite into polymer pieces in a biodegradable environment. However, in 1990 a class-action lawsuit forced producers to remove the degradable claim. The US Federal Trade Commission has since created guidelines 8 for environmental marketing claims related to degradability, biodegradability, photodegradability, compostability and recyclability.

xviii

Introduction

Recyclability is often confused by the public with biodegradability, the terms often being regarded as interchangeable. Obviously this is not the case, as recyclability refers to retrieving useful materials from waste via either mechanical or chemical breakdown. Recyclability of materials, however, is made publicly obvious through labelling techniques, in a way that biodegradability is not. The universal recycling symbol (Fig. I.1) means that the product is both recyclable and made of recycled materials. Manufacturers also use the symbol shown in Fig. I.2, developed by the Society of the Plastics Industry, to indicate the type of plastic used for the packaging; SPI code numbers range from 1 to 7.

I.1 Universal recycling symbol.

1

I.2 SPI symbol indicating source material for possible recycling: (1) poly(ethylene terephthalate); (2) high density poly(ethylene); (3) poly(vinyl chloride); (4) low density poly(ethylene); (5) poly(propylene); (6) poly(styrene); (7) other.

Recycling of polymers is on the increase and should be encouraged, but the process of both material and chemical recycling consumes a significant amount of energy, and, even if very efficient, could not cope with all polymers used. It is therefore very easy to understand the necessity for biodegradable polymeric fibres, which can be recycled by microorganisms. While in some ways biodegradable polymers and plastics recycling complement each other, there are concerns that widespread use of biodegradable polymers could be detrimental to recycling. The main concern is that the contamination of recycled polymers with biodegradable polymers could adversely affect the properties of recycled polymers. This is becoming a common concern for many newly developed polymers, biodegradable or not. Disposal of biodegradable polymers is most appropriately carried out by the public through a composting mechanism, but this system requires an

Introduction

xix

infrastructure, including collection systems and composting facilities. Germany has invested in compost infrastructure and more than 60% of all German households have been issued organic waste bins, whose contents are collected for composting. In 2001–2002 a successful pilot study was undertaken in Kassel, Germany to demonstrate the use of biodegradable packaging in connection with composting. The UK’s first certification scheme for compostable packaging was launched by the Composting Association9 in 2003. The scheme enables certification to the DIN V 54900, BS EN 13432 and ASTM D 6400 standards. In order to achieve certification materials, intermediates and additives are exhaustively tested in four different areas: 1. 2. 3. 4.

Chemical test (test for heavy metals). Complete biodegradation. Disintegration under compost conditions. Ecological test (plant toxicity).

In addition to ensuring compostability, certification enables biologically degradable products to be identified by way of clear labelling. The compostability mark (Fig. I.3) serves to inform both waste consumers and disposers and the product must bear the inscription ‘compostable’ as well as the registration number assigned to it during the certification process.

Compostable

I.3 Composting Association compostability mark.

Second-generation biodegradable polymers were commercially introduced around 1990 and are represented by the starch-based products offered by Novamont (Mater-Bi™) and by several families of polyesters. One of these polyesters, poly(e-caprolactone), has been commercially available for more than twenty years; other biodegradable polyesters, which have been commercialized very recently, include poly(lactic acid) and other aliphatic polyesters. As a result of plant investments made by Cargill-Dow LLC (now NatureWorks LLC) and others, biodegradable polyesters should become more affordable very soon. Just as with most other polymers, processability is an important parameter to commercial success for biodegradable polymers. For example, some grades of starch-based polymers can be processed on standard low-density poly(ethylene) extrusion equipment for making blown or cast film. Other grades

xx

Introduction

can be extruded on existing equipment with minor die modifications to make loose-fill foam. Poly(lactic acid) can be processed in ways similar to processing of polyolefins and can be extruded with modifications. Performance properties are also important parameters to commercial success.

Sustainable fibres Arguably more important than biodegradability is the concept of ‘sustainability’. By definition, sustainable living is taking no more potentially renewable resources from the natural world than can be replenished naturally and not overloading the capacity of the environment to cleanse and renew itself by natural processes.4 Resources are sustainable if they cannot be used up; for instance, oil resources are gradually decreasing whereas the wind can be harnessed to produce energy continuously. In terms of fibres, a sustainable fibre is one that ideally involves completely renewable chemicals10 in its production and non-fossil-fuel-derived energy in the production processes. Renewable sources of polymeric materials offer an answer to maintaining sustainable development of economically and ecologically attractive technology. Vink11 set out a number of factors that the ideal sustainable material should meet; it should: ∑ provide an equivalent function to the product it replaces, and perform as well as or better than the existing product; ∑ be available at a competitive or lower price; ∑ have a minimum environmental footprint for all the processes involved, including those up- and down-stream; ∑ be manufactured from renewable resources; ∑ use only ingredients that are safe to both humans and the environment; ∑ not have any negative impact on food supply or water. These criteria reflect a strong empathy with the need to address the environmental aspects, and Vink demonstrated the positive benefits that poly(lactic acid) could achieve, both in terms of the manufacturing process, as well as the waste management disposal options at the end of a product’s useful life. The most important concept in terms of a truly ‘green’ material (in terms of this book, a fibre) is the concept of a fully green life cycle of the product. This embraces innovations in the development of materials from biopolymers and other renewable resources; the preservation of fossil-based raw materials; the reduction of fossil fuels used in energy production for fibre processing; the reduction in the volume of waste; compostability in the natural cycle; complete biological degradability; protection of the climate through the reduction of carbon dioxide released; and the reduction and elimination of hazards and environmentally detrimental chemistry at any point in the life cycle.12 An idealised life cycle for a green fibre is given in Fig. I.4.

Introduction

xxi

Fibres

Fibre extrusion

Biodegradable waste CO2 H2O

Polymer production

Extracting renewable sources (starch, cellulose, etc.)

Composting

CO2 H2O Plants

Photosynthesis

I.4 Life cycle of compostable, biodegradable fibres.

The key measurement tool to assess the environmental sustainability of a product is Life Cycle Assessment (LCA). Life cycle inventory analysis accounts for all inputs and outputs for a particular product and is typically practiced on a cradle-to-grave basis. A key benefit of LCA is the opportunity to benchmark performance against competitor products and processes in the marketplace, both to justify performance claims and to identify operations appropriate for performance improvement efforts.

Recent developments in biodegradable polymers This book focuses on polymers and their fibres defined and detailed in Section 1.2. However, new biodegradable polymers are appearing frequently due to the demand and interest for this technology. Although these polymers have no specific fibre application at press they should be examined in future research and development to afford new biodegradable fibre opportunities and applications.13

References 1. Stevens, E.S., Green Plastics, Princeton University Press, Princeton, 2002. 2. Sharron, E., Global climate change and the challenges of stewardship: man and nature in the 21st Century, 2 June 2002, Climate Independent Media Center. Available from: http://www.climateconference.org. 3. Nir, M.M., Miltz, J. and Ram, A., Plastics Engineering, March 1993, 75. 4. http://ecology.org/biod/library/glos_index.html, visited December 2004. 5. Albertsson, A.-C. and Karlsson, S., in: Chemistry and Technology of Biodegradable Polymers, Blackie, Glasgow, 1994, 48. 6. Okada, M., Prog. Polym. Sci., 2002, 27, 87.

xxii

Introduction

7. Mohanty, A.K., Misra, M. and Hinrichsen, G., Macromol. Mater. Eng., 2000, 276/ 277, 1. 8. http://www.ftc.gov/bcp/grnrule/guides980427.htm. 9. http://www.compost.org.uk/. 10. Biopolymers from Renewable Resources; Macromolecular Systems – Materials Approach, Kaplan, D.L. (ed.), Springer-Verlag, Berlin, Heidelberg, 1998. 11. Vink, E.T.H., Rabago, K.R., Glassner, D.A. and Gruber, P.R., Polymer Degradation and Stability, 2003, 80 (3), 403. 12. Lörcks, J., Polym. Degrad. Stab., 1998, 59, 245. 13. Okada, M., Prog. Polym. Sci., 2002, 27, 87.

1 Microbial processes in the degradation of fibers P M F E D O R A K, University of Alberta, Canada

1.1

Introduction

As society becomes more concerned about environmental quality, there are moves toward producing and using materials that will not accumulate in the environment. For example, many plastics that are made from petrochemicals persist indefinitely after being discarded. Similarly, synthetic fibers such as nylon or polypropylene are extremely stable in dark environments. Thus, there is a trend toward producing and using fibers that will break down after their disposal. Depending upon the fiber, their breakdown may occur by biotic or abiotic processes. Biotic processes involve biochemical reactions that are typically mediated by microorganisms such as bacteria and fungi. Abiotic processes include chemical oxidation and hydrolysis, and photodegradation. This chapter will focus only on biotic processes. Of course, natural fibers like wool and cotton are broken down through biotic processes. Microorganisms have evolved enzymes that attack key bonds in these natural polymers, thereby releasing monomers that can be used as carbon and energy sources for microbial growth. In contrast, microorganisms lack enzymes to break down many synthetic fibers, thus these materials persist and accumulate in the environment. This chapter provides an overview of microbial processes involved in the degradation of natural and synthetic fibers, starting with an introduction of relevant terminology. It discusses the methods used to assess the microbial breakdown of fibers and gives examples of the sources of microbial communities and the methods of incubation that are used in these studies. Finally, it provides examples of the types of bonds that are susceptible to microbial attack.

1.2

Background and terminology

Microbial processes, that change the structure or form of any material, always occur at the molecular level. In a general sense, microorganisms such as 1

2


bacteria and fungi can be considered ‘bundles of enzymes’ or sources of enzymes that catalyze a diverse array of chemical reactions that break down or modify substrates. Microorganisms carry out these activities to provide energy and suitable smaller molecules for the production of new cellular material, and ultimately new cells. Thus, when considering the microbial attack on any substance, it is important to remember that the size and physical and chemical characteristics of the substance influence how the microbes attack it. In addition, because microorganisms are living entities, environmental conditions must be suitable for their survival and growth. In this section, the general characteristics of fibers, microorganisms, and microbial processes will be discussed and some important terms will be defined.

1.2.1

Fibers, textiles and films

Fiber is the basic element of fabrics and other textile structure [1]. A fiber is typically defined as a material having a length at least 100 times its diameter. These can be natural, such as cellulose or wool, or synthetic, such as nylon. A textile is any product made from fibers [1]. This includes nonwoven fabrics such as felt, in which wool fibers are physically interlocked by a suitable combination of mechanical work, chemical action, moisture, and heat [1]; and woven fabrics in which yarns are interlaced perpendicular to each other. Yarns are made of fibers twisted together in a continuous strand. Many of the so-called thermoplastic, biodegradable natural polymers, known as poly(hydroxyalkanoates), and some synthetic polymers such as poly(lactides) can be made into fibers by cold drawing, melt spinning or thermal drawing [2, 3]. These polymers can also be extruded as sheets rather than fibers. These sheets are known as films which are not true textiles because they are not made of fibers [1]. Nonetheless, these films of poly(hydroxyalkanoates) or poly(lactides) have the same chemical properties as the corresponding fibers, so for convenience, films are often used in biodegradation studies. Fibers are composed of polymeric molecules with different arrangements. These can be random or parallel [4]. Amorphous regions of a fiber are due to random or unorganized arrangement of the polymers. In contrast, a parallel, highly ordered arrangement of the polymers is referred to as a crystalline region [4]. Fibers generally contain both types of polymer arrangements (for example, cotton is about 30% amorphous and 70% crystalline) and, typically, fibers of a particular type display greater strength with an increasing proportion of crystalline regions. These different arrangements also affect the biodegradability of fiber; the amorphous regions are more susceptible to biodegradation than the crystalline regions (for example, amorphous cellulose is biodegraded more rapidly than crystalline cellulose [5]). Hydrogen bonding between chains of polymers is the major force that


3

contributes to crystallinity [4]; the sum of the myriad of weak hydrogen bonds between adjacent polymers yields a strong, tight structure in the crystalline region of a fiber. Covalent cross-linking also occurs in some fibers, most notable are wool and silk in which disulfide bonds form between amino acid residues; the greater the number of disulfide bonds, the tighter the fiber structure. ‘Tightness’ imparted by hydrogen bonding and crosslinking reduces the susceptibility of the fiber to biodegradation.

1.2.2

Biodegradation, mineralization and biomass formation

The term biodegradation may have different connotations for people in different situations. In the broadest sense, biodegradation is the biologically catalyzed reduction in the complexity of chemicals [6]. A simple example is the conversion of glucose to ethanol during yeast fermentation; ethanol is a less complex molecule than glucose. Mineralization can be considered complete biodegradation, leading to the conversion of organic forms of elements to inorganic forms, as shown in Table 1.1. In some cases, two products are shown Table 1.1, for example, under aerobic conditions, organic-N may be converted to NH3 or NO 3– , depending upon the environmental conditions and the structure of the microbial community, that is, if conditions are favorable for nitrifying bacteria, NH3 (the first product of mineralization) may be oxidized to NO 3– . An anomaly in Table 1.1 is the appearance of CH4 (an organic compound) as a mineralization product of organic-C. This occurs in methanogenic environments (see Section 1.3.2), where CH4 production occurs along with CO2 production; however, it is generally accepted that CH4 is a product of mineralization in these environments. In non-methanogenic environments, CO2 is the product of mineralization (Table 1.1). Table 1.1 Major products of microbial mineralization under aerobic or anaerobic conditions Conditions

Substrate

Inorganic products

Aerobic

Organic-C Organic-H Organic-N Organic-S

CO2 H2 O NH 3 , NO –3 SO 2– 4

Anaerobic

Organic-C Organic-H Organic-N Organic-S

CO2, CH4 H2 , H 2 O NH3 H 2S

An important product of microbial metabolism and biodegradation is biomass or new cell material. In heterotrophic microorganisms, new cell

4


material is formed by the incorporation of some of the carbon from the biodegradable organic substrates, while a portion of the organic carbon is mineralized to yield energy for biosynthesis of biomass. More energy is produced from the oxidation of an organic substrate in the presence of O2, than in the absence of O2 (see Section 1.2.5), thus, under aerobic conditions, more energy is available for biosynthesis and more substrate carbon is incorporated into biomass in the presence of O2. In general, about 50% of substrate carbon is assimilated into new biomass under aerobic conditions, whereas only about 10% of substrate carbon is assimilated into new biomass under anaerobic conditions.

1.2.3

Microorganisms

The two major groups of microorganisms associated with the breakdown of organic matter are bacteria and fungi, both groups are extremely diverse in form, habitat and activity. Bacteria are typically simple, unicellular, prokaryotic (having no nucleus) organisms. They are commonly 1 to 5 mm in size, and are invisible to the naked eye. Some bacteria grow in filamentous forms, and these masses of cells can be seen without a microscope. Some bacteria, known as strict aerobes, require O2 for growth, whereas others, known as strict anaerobes, are killed in the presence of O2. Others, known as facultative anaerobes, can grow in the presence or absence of O2. Many bacteria are heterotrophs that derive energy and carbon from organic matter and these play a major role in nutrient cycling in the environment. Fungi are more complex microorganisms. They are eukaryotic (having a nucleus) and are typically multicellular, making them much larger than bacteria. Their filamentous growth of hyphae yields structures that are often large enough to be visible to the naked eye (for example, mold growing on bread). However, soils can contain kilometers of fungal hyphae that are not readily visible. Many fungi produce spores that are spread by wind, and upon landing, these spores can remain dormant until conditions are favorable for their germination and growth. This form of dispersal can often be observed as mildew growth on plants or other materials. Fungi are heterotrophs and most are strict aerobes. The ability of fungal hyphae to rapidly penetrate into tissues and other organic materials makes these organisms important decomposers. This invasion puts the hyphae into intimate contact with potential food sources.

1.2.4

Growth requirements for heterotrophic microorganisms

Although the habitats, activities, and metabolism of bacteria and fungi are extremely diverse, there are certain requirements and conditions needed by


5

all of these microorganisms for growth and reproduction. Water is essential for all life; the typical microbial cell consists of about 70 to 85% water. The availability of water is expressed as water activity (aw), defined as the ratio of the vapor pressure over the substance or medium to the vapor pressure of pure water (at a given temperature) [7], thus, the aw of pure water is 1.00. The presence of salts or other solutes in the medium reduces the aw, as does the dryness (lack of water) of the environment. Most bacteria and fungi require aw > 0.90 for growth; however, some xerophiles (organisms that can grow under relatively dry conditions) are active at aw < 0.85. Xerophiles are typically molds and yeasts [7]. On a dry weight basis, a typical microbial cell contains about 50% carbon, 20% oxygen, 14% nitrogen, 8% hydrogen, 3% phosphorus, and 1% sulfur [8]. These elements, along with potassium, magnesium, sodium, calcium and iron are considered macronutrients required for cell growth [9]. In addition, micronutrients are required by some microorganisms, and these include cobalt, copper, manganese, molybdenum, nickel, selenium, tungsten, vanadium and zinc. These metals are typically present in cofactors or specific enzymes. Microorganisms absorb their nutrients, that is, in order to be taken into the cell, the nutrients must be soluble and small enough to pass though the cell wall and cytoplasmic membrane. The utilization of macromolecules presents a special problem because these molecules are far too large to be taken into the cell. Large polymeric molecules such as proteins and polysaccharides must be broken down outside of the cell, and then small subunits or monomers can be taken into the cell. Many fungi and bacteria produce extracellular enzymes that hydrolyze soluble or insoluble polymers, for example, cellulose (the most abundant organic compound in the biosphere) is made of glucose subunits joined by b-(1 Æ 4)-linkages. This insoluble polymer is the major component of cotton. Although the sizes of cellulose molecules differ, it has been estimated that there are about 1000 to 1500 glucose subunits in each cellulose molecule [10]. Extracellular cellulases hydrolyze the b-(1 Æ 4)-linkages and ultimately produce glucose monomers, summarized as follows: cellulose Æ oligomers Æ cellobiose Æ glucose Glucose is readily transported into microbial cells, and its subsequent metabolism provides energy and carbon for microbial growth and reproduction. Similarly, the peptide bonds in proteins (the major constituents of wool and silk) are hydrolyzed by proteolytic enzymes leading to the following sequence of products: protein Æ polypeptides Æ simple peptides Æ amino acids Amino acids are easily transported into microbial cells, and they can serve

6


as sources of energy, carbon, nitrogen and sulfur, depending on the amino acid structure, the metabolism of the microorganisms, and the environment in which the microorganisms are growing. In a microbial community, only a few microbes may actually produce the extracellular enzymes needed to hydrolyze insoluble polymers in fibers. Clearly, the microbes that produce these hydrolytic enzymes play a key role in the biodegradation of fibers. However, they do not have an exclusive role in the compete biodegradation of the hydrolysis products. Once a polymer is broken into soluble subunits, other microorganisms in the community can transport these small molecules into their cells and use them as sources of energy and substrates for growth. Thus, in diverse microbial communities, the microorganisms that produce the extracellular hydrolytic enzymes do not consume all of the organic constituents derived from the fibers.

1.2.5

Terminal electron acceptors

Monomers and other small molecules are oxidized by heterotrophic microorganisms to produce energy for biosynthesis and other cellular activities. During these oxidations, the electrons that are removed from a substrate serving as an energy source must be ultimately passed to a terminal electron acceptor. The diversity of microorganisms, and particularly bacteria, allows a variety of compounds or ions to serve as terminal electron acceptors. Table 1.2 summarizes several energy yielding reactions with acetate serving as the electron donor coupled to various terminal electron acceptors. The free energy is expressed as kJ mol–1 of acetate that is oxidized. Table 1.2 Energy yielding processes used by microorganisms to oxidize acetate in the presence of different terminal electron acceptors (after [11] and [12]) DG∞¢ (kJ mol–1 of acetate)

Microbial process

Reaction with acetate as the electron donor (energy source)

Aerobic respiration

CH3COO– + 2O2 Æ CO2 + HCO –3 + H2O

–849

Mn(IV) reduction

CH3COO– + 4MnO2 + 2HCO –3 + 3H+ Æ 4MnCO3 + 4H2O

–737

Denitrification

5CH3COO– + 8NO –3 Æ 4N2 + 5CO2 + 5HCO –3 + 8OH– + H2O

–733

Fe(III) reduction

CH3COO– + 24Fe(OH)3 Æ 8Fe3O4 + CO2 + HCO –3 + 37H2O

–712

Sulfate reduction

CH3COO– + SO 2– + H+ Æ HS– 4 + CO2 + HCO –3 + H2O

–52

Methanogenesis

CH3COO– + H2O Æ CH4 + HCO –3

–31


7

Typically, O 2 serves as the terminal electron acceptor in aerobic environments. Aerobic respiration gives the greatest energy yield per mol of acetate (–849 kJ mol–1, Table 1.2). Under anaerobic conditions, Mn(IV), nitrate, Fe(III), sulfate, and carbon dioxide are commonly used terminal electron acceptors. Based on the equations in Table 1.2, Mn(IV) reduction and nitrate reduction to N2 (known as denitrification) yield approximately the same amount of energy per mol of acetate. There is slightly less energy gained when Fe(III) is used as a terminal electron acceptor. Much less energy is available under sulfate-reducing or methanogenic conditions. If two or more potential terminal electron acceptors are available, the one that yields more energy for the microbial population or community will typically be used first. For example, Pseudomonas stutzeri is able to grow with either O2 or nitrate as a terminal electron acceptor. If P. stutzeri is incubated aerobically in the presence of nitrate, it will preferentially use O2 as its terminal electron acceptor because of the greater energy yield. As the concentration of O2 is depleted, P. stutzeri will synthesize the enzymes required to use nitrate as its terminal electron acceptor [13]. Similarly, if nitrate and sulfate are present in an environment that is devoid of O2, the activities of the microbial community are dominated by the nitrate reduction (denitrification). In this case, there is a competition between two groups of bacteria in the community, these are the nitrate-reducing bacteria and the sulfate-reducing bacteria. There are several reasons why the nitrate-reducing bacteria out-compete the sulfate-reducing bacteria [14], and one of these is the greater energy yield when nitrate is used as the terminal electron acceptor (Table 1.2). In anaerobic, dark environments in which Mn(IV), Fe(III), nitrate, and sulfate are scarce, methanogenesis becomes the dominant microbial process [15]. This often occurs in landfills and methanogenesis is exploited in the anaerobic digestion of sewage sludges. Methanogens have a very limited range of substrates that can be used as energy sources, including acetate (Table 1.2), formate, and H2 (other substrates used by methanogens are listed in Zinder [16]). Because of their limited range of substrates, methanogens occupy the terminal position in the decomposition of organic matter in anaerobic environments, and they depend on three other microbial processes to degrade complex polymers such as cellulose or protein. These include: (a) hydrolysis that yields monomers from polymers; (b) fermentative acidogenesis that produces acids (e.g. lactate, propionate, butyrate) and alcohols (e.g. ethanol, propanol, butanol) from the monomers; and (c) acetogenesis that forms acetate, H2 and CO2 from the products of acidogenesis. H2 is an important energy source for methanogens; using CO 2 as a terminal electron acceptor, hydrogenotrophic methanogens carry out the reaction in equation 1.1: CO2 + 4H2 Æ CH4 + 2H2O

DG∞¢ = –136 kJ mol–1 methane 1.1

8


This is a greater energy yield than derived from acetate (–31 kJ mol–1 methane, Table 1.2), and hydrogenotrophic methanogens grow much more quickly that acetate-utilizing methanogens. The passage of H2 from other microbial processes to methanogens is known as interspecies H2 transfer, and this process plays a key role in the overall thermodynamics of the anaerobic process. For example, the acetogenic conversion of butyrate to acetate and H2 is thermodynamically unfavorable (DG∞¢ is positive). However, this reaction can occur only if H2 is removed by a hydrogenotrophic methanogen [17] because the overall reaction carrier out by the syntrophic association of the acetogen and a methanogen is thermodynamically favorable (DG∞¢ is negative).

1.3

Incubation conditions used for studying biodegradation of fibers and films

In the environment, fibers or textiles can find their way into aerobic or anaerobic environments. Many aquatic environments and the top few centimeters of soil contain sufficient O2 to be aerobic. In contrast, anaerobic conditions exist in the deeper soils, water-logged soils, aquatic sediments, and landfills. Thus, biodegradation studies have assessed the fates of fibers and textiles under aerobic and anaerobic conditions.

1.3.1

Aerobic incubations

Experimentally, aerobic incubations are much easier to set up and maintain than anaerobic conditions. Accordingly, many of the studies assessing the biodegradation of fibers and films have been done under aerobic conditions. In addition, fungi often play an important role in the breakdown of polymers, and many of these are strict aerobes. Of course, O2 serves as the terminal electron acceptor under aerobic conditions. The major requirements of these aerobic studies are to ensure that the fibers are accessible to the microorganisms, and that there is an ample supply of O2. To fully evaluate the progress of the biodegradation, the experimental method must also allow the collection of the residual polymers or biodegradation products, which may include collection of carbon dioxide that is liberated during the mineralization of the fiber or film (Table 1.1). One of the simplest methods for aerobic culturing (if carbon dioxide is not collected) is to add the fiber or film and the microbial inoculum to a container with liquid medium. This is covered in some manner to allow air to get into the container while preventing foreign microorganisms from contaminating the culture system; this is typically achieved by means of a sterile foam plug in the neck of the container. The container may be placed on a rotary shaker to increase the rate of aeration. However, if the depth of


9

the liquid is low and the surface area of the liquid is high, there will be adequate O2 diffusion without shaking. The formulation of the medium depends on the goal of the aerobic experiment. Often the medium contains only inorganic salts (including phosphate and ammonium or nitrate), and the polymer is the sole source of carbon for the heterotrophic microorganisms. This type of medium was used by Modelli et al. [18] in their investigations of the biodegradation of flax fiber by the bacterium Cellvibrio fibrovorans; similarly, Wiegand et al. [19] used ammonium nitrate-containing medium to isolate bacteria that degrade polyester amide BAK 1095. If the fiber contains nitrogen and sulfur, such as in wool and silk, the inorganic nitrogen and sulfur sources may be omitted, so the fiber serves as the carbon, nitrogen and sulfur source. This approach is illustrated by the work of Stahl et al. [20, 21] who incubated the fungus Microsporum gypseum in nitrogen-free and sulfur-free medium with wool as the sole carbon, nitrogen and sulfur source. They observed mineralization of organic nitrogen and organic sulfur to ammonium and sulfate, respectively. Similarly, media devoid of nitrogen salts were used in study of chitosan-gallen and poly(L-lysine)gellan fibers by filamentous fungi [22], and these two nitrogen-containing polymers served as the carbon and nitrogen sources for growth. Some studies that investigate fungal biodegradation of fibers use growth medium that is supplemented with a readily utilizable carbon source, such as glucose. For example, wool biodegradation studies using the fungi Trichophyton simii and Aspergillus niger used nitrogen-free, glucose-containing liquid medium [23].

1.3.2

Anaerobic incubations

The methods required to create and maintain conditions that are suitable for growing anaerobic cultures are more difficult than those required for culturing aerobic cultures. Nonetheless, use of methods such as the serum bottle modification of the Hungate technique [24] is now routine in many laboratories. As discussed in the previous section, the formulation of the medium depends on whether the fiber or fabric is to serve as the sole source of carbon, nitrogen or sulfur. In addition, the formulation of the anaerobic culture medium depends upon which terminal electron acceptor is to be considered in the study. As shown in Table 1.2, the list of terminal electron acceptors includes Mn(IV), nitrate, Fe(III), and sulfate. In addition, bicarbonate (carbon dioxide) serves as the terminal electron acceptor for methanogenesis (equation 1.1). Supplementing the medium with an abundant supply of one of the terminal electron acceptors prescribes the microbial process that occurs in the cultures. Fermentation, in which some oxidized organic compound serves as the terminal electron acceptor, also occurs under anaerobic conditions.

10


It is well documented that microbial activities in landfills lead to the production of methane [25, 26]. However, lack of moisture in the landfill often slows this process [25, 27]. Given sufficient moisture, anaerobic microbial process, other than methanogenesis, can occur in landfills. The analysis of groundwater samples upgradient to and downgradient from a landfill indicated that the alkalinity was higher downgradient (consistent with the mineralization of organics to bicarbonate in the landfill), and that sulfate was depleted, suggesting that sulfate-reduction occurred in the landfill [28]. In addition, higher concentrations of soluble Fe(II) were found downgradient, likely a product of Fe(III) reduction in the landfill. From the examination of materials dug from a landfill, Gurijala and Suflita [27] reported that textiles appear to absorb and concentrate sulfate, thus, they would exist in a microenvironment that is rich in this terminal electron acceptor. Despite the variety of potential electron acceptors, most studies of fiber or film biodegradation have focused on methanogenic reactions, with a few investigations of nitrate reduction, fermentation and sulfate reduction. Poly(3-hydroxybutyrate) is a thermoplastic that can be spun or filmed by conventional processes [29], and the anaerobic biodegradation of this polymer has been studied extensively. Under certain growth conditions, poly(3hydroxybutyrate) accumulates as inclusion bodies that are bacterial carbon storage reservoirs; because poly(3-hydroxybutyrate) and related poly(hydroxyalkanoates) are simply bacterial carbon reserves, it is not surprising that they are readily biodegradable. In one investigation, Budwill et al. [30] supplemented anoxic activated sludge from a domestic sewage treatment plant and demonstrated that poly(3-hydroxybutyrate) is mineralized with nitrate as the terminal electron acceptor. The biodegradation of poly(hydroxyalkanoates) under sulfate-reducing conditions has also been demonstrated [31, 32]. A mixture of a sulfate-rich lake sediment and anoxic lake water served as the growth medium and source of microorganisms. This mixture was supplemented with different amounts of poly(hydroxyalkanoates) as the carbon and energy source [32]. Sulfide was produced during incubation, and the rate of sulfide production increased with the amount of poly(hydroxyalkanoate) added to the cultures. The addition of molybdate, and inhibitor of sulfate-reduction, decreased the rates of sulfide production and poly(hydroxyalkanoate) degradation, indicating that this biodegradation process was driven by sulfate reduction. In a pure-culture study, Janssen and Schink [33] elucidated the pathway for the depolymerization and subsequent fermentation of poly(3hydroxybutyrate) by Ilyobacter delafieldii; the products of fermentation were acetate, butyrate, and H2, with a molar ratio of acetate:butyrate of 2.32:1. In a co-culture of I. delafieldii and the H2-consuming, sulfate reducer Desulfovibrio vulgaris, the molar ratio of acetate:butyrate changed to 4.02:1. Accordingly, the amount of H2 detected in the co-culture was much lower in the presence


11

of the sulfate reducer, and sulfide production was observed [33]. This illustrates that even a simple mixture of microorganisms can alter the flow of carbon and electrons under anaerobic conditions. Biodegradation of poly(hydroxyalkanoates) under methanogenic conditions has been the focus of several studies [34, 35, 36, 37]; using medium in which carbon dioxide was the sole terminal electron acceptor, Budwill et al. [34] demonstrated rapid methanogenesis from three different poly (hydroxyalkanoates). Methane production from these substrates was observed after 3 to 4 days of incubation. Gartiser et al. [35] examined a variety of test methods to assess the methanogenic biodegradability of a poly(hydroxyalkanoate) co-polymer and several other polymers. They concluded that the use of media formulations with inadequate amounts of bicarbonate or carbon dioxide resulted in poor methanogenic biodegradation. Anaerobic degradation of other fibers has also been studied: when the anaerobic rumen fungus Piromonas communis was cultured on cotton fibers as its sole carbon source, it produced an extracellular cellulase that rapidly solubilized these fibers [38]. The cellulase enzyme system of P. communis was similar to that of another crystalline cellulose-degrading, anaerobic rumen fungus, Neocallimastix frontalis [39], and Nakashimada et al. [40] cultivated N. frontalis on cellulose in co-culture with two methanogens, Methanobacterium formicicum and Methanosaeta conncilii. Cellulose depolymerization and fermentation by the fungus yielded substrates, including acetate, formate, H2 and CO2, for the methanogens. M. formicicum produced methane from formate, H2 and CO2, whereas M. conncilii produced methane from acetate. These experiments demonstrated the breakdown of cellulose from cotton to produce methane in a defined microbial co-culture. The anaerobic breakdown of cotton has also been considered as a method to produce volatile fatty acids and methane [41]. When the pH of a growth medium that contained cotton fibers as the carbon and energy source for a methanogenic consortium was adjusted to different initial values, Tükenmez et al. [41] observed that the amounts of fermentation products were different. The molar ratios of acetic, propionic, and butyric and methane were altered in this manner, suggesting that processes for producing specific fermentation products from cotton could be developed. Using anaerobic, thermophilic incubation conditions, with wool as the major source of carbon in the growth medium, Riessen and Antranikian [42] isolated a new bacterium that was named Thermoanaerobacter keratinophilus. This isolate grows optimally at 70∞C, and produces an extracellular protease that is responsible for the degradation of native keratin in the wool. One of the goals of this work was to isolate a thermostable keratinolytic enzyme that might be used to modify wool fibers in the textile industry.

12

1.4


Sources of microorganisms and enzymes for laboratory incubations

There are many different types of studies that are done to assess the biodegradation of fibers, textiles and films. These range from experiments that use undefined, natural microbial communities, to pure cultures of microorganisms, to highly purified enzymes that depolymerize macromolecules. Table 1.3 summarizes some of the different types of studies, beginning with the broad diversity of microbial activities in communities from natural sources, to and ending with work using purified enzymes. Table 1.3 An overview of different types of studies that are done to assess the biodegradation of fibers, textiles and films Types or sources of microorganisms or depolymerizing materials

Type or purpose of experiment

Examples and comments

Entire microbial communities

To assess biodegradation in soil

Passive soil burial tests simply involve burying fibers or textiles in soil for some period of time and then examining them for signs of biodegradation. Peacock [43] used this method to study the changes in linen, cotton, silk and wool over a 32-week incubation time. Modelli et al. [18] buried flax fibers in soil amended with ammonium phosphate to provide inorganic nutrients and CO2 production was used to monitor the biodegradation of these fibers.

To assess in vitro biodegradation

These types of experiments provide conditions that would help stimulate microbial activities. Keller et al. [44] provided a continuous stream of oxygen through a column of soil that was supplemented with nitrate to stimulate biodegradation of fiber composites. Kasuya et al. [45] used various natural water samples supplemented with many inorganic nutrients and an ample supply of O2 to test the biodegradation of poly(3hydroxybutyrate) and related copolymers. The methanogenic degradation of these polymers by microbes in anaerobic sewage sludge was studied by Budwill et al. [34] and Abou-Zeid et al. [37] in culture


13

Table 1.3 Continued Types or sources of microorganisms or depolymerizing materials



systems that provided strict anaerobic conditions and all macro- and micronutrients. Enrichment or selective culture

To isolate microbes that degrade a specific fiber or film

An insoluble polymer is used as the sole carbon source to select microorganisms that will use the polymer for growth. Using strict anaerobic methods and serial transfers, Janssen and Harfoot [47] isolated the poly(3-hydroxybutyrate) fermenting bacterium Ilyobacter delafielkii. Chang et al. [48] isolated the bacterium, Chitinimonas taiwanensis, which depolymerizes and grows on chitin. Two strains of poly(Llactic acid)-degrading Amycolatopsis sp were isolated by detecting clear zones on the mineral agar plates containing poly(L-lactic acid) [49].

Pure cultures

To study the capability of a single species to degrade fibers or films

Crude preparations of depolymerizing material

To carry out biodegradation studies without viable microorganisms or purified enzymes

Pure cultures are obtained from a commercial or private culture collection for these studies. Shrivastava et al. [23] surveyed 10 fungal isolates to see which would degrade wool. Pranamuda and Tokiwa [50] obtained 25 strains of Amycolatopsis and tested their abilities to degrade poly(L-lactide). Ohkawa et al. [22] used seven fungal species to study the biodegradation of chitosan-gellan and poly(L-lysine)gellan. Mixtures or unpurified extracellular enzymes from various sources are used for degradation studies. Using cotton fibers and other substrates, Wilson and Wood [39] detected three different types of enzyme present in culture filtrates of Neocallimastix frontalis RK21. Haga et al. [51] used commercially available crude cellulases from the fungus Trichoderma viride to test the extents of biodegradation of fibers that had

14


Table 1.3 Continued Types or sources of microorganisms or depolymerizing materials



different mercerization treatments. Christeller [52] used extracts from the midgut of the larva of the brown house moth to study the degradation of wool. Purified enzymes

To understand the specificity of enzymes and the mechanisms of depolymerization, pure extracellular enzymes are studied

Enzymes are obtained by purifying from microbial cultures, or by purchasing from commercial sources. Ignatova et al. [53] purified a monomeric keratinase from a thermophilic actinomycete strain of Thermoactinomyces candidus that degrades wool. Pranamuda et al. [54] isolated a poly(L-lactide)-degrading enzyme produced by a Amycolatopsis sp. This enzyme also degrades silk powder. Wang et al. [55] purified a low molecular weight peptide from a cellulolytic fungus and studied its effects on cotton fibers. Arai et al. [56] purchased three proteolytic enzymes to study their degradation of silk fibers and films.

Passive soil burial experiments (Table 1.3) are often done under laboratory conditions, with soil and fibers incubated in closed containers. Typically, the soil and the fibers are moistened and the incubation container is sealed to ensure an adequate supply of water for microbial activity. For example, Peacock [43] maintained 65% relative humidity in incubation containers that contained linen, cotton, silk and wool in two different soil types. The goal of this work was to simulate wet archaeological burial environments to assess the decay of textile fabrics. The order of susceptibility to biodegradation was found to be linen > cotton > wool > silk. No nutrients were added to these soils [43]. In contrast, Modelli et al. [18] used the soil burial method (Table 1.3), but added ammonium and phosphate to stimulate microbial activity to ensure these essential nutrients were not limiting in the moistened soil. These were incubated under aerobic conditions for up to 180 days. Experiments to test in vitro biodegradation of fibers or films are very common, and these are often the first tests done to assess the biodegradability


15

of a polymer. These take advantage of the diversity of microbial communities from various environments and provide conditions to increase the likelihood of observing biodegradation. As shown in Table 1.3, experiments are typically done by providing important macro- and micronutrients and an ample supply of O2 for aerobic studies, or strict anaerobic conditions for anaerobic studies. The variety of sources of inocula can include soil [44], sediments [32], natural waters [45], aerobic sewage [46], and anaerobic sewage sludge [34, 37]. To gain a better understanding of biodegradation processes, researchers often isolate pure cultures of microorganisms for these studies. The selective or enrichment techniques used usually involve incubating a mixed microbial community from soil, or some other environment, in liquid culture with the selected fiber or film as the sole carbons sources (Table 1.3). After suitable incubation times, serial transfers are made into fresh medium with the same fiber or film as the growth substrate. After a few of these transfers, the culture will be enriched with those microorganisms that can degrade the provided carbons source. This method was used to isolate the poly(3hydroxybutyrate)-fermenting bacterium Ilyobacter delafielkii [47] and the chitin-degrading bacterium Chitinimonas taiwanensis [48]. Nakamura et al. [49] used a modification of the enrichment procedure in which insoluble poly(L-lactic acid) was added to mineral agar plates, and those microorganisms that grew and hydrolyzed the polymer to soluble products produced clear zones around the colonies (Table 1.3). This method allows easy selection of isolates that degrade an insoluble polymer. After pure cultures of bacteria or fungi have been isolated, they are typically stored in a researchers’ private culture collections or deposited in a commercial culture collection such as the American Type Culture Collection or the United Kingdom National Culture Collection. Some investigations that used pure cultures are summarized in Table 1.3. As an example, Shrivastava et al. [23] tested the abilities of 10 fungal isolates to degrade wool. Four of these isolates were Trichophyton simii strains, and they all degraded keratin from wool; six of the isolates were Aspergillus niger strains, and only two of these degraded keratin. Pranamuda and Tokiwa [50] obtained 25 strains of Amycolatopsis from various commercial culture collections and tested their abilities to degrade poly(L-lactide). Seven pure strains of fungi were used by Ohkawa et al. [22] to study the biodegradation of fibers composed of chitosangellan and poly(L-lysine)-gellan. Both fibers were biodegradable under aerobic conditions, with the greatest amount of degradation observed with Penicillium caseicolun and Aspergillus oryzae. In addition to monocultures studies, pure cultures can be combined to monitor the effects of a simply, defined microbial community. For instance, Nakashimada et al. [40] incubated the cellulosedegrading fungus, N. frontalis, with two methanogens and observed methane production.

16


The biodegradation of fibers and other polymers is generally initiated by extracellular enzymes, thus, if a microbial culture is degrading a polymer, the enzyme or enzymes that attack the complex substrate can often be detected in the culture supernatant, after the cells have been removed by centrifugation or ultrafiltration. Two examples of this are given in Table 1.3, including the work of Wilson and Wood [39] who detected three different types of enzymes in the culture filtrates of the fungus Neocallimastix frontalis strain RK21 grown on cotton fibers. Some crude enzyme preparations are commercially available, and Haga et al. [51] used one of these preparations of cellulases to examine how different mercerization treatments affected cotton biodegradation. Mercerization is the treatment of cotton fibers to improve their strength, uniformity, luster and affinity for dyes [1]. In general, the untreated cotton was less susceptible to enzymatic attack than the cotton that was treated with ammonia and/or sodium hydroxide, due to the increase in amorphous regions in the fiber as a result of the mercerization treatment. Extracts from sources other than microbial cultures can also be tested for fiber depolymerizing activity. This is illustrated by the work of Christeller [52] who used extracts from the midgut of the larva of Hofmannophila pseudospretella (brown house moth) to study the degradation of wool. These experiments were done under highly anaerobic conditions, and they demonstrated that the degradation of this fiber consisted of reduction and solubilization, followed by proteolysis of the wool protein. Finally, purified enzymes can be used to study their characteristics, specificity and the mechanisms of depolymerization of macromolecules (Table 1.3). Ignatova et al. [53] grew a thermophilic actinomycete strain of Thermoactinomyces candidus that produced a keratinase in liquid medium with wool as the sole source of carbon and nitrogen. The monomeric enzyme was purified 62-fold and it was found to have a molecular weight of 30 kDa, with the pH and temperature optima of 8.6 and 70∞C, respectively. The purified enzyme catalyzes the hydrolysis of a variety of proteins and the keratins in hair, feathers and horns. Pranamuda et al. [54] isolated a poly(Llactide)-degrading enzyme produced in a liquid culture of Amycolatopsis sp. (strain 41), the molecular weight of the enzyme was estimated to be between 40 and 42 kDa, with pH and temperature optima of 6.0 and 37 to 45∞C, respectively; this enzyme also degrades silk powder. The culture supernatant of the cellulolytic fungus Trichoderma pseudokoningii was the source of a low molecular weight peptide isolated by Wang et al. [55]; this peptide chelates Fe(III) and reduces it to Fe(II) in the presence of O2, generating a free radical that leads to the destruction of hydrogen bonds and cleavage of glycosidic bonds; these mechanisms produce short fibers of cellulose. Rather than isolating enzymes, Arai et al. [56] purchased three proteolytic enzymes to study their degradation of silk fibers and films (Table 1.3); these included collagenase type F, a-chymotrypsin type I-S, and protease type XXI. All three enzymes hydrolyzed silk, but the protease was the most active.


1.5

17

Analytical methods used to assess biodegradation of fibers and films

Microbial metabolism of a substrate causes chemical changes to that substrate. If the substrate has a microscopic or macroscopic form, the chemical changes brought about by the microbial metabolism can change the physical form or structure of the substrate. A variety of physical and chemical methods are available to follow the changes caused by microbial metabolism of a substrate; the methods used depend to a large extent on the size of the substrate. For example, if the substrate is a simple soluble molecule, such as an amino acid, in an aqueous growth medium, physical methods would have very limited applicability. However, chemical methods would provide ample evidence of biodegradation of the amino acid. A variety of chemical analyses could be used in this case, including monitoring the decrease in concentration of the substrate, measuring the release of ammonium or carbon dioxide from the biodegradation of the amino acid. On the other hand, if the substrate is large and insoluble in water, such as a fiber or a film of biodegradable material, physical measurements are commonly used to assess the initial microbial attack on the substrate. These methods include microscopic examination, measure of weight loss, or measure of the loss of mechanical strength. As the biodegradation of the fiber continues, individual small molecules are released and chemical analyses of monomers and products of mineralization can be detected. Often, several of these methods are used in a single study to confirm and characterize the biodegradation of the test material.

1.5.1

Detecting subtle changes in fiber structure or composition

Some of the initial reactions during the biodegradation of a fiber may produce only small changes in its structure or composition. These are typically observed using very sophisticated instrumental methods. Amass et al. [57] reviewed the use of many of these methods, so only a few examples are given here. Infrared spectroscopy or Fourier transformed infrared spectroscopy (FTIR) is often used to detect changes in crystallinity or minor chemical changes in a fiber. Wang et al. [55] used the ‘finger-print region’ of 1400 to 900 cm–1 to detect changes in the crystallinity of cotton fibers treated with a peptide that shortened the fibers. Their infrared data also indicated that the intermolecular hydrogen bonds were disrupted. Increases in specific IR absorbances can be used to indicate changes in fibers. For example, Khabbaz et al. [58] observed that microbial activity on poly(L-lactide) produced a new absorbance at 1600 cm–1, which they interpreted as being due to an increased number of carboxylate ions in the residual polymer. Similarly, Frisoni et al.

18


[59] observed increased absorbance at 1540 and 1650 cm–1, when acetylated cellulose fibers were examined after 13 days’ incubation with a cellulolytic bacterial strain. These absorbances are characteristic of the amide group, and Frisoni et al. [59] surmised that proteins were bound to the residual fibers. X-ray diffraction can be used to study changes in fabrics during biodegradation. Park et al. [60] contacted cellulose fabrics with soil microorganisms and measured changes in crystallinity by X-ray diffraction measurements. They observed that the crystallinity increased, during incubation; this was attributed to the initial preferential biodegradation of the amorphous regions of the fibers, leaving the more resistant crystalline regions. Solid-state cross-polarization/magic-angle-spinning (CP/MAS) 13C-nuclear magnetic resonance (NMR) spectrometry can also be used to detect change in crystallinity of polymers. Vishu Kumar et al. [61] treated chitosan with a serine protease producing low molecular weight, depolymerized chitosans. The native chitosan gave CP/MAS 13C-NMR peaks with narrower widths than those from the low molecular weight chitosans, indicating that depolymerization gave products with lower crystallinity than the native fibers.

1.5.2

Visual observations and microscopy

An early indication of biodegradation of a fiber or film is a change in its appearance. This may occur on a macroscopic scale, so the changes can be observed with the naked eye. Figure 1.1 shows the obvious changes in the appearance of a film of poly(3-hydroxybutyrate) after incubation in an anaerobic culture for 10 days [62]; the top strip was incubated in sterile medium, whereas the bottom strip was incubated in the viable culture. The depolymerization of fibers or films leads to the formation of watersoluble small molecules; this results in the dissolution of the fiber or film, accounting for the ‘holes’ in the film in Fig. 1.1. This type of biodegradation can also be observed by placing a polymer on the surface of an aqueous medium or in an agar plate. As an example, Ratajska et al. [46] floated chitosan (N-deacetylated chitin comprised of chains of D-glucosamine) in liquid medium containing mixed microbial community from a wastewater. Initially, the polymer covered essentially the entire surface of the liquid. Over a 5-week incubation period, dissolution caused the diameter of the polymer mat to decrease until it was about one-fifth of its original size. To isolate silk-degrading microorganisms, Tokiwa et al. [63] incorporated washed silk powder in agar plates. After inoculation and incubation, those microorganisms that utilized the silk by depolymerizing and solubilizing it were readily detected because they produced clear zones around the colonies. Subsequently, a silk-degrading, filamentous bacterium (an Amycolatopsis sp.) was shown to produce clear zones in a plate that contained poly(L-lactide).


19

1.1 Biodegradation of poly(3-hydroxybutyrate) films under methanogenic conditions. The top strip was incubated in sterile medium and the bottom strip was incubated in anaerobic cultures for 10 days at 35∞C. Initially, the strips were 0.016 mm thick and 1 cm ¥ 7 cm. Reprinted with permission from Budwil [62]. Copyright 1995 K. Budwill.

Microscopy is often used to observe microbial colonization and physical changes in fibers. Figure 1.2a shows the colonization of a poly(L-lysine)gellan fiber by the fungus Curvalaria sp. [22]. Fibers (1.5 to 2 cm in length) were incubated with this fungus in aqueous medium for 40 days prior to microscopic examination. Biodegradation led to the collapse of the fiber at the location shown by the arrow in Fig. 1.2a. Scanning electron microscopy is also used to view the effects of biodegradation; this provides much higher magnification than light microscopy. Scanning electron microscopy was used to observe the damage to flax fiber (Fig. 1.2b) incubated for 13 days with the cellulolytic bacterium Cellvibrio fibrivorans [18]. This photo shows that the fibers remained cylindrical but they were shortened by the microbial activity.

1.5.3

Measuring weight loss

Biodegradation of fibers or films result in the dissolution of part or all of the material, resulting in an overall weight loss of the material. Indeed, measuring weight loss is the common method for detecting biodegradation of a variety of insoluble fibers or films. For example, Shrivastava et al. [23] used weight loss as one means of detecting the biodegradation of wool by the fungal species Trichophyton simmi and Aspergillus niger. Each culture received

20


1.2 (a) Examination of a poly(L-lysine)-gellan fiber by light microscopy showing growth of the fungus Curvalaria sp. After 40 days’ incubation, biodegradation led to the fracture of the fiber shown by the arrow. Reprinted from Ohkawa et al. [22] with kind permission of the authors and Springer Science and Business Media (copyright 2000). (b) Scanning electron micrograph of flax fibers after 13 days’ incubation with Cellvibrio fibrivorans at 28∞C. The arrow shows the fracture of the fiber. Reprinted with permission from Modelli et al. [18]. Copyright 2004 American Chemical Society.

250 mg of wool, and after 4 weeks of incubation in unicultures, the weight losses were 58% and 22% in the T. simmi and A. niger cultures, respectively. Weight loss measurement was one of several methods used to study the biodegradation of cellulose fibers from flax by two strains of Cellvibrio [59]. These aerobic cultures initially contained 0.1 g of natural fiber or fibers with different degrees of acetylation. After 13 days of incubation, weight losses of 20% to 76% were recorded; these data showed that more highly acetylated fibers were more resistant to biodegradation. Measuring weight loss of polymers has also been used to follow biodegradation in anaerobic culture systems. Abou-Zeid et al. [37] incubated


21

individual films of poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3hydroxyvalerate), and poly(e-caprolactone) in methanogenic cultures. Over a 10-week incubation time, the weight loss of the first two polymers was much greater than that of the latter polymer.

1.5.4

Tensile properties (breaking load)

Hydrolysis of chemical bonds during microbial depolymerization of macromolecules weakens fibers, and this can be detected by measuring tensile properties, as illustrated in a study by Seves et al. [64]. These researchers buried pieces of silk fabric (1 cm ¥ 1 cm) in soil and these were incubated for up to 2 months. Yarns were removed from fabric and subjected to a breaking load test. Prior to burial, the mean breaking load of the warp yarns was 297 grams-force, whereas after 2 months’ incubation, this was drastically reduced to only 2 grams-force. Similarly, Arai et al. [56] studied the degradation of silk fibroin fibers by several proteolytic enzymes. They monitored loss of weight, tensile strength, changes in the infrared spectra, changes in the molecular weight distribution of fibroin, and in the amounts of soluble polypeptides released by the actions of the proteolytic enzyme. Of all the methods used, the tensile strength measurement was the most sensitive for detecting the onset of the silk fiber biodegradation.

1.5.5

Detecting the products of mineralization

Table 1.1 summarized the most common products of microbial mineralization. Heterotrophic metabolism leads to oxidation of some of the substrate carbon to carbon dioxide, if this activity occurs under methanogenic conditions; both carbon dioxide and methane are mineralization products. Thus, monitoring increases in the amounts of one or both of these products in a closed culture system provides sound evidence to demonstrate mineralization of the substrate being tested for its biodegradability. Although most studies monitor the products of organic carbon mineralization, detecting the products of the mineralization of organic sulfur and organic nitrogen from substrates also provides excellent evidence of microbial degradation of these compounds, for example, the fungal metabolism of wool led to the release of ammonium and sulfate as the mineralization products of the nitrogen and sulfur, respectively [20, 21]. Carbon dioxide formation was measured by Keller et al. [44] who studied the kinetics of aerobic biodegradation of fiber composites comprised of degummed hemp and a polyesteramide in a column that contained 300 g of soil mixed with a fiber composite. A constant stream of oxygen was passed through the column and the effluent gas was passed through a sodium hydroxide solution to trap the carbon dioxide resulting from mineralization of the fiber

22


composite. The amount of trapped carbon dioxide was determined by titration with hydrochloric acid. Biochemical oxygen demand tests with aqueous culture systems [45] can be used to measure the volume of carbon dioxide liberated from the mineralization of a substrate. Ohkawa et al. [22] used this method to follow the mineralization of fibers of the polyion complexes of chitosan-gellan and poly(L-lysine)-gellan. Filamentous fungi were used in this investigation and carbon dioxide release was observed. The methanogenic biodegradability of several polymers, including cotton, cellulose acetate fibers, poly(lactide), and poly(hydroxyalkanoates), was studied by Gartiser et al. [35]. The increase in gas volume (estimated from excess pressure in the culture headspace or collected in graduated collecting tubes) was used as the measure of biogas production, which is the sum of methane and carbon dioxide formed. Gartiser et al. [35] stated that the theoretical yield of biogas was 2.23 mL for each milligram of organic carbon in the growth medium. Most of the polymers yielded biogas, but widely different yields were obtained depending on the growth medium used and the polymer. Incubation times ranged from about 35 to 80 days. Methanogenic degradation of cotton typically yielded 40 to 75% of the theoretical biogas yield. In sharp contrast, the biogas yield from poly(lactide) was less than 10% of theoretical yield [35]. The amounts of methane and carbon dioxide produced from a substrate can be estimated by Buswell’s equation. A form of this equation for substrates that contain carbon, hydrogen, oxygen, nitrogen and sulfur [65] is shown in equation 1.2: a b 7c È ˘ C n H a O b N c Sd + Ín – Ê ˆ – Ê ˆ + Ê ˆ + Ê d ˆ ˙ H 2 O Æ 4 2 4 2 Ë ¯ Ë ¯ Ë ¯ Ë ¯ Î ˚

cNH4HCO3 + dH2S a b 5c È ˘ + Í Ê n ˆ – Ê ˆ + Ê ˆ – Ê ˆ + Ê d ˆ ˙ CO 2 2 8 4 8 4 Ë ¯ Ë ¯ Ë ¯ Ë ¯ Ë ¯ Î ˚ 3c a È ˘ + Í Ê n ˆ + Ê ˆ – Ê b ˆ – Ê ˆ – Ê d ˆ ˙ CH 4 ÎË 2 ¯ Ë 8 ¯ Ë 4 ¯ Ë 8 ¯ Ë 4 ¯ ˚

1.2

Of course, if the substrate contains no nitrogen or sulfur, the values of ‘c’ and ‘d’ are zero, and the number of moles of NH4HCO3 and H2S expected are zero. Similarly, the ‘c’ and ‘d’ terms in the calculation of the expected CO2 and CH4 are zero. Budwill et al. [34] studied the methanogenic degradation of poly(3hydroxyalkanoates). The poly(3-hydroxybutyrate) used in that study had a molecular weight of about 106 daltons, giving an empirical formula of


23

C 48000 H 72000 O 24000 . Based on Buswell’s equation, total gas volume measurements, and gas chromatography analyses, the yields of methane and carbon dioxide were 108% and 60%, respectively, of the predicted yields. The overall biogas recovery was 87% of predicted amount [34]. Of course, equation 1.2 does not account for carbon incorporation into biomass, and typically about 10% of the organic carbon in the substrate is converted to cellular material under methanogenic conditions. Thirteen percent of the polymer carbon was not found as biogas [34], and this proportion is consistent with the expected amount of carbon that would be incorporated into biomass under the growth conditions used in that study.

1.5.6

Detection of intermediates of biodegradation of fibers and films

Finding the products of mineralization demonstrates extensive or complete biodegradation of fibers or films, and the methods to detect these products are quite simple. However, partial degradation through depolymerization leads to the reduction in molecular weight of the polymer, and to the production of intermediates with various molecular sizes. These products are often more difficult to detect than those of mineralization, but several different methods can be used for this. The choice of the method is dictated by the size of the intermediates; when the intermediates are still polymeric, they are usually characterized by size exclusion (gel permeation) chromatography, often using high performance liquid chromatography (HPLC) methods. Size exclusion chromatographic analyses showed that a microbial community incubated with chitosan at 20∞C yielded polymers with a weight average molecular weight (Mw) of 7.8 kDa [46]; this was a sharp decrease from the initial Mw of the chitosan that was 261 kD. Based on size exclusion chromatographic analyses, Hakkarainen et al. [66] showed that the biodegradation of poly(L-lactide) led to lower molecular weight polymers; the initial Mw of the substrate was about 550 kDa, and after 4 weeks of incubation with compost microorganisms, the Mw decreased to about 350 kDa. In another study, Arai et al. [56] tested the abilities of three different commercial enzymes to degrade silk fibroin with Mw of 119.8 kDa. After 17 days’ incubation with collagenase type F, a-chymotrypsin type I-S, or protease type XXI, the Mw decreased to 94.5, 53.7, and 102.4 kDa respectively. Clearly, size exclusion chromatography can provide sound evidence of depolymerization. Further biodegradation of polymers yields oligomers and monomers. A variety of different analytical methods are used to detect these low molecular weight compounds. For example, Chang et al. [48] isolated a Gram-negative bacterium, Chitinimonas taiwanensis, that depolymerizes and grows on chitin (a homopolymer of b-(1 Æ 4)-linked N-acetyl-b-D-glucosamine). HPLC

24


analysis of a culture supernatant showed that the major intermediate was a chitotriose, consisting of three N-acetyl-b-D-glucosamine moieties. In another study, Wiegand et al. [19] used HPLC-mass spectrometry to detect and identify biodegradation products of polyester amide BAK 1095. They found monomers of adipic acid and aminocaproic acid, oligomers of these two acids in various proportions and more complex oligomers. Nakamura et al. [49] isolated and purified a poly(L-lactate)-degrading enzyme from an actinomycete. Mixtures of the purified enzyme and the polymer yielded the monomer, lactate, which was detected by thin layer chromatography.

1.6

Examples of types of bonds that are susceptible to enzymatic attack

Cellulose is a major component of some natural fibers; cotton contains about 94% cellulose [67] and the flax fibers contain about 60–80% cellulose [68]. The enzymatic hydrolysis of cellulose has been reviewed by Mansfield et al. [5] and Leschine [69]. Figure 1.3 shows the structure of cellulose (R = OH), which is a homopolymer of glucose moieties jointed via b-(1 Æ 4)-linkages. Depolymerization occurs at these glycoside bonds (shown by the arrow in Fig. 1.3). The cellulases that catalyze these hydrolyses fall into three groups: (a) endoglucanases; (b) exoglucanases; and (c) b-glucosidases. The endoglucanases randomly hydrolyze internal b-(1 Æ 4)-glycosidic bonds (where n and m are large integers) thereby quickly decreasing the polymer length but slowing increasing the concentration of reducing sugars [70]. In contrast, the concentration of reducing sugars increases rapidly through the activity of exoglucanases. These enzymes remove cellobiose from the nonreducing end of cellulose (n = 1, and m is a large integer). As a result, the polymer length decreases slowly, because only two glucose moieties are removed with each hydrolysis. b-Glucosidases hydrolyze cellobiose (n = m = 0) and short oligosaccharides (n = 0, and m is a small integer) to release

H OH H O

O HO

H H

H

H HO O

H

n

O HO

H H

H OH

H

H H O

H O

R H

R

H OH HO O

H

O

H O

R H

R

H OH

m

Reducing end

Non-reducing end H Cellulose R

OH

Chitin R

N H3C

Chitosan R O

NH2

1.3 Structures of cellulose, chitin and chitosan. The arrows indicate the location of enzymatic hydrolysis.


25

glucose. Some, but not all, cellulose-degrading microorganisms produce all three types of cellulases, and these are usually three different enzymes [70]. However, some cellulases can exhibit more than one activity as illustrated by the cellulase isolated and characterized by Han et al. [70]. This enzyme has both endo and exo activities. Figure 1.3 also shows the structure of chitin [R = NHC(O)CH3]. The hydrolysis of this polymer is similar to that of cellulose, involving chitinase that randomly hydrolyzes b-(1 Æ 4)-glycosidic bonds (where n and m are large integers), chitobiase that carries out the hydrolysis of non-reducing sugar, and chitobiohydrolase that removed dimeric units from the non-reducing end (n = 1, and m is a large integer) [71]. Chitin deacetylase hydrolyzes the N-acetamido bonds in chitin to yield chitosan (Fig. 1.3, R = NH2). Chitosan is depolymerized by chitosanases in a manner similar to the depolymerization of chitin. Type XXV serine protease from Streptomyces griseus also depolymerizes chitosan yielding low molecular weight chitosan (dimer to hexamer) and the monomer [61]. Protein is the major component of wool, comprising about 97%, with about 2% being lipids and 1% being mineral salts, carbohydrates and nucleic acid residues [72]. Hydrolysis of the peptide bonds in the wool protein yields 18 amino acids [73], including sulfur-containing amino acids, most notably cysteine (Fig. 1.4). Keratins (including wool) are distinguished by their high cystine content, which is approximately 500 mmol g–1 of wool. A disulfide bridge between two cysteine residues forms cystine (Fig. 1.4). Adjacent protein chains are often cross-linked through the disulfide bonds of the numerous cystine moieties [73]. Thus, the biodegradation of wool required the hydrolysis of peptide bonds (Fig. 1.4) and cleavage of disulfide bonds.

H

R H H

N

N

N O

p

O

H

R

O

OH

N R

H

R

O

q

Protein chain of wool or silk O

O

OH

O

OH

S

H2N

S

HO SH cysteine

NH2

NH2 cystine

1.4 General structure of a protein (different R groups represent various amino acids) and the structures of cysteine and cystine. The arrow indicates the location of enzymatic hydrolysis.

26


The biodegradation of keratins is hampered by the cross-linking of disulfide bonds because they hinder the accessibility of peptide bonds to proteinhydrolyzing enzymes. Under the low redox conditions in an anaerobic environment, it appears that the disulfide bonds are reduced to loosen the peptide chains (chain1–S–S–chain2 + 2H Æ chain1–SH + HS–chain2). Christeller [52] proposed a model for keratinolysis in the midgut of H. pseudospretella which included reduction of disulfide bonds in this manner. In vitro the addition of the reducing agent 1,4-dithiothreitol to split disulfide bonds stimulated the lysis of keratin by an extracellular keratinase isolated from a wool-degrading actinomycete [53]. Sulfitolysis is another means by which the disulfide bond is broken by some fungi. This reaction occurs in the presence of sulfite and under alkaline conditions. It cleaves the disulfide in cystine to S-sulfocysteine and cysteine [74, 75]. This causes the keratin chains to denature by releasing them from one another (chain1–S–S–chain2 + SO 32– Æ chain 1 –S–SO 3– + – S–chain 2 ). The fungal oxidation of thiols in the cysteine residues yields sulfide for this reaction, and the liberation of ammonium from the deamination of the amino acids creates the alkaline conditions to help drive this reaction [76]. After the structure of keratin in the wool is loosened by breaking the disulfide bonds, extracellular proteolytic enzymes hydrolyze the peptide bonds as shown in Fig. 1.4. The hydrolysis releases soluble peptides that are further hydrolyzed to amino acids. Silk is also a proteinaceous fiber. Raw silk from the silkworm Bombyx mori is made primarily of a filamentous protein called fibroin held together by a gum-like protein called sericin. Before weaving, the raw silk is degummed to remove sericin [64]. Fibroin comprises heavy-chain and light-chain proteins held together by disulfide bonds. Tanaka et al. [77] provide evidence that there is only one disulfide bond between each heavy-chain and light-chain molecule in silk from B. mori. Not surprisingly, the biodegradation of silk is similar to the biodegradation of wool, except that, because there are many few disulfide bonds in silk, they do not hinder proteases to any great extent. For example, Arai et al. [56] demonstrated the breakdown of silk by three commercially available proteases, without having to add a reducing agent to break the disulfide linkages. These proteases hydrolyze the peptide bonds shown in Fig. 1.4. Silk also contains a few small peptides and two of these have been shown to inhibit some bacterial and fungal proteinases [78]. This is a clever adaptation that the silkproducing insects have evolved to increase the longevity of their silk by inhibiting depolymerization. The structure of poly(L-lactide) is shown in Fig. 1.5. The L-lactic acid unit of this polymer is structurally very similar to L-alanine and glycine (Fig. 1.5). Silk from B. mori is rich in glycine (44.6 mol%) and L-alanine (29.5 mol%) [63]. Because of these structural similarities, Tokiwa et al. [63]

Microbial processes in the degradation of fibers H

O

CH3

H

H

O

CH3

O

O

O

O H

O

27

OH

CH3

O

n

H

CH3

m

poly(L-lactide) O

H

H N H

N H

CH3

O

L-alanine

glycine

HO carboxy terminus

H

O O

R1

OH O

i

hydroxy terminus

R2 j

k polyhydroxyalkanoic acids poly(3-hydroxybutyrate) R1 = R2 = CH3 copolymers R1 π R2

1.5 The structure of poly(L-lactide) compared to the structures of L-alanine and glycine moieties, and the general structure of poly(hydroxyalkanoic) acids. The arrows indicate the location of enzymatic hydrolysis.

postulated that silk-degrading microorganisms might be able to degrade poly(Llactide). They verified this by isolating a silk-degrading actinomycete that also degraded poly(L-lactide). Pranamuda and Tokiwa [50] surveyed 25 strains of Amycolatopsis, and found that 13 of these degraded silk. Of these, 12 also degraded poly(L-lactide). Tokiwa and Jarerat [79] reviewed the biodegradation of poly(L-lactide). They wrote that proteinase K and alkaline proteases from Bacillus are able to degrade poly(L-lactide). Studies also showed that a poly(L-lactide)-degrading enzyme isolated from a strain of Amycolatopsis was a serine type protease that also degrades silk fibroin; thus, there is considerable evidence that many enzymes that can hydrolyze peptide bonds (as illustrated by the bond between L-alanine and glycine in Fig. 1.5) are also able to hydrolyze the ester bond in poly(L-lactide) (Fig. 1.5). This hydrolysis ultimately releases lactic acid, which is readily metabolized by many microorganisms. Surprisingly, lipases do not cleave the ester bonds in poly(L-lactide) [79], but will hydrolyze the amorphous polymer poly(DL-lactide). Like poly(lactides), the monomers of poly(hydroxyalkanoates) are linked by ester bonds, as illustrated in Fig. 1.5. Among the most common

28


poly(hydroxyalkanoates) are poly(3-hydroxybutyrate) (shown in Fig. 1.5 where R1 = R2 = CH3) and simple co-polymers such as poly(3-hydroxybutyrate-3hydroxyvalerate) where R1 = CH3 and R2 = CH2CH3. In the latter case, the proportions of the 3-hydroxybutyrate and 3-hydroxyvalerate monomers incorporated into the co-polymer can vary, giving different values of the subscripts ‘i’ and ‘j’ in the general formula in Fig. 1.5. However, there are a wide variety of monomers (e.g. R1 and R2 in Fig. 1.5) that can be incorporated into different poly(hydroxyalkanoates). For example, Steinbüchel and Valentin [80] reported 91 different hydroxyalkanoic acids that are found in natural poly(hydroxyalkanoates). Poly(hydroxyalkanoates) are polyesters that exist inside of many bacterial cells as storage compounds of carbon and energy, and they also exist outside of the cells. Enzymes that degrade the intracellular (native) polyesters are different from those that degrade the extracellular polyesters [81]. This discussion will focus on the enzymes that depolymerize the extracellular poly(hydroxyalkanoates), which are more crystalline than the intracellular polyesters. Of course, man-made fibers and films made of poly(hydroxyalkanoates) exist outside of microbial cells, and therefore extracellular depolymerases are required for their biodegradation. Most, if not all, extracellular depolymerases have endo- and exohydrolase activities on poly(hydroxyalkanoates) [81]. The endohydrolase activity randomly cleaves ester bonds some distance from the terminus of the molecule, yielding smaller polymers. The exohydrolase activity cleaves ester bond near the terminus of the polymer, yielding monomers, monomers and dimers, or a mixture of oligomers. Shirakura et al. [82] demonstrated that the purified depolymerase from Alcaligenes faecalis strain T1 hydrolyzed insoluble poly(3hydroxybutyrate) and soluble trimer and larger oligomers of D-(–)-3hydroxybutyrate. This enzyme cleaved only the second and third ester linkages from the hydroxy terminus (Fig. 1.5, R1 = R2 = CH3 and j = 2 or 3) of the trimer and tetramer, but it acted as a endohydrolyase with the pentamer or higher oligomers. The biodegradable polymers shown in Figs 1.3 and 1.5 are all of biological origin. In contrast, the polyester amide BAK 1095, shown in Fig. 1.6, is chemically synthesized as a random co-polymer of 1,4-butanediol, 6aminocaproic acid and adipic acid. BAK 1095 contains 40% ester bonds and 60% amide bonds. Wiegand et al. [19] demonstrated that the ester bonds are hydrolyzed by a commercial esterase, and they isolated 15 bacterial strains that grew on this co-polymer. All of the isolates depolymerized BAK 1095 by hydrolyzing the ester bonds, but based on the analyses of accumulated products, there was no evidence to indicate that the isolates cleaved the amide bonds (indicated by a question mark in Fig. 1.6). Thus, oligoamides accumulated in the growth medium that contained ammonium nitrate as the nitrogen source. However, Wiegand et al. [19] cited several studies that have

Microbial processes in the degradation of fibers O

O OH

HO

O

NH2

HO

1,4-butanediol

OH

29

OH

6-aminocaproic acid

adipic acid

? O H

O

O

O

H N

O O

O

H n

polyester amide BAK 1095 – a random co-polymer

1.6 The general structure of the random polyester amide BAK1095. The molecule shown contains two moieties from 1,4-butanediol, one moiety from 6-aminocaproic acid and one moiety from adipic acid. The arrows show enzymatic cleavage sites. The question mark indicates that cleavage of the amide bond was not clearly established (Weigand et al.) [19].

demonstrated cleavage of amide bonds, so it is very likely that each of the bond types shown in Fig. 1.6 is broken. The presence of the readily available ammonium nitrate in the medium used by Wiegand et al. [19] would have precluded selection of microorganisms that split the amide bonds and extract the organic nitrogen for cell growth.

1.7

Future trends

The microbial processes involved in hydrolysis of biodegradable fibers are now very well understood, including environmental conditions under which biodegradation can occur and the types of chemical bonds that are amenable to microbial attack. In addition, a wide variety of procedures and analytical methods have been established to follow biodegradation of fibers, films and textiles. Thus, evaluation of the biodegradability of novel fibers introduced in the future will be straightforward. With the knowledge and experience gained from studying biodegradation and the enzymology associated with these processes, it is now possible to apply these to improve the treatments of natural materials to yield usable fibers with less impact on the environment, and to improve the qualities of the fibers and textiles. A few examples are given below. Many raw fibers require degumming before they can be used. Conventional processes that remove plant gum from ramie and hemp use hot alkaline solutions, and these methods have high energy demands, of raising concerns about disposal methods. As an alternative, Kapoor et al. [83] evaluated the use of a polygalacturonase from a bacterium (Bacillus sp. strain MG-cp-2) to degum ramie and hemp to produce cellulosic fibers. In experiments with ramie, a chemical treatment with 2% NaOH at 90∞C for 8 h released 4.1

30


mmol of reducing sugar per cm3, whereas an enzyme treatment at 50∞C for 9 h released 7.6 mmol of reducing sugar per cm3. In contrast, the enzyme treatment of hemp released less reducing sugar than did the chemical treatment. Kapoor et al. [83] concluded that sequential chemical and enzyme treatments provided a better fiber. Degumming is also applied to raw silk to remove the protein sericin from the fibroin filaments. The typical method for sericin removal involves dissolving this protein by boiling the raw silk in aqueous solutions containing alkali and detergents; this process consumes large amounts of water and energy. Seeking a ‘greener’ treatment, Freddi et al. [84] tested the abilities of four proteases (three from microorganisms and one from a plant) to act as degumming agents. The standard condition for chemical degumming was treating silk at 98∞C for 1 h in a solution of soap (10 g dm–3), and sodium carbonate (1 g dm–3). In contrast, no detergent was used with the enzyme treatments and the temperatures of the treatments were between 50 and 65∞C, with a pH of 3 to 10, depending on the enzyme. The alkaline and neutral proteases effectively degummed the silk, giving nearly complete removal of the sericin. The use of microbial and mammalian transglutaminases to modify wool textiles has recently been reported [85]. These enzymes cross-link proteins leading to increased protein stability and increased resistance to proteolytic and chemical degradation. Treating wool with these transglutaminases increased the tensile strength of yarns and reduced felting shrinkage in the fabric. The enzymatic treatment is more environmentally friendly than the conventional treatment of wool with chlorine to prevent felting shrinkage [85]. Tzanov et al. [86] investigated the preparation of cotton fabrics using a series of enzymatic treatments. To replace conventional alkaline boiling as a means of scouring (e.g. 0.2 M NaOH in 1 g dm–3 surfactant), they evaluated the use of commercial bacterial and fungal pectinases for ‘bio-scouring’ cotton. The presence of a surfactant improved the scouring with both enzymes, and these treatments at 40∞C and 55∞C for 2 h resulted in an absorbency and removal of fiber cuticle comparable to the conventional alkaline boiling. Hydrogen peroxide is commonly used to remove the natural gray color from cotton. Tzanov et al. [86] also explored the enzymatic production of hydrogen peroxide using glucose oxidase with glucose and oxygen as substrates. In this ‘bio-bleaching’ process, the rate of aeration was key to the success of the method because glucose and protein appeared to stabilize the hydrogen peroxide. Tzanov et al. [86] extended this work and established a closed loop process of scouring, bleaching and reusing starch contained in desizing baths. Advances made over the past few decades have improved our understanding of the biodegradation of fibers, films, and fabrics. Experience has shown which types of polymers are susceptible to biodegradation, and which types are not susceptible. Applying this solid background of information, new


31

fibers and composites can be developed that will be biodegradable and will not remain in the environment indefinitely. Building on our knowledge of microbial and enzymatic activities, new environmentally-friendly processes can be developed to provide more sustainable fiber and fabric production.

1.8

Acknowledgements

I thank Rose Fedorak for her enlightening discussions about fibers and fabrics. I am grateful to Drs Budwill, Ohkawa and Modelli for providing copies of the photographs included in this chapter. I also acknowledge the assistance of J. Scott.

1.9

References

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2 Bast fibres (flax, hemp, jute, ramie, kenaf, abaca) R K O Z L O W S K I, P B A R A N I E C K I and J B A R R I G A - B E D O Y A, Institute of Natural Fibres, Poland

2.1

Introduction

Nature in its abundance offers us a lot of material that can be called fibrous; fibres are found in plant leaves, fruits, seed covers and stalk. Fibres from these plants can be considered to be totally renewable and biodegradable. Bast fibres are soft, woody fibres obtained from stems of dicotyledonous plants (flowering plants with net-veined leaves). Such fibres, usually characterized by fineness and flexibility, are also known as ‘soft’ fibres, distinguishing them from the coarser, less flexible fibres of the leaf, or ‘hard’, fibre group. This chapter will discuss bast fibres from flax, hemp, jute, ramie, kenaf and abaca. Green fibres like flax, jute, sisal, kenaf and fibres of allied plants, which have been used for more than 8000 years, are the present and will be the future raw materials not only for the textile industry but also for modern ecofriendly composites used in different areas of application like building materials, particle boards, insulation boards, food, fodder and nourishment, friendly cosmetics, medicine and source for other bio-polymers ‘agro-fine chemicals and energy’. Potentially, under optimum cultivation conditions, they cause little or no detrimental effect on the ecosystem, they can grow in different climatic zones and they recycle the carbon dioxide in the Earth’s atmosphere. Global trends towards sustainable development have brought to light natural, renewable, biodegradable raw materials, among them bast fibres. Science and technology continue to extend their use in textile and other industries. Recent achievements and new applications of green fibres and associated products made of bast fibrous plants can provide the background for the following conclusions: ∑ A fast-growing population and eco- and health awareness will create large market for future expansion of other natural cellulosic fibres as alternatives to cotton. ∑ Green fibres/bast fibrous plants will also be used in growing quantity in 36


37

a wide spectrum of biocomposite materials. Being lignocellulosic they can be combined with man-made or natural polymers to provide a wide range of useful composites in textiles (including geotextiles and nonwovens), in particle and other boards, in goods containing chemical and thermosetting polymers, in filters, in transportation, in the building industry and agriculture. In the future all biocomposites will have to be recyclable and fully biodegradable. ∑ Green fibrous plants provide valuable by-products such as seeds, waxes, fragrances, and pigments. These may be used for food, fodder, pharmaceutics, cosmetics, and body-care items. Especially important are linseed/hemp seed. They contain substances indispensable for our brain and nervous system as well as antisclerotic/anticarcinogenic lignans and unsaturated fatty acids. The Institute of Natural Fibres (INF) has developed valuable food additives from linseed/hemp seed, which are in high demand together with cosmetics/body-care products. This is a good example of providing jobs and raising living standards by promoting agriculturally based industries. Resuscitation of these plants is very important because they provide a better agricultural balance on the Earth and they will reduce a deficit of cellulosic pulp for the twenty-first century when the population will grow to about 11.6 billion.

2.2

Flax

Flax (Linum ussitatissimum L.) has 15 pairs of chromosomes (2n = 30) and the spring cultivars of fibre and oil flax are economically important. Flax is the oldest textile fibre in the world, it has been grown since the beginning of civilization; the earliest trace of its use dates from 8000 BC. Linen fabrics have also been discovered in Egyptian tombs wrapped around the bodies of the pharaohs. The qualities of nobility and strength of this fibre were already renowned in 6000 BC. The Phoenicians, famous merchants and navigators, bought flax in Egypt to export it to Ireland and Britain. In this way the fibre of flax entered the European continent. Around 3000 BC flax was cultivated in Babylon and around 650 BC burial chambers depict cultivation and clothing from flax fibres. Hippocrates writes about using flax for the relief of gastrointestinal problems and Theophrastus recommends the use of flax mucilage as a cough remedy.

2.2.1

Economic importance of flax

Fibre flax is a plant grown mainly in wet temperate climates, and cultivated widely in Europe (Table 2.1). Outside Europe fibre flax is grown in China

38

Biodegradable and sustainable fibres Table 2.1 World fibre flax production in 2003 Country

Cultivation area (ha)

Total fibre yield (t/ha)

Argentina Belarus Belgium Chile China Czech Republic Egypt Estonia France Italy Lithuania Netherlands Poland Romania Russian Federation Slovakia Spain Ukraine United Kingdom Total (World)

2 70 19 2 143 7 7

0.70 1 900 0.37 26 000 NA NA 1.00 2 200 3.50 500 500 2.53 17 700 0.93 7 160 0.64 69 1.00 86 000 0.15 450 0.66 6 270 5.56 25 000 1.96 10 000 1.50 300 0.47 37 000 0.40 350 0.73 11 000 0.50 12 000 1.56 28 000 1.27 (avg yield) 771 899

86 3 9 4 5 79 15 24 18 497

700 000 250* 200 000 000 700 107 351 000 500 500 100 200 000 880 000 000 000 488

Total fibre production (t)

Source: FAO statistics (http:/apps.fao.org). *INF data; NA – not available.

and Egypt. Flax fibre is used today as a valuable, environmentally friendly raw material for the textile industry. It is used to produce woven and knitted fabrics with exceptional health, hygienic and aesthetic qualities which ensure that linen never goes out of fashion! Linen woven and knitted fabrics ‘breathe’ with the skin. They absorb sweat and are good heat exchangers which cool us down in hot weather. An additional quality of flax fibre is that they do not collect an electrostatic charge, which is beneficial for human health and provides psychophysical comfort. Traditionally, linen was used to produce table cloth, bed linen and upholstery fabrics. Recently, however, it is more and more frequently used as a raw material for clothes and for non-textile applications such as floor covering, geotextiles and in the automotive industry. The processing of flax, besides fibre, also yields shive (50% of the harvested biomass) which is a good raw material for manufacturing lignocellulosic boards used in the construction and furniture industry. Flax is a source of many other valuable raw materials used in the food and the pharmaceutical industries. Flax seed (linseed) can be added to bakery products (whole and ground) increasing its taste, nutritive value and prolonging its shelf-life. The linseed oil, containing polyunsaturated fatty acids, is a fast-drying oil and is a basic component in the production of paints and


39

varnishes. It is also a precious dietetic and curative product. It has a beneficial effect on the peristaltic action of the intestine, prevents skin problems and shows anti-sclerotic action. Oil flax is grown mainly in dry, warm and temperate climatic conditions. The major producers of oil flax are Canada, India, China, Argentina, USA and Europe (Table 2.2). The main crop is seed. The fibre is a secondary material used sometimes for quality pulp production and in non-textile applications. Today, flax is facing strong competition from cotton and man-made fibres. To meet this challenge breeders try to improve its qualities by creating better cultivars. To obtain new cultivars by the classical method usually takes about 16 years but this process can be accelerated by implementing modern breeding techniques, for instance from molecular biology and biotechnology. Research at the INF has succeeded in breeding haploid plants through anther culture, which shortens the breeding cycle to only 8 years. During the experiments a new variety of fibrous flax was developed and called Alba 2, which is characterized by higher resistance to Fusarium wilt in comparison with the initial variety Alba. Applying modern biotechnological methods in plant breeding will bring further development progress in the production of both food and other products. Continuous development of the research has resulted so far in three generations of genetically modified plants. To ensure successful breeding, regardless of the method used, breeders must have access to gene banks as the source of genetic variability indispensable for breeding. The best source is the IPGR gene bank, which holds 7934 accessions (47% is represented by fibre type, 10% by linseed type, 6% by combined type, 1% by other type and about 36% is not specified) [1].

2.2.2

Anatomy of the flax plant

Flax seed is oval in shape and flat (slightly convex) with one end round and the opposite end with a sharp beak-like tip. The colour of the seed is mostly brown (in a wide range of shades, from almost black to light brown). Some cultivars (mainly oil ones) have yellow (golden) seed. Flax has a tap root system reaching approximately to the depth corresponding to the height of the plant, namely 60–100 cm. The size of the root system depends strongly on the conditions the plant is growing in. In good soil/moisture conditions the root system is weaker. The stem is composed of three main parts: root neck, non-branched (in fibre flax) or branched (oil flax) stem and an inflorescence – panicle. Leaves are small, narrow, lanceolate, arranged spirally on the stem, without petioles. Usually there are fewer leaves at the top of the stem than at the bottom of the stem. Leaves have three main parallel nerves and are covered with a thin layer of wax.

40

Biodegradable and sustainable fibres Table 2.2 World linseed production in 2003 Country

Argentina Australia Bangladesh Belarus Belgium Brazil Canada Chile China Czech Republic Denmark Ecuador Egypt Eritrea Estonia Ethiopia France Germany Hungary India Iran, Islamic Rep of Italy Kazakhstan Kenya Kyrgyzstan Lithuania Netherlands New Zealand Pakistan Peru Poland Romania Russian Federation Slovakia Spain Sweden Tunisia Turkey Ukraine United Kingdom United States of America Uruguay Total (World)

Cultivation area (ha) 13 7 70 70 20 11 728 1 390 5

17

98 77 16 1 459 3

6 9 4 6 1 1 62 1 11 3 2 24 34 235

800 000 000 000 232 000 400 150 000 344 221 90 000 341 107 595 124 000 000 200 800 000 650 800 000 500 500 500 200 000 740 561 800 635 145 600 200 350 000 000 930

2 600 2 399 116

Source: FAO statistics (http://apps.fao.org).

Linseed yield (t/ha) 0.82 0.86 0.71 0.20 0.67 0.66 1.04 0.90 1.19 0.91 0.31 0.40 1.53 0.59 0.92 0.52 0.69 0.63 1.00 0.38 0.88 0.67 1.00 1.00 0.63 0.28 1.00 2.00 0.48 0.75 0.99 0.96 0.88 0.82 0.34 1.83 2.14 0.31 0.25 1.74 1.12

Linseed production (t) 11 6 50 14 13 7 754 1 466 4

250 000 000 000 460 300 400 035 000 848 68 36 26 000 200 98 51 052 52 941 10 000 1000 172 600 700 2 000 650 800 3 800 2 700 4 500 1 000 3 000 750 733 1 498 55 000 1 333 3 800 6 600 4 700 110 6 000 59 000 264 830

1.19 3 100 0.86 (avr. yield) 2 068 892


41

The cross-section of flax stem reveals eight main elements (Fig. 2.1 [2]): ∑ epidermis – an outer part of the stem formed by a single layer of epidermal cells covered with cuticle; ∑ primary cortex – a tissue formed of 2–7 layers of parenchyma cells; ∑ layer of bast fibres – bundles of fibres in the form of a loose ring; ∑ cortex – a layer of secondary cortex made of tiny cells forming vascular bundles; ∑ cambium – a layer of parenchyma cells separating cortex from xylem; ∑ xylem – a thick layer of thick-wall cells; ∑ core – inner part of the xylem; ∑ core channel – an empty space inside the middle part of the stem along the whole length of the stem. Bundle of fibers Epidermis Primary cortex Cortex

Cambium

Core channel

Xylem

2.1 Flax stem cross-section [2].

The clusters of fibre called fibre bundles are located in the layer of cortex. Fibre flax is characterized by a high number of bundles with compacted structure. The lower number of loosely arranged bundles is characteristic of oil flax. The inflorescence of flax is a panicle or a hanging bunch. The flower is self-pollinating with five petals, five-petal calyx and five-petal corona. The colour of the flower varies in different cultivars from white to different shades of blue. Five crochet-shaped, 2–5 mm stamens are joined at the bottom. The pistil is composed of a five-chamber ovary and five free necks, on top of each forming club-shaped stigma [3].

42


The following main development phases can be distinguished in flax [4]: 0 1 3 5 6 7 8 9

Germination. Leaf development, young plant elongation. Shoot development. Inflorescence emergence. Flowering. Development of flax capsules. Ripening of flax capsules. Senescence.

2.2.3

Cultivation of flax

Flax is a temperate climate plant; the best weather conditions for flax are high temperature and high relative air humidity. The sum of rainfalls during the vegetation period should lie within 110–130 mm (600–800 mm annually). Flax consumes 400–600 ml of water to produce 1 g of dry matter. The best soils for flax cultivation are loess soils in good culture, medium-heavy, claysandy and sand-loamy soils. The flax should be cultivated on soils of good structure, with the ability to hold water and release it in dry conditions. The best soils for flax cultivation are the soil of IInd and IIIrd class, sometimes IVa [5]. Flax is susceptible to the preceding crop. In intensive agriculture the best forecrops are cereals, while in more extensive conditions sugar beet, and on weaker soils legumes. It does not tolerate growing on the same field the following year. The suggested break is 6–7 years [5]. Important elements in tillage are mechanical treatments controlling weeds in spring and in autumn and reducing evaporation from the soil (in spring). Besides basic nutrients (N, P, K), more and more important is the application of magnesium and calcium and microelements (B, Zn, Cu, Mn and Mo). There are several diseases that are a problem in flax. The most dangerous ones are Fusarium wilt, anthracnose, gray mildew, polysporosis, rust, rhisoctoniosis, powdery mildew and pasmo [6], but to some extent the diseases (especially Fusarium wilt) can be controlled chemically with fungicides. The most dangerous insects for flax are flea beetles (two species) and thrips. Flea beetles are most dangerous from the beginning of germination till plants are about 5 cm tall. When the population of flea beetles reaches 5–10 insects m–2 the plants should be sprayed with insecticide. The optimal time for harvesting flax grown for fibre is the early yellow maturity of straw (green–yellow maturity). In this stage the stems are yellowish on approximately 1/3 of length (from the bottom of the plant), the stem has no leaves up to about a quarter of its length; the bolls begin to turn yellow. Flax grown for seeds should be harvested at yellow maturity when stems


43

are completely yellow, leaves have fallen off from 2/3 of the stem and the bolls are yellow. Seeds are fully developed and begin to turn brown. Harvesting of flax in the first stage of this operation is done by pulling machines. These are mostly self-propelled machines that pull out straw and swath it in the field for the retting process. When retting is half-advanced, the straw must be turned over to secure uniform retting. This operation is performed by special turning machines which usually also harvest seeds. Sometimes seeds are harvested during the pulling operation [7]. For the reasons mentioned in Chapter 1 on hemp, most of the cultivated flax is dewretted. When retting is completed straw is picked up from the field with rolling presses. Retted straw is delivered for processing which generally is similar to processing of hemp except that most flax straw, unlike hemp straw, is processed for long fibre. Therefore, the majority of flax straw is processed by scutching lines.

2.2.4

Degumming of flax and hemp

The process of loosening the bond between the fibre bundles and surrounding tissue (decomposing the natural adhesive pectin) is called degumming. There are several degumming methods (Fig. 2.2); however, only two are commonly used, water and dew retting. Degumming of bast fibrous (flax, hemp) plants

Water retting

Physical methods Dew retting

Chemical degumming

2.2 Degumming methods.

Water retting In the past flax and hemp were retted in industrial conditions in so-called retting mills. Straw was water retted in special basins (tanks) in warm or cold water. The retting process was conducted by bacteria (Bacillus amylobacter, Bacillus felsineus, Granulobacter pectinovorum, Clostridium felsineum, Bacillus comesii rossi) [8], in anaerobic conditions. Due to high costs of the process in terms of energy (required for heating water up to 30∞C) and waste water (20 tonnes of water per tonne of straw and 10 tonnes

44


for washing and rinsing) it is a considerable threat to the environment and it is not used any more in many countries. However, in hotter countries where no heat is required for warming water and with a more ‘flexible’ approach to environmental protection, water retting is still used. Dew retting Today, in most countries where flax and hemp straw is retted, it is by dew retting. This method involves the action of fungi, mainly Cladosporium herbarummand and also other species like Mucor stoloniter, Mucor hiemalis, Mucor plumbens, Aspergillus niger, Fusarium culmorum, Epicoccum nigrum and Rhizopus sp. [8]. Unlike water retting this process is aerobic. In rainy conditions the action of bacteria is also involved. In the retting process, regardless of which organisms are involved, the principle is to decompose the plant glue (pectin) that bind both the elementary fibres together and also conglomerates of fibres in the form of strands to woody fractions (shive, hurds). When dew retting is half-advanced, the straw should be turned over. This allows the straw to be retted evenly and prevents the unwanted vegetation to grow through the swath. The time to break the retting is crucial for the quality of the fibre, especially when it is to be used for clothing. If the process is stopped too soon the fibre will be rough, stiff and it will be difficult to separate from the hurds. If retting takes too long, the fibre becomes weak and breaks into shorter and shorter fragments causing losses. It may also finally rot if wet weather conditions persist. Both processes involve the action of enzymes used by retting organisms to dissolve pectin. Enzymes can also be used directly to degum the straw. For flax a pectolytic activity (∞PM) of 210 U cm–3 is required. Chemical degumming In chemical degumming straw is exposed to the action of chemicals, such as ethylene, oxalic acid, sulfuric acid, sodium hydroxide, acid and sodium carbonate, oxygenated water, soda, sodium sulfite, etc. Physical methods Several physical methods of straw degumming have been developed; all involve the application of physical techniques leading to the separation of fibre from surrounding tissue. Among these techniques the following are mostly used: ultrasound oscillation, electron radiation, steaming with application of pressure methods, steam explosion, flash hydrolysis, steam hydrolysis, osmosis degumming, high-power electromagnetic pulses.


45

Among these methods, osmosis degumming is especially worth mentioning as practically the only one that can be used more widely, at least as a laboratory method of retting for evaluation of content and quality of fibre, especially in the case of new cultivar breeding. In this method, the degumming of fibre from straw is based on the physical laws of nature: water diffusion, osmosis and osmotic pressure. In this process, fibrous plants are subjected to treatment with water which flows continuously in order to extract fibres from plants without influencing the natural properties of the fibre formed. The fibre obtained in the above method is characterized by a colour, delicacy and thinness appropriate to the raw input material. The best results are obtained under the following conditions: ∑ temperature: 25–35∞C, ∑ time of process: from 72 to 168 hours; ∑ constant, regular and controlled water flow.

2.2.5

Straw processing (flax and hemp)

When the retting is complete, the stems are collected and dried. The straw containing below 19% moisture can be processed. Straw can be supplied to the retting mill in four forms depending on the technology applied: 1. 2. 3. 4.

Whole threshed stems bound in bundles. Stems without tops cut into sections and baled. Baled decorticated fibre (bast). Raw non-retted straw.

Basically, the objective of primary processing of straw is to weaken the bond between the fibre and shive and to separate the fibre. This objective is achieved by carrying out a series of breaking, shaking and scutching operations repeated several times until the fibre is satisfactorily extracted and separated from the shive. Depending on what kind of fibre is to be obtained, the straw is processed on a scutching line producing long fibre and waste short fibre (tow) or on a tow producing unit. The scutching line comprises breaking machines which break the stem and crush the woody part of the stem thus causing preliminary separation of the fibre. Next, the crushed stems are scutched by scutching drums which hit the stems and knock off the shive. The tow line is used for producing tow, also called homomorphic fibre in this case as there are no two different kinds of fibre in this process. To achieve this objective the processing line is assembled of three main units: breakers, scutchers and shakers. Breakers, typically, are a set of riffled pairs of cylinders. The straw is inserted between them and crushed. It is the crushing action of the breakers that loosens the stem and allows the knocking down of the woody part of the

46


stem (shive) in the following stage of processing (scutching and shaking). Scutchers can be similar to the scutching drums used in the processing of flax straw or are built similarly to the breakers with specially shaped teeth. Shakers are basically the sets of needles and sieves allowing for separation of loosened hurds from the fibre still remaining after scutching. The technologies described above serve to process retted fibre. The raw straw needs much more intense operation (decortication) to loosen the bonds between hurds and fibre and yield fibre that requires extensive further processing to make it spinnable into yarns. This technology employs, as previously described, the decortication unit together with a set of several machines applying cyclically repeated actions of breaking, scutching and shaking (Fig. 2.3). The straw or bast obtained as a result of initial decortication should be dried to below a 15% moisture content before being fed into the aggregate. Feeding unit

Breaking unit

Fibre extraction unit

Press

Cleaning unit

Condensing unit

Residues collector

2.3 Scheme of decortication line.

Flax and hemp straw due to the specific structure of fibre finds its application mainly through the traditional flax spinning method. Linen and hemp textiles are usually thicker than cotton, wool or chemical fibres. Traditional flax spinning technology is highly labour intensive, inefficient and uneconomical. The physical parameters of simple flax and hemp fibres are similar to those of cotton. This, for quite a long time, has been a reason for the interest in the possible utilization of bast fibres, properly modified, for manufacturing yarns blended with cotton and chemical fibres. The principle of modification of flax and hemp fibres (cottonization) is to make hemp fibres resemble cotton fibres. The following three types of cottonized hemp fibres can be obtained: ∑ mechanically cottonized fibre; ∑ chemically cottonized fibre; ∑ enzymatically cottonized fibre.


47

Mechanical production of flax and hemp cottonized fibre for spinning by a cotton system involves the following operations: ∑ double breaking of fibre (tow or homomorphic fibre) on a breaker equipped with 32 pairs of rollers; ∑ fibre wetting and conditioning; ∑ forming reels; ∑ carding on a carding machine; ∑ cutting a sliver into 40 mm sections; ∑ single or double processing of cut sliver on a carding machine adopted for processing of flax and hemp by the Institute of Natural Fibres in Pozna´n. The chemically cottonized fibre can be produced using similar operations as mechanically cottonized fibre. The only difference is that the sliver is cut or broken into 60 mm sections and next boiled in a solution of sodium hydroxide followed by acidifying, rinsing, centrifuging, drying and double processing on a carding machine. The retted fibre, cottonized chemically, mechanically or enzymatically, may be used for production of thin, pure linen and hemp yarns or yarns blended with cotton, wool and other fibres. Such modified fibre allows for the production of yarns having 50–100 tex of linear mass by pneumomechanical spinning system.

2.2.6

Spinning flax

As mentioned earlier flax is spun mainly by flax spinning equipment. The spinning of flax comprises various operations that make it possible to transform the fibres into yarn. The techniques vary according to the raw materials used and type of yarn to be produced. In short, the spinning process involves drafting and doubling, or carding and drawing out the long or short fibres into sinuous ‘ribbons’ which are then plied together on spinning looms in various weights and thicknesses. The fine yarn is ‘wet spun’ to impart a smoother, shiny appearance. The tow is commonly ‘dry spun’ yielding a less regular and napped yarn. Linen can be used to make a very wide range of fabrics, for many applications. A few years ago, the weaving of linen was the work of a few specialist weavers; today the evolution in equipment and the quality of the yarn available on the market have allowed a great part of the textile sector to use linen in the development of increasingly competitive products. The development of products in pure linen or majority blended linen has continued to grow in response to the new trends in fashion and the expectation of consumers. Linen is known for numerous advantages, but has also some disadvantages, the most problematic being susceptibility to creasing. This disadvantage, however, can be overcome by special fabric finishing in liquid ammonia. The cellulosic clothing materials, such as linen or cotton, that are treated

48


serially with liquid ammonia show a remarkable shrink-proof or shape-retention property and crease resistance. This finishing changes the crystal structure of cellulose making it more elastic. A new quite promising finishing for linen and other lignocellulosic fibres is CORONA treatment. The CORONA discharge is produced between two electrodes, at high voltage with frequency of 20–40 Hz. It affects the surface of a substrate moving continuously between electrodes at ambient pressure and temperature. For natural fibres such as cotton and linen, superficial changes are high enough to obtain good effects in textile processing and the material’s properties. The processing of linen fabrics by CORONA treatment has an effect on some physical and mechanical properties of fabrics, namely in: 1. significant increase of water sorption (Fig. 2.4); and an 2. increase of hygroscopicity (Fig. 2.5).

Time taken for absorption (s)

1200

1000

Without CORONA With CORONA

800 600

400 200

0 23080 1/2B

3003 3G Fabric type

2.4 Speed of water absorption – drop method.

Apparel made of linen also ensures safety during sunny days, protecting us against hazardous ultraviolet radiation. Natural fibres containing natural pigments and lignin absorb ultraviolet rays very effectively. Linen and hemp fibres contain lignin as a part of their structure and therefore can be classified as excellent protectors against UV rays (Fig. 2.6). More dense structure of linen fabric with a lower level of clearance guarantees the user the best protection against ultraviolet radiation.


49

16

Hygroscopicity (%)

14 12 10 8 6 4 2 0 23080 1/2B

30033 G Fabric type

65% humidity of air, without CORONA 65% humidity of air, with CORONA 100% humidity of air without CORONA 100% humidity of air, with CORONA

%T

UVA

UVB 5

1+5 300 320 340 360 380 Wavelength (nm) Raw linen fabric; UPF = 15

%T

%T

2.5 Hygroscopicity of fabrics.

10 5

UVB

10 5 0

UVB

UVA

3+5 300 320 340 360 380 Wavelength (nm) Linen fabric after ammonia; UPF = 30

UVA

300

320 340 360 380 Wavelength (nm) Linen fabric after finishing with resins; UPF = 50

2.6 Protective effect of linen against UV radiation.

2.2.7

Applications of flax

Textiles Application of flax fibre in textiles cover both the clothing and non-clothing sectors. Flax is a well-known textile raw material used for exclusive and healthy clothing, woven and knitted. The non-clothing textile sector covers mainly table, bed linen and curtain fabrics. Some textiles may need fire retardant treatment. Fortunately cellulosic fibres, contrary to common belief, are quite safe in this respect as compared to other polymers (Figs. 2.7 and 2.8). Flax production and processing is the source of valuable by-products such as seeds, shive and waste fibres. Flax seed as a by-product of bast fibrous plant is a rich source of valuable fatty acids, lignans, cyclolinopeptides, mucilage, waxes and other ‘agro-fine

50


HRR [kW/m2]

30 20 10 0 0

100

200

300

Time [s] Cotton Hemp

Curaua Cabuya

Abaca Flax

HRR [kW/m2]

2.7 Heat release rate of cellulosic bast fibres (heat flux 35 kW m–2). 120 100 80 60 40 20 0

0

100

200

300

Time [s] Cotton Hemp

Curaua Cabuya

Abaca Flax

2.8 Heat release rate of cellulosic bast fibres (heat flux 50 kW m–2).

chemicals’ making it an excellent source of valuable nutrients (Table 2.3). Shive constitutes 50% of processed straw and has numerous applications. The most common are particle board with different densities (Table 2.4; Fig. 2.9). Flax fibres are used also for non-textile applications, for instance in floor covering, and more and more often used as filling or reinforcing fibre in composites. Flax fibres display high tensile strength and much better tenacity than glass fibre. These properties make them an excellent choice for the automotive industry. Additionally, composites reinforced with natural fibres become biocomposites, which are a lower burden on the environment. Table 2.3 Nutrient composition of flax seed Nutrient

% by mass

Ash Oil (90% polyunsaturated fatty acids) Protein Soluble dietary fibre (mucilage) Insoluble dietary fibre Total dietary fibre Total carbohydrates Lignans (phytoestrogens)

4–5 28–45 24–30 6–10 18–22 27–31 20–23 0.3


51

Table 2.4 Density range of particle boards from different annual plant residue Density (kg m–3)

Low density 300

400

Medium density 500

600

700

800

Flax Hemp Jute Bagasse Flax and sawdust Rape (canola) Source: Kozlowski, Mieleniak and Przepiera (1994) INF, Poland.

2.9 Lignocellulosic particle boards, left to right hemp, jute, vetiver root and flax.

2.3

Hemp

Hemp (Cannabis sativa L.) has ten chromosomes (2n = 20); it is an annual, dioecious, allogamic and wind-pollinating plant. Among hemp, three botanical varieties are found: var. vulgaris – regular hemp, var. indica – Indian hemp and var. ruderalis – wild hemp. Hemp is the only species found within the genus Cannabis; however, they are enormously rich in forms. Hemp can be divided into the following groups (types): northern hemp, middle European hemp (intermediate hemp) and southern hemp. The northern hemp is characterized by a short growing period (60–75 days), has high yields of seeds and low yields of poor quality fibre. The southern hemp gives high yields of vegetative biomass, including good quality fibre and low yields of

52


seeds; they also have a long growing period (over 150 days) [9]. The group of intermediate hemp is characterized by factors lying between these values.

2.3.1

Economical importance of hemp

Hemp is a cosmopolitan plant found all over the world. Hemp is indigenous in Middle Asia, from the foothills of the Himalayas, from where it migrated to Eastern and Southern Asia. Hemp was first cultivated in China 5000 years ago and from there it spread to the whole world. Hemp was grown and still is grown mainly for fibre and seed but also as a source of narcotics. It is or was grown in the recent past on all continents except Antarctica from the tropics to Northern Europe (as far as 70∞ north altitude). Today, hemp is grown for fibre mainly in China, Europe (Russia, France, Ukraine, United Kingdom, Germany, Poland, and Finland) and also in North America (Canada). The area of cultivation, however, is much smaller than for other crops (Table 2.5). Table 2.5 World hemp production in 2003 Country

Cultivation area (ha)

Total fibre yield (t/ha)

Total fibre production (t)

Bulgaria Chile China France Hungary Italy Korea, Dem. People’s Rep. Korea, Republic of Poland Romania Russian Federation Serbia and Montenegro Spain Ukraine

8 4 300 15 000 200 60 296 18 500 120 70 600 15 000 50 11 000 2 000

0.00 0.95 2.53 1.80 2.00 4.33 0.69 1.87 0.71 3.33 0.47 0.40 1.36 0.50

0 4 095 38 000 360 120 1 281 12 800 224 50 2 000 7 000 20 15 000 1 000

Source: FAO statistics (http://apps.fao.org).

In the past, before cheap fibres like cotton and, later, chemical fibres become available, hemp fibre was commonly used to produce technical items (ropes, sail fabric, twine, tarpaulin, etc.). After the Second World War, hemp cultivation was prohibited in the USA and Western Europe (except France) because of its narcotic properties. In recent years, because of the search for alternatives to food crops in Europe and alternative sources of renewable resources, hemp is again of interest. Besides the traditional textile application of hemp numerous new directions emerge:


53

∑ building and isolation materials – particle board, MDF boards, cement boards with additional fibre, acoustic insulation materials, thermal insulation materials, light structural elements, bricks, upholstery material, nonwovens, etc.; ∑ composite materials – pressed elements for the automotive industry, coatings for clutches and brakes, filling material for injection moulding; ∑ special cellulose – special papers such as banknote, cardboard, cartographic, paper, cigarette and tea bag tissue, cellulose ‘plastics’; ∑ clothes, home textiles – wear, bed linen, towels, tablecloths; ∑ technical textiles – for laminates, tarpaulin, sail fabric, ropes, nets, etc.; ∑ geotextiles and agricultural textiles – nonwovens, felt, mats, embankment reinforcement material; ∑ oil-based products – paints, varnish, lubricants, cosmetics, food oil, dietetic foods, nutraceuticals, etc., and essential oils; ∑ items for agriculture and horticulture – animal bedding, flower beds.

2.3.2

Anatomy of the hemp plant

Anatomically, the hemp nut consists of lignified ovary, seed shell, and embryo with cotyledons, apical bud, radicle and endosperm [9]. The main chemical components of a seed are: fats (25–38%), protein (about 25%) and carbohydrates (25%) [9]. Hemp has a strong tap root system, reaching 1.5– 2 m deep into the soil. For 80 cm of depth the roots branch strongly, following side branch levels, spreading perpendicularly reaching about 1 m from the main root. The main root mass is located about 20–40 cm from the top of the soil. The development of the root system depends on, among other things, ground water level; hence the depth of root system of hemp growing in peat soils is only 50–60 cm. A stem of hemp grown for fibre (at higher sowing densities) does not branch and has about 10–13 mm diameter. At low sowing densities, on seed plantations and especially in the case of single plants, the stem strongly branches. The height of plants varies from 150–200 cm to 400 cm and even more. The internodes on the stem produce one pair of leaves. The palmatipartite leaves grow on leaf petioles and are divided into lancet-shape sections. The number of sections (5–11) differs, depending on the location on the stem. The closer to the top of the stem, the lower the number of sections. On the top of the male plant, the leaves consist only of a single section. In the bottom and middle part of the stem, the leaves are arranged opposite, while within the inflorescence, alternately. The leaves fade gradually and fall off the plant as the plant matures [9]. The anatomy of hemp stems is characteristic of this species. It consists of a cortex, collenchymas, a ring of primary bast and rings of secondary bast

54


and wood. A layer of bast in hemp, which becomes fibre after proper processing, lies directly under the cortex. It is arranged in bundles of tidily adhering cells of bast glued with a pectin and joint with so-called anastomosises [9]. The fibre is not distributed regularly in the stem. The highest concentration of fibre is found in the middle part of the stem. At the very bottom of the stem, at the so-called root neck, it is strongly lignified and has no technological value. At the top of the stem, the concentration of fibre decreases and in the middle of the inflorescence it is so low that the top of the stem can be easily broken. Therefore, the section of the stem from the root neck to the middle of the inflorescence is called a ‘technical’ length to distinguish it from the total length of a hemp plant. The fibre in the stem is arranged in the form of rings of primary and secondary fibre. The latter can be found mainly in the bottom part of the stem, while there is no secondary fibre in the top part of the stem. The highest concentration of primary fibre can be found in the middle part of the stem (Fig. 2.10 [10]). Parenchyma Fibre bundles Meristem tissue

Epidermis

Cortex Pith channel Xylem

2.10 Hemp stem cross- and longitudinal section.

The secondary fibre is formed later in the development of the plant and decreases the technological value of the fibre while it is strongly lignified, stiff and difficult to divide. The quality and amount of fibre in the stem is an outcome of the effects of numerous natural and agricultural factors, mostly on the cultivar (genetic potential), type of soil and fertilizer supply, sowing density, time of harvesting, etc. The inflorescence of hemp is a panicle. Hemp, being naturally a dioecious species, is characterized by a clear dimorphism of sex. Male plants form loose, strongly branched panicles with a very low number of leaves while females produce compact panicles with lots of small leaves. A male flower has five stamens: anthers set on long filaments and five sepals. A female flower is located in green, rolled in the form of sheath bractlets and has a singlechamber ovary. The stigmas reach out from a narrow slit during flowering. Hemp belongs to the plants with a very intensive growth, compared to


55

other crops. In the fast growth phase it grows more than 10 mm a day making it very competitive to weeds which usually dispenses with the need for using herbicides in cultivation of hemp for fibre. The following development phases can be distinguished in hemp: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

germination; beginning of emergence; full emergence; fast growth phase; inflorescence buds formation; beginning of male flowers blooming or beginning of flowering (monoecious hemp); mass flowering of male plants or full flowering (monoecious hemp); milky maturity of seeds; waxy maturity of seeds; full (biological) maturity of seeds.

2.3.3

Cultivation of hemp

In Europe both mono- and dioecious cultivars are grown. Currently mostly Polish, Hungarian Ukrainian and French cultivars are available in the international market. Polish cultivars are well adapted to the climatic conditions of Poland. However, the warmer the summer, the higher the yields that can be obtained, provided there is sufficient water supply. Despite hemp being a warmth-liking species, Polish cultivars can stand light freezing conditions. It can survive the temperatures of –5 to –7∞C better than flax. Hemp has high soil requirements. The soils must be fertile, structural, with stabilized water conditions. Mostly the soils suitable for hemp cultivation are black earths, alluvial soils, loesses, lime soils rich in humus and wellmeliorated peats. Hemp is very sensitive to the pH of the soil. The optimum pH for hemp is 7.1–7.6. Therefore, the soil should be limed when the acidity drops below these values. Hemp should not be grown on soils where the pH is below 6.0 [5]. Hemp is also a good crop to grow on soils polluted with heavy metals. An experiment has shown [11] that providing the soil shows good agricultural quality, hemp can extract and accumulate substantial amounts of elements such as copper, lead, zinc and cadmium with no detrimental effect on the quantity and quality of the crop. The total calculated, extracted and fixed copper and lead can reach 377 g and 141 g per hectare, respectively [12]. This results in gradual remediation of the soil and eliminates the threat of introduction of heavy metals to the nutritive chain of humans and livestock. Hemp has no special requirements for the preceding crop. Hemp grows well after root crops grown on manure, legume crops; it even can be grown for a few years on the same field. When growing for seeds, special attention

56


should be paid to weeds, especially to Elymus repens (L.) Gould. Hemp, especially when grown for fibre (at high sowing densities and sown in narrow row spacing), is very competitive to weeds, giving no chances for any weed to grow. It also improves the structure of the soil (deep root system) and leaves very good conditions for the following crops [5]. When growing hemp for fibre potassium and calcium are especially important for plant cell formation, while when growing for seeds it is phosphorus, which is taking part in seed formation, that is important. The N:P:K ratio in seed production should be 1:8:1, while in fibre production 1:7:1.5 [5]. As mentioned before, hemp is often considered a crop free of pathogens. Some authors suggest that the presence of essential oils in hemp, especially a-pinene and limonene, are responsible for it or stipulate that it is the cannabinoids content, especially in narcotic type hemp, that keeps the insects away [13]. Some experiments show that insects forced to feed on hemp slow down their development or are even killed. Another explanation for treating hemp as a pathogen-free crop may also be the fact that for decades it was a marginal or non-existing commercial crop in farming. This situation may change dramatically when hemp is grown to a bigger acreage. Geographical differentiation of pathogens can also have a lot to do with the opinion of hemp as a pathogen-free crop. On the other hand, a considerable list of insects and also non-arthropod pests are mentioned that may be or actually are a problem in hemp [14, 15]. Some authors claim as many as 272 insects are associated with hemp and there is at least one of six arthropod classes depending on hemp to a greater or lesser extent [16]. Among the insects are the European corn borer (Ostrinia nubilalis) and the hemp borer (Grapholita delineana). These insects, or actually their larvae, feed inside the main stem and inside branches breaking the tops and wilting distal plant parts. These pests are especially dangerous as they destroy the stem and automatically a fibre which is currently the main purpose of hemp cultivation [15]. Another dangerous stem-boring pest in hemp can be the budworm (e.g. Heliothis armigera and Heliothis viriplaca) [15]. They can be a problem in hemp grown for seeds as they feed on flowering buds but have no effect on hemp grown only for fibre. Another insect threatening hemp, especially in the germination stage, is the hemp flea beetle (Psylliodes attenuata) [15]. This insect feeds on cotyledons and first true leaves. In years of heavy infestation it can destroy the plantation if not controlled. Hemp can also suffer from fungi and diseases like Fusarium wilt, septoriosis and gray mildew are found especially in weather conditions promoting these diseases. Sometimes, especially if hemp is grown several times on the same stand, in may suffer from a parasitic plant, branched broomrape (Orobranche ramosa L.) [14]. Virus diseases may also sometimes attack hemp.


57

The time of harvesting depends on the purpose of cultivation of hemp. Hemp grown only for fibre should be harvested at the beginning of flowering. This allows for obtaining delicate and quite strong fibre suitable for textile production. Delaying harvest increases the fibre yield. Also, its strength is increasing when harvesting is delayed; however, this is also connected with stronger and stronger lignification of the fibre and such fibre is not suitable for textile production – it can only be used for technical purposes and for pulp production. If hemp is grown for both fibre and seeds or only for seeds it should be harvested at full maturity phase, when the seeds in the middle part of the panicle are mature. The fibre obtained from hemp harvested at this time has no value for textile application. It can be used for twine and for other technical non-textile applications, including pulp. Due to its exceptional productivity and the high bulk volumes of biomass to be harvested, hemp is quite difficult to harvest. Currently there is no modern and efficient technology available for hemp, especially for fibre. Harvesting is done either manually (Asia) using simple tools or mechanically using quite out-dated machinery (eastern Europe). There are numerous trials in many research centres to develop efficient harvesting technology, but with no major commercial success [17].

2.3.4

Degumming of flax and hemp

The degumming (retting) of hemp is discussed in Section 2.2.4. However, research has been conducted on hemp fibre into new retting techniques. Enzyme degumming Within research on utilization of enzymatic processes for fibre extraction two preparations of different bacterial laccases (NS 51002 and NS 51003) were used and the third enzyme used was Pectinex 100L. The laccases are oxidoreductases which react directly with oxygen; it is an important oxidant used in the paper industry to eliminate lignin from the pulp. The research showed that optimal processing conditions during the application of laccase preparations for enzymatic degumming of raw hemp fibres required the following conditions: ∑ ∑ ∑ ∑

temperature pH time of treatment dispersing agent

37∞C 4.5 48 hours Sultafon UNS in amount of 1g dm–3 and hydromodulus 1:25.

The fibre thinness after enzymatic processing varies from 8.21–9.03 Tex

58


when treated with NS 51002 and 8.05–9.1 Tex when treated with NS 51003. The thinness for control treatment was 16 Tex. The enzymatic process allows shortening of the fibre from 269 mm to 144–172 mm when treated with NS 51002, and 153–177 mm when treated with NS 51003. Observed total mass loss shows that non-cellulosic components (pectin, hemicellulose, fats and waxes) are removed from fibre during enzymatic treatment, it has also been found that within the application of laccases followed by mechanical upgrading the mass loss is approximately twice as intensive as mechanical upgrading alone. The mass loss in a whole process is on the level of 20–21% for NS 51002 and 21–23% for NS 51003 while in mechanical processing it is only 11%. Pectinex 100L is a treatment obtained from a selected strain of fungus, Aspergillus niger. The active enzymes in this preparation are pectinotransferase, polygalacturonase, hemicellulase and arabanase. Tests on Pectinex 100L in conditions of variable temperature, time, pH, and concentration of the enzyme were conducted to determine the optimum parameters of the processing of hemp fibre. However, the thinness of hemp fibre after treatment did not differ significantly from the control and was 14–16 Tex. Also the length of the fibre and the mass loss remained unchanged; the latter was 16%, as in the control treatment. Electrolytic degumming The effect of an electrolyic process on hemp fibre degumming was tested at the Institute of Natural Fibres, Poland. Experiments were conducted using an oxidation process and using a reduction process. In both cases 0.01M and 0.1M solutions of hydrochloric acid and sodium hydroxide were used as electrolytes. The research found that application of sodium hydroxide in 0.1M concentration gave better results in both oxidation and reduction at the following treatment parameters: ∑ temperature ∑ time of treatment ∑ voltage

20–25∞C 48 hours 4.5 V

Hemp fibre thinness after electrolysis with this electrolyte varied from 8.51 to 10.32 Tex (control treatment – 16.35 Tex). The mass loss in both oxidation and reduction processes calculated as a total for electrolytic and mechanical processing was 18.77–21.56% (control – 12.89–14.19%).

2.3.5

Straw processing

Straw processing is carried out as described in Section 2.2.5.


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Table 2.6 Hemp fibre content in function of slenderness Slenderness Fibre content (%)

229 23.7

174 20.4

169 19.3

163 18.8

144 18.4

118 16.1

116 15.9

Besides yield, a feature that characterizes good hemp straw is slenderness (Table 2.6 [9]), namely the ratio of the technical length and thickness of the stem. Therefore the best results in processing are obtained when technical length of stem is 150–200 cm and thickness 4–8 mm. The quality of raw material depends also on the length of internodes. The long internodes contain the fibre of longer elementary cells. As mentioned before, the primary fibre (the most interesting material from a processing point of view) is distributed along the whole stem, with the highest concentration in the middle part of the stem. It is formed in the apex from the meristem before the end of flowering. The secondary fibre is contained mainly in the lower part of the stem and is formed later in the development. The parameters of the fibre and a comparison with flax fibre are presented in Tables 2.7–2.9 [9]. Table 2.7 Breaking strength of hemp fibre from different parts of the stem Part of the stem

Breaking strength of hemp fibre (kg)

Bottom Middle bottom Middle Middle upper Upper

11.3 42.7 31.6 32.8 36.2

Table 2.8 Length of elementary fibres in different parts of the stem

2.3.6

Part of the stem (cm)

Length of elementary fibres (mm)

0–15.5 75.5–95.5 105.5–135.5 155.3–175.5 195.5–215.5 235.0–245.0

5.1 10.2 9.1 8.9 8.3 3.9

Chemical composition of fibre

The most important components of fibre are cellulose, pectin and waxes. Pectin is found in the middle lamella of the fibre cells and glues the elementary

60

Biodegradable and sustainable fibres Table 2.9 Comparison of elementary fibre of flax and hemp Physical features

Flax

Hemp

Length of fibre* (mm) Average length (mm) Diameter of fibre (mm) Thickness of fibre (nm (metrical number)) Average thickness of fibre (nm (metrical number)) Self breaking (km) Average self breaking (km) Elongation of fibre – dry (%)

2–120 20 17–20 1500–9500 3620

2–90 15 10–30 800–8500 3460

23–72 49 1.5

27–63 46 1.5

*Maximum length of secondary fibres reaches 5 mm.

fibres to form bundles. The lignin is an incrusting component of the fibre. It is incrusting cellulose and contributes to the hardness and breakability of fibres. It is allocated in the middle lamellae and fibre cells. Other components of hemp fibres are tannin, resins, fats, proteins, etc. The composition of hemp fibre is presented in Table 2.10 [9]. The content of additional substances (other than cellulose) is much higher in hemp than in cotton. Therefore, the processing of those fibres requires different technology of processing. The higher the content of cellulose the better the quality of fibre. Table 2.10 The components of hemp fibre (%)

2.4

Component

% by mass

Water Ash Waxes Water solubles 0.5% HCI solubles Pectin A Total pectin Hemicellulose Lignin Cellulose Protein

6.60 1.2 1.47 2.45 0.89 1.35 1.42 6.08 1.75 77.89 2.65

Jute

Jute (Corchorus capsularis and Corchorus olitorius) is one of the best known and most extensively used textile fibres of plant origin, with the exception of cotton. Jute was regarded by many early botanists as of Chinese origin, but the most widely cultivated species apparently is indigenous to India. On the Indian subcontinent, the jute plant has been utilized from time immemorial both as a vegetable and as a source of textile fibre [18].


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After much debate, there seems to be agreement that white jute originated in the Indo-Burma region and tossa jute in Africa. China is also considered to be one of the places of jute origin. According to some academics, some provinces of the southern parts of China are the secondary centres of origin of tossa and white jute [19]. The various jute plants and hence the fibre and fabric made from it went under the name ‘pat’. The word ‘jute’ may be traced to ‘jhat’, a term still used in India which was doubtless derived from the Sanskrit word ‘yuta’ (fibre). Jute, a bast fibre, means the fibre known as pat, kosta, nalita, bimli or mesta [18]. Jute is a natural fibre obtained as an extract from the bark of the jute plant. This fibrous plant in earlier days was also used by the inhabitants as a delicacy, which accompanied their staple diet. Jute is a long, soft, shiny fibre that can be spun into coarse, strong threads. It is one of the cheapest natural fibres. Jute fibres are composed primarily of the plant materials: cellulose, lignin and pectin. Both the fibre and the plant from which it comes are called jute. It belongs to the family Tiliaceae, which belongs to the genus Corchorus. At present the major producers of jute, kenaf and roselle fibres are India, Bangladesh, China, Myanmar, Nepal and Thailand.

2.4.1

Economic importance of jute

The advent of jute as a commercial commodity dates back to the eighteenth century. In the beginning, the jute fibre was made into ropes that were extensively used in wind- and hand-driven sea vessels. Later, it was learnt that jute can be spun and woven for the manufacture of carpets. By 1838 newer technologies emerged and jute fibre was spun into better yarn and woven to make jute-cloth. Sacking bags and jute handbags were the initial developments and with that jute products started to enter people’s daily lives. Applications in carpet making and packaging has also been dominated by jute ever since jute started to be woven into fabric form [20]. Other properties of jute fibre started to be noticed in the middle of the last century. The jute fibre and its subsequent processing might find application in new areas of use and also newer products. Cultivation of jute plays an important role in maintaining soil quality; as the stem is cut during harvesting a major portion of the root remains within the soil. This in due course decomposes and disappears into the soil. Through this process of bio-degradation it enriches the soil by way of providing natural manure for other subsequent crops to be grown there. In countries like India, some products such as petrochemical products, especially plastic bags, have become an indiscriminate menace on the environmental front. Soil pollution, visual pollution, choking of drains and blockage of natural water streams have all added to the problem. Jute bags

62


and paper bags are thus regaining popularity. This alternative may not be as cheap as its plastic counterpart, but the price paid will still be cheap compared with the cost of saving the environment. Jute products have a good market in India as well as in other regions like USA, Australia and Europe. The products generally in demand are jute shopping bags, wall hangings and floor coverings. New markets are being explored through increased consumer awareness and product promotion. It will not be surprising to find this natural fibre product becoming much more regularly used by consumers all over the world. India, Bangladesh, China, Thailand, Myanmar and Nepal are the major producing countries with about 95% of the global production of jute and allied fibres (Table 2.11). India and Bangladesh produce mostly jute, China produces mostly kenaf, while Thailand produces kenaf and roselle. Table 2.11 Area and yield of jute and allied fibre production in major producing countries Country

Area (2002/2003) (’000 ha)

Yield (1998/99–2002/03) (Mt ha–1)

Bangladesh China India Myanmar Nepal Thailand

500 56 1000 58 11 19

1.79 2.53 1.86 0.85 1.13 1.54

Jute cultivation is labour-intensive, but it requires relatively small quantities of other input, such as fertilizers and pesticides, and can be carried out in smallholdings. Jute competes for land with food crops such as paddy rice in Bangladesh and India, and cassava in Thailand. In general, producers attempt to adopt a multi-cropping strategy with jute in rotation with paddy. Until the late 1990s, world production of jute fluctuated between 3 million and 3.7 million tonnes, with the notable exception of a record crop of over 6.0 million tonnes in 1985. Between 1998 and 2000, world production exhibited a marked decline to an average level of 2.6 million tonnes. This decline was the result of a decline in jute’s competitiveness relative to polypropylene fuelled by decreases in the latter’s price. Assuming that weather conditions and yield per hectare of jute follow their normal patterns, world production of jute is projected at 2.4 million tonnes per year by 2010, well below the average production level of the past decade. This decrease in production in the medium term is expected to result from a weakening in price incentives due to declining global demand for jute fibre. In the medium term, the area under jute cultivation in the Far East is expected to contract by 3.1% per annum from an average of 1.6 million


63

hectares in 1998–2000 to 1.2 million hectares in 2010 as producers adjust to market conditions through disinvestment. Production is expected to decline by 1.6% per annum from an average of 2.6 million tonnes during the years 1998–2000 to 2.3 million tonnes in 2010. India is projected to increase its dominance of global jute production, accounting for 66% of the world production by 2010, compared to 58% during the period 1998–2000. In the medium term, the area under jute cultivation in India is expected to contract by 2.7% per annum, although production is expected to remain at approximately 1.6 million tonnes due to increases in yield. Between 1990 and 2000, yields increased from 1.60 to 1.86 tonnes per hectare and are expected to continue increasing to 2.1 tonnes per hectare by 2010. In Bangladesh the area under jute cultivation is projected to contract from 447,000 hectares to 387,000 hectares by 2010, as producers respond to lower market prices and allocate land to competing food crops. The contraction of the land under jute will be partly offset by increased yields. Yields are projected to increase from the 1998–2000 average of 1.70 tonnes per hectare to 1.76 tonnes per hectare in 2010. As a consequence, production is expected to decline by 1.9% annually from 768,000 tonnes in 1998–2000 to 681,000 in 2010. The area and production of jute in China are projected to continue to contract. Production declined from 726,000 tonnes to 126,000 tonnes in the course of the 1990s and is expected to continue to decline to 7000 tonnes by 2010 as more land is given over to food crops. During the same period, production in Thailand is also projected to decline to 17,000 tonnes, while in Vietnam production is expected to remain stable at 12,000 tonnes [21].

2.4.2

Cultivation of jute

Jute needs a humid climate with temperatures of between 24∞C and 38∞C, the optimum being around 34∞C Celsius and humidity between 70% and 90%. This type of climate is prevailing in areas between 30∞ latitude north and south of the Earth. Minimum rainfall required for jute cultivation is 1000 mm. The most suitable growth for jute is in new grey alluvial soil of good depth receiving silt from annual floods. However, jute is grown widely in sandy loams and clay loams. Sandy soil and heavy clays are unsuitable for its cultivation. Loamy soil usually produces the best fibre. The clayey soil yields a short crop; the sandy soil produces coarse fibre. Jute plants, after the shedding of leaves, are immersed in water for retting; gently flowing clear and soft water is ideal. Incomplete submersion and retting in stagnant water produce inferior quality fibre; most defects are due to faulty retting. Over-retting results in lustreless and weak fibre. There are mainly two types of jute. They are capsularis (white) and olitorious (tossa), while the capsularis fibre is whitish in colour and olitorious

64


fibre is finer and stronger than the capsularis and is yellowish reddish, greyish in colour. Capsularis is normally sown between February and May while olitorious is sown between April and mid-June. Plants are cut from the bottom, leaves are stripped off from the top and accumulated in bundles. Structurally the jute stems are composed of epidermis, cortex, large phloem, cambium, wide xylem or wood and central pith tissues. The crop is harvested at different stages of maturity depending on various circumstances. The most common harvest stage is when 50% of the plants have produced pods. If the crop is harvested at this stage, both the yield and fibre quality are good.

2.4.3

Degumming of jute

Degumming (retting) is the process of extraction of the fibre whereby jute stalks or crude ribbons are immersed in water for a certain period of time. Bundles are then submerged in water for 7 to 10 days. Retting takes place due to the joint action of water, aquatic and plant surface organisms, mostly bacteria; in the beginning the process is aerobic, later anaerobic. The combined action of water and microorganisms results in the softening and removal of pectins, gums and other mucilaginous substances, thus making the fibre easy to separate from the woody stem portion and bark. The minimum ratio of plant material to water in stagnant water should be 1:20. The important conditions for good retting are: ∑ ∑ ∑ ∑

the water should be non-saline and clear; the volume of water should be enough to allow jute bundles to float; bundles, when immersed, should not touch the bottom; and the same retting tank or ditch should not be used when water becomes dirtier.

Retting has been used for a long time as a method of fibre extraction from jute and allied vegetable fibre plants. If the stems are retted, the process is called stem retting; if ribbons are retted it is called ribbon retting. Retting is an important step in the production of good quality fibre, the retting process depending on a country and the practices can be variable. The retting process is finished when the bast layer can be easily removed from the woody stem and the bark readily washed off, leaving the clean fibre. The process is completed in about 10–30 days depending upon the temperature and movement of water, the amount of retting organisms in the water, age and size of the stems. If the water temperature is around 30∞C, complete retting should take 10 to 12 days [18].

2.4.4

Processing jute straw

When retting is complete the straw is stripped, this process is accomplished by hand. The stripper holds the stem in one bunch and taps the root end


65

lightly with a mallet, this frees the fibre at the foot of the stalk. The fibre is then grasped by lashing and jerking the stem in the water while the rest of the fibre loosens and comes off. Other methods of hand stripping retted stems are used, depending upon local tradition. Jute fibres are kept hanging on makeshift hangers for drying; this process takes about 2 to 3 days. Grading becomes imperative depending on the fineness, colour, density, clearness etc., that are all taken into account. The crop then proceeds to the hands of buyer. Dried retted jute as produced is usually tied in small or large bundles when offered for sale. Jute is then accumulated at the baling centres where preliminary grading is made. The bundles are scanned and jute fibres are categorized as per grades. Gradewise these are stocked at separate locations. Graded bundles are subjected to machine press to convert them to bales. (The ropes used to tie the bales are prepared from the jute waste.) The bales are finally stored in the warehouse according to their grades, ready for sale. The ribbons, fibres, leaves and stick contents of different JAF plants vary considerably. The fibre content of the C. olitorius jute plant is the highest and that of H. sabdariffa the lowest. The green leaf content of H. sabdariffa plant is the highest. The dry ribbon contents of both C. capsularis and C. olitorius plants are higher than those of H. cannabinus (Table 2.12). Table 2.12 Physical compositions of different jute and allied fibre plants Crop

Plant (t/ha)

C. olitorius C. capsularis H. sabdariffa H. cannabinus Approx. avg

2.4.5

46 34 48 36 40

Ribbons %

Leaf %

Sticks %

Green Dry

Green Dry

Green Dry

Dry

38.7 40.2 35.2 34.0 37.0

11.0 15.9 16.3 14.2 14.2

50.3 44.2 48.5 57.8 48.8

6.8 5.9 4.8 4.9 5.5

11.7 11.2 9.6 9.5 10.3

2.7 3.9 3.6 3.3 3.3

16.6 12.5 15.0 15.9 15.2

Fibres %

Fibre characteristics

Among jute, ramie, flax, hemp and cotton, jute has the shortest fibre length (0.5–6.0 mm) and ramie has the longest fibre length (125–126 mm). In the case of jute the average length of fibres from outer parts of the wedge is 0.3– 2 mm and from the inner parts about 1–5 mm only (Table 2.13). The quality of jute fibre is judged by its suitability for the production of various types of yarn and its behaviour in the manufacturing process. The fibre that spins into the finest yarn is considered to be of very good quality. Jute fibre is marketed in bundles of fibre hanks. A fibre hank is composed of about 10–15 fibre reeds obtained from 10–15 plants. Each fibre reed is composed of thousands of fibre strands made of ultimate fibres with lignin and pectic substances, the cementing materials. Commercially, fibre

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Table 2.13 Physical properties of jute and other fibres Properties

Jute

Mesta

Roselle

Ramie

Flax

Ultimate cell Length (mm) Length ¥ breadth ratio

0.5–6.0 110

2–11 140

1.5–3.5 100

125–126 3500

26–65 1700

15–60 1300

Filament Tenacity 30–45 (g tex–1) Extension at 1.0–2 break (%)

30–45

25–40

40–65

45–55

20–45

3–4

2.5–3.5

6.5–7.5

1–2

1–1.8

Comparative value for cotton

quality is assessed by taking a hank out of a lot, spreading the individual reeds on the ground and then assessing the different characteristics by ‘look and touch’ method. The different characteristics of the fibre which are usually taken into consideration for the assessment of quality include root content, colour, lustre, strength, defects, etc. They are described below, and in Table 2.14 and Fig. 2.11 [22]: Table 2.14 Jute fibre measurements in comparison with other fibres Fibre origin

Technical fibres (bundle diameter)

Elementary fibres (single cell)

Surface area (S) (mm2)

Diameter (L) (mm)

Surface area (S) (mm2)

Diameter (L) (mm)

Curaua Sisal Abaca Flax Hemp Jute Cabuya Lufa

2 833–11 554 11 983–26 232 6 672–52 659 1 252–2 801 7 196–9 774 2 349–4 087 4 880–27 254 –

62–119 121–205 90–347 38–85 123 63–85 130–258 –

30–39 131–233 166–274 111–272 441–911 299–780 205–551 244–885

8–9 18–27 22–30 15–22 29–45 26–30 25–34 28–57

Wool Cotton Silk

18–60 12–42 13–25

∑ Root content. These are hardy, incompletely decomposed basal or root areas of the fibre. It occurs more in white jute than in tossa jute. For processing in the factories and before export, these, constituting about 12.5 to 39.0 cm, are cut away and separately sold at a much cheaper price. ∑ Length. Differences in quality are generally associated with differences in the total length of the fibre strands. With increase in their average length, the mass of unit length of strands increases. It has been observed


67

L[8] = 79 [um] L[4] = 17.59 [um] L[11] = 18.91 [um]

L[3] = 17.72 [um] L[5] = 17.72 [um]

L[10] = 21.33 [um]

L[7] = 74.33 [um]

L[9] = 75.51 [um] L[2] = 18.52 [um]

L[12] = 23.61 [um]

L[1] = 15.27 [um] L[6] = 11.13 [um] L[13] = 19.31 [um]

2.11 Jute stem cross-section.

that the fibre that feels heavier in the hand tends to have higher spinning quality. ∑ Colour and lustre. The natural fibre colour of white jute is creamy white and that of tossa jute is golden. Any adverse deviation from the natural colour is considered to be bad. Lustre is a characteristic of shine when light falls on the fibre; the more the shine, the better the quality. ∑ Strength. This is a very important characteristic: the stronger the fibre the better the quality. No product can be made from weak fibre. The strength is divided into five levels viz. very strong, strong, sound, average and any strength. ∑ Fineness. The finer and stronger the fibre, the better the quality.

2.4.6

Jute and jute products, uses and applications and environmental advantages

Jute can be used not only in its traditional materials, but also for the production of other value-added products such as pulp and paper, geotextiles, composites and also home textiles. Jute has been employed for centuries as a packaging material. In recent times it is found to be a valuable aid to sound environmental management. Jute is an annually renewable energy source with a high biomass production per unit land area, it is biodegradable and its products can be easily disposed of without causing environmental hazards. The roots of jute plants play a vital role in increasing the fertility of the soil; jute acts as a barrier to pests and diseases for other crops by rotating with other crops, and also provides

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a substantial amount of nutrients to other crops in the form of organic matter and micronutrients. The jute leaves after defoliation have fertilizer value and enrich the soil nutrients. Jute leaves are used as vegetables and have both nutritional as well as medicinal value. Jute sticks are used for fuel and shelter in jute-growing rural areas. Jute is a fast-growing seasonal crop. Because the biological efficiency of jute is much higher than that of wood plants, the use of jute instead of wood to make paper pulp will lower substantially the cost of production of pulp and paper and save forest resources. The production flow of jute agriculture involves: sowing, weeding/thinning, harvesting, defoliation, retting, fibre extraction, washing and drying. Processes of jute retting, fibre extraction and washing have drawn some concerns regarding solid residue and gaseous emissions that arise from such processes. Complaints about the unpleasant smell during retting are quite common, but the retting process is being improved using biotechnology (i.e. enzyme treatment) by volatile fatty acids (mainly butyric acid); the retting liquid contains the following volatile fatty acids: acetic, propionic, butyric, caproic and traces of caprylic acids. Butyric acid is the main component of retting liquor (circa 80%) [23]. Jute products manufacturing process involves several stages such as batching, softening with batching oil, carding, drawing, spinning, weaving and finishing. The use of mineral batching oils is being replaced in the case of bags for specific use like packaging of cacao and coffee for example. The experiments to convert jute fibre and whole jute plant into paper pulp have successfully produced good quality pulp and paper. The growing demand of pulp and paper worldwide on a continuous basis and the increase of public awareness of environmental issues have created conditions to check depletion of forest resources through using jute/kenaf for producing pulp and paper. Jute can be also used for the production of good quality writing and other paper. Figure 2.12 shows the world population, forest area and wood consumption (for years 1980–2050). Jute fibre also has the potential to compete with glass fibre as a reinforcing agent in composites and technologies exist that make it possible to incorporate jute fibre into polypropylene. Products made from jute-reinforced composites have the advantage of low cost, low density, renewability and biodegradability. These composites can be used in the packaging industry, i.e. the manufacturing of crates, boxes or cases used for storage and transportation of agricultural products; in the automobile industry, i.e. to replace glass fibre in car door panels; and as a construction material. Applications of jute-reinforced composites are expected to have a significant positive environmental impact. Particle board made from jute, or the mixture of bast fibre and the core fibre can find a wide application as a substitute for wood. The availability of the technologies for producing particle board and


69

12 10

Billions

8

Forest area [ha]; 0.5% annual loss

6 4

Wood consumption [m3], 1.3% annual growth

2

Population; 1.4% annual growth

0 1980

1990 2010 Years

2030

2050

2.12 World population, forest area and wood consumption, years 1980– 2050.

its high socio-economic value are solid arguments in favour of the future development of this product. Jute geo-textile is another product with a potentially large-scale application. It can have several uses: soil erosion control, vegetation consolidation, agromulching materials, and road pavement construction. Jute, for its versatility, rightfully deserves to be branded as the ‘fibre for the future’. It is another natural option for a cleaner and healthier environment. The main jute products are: ∑ Yarns. The types of jute yarn manufactured can be classified according to the application to which they are put, i.e. fine yarns, hessian yarns, carpet/ special yarns, sacking yarns, etc. These yarns can be further classified into warp and weft yarns, the warp yarns normally being superior to the weft yarns as they have to withstand the cycles of stress during weaving while the weft yarns act more as filler and undergo less strain during the weaving process. ∑ Twines, ropes and cordage are used for the purpose of tying, knotting and binding, particularly agricultural commodities. Jute yarns of various dimensions are plied together to make twines as per requirement and use. ∑ Fabrics, hessian, sacking, scrim, carpet backing cloth and canvas. ∑ The other areas where jute has started to find good markets are floor coverings, furnishing fabrics, handicraft, wall hangings and footwear. The application of jute in many diverse areas is opening new perspectives for its usefulness. The jute sector is able to provide sizeable employment especially in the rural areas and among women. It also contributes to keeping ethnic art forms alive. Depending on the perfection and creativity of the artisans and craftpersons, these products find their way to their respective markets.

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2.5


Ramie

Ramie* (Boehmeria nivea (L.) Gaud.- Beaup.) is the name of the product of one or more species of the genus Boelimeria, a member of the order Urticaceae and nearly allied to the stinging nettle genus (Urtica), from which, however, it differs in the absence of stinging hairs. Some confusion has arisen in the use of the various terms China-grass, ramie and rhea. Two plants are concerned: one, Boehmeria nivea, China-grass, has been cultivated by the Chinese from very early times under the name Tschou-ma; the other, probably a variety of the same species (Boehmeria nivea, var. tenacissima) though sometimes regarded as a distinct species (B. Tenacissima), is the ramie (Malay zamf) of the Malay Islands and the rhea of Assam. Ramie (Boehmeria nivea (L.) Gaud., Boehmeria nivea var. tenacissima) is also commonly known as white ramie; green ramie is one of the group referred to as the bast fibre crops. Ramie fibre is classified chemically as a cellulosic, just like cotton, linen, etc. Leading producers of ramie are China, Taiwan, Korea, the Philippines and Brazil. Until recently ramie has been unknown in the ready-to-wear market, but it is appearing in more garments. It is often blended with cotton and available in woven and knit fabrics from those that resemble fine linen to coarse canvas.

2.5.1

Economic importance of ramie

Ramie is a plant fibre that has been used since ancient times; long before the introduction of cotton, ramie was well established as one of the principal fibres of the Far East. It was used in mummy cloths in Egypt during the period 5000–3300 BC and has been grown in China for many centuries. In those days main areas of ramie cultivation were China, Indonesia, India, Algiers and Congo. Boehmeria cylindrica was used by Indians of the New World as twine to attach spear and arrow heads to shafts. Ramie was introduced into Europe in 1733; the first cultivation trials were undertaken in Holland in 1808–09. However, according to Karpowiczowa [24], the first records of ramie cultivation are dated 1786 in Bologna, Italy. The first attempts in mechanization of ramie processing were undertaken by the government of India in 1869 and then by the French government which led towards the design of a hand-fed raspador decorticator, patented in the United States in 1896. Floridians found that hydrogen peroxide and lime could be used for degumming; these are much less harmful than chlorine and sulfuric acid currently being used. In these regions ramie is locally spun and woven into *There are at least two acceptable pronunciations for the word. Some authorities call it ra-me (‘RAY-mee’) while others pronounce it ram-e (‘rah-mee’).


71

coarse cloth, often without degumming. In China it is also used to make fine oriental textiles. Brazil began production in the late 1930s with production peaking in 1971 with about 30,000 tonnes. Production in the Philippines began in the early 1950s, peaking in the mid-1960s with 5500 tonnes. Nowadays, the main producer countries are reported to be China, Brazil, Philippines, India, South Korea and Thailand, but the available statistics are not reliable. This is because production statistics for ramie are included in the Fibre Crops NES (Not Elsewhere Specified) in the official FAO Production Statistics (FAO 1995). Ramie usage in the US increased in the mid-1980s with the emphasis of fashion on natural fibres. The main importing countries are Japan, Germany, France and the United Kingdom. Only a small proportion of production enters international trade as most is used in the country of production. Ramie, and garments made of more than 50% ramie, entered the United States without import quota limits. Legislation was passed in 1986 eliminating the quota-free status of ramie. Advantages and disadvantages of ramie [25] (a) Advantages ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

Natural lignocellulosic fibres can be used as a substitute to flax and silk. Resistant to bacteria, mildew, and insect attack. Extremely absorbent. Dyes fairly easy. Increases in strength when wet. Withstands high water temperatures during laundering. Smooth lustrous appearance improves with washing. Keeps its shape and does not shrink. Can be bleached and dyed.

(b) Disadvantages of ramie ∑ ∑ ∑ ∑ ∑

Low in elasticity. Lacks resilience. Low abrasion resistance. Wrinkles easily. Stiff and brittle.

2.5.2

Anatomy of the ramie plant

Ramie is a member of the Urticaceae or nettle family and is a hardy perennial which produces a large number of unbranched stems from underground

72


rhizomes. Boehmeria nivea is a shrubby plant with the growth of the common nettle, but without stinging hairs, sending up each season a number of straight shoots from a perennial underground rootstock. The long-stalked leaves recall those of the nettle in their shape and serrated margin, but their backs are clothed with a downy substance and have a silvery appearance. The minute greenish flowers are closely arranged along a slender axis. The small seeds are dark brown in colour, ovate and produced in very large quantity. Ramie stems are slender, sometimes striated and range in diameter from 8 to 16 mm at the base. They may attain a height of 2 to 2.5 metres in 45 to 60 days under ideal growing conditions. They are usually hollow at maturity, being filled with dried pith and may be readily crushed between the fingers. The variety tenacissima differs in that it is more robust and has larger leaves, which are pale green on the face and a very much paler green on the back. They are not downy, however, and this affords a ready means of distinction from true China-grass. Boehmeria nivea is sometimes found wild in India, Malaya, China and Japan, and is probably a native of India and Malaya. China-grass and ramie are widely cultivated not only in China, Formosa and Japan, but also in Brazil, Mexico and the southern states of North America, and also in South Europe. Ramie fibres are found in the bark of the stalk; the fibre is very fine and silk-like, naturally white in colour and has a high lustre. The process of transforming ramie fibre into fabric is similar to processing linen from flax.

2.5.3

Cultivation of ramie

Ramie plant differs from the other bast fibre crops in several important characteristics. It attains a height of from 1 to 2.4 m, and is grown from seed, cuttings or layers, or division of the roots. It is easy to cultivate, and thrives in almost any soil, but especially in a naturally rich, moist, light, loamy soil. Ramie grows best in open type soils, and requires a rich, warm, sandy soil that is very well drained. Also suitable are soils of volcanic origin, including pumaceous types and friable sandy loams, but it is intolerant of wet soils. The plant does best in areas with high temperatures and high humidity plus a rainfall of 1100 cm evenly distributed throughout the year. It tolerates a pH in the range 4.3 to 7.3, but prefers slightly acid soil conditions. Calcareous soils are totally unsuitable despite the high demand of ramie for calcium. For the best growth a good and equally distributed rainfall is necessary. Sudden changes of weather result in irregularities in growth, and these have a tendency to produce plants whose fibres vary in strength. Ramie is a hardy perennial which under suitable conditions can be harvested up to six times a year. Also, the useful crop life ranges from 6 to 20 years. The bark contains gums and pectins which necessitate a chemical or enzymatic treatment to recover the bast fibres.


73

The Boehmeria nivea is a dicot and angiosperm, is adapted to moist tropical climates and deep soils, is a perennial plant, occupies the land all year round and, when under cultivation, it is without branches (apical dominance, apical buds pinched off). Two to four harvests per year are possible depending upon the climate but, as already mentioned, under good growing conditions it can be harvested up to six times per year; it is harvested as the stems turn brown. Harvesting is done just before or soon after the onset of flowering, since there is a decline in plant growth at this stage and maximum fibre content is achieved. The timing of the harvest of a particular stem is important as fibre yields are reduced if it is immature; additionally, there are difficulties in removing the fibre if the stem is over-mature. According to Buchanan, the plant is best harvested as the female flowers open [26]. The outer bark is removed and then the fibrous inner bark is taken off and boiled before being woven into thread [27]. Stems are harvested by cutting just above the lateral roots or the stem can be bent, to enable the core to be broken and the cortex can be stripped from the plant in situ. Mechanical harvesters have been developed but are not used commercially. After harvesting, stems are decorticated while the plants are fresh as the bark gets harder to remove as the plant dries out. The bark ribbons are dried as quickly as possible to prevent attack by bacteria or fungi. The dry weight of harvested stems from both tropical and temperate crops ranges from about 3.4 to 4.5 tonnes ha–1 year–1; a 4.5 tonne crop yields about 1600 kg ha–1 year–1of dry undegummed fibre. The weight loss during degumming can be up to 25% giving a yield of degummed fibre of about 1200 kg ha–1 year–1.

2.5.4

Degumming of ramie

Raw ramie fibre produced either by hand scraping or decortication contains a fairly large percentage of gums and non-fibrous cells, or parenchyma (30– 35%). These gums and cells are, for the most part, insoluble in water and must be removed before the fibre can be mechanically spun in fine count yarns. They are composed principally of arabans and xylans which are readily soluble in alkaline solutions. Actually, they all follow a similar pattern consisting of the following basic steps: (a) boiling of the fibre one or more times in an aqueous alkaline solution with or without pressure and agitation, and with or without penetrants or reducing agents; (b) washing with water and neutralizing; (c) bleaching with the dilute hypochlorite or hydrogen peroxide;

74


(d) washing with water and neutralizing; (e) oiling with a sulphonated hydrocarbon. This process may be carried out on the undried or dried fibre although Hoefer’s [28] findings indicate the latter is preferable. Most of the processes involve a treatment with caustic soda to dissolve the residual pectins and gums. Ramie fibre, when properly degummed, is long and strong vegetable fibre. It is lustrous, possesses high tensile strength, is remarkably absorbent and gains strength appreciably when moist. Although ramie fibre is usually degummed chemically, there have been promising developments in microbial degumming (retting). Additionally some researchers reports that the use of ultrasonic vibrations speeds up the degumming process. The enzymatic treatment seems to be promising. Chemical degumming Hot alkali is used to dissolve the pectic substances that bind the ultimate fibres into bundles. Some commercial processes, for reasons of cheapness, use sodium hydroxide almost exclusively as the alkali, though it is used sometimes in admixture with sodium carbonate. The main factors that affect the efficiency and economics of chemical degumming are: ∑ ∑ ∑ ∑ ∑

concentration; pH; volume and temperature of the alkaline liquor; the duration of degumming; agitation.

Degumming can be carried out both below (‘low temperature’) and above atmospheric pressure (‘high temperature’); during the latter the yield of degummed fibre tends to be low (about 75% on washed decorticated fibre). This is because at temperatures above boiling point not only the pectic substances and adhering epidermal tissues are removed, but also hemicelluloses and, sometimes, some of the true cellulose [28]. Two further aspects require consideration if chemically degummed fibre is to be suitable for spinning. Firstly, care must be taken to avoid tangling of the fibre during degumming and subsequent washing, especially if much mechanical agitation is used. Secondly, thorough washing after degumming is very important. Most of the published chemical degumming methods use water as either the sole or the final washing agent. Microbial degumming (retting) This method involves the utilization of several mixed bacterial cultures isolated from different sources. Each of the mixed cultures contained several bacterial


75

species, which grew in association with one another [29]. The researchers observed that attempts to separate an individual organism for isolation and identification failed since it did not grow separately, probably owing to its dependence on the metabolic products of other organisms in the mixed culture for nutrition and growth. Microbial degumming with mixed bacterial cultures is thus a good alternative to chemical degumming; this process involves several treatments to obtain good-quality fibres. The mixed degumming method is simple and economical in that less alkali is required, the treatment is less drastic, and such fibre properties as softness, feel, and lustre are also much improved. The combined microbial and chemical method is simpler and more economical.

2.5.5

Ramie straw processing

After the degumming process the degummed fibre, or filasse, is fairly white; if pure white fibres are required, the filasse must be bleached. In the past the fibre was bleached before being made into yarn or cloth but nowadays it may be possible to bleach cloth. The result of bleaching is a small loss of weight and of fibre strength [30]; it should be carried out only when absolutely necessary. Since degummed fibre may be stiff, harsh and dry, and not completely separated, it needs to be softened, before spinning, by the application of a suitable agent – for example glycerine, oil, fat, soap, paraffin, wax or tallow – and left for some time to condition. The fibres can be further softened and separated by passing them through a series of paired fluted rollers and then through a pair of smooth rollers; if necessary, they can be passed through several times [30].

2.5.6

Spinning ramie

In traditional spinning individual fibres are then spun together; the warp is spun on a spinning wheel; the weft is joined and given a slight twist by hand. Industrial spinning is a multi-stage process which consists of three basic steps: ∑ carding (combing the fibres into a mat); ∑ drawing (pulling the carded mat out to form a long thin strand, or sliver, of adhering parallel fibres); ∑ spinning (twisting the sliver so that the fibres lock each other into the strand). Ramie may be combed and spun by several methods. The finest yarns are produced on the spun silk system developed by the Japanese, but this system requires much labour. In Europe, Brazil and the Philippines, modification is

76


used. This produces coarser count yarn and much less labour is required. Ramie may also be spun on the worsted and long draft cotton systems, but in the latter case stapled noils are used and usually blended with cotton or synthetic fibres. Since ramie fibre is relatively coarse in comparison with cotton, it is never spun into fine count yarns on the cotton system. It appears that the main difficulty in spinning ramie results from the combination of high tensile strength with the long fibre length; the breaker cards cannot break the fibre into staple length suitable for subsequent spinning. Thus, it is usually necessary to pass the fibre through a stapling machine: this is reported to break, rather than cut, the fibre into the staple length. The advantages claimed for this method of stapling include: reduced fibre loss, easier spinning resulting from the more uniform length within a given staple, and smoother yarn resulting from feathered as opposed to blunt ends. The weaving of ramie yarn does not pose any problem and all kinds of linen and cotton looms are used for this purpose. The ramie can be dyed in tops, in yarn or in fabrics and its dyeing properties are similar to those of linen and cotton. Very rich in cellulose, ramie remains snow white after exposure to the sun. The textiles woven from ramie yarn show excellent wearing properties and cover a vast range, running from very fine shirting to heavy uniform suitings.

2.5.7

Properties of ramie fibre

The fibres are the longest known in the plant realm [31], the tensile strength is seven times that of silk and eight times that of cotton, and this is improved on wetting the fibre. The fibre of the ramie after the extraction and primary processing is strong and durable, ranking first among all vegetable fibres in this respect. It is the least affected by moisture and its tenacity even increases by 25% when wet. It has the special advantage of resisting rot when exposed to weather conditions or immersed in water. The fibre of ramie is exceptionally white, being comparable to bleached cotton, and has also a high lustre, exceeding linen in this respect. Ramie requires a different technological process to cotton; therefore it is difficult to compare directly the properties of those fibres. However, to give some idea about the advantages of ramie, some of the physical and chemical characteristics of ramie, flax, hemp and cotton are compared in Table 2.15 [32]. The ultimate fibres are exceptionally long and are claimed to be the longest of vegetable origin, with one report claiming the fibres range up to 580 mm, averaging about 125 mm. Another report describes the ultimate fibre as ranging between 48 and 290 mm in length. It was also reported that the range of bark fibre length was 5 to 36 mm and the fibre width was 41.8 microns. The inner structure of ramie differs from the other plant fibres in


77

Table 2.15 Ramie fibre properties in comparison with other fibres Characteristics

Ramie

Flax

Hemp

Cotton

Ultimate fibre length (mm) minimum most frequented maximum

5 120–150 620

1 13–14 130

5 15–25 55

9 20–30 63

Ultimate fibre diameter (mm) minimum most frequented maximum

13 40–60 126

5 17–20 40

10 15–30 50

12 14–16 20

Tensile strength (kg mm2)

95

78

83

45

Moisture regain (%)

12

12

12

8

Chemical composition (%) Cellulose Lignin Hemicellulose, pectin and others

*72–97 0–1 3–27

64–86 1–5 14–31

67–78 4–6 18–27

88–96 0 4–12

*Minimum and maximum cellulose contents refer to decorticated and degummed ramie respectively.

that the physical form of the cellulose is rigid and crystalline like linen, but is a more porous sieve-like form [33], providing it with even better absorbency than other cellulose fibres. In addition, it is softer with better dyeability. Ramie fibre is very durable, but like linen and cotton, ramie has poor resiliency and wrinkles easily; application of wrinkle-resistant finishes or blending with synthetic fibres can reduce the problem in woven fabrics. Because of its high absorbency, ramie is comfortable to wear, especially during warm weather. Other properties include resistance to alkali, rotting, light and mildew. Resistance to insects is good unless the fabric is heavily starched. Ramie is not harmed by mild acids but can be damaged by concentrated acids. The fibre has some natural stain-resisting ability with ease of stain/soil removal similar to that of linen, which is better than cotton. Dyes appear to have good wet-fastness in laundering but there can be a tendency for crocking in dark or saturated colours. Dark colours may lose their vibrancy over repeated launderings.

2.5.8

Applications of ramie and ramie fibres

In textile applications, blends are more common than pure ramie with the most being typically 55% ramie/45% cotton. The uneven linen texture is generally apparent in the blend, but the lustre is lost. When polyester and

78


other man-made fibres are included in the blend, it improves wrinkle resistance and helps provide easy care and shrinkage control. When used in admixture with wool, shrinkage is reported to be greatly reduced when compared with pure wool. Advantages and disadvantages of ramie as a fabric (a) Advantages: resistant to bacteria, mildew, and insect attack. Extremely absorbent. Dyes fairly easily. Increases in strength when wet. Withstands high water temperatures during laundering. Smooth lustrous appearance improves with washing. Keeps its shape and does not shrink. Can be bleached. (b) Disadvantatages: low in elasticity. Lacks resiliency. Low abrasion resistance. Wrinkles easily. Stiff and brittle. Medicinal uses Antiphlogistic, astringent, demulcent, diuretic, febrifuge, haemostatic, resolvent, vulnerary and women’s complaints. Used to prevent miscarriages and promote the drainage of pus [34, 35]. The leaves are astringent and resolvent [36, 37]. They are used in the treatment of fluxes and wounds. The root is antiabortifacient, cooling, demulcent, diuretic, resolvent and uterosedative.

2.6

Kenaf

Sources are not consistent regarding the origin of kenaf (Hibiscus cannabinus L.) except that Africa is the continent of origin, and it has been known and grown for over 4000 years. Due to its origin, the species is a short-day plant; however, there are also photo-insensitive cultivars available [38, 39].

2.6.1

Economic importance of kenaf

It is grown mainly in Thailand, China, India, Vietnam and Cuba (Table 2.16). Production of kenaf is fairly constant throughout recent years although many producers experienced a substantial decline in 1999. The biomass production ranges from 12–18 t ha–1 and the fibre content is 18–22% of the dry stalk weight. The fibre yield on average is 1–2 tonnes fibre ha–1, rising to 3–3.5 t ha–1 under favourable conditions [40]. Today’s interest in kenaf focuses on it as an alternative source of paper pulp although still on a very limited scale. Ultimately refined bast fibres measure on average 2.6 mm in length and resemble the best softwood fibres while core fibres are only about 0.6 mm long and are similar to hardwood


79

Table 2.16 Main producers of kenaf (thousand tons) Country

1998/ 1998

1999/ 2000

2000/ 2001

2001/ 2002

2002/ 2003

2003/ 2004

China India Indonesia Thailand Vietnam Cambodia Pakistan Brazil Cuba

248.00 182.16 7.20 47.21 14.60 1.10 1.90 8.60 10.00

164.00 198.18 7.50 29.72 9.40 0.30 1.90 7.90 10.00

126.00 198.00 7.00 29.69 11.30 0.20 2.20 7.30 10.00

163.00 203.40 10.20 56.00 14.60 0.20 2.20 7.20 10.00

155.00 202.14 6.80 41.00 20.50 0.50 2.20 10.20 10.00

165.00 198.70 7.00 57.00 12.50 0.50 2.20 10.90 10.00

Source: Compendium of Statistics on Sisal, Jute, Kenaf, Abaca and Coir. Consultation on Natural Fibres, FAO.

fibres [41]. It contains about 40% of cellulose in the fibre and over three times less lignin (10%) than southern pine, which makes it easily and quickly pulped and bleached with less toxic chemicals (hydrogen peroxide versus chlorine) [42]. This, in combination with its annual renewability, makes kenaf a much more environmentally sound raw material than commonly used timber. Today about 200 tonnes per year of kenaf pulp is produced in the USA and also in small mills in China, India, Thailand and Spain [41, 43]. Also, in Japan many paper companies use or have used kenaf in their products [44]. Although different pulping technologies have been tested the only one commercially used remains the sulfate (craft) process. Other uses of core kenaf fibre include also soil-less potting mixes, animal bedding, oil absorbents, packing material, organic filler for plastics, drilling mud binder, grass and flower mats, decorative fibres and insulation as well as animal feed and human food [41]. Bast fibre is also blended with cotton and used in textiles [41]. The bast fibre of kenaf can also be mixed with plastic for injection moulding.

2.6.2

Anatomy of the kenaf plant

Kenaf seeds are produced in 1.9 to 2.5 cm long and 1.3 to 1.9 cm in diameter fruits called seed capsules that contain many (20–26) small seeds, ranging in colour from brown to black [40, 45]. The plant has a long taproot system with relatively deep, wide-ranging lateral roots making the plant drought tolerant [46]. The stem is straight and poorly branched when cultivated for fibre and can reach up to 6 m in height in favourable conditions; usually the most desired height is 3–4 m. A cross-section of kenaf stem reveals three main parts: an epidermis; a thin layer of bast and wood filling; and the inner

80


part of the stem. The two latter tissues are considered the source of two different kinds of fibre, the bast and the ‘core’. The bast comprises roughly 40% and the core 60% of the stalk’s dry weight [41]. Leaves are individually stalked and lobate to a different degree, depending on the cultivar [40]. Kenaf plants produce large, light yellow or creamy coloured flowers that are bell-shaped and widely open. The flowers of many cultivars have a deep red or maroon coloured centre [45].

2.6.3

Cultivation of kenaf

Kenaf is cultivated for its fibre and also as a fodder plant and has a relatively wide range of adaptation to climate and soils. With the exception of some early types developed for the Asiatic regions of the former USSR, most of the current kenaf cultivars grow best under tropical and sub-tropical conditions where mean daily temperatures are greater than 20∞C [47]. Water demand varies but on average lies between 75 and 125 mm per month during the first 100 days of vegetation [41]. Kenaf can be successfully grown in a wide range of soils, from high organic peat soils to sandy desert soils [41]. The best effects are obtained on well-drained, fertile soils with a neutral pH; however, it can withstand late season flooding, low soil fertility, and a wide range of soil pH values [41]. Kenaf also has shown excellent tolerance to drought conditions. Although some studies show that kenaf requires no fertilizer supply, it seams reasonable to secure the proper nutrient balance for the entire crop rotation planned. The rates of N, P and K should be about 100–130, 35–50 and 110–140 kg ha–1 respectively, depending on the soil nutrient content [47]. Where local weather conditions make kenaf growing impossible due to cool weather it can be planted once the soil has warmed to 13∞C and the threat of frost is past [45]. Depending on local conditions, for fibre, 185,000 to 370,000 plants ha–1 or even 300,000 to 500,000 plants ha–1 is optimal, which can be accomplished sowing 8 to 25 kg ha–1 of seed [45, 47]. The row spacing should be about 50 cm with plants 3–4 cm apart within the row. The seeds should be planted 2–3 cm deep and about 1 cm deeper in moisture deficiency conditions [45, 48]. Kenaf is fairly resistant to pest and diseases; however, locally, cut worms, leaf miners, and other chewing/sucking insects are potential problems. The only pest problem most likely to be experienced almost anywhere kenaf is grown is plant parasitic nematodes, especially root-knot nematode (Meloidogyne spp.) [40, 48]. Kenaf can also be infected by anthracnose (Colletotrichum hibisci) [45]; however, choosing resistant cultivars can help solve this problem. In a large-scale plant production manual harvesting is an unacceptable


81

harvesting method. Although kenaf has gained the interest of market-oriented farmers around the world, few very efficient harvesting methods have been developed. Whole stalks can be harvested; plants are cut and laid down in an orderly fashion at right angles to the row, stalks are allowed to dry for around two weeks and are then gathered by a machine that picks up the stalks and arranges them in large bundles; the bundles are transferred to field trailers. Another method is to use forage choppers to harvest the crop. This method can be used in colder areas where the crop is allowed to dry after being killed by frost or by a desiccant. This method has been used in Mississippi [41]. The chopped kenaf is stored and transported in cotton modules with the same equipment used for harvesting cotton [48]. The crop may also be chopped and baled with forage equipment and, if covered, can be stored as large round or rectangular bales on field edges.

2.6.4

Processing of kenaf straw

Before straw is processed and fibres extracted, it has to be retted. In Asia, Africa and Latin America kenaf is still retted in ponds. However, this process is labour intensive and leads to serious contamination of waterways [47, 49], therefore, like many other bast fibre yielding crops, kenaf can be dew-retted. The second stage of processing involves a series of decortication machines that break the stem and separate core and bast fibres [50].

2.7

Abaca

Abaca or Manila hemp (Musa Textilis Nee) is a herbaceous plant that belongs to the family of Musaceae. Its appearance is similar to the banana plant, but it is completely different in its properties and uses. Abaca and other Musa Textilis mixtures, with different levels of quality and resistance, are produced and successfully marketed in several countries. Abaca is indigenous to the Philippines, but has been introduced to Borneo, Indonesia and Central and South Americas. The origin of the abaca plant is in the southern part of the Philippines where there are rainforests and highly humid atmospheric conditions. The Philippines is the world’s largest source and supplier of abaca fibre for cordage and pulp for specialist paper. While abaca fibre has been used in cordage manufacturing for many years now, fibre for pulp in specialist paper manufacturing came into commercial use only in the 1930s. In the Philippines, there are three regions that produce abaca on a commercial scale: Mindanao, Visayas and the Bicol regions. Each of these regions supplies different varieties and hybrids. Although each variety has certain advantageous qualities, not a single variety can be considered perfect. It was produced exclusively in this region until the Second World War, when the Japanese army cut off production and producers looked for

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new places to establish and grow it. An excellent place to grow abaca was found in countries such as Ecuador. Abaca is a versatile plant with several uses. Because its fibres are particularly resistant to saltwater, abaca has been commonly used for fishing nets. Abaca fibre is used mainly in the production of tea bags and meat casings; it is also a substitute for bark, which was once a primary source of cloth. In addition, it is considered an excellent raw material in the processing of security and high quality paper, diapers, napkins, machinery filters, hospital textiles (aprons, caps, gloves), and electrical conduction cables, as well as some 200 other different finished products. Fibres are removed from the abaca’s stalk to make ropes, clothing, and paper-based materials. These plants thrive well in shaded and cool habitats and resemble the banana plant in many respects.

2.7.1

Economic importance of abaca

Table 2.17 shows recent distribution of the global production of abaca, according to the Abaca Growers Corporation of Ecuador (CADE), whose production represents 42% of the national total; the Philippines has never lost its predominant place in the global production of abaca. Abaca in Ecuador is processed with special equipment that separates the raw material and fibre; in contrast, in the Philippines the process is still performed manually, resulting in a lower yield and quality. The quality of the abaca plant in Ecuador is 12.5% (1.1% hard fibre and 11.4% soft fibre or bagasse). Table 2.17 Abaca global production 1998 Country

Philippines Ecuador Costa Rica Indonesia Equatorial Guinea Kenya TOTAL

Distribution (%) 79.00 17.00 1.50 1.02 0.98 0.50 100.00

Production (Mt)

65 570 14 110 1 245 845 815 415 83.000

Source: Abaca Corporation of Ecuador (CADE).

While almost all production is sold directly from the producers to the exporters, a small amount is still sold through third parties. Before being exported, the fibre is taken to warehouses to be classified. All the companies have their own warehouses located in central locations. The maximum moisture allowed is 8% and the cellulose percentage ranges from 70 to 80%. The


83

classification system is based mainly on colour and measurement of the fibre’s diameter. Variation in fibre length, which normally goes from 1.8 to 6.0 metres, is considered in the classification. This classification has five and sometimes six grades, ranging from a white fibre, which would be considered grade 1, to a dark brown fibre, which would be considered grade 5. The rule for measuring quality based on the fibre’s diameter is simple: the thinner the better.

2.7.2

Anatomy of the abaca plant

The abaca plant can grow to more than 6 m in height and is found in several varieties. Not all varieties are grown commercially. Abaca has a perennial production cycle; at the beginning, it takes 18 to 24 months to produce fibre and after that, the product can be harvested every two to three months. Abaca should be grown in regions with optimum conditions in order to get the best results possible. The best regions to grow abaca are those with humid tropical weather and temperatures from 22 to 28∞C. Additionally, rainfall is extremely important; abaca must have from 1,800 mm to 2,500 mm of welldistributed rain during the year. One more element to consider is the altitude; the optimum altitude is between 100 and 140 metres above sea level. Rain and sunlight are both essential factors in the production of abaca: excess sunlight combined with a lack of rain may adversely affect the development of a regular plant to the point of making the production worthless. In a good growing environment, an abaca plantation can commercially produce fibre for 15 to 20 years.

2.7.3

Cultivation of abaca

The most appropriate time for planting is at the beginning of the winter season; however, abaca can be planted in another season if the soil has enough moisture. The abaca plant propagates itself through suckering, or the growing of shoots from the roots. When all the leaves have been formed from the stem, flower buds develop, at which time the plant has reached maturity and is then ready for harvest. The time that a plantation lasts, between planting and harvesting, depends on various factors such as the nature of the property, the variety of abaca planted, seed selection, weather conditions, maintenance activities, etc. In general, the time between planting and harvesting is 18 to 24 months. The adequate moment to initiate the harvest is when the flower begins to develop. The stalks are considered mature and are harvested when the flag leaf appears. Harvesting is not recommended before or after this stage because the quality and the amount of fibre is reduced.

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2.7.4


Processing abaca straw

The fibre extraction process is conducted by a shaving system; the operator surrounds the rod with one part of the tuxie (slice of the plant) while the other part is held between the blades. The blades are closed and with one movement or stretch, the fibre is separated from the tuxies. The other part of the tuxie that surrounds the rod is handled in the same way. A fibre extractor produces an average of 120 kg, with an optimum of 200 kg. In the Philippines, 90% of the fibre extraction is conducted manually by means of a set of knives placed in a special wood frame. The fibre that is obtained has a high percentage of moisture which makes it necessary to dry it at the farm in cane structures specially designed for this purpose. The drying period can last from a couple of hours to days, depending on weather conditions. At the same time that this activity is conducted, a preliminary classification is conducted depending on the colour that the fibre presents. After drying, the fibre is piled in dry places that can be covered and that have adequate ventilation, because even after drying the fibre holds a certain percentage of moisture and without ventilation, the fibre can change colour and lose quality.

2.7.5

Fibre physical properties and chemical composition

Abaca fibre measurements can be seen in Table 2.14 (see page 66) and in the cross-section of the fibre bundle (Fig. 2.13). Chemically, abaca comprises 76.6% cellulose, 14.6% hemicellulose, 8.4% lignin, 0.3% pectin and 0.1% wax and fat.

L[6] = 15.39 [um] L[1] = 19.42 [um] L[3] = 27.62 [um]

L[4] = 23.07 [um]

L[10] = 13.5 [um] L[2] = 14.79 [um]

L[5] = 27.27 [um]

L[8] = 28.91 [um]

L[1] = 18.7 [um]

L[7] = 24.74 [um]

2.13 Abaca stem cross-section.

Fibre

Flax Hemp Jute Ramie Kenaf Abaca

Elementary fibres (single cell) Surface area S (mm2)

Diameter (mm)

110–270 440–910 300–780 400–900 150–700 166–274

15–22 29–45 26–30 40–60 14–33 15–30

Ultimate stress (Mpa)

Elongation (%)

Young’s modulus (GPa)

Density (kg m–3)

Specific stress (MPa m3 kg–1)

Cellulose (%)

Lignin (%)

900–1200 400–700 400–700 800–1000 350–600 –

2.0–3.0 1.6–2.5 1.5–2 1.7–3 2.5–3.5 –

100 35 2.5–15 50–80 40 –

1540 1480 1450 1560 1500 –

0.58–0.80 0.27.0.47 0.28–0.48 0.51–0.64 0.22–0.40 –

71 70–88 63–70 70–80 75–90 77

2.2 3.0–4.0 12.0 0.5–1.0 – 8.4

Source: Institute of Natural Fibres, Poland, Department of Textile Raw Materials and Products Metrology.


Table 2.18 Comparison of physical parameters of natural fibres

85

86


Abaca is considered the strongest of natural fibres, being three times stronger than sisal fibre, and is far more resistant to saltwater decomposition than most of the vegetable fibres. Compared to synthetic fibres like rayon and nylon, abaca fibre possesses higher tensile strength and lower elongation in both wet and dry states.

2.8

Comparison of fibre properties

By way of summary, the physical properties of the six bast fibres discussed in this chapter are given in Table 2.18.

2.9

References

1. Pavelek, M., Stock of Flax Genetic Resources in Europe. Proceedings of the 3rd Global Workshop ‘Bast Fibrous Plants for Healthy Life’. Banka Luka, Bosnia and Herzegovina, Republic of Srpska, 24–28.10.2004. ¢ , M., Ontogenesis of Fiber Flax (Linum ussitatissimum 2. Heller, K. and Byczynska L.). Proceedings of the 3rd Global Workshop ‘Bast Fibrous Plants for Healthy Life’. Banka Luka, Bosnia and Herzegovina, Republic of Srpska, 24–28.10.2004. 3. Schilling, E. and Müller, W. (1951), Len [Flax]. PWT Warsaw, Poland. 4. Heller, K. and St. Rólski, The Effect of Agricultural Conditions on Weed Communities and Herbicide Efficacy in Fibre Flax Cultivation in Poland. Proceedings of the Second Global Workshop ‘Bast Plants in the New Millenium’. Borovets, Bulgaria, 3–6 June, 2001. 5. Poradnik plantatora lnu i konopi [Guide for flax and hemp growers]. PWRiL, Poznan¢ , Poland, 1994. ¢ 6. Truszkowska, W. et al. (1971), Nauka o chorobach i szkodnikach roslin . PWRiL, Warsaw, Poland. 7. Shekhar Sharma, H.S. (ed.) (1996), The Biology and Processing of Flax. M Publications, Belfast, Northern Ireland. 8. Marshall, G. (ed.) (1989), Flax: Breeding and Utilisation. Kluwer Academic Publishers, Dordrecht/Boston/London. 9. Szalkowski , Z. et al. (1965), Poradnik roszarnika [Retting Engineer Guidebook]. WPL, Warsaw, Poland. 10. Szalkowski , Z. (1967), Podstawy chemicznej technologii surowców i wlókien lykowych [Principles of Chemical Technology of Bast Raw Materials and Fibres]. Warsaw. 11. Grabowska, L. and Baraniecki, P. Three Year Results on Utilization of Soil Polluted by Copper-Produciln Industry for Cultivation of Industrial Crops. Natural FibresWlókna Naturalne, Special Edition, 123–131, Flax and Other Bast Plants Symposium 30.09.01.10.1997. Poznan. 12. Grzebisz, W., Chudzinski, B., Diatta J.B. and Barlóg, P. Phytoremediation of Soils Contaminated by Copper Smelter Activity. Part II. Usefulness of Non-Consumable Crops. Natural Fibres-Wlókna Naturalne, Special Edition, 118–122, Flax and Other Bast Plants Symposium 30.09–01.10.1997. Poznan. 13. Mediavilla, V. and Steinemann, S. (1997), Essential oil of Cannabis sativa L. strains. Journal of International Hemp Association, 4 (2). 82–84.


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14. McPartland, J.M., Clarke, R.C. and Watson, D.P. (2000), Hemp Diseases and Pests Management and Biological Control. CABI Publishing, New York, USA. 15. www.hempfood.com/IHA/iha03201.html 16. Mostafa, A.R. and Messenger, P.S. (1972), Insects and mites associated with plants of the genera Argemone, Cannabis, Glaucium, Erythroxylum, Eschscholtzia, Humulus, and Papaver. Unpublished manuscript, University of California, Berkeley, USA. 17. Kaniewski, R., Kubacki, A. and Konczewicz, W. (2001), The Technology of Mechanical Harvesting of Hemp for Fibre, Allowing for Separate Harvesting of Hemp Tops and for Seed Production. Second Global Workshop ‘Bast Plants in the New Millenium’ Natural Fibres, Special Edition, Poznan, Poland. 18. Dempsey, M. James (1963), Long Vegetable Fibre Development in South Vietnam and other Asian Countries, 1957–1962, USOM, Saigon. 19. Peikun Huang (1992), in http://www.jute.org/ 20. http://www.juteworld.com/ 21. http://www.fao.org/ 22. Institute of Natural Fibres, Poland, Department of Textile Raw Materials and Products Metrology. 23. Kozlowski R. (1970), Proces roszenia lnu z dodatkiem mocznika na tle klasycznego ˛ lotnych kwasów tluszczowych w plynie sposobu, ze szczególnym uwzglednieniem ¢ roszarniczym i w pazdzierzach , wydzielonych gazów i zwia˛zków azotowych. Zeszyty informacyjny, IKWN No. 4, r-6. 24. Karpowiczowa, L. (1954), Rami, Warszawa, PWN 1954. 25. http://www.ianr.unl.edu/pubs/textiles/nf45.htm 26. Buchanan, R. (1987), A Weavers Garden. Loveland, Colorado, Interwave Press, 230. 27. Stuart, Rev. G.A., Chinese Materia Medica. Taipei, Southern Materials Centre. 28. Hoefer, T.H., The Ramie Fibre Today, Melliand Textilberichte, no. 47. 29. Paul, N.B. and Bhattacharyya, S.K. (1979), The Microbial Degumming of Raw Ramie Fibre, J. Text. Inst., no.12. 30. Luniak, B. (1954), Ramie – Fibre Properties, Text. Q., 4. 31. Usher, G. (1974), A Dictionary of Plants Used by Man. Constable. 32. Ramie, F.A.O. (1977), Fibre Production and Manufacturing, Rome. 33. Angelini, L.G. and Lazzeri, A. et al. (2000), Ramie and Spanish Broom Fibres for Composite Materials: Agronomical Aspects, Morphology and Mechanical Properties. Industrial Crops and Products, March. 34. A Barefoot Doctor’s Manual. Running Press. 35. Stuart, Rev. G.A., Chinese Materia Medica. Taipei. Southern Materials Centre. 36. Duke, J.A. and Ayensu, E.S. (1985), Medicinal Plants of China, Reference Publications, Inc. 37. Chopra, R.N., Nayar, S.L. and Chopra, I.C. (1986), Glossary of Indian Medicinal Plants (Including the Supplement). Council of Scientific and Industrial Research, New Delhi. 38. Dempsey, J.M. (1975), Fibre Crops. The Univ. Presses of Florida, Gainesville. 39. Dryer, J.F. (1967), Kenaf seed varieties, pp. 44–46. Proc. First Conf. Kenaf for Pulp. Gainesville, FL. 40. www.thepack.co.jp/hp01/ecology2.html 41. www.visionpaper.com/kenaf2.html 42. Richard, M. Rowell and James, M. Han, Kenaf Properties, Processing and Products. www.fpl.fs.fed.us/documnts/pdf1999/rowel99a.pdf

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43. Thomas, A. Rymsza, Kenaf and the 21st Century, Current Developments and Trends. www.visionpaper.com/speeches_papers/992aks.html 44. Proceedings of the 2000 International Kenaf Symposium, 13–14.10.2000, Hiroshima, Japan. 45. Webber, C.L., III, Bhardwaj, H.L. and Bledsoe, V.K. (2002), Kenaf production: Fibre, feed, and seed, pp. 327–339 in Janick, J. and Whipkey, A. (eds) Trends in New Crops and New Uses. ASHS Press, Alexandria, VA. 46. Grower’s Handbook for Kenaf Production in the Lower Rio Grande Valley of Texas, USA (1989) Kenaf International with Rio Farms, Inc., McAllen, TX. 47. www.rirdc.gov.au/pub/handbook/plantfibre.html 48. Stricker, J.A., Prine, G.M. and Riddle, T.C. (2001), Kenaf – A Possible New Crop for Central Florida, University of Florida, Institute of Food and Agricultural Sciences, Gainesville, USA. 49. Webber, Charles L. III and Bledsoe, Venita K., Kenaf Yield Components and Plant Composition. www.hort.purdue.edu/newcrop/ncnu02/pdf/webber-348.pdf 50. www.greennaturalfibers.com

3 Alginate fibers J M M U R I and P J B R O W N Clemson University, USA

3.1

Introduction

Historically, the use of seaweed extracts for food and medicine can be traced back as far as 3000 BC. Alginate in one form or another is used in the food industry, pharmaceuticals and textiles.1 Stanford,2 a British scientist, is reputed as the pioneer in scientific literature of alginate. In 1880 he described a new process for the extraction of iodine and salts from seaweed. The residual material, after extraction, which was unaltered in appearance, was found to contain a substance that had not been isolated before. He named this substance ‘algin’. He obtained his first soluble ‘algin’ by extracting seaweed with sodium carbonate and a second insoluble product by acidifying a solution of the first product. He was convinced that his ‘algin’ was a nitrogen compound. In his later investigations of the ‘algin’, he realized that it behaved as an acid, i.e. forming salts with metals and liberating carbon dioxide from sodium carbonate, prompting him to rename it ‘alginic acid’.

3.1.1

Types of alginates

The most important source of commercial alginates is brown algae. Alginate is the main constituent of brown algae and is found in the cell wall and intercellular regions.3–6 However, only three types of brown algae are sufficiently abundant or suitable for commercial extraction of the alginic acid. In order of abundance, they are laminaria (British Isles, Norway, France, N. America, Japan), microcystis (USA), and ascophyllum (British Isles). The high viscosity alginate in commercial use has a molecular weight of about 150,000 and a degree of polymerization (DP) of about 750 but the average molecular weight of ordinary alginate is 15,000.7,8 Laminariales are the largest algae and most complex. They are composed of a lamina (frond), a stipe (stem) and a basal (roots). Plants of laminaria vary in length with age, reaching a maximum of 3 metres and since they present a large surface area to a turbulent environment, need to be firmly 89

90


anchored to the substratum. Marine algae in general may be said to be indifferent to the chemical composition of the substratum8 in that unlike surface plants, which absorb their nutrients from the soil, they absorb minerals directly from the sea over their whole surface.1,9 Harvesting is easy because most brown seaweed grow in shallow water. Table 3.1 gives the botanical sources of alginate used in the industry. Table 3.1 Industrial source of alginate1

3.1.2

Genus

Species

Country

Marine location

Microcystis

pyrifera

USA

Fixed by holfast in deep water

Microcystis

intergrifolia

Laminaria Laminaria Laminaria Laminaria

digitata saccharina flexicaulis stenphylla

Europe and Japan

Sulittoral

Ascophyllum Nereocystis Fucus Fucus Fucus

nodosum luetkeana vesiculatosus serratus spiralis

Europe

Littora

Ecklonia Pelvetia

maxima canaliculata

South Africa

Littora

Manufacturing process of alginate and alginate fibers

The manufacture of alginate fibers10 consists of the following steps. Seaweed is collected, dried and milled. The powdered seaweed is treated with aqueous sodium carbonate and sodium hydroxide, which convert the alginate present to the sodium salt. The pigment and the cellulose present in the seaweed are not dissolved. The viscous solution of sodium alginate is purified by sedimentation then bleached and sterilized by the addition of sodium hypochlorite. The alginic acid is then precipitated by acidification, which is later washed and reconverted to the pure sodium salt. The sodium alginate salt is made into a thick paste, dried and milled to make sodium alginate powder. A dilute solution of sodium alginate is made, filtered, then spun by the viscose spinning method into a coagulation bath (wet spinning) containing certain polyvalent cation salts (Ca++, Al+++, etc.) or inorganic acid solution; about 0.02N hydrochloric acid, emulsified oil, and a small quantity of a cationic surface agent. The water soluble sodium alginate is thus precipitated in filament form as an alginic acid metal salt, e.g. calcium alginate or alginic acid. The filaments are drawn together, washed, lubricated, dried and wound

Alginate fibers

91

onto bobbins or cut (sometimes stretched to break the fibers by varying the relative feed rates which control the degree of stretch-breaking effect) to the required staple length suitable for non-woven products. The process is a chemical reaction and proceeds as follows in Fig. 3.1. As calcium or hydrogen ions are exchanged with sodium ion, the reaction proceeds until the sodium alginate is converted to calcium alginate or alginic acid.11 With calcium chloride: O 2 alg

C

O

O + Ca

alg

C

O Na

C O

Ca

alg + 2 Na

O

solid With hydrochloric acid: O alg

C

O + H

alg

+ Na

C

O Na

OH solid

3.1 The precipitation process of sodium alginate/water solutions.

Speakman et al.12 carried out the successful development of alginic acid fibers. One of their products was a green coarse monofilament of chromium alginate. Later they made multifilament yarns of calcium alginate, which were white in color and silk-like in appearance. Their ready solubility in weak alkaline solution, including soap, made it difficult to develop them as fibers for commercial use. A flow chart of the alginate manufacturing process has previously been described11 and alginate can be wet spun in apparatus similar to those used for spinning regenerated cellulose fibers (Fig. 3.2). There are many possible technical designs of wet-spinning processes. The precipitation (coagulation) bath can be situated horizontally or vertically (fiber moving upwards or downwards). Further manufacturing operations (drawing, washing, drying, etc.) can be realized continuously or periodically as separate operations. The morphological structure of fibers is very sensitive to the composition and condition within the precipitating bath. A number of methods for producing conventional alginate fibers are described in literature.13–15 Typical calcium alginate fibers may be prepared as follows: a 6.4% by weight aqueous sodium alginate solution is extruded through a jet containing 20 holes into a bath containing 5% salt of calcium chloride, a 0.2% acetic acid and 0.05% cetyl pyridinium chloride (cation active compound) at 40∞C. By this process, it is possible to obtain alginate fibers which do not adhere to one another without

92

Biodegradable and sustainable fibres 1

8 5 6

7 2

3

4

3.2 Scheme of a horizontal wet spinning system. 1. inlet of the spinning dope; 2. spinneret; 3. spinning line; 4. coagulation bath; 5. take-godet; 6. and 7. inlet and outlet of the spinning bath.

addition of emulsified oil to the bath. The threads produced are 37% stretched by passing them over godets and reeled into skeins. They are then washed in a 0.1% solution of calcium chloride at 80∞C and then dried at room temperature and conditioned.

3.2

The chemical nature of alginate materials

3.2.1

Chemical structure and composition of alginates

The molecular structure of alginates may be considered from three different though interrelated points of view. There is the chemical structure which describes the nature and sequence of constituent units, the conformations which describe the arrangement in space and the intermolecular interactions. These are discussed in this section. Alginate is a natural occurring polymer and is the major matrix polysaccharide in brown seaweed. It occurs as an insoluble complex of potassium, sodium, calcium and magnesium.16 Dillon and McGuinness17 suggested that alginic acid was a polymer of mannuronic acid. In addition Lunde, Heen and Oy18 suggested that alginic acid had a pyranose structure. This view was supported by Hirst, Jones and Jones’s19 study of methylated alginic acid. Research work by Fischer and Dorfel20 showed that guluronic acid was also present in alginic acid. Alginic acid consists of two uronic acids, b-D-mannuronic acid and a-L–guluronic acid. Work carried out on the acetylation of alginic acid17 confirmed that the pyranose structure was correct and closely related to the cellulosic type structures. Similar to cellulose, alginic acid has a long linear chain structure consisting of a large number of pyranose rings turned through 180∞ to each other and linked by a b(1 Æ 4)

Alginate fibers

93

glycosidic bridge.21 The importance difference between these two polymers is the side groups attached to C5. Cellulose has a hydroxymethyl group (–CH2OH) whereas alginic acid has a carboxyl group (–COOH). In 1967 Haug et al.22 showed by free boundary electrophoresis that both heterogeneous and homogenous acid hydrolysis led to a splitting of the alginic acid molecule into chemically different fragments. It was found that alginic acid was a block copolymer containing long sequences of an alternating structure of both mannuronic acid and guluronic acid residues. The arrangement of the polymannuronic acid chain is similar to that found in cellulose in which the individual pyranose rings lie at a very small angle to the polymer chain in a zig-zag formation. The corresponding angle in polyguluronic acid is larger and in an alternating sequence, the repeating unit alternates between the two angles. For convenience the three blocks in the chain can be represented as: [GG]m [ MM]n

[MG]p

The values of n, m and p are not necessarily constant within a polymer chain. The conformation seems to be stabilized by the formation of intramolecular hydrogen bonds between the hydroxyl group on C3 of one unit and the ring oxygen atom of the next unit in the chain. The molecular chains are bonded into sheets by means of hydrogen bonds formed between the hydroxyl of the carboxyl group and the oxygen atom on C3 in the pyranose ring in parallel chains and between the axial hydroxyl group on C2 and the oxygen atom of the carboxyl group in anti-parallel chains.23 The physico-chemical and biological properties are dependent on the relative proportions of these blocks and their average lengths. The carboxylic acid group in the unit makes it more reactive than cellulose and contributes to its solubility in alkaline conditions and with some sequestering agents. The ratio of mannuronic to guluronic units in an alginate molecule is referred to as M/G ratio. The M/G ratios indicate, indirectly, the proportions of the blocks within an alginate molecule, which in turn give rise to the specific properties of that alginate molecule.

3.2.2

Chemical properties of alginic acid

For short periods of time, alginate is unaffected by acids and alkalis at room temperatures. On heating to 80∞C and at pH values above 9, depolymerization occurs and unsaturated derivatives are formed. At pH values below 2 the rate of hydrolysis depends on the dissociation of the carboxyl groups on the alginate monomers. Acid hydrolysis of alginate occurs in both strong and weak acids, but at a slower rate than most neutral polysaccharides. Alginate salts are soluble between pH 5–10 at room temperature, however, outside this pH range, degradation is considerable.23

94


Univalent metals such as sodium and potassium give salts of alginic acid which are soluble in water and form smooth solutions having good flow properties. Soluble alginates are easily and quickly dissolved in soft water with the aid of a stirrer. Hard water may result in precipitation of insoluble alginate salts. The addition of divalent cations to an alginic solution reduces the solubility of the polymer resulting in precipitation or, if the concentration and molecular weight are sufficiently high, gelation. The gel is not thermally reversible but can be re-dissolved by ion-exchange of the divalent for a monovalent cation. The salts of univalent metals (except silver) are soluble in water since no cross-links are formed. Some salts and derivatives of alginates and their special features are shown in Table 3.2. The most important and useful property of alginic acid is the ability to form water insoluble salts by reaction with multivalent metal ions. The insoluble alginic acid in water is easily brought into solution by neutralization with sodium hydroxide to form a highly ionised salt. The alginate poly-ion therefore carries a negative charge, which reacts with positive Table 3.2 Salts and derivatives of alginic acid1 Compound

Solvent

Special properties

Sodium alginate

Water

Available in a wide range of grades

Potassium alginate

Water

–

Magnesium alginate

Water

–

Ammonium alginate

Water

Low ash content

Calcium alginate

Calcium chelating agents

Gel formation

Aluminium alginate

Ammonium solution

–

Copper alginate

Ammonium solution

–

Zinc alginate

Ammonia and ammonium salts

–

Silver alginate

Ammonium solution

Darkens on exposure to light

Triethanolamine alginate

Water and 75% ethanol

Forms soft films

Propylene glycol alginate

Water and acidic solutions

–

Alginic acid acetate

Water

–

Alginic acid sulphate

Water

Low molecular weight ester have blood anticoagulant properties

Alginic acid amide

Hot water

Forms gel on cooling

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ions. In general, alginates are insoluble in non-aqueous solvents, but some amine salts of alginate are soluble in non-aqueous solvents. Pure organic solvents have no effect on alginates.11 The insolubility of polyvalent metal ion salts is due to association with two or more carboxyl groups on different chains; the molecule loses its freedom of movement and a close linked network structure is formed. In the process of precipitation, the polyvalent metal alginate tends to become aligned when extruded through orifices and can be obtained in the form of fibers. If the precipitated alginate is not subjected to extrusion, the polyvalent metal alginate precipitate forms a gel, in which a large amount of water is held in the insoluble precipitate (hydrocolloid gel). Alginate gels are randomly cross-linked networks with short extended segments in between metal ion junctions. The exact nature of the gel junction is not clear but evidence suggests that the association of the chains occurs by stacking with polyvalent ions packed between chains. Unlike the dilute solution properties where isolated molecules are considered, gelling is concerned with the interaction between alginate chains.11 Ion binding and gelling are closely linked properties of alginate and are important in understanding its behavior. Calcium ion chelation to alginate chains has been investigated by circular dichroism (c.d.) and by equilibrium dialysis in the presence of various concentrations of sodium chloride.24 These results together with the work on X-ray diffraction of alginate led to the theory of an ‘egg box’ structure where a cooperative interchain binding the calcium alginate gel is formed. Ion selectivities, in exchange reactions between monovalent and divalent metal ions, have been investigated by Haug and Smidsrod.25,26 Interaction between alkaline earth metal ions such as calcium and magnesium show that the extent of exchange increased as the content of guluronic acid residues increased. Guluronic acid rich alginates (low M/G ratios) therefore have a higher Ca++ binding activity than mannuronic acid rich alginate.5,25,27,28 A strong autocooperative binding of Ca++ ion occurs between the chains in the gel state. Compared to other acidic polysaccharides, this property of guluronic acid is unique. Work on the ion selectivity of polyuronides led to the speculation that the divalent ions, especially Ca++ are bound at two or more points on the uronic acid monomer.1

3.2.3

Properties of alginate solutions

The viscosity of a particular molecular weight of alginate solution is affected by temperature and concentration.7 The viscosity of salt alginate solutions decreases with increasing temperature and this effect is reversible unless temperatures are so high as to cause partial depolymerization of the molecules which then results in a viscosity decrease. In addition the viscosity of sodium alginate is pH sensitive. It is constant in the pH range 5–10. Below 4.5, a

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significant increase in viscosity occurs and below pH 3, insoluble alginic acid is precipitated. The concentration of sodium alginate affects its flow properties and for a 2.5% medium-viscosity sodium alginate solution (w/v) the behaviour is pseudoplastic at high shear rates, 10–10,000 sec–1, whereas a 0.5 % (w/v) solution of the same sodium alginate can behave in a Newtonian manner at low shear rates. Pseudoplastic behavior29 at this concentration is typically only seen at high shear rates, 1000–10,000 sec–1 The addition of increasing amounts of non-aqueous water miscible solvents such as glycol, acetone, etc. to alginate solution results in viscosity increases and eventual precipitation.

3.3

Physical properties of alginate-based materials

3.3.1

Structure property relationships

The modulus of calcium alginate materials is directly proportional to the level of cross-linking in the system. Microscopical work24 has shown that structural differences can be seen on alginate gels by varying the type of the metal ion used in gel formation. The modulus is also dependent on the type of divalent metal used, and increases with increasing affinity of the polyuronate for the divalent metal: Pb++ > Cu++ > Cd++ > Zn++, Ni++, Co++ > Mn++ The stiffness of the molecular chain of alginates has been compared with other polysaccharides by Smidsrod et al.26 Alginates are found to be stiffer than the similar polymers like carboxymethylcellulose (Brucker et al.30 and Smidsrod et al.26). The stiffness is also dependent upon polymer composition and L-guluronic acid rich alginates posses a more rigid chain conformation than D-mannuronic acid rich alginates. The molecular stiffness of the three types of alginate blocks in solution decreases in the order: (G)n > (M)n > (MG)n. Unlike calcium polyguluronate, calcium polymannuronate does not form gels with any significant rigidity. The number and strength of the gel network junctions has a marked effect on the modulus of rigidity of the resulting fibers. In nature alginate occurs in brown algae as a mixed salt in which Ca++ ions provide gel strength to the alginate for support issues.1 The most obvious difference observed between polymannuronate rich gel and polyguluronate rich gels is that the polymannuronate type forms voluminous, turbid slurries of aggregates whereas polyguluronate gels form rigid, transparent gels of high modulus. Gel formation has been associated with the occurrence of Lguluronic units in alginates. Alginate gels of the (MG)n type have been observed to have a low but measurable gel rigidity.1 The modulus of rigidity indicates that alginate gels are typical viscoelastic gels.

Alginate fibers

3.3.2

97

Alginate fibers

Useful polymeric fibers tend to have certain things in common at both the molecular and macromolecular scale. Even though fiber uses may vary tremendously the basic tenet for useful textile fibers is that that long-chain molecules lie in a somewhat parallel arrangement along the fiber length and lateral forces to hold the molecules together and give cohesion to the fiber structure. Additionally some measure of freedom of molecular movement is often a bonus as this gives fiber extensibility and accessibility for moisture absorption and uptake of other chemicals, for example dyestuffs.28 Alginic acid possesses a high degree of polymerization, is a linear polymer and has very reactive groups at close and regular intervals along the length of its constituent molecules. This allows hydrogen bonding and van der Waal’s forces to occur and thus alginic acid is a potential fiber forming material. The functionality of the alginate chains also means that it is possible to strengthen the lateral force in the fiber by the introduction of cross-links between two active groups with a suitable agent.12 Furthermore cross-links formed by using formaldehyde-based reagents can improve the fiber’s water resistant properties by progressively decreasing hydroxyl content of the system. The promise of such an alginate system has meant that many attempts have been made to spin fibers from other alginate salts in the hope of obtaining fibers that would be suitable for normal textile use. Some metallic alginates are sufficiently resistant to alkalis and laundering conditions. In this group are beryllium, chromium and aluminium. Beryllium alginate is toxic and brittle while chromium alginate is green in colour, which in addition to issues of chromium itself, limits restricts commercial applications.12 As a fiber, alginates, e.g., calcium alginate can be stretched to give a high degree of orientation and to enhance fiber crystallinity as shown by sharply defined X-ray diffraction patterns. In comparison to other natural fibers, however, the degree of orientation and crystallinity of alginate is lower than in cellulose even after stretching. This is due to cross-linking of the chains by the metal ions which takes place in the chemical coagulation bath immediately as the fibers are extruded through the spinneret. Hence, the cross-linking which is vital to gel formation also decreases molecular rearrangements and thus normal stretching less effective than in conventional wet spun fiber systems. Indeed, experience has shown that 30% stretch is about the maximum before breakage occurs.29 However, improvements can be obtained by drying the filaments under tension since they can then contract in only one direction, and straightening of the chain molecules will tend to take place. This is aided by high temperature and the presence of water, a plasticizer which allows the breaking, and reformation of bonds. Alkali resistant alginates (beryllium and chromium) have few ionic bonds but considerable coordinate links, which are more stable. These links decrease extensibility but increase tenacity of the fibers.

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Chamberlain et al. found that there is a relationship between metal content and tensile strength and that a 10% metal content gave the highest fiber strength. Moussavi2 reviewed the physical properties of alginate produced by other workers and the results are summarized in Table 3.3. Table 3.3 Physical properties of alginate fibers quoted by different workers Type of alginate

Tenacity (cN tex–1)

Extension at break (%)

Density (g cm–3)

Reference

Calcium Calcium Calcium Calcium Calcium Calcium Zinc

15.4 10.1 18.3 18.3 12.8 12.8 20.4

14.5 11.6 14.0 6.0 14.0 12.6 10.0

– – 1.78 1.77 1.75 1.68 1.77

31 32 33 34 35 2 2

When the metal content is kept constant, the tensile properties of alginate fibers are largely affected by the moisture content. Calcium alginate yarns have a dry strength comparable to that of viscose rayon (18–39 cN tex–1), but their wet strength is low (< 3 cN tex–1). Their extensibility is sufficiently high to meet most textile requirements both in use and in processing.35 The variation in physical properties with moisture content is shown in Table 3.4. Table 3.4 Variation of physical properties with humidity35 Atmosphere


Elongation at break (%)

Dry 65% relative humidity 100% relative humidity (saturated)

20.19 10.46 2.66

10 14 26

In some cases it has been found that the physical properties of some alginate fibers change during storage. Calcium-based and alginic acid yarns deteriorate whereas chromium- and beryllium-based yarns improve rather than deteriorate during storage under standard conditions.31

3.3.3

Moisture properties of alginic acid-based fibers

Dry alginic acid and its salts,7 when exposed to the air, will pick up moisture and attain equilibrium moisture content depending on the humidity of the atmosphere and this is similar to the behavior of cellulose, but the equilibrium moisture content is generally higher for the salts of alginic acid. Moisture

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content over a wide humidity range was determined by Chamberlain et al.31 although some difficulty was found in obtaining precise equilibrium uptake figures. Common alginate exists as an apparently dry solid even though it may contain up to 30% of water, and in a powder form readily forms clumps even in dry climates. Furthermore, insoluble alginate fibers when freshly precipitated from solutions retain large quantities of water, even when subjected to pressure. The amount of solvent (water) retained depends upon the composition of the salt and the concentration of the solutions from which it was precipitated. After drying, the insoluble alginate fibers will swell on absorption of water. Highly swelling insoluble alginates can be obtained using mixed salts of alkali metals.7

3.3.4

Thermal properties of alginic acid-based fibers

Very little published work has been carried out on the thermal properties of alginate fibers. On heating below 50∞C highly polymerized alginic acid depolymerizes to give a stable low molecular weight alginic acid. Sodium alginate with a degree of polymerization of 500 can be stored, without observable change, for three years at temperatures between 10∞C and 20∞C. But at temperatures above 50∞C, degradation of high molecular weight sodium alginate can occur. The presence of moisture increases the rate of degradation. Complete breakdown to uronic acids by heating is difficult, but can be achieved at low pH and breakdown of the polymer occurs with guluronic acid decomposing more rapidly than the mannuronic acid.7 At temperatures from 50–200∞C there is a breakdown of the uronic acid units. At temperatures above 200∞C there is rapid degradation and roughly one molecule of carbon dioxide is evolved for every uronic acid unit decomposed.36

3.3.5

Biodegradation of alginate fibers

Biodegradable polymers and fibers are renewable resources that can be used for the manufacture of polymers and fibers and are clearly of interest with increasing environmental concerns and in the long term dwindling petroleum resources. Degradation of alginate is due to a variety of factors, which include light, water, atmospheric composition, fungi and microorganisms. Moisture content plays an important role in the degradation process due to microorganisms and bacteria. The mammalian digestive system is unable to degrade alginate49 but when considered as a medical fiber, alginate is nontoxic, non-carcinogenic, biocompatible, sterilizable and offers cheap processing by nonwoven technologies. In general, it can be thought that the short-term degradation of textile materials is an undesirable property; however, this very property is useful for alginate products in disposable wound dressings.

100

3.4


Industrial applications of alginates

The UK was the only producer of alginate fiber until 1971. The fiber was made into soluble yarn, which would dissolve in the scouring process. It was also used to make fabric for supporting fine lace during manufacture, as a draw thread in knitting and a small proportion of alginate fiber was utilized in the medical field.1 In 1970 the soluble yarn market was taken over by polyvinyl alcohol thus leaving the medical field as the main outlet of alginate fibers other than fashion and fire resistant applications.1 The current applications of alginates are shown in Table 3.5. The unique properties of alginate and its derivatives have found applications where thickening, suspending, emulsifying, stabilizing and gel formation is required. The properties of sodium alginate allow a solution thickening effect and certain emulsifying qualities by reducing the sedimentation rates of suspended solids. An example of this is propylene glycol alginate, which has both lipophilic and hydrophilic agent. It is used to thicken dyestuff paste for textile printing because of its high solution viscosity at low concentration and it is easily removed by washing. The monovalent alkali metal derivatives are soluble and stable at low pH values and are used to emulsify acidic solutions. Similarly, alginates and alginate derivatives are Table 3.5 Applications of alginates Industry

Product

Food

Ice cream, frozen desserts, milk shake mixes, chocolate milk, cream cheese, cake filling and topping, bakery jellies, margarine, baking, French dressings, salad dressing, syrup and topping, fish preservation, meat preservation, sausage casings, beer foam stabilization, soft drinks, synthetic foods, etc.

Pharmaceutical and cosmetics

Suspensions, jellies, ointments, emulsions and encapsulation, tablet disintegrating agent, tablet binder, tooth paste, shaving cream, hand cream and lotions, liquid shampoos, dental impression materials, moulding compounds, active compound in antibody formation, etc.

Paper

Surface sizing, coatings, adhesives.

Textile

Textile printing, spinning and weaving of temporary fibres from calcium alginate, wound dressings such as: bandages, adhesive strip, pads of various kinds, surgical sponges, tampons, theatre curtains, etc.

Rubber products

Latex creaming, latex thickening.

Other industrial uses

Fixation of 90Sr and other divalent radioactive compounds in the bloodstream of mammals and seawater, paints, ceramics and refractory linings and moulds, insecticides, flocculants, liquid fertilizers, packaging, electrical insulating paper, anti-corrosion, etc.

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used in pharmaceutical and textile industries for suspending and emulsifying groups so one molecule has the properties of an emulsifier and a thickener. The polyelectrolyte and colloidal properties of alginates are used in the protective coatings of particles and granules (encapsulation) particularly in pharmaceutical and cosmetic industries. The action of alginate as a stabilizer is less well defined than other functions. Stabilization is assumed to depend on all the above-mentioned properties and prevents the formation of large ice crystals in many dairy products.1 Alginates can form strong films and these can be used to coat textiles and paper with a grease resistant surface, e.g. greaseproof paper. Gelation of sodium alginate on the addition of divalent salts is a property used frequently in the food industry. The calcium–sodium ion exchange properties of alginate are exploited to form gels of controlled strength. Calcium ions are usually introduced into the sodium alginate solution slowly and in a controlled manner, either by using a calcium salt of low solubility which has the necessary degree of ionization, selecting an appropriate pH to control the solubility of certain calcium salts or the use of a chelating agent to allow a slow release of calcium.1 The addition of low concentrations of sodium alginate to solutions of fertilizers and insecticides provides better adhesion to the soil and plant foliage, which in turn prolongs the effective life of the chemical. Alginate gel membranes can be used to desalinate and purify seawater and sewage effluent.37 Sodium alginate can also ‘fix’ radioactive metal ions from seawater.38 Another use of alginate is in the stabilisation of ocean bottom sediment. A recent innovation is the mixed gels of calcium alginate and gelatin.31 These mixed gels have been reported to have a better range of controlled melting points than the respective gels alone, a property needed in the food industry. It has also been used to separate the oxidant and reductant in fuel cells.39 The use of alginate to bind the components of solid rocket propellant has been described by Kaufman.32

3.5

Fabrication of alginates as useful flexible substrates in medical textile-based products

The type of use, the method of manufacture, the form and ways of disposal govern the choice of fibers used in the medical field. The fiber properties influence the final product through to the ultimate applications where the requirements may be absorbency, tenacity, flexibility and biodegradability. In medical use one can consider alginate fibers as non-toxic, noncarcinogenic, non-allergenic, highly absorbent, haemostatic, of reasonable strength, biocompatible, capable of being sterilized, manipulatable to incorporate medicants and using cheap nonwoven technology to process it. The incorporation of biological agents into the fiber used for nonwoven

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wound dressings provides a means for directly introducing such agents to the wound without a separate application and with no additional discomfort to the patient. In this regard, it may be used to introduce various therapeutic drugs for absorption through the skin or to diagnose topical skin reactivity to various agents. A recent discovery about alginate is their activity in antibody formation, antiviral properties 39 and hypocholesterolemic activity40,41 where the alginate was found to be the active ingredient and not just a suspender. The pharmaceutical and medical industries have known about the 43Ca and 90 Sr fixation properties of alginate for many years.36,42,43 Alginate has been shown to be effective in reducing the absorption of 90Sr from human intestines.44,45 Nonwoven alginate fabrics have attracted attention as disposable textiles especially in wound dressings. Shorter production cycles, high flexibility and versatility and low production cost are some of the claimed advantages. An ideal nonwoven wound dressing for medical textile must be haemostatic, have good integrity, good absorbency and excellent retention properties. The dressing must form a soft, moist, non-adherent, hydrocolloid gel upon contact with any fluid. A haemostatic action ensures that on contact with blood, calcium ions are released and exchanged for sodium ions and these interact with blood clotting cascade mechanism resulting in clot formation. High absorbency improves the efficiency of exudate uptake together with its associated toxins and other undesirable matter. A high retention capacity means that blood is retained within the dressing when it is removed from the wound and dripping of blood is minimized. Good integrity ensures that the fabric has sufficient strength when being handled. It must be flexible to provide conformability to all wound surfaces. The material must also be permeable to gases to allow sufficient oxygenation of tissues so as to promote natural healing while allowing wound gases to escape, thus preventing the wound from becoming malodorous.46,47 Calcium alginate is insoluble in water and therefore slow in its haemostatic action. Partial replacement of calcium ions by sodium ions (popularly known as ‘conversion’)48 makes the fabric more soluble and also increases the haemostatic action.49 Suitable medicaments are also incorporated into the fiber.46,47 These include those which aid wound recovery like antibacterial and angiogenesis promoting agents. Popular antibacterial medicament agents such as chlorhexidine are acetate or gluconate salts prepared by treating the fiber with an aqueous solution of the medicament or its salt. Hyaluronic acid (HA) is normally employed as a sodium salt and is an active agent in wound healing and angiogenesis.46,47 There are various types of nonwoven wound dressings comprising the alginates discussed in literature.50–55 Sorbsan (from the Maersk company) surgical dressing is a carded web of laid calcium alginate fibers and this is presented as a loose fiber ‘rope’ for cavity packing, or a ribbon for narrow

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103

wounds or sinuses; in addition flat nonwoven pads can be applied to large open wounds. Kaltostat (from ConvaTec) is a haemostatic wound dressing comprising a carded and needle-tacked web of alginate fibers and the dressing is used on surface wounds. In both cases these materials in the presence of body fluids containing sodium ions, the fibers absorb liquid and swell and calcium ions in the fibers are partially replaced by sodium ions. The dressings then appear gel-like in appearance and these structures provide microenvironments that are believed to facilitate wound healing. The swelling properties of both Sorbsan and Kaltostat products have recently been discussed by Yimin Qin86 in which he describes how high (M) alginate fibers exchange ions more easily than high (G) fibers. In addition, high (M) fibers have better gelling abilities than those of high (G) fibers, although the gelling ability and absorption capacity for high (G) fibers can be improved by the introduction of sodium ions into the substrate. In another development, dried tow is crimped, staple cut and converted to nonwoven fabric by conventional carding, cross-lapping and needle punching techniques.56 A highly absorbent nonwoven fabric has been produced by Mahoney.57 Staple alginate fibers are processed to provide a mat and fibers in the mat are entangled by means of barbed needles. The number of layers in the mat depends upon the desired basis weight. David Tong58 has described a process of producing a web of dry alginate fiber material suitable for making medical or surgical dressings. It is a continuous method of producing a unitary dried nonwoven web of non-bonded alginate fibers from a spinning dope containing an aqueous solution of a soluble alginate salt. A nonwoven alginate fabric has been prepared by spreading a tow of calcium alginate filaments in a flow of water and overfeeding the spread of filaments onto a water-pervious support so that the filaments cross over each other. When dried the filaments become bonded to each other at their points of contact.59 A wound dressing of alginate staple fibers of good integrity and capable of been lifted from a wound in one single piece and having little or no residue is claimed by Susan M. Cole.60 The integrity is imparted by subjecting the nonwoven web to a hydroentanglement process. Mahoney et al.61 have described a process of producing a nonwoven fabric from crimped and staple cut alginate fibers. The web from a conventional card is built up by the sequential laying of layers of fibers, over one another, until a web of the desired weight is achieved. The layers are joined together at a plurality of points throughout by needle punching and an embossed pattern produced on at least one of the major external surfaces of the fabric by calendaring. A detectable X-ray strip may be incorporated between the webs. When the basis weight of a nonwoven dressing is lower than 60 g m–2, reinforcing fibers of greater strength such as staple rayon are incorporated by blending with alginate fibers during web formation. The finished dressing

104


is packaged in a hermetically sealed envelope and sterilized with either ethylene oxide or by gamma-irradiation. Modifications of alginate to either improve or introduce novel functional properties in the last few years are detailed in literature.62–69 Much attention has been focused on absorption, retention properties, non-immunogenic, bioerodible implantation composition and incorporation of medicants to assist the natural haemostatic property of the fiber. In this regard, Mahoney et al.70 claim to have improved the absorbency of alginate fiber by 120 times its own weight. Blending of alginate with carboxymethyl cellulose with the object of improving the swelling and reducing the brittleness of alginate fibers has been attempted.62 Gilding has patented a porous fibrous material alginate which comprises of either zinc, copper, silver, cerium, manganese, cobalt cations and/or any cation which is an enzyme co-factor, the cation provides exchangeable ions which have wound healing properties and increased absorbency.71 Interesting developments have occurred in the fabrication of water soluble alginate fibers.54 Continuous filaments of a water-soluble alginate can be obtained by extruding an aqueous spinning solution of water-soluble alginate into a coagulating bath of a large quantity of a hydrophilic organic nonsolvent in which the sodium alginate is insoluble. The prompt displacement of the water in the dope with the non-solvent produces the fibers. Watersoluble alginates that may be used include, salts from sodium, lithium, potassium, magnesium or ammonium and organic amine salts and organic esters. It may contain different types of salt structures in the molecule, and can be used singly or as a mixture of two or more of the above. Sodium alginate is often preferred since it provides fibers of a high mechanical strength. The potassium salt or propylene glycol ester maybe used together with the sodium alginate. The aqueous dope may contain 5–10% by weight of the water-soluble alginate. When the concentration is too small, neither coagulation nor formation of alginate filaments takes place. The coagulation medium is an organic solvent that is water miscible but is also a nonsolvent for the water soluble alginate. The water in the fiber is displaced by the organic solvent to allow fiber formation. Typical organic non-solvents used are methanol, ethanol or isopropanol, acetone, dioxane, ethylene glycol monomethyl ether, dimethyl sulphoxide, dimethylformamide, dimethylacetamide, acetonitrile, methylethyl ketone or phenol. Among these, acetone is often chosen as it provides fibers of a reasonably high mechanical strength. These organic non-solvents may also be used singly or as a mixture. The coagulating bath inevitably contains water after an aqueous dope is extruded into it. The build-up of water should be controlled and ideally be in as small amount as possible and never above 20%. This can be achieved by connecting the coagulation bath to a circulation system in which the nonsolvent is dried by dehydration equipment and in addition more dehydrated

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105

solvent can be added from another reservoir to supply the bath with the dehydrated non-solvent continuously. The bath is maintained at a temperature between 20–100∞C depending upon the non-solvent used or other extrusion conditions. After the fibers are formed in the coagulating bath, they may be immersed in a second dehydrating non-solvent, and then either heated or airdried.

3.6

Alginates in bioengineering

Research papers, comprehensive as well as specialized reviews, have been published on some important aspects of tissue and cell immobilization technology. There are also specialized monograms, and conference proceedings pertaining to this field, which have excited microbiologists and bioengineers.72 Of interest in this chapter is an overview of alginate in the field, as it would be extremely difficult to include and cover every report on all the aspects of tissue and cell entrapments due to paucity of space, but clearly this is an area being actively pursued. Rapid advances in the field have lead to the development of novel materials, in which different types of cells and tissues are encapsulated or combined with a variety of biopolymers in an attempt to restore, maintain, or improve tissues or organ function. The cell, or the biological component of the cell– biomaterial combination, secretes special chemicals or hormones while the biomaterial component protects the cell from immune attacks while at the same time being biocompatible with the host tissue. These encapsulated live cells, organelles or tissues have led to the production of artificial organs suitable for implantation in mammalian bodies.73–77 In this class of biomaterials, alginate is becoming increasingly popular as a culture medium for many types of cell and tissue entrapment because of the requirement for mild conditions and the simplicity of the procedures used. For clarification purposes a transplant is regarded as the tissue or organ transferred from one body or body part to another whilst an implant is an inserted or embedded object or device by surgical means, e.g. a drug capsule or a pacemaker.78 Lim’s79 patent claims to have encapsulated tissue or individual cells in a manner that they remain viable and in a protected state within a membrane permeable to nutrients, ions, oxygen and other materials needed to both maintain the tissue and support its normal metabolic functions, but impermeable to bacteria, lymphocytes and large proteins of the type responsible for immunochemical reactions resulting in rejection. In their patent, Skjak-Breaek et al.80 have reviewed prior problems associated with encapsulated implant rejection and described a method of encapsulating cells in an alginate containing a high content of guluronic acid residue that would solve the problem. Their claim is that their microencapsulation provide transplant or implant in vivo which is non-immunogenic and non-fibroblast inducing. Further attempts to

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improve implant acceptance by the host body is described by Cochrum et al.81 In their patent they describe a method of preparing a multiple layer coating of biological tissue and cells for transplantation by multiple application of sodium alginate gelled with divalent cations. This gives a transplant with distinct structure where biological tissue or cell core is covered with the first layer alginate coat surrounded by an intermediate halo layer which is then covered by the outer coating. A subsequent patent82 describes a transplant with a core of a viable, physiologically active cells and non-fibrogenic coating of metal alginate having a high mannuronate to guluronate molar ratio and free from fibrogenic amounts of fucose, sulphate, phloroglucinol and protein moieties. They claim the coating has permeability sufficiently low and thickness sufficiently large to protect the tissue cell from host immunological agents after transplantation, but thin enough to be permitting diffusion of sufficient nutrients and cell products. Shah83 claims to have macroencapsulated somatic cells using ultrapurified sodium alginate and polysulfone hollow fibers. He used a high guluronate, low endotoxin, low divalent metal toxin contents and low protein impurities. The islet cells prior to being encapsulated were purified to reduce the bioburden of microorganism including viruses. Encapsulation was done in RPMI 1640 tissue culture fluid containing the necessary nutritional supplements and ATP source of energy. The open ends of the fiber were covered with a porous membrane. To further improve biocompatibility, the outer wall of the polysulfone was lightly gelled with alginate gel and sulfonic group to inhibit complement activation. He claims that such gelled, encapsulated fiber does not affect diffusion of glucose or insulin across the hollow fibers. These organic non-solvents may also be used singly or as a mixture. Recent studies in the area of sustained drug release are by Chi and Woo84 and by Steiner et al.85 Chi and Woo describe a carrier made of sodium alginate/xanthan gum and gel hydration accelerator, which is a mixture of hydoxypropyl metylcellulose and propylene glycol alginate. It is claimed that the carrier provides a constant drug level in blood for 24 hours or more owing to the fact that the drug release rate follows zero order kinetics and does not significantly vary with the degree of gastrointestinal motility due to rapid gel hydration without forming a non-gelated core. Steiner et al. describe a drug delivery to the pulmonary system, which is achieved by encapsulation of the drug in alginate among other biomaterials into microparticles having a size between 2 and 5mm.85 They claim that the microparticles formed release drug at a pH of 6.4 to 8.0 and that they have been modified to target specific cell types and to effect release only after reaching them. Of interest to this work is the engineering of an alginate encapsulation that can be recognized by the body as a normal part of the physiology and to precisely trigger healing and reconstruction processes. This will be achieved by designing receptor sites onto the material matrix or scaffold where cellular

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107

recognition and cell–cell signalling will be expected to trigger biological processes. The use of alginate in textile scaffolds has certain specialized uses. Flexibility provides versatility and thus alginate fiber systems are ideal for encouraging cells to recreate the tissue geometry in three dimensions. Scaffolds may be knitted, woven, nonwoven, braided, embroidered or a combination of these techniques. They may be modified to meet the different cell requirements by, say, altering the fiber diameter, length or even the extreme step of modifying the polymer.

3.7

References

1. Stokton, B. (1979), University of Leeds, PhD Thesis, ‘Study of alginate in larger brown algae with particular reference to attachment’. 2. Moussavi, P.H. (1991), University of Leeds, PhD Thesis, ‘Production of fibers based on alginate’. 3. Frei, E. and Preston, R.D. (1961), Nature, 192, 939. 4. Baardseth, E. (1961), Pro. V Int. Seaweeds Symposium in Halifax, Pergamon Press, 1966. 5. Haug, A. and Smidsrod, O. (1967), Nature, 215, 757. 6. Evans, L.V. and Holligan, M.S. (1972), New Phytologist, 71, 1161. 7. McDowell, R.H. (1963), Properties of alginate, London Press. 8. Mahoney, P.M.J. and Walker, K. (1999), Squibb_Bristol (UK), ‘Alginate fibers, manufacture and use’, US Pat. 5874100. 9. Lauring, T., Hoppe, H.A. and Schmid, O.J. (1969), Cram, de Gryster & Co., Hamburg, Marine algae, A survey of research and utilisation. 10. Golging, B. (1959), Polymers and Resins, Van Nostrand, New York, 219–220. 11. Tang, M.C. (1983), MSc Dissertation, University of Leeds, ‘Influence of spinning variables on the properties of calcium alginate fibers’. 12. Chamberlain, N.H., Cunningham, G.E. and Speakman, J.B. (1964), Nature, 158. 13. Hall, R.B. (1945), Courtaulds Ltd (UK), ‘Improvement in and relating to the manufacture of alginate threads’, GB Pat. 567641. 14. Tallis, E.E. (1945), Courtaulds Ltd (UK), ‘Improvement in and relating to the manufacture of threads, filaments, films and the likes from alginates’, GB Pat. 568177. 15. Tallis, E.E. (1949), Courtaulds Ltd (UK), ‘Improvement in and relating to the manufacture of alginate threads’, GB Pat. 624987. 16. Haug, A.J. (1965), Methods Carbohydr. Chem., 5, 69. 17. Dillon, T. and McGuinness, A. (1930), Pro. Roy. Soc. Dublin, 33, 20. 18. Lunde, K., Heen, I. and Oy, M. (1938), Kolloid-Z, 83. 19. Hirst, E.l., Jones, J.K.N. and Jones, W.O. (1939), B., Nature, 143. 20. Fischer, F.C. and Dorfel, H. (1955), Z. Physiol. Chem., 302, 186–203. 21. Wilkinson, M.F. Private communication. 22. Haug, A., Larsen, B. and Smidsrod, O. (1966), A study of constitution of alginic acid by partial hydrolysis, Acta. Chem. Scand., 20, 183–190. 23. Noy, N.J. (1976), PhD Thesis, University of Leeds, ‘Solution properties of alginates’.

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24. Thiele, H. (1967), ‘Studies on the sequence of uronic acid residues in alginic acid’, Acta. Chem. Scand., 21, 691–704. 25. Kohn, R., Furda, I., Haug, A. and Smidsrod, O. (1968), Acta. Chem. Scand., 22, 3098. 26. Smidsrod, O., Haug, A. and Lian, B. (1972), Acta. Chem. Scand., 26, 71. 27. Haug, A., (1964), Report No. 30, Norwegian Seaweeds Research, Trondheim. 28. Morton, W.E. and Hearle, J.W.S. (1962), Physical Properties of Textile Fibers, 22– 23. 29. Varta, A.G. (1968), German Pat. 1 264 556. 30. Brucker, R.F., Wormington, C.M. and Nakada, H.I. (1971), J. Macromol. Sci. Chem., A5, 1169. 31. Chamberlain, N.H., Johnson, A. and Speakman, J.B. (1945), J. Soc. Dyers and Colourists, 61, 13. 32. Kaufman, M.H. (1971), US Pat. 3613373. 33. Luniak, B. (1953), ‘The identification of textile fibers’, Sir Isaac Pitman & Sons Ltd. 34. Cook, J.G. (1968), Handbook of Textile Fibres (Man-Made Fibres), Merrow Publishing Co. Ltd, 600–650. 35. Aldred, F.C. and Moseley, C.R. (eds) (1980), EP 0344913. 36. Tanaka, Y. (1968), Can. Med. Ass. J., 98, 25. 37. Levy, W.J. (1966), British Pat. 1 045 599. 38. Lazorenko, G.E. (1970), Dopov. Akad. Nauk. Ukr. R.S.R: Ser. b, 32, 50, 445. 39. Claus, D. (1965), Bioch. Biophys. Research Communications, 20, 745. 40. Oshima, F. (1957), Japanese Circulation Journal, 21, 205. 41. Fahrenbach, F.B. (1966), Pro. Soc. Exp. Bio. Med., 123, 321. 42. Ichikawa, R. (1969), Kgaka To Seibatsu, 7, (4), 208–211. 43. Malkin, P.M. (1970), Radiobiologiya, 10 (4), 566. 44. Carr, T. (1969a), Nature, 224, 115. 45. Carr, T. (1969b), Nature, 224, 116. 46. Fenton, J.C., Griffiths, B. and Mahoney, P.M.J. (1993), WO 94/17227, PCT/GB94/ 00102. 47. Kobayashi, Y., Fukuko, S., Obika, H., Asaoka, T. and Tenma, K. (1991), EP 0439 339 A2. 48. Kenneth, J.F. and Keith, B. (1975), Medical Alginates Ltd, UK, ‘Calcium alginate conversion process’, GB Pat. 1394741. 49. Bonniksen, C.W. (1951), ‘An improved alginate medical or surgical preparation’, GB Pat. 653341. 50. Mahoney, P.M.J. (1991), EP 019476 756 A1. 51. Patel, H. (1995), Kendall & Co. (US), ‘Process for preparing the alginate-containing wound dressing’, US Pat. 5470576. 52. Pandit, A.S. (1996), The Kendall Company (US), ‘Hemostatic wound dressing’, US Pat. 5836970. 53. Scherr, George H. (1997), DeRoyal Industries Inc. US, ‘Alginate fibrous dressing and method of making the same’, US Pat. 5674524. 54. Wren, David, C. (1993), BritCair Limited (Aldershot, GB2), ‘Wound dressing’, US Pat. 5238685. 55. Kobayashi, Y., Fukuko, S., Obika, H., Asaoka, T. and Tenma, K. (1995), Sakai Chemical Industry Co. Ltd Agency of Industrial Science & Technology (both of Japan), ‘Water soluble algin fibers and production thereof’, US Pat. 5474781. 56. Wren, D.C. (1989), WO 89/12471, PCT/GB89/00706.

Alginate fibers

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57. Mahoney, P.M.J. (1993), BritCair Ltd (Aldershot, GB2), ‘Absorbency alginate fabric, use as wound and burn dressings, and a method for its preparation’, US Pat. 5256 477. 58. Tong, David P. (1983), ‘Process for production of alginate fiber material and products made therefrom’, EP 0072 680 A3. 59. Bahia, H.S. and James, J.R. (1994), EP 0616650. 60. Cole, Susan M. (1989), Minnesota mining and manufacturing company (US), ‘Alginate wound dressing of good integrity’, EP 0344913 AI. 61. Mahoney, P.M.J., Fenton, J.C. and Keys, A.F. (1992), C.V. Laboratories Ltd (GB), ‘Alginate fabric, its use in wound dressing and surgical haemostats and process for its manufcture’, WO 92/19802 or PCT/GB92/00792. 62. Carlo, D.J., Hendler. S.S. and Marchese, R. (1992), Viaderm Pharmaceuticals Inc. (US), ‘Alginate wound dressings’, WO Pat. 9222285. 63. Mahoney, P.M.J. and Walker, K. (1999), Bristol Squibb-Meyers (UK), ‘Alginate fibers, manufacture and use’, US Pat. 5874100. 64. Mahoney, P.M.J. and Howells, A.E. (1998), Squibb Bristol Myers Co. (US), ‘Alginate fibers, manufacture and use’, US Pat. 5820874. 65. Pandit, A.S. (1996), The Kendall Company (US), ‘Hemostatic wound dressing’, US Pat. 5836970. 66. Scherr, George H. (1998), Park Forest, IL 60466 US, ‘Alginate foam products’, US Pat. 5718916. 67. Capelli, Christopher, C. (1998), 4500 7th St, Kenosha, US, ‘Silver-based pharmaceutical compositions’, US Pat. 5744151. 68. Scherr, George H. (1997), DeRoyal Industries Inc. US, ‘Alginate fibrous dressing and method of making the same’, US Pat. 5674524. 69. Wren, David, C. (1993), BritCair Limited (Aldershot, GB2), ‘Wound dressing’, US Pat. 5238685. 70. Fenton, J.C., Griffiths, B. and Mahoney, P.M.J. (1993), WO 94/17227 or PCT/ GB94/00102. 71. Gilding, D.K. (1991), Beam_Tech Ltd (GB), ‘Alginate materials’, World Pat. WO9111206. 72. http://www.microlithe.com/microcap/microtn/Literature/Bioencapsulation.doc 73. National University of Ireland, National Centre for Biomedical Engineering Science, http://www.mis.nuigalway.ie/ncbes/biomaterials.htm 74. Ramakrishna, S.V. and Prakasham, R.S. http://www.ias.ac.in/currsci/jul10/ articles17.htm 75. Heriot-Watt University, Biomedical Textile Research Centre, http://www.hw.ac.uk/ 76. Chaplin, M. (1990), Enzyme Technology, Christopher Bucke (ed.), Cambridge University Press. 77. Kierstan, M. and Bucke, C. (1977), Biotechnology Bioengineering, 29, 387–397. 78. http://dictionary.reference.com/ 79. Lim, F. (1983), ‘Microcapsules containing viable cells’, US Pat. 4391909. 80. Skyjak-Braek, G., Espevik, T., Smidsrod, O., Soon-Shiong, P. and Otterlei, M. (1995), ‘Cells encapsulated in alginate containg a high content of a a-L-guluronic acid’, US Pat. 5459054. 81. Cochrum, K., Dorian, R. and Jemtrud, S. (1996), ‘Multiple layer alginate coatings of biologicaltissue for transplantation’, US Pat. 5578314. 82. Cochrum, K., Dorian, R. and Jemtrud, S. (1996), ‘Non-fibrogenic high mannuronate alginate coated transplants, processes for their manufacture, and methods for their use’, US Pat. 5693514.

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83. Shah, K.A. (1999), ‘Encapsulated cell device’, US Pat. 5976780. 84. Chi, M. and Woo, J. (2004), ‘Sustained release composition for oral administration of drugs’, World Pat. 2004037290. 85. Steiner, S.S., Lian, H., Shen, G.S., Feldstein, R. and Rhodes, C. (2004), ‘Method for drug delivery to the pulmonary system’, US Pat.2004096403. 86. Qin, Y. (2004), ‘Gel swelling properties of alginate fibers’, Journal of Applied Polymer Science, Vol. 91, 1641–1645.

4 Cellulosic fibres and fabric processing D C I E C H A Ñ S K A, Institute of Chemical Fibres, Poland and P N O U S I A I N E N, Tampere University of Technology, Finland

4.1

Introduction

In order to evaluate sustainability of products and processes, the life cycle assessment (LCA) concept is a method for industry to understand, manage and reduce the environmental, health and resource consumption impacts associated with processes, products and activities.1,2 Due to the well-known environmental impacts of cotton and its further chemical processing, LCA evaluations have been used to develop the sustainability of man-made fibres, as well.3 In the case of cellulosic fibres based on cellulose polymers produced by photosynthesis, biodegradability can be seen as a benefit to fit the criteria of sustainability. Recently, biotechnological methods of wood and pulp processing, polymer synthesis, biodegradation and biological transformation have been widely introduced into industry.4 Mechanical and chemo-mechanical methods have been increasingly replaced by chemical sulphate (Kraft) and sulphite pulping methods for raw materials of paper and cardboard.5 Enzyme technology has been successfully applied in the pulp and paper industries: the production of dissolving pulp, bleaching de-inking, removal of slime, removal of pitch, removal of shives, debarking, and retting of flax fibres can be done with enzymes.6 This trend is justified by numerous benefits resulting from the application of biotechnology, such as mild conditions of technological processes, low energy consumption, high safety, low amounts of by-products, renewable raw materials, economic aspects and others. The further development of enzymes in bio-processing of new applications and improved characteristics is based increasingly on gene isolation and transplantation technology.7,8 Nowadays, the biotechnological processing of pulps using selected types of enzymes seems to be a promising method for the production of the cellulosic products. For modification of pulps the cellulase and xylanase extracted by submerged fermentation from fungi strains such as Aspergillus niger and Trichoderma reesei have been applied.9,10 The modification of molecular, supermolecular and morphological structure occurring during enzymatic 111

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reaction has definitely improved the cellulose properties, i.e. solubility in alkaline solutions up to 100% (Celsol process), solubility in organic solvents (NMMO process) and chemical reactivity towards reactants.11 Methods of biological utilisation of polyester/cellulose textile blends are based exclusively on the application of cellulolytic enzymes, like cellulases, catalysing the hydrolytic degradation of cellulose to a sugar mixture like glucose or cellobiose. The recovered polyester components were tested to be recycled in the melt process.12 Microbial synthesis of cellulosic fibres affords the opportunity of obtaining products with unique properties suitable for practical application in medicine and the electronics industry. For synthesis of modified bacterial cellulose, the Acetobacter subsp. strains have been applied. The selection of suitable polyaminosaccharide modifiers allows the production of bacterial cellulose characterised by valuable mechanical, electro-acoustic and biological properties. Practical applications of this cellulosic composite material for manufacture of novel wound-healing dressings as well as diaphragms for loudspeakers have been tested.13 Enzyme treatments of textiles, typically cellulose materials such as cotton, viscose or lyocell fabrics, have widely been used in the textile industry since the 1980s. There are some of advantages resulting from the use of enzymes for finishing cellulosic fibres and fabrics, i.e. smooth surface, better appearance, anti-pilling properties, soft and delicate feel, improved wetability and dyeability.14 Enzymatic treatment of cotton in denim washing or other biopolishing is standard technique in industry. Various cellulases, e.g. purified T. reesei types, have been studied and optimised in cotton bioscouring and finishing.15 Flax fibres and linen fabrics containing higher amounts of pectin lignin need synergistic mixtures of cellulases, pectinases and ligninases or special flax retting enzymes.16

4.2

Life cycle assessment (LCA)

4.2.1

General procedure

Wood processing, fibre, and textile industries are continuously facing the need to increase their knowledge of the environmental impacts associated with production of pulp, paper, fibres, and textiles. Presently, companies are more or less transferred from the prevention of local risks to environmental management strategies. In general these strategies can be understood as the companies’ environmental attitudes and guidelines. The environmental strategies can be roughly divided into four levels: 1. Strategy based on environmental requirements and laws, where the company’s strategy is based on observance of environmental laws and other requirements.


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2. Strategy based on preventive actions, when the company is concentrating on the prevention of environmental hazards and risks. 3. Strategy based on ecological competitiveness, when the company is improving its competitiveness by an effective use of resources and by making use of the eco-marketing possibilities. 4. Strategy based on the principles of a sustainable development. The company’s environmental strategy pays attention to the social justice and to the rights of the future generation in addition to an effective ecological policy. The first LCA attempts were carried out in the USA in the late 1960s. These assessments were only about energy requirements from ‘cradle to grave’ concerning package production. Later in the 1990s the development of LCA and life cycle inventory (LCI) accelerated including throughout development work on LCA methodology and its standardisation, and were applied to other industries, as well.17 Studies for cotton fibre production, textile, and laundry industries were included more thoroughly in the late 1990s.3 The general applications of LCA methodologies are often used for eco-labelling, namely a company’s environmental claims on manufacturing and production stages as follows: ∑ identification of processes, ingredients and systems in order to minimise environmental impact; ∑ comparison of different options within product systems in order to minimise environmental impact; ∑ guidance in long-term strategic planning; ∑ evaluation of resource effects associated with particular products; ∑ training of product designers; and ∑ comparison of functionally equivalent products. The overall scheme for LCA is presented in Fig. 4.1. The Environmental Management and Audit Scheme (EMAS) has been standardised for the follow-up of environmental strategies, and as a tool in a company’s commitment to continous improvement of the achieved environmental status by using Best Available Technologies (BAT).

4.2.2

Life cycle assessment of cotton

The general life cycle scheme for cotton fibre is presented in Fig. 4.2. According to collected data of production for white terry sheets, the main negative impacts are related to consumption of water, use of fertilisers and pesticides, total nitrogen to waste water and, surprisingly, release of carbon dioxide during the whole life cycle.18,19 Because cotton is sensitive for attacks of various insects, such as boll weevil, pink ball worm, and many fungi, various pesticides and fungicides are used to improve its growth.20 Thus, a high

114

Biodegradable and sustainable fibres LCA framework

GOAL AND SCOPE DEFINITION – the application of the LCA – the functions of the system – the functional unit – the system boundaries – allocation procedures – data requirements – assumptions – limitations – the initial data quality INVENTORY ANALYSIS – includes data collection and calculation procedures of inputs and outputs such as use of re-sources and emissions to air, water and land – energy flow calculations including associated inputs and outputs IMPACT ASSESSMENTS – aimed at evaluating the significance of impacts – classification by assigning of inventory data to impact categories – characterisation – valuation

LCA INTERPRETATION – the findings from the inventory analysis and the impact assesment are combined together – conclusions and recommendations – may include a review of the scope of the LCA and data quality

DIRECT APPLICATIONS – product/process developments – strategic planning – public policy making – marketing – other

– reporting incl. the transparency of the results, data, methods, assumptions and limitations

4.1 The general overall scheme for LCA. Cotton seed

Growing, Energy protecting Chemicals Fertilisers against insects, etc. Resources Ginning, baling

Cotton fibres

– Solid waste Cotton seed Seed cakes Oil Seed shells

4.2 The life cycle scheme of cotton fibre.

share (25%) of pesticides and fungicides is used in cotton fields causing environmental problems in ground water and health problems for farmers. Gene transplantation technology is increasingly used to develop cotton species


115

resistant against biological attacks.21 However, a gene, e.g. of bacteria Bacillus thuringiensis controlling the production of pesticide-active protein, which is capable of destroying certain insects and some fungicides is still needed. Other methods for improving the sustainability of cotton include the organic cultivation of old species more resistant against insect attacks, lower use of fertilisers, and controlling the use of the irrigation water. The organic cultivation produces low yields of cotton and is presently not competitive with old or new types of non-organic methods.

4.2.3

Life cycle assessment of cellulosic and synthetic man-made fibres

Cellulosic fibres The production and manufacture of cellulosic fibres is based mainly on processing of dissolving pulp and cotton linters.22,23 This group of manmade fibres includes viscose, modal, cupro, lyocell and acetate as the main commercial products. Viscose has been produced by the xhanthogenate method for more than 115 years. Other technologies applying the same chemical reactions of dissolving pulp with alkaline-activated carbon disulphide include modal, viscose, and polynosic fibre processing.24 Compared with viscose, the modal and polynosic processes operate with cellulose of a higher alpha content (non-soluble in18% NaOH) and degree of polymerisation (DP), lower cellulose concentration, a higher degree of filtration and chemical additives in the spinning bath, and a lower spinning speed.25 Thus, the process for regular viscose fibres is more efficient regarding the consumption of raw materials. Solid waste production into water by unreacted hemicellulose and exhaust air by carbon disulphide are higher in the case of modal and polynosic, as well. All these processes have a high consumption of water due to the wet spinning, evaporating, and various fibre washing and finishing and drying stages. The cupro and lyocell processes are based on the application of direct dissolution systems. The copper ammonia complex is prepared from copper sulphate and sodium carbonate with sodium and ammonium hydroxides, as presented in the equation in Fig. 4.3. The dissolution process is carried out for cuprammonium via weakening the intra/intermolecular hydrogen bonds and complex formation. Cupro and especially lyocell processes consume lower amounts of water, but a similar magnitude of energy. According to a novel method, enzymatic modification of cellulose by increasing the alkaline solubility enables the manufacturing of regenerated fibres without any derivative or organic solvent.26 It has been proved that by using certain specific compositions of cellulolytic enzymes (a controlled ratio of endoglucanases to cellobiohydrolases) it is possible to obtain directly

116

Biodegradable and sustainable fibres A. Viscose process Xhantogenation cellulose–O–Na+ + CS2 Æ cellulose–OCS2– Na+

Regeneration 2 cellulose–O CS2 Na + H2SO4 Æ 2 cellulose–OH + Na2SO4 + 2CS2

Carbon disulphide hydrolysis 3CS2 + 6NaOH Æ 2Na2CS3 + Na2CO3 + 3H2O Na2CS3 + 6NaOH Æ 3Na2S + Na2CO3 + 3H2O

Hydrogen sulphide formation Na2CS3 + H2SO4 Æ CS2 + H2S + Na2 SO4 Na2S + H2SO4 Æ H2S + Na2SO4

B. Cupro process Copper hydroxide formation CuSO4 · 3Cu(OH)2 + 2NaOH = 4Cu(OH)2 + Na2SO4

Cuprammonium complex formation Cu(OH)2 + 4NH4OH = Cu(NH3)4(OH)2 + 4H2O

C. Lyocell process 40

O

30

Ce

30

Solution

e

40

los

Undissolved pulp fibres

CH3

N-methylmorpholine oxide

llu

O

20

60∞C 70∞C

N

10

Crystals 0 20 10 0 NMMO Water

4.3 Main chemistry in viscose (A), cupro (B) and Lyocell (C) fibre processes.

alkali soluble cellulose and regenerated fibres without essential loss in DP, as shown in Fig. 4.4.27 The method is further discussed in Section 4.5.1. Cellulose secondary acetate fibres are manufactured from cotton linters by steeping in glacial acetic acid and sulphuric acid-catalysed reaction with acetic anhydride. The reaction is exothermic and the final product in a maximum of 20 hours is cellulose triacetate, which is converted to secondary acetate by adding sufficient water. The hydrolysis is stopped when 1/6 of the acetate groups have been randomly changed to hydroxyl groups. The precipitated polymer flakes are dissoluted in acetone containing small amounts of water or alcohol. The chemical formula of cellulose triacetate and the diacetate fibre production chart are shown in Fig. 4.5.

Alkali solubility at –5∞C, %


117

120 100 80 60 40 20 0

0

0.9 Amount of zinc oxide, % Untreated

1.8

Enzyme treated

4.4 The solubility of untreated and enzyme-treated pine sulphate paper grade pulps with the aid of zinc oxide.

O

H

OCOCH3

C

C

CH2OCOCH3 C

H OCOCH3 H

C

C

H

C

H C

CH2OCOCH3

Wood pulp or cotton linters

O

H

C

OCOCH3 H

O

O

O

H

C

C

H

OCOCH3

Petroleum ‘cracking’

Cellulose Ethylene gas

Propylene gas

Activated cellulose

absorption

absorption

Acetylated cellulose

Acetic acid liquid ‘cracking’

hydrolysis

Other gases or chemicals

Acetone liquid ‘cracking’

Acetic anhydride liquid

Acetylated cellulose

Sulphuric acid catalyst

water Acetate flake water

washing drying

recovery and re-use weak acetic acid

Spinning solution

acetone exhaust to recovery and re-use

4.5 The chemical formula of cellulose triacetate and scheme for production of cellulose diacetate.

118

4.2.4


Properties and environmental costs of different processes of regenerated cellulosic fibres

According to a recent comparison of conventional and emerging processes, enzyme-based direct dissolving method (Celsol) and cellulose carbamate (CC) processes are capable for producing regular type regenerated cellulose fibres.28 These methods, however, are either in semi-industrial stage (CC) or in pilot stage (Celsol), and further improvement of the fibre properties can be foreseen. This is evident when making comparisons with the development of viscose fibres during 100 years. Very strong fibres can be produced by the lyocell method; however, the well-known fibrillilation tendency caused by highly oriented cellulose molecules demands special treatments to prevent any drawbacks in textile applications. The properties of selected regenerated cellulose fibres are presented in Table 4.1. Table 4.1 The comparison of properties of different cellulosic fibres Type of fibre

Titre (cN)

Tenacity (cN/tex)

Elongation (%)

WRV (%)

Viscose (regular) CC Celsol Lyocell

1.7–3.3 1.7–3.3 1.7–3.3 0.5–4.0

15–25 15–25 ~ 20 30–40

15–27 15–23 19–25 5–10

50–80 110–125 120–125 50–60

The life cycle assessment of regenerated cellulose fibres results in high water and energy consumption, as can be concluded from process descriptions. The recovery of regeneration cleavage products of viscose (CS2, H2S) and cellulose carbamate (N-compounds, urea) needs special units for the prevention of air and water pollution. Thus, the extra processing costs as ‘environmental costs’ are quite high, as presented in Fig. 4.6. The environmental protection processing cost distribution of the viscose process consists of active carbon recovery systems for CS2 along the whole production line: wet spinning into sulphuric acid liquor, staple fibre web formation, alkaline sulphur removal, washing, and finishing. The formation of sodium sulphate is high due to the stoichiometry, and about 50–60% of the salt must be precipitated and purified. This is generally carried out in a separate factory, a ‘salt factory’, and sodium sulphate can be sold and utilised in the chemical industry as pulp, medicine, glass, and textile dyeing, etc. Additional conventional waste from cellulosic fibre production from dissolving pulp is the loss of 4–6% of hemicellulose based on starting pulp. Various attempts have been made in order to concentrate and separate hemicellulose (xylane) for xylitol and derived products. The processing costs are, however, too high for economic production and hemicellulose is increasingly removed in wastewater treatment plants.


119

100 90

Percentage

80 70 60

CS2 H2 S Na2SO4 ZnS2 S

50 40

Na2SO4 residual urea

30

Na2SO4

20

Regeneration of NMMO

10 0

Viscose

CC

Celsol

Lyocell

4.6 Environmental protection costs of selected processes for regenerated cellulose fibres.32

The salt and hemicellulose formation are common in all systems based on neutralisation in viscose, modal, carbamate and Celsol processes, and to some extent in lyocell, as well. Wastewater treatment costs of hemicellulose are not included in calculations in Fig. 4.6.

4.2.5

Comparison of man-made fibres with cotton and cellulosics

The life cycle schemes of cellulose and polypropylene fibres, both representative of a polymer material based on a single monomer, are presented in Fig. 4.7. In both cases the actual fibre manufacturing process is only a part of the whole life cycle including raw material production and transportation, textile manufacture and use, and waste management by landfill (viscose) or incineration (polypropylene). The comparison of fibres gives advantages and drawbacks for both fibres, and it is very important to carry out the definition of goals and boundaries of the system to be considered, as presented in Fig. 4.8. The advantages for viscose are as follows: renewable raw material (through photosynthetic processes); good properties for clothing and hygienic purposes; and waste management. Furthermore, lyocell and cupro processes show lower environmental impacts, because of total recovery of solvents during processing. For polypropylene, in turn, advantages, such as the simplicity of the spinning process, low air and water emissions, low energy demand in drying, light weight, and the possibility for recycled products can be observed. General environmental profiles of cellulosic fibres depend on actual processes and parameters applied. The life cycle inventory analysis of cotton, organic cotton and polyester

120

Biodegradable and sustainable fibres Forest Extraction Felling Crude oil Wood Refining Wood chipper Naphta Chips Cracking Pulp prod.

Gas

Chemicals Propylene

Bleach Chemicals

Polymerisation Viscose prod.

Additives

Fibre prod.

Spin finish

Polypropylene

Chemicals

Fibre prod.

Spin finish

Recycling Viscose fibre

Nonwoven prod.

Nonwoven prod.

Use

Use Incineration

Waste

Landfill

Incineration

Waste

Landfill

4.7 Life cycles of viscose and polypropylene as single-monomer fibre materials.

has been compared in a study on hotel textile production and services.29 The detailed calculation, presented in Table 4.2, is divided into five important impact parameters: ∑ ∑ ∑ ∑ ∑

energy consumption; non-renewable resources; water consumption; emissions to air; emissions to water.

From the comparison in Table 4.2 it can be concluded that advantages of cotton are lower energy consumption, renewable resources, and possible (if known) lower emissions to water. Disadvantages are the high consumption of water, consumption of fertilisers and pesticides, and high emissions to air (particularly CO2). Organic cotton shows slightly lower energy consumption, renewable resources, and slightly lower emissions to air.

4.3

The mechanisms of enzymatic reactions on wood and cellulose

The structure of enzymes is usually described in terms of molecular mass, isoelectric point, amount of amino acid residues, and structural organisation,

DMT manufacture

Methanol manufacture

Melt phase polymerisation Natural gas processing

Crude oil production

Distillation/ desalting

Acetic acid manufacture

TPA manufacture

Olefins manufacture

Naphta reforming

PTA manufacture

Ethylene glycol

Paraxylene extraction

Other additives

Fibre filament production Continuous polymerisation

4.8 Life cycle scheme of polyester (PET) fibre materials as polymer based on two monomers’ polycondensation.

Staple


Natural gas

121

122


Table 4.2 Life cycle inventory analysis of cotton, organic cotton and polyester (PET) fibres Parameter

Unit/kg

Polyester

Cotton

Organic co. (Greenco)

Energy consumption Electricity Fossil fuel Other energy type Renewable fuel Inherent energy Non-renewable resources Natural gas (r) Natural gas, feedstock (r)* Crude oil (r) Crude oil, feedstock (r)* Coal (r) Coal, feedstock (r)* LP gas (r) Hydro power (MJel) (r) Natural uranium (r) Fertilisers Pesticides Other non-renewable resources (r) Water (r)

MJ MJ MJ MJ MJ MJ KG

97.4 15.2 82.2

59.8 12.1 47.7

53.6 13 40.6

2.4

1.4

1.3

g g

360 287

354

140

g g g g g MJ mg g g

410 867 141 373

527

567

524

564

32 1 14 457 16

34.6 1.1 15.1

0.41

KG

17.2

22200

24000

Emissions to air: CO2 CH4 SO2 NOx CH CO

g g g g g g

2310 0.1 0.2 19.4 39.5 18.2

4265 7.6 4 22.7 5 16.1

3913 6.1 4.2 22.7 5.3 17.2

Emissions to water: COD BOD Tot-P Tot-N

g g g g

3.2 1

*feedstock values included to energy consumption.

as schematically presented in Fig. 4.9. The enzyme molecule comprises, normally, a binding domain (CBD), linker protein chain and the main active catalytic domain for hydrolysis of the specific substrate, as presented in Fig. 4.10.30 The hydrolysis of wood fibres with enzymes comprises the action of a group of enzymes which hydrolyse by the specific catalytic action the different components present in the wood: cellulose, hemicelluloses and

Cellulosic fibres and fabric processing Enzyme

Family

Amino acid residues

Molecular mass kDa

Isoelectric point (pI)

EGI

7

437

50–55

4.6

EGII

5

397

48

5.5

EGIII

12

218

25

7.4

EGIV

61

326

(37)a

–

EGV

45

225

(23)a

2.8–3

95–105

5.6–6.8

Structural organisationb

368 36 34

unknown* unknown*

33 36

327

218 233

EGVI

123

CBHI

7

497

59–68

3.5–4.2

CBHII

6

447

50–58

5.1–6.3

56 37

166 23 36 unknown*

430 36 44

31 36 365

a The molecular mass calculated from amino acid sequence b ■, the catalytic domain; ■ , linker region; ■, CBD * gene not described

4.9 The composition and molecular characteristics of selected cellulases.

B

A

C

CBDs

Catalytic domains

4.10 Schematic representation of some of the domain arrangements found in cellulolytic enzymes.

lignin. The respective enzymes are called cellulases or endoglucanases (EG) for b-(1 Æ 4) bonds, cello-biohydrolases (CBH) for non-reducing molecular ends, b-glucosidases (bG) for cello-oligosaccharides and xylanases (EX) for b-(1 Æ 4) bonds between D-xylose residues of heteroxylans and xylooligosacchrides, in Fig. 4.11. The enzymatic hydrolysis of cellulose proceeds by the attack of EG, CBH, or bG on the b-(1 Æ 4)-glucosidic linkages.31 Since the structure of cellulose and hemicellulose is of complicated nature, their biodegradation requires a complex system of secreted enzymes with specific moles of action. The classification and mode of action of various glycoside hydrolases in molecular level is shown in Fig. 4.11.

HO

O OH

OH

O HO

OH O

HO O OH

OH O OH

O HO

OH O

OH O

OH

HO O

O

O

OH

HO

OH

OH O O

HO O

OH

OH

Endoglucanase

ßG CBH

HO

OH O HO

OH

OH HO HO

OH O

HO HO

OH O

OH

OH

OH

HO O

O

OH O

HO

OH

OH

OH O

OH

HO O

O

O

O

HO

OH

OH

OH

HO O

O

O OH

OH

OH

ßG

ßG CBH

OH O

HO HO

OH O

HO OH

HO

OH

OH

OH

OH

OH O

HO HO

OH

OH

HO OH O

HO HO

4.11 The enzymatic hydrolysis of cellulose.

O

HO

OH

OH

OH

HO O

OH O OH

O


HO

OH

HO O

124

OH O


125

Hemicelluloses present in a wood fibre are exposed to the cellulolytic complex of enzymes by the action of different xylanases, which catalyse the hydrolysis of the xylose backbone of heteroxylan chains layered on the heteroxylan microfibrills. The xylanases act at random locations on the heteroxylan chains that have themselves become exposed by the action of the action of the various debranching enzymes, as presented in Fig. 4.12. b-Xylosidase

HO O OH

O

HO HO

Acetylxylan CH 3 esterase C O

Xylanase OH O O HO

O OH O

HO O O H3C COOH

HO O

OH O HO O O O O OH C O

a-Glucuronidase

CH3

OH O

O O

O O

H2C O C O

O

HO O

OH

OH O O OH

a-Arabinofuranosidase Ferulic acid esterase

CH CH H3C O

OH

4.12 Enzymes involved in the hydrolysis of a hypothetical heteroxylan.

The supermolecular structure of cellulose consists of crystalline ordered regions formed by certain arrangement of hydrogen bonds between and inside the cellulose molecular chain. Due to the large number of hydroxyl and ether bonds of the chain, there are several possibilities for crystal formation, the main types classified as cellulose I and cellulose II, as schematically shown in Fig. 4.13. The formation of cellulose I is normally as the result of the bisynthesis process in nature, wheras cellulose II results in chemical swelling action of sodium hydroxide. There are also other types of crystalline cellulose, e.g. cellulose III generated during swelling with gaseous ammonia. The structure of isolated cellulose microfibrills of different origin (cotton linters and spruce sulphite pulp) is shown in Fig. 4.14. The practical application of cellulose pulps for different processes is connected with the selection of different methods for modifying this valuable raw material in order to improve its reactivity and solubility. The cellulose modification process can be conducted by chemical (alkalisation),32,33 physical (irradiation)34,35 or biochemical (enzymatic transformation)36,37 methods. Two major modified strains of Aspergillus niger38 and Trichoderma reesei39 have been selected as being the best producers of enzymes suitable for biotransformation of softwood- and hardwood-originated pulps. The enzymatic transformation of cellulose occurs in a heterogenic system.

126

Biodegradable and sustainable fibres Cellulose I

Cellulose II

01

01

c

06

c

03

06

03

02¢

05¢

02¢

05¢ 06¢

06¢ 04¢

a

04¢

a

4.13 Hydrogen bond patterns of cellulose amorphs.

a

mm mm

b

0.1mm 0.1 mm

4.14 The structure of isolated cellulose microfibrills of different origin: (a) cotton linters, (b) spruce sulphite pulp.

The course of biotransformation and the final product properties depend both on the type and composition of the enzyme complex and on the cellulose structure such as the average polymerisation degree and its distribution, crystallinity, capillary system and swelling behaviour as well as the physical– chemical parameters of processes such as ratio enzyme activity to cellulose content, time and temperature reactions. The increased susceptibility of cellulose chains is a result of the modified structure and developed intrinsic surface of cellulose obtained by using suitable pre-treatment methods, particularly mechanical processing.37,38


127

It is well known that direct contact between the enzymes and the substrate is a prerequisite to hydrolysis. Since cellulose is an insoluble and structurally complex substrate, this contact can be achieved by diffusion of the enzymes into the cellulose matrix. Any structural feature that limits the accessibility of cellulose to enzymes will diminish the susceptibility of cellulose to hydrolysis.40,41 After pulping, the softwood has a more porous structure than the hardwood because the lignin is removed during this process. The softwood pulp seems to be accessible enough to enzymes after mechanical pre-treatment, whereas the hardwood pulp is less affected and needed additional treatment. The main effect of mechanical shredding is to subdivide cellulosic material into fine particles which are highly susceptible to acid or enzymatic hydrolysis. The smaller particles have a larger surface-to-volume ratio, thus rendering the cellulose more accessible to hydrolysis. In the case of acid pre-hydrolysis of hardwood pulp the creation of micropores by removal of hemicelluloses, a change in crystallinity and reduction of DP have been discovered.42 The significant correlation between the decrease of an average polymerisation degree and increase of cellulose solubility degree in aqueous sodium hydroxide solution has been investigated. The effects of cellulase treatment on average polymerisation degree (DP) and solubility degree (Sa) of cellulose treated by cellulolytic complexes from Aspergillus and Trichoderma strains are presented in Figs 4.15 and 4.16.43

100

610

560 80 DP

Sa %

510

460 60 410

40

360 0

1

2

3

4

5

6

7

Time, h DP(E/S=48UCMC/g

DP(E/S=32UCMC/g)

Sa(E/S=48UCMC/g)

Sa(E/S=32UCMC/g)

4.15 Changes of average polymerisation degree D P and solubility degree Sa vs time of biotransformation process (for cellulase from Aspergillus niger).

128


610

100

DP(E/S = 105 UCMC/g) DP(E/S = 52 UCMC/g) Sa(E/S = 105 UCMC/g) Sa(E/S = 52 UCMC/g)

510

80

Sa %

DP

560

460 60 410 360 0

1

2

3

4

5

6

7

40

Time, h

4.16 Changes of average polymerisation degree D P and solubility degree Sa vs time of biotransformation process (for cellulase from Trichoderma reesei).

The quality and quantity of reducing sugars generated during cellulose biotransformation differ depending on the process parameters, i.e. mainly type of enzyme (mechanism of enzyme action) and time of reaction. The effect of the time of reaction on the amount of reducing sugars realised during the biotransformation process of cellulose using cellulolytic complexes from Trichoderma reesei and Aspergillus niger is demonstrated in Figs 4.17

Reducing sugar content mg/cm3

4 Xylose Glucose Cellobiose Total

2

0 2

4 Time, h

6

4.17 Changes of soluble sugars content vs time of biotransformation process (for cellulase from Trichoderma reesei).


129

and 4.18. The reducing sugars found in enzyme solutions after biotransformation with cellulolytic complex were as follows: ∑ for enzymes from Aspergillus niger: glucose (3.68–6.39 mg cm–3), xylose (0.45–1.68 mg cm–3), cellobiose (lack) and total amount (4.15–8.01 mg cm–3); ∑ for Trichoderma reseei: glucose (0.87–1.38 mg cm–3), xylose (0.15–0.68 mg cm–3), cellobiose (1.23–1.98 mg cm–3) and total amount (2.25–3.59 mg cm–3).

Reducing sugar content mg/cm3

8

Xylose Glucose Total

6

4

2

0 2

4 Time, h

6

4.18 Changes of soluble sugars content vs time of biotransformation process (for cellulase from Aspergillus niger).

The sugar content and composition of solubilised carbohydrates of cellulose treated by purified cellulases from Trichoderma reesei have been estimated (see Table 4.3).42 When the cellulases were compared at the same protein dosages, both endoglucanases (EGI and EGII) were able to hydrolyse pulp more efficiently than the cellobiohydrolases (CBHI and CBHII). With an enzyme dosage of 2.5 mg g–1 the hydrolysis yields of EGI and EGII were 1.6% and 1.4% of pulp, corresponding to 1.36% and 1.47% of initial cellulose solubilised, respectively (based on the original carbohydrate composition of the pulp). CBHI and CBHII solubilised 0.49% and 0.81% of pulp, respectively, with the same dosage (see Table 4.3). Neither overall degradation nor saccharification of cellulose is a required phenomenon in the biotransformation process of pulps. The controlled

130


Table 4.3 Composition of solubilised carbohydrates of the reference and enzymetreated cellulose Enzyme

Dosage (mg g–1)

Solubilised sugars (% of dry weight)

Sugar composition of solubilised carbohydrates (% of dry weight) Glucose

Xylose

EGI

0.1 0.5 2.5

0.19 0.70 1.60

0.12 0.54 1.33

0.07 0.16 0.27

EGII

0.1 0.5 2.5

0.22 0.65 1.44

0.16 0.62 1.44

0.06 0.03 0.00

CBHI

0.1 2.5 5.0

0.15 0.49 0.89

0.13 0.49 0.89

0.02 0.00 0.00

CBHII

0.1 2.5 5.0

0.21 0.81 1.28

0.16 0.77 1.28

0.05 0.04 0.00

Control

0.0

0.11

0.03

0.08

degradation of cellulose chain (reducing DP to below 400) and changes in supermolecular and morphological structure allow preparation of directly alkali-soluble cellulose applied in the Celsol process.41 Structural characteristics of biotransformed cellulose are presented in Tables 4.4 and 4.5. Table 4.4 Molecular weight distribution of enzyme-transformed pulp Type of pulp

Mn (kDa)

Mw (kDa)

DPw

Pd

Percentage content of DP fraction < 200

200–550

>550

Initial softwood pulp

47

119

740

2.6

17

39

44

Biotransformed pulp

25

64

395

2.5

40

42

18

The biotransformation of pulp causes, in comparison to the initial pulp, several changes as follows: decrease in the average polymerisation degree and crystalline degree, reduction of hydrogen bonds, increase in intrinsic surface and increase in the solubility in aqueous sodium hydroxide.


131

Table 4.5 Characteristic of supermolecular and morphological structure of enzymetransformed pulp Type of pulp

Crystalline Energy of degree hydrogen bond EH (kJ mol–1)

Total Total Intrinsic Solubility volume porosity surface degree of pores (%) (m2 mm–3) in 9% (mm3 g–1) NaOH (%)

Initial softwood pulp

0.52

20.0–22.0

253.8

6.09

4.9

17.0

Biotransformed pulp

0.48

12.9–15.1

316.4

9.17

9.6

99.5

4.4

Biodegradability of cellulose fibres in textile blends

Large quantities of textile wastes consisting of blended polyester and cellulose fibres such as cotton and viscose fibres are produced in the textile industry. Most of such waste is discarded or converted into wiping cloths of very little commercial value. On one hand, due to the storage of textile fibres the recycling and conversion of such wastes into useful products has become a very important problem. On the other hand, the reuse of such blends composed of polyester and cellulose fibres is a particularly difficult problem due to the unique properties of these two types of fibres. During recent years there has been a growing interest in the problem of textile waste utilisation involving polymeric blends, in particular of recycling and conversion of such wastes into useful products.44–46 Various studies performed in this field have been dictated, by ecological considerations, which are one of the most important technological problems in contemporary industry. Well-known methods of utilisation of textile wastes including polyester–cellulose blends such as chemical processing, storage in waste dumps, or burning are characterised by large hazards to the natural environment due to toxic gas emission or sewage seepage into the water table.47 The alternative method for utilisation of such wastes is a biotechnological method based on application of cellulolytic enzymes, catalysing the hydrolytic degradation of cellulose to mixture of mono and oligosaccharides.48 In the saccharification cellulose process, the cellulolytic complex originating from Trichoderma reesei fungi is especially advantageous. Because of the high specificity of the enzyme, it is possible to obtain a well-defined final product of decomposition of a natural polymer in the form of low-molecular products (glucose, cellobiose, oligosaccharides).49,50 These products could be applied in some chemical processes such as production of ethyl and methyl alcohol or methane.51,52 The rate of enzymatic hydrolysis depends on the structural features of cellulose, as well as on the composition of the cellulolytic complex. Structural features such as crystallinity and accessible surface area determine the

132


susceptibility of cellulose to enzymatic degradation. Macromolecules of cellulose of low-ordered regions are hydrolysed easier than those of highordered regions. As should be expected, the key to success in increasing the rate and yield of the biodegradation process is the breakdown of the crystalline structure of cellulose or surface displaying, making it accessible to enzyme action. It has been found that mechanical pre-treatment affects the increasing absorption ability of enzymes on the cellulose surface.53 An increase in accessibility is a result of the opening up of the cellulose structure and its internal surface enlargement. Although many chemical pre-treatments for enhancing biodegradation of cellulose are based on treatment with Cadoxen, sulphuric acid, phosphoric acid, sodium hydroxide54,55 or hydrogen peroxide56 generate partial recrystallisation of cellulose and influence an increase in efficiency of the enzymatic cellulose hydrolysis, these processes involve the application of harmful reagents and the necessity of by-product inactivation. Utilisation of textile blends is performed according to the scheme presented in Fig. 4.19.57 Polyester/cellulose textile waste blend

Mechanical/chemical treatment

recirculation Enzymatic degradation of cellulose component

Soluble sugars

Polyester component

Biotechnological processes

Recycling

Enzyme

4.19 Scheme of bioutilisation of textile waste blends composed of cellulose and polyester fibres.14

Application of scanning electron microscopy (SEM) allows the observation of appearance of fibres and their changes during the enzymatic degradation (see Fig. 4.20). As a result of the enzymatic degradation process for the polyester–cellulose fibrous blends, the degree of saccharification of cellulose fraction was achieved at a level up to 99 wt.%. The final product resulting from the biodegradation process was composed mainly of the polyester fibres with a relatively low content of residual cellulose fibres. The mechanical properties of the polyester fibres isolated from the blend after enzymatic treatment are comparable to the standard PET fibres


133

4.20 Scanning electron microscopic image of the polyester–cellulose blend: (a) initial blend, (b) after biodegradation of cellulose fibres.

(see Table 4.6). The results of experiments indicated that the cellulose biodegradation process did not essentially affect the properties of polyester fibres.58 Table 4.6 Some properties of isolated polyester fibres15 Parameter

Unit

PET fibres standard

Residual PET fibres

Average molecular weight Linear density Breaking force Tenacity Elongation Coefficient of variation of breaking force

g mol–1 dtex cN cN/tex % %

16 900 3.65 13.4 36.7 54.8 14.4

16 100 3.50 12.9 36.9 62.5 13.4

After separating the natural component from the blend, the remaining polyester may be garnered to obtain staple fibre products, for the manufacture of nonwoven products, or may be recycled in the melt spinning method. Formation of filament or films occurs without problems when the product of degradation is used as an addition to the standard polyester material.58

4.5

Biotechnology for manufacture and modification of cellulosic fibres

4.5.1

Bioprocessing of cellulose for new chemical fibres

An increasing interest in the protection of the natural environment was manifested at the close of the twentieth century and continues in the early twenty-first century. Many different industrial technologies are characterised by the emission of highly dangerous substances, e.g. carbon disulfide and hydrogen sulphide, which continue to poison the environment.

134


Due to the environmental risk caused by the use of carbon disulphide in the viscose process, attempts have been made to develop new types of processes for fibre production from cellulose, such as pulp directly dissolved in organic solvents, e.g. N-methylmorpholine-N-oxide (NMMO)59 application of cellulose derivatives such as cellulose carbamate60 and dissolving pulp in aqueous sodium hydroxide (biotransformed pulp).61 Currently the cellulose dissolving methods, which are used on an industrial scale, are cuprammonium, viscose and NMMO methods. In the two first methods, a certain cellulose derivative has to be formed prior to dissolution. Accordingly, regeneration is necessary for converting this cellulose derivative to the so-called regenerated cellulose. Both processes are energy intensive, which to a large extent cancels out the potential savings obtainable from the use of cellulose as a renewable raw material. Further, both the cuprammonium process and the viscose process produce effluent streams containing significant amounts of toxic materials, which must be removed before the effluent can be disposed of. In the NMMO process, cellulose is dissolved directly in Nmethylmorpholine-N-oxide at high temperature and the fibres are formed by extruding the solution into an aqueous solution of NMMO. The solvent is almost completely recycled making the process more ecological than the cuprammonium and viscose processes. However, the solvent is expensive and the plant requires high founding costs. Many cellulose-dissolving methods, which are not utilised industrially, have been discovered including the use of different metal complexes or organic solvents. However, these methods are not superior to the industrial methods because of the use of toxic components such as heavy metals and amines or complicated multi-component solvent systems, which are very expensive and from which the solvents are difficult to recover. Thus, these methods are disadvantageous from an environmental and economical viewpoint. Apparently, it is extremely difficult to dissolve cellulose safely in a simple and cheap solvent and accordingly, there exists a need for a process capable of manufacturing cellulosic products economically and in an environmentally acceptable manner. This is especially important for improving sustainability and for reducing environmental and health impacts of the industry producing shaped cellulosic articles. The benefits of biotransformed cellulose pulp (discussed in Section 4.3) in connection with its technological simplicity and economical effectiveness promote this biotechnological method as the most useful way of processing pulp into fibres. The modification of cellulose structure during biotransformation has definitely improved the main properties of cellulose pulp such as solubility in aqueous sodium hydroxide and chemical reactivity leading towards obtaining: ∑ direct soluble pulps (Celsol), with solubility in an aqueous sodium hydroxide; solution up to 99.5 wt %,62–64


135

∑ highly soluble pulps, with improved solubility in NMMO;65,66 ∑ highly reactive pulps with improved reactivity toward reactants such as acetic anhydride.67 There are great opportunities to obtain the most reactive biotransformed cellulose, which can be utilised for preparation of cellulosic products such as fibres, films, derivatives, sponges, etc. The enzymatic transformation of pulps into direct soluble cellulose (Celsol) is shown schematically in Fig. 4.21. Pulp

Biotransformation

Pretreatment

Recycled Enzyme Purification of product

Dissolving pulp Celsol

Technical products (fibres, foil, sponges, beads)

Derivatives

Nonwoven in NMMO

4.21 Scheme of biotransformation process.

From the samples of biotransformed cellulose characterised by a dissolution degree of cellulose in the aqueous sodium hydroxide of over 99%, alkali spinning solutions have been prepared. The characteristics of performed alkali solutions are presented in Table 4.7.68 Table 4.7 Some properties of alkaline solution of biotransformed pulp Parameter

Biotransformed pulp

a-cellulose content (wt%) Alkali content (wt%) Viscosity (s) Ripeness degree (∞H) Stability of solution at 5∞C (h)

5–7 7–8 40 – 100 8 – 10 48

The spinning of regenerated cellulose fibres (Celsol)69 using an alkaline solution of biotransformed pulp and acidic regeneration bath, to produce the

136


fibres with high purity and sorption capacity, produces fibres with a tenacity value of 19 cN/tex. The affectivity of fibre and film production using a biotransformation process is generally on a level of 93–95% to the weight of initial pulp. Some properties of regenerated cellulose fibres spun from alkali solutions of biotransformed pulp are presented in Table 4.8. The SEM photo of the fibre’s cross-section is shown in Fig. 4.22. The fibres are characterised by an oval cross-section with little developed boundary line; they are regular and show no sticking. There are great opportunities to improve this property during the development stage of the discussed method. It is well known that several parameters including spin additives, coagulation time and temperature, spin bath chemicals, spinneret type, jet draw, finishing and stretch have significant effect on the fibre properties. These parameters have not been developed sufficiently yet. Table 4.8 Some physical–mechanical properties of Celsol fibres Parameter

Celsol fibres

Titre (dtex) Breaking force (cN) Tenacity (cN/tex) Elongation at break (%)

2.55 4.95 19.4 13.0

4.22 SEM photos of Celsol fibres – cross-section.


4.5.2

137

Microbial synthesis of cellulosic fibres

The bacteria belonging to the genera of Acetobacter, Agrobacterium, Rhizobium, Pseudomonas or Alcaligenes show ability for cellulose biosynthesis (see Table 4.9).70–74 The synthesis of cellulose in Acetobacter xylinum – the best producer, occurs between the outer membrane and cytoplasma membrane by a cellulose-synthesising complex, which is in association with pores at the surface of the bacterium (see Fig. 4.23).75 The cellulose synthase is considered to be the most important enzyme in this process. A biochemical pathway from glucose to cellulose is presented in Fig. 4.24.76 The cellulose is produced in the form of microfibrils synthesised at the bacterial surface at sites external to the cell membrane. The microfibrils released outside the bacterial cells are joined together by strong hydrogen bonds and form a gelatinous film on the surface of the nutrient medium in the static method and an unwoven fabric in the dynamic method. The microfibrils of bacterial cellulose are composed of pure cellulose, which is devoid of lignin, hemicellulose, and other substances. Table 4.9 Bacterial cellulose producers75 Organisms (genus)

Cellulose produced

Biological role

Ref. in 75

Acetobacter

Extracellular pellicle Cellulose ribbons

To keep in aerobic environment

(1, 2, 8)

Achromobacter

Cellulose fibrils

Flocculation in wastewater

(1–3)

Aerobacter

Cellulose fibrils


(2, 3, 8)

Agrobacterium

Short fibrils

Attach to plant tissues

(2, 3, 8)

Alcaligenes

Cellulose fibrils


(1–3)

Pseudomonas

No distinct fibrils


(1–3, 8)

Rhizobium

Short fibrils

Attached to most plants

(1, 8)

Sarcina

Amorphous

Unknown

(5, 8)

Zoogloea

Not well defined


(2, 3)

Source: reprinted from Polymer Degradation and Stability, vol. 59, Rainer Jonas and Luiz F. Farah, ‘Production and application of microbial cellulose’, 101–106, copyright (1998), with permission from Elsevier

Two types of production methods are used at present: static, where the cellulose net is growing on a surface of nutrient solution, and dynamic, being under intensive studies. The method used for preparation of bacterial cellulose

138

Biodegradable and sustainable fibres 1.5 nm sub-elementary fibril

Ribbon

3.5 nm pore

LPS envelope Periplasmic space

10 nm particle

Plasma membrane b(1, 4) glucan polymerizing enzymes

4.23 Scheme for the formation of bacterial cellulose75 (reprinted from Polymer Degradation and Stability, vol. 59, Rainer Jonas and Luiz F. Farah, ‘Production and application of microbial cellulose’, 101–106, copyright (1998), with permission from Elsevier).

Membrane

Nonactivated cellulose synthase

2Pi PPi 2 GTP

PDE PDE A B Mg ++

Diguanylate 2Pi cyclase Mg ++ PPi pppGpG I

Ca++

pGpG

Activated cellulose synthase

pGpG 25¢ GMP III pGpG

II

UDP

4.24 Model for regulation of cellulose biosynthesis in A. xylinum76 (reprinted from Polymer Degradation and Stability, vol. 59, E.J. Vandamme, S. De Baets, A. Vanbaelen, K. Joris and P. De Wulf, ‘Improved production of bacterial cellulose and its application potential’, 93–99, copyright (1998), with permission from Elsevier).

(BC) depends on the strain type. Generally, the Acetobacter strains are most suitable for static (emers) cellulose biosynthesis. However, this method is not effective with respect to the yield of the final product. Therefore, the dynamic (submers fermentation) method has to be considered. The most important question of this method is to have stable bacteria strains because an unstable strain produces spontaneously inactive bacteria (Cel–). Therefore, the preparation of highly effective bacteria strains by mutagenisation and genetic manipulation improves the dynamic production of BC. The best pilot-scale world producer of BC is Weyerhauser Co., Tacoma, WA, USA in cooperation with Cetus Corp., Emerville, California, USA. The main benefits for BC, generally different from a native plant-produced


139

cellulose pulp, are connected with its extraordinary properties and behaviour summarised in the following: high purity and crystallinity, high sorption capacity and porosity, high intrinsic surface and adhesivity, as well as high tenacity and Young’s modulus. Mechanical properties of bacterial cellulose pellicle can exceed by far those of usual synthetic polymers and of regenerated cellulosic (cellophane) films.77 Based upon these special properties, BC is proposed to be used for several applications in papermaking, textile, food, electronic industries as well as medicine. Taking advantage of the high modulus, Sony Co. is using bacterial cellulose for manufacture of sound membranes. BC having non-allergic behaviour can be used as wound healing dressings or artificial skin. The highly developed intrinsic surface and sorption ability of BC allows the application of this material for special paper preparations or as a carrier in biotechnology. A list of potential applications of bacterial cellulose (most of them are patented) is presented in Table 4.10.75,78–92 Table 4.10 Application of bacterial cellulose75 Material

Application

Reference

Temporary artificial skin Biofill®, Bioprocess®, Gengiflex®

Therapy of burns, ulcers, dental implants

(54, 55)

Nonwoven paper or fabric

Improvement of latex or other binders Repairs of old documents

(56)

Sensitive diaphragms

Stereo headphones

(8)

Cellulose

Immobilisation of proteins, chromatographic techniques

(51, 58–60)

Edible cellulose

Addition to food

(61–63,67)

Cellulose

Stabiliser of emulsions in cosmetics, food

(64, 65)

Cellulose

Coating compositions

(66)

(57)

Source: reprinted from Polymer Degradation and Stability, vol. 59, Rainer Jonas and Luiz F. Farah, ‘Production and application of microbial cellulose’, 101–106, copyright (1998), with permission from Elsevier

In recent years, many different theories about cellulose biosynthesis can be observed and it has been found that cellulose is produced from a wide range of substrates, e.g. glucose, fructose, mono and disaccharides, starch hydrolysates, molasses, and others. The capacity for biosynthesis as well as the properties of biopolymers produced is strongly dependent on strain productivity, medium composition, and culture conditions. The biosynthesis of modified cellulose is carried out using culture media composed of

140


different substrates and other additives, i.e. natural and synthetic polymers and fibres. Blends of BC and chitosan and/or derivatives thereof have been studied quite intensively under various aspects, including determination of mechanical properties as well as swelling and water retention, development of pharmaceuticals and drug release formulations, design of biodegradable plastics, non-woven fabrics, fibres and membranes, for use in speciality paper manufacture and for applications in biotechnology. The novel bacterial cellulose with new functions (especially with chitinous properties) and higher susceptibility for chitinolytic enzymes is produced on the surface of a liquid medium by Acetobacter bacteria adapted to a medium containing oligo- and poly-aminosaccharides. The modification of bacterial cellulose consists in introducing the glucosamine monomeric unit into the cellulose chain, in consequence of the degradation of oligo- and polyaminosaccharides which appears in the culture medium, as well as in the association of these mentioned agents with the cellulose fibres.93,94 Comparing the FTIR spectra (see Fig. 4.25), the peaks for chitosan-modified bacterial cellulose at 1650 cm–1 (amide I) and 1560 cm–1 (amide II), attributed to amide groups characteristic for chitosan, have been found. SEM photographs of chitosan-modified bacterial cellulose are shown in Fig. 4.26. The chitosan-modified bacterial cellulose, characterised by unique properties, i.e. bioactivity, biodegradability, biocompatibility, no toxicity and non-allergic action, connected with good mechanical tenacity, has been found to be a great material for biomedical applications such as dressings for wound healing.95 The application of chitosan-modified bacterial cellulose for manufacturing of acoustic diaphragms allows the manufacture of new loudspeaker constructions with special, unique parameters, including: the mid-tweeter with a very wide sound transmission range (from 580 Hz up to 22 kHz), which replaces mid-range and tweeter loudspeakers. Practically, membranes made from chitosan-modified bacterial cellulose can be used in all types of electro-acoustic transducers, i.e. earphones, hearing aids, microphones, alarm buzzers, etc.96

4.6

Enzyme applications in fabric and dyestuff processing

4.6.1

Cellulose fibres

Cellulases are increasingly being used in the textile industry. Their most successful application is in producing the stone-washed look of denim garments.97–102 Other processes that improve fabric appearance by removing fuzz fibres and pills or deliver softening benefits have also been introduced.


141

0.6

0.4 0.3 0.2

Absorbance

0.5

0.1

1800

1750

1700

1650 1600 1550 Wave number [cm–1]

1500

1450

0 1400 1.4 1.0 0.8 0.6 0.4

Absorbance

1.2

0.2 3650

3150

2650 2150 1650 Wave number [cm–1]

1150

0.0 650

Unmodified bacterial cellulose Modified bacterial cellulose MBC/M Modified bacterial cellulose MBC/O

4.25 FTIR spectra of unmodified and modified bacterial cellulose (static culture).

4.26 SEM photographs of modified bacterial cellulose: (a) sample MBC/O and (b) sample MBC/M.

142


Increased use is also being made of cellulases in domestic fabric washing products where they are claimed to aid detergency and to clean fibre surfaces, improving appearance and colour brightness. Nowadays, these finishing and washing effects represent the largest market for cellulase enzymes worldwide. Scouring and bleaching of cotton fabrics are also attractive targets for enzyme-based processes, due to the severe environmental impact of these processes. These processes are also very energy demanding. Raw cotton contains about 10% of impurities. Pectins, waxes and coloured components can all be partially removed from raw cotton by enzymatic treatments but the residual seed coating remains a problem. Cellulase treatment of cellulose materials, such as cotton, viscose, lyocell, cupro or polynosic fabrics and their blends, has gained increasing interest with growing consumer and industrial concern about environmental issues. The best known applications of cellulases are in denim, garment washing, biostoning, as an alternative to stone washing and in surface modification of cotton fabrics, biofinishing, to improve the surface properties. Cellulase enzymes can replace the pumice stones and result in less damage to the cloths, machinery and environment. In biofinishing cellulases remove fuzz from the surface of cellulose fibres, which eliminates pilling, making the fabric smoother and cleaner-looking. This technique is particularly promising for the new generation of solvent spun cellulose fibres such as Tencel and lyocell. The exploitation of novel cellulase processes for the textile industry has been enabled by the development of molecular engineering techniques. Enzymatic treatment with amylases has replaced the harsh processes since the beginning of the twentieth century, and is still used as a BAT in desizing.103 The process characteristics are suitable for existing machinery and production speeds; however, research for the improvement of economics and consistency of the process including use of heat stable enzymes is continuously carried out. The greatest number of enzymatic treatments has been applied to industrial processing of cellulosic fibres for obtaining new finishing effects or to replace harsh chemicals used in conventional cotton processing. Conventional industrial processing of cotton may include several chemical steps during wet processes, which can be partly replaced by enzymatic processes, as shown in Fig. 4.27. The main categories additional to desizing include biopreparation, biopolishing, biofinishing, and biostoning of cotton and other natural cellulosic fibres. These processes are resulting in smoother, glossier or shaded appearances of the fabric. The loss of mechanical properties of fabrics can be reduced by using purified, special enzymes.104 Cotton processing provides several possibilities for enzyme applications. The removal of cotton impurities is essential for fabric manufacture and is generally carried out by scouring with alkali. However, environmentally harsh scouring can be replaced with an enzymatic treatment, in which cotton impurities, such as protein, wax, pectin and ash, can be efficiently removed

Cellulosic fibres and fabric processing Raw material

Dry processing

Fibres

Spinning

Yarn

Sizing Weaving Singeing

143

Wet processing Greige fabric

Desizing Scouring Bleaching Mercerisation

Finished fabric

Dyeing Finishing

Amylases Biopreparation

Biopolishing Biofinishing Biostoning

4.27 Stages of cotton processing.

prior to further processing.105 Cotton fibres have been treated with different pectinases, proteases, laccases and lipases whereafter the effect of the enzymatic action was analysed using HPLC, GC, ESCA, titration, wetability and metal content measurements. The bioscouring process using the most potential enzyme mixture has been compared to the traditional alkaline scouring with respect to yarn or fabric processability, brightness and strength. Enzymatic and simple buffer treatments in the presence of a non-ionic surfactant improve water wetability of fabrics to a level equal to alkaline scouring.106 Most fibres made from regenerated cellulose such as viscose, lyocell, and Celsol are characterised by stiffness as well as a fuzzy and uneven surface that makes fabrics susceptible to pilling, even over a short period of use. In order to modify the surface properties of cellulosic fibres and fabrics and to improve their quality biotechnological approaches based on specialised enzymes are widely used. Finishing processes, employing cellulases and xylanases, can replace a number of mechanical and chemical operations, which have been applied until now to improve comfort and quality of fibres and textiles. The principle of enzyme action in the finishing process is controlled hydrolysis of cellulose, in which impurities and fuzz are removed from the surface of fibres, without decreasing their mechanical tenacity or the elasticity of the fabric. Biofinishing of cellulosic fabrics is limited on controlled removal of fibre hairiness, pilling or other non-desired properties after dyeing. Enzymes – at present – are not capable for modifying or chemically cross-linking of cellulose polymers. Some effects on improvements on dimensional stability, however, have been observed. An overview of patents and research on the treatment of cotton with modified or purified cellulases is presented in Table 4.11. The

144

Biodegradable and sustainable fibres Table 4.11 An overview of the patents and research on denim with modified or purified cellulases15 Enzymes used

Analyses

Results

Trichoderma longibranchiatum free of CBH type components

Evaluating stone washing appearance

Reduced backstaining

T. reesei TC* + EGI, EGII, CBHI and CBHII

Weight loss, tensile strength, lightness units, blueness units and colour difference of denim fabrics

Elevated EGII contents increased stone washing effect

T. reesei, TC*, Humicola insolens, EGV, EGV-core Cellulomonas fimi, CenA-core

Staining levels, effect of mechanical action

Cellulases without CBDs and mechanical action caused less backstaining

a-amylase, EG V and EG III from e.g. Scytalidium (f. Humicola), Fusarium, Myceliophthora, Trichoderma

Evaluating streaks and stone-washing appearance

A one-step process for combined desizing and stone washing

*TC: total crude cellulase

biostoning process of denim can be carried out with some alternative enzyme preparations achieving reduced backstaining, increased effect, and combined effect of desizing and stone-washing, as presented in Table 4.12. Results of biofinishing and biopolishing are dependent on enzymes and process conditions applied. The effects of different dosages of various enzymes on cotton interlock knitting are shown in Table 4.12. The dosage and type of enzyme are important in treatments and effect on desired properties, such as lowering the pilling tendency, as seen in Table 4.13. The pilling value improvement of a cotton knitted fabric can be raised from 2–3 to 5. At the same time, however, weight and strength loss should be minimised. Purified cellulase EG II has been showing lowest damaging effect and highest improvement of the pilling value.

4.7

Hygienic and medical fibres

4.7.1

Cellulosic fibres for medical and health-care products

Viscose fibres are increasingly used for hygienic and medical applications. In Europe the share of hygienic and medical applications among nonwovens

Table 4.12 An overview of enzymatic textile applications investigated and their commercial status15 Microorganism

Enzymes

Aim of the study

Properties measured

Fibres

T. reesei

CBHI, CBHII, EGI

Effects on spinnability

Microscopic analysis, spinnability

CBHI, EGII

Evaluate fabric properties

Yarn evenness, tenacity hairiness, pilling

EGV + core

Effect of agitation, binding

Adsorption/desorption, weight loss

Yarns

Poplin fabric

Humicola insolens Cellulomonas fimi

CenA + core

Cellulomonas fimi

CBD

Binding

Dye affinity, washing fastness, migration, strength loss, reducing ends

Fabric

Trichoderma longibrachiatum

Cellulase mixture free of CBHI

Provide an improved cellulase composition; decrease strength loss

Strength loss, hand, appearance colour enhancement, softness, stone-washed appearance

Fabric

Fungal cellulase

CBH1enriched

Provide an improved cellulase composition; decrease strength loss

Strength loss, hand, appearance colour enhancement, softness stone-washed appearance

Fabric

Humicola insolens

Monocomponent, 43 kD endoglucanace

Improved pilling

Pilling, weight loss

145

Fabric


Substrate

146

Table 4.12 Continued Microorganism

Enzymes

Aim of the study

Properties measured

Fibres, linters

T. reesei

CBHI, CBHII, Endo-2

Synergistic action

Irpex lacteus Aspergillus niger

EX-1, En-1 Exo-A, EG-1

Reducing sugars, DP, thin-layer chromatography, electron microscopy

Fabric

T. reesei

TC*, CBH-rich, EG–rich

Adsorption

Cellulase activity, adsorption

Fabric Indigo dyed

T. reesei Humicola insolens Cellulomonas fimi

TC* EGV, EGV-core

Influence of cellulases on indigo backstaining

Staining levels, effect of mechanical action

CenA-core

Fabric, woven and knitted

T. reesei

Over-producing strains (CBH II rich or EG II rich) + purified EGI, EGII, CBHI and CBHII

Provide an improved cellulase composition for treating cellulose containing textiles

Pilling, weight loss, tensile strength, visual appearance, colour

Farbic, woven and knitted

T. reesei

TC*, EG-rich

Optimising the use of cellulases in finishing cellulosic fabrics

Pilling, weight loss, tensile strength, drapeability

Fabric knitted

Aspergillus

TC*, endoenriched, monocomponent acid endoglucanses

Optimisation of biopolishing

Pilling, weight loss, strength loss


Substrate

Table 4.12 Continued Microorganism

Enzymes

Aim of the study

Properties measured

Fabric woven

T. reesei

TC*, CBH-rich, EG-rich

Effects of agitation on the adsorption– desorption behaviour

Weight loss, strength loss, softness, shear and bending hysteresis, bound protein, reducing sugars

Fabric, woven and knitted

T. reesei

TC*, CBH-rich, EG-rich, genetically modified strains

Improve dimensional stability of fabrics

Weight loss, strength loss, dimensional stability


Substrate

147

148


Table 4.13 Effect of dosage of various cellulases on cotton interlock properties Sample

Dosage mg/g

Weight loss %

DNS % of d.w.

Strength loss %

Pilling value

EGII

5 2.5 1.0 0.5 0.25 0.1 0.01

1.9 (+/–0) 1.4 (+/–0.1) 1.0( +/–0) 0.7 (+/–0) 0.5 (+/–0.1) 0.4 (+/–0) 0.4 (+/–0.1)

0.27 0.20 0.17 0.13 0.10 0.07 0.04

13.7 5.4 6.4 1.4 4.6 2.4 1.8

5 5 5 5 4 4 3–4

Biotouch L

5 1.0 0.5 0.25 0.1 0.01

2.9 1.1 0.9 0.6 0.6 0.4

(+/–0.2) (+/–0.1) (+/–0) (+/–0.1) (+/–0.1) (+/–0.1)

1.53 0.71 0.47 0.28 0.19 0.05

8.9 8.2 5.5 4.0 4.3 3.7

5 4 4 3–4 3–4 2–3

Cellulase E

1.0 0.5 0.25 0.1 0.01

1.4 1.2 0.7 0.6 0.5

(+/–-0.3) (+/–0.3) (+/–o) (+/–0.1) (+/–0.1)

0.44 0.32 0.22 0.14 0.04

6.2 7.5 7.9 7.2 3.3

5 5 4–5 3–4 2–3

Cellulase F

1.0 0.5 0.25 0.1 0.01

(+/–0.1) 0.7 (+/–0.1) 0.6 (+/–0) 0.4 (+/–0.1) 0.3 (+/–0.1)

0.68 0.47 0.30 0.16 0.04

5.9 4.9 10.3 7.2 0.6

5 5 4 3–4 2–3

Ref.

–

0.2 (+/–0.1)

0.04

–

2–3

is around 39%.107 Main products need to show high water absorbency properties and safety when used in contact with skin. Thus, the main materials are viscose for water absorbency and polypropylene for bonding and reinforcing. Biodegradability and easy composting/incineration possibilities for waste handling offer further advantages for viscose and regenerated cellulose fibres. Main application areas can be classified as follows: ∑ Hygiene products: baby nappies, bandages, sanitary napkins, tampons, incontinence products, cleaning waddings, cleaning wipes: ∑ Medical products: absorbents, wound dressings, cleaning wipes. Microbial synthesis of cellulosic fibres affords the opportunities to create products with unique properties suitable for practical application in human and veterinary medicine. Due to its high water absorption capacity, high mechanical strength in the wet state, substantial permeability for liquids and gases, wet cellulose can be used as a temporary artificial skin to treat severe skin burns. Bacterial cellulose somehow seems to enhance the growth of human skin


149

cells. Biofill® and Gengiflex® are products of bacterial cellulose that now have wide applications in surgery and dental implants and realities in the human health-care sector.75,108 The authors documented the following advantages for Biofill in more than 300 treatments: immediate pain relief, close adhesion to the wound bed, diminished post-surgery discomfort, reduced infection rate, easiness of wound inspection (transparency), faster healing, improved exudates retention, spontaneous detachment reepithelisation, and reduced treatment time and costs. The disadvantage was limited elasticity in areas of great mobility. Gengiflex® was developed to recover periodontal tissues.109,110 The application of bacterial cellulose (Cellumed) in veterinary medicine to treat recent, large surface wounds on horses has been investigated.111 In experiments with dogs biosynthetic cellulose was also successfully applied to substitute the dura matter in the brain.112 Novel biological hydrogel wound healing dressings made from chitosanmodified bacterial cellulose has been performed.113,114 Bacterial cellulose modified with chitosan combines the beneficial, from a medical point of view, properties of the two biopolymers providing an excellent dressing material. Poly-aminosacharides including chitosan and its derivatives have excellent biostimulating properties which ease the recovery of infected tissue and prevent the formation of large scars. A specific feature of chitosan, essential for its medical application, is biological activity resulting from the susceptibility to degradation influenced by lysozyme, an enzyme present in tissue fluids. Mono- and oligo-aminosaccharides, being the products of enzymatic chitosan degradation, stimulate tissue granulation which, in turn, accelerates the wound healing. The dressings from modified bacterial cellulose have an advantage over similar products mainly in the antibacterial activity of protecting the wound against secondary infections. In addition to the bacterial cellulose method, a hydro-thermal method for manufacture of cellulose micro- and nano-scale fibres (fibrils) has been developed.115,116 This process is competitive with commercially applied processes for its simplicity and ecological advantage.

4.7.2

Tissue engineering

The worldwide market for biomaterials is estimated to be 725 billion, growing approximately 10 to 15% per annum.117 The application areas of biomaterials consist of low-tech biomaterial-based prosthetic devices used in the fields of orthopedics, dental, drug delivery, cardiovascular and ophthalmology and high-tech biomaterials with applications in tissue and bone regeneration, wound healing, bio-adhesives, radio-therapeutics and cosmetic surgery fields. The growth of cardiovascular biomaterial business has shown a positive tendency of 12–14% p.a., and the tissue engineering business has even higher growth figures of 10–25%.

150


The main present applications of biomaterials within tissue engineering are: ∑ Tissue regeneration: – bio-engineered tendons/nerves; – artificial skin. ∑ Wound management: – absorbents/wound dressings; – bio-films/membranes/barriers; – tissue regeneration agents; – artificial skin; – hemostatic agents. Instead of cellulose, collagens have many inherent properties that make these proteins ideally suited as a biomaterial for tissue engineering. In living organisms, cellulose as a high molecular weight fibril acts as a reinforcing material; however, in human tissues this is normally carried out by collagen fibrils. To date, only collagen type I has been used in tissue engineering applications. This is because it is the only collagen available in sufficient quantity and human material, as well as the rarer types of collagen, has been essentially unavailable. A recombinant collagen technology has been developed which allows, for the first time, use of the appropriate collagen for the tissues undergoing repair. Materials usually applied in tissue engineering are organic hydrogels of poly(N-isopropylacrylamide), poly(N-p-vinylbenzylO-b-D-galactopyranosyl-D-gluconamide), poly(glucosyloxyethyl methacrylate), collagens, gelatin, chitosans, and alginate.118 For the implantation applications of cellulose a high molecular weight fibrous materials are not possible due to be low biodegradability. However, some cellulose derivatives, nano-fibres, and biodegradable fibres of low molecular weight could possibly be used in composite 3-D structures for tissue engineering of skin, cartilage and bone with poly(lactic acid) (PLA), poly(glycolic acid) (PGA), collagen and inorganic additives.119,120 Some anti-inflammatory or anti-thrombogenic surface treatment with heparin or chitosan for various structures is often needed. Electrospinning is a technique for obtaining nano-scale fibres from various polymer solutions, including PLGA, poly(lactide-b-ethylene glycol) (PLA-b-PEG) diblock copolymer and PLA.121,122 Cellulose solutions are needed for manufacture of nano-scale structures and coatings on films or on fibrous materials enables the application of double-level structures for tissue engineering, as presented in Fig. 4.28.

4.8

Future trends

Due to the widespread availability of natural raw materials, and the present limitations of cotton cultivation, there is a high demand for simple, ecological


Electrode

Capillary

151

150–250 mm (d)

Single jet

HV power supply Taylor cone Collection screen

4.28 The principle of electrospinning and structure of nonwoven by SEM photographs: thick fibres viscose and nanofibres (150–200 nm) PVA.

and economic processes for production of cotton-type man-made cellulose fibres with similar or better properties. The production of synthetic fibres, especially polyesters and olefines, has reached a share of two-thirds of a total consumption of 100–120 million metric tons of textile fibres within 40 years. The petrochemical industry, however, has to reduce the consumption of crude oil for fuels in order to keep sufficient raw material reserves for chemicals and fibres. When managing it successfully, and taking into account a slight increase in cotton production, a demand of 10–15 million tons of cellulose fibres remains in order to produce all varieties of clothing textiles. It is very important to apply new technology and biotechnology for developing feasible processes for future cellulose fibres although the technology may yet return to its basic principles developed in the nineteenth century.

4.9

References

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51. Ranten, R. and Albrecht, R., Membrane Processes, J. Wiley & Sons, Chichester, 1989. 52. Lutzen, H.W., et al., Cellulases and Their Application in the Conversion of Lignocellulose to Fermentable Sugars, Phil. Irans. R. Soc., London, 300, 283, 2002. 53. Caufield, D.F. and Moore, W.E., Wood Science, 6, 375, 1974. 54. David, C., Fornasier, F. and Thiry, P., Eur. Polym. J., 22, 7, 515, 1986. 55. Sasaki, T. and Tanaka, T., Biotechnol. Bioeng., 21, 1031, 1979. 56. Gould, M., Biotechnol. Bioeng. 26, 46, 1984. ¢ , D., ‘Development of the Biological 57. Struszczyk, H., Wesotowska, E. and Ciechanska Utilisation of Textile Wastes. II. Specification of Biodegraded Product and its Recycling’, Fibres & Textiles in Eastern Europe, 8, 3(30), 73–5, 2000. ¢ , D., Application of Enzymatic 58. Struszczyk, H., Wesolowska, E. and Ciechanska Degradation in the Utilization of Textile Wastes. Characteristics of Biodegradation Product and its Practical Application, Fibres & Textiles in Eastern Europe, 5, 4(19), 40–2, 1997. 59. ‘Challenges in Cellulosic Man-Made Fibres’, Akzo Nobel Conference, Stockholm, Sweden, 1994. ¢ , J., Wawro, D., Bodek, A. and Urbanowski, 60. Starostka, P., Struszczyk, H., Józwicka A., ‘Method of Cellulose Carbamate Manufacturing’ Polish Patent 165 916, 1995. ¢ , D., et al. Polish Pat. 167519, Method 61. Struszczyk, H., Wawro, D. and Ciechanska for production of dissolving pulp, 1992. ¢ 62. Struszczyk, H., Ciechanska , D., Wawro, D., Nousiainen, P. and Matero, M., Direct Soluble Cellulose of Celsol: Properties and Behaviour, in Cellulose Derivatives: Physical–chemical Aspects and Industrial Applications, Kennedy, J.F. and Phillips, G.O., Woodhead Publishing Ltd, Cambridge 29–35, 1995. 63. Struszczyk, H., Alternative Wet Spinning Technologies for the Manufacture of Cellulose Fibres, in Cellulosic Men-made Fibres, Akzo Nobel Co., Singapore, 1997, chapter 18. 64. Vehviläinen, M. and Nousiainen, P., CELSOL–Modification of Pine Sulphate Paper Grade with Trichoderma reesei Cellulases for Fibre Spinning, in Cellulosic Manmade Fibres, Akzo Nobel Co., Singapore, 1997, chapter 17. 65. Schleicher, H., Struszczyk, H. and Wetzel, H., Das Papier, 1996, 12, 674–681. ¢ , D., Ger. Pat. Appl. 66. Schleicher, H., Wetzel, H., Struszczyk, H. and Ciechanska 19624866.3 ‘Verfahren zur Verbesserung der Reaktionsaktivität von Cellulose zur Herstellung von Celluloseacetat’, 1996. ¢ , D., Patent 67. Schleicher, H., Weigel, P., Wetzel, H., Struszczyk, H. and Ciechanska RFN DE 19624867 ‘Verfakren zur Verbesserung der Löslichkeit von Cellulose in wasserhaltigem Aminoxid’, 2001. ¢ , D., ‘Perspectives of Enzymes for Processing for 68. Struszczyk, H. and Ciechanska New Chemical Fibres’, W.A. Cavaco-Paulo, K.E.L. Eriksson (eds), ‘Enzyme Applications for Fiber Processing’, ACS Symposium Series, Washington, 1997. ¢ , D., Wawro, D., Struszczyk, H. and Steplewski, W., ‘Advanced technical 69. Ciechanska cellulosic products based on enzyme treated pulp’, 3rd CEC, Fibres-Grade Polymers, Chemical Fibres and Special Textiles, Portorose, Slowenia, 2003. 70. Deinema, M.H. and Zevenhuizen, L., Arch. Microbiol., 1971, 78, 42. 71. Fiedler, S., Füssel, M. and Sattler, K., Zentralbl. Microbiol., 1989, 144, 473. 72. Matthyse, A.G., J. Bacteriol., 1983, 154, 906. 73. Canale-Parola, E. and Wolfe, R.S., Biochim. Biophys. Acta, 1964, 82, 403.


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5 Lyocell fibres P W H I T E, M H A Y H U R S T, J T A Y L O R and A S L A T E R, Lenzing® Fibers Ltd, Derby, UK

5.1

Introduction

Lyocell is the first in a new generation of cellulosic fibres made by a solvent spinning process. A major driving force to its development was the demand for a process that was environmentally responsible and utilised renewable resources as their raw materials. The first samples were produced in 1984 and commercial production started in 1988. A wide range of attractive textile fabrics can be made from lyocell that are comfortable to wear and have good physical performance. This physical performance combined with its absorbency also make lyocell ideal for nonwoven fabrics and papers. Lyocell is a cellulosic fibre derived from wood pulp produced from sustainable managed forests. The wood pulp is dissolved in a solution of an ‘amine oxide’ (usually N-methylmorpholine-N-oxide (5.1)). The solution is spun into fibres and the solvent extracted as the fibres pass through a washing process. The manufacturing process recovers >99.5% of the solvent. The solvent itself is non-toxic and all the effluent produced is non-hazardous. O N H3C

O

5.1

It is the direct dissolution of the cellulose in an organic solvent without the formation of an intermediate compound that generically differentiates lyocell from other cellulosic fibres such as viscose. Lyocell has all the benefits of being a cellulosic fibre, in that it is fully biodegradable and absorbent. It has high strength in both the wet and dry state. It blends well with fibres such as cotton, linen and wool. In common with other highly oriented cellulosic fibres, lyocell fibrillates when the fibre 157

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is abraded in the wet state. Surface fibrils (small fibre-like structures) peel away from the main body of the fibre but remain attached. The fibrillation behaviour of the fibres is exploited to produce a wide range of attractive fabric aesthetics. This chapter will consider all aspects from the fibre manufacturing methods, the fibre properties including an assessment of its sustainability and biodegradation credentials. It will also consider the key applications of the fibre, concentrating on its application in textile apparel and nonwoven fabrics.

5.1.1

Historical background and production process

1939 1966–1968

1969–1979 1979 1983 1988 1989 1992 1997 1998 2004

Patent appears describing the dissolution of cellulose in amine oxide. D.L. Johnson, Eastman Kodak Inc., publishes series of papers discussing a range of compounds, including cellulose, which dissolve in amine oxide. American Enka/Akzona Inc. work on spinning fibre from a solution of cellulose in amine oxide but did not scale up. Courtaulds start research on the new cellulosic fibre, which was to become Tencel®. First pilot plant built in Coventry. Small commercial Tencel® plant at Grimsby, UK. BISFA agrees to new generic name – lyocell. Full-scale production Tencel® plant at Mobile, USA. Lenzing starts a production plant in Austria. Tencel® plant in Grimsby, UK. First non-fibrillating variant, A100, produced at full commercial scale. Lenzing AG acquire the Tencel® business.

Lyocell was first made in the lab and then in the pilot plant at Coventry during the early 1980s. It used technology that was significantly different to that developed by Enka. Whilst the process appears simple, major technological challenges had to be overcome to achieve an economically viable process – spinning, dissolution and solvent recovery all proved challenging. In 1988 a semi-commercial plant (capacity 30 tonnes/week) was started up at Grimsby, this plant is known as S25 because originally there were 25 spinning ends. The operation at S25 enabled the manufacturing process to be developed and proven and it provided enough staple fibre to initiate full-scale market development. The fibre was branded Tencel®. In May/June 1992 the first full-scale Tencel® factory (SL1) was commissioned in Mobile, Alabama. The £67m investment represented a significant risk to Courtaulds as neither the market nor the technology were proven when the plant design started – it was a three-year project. However, the success of producing and

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selling fibre from SL1 convinced the board to invest heavily in the future of Tencel®. The second Tencel® plant, SL2, started up alongside SL1 at Mobile in the summer of 1995 with a capacity of ª 20,000 tonnes/year. A third Tencel® plant was installed in UK (Grimsby) in 1998 with 30,000 tonnes/year capacity. Half of this capacity included facility to make a non-fibrillating variant called ‘A100’. Lenzing of Austria commenced production of their lyocell fibre in 1997. This rapid expansion led to a temporary overcapacity and no further production occurred until Lenzing expanded their capacity by 20,000 tonnes per year in 2004. Development of a continuous filament spinning process by AKZO/ Tencel® continued throughout the 1990s but has not yet been commercialised because of the decline in the market for filament rayon and the high investment cost required.

5.2

Process description

This section provides a description of the process steps required for making lyocell. A diagram of the process is shown in Fig 5.1. The principles are simple. Firstly, the pulp is wetted out with dilute aqueous amine oxide to fully penetrate the pulp fibres. The subsequent removal of the excess water under heat and vacuum is a very effective way of making a homogenous solution with a minimum of undissolved pulp particles and air bubbles. The solution is highly viscous at its operating temperature (90 to 120∞C) and must be processed in similar high pressure equipment to that used in melt Wood pulp

Amine oxide Mix Dissolve Filter Evaporate Spin Wash Purify Finish Dry Crimp

5.1 The lyocell process.

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polymer systems. The fibres are formed by spinning into an air gap and then coagulating in a water/amine oxide bath. They are then washed and dried and cut. The wash liquors are recovered, purified, concentrated then recycled. The process description below applies to the two commercial-scale operations of Tencel® and Lenzing. Variations in detail have been cited in patent applications and the literature but these are at a much smaller scale of operation.

5.2.1

Pulp and premix

Wood pulp is the principal raw material of the lyocell process in terms of cost and volume. The grade used is similar to the dissolving pulp used for viscose rayon but has a slightly lower degree of polymerisation (DP); Tencel® fibres have a DP of 500 to 550. The pulp is pulled from the reels into a shredder, which cuts the pulp into small pieces for mixing with the amine oxide solvent. The amount of pulp fed to the mixer has to be accurately measured so that the cellulose content in solution is closely controlled. The cut pulp is conveyed to vessels where it is mixed with a 76–78% amine oxide solution in water. A small quantity of a degradation inhibitor is also added to the mixer; other additives such as titanium dioxide (for producing matt fibre) can also be added. The mixing is achieved at 70 to 90∞C in a ploughshare mixer that contains a number of high-speed refiners to break the pulp down and aid solvent wetting. The resultant slurry consists of swollen pulp fibres and has the consistency of dough. This premix is dropped into an agitated storage hopper from which it is accurately metered to the next stage of the process.

5.2.2

Solution making

Premix is heated under vacuum to remove sufficient water to give a clear, dark amber-coloured viscous solution of the cellulose. Typically the solutions contain 10 to 18% cellulose. The evaporation of water from premix to make solution is achieved in a wiped thin film evaporator such as a Filmtruder (from Buss-SMS) (see Fig. 5.2). This is a long vertical cylindrical vessel with steam heating in jackets around the vessel. A shaft down the centre of the vessel with blades attached to its circumference is rotated to smear the material around the heated surface to promote the evaporation process and to transport the solution down the vessel. The evaporator vessel is operated under vacuum to reduce the temperature (circa 90 to 120∞C) at which the water evaporates. This is important because the amine oxide solvent in solution can undergo an exothermic degradation process if it is overheated.

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Rotor drive

Top rotor bearing

Vapour outlet

Upper part

Rotor

Feed

Heating jacket

Lower bearing

Discharge cone

Product discharge

5.2 A Buss-Luwa filmtruder (courtesy Buss-SMS).

5.2.3

Solution transport

The solution leaving the filmtruder is pumped by a number of specialised pumps in series through the transport system. The transport system consists of a solution cooler and a hydraulic ram buffer tank, which feed into the solution primary filters. Due to the viscous nature of solution the pressures

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involved in pumping solution can be as high as 180 bar. A complication to the process design is caused by the tendency of the amine oxide in solution to exothermically degrade. Exotherms can be caused by maloperation of equipment or chemical contamination of the solution; if an exotherm occurs, the temperature of the solution increases rapidly and it decomposes to volatile amines and water. This causes a very rapid increase in pressure that would be sufficient to rupture the high pressure equipment with very serious safety implications. To allow for this possibility bursting discs are provided at strategic positions throughout the plant to relieve pressure in the event of an exotherm. The bursting discs are of a special design to prevent any flow dead spots. The discs vent into disentrainment pots which separate the solid degradation products and allow the gases to be vented to the atmosphere. The understanding of how and why exotherms occur and the development of a safe way of venting exotherms when they do occur was one of the keys to scaling up the whole process.

5.2.4

Solution filtration

Prior to spinning it is necessary to filter various impurities out of the solution. Most of the impurities are introduced with the pulp feedstock, the principle ones being undissolved pulp fibres or inorganic compounds such as sand and ash. The solution is passed through two stages of filtration. The primary filtration is centrally located and consists of sets of sintered stainless steel media candle filter elements. The secondary stage filtration is achieved by candle filter elements associated with each spinning machine position. Filters are washed for reuse by an off-line process involving rinsing with hot amine oxide, chemically decomposing the residual compounds and then ultrasonic washing.

5.2.5

Spinning

For spinning, the solution is split into sub-streams, which serve a number of spinning positions. The solution is then supplied to each jet, via a filter, by a metering pump. It is then extruded and spun through an air gap into a spin bath containing dilute amine oxide solution. Each jet consists of thousands of tiny holes through which the solution is extruded into fibres. Just below each jet face is a small air gap across which air is blown by the cross-draught system to condition the fibres. After passing through the air gap the fibres, or tow, are pulled down through the spin bath where the cellulose is regenerated in dilute solvent. The fibres are drawn, or stretched, in the air gap by the pull of traction units, or godets. The design of the spinning assembly proved critical to successfully scaling up the process and achieving commercially attractive fibre properties and

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manufacturing costs. Spinning the solution directly into an aqueous bath would necessitate using very dilute cellulose solutions (causing high costs) and generates fibres with properties that are generally inferior to viscose. Extruding the solution into an air gap enables more economical, higher cellulose solutions to be spun. Furthermore, when the solution is extended or drawn in the air gap it is also orientated so that good strength and elongation properties can be imparted. A draw ratio of between 4 and 20 is typical – within this range fibre properties are similar. At lower ratios fibre tenacities are reduced and at higher ratios spinning stability deteriorates. When the solution is drawn in the air gap it will readily break unless it is also cooled by means of a gas flow. This tendency to rupture is worse at higher draw ratios and spinning speeds, but is reduced if highly viscous solutions are used. This necessitates the use of relatively small spinneret holes relative to the high polymer viscosity so special jets needed to be designed. The main limitation of air-gap spinning is the tendency of neighbouring filaments to touch and stick together. This limits the packing density of the spinneret and hence the productivity of the spinning machines. This is compounded by the need to adequately cool all the filaments using an air flow velocity sufficiently low that it does not disrupt the stable flow of the filaments. The two commercial processes overcame these constraints in different ways. Tencel® arranges the spinnerets into rectangular strips whereas Lenzing uses a circular array. Both processes use a controlled flow of gas across the filament arrays to stabilise and control the process.

5.2.6

Fibre washing

The fibre tows from each end are brought together into one large tow band for processing down the fibre line. The first process on the fibre line is washing; the solvent is washed from the fibre with hot demineralised water in a series of wash baths. In the Tencel® process the fibre is washed as a single large continuous tow through a series of wash troughs, each of which consists of a wide, shallow bath containing a number of wedges. These wedges deflect the tow band alternately up and down as it is pulled along the trough. This serves to allow dilute solvent into the tow band and then squeeze it out. Wash liquor leaving the wash line goes into the spin bath system. The washing water is fed counter-current to the tow band at a rate to keep the spin bath liquor concentration at the required level. In the Lenzing lyocell process the fibres are cut before washing – as in the viscose process.

5.2.7

Fibre treatments

After washing the fibre is treated in a number of ways:

164

∑ ∑ ∑ ∑


The fibre could be bleached if required. Soft finish is always applied to make further processing easier. Antistatic agent is applied. Other treatments to give specific fibre properties can also be performed – in particular various chemical treatments are used to control fibre fibrillation (e.g. as for the Tencel A100 and Lenzing lyocell LF grades).

5.2.8

Fibre drying

After washing and finishing the fibre is dried in a fibre drum dryer. These consist of a series of perforated drums that the fibres pass over. Steam heated air is sucked through the fibres as they pass over the drums.

5.2.9

Crimping, cutting and baling

In the Tencel® process, dry fibre is crimped before being fed to a radial blade cutter for cutting into staple. In the Lenzing process, the fibres emerge from the driers with a crimp imparted during the washing stage. The fibres are then baled and dispatched in the normal manner.

5.2.10 Solvent recovery Diluted amine oxide solvent from the spin baths on the spinning machine and various other process sections is collected to recover the relatively expensive solvent. Over 99.5% of the solvent used is recycled and recovered by the process. The amine oxide will slowly oxidise the cellulose during the process – particularly at the elevated temperatures used. The reaction will reduce the DP of the cellulose (giving poorer fibre properties) as well as generating coloured compounds that would detract from the whiteness of the fibres. The amine oxide degrades to N-methylmorpholine (5.2) plus other amines. The reaction is strongly catalysed by transition metals such as copper and iron. To control this it is essential that a stabiliser such as propyl gallate (5.3) is incorporated – this acts both as an antioxidant and a chelating agent. O

OH O

N

OH O OH

5.2

5.3

Lyocell fibres

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Solvent recovery consists of two main processes, ion exchange of the dilute solvent then evaporation of the excess water to a concentration required in premixing. The ion exchange process consists of cation and anion beds which remove various ions that would destabilise the solution and the colour contaminants that would otherwise build up in the solvent. The ion exchange resins and regeneration procedures have been developed especially for amine oxide. The amine oxide is re-concentrated in a steam heated multiple effect falling thin film evaporator. The tendency of the solvent to exothermically degrade means the process control of the operation has to be such that the amine oxide can never be overheated. The water overheads can be reused to wash the fibres so minimising the environmental impact of the process.

5.3

Lyocell sustainability

In addition to their well-documented consumer comfort, aesthetic and performance benefits, cellulosic fibres also offer the potential for very attractive environmental characteristics. Lyocell is the youngest member of the cellulosic fibre family. It was first commercialised, under the brand name Tencel®, in the early 1990s. One of the key development targets was to deliver a product offering significant benefits in terms of low environmental impact and sustainability. Lyocell is an excellent example of a ‘sustainable fibre’ because: ∑ The forests that provide the raw material for lyocell are always being replenished. ∑ Other materials used in the fibre production process are re-cycled with very little loss. ∑ The fibre is biodegradable. The following is a simplified representation of the life cycle of lyocell fibres, from creation through to ultimate disposal and biodegradation (see also Fig. 5.3). ∑ Lyocell fibre is manufactured from cellulose wood pulp, which is produced from trees grown in managed forests. ∑ The fibre production route is, chemically, very simple. The wood pulp is dissolved directly in a solvent and formed into fibres. There are no chemical by-products and the solvent is recycled at high levels of efficiency. ∑ Lyocell fibres are converted to a very wide range of textile and industrial products. In many areas, the particular properties of lyocell lead to environmental benefits for customers during product manufacture and use. ∑ At the end of their useful life, lyocell products are biodegradable. The


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Sustainable creation of lyocell

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166

Dry, cut Yarn, knit/weave, dye and finish garment make-up Nonwovens processing

Lyocell products

Lyocell fibre

5.3 Lyocell sustainability.

products of this biodegradation can be considered as contributing to photosynthesis, and hence the growth of new trees for future lyocell production. Each component of the lyocell life cycle is considered in more detail in the following sections.

5.3.1

Raw material

Lyocell cellulosic fibre is produced from special grades of wood pulp, a natural and sustainable resource that is derived from trees. By contrast, synthetic fibres such as polyester and nylon are oil-based. A very wide range of tree types, both softwood and hardwood, are suitable for conversion to wood pulp for lyocell (Fig. 5.4). The main species currently chosen as feedstock for commercial lyocell production is eucalyptus. Provided the forests are managed and harvested in a responsible manner, trees are an excellent source of renewable raw material. Lyocell manufacturers have paid particular attention to forestry management practices when developing partnerships with wood-pulp suppliers. Independent accreditation

Lyocell fibres

167

5.4 The trees used for Tencel® production (courtesy of Sappi Saiccor).

schemes, which have the support of environmental organisations such as Friends of the Earth, are now being adopted. A leading example is that offered by the Forestry Stewardship Council (FSC). The Forestry Stewardship Council aims to support ‘environmentally appropriate, socially beneficial and economically viable management of the world’s forests’. It offers a labelling scheme for forest products (including wood pulp) that provides ‘a credible guarantee that the product comes from a well managed forest’. The Principles and Criteria of FSC cover issues such as protecting water quality, managing the effects on threatened or endangered species and monitoring the effects of harvesting techniques. The primary supplier of wood pulp to Tencel has recently demonstrated compliance with all the requirements for FSC ‘Chain of Custody’ accreditation.

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Trees as a source of cellulose offer a number of potential advantages over most of the world’s cotton production. The cellulose yield per acre of land is much higher (up to 10 times more, dependent on conditions). They do not require ‘high grade’ agricultural land that could be used for food crops and require much less irrigation and pesticides. The trees are converted to wood pulp in pulp mills. Environmental performance of the pulp mill is an important consideration in the overall Life Cycle Analysis for lyocell. It is vital that byproducts from the pulping process are used in a sustainable manner and effluent is minimised. The class-leading mills which supply lyocell manufacturers adopt a number of strategies to achieve this: for example, byproducts are converted to specialist chemicals for sale and to produce energy. Bleaching is via either Elemental or Totally Chlorine Free processes (ECF, TCF).

5.3.2

Lyocell fibre

Details of the lyocell production process have been described earlier. This section will focus on the environmental attributes of the technology. Overall, current lyocell factories are designed and operated to achieve world-class low levels of emissions and minimise energy consumption. The solvent used to dissolve cellulose wood pulp, N-methylmorpholineN-oxide (‘amine oxide’), is not classified as corrosive or toxic and is biodegradable. The cellulose dissolves directly in the amine oxide. There is no ‘chemical conversion’, so no requirement for the complex emissions treatment associated with the release of by-products. After fibre formation, solvent is recovered from the fibre via multiple washing steps. It is then purified and returned to the concentration required to dissolve cellulose, for reuse at the beginning of the process. This is the so-called ‘closed-loop’ technology. By careful attention to detail at each stage of recycling, solvent recovery rates in excess of 99.5% are achieved. A range of design and operational features minimise the production of solid waste. Any fibrous waste produced is processed and used in non-textile applications. Lyocell has been awarded the right to use the Oeko-Tex 100 ‘Confidence in Textiles’ label, issued by the International Association for Researching and Testing in the field of Textile Technology. This independent testing confirms the purity of lyocell fibre. In addition, in December 2000, Lenzing Lyocell was awarded the ‘European Environment Award for Sustainable Development’ and in 2002 received the European Eco-label. The Eco-label is a voluntary scheme, sponsored by the European Union, designed to encourage and highlight examples of sustainable development.

Lyocell fibres

5.3.3

169

Fibre processing

There are many process steps involved in the conversion of fibres to finished articles. The particular properties of lyocell can assist customers in reducing environmental impacts of some of these steps, particularly in textile manufacture. Because lyocell is very clean, with virtually no impurities, bleaching for textile applications is not normally required. (Most of the cotton used for apparel application is bleached). Lyocell is also extremely efficient in terms of dye usage, allowing reduced dye-house effluent. Dyeing and finishing technology for lyocell is the subject of continuing intensive research and development. Recent technical breakthroughs in this area are expected to allow customers to further reduce chemical consumption, water consumption and energy usage. In nonwoven applications, the openness of Tencel® fibre in the bale makes it easier to process, giving lower energy consumption

5.3.4

Product use

The impact on overall life cycle of product use is often underestimated. For example, life cycle analyses of textiles have shown that most of the energy consumed during the life of a garment occurs during domestic laundering. This is accompanied by significant discharges of detergents to effluent. This is an area of the life cycle where fibre producers have relatively little direct influence. However, major retailers are now launching initiatives to reduce the environmental impact of laundering. For example, Marks & Spencer, the major UK retailer, has implemented a programme to encourage lower temperature laundering. They commissioned independent life cycle assessments of cotton and polyester garments which showed over 75% of the ‘life cycle extracted energy consumption’ is associated with consumer care. Further, the assessment concluded that a move from washing at 50∞C to 40∞C would reduce life cycle energy burden by ~10%. The inherent properties of lyocell fibres make them particularly suitable for less intensive laundering.

5.3.5

Product disposal

At the end of their useful lives, lyocell products are biodegradable. Biodegradation occurs through the action of enzymes created by living organisms, breaking a product down to carbon dioxide and water. Cellulosic fibres are known to be biodegradable, whereas synthetic fibres are not. There are a number of accepted techniques to quantify biodegradation (Fig. 5.5): ∑ Composting. Lyocell fibres have been found to degrade completely after six weeks in a static aerated compost pile. Synthetic fibres tested

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5.5 Biodegradability of lyocell fibre.

Lyocell fibres

171

(polyester, polypropylene, polyethylene) showed very little sign of degradation. ∑ Sewage treatment. Lyocell fibres degrade completely within 8 days in a typical sewage farm anaerobic digester, where the residence cycle is about 20 days. Synthetic fibres showed slight reductions in strength after 12 weeks. ∑ Landfill. Organic matter buried in the ground rots over a period of time by the bacterial process of anaerobic digestion. A landfill site is not easy to define or simulate, as it is somewhat heterogeneous. Soil burial tests (BS 6085/AATCC 30) are accepted methods of assessing the biodegradability of a product. Lyocell degrades completely within 12 weeks. Synthetic fibres gain weight initially and only show slight strength and weight loss after 24 weeks’ burial. Alternatively, if lyocell fibre were destroyed by incineration, it would burn readily, to carbon dioxide and water, with a heat of combustion of 15 kJ g–1. In a commercial incineration plant, this ‘waste energy’ can be put to good use.

5.3.6

The future

Lyocell is an excellent example of a sustainable fibre today. Building on this, lyocell manufacturers operate a policy of continuous improvement in partnership with key raw material suppliers, customers and retailers. Future developments are likely to further minimise the environmental impact of lyocell fibres in areas such as: ∑ Energy efficiency. Tencel® are working in partnership with the Carbon Trust to explore new energy-saving opportunities for lyocell production. ∑ The development of new fibre processing routes that require less energy and chemicals and consumer education to minimise environmental impact of consumer care.

5.4

Lyocell fibre properties

Tencel® fibre is characterised by its high strength both dry and wet as can be seen from Table 5.1. Tencel® shows a dry tenacity significantly higher than other cellulosics and approaching that of polyester. In the wet state, Tencel® retains 85% of its dry strength and is the only man-made cellulosic fibre to be stronger than cotton when wet. The high strength of the fibre translates into strong yarns and fabrics, and plays an important role in subsequent processing. In addition, Tencel® has a high modulus that leads to low shrinkage in water. Thus fabrics and garments demonstrate good stability when washed. Tencel® fibre

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Table 5.1 Lyocell fibre properties Property

Tencel®

Viscose

Cotton

Polyester

Titre (dtex) Dry tenacity (cN/tex) Dry elongation (%) Wet tenacity (cN/tex) Wet elongation (%)

1.7 38–42 14–16 34–38 16–18

1.7 22–26 20–25 10–15 25–30

– 20–24 7–9 26–30 12–14

1.7 55–60 25–30 54–58 25–30

fibrillates under certain conditions: in the wet state, through abrasive action, micro-fibrils develop on the surface of the fibre but, critically, remain attached to the main body of the fibre. The development of these micro-fibrils occurs mainly on the surface of a fabric and can be used to engineer a range of interesting fabric aesthetics.

5.5

Lyocell in textiles

5.5.1

Lyocell yarn conversion

Once lyocell fibre has been produced, either as cut staple fibre or continuous tow, it will be converted to yarns and fabrics by a range of conventional textile processes. The most common way of using lyocell fibre is as cut staple, with 1.4 and 1.7 dtex fibres cut to 38 mm and converted into a spun yarn using machinery developed over many years for handling cotton fibres that are similar in dtex and length to lyocell. The following comments apply to the processing of Tencel® fibres. Lenzing lyocell is made by a wet-cut route and has different processing characteristics. Yarn manufacture Lyocell can be processed via established yarn manufacturing routes, using, for example, 1.7 dtex 38 mm, 1.4 dtex 38 mm and 1.25 dtex 38 mm fibre for short staple routes. The fibre can be processed on conventional machinery, usually requiring a few setting changes in order to optimise processing performance. Lyocell’s processing performance is mostly influenced by the following properties: ∑ ∑ ∑ ∑ ∑ ∑

it is a cellulosic fibre and absorbs moisture readily; it has a smooth surface with a round cross-section; it possesses a non-durable crimp; it has a high modulus; it has a high tensile strength; the fibre is very open and there is little fibre entanglement;

Lyocell fibres

173

∑ the fibre supplied is usually lower in moisture content than cotton or viscose. Thus lyocell will open very easily with little nep formation. In sliver and roving, the fibres pack together, giving high cohesion and therefore requiring high drafting forces. Lyocell yields very regular yarns with high tensile strength and few imperfections. Lyocell blends well with other fibres, including cotton, viscose, linen, wool, silk, nylon and polyester. Lyocell adds strength to the yarn as well as enhancing the performance and aesthetic properties of the final fabrics. Important differences between the processing of viscose and lyocell are summarised as follows. The fibre is very open and it is important to minimise the number of opening points to avoid too much fibre breakage. Lyocell tends to dye slightly darker than viscose and cotton, hence it is important to take effective measures to guard against cross-contamination and airborne fly contamination during processing. Minimal carding power is required, as the fibre is very open. In drawing, sliver detectors may need to be re-set to adjust for the low bulk of the lyocell. In roving, the twist should be low to avoid too high a cohesion. Optimisation is very important at this stage of the process. Yarn steaming should be avoided wherever possible. Steaming cellulosic fibres, amongst other things affects fibre dye affinity, twist liveliness and splice strength. The dye affinity for cellulosic fibres reduces with increasing steam temperature and the influence on lyocell fibre is greater than for other cellulosics, such as cotton and viscose. Therefore steaming should be avoided unless this can be extremely well controlled. Twist liveliness can be reduced in other ways, such as by storing yarn on ringtube for 16–24 hours in a high humidity environment prior to winding.

5.5.2

Fabric manufacture

Weaving of lyocell fabrics can be successfully carried out on most conventional looms and in a wide range of constructions. The construction needs to be carefully engineered with the dyeing/finishing route to develop the best performance and aesthetics. Very tight constructions can give problems in dyeing and tend to give fabrics with poorer easy-care performance. Woven fabric structure The effects of fabric structure on fabric properties such as weight, bulk, warmth, flexibility, smoothness, cost, etc. are generally well appreciated in the trade. However, the fundamental influence of fabric structure on other important factors such as stability and susceptibility to creasing are often ignored or poorly understood.

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Fabric structure is particularly relevant for lyocell because of its high water absorption characteristics. Good absorption is, of course, highly desirable for comfort but the consequence of high absorption in lyocell is the development of powerful swelling forces. Swelling easily causes fibres and yarns to move. Such movement can cause creasing unless the fabric and finishing are designed with swelling in mind. Cotton, viscose and other cellulosic fabrics also swell and give similar effects but cotton swelling is smaller and viscose swelling is less powerful. Swelling and yarn crimp It should be appreciated that lyocell fibres are stable in length when wetted and dried (i.e. they do not shrink). Fabric shrinkage arises only from the fact that the fibres and yarns swell in diameter; swelling in diameter forces yarn crimp to increase so that the fabric will contract (shrink) and become thicker. Thickness is due to an increase in the yarn crimp amplitude. This, together with more ‘space’ within the fabric, creates useful fabric bulk. Contraction can be altered to some degree by dimensional control during finishing. However, it is the fabric structure and not finishing that largely determines the dimensions at which the fabric will be stable. Structure (ends/cm, picks/cm and yarn tex) can be adjusted to tailor fabrics to meet specific shrinkages. Contraction and shrinkage The total amount of wet fabric contraction depends directly on the number of yarns in warp and weft and the size of those yarns. For a fabric with similar warp and weft yarns, the distribution of the wet contraction between length and width depends on the relative numbers of warp yarns compared to weft. A predominance of warp yarn means that fabric length contraction will tend to be greater than that in the width. The greater the difference, the more the length will tend to contract compared to the width. Fabrics with equal numbers of yarns and similar counts in warp and weft will have more or less equal potential for warp and weft contraction. In practice, actual contraction may differ due to dimensional constraints such as warp tensions during processing. Residual shrinkages in such fabrics tend to differ correspondingly. Fabric contraction and ‘jamming’ Fabric shrinkage and contraction effects occur as a consequence of swelling in fibre diameter and the resulting increases in yarn crimp. There are three particular factors that affect the amount of yarn crimp development:

Lyocell fibres

175

∑ Processing tensions. For example, greater warp tensions will give either (a) more weft yarn crimp or (b) greater residual warp shrinkage (or both!). ∑ Frequency and diameter of yarns ‘yarn density’. Where yarns are more numerous and/or larger in diameter then yarn crimp will develop only to the point at which adjacent yarns just touch. When such yarns touch one another the fabric becomes ‘jammed’ or locked so that no more yarn crimp can develop without disrupting the alignment of yarns in that plane. ‘Jamming’ prevents further contraction in that plane but can readily force increased contraction (shrinkage) or fabric movement (creasing) elsewhere. ∑ Fibre and yarn swelling in diameter. Swelling adds to the ‘jamming’ effect by thickening the yarns. Fabrics that are apparently quite loosely woven when dry can become stiff and firm when wet. Again, this limits the amount of yarn crimp that can develop within the plane of the fabric (i.e. without buckling). Causes of fabric displacement and creasing Fabrics only exhibit creasing when one or more groups of yarns are displaced with respect to their neighbours. Yarns may become displaced to form creases either through handling or because of structural movements that are driven by swelling. Such movements already referred to above of course are contraction, shrinkage and yarn crimp development. However, such swellinginduced movements will only lead to creasing if wet yarns are unable to return to their original positions when dry. Minimising fabric movement and creasing There are several areas to consider in order to minimise wet fabric creasing: ∑ Limiting the scope for yarn movements by ‘setting’ the fabric. Unlike synthetics, lyocell fabrics are not thermoplastic and so cannot be ‘set’ with heat. However, there are wet finishing treatments that can fix yarns so that they will retain a memory of their crease-free state. Resin treatments help in this respect but caustic ‘setting’ is particularly relevant for lyocell. ∑ Limiting the effects of fibre/yarn swelling by creating ‘space’ inside the fabric. Fabric movement, and potential for creasing, is reduced if the finished fabric has sufficient room (‘space’) within the structure to accommodate yarn swelling without causing yarn displacement. Fabric construction is important. Generally fabrics should not be too tightly woven (to allow room for swelling). For tighter woven fabrics, caustic treatment is recommended. Caustic soda promotes increased lyocell swelling that causes additional useful ‘space’ to develop within the woven structure.

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∑ Adjusting the ‘balance’ between warp and weft yarn components. As mentioned earlier, the amount of fabric contraction is dependent on the numbers (and sizes) of the yarns in the structure. Where one set of yarns (say the warp) predominates over the other then this promotes greater contraction in that direction. Some inequality between proportions of warp and weft is acceptable but where differences are great then excessive contraction can develop in one direction, say the warp, together with negligible weft contraction. Imbalance can lead to excessive fabric movements in one direction and correspondingly greater tendency for creasing. Relatively well-balanced constructions are preferred. Although the foregoing is presented primarily with wet creasing in mind, these recommendations generally apply also to dry creasing. In this case the wet swelling issues are less relevant but fabric setting and the need for balance in the construction remain important. More detailed explanations of the interrelationships between these factors are beyond the scope of this chapter. However, on request Lenzing can provide its customers with more detailed and specific advice in respect of fabric constructions on a case-by-case basis. Knitting Lyocell fabrics are readily made on most commercially available knitting machines. The best quality fabrics are made with high quality yarns and slightly shorter stitch lengths to minimise the risk of work up and pilling during dyeing and subsequent use.

5.5.3

Dyeing and finishing of lyocell

Over the last twelve years since the commercial introduction of lyocell into the global marketplace, great progress has been made in dyeing and finishing technology. We should not forget that at that time lyocell fabrics could not be piece dyed on jet machinery at all, because of uncontrolled fibrillation leading to damage marks on the fabrics. The garment processing system has always been central to lyocell processing, because the effective re-orientation of the garment during the process allows fibrillation to be generated evenly throughout the piece. Conventional liquor jet machines will always be problematical because the fabric rope does not move sufficiently in running on the machine and uncontrolled localised fibrillation can develop. We have seen the introduction of air jet technology to help in rope re-orientation and hence with fibrillation control, together with the introduction of new processing routes to reduce the high cost of lyocell piece finishing. Fabric behaviour understanding has

Lyocell fibres

177

seen the development of new possibilities for lyocell with the importance of causticisation now being fully characterised. This has led to the development of Tencel® Natural Stretch, TOP+C and MAGIC processing which will be discussed later. Studies of lyocell fabrics processed with conventional open width techniques demonstrate the virtues of lyocell with low-cost production techniques and have shown the excellent easy care performance that lyocell can deliver in combination with resin finishing. The softness, fluidity and drape characteristics shine through even without fibrillation. The introduction of Tencel® A100 and Lenzing lyocell LF have further demonstrated that lyocell can offer positive performance attributes without fibrillation. Tencel® A100 also has a powerful colour story, with its reactive dye yield being higher than any other cellulosic fibre in the market, which has advantages in dye and chemical cost reductions as well as positive environmental messages from reducing colour in effluent. Fundamental fibre properties and their influence on dyeing and finishing The key fibre properties are swelling and fibrillation. The high lateral swelling in water causes fabrics to stiffen appreciably in cold water, giving difficulties in crease avoidance. The creases formed also are more susceptible to abrasion and hence fibrillation. Localised fibrillation on the creases gives rise to white lines and damage marks in the finished goods. Correct machinery selection and the use of fabric causticisation help to minimise these effects. Pre-treatment of the fabric in caustic soda can be very effective in improving both fabric performance and aesthetic. It is very widely used in commercial lyocell fabrics. In caustic soda, lyocell swells very significantly in its diameter but very little in length. The maximum effect is at 10% to 12% concentration, but mercerisation conditions can also be used. The swelling of the fibres will increase the diameter of the yarns in the fabric and this will cause the fabric to shrink. This step needs to be carried out with the fabric in open width form and if the fabric is allowed to shrink during caustic treatment its bulk will be significantly enhanced. Furthermore, the fibres become set into this new configuration when the fabric is subsequently washed. The fabric then has a greater bulk and flexibility – in particular the wet stiffness of the fabric is much reduced. The latter is important for further processing since it is less prone to crease damage marks during the dyeing processing. The caustic treatment also gives more rapid fibrillation removal in processing and a reduced tendency to fibrillation in domestic use. The fibrillation on the fabric surface changes the light reflectance behaviour. This gives rise to the ‘bloom’ or ‘frosted’ look on a peach-touch lyocell fabric. The degree of fibrillation will influence the apparent colour.

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Basic process for standard lyocell The conventional process is a three-stage system. Fibrillation is deliberately induced and then removed by a treatment in a cellulase enzyme. Secondary fibrillation is then created by further mechanical action, which can be simultaneous with dyeing if the fabric is not already pre-dyed. This threestage process is time consuming and a great deal of effort has been directed at new process development to reduce the time of or eliminate these steps. Final finishing on a rope tumbling machine is required to fully generate the fibrillated effect. Processing in garment form By its nature this technique will deliver a casual peach-touch result. Lyocell can be successfully processed in garment dye or garment wash systems (including indigo). The high level of garment movement and mechanical aggression in the machine make for rapid processing through the fibrillation and enzyme processing steps and even surface effects. The acceptance of lyocell by the garment processing industry was relatively straightforward as they were already used to enzyme systems for indigo denim. (It is also interesting to note that cellulase enzymes were only introduced for textiles in the mid-1980s so prior to that lyocell would have been impossible to process to a controlled fibrillated finish.) The Nidom development The early–mid-1990s saw a hybrid machine development in Japan. The Nidom is a garment type processing machine designed to process fabrics in piece form, rather than in made-up garments. It allows a garment-processed aesthetic to be generated on fabrics prior to garment construction. The aesthetic quality is exceptional, but the processing of short lengths in multiple chambers gives an extremely labour-intensive and hence expensive system. Piece processing on jet machines As has been mentioned previously conventional liquor jets do not have sufficient action to re-orientate the fabric rope. The result is damaged second quality fabric. Air jets are a relatively new development from the supply industry (again coming in the 1980s) that cause the fabric to ‘explode’ from the jet, giving excellent re-orientation and high first quality results. The three-stage process can be followed to give a peach-touch result, or the step order can be changed to give a clear fabric surface, but in this instance resin finishing needs to be applied to prevent secondary fibrillation being generated in domestic washing.

Lyocell fibres

179

Pre-causticisation of the fabric can help as it imparts wet softness to the fabric, which allows better running on the machine and decreases the risk of creasing and damage. Recent developments have seen the extension of garment process technology in combination with optional processing on the air jet, to knitted fabric constructions, to create a so-called ‘angelskin’ aesthetic. TOP+C processing Tencel® Oxidative Preparation plus Caustic is a technique that incorporates a peroxide bleach (preferably pad steam) prior to causticisation. The result is that the fabric requires much less mechanical action to generate the peach touch and the three-stage process is no longer required. In jet processing the action on the fabric of a reactive dyeing process followed by rope tumbling is sufficient. This can halve the time needed on the jet and eliminate the cost of the enzymes. It is also applicable for garment processing, again showing significant time-savings and chemical cost reductions, although enzymes can still have positive effects on finished fabric hand. MAGIC processing Not every finisher has the right sort of equipment for TOP+C. MAGIC only needs a pad mangle and stenter, possessed by 99% of fabric dyers. The pretreatment means that pre-fibrillation and enzyme can be omitted from the jet process delivering shorter, less complex processes such as TOP+C. Open width processing – processing without fibrillation Lyocell can be processed in conventional open width systems. Because the mechanical action on the fabric is virtually eliminated the result is a flat classical fabric. Aesthetical differentiation is reduced against cotton, but the lyocell (or blend) will still be softer and more fluid than a cotton comparison. No fibrillation occurs on lyocell fabrics that have been open width processed, but they still retain a soft handle with an underlying bounce and resilience. The high strength of lyocell means that aggressive surface treatments such as wet and dry emerising can be successfully used which enhances the aesthetical difference. Resin finishing on fabrics processed by this route is imperative to maintain appearance in washing. In fact, wash performance of lyocell is excellent with hand and appearance being retained exceptionally well on multiple washing. Open width processing enables cost-effective routes to production of lyocell fabrics for a wide range of garment types without the need for specialist processing equipment. This enables the production of garments with the touch and drape of lyocell extracting both performance and value from the fibre, which until now has needed special treatment.

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Tencel® natural stretch This is an extension of the open width processing principles in combination with causticisation. The fabric construction should be devised so that there is space to allow yarn crimp to develop. Causticisation will cause the crimp development and impart a memory which together with the fibre’s high modulus and resilience will allow stretch with high degrees of recovery, equal to that achievable with elastane containing fabrics. It should be borne in mind though, that this is a ‘comfort’ stretch rather than the ‘power’ stretch associated with elastomerics. Easy-care lyocell As with any fabric, chemical finishing is an important aspect of the process and this is especially true when considering the finishing of open-width processed lyocell fabrics. In such processing, resination is the method of controlling fibrillation. If too little resin is fixed then fabrics will fibrillate on subsequent washing, too much and physical performance deteriorates. It is also important to include appropriate softeners and auxiliary products into the chemical finish so that performance and handle are appropriate to the customer’s requirements. The application of ~2–3% omf (on mass of fibre) fixed resin appears to be optimal for easy-care properties, dependant on the fabric construction and weight. Application levels of 2% omf are needed to stop fibrillation on domestic washing. In addition to the resin, the choice of softener can have a large effect on the easy-care performance of fabrics, and it is important to consider the whole formulation and build it up to give the required performance. Silicone micro-emulsions penetrate yarns more than the macro-emulsions. Polyethylene dispersions aid sewing and build the handle of the fabric, whilst some soft acrylic-based chemicals can increase the abrasion resistance. It is also worth remembering that caustic soda or liquid ammonia treatment in preparation will help to increase the easy-care rating of lyocell fabrics. Tencel® A100 The normal glyoxal-based easy-care resins significantly impair the dyeability of cellulosic fibres so they cannot be used as a treatment prior to dyeing. To prevent a lyocell fabric from fibrillating during jet dyeing for example, other types of cross-link chemicals had to be considered to modify the fibre’s behaviour. TAHT (Triacroyl hexahydrotriazine (5.4)) will cross-link lyocell under alkaline conditions. Pre-treatment of the fibre using this chemical during the manufacturing process has proved successful. In the A100 process developed

Lyocell fibres

181

O N

O N

N O

5.4

by Tencel®, the TAHT chemical is applied to the washed tow in the fibre production line. It is then dried, crimped, cut and baled in the normal way. The A100 fibre has similar strength, elongation and modulus properties to the standard fibre and has an enhanced dye uptake that gives more economical dyestuff costs and strong, deep colouration. Its resistance to fibrillation means it can be processed on most dyeing machines and the fabrics produced have a good performance in subsequent washing. Tencel® A100 was launched in 1998 and has proved to be very popular and effective at extending the range of applications for Tencel® fibres. It is particularly beneficial for jersey knit applications because of its excellent colouration, attractive aesthetics and good wash/wear performance. The warm touch of jersey fabrics made from A100 complement the cooler touch of ‘peach skin’ fabrics made from standard lyocell. Wet processing of tencel® A100 The non-fibrillating nature makes dyeing on conventional liquor jets feasible. The drape and fluidity will generate more effectively than standard lyocell processed in an open width system and resination is an option rather than a necessity. The colour properties of Tencel® A100 are exceptional. Its uptake of reactive dyes is higher than any other cellulosic fibre. This means that either the same shade can be obtained more cost effectively (typically 40% less dye than modal or cotton), or unique deep shades can be obtained that are not possible on other fibres. The high efficiency of dyeing with Tencel® A100 (dye exhaustion and fixation) means that less unfixed colour remains to be removed, both reducing the water consumption in washing off and reducing the colour loading in the dyehouse effluent.

5.6

Lyocell – a versatile, high performance fibre for nonwovens

The early stages of the commercialisation of lyocell were focused towards the fashion textile apparel sector. However, this has changed during the first years of the twenty-first century so that lyocell is now targeted equally into the industrial sector, with particular emphasis on the key nonwovens markets

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of wipes, filters and feminine hygiene products. The key difference between traditional textile production and nonwovens production is the omission of the yarn stage from the production process. In nonwovens manufacture, the fibres are formed into a web and a fibre bonding or entangling process is used to impart integrity and control the function, hand and appearance of the resulting nonwovens’ substrate. Staple fibre grades are produced to suit carded dry laid, air laid and wet laid processes. The attributes of lyocell fibre are discussed with reference to each of these conversion technologies below.

5.6.1

Lyocell in carded processing

Staple fibres provide the starting point for many nonwoven processes. Because the staple-based technologies are so diverse in terms of machinery design and fibre processing requirements, a range of fibre types have been developed to suit individual needs and to ensure optimum conversion efficiencies and fabric properties. For carding-based technologies, Tencel® fibres have been designed to open easily and fully prior to the carding step. This gives uniform and nepfree webs. To ensure adequate web cohesion and high running speeds, high cohesion fibre surface finishes and high crimp fibre variants have been introduced, to provide for efficient web transfer and to minimise fibre fly. Fully optimised lyocell variants have been shown to run at greater than 250 m min–1 on commercial nonwoven carding lines. This is significantly faster than the speeds at which traditional cellulosic fibres can be run, and with greater web quality and control. The benefits in terms of maximum card speed achieved with Tencel® HS260, for instance, on the new generation of Thibeau cards are outlined below (Table 5.2). The webs formed in the trials were set at 30g m–2 weight. Table 5.2 Comparative carding speeds of different fibre types Fibre type

Maximum carding speed (m min–1)

Tencel® HS260 Polypropylene Polyester Viscose

250+ 250+ 200–250 ~150

The high crimp Tencel® HS260 grades are supplied in a form that is very similar to that of polyester or polypropylene staple, so blending with these fibre types is very efficient. However, lyocell is also available in a natural

Lyocell fibres

183

crimp variant, giving it similar processing characteristics to that of a standard viscose whilst maintaining the superior fibre strength of lyocell. This variant is selected for processing lines that have been designed from the outset to process viscose fibres.

5.6.2

Fabric properties

A large amount of work has been carried out over the last five years to understand and enhance the attributes of lyocell in nonwoven structures. These basic attributes are now being exploited to significant commercial advantage by an increasing range of nonwoven producers. Spunlacing The basic properties of lyocell fibre are particularly well-suited to the spunlacing (hydroentanglement) method of bonding. The fibre strength is very high, particularly in the wet state, being at least twice as strong as regular viscose. Lyocell also entangles efficiently, because of its smooth surface and because its fibrillar cellulose structure plasticises when wet. This results in very strong and stable spunlaced nonwovens, but it is still entirely possible to preserve the drape and softness of the final fabric through appropriate choice of bonding pressure profile, belt design and fabric basis weight. Fabric wet strengths can sometimes be higher than the dry values because the lyocell filaments swell predominantly in the diameter (Fig. 5.6). This causes the entangled fibres to lock together even more efficiently. Consequently very soft fabrics with adequate strength can be produced, by using relatively low water pressures. Also significant basis weight reductions can be achieved,

Dry state

Wet state

5.6 Radial swelling of lyocell fibres in water.

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either to generate cost savings, or to reach new markets such as thin, highly permeable coverstocks with basis weights below 20 g m–2 (10 g m–2 for some latex bonded structures). When using apertured entangling belts, fabric hole clarity is the best of any fibre, leading to a superior wiping action and an improved appearance. The combination of a strong fibre and efficient entanglement also results in exceptionally low lint levels, a key requirement for many nonwoven wipers in critical task situations. High fibre modulus and relatively low fibre elongation gives superior fabric stability, which improves the efficiency of conversion from fabric to finished product and acts as an excellent base material for many coating processes. The flexibility in the possible basis weights and bonding levels possible with lyocell permits an expanded range of absorbent fabric properties to be achieved. The working range of water pressure for lyocell is about 40 to 100 bar (compared to viscose rayon at 55 to 70 bar). The range of consolidation is therefore greater, and capacity, wicking, bulk and softness can be engineered within a much larger envelope. Needlepunching In needlepunching applications where bonding is achieved through the use of mechanical needling of the fibre web, very efficient bonding is seen with lyocell, even with what for this market sector is traditionally perceived as a low decitex product. For example, 1.7 decitex lyocell is now used routinely in needlepunched nonwovens, whereas the lowest practical decitex limit for traditional fibres is about 3.0 or above. This is achieved without significant fibre breakage and leads to softer, more absorbent structures, which have a unique ability to retain and release fluid. The high wet resiliency of the fibre allows the fabrics to retain bulk in the wet state, leading to superior handling aesthetics and wiping action. Where greater resilience or firmer fabric hand are required, 2.4 dtex and 3.3 dtex lyocell grades are available. Latex bonding In latex bonding, fabrics are created by bonding the webs with latex emulsions, starch sizes, or other adhesives. This is usually followed by a curing step. For this technology, which is still used extensively to produce dry wipes, the high fabric strength produced from lyocell enables binder levels to be reduced by as much as 50%. This enhances both fabric absorbency and softness. Binder levels can be reduced to such an extent that the flushability and biodegradability of the final fabric can be significantly enhanced. These are useful features in today’s environmentally aware marketplace. If binder levels are maintained, then very strong and stable fabrics can be obtained with good abrasion resistance and hence increased product lifetime.

Lyocell fibres

185

Lyocell in air-laying Air laying technology requires very different fibres to those used for the main staple processing routes described above. Here fibre webs are created using air dispersion, usually followed by deposition on a suction belt. For optimum air laying, low cohesion and static free fibre surfaces are advisable, combined with a short fibre cut length. The inherently open nature of a tow washed and mechanically crimped fibre like Tencel® makes it more suitable for web formation in an air stream than many other cellulosic fibres. Tailored fibre crimp can also be employed to enhance this performance and improve filament dispersion within a matrix of other fibre types. Substituting for modest percentages of normal fluff pulp with lyocell can significantly increase the softness, strength, bulk and absorbency of air laid fabrics. The longer lyocell fibres support the pulp matrix leading to higher tear and burst resistance, and better fluid transport because pores in the structure collapse less easily. Lyocell in wet-laying An alternative method of dispersing short fibres to form a fabric is to use water. This is the basis for wet laid nonwoven manufacture or traditional papermaking. When wet laying to form a fabric the high modulus of lyocell fibres allows relatively long length filaments to be processed without dispersion problems. This results in low defect webs with excellent tear and burst strength and good dimensional stability. In addition, the unique fibrillar, crystalline structure of lyocell can be broken down through mechanical wet abrasion, for example at the beating stage of a traditional papermaking process. This can be very readily achieved because the fibre swells so much radially when wet. The wet abrasion generates submicron diameter fibrils, which are retained within the fibre matrix, creating a micro-porous network, which is ideal for fine filtration (Fig. 5.7.).

Lightly fibrillated

Highly fibrillated

5.7 Fibrillated lyocell used in special paper applications.

186

5.6.3


Finished product benefits

Much of the impressive growth in the use of lyocell in nonwovens has coincided with the substantial growth in, and segmentation of the consumer and industrial wipes sectors. Lyocell is the ideal fibre for many categories of wiping. It is free of chemical odours because the manufacturing process is effectively a simple dissolution and regeneration process. This permits the production of fragrance-free wet wiping products, where the liberation of unpleasant odours have been a perennial problem when using other fibre types. Also for many wet-wiping applications, the use of lyocell means the absorbent properties of the wipe can be engineered to ensure that gravitational drainage in tubs and canisters can be reduced to a minimum and yet all the wiping layers are equally wetted out. Because relatively little liquid is trapped within the fibre structure, the volume of expensive lotion required for an effective wiping performance can be significantly reduced. Lyocell produces low lint fabrics because of its strength and durability as a fibre and the efficiency with which it can be bonded. This low linting feature is mandatory for many critical-task wiping applications. This includes clean-room use, surface preparation in the automotive, aerospace and printing industries and many medical wipes, gauzes and swabs. Chemical purity and a good regulatory history and compliance record commend the fibre’s use in personal hygiene and hospital products. Lyocell has excellent wet resilience, giving improved aesthetics and efficient skin exfoliation in cosmetic wipes. High fabric strength can extend the life of many wiping products. In particular, lyocell wipes stand up well to overnight bleaching, which is common in the food service industry. Also, as bonding levels can sometimes be reduced to take advantage of the filament strength, fabric flushability and biodegradability can be enhanced making disposal cheaper and easier. In non-wiping applications, lyocell fibres impart strength, stability and uniformity to synthetic leather substrates. Lyocell is ideal for food contact use in hot oil and beverage filtration, as a result of very uniform fabrics and good regulatory clearance. Papers containing fibrillated lyocell are an economic alternative to other sources of nanofibres in high efficiency air and liquid filtration.

5.6.4

Conclusion

The case for specifying lyocell fibres in nonwoven products is compelling. Priced cost effectively, with unique attributes and excellent environmental credentials, their future in absorbent nonwoven products is assured.

Lyocell fibres

5.7

187

Marketing

It’s a long run through the supply chain from a fibre to the consumer. The consumer either automatically chooses a fibre, such as cotton, because they always have or thinks again when they see a new fibre name on a label. But what comes to mind when they see a fibre name such as lyocell? Research tells us that the consumer thinks the fibre is man-made, or synthetic – the two terms being synonymous in their minds. The fact that lyocell is a cellulosic fibre and was invented to be environmentally sound is not something that easily comes into the consumer’s mind. So what does a fibre company do about this? In the case of Tencel®, the company has had a long and consistent approach to marketing its fibres and getting the natural/nature message across. The Tencel® brand has associated itself with all forms of ‘nature’ in developing communications platforms to and through the trade. More recently, the focus has been closer to the end consumer with a higher level of involvement with brands and retailers. The success of this increased focus is measured by the uptake of the Tencel® Branding Programme. Through direct research, consumers also tell us that they find on-garment labels helpful in quickly identifying a brand, a type of garment, a type of fit or a type of fibre – all important to the busy shopper who shops less frequently and with less time to spare. Marketing vehicles such as the Tencel® Branding Programme are extremely supportive for brands and retailers who understand the importance of making shopping easier. Tencel® provides, free of charge, a contemporary branded swing ticket, printed on recycled paper stock reflecting the consumer’s concern for the environment. With increasing recognition of the Tencel® brand comes an association and understanding that the garment will be several things: soft to the touch; comfortable to wear; easy to care for and easy to wear. The loyal Tencel® purchaser is an avid one and ultimately looks for the brand when shopping. Tencel® is a market-led company with a market-facing brand and prides itself on its understanding of customers through the supply chain. The company partners with key consumer brands and labels to reinforce a brand’s commitment to its own ultimate consumer. The company reinforces its supply chain partners by paralleling their marketing efforts to optimum effect. Business and brand growth for the future will come from increasing Tencel®s menswear market share and also in further developments within home textiles. In addition, special finishes and processing routes will enable Tencel® to work with a broader supply chain and expand its own capabilities into new sectors. Strategically, the company believes that Tencel® will be one of the most trusted fibres of the future as it continually reinforces the importance of sustainability and its natural roots. Link these benefits to the ultimate

188


needs of the primary consumer’s modern lifestyle: versatility, ease, comfort, style and value and you have the perfect solution to today’s modern wardrobe! Tencel® lyocell!

5.8

Future trends

The speed at which lyocell fibres have been accepted by both the textile apparel and the nonwoven markets has been a remarkable success story. In textile apparel, the ability of the fibre to fibrillate and create new and novel fabric appearances and handles, has allowed innovative textile mills to launch completely new and differentiated fabric types. In textiles, the newly developed processing systems, becoming ever less complex and cost effective is likely to see a continued market penetration of this fibre type into a widening range of fabric and product types. The use of lyocell fibres in the home textile market is expected to see significant growth, where the comfort properties can deliver tangible consumer benefits. The last few years have seen a huge growth of lyocell in nonwoven applications. The fibrillation property can deliver significant performance benefits and sustained growth is expected in the key areas of wet wipes and filtration. New fibre types are currently in development and the future will see significant launches of new fibre variants into the market. The introduction of the new non-fibrillating fibre types of Tencel® A100 and Lenzing lyocell LF have seen impressive growth since their introduction, and we can expect to see fibres with new and innovative functional properties being introduced over the next few years. The environmentally benign production process sets lyocell apart from other man-made cellulosic fibre types, and this will gain in importance over the coming years providing impetus for fibre production growth. Already Hanil of Korea have been producing fibre from a large pilot plant, and talk is of a significant investment in China to construct a largescale manufacturing facility. It is clear that great opportunities exist, and the life cycle of lyocell fibre has only just begun.

5.9

Sources of further information

Overview White, P.A., ‘Lyocell – The Production Process and Market Development’ in Regenerated Cellulose Fibres, C. Woodings (ed.).

Raw materials (trees, wood pulp) Forest Stewardship Council

fsc.org

Lyocell fibres Sustainable Forestry Initiative Sappi Saiccor wood pulp

189

aboutsfi.org sappi.com/sustainability

Fibre formation and structure Mortimer, S.A., Peguy, A.A. and Ball, R.C., ‘Influence of the physical process parameters on the structure formation of lyocell fibers’, Cellulose Chemistry and Technology, Vol. 30; Number 3/4251-266, 1996. Mortimer, S.A. and Peguy, A., ‘Spinning of fibres through the N-methylmorpholine-Noxide process’, 8th International cellucon conference, Cellulose and cellulose derivatives: physico-chemical aspects and industrial applications, Woodhead Publishing, 561– 567, 1995. Mortimer, S.A. and Peguy, A.A., ‘Methods for Reducing the Tendency of Lyocell Fibres to Fibrillate’, J. App. Pol. Sci., 60, 305–316, 1996. Mortimer, S.A. and Peguy, A.A., ‘The influence of air gap conditions on the structure formation of lyocell fibres’, J. App. Pol. Sci., 60, 1747–56, 1996. Coulsey, H.A. and Smith, S.B., ‘The formation and structure of a new cellulosic fibre’, Dornbirn Conference, Austria, 1995. Tencel tencel.com Lenzing lenzing.com Oeko-tex oeko-tex.com Eco-label http://europa.eu.int/comm/environment/ecolabel

Fabric treatment White, P.A., ‘Improvements in Processing of Tencel Fibres and Fabrics’, International Man-Made Fibres Congress, 41st, Dornbirn, Austria 1116+, 7 pp., 18–20 September 2002. Willmott, Nicola Jane, ‘TENCEL: the manipulation of fabric aesthetics and performance via the application of chemical finishes’, Book Pap. – Int. Conf. Exhib. AATCC, 459– 462, 1998. Burrow, T.R., ‘The textile applications of Courtaulds Lyocell fiber’, World Text. Congr. Nat. Nat.-Polym. Fibres, 68–77, University of Huddersfield, Queensgate UK, 1997. Burrow, Tom, ‘Recent results with lyocell fibers in textiles’, Lenzinger Ber., 78, 37–40, 1998. Taylor, Jim and Fairbrother, Andy, ‘Tencel – it’s more than just peachskin’, J. Soc. Dyers Colour., 116(12), 381–384, 2000. Morley, R.J. and Taylor, J.M., ‘Easy Care TENCEL: Best Practice in Fabric Construction and Finishing’, International Dyer 187, No. 4: 17+, 5 pp., April 2002. Gandhi, K., Burkinshaw, S.M., Taylor, J.M. and Collins G.W., ‘A novel route for obtaining a “peach skin effect” on lyocell and its blends’, AATCC Review 2/4, 48–52, 2002. Taylor, J.M., Alwis, P., Harrington, L. and Geubtner, M., ‘Easy Care Tencel Textiles’, Melliand Textilberichte/International Textile Reports (German Edition), 83, No. 3: E37+, 2 pages, March 2002. Taylor, J.M., ‘Tencel A100 – The Deep Dye Cellulosic Fibre’, Colourage 49, No. 9: 45+, 5 pages, September 2002. Taylor, J.M. and Harnden, A.L., ‘An introduction to Tencel A100’, Book Pap. – Int. Conf. Exhib. AATCC, 546–555, 1988.

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Non-wovens Macfarlane, Keith, ‘Nonwovens applications of lyocell fiber’, World Text. Congr. Nat. Nat.-Polym. Fibres, 131–139, 1997. Woodings, Calvin, ‘Technical applications of Courtaulds Lyocell’, Lenzinger Ber., 78, 65–68, 1998.

Product use Marks & Spencer

marksandspencer.com/thecompany/ourcommitmenttosociety/ environment

6 Poly(lactic acid) fibers D W F A R R I N G T O N, Consultant, UK, J L U N T, S D A V I E S, NatureWorks LLC, USA and R S B L A C K B U R N, University of Leeds, UK

6.1

Introduction

In a world that is becoming increasingly sensitive to the need to protect our environment, the ability to manufacture products from sustainable resources and which are fully compostable at the end of their useful life, is an exciting and attractive proposition. Poly(lactic acid) (PLA) is a linear aliphatic thermoplastic polyester derived from 100% renewable sources such as corn, and the polymer is compostable.1,2 However, most initial uses were limited to biomedical applications such as sutures3 and drug delivery systems4 due to availability and cost of manufacture. Over the past few years, NatureWorks LLC has developed large-scale operations for the economic production of PLA polymer used for packaging and fiber applications. It is important that PLA is used broadly in textile applications for several reasons. Polyesters currently used for apparel and related fiber applications, mainly poly(ethyleneterephthalate) (PET), account for over 40% of world textile consumption (second only to cotton) and their use is constantly increasing. Production of such polyesters consumes fossil fuel resources and disposal of the polymer adds to landfill sites as they are non-biodegradable and are not easily recycled. In contrast, PLA fiber is derived from annually renewable crops, it is 100% compostable and its life cycle potentially reduces the Earth’s carbon dioxide level. The recognition by the FTC in the USA and the EU commission that PLA fibers are a completely new generic class of synthetic fibers further reinforces the validity of this new approach to producing performance melt-spinnable fibers. This chapter will review the chemistry of PLA and will discuss the commercial manufacturing process that confirms its position as a viable material for many applications in both the fibers and the plastics industries. It will review the various properties of the polymer and consider the fiber properties that make it attractive to the commercial sectors into which it is being developed today. Recognizing that this is very much a new polymer with its own characteristics and processing requirements, the current status 191

192


of the downstream technologies in these sectors will be appraised. The environmental benefits of PLA will be discussed and compared with the current petroleum-based polymers. The potential for further improvements in the environmental impact will be considered as the technologies for this new polymer are advanced.

6.2

Chemistry and manufacture of PLA polymer resin

Conventional synthetic polymers rely on reserves of oil and gas for their monomer source and energy to manufacture. These reserves of fossil fuel take millions of years to regenerate and are a declining resource. In contrast, the monomer used to manufacture poly(lactic acid) is obtained from annually renewable crops. Energy from the sun promotes photosynthesis within the plant cells; carbon dioxide and water from the atmosphere are converted into starch. This starch is readily extracted from plant matter and converted to a fermentable sugar (e.g. glucose) by enzymatic hydrolysis. The carbon and other elements in these natural sugars are then converted to lactic acid through fermentation (Fig. 6.1).5,6 H OH CO2 + H2O

Photosynthesis

*

HO

O HO

H H

OH H

O * n

Starch

Enzyme hydrolysis

+ H2O

H OH O HO H

Fermentation

OH CH3

Lactic acid

HO

HO HO

H H

OH OH

H

Glucose

6.1 Production of lactic acid from renewable resources.

Presently, the cheapest and most abundant source of sugar is dextrose (glucose) from corn. The land mass necessary for feedstock production is minimal. Producing 500,000 tonnes of PLA requires less than 0.5% of the


193

annual US corn crop;7 since corn is a cheap dextrose source, the current feedstock supply is more than adequate to meet foreseeable demand. Furthermore, there are many other alternatives for the starch or sugar supply. As fermentation techniques improve, as PLA production improves, and if PLA production extends to other geographies, it is quite likely that other materials such as grass and even biomass could be used; there is no need to be reliant on food crops. PLA takes advantage of a biological system to do chemistry that traditional chemical techniques cannot.

6.2.1

Production of PLA

The polymer is formed either by (1) direct condensation of lactic acid or (2) via the cyclic intermediate dimer (lactide), through a ring opening process (Fig. 6.2).2 H2O

O

O

OH HO H

O * *

CH3

CH3 H Poly(lactic acid)

(1)

Lactic acid (2) O H2 O

H

O

CH3

H 3C

O H

O Lactide

6.2 Polymerization routes to poly(lactic acid).

Polycondensation of lactic acid This process involves the removal of water by condensation and the use of solvent under high vacuum and temperature; the approach was originally used by Carothers who discovered PLA in 1932. With this route, only low to intermediate molecular weight polymers can be produced, mainly due to the difficulties of removing water and impurities. Other disadvantages of this route are the relatively large reactor required, the need for evaporation, recovery of the solvent and increased color and racemization. Most work has focused on the ring-opening polymerization, although Mitsui Toatsu Chemicals have patented an azeotropic distillation using a high boiling solvent to drive the removal of water in the direct esterification process to obtain high molecular weight PLA.

194


Ring-opening polymerization This method is a better way to produce a high molecular weight polymer, and has now been adapted commercially due to advances in the fermentation of corn dextrose which have significantly reduced lactic acid production costs. The fermentation of sugar produces chiral lactic acid inexpensively in high yield. Chiral molecules exist as ‘mirror images’ or stereoisomers; lactic acid can exist as the L- or D-stereoisomer (Fig. 6.3). Chemically synthesized lactic acid gives the racemic mixture (50% D and 50% L), however, fermentation is very specific, allowing the production of essentially one major stereoisomer; fermentation derived lactic acid consists of 99.5% of the L-isomer and 0.5% of the D-isomer. O

O

HO H

HO OH

L-lactic

OH

H3C

CH3 acid

H

D-lactic

acid

6.3 The stereoisomers of lactic acid.

The process is based on removing water under milder conditions, without solvent, to produce a cyclic intermediate dimer, referred to as lactide. This monomer is readily purified under vacuum distillation. Ring-opening polymerization of the dimer is accomplished under heat, again without the need for solvent. By controlling the purity of the dimer, it is possible to produce a wide range of molecular weights. Production of the cyclic lactide dimer results in three potential forms: the D,D-lactide (called D-lactide), L,Llactide (called L-lactide) and L,D- or D,L-lactide (called meso-lactide) (Fig. 6.4). Meso-lactide has different properties from D- and L-lactide; D- and Llactide are optically active, meso- is not. Before polymerization, the lactide stream is split into a low D-lactide stream and a high D-/meso-lactide stream. Ring-opening polymerization of the optically active types of lactide can yield a ‘family’ of polymers with a range of molecular weights by varying the amount and the sequence of D-lactide in the polymer backbone. Polymers with high L-lactide levels can be used to produce crystalline polymers O O O

H3C

O

O CH3

O O

H3C O

O L-lactide

CH3

Meso-lactide

6.4 Dimeric lactide isomers.

CH3

O O

H3C O

D-lactide


195

while the higher D-lactide materials (>15%) are more amorphous. By controlling the purity of the lactide it is possible to produce a wide range of molecular weights and by varying the amount and sequence of D-lactic units in the polymer backbone, the product properties can be changed. These changes impact melt behavior, thermal properties, barrier properties, and ductility.8 Based on this lactide intermediate method, NatureWorks LLC has developed a patented, low cost continuous process for the production of lactic acidbased polymers.9 The process combines the substantial environmental and economic benefits of synthesizing both lactide and PLA in the melt rather than in solution and, for the first time, provides a commercially viable compostable commodity polymer made from annually renewable resources. The process starts with a continuous condensation reaction of aqueous lactic acid to produce low molecular weight PLA pre-polymer (Fig. 6.5). O

OH

HO

O

HO

O

OH

O n

O

O

Lactic acid High molecular weight PLA Condensation –H2O

Ring-opening polymerization

O O O

HO

O n

O

OH

Depolymerization

O O

O O

Prepolymer Mn ~ 5000

6.5 Production of high molecular weight PLA via prepolymer and lactide.

Next, the pre-polymer is converted into a mixture of lactide stereoisomers using tin catalysis to enhance the rate and selectivity of the intramolecular cyclization reaction. The molten lactide mixture is then purified by vacuum distillation. Finally, PLA high polymer is produced using a tin-catalyzed, ring-opening lactide polymerization in the melt, completely eliminating the use of costly and environmentally unfriendly solvents. After the polymerization is complete, any remaining monomer is removed under vacuum and recycled to the beginning of the process (Fig. 6.6).

196


Dextrose

Corn

Unconverted monomer PLA polymer

Lactide formation

Prepolymer

Meso lactide Polymerization Low D lactide

Distillation

Lactic acid

Distillation

Fermentation

6.6 Non-solvent process to prepare poly(lactic acid).

6.2.2

Catalytic polymerization of lactide

Many catalyst systems have been evaluated for the polymerization of lactide including complexes of aluminum, zinc, tin, and lanthanides. Metal alkoxides are the most common metal-containing species for the ring-opening polymerization of cyclic esters. Simple sodium, lithium, and potassium alkoxides are effective, however, the high basicity of these ionic species leads to side reactions such as epimerization of chiral centers in the polymer backbone. Alternatively, covalent metal alkoxides are much more selective and therefore widely used. Initiators such as Al-alkoxides,10 yttrium and lanthanide alkoxides,11 and recently iron alkoxides12 have been shown to give a controlled and living polymerization of lactides via a so-called coordination–insertion mechanism with ring opening of the lactide to add two lactic acid molecules to the growing end of the polymer chain. Depending on the catalyst system and reaction conditions, almost all conceivable mechanisms (cationic,13 anionic,14 coordination,15,16 etc.) have been proposed to explain the kinetics, side reactions, and nature of the end groups observed in lactide polymerization. Tin compounds, especially tin(II) bis-2-ethylhexanoic acid (Sn(Oct)2), are preferred for the bulk polymerization of lactide due to their solubility in molten lactide, high catalytic activity, and low rate of racemization of the polymer; the mechanism is also via a coordination– insertion mechanism7 (Fig. 6.7). Conversions of >90% and less than 1% racemization can be obtained while providing polymer with high molecular weight. High molecular weight polymer, good reaction rate, and low levels of racemization are observed with Sn(Oct)2 catalyzed polymerization of lactide. Typical conditions for polymerization are 180–210∞C, Sn(Oct)2 concentrations of 100–1000 ppm, and 2–5 hours to reach circa 95% conversion. The polymerization is first order in both catalyst and lactide. Frequently, hydroxyl-containing initiators such as 1-octanol are used to both control molecular weight and accelerate the reaction.

Poly(lactic acid) fibers O

O

O O

O

O

R–OH

O

H O Sn

O O

O O

O

Sn(Oct)2

O

O

O

O Sn

R

(Oct)2

O

H

R

197

O

H RO

Sn(Oct)2

O

O O

H

Sn(Oct)2

RO

(Oct)2

6.7 Generalized coordination–insertion chain growth mechanism of lactide to PLA; R = growing polymer chain.

Copolymers of lactide with other cyclic monomers such as e-caprolactone17 can be prepared using similar reaction conditions (Fig. 6.8). These monomers can be used to prepare random copolymers or block polymers because of the end growth polymerization mechanism. Cyclic carbonates, epoxides and morpohinediones have also been copolymerized with lactide. O O

O O O Lactide

O

O Sn(Oct)2

O

O On

m

e-Caprolactone

6.8 Copolymerization of lactide and caprolactone.

6.3

PLA fiber properties

PLA fiber has a number of characteristics that are similar to many other thermoplastic fibers, such as controlled crimp, smooth surface and low moisture regain. One unique property in comparison is that it is the only melt-processable fiber from annually renewable natural resources. The physical properties and structure have been studied by several researchers,18 and these works confirmed that this polymer has significant commercial potential as a textile fiber. Its mechanical properties are considered to be broadly similar to those of conventional PET,19 and, probably due to its lower melting and softening temperatures, comparisons to polypropylene are also appropriate20. A résumé of the properties is given, although further detail about specific properties will be covered, as appropriate in Section 6.4, ‘PLA Applications’: ∑ Appearance. Fibers are generally circular in cross-section and have a smooth surface. ∑ Density. The specific gravity is 1.25 g cm–3, lower than natural fibers and PET. ∑ Refractive index. The refractive index of 1.35–1.45 is lower than PET (1.54). Trilobal and other shapes can be made, and give improved antisoiling characteristics.

198


∑ Thermal properties. PLA is a stiff polymer at room temperature. The glass transition temperature (Tg) is typically between 55–65∞C. The melting temperature (Tm) of PLA containing either the L- or D-isomeric form alone, is between 160–170∞C. The DSC scans for PLA and PET are shown21 (Fig. 6.9). It can be seen that PLA exhibits an endothermic peak (Tm) at approximately 166∞C, whereas the Tm of PET is approximately 254∞C. This low melting point compared to PET underlines one of the main restrictions for PLA in developing suitable applications. However, as mentioned before, the properties of PLA can be modified by adjusting the ratio and the distribution of the D- and L-isomers in the polymer chain, and melting points as low as 130∞C and as high as 220∞C have been obtained. 0.5 0.0

Heat flow (J/g)

–0.5 –1.0 –1.5 –2.0 –2.5

PET yarn PLA yarn

–3.0 0

20

40

60

80 100 120 140 160 180 200 220 240 260 280 300 320 Temperature (∞C)

6.9 DSC scans of PET and PLA.

∑ Crimp. PLA can achieve good degree of crimp and good retention level through processing. ∑ Fibre types. Both filament yarns and spun yarns can be made, as with PET. ∑ Tenacity. The tenacity at break (32–36 cN tex–1) is higher than for natural fibers although, of course, it can be varied according to the degree of drawing that is applied to the undrawn yarn. It is relatively unaffected by changes in humidity at ambient temperature, though as with other manufactured fibers there is a small but measurable increase in elongation. As the temperature is increased the tenacity does reduce quite quickly with a concomitant increase in fiber extension, a feature commonly found in synthetic fibers. ∑ Tensile properties. The tensile properties of PLA fiber as used in staple form for textile processing are shown in Fig. 6.10. Clearly they are very


199

60 HT polyester


50

40

Lyocell Cotton

30

PLA Viscose

20 Wool 10 0 0

5

10

15

20

25 30 35 Extension (%)

40

45

50

55

60

6.10 Tenacity-extension curves for PLA and other common textile fibers (20∞C, 65% RH).

different from those of high tenacity polyester and more akin to wool with a high fiber extension when stressed and relatively low final tenacity. The initial modulus (at 2% extension) is very similar to many other textile fibers, but the yield point is very marked, the fibers (and spun yarns) stretching very easily once past this point. A consequence, however, of the high elongation is that the work of rupture is relatively high giving yarns and fabrics an acceptable performance in commercial use. Elastic recovery is affected by the yield point and is particularly good at low strains. At 2% strain, the recovery is 99.2% +/– 0.75%, and 92.6% +/– 1.60% at 5% strain, higher than for most other fibers. The unusual tensile properties could be expected to have an influence on some of the commercial applications in fiber blend developments. For example, intimate spun yarn blends with cotton (and lyocell) are relatively weak due to the contrast of fiber properties, and this might possibly limit applications to knitted fabrics. However, wool blends well with PLA, and with their load-elongation curves being very similar, this would enable the full properties of both fibers to be exploited. ∑ Moisture regain. At 0.4–0.6%, PLA has extremely low moisture regain, much lower than natural fibers and slightly higher than polyester. ∑ Flammability. Although PLA is not a non-flammable polymer, the fiber has good self-extinguishing characteristics; it burns for two minutes after a flame is removed, and burns with a white and a low smoke generation. PLA also has a higher LOI (limiting oxygen index) compared to most other fibers, meaning that it is more difficult to ignite as it requires a greater oxygen level. Table 6.1 compares PLA with standard PET.

200

Biodegradable and sustainable fibres Table 6.1 Comparison of PLA and PET flammability properties Fiber property

PLA

PET

Flammability

Continues to burn for two minutes after flame removed

Continues to burn for six minutes after flame is removed

Smoke generation

63 m2 kg–1

394 m2 kg–1

LOI

26%

22%

∑ UV resistance. Unlike other synthetic fibers, PLA does not absorb light in the visible region of the spectrum; this leads to very low strength loss compared to petroleum-based fibers when exposed to ultraviolet light. ∑ Moisture transport. PLA shows excellent wicking ability. This property and the additional properties of fast water spreading and rapid drying capability give the fiber a very positive inherent moisture management characteristic. ∑ Biological resistance. Although PLA fibers are not inherently ‘antimicrobial’ without suitable after-finish treatment, they do not provide a microbial food source. In addition, testing by Odor Science and Engineering showed that PLA fiber-based fabrics outperformed PET-based fabrics for low odor retention.22 ∑ Chemical resistance. As PLA is a linear aliphatic fiber, its resistance to hydrolysis is therefore relatively poor. This feature means that care must be taken in dyeing and finishing of the fiber. ∑ Solubility. With regard to other chemicals PLA has limited solubility and is unaffected by dry-cleaning solvents for example.

6.4

Applications

The ease of melt processing, coupled with the unique property spectrum and renewable resource origin, has led to PLA fibers finding increasing acceptance across a variety of commercial sectors. It would also seem that with the estimated global increase in fiber demand, there is an opportunity for viable alternative materials. The total fiber business in 2002 was estimated to be some 55 million tonnes, and projections suggest that this could increase to 83 million tonnes by 2015. Particularly relevant within these figures is the growth of man-made fibers from 33 million tonnes in 2002 to 57 million tonnes by 2015, and that the major fibers creating this increase will be synthetic materials, mainly PET.23 With approximately 50% of the fibers in the market today being synthetic materials from petrochemicals, it is clear that without synthetic fibers there would be insufficient fiber materials to meet the demands, particularly as natural fibers alone (mainly cotton) could not cover this. It is considered that


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cotton is already at near peak production, so as the world population and the fiber requirements increase, the opportunity to use such a product as PLA is both logical and in line with our need to address the environmental problems we face.

6.4.1

Apparel

The apparel fiber business in 2002 was estimated at approximately 30 million tonnes (Table 6.2) and projections for 2015 indicate that it will grow to 42 million tonnes. The sector is dominated by two fibers, cotton and PET, and as the total requirement increases, it is PET that is projected to become the majority material. Apparel is by some way the largest sector in the global fiber business, and is a highly technical, fast changing, and demanding industry in terms of design, fashion, color, aesthetic, and performance. However, despite the steady growth in the apparel fiber market size, there has been a reduction in the price points, therefore creating a real pressure on costs through the whole supply chain. In many respects, it would seem that the existing range of fibers could meet all the possible consumer needs, either by virtue of their inherent properties, or with some chemical or physical processing technique within the downstream processing. For a new fiber, the entry into the apparel market could therefore be seen as both a very exciting challenge, but also a very difficult one. Against this backdrop, the real opportunity for PLA lies in the combination of both the fiber’s inherent performance properties and the positive environmental advantages of being sourced from annually renewable natural resources, which, at the end of the product life, can be easily composted or recycled. Table 6.2 Apparel mill consumption by fiber type 2002 Fiber type

Million tonnes

%

Cotton Wool Polyester Acrylic Cellulosic Nylon Others Total

13.0 1.3 10.2 2.0 1.8 1.2 0.5 30.0

43 4 34 7 6 4 2 100

Kanebo, Inc. introduced a PLA fiber under the trade name Lactron™ fiber at the February 1998 Nagano Winter Olympics, under the theme of ‘Fashion for the Earth’. Kanebo exhibited several garments from PLA or PLA/natural fiber blends. More recently, in 2003, Cargill Dow LLC (now NatureWorks LLC) announced their PLA fiber brand Ingeo™ (ingredient from the earth),

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and this is now being adopted across a wide range of garment categories globally by leading garment brands. It is one of the features of PLA that it can be produced as both filament and spun yarns. Fabrics produced from spun yarn have a ‘natural’ hand and are considered to feel similar to cotton in this respect. Fabrics from filament yarns have a cool and soft hand and exhibit a high fluidity or drape with a degree of elasticity. NatureWorks LLC product development suggests, for example, that a 1.2 dpf PLA achieves the softness of a microdenier PET (e.g. 0.7 dpf). With the opportunity to create a range of fabrics with attractive aesthetic properties, there are a number of performance properties of PLA that are significant to its positioning and progress in the apparel sector: ∑ The moisture management properties of good wicking, faster moisture spreading and drying mean that garments are comfortable. This is an important consideration for next-to-skin garments and particularly in the sportswear market. In terms of comfort under normal and active wear conditions, independent laboratory testing by the Hohenstein Research Institute demonstrated that PLA fibers perform better than PET and cotton, either when combined with cotton or as 100% PLA fabric. This testing included a series of different tests, including thermal insulation, breathability, water vapor transport, and buffering capacity to liquid sweat (several tests in this series). The results of the Hohenstein testing concluded that wearers of PLA/cotton fabric would experience improved physiological comfort versus equivalent PET/cotton fabric.24 ∑ The elastic recovery and crimp retention properties provide excellent shape retention and crease resistance. ∑ Thermosetting capability of the fiber provides for controlled fabric stability, with garments having a low shrinkage through repeated washings. ∑ The flammability properties described earlier show the potential in specific segments of the apparel market. PLA fabrics with no flame retardant treatments have passed the US tests 16 CFR 1610, and have also achieved the standards specific for children’s sleepwear, 16 CFR 1615 and 16 CFR 1616. ∑ High resiliency and lower specific gravity than natural fibers can give fabrics a light and lofty feel. These properties have been used in garments to provide an effective wadded layer, e.g. ski jackets. ∑ The after-care properties of garments in washing are very positive. There is no damage in repeated laundering of PLA fabrics: testing has been carried out under simulated conditions in accordance with AATCC standards, with no degradation observed (Table 6.3).7 Also, the fabric appearance remains extremely good after washing, without creasing and very clean surface. It would seem that any fiber work-up, which is quite normal under such conditions, is removed during the washing cycles.


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Table 6.3 AATCC Test Method 61-1994 (35% PLA/65% cotton blend knitted shirt, simulates five washings) Simulated conditions

AATCC Test

Burst strength (psi)

% Dimensional change (width/length)

Mn

Mw

Control

83

–

57694

117970

Cold hand wash (40∞C)

79

0/–3.82

56343

107835

Same, no bleach

1A

82

0/–3.13

52123

108115

Cold machine wash (40∞C)

–

75

0/–4.17

56281

111206

Same, no bleach

–

78

0/–3.82

Warm machine wash (50∞C)

5A

74

+6.25/–7.98

57190

112036

Same, no bleach

2A

78

+7.64/–7.98

Hot machine wash (70∞C)

4A

74

+2.00/–6.25

58005

112510

Same, no bleach

3A

76

+2.00/–6.25

Additional to the above properties that could all be described as positive attributes, there are certain factors in this relatively early stage of technical and commercial development that are somewhat restrictive to the development across a full apparel spectrum: ∑ The melting point of the yarns that are commercially available today is relatively low at 170∞C. This does cause limitations in some of the downstream processing technologies, but the main concern is for the consumer after-care of garments. Garment pressing and ironing temperatures have to be lower than the popular fibers of cotton and PET, and despite the appropriate care labeling instructions being used, it is a fact that consumers often disregard these. ∑ Hydrolysis degradation of the polymer can occur, particularly under combined aqueous high temperature and alkaline conditions; the degree of hydrolysis is influenced by the time, temperature and pH. This is of particular significance in the dyeing and finishing processes, as it will cause a reduction in the molecular weight of the polymer and therefore the strength of a yarn or fabric, if the appropriate finishing conditions are not observed. Through all of the manufacturing stages of the supply chain – spinning, fabric formation, dyeing and finishing, and garment making – the existing machinery can be used. There is no need for any specialized capital investment when processing PLA. In many of the downstream processing technologies,

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PLA processes and products are compared and are comparable to PET; both are melt spun, both are available in filament and staple form, weaving and knitting set-up conditions are similar, fabrics can be heat treated to give dimensional stability, and they are both dyed with disperse dyes. However, both fibers are unique and they do behave differently. It is therefore important that processors acknowledge this fact, and that the operating details are refined accordingly. Yarn spinning There is now a wide selection of commercial yarns being produced. In spun yarns, commercial products currently span the range Ne 5 to Ne 60 while in filament production, typical products such as dtex 70–68, dtex 150–72, are being produced. For both spun and filament yarns spinners have confirmed the processability of the material. An interesting feature of PLA is that processing temperatures are more typical of polyolefins (approximately 240∞C), although the properties more typically combine many of the features found in rayon, nylons, acrylics and polyesters. PLA pellets are supplied with a maximum moisture content of 400 ppm. It is important to reduce the moisture content to 100 ppm during processing to prevent hydrolytic degradation, similar to PET and nylon products. For short staple yarns, because the characteristics of PLA are comparable to other thermoplastic fibers, the processing conditions are similar to those for PET. For long staple yarns also, processing can be carried out on conventional long staple equipment as used for thermoplastic fibers in blend with wool. Fabric formation There are no special requirements for knitting, and machine settings are comparable with those for PET. In weaving, the additional points for attention would be: if a size is being applied, to use a PVA or water-soluble size to avoid any need for strong alkali desizing, and to minimize tensions due to the high fiber extension. Woven filament fabrics give a very soft hand, and have a high fluidity/drape compared to PET. Garment making The low melting point may mean that precautions are taken at those stages where fiber–metal friction could generate sufficient heat, namely the fabric cutting and the garment panel sewing operations. Care is also needed at the final pressing stage.


205

Dyeing and finishing As is usual with any new fiber, the coloration and wet processing technologies demand significant attention. Color is so important, not only because it is the prime driver when purchasing a new garment, but also because the various applications and subsequent finishing treatments have the most chemical and physical effect on any fiber or polymer. Similar to PET, PLA is dyed with disperse dyes. However, dye selection is most important, as the individual dye behavior is quite different from dyeing on PET. In general terms, dyes show their maximum absorption at a shorter wavelength than on PET and tend to look brighter25 (Fig. 6.11). Dyes also show a much greater variation in exhaustion levels; Yang and Huda26 studied the exhaustion of ten disperse dyes on PLA and PET fabrics and found that the percentage dye exhaustion of all the dyes was lower on PLA than on PET. However, the color yield of the dyed PLA was higher than on PET because of the lower reflectance of PLA. 15 O

NH2 O

O

OH

K/S

10

5 PLA PET 0 400

500 600 Wavelength (nm)

700

6.11 K/S curves of C.I. Disperse Red 60 on dyed PLA and PET.

Several of the major dye manufacturers have studied their ranges of disperse dyes to provide the most appropriate selection for applying to PLA. DyStar GmbH & Co. have worked closely with NatureWorks LLC to further the commercial dyeing technology for PLA and have recently provided details for dyeing both 100% PLA as well as blends with cotton and wool.27 The optimum dyeing conditions recommended by DyStar for dyeing PLA is 110∞C for 30 minutes at pH 5. Practical experience has shown that the use of higher temperatures or longer times of dyeing can cause degradation of the polymer. One of the observations in the dyeing of PLA is that obtaining dark shades is more problematic, compared to PET. A reason for this is attributable to the

206


lower exhaustion levels, although, of course, dye selection has to balance many other factors including fastness requirements, reproducibility, and levelness. There are indications that the color exhaustion and color yields can be increased by modification of the basic PLA polymer, altering the proportions of the D-and L-isomers, and thereby changing the amorphous/crystalline ratios. Higher D-levels have more amorphous and less crystalline regions and allow for increased dye exhaustions.28 Comparison of PLA fabrics with varying Disomer content revealed differences in the enthalpy of fusion, and, hence, percentage crystallinity; high D-fibers have more amorphous and less crystalline regions in the polymer, with respect to low D-fibers. High D-fabrics display greater dye exhaustion and color strength with respect to low D-fabrics in all dyes and all concentrations as a result of the greater number of amorphous regions in high D-fibers with respect to low D-fibers. In application of a dye mixture for a black shade high D-fibers are able to be dyed to an excellent black shade, whereas low D-fibers appear very brown, due to less exhaustion of the blue component of the mix. In terms of wash fastness, there is very little difference between high D-fibers and low D-fibers; this is because the glass transition temperature for both fibers is very similar. Color fastness figures tend to be slightly lower than on PET. One reason for lower wet fastness is believed to be due to more movement of dye to the surface by thermomigration, during post-heat treatments. It may also be that the lower thermosetting temperature for PLA (130∞C) compared to PET (190∞C) means less sublimation of the dye from the surface.29 With the introduction of PLA into apparel, developments and commercial adoptions have included fabrics made from 100% PLA as well as in blend with other fibers. The main blends are either with cellulosic fibers (cotton, lyocell) or wool. Apart from any aesthetic or performance benefits, such blends also have the feature of being biodegradable compared to their PET counterparts. The wet processing of cellulosic blends need to be adapted to recognize the sensitivity of PLA to alkali treatments. This causes some limitations, as bleaching and dyeing systems for cellulosic fibers generally use alkaline processes. However, the potential significance of this blend has been recognized, and methods are available for all stages of wet processing. These include the use of neutral bleaching systems based on TAED (tetraacetylethylenediamine), and direct dyes, as well as the more conventional alkaline bleaches and selected reactive dyeing systems. A detailed study into the effect of the various wet processes on the molecular weight and physical strength of PLA was carried out at UMIST, and this confirmed the feasibility for suitable processing.30,31 Dyeing of PLA/wool blends does not present such a problem, as both fibers have the same characteristic with regard to alkali. Indeed, with their similar stress–strain profile, there would seem to be some benefits compared to PET/wool.


207

There is much to be still learnt about the dyeing of PLA but it would seem that processes are available to achieve commercial viability. In real terms, it is not that long ago that fibers such as lyocell and indeed PET were introduced to the textile world, with many technical difficulties in their processing. These are now well accepted fibers and it will be interesting to watch as PLA follows a similar course.

6.4.2

Homeware

Typical products encompassed in this segment range from pillows, duvets, blankets, mattress pads, carpet tiles, office panel fabrics, drapes to bonded fiber products such as mattresses. As in the other sectors, the unique origin of these fibers, being derived from natural sugars obtained from annually renewable resources, coupled with the performance benefits, is proving to have a strong appeal to consumers in various geographies. The resistance to UV and low flammability, low smoke generation and low toxic gas on burning are attractive properties for this market segment, which differentiate PLA fabrics manufactured from conventional petrochemical-based synthetics. Superior resilience found in crimped staple fiber products such as fiberfill, coupled with the natural wicking performance of the fiber are added features, which further enhance the scope of opportunities for PLA. Since this moisture wicking behavior is inherent to the fiber and achieved without the use of finishes this behavior should not decrease over time. Independent testing indicates the wicking behavior of untreated PLA fibers is superior to either untreated or treated polyester fibers.32 Laboratory UV resistance testing using a Xenon Arc33 indicates that in comparison with polyester and acrylic fibers, PLA fabrics have superior strength retention than polyester and far superior resistance to discoloration than acrylics. Independent testing by the Hohenstein Institute34 shows that PLA fibers when used in duvets provide a better microclimate between the body and the duvet. The superior wicking properties of PLA fibers, compared with polyester, results in the dynamic adjustment of the moisture level by dissipating moisture as the humidity level changes. In pillows, testing has shown that PLA fibers offer outstanding filling power and resilience after three years of simulated usage.35 Flammability testing indicates a Limiting Oxygen Index value for PLA fibers of 24–28 when tested according to ASTM D2863. This is superior to untreated PET fibers, cotton, rayon and acrylics and equivalent to natural protein-based fibers such as wool. In addition, flame propagation and time to extinguish are properties of considerable interest in the furnishings segment. Fabrics made from untreated PLA fibers show far shorter burn times than cotton and polyester fibers. Untreated PLA fibers meet the test criteria for UNI 8456(1986), UNI 9174(1987) and UNI 9174/A1 (1996) that measure

208


the time to self-extinguish (after the fabric is removed from the heat source). This test also measures the rate of flame propagation, char length, and time of flaming of any dripping materials to extinguish. The results enable 100% PLA fabrics to be classified as Category 1 as required by the Italian Drapery Industry.36 Fabrics made from PLA fibers have also passed flammability tests such as NFPA 701, which covers the fire safety requirements for textiles used in interior furnishings for public occupancy buildings.36 Typical products include drapery, cubicle curtains, wall covering and protective outdoor coverings such as tarpaulins and tents. Finally, a prototype mattress made from 100% PLA fibers successfully passed Cal 129 testing for furniture flammability.36 All these tests were conducted on 100% PLA without any flame retardant additives. However, individual results will depend upon fabric construction and the use of finishes or dyestuffs. The unique resistance to staining and soiling of PLA Bulk Continuous Filament (BCF) has led to significant interest in PLA fibers for use in carpets. Presently, PLA fibers are used in carpet mats for the hybrid Toyota Prius in Japan as well as in carpet tiles for domestic and institutional markets. In the technology development area PLA/PLA binder fibers are finding application in mattresses, and other bonded batting areas. As described earlier, PLA bicomponent technology utilizes the unique polymer properties induced by the two optically active forms of the lactic acid monomer. By controlling the ratio and distribution of the D- and Lisomers in the polymer chain, it is possible to induce different crystalline melting points during melt processing; this feature offers distinct benefits particularly in the binder fiber area. Bicomponent fibers with a sheath softening point of 60∞C (amorphous) up to a crystalline melting point of 175∞C can be produced (Fig. 6.12). As well as bonding to other PLA fibers or petroleumbased synthetics, PLA bicomponent binder fibers are increasingly being used to bond natural fibers such as jute and hemp in automotive applications for spare wheel covers and door panels. Bicomponent PLA technology is also finding applications in self-crimping and micro denier technologies. Technology growth areas are in sheath/core, side by side, segmented pie and islands in the sea structures. Overall, PLA binder fibers can replace existing synthetic polymers where renewable resource is a benefit or where additional performance such as controlled temperature bonding, controlled shrinkage or lower temperature processing is required. In addition, improved or reduced adhesion and alternative approaches to soluble/non-soluble island/sea combinations are all possible and under development.


209

180.0

Fiber melt temperature (celsius)

170.0

160.0

150.0

140.0

130.0

120.0

110.0 0

2

4

6 %D

8

10

12

6.12 PLA fiber melt temperature as a function of % D-isomer.

6.4.3

Nonwovens

Nonwoven products are a major application segment offering great potential for the unique benefits of PLA fibers. Outside of the fiberfill products the major markets are in spun bond, industrial and household wipes, hygiene and filtration areas. Spun bond PLA products can be produced on typical polyester spun bond lines in a variety of fabric weights. Filament velocities of 3500 m min–1 and above are needed to produce fabrics with the required low shrinkage performance. Key applications are in carpet backing, hygiene, and compostable geotextiles for soil erosion control and plant as well as crop protection. Under the correct conditions of temperature and humidity PLA fabrics are completely compostable and return naturally to the soil releasing carbon dioxide and water. Under normal storage conditions and use, however, the fabrics are durable enough to meet the various market and supply chain requirements. Industrial/household wipes In recent years there has been a significant growth in the wipes market, and in particular the wet wipes segment across a variety of applications. Most wet wipes comprise a blend of cellulose, viscose or rayon with a synthetic fiber such as polyester or polypropylene. Up to 50% of the wipe comprises

210


these synthetic fibers. Recent surveys indicate a 6.5% annual growth in personal care wipes versus 2.5% in baby wipes in the US.37 Similar trends are being seen in Europe and Japan. New product introductions in personal care and household cleaning markets are occurring rapidly. Typical new trends and applications for wet wipes include: ∑ ∑ ∑ ∑ ∑

Feminine hygiene, facial cleansing, hemorrhoid treatment, etc. Functional treatments (antimicrobial, cleaners, abrasives, etc.). Decorative patterns. Continued push toward cloth-like aesthetics. Environmentally friendly materials and processes.

Spun laced investments have recently been announced by Jacob Holm, Spuntech, Orlandi, & Green Bay Nonwovens. In addition, Japan has large spun lace capacity with markets that are seeking environmental solutions. Wipes converters globally are expanding capabilities and expediting line extensions. PLA fibers show superior wicking performance when compared with the petroleum-based synthetics used in these applications. This inherent property leads to increased rates of liquid absorption,32 in addition the appeal of an all-natural-based wipe has demonstrated strong consumer pull. The wipes market is, however, extremely price sensitive; recent price fluctuations in the price of oil coupled with the advances made in PLA fiber and resin production means that pricing differences between the petrochemical-based and PLA fibers continues to narrow. Hygiene PLA fibers, because of their natural wicking properties, are finding utility in the diaper and feminine hygiene markets, both in spun bond top sheets and acquisition/distribution layers. Again the ability to replace petroleum-based fibers with enhanced performance with a natural-based fiber has strong consumer appeal. An additional benefit is that since PLA fibers are fully compostable the products can be disposed of by composting if the infrastructure for disposal is in place. Filtration and separation Two areas of potential applicability of PLA polymers have appeared in the industry in recent years. ∑ Triboelectric media to improve filtration efficiency. ∑ Filters, for example in the automotive and chemical industries, as well as single use applications. The unique combination of properties displayed by PLA fibers offers promise


211

in these areas. Untreated PLA fibers are repelled by glass surfaces and are attracted to polypropylene, indicating that they are somewhat electropositive. Disposable filters are also an area of interest for PLA fibers.

6.4.4

Medical applications

Textile fibers can be used to cultivate different human organs. The process involves culturing and growing living cells, taken from human organs, on a textile scaffold, to the desired 2-dimensional and/or 3-dimensional shapes. The scaffold is made from biodegradable and resorbable fibers which are in turn produced from biocompatible and degradable polymers. The major bioresorbable fibers used in implants are PLA and PGA (polyglycolic acid – see Chapter 7). They can either be used as a single polymer or by blending a copolymer of PLA and PGA. Varying the proportions of PLA and PGA alters the degradation rate and strength retention time of the fiber. These properties can therefore be varied in this way according to the requirements of specific medical applications. During the process of degradation, fibrous connective tissues replace the degrading implant. The key advantage is that no further surgery is required to remove the products since they slowly degrade in the body without any side affects. The US Food and Drug Administration (FDA) has approved the use of PLA for certain human clinical applications. Also, PLA-based materials have been used for bone support splints.

6.4.5

PLA as a plastic

Although it is outside the scope of this chapter, it is important to recognize the tremendous scope for PLA as a plastic in several applications. The material has significant potential use in fresh food rigid packaging, bottles for beverages like milk, juice and non-carbonated water. PLC can also be made into an ideal film for use as labels, wrappers and windows.

6.5

Environmental sustainability

The previous sections of this chapter have examined the manufacturing of PLA, its various properties as a polymer and a fiber, and considered the potential for its use in a range of commercial applications. Additionally, the environmental aspects for producing and using PLA products must be appraised in any discussion about its significance as a sustainable commercial polymer material. Environmental sustainability is about making products that serve useful market and social functions with lower detrimental environmental impact than the currently available alternatives. The case for PLA in this respect

212


would seem to be very strong and needs to be appraised in any discussion about its potential as a significant polymer material. The ideal environmentally sustainable product provides equivalent function as the product it replaces and is available at competitive costs. It is made from renewable resources, can itself be constantly renewed without degradation in quality or performance, and has a minimum environmental impact. Such a product is made using only substances known to be safe for both humans and the environment. Ideally the life cycle of the sustainable product is in balance with the surrounding ecosystem.

6.5.1

Polymer processing and environmental measures

According to Vink et al.38 there are probably three items that are generally considered to have an increasing global importance with regard to environmental concerns: fossil energy use; greenhouse gas emissions; and water use. Fossil energy use The existing range of petrochemical-based plastics is diverse, specialized and mature, so that precise and exact comparisons with PLA, a single product performing multiple functions, are difficult, especially considering the great number of impact categories compared. Figure 6.13 plots the fossil energy requirement for these products. Data for the petroleum-based polymers was supplied by the Association of Plastics Manufacturers in Europe (APME). The data is valid for the polymers as produced in Europe. A key finding of the analysis is that the first generation polylactide production system (PLA1) uses 25 to 55% less fossil energy than the petroleum-based polymers.38,39 Process improvements are targeted by NatureWorks LLC for the near future involving the use of biomass (B) and wind power (WP) as energy sources in the PLA production process (PLA B/WP), and with these improvements the use of fossil energy can be reduced by more than 90% compared to any of the petroleum-based polymers being replaced. This also will give a significant reduction in fossil energy related air and water emissions. This comparison represents the outstanding potential for environmental benefits for polymers made from renewable resources. It needs to be recognized that the data for PLA1 and PLA B/WP represent engineering estimates. In addition, there is good reason to expect improvements in the actual performance versus the estimates.38 Despite years of development work, the commercial manufacturing process for PLA is in its infancy. If the experience from petrochemical-based polymers offers any instruction, it is that process improvements implemented in the early years of a technology typically lead to substantial cost improvements. This is because the pursuit


213

160.0 Fossil feedstock Fossil fuels

140.0

MJ kg–1 pellets

120.0 100.0 80.0 60.0 40.0

PLA B/WP

PLA 1

PP

PET AM

PET SSP

LDPE

GPPS

Cellophane

HIPS

PC

Nylon 66

0.0

Nylon 6

20.0

6.13 Fossil energy requirement for some petroleum-based polymers and polylactide. The cross-hashed part of the bars represent the fossil energy used as chemical feedstock (the fossil resource to build the polymer chain). The solid part of each bar represents the gross fossil energy use for the fuels and operations supplies used to drive the production processes. PC = Polycarbonate; HIPS = High impact polystyrene; GPPS = General purpose polystyrene; LDPE = Low density polyethylene; PET SSP = Polyethylene terepthalate solid state polymerization (bottle grade); PET AM = Polyethylene terepthalate Amorphous (fibers and film grade); PP = Polypropylene; PLA1 = Polylactide (first generation); PLA B/WP (Polylactide, biomass/ windpower scenario).

of cost improvements for competitive reasons often targets energy use due to its relatively high contribution to overall material costs. For example, through work on biocatalyst and lactic acid manufacturing process, NatureWorks LLC expects to achieve improvements that should further improve the performance of their production plant and simultaneously reduce energy demand. There is therefore good reason to expect a performance improvement trajectory for PLA1 that mirrors the experience from the current incumbent materials. Global climate change Global climate change has been identified as perhaps the most important environmental issue of this century.40 Greenhouse gas emissions are not exactly the same as combusted fossil fuel emissions, because several noncombustion gases can contribute to global climate change as well. For example,

214


methane (CH4) is a potent greenhouse gas that can emanate from natural gas system leaks, decomposition of biological materials, and chemical/industrial processes. However, greenhouse gas emissions are closely correlated to fossil fuel emissions because combustion of fossil fuels is the source of most anthropogenic greenhouse gases. NatureWorks LLC has undertaken a comparison of the contributions to global climate change from a range of petrochemical-based polymers as well as the two PLA cases described above.38 This comparison is depicted in Fig. 6.14. In conducting this analysis, Vink et al.38 relied upon the 100-year time horizon Global Warming Potentials for greenhouse gases, a time period generally accepted as the mean atmospheric residence time for the most volumetrically significant greenhouse gas, carbon dioxide. A check of the data revealed that use of the 20- and 500-year time horizons generates the same ranking among the products studied. According to the Intergovernmental Panel on Climate Change (IPCC) the relative global warming potentials of the three largest (volumetric) greenhouse gases are: CO2-1, CH4-21, and N2O-310.41 These factors were used in NatureWorks LLC’s analysis. As in the comparison of fossil energy use, the analysis compares conventional polymers with PLA from cradle to pellet (from raw materials to the point where the product is ready for shipment to a converter or fabricator). All emissions values were converted to CO2 equivalents in order to facilitate comparison. Nitrous oxide

Methane

PET SSP

PLA1

Carbon dioxide 8

PLA B/WP

6 4

PP

PET AM

LDPE

GPPS

HIPS

Cellophane

–4

PC

0 –2

Nylon 6

2

Nylon 66

kg CO2 eq. per kg polymer

10

6.14 Contributions to global climate change for some petrochemical polymers and the two polylactide polymers (for key to polymers see Figure 6.13)

The analysis demonstrates that the PLA1 production process enjoys a substantial advantage over most polymers, and is comparable to several others. Even more exciting are the greenhouse benefits that derive from the transition to biomass feedstocks and reliance on wind energy for the balance of plant energy requirements. The utilization of the lignin fraction of lignocellulosic feedstocks for process heat generation ‘closes the loop’ on carbon related to energy generation, and in combination with other factors


215

yields a negative greenhouse gas impact for PLA pellets. A most appealing result of the use of agricultural feedstocks for the PLA polymer production and most of the process energy requirement means that customers using PLA cannot only use PLA as a product, but as a component of their greenhouse gas reduction strategies. Life cycle assessment reveals that no petroleum-derived polymer can rival the greenhouse gas sink effect of the improved PLA process. Although disposal of PLA products – whether by combustion, composting or other conventional means – results in a return of carbon dioxide to the atmosphere, this advantage survives. Water use Vink et al. also studied the water use.38 Figure 6.15 gives the gross water use of the traditional polymers and the two PLA cases (PLA1 and PLA B/WP) as described above. The gross water use is the sum of public supply, river, canal, sea and well water and used as cooling water, process water and irrigation water. Despite the use of irrigation water during corn growing and the two water-based processes (dextrose and lactic acid production) the total amount of water required is competitive with the best performing petrochemical polymers. 800 Irrigation water Cooling water Process water

kg water per kg polymer

700 600 500 400 300 200

PLA

PP

PET AM

PET SSP

LDPE

GPPS

Cellophane

HIPS

PC

Nylon 6

Nylon 66

0

PLA B/WP

100

6.15 Gross water use by petrochemical polymers and the two PLA cases (for key to polymers see Figure 6.13).

6.5.2

Disposal options

The most common waste management options for the fossil fuel-based polymers are incineration, landfill and mechanical recycling. In addition to these

216


traditional processing routes, the PLA waste streams can also be processed using composting, chemical recycling and anaerobic digestion (see Chapter 1). Composting Composting is a beneficial waste management system, particularly where landfill sites are limited, and in more densely populated locations. It does require an appropriate infrastructure to be set up, but progress is being made, particularly in parts of Western Europe.42 Composting is a method of waste disposal that allows organic materials to be recycled into a product that can be used as a valuable soil amendment. The primary mechanism of degradation of PLA is hydrolysis, catalyzed by temperature, followed by bacterial attack on the fragmented residues. In composting, the moisture and the heat in the compost pile attacks the PLA polymer chains and splits them apart, creating smaller polymer fragments, and finally, lactic acid. Microorganisms, found in active compost piles, consume the smaller polymer fragments and lactic acid as energy source. Since lactic acid is widely found in nature, a large number of naturally occurring organisms metabolize lactic acid. At a minimum, bacteria and fungi are involved in PLA degradation. The end result of the process is carbon dioxide, water and some humus.7 In summary, via composting, the carbon dioxide which has been harnessed during corn growing flows back into the atmosphere and the short cycle carbon dioxide loop has been closed. The degradation process is temperature and humidity dependent. PLA is compostable at industrial composting facilities, but will not degrade sufficiently fast in domestic composting piles since the minimum required conditions are typically not present.43 A typical degradation curve of PLA under composting conditions is shown in Fig. 6.16. Chemical recycling Vink et al. describe43 the possibilities of chemical recycling as a promising future alternative waste disposal route. The PLA polyester polymer is formed from reversible polycondensation reactions and can be depolymerized by hydrolysis. This equilibrium results in recycling advantages for polyesters such as PLA. Manufacturing waste, converter waste, or post-consumer PLA materials can be recycled by chemical means to produce lactic acid monomers and oligomers. These materials can then be fed to the front end of a manufacturing process for making PLA lactide, ethyl lactate, or other lactide derivatives. The recycling can be carried out with water at a wide range of temperatures (100–250∞C). The reaction rate is enhanced by a catalyst such as nitric acid as is common in the PET recycling industry. The reactor residence times for PLA hydrolysis are in the order of hours, and depend on reactor temperature and catalyst level.

80

217 120

70 100 60 80 50 40

60

30

40

20 20 10 0 0

1

2

Mn

3

7

10 12 15 Time (days)

20

25

30

35

40

% Biodegradation (CO2 recovery)

Number average molecular weight, thousands


0

% biodegradation (CO2 recovery)

6.16 Biodegradation of PLA in compost at 60∞C + 95% humidity.

Chemical recycling should be considered in any waste management system for PLA, since from a life-cycle perspective, it represents a relatively small amount of net chemistry compared to the CO2-to-PLA cycles for incineration or composting. Simple hydrolysis can turn waste PLA back into fully functional lactic acid, at potentially low economic and environmental cost, contributing to the total sustainability of PLA production. The inferences from the studies of Vink et al.43 confirm the positive impact that PLA has in addressing the key environmental concerns of today when compared to petrochemical-based polymers. They also show the additional benefits that can be gained by making environmentally responsible modifications to the existing manufacturing process. The commercial producer of PLA, NatureWorks LLC, states that its commitment and ethos is based on ‘making plastics from annually renewable resources which meet the needs of today without compromising the earth’s ability to meet the needs of tomorrow’. Its philosophy is based on ‘reducing the environmental footprint’ and on ‘designing products with end-use disposal in mind’.44 The company is committed to environmental responsibility and therefore it would seem that the polymer market as we know it today will experience some very significant changes in the near future.

218

6.6


Future trends

The vision for PLA fibers is more than just developing new performance products; it encompasses the additional goal of reducing total environmental impact. The result is a product that is more sustainable than comparable polymers on the market today. PLA fibers have made significant steps towards creating more sustainable products with unique performance attributes. This technology allows one of the world’s most commonly used materials, plastics, to be made from simple plant sugars, which are then turned into fibers using conventional melt spinning equipment and processes. Made from annually renewable resources, PLA plastics and fibers use 20 to 50% less fossil fuel resources than is required by conventional petroleum-based resins. Fossil resource use in the manufacture of these materials will continue to decrease as plans are put into place to switch feed stocks to biomass (corn stalks, leaves, etc.) and possibly integrate alternative energy sources, such as windpower. With PLA, carbon dioxide is removed from the atmosphere when growing the feedstock crop and is returned to the earth when PLA is composted. Since this process recycles the earth’s carbon, PLA emits less CO2 compared to other petroleum-based fibers. Disposal of PLA fits within the existing disposal systems but also includes the additional option of composting. PLA fibers are increasingly penetrating markets traditionally occupied by petroleum-based synthetics. The rationale for this trend is the increasing public concern over the depletion of natural resources and the accompanying atmospheric pollution. Various studies indicate that reserves of oil and gas will eventually be depleted, although the actual time scale is a topic of considerable debate. It is evident, however, that the use of annually renewable crops as a means of producing the materials we need to sustain our everyday lives will continue to accelerate. PLA fibers offer the promise of ultimately reversing the damage we have imposed upon the earth while extending the usable life of the diminishing oil and natural gas reserves. Ultimately the technology of using natural crops to produce fibers and packaging will become widespread across the globe. The use of waste products, referred to as biomass, will also provide the fuel to drive the factories used to convert these polymers to the variety of products needed to sustain the ever-increasing demands for an improved quality of life. The key to this transformation lies in the economical manufacture of these renewable resource-based products coupled with meeting the performance demands of the targeted applications. PLA fibers and packaging products are well on the way to realizing this goal. Presently these fibers are produced from natural corn sugar. However, as the technology develops, any starch-based crop will be used. Advances in farming practices will enable higher crop yields with subsequent improved


219

economics. Additional improvements in the fermentation and polymerization processes along with economies of scale will also contribute to the improved cost basis for these products. PLA products have come a long way from the early work of Carothers in 1932. The ability to economically manufacture packaging and fibers products has led to rapid growth. This growth will continue. We are at the beginning of a new industrial revolution in which PLA fibers are playing a leading role. In addition, PLA fibers are still in their infancy: improvements in chemistry and downstream process development will lead to new applications and a bright future for these renewable resource-based melt-spinnable fibers.

6.7 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

22.

References Tsuji, H. and Ikada, Y., J. Appl. Polym. Sci., 1998, 67, 405. Drumright, R.E., Gruber, P.R. and Henton, D.E., Adv. Mater., 2000, 12 (23), 1841. Lipinsky, E.S. and Sinclair, R.G., Chem. Eng. Prog., 1986, 82 (8), 26. Vert, M., Schwacch, G. and Coudane, J., J. Macromol. Sci. Pure, 1995, A32, 787. Lunt, J., Polym. Degrad. Stabil., 1998, 59, 145. NatureWorks LLC, http://www.ingeofibers.com/ingeo/home.asp (accessed April 2004). Gruber, Pat and O’Brien, Michael, Biopolymer Volume 6, Chapter 8, Polylactides: NatureWorks® PLA, June 2001. Plastics Technology, January 1998, 13–15. Gruber, et al. USP 5 142 023. Mecerreyes, D. and Jerome, R., Macromolecular Chemistry and Physics, 1999, 200 (12), 2581. Stevels, W.M., Dijkstra, P.J. and Feijen, J., Trends in Polymer Science, 1997, 5 (9), 300. O’Keefe, B.J., Monnier, S.M., Hillmyer, M.A. and Tolman, W.B., J. Am. Chem. Soc., 2001, 123 (2), 339. Kricheldorf, H.R. and Krieser, I., Makromol. Chem., 1986, 187, 186. Kurcok, P., Kowalczuk, M., Hennek, K. and Jedlinski, Z., Macromolecules, 1992, 25 (7), 2017. Du, Y.J., Lemestra, P.J., Nijenhuis, A.J., Vanaert, H.A.M. and Bastiaansen, C., Macromolecules, 1995, 28 (7), 2124. Kricheldorf, H.R., Scharnagl, N. and Jedlinski, Z., Polymer, 1996, 37 (8), 1405. Sinclair, R.G., USP 4 045 418. Drumright, R.E., Gruber, P.R. and Henton, D.E., Advanced Materials, 2000, 12 (23), 1841. Lunt, J. and Bone, J., AATCC Rev., 2001, 1 (9), 20. Palade, L.I., Lehermeier, H.J. and Dorgan, J.R., Macromolecules, 2001, 34 (5), 1384. Suesat, J., ‘Investigation of the Influence of Fibre Morphology on the Dyeing and Fastness Properties of Poly(Lactic Acid)’, PhD Thesis, UMIST, Manchester, UK, 2004. Ingeo Fibres bring natural performance – low odor. Testing by Odor Science and Engineering Inc., Technical Bulletin 290904.

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23. Nash and Abel International Marketing, Base source: PCI World Synthetic Fibres Supply/Demand Report 2002. 24. Test Report no. ZO.4.3805, Comparison of physiological comfort of polyester (PET)/cotton and NatureWorks™ fibers/cotton fabrics, Forschungsinstitut Hohenstein 5/24/00. 25. Takashi Nakumara, International Textile Bulletin, 4/2003, 68. 26. Yang, Y.Q. and Huda, S., AATCC Rev., 2003, 3 (8), 56. 27. DyStar Textilfarben GmbH & Co. Deutschland KG, ‘Ingeo™ Fiber Coloration Pack’. 28. Blackburn, R.S., Zhao, X. and Farrington, D., Johnson, L., Dyes and Pigments, 2006, in press. 29. Phillips, D., Suesat, J., Taylor, J.A., Wilding, M., Farrington, D., Bone, J. and Dervan, S., Coloration Technology, 2004, 120 (5), 260. 30. Phillips, D., Suesat, J., Wilding, M., Farrington, D., Sandukas, S., Sawyer, D., Bone, J. and Dervan, S., Coloration Technology, 2004, 120 (1), 35. 31. Phillips, D., Suesat, J., Wilding, M., Farrington, D., Sandukas, S., Sawyer, D., Bone, J. and Dervan, S., Coloration Technology, 2004, 120 (1), 41. 32. Moisture Transport in Ingeo Fibers Non-Woven Fabrics. Ingeo Fibers Technical Bulletin 380904 (Source: www.ingeofibers.com). 33. Ingeo fiber-based fabric: UV resistance. Technical Bulletin 370904 (source: www.ingeofibers.com). 34. Ingeo fiber v high-end polyester in duvet/comforter testing. Technical Bulletin 130904 (source: www.ingeofibers.com). 35. Ingeo fiber v high-end polyester in pillow loft and support testing. Technical Bulletin 320904 (Source: www.ingeofibers.com). 36. Furnishings flammability characteristics. Technical Bulletin 110104 (source: www.ingeofibers.com). 37. John, R., Starr Associated Nonwovens market survey (source: www.johnrstarr.com). 38. Vink, E.T.H., Rabago, K.R., Glassner, D.A. and Gruber, P.R., Polymer Degradation and Stability, 2003, 80 (3), 403. 39. Glassner, D., Presented at the 23rd Symposium on Biotechnology for Fuels and Chemicals, Commercialization of polylactide polymers, 9 May 2001. 40. Sharron, E., Global climate change and the challenges of stewardship: man and nature in the 21st century, 2 June 2002, Climate Independent Media Center (available from: http://www.climateconference.org). 41. IPCC Intergovernmental Panel on Climate Change. Climate change 1995 – the science of climate change. Albritton, D., Derwent, R., Isaksen, I., Lal, M. and Wuebbles, D. In: Houghton, J.T., Meira Filho, L.G., Callander, B.A., Harris, N., Kattenberg, A. and Maskell, K. (eds), Radiative forcing of climate change. Cambridge (UK): Cambridge University Press, 1995, 119–21. 42. Bohlmann, Gregory M. with Toki, Goro, ‘CEH Marketing Research Report, Biodegradable Polymers’. 43. Vink, E.T.H., Rabago, K.R., Glassner, D.A., Springs, B., O’Connor, R.P., Kolstad, J. and Gruber, P.R., Macromol. Biosci., 2004, 4, 551–564. 44. Ingeo™ website (www.ingeofibers.com). Ingeo and NatureWorks are trademarks of NatureWorks LLC.

7 Poly(hydroxyalkanoates) and poly(caprolactone) I C H O D Á K, Polymer Institute of the Slovak Academy of Sciences, Slovakia and R S B L A C K B U R N, University of Leeds, UK

7.1

Introduction

Bacterial polyesters poly(hydroxyalkanoates) (PHAs), with poly(hydroxybutyrate) (PHB) (7.1) as the first homologue, belong to the most interesting, but also the most controversial, group of biodegradable polymers. Advantages include production from fully renewable resources, rather fast and complete biodegradability, biocompatibility, and excellent strength and stiffness, which favours this material as a polymer of the future.1 However, several serious drawbacks hinder its wider application, including rather high susceptibility towards thermal degradation, difficult processing related partially to thermal instability as well as to low melt elasticity, brittleness of the material resulting in low toughness (which increases further during storing due to an interesting phenomenon of physical ageing), and rather high price. These are the main reasons for low production volume and unsatisfactory number of applications.2 *

O *

R

H PHB R

O CH3

7.1

The low number of applications up to now seems to be also partially the reason for the high price of the polymer creating a vicious circle where the applications are not developing because of too high a price, while the price is not decreasing due to a low volume of produced polymer. Since fibres can be considered as a new prospective product, successful development of PHA fibres would also contribute significantly to the general spread of products from PHAs. Moreover, it is generally believed that after drawing, the properties of poly(hydroxyalkanoates) will improve including the increase of toughness. 221

222


Poly(caprolactone) (7.2) is a synthetic polymer prepared mainly by ring opening polymerisation of caprolactone. The polymer is, similar to PHAs, fully biodegradable, although the rate of biodegradation is lower compared to PHAs. This, together with a low melting temperature (of about 60∞C), is a reason that the polymer is used mainly either as a component of polymer blend or as a matrix for biodegradable composites. Among the latter, its mixture with starch is possibly best known under the trademark MaterBi produced by Novamont, Italy.3 Nevertheless, poly(caprolactone) has a number of interesting properties, such as good processability, high toughness and deformability, and good thermal stability. Therefore its wider application is expected in the future. O

*

*

O

7.2

7.2

PHA-based oriented structures

7.2.1

Materials and techniques available

Poly(hydroxyalkanoates) represent a number of materials with a broad range of properties. Generally all types described in scientific literature are produced by bacteria, although synthetic routes are also known and used.4 About 40 roots of bacteria exist able to produce polyester-type polymers.5 Although various bacteria differ in the conditions and the efficiency of the PHA production, they produce more or less the same products. The variation in the polymer produced is reached more by changes in the production conditions than by changing the root of bacteria; the most important from this point of view is the substrate for feeding the bacteria.6 By sophisticated selection of the substrate, PHB homopolymer, its copolymers with higher PHAs,7 even polyesters with branching8 functional groups (epoxy,9 aromatic structures10 chlorine,11 double bonds12) in the chain may be produced. The properties of the first homologue, poly(hydroxybutyrate) are similar to polypropylene, as seen in Table 7.1.2 While strength parameters (tensile strength, Young’s modulus) and the most physical properties (crystallinity, melting temperature, and glass transition temperature) are basically the same, the important difference consists in elongation at break and, consequently, toughness. While the ductile polypropylene breaks at elongation around 700%, PHB hardly exceeds 10%, with typical values between 1 and 3%; PHB copolymers with higher PHAs have higher elongation at break mainly due to much lower crystallinity. However, the preparation of these, while well mastered in laboratory conditions, results in much more expensive materials if industrial, large-scale production is considered. From this point of view, only PHB


223

Table 7.1 A comparison of physical properties of PHB, copolymers of PHB with higher PHAs, polypropylene (PP), and low-density polyethylene (LDPE) Property

PHB

20V1

6HA2

PP

LDPE

Melting temperature (∞C) Glass transition temperature (∞C) Crystallinity (%) Density (g cm–3) E modulus (Mpa) Tensile strength (MPa) Elongation at break (%)

175 4

145 –1

133 –8

176 –10

110 –30

ng3 ng 0.8 20 50

ng ng 0.2 17 680

50 0.91 1.5 38 400

50 0.92 0.2 10 600

60 1.25 3.5 40 5

1

poly(3-hydroxybutyrate-co-20 mol % hydroxyvalerate) poly(3-hydroxybutyrate-co-6 mol % hydroxyalkanoates) = 3% 3-hydroxydecanoate, 3% 3-hydroxydodecanoate, < 1% 3-hydroxyoctanoate, < 1% 5-hydroxydodecanoate 3 ng = negligible 2

copolymer with poly(hydroxyvalerate) (7.1 R = CH2CH3) could be considered to be suitable for some applications. Considering fibres based on poly(hydroxybutyrate) or higher PHAs, no successful process was reported for preparation of PHB fibres by conventional fibre processing technology, i.e. melt or gel spinning with subsequent hot drawing. Therefore, more sophisticated procedures have to be developed to achieve reasonable draw ratios, resulting in production of anisotropic material with important improvement of properties. To prepare fibres, it is usually advantageous to start with a polymeric precursor with molecular weight within certain limits. The molecular weight values should be as high as possible to achieve good drawability and high draw ratio. On the other hand, it should increase only to the values acceptable from the point of view of processing (spinning); in the case of very high molecular weights, extremely high draw ratios can be reached resulting in ultra-high modulus, e.g. for polyethylene,13 however, rather sophisticated preparation techniques, e.g. dry gel technology,14 have to be applied. In the case of PHB and generally PHAs, polymers with high molecular weight are also important concerning the rather high susceptibility of the polymer towards thermal degradation. Thus, starting with a polymer with high molecular weight may result in a polymer having the molecular weight still well above certain limits, even after rather demanding thermal treatment during processing and resulting decrease in the molecular weight due to thermal degradation. Fortunately, by a sophisticated selection of bacteria and preparation conditions, PHB or its copolymers with high MW can be produced. Kusaka et al.15 reported a preparation of P(3HB) with a final average molecular weight of between 1.1 to 11 million produced by Escherichia coli XL-1 Blue (pSYL105)16 containing a stable plasmid harbouring the Alcaligenes eutrophus

224


H16 (ATCC 17699) PHB biosynthesis geneoperon phaCAB. Two-step cultivation of the recombinant E. coli was applied for the production of high molecular weight P(3HB).17 Molecular weight of PHB produced within the cells was strongly dependent on the pH of the culture medium.17 Apparently, as mentioned above, the most important reason to synthesise high molecular weight PHB seems to be a high susceptibility of PHAs towards thermal degradation. When starting with HMW material, even after a substantial decrease in molecular weight, the polymer still has MW high enough to be processed to secure a product with acceptable properties. Another option to avoid these shortcomings might be to develop modified processing procedures, leading to a thermal treatment as short as possible or proceeding at a processing temperature below the degradation limits. Thus, solid state processing was suggested as a viable alternative for PHB processing with low thermal degradation.18 An extrusion of PHB powder at temperatures well below melting temperature was successfully preformed to products with improved mechanical properties. Compared to melt processed PHB, the ductility (and consequently toughness) improved significantly.

7.2.2

Processing/preparation

Generally, fibre-forming polymers can be processed either by drawing the preformed amorphous material at temperatures above, but near, the glass transition temperature (Tg) or by drawing a crystalline material below but near melting temperature. Thus, the easiest way to produce fibres should be melt spinning and, consequently cold or hot drawing. More sophisticated procedures involve so called gel-spinning, which was successfully applied for a production of ultra-high modulus polyethylene fibres13 and later for other polymers.19 These basic techniques have been tested and modified also for preparation of PHA fibres; other procedures have been suggested and investigated. Although the number of scientific papers dealing with PHB or other PHA fibres is much smaller compared to important synthetic polymers, the information on various spinning/drawing processes deserves reviewing, especially regarding differences between different processes and between properties of fibres prepared by different research teams. Melt spun fibres PHB melt spinning is from several points of view not as straightforward a process as for many other fibre-forming polymers. The problems include rather rapid thermal degradation of PHB at temperatures just above melting temperature, low melt elasticity, and slow crystallisation after spinning (which results in a formation of large crystallites leading to extremely brittle material). The brittleness increases during storing due to an interesting phenomenon of


225

physical ageing; some improvements can be achieved by an addition of plasticisers and nucleating agents; boron nitride was reported to be one of the most efficient nucleating agents for PHB.20 An important problem consists in the fact that crystalline PHAs and especially PHB are rather brittle materials, though the brittleness can be partially removed by compression moulding. The cold rolling of PHB at room temperature results in an increase in elongation at break from 8% up to 200%, if measured in tensile mode in the rolling direction, while no change (elongation 8%) was observed if measured in the perpendicular direction.21 Such pre-deformed material could be drawn at 125∞C leading to an increase in tensile strength by a factor of 5. A patent from 198421 claims the procedure of cold rolling and subsequent drawing at temperature in the range of 50 to 150∞C below the melting temperature of the respective polymer. Melt spinning may lead to an improvement of some of the features mentioned above. Gordeyev et al. reported already in 1977 that melt spinning followed by a pre-orientation may prevent the ageing process.22 However, the melt spinning is not so easy; several methods have been developed and described, differing in many details of the process. Yokouchi et al.23 and Nicholson et al.24 reported the procedure of the melt quenching below Tg and subsequent drawing. A more successful process seems to be the drawing of melt spun PHB immediately after spinning while the material is still hot, to obtain preoriented material. These pre-oriented fibres can be drawn to high draw ratios (DR) even after few weeks of storing at room temperature, under conditions when bulk PHB would turn to be extremely brittle as a result of physical ageing.22 A successful procedure leading to melt spun fibres with good properties was described by Gordeyev and Nekrasov.25 The authors suggested dissolving PHB with a molecular weight of about 300,000 and Tm = 180∞C (determined by DSC) in chloroform and filtering the solution before spinning to remove impurities as well as high molecular weight portions, although it is not explained why higher molecular weight portions are recommended for removal nor what portion, if any, of what molecular weight was actually removed. The spinning and pre-drawing step (DR = 2) was performed in an extruder heated in four regions between 170 up to 182∞C. Hot drawing proceeded at 110∞C and the DR achieved was about 8. Although only a modest rise of Young’s modulus was achieved, an increase in tensile strength by a factor 4 to 5 was found when compared to undrawn bulk PHB. Moreover, the fibres were rather elastic showing an elongation at break of about 50%, as seen in Table 7.2. More detailed study on the melt spinning/hot drawing of PHB was published by Yamane et al.26 Filaments, about 0.3 mm in diameter, were obtained by extruding. These fibres were drawn at 110∞C immediately after melt spinning; the maximum draw ratio achieved was about 6. The drawing immediately

226


Table 7.2 Mechanical properties of melt spun PHB fibres Sample

Draw ratio

Tensile strength (MPa)


Elongation at break (%)

As spun Hot drawn 110∞C Annealed 155∞C, 1 h

2 8 8

109 127 190

2.2 3.5 5.6

160 95 54

after melt spinning is important since after even short storing periods, PHB turns to a brittle material which is impossible to draw. A further requirement seems to be a presence of nucleating agents to increase the originally low nucleation rate of PHB. Boron nitride can be used as an additive; contaminants such as remnants of proteins and lipids from the culture media can also act as efficient nucleating agents.26 The authors refer to further improvement of mechanical properties of the fibres by annealing after drawing. It seems that to achieve the drawability, a certain structure of the polymer must be formed. The material has to be in crystalline form, but crystals must not be too large to be able to be deformed during drawing. Thus, the basic requirement for melt spinning/drawing seems to be the presence of nucleating agent on the one hand and the drawing or at least pre-drawing while the material has not developed a fully crystalline structure on the other. High speed melt spinning and spin drawing Certainly, a demonstration of an ability to draw fibres is of primary importance for further development. The possible industrial production depends not only on procedures enabling spinning and drawing the polymer producing fibres with good properties, but also on efficiency of the production, which is limited mainly by a development of a procedure for high speed melt spinning. Such a process was described by Schmack et al.27 A spinning line consisted of an extruder, spinning pump, heated godets, and two winders, enabling the speed in a range of 2000–6000 m min–1, comparable with the speed of production of synthetic industrial polymeric fibres. Exceptional attention was paid to a thorough drying of the PHB powder prior to spinning to minimise hydrolytic degradation. In spite of the extreme care regarding the moisture removal, the viscometric molecular weight of the PHB being 540,000 for virgin powder dropped down to 175,000 after spinning; this has to be attributed to thermal chain scission since the water content of dried pellets was only 0.01%. The process described seems to be fast enough to be considered for an efficient production of PHB fibres, especially since the mechanical properties of the fibres are satisfying, as seen in Table 7.3. The paper deals with the procedure in rather a detailed way, so that the effect of


227

Table 7.3 Mechanical properties (tensile strength (s), elongation at break (e) and sonic modulus (E)) of PHB fibres in dependence on spinning speed (v) and draw ratio (DR)27 v (m min–1)

s (Mpa)

e (%)

E (Gpa)

2000 3000 3500

228 281 250

72 48 26

5.8 7.1 7.6

DR (m min–1)

s (Mpa)

e (%)

E (Gpa)

T1/ T2* (∞C)

4.0 4.5 5.0 5.4 5.5 6.4 6.9

52 108 220 178 263 310 330

10 60 53 71 60 45 37

n.a n.a n.a. 5.2 5.6 6.8 7.7

40/50 40/50 40/50 45/60 40/50 45/60 45/60

* temperature of the first and second godet in the production line

changing preparation conditions on the ultimate properties of the fibres can be estimated. Gel spun fibres Gel spinning is an example of a special technique for preparation of fibres with unique properties. The well-known commercially available fibres prepared by this method are those of ultra-high molecular weight polyethylene.19 The gel spun material can be drawn to very high draw ratios, well above 100; fibres exhibit extremely high stiffness and strength, reaching the values of 3 to 6 GPa and 150 GPa for tensile strength and modulus, respectively.13 The typical procedure leading to gel spun PHB fibres is described by Gordeyev et al.28 It involves dissolving the PHB in a suitable liquid (1,2dichloromethane is recommended as the best solvent); a solution with a PHB concentration as high as possible should be prepared, which depends on the original molecular weight and is about 20 wt % for PHB with Mw about 300,000 g mol–1. Then a solid gel is prepared by evaporation of a part of the solvent; at this stage the concentration of the polymer is about 30 wt %. The gel was extrudable at about 170∞C. The extruded gel is consequently processed in three stages.28 For the first, so-called pre-conditioning stage, the fibre was wound on a speed-controlled drum – the optimal pre-conditioning draw ratio was estimated by comparing Tex values of the extruded and drawn fibres, respectively. For further efficient drawing the optimal draw ratio of the preconditioning step was found to be around 2. Continuous hot drawing between

228


two rollers was performed at 120∞C as the second step – the total draw ratio was around 10. Finally the fibres were stretched at room temperature to 180% of the length after the second step – they were fixed and annealed at 150∞C for 1 hour. ‘As-spun’ fibres from the stage 1 could be drawn via the necking process both at as well as above room temperature; the necking begins at a strain about 6–7%. The mechanical properties of the drawn fibres are shown in Table 7.4. Table 7.4 Tensile properties of gel spun fibres at room temperature28 Sample

Draw ratio

Tensile strength (MPa)

Static modulus (GPa)

Dynamic modulus GPa)

Strain at break (%)

As spun Hot drawn Annealed

2 10 10*

103 332 360

2.0 3.8 5.6

4.6 5.8 7.5

250 104 37

* after hot drawing

The drawability depends on drawing temperature to a certain extent, manifesting the highest draw ratio (about 5) at 120∞C. Although the first step (pre-conditioned drawing after spinning) has to be performed soon after spinning, the pre-conditioned fibres, once drawn, can be stored for several months without losing the ability to be drawn at stage 2. Surprisingly, the drawn material exhibited rubber-like elastic behaviour. Presumably, drawing may introduce changes to the brittle bulk PHB similar to those of cold rolled polyester,29 the effect on drawing is much more pronounced, obviously as a result of a much higher degree of chain orientation. It was reported that tensile strength of gel spun fibres is about double that of melt spun material of similar parameters.28 This behaviour was attributed to a lower degree of thermal degradation due to lower thermal treatment during three-stage processing. However, this assumption was not directly proved, e.g. by comparison of changes in molecular weights. Comparison of mechanical properties of gel spun fibres from28 with melt spun fibres prepared by the high spin procedure shows certain differences, but generally the properties are similar, being dependent mainly on the draw ratio. In the latter case the original molecular weight of powder 550,000 dropped down to 175,000 after processing; similar molecular weight of the fibres can be expected after the less detrimental gel spinning process from the original molecular weight around 330,000. These considerations suggest that the procedure itself does not affect the ultimate properties of the fibres if the draw ratio and molecular weight of the fibre-forming polymer is the same; the effect of the process consists in secondary phenomena, mainly the extent of thermal


229

treatment which is less detrimental in the case of gel spinning compared with melt spun material. It must be stressed also that for gel spun fibres annealing leads to a certain increase in stiffness while tensile strength does not change.28 It is important to note that mechanical properties do not change significantly during storing as demonstrated by strength and modulus values measured during a period of 120 days. The modulus increased by a factor of less than 1.2 while a decrease in tensile strength was observed to about 82% of the original value.28 Besides fibre formation, oriented PHB can be prepared also in the form of films. A patented procedure30 refers to PHB with MW higher than 500,000, which can be oriented at temperature 144–180∞C. As an example, PHB with Mn = 6 ¥ 106 was treated at 160∞C at draw ratio 6.2 to produce oriented film with Tm = 186∞C, Tg value 2.2∞C, crystallinity higher than 90%, Young’s modulus 1.7 GPa, tensile strength 80 MPa, and 70% elongation. The strength parameters are not extraordinarily impressive, but reasonable elongation indicates that the material may form a flexible foil which could be considered for packaging. In that case, strength and modulus are similar to polypropylene foils and are substantially higher compared to common foil-forming material such as low density polyethylene. Other procedures Kusaka et al.15 described a preparation of PHB fibres with a draw ratio higher than 6 via stretching solution-cast high molecular weight films in a silicon oil bath at 160∞C. PHB with molecular weight well above 106 was used. The mechanical properties of the fibres are shown in Table 7.5. Compared with solution-cast isotropic undrawn films, drawing results in higher tensile strength while modulus is the same or slightly lower. Elongation at break is substantially higher and it is almost unaffected by ageing, especially if annealing

Table 7.5 Physical properties of solution cast drawn UHMW poly(3-hydroxybutyrate) films with original molecular weight Mn = 6,000,000; Mw = 16,000,000 DR

tanneal

tageing (days)

C (%)

s (MPa)

e (%)

0 0 6.5 6.5 6.5 6.5

0 1 1 1 2 2

7 7 7 190 7 190

65 ± 65 ± 80 ± 75 ± > 85 > 85

41 ± 41 ± 62 ± 88 ± 77 ± 100 ±

7 6 58 30 67 67

sec sec sec hrs hrs

5 5 5 5

4 3 5 8 10 10

± ± ± ± ± ±

E (GPa)

3 3 1 1 1 2

2.3 2.4 1.1 2.5 1.8 2.5

± ± ± ± ± ±

0.5 0.3 0.1 0.2 0.3 0.2

DR = draw ratio; tanneal = annealing time at 160∞C; tageing = ageing time at RT; C = crystallinity (X-ray); s = tensile strength; e = elongaton at break; E = Young’s modulus

230


of the fibre is performed after drawing. The elongation values, the absence of the ageing phenomenon as well as a certain decrease in modulus seem to be of interest especially considering a clear increase in crystallinity due to drawing and a further rise resulting from annealing. Obviously, high crystallinity is not necessarily a reason for the high stiffness/low toughness of PHB; the changes in crystal morphology and also changes in the supramolecular structure of the amorphous phase seem to be much more important. Some authors reported a conformational transformation as a result of drawing (e.g. Orts et al. for PHBV stretched films);31 however, reflections indicative for helix to planar conformation were not observed for UHMW PHB by Kusaka et al.15 A centrifugation spinning process of PHB fibres preparation was also demonstrated32 to be an alternative of gel spinning. An entangled fibrous material was produced which resembles ‘cotton wool’. The fibres were found to possess various surface irregularities such as pores with a diameter in the range 1–15 mm. Copolymers of poly(hydroxybutyrate) with higher poly(hydroxyalkanoates) Most of the papers on PHA fibres deal with poly(hydroxybutyrate). Although sometimes a copolymer of PHB with a low amount of poly(hydroxyvalerate) is also referred to as PHB; in the literature quoted in this review such inconsistency is not expected. The reason for not using PHBV may consist in the fact that the copolymer has a much lower crystallinity and consequently, also, modulus is lower compared to PHB. Thus, the processing to fibres may be more difficult because of low crystalline content and also the properties of the fibres may be expected to be less impressive. Moreover, PHBV itself is much tougher compared to PHB, but also more expensive, so that the need of an improvement of toughness via drawing is not crucial while the price may be prohibitive if the expected modest improvement in the properties of fibres is considered. The procedure of preparation of fibres based on a copolymer of PHB with higher PHA is described by Fischer et al.33 A copolymer PHB with hydroxyhexanoate (PHBH) shows high elongation but low tensile strength; cold drawing is believed to improve the strength behaviour. Solvent cast films of PHB copolymer with 5 or 12% of hydroxyhexanoate were melted in a hot press and subsequently quenched in ice water. The more or less amorphous films were oriented by cold drawing to DR 2 to 5 and annealed at various temperatures (23–140∞C), then further drawing was applied at RT before annealing. PHBH films were easily drawn at low stress to DR = 5. Similar to PHB drawn fibres, also PHBH stretched films showed an elastic behaviour after the sample was released from the clamps of the stretching equipment.33 Therefore an annealing procedure was required for a fixation of the extended polymer. It is interesting to note that no changes in the molecular weight


231

were observed as a result of the drawing procedure: this may be attributed partially to rather mild thermal treatment as well as to higher thermal stability of PHBH as a result of steric hindrance of the chain scission due to a presence of the propyl side chains.34 Considering mechanical properties of the copolymer PHBH with 5% of H, an increase of tensile strength was observed from 25 MPa for isotropic film up to 75 MPa for DR 5. In the same range of DR, modulus increased from 400 MPa up to about double value, while elongation at break decreased from an original 250% (isotropic sample) down to less than 100 % for DR 2 and then a monotonic increase back to the original 250% at DR 5 was observed. The increase in the H-copolymer content to 12% leads to a substantial increase in elongation of isotropic film and a certain decrease in tensile strength, while the decrease in modulus is substantial (from 400 down to 100 MPa). Monotonous dependences of all parameters with rising draw ratio were observed (a decrease in elongation at break and increase in tensile strength and modulus values). The two-step drawing resulted in continuing tendencies for all parameters as seen in Table 7.6. Crystallinity changes (Table 7.6) correspond with the trends in mechanical properties of drawn films. Table 7.6 Mechanical properties (tensile strength (s); elongation at break (e); and modulus (E)) and crystallinity (C) of drawn PHBH films and dependence on the content of hydroxyhexanoate (H) and draw ratio (DR) H content (%)

DR

s (MPa)

e (%)

E (GPa)

C (%)

5

1 5 10

32 ± 2 80 ± 1 140 ± 20

267 ± 30 258 ± 10 116 ± 10

480 ± 70 870 ± 40 1480 ± 150

42 ± 5 47 ± 5 65 ± 5

12

1 5

23 ± 4 53 ± 4

871 ± 70 204 ± 18

90 ± 3 30 ± 1

31 ± 5 35 ± 5

Oriented blends of PHAs Besides drawing of PHB or its copolymers with other PHAs, several attempts were made to prepare fibres from blends of PHAs with other polymers. These attempts are mainly aimed to obtain fibres with different properties or lower price; the latter being connected with either a less expensive second component of the blend or with easier processing. Park et al.35 investigated preparation and properties of fibres made from poly(L-lactic acid) (PLLA) and PHB of two different molecular weights. The two polymers are immiscible in the whole concentration range. The films of the blend after preparation by solvent casting were uniaxially drawn at either 2∞C for PHB-rich blends (close to PHB’s Tg) or 60∞C for PLLA-rich blends (around PLLA’s Tg). It is of interest to note that while blends based on the

232


PHB matrix were impossible to draw above Tg of the matrix due to rapid stress relaxation, PLLA-rich blends have to be drawn above Tg of the matrix polymer. Although PLLA domains in normal molecular weight PHB matrix (600,000) remained almost unstretched during cold drawing, good interfacial adhesion was suggested considering good mechanical properties and the reinforcing effect of the PLLA presence. On the other hand, PLLA was found to be oriented if ultra-high molecular weight PHB (MW almost 6,000,000) was used as the matrix. Interfacial entanglements, which are much more numerous in blends with UHMW PHB, are suggested to be responsible for the differences in orientation of the minor component. As a result, the mechanical properties of blends with UHMW PHB matrix improved considerably with increasing PLLA content, as seen also in Table 7.7. Table 7.7 Mechanical properties of the blends PHB/PLLA depending on the MW of PHB and composition of the blends (data estimated from figures in Park et al.)35 PHB portion

DR

1

1 5 1 5 1 5 1 5 1 5 1 5

0.9 0.7 0.5 0.3 0

s (MPa)

E (%)

E (GPa)

NMW

UHMW

NMW

UHMW

NMW

UHMW

22 25 22 28 22 34 24 72 30 165 44 170

21 41 23 70 24 102 30 151 31 158 44 168

22 60 30 48 30 39 18 50 28 92 22 90

12 45 13 75 16 102 33 150 35 158 22 167

0.65 0.88 0.65 1.05 0.63 1.12 0.70 1.31 0.82 1.80 1.12 1.86

0.72 1.18 0.73 1.22 0.73 1.55 0.81 1.54 0.81 2.05 1.12 2.18

Oriented foils based on blends of copolymer PHB-co-hydroxyvalerate and polyalcohols are described by Cyras et al.36 The blends were prepared by solvent casting, and castor oil or polypropylene glycol were used as the polyalcohol component. Dynamic mechanical behaviour indicates a formation of the two-phase immiscible blend. The addition of polyalcohols leads to an increase of crystallinity but lower storage modulus was observed due to an addition of the amorphous compound.

7.3

Poly(caprolactone)-based fibres

Poly(e-caprolactone) (PCL) is a synthetic polymer which has many advantages: biodegradability; mechanical properties similar to polyolefins; hydrolysability similar to polyesters; compatibility with many other polymers; ease of melt


233

processability; and high thermal stability. However, low melting point (around 60∞C) and slow rate of degradation in vivo (2–3 years) hinder its use as a homopolymer in many cases. Therefore PCL is used more frequently as a component in blends or as a comonomer if copolymers are to be formed. Various methods have been described and used for PCL fibre production, some of them rather unconventional. Simple melt spinning is possible to apply.37 Due to low melting and crystallisation temperature of PCL, vertical direction of spinning, small distance between the die and cooling bath, and intensive cooling (ice water 5–10∞C) is recommended. The spinning temperature should be kept around 85–90∞C. At higher temperature (120∞C) fibres with uniform diameter could still be obtained but signs of capillary instability were observed.37 Fibre diameters were in the range 0.49–0.91 mm depending on the spinning conditions, e.g. ram speed, extrusion rate, take-up rate and the ratio of take-up to extrusion rate. Melt spinning of PCL with the additive content was performed to receive PCL fibres containing N-(3,4dimethoxycinnamoyl)-anthranilic acid, a drug suppressing the fibroblast hyperplasia.38 An interesting method of PCL fibre preparation is described by Smith and Lemstra14 as gravity spinning. The polymer is dissolved in a suitable solvent, in this case acetone to produce solutions containing 6–20 wt % of PCL. The solution was transferred into a vessel and allowed to flow out through a spineret placed in a bottom of the vessel. The polymer solution was forced by its gravity to flow into a non-solvent (methanol) forming a fibre. The ‘asspun’ fibre was taken up on a mandrel using a variable speed. At concentration 5% and lower the fibre was not formed. Within a concentration range of 6– 20% the production rate varied from 2.5 to 0.9 m min–1 and fibres with diameter between 0.19 to 0.15 mm were formed. Both the production rate and diameter of the fibres decreased with the increase in the solution concentration. The fibres were round in diameter and exhibited a rough, porous surface. This procedure can be used also for preparation of fibres containing various additives. Williamson et al.39,40 prepared PCL fibres with addition of ovalbumin as a hydrophilic macromolecule. The procedure consists in a preparation of 10% PCL solution in acetone and in situ formation of ovalbumin nanoparticles in the concentration 1 or 5% re PCL content. Addition of poly(vinylpyrrolidone) is recommended for obtaining better dispersion of nanoparticles. The fibres are then prepared by gravity spinning as described above. Progesterone as a lipophilic steroid was also shown to be incorporated into fibres in a concentration 0.625 and 1.25% using the above mentioned procedure. Zeng et al.41 investigated a preparation of ultrafine poly(caprolactone) fibres by electrospinning technique to achieve a biodegradable material with high surface area so that the rate of biodegradation could be substantially increased. The technique itself involves spinning the polymer from solution

234


in a strong electrostatic field through a syringe with a capillary jet outlet. When the voltage is over a certain threshold value, the electrostatic forces are higher than surface tension. As the jet moves towards a collecting metal screen it acts as a counter-electrode, the droplets of the solution split into small charged fibres or fibrils and the solvent evaporates.42 Nonwoven fabric is formed from the fibres produced. Chloroform solution of PCL was used; an addition of 1,2-dichloroethane resulted in an improvement of the process.41 The voltage depends on the capillary diameter and was in the range of 29 to 36 kV. The diameter of fibres was strongly dependent on the capillary thickness which was between 0.1 and 0.4 mm, while the fibre diameter was in the range 300 to 900 nm. The effect of the driving pressure in the capillary as well as ambient temperature and air flow were also investigated in detail. The mechanical strength of melt spun fibres was very low. The fibre could be easily stretched to DR over 20 without breaking; it was suggested that the spinning process introduced very little if any orientation along the fibre axis.37 After drawing to various DR (between 5 and 25) the strength and modulus increased and elongation at break decreased substantially. Unfortunately the authors do not present exact data on mechanical properties; for the highest DR = 25, the values 280 MPa and 450% for tensile strength and elongation at break, respectively, can be roughly estimated from the stress–strain curves presented. The tensile properties of the gravity spun fibres were rather low, characterised by a tensile modulus between 10 and 100 MPa, tensile strength 1.8 to 9.9 MPa and elongation at break 175 up to 600 for 6 and 20% PCL concentration, respectively. All values are substantially lower when compared to bulk PCL, obviously due to the porosity of the samples. Cold drawing of the fibres proceeded rather easily and resulted in an increase in strength properties and a decrease in elongation. Again, the properties depended on the concentration of the PCL solution during spinning. Fibres prepared from 6% solution were possible to draw up to DR 2, while DR = 5 could be achieved with the other fibres prepared from solutions containing 10 to 20% of the polymer. The highest values reached were 320 MPa for modulus, 39 MPa for tensile strength and 136% for elongation at break. These values are not at all impressive and do not differ substantially from values for bulk PCL. Similarly, only a marginal effect was observed regarding the changes in melting temperature and crystallinity in dependence on DR. Incorporation of additives (ovalbumin, progesterone) resulted in a decrease of tensile strength, modulus and elongation at break compared to fibres without additive. A continuous decrease in the properties was observed with increasing concentration of the additive.40 Similar concentration-dependent deterioration of mechanical properties was observed also if N-(3,4dimethoxycinnamoyl)-anthranilic acid was mixed into PCL fibres during melt spinning.38


235

As indicated, caprolactone can be used also as a comonomer for preparation of copolymers, in this case, transesterification reactions can also be considered. Poly(ethylene terephthalate) copolymers with poly(caprolactone) were prepared by reactive extrusion. In the presence of stannous octoate, ring opening polymerisation of caprolactone is initiated by hydroxyl end groups of molten PET. A block copolymer with rather low portion of transesterification was formed in twin screw extruder as a result of fast distributive mixing of caprolactone into high melt viscosity PET and short reaction time. The copolymer was directly fed into a spin pot and extruded filaments were spun and drawn to DR = 6.6. The interactions between PET and PCL segments may result in a formation of miscible phase with a single Tg being around 45∞C.

7.4

Structure of drawn fibres

Isotropic PHB crystallises in an orthorhombic lattice crystalline structure (a-form) with the chains in the left-handed 2/1 helix, as reported by Yokouchi et al.23 and Pazur et al.43 X-ray diffraction patterns indicating an orthorhombic crystal structure (a modification, 2/1 helix) were also described by Yamamoto et al.44 Changes in the crystalline structure may be expected as a result of drawing. Frequently, an additional crystal structure is observed, assigned to a zigzag conformation of a hexagonal b modification.31 Thus, appearance of both helical and planar conformation and their ratio depend on preparation conditions, mainly on draw ratio. A paper of Yamane et al.26 describes an appearance of higher crystalline orientation with increasing draw ratio with c-axis parallel to the fibre axis, which seems to be the most preferential orientation direction. A reflection at 2q = 19.7∞ was assigned to a reflection of the pseudohexagonal phase as proposed by Furuhashi et al.45 Using high speed spinning procedure, Schmack et al.27 reported no signs of hexagonal modification for high speed spinning up to draw ratio 4.0 and proposed lack of stress-induced crystallisation at that degree of orientation. However, an increase to 4.5 leads to WAXS patterns indicating a presence of both crystal structures. The observed effects are generally in accordance with mechanical properties, but the effect of extrusion speed is not clear from this study, i.e. whether stress-induced crystallisation would be sufficient for a formation of hexagonal modification if the DR would stay low at higher speed of extrusion. Similar orthorhombic structure was observed also for drawn fibres based on a copolymer PHB-co-hydroxyhezanoate.33 It was concluded that the higher hydroxyalkanoate units were excluded from the crystal structure. The surface of melt spun fibres consists of many large spherulites, as indicated by SEM observations.46 After drawing to DR = 6, the fibres have a fibrillar structure and their surface is fairly smooth. This fibrillar structure is formed mainly in surface areas of the fibres as indicated by the appearance

236


of fibres observed by SEM after various periods of enzymatic degradation. The annealing results in a higher portion of fibrillar structure which is formed also in core parts of the fibres. Annealing under tension leads to a formation of a more perfect fibrillar structure in the core and this effect seems to be increasing with the rising tension load during annealing.46 A similar structure was observed also for melt spun poly(caprolactone) fibres. SEM observations showed spherulites in the undrawn fibres which change to fibrillar stripes along the fibre axis in drawn fibres.47 Differences in the crystal structure were observed also with rising draw ratio.

7.5

Thermal properties

Generally thermal properties of polymers change after drawing. Melt temperature of PHB determined by DSC depends to certain extent on the material itself as well as on the measuring conditions; Yamane et al.26 reported Tm = 171∞C (second run) and Tc = 91∞C at cooling rate 5∞C min–1, while Tm of virgin powder was found to be 177∞C.27 Melting temperature depends also on the molecular weight, as revealed by Park et al.35 who determined Tm to be around 165 and 177∞C for PHB with molecular weight Mw 590,000 and 5,300,000, respectively. Equilibrium melting temperature was determined to be around 186∞C.48 ‘As-spun’ fibres showed a melting temperature of 176∞C irrespective on draw ratio,26 drawing results in a significant increase in the Tm. An increase from 177∞C for virgin PHB powder up to 181.6, and even to 188.9∞C, was reported,27 depending on the draw ratio being between 4.5 and 6.9. It is interesting that melt spinning/drawing at DR = 4 does not lead to any increase in Tm; also mechanical properties are not improving due to drawing, although a little higher DR, namely 4.5, was reached. It has also to be mentioned that fibres drawn to DR around 6 have a melting temperature higher than that reported for equilibrium temperature of isotropic PHB.27 Thorough drying itself leads to a substantial increase in the melting temperature, namely from 177.2∞C for virgin PHB powder up to 189.9∞C for pellets dried for 16 hours at 120∞C.27 Thorough drying may have certain effects on the melting temperature, changes in the supramolecular structure occurs after several hours treatment at 120∞C, even without the effect of losing moisture. Annealing leads to changes in thermal behaviour of drawn fibres. DSC melting peaks of annealed fibres are larger and tend to be sharper compared to unannealed fibres;26 both effects are more pronounced with rising annealing temperature. Annealing under tension results in even sharper peaks, the results indicate26 that annealing without tension leads to a recrystallisation of the material to more perfect a-form, while the molecules between a-form lamellae crystallise into b-form if tension is applied during annealing; the effect is more pronounced at higher annealing temperatures.


7.6

237

Enzymatic and hydrolytic degradation

Yamane et al.46 investigated the enzymatic degradation of melt spun fibres, PHA depolymerase from Commomonas testosteroni in potassium phosphate buffer was used at 37∞C for this study. The enzymatic degradation of PHB occurs on the surface of the material and the rate of degradation strongly depends on the structure.46 SEM observations of ‘as-spun’ fibres indicate that enzymatic degradation begins on the surface, preferentially in lessordered regions between spherulitic crystals. At the same time, the fibre diameter decreases as the degradation proceeds to the core of the fibre, similarly, degradation of drawn fibres started in less-ordered regions leaving the fibrillar structure to resist for longer time. Clear difference in the morphology of fibres drawn to various DR (4, 5, and 6) was not observed. After four days of enzymatic degradation, the drawn fibres changed to aggregates of small fibrous fragments while ‘as-spun’ fibres retained their fibre shape with a spongy structure, although the diameter significantly decreased. This difference was attributed mainly to the original thickness of the ‘as-spun’ fibres being thicker compared to drawn fibres while the degree of crystallinity was similar. It has to be noted that both the ‘as-spun’ as well as drawn fibres decomposed rather fast so that mechanical properties could not be measured after 24 hours of degradation. However, the annealed fibres (DR = 6) kept their consistency longer, so that the mechanical tests could be performed even after 50 hours of degradation. The resistance of fibres against enzymatic degradation increases with increasing temperature of annealing; higher tension during annealing has a retarding effect on degradation.46 WAXS study revealed that disordered b form is attacked by PHB depolymerase more rapidly than the more ordered a form. Similar tests were done with an oriented copolymer PHBpoly(hydroxyhexanoate) using PHB depolymerase purified from Ralstonia picketti T1.33 It was found that the rate of ezymatic degradation of two-step drawn films decreased with increasing DR, i.e. with increasing crystallinity, as expected. However, the decrease in the rate for one-step films was irrespective on draw ratio 3 or 5 and much more pronounced compared to the two-step drawn films even though the DR was significantly higher in the latter case (DR up to 10). The authors admit having difficulties with offering a reasonable explanation for this behaviour. Special centrifugally spun fibres as an alternative for gel spun fibres were tested regarding hydrolytic degradation (pH 10.6, 70∞C).32 Fibres degraded by gradual fragmentation and erosion to fibre fragments, particles and eventually monomer. Mammalian and human epithelial cells were used to investigate the cellular interactions with the fibres. To achieve good cell adhesion, the surface of the fibres has to be treated by alkali or acids; the introduction of hydroxyls or carbonyls on the surface is suggested as an

238


explanation of the effect. Neither cell line exhibited any cytotoxic response to the fibres. Although no more detailed studies were found on the environmental degradation of PHB fibres, the above-mentioned effects may be considered to be generally valid for the anisotropic materials depending on the draw ratio, structure parameters (content of a and b phase, etc.). The rate of enzymatic degradation by lipase of poly(caprolactone) fibres drawn to various ratios was dependent on draw ratios, suggesting that crystallinity and orientation degree are important parameters affecting the degradation kinetics.49 Degradation behaviour of fibres differing in details of the preparation procedure revealed the changes in degradation kinetics on supramolecular structure of the material. From SEM observation it is observed that enzyme preferentially attacks amorphous or less ordered regions.47 Differences in crystal structure were revealed by SEM consisting mainly in a portion of spherulites and fibrilles depending on the drawing conditions, these structural parameters affect the enzymatic degradation kinetics. However, in spite of different degradation rates in amorphous and crystalline regions, the crystalline part is also attacked by the enzymes and biodegrades.47 It is interesting to note that when investigating enzymatic degradation of films made from butylene succinate-co-ethylene succinate copolymer it was found that the rate of degradation depends on the crystallinity rather than on the primary chemical structure. Thus, the crystallinity degree seems to be the major rate-determining factor of biodegradation of solid polymers, while the crystalline structure seems to be an additional parameter.

7.7

Other biodegradable and sustainable polyesters

Poly(glycolic acid) (PGA; 7.3) is an aliphatic polyester that has been widely used in biomedical applications since the early 1970s50–54 and degrades via a simple hydrolysis mechanism (bulk degradation). However, the homopolymer has found little application as a fibre outside medicine, but such fibres do offer potential for the future and maybe an area worth further research. O *

O

*

7.3

Poly(trimethylene terephthalate) (PTT; 7.4) was first synthesised in the 1940s by Whinfield and Dickson,55 but has only recently received attention as a viable textile fibre as it can be synthesised via a more economical process.56 PTT is desirable because it has several unique properties, such as


239

O *

O

O * O

7.4

its force-elongation behaviour, resilience, and dyeing properties. It has outstanding elastic recovery; the fibre recovers 100% from approximately 120% strain.57 Although PTT is not a biodegradable fibre, it is worth mentioning as a fibre which offers a level of sustainability in the form of Sorona®. In 2004, DuPont and Tate & Lyle PLC announced a joint venture (DuPont Tate & Lyle BioProducts, LLC) to create products from renewable resources such as corn for numerous applications including clothing, interiors, engineered polymers and textile fibres. The company uses a proprietary fermentation and purification process to produce 1,3-propanediol (PDO), one of the two base chemicals for producing PTT. Rather than using PDO from petrochemical sources, in this process it is derived from renewable sugar. The resultant fibre formed from polymerisation of renewable PDO and terephthalic acid is Sorona®.57 However, the polymer is not fully sustainable due to the nonrenewable sources of terephthalic acid, so cannot be compared to poly(lactic acid) in terms of its sustainability. Nevertheless, this fibre is a demonstration of positive moves by multinational companies to reduce demands on fossil fuels.

7.8

Application of polyester-based biodegradable fibres

PHA fibres are frequently aimed at medical applications; a combination of biodegradability, hydrophobicity and biocompatibility seems to be of importance for many medical applications. Poly(hydroxybutyrate) fibres were considered to be mainly used for production of scaffolds.58 From the point of view of medical applications, an interesting paper of Schmack et al.59 deals with the effect of electron irradiation on properties and degradation of PHB fibres with the aim of estimating the consequences of sterilisation of medical devices via electron beam irradiation. In this paper, melt spun PHB fibres were drawn to DR 7 and textiles with a mesh size of 0.5 mm were produced using embroidery technology.60 Irradiation resulted in a decrease in tensile strength, while changes in modulus and elongation at break were negligible, as seen in Table 7.8; the changes can be attributed to a decrease in molecular weight, which is quite substantial. Hydrolytic degradation in a Sorensen buffer of irradiated fibres was found to be only a little more pronounced if

240


Table 7.8 The effect of irradiation dose on changes in average number Mn and molecular Mw weights, tensile properties (strength (s), elongation (e), modulus (E)) and relative change of Mw after in vitro degradation Dose (kGy)

s (MPa)

e(%)

E (GPa)

Mn (kg mol–1)

Mw M 84* n (kg mol–1)

M 84* w

0 5 10 15 25

307 295 248 257 236

41 41 39 39 39

4.5 4.3 4.3 4.1 4.4

77 65 61 53 44

157 145 126 111 92

0.84 – – – 0.74

0.87 – – – 0.82

* relative molecular weight (Mt /Mo) after 84 days in Sorensen buffer

the fibres were irradiated by a 25 kGy dose compared to non-irradiated fibres.59 Application as scaffolds was suggested also for centrifugally spun fibres.32 Although almost no cell adhesion was observed for unmodified fibres investigated by SEM, subsequent acid or alkali treatment resulted in a celladhesive material which may have a potential value as wound scaffold. In fact, most biodegradable polyester-type fibres are intended for medical purposes, although the application is not limited to scaffolds. Poly(caprolactone) gravity spun fibres were found to attach fibroblasts and myoblasts. Due to high fibre compliance and a potential for controlling the fibre surface architecture, the fibres can be recommended to be used as compliance-matched implants for soft tissue engineering.39 Poly(caprolactone) gravity spun fibres were used also as a carrier for ovalbumin or progesterone.40 The delayed drug release was observed; the rate depends on several factors, e.g. concentration and size of the protein particulates, which enables a programmed delivery of drugs or supporting agents for tissue engineering. Poly(caprolactone) was used as a matrix for a composite with poly(lactic acid) and poly(glycolic acid) long fibres. The composite was prepared by in situ polymerisation of caprolactone with dispersed fibres, and bioabsorbable composite material was obtained which is investigated for an application for craniofacial bone reconstruction. 61 PCL fibres containing N-(3,4dimethoxycinnamoyl)-anthranilic acid can release the agent which suppresses the fibroblast hyperplasia.38 The drug release rate was found to decrease with increasing draw ratio, obviously as a result of increased crystallinity of the polymeric matrix. An industrial application was suggested for copolymers of poly(caprolactone) and poly(ethylene terephthalate) as a material for elastic seat belts. In this case the impact on the passenger was damped by the increased elasticity of the belt, compared to the belt made of pure PET, decreasing the extent of injuries caused by contact with the seat belt.62


7.9

241

Future trends and concluding remarks

It can be said that the research on PHA fibre techniques is in quite an advanced stage so that a feasible technology could be designed if a request for a high amount of fibres arose. The high volume production, however, depends on two factors, the quality of the fibres and the ultimate price of the product. At the moment, apparently, the quality improvement resulting from drawing does not compensate for the rather high price of the fibre, caused mainly by the price of the poly(hydroxyalkanoates) as the parent materials. The reason for the high price consists partially in the low volume of production of PHAs; it can be hardly expected that a production of fibres can influence the volume of production to such an extent that the price would fall substantially. Thus, in the near future, the PHA fibres can be expected to be applied mainly as low volume special materials. From this point of view, an application in medicine is the most obvious. The biodegradability of the fibres is certainly considered in many applications outside of medicine, but at the current material price it can hardly be expected to be a decisive factor regarding the high volume applications of PHA fibres. Electrospinning may play a role in a spread of biodegradable fibres, especially if feasible ideas for an application of nonwoven biodegradable textiles would appear.63 Electrospinning of poly(caprolactone) was successfully accomplished at laboratory scale as mentioned above; poly(hydroxyalkanoate) electrospinning should be investigated as well, especially regarding PHB, electrospinning could bring some interesting, possibly surprising, results. Considering oriented biodegradable bacterial polyesters, attention should be paid to foils. Manufacturing the oriented or even biaxially oriented biodegradable foils could result in a material with high application potential in packaging. In this case the volume of the biodegradable plastics could be rather high so that also the price of the polymer could be affected. Thus, the success in this direction may have a positive influence on the future of biodegradable polyesters. The potential of oriented poly(caprolactone) seems to be lower compared to PHAs. Apparently it will be an important special polymer, but high volume applications are less probable. The main problem consists in its low melting temperature, although it can be improved to a certain extent via various modifications, especially transesterifications with poly(caprolactames)64 or other polymers. However, even in this case the consumption will stay at low level. The most promising way seems to be Novamont’s (Italy) attitude of mixing PCL with starch or other biodegradable species to prepare composites. This material (MaterBi) is used routinely for production of biodegradable foils of good quality.3 At the moment no data are available on drawing of the composites to produce fibres; neither does a need for such fibres seem to exist. Poly(caprolactone) can be used in quite high volume as a modifying

242


component in blends with other biodegradable plastics, including PHAs and PLA. In this case, its role consists in an improvement of toughness and the increase in drawability. The significance of such blends for biodegradable fibre production is obvious. In any case, biodegradable fibres based on polyesters while investigated and developed by researchers, should be considered seriously by industry and consumers. A knowledge concerning the properties and new techniques of production of the fibres should bring new ideas for applications of these materials followed by development of production technology.

7.10

References

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8 The route to synthetic silks F V O L L R A T H and A S P O N N E R, University of Oxford, UK

8.1

Introduction

Natural silks are extremely fine, tough, strong and extensible. Many spiders produce, from a widely diverse ‘battery’ of glands, a wide variety of silks (Fig. 8.1). Silkworms only have one type of gland. A single silk filament can be anything up to 1200 metres (as in silkworm silk reeled from a high quality cocoon) or up to 500 metres (as in spider dragline silk reeled directly from the immobilised animal). Industrialists have long dreamt of artificially producing silk fibres with the properties of spider dragline silk. To this end attempts have begun to express spider silk genes in organisms that are more easily and cheaply cultured than spiders ranging from micro-organisms to potatoes and even mammals [1–3]. While such a biotechnological approach in itself poses a range of problems, both with the transfer of genes as well as the expression and extraction of the relevant proteins, it emerges that the exceptional properties of a silk do not depend solely on the unique nature of the silk precursor-feedstock. There is now strong evidence that a spider’s (as well as a silkworm’s) spinning mechanism may be no less important in determining a filament’s material properties than the feedstock polymer (e.g. [4–6]).

8.2

Silk structures

Spider silk, like insect silk, was long thought to be a composite consisting of protein crystals embedded in a protein matrix with the crystals giving the silk biopolymer its strength and the matrix its elasticity [7–9]. This strength would largely be determined by the length, the diameter and the composition of the crystals while the elasticity is thought to derive from the entropic qualities of the matrix molecules [10]. However, it now appears that this simple model is far from correct for most silks. Indeed, the draglines of both the golden silk spider Nephila (Fig. 8.2) and of the garden spider Araneus are among the toughest silks so far investigated. Both are not simple 245

246


G

E–F

D

A

C

B

8.1 The female cross spider Araneus (like most other advanced orb weaving spiders) spins 7 different silks from as many different glands: soft silk, to enshroud prey and cradle the eggs in the cocoon or to balloon (A); tough silk for the egg sac ‘shell’ (G); cement silk to affix the web to support (B); strong but relatively stiff silk for the scaffolding of the web (C); additional scaffolding silk also used for the temporary support spiral (D); and, finally, very elastic sticky capture silk consisting of a core thread and a watery coat that forms the web’s droplets and glue (E–F).

homogeneous filaments (as was long thought) but resemble microscopic climbing ropes with nano-engineered strands of micro fibrils interspersed with filled inclusion channels and covered by several layers of coating [11]. Another interesting silk composite, the sticky capture silks of Nephila and Araneus, are complex, albeit microscopic, mechanical windlass systems that make good use of the physics of biological micro-engineering (Fig. 8.3). In the ‘windlass’ silk (which operates in the wet state) the elasticity is given by a combination of surface tension of the aqueous coat and recoil of the plasticised silk fibre [12–14] while adhesion is bestowed by a separate glycoprotein complex [12, 15]. The mechanical behaviour of radial and capture silks differs greatly. For example, the wet and soft sticky spiral of the Araneus diadematus garden spider absorbs energy by large extendibility (circa 500%) of the wetted thread which develops substantial force only after 100–200% extension with


247

B

A

C

D

8.2 Electron micrographs of Nephila dragline silk: (A) longitudinal section of a fully urea-swollen fibre – note the numerous electronlight canaliculi and the electron-dark outer coat (bar 1mm); (B) magnification of the coat showing the various levels (bar 100 nm); (C) cryo-section of an untreated fibre (bar 1 mm); (D) enzyme K treated fibre (bar 1 mm). (A/B modified from ref. 85; C/D: we thank W. Hu for help with the preparation.)

the thread breaking suddenly at around 400–500% extension [12, 16]. Nevertheless, the engineering strength of these Araneus capture threads is 1338 + 80 MN m–2 with a breaking energy of 163 J cm–3 (N = 6) and thus is comparable to that of the radial threads at 1153 + 144 MN m–2 with a breaking energy of 194 J cm–3. Finally, a third basic type of web silk employs very fine filaments of only nanometres in diameter which is combed into hackled bands of many threads onto supporting axial threads which are often sprung. In this kind of silk, which operates in the dry state the hackled bands provide some elasticity as well as tremendous adhesion presumably by electrostatic forces [16]. The mechanical behaviour of these threads resembles more that of the wet capture silks than that of the dry radial threads. The functional and developmental details of the two so very different elastic recoil mechanisms of the two types of capture silk micro-machines are interesting and deserve deeper studies. However, at present we do not even understand the interaction of form and function in the much more ‘typical’ spider silk fibre such as a dragline filament. Recent studies indicate that the toughness of spider dragline silk may depend on the complex hierarchical structure of the fibre [11] which in turn depends on a complex

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8.3 A capture thread of Araneus diadematus under increasing magnification. The windlass mechanism is seen in the lower picture where the core fibres after a large extension–contraction cycle have been reeled into a droplet (for details see ref. 12).

spinning process [17, 18], as well as on tuned dopants in the polymer feedstock [19]. Several factors may contribute to the toughness of a spider silk fibre: (a) the design of the protein monomers and their conformations and interactions; (b) the hierarchy of protofibrils and fibrils within the filament which provide several levels of energy dissipation; (c) numerous narrow, highly elongated channels in the silk that serve as crack deflectors or fluid filled shock absorbers; (d) a multi-layered coat to the filament surrounding a central core to prevent surface crazing.

8.3

Development of fibre: the feedstock

8.3.1

Biochemistry

Neither spider nor silkworm silks are a simple single protein bio-polymer. Indeed, the typical spider major ampullate dragline silk contains many different


249

organic and inorganic components like neurotransmitter proteins, glycoproteins, lipids, sugars, phosphates, calcium, potassium or sulphur [15, 20–28]. Notwithstanding the plethora of associated compounds in many silks, the main constituents of typical silks are a class of proteins called fibroins for insects and spidroins for spiders. These proteins are considered to be principally responsible for the mechanical properties of the silk fibres although the nonfibrous protein compounds (e.g. those mentioned above or the sericins of the Bombyx silks) also affect mechanical properties [28, 29]. However, strength, elasticity and toughness are not the only tasks that a spider’s silk must fulfil in nature. Like silkworm cocoon silks, spider silks also have to withstand microbial attack as well as other physiological/environmental challenges, therefore, some components of the spun thread may not be required for the mechanical properties; in fact, a proteinase inhibitory function could be demonstrated for smaller peptides found in some insect silks [30]. Additionally, some components found in spun silk may have their function in the formation of the spinning dope or the spinning process, but may not play any substantial role in the fibre. Quantitative amino acid analyses showed already nearly 100 years ago that silks like those of Bombyx mori and Nephila madagascariensis are very rich in glycine (8.1) and alanine (8.2) [31]. A predominance of amino acids with short side chains can be stated for almost all silks [32]. The chemical composition of spider silk, however, can vary between subsequent webs of the same animal in dependence of its diet and living conditions [20, 33]. Very different values have been reported for the molecular weights of spider spidroins derived from solved silk and the liquid spinning dope of the major ampullate glands, ranging between 30 to 740 kDa [34]. Various technical difficulties implied by the peculiar amino acid compositions and the nature of the proteins may interfere with different methods. Values derived from SDS-PAGE are reported to be 323.6 kDa [35], 195 and 220 kDa [36] for Nephila clavipes spidroins; the latter values were obtained under reducing and non-reducing conditions indicating covalent bonds via disulfide bridges. Disulfide bridges are also found in Bombyx mori where heavy and light chain fibroins are coupled [37]. O OH O H2N OH

8.1

NH2

8.2

Recent investigations of the gel-like protein content gathered and stored in the sac-like glands of the major ampullate glands revealed that the majority

250


of the protein fraction is of high molecular weight (>200 kDa) but migrates as multiple bands in SDS-PAGE. Partly this observation is explained by the presence of the two different major ampullate spidroins Masp1 and Masp2 (see Section 8.3.2). This is evident from both antibody staining [38, 39] and isoelectric focusing [39], but the results also show that multiple sizes exist from one protein species. Aside from splicing especially degradational processes are discussed as an explanation; and the question was put forward if and how this heterogeneity impinges on the spinning process and quality of the fibres.

8.3.2

Molecular biology

The first sequence data for spidroins were obtained from c-DNA clones of Nephila clavipes major ampullate glands that form the dragline [40]. Two different spidroins, Masp1 and Masp2, were identified and approximately 2000 base pairs from the 3¢ end of their reading frames were sequenced. Different values of the transcript sizes were published and the actual size remains somewhat doubtful (Chinali, personal communication). The highest values, however, indicate that only approximately 20–25% of the sequences might be known yet (Table 8.1). Table 8.1 Published transcript sizes of various spidroins (only the highest values are given) Gland

Species

Gene

Transcript size (¥103 bases)

Ref.

Major ampullate Major ampullate Major ampullate Major ampullate Minor ampullate Minor ampullate Minor ampullate Flagelliform Cylindrical

N. clavipes N. clavipes A. diadematus A. diadematus N. clavipes N. clavipes A. diadematus N. clavipes A. diadematus

MASp1 (NCF-1) MASp2 (NCF-2) ADF-3 ADF-4 MiSp1 MiSp2 ADF-1 Flag ADF-2

12.0 11.5 9.0 7.5 9.5 7.5 9.5 15.5 14

[47] [47] [46] [46] [45] [45] [46] [43] [46]

The sequences reveal that the genes are highly repetitive in their main parts [40]. Due to the codons of the most prominent amino acids alanine and glycine they are very rich in guanine (8.3) and cytosine (8.4) although these nucleotides are avoided in the third position. The latter is reflected by clusters of tRNA genes with the respective anticodons [41]. The repetitive parts of the protein sequences are made up of several peptide motifs that are iterated multiple times to form repetitive structural modules. These motifs are used in various combinations in the different


251

O HN

N

H2N

NH N

H2N N

8.3

O

N H

8.4

spidroins and are believed to confer the observed mechano-physical properties to the respective silk [42, 43] (see Fig. 8.4). b-spiral GPGXX

Spider silk modules ‘linker’ crystalline 310-helix b-sheet GP(S.Y.G) Ala-rich GGX

spacer

MaSp1

MaSp2

ADF-1

?

ADF-2

ADF-3

ADF-4

Flag

?

8.4 Distribution of structural motifs within different spidroins according to Hayashi et al.47 X indicates a residue that may vary.

These repetitive sections are framed by non-repetitive N- and C-termini [40, 44–51]. The N-terminal sequences contain putative signal sequences and might play a role in the secretion of the spidroins into the glandular lumen. However, although they are potentially very important for our understanding of silk production/storage, to date N-terminal sequences are known only from Flag spidroins and one minor ampullate spidroin [45, 50– 52]. The C-termini of the spidroins derived from ampullate glands are highly

252


conserved, not only between different spidroins but also between different spider species [44] and are considered to be of high functional importance [38]. Various functions have been proposed, for example, that they might play a role in the solubility of spidroins in the highly concentrated spinning dope. In addition (or alternatively) the C-termini might represent protopeptides with signalling capabilities, which are cleaved off after secretion. Studies of Bombyx mori fibroins have demonstrated emulsion formation and micellar structures from aqueous solutions of reconstituted silkworm silk fibroin [53]. The hydrophilic N- and C-terminal peptide sequences of B. mori might be required for this micellar-like organisation that subsequently leads to the highly concentrated gel-like state of the spinning dope. It may be speculated that spidroin C-termini, although substantially shorter, might fulfil a similar function [53]. New experimental evidence [54] indicates that the C-termini of Nephila clavipes major ampullate spidroins, which are present in the high molecular weight fractions of both the proteins derived from the secretions of the glands and the spun thread, are involved in the formation of disulfide bridges. However, it is rather unlikely that such covalent cross-linking has a strong impact on the material properties as the C-termini of the minor ampullate spidroins that form a very stiff fibre do not contain cysteines [50].

8.3.3

Molecular structure

Most structural investigations of silks on the molecular level use X-ray diffraction and crystallography. In this way, so far, over 100 different silks have been characterised and grouped according to their structures [32]. Most of these structures contain b-pleated sheets orientated either parallel or antiparallel to the fibre axis, but one can also find silks with cross b-pleated sheets, b- and/or 3(1)-helices or a-helical conformations. The a-helical silks have a relatively low content of glycine and are high in acidic residues. The dragline silks of spiders belong to the first group with b-pleated sheets. The investigation of major ampullate silks (MAS) of Nephila madagascariensis and Nephila clavipes revealed amorphous regions in which crystalline domains of antiparallel b-pleated sheets are interspersed [55]. The crystallinity was estimated to be 30–50% in Nephila clavipes MAS [56]. For the diameters of the crystallites a size in the range of 70–100 nm [28] and later 70–500 nm [57, 58] have been published. Calcium detected by EELS (Electron Energy Loss Spectroscopy) was found exclusively in the crystallites [56]. Hence it could be important either for the conformation or the generation of the crystalline structure. The b-pleated sheets are formed by the poly-alanine stretches. Solid state NMR revealed that about 78% of the alanine in Nephila clavipes MAS are included in the crystalline fraction. It consists of two types of alanine-rich


253

regions. About 40% make up a dense and highly orientated fraction. The second fraction, of lower density and less well orientated, is suspected to contain the GAG motifs on the flanks of the highly orientated poly-alanine stretches [59]. As was evident from REDOR (rotational-echo double-resonance) NMR data the sequence motif LGXQG (X = G, S, N) which is present in front of the poly-alanine stretches possibly is a b-turn of type I that could reverse the chain direction [60]. From the size of the crystallites as well as the measured intersheet spacing it was concluded that the crystalline domains must also include glycine rich sequence parts [56–58, 61]. The inclusion of GGX motifs, however, is still an open issue [62]. The molecular and functional organisation of spider silk has been compared to that of rubber [8]. It was proposed that the crystals formed by the bpleated sheets cross-link the fibroins into a polymer network and thus provides toughness and stiffness. The amorphous regions, made of randomly oriented chains, would account for the elastic properties. The glycine rich fraction that is thought to make up the amorphous matrix, however, seems to be much more orientated as NMR experiments with Nephila madagascariensis MAS indicate; it adopts a 3(1)-helical conformation that could contribute substantially to the mechanical properties by the high forces of hydrogen bonds [62–65]. The poly-alanine stretches in spidroin 2 are somewhat longer than those of spidroin 1 and therefore would be more suitable for the formation of bsheets. However, the proline residues in the glycine rich neighbourhood are rather detrimental to a formation of b-pleated sheets and would disrupt the integrity of the conformation. It was proposed that spidroin 2 should therefore be exclusively found in the amorphous matrix while solely motifs found in spidroin 1 are responsible for the formation of non-periodic lattices (NPL) in the crystalline fraction [57, 58]. Yet, attempts to investigate the conformation of spidroin 2 with NMR failed as no labelled proline could be found [66]. Thus the role of spidroin 2 in the molecular structure remains speculative.

8.3.4

Insect silks

The silks of insects like the silkworm Bombyx mori have been mentioned several times in this chapter; in fact, far more is known about the biochemistry, molecular biology and genetics of the silkworm than of any spider. There are many similarities between insect silks and spider silks, e.g. the predominance of amino acids with short side chains or the occurrence of some sequence motifs, which can help to understand the general principles involved in the generation of their material properties. Yet, there are also major differences that have to be taken into account. Most insect silks are used as cocoons or protective webs in larval stages and originate in labial glands [32]. In contrast,

254


in almost all spider species the silk glands are situated in the so-called opisthosoma, the abdominal body segment of these animals. Apart from the different phylogenetic as well as ontogenetic origin of the glands, there are far more different gland types in spiders and they are usually not restricted to a certain developmental stage. The main difference lies in the usage of the silks and the subsequent adaptations of the material. Threads used for protective shelters for instance do not require elasticity whereas the capture threads of spider webs would just not function without it. Bombyx mori silk fibres show a different morphology to spider fibres. The silkworm fibres (as we know them so well from commercial fabrics) actually do not represent the natural fibre but its core only, having been separated from its outer shell by a process called ‘degumming’. The coating layer is produced in the middle parts of the silk duct and added on top of the passing liquid core material so that, upon extrusion, a two-layered fibre is created. The shell represents an extensive layer of glycoproteins that are named sericins due to their high content in the amino acid serine (8.5). A high variability is observed in sericins in which splicing is involved [67–70]. The core of a silkworm silk consists of three different proteins: a protein called P25, the heavy chain (H-fibroin) and the light chain fibroins, which are exclusively produced in the posterior (PSG) section of silk glands. Hfibroin is a highly repetitive protein with a high molecular weight of 391.5 kDa, whereas P25 and L-fibroin with app. 25–30 and 25 kDa, respectively, are rather small. Such small proteins were not identified in spider silks. The heavy and light chains are linked by disulfide bonds and their assembly is essential for the efficient intracellular transport and secretion of fibroin as is the participation of P25 [37, 71, 72]. Analogues of these genes have also been found in other insect silks. O OH H 2N OH

8.5

Similar to spider silk, the fibre is composed of microcrystalline arrays alternating with amorphous regions. The crystalline arrays are composed of anti-parallel beta sheets that run parallel to the fibre axis. This conformation results from long (-Gly-Ser-Gly-Ala-Gly-Ala-)n stretches in the sequence of the H-fibroin which are interrupted by regions containing bulkier residues [73]. The GSGAGA motif is also found in the sequences of minor ampullate spidroins [50, 51]. Interestingly, the fibroin of some insects, for example,


255

Antheraea, which produces a tougher silk than Bombyx also contains polyalanine of the type (A)n [74].

8.4

Development of fibre: spinning

Clearly, in the dragline silks as in the capture silks, the different components and their interactions heavily depend on processing conditions during natural silk spinning. Silk is converted from the liquid feedstock in the gland into a solid thread inside a tapering tubular duct, which exits at the spigot [75]. The core and coat composite structure of the dragline thread is formed by the codrawing of at least two feedstocks through a single die and followed by further coating in the duct [11]. The spider modifies the mechanical properties of its silk not only on a medium-term basis (e.g. in response to starvation [76]) but also in rapid response to immediate static requirements of the web, which are complex structures with highly specific design characteristics relying heavily on silk properties [77, 78]. To be able to alter silk properties so rapidly, the animal cannot do it by changing the molecular structure because the silk feedstock dope is prepared well in advance of spinning. Instead the spider must use its ability to control the refolding of these molecules during the extrusion process, which would mean controlled modifications as the fibre is formed. Hence we must assume that a proper spider silk fibre is not self-assembling but instead forms during an assembly process that is highly controlled. Nevertheless, outside the animal, raw silk pre-cursor (as well as recombinantly produced silk peptides) can self-assemble to some degree into some sort of filament simply by drying out or if shared [34, 79, 80]. The details of this assistance are clearly important but as yet only poorly understood (Fig. 8.5). Nevertheless, we assume that control is asserted all along the spinning production process. Here the feedstock can be chemically modified for example by subtle pH alterations [81] or the rate by which salts and water is pumped in and out of the duct [5]. Moreover, the spider can influence the rate (i.e. the spinning speed) by which the dope moves through the duct (e.g. by walking or running [82]). This rate of flow through affects the mechanisms of flow elongation and flow shear [6, 83] with direct consequences on silk mechanics [11] via the degree of b-sheeting [5] and molecular order in general [84]. Furthermore, the coating of the fibres is important [85] and the action of the ratchet clamp has some effect, possibly as internal post-extrusion drawdown [11]. Finally, the post-processing external draw-down settles the silk’s mechanical behaviour [86–88] by locking the molecules into position. All of these are parameters and variables in a centrally controlled production system with considerable scope for feedback [76].

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8.5 Generalised spider silk gland based on typical major gland producing structural threads. The liquid crystalline dope solution produced in different zones of the gland is drawn through a tapering S-shaped duct that converts it into an elastomeric thread with a composite core-coat structure. A funnel (A) links gland and duct while reducing turbulence and mixing. The duct (B) removes water and adds auxiliary compounds facilitating shear stressing and has an internal draw-down. The ‘valve’ (C) is not a shaping device but a clamp for gripping and a ratchet to retrieve a broken thread. The spigot (D) strips off the last of the lumen solvents and surfactants. Magnifications (height), funnel 350 mm, draw-down 40 mm, valve 300 mm, spigot 190 mm. Both gland and duct have a complex histology. The so-called A-zone of the gland secretes the spidroin forming the core of the thread and the compounds filling the canaliculi while the B-zone secretes the mantle. The secretory granules of the A-zone contain finely granular/filamentous material while those of the Bzone contain polydomain hexagonal columnar liquid crystalline material. The thick cuticle of the short funnel reduces movement while the long and flexible spinning duct has a thin cuticle acting as an advanced hollow fibre dialysis membrane. The epithelium of the S-shaped duct increases in height from the first to the second and third limbs of the duct indicating increasing specialisation for pumping water and ions on the outflowing dope. Just before the draw-down taper, single flask-shaped gland cells contribute surfactants and extra coating. The final section of duct past the clamp is highly specialised for pumping with its tall cells full of mitochondria and covered with apical microvilli with a highly folded plasma membrane. Finally, the lips of the spigot seal the duct to the outside world (for further details see ref. 75).

8.5

Performance characteristics

The dry and very tough radius threads of orb web-building spiders such as Nephila spp. or Araneus diadematus show good extendibility (up to and


257

even above 40%) as well as high tensile strength (circa 1.2 GPa) and large hysteresis (circa 50%). Together all performance characteristics indicate that in the web these fibres function as both shock absorbers and structural elements [16]. For example, radial/dragline thread drawn from the major ampullate glands of mature females Nephila edulis (average weight 527 ± 103 mg, mean ± s.d.) at control spinning conditions (a drawing speed of 20 mm s–1 and a temperature of 25∞C) have an average silk diameter of 3.35 ± 0.63 mm with a normalised average breaking strain of 0.39 ± 0.08%, a breaking stress of 1.15 ± 0.20 GPa, an initial modulus of 7.87 ± 1.85 GPa, a yield stress of 0.153 ± 0.058 GPa and a breaking energy of 165 ± 28 kJ kg–1 [89]. Like most fibres, silk has a moderate positive Poisson ratio with a thinning ratio of circa 5% for each 10% of strain in a linear fashion until the maximum extensibility of 40% when the fibre typically breaks [89]. The mechanical properties of a fibre are greatly affected by the conditions of manufacture. For example, in Nephila dragline silk produced under highly controlled conditions, not only diameter but also most mechanical properties were affected significantly by both speed and temperature of spinning [89] (Fig. 8.6 and Fig. 8.7). Micro-X-ray diffraction shows that spinning speed and temperature both affect the molecular structure of a silk filament [87, 90], which in turn is responsible for the observed mechanical properties. In addition to these environmentally induced variablities, the mechanical properties of dragline silks show also large variability between individual spiders as well as differences between species [76] (Fig. 8.8). It can be strongly argued that silk is tuned for the average conditions that the different spiders would encounter in nature. Thus, as each silk of a particular species is optimised, in some cases the ability for a rapid (and temporary) adaptation to the environment could offer a great selective advantage [20]. Orb webs have been designed by evolution to take out-of-plane load in maximum deflection [78]. Their performance is greatly enhanced by incorporating into one web the mechanical properties of different types of silk [7]. Environmental conditions affect both the architecture of the web [77] and the mechanical properties of the silken threads [89]. Consequently the spider is under considerable and sustained selection pressure to modify web engineering including silk mechanics [4, 20] and silk genetics [42]. As shown earlier, orb weavers like the garden spider Araneus or the golden silk spider Nephila coat the capture threads with an aqueous solution that forms sticky droplets. Coat and droplets are crucial for the function of these capture threads as their elasticity derives largely from the high water content of the coat. However, water is important not only for these threads, but for many other types of thread as well, and the role of water as well as other solvents for understanding and manipulating the mechanical properties of spider silk cannot be understated (Fig. 8.9). This can be of special interest if we aim to produce bio-engineered silks with specific properties.

Biodegradable and sustainable fibres Normalised thread diameter [mm]

258

Reeling speed (mm per sec.) Temperature (∞C)

1.4

1.2

1.0 A 0.8

Normalised breaking energy

1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6

B

0.5 0.1

0.5 5–12

0.2 10 13–19 20–26

20 100 400 Speed 27–33 34–40 Temperature

8.6 The effect of drawing speed and abdominal temperature at spinning on normalised silk diameter (A) and normalised energy (B) required to break a thread. For non-normalised data under control conditions see Table 8.1. Reeling speeds (∑) are denoted in mm s–1; body temperatures ( ) are given in ∞C. Control temperature was 25∞C when reeling speed was varied, and control speed was 20 mm s–1 when body temperature was varied; 95% confidence intervals are given for each data point (after ref. 89). At 20 mm s–1 and 25∞C females with an average weight of 527 + 103 mg (mean + s.d., N = 4) had an average silk diameter of 3.35 + 0.63 mm with a normalised average breaking strain of 0.39 + 0.08, a breaking stress of 1.15 + 0.20 GPa, an initial modulus of 7.87 + 1.85 GPa, a yield stress of 0.153 + 0.058 GPa and a breaking energy of 165 + 28 kJ kg–1 (for details see ref. 89).

Many spider silks contract in water [91] and other small-molecular solvents [92]. Indeed some silks such as, for example, the typical Nephila and Araneus dragline threads, can super-contract up to 50%. The degree of super-contraction seems to be a function of the crystallinity of the material and can be used to study both the gross morphology of silks [93] and their molecular structure [92, 94]. The glass transition temperature of spider silk of about –75∞C suggests that at room temperature the molecular chains are held in place by intermolecular hydrogen bonds. As these bonds are gradually destroyed by

1.2 1.0 0.8 0.6 5

15 25 Temperature (∞C)

35

Thread diameter and breaking energy

1.7

1.4

1.6 1.4 1.2 1.0 0.8 0.6 0.4

Normalised breaking stress and strain

Normalised breaking strain and stress

1.7 1.5 1.3 1.1 0.9 0.7 0.5

Breaking energy Thread diameter

1.6 1.4 1.2 1.0 0.8 0.6 0.4

Normalised modulus and yield point

Thread diameter and breaking energy

1.6

Normalised modulus and yield point


2.5

Breaking energy Thread diameter .1 1 10 100 1000 Reeling speed (mm per sec.)

(A)

(D)

Breaking strain Breaking stress

5


35

Modulus Yield point

1.3 1.1 0.9 0.7 5


(C)

Breaking strain Breaking stress

.1 1 10 100 1000 Reeling speed (mm per sec.)

(E)

(B) 1.5

259

35

Modulus Yield point

1.5

0.5 .1 1 10 100 1000 Reeling speed (mm per sec.)

(F)

8.7 The effect of abdominal temperature (A, B, C) and drawing speed (D, E, F) on normalised silk parameters. For non-normalised data under control conditions see Fig. 8.6 Shown here are (A, D) thread diameter and breaking energy, (B, E) stress and strain at breaking as well as (C, D) initial Young’s modulus and point of yielding. Each point represents the average taken from several spiders, with 3 measurements for each animal; the vertical bars are 95% confidence intervals (for details see ref. 89).

the actions of the solvents, the molecular chains begin to ‘disorient’, i.e. lose order. Thus, in essence, the contraction of spider silk in water results from the disorientation of the molecular chains. The more hydrogen bonds are destroyed, the larger the shrinkage until, finally and in a strong solvent, the silk is totally dissolved. It has been shown that, in water, silk with high birefringence shrinks less than silk with low birefringence [95] and we may assume that birefringence is positively correlated with degree of hydrogen bonding. Thus, birefringence would also be positively correlated with molecular orientation and higher density, i.e. more so-called ‘b-sheet crystal areas’; one might hypothesise that such very dense areas keep water molecules out with the result of fewer broken hydrogen bonds and lesser shrinkage. Major ampullate dragline silk from Nephila spiders has high initial modulus

260

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1 3

Stress (GPa)

2 1.0

4

5

0.5

0 0

0.1

0.2 Strain (A)

0.3

0.4

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Spider 27 mm/s

1.0

20 mm/s 13 mm/s 0.5

0

Cocoon

0

0.1

0.2 Strain (B)

4 mm/s

0.3

0.4

8.8 (A) The mechanical properties of different spider dragline silks. Stress–strain characteristics of silk reeled from spiders belonging to widely diverging taxa: (1) Euprosthenops sp. (Pisauridae), (2) Cyrtophora citricola (Araneidae), (3) Steatoda bipunctata/ Latrodectus (Theridiidae), (4) Araneus diadematus (Araneidae) and (5) Nephila edulis (Tetragnathidae). The large inter-specific differences in drag and structural major ampullate threads which might correlate to web type: Euprosthenops, Leatrodectus and Cyrtophora build 3-D space knock-down webs that catch by breaking threads and that have a long active life (several months) whereas Araneus and Nephila build 2-D orb webs that catch by net-action and that have a short service life (a few days at most). Note the differences in strength, extendibility and yielding, all, of course, affecting toughness (for details see ref. 76). (B) Influence of reeling speed on the mechanical properties demonstrated with Bombyx mori cocoon silk. For comparison, the properties of spider dragline (spider) and cocoon silk (cocoon) are given.

(~14GPa), good tensile strength (~1.5Gpa) and high extensibility (~40%) even below 0∞C. However, unlike most synthetic filaments, this silk also shows remarkable toughness at temperatures below –60∞C as well as tough fracture behaviour even in liquid nitrogen [96]. Even more unusual, the elongation to break decreases with the increasing temperature and reaches a


261

7.9

3.0

4.9

0

2.4

Distance from exit spigot (mm) Motorized spindle Silk thread

Spider

Needle

Needle A

B

C

8.9 (A) Experimental set-up for in situ X-ray diffraction during forced silking. The spider is fixed by soft tape and mylar bandages to a metal support. The path of the thread from the spinnerets to the motorised reel is schematically indicated. Distance indications (to the spinneret exit) correspond to points where X-ray diffraction data were recorded. (B) Optical image of draw-down of N. edulis spider silk at a drawing speed of 20 mm s–1. (C) Diffraction pattern obtained at 23.5 ± .5∞C. Miller’s indices are indicated for selected reflections (for details see ref. 87).

minimum around +70∞C. Strength and toughness of this silk begins to decrease around +100∞C while the failure temperature lies around 370∞C [96]. Thus, this benchmark spider silk retains its exceptional mechanical properties over a temperature range from at least –66∞C and probably down to liquid nitrogen temperatures and up to about 100∞C while breaking up just below 400∞C. Clearly, spider silk can teach even the modern synthetic chemist a trick or two. A quantitative model for silk can be developed [84] by looking at silk from the perspective of a user. The spider uses silk threads to manage mechanical energy (without the silk actually breaking) for different tasks: to store elastic energy in supporting its own weight or for the structural framework of a web, and to absorb kinetic energy to capture flying insects. The mechanisms at a molecular level that dictate energy storage and dissipation in a polymer can be identified and analytical relations can be derived for the full range of possible mechanical properties in silk. Such relations can then be expressed in terms of a small number of energy-based parameters with a direct fundamental link to chemical composition and morphological order [84].

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Ideally, thus the key design principles in natural polymers fibres can be elucidated and compared to man-made polymers. Such comparison would offer the potential for the design of greatly improved synthetic biopolymers.

8.6

Applications

During Napoleonic times, in Madagascar and France, silk from the giant Nephila madagascariensis golden silk spider was collected and used to spin gloves, for example. This industry was not successful, not least due to the cannibalistic nature of the spiders. But ever since (and probably before, see the Greek story of Arachne) spider silks were seen as an exemplary fibre for many applications that required strength and toughness. Recently, spider fibres are often associated with bullet-proof vests, perhaps in analogy to the spider’s insect prey which can be seen as flying protein-bullets. However, the way in which both web and silk deal with the kinetic energy of the insect projectile is not readily transferable to a flak-jacket [7], certainly not if one does not want the bullet to dent the fabric by more than a few inches. After all, the toughness of spider silk depends on its extensibility. Hence artificial spider silks are best envisioned for requirements where stretch is required and where water is not likely to be a problem. But in addition to toughness spider silks seem to be bio-compatible [6], as well as being decorated for function [75], making them excellent biomedical materials. Accordingly, it seems that today much effort is spent looking for ways to develop spider silks for this application.

8.7

Future trends

Spider silks are bio-polymers with a wide range of interesting mechanical properties [97]. If these silks could be manufactured in quantity and quality and with comparably cheap and environmentally friendly production methods [6] then they could indeed become interesting alternative fibres to low-tech materials such as nylon or cotton (which are cheap, but environmentally costly) or hi-tech materials such as Kevlar or Twaron (which are expensive and environmentally costly). The worldwide production of synthetic fibres exceeds several million tons p.a. and requires an equal amount of fossil carbohydrates; the consumption of energy is not even considered [75]. Although decay of deposited synthetics is slow, in the end the degradation of these fibres will add to the overall balance of the greenhouse gas carbon dioxide. Recombinant spider silk on the other hand can be generated from sustainable resources and could be recycled since they are made of proteins and therefore are fully degradable [98]. Thus, even the replacement of low-tech fibres would be beneficial by lowering carbon dioxide output and saving valuable resources. However, the key to low-tech applications lies in the capabilities


263

to find cheap and efficient production methods to deliver the huge amount of material that is demanded. Initially, artificial dragline-type spider silk will probably find use in medicine [99] partly because of the traditionally high return on investment in this field, partly because here spider silks have already a long tradition as ad hoc emergency plaster. However, there would also be a potential future in other markets; it is likely that techno-silks in addition to replacing some now traditional man-made fibres might find a use in novel applications. Magnetic silk-fibre composites, for example, can be made by binding colloidal magnetite (Fe3O4) nanoparticles to threads of dragline spider silk [100]. Such mineralised fibres retain their high strength and elasticity but can be oriented by an external magnetic field. Finally, artificial silks could find profitable employment in lightweight composites where their toughness and good thermal stability might be rather desirable. In order to find an efficient way to produce spider silk proteins, a number of researchers and companies have attempted to express the relevant genes in a range of organisms that are relatively easily and cheaply cultured. This included transgenic plants (e.g. potato tubers) and mammals (e.g. goat’s milk), which could provide a substantial harvests in agricultural production systems [1–3]. These more ‘advanced’ and not necessarily more productive systems were used in addition to the more typical fermentation systems where spider silk genes were expressed in microorganisms such as E. coli and Pichia pastoris [38, 101–103]. Other host systems like MAC-T or BHK mammal cell lines were also used, but due to the high costs these were more of scientific interest [1]. Commercially more interesting could be the use of specialist spider or insect cell lines [104]. Products of sizes up to 150 kDa were successfully expressed in these systems and the applied gene cassette models are capable to extend the product size at wish. However, it remains unclear whether the natural protein size is a requirement for the fibre quality and, aside from molecular size, whether the heterogeneity observed in the sequence repeats might also be important. It is, of course, very much hoped that one or several of these production systems will be able to supply (some time in the not-too-distant future) sufficient amounts of raw materials to allow spinning silk-like fibres on a commercial scale. However, we must not forget the parallel development of appropriate ‘spinning’ extruders. Once a good and reliable expression system is up and running then we can test and optimise both the artificial spinning dopes and the spinning methods. Only by tuning both to act in synergy will we be able to manufacture fibres to match the spider’s threads and their millions of years of co-evolution of feedstocks and extrusion systems.

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8.8


Acknowledgements

For funding we thank the British EPSRC (grant GR/NO1538/01) and BBSRC (S12778), the European Commission (grants G5RD-CT-2002-00738 and MTKD-CT-2004-014533), the Danish SNF (grant 21-00-0485), the German BMBF (BMBF FKZ 0311130), the Thuringer Ministerium fuer Wissenschaft, Forschung und Kultur (TMWFK B307-99-001), and the AFSOR of the USA (grant F49620-03-1-0111).

8.9

References and sources of further information

8.9.1

Sources in the web and recommended reading

http://www.arachnology.org/arachnology/arachnology.html http://www.nexiabiotech.com http//www.spinox.net http://www.xs4all.nl/~ednieuw/spiders/spidhome.htm Craig, C., Spider webs and silks, 2004, Oxford: Oxford University Press. Porter, D., Vollrath, F. and Shao, Z., Predicting the mechanical properties of spider silk as a model nanostructured polymer. Eur. Phys. J. E. Soft Matter, February 2005, 16(2): 199–206. Vollrath, F. and Knight, D.P., Liquid crystalline spinning of spider silk. Nature, 2001, 410(6828): 541–548.

8.9.2

References

1. Lazaris, A., Arcidiacono, S., Huang, Y., Zhou, J.F., Duguay, F., Chretien, N., Welsh, E.A., Soares, J.W. and Karatzas, C.N., Spider silk fibers spun from soluble recombinant silk produced in mammalian cells. Science, 2002, 295(5554): 472– 476. 2. Scheller, J., Gührs, K.-H., Grosse, F. and Conrad, U., Production of spider silk proteins in tobacco and potato. Nature Biotechnology, 2001, 19: 573–577. 3. Scheller, J., Henggeler, D., Viviani, A. and Conrad, U., Purification of spider silkelastin from transgenic plants and application for human chondrocyte proliferation. Transgenic Research, 2004, 13(1): 51–57. 4. Vollrath, F., Coevolution of behaviour and material in the spider’s web, in Biomechanics in Animal Behaviour, P. Domenici (ed.), 2000, Bios: Oxford. 5. Knight, D.P., Knight, M.M. and Vollrath, F., Beta transition and stress-induced phase separation in the spinning of spider dragline silk. International Journal of Biological Macromolecules, 2000, 27(3): 205–210. 6. Vollrath, F.K., Yoshihauru, D., Biology and technology of silk production, in Handbook of Biopolymers, Steinbüchel, A. (ed.), 2003, Heidelberg and New York: Wiley-VCH, 25–46. 7. Vollrath, F., Spider webs and silks. Scientific American, 1992: 70–76. 8. Gosline, J.M., Denny, M.W. and Demont, M.E., Spider silk as rubber. Nature, 1984, 309(5968): 551–552.


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9 Biodegradable natural fiber composites A N N E T R A V A L I, Cornell University, USA

9.1

Introduction

9.1.1

Composite materials

Fiber reinforced polymeric composites have been used in many structural applications because of their high strength and low density giving them significant advantages over conventional metals. Initially developed for the defense and aerospace industries, high performance or ‘advanced’ composites are now commonly used in many applications from circuit boards to sports gear and from automotive parts to building materials. The use of composite materials has expanded at more than 10% per year in developed countries. In developing countries such as India and China the use of composites is growing at even faster pace. Fiber reinforced composites, depending on the properties needed, can be fabricated in three different ways. Very short fibers can be used as filler, short fibers can be organized with random orientation and long fibers can be laid in one direction to form unidirectional composites. Short staple fibers may also be twisted together to form continuous yarns to fabricate unidirectional composite laminates similar to those made using long fibers. Several unidirectional laminates may be combined by layering in different directions to form laminar composites. Yarns may also be woven or knitted into fabrics to form similar laminar composites. Most of the fibers and resins currently available on the market are derived from petroleum. There are two major problems associated with using petroleum as the feedstock for polymers. First, it is a non-renewable (non-sustainable) resource and at the current rate of consumption, by some estimates, it is expected to last for only 50–60 years [1]. Also, the current petroleum consumption rate is estimated to be 100,000 times the rate of natural generation rate [1]. Second, most fibers and resins made using petroleum are nondegradable. Although this is desirable in many applications from the durability point of view, at the end of their life, they are not easy to dispose of. Discarded 271

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in natural environment, these polymers and composite materials can last for several decades without decomposing. This has exacerbated the already existing ecological and environmental problems of waste building. Because composites are made using two dissimilar materials, they cannot be easily reused or recycled. Although some composites are incinerated, most composites end up in landfills. Both of these disposal alternatives are environmentally unsound, wasteful and expensive. In addition, landfills are decreasing in number, making less space available to discard waste. In the US alone, the number of landfills dropped from 8000 to 2314 between 1988 and 1998 [1]. As a result the tipping fees have been rising steadily. In recent years, the ever-growing litter problem has raised environmental consciousness among many activists, consumers as well as manufacturers, forcing them to act. Many governments, in response, have established laws to encourage recycling and the use of bio-based ‘green’ products [2]. Some governments have enforced stricter ‘take-back’ rules requiring manufacturers to take back packaging and products at the end of their life. These environmental concerns and depletion of petroleum resources have given birth to the concepts of sustainability, eco-efficiency, industrial ecology and cradle-to-cradle design. These concepts form the principles that have triggered the search for new generation of ‘green’ materials, many of them plant-based. Most manufacturers are working hard to make their products eco-efficient and ‘green’ to the fullest extent possible. Composite materials are no exception to this new paradigm. Undoubtedly, environment-friendly, fully degradable ‘green’ composites will play a major role in greening the products of tomorrow [3]. Many applications, e.g. secondary and tertiary structures and those used in consumer products for casings and packaging, do not require the high strength and stiffness of advanced composites. In these products plant-based lignocellulosic fibers have been a natural choice. Worldwide availability of inexpensive plant-based fibers has fueled their use in the past few years for reinforcing or filling polymers/plastics to make them greener. Natural fibers have several advantages besides being biodegradable. They are nonabrasive to processing equipment, can be incinerated and are CO2 neutral (when burned) [4]. In addition, because of their hollow tubular structure and cellular nature, most bast fibers, derived from plant stems, perform well as acoustic and thermal insulators [5]. Their hollow structure also reduces their bulk density, making them and their composites lightweight. There are plenty of examples in the literature where plant-based fibers have been used for reinforcing or filling non-degradable resins such as polypropylene (PP), polyethylene (PE), nylons, polyvinyl chloride (PVC), epoxies and polyurethanes (PU), etc. [6–15]. The bulk of plant-based fiber composites, however, are made using wood flour, a byproduct from saw mills, or wood fiber obtained from waste or used wood products, e.g. packaging pallets, old furniture, and construction wood scraps. These inexpensive


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composites, particularly the ones using recycled polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC), are used for applications ranging from decking (plastic lumber) to furniture and from rail road ties to window and door frames. Demand for such composites is predicted to double from 2001 to 2006 in North America [16]. Longer plant-based fibers, e.g. abaca, bamboo, flax, hemp, henequen, jute, kenaf, pineapple, ramie, sisal, etc., are being evaluated as a low cost replacement for glass fibers in composites. Most of these fibers are harvested annually as compared to wood which takes 20–25 years to grow before it may be cut and used. Some fibers, e.g. ramie, may be harvested a couple of times a year making their supply virtually inexhaustible [17]. Bamboo plants, belonging to the grass family, also grow very fast and may be harvested every 3–4 months [3]. Since these composites combine non-degradable resins with plant-based degradable fibers they can neither return to an industrial metabolism nor to a natural metabolism. Unfortunately, they cannot be food stock for either system. They can only be downcycled because of their property degradation during reprocessing or incinerated to recover the energy value.

9.1.2

Fully green composites

Significant research is currently being done to develop a new class of fully biodegradable and truly ‘green’ composites by combining plant-based cellulosic fibers with biodegradable resins, particularly those derived from plants [18– 35]. A variety of natural and synthetic biodegradable resins are available for use in green composites [31, 36–42]. At the end of their life they can be easily disposed of or composted without harming the environment. Being in its infancy, most of the current green composites technology is still in the research and development stage although a few scattered and niche examples of commercial products can be found [31, 33]. For example, NEC Corp., Japan, has developed kenaf fiber reinforced polylactic acid (PLA) in place of epoxy resin for encapsulating silicon chips and Fujitsu, another Japanese technology company, is using PLA for laptop casings. All major automobile makers are also involved in developing green polymers and composites for interior applications. Green composites may be used in many applications such as mass produced non-durable consumer products with short life cycles or products intended for one-time or short-term use before being disposed. However, most green composites may be used in indoor applications extending their useful life to several years, just like wood. One of the major factors that limits the use of green composites today is their high cost compared to conventional materials. However, as their applications increase and they begin to be mass produced, the cost is expected to drop, as always is the case.

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The following sections provide brief information regarding the plantbased fibers and describe some of the research efforts in developing green composites using plant-based fibers and soy protein-based resins.

9.2

Biodegradable fibers

Fibers used in most applications can be divided into two main categories; natural or synthetic. Much of the information about fibers regarding their chemistry, manufacture and properties can be found in general fiber handbooks [43]. Many fibers, natural, regenerated or synthetic, are biodegradable. Natural fibers can be subdivided into three main categories depending on the nature of their source: (1) vegetable, (2) animal or (3) mineral. While vegetable (plant-based) and animal fibers are fully biodegradable, mineral fibers are not.

9.2.1

Plant-based fibers

Many useful fibers have been obtained from various parts of the plants including the leaves, stems, (bast) and fruits/seeds. The lengths of these fibers depend mainly on their location within the plant, e.g., fibers from fruits/seeds are short (few centimeters) whereas fibers from the stem and leaves can be longer than one metre. Plants commonly produce cellulose, a linear polysaccharide, as the structural material. It is present in all parts of the plant, in different amounts, from roots to fruits. The other major constituents of fibers obtained from plants are hemicellulose and lignin. A small amount of pectin may also be present in many plant-based fibers. Cellulose is a condensation product consisting of a varying number of anhydroglucose monomeric units connected to each other by b-1,4-glycosidic linkages [44]. The degree of polymerization (DP) varies between 200 and 10,000, but 3000 is the acceptable average value for plant cellulose [45]. Three hydroxyl groups present in the glucose units form strong intramolecular as well as intermolecular hydrogen bonds between the adjacent cellulose molecules. Being linear, cellulose molecules can orient easily and form crystals that can organize into microfibrils, each of which orients at a specific angle in relation to the fiber axis. This microfibrillar angle has been found to vary with the fiber species [44]. Water molecules can penetrate through capillaries and spaces between fibrils and can chemically link to the groups present in cellulose molecule [46]. These water molecules can significantly reduce the rigidity of cellulose by forcing the cellulose molecules apart and acting as a plasticizer. Hemicellulose comprises a group of polysaccharides and it differs from cellulose in terms of high degree of chain branching and low DP [45]. It also contains several different sugar units [42, 47, 48]. As a result of the branching


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and presence of several different sugar units, it cannot crystallize. Lignin is a hydrocarbon polymer with a complex structure consisting of both aliphatic and aromatic constituents that forms the matrix sheath around fibrils and fibers [49]. It is responsible for binding cellulose microfibrils to form a composite structure. The exact chemistry of lignin is not well understood. Hemicellulose is thought to be responsible for the biodegradation, moisture absorption and thermal degradation of the fibers, whereas lignin is responsible for the UV degradation [50]. Cellulose with three hydroxyl groups on each glucose unit can also absorb moisture, but it is limited because of its high crystallinity. Pectins comprise a collection of heteropolysaccharides and are characterized by high uronic acid content [45, 51]. Pectin has higher DP than hemicellulose but much lower than cellulose itself [45]. The cellulose, hemicellulose and lignin contents in plant fibers vary depending on the plant species, origin, quality and conditioning [52]. A list of some plant-based fibers and their chemical compositions and microfibrillar angles are presented in Table 9.1. The mechanical properties of the fibers vary depending on their constitution and the amount of cellulose and the crystallinity. As mentioned earlier, they are also influenced by the DP of the cellulose and microfibrillar orientation. The part of the plant from which the fibers are derived is also a contributing factor in many cases. For example fruit/seed fibers, e.g., coir fibers obtained from coconut, are weaker than the fibers obtained from the stem of the plant, e.g., ramie, hemp or flax fibers. Table 9.2 gives the physical and mechanical properties and moisture content of some plant-based and viscose fibers. Microfibrillar angle, one of the internal structural parameters, and cellulose content determine the strength and stiffness of the fibers [53]. It is clear from Table 9.2 that hemp and ramie, both bast fibers, have high tensile strength and modulus, which are attributed to their low microfibrillar angle and high cellulose content, whereas coir fibers, obtained from coconut seed, have the least tensile strength resulting from their high microfibrillar angle and low cellulose content. It is, however, difficult to draw an exact relationship between the various factors. Yet, it is

Table 9.1 Chemical compositions and microfibrillar angles of some natural fibers Fiber

Cellulose (wt %)

Hemicellulose (wt %)

Lignin (wt %)

Pectin (wt %)

Microfibrillar angle (∞)

Bamboo Coir Flax Hemp Jute Ramie Sisal

60.8 36.0–43.0 71.0 70.2–74.4 61.0–71.5 68.6–76.2 67.0–78.0

– 0.2–0.3 18.6–20.6 17.9–22.4 13.6–20.4 13.1–16.7 10.0–14.2

32.2 41.0–45.0 2.2 3.7–5.7 12.0–13.0 0.6–0.7 8.0–11.0

– 3.0–4.0 2.3 0.9 0.2 1.9 10.0

2.0–10.0 41.0–45.0 10.0 6.2 8.0 7.5 20.0

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Table 9.2 Comparative properties of some plant-based and viscose fibers Fiber

Density (g cm–3)

Tensile stress (MPa)

Bamboo Coir Cotton Flax Hemp Jute Pineapple Ramie Sisal Softwood kraft Viscose (cord)

0.8 1.2 1.5 1.5 – 1.3–1.5 – 1.5 1.5 1.5 –

221–661 131–175 287–597 345–1100 690 393–773 413–1627 400–938 511–635 1000 593

Young’s modulus (GPa) 22.8–49.0 4.0–6.0 5.5–12.6 27.6 – 13.0–26.5 34.5–82.5 61.4–128 9.4–22.0 40.0 11.0

Fracture strain (%)

Moisture content (wt. %)

1.3 15.0–40.0 7.0–8.0 2.7–3.2 1.6 1.2–1.5 1.6 1.2–3.8 3.9–7.0 – 11.4

– 8.0 – 10.0 10.8 12.6 11.8 8.0 11.0 – –

well known that the strength of any fiber, including the ones obtained from plants, is more affected by defects than by structural elements per se. Many of these fibers are strong, as seen in Table 9.2, and are increasingly being used as reinforcement in both biodegradable and non-biodegradable polymers [3, 4, 37, 52]. Because of the low specific gravity of these fibers, the specific mechanical strength and modulus of some fibers are excellent and comparable to glass fibers. As a result, they have been used to replace glass fibers in some applications.

9.2.2

Protein fibers

Protein fibers are made up of polypeptide chains composed of various amino acids as primary (monomeric) units. Protein fibers can be categorized into four distinct groups based on the source: hair fibers obtained from animals, fibers formed by their secretion, fibers obtained from avian feathers, and fibers that are regenerated from vegetable or animal proteins. The most common hair fiber used in the textile industry is wool obtained from sheep [43, 45]. Hair obtained from other animals such as goats, camels, llama, alpaca, guanaco, ox, rabbit, etc. have been also used as specialty fibers. The protein forming hair and feather fibers is known as keratin. Most hair fibers are made up of several different amino acids as basic monomeric units and cannot crystallize well. As a result, hair fibers tend to have low mechanical properties compared to plant fibers and hence are not generally used as reinforcements for composites. However, they tend to have excellent thermal properties and are commonly used as sweaters and winter jackets. The most common fiber produced from animal (or insect) secretion is silk, which is secreted by the caterpillar (silkworm) of the moth Bombyx Mori [17, 43, 45]. Nearly the entire silk industry is based on the silkworm.


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The cocoon formed by the caterpillar contains continuous length of 800– 1200 m making silk the only continuous length natural fiber. Silk is also stronger than many of the animal hair fibers and has been used in reinforcing composites. However, silk production is expensive because of the laborintensive and time-consuming nature of sericulture. Several wild varieties of silk are also commercially produced. One of the more common wild silks, known as tussah silk, is the secretion of the caterpillar of the moth Antheraea Mylitta. Tussah silk, in general, is coarser and stronger than silk but contains shorter fibers. It also tends to be less uniform compared to silk and contains defects such as varying thicknesses along the length [17]. There has been a significant interest, in recent years, in spider silk, particularly the dragline silk produced by the golden orb spider because of its excellent mechanical properties. Much effort has been spent understanding the chemistry and structure of these fibers and using biotechnology (biomimicry) to produce such fibers commercially. Nexia Biotechnologies Inc. has been successful in introducing the spider silk gene into goats to obtain the dragline silk protein in their milk [54]. This technology is still not mature and will take a few years before spider silk fibers will be commercially available. Feathers of various bird species generate fibers with useful properties [55]. The annual US production of the most abundantly available feathers, a byproduct from poultry, is 2000 million kg [55]. Half of that weight is fibers, making this an abundant, readily harvestable agriculture resource. Use of feather fibers in thermal insulation in winter jackets and other applications is well known. Turkey feathers, with a length of approximately 3 cm, have also been spun into useful yarn. Other fibers have also been used to make functional, water repellent and filter paper, oil absorbing mats as well as composites. Some turkey fibers have also been processed into mats to be used for erosion control and other applications [56]. Since feather fibers generally don’t have good mechanical properties, they cannot be used in load bearing composites.

9.2.3

Regenerated and modified fibers

Since cellulose from plants is a renewable resource, there have been several efforts in developing regenerated cellulose fibers. A secondary reason for these efforts is that no plant fibers come in continuous form for easy weaving or knitting into fabrics. One of the most common regenerated cellulose fibers is viscose rayon (viscose), derived from purified wood pulp. Many varieties of viscose fibers, e.g. viscose, high tenacity viscose, etc., are available commercially. The latest version of regenerated fiber, ‘lyocell’ fiber commercialized by Acordis in England, and Lenzing in Austria, employs an intrinsically cleaner process by dissolving cellulose in a non-toxic solvent N-methyl morpholine N-oxide (NMMO). NMMO is environmentally harmless

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and biodegradable in waste water treatment. Also, most of the NMMO used in spinning the fibers is recovered and reused [57]. More information about this can be obtained elsewhere in this book. Chitosan, a copolymer of 2-amino-2-deoxy-D-glucopyranose and 2acetamido-2-deoxy-D-glucopyranose units, and alginate, a linear copolymer of b-(1,4)-linked D-mannuronic acid and a-(1,4)-linked L-guluronic acid units, have been spun into useful fibers. However, because of low mechanical properties they are mostly limited to medical and other non-load-bearing specialty applications [58]. Further information about chitosan fiber can be obtained elsewhere in this book.

9.2.4

Developments in fibers

There are several efforts in developing nano or submicron size fibers from cellulose and other polymers using electrospinning. eSpin Technologies, Inc., Chattanooga, TN, has been a leader in commercializing the electrospinning technology to spin fibers on a large scale. However, production on a large scale is still a thing of the future. Also, the current technology is still in its infancy and it is difficult to obtain molecular orientation and strong fibers. As a result, most of the current applications of electrospun fibers can only take advantage of their small diameters and large surface area per unit weight. Another significant effort in developing high strength spider silk proteinbased fibers under the trademark Biosteel® is by Nexia Biotechnologies Inc. [54]. Another development is to obtain microfibrillated cellulose (MFC) from inexpensive plant fibers. These microfibrillar structural units are comprised of oriented cellulose chains and exhibit excellent mechanical properties, almost comparable to high strength aramid fibers [30, 59–62]. MFC can be obtained through a process consisting of a mechanical treatment of pulp fibers, consisting of refining and high pressure homogenizing [59–62]. The refining process used is common in the paper industry and is accomplished using the refiner. In a disk refiner, the dilute fiber suspension is forced through a gap between rotor and stator disks. These disks have surfaces fitted with bars and grooves. During the process the pulp fibers get sheared into fibrils. In the homogenizing process, dilute slurries of fibrils obtained from refiner are pumped at high pressure and fed through a spring loaded valve assembly. As the valve opens and closes in rapid succession, the fibrils are subjected to large pressure drops with shearing impact forces. This combination forces the fibrils to further fibrillate into microfibrils. To obtain high degree of microfibrillation the homogenizing process may be repeated several times. The strength of MFC has been estimated at 2 GPa based on the experimental results of 1.7 GPa obtained for kraft pulp which mainly consists of cellulose microfibrils where 70 to 80% of the microfibrils are distributed


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parallel to the fiber axis [63]. The nanofibrillar cellulose strength, which is made up of cellulose whiskers, has been estimated between 2 GPa and 12 GPa [59–62]. The modulus of the cellulose microfibrils can reach 134 GPa with a density of 1.5 g/cc [64]. Cellulose is also secreted by some bacterial species. Bacterial cellulose (BC) is produced by Acetobactor species cultivated in a culture medium containing carbon and nitrogen sources [65]. This extremely fine and pure fiber network structure has very high mechanical strength. The network structure in the form of pellicle made up of random assembly of ribbonshaped fibrils, less than 100 nm wide, which are made up of a bundle of much finer nanofibrils, 2 to 4 nm in diameter. Unlike fibrillation of plant fibers, BC is produced by bacteria in a reverse way, synthesizing cellulose and building up bundles of nanofibrils. The BC fibrils are highly oriented and could be used in ‘green’ composites. In fact, Nishi et al. [66] have reported excellent dynamic modulus of about 30 GPa of sheets processed from BC pellicles. Nakagaito et al. [65] reported Young’s modulus of 28 GPa of BC-based composites made using phenol-formaldehyde resin.

9.3

Biodegradable resins

As mentioned earlier, rising oil prices, widespread awareness of nonsustainability of petroleum oil, and the ever increasing tipping fees for landfills and incineration costs of commodity plastics and composites have contributed to renewed interest in fully biodegradable, renewable and environment-friendly, green plastics. Although the current market is very small compared to conventional resins, there are a variety of green resins available in the market today. As in the case of fibers, the resins can be classified into two broad categories based on their origin: natural and synthetic resins [1]. This chapter is mainly devoted to the natural soy protein-based resin and its modifications and use in composites.

9.3.1

Natural resins

Most natural resins are derived from plants or animals. They may be modified chemically or blended with other materials to improve their mechanical, physical or thermal properties and to make them easier to process. There are two major categories of natural polymers: polysaccharide-based and proteinbased. Many of these polymers can be used as resins; a partial list of natural and synthetic biodegradable resins is presented in Table 9.3. Most of these resins degrade through enzymatic reactions in environments such as natural composting while some may degrade by hydrolysis in the presence of moisture under acidic or alkaline conditions. However, it is important that all intermediate and final degradation reaction products be environmentally benign for the

280


Table 9.3 Natural and synthetic biodegradable polymer resins Natural

Synthetic

1.

1. 2. 3. 4. 5. 6. 7.

2.

3.

4.

Polysaccharides ∑ Starch ∑ Cellulose ∑ Chitin ∑ Pullulan ∑ Levan ∑ Konjac Proteins ∑ Protein from grains ∑ Collagen/gelatin ∑ Casein, albumin, fibrogen, silks, elastin Polyesters ∑ Polyhydroxyalkanoates, copolymers Other polymers ∑ Lignin ∑ Shellac ∑ Natural rubber

8. 9. 10. 11.

Poly(amides) Poly(anhydrides) Poly(amide-enamines) Poly(vinyl alcohol) Poly(ethylene-co-vinyl alcohol) Poly(vinyl acetate) Polyesters ∑ Poly(glycolic acid) ∑ Poly(lactic acid) ∑ Poly(caprolactone) ∑ Poly(orho esters) Poly(ethylene oxide) Poly(urethanes) Poly(phosphazines) Poly(acrylates)

resins to be truly ‘green’. The remainder of this chapter discusses soy protein resins and some of the modifications carried out to improve their mechanical, physical and thermal properties as well as the moisture resistance as well as green composites made using these resins. Soy protein resins Soy protein, with proper processing, enjoys several advantages such as the ability to form a network structure for use as resin [67]. It can be processed into films for use as garbage and grocery bags [68], edible films [69, 70] and adhesives in particle board and plywood [71, 72]. Soy protein resin has also been combined with natural fibers to produce reinforced composites [19–24, 35, 36]. Some of these efforts are described later in this chapter. The use of soy protein, however, is not new; as early as in 1910s Henry Ford experimented with using agricultural materials to make parts of cars [1]. He tried many crops including wheat gluten, soy meal and soy oil and was successful in making various automobile parts such as coil cases with wheat gluten reinforced with asbestos fibers and glove-box doors, gear-shift knobs, horn buttons, accelerator pedals, distributor heads, interior trim, dashboard panels, etc. with soy meal reinforced with fibers. In 1941, a prototype ‘soybean’ plastic car was developed by Ford Motor Co. The body of this prototype consisted of 14 compression molded panels fixed to a tubular frame. Unfortunately, his efforts were interrupted by the


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Second World War. Later, with a variety of inexpensive polymers developed from petroleum, most efforts in the recent decades have been devoted to the use of non-degradable plastics and composites. However, renewed interest in environment and sustainability issues are bringing green materials to the forefront once again. Commercially available soy proteins are classified into three major groups based on the protein content: soy flour (SF), soy protein concentrate (SPC) and soy protein isolate (SPI) [73]. Approximate compositions of these three varieties are presented in Table 9.4 [74]. SF containing about 55% protein and 35% starch (carbohydrates) is the least refined form of soy protein and is prepared by grinding defatted soy bean flakes. SPC is prepared by eluting soluble components from defatted soy flour and contains about 70% protein and 18% carbohydrates. SPI is the purest form of them all and contains about 95% protein. Table 9.4 Typical compositions of commercially available soy protein varieties Component

Protein

Carbohydrates

Ash

Fiber

Fat

Soy flours (%) Soy protein concentrates (%) Soy protein isolates (%)

56.0 72.0

33.5 17.5

6.0 5.0

3.5 4.5

1.0 1.0

96.0

0.3

3.5

0.1

0.1

Soy protein contains about 18 different amino acids. Some of them contain acidic groups, e.g., aspartic acid and glutamic acid, some of them contain basic groups, e.g., lysine and arginine, etc., and others contain non-polar groups, e.g., alanine, leucine, isoleucine, etc. [75, 76]. The polar groups, both acidic and basic, are responsible for high water absorption by soy protein. The amino acids combine through a condensation reaction to form amide linkages and long polypeptide chains. Most of the soy protein is globulin and is soluble in salt water. Soy protein has been fractionated into various molecular weight components by their sedimentation constants. Four major fractions known as 2S, 7S, 11S and 15S, where S stands for Swedberg units, have been studied extensively. The numbers are nominal and stand for various molecular weights as follows: 2S = 8–22 kDa, 7S = 180–210 kDa, 11S ⯝ 350 kDa and 15S ⯝ 600 kDa [77]. Soy proteins are least soluble in water in their isoelectric region of pH between 4.2 and 4.6 [73, 76]. However, solubility, measured by nitrogen solubility index (NSI, %), sharply increases above and below the isoelectric point as the soy protein denatures and unfolds. This exposes the sulfhydryl groups which associate to form covalent disulfide intermolecular bonding. Under acidic conditions of pH 1–3, significant repulsive forces develop among positively charged soy polypeptide chains, resulting in less

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intermolecular cross-linking [78]. Because of the lower solubility in acidic conditions, various acids have been used to decrease the water absorption of soy proteins [79, 80]. Unfortunately, the acidic conditions do not tend to improve the mechanical properties of the resin. On the other hand, many alkalis, which have been utilized for the dissolution of soy protein, create bridges between the polypeptide chains to stabilize and strengthen the crosslinked network [81]. In such cases, smaller cations, e.g. Li+ and Na+, have been reported to perform better than larger cations, e.g. NH +4 , because of the steric hindrance. Gennadios et al. [82] who studied the effect of pH on the physical and mechanical properties of SPI films, reported that soy protein films prepared at pH of 6–11 had higher fracture stress and strain and lower water permeability than those prepared at pH 1–3. Hettiarchchy et al. [83] were able to improve the adhesive strength of SPI at pH of 10–11. These observations suggested that the SPI molecules open up in more in alkaline conditions resulting in better intermolecular interactions. Heat also has been shown to convert protein from its native state to an unfolded state allowing intermolecular interactions [84]. Mo and Sun [85] determined the denaturation temperatures of SPI through enthalpy changes using differential scanning calorimetry (DSC). They detected two peaks corresponding to the endothermic transitions of b-conglycinin and glycinin, at around 73 and 88∞C, respectively. It is well known that b-conglycinin and glycinin have different structures and functional properties. b-conglycinin is a trimeric glycoprotein of various combinations of three subunits [86]. Glycinin, on the other hand, is a hexamer composed of various combinations of five subunits [87]. Each subunit is made up of an acidic and a basic polypeptide component, which is linked by a single disulfide bond [88]. Thermal treatments also promote intra- and intermolecular cross-links within soy proteins [84, 89]. Such cross-links obviously contribute to the higher tensile strength and modulus of SPI films. However, at the same time they reduce the fracture strain and toughness. Liang et al. [67] showed that both strength and modulus increased with an increase in processing temperature. However, above 160∞C the soy protein starts to degrade and the properties begin to decrease. Thames and Zhou [35] showed using thermogravimetry that SPI starts to decompose above 190∞C. Nam [90] confirmed degradation of SPC when processed above 140∞C for longer exposures above 2 hr. Cured pure soy proteins tend to be brittle. They are also weak as a result of their low fracture strains. To reduce their brittleness, plasticizers are commonly added. The most commonly used plasticizers include polyols, mono-, di- or oligo-sachcharides, lipids and their derivatives. However, glycerin (1,2,3-propanetriol) is by far the most used plasticizer with soy proteins. As in the case of other plasticizers, glycerol decreases the mechanical properties, e.g. strength and modulus, of soy proteins. Glycerin is a relatively small hydrophilic molecule and can easily be inserted between polypeptide chains


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to establish hydrogen bonds with amide, hydroxyl and carboxyl groups. Because of its three hydroxyl groups, glycerin is also strongly linked to increased moisture absorption in the case of soy proteins [19–21, 32, 90, 91]. With further plasticization through the absorbed moisture, the mechanical properties decrease further. Mo et al. [72] showed that at moisture contents higher than 40%, cracks can be easily formed in straw-protein particle boards due to high water vapor pressure trapped in the composites. Jane et al. [79] also showed that the shape changed as water was lost through evaporation. Takagi et al. [92] found that the number of disulfide bonds formed in a protein is affected by the water absorption. The protein with higher number of disulfide bonds adsorbed less water than that with lower disulfide links. Kajiyama et al. [93] reported that the exposure of hydrophobic groups through the denaturation of soy protein molecules contributed to a reduction in water absorption. On this basis, various methods have been employed to improve the water vapor barrier properties of soy protein material, including changes in pH [82], heat treatment [80], enzymatic treatment with horseradish peroxidase [94] and treatment with formaldehyde and urea [95, 72]. Other attempts to increase moisture resistance and improve the mechanical and thermal properties of soy protein resins include cross-linking with maleinized tung oil (MTO) [35] and glutaraldehyde [32], internal plasticizing by stearic acid and forming a cross-linked complex with Phytagel® [19–21] and forming nano-composites with cloisite Na+ clay nanoparticles [96]. Lower water absorption also translates to higher mechanical properties because of less plasticization. These modifications are briefly described below. Soy protein modifications Thames and Zhou [35] used several cross-linking agents to improve the properties of SPI and wood fiber-based composites. Soy protein and wood fiber (50/50 parts by wt) were mixed in a laboratory mixer and cross-linking agents (8%) were individually added by spraying. The composites were molded (hot pressed) using a laboratory press, at temperatures up to 185∞C. Among the various cross-linking agents tried, MTO which was synthesized in their laboratory worked the best. The flexural strength of the composites molded at 165∞C increased from 33 to 58 MPa while the water absorption decreased from 87% to 31%. This was attributed to the intermolecular crosslinking of the SPI introduced by the MTO. However, the long fatty acid chain of the MTO was perhaps a factor in reducing the moisture absorption by the SPI and thus a factor in increasing the flexural strength. Several researchers have used glutaraldehyde (1,5 pentane-di-al) (GA), a colorless liquid with a boiling point of 101∞C and a specific gravity of 1.062, as a cross-linking agent for proteins and soft tissues [97–102, 3]. Figure 9.1 shows the structure of GA. GA can react with the amine groups from various

284

Biodegradable and sustainable fibres O

O

9.1 Structure of glutaraldehyde molecule.

amino acids present in the protein. Although there is no consensus, GA has been shown to react with a- and e-amino groups in lysine, with a-group in glycine and only partially with a-amino groups of histidine and tyrosine [98]. Different mechanisms have been reported for explaining the reactions between GA and proteins [99, 103]. Matsuda et al. [99] showed a simple condensation cross-linking reaction scheme between GA and gelatin. GA has also been used as a cross-linking agent for SPI [101, 102], soy dreg [102], SPC [3, 32] and SPC nanocomposites [96]. Soy protein resin processing Chabba and Netravali [32] used GA to cross-link SPC to improve its mechanical and thermal properties and increase its moisture resistance. The cross-linked (modified) SPC was used as a resin to fabricate green composites using flax yarns and fabrics. The processing of the unmodified SPC resin consisted of two steps: pre-curing and the final curing, simply referred to as curing. SPC powder was mixed with distilled deionized water in a ratio of 1:13 (by wt) in a beaker. In their studies glycerin was used as a plasticizer. The concentration of glycerin was varied from 5% to 20% (by wt of SPC) to study its plasticizing effect. This solution was homogenized using a magnetic stirrer for 15 minutes and then the pH of the mixture was adjusted to 11, using 1 N NaOH solution. SPC solution was again stirred for 15 minutes and then the beaker was transferred to a water bath maintained at 70∞C. The solution was stirred in the water bath for 30 minutes at 70∞C. This step was called pre-curing. To obtain cured SPC sheets, pre-cured SPC solution was cast on Teflon® coated glass plates and dried in an air circulating oven at 35∞C for 20 hrs. Finally the dried SPC sheets were hot pressed (cured) in a Carver hot press at 120∞C for 25 min under a pressure of 7 MPa. Similar processing steps were also utilized for processing SPI and SF by others. Glutaraldehyde modification of SPC The curing process for GA modified SPC (GA–SPC) was similar to that described above for SPC with a few modifications [3, 32]. In this case, distilled and deionized water was added in a ratio of 1:15 (by wt) and a required amount of glycerin was added as plasticizer. The concentration of glycerin was varied between 10% and 20% (by wt of SPC) to study its effect. The solution was homogenized for 15 minutes and the pH of the solution was adjusted to 11 ± 0.1 using 1 N NaOH solution. SPC solution was stirred


285

for 15 minutes and then the beaker containing the mixture was transferred to a water bath maintained at 70∞C. The solution was pre-cured for 27 minutes and then the desired amount of GA solution (25% concentration in water) was added. The solution was further stirred for 3 minutes in a water bath to uniformly disperse the GA. GA is very reactive and starts to react immediately increasing the viscosity of the resin. The GA solution content was varied between 5% and 50% (by wt of SPC), to study the effect of GA cross-linking on cured SPC polymer properties. After pre-curing, the resin solution was cast on Teflon® coated glass sheets, dried at room temperature for 36 hrs and cured in a hot press at 120∞C for 25 minutes under a pressure of 7 MPa. Table 9.5 presents the effect of GA % on the tensile properties of the SPC resin. This resin contained 15% glycerin. It is clear that both fracture stress and modulus increased with the GA content up to 10%. Beyond 10%, however, both stress and modulus dropped. This was because the –NH2 sites were available for the GA to react up to content of 10% GA. Any additional GA, that remained unreacted, resulted in plasticizing the SPC resin. Table 9.6 shows the effect of glycerin content on the moisture absorption of GA–SPC resin containing 10% GA. These data clearly show the positive correlation between the glycerin and moisture content. As stated earlier, this is very much expected because of the three –OH groups present in glycerin. It is also one of the main reasons to eliminate the use of glycerin as plasticizer. Table 9.5 Effect of GA% on the tensile properties of SPC resin containing 15% glycerin GA (%)

Fracture stress (MPa)

Young’s modulus (MPa)

Fracture strain (%)

0 5 10 30 40 50

16.9 17.7 18.4 19.6 19.9 19.5

367.6 374.2 402.1 447.5 484.2 480.9

21.9 25.6 25.4 21.5 20.9 21.9

(3.8)* (5.8) (5.4) (4.2) (6.4) (4.5)

*Figures in parentheses are CV%

Table 9.6 Effect of glycerin content on the moisture absorption of GA–SPC resin containing 40% GA Glycerin (% w/w of SPC)

Moisture content (%)

10 15 20

13.5 13.9 15.1

(3.3) (9.3) (7.9) (5.9) (6.7) (4.7)

(10.2) (11.8) (12.6) (13.0) (14.9) (15.3)

286


Figure 9.2 shows the thermograms for the SPC and GA–SPC resins obtained using a thermogravimetric analyzer (TGA). It is clear from Fig. 9.2 that within the entire temperature range, the weight loss for the GA–SPC resin is lower compared to SPC. Also, the GA–SPC resin starts final degradation around 270∞C compared to 235∞C for the SPC resin. Both of these observations indicate improved thermal stability for the GA–SPC resin compared to SPC. Dynamic mechanical analysis and differential scanning calorimetric (DSC) studies have further confirmed that the GA–SPC was more stable compared to SPC resin with higher glass transition temperature (Tg) than SPC [104]. Properties of composites made using SPC and GA–SPC resins and flax fabrics are discussed later in Section 9.4. 120

110

100

Weight (%)

90

80

70

60

50

SPC with 10% glycerin MSPC with 40% GA and 10% glycerin

40 0

100

200 Temperature (Celsius)

300

400

9.2 TGA thermograms of SPC and MSPC resin containing 40% GA and 10% glycerin.

Stearic acid modification The use of glycerin as an external plasticizer in soy protein has some disadvantages in addition to increased moisture absorption and a lowering of mechanical properties. As a small molecule, it can leach out easily over time making the resin brittle again. Since the glycerin containing soy protein absorbs more moisture, the leaching of glycerin is further facilitated. This


287

has prompted the use of internal plasticizers that will covalently bond to the polypeptide chains and thus will not leach out during use. Also, the use of a higher molecular weight plasticizer, compared to glycerin, could reduce the leaching rate. One such modification of SPI using stearic acid (n-octadecanoic acid) (SA) was carried out successfully by Lodha and Netravali [20, 105, 106]. The carboxylic group in SA can react with amine, imine and/or hydroxyl groups on the soy protein chain depending on the pH conditions to form amide and ester groups. Once reacted, SA acts as an internal plasticizer that is covalently bonded to the polypeptide molecule and cannot leach out. Its 18-carbon long non-polar hydrocarbon chain can also be helpful in reducing the moisture absorption and resulting in higher mechanical properties. The SA modifications were carried out under alkaline (pH 10) as well as near neutral pH conditions. Better improvements in mechanical properties were obtained with the modifications at near neutral pH conditions. Lodha and Netravali [20, 105, 106] studied the effects of both stearic acid and glycerol on the mechanical, thermal and moisture absorption properties of the resin. Best mechanical properties were obtained with 20% stearic acid. At that concentration no glycerin was needed as a plasticizer and the resin processing and handling was as easy as SPC or GA–SPC resins. This stearic acid modified SPI (SA–SPI) resin was then used to fabricate flax yarn reinforced composites. The pre-curing and curing process was similar to the one described earlier for the SPC resin except that the pre-curing was done at 90∞C and the final curing process was done at 110–120∞C and 11 MPa pressure. The effect of stearic acid content on tensile properties and moisture absorption of the SPI resin containing 30% glycerin is presented in Table 9.7. It is clear that with an increase in SA content up to 50% (on SPI wt basis) both fracture stress and modulus increased significantly whereas the fracture strain decreased. Further increase in SA content did not continue to increase fracture stress. It is also clear from the data in Table 9.7 that the moisture content decreased with an increase in SA, as was expected. The decreased moisture absorption was perhaps the main reason for improved mechanical properties. Some of the hydrophobic SA was observed to phase Table 9.7 Effect of stearic acid content on the tensile properties and moisture absorption of SPI resin containing 30% glycerol Stearic acid (%)



Fracture strain (%)


0 20 30 50 75

6.1a 6.1a 6.8b 6.6b 6.2a

124.7a 181.3b 212.2c 278.2d 307.2e

154.1a 64.5b 31.4c 10.6d 3.4e

16.3a 14.0b 13.9b 12.6c 12.8c

Means within a column with the same superscript did not show a statistically significant difference at a = 0.05.

288


separate from SPI, particularly at higher concentrations, and form crystals of small dimensions. This was confirmed by both X-ray diffraction (XRD) and DSC. The SA-SPI resin, containing well dispersed SA crystals, behaved somewhat like a nanocomposite, increasing the tortuosity of the polypeptide molecules and restricting their mobility when loaded, thus increasing its modulus. They concluded that the carboxyl group on the SA also reacted with some of the amine, imine and hydroxyl groups on polypeptide chains forming amide, and ester groups. Once grafted, the SA acts as an internal plasticizer, eliminating the need for the external plasticizer, glycerin. In addition, as the SA reacted along the polypeptide chain, it increased the molecular weight and increased the viscosity and the Tg of the resin. All these mechanisms together were responsible in making the SA–SPI resin stronger and stiffer. Lodha and Netravali [20, 105] also studied the effect of glycerin on the moisture content. Again, a similar positive relationship was found. As the glycerin content of the SA–SPI resin (with 20% SA) was lowered from 30% to 0%, the Young’s modulus and the fracture stress increased from 181 MPa to 1096 MPa and from 6.1 MPa to over 20 MPa, respectively. At the same time the fracture strain decreased from 64.5% to 2.8%. The SA–SPI resin containing 20% stearic acid and 0% glycerin was used for fabricating flax yarn reinforced unidirectional composites. The TGA and DSC studies of the SA–SPI resin further confirmed that the SA–SPI resin was thermally more stable than the SPI resin. The SEM photomicrographs of the SA–SPI resin, presented in Fig. 9.3, showed a significantly rougher and layered surface compared to SPI resin. A similar layered surface was also observed by Lodha and Netravali [105] for SA–SPI resin prepared under alkaline conditions. The formation of layered structure was also observed in the case of stearic acid modified zein protein by Lai et al. [107]. Phytagel® modification of SPI In another modification, Lodha and Netravali [106, 108] blended Phytagel®, a polycarboxylic compound, to improve the mechanical properties of SPI resin by forming a cross-linked complex structure. Phytagel® is produced by bacterial fermentation and is composed of glucuronic acid, rhamnose and glucose. It is commonly used as a gelling agent for electrophoresis to determine the molecular weights of DNA molecules as well as in detection of microbial contamination (MSDS by Sigma-Aldrich Co.). Phytagel® is known to form a strong gel via ionic cross-links at its glucuronic acid sites, using divalent cations naturally present in most plant tissue culture media. The carboxyl group in glucuronic acid is the main reactive group in Phytagel® which can react with amine and hydroxyl groups in SPI to form amide and ester groups,


289

10mm (a) SPI resin

10mm

(b) SAM–SPI resin

9.3 SEM photomicrographs of the fracture surfaces of (a) SPI and (b) SA-SPI resins.

respectively. The hydroxyl groups on rhamnose and glucose molecules may also react with carboxyl groups present on aspartic and glutamic acids in SPI, under suitable conditions, to form ester bonds. The carboxyl and hydroxyl groups can also interact with SPI via hydrogen bonds. All these possible reactions and interactions formed a complex, cross-linked and hydrogen bonded structure that was much stiffer than the SPI alone. Lodha and Netravali [106, 108] did the sol–gel analysis of these resins and found out that the

290


Phytagel® modified resins had a significantly higher gel fraction confirming the possibility of cross–linking between Phytagel® and SPI or formation of interpenetrating network (IPN) type structures. The resin pre-curing and curing processes for the Phytagel® modified SPI (PH–SPI) resin were essentially similar as used for SPI and required only minor modifications. To study the effect of Phytagel® content, Lodha and Netravali [106, 108] varied the amount of Phytagel® between 5 to 50% (by wt of SPI) prior to pre-curing. Water (15 times wt of SPI) and glycerin, in varying amount, were added to the mixture and pre-curing was done at 70∞C for 30 minutes. The final curing included 120∞C for 5 minutes at 2.8 MPa and 120∞C for 25 minutes at 11 MPa pressure. Table 9.8 presents the effect of Phytagel® content on the mechanical properties and moisture absorption of the PH–SPI resin. The tensile properties and moisture absorption of two formulations containing 20% Phytagel® (PH2–SPI) and 40% Phytagel® (PH4– SPI), both with 12.5% glycerin, are compared with those of SPI resin in Table 9.9. These resins were later used to fabricate flax yarn reinforced composites. Dynamic mechanical analyzer was used to characterize tan d, loss and storage moduli of the three resins. The data indicated a Tg of 115∞C for SPI whereas the Tgs for PH2–SPI and PH4–SPI resins were 171∞C and 177∞C, respectively, indicating much higher thermal stability. It should be noted that the pure Phytagel® material processed similarly did not show any Table 9.8 Effect of Phytagel® on the tensile properties and moisture absorption of SPI resin, containing 30% glycerol Phytagel®



Fracture strain (%)


0 5 7 10 20 30 40 50

6.0 10.5 12.7 14.9 22.4 29.7 31.8 28.9

98.7 123.6 136.3 146.3 225.8 277.0 388.7 337.2

206.4 53.4 51.7 42.4 35.5 33.9 20.4 20.0

19.2 19.0 18.5 18.2 17.2 16.2 17.2 17.2

Table 9.9 Tensile properties and moisture absorption of SPI, PH2–SPI and PH4–SPI resins Resin



Fracture strain (%)


SPI PH2–SPI PH4–SPI

6.0 42.6 60.0

98.7 657.6 896.5

206.4 28.9 19.5

19.15 12.7 12.4


291

glass transition of its own within the temperature range studied. Thermal degradation temperatures from TGA measurements also showed significant increases for PH2–SPI (247∞C), PH4–SPI (249∞C) compared to 208∞C for SPI resin. This again indicated that the Phytagel® modified SPI resins have higher cross-linking and hydrogen bonding compared to SPI resin which results in better thermal stability. The SEM photomicrographs of the fractured surfaces presented in Fig. 9.4 also showed a rougher and layered surface similar to those seen earlier for the SA–SPI resin.

10mm

(a)

10mm (b)

9.4 SEM micrographs of fractured surfaces of (a) PH2-SPI and (b) PH4-SPI resins.

292


Soy protein nanocomposite Blending nanoparticles with various types of polymers, particularly to form nanocomposites, have been shown to offer several advantages including higher mechanical properties [109]. As mentioned earlier, the nanoparticles increase the tortuosity of the polymer molecules and restrict their mobility under loading, thus increasing its modulus. This effect is significant when the particles are uniformly dispersed in the polymer [109]. Inclusion of nanoparticles, e.g. clay, have also been shown to improve the gas and liquid barrier properties of the polymers [109]. Simultaneous increases in glass transition temperatures (Tg) can also be seen due to the organosilicate– polymer interactions that restrict molecular motion. Thermo-mechanical properties such as yield strength, tensile modulus and heat distortion temperature (HDT) also show significant improvements with the introduction of nanoparticles [110]. However, toughness, elongation at break and impact strength, in general, may be lowered. Nanocomposites experiencing thermal degradation also show significant delay in weight loss indicating enhanced thermal stability. This arises due to the barrier effect of the silicates that prevents the escape of the volatile thermo-oxidation products and simultaneously reduces the rate of oxygen diffusion into the nanocomposite. Reduction in flammability of nanocomposites has also been reported in many cases [110]. The general nanocomposite flame retardant mechanism involves the build-up of a layer of carbonaceous silicate char on the surface during burning. This layer insulates the underlying material and slows the rate of mass loss of the byproducts formed during thermal degradation. Huang and Netravali [96] formed nanocomposites using SPC and GASPC resins by dispersing exfoliated Cloisite® Na+ clay (Southern Clay Products, Inc., TX) nanoparticles. Clay nanoparticles were first dispersed into distilled water using magnetic stirring and ultrasonication. Dispersed clay particle solution was then introduced into SPC and GA–SPC resins during the precuring process. This process has been described earlier for various resin modifications. The nanoclay particle dispersion in SPC and GA–SPC resins was evaluated using both XRD and transmission electron microscopy (TEM) using thin microtomed sections of the resins. Figure 9.5 shows the X-ray diffraction patterns of Cloisite™ NA+ clay powder (a) and SPC resin with different clay loadings (b). For all specimens the disappearance of the peak at 9∞ indicates that the particles were exfoliated and almost fully dispersed into SPC. Both XRD and TEM techniques indicated good dispersion, although some sections indicated agglomeration of nanoclay particle, particularly at higher clay loadings. Effects of nanoparticle loading on the mechanical properties and moisture absorption of the SPC resin are presented in Table 9.10. The SPC resin in this case contained 30% glycerin. From the data, it is clear that the nanoclay loading has a significant effect on the fracture stress and modulus of the SPC resin. For 30% nanoclay loading the modulus of the


293

4000 3500 3000 2500 2000 1500 1000 500 0

0

5

10

15

20

8000 Clay Clay Clay Clay Clay

7000 6000 5000

0% 5% 10% 20% 30%

4000 3000 2000 1000 0 0

10

20

30

40

9.5 XRD of Cloisite® Na+ clay powder (a) and XRD of SPC composites with different clay loadings (b) [Y-axis = % intensity, X-axis = goniometer].

Table 9.10 Effect of clay nanoparticle loading on the tensile properties and moisture absorption of the SPC resin Clay (%)

Stress at max. load (MPa)

Strain at max. load (%)

Modulus (MPa)


0 0.5 1 3 5 7 10 15 20 30

8.0 8.0 8.6 10.0 12.3 12.5 14.2 16.7 17.2 20.2

28.6 30.3 30.0 27.2 23.5 20.3 16.4 11.0 9.4 6.0

84.3 84.8 91.8 131.6 179.5 238.9 327.9 589.1 725.4 1023.9

22.0 21.4 20.6 20.4 20.0 19.6 19.5 18.0 18.0 17.1

294


resin was over 1 GPa compared to 84 MPa for resin without clay, a 1100% increase, and the fracture stress increased to 20 MPa from 8 MPa, a 150% increase. At the same time the resin became brittle as the fracture strain decreased from over 28% for resin without any nanoclay to 6% at 30% loading. Similar changes in mechanical properties have been observed for various polymer nanocomposite systems as well [109]. Of particular importance was the reduction in moisture absorption from 22% to 17.1%. This suggested that the nanoclay particles were also effective in blocking moisture from entering into the resin. The increases in the modulus and fracture stress are also a result of lower moisture absorption by the resin. The effect of GA content on the GA–SPC nanocomposite resin mechanical properties and moisture absorption are presented in Table 9.11. The SPC nanocomposite resin in this case contained 15% glycerin and 5% nanoclay. It is clear from these data that as the GA content increased from 0% to 10% the modulus increased from 778 MPa to over 1 GPa. However, any increase in GA content above 10% resulted in a drop in modulus. This is because of the unreacted GA acting as a plasticizer. Similar trends were also obtained for GA–SPC resins discussed earlier. The effect of clay loading on the thermal properties of the clay/SPC nanocomposites, containing 15% glycerin, is shown in Fig. 9.6. With an increase in nanoclay loading, the GA modified resin became increasingly stable at higher temperatures and their degradation temperatures increased significantly and the weight loss decreased along the entire temperature range. Part of this is due to the lower moisture absorption of the nanocomposite resins. It is interesting to note that the moisture content for all resins remained in a very narrow range in spite of increased crosslinking with increased GA content. This suggests that the nanoclay particles may be more effective for the moisture control than cross-linking with GA. Table 9.11 Effect of glutaraldehyde content on the tensile properties and moisture absorption of GA–SPC resin GA (%)

Stress at max. load (MPa)

Strain at max. load (MPa)

Modulus (MPa)


0 1.3 3.8 5.0 7.5 10.0 14.4

24.9 27.4 26.2 28.5 29.5 28.9 25.7

11.6 9.9 8.9 9.6 8.7 7.8 8.3

778.0 901.3 973.4 969.3 993.1 1043.0 838.8

14.7 14.6 14.4 14.3 14.1 14.0 14.5

Several of these modified SPI and SPC resins were used to fabricate fiber, yarn and fabric reinforced green composites. The fabrication process and properties of these composites are discussed in the next section. Data for green composites using nanoclay were, however, not available at this time.


295

Weight (%)

90

70

Load Load Load Load Load

50

30% 15% 10% 5% 0%

30 20

120

220 Temperature (∞C)

320

420 Universal V2.6D TA

9.6 Effect of nanoclay loading on thermal stability by TGA.

9.4

Soy protein-based green composites

As stated earlier, most plant fibers are termed ‘staple’, i.e. short length fibers. Fibers derived from the stem of the plant, e.g. ramie and flax, and those derived from some plant leaves, e.g. sisal and henequen, could be longer than 1 meter. Although it is easier to fabricate random, short fiber composites, with some manipulation, unidirectional composites of small dimensions can also be made using these fibers. Lodha and Netravali [19] used chopped ramie fibers and SPI resin to make random fiber green composites. Nam and Netravali [23, 90] fabricated unidirectional composites using SPC resin with ramie fibers. Flax yarn and fabric reinforced SPC and modified SPC resin composites have also been made and have shown to have excellent properties [20, 22, 32, 105, 106]. Fabrication of these composites and their properties are briefly discussed in the next subsections.

9.4.1

Fiber-reinforced composites

Short fiber composites Lodha and Netravali [19] studied the effect of fiber length and content on the short ramie fibers/SPI resin composite properties. The average diameter of the fibers was about 50 mm. The average fiber fracture stress and modulus were measured to be 620 MPa and 48 GPa, respectively. A wide variability in mechanical properties was found for these fibers as is the case for all natural plant-based fibers. To prepare random, short fiber composites, ramie fibers were chopped to 5, 10 and 15 mm lengths. These fiber lengths were based on the interfacial shear strength (IFSS) measured using the microbead

296


test [24, 40, 41]. For the ramie fiber/SPI resin system the IFSS was found to be in the range of 30 MPa. Based on this IFSS value the critical length of the fiber was calculated to be just over 2.5 mm. Based on theory of reinforcement, below this critical length, there should be no effect of fiber inclusion on the mechanical properties of the resin. Above this length, however, the fibers are expected to reinforce the resin and thus improve the strength and modulus. However, this theory applies mostly to fibers that are laid in the direction of the stress. To prepare short fiber composites, the SPI powder was mixed with 30% glycerin and 300% water (by wt of SPI) and stirred to make uniform dough. Predetermined amounts of chopped fibers were added to the dough in small increments and the mixture was stirred with compressed air to obtain uniform fiber distribution. However, they noted that the composites showed resin-rich and fiber-rich areas indicating uneven fiber distribution due to the high viscosity of the resin. Once all the fibers were added, the dough was made into small balls and subjected to hot pressing at 70∞C for 30 minutes, dried in air for 24 hr and cured by hot pressing at 110∞C for 2 hr under 5.5 to 7 MPa pressure to form composite sheets. These composites were then conditioned at ASTM conditions of 21∞C and 65% relative humidity (RH) prior to characterizing their properties. Table 9.12 presents ramie fiber/SPC short fiber composite fracture stress as a function of fiber content and length. Table 9.13 presents the modulus of Table 9.12 Fracture stress of ramie/SPC short fiber composites for various fiber lengths and contents Fiber content (% w/w) 0 10 20 30

Fracture stress (MPa) 5 mm

10 mm

15 mm

5.9 3.2 9.8 12.5

5.9 9.3 17.1 24.5

5.9 15.7 25.5 33.4*

*A large amount of delamination was observed for these specimens

Table 9.13 Young’s modulus of ramie/SPC short fiber composites for various fiber lengths and contents Fiber content (% w/w) 0 10 20 30

Young’s modulus (MPa) 5 mm

10 mm

15 mm

18.0 68.0 238.4 431.0

18.0 235.6 613.1 1173.1

18.0 521.2 1082.3 1654.3*

*A large amount of delamination was observed for these specimens


297

the composites as a function of fiber content and length. Typical loaddisplacement curves for various composites with 5 mm long ramie fibers and various fiber contents are shown in Fig. 9.7. These data confirm that both fracture strength and modulus are a strong function of the fiber content and fiber length and increase with both fiber length and content, as would be expected. However, at small length, 5 mm in this case, and at low fiber content, the fibers seemed to act as defects. As a result, instead of contributing to the strength, the fibers reduced the strength of the composites to lower than that of the resin. As mentioned earlier the critical length of 2.5 mm applies when the fibers are in the direction of the stress. Any fibers at an angle to this direction cannot contribute fully. Theoretical values of modulus were higher than the experimental values for all specimens. The differences between the theoretical and experimental values were much higher at smaller fiber lengths and volume content and narrowed down as the fiber length and volume increased. Several factors including defects such as the uneven distribution of the resin and fibers, lack of perfect randomness, voids, shrinkage during processing etc. were responsible for this behavior [19]. 12 0% Fiber 10% Fiber 20% Fiber 30% Fiber

Load (kg)

10 8 6 4 2 0 0

5

10

15 20 25 Displacement (mm)

30

35

40

9.7 Typical load-displacement plots of tensile test of various composite specimens with 5 mm long ramie fibers and various fiber contents.

Unidirectional composites Unidirectional composites using ramie fibers and SPC resin were prepared by Nam and Netravali [23, 90]. Composites with unidirectional lay up of fibers was possible because of the small dimensions of the specimens prepared. The fibers used in this case were between 600 and 1700 mm long. The average diameter of the ramie fibers was measured to be 123 mm. The average tensile stress and modulus were found to be 627 MPa and 31.8 GPa, respectively.

298


The fabrication of unidirectional composites was accomplished in two steps. During the first step, fibers, in a parallel bundle form, were soaked in the pre-cured SPC resin (30% glycerin) and the excess resin was squeezed out. This process was repeated a few times to ensure the full penetration of the resin in between fibers. The wet fibers were aligned on Teflon® coated glass plates layer by layer, in a parallel array, to make a 100 mm ¥ 100 mm sheet and dried for 48 hr at room temperature to form pre-impregnated sheet (prepreg). In the second step the prepreg was placed between two stainless steel plates and hot pressed at 120∞C for 2 hr at a pressure of 5 MPa. The cured ramie fiber/SPC ‘green’ composite was then allowed to cool and thereafter conditioned at ASTM conditions of 21∞C and 65% RH for 2 days prior to characterizing the mechanical properties. The total fiber content of these green composites, calculated from the fiber weight and the final composite weight, was 65%. Because of the higher density of the fibers compared to the resin, the volume content was slightly less. Table 9.14 compares the tensile properties of the ramie fiber/SPC unidirectional composites, in both longitudinal and transverse directions, with those of the SPC resin. Properties of SPC polymer are also included for comparison. Being unidirectional, the composites have significantly higher tensile modulus (4.9 GPa) and fracture stress (271 MPa) in the longitudinal direction compared to the modulus of 0.9 GPa and fracture strength of 7.4 MPa in the transverse direction. In unidirectional composites, the tensile properties in the longitudinal direction are controlled by the fiber properties whereas the transverse direction properties are controlled by the resin and/or the fiber/resin interface properties. In this case, the fracture strength in the transverse direction was controlled by the SPC resin strength. However, slightly higher values for both fracture stress and modulus in the transverse direction were because of not having a perfect alignment of the fibers as a result of being hand laid and having no control over them while hot pressing. In addition, ramie fibers, like any other plant-based fibers are fibrillar and fibrillate during processing. The protruding fibrils generally tend to go tangentially to the fiber axis, improving the interface strength. Any fiber Table 9.14 Tensile properties of ramie fiber/SPC unidirectional composites in longitudinal and transverse directions compared with SPC resin Material

Test direction

Tensile stress (MPa)


Composite*

Longitudinal Transverse

271.4 7.4

4.9 0.9

9.2 5.3

6.9

0.1

30.2

SPC resin *65% fiber volume fraction

Fracture strain (%)


299

misalignment reduces strength and modulus in the longitudinal direction and increases them in the transverse direction. Fiber fibrillation, a common property of plant-based fibers, is also believed to be partially contributing to this. The theoretical calculations of the tensile fracture stress and modulus in the longitudinal direction were calculated using the simple rule of mixtures as shown in equations 9.1 and 9.2 [111]

sc = s f V f + sm V m

9.1

Ec = Ef Vf + E mVm

9.2

where sc, sf and sm represent the tensile stress values for composite, fiber and matrix, respectively. Ec, Ef and Em represent the Young’s modulus values of the composite, fiber and matrix, respectively, and Vf and Vm are volume fractions of fiber and matrix, respectively. The calculated values of tensile stress and modulus of composite were 407 MPa and 24.3 GPa, respectively. The experimental values of 271 MPa and 4.9 GPa for fracture stress and modulus are significantly smaller than the theoretical values. This discrepancy was attributed to several reasons including resin shrinkage during curing, fibers no having perfect alignment, presence of voids and thermal degradation of the ramie fibers. As mentioned earlier, a significant amount of water is used in processing of the composites. As the water is dried during the curing process, the resin shrinks significantly. As a result, the fibers undergo longitudinal compression and lose their alignment resulting in lower modulus. In addition, the hand lay-up process is not so accurate and results in misalignment of the fibers as well. The voids, especially around the fibers, also contribute to the lower composite properties. The voids are commonly generated because of the water present in the resin as well as the ramie fibers which evaporates during curing at 120∞C. Garcia-Zetina et al. [112] used a correction factor to account for the void content to predict the strength of short fiber composites. However, in the case of ramie fibers/SPC composites it was difficult to estimate the void content and hence the correction factor could not be estimated. Although lower than the predicted, the ramie fiber/SPC green composite strength of 271 MPa is close to the strength of soft steel. The density of steel is about 7.75 g cm–3 which is over 5.5 times higher than the 1.35 g cm–3 estimated for the green composites. As a result, on strength per weight basis, the ramie fiber/SPC green composites are superior to steel by 5.5 times. Nam and Netravali [23, 90] also compared the mechanical properties of ramie fiber/SPC green composites to three varieties of wood [40, 41]. The fracture strength of the green composites in the longitudinal direction was almost twice and the modulus was between 5 to 10 times that of commonly used bass, cherry and walnut wood varieties. The properties in the transverse direction were comparable. The flexural strength and modulus of these unidirectional composites, in the longitudinal direction, were about 230 MPa

300


and 12.5 GPa, respectively, which are significantly better than the wood varieties. With their excellent mechanical properties, these composites may be used in many indoor structural applications as well. As mentioned earlier, trees take 20–25 years for their full growth before they can be harvested and used as wood. On the other hand, ramie and other fibers such as flax and hemp, etc., as well as soybeans, are yearly renewable. Yarn-reinforced composites Lodha and Netravali [20, 21, 106] fabricated unidirectional composites using flax yarns and SPI, PH2–SPI (containing 20% Phytagel®) and PH4–SPI (containing 40% Phytagel®) resins. As mentioned earlier, plant-based fibers are not continuous. However, they can be spun into continuous yarns for making unidirectional composites. To fabricate composites, flax yarns were aligned parallel by manually winding them on a metal frame and wetting by immersing the frame in the pre-cured resin solution. Winding gave two layers of parallel aligned layers of yarns to obtain the desired thickness of the composite specimens. Additional amount of pre-cured resin was also poured in between the layers to ensure good resin impregnation if needed. The resin impregnated flax yarn sheets were oven dried at 35∞C for 24 hr. The dried sheets were hot pressed at 120∞C for 5 minutes at a pressure of 2.8 MPa and for additional 25 minutes at a pressure of 11 MPa to form composites. All composites had 45% yarn content by wt. Tensile and flexural properties of the composites were measured after conditioning them for 72 hr. Table 9.15 summarizes the tensile properties of flax yarn reinforced SPI, PH2–SPI and PH4–SPI composites in the longitudinal direction. Both PH2– SPI and PH4–SPI resin composites showed higher modulus and lower fracture strain compared to composites prepared with SPI resin. However, within the Phytagel® modified resins, both fracture strength and modulus were higher for PH2–SPI composites when compared to PH4–SPI composites. It was reported that the yarn pull out lengths were higher in the cases of SPI and PH4–SPI resins. While the theoretical predictions for modulus values, based on the simple rule of mixture, were significantly higher than the experimentally obtained values, the fracture stress values were very close. As explained in Table 9.15 Tensile properties of flax yarn reinforced SPI, PH2–SPI and PH4–SPI composites in the longitudinal direction

Flax yarn/SPI composites Flax yarn/PH2–SPI composites Flax yarn/PH4–SPI composites



Fracture strain (%)

197.2 220.2 174.0

2.41 4.11 3.10

11.2 7.5 8.8


301

the case of ramie fiber/SPC composites, these resins were also prepared using significant amounts of water which is driven out during the drying and curing processes. As the resin dries, it shrinks pulling with it the yarns. The longitudinal shrinkage of the yarns significantly affects the modulus but not as much the fracture stress if the yarn does not kink. No kinking was observed in these composites. In the case of PH4–SPI composites, it was observed that the addition of 40% Phytagel® raised the resin viscosity significantly as it gelled quickly. The higher viscosity of the resin resulted in poor penetration and longer pull-out lengths and, consequently, lower modulus and strength values compared to PH2–SPI composites. Poor penetration of the resin was also a result of the high twist yarns used in the study. It was observed that the high twist in the yarn packs the fibers together leaving no space in between for the resin to penetrate. As a result, though bonded, the resin remained only on the yarn surface. Highly twisted yarns also increase obliquity factor (a function of the angle made by the fibers with the yarn axis) and making the yarn brittle and reducing the yarn strength, thus, resulting in lower composite strength. Lower twist yarns should perform much better in such cases. The flexural properties in terms of flexural stress, strain and chord modulus (between 0.25% and 0.75% of the yield point) of the flax yarn reinforced composites with SPI, PH2–SPI and PH4–SPI composites are presented in Fig. 9.8. As in the case of tensile properties, flax yarn reinforced PH2-SPI composites showed significantly higher chord modulus and flexural stress, 7.8 GPa and 105 MPa, respectively, than the SPI (2.8 GPa and 48.9 MPa) 120

Flexural stress, MPa

100

80

9

Flax yarn/PH2–SPI composites

8

Flax yarn/PH4–SPI composites

7 6

60

5 4

40

3 2

20

1

Flexural strain, %, Chord modulus, GPa

10 Flax yarn/SPI composites

0

0 Flexural stress

Flexural strain

Chord modulus

9.8 Flexural properties of flax yarn/SPI, flax yarn/PH2–SPI and flax yarn/PH4–SPI composites tested in lengthwise direction.

302


and PH4–SPI (4.5 GPa and 52.3 MPa) composites. The flexural strains for all three composites were within a narrow range. These results were consistent with the expectations. In another study Chabba et al. [113] modified soy flour (SF) with GA (GA–SF) and reinforced it with flax yarns. During the impregnation and fabrication of composites, the flax yarns were held under high stress to achieve good orientation and to counter the effects of resin shrinkage. The cured composites had 60% yarn content (by wt). These composites exhibited fracture stress and Young’s modulus of 260 MPa and 3.71 GPa, respectively. The flexural strength was about 174 MPa in the longitudinal direction. As in the case of ramie fibers/SPC composites, these flax yarn reinforced composites have excellent mechanical properties comparable to steel. These green composites could also be used for indoor structural applications. Fabric-reinforced composites Chabba and Netravali [22, 32, 104] fabricated two dimensional composite sheets using flax fabrics and GA–SPC resin. They prepared composites with either all layers oriented at 0∞ (warp direction, longitudinal) or 90∞ (weft direction, transverse). Strips of flax fabrics, 2.5 cm wide and 13 cm long, were cut in the desired directions (warp and weft). Four strips were layered to fabricate each composite specimen and the weight of the fabric strips was recorded. Fabric strips were held under tension in a glass container. Precured GA–SPC resin was poured over the strips and allowed to stand for 15 minutes at room temperature. The fabric strips were transferred to Teflon® coated glass plates. Further resin was added between the layers to assure good penetration and the specimens were allowed to dry in an oven at 35∞C for about 24 hr. The dried specimens were cured in a mold by hot pressing at 125∞C for 25 minutes at a pressure of 8 MPa. The fabric weight content was calculated to be 45% on the basis of final composite weight and initial weight of the fabric strips. The cured specimens were conditioned as per ASTM, prior to characterizing their properties. Table 9.16 summarizes the tensile properties of fabric reinforced GA– SPC composites as well as the flax fabric, in both longitudinal and transverse directions. As can be seen from the data in Table 9.16, both fracture stress and modulus of the composites were higher in the transverse directions, merely reflecting fabric properties in the two directions. However, it is clear that the composite fracture stress and modulus values are significantly higher than the fabric values, as expected. It was noticed that during the tensile testing of the composites, the resin at the surface begins to crack at different locations as the composite is strained. This was because of the higher crimp in the fabric that allowed it to be strained while the cross-linked GA–SPC resin was comparatively more brittle. As the resin cracked the load was


303

Table 9.16 Tensile properties of flax fabric reinforced GA–SPC composites and flax fabric Material

Test direction



Fracture strain (%)

Flax fabric

Longitudinal (warp)

50.3

1.01

21.0

Reinforced GA–SPC composite

Transverse (weft)

55.7

1.26

7.8

Flax fabric

Longitudinal (warp) Transverse (weft)

33.3 41.2

512.9 1017.9

17.7 7.1

transferred to the fabric layers and at some strain the composite fractured at one of these cracks. The fracture sequence in the longitudinal direction also showed similar resin cracking behavior, but at higher strains. The photographs also show some resin adhering to the yarns in the fabric indicating good adhesion. However, being brittle, most of the resin that cracked, seemed to separate from the fabric. Another reason for the resin separation from yarns was the highly twisted yarn used in the fabric which, as discussed earlier, limited the resin penetration. Theoretical analysis of flax fabric reinforced GA–SPC composites in both directions was carried out using pcGINA© (PC based Graphical Integrated Numerical Analysis) software. This software was created by Dr Y. Gowayed and his group at Auburn University, Auburn, USA, for Pratt & Whitney, NASA Lewis and GE and was obtained from Dr P.L.N. Murthy, NASA Glen [104, 114–116]. Table 9.17 shows the comparison of the experimental and theoretical values of fracture stress and modulus of the flax fabric reinforced GA–SPC composites. The fracture stress values predicted by pcGINA in both longitudinal and transverse directions were 48.4 MPa. The modulus values predicted in the longitudinal and transverse directions were 1.09 GPa and 1.11 GPa, respectively. It is clear that the theoretical and experimental Table 9.17 Comparison of experimental tensile properties of flax fabric reinforced GA–SPC composite with the theoretical predictions (by pcGINA©) Source



Experimental (L)* pcGINA© (L) Experimental (T)* pcGINA© (T)

50.3 48.35 55.7 48.35

1.01 1.09 1.26 1.11

L: Longitudinal, T: Transverse

304


values are not significantly different. This indicates that green composites using fabric- and soy-based resins could be designed to have required properties depending on the applications.

9.5

Conclusions and future trends

This chapter presents some of the recent research on environmentally friendly, fully biodegradable green composites made using various soy protein (SPI, SPC, SF) resins and their modified versions reinforced with random short ramie fibers, unidirectional ramie fibers and flax yarns as well as flax fabrics. The properties obtained, both tensile and flexural, are sufficient for many applications in packaging and as casings and other applications in consumer goods as well as in the automotive and housing industry. Some unidirectional composites using ramie fibers and flax yarns have sufficient strength for use as primary structural components in some applications. All the fibers and resins used in these composites are plant based and yearly renewable. At the end of their life these composites can be disposed of safely and easily or composted without harming the environment. Some of the modified SPC and SPI resins and the soy protein nanocomposite may be used as a replacement for petroleum-based non-degradable plastics. The green nanocomposite can be further reinforced with fibers, yarns or fabrics for use in many of the applications mentioned above. The SPC nanocomposites may be used in applications where higher thermal stability is desired. Most of the current research in green composites is based on plant-based fibers because of their ready availability. There are several other resins and fibers, not based on plants, which can be used to make useful green composites. Synthetic biodegradable fibers and resins are also being developed or existing ones modified for use in green composites with improved properties. Biotechnology will also play a key role in this development [3, 54]. Thus research is already in progress to develop ‘Advanced Green Composites’ with superior mechanical properties, thermal stability and better moisture resistance using fully sustainable materials. As the petroleum becomes more expensive and scarcer in the not so distant future, there will be no alternative but to accelerate these efforts.

9.6

Acknowledgements

Most of the research presented here was supported by the National Textile Center (NTC) and the College of Human Ecology at Cornell University. The help of my students Dr Preeti Lodha, Sunghyun Nam, Shitij Chabba, Xiaosong Huang and Yuzo Yamamoto in preparing this chapter is greatly appreciated. Without their hard work writing this chapter would not have been possible. Help from Professor Dotsevi Sogah, Dr Xiaoping Cheng, Cornell University,


305

Professor Yasser Govayed, Auburn University, and Dr P.L.N. Murthy, NASA Glen, in various aspects of this research is also acknowledged.

9.7

References

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10 Biodegradable nonwovens G B H A T, University of Tennessee, USA and H R O N G, Johnson Controls, Inc., USA

10.1

Introduction

Nonwoven fabrics are flat, porous sheets or web structures that are made directly from separate fibers or from molten plastics or from plastic films by entangling fibers or filaments mechanically, thermally or chemically. Nonwovens can be produced from both natural and synthetic fibers or directly from polymers by a variety of techniques that involve web formation and bonding. Different polymers/fibers are more suited for one process than the other. All of the different techniques available for web formation and bonding are discussed in sufficient detail. Nonwovens are the fastest growing sectors of textile materials, and they continue to grow all over the world. A significantly large share of these are used as single use, or short-life products, leading to disposability related problems; biodegradable or compostable nonwovens are the answer to the sustainability issues, especially in the long run. Studies done on processing, structure and properties of the nonwovens produced by different techniques from a variety of biodegradable polymers and fibers are discussed. Although the techniques used are similar to the ones used for other commonly used polymers/fibers such as polypropylene, polyester and cellulosics, some specific issues need to be addressed with newer polymer/fiber candidates. Since most of the biodegradable polymers and fibers are discussed in detail in other chapters, only a brief overview of the materials used in biodegradable nonwovens is provided, with detailed discussions about the processing of these materials into nonwoven webs. Structure and properties of such nonwovens from different biodegradable materials, especially with reference to their processing and performance are detailed. Included in the discussion are the fast growing applications of these nonwovens. As these fabrics are becoming more and more important, there is a lot of information and update one can obtain through important books, websites and professional organizations listed in this chapter. 310


10.2

311

Nonwoven fabrics

Unlike traditional textiles, nonwoven fabrics are not manufactured by the traditional processes of weaving or knitting, and converting of fibers to yarns is not required. Both natural and synthetic fibers, organic and inorganic, can be used for nonwoven fabrics. The fibers in these structures may be staple or continuous, or be formed in situ, and may be directionally or randomly oriented, depending on the nature of their production process. According to INDA [1], the association of the nonwovens fabrics industry: ‘Nonwovens are a sheet, web, or batt of natural and/or man-made fibers or filaments, excluding paper, that have not been converted into yarns, and that are bonded to each other by any of several means.’ European nonwovens association EDANA defines: ‘Nonwoven as a manufactured sheet, web or batt of directionally or randomly oriented fibers, bonded by friction, and/or cohesion and/or adhesion, excluding paper or products which are woven, knitted, tufted stitch bonded incorporating binding yarns or filaments, or felted by wet milling, whether or not additionally needled. The fibers may be of natural or man-made origin. They may be staple or continuous or be formed in situ.’ Nonwoven fabrics demonstrate specific characteristics such as strength, stretch, resilience, absorbency, liquid repellency, softness, flame-retardancy, cushioning, washability, filtering, bacterial barrier and sterility. Nonwoven fabrics can be used in a wide variety of applications, which may be limited life, single-use fabrics as disposable materials or durable fabrics for automotive and civil engineering applications [2, 3]. Demand for nonwoven materials in the US is expected to increase by 3.9% per year to nearly $5 billion in 2007. Nonwoven growth trend in the world and the growth trend for North American nonwovens market are shown in Figs 10.1 and 10.2. This increasing market share will be driven by the strong growth in many key disposable markets such as adult incontinence products, filters, and protective apparel, and key non-disposable markets such as geotextiles and battery separators. Disposable markets are still the majority of nonwoven demand in 2002, which account for a 64% share (Fig. 10.3) [4]. Disposable consumer products, which primarily include baby diapers, adult incontinence and feminine hygiene products, and wipes, were the largest market for nonwovens in 2002 [5]. Based on the data that is available, continued growth in nonwovens is more in the disposables area and the share of the short-life nonwovens is going to remain significantly large. Also, looking at the distribution of durables and disposables in terms of yardage or volume, the disposable share is four-fifths of the total nonwovens, making them much more visible in the waste stream. Considering the fact that large share of these important and growing materials are throw-away products, it is important that issues related to their disposal be carefully addressed.

312

Biodegradable and sustainable fibres ’000 of metric tonnes

1000

900

800

700

600

500 1905

1993

1995

1998

2000

2003

2005

2010

10.1 North American nonwovens market [4]. MM lbs/year 2.5

2.0

North America Western Europe Japan Rest of World

1.5

1.0

0.5

0.0 1993

1998

2003

10.2 Trend in world nonwovens production [4].

Nonwovens are used almost everywhere, in agriculture, construction, military, clothing, home furnishing, travel and leisure, health care, personal care and household applications. Of these many applications that continue to grow, more than two-thirds of them are disposables, mostly of single-use type.


313

1.004 million tonnes

22 billion square meters

Long life 34%

Short life 66%

Short life 82% Long life 18%

$4.1 billion

Hort life 64%

Long life 36%

Source: INDA estimates

10.3 North American nonwovens volume and sales [4].

The environmental impact of disposable products is becoming a major concern throughout the world in recent years [6–7]. These disposable products are usually produced from traditional thermoplastic resins, such as polypropylene (PP), polyethylene (PE), polyester (PET), polyamide (PA), polycarbonate (PC), which are not biodegradable. However, due to increasing environmental consciousness and demands of legislative authorities, the manufacture, use and removal of products made of such traditional polymers are considered more critically. The remedy to this problem could be found in the development of substitute products based on biodegradable, and ideally from natural and renewable materials. Natural fibers, such as cotton, kenaf, coir, jute, flax, sisal, hemp, and wood, etc., become the first choice due to their biodegradability. Some synthetic biodegradable fibers have also been used for nonwoven applications, including cellulose esters such as cellulose acetate, rayon, lyocell, etc., polyesters such as poly(lactic acid) (PLA), poly(caprolactone) (PCL), poly(hydroxybutyrate) (PHB), poly(hydroxybutyrate-co-valerate) (PHBV), Biomax, Biopol, polytetramethylene adipate-co-terephthalate (PTAT), etc., and water solubles such as poly(vinyl alcohol) (PVA), etc. Thus the target for biodegradable nonwovens is to replace synthetic fibers with biodegradable fibers in the disposable nonwovens. One group of disposable nonwovens is the wet laid pulp/polyester spunlaced fabrics mainly for industrial and professional wipe products. Another group of disposable nonwovens is

314


household and hygienic wipes, which are spunbonded or dry laid and then chemically or thermally bonded.

10.3

Fiber consumption in nonwovens

Fibers are the basic element of nonwovens; world consumption of fibers in nonwoven production is 63% polypropylene, 23% polyester, 8% viscose rayon, 2% acrylic, 1.5% polyamide and 3% other high performance fibers [8]. The data in Fig. 10.4 shows the market share of important polymers and fibers in the nonwovens market. Manufacturers of nonwoven products can make use of almost any kind of fibers. These include traditional textile fibers, as well as recently developed hi-tech fibers. Future advancements will be in bicomponent fibers, micro-fibers (split bicomponent fibers or meltblown nonwovens), nano-fibers, biodegradable fibers, super-absorbent fibers and high performance fibers. The selection of raw fibers, to a considerable degree, determines the properties of the final nonwoven products. The selection of fibers also depends on customer requirement, cost, processability, changes of properties because of web formation and consolidation. The fibers can be in the form of filament, staple fiber or even yarn. Nylon and other High density polyethylene

Polypropylene

Polyester

Other fibers Rayon

Polyester

Polypropylene

10.4 Consumption of polymers and fiber types in nonwovens [4].

Many different fiber types are used in the formation of nonwovens: ∑ Traditional textile fibers: – PET, polyolefin (PP/PE), nylon, cotton, rayon, wool, lyocell, modacrylic. ∑ Advanced fibers: – aramid (Nomex/Kevlar); – conductive nylon; – bicomponents (side-by-side, sheath-core, segmented pie, and islandsin-the-sea); – melamine (heat and flame resistant); – hollow fibers (polyetherketone, polyaniline); – Spandex fibers (polyether) – fusible co-PET fiber; – nylon 6 support/matrix fiber;


315

– glass micro-fiber; – chlorofiber; – antibacterial fiber; – stainless steel; – rubber thread; – poly(tetrafluoroethylene) (PTFE); ∑ Nanofibers: – carbon nanotubes; – electrospun polymeric nanofibers However, the major players continue to be polyolefins and polyesters, with rayons having a visible third place. As can be seen from the recent trend (Fig. 10.5), the rayon share is going down and share of polyolefins is continuing to increase. US producer’s shipments of staple to nonwovens (1989–1999) (million lb, % of total) 525 450 375 300 225 Rayon Polyester Olefin

150 75 0 1988

1990

1992

1994

1996

1998

2000

10.5 Trend in US shipment of nonwovens (4).

10.4

Web formation methods

The web formation in nonwoven production is a critical part of end-use product performance. Three basic methods used to form a web are: dry laid; wet laid; and polymer laid (spunlaid and melt blown). Webs, other than spunlaid, have little strength in their unbonded form. The web must therefore be consolidated in some way. There are three basic types of bonding: chemical; thermal; and mechanical. The nonwoven formation methods are summarized in Fig. 10.6.

10.4.1 Dry laid In the dry laid process, the conventional staple fibers are used, which are usually 12 to 200 mm or longer. The fibrous web is prepared using the classical textile carding machine or air laying machine to separate and orient

316

Biodegradable and sustainable fibres Dry laid

Wet laid

Random air laid Card laid Parallel Cross laid

Hydroentanglement

Polymer laid Spunbonded Mechanical Thermal

Mechanical bonding

Chemical bonding

Needle punching

Melt blown Laminated Sandwiched

Thermal bonding

Hydroentanglement

Worldwide production of nonwovens by process

10.6 Nonwoven web formation methods (from ref. 3).

the fiber mechanically. Carding is the most common process to produce nonwoven fabrics from staple fibers. The objective of carding is to separate the fiber stock into individual fibers with minimum fiber breakage. Thus, the carding process consists of opening and blending of different species of fibers thoroughly; carding is performed by the mechanical action in which the fibers are held by one surface while the other surface combs the fibers, causing the separation of individual fibers. In a normal carding process, the fibers are more oriented along machine direction than cross-direction. More random web structures can be obtained by cross-lapping, centrifugal dynamic random card system, or by using aerodynamic web formation (air lay) method [7]. The carded or air laid web usually has a basis weight ranging from 1 to 90 ounces per square yard. Typical end uses for dry laid nonwoven fabrics are the fabrics for carpet backing, interlinings for garments, apparel and upholstery backings, filter media, diaper coverstock, wipes, and personal hygiene products.

10.4.2 Wet laid Wet laid nonwovens are webs made by a modified papermaking process. First, the fibers are mixed with chemicals and suspended in water to make the slurry. Then, specialized paper machines are used to drain the water off the fibers to form a uniform sheet of material, which is then bonded and dried. Thus, three steps are needed for the wet laid process, the swelling and dispersion of the fiber in water, transporting the suspension onto a continuous traveling screen to form the continuous web, and drying and bonding of the web [9]. Short fibers, which are usually less than 10 mm, are needed for the wet laid process and the resulting fabric has a basis weight ranging from 10 to 540 g m–2. The wet laid process has advantages of high productivity, control of orientation of properties, and high uniformity at low basis weight


317

when compared with the air laid process. Typical applications for wet laid nonwovens include tea bags, wipes, surgical gowns and drapes, towels etc. [7].

10.4.3 Spunbonded Spunbonding is a one-step process, which involves polymer melting, filament extrusion, drawing, laydown and bonding of the web to impart strength, cohesiveness and integrity to it. A schematic of a spunbonding process is shown in Fig. 10.7 [9]. The spinning process is similar to the production of continuous filament yarns and similar extrusion conditions are used for a given polymer. Fibers are formed as the molten polymer exits the spinnerets and are quenched by cool air. Unlike in the typical fiber spinning process, there is no positive take-up and fibers are directly deposited on a moving collector to form a web. Before deposition on a moving belt or screen, the individual filaments must be attenuated to orient molecular chains within the fibers to increase fiber strength and decrease extensibility by rapidly stretching the plastic fibers immediately after exiting the spinneret either mechanically or pneumatically. Then the web is formed by the pneumatic deposition of the filament bundles onto the moving belt. The fibers have to be distributed on the belt using some type of randomization so that a fairly uniform random web is formed. The formed webs are bonded either by mechanical, chemical, or thermal method depending on the ultimate fabric applications [10]. Of the different options, thermal point bonding is the commercially popular method, 1 2 3 4 5 6 7 8 9 10 11 12

1 3 4 2 5 6

Feed hopper Extruder Gear pump Spinneret Cooling air Draw roll Air gun Porous belt Bonding oven Vacuum exhaust Fabric inspection Wind up

7

9

11

8

10

10.7 Schematic of a spunbonding process [10].

12

318


wherein the bond area is about 15%. At these bond points fiber surfaces are partially melted to form fusion between neighboring fibers, thereby imparting strength to the webs. Today spunbonded fabrics have been widely used throughout the automobile as backing for tufted automobile floor carpets, trim parts, trunkliners, interior door panel, and seat covers, etc. For the civil engineering applications, spunbond fabrics have been applied for erosion control, revetment protection, railroad beds stabilization, canal and reservoir lining protection, highway and airfield black top cracking prevention, roofing, etc. [11]. Spunbonded fabrics have also been widely used in sanitary, medical and packaging industries [12]. Spunbonding is one of the fastest growing processes as indicated in Fig. 10.8 [13]. 3000

Millions of tonnes

2500 2000

Spunbond Carded Air laid Wet laid

1500 1000 500 0 1991

1996

2001

2006

10.8 Worldwide nonwoven production by process [13].

10.4.4 Meltblown Melt blowing is one of the most popular processes to make super-fine fibers on the micron or sub-micron scale. In a melt blowing process a thermoplastic polymer is extruded through an extruder die which is rapidly attenuated by the hot air stream to form the extremely fine diameter fibers. The attenuated fibers are then blown by high-velocity air to a collector screen to form a fine fiberd, self-bonded web. The combination of fiber entanglement and fiberto-fiber bonding generally provides enough web cohesion so that the web can be used without further bonding. A schematic of a melt blowing process is shown in Fig. 10.9 [7]. Melt blown fibers generally have diameters in the range of 2 to 7 mm, although they may be as small as 0.1 mm and as large as 30 mm. Due to the large fiber surface area of the melt blown fabrics, they are used in filtration, insulation and liquid absorption applications. Because of the simplicity of the process, any thermoplastic fiber can be melt blown. However, the polymer should have very low melt viscosity, and it is an energy consuming process.


319

Air manifold

Extruder

Die

Gear pump

Winder Collector

10.9 Schematic of a melt blowing process.

10.5

Web bonding techniques

The web bonding techniques can be generally classified into three categories, mechanical, chemical, and thermal bonding, depending on the ultimate fabric applications and/or on the web formation method. Sometimes, in order to achieve products with certain properties, a combination of different bonding methods is applied.

10.5.1 Mechanical bonding Mechanical bonding can be further classified as needle punching, stitching, and spunlacing (hydraulic entangling). Needle punching is a process of bonding nonwoven web structures by mechanically interlocking the fibers through the web via the barbed needles. It is the only bonding method suitable for heavyweight nonwoven fabric bonding. The needle-punched fabrics are extensible, bulky, conformable, distortable and extremely absorbent. Both dry laid and polymer laid webs can be needle punched. Needle punched fabrics have been used as carpet backing fabrics, automobile carpets and headliners, blankets, and geotextile fabrics [7]. Stitch bonding is the process of bonding a web by using stitching yarns, filaments, fibers, or just the stitching needles themselves to do the bonding. Stitch bonded fabrics have taken the place of woven goods in many applications such as decorative fabrics for home furnishing, shoe fabrics, backing fabrics for artificial leather, etc. Spunlacing is a process of entangling individual fibers with each other by using high-pressure water jets, which cause the fibers to migrate and entangle. Spunlaced fabrics can be used as wipes, medical gowns, dust cloths, etc. [7]. The popularity of spunlaced fabrics is increasing and the share of this process continues to climb (Fig. 10.10).

10.5.2 Chemical bonding Bonding a web by means of a chemical is one of the most common methods of bonding. The polymer latex or a polymer solution is applied to the web

320

Biodegradable and sustainable fibres 70

% of total production

60

Needle punch

50 40

Thermal/resin bond

30 20

Spunlace

10 0 1991

1996

2001

2006

10.10 Market share by weight from different processes during 1991– 2006 [13].

and then cured thermally to obtain bonding. Several methods are used to apply the polymer latex/solution and these include saturation bonding, spray bonding, print bonding, and foam bonding. Chemically bonded fabrics have been widely used as wipes and towels, apparel interlinings, automotive trim, filter media, etc. [14]. Use of the right chemical binder depending on the fiber and the intended application is important since the binder stays in the fabric. Also, environmental issues while applying or curing the binders also need to be considered.

10.5.3 Thermal bonding Thermal bonding is the process of using heat to bond or stabilize a web structure that consists of a thermoplastic binder. It is the most popular method of bonding used in nonwovens, because of the favorable process economics, the absence of chemical binders, that is, environmentally friendly, the availability of new fibers and machinery, and process and product enhancement. The bonding is achieved by the direct action of heat and pressure by a calender, an oven, a radiant heat source, or an ultrasonic wave source. The thermoplastic binder can be in the form of fiber, web, or powder. There are four methods of thermal bonding. They are hot calendering, oven bonding, ultrasonic bonding, and radiant heat bonding. Hot calendering can be further classified as area or point bond hot calendering, and embossing hot calendering. Among the various types of thermal bonding methods, point bonding using embossing rolls is the most desired method, which is the leading method used by the cover-stock industry for baby diapers. It employs direct contact, with heat and pressure, to produce localized bonding in a nonwoven. Also it adds softness and flexibility to the fabric by the embossing rolls compared with smooth rolls used in area bond hot calendering. A schematic of a point bonding process is illustrated in Fig. 10.11 [15].


321

Nip width

Point-bonded web Input web

Nip time @ 10 ms Temperature @ softening point Load @ 30–300 kN/m Speed @ 4–10 m/s

10.11 Schematic of a thermal point bonding process [15].

10.6

Technology and relative production rate

One of the reasons for continuing growth of the nonwovens is the high production rates that are possible with the new technologies. The approximate production rates are listed in Table 10.1. Compared to only a few meters per minute possible with the woven or knitted fabrics, nonwovens can be produced at the rate of a few hundred to thousand meters per minute. This high production rate combined with the fact that the intermediate yarn formation is eliminated helps in keeping the cost of nonwovens very low. This low cost of roll goods production has helped the spurt in growth of nonwoven products.

Table 10.1 Fabric production rates from different technologies Technology

Weaving Knitting

Relative production rate (m/min) 1 –6 3 –16

Nonwovens – web forming: – Carding – Spunbond – Wet-laid

120 –400 200 –2000 2300

Nonwovens – bonding – Stitchbonding – Needling – Calendering – Hot air bonding

40 30 –500 2000 5000

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10.7


Recent research on biodegradable nonwovens

10.7.1 Natural cellulosic fiber nonwovens Natural fibers have come a long way; during the past several years, these soft, durable and biodegradable fibers have established a positive and highly regarded name for themselves in numerous nonwovens end use markets because of their reputation for being soft, durable, breathable and coming from renewable resources. These days, traditional natural fibers, including cotton, hemp, flax and jute, have been seeing more demand internationally, while other fibers such as hemp and milkweed are starting to emerge into more developed nonwovens areas. Many manufacturers predict that the use of these fibers will grow, as consumers become more aware of their advantages. In the meantime, manufacturers and university researchers are working on new innovations for all natural fibers. The total production of different natural cellulosic fibers is shown in Table 10.2. Obviously cotton is the most used fiber, again due to its popularity in apparel and other fabrics. Jute, kenaf and flax come next, with the rest of the fibers having only a small share. The cost of these fibers also varies and cotton is the most expensive of this group of fibers (Fig. 10.12) [16]. Although cotton is the most attractive fiber for many applications, the cost is the factor that has limited its growth. Table 10.2 Commercially important natural fiber sources [16] Fiber source

World production (103 tonnes)

Viable growing regions

Cotton (Gossypium sp.) Jute (Corchorus sp.) Kenaf (Hibiscus cannabinus) Flax (Linum usitatissimum) Sisal (Agave sisilana) Roselle (Hibiscus sabdariffa) Hemp (Cannabis sativa) Coir (Cocos nucifera) Ramie (Boehmeria nivea) Abaca (Musa textiles) Sunn hemp (Crotalaria juncea)

18,450 2300 970 830 378 250 214 100 100 70 70

E, W W E, W UK, E, W W W UK, E, W W W W W

UK = United Kingdom, E = Europe, W = World

10.7.2 Cotton nonwovens Just about everyone can recognize cotton as a durable, breathable and soft fiber. Perhaps no one recognizes the benefits of cotton as well as Cotton Incorporated, a Cary, NC (USA)-based non-profit organization dedicated to its advancement. Its report, ‘Cotton for Nonwovens: A Technical Guide’ [17, 18], sheds light on how powerful the name cotton has become. In 2000, the


323

Relative price 100 90 80 70 60 50 40 30 20 10 0

Cotton

Flax

Abaca

Jute

Sisal

Coir

Wood

Straw

10.12 Relative cost of fibers from different sources (cotton = 100) [16].

US apparel and home fabrics markets purchased the equivalent of 15.1 million bales of cotton, while the global nonwovens market used the equivalent of 14.7 million bales of fibers; between 1996 and 2000 global consumption of bleached cotton fiber rose by 6%. Cotton’s current share of the nonwovens market is 7.8% globally and 2.8% in North America, and in the major consumer markets of North America, Western Europe and Japan, growth of cotton usage in nonwovens is projected at 3–6% per year for the next few years. A study, conducted by Cotton Incorporated in six cities across the US, tested consumers’ perceptions of fiber content in nonwoven products and how these perceptions affected purchasing preferences. One thousand women aged 18 to 49 took part in a study of four product categories: feminine napkins; tampons; baby wipes; and disposable diapers. The women were all shown pictures of well-known brands with and without the Cotton Seal. In each category, the Cotton Seal significantly influenced consumers’ purchasing preference. Moreover, 66% of consumers perceived personal care products with this seal to be of higher quality. Fifty-nine percent agreed with the statement, ‘I expect to pay more for products with the Cotton Seal’, and 57% said they were willing to pay more. Although cotton in its pure, untouched state is widely used and accepted, cotton can also have special properties applied to it, thereby paving a way for new uses and markets. One of these properties relates to bleaching. Barnhardt Manufacturing Company, Charlotte, NC (USA), produces bleached cotton fibers for carded web products, chemically bonded fabrics, and spunlaced and needled fabrics, with approximately 95% of the company’s bleached fibers targeting nonwovens, due to increasing interest of bleached fibers,

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among nonwoven manufacturers. Cotton, when bleached, is also more aesthetic to consumers who appreciate the snow-white quality of bleached cotton. Also, when a natural fiber such as cotton is dyed, the colors tend to be softer and pastel, unlike synthetic fibers that produce much shinier and usually glare-like effects. Cotton fibers give nonwoven fabrics unique characteristics that synthetic fibers cannot duplicate easily. Synthetic fibers are currently being used more in nonwoven fabrics than cotton because of misconceptions regarding cotton’s processability. With improved bleaching techniques and the development of new finish applications, cotton can be processed at speeds comparable to that used with synthetics while providing the superior attributes of cotton to the nonwoven. Most consumer data suggests that consumers prefer cotton fibers. Additional advantages of cotton and other natural fibers include superior wet strength as well as a quick dry surface, notably in wipes. Bleached cotton fibers have high levels of absorbency and are soft to the touch, breathable and biodegradable. One quickly-growing area, especially throughout Europe and Japan, is spunlaced cotton used for cosmetic wipes and other disposable products; these trends are likely to spread to other markets as well. Consumer demands for cotton are well documented, but because nonwovens are not required to list fiber content in products, consumers often don’t know what they are purchasing. There is definitely an opportunity to increase market share by adding the fiber content as being cotton, since there is consumer preference to purchase cotton-containing products [18]. Although cotton, with all its varying attributes, can tend to dominate the natural fibers market, hemp, jute, flax and milkweed are some other examples of fibers that are used not only in nonwovens, but are also growing in popularity in many different applications. As companies become more familiar with the benefits and uses of these fibers, new innovations will be developed in the future.

10.7.3 Hemp Hemp fibers are not as well known as cotton, but they certainly have proven themselves for Hempline, Delaware, Ontario, Canada. Hempline is a large supplier of hemp fiber to the nonwovens industry, primarily supplying hemp as a reinforcing fiber for substrates. With 50% of the company’s sales conducted in the nonwovens industry, Hempline is noticing a rapid increase in demand for its products, especially its reinforcement fibers: ‘Hemp fiber has found its way into more vehicles in the past year, and, chances are good that many people have a natural nonwoven fiber product in their car and don’t even know it. Hemp fiber has been found to be very cost-effective, with high strength and can be used as an excellent reinforcement fiber for replacing glass fiber, at a much lower price. The increasing commercial availability of


325

hemp fiber and the demand for low cost, high strength fibers has resulted in new applications for hemp, particularly in automotive and construction products.’ [19] Aside from its high strength, hemp has been recognized for its elasticity, ease of processing and recycling. However, there are a few setbacks, the main one being consumers’ unfamiliarity with hemp fiber. There is need for consumer education as far as the benefits of hemp are concerned. Key advantages of hemp fiber are its high strength and low cost, and there are many markets still awaiting the use of this fiber as it slowly makes its way into becoming another option for manufacturers. Also, hemp fiber’s staple length and strength can be modified according to the needs of the consumer. Although the market is price-conscious, using better qualities of natural fibers results in lower price rejects, reduces downtime on the equipment, minimizes loss of fiber during processing and, overall, makes better economic sense.

10.7.4 Other natural fibers Another natural fiber increasing its role in the nonwovens industry is milkweed. Milkweed floss is a silky white seed with a resilient hollow tube that looks similar to a straw. It is similar to high quality down and is a hydrophobic, cellulose fiber with a high chemical resistance and the ability to be dyed readily. Some properties milkweed floss can provide nonwovens include super absorbency, softening, hydrophobicity, paper-strength, bulking, selfbonding and tactile-change. Milkweed floss fiber from advanced agricultural production has the ability to compete in nonwoven applications, especially in filtration, absorbent products and thermal and sound insulation products. Natural Fibers Corporation has introduced a 75% recycled cotton and 25% milkweed fiber mattress pad through its subsidiary, Ogallala Comfort Company, Ogallala, NE [19]. Although technology is available to use many of the natural fibers in nonwovens, the industry will have to wait for a number of things to happen, including an improved economic climate, which may possibly change people’s willingness to pay for improvements. If anything, this industry is growing internationally, which may force manufacturers and consumers alike to keep up with the competition. Additionally, the natural fiber, particularly cotton, market is currently expanding globally, as it hits uncharted territory around the globe.

10.7.5 Cotton-based fully biodegradable nonwovens Research on cotton-based nonwovens has been carried out at the University of Tennessee since 1987 by applying different kinds of binder fibers through

326


carding and thermal calendering processes. Cellulose acetate (CA) fiber has first been applied most successfully as the binder fiber since it is thermoplastic, hydrophilic and biodegradable [20–25]. Eastar Bio ® GP copolyester unicomponent and bicomponent (Eastar/PP) fibers were further selected as the binder fibers in recent studies [26–30]. Five different kinds of fibers were used for the study. Cotton fiber is the base fiber, and four types of binder fibers, ordinary cellulose acetate (OCA), plasticized cellulose acetate (PCA), Eastar Bio® copolyester unicomponent (Eastar), and Eastar Bio® copolyester bicomponent (Eastar/PP) fibers. The chemical name of Eastar Bio® copolyester is poly(tetramethylene adipateco-terephthalate) (PTAT). The cotton fiber used in this research as the carrier fiber was supplied by Cotton Incorporated, Cary, NC. The scoured and bleached commodity cotton fiber had a moisture content of 5.2%, a micronaire value of 5.4 and an upper-half-mean fiber length of 24.4 mm. Both the OCA and PCA binder fibers were provided by Celanese Corporation, Charlotte, NC; while the Eastar and Eastar/PP bicomponent binder fibers selected for this study were produced by Eastman Chemical Company, Kingsport, TN. The plasticizer used in PCA binder fiber is triethyl citrate ester (C12H20O7) with a weight concentration around 2%. The bicomponent Eastar/PP has a sheath core structure with Eastar as the sheath and PP as the core. The properties of these selected fibers are listed in Table 10.3. Table 10.3 Properties of selected fibers Property Filament density (g cm–3) Filament denier (denier) Filament tenacity (g den–1) Peak extension (%) Staple length (cm) Crimps per cm Softening temperature (∞C) Contact angle (∞)

Cotton 1.5 2.2 1.8 7.8 2.44[a] [b]

– 31.90

OCA

PCA

Eastar

Eastar/PP

1.3 1.1 1.2 25.0 4.32 more ~190 –

1.3 2.9 1.3 50.6 4.57 13 ~110 43.04

1.2 4.0 1.6 296.1 2.54

1.1 4.0 2.2 148.7 3.81 18 ~80[d] 63.02

[c]

~80 56.75

[a] upper-half-mean fiber length; [b] cotton has natural convolution; [c] not measurable; [d] softening temperature of sheath

Processing The nonwoven fabrics in this research were produced by first carding of cotton and the binder fiber and then thermally bonding the carded webs. The fiber components were prepared by separately opening and then hand mixing of the two fiber types for homogeneity. The fiber blend was then carded to form a web using a modified Hollingsworth card with the conventional flats


327

installed at the licker-end of the machine. The resulting carded webs had the basis weights of about 40 g m–2. After carding, acetone solvent or water dipnip treatment was applied to some of the carded webs. Then the treated or untreated webs were fed for thermally point-bonding using a Ramisch Kleinewefers 60 cm-wide calender. The embossed roll had a diamond pattern, covering approximately 16.6% of the surface area, i.e., the bonded area was around 16.6%. Cotton/cellulose acetate biodegradable nonwovens The first studied biodegradable cotton-based nonwoven fabrics were produced by cotton and ordinary cellulose acetate (OCA) fiber. Bonding temperatures used here for thermal calendering are 150∞C, 170∞C, and 190∞C based on the ordinary cellulose acetate’s high softening temperature (Ts: 180–205∞C). The tensile strengths along machine direction of the bonded fabrics are listed in Table 10.4. However, the tensile strength of the nonwoven fabric made with cotton/cellulose acetate nonwoven blend is quite low and is not suitable for consumer application when it is processed under the temperatures associated with cellulose acetate’s softening temperature (180∞C–205∞C). Solvent treatment has been introduced in order to modify the softening temperature of cellulose acetate fiber and to lower the calendering temperature, while maintaining enhanced tensile properties. Acetone, a good solvent for cellulose fiber, was considered a choice in the solvent pre-treatment. Twenty percent Table 10.4 Peak load for cotton/cellulose nonwovens (kg) Bonding temperature (∞C)

Binder component 25%

Binder component 50%

Cotton/OCA (No treatment)

150 170 190

0.10 0.09 0.09

0.03 0.03 0.09

Cotton/OCA (With 20% acetone solvent treatment)

150 170 190

0.21 0.34 0.69

0.20 0.47 0.65

Cotton/OCA (With water dip-nip treatment)

150 170 190

0.21 0.37 0.81

0.25 0.44 0.77

Cotton/PCA (No treatment)

150 170 190

0.17 0.33 0.63

0.25 0.42 0.90

Cotton/PCA (With water dip-nip treatment)

150 170 190

0.52 0.80 0.81

0.57 0.86 0.87

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acetone solvent pre-treatment was then applied for cotton/cellulose acetate nonwovens to decrease the softening temperature and further lower the calendering temperature [24]. The results showed that these solvent treatments can decrease the softening temperature of cellulose acetate fiber and produce comparatively high tensile strengths as shown in the data in Table 10.4. However, from a practical standpoint, manufacturers do not like to have a process involving the use of acetone since acetone evaporates easily, and is flammable and toxic. These detrimental factors may cause big problems in manufacturing and pollute the working environment. Also, consumers may not prefer to buy acetone treated products. Thus, two alternative methods were further applied for cotton/cellulose acetate nonwovens [25]. Water dip-nip treatment was further used instead of acetone solvent pre-treatment to make the process more environment friendly. Comparing the effect of water dip-nip treatment with acetone solvent treatment, it can be found that there is no significant difference between water dip-nip treatment and 20% acetone solvent treatment and peak load of cotton/cellulose acetate thermally bonded webs are enhanced by both the treatments. Based on these data, water can be used as an external plasticizer instead of 20% acetone solvent without compromising web strength and the process is environment friendly. From the point of energy concern, it is better to make the whole process as simple as possible. So a plasticized cellulose acetate fiber, wherein an internal plasticizer was added during fiber manufacture to lower the softening temperature of ordinary cellulose acetate and further lower the bonding temperature during thermal calendering process. It can be clearly seen from the data (Table 10.4) that peak load has been improved by using PCA instead of OCA, especially at higher bonding temperature. Further comparison of external plasticizer (water) and internal plasticizer shows that there is no significant difference between using external plasticizer and internal plasticizer. Thus, it is evident that an internal plasticizer (PCA) can be used in place of the external plasticizer (water) without compromising web strength, and the process is more economical. Based on the above analysis, it seems that the optimal processing conditions are either for cotton/OCA with water dip-nip treatment or cotton/PCA without treatment bonded at 190∞C for both the blend ratios. The optimal strength of the biodegradable nonwovens is around 0.8 kg. Cotton/Eastar biodegradable nonwovens The desired calendering temperature of PCA bonded nonwovens is still much higher for achieving good tensile properties. So, a newly introduced biodegradable copolyester unicomponent (Eastar) fiber, which has a relatively low softening temperature (~80∞C), was further selected as a binder fiber instead of cellulose acetate fiber. It has been reported that this binder fiber


329

can be totally degraded into CO2, H2O and biomass [30]. Because of the low softening temperature of the binder fiber (Ts: ~80∞C), bonding temperatures used are 90∞C, 100∞C, 110∞C, and 120∞C. The tensile strengths along machine direction of the cotton/Eastar fabrics are shown in Fig. 10.13. It can be seen that these strength values are higher than those of cotton/OCA nonwovens but much lower than those of cotton/PCA nonwovens as listed in Table 10.4. 0.4

0.35

0.3

Peak load (kg)

0.25

0.2

0.15

0.1 Cotton/Eastar = 70/30 Cotton/Eastar = 50/50 Cotton/Eastar = 85/15

0.05

0 80

90

100 110 Bonding temperature (∞C)

120

130

10.13 Peak strength of cotton/Eastar nonwovens.

Unicomponent Eastar Bio® GP copolyester fibers are soft and somewhat difficult to crimp due to the high elasticity of the fiber. For carding process, relatively stiffer fibers are preferred. One disadvantage in using Eastar as a binder fiber is that it is hard to get the binder fibers well distributed, which may cause the low tensile properties of the final calendered nonwoven fabrics. Thus, a bicomponent fiber with Eastar Bio® GP copolyester as a sheath on a stiffer PP core was produced by Eastman Chemical Company, Kingsport, TN to offer more stiffness than a 100% unicomponent Eastar Bio® GP copolyester fibers and to further improve the tensile properties of the nonwoven fabrics. This bicomponent binder fiber has higher tenacity, higher crimps, and lower peak extension compared to that of Eastar unicomponent binder fiber as listed in Table 10.3. These properties are preferred for the carding

330


process. The tensile strengths of cotton/(Eastar/PP) nonwovens are shown in Fig. 10.14. It can be clearly seen that the tensile strengths of the fabrics are much higher than those of cotton/Eastar nonwovens shown in Fig. 10.4, and even higher than cotton/PCA nonwovens. The optimal cotton/(Eastar/PP) web has a peak load value of 1.21 kg or 1.15 kg for the binder fiber component of 50% at bonding temperature of 110∞C or 100∞C respectively. Therefore, using Eastar/PP bicomponent fiber as a binder fiber can improve the tensile properties of cotton/Eastar nonwoven fabrics. The optimal thermal calendering temperature is relatively lower than that for cotton/cellulose nonwovens, which means that the cost of the process can be greatly reduced by using Eastar/PP bicomponent fiber as the binder fiber for the cotton-based biodegradable nonwovens. 1.4 Cotton/Eastar PP = 70/30 Cotton/Eastar PP = 50/50 Cotton/Eastar PP = 85/15

1.2

Peak load (kg)

1

0.8

0.6

0.4

0.2

0 80

90

100 110 Bonding temperature (∞C)

120

130

10.14 Peak strength of cotton/(Eastar/PP) nonwovens.

Flexural rigidity and absorption properties of the cotton/(Eastar/PP) nonwovens were also studied. Results show that the nonwovens have good flexural rigidity and absorbency, which indicate that the nonwoven materials may be used for medical and sanitary applications. However, one has to remember that PP component in the bicomponent fiber is not biodegradable; this puts this fabric in the category of many other cotton/binder nonwovens that may have PP or PET as binder fibers. The results obtained from these


331

studies suggest different routes for producing high strength nonwoven webs from cotton fibers with a thermoplastic binder fiber. Newer thermoplastic polymers are good candidates for such applications. In fact, recent work in our laboratory has shown that PLA can be used as a binder fiber to produce strong point bonded nonwoven webs. Another advantage is that PLA requires a relatively lower bonding temperature.

10.7.6 Wet laid disposable nonwovens with flax fiber The use of bleached elementary flax fiber in modern disposable nonwoven products was recently studied by Van Roekel et al. [8]. Due to the long elementary fiber length and high cellulose content of flax bast fibers, they become an excellent substitution for synthetic fibers in disposable nonwovens. Wet laid nonwoven sheets were produced and spunlaced on a pilot unit, however, further improvement are reported to be needed for the process. Usually, wet laid disposable nonwovens are manufactured on fourdriniertype paper machines, stock preparation and headbox are modified for long fibers, and surfactants are applied to help disperse the long fibers in the primary water cycle. The machine for wet laying flax nonwovens needs to be fast rewetting, easy dispersion in the existing stock preparation system and homogeneous formation. Various blends of 18 mm cut flax and PET fiber, supplemented with fluff pulp fillers were produced; no finishing was applied for the flax fiber for the process. A 1.5 m wide, 80 g m–2 web at about 100 m min–1 was formed. The properties of the resultant wet laid nonwovens are listed in Table 10.5. It was observed that the strength properties of the web disappear completely with the increase of flax content. When extrapolated to 40% flax content, strength can be fully attributed to the fluff pulp, and the strength of the web was not improved by adding more flax. Since the individual flax fiber has sufficient strength, the absence of tensile strength in the web was believed to be from the poor formation and bonding properties of the web. Therefore, further improvement of the wet laid process is needed either by using shorter flax fiber or applying finish to flax fiber to improve its dispersion. Table 10.5 Wet laid nonwoven properties [8] Property

Unit

Run 1

Run 2

Run 3

Flax Synthetic fiber Fluff pulp Weight Density Tensile dry Tensile wet Elongation

% w/w % w/w % w/w g m–2 kg m–3 Nm g–1 Nm g–1 % M/C

10 20 60 98.8 204 15 5.3 40

20 20 60 79.2 213 14 3.0 32

30 10 60 62.2 204 9 1.5 24

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10.7.7 Nonwovens from animal fibers Wool Wool has been one of the most widely used animal fibers. The first nonwovens were produced from wool fibers as felts by mechanically interlocking the woolen fibers, taking advantage of their natural surface scales. Wool has excellent thermal properties and is one of the best insulating fibers. Because it is more expensive than many of the synthetic fibers used in nonwovens, it has not been one of the popular fibers. Lately, there is increasing effort to incorporate wool fibers in special nonwoven applications. Using nonwoven processes, it is possible to produce low-cost lightweight woolen fabrics with high stretch. Recent work [31] has shown that nonwoven fabrics from wool can be produced with properties that are not possible to achieve by knitting and weaving. Some of the apparel products that are produced from merino wool include three-dimensional coating fabrics, stretch fabrics, windproof fabrics, and footwear accessory fabrics. Thermal blankets produced from wool fibers have excellent insulation and comfort properties. Also, these are waterproof and pack into a small volume, making them suitable for lightweight blankets for search and rescue operations. The combination of properties such as wicking ability, moisture and sound absorption, resiliency, and thermal insulation makes wool and wool-blend nonwovens suitable for many automotive uses. Thus, there is increasing effort to take advantage of wool’s properties in many emerging applications. One such example is blending 20–35% wool with rayon to produce affordable WoolFelt® nonwovens by Natural Nonwovens [32]. These fabrics can be colored or textured as desired, and are considered as fabrics of choice for heritage quilts, penny rugs and heirloom crafts. Silk Silk, considered the queen of fibers, is an expensive fiber with many rich properties and being a natural protein fiber, it is known to be biodegradable. Because of the cost, this is not a fiber targeted for nonwovens. However, there have been efforts to produce silk nonwovens for niche applications; one advantage is that waste and poor quality silk can be used to produce many of the nonwoven products, thereby helping control the cost. Recently, spunlaced silk nonwovens with very low basis weight of 25 g m–2 have been developed using the Jetlace 2000 water jet machine form Rieter Perfojet [33]. These lightweight nonwovens are targeted for sanitary materials and medical applications such as gauze and wound dressings, cosmetics and skin care products, where the property demand might be stringent. Also, by using the hydroentangling process, using any other chemical additive is avoided. These fabrics have softness, elasticity, moisture absorption, heat preservation,


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breathability and are not harmful to the body in any way. Some of these nonwovens can also be used as high value garments as liner for overcoats, jackets, suits or fashion fabrics. There is likely to be continuing research and development in this area as the market realizes the potential for such fabrics and with the simultaneous efforts of reducing the cost by using waste silk. Chitin Chitin is a safe natural substance found in the shells of crabs, shrimp and lobster, and in the wings of butterflies, ladybugs etc. Chitin is one of the three most abundant polysaccharides in nature, in addition to glucose and starch. It ranks second to cellulose as the most abundant organic compound on earth. Chitin and its derivatives, chitosan, chitin oligosaccharide, and chitosan oligosaccharide, have many useful properties that make them suitable for a wide variety of health-related applications. Also, chitin products are known to be anti-bacterial, anti-fungal, anti-viral, non-toxic and non-allergic. Nonwoven webs can be formed from chitin fibers for use in medical applications. The chitin artificial skin is a newly developed patented product produced by a process technology [34]. The chitin fiber is produced by special wet laid spinning process with the selected chitin; it has the properties of three-dimensional structure, soft handle, absorbency, breathability, nonchemical additive, compact texture, softness and smoothness, thus it is the ideal dressing for extensive burn, scald and other traumas. Main features are: inhibition of bacterial growth avoiding cross-infection and control of the loss of the exudates, good biocompatibility, excellent bioactivity, stimulation of new skin cell growth, accelerated wound healing, no adverse reaction of abnormal immunity, repelling and irritation. As well as artificial skin, other products include wound protective bandages, wound dressings, and skin beauty packs. Chicken feather Feather products have been used in bedding and some outerwear for cold climates. Nonwoven battings made from chicken feather fibers have been evaluated as possible insulating materials. When compared with goose and synthetic fibers, chicken feather batts show insulating properties better than that of synthetic fibers and close to that of downs. Also, the chicken feather battings have good resiliency, which is important for insulation battings. One disadvantage is that the properties of chicken feather, both size and tenacity, vary depending on how they are separated from the quill [35]. This introduces further non-uniformity and the process has to be very well controlled to compensate for this.

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10.7.8 Spunbond biodegradable nonwovens Spunbond PTAT nonwovens The Eastar Bio® GP copolyester (PTAT) can be melt spun into spunbond and melt blown fabrics. It has been reported that uniform spunbond fabrics have been produced on Ason spunbond equipment using slotted air technology and Reifenhauser Reicofil equipment at conventional spinning speeds [36]. Fabrics with finer fibers, higher throughputs, higher spinning speeds (> 4500 m min–1), and basis weight ranges from 14 to 130 g m–2 have been successfully obtained. The resultant spunbonded fabrics are semi-crystalline with good drapeability, soft hand, and elastic properties. The fabrics can be gamma radiation sterilized; radio frequency bonded, and ultrasonically sealed, which make the fabrics suitable for medical applications, such as hospital surgical packs, wipes, bondages, face masks, etc. The fabrics can also be used for agricultural and other absorbent disposable products like diapers, seed mats, ground cover, etc. Spunbond PLA nonwovens Polylactic acid (PLA) first received considerable attention because of its biodegradability and biocompatibility; in recent years, researchers are paying more attention on biodegradable nonwoven products. Polylactic acid (PLA) was spunbonded and melt blown at the University of Tennessee, Knoxville in 1993 [37]. Later, Kanebo, a Japanese company, introduced Lactron® (polyL-lactide) fiber and spunlaid nonwovens in 1994. Biesheim-based Fibreweb (France) has developed nonwoven webs and laminates made of 100% PLA in 1997 and introduced a range of melt blown and spunlaid PLA fabrics under the brand name of Deposa™ [38]. The composite structures, described in US Patent 5,702,826 [39], comprise one or more plies of nonwoven laminated to a film, where all the plies were totally manufactured from polymers derived form lactic acid. Each ply provides mechanical, barrier-effect, absorption, filtration and thermal insulation properties that can be adapted to each application by selecting the suitable composition of the nonwovens and of the films based on polylactic acid. The spunbonded nonwoven layer is used as the support, the film provides impermeability and barrier effect, and another spunbond or melt blown nonwoven layer is added to offer filtration/absorption and thermal insulation properties. Depending on the application, a derivative of lactic acid chosen from D-lactic acid, L-lactic acid, or DL-lactic acid may be used. The PLA polymers are processed using conventional spunbond or melt blown techniques. The plies of the nonwovens can either be hot calendered, needle punched, hydroentangled, or chemical bonded. They are intended for disposable hygiene, agriculture, and medical applications such as diapers,


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sanitary napkins, protective clothing, surgical masks and drapes. An example of the three-ply nonwoven for medical application consists of the first spunbond web with a basis weight of 10–20 g m–2 and the linear density of the fibers between 1.5–2.5 dtex, the melt blown web with basis weight of 5–15 g m–2 and the linear density of the fibers between 0.1–0.3 dtex, and the second spunbond with a basis weight of 10–20 g m–2 and the linear density of the fibers between 1.5–3.0 dtex. The total weight is from 25–55 g m–2. The calendering temperature for the laminate is between 65–120∞C depending on the type of raw material and the calendering speed. The laminate has a bonded surface area of 8–15%. The strength of the composite is between 40– 100 N and the elongation at break is from 30–60%.

10.7.9 Melt blown biodegradable nonwovens and laminates In JP Patent 11,117,164 [40] a kind of biodegradable nonwoven laminate of melt blown nonwoven fabrics of aliphatic polyester fibers and spunbonded nonwoven fabrics of urethane bond-containing butylene succinate copolymer fibers was described. The biodegradable laminates were prepared by sandwiching melt blown nonwoven fabrics of biodegradable aliphatic polyester fibers with diameter of 0.5–2.0 mm between spunbonded nonwoven fabrics of long fibers consisting of polymers containing 1,4-butanediol units and succinic acid units and having urethane bonds to give laminates with melt blown nonwoven fabric content of 10–30%. The fabrics are useful for medical care materials and hygienic materials. Bionolle 1030 (butylene succinate copolymer containing urethane bonds) was melt spun at 190∞C, passed through an ejecter, and piled on a screen to give spunbonded nonwoven fabric. Bionolle 3300 (butylene succinate copolymer containing 20 mol% adipic acid units and urethane bonds) was melt spun by a melt blowing method, sandwiched between two spunbonded nonwoven layers of Bionolle 3300 fibers, and embossed at 105∞C to give a laminated nonwoven fabric with tensile strength 151 N and softness rating 98 and exhibiting weight loss more than 50% on embedding the nonwoven fabric in soil for 6 months.

10.7.10 Wet laid nonwoven fabrics with PLA fiber In JP Patent 2003,268,691 [41], wet laid nonwoven fabrics comprising biodegradable fibers consisting of biodegradable polymers derived from sources other than wood and petroleum were described. The wet laid nonwoven fabrics comprise more than 90% biodegradable fibers consisting of PLA, or the wet laid nonwoven fabrics comprise more than one portion of the biodegradable fibers comprising fibrillated fibers. The wet laid nonwoven fabrics are reported to be useful for packaging paper, corrugated cardboard,

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tissue paper, printing paper, wiping paper, toilet paper, and filter paper and for agriculture. Thus, 5:95 D-lactic acid:L-lactic acid copolymer chips were melt spun at 260∞C and wound to give undrawn fibers. The wound fibers were drawn at hot roll temperature of 80∞C and heat-setting temperature of 118∞C to give drawn fibers with elongation 40% and denier per filament 2.2 dtex. The undrawn fibers and drawn fibers were crimped and cut to give undrawn staple fibers with 5.24 crimps cm–1 and drawn staple fibers with 5.20 crimps cm–1. The drawn staple fibers were passed through an orifice at high speed and impacted against a wall at the exit of the orifice to give fibrillated fibers. A dispersion contains 60:15:25 mixture of undrawn staple fibers:drawn staple fibers:fibrillated ultrafine staple fibers were made into a wet laid nonwoven fabric using a mesh drum, dried at dryer surface temperature of 110∞C, and calendered at 140∞C to give a biodegradable nonwoven fabric showing complete fiber degradation on embedding the nonwoven fabric in a compost for 50 days.

10.8

Applications of biodegradable nonwovens

Biodegradable nonwovens can be used for almost all the areas of nonwoven applications. In sanitary and medical industries, a hair cap made of a poly(Llactic acid)-based thermoplastic resin nonwoven fabric showed good haircapturing property according to JP Patent 2002345541 [42]; breathable, biodegradable/compostable disposable personal care product was produced from Bionolle 3001 nonwovens reported from WO Patent 2002053376 and JP Patent 2002035037 [43, 44]. Natural coconut fibers (coir) have been applied for biodegradable erosion control mats by Landlok [45] in the geotextile industry. In the automotive industry, most of the European automotive producers already use car interiors made of natural fibers. In Germany, in 1996, 3630 tonnes of flax, sisal and jute were used for car interiors, and in 1999 this figure increased to 11,800 tonnes. The absolute figure of the production at the moment is not very high, but the average annual growth, which is round about 50%, is promising [46]. Nonwovens made from kenaf fiber [47] offer good sound insulation property for automobile interiors. Yachmenev et al. [48] reported that a variety of moldable, cellulosic-based nonwoven composites for automotive applications with excellent thermal insulation properties were fabricated from kenaf, jute, flax, and waste cotton using recycled polyester and substandard polypropylene. In filtration industry, refuse bag and drain filter have been made by using fine denier biodegradable polylactic acid nonwovens for the application of sink drain [49]; biodegradable pleated filter material and filter unit for air purification and liquid filtration are produced [50].


10.9

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Flushable nonwovens

Liquid waste system disposal is quite attractive compared to solid waste disposal, where infrastructure is not well developed for the latter. Also, in many instances, landfill and solid waste disposal techniques have some other environmental related problems. Considering this, the wastewater system is more convenient, hygienic and environmentally sound; already there is a massive infrastructure in place as wastes from houses go to industrial biodegraders in the form of sewage farms, or local biodegraders or septic tanks. In such situations, many disposable products can be flushed in the system rather than thrown away as solid waste [51]. Flushable nonwoven diapers, liners and wipes have been in the market for a while. For such products, flushability is desirable and technically it is possible to develop such products. However, lack of convenience and cost issues have driven the market towards non-degradable plastics in diapers as well as feminine hygiene products. In designing and developing flushable nonwovens, one of the challenging requirements is that the products have to be strong enough to be stored and/or used when wet, but at the same time, should be weak enough to break down in the sewage system. Flushability itself is not well defined and there is no accepted standard method to evaluate and certify flushability of such products. The current efforts involve comparing fabrics by agitating them in a standard volume of water for a standard time and observe the fragmentation degree or determine the time for achieving full dispersion. For a nonwoven material to be claimed flushable, the fabric must break up immediately in a toilet bowl and be small enough to be transported from the toilet bowl to the sewage system in a single flush. It should not lead to blocking of pipe work and there should not be any accumulation in subsequent flushes. There have been consumer studies that have shown that many flushable wipes in the market lead to clogging of pipes. In addition to the fact that they have to break down, they should not contain any chemicals that might affect the functioning of sewage farms or the quality of the treated water. This means that all the materials used have to be biodegradable. With these stringent requirements, some of the means of achieving these are: ∑ Hydrogen bonded cellulose without other bonding as in toilet tissue paper. These will consist of cellulosic fibers, refined pulp and fibrillated fibers that give stronger products that can be dispersed in water. However, these are likely to be stiffer in nature. ∑ Hydrogen and friction bonded cellulose wherein carded, air laid or wet laid fiber web is hydroentangled. By controlling the process conditions, it should be possible to develop a structure that is dispersible in water under the suggested conditions.

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∑ Fibers bonded with water-soluble polymers such as starches, carboxymethyl cellulose, polyethylene oxides, polyvinyl alcohols, polyacrylates, etc. This may involve bonding of biodegradable fibers or films or other structures that will break down in the flush. In many of these situations, the enhancement of wet strength will retard flushability. There has been more focus on development of systems where wet strength is enhanced for storage and use, but not in the sewage system. In most of the systems, there seems to be a compromise where the performance has to be sacrificed to achieve flushability. Some of the approaches have been to employ water-sensitive binders with salts, which enhance the solubility of the binder in the solution. For wet wipes, other alternatives suggested are to use a system where the wipes remain dry till they are ready to use, wherein a wet additive is incorporated just as it is being dispensed. Another suggestion is, possibly, to modify the toilets and flushing systems to handle new materials, where the breakdown is accelerated either by additional chemical or mechanical action [51]. When one looks at all the available materials and processing technologies, air laid and wet laid systems are more suitable. The problem with spunbond types is the difficulty of breaking down continuous fibers; using short fibers to form webs and binding them with fibers that are biodegradable or water soluble will be the best approach. There is a lot of patent activity indicating interest and inclination in the industry to develop such products. One such example is the introduction of a new biodegradable polymer Nodax® (a polyhydroxyalkanoate), which may be used for flushable nonwoven products, and other biodegradable nonwoven materials as well.

10.10 Leading producers of nonwovens There are many companies both big and small involved in various aspects of nonwovens, from producing polymers, fibers, additives, making machinery, producing nonwoven roll goods and converting the nonwovens into final products. Table 10.6 shows companies that are leading producers of nonwovens, and although there may be slight shift in the ranking of a few companies over the years, overall these companies continue to be major players [52].

10.11 Sources of further information and advice As the nonwovens industry is continuing to grow, there is a lot of new information available with new materials and products coming into the market continuously. It is advised that the readers refer to some of the valuable resources for latest updates. Some of the important organizations and other resources are listed below with their url information.


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Table 10.6 Leading producers of nonwovens [52] Company

Worldwide sales in 2003 (millions USD)

Freudenberg DuPont Kimberly-Clark BBA Group PGI Ahlstrom Johns Manville Colbond Buckeye Technologies Japan Vilene Asahi Kasei Hollingsworth & Vose Lohman Foss Manufacturing British Vita

1,400 1,200 925 900 730 728 500 250 217 185 167 165 158 157 154

10.11.1 Important organizations Association of the Nonwovens Fabrics Industry, Cary, NC, USA – INDA (www.inda.org) European Disposables and Nonwovens Association – EDANA (www.edana.org) Technical Association of the Pulp, Paper and Converting Industry – TAPPI (www.tappi.org) China Nonwovens Technical Association – CNTA (www.cnta.org) Technical Textiles and Nonwovens Association – TTNA (www.ttna.com.au)

10.11.2 Prominent university centers Nonwovens Cooperative Research Center, North Carolina State University, Raleigh, NC – NCRC (www.tx.ncsu.edu/ncrc) Textiles and Nonwovens Development Center, University of Tennessee, Knoxville – TANDEC (web.utk.edu/~tandec) Nonwovens Research Group, Department of Textiles, Leeds, UK – NRG (www.nonwovens.leeds.ac.uk)

10.11.3 Books and other publications Nonwovens Handbook, edited by Russell, S. Woodhead Publishers, 2005. Nonwoven Textiles, by Jirsak, O. and Wadsworth, L.C. Carolina Academic Press, Durham NC, 1999. Nonwoven Fabrics – Raw Materials, Manufacture, Applications,

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Characteristics, Testing Processes, by Albrecht, W., Fuchs, H. and Kittelmann, W. Wiley-VCH, Weinheim, 2003. Nonwoven Bonded Fabrics, by Lunenschloss, J. and Albrecht, W., Ellis Horwood Limited (John Wiley), 1985. TANDEC Conference Proceedings. Asia: Nonwovens Factbook and Dictionary. Nonwovens Market International Company Profiles. Nonwovens Market International Factbook & Directory. Publications from INDA, TAPPI and EDANA.

10.11.4 Research and trade journals and useful websites International Nonwovens Journal – ww.inda.org Nonwovens Industry – www.nonwovens-industry.com Nonwovens World – www.marketingtechnologyservice.com/publications Nonwovens information and business network – www.nonwovens.com Nonwovens Report International – www.nonwovens-report.com

10.12 References 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16.

http://www.inda.org/category/nwn_index.html Tom, H., What are Nonwovens – Again?, Nonwoven Industry, March 1989. Turbak, A.F., Introduction to Nonwovens, TAPPI Press, Atlanta, GA 1998. Nonwovens Handbook, INDA Publications, Cary, NC, 2004. http://www.the-infoshop.com/study/fd16116_nonwovens.html Woodings, C., ‘New Developments in Biodegradable Nonwovens’, jttp:// www.technical.net/NF/NF3/biodegradable.htm Hansen, S.M., Nonwoven Engineering Principles, Nonwovens – Theory, Process, Performance and Testing’, edited by Turbak, A.F., TAPPI Press, Atlanta, GA, 1993. Van Roekel, Jr, G.J. and De Jong, E. (ed.), ‘Elementary Flax Fibers for Disposable Nonwovens’, TAPPI Pulping Conference, 1999 Proceedings, 31 October – 4 November, 1999, Rance Orlando, Orlando, Florida, 677–682. Bhat, G.S. and Malkan, S.R., ‘Extruded Continuous Filament Nonwovens: Advances in Scientific Aspects’, J. of Appl. Polym. Sci., Vol. 83, 572–585, 2002. Lunenschloss, J. and Albrecht, W., Nonwoven Bonded Fabrics, Ellis Horwood Limited, 1985, 317. Poter, K., Encyclopedia of Chemical Technology, 3rd edn, 16, 72–104, 1976. Smorada, R.L., ‘Encyclopedia of Polymer Science and Engineering, New York, Vol. 10, 227–253, 1985. Watzl, A., ‘Growth and Prospects for Hydroentangled Nonwovens’, Nonwovens World, 13, #5, 55–58, 2004. http://www.apparelsearch.com/Apparel_Search_2.htm Dharmadhikary, R.K., Gilmore, T.F., Davis, H.A. and Batra, S.K., ‘Thermal Bonding of Nonwoven Fabrics’, Textile Progress, 26 (2), 1995. Robson, D. and Hague, J., ‘A Comparison of Wood and Plant Fibre Properties’, in Wood Fibre Plastics Conference Proceedings, Madison WI, 41–46, 1995.


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17. Bhat, G.S., ‘Overview of Cotton-Based Nonwovens’, Beltwide Conference Proceedings, New Orleans, LA, January 2005. 18. ‘Cotton for Nonwovens: A Technical Guide’, Cotton Incorporated, http:// www.cottoninc.com/Nonwovens 19. Wubbe, E., ‘Harvesting the Benefits of Natural Fibers’, http://www.nonwovensindustry.com/June021.htm 20. Duckett, K.E., Bhat, G.S. and Suh, H., ‘Compostable Nonwovens from Cotton/ Cellulose Acetate Blends’, Proceedings of the 1995 TAPPI Nonwovens Conference, 1995. 21. Suh, H., Duckett, K.E. and Bhat, G.S., ‘Biodegradable and Tensile Properties of Cotton/Cellulose Acetate Nonwovens’, Textile Research Journal, 66 (4), 230–237 1996. 22. Bhat, G.S., Duckett, K.E., Heismeyer, G.M., Gao, X. and Mac McLean, ‘Processing and Properties of Cotton-Based Nonwovens’, Proceedings of the 9th TANDEC Conference 1999. 23. Duckett, K.E., Bhat, G.S. and Hagen, S., ‘Compostable/Biodegradable Nonwovens’, US Patent 5,783,505 (issued July 21, 1998). 24. Duckett, K.E., Bhat, G.S., Giao, X. and Haoming, R., ‘Characterization of Cotton/ Cellulose Acetate Nonwovens of Untreated and Aqueous Pretreated Webs prior to Thermal Bonding’, Proceedings of the INTC 2000. 25. Gao, X., Duckett, K.E. Bhat, G.S. and Rong, H., ‘Effects of Water Treatment on Processing and Properties of Cotton/Cellulose Acetate Nonwovens’, International Nonwovens Journal, 10(2), 21–25, 2001. 26. Bhat, G.S. Rong, H.M. and Mclean, M., ‘Biodegradable/Compostable Nonwovens from Cotton-based Compositions’, INTC 2003 Proceedings, Baltimore, Maryland, September 15–18, 2003. 27. Rong, H.M., ‘Structure and Properties of Cotton-based Biodegradable/Compostable Nonwovens’, Thesis, University of Tennessee, 2004. 28. Rong, H.M. and Bhat, G.S., ‘Preparation and Properties of Cotton-Eastar Nonwovens’, Int. Nonwovens J., 12(2), 53–57, 2003. 29. Rong, H.M. and Bhat, G.S., ‘Preparation and Properties of Cotton-Eastar Biodegradable/ compostable Nonwovens’, Proc. Nonwovens Conference Beltwide 2003, Nashville, TN, January 6–10, 2003. 30. Haile, W.A., Tincher, M.E. and Williams, F.W., ‘New Biodegradable Copolyester for Fibers and Nonwovens’, Proceedings of INTC 2001 Conference, 2001. 31. ‘Nonwovens made from Wool’, http://www.canesis.com/annualreports/ 2003_textile_highlights.shtm 32. www.naturalnonwovens.com/pr.htm 33. http://static.highbeam.comn/nonwovensindustry/August012000 34. http://www.xlwf-gz.com/english/chanpin/new/jiakezhi.htm 35. Ye, W. and Broughton, R, Jr, ‘Chicken Feather as a Fibre Source for Nonwoven Insulation’, International Nonwovens Journal, Vol. 8, #1, 112–120, 1999. 36. Haile, W.A., Tincher, M.E. and Williams, F.W., ‘A Biodegradable Copolyester for Binder Fibers in Nonwovens’, Proceedings of Insight 2001 International Conference, 2001. 37. Wadsworth, L. et al., ‘Melt Processing of PLA Resin into Nonwovens’, 3rd Annual TANDEC Conference, Knoxville, 1993. 38. Ehret, P., ‘Deposa Nonwovens: Deposable disposables’, INSIGHT 96, San Antonio. 39. US Patent 5,702,826, ‘Laminated nonwoven webs derived from polymers of lactic acid and process for producing’, Ehret, P. et al., 1997, assigned to Fibreweb.

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40. JP Patent 11,117,164, ‘Biodegradable nonwoven laminates of melt-blown nonwoven fabrics of aliphatic polyester fibers and spunbonded nonwoven fabrics of urethane bond-containing butylene succinate copolymer fibers’, Kawano, Akitaka; Kin, Kasue, 1999. 41. JP Patent 2003,268,691, ‘Wet-laid nonwoven fabrics comprising biodegradable fibers consisting of biodegradable polymers derived from sources other than wood and petroleum’, Nakahara, Makoto, 2003. 42. JP Patent 2002345541, ‘Biodegradable, dust-capturing hair caps’, Takahashi, Masanori, 2002. 43. WO Patent 2002053376, ‘Breathable, biodegradable/compostable laminate for disposable personal care product’, Tsai, Fu-Jya Daniel; Balogh, Bridget A, 2002. 44. JP Patent 2002035037, ‘Biodegradable sanitary products containing galactomannanbased water absorbents’, Kawanaka, Satoshi; Ueda, Atsuko; Miyake, Munehiro, 2002. 45. http://www.permathene.co.nz/htm/straw&coir.htm 46. Mueller, Dieter, H., ‘Biodegradable nonwovens – natural and polymer fibers, technology, properties’, INTC 2003 Proceedings, Baltimore, Maryland, September 15–18, 2003. 47. Parikh, D.V. and Calamari, T.A., ‘Performance of Nonwoven Cellulosic Composites for Automotive Interiors’, Int. Nonwovens J., 9(2), 83–85, 2000. 48. Yachmenev, V.G., Parikh, D.V. and Calamari, T.A. Jr, ‘Thermal Insulation Properties of Biodegradable, Cellulosic-based Nonwoven Composites for Automotive Application’, Journal of Industrial Textiles, 31(4), 283–296, 2002. 49. JP Patent 2000034657, ‘Biodegradable nonwoven fabric filtering material for sink drain’, Matsunaga, Mamiko; Matsunaga, Atsushi, 2000. 50. JP Patent 2003299924, ‘Biodegradable pleated filter material and filter unit for air purification and liquid filtration’, Omori, Taira, 2003. 51. Woodings, C., ‘Flushability’, http://www.nonwovens.co.uk/reports/flushability.htm 52. http://www.nonwovens-industry.com/sept042.htm

11 Natural geotextiles C L A W R E N C E, University of Leeds, and B C O L L I E R, University of Tennessee, USA

11.1

Introduction

Geotextiles is a compound word, ‘geo’ and ‘textile’, which means fabrics used in association with soils during ground engineering. Natural geotextiles is a shortening of the phrase ‘natural-fibre geotextiles’, meaning, therefore, textiles that are made from natural fibres and used in association with soils. It is believed that mankind’s cultural history began around 10,000 to 12,000 years ago [1], when nomadic tribes developed from living the forager– hunter existence, following the natural migration of wild herds, to establishing early farming practices of domesticating animals and cultivating plants. Besides being used for fuel in cooking and keeping warm, fibrous materials would have then found particular applications, not only in the building of shelters and fences, but also in the construction of pathways, where soft soils would have been reinforced with logs of wood, branches hand woven together, bamboo, straw, reeds and stones. It could well be argued that the use of bamboo, straw and reeds woven into sheet materials was one early application of primitive forms of natural geotextiles. It is believed that the biblical ‘tower of Babel’ was built on a substrate, reinforced by woven river reeds [2]. Records also show that as far back as the fifth millennium BC, the then Persians used fibrous sheet materials for soil reinforcement to increase the stability and longevity of pathways [3]. The construction of reinforced clay installed along the banks of the Tigris and Euphrates occurred in the third millennium BC. Some 3,000 years ago in ancient Egypt straw was also used to reinforce clay for building walls and footpaths. Hemp is one fibre type used today for producing modern natural geotextile products; this plant is considered native to Central Asia and was subsequently introduced into China where for over 4,500 years it was grown for a wide range of end-uses including geotechnical applications. Thus, in their various primitive forms, natural geotextiles existed for many years. The systematic application of geotextiles began around the late 1950s, largely with the development of synthetic fibres. Initially, woven fabrics 343

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were mainly employed in the construction of river and waterway bankings to prevent erosion of sandy soils behind stone barriers. In the mid-1960s other textile materials, referred to as nonwoven fabrics, became important and widened the end-uses for geotextiles. Today, for many geotechnical applications, these various textile forms are employed in combination with plastic sheet materials, termed geomembranes, in particular high density polyethylene (HDPE). The majority of textile fibres utilised are synthetics, mostly poly(propylene) (PP), poly(ethylene) (PE), and polyester (PET). Thus, the term geosynthetics is used to distinguish between materials made from synthetic fibres and polymer sheets and those made from natural fibres. The biodegradability of natural fibres has led to a resurgence of their use in geotechnical applications where non-permanent man-made structures are needed to temporarily protect and enable the natural growth of local foliage in land and waterway reclamation, restoration or development. Natural geotextiles are being widely used for short-term (6-months to 10-year) applications where biodegradability is a positive attribute. This chapter describes the types of natural fibres that are used for biodegradable geotextiles, the important properties of these fibres, the process and structures of the textile forms into which they are converted, and most importantly their geotechnical end-uses. First, however, it is necessary to understand what geotextiles are, and also their primary functions.

11.2

Fundamental aspects of geotextiles

11.2.1 Definition and terminology Geotextiles are permeable textiles used with foundation soil, rock, earth or other geotechnical engineering related materials as an integral part of a manmade product, structure or system [4]; they are made from both natural and synthetic fibres. In contrast to geotextiles, geomembranes are synthetic liners which are impermeable to both fluids and particles [5]. Between these two extremes are grids, nets and mesh structures, and laminated combinations of the various groups that are called geocomposites. Since here we are only concerned with natural geotextiles, readers interested in the wider subject of flexible geomaterials are referred to references [6,7].

11.2.2 Functions of geotextiles Geotextiles essentially have five functions: 1. Fluid transmission: a geotextile can be constructed to enable a liquid or a gas to flow, usually lengthways, within the plane of the fabric. A typical example is the use of a geotextile to drain excess water in soils to reduce

Natural geotextiles

2.

3.

4.

5.

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the pore water pressure and increase the loading capacity or rigidity of the soil, i.e. the compressive modulus. The important property of a geotextile for fluid transmission is its in-plane water permeability, more technically referred to as its transmissivity. Filtration: geotextiles may be used as filters by permitting liquids to flow through the plane of the structure (the flow is assumed normal to the fabric surface), whilst preventing soil particles from being carried away by the liquid flow, i.e. stopping liquid transmission of particles through the fabric. The through-plane permeability, more technically termed the permittivity, and the pore size characteristics are indicators of a fabric’s effectiveness as a geotechnical filter. Reinforcement: compacted soils and aggregates have good compressive modulus but poor tensile modulus and consequently can be readily separated when subjected to sizeable tensile loads. Employing fibres of appropriate tensile moduli, geotextiles become effective reinforcing tension elements when embedded in compacted soils and aggregates. Separation: placed between two layers of soil or granular material of differing particle sizes, a geotextile with suitable pore size characteristics would form a barrier to the migration of particles from one layer to another. Protection: a geotextile can be constructed to have good resilience and thermal insulation. Its compressive properties are often utilised in puncture protection of membranes by placing the geotextile as an intermediate layer between, say, a stony soil base and a membrane covering. When used in the control of surface soil erosion a geotextile, in addition to other functions, provides protection to seedlings until the vegetation becomes established.

Generally in any given application, a geotextile performs one or more of the functions described, and illustrations are given for natural geotextiles later on in this chapter.

11.3

Fibres used for natural geotextile products

The fibres used for natural geotextile products are plant or vegetable fibres, although some research and development studies have considered the use of very low grade sheep’s wool and wool waste. Figure 11.1 shows a classification of well known natural fibres. However, the only ones that meet with the technical requirements, cost effectively, are the bast fibres: jute, hemp, kenaf and flax (also called soft fibres because they are from the softer region of the plants), and the hard (or leaf) and fruit fibres: sisal and coir. Others of the remaining fibres, e.g. wood fibres, are sometimes used as fillers within the textile structure.

346

Biodegradable and sustainable fibres Categories Seed

Bast (soft fibres)

Hard

Fruit

Wood

Fibre types cotton kapok ochroma milkweed akund floss

jute hemp kenaf flax ramie

Coir

sisal yucca pineapple manila hemp

11.1 Classification of natural organic fibres.

11.3.1 Morphology All plant species are built up of cells and when such cells are very long in relation to their width they tend to form fibres. The cells of the fibres used in natural geotextiles are like microscopic tubes, having a thick wall surrounding a central void termed a lumen. Figure 11.2 illustrates some cross-sections of natural geotextile fibres and the microscopic tubular shape of the cell with lumen centres can be seen, particularly in respect of the bast fibres. Strictly, the cells are polygonal in outline having sharply defined angles, depending on fibre type; for example, jute and kenaf compared with flax and hemp are more angular. Table 11.1 gives examples of cell and fibre dimensions. From Fig. 11.2 and the data in Table 11.1, it can be understood that fibres comprise aggregates or bundles of cells held together by median layers of lignin, the Lumen

Flax

Longitudinal section of jute

Kenaf

Median layers of lignin

Jute

Hemp

Interruption in lumen Longitudinal section of kenaf

11.2 Illustration of the cross-sectional features of bast fibres.

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347

Table 11.1 Fibre dimensions Fiber type

Cell width (m)

Cell length (mm)

Mean aspect ratio

Jute Hemp Kenaf Flax Sisal Coir

20–25 16–50 14–33 11–20 10–48 12–24

1.3–4.8 4.8–54.9 2.0–6.1 10.9–37.8 1.0–4.8 0.3–1.0

90 1000 200 1000 100 30

Fibre width (m)

10–100 4–600

Fibre length (mm) 1500–3600 1000–4000 1500–1800 250–1200 150–300

fibre lengths being formed by overlapping cell lengths. The number of bast cells in an aggregate governs the fibre thickness or diameter and may vary from 3 to 20 depending on fibre type. However, the dimensions of the fibres are not only dependent on the cell dimensions, but also on the processes used to extract the fibres from the original plant and convert them into a useable form. These processes are described later. Fibres can be processed to be finer and shorter than the figures given in Table 11.1, so as to meet end-use requirements. The ultimate fineness and length would approximate closely the cell dimensions, suitable as papermaking pulp. Thus, the average ultimate fibre length of flax would be 25 to 35 mm; jute, on the other hand, would be 2 to 3 mm. Generally, raw materials of finer and longer fibres, with narrow fineness and length distributions, are more beneficial to the manufacturing processes used to produce textile fabrics, and also the overall quality of the fabrics produced is better [8, 9, 10]. For example, the finer and longer the fibres are, the finer and stronger the yarns that can be spun to produce lighter and/or stronger woven fabrics. This is because, in addition to the twist inserted to make a yarn, the number of fibres in the yarn cross-section and the overlapping lengths of such fibres govern the yarn tensile properties [8]. Thus, the monetary value of a given raw material is determined by the dimensions and uniformity of the mass of fibre extracted from the plants. Where fibres are used to make a nonwoven sheet or mat, then for a given fibre type, coarser and therefore cheaper fibres may be more cost effective.

11.3.2 Chemical and physical properties The fibres used for natural geotextiles are effectively cellulose reinforced materials as their physical structures comprise cellulosic microfibrils, running along the fibre cell length, contained in an amorphous matrix of hemicellulose and/or lignin; from the viewpoint of their chemical grouping, they are called lignocellulosic fibres. The cellulosic polymer structure and the hydrogen bonds linking the polymer chains give strength and stiffness to the fibres.

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The lignin and hemicellulose polymers are responsible for such physical and chemical properties as biodegradability, flammability, sensitivity towards moisture, thermoplasticity, and degradability by UV-light [11]. (See Table 11.2). Table 11.2 Chemical constituents controlling fibre physical properties [11] Constituent

Physical properties

Cellulose

Tensile Moisture absorption

Hemicellulose

Biodegradability Thermal degradability Moisture absorption

Lignin

Thermal stability UV degradability

Source: ref. 11

Table 11.3 gives the relative proportions of the chemical constituents for each type of natural fibre commonly used for geotextiles. These fibres can have moisture contents of up to 20% by weight. The constituents of the fibres determine the interaction between the geotextiles and their immediate environment and therefore the longevity of the geotextiles will depend on the amount of cellulose and lignin present in the constituent fibres; the greater the amount, the higher the endurance. Coir has the highest resistance to biodegradation. Table 11.3 Comparative chemical composition Fibre

Cellulose (%)

Hemicellulose (%)

Lignin (%)

Bast fibres jute hemp kenaf flax

70–75 68–85 60.8 70–78

12–15 10–17 20.3 9–10

10–15 10.6 11 3–8

Hard fibres Sisal

73

11

Fruit fibre Coir

35–45

40–45

Source: Danforth International and TAPPI

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349

Figure 11.3 depicts the tensile behaviour of some commonly known natural and man-made fibres. The bast fibres (flax and ramie) show that a high specific stress is required just to obtain a relatively small strain, whereas wool, say, extends readily with much lower specific stress. The bast fibres show no yield point and brittle fracture. Wool in contrast shows a definite yield point. Clearly the higher resistance to extension is useful for temporary reinforcement of soils, where the intention is for vegetation growth to ultimately replace this reinforcement function of a geotextile embedded in the soil. Until the vegetation is established, the geotextile will prevent significant movement of the soil under load; for example, slippage of newly laid soil on a slope. An important property in such applications is the initial modulus (Emodulus) of the natural fibre, and Table 11.4 enables a comparison to be made with E-glass, which is widely used as a reinforcing fibre for industrial applications. It is evident that excepting for coir, the tensile properties of the other fibres used in geotechnical applications compare favourably with those of glass, in particular their specific modulus. This means that for the same fabric structure and mass per unit area, these natural fibres should be at least as good as glass as reinforcing materials, in fact flax would appear to outperform glass. However, the usefulness of these fibre properties will depend on the effectiveness of the interfacial bonding between the reinforcing fibre and the matrix. For soils, the moisture regain of a fibre is a useful property for initiating good interfacial bonding, and where restoration of vegetation is also a requirement, retention of moisture by the geotextile is advantageous. Flax Ramie Durafil

Specific stress (g wt/tex)

50

St Vincent cotton 40

Nylon Silk

Uppers cotton

30 Fibro 20

Viscose rayon Wool

Acetate rayon 10

Lanital

0

5

10

15

20 25 Strain (%)

30

35

11.3 Examples of stress strain behaviour of natural and manufactured fibres.

40

350

Property

Density (g cm–3) E-modulus (GPa) Specific modulus (N/tex) Tensile strength* (MPa) Specific strength (cN/tex) Elongation at failure (%) Moisture absorption (% by wt) Price/kg ($), raw

Fibre type E-glass

kenaf

2.55

1.4

flax

hemp

jute

ramie

coir

sisal

cotton

1.4

1.48

1.46

1.5

1.25

1.33

1.51

60–80 43–57

70 47

10–30 7–21

44 29

6 5

38 29

12 8

800–1500

550–900

400–800

500

220

600–700

400

57–107

37–61

27–55

33

18

45–53

27

3

1.2–1.6

1.6

1.8

2

15–25

2–3

3–10

–

7

8

12

12–17

10

11

8–25

1.3

0.5–1.5

0.6–1.8

0.35

1.5–2.5

0.25–0.5

0.6–0.7

1.5–2.2

73 29

53 38

2400

930

94

66

*Influenced by number of cells in cross-section of industrial fibre


Table 11.4 Comparison of fibre properties and estimated cost

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351

The data in Table 11.4 show that moisture absorption (regain [12]) of the fibres concerned is within the measured range for various varieties of cotton. In this respect, ramie would technically seem to be a useful fibre for geotechnical application. However, its material cost would appear to preclude it from such end-uses.

11.4

Fibre extraction and preparation

The fibres that were described above as being used for natural geotextiles, are usually extracted from four varieties of the woody-stemmed herbaceous dicotyledons (i.e. the bast fibres flax, jute, hemp and kenaf), one of the monocotyledonous plants (i.e. sisal) and one of Palmaceae (palms) (i.e. coir) [13, 14].

11.4.1 The bast fibres Figure 11.4 illustrates the typical cross-section of annually grown dicotyledonous plants. Bast fibres are obtained from the fibrovascular bundle region, known as the bast (or phloem), located between the bark (or epidermis) and the hurd (or pith). Each plant is made up of approximately 30% bast and 60% hurd; the bark, cortex and cambium accounting for the remaining 10% [15]. As described earlier, a bast fibre comprises overlapping bast (or sclerechymous) cells, thereby effectively forming a continuous filament. These fibre-containing plants have their own distinctive attributes and Table 11.5 gives a short account of these, also the principal producing countries and the relative percentages of the global production of natural organic fibres used commercially. All bast fibres are extracted from their plants by a retting process which frees the fibre from the hurd and is then followed by scutching to separate and remove the fibres from the bark and hurd. Retting is the bacterial decomposing of the natural glues that adhere the bast fibre to the hurd. Traditionally this is accomplished in one of two ways; either dew-retting or water retting. With the former, the swath of stem material, after mechanical harvesting, is left for about 4–6 weeks in the field for dew and rainfall to affect the process; however, prolonged excessively wet conditions can turn retting to rotting. Owing to the vagaries of weather and the need to speed up the process water retting was developed. Here, the sheaves of cut plants are immersed, root downwards, into tanks and covered. The water is kept at approximately 35∞C and circulated through the mass of material. After retting is completed the sheaves are removed, drained and left to dry in the field, termed ‘gassing’. When the crop is dried to less than 10% moisture content it can be stored ready for scutching. It is claimed that water retting produces a more uniform and higher quality fibre but the process is

352


Epidermis: thin outer protective Bast (phloem): layer containing bast fibres Pith (hurd): layer of thick woody tissues

Cortex: thin layer of wall cells containing chlorophyll but no fibre Cambium: growth area – produces hurds on inside and bast on outside

Hollow core

11.4 Morphology of bast fibre plant.

time consuming and costly and can pollute the body of water being used for the process. Various newer technologies have been developed to speed up the process of fibre extraction such as chemical retting, using glyphosate and Diquat, and mechanical retting or decortication which mechanically separates the bast from the hurd without the additional scutching. Steam explosion, detergents and ultrasound are other methods being tried. Scutching is a process in which the retted plant is separated or ‘transformed’ into its basic parts: the hurd and the bast fibre. While transforming the plant, the fibres are kept at full length so at the end they can be cut to the length required for further processing (i.e. length needed for paper making, spinning/ weaving, or nonwovens used in composites and geotextiles). The scutching operation usually involves four stages: rough scutching, shaking, fine scutching, and cutting and baling [16]. In the first stage of the process line, layers of the retted plant are presented via feed rolls to a

Table 11.5 Fibre-containing dicotyledonous plants Dicotyledon species

Principal producing countries

Attributes

Percentage of global natural organic fibre production (%)**

Flax

Linum usitatissimum

China, France, Romania, Belarus, Netherlands

Plant reaches 1 m in height [2] with a slender stem and sky blue or white flowers

3.2

Jute

Corchorus capsularis China, India, Bangladesh (also called Jew’s mallow)

Plants range from 9–19 mm in diameter and 1.5–3.7 m in height, with small yellow flowers

14

Hemp*

Cannabis sativa

Russia, Ukraine, Poland, Hungary, Yugoslavia, Romania, France, Italy, UK, China, Germany, Africa, Canada

The plant is cultivated similar to flax and reaches a height of 1.2–3.7 m with yellowish green flowers

0.09

Kenaf

Hibiscus cannabinus

Indigenous to Africa, but also grown in India, China, The Commonwealth of Independent States, Iran, Thailand and the USA

Mature plants grow to 2.5–6 m in height with yellowish flowers

*Conventional plant breeding has enabled the virtual elimination of the psychoactive drug THC, tetrahydrocannabinol, from the genus **Cotton accounts for 71.8%, wool 8.10%, silk 0.04% and ramie 0.04%

Natural geotextiles

Fibre type

353

354


rotating drum with spring steel blades mounted on its outer circumference. The circumference of the feed rollers is profiled to crimp the plant along its length, thereby breaking up the hurd without damaging the fibres. Then, as the material passes the steel blades while moving with the rotating drum, the pieces of broken-up hurd (or shives) are scraped from the fibres; on the whole fibres suffer little damage. The second stage involves the use of shaking screens which, as the name implies, shake up the material while transporting it to the third stage with the result that loose shives fall out from the fibres. At the third stage, a similar scutching unit to the first stage is used. The steel blades are, however, set closer to the drum, so that now the finer fragments can be scrapped from the fibres. The cleaned fibres would still retain their full lengths, so that at the final stage they can be cut to size, as required. After cutting, the fibres are baled by an automatic hydraulic baling press ready for the subsequent manufacturing processes. Grading of the bast fibres, as well as of sisal and coir, is well explained in the cited literature [12, 13, 14]. However, in general, there are three classes of fibre [17]: 1. Primary fibres: these are long and low in lignin and are the strongest and most valuable. 2. Secondary fibres: medium in length and higher in lignin. 3. Very short fibres: this type of fibre is often referred to as tow. The primary class is seen also as the ultra-cleaned material. Cut lengths can be from 13 mm to 152 mm long, and can be suitably converted into nonwovens, woven textiles and composites products for the automotive, furniture, and construction industries. The secondary and very short fibre classes are general purpose grades having 50–70% cleaned fibre. These are used for hydro mulch, as cement fillers, in insulation and for geo-matting.

11.4.2 Hard fibres Hard fibres (also termed leaf and structural fibres) are larger and stiffer than bast fibres, hence the name. They grow throughout the leaves or stem of monocotyledonous plants and like the bast fibres they give rigidity to the plant and also transport water and plant food from one part of the plant to another. Sisal (Agave sisalana) is the most important species of hard fibre and is used for natural geotextiles as well as other industrial products. It is grown in Java, Africa and Haiti and accounts for about 1.5% of the world total natural fibre production. The leaves are harvested and subjected to a decortication process in which the epidermis and pulp are scraped from the fibre whilst simultaneously being washed. The resulting fibre mass is then dried and baled.

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355

11.4.3 Fruit fibres Coir fibres (Cocus nucifera) are obtained from the shells of coconuts. Traditionally, the nuts are firstly soaked in sea water, which softens the husks, after which they are manually beaten and washed with fresh water. The residual reddish brown fibrous mass is decorticated by tearing and hackling it into fibres, about 250 mm in length. The industrial process involves initially splitting the fruit into quarters and weighing down these parts into large holding tanks of water for around 5 days. The husks are then removed and run through a machine with corrugated crushing rollers, called a ‘breaker’. The crushed fibrous mass is passed to a second stage where two machines in sequence, called ‘drums’, separate the fibres from the woody husks. The drums, 1 m diameter rollers ¥ 355 mm width, are studded with spikes. The husks are held against these rotating drums enabling the spikes to remove the woody part, leaving the fibres to pass through the process line. The first drum would have the spikes spaced to give a coarse treatment, i.e. removal of larger broken pieces of husks, whilst the spikes of the second drum are more closely spaced for removing the finer husk fragments. The fibres are subsequently washed and dried and further hackled by combing with steel spikes and converted into hanks of tows. The tows are baled by a hydraulic press ready for transport to the spinning and weaving mill or the nonwoven plant.

11.5

Production of natural geotextile products

Natural geotextile products are usually manufactured in the form of a nonwoven matting (felt or mattress), or as a woven sheet of fabric or netting, and also as combinations of woven and nonwoven. For certain requirements a woven net may be converted into a cylindrical mesh which is then stuffed with fibres to form a geotextiles log. These logs or rolls are often used for erosion control of river embankments, as they provide initial structural stability by resisting wave action and flow velocity; also young seedlings may be planted in them for restoration of river-side vegetation. By slowing the water flow near the banking, sediment is deposited in and around these rolls to create an environment for vegetation growth. Having the slowest rate of biodegradation, coir is a suitable fibre from which to produce such bio rolls or bio logs. The process technologies for producing nonwoven and woven fabric are already well described in the widely available technical literature on textile technology or textile engineering. It is therefore appropriate to restrict our considerations in this chapter to a general overview for the reader unfamiliar with the subject area, while citing suitable references for further study.

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11.5.1 Production of nonwoven mattings A nonwoven is a textile structure in the form of a flexible sheet that principally consists of a combination of several layered webs of fibres, which is given cohesion by some means of bonding the fibres together [18, 19, 20]. For the vast majority of natural geotextile mattings, bonding is achieved through mechanical entanglement of the constituent fibres comprising the structure, thereby utilising inter-fibre friction to obtain the cohesion. Figure 11.5 illustrates the process sequence for producing natural geotextile mattings. Following the opening of fibre bales, a web of fibres is produced at an early stage in the process line known as carding. The card consists of a series of paired large and small rotating rollers; each covered with sharp points, the purpose of which is to separate the fibre mass into individual Process sequence Fibre opening

Fibre opening

Fibre path

Weigh pan Feed lattice Conveyor lattice

Feed lattice

Carding

Carding

Cross-lapping (not shown) Output of fibre web sheet

Fiber mass feed Needle Needling

Needling

Layered webs Nowoven matting Barb on needle for pulling fibres

11.5 Illustration of the process stages for nonwoven matting.

Natural geotextiles

357

fibres so as to produce the web of fibres at the output section of the card. A subsequent stage, termed cross-lapping, forms the layered structure with the fibre web. Fibre entanglement is achieved by rows of long barbed needles repeatedly penetrating the thickness of the layered webs, and in so doing, each needle pulls fibre lengths from an upper layer through the layers beneath, with the ends of these lengths finally protruding out of the bottom surface of the sheet. This final stage is known as needle punching or needling, and is performed on what is called a needle-loom. Needling not only gives cohesion and strength to the nonwoven, but also decreases the fabric thickness and increases its density through greater compaction of the fibres. Thus, the degree of needling, specified as punches per unit area, along with fibre length and fineness, and the mass per unit volume of the cross-laid intermediate are important process parameters. Needling can be accomplished from both top and bottom surfaces of the layered fibre mass to give greater cohesion, compaction and stiffness to the resulting fabric. If the nonwoven geotextile is to be used as a mulch,1 then plant seeds can be introduced into the fibrous web prior to needling. The fabric would thereby act as a vehicle for the distribution of seeds in the soil and prevent erosion during seed establishment and early plant growth. Suggested insertion rates range from 2000 to 4000 wildflower seeds per m2 for fabrics of 340 to 680 g m–2.

11.5.2 Production of woven fabrics The fibres must first be spun into yarns [21, 22]. To do so the baled material is opened and carded in a similar manner as described above, but instead of the carded web being cross-lapped, it is consolidated into the form of a twistless rope called a sliver. Usually two carding stages are used: the sliver from the first, so-called breaker card is then fed to the second, ‘finisher’, card. A two-carding operation enables gentle treatment of the fibres with effective extraction of any remaining shive fragments whilst maintaining a high production rate. Figure 11.6, which is a flowchart of the process sequence, shows that the carded sliver passes through what is termed ‘drawing’ or ‘gilling’. Often three drawing stages are used. This basically involves three machines (drawframes or gill boxes) that attenuate a group of slivers, each of the same known mass per unit length, down to one output sliver having the equivalent mass per unit length as an input sliver. Thus, if eight slivers were fed to a gill box the attenuation (i.e. ‘the draft’) would be eight. The figure shows the principle of how the gill box attenuates the material. Essentially, the output 1

A mulch is defined as ‘an application or creation of any soil cover that contributes a barrier to the exchange of heat or vapour’.

358

Biodegradable and sustainable fibres Fibre opening and carding

Gill box Top fallers

Feed rollers

Delivery rollers Gill sliver out

Vb Vc

Slivers in

Drawing

Va

Vc >> Vb > Va Fallers bed Control roller Bottom fallers Ratchet

Input sliver Back rollers Tumblers

Top front roller

Yarn spinning

Carriers Bottom front roller

Flyer spinning

Slubbing Flyer Twizzle Bobbin Washers Lifter-plate Spindle Spindle drive Weaving Drop wires

Weaving loom Back rest

Warp

Weft yarns

W ar

p

Warp beam

fel

l

Dri

ve

th

Cloth

Clo

Cloth roll

11.6 Conversion of fibre to woven fabric.

Warp yarns

Natural geotextiles

359

front rollers are made to rotate with a surface speed eight times as fast as that of the input, back rollers. The front rollers are positioned so that their distance from the back rollers (termed the ratchet) is slightly longer than the maximum fibre length. Thus, as fibres are fed towards the front rollers, the leading ends of those reaching the nip of the front rollers will have their trailing ends released from the nip of the back rollers; these fibres can be pulled away by the front rollers at eight times the speed of those still nipped by the back rollers. The action is called drafting and this causes the attenuation. The movement of the fibres during the drafting process is controlled by gill pins, hence the term ‘gilling’. In a three-stage drawing sequence, then, eight output slivers from the first drawing stage would be fed to the second, intermediate, drawing stage and eight from the second stage to the third. It is evident that at the first stage the output sliver comprises one-eighth of each input sliver, and that essentially these are combined to give the output sliver; the effect is referred to as a doubling of eight. This gives a mixing of the input slivers, so the result is termed blending by doubling. A simple calculation based on this eight-sliver feed would show that with three-stage drawing, a doubling of 512 would be obtained, giving a high degree of blending. Owing to the variation in properties between even fibres of the same type, blending is important in order to achieve consistent yarn properties. The output sliver from the third, or finisher, drawing stage is passed to the spinning frame where it is attenuated, without doubling, at a very high draft needed to attain the much lower specified mass per unit length for the yarn. As illustrated in Fig. 11.6, drafting is now carried out with two pairs of small rollers, called tumblers and carriers, controlling the fibre movement in the drafting zone, replacing the gill pins, so that high drafts can be applied effectively. Twist is inserted into the ribbon of fibres issuing from the front roller of the drafting system to form the spun yarn. The twisting mechanism widely used for spinning these plant fibres is referred to as the flyer and the action as flyer twisting or flyer spinning. The illustration in Fig. 11.6 is one example of flyer spinning. It can be seen that the newly formed length of yarn passes around one leg of the fly (mounted on a spindle), through a twizzle in the flyer leg and then onto a bobbin. As the flyer rotates twist propagates up the already formed yarn travelling down the flyer leg and into the fibre ribbon; one flyer rotation inserts one turn of twist. The spun yarn is simultaneously wound onto the bobbin through a combination of bobbin inertia and the frictional drag of the felt washers, positioned at the bottom end of the bobbins. If required, two yarns may be twisted together to achieve increased yarn strength and to produce a heavier woven fabric. The resultant yarn is called a two-fold, a doubled or a plied yarn and its linear density would be twice that of the individual component yarns.

360


Figure 11.6 also illustrates how the spun yarns are employed in weaving a fabric. One set of yarns would be used as a warp (i.e. giving the length of the fabric) and the other as the weft (i.e. giving the width). The mechanical action of looms is well reported in the cited literature [23, 24]. Here, we will therefore consider certain basic parameters of importance to the properties of woven natural geotextiles. It should be evident that from the view point of the fabric construction, the number of warp yarns per unit length across the fabric width (referred to as ends per cm) will govern the fabric strength in the length direction. Similarly, the strength in the width direction is dependent on the number of weft yarns per unit length (picks per cm). The generally used terminology is to call the number of warp yarns the number of ‘ends’ and the number of weft yarns, the number of ‘picks’. Thus, with regard to fabric construction the number of ends and picks per cm squared is of importance to the strength of the fabric. The strength of the fibres must be, of course, effectively utilised by the spun yarn. A yarn can never have the same strength as its constituent fibres because of the fact that to withstand an applied load, the yarn depends on the frictional contact between the overlapping lengths of fibres to transfer the load to all the fibres. The higher the twist the better the frictional contact and the stronger the yarn, but for each fibre type there will be an optimum twist to obtain maximum yarn strength [8]. In addition to tensile strength, as explained in Section 11.3.2, the tensile modulus of the fabric can be equally important. Contradictorily, as twist is increased to achieve better yarn strength, the effective conversion of fibre modulus to yarn modulus becomes poorer. The yarn modulus therefore decreases with twist and this will be reflected in the fabric. Another important factor is the fabric crimp.2 This is the undulations a weft yarn, say, has owing to its interlace with the warp yarns, and it would be called the weft crimp of the fabric. The crimp means the constituent yarn lengths are greater than the linear fabric dimensions. Thus, if L is the measured weft yarn length in a metre square sample of a fabric, then the increased length as a percentage of the sample width is called the percentage crimp and is calculated by equation: % crimp = (L – 1) 100

11.1

Both weft and warp crimp of a fabric are important to the fabric modulus [25], in that the higher the crimp is, the lower the modulus will be. From what has been discussed above it can be reasoned that the fabric mass per unit area will be governed by the number of ends and picks per unit area; the value of weft and warp crimp; and by the mass per unit length of 2

The waviness or distortion of a yarn that is due to interlacing in the fabric.

Natural geotextiles

361

the weft and warp yarns. The latter parameters are commonly called the yarn count. The International Standard unit for yarn count is the tex, which is the number of grams that a 1000 metre-length of yarn weighs. If Np, Cp, Nt, Ct, are the above warp and weft parameters, then the total length of the warp yarns in a metre square fabric sample is: = Np (0.01Cp + 1)

11.2

and the total length of weft yarns NtL = Nt (0.01Ct + 1)

11.3

Let Tp and Tt be the counts of the warp and weft. The mass per square metre of the fabric (i.e. the basis weight in g m–2) is equal to the sum of total lengths of warp and weft yarns, and can be calculated by Fabric Basis Weight = TpNpL + TtNtL

11.4

The main purpose of a mulch is to control the environmental factors that affect seedling survival; e.g. moisture, temperature, light, chemical presence or absence, weeds and mechanical damage. The mulch also acts as a soil insulator to keep the soil warm in the early and late part of the growing season. It is also a vapour block to suppress evaporation and a weed suppressor so that saplings can make full use of light, moisture, and nutrients. A mulch must therefore be as opaque as possible to prevent weed growth, possess good insulation characteristics and be sufficiently porous for water infiltration, yet retard water loss from underneath it. It needs to be strong and durable enough to last until seedlings are well established, usually about 3 years. The open spaces of a fabric, formed by the interlacing of the yarn, are called the interstices and it is evident that the sizes of these depend on the spacing of the picks and ends, and the yarn diameters. The importance of the interstices is that they enable soil in which the geotextile is laid to become embedded into the fabric, thereby anchoring the reinforcing fabric. If the fabric is to be used as mulch for protection against soil erosion, then the interstices help to disrupt surface water flow and enable vegetation to grow through the fabric. In practice, it is the area fraction covered by the yarns in a square metre sample of fabric that is specified and is termed the cover factor, K. The total open space of the fabric sample is then equal to 1–K. The cover factor is calculated according to K=

Dp Dp Dt D + t – Sp St Sp St

11.5

where Dp and Dt are respectively the warp and weft yarn diameters, and Sp and St the average spacing between two ends and between two picks [24].

362


The rate of moisture absorption and biodegradability is primarily dependent on the chemistry of the fibre type, as indicated by Table 11.2. However, the twist of the yarn and the basis weight of the fabric can be used as moderating factors. The higher the twist, the slower the rate of absorption, and the greater the fabric weight, the lower the rate of biodegradation.

11.6

Measurement of the properties of natural geotextiles

From the point of view of product specification and quality assurance3 the general practice is to adopt measurement procedures described in international standards (International Organisation for Standardisation – ISO) and European standards (BS EN), rather than national standards, excepting, that is, for the American Standard Test Measurements (ASTM) which, it may be argued, continue to take the lead in developing reproducible test procedures for geosynthetics, generally. The fundamental issues surrounding the use of standards for geosynthetics as a whole, including the sampling and preparation of test specimens (test standard BS EN 963: 1995), are well discussed by Koerner [6]. We will, therefore, only consider here the basics of the tests relevant to natural geotextiles in woven and nonwoven forms. Based on the US Erosion Control Technology Council (ECTC) Guidelines and on published studies of natural geotextiles, the tests listed in Table 11.6 are important ones used in evaluating such products. Table 11.6 Test methods for natural geotextiles ISO

10319: 1996*

12958:1999*

BS EN

ASTM

Description

964-1:1995 965:1995

D D D D D D D

Nominal thickness Mass per unit area Breaking force and elongation Apparent opening size Shear friction Water permeability Erosion control blanket performance in protecting hillslopes from rainfall-induced erosion Exposure to ultraviolet light and water Aerobic biodegradation under controlled composting conditions

5199 5261 5035 4751 5321 4491 6459

D 4355 D 5988

* BS EN ISO – i.e. European standard adopted by ISO

3

The verification of the conformance of materials and methods of application to the governing specifications, in order to achieve the desired result [25].

Natural geotextiles

363

11.6.1 Nominal thickness and mass per unit area The thickness of a geotextile is accepted as the calculated average of 10 measured specimens, each of 70 mm diameter taken randomly across the width of the fabric. Each specimen is, in turn, placed between two flat steel plates and the measurement made following an applied pressure of 2 kPa. Further measurements may be made at 20 and 200 kPa, and the three sets of values plotted so that the thickness under zero pressure can be extrapolated. The specimens used for the thickness measurements may be weighed and the average value used to calculate the basis weight. However, it is more common to use 100 mm diameter specimens.

11.6.2 Breaking force and elongation A basic tensile test is carried out on 200 mm wide specimens, using a rate of extension of 20% min–1. The load-extension curve enables an understanding of the likely behaviour of the woven or nonwoven material, in respect of its initial modulus.

11.6.3 Apparent opening size Several terms are used in geotechnical engineering to describe the pore size of fabrics, namely: apparent opening size (AOS), equivalent opening size (EOS) and filtration opening size (FOS). The terms are used with the symbol Ox, which refers to the pore size diameter which is greater than x% of the pore diameters contained within the fabric. Therefore O95 refers to the measured value of the ‘near largest’ pore diameter in the fabric; for the purpose of filtration it is necessary that O95 pore size enables adequate flow capacity, but yet prevents fabric penetration and migration of the majority of soil particles and minimises any tendency for clogging of pores. Consequently, the criterion for soil retention is commonly set as a ratio of the AOS to the particle diameter of D90, D85 or D50; for example, O95/D85 < 1. Koerner [6] in a review paper on the subject summarises various soil retention design criteria. The standard method used for determining O95 is referred to as the ‘dry sieving method’. The fabric is clamped in a sieve frame, attached to a shaker, and 50 g of glass beads of a known diameter are sieved for 10 mins. The quantity of beads that passes through the fabric and that have remained on top of it are weighed; the difference indicates the amount trapped within the fabric. The procedure is repeated using successively larger diameter beads, until the mass that has passed through the fabric is 5%. The related diameter is equivalent to O95.

364


11.6.4 Shear friction For situations involving soil–fabric interaction on sloping surfaces it is often necessary to determine the soil-to-fabric interfacial strength. The test instrument is an adaptation of the direct shear box used in soil friction tests [6]. This consists of two parts, an upper and a lower box. The test specimen covers the face of the lower box and is securely held in place. The upper box initially rests on top of the lower box and is filled with soil which then makes full contact with the fabric surface. A normal pressure is uniformly applied to the soil in the upper box, and whilst the upper box remains stationary, the lower one is moved horizontally, under the applied normal pressure; this generates shear forces at the soil–fabric interface. To move the box, the force applied initially increases to a peak value and then decreases to a constant level with the displacement of the lower box. By repeating the procedure using increased normal pressures, a straight-line graph of peak shear force versus normal loading (i.e. the Mohr–Columb Envelope) is plotted to obtain the apparent cohesion and the friction angle (the tan Q of the line gradient) [6].

11.6.5 Water permeability Darcy’s law is recognised as a principle governing liquid movement through a porous medium under conditions of laminar flow. The basic characteristic parameters of water permeability for geotextiles are permittivity and transmissivity, which are, respectively, the ease of flow of water, at normal angle, through the fabric surface, and the ease of flow within the plane of the fabric [6]. Water permeability is a principal property of nonwoven geotextiles used for filtration and drainage; however, when woven or nonwoven fabrics are used for mulches the permeability behaviour is less straightforward. Rainfall effects on slopes are of particular interest and the permeability of a fabric should relate to the amount of rainfall immediately available to seedlings. Performance tests may be undertaken for a 30∞ slope angle. The test apparatus consists of a regulated sprinkler head and two tanks with tops cut at a 30∞ angle, one tank collects the water that passes through the mulch specimen of dimensions 28 cm ¥ 28 cm, and is therefore covered with a 12 mm screen to support the specimen. The other tank collects the water runoff. The sprinkler head is set to a spray rate of 50 g min–1. The basis weight of the mulch is determined before and after a set duration of spraying, and the water content in the two tanks is also measured, water not accounted for is designated ‘splash’. The permeability of the mulch is then a measure of the amount of water that had passed through the fabric and would therefore have been available to seedlings; high run-off levels would indicate low water availability. Generally, most effective mulches have permeability values within the range of 25–60% water flow-through.

Natural geotextiles

365

11.6.6 Erosion control blanket performance Sediment resulting from water run-off on steep slopes can be detrimental to the free flow of waterways; most commercial natural geotextiles target this problem. One commonly accepted method for measuring the effect of a geotextile on erosion control involves a modification to the above procedure. A 1.2 m ¥ 1.2 m specimen of the geotextile is placed on a soil slope and a measured amount of artificial rain applied to the slope. Sediment and water run-off are then collected and measured. This test is best conducted in the field where site-specific conditions, such as inherent soil type, can be considered.

11.6.7 Exposure to UV radiation The UV spectrum may be subdivided into three wavelength bands: UV-A (400 to 315 nm), UV-B (315 to 280 nm) and U-C (280 to 100 nm). The radiation of the first causes a small amount of polymer degradation; that of the third is absorbed by the upper atmosphere, but the radiation of the second band is very damaging, breaking polymer bonds and weakening fibres. Tests for the resistance to UV degradation involve exposing geotextile specimens to light from a xenon-arc lamp, simulating sunshine. Specimens are exposed for 150, 300 and 500 hours and the degradation of measured properties (e.g. strength and extension) determined. The test follows 120 minute cycle of which 102 minutes is exposure under dry conditions and 18 minutes under wet conditions.

11.6.8 Biodegradation Soil contact test: here a defined sand/soil/mature compost matrix is employed to provide a consortium of mesophilic and thermophilic bacteria and fungi. Biodegradation is based on the amount of material carbon converted to gaseous carbon (CO). Materials that rapidly biodegrade can be completely tested in 30 to 60 days.

11.7 1. 2. 3. 4.

References

Bronowski, J., The Ascent of Man, Book Clun Associates, London, 1976. Coppelstone, T., World Architecture, Hamlyn, Essex, 1963, 178. Ingold, T.D., Geotextiles and Geomembranes Manual, Elsevier, Oxford, 1994, 2. ASTM D4439-87: Standard Terminology for Geotextiles, American Society for Testing and Materials, 1987. 5. Giroud, J.P. and Frobel, R.K., Geomembrane Products, Geotechnical Fabrics Report, Fall, 1983, 38–42. 6. Koerner, R.M., Designing with Geosynthetics, 4th edn, Prentice Hall, New Jersey, ISBN 0-13-726175-6, 1997.

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7. Ingold, T.D., Geotextiles and Geomembranes Manual, Elsevier, Oxford, 1994. 8. Lawrence, C.A., Fundamentals of Spun Yarn Technology, CRC Press, ISBN 1-56676821-7, 2003, 25–31. 9. Goswami, B.C., Martindale, J.G. and Scardino, F.L., Textile Yarns, John Wiley & Sons, New York, ISBN 0-471-31900-7, 1977, 17–25. 10. Robinson, A.T.C. and Marks, R., Woven Cloth Construction, Textile Institute ISBN 0-900739-04-5, 1973. 11. Nabi, Saheb, D. and Jog, J.P., Natural Fibre Polymer Composites: A Review, Advances in Polymer Technology, Vol. 18, No. 4, 1999, 351–363. 12. Morton, W.E. and Hearle, J.W.S., Physical Properties of Textile Fibres, Butterworths, London, 1962, 155. 13. Stout, H.P., Fibre and Yarn Quality in Jute Spinning, The Textile Institute, Manchester, ISBN 1-870812-09-3, 1988, 2–3. 14. Mauersberger, H.R. (ed.), Matthew’s Textile Fibres, Their Physical, Microscopical, and Chemical Properties, 5th edn, John Wiley & Sons, New York, Chapman & Hall, London, 1947, Chapters 9–11, 305–449. 15. Sellers, Terry, Miller, G.D. and Fuller, M.J., Kenaf Core as a Board Raw Material, Forest Products Journal, 1993, 43: 7–8, 69–71; 16 ref. 16. Demtec, [email protected] 17. http://www.hempandcompany.com/hemp.php http://www.kenaf.com.au/crop.html http://www.hempology.org/CURRENT%20HISTORY/ 1996%20HEMP%20COMPOSITES.html 18. McIntyre, J.E., Textile Terms and Definitions, 10th edn, ISBN-1-87012-77-8, 1997, 227–230. 19. Ramsey, N.J., Nonwovens Industry, Rodman Publications. 20. Lord, P.R., Economics, Science and Technology of Yarn Production, North Carolina State University, 1981. 21. Oxtoby, E., Spun Yarn Technology, Butterworths-Heinemann, Boston MA, 1987. 22. Lord, P.R. and Mohamed, M.H., Weaving: Conversion of Yarn to Fabric, 2nd edn, Merrow, ISBN 0-900-54178-4, 1988. 23. Mohamed, M., Weaving Technology and Woven Fabrics, Woodhead Publishing, 2005. 24. Hearle, J.W.S., Grosberg, P. and Backer, S., Structural Mechanics of Fibres, Yarns and Fabrics, Vol. 1, Chap. 3, Wiley-Interscience, New York, 148, 1969. 25. Frobel, R.K., (ed.), Geosynthetics Terminology, Industrial Fabrics Association, 1987, 87.

12 Conversion of cellulose, chitin and chitosan to filaments with simple salt solutions H S W H A N G, N A M I N U D D I N, Fiber and Polymer Science Program, USA, M F R E Y, Cornell University, USA, S M H U D S O N and J A C U C U L O, Fiber and Polymer Science Program, USA

12.1

Introduction

The biomaterials available in the largest quantities on the Earth are cellulose and chitin, being easily isolated and processed into many articles and chemical forms. The manufacture of wood pulp is a worldwide business and the conversion of shellfish waste into chemical chitin is now an established industry. These materials are closely related polysaccharides and are found in nature as structural materials; cellulose is found in the cell walls of plants; and chitin is found in the shells of marine crustaceans. These structures are indicated in Fig. 12.1. OH O

O HO

OH HO O

O OH

OH

n

(a)

CH3 O C 4

OH O

6

O HO

5 3

2

NH

3

HO O

1

NH2

4

n (b)

5 6

2

O OH

1

m

12.1 The structural comparison of chitin and chitosan to cellulose. The structure of cellulose is shown in (a). In (b), when m > n, the structure is not soluble in aqueous acid and is called chitin. When n > m, and the copolymer is soluble in aqueous acid, then it is called chitosan.

The conversion of cellulosic wood pulp into a textile fiber dates to the earliest periods of synthetic fiber manufacture; viscose rayon and lyocell processes are well known. However, it is still conceivable to improve upon 367

368


the environmental impact of these existing methods by the development of new solvent systems which are less toxic and more easily recycled. In the case of chitin, and its deacetylated form, chitosan, much more work is needed to find a truly suitable method for the manufacture of filaments from nontoxic systems and that have adequate mechanical properties. In this chapter we describe the use of simple ionic liquids based on ammonium salts with ammonia or other amino compounds, to dissolve cellulose and extrude filament. In the second section, the conversion of chitin and chitosan to filaments with other salt solutions is described. Both of these methods involve the direct dissolution of the polymer without derivatization. The solvent components include simple inorganic salts which can either be recovered and recycled back into the spinning process or used as fertilizer.

12.2

Cellulose in liquid ammonia/ammonium thiocyanate solutions

Cellulose is a natural, high molecular weight polymer. As well as being renewable and biodegradable, it is the most abundant naturally occurring organic polymer. Its b-(1 Æ 4)-glucopyranose structure favors close packing of the chains to form fibrous crystals. Due to its complex crystalline and amorphous morphology, considerable hydrogen bonding, and very high molecular weight, cellulose does not melt nor does it dissolve readily in many solvents. For this reason, cellulose has not been exploited to its fullest potential. Hence, any process which affects, simplifies or hastens the dissolution of cellulose represents a significant step forward in the development of cellulose as a viable, ecologically favorable polymer source. Furthermore, the ability of cellulose and its derivatives to form liquid crystalline solutions in certain solvents has resulted in many attempts to develop high-performance cellulosic fibers. The terms ‘liquid crystal’ and ‘mesophase’ are interchangeable. Meso, in Greek, means ‘between’, so a fluid can be called a mesophase if it has some properties that are characteristic of crystals. The ability of the fluid to form a liquid crystal is due to the molecules’ ability to align with each other and create local ordering. So, liquid crystalline polymers are those polymers that form liquid crystalline phases either in solution or in the melt. Molecules that form a mesophase are usually rod-like or disc-like. In the case of rodlike polymers, such as poly(p-phenyleneterephthalamide) (PPTA), the rigidity of the backbone is primarily responsible for the formation of a mesophase. The rigidity is, of course, dependent on a variety of factors such as the nature of the solvent used, the temperature of the solution, and the chemical structure of the molecule. Presently there are only a few solvents that can directly dissolve cellulose without involving a chemical derivative of cellulose: N-methyl morpholine-

Conversion of cellulose, chitin and chitosan to filaments

369

N-oxide/water (NMMO/H2O); lithium chloride/dimethyl acetamide (LiCl/ DMAc); trifluoroacetic acid/methylene chloride (TFA/CH2Cl2); calcium thiocyanate/water (Ca(SCN)2/H2O); ammonia/ammonium thiocyanate (NH3/ NH4SCN); zinc chloride/water (ZnCl2/H2O); and sodium hydroxide/water (NaOH/H2O). Two newly reported solvents are also of interest: one based on an ionic liquid, namely, 1-butyl-3-methylimidazolium chloride [1]; and a polar fluid/salt solvent, ethylene diamine/KSCN, discussed in Section 12.4.

12.2.1 Preparation of filament Ammonia/ammonium thiocyanate (NH3/NH4SCN) is a powerful solvent for cellulose. Scherer [2] was first to observe the dissolution of cellulose in this mixture of liquid ammonia and ammonium salt, and this solvent was later rediscovered by Hudson and Cuculo [3, 4], which effectively dissolves cellulose by taking advantage of the powerful swelling character of liquid ammonia on cellulose. Cuculo et al. [5, 6] investigated the mechanism of dissolution and observed the transformation of cellulose polymorphs during dissolution. They observed during dissolution of cellulose I, that it experienced polymorph transformation from cellulose I Æ II Æ III Æ amorphous in solution. Once in solution, the cellulose can attain the most extended chain conformation to accommodate the formation of the anisotropic phase [7]. The anisotropic phase has been shown to occur in solution with cellulose concentration above 12% (w/w). Another attribute of the solvent is that it neither reacts with nor degrades cellulose [7]. These attributes are important; this solvent system can potentially form cellulose fibers of exceptional properties via the anisotropic solution without the need for regeneration of cellulose and with no degradation of the cellulose itself. The formation of high concentration cellulose solutions in this solvent, however, may be somewhat impeded by the presence of gel which in turn affects the anisotropic phase formation. Fortunately, the gelation is thermoreversible and it has been possible to form gel-free anisotropic solutions [8]. Frey et al. [9, 10] investigated the mechanism of gelation of cellulose in this solvent. Gelation usually forms in solutions with cellulose concentration greater than 10% (w/w) and it is rapid at temperatures below the gel melting point (circa 28∞C). The cellulose gel consists of a three-dimensional fibrillar network, stabilized by hydrogen bonds which can be broken at temperatures above the gel melting point. Therefore, the anisotropic solution can be obtained gel-free at temperatures above the gel melting point. Also, the solvent can tolerate significant amounts of water without losing its dissolving power. Solvent, with addition of water, representing as much as 15% by weight of cellulose, was shown to dissolve cellulose completely. This is an interesting observation considering that water is a non-solvent for cellulose. Previous work has shown that the formation of anisotropic solutions is

370


thermodynamically driven [11], thus, the anisotropic phase formation is expected to be instantaneous with dissolution. The anisotropic phase formation of the cellulose/NH3/NH4SCN system was found to be consistent with this expectation; the anisotropic phase formed instantaneously, without any storage or aging period. Frey [12] constructed a theoretical phase diagram for this cellulose-solvent system, shown in Fig. 12.2. Solutions with cellulose concentration at 12% were observed to be birefringent. Observation under a polarizing microscope of this cellulose solution revealed isotropic regions with a few anisotropic domains. As the cellulose concentration was increased to 14% and 16%, the number of anisotropic domains increased. These observations are consistent with the predicted phase diagram; this concentration range is located within the narrow biphasic chimney region [12]. 0.20

Spinodal 2 phase/gel

Anisotropic anisotropic binodal

Isotropic anisotropic binodal

0.00

Isotropic

c

0.10

Anisotropic

–0.10

–0.20 0.00

0.20

0.40

0.60

0.80

1.00

v2

12.2 Phase diagram for cellulose in 24.5/75.5 w/w NH3/NH4SCN solvent. Rigid rod model x = 61.4.

Measurements of water content showed that undried cellulose pulp contained approximately 7% bound water. Cellulose containing 7% of both added and bound water can form high concentration and anisotropic cellulose solutions. The flow rate behavior of cellulose solutions, containing 7% water (both added and bound water), were similar to that of solutions prepared with predried cellulose. This means that the presence of water up to 7% did not affect


371

the flow rate behavior, hence viscosity, of the cellulose solutions. This cellulose/ NH3/NH4SCN system has demonstrated a high potential as a viable commercial solvent: cellulose can be dissolved easily to prepare gel-free concentrated, anisotropic solutions. Anhydrous conditions are not required. Measurements of the steady shear viscosity in the Newtonian or linear regime yields valuable information on molecular interactions. The molecular weight (Mw) and polymer concentration (C) dependence of the zero-shear viscosity, ho, of cellulose solutions, exhibits two distinct regions. The dilute regime shows a linear increase of zero-shear viscosity with respect to CMW. In the semi-dilute region, the CMW is no longer linearly proportional to ho. It is well established for linear, flexible polymer chains that ho is proportional to (CMW)3.4 [13]. This proportionality for cellulose in the NH3/NH4SCN solvent is different than that of a flexible polymer. The result is similar to that of a rod-like polymer, which further supports that cellulose behaves as a semi-rigid polymer in the NH3/NH4SCN solvent. A dynamic rheological investigation by Cuculo et al. [9], Frey et al. [10] on the formation and the behavior of the gel has shed some light on the socalled ‘window of spinnability’ of the anisotropic cellulose in this solvent. Steady state shear rheology shows that cellulose solutions possess a linear region at low shear rates and this is followed by a transition to a non-linear region at higher shear rates. This behavior shows that the cellulose in this solvent behaves as a true polymer solution. The zero-shear rate viscosity dependence on concentration and molecular weight further confirms that the cellulose in this solvent behaves as a semi-rigid polymer. The steady shear rate viscosity of cellulose solutions as a function of shear rate for degree of polymerization (DP) 760 is shown in Fig. 12.3. A region of Newtonian behavior is observed for solutions with lower concentrations. In this region, the viscosity is independent of the shear rate. The viscosity continues to decrease monotonically with increasing shear rate and this region is called the non-Newtonian region. The transition point between the linear and nonlinear regions corresponds to the onset of molecular orientation and the break-up of entanglements. This point is described by the characteristic relaxation time, l [14]. The plots in Fig. 12.4 show that under the extant conditions of cellulose DP and concentrations and solvent composition, liquid crystal formation occurs at circa 10% concentration. Near this point the cellulose/NH3/NH4SCN system starts to experience the drop in viscosity with increasing cellulose concentration. Observe the lower viscosity of the 10%, 12% and 14% solutions relative to that of the 9% solution and also observe that this behavior became evident only at the high shear rate range. Another advantage of using anisotropic solutions is the molecular ordering in the solution from which fiber can be obtained. This molecular chain ordering translates to superior mechanical fiber properties.

372


1e+5

Viscosity (poise)

1e+4

1e+3

1% 2% 4% 6% 8% 9% 10%

1e+2

1e+1

1e+0 1e-4

1e-3

1e-2

1e-1 1e+0 Shear rate (s–1)

1e+1

12.3 The viscosity dependence on shear rate for cellulose DP760 in NH3/NH4SCN at various concentrations.

1e+5

Viscosity (poise)

1e+4

1e+3

1% 2% 4% 6% 8% 9% 10% 12% 14%

1e+2

1e+1

1e+0 1e-4

1e-3

1e-2

1e-1

1e+0 1e+1 1e+2 Shear rate (s–1)

1e+3

1e+4

12.4 The viscosity in the linear and non-linear regions for cellulose solutions at different concentrations.

The physical properties of fibers produced from precursor liquid crystalline solutions are generally superior to those obtained from the corresponding isotropic solutions. Probably the most well-known commercial fiber derived from a lyotropic system is Kevlar®, produced by Du Pont. Cellulose fibers have not yet been produced commercially from mesophase solutions using the direct solvent route [15]. Tencel®, the commercialized cellulose fibers by


373

Courtauld, however, have shown marked improvement in physical properties as compared to its predecessor, rayon. Tencel® has a tensile strength of circa 4.5 g den–1. The difficulty associated with extruding mesophase cellulose solutions to produce high-strength fibers remains a perplexing problem. Navard and Haudin [16] indicated that one of the problems of spinning liquid crystalline cellulose solutions in the NMMO/water system was the instability or solution fracture of the solutions during extrusion. This instability resulted in uneven fiber dimensions with correspondingly poor physical properties. In 1980, Chanzy et al. [17] also reported fiber formation from lyotropic cellulose solutions in NMMO/H2O. Fibers obtained from a coagulated polymer solution, may be produced by two spinning methods, namely, wet spinning or dry-jet wet spinning. In the former, the solutions are forced through the spinneret directly into a coagulant. The coagulant is a nonsolvent for the polymer. In the coagulant bath the solvents diffuse out of the extrudate while the fiber is further stretched and washed and post treated. In the dry-jet wet spinning method, the solution is extruded into air and immediately proceeds into the coagulant. Post-treatments are then applied to the resulting fibers. The purpose of the air gap is to provide extensional flow to the extrudate to increase orientation across the fiber cross-section. The air gap also allows the temperature of the polymer solution to be different (usually higher) from that of the coagulant, as is the case in the production of Kevlar ®. Fibers from the anisotropic phase of cellulose/NH3/NH4SCN solution have been spun. High mechanical properties have been obtained with these cellulose fibers giving tenacity above 4.5 g den–1 and modulus above 200 g den–1. The solutions were extruded by the dry-jet wet spinning method. A schematic of this process is displayed in Fig. 12.5. The temperature of the solution was adjusted to 30∞C by using a heater block and heater sleeves. Cellulose solutions

Washing and post-treatment

Air gap

12.5 Dry-jet wet spinning process.

374


Three spinnerets were used in this study. They all had a single hole with 60∞ convergence angle and with capillary diameter of 0.127 mm but each spinneret had a different length. The spinnerets used had a ratio of length to diameter (L/D) of 2, 5 and 10. The temperature of the coagulant was held at –4∞C and the washing stage was done by two methods; (a) online washing; (b) no tension washing. Water at ambient temperature (circa 21∞C) was used in both washing methods. For the online washing method, a second bath containing water was used in the threadline. The fiber was washed immediately, under tension, prior to winding on the bobbin. The bobbin was then immersed in water overnight to further remove any solvent left on the fiber. For the no tension washing, the fiber was collected on the bobbin immediately after the coagulation bath. The bobbin was then immersed in water overnight. Finally, fibers collected by both washing methods were dried at room temperature for 24 hours. One of the most important aspects in solution spinning is the choice of coagulant. In this study, methanol was primarily used. Liu et al. [18] reported that this coagulant at low temperature would provide a circular cross-section, a denser fiber and higher initial modulus for cellulose in this solvent system. Thus the temperature of the coagulant was kept at –4∞C. The length of the air gap was maintained constant at 25.4 mm (1 inch), ambient conditions and no shroud was used. The length and the atmospheric condition of the air gap are influential in the cellulose/NMMO/water system of lyocell production affecting the defibrillation of the fibre. In this study attention is focused on obtaining the best mechanical properties of fibers, thus, variation of the air gap to affect the fiber properties was not a priority. The draw-down ratio was varied in attempts to enhance the mechanical properties of the fibers. The draw-down ratio, Dr, affects the linear density of the fiber, which has effects on the tenacity and the initial modulus of the fiber. Dr, is the ratio of the take-up speed, Vi, to the extrusion speed, Vo. Therefore, Dr was changed by varying either Vo or Vi. The fiber properties that were obtained for these different spinning conditions are discussed below.

12.2.2 Fiber performance and characterization The processing of the solution affects the morphology of the fiber. This in turn influences the mechanical properties of the fiber. The morphology of cellulose solutions has been investigated by Frey et al. [10]. In their study of the gelled morphology of cellulose/NH3/NH4SCN solutions, they observed spheroid structures connected to fibrils. They credited this morphology formation to the removal of the solvent by ethanol during the samples’ preparation via critical point drying. The morphology was formed by the phase separation of the cellulose solution during the removal of the solvent into solvent-rich isotropic and polymer-rich anisotropic phases. Apparently,


375

the spheroid nodules were thought to form during the ethanol exchange where the cellulose in the isotropic phases collapsed due to the removal of the NH3/NH4SCN solvent. The fibrils, however, were formed due to the molecular alignment in the anisotropic phases. It was also observed that these spheroid nodules were connected to the fibrils because the long cellulose chains are able to participate in both isotopic and anisotropic phases. Note that the preparation of the gelled samples was conducted without any tension or stress applied on the samples. Similar morphology, Fig. 12.6, was observed on the cross-section of the extrudate obtained from the spinning of the solutions, without any tension or stress applied along the threadline. The fiber’s cross-section shows a defect zone which consists of elongated fibrils connected to spheroid nodules with average diameter in the range of 0.3–0.5 mm. These nodules are evenly distributed along the fiber’s cross-section. The formation of the nodules may be attributed to the collapse of the cellulose structures in the isotropic phase during the removal of the solvent by the coagulant. These structures would contribute to the fiber’s low tenacity. Also, instability of the extrudate is thought to originate in the entrance zone of the spinneret. In this zone the geometry of the entrance and the stress applied on the solutions provide no opportunity for alignment of the chains thus, resulting in the flow disturbance at the entrance. This instability is retained along the capillary and in the extrudate. The instability precludes application of any tension intended to

12.6 SEM micrograph of a fiber cross-section exhibiting the defect zone, due to the spinning instability. The fiber was obtained from spinning a 9% (w/w) cellulose solution at 30∞C into methanol coagulant bath kept at –4∞C with Dr above 1.5. Scale bar equals 10 mm.

376


induce chain alignment. Therefore, the flow disturbance at the entrance zone of the capillary is the original cause responsible for the poor mechanical properties of the fibers, albeit an anisotropic solution was used for the fiber formation. The extrudates spun below the critical stress have been shown to have a smooth surface structure without any evidence of instability along its length. In the absence of the instability, extrudate can be drawn to induce chain alignment. This would allow for elongation to take place and thus accommodate chain alignment along the fiber axis. This alignment of the chains would allow for a supporting network that would prevent collapse of the cellulose chains during the removal of the solvent. This is evident from the crosssection of the fiber displayed in Fig. 12.7 by the absence of the spheroid nodules. The cross-section shows elongated fibrils that are aligned along the fiber axis. The ribbon-like fibrils shown in Fig. 12.7 have an average diameter of 0.2 mm.

12.7 SEM micrograph showing the cross-section of fiber exhibiting the elongated fibrils. The fiber was obtained from spinning a 9% (w/w) cellulose solution at 30∞C into methanol coagulant bath kept at –4∞C with Dr above 2. Scale bar equals 10 mm.

Therefore, two conclusions may be drawn from the instability of the extrudate during spinning. First, the instability of extrudate prevented any drawing or stretching of the extrudate thus eliminating any chance to induce alignment of the chains or anisotropic phase. These chains are then able to support each other to prevent collapsing of the cellulose chains during the removal of solvent. Secondly, the absence of tension along the threadline and hence absence of chain alignment, might also have resulted in the formation


377

of the spheroid nodules due to the aggregation of the cellulose chains during solvent removal. This sequence of events then contributed to the poor mechanical properties of the fiber. The general trends of the mechanical properties of the fibers are displayed in Table 12.1. Higher cellulose concentration leads to better mechanical properties. As has been mentioned earlier, the processing conditions for fiber formation, of course, also directly influence the mechanical properties of the fibers. The type of coagulant affects the structural formation of the fibers as well as the properties. The type of appropriate coagulant has been discussed elsewhere [18]. The method of washing of the fiber also directly influences the properties. Washing under tension enhances the properties of the fibers, but reduces elongation. The draw-down ratio, that is, the ratio of the speed of the take-up to that of the extrusion, also influences the properties of the fibers. This ratio affects not only the linear density of the fiber but also the orientation and the crystallinity index. These, then, affect the mechanical properties such as the tenacity and the initial modulus. The initial modulus of the fiber is a measure of the fiber’s stiffness. It is calculated from the slope of the, initial, linear portion of the stress–strain curve. It is related closely to the molecular ordering of the fiber as well as the crystallinity. As shown in Table 12.1 the initial modulus of the fiber is higher for fiber spun from biphasic solutions (12% (w/w) cellulose concentration). This suggests that the pre-ordering in the anisotropic spinning solution contributes importantly to the final molecular ordering in the fiber. The molecular ordering in the solution appears to be retained in the fiber. Similar findings were reported by O’Brien [19] and Kwolek [20] in their spinning of cellulose triacetate and polyamide fibers, respectively. Another way to induce ordering in the fiber is to draw or stretch the fiber. The draw-down ratio, Dr, extant during the spinning process affects the physical properties of the fiber. In general an increase of Dr is accompanied by an increase in tenacity and initial modulus. This is because the drawing or stretching of the extrudate induced molecular alignment or crystallization. This is evidenced by its effect in increasing the crystal orientation factor, fc. X-ray diffraction analysis indicates that the cellulose fibers formed from this solvent system exist in the cellulose III polymorph conformation. This polymorph structure is revealed by X-ray diffraction peaks at circa 20.8∞, which correspond to both the (002) and (101) planes and another at circa 12.1∞ which corresponds to the (101) plane. The intensity of these X-ray diffraction peaks of the fiber suggests that it consists of a crystalline structure, in this case cellulose III crystals. As displayed in Table 12.2, the d(002) and d(101) are similar to those of cellulose III. These interplanar spacings are the average distance between the crystalline unit planes, and they are different from one cellulose polymorph to another. This suggests that the CH2OH moiety of the cellulose polymers are in the ‘gg’ conformation and are free of

378

Cellulose concentration (% w/w)

Drawdown ratio

Linear density (denier)

Tenacity (g den–1)

Elongation (%)

Initial modulus (g den–1)

Second bath

Crystallinity index (%)

Apparent crystal sizes (Å)

Crystal orientation factor (fC)

0.634 0.684

8

2.13

3.55

2.87

3.53

169.23

Water

65.49

9

0.75 0.80 1.01 1.89 2.14

10.14 10.40 8.97 3.68 3.97

2.09 2.02 2.29 2.67 2.65

7.25 7.59 7.32 3.47 4.74

73.28 73.93 91.09 108.51 125.2

n/a n/a n/a Water Water

40.82 39.27 44.62 60.72 62.03

9.32 9.71 10.52 14.62 15.84

10

2.03 2.53

3.62 3.27

3.44 3.66

5.94 5.46

144.22 153.13

Water Water

63.33 64.63

13.42 13.54

12

5.77 3.15 3.15

3.68 4.37 5.45

3.00 3.77 4.65

9.60 10.96 4.82

122.00 138.91 212.12

n/a n/a Water

62.32 63.68 88.61

13.52 15.23 19.13

0.621 0.672 0.826

1.50

4.51

12.92

132.27

85.11

20.01

0.756

lyocell


Table 12.1 The mechanical properties of cellulose fibers from NH3 /NH4SCN solvent system and lyocell


379

Table 12.2 The diffraction planes of cellulose polymorphs measured by WAXS Polymorph

Cellulose I Cellulose II Cellulose III Cellulose fibers from NH3/NH4SCN Lyocell

Diffraction angle (2q;∞)

Interplanar spacing (Å)

(101)

(101)

(002)

d(101)

d(101)

d(002)

15.30 12.10 12.18 12.71

16.32 20.45 20.83 20.91

22.76 21.65 20.83 20.91

5.86 7.38 7.00 7.00

5.43 4.35 4.26 4.24

3.90 4.10 4.26 4.24

12.75

21.00

21.65

6.94

4.23

4.11

the inter- and intramolecular hydrogen-bonding. The structural conformation of lyocell, on the other hand, possesses the cellulose II conformation. Cellulose fibers with high mechanical properties have been produced from the cellulose/NH3/NH4SCN solvent system. The fiber spun from the biphasic solution has been shown to have superior properties to those from the isotropic solutions. Processing of the solutions is very important in attaining the high mechanical property fibers. Any instability in the extrudate resulted in fibers with low mechanical properties. The solvent exchange in methanol or ethanol during the coagulation process produced a stretchable gel extrudate which formed fibers with circular crosssection. The stretching or extension of the extrudate was done by setting the speed of the take-up godet higher than that of the extrusion. The velocity ratio of the godet rate to the extrusion rate is known as the draw-down ratio, Dr. This ratio controls the linear density of the fiber and hence the morphology and the crystallinity of the fiber. The orientation of the chains and crystalline regions influence the tenacity and the initial modulus. The method of washing during the solution spinning also affects the mechanical properties of the fibers. The washing (or further solvent removal) of the fiber during the spinning process allowed for washing under tension. The tension on the fibers in the second bath would maintain the molecular orientation attained in the coagulant bath, while removing the remaining solvent. Thus, fibers with high cystallinity index as well as high crystalline orientation factor could be obtained. The fibers made from this cellulose-solvent system by the above method have tenacity of 4.65 g den–1 and initial modulus of 212 g den–1. The tenacity of this cellulose fiber is very similar to that of lyocell, circa 4.50 g den–1. The initial modulus of this cellulose fiber is much higher than that of lyocell, circa 132 g den–1. The extensional ratio and extrusion speed, however, is lower than that of lyocell. A cellulose fiber with mechanical properties better than that of lyocell has been made. This cellulose fiber from the NH3/NH4SCN solvent system provides an excellent alternate in producing cellulose fibers simply, quickly and inexpensively as well as attaining high mechanical properties.

380

12.3


Fibers from chitin and chitosan

Chitin, poly-(1Æ4)-2-acetamide-2-deoxy-b-D-glucose, is the second most abundant natural polysaccharide and has a molecular structure that is very similar to cellulose, see Fig. 12.1. Chitin can be found in a wide variety of species of lower animals and plants where it is used as cell wall reinforcement. Arthropod shells (exoskeletons) are the most easily accessible sources of chitin; these shells contain 20–50% chitin on a dry weight basis. From a practical viewpoint, shells of crustaceans such as crabs and shrimps are conveniently available as wastes from seafood processing industries, other potential sources for chitin production include krill, crayfish, insects, clams, oysters, jellyfish, algae, and fungi. Squid pens also contain chitin that is classified as b-chitin, this material is distinguished from the ordinary achitin in crustacean shells based on the difference in crystalline structure. bchitin has weaker intermolecular forces and is quite attractive as another form of chitin having some characteristics considerably different from those of a-chitin. The chemistry of b-chitin is rapidly advancing, although this starting material is less abundant and is not yet produced commercially. Chitosan, poly-(1Æ4)-2-amino-2-deoxy-b-D-glucose, is the deacetylated product of chitin which contains one free amino group for each glucose building unit. However, chitosan is a copolymer composed of the two sugar residues N-acetyl-D-glucosamine (GlcNAc) and D-glucosamine (GlcN). Chitosan is one of a few natural cationic polysaccharides that can be derived from crustaceans or various fungi. Practically, however, chitosan is more easily produced from chitin by deacetylation with concentrated alkali solutions at elevated temperatures [21]. Both polymers cannot be melt-processed because extensive hydrogen bonding restricts relaxation of the polymer chains, so that on heating, the polymer decomposes instead of melting. Measurement of water loss for the polymers requires about ten times the amount of energy required to vaporize an equivalent amount of free water, indicating that the water in chitin and chitosan is tightly bonded, hence, dry spinning may not be an option. Therefore, conversion of the polymers to fiber without polymer degradation requires a wet spinning process. Both polymers have attracted much attention due to their unique properties such as biocompatability, biodegradability, and nontoxicity; chitin and chitosan have been utilized in various fields including food science, cosmetics, water engineering, agriculture, medicine, pharmaceutics, textiles, and their applications are still expanding.

12.3.1 Preparation of filament As mentioned before, it is essential that a stable chitin solution form in order to spin the chitin fibers. There is a long history of attempts to find stable


381

solvents of chitin, dating back to the work of Von Weimarn in 1926; he reported the first solutions of chitin that could be formed into a ‘ropy-plastic’ state though no tensile properties were evaluated on these ropy materials [22]. Since then, numerous solvent spin systems have been extensively studied. However, the major difficulty has been to find a stable solvent because of the relatively inert chemical structure of chitin and its rigid crystalline structure. The discovery of various new solvents by Austin and co-workers in the 1970s stimulated new interest in the problem. Austin suggested organic solvents containing acids such as chloroethanol, sulfuric acid and trichloroacetic acid (TCA) for the direct dissolution of chitin in 1975 [23]. A filament was extruded from the spin solution made with chitin in a mixture of 40% TCA, 40% chloral hydrate, and 20% methylene chloride. The filaments were solidified in the coagulating bath containing acetone, neutralized with potassium hydroxide (KOH) in 2-propanol followed by washing in deionized water, and, then, cold drawn. Other solvent systems for chitin have been proposed [24]. However, they require fairly drastic conditions to dissolve chitin and some are very corrosive or expensive. It is hard to maintain the chitin molecule without depolymerization during the dissolution process. Chitosan, on the other hand, is readily soluble in dilute acid solutions, offering a convenient and relatively inexpensive solvent for fiber production. As early as 1926, Kunike pointed out that chitosan was easily soluble in dilute acids such as dilute aqueous acetic acid [25]. Despite this advantage over chitin, the study of the production of fibers from chitosan has lagged behind that of chitin, the first significant description of chitosan fiber production being as recent as 1981 [26]. Later, Tokura et al. reported that chitosan fibers for various degree of deacetylation can be prepared by extruding dopes in 4% aqueous acetic acid into coagulation bath containing CuSO4/NH4OH or CuSO4/H2SO4. A complex of chitosan fibers and copper were obtained, but the copper can be removed afterwards [27]. Solvent spin systems as mentioned above were too drastic and harmful to apply for biomedical purpose due to toxicity concerns as well as environmental regulations. Therefore, the development of mild spinning solvents as well as coagulating solvents is required for the preparation of chitin and chitosan fibers and their derived fibers. Tokura and coworkers were attracted to a calcium halides/saturated alcohol solvent system as a solvent for chitin [28]. They investigated the solubility of chitin in various metal salts of Group II of the Periodic Table in combination with alcohols such as methanol, ethanol, n-propanol, isopropanol, etc., they reported that the calcium chloride dihydrate saturated methanol (saturated CaCl2·2H2O/MeOH) was the most effective system. This may be due to the fact that the 3-d hydrogen bonding involving the amide and hydroxyl groups of chitin is destroyed by this solvent system without disruption of the glycoside linkage. Among other alcohols, methanol was the best solvent for chitin with calcium chloride dihydrate.

382


In terms of solubility, there is a clear difference between a- and b-chitin. Lower solubility was found for b-chitin in spite of a looser crystalline structure than that of a-chitin. Since the gelation of b-chitin solution started at a lower chitin concentration than that of a-chitin, the gelation mechanism might be related significantly with the dissolution mechanism. They further investigated the dependence of degree of deacetylation on solubility of chitin for the calcium chloride dihydrate saturated methanol solution using various deacetylated chitins with different molecular weights. It turns out that there is a strong dependency of the molecular weight and the molecular conformation of chitin on the solubility. The better solubility is given by the smaller molecular weight fractions at room temperature. This may be due to the fact that chitin is dissolved by an interaction between a calcium ion and an acetamide group. Also, better solubility is given by deacetylated chitin from that of natural chitin even if of similar molecular weight. Since the deacetylated chitin has a less crystalline structure than that of natural chitin, calcium ions would form a complex with chitin molecule much easier [28]. While calcium chloride dihydrate saturated methanol solution was found to be a mild solvent for chitin, chitosan solutions were found to be coagulated by calcium ion [29]. This system would have an advantage for biomedical applications due to such a potentially non-harmful spinning system. Chitosan solution in 10% acetic acid was spun through a spinneret into the first coagulation bath containing calcium chloride or calcium acetate saturated aqueous ethanol. The chitosan filament in the first coagulation bath was solidified through calcium chelation with the amino group of chitosan. Then, from the biomedical point of view, ethanol was applied to the coagulated chitosan filament instead of methanol, washed with distilled water, and dried in air. A post spinning treatment of the filament is required since the filament is water soluble and very weak. The filaments become insoluble under treatment with tetrasodium ethylenediaminetetraacetic acid for 5 hours or sodium hydrogen phosphate for 5 hours, or sodium hydroxide (NaOH) for 48 hours. The results show that the treatment with diluted NaOH aqueous solution improved the mechanical properties by elimination of chelated calcium ion. The tensile strength of filament treated with NaOH in the dry state was 1.33 g den–1 compared with untreated original filament (0.44 g den–1). The remarkable improvement of Young’s modulus was shown in the filament treated with NaOH (57 g den–1) compared with the untreated original filament (17 g den–1). Hence, the results suggest that elimination of calcium from the filament is essential to improve the fiber mechanical properties [29]. Chitosan has also been used to prepare coated or bicomponent filaments. A chitosan-coated alginate filament was successfully prepared by the addition of a small amount of chitosan to the first coagulation bath containing the calcium chloride dihydrate saturated methanol solution. Touch and feel as well as mechanical properties of the alginate filament were improved by


383

coating the alginate filament. Additionally, water insolubility of the resultant filament indicates that the free amino group on the glucosamine residues had an ionic interaction with the carboxylate group on alginate [30]. Over the past 7–8 years, Hirano and co-workers have extensively studied a series of novel functional fibers based on chitin and chitosan. Acylation of chitosan with acid anhydrides such as acetic and propionic anhydride as the reactant introduces amido groups at the chitosan nitrogen. Acetic anhydride affords fully acetylated chitosan. Linear aliphatic N-acyl groups above propionic anhydride permits rapid acylation of hydroxyl groups [31]. Chitosan and aldehyde produce N-alkyl chitosan upon hydrogenation of the resulting Schiff base. The presence of the more or less bulky substituent weakens the hydrogen bonds of chitosan. Therefore, N-alkyl chitosan swells in water in spite of the hydrophobicity of the alkyl chains. N-alkylation, N-alkylidene and N-arylidene chitosan fibers were prepared by the post-treatment of chitosan fibers with aldehydes including vanillin [32]. In the case of N-acylchitosan and their cellulosic composite fibers, the spinning dope was prepared by mixing aqueous 14% NaOH solution of sodium N-acetyl chitosan xanthate with sodium cellulose xanthate. The filaments were spun at 45–50∞C through a viscose-type spinneret into a coagulating bath containing aqueous 10% H2SO4, 32% Na2SO4, and 1.3% ZnSO4 and collected on a roller. The wet fiber obtained was treated in boiling water for 10–20 minutes and in aqueous 0.5% NaOH at 60–70∞C for a few minutes, washed with deionized water, and pressed for dehydration. The chitin–cellulose fiber has been commercialized in the Japanese textile industry as a natural functional fiber for socks and underwear, under the trade name of Chitopoly™ by the Fujibo Spinning Co. The choice of an economical solution for fiber spinning and coagulating is essential for industrial fiber manufacturing. In addition, biomedical fibers should be free of trace harmful elements including sulfur, copper, and organic solvents. Therefore, it is necessary to develop a method for the preparation of N-acetylchitosan fibers without using the harmful CS2. An aqueous solution of sodium Nacetylchitosan salt in aqueous 14% NaOH was spun through a spinneret into a coagulating bath containing aqueous 10% H2SO4, 32% Na2SO4, and 1.3% ZnSO4 [33]. Other researchers in addition to Hirano have extensively studied the production of various chitin, chitosan, their derivatives, and composite fibers with other polymer materials, since 1994. Their spinning solution, coagulating solution, and polymer materials used in the dope are summarized in Table 12.3. Rathke and Hudson’s review concerning chitin, chitosan, and their derivatives covers all of the reported information up to 1994 [24].

384

Table 12.3 Spinning and coagulation solutions of various fibers based on chitin and chitosan References

Fibers

Polymer used in the dope

Spinning solution

Coagulation solution

Hirano, et al.

32

Chitosan

Chitosan acetate

aq. 2% acetic acid

aq. 10% NaOH and 30% Na2SO4 aq. 10% NaOH and 40–43% (NH4)2SO4 aq. ca. 5% ammonia and 40–43% (NH4)2SO4 aq. 10% H2SO4, 32% Na2SO4 and 1.3% ZnSO4 aq. 10% H2SO4 and 40–43% (NH4)2SO4 aq. 10% H2SO4 and 25% Na2SO4 aq. 10% H2SO4 and 40–43% (NH4)2SO4

aq. 2% acetic acid

32

Chitin

Sodium chitin salt

Sodium chitin xanthate

32,33, 58, 59 31,32

60 61

N-acylation*

Chitosan fiber

N-alkylation* N-alkylidene* N-arylidene* N-(carboxyacyl)* Chitosantropocollagen

Chitosan fiber

Chitosan fiber Chitosan acetatecollagen

aq. 2% acetic acid-MeOH aq. 14% NaOH

aq. 14% NaOH

aq. 2% acetic acid aq. 2% acetic acid-MeOH

61 62

Chitintropocollagen* Chitosan-silk fibroin

Chitosantropocollagen fiber Chitosan acetatesilk fibroin

aq. 2% acetic acid

aq. 10% NaOH and 40–43% (NH4)2SO4 aq. ca. 5% ammonia and 40–43% (NH4)2SO4

aq. 10% NaOH and 40–43% (NH4)2SO4


Authors

Table 12.3 Continued Authors

References

63 32,33,34, 58, 59

Chitin-silk fibroin Cellulose-silk fibroin N-acylationcellulose


Sodium chitin xanthate-silk fibroin Sodium cellulose xanthate-silk fibroin Sodium N-acetyl xanthate-sodium cellulose xanthate Sodium N-aceylation salts-sodium cellulose xanthate

Spinning solution


aq. 2% acetic acid-MeOH aq. 5% NaOH

aq. ca. 5% ammonia and 40–43% (NH4)2SO4 aq. 10% H2SO4 and 40–43% (NH4)2SO4 aq. 10% H2SO4 and 40–43% (NH4)2SO4 aq. 10% H2SO4 and 25% Na2SO4

aq. 5% NaOH aq. 5% NaOH

aq. 5% NaOH

aq. 10% H2SO4 and 25% Na2SO4

Nam, et al.

64,65

Chitosan derivative-PAN

N-(2-Hydroxy)propyl3-trimethyl-ammonium chitosan chloride or water soluble chitosan derivative / Polyacrylonitrile

aq. 46% (w/w) NaSCN

aq. 10% NaSCN

Sun, et al.

66

Chitosan- PEG

Chitosan-poly (propylene glycol)

aq. 2% CH3COOH

aq. 2% NaOH

Nousiainen, et al.

67

Viscose-chitosan

Microcrystalline chitosan gel/sodium alginate (2:1)-sodium cellulose xanthate

Zheng, et al.

68

Chitosan-PVA

Chitosan-poly(vinyl alcohol)

90 g/L H2SO4

2 wt % CH3COOH


62

Fibers

aq. NaOH and C2H5OH

385

386

Authors

References

Fibers


Li, et al.

69

Chitosan derivativeViscose rayon

N,O-carboxylated chitosan-Viscose rayon

Hirano, et al.

59

Chitin-cellulosesilk fibroin

Sodium chitin xanthate-sodium cellulose xanthatesilk fibroin

* Post-chemical modification at the fiber state.

Spinning solution


aq. 5% NaOH

aq. 10% H2SO4 and 40–43% (NH4)2SO4


Table 12.3 Continued


387

12.3.2 Fiber performance and characterization Chitin, chitosan, and their derivative fibers of various tensile and mechanical properties such as toughness, flexibility, and tensile strength can be made under a variety of spinning conditions rather than by their chemical nature. Chitin is a highly crystalline polymer because of the ability of the acetamide groups to form hydrogen bonds. The increase of both the dry and wet strengths with the increase in the degree of acetylation is a reflection of the increase in the interchain forces and the increase in the degree of crystallinity. However, chitosan is a more preferred form of the polymer for formation of fiber rather than chitin, although chitosan is difficult to deacetylate fully. The copolymer structure of partially deacetylated chitosan generally lowers the dry/wet strength properties of chitosan fibers. This may be due to the fact that for the partially deacetylated fibers, the polymer structure is a random copolymer consisting of glucosamines and N-acetylglucosamines. The irregular structure inhibits crystallization and reduces the strength of the fiber. As a result, chitosan has enhanced hydrophilicity compared to chitin, which results in a considerable loss of tensile strength when wet [35]. Several approaches have been developed to improve the mechanical properties of chitosan fibers. The first approach is the application of draw to the fibers, which is a physical process. Fibers can have improved tensile and mechanical properties by further processing. The mechanical properties of fibers can be improved by applying a stretch to the yarn in the coagulation step in the wet spinning, or after coagulation while it is still wet and swollen with water, or after it is dried and just prior to being wound up [36]. East and Qin reported the effects on mechanical properties of chitosan fibers by increasing jet stretch ratio in relation to the maximum draw ratio in the wash and draw bath. They demonstrated a decrease in the maximum draw ratio with increasing jet stretch ratio. They also reported the results for mechanical properties of fibers with variations of the jet stretch ratio at a fixed draw ratio and the results of variations of the draw ratio at a fixed jet stretch ratio. The results indicates that a tremendous decline in breaking strength was shown with increasing jet stretch and draw ratios. On the other hand, only minimal changes in modulus were shown [37]. Knaul et al. discussed the improvement of the drying system employed following the coagulation step to remove excess water from the chitosan yarn. The drying step is essential to the spinning process because the filaments are prepared for winding and individual filaments in the wound yarn need to be well separated. Various drying techniques including direct and radiant heat, forced air, and chemical drying agents were considered and the corresponding impact on the mechanical properties of the product chitosan fibers was analyzed. Mechanical properties of methanol-dried chitosan fibers were superior to those properties of fibers dried using forced air, heat, or

388


other tested drying agents. The elongation at break for these fibers remained constant or decreased in both the dry and wet states. The wet mechanical values were: modulus of 28.2 g den–1; a tenacity of 1.05 g den–1; and an elongation at break of 18.5% in the methanol-dried chitosan fibers [38]. The second approach to improving mechanical properties is the chemical cross-linking of chitosan fibers. Fiber mechanical properties can be altered by reaction with another compound which involves crosslinking the polymer chains. Wei et al. cross-linked chitosan fibers using epichlorohydrin [35]. Results demonstrated a significant increase in wet mechanical properties with increasing reaction time, reaction temperature, and concentration of epichlorohydrin. Dry mechanical properties improved only slightly under the same reaction conditions although the tenacity and modulus were decreased in some cases. Elongation at break remained constant or decreased when determined for both the dry and wet fiber. The dry mechanical values exhibited a modulus of 48.1–73.4 g den–1, a tenacity of 1.19–1.46 g den–1, and a elongation at break of 11.2–7.5% [35]. Knaul et al. reported on the changes in the mechanical properties of chitosan fibers using buffered solutions based on potassium dihydrogen phosphate and potassium hydrogen phthalate. The variations were pH, immersion time, and temperature [39]. Hudson et al. discussed improvements to dry mechanical properties through post-spinning application of glyoxal and glutaraldehyde to chitosan fibers. Fiber samples were wrapped onto bent glass rods and submerged into 100 cm3 of either aqueous glyoxal or glutaradehyde solution for one hour at 25∞C. After removal from the dialdehyde solution, the fiber was rinsed with distilled water and dried in an oven at 50∞C for 16 hours. The variance of initial modulus, tenacity, and elongation-at-break, with increasing concentration of dialdehyde was determined. With increasing amount of dialdehyde, the elongation-at-break decreases. On the other hand, the modulus and tenacity increase to a certain point, but then decrease again with increasing amount of dialdehyde. This suggests that the dialdehyde will degrade the molecular structure at relatively high concentrations or cause stress concentrations in the fiber. This result was even more evident in the stress–strain curve, which displayed an increase in the elastic portion and a corresponding decrease in the plastic region with increasing amounts of cross-linking agent. The disappearance of the plastic region with increasing modulus is a characteristic property of a fiber that is becoming increasingly brittle [40]. This phenomenon was reported by Wei et al. for increasing cross-linked density with epichlorohydrin [35]. Lee et al. reported on the effects of the concentration of cross-linking agent, epichlorohydrin (ECH), in the spinning dope on the mechanical properties of chitosan fiber. Chitosan fibers are prepared with ECH concentrations from 0.5 to 25.0 ¥ 10–2 M. The wet tenacity increases somewhat as the concentration of ECH increases in the spinning dope. However, the


389

dry tenacity decreases significantly with an increasing concentration of ECH. The elongation decreases with the addition of ECH for the dry fibers. There is no deleterious effect on the wet fiber properties as a result of crosslinking. However, the work of rupture decreases with the addition of ECH. The saturated concentration of ECH is about 5 ¥ 10–2 M [41]. A third approach for improving fiber performance is the use of chitosan as a fiber coating. Ionic interaction is a convenient way to form tight interactions between molecules. Chitosan is a cationic polysaccharide consisting of glucosamine residues. A tight interaction between chitosan and an anionic polymer such as alginate is expected to improve the tensile strength through ionic interaction. Tamura et al. proposed to prepare chitosan-coated alginate filaments for application as a wound healing material. The coating of the alginate filament was achieved by addition of a minimal amount of chitosan solution dissolved in acetic acid to the aqueous calcium chloride solution in the coagulation bath. Coagulated filament was wound up by a stretching procedure in the wet state and dried in air following extensive washing with ethanol. A significant increase of the wet/dry ratio of knot strength is observed and this suggests the tight ionic interaction of chitosan to the alginate filament and the improvement of filament flexibility. Although knot strength in the dry state decreased with the increase of chitosan concentration in the coagulation bath, those in the wet state (about 0.57 g den–1) are independent of the chitosan concentration. In general, these filaments are stronger in the dry condition than in the wet condition. However, the present chitosan-coated alginate filament showed higher wet strength, especially in knot strength. There is also a significant molecular weight dependency for the tensile strength of chitosan-coated alginate filament especially when wet. When chitosan of molecular weight 1.6 ¥ 105 g mol–1 is added to the coagulation bath, tensile strengths in dry state (1.13–1.81 g den–1) and wet state (0.7–0.9 g den–1) are improved compared with the filaments coated by chitosan of low molecular weight. This suggests the tight interaction of chitosan with alginate filament. In addition, the coagulation effect of calcium chloride might play a role for the improvement of the filament strength [30]. Mechanical properties of chitin, chitosan, and their derivatives are reported up to 1994 in Rathke and Hudson’s review paper [24]. Table 12.4 shows the mechanical properties of various fibers based on chitin and chitosan obtained between 1994 and 2004.

12.3.3 Applications of chitin and chitosan fibers The antimicrobial activity of chitosan against microorganisms of a wide variety of bacteria and fungi has long been recognized [42]. This unique property has led to many potential applications to food science, agriculture, paper, medicine, pharmaceuticals, and textiles. Mechanisms behind the

390


Table 12.4 Mechanical properties of fibers based on chitin and chitosan Fibers

Titer (denier)

Tenacity (g den–1)

Elongation (%)

Chitin Chitosan N-acetylation N-alkylidene N-arylidene N-(carboxyacyl)

3.75–7.89 4.50 3.08/4.35 6.2

1.15–1.25 1.29 1.30/0.87 1.41

3.6–8.4 8.2–10.4 11.2/6.9 12.9

4.09–5.85

0.37–0.90

7.6–11.7

Chitosan:tropocollagen (90:10) (70:30) (50:50)

11.3 16.3 17.7

1.11 1.08 1.15

14.4 15.7 10.9

Chitosan:silk fibroin (94:6) (80:17) (67:33)

3.24 9.70 18.5

1.05 0.67 0.10

8.44 4.20 0.17

Chitin:silk fibroin (83:17) (94:6)

9.70 3.24

0.67 1.05

4.2 8.4

Cellulose:silk fibroin (47:53) (90:10) (100:0)

19.7 9.9 4.1

0.15 1.08 1.27

0.8 35.0 39.2

N-acylation:cellulose (67:33) (50:50) (33:67)

5.31 6.96 3.62

0.18 (dry) 0.26 (dry) 0.53 (dry)

4.8 (dry) 4.5 (dry) 19.1 (dry)

Chitosan derivative:PAN (1:99) (10:90) (20:80) Viscose-chitosan

94 83 92 4.5

1.05 1.03 1.16 1.4

15.7 8.8 7.9 20

Chitosan:PVA (90:10) (70:30) (50:50)

1.9/0.82 (dry/wet) 1.9/0.82 (dry/wet) 1.65/0.68 (dry/wet)

16/27.3 (dry/wet) 16/30 (dry/wet) 14.5/24 (dry/wet)

Chitosan:chitosan derivative:viscose rayon (%) (0.4:0.1:99.5) (1.6:0.4:98.0)

1.64/0.88 (dry/wet) 1.53/0.80 (dry/wet)

21.6 19.5

0.93 0.70 0.85

25.0 20.6 28.6

Chitin:cellulose:silk fibroin (%) (9:80:11) (16:41:43) (77:11:12)

5.0 4.8 3.9


391

antimicrobial activity of chitosan have been proposed. One of the most plausible mechanisms is that the antimicrobial activity of chitosan originates from the polycationic nature of chitosan that can bind with anionic sites in proteins, hence, resulting in selective antimicrobial activities toward fungi or bacteria. The antimicrobial activities, which are not found in chitin, mainly depend on the type of functional groups in chitosan and the molecular weight of the base chitosan. The antimicrobial activities are generated from the protonated amino groups in chitosan in an aqueous acidic environment. Another mechanism is that the positively charged oligomer chitosan penetrates into the cells of microorganisms and prevents the growth of cells by inhibiting the production of RNA. In this mechanism, chitosan must be hydrolyzed to a low MW to penetrate into the cell of microorganisms, though this mechanism is controversial. A large number of fibers has been used or developed for various applications such as wound dressing materials, sutures, scaffolds in tissue engineering, functional textile, and air filters. Over the past several years, the biomedical application interests for chitin and chitosan have drawn much attention. This is due to the fact that chitin and chitosan are biodegradable, biocompatible, and non-toxic. Wound dressing As far as chitin and chitosan based commercial wound dressings are concerned, a Japanese manufacturer, Unitika Ltd, is producing a wound dressing from highly purified chitin, which is available in nonwoven, sponge, and fleece forms. Nara et al. patented a wound dressing comprising a nonwoven fabric composed of chitin fibers [43]. Another patent from the British Textile Technology Group (BTTG) reported a procedure for making a chitin-based fibrous dressing. They developed a novel method, which uses a non-animal source, microfungal mycelia, as the raw material, and the resulting microfungal fibers are different from the normal spun fibers [44]. Hirano et al. introduced a novel chitin-acid glycosaminoglycan fiber for use as a biocompatible dressing material in the veterinary and clinical fields. Though these fibers are unfit for a textile material because of their mechanical weakness, they are usable as a wound-dressing material in the clinical and veterinary fields because of their ease in handling, soft feel, and protective properties [45]. Sutures Sutures are either monofilament or monofilament threads in surgery for wound closure. Chitin sutures resist attack in bile, urine and pancreatic juice, which are problem areas with other absorbable sutures [46].

392


Scaffolds in tissue engineering Iwasaki and co-workers produced an alginate-based chitosan hybrid polymer fiber using the coagulation system containing the calcium chloride dihydrate saturated methanol solution. They suggest that the fiber has considerable potential as a desirable biomaterial for cartilage tissue scaffolds. In order to maintain the number of attached chondrocytes, high cellular adhesivity is a requirement for scaffold materials in cartilage tissue engineering. In their report, the adhesivity of chondrocytes was significantly higher on the alginatebased chitosan hybrid polymer fiber than on the alginate polymer fiber with its anionic nature. This is due to the fact that the cationic chitosan allows for electronic interaction with anionic glycosaminoglycans (GAGs), proteoglycans, and other negatively charged species. In addition, adequate mechanical strength to maintain the initial shape of the implanted scaffold is necessary. In order to achieve this requirement, they have developed a 3D fabricated scaffold using an apparatus formed from the novel fibers [47]. Tuzlakoglu et al. developed 3-D chitosan fiber meshes for potential use in tissue engineering applications. Chitosan solution dissolved in aqueous acetic acid was diluted with methanol, injected into a coagulation bath containing a mixture of 30% 0.5 M Na2SO4, 10% 1 M NaOH, and 60% distilled water, and washed with distilled water before being suspended in 50% methanol for 1 hour. A subsequent drying treatment with methanol was used to improve the mechanical strength of the fibers so that the fibers had enough tensile strength to be used for scaffolds. Using a short-term MEM extraction test, the fiber was found to be non-cytotoxic to fibroblasts. Furthermore, osteoblasts directly cultured over the chitosan fiber mesh scaffolds. There was no inhibition of cell proliferation [48]. Controlled release of drug Liu et al. prepared chitosan coated cotton fiber by the reaction between aqueous chitosan acetic acid and oxidized cotton fiber. Since the chemical reaction activity of the amino group is greater than the hydroxyl group of cellulose, the fiber has potential for still more chemical modification. They have tried the control release of the herb medicine shikonin and obtained a good result. Potential usefulness of this fiber as a support for the controlled release of drugs is suggested [49]. Functional textiles Fuji Spinning Co. Ltd have developed and commercialized the hybrid fiber ‘Chitopoly’ from chitosan. Originally, Chitopoly was developed for use in atheletic socks and hospital gowns to prevent the spread of disease. Nowadays,


393

Chitopoly has been widely acknowledged as an undergarment for people with symptoms of atopic allergy [50]. Also, a chitin-cellulose blend fiber, named Crabyon, has been commercialized as a functional textile material for underwear, towels, sportswear, and socks [51]. Industrial materials Hirano and Hayashi have introduced a novel fragrant fiber and yarn based on chitosan. A bundle of two silk fibroin filaments was coated with a layer of Nmodified chitosan using fragrant aldehydes such as cinnamaldehyde. A portion of chitosan fiber was suspended in methanol and a fragrant aldehyde was added. The mixture was kept at room temperature for 18 hours, washed with methanol, filtered, and air-dried. The fragrant derivatives, fibers, and yarns are suggested to be useful as a novel biomaterials in a wide field of applications, including air-filters, cosmetics, and textiles [52].

12.4

Future trends

Improvements in cellulosic fibers are continuing along several paths. Cellulose is a sustainable resource and widely available worldwide. It has many desirable attributes for use in textile fibers, including ease of dyeing, comfort and biodegradability. The search continues for solvent components which are easier to handle. In the case of the NH3/NH4SCN solvent system, it may be possible to substitute the ammonia for a material with a higher boiling point such as hydrazine or ethylene diamine. Hattori and Cuculo began an investigation of analogs to NH3 including hydrazine, hydrazine hydrate and ethylene diamine [53–55]. These systems have shown similar behavior to the NH3/NH4SCN in early dissolution studies. One of the solvents, the ethylene diamine/salt system, shows promise as a commercially viable solvent for cellulose processing. This solvent system has great flexibility because salt type and concentration can be varied. Also, formation of ultra fine fibers via electrospinning is possible from the ethylene diamine/salt solvent [56]. Chan et al. have reported an investigation of cellulose/ethylene diamine/ salt solution rheology [57]. Solutions of cellulose in ethylene diamine/KSCN solvents were found to be strongly shear thinning, approaching Bingham plastic behavior. Shear viscosity was found to be highly dependent on both cellulose and salt concentration as seen in Fig. 12.8. Decreases in viscosity with increasing cellulose or salt concentration indicate onset of liquid crystal formation and, at higher concentrations, phase separation into polymer rich and solvent rich phases. At further increased concentrations, stiff clear gels similar to those observed in the cellulose/NH3/NH4SCN system form. Preliminary coagulation studies were performed using a dry-jet wet spinning

394

Biodegradable and sustainable fibres KSCN – 40% of saturation in ethylene diamine

3

KSCN – 45% of saturation in ethylene diamine

1 Hz 10 Hz 100 Hz

2.5

4 1 Hz 10 Hz 100 Hz 3

h*(Pa*s)

h*(Pa*s)

2

1.5

2

1 1 0.5

0

0 4

5

6 7 % w/w cellulose (a)

8

9

4

5

6 7 8 % w/w cellulose (b)

9

12.8 Effect of cellulose concentration on complex viscosity of samples with KSCN concentration at 40% and 45% of saturation in ethylene diamine [57].

technique. Ethanol and methanol were both found to successfully coagulate fibers. Fiber properties have not yet been optimized. Chitosan is called the last biomass of the twentieth century, and is a material that waits further development as an extraordinary biomaterial in the twenty-first century. Chitin and chitosan, derived from shellfish waste, continue to be underutilized resources. We believe that to further their development as a resource, that there are needs for products of intermediate value; easy to produce and which capitalize upon the unique features of chitin and chitosan. Fibers and films based on chitin or chitosan have considerable promise for medical textile applications. The commercialization of chitosan-based hemostatic bandages, such as Hemcon™, represents a significant accomplishment in gaining the acceptance of chitosan as a biomaterial.

12.5

Sources of further information

‘Solvent Spun Cellulose Fibers’, Cuculo, J.A., Aminuddin, N. and Frey, M.W., in Structure Formation in Polymeric Fibers, 296–328, D.R. Salem (ed.), Hanser Publishers: Munich (2000). ‘Chitin and Chitosan’, Jenkins, D. and Hudson, S.M., in Polymer Encyclopedia of Polymer Science and Technology, Czekaj, C. (ed.), Wiley Interscience, 3rd edn (2001). Handbook of Fiber Chemistry, edited by Lewin, Menachem and Pearce, Eli M., International Fiber Science and Technology Series, #15.


12.6

395

References

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13 Soya bean protein fibres – past, present and future M M B R O O K S, University of Southampton, UK

13.1

Introduction

Creating a warm, soft, comfortable and economically viable man-made fibre equivalent to wool, a natural protein fibre, has long been a goal for textile scientists and manufacturers. Developments in biodegradable fibres from renewable resources in the late twentieth century have revived interest in these fibres. The development of a wool-like fibre from soya beans is a story of technological innovation (and failure) interlinked with changing political and ecological priorities. Soya bean protein fibres were one of several innovative and pioneering regenerated protein fibres which were developed in the midtwentieth century. However, technical problems meant that the resulting fibres could not compete with either natural fibres or the newly developed synthetic fibres and so having limited commercial application failed to become mainstream fibres and were almost totally forgotten. Spasmodic interest continued, but these fibres have only recently become the focus of renewed interest and commercial activity with research taking off again in the last decades of the twentieth century with the development of new processing methods and fibre structures. This chapter explores the two phases of the development of soya bean fibre using contemporaneous documentary evidence such as patents, technical journals, research papers and home economics literature. Technical data about the different fibres is presented where this is available. Samples of the mid-twentieth century soya bean protein fibres have not yet been located so textual evidence cannot be confirmed through analysis. Conversely, contemporary fibres can be acquired, but relatively little data is yet available in this rapidly developing area.

13.2

The soya bean plant

The soya bean plant (genus Glycine; species Max; family Leguminosae; subfamily Papilionoideae) is a bush-like annual, growing about 1.8 m tall and bearing pods which each contain several smooth seeds – the soya beans. It 398


399

was grown in the Yellow River valley, North China as early as 3000 BC and remains an important crop in China and Japan. Missionaries brought seeds to Europe in the eighteenth century and soya beans were first cultivated in the USA in the nineteenth century. By the late 1990s, 70 million hectares worldwide were being used for soya bean cultivation, mainly in Argentina, Brazil, China, India and the USA. Thousands of varieties of soya bean are known and intensive research has been undertaken into soya bean breeding for a variety of applications. For example, research in Australia has explored the development of improved soya bean genotypes suitable for use in poultry diets.1 Soya beans became a significant crop in the USA in the early twentieth century in a linked producer/processor development known as the ‘American soya complex’ and were well established by the 1930s. Thousands of new varieties were introduced,2 and by 1933 over 1,400,000 hectares were being used for soya bean production, producing over 600,000 kg of soya bean oil and equally large quantities of soya bean meal.3 In 2002, 29.6 million hectares were planted with soya beans producing about 110 million metric tonnes annually; USA soya beans have an export value of $7.2 billion, the major markets being Europe, China, Mexico and Japan.4 Research into improving plant and bean characteristics has been undertaken in America since the 1930s. For example, two bulletins published by the Agricultural Experiment Station, Kansas State College of Agriculture & Applied Science5,6 explored methods of soya bean cultivation and reported on tests on over 60 varieties of soya beans. The US Regional Soybean Industrial Products Laboratory (now the National Soybean Research Laboratory) was established at Urbana, Illinois in 1936, developing soya bean varieties, disease management and different end uses. Since the 1980s, the US Department of Agricultural Research Service has developed 66 different soya bean types. Genetic modification techniques are now being used to produce beans for very specific needs, but concern has been expressed that soya bean breeding has led to ‘a dangerously narrow genetic base’ and US gene banks have sought to introduce Chinese soya bean genetic materials into American strains.7 Du Pont researchers have been experimenting with modifying soya beans to give beans with a higher oil or protein content.8 Monsanto have produced a genetically modified soya bean cultivar, Roundup Ready™. This variety has been designed to be resistant to their Roundup© herbicide although the implications of the development of patented seeds is causing concern to Chinese farmers.9 Soya beans contain oil (180–220 g kg–1) and approximately 35–45% protein (370–420 g kg–1). The amino acid content differs significantly from that of wool and silk protein (see Table 13.1). Soya bean protein content is higher than that of peanuts (approximately 25% protein) and maize (approximately 10% protein).10 The principal components of this protein are

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13.1 Soya bean fibres made by the Ford Motor Company, early 1940s. From: America’s Fabrics reproduced in Kiplinger, J. 2003. Meet the Azlons from A–Z: Regenerated & Rejuvenated. www.fabrics.net/joan103.asp. Accessed 16 November 2004.

b-conglycinin (trimeric structure) and glycinin (six subunits, each a basic polypeptide and an acid polypeptide connected with a disulphide bond) with other proteins in lesser quantities such as trypsin inhibitors, lipoxygenases and pectins. The complex association–dissociation behaviours of b-conglycinin and glycinin, resulting in the formation of soluble aggregates, are a critical factor in the processing of soya bean protein for the formation of fibres.11,12

13.3

Naming regenerated protein fibres

In the 1940s, there was considerable uncertainty over how to categorise and name regenerated protein fibres. Researchers were experimenting with a variety of animal and vegetable protein sources to create a wool-like fibre


401

Table 13.1 Percentage of amino acid content in soya bean protein compared with that of wool and silk Amino Acid

Soya

Wool

Silk

Alanine Arginine Aspartic acid Cystine (sulphur containing) Glutamic Acid Glycine Histidine Isoleucine Leucine Lysine Methionine (sulphur containing) Phenylalanine Proline Serine Threonine Tryptophan Tyrosine Valine Ammonia Hydroxylysine

4.12 (1.7) 5.80 (8.3) 3.86 (5.7) 1.00 (1.1) 19.46 (19) 0.23 (0.7) 2.30 (2.2) 4.00 (2.4) 8.40 5.40 (5.4) 2.00 (1.8) 5.30 (4.3) 3.04 (4.3) 6.00 4.00 (2.1) 1.50 (1.7) 4.30 (3.9) 4.50 (1.6) – –

4.10 3.60 7.27 11.30 16.00 6.50 0.70 – 9.70 2.50 0.35 1.60 7.20 9.50 6.60 0.70 6.10 5.50 1.18 0.10

26.40 1.05 2.00 – 2.03 43.80 0.47 1.37 0.80 0.88 – 1.50 1.50 12.60 1.50 – 10.60 3.20 – –

Data from Traill 1951, 258 with comparative data (in brackets) from www.nnfcc.co.uk/ crops/info/soya.htm

which would parallel the silk-like fibres derived from regenerated cellulosic sources. Francis Atwood gave a paper in 1941 to the American Chemical Society and proposed the term ‘prolon’, combining ‘pro’ from protein and ‘on’ from nylon and cotton;13 this had brief currency, but the term ‘azlon’ was eventually proposed and accepted; its derivation is unclear. The American Federal Trade Commission and the Textile Fibre Products Identification Act define azlon as ‘A manufactured fibre in which the fibre forming substance is composed of any regenerated, naturally occurring protein’, although other definitions extend this to include both regenerated protein and cellulosic fibres.14

13.4

The need for new fibre sources

13.4.1 The context for mid-twentieth century research into alternative protein fibre sources The impetus behind research into alternative fibre sources in the 1930s and 1940s was fuelled by the desire to produce economically viable wool-like fibres which could compete with, or complement, natural wool fibres. It is

402


clear that a fear of shortages of natural fibres during wartime was an increasingly strong factor in encouraging this research in America. By the mid-1930s, almost half of USA’s wool requirement was imported; over 112 million kg in 1936. This dependence on overseas wool sources, particularly from Australasia, meant the textile industry was subjected to the effect of considerable price fluctuations. Increasing domestic production of a wool substitute had obvious benefits, particularly as preparations for war intensified; wool was needed in large quantities for military uniforms and equipment.

13.2 Blended regenerated protein fibres. This is an upholstery fabric containing soya bean fibre. (Photograph courtesy of Kansas State University Agricultural Experiment Station and Cooperative Extension Service, from the publication Synthetic Fibers and Textiles (Bulletin 300), Fletcher H.M., 1942.)


403

American textile manufacturers were clearly aware of the impact of war on both the supply of unprocessed fibres and the type of textiles required. Wickliffe Rose of the American Viscose Corporation made a speech to the 1944 American Association of Textile Chemists and Colorists (AATCC) convention in which he identified three aspects of this process: substitution of man-made for natural fibres, modification of industrial practice as a result of shortages of natural fibres and intense research into new fibres to satisfy military requirements, which had the effect of restricting supplies available for the civilian market.15 What was available was of poorer quality. A 1944 survey undertaken by the Bureau of Human Nutrition and Home Economics showed ‘how essential fabrics were downgraded during the war’.16 In the 1930s and 1940s, American, European and Japanese researchers and textile specialists were exploring the possibilities of making fibres from a variety of novel protein sources. O’Brian lists the range of sources being considered: ‘Textile fibres from the redwoods and from yuccas are being studied. Fibres from milk casein, from soybeans, and many other sources are making their appearance’.17 Regenerated protein fibres from animal sources which were produced commercially included ‘Aralac’ (USA) and ‘Lanital’ (Italy) made of milk. The American military experimented with the use of chicken feathers for blankets and some ladies’ suits, seemingly of a rough tweed-like fabric, are said to have been made with feathers. Other researchers explored the possibility of making fibres from egg white and slaughterhouse waste such as horns, hooves and gelatine. Research into fibres made from vegetable protein sources resulted in the commercial development of ‘Ardil’ (UK) and ‘Sarelon’ (USA) from peanuts and ‘Vicara’ (USA) from zein protein in maize. Lundgren and O’Connell, two researchers at the US Department of Agriculture’s Western Regional Research Laboratory (Albany, California), noted that ‘Interest in the formation of artificial fibres from proteins has been stimulated by the war emergency’.18 An anonymous writer in Rayon Textile Monthly echoed this view: ‘The nation’s war effort has greatly accelerated the tempo of American skill and ingenuity in fibre and fabric creation’.19

13.4.2 The context for mid-twentieth century research into soya bean fibres To be suitable for use as source for fibres, a vegetable protein needs to be either colour free or bleachable and readily available in large quantities at an economic cost. Soya beans were produced in large quantities because of their established uses in agriculture. Methods for extracting soya bean protein were already established, together with a tradition of exploring its potential in industrial contexts; it made sense to see whether it could also be used to create an effective fibre. Nevertheless, it is worth noting that soya bean

404


13.3 Henry Ford wearing his suit made from fabric containing soya bean protein, circa 1941. From the collections of Henry Ford (P.188.29410).

protein was considered as the source for a viable regenerated protein fibre for a relatively brief period during the mid-twentieth century. By 1942, Fletcher noted that technical problems were preventing the production of ‘firm, tough protein filaments which will resist wear and deterioration like the natural and other synthetic fibres’, although she implies that soya bean fibre is being manufactured.20 Sherman and Sherman’s comparisons of soya bean fibres manufactured between 1939 and 1944 indicate that the fibre was still in development and that quality was still being improved.21 They note that ‘the new protein fibre [soya bean protein fibre] should sell at around the


405

same price level as casein fibre and find its principal market in blends with cotton, wool, and rayon’;22 however, there is no indication that any trade name was registered. It is unclear whether soya bean fibre was made into commercially available woven fabrics in the 1940s. One company, Drackett, produced the fibre in considerable quantities but its main commercial use seems to have been by hat manufacturers in unwoven felt (Fig. 13.4). There is no evidence for a marketing campaign to introduce soya bean fibre to the public comparable to that run by Atlantic Research Associates for their milk protein fibre ‘Aralac’. Examples of surviving garments or hats have yet to be identified. Need stimulated by war was affected by the arrival of peace and by 1945 the US government held over 1,800 million kg of wool suitable for clothing stockpiled during the war. It was initially expected that disposal of these stocks would take up to thirteen years; however, high home demand, reconstruction requirements in Europe and a drop in world wool production coupled with a slump in wool prices meant this stockpile was consumed in about ten years. This demand for wool was expected to stimulate research and production of regenerated protein fibres. However, their technological weaknesses meant that they failed to compete effectively with natural or synthetic fibres, even when used as a blend. It seemed that the regenerated protein fibres had no place in a world supplied with an increasingly wide range of effective and well-priced synthetic fibres. Some patents for soya bean fibres were filed in the 1950s but interest seems to have faded more quickly in this fibre source than in other possible protein fibre sources. Patents often list a range of protein sources. For example, patent GB 634,812 for improving ‘the properties of protein spinning products’ focuses primarily on fibres from milk casein but also cites soya bean or peanut protein as alternative sources (see Table 13.2). Specific commercial applications for soya bean fibres do not seem to have developed and research interests shifted fairly rapidly to fibres that appeared to have greater potential. For example, Wormell, a researcher closely involved in the development of regenerated protein fibres, does not include soya bean protein fibre in his 1954 X-ray diffraction studies. Post-war, there was also concern about using scarce resources for fibres: ‘Why use good food to make poor wool?’23 By 1966, such fibres only merited a fleeting comment in standard textile textbooks. Traill acknowledges the technical problems, but, drawing inspiration from the improved quality of the regenerated cellulose fibres, he expresses the hope that ‘regenerated protein fibres may become different from those we now know’.24 Nevertheless, the general reduction in interest suggests that few researchers retained Moncreiff’s belief that this generation of regenerated protein fibres were ‘the pioneers of those man-made, probably scientific, protein fibres that will one day surely come, and in this sense they have played a part in the advance of fibre science’.25

406

Table 13.2 Selected patents for regenerated protein fibres using soya bean protein Patent number Date

Patentee/s

Summary

Specification published 7 May 2003 Application date 10 December 2002

Spinning dope of synthetic fibre of phytoprotein and its producing method

Hu Zongshan and Song Huiyuan

Method for making spinning solution from plant protein copolymerised with an aqueous solution of poly(vinyl alcohol) with sodium or potassium sulphite

Specification published 10 July 2003 Application date 31 December 2002

Phytoprotein synthetic fibre and the method of making the same

Li Guanqi

Method for making a phytoprotein synthetic fibre made from vegetable protein and poly(vinyl alcohol)

France 827,992

6 May 1938

Textile fibres made from soya bean protein

Nippon Kari Kogyo K K Japan

Method proposed is similar to that of manufacture of Lanital from milk casein

828, 075

10 May 1938

Textile fibres made from soya bean protein

Nippon Kari Kogyo K K Japan

Method proposed is similar to that of manufacture of Lanital from milk casein

Germany H 153,501 .IV c/12p

Filed 1 August 1939

China CN 141, 5646

WO 030, 56076

Great Britain GB 525, Application 23 Feb 577 1939 Specification published 30 August 1940

Sulfurized protein. After treatment with Igepon and Stocko Improvements in and relating to the production of textile threads

Donald Leonard Wilson and Courtaulds Ltd, UK

Method for partial hardening using formaldehyde and heat treatment for fibres from casein, soya bean and peanut protein solutions


Title

Table 13.2 Continued Patentee/s

Summary

GB 536,841

Application 24 November 1939 Specification published 29 May 1941

A process of treating a product synthetically formed from protein material to improve or modify the dyeing affinities, resistance to water and chemicals, and other properties

Atlantic Research Associates, USA

Method for stabilising fibres formed from milk or soya bean protein using an acylating agent such as acetic anhydride and ketenes which may be in gaseous form

GB 543,586

Application 29 August 1940 Specification published 4 March 1942

Improvements in or relating to the manufacture of filaments from vegetable globulin

ICI Ltd, David Traill, UK

Method for using sodium chloride solutions to harden fibres formed from peanuts, hemp-seed, castor oil seed or soya beans

GB 539,985

Application 30 March 1940 Specification published 1 October 1941

Improvements in or relating to the manufacture of wet spun protein fibres

Antonio Ferretti

Method for using chromium salt and formaldehyde to harden and insolubilise fibres formed from milk or soya casein

GB 605,830

Application 7 Jan 1946 Specification published 30 July 1948

Improvements in or relating to the insolubilising treatment of films, filaments, fibres and like-shaped articles made from protein solutions

Andrew Mclean, ICI Ltd, David Traill

Method for improving the dyeing of protein fibres of either peanut or soya bean fibres with acid wool dyes using alkali metal sulphate solutions acidified with sulphuric acid as the coagulating bath

407

Title


Patent number Date

408

Table 13.2 Continued Title

Patentee/s

Summary

GB 634,812

Application 23 July 1947 Specification published 29 March 1950

Process for improving the properties of protein spinning products

Onderzoekings Instituut Research, Holland

Method for using precondensation products of formaldehyde and resorcin to improve resistant to hot dilute acid liquids such as acid dye baths without loss of flexibility

GB 638,356

Application 7 October 1946 Specification published 7 June 1950

Improvements in regenerated protein fibres and process for preparation thereof

Jack Jay Press, New York, USA

Method to produce a regenerated protein fibre with improved resistance to aqueous processing by forming insoluble condensation bodies within the fibre. Protein sources cited include casein, soya beans, peanuts, zein, silk waste and fish albumen

GB 665,462

Application 3 August 1948 Specification published 23 January 1952

Improvements in or relating to a method for improving the strength of artificial insolubilised protein filaments or fibres

George Kirkwood Simpson, ICI Ltd.

Method for metallic salt solutions, usually with formaldehyde, to improve the processing strength of fibres produced from ‘alkaline solutions of casein and vegetable globulins such as peanut and soya bean globulin’


Patent number Date

Table 13.2 Continued Patentee/s

Summary

GB 667,115

Application 13 April 1949 Specification published 27 Feb 1952

Improvements in and relating to the production of artificial protein threads, fibres, filaments, yarns and the like

Courtaulds Ltd, Robert Louis Wormell

Method for producing fibres using a solution of keratin from wool waste, horn or hoof with casein derived from milk, peanuts, castor beans or soya beans. The resulting fibres were formed in a coagulating bath of sodium sulphate before hardening and stretching

GB 673,676

Application 31 May 1949 Completed specification filed 30 May 1950 Specification published June 11, 1952

Improvements in and relating to the production of artificial protein threads, filaments and the like

Courtaulds Ltd, Frank Happey and Robert Louis Wormell

Method for producing fibres from ‘solutions of proteins such as lactic casein and vegetable seed proteins, otherwise known as vegetable casein, such as soya bean protein and peanut protein’ and improving tenacity by denaturing and stretching the fibre at high temperature

GB 674,755

Application 4 August 1949 Completed specification filed 8 Aug 1950 Specification published 2 July 1952

Improvements in and relating to the production of artificial protein fibres

Courtaulds Ltd, and Robert Louis Wormell

Method for producing ‘a cheap soluble protein fibre having the requisite strength for use in protein processes’. This patent is intentionally seeking to design a soluble fibre which could be dissolved

409

Title


Patent number Date

410

Table 13.2 Continued Patent number Date

Title

Patentee/s

Summary

GB 862,428

Application 30 May 1957 Specification published 8 March 1961

United States of America US 2,191, Application 8 Sept 1937 194 Patented 5 March 1940

Method and apparatus for forming fibres

American Viscose Corp.

Method for forming fibres from thermoplastic macromolecular substances by discharging molten material into stream of high velocity gas to form fibres

Process for manufacturing artificial fiber from protein contained in soya bean

Assigned to Showa Sgangyo KK, Yokohama

Application applied for by Toshiji Kajita and Ryohei Inoue of Japan

US 2,342, 634

Application 23 August 1939 Patented 29 February 1944

Method of treating fibrous material and produce resulting therefrom

Francis Clarke Atwood, Newton, Mass., assignor to National Dairy Products Corporation, New York,

Method of forming fibres from casein and soya bean proteins using acylation to improve fibre stability

US 2,309, 113

Application 13 May 1940 Patented 26 January 1943

Treatment of artificial protein films and filaments

Oskar Huppert, Chicago, Ill., assignor to the Glidden Company, Cleveland, Ohio

After treatment using polyhydric alcohols (glycerol, glycol, glycol ethers), thiogelatine, controlled drying and heat setting. Twenty-two


out of a constructed fabric. The protein source could be casein (milk-based protein) or soya bean, peanut or castor bean proteins

Table 13.2 Continued Patent number Date

Title

Patentee/s

Summary

Application 23 August 1939 Patented 29 February 1944

Method of treating fibrous material and produce resulting therefrom

Francis Clarke Atwood, MA, assignor to National Dairy Products Corp, New York

Acylation of synthetic protein materials to make them more resistant to water, acids and alkaline solutions

US 2,372, 622

Application 18 November 1942 Patented 27 March 1945 (in Great Britain 28 January 1943)

Manufacture and production of artificial threads, filaments and the like

Robert Louis Wormell, Coventry, England, assignor to Courtaulds Ltd, London, England

Method for improving resistance of threads from milk casein or vegetable seed caseins such as soya beans or peanuts to hot water and hot dilute acid by the use of formaldehyde and dilute sulphuric acid sufficient to form sodium bisulphate

US 2,377, 853

Application 10 May 1941 Patent 12 June 1945

Protein manufacture

Robert A. Boyer, Joseph Crupi, and William T. Atkinson, assignors to Ford Motor Co., Dearborn, Michigan

Method for improved protein manufacturing to produce ‘a purer, more economical and more usable product’ using a slow-freezing process, to dehydrate the protein and thus improve physical characteristics of fibres and paints, glues, sizes, etc.

411

US 2,342, 634


variations listed, including versions for casein (milkbased protein) and zein (maizebased protein)

412

Patent number Date

Title

Patentee/s

Summary

US 2,377, 854

Application 7 June 1941 Patented 12 June 1945

Artificial fibres and manufacture thereof

Robert A. Boyer, William T. Atkinson and Charles F. Robinette, assignors to Ford Motor Co., Dearborn, Michigan

Method of spinning fibres from soya bean protein

US 2,377, 885

Application 20 December 1939 Patented 12 June 1945

Process of manufacture of synthetic wool from soya bean protein

Oskar Huppert, Chicago, Ill., assignor to the Glidden Company, Cleveland, Ohio

Process for spinning an alkaline solution of soya protein into an acid coagulating bath improved by hydrolysing the soya protein with pepsin in a hydrochloric acid solution so producing a controlled ageing of the alkaline solution


Table 13.2 Continued


13.5

413

Generalised method for producing soya bean fibre in the mid-twentieth century

The fundamental requirement in creating a fibre from soya bean protein is forcing a globular protein to become a fibre-forming protein. Wormell noted that in contrast to ‘cellulosic fibres [which] are regenerated immediately on coagulation … protein filaments have to be cross-linked if fibrous products are to be obtained’.26 Five main production stages can be identified: 1. Separation: ‘clarifying’ the soya bean meal and precipitating out the protein. 2. Solubilisation: dissolving the resulting washed and dried curd to form the ‘spinning’ solution. 3. Hardening: forcing this solution, when sufficiently ripened, through spinneretes into a coagulating bath resulting in the formation of fibres. 4. Insolubilising: stretching and hardening this fibres, often using formaldehyde. 5. Controlled washing and drying followed by cutting into staple lengths.

13.4 Robert Boyer and H.R. Drackett with soya bean protein fibre tow. From: Anon. 1944. Drackett Co. produces new soy bean textile fiber. Rayon Textile Monthly, 85 (37).

Each stage of the fibre manufacturing process presented a variety of complex technical challenges. The numerous patents filed by researchers such as Atwood, Boyer, Huppert and Wormell amongst others reveal both the problems experienced in processing these fibres as well as the techniques explored in attempts to resolve them.

414


13.5.1 Producing the soya bean protein curd The production of a suitable oil-free soya bean protein was generally agreed to be critical to the success of the resulting fibres. An unpublished Ford Motor Company typescript notes that extracting the protein from the soya was ‘the most difficult’ stage.27 The normal procedure was to flake the soya beans and then extract the oils and other fatty substances by mechanical or solvent extraction. Traill argues that the choice of solvent influenced the degree of denaturation; hexane resulted in 87% peptisation of nitrogen while ethyl alcohol only resulted in 57% peptisation.28 Heat was also a critical factor, influencing the degree of denaturation obtained; exposure to incorrect temperatures could result in less soluble or darkened proteins.29 Bergen notes that lower temperatures were used when preparing soya for use in fibres.30 The protein was then extracted by dissolving the resulting oil-free substance in weak aqueous solutions of alkali with a pH ranging between 7

13.5 H.R. Drackett inspecting a batch of soya bean fibre as it emerges from the spinnerets. From: Anon. 1944. Drackett Co. produces new soy bean textile fiber. Rayon Textile Monthly, 85 (37).


415

and 12. Reducing agents such as sodium sulphide or sodium sulphite (0.1%) could be added to prevent oxidation during this extraction process.31 After clarification by centrifuging or filtering, an acid was added to precipitate the protein in the form of a soft curd, which was washed to remove soluble salts and excess acids. The curd was then drained through filtering cloths to obtain a protein cake with a solids content of at least 60%; it could then be grated and dried, either at room temperature or under vacuum. Great care had to be taken in controlling the temperature and pH in order to obtain a curd that could be handled. Wormell32 notes that both the solid and liquid byproducts of this process could be treated for use as animal fodder or yeast extract so they too had an economic value. Considerable research was undertaken into methods of improving the quality of the protein curd. Denatured soya bean protein tends to aggregate rather than crystallise so many of the modifications were intended to overcome this adhesive-like behaviour which gave rise to a variety of technical problems.33 US Patent 2,112,210 describes a process in which the protein was solubilised in sodium hydroxide, treated with carbon disulphide and oxidised with air. It was hoped that this method would result in a fibre with better spinning viscosity, stability and tensile strength.

13.5.2 Producing the spinning solution The soya protein was then dissolved again to form a viscous solution, often referred to as ‘dope’, with the consistency of molasses. A high viscosity spinning solution was needed to obtain a fibre, ideally with a high solids content of up to 20%. Proteins tend to gel in high concentrations so problems were experienced with forming fibres from the spinning solution. Atwood experimented with dissolving the protein in caustic soda (US 142,574; see Table 13.2). Astbury and his co-workers also explored the potential of using aqueous urea solutions to solubilise ‘corpuscular’ proteins (GB 467, 704 and GB 467,812). Lundgren, arguing that neither was effective, recommended the use of synthetic detergents.34 The resulting soya protein was then slightly hydrolysed by pepsin or more fully hydrolysed by alkalis (US 2,309,113; see Table 13.2). Huppert observed that it was more difficult to form a spinnable solution from soya bean protein than from casein protein because the spherical soya protein ‘particles’, despite having larger molecules, form tri-dimensional peptide chains with low viscosity. He recommended treating soya protein with pepsin in hydrochloric acid to form long folded peptide chains running parallel with the length of the micelles (US 2,377,685). However obtained, the solution was then allowed to age or ‘ripen’ to achieve the required high viscosity and ‘stringiness’. The nature of the changes undergone by the soya bean protein during denaturation was studied by a number of researchers including Traill35 and Wormell.36 Traill notes this vital denaturation process,

416


in which the long-chain molecules opened out into an extended form, could be monitored by measuring the quantity of thiol (-SH) groups. 37 The control of enzymes and bacteria in the solution was also a concern. This process is now seen as consisting of two stages: degradation in which peptide bonds break down and denaturation in which the conformation of the molecules changes, in this case from the original folded and globular state to a random state. Later researchers have studied different factors influencing the viscosity of this solution, including pH levels and the effect of sodium sulfite and sodium hydroxide.38,39

13.5.3 Extruding and insolubilising fibres Once sufficiently mature, this solution was ‘wet spun’ (actually extruded) into filaments, sometimes called ‘tow’, by extruding it through fine spinnerets into a precipitation bath (also termed a coagulation bath) (Fig. 13.5). The next stage was to ‘set’ or insolublise the fibres, sometimes termed ‘tanning’ or ‘hardening’. Wormell notes that tanning methods using chromium and aluminium salts were relevant for developing processes to form regenerated protein fibres.40 As a result of Ferretti’s innovative work into the formation of milk casein fibres, the coagulation bath was usually a salt and acid bath such as sodium, aluminium or magnesium sulphate and sulphuric acid.41 The salt had multiple functions. The osmotic pressure created by its presence caused the diameter of the newly extruded filaments to shrink, strengthening them and minimising their tendency to clump together. Insolubilisation was usually achieved by immersing the newly coagulated fibres in a formaldehyde

13.6 Soya beans and soya bean fibre. Harvester SPF Textile Co. Ltd.


417

bath under acid conditions. Traill summarises research into the processes going on during this treatment, but considered these were not clearly understood.42 The formaldehyde reacts with lysine side chain amino acids while cyclic methylene complexes connect other side-chain amino groups through a secondary condensation reaction. The aim was to enable the formation of a complex network between the protein chains with sufficient cross-links to improve the wet strength of the resulting fibre but not so many as to create an over-rigid structure.43 Organic acids such as formic, acetic or lactic acids, all of which are solvents for proteins, could be added to the bath to improve the flow of the solution. Formaldehyde, synthetic tanning agents (‘syntans’) or other spinning aids, such as cation-active agents or anion-active soaps, could be used to reduce the potential for the newly formed fibres to stick to each other or to the processing equipment.44 Wormell also notes that DDT could be added to control behaviour during extrusion.45 Atwood recommends using a softening agent, such as a soap solution, during the neutralisation of the fibre after coagulation in an acid solution (US 2,342,634; see Table 13.2). Lack of strength in the hot baths required for processing was a persistent problem. One patent noted that the filaments ‘tend to stick together, or even to dissolve’ in boiling water or hot dilute acids (US 2,372,622; see Table 13.2). Numerous modifications were developed to try to overcome these technical challenges. A delicate balance of acidity level and temperature was required to improve fibre resistance to boiling water without damaging the fibre’s physical appearance or properties. Methods for improving stability during processing included the use of different stabilising baths. Solutions proposed included formaldehyde and sulphuric acid, formaldehyde, formaldehyde and chromium salt (Ferretti GB 539,985; see Table 13.2), alkali metal bisulphate or sulphate and sulphuric acid sufficient to form a bisulphate (Wormell and Knight US 440,116) or aldehyde and sulphuric acid (US 2,293,986). Wormell’s 1945 patent sought to improve stability by using a strong sulphuric acid solution with formaldehyde and sufficient sodium sulphate to form sodium bisulphate (US 2,372,622; see Table 13.2). Press’s patent explored the potential of forming insoluble condensation products within the regenerated protein fibre itself in order to improve aqueous processing abilities (GB 638,356; see Table 13.2). Others explored techniques to improve the elasticity and flexibility of the fibre after washing (US 2,309, 113; see Table 13.2). Acetylation, sometimes using acetic anhydride at temperatures of 80∞C or above, could be used to improve colour, handle and dyeing performance.46 Atlantic Research Associates used this approach to stabilising fibres formed from either milk or soya bean protein. The acylating agent, which could be in gaseous form, was used to make the newly formed fibres more resistant to water, acids and alkalis (GB

418


13.7 Bleached soya bean fibre top. Harvester SPF Textile Co. Ltd.

536,841; see Table 13.2). Atwood also explored the use of acetylating agents, such as acetic anhydride, ketene, keto-ketenes or ketenes of the lower fatty acids (butyl, proponyl and amyl), applied after the fibre had been partially dried. He considered this made the protein ‘more resistant to oxidation and less reactive chemically’ as well as being more economical (US 2,342,634; see Table 13.2). Although such an acetylation process removed the need to treat the fibre with formaldehyde to improve fibre characteristics, some treatment was still needed to avoid fibres sticking together and embrittling during drying. Atwood proposed using a dehydrating agent such as acetone rather than formaldehyde (US 2,342,634; see Table 13.2). Wormell reported on an alternative method of insolubilising protein fibres using nitrous acid, although he acknowledged that this added a stage to the process.47

13.5.4 Orientation of the fibres through tensioning At this stage the tow, although slightly hardened, was still plastic, vividly described by Traill as ‘flabby’ when wet, soluble in saline solutions, acid and alkali and then becoming brittle when dry.48 Bobbins or reels were used to collect the filaments which were then pulled through a bath over two glass pulleys or wheels, sometimes referred to as godet wheels.49 One of these pulleys revolved faster than the other so the filaments were tensioned or stretched. This process aimed to improve the orientation of the molecules parallel to the fibre length resulting in ‘greater wet and dry strength, and a greater wet–dry strength ratio’ (Atwood US 2,342,634; see Table 13.2).


419

13.5.5 Washing, drying and crimping The fibres were then washed and dried, a complex process requiring careful control of temperature and humidity, but which could be modified to create straight or crimped effects. Crimp frequency could vary from 0–4.7 per cm. Different after-treatments, including bleaching, could be applied before the fibre was subjected to controlled drying. Many patents registered techniques to improve the elasticity and flexibility of protein fibres after washing (e.g. US 2, 309, 113; see Table 13.2). Soya bean protein fibre was manufactured as continuous filaments with a fibre width between 13–27 mm but was normally cut into staple fibres ranging from 0.64–15.2 cm long.50

13.5.6 Dyeing Soya bean fibres had an affinity for the acid and chrome dyes used on wool, but became very harsh and brittle if dyed at below pH 3.51 Atwood discussed dyeing synthetic protein fibres in US patent 2,342,634 (see Table 13.2). He noted that achieving even dyeing of blended fibres requires the regenerated protein fibre to behave in the same way as a natural protein fibre. As wool and silk dyeing then required hot processing, either in acid or aqueous baths, the ability of regenerated protein fibres to withstand such processing was crucial. An alternative, proposed by Boyer, was to add dyestuff directly to the spinning solution.52

13.5.7 Manufacturing requirements: spinning, blending and weaving Once a relatively stable thread was formed, standard textile manufacturing methods and machinery could be used. The threads could be plied, passed through a picker and blender, carded, and then twisted on a normal warping machine. The soya bean protein yarn could be blended with wool, rayon or silk yarn at this point. The yarns were then ready for weaving. Ramseyer53 reported that equipment designed for spinning rayon (regenerated cellulose) could be used while Wormell54 noted that regenerated proteins could be processed on cotton spinning machinery. Boyer stated the fibre blended well with wool, cotton and rayon and could be processed using either cotton or worsted wool fibre equipment.55 Blending regenerated protein fibres with poorer quality woollen-spun wool yarns improved the qualities of both (Fig. 13.2 and Fig. 13.3).

13.5.8 After care The fibres were said to be stable to dry cleaning solvents but shrank in boiling water.

420


13.8 Soya bean protein fibre yarns. Meedoo Textile Co. Ltd.

13.6

Contemporary research into alternative protein fibre sources

Soya bean protein fibre, now often referred to as SPF, is again being explored as a source for commercially viable fibres. This is a rapidly developing area with research being undertaken in several countries, primarily America and China (Fig. 13.6). The impetus behind the late twentieth and early twentiethfirst century research into regenerated protein fibres is to do with reducing the ecological impact of large-scale production and consumption rather than seeking substitute fibres in a time of shortages. Wormell had raised the issue of ecological damage caused by merino sheep as early as 1954, arguing that animals were poor converters of food into protein fibre.56 Regenerated protein fibres, probably produced in combination with synthetic polymers, could become technically and economically viable fibres which have less environmental impact than purely oil-based synthetic fibres. The use of renewable resources, reduced environmental impact of processing chemicals and the biodegradability of the resulting fibre all needed to be considered in looking at the life-cycle impact of these new regenerated protein fibres. SPF is being promoted as a healthy and comfortable fibre which also has ecological


421

13.9 Soya bean protein fabric. Meedoo Textile Co. Ltd.

benefits. However, serious issues have been raised for some time about the impact of large-scale intensive soya bean farming on the environment. Argentina, for example, is suffering deforestation, estimated at the rate of 10,000 hectares per year, with a consequent impact on the lives of indigenous communities. There are also concerns about environmental and economic effects of large-scale use of patented or licensed genetically-engineered soya bean seeds and related herbicides supported by bio-technology companies.57,58 Conversely, the Chinese are arguing that the development of soya fibre could be environmentally beneficial. Throughout the 1990s Chinese researchers, supported by the State Economic & Trade Commission’s national technology innovation programme, were exploring methods of developing fibres from soya bean protein. New bioengineering approaches using enzymes seem to have been the key to renewing the potential of soya bean protein as a fibre source. The aim is to produce soya bean fibres with a soft handle and attractive lustre which could become a replacement for cashmere fibres. China is a world leader in the production of cashmere, producing 80% of the world’s annual production (10,000 tons/9,071.8 metric tons). However, the cashmere goats are highly destructive of grassland on which they graze and have been accelerating the process of desertification. A move to a commercially viable soya-based fibre could help preserve the environment.59,60 Alternative methods

422


of producing soya bean protein fibres have also been researched in America, partly funded by the United Soybean Board and the US Department of Agriculture. Their research has tended to focus on the development of bicomponent fibres combining soya protein with synthetics.

13.7

Contemporary methods for producing fibres from soya bean protein

Two innovations appear central to the development of the soya protein fibres (SPF). Biochemistry is being used in the production process to modify the structure of soya bean protein while strength is added to the fibre by incorporating polyvinyl alcohol (PVA; Fig. 13.10) although methods for achieving this vary. PVA offers the benefits of higher strength and modulus and, like SPF, is soluble in water and exhibits hydrogen bonding so the same processing methods can be used. Fibres from water soluble PVA are said to be biodegradable in soil. In contrast with the mid-twentieth century producers, Chinese companies making soya bean fibres generally have expertise in fibre and textiles manufacture. In addition to producing a competitive fibre to compete or complement with wool and cashmere, SPF fibres are being promoted as having health benefits through their beneficial impact on skin.61 O *

*

13.10

13.7.1 Generalised methods for producing SPF Similar to the processes used by mid-twentieth century manufacturers, the protein is first isolated from soya bean meal from which the oil has been extracted. The Chinese method then uses bioengineering techniques to change the structure of the spherical soya bean protein using enzymes and an unidentified ‘functional auxiliary’.62 PVA may be incorporated into the heated spinning solution. The fibre is ‘wet spun’ (extruded) before stabilisation through acetylation, curling and thermoforming before cutting into short staple lengths. Careful pre-treatment, dyeing and finishing of SPF yarn appears to be necessary to obtain and retain the desired fibre characteristics. Yi-you describes the use of ‘hydroformylation’ to stabilise the fibres before they are wet spun, heat set and cut.63 American researchers seem to be favouring an approach that modifies the fibre structure itself by creating a bi-component fibre with a PVA core surrounded by an outer sheath of soya protein. This is said to add strength and stability to the fibre whilst retaining lustre and soft hand, although problems have been experienced with the drawability of the fibre.64


423

Drying Drying has to be carefully controlled to maintain a good hand. It should be carried out with as little tension as possible and at temperatures below 100∞C.65 Bleaching and dyeing properties SPF fibres are usually light yellow in colour. They are stable with both hydrogen peroxide and reduction bleaching. A whitening or brightening treatment may be necessary for purer whiteness (Fig. 13.7). When bleaching an SPF blended with another fibre, care must be taken to use a method suitable for both fibres. Either acid or reactive dyes may be used; dye fastness compares well with that of silk.66

13.11 Tee-shirt in ‘Luxury Soy’ 55% soy, 40% cotton, 5% Lycra®. Colorado Trading and Clothing. Photograph © Textile Conservation Centre, University of Southampton.

424


Finishes A variety of finishes can be used to soften the fibre and give anti-wrinkle properties. The fibre is being promoted as health-giving with natural antibacterial properties. However, antibacterial finishes are also being applied. Yi-You notes that ‘the addition of Chinese herbal medicine’ with sterilising and anti-inflammatory properties can be added during the production of the fibre. He argues that this gives the resulting fibre medical properties which are more long-lasting than those obtained from after-treatments.67 Manufacturing requirements: spinning, weaving and blends The resulting fibres can be spun on cotton or worsted machinery and blended with cashmere (80% soya to 20% cashmere) or in 50% blends with wool, silk, cotton (Fig. 13.8). It can be woven into high quality fabrics with high weave counts (Fig. 13.9). After-care SPF has a high modulus resulting in low shrinkage in hot water.68 It is therefore said to have stable to normal washing requirements; it is also claimed to be a fast drying fibre. However, Colorado Trading recommends cold machine washing with low temperature ironing and non-chlorine bleach only for its SPF fibre blend tee-shirts.

13.7.2 Contemporary commercial availability Soya bean fibre is being marketed commercially, sometimes as ‘vegetable cashmere’, as yarns and as garments including underwear as well as in bedding.69 Some soya textiles are being marketed with organic certification although SPF is currently more expensive than other organic fibres, costing approximately 30% more than organic cotton.70 Yarns The South West Trading Company (2004) is the North American distributor for a range of soya bean protein fibre yarns marketed under the trademark Soy Silk™. This is being sold to hand spinners, weavers and knitters as an ‘environmentally friendly fibre’ made from the waste of the tofu manufacturing process. Yarns available include worsted ribbon yarn, sport yarn, lace weight yarn, chenille and blends with wool or cashmere. Staple threads for spinning are also sold in natural or white which can be home-dyed using acid dyes. Staple fibres are also available in the UK and are similarly being marketed to spinners and feltmakers.


425

Garments SPF is being used in an increasingly wide range of garments, tee-shirts and sweaters and underwear, sometimes in blends with bamboo fibre and silk. The Sichuan Silk Corporation exhibited knitted soya bean fibre shirts at the China Export Commodities Fair, Guangzhou, Guangdon Province in 2001. Over ten overseas importers were reported as signing letters of intent to promote these garments.71 The Nanjing Textiles Import/Export Co. showed SPF underwear at the October 2002 New York International Fashion Fabric Exhibition.72 Soya fibre appears to have been introduced to the American market in 2003 at the MAGIC fashion trade show in Las Vegas. Colorado Trading and Clothing (Boulder, Colorado, USA) exhibited clothes made from soy fibre blends, including SoyBu™, a soya bean/bamboo mix, arguing that this makes an expensive new fibre more widely available to more people. Their marketing stresses that the fibre is ‘eco-conscious’ and ‘naturally antimicrobial’.73 They market tee-shirts which are a soya protein, cotton and Lycra® blend (Fig. 13.11) as well as robes and throws in soya protein fibre and polymicro chenille. Other companies, including Of The Earth and Under the Canopy, are also offering SPF clothing as part of a range of garments made from organic or renewable textile fibres.

13.8

Fibre characteristics

13.8.1 Mid-twentieth century soya bean protein fibres Under magnification, these soya bean fibres were translucent with a smooth surface although some granulation and streakiness was often visible. The cross-section was almost circular.74,75 Wormell noted that fibres from ‘seed proteins’ were brownish or yellowish76 although Ford’s process apparently resulted in a white fibre (Fig. 13.1) whereas Drackett’s fibre was light tan to white. The staple fibre was described as ‘a loose, fluffy mass with a resemblance to scoured wool’, soft to touch and with good resiliency.77 Its high moisture absorption and high heat of wetting made it warm and comfortable to wear. Under standard conditions (65% RH and 21∞C), soya bean fibre showed a 16.1% regain when coming to equilibrium from the wet state and 12.9% regain from the dry state. This is similar to the hysteresis exhibited by wool.78 Despite this superficial resemblance, soya bean protein fibres had lower wet and dry strength and elasticity than wool (see Table 13.3). All midtwentieth century manufacturers experienced problems with the low tensile strength and poor wet strength of soya bean fibres. Sherman and Sherman stated that ‘its wet strength [was] so inferior as to constitute a distinct handicap’.79 Dry soya bean fibre had a tensile strength of about 55% that of

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Table 13.3 Tensile strength of soya bean fibre compared with wool of the same grade. Bundle test at 65% relative humidity and 21∞C Property

Soya bean staple 1944

Wool top 625

Dry tensile strength (kg cm–2) Wet tensile strength (kg cm–2) Strength, loss from dry (%)

805 298 63.0

1476 1244 15.8

Sherman, J.V. and Sherman, S.L. (1946) The New Fibers, New York, D. van Nostrand Company, 185

wool but, when wet, the tensile strength dropped to about 24% the strength of wool. Soya bean fibres’ loss of strength when wet is particularly noticeable, being about 35–50% of its dry strength. This compared with wool which retains about 85% of its dry strength when wet.80 When wet, soya bean protein fibres were 76% weaker than similar quality wool fibres. Boyer reports that the fibre produced at Ford had 80% of the strength of wool with greater wet and dry elongation.81 Drackett claimed in 1944 that this weakness had been overcome, but this seems to have been over-optimistic; lower strength usually results in greater extensibility. Soya bean fibres, like wool and casein fibres, had high extensibility and similar moisture absorption (10–12%), heat-insulating characteristics, felting and dyeing characteristics to wool. In comparison with other regenerated protein fibres, soya bean fibres were ‘intermediate in properties between peanut fibres and those from milk casein, reflecting intermediate values for basic amino-acid constituents and amide nitrogen’82 (see Table 13.4). When blended with other fibres which provided much-needed strength, such as rayon or cotton, soya bean protein fibres provided warmth and softness.

13.8.2 Contemporary soya bean protein fibres Contemporary SPF differs considerably from that of the mid-twentieth century fibres. The Chinese version has a grooved surface and a dumb-bell crosssection with a microporous structure. American versions tend to have a circular cross-section with a central synthetic core. The fibre is naturally coloured light yellow so requires bleaching prior to dying and has a soft lustre similar to that of silk. Like the mid-twentieth century soya bean protein fibre, woven SPF is said to be comfortable to wear with a soft hand and good drape, similar to that of silk; Chinese manufacturers compare it to cashmere.83 The fibre is said to have good warmth retention and better moisture transmission than cotton, making it comfortable and healthy to wear. Huakangtianyarn Ltd state their fibre has 8.6% moisture regain, again similar to that of cotton. However, problems with tenacity and wet strength appear to remain. They

Table 13.4 Characteristics of soya bean fibre in comparison with other fibres Soya bean

Casein milk fibre

Wool

Silk (degummed)

Nylon

Specific gravity (g cm3) Dry tenacity (g den–1) Wet tenacity (g den–1) Tensile strength (kg cm–2) Dry extensibility at break (%) Wet extensibility at break (%) Residual elongation, wet (%) Residual elongation, dry (%)

1.31 0.6–0.7 0.35–0.50 700–840 30–40 60–70 8.0 Fibre broke before 20% extension reached 0.006 0.28 0.05% Fibre broke before 20% extension reached

1.29 0.6–0.7 40–50 700–910 30–50 85–120 9.0 15.5

1.32 1.2–1.7 80–90 1400–2000 30–50 30–60 0.0 12.0

1.14 4.5–5.7 84–90 4570–8220 12–20 13–26 Not tested Not tested

0.016 0.24 0.11% 0.44%

0.10 0.22 0.32% 0.72%

1.25 2.8–5.00 75–90 3100–5600 13–20 – 13.6% Fibre broke before 20% extension reached 0.26 0.74 3.27 Fibre broke before 20% extension reached

Young’s modulus, wet (%) Young’s modulus, dry (%) Load at 20% elongation, wet Load at 20% elongation, dry

Not Not Not Not

tested tested tested tested

Data derived from Harris and Brown, Natural and synthetic protein fibres, Textile Research Journal, XVII, 6, 323–330, 1947. Harris and Brown note that soya bean fibre was an experimental fibre and that these are not representative figures as fibre properties change with different treatments.


Property

427

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also claim that the dry strength of their fibre is higher than that of wool, cotton and silk at 3.0 cN dtex–1 while the wet strength is similar to cotton at 2.5–2.0 cN dtex–1. Data about the characteristics of the Winshow SPF is available on their website.84 The fibre is said to have a higher breaking strength than wool, cotton and silk (over 3.0 cN dtex–1) and a high modulus with low shrinkage in boiling water making it stable to normal domestic washing. Results of published tests indicate that SPF has reasonable wet permeability, better moisture transmission properties than silk and is better than silk in retaining warmth although less well than wool. Its low friction coefficient results in a good hand which is combined with low pilling. Additionally, it is said to have ‘natural bacteria resistance’ to coli bacillus, Staph. aureus and Candida albicans.85

13.9

Identifying soya bean protein fibres

13.9.1 Mid-twentieth century fibres These fibres have few surface features and a circular cross-section – see Fig. 13.12(a) and (b). However, these features cannot be used as the sole basis for identification. In common with other man-made fibres, such characteristics may be influenced by the temperature of extrusion, the viscosity of the spinning solution or pressure exerted by processing equipment, depending upon the degree of plasticity in different stages of production. Fine marks and striations may therefore be visible on the surface of all such fibres.86 Fletcher reports that soya bean fibres responded like wool to chemical and burning tests.87 Press describes their behaviour in burn tests: soya bean protein fibres melt away from the flame before touching the flame and melt and burn in the flame with a smell of burning feathers although they do not combust easily, tending to melt before burning.88 The black ash is said to be brittle, puffy and easily crushable. Williams and Tonn applied a range of standard stain methods to enable soya bean fibres to be distinguished from other regenerated or natural protein fibres (see Table 13.5).89 They used soya bean fibre samples from Ford Motor Company, the Glidden Company, Cleveland, and the United States Soybean Laboratory, Urbana, Illinois (see Table 13.5). Although unsuccessful in distinguishing between different soya bean protein fibres, these tests did distinguish regenerated protein fibres from natural fibres. Wormell outlines a method to estimate the amount of a regenerated protein fibre in blends with natural protein fibres dependent upon establishing the relative amounts of phosphorus while also noting that the soya bean fibres test positive for tryptophan when soaked in concentrated hydrochloric acid.90


429

(a)

(b)

13.12 (a) Longitudinal view (¥75) of mid-twentieth century soya bean protein fibre. From: Harris, M. (1954). Handbook of Textile Fibers. Washington: Harris Research Laboratories, 82. (b) Cross-sectional view (¥380) of mid-twentieth century soya bean protein fibre. From: Harris, M. (1954). Handbook of Textile Fibers. Washington: Harris Research Laboratories, 82.

13.9.2 Late twentieth and early twenty-first century fibres SPF has irregular striations running longitudinally along the surface which are said to contribute to the fibre’s moisture-absorbing properties (Fig. 13.13).

430

Table 13.5 Stain tests a-napthol hypobromite test for arginine

Ninhydrin test for b-alanine

Adamciewicz test for tryptophane

Vanilla test for tryptophane

Morse test for hydroxyproline

Solubility in 18% NaOH (1 hr hot)

Sulphur test for cystine

Ford soya bean

Deep red

Colourless

Deep wine purple

Purple

Colourless

Soluble

Black

Glidden soya bean

Deep red

Colourless

Brown purple

Purple

Colourless

Soluble

Black

USDA soya bean*

Deep red

Colourless

Deep wine purple

Purple

Colourless

Soluble

Black

Non-pigmented Aralac (USA milk-based fibre)

Orange

Blue purple

Colourless

Light brown

Rose-red

Disintegrated but not dissolved

Black

Pigmented Aralac (USA milk-based fibre)

Faintly orange

Colourless

Colourless to faint lavender

Light brown

Rose-red

Undissolved

Black

Lanital (Italian milk-based fibre)

Orange

Blue purple

Colourless

Light brown

Rose-red

Disintegrated but not dissolved

Black

Wool

Deep red

Blue purple

Colourless

Light purple

Colourless

Soluble

Black

Silk

Deep red

Colourless

Slightly yellowed

Purple (disintegrates)

Colourless

Soluble

Colourless

Nylon

Yellowed

Blue purple

Dissolves

Dissolves

Colourless

Soluble

Colourless

From: Williams S. and Tonn W.H., Qualitative methods of identifying soybean fibres in mixtures of casein fibre, wool or other textile fibre, Rayon Textile Monthly September, 63–64 (523–524), 1941. * Soya bean protein fibre from the United States Soybean Laboratory, Urbana, Illinois obtained through A.E. Stanley Manufacturing Company


Fibres


431

The cross-section of the Chinese fibre takes the form of irregular dumb-bells with micro-pores sometimes described as ‘islands-in-a-sea’. This makes the fibre permeable to air and moisture.91 Spectral analysis of several samples of modern soya fibre showed the presence of protein (two small peaks at 1640 and 1530 cm–1, the amide I and II bands) along with a larger quantity of cellulose or polysaccharide material (Fig. 13.14).

13.10 Degradation behaviour 13.10.1 Mid-twentieth century fibres Soya bean fibre was easily degraded by alkali and yellowed considerably when placed in a conditioning oven and exposed to a temperature of 220∞F. This is comparable to the behaviour by casein fibres.92 Fletcher reports that soya bean fibres mildewed less easily than natural and casein fibres but more easily than the synthetic fibres.93 Views diverge on the susceptibility of regenerated protein fibres to biological attack. Wormell argues that ‘the more a protein molecule is changed by chemical and tanning [hardening] processes, the less likely is it to suffer biological attack’.94 Others contended that regenerated protein fibres, including soya bean fibres, were subject to attack by moths.95 An anonymous writer in Rayon Textile Monthly notes that preventing such damage is a major issue ‘when fibre conservation is of such primary concern on both military and civilian fronts’.96 It should be noted, however, that the author recommends the use of Merck & Co.’s anti-moth treatment ‘Amuno’ and the nature of the article is such that it seems possible that it is a promotional piece. However, the fibre’s lack of strength when wet appears to have been the major route for degradation.

13.10.2 Late twentieth and early twenty-first century fibres SPF is being actively promoted as a biodegradable fibre. It is said to wear well, being resistant to acid, alkali, perspiration and light, including UV light, although little reliable quantitative data appears to be available.97 Two months’ outdoor exposure of the Huakangtianyuan SPF fibre resulted in little fading, 11% strength loss and no fungal formation. Exposure to UV for 120 hours resulted in a 9.8% strength loss.98 It has good acid and alkali resistance. Exposure to dry heat caused the fibre to become yellow and sticky.99 The fibre itself is said to be biodegradable in landfill100 and it seems likely that biodegradation processes would be initiated through exposure to water.

432


(a)

(b)

13.13 (a) Longitudinal SEM view of contemporary soya bean protein fibre. Note the irregular grooves. From: Senshoku Keizai Shimbun, 2004, Physical characteristics and processing method of Chinese soybean fiber. Textileinfo.com. (b) Cross-sectional SEM view of contemporary soya bean protein fibre. Note the irregular dumbbell shape with the so-called ‘islands-in-the-sea’ structure. From: Senshoku Keizai Shimbun, 2004, Physical characteristics and processing method of Chinese soybean fiber. Textileinfo.com.


433

Tee shirt Colorado Clothing Company (made in China)

Soy fibre (retailer Paper Shed, Yorkshire, UK)

Soy fibre (retailer Winghan Wool Work, Yorkshire)

Cotton reference

Polyester polyurethane reference

Degummed undyed Habutai silk

4000

3500

3000

2500

2000

1500

1000

500

13.14 Comparative ATR spectra of soya bean protein fibres and natural and synthetic fibres. © Textile Conservation Centre, University of Southampton.

434


13.11 A truly biodegradable and ecological fibre? Biodegradable fibres degrade safely and relatively quickly through biological processes, returning to their source materials. The mid-twentieth century fibre appears to have been all too successful in achieving this goal. The status of modern fibres is rather more complex. Pure SPF is being promoted as being as a biodegradable fibre; however, the impact of finishes on this process need to be considered as does the biodegradability of the PVA used in bi-component fibres. Whether or not it can be considered as an ecological fibre is not straightforward. Soya beans are a renewable resource although the environmental impact of their production is increasingly being questioned; the production method is said not to be environmentally damaging. Manufacturers such as Huakangtianyarn Ltd 101 stress the ecological acceptability of the process: the agents used in production are said to be nontoxic while other auxiliaries can be recycled; the residues of the soya beans may be used as animal fodder once the protein has been extracted.102,103 However, the wider environmental impact of large-scale soya bean farming needs to be factored into an overall evaluation of the environmental impact of SPF.

13.12 Conclusion The future of soya protein fibres is related to the availability of soya bean protein, which is influenced by political, economic and ecological issues. Commandeur et al.104 argue that sustainable development, with reduced animal production, less dependence on agrochemicals and diversification of the vegetable oil sector might act to reduce soya production. The future of genetically modified soya beans will also have an impact on production. However, the future of the fibre itself seems promising. Li Jinbao, Director of the Textile Science and Technology Centre, China Textile Industry Association is optimistic that problems with flexibility can be overcome.105 The attraction of renewable and organic textiles is growing; Magruder, director of Fabrikology International notes ‘The whole category of renewable resourcebase fibres is going to be huge’.106 Echoing Henry Ford, Yi-you argues that soya bean fibres have the potential to create a new range of products which additionally ‘will be beneficial to the industrialisation of agriculture’.107 Clearly, SPF is being continually improved with the focus on developing yarns, spinning technology and weaving methods and manufacturers are developing confidence in the properties of the new fibre. Introducing consumers to the fibre is as critical as the performance of the fibre. SPF could remain a niche fibre, marketed for ecologically conscious and probably better-off consumers, or could develop into a mainstream fibre, effectively competing with natural and synthetic fibres. Whether it achieves acceptability as an


435

economically competitive fibre, capable of holding its own in the marketplace with natural and synthetic fibres and with the added benefit of being a biodegradable fibre, is an unfolding story.

13.13 Acknowledgements Much of the initial research for this paper was undertaken at the Getty Conservation Institute (GCI), Los Angeles as a GCI Scholar in 2002. I am most grateful for the Institute’s generous support and particularly for the input of their research librarians, especially Luke Swetland and Mitchel Bishop. I would also like to acknowledge the help of colleagues at the Textile Conservation Centre, University of Southampton, particularly Dinah Eastop, Leo Dokos, Paul Garside for analysis and Michael Halliwell for imaging. Denise Buhr (Research Librarian, Central Soya, Indiana) and Patricia Starrett (Archivist, ICI) kindly searched company archives on my behalf. Finally, I would like to thank Nell Hoare, Director, Textile Conservation Centre, University of Southampton for permission to publish.

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78. Sherman, J.V. and Sherman, S.L. (1946), The New Fibers, New York, D van Nostrand Company, 186. 79. Sherman, J.V. and Sherman, S.L. (1946), The New Fibers, New York, D van Nostrand Company, 183. 80. Sherman, J.V. and Sherman, S.L. (1946), The New Fibers, New York, D van Nostrand Company, 22. 81. Boyer, R.A. (1940), ‘Soybean protein fibers: Experimental production’, Industrial and Engineering Chemistry, 32 (12), 1549–1551, summarised in Shurtleff, W. and Aoyagi, A. (1994), Henry Ford and his Researchers’ Work with Soybeans, Soyfoods, and Chemurgy – Bibliography and Sourcebook, 1921 to 1993, Lafayette CA, Soyfoods Center, 53. 82. Wormell, R.L. (1954), New Fibres from Proteins, London, Butterworths Scientific Publications, 153. 83. Yi-you, L. (2004), ‘The soybean protein fibre – a healthy and comfortable fibre for the 21st century’, Fibres and Textiles in Eastern Europe, 12 (2), [46], 8–9. 84. Huakangtianyuan High-Tech Co. Ltd (n.d.), Introduction to SPF, www.soybeanfibre.com, accessed 28 August 2004. 85. Swicofil, A.G. Textile Services (n.d.), Soybean Protein Fiber, www.swicofil, accessed 28 August 2004. 86. Wormell, R.L. (1954), New Fibres from Proteins, London, Butterworths Scientific Publications, 170. 87. Fletcher, H.M. (1942), Synthetic Fibers and Textiles. Agricultural Experiment Station Bulletin 300, Kansas, Kansas State College of Agriculture and Applied Science, 16. 88. Press, J.J. (1959), Man-made Textile Encyclopedia, New York, Textile Book Publishers, 143. 89. Williams, S. and Tonn, W.H. (1941), ‘Qualitative methods of identifying soybean fibers in mixtures of casein fiber, wool, or other textile fiber’, Rayon Textile Monthly, XXII, 11, 63–64. 90. Wormell, R.L. (1954), New Fibres from Proteins, London, Butterworths Scientific Publications, 153 and 175–176. 91. Shimbun, S.K. (1999), Physical Characteristics and Processing Method of Chinese Soybean Fiber, www.Textileinfo.com, accessed 3 March 2004. 92. Sherman, J.V. and Sherman, S.L. (1946), The New Fibers, New York, D van Nostrand Company, 186. 93. Fletcher, H.M. (1942), Synthetic Fibers and Textiles. Agricultural Experiment Station Bulletin 300, Kansas, Kansas State College of Agriculture and Applied Science, 17. 94. Wormell, R.L. (1954), New Fibres from Proteins, London, Butterworths Scientific Publications, 145. 95. Dooley, W.H. (1943), Textiles (new revised edition with experiments), Boston, Heath & Co., 699. 96. Anon. (1942), ‘Beetles and moths will attack all protein fibres’, Rayon Textile Monthly, XXIII (9), 109. 97. Yi-you, L. (2004), ‘The soybean protein fibre – a healthy and comfortable fibre for the 21st century’, Fibres and Textiles in Eastern Europe, 12 (2), [46], 9. 98. Huakangtianyuan High-Tech Co. Ltd (n.d.), Introduction to SPF, www.soybeanfibre.com, accessed 28 August 2004. 99. Anon. (2003), ‘China develops soyabean fibre’, Textiles Magazine, 30 (2), 4.

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Index

abaca 81–6 anatomy of the plant 83 applications 82 cultivation 83 economic importance 82–3 fibre physical properties and chemical composition 84–6 straw processing 84 abiotic processes 1 acetate 116, 117 Acetobacter xylinum 137, 138 acetogenesis 7 acetone 104, 327–8, 418 acetylation 417–18 acoustic diaphragms 139, 140 additives 233, 234 advanced fibres 314–15 aerobic incubations 8–9 after care see laundering air-gap spinning 162–3 air jet processing 178–9 air-laying 185, 315–16 alanine 249, 252–3 L-alanine 26–7 aldehydes, fragrant 393 alginates 89–110, 278 alginate-based chitosan hybrid polymer fibre 392 biodegradation 99 in bioengineering 105–7 chemical properties of alginic acid 93–5 chemical structure and composition 92–3 chitosan-coated alginate filament 382–3, 389 as flexible substrates in medical textile-

based products 101–5 industrial applications 100–1 manufacturing process 90–2 moisture properties of alginic acidbased fibres 98–9 physical properties of alginate fibres 97–8 properties of alginate solutions 95–6 structure property relationships 96 thermal properties of alginic acid-based fibres 99 types of 89–90 alginic acid 91, 92–3 a-chitin 380, 382 a-helical silks 252 American soya complex 399 amide bonds 28–9 amine oxide 157, 158, 168 exothermic degradation 162 recovery 164–5 transport 161–2 see also N-methylmorpholine-N-oxide amino acids 5–6, 249 soya bean protein 281, 399, 401 ammonia/ammonium thiocyanate solutions 368–79 amorphous regions 2 amylases 142 analytical methods 17–24 anaerobic incubations 8, 9–11 angiogenesis 102 animal fibres 276–7 chicken feather fibres 277, 333, 403 nonwovens from 332–3 see also chitin; silk; spider silks; wool annealing 236 antibacterial medicament agents 102

441

442

Index

antimicrobial activity of chitosan 389–91 apparel see garments apparent opening size (AOS) 362, 363 Aralac 403, 405 Araneus (garden spider) silk 245–6 Ardil 403 Argentina 421 arthropod shells 380 artificial organs 105–6 artificial skin 139, 148–9, 333 ascophyllum 89–90 Aspergillus niger 125–9 automotive interiors 326 azlon 401 bacteria 4 see also microbial processes bacterial cellulose (BC) 112, 137–40, 279 chitosan-modified 140, 141, 149 bacterial polyesters see polyhydroxyalkanoates BAK 1095 28–9 baling 164 bamboo 273, 343 Bangladesh 62, 63 bast fibres 36–88, 345, 346, 351–4 abaca 81–6 comparison of fibre properties 85, 86, 346–7 extraction and preparation 351–4 flax see flax hemp see hemp jute 60–9, 85, 346, 353 kenaf 78–81, 85, 346, 353 ramie see ramie beryllium alginate 97 best available technologies (BAT) 113 b-chitin 380, 382 b-conglycinin 282, 400 b-glucosidases (bG) 24–5, 123, 124 b-pleated sheets 252 bicomponent fibres 208, 209 binder fibres 208 bio rolls/logs 355 biodegradability xvi-xvii SPF 431, 434 biodegradable resins 279–95 biodegradation 3–4 alginate fibres 99 detection of intermediates of 23–4

lyocell 169–71 microbial processes see microbial processes SPF 431 testing geotextiles 362, 365 Biofill 149 biofinishing 142–4 biogas 22 biological resistance 200 biomass 218 biomass and wind power PLA process 212–13, 214–15 formation 3–4 Bionolle 1030 335 Bionolle 3300 335 biopolishing 142–4 Biosteel 278 biostoning 142, 144 biotechnology alginates in bioengineering 105–7 manufacture and modification of cellulosic fibres 133–40 bioprocessing of cellulose 133–6 microbial synthesis 137–40 soya protein fibre 421, 422 biotic processes 1 see also microbial processes biotransformed cellulose pulp 120–31, 134–6 birefringence 259 bleaching 30 cotton 30, 142, 323–4 lyocell production 168, 169 ramie 75 soya bean protein fibre 423 blending/blends biodegradability of cellulose fibres in 131–3 PHAs 231–2 PLA 206 soya bean protein fibre 419, 424 Bombyx mori (silkworm) silk 245, 253, 254, 276–7 bonding techniques 315, 316, 319–21 bonds crosslinking see crosslinking hydrogen see hydrogen bonds susceptible to enzymatic attack 24–9 branding 187–8 breakers 45–6

Index breaking force 21, 59, 362, 363 brown algae 89–90 see also alginates budworm 56 bullet-proof vests 262 bursting discs 162 Buss filmtruder 160–1 Buswell’s equation 22 C-terminal sequences 251–2 calcium alginate 91–2, 101, 102 calcium chloride dihydrate saturated methanol 381–2 caprolactone 197 capture silk 246–7, 257 carbon dioxide 214, 218 mineralisation 3 measuring 21–3 carbon disulphide 133–4 carding 75, 182–3, 316, 356–7 carpets 208 casein fibres 403, 426, 427 cashmere 421 catalytic polymerisation 196–7 caustic soda treatment 175, 177, 180 cellobiohydrolases (CBH) 123, 124, 129, 130 cellulases 24–5, 123, 124, 129, 130 applications in fabric and dyestuff processing 140–4, 145–7, 148 cellulose 92–3 composition of plant-based fibres 60, 85, 274–5, 347–8 flushable nonwovens 337 microbial processes 5, 24–5 see also cellulosic fibres cellulose acetate (CA) 326 cotton/cellulose acetate biodegradable nonwovens 327–8 cellulose carbamate (CC) 118–19 cellulose diacetate 116, 117 cellulose III polymorph conformation 377 cellulose triacetate 116, 117 cellulosic fibres 111–56 biodegradability in textile blends 131–3 biotechnology for manufacture and modification of 133–40 bioprocessing of cellulose 133–6 microbial synthesis 137–40

443

enzyme applications in fabric and dyestuff processing 140–4, 145–7, 148 future trends 150–1 hygienic and medical fibres 144–50 medical and health-care products 144–9 tissue engineering 149–50 life cycle assessment 115–17 comparison of man-made fibres with cotton and cellulosics 119–20, 121, 122 properties and environmental costs of regenerated cellulosic fibres 118–19 mechanisms of enzymatic reactions 120–31 nonwovens 322–5 salt solution processes 367–8 ethylene diamine/salt system 393–4 fibre performance and characterisation 374–9 liquid ammonia/ammonium thiocyanate solutions 368–79, 393 preparation of filament 369–74 see also under individual fibres Celsol 118–19, 134, 135–6 centrifugation spinning 230 chemical bonding 319–20 chemical cottonisation 46–7 chemical degumming/retting 44, 74, 352 chemical recycling 216–17 chemical resistance 200 chicken feather fibres 277, 333, 403 China 62, 63, 406, 421 China-grass 70 see also ramie chitin 24, 25, 333, 367–8 fibres from 380–93 applications of chitin fibres 389–93 fibre performance and characterisation 387–9, 390 future trends 394 preparation of filament 380–6 chitin-acid glycosaminoglycan fibre 391 chitin deacetylase 25 chitinase 25 chitobiase 25 chitobiohydrolase 25

444

Index

Chitopoly 383, 392–3 chitosan 24, 25, 278, 367–8 fibres from 380–93 applications of chitosan fibres 389–93 fibre performance and characterisation 387–9, 390 future trends 394 preparation of filament 380–6 chitosan-modified bacterial cellulose 140, 141, 149 chloral hydrate 381 chlorhexidine 102 chromium alginate 91, 97 climate change, global xvi, 213–15 ‘closed-loop’ technology 168 coagulation bath 90, 91, 413, 416 coir 355 collagens 150 Colorado Trading and Clothing 425 colour 67 colour fastness 206 commercial culture collections 13, 15 composite materials 271–3 natural fibre composites see natural fibre composites composition, detecting changes in 17–18 composting xviii-xix lyocell 169–71 PLA 216, 217 Composting Association xix condensation 193 conglycinin 282, 400 contraction 174–5 controlled drug release 106–7, 240, 392 coordination-insertion mechanism 196, 197 copolymers lactide and caprolactone 197 PHB with higher PHAs 223, 230–1 PET and PCL 235, 240 corn 192–3 Corona discharge treatment 48 cost xvii, 322, 323 cotton 11, 66, 77, 172 chitosan-coated cotton fibre 392 enzymatic treatments 30 enzyme applications in processing 142–4, 145–7, 148 life cycle assessment 113–15

comparison of man-made fibres with cellulosics and 119–20, 122 nonwovens 322–4 cotton-based biodegradable nonwovens 325–31 cotton/cellulose acetate 327–8 cotton/Eastar 328–31 processing 326–7 Cotton Incorporated 322 Cotton Seal 323 cottonisation 46–7 cover factor 361 Crabyon 393 creasing 175–6 crimp 164, 174–5, 198, 360, 419 crosslinking 3 alginates 95, 97 chitosan fibres 388–9 cross polarisation/magic-angle spinning (CP/MAS) 13C-nuclear magnetic resonance (NMR) spectroscopy 18 crude enzyme preparations 13–14, 16 crystalline regions 2 cellulose 125, 126 crystalline arrays in silks 252–3, 254 crystalline structure of PHB 235 cupro (cuprammonium) process 115, 116, 134 curd, soya bean 414–15 cutting 164, 413, 419 cysteine 25 cystine 25 cytosine 250, 251 DDT 417 decortication 46, 81 defect zone 374–7 degumming/retting 29–30, 351–2 flax 43–5 hemp 29–30, 43–5, 57–8 jute 64, 68 kenaf 81 ramie 29–30, 73–5 denaturation 415–16 denim 141, 142, 143 density alginate fibres 98 bast fibres 85 PLA 197

Index Deposa 334 dew retting 44, 351 diffraction planes 377–9 diseases, in bast fibre plants 42, 56 displacement, fabric 175–6 disposable nonwovens 311, 313–14 flushable 337–8 wet laid with flax fibre 331 disposal see waste disposal disulphide bonds 25–6 dragline silk 245–7, 277 mechanical properties 256–7, 258, 259 draw-down ratio 374, 377, 379, 387 drawing 75, 357–9 PHAs 226–7 structure of drawn fibres 235–6 drugs, controlled release of 106–7, 240, 392 dry-jet wet spinning 373–4 dry laid process 185, 315–16 dry sieving method 363 drying abaca 84 chitosan fibres 387–8 lyocell fibre 164 soya bean protein fibre 413, 419, 423 duvets 207 dyeing enzyme applications in fabric and dyestuff processing 140–4, 145–7, 148 lyocell 169, 176–81 PLA 205–7 soya bean protein fibre 419, 423 dynamic cellulose biosynthesis 137–8 DyStar 205 Eastar (PTAT) 326, 334 cotton/Eastar biodegradable nonwovens 328–31 easy care lyocell 180 Eco-label 168 Ecuador 82 electrolytic degumming 58 electron beam irradiation 239–40 electrospinning 150, 151, 241, 278 PCL 233–4 elementary fibres 59, 66, 85 elongation 85, 362, 363 emulsification 100–1

445

encapsulation 101 alginates 105–7 endoglucanases 24–5, 123, 124, 129, 130 ends per cm 360 energy efficiency 171 energy use xv-xvi, 212–13, 218 enrichment/selective cultures 13, 15 environmental impact of SPF 420–2 Environmental Management and Audit Scheme (EMAS) 113 environmental management strategies 112–13 environmental protection costs 118–19 enzymatic degumming 57–8 enzymatic reactions degradation of PCL 238 degradation of PHB 237–8 modification of cellullose 115–16, 117 on wood and cellulose 120–31 see also microbial processes enzymes, extracellular 5–6 crude 13–14, 16 purified 14, 16 sources of for laboratory incubations 12–16 epichlorohydrin (ECH) 388–9 erosion control 336, 355 erosion control blanket performance 362, 365 ester bonds 27–8 ethyl alcohol 414 ethylene diamine/salt solvent system 393–4 eucalyptus 166 European corn borer 56 European Union Eco-label 168 exoglucanases 24–5 exotherms 162 extension at break 66 alginate fibres 97, 98 extension curves 198–9 extracellular enzymes 5–6 crude 13–14, 16 purified 14, 16 extrudate instability 374–7 extruders, ‘spinning’ 263 fabric basis weight 361 fabric-reinforced composites 271 natural fibre 302–4

446

Index

fabrics see textiles facultative anaerobes 4 feather fibres 277, 333, 403 feedstock for synthetic silks 248–55 fermentation 9 fermentative acidogenesis 7 fibre hanks 65 fibres, definition of 2–3 fibrillation 137, 157–8, 235–6, 299 cellulose 374–7 lyocell 171–2, 176, 177, 178, 185 fibroins 26, 249, 254–5 filament yarn fabrics 202 filaments preparation from cellulose 369–74 preparation from chitin and chitosan 380–6 films 2–3 film PHB 229 filmtruder 160–1 filtration applications of nonwovens 336 geotextiles 345 PLA applications 210–11 solution filtration (lyocell) 162 fineness 67 finishing lyocell 163–4, 176–81 PLA 205–7 soya protein fibre 424 flammability 199–200, 202, 207–8 flax 37–51, 66, 77, 85, 346, 353 anatomy of the plant 39–42 applications 49–51 cultivation 42–3 degumming 43–5 economic importance 37–9, 40 fibre compared with hemp fibre 59 soy protein-based green composites 300–4 spinning 46, 47–9 straw processing 43, 45–7 wet laid disposable nonwovens with flax fibre 331 flaxseed (linseed) 37, 38–9, 40, 49–50 flea beetles 42, 56 flexural properties 301–2 fluid transmission 344–5 flushable nonwovens 337–8 flyer spinning 359

foils 229, 232, 241 forage choppers 81 Ford, Henry 280–1, 404 forestry management 166–8 Forestry Stewardship Council (FSC) 167 formaldehyde 413, 416–17 fossil energy use xv-xvi, 212–13, 218 Fourier transformed infrared spectroscopy (FTIR) 17–18 fragrant fibre/yarn 393 France 406 fruit fibres 345, 346, 355 Fujitsu 273 functional textiles 392–3 fungi 4 see also microbial processes furnishings 207–8 garments PLA 201–7 garment making 204 processing lyocell in garment form 178 SPF 425 gel spinning 227–9 gelatin 101 gelation alginates 94, 95, 96, 101 cellulose 369 Gengiflex 149 geomembranes 344 geotextiles 69, 336, 343–66 definition and teminology 344 fibre extraction and preparation 351–5 bast fibres 351–4 fruit fibres 355 hard fibres 354 fibres used for natural geotextile products 345–51 chemical and physical properties 347–51 morphology 346–7 functions of 344–5 measurement of properties of natural geotextiles 362–5 production of natural geotextile products 355–62 nonwoven mattings 355, 356–7 woven fabrics 355, 357–62 Germany xix, 406 glass fibre 68

Index global climate change xvi, 213–15 glucose 5, 192–3 glucosidases 24–5, 123, 124 glutaraldehyde (GA) 283–4, 388 GA-SPC nanocomposite resin 294 GA-SPC reinforced with flax fabric 302–4 modification of SPC (GA-SPC) 284–6 glycerin 282–3, 284–5, 286 glycine 26–7, 249, 253 glycinin 282, 400 glyoxal 388 Godet wheels 418 grading of bast fibres 65, 82–3, 354 gravity spinning 233, 234 Great Britain 406–10 greenhouse gas emissions xv-xvi, 213–15 growth requirements for heterotrophic microorganisms 4–6 guanine 250, 251 guluronic acid 92, 93, 95 hackled bands 247 haemostatic dressings 102–3 hair fibres 276 see also wool hard fibres 345, 346, 354 hardening 413, 416 harvesting bast fibres 42–3, 56–7, 64, 73, 80–1, 83 hemicellulose (xylane) 118–19, 125, 274–5, 347–8 Hemcon 394 hemp 51–60, 77, 85, 343, 346, 353 anatomy of the plant 53–5 applications 53 chemical composition of fibre 60 cultivation 55–7 degumming 29–30, 43–5, 57–8 economic importance 52–3 nonwovens 324–5 straw processing 45–7, 58–9 hemp borer 56 hemp seed 37 Hempline 324 heterotrophs, growth requirements for 4–6 hexane 414 high density polyethylene (HDPE) 344 high molecular weight PHB 223–4

447

high performance liquid chromatography (HPLC) 23–4 high speed melt spinning 226–7 homeware 207–9 hot calendering 320 household textiles 207–9 humidity 98–9 hyaluronic acid (HA) 102 hydrogel wound healing dressings 149 hydrogen bonds 2–3 cellulose 125, 126 silks 258–9 hydrolysis 7 PHB degradation 237–8 PLA 203, 216–17 hydroxyhexanoate 230–1 hygiene products 144–9, 210 hygroscopicity 48, 49 implants 105–6, 211 in vitro biodegradation tests 12–13, 14–15 incineration 171 incubations conditions 8–11 sources of microorganisms and enzymes for laboratory incubations 12–16 India 62–3 infrared spectroscopy 17 Ingeo 201–2 insect cell lines 263 insect silks 253–5 insects (pests) 42, 56, 80 insolubilising 413, 416–18 intermediates, detection of 23–4 interplanar spacings 377–9 interspecies hydrogen transfer 8 interstices 361 ion binding 95 ionic interaction 389 iron 6, 7 ‘islands-in-the-sea’ structure 431, 432 ‘jamming’ 174–5 jet processing 178–9 jet stretch ratio 387 jute 60–9, 85, 346, 353 cultivation 63–4 degumming 64

448

Index

economic importance 61–3 fibre characteristics 65–7 products and applications 61, 67–9 straw processing 64–5 Kaltostat 103 kenaf 78–81, 85, 346, 353 anatomy of the plant 79–80 cultivation 80–1 economic importance 78–9 straw processing 81 keratin 25–6, 276 Kevlar 372, 373 knitting 176, 204 lactic acid 192 chemical recycling of PLA 216–17 polycondensation of 193 stereoisomers 194 lactide 193, 194–6 catalytic polymerisation 196–7 Lactron 201, 334 laminaria 89–90 laminates, melt blown 335 landfill xvi, 171, 272 Lanital 403 latex bonding 184 laundering lyocell 169 PLA fabrics 202–3 soya protein fibres 419, 424 Lenzing 158, 159, 160, 163, 164, 168, 172 see also lyocell Lenzing lyocell LF 177, 188 life cycle assessment (LCA) xxi, 111, 112–20 cellulosic fibres 115–17 comparison of man-made fibres with cotton and cellulosics 119–20, 121, 122 cotton 113–15 general procedure 112–13, 114 lyocell 165–71 properties and environmental costs of regenerated cellulosic fibres 118–19 life cycle for a green fibre xx-xxi lignin 85, 274–5, 347–8 linen 37, 38, 47–9 see also flax

linseed (flaxseed) 37, 38–9, 40, 49–50 linseed oil 38–9 liquid ammonia 47–8 cellulose in liquid ammonia/ammonium thiocyanate solutions 368–79 liquid crystalline solutions 368–73 low-density polyethylene (LDPE) 223 lumen 346 lustre 67 lyocell 115, 118–19, 157–90, 207, 277–8, 378, 379 fibre properties 171–2 future trends 188 high performance fibre for nonwovens 181–6 carded processing 182–3 fabric properties 183–5 finished product benefits 186 historical background 158–9 marketing 187–8 process 159–65 crimping, cutting and baling 164 fibre drying 164 fibre treatments 163–4 fibre washing 163 pulp and premix 160 solution filtration 162 solution making 160–1 solution transport 161–2 solvent recovery 164–5, 168 spinning 162–3 sustainability 165–71 fibre processing 169 future 171 lyocell fibre 168 product disposal 169–71 product use 169 raw material 166–8 in textiles 172–81 dyeing and finishing 176–81 fabric manufacture 173–6 yarn conversion 172–3 M/G ratio 93 macronutrients 5 MAGIC processing 179 magnetic silk-fibre composites 263 maleinised tung oil (MTO) 283 mammal cell lines 263 manganese nitrate 6, 7

Index Manila hemp see abaca mannuronic acid 92, 93 marketing 187–8 Marks and Spencer 169 mass per unit area 362–3 MaterBi 222, 241 mechanical bonding 319 mechanical cottonisation 46–7 mechanical properties cellulose fibres from solvent system 377–9 chitin and chitosan fibres 387–9, 390 PCL 234 PHAs 222–3, 225–7, 228–30, 231, 232 effect of irradiation 239–40 plant-based fibres 58–9, 65–7, 76–7, 84–6, 275–6 spider silk 256–62 see also physical properties; tensile properties mechanical retting 44–5, 352 medical applications alginates 100, 101–5 cellulosic fibre products 144–9 PLA 211 polyester-based biodegradable fibres 239–40 ramie 78 spider silks 262, 263 melt blowing 316, 318–19 meltblown biodegradable nonwovens and laminates 335 melt spinning PCL 233, 234 PHAs 224–6 high speed melt spinning 226–7 melting point 198, 203 drawn fibres 236 mercerisation 16 mesophase (liquid crystal) 368–73 mesta 66 metal alginates 94–5 metal alkoxides 196–7 methane 214 mineralisation 3 measuring 21–3 methanogenesis 6, 7–8 methylene chloride 381 microbial degumming 74–5 microbial processes 1–35

449

analytical methods 17–24 intermediates of biodegradation 23–4 mineralisation products 21–3 subtle changes in fibre structure or composition 17–18 tensile properties 21 visual observation and microscopy 18–19, 20 weight loss 19–21 biodegradation, mineralisation and biomass formation 3–4 future trends 29–31 growth requirements for heterotrophic microorganisms 4–6 incubation conditions 8–11 aerobic incubations 8–9 anaerobic incubations 8, 9–11 microorganisms 4 sources of microorganisms and enzymes for laboratory incubations 12–16 terminal electron acceptors 6–8, 9 types of bonds susceptible to enzymatic attack 24–9 microbial synthesis of cellulosic fibres 137–40 microcystis 89–90 microfibrillar angle 274, 275 microfibrillated cellulose (MFC) 278–9 microfungal chitin fibres 391 micronutrients 5 microscopy 19, 20 milk protein fibres 403, 426, 427 milkweed 325 mineralisation 3–4 detecting the products of 21–3 mixed gels 101 modal 115 modulus (stiffness) 85, 96, 377, 378 moisture absorption alginates 104 soy protein resins 285, 287–8, 290, 293, 294 speed of water absorption 48 moisture content 98–9 moisture management 200, 202, 207 moisture regain PLA 199 SPF 425, 426

450

Index

monomers 23–4 moths, attack by 431 mulch 357, 361 N-acylchitosan 383 N-methylmorpholine 164 N-methylmorpholine-N-oxide (NMMO) 134, 157, 168, 277–8 see also amine oxide N-terminal sequences 251–2 nanocomposites, soy protein 292–5 nanofibres 150, 151, 315 Natural Fibers Corporation 325 natural fibre composites 271–309 biodegradable fibres 274–9 developments in fibres 278–9 plant-based fibres 274–6 protein fibres 276–7 regenerated and modified fibres 277–8 biodegradable resins 279–95 composite materials 271–3 fully green composites 273–4 future trends 304 soy protein-based green composites 295–304 fabric-reinforced composites 302–4 short fibre composites 295–7 unidirectional composites 297–300 yarn-reinforced composites 300–2 natural fibres bast fibres see bast fibres classification 345–6 nonwovens 322–5 see also under individual fibres natural geotextiles see geotextiles natural resins 279–80 see also soy protein resins NatureWorks 191, 195, 211, 212, 213, 217 NEC Corporation 273 needle punching (needling) 184, 319, 320, 357 Neocallimastix frontalis 11 Nephila (golden silk spider) silk 245–6, 247, 259–61 Nidom 178 nitrous acid 418 nitrous oxide 214 Nodax 338 nominal thickness 362–3

non-Newtonian region 371 nonwoven fabrics 2, 310–42 alginate fabrics 101–4 applications 312 biodegradable 322–36 animal fibre-based nonwovens 332–3 applications 336 cotton-based fully biodegradable nonwovens 325–31 cotton nonwovens 322–4 hemp 324–5 meltblown nonwovens 335 natural cellulosic fibre nonwovens 322 other natural fibres 325 spunbond nonwovens 334–5 wet laid disposable nonwovens with flax fibre 331 wet laid nonwovens with PLA fibre 335–6 demand 311, 312, 313 fibre consumption 314–15 flushable nonwovens 337–8 geotextile mattings 355, 356–7 leading producers of nonwovens 338, 339 lyocell 181–6 PLA 209–11 production trends 311, 312 technology and relative production rate 321 web bonding techniques 315, 316, 319–21 web formation methods 315–19 nylon 427 Oeko-Tex 100 ‘Confidence in Textiles’ label 168 Of The Earth 425 Ogallala Comfort Company 325 oil consumption xv oligomers 23–4 open width processing 179 organic acids 417 organic cotton 119–20, 122 orientation through tensioning 418 osmosis degumming 45 ovalbumin 233, 234, 240 oxygen 6, 7

Index P25 254 paper pulp 68 particleboard 50, 51, 68–9 passive soil burial tests 12, 14, 170, 171 patents 405, 406–12 pectin 44, 60, 274, 275 Pectinex 100L 58 pepsin 415 peptide motifs 250–1 permeability 362, 364 permittivity 364 pests 42, 56, 80 petroleum xv as feedstock for polymers 271–2 phase diagram 370 PHBH copolymer 230–1 Philippines 81, 82 photosynthesis 192 physical degumming 44–5, 352 physical properties alginates 97–8 bast fibres 58–9, 65–7, 76–7, 84–6 PHAs 222–3 see also mechanical properties; tensile properties; thermal properties Phytagel modified SPI (PH-SPI) 288–91 fibre-reinforced composites 300–2 picks per cm 360 pilling value 144, 148 pillows 207 Piromonas communis 11 plant-based fibres 274–6 bast fibres see bast fibres in composites 272–3 used for natural geotextile products 345–51 extraction and preparation 351–5 see also cotton; natural fibre composites plant breeding 39 plasticisers 282–3 point bonding 320–1 polyalcohols 232 polycaprolactone (PCL) xix, 222, 232–5, 236, 238, 240, 241–2 polycondensation 193 polyester (PET) 119–20, 122, 172, 191 consumption in nonwovens 314, 315 polyester-cellulose blends 131–3

451

polyester-based biodegradable fibres 221–44 applications 239–40 enzymatic and hydrolytic degradation 237–8 future trends 241–2 PCL-based fibres 232–5 PGA 238 PHA-based oriented structures 222–32 PTT 238–9 Sorona 239 structure of drawn fibres 235–6 thermal properties 236 polyethylene (PE) 272–3 polyethyleneterephthalate (PET) 191, 201, 207 copolymers with PCL 235, 240 flammability 199–200 polyglycolic acid (PGA) 211, 238 polyhydroxyalkanoates (PHAs) 2, 10, 11, 27–8, 221, 222–32 applications 239–40 enzymatic and hydrolytic degradation 237–8 future trends 241–2 materials and techniques 222–4 processing/preparation 224–32 copolymers of PHB with higher PHAs 223, 230–1 gel spinning 227–9 high speed melt spinning and spin drawing 226–7 melt spinning 224–6 oriented blends of PHAs 231–2 structure of drawn fibres 235–6 thermal properties 236 polyhydroxybutyrate (PHB) 10–11, 27–8, 221, 222–32 copolymers of PHB with higher PHAs 223, 230–1 processing/preparation 224–32 polylactic acid (PLA) xix, xx, 191–220, 273 applications 200–11 apparel 201–7 homeware 207–9 medical applications 211 nonwovens 209–11 PLA as a plastic 211

452

Index

chemistry and manufacture of PLA polymer resin 192–7 catalytic polymerisation of lactide 196–7 production of PLA 193–6 future trends 218–19 PLA fibre properties 197–200 poly-L-lactic acid (PLLA) 231–2 spunbond PLA nonwovens 334–5 sustainability 211–17 disposal options 215–17 polymer processing and environmental measures 212–15 wet laid nonwovens with PLA fibre 335–6 polylactides 2 biodegradation 26–7 polymerisation catalytic 196–7 ring-opening 193, 194–6 polymerisation degree 127–8 polyolefins 314, 315 see also polyester; polypropylene polypropylene (PP) 119, 120, 148, 222, 223, 272–3, 314 cotton/Eastar/PP nonwovens 329–30 Eastar/PP 326 polytetramethylene adipate-coterephthalate (PTAT) see Eastar polytrimethylene terephthalate (PTT) 238–9 polyvalent metal alginates 94–5 polyvinyl alcohol (PVA) 100 soya bean protein fibres 422 polyvinyl chloride (PVC) 272–3 primary fibres 354 private culture collections 13, 15 production rates 321 progesterone 233, 234, 240 propanediol (PDO) 239 propyl gallate 164 propylene glycol alginate 100 protection, geotextiles and 345 protein fibres 276–7 see also animal fibres; soya protein fibres proteins 5–6, 25–6 extraction of protein from soya bean 414–15 Pseudomonas stutzeri 7

pulling machines 43 pulp mills 168 pulp and paper industries 111 pure cultures 13, 15 purified enzymes 14, 16 radial silk see dragline silk ramie 65, 66, 70–8, 85, 273 advantages and disadvantages 71 anatomy of the plant 71–2 applications 77–8 cultivation 72–3 degumming 29–30, 73–5 economic importance 70–1 properties of ramie fibre 76–7 soy protein-based green composites 295–300 spinning 75–6 straw processing 75 rayon 314 recombinant collagen technology 150 recycling xviii chemical recycling of PLA 216–17 reducing sugars 128–9 reeds 343 refractive index 197 regenerated fibres cellulosic see cellulosic fibres chitosan see chitosan protein contemporary research 420–2 naming 400–1 patents 405, 406–12 soya protein fibres see soya protein fibres reinforcement 345 resination 180 resins, biodegradable 279–95 retting see degumming/retting rhea 70 see also ramie ribbon retting 64 ring-opening polymerisation 193, 194–6 root content 66 roselle 66 Roundup Ready soya bean 399 salt solution processes 369–97 conversion of cellulose to filaments 367–79, 393–4

Index fibres from chitin and chitosan 380–93, 394 future trends 393–4 Sarelon 403 scaffolds, textile 106–7, 211, 239–40, 392 scanning electron microscopy 19, 20 scouring 30, 142–3 scutching 45, 46, 352–4 seat belts 240 seaweed see alginates secondary fibres 354 secretion, fibres from 276–7 see also spider silks; silk selective/enrichment cultures 13, 15 separation 210–11 geotextiles 345 soya protein fibre 413, 414–15 sericins 30, 254 serine 254 ‘setting’ fabric 175 sewage system 171, 337–8 shakers 46 shear friction 362, 364 shear rate 371, 372 shive 38 short fibre composites 271 natural fibre 295–7 shrinkage 174–5 Sichuan Silk Corporation 425 silk 26, 276–7, 427 degumming 30 nonwovens from 332–3 spider silks see spider silks structure of silks 245–8 synthetic silks see synthetic silks silkworm 245, 253, 254, 276–7 sisal 354 size exclusion (gel permeation) chromatography 23 Society of the Plastics Industry (SPI) xviii sodium alginate 90–1, 100, 101, 104 sodium sulphate 118 softening agents 75, 417 soil burial tests 12, 14, 170, 171 soil contact test 362, 365 solubilisation 413, 415–16 solubility chitin 382 PLA 200 soy proteins 281–2

453

solubility degree 127–8 soluble yarn 100 solution-cast drawn UHMW PHB 229–30 solution transport 161–2 solvent recovery 164–5, 168 solvent spinning lyocell 159–65 salt solution processes see salt solution processes SPF 413, 415–16 Sorbsan surgical dressing 102–3 Sorona 239 sound membranes 139, 140 soy flour (SF) 281 GA-modified soy flour reinforced with flax 302 soy protein-based green composites 295–304 soy protein concentrate (SPC) 281 fibre-reinforced composites 295–300 glutaraldehyde modification of (GASPC) 284–6 fabric-reinforced GA-SPC 302–4 GA-SPC nanocomposite resin 294 nanocomposites 292–4 processing 284 soy protein isolate (SPI) 281 fibre-reinforced composites 300–2 modified with MTO 283 Phytagel modification (PH-SPI) 288–91 stearic acid modification (SA-SPI) 286–8, 289 soy protein resins 280–95 modifications 283–95 processing 284 Soy Silk 424 soya protein fibres (SPF) 398–440 biodegradability 434 commercial availability 424–5 context for research into 403–12 degradation behaviour 431 late 20th and early 21st century fibres 431 mid-20th century fibres 431 fibre characteristics 425–8 contemporary fibres 426–8 mid-20th century fibres 425–6, 427 identifying 428–31 late 20th and early 21st century fibres 429–31, 432, 433

454

Index

mid-20th century fibres 428–9, 430 patents for regenerated protein fibres 405, 406–12 production methods 413–19 after care 419, 424 bleaching 423 contemporary methods 422–5 curd production 414–15 drying 413, 419, 423 dyeing 419, 423 extruding and insolubilising fibres 413, 416–18 finishes 424 orientation of fibres through tensioning 418 spinning, blending and weaving 419, 424 spinning solution production 415–16 washing and crimping 419 research into alternative protein fibre sources 420–2 soya bean plant 398–400 SoyBu 425 specific stress 85 spectral analysis 431, 433 spheroid nodules 374–7 spider cell lines 263 spider silks 245–70, 277 applications 262 biochemistry 248–50 future trends 262–3 molecular biology 250–2 molecular structure 252–3 performance characteristics 256–62 spinning 255–6 structures 245–8 spidroins 249–53 spinning air-gap spinning 162–3 dry-jet wet spinning 373–4 electrospinning 150, 151, 233–4, 241, 278 flax 46, 47–9 gel spinning 227–9 gravity spinning 233, 234 melt spinning 224–7, 233, 234 PLA 204 ramie 75–6 silks 255–6

solvent spinning see solvent spinning soya protein fibre 419, 424 wet spinning 90–2, 373, 416 spinning aids 417 ‘spinning’ extruders 263 spinning speed 257, 258, 259 spores 4 spun yarn fabrics 202 spunbonding process 316, 317–18 biodegradable nonwovens 334–5 spunbond PLA products 209 spunlacing (hydroentanglement) 183–4, 319, 320 stabilisation 101 stabilising baths 417 stain tests 428, 430 stapling 76 starch 192 static cellulose biosynthesis 137–8 steaming 173 stearic acid modified soy protein isolate (SA-SPI) 286–8, 289 steel 299 stem retting 64 stereoisomers 194 stiffness (modulus) 85, 96, 377, 378 stitch bonding (stitching) 319 ‘stone-washed’ denim 140, 142 straw 343 straw processing abaca 84 flax 43, 45–7 hemp 45–7, 58–9 jute 64–5 ramie 75 strength see tenacity stress-strain characteristics 260, 349 striations 429, 432 strict aerobes 4 strict anaerobes 4 structure, detecting changes in 17–18 sulphate 6, 7 sulphitolysis 26 sustainability xx-xxi, 113 lyocell 165–71 PLA 211–17 sutures 391 swelling 104, 174–5, 177 synthetic biodegradable resins 279, 280 synthetic silks 245–70

Index applications 262 feedstock 248–55 biochemistry 248–50 insect silks 253–5 molecular biology 250–2 molecular structure 252–3 future trends 262–3 performance characteristics 256–62 silk structures 245–8 spinning 255–6 synthetic tanning agents (syntans) 417 TAED (tetraacetylethylenediamine) 206 TAHT (triacroyl hexahydrotriazine) 180–1 ‘take-back’ rules xvi, 272 tanning (hardening) 413, 416 temperature spinning temperature and properties of silk 257, 258, 259 see also thermal properties temporary artificial skin 139, 148–9, 333 tenacity alginate fibres 97–8 lyocell 171–2 PLA 198 plant-based fibres 66, 67, 76, 77 Tencel 158–9, 160, 165, 172, 372–3 carding 182 crimping 164 energy efficiency 171 fibre properties 171–2 marketing 187–8 spinning 163 see also lyocell Tencel A100 158, 159, 177, 180–1, 188 wet processing 181 Tencel natural stretch 180 tensile properties 21 biodegradable nonwovens 329–30 natural geotextiles 349–51, 360 PLA 198–9 soy protein-based green composites 296–7, 298–1, 302–4 soy protein resins 285, 287, 290, 292–4 SPF 425–8 see also mechanical properties; physical properties tensioning 418 tensions, processing 175 terminal electron acceptors 6–8, 9

455

test procedures for geotextiles 362–5 textile blends see blending/blends textile scaffolds 106–7, 211, 239–40, 392 textiles 2–3, 50 application of flax 49 applications of cellulases in textile industry 140–4, 145–7, 148 fabric formation from PLA 204 household textiles 207–9 jute 69 lyocell in 172–81 nonwoven see nonwoven fabrics woven see woven fabrics Thailand 62, 63 thermal blankets 332 thermal bonding 320–1 thermal degradation 224 thermal point bonding 317–18, 320–1 thermal properties alginic acid-based fibres 99 PHB 236 PLA 198 soy protein resins 286, 291, 294, 295 Thermoanaerobacter keratinophilus 11 thickening 100–1 three-dimensional chitosan fibre meshes 392 tin alkoxide 196–7 tissue engineering 149–50, 151, 211, 239–40, 392 TOP+C processing (Tencel Oxidative Preparation plus Caustic) 179 tossa jute 63, 67 tow (very short fibres) 354 tow line 45–6 transesterification 235 transgenic plants and mammals 263 transglutaminases 30 transmissivity 364 transplants 105–6 treatments see finishing trichloroacetic acid 381 Trichoderma reesei 125–9 tussah silk 277 twines 69 ultimate fibre length 59, 65, 66–7, 76, 77 ultimate stress 85 ultra high molecular weight (UHMW) PHB 229–30, 232

456

Index

ultraviolet radiation exposure to 362, 365 protective effect of linen 48, 49 UV resistance of PLA 200 Under the Canopy 425 unidirectional composites 271 natural fibre 297–300 United States (USA) 410–12, 422 very short fibres (tow) 354 Vicara 403 viscose 119, 120, 172, 183, 277 applications 144–8 process 115, 116, 118–19, 134 viscosity 95–6, 371, 372 visual observation 18, 19 war 402–3 warp 358, 360 balance with weft 176 washing of fibres lyocell 163 SPF 413, 419 waste disposal xvi, 271–2 lyocell 169–71 PLA 215–17 water spider silk and 257–9, 261 use in PLA production 215 water absorption see moisture absorption water activity 5 water dip-nip treatment 328 water permeability 362, 364 water retting 43–4, 351–2 water soluble fibres 104–5, 338 weaving lyocell 173–6 PLA 204 soya bean protein fibre 419, 424 web bonding techniques 315, 316, 319–21 web formation 315–19 dry laid 185, 315–16 meltblown 316, 318–19, 335 spunbonded 209, 316, 317–18, 334–5 wet laid 185, 316–17, 331, 335–6 webs 257 see also spider silks weft 358, 360 balance with warp 176

weight loss, measuring 19–21 wet laid process 316–17 disposal nonwovens with flax fibre 331 lyocell in wet-laying 185 nonwovens with PLA fibre 335–6 wet spinning 90–2, 373, 416 wet tensile strength 426–8 wet wipes 186, 209–10 white jute 63, 67 wicking 200, 202, 207 wind power and biomass PLA process 212–13, 214–15 windlass mechanism 246, 248 wipes 186, 209–10, 313–14 wood 299–300 mechanisms of enzymatic reactions on 120–31 plant-based fibre composites 272–3 wood pulp and lyocell production 160, 165, 166–8 wool 425–6, 427 biodegradation 11, 16, 25–6 nonwovens from 332 and research into alternative protein fibres 401–2 stockpile after the war 405 treatment with transglutaminases 30 WoolFelt nonwovens 332 World War Two 402–3 wound dressings 101–5, 139, 149, 391 woven fabrics 2 geotextile products 355, 357–62 lyocell 173–6 X-ray diffraction 18, 377–9 xylanases 123, 125 xylane see hemicellulose yarn count 360–1 yarn crimp 164, 174–5, 198, 360, 419 yarn-reinforced composites 271 natural fibres 300–2 yarns 2 conversion of lyocell to yarn 172–3 jute 69 soya bean protein fibre 424 spinning PLA 204 Young’s modulus 85, 96, 377, 378

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