Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications (Wiley Series in Renewable Resource)

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Industrial Applications of Natural Fibres Structure, Properties and Technical Applications

Edited by

¨ ¨ JORG MUSSIG Department of Biomimetics, Hochschule Bremen – University of Applied Sciences, Bremen, Germany

A John Wiley and Sons, Ltd., Publication

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Industrial Applications of Natural Fibres

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Wiley Series in Renewable Resources

Series Editor Christian V. Stevens, Department of Organic Chemistry, Ghent University, Belgium

Titles in the Series Wood Modification: Chemical, Thermal and Other Processes Callum A.S. Hill Renewables-Based Technology: Sustainability Assessment Jo Dewulf & Herman Van Langenhove Introduction to Chemicals from Biomass James H. Clark & Fabien E.I. Deswarte Biofuels Wim Soetaert & Erick J. Vandamme Handbook of Natural Colorants Thomas Bechtold & Rita Mussak Surfactants from Renewable Resources Mikael Kjellin & Ingeg¨ard Johansson Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications J¨org M¨ussig

Forthcoming Titles Thermochemical Processing of Biomass Robert C. Brown

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Industrial Applications of Natural Fibres Structure, Properties and Technical Applications

Edited by

¨ ¨ JORG MUSSIG Department of Biomimetics, Hochschule Bremen – University of Applied Sciences, Bremen, Germany

A John Wiley and Sons, Ltd., Publication

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This edition first published 2010
C 2010 John Wiley & Sons, Ltd Registered office John Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com. The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the publisher nor the author shall be liable for any damages arising herefrom. Library of Congress Cataloging-in-Publication Data Industrial applications of natural fibres: structure, properties and technical applications / edited by J¨org M¨ussig. p. cm. – (Wiley series in renewable resources) Includes bibliographical references and index. ISBN 978-0-470-69508-1 (cloth) 1. Plant fibers–Industrial applications. 2. Animal fibers–Industrial applications. I. M¨ussig, J¨org. TS1540.I528 2010 677–dc22 2009049249 A catalogue record for this book is available from the British Library. ISBN 978-0-470-69501-1 Set in 10/12pt Times by Aptara Inc., New Delhi, India Printed and bound in Great Britain by CPI Antony Rowe, Chippenham, Wiltshire

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Dedication In recent years, natural fibres have become increasingly popular for use in industrial applications, e.g. as reinforcement for plastics. This approach is also of growing interest in light of the discussion about sustainability and environmental issues. These aspects are commonly not included in the regular university education for engineers and natural scientists. This book will examine the value-added chain of natural fibres in order to bring more detailed information about this complex topic to students as well as to industry and research. The book will enable the reader to gain a fundamental understanding of the sometimes complex transformation of a natural fibre to final technical product. This book is dedicated to professional industrial researchers working in production processing (from fibre separation to the final product – textiles and composites), in fibre characterisation and in standardisation and harmonisation, to academics researching in the field of technical applications of natural fibres, as well as to postgraduates on specific courses and research projects in the above areas.

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Contents Series Preface Preface

xi xiii

Foreword

xv

List of Contributors

xvii

List of Illustrators

xxiii

PART I BACKGROUND 1 2

3

Historic Usage and Preservation of Cultural Heritage Fenella G. France What Are Natural Fibres? 2.1 Chemistry of Plant Fibres Danny E. Akin 2.2 Natural Fibres – Function in Nature Michaela Eder and Ingo Burgert 2.3 Types of Fibre J¨org M¨ussig and Tanja Slootmaker

3 11 13 23 41

Economic Aspects 3.1 Grades and Standards Axel Drieling and J¨org M¨ussig 3.2 Technical Applications of Natural Fibres: An Overview Nina Graupner and J¨org M¨ussig

49 51

3.3

73

Natural Fibres in Technical Applications: Market and Trends Stephan Piotrowski and Michael Carus

63

PART II VEGETABLE FIBRES 4 5

Flax – Structure, Chemistry, Retting and Processing Danny E. Akin Hemp – Cultivation, Extraction and Processing Stefano Amaducci and Hans-J¨org Gusovius

89 109

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Contents

6

Jute – A Versatile Natural Fibre. Cultivation, Extraction and Processing Md. Siddiqur Rahman

135

7

Abac´a – Cultivation, Extraction and Processing Friedhelm G¨oltenboth and Werner M¨uhlbauer Sisal – Cultivation, Processing and Products Rajesh D. Anandjiwala and Maya John

163

8 9

Coir – Coconut Cultivation, Extraction and Processing of Coir Chitrangani Jayasekara and Nalinie Amarasinghe 10 Cotton Production and Processing Muhammed Rafiq Chaudhry

181 197 219

PART III ANIMAL FIBRES 11

Mulberry Silk, Spider Dragline and Recombinant Silks Anja Gliˇsovi´c and Fritz Vollrath

12

Wool – Structure, Mechanical Properties and Technical Products based on Animal Fibres Crisan Popescu and Franz-Josef Wortmann

237

255

PART IV TESTING AND QUALITY MANAGEMENT 13

14 15 16 17 18

Testing Methods for Measuring Physical and Mechanical Fibre Properties (Plant and Animal Fibres) J¨org M¨ussig, Holger Fischer, Nina Graupner and Axel Drieling SEM Catalogue for Animal and Plant Fibres Tanja Slootmaker and J¨org M¨ussig Combined (In Situ) Methods Ingo Burgert and Michaela Eder DNA-Analytical Identification of Species and Genetic Modifications in Natural Fibres Lothar Kruse Cotton/Worldwide Harmonisation Axel Drieling and Jean-Paul Gourlot Flax – ASTM Standardisation and Harmonisation Danny E. Akin

269 311 337 345 353 371

PART V APPLICATIONS: CURRENT AND POTENTIAL 19

Composites 19.1 Historical, Contemporary and Future Applications Tuomas H¨anninen and Mark Hughes 19.2 Design, Material Properties and Databases Erwin Baur and Frank Otremba

383 385

19.3

407

19.4

Natural Fibre Composite Processing: A Technical Overview Tim Huber, Nina Graupner and J¨org M¨ussig Natural Fibre-Reinforced Polymers in Automotive Interior Applications Eugen Pr¨omper

397

423

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Contents

19.5

Composites Based on Natural Resources Martien van den Oever and Harri¨ette Bos

19.6 20 21

Cellulose Nanocomposites Sanchita Bandyopadhyay-Ghosh, Subrata Bandhu Ghosh and Mohini Sain Insulation Materials Based on Natural Fibres Franz Neubauer

Natural Fibres in Geotextiles for Soil Protection and Erosion Control Gero Leson, Michael V. Harding, and Klaus Dippon

Index

ix

437 459 481 509

523

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Series Preface Renewable resources, their use and modification are involved in a multitude of important processes with a major influence on our everyday lives. Applications can be found in the energy sector, chemistry, pharmacy, the textile industry, paints and coatings, to name but a few. The area interconnects several scientific disciplines (agriculture, biochemistry, chemistry, technology, environmental sciences, forestry, . . . ), which makes it very difficult to have an expert view on the complicated interaction. Therefore, the idea to create a series of scientific books, focussing on specific topics concerning renewable resources, has been very opportune and can help to clarify some of the underlying connections in this area. In a very fast changing world, trends are not only characteristic of fashion and political standpoints, also science is not free from hypes and buzzwords. The use of renewable resources is again more important nowadays; however, it is not part of a hype or a fashion. As the lively discussions among scientists continue about how many years we will still be able to use fossil fuels, with opinions ranging from 50 years to 500 years, they do agree that the reserve is limited and that it is essential not only to search for new energy carriers but also for new material sources. In this respect, renewable resources are a crucial area in the search for alternatives for fossil-based raw materials and energy. In the field of energy supply, biomass and renewable-based resources will be part of the solution, alongside other alternatives such as solar energy, wind energy, hydraulic power, hydrogen technology and nuclear energy. In the field of material sciences, the impact of renewable resources will probably be even greater. Integral utilisation of crops and the use of waste streams in certain industries will grow in importance, leading to a more sustainable way of producing materials. Although our society was much more (almost exclusively) based on renewable resources centuries ago, this disappeared in the Western world in the nineteenth century. Now it is time to focus again on this field of research. However, this should not mean a ‘retour a` la nature’ but should be a multidisciplinary effort on a highly technological level to perform research into the development of new crops and products from renewable resources. This will be essential to guarantee a level of comfort for a growing number of people living on our planet. It is the challenge for the coming generations of scientists to develop more sustainable ways to create prosperity and to fight poverty and hunger in the world. A global approach is certainly favoured. This challenge can only be dealt with if scientists are attracted to this area and are recognised for their efforts in this interdisciplinary field. It is therefore also essential that consumers recognise the fate of renewable resources in a number of products. Furthermore, scientists do need to communicate and discuss the relevance of their work. The use and modification of renewable resources may not follow the path of the genetic engineering concept in view of consumer acceptance in Europe. In this regard, the series will certainly help to increase the visibility of the importance of renewable resources.

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Series Preface

Being convinced of the value of the renewables approach for the industrial world, as well as for developing countries, I was myself delighted to collaborate on this series of books focusing on different aspects of renewable resources. I hope that readers become aware of the complexity, the interaction and interconnections and the challenges of this field, and that they will help to communicate the importance of renewable resources. I certainly wish to thank the people at John Wiley & Sons, Chichester, especially David Hughes, Jenny Cossham and Lyn Roberts, in seeing the need for such a series of books on renewable resources, for initiating and supporting it and for helping to carry the project through to the end. Last but not least, I would like to thank my family, especially my wife Hilde and my children Paulien and Pieter-Jan, for their patience and for giving me the time to work on the series when other activities seemed to be more inviting. Christian V. Stevens Faculty of Bioscience Engineering, Ghent University, Belgium Series Editor Renewable Resources June 2005

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Preface What makes natural fibres so fascinating? Representatives of different professional disciplines, like biologists, chemists, agrononomical scientists, process engineers or preservation scientists, would certainly each answer this question quite differently, according to their own scientific interest and research. As a material scientist, I would like to describe my own perception and at the same time outline the leading thoughts of this book. Material discoveries and material developments have in the history of mankind led to great progress in innovation, with far-reaching consequences for technology, economy and culture. The periodical division of prehistory and early history of mankind is mainly determined by the materials used in these periods (Stone Age, Bronze Age and Iron Age). Although the utilisation of natural fibres is verifiable in early archaeological cultures, it has not resulted in the naming of an epoch. There is no ‘natural fibre age’, although in history the usage of natural fibre has been quite varied and has repeatedly generated culturally significant innovations. Clothing textiles as well as technical textiles (e.g. nets) or composite materials (e.g. natural fibre compounded clay) are examples of such innovations. In this book these historical aspects of natural fibre usage are combined with possible future products. In our progressively globalised world with unforeseeable demographic, economic and ecological challenges, management of resources and sustainability are increasingly becoming the focus of debate and discussion. The utilisation of materials is a key factor, and natural fibres in particular, being a natural resource, provide opportunities for technical innovation and sustainability. The use of natural fibres, e.g. in technical applications, needs to be in line with the three essential pillars of sustainability – economy, ecology and society. To ensure that this remains so now and in the future, the worldwide raw material turnaround and its effects on the selection of materials must be critically examined on the basis of sustainability criteria. The main argument against the industrial use of natural fibres is often that the quality of the fibres depends on the year in which they were grown. It is nevertheless possible to obtain fibres of consistent quality, as well as reliable data, enhancing the predictability of the properties of natural fibre products by using a quality management system that starts for plant fibres at the cultivation stage and that is based on reproducible proof of origin and harvesting parameters. This book will combine the different steps of processing, from agriculture, fibre separation and fibre processing to the manufacture of the final product. Each step will be linked to the fibre properties, the possibilities to characterise them, and how the different natural fibres will influence the product properties. In order to understand why and how a natural fibre influences a product property, their chemical as well as structural qualities are thoroughly described. The fundamental understanding of the hierarchy and construction of natural fibre structures allow for a specific and selective design of natural fibre products. However, natural fibres and their function in biological systems also offer an exceedingly interesting model for the development of biomimetic and bio-inspired materials. Here, also, a fundamental understanding of the functions enhances the transfer from biological system to technological appliance.

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Preface

The subject of natural fibres is an interdisciplinary field of research and, among others, touches the fields of cultivation, biochemistry, agricultural science, biology, material science and engineering. The aim and objective of writing this book was to provide a substantiated overview of the status of current research on the subject of natural fibres and technical natural fibre usage, including the perspectives of other disciplines. I would like to thank the authors, who have shown great interest in this interdisciplinary book project. As a combination of different areas of research may cause problems of understanding, there has been great emphasis on using consistent terminology. This will enhance understanding across the borders of scientific fields. In this context, I would again like to thank the authors, who worked very cooperatively in this project. A special focus was to present the graphic elements in this book consistently and appealingly. Using mainly handwritten graphics and diagrams, we have attempted a new way of illustration in this book. My special thanks to Tanja Slootmaker and Anja M¨ussig for their creative work. I would like to thank the staff at John Wiley & Sons, Chichester, especially Richard Davies, Sarah Hall and Jenny Cossham, for supporting the book project through to the end. I would also like to thank my family and friends for their patience and the time they have given me for the conception and writing of this book. I hope while reading this book you will experience some of the fascination of ‘natural fibres’ that I have been experiencing for years now, being engaged in this highly interesting area of research. J¨org M¨ussig Hochschule Bremen – University of Applied Sciences, Faculty 5 – Department of Biomimetics, Professorship Biological Materials, Bremen, Germany Editor Industrial Applications of Natural Fibres January 2010

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Foreword In the past, when synthetics became used as alternative construction materials instead of metals, a lot of damage happened to different components. This resulted in a very negative estimation of the performance of synthetics. Soon it became clear that metals could not simply be replaced by synthetics and the designer had to learn how to deal with these new materials. This time of apprenticeship was injurious to the image and the reputation of synthetics, and as a consequence similar developments have to be avoided, if new materials like natural fibres are to be technically applied in the future. Thus, knowledge of structure and properties as well as interconnection with shaping is necessary for material selection. It is therefore highly appreciated that the publishers John Wiley & Sons, Chichester, have initiated a series of scientific books on special subjects of renewable resources. This particular volume “Industrial Applications of Natural Fibres” is edited by J¨org M¨ussig, a very active young Professor of Biological Materials. He is both the initiator and scientific head of numerous research projects on the value-added chain of natural fibres in the field of technical applications, starting from agriculture and ending with the final product. Bulk properties of materials are mainly determined by their chemical composition and atomic structure. Technically, geometrical and test conditions additionally influence parameters of construction materials. As all of them have their own life history, these facts have to be known if materials are to be used sustainably in industrial applications. This means that modern procedures using statistical methods of testing and evaluation are necessary. Particularly in the case of natural fibres, the whole distribution of property should be known. Thanks to the thorough and extensive activities of the editor, a great number of internationally well-known experts in the field of natural fibres have contributed their expertise, writing articles on this interdisciplinary field of research and application, and thus making a comprehensive compendium available. Many of the chapters refer to the requirements mentioned above. The uniformity of the structure of each chapter, the well coordinated contents with links to corresponding chapters and the consistent terminology of the combined contributions will be of great advantage for every reader. Of particular note are the handwritten graphics and diagrams. They are very informative, and in combination with historical drawings of plants, the information presented becomes clear and vivid. The reader not only gets general information but also detailed facts on a scientific basis with links to comprehensive lists of well investigated current publications. It was a great pleasure to read the manuscript and hopefully many students, as well as academic and industrial researchers in the field of technical applications of natural fibres will contribute to the development of these advanced materials by studying this highly professional compendium. I congratulate and thank the editor and the authors for their ambitious work. Helmuth Harig Professor of Materials (retired) Universit¨at Bremen/Faserinstitut Bremen Berlin, January 2010

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List of Contributors Danny E. Akin Athens, Georgia, USA. Dr. Akin (PhD in Microbiology); retired in January 2008, after a 37 year career with the US Department of Agriculture; currently associated with the consulting firm Light Light Solutions, LLC, in Athens, Georgia, USA. Stefano Amaducci Istituto di Agronomia, Universit`a Cattolica del Sacro Cuore, Piacenza, Italy. Dr. Amaducci; researcher and teaches the course of Field Crops at Universit`a Cattolica del Sacro Cuore; research focus: agronomic evaluation of industrial crops, particularly for fibre and biomass production. Nalinie Amarasinghe Industrial Technology Institute, Colombo, Sri Lanka. MSc Amarasinghe (Diploma in Technology, University of Moratuwa, Sri Lanka; Post Graduate Diploma and MSc in Chemical Engineering, University of Bradford, UK); Project Director at the ITI ‘Coir Processing and Quality Control.’ Rajesh D. Anandjiwala CSIR Materials Science and Manufacturing, Port Elizabeth, South Africa, and Department of Textile Science, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa. Dr. Anandjiwala (Doctor of Philosophy; University of Leeds, UK in Textile Engineering); Chief Researcher and Research Group Leader at the CSIR; Adjunct Professor, Nelson Mandela Metropolitan University. Subrata Bandhu Ghosh Center for Biocomposites and Biomaterials Processing, Faculty of Forestry, University of Toronto, Toronto, Canada. Dr. Ghosh (PhD, Department of Engineering Materials, University of Sheffield, UK); currently a Post-doctoral Research Fellow at the University of Toronto; research focus: biobased foams and biocomposites. Sanchita Bandyopadhyay-Ghosh Center for Biocomposites and Biomaterials Processing, Faculty of Forestry, University of Toronto, Toronto, Canada. Dr. Bandyopadhyay-Ghosh (PhD, Department of Engineering Materials, University of Sheffield, UK); currently a Post-doctoral Research Fellow at the University of Toronto; research focus: biopolyol, biofoam and cellulose nanofibres. Erwin Baur M-Base Engineering + Software GmbH, Aachen, Germany. Dr.-Ing. Baur (Graduated in Mechanical Engineering, specialised in Plastics Technology, Technical University of Aachen (RWTH), Aachen, Germany); Managing Director of M-Base Engineering + Software GmbH in Aachen.

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List of Contributors

Harri¨ette Bos Wageningen University and Research Centre, Food and Biobased Research, Department of Fibre and Paper Technology, Wageningen, The Netherlands. Dr. Bos (PhD, Eindhoven University; graduated in Physical Chemistry, University of Groningen, The Netherlands); currently responsible for the policy support research program on Biobased Economy from the Ministry of Agriculture. Ingo Burgert Max-Planck-Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany. Dr. Burgert (Wood Science and Technology, University of Hamburg, Germany); currently research group leader “Plant Biomechanics and Biomimetics”; research focus: plant cell walls, nanostructure and micromechanical properties, biomimetics. Michael Carus nova-Institut, H¨urth, Germany. Diplom-Physiker Michael Carus (Advanced degree in Physics, University of Cologne, Germany); currently Managing Director of nova-Institut and head of the field “Renewable resources/market research.” Muhammad Rafiq Chaudhry International Cotton Advisory Committee, Washington, DC, USA. Dr. Chaudhry (PhD in Cotton Breeding and Genetics, Uzbekistan); currently head of the Technical Information Section of the ICAC; author of the book ‘Cotton Facts’ and Editor of the THE ICAC RECORDER. Klaus Dippon Bio-Composites And More GmbH, Ipsheim, Germany. Dr. Dippon (PhD in Agricultural Engineering, University of Stuttgart-Hohenheim, Germany); Vice President to a start-up firm that produced high quality erosion control products from coir; currently Managing Director of B.A.M. Axel Drieling Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany. Dipl.-Ing. Drieling (Degree in Production Engineering, University of Bremen, Germany); currently head of the Testing Methods Department at FIBRE; research focus: harmonisation of fibre testing (ITMF, CSITC & INTERWOOLLABS). Michaela Eder Max-Planck-Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany. Dr. Eder (Wood Science and Technology at BOKU University, Vienna, Austria); currently post-doctoral fellow at the Department of Biomaterials; research focus: mechanical performance of plant cell wall properties. Holger Fischer Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany. Dr. Fischer (Dr. rer. nat. in Chemistry, University of Bremen, Germany); currently Senior Research Fellow at the FIBRE; research focus: enzymatic fibre modification, fibre characterisation, fibre surface modification, biocomposites. Fenella G. France Preservation Research and Testing Division, Library of Congress, Washington, DC, USA. Dr. France (PhD from Otago University, New Zealand); currently preservation scientist in the Library of Congress Preservation Directorate; research focus: polymer aging, polymer and textile deterioration. Anja Gliˇsovi´c Fraunhofer Institut f¨ur Fertigungstechnik und Angewandte Materialforschung (IFAM), Bremen, Germany. Dr. Gliˇsovi´c (PhD in Physics, Georg-August-Universit¨at G¨ottingen, Germany); currently project manager at the IFAM; research focus: development and industrial application of biopolymers and nature-inspired biomaterials.

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List of Contributors

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Friedhelm G¨oltenboth Institute for Plant Production and Agroecology in the Tropics and Subtropics, University of Hohenheim, Stuttgart, Germany. Prof. Dr. G¨oltenboth (PhD in Genetics, Ruhruniversity Bochum, Germany); Honorary Professor for Tropical Agro-Ecology, University of Hohenheim; research focus: tropical agro-ecology in indonesia, Papua New Guinea and Philippines. Jean-Paul Gourlot CIRAD PERSYST LTC, Montpellier, France. Dr. Gourlot (PhD in Sciences for Engineer); Head of the Cotton Technology Laboratory at CIRAD; research focus: cotton testing and standardisation, ‘Commercial Standardized Instrument Testing for Cotton Task Force.’ Nina Graupner Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany. Dipl.-Ing. (FH) Graupner (Degree in Renewable Resources, University of Applied Sciences, Hanover, Germany); currently affiliated with the Hochschule Bremen; research focus: biopolymer composites and fibre/matrix interaction. Hans-J¨org Gusovius Leibniz-Institut f¨ur Agrartechnik Potsdam-Bornim e.V., Potsdam, Germany. Dr. Gusovius (Dr.-Ing. Agriculture, Humboldt-University, Berlin, Germany); currently member of staff at Leibniz-Institute for Agricultural Engineering: research focus: development of highly effective harvesting machinery for hemp. Tuomas H¨anninen Department of Forest Products Technology, Aalto University, Helsinki, Finland. MSc H¨anninen (Wood Chemistry, Helsinki University of Technology, Finland); currently PhD at the Department of Forest Products Technology; research focus: ultrastructural characteristics of natural fibres, Raman spectroscopy. Michael V. Harding Great Circle International, Inc., San Diego, CA, USA. Michael Harding (graduate from Purdue University) Director of the San Diego State University Soil Erosion Research Lab. and President of the IECA: research focus: development and implementation of test methods for EC products. Tim Huber University of Canterbury, Department of Mechanical Engineering, Christchurch, New Zealand. BSc Tim Huber (University of Applied Sciences, Bremen, Germany); currently PhD at the Canterbury University, Christchurch, New Zealand; research focus: biocomposites and processing of novel all-cellulose composites. Mark Hughes Department of Forest Products Technology, Aalto University, Helsinki, Finland. Prof. Dr. Hughes (PhD in Wood Science); currently Professor of Wood Technology at the Aalto University; research focus: wood and non-wood fibre reinforced composites, experimental mechanics and micromechanics. Chitrangani Jayasekara Coconut Research Institute, Lunuwila, Sri Lanka. Dr. Jayasekara (PhD University of Queensland, Australia); currently Director of the Coconut Research Institute of Sri Lanka; research focus: retting of coir, development of coir based new products for agricultural applications. Maya John CSIR Materials Science and Manufacturing, Port Elizabeth, South Africa, and Department of Textile Science, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa. Dr. John (PhD, Mahatma Gandhi University, India); currently Senior Researcher at the CSIR; research focus: hybrid natural fibre composites, lignocellulosic fibre reinforced composites and biopolymer systems.

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List of Contributors

Lothar Kruse Impetus GmbH & Co. Bioscience KG, Bremerhaven, Germany. Dr. Kruse (PhD in Molecular Biology, University of Bremen, Germany); Managing Director of Impetus; research focus: test systems and analyses for the DNA-based identification of species and genetic modifications in food, feed, seed and fibres. Gero Leson Leson & Associates, Berkeley, CA, USA. Dr. Leson (Physicist and Environmental Scientist); project coordinator for the sustainable production of organic and fair trade raw materials (coconut oil, palm oil) for use in the production of Dr. Bronner’s natural soaps and as foods. ¨ Werner Muhlbauer Institute for Agricultural Engineering, University of Hohenheim, Stuttgart, Germany. Prof. Dr.-Ing. Dr. h.c. M¨uhlbauer (Stuttgart University, Germany); Managing Director of the Institute at Hohenheim University until his retirement in 2004; initiated and implemented the abac´a PPP-Project with Daimler AG. ¨ J¨org Mussig Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany. Prof. Dr.-Ing. J¨org M¨ussig (Dr.-Ing. University of Bremen, Germany); currently Professor of Biological Materials at the Hochschule Bremen; research focus: bio-inspired materials, natural fibres and natural fibre composites. Franz Neubauer ECOLABOR e.U., Accredited Testing Laboratory and Inspection Agency for Thermal-, Moisture-, Sound- and Fire Protection, Stainz, Austria. Dipl.-Ing. Neubauer (University of Technology of Graz, Austria); founder of the ECOLABOR e.U., member of standardization committees; research focus: thermal conductivity and water-vapour transmission property. Frank Otremba M-Base Engineering + Software GmbH, Aachen, Germany. Dipl.-Ing. Otremba (Technical University of Aachen, Germany); 2001–2009 simulation engineer and project manager at M-Base, currently simulation specialist (theory group) of Enrichtemnet Technolgy Company Ltd, J¨ulich, Germany. Stephan Piotrowski nova-Institut, Department of Economics and Resource Management, H¨urth, Germany. Dr. Piotrowski (PhD Agricultural Economics, University of Stuttgart-Hohenheim); currently working at the nova-Institut; research focus: land use competition between food and energy crops, renewable raw materials for material uses. Crisan Popescu DWI an der RWTH Aachen e.V., Aachen, Germany. Prof. Dr. Popescu (Doctorate in Physical Chemistry, University of Bucharest); Professor of Textile Chemistry, University ‘Aurel Vlaicu’, Arad, Romania; currently scientist at DWI; research focus: keratin fibres, biomaterials and chemistry of proteins. Eugen Pr¨omper Johnson Controls, Burscheid, Germany. Dr. rer. nat. Pr¨omper (Polymer Chemistry, Technical University of Aachen, Germany); department leader for material research and testing at different automotive suppliers; currently associated with Pr¨omper-Consulting, Viersen, Germany. Siddiqur Rahman International Jute Study, Dhaka, Bangladesh. MSc Rahman (Degree in Applied Physics, University of Dhaka, Bangladesh); currently working in the International Jute Study Group (IJSG), an intergovernmental group which works for the development of world jute economy.

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Mohini Sain Center for Biocomposites and Biomaterials Processing, Faculty of Forestry, University of Toronto, Toronto, Canada. Prof. Dr. Sain is a Professor of the Faculty of Forestry and Director of the Centre for Biocomposites and Biomaterials Processing; research focus: cellulose based micro and nano composite, biomaterials and biocomposites. Tanja Slootmaker Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany. Mrs. Slootmaker (physical-technical assistant) at the FIBRE; currently responsible for the administration of international wool standards and round trials; research focus: identification and differentiation of natural fibres. Martien van den Oever Wageningen University and Research Centre, Food and Biobased Research, Wageningen, The Netherlands. MSc van den Oever (Chemical Engineering, Eindhoven University, The Netherlands); Project Manager at the Research Institute F & BR; research focus: fibre reinforced polymers, panel and board materials, fibre based foams and films, and textiles. Fritz Vollrath Department of Zoology, Oxford University, Oxford, UK. Prof. Dr. Vollrath (PhD, University of Freiburg, Germany); currently a Senior Research Fellow at the Department of Zoology, University of Oxford; research focus: silks and silk-structures as well as animal decision-making. Franz-Josef Wortmann Textiles & Paper, School of Materials, University of Manchester, UK. Prof. Dr. Wortmann (PhD in Polymer Chemistry at DWI, Aachen, Germany) currently Professor of Fibre and Textile Technology at the University of Manchester; research focus: chemical and physical properties of animal fibres.

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List of Illustrators ¨ Anja Mussig schnittreif, Bremen, Germany. Dipl.-Ing. (FH) Anja M¨ussig (University of Applied Sciences Niederrhein, Germany); during her industry career, strong focus on construction and pattern design; currently free-lancer in the clothing business and design of ‘schnittreif.’ Tanja Slootmaker Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany. Beside her expertise in identification and differentiation of natural fibres, she has a strong affinity towards art and design. She combines the topics fibre technology and fibre science with arts in this publication.

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PART I BACKGROUND

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1 Historic Usage and Preservation of Cultural Heritage1 Fenella G. France Preservation Research and Testing Division, Library of Congress, Washington, DC, USA

1.1 Introduction The responsibility of preserving our material heritage heightens our awareness of our cultural history. Historical textiles are made from natural fibres and serve to create a special link between the natural environment and the social environment that underlies all our lives, from the everyday textile to patriotic to ceremonial. Understanding and identifying natural historic textile materials helps assure that these textiles are preserved for future generations. The application of a range of scientific techniques to fibre analysis provides a wealth of information for textile preservation (France, 2005a). A historical textile consists not only of the material itself but also of all the historical evidence collected upon and within it over years of use. Scientific analyses can establish whether surface contaminants and soiling have historical significance or are potential sources of degradation. For cultural heritage institutions (including museums, libraries, archives and historic house collections) this involves additional critical details concerning display, storage, exhibition and treatments, including details about soiling, deterioration and the effects of environmental conditions. Techniques such as scanning electron microscopy, X-ray analysis, confocal microscopy, gas chromatography, mechanical testing and chemical analyses allow investigations into internal and external aspects of the fibre structure, identification of surface contaminants and the opportunity to learn about the impact of treatments and display environments on textile deterioration. This microscopic-level examination in turn reveals macro-level information pertaining to the condition of the entire textile.

1

The views presented in this chapter reflect the opinion of the author and not the Library of Congress.

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Preservation of Cultural Heritage

The historic use of natural fibres is much broader than clothing and tapestries, as they heralded patriotism, sovereignty, peace and all too often war when structured into woven fabrics. Finely woven wool fabrics were utilised as banners and large flags, as they were robust and could be dyed to required colours, with the loose-weave structure allowing them to brandish the symbol of the message, even in a large format. Preserving our past requires knowledge of the properties of the textiles from which they are formed (France, 2005b). The ubiquitous nature and functionality of textiles give them a unique place in our cultural heritage. Unlike works of art and other items, textiles are generally made to be functional, so that, after normal wear and tear, they enter our museums already in a fragile state. This also has implications for their documentation, or lack thereof. Gaining knowledge of the unique natural fibre textile properties and structure can be accomplished with a range of scientific analyses and techniques. While highly significant and recognisable items such as the United States’ Star-Spangled Banner are noted to have been present at certain events – in this case the battle at Fort McHenry in Baltimore harbour in September 1814 – detailed knowledge of their history is often sketchy. This historic flag was commissioned by Major George Armistead, commander of Fort McHenry, and was raised over Fort McHenry on the morning of 14 September 1814 to signal American victory over the British in the Battle of Baltimore. It was this event that inspired Francis Scott Key to write ‘The Star-Spangled Banner’, the song that became the United States’ national anthem. The original flag was 9.1 m by 12.8 m (30 ft by 42 ft) and made of high-quality single-weave wool bunting and cotton. Each of the fifteen stripes in red and white (undyed) wool were 0.6 m (24 in) wide, the same width for the wool in the blue canton, and the fifteen large cotton stars measured 0.6 m (2 ft) point to point. The history of this woollen flag fabric can be traced back to a cottage industry in Sudbury, Suffolk, England, in the late eighteenth century (France, 2007). This artefact provides us with an excellent example of the impact, potential and challenges offered by science and technology for studies into the textile structure and properties of cultural objects. Scientific techniques and new technologies are proving critical for providing previously lost information, information that informs us as to both the current state of the artefact and the main sources of degradation, as this information is paramount for establishing the optimum environmental conditions to ensure the long-term preservation of a historic technical textile such as a flag. The determination of chemical and mechanical properties and fabric, yarn and fibre morphology start to provide this knowledge. A historical textile consists not only of the material itself but also of all the historical evidence collected over its years of use. Determining the probable source of surface contaminants is critical, as soiling offers curators evidence linking a textile to a particular geographical location, or can reveal trace elements from a particular historic event, such as the War of 1812. Scientific analysis supported by microscopy helps establish whether surface particulates, contaminants and soiling have historical significance, and aids in critical decisions regarding possible degradation from surface contaminants that could reduce the artefact’s life. Assessment of the amino acid content of the wool fibres allows identification of the specific amino acid composition characteristic of the specific breed of sheep or domestic animal (see Chapters 12 and 16). Historically, many cultural heritage items had pieces removed over the years. For example, when soldiers who fought at Fort McHenry died, their widows wrote to the daughter of the commander of the fort, requesting a piece of the Star-Spangled Banner to be buried with their husband. Amino acid analyses of samples found later in various locations were tested to assess and confirm their provenance against the amino acid composition of the flag keratin. Changes in specific amino acid analyses can also confirm the main agent of deterioration (e.g. light or temperature), as specific amino acids will degrade under certain conditions while others are left unchanged. Scanning electron microscopy (SEM) and elemental analysis are pertinent techniques for assessing the effects of surface deterioration to support curatorial decisions. SEM provides insights into fabric, yarn and fibre fracture morphology, which illustrates changes due to photodegradation, through high-resolution high-magnification images. At the fibre level, these highly magnified visible changes, linked with specific

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Figure 1.1 Scanning electron micrograph of an aged wool fibre dated ca. 1800, approximately 47 µm in diameter (2000× magnification).

mechanical and chemical behaviour in the keratin structure, lead to a commensurate decrease in the textile’s mechanical strength, as the fibre fracture is a direct manifestation of changes in the internal structure that reduce the mechanical properties. Changes in fibre surface morphology evident in SEM images provide strong indicators of the effects of various degradative environmental influences – light, relative humidity, biological and soiling. This fibre degradation is indicated by the presence of microfractures and cracking from relative humidity fluctuations, abrasion from particulates and/or damage from biological organisms. Analyses by SEM allow confirmation of the high sulphur content characteristic of wool, as well as determination of any surface contaminants that can provide further historic information relating to the historic context, sometimes geographical information and size and composition of degradative particulates and soiling (France, 2003a). The SEM micrograph in Figure 1.1 shows a relatively smooth fracture surface, indicating light damage, and a lack of scale structure, indicating both age and damage from usage as a technical textile. Further morphological details illustrate the presence of microfractures in the fibre, probably owing to environmental fluctuations and the expansion and contraction of organic natural fibres from moisture changes, and the lodging of small particulates in these microfractures, which exacerbates the fracture and leads to breaks and deterioration of the textile. The basic theory regarding fibre fracture in extension involves the propagation of a crack from a flaw (Andrews, 1964). The influence of flaws on the tensile properties of natural fibres will be discussed in more detail in Chapter 13. In aged wool fibres, deterioration has already occurred owing to the effects of use and exposure to the environment. Changes in relative humidity cause small changes in fibre dimensions, which, when constantly repeated, slowly generate microscopic flaws in the wool fibres. Modern fracture mechanics has established that fibre breaks can initiate from a microscopic flaw present in the fibre structure, with axial shear deformation playing an important role in the initiation and propagation of cracks. Figure 1.2 illustrates the soiling that is prevalent with historic natural wool fibres, but also the fact that, through shielding within the textile yarn structure, some fibres may retain scale formation. Therefore, it should be noted that, while some fibres may be so degraded as to require amino acid analysis or chemical testing to confirm their substrate, there can be a range of fibre morphologies within natural historic fibre assemblies. However, the microfractures – albeit smaller – are still in evidence, with small particulate material lodging in the fractures and leading to exacerbated damage and deterioration of the natural fibres.

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Figure 1.2 Scanning electron micrograph of an aged wool fibre dated ca. 1800, approximately 50 µm in diameter (2000× magnification).

For conservation specialists and cultural heritage collections, preventive conservation requires analyses that include additional critical details about display, storage, exhibition and treatments. These investigations must include information about soiling, patterns and levels of deterioration and the effects of environmental conditions – such as relative humidity, light levels and pollution control. The application of a range of scientific techniques to fibre analysis provides a wealth of information for textile preservation. Techniques such as SEM, elemental analysis, confocal microscopy, light microscopy, gas chromatography (GC-MS), mechanical testing and chemical analyses allow investigations into internal and external aspects of the fibre structure, identification of surface contaminants and the opportunity to learn about the impact of various treatments and display environments on textile deterioration (France, 2004). Linking chemical and mechanical properties allows changes in the fibre properties to be associated with physical changes in the technical textile. While much attention is paid to temperature, organic materials are highly susceptible to changes in relative humidity, as indicated by the micrograph in Figure 1.2. Wayne (1970) noted the basis for distinguishing between photochemical and thermal effects. This can be highlighted by the example of a bond-breaking reaction that requires energy of 251 kJ/mol, typical of many covalent bonds in wool fibres. The excitation energy to break this bond can be induced photochemically by a single quantum of light of about 450 nm (i.e. green light). In contrast, at ambient temperatures the thermal energy available for bond cleavage is essentially zero (4 × 10−46 ). The state of historic wool fibres is dependent upon the extent to which the textile item has been used, and the conditions to which they have been exposed: light, water, oxygen and temperature. A study of keratin fibres taken from tombs in Egypt dated at between 1500 and 4000 years old showed that these retained as much as 20% of the strength and 10% of the extensibility of modern unaged wool fibres (Massa et al., 1980). These values were also comparable with those of wool fibres only 200 years old from textiles that had been used as working textiles, and that had spent a portion of their recent history in museum environments. To gain an accurate assessment of the state of deterioration of historic natural fibre assemblies, the use of the ‘energy of rupture’ measure provides a combination of both the loss of strength and the loss of extensibility of the aged fibres. As shown in Figure 1.3, pre-Columbian wool textile fibres (ca. 1500) that had been buried under conditions of constant relative humidity, low oxygen and no light were shown to retain up to 50% of the strength of unaged wool fibres, as compared with textiles from 1800 that had been exposed to environmental fluctuations. This shows the significant effect of environmental

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Figure 1.3

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Energy of rupture degradation curve of wool fibres from ca. 1500 AD and 1800 AD after different irradiation times.

parameters on historic wool textiles. As is evident from the steep initial portion of the curve, extensive degradation occurs early in the life of a textile; therefore, as regards the degradation process of natural fibres, the preservation of modern natural textiles needs to be carefully considered in terms of their exposure to degradative environmental influences (France et al., 2005). The primary goal of conservation is the preservation of cultural property, with current preventive conservation focusing on non-intervention techniques if possible. An important consideration is whether stabilisation of conditions alone can confer enough of a benefit to offset the requirement for treatment to the historic textile artefact. If treatment of the textile is required to remove harmful contaminants, an evaluation of the treatment is necessary to ensure that it both confers a benefit by removing soiling and particulate matter and does no harm through fracturing or decreasing mechanical properties. In order to create a baseline for many of these tests, samples that have undergone accelerated ageing are usually utilised to assess the various potential environmental conditions and treatments. In these cases, removing samples from an already fragile historic textile for assessment is an ethical dilemma. Very small samples that can directly answer critical questions for the long-term preservation of the item may be permitted; however, preservation scientists are constantly developing non-destructive, non-invasive techniques that can provide the same level of information without any impact on an item of significant cultural heritage (France, 2003b). This assessment of treatments and environmental parameters supports critical cost-benefit decisions while providing a more comprehensive overview of preservation requirements. Characterisation of natural fibres is a critical component in assessing the overall properties of the product, as what occurs at a micro level can have a significant impact on the effects observed at a macro level and the applicability of the fibre for specific uses. The determination of chemical and mechanical properties is critical for cultural heritage, as often there is little documentation and it is only through scientific analyses of the fibre and fibre structures that a historical profile can be recreated. Many techniques in conservation science focus upon microsampling as non-invasively as possible, causing minimal disturbances to an already fragile textile. Changes in chemical properties can give a good assessment of structural damage and deterioration of the natural fibres, particularly when these are linked with physical markers that integrate this micro-level information with the macro-level manifestation. Significant interest is again being shown in the analysis of both animal and plant natural fibres because of their inherent properties, providing a template for the creation of man-made fibres. These can include bicomponent structures such as wool, or specific moisture, strength and extensibility properties such as those for cellulose and protein fibres. The utilisation of these natural fibres in long-term applications such as technical textiles will continue to play a significant role in the preservation and understanding of our cultural heritage as well as in future developments for sustainable and environmentally compatible textiles.

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Figure 1.4

Scanning electron micrograph of ca. 1900 AD single bast fibres with a diameter of approximately 20 µm.

As mentioned previously, the stars of the Star-Spangled Banner were made from cotton fabric, and these exhibited a higher level of acidity than the wool fabric. Stabilisation of the cotton fabric was part of the conservation treatment, as well as removal of a linen backing attached in 1913 to stabilise the flag. The linen backing and Amelia Fowler stitching stabilisation undertaken in 1913 was removed, as the bast fibres (Figure 1.4) were degrading at a different rate to the wool fabric and causing destabilisation of the wool and cotton flag structure. It should be noted that all natural fibres have different properties with regard to strength and extensibility, so composite fabrics – whether historic or modern technical textiles – need careful attention as to the combination of fibre properties.

1.3

Conclusion

Studies of the history of historic textiles and their natural fibres provide additional insights into the technical applications of the textiles over the years. The stewardship of historic textiles, in common with all cultural heritage items, requires the best preservation techniques possible to ensure their longevity based on current information and resources. This requires an understanding of the history of the textile in terms of its lifetime of usage, display and storage environments, technical application and the effects of treatments and conditions. The current state of a natural fibre textile will be entirely dependent upon this history. As this is often not documented, a range of analytical techniques are essential to provide the missing information, including by testing the properties of the natural fibres. This critical information should include not only the mechanical and chemical state of the textile, together with fibre and dye analysis and identification, but also the level and type of environmental degradation that have occurred. Identification of soils and contaminants, as well as an assessment of those treatments that are most beneficial for the preservation and understanding of this textile, is also important. While a wide variety of analytical techniques are available, a clear understanding of the information required for conservation of each textile should be established, so as to utilise the most appropriate technique to answer the conservation questions and determine the optimum preservation outcome or treatment. Conservation requires scientific analyses for conservation specialists and museum collections that provide critical details for identifying a natural historic textile – history, display, storage, exhibition and treatments.

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These investigations must include information about soiling, patterns and levels of deterioration and the effects of environmental conditions – light levels, relative humidity and pollution control. The application of a range of scientific techniques to textile fibre analysis provides a wealth of information for textile preservation. The advanced precision in techniques such as mechanical testing, chemical analyses and microscopy allows investigations into internal and external aspects of the fibre structure, identification of surface contaminants and assessment of textile deterioration linked to environment or treatment. In the museum setting, conditions can be controlled and monitored to minimise the effects of environmental factors. The primary goal of conservation is the preservation of cultural property, with preventive conservation focusing on non-intervention techniques if possible. An important consideration is whether stabilisation of conditions alone can confer enough of a benefit to offset the requirement for treatment of a historic artefact. If treatment of a textile is required to remove harmful contaminants, an evaluation of the treatment is necessary to ensure that it both confers a benefit by removing soiling and particulate matter and does no harm through fracturing or decreasing mechanical properties. Preventive textile conservation extends the life of a textile with the best care available. This involves making decisions about exhibition and storage conditions, monitoring and controlling the environment and treating or cleaning the textile. Critical information is necessary to make these informed decisions. The use of available and well-developed scientific techniques provides those involved in conservation activities with the empirical information needed to understand the properties of the natural fibres and to make these critical decisions about preservation.

References Andrews, M.W. (1964) The fracture mechanism of wool fibers under tension. Text. Res. J., 34, 831–835. France, F.G. (2003a) Beneath the grime: measuring the effects of preservation treatments for textiles, in Textile Specialty Group Postprints. Textile Specialty Group, American Institute for Conservation, Washington, DC, pp. 45–52. France, F.G. (2003b) Creating a standard vocabulary for defining levels of deterioration, in Development of a WebAccessible Reference Library of Deteriorated Fibers Using Digital Imaging and Image Analysis, ed. by Merritt, J. Proceedings of Conference, Harpers Ferry, WV, 3–6 April 2003. Harpers Ferry Center, National Park Service, US Department of the Interior, pp. 77–86; available at: http://www.nps.gov/hfc/products/cons/con-fiber.htm (accessed 17 December 2009). France, F.G. (2004) Preservation of textile cultural heritage, in Quality Textiles for Quality Life. Proceedings of the Textile Institute’s 83rd World Conference, 23–27 May 2004. College of Textiles, Donghua University, Shanghai, China/Textile Institute, Manchester, UK, pp. 1583–1587. France, F.G. (2005a) Scientific analysis in the identification of textile materials, in Scientific Analysis of Ancient and Historic Textiles: Informing Preservation, Display and Interpretation: Postprints, ed. by Janaway, R. and Wyeth, P. Archetype Publications, London, UK, pp. 3–11. France, F.G. (2005b) Andean to banners, in Proceedings of the 11th International Wool Research Conference, University of Leeds, 4–9 September 2005. Department of Colour and Polymer Chemistry, University of Leeds, Leeds, UK, CD ROM. France, F.G. (2007), Weaving independence from a distant cottage industry, in Textile Narratives + Conversations, ed. by Bier, C. and Perlman, A.S. 10th Biennial Symposium, Textile Society of America, Earleville, MD, CD-ROM. France, F.G., Roussakis, V., Lissa, P., Xanena, M., Santillan, P., Campero de Larran, M., Dona, G. and Ammrati, C. (2005) Textile treasures of Llullaillaco, in Recovering the Past: the Conservation of Archaelogical and Ethnographic Textiles. 5th Biennial North American Textile Conservation Conference, Mexico City, Mexico, 9–11 November 2005, pp. 31–34. Massa, E.R., Masali, M. and Fuhrman, A.M.C. (1980) Early Egyptian mummy hairs: tensile strength tests, optical and scanning electron microscope observation. A paleo-biological research. J. Hum. Evolution, 9, 133. Wayne, R.P. (1970) Photochemistry. Butterworth & Co. Ltd, London, UK.

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2 What Are Natural Fibres?

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2.1 Chemistry of Plant Fibres Danny E. Akin Athens, Georgia, USA

2.1.1 Introduction Natural plant fibres are cell walls that occur in stem and leaf parts and are comprised of cellulose, hemicelluloses, lignins and aromatics, waxes and other lipids, ash and water-soluble compounds. Figure 2.1.1 shows a ‘typical’ cell wall with major components and a schematic representation of their organisation. The chemistry and structure of fibres determine their characteristics, functionalities and processing efficiencies. The following information briefly describes the major components in these natural fibres as it relates to fibre applications.

2.1.2 Cellulose Cellulose is a linear polymer of glucose (Nelson and Cox, 2000; Ljungdahl, 1990; Focher, 1992). In its simplest form, cellulose is a linear carbohydrate polymer of β-1,4-linked glucose units. However, the basic repeating unit of cellulose is the dimer cellobiose, which comprises two glucose units bound by the β-1,4 linkage as well as intermolecular hydrogen bonds. A typical structure of cellulose is shown in Figure 2.1.2. The structure of how glucose is bound in the linear polymer determines the properties of cellulose. Cellulose can take many forms, a phenomenon that is the basis for numerous in-depth reviews of this important natural polymer (Ljungdahl, 1990; Focher, 1992). Briefly, cellulose, which consists of thousands of glucose units, can stack to form crystalline forms with intramolecular hydrogen bonds providing a stable, hydrophobic polymer with high tensile strength. Cellulose occurs in plant cell walls as microfibrils (e.g. 2–20 nm diameter and 100–40 000 nm long) providing a linear and structurally strong framework (see Figure 2.1.1). Several models have been proposed of the packing of microfibrils within the cellulosic fibre. In addition to the more ordered or crystalline regions of cellulose, there are other regions of less order, or non-crystalline regions. These differences can have enormous influence on characteristics and functionalities.

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
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Figure 2.1.1 Schematic diagram of the hierarchy of a ‘typical’ plant cell wall, from cellulose synthesis to a simplified model of a primary cell wall and from microfibril structure to crystalline cellulose to the cellulose molecule with the monomer unit glucose. Adapted with permission from AAAS, C. Somerville et al. Science, 306, 2206–11 (2004) and U.S. Department of Energy Genome Programs, http://genomics.energy.gov.

The structure of cellulose results in a complicated situation for enzymatic degradation. Classically, three cellulases are required to degrade cellulose: exocellulase (exocellobiose hydrolase), endocellulase and cellobiase (Ljungdahl, 1990). Much detailed work has elucidated some of the complexity of cellulolysis, and in general terms exocellulase knocks off cellobiose units from an end of the polymer, endocellulase randomly breaks β linkages and the cellobiase degrades the dimer into glucose units. It is known that enzymes can degrade cellulose by other mechanisms. While a large amount of information exists on this very important polymer, there are characteristics not well understood for particular fibres. Such characteristics may be further influenced by various amounts of other sugars or components as integral parts of ‘cellulose’ (Focher, 1992). A practical example of this differential chemistry is shown in retting of cellulosic bast fibres such as flax. In the bast tissues, the cellulosic structure of the fibre is more resistant than other components, such as the matrix components, to the enzyme consortium of retting microorganisms, allowing for a separation of the (primarily) cellulosic fibres (Figure 4.4 in Chapter 4).

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Figure 2.1.2 Typical schematic of cellulose, showing the linear polymer nature of glucose units: (A) cellulose in Haworth notation; (B) structure of the dimer cellobiose; (C) cellulose molecule with β-1,4 linkage between C atoms 1 and 4.

2.1.3

Hemicellulose

After cellulose, hemicellulose is reported to be the second most abundant carbohydrate of plant cell walls (Deblois and Wiegel, 1990). Hemicellulose is a collective term for an extremely heterogeneous group of polysaccharides, differing in both composition and structure and depending on their origins. Non-cellulosic polysaccharides such as glucans (a polymer of d-glucose monomers – C6 H12 O6 ), mannans (a polymer of the sugar mannose – C6 H12 O6 ), galactans (a polymer of the sugar galactose – C6 H12 O6 ), arabinans (a polymer of arabinose – C5 H10 O5 ) and xylans (a highly complex polymer of the pentose sugar xylose – C5 H10 O5 ) comprise hemicelluloses. Hemicelluloses, which are not linear, are associated with pectins, cellulose and aromatic constituents within plant cell walls (see Figure 2.1.3). Hemicelluloses are often referred to as matrix components and may be found in the middle lamellae that bind cell walls of fibres, in the primary wall regions and in the thicker, cellulose-rich, secondary layer of the plant cell wall (Focher, 1992). This latter situation exists in multilayered regions, at times with hemicelluloses bonded with cellulose and lignin. The heterogeneity of hemicelluloses extends to branching polymers (side units of a linear-backbone polymer), thus giving new dimensions and complexities within the cell walls. Often, for research, xylans are used as representatives of hemicellulose, with degrading enzymes listed similarly to cellulases, such as exoxylanases, endoxylanases and xylosidases. Branching within hemicellulosic structures, however, requires many types of enzyme to degrade the variously substituted and/or branched polymers (Deblois and Wiegel, 1990), including xylanases, arabinases, mannanases and non-cellulosic glucanases. While cellulose makes up most of the cotton fibre (once it is scoured to remove other components) and little hemicellulose is found, in contrast hemicellulose comprises a much greater percentage of bast fibres (Table 13.9 in Chapter 13). For example, flax fibre is only 60–80% cellulose, with hemicellulosic sugars comprising a significant amount (Focher et al., 1992; Akin et al., 1996). Galactose and mannose specifically are prevalent in retted flax fibre (Akin et al., 1996), and galactomannans and glucomannans have been reported to be integral components of flax fibres (Focher, 1992). Glucomannan, mainly a straight-chain polymer, with a small amount of branching, consists of β-(1,4)-linked d-mannose and d-glucose sugars (Katsuraya et al., 2003).

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Figure 2.1.3 Scheme of the cell wall of an onion. Cellulose and hemicellulose (most of the hemicellulose crosslinks are not shown to simplify the figure) are arranged into layers in a matrix of pectin polymers. Adapted from M. McCann and K.R. Roberts, 1991, in C. Lloyd, ed., The Cytoskeletal Basis of Plant Growth and Form, p. 126.

Hemicelluloses of flax and other natural fibres have undergone considerable research owing to their extreme importance in processing and functional attributes. Characteristics, such as the high moisture absorption of flax, may be due to the presence of hemicelluloses with cellulose. While retting of flax separates the primarily cellulosic fibres from non-fibre components, non-cellulosic carbohydrates along with other materials may remain on the fibre surface and impart undesirable features, such as uneven dyeing.

2.1.4 Pectin Pectins, as with hemicelluloses, are a diverse group of substances associated with cell walls and natural fibres (Sakai et al., 1993). Galacturonic acid residues linked through α bonds are a major component of many pectins, and rhamnose and galactose units are highly representative of pectins (see Figures 2.1.4 and 2.1.5). Pectin amounts are often low (Table 13.9 in Chapter 13) in natural plant fibres, but they are strategically located within the plant tissues. Pectins, along with hemicelluloses, are called matrix polysaccharides in plants and hold tissues, including fibres, together (see Figure 2.1.3). Significant properties in plants can be determined from the carboxyl group (–COOH) of the galacturonic residue structure, specifically whether it exists in acid form or with a methoxy group (–O–CH3 ) on carbon 6. When in the acid form (dissociation of H+ from the COOH group (see Figure 2.1.4(B)), divalent ions, and particularly Ca2+ , may bridge galacturonic acid residues and provide stability within the plant tissue. Imaging microspectroscopy as well as tagging with monoclonal antibodies have revealed that different pectin structures reside in different regions of the plant (Himmelsbach et al., 1998; Andeme-Onzighi et al., 2000). Even though small in amount, pectins have major importance for processing fibre. For example, the cotton fibre is covered with a waxy protective layer called a cuticle. Pectin is the matrix material within the primary cell wall that resides just underneath the cuticle and holds this waxy barrier layer to the cellulosic cotton fibre. Scouring, using NaOH or more recently using pectinolytic enzymes, is a method used to degrade pectin and allow separation of the cuticle so that the cotton fibres can be processed and dyed consistently. Pectate lyase has been marketed as part of an enzymatic scouring method for cotton, with very good results (Akin et al., 2007).

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Figure 2.1.4 (A) D-Galacturonic acid as an open chain in Fischer projection, (B) D-galacturonic acid in ring form (α pyranose), (C) D- and L-rhamnose as an open chain in Fischer projection and (D) D- and L-galactose as an open chain in Fischer projection.

Figure 2.1.5 (A) Fragment of the backbone of a poly-α-1,4-galacturonic acid, and (B) rhamnogalacturonan backbone with a kink induced by the integrated rhamnose.

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Flax and other bast fibres undergo a process called retting to separate non-fibrous materials from the fibres (see Chapter 4). Degradation of matrix materials to separate fibres from non-fibrous components is the primary focus in retting. As most practical retting methods are microbial, pectinolytic enzymes are central in retting. Earlier, a mixed enzyme consortium of pectinases, cellulases and hemicellulases was considered desirable for retting, so as to have a partial degradation of the complex plant material to release fibres. Recent research, however, has shown that just a pectinase, either a pectate lyase (belonging to the family of lyases – enzymes that catalyse the breaking of chemical bonds not by hydrolysis or oxidation but, for example, by forming new double bonds or new ring structures) or an endopolygalacturonase, can satisfactorily separate fibres (Akin et al., 2007). In both cotton and flax, Ca2+ is removed from the intermolecular bridges with chelators, such as ethylenediaminetetraacidic acid (EDTA), to destabilise the pectin and promote enzyme action.

2.1.5 Lignin and Aromatic Compounds The aromatic ring structure is the primary chemical constituent in lignin and other aromatics. These compounds are extremely diverse and present in many forms within plants and plant cell walls. Lignin is classically defined as a polyphenylpropanoid complex that arises from one or more of the following alcohols (see Figure 2.1.6): p-coumaryl (no methoxyl groups (–O–CH3 ) on the aromatic ring), coniferyl (one methoxyl group at the 3 position) or sinaple (two methoxyl groups at 3 and 5 positions) (Sarkanen and Ludwig, 1971). Lignin is reported to be the second most abundant material in plants and responsible for strength, rigidity and protection against microbial pathogens of cell walls. The amount of lignin in plants is often related to the method of analysis, and amounts can be quite varied. For example, Klason lignin is based on the residue after strong acid treatment, whereas acetyl bromide lignin is aromatic material that is solubilised by the chemical. It is not surprising, then, that different amounts and constituents comprise ‘lignin’ of a fibrous material. Lignins are intimately associated with hemicellulose and cellulose within plant cell walls. Covalent linkages occur between hemicellulose and lignin, and often the association with cellulose is a masking by lignin or the lignin/hemicellulose complex. Different types of lignin (i.e. syringyl or coniferyl lignins) predominate in different plants and in different tissues within plants (Sarkanen and Ludwig, 1971; Akin 2008). These differences can have profound effects on plants. The type of lignin within tissues influences the degree of biodegradation, susceptibility to microbial action and response to chemical treatments. For example, softwoods have mostly coniferyl lignin and are less biodegradable, while hardwoods tend to have more syringyl lignin than softwoods.

Figure 2.1.6

Chemical structure of coumaryl, coniferyl and sinapyl (syringyl) alcohols, which form different types of lignin.

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Figure 2.1.7

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Chemical structure of ferulic and p-coumaric acids.

Other aromatic compounds occur in plants and impart particular characteristics. Condensed tannins, which are phenolic complexes arising from the condensation of leucoanathocyanidin and catechin, are associated with antiquality factors in forage plants, e.g. limit intake by ruminants (Windham et al., 1990). The large number of hydroxyl groups in the tannin molecule provide many points of attachment for protein–tannin complexes. Another group of aromatics associated with plant cell walls are low-molecular-weight phenolic acids, especially ferulic and p-coumaric acids (see Figure 2.1.7). Grass cell walls are particularly rich in phenolic acids, which may be ester linked to arabinose within hemicelluloses and/or ether linked to other phenolic entities of lignin. These compounds, as is typical for other aromatic compounds, are inhibitory to microorganisms and often have a particular attribute in binding to proteins and in antimicrobial activities. The special epidermal cell that comprises the cotton fibre is attached to the seed coat. Imaging spectroscopy reveals the presence of tannin-like compounds around the base of the cotton fibre, which may, upon polymerisation, increase fibre/seed coat strength (Himmelsbach et al., 2003). Strongly attached fibres often cause portions of the seed coat to be removed during ginning, resulting in fibre trash. Further work is required fully to assess the relationship of aromatics and fibre/seed coat strength. Bast fibre plants pose an interesting situation pertinent to lignin. With flax, for example, most of the lignin resides in the core tissues towards the centre of the stem. This tissue contains the xylem and other structural cell walls for water conduction. The bast tissue, i.e. outer epidermal and bast fibre layer, has little lignin or aromatics (Akin et al., 1996) (Table 13.9 in Chapter 13). The small amount of aromatics in flax fibre is located in the cell corners of the middle lamella and does not appear to prevent fibre separation. Some research (Love et al., 1994) indicated that aromatics in the bast fibres appeared to be mostly an anthocyanin other than lignin. Similarly, ramie has virtually no aromatics within the bast fibres. In contrast, hemp appears to have more lignin than flax or ramie in the bast region, which may be related to the fact that enzymatic retting methods which work for flax are not as successful with hemp (Fischer et al., 2005). Kenaf fibres, in contrast to flax, ramie and hemp, have lignin throughout the fibre cell wall and in the middle lamella, as shown by chemistry, histochemistry and ultraviolet absorption microspectrophotometry (Morrison et al., 1996). These chemical differences in lignin amount and location have profound effects on retting methods, retting efficiencies and end-product use of the fibre. Sisal (Agave spp.), which provides fibres from the leaf sheath (see Table 13.9 in Chapter 13), gives a positive reaction for aromatics with chlorine-sulphite (chlorine water followed by sodium sulphite) but not with acid phloroglucinol (Akin, 1987), suggesting a prevalence of syringyl or other aromatics within the cell wall. Anaerobic, rumen fungi, which are known to attack lignified grass cell walls via enzymes with phenolic acid esterase activity (Akin, 2008), also attack sisal, and in fact sisal is an important substrate for isolation of these fungi (Akin, 1987). Therefore, evaluation of lignin and aromatics in this leaf sheath fibre may require a broad study of these compounds, other than just lignin, to understand their influence on fibre processing, quality and application.

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2.1.6

Fats, Waxes and Lipids

These hydrocarbons are a diverse group with the common and defining feature of insolubility in water (Nelson and Cox, 2000). Their biological functions are equally diverse. Fats and oils are the principal storage forms of energy in many organisms. Phospholipids and sterols are structural compounds of membranes. Other lipids play roles as diverse as enzyme cofactors, electron carriers and light-absorbing pigments. Biological waxes are esters of long-chain (carbon 14 and upwards to carbon greater than 50) alcohols, with separation based on different melting points. The amounts of these compounds are relatively low in cotton and bast fibre plants, but higher in grasses (bagasse and cereal) (Table 13.9 in Chapter 13). Lipids are especially important on the outside of plants or plant parts, especially fibres (Stern et al., 2003). The wax accumulation on the cuticle provides a protective barrier that prevents drying and microbial entry inside the plant. This waxy cuticle influences the processing and quality of natural fibres. For cotton and bast plants, removal of the cuticle during processing, i.e. scouring and retting respectively, is required to obtain quality cellulosic fibres of industrial importance in textiles. Cotton fibres, which are exposed to the weather and climate upon opening of the bolls, have a protective waxy cuticle that appears to be non-uniform over the surface. The wax is more heavily deposited nearer to the site of attachment of the fibre to the seed coat (Himmelsbach et al., 2003). Scouring of cotton is undertaken to remove the cuticle and thereby expose the cellulosic structures. Wax content on fibre surfaces influences the processing of cotton (Brushwood, 2004). A similar situation exists with flax during retting, except that the cuticle is outside the stem and little wax occurs on the fibre surface per se. In retting, the cuticular/epidermal layer of the stem is separated from the bast fibres, as are the lignified core tissues. A variety of lipid compounds have been identified in flax cuticle, including long-chain fatty acids (carboxylic acid with an unbranched aliphatic chain), fatty alcohols (aliphatic alcohols) and wax esters (esters of long-chain fatty alcohols with long-chain fatty acids) (Morrison et al., 2006). Some work indicates that this epidermal/cuticle barrier is more difficult to remove than the core tissues during retting and constitutes a much greater problem than core tissue (i.e. shive) in poorly processed fibre.

2.1.7

Ash

Plant material is burned in a furnace, with remaining mineral materials determined gravimetrically as the ash content (Archibald, 1992). The insoluble mineral content of plant material can be determined by a variety of methods, e.g. atomic absorption or inductive coupling plasma (ICP) emission spectroscopy, to provide quantification of several elements. The ash % from a variety of fibres (Table 13.9 in Chapter 13) showed less than 2% for most plant fibres, but substantially higher levels in grasses such as rice and wheat straw. These higher levels in grasses are probably due to silica (SiO2 ) content, which is deposited in grass cuticle as silica bodies and in trichomes and other features of leaves (Stern et al., 2003). The quantity and type of elements in plants may influence fibre processing, particularly spinning efficiency or metal abrasion. For example, in a study of selected cotton samples (Brushwood, 2004), correlations were reported of the potassium, calcium and magnesium contents with various fibre and yarn properties, including strength, neps and fibre yellowness. In some cases, the elemental content may be significant in particular tissues related to processing or quality. Calcium content has shown a correlation with increased processing wastes in cotton (Brushwood, 2004). Furthermore, calcium content seems to have a particular relationship in flax bast tissue. ICP determination of calcium content in flax stem components showed an almost 6-fold higher level in epidermis/cuticle compared with fibre tissue. This finding is associated with a higher acidic pectin content in this tissue, indicating the presence of a more resistant pectin (bound with calcium bridges) to retting, and helps explain the difficulty in removing cuticle from fibre. While the mineral content is usually low in natural fibre plants, there is evidence of accumulation of heavy metals such as lead (Pb), copper (Cu), zinc (Zn) and cadmium (Cd) in plants such as flax, hemp and cotton (Angelova et al., 2004). Flax is particularly efficient in sequestering heavy metals, and some interest exists

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in photobioremediation. Metal analyses of natural fibres, then, must consider the source of production for health and processing concerns.

2.1.8 Water-Soluble Material This material may consist of soluble sugars, such as hemicellulose and pectin, tannins, vitamins, etc. Reported values (Table 13.9 in Chapter 13) show a wide range in various natural plant fibres. The large difference in flax and cotton may reflect the fact that cotton is mostly cellulose, while flax fibres have a higher level of hemicellulose and pectins. In water retting, often a preliminary warm water leaching was carried out before the retting process began (Sharma and Van Sumere, 1992.) Possibly, some antimicrobial compounds were removed to promote bacterial activity.

2.1.9

Conclusion

As with plant cell walls generally, natural fibres have a complex structure and chemistry that impart characteristic features. These features have been recognised and put to use for mankind for many years in diverse industrial applications. The future need for natural fibres of all kinds is projected to increase both in historic uses and in new applications, possibly replacing and reducing the use of petroleum-based fibres. Better understanding of the chemistry of the fibres, and the manner in which the chemical molecules are arranged, will result in more efficient processing, improved standards for quality and new and advanced applications. Not to be ignored is the need to expand the chemical knowledge of all parts of the fibre plants that may improve the economic benefits of natural fibres through coproducts.

References Akin, D.E (1987) Association of rumen fungi with various forage grasses. Anim. Feed Sci. Technol. 16, 273–285. Akin, D.E. (2008) Plant cell wall aromatics: influence on degradation of biomass. Biofuels, Bioproducts, and Bioprocess., 2, 288–303. Akin, D.E., Condon, B., Sohn, M., Foulk, J.A., Dodd, R.B. and Rigsby, L.L. (2007) Optimization for enzyme-retting of flax with pectate lyase. Ind. Crops Prod., 25, 136–146. Akin, D.E., Gamble, G.R., Morrison III, W.H., Rigsby, L. L. and Dodd, R.B. (1996) Chemical and structural analysis of fibre and core tissues from flax. J. Sci. Food Agric., 72, 155–165. Andeme-Onzighi, C., Girault, R., His, I., Morvan, C. and Driouich, A. (2000) Immunocytochemical characterization of early-developing flax fibre cells. Protoplasma, 213, 235–245. Angelova, V., Ivanova, R., Delibaltova, V. and Ivanov, K. (2004) Bio-accumulation and distribution of heavy metals in fibre crops (flax, cotton and hemp). Ind. Crops Prod., 19, 197–205. Archibald, L.B. (1992) Quality in flax fibre, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F. M. Publications, Belfast, UK, pp. 297–309. Brushwood, D.E. (2004) The influence of cotton noncellulosic naturally occurring materials on yarn processing properties. Trans. ASAE, 47, 995–1002. Deblois, S. and Wiegel, J. (1990) Hemicellulases in lignocellulose degradation, in Microbial and Plant Opportunties to Improve Lignocellulose Utilization by Ruminants, ed. by Akin, D.E., Ljungdahl, L.G., Wilson, J.R. and Harris, P.J. Elsevier, New York, NY, pp. 275–287. Fischer, H., M¨ussig, J. and Bluhm, C. (2005) Enzymatic modification of hemp fibres for sustainable production of high quality materials: influence of processing parameters. J. Nat. Fibr., 3, 39–53. Focher, B. (1992) Physical characteristics of flax fibre, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere C.F., M. Publications, Belfast, UK, pp. 11–32.

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Focher, B., Marzetti, A. and Sharma, H.S.S. (1992) Changes in the structure and properties of flax fibre during processing, in The Biology and Processing of Flax, ed. by Sharma, H. S. S. and Van Sumere C.F. M. Publications, Belfast, UK, pp. 329–342. GMIS (2007) Biofuels Primer – Part 2: Understanding Biomass: Plant Cell Walls, US Department of Energy GenomeProgram’s Genome Management Information System (GMIS) (May 2007); available at: http://genomicsgtl.energy. gov/biofuels/placemat.shtml (accessed 27 July 2009). Himmelsbach, D.S., Akin, D.E., Kim, J. and Hardin, I.R. (2003) Chemical structural investigation of the cotton fibre base and associated seed coat: Fourier-transform infrared mapping and histochemistry. Text. Res. J., 73, 281–288. Himmelsbach, D.S., Khalili, S. and Akin, D.E. (1998) FT-IR microspectroscopic imaging of flax (Linum usitatissimum L.) stems. Cell. Molec. Biol., 44, 99–108. Katsuraya, K., Okuyamab, K., Hatanakab, K., Oshimab, R., Satoc, T. and Matsuzakic, K. (2003) Constitution of konjac glucomannan: chemical analysis and 13 C NMR spectroscopy. Carbohydrate Polym., 53(2), 183–189. Ljungdahl, L.G. (1990) Cellulases and the cellulosome concept, in Microbial and Plant Opportunties to Improve Lignocellulose Utilization by Ruminants, ed. by Akin, D.E., Ljungdahl, L.G., Wilson, J.R. and Harris, P.J. Elsevier, New York, NY, pp. 265–273. Love, G.D., Snape, C.E., Jarvis, M.C., and Morrison, I.M. (1994) Determination of phenolic structures in flax fibre by solid-state 13 C NMR. Phytochemistry, 35, 489–491. McCann, M. and Roberts, K.R. (1991) in The Cytoskeletal Basis of Plant Growth and Form, ed. by Lloyd, C.W. Academic Press, London, UK, p. 126. Morrison III, W.H., Akin, D.E., Ramaswamy, G. and Baldwin, B. (1996) Evaluating chemically retted kenaf using chemical, histochemical, and microspectrophotometric analyses. Text. Res. J., 66, 651–656. Morrison III, W.H., Holser, R. and Akin, D.E. (2006) Cuticular wax from flax processing waste with hexane and super critical carbon dioxide extractions. Ind. Crops Prod., 24, 119–122. Nelson, D.L. and Cox, M.M. (2000) Lehninger Principles of Biochemistry, 3rd edition. Worth Publishers, New York, NY. Sakai, T., Sakamoto, T., Hallaert, J. and Vandamme, E.J. (1993) Pectin, pectinase, and protopectinase: production, properties, and applications, in Advances in Applied Microbiology, ed. by Neidleman, S. and Laskin, A.I. Academic Press, New York, pp. 213–294. Sarkanen, K.V. and Ludwig, C.H. (1971) Lignins: Occurrence, Formation, Structure, and Reactions. Wiley-Interscience, New York, NY, pp. 1–18. Sharma, H.S.S. and Van Sumere, C.F. (1992) Enzyme treatment of flax. Genet. Eng. Technol., 12, 19–23. Somerville, C., Bauer, S., Brininstool, G., Facette, M., Hamann, T., Milne, J., Osborne, E., Paredez, A., Persson, S., Raab, T., Vorwerk, S. and Youngs, H. (2004) Toward a systems approach to understanding plant cell walls. Science, 306 (December), 2206–2211. Stern, K.R., Jansky, S. and Bidlack, J.E. (2003) Introductory Plant Biology, 9th edition. McGraw-Hill, New York, NY, 624 pp. Windham, W.R., Petersen, J.C. and Terrill, T.H. (1990) Tannins as anti-quality factors in forage, in Microbial and Plant Opportunities to Improve Lignocellulose Utilization by Ruminants, ed. by Akin, D.E., Ljungdahl, L.G., Wilson, J.R. and Harris P.J. Elsevier, New York, NY, pp. 127–135.

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2.2 Natural Fibres – Function in Nature Michaela Eder and Ingo Burgert Max-Planck-Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany

2.2.1 Introduction Natural fibres are nature’s solution for adapting plant material properties to mechanical constraints that the plant body has to cope with during its lifetime. In this sense the natural plant fibre is an optimised structure, proved by evolution, which for that reason has been technically utilised from the beginning of mankind. This chapter provides insight into the in vivo function of plant fibres in the living organism for a fundamental understanding of the structure–function relationships of plant fibres. These underlying principles are a prerequisite to understanding and improving plant fibre performance in established fibre-based composites and to creating new innovative fibre-based materials like insulation products or geotextiles. Different plant fibre types are introduced, with a focus on fibres that are relevant for technical applications. Their arrangement in the plant body and their anatomy are presented, highlighting how plants are able to adapt their mechanical properties by a specific cell wall organisation. For a general overview, the resulting tensile properties of various plant fibres are listed in a fairly comprehensive table. Possible reasons for the large variation in mechanical properties within and between fibre types are identified. Both variations related to mechanical testing techniques and the high natural variability of plant fibre properties caused by micro- and nanostructural features, in particular cellulose fibril orientation, are discussed. Finally, the generation of growth stresses in wood fibres is presented as an efficient concept to prestress the natural fibre composite, which serves both to compensate for the low compressive strength of wood and to perform directed bending movements.

2.2.2 Fibre Types and Anatomy Plant fibres are defined on the one hand with respect to their function in the plant and on the other hand with respect to their size and shape, in particular the aspect ratio. From a plant biological science perspective, fibres belong to the so-called sclerenchyma, referring to cells that consist of mostly lignified secondary cell walls and serve for mechanical stability of the plant body. Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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2.2.2.1

Fibre Types and Arrangement in the Plant

There is a multitude of plant fibres that are used for technical applications or have the potential to be utilised. In the plant body they can appear as the dominating tissue type, in a ring-like fashion, or as separate clusters, organised in a characteristic pattern or just randomly distributed. It is beyond the scope of this chapter to draw a comprehensive picture of where fibres can be located in various plant bodies; however, a schematic drawing is provided in Figure 2.2.1. This is intended to provide a rough scheme of where technically utilised fibres can be found in plant stems. Readers who show further interest are referred to excellent textbooks such as Esau’s Plant Anatomy (Evert, 2006). In terms of pulp and paper, mainly wood fibres are utilised (Figure 2.2.1(c)). Botanically speaking, these are secondary xylem fibres. The xylem is the tissue used to conduct water and nutrient salts, which mostly can also serves for mechanical stability of the organ. Primary and secondary xylems are distinguished according to the meristematic tissue that forms the cells are derived from. This can be the apical meristems or the (secondary) cambial meristem. From an evolutionary perspective, wood fibres are the consequence of an ongoing specialisation of tissue types. For instance, the wood of the evolutionary older softwood species consists of up to ∼95% of one cell type, the so-called tracheid, which has to serve for water transport and mechanical stability. In highly developed angiosperm plants, these two functions are covered by two specialised cell types, vessels with large lumina and thin cell walls and fibres with small lumina and thick cell walls respectively. Consequently, in botanical terms, softwood tracheids are not regarded as plant fibres, although they are included in this chapter as technically they play a crucial role as ‘softwood fibres’. Another important group of utilised plant fibres are the so-called phloem or bast fibres. Phloem or bast stands for those tissues that conduct the organic nutrients (photosynthesis products). Similarly to xylem, a primary and a secondary phloem can be distinguished. Most hardwood species possess primary and secondary phloem fibres, whereas only a few softwood species contain secondary phloem fibres. However, phloem fibres of trees are only marginally technically utilised. In terms of fibre-based composites, phloem fibres extracted from stems of annual plants such as hemp (Cannabis sativa L.) (see Chapter 5) or flax (Linum usitatissimum L.) (see Chapter 4) are more prominent (Figure 2.2.1(b)). These fibres are arranged in bundles, several fibres thick, or form fibre clusters. Another class of fibres that are categorised as extraxylary in Esau’s Plant Anatomy (Evert, 2006) (as with the phloem fibres) are the fibres of monocots such as sisal (Agave sisalana P.) (see Chapter 8) or abac´a (Musa textiles NEE ` ) (see Chapter 7). Here, fibres appear either connected with the vascular bundles or distributed in small clusters or assembled into a ring structure (Figure 2.2.1(a)). Fibres are also prominent in monocot leaves, in which they stabilise the plane shapes by being located mainly below the epidermis. A further fibre class are seed hairs, to which cotton (Gossypium sp.) (Chapter 10), one of the most widely used fibres in the textile and clothing industry, belongs. Botanically speaking, seed hairs are not fibres but trichomes. A second, technically used seed hair is kapok (Ceiba pentandra L.). In the living plant, seed hairs

Figure 2.2.1 Schematic drawing showing possible locations of fibres within stems of monocotyledonous and dicotyledonous plants and trees (both dicotyledonous and gymnosperm trees).

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function as parts of the seed dispersal units because they facilitate seed transport over larger distances by, for example, wind or animals. The last category of technical plant fibres worth mentioning are the so-called mesocarp fibres. A prominent example is coir, which is extracted from the mesocarp of ripe or unripe coconut fruits (Cocos nucifera L.) (see Chapter 9). 2.2.2.2

Size and Shape of the Various Plant Fibres

Taking the various arrangements of fibres in the plant into consideration, it is no surprise that fibre size and shape are highly variable. With the exception of seed hair fibres, all technically used plant fibres are directly derived from meristematic tissues, which has consequences for their formation and growth. Softwood fibres increase their lengths by ∼10% after formation (Br¨andstr¨om, 2001) and belong to the group of fibres with a small subsequent elongation. In hardwoods, fibres can double their lengths (Ridoutt and Sands, 1993). The final lengths of oil palm fibres can exceed those of their cambium initials fivefold (Khalil et al., 2008). An example of extraordinary cell elongation after formation was reported by Aldaba (1927). Ramie fibres (Boehmeria nivea L.) start their growth as 20 µm short elements when derived from meristematic initials, and in extreme cases they can reach a length of up to 550 mm, which corresponds to a 27 000-fold elongation. Extreme fibre lengths of some bast fibres are accomplished with two ways of cell elongation: so-called coordinated growth and intrusive growth. Both Gorshkova et al. (2003) and Ageeva et al. (2005) investigated coordinated and intrusive growth of flax phloem fibres, and observed that fibres elongate rather by diffuse growth than by tip growth. Further information on the growth forms can be found in an excellent review by Wasteneys and Galway (2003). Gorshkova’s studies also revealed that fibre elongation in flax ceases after 2–4 days, whereas secondary cell wall deposition lasts for ∼60 days, resulting in cell death for fibres with very small lumina. The ratio between the length and the diameter of a plant fibre, the so-called aspect ratio, plays an important role in its performance in a fibre composite material. Figure 2.2.2 provides an overview of fibre lengths and diameters of common technically used fibres. The symbols represent estimated mean values, and related dotted lines show associated ranges of the literature data. The plotted linear curve depicts an aspect ratio of 100, frequently found for plant fibres (see Table 13.6 in Chapter 13). The size and shape and thereby the aspect ratio of plant fibres depend on environmental conditions such as growth season, soil, wind, etc. Additionally, characteristic differences between fibre types (wood fibres, bast fibres, fibres of monocots, fruit and seed fibres) can be identified, which are described in the following. According to Figure 2.2.2, the mean aspect ratios of wood fibres are quite close to 100. The wood fibres can be considered as hollow tubes with tapered ends, 0.5–4 mm long and 15–40 µm in diameter. The walls of the rectangular, hexagonal, round or irregular-shaped tubes are 0.5–10 µm thick. Differences in fibre geometry are seen between the various tree species and between specific locations within each tree. Large variations in the aspect ratios of softwood fibres within small volume elements of a stem (as indicated in Figure 2.2.2) can be explained by their ‘multifunctionality’, e.g. maintenance of water transport and mechanical stability. The softwood fibres of the stem show only comparatively little variation in their length but large variability in their cross-sectional dimensions across a growth ring (as indicated in the SEM micrographs of softwood and softwood tracheids in Chapter 14). In the early spring, fibres with large cross-sections and thin cell walls are formed to ensure efficient water transport. In summer, fibres with small cross-sections and thick walls are formed that contribute mainly to the mechanical stability of the tree. As hardwood fibres serve predominantly for mechanical stability, their aspect ratios are less variable, with lengths of up to 2 mm and diameters of ∼20 µm. The described variations in size and shape of wood fibres appear rather low when surveying bast fibres (Figure 2.2.2). The lengths of the mostly irregular-shaped bast fibres vary extraordinarily between species,

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Figure 2.2.2 Fibre diameters and lengths. Symbols (black triangles – wood fibres; white triangle – mesocarp fibres; black circles – seed hairs; white circles – fibres of monocots; white rectangles – bast fibres) represent estimated mean values, and the dotted lines show ranges of literature data. The linear curve represents an aspect ratio of 100. Data were taken from Ageeva et al. (2005), Aldaba (1927), Angelini et al. (2000), Ashori et al. (2006), Baley (2002), Jarman and Laws (1965), Khalil et al. (2008), Kozlowski et al. (2005), Kundu (1956), Leupin (2001), McDougall et al. (1993), Morvan et al. (2003), Mukherjee and Satyanarayana (1986) and Ruys et al. (2002).

and also variations within species are extremely high. Whereas kenaf (Hibiscus cannabinus L.) or jute fibres (Corchorus olitorius or Corchorus capsularis) are rather short (∼2–3 mm), ramie fibres can reach lengths of up to 550 mm (Albada, 1927). Bast fibre aspect ratios of ∼100 are as conceivable as aspect ratios of >6000. One possible explanation for the large variations in fibre geometries within a plant might be that both primary and secondary phloem fibres are pooled. For instance, in jute the outer primary phloem fibres are ∼3.2 mm long, whereas the inner secondary phloem fibres are only ∼1.5 mm long (Kundu, 1956). The differences in length between primary and secondary bast fibres are even more pronounced in hemp, where primary fibres reach lengths of up to ∼25 mm and secondary fibres reach lengths of up to ∼2 mm (Cronier et al., 2005). In monocotyledonous plants, fibres extracted from leaves and fibres extracted from stems can be distinguished. According to the data shown in Figure 2.2.2, the fibres extracted from the stem have an aspect ratio smaller than 100, e.g. oil palm trunk fibres (Elaeis guineensis Jacq.) grow to a maximum length of ∼0.66 mm, and their diameters do not exceed ∼16.6 µm (Khalil et al., 2008). In contrast, it appears that the aspect ratio of the technically more widely used fibres of leaves tends to be higher than 100, although more data are needed to confirm this. Fibrous cells of monocotyledonous plants can stay alive and increase cell wall thickness throughout the lifetime of the plant. In contrast to the previously described fibre types, seed hairs are not directly derived from meristematic tissue. They are outgrowths of single epidermal cells of the parenchymatous seed coat. For cotton, the fibre cross-sections appear kidney shaped, with a final fibre diameter of 20–25 µm, which is reached soon after growth initiation, whereas it takes 15–20 days to reach the final length of up to 6–7 mm (Anderson and Kerr, 1938). Afterwards, cell wall thickening takes place. After cell senescence, the cotton seed hairs dry out (Maxwell et al., 2003; Rollins and Tripp, 1954). The very high aspect ratio of the cotton seed hairs (Figure 2.2.2) makes them suitable for spinning technologies. Coir, fibre bundles of the mesocarp of coconut, is composed of 30–300 single fibres with lengths of ∼1 mm and diameters of ∼15 µm (Jarman and Laws, 1965).

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The size and shape of the above-described fibres are visualised by scanning electron micrographs shown in Chapter 14.

2.2.2.3

Cell Wall Structure

As with other plant cells, fibres are glued together by a pectin and/or lignin-rich middle lamella to form specialised tissues that fulfil specific requirements of the organism. Compared with animal cells, plant cells have a cell wall, which accounts for mechanical stability. Stiff cellulose fibrils, the major load-bearing components in plant cell walls, are embedded in matrices of complex macromolecules, e.g. hemicelluloses, pectin and/or lignin (see also Chapter 2.1). Cellulose itself is a linear polymer composed of β-d-glucose units, linked together by β-1,4-glycosidic bonds to generate a linear polymer chain. The degree of polymerisation of cellulose varies according to the plant species (7000– 15 000). The cellulose chains agglomerate to ∼3–5 nm (Cosgrove, 2005; Fahlen and Salm´en, 2005) thick microfibrils with crystalline and non-crystalline regions. Hemicelluloses are assumed to be the mediators between cellulose and lignin, as they can bind to cellulose via hydrogen bonds and even covalently to lignin (Fengel and Wegener, 1989). Chemically, the hemicelluloses are heteropolymers, built up of neutral sugars, e.g. glucose, mannose, galactose, xylose and arabinose. Some polymers also contain uronic acids. The degree of polymerisation of the side-branched chains is much lower than that of cellulose. Lignin, a macromolecule of phenylpropane units, is incorporated in the cell wall after cellulose and hemicelluloses have built up the basic cell wall assembly. Pectins, which are mainly found in the middle lamella and in primary cell walls, are a group of heterogeneous, strongly hydrophilic polysaccharides. The backbones of the side-branched molecules can either be linear (e.g. homogalacturonan) or consist of repetitive building blocks (e.g. rhamnogalacturonan is composed of repeating disaccharides). Depending on its developmental stage, the plant cell wall is composed of one or several layers with different thicknesses and chemical organisation. The protoplasm of the growing cell is usually enclosed by a 0.1–1 µm thin primary cell wall. The special organisation of primary cell wall components (cellulose, hemicelluloses, pectins and structural proteins) and the ongoing modifications of the macromolecular bonding patterns during growth (Cosgrove, 2005; Schopfer, 2001) allow for cell wall expansion without rupture. After the fibre has reached its final size and shape, the cell starts to synthesise a comparatively thick secondary cell wall. The predominance of the secondary cell wall certainly explains its important role in adapting the (mechanical) cell wall properties. Within the secondary cell wall, the stiff cellulose fibrils are winding in an S- or Z-helix parallel to each other around the cell. The angle of the parallel cellulose microfibrils to the longitudinal cell axis is called the cellulose microfibril angle (MFA). The thickness of the cell wall layers and their cellulose fibril orientations play a dominant role for the mechanical properties of plant fibres (see Section 2.2.3). Figure 2.2.3 illustrates the cell wall organisation of different fibre types by schematic drawings, in particular the arrangement of cell wall layers and the orientation of the cellulose microfibril angles within the layers. The cell wall of a wood fibre is composed of a primary cell wall and a secondary cell wall which consists of three layers, S1, S2 and S3, with a dominating S2 layer (Figure 2.2.3(A)). Depending on the wood fibre type, the thickness of the S2 layer may vary considerably. Literature values given for the MFA normally describe the angle of the microfibrils within the S2 layer. Adult wood fibres of the stem usually possess a microfibril angle of between ∼0 and 20◦ . An extensive review of MFAs in wood is given by Barnett and Bonham (2004). A speculative model of the cell wall structure of a bast fibre is shown in Figure 2.2.3(B). Micrographs of bast fibre cross-sections indicate that the secondary cell wall is composed of multiple layers (Blake et al., 2008; Romhany et al., 2003). MFAs reported in the literature are below 10◦ for most of the bast fibre types, indicating that their orientation might be similar within all secondary cell wall layers. Compared with wood fibres, bast fibres stay alive for a longer period, and cell wall thicknesses often increase until the lumen has almost disappeared.

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Figure 2.2.3 Schematics of possible cell wall organisation in (A) wood fibres, (B) bast fibres, (C) monocotyledonous plant fibres (bamboo, after Parameswaran and Liese, 1976) and (D) seed fibres. Black lines indicate orientation of cellulose microfibrils.

Fibres of some of the monocotyledonous plants, e.g. palms or bamboo, stay alive during the whole lifetime of the plant (Tomlinson, 2006). Consecutive secondary cell wall layers with different microfibril angles (Z-helix) are deposited while ageing, resulting in a multilamellar structure of the cell wall (Fig. 2.2.3(C)). This allows for continuous adaptation at the cell level throughout the lifetime of the plant. For rattans (climbing palms) and bamboo it has been shown that thick and thin layers alternate (Bhat et al., 1990; Liese, 1987; Parameswaran and Liese, 1976). The schematic drawing in Fig. 2.2.3(D) shows a model of the cell wall of cotton seed hairs. The observed multilamellar structure of the secondary cell wall is a result of diurnal growth (Anderson and Kerr, 1938; Balls and Hancock, 1922). Within one layer, the parallel orientation of the cellulose fibrils is changing frequently, and both S- and Z-helical arrangements of the cellulose microfibrils have been found. Anderson and Kerr (1938) described two patterns of so-called reversals: (i) the orientation of cellulose fibrils changes direction by following an arc; (ii) at a position where a set of spiral cellulose strands ends, a new set of spiral strands starts growing in the opposite direction. The cross-sections of fibres often show a laminate structure, which is caused by changing microfibril angles and density variations. According to Jarman and Laws (1965), the secondary cell wall of coir fibres is composed of a thick S1 layer, in which the cellulose microfibrils are oriented in an S-helix at rather high microfibril angles. In the S2 layer, with a similar cell wall thickness, microfibrils are arranged in a Z-helix. The inner S3 layer is very thin and has not been studied in detail yet.

2.2.3 2.2.3.1

Structure–Function (Property) Relationships of Plant Fibres Cellulose Microfibril Angle and Tensile Behaviour of Fibres

The mechanical properties of individual plant fibres depend decisively on the polymer organisation and the molecular interactions in their cell walls (Fratzl et al., 2004a; Burgert, 2006). As indicated in the schematic drawings in Figure 2.2.4, the most crucial parameters are the cell wall/lumen ratio (density of the fibre) and the cellulose orientations in the dominant cell wall layers. Obviously, a fibre with higher density is stiffer and stronger compared with one with a lower density when mechanical properties are related to the overall fibre cross-section (Figure 2.2.4(A)). The interdependency

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Figure 2.2.4 Schematic stress–strain curves of (A) low- and high-density fibres with constant MFA and (B) fibres with different microfibril angles.

between cellulose microfibril angle and axial tensile properties is more complex and has been investigated experimentally in various single-fibre studies (Page and El-Hosseiny, 1983; Burgert et al., 2002; Groom et al., 2002a and 2002b). The schematic curves in Figure 2.2.4(B) show that the modulus of elasticity is highly dependent on the orientation of the parallel cellulose fibrils. A small microfibril angle (cellulose fibrils are oriented almost parallel to the axial direction) leads to a high modulus of elasticity, whereas the stiffness is considerably reduced for higher microfibril angles. The shape of the schematic stress–strain curves for single fibres with small microfibril angles shows a very stiff and almost fully elastic response with a brittle fracture. For large cellulose microfibril angles the interaction of the cellulose fibrils with the matrix macromolecules becomes more crucial for the overall mechanical behaviour of the cell wall. Typically, the stress–strain curves of tissues and fibres with high microfibril angles show a biphasic or triphasic behaviour (Page et al., 1971; Bodig and Jayne, 1993; Navi et al., 1995; K¨ohler and Spatz, 2002; Keckes et al., 2003; Martinschitz et al., 2008) with a short initial elastic phase and a large plastic deformation after yield, which results in a high toughness. At the yield point a critical shear stress in the matrix is exceeded, which results in a viscous flow of the matrix and a gliding of the cellulose fibrils (Spatz et al., 1999; Keckes et al., 2003; Altaner and Jarvis, 2008). Concerning the experimental determination of mechanical properties of biological materials, it is well known that testing conditions, e.g. test speed, influence the response to load. For further details, see Chapter 13.

2.2.3.2

Mechanical properties of different plant fibres

Table 2.2.1 provides an overview of the tensile properties and the cellulose fibril orientations of the previously introduced and various technically utilised fibre types. The mechanical data of the different fibre types from different sources clearly show why there are crucial concerns about the large variation in mechanical properties of plant fibres when it comes to technical utilisation. The mechanical properties show large variability both within and between fibre types. This variation can be partly explained by natural variation, but it is also due to technical factors.

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Table 2.2.1 Literature data of mechanical properties (tensile stiffness and ultimate tensile stress) and measured microfibril angles of different plant fibres. The * indicates that data from these references were taken from tables of collected data coming originally from other sources

Fibre type

Isolation procedure/fibre type

Wood fibres Softwood, stem, juvenile and adult wood (Pinus taeda)

Chemical/ single fibre

Softwood, stem, adult wood (Picea abies) Softwood, branch (Picea abies, Juniperus virginiana, Taxus baccata, Ginko biloba) Hardwood, stem, adult wood (Populus sp.) Bast fibres Flax (Linum usitatissimum)

Hemp (Cannabis sativa)

Mechanical/ single fibre Mechanical/ single fibre

Tensile stiffness in GPa

Ultimate stress in MPa

5.1–27.5

410–1422

7.4–13.6

16.1–26

386–930

30–45

0.5-1

MFA in deg

30.7 ± 4.6

Mechanical/ single fibre Mechanical/ single fibre Single fibre Retted, single fibre Unknown

10

Jute (Corchorus capsularis and C. olitorius)

Ramie (Bohemeria nivea)

Kenaf (Hibiscus cannabinus)

Spanish broom (Spartium junceum) Nettle (Urtica dioica)

866 ± 246

Eder, 2007

1834 ± 900

Bos et al., 2002 Baley, 2002 Davies and Bruce, 1998 Tables* in Bledzki and Gassan, 1999; Gassan et al., 2001 Thygesen et al., 2007

54.1 ± 15.1 51.7 ± 18.2

1339 ± 486 621 ± 295

6–10

27.6

345–1035

Mechanical, single fibres Fibre bundles

24.9 ± 10.6

1735 ± 723

37.5 ± 3.4

594 ± 106

Fibre bundles

17.2 ± 9.2

315.6 ± 177.8 690

Unknown

6.2

30–60

310–750

Unknown

7–9

2.5–13

533

Unknown

8

26.5

393–773

8; 7.5

65 61.4–128

938 950 400–938

Single fibre Fibre bundles Unknown

Fibre bundles

24.6 ± 11.7

Fibre bundles Fibre bundles

20

418.1 ± 195.3 215 700

25.5 ± 6.8

368 ± 195

Retted, single fibres

Groom et al., 2002a; Groom et al., 2002b; Mott et al., 2002 Eder et al., 2009 Burgert et al., 2004

10–11

6

Sources

Mwaikambo and Ansell, 2006b Schledjewski et al., 2006 Tables* in Bledzki and Gassan, 1999; Gassan et al., 2001 Tables* in Mwaikambo and Ansell, 2006b Table* in Bisanda and Ansell, 1992 Table* in Bledzki and Gassan, 1999 Li et al., 2008 Angelini et al., 2000 Tables* in Bledzki and Gassan, 1999; Gassan et al., 2001 Schledjewski et al., 2006 Edeerozey et al., 2007 Angelini et al., 2000 Davies and Bruce, 1998

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(Continued) Isolation procedure/fibre type

Fibre type

Tensile stiffness in GPa

Ultimate stress in MPa

26–35

452–581

19.7

13–28

450–820

18–22

5.3–29.1

463–840

Unknown

10–25

9.4–22

511–635

Fibre bundles Unknown

11–12 11

30 7.7–20.8

800 641

Fibre bundles Fibre bundles

5–8 6.2

81–315

Fibre bundles

16–36

362–747

34–82

1020

5.5–12.6

287–597

MFA in deg

Fibres from monocotyledonous plants (leaves and stems) Sisal (Agave sisalana) Fibre bundles Fibre bundles

Banana (Musa sepientum)

Caroa (Neoglaziovia variegata) Piassava (Attalea funifera) Pineapple PALF (Ananas comosus)

8–14

Unknown

14–80

Seed hairs Cotton (Gossypium sp.)

Sources

Chand and Hashmi, 1993 Mwaikambo and Ansell, 2006a Bisanda and Ansell, 1992 Tables* in Bledzki and Gassan, 1999; Gassan et al., 2001 Kulkarni et al., 1983 Table* in Bisanda and Ansell, 1992 d’Almeida et al., 2008 d’Almeida et al., 2006 Mukherjee and Satyanarayana, 1986 Tables* in Bledzki and Gassan, 1999; Gassan et al., 2001 Table* in Bisanda and Ansell, 1992 Table* in Bledzki and Gassan, 1999; Gassan et al., 2001

Kapok (Eriodendron anfractuosum, Bombax malarbaricum) Mesocarp fibres Coir (Cocos nucifera)

2.2.3.3

Fibre bundles Unknown

40–47 45

3.3–5 4–6

175

Unknown

30–49

4–6

153

Fibre bundles

43

160–250

Kulkarni et al., 1981 Tables* in Bledzki and Gassan, 1999; Gassan et al., 2001 Table* in Bisanda and Ansell, 1992 Martinschitz et al., 2008

Technical Aspects of Plant Fibre Variability

A limitation in comparing the mechanical properties of plant fibres from different sources is that different mechanical tests are applied. Even various tensile testing methods for single fibres exist (Page and El-Hosseiny, 1983; Groom et al., 2002a and 2002b; Burgert et al., 2003). However, the main problem is that the terminology is inconsistent. The term ‘fibre’ is misleadingly used for both a fibre bundle and a single fibre. Additionally, the interpretation of data is impeded when fibre bundles are measured instead of single fibres without clearly being described as such. The properties of fibre bundles can differ quite substantially from

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those of individual fibres. Important parameters are the span length, the fibre length, the bonding characteristics between the fibres and the orientation of the fibres within the bundles, but also the cellulose microfibril angle in the cell walls. In terms of the latter, this is particularly critical for fibres with intermediate or large microfibril angles in dominating S2 layers, because of the characteristic connection between adjacent fibres. Usually, all fibres of a fibre bundle possess the same rotational direction of cellulose fibrils in their cell walls (Meylan and Butterfield, 1978) (Figure 2.2.3). In consequence, two adjacent fibres have an opposite cellulose fibril orientation in the connected cell walls (Booker, 1996). If the interlocking of the fibres is tight, a fibre bundle with a high cellulose microfibril angle will appear stiffer than single fibres, as it superimposes the elastic responses of the pure cell wall. In terms of axial tensile tests on single fibres, these tend to rotate owing to the spiral orientation of the microfibrils (Mark and Gillis, 1970). It is also worth noting that the single fibre test cannot be regarded as a direct measure of cell wall properties, because fibre geometry and size influence the material response (Eder et al., 2008a).

2.2.3.4

Natural Variability of Plant Fibre Properties

Some of the natural variability of plant fibre properties comes from the adaptational growth of plants, with the consequence that the within-plant variation in fibre properties is as large as the between-plant variations in fibre properties. Plants utilise various concepts to adjust the mechanical performance of their fibres and thereby the macroscopic properties of their organs. For instance, trees change the orientation of cellulose fibrils in the cell walls to adjust mechanical properties (Lichtenegger et al., 1999), which can be followed during the ontogeny of the organism. Young trees form so-called juvenile wood fibres, which possess cell walls with a rather large MFA (Lindstr¨om et al., 1998; Bonham and Barnett, 2001). By making their wood flexible, young trees support the strategy of reducing the impact of wind loads by streamlining. In contrast, mature trees build wood fibres with rather small microfibril angles in their cell walls, which make the adult wood stiff and help to withstand wind loads. Recently it has been shown for a palm tree (Mexican fan palm, Washingtonia robusta) that palms might follow a different strategy of adjusting the mechanical performance in their fibres in the vascular fibre caps (R¨uggeberg et al., 2008). As the palm tree fibres remain alive in the plant body, they can continuously add new cell wall layers until the whole cell lumen is filled. Furthermore, the fibres can increase the degree of lignification of the cell wall. Hence, the mechanical performance of these fibres depends decisively on their developmental stage. An ongoing lignification might prove to increase not only the compressive strength and buckling resistance of the fibres but also their tensile stiffness. The measurements by R¨uggeberg et al. (2008) showed that one prominent type of vascular bundle in the investigated palm possesses a stiffness gradient across the fibre cap. In contrast to trees, this gradient was not achieved by changing the cellulose fibril orientation, because a constant but comparably high cellulose microfibril angle was found across the fibre cap. Instead, the degree of lignification is changed across the fibre cap, and therewith the shear stiffness and strength of the matrix, which has a pronounced influence on the axial mechanical response of cell walls with large microfibril angles (Fratzl et al., 2004b) (see Figure 2.2.4(B)). However, the natural variability of plant fibres is not only due to adaptational growth of the long-living plants but also a result of the specific cell wall organisation of fibres. The ranges of tensile stiffness and strength, as well as measured microfibril angles, as shown in Table 2.2.1, are fairly wide for all mentioned fibre types and plant species. As shown in the previous sections on shape and structure, plant fibres in general are individual units and show both large differences among themselves and along the single fibre. Therefore, it is not only the variation between different plant fibres of the same species but also the intrafibre cell wall variation that have to be considered. For instance, large variations in cellulose microfibril angles along the fibre have been recorded (Anagnost et al., 2002; Sedighi-Gilani et al., 2006). These can be related to structural

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defects, so-called dislocations (see Chapter 15), or to structurally inherent patterns, e.g. pits and pitfields. In the living plant the pits allow for water and nutrient transport between fibres, as well as parenchyma cells. As the features are structurally inherent in the plant fibre, they have been optimised by the plant. Pits are reinforced by the surrounding cellulose fibrils (Bergander and Salm´en, 2000), and it seems that bordered pits do not alter the elastic properties of the fibre (Eder et al., 2008a). Another reason for natural variability are fibrenodes, which are regions in the fibre cell wall where the cellulose microfibril orientation in a short segment differs substantially from the cellulose orientation of the surrounding segments of the fibre cell wall (Nyholm et al., 2001). Fibrenodes are reported for a multitude of plant fibres such as wood, flax or hemp and are inherent in the living plant body, but they can also be induced during fibre processing (Bos and Donald, 1999). Possible origins in the living plant might be internal stresses or the swaying of the stem in the wind (Koch et al., 1996). The influence of fibrenodes on the mechanical properties of individual plant fibres has been investigated in several studies (Davies and Bruce, 1998; Bos et al., 2002; Baily, 2004), but no strong correlation has been found between either the number or the volume fraction of fibrenodes and the stiffness or strength. This might be due to the realignment of distorted cellulose fibrils in single-fibre tensile studies, as shown by an in situ study on hemp fibres (Thygesen et al., 2007; see Chapter 15) and single-fibre tests of buckled spruce fibres (Eder et al., 2008b). However, in terms of processing, dislocations might expose the cell wall for chemical and/or enzymatic treatments, so that they act as initiation points for fibre fracture (Ander and Daniel, 2006; see Chapter 4).

2.2.4

Stress Generation in Wood Fibres

Another feature that could be potentially used in technical applications is the ability of plant fibres to generate prestresses during their differentiation process. Interestingly, both tensile and compressive stresses can be generated in wood fibres, just by changing some micro- and nanostructural features of the cell wall. With this final section we intend to stimulate approaches that utilise this specific capacity of plant fibres for composite design, referring also to recent own work on the mechanisms of stress generation. Plants have developed the ability to prestress their tissues in order to improve the mechanical performance of organs and to control and change the direction of growth. In particular, trees are highly dependent on this mechanism. Because the strength of wood under compression is only half that under tension, trees would be prone to fail under high wind forces. The compensation strategy is to prestress the outer parts of the trunk in tension and thereby protect the compressive side when the stem is bent under wind loads (Boyd, 1950a; Kubler, 1987; Mattheck and Kubler, 1996). The tissue stresses in trees are generated during the cell wall formation of each individual fibre while it is differentiating. In the course of the cell wall formation, lignification follows cellulose and hemicellulose agglomeration (Boerjan et al., 2003). This retarded insertion of the wall constituents is believed to play a crucial role in stress generation processes. However, controversy exists about its implications, e.g. a volume increase of the cell wall during lignification (Boyd, 1950b) or an influence of lignin insertion on cellulose crystallisation (Bamber, 1979) or a combination of both (Yamamoto, 1998).

2.2.4.1

Stress Generation in Softwood Fibres

Softwoods are able to generate either tensile or compressive stresses in their fibres. In a straight trunk, the fibres are prestressed in tension in the longitudinal direction during differentiation. However, when reaction wood, so called compression wood, is formed at the underside of leaning stems and branches in order to maintain branches in their horizontal position or to reorient leaning stems, compressive stresses are generated (Wardrop, 1965).

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Normal and compression wood fibres differ in various structural and chemical features. Normal wood fibres in the straight trunk have an almost rectangular cross-section, whereas compression wood fibres show a round shape in the cross-section. Compression wood fibres lack the S3 layer and have higher lignin contents as well as altered hemicellulose composition compared with normal wood fibre (Cˆote and Day, 1965; Timell, 1982). The microfibril angle varies between ∼0 and 20◦ in normal adult wood fibres, whereas it is typically above 30◦ in compression wood fibres. The basic principle of stress generation in softwoods has not yet been clarified. However, the structural features indicate that, similarly to other plant actuation systems, the microfibril angle in the secondary cell wall may play a major role (Burgert and Fratzl, 2009). Yamamoto (1998) modelled the deformation of softwood fibres as a function of cellulose orientation and showed that, below a microfibril angle of ∼30◦ , fibres tend to contract during differentiation and produce tensile stresses, whereas, above ∼30◦ , fibres tend to elongate longitudinally and produce compressive stresses. In a recent study, measurements of the elongation of normal and compression wood fibres were performed (Burgert et al., 2007). Similarly to the stress generation capacities of the tissues, wet compression wood fibres increased in longitudinal length after a swelling treatment with sodium iodide (NaI), whereas wet normal wood fibres shrank in the longitudinal direction. Interestingly, opposite responses of the fibres occurred, in spite of the fact that the cell wall was swelling in both cases, which indicates that the sense of deformation must be related to the different structural designs of the two fibre types. Therefore, it has been proposed that softwoods might be able to control the generation of either tensile or compressive stresses simply by slightly modulating cell geometry and cell wall ultrastructure by means of the orientation of the cellulose microfibrils.

2.2.4.2

Stress Generation in Hardwood Fibres

In contrast to softwoods, hardwoods generate high tensile stresses in their reaction wood, so-called tension wood, and pull leaning stems and branches upwards. The tension wood fibres generate very high longitudinal tensile stresses during differentiation (Okuyama et al., 1994). In many hardwood species they show an additional characteristic cell wall layer compared with regular fibres. This so-called G-layer can be regarded as the operative part of the tension wood fibre (Cˆote and Day, 1965; Clair et al., 2003), although many hardwood species are able to bend their organs without its formation (Fisher and Stevenson, 1981; Clair et al., 2006; Qiu et al., 2008). The G-layer can fill the whole lumen of the tension wood fibre and consists of almost pure cellulose, oriented parallel to the axial direction with a comparatively high crystallinity, and some xyloglucan, as well as traces of monolignols or syringyl units (Cˆote and Day, 1965; Norberg and Meier, 1966; Coutand et al., 2004; Joseleau et al., 2004; Gierlinger and Schwanninger, 2006; Nishikubo et al., 2007; Lehringer et al., 2008). Several hypotheses on the underlying stress generation mechanisms have been proposed. M¨unch (1938) was the first to mention that the contraction of the tension wood fibres might not be caused by the G-layer directly, but rather be the result of an interaction with the surrounding secondary cell wall. Yamamoto (2004) modelled the deformation of the tension wood fibre as a function of different microfibril angles in the cell wall layers. Recently, Nishikubo et al. (2007), as well as Mellerowicz et al. (2008), suggested a persisting activity of xyloglucan endotransglycosylase (XET). Goswami et al. (2008) proposed a mechanism that deals with the question as to how a tension wood fibre can be actuated to shrink axially by a G-layer consisting of axially oriented almost incontractible cellulose fibrils. The G-layer is very stiff in the axial direction of the cellulose microfibrils, but highly swellable in the lateral direction. As the microfibril angle in the surrounding secondary cell wall of a tension wood fibre is comparatively large (M¨uller et al., 2006; Goswami et al., 2008), this layer can be rather easily axially deformed. The mechanical model by Goswami et al., (2008) proposes that the radial swelling of the G-layer generates radial stresses that can be transformed into a longitudinal contraction of the surrounding S2 layer of

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the cell wall, resulting in a contraction of the entire fibre. Accordingly, it could be the bilamellar structure of the tension wood fibres with the specific structural and chemical features of the G-layer and the surrounding secondary cell wall that conjointly merge into a stiff but contractible composite structure.

2.2.5

Conclusion

A multitude of fibre types from different plant species and different positions in plant bodies can be technically utilised in natural fibre-based composites. Depending on their function in the living plant, fibres vary considerably in their structural features, mechanical properties and prestressed conditions. The aspect ratio, the cell diameter/cell wall thickness ratio and the orientation of cellulose microfibril angles in the secondary cell wall layers of the fibres are the main parameters to be considered in terms of technical application. One crucial concern about plant fibre utilisation in fibre composites and other technical applications is the large natural variability of the mechanical performance of plant fibres from the same species or even the same plant. On the one hand, these variations are due to adaptational growth of the plant, meaning that each fibre as an individual unit can be optimised in the plant to serve a specific function with adjusted micro- and nanostructure and mechanical properties. This can be related to the age of the plant, the position of fibres in a certain organ and changing environmental conditions. On the other hand, a certain natural variability emerges from specific structural features of the cell wall itself that inherently maintain the multifunctionality of the fibre (e.g. pits, etc.) or from cell wall damage (dislocations) already appearing in the living plant. Hence to homogenise the mechanical properties of a specific plant fibre type and thereby increase fibre quality and reliability, we need better to understand the underlying structure–function relationships in the plant. Moreover, systematic studies on the influence of growing conditions, age, processing, etc., on the mechanical performance of fibres are required to establish plant fibre sustainability in fibre-based composites and technical products like geotextiles or insulation products.

Acknowledgement We wish to thank John Dunlop for linguistic revision of the manuscript and for his critical comments.

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2.3 Types of Fibre ¨ J¨org Mussig Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany

Tanja Slootmaker Faserinstitut Bremen e.V. (Fibre), Bremen, Germany

2.3.1 Introduction This chapter gives an introduction to and a brief overview of natural fibres, which will be described in more detail in the course of the book. Some historical botanical drawings are included to illustrate the origin of the natural fibres. From this short description, the reader will get to know the different aspects and the scope of this book – from plant to final product.

2.3.2

Natural Fibres

In the terminology of Schnegelsberg (1999), a fibre is an entity that is elementary and linear, has a characteristic longitudinal and cross-sectional shape and consists of a primary chemical substance. The wool fibre, for example, is mainly composed of keratin – a fibrous structural protein – while the cotton fibre basically is made up of cellulose – a linear polysaccharide consisting of β-(1, 4)-linked d-glucose units. In plant structure, a fibre is the smallest intact, identifiable unit of a cell wall (Vincent, 2000). Vincent pointed out that fibres are rarely found as individual cells, but mostly assembled into bundles. In this context it is important to use an exact nomenclature and to differentiate, for example, between a fibre and a fibre bundle. M¨ussig and Martens (2003) advise that testing a fibre, a fibre bundle or a collective of bundles results in different mechanical values. More detailed information regarding this topic can be found in Chapter 13. Based on this knowledge, it is essential to distinguish between a fibre and a fibre bundle, for example, in the field of decortication and separation processes. In most cases within this book, the differentiation between fibre and fibre bundles is made clear. In some parts of the text the authors use the phrase ‘natural fibre’ in a general context. Some authors use the phrase ‘sisal’, ‘jute’ or ‘coir’ if they are talking about either fibres or fibre bundles from the plants Agave sisalana P., Corchorus olitorius L. (‘tossa jute’) and Corchorus capsularis (‘white jute’) Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
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or Cocos nucifera L., respectively, in a general context. If the differentiation between fibres and bundles is necessary, the term ‘fibre’ or ‘fibre bundle’ will be stated clearly. The variety of fibres in nature is enormous. Depending on their function within the plant, fibres may be located in different regions of the plant. As described in Chapter 2.2, the location of fibres in the stem depends on the species. Plant fibres can be found, for example, within stems of monocotyledonous and dicotyledonous plants and dicotyledonous and gymnosperm trees at different positions. As described in Chapter 2.2, natural fibres used for technical applications range from secondary xylem fibres such as wood fibres for paper production, phloem fibres such as hemp bast fibres (Cannabis sativa L.), extra-xylary fibres such as sisal (Agave sisalana P.) and seed hairs such as cotton (Gossypium sp.) to mesocarp fibres such as coir (Cocos nucifera L.). Figure 2.3.1 gives an overview of the broad range of organic and inorganic natural fibres. The scheme subdivides the organic fibres into plant and animal fibres.

Figure 2.3.1

Overview of natural fibres. Adapted from J. Mussig, 2001. ¨

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To give the reader a visual impression of the origins of the natural fibres that are part of this book, we have prepared collages of botanical drawings from historical books. On the one hand these entries symbolise the long history of natural fibres as a resource for human beings and on the other hand they show the importance of older publications (and not only publications from the last 5 years), which are often worth reading and studying for important information on future applications. In Figures 2.3.2 and 2.3.3, the origins of the natural fibres that are described in the book are shown.

2.3.3 2.3.3.1

Hairs and Threads Spider Silk

The spider Nephila plumipes is shown in Figure 2.3.2. As described in Chapter 11, such spiders produce gland fibres that originate as paired fibres (threads). In Chapter 14 (Figure 14.5), a scanning electron microscope (SEM) observation of spider threads of Nephila senegalensis is shown.

2.3.3.2

Mulberry Silk (Bombyx Silk)

Mulberry silk is produced by the silkworm of the domesticated silkmoth Bombyx mori (see Figure 2.3.2 and Chapter 11). SEM micrographs of mulberry silk can be found in Figure 14.6 in Chapter 14).

2.3.3.3

Tussah Silk

Tussah silk is produced by the feral tussah spinner Antheraea perny or Antheraea yamami (see Chapter 11). Characteristic length and cross-sectional views are shown in Figure 14.7 in Chapter 14.

2.3.3.4

Wool Fibres

Sheep (Ovis aries) produce keratin fibres (see Figure 2.3.2). Micrographs of wool top standards are shown in Figures 14.8 and 14.9 in Chapter 14. The structure, the mechanical properties and technical products of wool fibres are described in more detail in Chapter 12.

2.3.3.5

Cashmere

Cashmere is the hair of the cashmere goat (Capra hircus laniger). The differences in the amino acid composition of wool, cashmere and yak fibres are pointed out in Table 12.2 in Chapter 12. The morphological differences between guard and bottom hair are shown in Figure 14.10 in Chapter 14. 2.3.3.6

Yak

Yak (Bos mutus) is a bovine that lives in the Himalayan region. As discussed in Chapter 14 (Figure 14.11), the differentiation between cashmere and fine yak fibres is often complicated. Chapter 16 presents an analytical DNA-based procedure for the identification of yak admixture in a cashmere sample.

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Figure 2.3.2 Collage of pictures of origins for natural fibres – part I. Image reproduced with permission from Klassik Stiftung C 1995–2009 http://www.botanicus.org. Weimar and Missouri Botanical Garden


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C 1995–2009 Figure 2.3.3 Collage of pictures of origins for natural fibres – part II. Image courtesy Missouri Botanical Garden
http://www.botanicus.org.

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2.3.4 2.3.4.1

Plant Fibres Poplar Wood

The leaves of Populus alba are shown in Figure 2.3.2. The micrographs in Figure 14.12 (Chapter 14) show cross-sections of a poplar hardwood stem and macerated fibrous cells. The samples were made available by Michaela Eder from MPIKG, Potsdam, Germany. Details about structure and function of plant fibres, for example in hardwood, can be found in Chapter 2.2.

2.3.4.2

Spruce Wood

The cone and the branch of Norway spruce (Picea abies) is shown in Figure 2.3.2. The SEM micrographs in Figure 14.13 (Chapter 14) present spruce wood (softwood stem) structures and length views of macerated fibrous cells of early and late spruce wood. The samples were made available by Michaela Eder, MPIKG, Potsdam, Germany.

2.3.4.3

Flax

In Figure 2.3.3 the stem of Linum usitatissimum L. is shown. Details about processing of flax are given in Chapter 4. SEM micrographs of IFS flax standard ‘C’ and ‘J’ are presented in Figures 14.14 and 14.15 in Chapter 14. Chapter 18 deals with the use of the IFS flax standards as a reference material for testing.

2.3.4.4

Hemp

Figure 2.3.3 shows the botanical drawing of a hemp stem (Cannabis sativa L.). Chapter 5 gives detailed information about the cultivation, extraction and processing of hemp. The influence of mechanical and physicochemical separation can be seen in Figures 14.16 and 14.17 (Chapter 14).

2.3.4.5

Jute

The stems and fruits of ‘tossa jute’ (Corchorus olitorius L.) and ‘white jute’ (Corchorus capsularis L.) are shown in Figure 2.3.3. Chapter 6 deals with the cultivation, extraction and processing of jute. The morphological differences of ‘tossa jute’ and ‘white jute’ are pointed out in Figures 14.18 and 14.19 in Chapter 14.

2.3.4.6

Kenaf

Kenaf (Hibiscus cannabinus L.) is a plant in the Malvaceae family. The structure of the fibre bundles can be seen in Figures 14.20 and 14.21 in Chapter 14. Similarities of kenaf and jute are described in Chapter 6.

2.3.4.7

Cotton

Figure 2.3.2 displays a historical botanical drawing of cotton. Cotton ‘fibres’ are the seed hairs of the cotton plant Gossypium. Different leaf shapes in the cotton plant are presented in Figure 10.1, Chapter 10. In the

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same chapter the details of cotton production and processing are broadly discussed. As presented in Chapter 17, cotton standardisation is an example of a worldwide well-developed testing standardisation system. Micrographs of the USDA calibration cotton are shown in Figures 14.22 and 14.23 in Chapter 14.

2.3.4.8

Abac´a

A picture of Musa textilis N´ee is given in Figure 2.3.3. Coarse and fine grades of abac´a are shown in Figures 14.24 and 14.25 in Chapter 14. The cultivation, extraction, and processing of abac´a is depicted in Chapter 7.

2.3.4.9

Sisal

The historical drawing of the agave plant (Agave sisalana P.), which is a monocotyledon of the family Agavaceae, can be found in Figure 2.3.3. The cultivation, processing and also the production of sisal composites are itemised in Chapter 8. The influence of mechanical and chemical separation on the sisal fibre bundle structure is highlighted in Figures 14.26 and 14.27.

2.3.4.10

Coir

Coir fibre bundles are obtained from the outer layer (coconut husk) of the fruit of the coconut palm (Cocos nucifera L.). Such a palm can be seen in Figure 2.3.2. The coconut cultivation and the extraction and the processing of coir are specified in Chapter 9. Coir is an interesting example of grading of raw fibre bundles, semi-manufactured products and final products. Details can be found in Chapter 3.1. SEM micrographs of two different grades named ‘mattress’ and ‘bristle’ are presented in Figures 14.28 and 14.29 in Chapter 14.

2.3.5

Conclusion

We hope that the reader has found here an essential starting point for the following chapters. The intention of this book on fibres is to link the single stages from the plant to the final product. The important steps are: r from cultivation to harvesting; r from separation to processing; r from fibre testing to standardisation; r from processing to the final technical product.

References Bertuch, F.J. (1795) Bilderbuch f¨ur Kinder:..., Book 2, No. 26, Ovis aries, Figure 3 ‘Widder’ and Figure 4 ‘Schaaf’, Naturwissenschaftliche Sammlung der HAAB Weimar, Klassik Stiftung Weimar, Herzogin Anna Amalia Bibliothek, Germany, online resource http://ora-web.swkk.de/digimo online/digimo.entry?source= digimo.Digitalisat anzeigen&a id=692 Curtis, S. (publ.) (1828) Curtis’s Botanical Magazine, Illustration of Corchorus olitorius, Vol. 55, New Series 2, Tab. 2810, Image courtesy of Missouri Botanical Garden © 1995–2009, http://www.botanicus.org Dietrich, D. (1838) Forstflora oder Abbildung und Beschreibung der f¨ur den Forstmann wichtigen B¨aume und Str¨aucher, welche in Deutschland wild wachsen, so wie der ausl¨andischen, daselbst im Freien ausdauernden, 2nd

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edition, Populus alba: Book 1, Tab. 23, Pinus abies: Book 1, Tab. 121, Naturwissenschaftliche Sammlung der HAAB Weimar, Klassik Stiftung Weimar, Herzogin Anna Amalia Bibliothek, Germany, online resource http://oraweb.swkk.de/digimo online/digimo.entry?source=digimo.Digitalisat anzeigen&a id=211 Hoffmann, C. (1884) Botanischer Bilder-Atlas nach de Candolle’s Nat¨urlichem Pflanzensystem, with over 500 full-scale pictures of plants on 85 fine colour plates and explanatory text, Cocos nucifera, Plate 75, 431a/b, J. Hoffmann, Stuttgart, Germany, online resource, Kurt Stueber (2003) http://caliban.mpiz-koeln.mpg.de/˜stueber/hoffmann/index.html Meyers (1906) Großes Konversations-Lexikon. Band 6. Faserpflanzen I. und Faserpflanzen II. Bibliographisches Institut, Wien/Leipzig, Austria/Germany, online resource, Editura Gesellschaft f¨ur Verlagsdienstleistungen mbH (2009), http://www.zeno.org M¨ussig, J. (2001) Untersuchung der Eignung heimischer Pflanzenfasern f¨ur die Herstellung von naturfaserverst¨arkten Duroplasten – vom Anbau zum Verbundwerkstoff. VDI Verlag GmbH, D¨usseldorf, Germany. M¨ussig, J. and Martens, R. (2003) Quality aspects in hemp fibre production – influence of cultivation, harvesting and retting. J. Ind. Hemp, 8(1), 11–31. Oken, L. (1843) Oken’s allgemeiner Naturgeschichte f¨ur alle St¨ande, Zoologie, Bombyx mori, Tafel 37, Stuttgart, Germany, Hoffmann’sche Verlags-Buchhandlung, Naturwissenschaftliche Sammlung der HAAB Weimar, Klassik Stiftung Weimar, Herzogin Anna Amalia Bibliothek, Germany, online resource http://oraweb.swkk.de/digimo online/digimo.entry?source=digimo.Digitalisat anzeigen&a id=227 Pabst, G. (ed.) (1887) K¨ohler’s Medizinal-Pflanzen in naturgetreuen Abbildungen mit kurz erl¨auterndem Texte, Band I, Cannabis sativa L., Tafel 13, Orginalzeichnungen von Walther M¨uller in Gera, Gera-Untermhaus, Germany, Verlag von Fr. Eugen K¨ohler, Image courtesy of Missouri Botanical Garden © 1995–2009, http://www.botanicus.org Schnegelsberg, G. (1999) Handbuch der Faser – Theorie und Systematik der Faser. Deutscher Fachverlag, Frankfurt am Main, Germany. Vincent, J.F.V. (2000) A unified nomenclature for plant fibres for industrial use. Appl. Compos. Mater., 7, 269–271. Wettstein, R. (1924) Handbuch der Systematischen Botanik, online resource http://edocs.ub.uni-frankfurt.de/ volltexte/2006/50229/, p. 710. Wilder, B.G. (1865) On the Nephila Plumipes: or Silk Spider of South Carolina, Late Surgeon 55th Mass. Vols, from the Proceedings of the Boston Society of Natural History, 4 October 1865, p. 11.

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3 Economic Aspects

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3.1 Grades and Standards Axel Drieling Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany

¨ J¨org Mussig Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany

3.1.1 Introduction There are many different grading systems specifically introduced for different kinds of fibre. These systems show some differences, and at the same time many similarities. When the similarities and differences of the grading systems are understood, then the existing systems given for some fibres can be used to develop or improve grading systems for other fibres. With this in mind, this chapter will start with a systematic explanation of the differentiations, and will then show existing grading examples.

3.1.2

Background

The quality of the produced fibres or fibre bundles depends on the given boundary conditions. For example, vegetable fibres are highly responsive to growing conditions in terms of the climate (temperature and water) or input applications. The cultivation of fibres in different countries implies different varieties in use and different conditions for growing and harvesting. In order to fix their economic value, fibres and fibre bundles have to be inspected visually and/or tested with suitable instruments. For trading with the fibres and fibre bundles, the classification systems and grades or the testing methods and their calibration have to be fixed and agreed between the trade partners. For many natural fibres, specific classification schemes exist in parallel, e.g. based on different countries, based on different production and processing, based on different applications or based on the current status of processing. Fibre classification or testing directly leads to a corresponding pricing structure.

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
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Figure 3.1.1 (a) Chequered tabulation of the grading criteria with the corresponding ratings; (b) fibres with unreserved use, equivalent to orientation quality and the basic price; (c) fibres labelled as reject – the effort for classifying is reduced to a minimum; (d) example of a traded sample with different properties. Adapted with permission from G. Schnegelsberg, Handbuch der Faser – Theorie und Systematik der Faser, Deutscher Fachverlag GmbH, 1999.

Comparison of the usual quality rating systems for trading units of natural fibres shows differences not only between different kinds of fibre but also between varieties, provenances, areas and countries. The trading unit is understood to be, for example, kg/bale, kg/container and amount/kg (ropes, cocoons). According to Schnegelsberg (1999), a standardisation across all kinds of fibre or fibre bundle for a quality rating system for trading units will be sought. Trading units are subdivided into classes; this division calls for classification (cotton or wool) or a grading. A class (grade of quality) consists of a valued grade sorted by equal, selected characteristics. A scheme of how a grading system can work is shown in Figure 3.1.1. According to Schnegelsberg (1999), such a system should be based on six grading criteria with five corresponding ratings. The grading ranges can be described as follows: r r r r r r

L: length; C: colour; B: quality restrictions like spots or mildew contamination; D: degree of cleanness, for example impurities, grease, humidity; P: packaging of a trading good, for example oriented or scattered; F: fineness. The ratings can be described as follows:

r r r r r

1: very good; selection; this quality will get a price surcharge compared with rating 2. 2: good; orientation quality; this quality defines the basic price. 3: ordinary; mean; this quality will get a markdown compared with rating 2. 4: base; less valuable; this quality will get a markdown compared with rating 3. 5: bad; reject; this quality will get a markdown compared with rating 4.

The last rating (5) comprises unsorted fibre types. Figure 3.1.1 presents some examples of an evaluation scheme. Example (d) in Figure 3.1.1 stands for a fibre lot with the following grading criteria and the corresponding ratings: selected fibre length; predominantly light-coloured with a few brown-coloured fibres; the brown-coloured fibres will slightly lower the quality because of spottiness; the degree of cleanness is mean to less valuable; the fineness is ordinary. These grading criteria result in the following price scenario: a price surcharge for length (L); the markdown for C5 will be reduced because of the good colour C2; while the markdown for F3 will be small, the degree of cleanness (D3 + 4) will lead to a higher markdown. The chequered tabulation of the grading criteria with the corresponding ratings allows an easy and practical evaluation system for quality rating and price definition.

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Organoleptic inspection versus instrument testing; the use of natural fibres as grades or as reference material.

3.1.3 Important Distinctions regarding Classification The term ‘classification’ refers to the application of official standards and standardised procedures developed for measuring those physical attributes of the fibres that affect the quality of the finished product and/or manufacturing efficiency (USDA, 2001). A structured description of the different aspects of classification is given in Figure 3.1.2. (a) The kind of classification. Principally, classification can be divided into organoleptic inspection by classers (often referred to as manual classification, manual classing or manual grading) and into instrument testing in laboratories (often referred to as testing, but sometimes mentioned as instrument classification or instrument classing). (b) Reference. Each classification or testing method has to be based on given references. For manual classification, the typical reference is given with grades. For instrument testing, the instrument can either be calibrated on the basis of a physical base unit calibration (e.g. force, length, time), or, which is more usual, calibrated on the basis of a reference material, mostly consisting of prepared, homogeneous fibre samples. For all reference material in use, the traceability to the nationally or internationally given reference has to be assured. (c) Types of variable. Variables are divided into qualitative (descriptive) and quantitative variables. Usually, quantitative variables are preferable. Quantitative variables are subdivided into continuous and discrete variables. The word ‘grading’ typically refers to descriptive or to discrete variables, but most instrumentally tested parameters are continuous. A typical example of manual classification is the colour grade of cotton, fixed in the Universal Cotton Standards Agreement. Classers compare cotton samples with given colour grades and fix the best-fitting grade (e.g. ‘strict middling, light spotted’, see Section 3.1.4.1). Based on the given structure, this system is an organoleptic inspection, and the reference is the set of official grade standards. The parameter is descriptive/ discrete.

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The diameter testing of wool is a suitable example of instrument testing. Wool tops are measured for diameter with one of the four suitable diameter testing methods (projection microscope, airflow method, OFDA, or laserscan). The diameter parameter is continuous. The projection microscope is calibrated with length scales for microscopes, which are traceable to the length SI unit; no natural fibre material is used for this calibration. The other three methods are calibrated with diameter reference material provided by INTERWOOLLABS (West Yorkshire, UK). In the case of the airflow method, a calibration curve is calculated for each instrument on the basis of the measurement of eight standard tops for wool diameter with a regression curve between the nominal values and the measured flow rates of the air. The development of classification is today changing from manual classification to instrument testing, with partly parallel manual and instrument classification for the same properties. In the case of cotton, in many countries the colour is classed manually with grades, but it is tested for reflectance and yellowness using high-volume testing devices (see Chapter 17). Manual classing and instrument testing can complement one another, e.g. for cotton classification in the USA, with: r instrument testing for length, length uniformity, strength, fineness/micronaire and colour; r manual classification for preparation and extraneous matter; r parallel instrument and manual classification for trash/leaf grade.

3.1.4 Classification Systems for Different Kinds of Fibre The classification systems for a choice of fibres are described in this chapter. From Figure 3.1.3 it can be seen that some fibres or fibre bundles are used as grades for classing and some as reference materials for testing. Cotton and flax, for example, are used for both. More details will be given in the following sections for the natural fibres mentioned in Figure 3.1.3. We will focus on whether the fibre or fibre bundles will be used as reference material or as grades for trading. We could not prepare a complete elaboration for all natural fibres, but we have selected the given examples to show the differences and specialities in the grading and standardisation world of natural fibres.

Figure 3.1.3

Grades and reference materials used for testing or classing.

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Cotton

Cotton classification is divided into the classification of the seed cotton and the classification of the ginned cotton lint. Seed cotton classification is mainly suitable for estimating the quality of the cotton delivered by the farmers, and to allow payments based on quality. Usually, seed cotton grades give only basic distinctions, as in the case of Burkina Faso: Grade

Description

First Second Third

Clean and sorted cotton Unsorted cotton Low quality

Seed cotton classification systems are specific to every country. In ICAC (1998), 19 different seed cotton grading systems are described. Most of the cotton-producing countries are using a seed cotton classification system. For trading purposes, only cotton lint is considered. The most important step for classification was the development of the Universal Cotton Standards. For this, the United States Cotton Standards Act was passed in 1923 for the primary purpose of establishing and promoting the use of the official cotton standards of the United States in interstate and foreign trade. Under the provision of the United States Cotton Standards Act, the Universal Cotton Standards Agreement was put into effect in 1923 between the United States Department of Agriculture (USDA) and overseas cotton associations for the trading of US cotton. It has been signed by 24 cotton associations in 21 different countries (ICAC, 1998). The official colour grades of US upland cotton are given in Table 3.1.1. Independently of the colour grade, the leaf grade is defined between 1 and 8. Based on this development, the Universal Upland Grade Standards, distributed by the USDA, are the most recognised and widely used grade standards. → This system is directly followed in some countries like Israel. → This system is used, although some variations may exist, particularly with respect to separation of colour and leaf grade, in Australia and Colombia, for example. → This system is principally used, but under a different name, in additional countries. → However, most countries use their own system, and efforts have been made to compare local standards with the US Universal Cotton Standards. The USDA cotton classing system in total consists of leaf grade and extraneous matter based on manual classification, and high-volume testing for fibre length and length uniformity, micronaire, strength, colour Table 3.1.1 Official colour grades of US upland cotton. Adapted with permission from International Trade Center UNCTAD/WTO: Cotton Exporter’s Guide, ITC, Geneva, 2007

Good middling Strict middling Middling Strict low middling Low middling Strict good ordinary Good ordinary Below grade

White

Light spotted

Spotted

Tinged

Yellow stained

11 21 31 41 51 61 71 81

12 22 32 42 52 62 — 82

12 23 33 43 53 63 — 83

— 24 34 44 54 — — 84

— 25 35 — — — — 85

Physical standards in bold, descriptive standards in italics.

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(reflectance and yellowness) and trash. Manual classing and instrument testing are done on every sample. Transition to instrument testing for all properties is aspired to and will be done ‘as quickly as the technology can be developed and instruments are sufficiently refined’ (USDA, 2001). Other countries are partly following the change towards instrument testing, but with different degrees of realisation. 3.1.4.2

Flax

The traditional flax industry of Europe has not actively promoted the development of objective standards and continues to rely upon organoleptic ways of characterisation. Various classification schemes exist within an industry segment and include criteria such as the source (e.g. Belgium, France, Russia or China), the processing history (e.g. water or dew retted) or the application (e.g. warp or weft yarn). Within particular countries (e.g. Czech Republic, Germany, Poland or Russia), measurement of flax fibres is done by more or less consistent means, and therefore a limited classification system exists. More information about this topic is given in Chapter 18. A suitable example for instrument testing of flax is given with the measurement of fineness. Fineness is usually considered as the most important quality characteristic of flax. The fineness of flax can be measured by different methods, the most common one, and the only internationally standardised one before the year 2000, being the permeametric method defined in the ISO 2370 Standard Test Method: ‘Textiles – Determination of fineness of flax fibres – Permeametric methods’ from 1980. ISO 2370 includes two different methods, a reference method considering different densities, and a simplified method neglecting possible variations in fibre density. As permeametric methods for fineness do not allow a physical base unit calibration, the method mentions a range of 10 reference flax batches with fixed characteristics for calibration. Unfortunately, these reference batches cover fineness results up to a maximum 80 IFS (see Chapter 18, Table 18.2), which is not sufficiently coarse to cover the whole range of technically used bast fibre bundles. Additional efforts towards the development of international standard test methods for flax are described in Chapter 18. 3.1.4.3

Jute

The stembast called jute is gained mainly from the tiliacea plants Corchorus capsularis L. and Corchorus olitorius L. (Schnegelsberg, 1999). These are the only two species that are cultivated for commercial purposes. Of these, the fibres of Corchorus capsularis L. are referred to as ‘white jute’ and those of Corchorus olitorius L. as ‘tossa jute’ (Pan et al., 2000). As Pan et al. mention, various qualities are used in trade that differ particularly in the properties of colour, fineness, strength, density, root proportion and tendering. Both types are sorted into a total of eight categories in India: ‘tossa jute’ (TD1 to TD8) and ‘white jute’ (W1 to W8). According to Rowell and Stout (1998), the classification of fibres still takes place using organoleptic methods. For this reason, an international comparison is difficult. In Bangladesh, for instance, ‘white jute’ and ‘tossa jute’ are divided into six ‘export’ classes (Special, A, B, C, D and E; see Table 6.5, Chapter 6). ‘Bangla white special’ and ‘Bangla tossa special’ are the highest grades of Bangladesh jute. They are of the finest texture, very strong and with high lustre. They are free of defects, well hackled and clean cut. The lower grades of jute are weaker and coarser and with bark and specks (Jarman, 1998). As described in more detail in Chapter 6, grading in Bangladesh is done for: r home trade (kutcha grade: see Chapter 6, Table 6.4); r export trade (pucca grade: see Chapter 6, Table 6.5). In India there is only one grading system (Chapter 6, Table 6.6). The grading and classification of jute are still carried out subjectively by hand and eye. The Bangladesh Standards and Testing Institution (BSTI,

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Dhaka, Bangladesh) and the Bureau of Indian standards (BIS, New Delhi, India) are government authorities for developing standards. Jute is mainly cultivated in India, Bangladesh, China, Nepal, Thailand, Indonesia and a few other SouthEast-Asian countries (Pan et al., 2000). Detailed information about the jute grading system can be found on the website of the International Jute Study Group (IJSG, Dhaka, Bangladesh) (IJSG, 2003). The existing jute grading systems and the names of the different grades, as practised in the major producing countries (Bangladesh, India, China, Indonesia and Nepal), are accurately described. 3.1.4.4

Coir

According to Schnegelsberg (1999), coir fibre bundles can be divided into: r ‘white coir’ (fibre bundles extracted from retted green husks); r ‘brown coir’ (fibre bundles extracted from dry husks of ripe coconuts after soaking the husks in water). As described in Chapter 9, in Sri Lanka there are essentially four main categories of coir grades: r r r r

‘bristle’ (type of long, parallel, clean fibre bundles: >135 mm); ‘omat’ (type of medium-length fibre bundles: 70–135 mm); ‘mattress’ (short fibre bundle fraction: between 30 and 69 mm); ‘mixed’ (fibre bundles extracted from matured green husk or brown husks, with average lengths between 36 and 119 mm).

Coir is either sold as raw fibre bundles or processed into products such as brooms, brushes, twine, matting, geotextiles, rubberised coir mattresses or upholstery (WCIC, 2009; Chapter 9). The Coir Council International (CCI, Etul Kotte, Sri Lanka) has been the umbrella association of the coir industry since 2003 in Sri Lanka. One aim is to improve the competitiveness of the coir industry by developing coir fibre standards (New Agriculturist, 2009). The coir classification in Sri Lanka is given in detail in Chapter 9. Jose (2009) gives a broad overview about the standardisation activity of coir and coir products in India. The grade specifications for ‘white coir’ or retted coir fibre bundles (IS 898, 1985) can be summarised as follows. 3.1.4.4.1

‘White Coir’ or Retted Coir Fibre Bundles

According to Jose (2009), coir shall be graded in accordance with colour and maximum permissible impurities, as shown in Table 3.1.2. ‘White coir’ can be classed into the following grades according to length (Jose, 2009): r r r r

‘long’: over 15 cm; ‘medium’: over 10 cm and up to 15 cm; ‘short’: over 5 cm and up to 10 cm; ‘bit’: up to and including 5 cm.

Where no agreement between buyer and supplier exists, the proportion by mass of ‘long’, ‘medium’, ‘short’ and ‘bit’ fibre bundles shall be higher than 50% ‘long’ and no more than 5% ‘bit’, the rest consisting of ‘medium’ and ‘short’ fibre bundles (Jose, 2009). Other important characteristics are as follows: r the salt content in fibre bundles (sodium chloride, NaCl) should not exceed 4%; r the moisture content should not exceed 15%.

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Industrial Applications of Natural Fibres Table 3.1.2 Characteristics of white coir of various grades. Adapted from A.C. Jose, Constructional details of coir and coir products, Coir Board, KOCHI – 682 016, 2009

Colour

Maximum impurities in % by mass

Natural bright Natural light brown and/or light grey Natural brown and /or grey Natural dark brown and/or dark grey

2.0 3.0 5.0 7.0

Grade 1 2 3 4

As mentioned by Jose (2009), ‘white coir’ should be packed in bales or as agreed between buyer and supplier. The bales or packages need to labelled with the following information: (i) (ii) (iii) (iv)

grade number; designation; net mass of the bale; any other information required by the buyer.

3.1.4.4.2

‘Brown coir’ or Mechanically Extracted Coir Fibre Bundles

The grade specifications for ‘brown coir’ or ‘mechanically extracted coir fibre bundles’ is given in IS 9308 (1987). According to Jose (2009), ‘brown coir’ is mechanically extracted from the dry husks of matured and ripe coconut after soaking in water. The requirements of three commercial grades of coir fibre bundles are as follows: r Bristle coir. The two grades of ‘bristle coir fibre bundles’ should be comparatively long and stiff. The texture needs to be firm and stiff and should not be brittle. The colour should be cinnamon brown. r Mattress coir. The ‘mattress coir fibre bundles’ are comparatively short and resilient. r Decorticated coir. The ‘decorticated coir fibre bundles’ are also called ‘mixed coir’. Grade I fibre needs to be strong, not brittle and springy. Grade II should be softer compared with grade I, but harder and more springy than ‘mattress coir’, and not be brittle. The mechanically extracted coir fibre bundles can be grouped into length classes as shown in Table 3.1.3 (Jose, 2009): r long fibres bundles: above 200 mm; r medium fibre bundles: above 150 mm and up to 200 mm; r short fibre bundles: above 50 mm and up to 150 mm. Table 3.1.3 Fibre bundle length and impurity content in bristle, mattress and decorticated coir. Adapted from A.C. Jose, Constructional details of coir and coir products, Coir Board, KOCHI – 682 016, 2009

Grade ‘Bristle coir’ grade I ‘Bristle coir’ grade II ‘Mattress coir’ ‘Decorticated coir’ grade I ‘Decorticated coir’ grade II

Long fibre Medium fibre Short fibre bundles bundles bundles Impurities min. content max. content max. content max. content 50 40 20 20

*Long/medium fibre bundles minimum 10% by mass.

30 25 10* 30 25

20 35 90 50 55

4 5 20 7 12

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According to Jose (2009), other important characteristics for ‘brown coir’ are as follows: r the salt content (NaCl) in the fibre bundles should not exceed 0.6% by mass; r the sulphate content (SO4 2− ) needs to be lower than 0.25% by mass; r the moisture content should not exceed 15%. ‘Brown coir’ should be packed in bales or as agreed between buyer and supplier. The bales or packages need to be labelled with the following information (Jose, 2009): (i) (ii) (iii) (iv) (v)

manufacturer’s name, initials or trade mark; name of the material; net mass of the bale; grade number; month and year of production.

Jose (2009) gives a detailed list of ‘Indian Standards on Coir’ and describes and names the different products and grades of: r ‘white coir’ and products such as yarn, fabrics, matting, carpets, mourzouks, fleeces and needle felts, etc.; r ‘brown coir’ and products such as yarn, fabrics, matting, coir fenders, rope and rubberised coir products. 3.1.4.4.3

Abac´a

Abac´a raw fibre bundles will be extracted from the pseudostem of the plant Musa textiles. Fibre bundles from the outer leaf sheath are, in general, coarser than the fibre bundles from the inner part of the pseudostem. From the outer leaf sheath, coarse bundles, so-called Bandala, can be separated. These fibre bundles are used, for example, for ropes. Fibre bundles from leaf sheath in the middle of the pseudostem can be spun to yarns. For coarse yarns and fabrics, fibre bundles called Lupis can be used, and for finer yarns the so-called Quilot abac´a bundles. To extract the finest abac´a bundles from the leaf sheath from the inner part of the pseudostem, special extraction techniques are used. These fibre bundles are called Tupoz. According to the different fineness of the extracted fibre bundles, the following types of abac´a are traded (Schnegelsberg, 1999): r r r r

coarse abac´a bundles: Bandala; medium coarse abac´a bundles: Lupis; medium fine abac´a bundles: Quilot; fine abac´a bundles: Tupoz.

On behalf of the Department of Commerce and Industry, the Bureau of Fibre Inspection Service on the Philippines has developed a classification system for abac´a. The main characterisation aspects are strength, cleanness and colour, and further characteristics are length and texture (BFIS, 1978). Table 7.1 in Chapter 7 shows the standard grades of abac´a fibre bundles in the Philippines as formulated by the Fibre Industry Development Authority (FiDA, Quezon City, The Philippines). There are ten normal grades, four residual grades and one unclassified grade. Abac´a grading in Ecuador is far simpler compared with the system in the Philippines. According to Jarman (1998), this is possible because all fibre bundles are produced by spindle stripping (see Chapter 7).

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3.1.4.4.4

Sisal

For Sisal (Agave sisalana), different grading systems are given in East Africa and in Brazil. Sisal fibres are graded according to length and colour (Jarman, 1998). As described in Chapter 8, sisal is graded according to the country of growing and is further subdivided into class and/or type according to colour, cleanness and length. The classes are: (i) long (length over 0.90 m); (ii) medium (length between 0.71 and 0.90 m); (iii) short (length between 0.60 and 0.70 m). In Brazil, the quality of sisal is categorised in accordance with Law 71 dated 16 March 16 1993 of the Ministry of Agriculture (details can be found in Chapter 8). The East Africa Sisal Growers Association (Nairobi, Kenya) and the London Sisal Association (London, UK) define different sisal grades, given in more detail in Chapter 8, Table 8.1.

3.1.4.4.5

Wool

Wool is classed as raw wool and as combed wool top. Classing of the raw wool is done under national regulations. For Australia, the Australian Wool Exchange (AWEX) has released an industry-revised Woolclasser’s Code of Practice, with the latest version 2007 to 2009 (AWEX, 2009). It includes in-shed management, in-store lotting and product description for the end-user of the wool. Therefore, not only objectively measured characteristics such as the average length but also the sheep breed group (e.g. ‘AAA M BLS’ for Merino bellies), the breeding background, details for calculation of dark or medullated fibre risk and the burr/seed content are specified. Wool tops are tested in wool testing laboratories. The most important parameters for wool tops are the fibre length, with its distribution, and the fibre diameter. The test methods are specified by the International Wool Textile Organisation (IWTO, 2009) and are used worldwide. The International Association of Wool Textile Laboratories (INTERWOOLLABS), which was set up in 1969, is checking its approximately 100 member laboratories from 33 countries on the basis of their participation in round tests (INTERWOOLLABS, 2009). Only laboratories that meet the demanded level of accuracy in the IWTO test methods are accredited and given a yearly valid stamp. This stamp is used on the laboratories’ test certificates to state the INTERWOOLLABS accreditation and therefore the qualification of the laboratory to produce accurate results. The limits that have to be fulfilled for airflow, laserscan and OFDA as the most important diameter measurements are: r r r r r r

<20 µm wool top fineness: limit ±0.30 µm; 20.01–24 µm wool top fineness: limit ±0.40 µm; 24.01–28 µm wool top fineness: limit ±0.50 µm; 28.01–32 µm wool top fineness: limit ±0.60 µm; 32.01–36 µm wool top fineness: limit ±0.70 µm; >36.01 µm wool top fineness: limit ±0.90 µm.

For the calibration of these instruments, INTERWOOLLABS is providing calibration tops as reference material. These calibration tops are based on repeated participation in the INTERWOOLLABS round trials with airflow as well as with the base unit calibrated projection microscope.

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For length measurement, the Almeter is used (see Chapter 13, Figure 13.10). Wool tops are not necessary for calibrating the Almeter instrument. The limits for the average lengths are: r <40 mm hauteur: limit 1.0 mm hauteur1 /1.3 mm barbe;2 r 40.01–50 mm hauteur: limit 1.1 mm hauteur/1.4 mm barbe; r 50.1–65 mm hauteur: limit 1.4 mm hauteur/1.8 mm barbe; r 65.1–80 mm hauteur: limit 1.8 mm hauteur/2.3 mm barbe; r >80 mm hauteur: limit 2.3 mm hauteur/2.9 mm barbe.

3.1.5

Conclusion

For each kind of fibre, for different processing stages, for different varieties and for different countries, specific schemes for classification are given. The given classification schemes show large differences in complexity and in stage of development. Generally the trend is moving from manual classing to instrument classing. A suitable system for classing high numbers of bales is given for cotton, where nearly all 100 million bales are classed manually, and the share of instrumentally tested bales is moving towards 60%. Fast and reliable testing instruments are essential for this. An advantageous system for assuring accurate test results is given for wool, where the laboratories are periodically checked for the accuracy of their test results. Directions for the future development of classification schemes are as follows: r standardisation of classing for each kind of fibre; r replacement of manual classing with instrument testing as soon as the test results prove to be sufficiently reliable; r harmonisation of classing by providing suitable calibration material; r checking of harmonisation by periodical round trial schemes, as for cotton and wool. Additionally, all efforts should be directed at achieving comparable test results across all kinds of fibre, as different kinds of fibre may be used for similar purposes.

References AWEX (2009) Industry-revised Woolclasser’s Code of Practice with the latest version 2007 to 2009; available at: www.wool.com.au and www.awex.com.au (accessed 13 July 2009). BFIS (1978) Official Standard Grades, Republic of the Philippines, Department of Commerce and Industry, Bureau of Fibre Inspection Service (BFIS), Manila, the Philippines. ICAC (1998), Classing and grading of cotton – report by the Technical Information Section of the ICAC, ICAC, Washington, DC. IJSG (2003), Fibre grading for tossa and white jute in Bangladesh, The International Jute Study Group (IJSG), Dhaka, Bangladesh; available at: http://www.jute.org (accessed 29 June 2009). INTERWOOLLABS (2009), International Association of Wool Textile Laboratories; available at: http://www.interwool labs.org (accessed 30 June 2009). IS 898 (1985), Specification for retted coir fibre, 2nd revision, Indian Standards.

1

In wool, hauteur stands for the length distribution by number or by cross-section. In wool, barbe stands for the length distribution by length or by mass; barbe length results are usually higher than the hauteur results. 2

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IS 9308 (1987), The grade specifications for mechanically extracted coir fiber, 1st revision, Indian Standards, Parts 1 to 3. ISO 2370 (1980) Textiles – determination of fineness of flax fibres – permeametric methods, International Organisation for Standardisation. ITC (2007) International Trade Center UNCTAD/WTO, Cotton Exporter’s Guide, ITC, Geneva. IWTO (2009), International Wool Textile Organisation; available at: www.iwto.org (accessed 30 June 2009). Jarman, C. (1998) Plant Fibre Processing – A Handbook. Intermediate Technology Publications, London, UK. Jose, A.C. (2009) Constructional details of coir and coir products, Coir Board, KOCHI – 682 016; available at: http:// coirboard.gov.in/about products-4.htm (accessed 23 June 2009). New Agriculturist (2009), Coir – a fibre for the future? May 2009; available at: http://www.new-ag.info/09/03/ focuson/focuson2.php (accessed 29 June 2009). Pan, N.C., Day, A and Mahalanabis, K.K. (2000) Properties of jute – an overview of jute from fibre to application. Indian Text. J., 110(5), 16–23. Rowell, R.M. and Stout, H.P. (1998) Jute and kenaf, in Handbook of Fiber Chemistry, 2nd edition, ed. by Lewin, M. and Pearce, E.M. International Fiber Science and Technology Series, 15, Marcel Dekker, New York, NY, pp. 465–504. Schnegelsberg, G. (1999) Handbuch der Faser – Theorie und Systematik der Faser. Deutscher Fachverlag, Frankfurt am Main, Germany. USDA (2001), The Classification of Cotton, Agricultural Handbook 566, Prepared by Cotton Program, Agricultural Marketing Service (AMS), US Department of Agriculture, Washington, DC. WCIC (2009), The Women’s Chamber of Industry and Commerce (WCIC), Coir Sector in Sri Lanka, http://www. wcicsl.org (accessed 23 June 2009).

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3.2 Technical Applications of Natural Fibres: An Overview ¨ Nina Graupner and J¨org Mussig Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany

3.2.1 Introduction The demand for natural fibres and natural-fibre-based products has strongly increased in recent years, with a projected rising trend for future years (Karus and Kaup, 2002; Anandjiwala and Blouw, 2007). The development of new products has been additionally motivated by the shortage of crude oil and the increase in environmental awareness (Wambua et al., 2003). Currently, natural fibres of various origins are processed using different procedures (see Chapters 4 to 10). Owing to different mechanical and physical properties (which are described in Chapter 13), natural fibres are suitable for a variety of applications (Riedel and Nickel, 1999). A wide range of products has already been developed to date. The areas of application include the use of raw fibres in the sealing of pipes (e.g. hemp), as a filling material for seat upholstery (e.g. coir fibres), life preservers (e.g. kapok), technical textiles, transport packaging and geotextiles through to complex construction materials used in the automotive and furniture industries and materials used as reinforcement in cement (Kozlowski et al., 2004; Coutts, 2005; Nabi Saheb and Jog, 1999; Riedel and Nickel, 1999; Lekha, 2004). Figure 3.2.1 gives an overview of the different application areas of natural fibres, involving various technical uses. Ropes are produced predominantly from flax and jute. Fabrics, knitted goods and knotted nets can be produced from yarns, and wadding, fleeces and felts are produced by aerodynamic or mechanical laying of fibres. Fabrics based on one or more kinds of yarn, for example from flax, hemp, nettle, cotton, sisal, jute and abac´a, can be used to produce goods such as tarpaulin, geotextiles, bags, carpets or furniture materials. Knitted goods based, for example, on sisal or jute are used for transport packaging or geotextiles. Knotted nets have found applications in transport nets or curtains. Wadding made from cotton or kapok is used in upholstery by the furniture industry; fleeces and needle felts made of hemp and/or flax are used by the thermal insulating industry and in geotextiles. Needle felts made from coir fibre can be used as an impact Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Figure 3.2.1

Different uses of natural fibres in technical applications.

sound insulation in buildings. Composites are produced by the addition of a polymer matrix to raw fibres or to semi-finished products such as fibre pellets, fabrics, knitted goods, fleeces or needle felts. These can be processed by means of different production procedures (see Chapter 19.3) into semi-finished products such as, for example, interior and exterior parts in the automotive industry, decking and furniture, tableware and flower pots, office equipment or containers and packaging goods. In the following, three important ranges of application areas for natural fibres are described briefly, namely insulating materials, geotextiles and composites. These will be focused upon in more detail in Chapters 19 to 21.

3.2.2

Insulating Materials Made from Natural Fibres

Insulating materials made from natural fibres provide good summer heat accumulation properties and winter heat insulation, as well as good sound absorption. In addition, these insulating materials have a high sorption capacity providing a climate balance between the living space and surrounding areas (Brandhorst et al., 2006). Nevertheless, they are naturally flammable, and the use of flame inhibitors such as borates is often essential. Therefore, these insulating materials cannot as yet be used in areas demanding high fire standards.

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Figure 3.2.2

65

Example of a thermal insulation system for a house wall based on natural fibres.

A wide range of insulation materials made from natural fibres is already being produced for different application areas. An example is given in Figure 3.2.2. Insulating fleeces and felts, panel absorbers, blowing insulation, pouring insulation, impact sound insulation materials and ceiling panels are used for thermal insulation and acoustic soundproofing (see Figure 3.2.1). Insulating fleeces involve fitted products. Processing of the fibres is carried out by a roller carding machine or an aerodynamic laying machine. Morphologically, the product is a planar-arranged entity that is bonded by a binder or other vinculat (derived from the Latin word vinculare: to bind) to a fleece. The fleeces have a low compressive strength and good compressibility. However, their strength is sufficient for them to self-support and self-adhere to form a filling or stand wedged between constructions. They have good thermal insulating properties and acoustic sound dampening advantages. In Europe, primarily wood, wool, hemp and/or flax fibres are processed into insulating fleeces (Brandhorst et al., 2006). Insulating felts are provided as fitted products. Compared with fleeces, the felts are thin. The fibres are oriented by a roller carding machine or an aerodynamic laying machine and processed into a planar-arranged entity. The fibres are then hardened to a needle felt by means of a needle process using fibre tangles and adjustable needle density. Insulating felts are predominantly used as impact sound insulation or insulating wallpaper. Used as impact sound insulation, the felts must have a certain elasticity to return to their initial state after load. Suitable fibres are wool, cotton, wood, hemp, flax, jute and coir. If they are to be used as an insulating wallpaper, they must be further hardened by the needle process, because a very firm surface is required (Brandhorst et al., 2006). Insulating boards are also offered as fitted products. Nevertheless, they are substantially more pressure resistant and only slightly compressible compared with insulating fleeces. On account of these characteristics, they are ideally suited as facade insulation. In application in wooden constructions, insulating board follows the shrinking of wood very poorly in comparison with insulating fleeces and should only be used in constructions with negligible or no shrinking. Softwood fibres or poplar wood planing chips of up to 500 mm in length (often used as wooden fill fibres with a diameter of 0.08–0.3 mm) are primarily processed into insulating boards. The wooden rests are frayed out and mixed with water to a wooden mash. The mash is then fed through long sieves and role pressings for draining and forming. In a dry procedure, the components are mixed, dispersed and hot formed. The insulating qualities can vary according to density and application area. In addition, thermal insulating boards are also suitable to use as acoustic sound absorption (Brandhorst et al., 2006; Pfriem et al., 2005).

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For blowing insulation materials, recycled paper and cellulose fibres are primarily used. Special machines are required for the blowing of insulation materials such as wood and cotton fibres in order to fill the hollows in cavities. Boundary materials for the delineation of the blowing pressure must be well secured. Owing to the blowing pressure, the fibres behave like a compressed wadding. The fibres become caught up together against the boundary material (Hersener and Keller, 2002; Brandhorst et al., 2006). Insulating materials that are to be poured are produced from recycled paper, cellulose fibres, wood chips, cotton and shives from hemp plant stalks. The hollow cavity to be insulated is dammed, and then the insulating material is poured in to fill it. Darning wadding from hemp, flax, wool or cotton is used for the insulation of joints, for example in windows, where it is stuffed into the hollow cavities (Brandhorst et al., 2006). More details about the use of natural fibres in the world of insulation materials are given in Chapter 20.

3.2.3

Geotextiles from Natural Fibres

The concept of geotextile fabrics includes knotted nets as well as fleece and felt materials. These products are produced by weaving, knitting, fleece and needle felt production (Smith, 2000). On account of the adaptable three-dimensional structure, fleeces and felts are used primarily in building construction, while small elastic fabrics are used predominantly for ground solidification. The application area of geotextiles covers significantly different operational areas. Their tasks consist of the separation of different layers of soil or fillers, the improvement of the mechanical properties of soil layers, the prevention of damage to a geotechnical system and their filter effect by the retention of soil particles while liquid transportation is still guaranteed. They are also used for draining by collection of precipitation or groundwater and then the forwarding of it at the level of the geotextiles. They can be used primarily for earth construction – embankment and erosion protection, renovation of ski runways and slopes and as carrier felts for rolling lawn (cultivation felt) in the form of needle felts or fabrics, as well as for water constructions (Schmalz and B¨ottcher, 1999; Lekha, 2004; Prabakar and Sridhar, 2002; Ghosh et al., 2005; Davies et al., 2006; Mwasha, 2009; Rowell and Stout, 2007). An example of a geotextile construction is given in Figure 3.2.3. Natural fibre-based geotextiles are smooth and can adapt themselves very well to ground unevenness. They are made predominantly from coir, jute and sisal (Dippon, 1999). In comparison with petrochemical-based geotextiles, they have the advantage that, when they remain in the earth after they have done their job, they decompose after some years, leaving no remains. Plants are able to penetrate the textiles with their roots without limiting the function of the textiles and without the plants being constricted by the textiles. The textiles should be able to last long enough for the plant roots to perform the job of ground solidification. Thereby,

Figure 3.2.3

Application of a geotextile (e.g. fabric or a felt).

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a construction method close to nature is provided in landscaping and the plants can develop unhindered (Schmalz and B¨ottcher, 1999; Mwasha, 2009). Natural fibres, through their good water sorption capacity, can bind and store water very well, with the underlying seed bed achieving a good microclimate. According to different requirements, and depending on how long the textiles should be preserved, the rotting process can be accelerated or delayed by a suitable fibre choice. Thus, owing to the higher lignin content, coir is preserved longer in the ground than, for example, jute or hemp, and it offers erosion protection. Certain grain dimensions can be filtered specifically according to the choice of the fibre fineness in a fleece or felt. Specific grain dimension areas can be filtered by the combination of several kinds of fibre with different fineness. Elastic geotextiles made from elastic fibres with adjusted process parameters can be used for applications with lower requirements, which can be well adapted for use in earth movement areas without breaking (Schmalz and B¨ottcher, 1999; Lekha, 2004; Mwasha, 2009; Rowell and Stout, 2007). More information on the use of natural fibres in geotextiles and the markets is given in Chapter 21.

3.2.4

Composites of Natural Fibres

Natural fibres are just as able as petrochemical-based fibres to be processed into composites with a polymer matrix in different production procedures (see Chapter 19.3 and Nabi Saheb and Jog, 1999; Rowell et al., 1997; Riedel and Nickel, 1999; Biagotti et al., 2004; Pickering, 2008). The classical fibre composite construction (see Figure 3.2.4) makes it possible to generate highly loaded structures with a variable fibre orientation and directional characteristics (Wambua et al., 2003). In addition to their environmental friendliness, the other advantages of natural fibres include a good stiffness and strength, with at the same time a low density compared with glass fibre. The specific Young’s modulus of natural-fibre-reinforced composites is comparable with that of glass-fibre composites. Naturalfibre-reinforced composites have good lightweight construction potential and show positive break behaviour, i.e. they break without rough edges and the components do not splinter. Disadvantages are their moisture expansion characteristics, their flammability and their variable quality. There are also significant problems in the fibre–matrix interaction, which reduces the mechanical characteristics (Rowell et al., 1997; Pickering, 2008; Drzal et al., 2003). The uses of natural-fibre-reinforced composites are many and range from small mechanically loaded structures such as biodegradable flowerpots, disposable articles such as golf tees, through to articles with more demanding specifications, from grinding disc holders, briefcases and instrument cases and tableware up to structural components in the furniture industry such as tabletops, chairs and bookshelves, and also exterior uses such as garden furniture, patio planking or structural components in the automotive industry.

Figure 3.2.4

Principle of a fibre-reinforced polymer.

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The automotive industry is currently taking a leading role in the processing of natural-fibre-reinforced composites. Semi-finished parts are made predominantly for the automotive interior, such as indoor panels or instrument boards (Karus et al., 2006; Bhat et al., 2004; Pr¨omper et al., 2004; Karus and Kaup, 2002). Moreover, automotive manufacturers have also started to use natural-fibre-reinforced plastics in exterior uses for carbody components. Examples are the underbody panelling of the Daimler A class (Knothe and Schl¨osser, 2000), gear encapsulation in an urban bus (Knothe and Schl¨osser, 2000), a prototype of the middle section between the headlights above the bumper of a passenger bus (M¨ussig et al., 2006), the spare wheel cover of the Toyota RAUM (Anonymous, 2007), as well as the bonnet, boot lid and roof of the BioConcept car (Anonymous, 2007). A detailed representation of the development of natural-fibre-reinforced composites in the automotive industry is given in Chapter 19.4. Although rail construction places very high requirements on security and fire prevention, prototypes of natural-fibre-reinforced seat box and seat back cladding have been successfully realised and used in the high-level road DT 4.5 in Hamburg, Germany (Riedel, 2003). In addition, there have been several attempts showing that natural-fibre-reinforced composites could also be used in higher-loaded structural components. It has been shown through the example of a 3 m long rotor sheet for a wind generator that it is possible to build wide structural components based on flax fibres as reinforcement (Sedlacik, 2004). The aircraft construction industry, which demands the highest requirements of mechanical and fire prevention characteristics, is also anxious to use natural fibres in aeroplane interiors. At the moment, market entry is still prevented by the failure to meet high fire standards. Nevertheless, the past demonstrates that it is in principle possible to use natural fibres in aircraft construction. As early as in the 1920s and 1930s the first composites in aircraft construction were made from natural fibres with the aim of realising lighter components for primary structures in the aeroplane (McMullen, 1984). Construction with natural-fibre-reinforced composites is very challenging owing to the variable quality of the fibres. Only standard construction design guidelines taken from the glass or carbon fibre industry can be used. A special adjustment and new simulation methods must be created especially for natural-fibre-reinforced composites. Detailed information about this topic is given in Chapter 19.2. With the rediscovery of natural fibres in the 1990s, scientific activity in the area of natural-fibre-reinforced composites has accelerated internationally. Intensive work, in particular in the area of basic research, has led to numerous publications in approved international journals. A comprehensive listing of these works would be overwhelming in this context, but some important overview articles can be named here: Youngquist et al. (1994), Mohanty and Misra (1995), Hanselka and Herrmann (1999), Nabi Saheb and Jog (1999), Bledzki and Gassan (1999), Joseph et al. (1999), Mohanty et al. (2001), Eichhorn et al. (2001) and M¨ussig et al. (2005). For the area of biodegradable natural-fibre-reinforced composites, see Hanselka and Herrmann (1995), Mohanty et al. (2000) and Patel et al. (2004). This strong increase in scientific activity has also led to respected publishing companies such as Springer, CRC, Woodhead Publishing or RAPRA publishing the first books on the topic of natural-fibre-reinforced composites. These include the following works: Harig and M¨ussig (1999), Bledzki et al. (2002), Baillie (2004), Franck (2005), Mohanty et al. (2005), Fakirov and Bhattacharyya (2007) and Pickering (2008). More details about natural-fibre-reinforced composites can be found in Chapter 19.

3.2.5

Summary and Outlook

Products based on natural fibres have already conquered a wide market. Their operational areas range from the use of raw fibres in technical textiles to their application as composites. While the good sorption capacity of natural fibres provides an advantage for their application in geotextiles and in insulating materials, as it allows for a good ground and space climate, this ability is a disadvantage for their application in composites owing to source compatibility. Mechanical characteristics such as, for

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example, the low impact strength of bast-fibre-reinforced composites or physical properties such as their low fire resistance are the most important limiting factors for a continuing application in composites. However, there are also great advantages, such as their high stiffness and their ability to break without leaving rough edges. In comparison with the processing of glass or carbon fibres, their positive environmental impact and industrial safety should also be emphasised. Existing drawbacks such as flammability, the variable qualities of products made from natural fibres and partially bad interactions between fibres and matrix must be resolved by future research. The elimination of these problems would lead to more technically advanced products and thereby an increase in demand for natural fibres.

References Anandjiwala, R.D. and Blouw, S. (2007) Composites from bast fibres – prospects and potential in the changing market environment. J. Nat. Fibr., 4(2), 91–109. Anonymous (2007) Bioplastics in automotive applications, Bioplast. Mag., 2, 14–18. Baillie, C. (ed.) (2004) Green Composites – Polymer Composites and the Environment. Woodhead Publishing, Cambridge, UK. Bhat, G., Kamath, G., Mueller, D., McLean, M. and Parikh, D.V. (2004) Cotton-based composites for automotive applications, GPEC paper. Biagotti, J., Puglia, D. and Kenny, J.M. (2004) A review on natural fibre-based composites – Part I: structure, processing and properties of vegetable fibres. J. Nat. Fibr., 1(2), 37–68. Bledzki, A.K. and Gassan, J. (1999) Composites reinforced with cellulose based fibres. Prog. Polym. Sci., 24, 221–274. Bledzki, A.K., Sperber, V.E. and Faruk, O. (2002) Natural and wood fibre reinforcement in polymers. Rapra Rev. Rep., 13(8), Report 152. Brandhorst, J., Spritzendorfer, J. and Gildhorn, K. (2006) D¨ammstoffe aus Nachwachsenden Rohstoffen. Fachagentur Nachwachsender Rohstoffe (FNR), G¨ulzow, Germany. Coutts, R.S.P. (2005) A review of Australian research into natural fibre cement composites. Cem. Concr. Compos., 27, 518–526. Davies, K., Fullen, M.A. and Booth, C.A. (2006) A pilot project on the potential contribution of palm-mat geotextiles to soil conservation. Earth Surf. Processes Landforms, 31, 561–569. Dippon, K. (1999) Geotextilien aus Naturfasern f¨ur die Renaturierung von Gew¨assern am Beispiel Japan, in Marktinnovation Hanf Geo- und Agrartextilien aus Hanffasern ‘Technik’ Fachseminar Technik, Faserinstitut Bremen e.V./Bremer Baumwollb¨orse, Bremen, Germany, 27 October. Drzal, L.T., Mohanty, A.K., Wibowo, A., Misra, M. and Seiler, B.D. (2003) Hemp fiber-reinforced cellulosic plasticbased bio-composites: physical-mechanical and morphological properties evaluation, in 7th International Conference on Woodfiber–Plastic Composites, Forest Products Society, Madison, WI. Eichhorn, S.J., Baillie, C.A., Zafeiropoulos, N., Mwaikambo, L.Y., Ansell, M.P., Dufresne, A., Entwistle, K.M., HerreraFranco, P.J., Escamilla, G.C., Groom, L., Hughes, M., Hill, C., Rials, T.G. and Wild, P.M. (2001) Review: current international research into cellulosic fibres and composites. J. Mater. Sci., 36(9), 2107–2131. Fakirov, S. and Bhattacharyya, D. (ed.) (2007) Handbook of Engineering Biopolymers: Homopolymers, Blends and Composites. Carl Hanser, Munich, Germany. Franck, R.R. (ed.) (2005) Bast and Other Plant Fibres. Woodhead Publishing, Cambridge, UK. Ghosh, A., Ghosh, A. and Bera, A.K. (2005) Bearing capacity of square footing on pond ash reinforced with jute-geotextile. Geotext. Geomembranes, 23, 144–173. Hanselka, H. and Herrmann, A.S. (1995) Kompostierbare Faserverbund-Bauteile aus nachwachsenden Rohstoffen, in 7th Int. Techtextil Symp., Neue Verbundtextilien und Composites, Textilarmierte Werkstoffe Teil 2, Frankfurt am Main, Germany. Hanselka, H. and Herrmann, A.S. (1999) Technischer Leitfaden zur Anwendung von o¨ kologisch vorteilhaften Faserverbundwerkstoffen – aus nachwachsenden Rohstoffen am Beispiel eines Kastentr¨agers als Prototyp f¨ur hochbelastbare Baugruppen. Shaker Verlag, Aachen, Germany.

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Harig, H. and M¨ussig, J. (1999) Heimische Pflanzenfasern f¨ur das Automobil, in Neue Materialien f¨ur innovative Produkte – Entwicklungstrends und gesellschaftliche Relevanz. Bund 3. Wissenschaftsethik und Technik und Technikfolgenbeurteilung, ed. by Harig, H. and Langenbach, C.J. Springer Verlag, Berlin, Germany. Hersener, J.-L. and Keller, A. (2002) Einblasd¨ammstoff aus Faserhanf und Grasfasern, in Jahresbericht 2002, Eidgen¨ossische Forschungsanstalt f¨ur Agrarwirtschaft und Landtechnik (FAT), Ettenhausen, Switzerland. Joseph, K., Tolˆedo Filho, R.D., James, B., Thomas, S. and Hecker de Carvalho, L. (1999) A review on sisal fiber reinforced polymer composites. Revista Brasileira de Engenharia Agr´ıcola e Ambiental, 3, 367–379. Karus, M. and Kaup, M. (2002) Natural fibers in the European automotive industry. J. Ind. Hemp, 7(1), 119–131. Karus, M., Ortmann, S., Gahle, C. and Pendarovski, C. (2006) Use of natural fibres in composites for the German automotive production from 1999–2005, in NF-Market Study. nova Institut, H¨urth, Germany. Knothe, J. and Schl¨osser, T. (2000) Natural fibre reinforced plastics in automotive exterior applications, in 3rd International Wood and Natural Fibre Composites Symposium, Kassel, Germany. Kozlowski, R., Muzyczek, M. and Mieleniak, B. (2004) Upholstery fire barriers based on natural fibers. J. Nat. Fibr., 1(1), 85–95. Lekha, K.R. (2004) Field instrumentation and monitoring of soil erosion in coir geotextile stabilised slopes – a case study. Geotext. Geomembranes, 22, 399–413. McMullen, P. (1984) Fibre/resin composites for aircraft primary structures: a short history 1936–1984. Composites, 15(3), 222–230. Mohanty, A.K. and Misra, M. (1995) Studies on jute composites – a literature review. Polym.-Plast. Technol. Eng., 34(5), 729–792. Mohanty, A.K., Misra, M. and Drzal, L.T. (2001) Surface modifications of natural fibers and performance of the resulting biocomposites: an overview. Macromolec. Mater. Eng., 276(3–4), 1–24. Mohanty, A.K., Misra, M. and Drzal, L.T. (2005) Natural Fibers, Biopolymers, and Biocomposites. CRC Press, Boca Raton, FL. Mohanty, A.K., Misra, M. and Hinrichsen, G. (2000) Biofibres, biodegradable polymers and biocomposites: an overview. Macromolec. Mater. and Eng., 276(27), 1–24. M¨ussig, J., Karus, M. and Franck, R.R. (2005) Bast and leaf fibre composite materials, in Bast and Other Plant fibres, ed. by Franck R.R. Woodhead Publishing, Cambridge, UK. M¨ussig, J., Schmehl, M., von Buttlar, H.B., Sch¨onfeld, U. and Arndt, K. (2006) Exterior components based on renewable resources produced with SMC technology – considering a bus component as example. Ind. Crops Prod., 24, 132–145. Mwasha, A. (2009) Using environmentally friendly geotextiles for soil reinforcement: a parametric study. J. Mater. Des., 30(5), 1798–1803. Nabi Saheb, D. and Jog, J.P. (1999) Natural fiber polymer composites: a review. Adv. in Polym. Technol., 18(4), 351–363. Patel, M., Bastioli, C., Marini, L. and W¨urdinger, E. (2004) Environmental assessment of bio-based polymers and natural fibres. Utrecht University, Department of Science, Technology and Society (STS), Utrecht, The Netherlands. Pfriem, A., Wagenf¨uhr, A. and M¨uller, M. (2005) Ultra-light fibre boards as insulating and lightweight construction material, in 5th International Symposium on Materials Made from Renewable Resources, naro.tech, Erfurt, Germany. Pickering, K. (ed.) (2008) Properties and Performance of Natural-fibre Composites. Woodhead Publishing Limited, Cambridge, UK. Prabakar, J. and Sridhar, R.S. (2002) Effect of random inclusion of sisal fibre on strength behaviour of soil. Constr. Build. Mater., 16, 123–131. Pr¨omper, E., Bogdanov, N. and Kreitlow, R. (2004) New automotive interior parts from natural fiber materials, in 7th International AVK-TV Conference, AVK, Baden-Baden, Germany. Riedel, U. (2003) Biocomposites in rail vehicle applications, in 4th International Symposium on Materials Made from Renewable Resources, naro.tech, Erfurt, Germany. Riedel, U. and Nickel, J. (1999) Natural fibre-reinforced biopolymers as construction materials – new discoveries. Angew. Makromolek. Chem., 272, 34–40. Rowell, R.M., Sanadi, A.R., Caulfield, D.F. and Jacobson, R.E. (1997) Utilization of natural fibers in plastic composites: problems and opportunities, in Lignocellulosic– Plastics Composites, ed. by Leao, A.L., Carvalho, F.X. and Frollini, E. University of Rio de Janeiro, USP and UNESP, Rio de Janeiro, Brazil. Rowell, R.M. and Stout, H.P. (2007) Jute and kenaf, in Handbook of Fiber Chemistry, 3rd edition, ed. by Lewin, M. Taylor & Francis Group, CRC Press, Boca Raton, London/New York.

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Schmalz, E and B¨ottcher, P. (1999) M¨arkte und Einsatzgebiete von Geotextilien aus synthetischen Fasern und Naturfasern, ¨ in Geo und Agrartextilien aus Hanffasern – M¨arkte & Okonomie, S¨achsisches Textilforschungsinstitut, Chemnitz, Rheine, Germany. Sedlacik, G. (2004) Beitrag zum Einsatz von unidirektional naturfaserverst¨arkten thermoplastischen Kunststoffen als Werkstoff f¨ur großfl¨achige Strukturbauteile, Dissertation at the Faculty of Mechanical Engineering at the Technical University Chemnitz, Chemnitz, Germany. Smith, R. (2000) The potential market for sisal and henequen geotextiles, in Proceedings of a Seminar held by the Food and Agriculture Organization of the UN (FAO) and the Common Fund for Commodities (CFC), Rome, Italy. Wambua, P., Ivens, J. and Verpoest, I. (2003) Natural fibres – can they replace glass in fibre reinforced plastics? Compos. Sci. Technol., 63, 1259–1264. Youngquist, J.A., English, B.E., Scharmer, R.C., Chow, P. and Shook, S.R. (1994) Literature review on use of nonwood plant fibers for building materials and panels, in General Technical Report FPL-GTR-80, US Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI.

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3.3 Natural Fibres in Technical Applications: Market and Trends Stephan Piotrowski and Michael Carus nova-Institut, H¨urth, Germany

3.3.1 Introduction Out of all the natural fibres (see Figure 2.3.1 in Chapter 2.3), this chapter, addressing the market for natural fibres, is only concerned with those of plant origin. Quantitatively, by far the most important plant fibre is cotton. However, cotton is only discussed briefly in this chapter, which will concentrate on those plant fibres of secondary importance, which all have their particular technical properties and applications. Most of these are leaf fibres (sisal, abac´a) and bast fibres (hemp, flax, jute, kenaf). An exception is coir, which is extracted as fibre bundles from the fibrous middle layer of the coconut and is therefore a fruit fibre. Sections 3.3.2 to 3.3.4 will provide an overview of the world markets, main technical applications and the price development for these natural fibres. Sections 3.3.5 and 3.3.6 will give an outlook on future trends and perspectives for natural fibres and draw main conclusions to summarise the whole chapter.

3.3.2

Recent World Market Data on Cultivation and Production of Natural Fibres

The main source for global production data on natural fibres is the FAO (Food and Agriculture Organisation of the United Nations). Most of the available data from the FAOSTAT database (FAOSTAT, 2009) only go up to 2005, except for jute and flax, but another FAO publication provides more recent production data for jute, kenaf, sisal, abac´a, coir and allied fibres (FAO, 2008). In many cases, though, inconsistencies in the available data are apparent and call for an improvement in the coverage of natural fibres in official statistics. For the case of European cultivation and production of hemp and flax, the data from the European Commission (2008) are considered to be more reliable. A particular problem in the coverage of natural fibres arises from the use of misleading common names for the fibre plants. As an example, Schnegelsberg (1996) shows in his study that there are more than Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
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60 unconventional trade names for bast and leaf fibres that contain the word ‘hemp’ but that have nothing to do with Cannabis sativa L. The polysemic use of the word ‘hemp’ leads to misunderstandings, for instance in import and export statistics and production data of plant fibres. One example is the unconventional term ‘manila hemp’ for the fibres of the monocotylic plant Musa textilis N´ee. The use of the term abac´a for Musa textilis, on the other hand, is unambiguous. The same problem applies to the confusing name ‘New Zealand flax’ for the fibres of the monocotylic plant Phormium tenax. The term Phormium (Phormium tenax J.R. Forst. et G. Forst.) is, on the other hand, unambiguous and does not cause confusion, in trade statistics for instance. Regarding most natural fibres, South American and East Asian developing and newly industrialising countries dominate. Only in the case of flax and hemp, countries in temperate zones also have significant market shares. This has a direct impact on production structures, social conditions and the quality of processing techniques. Mostly, natural fibres are small farmers’ crops, although there are regional differences. For example, abac´a is grown on large estates in Ecuador, while it is a smallholder crop in the Philippines. The ecological impact of fibre crop cultivation and its competition with other land uses cannot be generalised. Some natural fibre crops are not very demanding as regards soil and climate conditions, especially sisal, which is well adapted to harsh, meagre environments. The cultivation of sisal therefore presents little or no competition to other land uses. Hemp is well known for needing little or no pesticide applications (Fortenbery and Bennett, 2004), and both hemp and flax need low to medium quantities of fertiliser. On the other hand, there are also very resource-intensive fibre crops like cotton, which alone demands about 6–10% of global pesticide use (Paulitsch et al., 2004). Global production of natural fibres of plant and animal origin amounts to about 32 million t annually. Among these, cotton is the undisputed number one natural fibre, with a market share of around 75%. All other natural fibres of plant origin make up about 20%, wool less than 5% and rare animal natural fibres only around 0.1% (Figure 3.3.1). Owing to the overwhelming importance of cotton, it is included in this section on world markets, while the rest of the chapter is dedicated to the quantitatively minor fibre crops.

3.3.2.1

Cotton

Global production of cotton amounted to 24.6 million t in 2008 (USDA, 2009). The three largest cotton producers are China, India and the USA. With a share of 32% of global production, China dominates, followed by India (22%) and the USA (12%). World cotton production decreased in 2008 by around 2 million t from the previous year, and also the processing quantity decreased by 7%. The two main reasons were the stagnating global economy, which caused a decline in demand as well as a credit crunch of processors, and the competition from more profitable crops like maize (Zea mays) and soybeans (Glycine max) used for energy and feed. This was particularly apparent in the USA, where the cotton area declined by 30% to 3 million ha, which was the lowest cotton acreage since 1983 (USDA, 2009). Another long-term reason for the decline in cotton production was replacement by synthetic fibres. Until the early 1990s, global production of cotton and man-made fibres (synthetic and cellulose fibres) was about head to head, but then, up to 2007, the production of man-made fibres increased to about 175% of cotton production (CIRFS, 2008).

3.3.2.2

Jute

Tossa jute (Corchorus olitorius L.) and white jute (Corchorus capsularis L.) – see Chapter 6 – dominate the world market for natural fibres behind cotton. Acreages as well as total production of jute approximately add up to those of all other plant natural fibres together, excluding cotton. World production of jute fibre amounted to 2.7 million t in 2007/08 and fluctuated in the past decade around 2.5 million t (FAO, 2008). The largest

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Worldwide production of natural fibres, in million t (Sources: FAOSTAT, 2009 and FAO, 2009).

producer is India, with more than 60% of this production (around 1.6 million t). All other jute is produced in other Asian developing countries, namely Bangladesh (0.9 million t), the largest producer behind India, Myanmar (0.04 million t) and Nepal (0.02 million t). Kenaf (Hibiscus cannabinus L.) is a plant very similar to jute, and in production statistics it is therefore often confused with jute. Moreover, as yet another example of the polysemic use of the word ‘hemp’, kenaf is sometimes also referred to as ambary hemp. According to FAO (2008), world production of kenaf and allied fibres was around 340 000 t in 2007/08, down from 430 000 t in 2002/03. The largest producers in 2007/08 were India (140 000 t), followed by China (87 000 t) and Thailand (36 000 t), and in all three countries kenaf is on the decline. According to news reports, kenaf may experience a renaissance nonetheless, in particular in technical applications (Adnan, 2009; Inteletex, 2009), as there is a specific problem with jute for an exposed use, for example in the car interior, which makes kenaf an ideal substitute. In order to make the jute fibres more elastic and soft, which improves the carding and spinning process, they are often pretreated with a mineral-oil-based so-called jute batching oil (JBO), which has a lot of potential for migration, leads to an oily or fishy smell and has been reported to be carcinogenic (Mehrotra et al., 1988). However, this problem is only relevant in technical applications, because in textiles the oil is eventually washed out during the production process. In technical applications, for example in interior automobile materials, the JBO can lead to high fogging values1 (nova-Institut, 2004).

1 ‘Fogging tests measure the tendency for plastics or elastomeric materials to volatise substances which can condense and collect on other surfaces when in use’ (PTL, 2005).

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The average yield of jute, kenaf and allied fibres varied between 0.8 t/ha in Myanmar and 2.71 t/ha in China (in the 2007/08 season), although in the case of China these were mostly fibres other than jute (FAO, 2008). 3.3.2.3

Flax

As mentioned above, estimates of the global production of flax (fibre bundles from the plant Linum usitatissimum L.) vary. Mackiewicz-Talarczyk et al. (2008) report a production of 751 000 t, produced on about 400 000 ha, while the FAO (2008) estimates for 2007 a production of about 1 million t, produced on 450 000 ha. According to the European Commission, the flax area in the European Union has declined steadily from about 120 000 ha in 2004 to about 80 000 ha in 2008 (with a yield of about 0.95 t/ha), and a further decline by 20 000 ha is expected for 2009 (European Commission, 2008). Reasons for this decline are marketing problems and competition from other crops like maize (Zea mays) for bioenergy use. Although China, with about 30 000 t of flax, produced on an estimated 130 000 ha (European Commission, 2008), is the second largest producer of flax after the EU, the European Union still exports about 70–80% of its long flax fibre bundle production to China (Carus et al., 2008). The reason is that China has a market share of about 70% in the global flax spinning sector, more than 50% in the weaving sector and 40% in the textiles sector. The quality of Chinese long flax fibre bundles is, however, not good enough for fine linen fabric (Mackiewicz-Talarczyk et al., 2008). The EU, on the other hand, only has market shares in these sectors of between 10 and 20% (European Commission, 2008). There is hardly any future for a European flax textile industry, and European processors need to focus on value-added alternatives (Wilson, 2008). 3.3.2.4

Sisal

The sisal fibre bundles are extracted from the plant Agave sisalana (see Chapter 8), while fibres of other agaves carry different names, e.g. henequen, which is extracted from Agave fourcroydes. In 2007, Brazil produced 113 300 t of sisal, which equates to 47% of the global production. The plant originates from Central America and was introduced to today’s Tanzania and Kenya at the end of the nineteenth century (Moir, 2006). Even today, Tanzania (36 900 t) is the second largest producer behind Brazil, followed by China (35 000 t), Kenya (27 600 t) and Venezuela (10 000 t). Global production has been quite stable in the last decade at 240 000– 250 000 t, but shifts in production are taking place between countries. Notably, China is reducing its acreage, because of concerns over the priority given to food production, while it is expanding in Brazil (FAO, 2008). Global production of henequen amounted to 21 900 t, of which Mexico produced the largest share (17 000 t). Henequen produces a fibre that is almost identical to sisal but of lower quality. Similar hard fibres add up to another 34 800 t (FAO, 2008). For a long time, agricultural ropes and twines made up the largest share of sisal and henequen consumption (Landon, 2000). Recently, there has been a growing demand for natural fibres, mainly sisal, as metal polishing material, especially from China (Carus et al., 2008). 3.3.2.5

Abac´a

As explained above, abac´a is an example of a plant that is erroneously sometimes thought to be related to Cannabis sativa. Although the plant abac´a (Musa textiles N´ee) is also known as manila hemp, it belongs to the family of Musaceae, of which banana (Musa musa) is the most prominent exponent. Global production in 2007 amounted to about 73 000 t, produced on 130 000 ha, down from a high of 89 000 t in 2004, which is a decline of almost 20% (FAO, 2008). By far the largest producer is the Philippines, with about 80% of total production (the common name ‘manila hemp’ speaks for itself). Ecuador is the second largest producer, with 10 000 t. Abac´a is grown in several other South-East-Asian countries, but none with significant market shares.

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There are accounts of a comeback by abac´a, spurred by the International Year of Natural Fibres (IYNF) and the fashion towards ecofriendly textiles. Furthermore, it is an excellent technical fibre and is used in composites in the automotive industry, for example in the Mercedes A- and B-class, but it is relatively expensive. Abac´a had been a major export good in the nineteenth century but declined with the advent of synthetics. Currently, the Philippines is eager to promote abac´a, not least because a reported 1.5 million Filipinos directly or indirectly depend on the abac´a industry for their livelihood (Manila Bulletin, 2009). 3.3.2.6

Hemp

The FAO reported a global hemp fibre (fibres from the plant Cannabis sativa L.) production for 2005 of about 90 000 t, produced on 60 000 ha. According to these statistics, the largest producers were China (41 000 t) and Europe (30 000 t). Also, Canada has a significant, though very volatile, production of hemp, fluctuating between 19 000 ha in 2006 and 6 000 ha in 2007. However, the main outlet for Canadian hemp is currently the export of seeds to the United States (Brook, 2008). Only a very small quantity of the hemp fibres from Canadian production is actually used. As indicated above, the figures on the acreages of hemp are riddled with uncertainties. Contrary to the FAO, global acreage has been estimated to be around 115 000 ha (Carus et al., 2008). In particular, the figure for China varies from 20 000 ha according to the FAO to 80 000 ha according to Carus et al. (2008). Adding to this uncertainty, a government programme to produce army clothes from hemp reportedly led to a one-time increase in the Chinese acreage to 250 000 ha in 2008 (Hertel, R., HempAge AG, Adelsdorf, Germany, 2008, private communication). In 2008, hemp in the EU27 was cultivated on about 13 000 ha (European Commission, 2008). More than 50% of total production takes place in France. Other important producers are the UK, Germany and the Netherlands, and minor acreages can be found in Poland, the Czech Republic, Austria and Italy. The European Commission expected a significant rise in hemp cultivation in 2009, to more than 18 000 ha. The reason for this development is on the one hand the low wheat price in February 2009 (225 $US/t compared with 425 $US/t a year earlier), which made hemp an attractive crop, and on the other hand the high demand for shives as animal bedding, in particular in the UK (see Section 3.3.3.6). 3.3.2.7

Coir

In only six of the over 90 countries with coconut cultivation does a noticeable production of coir fibre bundles takes place, namely in India, Vietnam, Sri Lanka, Thailand, Ghana and Malaysia. According to FAO figures, the global production of coir amounted to about 1 million t in 2005. This coir is mainly used for making floor mats and mattresses. The coconut consists of about 42% fibrous mesocarp. Of this mesocarp, 30% coir and 70% pith and short fibre bundles can be obtained (Arancon, R.N., Executive Director, Asian and Pacific Coconut Community, Jakarta, Indonesia, 2009, private communication). Given the global production of coconuts of around 50 million t, the production of coir could therefore be theoretically extended to a maximum of about 6 million t. Currently, the largest share of the fibrous mesocarp is used for process heat in copra production, which is the dried coconut meat.

3.3.3 3.3.3.1

Main Technical Applications for Natural Fibres Yarns, Twines and Other Traditional Uses

The largest share of most natural fibres is still used for traditional applications. As Franck (2005) reports, about half the jute from Indian production is used for sackings, and also other applications are mainly traditional uses such as yarns, twines and cloth.

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Flax produces long fibre bundles (long flax, see Chapter 4), which are mainly used for textiles, and short fibre bundles (tow), which are used for textiles but also for various technical purposes. Long flax for textiles makes up by far the highest turnover of flax. According to a market study from 2005 (ANDI, 2005), all other applications (textiles short fibre bundles, speciality and technical paper, non-wovens and composites) each comprises less than 5% of the turnover. According to Moir (2006), the share of traditional uses of sisal (baler twines and other cordage applications) was around 90% in the early 1970s but was then pushed back to 40% by synthetic products and growth of new applications for sisal-like polishing cloths. Three types of coir qualities need to be differentiated, namely bristle, twisted and mattress quality (see Chapter 9). Furthermore, there are mixed fibre bundles produced by decortication of the whole green or retted brown husk. These qualities are used in different, but mostly traditional applications. Twisted coir is used for automotive upholstery, ropes and geotextiles, but also in mattresses. Bristle coir is used mainly for brushes and brooms. The mattress fibre bundles are used for all kinds of bedding, cushioning and filling material. By coating the coir with natural latex, rubberised coir is produced, which improves its use for mats but also for high-quality automobile seats (see Chapter 19.4).

3.3.3.2

Pulp and Paper

The pulp of natural fibres is particularly suited for making speciality paper, e.g. for cigarettes, bank notes and technical filters, where thinness needs to be combined with exceptional strength and durability. This is a specialised market, with no significant potential for market growth or innovations, but constitutes a large portion of total utilisation for many natural fibres, e.g. 75% of hemp. A very large portion of cigarette paper is made from flax and hemp. Regarding sisal used for pulp and paper, Landon (2000) reports a share of 28%, while Andrade et al. (2006) indicate a share of 60% for the Brazilian market. Ultimately, the shares are difficult to estimate because, in the end-product, different types of fibre may be combined, and the input share of a particular fibre like sisal depends on its price. The importance of the pulp and paper market is also indicated by abac´a export statistics from the Philippines. In the Philippines, abac´a is an export-oriented crop. From the total average export earnings of $US 80 million between 1996 and 2000, pulp constituted the largest share of $US 36 million, followed by raw fibres, fibre crafts, cordage, ropes, twines, yarns and fabrics (Fiber Industry Development Authority, 2009). Tea bags are almost exclusively made of abac´a owing to the fibre’s exceptional water resistance.

3.3.3.3

Composites

A still small but growing segment for the use of natural fibres comprises natural-fibre-reinforced plastics used, for example, in the automotive industry. About 12% of European hemp and 23% of European short flax production (9% of all flax produced in Europe) are used in composites (EIHA, 2007; ANDI, 2005). Natural-fibre-reinforced composites are characterised by a low density, which allows mass reduction, and favourable mechanical, acoustic and processing properties, which make them particularly suitable for use in automobile parts. According to a study by the nova-Institut (Karus et al., 2006), the quantity of natural-fibre-reinforced composites used in German automotive production amounted to 30 000 t in 2005 (without cotton and wood fibres). Of these, about two-thirds were thermoplastics and one-third thermosetting plastics. Figure 3.3.2 shows that the use of natural fibres in the German automotive production increased in the years 2004 and 2005 – albeit with slowed growth rates of less than 3%. This growth was primarily based on the rising use of the press flow moulding and injection moulding techniques (both new to natural fibres), while the established compression moulding was stagnating (more details about the natural fibre composite processing techniques

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Figure 3.3.2

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Use of natural fibres for composites in the German Automotive Industry 1999–2005 (Source: Karus et al., 2006).

can be found in Chapter 19.3). In 2005, for the first time, 19 000 t of natural fibres (without wood and cotton) were used in automotive composites. At the same time, the shares of natural fibres used have changed. While the consumption of natural fibres like jute and kenaf, sisal, coir and abac´a increased substantially between 2000 and 2004, both on a percentage basis and absolutely, there has been a stagnation ever since. This is directly linked to the prices of European flax, which were quite high in this period but have decreased since 2004. Simultaneously, in recent years there have been significant price increases for jute and kenaf on the world market. Accordingly, flax was able to expand its market position in the years 2004 and 2005. The shares of hemp are mainly determined by the short supply. Of the 19 000 t in 2005, flax accounted for more than 60%. These quantities are almost exclusively produced in Europe, in most cases as a byproduct of textile long flax production. Hemp fibre bundles, also almost exclusively from European production, show a market share of just under 10%. Jute and kenaf make up 11%, and sisal around 7%. For the segment of composite materials, the bast fibres flax, hemp, jute and kenaf are technically more or less interchangeable and are therefore competitors. Sisal can only be considered for composite materials after appropriate conditioning. The use of wood and cotton composites each exceeds the quantity of all other natural fibre composites. Karus and Ortmann (2004) report for the year 2003 about 25 000 t of wood fibres (equating to 36 000 t of wood fibre composites) and 45 000 t of cotton fibres (equating to 79 000 t of cotton fibre composites) used in the German automotive industry. The most common processing technology for natural fibres (without wood and cotton) in the automotive industry is thermoplastic press moulding (61%), followed by thermosetting plastic press moulding (35%), press flow moulding (2%) and injection moulding (2%). The latter two are relatively new technologies for natural fibres that are expected to gain in importance in coming years. Natural fibre compression moulding is an established and proven technique for the production of extensive, lightweight and high-class interior parts in medium- and luxury-class cars. Injection moulding, on the other hand, is more suitable for inexpensive door concepts with a high part integration. The fibre volume fraction is higher in thermosetting plastics (55%) than in thermoplastics (46%), and, based on these results and an average edge trim of 20%, the amount of natural-fibre-reinforced plastics used in the automotive industry was calculated by Karus et al. (2006) to have been around 30 000 t in 2005.

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In small quantities, suppliers also produce composites for other industries, mainly with polypropylene in the form of PP–NF granulates. Successful examples are sanding discs made of an injection-moulded hemp–polypropylene composite (Carus et al., 2008). 3.3.3.4

Geo- and Agrotextiles

Geotextiles stabilise soils against wind and water erosion (see Chapter 21). Coir has also been shown to reduce the swelling behaviour of expansive soils and thus to improve their engineering properties (Sivakumar Babu et al., 2008). The varying degradability of different natural fibres can be exploited when using geotextiles. For instance, coir geotextiles need much longer to degrade than other natural fibres, which is due to their high lignin content of 40–50%. Hemp and flax fibres with a lignin content of only around 4% are, on the other hand, not suited as geotextiles in aqueous, earthy environments (Carus et al., 2008). In a study on the suitability of a coir geotextile for watershed protection in Kerala, India, Vishnudas et al. (2005) found the coir fibre to retain 19% of its original tensile strength after 9 months. This period was sufficient to stabilise the slopes of the watershed banks through natural vegetation. In a previous study, cited by Vishnudas et al. (2005), on the degradability of natural fibres in highly humid soils, coir retained 20% of its strength after 1 year, while jute degraded within 8 weeks and cotton within 6 weeks. Depending on the purpose, any time span may be preferred over the other. Geotextiles made from natural fibres are estimated to be able to cover about 5% of the market, which is currently dominated by synthetics. Information deficits on the part of decision-makers are seen as the main barrier to the exploitation of this potential (Karus et al., 2003). Agrotextiles made from natural fibres can substitute for plastic mulch for the protection of young plantings or serve as a base layer, e.g. made of hemp or flax fibre bundles, in cress production and other types of greenhouse culture. In Germany, the market share of hemp and flax fibres for garden cress cultivation is on average 70%, and even up to 90% for punneted cress. Apart from their biodegradability (the layers can be disposed of together with the plant residues), natural fibres have a number of advantages in this niche market. Under optimal conditions, the cress reaches marketability faster than with the conventional substrate perlite, the seedlings do not need additional fertiliser and production costs for the natural-fibre-based substrate are barely higher (Carus et al., 2008). 3.3.3.5

Insulation Products

Dominating raw materials on the market for insulation products are glass- and mineral-fibre products, polystyrene and polyurethane. Natural-fibre-based insulation products are typically made of ‘non-wovens’, which are textiles such as (i) felts, which are neither knitted nor woven but bonded together mechanically (by needle punching), or (ii) fleeces, which are bonded by a vinculat like binders or glues. ‘Non-wovens’ also find applications as felts for various other uses such as in shoes, blankets or filters. In the European flax industry in 2003, ‘non-wovens’ made up only 1%, while fleeces and felts for insulation products is a more significant sector for hemp, with a share of 20% in volume (EIHA, 2007; ANDI, 2005). Overall, the market for insulation products made from renewable raw materials is still small but has enjoyed success, especially in Germany, France and the UK. In 2005, renewable raw materials in total (of which about 9% were flax and hemp products) covered about 5% of the German market (Carus et al., 2008). The use of natural fibres in insulation products is described in more detail in Chapter 20. 3.3.3.6

Byproducts

As a byproduct of bast fibres in the process of extracting the fibre bundles, shives (or hurds) are retained. For most fibres, high-quality applications currently do not exist for this byproduct. In comparison, hemp

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shives are used extensively for animal bedding, in particular in the equestrian sector. Here, the high water absorptive capacity of hemp shives is beneficial. Shives make up 50–60% of the hemp stalks, and, according to EIHA (2007), 70% of European hemp shive production was used for animal bedding, 17% as construction material and the rest for gardening, boards and as an energy resource. Hemp–lime materials used in housing construction in the UK and France are a prominent and successful example (Carus et al., 2008).

3.3.4

Price Development of Main Technical Natural Fibres

World market prices are available, for example, for jute, sisal, abac´a and coir. For flax and hemp, European price indices are shown that have been surveyed on a monthly basis by the nova-Institut, H¨urth, Germany, since March 2003. Figure 3.3.3 shows these prices as monthly indices, together with an index of wheat prices in order better to facilitate a comparison, choosing March 2003 as the base. As a general trend, prices of natural fibres fell slightly from 2000 to the end of 2003. Only with rising oil prices and increasing demand for natural fibres, especially from China, did prices increase again from 2004 onwards, in particular for jute and sisal, which approximately doubled, and for abac´a, which increased in the second half of 2008 to almost 250% compared with March 2003. Owing to its excellent technical properties, demand for abac´a surged on world markets. Jute dominates all other technical natural fibres (apart from cotton) in terms of quantity, so the jute price is a significant determinant for the prices of the other technical fibres. Price increases for jute and sisal followed the wheat price explosion in spring 2007 (reaching a peak of $US 440/t in March 2008), with a certain lag, but have still been relatively modest by comparison. The stronger correlation with the wheat price and, more directly, with the oil price can be explained by the fact that, for sisal and jute, traditional applications such as sacking and bags dominate, which are substitutes for oil-based (e.g. polypropylene) products (Carus et al., 2008). By comparison, price increases for hemp and flax have been very stable and lie in the 5–10% range.

Figure 3.3.3 Price indices for natural fibres and wheat (Sources: FAO, 2008, nova-Institut, 2008, and World Bank, 2009). Flax and hemp: European technical short fibre bundles (EUR = euros). Wheat, jute, coir, sisal and abaca: ´ world market prices (USD = US dollars).

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Figure 3.3.4 Normal distributions of natural fibre prices (in $US/t) (Sources: FAO (2008), nova-Institut (2008) and World Bank (2009). Flax and hemp: European technical short fibre bundles. Wheat, jute, coir, sisal and abaca: ´ world market prices).

Coir prices are only available from Sri Lanka until August 2008 for the three qualities bristle, twisted and mattress coir. For the price index, the twisted fibre bundles price is used, as it lies approximately between the mattress coir and the bristle coir prices. The price for bristle coir (around $US 450/t) is about 100% higher than for mattress coir and 30% higher than for twisted fibre bundles. Over the years, coir prices have been relatively stable and even decreased, especially in the case of bristle coir, but have shown high volatility in the short term. The main reason is that coir is in excess supply, so that any changes in demand can in principle easily be met by increasing the rate of utilisation without the need to expand cultivated areas. Figure 3.3.3 also shows that abac´a, jute and sisal have followed the rapid decline in prices since the second half of 2008, while the development of coir, flax and hemp is, as expected, less volatile. However, it must be stressed that the prices cannot be directly compared, as these are European prices for hemp and flax and world market prices (f.o.b. prices at the major ports of the producing countries) for the other fibres. Figure 3.3.4 displays the normal distributions of monthly fibre prices since January 2000 (and since March 2003 for hemp and flax). From this visualisation it becomes clear that in fact abac´a tends to be the most expensive of the fibres described in this chapter. Its mean price (of grade ‘G’, which stands for good quality and lies in between ‘S2’, which stands for excellent, and ‘JK’, which stands for fair) is three times higher than that of jute and coir and almost twice as high as that of hemp and flax. This situation reflects the fact that abac´a has unique properties that make it a fibre used mainly in premium products such as speciality paper and currencies. Furthermore, abac´a prices are characterised by a high standard deviation, which is caused by the steep surge and decline in prices from 2008 to 2009. By contrast, coir shows the smallest standard deviation and lies in about the same price range as jute.2 A comparison of actual procurement costs for natural fibres in Europe at one point in time provides Figure 3.3.5, which shows price ranges for hemp, flax, jute, sisal and coir used in the German automotive industry, as reported from purchase contracts at the beginning of 2007. It confirms that coir was cheapest, not least

2 Flax and hemp prices, which are surveyed on €/t basis, are converted to $US/t to facilitate the comparison in Figure 3.3.4. On the €/t basis, flax and hemp show the smallest standard deviation.

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Figure 3.3.5 Procurement prices for technical short fibres, summer 2007 (qualities suited for use in the automotive industry); adapted from Mussig and Carus (2007). ¨

because its profile does not compete with other natural fibres and does not play a role in composites (except for high-quality car seats).

3.3.5 Future Trends The year 2009 was the International Year of Natural Fibres (IYNF), as proclaimed by the UN/FAO. Its specific objectives were to ‘promote the efficiency and sustainability of the natural fibres industries; encourage appropriate policy responses from governments to the problems faced by natural fibre industries; foster an effective and enduring international partnership among the various natural fibres industries’ (FAO, 2009). Currently, there is no single organisation or association of international organisations representing the interests of natural fibres as a whole. A closer cooperation between the numerous organisations of the different natural fibres is therefore a more direct expected outcome of the IYNF (Moir and Plastina, 2009). In fact, the natural fibres described in this chapter only compete partly among each other, as each has its own particular applications and niche markets. For jute, the technical textile market (no clothing textiles and fleeces and felts) is very dominant, so that the overlap with flax and hemp, which have a higher competitive advantage in the market for clothing textiles and insulation material, is quite limited. Only with regard to the small market for composites does jute constitute a relevant competitor, especially as the bast fibres – flax, hemp, jute and kenaf – are easily substitutable in this sector. Sisal, on the other hand, with its long, coarse leaf fibre bundles, is only applicable for composites after appropriate conditioning. The market for speciality paper, again, traditionally prefers certain fibres, e.g. abac´a for tea bags, cotton linters for currency and flax and hemp for cigarette paper. In the case of geotextiles, different natural fibres are suitable for different applications, depending on their degradability – determined mainly by their lignin content – so that also in this market each of the fibres has its particular edge. Another interesting technical development is the combination of natural fibres with biopolymers in naturalfibre-reinforced plastics, e.g. based on kenaf in the automotive industry, electronic products and other biobased industry materials in Japan (Serizawa et al., 2006; Nishimura, 2006). Natural-fibre-reinforced biopolymers often exhibit improved properties at lower prices. As in other sectors of the global economy, China has come to be a major player in the world fibre markets, and shifts in its domestic market and politically set priorities have enormous impacts elsewhere. In particular, China’s textile industry demands a high share of the world natural fibre market. For example, in 2005, 49%

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of global flax imports went to China, even though the country itself is already the largest producer. The 10% reduction in cotton milling in China – in the wake of the financial crisis – in 2008/09 compared with the previous year caused turbulence on the world market. Recently, announced plans to expand industrial hemp cultivation from an estimated 20 000 ha to as much as 1.3 million ha in order to reduce dependency on cotton have highlighted the country’s ambition to play an important role in natural fibre markets in the future (FAO, 2009). Furthermore, the expansion of hemp in China is intended to take place in hilly, less fertile regions, which is hoped to free some of the cotton-planted areas for food production. There is still much scope for further development of the technical applications of natural fibres, but the most pressing need for improvements and innovation potential is seen by the FAO (2009) to be in cultivation and processing techniques. For example, the technique for the extraction of sisal fibre bundles has not changed much in the last 100 years. Such improvements will need investments, however, and these will only take place in favourable market conditions. Another recent development that could foster such innovations is that the international trading structures have become less dependent on intermediaries owing to Internet and email communication and cheap international flights. This has made it easier for market partners to communicate their demands and steer the production accordingly. Compared with food and feed crops, yield increases of most natural fibres have been small in the past. While average wheat yield is today at a level of 250% compared to the early 1960s, jute lies at about 150%, abac´a at 130% and sisal has basically experienced no yield increases at all (FAOSTAT, 2009). Breeding efforts to increase fibre yields should therefore be undertaken. A favourable political framework could help biomaterials, and, among these, materials based on natural fibres are experiencing considerable growth. For example, forced measures for a reduction in CO2 emissions must be mentioned here. In this sector, natural fibres can score particularly well – their production is 10 times less energy intensive than the production of glass fibres (Carus et al., 2008). Furthermore, by far the largest share of natural fibres is produced in developing countries, providing the major source of income for millions of smallholder farmers. According to Datta (2007), the Indian jute industry supports four million farmers, 250 000 workers and about 1 million traders. The significance of these industries for maintaining a large number of livelihoods makes clear that governments and NGOs have a keen interest in maintaining their market shares. A prominent example of protectionism is the mandatory use of jute in food-grade bags in India (Dey, 2005). At present, however, most natural fibres face a difficult future, as the FAO (2009) states. In Europe in particular, flax and hemp depend on processing subsidies, which will be phased out by 2013, and need to be established in non-traditional segments such as paper and reinforced plastic composites.

3.3.6

Conclusions

This chapter has given an overview of current natural fibre production and market prospects. Evaluating the available data suggests that the most promising prospects for natural fibres lie in relatively new applications such as in insulation products, composites and geotextiles. Traditional products like yarns, twines and cloth still make up the highest turnover for most natural fibres such as jute, sisal, flax and coir, but no significant growth can be expected in these markets. However, as cotton is currently under enormous pressure for various reasons (see Section 3.3.2.1), certain market shifts are likely to take place in the quest for alternative natural fibres. Some natural fibres already benefit from quite stable demand in niche markets, like abac´a, while others, like kenaf, are just on the verge of conquering new markets. With the rapid decline in the crude oil price in 2008 and 2009, the discussion about the need to shift the industrial raw material base away from oil has cooled down, but there is evidence that the oil price will rise again as soon as the world economy regains momentum, because of insufficient investments in technology and new explorations (IEA, 2008). When this will take place and how drastic price rises will be is uncertain, but in the long run there is no alternative to an industry based on renewable resources and biomaterials.

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Together with wood, other plant resources and new biopolymers, natural fibres will have to find their place in the ongoing and dynamic shift towards renewable raw materials.

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Mehrotra, N.K., Kumar, S. and Anthony, M. (1988) Carcinogenic property of JBO(P) variety of jute batching oil. Drug Chem. Toxic., 11(2), 181–193. Moir, B. (2006) FAO – the world sisal economy, in Brazilian Sisal, ed. by Andrade, W., Rodrigues, R., Wagner, J. and Quir´os, J. Sindifibras (Sindicato das Ind´ustrias de Fibras Vegetais do Estado da Bahia), Brazil. Moir, B. and Plastina, A. (2009) 2009 International Year of Natural Fibers. Cotton Promotion Bull., 20. M¨ussig, J. and Carus, M. (2007) Bio-Polymerwerkstoffe sowie holz- und naturfaserverst¨arkte Kunststoffe, in Marktanalyse – Nachwachsende Rohstoffe Teil II. FNR, G¨ulzow, Germany. Nishimura, T. (2006) Development of car components using kenaf and a new evolution in biomaterials (in Japanese, abstract in English). J. Soc. Automot. Engrs Jap., 60(1), 100–104. ¨ nova-Institut (2004) Marktreife von PP-NF-Spritzguss – Uberblick u¨ ber die PP-NF-Spritzguss-Technologie und ihre Eigenschaften. nova-Institut, H¨urth, Germany. nova-Institut (2008) Regular census of hemp and flax fibre prices in the German market by researchers of the nova-Institut. nova-Institut, H¨urth, Germany. Paulitsch, K., Baedeker, C. and Burdick, B. (2004) Am Beispiel Baumwolle: Fl¨achennutzungskonkurrenz durch exportorientierte Landwirtschaft. Wuppertal Institut f¨ur Klima, Umwelt, Energie, Wuppertal, Germany. PTL (Plastics Technology Laboratories, Inc.) (2005) Fogging characteristics of interior automotive materials; available at: www.ptli.com/testlopedia/tests/Fogging SAE.asp (accessed 10 May 2005). Schnegelsberg, G. (1996) Was ist Hanf? Ein Beitrag zur begrifflichen Kl¨arung, in Hanf & Co.: Die Renaissance der heimischen Faserpflanzen, 2nd edition, ed. by Waskow, F. Die Werkstatt, G¨ottingen, Germany, pp. 205–215. Serizawa, S., Inoue, K. and Iji, M. (2006) Kenaf-fiber-reinforced poly (lactic acid) used for electronic products. J. Appl. Polym. Sci., 100(1), 618–624. Sivakumar Babu, G.L., Vasudevan, A.K. and Sayida, M.K. (2008) Use of coir fibers for improving the engineering properties of expansive soils. J. Nat. Fibr., 5(1), 61–75. USDA (United States Department of Agriculture) (2009) Foreign Agricultural Service – Production, Supply and Distribution Online; available at: http://www.fas.usda.gov/psdonline/ psdHome.aspx (accessed 28 April 2009). Vishnudas, S., Savenije, H.H.G., van der Zaag, P., Anil, K.R., and Balan, K. (2005) Experimental study using coir geotextiles in watershed management. Hydrol. Earth Syst. Sci. Discuss., 2, 2327–2348. Wilson, A. (2008) Processors of European flax and hemp fibres need value-added alternatives to conventional textiles. Are composites the answer? (June 2008); available at: http://textile.2456.com/eng/epub/n details.asp?e=1&epubiid= 4&id=2775 (accessed 28 April 2009). World Bank (2009) Commodity price data; available at: http://econ.worldbank.org/WBSITE/EXTERNAL/EXTDEC/ EXTDECPROSPECTS/0,,contentMDK:21148472∼menuPK:556802∼pagePK:64165401∼piPK:64165026∼theSite PK: 476883,00.html (accessed 16 April 2009).

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PART II VEGETABLE FIBRES

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4 Flax – Structure, Chemistry, Retting and Processing Danny E. Akin Athens, Georgia, USA

4.1 Introduction 4.1.1

Flax for Fibre and Oilseed

The scientific name of flax (Linum usitatissimum L.) is literally translated as ‘linen most useful’ (Borland, 2002). This designation is supported by the fact that flax supplies high-quality fibre for textiles and other applications, as well as seeds and oil for industrial and nutraceutical purposes. Furthermore, this claim is borne out by the crop’s production in a variety of climates throughout the world. Flax is a temperate weather crop, generally cultivated in areas where the daily temperature remains below 30 ◦ C (Sultana, 1992a). Typically, in regions for high-quality fibre production in western Europe, flax is planted in March to April for harvest in mid-July to August (Sultana, 1992a). In warm climates, flax is a winter crop, with seeds sown in November for a May harvest (El-Hariri, 1994; Frederick et al., 1993; Foulk et al., 2002). Production of flax is environmentally friendly in that few chemicals are required for crop production. In a life cycle analysis study, the impacts of the western European flax scenario (dew retting) and central European hemp scenario (warm water retting) were similar, except that pesticide use was higher for flax and water use during processing was higher for hemp (van der Werf and Turunen, 2008). The important aspect of fibre/yarn quality, however, was not considered in this study along with the environmental impacts. In traditional linen production, flax is rotated among fields to reduce fungal pathogens, often with a 7 year rotation. Flax is used in a rotation system with grasses or vegetable crops, which allows for pathogen reduction or use of various kinds of herbicide to control unwanted plants. For fibre production, seeds from high-fibre varieties are densely sown to give a final plant density of about 2000 plants per square metre (Sultana, 1992a). Planting in this way, and harvesting before full seed maturity, produces thin-stemmed, straight and tall plants that provide fibre of high yield and excellent properties. Fibre yields, as well as quality, vary with cultivar, environment and agronomic practices, but total fibre yields of 25–30% of straw dry mass are possible (Stephens, 1997). Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications  C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Figure 4.1 Photograph of a retted flax stem, showing bast fibres (bf) as they separate (↓) from inner, core tissues that make up shive (s) in processing waste.

In contrast to flax fibre production, linseed varieties are sown in low densities (about 750 plants per square metre) to maximise branching for greater seed production. When harvested at full seed maturity, linseed plants often have thick stems that are low in fibre yield and quality. The idea of dual use for flax, i.e. fibre and seed, has been explored, and certain varieties do provide a balance. Flax is a bast fibre plant, a category that includes other industrially important plants such as hemp, kenaf and ramie (Sultana, 1992a; van Dam et al., 1994; Sharma and Van Sumere, 1992a) (see Chapter 2.3). The industrially important bast fibres are produced in the plant stalks or stems and are freed from non-fibre tissues by a process termed retting (Figure 4.1). The structure and the chemistry of the flax stalk influences retting quality, fibre properties and ultimately the end-uses.

4.2

Structure of Flax Stems

The anatomy of flax is shown in Figure 4.2. The outermost layer, the cuticle, is a thin waxy layer that affords protection against pathogen entry and water loss. A single layer of epidermis is just under the cuticle. The cuticle and epidermis maintain a close association and constitute a contaminant if this fraction remains

Figure 4.2 Light micrograph of a cross-section of a flax stem, showing the arrangement of structures: c = cuticle, e = epidermis, f = fibres developing in bundles, ↓ = cambium, s = shive, which comprises the innermost, lignified vascular tissues. Adapted from D.E. Akin, Plant cell wall aromatics: influence on degradation of biomass, Biofuels, Bioproducts, and Biorefining, 2, 288–303, 2008.

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Figure 4.3 Polarised light micrograph showing the birefringence of flax fibres and the intact structure after incubation with a pectinolytic enzyme. Multiple nodes (↓) are observed perpendicular to the fibre axis. Reproduced from J.A. Foulk, D.E. Akin and R.B. Dodd, Influence of pectinolytic enzymes on retting effectiveness and resultant fiber properties, Bioresources, 3, 155–169, 2008.

with the fibre after processing. Thin-walled parenchyma cells separate the epidermis from the bast fibres. The industrially important bast fibres are long, specialised, cellulose-rich fibre cells that develop in bundles in the cortex region, which is between the outermost cuticularised epidermis and the innermost, woody, core cells. Where fibre bundles are discontinuous, parenchyma cells exist. A thin cambium layer separates fibre bundles from the woody core cells. Botanically, the cambium produces secondary tissues for support and conduction in plants (Stern et al., 2003). The central, core tissues comprise lignified woody cells, providing support and conduction. These highly lignified cells constitute primarily the ‘shive’ fraction, which is a main contaminant of processed fibre that is produced during cleaning. Bast fibres exist in bundles of ultimate, or individual, fibres (Figure 4.2) that form a ring, which in turn encircles the innermost, lignified, core tissues. About 20–50 bundles form in cross-sections of flax stems, with 10–40 spindle-shaped ultimate fibres of 2–3 cm length and 15–20 µm diameter per bundle (Hamilton, 1986; Van Sumere, 1992). Fibres vary in length with position on the stem. Microscopically, the separated fibres and fibre bundles appear stiff and brittle, having little elongation (Figure 4.3). Nodes, or ‘fibrenodes’ (Khalili et al., 2002), are dislocations perpendicular to the fibre axis that appear as horizontal bands in the fibres and bundles and are easily recognised (Figure 4.3, ↓). These dislocations, whose origin is not fully understood, are regions where moisture, dyes and enzymes more easily penetrate and influence fibre properties (BuschleDiller et al., 1994; Focher et al., 1992; Peters, 1963). They also represent weak points in the fibres and appear to be the point of fracture in strength tests, giving a blunt and distinct appearance at the fibre breaks. Kink bands, whose appearance is similar to nodes, arise from processing methods or other physical means and have been implicated in failures of compression tests (Bos et al., 2002). Flax variety, climate and production practices influence the stem and fibre anatomies.

4.3

Chemistry

The chemical composition and the location of constituents within the flax stem define properties and applications of flax. Furthermore, these chemical entities influence processing efficiency (including retting) and ultimate quality of the bast fibres. From a general perspective, flax bast fibre is rich in cellulose, and shive material is rich in aromatics (Table 4.1) (Akin et al., 1996; Akin et al., 1997). Therefore, in simplest form, the cellulosic fibres find application in textiles, reinforcement of composites and paper/pulp. The shive, which is a byproduct of fibre processing, has value, however, in the overall processing system, finding application in low-value uses typical of lignocelluloses, such as animal bedding, mulch or burning. These additional sources of revenue provided by selling the shive are essential to guarantee competitive fibre prices and provide a positive economic position of the flax processing facility.

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Table 4.1

Chemical composition of flax fractionsa

Plant Fraction

Retting condition

Uronic acids in mg/g

Shives

Unretted Dew retted Unretted Dew-retted

NDb NDb 0.21 0.08

Fibre

Non-cellulosic polysaccharides in mg/g

Glucose in mg/g

Aromatics in mg/g

179 158 104 94

277 247 434 650

18 13 7 Trace

a

Commercial cv. ‘Ariane’ flax stems supplied by Van De Bilt Zaden, Sluiskil BV, The Netherlands. Not determined. Data adapted from J. Sci. Food Agri. 72: 155–165 (1996) and Textile Res. J. 67, 279–287 (1997). b

4.3.1

Waxes and Related Compounds

The cuticle of the stem (Figure 4.2, c) contains waxes, cutin and aromatics which provide a protective barrier to plants (Stern et al., 2003; Akin et al., 1996; Morrison III, 1999a) The cuticle closely associates with the epidermis, and the chemical nature of this cuticularised epidermal fragment constitutes a rigid and formidable structure that prevents water loss, limits entry of pathogens and restricts fibre separation (retting) by microorganisms. Underneath the cuticularised epidermis, the presence of a pectin-rich layer is more amenable to microbial and enzymatic activity (see below), thus allowing removal of the cuticular barrier to internal tissues. The waxy nature of the cuticle is shown histochemically with oil red, which has been shown to indicate the presence of waxes in a variety of cuticular layers (Akin et al., 2004b; Achwal and Roy, 1985). Cotton, another natural cellulosic fibre known to have a waxy surface, stains red with oil red along the fibre axis, with heavier staining near the site of attachment to the seed coat. Flax fibres, however, do not appear to contain this heavy waxy layer (Akin et al., 2004b), allowing oil red to be used as a visible indicator of cuticle contamination in cleaned fibre. Analytical studies of fibres manually separated and cleaned of all other tissues confirmed the presence of low levels of waxes, cutins and sterols, with amounts of about 0.2% of fibre dry mass and one-twentieth or less of levels in the cuticularised epidermis (Morrison III and Akin, 2001). 4.3.2

Pectins and Related Compounds in Fibres

Parenchyma, cambium and the middle lamella-binding fibres in the bundles are rich in pectins, hemicelluloses and other matrix polysaccharides, as shown by the response of these tissues to pectinolytic enzymes (Akin et al., 1997). The separation of fibres from the woody core occurs at the cambium and is facilitated when stems have been stored in dry climates for an extended time. It is the pectin-containing regions that are of prime importance in retting, with their degradation resulting in tissue separation and freeing of cellulosic fibres from the cuticularised epidermis and woody shive. Pectin is a complex and diverse sugar component of many plant cell walls and plant tissues (see Chapter 2.1.4). Pectin is strategically located and binds cell walls, like cement for bricks, within plants. While pectin is, therefore, particularly important in maintaining the structure of flax stems, its degradation is of fundamental importance for retting and resulting quality of flax fibres (Van Sumere, 1992). Chemically, pectin is a heteropolysaccharide consisting mainly of 1,4-linked α-d-galacturonic acid, with various degrees of methylesterification at the carboxyl position and with various attached side chains (Sakai et al., 1993). In some cases, pectin in primary plant cell walls may have a high proportion of oligosaccharide chains on the backbone and longer chains than the pectin in the middle lamellae (Sakai et al., 1993). A rhamnogalacturonan structure of type I pectin, which is a prominent form in plants, probably forms the backbone of the high-molecular-weight polysaccharides in flax fibre, as shown by nuclear magnetic resonance spectrometry (Davis et al., 1990). The degree of substitution and the presence of side chains to the backbone of pectin

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Inductive coupling plasma (ICP) emission spectroscopy of calcium concentrations in flax

Flax sample

Tissue type (manually separated)

Ariane fibre type

Total intact bast tissue Epidermis/cuticle (outermost tissue) Bast fibres

Calcium amount of plant dry mass in mmol/kg 68.0 ± 17.9 bc 299.3 ± 26.5 a 54.3 ± 10.1 c

Different lower-case letters (a–c) within columns differ at P < 0.5. Data adapted from J. Nat. Fibres, 1, 21–47 (2004).

molecules influence enzymatic degradation and the types of enzymes required to break bonds and degrade pectin. Regarding retting of flax, pectin degradation was reported to be faster in flax harvested during flowering than in mature flax stems, and a residual pectin level of 7–10 g/kg remained after retting (Meijer et al., 1995). A study of mapping with mid-infrared microspectroscopy of different varieties of mature flax fibre confirmed that pectin types varied among plant types and regions (Himmelsbach et al., 1998), with the potential to influence retting efficiency. Immunocytochemical staining methods, using gold-labelled antibodies against specific pectin structures, provide further indications of variations in sites of specific pectin types within areas and even layers of flax fibres (Andeme-Onzighi et al., 2000; His et al., 2001). Non-methoxylated carboxyl groups on galacturonic acid, a major component of pectins, are often crosslinked by Ca++ or other cations that form stable bridges across pectin molecules (Sakai et al., 1993). Inductive coupling plasma (ICP) emission spectrometry showed that calcium levels in the various cuticularised epidermis tissues were 5.5-fold greater than in the fibres of ‘Ariane’ flax (Table 4.2) (Akin et al., 2004a). Both non-methoxylated pectin and calcium levels are higher in the epidermal regions of the flax stem and lower in the fibres (Jauneau et al., 1997). High amounts of calcium in the rigid cuticularised epidermis further stabilise an already formidable barrier to retting in flax stalks. Endopolygalacturonase, a pectinolytic enzyme that is present in many enzyme mixtures, was reportedly inhibited by steric hindrance through calcium linkages in pectin (Jauneau et al., 1994; Rihouey et al., 1995). Specific determination of pectin content in flax is difficult owing to several factors including complexity of pectin structure, different analytical methods, variations among cultivars, positions on the stem and fibre quality. Pectin content in decorticated flax was reported to be 20.5% (Ansari et al., 1990). The content of pectic substances of cell walls for various flax cultivars ranged from 26 to 34% (Brown et al., 1986). Based on hyrolysis with dilute hydrochloric acid followed by ammonium citrate, the pectin content of flax fibres was reported to be only 1.6% (Bochek et al., 2002).

4.3.3

Bast Cellulosic Fibres

The commercially important flax bast fibres primarily are comprised of cellulose, but pectins, hemicellulose and phenolic compounds are also present (Akin et al., 1996; Akin et al., 1997). In comparison, cotton fibres are typically about 95% cellulose (Wakelyn et al., 1998), while flax has a lower percentage of cellulose (65–80%) and higher levels of pectin and hemicellulose (Focher et al., 1992). For example, in retted ‘Ariane’ flax, glucose was the predominant sugar (650 mg/g dry mass), followed by mannose (39.2 mg/g) and galactose (35.0 mg/g); rhamnose, xylose, arabinose and uronic acids were also present (Akin et al., 1996). Analyses of retted fibre showed a high level of glucose, indicative of cellulose, but also considerable amounts of mannose and galactose, suggesting a close involvement of non-cellulosic sugars in the secondary cell walls of flax fibre. Hemicellulosic constituents such as galactoglucomannans and xylose are often reported as substantial components in flax fibres (Focher, 1992; Gorshkova et al., 1996; Stewart et al., 1995). Distinguishing characteristics of flax, such as high moisture regain, may be influenced by the presence of these non-cellulosic

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carbohydrates with the cellulose. Proteins and proteoglycans are associated with flax fibre secondary walls and possibly provide structure (Girault et al., 2000). X-ray diffractometry shows a higher region of order for flax fibre than for some other natural fibres (Focher et al., 2001). Therefore, while flax fibre is primarily a cellulosic fibre, its chemistry and characteristics differ from those of cotton and many other natural fibres.

4.3.4

Lignin and Aromatics in Flax Tissues

The woody core tissues in the central region of the stem are the most highly lignified cells in flax (Akin et al., 1996). Lignin, consisting of recalcitrant compounds with a complex polyphenylpropanoid structure, is a major limitation generally to microbial degradation of plant carbohydrates (Akin, 1989; Eriksson et al., 1990). From a general perspective, lignin imparts rigidity and strength to plant tissues. Compositional analyses of flax tissues have shown that stems have high levels of aromatics, including both guaiacyl (monomethoxylated) and syringyl (dimethoxylated) groups, as well as small amounts of phenolic acids in particular cultivars (Akin et al., 1996). Guaiacyl lignin was more prevalent in some cultivars examined. These lignified core cell walls were little affected by the microorganisms during dew retting, indicating the recalcitrance of the entire lignified cell wall to fungal attack. Solid-phase 13 C nuclear magnetic resonance (NMR) spectrometry indicated that aromatic material in flax fibres was predominately an anthocyanin, rather than lignin (Love et al., 1994). Extraction of flax bast tissue with a series of organic solvents (i.e. hexane, propanol and methanol) and analysis by reverse-phase high-pressure liquid chromatography (HPLC) and 13 C NMR showed the presence of a variety of aromatic constituents including flavonoids and hydroxy-methoxy cinnamic acids (Gamble et al., 2000). The water extract from these flax samples contained a complex mixture of compounds, including sugars and aromatics representative of the type found in intact plants (Akin et al., 1996). Studies to localise sites of lignin and aromatic compounds in bast fibres, using histochemical stains (Akin et al., 1996; Gorshkova et al., 2000) and ultraviolet absorption microspectrophotometry (Akin et al., 1996), showed that these compounds occurred non-uniformly in middle lamellae between fibres, with the greatest levels in cell corners. Lignin, however, did not appear to impede fibre separation from the core cells (Akin et al., 2001b), particularly with subsequent processing to clean fibre. Heavily localised areas of aromatics that remain on retted fibre, however, could influence properties (Sharma and Van Sumere, 1992b) or reduce processing efficiency.

4.4

Retting

Flax, like other bast fibre plants, undergoes a process called retting, which is usually microbial in nature, to loosen and separate the bast fibre bundles from the non-fibre fractions of the flax stem (Figure 4.4). During retting, microorganisms colonise the stem tissues and partially degrade plant constituents to separate bast fibres from non-fibre fractions. Proper retting is a major problem in processing flax. Plant development and weather influence the quality of retting, which in turn determines both fibre yield and quality (Sharma and Van Sumere, 1992b). Underretting, i.e. incomplete degradation of matrix components (i.e. pectin and hemicelluloses), leaves woody core cells and cuticularised epidermis still associated with fibre, reducing processing efficiency and fibre quality. The resulting flax fibre bundles are coarse and contaminated with stiff shive particles. Shive constitutes the major trash component associated with flax fibres, as the stem is about 70% woody core material. In addition to shive, underretting leaves a coarse fibre bundle consisting of fragments of cuticularised epidermis bound to several fibre bundles. Amounts of cuticular fragments have been shown to be inversely related to quality of yarn and fibre (Morrison III et al., 1999b), and the cuticle is particularly problematic in retting mature or seed flax stems (Gorshkova et al., 2000). In some cases, this cuticular fragment appears more of a contaminant than shive with fibre. Considerable cleaning is required to remove shive and other trash components from the fibre for industrial uses (Sultana, 1992b). Conversely,

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Figure 4.4 Scanning electron micrograph of the cross-section of a retted flax stem, showing fibres and fibre bundles (f) separated from the cuticularised epidermis (c-e) and inner core cells that will become shive (s). Reproduced from D.E. Akin, G.R. Gamble, W.H. Morrison, L.L. Rigsby and R.B. Dodd, Chemical and structural analysis of fiber and core tissues from flax, J. Sci. Food Agri., 72, 155–165, 1996.

overretting occurs from excessive microbial degradation, where fibre strength is reduced owing to excessive thinning of bundles and/or degradation of fibre cellulose. Such fibre can be of little use in some applications, such as textiles or composites. Two primary methods for retting, namely water and dew retting, have been traditionally used to extract fibres for commercial applications.

4.4.1

Water Retting

This method reportedly produces the highest-quality flax. Water retting depends on colonisation of flax stalks and fermentation by anaerobic bacteria, e.g. Clostridium felsinium, to degrade pectins and other matrix substances and free bast fibres (Van Sumere, 1992). A study of the microbiology involved in water retting and methods to improve the process have been reported in a comprehensive book on flax (Sharma and Van Sumere, 1992a). In early times, harvested flax stalks were bundled and submerged in natural bodies of running or still water (e.g. lakes, rivers, dams) for 5–7 days and then dried in the field for 1–2 weeks. Retting pits or tanks were constructed to be flushed with an initial rinse water to remove contaminants, heated to controlled temperatures and even inoculated with specific microorganisms. Aeration of the tanks has been attempted to modify the microbial species and subsequently the anaerobic metabolism (i.e. reduce acidity and toxins to retting microorganisms). Different microbial consortia and more complete oxidation of organic materials result from aerated conditions. Water-retted stems are then sun bleached and dried naturally. Van Sumere (1992) has given a historical perspective of retting, and Sharma et al. (1992) have reviewed details of the microbiology in retting. Butyric acid and other fermentation products resulting from water retting produced a stench and polluted the waters employed in this retting process where water retting was extensively practised. In spite of the quality of fibre, water retting was largely discontinued in western Europe in the 1950s owing to high costs and the pollution arising from fermentation of the plant material. Fermentation products absorbed by the fibres during water retting also imposed an unpleasant odour (Van Sumere, 1992). Water retting has been mostly replaced by field or dew retting, but water retting is still carried out in some places (Daenekindt, 2004) and water-retted fibre is still marketed (Kozlowski, 2001).

4.4.2

Dew Retting

Dew retting is reportedly the oldest method of retting, having been used by Egyptians for millenia (Van Sumere, 1992). Even though the flax produced is of lower quality than that from water retting, lower labour costs and

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higher fibre yields make dew retting attractive and sustainable. Stems are pulled or mowed, spread in uniform and thin non-overlapping swaths and left in the field, where the moisture and temperature encourage microbial colonisation and partial degradation of flax stems. Flax plants are turned over on a regular basis to produce more uniform retting. Primarily, indigenous fungi effect dew retting, and successions of various species and groups occur during the process (Sharma and Van Sumere, 1992b). Typical saprophytic soil fungi are the major components of these consortia, including species of Aspergillus, Cladosporium, Fusarium, Rhizopus and Trichoderma (Van Sumere, 1992; Fila et al., 2001; Henriksson et al., 1997a). Secondary colonists, notably Epicoccus nigrum (Sharma et al., 1992; Fila et al., 2001; Henriksson et al., 1997a; Brown, 1984), produce the most cellulase, weaken the fibre and reduce quality (Sharma and Van Sumere, 1992b). Van Sumere (1992) noted particular fungi with retting periods, e.g. Cladosporium herbarum for summer retting, Mucor stolonifer for autumn retting and M. hiemalis for snow retting. Reports of the mycological consortia are often from the United Kingdom and western Europe, where flax has been grown over long periods of time. Possibly, other microorganisms dominate in different regions and affect various fibre parameters. Flax bales from different regions vary in colour, suggesting among other factors the possible variation in dominant retting microorganisms. In this regard, Henriksson et al. (1997a) isolated fungi from winter-grown flax that was dew retted in South Carolina, USA. The most prevalent species was Rhizopus oryzae, which in laboratory studies effectively retted flax without the loss in fibre strength noted for some other fungi (Akin et al., 1998; Henriksson et al., 1999). While problems of fermentative activity are avoided, dew retting suffers from several disadvantages (Van Sumere, 1992): 1. Since dew retting replaced water retting, overall fibre quality for the industry has reportedly been poor and, perhaps more importantly, inconsistent (Sharma and Faughey, 1999). Because of contact with the soil and the fungal growth, the fibre produced in this manner is dark (rather than light as in water-retted fibre) and very dirty. 2. Dew retting occupies agricultural fields for several weeks and is restricted to geographical regions with appropriate moisture and temperature for effective fungal growth (Van Sumere, 1992; Brown, 1984). Several geographic areas formerly of prime importance, but that lack conditions suitable for consistent fungal growth, no longer produce flax for linen. 3. In western Europe, which reportedly produces the highest-quality linen owing to a favourable climate for dew retting (Hamilton, 1986), substantial crop losses still frequently occur. Too much rain and lack of sufficient time for drying further contribute to failed harvests of flax fibre. In other regions, dry weather after harvest prevents microbial growth and, therefore, proper retting. So, while dew retting remains the method of choice for most flax fibre production, other methods are sought. 4.4.3

Stand Retting

Another method of retting in the field with indigenous fungi for the most part was attempted in the 1960s to 1970s to overcome limitations of dew retting in Northern Ireland (Brown, 1984). In these trials, glyphosate (N-phosphonomethyl glycine) was used as a preharvest desiccant to facilitate retting (Easson and Long, 1992). Stand-retted fibre pretreated with glyphosate retained more strength than dew-retted flax, although fungal colonisation and retting were slower (Sharma, 1986). Dry weather during production and harvest, however, proved problematic for use of glyphosate as an aid to retting (Van Sumere, 1992; Sharma, 1986). Recent reports indicate a continuing interest, however, in glyphosate treatment and stand retting (Goodman et al., 2002). 4.4.4

Chemical Retting

Considerable research has been undertaken to find a replacement for dew retting. Chemical retting has been evaluated using a variety of methods, including ethylenediaminetetraacetic acid (EDTA) or other chemical

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chelators at high pH, detergents, strong alkali and steam explosion (Adamsen et al., 2002a; Henriksson et al., 1998; Sharma, 1987a). Sharma (1988) patented a chemical retting method using chelating agents that removed divalent cations, such as Ca2+ , and helped separate fibre from non-fibre components. Autoclaving flax straw with the chelators EDTA and oxalate was reported to be an effective method for breeding programmes in increasing the extractability of unretted flax fibres to the level of warm-water-retted flax (van den Oever et al., 2003). Other chemical methods, sometimes combined with physical methods, have been reported to separate fibres from non-fibre components successfully. A mechanical process to produce fibre strip, followed by a chemical/cooking process under pressure, has been patented to separate fibres (Costard, 1997). Ultrasonic treatment, following decortication and opening of green flax or hemp stalks, has been used to obtain fibres from diverse sources without the use of chemicals (Zimmer and Kloss, 1995). Flash hydrolysis or steam explosion treatment, with or without impregnation before steam treatment, has been used to remove pectins and hemicelluloses from decorticated flax for small bundles and ultimate fibres (Tubach and Kessler, 1994; Sotton and Ferrari, (1989); Kessler et al., 1998). Chemical separation has resulted in successful laboratory results, but at times fibre properties are less satisfactory than those from other methods. Efforts are reported to be under way in using some of these physical and chemical methods for separating fibre.

4.4.5

Enzyme Retting

Enzyme retting has long been considered as a potential replacement for the dew retting of flax. Successful enzyme retting could provide far-reaching advantages including: (1) high- and consistent-quality flax fibre, (2) tailored properties for specific applications and (3) broadened geographic regions for production of flax and linen. Such potential for enzyme retting has prompted in-depth research projects to develop effective processes. While the presence of diverse, multiple hemicellulases reportedly contributes to effective retting, early work with water- and dew-retting microorganisms showed conclusively that pectinases were a primary requirement for effective retting (Van Sumere, 1992). Most enzyme retting projects have highlighted pectinases. A major research effort took place in Europe in the 1980s to develop enzyme retting as a replacement for dew retting to produce long fibre bundles for linen (Van Sumere, 1992). The strategy was to submerge pulled flax stems in an enzyme preparation containing pectinases, hemicellulases and cellulases and simulate water retting by replacing bacteria with cell-free enzymes. Several commercial enzyme mixtures were evaluated. A pilot-plant-scale study (Sharma, 1987b) using 80 kg of flax stems submerged in SP 249 (Novo Nordisk, Copenhagen, Denmark) at 0.3% v/v (11:1 liquid to fibre ratio, 45 ◦ C, 24 h) carried out in Europe in the 1980s produced fibres of equal yield and quality to that from water and chemical retting in the same tests. Lost fibre strength due to the continued activity of the cellulases, which are usually inherently present in supernatants of fungal growth, was a potential problem. Oxidising agents, such as sodium hypochlorite, or reagents giving a high pH were used to denature the enzymes and prevent the continuing activity of the cellulases present in the mixture. Flaxzyme, a commercial enzyme mixture developed from Aspergillus species (Novo Nordisk, Copenhagen, Denmark) (Sharma and Van Sumere, 1992b), produced fibre with yield, strength and fineness equal to water-retted fibre (Van Sumere and Sharma, 1991). In addition to Flaxzyme, Lyvelin (Lyven, Caen, France), which is reportedly a pectinase from Aspergillus niger, is marketed especially for the retting of flax. In spite of positive results for flax retting from this work (Van Sumere, 1992), a commercial process for enzyme retting was not established. Flaxzyme, by this name, is no longer available. The importance of flax and linen to the US textile market in the 1990s led to a research project on enzyme retting that was initiated by the Agricultural Research Service, US Department of Agriculture. Results from the previous project in Europe served as a basis for the US project, but some differences were incorporated to reduce costs, test other enzymes and expand the sources of flax. Because the US textile industry is based on staple fibres (primarily cotton), the production of traditional long-fibre bundles, or longitudinal flax, was not the primary goal as in the European work. Total bast fibre content, particularly fibre from linseed straw, was investigated, as North America is a major grower of linseed. Existing enzyme products, new enzymes

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and enzyme/chemical formulations were evaluated. Initially, retting was evaluated by microscopic means and the in vitro Frieds test for fibre separation from core cells (Van Sumere, 1992). Methods of cleaning and evaluating flax fibre did not exist in the USA at that time. In order to carry out the project, a flax fibre pilot plant based on the ‘unified line’ (Ceskomoravsky len, Humpolec, Czech Republic) for total flax fibre was designed for about 5–20 kg samples, set up at the ARS-USDA research station in Clemson, SC, and tested with enzyme-retted and dew-retted flax (Akin et al., 2005). Fibre properties were characterised using existing and modified methods (primarily from the cotton industry), and some fibre quality standards were developed. The methods primarily used to compare fibres produced by different enzymes or retting conditions included: fine fibre yield (pilot-plant cleaning plus one pass through the Shirley Analyzer, SDL Atlas LLC, Charlotte, NC), fineness (airflow method), elongation and strength (Stelometer, SDL Atlas Ltd, Stockport, UK), (see Chapter 13.6.3) and shive level (near-infrared method) (see Chapter 18.5.4). Several available pectinase-rich enzyme mixtures, which are generally supernatants from plant-cell-walldegrading fungi, were initially tested for retting efficiency (Akin et al., 1997). Viscozyme L (Novozymes North America, Inc., Franklinton, NC), reported to be similar to Flaxzyme, proved to be an effective retting enzyme. A related product, SP 249 (Novozymes North America, Inc., Franklinton, NC), was equally effective and probably was similar to Novozym 249 (Novo Nordisk, Copenhagen, Denmark) used by Sharma (1987b). Both of these enzyme mixtures separated fibre from non-fibre materials, as shown by microscopy and by the Fried test, which assesses the degree of fibre separation by comparison with standard images (Van Sumere, 1992). Chelators such as EDTA or oxalic acid in formulations with enzymes reduced the level of enzyme required for effective retting by about 50-fold using the Fried test (Henriksson et al., 1997b). Ca2+ is thought to play a major role in stabilising the pectic polymers (Goldberg et al., 1996). EDTA was more effective than oxalic acid in enzyme retting, as indicated by the Fried test (Henriksson, 1997b). In a comparison of chelators of various chemical types (e.g. polyphosphates, phosphonic acids and aminopolycarboxylic acids), EDTA was the most effective chelator for binding Ca2+ , even at a pH of 5–6, which is required for certain pectinases like polygalacturonases (Adamsen et al., 2002a; Adamsen et al., 2002b). The loss of calcium per se from stems, however, is not an indicator of retting efficiency, as the stronger, dew-retted flax had significantly higher calcium amounts than enzyme–chelator-retted flax (Akin et al., 2004a). Through laboratory and small pilot-scale (10 kg) tests, a ‘spray enzyme retting’ (SER) method was developed using enzymes and chelators. The features of this protocol that vary from previous methods are: (1) physical crimping of flax stems to disrupt the cuticle barrier, (2) inclusion of chelators with enzymes at pH 5 or 6 (depending on the enzyme employed) in specific formulations from 0.05 to 0.3% of product as supplied and (3) spraying of formulations (or, later, briefly soaking for 2 min) to saturate stems at a low liquid:fibre ratio of about 2–3:1. Crimping of stems physically to disrupt barriers by passing them through fluted rollers was more effective for increasing liquid absorbance than either increasing atmospheric pressure of soaked stalks to 310 kPa or imposing a vacuum to about 88 kPa (Foulk et al., 2001). A series of samples, both fibre and linseed flax, was spray enzyme retted and commercially cleaned using the unified line (Ceskomoravsky len, Humpolec, Czech Republic), and the fibres were evaluated by textile fibre tests (Table 4.3) (Akin et al., 2001a). Enzyme retting in these first tests produced fibres/fibre bundles from both fibre and linseed flax, showed the value of cotton testing equipment for characterisation of fibre properties and revealed loss of strength with higher enzyme levels. The commercial products Viscozyme L and Mayoquest 200 (Lynx Chemical Group, LLC, Dalton, GA), which contained about 36–38% EDTA, comprised the formulation used more or less as the ‘standard’ in the US project for other comparisons. A series of studies was carried out using enzyme levels of 0.05–0.3% and chelator levels of 0.4–1.8%, with fibre yield and properties as criteria for quality (Table 4.4) (Akin et al., 2002a). Stelometer strength was inversely related to enzyme level, as shown before, and not affected by chelator level. Fibre bundle was finer, by airflow tests, with higher enzyme levels, and within a single enzyme level a higher chelator amount tended to produce finer fibre bundles. The cuticularised epidermis, which contains a high level of calcium-bound pectins, was probably degraded from the fibres more effectively with the addition of chelator to the enzyme. The combination of enzyme and chelator retted flax most effectively,

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Flax – Structure, Chemistry, Retting and Processing Table 4.3

Properties of spray-enzyme-retted and commercially cleaned seed and fibre flaxa

Sample

Retting treatment

Seed flax Fibre flax Fibre flax Fibre flax

Enzyme (0.05)–EDTA Enzyme (0.05)–EDTA Enzyme (0.3)–EDTA Dew-retted

Fineness in air flow

Strength in g/tex

6.0 5.7 4.6 5.3

19.6 20.9 15.8 36.2

99

Elongation in % Length in UQL Fine fibre in % 1.7 2.0 1.8 2.3

1.2 1.4 1.2 1.3

23.6 37.9 58.7 43.0

a Experimentally spray enzyme retted with Viscozyme L and EDTA and cleaned through the commercial unified line decortication and La Roche cottonising system at Ceskomoravsky len, Czech Republic. Properties analysed at the Cotton Quality Research Station, ARS-USDA, Clemson, SC, using standard or modified cotton methods as follows: fineness by modified airflow method, strength and elongation by Stelometer (collective of flax bundles and fibre), length by array method. UQL is the upper quartile length. Fine fibre yield is the amount of fibre obtained by passing cleaned, cottonised fibre through the Shirley Analyzer (SDL America, Inc., Charlotte, NC). Data adapted from J. Biotechnol. 89, 193–203 (2001).

based on fine fibre yield and other properties. Furthermore, these results indicated that variations were imparted to fibres with various retting formulations. The use of a commercial cleaning system integrated in the retting system is required, however, to determine ultimate yield and fully compare costs and properties, and to optimise enzyme retting for commercialisation. Many commercial, pectinase-rich enzymes, e.g. Viscozyme and Lyvelin, which contain a highly active fungal endopolygalacturonase, are available for retting. However, fungal supernatants generally contain cellulases as a component of the consortia of enzymes to degrade plant cell walls (Foulk et al., 2008). The nodes of bast fibres in flax are particularly sensitive to attack by cellulases, resulting in a weakened fibre (Foulk et al., 2008) (Figure 4.5). Experimental work showed that polygalacturonase enzyme alone was sufficient to ret flax fibres, without any added benefit from cellulases, hemicellulases or other plant-cell-wall-degrading enzymes generally present in fungal supernatants (Akin et al., 2004a; Evans et al., 2002; Zhang et al., 2000; Akin et al., 2002b). If availability and cost were not issues, the use of pure pectinases for retting would maintain fibre strength. Furthermore, with specific knowledge of the composition of the enzyme mixture, enzyme retting could be used to tailor fibres/fibre bundles with particular properties, such as strength and fineness, and for specific applications. Several mixed and pure enzymes were tested in SER studies, and fibre properties determined Table 4.4

Spray enzyme retting with Viscozyme L and Mayoquest 200 at various levels and properties of flax fibre

Enzyme/chelatora in % 0/0 0.05/0.4 0.05/0.7 0.05/1.8 0.1/0.4 0.1/0.7 0.1/1.8 0.2/0.4 0.2/0.7 0.2/1.8 0.3/0.4 0.3/0.7 0.3/1.8 a

Fine fibre yieldb in % 4.3 ± 1.7 f 5.4 ± 2.2 ef 7.0 ± 1.8 bcde 8.5 ± 0.6 abc 6.2 ± 1.3 def 7.3 ± 1.8 bcde 7.9 ± 1.2 abcd 6.7 ± 0.9 cde 7.9 ± 1.3 abcd 8.9 ± 2.1 ab 5.5 ± 0.9 ef 7.3 ± 1.1 bcde 9.8 ± 0.8 a

Strengthc in g/tex 26.9 ± 0.8 a 24.0 ± 1.4 abc 23.9 ± 5.5 abc 24.6 ± 2.3 ab 20.3 ± 2.5 bcd 17.9 ± 2.3 de 20.3 ± 1.8 bcd 18.1 ± 0.6 de 17.6 ± 0 de 17.7 ± 1.4 de 15.3 ± 0.5 e 18.1 ± 1.3 de 19.5 ± 0.7 cde

Finenessd in airflow 8.0 ± 0 a 7.7 ± 0.1 ab 7.9 ± 0 ab 7.7 ± 0.1 abc 7.6 ± 0.1 abc 7.6 ± 0.1 abc 7.1 ± 0 cde 7.4 ± 0.1 bcde 7.0 ± 0.5 de 6.9 ± 0.4 e 7.5 ± 0.7 abcd 6.9 ± 0.1 e 6.9 ± 0 e

Viscozyme and Mayoquest 200 (ca 38% EDTA) as percentage of commercially available product. Enzyme-retted straw passed twice through a hand card and once through the Shirley Analyzer. c Two replicates of Shirley-cleaned fibre, each an average of six tests by Stelometer (collective of flax bundles and fibre). d Two replicates of Shirley-cleaned fibre by airflow using approximately 5 g. Values within columns followed by different lower-case letters differ at P < 0.05. Data adapted from Textile Res. J. 72, 510–514 (2002). b

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Figure 4.5 Polarised light micrograph of the partial degradation of fibres after incubation with Viscozyme, showing attack at the fibrenodes (↓) by the cellulase component in the enzyme mixture. Attack and degradation at nodes of fibres resembles that by cellulase. Reproduced from J.A. Foulk, D.E. Akin and R.B. Dodd, Influence of pectinolytic enzymes on retting effectiveness and resultant fiber properties, Bioresources, 3, 155–169, 2008.

(Table 4.5). Fibre properties were modified according to the enzyme and the level used. Strength, which is of major concern in many applications, was preserved by retting with relatively pure pectinases, either pectate lyase or polygalacturonase. A further comparison with two linseed varieties retted with Viscozyme or pectate lyase, plus EDTA, confirmed the maintenance of strength and slightly higher fine fibre yields with pectate lyase (Table 4.6 (Akin et al., 2007). However, the use of mixed enzyme preparations containing cellulases could be used for advantage in applications where the fibres will be shortened, such as for paper/pulp or injection moulding (Foulk et al., 2008) (see Chapter 19.3). The final application could determine the retting formulations. Table 4.5

Fibre properties of flax enzymatically retted with various commercial enzymesa

Treatment Control Viscozyme (0.05%)e Lyvelin (0.05%)e Lyvelin (0.1%)e Pectate lyase (0.05%)e,f Pectate lyase (0.01%)e,f Pectate lyase (0.05%) + STPPf,g PGase I (A. niger)e,h PGaseII (Rhizopus sp.)e,g,i

Fine fibre yieldb in %

Strengthc in g/tex

Finenessd in airflow

13.5 ± 2.3 c 19.5 ± 3.4 ab 10.4 ± 0.7 c 12.7 ± 1.1 c 17.1 ± 0.7 b 13.4 ± 1.4 c 18.1 ± 1.6 b 22.7 ± 2.5 a 13.4 ± 2.8 c

43.6 ± 4.4 a 33.3 ± 2.5 c 29.4 ± 3.0 d 24.5 ± 1.0 e 41.0 ± 0.8 ab 39.9 ± 1.3 ab 39.7 ± 0.1 b 40.6 ± 1.9 ab 23.4 ± 1.5 e

8.0 ± 0 a 7.1 ± 0.6 bc 6.6 ± 0.2 c 5.7 ± 0.2 d 8.0 ± 0 a 8.0 ± 0 a 7.7 ± 0.5 ab 6.8 ± 0.7 c 7.2 ± 0.3 bc

a Triplicate 50 g samples of crimped, Ariane (SC 99) harvested as a mature crop were soaked for 2 min in retting solutions, incubated at 50 ◦ C (40 ◦ C for Viscozyme) for 24 h, washed and dried. b Fibre collected after one pass through a Shirley Analyzer and calculated as % of straw mass. c Average and standard deviation of three replicates, each an average of six tests by Stelometer (collective of flax bundles and fibres). d Determined by airflow readings based on Micronaire but modified using flax calibration standards (Institut Textile de France, Lille, France) and 5.0 g samples. Average and standard deviation of three replicates, each replicate an average of two tests. e Mayoquest 200 was used to provide 20 mM EDTA. f Pectate lyase in % BioPrep. g Sodium tripolyphosphate (100 mM) as chelator. h Experimental polygalacturonase without cellulase activity. i Experimental polygalacturonase containing cellulase activity. Values within columns followed by different lower-case letters differ at P < 0.05. Data adapted from J. Nat. Fibres, 1, 21–47 (2004) and Bioresources, 3, 155–169 (2008).

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Table 4.6 Fibre yield and properties of Hermes and Omega oilseed flax enzyme retted with pectate lyase or Viscozyme and chelator

Cultivar

Formulationa

Hermes

PL; C Viscozyme + C PL; C Viscozyme + C

Omega

Fine fibre yieldb in % of straw

Strength in g/tex

Fineness in airflow

5.9 ± 0.3 bc 5.0 ± 0.6 c 8.4 ± 0.3 a 6.3 ± 0 b

36.7 ± 0.9 a 21.3 ± 1.8 c 30.5 ± 0.1 b 20.7 ± 1.5 c

4.1 ± 0.2 a 3.0 ± 0.1 b 1.1 ± 0.1 a 1.2 ± 0 c

a Flax fibres were crimped through a nine-roller calender, and 150 g of crimped fibres was enzyme retted in duplicate samples for each variety and each enzyme. Stems then were soaked for 2 min in pectate lyase (PL) in 0.1% BioPrep in 0.5 mM sodium borate, pH 8.74, at 50 ◦ C for 1 h. Then, without washing, PL-saturated flax was soaked for 2 min in chelator (C) (Mayoquest 200, 18 mM EDTA), pH 12.0, and incubated at 50–57 ◦ C for a total of 24 h. Viscozyme-retted fibres were soaked in 0.1% Viscozyme L plus chelator (C), pH 5.0, at 40 ◦ C for 24 h. b The yield from stems cleaned through the USDA flax fibre pilot plant and the Shirley Analyzer. Values within columns followed by different lower-case letters differ at P ≤ 0.05. Data adapted from Ind. Crops Prod. 25, 136–146 (2007).

Pectate lyase and polygalacturonase both degrade pectin, and examples of both enzyme types showed the ability to ret flax (Table 4.5). Pectate lyase and polygalacturonase are depolymerising enzymes for pectin; pectate lyase carries out a non-hydrolytic breakdown of pectates and pectinates by a transelimination split of the pectic polymer, while polygalacturonase catalyses random hydrolysis of α-1,4-polygalacturonic acid (Sakai et al., 1993). These pectinases vary in the optimal conditions for activity, and these characteristics may help determine their application. Pectate lyase is more active at high pH (e.g. 8–9) and high temperature (about 55 ◦ C), while polygalacturonase hydrolyses pectin better at a pH of 5–6 and around 40 ◦ C. Biotechnology has resulted in the commercial availability of specific pectinases in recent years. A pectate lyase was developed as a replacement for chemical scouring of cotton (Akin et al., 2007). The product was made through multiple copies of the native gene for the enzyme inserted into the original Bacillus lichiniformis bacterium, thereby providing a high level of pectinase without cellulases. The commercial product, called BioPrep 3000L, was developed by Novozymes North America, Inc. (Franklinton, NC, USA) as a liquid commercial alkaline pectate lyase (PL) with a reported activity of 3000 alkaline pectinase standard units (APSU)/g. It is marketed by Dexter Chemical LLC (Bronx, NY, USA) under the trade name Dextrol Bioscour 3000. BioPrep was the first commercially available pectate lyase and was isolated and produced for its unique ability to degrade the pectin layer between the waxy cuticle and cellulosic fibre of cotton. BioPrep 3000 performed well as a replacement for alkaline cotton scouring (Durden et al., 2001; Etters et al., 2001). Pectins are complex molecules that bind plant tissues together in both cotton and flax. In cotton, pectic substances, determined by response to chemical extractives, are reported to range from 0.7 to 1.2% of fibre dry mass (Wakelyn et al., 1998). It is clear that the amount of non-cellulosic carbohydrates, including pectins, in the bast regions of flax is considerably higher than these constituents in cotton fibres (Ansari et al., 1990; Brown et al., 1986). Modifications to methods used for cotton scouring and enzyme retting with Viscozyme are required effectively to use pectate lyases to ret flax. The binding capacity of EDTA for Ca2+ is considerably greater at alkaline pH (Adamsen et al., 2002a; Adamsen et al., 2002b), and therefore a higher pH should be more efficient in retting with an enzyme–chelator formulation. Pectate lyase, however, requires Ca2+ for activity (Sakai et al., 1993). With this requirement in mind, the suggested method for cotton scouring with BioPrep is to apply the enzyme first and later apply the chelator (Salmon, S., Novozymes, private communication). Table 4.7 shows results from a series of various tests carried out to evaluate enzyme level under optimal conditions for activity. The sequential treatment with enzyme followed by chelator was more effective (i.e. higher fibre yield and cleaner fibre) for retting flax than enzyme alone or enzyme plus chelator in a combined solution. Stelometer strength was maintained at all levels with pectate lyase, and a level of about 1.5–2% of the commercial product provided sufficient enzyme activity. Based on these data and other preliminary studies on chelator level and incubation times, the SER method most effective for producing fine,

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Table 4.7

Properties of cleaned seedflax fibrea enzyme retted with various levels of pectate lyase

Enzyme formulationb

Fine fibre yield in %

Shive contentc in %

Fineness in airflow

Strength in g/tex

0.1% PL ff chelator 0.1% PL + chelator 0.1% PL no chelator 0.5% PL ff chelator 0.5% PL + chelator 0.5% PL no chelator 1.0% PL ff chelator 1.0% PL + chelator 1.0% PL no chelator 1.5% PL ff chelator 1.5% PL + chelator 1.5% PL no chelator 2.0% PL ff chelator 2.0% PL + chelator 2.0% PL no chelator 5.0% PL ff chelator 5.0% PL + chelator 5.0% PL no chelator

10.0 ± 2.9 bcd 6.3 ± 0.2 ef 5.6 ± 0.4 f 10.7 ± 2.5 bcd 9.3 ± 2.7 bcdef 8.5 ± 1.0 cdef 11.8 ± 2.2 abc 9.0 ± 1.3 bcdef 9.5 ± 0.3 bcde 11.8 ± 2.1 abc 8.1 ± 0.1 def Not determined 10.2 ± 2.0 bcd 7.4 ± 1.1 def Not determined 11.7 ± 2.9 abc 13.2 ± 5.2 a 12.7 ± 2.4 ab

5.1 ± 1.9 bc 4.6 ± 0.9 bc 11.4 ± 2.6 a 2.0 ± 1.1 def 3.9 ± 1.9 bcd 5.7 ± 1.7 b 1.7 ± 0.9 ef 3.6 ± 1.4 bcdef 3.7 ± 1.5 bcde 1.5 ± 0.2 f 3.0 ± 1.1 cdef Not determined 2.3 ± 1.2 def 2.9 ± 1.3 cdef Not determined 1.6 ± 0.5 ef 2.0 ± 0.4 def 2.3 ± 0.8 def

4.5 ± 0.1 ab 4.5 ± 0.1 ab Not determined 4.5 ± 0.1 ab 4.5 ± 0.1 a Not determined 4.3 ± 0.1 c 4.4 ± 0.1 abc Not determined 4.1 ± 0.1 d 4.4 ± 0.1 abc Not determined 4.1 ± 0.1 d 4.4 ± 0.1 bc Not determined 4.2 ± 0.1 de 4.1 ± 0.1 d 4.2 ± 0.1 de

34.7 ± 2.1 a 31.8 ± 1.5 a Not determined 36.1 ± 3.6 a 33.3 ± 1.4 a Not determined 32.1 ± 0.7 a 30.6 ± 1.1 a Not determined 29.8 ± 6.8 a 33.2 ± 0.6 a Not determined 32.6 ± 0.9 a 31.6 ± 0.6 a Not determined 32.6 ± 1.3 a 33.9 ± 0.8 a 29.8 ± 3.6 a

a Hermes was grown to full seed maturity at Carrington, ND, USA, in 2004, and used for all tests. Enzyme-retted flax cleaned through the flax pilot plant and passed once through a Shirley Analyzer to obtain fine, cleaned fibre/fibre bundles. b Pectate lyase (PL) in % BioPrep and 1.83% Mayoquest 200 as chelator applied as follows: ff = enzyme followed by chelator, + = enzyme and chelator combined in one formulation. Triplicate samples of 150 g. c Determined by near-infrared spectroscopy. Values within columns followed by different lower-case letters differ at P ≤ 0.05.

clean fibre/fibre bundles with pectate lyase was as follows: (1) saturate crimped flax stems with pectate lyase at 2% of the commercial product at pH 8.5, (2) incubate for 1 h at 55 ◦ C, (3) without washing, resoak with 18 mm EDTA at pH 12, (4) continue incubation at 55 ◦ C for about 24 h total time, and (5) wash and dry fibre in preparation for mechanical cleaning. Enzyme retting of bast plants is increasing in interest, as shown by its emphasis at major international conferences (G¨ubitz and Cavaco-Paulo, 2001; Hardin et al., 2002; Kozlowski et al., 2005). Use of enzymes for retting bast fibres, including flax, is a process still undergoing development and evaluation. Several commercial products are now available with pure, or at least cellulase-free, pectinases. Inotex (Dvur Kralove n.L., Czech Republic) has carried out research on field spraying of proprietary cellulase-free pectinases on flax and other bast plants (Antonov et al., 2007). Research with enzyme retting has expanded to other bast plants (Fischer et al., 2005; Br¨uhlmann et al., 2000), where work with hemp has had some success but has shown that different enzymes or protocols from those with flax are needed. There is also considerable interest for enzymes, some of which are similar to retting enzymes, further to clean or impart specific properties to already prepared fibres (Marek et al., 2008). Successful, cost-effective and commercial technologies will probably have requirements such as: selected flax material, precisely identified enzyme formulations and conditions, integrated cleaning procedures and knowledge of specific end-use of fibres.

4.5

Mechanical Cleaning

After retting, mechanical cleaning follows to remove shive and cuticularised epidermis from the fibre. In traditional linen production, typically a large round bale of deseeded flax stems enters a bale opener to begin the cleaning process. The first phase of cleaning breaks the stems by passage through fluted rollers, and then the scutching blades beat and stroke the fibre bundle to remove shive (Sultana, 2002b). The quality of

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Figure 4.6 A schematic overview of the systematic nomenclature used in traditional flax processing products. From hemp (Schegelsberg, 1996) adapted to flax (Mussig, 2001). ¨

retting determines the quantity and quality of the fibre after scutching, as residual shive and cuticle fragments constitute a detriment to fibre quality. From the primitive manual tools such as hammers and boards used to scutch flax, modern equipment, although automated, scutches flax more or less by the same methods. Figure 4.6 gives an overview of the systematic nomenclature of the traditional flax processing products, showing the collection of long line fibre bundles for traditional linen spinning, short tow fibre bundles and shive (Schnegelsberg, 1996; M¨ussig, 2001). While in the traditional processing of long flax the orientation of the stems and the fibre bundles needs to be maintained during the complete value-added chain from harvesting to the final yarn, modern processing lines pass on this strict orientation. The length of the bundles is not the key issue, but rather the orientation. According to M¨ussig (2001), the following definitions help to distinguish between the processing techniques: (1) longitudinal flax, which is flax with fibres and fibre bundles particularly oriented in only one direction, and (2) disordered flax, which is flax with fibres and fibre bundles having no preferred orientation. Scutching mills clean long fibre bundles by gripping the broken stems and beating first the top portion and then the lower portion with paddles or blades. As the long fibre bundles are beaten, short fibre bundles, called tow, are sorted out along with contaminants and cleaned separately. Before breaking the stems, modern mills may align and carry out other processes to improve the efficiency of scutching. During these mechanical cleaning processes, an effort is made to maintain the integrity of the long flax bundles, which are to be spun into high-value flax yarn. Scutched flax is then cleaned using a combing action called hackling, which removes smaller contaminants, disentangles and aligns the long fibre bundles and separates the bundles without destroying length (Ross, 1992). A short fibre bundle fraction, termed hackling tow, is produced as a byproduct of the long flax. Automated hackling systems with progressively finer and finer pinned rollers comb through scutched flax to produce the long, hackled fibre bundles for traditional linen textiles. As in the scutching process, the integrity of the long flax is maintained. Fibre bundles are then processed into sliver (a continuous strand of loosely assembled fibres/bundles) and then roving (sliver with reduced diameter and a slight twist to hold fibres together). From this material, yarns are made using a wet, ring spinning system that is relatively slow and expensive in comparison with cotton spinning.

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The shorter, scutching and hackling tow is cleaned and refined to cottonised flax, blended with cotton or other fibres and spun on efficient dry ring or rotor spinning systems. Tow is also used in various non-textile industrial applications, such as composites, geotextiles and insulation materials (see Chapters 19 to 21). Disordered flax processing, sometimes termed ‘total fibre processing’, can be carried out to process only one type of fibre from the flax stems rather than long flax and shorter tow. This process is simpler than that for traditional linen (or longitudinal flax) in that alignment of stems is not as critical for processing, and nontraditional sources of fibre (e.g. linseed straw) may be used. Equipment is often quite expensive for refining and shortening clean flax for blending with cotton and processing on short staple equipment. Generally, for disordered flax processing, the retted stems are broken and then cleaned of shive and contaminants, e.g. through a beating or carding action, where fibre bundles of non-uniform length result. In some applications, fibre bundles are chopped to uniform length or reduced in size in some way and further refined and cleaned for cottonised flax. Clean, fine flax, now similar in length to cotton, can be blended with cotton or other staple length fibres and spun on high-efficiency, short staple spinning systems, as indicated earlier. Shorter flax fibre bundles, such as tow, originating from cleaned longitudinal flax processing or from disordered flax processing, do not have the properties or generally bring the high price of long flax used in traditional linen mills (Kozlowski, 2001). Research has been carried out over some time to develop equipment for more efficient decortication (e.g. removal of shive from fibre bundes) of linseed straw for fibres of lower technical grade where traditional linen is not the object. Depending on the application, fibre from disordered flax processing of various levels of cleanliness (i.e. amount of shive remaining with fibres) can be produced in these systems for use in composites, insulation materials and geotextiles. A major challenge is to produce textile-grade fibre efficiently and sustainably from linseed or other non-traditional linen sources. Still challenging but perhaps a more timely scenario is the sourcing of large amounts of clean, technical-grade flax, sustainably provided and with consistent quality, which are much desired in the automotive, insulation and other composite industries.

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Foulk, J.A., Akin, D.E. and Dodd, R.B. (2001) Processing techniques for improving enzyme-retting of flax. Ind. Crops Prod., 13, 239–248. Foulk, J.A., Akin, D.E. and Dodd, R.B. (2008) Influence of pectinolytic enzymes on retting effectiveness and resultant fiber properties. Bioresources, 3, 155–169. Foulk, J.A., Akin, D.E., Dodd, R.B. and Frederick, J.R. (2002) Cultural practices of flax in the Southeastern United States, in Proceedings of the 59th Flax Institute of the United States, Fargo, ND, 2002, pp.124–132. Frederick, J.R., Porter, P.M., Murdock, E.C., Dodd, R.B. and Todd, M.A. (1993) Growing Flax in South Carolina. Clemson University Cooperative Extension Service, Clemson, SC. Gamble, G.R., Snook, M.E., Henriksson, G. and Akin, D.E. (2000) Phenolic constituents in flax bast tissue and inhibition of cellulase and pectinase. Biotechnol. Lett. V, 22, 741–746. Girault, R., His, I., Andeme-Onzighi, C., Driouich, A. and Morvan, C. (2000) Identification and partial characterization of proteins and proteoglycans encrusting secondary walls of flax fibres. Planta 211, 256–264. Goldberg, R., Morvan, C., Jauneau, C. and Jarvis, M.C. (1996) Methyl-esterification, de-esterification, and gelation of pectins in the primary cell walls, in Pectins and Pectinases, ed. by Visser, I. and Voragen, A.G.J. Elsevier, New York, NY, pp. 151–172. Goodman, A.M., Ennos, A.R. and Booth, I. (2002) A mechanical study of retting in glyphosate treated flax stems (Linum usitatissimum), Ind. Crops Prod., 15, 169–177. Gorshkova, T.A., Salnikov, V.V., Pogodina, N.M., Chemikosova, S.B., Yablokova, E.V., Ulanov, A.V., Ageeva, M.V., van Dam, J.E.G. and Lozovaya, V.V. (2000) Composition and distribution of cell wall phenolic compounds in flax (Linum usitatissimum L.) stem tissues. Ann. Bot., 85, 477–486. Gorshkova, T.A., Wyatt, S.E., Salnikov, V.V., Gibeaut, D.M., Ibragimov, M.R., Lozovaya, V.V. and Carpita, N.C. (1996) Cell-wall polysaccharides of developing flax plants. Plant Physiol., 110, 721–729. G¨ubitz, G.M. and Cavaco-Paulo, A. (eds) (2001) Biotechnology in the Textile Industry – Perspectives for the New Millenium. Special Issue of J. Biotechnol., 89, 89–312. Hamilton, I.T. (1986) Linen. Textiles, 15, 30–34. Hardin, I.R., Akin, D.E. and Wilson, S.J. (eds) (2002) Advances in Biotechnology for Textile Processing. Department of Textiles, Mechandising and Interiors, University of Georgia, Athens, GA, USA. Henriksson, G., Akin, D.E., Hanlin, R.T., Rodriguez, C., Archibald, D.D., Rigsby, L.L. and Eriksson, K.-E.L. (1997a) Identification and retting efficiencies of fungi isolated from dew-retted flax in the United States and Europe. App. Environ. Microbiol., 63, 3950–3956. Henriksson, G., Akin, D.E., Rigsby, L.L., Patel, N. and Eriksson, K.-E.L. (1997b) Influence of chelating agents and mechanical pretreatment on enzymatic retting of flax. Text. Res. J., 67, 829–836. Henriksson, G., Akin, D.E., Slomczynski, D. and Eriksson, K.-E.L. (1999) Production of highly efficient enzymes for flax retting by Rhizomucor pusillus. J. Biotechnol., 68, 115–123. Henriksson, G., Eriksson, K.-E.L., Kimmel, L. and Akin, D.E. (1998) Chemical/physical retting of flax using detergent and oxalic acid at high pH. Text. Res. J., 68, 942–947. Himmelsbach, D.S., Khalili, S. and Akin, D.E. (1998) FT-IR microspectroscopic imaging of flax (Linum usitatissimum L.) stems. Cell. Molec. Biol., 44, 99–108. His, I., Andeme-Onzighi, C., Morvan, C. and Driouich, A. (2001) Microscopical analysis of mature flax fibres embedded in London Resin White: immunogold localisation of cell wall matrix polysaccharides. J. Histochem. Cytochem., 49, 1525–1535. Jauneau, A., Cabin-Flaman, A., Verdus, M.-C., Ripoll, C. and Thellier, M. (1994) Involvement of calcium in the inhibition of endopolygalacturonase. Plant Physiol. Biochem., 32, 839–846. Jauneau, A., Quentin, M. and Driouich, A. (1997) Micro-heterogeneity of pectins and calcium distribution in the epidermal and cortical parenchyma cell walls of flax hypocotyl. Protoplasma, 198, 9–19. Kessler, R.W., Becker, U., Kohler, R. and Goth, B. (1998) Steam explosion of flax – a superior technique for upgrading fibre value. Biomass Bioenergy, 14, 237–249. Khalili, S., Akin, D.E., Pettersson, B. and Henriksson, G. (2002) Fibernodes in flax and other bast fibers. J. Appl. Bot., 76, 133–138. Kozlowski, R. (ed.) (2001) Euroflax Newsletter No. 16, Institute of Natural Fibres, Poznan, Poland. Kozlowski, R., Batog, J., Konczewicz, W., Mackiewicz-Talarczyk, M., Muzyczek, M., Sedelnik, N. and Tanska, B. (2005) Latest state-of-art in bast fibres bioprocessing, in Proceedings of the 11th Conference for Renewable Resources and Plant Biotechnology, Institute of Natural Fibres, Poznan, Poland.

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Love, G.D., Snape, C.E., Jarvis, M.C. and Morrison, I.M. (1994) Determination of phenolic structures in flax fibre by solid state 13 C NMR. Phytochemistry, 35, 489–491. Marek, J., Antonov, V., Bjelkova, M., Smirous, P., Fischer, H. and Janosik, S. (2008) Enzymatic bioprocessing – new tool for extensive natural fibre source utilization, in 2008 International Conference on Flax and Other Bast Plants, Saskatoon, Sakatchewan, Canada, 21–23 July 2008, pp. 159–169. Meijer, W.J.M., Vertregt, N., Rutgers, B. and van de Waart, M. (1995) The pectin content as a measure of the retting and rettability of flax. Ind. Crops Prod., 4, 273–284. Morrison III, W.H. and Akin, D.E. (2001) Chemical composition of components comprising bast tissue in flax. J. Agric. Food Chem., 49, 2333–2338. Morrison III, W.H., Akin, D.E., Himmelsbach, D.S. and Gamble, G.R. (1999a) Chemical, microscopic, and instrumental analysis of graded flax fiber and yarn. J. Sci. Food Agric., 79, 3–10. Morrison III, W.H., Akin, D.E., Himmelsbach, D.S. and Gamble, G.R. (1999b) Chemical, microscopic, and instrumental analysis of graded flax fiber and yarn. J. Sci. Food Agric., 79, 3–10. M¨ussig, J. (2001) Untersuchung der Eignung heimischer Pflanzenfasern f¨ur die Herstellung von naturfaserverst¨arkten Duroplasten – vom Anbau zum Verbundwerkstoff. VDI Verlag GmbH, D¨usseldorf (Fortschritt-Bericht VDI, Reihe 5, Grund- und Werkstoffe/Kunststoffe, No. 630). Peters, R.H. (1963) The chemistry of fibers. Text. Chem., 1, 168–174. Rihouey, C., Morvan, C., Borissova, I., Jauneau, A., Demarty, M. and Jarvis, M. (1995) Structural feature of EDTA-soluble pectins from flax hypocotyls. Carbohydr. Polym. 28, 159–166. Ross, T. (1992) Preparation and spinning of flax fibre, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F. M. Publications, Belfast, UK, pp. 275–296. Sakai, T., Sakamoto, T., Hallaert, J. and Vandamme, E.J. (1993) Pectin, pectinase, and protopectinase: production, properties, and applications, in Advances in Applied Microbiology, ed. by Neidleman, S. and Laskin, A.I. Academic Press, New York, NY, pp. 213–294. Schnegelsberg, G. (1996) Was ist Hanf? – Ein Beitrag zur begrifflichen Kl¨arung, in Hanf & Co.: Die Renaissance der heimischen Faserpflanzen, 2nd edition, ed. by Waskow, F. Die Werkstatt, G¨ottingen, Germany, pp. 205–221. Sharma, H.S.S. (1986) The role of bacteria in retting of desiccated flax during damp weather. Appl. Microbiol. Biotechnol., 24, 463–467. Sharma, H.S.S. (1987a) Studies on chemical and enzyme retting of flax on a semi-industrial scale and analysis of the effluents for their physico-chemical components. Int. Biodeterioration, 23, 329–342. Sharma, H.S.S. (1987b) Screening of polysaccharide-degrading enzymes for retting flax stem. Int. Biodeterioration, 23, 181–186. Sharma, H.S.S. (1988) Chemical retting of flax using chelating compounds. Ann. Appl. Biol., 113, 159–165. Sharma, H.S.S. and Faughey, G.J. (1999) Comparison of subjective and objective methods to assess flax straw cultivars and fibre quality after dew-retting. Ann. Appl. Biol., 135, 495–501. Sharma, H.S.S., Lefevre, J. and Boucaud, J. (1992) Role of microbial enzymes during retting and their effect on fibre characteristics, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F. M. Publications, Belfast, UK, pp. 199–212. Sharma, H.S.S. and Van Sumere, C.F. (eds) (1992a) The Biology and Processing of Flax. M. Publications, Belfast, UK, 1992, 576 pp. Sharma, H.S.S. and Van Sumere, C.F. (1992b) Enzyme treatment of flax. Genet. Engng Biotechnol., 12, 19–23. Sotton, M. and Ferrari, M. (1989) Le lin ultra-affine par le traitement hydrolyse flash. L’Ind. Text., 1197, 58–60. Stephens, G.R. (1997) Connecticut fiber flax trials 1992–93, Bulletin 946, The Connecticut Agricultural Experiment Station, New Haven, CT, October. Stern, K.R., Jansky, S. and Bidlack, J.E. (1986) Introductory Plant Biology, 9th edition. McGraw-Hill, New York, NY, 624 pp. Stewart, D., McDougall, G.J. and Baty, A. (1995) Fourier-transform infrared microspectroscopy of anatomically different cells of flax (Linum usitatissimum) stems during development. J. Agric. Food Chem., 43, 1853–1858. Sultana, C. (1992a) Growing and harvesting flax, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F. M. Publications, Belfast, UK, pp. 83–109. Sultana, C. (1992b) Scutching of retted-flax straw, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F. M. Publications, Belfast, UK, pp. 261–274.

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5 Hemp – Cultivation, Extraction and Processing Stefano Amaducci Istituto di Agronomia, Genetica e Coltivazioni erbacee, Universit`a Cattolica del Sacro Cuore, Piacenza, Italy

Hans-J¨org Gusovius Leibniz-Institut f¨ur Agrartechnik Potsdam-Bornim e.V., Potsdam, Germany

5.1 Introduction Hemp (Cannabis sativa L.) is a multiuse, multifunctional crop that can provide valuable raw material to a large number of non-food industrial applications. The environmentally friendly cultivation and the sustainability of its products are the main drivers for a future expansion of the hemp crop. In this chapter we will tackle the main technical issues encountered along the hemp production chains, from cultivation to fibre processing, for its utilisation in textile (longitudinal) and non-textile (disordered) applications.

5.2

Background

Hemp (Cannabis sativa L.), considered one of the oldest crops known to man, was defined by Schultes (1970) as a green, very abundant and ubiquitous plant, economically valuable, possibly dangerous and certainly in many ways mysterious. This definition, though simple, is effective in describing the valuable but contradictory nature of this important industrial crop. Rendering it industrially important is the large yield potential that can be achieved in a relatively short cropping cycle; in Europe, production of up to 20 t/ha of dry biomass has been reported (Struik et al., 2000). Hemp is also very adaptable and can be grown in a large array of environments, from northern latitudes to tropical climates. The economical value of hemp can certainly be related to the possibility of growing it for a very large number of end-use applications. Hemp is traditionally cultivated for its three main products: the fibre, the seeds and the psychoactive substances that accumulate particularly in the female inflorescence. Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Figure 5.1

Main uses of the hemp plant and its fractions.

The principal uses of these three products are shown in Figure 5.1. Note that the fibre is divided into three fractions during mechanical processing: the short and long bast fibre bundles and the woody core (also referred to as hurds or shives). Traditionally, bast fibres were used for making ropes, twines, bags and hard-wearing fabrics, while the hurds would often be burnt for heating or to fuel the steam engines that operated the first scutching machines (Sponner et al., 2005). More recently, the potential application of the fibres has widened. In addition to textile destinations (Amaducci, 2003 and 2005), the bast fibres are used to produce speciality paper (Karus and Vogt, 2004), and studies were carried out to evaluate the possibility of using the entire stem to produce paper pulp (de Meijer, 1994). A promising destination for hemp bast fibres is the production of insulating products and fibre-reinforced composites (M¨ussig et al., 2005), the latter currently being used in the automotive industry. The woody core is mainly used as horse bedding, with minor applications in the construction sector (Karus and Vogt, 2004). To return to hemp’s other main products, hemp seeds are most commonly used as animal feed, but various food and industrial applications are emerging (Karus and Vogt, 2004). Recently, the oil extracted from the seeds has been considered for skin care and cosmetic applications owing to its high polyunsaturated fatty acid content (Vogl et al., 2004). Finally, the use of cannabinoids by the pharmaceutical industry could turn one of the major factors limiting hemp cultivation into a resource (Hollister, 2001). Besides its multiuse applications, many other features make hemp a suitable crop for modern agricultural systems, where low input and environmentally friendly production systems are to be promoted. In Italian regions where it was traditionally cultivated, hemp always had a very important position in crop rotations because of its beneficial effect on following crops (Venturi and Amaducci, 1999). Wheat grown after hemp was reported to have an increased yield, for example (Gorchs et al., 2000), and herbicides were not considered necessary because of hemp’s fast growth, which results in weed suppression (Lotz et al., 1991; Berger, 1969). Other interesting aspects are that hemp can be grown on polluted soil and can contribute to phytodepuration (Kozlowski et al., 1995; Ciurli et al., 2002; Giovanardi et al., 2002), and, because of its limited input needs (Venturi and Bentini, 2001), positive impact on the landscape (Biewinga and van der Bijl, 1996) and high root biomass accumulation (Amaducci et al., 2008e), hemp can be considered a suitable biomass crop for energy production. All the above-mentioned characteristics of hemp also render it suitable for organic agriculture (Stickland, 1995).

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Unfortunately, in spite of hemp’s many positive features, its cultivation worldwide steadily decreased during the last century. Only in recent years has the surge of environmental movements and the need for more sustainable production systems brought a renewed interest in this crop, but, before hemp regains its role as an industrial crop, a series of bottlenecks must be tackled (Venturi and Amaducci, 2004): r legislation related to THC content in the plant; r availability of seeds from improved genotypes; r revised agrotechnique for innovative production systems; r improved harvesting systems; r mechanical and biological processing for fibre separation; r development and marketing of new products. The first bottleneck is linked to the delta-9-tetrahydrocannabinol (THC) content of the plant, which is still limiting the possibility of growing hemp in many countries. In Europe, hemp cultivation is possible provided that low THC varieties are employed, but the progressive lowering of the legal THC limit (from 0.3 to 0.2%) and the not always uniform application of the EU regulation (No. 796/2004) lead to difficulties of various nature. In particular, well-established dioecious cultivars from eastern Europe and new oilseed ones have been excluded from the European variety list because the THC content exceeded the new limit, a limit that is probably unnecessarily restrictive considering that ‘drug’ genotypes generally have 5–10% THC content (Callaway, 2008). Moreover, the restriction enforced by the EC regulation has diverted the limited resources devoted to breeding towards a reduction of THC level in the plant, when increasing the yield and quality is what would be necessary to sustain hemp cultivation. The second bottleneck is related to the commercial availability of genotypes suitable for specific locations and end-use destinations. As already pointed out, THC restrictions in Europe have reduced the number of varieties available, and those included on the variety list are not all available in commercial quantities. Loss of traditional genotypes and difficulties in registering new ones limit the possibility of expanding hemp cultivation. What is now needed is a selection of new genotypes, suited to a range of environments, that have an improved fibre content and specific fibre quality adapted to innovative end-uses. The third bottleneck is related to the influence of agronomic factors on fibre yield and quality and on the need to revise the cultivation techniques to meet the demands of new production systems (M¨ussig and Martens, 2003), as a function of mechanisation (Venturi et al., 2007) and end-use destinations (i.e. textile) (Amaducci et al., 2008b). Limited information is available on the effect of interaction between more factors, like nitrogen dose, sowing density and harvesting time (Grabowska and Koziara, 2005). The fourth bottleneck refers to the need for improved harvesting systems that have to cut and prepare hemp for subsequent storage and processing. A large number of prototypes have been developed in recent years, in particular to harvest hemp for technical purposes (i.e. disordered hemp) (M¨ussig, 2001), and satisfactory results have been achieved, as will be described later in the chapter. There is a more critical situation, however, with regard to the production chain for longitudinal hemp (i.e. for wet spinning), where hemp stems have to be kept parallel until the scutching and hackling phase. For harvesting hemp seeds, combine harvesters are used for monoecious cultivars, although for dioecious cultivars high labour involvement is still largely required and only a few prototypes have been proposed for the purpose (Kaniewski and Banach, 2008; Burczyk and Kaniewski, 2005). In general, hemp fibre production chains include a mechanical and biological processing step for fibre separation, and this has a large impact on fibre yield and quality. It is critical to evaluate the effect of different processing methods in combination with cultivation technique and harvesting, but information on this aspect is limited (M¨ussig and Martens, 2003), thus creating the fifth bottleneck. When the retting process, necessary to separate bast fibres from woody core and to refine fibre bundles, takes place on field, it is difficult to control fibre quality (Toonen et al., 2007). Controlled microbiological or enzymatic retting guarantees higher quality, but being costly it can only be justified for high-added-value applications (i.e. textile or odour-optimised fibres for insulation or composite applications). Various systems to carry out mechanical separation of the fibre

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bundles have recently been developed, often adapting existing technology (i.e. for flax) to hemp. Adjustments are still needed to optimise these systems, particularly to improve cost efficiency (see Chapter 4). Technical, economic and ecological advantages of natural fibres, and in particular of hemp fibre, are the main driver for the increased interest in this crop, which has resulted in many scientific projects and entrepreneurial activities. The success of current hemp production chains and their future development depends strongly on the market demand for hemp products, which is not currently very strong and suffers from the competition from other natural fibres (see Chapter 3.3). Marketing of new hemp products should be improved to communicate all their values, thereby eliminating the sixth bottleneck. In the following sections, the steps, from sowing to industrial processing, of the main hemp production chains will be presented and discussed, highlighting possible solutions to the aforementioned bottlenecks.

5.3 Hemp Cultivation and Improvement Hemp can be cultivated for the seeds and for psychoactive substances, but its main product is the bast fibre. Hemp fibre can be easily identified in a cross-section of mature hemp stems (Figure 5.2). The primary function of fibres is to provide structural support to the plant, offering resistance to disruptive forces. Hemp bast fibre cells, forming a sclerenchymatic tissue with a mechanical function, die because of the thickness and the extent of lignification of the cell wall (the lignification blocking the passage of water and solutes). As indicated in Figure 5.2, different types of fibre can be identified in a stem cross-section: xylary (or wood) fibres and extraxylary (or bast) fibres. Extraxylary fibres, on the basis of their origin or location in the plant, have been classified as cortical, pericyclic, phloematic and leaf fibres (Dickison, 2000). Classification of hemp fibres is ambiguous; they have been considered as cortical fibres (primary fibres) and phloem fibres (secondary fibres) (Dickison, 2000), or as originating partially from secondary phloem (secondary fibres) and partially from the pericycle (primary fibres) (McDougall et al., 1993; Hayward, 1951). To avoid confusion, it can simply be stated that hemp has primary and secondary extraxylary fibres (Amaducci et al., 2005). The primary fibres are formed by the apical meristem. Their number in each internode is established at the moment of their formation, and they stretch during internode elongation until their final length is reached. It was found that fibre length varies with internode length (Briosi and Tognini, 1894), longer fibres being found in the longer internodes (Kundu, 1942). Consequently, factors affecting internode length (i.e. plant population) could have an effect on primary fibre length (Amaducci et al., 2002b). When the elongation of the internode is complete, secondary growth starts, with the cambium producing secondary phloem and xylem. Secondary fibres are thinner and shorter than the larger and longer primary fibres, and their cell walls are strongly lignified, which makes them less suitable for many industrial applications. Selecting a harvest time at which secondary growth is minimised would therefore improve fibre quality (Amaducci et al., 2008a; Amaducci

Figure 5.2

Cross-section of a basal internode of a mature hemp plant.

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et al., 2005). The reported values for primary bast fibre length range between 3 and 55 mm (Kundu, 1942), with an average of 20–28 mm (Solaro, 1914; Hoffmann, 1957), whereas the average length of secondary fibres is 2 mm (van der Werf et al., 1994). Their diameter has been reported to be 32 µm (Solaro, 1914), 34 µm (Kundu, 1942) and 22 µm (Hayward, 1951). In spite of the differences reported, primary fibres have a larger average diameter than secondary fibres (17 µm) (Kundu, 1942). Wood fibres seem to be less variable; length ranges from 0.433 to 0.613 mm, and the width ranges from 24 to 41 µm (de Meijer, 1994). An important parameter for evaluating suitability of long fibre for textile applications is the length/diameter ratio (Rowell et al., 2000), as is the degree of filling of the cell lumen (Rowell et al., 1997); the latter can also be considered as the degree of maturation of the fibre. Shortly after its formation, the primary wall of single fibre cells starts to lignify, after which maturation proceeds with the deposition of layers of secondary wall until the lumen is reduced to a small cavity. At complete fibre maturity, Hayward (1951) described the lumen as being oval and elongated and occupying about one-third of the transsection of the cell. Fibre maturation progresses unevenly among the fibre cells of the same internode (with fibres from the outer layers maturing earlier than those from the inner ones) (Amaducci et al., 2005), and fibre cells from lower internodes mature sooner than in higher ones, even though the rate of maturation seems faster in higher internodes (Amaducci et al., 2005). For this reason, fibre characteristics vary with harvest time and plant portion (Amaducci et al., 2005; Amaducci et al., 2008a; Mediavilla et al., 2001), and harvest time and harvest method should therefore be adjusted to obtain specific fibre quality. For example, Rowell et al. (1997) state that fibres from young plants are ‘silk-like, fine textured, very flexible and thin’ and are therefore more suitable for fine textile applications. On the contrary, determinations carried out on ‘baby hemp’ (hemp cultivated at high density and harvested when plants are immature) showed poor and uneven quality for textile destination (Liberalato, 2003). Agronomic factors that have a major influence on fibre yield, both in terms of quality and in terms of quantity, are plant density, harvest time and cultivar choice.

5.3.1

Cultivar Choice

Until the onset of flowering, the crop growth rate (CGR) and radiation use efficiency (RUE) have similar and relatively high values for most hemp cultivars; after flowering, they decrease (van der Werf et al., 1996), and biomass accumulation stops with seed maturity and plant senescence. For this reason, in a specific environment, stem and fibre yield are maximised when genotypes with a long vegetative cycle are cultivated (van der Werf et al., 1995). Genotypes selected in southern environments tend to have a long vegetative cycle and high biomass production when grown at northern latitudes (Amaducci et al., 2008a; De Meijer and Keizer, 1994). On the contrary, when genotypes that are bred at higher latitudes are cultivated in southern environments, they show short vegetative growth and limited biomass accumulation. This was acknowledged in Italy in the past, for example when exceptionally high demand for local genotypes could not be met and the use of imported seeds resulted in ‘preflowered’ crops (Barbieri, 1952), and when monoecious varieties bred in France were used (Venturi, 1967 and 1969). Hemp is naturally dioecious, but it is relatively simple to breed monoecious varieties, which have the advantage of producing, firstly, a more homogeneous crop (all the plants are similar, while in a dioecious crop male plants have finer fibre, they flower sooner and dry in the field, while female plants bear ripening seeds) and, secondly, more seeds (as all the plants bare seeds, while only 50% bare seeds in a dioecious crop). For these reasons, most of the new varieties released on the market in the last few decades have been monoecious. Regarding fibre quality, little research has been carried out in the last few years to develop or even identify superior genotypes, and information from old experimental work refers to cultivars that are no longer available. Multiple systems and methods to determine fibre quality exist, and results presented in the literature are often difficult to compare. It is important to note that the concept of quality is relative to the end destination of the fibre (see Chapter 13). For pulp and paper production, low lignified fibres are needed, and therefore genotypes with limited accumulation of secondary bast fibre (van der Werf et al., 1994), limited production of woody

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core and a higher bast to xylem fibre ratio should be selected and cultivated (De Meijer and Keizer, 1994). Nowadays, the most desired characteristics for a hemp genotype are limited to high stem yield potential and high fibre content in the stem (van der Werf et al., 1996). Fibre content, often referred to as pure fibre content and determined by chemical extraction (Bredemann, 1942), ranges between 15 and 25% in commercial cultivars, but can be higher than 30% in Elite plants. When fibre content is determined via enzymatic retting or by mechanical extraction, the fibre/fibre bundle content reported is higher than the pure fibre content (H¨oppner and Menge-Hartmann, 1995). Information to discriminate genotypes on the basis of fibre content is limited by the lack of homogeneity of the methodologies and by contrasting results. Experiments carried out in north-central Europe showed that the dioecious Kompolti HTC has a higher fibre content than monoecious cultivars of French origin (Sankari, 2000; H¨oppner and Menge-Hartmann, 1995; van der Werf et al., 1994). A recent experiment carried out in southern Europe showed that the monoecious French cultivar Futura had a greater pure fibre content (but coarser fibres) than the dioecious Hungarian cultivar Tiborszallasi (Amaducci et al., 2008b). Besides fibre yield and fibre quality, other features related to crop processing should be considered when choosing a cultivar; for example, a new cultivar recently selected in the Netherlands (Toonen et al., 2004) has the peculiar quality of being easy to decorticate, i.e. limited energy is needed to efficiently separate the woody core from the bast fibre bundles, with the positive consequence of having cleaner fibre, reduced energy consumption and less need for retting. When hemp is grown for seed production, it is very important to choose genotypes that can complete flowering and seed ripening before weather conditions become adverse, especially at extreme latitudes (Sankari, 2000).

5.3.2

Plant Density

Seed rate (kg/ha), and therefore plant density (number/m2 ), is considered an important factor of the agronomic technique, affecting fibre yield and fibre quality. Considering that stem yield is not affected by plant population and in some cases reduced biomass yields are noticed when plant population is high, probably as a consequence of competition and high plant mortality at higher stands (Grabowska and Koziara, 2005; Amaducci et al., 2002a,b; Cromack, 1998; Di Candilo et al., 1996; van der Werf et al., 1995), the supposed advantage of high plant density should depend on higher fibre content in the stem, finer fibre cells (Amaducci et al., 2002b; van der Werf et al., 1995; Jakobey, 1965) and lower presence of lignified secondary bast fibres (Amaducci et al., 2008b; Sch¨afer and Honermeier, 2006; van der Werf et al., 1994). Results on the effect of plant density on fibre content, however, are ambiguous, and some authors report higher fibre content at higher plant population (van der Werf et al., 1995; Jakobey, 1965), while others found no significant effect of this factor (Amaducci et al., 2008b; Grabowska and Koziara, 2005; H¨oppner and Menge-Hartmann, 1995). Single fibre cells of plants grown at higher densities have a smaller diameter (Amaducci et al., 2008b; Sch¨afer and Honermeier, 2006), which should result in finer fibre bundles better suited to textile applications. The uncertain effect of plant population on fibre yield is also reflected in the various seed rates that are suggested in the literature, something that also reflects, however, different end-uses of the plant. The optimal density for cultivation of drug hemp seems to be 10 plants/m2 (Rosenthal, 1987); for seed production the optimal density ranges from 30 to 75 plants/m2 (Venturi, 1965; Hennink et al., 1994; van der Werf, 1994); for fibre production the densities are very variable, from 50 to 750 plants/m2 (Dempsey, 1975), with higher rates for textile destinations, e.g. 150–200 plants/m2 (Jaranowska, 1966) and 250–350 plants/m2 (Starcevic, 1996), and lower rates for non-textile destinations, e.g. 90 plants/m2 for paper pulp (Martinov et al., 1996). In Italy, hemp crops for textile destination traditionally had 90–100 plants/m2 (Bruna, 1955; Venturi, 1967; Venturi and Amaducci, 1999). Although the effect of plant population on fibre yield is debatable, its effect on plant biometrics is evident. Increasing plant population results in shorter and thinner stems (Amaducci et al., 2008b; Sch¨afer and

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Honermeier, 2006; Grabowska and Koziara, 2005; Amaducci et al., 2002a,b; H¨oppner and Menge-Hartmann, 1995; van der Werf et al., 1994) and longer basal internodes (Amaducci et al., 2002b) bearing longer and thinner fibre cells. Plant density can therefore be adjusted to control stem height and stem diameter for optimal harvesting (Venturi et al., 2007), and to improve fibre extraction, considering that thinner stems are more difficult to decorticate, especially with no or minimal retting (Amaducci et al., 2008c).

5.3.3

Harvesting Time

Hemp crops for fibre production were traditionally harvested at full flowering of male plants (B`ocsa and Karus, 1998), which is when primary bast fibre yield reaches its maximum (Amaducci et al., 2008b; Mediavilla et al., 2001). Pure fibre content of the stem (expressed as percentage of stem biomass) tends to decrease along the cropping cycle and after flowering owing to continuous accumulation of secondary fibres and xylem (Amaducci et al., 2008b; Keller et al., 2001; Mediavilla et al., 2001). Hence, postponing harvesting time after the end of flowering, as is necessary when seed production is requested, results in a higher proportion of lignified fibre. Keller et al. (2001) reported that delaying harvesting after the end of flowering resulted in improved decorticability of the stem, which is of course a positive factor. Amaducci et al. (2008b) found that fibre production increased by 25% between the beginning of flowering and full flowering, mainly because fibre maturation at higher internodes was more advanced. The authors therefore concluded that the time of harvesting should be set at full flowering to maximise fibre yield but also fibre homogeneity. Furthermore, to establish the beginning of flowering for harvesting can be misleading because of ‘preflowering’ in unfavourable environmental conditions and in certain genotypes (Amaducci et al., 2008d).

5.3.4

Breeding

Fibre hemp varieties descend from a limited number of ancestors, with three gene pool sections: northern and central European, southern European and East Asian ecotypes (De Meijer, 1995). Hemp farmers over thousands of years of cultivation have selected the local landraces from which genetists have selected modern cultivars on the basis of length of vegetative period, stem biometrics and seed yield (Ranalli, 2004). In a subsequent phase, fibre content increase became one of the main objectives of crop improvement, with a methodology set up by Bredemann (1924), together with resistance to pathogens, lodging and suitability to different environments (Allavena, 1961). Discovery of monoecious plants set the basis for a new frontier in hemp breeding, and many monoecious cultivars were registered in Europe. The first hemp hybrid was probably that obtained by crossing the Italian ecotype Ferrara with the cultivar Kymington (Dewey, 1927), but it was only in Hungary that heterosis was exploited and hybrids like Kompolti HTC were produced (B`osca and Karus, 1998). In the last few decades, breeding activities have been reduced, reflecting the worldwide decrease in hemp acreage. Much effort has been devoted to reducing THC in the plant to meet European regulations, and extensive research has been carried out to understand the genetic control of cannabionoid metabolism. Two genetic markers, one associated with male plants (Mandolino et al., 1999) and the other associated with chemotype (Mandolino et al., 2003) have been identified and can be used in assisted selection. The future challenge in hemp breeding is the selection of genotypes with high fibre production and specific fibre qualities. High fibre production can be achieved by increasing both stem yield and stem fibre content. Increased stem yield is strongly correlated with the length of the vegetative cycle (van der Werf et al., 1996); late-blooming cultivars with lower sensitivity to photoperiod should therefore be selected. Genetic control of hemp development is poorly understood and should be further studied to support breeding activities for cultivars adapted to a variety of latitudes, and also to improve forecasts of flowering time (Amaducci et al., 2008a). Increasing fibre content following the methodology of Bredemann (1924) has resulted in an increase in secondary fibres, with a negative impact on quality characteristics (Hoffmann, 1957). Unravelling

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the complexity of fibre biosynthesis and understanding its genetic control are major challenges to be met to improve fibre quality via genomics. Toonen et al. (2007) exemplify how biotechnological approaches could improve important aspects of fibre quality for composite and textile applications, such as altering fibre surface properties, extending fibre length, increasing expansion expression and producing biodegradable polymers inside natural fibres. Until now, limited information on cell wall biosynthesis has been available, and extensive research is needed before biotech (genetically modified) hemp cultivar will be released (see also Chapter 16).

5.4

Hemp Harvesting

Hemp harvesting has been developed and optimised according to the end-use of the crop and the cultivation practices used (Venturi et al., 2007). Very different harvesting techniques are needed when hemp is grown to obtain the long fibre bundles for, for example, wet spun yarn, or the short fibre bundles for, for example, needle felts for composite production, as well as when hemp is cultivated both for the seeds and the fibre. Regardless of the production chain in question, technological developments in harvesting systems were aimed at: r obtaining a uniform field drying and retting (dew) after mowing; r having a low susceptibility to weather conditions; r enabling efficient swath handling (turning and windrowing) including pick-up for baling without fibre and shive loss; r preparing the stems for the further steps of processing and manufacturing (quality management starts with an optimal arrangement of field operations) (Figure 5.3).

5.4.1

Longitudinal Hemp Harvesting

The main products obtained at the end of the traditional hemp production chain are parallel fibre bundles, so-called long hemp (see also Figure 4.6 in Chapter 4), that, after hackling, can be spun into a yarn by wet spinning. For this specific purpose, harvesters are built to mimic the operation, once carried out by hand, of creating stem hoods. In a single passage, a reaper and binder cuts the stems, ties them into bunches and drops them onto the field where people arrange them for efficient drying. Other machines cut and lay the stem in an ordered swath, and a second machine passes to form bunches or bales. In current hemp clothing textile production, substantial developments in harvesting mechanisation are needed to reduce hand labour and to minimise the cost of stem processing. Traditional processing lines, now

Figure 5.3 Information flow along the production chain. Adapted with permission from J. Mussig and H. Harig, Filze und ¨ Vliese aus Hanffasern, Proceedings of the second international symposium “Biorohstoff Hanf,” nova-Institut, 1997.

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very old and inefficient, accept entire stems and produce long ribbons of fibre bundles that need to be divided in portions of 0.6–0.7 m (Sponner et al., 2005) in order to be subsequently hackled in flax machines. To eliminate the necessity of cutting the stems by hand, recent strategies for long hemp production have adapted cultivation or harvesting techniques so that the hemp stems fit modern flax scutching lines (that accept stems of maximum 1 m long). In the case of ‘baby hemp’, the crop is desiccated when it reaches a height of 1–1.4 m and is subsequently harvested with flax lifters. These machines, working on a front of 1 or 2 m wide, can harvest up to 0.4 ha/h, pulling the plants and lying them in ordered swaths on the soil. Thereafter, stems are baled with flax balers (Venturi et al., 2007; Amaducci, 2005). Following this strategy, which adapts the plants to the production chain, biomass yield is strongly limited and fibre quality is not homogeneous (Liberalato, 2003), probably because of the uneven development of the fibres in immature stems (Amaducci et al., 2008b; Amaducci et al., 2005). An alternative strategy that aims to exploit the full potential of hemp production is based on harvesting the crop at full flowering (when stems exceed 2 m) using an innovative machine manufactured by the company Kranemann (Klocksin, Germany) that cuts the stems into two portions of approximately 1 m that are then laid on the ground in two ordered swaths. A subsequent passage with another machine will be needed to divide stems in two portions of 1 m each. After drying, the stems can be baled with common flax balers (Amaducci, 2003). This system, developed at prototype level, has the advantage that it separates the bottom and top stem portions, thus potentially improving fibre homogeneity (Venturi et al., 2007; Amaducci et al., 2002b), even though preliminary results from industrial validation do not confirm this hypothesis because of the high loss of finer fibres/fibre bundles in the top portion during scutching (Amaducci et al., 2008c).

5.4.2

Disordered Hemp Harvesting

Revival of the industrial utilisation of bast fibres in technical applications has led to a multitude of technical developments that enable and improve the supply of raw material according to the quality requirements of the successive processer. A critical step in preserving quality is harvest. The most common technology in Europe and probably worldwide is based on the principle of the ‘one-knife cutting drum’, as engineered in the machine system HempFlax/HempCut 3000/4500 (Figure 5.4). Originally developed and introduced by the company HempFlax (Oude Pekela, The Netherlands) it is mounted onto tractors with a rear driving system. The system consists of a header (conventionally a row independent rotary crop header made by Kemper (Stadtlohn, Germany) and an adapted one-knife cutting drum with mass balancing. The hemp stalks are fed lengthwise into the chopping drum, cut into 600–700 mm long pieces and placed onto the field directly under the drum. This system was further developed by the company Wittrock (Rhede/Brual, Germany) and is now sold worldwide as HempCut 3000 or 4500 according to the working width of the adapted Kemper header.

Figure 5.4 Harvesting system HempCut 3000/4500 (left) and the one-knife cutting drum (right). Adapted with permission from B. Wittrock, Wittrock harvesting system for hemp, European Industrial Hemp Association, 2004.

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The technological parameters of the harvester can be calculated on the basis of the working width and the average working speed: r r r r r

maximum working width: 3000 or 4500 mm; working speed: 5–10 km/h; area performance: up to 2.1 or 3.2 ha/h; swath width: approximately 800 mm; power requirement: 250 kW (Claas Jaguar 830).

Field experiments have shown that the technology enables an adequate retting and drying behaviour of the mowed and cut plant material, although some concerns about the lack of uniformity of these naturally occurring processes have been expressed (Mastel et al., 1998; M¨ussig and Martens, 2003). The concern is that areas with prolonged humidity form within the relatively compact swath, and repeated swath turning is recommended to minimise this. Another important development of the late 1990s was the machine system Bluecher 02/03 developed and manufactured by the company Kranemann (Klocksin, Germany) (Figure 5.5). The main idea is to preserve the original array of the hemp plant until it is cut into pieces of 600–700 mm. After the plant is cut, eccentric steered conveyor elements snatch up the hemp stalks in a vertical position. The cutting units (discs) located at fixed positions on the drum chop the upright stalks several times before re-laying them on the ground in a swath. Because the cutting discs can be interchanged (blades for fresh green crops and fine-toothed knifes for more lignified plants), this harvesting system is well adapted for almost all field and weather conditions. Reduced ground clearance of the bearer vehicle compromises the deposition of the cut plant into the intended fan-shape swath, and thus the intended even retting and drying are not fully achieved. The following performance parameters were reported, based on manufacturer’s data and operator experience: r r r r r

maximum working width: 3500 mm; working speed: 5–12 km/h; area performance: up to 2.9 ha/h; swath width: approximately 800 mm; power requirement: minimum 100 kW.

Simple cutter bars have been used for decades to harvest industrial hemp, especially in France where these systems are available at almost all single farms and are utilised to cut remaining hemp stalks after seeds have been gathered with combine harvesters. Such cutter bar systems have also been developed to cut the naturally long hemp stalks into shorter pieces to satisfy the requirements of the processing facilities. A prototype of a two-level double-knife mower with a fixed assignment of two cutter bars in a frame, without staggered offset

Figure 5.5

Harvesting system Bluecher 02/03. Reproduced by permission of Kranemann GmbH.

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Figure 5.6 Multiple-level cutter bar harvesters: prototype ATB (left); HMG 4-2400 (right) Adapated from J. Paulitz, Presentation at annual meeting of the German Natural Fibre Association, Werlte, 2008.

of the bars, was constructed at the Leibniz-Institute of Agricultural Engineering Potsdam-Bornim (Ehlert, 1997). The companies Scholz GmbH (Dresden, Germany) and Huster GmbH (Chemnitz, Germany) have continually improved the cutter bar system as a tractor side attachment since 1996. To date, more than 10 systems with different mounting solutions and different working widths exist in Europe (SMU-2/SMU-3-210, HMG 4-240) (Figure 5.6). Additional level cutter bar harvesters were further developed and improved to overcome the limited working width by constructing cutter bars longer than 2400 mm. The new HMG 4-5000 prototype was finished and successfully tested in the 2008 harvesting period. The technological parameters were as follows (Paulitz, 2008): r r r r r

maximum working width: 5000 mm; working speed: 10–16 km/h; area performance: up to 4.8 ha/h; swath width: up to 5000 mm; power requirement: approximately 2.5 kW/m working width.

Several results, as partly already reported in the literature (M¨ussig et al., 1999; Gusovius 2002) can also be underlined for the new prototype: r r r r r

increased effectiveness and reduced running costs; reduced power demand for hydraulic mowing motors of approximately 50 kW; used the full field area for fast and homogeneous retting and field drying in wide swaths; reduced raw material losses (shives) compared with other technologies; reduced the level of fibre damage (which is also related to retting intensity).

Manufacturers from France and Czech Republic have also assumed these basic principles and are offering similar technical solutions. After harvesting, swath turning is recommended for even retting and drying of the stems. Windrowing is carried out to prepare the straw for baling. Common swathers and windrowers can be used as far as they are mechanically robust. In some cases, specified pick-up systems are employed for swath turning. Standard agricultural round or square balers, with only small modifications to prevent fibre wrapping, are in use. For transport and storage, biomass moisture should not exceed 15%. A lot of research was carried out to determine the weather-related risk of different harvesting technologies, as well as their influence on several straw and fibre quality parameters (Ehlert, 1997; Basetti et al., 1998; M¨ussig et al., 1999; Gusovius et al., 2000; Mastel and Stolzenburg, 2002; Gusovius, 2002; M¨ussig and Martens, 2003; R¨ohricht and Schulz, 2003). It could be shown that particularly the multiple-level cutter bar system is improving the retting behaviour (Table 5.1).

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Table 5.1 Influence of selected harvesting technologies on the degree of retting of hemp straw (data based on Mussig and ¨ Martens, 2003)

Double-knife cutter bar

Rotor disc mower

HempFlax harvester

Swath mower, system Fortschritt

Degree of retting, Degree of retting, Degree of retting, Degree of retting, A 1000 value Std dev. A 1000 value Std dev. A 1000 value Std dev. A 1000 value Std dev.

Date 23.09.1997 02.10.1997 09.10.1997 16.10.1997

0.746 1.380 2.620 2.465

0.029 0.121 0.068 0.116

0.611 1.347 2.120 2.297

0.093 0.139 0.160 0.144

0.939 1.548 2.533 1.850

0.132 0.071 0.314 0.107

0.989 1.766 2.664 2.235

0.364 0.092 0.159 0.151

Gusovius (2002) confirmed the above-mentioned results with regard to field drying based on 4 year field experiments. The swathless stacking of the plant material in a comparatively thin layer (as obtained with cutting bars) enabled a much more even retting of the stalks in contrast to the compact swath of the HempCut or Bluecher technologies. The utilisation of harvesting technologies with high mechanical impact, like field processing, shortens the retting and drying period. However, strong mechanical impact on hemp stalks reduces the mechanical properties of the fibre (Hahn et al., 2000; M¨ussig and Martens, 2003) (Figure 5.7). Furthermore, Chen et al. (2004) carried out fundamental laboratory experiments to evaluate the power requirements for hemp stalk cutting and conditioning.

5.4.3

Harvesting for Seed and Fibre

Especially in the Champagne region of central France, the harvest of hemp seeds for reproduction prior to the utilisation of the stem part of the plant has a long tradition. Typically, with conventional grain combiners the upper part of the hemp plant is cut and threshed. The combine should be equipped with so-called hill master equipment to enable the lifting of the cutter header to a maximum height position. Rotating parts within the combiner have to be protected to prevent loosened hemp fibre bundles from winding around them.

Figure 5.7

Influence of harvesting technology on mechanical properties of hemp fibres (adapted from Gusovius, 2002).

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Figure 5.8 Hemp combine harvester developed by Deutz-Fahr Erntesysteme GmbH (Lauingen) and Gerhard Gotz ¨ GmbH (Buhl). Adapted with permission from K. Mastel and K. Stolzenburg, Prufung des Prototyps einer Maschine zur Ernte von ¨ ¨ Hanfstroh und Hanfkornern (Cannabis sativa L.). Informationen fur ¨ ¨ die Pflanzenproduktion, LTZ Augustenberg, 2002.

Chen and Liu (2003) tackled the problem of deferred maturity of the seeds, proposing a special solution for harvesting dual-purpose hemp crops. The ‘two-windrow concept’ is based on the separate cutting of the plant head with a forage harvester, depositing this part of the plant in an extra swath. Prototype harvesters have recently been proposed following a similar concept, but in this case plant tops are collected in a trailer (Burczyk and Kaniewski, 2005; Kaniewski and Banach, 2008). These methods seem particularly suitable for harvesting sowing seed because plants tops can be harvested at physiological maturity. Until the year 2000, the companies Deutz-Fahr Erntesysteme GmbH (Lauingen, Germany) and Gerhard G¨otz GmbH (B¨uhl, Germany), supported by the processing company Bafa GmbH (Malsch, Germany), developed and manufactured the prototype of a new hemp combine harvester (Figure 5.8). With this machine, the hemp plant is mowed from stubble, cut into 600 mm pieces, threshed to separate seeds and stacked in a swath on the field. The hemp harvesting combine was examined closely in a 2 year field experiment (Mastel and Stolzenburg, 2002). The results are summarised below: r The problem of deferred maturity of the seeds is still obvious; overmature seeds are lost owing to mechanical shaking of the hemp plant by the combine header before being fed into the machine. r The hemp straw is partly decorticated by passage of the whole crop through all working elements of the combine; raw material losses (fibres, shives) can therefore be expected. r Owing to partial crushing, the remaining retting is faster.

5.4.4

Alternative Harvesting Procedures

In recent years, several attempts were made to develop and improve systems for on-field decortication of hemp (and other bast fibre crops) with the main purpose of bringing more added value to primary producers (Venturi and Amaducci, 2004; Venturi et al., 2007). The German textile and agricultural machinery manufacturers Bahmer (S¨ohnstetten) and Claas (Harsewinkel) developed prototypes for on-field decortication of flax in the early 1990s (Gschoßmann, 1993; Weigelt, 1993), as did the University of Bonn (Heintges, 2000) and the German farmer N¨olke (Gusovius, 2002). All these developments were stopped after an evaluation phase. The idea of carrying out decortication on fresh stems was already realised in the 1930s (Peglion, 1937), and it was evaluated as a promising technique when hemp was used for papermaking (Venturi, 1970; de Maeyer and Huismann, 1994). Nowadays, the need to reduce transport costs, reflecting the prolonged distances between the area of cropping and the location of processing, is underlining the relevance of concepts such as on-field decortication.

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The damaging of the plant material in the fresh green state by the working elements of the machines as well as the resulting insufficient fibre was described as the main obstacle (M¨ussig et al. 1999; Gusovius et al., 2000; Gusovius, 2002; M¨ussig and Martens, 2003). Another main problem of almost all concepts for on-field decortication is the loss of a large part of the economically viable biomass, the shives, in the field. Technical solutions to overcome this obstacle (Voskens, 1993) could not be established in agricultural practice so far. A few years ago the company Ecofibre Industry Ltd, Maleny, Australia, instead established a totally different supply chain for hemp fibre, using machinery concepts from cotton harvest and storage. The key machine is a field processing unit (FPU) that decorticates the dried hemp straw before it is stored in a block stack (module) at field boundaries (Warner, 2008).

5.4.5

Harvesting for ‘Wet Line’

In the 1990s, Dutch scientists and engineers were investigating known storage systems similar to animal fodder (‘silage’) in order to improve the supply of high-quality raw materials originating from bast fibre crops for the pulp and paper industry (de Maeyer and Huismann, 1994; Huismann et al., 1995). Harvesting was carried out similarly to field processing with a mobile roller breaker system. Further scientific work was carried out on anaerobic storage of hemp and its utilisation in composite materials (von Buttlar et al., 1997; Scheffer et al., 1998; Einsiedel et al., 1998). The Leibniz-Institute of Agricultural Engineering, Potsdam-Bornim, Germany, started scientific research on the utilisation of the whole crop via an alternative supply chain at the end of the 1990s. The hemp plant is harvested with a moisture content of less than 40% with a conventional forage chopper with minor modifications in the type of cutter header (Pecenka et al., 2008b). Afterwards, the plant material, chopped to lengths of approximately 20–50 mm, is stored under anaerobic conditions until being processed into (intermediate) products.

5.5

Hemp Processing

Fibre extraction is normally carried out in industrial facilities following two main processing routes. Long hemp (longitudinal hemp) for clothing textile destination can be obtained in processing lines that always keep fibre bundles aligned, avoiding tangle which would dramatically reduce fibre yield during the hackling phase. For this purpose, old hemp processing lines are still available in eastern European countries that can scutch entire stems, yielding hemp fibre bundles as long as the stems. Alternatively, flax scutching lines can be used to process short hemp crops (i.e. baby hemp) or hemp stems cut to an appropriate length (approximately 1 m). Processing for disordered hemp (hemp fibre collectives whose fibres and fibre bundles have no preferred orientation) is actually the most common destination for hemp crops in Europe, and various lines have been developed for the purpose. Irrespective of the processing route, hemp stems or fibre bundles must go through a retting process, which facilitates the separation of the bark from the core and loosens the binds between single fibres so that, after extraction, cleaner and finer fibre bundles are obtained. Traditional retting of hemp stems carried out in ponds and rivers produced high-quality fibre bundles. Recently, most of the hemp cultivations in central and northern Europe have been dew retted: enzymes released by microorganisms (particularly fungi) degrade cell wall components in a process that is highly weather dependent but guarantees homogeneous and reproducible hemp quality (see Chapter 4). Enzymatic, microbiological and chemical methods that have been developed at a prototype scale to separate fibre bundles in controlled conditions have as yet not proved to be economically viable for industrial upscale.

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Processing of Longitudinal Hemp (for Textile Clothing Destination)

Hemp is considered to be a sustainable surrogate for natural and synthetic fibres, and high-added-value textile applications could provide a fundamental economical support for new hemp production chains. For these reasons, and considering that nowadays the large majority of hemp textiles for clothing are produced in Asia, various projects have recently studied the possibility of developing an all-European hemp clothing industry (Amaducci, 2003; Riddlestone, 2006). Two main production options have been identified: the first is the long hemp line (longitudinal hemp), the same as that employed for linen processing, which produces hackled long fibre bundles suitable for wet spinning; the second is the disordered hemp line, similar to those described for technical applications, which produces carded and cottonised fibre bundles or fibres (see Figure 4.6 in Chapter 4). Long fibre bundles are produced both in systems that can accept full stems, such as traditional scutching lines, and in linen processing lines that accept baby hemp and stem portions. In both systems, the stems are first fed into several pairs of breaking rolls which crush the stems and break and partially remove the shives. Subsequently, fibre bundles, held by grippers, are passed through scutching turbines, which soften and refine fibre bundles and also remove shives that are still attached to the fibre bundles. The efficiency of the process increases with the degree of retting of the stems; after water or dew retting, stems are easier to decorticate, and the fibre bundles obtained are cleaner than those obtained from non-retted stems (Sponner et al., 2005). Scutching produces parallel long fibre bundles, short fibre bundles (tow), shives and dust. Tow has similar characteristics to the short fibre bundles produced on disordered production lines used in most cases for technical applications (described in the following section), and it can in fact be used for technical applications and paper pulp production, as well as for low-value textiles (after carding or cottonisation). Inclusion of a retting step is essential for obtaining high-quality fibre suitable for high-added-value textiles. Traditionally, stems were retted prior to scutching, but, within the framework of the European project HempSys, this process was inverted. Stem portions of approximately 1 m were scutched in modern flax machines and microbiological retting of long fibre bundles was carried out afterwards (Amaducci et al., 2008c). Other projects have supported similar ideas, on short or not aligned fibre bundles, but using chemicals or enzymes to upgrade hemp quality (Riddlestone, 2006). All these approaches guarantee higher fibre quality and homogeneity compared with weather-dependent field retting but, because of high processing costs, can only be used for high-value niche productions. 5.5.2

Processing of Disordered Hemp

A long period with limited interest in hemp and in native bast fibres generally for technical applications has led to a lack of technological development in processing technologies. In the 1980s and 1990s, several machine manufacturers came into the market to establish new fibre processing facilities based on a renewed interest in flax and hemp (Gschoßmann, 1993; Charle and Wolpers, 1997; Desmadryl, 1998; Morgner, 1999; Poillet, 2000). Figure 5.9 illustrates alternative solutions for the four main steps involved in the processing of baled hemp stems. Bale opening technology (see Figure 5.10) depends mainly on the type of bale fed into the system (round or big square/rectangular). The purpose of this first step is to loosen and open the bale to enable a continuous flow of raw material to the following processing steps. The length of the hemp stem portions and the efficiency of the bale opener determine the throughput of the whole plant (Munder et al., 2005, Pecenka, 2008). After the detection and removal of metal and/or large mineral impurities, the stalks are carried along to the primary processing step. Depending on the type of decortication machine and the characteristics of the feed straw, it is necessary to adjust the material flow with a metering system (Munder et al., 2005). The main objective of decortication is the mechanical separation of bast fibres and shives and the refinement of hemp via the separation of fibre bundles. This can be realised by means of break and kink forces or by means of impact and shear forces. Therefore, the specific characteristics of the two main components (fibres

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Figure 5.9 Main process steps in industrial hemp processing. Adapted from R. Pecenka, Optimieren der Reinigung von Hanffasern auf Kammschutteln. Dissertation. Brandenburg Technical University Cottbus, 2008. ¨

are stronger and more flexible than shives) are utilised. Three different main principles of decortication can be distinguished (Figure 5.11). The most traditional systems of decortication, employing breaker rollers, have already been used for a very long time in the textile flax processing. Some machine manufacturers engineered this principle of decortication for hemp as well (Gschoßmann, 1995; Poilett, 2000). Most of these installations were stopped because of limited throughout capacity and other technical problems. Hammer mills decorticating hemp straw for the pulp and paper industry have already been in operation for several decades, especially in France. The disintegration effect to separate fibre bundles and shives is quite effective owing to the acting forces of the steel beaters on the plant material. However, modifications are needed to prevent fibre damage. The actual working capacity of such milling systems ranges from 5 to 10 t/h. Further developments in this area have led to technological progress in the whole natural fibre industry. Van Dommele (Gullegem-Wevelgem, Belgium) is offering a new concept for the decortication of bast fibre plants based on hammer mills (Declerck et al., 2008). Already running processing lines implementing this concept in England (Figure 5.12) and France demonstrate a new level of efficiency, with minimum throughput rates of 6 t/h. The necessary cleaning and refining equipment (by Temafa) is installed in a parallel double line.

Figure 5.10 Different types of bale opening technology. Adapted from R. Pecenka, Optimieren der Reinigung von Hanffasern auf Kammschutteln. Dissertation. Brandenburg Technical University Cottbus, 2008. ¨

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Figure 5.11 Decortication systems. (Left) Adapted with permission from F. Thronicke et al., Maschinentechnische Grundlagen in der Textilindustrie, 1989. Copyright Carl Hanser Verlag GmbH. (Right) Adapted with permission from Demaitere b.v.b.a., 2009, http://www.demtec.eu.com.

Based on scientific research, a pilot plant was developed and built at the Institute of Agricultural Engineering Potsdam-Bornim, Germany, in early 2000 (Figure 5.13). The main innovation is a decorticator based on the working principle of a hammer mill for the first time with an integrated cleaning effect. The decorticator is equipped with a sieve at the bottom of the process room to remove the already loosened shives from the plant material. Specific tools at the inner top of the machine improve the disintegration effect and reduce negative effects of impact stress on the fibre bundles (Munder et al., 2004; F¨urll et al., 2008). Generally, both conventional and adapted milling systems are less affected by the degree of retting of the stems and, within certain limits, by the moisture content of the straw. The improved detachment effect of fibre bundles and shives enables a reduced effort and complexity of the following cleaning process steps.

Figure 5.12 Scheme of a processing plant for hemp straw with a throughput of 6 t/h. R. Pecenka, C. Furll, H.-J. Gusovius ¨ and T. Hoffmann, Optimal plant lay-out for profitable bast fibre production in Europe with a novel processing technology, J. Biobased Mater. Bioenergy, in press (2009).

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Figure 5.13 Layout of the ATB pilot plant. Reproduced with permission from C. Furll ¨ et al., The Potsdam-Bornim plant for separation of natural fibre a new technology for cleaning and sizing of shives, 2008 International Conference on Flax and Other Bast Plants.

Another decortication principle – scutching – was also formerly engineered to decorticate fibre plants (Charle and Wolpers, 1997). The plant material is fixed by feeding rollers, and specified tools of the rotating scutching drum beat to disintegrate the bond between fibre bundles and shives (see Figure 5.14). Loosened shives and dust are able to pass the subjacent screen. The company Demaitere (Moorslede, Belgium), a longtime expert in flax processing technology, is recommending different types of this technology for successive steps in a complete scutching line (Demaitere, 2009). In the subsequent step(s) of cleaning, the shives, which are loosened but still in the fibrous mixture, have to be removed. In most existing processing lines, three main types of cleaning technology are used. All of these principles are used in modern bast fibre processing lines. Ultracleaners and scutching turbines (also known as tambours) are considered very effective with regard to working quality and throughput. While scutching machines are used exclusively after breaker roller decortication, nowadays drum sieves play a major role in fibre bundle cleaning as well. Comb shakers are often characterised as having a limited throughput and are mechanically fragile. A step forward could be realised by scientific research leading to an innovative advancement of this concept (Pecenka 2004; Pecenka, 2008). Results show that, because of the

Comb shaker Above or under a grid, attached combs shake and move the fibre bundles by active access of the comb pins; basically, good break-up of fibre–shive fluff is possible (adapted from Herzog, 1927)

Multiple ultracleaner/step cleaner Generates very intensive loosening of the fibre–shive mixture by the roller beaters and the additionally created airflow (adapted from Team of authors, 1988)

Scutching turbine Fibre bundles are clamped by feeding elements and beaten to loosen and separate shives passing the screen (adapted from Pecenka, 2008)

Figure 5.14 Different technologies employed to clean fibre bundles from shives (and other non-fibrous materials). (A) Adapted with permission from O. Herzog, Technologie der Textilfasern, Hanf und Hartfasern, Springer, 1927. (B) Adapted with permission from Technologie der Garn- und Zwirnherstellung, Autorenkollektiv, 1988. Copyright Carl Hanser Verlag GmbH. (C) R. Pecenka, Optimieren der Reinigung von Hanffasern auf Kammschutteln. Dissertation. Brandenburg Technical University Cottbus, 2008. ¨

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Opener

Carding machine

Used a horizontal or vertical design, clamped fibre bundles are treated by a rotating tambour drum equipped with needles or fine teeth; fine particles, tissue and dust passing the subjacent screen are removed (adapted from Charle, 2009)

With a working principle similar to that of openers, a larger, fast-running tambour additionally surrounded by smaller work and turning rollers; fibre bundles are parallelised and refined by means of different roller/drum speeds; commonly used for felts and fleeces (adapted from Team of authors, 1988)

127

Figure 5.15 Technological principles of fibre opening. (Left) Adapted with permission from Charle & Co bvba, 2009, http://www.charle.be. (Right) Adapted with permission from Technologie der Garn- und Zwirnherstellung, Autorenkollektiv, 1988. Copyright Carl Hanser Verlag GmbH.

reduced remaining shives content, this concept offers higher capacity. Constructional adaptations improving the mechanical stability of a comb shaker machine were proposed. Other working principles, like the pedal shaker (Morgner, 1999; Charle, 2009) and agricultural straw walkers, as well as pin, needle or tooth rollers, are known, but are not common in modern bast fibre lines. The final fibre bundle refining step is necessary to split the naturally bound fibre bundles into finer units. Within the cleaning step, remaining shives and other non-fibrous tissue matters are removed from fibre bundles. Depending on the previous processing steps (especially retting or other non-mechanical procedures), the bundles can be refined down to single fibre elements. Owing to morphological phenomena, fibre bundle refining always affects both fineness and length. Two working principles are in use for bast fibre bundle refining (Figure 5.15). Fibre bundle refining can be performed in a multistep procedure (coarse, middle and fine) to achieve end-user-requested qualities. All shives, separated at different stages of a bast fibre processing line, are commonly collected for further handling. Usually, shives have to be cleaned from impurities (fine fibre bundles, dust, etc.). The most common machine for this purpose is the so-called Duvex (Van Dommele, Gullegem-Wevelgem, Belgium), which works on the principle of a drum sieve (see Figure 5.16). The shives are moved in this sieve drum by means of rotating paddles. Dust, small fibres and smaller shives can be separated from the remaining throughput material. A similar technology is used to clean the fibre–shive mixture after hammer mill decortication (Declerck et al., 2008). The axial fractioner – a new solution combining the removal of remaining fibres, dedusting and fractionation of different shive sizes – was developed by scientists from the Institute of Agricultural Engineering PotsdamBornim (F¨urll et al., 2008). Shives and fibres are separated from each other with a rotor at high circumferential speed with special tools (paddles and sickles) after the fed fluff is dispersed. Additional components of a modern bast fibre processing line are transport devices (mechanical conveyors, or pneumatic devices using airstream in pipelines) installed between the different stages. Also, machines for packaging of the resulting products (fibre baling, shives baling or bagging) are necessary. Special attention has to be paid to dust abatement throughout the factory. The most common systems are rough and fine drum filters and downstream baghouse filters (Munder et al., 2005; Declerck et al., 2008; F¨urll et al., 2008). The collected dust, including mineral and organic particles, can be utilised as compost or compacted as a combustible fuel. Detailed performance and economic results of former or recent industrial processing lines are only available to a limited extent (Pecenka, 2004; Munder et al., 2005; F¨urll et al., 2008), also because existing lines undergo

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Figure 5.16 Drum sieve ‘Duvex’ (A) – Adapted from Van Dommele, 2009, http://www.vandommele.be and two-step axial fractioner (B) – Reproduced with permission from C. Furll ¨ et al., The Potsdam-Bornim plant for separation of natural fibre a new technology for cleaning and sizing of shives, 2008 International Conference on Flax and Other Bast Plants.

frequent technical revisions and/or modifications. Furthermore, the economic performance depends on the quality and quantity of fed raw material. Basically, every benchmarking of a processing facility or assessment of a concept has to be carried out under the existing conditions in the given proximity. Based on 13 years of experience and previous economic analyses (Munder et al., 2004; Pecenka, 2004; Steger, 2004; Carus et al., 2008), some very basic demands for the configuration of modern and long-term competitive processing lines can be summarised (Pecenka, 2008). This primarily includes the need for a minimum throughput of 2 t/h hemp straw. Higher investments can result in a reduction in labour costs through a higher automation level and the achievement of a minimum technological availability (production time) of 80–90%. Hemp fibre produced by such processing lines is suitable for the production of insulation products, geotextiles and composites (see Chapters 19 to 21). 5.5.3

Other Processing Systems

Other non-mechanical fibre processing technologies, such as chemical, ultrasound (von Drach et al., 1999), enzyme (Leupin, 1998; Dreyer et al., 2003; Fischer et al., 2006) or steam explosion treatment (Kessler and Kohler, 1996; Kessler et al., 1998) have been investigated over recent years but are not in use for the production of fibres for technical applications. Strong competition to other fibre resources in non-textile applications and comparatively high costs of such procedures have impeded their widespread use hitherto. Further developments and industrial applications of chemical and/or biological treatment procedures are being carried out in China, but the corresponding scientific documentation is not yet available. von Buttlar et al. (1997) reported on research results evaluating and utilising hemp from a silage process for technical applications. They were able to show that some fibre characteristics (geometry characteristics such as fineness) generate an interesting spectrum of applications, while others have to be recognised as problematic (mechanical characteristics such as strength, or organoleptic characteristics such as odour). An innovative processing route utilising the whole plant has been investigated on a pilot-plant scale at the Institute of Agricultural Engineering Potsdam-Bornim, Germany, since 2006. Within the patented procedure, wet preserved fibre plants are milled to reduce and homogenise the particle size with an extruder and a disc mill. The resulting fibrous material is dried, mixed with binders and laid to a fleece. Finally, the mixture is pressed into fibreboards. An adequate product quality can be achieved even at this early stage of development. The first boards produced, glued with synthetic binder systems, fulfilled the minimum standards of mechanical stability (Radosavljevic et al., 2008). The specific odour of the raw material and, to a certain extent, of the fibrous material is caused by some typical organic acids that arise in anaerobic storage. Although still under investigation, the

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problem seems solvable because the organic acids in question are thermally unstable to the drying and hot pressing of the fibrous material (Pecenka et al., 2008a).

5.6

Conclusions

Hemp is a valuable crop with positive agronomic characteristics that provides raw materials suitable for multiple industrial applications such as insulation products and composites. Several bottlenecks hamper production chain development, especially in the case of ‘longitudinal’ hemp, and major limitations are found in the harvesting and processing steps. Recently, harvesting equipment and industrial lines to process ‘disordered’ hemp have been developed and are now in use, while technologies for longitudinal hemp processing are only present on a prototype scale. Optimisation in actual and potential hemp production is possible through industrial research to define the effect of the different productions steps, from cultivation to processing, on fibre yield and quality. Environmental sustainability and improved quality are the key to the success and expansion of the multitude of industrial hemp fibre applications that are already available (e.g. in the automotive industry, building and insulation materials) around the world.

References Allavena, D. (1961) Fibranova: nuova variet`a di canapa ad alto contenuto di fibra. Sementi Elette, 5, 34–44. Amaducci, S. (2003) HEMP-SYS: design, development and up-scaling of a sustainable production system for hemp textiles – an integrated quality system approach. J. Ind. Hemp, 8(2), 79–83. Amaducci, S. (2005) Hemp production in Italy. J. Ind. Hemp, 10, 109–115. Amaducci, S., Colauzzi, M., Bellocchi, G. and Venturi, G. (2008a) Modelling post-emergent hemp phenology (Cannabis sativa L.): therory and evaluation. Eur. J. Agron., 28, 90–102. Amaducci, S., Colauzzi, M., Zatta, A., Venturi, G. (2008d) Flowering dynamics in monoecious and dioecious hemp genotypes. J. Ind. Hemp, 13(1), 5–19. Amaducci, S., Errani, M. and Venturi, G. (2002a) Response of hemp to plant population and nitrogen fertilisation. Ital. J. Agron., 6(2), 103–111. Amaducci, S., Errani, M. and Venturi, G. (2002b) Plant population effects on fibre hemp morphology and production. J. Ind. Hemp, 7(2), 33–60. Amaducci, S., M¨ussig, J., Zatta, A. and Venturi, G. (2008c) An innovative production system for hemp fibre for textile destinations: from laboratory results to industrial validation, in 2008 International Conference on Flax and Other Bast Plants, Saskatoon, Canada, 21–24 July 2008. Saskatchewan Flax Development Commission, Saskatoon, Canada; Crop Fibers Canada, Canada; FAO/ESCORENA – European Cooperative Research Network on Flax and Other Bast Plants – Rome, Italy; International Institute of Natural Fibres, Poznan, Poland, pp. 104–117. Amaducci, S., Pelatti, F. and Medeghini Bonatti, P. (2005) Fibre development in hemp (Cannabis sativa L.) as affected by agrotechnique: preliminary results of a microscopic study. J. Ind. Hemp, 10(1), 31–48. Amaducci, S., Zatta, A., Pelatti, F. and Venturi, G. (2008b) Influence of agronomic factors on yield and quality of hemp (Cannabis sativa L.) fibre and implication for an innovative production system. Field Crops Res., 107, 161–169. Amaducci, S., Zatta, A., Raffanini, M. and G. Venturi (2008e) Characterisation of hemp (Cannabis sativa L.) roots under different growing conditions. Plant and Soil, 313, 227–235. Barbieri, R. (1952) La ‘Prefioritura’ della canapa in Campania nell’annata. Agricoltura Napoletana, 7–9. Bassetti, P., Mediavilla, V., Spiess, E., Ammann, H., Strasser, H. and Mosimann, E. (1998) Hanfanbau in der SchweizGeschichte, aktuelle Situation, Sorten, Anbau- und Erntetechnik, wirtschaftliche Aspekte und Perspektiven. FATBerichte 1998 No. 516, Eidg. Forschungsanstalt f¨ur Agrarwirtschaft und Landtechnik (FAT), T¨anikon, Switzerland. Berger, J. (1969) The world’s major fibre crops: their cultivation and manuring. Centre D’Etude de l’Azote, Zurich, Switzerland. Biewinga, E.E. and van der Bijl, G. (1996) Sustainability of energy crops in Europe. Centre for Agriculture and Environment, CLM 234, Utrecht, The Netherlands, 1996. B`ocsa, I. and Karus, M. (1998) The cultivation of hemp: botany, varieties, cultivation and harvesting. Hemptech, Sebastopol, CA, USA.

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6 Jute – A Versatile Natural Fibre. Cultivation, Extraction and Processing Md. Siddiqur Rahman International Jute Study Group, Tejgaon, Dhaka, Bangladesh

6.1 Introduction Jute is an annually grown natural fibre. It is biodegradable and environmentally friendly. It provides sustenance to millions of people. It has a wide range of usage. Besides being used as packaging material worldwide, it is now widely used as floor coverings, home textiles, decorative fabrics, shopping bags, carrier bags, handicrafts, cushion covers, curtains, blankets, nursery pots, insulation material, soil saver, jute-based composites, etc. It has the potential to be used on a large scale as a geotextile in various applications such as soil stabilisation, erosion control, etc. It could be a good source of raw material for making pulp and paper. This chapter is intended for all kinds of reader and will present basic information about jute, providing a comprehensive overview of this versatile natural fibre. Information will be given on the identity of jute, how it is grown and the fibre obtained, the physical properties that make it suitable for diverse applications, the processes involved in manufacturing jute products and their uses. With the growing global awareness of the need for a pollution-free environment, jute is poised to become the fibre for the future for various end-uses and applications.

6.2

Background

Jute is the common name given to the fibre extracted from the stems of plants belonging to the genus Corchorus, family Tiliaceae. Kenaf is the name given to the rather similar fibre obtained from the stems of plants belonging to the genus Hibiscus, family Malvaceae, especially the species H. cannabinus L., while H. sabdariffa L. is known as mesta. About 40 species of Corchorus are known throughout the world, but C. capsularis (white jute) and C. olitorius (tossa jute) are the ones that are cultivated for their fibre (Kundu, 1956; Atkinson, 1965). Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Industrial Applications of Natural Fibres Table 6.1 World production of jute, kenaf and allied fibres (2006/2007–2007/2008) in thousand t 2006/2007 World Bangladesh China India Myanmar Nepal Thailand Vietnam

2007/2008

3051.31

2994.52

* 1186.00

* 1237.00

86.80

86.80

** 1800.00

** 1782.00

43.60 17.10 36.00 10.50

40.00 17.10 36.00 10.50

Data from FAO Statistics on Jute, Kenaf and Allied Fibres, June 2008 and *Ministry of Textiles & Jute, Government of Bangladesh & **Office of the Jute Commissioner, Government of India.

In general appearance, C. capsularis and C. olitorius are similar, having long straight stems about 3 cm in circumference, unbranched except at the top. The main difference between the two species is in their fruits: C. capsularis has a rough, wrinkled, spherical seed box of about 0.75 cm diameter, while C. olitorius has an elongated pod like a miniature cucumber about 5 cm long (see Figure 2.3.3 in Chapter 2.3). Moreover, C. capsularis tends to be shorter than C. olitorius. C. capsularis is grown on lower-lying ground, while C. olitorius is grown on higher ground. Tossa jute has a higher yield per hectare and commands a better price. Jute is grown in the rainy season at temperatures of 21–38◦ C with a relative humidity of 65–95% (Atkinson, 1965). It requires a rainfall of at least 1000 mm spread evenly over the 4 months of the growing season (Sobhan, 2010). The world jute, kenaf and allied fibres production appears to hover around 3 million t (Table 6.1). India is the largest raw jute producer and also the largest consumer. Bangladesh is the second largest raw jute producer and is the main raw jute exporter (Table 6.2). It is also apparent that world export of raw jute has increased, indicating a higher demand for raw jute in the world market. There does not appear to be a significant change in world export of jute products (Table 6.3). Most of the plants cultivated for fibre, including jute and kenaf, are grown from seed annually, but a few are grown as perennials. Jute is the most important fibre of this type, and it is probable that, in the industrial and engineering uses of textiles, more jute fibre is used than any other fibre (Rowell et al., 1998). Jute felt, jute webbing, etc., have industrial applications, while jute ropes, twines, sacks and hessian cloths are used as agrotextiles and build-techs. Kenaf is used in many countries where it is grown, but its international market is much smaller than that for jute. In many marketing statistics, figures given for the production or utilisation

Table 6.2 World exports of raw jute, kenaf and allied fibres in thousand t Country World Bangladesh Myanmar Thailand Others

2006/2007

2007/2008

468.1

533.8

* 439.7

* 517.0

9.0 1.0 18.8

10.0 1.0 5.8

Data from FAO Statistics on Jute, Kenaf and Allied Fibres, June 2008 and *Ministry of Textiles & Jute, Government of Bangladesh.

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Table 6.3 World exports of products of jute, kenaf and allied fibres in thousand t

World Bangladesh China India Nepal Thailand EC (27) Others

2006

2007 (provisional)

766.3 478.9 18.5 189.5 13.0 7.0 42.2 17.2

754.0 477.0 20.0 175.6 13.0 7.0 41.4 20.0

Data from FAO Statistics on Jute, Kenaf and Allied Fibres, 2008. Reproduced with permission from the Food and Agriculture Organization of the United Nations.

of ‘jute and allied fibres’ include all the fibres in this group. ‘Allied fibres’ are suitable for processing on jute spinning systems. Favourable conditions for jute cultivation are found in the deltas of the great rivers of the tropics and subtropics – the Ganges, the Irrawaddy, the Amazon and the Yangtze, for example – where irrigation, often by extensive flooding, and alluvial soils combine with long day lengths to provide the opportunity for considerable vegetative growth before flowering (Sen, 2009). Large-scale jute cultivation in Bangladesh and India was started about 200 years back. In view of the agroclimatic requirements, the cultivation of jute was mainly concentrated in the Ganges–Brahmaputra–Meghna delta areas, and also in adjoining areas covering Bangladesh and the adjoining states of India. Initially, jute was considered as a source of raw material for the packaging industry only, but now it has emerged as a versatile raw material for diverse applications (Biswas, 2004). Although there has been little change in the technical nature of the fibre, considerable developments have taken place in the techniques of conversion to yarn and fabrics, and in the end-uses for these products. Scientific studies began around 60 years ago, and, although they never received publicity on the scale given to studies of cotton and wool, the broad features of the internal structure and physical characteristics of these fibres were elucidated sufficiently long ago for a great deal of common knowledge to have been built up. Jute is predominantly a rain-fed annual crop. Its cultivation is labour intensive, but it requires relatively small quantities of other inputs, such as fertiliser and pesticides, and can be carried out in small holdings. For these reasons, jute production is increasingly concentrated in Bangladesh, India, Nepal, China and Thailand, which together account for more than 95% of the world production. Jute competes for land with food crops such as paddy (Oryza sativa L.) in Bangladesh and India, and cassava (Manihot esculenta Crantz) in Thailand. Land allocation between paddy and jute depends on the relativity of price levels and price variability. In general, producers attempt to adopt a multicropping strategy, with jute in rotation with paddy. Nevertheless, substitution between the two crops does take place, as producers attempt to minimise the risk of lower paddy yields resulting from delayed paddy transplanting. However, depending on the region, the possibilities of substituting paddy for jute may be limited on account of flooding (FAO, 2003) Jute and kenaf are versatile textile fibres. The fibres are biodegradable, environmentally benign and renewable and provide reliable employment in many rural areas. Jute fibre bundles are used to manufacture colourful carpets, carpet backing cloth, cordage, decorations, clothing fabrics, blankets, geo- and agrotextiles, ‘non-woven’ materials, industrial fabrics, thermal insulation and numerous utility items in a range of traditional and innovative uses. When used as a source of biomass fuel, jute and kenaf production helps to conserve tree cover and natural forests. Moreover, leaf and crop trash remains in the field to be recycled as organic materials, thereby reducing demand for supplementary chemical fertilisers for subsequent crops.

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6.3 Jute Agriculture 6.3.1

Land Preparation

The land has to be prepared and made suitable for growing of jute. The land is ploughed, and laddering is done 3–4 times to loosen the soil and make it suitable for growing of the jute plants.

6.3.2

Sowing

Sowing is done in the month of April by the broadcast method at a rate of 5.4 kg of seed per hectare for C. capsularis and 5.0 kg per hectare for C. olitorius. The sower walks across the field, scattering the seeds to either side. When the ground has been covered in one direction, the process is repeated by walking at right angles to the original line. In this way, a uniform distribution of seeds can be achieved. A light covering of earth is then drawn over the seeds and the surface is consolidated by laddering. Line sowing, which gives a better yield of fibre, is encouraged, but most seed is sown by the broadcast method (Atkinson, 1965). The seeds germinate within 2–3 days, and a few million plants are formed per hectare. Weeding and thinning are carried out manually in two stages until a final count of around 370 000 plants are left spaced 10–15 cm apart.

6.3.3

Formation of Fibre

Jute fibre develops in the phloem, or bast region, of the stem of the plants, in transverse sections of the stem (see Chapter 2.2 for more details). They appear as wedge-shaped bundles of cells intermingled with parenchyma cells and other soft tissue. In the growing part of the stem, a circumferential layer of primary fibres develops from the protophloem, but, as vertical growth ceases in the lower parts, secondary phloem fibres develop as a result of cambial activity. The secondary fibre accounts for about 90% of the total fibre bundles. The plants pass from vegetative to reproductive phase when the day length falls below 12.5 h. Vertical growth then ceases, and cambial activity declines. The production of cell bundles is much reduced, but at the same time the secondary fibre cells begin to mature rapidly. Their walls, which have remained thin during the vegetative period, become thicker, and they increase in mass and strength (Rowell and Stout, 1998).

6.3.4

Harvesting

Jute reaches about 2.5–3.5 m in height at maturity. Harvesting of the plants at the correct time is most important and requires long experience. The correct time for harvesting jute is judged to be when the plants are in the small pod stage. Harvesting before flowering generally results in lower yields and weaker fibre; and if the seeds are allowed to mature, the fibre becomes harsh and coarse and difficult to extract from the plant. Figure 6.1 shows a typical jute field ready for harvest. The normal age of the crop for good harvest is 110–120 days, but very often it has been observed that the crop age at harvest time varies from 100 to 135 days (Sobhan, 2010; Jarman, 1985).

6.4 Fibre Extraction The single jute fibre is only 1–6 mm long and 5–30 µm wide. For most products, fibre bundles are used, which can be 200 µm or more thick and as long as the complete jute stalk. The quality of fibre determines

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Jute – A Versatile Natural Fibre. Cultivation, Extraction and Processing

Figure 6.1

139

Jute field.

the end-uses. Jute is a bast fibre crop, and the fibre is obtained by extraction from retted plants. Retting is the process of separation and extraction of fibre from the woody part of the stem through dissolution and decomposition of pectin, gums and other mucilaginous substances. The retting process is one of the most important factors governing the quality of fibre and fibre bundles. A comparison between stem and ribbon retting and the influence on fibre quality can be found in Mitra (1999).

6.4.1

Retting Methods

Retting can be done either by (1) the chemical method or (2) the biological method.

6.4.1.1

Chemical Retting

In the chemical method, ammonium oxalate (N2 H8 C2 O4 ), sodium sulphite (Na2 SO3 ), etc., are used. The process is, however, costly, and therefore not practised by the farmers of Bangladesh and India (Biswas, 2004).

6.4.1.2

Biological Retting

The biological retting method is usually practised in Bangladesh and India. It consists of: (i) the stack method, (ii) ribbon retting and (iii) the steep method: (i) Stack method. In the stack method, the bunches of plants are stacked and some fungal culture is used for retting.

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(ii) Ribbon retting. Ribbon retting consists of ribboning of the bark from the stem. Manual or mechanical ribboners have been developed both in Bangladesh and in India. Ribbons are retted mainly by the biological method. Ribbons can also be retted by the chemical method. Ribbon retting technology is not, however, widely practised by the farmers. A major disadvantage of ribboning by decorticators, from the perspective of the farmers, is the damage to the jute stem. Decorticators fracture the stem into short pieces in separating the ribbons of bark from the stem. (iii) Steep method. The steep method of retting, usually seen as the traditional and most common method of retting, is practised in Bangladesh and India. In biological retting, the harvested plants are tied into bunches and the bunches are submerged in water – lakes, rivers, ditches, canals and ponds – for retting. The action involves water, microorganisms and enzymes, and retting takes about 15–20 days, depending on the temperature of the water. Various microbiological activities occur during the retting process, by which the fibres and fibre bundles are loosened and then separated out. The bunches are placed in retting water side by side, usually in 2–3 layers, to form a sort of platform called a ‘float’ or ‘jak’. The ‘jak’ is then covered with water hyacinth (Eichhornia crassipes) or any weed. The covering material should be such that it does not release tannin or iron, as these are likely to have an adverse effect on the quality and lustre of the jute. 6.4.1.3

Weighting-Down Material

The ‘jak’ is then weighted down with bamboo or a wooden log or coconut stem, or such other materials, and is kept submerged below the surface water. Clods or banana stem should not be used because these would damage the quality and colour of the fibres. Retting being a microbiological process, the completion of retting is determined by inspecting a few plants from day 10 onwards. If the fibre bundles readily slip off the wood on pressure from the thumb and finger, then retting is considered complete. Constant supervision is required, and the time of removal is critical, because, if the degree of retting is insufficient, the fibre bundles cannot easily be stripped from the woody core and may be contaminated with cortical cells; and if retting proceeds too far, the fibre cells themselves may be attacked and weakened by microorganisms. After retting, the fibre bundles are extracted manually, washed in clean water and sun dried before marketing. Figure 6.2 shows how fibres are extracted from the retted plants. The extracted fibres will be washed and dried. 6.4.1.4

Factors Affecting Retting

This method of retting is affected by various factors. Retting is a microbiological process by which the fibre bundles from the stems are loosened. Bacteria and fungi act on the soft tissue of the stem, which on dissolution makes it easy to separate the fibre bundles from the stem. Various factors affecting retting are: (i) the nature and volume of water; (ii) temperature; (iii) pH; (iv) the age of the plants: (v) the fertiliser applied to the crop; (vi) the activator used for retting; (vii) the weighting-down material, etc. (Biswas, 2004): (i) Nature and volume of water. Slow-moving, clear water is ideal for proper retting (Biswas, 2004). Moreover, the quantity of water is also very important. The ratio of ‘jak’ and water should be 1:20 (by volume). The enzymes of bacteria metabolise protein and release pectin, tannin and other gummy materials. If slow-moving water is there, these impurities are removed. Soft water is good for retting and produces better-quality fibre compared with saline or hard water. In hard water, lustre is affected and fibre becomes dazed. (ii) Temperature. The optimum temperature of retting water is 34◦ C (the higher the temperature, the faster the retting; the lower the temperature, the slower the retting). At 34◦ C temperature, the retting is completed in about 15 days.

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Figure 6.2

141

Extraction of jute.

(iii) pH value. The optimum pH value for retting of jute is 6.5–7.0. However, in the natural retting water, the pH value usually varies from 6.0 to 8.0. (iv) Age of the crop. The earlier-harvested crop rets faster than the crop harvested at a late stage. As the jute plants grow older, the tissue become more and more mature (Jarman, 1985). (v) Activator used for retting. Various activators may be used for enhancing the retting process (Rowell and Stout, 1998). Natural activators like dhaincha (Sesbania aculeata) and sunn hemp (Crotalaria juncea L.) may be used for enhancing the retting process (Halder and Kundu, 1957; Ali et al., 1972). These leguminous plants, being rich in nitrogen content, help the growth and activity of retting microbes by supplying additional nutrients to them (Ahmed and Akhter, 2001). Moreover, urea (organic compound (NH2 )2 CO) or ammonium sulphate (inorganic salt (NH4 )2 SO4 ) may be used as an activator in the retting process (Biswas, 2004).

6.4.1.5

Fungal Cultures

The Bangladesh Jute Research Institute (BJRI), Dhaka, Bangladesh, has screened fungi of different origins and found that the saprophytic fungus (Sporotricchum) is capable of retting dry ribbons of jute satisfactorily, under laboratory conditions. Post-retting treatments with the use of fungal cultures were also examined to minimise the effect of cuttings on the fibres by removing the hard and barky bottom portion without adversely affecting other fibre qualities. Aspergillus sp. was found to be beneficial in improving the quality of fibres produced by one or two grades (Ahmed and Akhter, 2001).

6.4.1.6

Retting and Fish Farming

Community retting-cum-pisciculture centres could be established, and fish could be raised there along with retting. Retting is predominantly anaerobic, and severe depletion of oxygen takes place, so the production of fish at retting time has to be restricted to air-breathing species (FAO, 1998).

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Industrial Applications of Natural Fibres

6.5

The Retting Mechanism

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The plants absorb water when steeped in water, and the soluble components come out of the plants, creating a surrounding environment that is a good starter medium for the growth of microorganisms present in the water as well as in the plants. These organisms gradually develop and multiply by utilising the free sugars, pectin, hemicelluloses, proteins, etc., of the plants as nutrients. Specific enzymes secreted by the organisms first cause degradation of the complex organic materials to simpler compounds, which are then metabolised for their life processes. A series of biochemical reactions thus proceed, as a result of which the chemical composition and pH value of the retting water continually change during the retting process. Decomposition of free sugars takes place at early stages of the process, followed by the breakdown of pectin. Decomposable hemicelluloses and nitrogenous compounds, chiefly proteins, are degraded at the later stage. Periodic analyses of retting water revealed the presence of several decomposition products such as organic acids (acetic, lactic, butyric), acetone (propan-2-one: OC(CH3 )2 ), ethyl alcohol (ethanol: C2 H5 OH), butyl alcohol (butanol: C4 H9 OH) and various gases. If, however, retting is allowed to continue beyond the optimum period, microorganisms begin to degrade the fibre cellulose. Such a condition is known as overretting (see Chapter 4 on flax).

6.5.1

Retting Microorganisms

Various attempts have been made to identify the microorganisms associated with the retting process. Among the fungi, Aspergillus niger, Macrophomina phaseoli, Mucor abundans, Chaetomium sp., Phoma sp. and several Pencillium sp. have been found to be good retting agents. Several aerobic bacteria of the genus Bacillus, namely B. subtilis, B. Polymyxa, B. mesentericus and B. macerans, and anaerobic bacteria of the genus Clostridium, namely C. tertium, C. aurantibutyricum, C. felsineum, etc., have been isolated from retting waters. The aerobic organisms grow first and consume most of the dissolved oxygen, ultimately creating an environment favourable for the growth of anaerobes. It has been stated that the greater part of decomposition is carried out by anaerobic species (Biswas, 2004). Retting and extraction processes have a profound effect on the quality of the fibre bundles produced, and on the cost of jute production. They affect the efficiency of manufacturing, the quality of the end-products and their competitiveness in the market. Ultimately, they determine the level of earnings for the industry and the returns for the growers. Given the severe levels of competition in fibre markets, jute producers are keenly aware of the need to improve retting and extraction processes, decrease their reliance on water, become less labour-intensive, lower costs and, above all, enhance the quality of the fibre bundles produced. The conventional retting method, which is easy and convenient, is practised throughout Bangladesh and India. As the factors influencing retting are not available in optimum condition, production of good-quality jute through conventional retting technology is affected. But where the factors are available in optimum condition, it is possible to produce good-quality jute with conventional retting technology. Recent developments in retting methods are summarised in Krishnan et al. (2005).

6.6 Grading of Jute The factors that are taken into account during grading are colour, length, fineness of fibre bundles, lustre, strength, cleanness, defects and the amount of root end that will have to be cut off. A strong fibre bundle with good length, even colour, high lustre, no defects and little root is considered to be of good quality. Some of the defects are as follows: (1) runners: long strips of bark adhering to the stem owing to inadequate retting; (2) rootiness: tough, hard, stiff pieces of bark sticking to the lower end of the fibre bundle;

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Jute – A Versatile Natural Fibre. Cultivation, Extraction and Processing Table 6.4

143

Standardisation of ‘kutcha grades’ – ‘white jute’ (Corchorus capsularis). Adapted from BSTI (1967)

Grade

Definition

White Top

White Middle

White B.bottom

White C.bottom

White X.bottom

Very strong fibre bundles; fairly good length; white/creamy white/bright light-golden colour; excellent lustre; free from any defect; maximum proportion of cutting 20% (by mass); would correspond to Bangla-White Special when converted to ‘pucca grade’ Strong and sound fibre bundles; fair length; white/light-cream/reddish colour; good lustre; free from specks, runners and harsh crop ends; maximum proportion of cutting 25% (by mass); would correspond to Bangla-White A when converted to ‘pucca grade’ Sound fibre bundles; fair length; silver white/light-cream to straw colour; good lustre; free from dark-grey and weak fibre; free from specks and runners; maximum proportion of cutting 25% (by mass); would correspond to Bangla-White B when converted to ‘pucca grade’ Average strength; any colour average lustre; free from hard-centred jute and runners and hard gummy tops; maximum proportion of cutting 33% (by mass); would correspond to Bangla-White C when converted to ‘pucca grade’ Any strength; any colour; occasional croppy and gummy tops, with barks and specks and hard-centred jute; maximum proportion of cutting 40% (by mass); would mostly correspond to Bangla-White D when converted to ‘pucca grade’

(3) croppy: gummy harsh top ends of the fibre bundle; (4) specky: small, black pieces of bark sticking to the fibre bundle; (5) dazed: dull, weak fibre bundle. In Bangladesh, grading is done in two stages – one for the home trade (kutcha grade, Table 6.4) and one for the export trade (pucca grade, Table 6.5). In India there is only one grading system (Table 6.6). The grading and classification of jute has a long history but is still carried out subjectively by hand and eye. Nevertheless, a degree of consistency is achieved, particularly for export purposes, and experienced buyers and sellers do not find it too difficult to agree on whether the grade assigned to a particular consignment of fibre is correct. The Bangladesh Standards and Testing Institution (BSTI), Dhaka, Bangladesh, and the Bureau of Indian Standards (BIS), New Delhi, India, are government authorities for developing standards.

Table 6.5

Export grades (‘pucca grades’) of ‘white jute’ (Corchorus capsularis) of Bangladesh. Adapted from BSTI (1967)

Grade Bangla-White Special Bangla-White A Bangla-White B Bangla-White C

Bangla-White D

Bangla-White E

Definition

Abbreviation

White/creamy white jute of the texture; very strong and very good lustre; completely free from any defect; clean cut and well hackled and entirely free from red ends White to light cream; jute of fine texture; strong and very good lustre; completely free from any blemish; clean cut, well hackled and entirely free from red ends Light cream to straw colour; jute of good texture; strong and good lustre; free from blemish; clean cut and well hackled, red ends excluded Light grey/light reddish to straw colour; clean jute of sound strength and average lustre; free from hard specks and croppy or hard gummy tops; well hackled, free from black roots, red soft ends permissible Any colour; average strength; occasional bark and specks permissible; slightly croppy and gummy tops permissible; well cut on the hard and hackled, red ends permissible Any colour; any strength but free from perished fibre bundle; free from any unretted jute and stick, but bark and hard centre permissible; rough cut on the hard and hackled

BW-Special

BW-A BW-B BW-C

BW-D

BW-E

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Table 6.6 Fibre characteristics and scoring scheme for different grades of ‘white jute’ (Corchorus capsularis). Figures in parentheses indicate score marks. Adapted from Indian Standard Specification No. IS 271 (1975)

Grade

Strength

W1

W7

Very good (26) Free from major and minor defects (22) Good (22) Free from major and minor defects (22) Fairly good (18) Free from major defects except a few specks (18) Free from major defects Fair average (14) and substantially free from specks (14) Average (10) Free from major defects (10) Average (10) Free from centre root and dazed/overretted fibre and reasonably free from entangled sticks (4) Weak mixed (3) —

W8

Entangled or any other jute not suitable for any of the above grades but of commercial value.

W2 W3 W4

W5 W6

Defects

Maximum root content in mass % Colour 10 (33) 15 (28) 20 (24) 26 (20)

Fineness

Density

Total score

Very good (12) Very fine (5)

Heavy bodied (100) (2) Good (9) Fine (2) Heavy bodied (85) (2) Fairly good (7) Fibres well Medium (69) separated (1) bodied (1) Fair average (4) Fibres well Medium (54) separated (1) bodied (1)

36 (16)

Average (3)

—

—

(39)

46 (12)

—

—

—

(26)

57 (9)

—

—

—

(12)

In the ‘kutcha’ grading method, the whole fibre bundles as they come to village markets are categorised into five grades. These are Top, Middle, B.Bottom, C.Bottom and X.Bottom. In the ‘pucca’ grading method, fibres whose basal parts have been cut away as cuttings are categorised into six grades. These are BW-Special, BW-A, BW-B, BW-C, BW-D and BW-E for white jute, and BT-Special, BT-A, BT-B, BT-C, BT-D and BT-E for ‘tossa jute’. The grades Special and A to E stand in order of merit and thus indicate degree of quality.

6.6.1

Jute Grading (Bangladesh)

The standardisation of kutcha grades – ‘white jute’ (Corchorus capsularis) – is shown in Table 6.4 (BSTI, 1967). Kutcha grading of ‘tossa jute’ is done similarly. Table 6.5 gives an overview of the ‘white jute’ export grades (‘pucca grades’) of Bangladesh. ‘Tossa jute’ (Corchorus olitorius) of Bangladesh is classified into six grades, similar to ‘white jute’ with slightly different definitions (IJSG, 2003).

6.6.2

Jute Grading (India)

Table 6.6 gives a detailed overview of the fibre characteristics and scoring scheme for different grades of ‘white jute’ (Corchorus capsularis) according to the Indian Standard Specification No. IS 271-1975. ‘Tossa jute’ (Corchorus olitorius) of India is classified into eight grades, similar to ‘white jute’ (Corchorus capsularis) (IJSG, 2003). Some important physical properties of jute are given in Table 6.7, which shows the possibility of various end-uses for this fibre. Tables 6.8 and 6.9 give an overview of jute properties compared with other natural and man-made fibres. A broader comparison can be found in Tables 13.6 to 13.10 in Chapter 13.

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Table 6.7 Important physical properties of jute fibre. Adapted with permission from A.B.M. Abdullah, L.B. Lutfar and N. Matin, An introduction to jute/allied fibres properties and processing, International Jute Organization, Dhaka, Bangladesh, 1992 1. Jute cell ultimate width (range) 2. Jute cell ultimate length (range) 3. Jute cell ultimate width (average) 4. Jute cell ultimate length (average) 5. Tenacity 6. Specific gravity 7. Moisture regain at 65% RH, 22◦ C 8. Fineness (g/1000 m) per single fibre 9. Breaking elongation 10. Refractive index (parallel) 11. Refractive index (perpendicular) 12. Young’s modulus (a) white jute (b) tossa jute 13. Modulus of rigidity 14. Heat of combustion

6.6.3

15–20 µm 1–6 mm 18 µm 2.5 mm 27–53 cN/tex 1.48 g/cm3 13.8% 0.26–0.46 tex 0.8–1.8% 1.577 1.536 8.6–17.4 GPa 9.6–19.4 GPa 0.442 GPa 17.46 J/g

Moisture Content and Moisture Regain

The amount of moisture held by jute can be expressed in two ways, by moisture content or moisture regain: mass of moisture × 100 Moisture content (%) = total mass of sample weight of moisture present Moisture regain (%) = mass of kiln-dry fibre sample × 100

6.7

Mechanical Processing

Jute being a coarse, hard and mechanically stable fibre bundle has always been preferred as packaging material, i.e. hessian, sacking cloth, etc., for its main commercial use. A mechanical processing system somewhere between those for wool and cotton has been developed for jute. In its raw state, jute consists of single fibres connected by cementing elements to a fibre bundle. The form of fibre bundles is complex and alters with mechanical treatment. Jute is an entity that, although appearing simple to the eye, is in reality a bundle of ultimate fibres. Fibre bundle opening is the very basis of jute processing. The type of opening required is completely different from most others, as jute fibres are not simply entangled but are actually joined. The degree of complexity in the entities can be altered by processing, so that a regular yarn might be produced by breaking down the entities to thinner bundles. The conventional jute process has two cards, i.e. breaker and finisher cards, followed by three drawing operations, a spinning frame, a weaving loom, etc. The entire conventional system of jute processing for the manufacture of goods is shown in the processing chart in Figure 6.3.

6.7.1

Process Stages

The mechanical processing of jute involves a number of stages as set out below.

***

cN/tex = (g/tex)/1.02.

Fineness in tex Breaking strength in cN/tex*** Breaking elongation in % Young’s modulus in cN/tex Toughness in J/m3

2.2 37 1.2 3088 0.18

0.2 31 8.0 386 1.44

0.7 62 4.4 1404 0.81

Ramie 1.3 53 2.0 2647 0.63

Flax 1.4 56 2.2 2526 0.36

Hemp 9.4 35 2.2 1604 0.36

Aloe 3.0 33 1.1 2968 0.18

Mesta 0.15 35 20.0 176 3.15

Silk

0.7 15 33.0 45 1.98

Wool

0.5 27 15.0 176 1.98

Viscose

0.3 45 15.0 294 2.25

Nylon

5.0 31 8.0 386 —

Steel

6.0 3.6 20.0 18 —

Copper

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Table 6.8 Comparative fineness and tensile properties of jute and certain other fibres and materials. Adapted with permission from A.B.M. Abdullah, L.B. Lutfar and N. Matin, An introduction to jute/allied fibres properties and processing, International Jute Organization, Dhaka, Bangladesh, 1992

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1.48 1.55 1.55 1.50 1.48 1.35 1.32

1.50 1.14

Fibres

Jute Cotton Ramie Flax Hemp Silk Wool

Viscose Nylon

12.0 4.0

13.0 8.5 8.0 — — 10.0 16.0 65 3.2

45 21 37 47 — 19 26 505 —

0.37 1.1 0.67 — — 1.5 1.2

Longitudinal swelling in % 0.46 0.22 — — — 0.52 0.11 (along scale) 0.15 (opposite scale) 0.43 0.47 0.324 —

0.324 0.319 — 0.322 0.323 0.331 0.326

Specific heat Coefficient of friction in cal/g ◦ C

8.4 3.9

1.1 × 108 — — —

2.8 4.7 — — — — 5.5

5 × 1010 5.6 × 107 3.7 × 106 6.3 × 106 1.1 × 107 6.4 × 109 3.8 × 1011

0.91 × 10−4 1.67 × 10−4 — 1.07 × 10−4 — 1.18 × 10−4 1.10 × 10−4

Dielectric constant at 2 kHz, 65% RH

Electrical resistance in ohm

Thermal conductivity in cal/s cm ◦ C/cm

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Table 6.9 Comparative physical properties of jute and certain other fibres. Adapted with permission from A.B.M. Abdullah, L.B. Lutfar and N. Matin, An introduction to jute/allied fibres properties and processing, International Jute Organization, Dhaka, Bangladesh, 1992

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Figure 6.3

6.7.1.1

Jute processing chart. Reproduced with permission from Jute Division, Birla Corporation Limited, Kolkata, India.

Selection of Jute for a Batch

A number of bales of jute selected for the purpose of manufacturing a particular type of yarn is known as a ‘batch’. The bales are opened by experienced workers to find any defect and remove the defective portion. A batch is selected with due consideration of prices, available stock and suitability for spinning and weaving, etc. Fibre bundle qualities for a batch depend mostly on the quality of yarn to be spun.

6.7.1.2

Piecing-up

In ‘pucca bales’, layers of jute comprising the bales are very hard owing to the tremendous pressure received by the jute during baling. The operation of taking layers of jute (2–3 lb equates to 0.91–1.36 kg) from the bales and loosening them by beating is known as piecing-up of jute.

6.7.1.3

Softening and Lubricating

Jute fibre bundles often need to be softened, i.e. their resistance to bending must be reduced to make them soft and pliable. This is done by adding oil and water (emulsion), which is known as batching. The water softens the fibre bundle and increases its extensibility, makes it easier for the fibre bundles to bend around the pins and rollers and reduces waste losses. Oil (jute batching oil) lubricates the fibre bundles and thereby reduces the frictional forces during processing. It also provides cohesion to the slivers, helping them to be drafted properly. The emulsion is applied while the material is passing through a unit known as a softener machine. Improved softening of low-grade jute can also be achieved by controlled application of certain selected enzymes along with the oil-in-water emulsion.

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The oil used in the emulsion is the jute batching oil (JBO), which is obtained as a straight-run product by distillation of crude oil in petroleum refineries. As some jute fabrics are used in packaging food materials, the presence of JBO may give a kerosene-like odour, and hence it is not desirable for such use. As such, the use of mineral-oil-based batching oil in the process of manufacturing jute bags used for packaging food-grade materials has been a matter of concern for end-consumers owing to alleged and possible hydrocarbon contamination of such bags. In order to resolve the issue to the satisfaction of all concerned, the International Jute Organisation (IJO) organised an international workshop in February 1998 in Calcutta, which was attended by the producers and buyers of jute bags and also by end-user industries of food-grade materials such as cocoa beans, coffee beans and shelled nuts. The workshop resulted in the elaboration of a draft standard specification for jute bags used in the packaging of such food-grade materials, including the establishment of maximum tolerance levels for hydrocarbon residues. The standard arrived at stipulates that the bags should not contain unsaponifiables exceeding 1250 mg/kg. The International Jute Council (IJC) accepted the recommendation of this workshop in March 1998 and adopted the specifications as IJO Standard 98/01. The standard came into force on 1 October 1998. The standard was accepted by the International Cocoa Organisation in May 1998 (IJSG, 2003).

6.7.1.4

Piling and Pile Breaking

The softener machine output material is carried to the piling place for piling. During piling, superficial moisture penetrates inside the fibre bundle and ‘thermophilic’ action takes place which softens the jute bundle. After maturing for a certain period of time (48–72 h), the piled jute is subjected to carding processes. Generally, the root cutting is done after piling near the hand-feed breaker carding machine.

6.7.1.5

Carding

Carding is a combining operation where jute reeds are split and extraneous matters are removed. Jute fibre bundles are formed into ribbons called ‘slivers’. There are mainly two different carding sections: (i) breaker carding and (ii) finisher carding: (i) Breaker carding. In different jute mills, the carding operation is carried out in two ways: (a) hand-feed breaker carding; (b) roll-feed breaker carding. In the breaker carding machine (see Figure 6.4), softened jute, after piling, is fed by hand in suitable mass. By action with different rollers, the machine turns out raw jute in the form of jute slivers for finisher carding. In this process, root cutting is necessary before feeding the material to the hand-feed breaker carding machine. (ii) Finisher card. One of the main objects of the finisher card (Figure 6.5) is to give more carding and cleaning action to the slivers from the breaker card. Thus, the material is again subjected to a carding action, but to a much greater degree, as the pinning and setting of rollers are finer. This splits up the fibre bundles into much finer form and tends to equalise the length of the bundles in the resultant slivers, thereby attaining a greater degree of regularity of sliver mass per unit length, which is further improved by doubling of the slivers in order to minimise thick and thin places in the sliver. Another objective of the finisher card is to blend different grades and varieties of jute with a view to reducing the cost of the batch, obtaining improved performance at spinning and incorporating visual effects in the resultant yarns and fabrics. Blending of jute with other fibres like wool, stapled rayon, etc., can also be done at the finisher card by feeding slivers from different fibres in the required proportion to the finisher

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Figure 6.4

Breaker card – schematic diagram. Adapted from Atkinson (1965) and Mukherjee and Ray (in press).

card input. Yet another objective of the finisher card is to improve parallelisation of fibre bundles in the output slivers. The most important carding variables are pin density, draft, speed of the machine, loading and setting between rollers. The intensity of carding depends on both cylinder speed and draft.

6.7.1.6

Drawing

The functions of the drawing stage are: (1) to draft the finisher card slivers to a count suitable for feeding the spinning frames; (2) to reduce mass irregularities by doubling; (3) to straighten the fibre bundles and lay them along the sliver axis so that, when they come to be spun on the spinning frame, they will be evenly drafted and twisted to form an acceptable yarn (Mukherjee and Ray, 2010).

Figure 6.5

Finisher card – schematic diagram. Adapted from Atkinson (1965) and Mukherjee and Ray (in press).

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Drawing is a process for reducing sliver width and thickness by simultaneously mixing several slivers together. There are three types of ‘drawing frame’ machine. In most mills, three drawing passes are used for hessian and two drawing passes for sacking: r First drawing. The slivers obtained from the finisher carding machine are fed in the required number to the first drawing frame machine. This machine carries out blending, equalising the slivers and doubling two or more of them, and provides quality and colour. It includes a delivery roller, a pressing roller, a retaining roller, faller screw sliders, a check spring, a back spring, a crimpling box, etc. r Second drawing. The second drawing frame machine receives the slivers from the first drawing machine and uses six slivers and deliveries per head. The second drawing machine makes more uniform slivers and reduces them to a suitable size for the third drawing operation. r Third drawing. The third drawing frame machine uses the slivers from the second drawing machine. It is high speed, and makes the slivers more crimpled and suitable for spinning.

6.7.1.7

Spinning

Spinning is essentially a simple operation of two steps carried out almost simultaneously. It consists of drawing fibres out of a mass and twisting them into a continuous yarn. Spinning is the process of producing yarns from the slivers received from the third drawing machine. Figure 6.6 shows a spinning frame in operation, attended by a lady worker. In the spinning process, slivers are elongated and fibre bundles are twisted into yarn to impart strength. Spun yarns in the spinning process are wound onto bobbins. The machine is stopped in order to replace these bobbins with empty bobbins. The entire time is called the average cycle time. Spinning of several types of yarn is done on a spinning frame machine. Yarn twist is inserted by rotating the lower end of the yarn about the upper end, and the twist actually ascends from below into the upper portions of the yarn and in this way runs up towards the drawing nip.

Figure 6.6

Spinning frame in operation.

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Industrial Applications of Natural Fibres Table 6.10 Range of yarns spun from jute (adapted from Atkinson, 1965) tex Finest yarn Fine yarns Hessian warp Hessian weft Sacking warp Sacking weft Carpet yarns

60–103 120–200 240–300 240–400 275–350 700–1400 480–820

lb/spy 1.75–3.0 3.5–6.0 7–9 7–12 8–10 20–40 14–24

The twist is transmitted by the lower fibre bundles taking up a spiral formation and forcing those above them to conform to the same configuration, i.e. to the same twist angles.

6.7.1.8

Jute Spinning

In general, the types of jute yarn manufactured can be classified according to the use to which they will be put, i.e. fine yarns, hessian qualities, carpet yarns, etc. They can be further divided into warp and weft qualities, the warp being superior to the weft, as it must withstand the tensions of weaving, while the weft acts more as a filler and undergoes less strain. Jute yarns of different counts for making various types of jute fabric could be spun. The range of yarns spun from jute is given in Table 6.10. Previously, jute count was expressed as pounds per spindle, which is a measure of the mass of 14 400 yards (ca 13 167 m) of yarn in pounds (lb/spy). Nowadays, tex is used, which is a measure of the mass in g of 1000 m of yarn.

6.7.1.9

Winding

Winding is a process that provides yarn as spools and cops for the requirement of beaming and weaving operations. There are two types of winding: (i) spool winding and (ii) cop winding: (i) Spool winding. In spool winding, yarn is produced for warp (the longitudinal yarn). The spool winding machine consists of a number of spindles. There is wide variation in the number of spindles per machine from one make to another. The productivity of spool winding depends on the surface speed of the spindle and machine utilisation. The spool winding machine uses bobbins containing a smaller length of yarn. This machine winds the yarn into bigger packages known as ‘spools’. The spools are used in making sheets of yarn to form the warp portion used during the interlacement of weaving. (ii) Cop winding. The cop winding machine obtains yarns from the spinning machines. The spinning bobbins are placed on a suitable pin on top of the cop machine, and yarn tension is maintained by means of a small lever. The yarns on the bobbins are converted into hollow cylindrical packages called cops.

6.7.1.10

Beaming

The beaming process follows spool winding. In the beaming operation, yarn from the spool is wound over a beam of proper width and correct number of ends to weave jute fabrics. To increase the quality of the

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woven product and the weaving efficiency, the warp yarns are coated with starch paste. Adequate moisture is essential in this process. Starch solution in water contains tamarind kernel powder (TKP, which is derived from the plant Tamarindus indica) and the antiseptic sodium silica fluoride (IUPAC: disodium hexafluorosilicon; Na2 SiF6 ), and its concentration varies with the quality of yarn. Warp yarns are supplied to a weaving loom through a warp beam. These warp yarns undergo cyclic tensioning as well as repeated abrasion, due to frictional contacts between the adjacent yarns and also between yarns and loom parts, during the weaving cycles. To reduce the incidence of warp yarn breakage as a result of higher tensioning and/or abrasive damage during weaving, i.e. to achieve a satisfactory weaving efficiency, the yarns are coated or sized with paste of some gummy materials. This coating helps in laying down the protruding fibres on the yarn surface and makes the yarn more resistant to abrasive damage. On the other hand, some penetration of sizing material into the yarn makes the yarn strong enough to withstand higher weaving tension. Thus, the sizing is meant to improve the weavability of the warp yarns and thereby yield the desired level of weaving production. The efficiency of the weaving shed is dependent on a good-quality warp beam. A good warp beam (preferably sized or dressed) ensures lower warp breakage, lower wastage, higher weaving efficiency, better fabric quality and lower weaver fatigue. The sheet of yarn passes through the size liquor kept in the sow box. The size liquor consists of adhesive, along with antiseptic, softening and wetting agents, etc., for various functions. After passing through the size liquor, the yarn sheet passes through the nipping roller for removal of surplus size solution. The sized yarn sheet then passes around a number of drying cylinders to evaporate the water. The yarn sheet is then wound on a weaver’s beam. Heating and cooling assist the binder to settle down in the yarn. Moisture retention plays an important role. Too much drying or too little drying makes the yarn either brittle or sticky, causing problems on the loom. Moreover, stretching of yarns during its movement before passing through the sow box on to the winding head also plays an important role in the case of jute owing to its low elongation.

6.7.1.11

Weaving

Weaving is a process of interlacing two sets of threads, called the ‘warp’ and ‘weft’ yarns, at right angles to each other to produce fabric of the desired quality. The warp is oriented in the direction of the length of the fabric, and the weft in the direction of its width. Individual warp and weft yarns are called ends or porters and picks or shots. Interlacing of the ends and picks with each other produces a coherent structure. The repeating pattern of interlacing is called the weave, or fabric structure. There are separate looms for hessian and sacking in the weaving section. In the hessian looms, a shuttle containing cops (weft yarn) is manually changed. The sacking looms are equipped with an ecoloader to load cops automatically into the shuttle. Plain weave has the simplest possible pattern of interlacing. It also has the maximum possible frequency of interlacing, from which it follows that the yarns in a plain-weave fabric are not easily displaced; the fabric tends to be firm and to resist slipping of yarns. Plain weave is mainly used in constructing conventional jute fabrics, e.g. hessian, bagging, tarpaulin or jute canvas, etc., whereas sacking is constructed mainly of twill weave. The specification of a cloth generally states the type of cloth, its width in inches and the mass per yard of the fabric in its finished state. Hessian of specification 11 × 12 × 40 – 10 oz (ca 0.2835 kg) in its finished state means that it is an 11 porter cloth, contains 12 shots per inch (0.0254 m) and is 40 inches wide (1.016 m), and every yard (ca 0.834 m) weighs 10 oz (ca 0.2835 kg). Detailed information about the specification of handloom woven jute and jute blended fabrics, as well as details about technical textiles based on jute fabrics, can be found in Krishnan et al. (2005).

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6.8 Damping Damping is the process in which the rolled woven cloth is unrolled and water is sprinkled on it continuously to provide desired moisture. Each roll is generally 104 yards (or 95.976 m). Damping is done manually.

6.9

Calendering

Calendering is a process similar to ironing of a fabric. After damping, the damped fabric passes through pairs of heavy rollers which flatten the threads in the fabric and improve the quality and appearance.

6.10

Lapping

Lapping is the process in which Hessian fabrics are folded into the required size for the ‘bale press’ operation on the lapping machine.

6.11 Bailing Bags or bale processing cloths are pressed compactly according to the buyer’s needs.

6.12 Cutting Cutting is the process where the sacking cloth is cut to the required length for making bags of different sizes.

6.13 Hemming In the hemming process, the raw edges of sacking-cloth cut pieces are sewn by a sewing machine.

6.14

Herackele Sewing

In herackele sewing, the sides of sacking-cloth cut pieces are sewn to make a complete bag.

6.15 Finishing (Woolenisation) Other important steps in jute processing are dyeing and finishing. Krishnan et al. (2005) give an overview of modern developments in the chemical finishing of jute products. An example is given here for the woolenisation of jute. Jute fibre bundles undergo many changes when treated with a strong chemical base (such as sodium hydroxide NaOH). Lateral swelling occurs, coupled with considerable shrinkage in length. The fibre bundles become soft to the touch and develop a high degree of crimp or waviness. This gives a wool-like appearance and, when the fibre bundle is stretched to break, the crimp is straightened and thereby the extensibility of the fibre bundle is increased. The effect is small at low alkali concentrations of up to about 10%, but the extensibility increases rapidly at concentrations of 15% and above and may reach 8 or 9%. The developed

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crimp is not very stable, and no elasticity is conferred on the fibre by this treatment. The appearance and feel of jute fabrics is much improved by the woolenising process, and bleached and dyed fabrics might have commercial possibilities (Rowel and Stout, 1998). Jute yarn is converted to a wool-like appearance and feel by chemical modification, dyeing and finishing. It may be used for making sweaters, cardigans, etc. It provides adequate warmth and is cheaper than wool and acrylic-based products.

6.16

Fire Retardancy and Rot Proofing

Jute fabrics are made fireproof by treating them in a composite solution admixed with different fire-retardant chemicals like monoammonium hydrogen phosphate (NH4 HPO4 ) or diammonium hydrogen phosphate (NH4 )2 HPO4 along with some additives. This fulfils the requirement of flameproof fabrics. These fabrics are suitable for all specialised uses wherever fireproofing or insulation is desirable. Jute fibre and fabrics can be made resistant to rotting by appropriate chemical treatment with copper compounds like copper sulphate (CuSO4 ), copper ammonium sulphate (Cu(NH3 )4 SO4 .H2 O), copper ammonium carbonate (Cu(NH3 )4 CO3 ), copper acetate (Cu(C2 H3 O2 )2 ) and copper naphthenate ((C11 H7 O2 )Cu.H2 O) (IJSG, 2003; Bhuiyan et al., 1968).

6.17

Uses

Jute is a plant fibre second only to cotton in production. It is cultivated for fibre that has various end-uses (see Figure 3.2.1 in Chapter 3.3). According to the International Jute Study Group (IJSG, 2003), traditionally jute has been used to manufacture packaging materials such as hessian, sacking, ropes, twines, carpet backing cloth, etc. Hessian is lighter than sacking and is used for bags, wrappers, wall coverings, upholstery and home furnishings. Sacking is used as a packaging material for transportation of agricultural products. Carpet backing cloth (CBC) is of two types. Primary CBC provides a tufting surface, while secondary CBC is bonded onto the primary backing for an overlay. To overcome the declining market for these conventional products of jute, new technologies have been evolved for the bulk use of jute as a raw material in the production of high-value-added and price-competitive intermediaries or final products. A host of innovative new products have been developed with high value addition. These products for new, alternative and non-traditional use of jute are generally referred to as ‘diversified jute products’ (IJSG, 2003). Figure 6.7 shows the processing steps for producing various diversified jute products. Among the various diversified jute products, floor coverings, home textiles, technical textiles, geotextiles, jute-reinforced composites, pulp and paper, particle boards, shopping bags, handicrafts, fashion accessories, espadrilles, clothing, etc., have potential for wider use and application (IJSG, 2003). The major breakthrough in the uses of jute came when the automobile, pulp and paper and the furniture and bedding industries started to use jute and its allied fibres with their ‘non-woven’ and composite technology in order to manufacture ‘non-wovens’ and other technical textiles as well as composites. Therefore, jute has changed its textile fibre outlook and is steadily heading towards a new identity as a versatile natural fibre. Normally, better-quality jute is used in making fleeces and felts, but it mainly depends on the end-use (IJSG, 2003). 6.17.1

Home Textiles

Jute has many advantages as a home textile, either replacing cotton or in blend with it. It is a strong, durable, colour- and light-fast fibre. Its UV protection, sound and heat insulation, low thermal conduction

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Figure 6.7

Processing steps for producing various diversified jute products (adapted from IJSG, 2003).

and antistatic properties make it a wise choice in home d´ecor. These properties indicate that jute can be used in high-performance technical textiles. Tablecloths, cushion covers, sofa covers, bed covers, curtain cloths, table mats, prayer mats, napkins, aprons, blankets, etc., are made of jute or jute in blend with other textile fibres. Jute can be blended with cotton for home textile applications. Attractive fabrics for use as upholstery and tapestry have already been developed and are in the market. These fabrics are strong, durable, both light- and colour-fast, attractive and cheaper than most fabrics made from other fibres. They are antistatic, UV protective, carbon dioxide neutral and naturally decomposable, i.e. free from health hazards. They are also excellent raw materials for different kinds of bag (IJSG, 2003).

6.17.2

Bags for Various Purposes

Travel bags, beach bags, fancy bags, ladies’ bags, school bags, shopping bags, carrier bags and a range of different bags made of jute are available for use. For shopping bags and other similar uses, attempts are also being made to produce bags of a jute/paper combination. Figure 6.8 shows jute bags of different designs.

6.17.3

Floor Coverings

Jute floor coverings consist of woven, tufted and piled carpets, rugs, runners, floormats, mattings, braided carpets, durries, etc., of jute alone or blended with other textile fibres. Woven jute floor covering and matting of continuous length can be woven in solid and fancy shades, and in different weaves such as ‘boucle’, ‘panama’, ‘herringbone’, etc. Jute floor coverings and rugs are made both on power looms and handlooms. The traditional Satranji carpet is becoming very popular in home d´ecor. Jute ‘non-wovens’ and composites

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Figure 6.8

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Jute bags.

can be used for underlay, linoleum substrate and backing for stick-down carpet tiles and specialised underlay for wood-block floorings.

6.17.4

Technical Textiles

Technical textiles can be summarised as textiles that are used for specific applications in agriculture or in sectors such as the automobile, aviation, civil engineering, chemical, electrical, leather, medical and transportation industries, or for environmental protection. The main features of technical textiles are rated on specific performance parameters and not on aesthetics. Jute fibre has some unique physical properties such as high tenacity, bulkiness, sound and heat insulation properties, low thermal conductivity and antistatic properties. On account of these qualities, jute fibre is more suited for the manufacture of technical textiles in certain specific areas. Nowadays, jute is being used in high-performance applications such as protective textiles, composites and automotive textiles.

6.17.5

Nursery Pots

Jute fabrics can be used as nursery pots owing to the biodegradable nature of jute (see Figure 6.9). Young trees can be planted directly with the container without disturbing the roots, and, for land restoration, jute cloth prevents erosion occurring while natural vegetation becomes established.

6.17.6

Jute Geotextiles

Jute geotextiles are among the most important diversified jute products, with a potentially largescale application. They may have several applications: soil erosion control, vegetation consolidation,

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Figure 6.9

Nursery pot.

agromulching, reinforcement and protection of riverbanks and embankments, land reclamation and road construction. The demand for jute geotextiles is increasing in various parts of the world. However, the absence of adequate awareness and standards and specifications seems to be affecting the possible expansion of the market. Jute geotextiles have the advantages of abundant supply, superior drapability, higher moisture retention capacity, ease of installation, etc. Figure 6.10 shows the application of jute geotextiles in road construction. Both fabrics and ‘non-wovens’ could be used as geotextiles, depending on the purpose of application. The principles of construction and properties of some jute soil-saver fabrics meant for different applications are given in Arun (2000); Ramaswamy (2003); Banerjee and Ghosh (2003) and Rickson (2003).

6.17.7

Jute ‘Non-Wovens’

A fabric consisting of an assembly of jute fibre bundles (oriented or in a random manner) held together (1) by mechanical interlocking, (2) by fusing of thermoplastic fibres or (3) by bonding with rubber, starch, glue, casein, latex or a cellulose derivative or synthetic resin is referred to as ‘non-woven’ fabric. If the jute fibre bundles are mechanically strengthened (1), then the textile product is called felt. If the bonding is realised by thermoplastic melted fibres (2) or glues, etc. (3), then the final textile product is a fleece. Jute ‘non-wovens’ are produced by using one or a combination of the above-mentioned basic techniques of fibre entanglement and bonding such as thermal bonding, needle punching, stitch bonding or hydroentanglement. For the use of ‘non-woven’ jute products, the jute fibre bundles are processed, as explained in Chapter 3.2, aerodynamically or mechanically by airlaid systems or carding machines. In the case of felts, the laid jute

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Figure 6.10

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Application of jute geotextile for stabilisation of a rural road.

fibre bundles are strengthened by needle punching (see, for example, Sengupta et al. (1985), Debnath et al. (1996) or Ganguly et al. (1999)). The production of jute needle felts is similar to the production of flax and hemp felts. The process is briefly described in Chapter 20. Jute felts are often value-added products made from jute fibre bundles that are sorted during jute processing in the spinning industry. Jute needle felts are used as insulation material (see Chapter 20) and soundproofing products. They may also be used as underlay of carpets, cushions and floormats, as well as in the footwear industry (IJSG, 2003). Especially for use in the automotive industry (see Chapter 19.4), jute needle felts are broadly used as semi-finished textile products to reinforce polymer materials.

6.17.8

Jute Composites

Detailed information about natural fibre composites can be found in Chapter 19.1 to 19.6. Jute composites are emerging as true substitutes for wood. A range of products that are presently being produced from jute composites are, for example, sheets/boards, doors, window frames, furniture, corrugated sheets and chequered boards (IJSG, 2003). An excellent overview of jute composites is given by Mohanty and Misra (1995).

6.17.9

Pulp and Paper

According to the International Jute Study Group (IJSG, 2003), the demand for pulp and paper is increasing globally and is expected to grow further. A drastic reduction in the supply of wood and bamboo pulp the world over, coupled with increasing concerns regarding reduced forest resources, has forced many countries to search for alternatives for making paper from so-called ‘tree-free’ pulp. Jute and kenaf plants are annually renewable resources, requiring only 120–180 days for growth. Jute and kenaf, containing cellulose like other raw materials used for paper pulp, have been found to be excellent raw materials for making good-quality pulp and paper. The technologies for making pulp and paper from whole jute as well as from jute fibre have been successfully developed (IJSG, 2003).

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6.17.10

Jute Particle Boards

Particle boards made from the wooden part of the jute stem find wide applications as substitutes for wood. The availability of the technologies for the production of particle boards and their high socioeconomic value are arguments in favour of the future development of this jute product. The use of jute particle boards has been found to be quite acceptable in terms of both quality and price (IJSG, 2003). The latest trends in particle boards based on natural fibres are given in Chapter 19.5.

6.17.11

Jute Products and Some Ecological Aspects

Fabrics made of jute fibres are nearly carbon dioxide neutral and naturally decomposable. One hectare of jute plants can consume about 15 t of carbon dioxide (CO2 ) and releases about 11 t of oxygen (O2 ). Studies also reveal that the CO2 assimilation rate of jute is several times higher than that of trees (Inagaki, 2000). Thus, jute is an environmentally friendly fibre, starting from seed to fibre, as the used fibres can be recycled. As jute is a natural fibre, it sequesters a significant amount of carbon during its agricultural stage. Thus, the greenhouse gas emission of jute was found to be negative (Jute Ecolabel, 2006).

6.18

Conclusion

Research and development activities are being continually carried out in the producing as well as in the consuming countries to develop new technologies and new jute-based products. Global awareness about a pollution-free environment is being built up, and people in general are becoming more inclined to use natural fibre products, which are not only environmentally friendly but also would serve the intended purpose. Thus, concerted efforts are required for the sustainable development of natural fibre industries. The profile of natural fibres must be enhanced in order to stimulate demand in the world market.

References Abdullah, A.B.M., Lutfar, L.B. and Matin, N. (1992) An Introduction to Jute/Allied Fibres Properties and Processing. International Jute Organization, Dhaka, Bangladesh. Ahmed, Z. and Akhter, F. (2001) Jute retting: an overview. Online J. Biol. Sci., 1(7), 685–688. Ali, M.M., Sayem, A.Z.M. and Eshaque, A.K.M. (1972) Effect of neutralization of retting liquor on the progress of retting and quality of fibre. Sci. Ind., 7, 124–136. Arun, N. (2000) Implementation of jute in GeoTech. Man-made Text. in India, XLLIII(6), p. 262. Atkinson, R.R. (1965) Jute – Fibre to Yarn. B.I. Publications, Bombay, India. Banerjee, P.K. and Ghosh, M. (2003) Jute geotextiles in rural road application, in Indian Jute – New Symphony, ed. by Sur, D. JMDC, Kolkata, India. Bhuiyan, A.M. et al. (1968) Rot-proofing of jute materials. Part 1 – sand bags. Jute Fabrics, Pakistan, VII, 6. Biswas, S.K. (2004) Retting technologies of India – an appraisal. Paper presented at a Workshop on ‘Modern Technologies of Retting of Jute’, held on 15 September 2004 and organised by the IJSG Secretariat at its Headquarters, Dhaka, Bangladesh. BSTI (1967) Standardisation of Kutcha grades – ‘white jute’ (Corchorus capsularis), Bangladesh Standards and Testing Institutions. Debnath, C.R., Roy, A.N., Ghosh, S.N. and Mukhopadhyay, B.N. (1996) Anisotropic behaviour of needle-punched parallel-laid jute nonwoven. Indian J. Fibr. Text. Res., 21(December), 244–250. FAO (1998) Improved retting and extraction of jute. IJO project AG:GCP/RAS/122/IJO, terminal report FAO/Government Cooperative Programme.

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FAO (2003) Medium term prospects for agricultural commodities. FAO Corporate Document Repository, Food and Agriculture Organisation of the United Nations, Rome, Italy; available at: http://www.fao.org/docrep/006/y5143e/ y5143e1g.htm#bm52 (accessed 8 July 2009). FAO (2008) Statistics on jute, kenaf and allied fibres, June 2008. Ganguly, P.K., Sengupta, S. and Samajpati, S. (1999) Mechanical behaviour of jute and polypropylene blended needlepunched fabrics. Indian J. Fibr. Text. Res., 24(March), 34–40. Halder, H.G. and Kundu, A.K. (1957) Effect of natural activators (Dhaincha – Sesbania aculeate and sunnhemp – Crotalaria juncea) in reducing the retting period of jute, in Proceedings of Indian Sci. Congr. III, p. 384. IJSG (2003) International Jute Study Group, Dhaka, Bangladesh; available at: http://www.jute.org (accessed 9 July 2009). Inagaki, H. (2000) Progress on Kenaf in Japan, Third Annual Conference. American Kenaf Society, Texas, USA. IS 271 (1975) Grading of white jute (Corchorus capsularis). Bureau of Indian Standards. Jarman, C.G. (1985) The retting of jute. FAO Agricultural Services Bulletin 60, FAO, Rome, Italy, 64 pp. Jute Division (2009) Jute processing chart, Birla Corporation Limited, Kolkata, India, (n.d.); available at: http:// www.birlacorporation.com/jute/jutegeneral.html (accessed 8 July 2009). Jute Ecolabel (2006) Life cycle assessment of jute products by Price Waterhouse Coopers, May 2006; available at: www.jute.com/ecolabel. Krishnan, K.B., Doraiswamy, I. and Chellamani, K.P. (2005) Jute, in Bast and Other plant fibres, ed. by Franck, R.R. Woodhead Publishing, Cambridge, UK, pp. 24–93. Kundu, B.C. (1956) Jute – world’s foremost bast fibre. 1. Botany, agronomy, pests and diseases. Econ. Bot., 10, 103–133. Ministry of Textiles and Jute, Government of Bangladesh (2009) Production of jute, kenaf and allied fibres (2006/2007–2007/2008). Mitra, B.C. (1999) Data Book on Jute. National Institute of Research on Jute and Allied Fibre Technology (NIRJAFT), Kolata, India. Mohanty, A. K. and Misra, M. (1995) Studies on jute composites – a literature review. Polym.-Plast. Technol. Eng., 34(5), 729–792. Mukherjee, A. and Ray, P. (2010) Mechanical processing of jute. Paper accepted for a book on jute to be published by the International Jute Study Group (IJSG), Dhaka, Bangladesh. Office of the Jute Commissioner, Government of India (2009) Production of jute, kenaf and allied fibres (2006/2007–2007/2008). Ramaswamy, S.D. (2003) Emerging user requirements for jute geotextiles, in Indian Jute – New Symphony, ed. by Sur, D. JMDC, Kolkata, India. Rickson, R.J. (2003) The use of jute based products as geotextiles, in Indian Jute – New Symphony, ed. by Sur, D. JMDC, Kolkata, India. Rowell, R.M. and Stout, H.P. (1998) Jute and kenaf, in Handbook of Fiber Chemistry, International Fiber Science and Technology Series 15, 2nd edition, ed. by Lewin, M. and Pearce, E.M. Marcel Dekker, New York, NY; available at: http://www.fpl.fs.fed.us/documnts/pdf1998/rowel98e.pdf (accessed 8 July 2009). Sen, H.S. (2009) Quality improvement in jute and kenaf fibre. Paper presented at the International Conference in Dhaka, Bangladesh organised by the IJSG in February 2009. Sengupta, A.K., Sinha, A.K. and Debnath, C.R. (1985) Needle-punched non-woven jute floor coverings. Part I – influence of fibre and process variables on tensile properties of fabrics. Indian J. Text. Res., 10(September), 91–96. Sobhan, M.A. (2010) Jute agriculture. Paper accepted for a book on jute to be published by the International Jute Study Group (IJSG), Dhaka, Bangladesh.

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7 Abac´a – Cultivation, Extraction and Processing Friedhelm G¨oltenboth Institute for Plant Production and Agroecology in the Tropics and Subtropics, University of Hohenheim, Stuttgart, Germany

¨ Werner Muhlbauer Institute for Agricultural Engineering, University of Hohenheim, Stuttgart, Germany

7.1 Introduction In comparison with other natural fibres, abac´a production, at about 80 000 t per year, is marginal. Abac´a is produced in only two countries, primarily in the Philippines and to a small extent in Ecuador. It is cultivated in the mid-mountain regions up to about 500 m above sea level, mainly as integrated culture, and does not usually compete with food crops. In general, no additional inputs in the form of mineral fertiliser and pesticides are required. For fibre extraction, no water is needed, and only a minimal energy input. The high tensile strength of the abac´a fibre favours its present use for the production of special papers such as bank notes, cigarette paper and tea bags. Furthermore, a new promising field is as a substitute for glass fibres in composite materials. In this chapter, the biology, cultivation and extraction technologies will be described, with special reference to the latest technical developments concerning stripping and high-quality production of abac´a fibres.

7.2

Background

Abac´a, Musa textilis N´ee, is an indigenous plant of the Philippine archipelago and Northern Borneo. The Philippines are considered to be the centre of origin, and the Filipinos have domesticated this prominent fibre-producing plant.

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Abac´a is considered to be one of the strongest natural fibres, and is tolerant to salt water. Until after World War II, it was a monopoly of the Philippines. Today, the major product from abac´a is pulp, followed by fibrecrafts, raw fibres, cordage, including ropes and twines, yarns and fabrics. Currently, about 85% of the world’s abac´a is supplied by the Philippines, the rest by Ecuador (Halos, 2008).

7.3

Cultivation

In 2008, a cultivation area of about 140 000 ha in 52 provinces of the Philippines was planted with abac´a. It is one of the cash crops that can grow with relatively little input compared with other crops in steep areas.

7.4

Distribution

Abac´a is currently cultivated in almost all provinces in the Philippines, with the exception of Ilocos, Cagayan, Region 3, Cavite and Batangas. However, it was introduced in many countries with a climate similar to the Philippines by the US government at a time when the US Navy relied solely on abac´a as the source of marine cordage (Spencer, 1953). By 1923, abac´a had been introduced in more than 20 Central and South American countries, including some Caribbean Islands (Dempsey, 1963). By 1925 it had been established in Sumatra, West and East Malaysia, New Caledonia and Queensland in Australia. There were also attempts to introduce the plant in India, East Africa, Florida and Vietnam. But today only the Philippines and Ecuador are supplying the world market with abac´a.

7.5 Biology and Cultivation 7.5.1

Main Varieties and Seedling Production

Abac´a and the banana belong to the family Musaceae of the order Zingiberales. This botanical family has two genera, Musa and Ensete. These two genera are composed of 45 species. The genus Musa is so diverse that it is further subdivided into four sections: Callimusa and Eumusa, where edible bananas belong, Australimusa, to which abac´a belongs, and Rhodachlamys. The Philippines has six indigenous species of Musaceae, including abac´a. There are numerous vernacular names for abac´a, indicating its widespread origin in the country. Abac´a is the common name in the Philippines, but, for example, Inosa is the vernacular name in Leyte and Davao. As many as 200 varieties and about 20 cultivars are presently known. Varieties usually bear the name given at their place of origin (Moreno and Parc, 1995). Abac´a or Musa textilis N´ee (Figure 7.1) has a basic chromosome number n = 10. The abac´a variety Inosa is found with chromosome numbers varying from 2n = 17 to 2n = 23 (Javier and Orazion, 1988). According to Brewbaker et al. (1956) and Tabora and Carlos (unpublished), the plant stools freely and produces 7–51 suckers per hill. The pseudostem can grow up to 7.6 m. The stalk is glossy, and the colour varies from greyish purple to blood red. The base circumference ranges from 12 to 63 cm, and the leaf sheaths range from 10 to 25 per stalk. Leaf sheaths and petioles are devoid of wax. The pseudostem consists of 12–30 tightly packed, long, concave-shaped sheaths that grow from the central core. About 90% of the pseudostem is water and sap, 2–5% is fibre material and the rest is a soft parenchyma tissue (Sinon, 2008). Leaf blades are oblong, narrowing towards the apex, rounded at the base and truncated at the tip. They can reach 1.45–2.95 m in length. The petioles are 30–62 cm long, holding the leaves at a high angle. The sterile bract varies in colour like the pseudostem. The basal female flower is organised in 3–9 hands. The male flowers, 8–12 per bract, sit in two rows. The female flower blooms for 4–6 days. The uppermost

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Figure 7.1 Habitus of abaca, ´ Musa textilis Nee, ´ family Musaceae. Adapted with permission from C.P. Hutter, F. Goltenboth ¨ and M. Hanssler, Paths to sustainable development: New experiences in the Philippines, S. Hirzel Verlag, 2003.

cluster at the base of the flower stalk opens first and then blooming continues downwards. All flowers in a cluster open simultaneously. The blooming of the male flower is longer. The first bracts to open contain only female flowers from which the fruits develop. The inner bracts that open afterwards contain only male flowers. This makes the abac´a plant strictly cross-pollinated (Torres and Garrido, 1939). Being a sucker plant, the plants within one hill can cross-pollinate, producing seeds that are virtually inbred. Insects and three nectivorous Philippine bats, Macroglosus minimus, Eonycteris speleae and Rousettus amplexicaudatus, aid in pollination (Heidemann and Utzurrum, 2003). It takes about 150 days from pollination to fruit maturity. On average the fruit is 3.7–7.0 cm long, including the pedicule. It usually shows a diameter of 2.0–7.0 cm in the middle. About 36–160 seeds can be found per fruit. They are very irregular and usually higher than broad. Abac´a seeds do not have a dormancy period and germinate within a period of 10–52 days after sowing (Spencer, 1953). Germination ranges from 20 to 90%, depending on the variety. Seeds from different bunches show varying degrees of viability. The germination percentage can usually be increased by air drying the seeds for at least 24 h. Sun drying or soaking them in hot water kills the seeds. The germination process is delayed up to 150 days by burying the seeds. Today, seeds are no longer used for plant production in commercial plantations. Vegetative propagation methods, including suckers, corms and tissue culture material, are now used for the production of planting material.

7.6 Seedling Production In principle, four types of planting material are available: seeds, corm, suckers and tissue-cultured plants: r Seeds are no longer used, except for breeding research. r Corm may be used whole or divided into sections, called seed bits, of about 10–15 cm diameter, usually containing two or more healthy eyebuds (FIDA, 2003). r Traditionally, suckers of about 1 m height are in use. r Today, the use of disease-free tissue-cultured planting material is recommended and progressing.

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The preparation of the shoot tip explants includes cleaning, trimming and washing of sword suckers of respective abac´a varieties with soap and tap water. The shoot tips are then excised to 4 cm × 3 cm and soaked overnight (16 h) in 0.5 g L−1 solution of chloramphenicol, a bacteriostatic antimicrobial (C11 H12 Cl2 N2 O5 ). They are then surface sterilised with a 70% solution of chlorine bleach (Philippines), a solution of approximately 3–6% sodium hypochlorite (NaClO), with Tween 20 polysorbate surfactant (polyoxyethylene sorbitane monolaurate; C58 H114 O26 ). After 15 min, the solution is decanted and the shoot tips are rinsed 4 times with sterile aqua dest. The chlorine treatment is repeated using a 50% solution for 15 min, followed by final rinsing with sterile aqua dest. The surface-sterilised shoot tips are aseptically trimmed to 0.5 cm × 0.5 cm. One shoot tip serves as one explant.

7.6.1

Culture Initiation (S0 )

The explants are inoculated into modified MS medium containing 30 g L−1 sugar, 150 mL L−1 coconut water, 30 mg L−1 thiamine, a water-soluble vitamin of the B-complex (vitamin B1; C12 H17 N4 OS+), 3.0 mg L−1 benzyl adenine, a plant growth regulator (IUPAC: N-benzyl-1H-purin-6-amine), 0.2 mg L−1 indole acetic acid, a heterocyclic compound that is an phytohormone called auxin (indole-3-acetic acid; C10 H9 NO2 ) and 7.0 g L−1 agar. The pH value is adjusted to 5.8 prior to autoclaving at 1.1 kg cm−2 (121 ◦ C) for 20 min. The inoculated shoot tips are incubated in the dark for 4 weeks.

7.6.2

Subculture Stage (S1 )

The black phenolic substance present in the corm tissue is scraped off the explants. Then the explants are subcultured to fresh medium for 1 month. During S1 , the primary shoot emerges. The shoot cultures are exposed to a 16 h photoperiod provided by a 40 W fluorescent light at 25 ◦ C. The explant is than split into two equal parts.

7.6.3

Subculture Stage (S2 )

The halved explants are transferred to fresh medium and incubated under 16 h of light for 1 month. About 1–2 adventitious shoots emerge.

7.6.4

Micropropagation Cycle (M1 –M7 )

The vegetative portion is removed and splitting of the explant is done. The micropropagated shoots are exposed to 16 h of light. This process is repeated 7 times, every 1 month cycle, to induce rapid multiplication of shoots. Shoots with 2–3 open leaves and a shoot base of at least 3–4 mm diameter are selected and transferred into semi-solid MS medium (Murashige and Skoog basal medium supplemented with 3 mg L−1 benzylamonopurine as a source of cytokinin and 3% sucrose + 0.3% activated charcoal). Using this method, up to 30 shoots per one original shoot tip explant can be achieved within 4–5 months.

7.6.5

In Vitro Rooting

Shoots with at least three leaves are individually separated from the cluster of shoots and transferred to a semi-solid half-strength MS medium containing 30 g L−1 sugar, 1.0 mg L−1 indole butyric acid (IBA;

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Explant preparation 2 days

Culture initiation S0 1 month

Subculture stages (S1-2) series of two transfers; 2 months

Micropropagation stages, Cycles 1-7 7 months

Laboratory, 11 months (up to micropropagation cycle 7)

In vitro rooting 1 month

167

Acclimatisation potting & hardening, 1 month Nursery, 1 month

Figure 7.2 General process flow of tissue culture production of virus-free abaca´ plantlets. Adapted from M. Belarmino and F.M. Duatin, Virus free seedlings production and ecological production of Abaca fibers, PPProject Phase II, Utilization of Abaca fiber in the automotive Industry, Terminal Report, 2008.

indole-3-buturic acid is a plant hormone in the auxin family; molecular formula C10 H13 NO2 ) and 1.0 mg L−1 1-naphthalene acetic acid (a plant hormone in the auxin family; formula C10 H7 CH2 CO2 H) at pH 5.7–5.8 to induce rooting. The cultures in glass jars are incubated in a culture room provided with 16 h light at 25–26 ◦ C. After 3 weeks the shoots are fully rooted.

7.6.6

Acclimatisation

Well-rooted plantlets inside culture bottles are taken out of the culture room and placed at room conditions (28–32 ◦ C) under continuous lighting for 1 week to acclimatise. The plantlets are then removed from the medium, washed with tap water, treated with 0.1% (w/v) fungicide solution (e.g. Benlate, Bayer, Germany) for 5 min and rinsed with tap water. They are then potted in soil and ready for transfer to a nursery or the field (Figures 7.2 and 7.3) (Belarmino and Duatin, 2008).

7.7 Environmental Requirements and Conditions It is well documented that abac´a can grow in a wide variety of conditions, but there is a marked difference in productivity of fibres and their quality (Spencer, 1953; Dempsey, 1963; Armecin & Ferraren, 2001). Excessive heat, water stress and strong winds hamper the growth performance of abac´a. Abac´a growth is best in shaded areas, and at about 50% shade the growth performance is significantly better than with less shade or more shade (Bande, 2009, private communication). A plant with a leaf area of about 1750 cm2 will lose 630 g of water per average sunny day (Copeland, 1911). Most of the roots are found between 15 and 25 cm below the ground, making the abac´a plant a typical surface feeder (Bande, 2009, private communication). Depending on the variety, suckers emerge from the main corm about 4–5 months after planting (Ricahuerta, 1952). Plants must be harvested when the flag leaf appears, because plants that are about to flower yield almost twice as much fibre as plants that have already borne flowers and fruits (Oyardo and Cecilio, 1974). Different varieties and hybrids attain maturity at different times. While the hybrid Putomag 22 (Puti-tumatagacan × Maguindanao) takes only 14 months to mature, Laylay needs 24 months. Late-maturing varieties are usually taller and larger than the others (Tabora and Carlos, unpublished). Regions with more than 2000 mm precipitation, a high humidity of up to 78–88% and temperatures of 22–26 ◦ C are optimal for abac´a growing. If rainfall is uniformly distributed without a prolonged dry season, abac´a will grow well. A moisture saturation of 60–80% is optimal. It prefers sandy or sandy loam soil of recent volcanic or alluvial origin with good fertility, good moisture retention, aeration and drainage. The soil should be at least 50 cm in depth with no hard pan. The plant cannot withstand a high water table. It grows best on neutral or slightly alkaline soils, but tolerates pH values of 4–8. Abac´a needs a fairly high amount of potash (salts that contain potassium (K) in water-soluble form), calcium (Ca) and magnesium (Mg). High organic

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Figure 7.3 Sequential steps in the production of tissue-cultured abaca´ plantlets. Adapted from M. Belarmino and F.M. Duatin, Virus free seedlings production and ecological production of Abaca fibers, PPProject Phase II, Utilization of Abaca fiber in the automotive Industry, Terminal Report, 2008.

matter content is preferable for abac´a. It is known to grow at up to 1200 m above sea level, but the temperature may be limiting. To help control the weeds, condition the soil and prevent soil erosion, cover crops are recommended. Traditionally, Pueraria javanica Benth., Vigna sinensis (L.) hassk. and Ipomea batatas (L.) Lam are used. In relatively steep areas, cover crops like Viginia sinensis, Pueraria javanica, Ipomea batatas and Centrosema pubescens may be found. Also effective are Desmodium heterocarpon (L.) DC. subsp. Ovalifolium (Prain) Ohashi and Calopogonium muconoides Desv. (Armecin et al., 2005). In particular, C. muconoides

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Desv. increases the yield of abac´a fibres significantly. It serves further as a host to the adult Braconid parasite of the destructive slug caterpillar (Thosea sinensis Wlk.) of abac´a (Sison, 1932). Often intercropped are annuals such as cowpea, peanut, rice and corn. Corn cannot be recommended because it is an alternative host to the abac´a mosaic virus and aphids that may carry the mosaic virus (Halos, 2008). Perennials such as mango, coconuts, coffee, durian and cacao are also combined with abac´a. As shade trees, Erythrina fusca Lour., Dracontomelon dao (Blanco) Merr&Rolfe, Leucaena leucocephala (Lam) de Wit, Afzelia rhomboidea (Blco.) Vidal, Cassia javanica L., Sesbania grandiflora (L.) Poiret and Pterocarpus indicus (Wild.) are commonly used.

7.8 Cultivation Methods 7.8.1

Traditional and Integrated Culture

In the Philippines, abac´a is mainly produced by small farmers with average landholdings of about 2 ha (Dargantes and Koch, 1994). Abac´a plantations remain productive until the twentieth year, but the production peaks from the second to the fifteenth year (Tirona and Arguelles, 1933).

7.8.2

Preparation of Land

Traditional land preparation included the removal of all wild plants, loosening of the soil and burning of the leftovers during the dry season. Only large trees were usually left as temporary shade. Today, land preparation is performed more sensitively to avoid soil exposure to sun and rain. Ring weeding instead of total removal of all wild plants is recommended. The growth of shade trees should be started well before the first abac´a seedlings are planted. Intercropping with coconuts is recommended (Figure 7.4). Abac´a planted as a monocrop gives significantly lower yield than when planted with a legume intercrop. The crop should be harvestable before the canopy is closing, and should help enrich the soil. The choice of varieties is critical to the successful establishment of an abac´a plantation. The Fibre Industry Development Authority (FIDA), Quezon City, Metro Manila, the Philippines, recommends different varieties for different regions in the Philippines (FIDA, 2003).

7.8.3

Time and Planting Distance

The recommended distance for planting is 2 m × 2 m at the beginning of the rainy season. Mechanical weeding needs to be performed where necessary to obtain optimal yields.

7.8.4

Fertilisation

Abac´a is very responsive to organic fertiliser (Macarayan, 2004). Therefore, leaving the organic material produced during the stripping process in the plantation is of the utmost importance. Additional application of fertiliser is not generally practised in the Philippines, but certainly it will have an effect on the growth performance of the plants. A quantity of 100 t of fresh abac´a removes as much as 280 kg nitrogen (N), about 30 kg phosphorus (P), about 517 kg potassium (K) and about 124 kg calcium (Ca) per ha (Halos, 2008).

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Figure 7.4

7.9

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Intercropping of abaca´ (Musa textilis Nee) ´ with coconuts (Cocos nucifera).

Plant Diseases

Poor yields and decimation of plants are mainly attributed to five major pest groups: (i) three major virus diseases, (ii) four fungal diseases, (iii) one bacterial disease, (iv) four groups of insects attacking the plants and (v) nematodes: (i) Major virus-induced diseases. In all cases, infected plants need to be eradicated and the transmitting vectors need to be killed by insecticides. r Abac´a bunchy top is caused by a DNA virus similar to the banana bunchy top virus (BBTV) (Furuya et al., 2006). It is transmitted by the aphid Petalonia nigronervosa Coq., and is vegetative through corms, suckers and movement of infected plants from place to place. Infected plants never recover. The first symptom is the presence of yellowish-white chlorotic areas on the laminae and margins of youngest furled leaves (Gonzal, 2008). Advanced stages are characterised by resetting or bunching of almost bladeless leaves. r Abac´a mosaic virus is a non-persistent potyvirus transmitted by corn aphids (Rhopalosiphum maidis Fitch), Water lily aphid (R. nymphaeae L.), apple grain aphid (R. prunifoleae Fitch), cotton aphid (Aphis gossypii Glover) and green bag aphid (Shizaphis graminum). Chlorotic streaks, necrotic lesions and upward cupping of the leaf margins are typical symptoms of infection. r Abac´a bract mosaic virus is caused by a non-persistent potyvirus. This virus was first reported in the Philippines in 2000 (Sharman et al., 2000). Symptoms of infection include stringing of young leaves with broad chlorotic stripes, greenish to yellowish streaks on the petioles, dark-coloured mosaic patterns and stripes or spindle-shaped streaks on pseudostems under dead outer leaf sheaths (Furuya et al., 2006). The virus is transmitted mainly by Pentalonia nigronervosa Coq., Aphis gossypii Glover and Rhapalosiphum maidis Fitch.

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(ii) Major fungal diseases: r Fusarium wilt is caused by the fungus Fusarium oxysporium f.c. cubense. Yellowing and wilting of the leaves, rotting and blackening at the base of the pseudostem are the main symptoms. Fungal propagules are carried by rain water and eroding soil. The airborne spores are transported by strong winds. Eradication and burning of diseased plants and fungicides are used as prevention and control measures. r Pseudostem heart rot is caused by Fusarium moniliforme. Yellowing of leaves, rotting of the central cylinder of the pseudostem and failure of the youngest leaf to emerge are symptoms of infection. Airborne spores and water-transported propagules are, besides contaminated farm tools, the main vectors for transmission. r Marasmius dry sheath rot is mostly seen in damp conditions and poorly drained soil. It is caused by Marasmius senuistus and M. inoderma. Symptoms are brown leaf sheaths showing mycelia and mushroom-like fruiting bodies on the stalk. Airborne spores, mechanical transmission and watertransported propagules are the main means of transmission. r Deightoniella pseudostem rot and leaf spot are two forms of a disease caused by Deightoniella torulosa (Syd.) Ellis. Enlarging lesions on the leaf sheaths are typical symptoms. The roguing of diseased plants and proper irrigation of fields during the dry season are means of control. Two diseases are of little economic importance because they are usually only destructive to the fruits of abac´a plants or seedlings grown from seeds. Anthracnose of abac´a is caused by Gloesporium musarum Cooke & Massee, and blight and root rot of abac´a seedlings is caused by the soil-inhabiting fungus Pythium sp. and Deightoniella torulosa (Syd.) Ellis. (Jones and Stover, 2000). (iii) Bacterial diseases: r Bacterial wilt is caused by Ralstonia solanacearum and appears as brown streaks on the leaves, followed by yellowing, wilting and drying. Transmission can take place mechanically through contaminated tools, rainwater and the use of contaminated planting material. Infected plants need to be burned, and the plantation needs to be kept as clean as possible. (iv) Insect pests: r Aphids are the most common pest organism. Direct feeding does not usually cause serious damage but spreads viral diseases. r The corm weevil (Cosmopolites sordidus German) is the most destructive insect pest. The eggs are laid by the blackish insect in pouches on the surface of the leaf sheath. The larvae hatch within 6 days and feed on the corm and central cylinder of the pseudostem for the next 42–45 days before they go into the pupal stage. Adults emerge from the pupae 5–6 days later. Infected plants die, and only the use of insecticides and the application of ash around the plant to suffocate the weevil will control this pest. r The slug caterpillar (Thosea sinensis Wlk.) (Lepidoptera: Limacodidae). The larvae feed on the leaves. Biological control agents are spiders and birds. r Abac´a leafroller (Erionota thrax) feeds on the leaves and rolls up a portion of the leaves. Removal of the infested leaves is recommended, and multiplication of natural enemies such as spiders and birds.

7.10 Fibre Bundle Extraction 7.10.1

Harvesting

Abac´a matures within 18–24 months of planting. Subsequent harvesting is carried out every 3–4 months. Harvesting is performed when the plant reaches its full maturity, as indicated by the appearance of the flag leaf, the smallest leaf that comes before the inflorescence. At this stage all leaves have reached full maturity and the pseudostems possess ideal properties. The optimum time of harvest greatly affects the quality and recovery

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Figure 7.5 Cross-section of an abaca´ leaf sheath (A) and pseudostem (B) Adapted with permission from F.G. Sinon, Optimization of stripping technologies for the production of high quality Abaca fiber. Copyright 2008 Shaker Verlag GmbH: 1, outer group of leaf sheath; 2, middle group; 3, inner group; 4, pith.

rate of the fibres. Immature pseudostems contain underdeveloped fibres that are weak, while overmature pseudostems contain a higher percentage of coarse and brownish fibres. Abac´a is harvested by cutting the stalk and removing the leaves using a sharp machete. 7.10.2

Tuxying

Abac´a fibre bundles are dispersed randomly in the outer and middle layers of the abac´a leaf sheaths (Figure 7.5). Before extraction of the fibre bundles from the pseudostem, the outer layer of the leaf sheath has to be removed. The separation of the outer layer containing the primary fibre from the inner layer is called tuxying. For tuxying, a small and thin knife is inserted between the outer and inner layers of the leaf sheath and then pulled off the entire length to separate the layers completely. Each leaf sheath produces 2–4 tuxies, 5–10 cm in width. The secondary fibres, which are located in the inner part of the leaf sheath, currently cannot be extracted and therefore are only used as organic fertiliser. Tuxies from the different leaf sheaths produce fibre bundles of varying colour, length, texture and strength. The outer 5–7 sheaths represent about 5% of the mass of the stalk and are dark in colour, which limits later use of the fibre. The middle and inner sheaths contain fibre bundles with high strength and an ivory white colour. Tuxying is necessary to lessen the required force in pulling, to separate the primary from the secondary fibre bundles for homogeneity purposes and to lighten the raw material, especially in transporting them from the field to the stripping centre. After tuxying, the abac´a leaves and the inner leaf sheaths are cut into pieces and spread around the abac´a plant as an organic fertiliser. 7.10.3

Stripping

Before the fibre bundle can be used for different applications, it has to be extracted from the stem. The mature abac´a fibre bundle has a diameter of 400 µm and a length of 2500–3500 mm and consists of 20–60 single fibres of different sizes and shapes. The single fibres are 20 µm thick and 6 mm in length. Unlike bast fibre from flax and hemp, the fibre of abac´a cannot be removed by biological retting alone. Stripping is the most common method for extraction of abac´a fibre. The basic principles are indicated in Figure 7.6. The tuxies are clamped between two fixed blades and pulled through the distance between the blades either manually or by a rotating spindle to remove the vascular tissues and parenchyma cells. To reduce the labour input,

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Figure 7.6 Basic stripping principles. Adapted with permission from F.G. Sinon, Optimization of stripping technologies for the production of high quality Abaca fiber. Copyright 2008 Shaker Verlag GmbH.

decortication machines with rotating blades were developed. The leaf sheath is fixed on the feeding table while the rotating blades provide impact force to the material, softening the vascular tissues and parenchyma cells, the continuous rotating motion of the blades providing a scraping action on the fibre in a curvilinear motion.

7.10.4

Hand Stripping

Owing to low investment in the equipment required, 80% of the abac´a fibre produced in the Philippines is still stripped by the manual method. Hand stripping equipment basically consists of a stripping serrated blade, a stripping block, a source of blade pressure and a pedal lever (Figure 7.7). A small piece of wood is used as a pulling aid. To strip, the pedal lever is first pressed and then released once the tuxy has been clamped between the blade and the stripping block at about mid-length. Most manual stripping knives used for hand stripping have serrations (number of teeth per inch) on their edge. The fewer the serrations, the coarser is the fibre bundle produced and the lower is the quality. Manual stripping is laborious work. The output capacity ranges from 10 to 20 kg of dry fibre bundles per day. This capacity depends upon the serration of the stripping knife used. As some vascular and parenchyma cells are still attached to the fibre, hand stripping produces fibre bundles of lower grade that commands a lower price in the market and limits use.

Figure 7.7 Manual stripping device. Adapted with permission from F.G. Sinon, Optimization of stripping technologies for the production of high quality Abaca fiber. Copyright 2008 Shaker Verlag GmbH.

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Figure 7.8 Traditional stationary spindle stripping machine. Adapted with permission from F.G. Sinon, Optimization of stripping technologies for the production of high quality Abaca fiber. Copyright 2008 Shaker Verlag GmbH.

7.10.5

Spindle Stripping

Spindle stripping is a semi-mechanised improvement on the hand stripping technique (Figure 7.8). Similarly to the manual method, the spindle stripper consists of pressure control, pedal control and blade-frame assemblies. Instead of pulling the tuxies manually, the rotating spindle pulls them. The tuxies are placed between the blade and stripping block. Stripping is done by winding (at least twice) the tip end of the tuxies around the spindle. With a slight tension, the spindle, through friction, draws the tuxies, thereby extracting the fibre bundles. Afterwards, the unstripped side of the tuxies is reversed. The tuxies are clamped again near the middle portion in between the blade and stripping block. Stripping is completed by winding the stripped side of the tuxies around the rotating spindle to pull the tuxies, thereby, scraping off the water, vascular tissues and parenchyma cells from the fibre bundles. In spindle stripping, 4–6 tuxies are stripped at once, while in manual stripping, only 2–3 tuxies at a time can be extracted with a serrated blade. For spindle stripping, only 10–20% of the force

Figure 7.9 Stationary spindle stripping machine developed by NARC. Adapted with permission from F.G. Sinon, Optimization of stripping technologies for the production of high quality Abaca fiber. Copyright 2008 Shaker Verlag GmbH.

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Figure 7.10 Isometric drawing of the designed potable stripping machine. Adapted with permission from F.G. Sinon, Optimization of stripping technologies for the production of high quality Abaca fiber. Copyright 2008 Shaker Verlag GmbH.

is exerted by the operator to provide friction between the tuxy and the spindle, which significantly reduces the drudgery of the work. Furthermore, quality improvement of the fibre bundles can be obtained by using nonserrated blades. The traditional fixed-type stripping machine consists of a 90–150 kg flywheel and a 9 m long flat belt, and is operated by a 2–4 kW kerosene engine. The machine weighs about 700 kg and can produce 100–120 kg of good-quality abac´a at a 1.5–2% recovery rate. Owing to its size and mass, the spindle stripping machine can only be used in centralised operations. This limits its usage to abac´a farms 2–6 km away from the centre. An improved version of the stationary spindle stripper was developed by the National Abac´a Research Centre (NARC), Baybay, Leyte, the Philippines, in order to reduce the cost and mass of the traditional machine. The machine has a 60 kg flywheel and the frame is fabricated from angular steel bars. Instead of a long flat belt, the model uses a V-belt equipped with an idler mechanism to make the machine more compact. The NARC stripper weighs only 220 kg. The machine yields 100–120 kg of high-quality abac´a per day, which is achieved by the use of a small and compact 5–7 kW diesel engine requiring 6–8 L/day (Figure 7.9). To enable farmers located in mountainous regions to produce high-quality fibre bundles, a portable stripping machine with a total mass of about 90 kg was developed. Using a 2.2 kW gasoline engine, average stripping capacity reaches up to 110 kg fibre bundle per day at an average fibre bundle recovery of 1.7% and a fuel requirement of 5 L per 100 kg of dried fibre bundles. For transportation, the stripping machine can easily be dismantled and transported by three people, even in hilly areas. In contrast to stationary stripping machines, all organic wastes remain in the plantation, which is a prerequisite for a sustainable abac´a farming system (Figure 7.10).

7.10.6

Decortication

The multifibre bundle decorticating machine using moving blades can be used to extract fibre bundles from abac´a, banana, maguey, sisal and ramie. Tuxying is not necessary for operation of the decortication machine. The leaf sheaths are fed manually, and primary and secondary fibres are extracted. The machine, driven by a 4–5 kW engine, is fixed on a trailer and can be moved either by tractor or by draught animals. The decortication machine produces only low-quality fibre bundles comprising a mixture of primary and secondary fibres (Figure 7.11).

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Figure 7.11 Mobile decortication machine. Adapted with permission from F.G. Sinon, Optimization of stripping technologies for the production of high quality Abaca fiber. Copyright 2008 Shaker Verlag GmbH.

7.10.7

Drying

After stripping, the abac´a fibre bundles have a moisture content of 55–60%. To prevent discolouration and degradation, the bundles have to be dried immediately after extraction to a maximum moisture content of 14%. During sunny days, the fibre bundles are hung on horizontal bamboo poles and exposed to sun and wind. To achieve the desired uniform drying, the fibres have to be arranged in a thin layer and evenly distributed. Under prevailing weather conditions, the fibre bundles are dried within 2–4 h. During night-time and the rainy season, drying time is extended to 1–3 days, which results in low fibre quality, especially when drying hand-stripped fibre bundles which contain a certain percentage of impurities. Mechanical dryers are currently not in use owing to the high investment required.

7.11 7.11.1

Marketing Fibre Trade

In the traditional fibre trade flow the producer links up to the end-user through a series of intermediaries. Abac´a fibre trading starts with the village dealer, who buys the fibre bundles directly from the farmers on an ‘all-in’ basis. At this level, fibre bundles are sold ungraded because of the farmers’ lack of knowledge of the grading and classification system. The village dealer’s purchases then go to the town trader, who accumulates the fibre and delivers it to the grading and baling establishments (GBEs), where final drying, classification and grading are done. The GBEs have the option of selling the classified fibre bundles to foreign traders or to local processors. Hence, alternative options for trading that would shorten the fibre trade route are under development. Linking the abac´a farmers directly to GBEs and domestic processors through farmer associations or cooperatives strengthens their bargaining power, establishes closer links with end-users, assures them of renumerative prices for their fibre through collective selling, provides producers with a steady market for their fibre bundles and provides end-users with a steady supply at a reasonable price. 7.11.2

Quality Standards

To meet the requirements of the industry, abac´a fibre bundles have to be graded and classified (see Chapter 3.1). The standard grades of abac´a in the Philippines were formulated by the Fibre Industry Development

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Authority (FIDA) (FIDA, 2003). Major quality criteria are colour, cleanness, texture, tensile strength and length of the fibre bundles. Bundles from outer leaf sheaths are brown, red or even black, while fibre bundles from inner leaf sheaths have an ivory white colour. The thickness of the fibre bundles is mainly influenced by the stripping method. Machine stripping using zero-serrated blades produces thin and clean bundles, while hand stripping using serrated blades results in thick fibre bundles with a high percentage of impurities. Superior Current (AD) and Mid-Current (EF) are classified under Streaky Two (S2). These grades are obtained from the inner and middle tuxies, respectively, and machine stripped using zero-serrated blades. These are excellent, pure fibre bundles, with soft texture, ivory white to very light brown in colour and normal to long. Streaky Three (S3) are also excellent, pure fibre bundles; however, they come from the outer group of leaf sheaths, stripped with a zero-serrated blade, which are dark red to dark brown in colour and short to normal in length. Current (I), Soft Seconds (G) and Soft Brown (H) are bundles with good cleaning that come from the inner, middle and outer groups of leaf sheaths respectively. The fibre bundles range from 0.5 to 0.75 mm in diameter, extracted using a stripping blade with 24 serrations (the width of a single fibre is 10–46 µm). Fibre bundles stripped using 17 blade serrations are classified into two grades, Seconds (JK) and Medium Brown (M1), that come from the inner to middle and outer groups of leaf sheaths respectively. These bundles have fair cleaning, with a diameter of 1–1.5 mm. Bundles stripped using 14 blade serrations have only one classification, Coarse (L), with diameters of 1 – 2 mm. Damage Fine (Y1) are residual fibre bundles from AD, EF, S2, S3, I and G, while Damage Medium-Coarse (Y2) are residual grades from H, JK, M1 and L. Strings (O) are bundles twisted into short strings used to tie up bales. Lastly comes the Tow (T), which are the reject unstripped tips of fibre bundles, less than 60 cm long. Other fibre bundles not falling under any of the 14 different grades are classified under Wide Strips (WS) (Table 7.1).

Table 7.1 Quality standards for abaca´ fibre bundles. Adapted with permission from F.G. Sinon, Optimization of stripping technologies for the production of high quality Abaca fiber. Copyright 2008 Shaker Verlag GmbH.

Normal grades

Cleaning

Texture

Length

Colour

Group of leaf sheaths

Knife serration

AD Superior Current EF Mid-Current

Excellent Pure fibre

Soft

Long

Ivory white to white

Inner

0

Normal-long

Middle

0

Next to outer

0

Outer

0

S2 Streaky Two

Normal

S3 Streaky Three I Current

Short-normal

Light ivory to very light brown Light ochre to very light brown Dark red to dark brown

Normal-long

Light to very light brown

Inner and middle

24

Dingy white Light brown Dull brown Dark brown colour of stalk

Next to outer

24

Outer

24

Inner middle next to outer —

17

Short-Normal

Dull brown to dingy light brown Nearly black

17

Normal-long

Brownish

—

14

Good HS 0.5 mm

Medium to soft

G Soft Seconds

Good SS

Normal

H Soft Brown

Good 0.75 mm

Short-normal

JK Seconds

Fair Medium HS 1.0 mm Fair Medium SS 1.5 mm Coarse Harsh HS 1–1.5 mm

Normal-long

M1 Medium Brown L Coarse

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Classification

About 80% of the abac´a fibre bundles produced are still hand stripped and sold by the farmers on an ‘all-in’ basis. However, mixing of fibre bundles extracted from the outer leaf sheaths with those taken from the middle and inner sheaths causes downgrading to a much lower grade. Abac´a delivered by local traders to the GBE undergoes several procedures in grading and classification. Owing to the lack of instruments for an objective measurement of fibre quality, grading and classification have to be done by visual inspection, which is time consuming and costly. Sorting serves as the preliminary classification of fibre bundles. Bunches of fibre bundles are opened and taken out for drying. Coloured (black, brown, red) fibre bundles are separated from the white bundles. Afterwards, all discoloured tips are cut off. Sorted fibre bundles are classified into specific grades based on cleanliness, colour and strength according to the official quality standards. Classified abac´a of similar grade is weighed and baled using a hydraulic press. The rectangular bales have a net mass of 125 kg. The bales are stored in a warehouse until they are sold to domestic processors or foreign brokers.

7.11.4

Fibre Market

The Philippines is the major producer of abac´a fibre, producing 72 000 t, which represents 85% of total world production, and 13 000 t is supplied by Ecuador. In the Philippines, abac´a is produced on about 106 000 ha on small farms with an acreage of 1–2 ha. Depending on the variety, soil fertility and planting density, the yields range between 600 and 1200 kg of dry fibre per hectare. A quantity of 24 000 t of fibre bundles is processed by local industry into pulp, cordage, textiles and handicrafts. High-quality fibres are used for the production of tea bags, cigarette wrappers and bank notes. A quantity of 48 000 t is exported mainly to the USA, the EU and Japan as raw material for the pulp and paper industry (FIDA, 2008).

7.12

Conclusion

Owing to virus infections such as bunchy top, the production of abac´a fibres has currently been reduced, leading to rising prices. Therefore, any efforts to avoid further spreading of the virus diseases are of crucial importance. Abac´a production has a high potential for extension if integrated, for example, in existing coconut plantations, making it more efficient and ecologically sound. However, virus-free suckers and corms are not available in sufficient quantities. Therefore, great efforts have to be made for tissue-based seedling production. The development and commercialisation of a portable and easy-to-handle stripping machine provide farmers, even in remote mountainous regions, with the opportunity to produce high-quality fibre bundles, a prerequisite for the utilisation of abac´a fibres in, for example, composites. The joint efforts of scientific and technical application research in recent years have resulted in abac´a fibre being used successfully even in the exterior parts of cars.

References Armecin, R.B. and Ferraren, A.S.A. (2001) Diagnosis of nutrient constraints to abac´a (Musa textilis N´ee) among selected soils in Eastern Visayas (Philippines) by nutrient omissions pot trial, in Proc. Asian Agric. Congress, Manila, the Phillipines, p. 314. Armecin, R.B., Seco, M.H.P., Caintic, P.S. and Milleza, E.J.M. (2005) Effect of leguminouse cover crops on the growth and yield of abac´a (Musa textilis N´ee), Ind. Crops Prod., 21, 317–323. Bande, M. (2009) Physiological response of abac´a (Musa textiles N´ee) to light, water and nutrient availability in volcanic soils of Leyte Island, Philippines (unpublished).

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Belarmino, M. and Duatin, F.M. (2008) Virus free seedlings production and ecological production of abac´a fibers, in PP Project Phase II. Utilization of Abac´a Fiber in the Automotive Industry, Terminal report ed. by M¨uhlbauer, W., G¨oltenboth, F., Milan, P.P. and Guarte R. (unpublished). Brewbaker, J.L., Gorres, D.D. and Umali, D.L. (1956) Classification of Philippine Musa II. Canton and Minay, Putative hybrid forms of Musa textilis N´ee and Musa balbisiana Colla, Phil. Agric., 40, 242–257. Copeland, E.B. Abac´a, (1911) Phil. Agric. For., 1, 64–73. Dargantes, B.B. and Koch, W. (1994) Case studies on the occupation and cultivation of the forest lands of Leyte, Philippines. Ann. Trop. Res., 16(2), 18–29. Dempsey, J.M. (1963) Long fiber development in South Vietnam and other Asian countries, 1957–62. US Department of Commerce, Washington, DC. FIDA (2003) Technoguide on abac´a, Musa textilis N´ee; available at: http://www.fida.gov.ph. (accessed 3 July 2009). FIDA (2008) Abac´a Fiber Statistics. Fiber Industry Development Agency, Manila, Philppines. Furuya, N., Dizon, T.O. and Natsuaki, K.T. (2006) Molecular characterization of banana bunchy top virus and cucumber mosaic virus from abac´a (Musa textilis Ne´e). J. Agric. Sci., 51, 92–101. Gonzal, L.R. (2008) Abac´a diseases and insect pests, PPP Abac´a Project in the Philippines: application of abac´a fiber in industry, Visayas State University, Visca, Baybay, the Philippines. Halos, S.C. (2008) The Abac´a. Department of Agriculture – Biotech. Progr. Office, Biotech. Coalition Phil., Inc. ,Quezon City, the Philippines. Heidemann, P.D. and Utzurrum, R.C.B. (2003) Seasonality and synchrony of reproduction in three species of nectivorous Philippine bats. BMC Ecol., 3, 11. Hutter, C.P., G¨oltenboth, F. and Hanssler, M. (2003) Paths to Sustainable Development. New experiences in the Philippines, Vol. 1, Euronatur edition. S. Hirzel Verlag, Stuttgart/Leipzig, Germany, 80 pp. Javier, D.F. and Orazion, M.Z. (1988) Cytology and pollen viability of two abac´a (Musa textilis N´ee) varieties found in Leyte (Philippines). Phil. J. Crop. Sci. (Philippines), (Suppl. No. 1), 13–15. Jones, D.R. and Stover, R.H. (2000) Fungal diseases of the root, corm and pseudostem: damping-off of Musa seedling, in Diseases of Banana, Abac´a and Enset, ed. by Jones D.R. CABI Publishing, Wallingford, UK, pp. 160–161. Kohler, H. (2005) Abac´a project a success: use of natural fibers in A-Class honored with SPE Automotive Award. 4th Environmental Forum, Magdeburg; http://www.environment-forum.com/05/press mantel.asp?pm=en abaca (accessed 18 December 2009). Macarayan, O.B. (2004) Growth and yield response of abac´a to the application of Genica dry organic fertilizer. Phil. J. Crop Sci., 29(Suppl. No. 1), 117. Moreno, I.O. and Parc, A.A. (1995) Promising abac´a accessions in the ViSCA germplasm collection. Phil. J. Crop. Sci., 20(Suppl. No. 1), 512. Oyardo, E.O. and Cecilio, Z.O. (1974) A study of the critical age of harvesting abac´a. Phil. J. Plant. Ind., 39, 127–141. Ricahuerta, J.R. (1952) Germination and variability study of seven abac´a varieties. Phil. Agric., 35, 504–511. Sharman, M., Gambley, C.F., Oloteo, E.O., Abgona, R.V.J. and Thomas, J.E. (2000) First record of natural infection of abac´a (Musa textilis N´ee) with banana bract mosaic potyvirus in the Philippines. Australasian Plant Pathology, 29, 69. Sinon, F.G. (2008) Optimization of Stripping Technologies for the Production of High Quality Abac´a Fiber. Shaker Verlag, Aachen, Germany. Sison, P. (1932) The slug caterpillar on abac´a (Thosea sinensis Wlk.), its life history and habits as observed in Davao, and suggestions for control. Phil. J. Argic., 3, 163–186. Spencer, J.E. (1953) The abac´a plant and its fiber, Manila hemp. Econ. Bot., 7 (3), 195–213. Tirona, M. and Arguelles, A.S. (1933) The soils of renovated abac´a (Musa textilis N´ee) fields in Davao and the reported inferior growth of this plant therein. Phil. J. Sci., 52, 79–87. Torres, J.P. and Garrido, T.G. (1939) Progress report on the breeding of abac´a (Musa textilis N´ee). Phil. J. Agric., 10, 211–230.

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8 Sisal – Cultivation, Processing and Products Rajesh D. Anandjiwala and Maya John CSIR Materials Science and Manufacturing, Port Elizabeth, South Africa, and Department of Textile Science, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa

8.1 Introduction In the production of natural fibres, sisal plays a dominant role in the field of leaf fibres. A total of 0.33 million t of sisal is produced worldwide (0.01% of the production of all natural fibres). Sisal is well suited for technical applications because of the interesting properties of the fibres. This chapter deals with the main aspects of cultivation and processing of sisal fibres. The different classifications of sisal fibres will be described. The properties and uses of sisal fibres and its technical products will be discussed. The focus will be on sisal-fibrereinforced plastics and the use of sisal fibres in different matrices (elastomer, thermoplastic and thermoset). Finally, new developments dealing with the processing of sisal fibre composites will be presented.

8.2

Historic Background

Sisal is a leaf fibre derived from a plant generally considered indigenous to Central and South America, where it was cultivated, harvested and used in spinning coarse yarns, twine and cordages for several centuries. During the late eighteenth century to early nineteenth century, the planting of sisal was also started on the African and Asian continents owing to its potential to grow under diverse ecological and climatic conditions, ranging from the hot and humid conditions of Kenya and Tanzania to the tropical conditions of Brazil, the coasts of Florida and Hawaii in the USA and the Caribbean islands (Brown, 2002). More recently, China has become a sisal-producing country, with an annual production of about 20 000 metric t in 2006 according to the UN Food and Agriculture Organisation (FAOSTAT, 2009). Consumer preferences shifted from natural to synthetic fibres with the introduction of synthetic fibres in the 1940s, and as a result the demand for sisal fibres in

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Figure 8.1 Different species of Agave: (A) Agave americana (the century plant), mediopicta habit at Enchanting Floral Gardens of Kula, Maui, Hawaii, USA, 24 October 2007; (B) and (C) Agave americana, habit at Omaopio, Maui, Hawaii, USA, 13 March 2007; (D) Agave sisalana, habit at La Perouse, Maui, Hawaii, USA, 24 January 2007; (E) Agave attenuate, habit at Olinda, Maui, Hawaii, USA, 25 December 2006; (F) Agave vivipara, habit at Enchanting Floral Gardens of Kula, Maui, Hawaii, USA, 24 October 2007. Photos courtesy of Forest & Kim Starr, 2009, Hawaii, USA.

the world market started falling gradually, and developing countries, such as Kenya and Tanzania, witnessed substantial erosion in their export market (Kimaro, 1994). 8.2.1

Classifications and Cultivars

Sisal plants are sterile hybrids of uncertain origin (Gentry, 1982) and derive their name from the port of Sisal in Yucat´an in Mexico, from where they were shipped in ancient times (UNIDO, 2009). However, sisal does not originate from there. The lifespan of a typical sisal plant varies from 7 to 15 years, depending upon cultivars, genetic species, climate, growing conditions and soil quality. It is classified within the family Agavaceae under genus Agave, and further subdivided into two subgenera, namely Agave and Littaea (Berger, 1915; Gentry, 2004; Webster, 1970; Rocha, 2005; The Plants Database, 2000; Franck, 2005). There are several botanical species identified. However, some commonly found species include Agave sisalana, Agave americana var. ‘marginata’ (the century plant), Agave americana var. ‘americana’, Agave attenuata, Agave vivipara are shown in Figures 8.1A to F.

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According to Schnegelsberg (1999), the term agave fibre stands for fibre bundles that are separated from the vascular bundles of the leaf vein of different Agave species. One can distinguish, for example, between: r Agave vivipara: kerratto fibre r Agave cantala: cantala fibre r Agave cocui: dispopo fibre r Agave decipiens: false sisal fibre r Agave falcata: guapilla fibre r Agave fourcroydes: henequen fibre r Agave heteracantha, A. tequilana, A. ixtle, A. lophantha: ixtle fibre r Agave letonae: letona fibre r Agave americana: pita fibre (synonym: magoe or maguey fibre = zambora fibre) r Agave sisalana, Agave rigida var. sisalana: sisal fibre r Agave zapupe: zapupe fibre Zander et al. (2002) distinguished between 51 families predominantly found in Mexico. According to a summary of the FAO, in Tanzania 27 families and in Kenya 28 families can be differentiated. Furthermore, it should be mentioned that more than 85 hybrids have been bred that are named by numbers, e.g. ‘Agave hybrid 11648’ or ‘Agave hybrid 1300’, etc. (Schnegelsberg, 1999).

8.2.1.1

Agave sisalana

Agave sisalana (see Figure 8.1(D)) is the most commonly referred to species of the Agave family. It is a rosette-forming succulent plant mainly cultivated for its fibres, which are extracted from the leaves. The plant has large, sword-shaped, thick, fleshy leaves, apparently emanating from the root, with a sharp-pointed end and generally with a spiny margin and short but stout stem. Each rosette grows slowly over the period of its lifespan and flowers only once. During flowering, a tall stem or pole grows from the centre of the leaf rosette and bears a large number of short and tubular flowers, as shown in Figure 8.2. After development of the fruit, the original plant withers and eventually dies, but suckers are frequently produced around the stem from the root below ground, which become new plants (Figure 8.3). During its lifespan of 7–10 years, the sisal plant is usually harvested first after 2–3 years and then at 6–12 month intervals. A typical plant will provide about 200–250 commercially usable leaves in its entire lifetime (some hybrid varieties may provide up to 400–450 leaves), and each leaf contains an average of around 1000 fibre bundles (Brown, 2002; Mukherjee and Satyanarayana, 1984; Li et al., 2000).

8.2.1.2

Agave americana (the Century Plant)

There are a number of varieties of Agave americana (see Figures 8.1 and 8.2) available. It is also sometimes referred to as the century plant, and is now widely cultivated for its handsome appearance. The leaf in its variegated form has a white or yellow marginal or central stripe from base to apex, thus offering an ornamental appearance. As the leaves open up from the centre of the rosette, the impression of the spines on the edge is clearly visible on the still erect younger leaves. The leaves mature very slowly and die after flowering, but they are easily propagated by the offshoots from the base of the stem. This is a native of tropical America. Common names include the century plant, maguey (in Mexico) and American aloe. The name ‘century plant’ relates to the long time it takes to flower, although the number of years before flowering depends on the vigour of the individual plant, the richness of the soil and the climate. During the years of growth, the plant stores the nourishment required for the effort of flowering in its fleshy leaves.

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Figure 8.2 Left: flowering of Agava Americana L. Adapted from Hoffmann, 1884; right: Agave rigida Mill. Adapted from Meyers Großes Konversations-Lexikon, 1906.

Figure 8.3

Suckers on the roots of an older sisal plant.

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There are a number of Agave americana plant varieties that are not like the century plant but show greater similarity to Agave sisalana, with spikey leaves, as shown in Figure 8.1(B and C), that provide agave juice, fibres and other medicinal extracts. One variation of Agave americana is also called Agave tequilana, as tequila is made from the juice extracted from it.

8.2.1.3

Agave attenuata

Agave attenuata (see Figure 8.1(E)) is also commonly grown as a garden plant. Unlike many other species of the Agave family, Agave attenuata has no teeth or terminal spines, making it an ideal plant for areas adjacent to footpaths and railroads.

8.2.1.4

Agave chiapensis

Agave chiapensis has all the common features found in other species, only some of it is edible, i.e. the flowers, the leaves, the stalks or basal rosettes and the sap (called honey water). The plant produces several pounds of edible flowers, rich in sap, for eating. During the development of the inflorescence there is a rush of sap to the base of the young flower stalk. Native uses of different Agave species are as follows (Banerjee, 1972; Bos and Lensing, 1973): r The leaves of several species yield fibre, for example: Agave rigida var. longifolia (synonym: Agave

r r r r

fourcroydes (henequen fibre)); Agave rigida var. sisalana (synonym: Agave sisalana (sisal fibre)); Agave decipiens (false sisal fibre), etc. A variation of Agave americana (pita fibre) is also a source of fibre and is used as a fibre plant in Mexico and the West Indies. When dried and cut into slices, the flowering stem forms natural razor strops, and the expressed juice of the leaves will lather in water like soap. The natives of Mexico used the Agave to make pens, nails and needles, as well as string to sew and make weavings. In India the plant is extensively used for hedges along railroads. Agave syrup (also called agave nectar) is used as an alternative to sugar in cooking. When dried out, the stalks can be used to make didgeridoos.

The juice from many species of Agave may lead to blistering lasting several weeks. Episodes of itching may recur up to a year thereafter, even though there is no longer a visible rash. Irritation is, in part, caused by calcium oxalate raphides. Dried parts of the plants can be handled with bare hands with little or no itching effect.

8.2.2

Production of Sisal

Sisal is the main leaf fibre produced in the world, accounting for approximately 70% of the commercial production of all such fibres. The worldwide production of sisal is estimated at about 427 000 t according to the Food and Agriculture Organisation Statistics published in 2006 (scattering from 1999 till 2006, between ca. 300 000 and 430 000 t). Brazil contributed the largest production of 247 000 t, which is almost 58% of the worldwide production, as shown in Figure 8.4. Together with Brazil, almost 75% of the worldwide production of sisal is contributed by the Southern American continent comprising Brazil, Mexico, Columbia, Cuba, Haiti and Nicaragua, some 10% is contributed by Tanzania and Kenya on the African continent and about 5% from China. It is estimated that more than 80% of the production of sisal in Brazil is exported to over 50 countries, the United States, China, Mexico and Portugal being the main importers.

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Figure 8.4

8.2.3

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World production of sisal and other Agave fibres (FAOSTAT, 2006)

Cultivation and Harvesting of Sisal

Cultivation of sisal plants in the nursery can be done by suckers from the plant or from bulbils obtained by shaking the pole and collecting them when fallen on the ground. Once suckers or bulbils have been carefully planted in a well-nourished soil in the nursery, they are nurtured until they reach a height of 15 cm and then transferred to the field in marked-out rows by hand (Gentry, 1982). For planting the seedlings, the land must be free of weeds. Thus, soil preparation may be done by ploughing or tractor using a disc plow or a heavy grill. The operation may be completed by harrowing with mild-grade soil. It is recommended that the planted rows are directed north–south, to avoid shading between plants. The density of plants per unit area varies according to the climate conditions and the soil of the region. The system of planting most widely used is that of rows with a simple spacing of 2 m × 1 m and a population of 5 thousand plants per hectare. Usually, a double-row planting is preferred to allow easy access to the field at all times. It is advantageous to keep fields free of weed so as to plant another crop between rows of sisal. This is economically beneficial because the average lifecycle of the sisal plant varies from 10 to 14 years, and no productive yield is received for the initial 2–5 years, depending upon the location, conditions and climate under which it is growing. The sisal is a tropical plant that survives in environments with little rain and high sunshine. It requires at least 400 mm of rainfall per year, equivalent to 4000 m3 per hectare per year. Its optimum annual temperature averages 20–28 ◦ C (Nobel, 1994; Lock, 1969). Normally, the first harvesting is performed 3–5 years after the crop is established. Almost 120–125 leaves, in about 1.5 m of height, are available for harvesting. The harvesting of sisal is a field operation in which the outer leaves are cut such that about 30 leaves are left on the plant so as to allow the plant to grow until the next harvesting cycle due after 15–18 months (see Figure 8.3). This process of harvesting is continued until the end of the life of the plant (Gentry, 1982; Yayock et al., 1988).

8.2.4

Decortication

The cut leaves are bunched together so as to load them in a trailer and take them to the factory for decortication. A large-scale automatic decortication machine, which can treat almost 25 000 leaves or 10–20 t of leaf per hour, is ideal for a large plantation, but, on a small scale or for pilot trials, hand decorticators are suitable for

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producing about 150–200 kg of dry fibre bundles in a shift of 10 h. About 20–30% of the fibres are wasted in such small-scale operation, and it is highly labour intensive. The process of decortication is aimed at removing the pulp fibre by mechanical scrapping of the leaves by passing them through rotors run by a diesel engine (Snyder et al., 2006). The sisal leaves enter the machine cross-wise, and decortication takes place in one stroke along the fibre bundle in a single direction so that all leaf tissues are crushed and rasped off fibres in one step. After the decortication process, fibre bundles are immersed in tanks filled with water for 8–12 h to remove mucilage, pectic substances and chlorophyll sap. The fibre bundles are then dried on drying lines under natural sunlight. After drying, the bundles are cleaned by beating or combing in whisks, either manually or in machines equipped with a rotor with blades that can remove tissues attached to fibre bundles and shake off dust. The recovery of fibre bundles from the leaves is approximately 3–5%, depending on the method employed. In this operation, short bundles are also removed, resulting in a clean, bright and soft product (Yayock et al., 1988; Lock, 1969).

8.2.5

Grading of Fibres

In view of marketing, the fibres are graded (as described in more detail in Chapter 3.1) in accordance with the international standards, because different quality attracts different prices. In the grading process, different fibres are separated into various categories according to length, colour and presence of impurities. Sisal is graded according to the country of growing and is further subdivided in class and/or type according to colour, cleanness and length. The classes are: (i) long (length over 0.90 m), (ii) medium (length between 0.71 and 0.90 m) and (iii) short (length between 0.60 and 0.70 m). The types are: (i) type 1, (ii) type 2 and (iii) type 3. The quality of sisal is broadly categorised as follows according to the Brazil classification in accordance with Law 71 dated 16 March 1993 of the Ministry of Agriculture: r Superior. Material composed of fibre bundles washed, dried and well beaten or brushed, cream-coloured to clear, in a great degree of ripeness, with softness, shiny, strong and sharp, maximum moisture 13.5%, and loose and clear, free from impurities, pectic substances, knots, fragments of leaves and bark and other defects. r Type 1. Consisting of fibre bundles dried and well beaten or brushed, cream-coloured or yellowish clear, in a greater degree of ripeness, with softness, shiny and normal strength, spots with little variation in relation to colour, regain percentage up to 13.5%, loose, free from impurities, pectic substances, knots, fragments of leaves and bark and other defects. r Type 2. Fibre bundles with perfect decortication, washed, natural gloss, cream-coloured to clear, uniform, dry, with regain percentage of 13.5%, with normal amounts of fragments of flesh adhering to the fibre bundles, selected as the class, and that, after undergoing the process of brushing or beating, in normal (proper) storage and time, fall in the upper type and/or type 1 of the specifications approved by resolution. r Type 3. Fibre bundles with perfect decortication, natural brightness, colour cream to clear or yellowish, dry, with regain percentage not exceeding 13.5%, with normal amounts of fragments of flesh adhering to the fibre bundles, selected as the class, and that, after undergoing the process of brushing or beating, in normal (proper) storage and time, fall in the type 1 and/or type 2 of the specifications approved by resolution. A list of grades as defined by the East Africa Sisal Growers Association (Nairobi, Kenya) and the London Sisal Association (London, UK) is given in Table 8.1. In the rule of the trade, a tolerance of 20% regarding the length of the fibre bundles is accepted, and therefore long fibre bundles may contain up to 20% of medium and short bundles. After brushing and classification, the fibre bundles are compressed into bales for transportation to the next processing stage, e.g. spinning. The bales are prepared in mechanical or hydraulic presses equipped with medium-sized boxes of 150 × 50 × 70 cm, varying between 200 and 250 kg. The following information should be attached to the bale: type of product, crop, batch, bale number, the name of the press, class, type, mass, place of pressing, city, state and date of baling.

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Table 8.1 Sisal fibre grading based on international grading regulations (East Africa Sisal Growers Association, Nairobi, Kenya and London Sisal Association, London, UK) Grade

Description r Length from 910 mm upwards, average 1065 mm r Free of defective decortication, properly brushed, free of tow, tousled and bunchy ends, knots and harshness r Colour creamy white to cream

No.1

r Length from 760 mm upwards r Free of defective decortication, properly brushed, free of tow, tousled and bunchy ends, knots and harshness r Colour creamy white to cream

No.2

r r r r

No.3

3L (3Long)

UG (Under Grade)

S.C.W.F. (Short Clean White Fibres)

U.H.D.S (Unwashed hand decorticated sisal) Tow 1

8.2.6

r r r r

Length from 610 mm (2 ft) upwards Brushed fibre bundle, with minor defects in cleaning permissible Must be free of tow, knots and barky or undecorated fibre/bundle Colour may vary from creamy white to yellowish, but a higher proportion of spotted or discoloured fibre bundle is permissible Length from 915 mm (3 ft) upwards Brushed fibre bundle with minor defects in cleaning It must be free of tow, knots and barky or undecorated fibre/bundle Colour may vary from creamy white to yellowish, but a higher proportion of spotted or discoloured fibre bundle is permissible

r Minimum length from 610 mm (2 ft) r Brushed fibre bundles r Fibre bundle that does not conform to the above-mentioned grades as regards colour, cleaning and length. However, the colour of this grade may not differ too much, and it may not be brown or black. Although defects in cleaning are allowable and some imperfectly decorticated fibre bundles or barky runners are permissible, it must be free from undecorated leaf and knots r Length from 450 to 610 mm r Free of defective decortication, properly brushed, free of tow, tousled and bunchy ends, knots and hardness r Colour creamy white to cream r Shall not be graded in accordance with sisal grading definitions, but shall be sold by sample r Proper tow from brushing machines r Free of fine fibre bundle cuttings and dirt, and reasonably free of dust, but entirely free of sweepings, knots and barky or undecorated fibre bundles r Colour creamy white to creamy

Structure and Properties

Most plant fibres, including sisal, are composed of mainly cellulose and lignin, but a number of other minor constituents, such as pectin, wax, inorganic salts, nitrogenous substance and pigments, etc., are also found in them (Chand et al., 1988). The sisal leaf consists of roughly 4% fibre, 0.75% cuticle, 8% dry matter and 87.25% water (Mukherjee and Satyanarayana, 1984; Bisanda and Ansell, 1992).

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Figure 8.5 Cross-section of a sisal leaf. With kind permission from Springer Science+Business Media: Journal of Materials Science, Properties of Sisal – CNSL Composites, 27, 1992, 1690–1700, E.T.N. Bisanda and M.P. Ansell.

The fibres are multicellular, with small individual cells bonded together (Li et al., 2000; de Andrade Silva et al., 2008). As shown in Figure 2.2.2 in Chapter 2.2 and in Table 13.6 in Chapter 13, the single fibre is on average about 20 µm thick and 3 mm long, and the fibre bundle can reach values of more than 400 µm in thickness and more than 1000 mm in length. These cells act as reinforcements for hemicellulose and lignin matrices. Therefore, the cell wall is a composite structure of lignocellulosic material reinforced by helical microfibrillar bands of cellulose (Li et al., 2000; Chapter 2.2). The total cellulose and lignin contents of sisal fibres are about 67 and 12% respectively (Mukherjee and Satyanarayanan, 1984; Table 13.9 in Chapter 13). Fibres of different age, from different sources and from different parts of the sisal plant exhibit different structures, and hence their properties are also different. The sisal leaf contains three types of fibre, namely mechanical, ribbon and xylem, as shown in Figure 8.5 (de Andrade Silva et al., 2008; Li et al., 2000; Mwaikambo and Ansell, 1999). The mechanical fibres, also known as structural fibres, are mostly found around the periphery of the leaf, whereas the ribbon fibres are oriented with the conducting tissues in the median line of the leaf, and the xylem fibres are oriented against the ribbon fibres (Bisanda and Ansell, 1992; Martins and Joekes, 2003). The mechanical fibres are rarely circular but mainly thick and horseshoe-shaped in cross-section, and they are difficult to separate in the extraction process. The mechanical fibres are finer and important to the grading of sisal fibres. Ribbon fibre bundles, also known as arch fibre bundles, run from the base to the tip of the plant, and they grow with the conducting tissues along the median line of the leaf, as shown in the cross-section shown in Figure 8.5. The structure of the ribbon fibres provides them with good mechanical strength, and they are also the longest fibre bundles and can be readily separated longitudinally during the extraction process (Bisanda and Ansell, 1992). Xylem fibres form a composite bundle across the median line opposite the ribbon fibres, and they have an irregular shape when separated from the vascular bundles. Xylem fibres have thin cell walls and hence are easily broken in the extraction process (Mwaikambo and Ansell, 1999). The length of extracted sisal fibre bundles ranges from 1.0 to 1.5 m, and the diameters are 100–300 µm, with an average fibre density of 1.26 ± 0.03 g/cm3 (Li et al., 2000; Martins and Joekes, 2004; Figure 13.23 in Chapter 13), whereas other authors have reported a sisal fibre density of 1.45 g/cm3 (Mukherjee and Satyanarayana, 1984; Table 13.8 in Chapter 13). The mechanical properties of the sisal fibres and fibre bundles vary from plant to plant and from cultivar to cultivar, and they also depend on the climatic conditions and soil quality of the region where the sisal is grown. Table 8.2 shows a compilation of physical and mechanical properties of sisal fibre bundles reported by several authors (Mukeherjee and Satyanarayana, 1984; Chakravorthy, 1969; Satyanarayana et al., 1982). Mukeherjee and Satyanarayana (1984) and de Andrade Silva et al. (2008) have carried out systematic studies on the tensile properties of sisal fibres at various testing speeds, and results have been explained in terms of the fine structure of the fibre, such as cell structure, microfibrillar angle and defects, using scanning electron microscopy. The failure mechanism of the sisal fibre bundle under tensile mode was attributed to the uncoiling of microfibrils, accompanied with pull-out and tearing of cell walls (Mukherjee and Satyanarayana, 1984). Mechanical properties of sisal fibre bundles, such as tensile strength, elongation, toughness and modulus, were tested at elevated temperatures by Chand and Hashmi (1993), and they reported that the tensile strength,

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Table 8.2 Compilation of the range of physical and mechanical properties of sisal fibre bundles (Mukherjee and Satyanarayana, 1984; Chakravorthy, 1969; Satyanarayana et al., 1982) Fibre bundle diameter in µm 50–200

Density in g/cm3

Moisture content at 65% RH in %

Tensile strength in N/mm2

Young’s modulus in N/mm2

Elongation in %

Flexural Modulus in N/m2

1.45

11

412–640

9400–15 200

2.5–5.0

12.5–17.6

Chemical constituents in % Cellulose

Lignin

66–78

8–14

modulus and toughness values of sisal fibre decreased with an increase in temperature. The crystalline character of Agave americana L. (pita) fibre bundles and its relationship with mechanical properties were reported by El Oudiani et al. (2008). These authors concluded that differences in the molecular and fine structure resulted in different mechanical properties. High tenacity and initial modulus values were observed for raw fibres, which were found to have the highest crystallinity. On the other hand, the low tenacity and higher extensibility observed in seawater extracted fibres were attributed to their amorphous character and to the increased unit cell dimensions (El Oudiani et al., 2008). El Oudiani et al. (2009) also studied elastic and viscoelastic deformation in Agave americana L. fibre bundles, and showed that elastic recovery values were influenced by the test conditions, such as the type of fibre extraction, relative humidity and the time for which the specimen was held at constant strain.

8.3

Processing and Uses of Sisal Fibres

The processing technique employed is obviously dependent upon the end-use (see Figure 3.2.1 in Chapter 3.2). The sisal fibre bundles can be spun into yarns by using traditional spinning processes, which include bale opening, mixing, cutting into staple fibre bundles, carding, drawing and sliver formation and spinning. The yarns can be then plied and twisted to form ropes and twines, as well as to produce woven fabrics for gunny bags. Long sisal fibre bundles can also be used directly in making ropes from slivers. The sisal fibre bundles are nowadays converted into needle-punched non-wovens using carding, crosslapping and needle-punching processes. These needle felts can be used for reinforcement in composites, as geotextiles for road and rail tracks, slope stabilisation, soil erosion control and drainage, in addition to horticultural uses (see Chapter 21).

8.3.1

Traditional Uses

Traditionally, the main uses of sisal fibre bundles are in ropes and twines, mainly produced through cottage industries. Sisal is converted into yarn, string, ropes, floor mats, bags, floor and wall coverings and different handicrafts. The sisal fibre can also be used in the manufacture of cellulose pulp to manufacture craft paper of high resistance and other types of thin paper. It can also be used in manufacturing reinforced composites for applications in the automotive industry, furniture and appliances and in building construction.

8.3.2

Pulp for Papermaking

As fibre yield from the biomass of the sisal plant is quite low (about 4%), the major application of the remaining biomass, which contains a large proportion of cellulose, is in manufacturing pulp for paper. Fibre extracted from the leaves of the Agave sisalana plant and its hybrids can be used to produce high-quality pulp for papermaking. Sisal pulp exhibits certain characteristics, such as high tear resistance, high alpha cellulose

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content, high porosity, high bulk, high absorbency and high folding endurance, which make sisal pulp suitable for many speciality papers (Hurter, 2001).

8.3.3

Sisal-Fibre-Reinforced Composites

Sisal fibre reinforced composites have found applications in several fields such as housing and automotive sectors. Roofing is an important problem in housing that requires a performance product. The replacement of asbestos with natural fibres is seen as an attractive option, and sisal–cement tiles have made an entry in several countries. Pulp from eucalyptus waste and residual sisal fibres have also been studied as a replacement for asbestos in roofing components (Agopyan et al., 2005). Sisal fibre bundles are also used in reinforcing soil to produce building material (Mattone, 2005). The potential for using sisal as reinforcement in composites has been much researched by the scientific community (Li et al., 2000; Joseph et al., 1999). The different types of matrix used range from thermoset, thermoplastics and rubbers to cement and gypsum. Detailed information about the processing techniques in the field of natural fibre composites (thermosets and thermoplastics) can be found in Chapter 19.3.

8.3.4

Thermoset Composites

A widely used thermosetting matrix is polyester (Pavithran et al., 1987; Fonseca et al., 2004) Compression moulding is the most convenient method for manufacturing thermoset natural fibre composites. Other conventional techniques include hand lay-up and vacuum bagging, resin transfer moulding, pultrusion and filament winding. In pultrusion, the predried fibres are mixed with the resin mix and pulled through to produce composites in the form of rods. Continuous profiles of any dimension can be made by this technique (see Figure 19.3.2 in Chapter 19.3). In filament winding, the fibres are impregnated in a mix and then wound on a rotating mandrel. Sisal–epoxy composites have been successfully fabricated by this method to prepare cylinders with helical reinforcements (Satyanarayana et al., 1990). In a recent study, Sreekumar et al. (2007) compared the mechanical properties of sisal fibre–polyester composites prepared by resin transfer moulding (RTM) and compression moulding (CM). The void content and water absorption characteristics of compression-moulded composites were found to be higher than those prepared by RTM. This was attributed to the higher interfacial adhesion in composites made from RTM. Table 8.3 shows the flexural properties of composites at different fibre bundle lengths. It can be seen that there is a definite distinction in properties for the different processing techniques, namely RTM and CM; the former resulted in better properties. This is attributed to good wettability and therefore better interfacial adhesion in RTM. There is also a substantial increase in flexural strength when the fibre bundle length increases from 20 to 30 mm for RTM, which is absent in CM, indicating that good wetting occurs between fibre bundle and matrix in the RTM process.

8.3.5

Thermoplastic Composites

Thermoplastics are favoured over thermosets owing to their low cost, ease of compounding and recyclable properties. Polyethylene and polypropylene are the most common thermoplastic matrix materials used. The mixing methods used for the sisal fibre and matrix are usually melt mixing and solution mixing, out of which melt mixing dominates the industrial sector. In melt mixing, the fibre is added to a melt of thermoplastics and mixing is performed at a specific temperature and speed for a specified time. The mix is extruded using an injection moulding machine as test specimens. In solution mixing, the fibres are added to a

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Table 8.3

Effect of processing technique on flexural resistance of sisal fibre composites (Sreekumar et al., 2007) Resin transfer moulding

Fibre bundle length in mm 10 20 30 40

Compression moulding

Flexural strength in MPa

Flexural modulus in MPa

Flexural strength in MPa

Flexural modulus in MPa

54 ± 3.5 60 ± 0.6 80 ± 0.3 70 ± 2.6

2490 ± 0.85 2814 ± 4.37 3434 ± 4.43 3010 ± 2.94

52 ± 4.9 58 ± 1.6 60 ± 3.6 59 ± 3.2

2152 ± 2.53 2583 ± 6.42 3283 ± 3.2 2873 ± 2.2

viscous solution of thermoplastics in a solvent. This mixture is then transferred into a vacuum oven to remove the solvent. The solution mixing procedure avoids the fibre damage that normally occurs during blending of fibre and thermoplastics by melt mixing. A preimpregnation technique was introduced by Fung et al. (2003) for the injection moulding of sisalfibre-reinforced polypropylene composites. In this study, the sisal fibres were coated with MA-g-PP before moulding. The compatabiliser functioned as a wax lubricant to enhance the flow and dispersion of the sisal fibres inside the polypropylene matrix. One of the main advantages of the preimpregnation technique was that the composites could be moulded with a low barrel temperature, and thermal degradation of the fibres could be reduced.

8.3.6

Natural Rubber Composites

A literature survey has shown that rubber is the second most widely used matrix for sisal composites after polyethylene. The rubber matrices used include natural rubber and styrene–butadiene rubber. Rubber composites are usually processed by milling followed by compression moulding. The first step in milling is to oven-dry the whole fibre bundles to reduce moisture to below 0.1%. The fibres can also be modified by chemical treatments to make them more compatible with the rubber matrix. The second step is the mixing of the treated fibre into the rubber formulation during the rubber compounding operation in a Banbury mixer or two-roll mill (Barlow, 1993). The product from this step is a homogeneous rubber compound reinforced with fibres. The compound is heated on a mill roll into manageable sheets for handling. The final process step is compression moulding at elevated temperature and pressure to cure the rubber. The reinforcement effect of sisal fibre bundles in natural rubber was investigated by Varghese et al. (1994). They observed that the tensile strength decreased up to 17.5% volume loading and then increased. The tear strength and modulus values, however, showed a consistent increase with loading. Prasanthakumar (1992) studied the effect of sisal fibre surface modifications, NaOH treatment, acetylation and benzoylation on the interfacial adhesion of sisal fibre and styrene–butadiene rubber (SBR) matrix. The author also analysed the dynamic mechanical behaviour of the composites. He found that the storage modulus of the composites increased with increasing fibre volume fraction, fibre surface modifications and the use of bonding agent. In another study, a unique combination of sisal and oil palm fibre bundles in natural rubber was utilised to design hybrid biocomposites. It was seen that the incorporation of fibres resulted in increased modulus. Alkali modification of sisal and oil palm fibres resulted in increased adhesion and properties (Jacob et al., 2004a and 2004b). Researchers have also designed novel rubber biocomposites by using a combination of leaf and fruit fibre in natural rubber (Haseena and Unnikrishnan, 2005a and 2005b). The incorporation of sisal and coir fibre bundles in natural rubber was seen to increase the dielectric constant of the composites. These hybrid biocomposites were found to have enormous applications as antistatic agents.

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Newer Composite Processing Techniques

The conventional techniques enumerated above for thermoplastic composites comprise two or more stages of processing. Such two-stage processing techniques expose natural fibres to high shear and thus damage them. The best use of natural fibres occurs when processing methods reduce or eliminate fibre damage (e.g. in the one-stage LFT process, as described in Chapter 19.3). Alternative processing of natural fibre–thermoplastic composites has become an important task for scientists. One such avenue is the application of powder impregnation technology in the fabrication of natural fibre composites. Mohanty et al. (2002) investigated the importance of a new environmentally benign powder impregnation processing technique for natural fibre–polypropylene composites. Chopped kenaf fibre bundles and polypropylene powder were mechanically mixed and subjected to compression moulding to make composite panels. A hybrid coupling agent was also adopted for this study, which resulted in improved properties. Another interesting approach is the commingling technique, where the polymer fibre and reinforcement fibre are intermingled together. Paul et al. (2008) attempted to use the commingling technique to make a banana–PP composite from PP fibres and banana fibres. Heating and consolidation of these fibres involved the melting of the dispersed polymer fibres and the subsequent formation of a continuous polymer matrix around the reinforcement fibres. The advantage of this method is that the reinforcement fibres are not subjected to shear forces as in melt mixing. No solvents are required for mixing the polymer with reinforcement fibre as in solution mixing, and the fibre loading can be increased up to 60%.

8.4

Conclusion

This chapter gives a detailed account of the cultivation and production aspects of sisal fibres. Brazil dominates the world market in terms of sisal production. The different species under the Agave family and the different classifications have been outlined. Sisal fibre is one of the most commonly used leaf fibres in composites owing to its interesting mechanical properties (e.g. strength/elongation characteristics) and has been traditionally used in ropes and carpets, but currently it is being studied as a replacement for asbestos in roofing components.

Acknowledgement The authors would like to acknowledge Professor Rasiah Ladchumananandasivam of the Centre of Technology, Department of Textile Engineering, Universidade Federal do Rio Grande do Norte – UFRN, for providing the literature translated from Portuguese.

References Agopyan, V., Savastano, H., John, V.M. and Cincotto, M.A. (2005) Developments on vegetable fibre–cement based materials in S˜ao Paulo, Brazil: an overview. Cem. Concr. Compos., 27, 527–536. Banerjee, A.K. (1972) Trial of Agave species in Lateritic areas of West Bengal. Indian For., 98(7), 432–436. Barlow, F.W. (1993) Rubber Compounding, Principles, Materials and Techniques, 2nd edition. Marcel Dekker, New York, NY, USA. Berger, A. (1915) Die Agaven – Beitr¨age zu einer Monographie. Verlag von Gustav Fischer, Jena, Germany. Bisanda, E.T.N. and Ansell, M.P. (1992) Properties of sisal – CNSL composites. J. Mater. Sci., 27, 1690-1700 Bos, J.J. and Lensing, F.H.G. (1973) A new cultivar in sisal from East Africa: Agave sisalana Perr. ex Engelm. cv. hildana. East Afr. Agric. For. J., 39(1), 17–25. Brown, K. (2002) Agave sisalana Perrine. Wildland Weeds, 5(20), 18–20. Chakravorthy, A.C. (1969) Observation of transverse compression of some plant fibers. Text. Res. J., 39, 878–881.

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Chand N. and Hashmi, S.A.R. (1993) Mechanical properties of sisal fibre at elevated temperatures. J. Mater. Sci., 28, 6724–6728. Chand, N., Tiwary, R.K. and Rohatgi, P.K. (1988) Bibliography resource structure properties of natural cellulosic fibres – an annotated bibliography. J. Mater. Sci., 23381–387. de Andrade Silva, F., Chawla, N. and de Toledo Filhos, R.D. (2008) Tensile behaviour of high performance natural (sisal) fibers. Compos. Sci. Technol., 68, 3438–3443. El Oudiani, A., Chaabouni, Y., Msahli, S. and Sakli, F. (2008) Crystalline character of Agave americana L. fibers. Text. Res. J., 78, 631–644. El Oudiani, A., Chaabouni, Y., Msahli, S. and Sakli, F. (2009) Elastic recovery and viscoelastic behavior of Agave americana L. fibers. Text. Res. J., 79, 166–178. FAOSTAT (2006) Food and Agriculture Organisation of the United Nations (February 2009); available at: http://faostat.fao.org/ (accessed 8 June 2009). FAOSTAT (2009) Food and Agriculture Organisation of the United Nations (February 2009); available at: http://faostat.fao.org/ (accessed 8 June 2009). Fonseca, V.M., Fernandas, V.J., Jr, De-Carvalho, L.H. and D’Almeida, J.R.M. (2004) Evaluation of the mechanical properties of sisal–polyester composites as a function of the polyester matrix formulation. J. Appl. Polym. Sci., 94, 1209–1217. Franck, R.R. (editor) (2005) Bast and Other Plant Fibres. Woodhouse Publishing, Cambridge, UK. Fung, K.L., Xing, X.S., Li, R.K.Y., Tjong, S.C. and Mai, Y.-W. (2003) An investigation on the processing of sisal fibre reinforced polypropylene composites. Compos. Sci. Technol., 63, 1255–1258. Gentry, H.S. (1982) Agaves of Continental North America. University of Arizona Press, Tucson, AZ, USA. Gentry, H.S. (2004) Agaves of Continental North America. University of Arizona Press, Tucson, AZ, USA. Haseena, P. and Unnikrishnan, G. (2005a) Tensile studies on short sisal/coir hybrid fibre reinforced natural rubber composites, in Proceedings of International Conference on Advances in Polymer Blends and Composites (ICBC 2005), Kerala, India, 21–23 March, p. 51. Haseena, P. and Unnikrishnan, G. (2005b) Dielectric properties of short sisal/coir hybrid fibre reinforced natural rubber composites, in Proceedings of International Conference on Advances in Polymer Blends and Composites (ICBC 2005), Kerala, India, 21–23 March, p. 61. Hoffmann, C. (1884) Botanischer Bilder-Atlas nach de Candolle’s Nat¨urlichem Pflanzensystem: mit u¨ ber 500 naturgetreuen Pflanzenbildern auf 85 fein kolorierten Tafeln und erl¨auterndem Text von Carl Hoffmann. Verlag von Julius Hoffmann (K. Thienemanns Verlag), Stuttgart, Germany, p. 74, Fig. 426; online resource http://edocs.ub.unifrankfurt.de/volltexte/2006/50096/. Hurter, R.W. (2001) Sisal fibre: market opportunities in the pulp and paper industry. Presented at Alternative Applications for Sisal and Henequen, a joint FAO/CFC seminar, Rome, Italy, 13 December 2000, FAO/CFC Technical Paper No. 14, pp. 61–74. Jacob, M., Varghese, K.T. and Thomas, S. (2004a) Natural rubber composites reinforced with sisal/oil palm hybrid fibers: tensile and cure characteristics. J. Appl. Polym. Sci., 93, 2305–2312. Jacob, M., Varghese, K.T. and Thomas, S. (2004b) Mechanical properties of sisal/oil palm hybrid fiber reinforced natural rubber composites. Compos. Sci. Technol., 64, 955–965. Joseph, K., Toledo Filho, R.D., James, B., Thomas, S. and Hecker de Carvalho, L. (1999) A review on sisal fibre reinforced polymer composites. Revista Brasileira de Engenharia Agricola e Ambiental, 3, 367–379. Kimaro, D.N., Msanya, B.M. and Takamura, Y. (1994) Review of sisal production and research in Tanzania. Afr. Study Monogr., 15, 227–242. Li, Y., Mai, Y. and Ye, L. (2000) Sisal fibre and its composites: a review of recent developments. Compos. Sci. Technol., 60, 2037–2055. Lock, G.W. (1969) Sisal, Thirty Years’ Sisal Research in Tanzania. Longmans, London, UK, pp. 25–315. Martins, M.A. and Joekes, I. (2003) Tire rubber–sisal composites: effect of mercerization and acetylation on reinforcement. J. Appl. Polym. Sci., 89, 2507. Mattone, R. (2005) Sisal fibre reinforced soil with cement or cactus pulp in bahareque technique. Cem. Concr. Compos., 27, 611–616. Meyers Großes Konversations-Lexikon, Band 6., Faserpflanzen I. Bibliographisches Institut, Wien/Leipzig, Austria/Germany, 1906.

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Mohanty, A.K., Drzal, L.T. and Misra, M. (2002) Novel hybrid coupling agent as an adhesion promoter in natural fiber reinforced powder polypropylene composites. J. Mater. Sci. Lett., 21, 1885–1888. Mukherjee, P.S. and Satyanarayan, K.G. (1984) Structure and properties of some vegetable fibres. Part 1: Sisal fibre. J. Mater. Sci., 19, 3925–3934. Mwaikambo, L.Y. and Ansell, M.P. (1999) The effect of chemical treatment on the properties of hemp, sisal, jute and kapok for composite reinforcement. Angew. Makromol. Chem., 272, 108. Nobel, P.S. (1994) Remarkable Agaves and Cacti. Oxford University Press, Oxford, UK. Paul, S.A., Boudenne, A., Ibos, L., Candau, Y., Joseph, K. and Thomas, S. (2008) Thermophysical properties of natural fibre reinforced polyester composites. Compos. Part A, 39, 1582–1588. Pavithran, C., Mukherjee, P.S., Brahamakumar, M. and Damodaran, A.D. (1987) Impact properties of natural fibre composites. J. Mater. Sci. Lett., 7, 825–826. Prasanthakumar, R. (1992) PhD Thesis. Mahatma Gandhi University, Kottayam, Kerala, India. Rocha, M., Valera, A. and Eguiarte, L.E. (2005) Reproductive ecology of five sympatric Agave littaea (Agavaceae) species in central Mexico. Am. J. Bot., 92, 1330–1341. Satyanarayana, K.G., Pai, B.C., Sukumaran, K. and Pillai, S.G.K. (1990) Handbook of Ceramics and Composites. Marcel Dekker, New York, NY, USA, Ch. 12, pp. 339–386. Satyanarayana, K.G., Pillai, C.K.S., Sukumaran, K., Pillai, S.G.K., Rohatgi, P.K. and Vijayan, K. (1982) Structure property studies of fibres from various parts of the coconut tree. J. Mater. Sci., 17, 2453–2462. Schnegelsberg, G. (1999) Handbuch der Faser – Theorie und Systematik der Faser. Deutscher Fachverlag, Frankfurt am Main, Germany. Snyder, B.J., Bussard, J., Dolak, J. and Weiser, T. (2006) A portable sisal decorticator for Kenyan farmers. Int. J. Serv. Learning Eng., 2(1), 92–116. Sreekumar, P.A., Joseph, K., Unnikrishnan, G. and Thomas, S. (2007) A comparative study on mechanical properties of sisal-leaf fibre-reinforced polyester composites prepared by resin transfer and compression moulding techniques. Compos. Sci. Technol., 67, 453–461. Starr, F. and Starr, K. (2002) Plants of Hawaii – a collection of images, maps, and reports for plants found in Hawaii (1 November 2002); available at: http://www.hear.org/starr/plants/ (accessed 8 June 2009). The Plants Database (2000) Database (version 5.1.1). National Plant Data Center, NRCS, USDA, Baton Rouge, LA, USA. UNIDO (2009) Creating opportunities in the Sisal industry. United Nations Industrial Development Organization (12 February 2009); available at: http://www.unido.org/index.php?id=o8447 (accessed 8 June 2009). Varghese, S., Kuriakose, B., Thomas, S. and Koshy, A.T. (1994) Mechanical and viscoelastic properties of short fiber reinforced natural rubber composites: effects of interfacial adhesion, fiber loading, and orientation. J. Adhes. Sci. Technol., 8, 235–248. Webster, C.C. (1970) Expl Agric., 6(1), p. 80. Yayock, J.Y., Lombin, G. and Owonubi, J.J. (1988) Crop Science and Production in Warm Climates. Macmillan Publishers Limited, London and Basingstoke, UK. Zander, R., Erhardt, W., G¨otz, E. and B¨odeker, N. (2002) Handw¨orterbuch der Pflanzennamen. Dictionary of Plant Names. Dictionnaire des Noms de Plantes. Ulmer (Eugen), Stuttgart, Germany.

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9 Coir – Coconut Cultivation, Extraction and Processing of Coir Chitrangani Jayasekara Coconut Research Institute, Lunuwila, Sri Lanka

Nalinie Amarasinghe Industrial Technology Institute, Colombo, Sri Lanka

9.1 Introduction Coir is a natural fibre extracted as fibre bundles from the husk surrounding the seed of a coconut. The seed is separated from the husk for the extraction of the oil-rich kernel for various food products such as fresh kernel, copra and desiccated coconut. The husk is thus a byproduct of copra or desiccated coconut production. Originally, coir extraction was a domestic industry that perhaps originated centuries ago along the east coast of South India and along the southern and the north-western coastal belt of Sri Lanka. Traditionally, coir was extracted from husks that had been soaked for 6–9 months (retted) in sea water or lagoon water, and then beaten with a wooden mallet. With time, the coir extraction processes have improved significantly, the quality coir fibre (bundles) being extracted either by wet processing (retting procedures) or mechanical decortication without soaking. Depending on the extraction process, the quality as well as the quantity of fibre/fibre bundles extracted from a given number of husks may vary. In the past, coir has been considered as a low-quality, low-value product, with its main uses being as coir yarn, coir nettings, white coir for yarn making for doormats and floor coverings, brown coir for rubberised pads and mattress and bristle coir for brooms and brushes. Coconut coir has outstanding resistance to sea water, and the cordage therefore has great value for marine uses (shipping and fisheries) (Barker, 1933). The introduction of synthetic polymer products gradually took over the demand for natural hard fibres in cordage and twine application. In the last three decades the application of coir has expanded tremendously for the manufacture of rubberised coir products for automobiles and upholstery and subsequently as woven and knitted geotextiles for erosion control and as a base for binding earth on sloping lands.

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Advances in coir research in the past few decades has led to a clearer understanding of the structural, chemical and mechanical properties of coir. A single coir fibre has a mean width of 0.02 mm and a mean length of 2.5 mm, and these short fibres exist as aggregated fibre bundles. Such a bundle would in common usage be referred to as a ‘strand’. Coir fibre has the highest lignin content when compared with other commonly used agro-based fibres (Rowell and Jacobson, 2002). Some advantages of coir are that it is non-abrasive, stiff, resilient, hydroscopic, viscoelastic, biodegradable, compostable and combustible, and a natural product amenable to chemical changes. As a fibre, coir has a high aspect ratio and a high strength-to-mass ratio, is low in energy conversion and has good insulation properties (Rowell and Jacobson, 2002). Nevertheless, a major disadvantage of coir is the great thickness of fibre bundles when compared with other fibres such as flax, hemp and wool. It therefore requires large volumes in insulation applications. High stiffness, resilience, stability and elasticity of twisted coir are some advantages of coir cordage. Recent advances in material sciences through research in European countries and in the Philippines have enabled highly technical and sophisticated applications of coir as coir composites, fibreboards, coir-based building materials, insulation products, etc. With the realisation of the value of ecofriendly natural products, the world is progressively moving towards the increased use of renewable natural resources/materials. Coir is a product available throughout the year, and cheaper in cost than other agro-based fibres. Easy extraction methods, modern machinery introduced over recent decades and new and versatile applications of coir as a result of technological advances have been the main milestones enabling this industry to progress from its humble beginnings. Furthermore, these achievements paved the way for the coir industry to expand significantly in the past 25 years and to maintain its position in the global hard fibre trade.

9.2 The Coconut Palm as a Crop and Its Cultivation The coconut palm (Cocos nucifera L.) holds pride of place among the palms growing in the tropical region of the world because of its versatility and myriad uses. It is the most economically important cultivated palm in over 93 countries falling in the tropical coastal ecosystem of the world, providing more than 200 products or byproducts for human use. It occupies an area of approximately 12.17 million hectares globally, with an annual production of around 57 billion nuts. The Philippines is the principal coconut producer in the world, with Indonesia, India and Sri Lanka holding second, third and fourth places respectively. The total exports of unfinished coir and coir products from producing countries in 2003 and 2004 were 172 928 million t and 194.926 million t respectively (APCC, 2006). World exports of coir and coir products in 2005 and 2006 were 177 527 million t and 204 863 million t respectively (Yogaratnam, 2009). India is the major coir producer in the world, while Sri Lanka and Thailand maintain second and third positions respectively. The Philippines, Indonesia, China, Vietnam, Mexico, Venezuela and Tanzania are the other coir-producing countries in the world. About 67 countries all over the world import coir and coir products. The major importing countries of coir and coir products are the United States, the European Union, Canada, Australia, Japan, the Middle East and Korea. Value addition to coir is carried out in some importing countries. The Netherlands and Germany are leading coir product manufacturing countries in Europe (Fernandez, 1999; Chand, 1996). Of the total extent and production of coconut in the world, 90% is accounted for by Asia and the Pacific region (APCC, 2007). Nevertheless, the use of coconut husk for coir extraction is limited to a few countries, while the others use coconut mainly for its kernel products, such as copra, oil or desiccated coconut. Major coconut-producing countries like the Philippines, Indonesia and Pacific countries like Papua New Guinea and Fiji use coconut husks and shells as an energy source for the manufacture of copra. Because of the economic importance and versatility of coconut and its many uses as a food, oil, fibre and timber crop, and one with much aesthetic value, it is popularly known as the ‘tree of life’ or ‘tree of heaven’. It does not thrive well

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in extreme temperatures and grows particularly well along coastal belts. The climatic requirements of the coconut palm are given below: r Latitude and elevation. The coconut palm thrives well within 23 ◦ N and 23 ◦ S latitude. This encompasses the five main coconut-growing countries – the Philippines, Indonesia, India, Sri Lanka and Thailand. Coconut grows from sea level to an elevation of about 600 m. r Temperature. The temperature requirement of the coconut palm is in the range 20–32 ◦ C, and the optimum temperature is around 27 ◦ C for maximum growth and nut production. r Rainfall. The coconut palm thrives well over a wide range in terms of distribution and intensity of rainfall. To give a good yield it requires 1500–2500 mm of rainfall. The tropical region of the world has a largely bimodal monsoonal rainfall pattern. The palm is adversely affected by drought periods extending beyond about 3 months.

9.3

Structure of the Coconut as a Drupe

The fruit of the palm is botanically described as a fibrous ‘drupe’. As shown in Figure 9.1, it has a large seed in the middle. The seed has high commercial value and it is surrounded by a fibrous fruit coat or husk. The husk consists of an outer skin (exocarp) and a fibrous mesocarp. The hard shell constitutes the endocarp. The thickness of the husks can vary between 2 and 5 cm at the narrowest point, depending on the variety. As the seed is surrounded by the fibrous husk, it is well protected (Sampson, 1923). The mature fruit is used as a source of food. The kernel of mature nuts is processed into two commercially important commodities, namely copra and desiccated coconut. The liquid endosperm of the tender fruit is used as a source of natural

Figure 9.1

Cross-section of a coconut.

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Figure 9.2

The range of products manufactured from the processed coconut husk.

healthy drink. The thick shell of the seed is used for the production of charcoal, which is further processed to yield high-quality activated carbon (Liyanage and Jayasekara, 2000). The name ‘coir’ for coconut fibre bundles is derived from the Tamil and Malayalam word ‘Kavur’, which means cord. However, the term is now used to refer to the fibre derived from the husk of coconut (Sampson, 1923). The coarse coir fibre bundles (up to approximately 500 µm width and approximately 330 mm length) extracted from the husk are unique in being the only commercially derived fibre from a fruit of a plant. In the extraction process, a large amount of pith tissue results. This material, either in raw form or composted, is a valuable material in soil amelioration or for use in potting mixtures. The best-quality coir is made from the husk of ripe nuts that have been harvested before the husk has completely dried. The coconut husk contains about 70% pith tissues and 30% fibre bundles on a dry mass basis. The ratio of yield of long, medium and short fibre bundles is on average 60:30:10 respectively (Piyasekara, 1997). Coir extracted from the coconut husk is used for the manufacture of a range of products, as given in Figure 9.2. The pith or coir dust currently finds use as a substitute for peat moss in horticultural applications.

9.4

Extraction of Coir Fibre from Coconut Husks

Extraction of coir is mainly carried out in fibre mills, and the following steps are involved in the traditional process involving pretreatment of husks by retting: (i) (ii) (iii) (iv) (v)

retting; extraction of coir fibre bundles; removal of pith and coir waste from coir; drying; packing (balloting).

9.4.1

Retting Process

Retting – a microbial separation process (explained in more detail in Chapters 4 and 6) – consists essentially of soaking the husk in water for a period. Depending on the condition of the husks and the nature of the water, retting duration can vary from 6 to 9 months in the traditional process. When the husks are mature and dry, the retting process takes nearly 6–9 months. Fresh green husks require 2–3 months.

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Figure 9.3 Optimum retting period for mattress and omat fibre bundles. Modern Mill Project, Dunkannawa, Sri Lanka. Reproduced with permission from Common Fund for Commodities.

However, experiments conducted recently under the project ‘Pilot Facility for Efficient Coir Processing and Quality Control’ revealed that the optimum retting period for uncrushed coconut husks is 2 months, as shown in Figure 9.3. In the improved retting process, the husks are crushed before soaking, and then very short periods of 2–3 weeks are required to get the same quantity and quality of coir if extracted without crushing (Figure 9.4). Concrete tanks are used to soak crushed husks; otherwise, retting is carried out in large open pits or lagoons. These pits can be 2–3 m deep. Water used for retting can be either fresh water, brackish water or saline water. During the retting process, anaerobic fermentation helps to soften and loosen the fibre bundles from the pith tissues. By crushing the husks, the surface area in contact with the water increases, and this accelerates the action of bacteria separating the fibre bundles from pith tissues. When the water in retting tanks or pits becomes a

Figure 9.4 Modern cemented retting tank and husk crusher. Modern Mill Project, Dunkannawa, Sri Lanka. Reproduced with permission from Common Fund for Commodities.

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Industrial Applications of Natural Fibres Table 9.1

Different coir extraction technologies and the grades of fibre obtained

Technology

Coir type (fibre bundles) Application

Traditional needle drum Traditional needle drum Defibreing machine Defibreing machine D-1 machine Decorticator

Bristle Mattress Omat/Bristle Mattress Mixed Mixed

Brushes, brooms, high strength twine Rubberised coir mattress High quality twine, yarn and machine twisted fibre Rubberised coir mattress Yarn & low quality twine machine twisted fibre Yarn & low quality twine

blackish red colour, it is important to renew it with fresh water. No waste water treatments are being practised in coir mills, but the water changes mainly through flooding, which dilutes the polluting constituents. The Central Coir Research Institute in Alleppey, Kerala, India, has innovated a process to treat effluent generated in soaking tanks. By this method, treated water can be recycled (Sarma, 2008).

9.4.2

Extraction Process

After retting, the next stage is the extraction of fibre bundles from the husk. The extraction of coir involves the breakdown and the separation of the coir fibre bundles from the connecting tissues or pith in between the fibre bundles and also the outer exocarp. Extraction of coir fibre by manual beating of retted husks has been a cottage industry in the southern part of Sri Lanka and in Kerala, India, since ancient times. The resulting fibre bundles are softer, longer and thus more pliable. Mechanised coir extraction from retted husks in Sri Lanka uses specially designed machines called traditional drum pairs or defibreing machines developed in India. Traditional drum devices gave Sri Lanka the advantage of producing high-quality bristle coir. Basically four technologies are used in fibre extraction, and all four have both advantages and disadvantages. Different grades of fibre produced by each technology and their applications are shown in Table 9.1.

9.4.2.1

Traditional Needle Drums or Ceylon Drum Pair

The traditional needle drum is the oldest Sri Lankan mechanical device for the extraction of coir. This machine has a set of two wheels with needles on the circumference to comb the retted husk and extract the fibre bundles. The first drum, called the ‘breaker drum’, has a set of needles that are coarse and serve to remove the exocarp. The husk segments are first fed to the breaker drum by hand. The second drum, called the ‘cleaner drum’, has needles that are finer and set closer to remove the short fibre bundles and the dust. The husk segments are held manually during this process against the thrust of the rotating wheels/drums. The drum consists of a wooden wheel of 0.9 m diameter with treads 0.3 cm wide and 15 cm long, into which iron nails are fitted, 3.8–5.0 cm apart. The wheel is protected by a wooden casing, with an opening of about 30 cm width protected by a pair of iron bars. The lower part of the casing of the wheel works like a chute and discharges the extracted fibre bundles to the ground. As the wheel revolves, one end of the husk segment is fed to the feeder drum by pressing in between fluted iron rollers. The fluted rollers assist the feeding of the husk, but it has to be held firmly with a force equal to that being exerted by the revolving drum. The nails of the rotating wheel tear away the short fibre bundles, which pass down the chute to the floor, leaving the husk with half-cleaned coir at one end. Then the other side of the husk is also fed through the iron rollers, leading to separation of the long bristle fibre bundles from the mattress coir. This bristle coir is then kept aside for another operator to feed into the cleaner drum. The cleaning drum contains finer nails fitted 2.5–3.8 cm apart. When the half-cleaned bristle fibre bundles from

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Figure 9.5

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Traditional needle drum and Indian-origin defibreing machine.

the breaker drum are fed into the cleaner drum, the remaining finer short fibre bundles and pith adhering to the bundles are combed out, leaving only the long bristle coir in the operator’s hands. Generally, one pair of drums can process 2000 husks per day of eight working hours, and it can produce 300–400 kg of bristle fibre bundles and 600–800 kg of mattress fibre bundles per working day. Feeding the husk through two iron rollers to the rotating drum is a very risky operation, and hands get injured frequently. Hence, finding skilled operators for this job is proving to be increasingly difficult. 9.4.2.2

Defibreing Machine (Indian Method)

The defibreing machine was developed in India and introduced into Sri Lanka in the 1980s. This machine is effective in separating the long fibre bundles of the retted brown husk from the shorter fibre bundles and pith, and is superior to the traditional needle drum (Ceylon drum) in terms of safety and production capacity. One machine could replace three sets of traditional needle drums with no reduction in processing capacity. In the defibreing machine, the husk segments are gripped at the periphery of a large wheel by another wheel placed eccentrically, so that the gap reduces as the husk moves towards the picker drum. The sharp pins of this drum remove the mattress fibre bundles and pith, leaving the bristle coir (Figure 9.5). The first drum defibres half of the husk segment, which is then transferred to a second wheel while the defibred part of the husk is held firmly by a conveyor chain. The defibreing is completed by the second picker drum. The quality of coir obtained from this machine is poor, and it needs to pass through a cleaner drum or wash to remove the pith adhering to the bundles. The maintenance cost of this machine is high, as needles have to be replaced almost every month. 9.4.2.3

Decorticator

The decorticator is a machine first developed by Downs in 1950 in England. The locally fabricated ‘Nugeng’ decorticator operates on the same principle as the Downs decorticator and is used for the dry processing of coir (Robbins et al., 1978). The advantages of this machine is that coir can be extracted from fresh husks or husks that have been soaked for a few hours, and it thus helps to reduce environmental pollution and enables extraction of potassium-rich pith tissues for recycling within coconut plantations. This machine is a high-powered, high-strength turbo cleaner into which green husks or moistened husks are directly fed without any preliminary processing. The husks are mechanically beaten against a cylindrical

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cage made out of 2.5 cm diameter torr steel. The rotary shaft, consisting of sharp-edged arms, facilitates the holding and hammering of the husk. The disadvantage of this machine is the inability to produce long fibre bundles. Also, the fibre output per husk is low. Only mixed-grade coir is produced by this machine. However, this is a cheaper option than the defibreing machine and occupies far less space.

9.4.2.4

D-1 Machine

This machine was developed in Sri Lanka to combine the effectiveness of traditional Ceylon drums, the safety of the defibreing machine and the affordability of the decorticator. Therefore it is called the ‘modified decorticator’ or ‘D-1 machine’. The nail drum acts as the opener, and the husk is fed through rotating rollers. Unlike with the traditional needle drums, it is not necessary to hold the husk manually for the fibre bundles to be combed out. The pith and exocarp of the husk are partly removed by the nail drum, and it is then automatically transferred to the turbo or a section similar to the decorticator for further removal of pith by mechanical beating. The machine is capable of using green husks, retted brown husks or wetted husks and produces mixed fibre bundles that are superior to the fibre bundles extracted by the decorticator alone. The machine has slowly gained in popularity where markets are mainly for mixed coir intended to be used for twine and yarn spinning. Several manufacturers, millers and village workshops have developed their own versions of this combined machine, with varying degrees of success.

9.4.3

Removal of Pith and Coir Waste from Coir

Different methods are being used for cleaning of coir fibre bundles. The cleaning of bristle coir is different from that of mattress coir bundles. The bristle coir or long fibre bundles have a smaller amount of residual pith. Therefore, bristle fibre bundles are cleaned (hackled) by combing through a set of steel spikes. The bristle coir is sometimes, but not always, washed in clean water and dried to give a better appearance and dyeability. Fibre bundles that have been extracted from the defibreing machine and mattress coir (medium and short fibre bundles) collected from the traditional drum pairs are fed into a cone-shaped revolving screen sifter. By gravitational action, fibre bundles are separated from the pith tissues. The coir is then fed into a turbocleaner, which consists of radially fixed iron rods rotating at a high speed, for further cleaning. By centrifugal action, remaining pith tissues and other debris attached to the fibre bundles are removed by this mechanical process, and better-quality coir is obtained.

9.4.4

Drying of Coir

Cleaned fibre bundles are dried under the sun to reduce the moisture content to about 15%. Modern mills use cemented or tarred drying yards to dry coir by exposure in good sunny conditions. Drying takes approximately 6 h, during which time the coir is turned over several times to ensure a uniformly dried product. In general, 1 or 2 days are required for sun drying of coir, and both bristle and mattress coir are dried in a similar manner.

9.4.5

Packing of Coir (Balloting)

Bristle coir is either packed in hydraulically pressed and hoop-bound bales of 152–203 kg or in ballots of 13 kg. ‘One-tie’ bristle coir as a rule is shipped in bales without an outer cover. The ballot is a rectangularshaped package prepared using a hydraulic press. Mattress coir is compressed using hand-operated presses into small ballots with dimensions of 60 × 36 × 18 cm and weighing 5 kg.

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At the exporters’ depots, both mattress and bristle coir are graded. Bristle coir is graded according to length, colour and stiffness, whereas mattress fibre is generally graded according to colour, resilience and cleanliness, based in particular on the quantity of pith present (Robbins et al., 1978). 9.4.6 9.4.6.1

Differences between White and Brown Coir Extraction Processes White Fibre Bundles

Originally, the manufacture of white coir was mostly carried out in the coastal belt of the southern part of Sri Lanka and along the east coast (Kerala Province) of India. In this process, fresh green husks are retted in pits in brackish water or in pits that are prepared in lagoons or rivers for 6–8 months. Retted husks are taken out of the water, their outer skin is peeled off and they are then beaten using a wooden mallet to separate fibre bundles from the pith tissues. The quality of white coir obtained by this method is of superior quality (Piyasekara, 1997). Today, mechanical methods are available for the extraction of white coir. Green husks are crushed, moistened by spraying water and then kept for 5–30 min. The wet husks are then fed into a decorticator or D-I machine. This method produces a mixture of long and short white coir (Piyasekara, 1997). After extraction from the husk, the coir fibre bundles undergo colour changes from white to a reddish brown colour, depending on the time for which the fibre bundles are exposed to the atmosphere. When exposed to air, the colourless phenolic substances in the product are converted to quinones owing to polyphenol oxidase activity. Hence, the coir acquires a dark-brown colour. 9.4.6.2

Brown Fibre Bundles

Extraction of brown fibre bundles is mainly carried out in Sri Lanka. Brown fibre bundles are effectively a byproduct of the copra and desiccated coconut production process, for which it is essential that the nuts are harvested fully mature (11–12 months old). Harvested nuts are stored for another 2–4 weeks in heaps for seasoning. Then the brown husks are soaked in retting pits for ‘wet milling’ in traditional needle drums (Ceylon drums) or a decorticator. Resulting fibre bundles are mostly brown coloured. New, environmentally friendly methods of brown fibre bundle production are now available. The Coir Board of India (Ravindranath and Sarma, 1995; Ravindranath and Bhosle, 1999) and the Coconut Research Institute of Sri Lanka (Fernando et al., 2008) have identified a consortium of microorganisms that reduce retting time substantially from 3–6 months to 3–4 weeks.

9.5 Different Types of Coir Fibre There are four different types of coir fibre: (i) (ii) (iii) (iv)

bristle fibre bundles; omat fibre bundles; mattress fibre bundles; mixed fibre bundles.

9.5.1

Bristle Coir

The type of long, parallel, clean fibre bundles produced from retted coir husks on the traditional needle drum or on the defibreing machine. Bristle coir has unique features and is still competitive with other coir types

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produced in other countries. It is used mostly for the manufacture of brushes, rubberised coir, nettings and high-strength twine. The classification of coir as described in the Sri Lanka Coir Standards (2008) is given below. Bristle coir is further subdivided into: (a) non hackled bristle coir – one tie, with an average length of 135 mm; (b) hackled bristle coir – two ties, with over 65% of fibre bundles by mass having length values of more than 221 mm; (c) hackled bristle coir – three ties, with over 70% of fibre bundles by mass having length values of more than 221 mm.

9.5.2

Omat Coir

The type of medium-length fibre bundles produced on the traditional needle drum and the defibreing machine and having an average length of between 70 and 135 mm. Omat fibre bundles are further subdivided into two groups: (a) omat coir ordinary – with a minimum 23% of fibre bundles by mass having length values above 200 mm, a minimum 38% having length values between 101 and 200 mm and a maximum 25% having length values below 100 mm; (b) omat coir superior – with a minimum 38% of fibre bundles by mass having length values above 200 mm, a minimum 45% having length values between 101 and 200 mm and a maximum 16% having length values below 100 mm.

9.5.3

Mattress Coir

The short fibre bundle fraction produced on the traditional needle drum or on the defibreing machine and having an average length of between 30 and 69 mm. This category is subdivided into two groups. (a) Mattress coir ordinary – produced on the defibreing machine, with 2% of fibre bundles by mass having length values above 200 mm, a minimum 33% having length values between 101 and 200 mm and 60% of fibre bundles having length values below 100 mm. (b) mattress coir superior – produced on the traditional needle drum set, with a minimum 11% by mass of fibre bundles having length values above 200 mm, 40% having length values between 101 and 200 mm and a maximum 48% of fibre bundles having length values below 100 mm.

9.5.4

Mixed Coir

Fibre bundles extracted from matured green husk or brown husks, with average lengths of between 36 and 119 mm. Decorticators or D1 machines produce mixed fibre bundles.

9.6

Characteristics of Coir as a Natural Fibre

Depending on the extraction process, quality as well as the yield of the fibre bundles varies. The quality is described on the basis of physical, mechanical and chemical properties. The utility of coir for a commercial application is determined by its physical properties, such as colour, texture, resistance to decay,

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length, linear density, fineness and moisture content. Detailed information about fibre properties and fibre testing methods can be found in Chapter 13. In the following, we will focus on some important coir properties. The most important mechanical properties of coir fibre bundles are their strength, elasticity, elongation and torsion rigidity. These, along with length and fineness, are the important physical properties determining the spinnability and commercial utility of coir fibre bundles (Ratnayake, 1996).

9.6.1 9.6.1.1

Physical Properties Colour

Colour of coir varies from pale yellow to dark brown. It mainly depends on the coconut variety, maturity of nuts, time lapsing between husking and retting, quality of water used for retting and the duration of retting.

9.6.1.2

Impurities

Anything other than coir, including husk pieces, pith and foreign particles, is considered as an impurity in coir. The degree of impurities in mattress and mixed coir is based on the contractual agreement between the supplier and the buyer.

9.6.1.3

Texture

Coir fibre bundles are stiff, resilient, spongy and pliable. Resilience expresses the amount of energy stored up in a body when one unit volume is stressed or compressed. It is measured as a percentage deviation from the original volume after releasing the compression force.

9.6.1.4

Resistance to Decay

Coir is a coarse fibre bundle and, in comparison with other natural fibres, has a high lignin content (see Chapter 13, Table 13.9). Coir is highly resistant to microbial attacks and to sea water (Barker, 1933). The impervious nature of coir contributes to these properties.

9.6.1.5

Length and Gravimetric Fineness (Linear Density)

The length of coir fibre bundles varies greatly within a sample. The size of a coconut varies with variety, with the location in which it is grown and also with the environmental conditions. As for all natural fibres (see Chapter 13), it is statistically more accurate to express the length of coir fibre bundles within a range. There is a significant variation in length distribution with type of extraction technology used and fibre grade/s produced. A total of 83 fibre samples covering all four technologies were analysed according to the test methods described in the Sri Lanka Standard No. 115 Part 1 (SLS115, 2009). The length distribution of different coir fibre grades is given in Figure 9.6. Bristle coir produced with the traditional needle drum has more long fibre bundles in length categories 150 and 250 mm compared with the three other coir grades shown in Figure 9.6. The length of fibre bundles is a very important property that determines the spinnability, twisting and commercial utility of coir.

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Figure 9.6 Variation in coir fibre bundle length. Modern Mill Project, Dunkannawa, Sri Lanka. Reproduced with permission from Common Fund for Commodities.

The gravimetrc fineness (linear density) of fibre bundles is defined as mass per unit length; a quotient obtained by dividing mass by length (1 tex = 1 g/1000 m). The gravimetric fineness (expressed in tex) of coir fibre bundles ranges from 5 to 181 tex, and their average linear density is 63 tex, with a standard deviation of 47 tex and a coefficient of variation of 65%. According to Nawaratne (2002), the gravimetric fineness of coir fibre bundles is 65.4 tex. A comparison of other natural fibres is given in Chapter 13, Table 13.6. The linear density distribution of 1000 measured coir fibre bundles is given in Figure 9.7.

9.6.1.6

Fibre Width (Diameter)

The fineness of fibre bundles can be expressed by their diameter in microns. The compactness and strength of a yarn or cord depends on the cohesion between fibre bundles and the friction between bundles. The smaller the diameter of the fibre bundles, the higher is the surface area per unit mass. As in the case of length, there is a large variation in the diameter of coir fibre bundles even within the same husk. Among the natural fibres, cotton and wool fibre bundles have diameters of 10–40 µm and 18–40 µm

Figure 9.7 Variation in the gravimetric fineness of coir fibre bundles. Modern Mill Project, Dunkannawa, Sri Lanka. Reproduced with permission from Common Fund for Commodities.

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Figure 9.8 Average diameter of different fibre types. Modern Mill Project, Dunkannawa, Sri Lanka. Reproduced with permission from Common Fund for Commodities.

respectively. Coir fibre bundles are coarser, and their diameters vary from 50 to 200 µm (Leson, 2002). Weighted average diameter at the mid-point of a coir fibre strand also varies with fibre type, as shown in Figure 9.8.

9.6.1.7

Moisture Absorption and Moisture Content

The capacity to absorb moisture from the surroundings is a valuable feature of coir fibre. The absorption of moisture changes the properties of coir such as tensile strength, elastic recovery, electrical resistance, rigidity, etc. As a result of absorption of water, the fibres and the fibre bundles tend to swell, altering their dimensions, and thus causing changes in the size, shape, stiffness and permeability of products such as yarn or ropes. When dry fibre or fibre bundles are exposed to the atmosphere, they will take up moisture from the surroundings and reach an equilibrium. Similarly, when exposed to a dry atmosphere, moisture is lost to the surroundings to establish a new equilibrium. The amount of water in coir fibre could be expressed in terms of moisture content or moisture regained. Nawaratne (2002) found that the moisture content of fresh-water-retted fibre samples was 10.20% with a moisture regain of 11.31%, while the moisture content in sea-water-retted coir was 7.92% with a moisture regain of 8.60%. Thus, retting methods have some effect on the moisture absorption of coir fibre bundles. A comparison between coir and other natural fibres in respect of their behaviour towards moisture is given in Chapter 13, Table 13.10.

9.6.2

Chemical Properties of Coir

As with other vegetable fibres, the main chemical components of coconut fibre bundles are cellulose, hemicellulose, pectin and lignin. In most plant fibres, more than 70% consists of cellulose (see Chapter 13, Table 13.9). In contrast, coir has a high lignin content and a lower amount of cellulose. The chemical composition of coir fibre and other plant fibres is given in Table 9.2 (van Dam, 2002). Cellulose is a metabolically inactive structural carbohydrate, a polysaccharide consisting of a linear chain of β-(1:4)-linked d-glucose units (Bidwell, 1979). Its empirical formula is (C6 H10 O5 )n . Hemicellulose is

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Table 9.2 Chemical composition of plant fibres. Reproduced with permission from J.E.G. van Dam, Coir Processing Technologies, Improvement of drying, softening, bleaching and dyeing coir fibre/yarn and printing coir floor coverings, CFC Technical paper No. 6, 2002. Chemical composition of plant fibres in (% of dry mass) Fibre Cotton Flax Hemp Jute Coir (brown) Coir (white) Coir (pith) Sisal Abac´a

Cellulose

Hemicellulose

Pectin

Lignin

Extractives

Fat and waxes

91.8 71.2 78.3 71.5 35.6 36.7 19.9 73.1 70.2

6.3 18.5 5.4 13.3 15.4 15.2 11.9 13.3 21.7

— 2.0 2.5 0.2 5.1 4.7 7.0 0.9 0.6

— 2.2 2.9 13.1 32.7 32.5 53.3 11.0 5.6

1.1 4.3 — 1.2 3.0 3.1 0.3 1.3 1.6

0.7 1.6 — 0.6 — — — 0.3 0.2

another polysaccharide with a random, amorphous structure, contains many different sugar monomers and is associated with cellulose. The combination of cellulose and hemicellulose is known as holocellulose. Lignin is a polymer of phenylpropanoid units. Lignins are formed from three different phenyl propane alcohols (monolignol monomers) known as coumaryl alcohol, coniferyl alcohol and sinapyl alcohol (Bidwell, 1979). According to Bhowmick and Debnath (1994), coir is a cellulose lignin complex. Satyanarayana et al. (1981) found that cellulose content varies from 33 to 43%, lignin from 41 to 46%, hemicellulose from 0.15 to 0.25% and pectins from 2 to 4%. However, these compounds can vary with the maturity of the fruit, as well as with the coconut variety. Lignin in the cell walls gives rigidity and colour. It also reduces permeation of water across the cell walls. Lignin also plays an important role in the transport of water, nutrients and metabolites in the vascular system of plants. Furthermore, lignin in plant cells helps to maintain rigidity of the cell wall and gives resistance towards compression and bending, as well as a protection against attack by microorganisms (Barker, 1933). Tests conducted by the Federal Institute for Materials Research and Testing, Berlin, Germany, on natural soil amendments over a prolonged period in highly fertile soil and under high humidity (90%) and moderate temperature (30 ◦ C) have revealed that: (a) cotton degrades totally within 6 weeks, and jute within 8 weeks, whereas coir retains 2% of its strength even after 1 year; (b) coir takes 15 times longer than cotton and 7 times longer than jute to degrade. The property of resistance of coir to degradation when buried has been attributed to its high content of lignin (30% or more) compared with other plant fibres such as cotton and jute (Rao, 2002).

9.6.3

Mechanical Properties of Coir

Detailed information about the mechanical testing of fibres is given in Chapter 13. The following section describes some important mechanical properties of coir.

9.6.3.1

Elongation

Elongation is defined as the amount of work that could be performed by a material within the limits of its breaking load. It is expressed as the percentage extension of the original length of the specimen.

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Breaking Load

The breaking load is the maximum force at which a specimen breaks under tension. The range of the average breaking load of tested coir fibre bundles is given as follows: r r r r r

5.5–7.5 N for hackled bristle fibre bundles (two-tie coir); 3.0–6.5 N for bristle fibre bundles; 2.7–5.6 N for omat fibre bundles; 1.6–5.2 N for mattress fibre bundles; 2.8–3.6 N for mixed fibres.

The minimum average breaking loads for different types of coir fibre are specified in the Sri Lanka Standards as: r r r r r

4.0 N for hackled bristle fibre bundles (two-tie coir); 3.8 N for non-hackled bristle fibre bundles; 2.6 N for omat fibre bundles; 1.6 N for mattress fibre bundles; 2.2 N for mixed fibre bundles.

9.6.3.3

Tensile Strength

The strength of a fibre is determined by its ability to resist strain or rupture induced by tension. Tensile strength is expressed as the breaking load per unit cross-sectional area of the test specimen. It is an important physical property for fibres or fibre bundles in textile applications because the properties of textile structures such as ropes or geotextiles depend on a complex interrelation between fibre arrangement and fibre properties. The strength of a fibre is related to its internal molecular arrangement (see Chapter 2.2). The ultimate tensile strength within Sri Lankan coconut varieties ranges from 99 to 123 N/mm2 (Nanayakkara, 2004). A detailed comparison with other natural fibres is given in Chapter 13, Table 13.7. The variation in the tenacity of coir fibre bundles and the distribution frequency are given in Table 9.3. 9.6.3.4

Initial Modulus

The initial modulus of coir fibre bundles varies from 120 to 1140 cN/tex. The average value is 397 cN/tex, with a standard deviation of 168 cN/tex and a coefficient of variation of 42% (Nawaratne, 2002). The initial modulus for coir fibre bundles and the distribution frequency are given in Table 9.4. Table 9.3 Variation in the tenacity of coir and the distribution frequency. Reproduced with permission from N.S. Nawaratne, Use of coir fibre as a raw material for geotextiles, University of Moratuwa, Sri Lanka, 2002. Class intervals of tenacity of fibre bundles in cN/tex 3–9 9–15 15–21 21–27 27–33 33–39 39–45

Frequency 132 217 100 33 12 2 4

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9.7

153 205 113 26 2

Major Uses of Coir

In the past, coir has been considered as a low-quality, low-value natural fibre that can be used for spinning of yarn, cordage, doormats, mattings, carpets, rugs, brushes and brooms. Even though man-made synthetic fibres have taken over the market of natural fibres, hard fibres have some advantages over synthetics fibres, as they possess properties that cannot be matched or copied easily. The current trends in enhanced utilisation of ecologically friendly bioproducts have paved the way for more diversified applications for coir and value-added products, providing greater potential for improving the rural economy in major coir-producing countries. Coir is a renewable, versatile, non-abrasive, porous, hydroscopic, viscoelastic, biodegradable, combustible and compostable natural product. Compared with other natural fibres, coir fibre bundles have medium strength but interesting elongation properties, which makes them attractive for certain technical applications. The energy conversion of coir is low (Rowell and Jacobson, 2002). With its high percentage of lignin, coir fibre possesses natural resistance to soiling and dampness. In hot climates it gives cool comfort and in cold weather it retains warmth. It also has good stretching and shrinking ability. Because of these favourable properties, it is widely used for floor coverings all over the world. Coir can be dyed and printed easily to get the desired colours and designs with a lasting finish. It can also absorb sound waves, and because of its superior acoustic qualities it is frequently used for wall panellings and floor coverings in auditoria and concert halls (see Chapter 20).

9.7.1 9.7.1.1

Coir-Based Traditional Products Coir Yarn

Coir yarn is made either by hand spinning or wheel spinning. Hand spinning is the oldest method. In this method the clean fibre bundles are rolled between the palms in a clockwise direction to twist into strands of shorter length. These short strands are taken into pairs and twisted together in the opposite direction to form two-ply yarn. By this method, one worker can produce 2–2.5 kg of yarn per day. Hand-spun yarn is soft and uniform in thickness, but the production is a tedious and poorly rewarded task. Wheel spinning of yarn is carried out using a set of spinning wheels. Three workers are required to operate one set of spinning wheels. Two are employed in making the strands while the one rotates the spinning wheel. Such a group of three operating a single set of spinning wheels can produce about 15–18 kg of yarn in 8 h. Motorised wheels can greatly improve the efficiency of coir yarn spinning. A mechanised yarn spinning machine would have an output of 15–60 kg per hour, depending on the quality of yarn and coir used for spinning.

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Yarn is spun using either white or brown fibre bundles. There are more than 60 grades of yarn produced in India and in Sri Lanka. The quality of yarn depends on the quality of coir used for spinning, such as white, brown, retted or unretted coir. In general, yarn should be free from extraneous matter, moisture and impurities and be of reasonably uniform construction thickness and colour. Long fibre bundles can be twisted easily. The compactness and strength of a yarn depends on the cohesion and friction between individual fibre bundles. Fine fibre bundles have a larger surface area per unit mass and will give a stronger yarn. Yarn and ropes made from coir fibre bundles are used for weaving floor coverings (carpets) and doormats and for horticultural applications such as in vineyards and the hop industry, as well as for making supports for oyster cultivation. Cord may also find use as ropeways (rope ladders) in date plantations (date palm: genus Phoenix), where several visits to the crown are necessary each season for maximising productivity.

9.7.1.2

Brushes and Brooms

Because of their rigidity and stiffness, bristle fibre bundles are mainly used for the manufacture of brushes and brooms. Brushes are mainly produced by fixing bristle coir on to a wooden or synthetic base. New technologies and industrial-scale machines are now available for the manufacture of brushes. Fibre cleaning, flagging, drilling, filling and trimming are carried out for improved quality of the finished products. Twistedin-wire (‘thawashi’) brushes (for the Japanese market) are made by inserting cut bristle fibre bundles between two wires that are twisted together by machine to grip the tufts. The size of the spiral and shapes vary depending on the required application. Different types of brush made out of bristle coir are shown in Figure 9.9.

9.7.1.3

Floor Mats and Mattings

Dyed and combed bristle fibre bundles are used to manufacture a great variety of floor mats and mattings. The three main types are (a) corridor mats, which are made using simple equipment, (b) woven matting made on a hand loom and (c) pile mats made on a power loom (Robbins et al., 1978). These floor mats are used as floor coverings, carpets and doormats for domestic or industrial use. New technologies and machinery are available for the manufacture of pile weave carpets.

Figure 9.9

Variety of uses for Bristle coir.

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9.7.1.4

Rubberised Coir Products

Rubberised coir products include mattresses, pillows, cushion material for the automobile industry, mainly for the seats of cars, buses and railway coaches, upholstery, air filters and packing materials. In the process of rubberising, coir fibre bundles are first subjected to a cleaning step, followed by curling and twisting to give a permanent resilience by special fibre cleaning and coir curling machines. On curling, the coir fibre bundles form highly resilient small coils that act as permanent springs with high elasticity. These coir springs are then bonded with atomised sprays of high-quality rubber latex. After spraying the latex, the coir products are dried, pressed, vulcanised and cut into the required shapes. In the manufacture of rubberised mattress pads, the loosened coir is passed to a machine that automatically feeds a constant mass of fibre that is thrown onto a conveyor belt to form an air-laid web. This air-laid web is passed to the spraying station where latex solution is applied to the top surface of the sheet. Then it is passed through a drying oven, turned over, sprayed and returned to the oven to dry again. Several layers of rubberised sheets are bonded together with latex. The multilayered rubberised fleece is placed in a press and then vulcanised in a drying oven. Finally, the mattress pads are trimmed and enclosed in cotton cloth covers. Rubberised coir is used to manufacture garden products such as coco pots, liners for hanging baskets, weed control fleeces and upholstery for the automotive industry. The latest development in the production of upholstery for the automotive industry is the automated ‘FaserTec’ process. This process is described in more detail in Chapter 19.4.3.4.

9.7.2

Novel Technical Applications for Coir

Globally, plants play a major role in the conversion of carbon dioxide into oxygen. At the same time, to meet the ever-increasing population demand, we need more land for housing and agriculture, more food and feed, more energy, more wood and fibre and so on. The coconut palm is an ecofriendly tree providing food, energy and other important resources such as fibre and timber. Coconut is a non-seasonal crop, and its products are available throughout the year. The pith tissue or coco-peat extracted as a byproduct during the process of extraction of coir is an excellent material for soil amelioration. In the past, coir fibre bundles have been used mainly in low-value products, such as doormats, brooms and brushes. With the realisation of the vast potential that exists for natural materials as bio-based composites, increased utilisation of coir fibre in diversified applications, such as in many new composite products, has been seen in the past two decades. The use of coir fibre for composites has many advantages over the use of other natural fibres owing to its special properties, such as its resistance to decay (Piyasena, 2008). Furthermore, recent work has proven that any limitation in performance of coir-based composites can be remedied through simple chemical modifications. A few novel technical applications of coir are as geotextiles, filters, sorbents, structural composites, nonstructural composites, natural hybrid composite material, moulded products and packaging materials (Rowell and Jacobson, 2002).

9.7.2.1

Geotextiles

Coir geotextiles are thin, permeable textiles used primarily in civil engineering applications to improve the structural properties of soils for various applications such as road development. As described in more detail in Chapter 21, the use of natural-fibre-based geotextiles has gained popularity in the past 20 years because of their environmentally friendly properties. Natural fibre geotextiles are mainly manufactured from jute, coir and blends of coir and jute. Depending on the method of manufacture, geotextiles can be categorised into three groups:

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Table 9.5 Geotextile applications and functions. Reproduced with permission from N.S. Nawaratne, Use of coir fibre as a raw material for geotextiles, University of Moratuwa, Sri Lanka, 2002. Geotextile function Application

Separation

Filtration

xxx

x

x x

xxx

Road, railway subgrade stabilisation Drainage Wet-fill embankment, lagoons Coastal river protection Land reclamation Asphalt reinforcement Soil reinforcement Marine causeways

xxx x x xxx

Drainage

x xxx xxx

xxx xxx x x

Reinforcement

x

x x x xxx xxx x

r Woven fabrics. These geotextiles are manufactured using weaving techniques adapted from other textiles. The weaving process gives these geotextiles the characteristic appearance of two sets of parallel strands interlaced at right angles to one another. r Knitted goods. The knitting of geotextiles involves interlocking a series of loops of yarn with one another. r Felts and fleeces. This type of geotextile is manufactured by thermal, chemical or mechanical bonding of coir fibre bundles. The properties of geotextiles used for various applications are different and depend on the purpose for which they are intended. The major functions of geotextiles are (a) separation, (b) filtration, (c) drainage and (d) reinforcement. Geotextiles almost always serve more than a single function. Coir-based geotextiles are widely used in erosion control, reinforcement and filtration, and these applications have found growing demand. Special applications of geotextiles and their functional attributes are given in Table 9.5. In addition to geotextiles, stitched blankets of coir are used for soil stabilisation along river banks and waterways, as shown in Figure 9.10. High- and medium-density coir fibre fleeces or felts can be used as natural mulch around plants, controlling the release of fertiliser. Medium-density coir fleeces pregerminated with grass are now available as a substitute for turf as soil stabilisation, and to establish grass rapidly. 9.7.2.2

Filters and Sorbants

Air filters to remove particulates can be made from medium- and high-density coir fleeces and felts to remove particulates. Fibre fillers can be impregnated with various chemicals as air fresheners or cleansers. Furthermore, medium- and high-density coir textiles such as felts can be used for the removal of oil spills and dyes and in the purification of solvents. 9.7.2.3

Composites

Structural composites are required to carry loads when in use. Conventional panel-type composites are particle boards, fibreboards and insulation boards. For the manufacture of coir-fibre-reinforced composites and fibreboards, low-cost organic binding material such as lignin or tannin can be used (see Chapter 19.5). Lignocelluloses serve as the main ingredient of such a composite. In the housing industry, such composites are used, for example, as ceiling panels for roof systems, partition boards and framing components. Coconuthusk-based binderless boards produced recently have shown better performance than MDF boards in terms

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Figure 9.10 Use of geotextile stitched blankets for soil stabilisation. I. Piyasena, New Technologies, processing methods, equipment/machineries and quality standards on the production of coir products, Proceedings of XLIII Cocotech meeting, 405–423, 2008. Reproduced with permission from Asian and Pacific Coconut Community.

of flexural properties (Keijsers, 2009). The latest trends in coir composites and coir boards are given in Chapter 19.5.

9.8

Conclusion

The coir industry is one of the oldest traditional, export-oriented and agro-based industries concentrated along the coastal belt of South and East Asian countries. This industry provides livelihood opportunities for poor rural people without gender bias. Several decades ago, coir was exported from producing countries as raw material in the form of ballots and bales of mattress, bristle or omat fibre bundles or as yarn, and the value addition took place in the coir-utilising countries. Gradual technological advancements achieved through research and innovation, cooperation extended from countries where value addition and utilisation took place and intervention from international agencies such as the Food and Agriculture Organisation (FAO) and the Common Fund for Commodities (CFC) have resulted in quality improvement and versatility of coir-based products. As a consequence, in the past two decades the coir industry has grown significantly, earning more foreign exchange and generating more employment opportunities with least damage to the environment. Coir is an ecofriendly, natural, biodegradable, durable, renewable, low-cost and sustainable product. Its special qualities cannot be matched easily by the synthetic fibres competing with natural fibres. Novel applications of coir as natural geotextiles for the protection of soils, woven fabrics and non-woven blankets, geo rolls or vegetation fascines for soil bioengineering applications, composites as a substitute for wood, plywood and MDF boards, filters, sorbants, packing material and insulation have opened up new opportunities for further technological advancements.

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References APCC (2006) Coconut Statistical Yearbook. Asia and Pacific Coconut Community, Jakarta, Indonesia. APCC (2007) Coconut Statistical Yearbook. Asia and Pacific Coconut Community, Jakarta, Indonesia. Barker, S.G. (1933) COIR – report on the attributes and properties of coconut fiber. Her Majesty Stationery Office, pp. 36–39. Bhowmick, B.B. and Debnath, C.R. (1984) Coir fibre – Part 1. Indian Coconut J., 15(5), 11–14. Bidwell, R.G.S. (1979) Plant Physiology, 2nd edition. Macmillan Publishing Co. Inc., New York, NY, 29, 241. Chand, V. (1996) Implications for the coir industry of market developments in Western Europe. Cocoinfo Int., 3(2), 5–10. Fernandez, C. (1999) Promoting coir products in the world market, in Proceedings of the 36th COCOTECH Meeting, Phuket,Thailand, 21–25 June 1999, pp. 234–242. Fernando, K., Weerasinghe, T.M.S.G., Mallawarachchi, S.M., Jayasekara, C. and Marikkar, J.M.A. (2008) Development of a technology for coconut coir retting using consortium of microorganisms, in Proceedings of the Second Symposium on Plantation Crop Research – Export Competitiveness through Quality Improvement, Colombo, Sri Lanka, pp. 45–54. Keijsers, E.R.P. (2009) Ecocoboard – processing technology and quality standardization. Cocoinfo Int., 16(1), 7–10. Leson, G. (2002) Thermal insulation materials from coir: opportunities and challenges, in Proceedings of the International Coir Convention, Common Fund for Commodities, Colombo, Sri Lanka, 13–14 June 2002. Liyanage, M. de S. and Jayasekara, C. (2000) Recent advances in coconut production and processing, in Plantation Management in the New Millennium, ed. by Sivaram, B., National Institute of Plantation Management, Aturugiriya, Sri Lanka, pp. 161–190. Nanayakkara, N.H.A. Sepa Y. (2004) Characterization and determination of properties of Sri Lankan coconut fibre. M. Phil. Thesis, University of Colombo, Sri Lanka, pp. 19–50. Nawaratne, N.S. (2002) Use of coir fibre as a raw material for geotextiles. MSc Thesis, University of Moratuwa, Sri Lanka, pp. 28–57. Piyasekara, S. (1997) Mill Fibre Industry of Sri Lanka – Problems Involved and Suggestions for Improvement. A Technical Handbook of the Industrial Development Board, Government of Sri Lanka, pp. 79–166. Piyasena, I. (2008) New technologies, processing methods, equipment/machineries and quality standards on the production of coir products, in Proceedings of XLIII Cocotech Meeting, Manado, Indonesia, pp. 405–423. Rao, G.V. (2002) Coir geotextiles – strategic management initiatives, in Proceedings of the International Coir Convention, Common Fund for Commodities, Colombo, Sri Lanka, 13–14 June 2002. Ratnayake, S.B. (1996) Mattress and bristle fibre. Cocoinfo Int., 3(2), 25–27. Ravindranath, A.D. and Bhosle, S. (1999) Bacterial consortia for retting of coconut husks in tanks. CORD, 15(1), 26–32. Ravindranath, A.D. and Sarma, U.S. (1995) Bioinoculants for coir retting, CORD, XI, 30–40. Robbins, S.R.J., Jarman, C.G. and Nichols, W. (1978) Report to the Government of Sri Lanka on the prospects for development of the local coir fiber industry. Rowell, R.M. and Jacobson, R.E. (2002) Use of coir in composite materials, in Proceedings of the International Coir Convention, Common Fund for Commodities, Colombo, Sri Lanka, 13–14 June 2002. Sampson, H.C. (1923) The Coconut Palm. The Science and Practice of Coconut Cultivation. Oxford House, UK, pp. 10–25. Sarma, U.S. (2008) Value addition in coconut husk. Indian Coconut J., 7, 8–11. Satyanarayana, K.G., Kulkarni, A.G. and Rohatgi, P.K. (1981) Potential of natural fibres as a resource for industrial materials in Kerala. J. Sci. Ind. Res., 40(4), 222–237. SLS115 (2009) Specification for coconut fibre (coir fiber). Part 1: Brown fibre and mixed fiber, 2nd revision, Sri Lanka Standards 115. Sri Lanka Coir Standards (2008) Sri Lanka Standards Institution (SLSI), National Standards Body of Sri Lanka, Ministry of Science and Technology, Colombo, Sri Lanka. van Dam, J.E.G. (2002) Coir processing technologies, improvement of drying, softening, bleaching and dyeing coir fibre/yarn and printing coir floor coverings. CFC Technical Paper No. 6, pp. 1–8. Yogaratnam, N. (2009) Tapping the coconut coir fibre industry’s vast potential. The Nation Economist, 1 February, p. 4.

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10 Cotton Production and Processing Muhammed Rafiq Chaudhry International Cotton Advisory Committee, Washington, DC, USA

10.1 Introduction Cotton is the most important of all natural fibre crops. In 1960, cotton represented 68% of all the fibre consumed in the world, and, although non-cotton fibres have benefited from recent technological developments and managed to erode cotton’s share of the market, as of 2009 cotton continued to account for no less than 38% of all the fibre consumed at the end-use level. Man-made industrial fibres can now be manufactured and sold at prices considerably below the price of cotton. That, plus improvements in their quality characteristics, has made man-made fibres more attractive than they once were. However, there are a number of features that are highly prized by consumers and are found exclusively in cotton. Cotton is unique in features such as its biodegradability, water absorbency, comfort and thermostatic capacity. The man-made fibre industry is coming up with new quality characteristics and producing renewable resource polyesters like PLA, but so far man-made fibres have not been able to match cotton features and, in all probability, will hardly be capable of surpassing cotton in those areas. More than 50 countries plant cotton on at least 10 000 hectares every year. Only about 13% of the cotton area is located in developed countries, so cotton is truly a developing country crop. The International Cotton Advisory Committee (ICAC), an intergovernmental organisation established in 1939, maintains world cotton statistics on area, production, yields, trade and prices. The data on the area planted to cotton, available since 1920/21, indicate that cotton has never been planted on more than 37 million hectares. In fact, world cotton area has surpassed 36 million hectares on only two occasions since the ICAC started compiling cotton statistics. On the other hand, after the 1950/51 season, cotton was planted on less than 30 million hectares only once, in 1986/87. So, in the intervening years, between the 1950/51 season and the present, the world cotton area has remained between 30 and 36 million hectares (ICAC, 2008a). In the same period, cotton production increased from 6.5 million t in 1950/51 to 26.3 million t in 2007/08. As the cotton area remained constant, all increases in production – 400% over 57 years – may be attributed to increases in yields. Owing to a more thorough understanding of the way the cotton plant develops and of how best to meet its needs, cotton yields have increased in all regions and countries. Research continues to Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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improve the technology for better harvests, but the toughest challenges still await cotton in the future, for even the highest-yielding countries in the world have attained no more than 50% of the genetic potential of current varieties. New technologies are continuously narrowing the gap between genetic and recoverable potential in all fibre crops, and thus in cotton. Biotech cotton as an important component of integrated pest management has contributed to increasing yields, lowering the cost of production and producing cotton with minimum use of toxic insecticides. Cotton still uses more insecticides (by value) compared with other field crops, but, according to Cropnosis Ltd (2009), a recent trend shows that the share of plant protection chemicals, particularly insecticides (by sale value), used on cotton has been on the decline since 2000. The world cotton industry is conscious of the fact that the industry needs to continue improving the sustainability of cotton production. Not only do the production practices have to be sustainable, but the processing of cotton, starting from ginning through to finished products, should also be environment friendly. Efforts are under way to improve economic, environmental and social sustainability of cotton production and consumption. Organic cotton production is seen as one way of improving sustainability, along with no-tillage and minimum-tillage techniques. Cotton consumption has increased at the same pace as production. Back in the 1950s, only about 3 million t of all cotton production was sold in the international markets. International trade of raw cotton increased to 9.8 million t in 2005/06. Shrinking mill use in the European Community and the United States, combined with rising mill use in others, has increased the amount of cotton traded on the international market. Mill use of cotton has increased significantly in China, India, Pakistan and Turkey in the last two decades. Apart from traditional uses of cotton, cotton use can be increased through composites. M¨ussig (2008) has presented a review of cotton mixing with other natural fibres for various composites used.

10.2 Origin and History The origin of the word cotton is still a mystery. However, there is a consensus in the specialised literature that cotton was derived from the Arabic word al qatan. The oldest written record of the use of cotton is found in a sacred Hindu text known as the Rig-Veda. Excavations at Mohenjo-daro, in Pakistan, show that human beings were using cotton fabric as far back as 3000 bc (Gulati and Turner, 1928). Other discoveries in Peru show that people there were using cotton over 4500 years ago. It seems evident that the drive to adopt cotton as a fibre crop stems from peoples’ search for a material from which to make clothing. It is reasonable to believe that diploid cottons were used in the Indian subcontinent whereas tetraploid cottons prevailed in South America (in Peru and Mexico) before they spread to other parts of the world. Flax, silk and wool were used long before cotton, and the literature shows that, in the earliest times, the word cotton was used for many different kinds of fibre. There is no one unanimous opinion on how cotton was domesticated by man and brought into widespread use, but most of the work on the origin of cotton is in agreement that man has transformed cultivated cotton species. Fryxell (1979), however, considers it probable that lint production and species differentiation came about before man took any real interest in the commercial production of cotton. He also believes that the species were defined independently of each other. According to Stewart (1994, updated in 2009), 50 species have been discovered so far, of which 19 have not yet been fully defined. Some of them are quite difficult to propagate, and many species do not even have the outgrowth (fibre) on the seed coat at all (Table 10.1). In spite of the many known species, there are only four recognised cultivated species of cotton: two diploid, Gossypium arboreum and Gossypium herbaceum, and two tetraploid, Gossypium hirsutum and Gossypium barbadense. The 2n = 26 diploid cottons are also called short staple cottons. G. hirsutum is usually referred to as upland cotton, and all extra-long staple/extra-fine cottons (also known as pima or giza types in Egypt) belong to G. barbadense. (The species known as Sea Island cotton also belongs to G. barbadense). Almost 97% of all cotton produced around the world is accounted for by upland cotton, with the remainder – only 3% of world production – made up by all other species. Interspecific and intraspecific commercial

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Table 10.1 Recognised Gossypium species, organised by germplasm pools. Compiled by Professor James McD. Stewart, University of Arkansas, USA, 2009. Reproduced with permission Species

Genomea

Notes

Primary (1◦ ) germplasm pool G. hirsutum G. barbadense G. tomentosum G. mustelinum G. darwinii

AD1 AD2 AD3 AD4 AD5

Current and obsolete cultivars, breeding stocks, primitive and wild accessions Current and obsolete cultivars, breeding stocks, primitive and wild accessions Wild, Hawaiian Islands Wild, north-east Brazil Wild, Galapagos Islands

Secondary (2◦ ) germplasm pool G. herbaceum G. arboreum G. anomalum G. triphyllum G. capitis-viridis G. trifurcatum G. longicalyx G. thurberi G. armourianum G. harknessii G. davidsonii G. klotschianum G. aridum G. raimondii G. gossypioides G. lobatum G. trilobum G. laxum G. turneri G. schwendimanii G. sp. nov.

A1 A2 B1 B2 B3 (B) F1 D1 D2-1 D2-2 D3-d D3-k D4 D5 D6 D7 D8 D9 D10 D11 (D)

Cultivars and land races of Africa and Asia Minor; one wild from Southern Africa Cultivars and land races from Asia Minor to south-east Asia and China; some African Wild, two subspecies from Sahel and south-west Africa Wild, south-west Africa Wild, Cape Verde Islands Wild, Somalia Wild, trailing shrub, east-central Africa Wild, Sonora Desert, USA Wild, Baja California (San Marcos Island), USA Wild, Baja California, USA Wild, Baja California Sur, USA Wild, Galapagos Islands Wild, arborescent, Pacific slopes of Mexico Wild, Pacific slopes of Peru Wild, south-central Oaxaca, Mexico Wild, arborescent, central to eastern Michoac´an Wild, west-central Mexico Wild, arborescent, central Guerrero, Mexico Wild, north-west Mexico Wild, arborescent, south-central Michoac´an and eastern Guerrero, Mexico Eastern Guerrero, Mexico

Tertiary (3◦ ) Germplasm Pool G. sturtianum G. robinsonii G. bickii G. australe G. nelsonii G. anapoides (new) G. costulatum G. cunninghamii G. enthyle G. exgiuum G. londonerriense G. marchantii G. nobile G. pilosum G. populifolium G. pulchellum G. rotundifolium G. stocksii G. somalense G. areysianum G. incanum G. bricchettii G. benadirense G. vollensenii

C1 C2 G1 (G) (G) (K) (K) (K) (K) (K) (K) (K) (K) (K) (K) (K) (K) E1 E2 E3 E4 (E) (E) (E)

Wild, ornamental, central Australia Wild, Western Australia Wild, central Australia Wild, northern Transaustralia Wild, central Australia Wild, erect, North Kimberleys, Australia Wild, ascending, west-coast North Kimberleys, Australia Wild, ascending, northern tip of Northern Territory, Australia Wild, erect, North Kimberleys, Australia Wild, prostrate, North Kimberleys, Australia Wild, ascending, North Kimberleys, Australia Wild decumbent, Australia Wild, erect, North Kimberleys, Australia Wild, ascending, North Kimberleys, Australia Wild, ascending, North Kimberleys, Australia Wild, erect, North Kimberleys, Australia Wild, prostrate, North Kimberleys, Australia Wild, Arabian Peninsula and Horn of Africa Horn of Africa and Sudan Arabian Peninsula Arabian Peninsula Somalia Somalia, Ethiopia, Kenya Somalia

a The genomic grouping of the Australian species is under study. Where used, ( ) indicates provisional genomic placement for the species in question

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cotton hybrids are grown on over 4 million hectares in India. Hybrids are also grown on a significant scale in China, Vietnam and some other countries, but their share in world production is hard to establish. Diploid species are grown in Bangladesh, India, Iran, Myanmar, Pakistan and Thailand, but India is the only country where all four cultivated species are grown on a commercial scale. World cotton statistics indicate that about 90% of all cotton is grown in the Northern Hemisphere, and about 10% in the Southern Hemisphere.

10.3

Phenology

The cotton plant is a perennial tree, but domesticated varieties were bred to grow as annuals. It has a tap root system, and its fibrous roots may penetrate into the soil as deep as 1.5–1.8 m in search of nutrients and water (Chaudhry and Guitchounts, 2003). Owing to its abundant root system, cotton can manage to survive in water-deficit conditions, and that is why it is generally considered to be a dryland crop. Cotton is usually planted in late spring, allowed to remain in the field during the harsh summer months and harvested in early autumn. After harvesting, the cotton seed has a dormancy period of about 1 month. The seed germinates well in soils at a temperature of over 15 ◦ C and sufficient moisture in the soil for the seed to absorb and burst, allowing the root and shoot to be formed. Under normal conditions, the seed germinates at 5–6 days after planting, and, if by the tenth day the germination rate is inadequate, the decision to replant can be made. Radicular growth is the first to start, forming the root even before the plumule breaks through the surface to emerge from the soil and form two cotyledonary leaves. At least 50–60 heat units are required for a seedling to break the surface of the soil (Kerby and Hake, 1996). In cotton, the cotyledonary leaves have a maximum life of 40 days and are different in shape from true leaves. The first true branch usually emerges on the fifth to sixth node. The first branches on the cotton plant are monopodial branches, sometimes also called vegetative branches. Monopodial branches are few in number, no more than 5–6, and sometimes they may be merely rudimentary with only sympodial branches visible on the plant. Monopodial branches do not bear fruit directly and give the plant a more voluminous look by comparison with a sympodial type of plant (Fryxell, 1984). The formation of monopodial branches ceases as soon as the first sympodial branch appears on the plant. As a result of the fruiting function of the secondary and tertiary branches, monopodial plants are usually characterised by late maturity. The cotton plant has a palmate leaf with well-developed mid-rib and lobes. Deeper cuts in lobes may turn the leaf shape into okra and superokra types, which are genetic characters (see Figure 10.1). The okra leaf shape, controlled by a single, partially dominant gene, was once thought to have a negative correlation with yield, but this linkage has been broken or disproved, and okra leaf varieties are successfully grown on a commercial scale, although in a limited number of countries. The leaf mid-rib may have a nectary that secretes a sugary juice (food for insects), or the leaf may have no nectary at all.

Figure 10.1

Different leaf shapes in cotton.

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Figure 10.2

223

Open cotton flower.

The cotton plant has a complete flower with well-defined calyx, corolla, androcium and gynaecium. The five sepals are fused, and the bud/flower is covered by three heart-shaped brackets/bracteoles with deep cuts. The base of each bracket also has a nectary. Sepals, fused to form the calyx, do not grow in size along with the petals (corolla) and style, but do anchor a large number of anther filaments. The five petals are tightly folded and become visible through the bracteoles only a day before the flower opens. The bud usually blooms into an open white flower 3–4 h before noon, and by then the anther dehiscence has already taken place, assuring self-pollination. Although the stigma is still receptive at the time of petal opening, cross-pollination can take place only in the presence of anthers. The pollen grains have spikes and are too heavy to be carried by the wind, and therefore they have to be transported either by insects or manually. Technically, cotton is a cross-pollinated crop, but under most conditions it behaves like a self-pollinated crop (Afzal and Ali, 1983; Munro, 1987). A 2% rate of natural outcrossing is common under most conditions whenever two varieties are planted in close proximity (Chaudhry and Guitchounts, 2003; Afzal and Ali, 1983). A separation of about 50 m between varieties is usually considered enough to avoid any outcrossing. Outcrossing may be higher – up to 50% or even greater – but it depends on the time of anther dehiscence, petal opening and insect activity in the field. Pollination takes place immediately as pollen grains are shed, but fertilisation may take 12–20 h. The ovary is superior with 4–5 carpels that ultimately become locks/lobes in an open boll. The ovules are linearly placed in two rows in each lobe, and each ovule must be fertilised to form a seed. Some diploid species may have only three carpels or lobes, and exceptionally some genotypes may have up to six lobes, but no upland varieties with such characters are in commercial cultivation anywhere in the world. On the day following anthesis, the petals turn pink. The next day, they turn dark purple, start withering and ultimately shed, leaving the young green bolls exposed to the vagaries of the weather. Unfertilised flowers inevitably drop off. At about 35 days after planting, the first flowers can be seen in the field, and it takes another 25 days for the buds to bloom into open flowers (see Figure 10.2). Technically, flowers may be referred to as bolls as soon as they have been fertilised, but the actual boll becomes visible only after the petals have been shed.

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A freshly fertilised flower takes another 45–50 days to become an open boll ready to be picked. All these timespans will ultimately be determined by the number of degree-days and other abiotic growing conditions. There are, of course, differences among varieties, but they do not play decisive roles. Early bud formation, together with the rate of horizontal and vertical flower/boll formation, will determine crop maturity or earliness.

10.4 Plant Nutrition Soil is composed of five components: air, water, organic matter, inorganic matter, or nutrients, and microorganisms. The relative quality of soil depends on the proportions of these constituents. The addition of organic matter increases the air and water retention capacity of soil and provides more favourable conditions for microorganisms to survive and flourish. Cotton can best be grown on sandy loam soils with a pH ranging from 6 to 8. The five elements that are applied to cotton are nitrogen (N), phosphorus (P), potassium (K), boron (B) and sulphur (S). Zinc (Zn) and other micronutrients are applied to cotton in fewer than five countries in the world (Chaudhry, 2008). In others, like Ethiopia and Tanzania, farmers may not apply nitrogen to cotton because they cannot afford it, but nitrogen is applied to cotton almost everywhere else. In countries such as Argentina, where it is still not applied commonly, research has established a positive effect on yield (Chaudhry, 2008). The continuous use of nitrogen for over five decades has reduced the cost-benefit ratio compared with the early days of the introduction of nitrogen. Nitrogen is applied to cotton in various forms, but all of it is taken up from the soil by the plant in nitrate form (NO3 − ). Nitrogen is usually split into 2–3 doses and applied prior to planting, the bud formation stage and at mid-boll formation stage (ICAC, 2008b). Nitrification must take into account losses into the air in the form of nitrogen gas, leaching into the soil or utilisation by diverse microorganisms before the plant manages to absorb it. Nitrogen is a must for healthy plant growth, but too much nitrogen may result in an imbalance between reproductive and vegetative growth. Phosphorus is used by the plant as a growth regulator, and consequently its impact on fibre quality is minimal. Phosphorus does not move in the soil, and the standard recommendation is always to apply it before or during planting and work it well into the soil. Phosphorus deficiency is more likely to occur in soils with a pH over 7.5, and phosphorus-deficient crops take on a dark-green colour and show stunted growth. A severe shortage may result in reddish-purple leaves, reduced flowering and delayed maturity of set bolls. Older cotton leaves quickly translocate phosphorus to younger bolls, so older leaves are more likely to show phosphorus deficiency symptoms (Oosterhuis and Howard, 2008). Potassium may or may not have an effect on yield. Potassium deficiency symptoms usually appear in the form of yellowish-white mottling in the area between leaf veins or on the leaf margins. In cases of severe deficiency, the leaves may be bronzed and curled downwards, but symptoms always proceed from the bottom to the top of the plant. The symptoms will depend on the availability of potassium in the soil, so, if potassium has been added to the preceding wheat crop, it is usually recommended to skip any application of potassium on cotton. Potassium is most needed by the plant at the boll maturing stage. Leaves and stem continue accumulating potassium during the vegetative growth period, and leaves quickly give up their potassium to the maturing bolls. Of all the parts of the cotton plant, bolls have the highest concentration of potassium (Awan, 1988) (Table 10.2). The addition of boron improves boll retention and boll opening by moving carbohydrates from the leaves to the bolls. Boron also affects root tip growth, synthesis of DNA and RNA and plays an important role in the elongation of the pollen tube, thus enhancing seed setting. Soils with less that 1.5% organic matter (sandy soils) are usually deficient in boron. All the boron is taken up by the plant in the form of boric acid. Some soils may be naturally rich in boron and never require boron application. Sulphur is another micronutrient used on cotton in some countries. Organic matter is the primary storehouse of sulphur in the soil; thus, soils low in organic matter may possibly require sulphur applications

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Table 10.2 Chemical composition of the cotton plant parts. Reproduced from A.M. Nawaz, Cotton Soils and Fertilizers, Cotton in Pakistan, Pakistan Central Cotton Committee, Ministry of Food and Agriculture, Pakistan, December, 1988 Plant part

Nitrogen in %

Phosphorus in %

Potassium in %

Calcium in %

Magnesium in %

Sulphur in %

0.82 1.06 2.30 1.08 3.13 0.18

0.12 0.08 0.15 0.18 0.43 0.28

1.06 1.60 2.49 3.50 2.10 2.28

0.45 0.69 3.15 1.28 0.18 0.08

0.25 0.25 0.52 0.26 0.33 0.05

0.06 0.05 0.42 0.17 0.33 0.05

Root Stem and branches Leaves Burr Seed Fibre

(Chaudhry, 1999). Symptoms of calcium (Ca), magnesium (Mg), molybdenum (Mo), copper (Cu), manganese (Mn) and zinc (Zn) deficiency are complicated and hard to differentiate from nitrogen and other nutrient deficiencies. They are used only in very rare situations.

10.5

Physiology

The cotton plant, like all other plants, absorbs carbon, hydrogen and oxygen from the air. So there is no dearth of these elements for the cotton plant to carry on photosynthesis and grow. Carbohydrates are formed during photosynthesis, and some plant species have the potential to utilise almost all the carbohydrates formed during photosynthesis: these are known as C4 plants. Owing to their ready access to an abundant supply of carbohydrates, C4 plants have high growth rates. The cotton plant is unable to utilise all available carbohydrates and tends to burn or release some part into the air by photorespiration. Cotton photorespires about 30% of the photosynthetic rate and thus belongs to the category of C3 plants (Cothren, 1999). Photorespiration in cotton is known to be catalysed by the same enzyme that catalyses the fixation of carbohydrates in the first position. Thus, elimination of photorespiration to convert cotton to the C4 category does not seem to be possible. Of all the approaches used to try to minimise the photorespiration rate in cotton, only two are worth mentioning: application of methanol and CO2 enrichment (Mauney et al., 1992). Both methods showed promise in the early experimental stages, but neither could be successfully commercialised in any country. The cotton plant produces many times more leaves than it does bolls. Genetically, each and every axil of a leaf, on the main stem or on the branches, is supposed to bear either a branch or a fruiting bud. This is, in fact, the case, but most fruiting buds are shed even before they become visible to the naked eye. Thus, the real number of bolls that will remain on the plant to maturity is only a small percentage of the actual boll spots occurring on the plant. The shedding of fruit forms is inevitable in cotton because of many factors. There are two physiological theories of fruit shedding in cotton: 1. The balance between auxin and growth-retarding hormones is disturbed and the result is shedding. The anti-auxin hormones increase in quantity and become more active, signalling the plant to form bolls at a slower rate or even inhibiting the formation of any more bolls on the plant. Guin (1986) has discussed in detail the role of hormones in abscission during reproduction. 2. The number of bolls increases beyond a certain limit determined by the vegetative mass of the plant, thus reducing the availability of carbohydrates and inhibiting the formation of bolls. Extremely high temperatures can hamper fertilisation, and that too results in shedding. Insect pressure and various other types of stress also cause shedding, but these factors affect buds more than anything else. Fertilised flowers are rarely shed. When bolls are shed, more often than they should, it will be due to abiotic stresses, such as water shortages, nutrient deficiencies and insect damage.

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Growth and fruiting of cotton

Stage

Period

Plant age

Planting to emergence Emergence to first true leaf Emergence to second true leaf Second true leaf to pinhead square (seventh node) Pinhead square to matchhead square Matchhead square to first one-third grown square First one-third grown square to first white bloom First white bloom to first open bolls Harvest bolls set in first 4 weeks of blooming

4–10 days 8 days 9 days 18–21 days 9–10 days 3–6 days 12–16 days 40–60 days 96%

4–10 days 12–18 days 21–27 days 39–48 days 48–58 days 51–64 days 63–80 days 103–140 days 91–128 days

The leaf is the most important part of the cotton plant. The leaves are the food factory for the plant, and in cotton only healthy leaves can bear fruiting buds, flowers and bolls. Physiologists have determined that leaves on the fifth node from the top are the most active on the plant (see details in Chaudhry, 2002). Leaf condition is a good indicator of plant health, nutrient status, water deficiency and insect pest damage, particularly by sucking insects, as well as of most diseases. Leaves that are affected by insect pests become incapable of retaining buds and shed them at a very early stage. Leaves must be healthy to have a good harvest. As in all deciduous trees, cotton leaves mature and reach their natural shedding stage. As the leaves age, an abscission layer, which is carbohydrate in nature, is formed between the leaf petiole and the stem or branch. The process of abscission layer formation is enhanced in cotton by the application of desiccants and defoliants. Defoliation is a prerequisite for machine picking of cotton, but, when harvest aids are applied too early in the cycle of the plant, i.e. when less than 60–70% of the bolls have opened, it reduces the yield and also affects fibre quality. Leaves may be hairy or non-hairy, and they come in various shades of green or red. Average growth and fruiting period is given in Table 10.3.

10.6

Insect Pests and Their Control

The cotton plant is naturally vulnerable to damage by a number of insect pests. About 17% (by value) of all insecticides used worldwide are sprayed on cotton, making it the top insecticide consumer among all field crops. On the other hand, cotton’s share of pesticide use (by value) is less than 8%, and it has been declining steadily over the last 10 years (various reports from Cropnosis Ltd, Edinburgh, UK). The Mexican boll weevil, Anthonomus grandis, which is limited to the Americas, is the most destructive pest in the Western Hemisphere. Elsewhere in the world, the American bollworm, Helicoverpa armigera, is the most widespread and most commonly occurring pest on cotton. The pink bollworm, Pectinophora gossypiella, was once a more serious pest, particularly in China, India and Pakistan, but now the American bollworm has taken the lead. Among sucking insects, the whitefly, Bemisia tabaci, is the most widely occurring and serious pest on cotton. The whitefly has spread to many countries in the last two decades. The American bollworm and the whitefly are notorious for developing resistance to insecticides. Repeated use of a particular chemical product on multiple generations year after year stimulates the insect’s ability to tolerate higher doses of insecticide. The basic resistance development mechanisms reported in cotton insects are: reduced penetration through the cuticle, ability to metabolise and excrete toxic chemicals, insensitivity of the target site (nervous system) and development by the insect of resistant genes that are passed on to subsequent generations (Russell, 2005; Kranthi, 2004). Australia, China, India, Pakistan and many West African countries have had to deal with this situation because of the indiscriminate use of insecticides. The problem has been resolved through the application of a wise rule of thumb: use insecticides only as a last resort. Other useful recommendations are: avoid using

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a single class of chemical over a long period of time; programme insecticide applications to hit the most susceptible stage in the life cycle of the insect; do not underdose or overdose insecticides; spray properly at the recommended thresholds and, when choices are available, use different classes of chemicals every year. Insecticide use is on the decline in most countries, and the future of pest control in cotton lies in integrated pest management, wherein biotech cotton (see Chapter 16) would be an important component. External control through chemicals will ultimately be replaced by a combination of control measures, including biological control (natural and specially introduced), host plant resistance, cultural control, legislative control, special control (e.g. male sterility) and, above all, biotech control. Among the pests affecting cotton are arthropods, mites, pathogens and weeds. Research in various countries has shown that it is extremely important to control weeds in order to get the maximum benefit from fertilisers and insect and disease control. Cultural control of weeds is the most widespread method, but it is slowly being replaced by herbicides. Biological control was tried for decades against various weeds but never attained commercial scale. Biological weed control is the intentional release of pathogens to attack specific weeds, but, when there are a wide variety of weeds (broad-leaf and grasses) occurring at the same time, it becomes impossible to control all of them with a single pathogen. Furthermore, the high cost of pathogen augmentation, environmental impact on pathogen activity (including low weed population), negative effects on the cotton plant and poor effectiveness in getting rid of weeds at early stages are some of the other difficulties that worked against the use of biological methods to control weeds. The only non-chemical control method that is gaining ground is conservation tillage, but it is impractical when the amount of land is limited, as is the case in small-scale farming systems. Herbicides have their own consequences, but, because of their ability to control weeds more effectively, herbicide use will continue to spread to more countries. The important diseases affecting cotton are Fusarium wilt, caused by Fusarium oxisporum, Verticilium wilt, caused by Verticillium dahliae, seedling damping-off, caused by Rhizoctonia solani, and bacterial blight, caused by Xanthomonas campestris pv. malvacearum. Leaf curl virus disease caused by geminiviruses has been wreaking havoc in Pakistan since 1992/93. The whitefly is the primary vector of cotton leaf curl virus (CLCV) disease, which was already an established pest in Pakistan. The disease also spread to India and has been detected in China. In the past, the cotton leaf curl virus disease caused damage in Sudan, but has never been as big a threat as it is now. Most diseases in the world are controlled either through seed treatment or by cultural means if the genetic resistance to the pathogen/disease is not available in the germplasm. Chemical control has been reported in a number of countries, but cotton rarely receives consecutive chemical sprays against diseases in any country. A latent threat from diseases will always exist.

10.7

Biotech Cotton

Herbicide-resistant biotech cotton was planted on a commercial scale for the first time in the United States in 1995/96. The following year, biotech cotton resistant to Lepidoptera was planted on a commercial scale in Australia and the USA. Herbicide- plus insect-resistant stacked gene biotech cotton became commercially available in 1997/98. By 2008/09, 11 countries had approved biotech cotton. Many genes were identified and inserted into the cotton genome in the first 14 years after the adoption of biotech cotton in the 1990s, but cry1Ac (Mon 531) has continued to be the leading gene for over 15 years in the area of insect control. Biotechnology is the technology with the fastest rate of adoption in the history of agriculture. Among its major benefits are higher yields, lower insecticide requirements, environmental safety, human safety and lower cost of production. The benefits are not uniform and vary greatly from country to country, from environmental safety alone to the whole range of the above-mentioned benefits. The technology also came with a number of conditions: producers had to plant refuge crops, and biosafety regulations had to be adopted, along with suspicions, especially in the context of future events. By the 2008/09 season, Argentina, Australia, Brazil, Burkina Faso, China, Colombia, India, Indonesia, Mexico, South Africa and the United States had already commercialised biotech cotton. Burkina Faso

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commercialised biotech cotton in June 2008, and many other countries are experimenting with it. Biosafety regulations/protocols and technology fees are the two main hurdles standing in the way of the adoption of biotech cotton. Biotechnology research, not only in cotton but also in general, received a big boost from the adoption of biotech crops. Many countries have invested heavily in biotechnology research because they believe that the future of crop improvement lies in biotechnology applications. In spite of the advantages of biotechnology, the field is not free of controversy, particularly with respect to food crops. A clear distinction must be made between the technology and the product. The technology is the ability to isolate and insert genes into non-related species, whereas the product or products developed through biotechnology are the insect- and herbicide-resistant cottons currently in use. A given product may or may not be good, but the technology cannot be denied or discarded. There is no doubt that biotechnology can be misused, as was the case with the Technology Protection System in cotton, but that threat is not exclusive to biotechnology. The biotech transgenes currently available in cotton, as well as in many other crops and fields of endeavour, are just the beginning. They are the vanguard of a great many things yet to come. Fertiliser efficiency, gene silencing, higher photosynthetic rates (for higher yields), transgene breeding within Gossypium species, enhanced oil content in the seed and many more aspects are already being explored. SmartStaxT (Monsanto, St Louis, MO, USA and Dow AgroSciences LLC, Indianapolis, IN, USA), the biotechnology industry’s first-ever eight-gene stacked combination in corn, is already close to becoming a reality. Crop breeding is now moving towards ‘directed breeding’ and the development of custom-tailored genotypes. Biotechnology has the potential to improve water, energy and stress management, as well as fibre quality. In cotton, the second generation of products will come in the form of drought-tolerant and fertiliser-efficient plants. The focus is shifting from altered agronomic traits to management of abiotic stresses. The possibility also exists of using the cotton plant as a biofactory in which to isolate the genes that encode the required biosynthetic enzymes and then returning them to the cotton plant to produce the compounds of interest. The third generation of biotech products will probably address the issue of improved lint quality, particularly in the form of longer and stronger fibres.

10.8 Cotton Harvesting and Ginning Most of the world’s cotton is hand picked. Cotton is picked entirely by machine only in Australia, the Brazilian savannas, Greece, Israel, Spain and the USA. The decision to employ hand pickers or mechanised picking is determined exclusively by labour availability and cost. Hand picking is a gentler way of picking cotton and thus technically preferred over machine picking. Hand picking preserves fibre quality and also does not require extensive cleaning during ginning. There are only two kinds of seed cotton harvester, pickers and strippers. Pickers, also called spindle pickers, were introduced in the USA in 1942 (Baker and Griffin, 1984). Multiple columns of rotating spindles are arranged around a rotating drum that projects the spindles towards the open bolls on the plant. The rotating spindles wrap the seed cotton around them; then the rotary or stripped doffer brushes the seed cotton away from the spindles. The seed cotton thus removed is collected in a basket behind the tractor. Pickers are now available that can pick cotton at variable row distances, and the seed cotton can be packed into small bales and heaped in the field. Mechanised baling of seed cotton as part of the picking process saves time and eliminates the need to stop the picking operation to allow time to empty the basket. The stripper harvester strips the entire plant, carrying away open bolls, non-open bolls and a great deal of plant material. The big difference between machine picking (see Figure 10.3) and hand picking (aside from the greater stress the fibre suffers) is the quantity of trash in the seed cotton (see Chapter 17). Hand picking has the least trash, followed by picker harvesting. Picker harvesting may have 6–8 % trash, depending on the extent of defoliation, but about 10–12% of the seed cotton is lost in picking. Most of the lost seed cotton falls to the ground, but some remains on the plant. Stripper-harvested cotton may have up to 25% trash. Moreover,

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Figure 10.3

229

Cotton field ready for picking (left); machine picking of cotton (right).

stripped cotton may have a higher nep content, a higher short fibre content, a shorter staple length and a lower uniformity ratio compared with spindle-picked cotton. Stripper picking is less expensive than spindle picking. Mechanically harvested cotton is usually stored in modules in the field before being taken to the gin. Ginning is the process of separating lint from seed. Basically, only two types of gin are available in the world, roller gins and saw gins. In roller ginning, fibres are held between the rollers and pulled away from the seed coat. Roller ginning is a relatively slow process, but it preserves fibre quality better than saw ginning. On the other hand, roller ginning allows trash, motes and immature seeds to be carried through the rollers and in with the lint. The trend in roller ginning today is towards aggressive cleaning of the seed cotton and gentle cleaning of the lint to limit fibre damage (Whitelock et al., 2007). Recently, more efficient roller gins have been developed, but they are still not as efficient as saw gins. The development of the saw gin by Eli Whitney in 1793 (see Figure 10.4) revolutionised cotton ginning and paved the way for large-scale production and processing of cotton (see Figure 10.5).

Figure 10.4 The saw gin by Eli Whitney. Adapted from Eli Whitney, Patent for the Cotton Gin, 1794, Records of the Patent and Trademark Office, Record Group 241, National Archives.

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Figure 10.5 Schematic drawing of a ginning process. Adapted with permission from Schematic Drawing of a Ginning Process, Continental Eagle Corporation, Prattville, USA, 2009.

Now, about 85% of all the cotton in the world is ginned on saw gins. Most countries have either saw gins or roller gins, except in Egypt where only roller ginning is done because the country grows only long staple and extra-long staple cotton, i.e. G. barbadense. Countries like India, Sudan, the USA and some Central Asian countries where G. hirsutum and G. barbadense are produced at the same time have saw gins as well as roller gins. Some African countries, particularly in Eastern Africa, gin medium staple cotton, i.e. G. hirsutum, on roller gins. The saw gin process is relatively harsh. It involves pulling as well as beating actions. Modern saw gins vary greatly in their design, but in practical terms they all operate on the same basic ginning principle. Depending upon its trash content, seed cotton may pass through multiple processing stages to eliminate bolls, sticks and other trash before the seed cotton reaches an actual gin stand. The gin saws, rotating at a high speed, grasp the seed cotton and draw it through huller ribs spaced between the saws. Fibres are easily drawn through the closely spaced ginning ribs, but the ribs are placed in such a way that the seeds cannot pass through the spaces between them. The saws, moving in a clockwise direction, push the fibres through to the back side of the gin stand while the seeds are collected at the bottom. The fibres are then carried by an air stream to the lint cleaners for further processing. The lint comes out fluffy, and part of the trash is automatically eliminated during ginning. The ginning process may be harsh, but it is still sensitive to the moisture content of the seed cotton. Fibre quality can best be preserved when the seed cotton is ginned at an 8% moisture content. A seed cotton moisture level below 8% improves lint grade but reduces lint colour and increases short fibre content. Thus, the ginning process comprises cleaning of seed cotton and lint, along with separation of the lint from the seeds, all in a rigorous sequence of precise actions, including adjustment of humidity and temperature. Recently, a sensor-based ginning system called ‘IntelliGin’ was developed. It automatically adjusts humidity, temperature and cleaning to achieve the best ginning results (Yankey, 1999). The thermopneumatic and mechanical processing of cotton does not affect many fibre qualities but does seriously affect fibre length. Fibre breakage is much higher in saw ginning, resulting in shorter fibre length and

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higher short fibre content. Length uniformity and strength are both slightly compromised as a consequence of the shorter fibres, but micronaire and colour remain intact (see Chapter 17). Unginned, hand-picked cotton does not have neps, so neps are attributed to machine picking and ginning processes. Saw ginning produces more neps than roller ginning. Lint samples are drawn for quality testing before the cotton is pressed and formed into bales. Most experts recommend wrapping bales in cotton fabric and pressing them at a standard density, but bale density and size continue to vary greatly among countries (ICAC, 2008b). Bales must be stored at 7.5% moisture in order to avoid lint colour degradation. Torn between the facts that roller ginning is comparatively slow and saw ginning is relatively harsh, the industry has been exploring a number of new ginning technologies, including differential ginning, cage ginning and Templeton rotary ginning (Chaudhry, 1997). The objective is to preserve the quality of the roller gin and attain the speed of the saw gin. Humidity and temperature adjustments, along with the cleaning process, also require improvement. So, the third goal in ginning research is minimal processing of cotton without sacrificing cleanliness and quality.

10.9 Organic Cotton Cotton grown without the use of synthetically compounded chemicals, such as pesticides, growth regulators, defoliants, fertilisers, etc., is called organic cotton. Organic farming started in England, based on the theories developed by Albert Howard in An Agricultural Testament; biodynamic agriculture developed from the teachings of Rudolf Steiner in Germany in the 1920s; and biological agriculture was started in Switzerland by Hans-Peter Rusch (Wakelyn and Chaudhry, 2007). Other terms used interchangeably to describe organic cotton are ‘green’, ‘biological’ ,’clean’, ‘natural’ and ‘ecological’ cotton. The United States National Organic Standards Board defined organic production as follows: ‘Organic agriculture is an ecological production management system that promotes and enhances biodiversity, biological cycles and soil biological activity. It is based on minimal use of off-farm inputs and on management practices that restore, maintain and enhance ecological harmony’ (AMS, 2009). The basic standards of the International Federation of Organic Agriculture Movement (IFOAM) state that ‘Organic agriculture [also known as ‘Biological’ or ‘Ecological’ agriculture or protected equivalent forms of these words (in other languages)] is a whole-system approach based upon a set of processes resulting in a sustainable ecosystem, safe food, good nutrition, animal welfare and social justice. Organic production therefore is more than a system of production that includes or excludes certain inputs’ (IFOAM, 2002). Organic cotton may also be defined in many other ways, but all organic cotton production must comply with the requirements of ‘certified organic’ cotton. For cotton to be sold as ‘organic cotton’, it must be certified by an independent organisation that verifies that it meets or exceeds defined organic agricultural production standards. Each certifying company may have its own standards and list of allowed and prohibited products, but all agree that biotech cotton, in any form, is not eligible for certification as organic cotton. Extensive use of biotech varieties has affected the organic cotton area in the USA. Many countries have defined criteria for organic labelling, and, in spite of all deviations between different regulations, one clear agreement can be identified – the absolute ban on genetic engineering (see Chapter 16). Commercial production of organic cotton started in the early 1990s in Egypt and the United States. About 12 000 t of organic cotton was produced in the world in 1995/96, and the USA’s share was about 65%. However, since the adoption of biotech cotton, organic production in the USA declined to only 2% of world production in 2007/08. Twenty-one countries produced organic cotton in 2007/08, and production reached over 140 000 t (Figure 10.6). The three major post-ginning processes in the conversion of raw cotton fibre into a finished fabric are spinning, fabric manufacturing (weaving and knitting) and dyeing and finishing. To produce organic textiles, certified organically produced cotton must be processed according to processes certified as organic. All wet processing facilities should have water conservation and resource management in place and should conform to waste water disposal standards.

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Figure 10.6

Organic cotton production in the world.

The term organic is a labelling or marketing tool. Organic producers and promoters assert that organic production is more sustainable and environmentally friendly than conventional production. No one can doubt that conventional production uses more toxic chemicals, but either method can be sustainable. The one is more environmentally friendly while the other may be more economically friendly. The United Nations Commission on Environment and Development defines sustainability as follows: ‘Sustainable development is a development that meets the needs of the present without compromising the ability of future generations to meet their own needs’ (United Nations, 1987). The three fundamentals of sustainability are environmental, social and economical, and organic cotton as well as conventional cotton can meet these criteria. Organic production is at least as technical as conventional production, if not more so. People started producing organic cotton without any extensive research to determine the most suitable varieties for organic production, weed control methods and so on. Unfortunately, there is still not enough research in the area of organic cotton for efficient production. With the increased emphasis on minimal use of agrochemicals, particularly pesticides for sustained production of conventional cotton, production of organic cotton would appear to have a future. Organic cotton will continue to hold a small niche in the market, but it must be developed into a producer-driven initiative rather than a labelling or a marketing tool. Less expensive and more effective means of pest control, soil enrichment and harvest aids (in the case of mechanical picking) must be explored and implemented within the organic standards.

10.10

Conclusion

Cotton is the most important natural fibre. Many species of cotton are known, but only four of them are grown on a commercial scale. Approximately 97% of all cotton produced belongs to the species G. hirsutum, 3% to G. barbadense and less that 1% are G. arboreum and G. herbaceum. The cotton plant is naturally vulnerable to a variety of pests, particularly insects. After over two decades of extensive insecticide use in cotton, integrated pest management is reducing the number of insecticide applications on cotton. Insect-resistant biotech cotton has emerged as a successful alternative for controlling lepidopteran insects. The awareness to produce and process cotton using sustainable methods is growing. Almost half of the cotton produced in the world gets assured irrigation, while the other half comes from rain-fed conditions. Most cotton is still picked by hand, machine picking being adopted only if labour is not available or expensive. Organic cotton is less than 1% of world production and will continue as a niche market. There is a great need to find alternative uses of cotton for enhancing cotton consumption in the world. High man-made fibre prices and

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recent technological developments have opened new avenues for cotton to be used in making ‘non-wovens’ and to be mixed with other natural fibres for producing biodegradable composites. Cotton can also be mixed with man-made fibres, depending on the mechanical and morphological properties of the mixing materials and resultant products.

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M¨ussig, J. (2008) Cotton fibre-reinforced thermosets versus Ramie composites – a comparative study using petrochemicaland agro-based resins. J. Polym. Environ., 16(2), 94–102. Nawaz, A.M. (1988) Cotton soils and fertilizers, in Cotton in Pakistan (in Urdu). Pakistan Central Cotton Committee, Ministry of Food and Agriculture, Pakistan, December. Oosterhuis, D.M. and Howard, D.D. (2008) Evaluation of slow-release nitrogen and potassium fertilizers for cotton production. Afr. J. Agric. Res., 3(1), 068–073. Russell, D. (2005) Insecticides in sustainable control of the cotton bollworm (Helicoverpa armigera) in small-scale cotton production system. The ICAC Recorder, XXIII, No. 4 (December). Stewart, J.McD. (1994, updated in 2009) Potential for crop improvement with exotic germplasm and genetic engineering, in Challenging the Future, Proc. World Cotton Research Conference – 1, ed. by Constable, G.A. and Forrester, N.W. CSIRO, Melbourne, Australia, pp. 297–327. United Nations (1987) Report of the World Commission on Environment and Development: Our common future; available at: http://www.un-documents.net/wced-ocf.htm (accessed 10 May 2009). Wakelyn, P.J. and Chaudhry, M.R. (2007) Organic cotton, in Cotton: Science and Technology, ed. by Gordon, S. and Hsieh, Y.-L. Woodhead Publishing Limited, Cambridge, UK. Whitelock, D.P., Armijo, C.B., Gamble, G.R. and Hughs, S.E. (2007) Survey of seed-cotton and lint cleaning equipment in U.S. roller gins. J. Cott. Sci., 11, 128–140. Yankey, J.M. (1999) Improved fibre quality from gin process control, in Proceedings of Beltwide Cotton Conferences, National Cotton Council, Memphis, TN, USA, pp. 676–679.

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PART III ANIMAL FIBRES

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11 Mulberry Silk, Spider Dragline and Recombinant Silks Anja Gliˇsovi´c Fraunhofer Institut f¨ur Fertigungstechnik und Angewandte, Materialforschung (IFAM), Bremen, Germany

Fritz Vollrath Department of Zoology, Oxford University, Oxford, UK

11.1 Introduction Silks stand out among the natural fibres. Consisting primarily of highly structured proteins, they exhibit a wide range of different properties, from high tensile strength to extreme extensibility and from chemical resistance to bioactivity. In some of their properties, specifically in toughness and biocompatibility, silks can easily outperform most natural and synthetic fibres. Best of all, the diversity in silk types and range of properties is vast. Most of the larvae of the approximately 140 000 Lepidoptera (butterflies) species, as well as the members of several other insect orders, produce silk typically for shelter and protective cocoons during metamorphosis (Kaplan et al., 1994; Beccaloni et al., 2003) (see Table 11.1 and Figure 11.1). In addition, there are over 40 000 known spider species that use up to seven different types of silk throughout their entire lifespan for shelter, protection, prey capture and reproduction and as dragline (Foelix, 1996) (Figure 11.2). Although often similar, every one of these thousands of silk types is unique in composition and structure and highly adapted to the animal’s natural environmental conditions (Vollrath, 1992; Gatesy et al., 2001). It appears that modern biotechnological production methods (creating recombinant or regenerated silk) might one day be able to bridge – for certain silk proteins – the border between natural and synthetic fibres. Such routes to the technological (rather than natural) manufacture of silk will hopefully ‘revive’ a more widespread use of silks, and, equally importantly, offer the prospect of designer fibres that are inspired by natural examples but are artificial in production and processing and highly adapted for specific human needs and purposes (see Section 11.3).

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Table 11.1 Selected silk-producing species, the name of their silk, if commercially available, and their diet and grade of domestication Species

Silk

Diet

Domestication

Literature

Bombyx mori (silkworm)

Mulberry (cocoon)

Yes

(Chang, 1986)

Antheraea pernyi (Chinese oak silk moth) Antheraea assamensis

Tussah (cocoon)

Semi

(Lucas et al., 1995)

Muga (cocoon)

Semi

(Lucas et al., 1995)

Philosamia cynthia ricini

Eri (cocoon)

Yes

(Lucas et al., 1995)

Nephila sp. (Golden orbweaver) Araneus sp. (European garden spider) Trichoptera sp. (Caddiesfly)

Dragline (among others) Dragline (among others) Part of cases

Morus spp. (mulberry) exclusively Quercus spp. (oak) exclusively Machilus bombycina Latsaea polyantha Ricinus communis (castor) exclusively Insects

No

(Foelix, 1996)

Insects

No

(Foelix, 1996)

Herbivorous Carnivorous

No

(Engster, 1976)

In order to cover the different production and processing routes, we will focus on mulberry silk from the silkworm Bombyx mori and the dragline of the tropical spider genus Nephila (Figure 11.3). The Nephila dragline has especially remarkable properties, with a tensile strength between that of polyamide 6.6 (Nylon; DuPont de Nemours) and steel and a toughness better than that of aramide fibres. It cannot be produced in large quantities by husbandry as all spider silks (Gosline et al., 1999) (see Table 11.2). Mulberry silk has for several millennia been the ‘key’ silk in commercial applications and today is still the only silk in commercial applications (Altman et al., 2003), albeit sometimes with considerable modifications.

Figure 11.1

Cocoons and raw silk top of Bombyx mori. Photograph kindly taken by Dagmar Fischer, Fraunhofer IFAM.

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Figure 11.2 The different types of silk that a Nephila spider can produce and its spinning apparatus. 1. Gl. pirifomes, viscous silk for the attachment disk; 2. Gl. ampullacea, solid fibre, dragline and frame; 3. Gl. aggregatae, viscous glue of the capture spiral; 4. Gl. flagelliformes, solid core of the capture spiral; 5. Gl. tubuliformes, solid, cocoon silk; 6. Gl. aciniformes, solid fibre, egg sac’s outer wall and sperm web; 7. posterior spinneret; 8. median spinneret; 9. anterior spinneret. Adapted with permission ¨ from H.M. Peters, Uber den Spinnapparat von Nephila madagascariensis, Zeitschrift fur ¨ Naturforschung, 10b, 396–400, 1955.

Figure 11.3 Nephila species can be found throughout the world’s tropical regions. Left: Nephila clavipes from Florida, USA. Right: Nephila senegalensis from West Africa. Nephila species are easy to keep. They have a body length of 4–5 cm and a pacific character (Glisovic, 2007).

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Table 11.2 Tensile properties of selected man-made and natural fibres. Fibres were not necessarily measured under equivalent conditions, which limits the comparability of the values (see Chapter 13)

Material High-tensile steel Aramide (Kevlar, DuPont) Polyamide 6.6 (Nylon, DuPont) Mulberry silk (Bombyx mori) Dragline (Nephila)

Tensile strength in GPa

Extensibility in (% of initial length)

1.5 47 0.95 0.6 1.1

0.8 3.6 18 18 30

Young’s modulus Toughness in in GPa MJ/m3 Literature 200 2.7 4 6 20

6 130 80 70 170

(Gosline et al., 1999) (Gosline et al., 1999) (Gosline et al., 1999) (Gosline et al., 1999) (Vehoff et al., 2007)

It will be reviewed here first, starting with the structure and properties of the material, followed by its production and finally its processing and applications as biomaterial.

11.2

Mulberry Silk (Bombyx mori)

Mulberry silk is used by the larvae of the silk moth (Bombyx mori) as part of the cocoon, which gives shelter and protection during pupation (see Figure 11.1). The silk fibroin (i.e. the protein that makes the bulk of the actual fibre) is secreted in the main body of the larva’s silk gland. During its flow through the fibre-forming duct, it is then coated with a wide range of additional sericin proteins, which commonly are referred to as silk gum and have to be washed off for the collection and textile use of the fibres. Sericins act as an adhesive and interconnect the different layers of the continuous silk fibre. About one-quarter to one-third of a Bombyx cocoon consists of sericin gum, with the rest being the actual fibroin fibre. 11.2.1

Structure

Mulberry silk fibres are semi-crystalline polymers consisting of β-sheet crystallites embedded in an amorphous protein matrix. The fibre-forming fibroin consists of two proteins: the light chain containing 262 amino acids, and the heavy chain with 5263 amino acids, which are present with a 1:1 fraction in the fibre (Zhou et al., 2000; Yamaguchi et al., 1989). The heavy chain consists of 12 repetitive domains that form the crystalline regions of the fibre which are interspersed with less organised domains that form the amorphous parts. The crystallite-forming domains consist of approximately 381 amino acids with a repetition of glycine–alanine–xxx–glycine–xxx motive, where xxx stands for either alanine, serine or tyrosine.1 The amino acid composition is 43% glycine, 30% alanine and 12% serine (Kaplan, 1998). The ratio of crystalline to amorphous matrix varies between 3:7 and 4:6, with a higher alanine content in the crystalline regions (Marsh et al., 1955). A typical β-sheet crystallite in mulberry silk is rectangular (Bhat and Nadinger, 1980). The coordinate system is defined as follows: x-axis along the amino acid side chains, ˚ and z-axis ˚ 2 y-axis in the direction of the H-bonds, lattice constant b = 9.44 A; lattice constant a = 9.2 A, ˚ (Warwicker, 1960). A schematic representation is given along the peptide bonds, lattice constant c = 6.95 A in Figure 11.4.

1

All amino acids have the same nitrogen, carbon and carbon backbone and are distinguished by the side group attached to the middle carbon atom. Glycine, C2 H5 NO2 , 2-aminoacetic acid, side group: hydrogen (-H); alanine, C3 H7 NO2 , (S)-2-aminopropanoic acid, side group: methyl (-OH); serine, C3 H7 NO3 , (S)-2-amino-3-hydroxypropanoic acid, side group: (-OH); tyrosine, C9 H11 NO3 , (S)-2-amino-3-(4-hydroxyphenyl)-propanoic acid, side group: (-hydroxyphenyl); see more details in Chapter 12, Table 12.1. 2 Angstr¨ ˚ om: a non-SI unit of length equal to 0.1 nm or 1 × 10−10 m.

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Figure 11.4 Structure of semi-crystalline silk: on micrometre scale – the fibre; on nanometre scale – the amorphous matrix and β-sheet crystallites. Dimensions are given for Nephila dragline. The microfibrillar substructure is not shown here for the sake of simplicity. Adapted with permission from A. Glisovic, T. Vehoff, R.J. Davies and T. Salditt, Strain dependent structural changes of spider dragline silk, Macromol., 41, 390–398. Copyright 2008, American Chemical Society.

Mulberry silk also shows a kind of submicron-scale fibrillar substructure; with diameters of approximately 120 nm, the fibrils represent an additional hierarchy level between the nanometer-sized β-sheet crystallites and the fibre with a diameter of 12 µm (Putthanarat et al., 2000). While the chemical properties are strongly predefined by the protein’s amino acid sequence (primary structure), the mechanical properties are mainly influenced by the size, orientation and fraction of the β-sheet crystallites. Importantly, the characteristics of the crystalline fraction strongly depend on the interaction between the primary structure and the conditions during fibre extrusion. For example, it is possible to achieve improved mechanical properties in Bombyx silk fibres (approaching that of spider silks) when the fibres are directly reeled from the silkworm’s mouthparts (and thus glands) rather than unravelled from the cocoons (Shao and Vollrath, 2002).

11.2.2

Properties

The cocoon is intended to give the larvae shelter during pupation. Accordingly, the silk fibroin is very well suited to withstand the elements. This is reflected in its chemical stability. Mulberry silk is insoluble in most alcohols or acetone (CH3 COCH3 , propan-2-one), shows very moderate swelling in water and only light water uptake and is resistant against mild acids. Even hydrochloric acid requires a couple of hours of exposure for hydrolysis, which preferentially takes place in the amorphous parts (Bhat and Nadinger, 1980). The fibres are stable up to temperatures of 150 ◦ C, above which thermal denaturation starts (Magoshi et al., 1994) (see Table 11.3).

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Table 11.3

Properties of mulberry and Nephila dragline silk

Property

Bombyx mori

Nephila dragline

Literature

Degree of crystallinity in % Density in g/cm3 Crystallite size in nm

38–66 1.35–1.42 1.0–2.5

20–45

Index of refraction

1.591 parallel to fibre

Maximum use temperature in ◦ C

170

1.538 perpendicular to fibre 150

(Bhat and Nadinger, 1980) (Fossey and Kaplan, 1999) (Bhat and Nadinger, 1980) (Glisovic and Salditt, 2007; Glisovic et al., 2008) (Fossey and Kaplan, 1999)

Thermal degradation in ◦ C

250

234

Heat capacity in J/g K Glass transition temperature Supercontraction in water

1.38 178 ◦ C at 0% RH No

39 ◦ C at 75% RH ∼50%

4.7 × 5.3 × 6.0

(Magoshi et al., 1994), (Cunniff et al., 1994), (Glisovic and Salditt, 2007) (Magoshi et al., 1994), (Cunniff et al., 1994), (Glisovic and Salditt, 2007) (Bhat and Nadinger, 1980) (Fossey and Kaplan, 1999) (Bell et al., 2002)

The mechanical properties, with a tensile strength of 0.6 GPa and an extensibility of 18%, are moderate and in the same range as the mechanical properties of, for example, wool (Gosline et al., 1999). The degummed fibres (brins) are shiny, white and approximately 1 km in length. They tend to have a smooth surface with roughness at the nano scale (Periyasamy et al., 2007) (see Chapter 14, Figure 14.6). 11.2.3

Production

Mulberry silkworms have been domesticated for over 5000 years (Chang, 1986). As a result, the silkworm Bombyx mori cannot survive without human support. The eggs are normally obtained from breeding stations and kept under special humidity and temperature conditions until the larvae hatch (Sericulture, 2005). Over the course of the following 6–8 weeks, the growing larvae are fed by an exclusive diet of mulberry leaves (Morus alba) or a leaf paste. A single larva consumes between 2 and 5 kg of leaves (wet mass), and, with approaching pupation, more than 40% of a larva’s body mass is made up of the silk glands. Commercial silk production capacity appears to be limited more by the availability of manpower, as well as of mulberry leaves, than by space for the larvae. For each kilogram of raw silk, about 4 kg of cocoons are necessary; 10 kg of Bombyx cocoons requires about 2000 individual larvae, which consume between 4 and 10 t of mulberry leaves. Harvesting the silk is quite simple: the cocoons are collected and the larvae killed (for example by overheating) before the cocoons are stored for further processing, which consists of gentle boiling in a mild soap solution while the silk fibres are unravelled (Sericulture, 2005). Boiling the cocoons dissolves the sericin gum, which binds the fibres. As they are unglued, the fibres can be reeled onto wheels, generally in collectives of nine threads, each thread containing 2 fibres. The process of degumming and fibre collection is followed by consecutive combing and washing steps in order to clean the raw silk from remaining sericin and other contaminations (Zhang, 2002). 11.2.4

Processing and Applications

For textile uses, the raw silk has to be spun, woven and dyed. These are processes that have been evolved and optimised since the first use of silk for garments about 5000 years ago. And, indeed, even today mulberry silk’s principal application is in the textile industry.

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Table 11.4 Silk solvents and dissolution conditions. EtOH: ethanol (C2 H6 O), MeOH: methanol (CH3 OH), NMMO: N-methylmorpoline N-oxide (C5 H11 NO2 ), HFIP: 1,1,1,3,3,3-hexafluoro-2-propanol (C3 H2 F6 O)

Solvent Carbamimidoylazanium chloride (CH6 ClN3 ; trivial: guanidine hydrochloride) 9.3 mol in H2 O MU-Solvent LiBr/EtOH/H2 O 45/52/13 wt %

Silk content in mass %

Temp. in ◦ C

Time in min

20

20

20

Dialysis

Processing

Lit.

180

Yes

Aqueous silk (Shao et al., solution Wet fibre 2003) formation Electro fibre formation

70

30

Yes

70

180

yes

(Ki et al., 2007)

Ca(NO3 )2 4.H2 O/MeOH 79/21 wt %

10

69

60

Yes

Methanoic acid/phosphoric acid 70/30 wt % NMMO/H2 O 87/13 wt % HFIP

20

RT

30

No

Aqueous silk solution Wet fibre formation Electro fibre formation Aqueous silk solution Wet fibre formation Electro fibre formation Aqueous silk solution Wet fibre formation Electro fibre formation Wet fibre formation

25

70–100

120

No

Wet fibre formation

(Yingxu, 2005)

30

RT

No

Wet fibre formation

(Lock, 1992 and 1993)

CaCl2 /H2 O/EtOH 32/41/27 wt %

(Matsumoto and Uejima, 1997) (Min et al., 2004)

(Ha et al., 2003)

However, aside from the textile industry, there are other notable uses, for example with silk as an ingredient in cosmetic products or as medical suture material (Vepari and Kaplan, 2007). Mulberry silk fibroin powder is often a key ingredient in cosmetic face powders. It absorbs moisture and fat and is supposed to lend a sheen to human skin (Takeshita et al., 2000). Free of sericin, it is non-allergic and highly biocompatible. The traditional myth of silk as an exotic product promotes marketing as well. Unfortunately, mulberry silk in its natural fibrous form is hard to mill (Yoshimizu and Asakura, 1990). As a result, the production of mulberry silk powder often involves dissolution and regeneration (formation of β-sheet crystallites) (Rajkhowa et al., 2008), which is far from environmentally friendly. Mulberry silk, spider silk and recombinant silk are all very similar in their dissolution behaviour. Therefore, a solvent found suitable for one type of silk can often be applied to the others. For the production of silk powder, two groups of solvents are used. One is based on alcohol (methanol, ethanol), alkaline metals and alkaline-earth metals (Ca, Li), and the other is based on guanidine (CH5 N3 ), a protein denaturant (see Table 11.4). Both can dissolve up to 20% silk by mass within 1 h (Shao et al., 2003; Matsumoto and Uejima, 1997). The metal ions and the guanidine have to be removed from the solution after complete dissolution, as they hinder the silk regeneration process (Ha et al., 2003). For this purpose, the solution is either dialysed against water or undergoes ultrafiltration. The dialysed aqueous solution can then be lyophilised for silk powder or concentrated for further processing by another dialysis against polyethylene glycol (Min et al., 2004; Li et al., 2006). This very elaborate procedure partly explains the high cost of silk-containing cosmetic products. Silk sutures are the other major industrial-scale application of silk. Currently, they are exclusively made from mulberry silk. Production of sutures requires the silk fibres to be washed thoroughly to remove sericin and other contaminants. After washing, the fibres are dyed to enhance visibility, waxed to enhance knot

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strength and finally braided to improve stability (Flury, 2002). If sericin is not completely removed, wound inflammation and allergic reactions can occur, which has been a major limitation for silk sutures in the past. Under the precondition of complete sericin removal, silk sutures cause virtually no irritation of the surrounding tissue (Meinel et al., 2005). The great biocompatibility of degummed silk has also resulted in the emerging applications of silk in cell culture and tissue engineering (Meinel et al., 2005; Li et al., 2006). Tissue engineering uses cell culture methods to engineer tissue artificially for transplants. Most cells grow attached to the extracellular matrix (in vivo) or a suitable scaffold (in vitro) (Meinel et al., 2005). The requirements of such a scaffold are biocompatibility, support of cell proliferation and mechanical stability (Li et al., 2006). This applies to electrostatically formed fibre mats made of regenerated silk materials (silk ‘non-wovens’). Silk ‘non-wovens’ are extremely interesting owing to their bioactivity rather than their mechanical properties. Their special nanofibre structure enhances cell proliferation, and, together with the high biocompatibility of silk, they provide interesting materials for applications in cell culture and tissue engineering (Min et al., 2004). Such materials have been successfully used to support a variety of human cells (Vepari and Kaplan, 2007), such as human bone marrow cells, osteoblasts and fibroblasts, which all show excellent proliferation on these substrates (Altman et al., 2003; Min et al., 2004). A typical lab-scale electrostatic fibre formation set-up consists of a hypodermic syringe needle connected to a high-voltage (10–15 kV) power supply and a grounded collector plate. The electrostatic charge repulsion along the extruded polymer jet causes the formation of nanometre- to micrometre-sized fibres. For example, regenerated silk fibre mats with a fibre diameter of 80 nm and a porosity of 68% have been obtained from formic acid solutions (Ohgo et al., 2003; Min et al., 2004). The cohesion of the fibre-forming solution has to be high enough to maintain a constant stream, otherwise the material is electrosprayed (Min et al., 2004). Solvent evaporation at such small diameters is so fast that the fibres deposited on the collector plate are nearly dry. Industrial-scale electrostatic fibre formation setups operate by the same principle but normally comprise cylinders as electrodes. They ensure a continuous process, as necessary for industrial production, by rotating through a reservoir of polymer solution. In this case the collector electrode is above the cylinder electrode (Yoshihiro, 2008). Except for the inverted geometry, the processes are comparable, and in both cases a random non-woven network of fibres is produced. Although silk scaffolds show promising results in the lab, as far as we know none of them has yet successfully run the full gauntlet required to achieve medical approval for clinical use. However, some seem to be in the pipeline, such as SeriACLTM , which is a fully implantable, bioresorbable, mulberry-silkbased replacement for the anterior cruciate ligament, produced by Serica Technologies, Medford, MA, by knitting and braiding reconstituted and regenerated silk fibres (Horan et al., 2009). According to the US National Institute of Health’s Clinical Trials Information Service, SeriACLTM is currently in phase I of the two phases of the US clinical trials under identifier NCT00490594 and (if successful) is expected to be commercially available in Europe in late 2010 and in the USA in 2014, according to an information request at Serica Technologies. Other biomedical applications for silks that are currently under development are novel suture threads, novel nerve repair kits and meniscal implants (Oxford Biomaterials Ltd, Newbury, UK) and recombinant silk raw materials (AMSilk GmbH, Munich, Germany).

11.3 Dragline Silk (Nephila) In contrast to insects, spiders use up to seven different types of silk throughout their lifetime for shelter, protection, prey capture and reproduction. Each of these different silk varieties is highly adapted to its special purpose and produced in a specialised gland (Foelix, 1996) (see Figure 11.2). Dragline silk, for example, is produced in the Glandula ampullaceae major and serves as the spider’s lifeline, arresting the animal safely after free fall when the spider drops from its web in moments of danger. With a diameter of around 3–5 µm, it is the biggest and toughest fibre in the Nephila spider’s silk set. The dragline’s resilient combination of

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extensibility and tensile strength has triggered human fascination and our desire to produce fibres with these qualities in commercial quantities. Unfortunately, this is a task that cannot be accomplished by conventional methods such as sericulture, as spiders (unlike silkworms which are strictly vegetarian) are carnivores and also cannibals, which makes culturing them in large quantities totally impracticable. However, progress in protenomics and genetic engineering has opened up new production routes for these silk proteins, which in turn provide not only novel opportunities in material design but also new technical challenges, as we will now discuss.

11.3.1

Structure

Dragline silk is, like mulberry silk, a semi-crystalline polymer consisting of β-sheet crystallites embedded in an amorphous protein matrix. Both amorphous and crystalline parts are to some extent predefined in the silk polymer’s primary structure. Dragline silk consists of two main proteins: spidroin I and spidroin II (Xu and Lewis, 1990; Hinman and Lewis, 1992). Perhaps they are the only two, but as yet we cannot be sure. Spidroins (the spider’s ‘fibroins’) are highly repetitive in structure, consisting of a periodic arrangement of similar amino acid motives followed by a non-repetitive tail, which influences solubility. For example, N. clavipes spidroin I is built of 747 amino acids with 25 motive repeats, and spidroin II is built of 627 amino acids with 15 motive repeats. A motive consists of a glycine-rich block of 20–30 amino acids, followed by an alanine block 5–8 amino acids long. Both spidroins are therefore diblock copolymers (Xu and Lewis, 1990; Hinman and Lewis, 1992). The alanine blocks dominate the crystalline parts, while glycine dominates the amorphous matrix. The ratio of crystalline to amorphous matrix varies between 3:7 to 2:8. The size of the crystallites varies slightly between the species. Typically, an average crystallite of Nephila draglines is rectangular with approximate ˚ (Glisovic and Salditt, 2007; Glisovic et al., 2008). dimensions of 53 × 47 × 60 A The coordinate system is defined as for mulberry silk: x-axis along the amino acid side chains, lattice ˚ (in mulberry silk a = 9.2 A); ˚ y-axis in the direction of the H-bonds, lattice constant b = constant a = 10.6 A ˚ and z-axis along the peptide bonds, lattice constant c = 6.95 A. ˚ 9.44 A; Larger but less periodic crystallites have also been suggested but have not been experimentally verified yet (Thiel et al., 1997). The z-axis of the crystallites is well aligned along the fibre axis, while the x- and y-axes are randomly distributed (see Figure 11.4). This preferential arrangement along the fibre axis in Nephila dragline makes it a nematic elastomer.3 The crystallites are densely arranged. The mean distance between two crystallites along the fibre axis was ˚ (Riekel and Vollrath, 2001; Sapede et al., 2005). The crystallite mean found to vary between 70 and 80 A spacing perpendicular to the fibre axis has not yet been experimentally determined. However, calculations on ˚ lead to a perpendicular a crystalline proportion of 20–30% and a given crystallite size of 53 × 47 × 60 A ˚ mean distance of 50–60 A. The diameter of dragline fibres from adult Nephila females ranges from 2 to 9 µm and depends on the spider’s size and species. Natural spider dragline, as occurring in webs, is in a double-fibre conformation (spider thread) owing to the symmetrical arrangement of the two major ampullate glands along the spider’s abdominal centre-line (see Figure 14.5 in Chapter 14). Similar compositions and structures can be found in the dragline of other orb-weaving spiders such as the European garden spider (Araneus) (Vollrath, 2000).

3 The term ‘nematic’ describes the preferential orientation of particles with their longest axis parallel to each other while their positions have no correlation at all. It is often found in liquid crystals (Warner and Terentjev, 2003).

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Properties

The mechanical properties of Nephila dragline are remarkable, and its specific combination of tensile strength and extensibility gives it a unique position among fibre materials. Single fibre measurements show that dragline can achieve tensile strengths of up to 1.1 GPa, which is better than that of polyamide 6.6 (0.95 GPa) but worse than that of high-tensile steel (1.5 GPa), although, of course, silk has a much lower density. At the same time, dragline silk outperforms both materials in extensibility, which is ≥30% for dragline and approximately 18% for polyamide 6.6 and 0.8% for steel (Vehoff et al., 2007; Gosline et al., 1999). The combination of tensile strength and extensibility gives rise to a toughness (total energy per volume upon failure) that, at 170 MJ/m3 , is better than that of aramide fibres (Kevlar, DuPont de Nemours) (see Table 11.2).

11.3.3

Energy Dissipation and Hysteresis

Stressing a fibre short of breaking allows us to study its energy dissipation, and mechanical hysteresis can be observed. Energy dissipation is a key feature for dragline silk. Without energy dissipation in the fibre, neither the spider nor its prey would be caught, but instead catapulted back in a spring-like fashion. Approximately 65–68% of the stretching energy is dissipated, while only 32–35% is elastically stored (Vehoff et al., 2007) (see Figure 11.5). Additionally spider dragline, and to a lesser extent mulberry and other silks, exhibit mechanical hysteresis. A stretched fibre contracts to its original shape, but the elongation is ‘stored’ in the fibre. As a result, subsequent strain causes less stress than the original up to the maximum original strain. From that point on, the initial unstretched fibre stress–strain behaviour is observed (Vehoff et al., 2007) (see Figure 11.5).

11.3.4

Influence of Water and Temperature

This ‘elongation history’ of dragline can be largely ‘erased’ by immersion in water followed by drying in the relaxed state. Wetted, the fibre contracts to approximately 50% of its original length and softens, but tensile strength and toughness do not diminish significantly. This phenomenon is called supercontraction (Vollrath and Edmonds, 1989; Eles and Michal, 2004; Bell et al., 2002). After drying, the fibre returns to its original state as before extension (Liu et al., 2005a; Elices et al., 2004). Depending on the relative humidity, a dragline fibre’s mechanical characteristics lie intermediate between the state of supercontraction and the dry state, with details depending on a combination of molecular composition and fibre formation conditions (Vehoff et al., 2007; Liu et al., 2005a; Liu et al., 2008). The mechanical properties of dragline silks are nearly constant over a wide temperature range and show an astonishing response to low temperatures (Porter et al., 2005). They decrease slightly at temperatures rising from freezing point to approximately 150 ◦ C, where degradation starts (Glisovic, 2007). This behaviour is comparable with that of polyamide 6.6 and mulberry silk. However, the mechanical properties greatly improve with temperatures falling below freezing point. At −60 ◦ C the extensibility reaches nearly 45%, while the tensile strength approaches that of steel at around 1.5 GPa (Yang et al., 2005). Our own qualitative observations have shown that, even in liquid nitrogen, dragline silk does not show signs of embrittlement (unpublished observations).

11.3.5

Production

The methods of sericulture do not apply to spiders. Spiders can be highly territorial and they are always carnivorous, and often also cannibalistic. They must be fed with living prey, which they catch in their webs,

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Figure 11.5 (a) A typical stress–strain curve of spider dragline silk up to an elongation of 10% of the original length and relaxation. The area between elongation and relaxation represents the energy dissipated in the fibre, while the energy elastically stored is represented by the area beneath the relaxation curve. (b) Subsequent set of elongation and relaxation curves (hysteresis). Every cycle starts after a threshold force of 1.5 mN is reached and is shifted to a higher starting length than the one before. The hysteresis cycles add up to an envelope corresponding to the stress–strain curve without relaxation cycles. Adapted from Biophy. J., 93, T. Vehoff, A. Glisovic, H. Schollmeyer, A. Zippelius and T. Salditt, Mechanical properties of spider dragline silk: humidity, hysteresis and relaxation, 4425–4432. Copyright 2007, with permission from Elsevier.

and kept separately (Foelix, 1996). Water has to be given by spraying the webs. This is very space and time intensive, making it practical only in laboratories where the number of spiders and amount of required silk are small. Additionally, collecting the spider silk is labour intensive. The spider must be sedated, which is usually done by exposure to cool temperatures. The sedated spider is then fixed upside down to expose the spinnerets (see Figure 11.2). These are stimulated with a small brush to provoke fibre production. A Nephila spider, for example, attaches nearly all types of her silks to the brush, so they have to be separated under a microscope and can then be reeled up (Work and Emerson, 1982; Glisovic and Salditt, 2007). This has to be done for each spider, which is uneconomic for industrial-scale production of spider silk.

11.3.6

Spider Silk Proteins – Production in Genetically Modified Organisms

Progress in proteomics and biotechnology over recent years is beginning to make it possible to produce practical amounts of spider silk peptides economically, for example by using genetically engineered organisms (Po Foo and Kaplan, 2002). Host organisms successfully used for spider silk expression (recombinant silk)

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Table 11.5

List of organisms successfully modified to produce spider silk protein

Species

Year-round production

Advantage

Disadvantage

Literature

High space consumption

(Wang et al., 2002)

Only 30 kDa proteins

(Scheibel, 2004)

Capra aegagrus hircus (domestic goat) Escherichia coli

Yes

Nicotiana tobaccum (tobacco)

No, seasonal

Easy to keep and can be constrained in stables Can be kept constrained in high densities 100 kDa proteins

Solanum tuberosum (potato)

No, seasonal

100 kDa proteins

Bombyx mori (silkworm)

No, seasonal

Produces fibres and raw protein, easy to keep and to keep constrained

Yes

Poor acceptance in Europe (Scheller et al., 2001; (genetic engineering for Scheller et al., 2004) agriculture) Poor acceptance in Europe (Scheller et al., 2001) (genetic engineering for agriculture) Fibres are not pure spider (Prudhomme and silk protein, exclusive diet Couble, 2002) of mulberry leaves = mulberry plantage required

include plants, insects, mammals and bacteria (see Table 11.5). Each host has different advantages and disadvantages that are not only technical. The release of genetically modified organisms, especially plants, is highly controversial and even rejected by the public in Europe. A safe containment of the host organisms, without violation of animal rights, is therefore a core feature for the successful introduction of this production pathway in Europe (see Table 11.5). Moreover, technical problems had and have to be overcome, one of the most important being the length of the recombinant proteins. Most host organisms are limited by the size of the proteins that their cells can produce. Normally, the upper limits lie around 30 kDa4 for bacteria and 100 kDa for plants, while natural spider silk proteins such as Nephila spidroins are approximately 300 kDa (Sponner et al., 2005). In order to overcome that problem, protein analogues are expressed in host organisms, which are smaller in size but retain the characteristic repetitive structure of the original protein (Scheibel, 2004; Scheller et al., 2004). Here, the borderline vanishes between the artificial production of a peptide, a natural protein and a designer protein. Take, for example, ADF 3 and ADF 4, two silk analogues derived from Araneus diadematus (European garden spider) spidroin in the group of Thomas Scheibel, University Bayreuth, Germany. They represent sections of the original protein but show totally different solubility and are therefore used to study the assembly process of silk in vitro (Huemmerich et al., 2004; Scheibel, 2004; Rammensee et al., 2008). In contrast to this is SO1-ELP, developed at IPK Gattersleben, Germany, explicitly designed to combine the high tensile strength of dragline silk with the elasticity of elastine (Scheller et al., 2004). The core is a dragline amino acid sequence enhanced by elastine sequences at both ends. It is expressed in Nicotiana tobaccum (tobacco plants), which allows the production of recombinant proteins of up to 100 kDa with a yield of 80 mg/kg tobacco leaves (Scheller et al., 2001).

4

Dalton (Da) or unified atomic mass unit (u): 1 Da = 1 u = 1.660538782(83) × 10−27 kg; 1 kDa = 1000 Da.

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Processing

The silk production route in genetically engineered organisms finally yields to a pure silk protein powder that has to be further processed into fibres. This is done in two steps: dissolution to obtain a processable silk protein solution, and regeneration (induction of β-sheet crystallite formation) in desired shape and properties. The processing method and conditions are crucial for forming a usable silk-based material with the desired properties (Termonia, 1994 and 1995). They both play a role in crystallite formation, proportion, size and orientation. As outlined in the previous sections, these aspects define the mechanical properties of the final product (Porter et al., 2005). A post-formation treatment of stretching and heating can improve the fibre properties by additional molecular alignment but can never replace the ideal alignment during the formation process (Corsini et al., 2007; Liu et al., 2005b).

11.3.8

Natural Fibre Formation Process

The silk glands of spiders exit in the abdomen (see Figure 11.2), while the silkworm’s silk glands exit in the head. In spite of these differences, the process of fibre formation is comparable in insects and spiders (Jin and Kaplan, 2003). Each silk gland comprises a lumen and a duct, which becomes smaller towards the external part of the gland. The lumen serves as a reservoir for the aqueous silk solution secreted by the endoplasmatic reticulum of the luminal cells. It is highly viscous, with protein concentrations of up to 50 mass % (Hijirida et al., 1996). The formation of the fibre takes place in the duct by a combination of four processes occurring in parallel. During the passage of the protein solution through the duct, the solution undergoes a phase transition from liquid to solid fibre, induced by flow elongation, shear and a decreasing pH value. The flow elongation stretches and aligns the protein chains, while shear and pH gradient induce the formation of β-sheet crystallites, which interconnect the protein strands in the solution (Knight and Vollrath, 1999; Vollrath et al., 2001; Vollrath and Knight, 2001a). Water extraction at the end of the duct further solidifies the fibre. Additionally, most of the time the silk fibre is not extruded but drawn (pull-trusion rather than push-trusion) from the duct. It assists the alignment of the β-sheet crystallites and of the protein chains. The drawing speed also determines the reaction time in the duct and thereby influences the content and the size of the crystallites (Vollrath et al., 2001; Vollrath and Knight, 2001a). The degree of this effect is higher in some silks than in others, with mulberry silk as a prime example. By reeling the silk directly from the silkworm’s glands, it is possible to achieve mechanical properties closer to those of dragline silk (Shao and Vollrath, 2002).

11.3.9

Wet Fibre Formation

The natural fibre formation process can be considered a wet fibre formation process, in which a fibre is formed by solvent extraction from a polymer solution. Similar techniques are widely established in industry, for example rayon and lyocell are produced in this way in thousands of tons every year (Liu and Hu, 2006). In a technical wet fibre formation process, the silk solution is extruded through a nozzle into a methanol coagulation bath (Philips et al., 2005). In the coagulation bath, the solvent is extracted and the formation of β-sheet crystallites is induced. The fibre diameter is defined by the nozzle size and the solution’s silk content. The higher the silk concentration, the closer is the diameter to the opening of the nozzle (Corsini et al., 2007). Additionally, the fibre is stretched and guided by rolls in the bath to further align the polymer chains and β-sheet crystallites. The strength of the coagulation bath and the movement speed of the fibre have to be carefully matched to prevent rupture and an insufficient regeneration. Finally, the fibres are washed, reeled up and dried.

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The best fibres have been obtained by using 1,1,1,3,3,3-hexafluoro-2-propanol (C3 H2 F6 O, HFIP) as solvent and methanol as coagulation bath. HFIP can dissolve 30% by mass of recombinant silk or regenerated silk, leading to solutions that can be directly processed in a wet fibre formation process (see Table 11.4). These recombinant fibres have nearly the same mechanical properties as the natural fibres (Lock, 1992 and 1993). Unfortunately, the high price of HFIP, in combination with the still quite high price for the production of recombinant silk, has prevented large-scale industrial use. Solvents that do not suffer from this limitation are N-methylmorpoline N-oxide (C5 H11 NO2 , NMMO) and FPA solvent, a mixture of 70% methanoic acid (CH2 OH; trivial: formic acid) and 30% phosphoric acid (H3 PO4 ) (see Table 11.4). NMMO is already used for the industrial manufacture of lyocell.5 Lenzing AG alone produces more than 40 000 metric t of this cellulose fibre per year in a wet fibre formation process (Lenzing, 2007). NMMO can also dissolve more than 25% of mulberry silk by mass, and can be used directly for wet fibre formation (Marsano et al., 2005; Yingxu, 2005). The FPA solvent demonstrates the strong analogy of silks and polyamides (O’Brien et al., 1998). Polyamides can be easily dissolved in methanoic acid (Burke and Orifina, 1969). Silk does not dissolve very well in methanoic acid, but with the addition of phosphoric acid a notable solvent is obtained, which can dissolve approximately 20% mulberry silk by mass (Ki et al., 2007). Although the surface quality of NMMO and FPA fibres is comparable with that of natural fibres, the β-sheet crystallite orientation is not, and therefore only 25% of the tensile strength and extensibility of natural silk can be reached (Marsano et al., 2005; Corsini et al., 2007; Lock, 1992 and 1993; Lazaris et al., 2002). When these problems have been overcome, they will be suitable solvent systems for the processing of recombinant (regenerated) silk fibres. 11.3.10

Microfluidic Extrusion Nozzle Heads

A variety of standard wet fibre formation set-ups contain microfluidic extrusion nozzle heads. Over the last two decades, ‘lab-on-a-chip’ systems, which use microfluidic set-ups to perform complex chemical reactions with microvolumes of liquids, have become increasingly sophisticated and reliable (Erickson and Li, 2004). This field of technology has opened up the possibility of mimicking the natural fibre formation process in the same dimensions as those found in vivo (Vollrath and Knight, 2001b). Accordingly, in the past few years, attempts have been made to reproduce in microfluidic devices the pH change that induces β-sheet formation, the elongational flow that aligns the protein strands and the ion exchange that is found in natural silk glands. Currently, multichannel set-ups of such devices are being developed, although so far none has been able to produce fibres with noteworthy properties (Martel, 2008; Rammensee et al., 2008). In a standard microfluidic silk processing set-up, three stock channels end in a reaction channel that narrows to provide elongational flow. The middle of the three stock channels contains the silk solution, while the two side channels contain an acidic potassium phosphate solution. These three streams are brought together at the reaction channel. The streams are not mixed, but instead flow in parallel layers to each other (laminar flow). Ions from the side streams diffuse into the silk solution and induce β-sheet formation (Martel, 2008). At the same time, the narrowing reaction channel diameter causes elongational flow, which stretches the protein strands and aligns the β-sheets. In addition, the narrowing diameter causes shear, which also stimulates β-sheet formation (Vollrath, 2000). In this way microfibres from recombinant silk can be produced. Currently, the recombinant fibres produced in this way are too short for a proper analysis of the mechanical properties. Nevertheless, this technique is promising for the production of performance fibres from silk solutions once all the details of both silk-stock solution and solution fibre-forming process combined with post-extrusion draw-stretching have been sorted out. 5

Lyocell is a regenerated cellulosic fibre that is directly formed from the solvent and solidifies by solvent extraction.

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Applications

The most promising applications for the widespread use of dragline and recombinant silk are, comparably with mulberry silk, applications as scaffolds in tissue engineering. Tissue engineering scaffolds require biocompatibility, support of cell proliferation and mechanical stability, properties that are even stronger in spider than in mulberry silk (Li et al., 2006; Hakimi et al., 2007; Dicko et al., 2008). For example, elastic cartilage, as found in the ear, contains collagen and elastin, which give it strength and elasticity (de Chalain et al., 1999). This combination is perfectly matched by SO1-ELP. Additionally, SO1-ELP substrates show cell proliferation of human chondrocytes (cartilage cells) and might one day find application for the tissue engineering of elastic cartilage. Even more interesting is the potential of dragline silk in nerve regeneration. Nerves regenerate slowly and need a guide for directional growth. Normally, when a segment of a nerve is damaged, scar tissue fills the space previously occupied by the nerve and regrowth from the unharmed nerve parts is hindered. Replacement nerves can be grown on a fibrous substrate and then transplanted to the injury to enhance cell proliferation and provide guidance. Nephila dragline is biocompatible, strong and flexible, and has about the same size as most nerves. Easy to transplant, it provides ample stability for transplanted nerves. These qualities make it the best material currently available for this purpose (Allmeling et al., 2006), and analogues are presently on trial.

11.4

Conclusion

Silk materials will be able to combine 5000-year-old, traditional techniques of agriculture with modern biotechnology and material design in a truly unique way. The potential and attraction of recombinant (regenerated) silks is undisputable. It is of course difficult to judge the potential of such a young field of technology. However, considering that regenerated, recombinant and natural silks can be used in cell culture applications and that the US cell culture market alone was $US 714.5 million in 2005, an economic impact of some hundreds of millions should be expected (Frost & Sullivan, 2006). Other fields also stand to benefit from the outstanding mechanical properties found in silks. From highperformance textiles, impact protection foils and composite materials to rope safety-cores, nanotechnology and even cryotechnology – the potential for silk materials has yet to be realised (Scheibel, 2005; Singh et al., 2007). One thing is certain: the success of future silk materials will depend firmly on the development of practical fibre formation and re-formation processes.

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Sapede, D., Seydel, T., Forsyth, V.T. et al. (2005) Nanofibrillar structure and molecular mobility in spider dragline silk. Macromolecules, 38, 8447–8453. Scheibel, T. (2004) Spider silks: recombinant synthesis, assembly, spinning and engineering of sythetic proteins. Microbiol. Cell Fact., 3, 14. Scheibel, T. (2005) Protein fibers as performance proteins: new technologies and applications. Curr. Opinion Biotechnol., 16, 427–433. Scheller, J., G¨uhrs, K.H., Grosse, F. and Conrad, U. (2001) Production of spider silk proteins in tobacco and potato. Nat. Biotechnol., 19, 573–577. Scheller, J., Henggeler, D., Viviani, A. and Conrad, U. (2004) Purification of spider silk-elastin from transgenic plants and application for human chondorcyte proliferation. Transgenet. Res., 13, 51–57. Sericulture (2005) Sericulture Manual: Standard Operating Procedures. Directorate of Sericulture, Khanapara, Assam. Shao, Z. and Vollrath, F. (2002) Surprising strength of silkworm silk. Nature, 418, 741–741. Shao, Z., Vollrath, F., Yang, Y. and Thøgersen, H.C. (2003) Structure and behavior of regenerated spider silk. Macromolecules, 36, 1157–1161. Singh, A., Hede, S. and Sastry, M. (2007) Spider silk as an active scaffold in the assembly of gold nanoparticles and application of the gold–silk bioconjugate in vapor sensing. Small, 3, 466–473. Sponner, A., Schlott, B., Vollrath, F. et al. (2005) Characterization of the protein components of Nephila clavipes dragline silk. Biochemistry, 44, 4727–4736. Takeshita, H., Ishida, K., Kamiishi, Y. et al. (2000) Production of fine powder from silk by radiation. Macromolec. Mater. Eng., 283, 126–131. Termonia, Y. (1994) Molecular modeling of spider silk elasticity. Macromolecules, 27, 7378–7381. Termonia, Y. (1995) Chain conformation at semicrystalline interphases. Macromolecules, 28, 7667–7670. Thiel, B.L., Guess, K.B. and Viney, C. (1997) Non-periodic lattice crystals in the hierarchical microstructure of spider (major ampullate) silk. Biopolymers, 41, 703–719. Vehoff, T., Glisovic, A., Schollmeyer, H., Zippelius, A. and Salditt, T. (2007) Mechanical properties of spider dragline silk: humidity, hysteresis and relaxation. Biophys. J., 93, 4425–4432. Vepari, C. and Kaplan, D.L. (2007) Silk as a biomaterial. Prog. Polym. Sci., 32, 991–1007. Vollrath, F. (1992) Spider webs and silks. Sci. Am., March, 52–54. Vollrath, F. (2000) Strength and structure of spiders’ silks. Rev. Molec. Biotechnol., 74, 67–83. Vollrath, F. and Edmonds, D. (1989) Modulation of normal spider silk by coating with water. Nature, 340, 305–307. Vollrath, F. and Knight, D.P. (2001a) Liquid crystal silk spinning in nature. Nature, 410, 541–548. Vollrath, F. and Knight, D.P. (2001b) Apparatus and method for forming materials. International Patent PCT/GB00/04489. Vollrath, F., Madsen, B. and Shao, Z. (2001) The effect of spinning conditions on the mechanics of a spider’s dragline silk. Proc. R. Soc. Lond. B., 268, 2339–2346. Wang, B., Baldassarre, H., Tao, T. et al. (2002) Transgenic goats produced by DNA pronuclear microinjection of in vitro derived zygotes. Molec. Reprod. Dev., 63, 437–443. Warner, M. and Terentjev, E.M. (2003) Liquid Crystal Elastomers. Oxford University Press, Oxford, UK. Warwicker, J.O. (1960) Comparative studies of fibroins – ii. The crystal structures of various fibroins. J. Molec. Biol., 2, 350–362. Work, R.W. and Emerson, P.D. (1982) An apparatus for the forcible silking of spiders. J. Arachnol., 10, 1–10. Xu, M. and Lewis, R.V. (1990) Structure of a protein superfibre: spider dragline silk. PNAS, 87, 7120–7124. Yamaguchi, K., Kikuchi, Y., Takagi, T. et al. (1989) Primary structure of the silk fibroin light chain determined by cDNA sequencing and peptide analysis. J. Molec. Biol., 210, 127–139. Yang, Y., Chen, X., Shao, Z. et al. (2005) Toughness of spider silk at high and low temperatures. Adv. Mater., 17, 84–88. Yingxu, H. (2005) Studies on spinning and rheological behaviors of regenerated silk fibroin/N-methylmorpholine-Noxide.H2 O. J. Mater. Sci., 40, 5355–5358. Yoshihiro, Y. (2008) Current state of nanofiber produced by electrospinning and prospects of mass production. J. Text. Eng., 54, 199–205. Yoshimizu, H. and Asakura, T. (1990) Preparation and characterization of silk fibroin powder and its application to enzyme immobilization. J. Appl. Polym. Sci., 40, 127–134. Zhang, Y.Q. (2002) Applications of natural silk protein sericin in biomaterials. Biotech. Adv., 20, 91–100. Zhou, C.Z., Confaloniere, F., Medina N. et al. (2000) Fine organization of B. mori fibroin heavy chain gene. Nucleic Acids Res., 28, 2413–2419.

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12 Wool – Structure, Mechanical Properties and Technical Products based on Animal Fibres Crisan Popescu DWI an der RWTH Aachen e.V., Aachen, Germany

Franz-Josef Wortmann Textiles & Paper, School of Materials, University of Manchester, UK

12.1 Introduction Wool, a proteinaceous fibre with a high hierarchical organisation at each level, is often referred to as ‘God’s gift’ for its unique properties and comfort performance. Wool serves as a model for the polymer chemists developing new fibres and for the protein scientists searching the secrets of nature. This chapter, dealing with the structure and morphology, mechanics and chemistry and end-usage of wool fibre, aims to discuss some of the facets that make it a premium fibre of the textile industry.

12.2

Historic Background

Wool, the fibrous appendage of sheep skin, is probably one of the first fibres used for making textiles – a wool carpet from 500 bc (the Pazyryk carpet), found in a Siberian tomb, is still on display in the Hermitage Museum in St Petersburg. Wool has accompanied mankind throughout history, and the role of wool is underlined by its occurrence in proverbs (to give an example, there are 19 entries for wool and 214 for sheep evenly spread through the books of the Bible) and legends (Golden Fleece, Penelope’s web, Cleopatra’s carpet). The modern history of wool is also strongly related to social events. During the Middle Ages the merino, the sheep with fine wool, was such a precious treasure of the crown of Spain that trade in sheep was forbidden and contraventions Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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were punished with death. Towards the mid-eighteenth century, the King of Spain sent a small merino flock to his cousin, the Elector of Saxony, allowing the formation of the Saxon merino breed; a few years later, the King of Spain sent, also as a present, a few merino sheep and rams to the King of France, which were accommodated at Rambouillet to nucleate the Rambouillet merino. Not too much later, DuPont de Nemour, escaping from the French revolution to the USA, carried the first merino (from Rambouillet) to America. The Napoleonic wars destroyed the merino farms in Spain. The Dutch government sent the first merino rams to the Dutch Cape colony (modern South Africa), and Captain MacArthur took the first 100 merino sheep and rams to Australia. During 50 years of European convulsions, the Spanish monopoly of fine wool ceased, and merino nuclei were already spread all over the world. The heritage of wool products is traceable worldwide, from the woolsack, the chair stuffed with English wool and used since the fourteenth century for the seat of the Lord Chancellor (now Lord Speaker) of England, to the American flags of the nineteenth century (see Chapter 1).

12.3

Chemistry and Morphology

Wool is the best-known representative of the large class of animal hairs, also described as α-keratin fibres, which share a common chemistry, structure and morphology. Their elemental analysis shows fairly similar percentages of carbon (around 50 mass %), hydrogen (7 mass %), oxygen (22 mass %), nitrogen (16 mass %) and sulphur (5 mass %) (Popescu and H¨ocker, 2007). The high amount of sulphur found in wool and the other animal hairs comes mainly from the high cystine content of these fibres. In addition, trace elements are detected. The total ash content of keratin fibres ranges from 0.3 to 0.9%. The most frequent trace metals found are Ca, Cd, Cr, Cu, Hg, Zn, Pb, Fe, As and Si, incorporated in keratin from extraneous sources (Sukumar and Subramanian, 2003). Total hydrolysis of the peptide bonds in proteins yields the 20 common natural α-amino acids (see the general structure in Figure 12.1) given in Table 12.1 (cystine, thiocysteine and cysteine are considered, being faces of the same amino acid) (Zahn et al., 2003). More than 100 amino acids bind each other to form the protein chains. As they contain both cationic and anionic groups, the fibres are amphoteric. The cationic character is due to the protonated side groups of arginine, lysine and histidine, and free terminal amino groups. Anionic groups are present as dissociated side groups of aspartic and glutamic acid residues and as carboxyl end groups. The amounts of amino acids differ slightly in α-keratin fibres, as shown by the data in Table 12.2. The peptide arrangement in the protein fibre has been investigated since the first half of the twentieth century. Astbury and Street (1931) and Astbury and Woods (1934) used X-rays to demonstrate the nature

Figure 12.1 α-Amino acid is a molecule containing amine and carboxyl functional groups attached to the same carbon (α-carbon). The various alpha amino acids differ in their side chain attached to the alpha carbon.

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The 20 common natural α-amino acids found in α-keratin fibres

Group

Name

Side chain

‘Acidic’ amino acids and their ω-amides

Aspartic acid Glutamic acid Asparagine Glutamine Arginine Lysine Histidine

–CH2 –COOH –(CH2 )2 –COOH –CH2 –CONH2 –(CH2 )2 –CONH2 –(CH2 )3 –NH–C(NH2 )=NH – (CH2 )4 –NH2 H CH2 N

‘Basic’ amino acids and tryptophan

N CH2

Tryptophan

NH Amino acids with hydroxyl groups in the side chain

Sulphur-containing amino acids

Amino acids without reactive groups in the side chain

Table 12.2

Serine Threonine Tyrosine Cysteine Thiocysteine Cystine Methionine Glycine Alanine Valine Proline

–CH2 –OH –CH(CH2 ) –OH –CH2 –C6 H4 –OH –CH2 –SH –CH2 –S–SH –CH2 –S–S–CH2 – –(CH2 )2 –S–CH3 –H –CH3 –CH(CH3 )2 –CH2 CH2 –CH2

Leucine Isoleucine Phenylalanine

–CH2 –CH(CH2 )2 –CH(CH2 )–CH2 –CH3 –CH2 –C6 H5

Amino acid composition of wool (Popescu and Hocker, 2007) and of cashmere and yak fibres ¨

Amino acid in mol %

Wool

Cashmere

Yak

Glycine Alanine Serine Glutamine + glutamic acid Cystine Proline Arginine Leucine Threonine Asparagine + aspartic acid Valine Tyrosine Isoleucine Phenylalanine Lysine Triptofan Histidine Methionine

8.1 5.0 10.2 12.1 11.2 7.5 7.2 6.9 6.5 6.0 5.1 4.2 2.8 2.5 2.3 1.2 0.7 0.5

9.9 5.8 12.2 12.4 6.0 6.7 7.0 7.5 6.6 6.2 5.5 3.5 3.2 2.8 2.8 — 1.2 0.5

9.8 5.6 10.0 12.5 6.4 6.6 7.1 8.3 6.6 6.7 5.9 3.4 3.5 3.0 3.0 — 1.0 0.4

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Figure 12.2 The architecture of intermediate filaments in wool. (A) The structure of the monomer α-helix and the coiled-coil dimer. The letters may be replaced by any amino acid residue from Table 12.2, the only requirement being that amino acids a and d are hydrophobic ones. (B) Structure of the intermediate filaments. A and B: helical domains; L: non-helical linkers; N, C: N and C termini respectively. Adapted from Pauling et al. (1951), Zahn et al. (2003) and Popescu & Hocker (2007). ¨

of a crystalline phase in hair. The X-ray diffraction pattern of animal hairs shows a meridian reflection at 0.51 nm and an equatorial reflection at 0.98 nm. Interpreting these results, Pauling et al. (1951) proposed a α-helix structure to account for the secondary structure of the keratin fibre, shown in Figure 12.2A. The α-helix contains 18 amino acid residues in five turns, i.e. 3.6 amino acid residues per turn. To ensure a distance between successive turns of the helix that leads to the observed meridian reflection (0.51 nm), the helical chain must itself be slightly coiled (superhelix, coiled coil (Crick, 1952)). Two superhelices combine to form a left-handed two-stranded rope-like assembly in which the superhelices are arranged in such a way that the hydrophobic side groups at the outside of the helices interlink to form a stable ‘buttonhole’ structure (Crick and Kendrew, 1957). These dimers are the actual structural subunits of the microfibrils, and can be termed ‘molecular twins’. The force that keeps two α-helices together in the coiled-coil dimer (the ‘brick’ of the intermediate filament rod) is given by the geometry of the arrangement of amino acid residues in the polypeptide chain and by the hydrophobic effect. The geometry requires a repeating sequence of seven amino acids (abcdefg), a heptade, with residues a and d representing hydrophobic ones, as shown in Figure 12.2A (Zahn et al., 2003). The further organisation of the α-helices in protofilaments, protofibrils to microfibrils or intermediary filaments follows a pairing rule illustrated in Figure 12.2B.

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Figure 12.3 The architecture of the cuticle. The layers are shown in the left-hand picture. The zoom shows the organisation of the epicuticle.

Morphologically, the fibres are composed of the cortex and the cuticle. Each of the two components is formed of various other morphological components. The cortex contains cortical cells and the cell membrane complex. The cortical cell is further composed of macrofibrils and intermacrofibrillar material. The macrofibrils consist of microfibrils and intermicrofibrillar matrix. In summary, the cortex is formed of microfibrils (intermediate filament (IF) or keratin proteins (KP)) and keratin-associated proteins (IFAP or KAP) which compose the intermicrofibrillar matrix containing cytoplasmatic and nuclear remnants. This ensemble is wrapped up in the cuticle as an external sheath that also has its own architecture, being formed of four layers: the epicuticle, the a-layer, the exocuticle and the endocuticle (see Figure 12.3). The epicuticle has a peculiar structure and is the layer responsible for the keratin fibre paradox (Popescu and H¨ocker, 2007): a hydrophobic surface wrapping a hydrophilic core. The reason for this is the presence of the 18-methyl eicosanoic acid (18 MEA) anchored by an ester bond (a bond between an acid and an alcohol, or a thiol) to a protein matrix, as shown in Figure 12.3 (detail). Summing up, the α-keratin fibre is an example of a natural composite system having a complex dual structure at all levels (Table 12.3). When α-keratins are stretched in a wet environment, metastably they achieve a new arrangement called the β-sheet (Astbury, 1933), which is the natural form in feather, silk or spider silk (see also Chapter 11). Table 12.3

Animal fibre structure

Composite

Type

Component 1

Component 2

α-Keratin fibre Cortex Cortex cell Macrofibril

Ring/core Filament in matrix Filament in matrix Filament in matrix

Cuticle Cortex cells (spindle shape) 5–8 macrofibrils 500–800 microfibrils (IFs)

Cortex Cell membrane complex Intermacrofibrillar matrix Intermicrofibrillar matrix

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Figure 12.4

Wool fibre surface showing the scales.

The animal fibres have a slightly elliptic cross-section and are protected by the scales arranged on their surface as tiles on a roof (see Figure 12.4 and the SEM micrographs of animal fibres in Chapter 14).

12.4

Mechanics of the Fibre

The complex morphological and molecular structure of α-keratins echoes the construction principle of biological composite structures in general, namely to combine components with different properties in one material so as to maximise suitability for its purpose. Table 12.3 shows the stepwise differentiation of the morphological structure of wool into the most important two-phase structures, leading ultimately to the so-called two-phase model. Rigorous simplification of the complex morphological structure leads to the cortex being considered effectively a nano-scaled, axially oriented filament/matrix composite. The dominant morphological components are the α-helical intermediate filaments (IFs) embedded, largely with axial orientation, in an amorphous matrix (Feughelman, 1959). The contribution of the cuticle, the outer protective sheet, to the mechanical properties of wool is considered to be largely negligible (Feughelman and Haly, 1960; Bendit and Feughelman, 1968). The α-helical, central, rod-like domain in the IFs is regarded as a crystalline (or, according to the actual terminology, a paracrystalline) domain; this phase accounts for 25–30% of the dry fibre (Bendit, 1968; Wortmann and Deutz, 1993). The other components make up the ‘matrix’ phase, which includes the cuticle, the cell membrane complex, the intermacrofibrillar material, the interfilament material and 40% of the IFs, i.e. the non-helical linkers and the ends of the IF monomers (Wortmann, 1992). This characteristic structure leads to the marked anisotropic properties of wool fibre, among which the pronounced differences between axial and lateral swelling, as well as between extensional and torsional properties, are the most relevant for practical purposes (Onions, 1962). The stress–strain curves recorded for wool fibre at two different humidities are illustrated in Figure 12.5 as representatives of the behaviour at any relative humidity. Any of the curves can be decomposed into three regions that are affected differently by humidity. After decrimping (not shown in Figure 12.5), the first region shows the tension in the fibre increasing fairly linearly up to a strain of 1–2%. Above this, the elongation increases rapidly for small increases in stress. This section of the curve is known as the yield region and ends between 25 and 30% elongation. The third region of the stress–strain curve is called the post-yield region, which terminates on rupture of the fibre. The three slopes of the initial, yield and post-yield regions, respectively, are in the approximate ratio of 100:1:10.

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Figure 12.5 Stress–strain curve of wool recorded at 20 ◦ C under standard relative humidity (55%) and under wet conditions (RH 100%).

Of particular interest is the recovery of the fibre after a strain of up to 30%. There are several models proposed for describing the stress–strain curve of wool, and the hysteresis behaviour, with Feughelman’s model (Feughelman, 1959 and 1994) and the Chapman–Hearle model (Hearle and Chapman, 1968) being the most known. In both cases the mechanical effort is seen as being distributed between the α-helix of the crystalline region of the fibre (IFs) and the amorphous matrix. Their contributions depend strongly on the moisture content of the fibre, with the matrix effect almost vanishing at 100% relative humidity. The difference between the two models is mainly in the way the crystalline and amorphous phases are connected: while Feughelman’s model (Feughelman, 1994) does not consider any link between the two phases, the Chapman–Hearle model (Hearle and Chapman, 1968) assumes that they are bridged by the disulphide bonds. The moisture content plays a very important role in the mechanical behaviour of the fibre. The curves in Figure 12.6 show the swelling of wool fibres from dry to wet with increasing regain. In the follicle, the keratin fibre is produced under wet conditions. Upon drying, the paracrystalline filaments will resist shrinkage, so that, on rewetting or swelling from the dry state, the length change is, at 1–2%, rather small. Radial swelling is a property of the amorphous matrix only. The effect is 16% between dry and wet, rather than large, and reflects the strong tendency of the matrix to absorb water. This stability of the α-helical fraction in the IFs is an interesting phenomenon in view of the generally assumed sensitivity of the α-helix against water, which

Figure 12.6

The anisotropic swelling of wool fibre.

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Industrial Applications of Natural Fibres Table 12.4 Mechanical properties of wool fibre at 22 ◦ C (adapted from Bobeth, 1993) Breaking stress Dry Wet Strength loss when wet Breaking strain Dry Wet Elasticity modulus Dry Wet Recovery at strain 2% 5% 10% Bending modulus Stretching modulus Torsion modulus parallel Stretching modulus/torsion modulus Shear modulus in torsion Dry Wet

250–350 MPa 100–200 MPa 20% 28–48% 40–61% 4.0–5.0 GPa 2.0–3.0 GPa 95–99% 60–70% 40–50% 4.0–5.5 GPa 5.0–6.0 GPa 1.1–1.3 GPa 3.0–4.0 GPa 1.2 GPa 0.1 GPa

Note: ‘dry’ refers to 65% relative humidity (RH); ‘wet’ refers to 100% RH.

in turn is attributed to the extensive hydrophobic interactions within the coiled-coil structure and the high degree of paracrystalline aggregation (Bendit and Feughelman, 1968; Feughelman, 1989). The tendency of keratin fibres to absorb and to be effectively plasticised by water is reflected in the decrease in the elastic modulus with humidity from dry to wet (see Table 12.4). The effect is limited to a factor of 2.7 owing to the humidity-invariant modulus of the filaments. Accordingly, the decrease is substantially more pronounced for torsion, which is a property of the matrix only, with a factor of about 15 (Bendit and Feughelman, 1968; Speakman, 1930). Similar factors have been observed in extensional relaxation experiments of wool fibres, when separating the elastic and viscoelastic contributions of filaments and matrix respectively (Wortmann and DeJong, 1985). The elastic and viscoelastic properties of wool fibres, as determined by the mechanical properties of IFs and the matrix, largely determine the crease resistance, dimensional stability, drape and handle of wool fabrics. A compilation of data on mechanical properties is provided in Table 12.4. A comparison with other natural fibres is given in Chapter 13 (Table 13.7). Owing to the protein structure, wool absorbs a large amount of moisture, which binds by hydrogen bonds to the amino acids, reaching 33% of its dry mass, the highest level among the natural fibres. As the crystalline region is water impermeable, the amorphous matrix can absorb water up to 45% of its dry mass (for a crystallinity of 25%) without feeling wet. The moisture absorption by wool is accompanied with dissipation of the corresponding heat, which is around 110 J/g fibre for 18% moisture (the amount absorbed at a relative humidity of 65% and 25 ◦ C) (Popescu and H¨ocker, 2007). The desorption of moisture follows a different path, which is shifted upwards compared with the sorption curve, with about 2% at its largest part (at the same 65% relative humidity) at 25 ◦ C. This gel-like behaviour makes wool fibre a unique comfort-providing material.

12.5

Characteristic Temperatures

Being a crystalline–amorphous composite fibre, wool also shows the viscoelastic transitions that are characteristic of semi-crystalline polymers, such as a low-temperature β-transition and a high-temperature α-transition

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(also known as ‘glass transition’). As temperature and low-molecular-weight plasticisers have similar effects on the mobility of polymer chains, it is to be expected that the glass transition temperature (T g ) will also depend on water content. King (1926) investigated the density and specific volume of wool with different water contents at 25 ◦ C, yielding, at a water fraction of ∼17.5 mass %, the turnover between the classical linear changes for polymers above and below the glass transition. Rosenbaum (1970) investigated the extent to which the Flory–Huggins equation for polymer solutions (Flory, 1953) could be applied for the description of the sorption isotherm of wool. The glass transition temperature was determined as the lower limiting temperature of applicability, beyond which the wool/water system transfers into the glassy state. Owing to its particular importance for the setting of wool fibres, the glass transition temperature of wool with various amounts of water content was measured several times. Fairly comprehensive data were acquired by investigating the torsional recovery of wool fibres for various temperature/humidity combinations (Wortmann et al., 1984). These investigations were followed by measurement using differential scanning calorimetry (DSC) (Phillips, 1985; Huson, 1991; Kure et al., 1997). By combining the values from various experiments, it was shown that wool follows well the Fox equation (Fox, 1956) for describing the effect of water (Wortmann et al., 1984): 1/Tg = w1 /Tg1 + w2 /Tg2 where w is the mass fraction, and subscripts 1 and 2 refer to dry wool and pure water respectively. The equation fits the data well, without any assumptions about the glass transition temperatures of the pure components, yielding, through extrapolation, T g1,wool = 447 K (174 ◦ C) and T g2,water = 125 K (−148 ◦ C) (Wortmann et al., 1984), in good agreement with the results of other experiments (Menefee and Yee, 1965; Kalichevsky et al., 1992). Between ‘dry’ and ‘wet’ the glass transition temperature of wool changes by 180 ◦ C from 170–180 ◦ C to −5 ◦ C. At 65% RH, with a water content of about 15%, the glass transition temperature is around 50–60 ◦ C, which means that, under normal climate conditions, wool is a semi-crystalline, glassy polymer. Similar behaviour is shown by the temperature of the β-transition, but this is more of academic significance because of its low value (for dry wool, Tβ ,wool = 224 K (−49 ◦ C)). The humidity and temperature range between the β- and the α-transition defines the range where the phenomenon of physical ageing occurs (Struik, 1978). This plays an important role in the understanding of various aspects of appearance retention of wool fabrics, such as wrinkling (Wortmann and De Jong, 1985; Chapman, 1975a and 1975b). Another characteristic temperature of importance for polymers is that of phase transition (melting). When heated in DSC, wool exhibits an endothermic effect around 230 ◦ C that is attributed to a melting-like process of its crystalline phase, known in protein chemistry as the thermal denaturation temperature, T D (Haly and Snaith, 1967). The temperature at which the endothermic effect occurs is related to the degree of crosslinking of the matrix (the temperature is shifted downwards almost 100 deg by softening of the matrix with water) (Haly and Snaith, 1967; Crighton and Hole, 1985). The enthalpy of the effect (the area of the endothermic peak) relates to the amount of α-helix (Wortmann and Deutz, 1993; Spei and Holzem, 1989). Although without much practical importance for the industrial processing of wool, the denaturation effect (denaturation temperature and enthalpy) serves as a good analytic indicator of the degree of fibre damage.

12.6

Fibre Availability

The keratin fibres are available almost everywhere in the world. Wool, produced by sheep, is by far the most widely used keratin fibre, while shatoosh (Tibetan antelope, Pantholops hodgsonii) produces the most expensive fibre and is found only on the high peaks of Himalaya. Among all keratin fibre producers, the sheep is the most important. Economically, the sheep can be seen as a ‘factory’ without wastes. The byproducts are defined from the point of view of the down-processing industry. For example, the textile industry regards milk and meat (lambs) as secondary products.

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Table 12.5 Composition in % of greasy wool. The micron limits for the three wool types are only informative. Adapted from W. Bobeth, Textile Faseratoffe, Beachaffenheit und Eigenschaften, Springer-Verlag Berlin, 1993 Wool type Merino (<25 µm) Cross-bred (25–33 µm) Long wool (>33 µm)

Grease and suint

Sand and dirt

Vegetable matters

Fibre

15–30 15–30 5–15

5–40 5–20 5–10

0.5–10 1–5 0–2

30–60 40–65 60–75

Compared with other keratin producers, the sheep gives most fibre from a given pasture surface. Owing to the way the sheep eats grass, the pasture regenerates quickly after sheep passage – compared with the cashmere goat, after the passage of which at least 2–3 seasons are required for regenerating the pasture. Very roughly, a sheep produces 1 kg of greasy wool (or some 0.6 kg of clean wool) annually from 1 ha of average pasture. Since 1950, when synthetic fibres commenced their offensive, the amount of wool production has fluctuated. After a steady increase until 1990, when the amount was almost double that of 1950, wool production declined. Its 2008 level, with about 1.200 Gkg clean wool produced, was only 20% greater than the amount produced in 1950 and made up 2.5% of the total yearly fibre consumption. Bringing wool fibres to the textile industry is a labour-intensive process. The shearing and collecting of greasy wool are manual operations, as are skirting (selecting the parts of the fleece) and classing. The collected greasy (raw) wool contains various amounts of different impurities, as detailed in Table 12.5. To deliver clean fibres to the industry, the greasy wool is scoured. The discharged waters from this operation may be further used for extracting lanolin by an Alfa Laval process (Stewart, 1985). The vegetable matter, if more than 2–3%, also has to be removed before going to further processing. The operation, known as ‘carbonisation’, makes use of the good resistance of wool to strong acids (particularly sulphuric acid) and of the hydrolysis of cellulose by the same environment.

12.7

Products Based on Wool Fibres

Wool fibres are traditional raw materials for textiles; according to the fibre diameter, they are designated to the clothing or interior textiles industry. The end-products exploit the wool fibre’s excellent mechanical and comfort properties, and many high-quality textiles are wool made. This is supported by the versatility of wool fibre, which can be dyed with dye of almost any class and any colour, making wool products suitable items for any fashion show. Over recent decades, because of the increasing use of washing machines, the topography of the fibre surface has received increased attention. The surface scales (Figure 12.4) give the fibres a directional friction coefficient, which induces a preferential direction for fibre movement. As a result, the fibres felt, and, while this property is desired to a certain extent for producing the unique wool fabric surface, it leads to undesired shrinkage of the material after washing if not properly handled. The treatment for stopping the felting propensity is generally a two-step one, with scale eroded by a chemical (chlorine, O3 ), plasma or enzymatic attack, followed by a resin coating (Zahn et al., 2003). This produces merchandise that can eventually be washed in a household washing machine and dried in a tumbler, fulfilling the claim of total easy care (TEC). Technical textile is the field in which wool has made significant gains in recent years, building up on its various special characteristics. A major advantage of using wool for technical purposes is that the fibre diameter plays a minor role here, and this allows also cheap wool, or even fibres from recycled textiles, to be embedded into various products. The particular chemical structure of wool makes it suitable for sequestering the cations of heavy metals. This property has been exploited since ancient times by shepherds for recovering gold from springs passing gold ores, a habit apparently at the root of the Golden Fleece legend. In modern times, wool is used to retain

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iron or lead from supply waters, for example in the food industry or nuclear plants, which makes wool a useful component of water filters (Katoh et al., 2004). The protein structure also helps wool retain noxious gases, a property that may be used for cleaning the surrounding atmosphere (H¨ocker and Wortmann, 2003). This has promoted the use of wool in products for the building industry (e.g. building blocks, technical insulation – see Chapter 20), which, besides thermal and acoustic protection, help to keep the indoor level of formaldehyde below the World Health Organisation security limit of 0.05 ppm (H¨ocker and Wortmann, 2003). The amino acids of the fibre can be used as soil fertiliser. Experiments on grass with and without wool wastes buried in the soil indicated the positive role of wool in assisting growth of the grass (H¨ocker and Wortmann, 2003).

12.8

Conclusion

After thousands of years of history, wool fibres appear to be as promising as ever. Our understanding of the structure of wool and the relationship between its sophisticated morphology and properties has made great strides in recent decades, which has been mirrored by new applications. Non-textile usage of wool is only just beginning, but the potential of its chemistry and structure, looking beyond the fibre, bodes very well for the future. Summing up, between luxury clothing and technical textiles there are innovative applications for every type of wool, and wool fibre is being used in more everyday products than a century ago.

References Astbury, W.T. (1933) Some problems in the X-ray analysis of the structure of animal hairs and other protein fibers. Trans. Faraday Soc., 29, 193–211. Astbury, W.T. and Street, A. (1931) X-ray studies of the structure of hair, wool and related fibres. I: general. Phil. Trans. R. Lond. Soc., A230, 75–101. Astbury, W.T. and Woods, H.J. (1934) X-ray studies of the structures of hair, wool and related fibres. II. The molecular structure and elastic properties of hair keratin. Phil. Trans. R. Lond. Soc., A232, 333–394. Bendit, E.G. (1968) The distribution of high- and low-sulfur fractions in alpha-keratin. Text. Res. J., 38, 15–21. Bendit, E.G. and Feughelman, M. (1968) Keratin. Encycl. Polym. Sci. Technol., 8, 1–44. Bobeth, W. (1993) Textile Faserstoffe, Beschaffenheit und Eigenschaften. Springer-Verlag, Berlin, Germany. Chapman, B.M. (1975a) The rheological behaviour of keratin during the ageing process. Rheol. Acta, 14, 466–470. Chapman, B.M. (1975b) The ageing of wool. Part I: ageing at various temperatures. J. Text. Inst., 66, 339–342. Crick, F.H.C. (1952) Is alpha-keratin a coiled coil? Nature, 170, 882–883. Crick, F.H.C. and Kendrew, J.C. (1957) X-ray analysis and protein structure. Adv. Protein Chem., 12, 133–214. Crighton, J.S. and Hole, E.R. (1985) A study of wool in aqueous media by high pressure differential analysis, in Proceedings of the 7th International Wool Textile Research Conference, Tokyo, Japan, Vol. I, pp. 283–292. Feughelman, M. (1959) A two-phase structure for keratin fibers. Text. Res. J., 29, 223–228. Feughelman, M. (1989) A note on the water-impenetrable component of α-keratin fibers. Text. Res. J., 59, 739–742. Feughelmann, M. (1994) A model for the mechanical properties of the α-keratin cortex. Text. Res. J., 64, 236–239. Feughelman, M. and Haly, A.R. (1960) The mechanical properties of the ortho- and para-like components of Lincoln wool fibers. Text. Res. J., 30, 897–900. Flory, P.J. (1953) Principles of Polymer Chemistry. Cornell University Press, New York, NY, USA. Fox, T.G. (1956) Influence of diluent and of copolymer composition on the glass temperature of a polymer system. Bull. Am. Phys. Soc., 1, 123. Haly, A.R. and Snaith, J.W. (1967) Differentielle Thermoanalyse von Wolle. Die Phasenumwandlungsendotherme unter verschiedenen Bedingungen. Text. Res. J., 37, 898–907. Hearle, J.W.S. and Chapman, B.M. (1968) On polymeric materials containing fibrils with a phase transition. I. General discussion of mechanics applied particularly to wool fibers. J. Macromolec. Sci. Phys. B, 2, 663–695.

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Hoecker, H. and Wortmann, G. (2003) Unconventional uses of wool. IWTO Meeting, Buenos Aires, Argentina. Huson, M.G. (1991) DSC investigation of the physical ageing and deageing of wool. Polym. Int., 26, 157–161. Kalichevsky, M.T., Jaroszkiewics, E.M. and Blanshard, J.M.V. (1992) Glass transition of gluten. 1: gluten and gluten–sugar mixtures. Int. J. Biol. Macromolec., 14, 257–266. Katoh, K., Shibayama, M., Tanabe, T. and Yamauchi, K. (2004) Preparation and properties of keratin-poly(vinyl alcohol) blend fiber. J. Appl. Polym. Sci., 91, 756–762. King, A.T. (1926) The specific gravity of wool and its relation to swelling and sorption in water and other liquids. J. Text. Inst., 17, T53–T67. Kure, J.M., Pierlot, A.P., Russell, I.M. and Shanks, R.A. (1997) The glass transition of wool: an improved determination using DSC. Text. Res. J., 67, 18–22. Menefee, E. and Yee, G. (1965) Thermally-induced structural changes in wool. Text. Res. J., 35, 801–812. Onions, W.J. (1962) Wool. An Introduction to its Properties, Varieties, Uses and Production. Ernest Benn Ltd, London, UK. Pauling, L., Corey R.B. and Branson, H.R. (1951) The structure of proteins: two hydrogen-bonded helical configurations of the polypeptide chain. Proc. Natl Acad. Sci. USA, 37, 205–211. Phillips, D.G. (1985) Detecting a glass transition in wool by differential scanning calorimetry. Text. Res. J., 55, 171–174. Popescu, C. and H¨ocker, H. (2007) Hair – the most sophisticated biological composite material. Chem. Soc. Rev., 36, 1282–1291. Rosenbaum, S. (1970) Solution of water in polymers: the keratin–water isotherm. J. Polym. Sci., C31, 45–55. Speakman, J.B. (1930) Adsorption of water by wool. J. Soc. Chem. Ind., 49, 209T–213T. Spei, M. and Holzem, R. (1989) Thermoanalytical determination of the relative helix content of keratins. Colloid. Polym. Sci., 267, 549–551. Stewart, R.G. (1985) Wool Scouring and Allied Technology, 2nd edition. Caxton Press, Christchurch, New Zealand. Struik, L.C.E. (1978) Physical Ageing in Amorphous Polymers and Other Materials. Elsevier, Amsterdam, The Netherlands. Sukumar, A. and Subramanian, R. (2003) Elements in the hair of workers at a workshop, foundry, a match factory. Ind. Health., 41, 63–68. Wortmann, F.-J. (1992) Thermo- und hydroplastische Eigenschaften von Wollfasern. Westdeutscher Verlag, Opladen, Germany. Wortmann, F.-J. and DeJong, S. (1985) Analysis of the humidity–time superposition for wool fibers. Text. Res. J., 55, 750–756. Wortmann, F.–J. and Deutz, H. (1993) Characterising keratins using high-pressure differential scanning calorimetry (HPDSC). J. Appl. Polym. Sci., 48, 137–150. Wortmann, F.-J., Rigby, B.J. and Phillips, D.G. (1984) Glass transition temperature of wool as a function of regain. Text. Res. J., 54, 6–8. Zahn, H., Schaeffer, K. and Popescu, C. (2003) Wool from animal sources, in Biopolymers. Vol. 8. Polyamides and Complex Proteinaceous Materials II, ed. by Steinb¨uchel, A. and Fahnestock, S.R. Wiley-VCH, Weinheim, Germany.

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PART IV TESTING AND QUALITY MANAGEMENT

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13 Testing Methods for Measuring Physical and Mechanical Fibre Properties (Plant and Animal Fibres) ¨ J¨org Mussig Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany

Holger Fischer Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany

Nina Graupner Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany

Axel Drieling Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany

13.1 Introduction In general there are more reasons for fibre testing than stakeholders in the value-added chain of natural fibres in industrial production. Consequently, potential customers for testing results can be found in each step of this chain, starting with fibre production/cultivation of plants. Here, there is a need for optimisation of plant breeding, driven by the strong interest in realising a price that reflects the fibre quality. In the subsequent step of fibre separation, testing is necessary for process control and optimisation. Also, in fibre trading there is great interest in buying fibres according to objective fibre quality and realising prices according to objective fibre quality in sale, combined with the possibility of offering special grades and fibre properties. Product creation is the next step in the chain, where fibre testing becomes essential for selecting appropriate fibre lots, minimising fibre loss during processing and enabling a failure-free production process. A controlled Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Figure 13.1 Effects of blending by controlled bale lay-down on homogeneity and long-time product quality. USTER news bulletin, 39, 08/1993. Adapted with permission from Uster Technologies AG.

bale lay-down based on bale-specific quality information will ensure homogenisation of product quality at the desired level and avoid variations in quality by blending (see Figure 13.1). This allows producers to guarantee product homogeneity and quality to their customers. Another important topic in production is the correlation of fibre and product properties (quality prediction) using either empirical or model-based methods, which is only possible if reliable fibre quality data exist. The aim of this chapter is to give the reader information about the wide range of fibre properties and how to structure them. This is accompanied with a discussion about single-point data and multipoint data and the fundamentals of statistics and distributions, which are prerequisites to understanding the evaluation of natural fibre measurements. This is followed by a description of accuracy and precision in order to understand the need to find a suitable method and to underline the importance of calibration. Subsequently, recommendations are given as to what measurements are reasonable at which stage of processing and which methods are reasonable in each case. Our recommendations for each case comprise one simple method with minimal equipment demands and another method supplying more elaborate data and distributions, useful for research or modelling. We describe the methods briefly and give examples of the broad range of property distributions for selected natural fibres. As far as possible, we have selected the same fibre lots as used in other chapters of this book. Finally, a compilation of the most important typical fibre properties is given in fibre tables based upon data collected from the scientific literature.

13.2

Fibre Properties

When considering measurable fibre properties, one should distinguish between intrinsic properties and the conditions of fibres. Intrinsic properties, for example length, strength, fineness and colour, are directly related to the material quality. Properties based on the condition of the fibre, for example shive content, dust, stickiness, mildew or microbacteria contamination, neps and other impurities are caused or influenced by production, transport or processing.

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The following tables show a possible structuring of important fibre or bundle properties, adapted and extended from the structure scheme of cotton properties. Owing to the very large number of possible fibre types, properties and conditions, we can only give an overview of the most relevant properties and point out typical examples. The intrinsic fibre properties listed in Table 13.1 are grouped into geometric properties, mechanical behaviour, physical and chemical properties and behaviour in different ambient conditions. Many of them are subgrouped, and details or examples are listed. Table 13.2 gives a list of fibre conditions grouped into impurities, processing state and processing behaviour. They cannot all be measured directly. For example, in the processing behaviour group there are many parameters that are recorded as sum parameters by using small-scale process-simulating equipment, e.g. testing by the Rotor-Ring device (ITV, Denkendorf, Germany) in cotton spinning. These tables give an overview of fibre properties in general. Please do not expect to measure all of them – it will take too much resource and effort. For this reason, Sections 13.5 and 13.6 give recommendations as to which properties should be measured in which case.

13.3

Characteristic Values and Statistics

Classifying a natural product like fibre in a scientifically correct way requires the interaction of reproducible test equipment and methods with a statistical evaluation of the measured values. While some methods give only a sum parameter of a special fibre property, other methods give a complete distribution of the tested properties. In the following section, these aspects will be discussed with the focus on fibre fineness testing. Fibre geometry, for example, can be characterised by different techniques. Measuring the mass-related fineness in tex by hand is a common method with a small requirement for instrument. In order to get reproducible results, a large number of measurements must be taken. Consequently, the method of gravimetric measurement is very time consuming. The main disadvantage of this method is that only a characteristic result like the mean value, but not the distribution of fineness, is recorded (Drieling et al., 1999). The result also depends on sample preparation, along with the experience and interpretation of the operator (Simor, 1959). Measuring the fibre surface by airflow is an indirect method to characterise the fineness of the fibre (K¨ob and Stiepel, 1951; Hadwich, 1975). K¨ob and Stiepel (1951) report the activities in the USA in the 1940s to correlate the fineness of cotton fibres with the resistance to airflow through samples. Information about cotton fibre fineness and maturity as separate results is available from airflow measurements with the Shirley IIC fineness-maturity tester (Stephens, 1977). The further development of the airflow method to an ASTM standard for testing flax fineness is described in Chapter 18. With the airflow method, a large number of fibres are tested in each sample. Based on this, the results are very reproducible, but no information about the measurement distribution is available. Mean values or other characteristic values alone supply no information on property distributions. Different distributions can lead to the same mean values or characteristic values. An example is given in Figure 13.2. Methods that measure quickly and reproducibly the sum parameter of a fibre sample as a mean value are attractive for grading and classification but not suitable for the evaluation of product quality if it is influenced by property distribution. The shape of the distribution has a strong influence on the product properties. Typical examples are as follows: r The width of the elongation distribution tested on single fibres influences strongly the strength of a collective of fibres and the strength of a yarn. r The width of the maturity distribution of cotton fibres considerably influences the colourability of a yarn or a fabric and the creation of neps during the carding process. r The width of the fineness distribution of hemp fibre bundles influences the mechanical properties of the resulting composite.

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Table 13.1

Proposed structuring for intrinsic fibre1 properties

Type

Subtype

Detail/example

1. Geometric properties

a. Length-related properties

– Average length – Average length of long-fibre fraction – Uniformity/length histogram – Short-fibre content – Fineness in terms of: ◦ length-related mass or ◦ cross-sectional area or ◦ diameter or width – Shape of cross-section, e.g.: ◦ cotton: maturity (average or share of immature fibres) ◦ wool: medullation ◦ bast fibres: lumen ◦ number of elementary fibres in cross-section ◦ hemp: polygonal or ribbon-shaped – Length difference between stretched and crimped fibre – Crimp length, height or geometry – Smooth (e.g. spider silk) – Structured e.g.: ◦ wool: scales ◦ silk: fibre surface torn open ◦ cotton: fibrillated

b. Cross-section-related properties

c. Crimp d. Surface shape and structure

2. Mechanical behaviour

e. Length-related irregularities

– Cotton: convolutions – Wool: variation in diameter – Flax: internodes

a. Tensile test (collective)

– Tensile strength – Breaking extension – Additional parameters like: ◦ Young’s modulus ◦ tensile energy absorption ◦ etc.

b. Tensile test (single element)

– Single-element tensile strength and its distribution – Single-element breaking elongation and its distribution – Stress relaxation (creep testing) – Additional parameters and their distribution like: ◦ Young’s modulus ◦ tensile energy absorption ◦ Poisson’s ratio ◦ etc.

c. Wet tensile test d. Flexural behaviour e. Torsional behaviour f. Behaviour in long-term tensile test g. Behaviour in cyclic (fatigue) testing 3. Density

a. Material density b. Apparent density

4. Colour

a. Physical colour assessment

b. Grading against fibre colour standards

– LAB values (colorimetry) – Spectroscopy: ◦ hemp/flax: degree of retting – Cotton: colour classing – Hemp/flax: degree of retting as colour scale

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(Continued)

Type

Subtype

5. Chemical fibre composition

a. Degree of polymerisation b. Content of constituents

Detail/example

– Cellulose-based fibres: content of cellulose, lignin, pectin, etc. – Wool: content of specific amino acids

6. Physical fibre composition a. Degree of crystallinity b. Fibrils arrangement (MFA – microfibril angle) 7. Frictional behaviour

a. Fibre/fibre friction b. Fibre/metal friction c. Friction against other materials (e.g. ceramics)

8. Electrostatic behaviour 9. Behaviour towards moisture

a. Moisture content b. Water absorption/water retention c. Fibre swelling – Determination of degree of fibre damage – Determination of dimensional changes

10. Behaviour in specific conditions

a. Thermal and hydrothermal behaviour (e.g. glass transition temperature) b. Acids/bases c. Oxidising agents d. Irradiation, e.g. light, UV, etc. e. Microbial attack

1

Fibre is used in this table as a synonym for both fibre or fibre bundle.

Devices measuring fibre properties such as diameter, fibre width or cross-section and their distributions are, for instance, laser-based devices like AFIS1 (Chu and Shofner, 1992) or microscopes in combination with image analysis techniques (Thibodeaux and Evans, 1986). While some non-automated techniques like cross-section measurement are relatively time consuming and complex to handle, fast-operating automated systems for width testing, like AFIS, OFDA2 and Laserscan, are cost intensive. The ‘Fibreshape’ width analyser is for some purposes a cost-efficient, easy-to-apply and reliable solution. More details on the theory and the means of system calibration can be found in Schmid (1999), Schmid et al. (2002) and M¨ussig and Schmid (2004). The Fibreshape system is a combination of a high-resolution slide scanner and a specialised, fully automatic image analysis system. In view of the importance of property distribution in fibre measurements, some important statistical information will now be discussed. Figure 13.3 shows parameters in descriptive statistics to describe the principal characteristic of a distribution. Based on the three values (mean, median and mode), the shape of the distribution curve can be evaluated (see Figure 13.4): r Right-skewed distribution: – the most common parameter value (the ‘mode’) is located far left, the mode is the smallest value of the three parameters; – the median cuts the distribution curve in half and is higher than the mode;

1 2

Advanced Fibre Information System. Optical-based Fibre Diameter Analyser (Baxter et al., 1992).

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Table 13.2

Proposed structuring for fibre1 conditions

Type

Subtype

1. Impurities

a. Fibre anomalities

– Cotton: neps – Fibre fragments b. Fibre surface constituents – Cotton: wax – Wool: grease – Silk: silk bast – Bast and leaf fibres: parenchyma cells c. Plant- or animal-related material – Bast fibres: shives on fibre surface – Cotton e.g.: ◦ leaves ◦ stem fragments ◦ seed coat fragments ◦ seed oil – Coir: pod fragments – Plant fibres in general: non-fibrous plant abrasion particles – Animal fibres in general: excrements d. External material on fibre – Cotton: honeydew surface e. External impurities caused by – Mineral dust cultivation or breeding – Vegetable matter – Microbial contamination f. External impurities caused by – Other fibres later processing – Yarn – Fabric – Paper – Oil – Metal – etc.

2. Processing state

a. Bast fibres: degree of retting with influence, for example, on: colour, fineness, length, processability, etc. b. For example, bast and wool fibres: fibre damage c. For example, silk: thread separation

3. Processing behaviour

a. In fibre extraction

– Bast fibres e.g.: ◦ extractability/decorticatability ◦ separatability

b. In fibre processing

– Examples for textile production: ◦ cotton: cleanability ◦ cotton: sum parameters in simplified standardised simulation of process, e.g. test in Rotor-Ring device ◦ cotton: spinning limit ◦ bast fibres: spinnability ◦ bast: sliver evenness – Examples for production of fibre-reinforced plastics: ◦ bast fibres: abrasivity ◦ bast fibres: ability to trickle – Examples for production of insulation materials: ◦ loss in roller-carding – Fibre-reinforced composites: e.g. fibre–matrix interaction – Insulation materials: e.g. heat conductivity – Geotextiles: e.g. compostability

c. Influence of fibre property on product behaviour

1

Fibre is used in this table as a synonym for both fibre or fibre bundle.

Detail

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Figure 13.2

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Three different distributions, same mean value. Adapted from: Sokal & Rohlf, 1995. Copyright Palgrave Macmillan.

Figure 13.3 Position of the mean value (the arithmetic average of a set of values), the median (the median cuts the dataset in half) and the mode (the value that occurs most frequently in the dataset) in a left-skewed distribution curve.

Figure 13.4 Three different types of skewness. Adapted with permission from C.-D. Schonwiese, Praktische Statistik fur ¨ ¨ Meteorologen und Geowissenschaftler, 2. verbesserte Auflage. Copyright Borntraeger Gebrueder, 1992.

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Figure 13.5 Three different types of distribution (unimodal, bimodal and multimodal). Adapted with permission from C.-D. Schonwiese, Praktische Statistik fur ¨ ¨ Meteorologen und Geowissenschaftler, 2. verbesserte Auflage. Copyright Borntraeger Gebrueder, 1992.

– the arithmetic mean is more sensitive to outliers, is positioned on the right side of the curve and is higher than the median. r Left-skewed distribution: changed ordering compared with the right-skewed distribution. r Symmetrical distribution: mean, median and mode have the same value. Different kinds of probability distribution are possible when natural fibres and fibre mixtures are characterised (see Figure 13.5). A unimodal distribution is a probability distribution whose cumulative distribution function is convex up to the mode value for x < m and concave for values x > m. This kind of distribution can be found, for example, in the fineness distribution of cotton (Figure 13.14). A bimodal distribution is a continuous probability distribution with two different mode values. This kind of distribution is typical for mixtures of fibres, e.g. a mix of very fine bottom hair with much coarser guard hair of the cashmere goat (see SEM micrographs of cashmere in Figure 14.10 in Chapter 14). As can be seen in Figure 13.5, a multimodal distribution is a distribution that has several relative maxima. Figure 13.6 shows the distribution of hemp fibre bundles after mechanical and enzymatic separation. In the graph, the frequency ratio and the cumulative frequency are given. Box-and-whisker diagrams can be constructed, based on the values decile (0.90 and 0.10), quartile (first 0.25 and third 0.75) and median. The first quartile cuts off the lowest 25% of the data, the second quartile is equal to the median and the third quartile cuts off 75% of the data (shown as x0.75 ). The box-and-whisker diagram allows a fast and easy comparison of different fibres/fibre bundles with very different distributions (see Figure 13.7). It is not enough to compare the properties of two groups of fibres without testing to significant differences (probability of chance). To make statistical decisions using experimental fibre data, statistical hypothesis

Figure 13.6 Fibre width distribution – frequency ratio and cumulative frequency for hemp fibre bundles (mechanical and enzymatic separation).

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Figure 13.7 system.

277

Box-and-whisker plots of fibre width distribution of different fibres and fibre bundles measured with the Fibreshape

tests are a useful tool. We want to highlight the importance of statistical evaluation, and refer at this point to scientific literature, e.g. Lehmann and Romano (2005) or Sachs (2004). To evaluate the accuracy and the precision of the measured values, the following aspects should be kept in mind (see Figure 13.8). In general the precision indicates how close the measured values are to each other, or, in other words, how repeatable the results are. A precise fibre measuring device will give nearly the same result each time it is used. There are different ways to report the precision of measured values, for example: r range: the difference between the highest and the lowest value, often reported as a plus/minus deviation from the average; r standard deviation; r difference between two defined quantiles (like x0.9 decile minus x0.1 decile).

Figure 13.8 Left: trueness is composed of accuracy and precision; resolution of the measurement device. Right: distinction between accuracy and precision. Widely used schematic → no copyrigth needed, no citation required.

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According to Figure 13.8, accuracy indicates how close a measured value is to the accepted value (the bull’s eye on the target). Accuracy is the degree of closeness of a measured or calculated value to its true value. A fibre testing device is called accurate if it is capable of providing a correct measurement. A fibre measurement system is called valid if it is accurate as well as precise. Regarding the analysis and the interpretation of fibre measurements, it is important to realise that a perfect measurement is impossible. The goal must be to come as close as possible to the true value within the limitations of the instruments used. In this context it is important to distinguish between accuracy and precision. In Figure 13.8, four different variations are given. Four targets show the results of four different shooters trying to hit the centre of each target. According to Taylor (1999), experimental uncertainties that can be revealed by repeating the fibre measurement are called random errors. Uncertainties that cannot be revealed in this way are called systematic errors. An example can be given for the length measurement of fibres or bundles using tweezers combined with a ruler. A source for a random error can be the need to interpolate between scale markings. During interpolation, the tester is probably just as likely to overestimate as to underestimate the fibre length. The source for a systematic error can be the ruler itself. If the ruler is distorted and stretched, the tester always underestimates length values, and, if the ruler has shrunk, the tester always overestimates the fibre length (adapted from Taylor, 1999). The results illustrated in Figure 13.8 can be explained in terms of random and systematic errors as follows: 1. 2. 3. 4.

Precise & Accurate: a small systematic and a small random error. Precise, Not Accurate: a large systematic and a small random error. Accurate, Not Precise: a small systematic and a large random error. Neither Precise Nor Accurate: a large systematic and a large random error.

The concept of precision is strongly related to the concept of random error. The smaller the standard deviation (small fluctuations of values), the higher is the precision of the fibre testing device. Random errors are always present in measurements. They are caused by fluctuations in the readings of a measurement device or in the tester’s interpretation of the instrumental readings, such as the length scales. By examining the distribution of the measured fibre values, it is easy to assess the random errors but not the systematic errors (Taylor, 1999). Systematic uncertainties are usually hard to evaluate and even harder to detect. The researcher has to learn to anticipate the possible sources of systematic errors and to make sure that all systematic errors are much smaller than the required precision. A possible solution is to adjust the device to accepted standards and correct them or replace them with gauged standards (Taylor, 1999). Based on the knowledge that systematic errors are caused by: r imperfect calibration of the fibre testing instruments, or r imperfect methods of fibre examination, or r interference of the environment with the fibre and the measuring process, we will focus our explanation in this chapter on the suitability of the measurement method and proceed with the idea of systematic measurements in Chapters 3.1, 17 and 18.

13.4

Significance of Fibre Testing Methods

To evaluate which fibre characteristic testing methods and testing equipment are suitable for a special research issue or problem, it is necessary to specify what is required, and this must be dicussed prior to starting measurements. Some ideas follow, based on experience with cotton (Drieling, 2008), and are

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summarised as a list containing questions and examples with regard to the topic ‘Significance of Fibre Testing Methods’: r Does the requested information describe an intrinsic fibre property? r Does the requested information describe a fibre condition, e.g. processing state or behaviour? r Which fibre properties are important: – for the process? – for the product? r Which of the important fibre properties are measurable? Does the measurement show systematic differences between the different samples? r In which form are the fibre properties connected? r Which unmeasured fibre characteristics influence the test result (e.g. the influence of the fibre stiffness on the strength testing)? r Which origin-based characteristics impede the comparison of testing results from different samples or result in a bias? r How are the measured characteristics associated with the desired information? In this context it is important to know that a 1:1 transfer of results to the desired information is not always possible. 1. Creation of neps and maturity: ◦ desired information: are there problems to be expected in nep creation during processing? ◦ measured properties: typically fibre fineness and maturity. 2. Yarn fineness, fibre fineness and fibre length: ◦ desired information: how can the yarn fineness be increased? ◦ measured properties: typically fibre length and fineness. r What are the typical ranges of values? r What are the influences of fibre property variations on processing and product behaviour? How strong is the effect? r How many measurements will I need to take to get the required level of precision, accuracy and statistical significance, and is this within my time and/or (financial) budget for the project? r . . .? Inspired by this overview, one can create one’s own list for a special research issue or problem specification. Although many fibre properties can be tested, there are more aspects to be considered in fibre processing and product manufacturing. On the one hand there are still properties that cannot be tested, and on the other hand the processing behaviour can sometimes not be explained by a single tested value. For technical applications, for example composites, natural fibres should undergo systematic testing to determine key properties reliably. Apart from the intrinsic properties, the condition, for example the processing state or behaviour, should also be monitored. Some selected quality characteristics governing the use of natural fibres for composites are shown in Table 13.3.

13.5 Suitability of the Measurement Method In the course of history, different measuring systems and testing specifications for cotton, wool, bast fibres and industrially produced fibres have come into use in the textile industry. Each method has been developed to determine the properties of one kind of fibre. Thus, it cannot simply be applied to other fibres/fibre bundles. We have carried out an evaluation of the present measuring methods with regard to their usability for quality control in mass production, and have applied the following criteria: r objectiveness; r suitability for incoming control tests and/or numerical simulation of properties;

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Table 13.3 Key properties and their industrial importance. G. Cescutti and J. Mussig, Industrial Quality Management: Natural ¨ Fibres, Kunststoffe plast europe, 1, 1–4, 2005. Adapted with permission from Carl Hanser Verlag GmbH & Co., Germany Industrial relevance/ application example

Comment

Fineness and shape of the cross-section Length Strength, Young’s modulus, etc.

Processing, simulation, mechanical behaviour

Detailed information is given in Sections 13.5 and 13.6 and in Chapter 14

Colour

CIELAB1 values

Design application

Detailed information is given in Chapter 18.5.2

Chemical composition, accompanying substances

Fogging

Automotive industry

DIN 75201-G

Odour

Car interior applications

Detailed information is given in Section 13.6.5

Shive content

Mechanical behaviour, processing, design

Retting degree

Processing, mechanical behaviour

Possible methods are: (a) separation and gravimetric analysis, (b) image analysis and (c) NIR spectroscopy (Chapter 18.5.4) Possible methods are: (a) visual assessment against colour grade scale, (b) CIELAB measurement and (c) NIR spectroscopy (see Chapter 18.5.4)

Categories

Properties

Morphological properties

Mechanical behaviour

Harvesting, mechanical decortication and separation

1 CIE L* a* b* ; Commission Internationale d’Eclairage – CIE, Vienna, Austria (International Commission on Illumination); Lab colour space with dimension L* for lightness (black to white) and a* (green to magenta) and b* (blue to yellow).

r flexibility and comparability of the results; r economic aspects such as cost per measurement or cost of the testing device. Care was taken to recommend only flexible methods that are in widespread use. Table 13.4 gives an overview of the evaluated methods.

13.6

Recommended Methods

To reduce the varying effect of the environment with the fibres and with the measuring process, and to reduce possible systematic errors, all fibre trials have to take place in a standard climate at 20 ◦ C and 65% relative humidity according to DIN EN ISO 139 (2005) or similar standards like ASTM D-1776. With increasing production of natural fibre composites, the demand for adequate fibre testing methods evolved. As an orientation aid for industry in this sector, recommendations have been set up as part of the N-FibreBase project (www.n-fibrebase.net). These test recommendations are listed in Table 13.5. They were developed in close coordination with the working group on natural-fibre-reinforced polymers of the German Federation for Composite Materials (AVK-TV). Some more of the methods listed in Table 13.4 are used as standard testing methods in different fields. These are described in the corresponding chapters, for example, the further development of the airflow method to an ASTM standard for testing flax fineness in Chapter 18.5.3 and the HVI method in Chapter 17.10. As shown in Table 13.5, two methods are chosen for each fibre characteristic: r a reference method giving data suitable for numerical simulation; r a reliable method without expensive equipment. These methods are briefly described in the following sections.

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Table 13.4 Qualitative evaluation of testing methods for measuring physical and mechanical fibre properties (adapted from Mussig et al., 2006) ¨ Remarks, method suitable fora

Fibre property

Equipment (method)

Strength and Young’s modulus

Tensile tester (single-element test)

NF

Stelometer (collective test)

No extension/ force diagram No Young’s modulus, NF NF

Dia-stron, Textechno Favimat (single-element test) HVI (collective test) Fineness

Scanner/Fibreshape (single-element test) Laboratory balance (collective test) OFDA/Laserscan (single-element test)

Airflow

Length

Almeter Tweezers, scale (single-element test) Imaging system, software (single-element test)

a b

Equipment Measurement Usual Measurement of costb costb method?b distribution −

◦

++

Yes

++

++

+

No

−

◦

−

Yes

–

++

++

No

Quick analysis, not yet widespread, NF High time demand, NF

++

+

−

Yes

++

−

++

No

Quick analysis, good reproducibility. Problems with thick fibres/fibre bundles, Wo (B) Indirect measurement (results not fully comparable), NF

−

+

◦

Yes

++

++

++

No

−

++

−

Yes

++

–

+

Yes

◦

−

◦

Yes

Co only

Maximum length 250 mm, Co, Wo, (NF) Extremely high time demand, NF High time demand, not yet fully automatic, NF

Abbreviations: Co = cotton fibres, B = bast fibres, Wo = wool fibres, NF = all kinds of natural fibre. Evaluation scheme (–, -, ◦, +, ++).

13.6.1 13.6.1.1

Fibre Fineness Gravimetric Fineness Measurement

Parallelised fibres or bundles are cut to a length of, for example, 20 mm (depending on the type of fibre) and separated by using tweezers. A minimum of 500 fibres or bundles are counted and weighed as one sample. The gravimetric fineness gF (in tex) is calculated by the following equation: gF =

M × 10−3 × 106 N B · l¯B
  =1000

where NB is the number of counted fibres, M is the mass of collected fibres (mg) and lB is the length of the cut fibres (mm) (here 20 mm). When using fibre bundles as a sample, it is important not to separate them during combing. For details of preparation, see DIN EN ISO 1973 (1995).

1

1

1

Length

Strength

Collective tensile test/Stelometer

Strength (cN/tex) (mean value, standard deviation), elongation (%), Young’s modulus (cN/tex), conversion to N/mm2 possible Strength (cN/tex) (maximum value, standard deviation), elongation (%)

Length (mm) (distribution)

Manual test process (tweezer process)

Single-element test

Length (mm) (cross-section related distribution)

Gravimetric measurements (dtex) (mean value)

Gravimetric fineness/balance

ISO 3060 (1974)

ISO 5079 (1995)

Wool testing standard IWTO 17-04 (2004), cotton (manufacturer’s data), bast (internal test specification) DIN 53808 (2003)

Internal test method (defined measurement mask, etc.) DIN EN ISO 1973 (1995)

Clamping material, clamping length, mass of tested collective, number index

Clamping length, test speed, test structure, material used for clamping material

Number of elements, operator’s influence

Cross-section-related, preparation (A- beard), number of measurements

Staple length, number of measured fibres

Resolution (dpi), number of measured fibres

Test information

X

X

X

X

X

X

X

X

Data suitable for numerical Reception simulation control

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Capacitive measurement/Almeter

Width (µm) (complete distribution)

Scanner/Fibreshape

Measurements

Type of standard or instructions for testing

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Property

Method Equipment alternative (method)

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283

Fibre Width Distribution with Fibreshape

An image analysis system called Fibreshape has been developed to describe, for example, the quality of the separation process of bast fibre bundles (M¨ussig and Schmid, 2004). Measurement is done by preparing snippets (ca 5 mm in length) in glass slides (type 69 01; Gepe, Zug, Switzerland). The slides are scanned in a high-resolution slide scanner (e.g. 4800 dpi). The images are analysed by the image analysis software Fibreshape (IST AG, Vilters, Switzerland). The scanner needs to be calibrated by using the USAF-1951 test target to check the real resolution. In addition, the Fibreshape system can be calibrated using, for example, the IWTO wool standards (see Chapter 3.1) or the IFS flax standards (see Chapter 18).

13.6.2

Fibre Length

13.6.2.1

Length Measurement Using Tweezers

With this method, the length of the whole fibre or fibre bundle is measured in the straightened state (DIN 53808-1 2003). The set-up is displayed in Figure 13.9. The fibre is softly drawn using tweezers with low force to avoid elongation. When the end of the fibre reaches the start point, the length can be read from the length class line. The distribution can be calculated from the number of fibres in each length class (Figure 13.9).

13.6.2.2

Length Measurement Using an Almeter

The Almeter was developed to measure the length distribution of wool fibres. Detailed information can be found in Grignet (1981) and IWTO 17-04 (2004). Samples have to be prepared in a ‘Fibroliner’ machine to end-align the fibres (all of them arranged with one end at the same position). For bast fibre bundles like hemp, the Fibroliner has to be adapted. The number of needles per comb must be reduced from 153 for wool to 75 for bast. As shown in Figure 13.10, the Almeter consists of two parts: a device for measuring the local sum of the cross-sectional areas of the fibre specimen and a unit to calculate the length distribution. The measuring device consists of a rectangular plate condenser, which allows a detailed scanning of the local sum of the cross-sectional areas in the longitudinal direction. The test specimen is drawn with constant speed between the electrodes. For bast fibre or bundles, we propose three measurements with approximately 0.6 g of sample per test. The test specimens have to be prepared carefully to avoid shortening by the Fibroliner. After one passage, the so-called ‘A-beard’ is transferred into the sample slide of the Almeter and measured. Only the

Figure 13.9

Length measurement using tweezers; fibres separated into length classes.

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Figure 13.10

Schematic representation of the Almeter system.

cross-section-related length distribution is valid, for example for bast fibres and bundles. The number-related length distribution is calculated on the basis of the assumption that all fibres have the same diameter. 13.6.2.3

Length Measurement Using Fibreshape

An adapted set-up has been developed for the image analysis system Fibreshape to measure the length distribution of short fibres or fibre bundles in a range of a few millimetres (e.g. for injection moulding). Measurement of the length can be done in the same way as described in Section 13.6.1.2. Further details can be found in Bos et al. (2006) and Cescutti et al. (2006). 13.6.3

Fibre Strength

Tests for the tensile strength of hemp fibre and bundles are not standardised, and in the literature very different preparations and methods are used. A comparison of the values found in the literature is hard to undertake and sometimes not possible because very different types of fibre arrangement can be used for testing (Figure 13.11). A detailed discussion about the influences of sample type is given in Section 13.7.3.4.

Figure 13.11 Various forms of hemp fibres/fibre bundles (Mussig, 2001). The illustrations show the difference between a ¨ single element (e.g. a single fibre or a single fibre bundle) and a collective (e.g. a collective of single fibres or a collective of single fibre bundles) (single bundle scheme adapted from Herzog, 1926). J. Mussig, Untersuchung der Eignung heimischer ¨ Pflanzenfasern fur Duroplasten – vom Anbau zum Verbundwerkstoff –, VDI Verlag ¨ die Herstellung von naturfaserverstarkten ¨ GmbH, 2001.

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285

Collective Strength/Stelometer

For the collective test by Stelometer, the samples have to be clamped in a Pressley clamp coated with plexiglass at a gauge length of 3.2 mm. The Stelometer is adjusted according to ASTM D 1445 (2008). To obtain a representative set of results, we propose testing more than 20 collectives for bast fibre bundles. The strength of fibre and fibre bundle collectives (in cN/tex) can be calculated from the mass of the bundle collective tested (in kg) divided by its mass-related fineness (in tex). As described in Section 13.7.3.4, the Stelometer results are only comparable if the samples have a similar number of tested elements in the collective. 13.6.3.2

Single-Element Strength/Dia-stron

Testing the strength of single fibres in general is described in ISO 5079 (1995). According to Nechwatal et al. (2003), the single-element test is important in determining the tensile properties of fibres. Difficulties for such tests are: r r r r

the influence of the clamping mechanism and of fibre slip in the clamp; various fibre gauge lengths and taking this influence into account (see Section 13.7.3.3); the determination of the fibre or fibre bundle cross-section surfaces; the calculation of the fibre modulus.

In order to solve the problems mentioned above, different approaches have been taken (e.g. an adapted universal tensile tester, Favigraph (Textechno, M¨onchengladbach, Germany), etc.). The problem of fibre slip can best be solved by gluing the elements into the clamping system. This approach was realised in the Dia-stron system (Dia-stron Ltd, Andover, UK). The cross-sectional surface area of each element is measured by means of a laser beam. The sample is then automatically transferred to the tensile testing system, and evaluation is based on the cross-section of the specimen actually tested. The software allows the determination both of tensile strength as a function of the fibre and fibre bundle cross-sections (in MPa) and of fineness-related values (in cN/tex), as well as further extensive evaluation of the data. 13.6.4

Fibre Density

For measuring the specific density of cellulosic fibres, we propose the flotation method. The dried sample (3 h at 105 ◦ C) is divided into three subsamples, and each of them is immersed in tetrachloromethane (CCl4 ), taking care that no air bubbles remain on the sample surface. Owing to the high density of the liquid (1.59 g/cm3 ), the sample stays at the surface. In the next step, the sample is transferred into a mixture of 90% CCl4 /10% xylene (C8 H10 ) (density 1.52 g/cm3 ). If it stays at the surface, it is again transferred into a mixture of 80% CCl4 /20% xylene (density 1.45 g/cm3 ), and so on until the density of the liquid is low enough to allow the sample to sink. When the sample is at the bottom, the density of the liquid is increased by adding CCl4 drop by drop until the sample is able to float in the middle of the beaker. In this state, the liquid and the sample have identical densities, and the density of the CCl4 /xylene mixture can be easily measured by means of a pycnometer with an accuracy of ±0.005 g/cm3 . It has to be mentioned that the result is the apparent density of the sample, i.e. the density of the specimen including closed cavities (e.g. lumen in fibre cells). The true density of the material may be significantly higher. 13.6.5

Determination of the Odour of Natural Fibres

The comparatively new standard DIN EN 13725 (2006) details the evaluation of odour intensity by a panel of test persons using an olfactometer. If hedonic criteria shall be assessed, this should be performed following

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VDI 3882 (1994). Owing to the short time since the publication of DIN EN 13725, there are markedly different traditional methods already established in different industrial sectors. Interior parts for cars in Germany, for example, are analysed according to the VDA 270 (1992) standard test method using an inconsistent scale for the evaluation of the samples, starting with objective criteria for low-odour samples and changing to hedonic criteria for high-odour samples. An improved version of the VDA 270 method using a consistent ‘intensity only’ scale has been proven to give comparable results with DIN EN 13725. As the human nose has exponential detection characteristics, there was a linear fit in the logarithmic plot of odour intensity (DIN EN 13725) versus intensity note (improved VDA 270) (Fischer and Lohmeyer, 2009).

13.7

Properties of Natural Fibres

In the following sections, the properties of natural fibres are discussed, based on typical examples. Attention is focused on the methods described in the previous section. For each property and proposed method, the range and possible distributions are displayed. This is followed by a fibre tables section, which gives a detailed overview of literature-based fibre characteristics. We used the following fibres or fibre bundles for evaluation of the testing methods that are described in this chapter: r Cotton US Pima: the cotton fibres (Gossypium barbadense L., species: US Pima) were provided by Faserinstitut Bremen eV., Bremen, Germany (FIBRE, 1994). r Coir mattress: the coir fibre bundles were obtained from Hayleys Exports Ltd, Ekala, Ja Ela, Sri Lanka, in form of a miniature bale of 500 g of mattress coir in 2001. r Flax B: the flax fibre bundles (Linum usitatissimum L.) (HL 04 01b) were provided by the company Holstein Flachs GmbH, Mielsdorf, Germany, in 2004. The flax was grown near Mielsdorf in 2003. After harvesting, the stems were field retted and baled. The fibre bundles were separated from the very homogeneously retted long flax during the scutching process in the separation plant at Holstein Flachs GmbH. r Hemp GDE02 and KGE02: raw hemp fibre samples (variety Fedora) obtained from NAFGO GmbH in D¨otlingen-Neerstedt (Oldenburg region, Germany). The lots GDE02 and KGE02 were grown, field retted and harvested in 2001, and coarse separated in 2002. The stems were coarse separated by NAFGO GmbH R line with four drums (Demaitre B.V., Belgium). using a DEMTEC
r Hemp samples RS, GA, MA, FA, GAD and GADO: for this trial hemp (Cannabis sativa L.) variety Felina 34 was used. The hemp had been grown in Klagenfurt, Austria. A detailed description of the harvesting process of the used hemp is given in M¨ussig (2001). The peeled bast of the thin stems was labelled as RS. Coarsely separated (GA), medium (MA) and finely separated (FA) fibre bundles were produced. After the coarse separator, a part of the material was removed from the line to be separated in a steam explosion process (GAD) and afterwards opened with an opener (GADO). r Jute v.S: the jute fibre bundles (Corchorus olitorius L.) (Schilgen) without any avivages on the surface were provided by NAFGO GmbH, Neerstedt, Germany, in 2001. r Sisal Or.: the sisal fibre bundles (Agave sisalana Perr. et Engelm.) were provided by NAFGO GmbH, Neerstedt, Germany, in 2001.

13.7.1

Fineness

The fineness of natural fibres ranges widely from ultrafine to ultracoarse. This is represented by an ultrafine microcellulose in the nanometre range at one extreme and an ultracoarse fibre, like horsehair, in nearly millimetre scale at the other. Figure 13.12 gives an overview of the whole range and the terminology of fibre or fibre bundle fineness.

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Figure 13.12 Determination of fibre (coir: bundle) fineness. Adapted from G. Schnegelsberg, Handbuch der Faser – Theorie und Systematik der Faser. Copyright Deutscher Fachverlag GmbH, 1999.

Typical mean fineness values measured with the Fibreshape system for fibres discussed in this book are presented in Figure 13.13. Besides the Fibreshape values, the calculation of gravimetric fineness (in tex), based on fibre width values, is displayed in the same diagram. Most of the fibres and fibre bundles described here are also presented in Chapter 14. Figure 13.14 displays the range of fibre and fibre bundle width distributions, starting with cotton (narrow and nearly normal distribution), moving on to flax and hemp (broader and left-skewed distribution) and ending with coir (extreme broad distribution). It also demonstrates the large measurement range of the Fibreshape system. So far we have given data on the intrinsic fibre fineness of typical raw materials. Bearing in mind subsequent fibre processing, the processing stage has a great influence on fineness. This was discussed in depth in Chapter 4. For example, coarse bast fibre bundles like hemp (see Figures 13.13 and 13.14, sample hemp GDE02) can be separated into smaller bundles by different techniques. Figure 13.15 gives an overview of how coarse hemp fibre bundles (RS) can be refined to very fine bundles and single fibres (GADO). These values were recorded with a relatively small experimental set-up (see Section 13.6.1.1) apart from the processing procedures. Results are given as gravimetric fineness in tex.

13.7.2

Length

The two recommended methods discussed in Section 13.6.2 give virtually identical length distribution (histograms). For this reason, we display here only results of the tweezers method. As for fibre fineness, there is a broad range of length distributions, as shown in Figure 13.16. The hemp, sisal and jute lots presented here are fibre bundles for needle felt production. For this reason, the lots were shortened during preceding processing to be suitable for carding. The resulting length distribution is thus a result of processing and not an intrinsic fibre property. Of the fibres compared above, cotton as a seed hair has the shortest fibre length and an even distribution. Hemp, sisal and jute show a much broader and left-skewed distribution and a fibre bundle length value

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Figure 13.13 Fibreshape width values for a wide range of natural fibres – from nanocellulose to horsehair; calculation of gravimetric fineness (in tex) based on fibre width values.3 More information on the used fibres and fibre bundles can be found in Chapter 14 and in the list at the beginning of Section 13.7.

higher than that of cotton, but much shorter compared with the possible bundle length values found in the plants. 13.7.3

Strength

To display the differences in the structure of the materials, results of single-element force obtained by Diastron measurements are given in Figure 13.17 for cotton, wool, flax and coir. We present the raw data here, resulting in very different direct force values caused by the extremely different cross-sections. Taking the cross-sections into account, the four fibre types show the following strength ranking: flax > cotton > coir > wool. Flax, with a low microfibril angle (MFA) in the S2 layer (see Chapter 2.2), shows an almost linear elastic behaviour. In contrast to flax, cotton and coir initially show a nearly linear elastic behaviour, followed by a second phase with less increase in force, resulting in a much higher total elongation at break. For wool the force–elongation diagram displays three phases. According to Wortmann and Zahn (1994), these three phases are better distinguished by measurement in water and can be related to ‘Hooke’s region’ up 3

The formula is valid for circular cross-sections. Values are given only to highlight the wide fineness range.

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Figure 13.14 Fibreshape width values for a wide range of natural fibres/fibre bundles – from cotton to coir. Cotton US Pima, hemp GDE02, flax B and coir mattress.

to 2% elongation, followed by a ‘yield region’ up to 20–30% elongation and a ‘post-yield region’ up to fibre break at 60–70% elongation. The slope of the curve in these regions is in a ratio of about 100:1:10 (see Chapter 12). The three phases of keratin-type fibres are better distinguished in the wet state, so it can be recommended that tests on such fibres are done in the wet state. This is to simplify humidity control, as well as the fact that differences between fibre groups are always much larger in the wet state. Testing hair at 65% RH will always yield less sensitive results than at 100% RH. Following the remarks made in Section 13.6.3, it is essential to bear in mind four important factors for the interpretation of fibre strength measurements: (i) the testing speed, (ii) the diameter, (iii) the clamping length (gauge length) and (iv) the number of tested elements. All of them can influence the results strongly. Thus, they are discussed separately in the following sections.

Figure 13.15 Gravimetric fineness measurements for hemp fibre bundles (Mussig, 2001); RS: bundle fineness after decortica¨ tion; GA: after mechanical coarse separation; MA: after mechanical medium separation; FA: after mechanical fine separation; GAD: after steam explosion; GADO: after steam explosion and an additional separation by carding. J. Mussig, Untersuchung ¨ der Eignung heimischer Pflanzenfasern fur Duroplasten – vom Anbau zum Verbundw¨ die Herstellung von naturfaserverstarkten ¨ erkstoff –, VDI Verlag GmbH, 2001.

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Figure 13.16 Results of fibre and fibre bundle length measurement with the ‘tweezers method’. Cotton US Pima, hemp GDE02, sisal Or. and jute v.S.

13.7.3.1

Influence of Testing Speed

The interference of the environment on a polymer during a tensile test or the testing conditions themselves influence dramatically the deformation behaviour of a polymer, because the relaxation and retardation behaviour of a polymer are strongly influenced by temperature and strain rate. The stress–strain curves of a ductile thermoplastic polymer tested at various strain rates are schematically given in Figure 13.18 (left). With

Figure 13.17 Results of fibre and fibre bundle strength tests with the Dia-stron system. Force–elongation curves of tested single elements (single fibre for wool and cotton; single fibre bundle for flax and coir); the free clamping length was 3.2 mm and the test speed was 2 mm/min. Cotton US Pima, wool, coir mattress, flax B.

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Figure 13.18 Left: schematic stress–strain curves of a thermoplastic polymer as a function of the test speed in the tensile test. W. Grellmann and S. Seidler, Kunststoffprufung, 2005. Reproduced with permission from Carl Hanser Verlag GmbH & ¨ Co., Germany. Right: stress–strain curves of a 250 µm diameter coir fibre bundle tested at various strain rates. Modified with permission from A.G. Kulkarni et al, Mechanical behaviour of coir fibres under tensile load, Journal of Materials Science, 16, 905–914. Copyright 1981, Springer Science+Business Media; the scheme of a viscoelastic model (Maxwell model) is given; the crystalline region is represented by the spring in the model.

increasing testing speed (or with decreasing temperature), the tensile strength increases and the elongation at break decreases. It is obvious that the shape of the stress–strain curve changes noticeably (Grellmann and Seidler, 2005). This underlines the importance of the stress relaxation mechanism, which can be measured as a unique property of a fibre or a fibre bundle. Relaxation mechanisms are also important in bend and torsion measurements. The influence of strain rate variation on the stress–strain behaviour of natural fibres was observed on sisal (Agave sisalana P.), coir (Cocos nucifera L.) and musa (Musa sapientum L.) fibre bundles. Figure 13.18 (right) shows for coir (Cocos nucifera L.) that the stress–strain curve is strain-rate dependent within the strain rates used. At a specified stress value, higher strain values are observed as the strain rate is decreased. This indicates the viscoelastic nature of the coir fibre bundle (Kulkarni et al., 1981). Mukherjee and Satyanarayana (1984) investigated the behaviour of sisal (Agave sisalana P.) in a tensile test at different testing speeds. The stress–strain curve for single sisal fibre bundles (having both crystalline and amorphous components) is characterised by an initial linear region followed by a non-linear curve. The observed variation in the strength values at testing speeds of 1, 2, 10 and 50 mm/min is explained from the viscoelastic model, which shows that, for a high strain rate, the fibre behaves more like an elastic body. This means that the crystalline region mainly shares the applied load, which results in higher strength values. When the strain rate decreases, the load will be shared increasingly by the amorphous region, which will result in lower strength values. According to Mukherjee and Satyanarayana (1984), for very low strain rates the fibre behaves like a viscous liquid and the major portion of the applied load is shared by the amorphous

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regions, which results in low modulus and strength values. No significant differences were observed between strain rate and elongation at break. Kulkarni et al. (1983) observed the behaviour of musa (Musa sapientum L.) fibre bundles using tensile tests at different strain rates. The breaking strength of the bundles increases as the speed of testing increases from 0.5 to 100 mm/min. For higher testing speeds, the strength showed a decrease. There was little influence of strain rate on elongation at break.

13.7.3.2

Influence of Fibre Diameter

As described by Katz (1999), in a weakest-link model for brittle materials, a microscopic failure causes the failure of a macroscopic specimen, ‘just as a chain fails if one link fails’. In the early 1920s, Griffith described the influence of small cracks reducing the strength of testpieces. Griffith pointed out the relation between diameter and strength for glass fibres. In the experiments, glass fibre of various diameters were tested, with the result that the strength increased with diminishing diameter (Griffith, 1921, Chapter 6, The strength of thin fibres). Finer glass fibres were considerably stronger than coarser fibres. In a weakest-link model, this can be explained by the presence of more flaws in the coarser fibres (Katz, 1999). A statistical evaluation of the theories of dependence of the strength of specimens on their volume or length can be found in Epstein (1948). The general relation between diameter and strength for brittle fibres is given in Figure 13.19. The relationship between diameter and mechanical properties (strength and Young’s modulus) for flax, jute, abac´a and sisal is described by Peponi et al. (2008). They present an advanced statistical approach to evaluating the dimensional and mechanical properties of natural fibres. Their new statistical approach interpolates experimental data well and correlates geometrical properties with mechanical properties for the observed natural fibres and fibre bundles. For sisal (Agave sisalana P.), Mukherjee and Satyanarayana (1984) observed that the mechanical parameters do not show any appreciable change with increase in diameter from 100 to 300 µm. Kulkarni et al. (1983) observed the behaviour of musa (Musa sapientum L.) and found no appreciable change in the mechanical properties of the fibres with an increase in the diameter in the range investigated (50–250 µm). Kulkarni et al. (1981) report that the strength of coir (Cocos nucifera L.) seems to increase up to a diameter of 200 µm, after which the properties remain almost constant. On the other hand, the initial modulus seems gradually to decrease with an increase in the diameter of the fibres in the entire investigated range between 100 µm and 450 µm.

Figure 13.19 Fibre strength plots against fibre diameter of ceramic fibres. Adapted with permission from D. Koch, Fibre strength plots against fibre diameter, Ceramics Institute, University of Bremen, 2008.

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Xia et al. (2009) observed a decrease in the strength of single jute fibre bundles (Corchorus olitorius) as the diameter variation increased. M¨ussig et al. (2006) examined the dependence of the strength of natural fibre specimens – cotton (Gossypium barbadense), ramie (Boehmeria nivea H. et A.), hemp (Cannabis sativa L.) and sisal (Agave sisalana P.) – in a single-element test on their cross-sectional area, and saw the same trend as Peponi et al. (2008) of increasing strength with diminishing diameter. While this trend is obvious for cotton, ramie and hemp, the sisal seems to be more independent of bundle width in the measured range.

13.7.3.3

Influence of Clamping Length

The influence of the clamping length on tensile strength needs to be discussed with the phenomenon of the weakest link. A broadly used statistical tool to describe the variability in tensile strength in, for example, ceramic brittle materials is the Weibull distribution. Weibull (1951) discussed in his paper, among other things, the statistical evaluation of strength data tested on Indian cotton. As described by Nechwatal et al. (2003), for instance, the dependence of the tensile strength on the gauge or clamping length is a well-known effect for different kinds of fibre. The longer the clamping distance of a tested element (single fibre or single fibre bundle), the more voids or flaws there are in the stressed fibre segment; this weakens the structure. With more flaws, automatically the probability of major defects increases. Thus, the strength decreases with increasing clamping length. This effect is well known for glass fibres (see, for example, Pardini et al. (2002) or Andersons et al. (2002)). The influence of gauge length variation on the stress–strain behaviour of natural fibres and bundles tested as a single element has been observed, for example, for: r r r r

coir (Cocos nucifera L.) by Kulkarni et al. (1981); musa (Musa sapientum L.) by Kulkarni et al. (1983); sisal (Agave sisalana P.) by e.g. Mukherjee and Satyanarayana (1984) and Nechwatal et al. (2003); flax (Linum usitatissimum L.) by Bos et al. (2002), Nechwatal et al. (2003), Andersons et al. (2005) and Peponi et al. (2008); r hemp (Cannabis sativa L.) by Nechwatal et al. (2003); r jute (Corchorus olitorius) by Xia et al. (2009); r cotton (Gossypium) by Xia et al. (2009). Kulkarni et al. (1981) observed coir at various gauge lengths and identified that the strength and percentage strain at fracture decrease with an increase in the gauge length. They attributed the decrease in strength to the fact that the probability of defects and weak links increases with the length of the single coir bundles. Mechanical tests of elementary flax fibres by Andersons et al. (2005) revealed that fibre strength is reasonably well approximated by the two-parameter Weibull distribution (see, for example, Joffe et al. (2003) or Bos et al. (2002)). Peponi et al. (2008) demonstrated the importance of the gauge length on the tensile strength for flax, which is composed of microfibrils, and the greater presence of flaws or voids in a longer fibre/fibre bundle. Their results show that a flax specimen with a gauge length of 5 mm is much stronger than one with a gauge length of 30 mm. In this context it is important to complete a detailed analysis of single fibres, fibre bundles and fracture mechanism as standard pratice (see, for example, Bos et al. (2002)). Bos et al. (2002) pointed out that the strength of single flax bundles is constant – about 500 MPa – from 100 mm down to a gauge length of 25 mm. For clamping lengths lower than 25 mm, the strength increases towards values of ca. 850 MPa at a gauge length of 3 mm. In their discussion it is suggested that it is unlikely that the relationship between strength and clamping length is governed only by the decreasing chance of the presence of critical voids or flaws. Fibre bundles are composed of shorter single fibres. At large gauge lengths, bundle failure takes place through the relatively weak middle lamellae that bond the single fibres

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Figure 13.20 Dependence of fibre strength on clamping length. Adapted from Composites Science and Technology, 63, A. Nechwatal, K.-P. Mieck and T. Reußmann, Developments in the characterization of natural fibre properties and in the use of natural fibres for composites, 1273–1279. Copyright 2003, with permission from Elsevier.

together. Bos et al. pointed out that the pectin interphase is oriented mainly in the longitudinal direction of the fibre bundle and fails by shear stress almost instantaneously. At shorter gauge lengths, a change in failure mechanism takes place and causes an increase in tensile strength of the single fibre bundle. If the gauge length decreases to values below the length of the single fibre, the pectin interphase is no longer the location of failure – the influence of critical flaws is reduced. At clamping lengths shorter than the single cell, the crack must run through the cell wall of the single fibres. The fact that the increase in tensile strength starts below a clamping length of 25 mm supports the concept of failure as described by Bos et al. (2002): single flax fibres show length values between 20 and 50 mm, with a mean value around 30 mm. Xia et al. (2009) examined the breaking strength of jute (single fibre bundles) and found that it was less sensitive to gauge length (5–20 mm) than cotton fibre (single fibre) because the breaking of jute bundles involves ultimate cells breaking repeatedly and matrix cracking. As can be seen in Figure 13.20, the strength values of tested single elements for hemp, flax and sisal are influenced by the clamping length. Going to the extreme (clamping length 0), the result changes from tenacity of the flaws to tenacity of the material structure. The influence of gauge length variation on the stress–strain behaviour of natural fibres tested as a collective has been observed, for example, by Kohler and Wedler (1996) for flax (different separation grades) and glass. While the glass collective shows only a small decrease in strength with increasing gauge length from 0 to 20 mm, the strength of the physicochemically separated flax decreased from over 50 cN/tex to below 20 cN/tex. Martens and M¨ussig (1999) compared glass fibres in a single-element test with a collective test. While the single glass fibre test shows a decrease in strength with an increase in gauge length from 0 to 20 mm, the decrease in strength for the glass collective was negligible. The collective test of hemp bundles shows a decrease in strength with increasing clamping length from 0 to 10 mm.

13.7.3.4

Single-Element Test versus Collective Test

According to B¨aumer et al. (1996), the typical specifications for tensile tests on single elements or on collectives are different (see Figure 13.21). In brief, some important points to be considered are:

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Single-element test: r determination of material constants (e.g. strength, elongation, Young’s modulus); r the mean single-element strength is higher than the mean collective strength; r often applied to man-made fibres; r cross-sectional and fineness-related values (MPa and cN/tex); r inspection effort is high, and the number of samples to be tested is large on account of the scattering values; r an automation of the testing procedure is complex, but possible (see Section 13.6.3.2). Collective test: r The characteristic value of this test is affected by the single-element strength but is significantly influenced by the single-element elongation and its distribution, as well as by the crimp and the crimp distribution of the fibres/fibre bundles after clamping (or their slack). r The collective strength is lower than the single-element strength. r The collective strength is strongly influenced by the number of elements in the tested collective. r Indication of value as cN/tex. r The test procedure is much faster than a single-element test. r Fewer measurements are necessary to access statistically reliable results. r The mechanical properties of the collective has a great influence on the strength values. r The results are strongly influenced by the method of collective preparation and the collective thickness and width, as well as by the clamping conditions, which are much more complicated compared with the single-element test. r A standardised preparation method is extremely important, and specialised staff are essential. Testing the strength of natural fibres, e.g. cotton, is not trivial. In principle, cotton can be tested for strength and elongation in a single-element test or in a collective test. The major difference between the single-element test and the collective test can be explained by means of the concept given in Figure 13.21. As can be seen in Figure 13.21a, the results from a single-element test show a scatterplot, and the values for elongation and strength of the tested fibres differ noticeably. Based on the values of the single-element test, a collective curve is mathematically constructible. As the work from Harig et al. (1994) shows, fundamental differences between the sums of the force values of the single fibres and the maximum value of the collective force appear. According to Figure 13.21b, only 20% of all fibres are broken at the maximum force point of the collective at 8% elongation. This can be proven, for example, with an acoustic emission analysis during collective fibre strength tests (Drieling, 2002). Fibres with breaking elongation values higher than 8% can only contribute to the maximum force to a limited extent. The maximum force of the collective with a value

Figure 13.21 Force–elongation curve of a cotton collective based on a single-element strength test. Adapted from H. Harig et al, Wie zuverlassig laßt von Rohbaumwolle bestimmen, Melliand Textilberichte, 12, 966–970. ¨ ¨ sich die Bundelfestigkeit ¨ Copyright Deutscher Fachverlag GmbH, 1994.

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Figure 13.22 Collective efficiency. Adapted from M.W. Suh, X. Cui and P.E. Sasser, New Understanding on HVI Tensile Data based on Mantis Single Fiber Test Results, Beltwide Cotton Conferences, Proceedings Vol. 3, 1400–1403, 1994.

of 120 cN is remarkably lower than the sum of the single element forces with a value of 200 cN. Harig et al. (1994) pointed out that the distribution of the elongation values of single elements has an extreme influence on the strength of a collective. Assuming that all elements in a collective have exactly the same elongation at break, and that the preparation of the collective is perfect, as no inequalities are present, the strength of the collective would reach values close to the mean value of the single-element strength. This effect was reported by Suh et al. (1994) for cotton. The authors present the influence of the amount of elements in a collective on the collective strength. While the collective force increases with an increasing number of single elements, the collective efficiency (collective force divided by the sum of the forces of the single elements that form the collective) decreases (Suh et al., 1994). Even more important than the breaking elongation of the single fibres is the present crimp of the fibres or their given slack. Fibres without crimp add to the force from the beginning onwards, whereas fibres with high crimp do not add to the force before the collective elongation is higher than their slack. This can be proven by shifting the single-fibre force–elongation curves of the participating fibres in a simulated collective to different starting elongations. With an increasing difference in fibre slack, the maximum force of the collective decreases (Drieling, 2002). The collective efficiency as a function of the amount of elements in a collective is given in Figure 13.22. Testing a single element, the collective efficiency corresponds approximately to a value of 1. The results illustrated in Figure 13.22 show (i) that the collective efficiency drops rapidly for the first 50 elements in a collective, reaching an almost constant value for a number of elements between 50 and 300. This socalled collective effect is mainly caused by different breaking elongations of the single fibres. The collective efficiency is (ii) strongly influenced by the kind of collective preparation (homogeneously or inhomogeneously strained) – this is mainly based on different slacks or pre-tensions of the fibres. Attention should be paid to the collective efficiency not only for cotton. It must be pointed out that two collective strength values from, for example, two different flax samples are only comparable if the amount of elements (fibres or bundles) in the collective is the same (M¨ussig, 2001). We can thus confirm that it is extremely important to distinguish between a single-element and a collective test, as they result in different values. Bobeth and Martin (1961) are critical of the fact that, especially in test result summaries, information about the tested entity is often missing.

13.7.3.5

Fibre Density

Different types of cellulosic fibre typically have densities in the region 1.40–1.55 g/cm3 . Synthetic fibres like polyethylene or polypropylene have densities below 1 g/cm3 , while the densities of glass fibres and alumina

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Figure 13.23 Apparent density of different natural fibres using the flotation method compared with typical man-made fibres. Sisal Or., jute v.S., flax B, hemp GDE02, cotton US Pima, ramie: own results – detailed information can be found in Chapter 14 and in the list at the beginning of Section 13.7; other fibre data from Bobeth (1993).

are around 2.6 g/cm3 . Steel has a much higher density of ca. 8 g/cm3 . Figure 13.23 gives a brief overview of the materials mentioned here; more detailed information can be found in Section 13.8. The true density of fibres depends more on the structure than on the chemical composition. Bobeth (1993) has pointed out that various factors, such as the presence of closed cavities (lumen), affect the measurement. Typical examples with large differences between true and apparent densities are given in Figure 13.24. The SEM picture of kapok cross-sections demonstrates the large share of lumen space causing the very low apparent density. 13.7.3.6

Odour

Most natural fibres typically have an odour of low intensity. Problems with malodorant (natural) fibre lots occur from time to time in industrial production. These are mostly caused either by mildew contamination or by overheating during fibre processing (Fischer et al., 2008). When fibres are processed at higher temperatures, it is not the cellulose itself that is the most sensitive compound, but actually accompanying substances, for example pectins. Thus, the removal of pectins by enzymatic treatment was found to result in higher thermal stability of hemp fibre bundles (Fischer et al., 2004); as shown in Figure 13.25, the odour of hemp treated

Figure 13.24 Left: comparison of true and apparent density of selected fibres and coir fibre bundles (values based on Bobeth, 1993, p. 171). Right: SEM micrograph of a kapok fibre cross-section with the typical hollow structure.

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Figure 13.25 Influence of enzymatic treatment on the odour behaviour of hemp GDE02 – detailed information can be found in the list at the beginning of Section 13.7. H. Fischer et al. 2004. Adapted with permission from AVK.

enzymatically is much lower than the odour of raw hemp which contains more pectins. At higher temperatures (200 ◦ C), decomposition of the cellulose begins, resulting in nearly identical odour intensity for both variants. In addition, enzymatic treatment has been shown to reduce the odour emission of mildew-contaminated hemp lots to that of normal raw fibre bundles (Fischer et al., 2008). Another important factor is the specific surface of the fibres/fibre bundles: odour in general is the emission of gaseous substances from a sample, a surface-dependent effect. Increasing the specific surface of hemp fibre bundles by pure mechanical processing (coarse separation, roller carding) results in a near-linear increase in odour intensity (Figure 13.26). This effect was not stable over time: after a period of 3 months, the odour intensity of the samples described here fell back almost to its initial level. To sum up, the odour problem occurring in fibre processing can be solved, for example, by enzymatic treatment, resulting in removal of mildew contamination (direct odour reduction) and in additional improvement

Figure 13.26 Influence of mechanical treatment on the odour intensity of hemp GDE02 and KGE02 – detailed information can be found in the list at the beginning of Section 13.7. H. Fischer and B. Lohmeyer, 2009. Adapted with permission from Institut fur ¨ Werkstofftechnik.

1.7–76

5–30

4.6–126

12–50 12–126

Flax

Jute

Ramie

Kenaf Nettle Sunn hemp Softwood Hardwood Cotton

Kapok

Seed fibre

Fruit hair

10–35

/

/

30

40–90

16–904

0.1–0.4

0.1–0.4

1.9–2.2

0.2–8

0.2–1.3

0.1–0.5

0.3–3

single fibre

/

/

5.5 8–10

1.4–25

0.4–10

5.4–40

fibre bundle

7–35

2–87 2–14 3.6 1.2 10–64

1.5–11

40-260

1–6

4–140

8.3–55

single fibre

19–215 750–1500

750–1800

800–2000

150–3600

100–1500

650–5000

fibre bundle

Length in mm

(Continued )

1; 2; 3; 5; 8; 9; 6; 11; 14; 15; 21; 22; 30 1; 2; 3; 5; 8; 9; 10; 6; 11; 13; 14; 15; 21; 22; 32 1; 2; 3; 5; 5; 7; 8; 9; 10; 6; 11; 14; 15; 21; 22; 26; 32 1; 5; 8; 9; 10; 6; 11; 14; 15; 22 1; 5; 6; 11; 14; 22; 26; 32 1; 31; 32 5; 6; 11; 26 26 26 1; 2; 5; 8; 9; 6; 3; 11; 14; 21; 22; 26;27;28 8; 9; 10; 22

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25–200

40–620

25–500

fibre bundle

Fineness in tex

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35 25 12–38

3–51

Hemp

Dicotyledonous plants

Stem fibre

single fibre

Kind of fibre

Fineness (diameter/fibre width) in µm

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Table 13.6

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7–16.4 15.4–16.4 8–33 25–40 13–23 20

Pineapple Phormium Henequen Bamboo Wheat straw Oil palm EFB

13–70 14–90 9–40 3.6–25 11–15

Bagasse

Wool

Mohair Camel Spider silk Angora

Other

Protein-based animal fibre

18–20

11–81

Banana

200–400

150–500

20–500

20–200

20–500

10–1000

0.014

0.7

1.5–2.3

1–4.6

single fibre

2.5–5.5

4.2–44.4

15–20

15–50

fibre bundle

150–300 50–80 3 000 000–4 000 000 30–60

55–500

0.8–2.8

1.6

1.5–4

5–5.7

3–10

0.9–5.5

2–12

0.5–8

0.3–1.2

single fibre

900–1500

10–300

60–2500

40–1250

36–330

fibre bundle

Length in mm

8 8 8; 3; 25 10

8; 3; 25

5; 14; 22; 26

21; 26 2; 11

1; 2; 5; 8; 6; 11; 14; 19; 22 2; 5; 8; 10; 6; 1; 11; 14; 15; 18; 19; 22; 26 15; 5; 6; 14; 22; 9; 10; 21; 32 1; 2; 5; 6; 11; 14; 19; 22 5; 11; 14; 22; 2; 1 22 6; 14 21; 22; 26

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Bagasse

6–46

Abac´a

9–460

50–460

fibre bundle

Fineness in tex

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Halm fibre

4–47

Sisal

Leaf sheath fibres

12-24

Coir

Monocotyledonous plants

Fruit fibre

single fibre

Fineness (diameter/fibre width) in µm

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(Continued)

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Table 13.7 Mechanical behaviour of selected natural fibres. Values are given for single fibre or bundle tests, but not collective tests Tensile strength (MPa) range (most frequently published)

Kind of fibre Dicotyledonous plants

Stem fibre

Seed fibre

Monocotyledonous plants

Protein-based animal fibre

Elongation at break (%) range (most frequently published) References

Flax

343–1500 (700) 8–100 (70)

1.2–4 (3)

Hemp

310–1110 (800) 3–90 (65)

1.3–6 (3)

Jute

187–800 (500)

0.2–3.1 (1.8)

Ramie

290–1060 (800) 5–128 (65)

1.2–4.6 (3)

Kenaf

180–1191 (700) 22–128 (55)

1.6–6.9 (3)

Nettle Sunn hemp Soft wood Cotton

650 (650) 389–440 (400) 1000 (1000) 220–840 (450)

1.7 (1.7) 5.5 (5.5)

3–64 (30)

38 (38) 35 (35) 40 (40) 4.5–12.6 (8)

2–10 (8)

1; 2; 3; 5; 7; 8; 10; 11; 12; 13; 14; 15; 17; 20; 22 1; 2; 5; 7; 8; 10; 11; 13; 14; 15; 17; 20; 22 1; 2; 3; 5; 5; 7; 8; 10; 11; 12; 13; 14; 15; 17; 20; 22 1; 2; 5; 7; 8; 11; 13; 14; 15; 17; 20; 22 1; 2; 5; 14; 20; 22 2; 20 5; 11; 12 13; 17 1; 2; 5; 7; 8; 10; 3; 11; 13; 14; 17; 20; 22 22, 29

Fruit hair

Kapok

45–93 (60)

1.7–4 (2.9)

1.2–4 (2)

Fruit fibre

Coir

95–270 (200)

2.8–6 (5)

15–51.4 (30)

Leaf sheat fibres

Sisal

80–855 (600)

9–38 (12)

1.9–14 (3)

Abac´a Banana

12–980 (600) 430–914 (600)

12–72 (50) 7.7–42 (20)

1–12 (4) 1–10 (4)

Pineapple

170–1627 (750) 6.2–82.5 (40)

0.8–3 (2)

Henequen Curaua Piassava Alfa Bamboo Date palm Oil palm EFB

430–580 (500) 439–495 (460) 134–143 (140) 350 (350) 140–1000 (500) 97–196 (150) 248 (248)

10.1–16.3 (13) 10.5 (10.5) 1.07–4.59 (3) 22 (22) 11–89 (30) 2.5–5.4 (5) 3.2–6.7 (4.5)

3–5.9 (4) 1.3–4.5 (3) 21.9 (22) 5.8 (2.8) 2–4.5 (3.3) 14–25 (20)

1; 2; 5; 7; 8; 11; 13; 14; 17; 20; 22 1; 2; 5; 7; 8; 10; 11; 12; 13; 14; 15; 17; 18; 19; 20; 22; 25; 27; 29 1; 5; 14; 20; 22 2; 5; 1; 11; 14; 19; 20; 22 1; 5; 11; 12; 13; 14; 22; 2 2; 6; 14; 20 11 20 20 13; 20; 22 20 2; 11; 20

Bagasse

20–290 (170)

2.7–17 (15)

0.9–1.1 (1)

5; 11; 14; 20

Wool

180–240 (210)

25–45 (35)

8; 10; 3

Silk Angora

340–620 (430) 500–1150 (875) 11.8 (11.8)

18–34 (26) 3.7–4.3 (4)

8; 3 20

Halm fibre

Other

Young’s modulus (GPa) range (most frequently published)

Bagasse

1.32 1.37 1.4

Wool Silk Angora

Protein-based animal fibre

0.45–1.25

0.92 1.4 0.6–0.89 0.6–1.5 1–1.2 0.7–1.55

Curaua Piassava Alfa Bamboo Date palm Oil palm EFB Bagasse

0.8–1.6

Pineapple

Other

Halm fibre

1.4–1.5 1.3–1.35

Abac´a Banana

1.47

Kapok

1.0–1.5

1.2–1.4 1.53 1.5 1.5–1.6

Kenaf Sunn hemp Soft wood Cotton

Sisal

1.5–1.56

Ramie

1.15–1.5

1.3–1.5

Jute

Coir

1.4–1.6

Hemp

1.1–1.2 0.86–0.62

1.2

0.384

1.35

1.44

1.23

1.38

17–21 35–53

17

12

7.5

14–15

10.7

47

75.6

55–75

55–70.9

45.75

78.47

87.87

Crystallinity (%)

2160

7000

2100–6500

1920

2200

2300–8000

Degree of polymerisation

42–46

2–10 85–90

6–18

10–12

10–25

30–49

20–30

9–10

7.5–12

7–10

2–6.2

5–10

Fibril angle (deg) (main cell wall) components)

8 20

10

5; 11; 14; 20; 22

1; 2; 7; 8; 11; 13; 14; 16; 17; 19; 20; 22 1; 2; 5; 7; 10; 6; 11; 13; 14; 15; 16; 17; 18; 19; 20; 22; 25; 29 1; 5; 6; 14; 20; 22 5; 6; 1; 11; 14; 16; 19; 20; 22 1; 5; 11; 14; 20; 22; 2; 14; 20 11 20 16; 20 13; 20; 22; 23; 24 20 2; 11; 20

1; 2; 3; 5; 7; 8; 10; 6; 11; 13; 14; 15; 17; 20; 22 1; 2; 3; 5; 7; 8; 10; 11; 14; 15; 17; 18; 20; 22 1; 2; 3; 5; 5; 7; 8; 10; 6; 11; 13; 14; 15; 16; 17; 18; 20; 22 [1; 2; 5; 7; 8; 6; 11; 13; 14; 15; 20; 22] 5; 14; 20; 22 6; 11 13 1; 2; 5; 7; 8; 10; 3; 11; 13; 14; 17; 20; 22 18; 22; 29

References

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Fruit hair

1.4–1.52

Flax

Porosity (%)

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Seed fibre

Stem fibres

Apparent density (g/cm3 )

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Dicotyledonous plants

Kind of fibre

Density (g/cm3 )

Physical characteristics of selected natural fibres

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Kind of fibre

57–92 (70)

51–84 (65)

53–86 (80) 68–85 (72)

36–72 (44) 41–78 (60) 45–50 (47) 82–96 (90)

Hemp

Jute

Nettle Ramie

Kenaf Sunn hemp Wood Cotton

Kapok

Seed fibres

Fruit hair

32

8–19 (16) 23–30 (25) 2–6 (4)

20–21 (21)

10 (10) 3–17 (14)

0.7–0.8 (0.7) 0.7–3 (1.5)

1.3–1.7 (1.5) 1.5 (1.5)

2 (2)

15–21 (18) 7 (7)

3.5–22 (5) 0.3 (0.3) 27 (27) 0–1.6 (0.7) 0–7 (4)

9–19 (18)

Ash (%) range (most frequently published)

0.8–2.1 (1)

3.9–10.5 (6)

0.6 (0.6)

0.4 (0.4)

0.8–2 (1.4)

11–14 (12.5)

0.3 (0.3)

0.4–1 (0.7)

1.4–3 (2.2)

5.5–6.4 (6)

(Continued )

1; 2; 3; 4; 5; 7; 8; 9; 10; 6; 11; 12; 13; 14; 15; 17; 20; 21; 22 1; 2; 3; 4; 5; 7; 8; 9; 10; 6; 11; 13; 14; 15; 17; 18; 20; 21; 22 1; 2; 3; 5; 5; 7; 8; 9; 10; 6; 11; 12; 13; 14; 15; 16; 17; 18; 20; 21; 22 1; 2; 20 1; 2; 5; 7; 8; 9; 10; 6; 11; 13; 14; 15; 17; 20; 22 1; 2; 5; 6; 11; 14; 20; 22 5; 6; 11; 12; 20 5; 14; 22 1; 2; 5; 7; 8; 9; 10; 6; 3; 11; 13; 14; 17; 20; 21; 22 5; 8; 9; 10; 18; 22; 29

Water solubles (%) range (most frequently published) References

0.2–4.5 (1.5) 0.4–0.8 (0.5) 0.17–0.7 (0.4) 0.5–2 (1)

0.8–2.5 (1)

0.9–3.8 (2)

Fat/wax (%) range (most frequently published)

0.5 (0.5) 0.9–4.8 (2.5) 4 (4) 0.5–1 (0.7) 1.9–2.1 (2) 0.3 (0.3)

5–14 (10)

2.8–13 (6)

2–5 (2.5)

Pectin (%) range (most frequently published)

22:49

12–24 (15)

6–22 (16)

14–21 (17)

Lignin (%) range (most frequently published)

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13–43 (30)

60–81 (70)

Flax

Stem fibres

Hemi-cellulose (%) range (most frequently published)

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Cellulose (%) range (most frequently published)

Chemical composition of selected cellulosic fibres (data for silk and wool are given in Chapters 11 and 12)

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65 (65) 73.6 (73.6) 28–70 (49) 29–51 (40) 38–45 (42)

Oil palm EFB Curaua Rice straw Wheat straw Cereal straw

Bagasse

32–55 (44)

7.5 (7.5) 12–16 (14) 12–25 (20) 12–20 (16) 8 (8)

60–78 (69) 67 (67) 45 (45) 29 (29) 26–43 (35)

Henequen Phormium Alfa Piassava Bamboo

Bagasse

19–29 (24)

50–68 (64)

Banana

Halm fibre

56–68 (60)

Abac´a

16–30 (23)

15–31 (25) 15–31 (23)

10–21 (15)

4–28 (16) 30 (30) 39 (39) 26 (26) 15–30 (22)

6–30 (15)

19–25 (21)

3–5 (4)

19–34 (23) 10 (10)

2 (2)

0.5 (0.5)

4.5 (4.5)

1.1–5 (3)

0.9 (0.9) 15–20 (18) 4.5–9 (6.7)

2 (2)

0.7–3.5 (2.1)

2.5–2.8 (2.7) 9–14 (12)

2.4 (2.4)

1.4 (1.4)

4 (4)

5; 11; 14; 20; 22

11; 20 11 11; 20; 21 2

1; 2; 5; 7; 8; 6; 11; 13; 14; 16; 17; 19; 20; 22 1; 2; 5; 7; 8; 10; 6; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 22; 29 1; 5; 11; 12; 13; 14; 20; 22 1; 2; 5; 6; 9; 10; 11; 12; 13; 14; 20; 21: 22 1; 2; 5; 6; 11; 14; 16; 19; 20; 22 2; 6; 14; 20 22 16; 20 20 13; 20; 21; 22; 23 2; 11; 20

Water solubles (%) range (most frequently published) References

0.14–0.55 (0.3) 1.2–6 (3.5)

Ash (%) range (most frequently published)

10–11 (10.5) 1.2 (1.2)

0.5–1 (0.8) 0.2–3 (1.4)

2–3 (2.5)

8–13 (10) 3–4 (3.5) 11 (11) 14.9 (14.9) 45 (45) 21–31 (26)

5–18 (9)

5–13 (10)

5–13 (12)

0.2–2 (1)

Fat/wax (%) range (most frequently published)

22:49

16–19 (17)

0.5–10 (2)

80–83 (81)

4–14 (10)

Pineapple

10–15 (12)

43–88 (66)

40–45 (43) 3–4 (3)

Sisal

0.2–0.3 (0.2)

Leaf sheat fibres

32–53 (40)

Coir

Pectin (%) range (most frequently published)

Fruit fibre

Lignin (%) range (most frequently published)

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Monocotyledonous plants

Hemi-cellulose (%) range (most frequently published)

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Kind of fibre

Cellulose (%) range (most frequently published)

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in stability towards thermal stress (indirect odour prevention). If a process is accompanied with increase in surface, the natural fibre odour is increased as a temporary effect. Table 13.10

Behaviour towards moisture of selected natural fibres Absorption regain (%) at Transverse Axial Volume Water 65% relative swelling swelling swelling retention humidity, 20 ◦ C (%) (%) (%) (%) References

Kind of fibre Dicotyledonous plants

Stem fibres

13.8

7–12

20–25

Hemp

6–12

Jute

8.5–17

20–22

Nettle Ramie

11–17 7.5–17

12–15

0.05–0.2 29.5

50–55 50–55

0.37

44–45

25–35

32

Seed fibre

Kenaf 17 Sunn hemp 10 Cotton 7–25

Fruit fibre

Kapok

10–11

Coir

2–13

6–15

Sisal

10–22

18–20

Abac´a Banana Pineapple

5–14 1–15 11.8

16–20 18–20

Wool

10.5–18

0.3–3

Silk

10.5

1.3–1.7 30–43.2 30–50

Monocotyledonous Fruit fibre plants Leaf sheath fibres

Protein-based animal fibres

Flax

18–20

45.4 1.1–2.8

45–50

39.5

30–45

42

36–41

40–45

1; 2; 3; 5; 7; 8; 9; 10; 6; 11; 13; 15; 22 1; 2; 3; 5; 8. 10; 6; 13; 15; 22 1; 2; 3; 5; 8; 10; 6; 11; 13; 15; 22 2 1; 2; 8; 9; 6; 11; 13; 15; 22 22 5; 6; 11 1; 2; 5; 8; 10; 6; 3; 22 10; 22 1; 2; 5; 8; 6; 11; 13; 22 1; 2; 10; 6; 11; 13; 19; 22 2; 6; 22; 5 2; 5; 6; 11 11; 13 8; 10; 6 8; 10

Fibre Tables and Summary

Bearing in mind the information given in this chapter – the great influence of testing methods and parameters, which are rarely published in the necessary depth – it seems to be nearly impossible to compare literature data with one’s own results. Consequently, no tables with ‘typical’ fibre data should be published. On the other hand, such tables are a useful aid to getting a first orientation and classifying one’s own results. After long discussion, we finally decided to include fibre tables here, which are based on intensive search in the literature. We sum up data concerning the mechanical behaviour, physical properties, geometrical properties, chemical composition and behaviour towards moisture as a selection of important fibre characteristics in Table 13.6 to Table 13.10. The data are based on single elements (‘single fibre’ or ‘fibre bundle’ is often not differentiated in the literature, but not collective tests). Nevertheless, the reader should bear in mind the following general aspects before using table values in direct comparison with his/her own results: r Comparable testing method? r Comparable testing parameters (speed, length, etc.)? r Comparable testing conditions? r (Type of) calibration?

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When these aspects are considered, direct comparison of data should be no problem. If data from the literature were recorded in different conditions, the comparison of trends may still be possible. Hopefully, we have given a helpful introduction to the complex field of natural fibre testing that will enable a better understanding and interpretation of fibre data, as well as good practice in planning one’s own experiments.

References to Fibre Tables 1. Franck, R.R. (2005) Overview, in Bast and Other Plant Fibres, ed. by Franck, R.R. Woodhead Publishing, Cambridge, UK, pp. 1–23. 2. Bismarck, A., Mishra, S. and Lampke, T. (2005) Plant fibers as reinforcement for green composites, in Natural Fibers. Biopolymers and Biocomposites, ed. by Mohanty, A.K., Misra, M. and Drzal, L.T. CRC Press/Taylor & Francis Group, Boca Raton, FL, USA, pp. 37–108. 3. Wilbrandt, G., Tunger, S., Geringswald, F., Hansick, H. and Kr¨ugel, D. (eds) (1972) Faserstofflehre, 2nd edition. VEB Fachbuchverlag Leipzig, Leipzig, Germany. 4. Kr¨assig, H., Schurz, R.G., Steadman, K., Schliefer, K. and Albrecht, W. Ullmann’s Encyclopaedia of Industrial Chemistry, Vol. A28. VCH, Weinheim, Germany. 5. Rowell, R.M. and Stout, H.P. (2007) Jute and kenaf, in Handbook of Fiber Chemistry, 3rd edition, ed. by Lewin, M. CRC Press/Taylor & Francis Group, Boca Raton. FL, USA, pp. 405–452. 6. Batra, S.K. (2007) Other long vegetable fibers: abac´a, banana, sisal, henequen, flax, ramie, hemp, sunn and coir, in Handbook of Fiber Chemistry, 3rd edition, ed. by Lewin, M. CRC Press/Taylor & Francis Group, Boca Raton, FL, USA, pp. 405–452. 7. Bledzki, A.K. and Gassan, J. (1999) Composites reinforced with cellulose based fibres. Prog. Polym. Sci., 24, 221–274. 8. Bobeth, W., Berger, W., Faulstich, H., Fischer, P., Heger, A., Jacobash, H.-J., Mally, A. and Mikut, I. (1993) Textile Faserstoffe: Beschaffenheit und Eigenschaften, ed. by Bobeth, W. Springer-Verlag, Berlin/Heidelberg/New York. 9. Meredith, R. (ed.) (1956) Cellulose fibres, in The Mechanical Properties of Textile Fibres. North-Holland, Amsterdam, The Netherlands, pp. 23–37. 10. Schenek, A. (ed.) (2000) Naturfaserlexikon. Deutscher Fachverlag, Frankfurt am Main, Germany. 11. Satyanarayana K.G. and Wypich, F. (2007) Characterization of natural fibers, in Handbook of Engineering Biopolymers: Homopolymers, Blends and Composites, ed. by Fakirov, S. and Bhattachayya, D. Hanser Verlag, M¨unchen, Germany, pp. 3–47. 12. Saheb, D.N. and Jog, J.P. (1999) Natural fiber polymer composites: a review. Adv. Polym. Technol., 18(4), 351–363. 13. Netravali, A.N. and Chabba, S. (2003) Composites get greener. Mater. Today, 23–29. 14. Biagotti, J., Puglia, D. and Kenny, J.M. (2004) A review on natural fibre-based composites. Part I: Structure, processing and properties of vegetable fibres. J. Nat. Fibr., 1(2), 37–68. 15. Hanselka, H. (1999) Fibre composites of raw renewable materials for the ecological lightweight design. Materialwissenschaften und Werkstofftechnik, 29, 300–311. 16. Jain, S., Kumar, R. and Jindal, U.C. (1992) Mechanical behavior of bamboo and bamboo composite. J. Mater. Sci., 27, 4598–4604. 17. Eichhorn, S.J., Baillie, C.A., Zafeiropoulos, N., Mwaikambo, L.Y., Ansell, M.P., Dufresne, A., Entwistle, K.M., Herrera-Franco, P.J., Escamilla, G.C., Groom, L., Hughes, M., Hill, C., Rials, T.G. and Wild, P.M. (2001) Review: current international research into cellulosic fibres and composites. J. Mater. Sci., 36(9), 2107–2131. 18. Mweikambo, L.Y. and Ansell, M.P. (2001) The effect of chemical treatment on the properties of hemp, sisal, jute and kapok for composite reinforcement. Angew. Makromolek. Chem., 272, 108–116. 19. Mukherjee, P.S. and Satyanarayana, K.G. (1984) Structure and properties of some vegetable fibres. Part I: Sisal fibre. J. Mater. Sci., 19, 3925–3934. 20. John, M.J. and Anandjiwala, R.D. (2008) Recent developments in chemical modification and characterization of natural fiber-reinforced composites. Polym. Compos., 187–207. 21. Rowell, R.M., Sanadi, A.R., Caulfield, D.F. and Jacobson, R.E. (1997) Utilization of natural fibers in plastic composites: problems and opportunities, in Lignocellulosic–Plastic Composites, ed. by Leao, A.L., Carvalho, F.X. and Frollini, E. University of Rio de Janeiro, USP and UNESP, Rio de Janeiro, Brazil.

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22. Mweikambo, L.Y. (2006) Review of the history and application of plant fibre. Afr. J. Sci. Technol., 7(2), 120–133. 23. Paramesrawan, N. and Liese, W. (1976) On the fine structure of bamboo fibres. Wood Sci. Technol., 10, 231– 246. 24. Mark, R.E. (2002) Mechanical properties of fibres, in Handbook of Physical Testing of Paper, Vol. 1, 2nd edition, ed. by Mark, R.D., Habeger, C.C., Borch, J. and Lyne, M.B. Marcel Dekker, New York, NY, USA. 25. Frollini, E., Paiva, J.M.F., Trindade, W.G., Tanaka Razera, I.A. and Tita, S.P. (2004) Plastics and composites from lignophenols, in Natural Fibers. Plastics and Composites, ed. by Wallenberger, F.T. and Weston, N. Springer, Berlin/New York. 26. Horn, R.A. and Setterholm, V.C. (1990) Fiber morphology and new crops, in Advances in New Crops, ed. by Janick, J. and Simon, J.E. Timber Press, Portland, OR, USA. 27. Wulfhorst, B. and K¨ulter, H. (1989) Faserstoff-Tabelle: Cotton. Chemiefasern/Textilind., 39(91), E133–E155. 28. FIBRE, Bremen Cotton Round Trial (Bremer Baumwollrundtest) – evaluation of the test results. Faserinstitut Bremen e.V. (FIBRE). Bremer Baumwollb¨orse, Bremen, Germany. 29. Venkata Reddy, G., Venkata Naidu, S. and Shobha Rani, T. (2009) A study on hardness and flexural properties of kapok/sisal composites. J. Reinf. Plast. Compos., 28(16), 2035–2044. 30. M¨ussig, J., Cescutti, G. and Fischer, H. (2006) Le management de la qualit´e appliqu´e a` l’emploi des fibres naturelles dans l’industrie, in Le chanvre industriel - production et utilisations, ed. by Bouloc, P., Groupe France Agricole, 2006, (Editions France Agricole) Paris, France, 235–269. 31. L¨utzkendorf, R., Mieck, K.-P., Reußmann, T., Dreyer, J. and L¨uck, M. (2000) Nesselfaser-Verbundwerkstoffe f¨ur Fahrzeuginnenteile – Was k¨onnen sie? Technische Textilien, 43, 30–32. 32. Sch¨onfeld, H. (1955) Bastfasern – Eine Faserstofflehre, Fachbuchverlag Leipzig, Leipzig, Germany.

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Lehmann, E.L. and Romano, J.P. (2005) Testing Statistical Hypotheses, 3rd edition. Springer, New York, NY. Martens, R. and M¨ussig, J. (1999) Untersuchung der Qualit¨aten von Naturfasern – Hanf, in Leistungs- und Qualit¨atspotential von Naturfasern zur technischen Verwendung. 4 Bonner Naturfasertag, Bonn, Germany, 10 August 1999, 1st edition, ed. by Heier, L., L´eon, J. and Kromer, K.-H. Eigenverlag, Bonn, Germany (Arbeiten aus dem Institut f¨ur Landtechnik der Rheinischen Friedrich-Wilhelms-Universit¨at Bonn, Heft 28), 56–65. Mukherjee, P.S. and Satyanarayana, K.G. (1984) Structure and properties of some vegetable fibres – Part 1: Sisal fibre. J. Mater. Sci., 19(12), 3925–3934. M¨ussig, J. (2001) Untersuchung der Eignung heimischer Pflanzenfasern f¨ur die Herstellung von naturfaserverst¨arkten Duroplasten – Vom Anbau zum Verbundwerkstoff. VDI Verlag GmbH, D¨usseldorf, Germany. M¨ussig, J., Cescutti, G. and Fischer, H. (2006) Le management de la qualit´e appliqu`e a` l’emploi des fibres naturelles dans l’industrie, in Le Chanvre Industriel – Production et Utilisations, ed. by Bouloc, P. Groupe France Agricole, Paris, France, pp. 235–269. M¨ussig, J. and Schmid, H.G. (2004) Quality control of fibers along the value added chain by using scanning technique – from fibers to the final product, in Microscopy and Microanalysis 2004, Savannah, GA, 1–5 August 2004, ed. by Anderson, I.M., Price, R., Clark, E. and McKernan, S. Press Syndicate of the University of Cambridge, Cambridge/New York/Melbourne, 2004 (Proc. Conf. Microscopy and Microanalysis, 2004, Vol. 10, Suppl. 2), 1332CD–1333CD. Nechwatal, A., Mieck, K.-P. and Reußmann, T. (2003) Developments in the characterization of natural fibre properties and in the use of natural fibres for composites. Compos. Sci. Technol., 63, 1273–1279. Pardini, L.C. and Mangani, L.G.B. (2002) Influence of the testing gage length on the strength. Young’s modulus and Weibull modulus of carbon fibres and glass fibres. Mat. Res., 5(4), 411–420. Peponi, L., Biagiotti, J., Torre, L., Kenny, J. and Mondrag`on, I. (2008) Statistical analysis of the mechanical properties of natural fibers and their composite materials. I: Natural fibers. Polym. Compos., 29(3), 313–320. Sachs, L. (2004) Angewandte Statistik. Anwendung statistischer Methoden, 11th edition. Springer, Berlin, Germany. Schmid, H.G. (1999) Image analysis for quality control of diamonds. Diamante Applic. Technol., 18, 112–120. Schmid, H.G., M¨ussig, J. and Gerardi, H. (2002) Image scanning for measurement of cotton fibre width, in Proceedings of the General Assembly, Bremen, 12–13 March 2002, Zurich, Switzerland. ITMF International Committee on Cotton Testing Methods (Committee Proceedings CD version), Working Group: Fineness and Maturity, Appendix: FM-10, pp. 156–162. Schnegelsberg, G. (1999) Handbuch der Faser – Theorie und Systematik der Faser. Deutscher Fachverlag, Frankfurt am Main, Germany. Sch¨onwiese, C.-D. (1992) Praktische Statistik f¨ur Meteorologen und Geowissenschaftler, 2nd edition. Gebr¨ueder Borntr¨ager, Stuttgart, Germany. Simor, P. (1959) Untersuchungen der Bastfasern mittels Pr¨ufger¨aten. Melliand Textilber., 40(2), 134–137. Sokal, R.R. and Rohlf, F.J. (1995) Biometry: The Principles and Practice of Statistics in Biological Research, 3rd edition. Palgrave Macmillan, Freeman, Houndmills, Basingstoke, Hants, UK. Stephens, S.G. (1977) Estimation of cotton fibre dimensions from Shirley I.I.C. finess/maturity tester readings. Text. Res. J., 47(8), 526–530. Suh, M.W., Cui, X. and Sasser, P.E. (1994) New understanding on HVI tensile data based on Mantis single fiber test results, in Proceedings of Beltwide Cotton Conference, San Diego, CA, 5–8 January 1994, Vol. 3, pp. 1400–1403. Taylor, J.R. (1999) An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements, 2nd edition. University Science Books, Sausalito, CA, USA. Thibodeaux, D.P. and Evans, J.P. (1986) Cotton fibre maturity by image analysis. Text. Res. J., 56(2), 130–139. USTER (1993) USTER News Bull., 39(08/1993). VDA 270 (1992) Determination of the odour characteristics of trim materials in motor vehicles. Verband der Automobilindustrie, Frankfurt am Main, Germany, 1992–10. VDI 3882 (1994) Blatt 2, Olfaktometrie – Bestimmung der hedonischen Geruchswirkung. Verein Deutscher Ingenieure e.V., D¨usseldorf, Germany, 1994-09. Weibull, W. (1951) A statistical distribution function of wide applicability. J. Appl. Mechanics, 18, 293–297. Wortmann, E.-J. and Zahn, H. (1994) The stress/strain curve of α-keratin fibers and the structure of the intermediate filament. Text. Res. J., 46, 347–737. Xia, Z.P., Yu, J.Y., Cheng, L.D., Liu, L.F. and Wang, W.M. (2009) Study on the breaking strength of jute fibres using modified Weibull distribution. Compos. Part A: Appl. Sci. Mfg, 40(1), 54–59.

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14 SEM Catalogue for Animal and Plant Fibres Tanja Slootmaker Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany

¨ J¨org Mussig Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany

14.1 Introduction The intention of this chapter is to give a detailed overview, using scanning electron microscopy (SEM), of the morphological structure of the important natural fibres described in this book. The same fibres have been selected for SEM as those used for measurements summarised in other chapters or that are described in detail in Chapters 4 to 12. We have introduced links to the corresponding chapters, and vice versa. The aim is also to show the variations among fibre types and to indicate features in separated or processed fibres that are otherwise difficult to determine. For example, animal hairs from even one single animal show different scale patterns. We want to identify salient features that can be determined by SEM observation of these natural fibres. This chapter can be seen as a starting point for identification of natural fibres with SEM, but we do not want to create the illusion that information contained herein will allow identification of difficult mixtures, e.g. a fine yak fibre in a cashmere sample. Extensive experience is necessary for identification of fibres, and only a few fibre specialists in the world are able to decide if yak and cashmere hairs are mixed. The combination of SEM with other techniques, e.g. DNA analysis, will give more accurate results. The DNA technique is described in Chapter 16. Our objective is to distinguish between plant and animal fibres and to describe the morphology and diversity of fibres or bundles that are processed in various ways. It is intended that the information in this chapter will be useful to others in investigating fibre types.

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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14.2 SEM Principle In this section, we will give a brief introduction to the SEM method. We will explain in more detail the method used to examine natural fibres with the SEM. An introduction to SEM and detailed information can be found in other works, e.g. Flegler et al. (1993), Goldstein et al. (2005), Goodhew and Humphres (1998) and Lyman et al. (1990). SEM is a further development of the transmission electron microscope (TEM), which was invented in the early 1930s by Ernst Ruska and Max Knoll (Berlin, Germany). SEM images are often easier to interpret than TEM images, and, for fibres, sample preparation for secondary electron imaging (see below) in the SEM is fairly straightforward. A big advantage of SEM is the large depth of field that can be achieved by using this microscopy technique (Table 14.1). Depending on the instrument and conditions, the resolution (broadly defined as the ability to see fine detail and technically as the smallest distance between two distinguishable points) can be less than 1 nm. With state-of-the-art SEM, using conventional tungsten sources, a resolution of 3.0 nm at 30 kV is achievable (JEOL Ltd, Tokyo, Japan). A resolution of 3.0 nm is achievable with highvacuum equipment of <1 × 10−2 Pa (EOS, Dortmund, Germany). Improvements in the shielding system for reduced electromagnetic and acoustic interferences, in conjunction with a completely dry vacuum system, have led to resolutions of 0.4 nm at 30 kV and 1.6 nm at 1 kV (Hitachi High Technologies America, Inc., Pleasanton, CA). In comparison with a standard optical (light) microscope, where visible light is used to image specimens, the SEM provides advantages of depth of focus and high resolution (Table 14.1) For the SEM, thermionic emission is used. From a heated tungsten cathode, electrons are emitted at temperatures of about 2700 K/2.7 A, producing an electron beam. In SEM, a sample under vacuum is scanned by a beam of electrons generated as described above. An electrical field is set up between the anode and cathode, leading to an acceleration of the electrons towards the anode. The electron beam is then concentrated by a Wehnelt cylinder and focused by electromagnetic lenses until the beam has a maximum fineness in order to give a clear image (see Figure 14.1). The specimen is scanned with the primary electron beam, which generates several reactions. Figure 14.2 shows examples of the reactions of a primary electron beam interacting with the atoms of the sample. The electron beam generates emissions of electrons from the specimen that depend on the topography and chemical composition of the sample. These electrons are collected and amplified by a photomultiplier and converted into a signal visible on a monitor screen. Magnification is achieved by reducing the scanning area of the electron beam. The resolution is dependent on the size of the electron beam, i.e. the ‘spot size’ (Sawbridge and Ford, 1987). When working with high resolutions, the operator decreases the spot size, affecting the signal-to-noise ratio. The black-and-white SEM images generated with secondary electrons contain various tones of grey based on the emission of electrons from the material. The primary electron beam intrudes into the sample and leads to interactions between primary electrons and the shell electrons or the kernel of the atom. Owing to this interaction, different signals are created: backscattered electrons, Auger electrons, secondary electrons, characteristic X-ray emission, Bremsstrahlung and X-ray fluorescence (see Figure 14.3). The interaction processes also consist of the primary electrons

Table 14.1 Relevant optical parameters for the performance of an optical microscope (OM; a type of microscope that uses visible light) versus a scanning electron microscope (SEM; a conventional tungsten scanning electron microscope). Adapted from P.M. Latzke and R. Hesse, 1988 Optical parameters

OM

SEM

Depth of focus in µm

0.1 at 1000×

Resolution in nm

ca 200–400

30 at 1000 × 10 at 10 000× ca 3–10

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Figure 14.1

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Schematic view of a scanning electron microscope.

being scattered inside the specimen, so that signals are not only produced from the surface but also from areas inside the specimen. All these different signals can be used to gain detailed information about the specimen, such as topographic contrasts or material contrasts. An overview of the different interactions as a function of sample depth is given in Figure 14.3. While the backscattered electrons characterise the sample in a deeper zone (up to 100 nm), the secondary electrons are used for topographic analysis of the sample. Because the interaction takes place in a very thin area (10 nm), the resolution of the secondary electrons is much higher compared with the resolution that can be obtained using backscattered electrons. A certain upper surface is ‘transparent’ for the backscattered electrons. Owing to the substantially greater emission depth, particles just under the surface appear in the image. The backscattered electron image contains more depth information and has a lower resolution than a secondary electron image. In summary, depending on surface structures, the angle of incidence, the sample tilt and the atomic number of the chemical elements inside the specimen, different signals are obtained to achieve an image. For the observation of natural fibres, secondary electrons are used to characterise the topography of the samples. To improve the image and to eliminate charging or build-up of electrostatic charges of non-metallic surfaces

Figure 14.2 Primary electrons creating different emissions after interaction with the sample. B. Heine, Werkstoffprufung – ¨ Ermittlung von Werkstoffeigenschaften, 2003. Adapted with permission from Carl Hanser Verlag GmbH & Co., Germany.

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Figure 14.3 Electron–specimen interaction as a function of sample depth. B. Heine, Werkstoffprufung – Ermittlung von ¨ Werkstoffeigenschaften, 2003. Adapted with permission from Carl Hanser Verlag GmbH & Co., Germany.

like natural fibres, the surface needs to be conductive. This can be done by sputter coating the sample with a thin layer of metal, e.g. gold, of approximately 20–50 nm thickness (Latzke and Hesse, 1988). This coating allows for good electrical conductivity and a proper contrast without charging of the fibre surface.

14.3 Sample Preparation It is our intention to show an easy and quick way to prepare cross-sections of fibres without the cost and time required in resin infiltration and microtoming of samples. 14.3.1

Preparation of Cross-Sections

R -Set The fibres and fibre bundles are parallelised and inserted into a heat-shrinkable tube (type: DERAY
2000, red (1.6 mm in diameter), blue (3.2 mm in diameter); Conrad Electronic SE, Hirschau, Germany). The fibre ends outside the tube are cut off with an industrial blade or with a razor blade. Then the tube is carefully heated with a conventional hairdryer until it shrinks tightly around the fibre or fibre bundle collective. After cooling and hardening, small cross-sections are then cut with an unused industrial razor blade (single-edge industrial blades with steel backs, supplied by Plano GmbH, Wetzlar, Germany). A new blade is required after three cuts maximum. The small slices are carefully mounted with tweezers onto a sample stub with double-sided electrically conductive adhesive tape. A schematic representation of this method is given in Figure 14.4. Where it is impossible to use the shrinkable tube, it is recommended to cut single fibres or bundles with an unused razor blade and mount them perpendicularly on a right-angled adhesive conductive tape. The SEM stubs are then sputter coated with a thin layer of electrically conducting substances such as gold, gold/palladium alloy, platinum or graphite. The coating prevents the accumulation of static electric charge on the specimen during observation and should be repeated if electrostatic charging still occurs. A bad

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Figure 14.4 Sample preparation steps for cross-sections; parallelised fibres or fibre bundles are inserted into a heat-shrinkable tube and cross-sections of fibre or fibre bundles are cut and prepared for SEM observation.

conductivity leads to ‘charging’ or a streakiness of the picture (sudden unloading), and this situation can be improved by conductive varnish to improve the conductivity between object and stub.

14.3.2

Length Views

The fibres or fibre bundles are parallelised by hand and mounted on the stub using adhesive electrically conductive tape. The fibres can be fixed with adhesive and conductive tape on both ends if necessary to ensure that no loose fibres get into the SEM chamber.

14.4

SEM Observation of Natural Fibres

The SEM photographs for this chapter were taken with a CamScan CS 24 scanning electron microscope manufactured by Obducat CamScan Ltd (Cambridgeshire, UK). Image acquisition was achieved with an R system by Olympus Soft Imaging Solutions GmbH (M¨unster, Germany). A prerequisite for analySIS
examining natural fibres with the SEM is having dry samples and making the samples electrically conductive. All specimens were mounted on aluminium stubs using double-sided electrically conducting carbon adhesive tabs prior to analysis and sputter coated with a layer of gold prior to SEM observations. Sputter coating was carried out with an EDWARDS Sputter Coater S150B (Edwards GmbH, Kirchheim, Germany). The SEM parameters for observation of the natural fibres presented in the following section were as follows: r r r r r r r

sputter coating time: 30–120 s if necessary (too much coating obscures details and reduces resolution); sputter coating voltage and current: approximately 1.4 kV; approximately 25 mA; magnification: ca. 100–2000×, depending on the sample; acceleration voltage: 20 kV; working distance: 25–30 mm; stubs: alumina, 12.5 mm in diameter (Plano GmbH, Wetzlar, Germany); mounting material: electrically conductive alumina tape and conductive charcoal tabs (Plano GmbH, Wetzlar, Germany).

During observation of natural fibres, the high-energy electrons from the SEM beam can ‘burn in’ if high magnifications are chosen. This ‘burning in’ by the primary electron beam produces artefacts on the sample surface and sets limits on the chosen magnification and the picture scanning time. Following this brief introduction of the application of the SEM, we will now present our practical work on the evaluation of natural fibre. Links are indicated to the chapter in which further information can be obtained concerning the investigated fibres.

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Scanning Electron Microscopy of Hairs and Threads Spider Silk

As shown in Figures 14.5A and B, the spider thread of Nephila senegalensis consists of two fibres. As described in Chapter 11, insect-produced gland fibres originate as paired fibres. All fibre-producing spiders form their ground thread from a minimum of two gland fibres, which constitute the final thread. This phenomenon is in agreement with the definition of a thread according to Schnegelsberg (1999), who defined a thread, morphologically seen, as ‘a filiform entity with a dominant one-dimensional extension (evenness in longitudinal extension) composed of fibres’. The fibres shown in Figure 14.5 have an extremely smooth surface. Spider silk tends to catch dust, and some dust particles are present on the fibre surface in Figures 14.5C and D. The spider’s silk tends to split parts off in winding, which also happens if fibres from other glands are attached to a thread. For example, the tangle in Figure 14.5A is the silk from another gland that has split off from the main thread. In Figures 14.5C and D, the diameter is nearly circular and very even. According to Chapter 11, the diameter of Nephila dragline fibres ranges from 4 to 9 µm and depends on the spider’s size and species. The diameter of the fibre in Figure 14.5B was measured to be ca. 6.5 µm. 14.5.2

Mulberry Silk (Bombyx Silk)

Mulberry silk (synonym: Bombyx silk) are threads that are spun from the silkworm of the domesticated silkmoth Bombyx mori. Their main food is the leaves of the white mulberry tree. A silk breeder requires undamaged cocoons in order to separate and acquire intact fibres. Hot water loosens the sericin (a glycoprotein glueing the two fibres to a thread), and then the silk threads can be extracted. When the sericin is not fully

Figure 14.5

Spider threads of Nephila senegalensis. A and B – length view at different magnifications; C and D – cross-sections.

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Figure 14.6

317

Degummed mulberry silk fibres of the silk worm Bombyx mori. A and B – cross-sections; C and D – length views.

removed (i.e. not adequately degummed), two fibres are held together by this substance as a thread. Particles of sericine are found on the fibre surface when the degumming process is not complete. As shown in Figures 14.6A and B, the typical shape of the fibre cross-section varies from polygonal to oval and at times is nearly triangular. In some unknown fibre mixtures, cultivated silk fibres can hardly be distinguished by SEM from synthetic fibres owing to their sometimes similar even shape and surface. The fibre diameter in Figure 14.5D ranges from ca. 9 to 21 µm. Latzke and Hesse (1988) report a variation in fibre diameter from 7 to 20 µm. According to the information in Chapter 11, the diameter of mulberry silk fibre is around 12 µm and the length is about 800 m. Figures 14.6C and D show the longitudinal view with a generally smooth and clean surface.

14.5.3

Tussah Silk

Saturniid moths of the genus Antharea provide tussah silk. Tussah silk is obtained by collecting cocoons of the feral tussah spinner (Antheraea perny, Antheraea yamami) from trees and bushes. More information about tussah silk is given in Chapter 11. Tussah silk is coarser than cultivated silk. Corrugations on the fibre surfaces and the ribbon-like shapes in the length view are typical (see Figures 14.7C and D). The surface is fibrillated to striated, partly with pronounced fissures, and at times convolutions can be found (Figure 14.7C). In the cross-section (see Figures 14.7A and B), this ribbon-like shape can be seen. Deep fissures appear as clefts, and the fibres are uneven in width. Sometimes the cross-sections show a brittle appearance. The measured widths of the broad sides of the fibres in Figure 14.7B range from ca. 27 to 44 µm. The thicknesses of the flat sides show values of ca. 4–13 µm. In laboratory practice, tussah silk noils are sometimes mixed with viscose, cotton, wool, angora rabbit hair and bast fibres. Besides light microscopy and histochemical staining, SEM is an adequate tool for analysing these mixtures.

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Figure 14.7

14.5.4

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Tussah silk after degumming. A and B – cross-sections; C and D – length views.

Wool Fibres (INTERWOOLLABS Standard 1 PM, Series 17)

Wool is the hair from various breeds of sheep (Ovis aries). Wool is a keratin fibre and is similar to human hair. More details on the structure of wool can be found in Chapter 12. Figures 14.8A and B show the cross-sections with round to oval forms, and no medulla can be identified for this fine wool. The cross-section shows a distribution in fibre diameters. The typical scales with fibrillated cuticle are shown in the longitudinal view (Figures 14.8C and D). According to Latzke and Hesse (1988), the scale arrangement forms a type of regular mosaic with distinct edges. The INTERWOOLLABS Standard 1 PM is an extremely fine standard with a mean value of 15.7 µm measured with the projection microscope in various international round trials. The measured width values in Figure 14.8D show a range from 11 to 25 µm. These wool standards are described in more detail in Chapter 3.1. The IWTO standards are used to calibrate image analysis systems to measure fibre width distribution (see Chapter 13). Fine wool is often mixed with cashmere fibres. One criterion for differentiating cashmere and wool fibres is the height of the single scale, which is about 0.4 µm for speciality fibres (like cashmere, camel, yak) and about 0.8 µm for sheep’s wool. It can be seen that the scale structure varies, especially between coarse and fine fibres.

14.5.5

Wool Fibres (INTERWOOLLABS Standard 8, Series 17)

Figure 14.9 shows cross-sections (A and B) and longitudinal views (C and D) of an INTERWOOLLABS Standard Series 17, No. 8, which is the coarsest standard with a mean diameter of 36.8 µm measured with the projection microscope in various international round trials (see Chapter 3.1). Typical is the presence of medullated fibres, which occur more frequently in coarse wool qualities. The scales form a kind of regular or

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Figure 14.8

INTERWOOLLABS Standard 1 PM (Series 17). A and B – cross-sections; C and D – length views.

Figure 14.9

INTERWOOLLABS Standard 8 (Series 17). A and B – cross-sections; C and D – length views.

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Figure 14.10

Cashmere hairs, coarse and fine (not dehaired). A and B – cross-sections; C and D – length views.

roof-tile mosaic. In fact, this mosaic structure varies. The scales overlap from the root to the tip of each hair. In Figure 14.9D the former root of both given wool hairs is oriented to the left bottom of the micrograph. 14.5.6

Cashmere

Cashmere is the hair of the cashmere goat (Capra hircus laniger). The fine underhair is one of the softest and most fleecy-feeling hair available. Cashmere is demanded worldwide for producing luxurious textiles. This is also the reason for mixing the expensive cashmere fibres with cheaper yak hairs or ultrafine sheep’s wool. Only a few experts worldwide can identify these mixtures. A combination of an SEM method with a DNA analysis gives more secure results concerning the identification (see Chapter 16). Besides the very low individual finenesses of approximately 8–24 µm for cashmere (Phan, 1994), another typical feature seen in the SEM is the extremely regular scale structure and the low scale height. Figure 14.10D shows a variation in fibre thicknesses of ca. 21–83 µm. As can be seen in Figures 14.10A and B, the cross-sections of the fine fibres (bottom hair) are round to oval and not medullated. The much coarser guard hair shows a strong medullation and an oval to bean-shaped form. The fibres in the longitudinal view in Figures 14.10C and D show a scaled surface. The fine fibre in Figure 14.10C has smoothed overlapping edges that encircle the whole fibre perimeter. 14.5.7

Yak

Yak (Bos mutus) is a bovine that, similarly to the cashmere goats, lives in the Himalayan region. Yak hairs (bottom hair) are often blended with cashmere bottom hairs. Identification is difficult owing to similarities of

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Figure 14.11

321

Yak hairs, coarse and fine. A and B – cross-sections; C and D – length views.

fineness and scale structure with cashmere hairs. Because of these similarities in structure, some researchers have focused their work on new image analysis tools (e.g. Shi and Yu, 2008). Other scientists have focused their work on developing DNA analysis systems to detect yak admixtures in a ‘pure’ cashmere sample. In Chapter 16, an analytical DNA-based procedure for the identification of yak in a cashmere sample is presented. The hairs shown above were taken directly from the animal and are not dehaired. The average fineness of yak hair is reported to range between 19 and 23 µm (Phan, 1994). The measured fibre widths in Figure 14.11A show a range of ca. 7–91 µm. As shown in Figures 14.11A and B, the cross-sections of the bottom hair are round to oval. While no medullation can be found in the fine fibres, the guard hair shows stronger medullation and an oval shape. The fibres in the longitudinal view (Figures 14.11C and D) show a scaled surface. The fine yak fibre (Figure 14.11C, left) has smooth overlapping edges encircling the whole fibre, and its morphological characteristics might be mistaken for those of the cashmere fibre in Figure 14.10C.

14.6 14.6.1

Scanning Electron Microscopy of Plant Fibres Poplar Wood

The micrographs in Figures 14.12A and B show cross-sections of a hardwood stem (poplar – genus Populus). Large and prominent vessels are surrounded by fibrous cells (libriform fibres and tracheids) with diameters of 10–30 µm and cell wall thicknesses of ∼2 µm. Chemically extracted poplar fibres with lengths of up to 1.5 mm are shown in Figures 14.12C and D. Pits (connection between cells in the tissue) appear as small holes in the fibres. The orientation of cellulose microfibrils in the S2 layer can be seen at higher magnifications (Figure 14.12D).

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Figure 14.12

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Poplar wood. A and B – cross-sections; C and D – length views of macerated fibrous cells.

Spruce Wood

Figures 14.13A and B show cross-sections of a softwood stem (Norway spruce – Picea abies). In contrast to hardwood, softwood is mainly composed of about 1–3 mm long tracheids with different cross-sectional dimensions across a growth ring. Fibre diameters range from ca. 10 to 40 µm, with cell wall thickness increasing from ∼1 µm (earlywood) up to 10 µm (latewood). Chemically isolated fibres are shown in Figures 14.13C and D. Pulp fibres are mostly extracted from wood. Structurally they can be considered as hollow tubes with closed ends. Their geometry is strongly dependent on tree species and location within the tree, and ranges from rectangular to round cross-sections with diameters of 5–50 µm and different wall thicknesses. Their lengths range from <1 mm to >3 mm. Further details about the structure and properties of wood fibres can be found in Chapter 2.2.

14.6.3

Flax (IFS Standard ‘C’ with an ISO Fineness of 23.5)

Flax (Linum usitatissimum L.) is a member of the family Linaceae. Flax is one of the oldest agricultural fibre crops grown worldwide. It is used as a fibre or fibre bundle for clothing, home textiles, furniture, technical textiles and composites. Figure 14.14 shows SEM micrographs of the IFS Standard ‘C’ with an ISO fineness of 23.5 from the Institut Franc¸ais du textile et de l’habillement, France. The IFS flax standards are described in greater detail in Chapter 18. The micrographs of the fine flax standard show well-separated fibres, but smaller bundles are still present in the sample (see Figures 14.14A and B). According to Latzke and Hesse (1988), the fibre diameter ranges from 5 to 20 µm. The cross-sections are described as being separate individual fibres with

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Figure 14.13 Spruce wood. A and B – cross-sections; C and D – length views of macerated fibrous cells of spruce earlywood (C) and latewood (D).

Figure 14.14 Fine IFS flax standard ‘C’ with an ISO fineness of 23.5 (from Institut Francais Textile, Habillement, France). A and B – cross-sections; C and D – length views.

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sharply polygonal shapes containing thin lumens. Figure 14.14B shows width values ranging from 12 µm for fibres up to 114 µm for fibre bundles. In the longitudinal view in Figures 14.14C and D, well-separated fibres with smooth surfaces can be found. The fibres show fibrenodes, which is a characteristic of flax. 14.6.4

Flax (IFS Standard ‘J’ with an ISO Fineness of 72.1)

The difference between the fine IFS flax standard ‘C’ shown in Figure 14.14 and the coarsest IFS standard ‘J’ in Figure 14.15 can be clearly seen. The fine flax standard shows well-separated single fibres and only a few small bundles, whereas thick bundles are found in the coarse standard. In these thick agglomerations of fibres it is hardly possible to identify the single fibres. Figure 14.15A shows bundle widths of up to ca. 180 µm. The length view (Figure 14.15C) illustrates that most of the fibre bundles are not fully separated into fibres. There is a morphological similarity to hemp fibres, which makes optical identification by microscope extremely doubtful. Compared with the hemp micrographs in this chapter (Figures 14.16 and 14.17), the micrographs show morphological features that differentiate hemp and flax. In practice this is not always the case. A secure classification can sometimes only be achieved by DNA analysis (see Chapter 16). 14.6.5

Hemp (4cGA – Mechanically Coarse Separated)

Hemp (Cannabis sativa L.) is an annual plant of the natural order Cannabinaceae. As described in greater detail in Chapter 5, the fibre bundles and fibres are located in the outer part of the stem. The fibre bundles can be removed from the stem by different techniques. Depending on the separation process, a wide range

Figure 14.15 Coarse IFS flax standard ‘J’ with an ISO fineness of 72.1 (from the Institut Francais Textile, Habillement, France). A and B – cross-sections; C and D – length views.

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Hemp ‘GA’ mechanically coarse separated. A and B – cross-sections; C and D – length views.

of possible fineness values can be achieved. Figure 14.16 shows the fibre bundles of a hemp stem coarsely separated by mechanical means; only a few single fibres can be seen. According to Latzke and Hesse (1988), single fibre width is about 5–40 µm; the fibre lengths are described as varying from 10 to 30 mm. In Figures 14.16A and B, some single cells forming the bundles can be identified. In the longitudinal views of Figures 14.16C and D, soft tissue is still attached to the bundles. Fibre to bundle width shows a very broad distribution.

14.6.6

Hemp (4cGADO – Chemically Separated)

Figure 14.17 illustrates the effect of a physicochemical separation process on the bundle structure of hemp. Mechanically coarse separated hemp (shown in Figure 14.16) was treated in a steam-explosion process and carded with an opener afterwards. It is clear that, compared with the coarse quality, the chemical treatment improves the separation of the fibre bundles. The differences in fineness become larger by this bundle opening (see Chapter 13, Figure 13.15). The presence of small bundles and single fibres indicates that a very efficient separation process has been achieved. Latzke and Hesse (1988) describe the cross-sections of single fibres (Figures 14.17A and B) as being polygonal or ribbon-shape forms and hollow, with different sizes and forms of lumen and different wall thicknesses. Figure 14.17A shows elliptical forms of the single cells with thickness ranges of ca. 4–60 µm for the bundle. The single fibre and bundle widths range from ca. 20 µm to 114 µm. The different appearances of the single fibres, in addition to the different fibre opening methods used, make an identification of bast fibre mixtures difficult and sometimes impossible. Mislabelling of mixtures of hemp with flax or other bast fibres is well known. Highly processed fibres or bundles in fabrics can hardly be identified. In these cases, a combination of microscopy and genetic analysis is the only choice for analysis (see Chapter 16).

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Figure 14.17

14.6.7

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Hemp (4cGADO) physicochemically opened. A and B – cross-sections; C and D – length views.

Tossa Jute (Bangladesh Grade BTD)

Tossa jute (Corchorus olitorius) is a bast fibre used worldwide and is the second most important vegetable fibre after cotton. It is produced from plants of the family Tiliaceae. The jute fibre comes from the outer skin of the stem. The fibre bundles are extracted by retting. In Bangladesh, six different grades of tossa jute can be distinguished: BT Special, BTA, BTB, BTC, BTD and BTE. The BTD grade shown in Figure 14.18 is characterised as follows: ‘light to medium grey/coppery grey; clean, sound fibre of good texture and good average lustre from blemish, clean-cut and well hackled’ (IJSG, 2003). More information can be found in Chapter 6. Jute is frequently used for the production of sacks, bags and packaging materials. It is sometimes known as a contaminant in wool production owing to the use of jute cord for packing material of wool bales. Latzke and Hesse (1988) describe the cross-sections of single fibres as round to polygonal in shape, with irregular lumen sizes and thick-walled cells. Figure 14.18B shows bundle widths between 41 µm and 79 µm. The length views (Figures 14.18C and D) show that the fibres mostly adhere to each other in a bundle. The fibres in Figure 14.18D have a striated or fibrillated surface, and many fibrenodes are visible.

14.6.8

White Jute (Bangladesh Grade BWB)

White jute (Corchorus capsularis) belongs to the family Tiliaceae. Chapter 6 provides more details. White jute originated in the Indo-Burma region, and tossa jute in Africa. In Bangladesh, the white jute is differentiated into six grades, namely BW Special, BWA, BWB, BWC, BWD and BWE. The characteristics of the BWB grade illustrated in Figure 14.19 is described as follows: ‘light cream to straw colour; fibre of good texture,

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Figure 14.18

Tossa jute, grade BTD. A and B – cross-sections; C and D – length views.

Figure 14.19

White jute, grade BWB. A and B – cross-sections; C and D – length views.

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Figure 14.20

Kenaf (GA) mechanically coarse separated. A and B – cross-sections; C and D – length views.

strong and good lustre, free from blemish; clean cut and well hackled excluding red ends’ (IJSG, 2003). Sawbridge and Ford (1987) reported that white jute is generally 20–25 µm in diameter (single fibre). The fibre lengths are reported to be only 1–5 mm, so that commercial use depends on overlapping fibre bundles of 10 or more individual cells. Figure 14.19A shows fibre bundle widths ranging from 18 to 106 µm. The single cells show a broad distribution in lumen size, cell wall thickness and shape. 14.6.9

Kenaf (LWK-GA – Mechanically Coarse Separated)

Kenaf (Hibiscus cannabinus L.) is a plant in the Malvaceae family. The kenaf fibre bundles shown in Figure 14.20 were mechanically separated from stems cultivated in a field experiment in Wehnen, Germany. The stems were non-retted. As can be seen in Figures 14.20C and D, the absence of retting leads to many, still attached parenchyma cells and very coarse bundles. In Figures 14.20A and B, some single fibres are seen. According to Sawbridge and Ford (1987), the diameter of kenaf fibres ranges from 12 to 36 µm. The length is reported to range from 2 to 6 mm. The cross-sections in Figures 14.20A and B show a variety of fibre forms, with round to polygonal shapes and irregular lumen sizes and wall thicknesses. Large fibre bundles as well as single fibres can be detected. The measured fibre bundle widths in Figure 14.20B range between ca. 30 and 247 µm. 14.6.10

Kenaf (Bangladesh HSF)

Parallelised, cleaned and chopped (70 mm) kenaf bundles from Bangladesh were used for SEM observation. In the length views in Figures 14.21C and D, the fibre bundles are much better separated compared with the unretted kenaf shown in Figure 14.20. The bundles are better cleaned, and more or less even surfaces

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Kenaf (HSF 1). A and B – cross-sections; C and D – length views.

can be seen. There is still a great variety in shape and size of the bundles, as shown in the cross-sectional views (Figures 14.21A and B). In Figure 14.21A, fibre and fibre bundle widths are measured between 16 and 224 µm. SEM clearly shows different appearances for fibre material taken from various retting, opening and treatment methods. 14.6.11

Cotton (USDA Calibration Cotton 32266 ‘Long/Strong’)

Cotton fibres are the seed hairs of the cotton plant Gossypium. As introduced in Chapter 10, there are only four recognised cultivar species of cotton: two diploid, Gossypium arboreum and G. herbaceum, and two tetraploid, G. hirsutum and G. barbadense. The diploid cottons are also called short staple cottons. G. hirsutum is usually referred to as upland cotton, while all extra-long staple/extra-fine cottons (also known as pima or giza types in Egypt) belong to G. barbadense. The USDA calibration cotton 32266 ‘long/strong’ (upland cotton) belongs to Gossypium hirsutum. The fibres are removed from the seeds by the ginning process. As can be seen in Figures 14.22A and B, the cotton fibre is bean- or U-shaped with a lumen inside the fibre. Twists of the fibres and the fibrils are typical for cotton fibres and are visible at higher magnifications of the fibre surface (Figure 14.22D). The irregular helical convolutions in the longitudinal views are specific to cotton. 14.6.12

Cotton (USDA Calibration Cotton 31977 ‘Short/Weak’)

Two different kinds of upland cotton are shown in Figures 14.22 and 14.23, which are used in the calibration of high-volume-instruments of quick instrumental testing of various cotton fibre qualities. Further

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Figure 14.22

USDA calibration cotton 32266 ‘long/strong’ (upland cotton). A and B – cross-sections; C and D – length views.

Figure 14.23

USDA calibration cotton 31977 ‘short/weak’ (upland cotton). A and B – cross-sections; C and D – length views.

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information on USDA calibration cotton is given in Chapter 17. The shapes of mature fibres are nearly round, whereas immature fibres show a very thin, flat and ribbon-like appearance. Owing to electron effects, these fibres appear brighter in the SEM – especially in longitudinal views (see Figure 14.23D). These immature fibres tend to tangle together and form neps, which are troubling in the spinning process. These neps can also be seen on the surfaces of textiles and are visible to the naked eye owing to their different dye uptake. With the SEM it is not possible to determine the provenance of the fibres or the cotton variety. It is also not possible to verify genetically modified (GM) cottons using the SEM. Such characterisation can be done using DNA analytical methods as described in Chapter 16. In mixes with other fibres it is easy to identify the cotton fibres owing to their distinct appearance. 14.6.13

Abac´a (Philippine Grade M1)

As reported in Chapter 7, abac´a is a perennial plant. Abac´a fibres originate from a certain banana plant (Musa textiles N´ee), which grows in eastern Asia and especially in the Philippines. It is also called ‘manila hemp’, but there is no botanical relationship to hemp (Cannabis sativa L.). Therefore, it is suggested that the name ‘manila hemp’ not be used in a systematic nomenclature. A mature abac´a plant consists of a group of stalks (pseudostems). These pseudostems are made up of a central core that is encircled by overlapping leaf sheaths. The fibre bundles are separated from the leaf sheaths. Fibre bundles from the outer leaf sheath are in general coarser than the fibre bundles from the inner part of the pseudostem. As described in Chapter 3.1, abac´a fibres are classified into grades. Grade M1 is hand-stripped bundles from the outer leaf sheath with a cleaning grade of fair. The coarse grade M1 is described as ‘medium brown’ and of so-called ‘short normal’ length (Guarte and Sinon, 2004). Sawbridge and Ford (1987) report that the separated single fibres are 3–12 mm long and 16–32 µm in diameter. Figures 14.24A and B show the cross-section of the abac´a bundles. The bundles are

Figure 14.24

Abaca, ´ coarse grade M1. A and B – cross-sections; C and D – length views.

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Figure 14.25

Abaca, ´ fine grade S3. A and B – cross-sections; C and D – length views.

composed of single fibres that have a polygonal to round shape with an irregular lumen. From the longitudinal views in Figures 14.24C and D it is apparent that a large amount of the soft tissue of the leaf is still connected to the coarse bundles.

14.6.14

Abac´a (Philippine Grade S3)

Abac´a grade S3 is defined as hand-stripped bundles from the outer leaf sheath with a cleaning grade of excellent. The fine grade S3 is described as ‘streaky three’ and of so-called ‘short normal length’ (Guarte and Sinon, 2004). In comparison with the coarse standard shown in Figure 14.24A to D, the bundles are finer and the surface is better cleaned from leaf soft tissue (Figures 14.25C and D). The cross-sections in Figure 14.25A and B are of clear shape, and the bundles are much finer compared with grade M1. The measured bundle widths in Figure 14.25C range from ca. 45 to 181 µm.

14.6.15

Sisal (Mechanically Coarse Separated)

Sisal is a leaf fibre from the agave plant (Agave sisalana P.), which is a monocotyledon of the family Agavaceae. It is often used for the production of ropes and cords and coarse yarns, as well as geotextiles and composites. The plant grows in tropical and subtropical areas such as Brazil, Tanzania, Kenya and the People’s Republic of China (major producers). The fibre bundles are extracted from the sisal leafs as described in Chapter 8. As shown in Figures 14.26A and B, the bundles are pieced together from single fibres.

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Figure 14.26

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Sisal, mechanically treated. A and B – cross-sections; C and D – length views.

The fibres show a polygonal to round shape, with irregular round or oval-shaped lumens. Latzke and Hesse (1988) reported a bundle diameter of ca. 80 µm and a fibre diameter of 5–20 µm. Figure 14.26B shows bundle widths in a range of ca. 114–288 µm. The longitudinal appearance of mechanically separated sisal bundles is shown in Figures 14.26C and D. Soft tissue of the leaf is still attached to the bundle and causes a rough bundle surface.

14.6.16

Sisal (Chemically Separated)

There is a remarkable difference in the appearance of the sisal fibres and bundles from different stages of treatment. As shown in Figs 14.27A and B, the cross-sections of the chemically treated sisal seem partly disintegrated. By comparing the length views of mechanically treated (Figures 14.26C and D) and chemically treated fibres (Figures 14.27C and D), chemical treatment is shown to change the surface of the fibres dramatically. Some fibres in Figure 14.27C show a cotton-like structure. This feature sometimes complicates a reliable identification of bast fibres. Figure 14.27C shows fibre widths of 7 µm up to bundle widths of 127 µm.

14.6.17

Coir (Mattress Fibres)

Coir fibre is obtained from the outer layer (coconut husk) of the fruit of the coconut palm (Cocos nucifera L.). As detailed in Chapter 9, the coconut husk consists of the epicarp (water-resistant outer skin) and the

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Figure 14.27

Sisal, chemically treated. A and B – cross-sections; C and D – length views.

mesocarp (fibrous zone). This fibrous zone consists of fibre bundles embedded in non-fibrous parenchyma tissue. There are two types: white and brown coir. White coir fibre bundles originate from immature green coconut. Brown coir is obtained from ripened coconuts that have lost their green colour. Besides ‘omatt fibres’ (fibre waste sorted during processing), brown ‘bristle’ and ‘mattress’ coir can be produced. Mattress fibres are described as short and ‘brown’ fibre bundles that are not spinnable and are used as bristles for brushes (Schnegelsberg, 1999). Latzke and Hesse (1988) describe the cross-sections of the individual cells as round to oval, with a prominent lumen and a width of 10–30 µm. Figures 14.28A and B show cross-sections of bundles consisting of flattened single fibres, sometimes with thin cell walls. The measured bundle widths in Figure 14.28C range from ca. 140 to 333 µm. The longitudinal views in Figures 14.28C and D show the typical appearance of coir bundles with cavities on the surface.

14.6.18

Coir (Bristle Fibres)

Bristle fibres are long and ‘brown’ fibre bundles that are separated from immature husks. The fibre bundles are spinnable (Schnegelsberg, 1999). Sawbridge and Ford (1987) report the cell walls as being thick and irregular in shape. According to Latzke and Hesse (1988), the surface is smooth to fibrillated and shows slightly transverse scratches. The bundle length is described as 150–300 mm. The width of the single cell is described as ranging from 10 to 30 µm. Figure 14.29C shows bundle widths between ca. 91 and 426 µm.

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Figure 14.28

Coir mattress fibres. A and B – cross-sections; C and D – length views.

Figure 14.29

Coir twisted fibres. A and B – cross-sections; C and D – length views.

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Summary

The reason for working on reliable and secure identification methods for natural fibres is their great number in a historical sense and also their modern applications. In this context, the use of the SEM for identification is very important. An interesting application is described in Chapter 1. Identification of cotton and wool using SEM is quite easy. In addition, SEM is valuable in the assessment of fibre damage. Even in the field of modern composites, it is possible to identify natural fibres, e.g. jute or flax in a composite. When dealing with a needle felt of a hemp/flax mixture or a cashmere/yak mix, SEM is limited. In these cases, combinations of microscopy and genetic analyses become the methods of choice. When dealing with the identification of natural fibres, the build-up of picture databases, including the widespread morphologies of different fibres, is extremely helpful. A strong recommendation is to collect and use reliable reference material rather than material from unknown sources. For this reason, all pictures shown herein are based on reference materials. We are confident that these ideas will make your job easier and give you inspiration to deepen your knowledge in the field of fibre identification.

References Flegler, S.L., Heckman, J.W. and Klomparens, K.L. (1993) Scanning and Transmission Electron Microscopy – An Introduction. Oxford University Press, Oxford, UK. Goldstein, J., Newbury, D.E., Joy, D.C., Lyman, C.E., Echlin, P., Lifshin, E., Sawyer, L.C. and Michael, J.R. (2005) Scanning Electron Microscopy and X-Ray Microanalysis. 3rd edition, Springer, Heidelberg/Berlin/New York. Goodhew, P.J. and Humphres, F.J. (1998) Electron Microscopy and Analysis, 2nd edition. Taylor & Francis, London, UK. Guarte, R.C. and Sinon, F.G. (2004) Processing of high quality abac´a fiber for industrial application. Department of Agricultural Engineering, Abac´a Research Center, Leyte State University, the Philippines. Heine, B. (2003) Werkstoffpr¨ufung – Ermittlung von Werkstoffeigenschaften. Fachbuchverlag Leipzig im Carl Hanser Verlag, Munich/Germany/Vienna, Australia. IJSG (2003) Fibre grading for tossa and white jute in Bangladesh. The International Jute Study Group (IJSG), Dhaka, Bangladesh; available at: http://www.jute.org (accessed 1 March 2009). Latzke, P.M. and Hesse, R. (1988) Textile Fasern: Rasterelektronenmikroskopie der Chemie- und Naturfasern; Analysieren, Klassifizieren, Zitieren, Ordnen. Deutscher Fachverlag, Frankfurt am Main, Germany. Lyman, C.E., Newbury, D.E., Goldstein, J., Williams, D.B., Romig, A.D., Jr, Armstrong, J., Echlin, P., Fiori, C., Joy, D.C., Lifshin, E. and Peter, K.-R. (1990) Scanning Electron Microscopy, X-Ray Microanalysis and Analytical Electron Microscopy: A Laboratory Workbook. Springer, Heidelberg/Berlin/New York. Phan, K.H. (1994) Neue Erkenntnisse u¨ ber die Morphologie von Keratinfasern mit Hilfe der Elektronenmikroskopie. Dissertation, RWTH Aachen, Aachen, Germany. Sawbridge, M. and Ford, J.E. (1987) Textile Fibres under the Microscope. Shirley Institute Publications, Didsbury, UK. Schnegelsberg, G. (1999) Handbuch der Faser – Theorie und Systematik der Faser. Deutscher Fachverlag, Frankfurt am Main, Germany. Shi, X.-J. and Yu, W.-D. (2008) Identification of animal fiber based on scale shape. Congress on Image and Signal Processing (CISP 2008), Sanya, Hainan, China.

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15 Combined (In Situ) Methods Ingo Burgert and Michaela Eder Max-Planck-Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany

15.1 Introduction A powerful approach to gaining access to deformation mechanisms of plant fibres at the cell and cell wall level are in situ methods that simultaneously combine mechanical loading and monitoring of structural deformation. By examining the plant fibre deforming, micro- and nanostructural deformation patterns and fracture events can be studied. At the micro scale, the influence of fibre shape and size and of pits or dislocations on the fibre properties can be analysed. Simultaneous observation of deformations at the nano scale helps to identify load-bearing components and networks in the cell wall and provide further knowledge on the underlying macromolecular interactions that determine the mechanical performance of the plant fibres. Four in situ approaches that should demonstrate the powerful strategy of combined (in situ) methods are introduced in this chapter. The purpose and interpretation of the studies, as well as the methodical background, are briefly described. The first two methods focus on the micro scale of plant fibres, following crack propagation and elucidating deformation mechanisms of the entire cell wall. The two other methods reveal nanodeformation of cell wall polymers, mainly of cellulose fibrils.

15.2

In Situ Micromechanical Tests Combined with Polarised Light Microscopy

The deformation of plant material upon external straining can be studied by micromechanical studies combined with light microscopy (Badel and Perr´e, 1999). However, at the single fibre level, the resolution might not be high enough to follow events at the cell wall level. If polarised light microscopy is utilised, then the mechanical response of specific cell wall features of plant fibres, can be observed. All natural fibres, such as xylem fibres (e.g. wood) or bast fibres (e.g. flax, hemp), contain fibrenodes, i.e. local spots in the cell wall at which the orientation of cellulose microfibrils differs from the surrounding cell wall (Bos and Donald, 1999; Nyholm et al., 2001). For instance, hemp fibres possess cellulose fibrils oriented almost parallel to the fibre axis (small microfibril angle), whereas in regions of fibrenodes much higher microfibril angles are observed. The influence of these fibrenodes on the mechanical properties of bast fibres is a matter of interest (Bos et al., 2002; Baily, 2004; Thygesen et al., 2007). Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Figure 15.1 Force–strain curve of a hemp fibre combined with polarised light microscopy images at different load levels, showing fibrenodes gradually disappearing during stretching. In polarised light the bulk fibre appears dark, whereas fibrenodes are illuminated. Adapted from L.G. Thygesen, M. Eder and I. Burgert, Dislocations in single hemp fibres – Investigations into the relationship of structural distortions and tensile properties at the cell wall level, Journal of Materials Science, 42, 2007, 558–564.

Because cellulose is birefringent, fibrenodes can be visualised by using polarised light microscopy (PLM). The bulk fibre wall appears dark, whereas fibrenodes light up if a fibre is viewed under crossed polarisers and properly oriented with respect to the direction of polarisation of one of the two filters (see Chapter 4, Figures 4.3 and 4.5). The shown in situ tensile experiment under polarised light (Figure 15.1) was performed on a hemp fibre in order to observe the fibrenode-related deformation processes upon tensile straining (Thygesen et al., 2007). Figure 15.1 shows the force–strain curve of an individual hemp fibre, accompanied with corresponding polarised light images at different force levels. During the tensile straining of the hemp fibre, the fibrenodes disappear gradually, and they are no longer visible at around 50% of the fracture strength. It can be seen that, upon stretching, the cellulose microfibrils in the fibrenode areas are shifted towards a parallel alignment to the fibre axis, which may explain why no direct influence of fibrenodes on mechanical properties has yet been observed.

15.3 In Situ Micromechanical Tests Combined with Scanning Electron Microscopy Crack initiation and propagation studies on individual plant fibres at higher magnifications than light microscopy can be performed with mechanical testing setups that are operated in the chamber of a scanning electron microscope (Bodner et al., 1996 and 1997). It is of particular advantage when the SEM can be operated in a ‘low-vacuum’ mode. This mode does not require a conductive coating of biological materials as known for classical ‘high-vacuum’ studies (see Chapter 14). Hence, the new surfaces created in the deformation process will not be charged, thus permitting in situ microfracture studies of plant fibres (Mott et al., 1995; Fr¨uhmann et al., 2003). Furthermore, in the ‘wet mode’ of an environmental scanning electron microscope (ESEM), the water content of plant fibres can be controlled, which allows for mechanical probing under different moisture contents or even fully hydrated conditions. In order to control the water content of the plant fibre, it is necessary to keep the sample cool to compensate for the low pressure in the ESEM chamber. This can be achieved by mechanical testing devices, which are additionally equipped with a Peltier cooling stage located directly below the plant fibre (Eder et al., 2008). Figure 15.2 shows an example of a microtensile testing device, which was specially designed for in situ ESEM experiments, and a spruce wood fibre before and after fracture.

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Figure 15.2 (A) Microtensile testing device specially designed for in situ ESEM experiments on plant fibres: A – foliar frame with sample; B – load cell (with pin); C – Peltier cooling stage; D – water cooling stage; E – step motor. (B) Spruce wood fibre before and after fracture; scale bar 50 µm. Reproduced with permission from M. Eder, S.E. Stanzl-Tschegg and I. Burgert, The fracture behaviour of single wood fibres is governed by geometrical constraints – In-situ ESEM studies on three fibre types, Wood Science Technology, 42, 2008, 679–689.

15.4

In Situ Micromechanical Tests Combined with X-Ray Diffraction

As shown in the previous section, polarised light microscopy can be used to track changes in the orientation of cellulose fibrils upon stretching. However, this method provides only a qualitative proof of the deformation mechanisms in the cell wall. For direct monitoring of changes in the cell wall assembly, nanostructural observation techniques need to be combined with micromechanical tests. Powerful techniques to visualise changes in the crystal lattice of cellulose or changes in the orientation of cellulose microfibrils upon tensile straining are X-ray scattering and diffraction methods (K¨ohler and Spatz, 2002; Keckes et al., 2003; Kamiyama et al., 2005; K¨olln et al., 2005; Gindl et al., 2006; Gindl et al., 2008; Peura et al., 2005; Peura et al., 2006; Peura et al., 2007; Martinschitz et al., 2008). Small-angle X-ray scattering (SAXS) takes advantage of the electron density contrast between the crystalline parts of cellulose microfibrils and matrix polymers, whereas wide-angle X-ray diffraction (WAXD) elucidates the periodic structures of the crystalline parts of the cellulose microfibrils (Lichtenegger et al., 1998). The cellulose microfibril angle in plant cell walls can be studied by both methods, which have been extensively used in the last decades (Kantola and K¨ahk¨onen, 1963; Evans, 1994; Cave, 1997; M¨uller et al., 1998; Reiterer et al., 1998; Saren et al., 2001). A concern about in situ measurements of changes in cellulose microfibril angle or the crystal lattice of cellulose are the relaxation processes in the biological material during data acquisition. Therefore, the data acquisition times should be as short as possible, which can be achieved with the high brilliance of an X-ray beam at a synchrotron source. In Figure 15.3 the schematic set-up for an in situ X-ray test, an example

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Figure 15.3 (A) Schematic drawing of the in situ test set-up. A plant fibre bundle is stretched in a tensile tester, and the scattered signal is collected by a 2D CCD detector. Reproduced with permission from I. Burgert, J. Keckes and P. Fratzl, Mechanics of the wood cell wall, Characterization of the cellulosic cell wall, D.D. Stokke, L.H. and Groom, pp 30–37. Copyright 2006 John Wiley & Sons Ltd. (B) Stress–strain curve of a coir fibre bundle and the change in microfibril orientation upon straining. Adapted from K.J. Martinschitz, P. Boesecke, C.J. Garvey, W. Gindl and J. Keckes, Changes in microfibril angle in cyclically deformed dry coir fibers studied by in-situ synchrotron X-ray diffraction, J. Mat. Sci., 43, 350–356 (2008).

stress–strain curve and the detected changes in microfibril angle upon tensile stretching of a coir fibre bundle are shown (Martinschitz et al., 2008). The stress–strain curve of the coir fibre bundle in Figure 15.3 displays a typical biphasic behaviour known for plant tissues with rather large microfibril angles in their cell walls (see Chapter 2.2). During straining, the cellulose fibrils are continuously shifted towards the cell axis as the cellulose microfibril angle decreases almost linearly from an angle of ∼43◦ in an unstressed state to an angle of ∼28◦ just before sample rupture.

15.5

Micromechanical Tests Combined with Raman or FT-IR Spectroscopy

Another methodical approach that is well suited to in situ tests is the combination of mechanical loading with spectroscopic analysis, e.g. FT-IR spectroscopy and Raman spectroscopy. Both methods can detect changes that occur at the molecular level when samples are subjected to external strain, and thereby make it possible to identify the load-bearing components of the composite structure (Eichhorn et al., 2001; Salm´en et al., 2005; Gierlinger et al., 2006). Raman spectroscopy is well suited to studying the deformation of molecular bonds in a plant, particularly bonds of cellulose (Eichhorn et al., 2001; Gierlinger et al., 2006). A strong, sharp band deriving from C–C and C–O–C stretching of cellulose is located at a wave number of ∼1097 cm−1 (Agarwal, 1999). This characteristic band shows changes in peak shape, peak intensity and peak position in the spectra acquired during external straining of plant fibres or tissues. In the given example, a single normal wood spruce fibre was stretched in a microtensile tester that was combined with a confocal Raman microscope equipped with a linear polarised laser (Gierlinger et al., 2006). The spectra acquired during the fibre stretching allow comparison of the stress–strain curve of the fibre and the calculated wave number shifts of the band at 1097 cm−1 (Figure 15.4). With increasing stress and strain in the fibre, the peak of the band at 1097 cm−1 is progressively shifted towards shorter wave numbers. This indicates that the C–O–C bond in the cellulose molecule is uniformly stretched by the external loading. This deformation is purely elastic, as the fracture of the fibre results in a rebounding of the peak to the initial peak position.

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Figure 15.4 (A) Tensile tester combined with a Raman spectroscope for in situ tests; (B) stress–strain curve of a single spruce latewood fibre and the calculated peak shift of the band at 1097 cm−1 (grey squares) representing the stretching of cellulose. Adapted with permission from N. Gierlinger, M. Schwanninger, A. Reinecke and I. Burgert, Molecular changes during tensile deformation of single wood fibers followed by Raman microscopy, Biomacromolecules, 7, 2077–2081. Copyright 2006 American Chemical Society.

These findings are in good agreement with dynamic FT-IR measurements on various cellulosic materials, which also indicate that cellulose fibrils are the main load-bearing structure (Salm´en et al., 2005). The same response has been observed for the C–O–C bond and the intramolecular hydrogen bond 3-OH···O-5, which both axially connect the glucose units in the macromolecular chain (Hinterstoisser et al., 2001). Moreover, the dynamic FT-IR measurements help to elucidate the interaction of cellulose with other cell wall macro˚ molecules during stretching and thereby provide valuable information on the cell wall assembly. Akerholm and Salm´en (2001) could distinguish between the specific interactions of different types of hemicellulose in softwood. The dynamic FT-IR analysis indicated that cellulose microfibrils are surrounded by glucomannan, whereas lignin is associated with xylan.

15.6

Conclusion

The introduced in situ methods allow monitoring of structural deformation of biological samples upon controlled mechanical loading. The unique combination of sophisticated micromechanical testing devices and micro- and nanostructural analysis provides access to cell wall deformations, polymer reorientations and fracture events. Hence, these in situ methods present a powerful approach to studying deformation patterns at the cell and cell wall level, and to gaining access to the underlying mechanisms that determine the mechanical response of a plant fibre.

Acknowledgement The authors would like to thank all those colleagues who performed or substantially contributed to the in situ methods shown, in particular N. Gierlinger, L. Thygesen, S.E. Stanzl-Tschegg, J. Keckes and P. Fratzl.

References Agarwal, U.P. (1999) An overview of Raman spectroscopy as applied to lignocellulosic materials, in Advances in Lignocellulosics Characterization, ed. by Argyropoulos, D.S. TAPPI Press, Atlanta, GA, USA, pp. 209–225.

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˚ Akerholm, M. and Salm´en, L. (2001) Interactions between wood polymers studied by dynamic FT-IR spectroscopy. Polymer, 42, 963–969. Badel, E. and Perr´e, P. (1999) D´etermination des propri´et´es e´ lastiques d’´el´ements individuels du plan ligneux du chˆene par des essais de traction sur micro-´eprouvettes. Ann. Sci. For., 56, 467–478. Baily, C. (2004) Influence of kink bands on the tensile strength of flax fibers. J. Mater. Sci., 39, 331. Bodner, J., Gr¨ull, G. and Schlag, M. (1996) In situ fracturing of wood in the scanning electron microscope. Holzforschung, 50, 487–490. Bodner, J., Gr¨ull, G. and Schlag, M. (1997) Fracture initiation and progress in wood specimens stressed in tension. Holzforschung, 51, 487–490. Bos, H.L. and Donald, A.M. (1999) In situ ESEM study of the deformation of elementary flax fibres. J. Mater. Sci., 34, 3029–3034. Bos, H.L., van den Oever, M.J.A. and Peters, O.C.J.J. (2002) Tensile and compressive properties of flax fibres for natural fibre reinforced composites. J. Mater. Sci., 37, 1683–1692. Burgert, I., Keckes, J. and Fratzl, P. (2006) Mechanics of the wood cell wall, in Characterization of the Cellulosic Cell Wall, ed. by Stokke, D.D. and Groom, L.H. Blackwell, Oxford, UK, pp. 30–37. Cave, I.D. (1997) Theory of X-ray measurement of microfibril angle in wood, I and II. Wood Sci. Technol., 31, 143–152 and 225–234. Eder, M., Stanzl-Tschegg, S.E. and Burgert, I. (2008) The fracture behaviour of single wood fibres is governed by geometrical constraints – in-situ ESEM studies on three fibre types. Wood Sci. Technol., 42(8), 679–689. Eichhorn, S.J., Sirichaisit, J. and Young, R.J. (2001) Deformation mechanisms in cellulose fibres, paper and wood. J. Mater. Sci., 36, 3129–3135. Evans, R. (1994) Rapid measurement of the transverse dimensions of tracheids in radial wood sections from Pinus radiate. Holzforschung, 48, 168–172. Fr¨uhmann, K., Burgert, I., Tschegg, E.K. and Stanzl-Tschegg, S.E. (2003) Detection of the fracture path under tensile loads through in situ tests in an ESEM chamber. Holzforschung, 57, 326–332. Gierlinger, N., Schwanninger, M., Reinecke, A. and Burgert, I. (2006) Molecular changes during tensile deformation of single wood fibers followed by Raman microscopy. Biomacromolecules, 7, 2077–2081. Gindl, W., Martinschitz, K.J., Boesecke, P. and Keckes, J. (2006) Strucural changes during tensile testing of an all-cellulose composite by in situ synchrotron X-ray diffraction. Compos. Sci. Technol., 66, 2639–2647. Gindl, W., Reifferscheid, M., Martinschitz, K.J., Boesecke, P. and Keckes, J. (2008) Reorientation of crystalline and noncrystalline regions in regenerated cellulose fibers and films tested in uniaxial tension. J. Polym. Sci. Part B – Polym. Phys., 46, 297–304. Hinterstoisser, B., Akerholm, M. and Salm´en, L. (2001) Effect of fiber orientation in dynamic FTIR study on native cellulose. Carbohydr. Res., 334, 27–37. Kamiyama, T., Suzuki, H. and Sugiyama, J. (2005) Studies of the structural change during deformation in Cryptomeria japonica by time-resolved synchrotron small-angle X-ray scattering. J. Struct. Biol., 151, 1–11. Kantola, M. and K¨ahk¨onen, H. (1963) Small-angle X-ray investigation of the orientation of crystallites in Finnish coniferous and deciduous wood fibers. Ann. Acad. Sci. Fenn., 37, 3–14. Keckes, J., Burgert, I., Fr¨uhmann, K., M¨uller, M., K¨olln, K., Hamilton, M., Burghammer, M., Roth, S.V., Stanzl-Tschegg, S.E. and Fratzl, P. (2003) Cell-wall recovery after irreversible deformation of wood. Nat. Mater., 2, 810–814. K¨ohler, L. and Spatz, H.C. (2002) Micromechanics of plant tissues beyond the linear-elastic range. Planta, 215, 33–40. K¨olln, K., Grotkopp, I., Burghammer, M., Roth, S.V., Funari, S.S., Dommach, M. and M¨uller, M. (2005) Mechanical properties of cellulose fibres and wood. Orientational aspects in situ investigated with synchrotron radiation. J. Synchrotron Rad., 12, 739–744. Lichtenegger, H., Reiterer, A., Tschegg, S. and Fratzl, P. (1998) Determination of spiral angles of elementary fibrils in the wood cell wall: comparison of small-angle X-ray scattering and wide-angle X-ray diffraction, in Microfibril Angle in Wood, ed. by Butterfield, B.G. IAWA Press, Christchurch, New Zealand, pp. 140–156. Martinschitz, K.J., Boesecke, P., Garvey, C.J., Gindl, W. and Keckes, J. (2008) Changes in microfibril angle in cyclically deformed dry coir fibers studied by in-situ synchrotron X-ray diffraction. J. Mater. Sci., 43, 350–356. Mott, L., Shaler, S.M., Groom, L.H. and Liang, B.H. (1995) The tensile testing of individual wood fibre using environmental scanning electron microscopy and video image analysis. Tappi, 78, 143–148. M¨uller, M., Czihak, C., Vogl, G., Fratzl, P., Schober, H. and Riekel, C. (1998) Direct observation of microfibril arrangement in a single native cellulose fibre by small-angle X-ray scattering. Macromolecules, 31, 3953–3957.

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Nyholm, K., Ander, P., Bardage, S. and Daniel, G. (2001) Dislocations in pulp fibres – their origin, characteristics and importance – a review. Nordic Pulp Paper Res. J., 4, 376–384. Peura, M., Grotkopp, I., Lemke, H., Vikkula, A., Laine, J., M¨uller, M. and Serimaa, R. (2006) Negative Poisson ratio of crystalline cellulose in kraft cooked Norway spruce. Biomacromolecules, 7, 1521–1528. Peura, M., K¨olln, K., Grotkopp, I., Saranp¨aa¨ , P., M¨uller, M. and Serimaa, R. (2007) The effect of axial strain on crystalline cellulose in Norway spruce. Wood Sci. Technol., 41, 565–583. Peura, M., M¨uller, M., Serimaa, R., Vainio, U., Sar´en, M.P., Saranp¨aa¨ , P. and Burghammer, M. (2005) Structural studies of single wood cell walls by synchrotron X-ray microdiffraction and polarised light microscopy. Nucl. Instrum. Meth. Phys. Res. B, 238, 16–20. Reiterer, A., Jakob, H.F., Stanzl-Tschegg, S.E. and Fratzl, P. (1998) Spiral angle of elementary cellulose fibrils in cell walls of Picea abies determined by small-angle X-ray scattering. Wood Sci. Technol., 32, 335–345. Salm´en, L., Akerholm, M. and Hinterstoisser, B. (2005) Two-dimensional Fourier transform infrared spectroscopy applied to cellulose and paper, in Polysaccharides – Structural Diversity and Functional Versatility, 2nd edition, ed. by Dumitriu, S. Marcel Dekker, New York, NY, USA, pp. 159–187. Saren, M.P., Serimaa, R., Andersson, S., Paakkari, T., Saranpaa, P. and Pesonen, E. (2001) Structural variation of tracheids in Norway spruce (Picea abies (L.) Karst.), J. Struct. Biol., 136, 101–109. Thygesen, L.G., Eder, M. and Burgert, I. (2007) Dislocations in single hemp fibres – investigations into the relationship of structural distortions and tensile properties at the cell wall level. J. Mater. Sci., 42, 558–564.

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16 DNA-Analytical Identification of Species and Genetic Modifications in Natural Fibres Lothar Kruse Impetus GmbH & Co. Bioscience KG, Bremerhaven, Germany

16.1 Introduction The availability of reliable methods to identify species and genetic modifications in plant and animal hair fibres is becoming increasingly important for many reasons. In this chapter the DNA-based analytical procedure we have established in our laboratory for the identification of species and genetic modifications in fibres is presented. Information will be given about the basic structure of DNA and how to extract this molecule from natural animal and plant fibres. The reader will become familiar with the polymerase chain reaction (PCR), an extremely sensitive method for amplifying tiny traces of DNA, and gain insight into the procedure of detection and identification of genetic modifications of cotton.

16.1.1

Natural Fibres

In Chapter 1, Fenella France describes how the first US star-spangled banner was made of cotton and wool. But it is not only the American Nation that relies on natural fibres. They also play an important role in many branches of industry. Well-known animal fibres are sheep’s wool, cashmere, alpaca, angora, mohair and camel. Commercially relevant plant fibres are, among others, cotton, jute, flax, coir, sisal, abac´a, hemp and kenaf. Speciality fibres, i.e. precious animal hair fibres not derived from sheep’s wool, have frequently been found to be adulterated with much cheaper animal fibres such as sheep’s wool or yak hair. Cashmere, one of the finest and most expensive animal hair fibres, is frequently the object of fraud. The increasing demand for Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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speciality fibres, high prices and the very restricted availability cause adulteration or false declaration (Phan and Wortmann, 2004). The only method so far established for the identification of animal fibres is scanning electron microscopy (see Chapter 14). However, this method is very time consuming and expensive and strongly dependent on the experience of individual microscopists. According to international standards, a large number of fibres, i.e. several hundreds to a thousand, should be manually identified and measured according to their fineness. But this method does not allow the differentiation of certain fibre combinations, e.g. fine yak hair and brown cashmere. Processing steps commonly used in the textile industry cause additional problems. As SEM has to rely on physical parameters such as internal morphology, fibre diameter and cuticle scale height, all influences (e.g. superwash) masking those visible structures complicate the analysis. Often, additional chemical analyses or swelling tests are unevitable, so that SEM seems not to be an optimal tool for mass screening. Microscopy sometimes cannot be used for the differentiation of plant fibres owing to the lack of visible morphological features. In particular, blends of flax and hemp can hardly ever be distinguished visually.

16.1.2

Genetically Modified Cotton

Genetically modified plants have been cultivated for more than 20 years (see also Chapter 10). Compared with soya (Glycine max), maize (Zea mays) and rape (Brassica napus), which are of enormous importance for global food and feed production, cotton is of minor interest for this industry. But it is still the most important natural fibre – 40% of the textiles produced worldwide are made of it. Alhough there have been a few field trials with genetically modified flax and hemp, a commercially relevant cultivation has not yet begun. To date, cotton is the only genetically modified plant widely used in the fibre and textile industry. The global cultivation of genetically modified cotton has increased enormously. In 2008, on nearly 50% of the global cotton cultivation area, approximately 20 different genetically modified cotton varieties were cultivated (Agbios GM Database, 2009). The primary objectives of the modifications are: r r r r

resistance against pests, e.g. bollworm (Bt cotton); tolerance against herbicides, e.g. glyphosate (RoundUp Ready) or glufosinate (Liberty Link); adaptation to abiotic factors like cold, heat, dryness and salt; improved fibre qualities, e.g. optimisation of length and strength (Agbios GM Database, 2009).

The proportion of cotton produced according to organic criteria is still very low – worldwide approximately 1% – but a growing demand for organic cotton can be observed (for more details, see Chapter 10). Many countries have defined criteria for organic labelling, and, in spite of all deviations between different regulations, one clear agreement can be identified – the absolute ban on genetic engineering. It is very difficult, if not impossible, to avoid unintended mixing of organic cotton and genetically modified cotton. Furthermore, adulteration cannot be excluded because of the increasing demand for organic cotton, its higher price and the very restricted availability. Certification programmes and standards that guarantee organic status still rely on documents and do not yet use analytical testing (Soth, J., Helvetas, 2008, private communication).

16.2 DNA Analysis Many areas of biomedical research, diagnostics and routine examinations are no longer imaginable without molecular biological methods. These are also becoming increasingly important in analytical chemistry for food and feed testing, especially the polymerase chain reaction (PCR), a method explained later in this chapter. For

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instance, in most cases it would be impossible to detect evidence of genetically modified organisms (GMOs) without PCR, which is why this is made imperative in national and international regulations.

16.2.1

DNA

The genetic make-up of all organisms consists of DNA (deoxyribonucleic acid). To understand the basic principle of important molecular biological techniques, e.g. PCR, one should be familiar with some characteristics of the DNA. It is a very long double-helical molecule consisting of a repetition of just four basic nucleotide building blocks – adenine, thymine, cytosine and guanine. Under physiological conditions, the two single-stranded DNA molecules are held together by hydrogen bonds between the nitrogenous bases stabilising the double helix. Only two base pairs are possible – adenine can only pair with thymine and cytosine can only pair with guanine. High temperatures will disrupt the hydrogen bonds, i.e. single-stranded DNA will be generated. Cooling down the solution will result in renaturation, i.e. the double-stranded structure will be restored. The precise sequence of the DNA nucleotides differs from organism to organism. The more remote the taxonomic relationship between organisms, the greater the difference in their DNA sequence becomes. These differences represent the basis for identifying different species or even individuals. Independently of the nature of the sample to be analysed, the examination generally comprises three basic steps: (1) extraction of the DNA, (2) the polymerase chain reaction and (3) detection. The three basic steps are explained in more detail as follows:

16.2.1.1

Extraction of the DNA

A crucial factor influencing the success of any analytical examination of the DNA is the availability of sufficient quantities of DNA in the purest possible form. The quality and quantity of the DNA isolated from a parent material depends directly on the degree to which it is processed. The more strongly a material is processed, the shorter the average fragment length of the originally very long DNA. Of great importance is also the source of DNA, i.e. the tissue from which the DNA is to be extracted. It is easy to extract large amounts of undegraded DNA from fresh liver or muscle because these tissues contain more DNA than nails or hairs. It is not difficult to extract DNA from fresh plant leaves, but it is harder to get DNA from fibres (Taliercio et al., 2005). The DNA content of hair shafts is typically low (Allen et al., 1998). Although the keratinisation process probably causes cytolysis and subsequent loss of nuclear DNA in the hair cortex (Ackermann et al., 1993), it has been shown that extraction of nuclear DNA from shafts is still possible (Schreiber et al., 1988; Kalbe et al., 1988; Heywood et al., 2003). The extraction of mitochondrial DNA from shafts generally does not cause problems because this DNA remains relatively intact during keratinisation (Linch et al., 2001), and, in contrast to nuclear DNA, mitochondrial DNA exists in high copy numbers in each cell. Another complication is presented by inhibitory agents, which could interfere with subsequent enzymatic steps, e.g. PCR, or interact with single-stranded and double-stranded DNA. Well-known inhibitors are natural cellular substances like melanin and keratin (Eckhart et al., 2000; Satayamoorthy et al., 2002; Thomas et al., 2004; Yoshii et al., 1993), located in hairs, or complex polysaccharides and phenolic compounds commonly found in plant tissues (Monteiro et al., 1997; Demeke and Adams, 1992; Katcher and Schwartz, 1994). If the concentration of coextracted interfering substances is too high, subsequent purification steps should be added. There is no extraction or purification procedure suitable for all matrices, because the numerous potential natural inhibitors present in cells and tissues and artificial inhibitory substances, i.e. materials used in processing steps, e.g. bleaching chemicals or dyes, are extremely diverse.

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Figure 16.1

PCR cycle.

The presence of compounds coextracted with DNA is the reason why the determination of DNA concentration by UV absorbance is a very critical step. DNA extracted from complex matrices will not yield a spectrum typical of DNA, i.e. a reliable measurement is almost impossible. A simple method for determining the quality, i.e. the size of the DNA fragments, and estimating the quantity is agarose gel electrophoresis. This technique uses a gel made of a natural polymer that acts as a molecular sieve to separate nucleic acids on the basis of size. Negatively charged DNA migrates toward the positive pole in an electric field. Direct visualisation of DNA is possible after staining with a specific dye, e.g. ethidiumbromide (IUPAC: 2,7-diamino-10-ethyl-6-phenylphenanthridinium bromide) and subsequent ultraviolet illumination. Although there are many DNA extraction kits commercially available, we prefer the classic method. Fibres are extensively disrupted, i.e. mechanically homogenised, and the DNA is then solubilised in cetyltrimethylammoniumbromide (CTAB, IUCAP: hexadecyl-trimethyl-azanium) and finally recovered by precipitation (Kerkhoff et al., 2009). If further purification of DNA is necessary, we use commercially available spin columns, i.e. microcentrifugation tubes filled with silica to which the DNA is bound and then recovered by elution in a low-salt buffer.

16.2.1.2

Polymerase Chain Reaction (PCR)

The PCR is an enzymatic method that can generate a billion identical molecules (amplification) of a desired segment of the DNA within about 2 h. A typical PCR reaction comprises 35–50 amplification cycles, each cycle consisting of three successive steps (Figure 16.1): r Denaturation. In this initial step, a sealed reaction vessel containing the reaction buffer with all the components necessary to synthesise new DNA – the target DNA, a heat-stable DNA polymerase, specific primers and an equimolar mixture of the four DNA nucleotides – is heated to about 95 ◦ C. This denatures the double-stranded DNA, or in other words dismantles it into two individual strands. r Annealing. In step 2, the reaction solution is cooled down to temperatures between 50 and 65 ◦ C, depending on the sequence of the primers. These are short, single-stranded DNA molecules that bind highly specifically at defined sections of the denatured sample DNA – provided they are contained in the sample to start with. A fundamental condition for producing specific primers is the availability of DNA sequence information. In other words, it must be known what sequences are typical for certain species, orders, classes, genetic modifications, etc. In many cases, such information can be called up in international databases like the National Centre for Biotechnology Information (NCBI) or the Barcode of Life Data System (BOL).

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r DNA synthesis. In the last step, the enzyme – the polymerase – recognises the short piece of double-stranded DNA formed by the denatured target DNA and the primer binding to it. The new DNA strand is synthesised from this starting point, with the complementary individual strand serving as a template. In theory, all subsequent cycles will exponentially amplify a specific DNA fragment, yielding trillions of identical molecules after 50 cycles. In practice, this amplification rate cannot be reached because the primers and nucleotides will be used up. Another complication might be the presence of disturbing substances that could interfere with the enzymatic activity of the polymerase or block primer binding sites. Nonetheless, in most cases the amplification rate will be sufficient to produce billions of new identical DNA fragments.

16.2.1.3

Detection

There are two usual ways to detect specifically amplified PCR products. Most common is direct visualisation by using agarose gel electrophoresis. In other words, after the fractionation of the PCR amplicons on a gel, their size is compared with a positive reference control. An indirect method for the detection of amplicons is real-time PCR. All systems using this principle are based on fluorescent dye systems (Higuchi et al., 1992). In real-time PCR, the amplification process is linked to a second process that generates a single fluorescent reporter molecule for every specific amplicon that is generated during PCR. Well established is the so-called TaqMan system, which is based on the detection and quantitation of a fluorescent reporter bound to TaqMan probes, i.e. single-stranded oligonucleotides slightly longer than the primers. The signal of the fluorescent dye increases in direct proportion to the amount of PCR product in a reaction (Holland et al., 1991). By recording the amount of fluorescence emission at each cycle, it is possible to monitor the PCR reaction during exponential phase where the first significant increase in the amount of PCR product correlates with the initial amount of target template. The higher the starting copy number of the nucleic acid target, the sooner a significant increase in fluorescence is observed.

16.3 Identification of Genetically Modified Plants To date, the identification of genetic modifications has relied almost exclusively on PCR. During a process called transformation, new genetic information, i.e. DNA sequences originating from other species, is introduced into the genome of a host organism. Stable integration of this foreign DNA will result in new properties. As for the identification of species, an absolute prerequisite to create a specific detection system is the reliable sequence information to design the primers. Genetic modifications can be detected on three different levels (Figure 16.2): r Screening. Genetically modified plants (GM plants) contain foreign genes that provide the plants with new properties such as herbicide tolerance or insect resistance. The function of genes depends on certain regulatory DNA sequences, so-called promoters and terminators. All GM plants approved in the EU and most of the GM plants cultivated worldwide contain such regulatory sequences. The most common ones are the 35S promoter from the cauliflower mosaic virus (CaMV) and the nos terminator, i.e. the 3 untranslated region of the nopaline synthase gene from agrobacterium tumefaciens. As many different GM plants contain identical regulatory sequences, a positive screening result strongly indicates the presence of some genetic modification but does not allow the unambiguous identification of a specific GM plant. r Construct-specific detection. One gene or several genes together with the necessary regulatory sequences is called a gene construct. Often, identical constructs have been inserted into different plant species. The construct shown in Figure 16.2 can be found in such different genetically modified plants as cotton, soya,

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Figure 16.2

Detection and identification of genetically modified plants.

maize, rape or sugar beet (Beta vulgaris L.). So the detection of a particular construct may prove an exact genetic modification without giving any clues as to the specific GM plant. r Event-specific detection. An event is each plant that has been successfully transformed, i.e. has stably integrated a genetically engineered gene construct. An event-specific analysis searches for the presence of a DNA sequence unique to a certain GM plant, usually the junction between the gene construct and the plant’s original DNA. This approach is ideal for precise identification of a specific GM plant.

16.4

Conclusion

In view of international trading agreements, consumer protection, improvement of quality control and reduction of adulteration and false declaration, it should be helpful to have a quick, reliable and sensitive method to identify species and genetic modifications in natural fibres. We have found that PCR is a very useful tool for the analysis of natural fibres. So far we have analysed hundreds of samples, not only raw materials but also processed textiles like yarns and bleached and stained garments. In most cases, identification of the species or genetic modifications was possible. In those cases where a successful analysis failed, impurities caused by coextracted substances like dyes posed the major problem. Sometimes the length of the extracted DNA fragments caused problems, for example so-called recyclates, i.e. recycled and chemically treated wool.

16.4.1

Animal Fibres

So far we have developed identification systems for the species sheep, goat, dromedary, Bactrian camel and yak. The specificity was proven in extensive mixing experiments, i.e. no cross-reactivities could be observed. The sensitivity of all systems is approximately 1%. In other words, in fibre blends, the detection limit for each of the above-mentioned species is as low as 1%. These results have been achieved with untreated and washed fibres, so the detection limit for bleached or dyed fibres will probably be higher because of inhibiting influences described earlier. It is not possible to give a general limit of detection owing to the enormous variability of processing steps commonly applied in the textile industry. But a direct comparison of our results with the data of microscopic techniques shows that, in more than 90% of all bleached and/or dyed samples we have analysed so far, as little as 3% of yak, sheep and goat fibres could be detected (Kerkhoff et al., 2009).

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351

Plant Fibres

Although the detection of species in blends of plant fibres is more difficult than the analysis of animal fibres owing to the more complex chemical composition of plant fibres, we have succeeded in developing specific identification systems for hemp, flax, cotton, nettle, ramie and kenaf. Hemp, flax and cotton seem to be more resistant against textile processing steps than nettle, ramie and kenaf. The detection of species-specific DNA in mechanically and chemically treated, bleached and dyed hemp, flax and cotton fibres is still possible, whereas bleaching and dyeing of the other fibres will prevent their analysis in most cases (Kerkhoff et al., 2009). In practice, this has proved to be a minor problem because cotton, hemp and flax are of much greater commercial interest. 16.4.3

Genetically Modified Cotton

Of all plants used in the textile industry, cotton is the only one that has been genetically modified (GM) and cultivated on a large scale for more than 10 years. Although cotton is the most important natural fibre, it plays only a minor role in the food and feed industry. Nevertheless, the protein- and oil-rich seeds are processed into various side-products such as cooking and frying oil, cottonseed meal, thickeners, stabilisers or emulsifiers. Therefore, the labelling conditions and thresholds defined in the European regulation 1829/2003 for GM food and feed can be applied to cotton, and methods to detect many GM cotton events have been published. So the problem was not to develop specific identification systems for GM cotton but to extract sufficient amounts of DNA from fibres. In the meantime, we are able to analyse not only raw cotton but also bleached and dyed fibres, yarns and garments. The limit of detection is 0.1% for raw and slightly processed materials. A limit of detection for garments cannot yet be given owing to the lack of reference material (Kruse and R¨uggeberg, 2008).

16.5

Prospects

The technology presented here is a valuable tool for the specific and sensitive identification of species and genetic modifications in animal and plant fibres. Notwithstanding these successful developments, more work needs to be done. Further optimisation of the DNA extraction and purification procedure is an important objective to improve the quality and yield of DNA. This is an essential prerequisite for the development of quantitative systems, which are necessary to distinguish between unavoidable contaminations and adulteration. According to the German textile labelling regulation, an admixture of 2% of fibres other than those labelled will be tolerated. Although all organic cotton standards exclude genetic modifications, there is as yet no analogous threshold in the textile industry. In other words, the so-called zero tolerance is still valid. Against the background of the global movement of goods, it is worth considering the definition of a threshold for technically unavoidable contaminations. We have already developed a PCR technique that allows an exact quantification of genetically modified cotton in raw, washed and stained fibres and also slightly processed materials like yarns. Currently, we are working on the development of PCR methods allowing the quantitative analysis of goat, sheep and yak fibres in textile blends.

References Ackermann, A.B., de Viragh, P.A. and Chongchitnant, N. (1993) Anatomic, histologic and biological aspects, in: Neoplasms with Follicular Differentiation (Ackerman’s Histologic Diagnosis of Neoplastic Skin Diseases: A Method by Pattern Analysis), ed. by Ackermann, P.A., de Viragh, A. and Chongchitnant, N. Lea&Febiger, Philadelphia, PA, USA.

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Agbios GM Database (2009) Information on GM approved products; available at: www.agbios.com. Allen, M., Engstr¨om, A.S., Meyer, S., Handt, O., Saldeen, T., von Haeseler, A., P¨aa¨ bo, S. and Gyllensten, U. (1998) Mitochondrial DNA sequencing of shed hairs and saliva on robbery caps: sensitivity and matching probabilities. J. Forensic Sci., 43, 453–464. Demeke, T. and Adams, R.P. (1992) The effects of plant polysaccharides and buffer additives on PCR. Biotechniques, 12, 332–334. Eckhart, L., Bach, J., Ban, J. and Tschachler, E. (2000) Melanin binds reversibly to thermostable DNA polymerase and inhibits its activity. Biochem. Biophys. Res. Commun., 271, 726–730. Heywood, D.M., Skinner, R. and Cornwell, P.A. (2003) Analysis of DNA in hair fibres. J. Cosmet. Sci., 54, 21–27. Higuchi, R., Dollinger, G., Walsh, P.S. and Griffith, R. (1992) Simultaneous amplification and detection of specific DNA sequences. Biotechnology, 10, 413–417. Holland, P.M., Abramson, A.D., Watson, R. and Gelfand, D.H. (1991) Detection of the specific polymerase chain reaction product by utilizing the 5 -3 exonuclease activity of Thermus aquaticus DNA polymerase. Proc Natl Acad. Sci. USA, 88 (16), 7276–7280. Kalbe, J., Kuropka, R., Meyer-Stork, L.S., Sauter, S.L., Loss, P. and Henco, K. (1988) Isolation and characterization of high-molecular mass DNA from hair shafts. Biol. Chem., 369, 413–416. Katcher, H.L. and Schwartz, I. (1994) A distinctive property of Tth DNA polymerase: enzymatic amplification in the presence of phenol. Biotechniques, 16, 84–92. Kerkhoff, K., Cescutti, G., Kruse, L. and M¨ussig, J. (2009) Development of a DNA-analytical method for the identification of animal hair fibres in textiles. Text. Res. J., 79(1), 69–75. Kruse, L. and R¨uggeberg, H. (2008) Detection of genetically modified cotton in raw material and cotton products, in 29th International Cotton Conference, Bremen, Germany, 2–5 April 2008. Bremer Baumwollb¨orse and Faserinstitut Bremen e.V., Bremen, Germany. Linch, A., Whiting, D.A. and Holland, M.M. (2001) Human hair histogenesis for the mitochondrial DNA forensic scientist. J. Forensic Sci., 46(4), 844–853. Monteiro, L. et al. (1997) Complex polysaccharides as PCR inhibitors in feces. Helicobacter pylori model. J. Clin. Microbiol., 35, 995–998. Phan, K.H. and Wortmann, F.J. (2004) Cashmere definition and analysis: scientific and technical status. Presentation at the 3rd International Symposium on Speciality Fibres, DWI Aachen, Germany. Satayamoorthy, K., Li, G., van Belle, P.A., Elder, E. and Herlyn, M. (2002) A versatile method for the removal of melanin from ribonucleic acids in melanocytic cells. Melanoma Res., 12, 449–453. Schreiber, A., Amtmann, E., Storch, V. and Sauer, G. (1988) The extraction of high-molecular mass DNA from hair shafts. FEBS Lett., 230, 209–211. Taliercio, E., Hendrix, B. and McStewart, J. (2005) DNA content and expression of genes related to cell cycling in developing Gossypium hirsutum (Malvaceae) fibres. Am. J. Bot., 92(12), 1942–1947. Thomas, M., Gilbert, P., Wilson, A.S., Bunce, M., Hansen, A.J., Willerslev, E., Shapiro, B., Higman, T.F.G., Richards, M.P., O’Connell, T.C., Tobin, D.J., Janaway, R.C. and Cooper, A. (2004) Ancient mitochondrial DNA from hair. Curr. Biol., 14, R463–R464. Yoshii, T., Tamura, K., Taniguchi, T., Akiyama, K. and Ishiyama, I. (1993) Water-soluble eumelanin as a PCR-inhibitor and a simple method for its removal. Nihon Hoigaku Zasshi, 47, 323–329.

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17 Cotton/Worldwide Harmonisation Axel Drieling Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany

Jean-Paul Gourlot CIRAD PERSYST LTC, Montpellier, France

17.1 Introduction With approximately 25 million tons of cotton lint produced each year, cotton is the most important natural fibre. The large amounts of cotton, combined with the worldwide trade, mean that great efforts have been and are being made to develop a worldwide standardised and accepted classification and testing system. This chapter covers manual classing as well as instrument testing, with a focus on the growing importance of instrument testing. Seed cotton classification is only mentioned marginally, as the classification for trading is done on cotton lint. For manual classing, as well as for instrument testing, harmonisation is essential. Harmonisation work for manual classing started at the beginning of the twentieth century. This chapter covers the harmonisation activity up to the latest developments for instrument testing as well as for manual classing.

17.2 Definition of Harmonisation Classing and testing must always be accompanied with measures striving for reliable results that are comparable between different classing or testing locations. These measures are defined in different ways, but in the authors’ view a suitable definition of the overall activity, including the different measures taken, is harmonisation in general. Harmonisation in general may be broken down into three parts: r standardisation; r harmonisation – specific measures; r check of harmonisation Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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For the example of instrument testing, standardisation as the first step concerns the testing procedure. The testing procedure must be standardised and accepted as a testing method. Usually, standardised test methods are given, for example, by the ISO (International Standardisation Organisation) or the American Society for Testing and Materials International (ASTM International). The second step is the harmonisation of testing and testing equipment itself. It is achieved mainly by calibrating the equipment with valid reference material, provided by a central body that is internationally agreed to be the responsible body for its production. This central body has to assure that the result level is kept constant for the whole reference material production as well as over time. For cotton, there are currently cotton reference materials for strength, length and micronaire measurements, produced by the US Department of Agriculture (USDA) (see Figures 17.4, 17.5 and 17.6). Additionally, colour tiles are given for colour calibration and trash tiles for trash calibration. Other production-relevant properties such as stickiness, neps, short fibre content and maturity are tested, but to date no reference material has been provided. Test methods that are based on a physical calibration avoid cotton-based reference material, which typically shows variability and concomitant errors. The third step is the check of harmonisation. The check of harmonisation is usually done with either round trials or with retests in other laboratories. Round trials are conducted in order to check the harmonisation between the individual laboratories and to help the laboratories to reduce deviations. Samples with low variation in their properties are sent out from one central body to all interested laboratories, and tested there. The results are then analysed and compared. Every laboratory is notified of the extent of deviation of its results from the mean values of all results or a given reference result, and can adapt or improve its testing routine accordingly. The target of the round tests is to reduce the interlaboratory variation in results. For retests, participating labs must agree on an independent retest laboratory. Typically, a randomly chosen subset of all samples from one lab is sent to the retest laboratory and is retested with more intense tests, usually double tests. A well-known example for retests is given by the USDA-AMS in Memphis for checking the 12 classing stations of the USA.

17.3

Seed Cotton Classification

Seed cotton is classified to ensure that it is clean enough for gentle processing in subsequent steps and to ensure an appropriate income to the producer, fostering a better quality. Thus, the criteria and the procedure for evaluating the quality are defined jointly between the farmer representatives and the ginner representatives. One visible and practical output of this agreement is the creation of seed cotton reference boxes and the use of these by trained classers. As shown in Figure 17.1, it is easy to compare the actual evaluated seed cotton with several typical seed cotton samples (two grades or more) given in a comparison box, and assign a seed cotton grade to it. Usually seed cotton grades give only basic distinctions, as in the case of Burkina Faso: Grade

Description

First Second Third

Clean and sorted cotton Unsorted cotton Low quality

According to the ICAC (1998), not all countries classify their seed cotton. Each of the countries where seed cotton classification is performed defines its own set of reference seed cotton grading boxes; there is no international harmonisation, as seed cotton is not traded internationally. Seed cotton samples are collected

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Figure 17.1

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Seed cotton classing in Chad (photo: J.P. Gourlot).

at the market level and evaluated in comparison with the reference material boxes on site and/or at the gin1 level, where this information may be used: r to assemble the seed cotton lots into homogeneous batches at the feeding stage of the gins; r to set the gentlest possible ginning process for this seed cotton quality; r to calculate the corresponding financial value for the grower.

17.4 Manual Classification of Cotton Lint Manual classification of cotton lint was first developed to assess cotton quality in order to predict textile behaviour of the fibres when instruments did not exist in the nineteenth century. It became clear that the appearance of fibres was an important factor, as well as their estimated length. Criteria were defined and approved for the quality control of the traded fibres, such as colour, foreign matter content and preparation. In practice, cotton samples are compared with existing approved (at national, regional or international levels) reference materials in order to assess their level of quality. Those approved reference materials are distributed to all users, such as classers, laboratories and Cotton Associations worldwide, and to arbitration bodies to serve as reference for trading (see Chapter 3.1).

17.5

Manual Classification – the US System

In the United States of America, cotton samples are drawn from both sides of each bale of ginned lint. Each sample is sent to and evaluated in one of 12 laboratories located across the US cotton-growing belt. Since

1

More details concerning the ginning process of cotton can be found in Chapter 10.

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Table 17.1 Colour grades of upland cotton. Adapted with permission from International Trade Center UNCTAD/WTO: Cotton Exporter’s Guide, ITC, Geneva, 2007

Good middling Strict middling Middling Strict low middling Low middling Strict good ordinary Good ordinary Below grade * Physical

White

Light spotted

Spotted

Tinged

Yellow stained

11* 21* 31* 41* 51* 61*

12 22 32 42 52 62

12 23* 33* 43* 53* 63*

— 24 34* 44* 54* —

— 25 35 — — —

71* 81

— 82

— 83

— 84

— 85

standards. All others descriptive.

1923, procedures and standards have existed and are periodically refined for better assessment of the quality of cotton samples and their corresponding originating bales. Both manual and instrument classification are performed in parallel to provide a fully complementary and comprehensive cotton classification. The actual procedure allows the assessment of colour according to 25 official colour grades for American upland cotton,2 plus five categories of below-grade colour, as shown in Table 17.1. The United States Department of Agriculture (USDA) maintains physical standards for 15 of the colour grades; the others are descriptive standards (see Figure 17.2). In addition to colour, the USDA white physical standards also represent official leaf grades 1 to 7 (leaf grade 8 is used as a below-grade description). As colour classification is now performed by instrument in the USDA, the visual classing of cotton only concerns the leaf content and extraneous matter (USDA-AMS, 2001). Although HVI trash measurement is provided by the USDA, the traditional method of classers’ determination for leaf grade and extraneous matter remains included as part of the USDA’s official cotton classification. The third component of the grade is the ‘preparation’ of the fibres. Preparation corresponds to the degree of smoothness or roughness of the ginned cotton lint. At the present time, no instrument is available for assessing this criterion, so only the human eye is currently suitable. These manual classification results are grouped together with instrument results into a database for further use during the trading of the cotton bales. Based on the Universal Cotton Standards Agreement from 1923, which was signed by 24 cotton associations worldwide, the standard boxes are accepted and used worldwide. Delegates from all cotton associations meet every 3 years in order jointly to consider any changes to the standards.

17.6

Manual Classification – Other Countries

Many countries (see ICAC, 1998) develop their own reference material grading systems implemented in reference material boxes for colour assessment, with some levels occasionally corresponding to the USDA’s grade standards. Not all countries develop procedures for estimating the foreign matter content in lint. Consequently, the manual classification of these countries assesses the colour of the cotton, its foreign matter content and its preparation, while the USDA assesses only the leaf content. In addition to this evaluation, some countries use the pulling method for assessing fibre commercial length, known as the staple length. A fibre beard is made manually by successive pulling on the fibres (with as little breakage as possible) and removal of the loose fibres. Then, classers assess the staple length in comparison with reference material.

2

More information to the species of cotton can be found in Chapter 10.

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Figure 17.2

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USDA standard boxes. Photo courtesy W. Neumann, Bremen Cotton Exchange, Bremen, Germany, 2009.

The manual classification results help in constituting homogeneous lots in terms of quality.

17.7

Manual Classification – Harmonisation Developments in Africa

A harmonisation process is under way in Africa through the adoption of the African Standards (2006). A UNIDO project helped Africa to develop a common grading system across its countries. The African Standards are correlated with most of the given country-specific grades (see Table 17.2). Thus, any classed bale may carry the grade given under the national and under the African system. Table 17.2 Table relating African grades to national grades in Africa. Reproduced from Amadou Soule et al, Manuel qualite´ pour les filieres cotonnieres UEMOA, Guide technique n◦ 3, version 1, UNIDO, 2006 ` ` Standard ‘Africa’ Class 0

1

2

3

4

Domestic sale types of different African countries

Subclass Benin

Burkina Faso Cote d’Ivoire Guinea-Bissau Mali

1 2 3 4

KABA/S

1 2 3 4

BELA

1 2 3 4

BELA/T

1 2 3 4

RADA ZANA/C BUFA ZANA/T DARO KENE BABU

1 2 3 4

BATI

KABA

BELA/C

MIKO BOBY/S BOLA/S

SARAMA LAZA/S JULI/S BANI NERE JULI

MANBO/S MANBO/N

BOBY BOLA

Niger

BELA/1 (GB) KATI

Senegal Togo SIGAL/S SEKA

MASA SIGAL MAKO (NIG)

OTI ALTO/S

PLEBE IRMA/S

ALTO

PLINE PLOBE IRMA-IRFO

MANBO/C KATI/C

ZANA

TOMA RUDY VOTA VIVA

BOBO/3 BOBO/4

BEMA/N

BELA/2 (GB)

LIBA

BEMA/C

LIBA/C

BILO

KOLA

Cameroon

TAMA

SAVAL

TANGO OGOU IRIS IGOR GOTO IRVI

SEVE

TOLE

IROL-SULI

PAVO MAKO

BUTO

GARU KCCA

GNOMA CORE

LUKO GALA

BUKA

KAMI LAGO BATA FAKO

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Furthermore, Africa is moving towards instrument testing to follow the recommendations of the Task Force for Commercial Standardisation of Instrument Testing for Cotton (CSITC TF), and to answer the demand from the trade. A project financed by the Common Fund for Commodities and the European Commission (CFC/ICAC/33) is ongoing to implement instrument testing for the classification of cotton in Africa.

17.8 Instrument Testing and its Future Directions Developments in cotton testing are driven by market demands, so it is important to have a look at the different stages of the cotton value-added chain in order to identify the given demands on cotton testing (Drieling, 2007). Instrument cotton testing is solely done on cotton lint. It is used for all stakeholders in the cotton value-added chain. In cotton breeding, testing is used for choosing varieties with assured or improved cotton quality. In cotton production, testing is used in order to establish the monetary value of the cotton. This should preferably be done on every bale, so that high-volume testing is necessary. For cotton ginning, testing makes it possible to control the ginning process and optimise the fibre quality. Instrument testing for trading is meant to give objective and reliable information about the cotton quality. Only those parameters that are reliable enough for trading may be tested. Results have to be given on the internationally accepted level (based on universal standard material), and have to show a variability between laboratories (interlaboratory variability) that is small enough to fix properties in trade contracts. High-volume testing is necessary in order to change from manual classing to instrument testing. In cotton processing, the many different reasons for testing lead to different directions of testing: to buy cotton varieties/bales that correspond to the envisaged yarn quality, to optimise bale laydowns on the basis of results for every single bale (see Figure 13.1, Chapter 13), preferably without testing the bales a second time, to assure sufficient quality of cotton as the input material over the entire processing time without tolerating oversized quality, to optimise machine settings on the basis of the maximum possible information and to control the process automatically on the basis of continuous quality parameters given by on-line testing. The current situation with cotton testing is as follows. For high-volume testing, the established, accepted and sufficiently reliable parameters are: r r r r

micronaire as a combination of fineness and maturity; strength; length and length uniformity; colour reflectance (Rd) and yellowness (+b).

Other properties such as elongation, short fibre index, fineness, maturity, moisture content and trash are included in integrated high-volume instruments, but are not fully accepted, as they are not sufficiently reliable. Low-volume testing or detailed testing is done for many properties, and there are reliable test methods for many additional properties that are not included in high-volume testing, such as the staple length distribution. But it is not possible to state which test method and which parameters are established and which are not; this is largely dependent on the interests of the individual users. Nevertheless, there is a strong demand for testing some parameters more reliably. The objectives of future developments in cotton testing are: r r r r

more reliable testing; speeding up of testing for testing of every bale; more information about cotton properties for its processing; more information about the variability of the property, e.g. based on single fibre property distributions.

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On the other hand, there are different directions that can be followed: r high-volume testing mainly for trading purposes; r detailed testing/low-volume testing mainly for processing purposes; r on-line measurements mainly for processing purposes. Harmonisation measures are primarily necessary for trading purposes, so today are mainly for high-volume testing.

17.9

Micronaire Measurement

Micronaire is a measure of fibre fineness and maturity. An airflow instrument is used to measure the air permeability of a constant mass of cotton fibres compressed to a fixed volume. The principle is given in Figure 17.3. It is implemented as a stand-alone instrument as well as being a module in high-volume instruments. The micronaire scale has been established empirically and is not linear. The typical ranges of micronaire results are as follows: Micronaire

Description

<3.0 3.1–3.6 3.7–4.7 4.8–5.4 >5.4

Very fine Fine Medium Coarse Very coarse

Unfortunately, the micronaire result is simultaneously dependent on fineness and maturity. So a fine and immature cotton gives a low micronaire, and a cotton with higher micronaire results may be either more coarse or more mature. Therefore, another view of micronaire readings on US upland cottons shows a premium range of 3.7–4.2, and lower values with higher or lower results (USDA-AMS, 2001). Micronaire is calibrated in a first step with orifices, and in a second step with universal cotton calibration standard material, prepared and provided by the USDA. The usage of the universal calibration material is accepted and applied worldwide. Calibration is done with two cottons in a wide range between approximately micronaire 2.6 and 5.5.

Figure 17.3 2008.

Measurement principle for micronaire. Adapted with permission from Uster Technologies, training documentation,

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The measurement uncertainty of the micronaire results may best be described on the basis of its variation in the CSITC Round Trials, which are explained below. Based on the results of 36 US upland cottons and approximately 80 participating instruments for each round trial, the standard deviation between results from different laboratories is: r interlab standard deviation SD (micronaire) = 0.092; based on single tests, corresponding to a coefficient of variation CV% (micronaire) = 2.2%; r interlab standard deviation SD = 0.076, based on 30 tests per lab. The micronaire measurement is well established and typically used in cotton contracts. The usual limit of acceptance for micronaire results is ±0.3 units. The cotton properties behind the micronaire results are, in spite of the widespread use of micronaire itself, not used for cotton trading. Fineness is measured with instruments such as the Uster AFIS or the fineness and maturity tester (FMT), but there is no test method that is sufficiently fast and sufficiently reliable for testing every bale. Furthermore, no calibration material is available for cotton fineness. Fibre maturity is measured, for example, by the fineness and maturity tester (FMT) or AFIS or microscopic methods, but again no test method is suitable for testing every bale, and no calibration material is available. Based on the Bremen Cotton Round Tests, the typical interlaboratory coefficient of variation for fineness and maturity with FMT instruments is 8–10% (FIBRE, 2009a). New-generation high-volume testing instruments offer maturity results, but these are not based on a measurement but rather on a calculation based on micronaire and strength and elongation readings and on their correlation with maturity results.

17.10 Strength For strength measurement with high-volume testing instruments, fibre collectives, called ‘fibre beards’, are prepared automatically or semi-automatically. The amount of fibres is then measured optically when the beard is shifted into the optical system, the fibre collective is clamped in two sets of jaws, spaced 3.2 mm apart, and finally the fibres are broken, and the strength is calculated from the force and the optical amount. At the same time, the breaking elongation is measured. The principle is shown in Figure 17.4. To obtain the actual strength, the results have to be calculated on the basis of calibration cottons. For calibration, the currently agreed level is the U-HVICCS level, with the reference cottons provided by the USDA. Usually, two cottons, namely upland short weak (ca 23 g/tex* ) and upland long strong (ca 33 g/tex) are used (SEM micrographs of these cotton standards can be found in Figures 14.22 and 14.23 in Chapter 14). The typical strength result levels are as follows (USDA-AMS, 2001):

∗

Strength

Description

≤23 g/tex 24–25 g/tex 26–28 g/tex 29–30 g/tex ≥31 g/tex

Weak Intermediate Average Strong Very strong

Conversion in cN/tex: cN/tex = (g/tex)/1.02

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Figure 17.4 Measurement principle for fibre length and strength in Uster high-volume instruments. Adapted with permission from Uster Technologies, training documentation, 2008.

The typical elongation result level is between 5 and 8%. As the results are based on the testing of fibre collectives, they cannot be directly related to single fibre strength and elongation results (see the detailed discussion in Chapter 13.7.3). The measurement uncertainty of the strength results can be estimated on the basis of strength variation in the CSITC Round Trials:3 r interlab standard deviation SD (strength) = 1.4 g/tex, based on single tests corresponding to a coefficient of variation CV% (strength) = 4.8%; r interlab standard deviation SD = 1.1 g/tex, based on 30 tests per lab.

For trading, there is at this stage no agreed level of acceptance. Besides the internationally agreed U-HVICCS level mentioned above, there are some other, outdated but still used test methods and calibrations in use for cotton, such as Pressley (0), Pressley (1/8 ), Stelometer and HVI on the ICCS level. Elongation is not used for trading, as the interlaboratory variation shows a CV% of 16% in Bremen Cotton Round Tests (FIBRE, 2009a), whereas a maximum CV% level of 5% is a typical threshold accepted by the ITMF ICCTM (see below).

17.11 Length and Uniformity For length measurement, the most obvious test method is to measure the total length of the fibres. Unfortunately, this can only be achieved by either doing single fibre tests or by sorting the fibres to an end-aligned beard (see Figure 17.5). As both are time consuming, high-volume testing is done on non-end-aligned beards, so that span lengths or partial fibre lengths are measured. The measurement is done in combination with the high-volume strength measurement system described above (see Figure 17.4). As a first result, the fibrogram is given (see Figure 17.6). Based on the diagram results, the following parameters are defined in the U-HVICCS system: r upper half mean length (UHML) – as the main parameter for cotton fibre length; r mean length (ML) – only used for calculation purposes; 3 Based on CSITC Round Trials on US upland cottons from 2007-1 to 2009-1, 36 cottons, approximately 80 instruments (ICAC, 2009a).

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Figure 17.5 Staple length/total length results based on end-aligned fibres, compared with span length/partial fibre length results based on non-aligned beards for the same fibres.

r uniformity index, defined as ML/UHML times 100% – as the parameter for the homogeneity of the fibre lengths in a sample. The calibration is done in combination with the strength calibration (see above). The typical length result levels (upper half mean length) are as follows: Length description Short Medium Medium to long Long Extra long

Length in inches 0.8–0.95 0.96–1.1 1.11–1.2 1.2–1.35 >1.35

For the typical length range, the UHML results are on the same level as manual classing results. For lengths beyond 1.25 , UHML starts deviating, showing lower results. Length results are usually given in inches or in millimetres. Another (US internal) system is to use multiples of 1/32 in order to correspond to manual classing length results.

Figure 17.6 Fibrogram (span length diagram) of non-end-aligned fibres, and the corresponding definition of the length parameters. Adapted with permission from Uster Technologies, training documentation, 2008.

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The typical uniformity result levels (uniformity indices) are as follows (USDA-AMS, 2001): Degree of uniformity Very low Low Intermediate High Very high

Uniformity index <77 77–79 80–82 83–85 >85

The measurement uncertainty of the length (UHML and uniformity index) results can be estimated on the basis of its variation in the CSITC Round Trials:4 r interlab standard deviation SD (UHML) = 0.017 , based on single tests corresponding to a coefficient of variation: CV% (UHML) = 1.6%; r interlab standard deviation SD (UHML) = 0.012 , based on 30 tests per lab; r interlab standard deviation SD (uniformity index) = 0.81, based on single tests corresponding to a coefficient of variation CV% (uniformity index) = 1%; r interlab standard deviation SD (uniformity index) = 0.52, based on 30 tests per lab. It can be seen that the length and uniformity results are highly reproducible. Problems in length testing occur for the measurement of the short fibre content (SFC or SFI). Depending on the instrument used, the interlaboratory variation shows a CV% between 20 and 40% (FIBRE, 2009a). With this, the short fibre content cannot be used for comparing results between labs, although it is important for spinners. Besides high-volume test instruments, other instruments, such as Uster AFIS, Premier aQura or the Almeter, also measure length (see Figure 13.10, Chapter 13). All named systems evaluate the total length of the fibres/the staple length distribution. Correlations are given to the high volume testing UHML results, but on a different level.

17.12 Colour and Trash Colour is measured with an optical system called a colorimeter, in units of reflectance (Rd) and yellowness (+b). Since 1999, the colour classification has shifted in the US system from manual classing to instrument testing. As trading is still oriented towards manual colour grades, the Nickerson–Hunter diagram, given in Figure 17.7, shows the relationship between manual grading results (GM, SM, M, etc., and White, Lt. Sp., etc. – see Section 17.5) and the instrument results for reflectance (Rd) and yellowness (+b). A similar figure to that given in Figure 17.7 is available for American Pima cottons. Principally, additional diagrams can be developed for each origin of cotton, as the best or most white cotton may be different in each country. Cotton colour is not calibrated with cotton-based reference material but with five colour tiles, as cotton changes its colour with storage. From a positive aspect, this avoids variation in cotton calibration material. On the other hand, some effects influencing cotton colour measurement, e.g. the pressure on the cotton, cannot be compensated for by tile-based calibration. To reduce these difficulties, cotton based HVI colour standards are available for verification. 4 Based on CSITC Round Trials on US upland cottons from 2007-1 to 2009-1, 36 cottons, approximately 80 instruments (ICAC, 2009a).

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Figure 17.7 HVI colour diagram for American upland cotton. Adapted with permission from International Trade Center UNCTAD/WTO: Cotton Exporter’s Guide, ITC, Geneva, 2007.

The measurement uncertainty of the colour results (reflectance and yellowness) can be estimated on the basis of colour variation in the CSITC Round Trials:5 r interlab standard deviation SD (Rd) = 1.04, based on single tests corresponding to a coefficient of variation CV% (Rd) = 1.4%; r interlab standard deviation SD (Rd) = 0.96, based on 30 tests per lab;

5 Based on CSITC Round Trials on US upland cottons from 2007-1 to 2009-1, 36 cottons, approximately 80 instruments (ICAC, 2009a).

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r interlab standard deviation SD (+b) = 0.39, based on single tests corresponding to a coefficient of variation CV% (+b) = 3.5%; r interlab standard deviation SD (+b) = 0.34, based on 30 tests per lab. With the optical colour module, the trash content (given as trash area) is measured at the same time. The defined correlation between classer’s leaf grade and HVI trash area is (USDA-AMS, 2001): HVI trash area 0.13 0.20 0.34 0.51 0.72 1.00 1.25 1.57

Classer’s leaf grade 1 2 3 4 5 6 7 8

Trash is calibrated with one trash calibration tile. The interlaboratory variation in the instrumentally measured trash content shows a CV% of more than 50% in Bremen Cotton Round Tests (FIBRE, 2009a), which definitely shows that the trash content is not suitable for comparing results between laboratories at this stage. Therefore, leaf/trash grading is still based on manual classification worldwide. Other test instruments measure the trash content with mechanical systems, which clean the cotton and weigh the mass of the trash that has been cleaned out.

17.13 Single Fibre Results and Fibre Collective Results Even in very homogeneous samples, the single fibres show a broad distribution. Figure 17.8 shows, for example, the maturity and fineness distribution of 3000 single fibres in a coarse/mature and in a fine/immature sample, measured at the Faserinstitut Bremen using an image analytical method. The width and the shape of the single fibre distributions are important for the processing behaviour of the cottons, so the measurement of only mean values or characteristic values is not sufficient to describe samples properly (see the detailed discussion of this topic in Chapter 13). With the given measurement principles, the time required for single fibre tests for all properties is too long to be considered for the daily characterisation of cotton samples. Additionally, because of the variation, the harmonisation of distribution results is far more difficult than the harmonisation of just a mean value. Nevertheless, the measurement of distribution characteristics such as the Uniformity Index for fibre length or the Short Fibre Content represents a first approach for considering distribution characteristics based on fibre collective measurements.

17.14

Provision of Reference Material

In order to get similar results worldwide for both manual and instrument classing for any given cotton sample, the cotton industry must rely on reference materials. For manual classing, samples are matched against reference materials to derive their own value. For instrument classing, it would have been best to set the instruments according to the International System Organisation units (e.g. metre, gram). However, as this is very difficult to achieve for cotton fibre characterisation, it has been decided to calibrate equipment

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Figure 17.8 Maturity (theta) distribution and fineness distribution for 3000 fibres of a coarse and mature cotton (left) and a fine and immature cotton (right). Adapted from FIBRE 2009b.

using reference cottons known for their established characteristics (length, micronaire, strength, etc.). Each single instrument or each module of an instrument normally uses two specific materials for each property, for instance two cottons covering a range of micronaire, to establish the relationship between measured and established data. Then, as results for reference materials are obtained at the established level, all results on any sample are assumed to be obtained at that agreed level as long as the calibration is considered valid. Calibration is done to maintain the relationship between measured and established results on reference materials in somewhat variable conditions. If one instrument is used by two different operators, as when working in shifts, for instance, it is required to check the validity of the relationship by testing the reference materials periodically. The same is necessary within a given shift to ensure that all results obtained during the shift are valid all the time. As a consequence of this intense testing, the reference material becomes exhausted periodically, and the stock of reference material needs to be renewed in order to avoid any doubt concerning the results obtained from a particular laboratory. Thus, to maintain the required testing and reading level for cotton sample characterisation, laboratories need to purchase reference materials on a constant and regular basis. Before being used in the laboratories, the nature of these reference materials is approved under national, regional and/or international agreements. Then, reference values of these materials are obtained by intense testing procedures before they are distributed to laboratories for use. Designated labs from different countries around the world are included in the process of obtaining reference results, so as to ensure independence and international acceptance.

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For manual classing, reference materials should be procured at the appropriate cotton associations; for instance, someone purchasing African cotton should procure the African cotton standards in order to compare evaluations of the cotton on the same basis as the classers who defined its quality. For rapid or high-volume instrument testing, a universal agreement specifies that only universal highvolume instrument calibration cottons shall be used to calibrate instruments for length, length uniformity and strength. The same agreement states that only universal micronaire calibration cottons shall be used. For colour calibration, calibration tiles are given, and universal HVI colour standards are provided for verification. The calibration material is produced by the USDA in Memphis, USA in accordance with this universal agreement, and can be ordered at the USDA-AMS (2009). For transparency of the value establishment work, an American Society for Testing and Materials International (ASTMI) standard, ‘The Value Establishment of Calibration Cottons’, is currently under development to fix all the procedures that are necessary to produce these reference materials, as well as to establish their reference characteristics. Based on this standard, it will be possible for other countries and bodies to produce reference material without deviating from the established and internationally accepted universal standard level.

17.15

Round Trials for Interlaboratory Comparisons

The main target of round trials is to reduce the variability of results between different laboratories on similar material. The principal system of round trials (officially recognised as proficiency testing schemes) is to send similar samples from one central body to all interested laboratories, test them at the laboratories and evaluate the results sent back to the central body. ISO Guide 43 (1997) proposes general prerequisites for executing these tests. Round trials are beneficial for laboratories, as they can check their results in comparison with other laboratories; this will help laboratories to achieve more accurate results. Round trial results of single labs are usually kept confidential. At the same time, round trials are beneficial for the whole cotton value-added chain, as they show the interlaboratory variation in test results. Based on this, suitable commercial trade limits may be fixed, for example. For cotton, the three important and regularly conducted international round trials are: r the Bremen Cotton Round Test, conducted by the Faserinstitut Bremen e.V., Bremen, Germany; r the USDA HVI Checktest, conducted by the USDA-AMS, Washington, DC, USA; r the CSITC Round Trial, hosted by the ICAC and conducted with cooperation between the Faserinstitut Bremen e.V. and the USDA-AMS. Each of these round trials has its own unique benefits, so that they cannot replace each other (see Table 17.3). The Bremen Cotton Round Test has got the longest history, starting in the 1950s. The main benefit is the inclusion of all common cotton test methods, whereas the other round trials are designed only for high-volume testing instruments. Participation is free of charge, so that taking part in this harmonisation measure is not hindered by cost factors. Based on these two factors, it is, for example, possible to compare different test methods according to statistically highly assured data for each method. The USDA HVI Checktest is designed for high-volume instruments and allows a monthly comparison of two samples on these instruments. The latest is the CSITC Round Trial, designed for the specific purposes of the Task Force on Commercial Standardisation of Instrument Testing of Cotton (CSITC – see below) (Drieling and Knowlton, 2007; Drieling, 2008). It is the most intense round trial series, with five samples for each conducted round trial, and with

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Table 17.3

Cotton round trials and their attributes; unique characteristics are highlighted

Attribute

USDA HVI Checktest

Bremen Round Trial

CSITC Round Trial

Number of participants Start Region Kinds of instrument Costs for participation Cottons: origin and type Frequency Number of samples Aim

50–80 ? Worldwide High-volume Testing Charged USA; upland 12 times/year 2 samples Information for the lab

180 labs (100–120 HVIs) 1950s Worldwide All kinds/all properties Free World; broad range 4 times/year 1 sample Information for the lab

Evaluation of Evaluation of trueness Evaluation of precision Detailed evaluation

Laboratory average Yes No No

Laboratory average Yes No No

80 2007 Worldwide High-volume testing Subsidised charge USA/world; upland or similar 4 times/year 4/5 samples Official lab evaluation; detailed analysis for lab Laboratory average and all single data Yes Yes Yes

30 tests on each of the five samples. All single data are collected, so that an evaluation of precision is possible. The three aims of CSITC Round Trials are as follows: (a) Evaluation of result variability. Very stable interlaboratory variation results form the basis of commercial trade limits. Additionally, typical intralaboratory variations are obtained as a basis for the evaluation of precision. (b) Rating of the participating laboratories. Each laboratory receives a certificate stating the accuracy of its results in comparison with the accuracy distribution of all other laboratories. This information is essential for laboratories to be able to advertise their true performance, and is beneficial to their customers, as this is the first time that an objective rating of the labs’ performance has been given. (c) Detailed analysis of the labs’ results. Based on a detailed analysis, it is possible for the laboratory to detect its major shortcomings in performance and possible reasons for them. The labs are contacted directly for additional advice. CSITC Round Trial results are published online (ICAC, 2009a).

17.16 ITMF Activities Since 1980, the International Textile Manufacturers Federation (ITMF), Z¨urich, Switzerland, has formed the International Committee on Cotton Testing Methods (ICCTM). The functions of the committee are: r r r r r r

to support the standardisation, harmonisation and check of harmonisation of cotton test methods; to recognise suitable instruments and cotton test methods; to encourage the development of improved testing methods; to identify suitable reference methods (which are necessary to create reference material); to encourage research into the basic science needed to develop commercially useful test methods; to bring together researchers for the topics above.

The focus of the committee is testing for cotton processing. The work of this ITMF committee is synchronised with the commercial standardisation activities of the CSITC Task Force.

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17.17 CSITC Activities The Task Force for Commercial Standardisation of Instrument Testing of Cotton (CSITC Task Force) was established by the International Cotton Advisory Committee (ICAC), Washington, DC, USA, in 2003 for the harmonisation of trade-related cotton testing. Its aims are to facilitate the changeover from manual classing to instrument testing as the basis for cotton trading, to facilitate the widespread use of instrument testing systems at the producer level, to standardise instrument testing and to uphold and improve the reliability of instrument testing. Therefore, the only focus is high-volume testing, which is capable of testing every bale. Important decisions and measures of the CSITC Task Force up to now are as follows (ICAC, 2009b): r fixing the choice of the six cotton properties that can be measured sufficiently reliably with high-volume testing for trading purposes; r fixing of the test methods, the calibration and calibration material, the definition of the test parameters and the sampling method; r fostering of the inclusion of instrument testing in cotton trade rules of the cotton associations, and definition of arbitration based on instrument testing. For checking of the harmonisation of the test results, the CSITC Task Force introduced the international CSITC Round Trial, which is described above. Additionally, the Task Force developed a system of regional support to laboratories. This is done by regional technical centres (Gourlot and Drieling, 2007). Regional technical centres were set up in the two important cotton-producing regions in Africa, based on a project funded by the Common Fund for Commodities and the European Commission (CFC/ICAC/33). Other regions, such as Central Asia, are in the process of setting up additional regional technical centres without project support. Testing-related research is not considered by the CSITC Task Force, but is regarded through cooperation with the ITMF ICCTM.

17.18

Conclusion

In this chapter, the different standardisation, harmonisation and harmonisation check measures for cotton testing are described, based on the current development from manual cotton classing to instrument testing. Besides national solutions, cotton classing is based on reference material that has been internationally accepted since the beginning of the twentieth century. For instrument testing there is worldwide agreement on universal calibration standard material produced by the USDA-AMS. An ASTMI standard method for assuring comparable results from reference material that is not produced in the USDA is in development. Harmonisation checks are done with three different international round trials, which have recently included the rating of laboratories and direct support for improving results. With the ITMF International Committee on Cotton Testing Methods (ICCTM) and with the ICAC Task Force on Commercial Standardisation of Instrument Testing of Cotton, two international boards are in place to foster cotton testing and harmonisation activities. In summary, cotton is a good example of the harmonisation of classing and testing. Nevertheless, the system is improving continually.

References Amadou Soule, A., Bachelier, B. et al. (2006) Manuel qualit´e pour les fili`eres cotonni`eres UEMOA, standards ‘Afrique’ de qualit´e du coton fibre. Guide Technique No. 3, Version 1. UNIDO Publication. Drieling, A. (2007) The future of cotton testing, in Proceedings of the 66th ICAC Plenary Meeting, Izmir, Turkey, 22–26 October 2007. International Cotton Advisory Committee (ICAC), Washington, DC, USA.

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Drieling, A. (2008) Results of the first year of implementation of the CSITC Round Trial, in Proceedings of International Cotton Conference, Bremen, 2–5 April 2008. Faserinstitut Bremen e.V. and Bremer Baumwollb¨orse, Bremen, Germany. Drieling, A. and Knowlton, J.L. (2007) Development of a regular CSITC Round Trial, in Proceedings of Cotton Quality Measurement Conference, New Orleans, LA, 9–12 January 2007. Beltwide Cotton Conferences, National Cotton Council of America, Memphis, TN, USA. FIBRE (2009a) Results of the Bremen Cotton Round Tests, available at Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany. FIBRE (2009b) Maturity (theta) distribution and fineness distribution for 3000 fibres of a coarse and mature cotton, and a fine and immature cotton, unpublished results from Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany. Gourlot, J.P. and Drieling, A. (2007) CSITC activities for assuring the reliability of cotton instrument testing in Africa, in Proceedings of Cotton Quality Measurement Conference, New Orleans, LA, 9–12 January 2007. Beltwide Cotton Conferences, National Cotton Council of America, Memphis, TN, USA. ICAC (1998) Classing and grading of cotton, Report by the Technical Information Section of the ICAC, ICAC, Washington, DC, USA. ICAC (2009a) CSITC Round Trials on US upland cottons from 2007-1 to 2009-1, 36 cottons, approx. 80 instruments; available at: http://www.icac.org (accessed 17 July 2009). ICAC (2009b) Instrument testing; available at: http://www.icac.org/csitc/english.html (accessed 6 July 2009). ISO Guide 43 (1997) Proficiency testing by interlaboratory comparisons, ISO. ITC (2007) International Trade Center UNCTAD/WTO: Cotton Exporter’s Guide. ITC, Geneva, Switzerland. Neumann, W. (2009) Photo of USDA standard boxes, photo courtesy of W. Neumann, Bremen Cotton Exchange, Bremen, Germany. USDA (2009) Ordering cotton calibration material – standard order form; available at: www.ams.usda.gov (accessed 23 July 2009). USDA-AMS (2001) The Classification of Cotton, Agricultural Handbook 566. Cotton Program, Agricultural Marketing Service, US Department of Agriculture, Washington, DC, USA. Uster (2008) Fiber Testing Training Course. Uster Technologies, Inc., Uster, Switzerland.

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18 Flax – ASTM Standardisation and Harmonisation Danny E. Akin Athens, Georgia, USA

18.1 Introduction For flax, the need for global agreement on standards, i.e. standards ‘set up and established by authority as a rule for the measure of quantity’ (Merriam-Webster, 1999), is well recognised. Standards, based on objective methods of assessment and agreed upon by sellers, buyers and users, are needed for trade and by manufacturers of fibre products. The global business and economic trade benefits that have arisen from cotton standards provide a worthy example for expanding the applications for flax fibres. Without standards, users of fibres are unsure of product quality and desired applications of diverse materials, and manufacturers are without knowledge of how to set equipment for optimum production and reduced downtime. With a global economy, natural fibres will be produced in extremely different climates and under myriad production systems, further contributing to variations in fibre properties and quality. While progress has been made, further use of new and existing methods of analysis need to be applied under strict guidelines for internationally accepted standards of flax fibre properties.

18.2

Historical Perspective

Flax fibre for traditional spinning with long fibre bundles requires exceptional properties for spinning and quality clothing. In spite of flax’s long and interesting past for clothing and other textiles (Sharma and Van Sumere, 1992), long flax (longitudinal flax processing) has been traditionally bought and sold by the subjective judgement of experienced graders who appraise properties by look and feel, so-called organoleptic methods, for fineness, strength, colour and many other properties (Ross, 1992). The traditional flax industry of Europe has not actively promoted the development of objective standards and continues to rely upon these subjective means for characterisation. Various classification schemes exist within an industry segment and include criteria Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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such as the source (e.g. Belgium, France, Russia or China), processing history (e.g. water or dew retted), or application (e.g. warp or weft yarn). Within particular countries (e.g. Czech Republic, Germany, Poland, Russia), measurement of flax fibres is done by more or less consistent means, and therefore a limited classification system exists. For example, in past years, Russia used an elaborate judging and grading system for commerce and processing of flax (Pfefferkorn, 1944). Grades of flax fibres for specific applications (e.g. cottonised fibres) are identified for marketing within a company. Methods for characterising flax are known and listed in a comprehensive book on flax (van Langenhove and Bruggeman, 1992). Other methods and instruments that objectively and rapidly analyse cotton fibres have been tested for application to flax fibre (Beckmann and Kromer, 1995). Some success has been had with modifications in hardware and software of the cotton HVI (high-volume instrument) fibre equipment, but the performance required has not been reached. In order to measure flax fibres/fibre bundles successfully with cotton equipment, a major redesign in the mechanics and software of instruments, such as the automated fibre information system (AFIS), is needed. The amount of development necessary, along with predicted small market size and lack of standards, has caused Zellweger Uster to discontinue work (Anja Schleth, Zellweger Uster, private communication). Other groups (e.g. Faserinstitut Bremen e.V. (FIBRE), Bremen, Germany; IAF, Reutlingen, Germany; Applied Science Division, Department of Agriculture, Northern Ireland) continue to research rapid methods for flax fibre assessment. Of particular note is the work on modifications to the equipment for strength testing, the optical fibre fineness analyser (OFDA) for fineness and development of an image-analytical test method for length of fibre bundles (Drieling et al., 1999).

18.3

Factors Influencing Testing and Standards in Flax Fibres

Natural fibres such as flax are by their nature variable. While a few natural fibres such as cotton and wool exist as individual units, flax occurs as ultimate fibres connected in bundles (Van Sumere, 1992) (see Chapter 4). The commercially useful fibres of flax (see Chapter 2.2) develop in bundles of various sizes in the bast (cortical) region of the stem and are associated with the outer epidermal cell layers and lignified inner core tissues. Harvested bast plants are retted to obtain fibre bundles (Van Sumere, 1992). Retting is usually a microbial process where plant pectin is degraded, and bast fibres consequently separate from non-fibre materials. The quality of retting exerts a major influence on the yield and quality of the resulting fibre bundles and fibres. Dew retting is the most widely used method today, and it is generally agreed that the best dew-retted fibre is produced in Normandy (northern France), Belgium and the Netherlands (Hamilton, 1986). Because of the climate and the producers’ skill and expertise, the quality is prized and rewarded, but, even under these conditions, fibre quality is variable and crop losses occur about one-third of the time. Most of the flax fibre is produced under less ideal conditions, and as a consequence commercial flax fibre can be extremely variable in quality. In China, which is increasing flax production, 80% of the flax fibre production is reported to be by warm water retting, with low yields and moderate quality (INF, 2004). Because of problems in both water and dew retting, research is continuing on developing a chemical or enzymatic retting method (Beckmann and Kromer, 1995; Akin et al., 2004; Antonov et al., 2007), and suitable alternative methods may become commercially used. Fibres produced by these newer methods may have different properties than those from traditional methods, with specific methods used to tailor fibre properties (Akin et al., 2007). Traditional flax consists of long flax arising from scutching and hackling operations (Sharma and Van Sumere, 1992). Tow, which is a byproduct of longitudinal flax processing cleaning methods, provides an important short fibre bundle for technical applications like insulation material and composites and for textile blends. Processing lines such as the unified line of Czech flax machinery (Merin, Czech Republic) or the Temafa Lin line (Bergisch Gladbach, Germany) produce short fibre bundles (disordered flax processing) from flax stems without traditional long flax and tow products. Flax fibre bundles may be ‘cottonised’, i.e. cleaned and shortened, for use in short stable spinning or for other applications (McAlister III et al., 2002). With

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burgeoning interest in the use of natural fibres for composites in the automotive sector (see Chapters 3.3 and 19.4) and other large industrial users, flax fibres will come from very diverse, non-traditional sources. For example, linseed straw, which is left after seed harvest, is available in large amounts, and currently only a small percentage is used for speciality paper and some composites. While considered inferior in quality to fibre flax for textiles and some composites, improved processing could provide fibres from linseed straw of various grades for particular applications. Fibre quality, as well as yield, is also influenced by cultivar and production practice. Marketing and utilisation of flax fibres from these diverse sources indicate the necessity for internationally recognised, uniform and objective standards for judging fibre quality for trade, optimal processing and expanded uses.

18.4

Current Status for Flax Standards

The potential for expanded uses in a variety of industries, along with variation in sources, has resulted in considerable interest and new calls for development of standards for flax fibres, and bast fibres generally (van Dam et al., 1994). Before 2000, only one international standard, i.e. ISO 2370 for flax fibre fineness, has been available over a long time period (ISO, 1980). For example, the European Cooperation in the Field of Scientific and Technical Research (COST) Action 8475 of the European Union (Textile Quality and Biotechnology), which operated until 2005, stated an objective of acquiring knowledge ‘to set up quality standards for assessing flax fibre’ (INF, 2002). However, standards were not developed during the lifetime of this project. In the late 1990s, representatives of government, industry and academia actively began the development of standards for flax fibre through ASTM International. Subcommittee D 13.17 (Flax and Linen) was officially formed in 1999 and began biannual meetings as part of the Textile Committee of ASTM International. From research among various collaborators and D 13.17’s actions, a series of standards has been approved as test methods for flax (Akin, 2005). At the time of writing, Subcommittee D 13.17 is active and meets regularly during ASTM’s ‘Textile Committee Week’.

18.5 18.5.1

Current ASTM Flax Fibre Standards Terminology

Under current rules, all subcommittees in ASTM International must have a terminology standard related to their subject. Lengthy discussions over several meetings were required to reach agreement on precise terminology for flax. Research was undertaken to assure that terms were not counter to accepted language in Europe or other regions with a long history of flax. ‘Standard Terminology Relating to Flax and Linen’ D 6798-02 was approved in 2002 as the first standard under D 13.17 (ASTM International, 2009). 18.5.2

Colour

The natural colour of flax fibre is light amber. Retting methods, however, influence the colour of processed fibres (Akin et al., 2000) (Table 18.1). The use of CIELAB measurements provided an established means for objective colour determination using three factors: black to white (L* value), green to red (a* value) and blue to yellow (b* value) (Epps et al., 2001) (see Chapter 13). Water retting results in a light-coloured fibre. Dew retting, in contrast, imparts varying degrees of grey to black to the fibres, depending upon the extent of retting, among other factors. Experimentally produced enzyme-retted or chemical-retted flax is very light owing to some bleaching action of the chemicals. In addition to lightness, colour measurement systems can show other colour scales, such as red-green and yellow-blue, and thereby provide additional information. In one study, water- and dewretted fibres differed in yellow values (Table 18.1). In practical use, much of the flax fibre is blended among

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CIELAB colour values of flax fibres retted by various means

Samples Dew retted (‘Natasja’) Dew retted (‘Viking’) Water-retted No. 1 Water-retted No. 2 Enzyme retted (‘Ariane’) Enzyme-retted seed flax

L*

a*

b*

59.92 57.88 66.80 68.28 76.20 67.81

2.50 2.27 2.67 2.53 2.31 4.61

10.26 9.99 14.20 14.89 14.03 17.05

Adapted from Textile Res. J. 70, 852–858 (2000).

harvests and especially during processing to have consistent colour in the final product. The use of a standard method for colour values could help in blending for particular properties of a fibre sample arising from a variety of sources and processing methods. With this method, problems related to colour matching can be more objectively addressed to provide better use of flax from a broad production system. Objective colour measurement is important for communication among fibre suppliers (M¨ussig, 2003) and can play an important role in design applications, for example in the automotive sector (Cescutti and M¨ussig, 2005). ‘Standard Test Method for Color Measurement of Flax Fibre’ D-6961-03 was approved in 2003 (ASTM International, 2009).

18.5.3

Fineness

Fineness is one of the most important properties for textile fibres (see Chapter 13). ISO 2370 (ISO, 1980), which was developed in the 1970s, used airflow to estimate fineness. The separation efficiency of hackling of bast fibres (Simor, 1965) and measurement of parallel flax fibre bundles (Otto and Rohs, 1969) have been tested using airflow methods. A standard airflow test, based on a modified cotton micronaire system (ASTM D 1448-97) was developed (Akin et al., 1999b) using a series of flax fibre grades purchased from the Institut Francais Textile – Habillement, Lille, France. This test provides a number as a comparative score for ranking fibres, but does not permit the same units and use as cotton micronaire. This ranking showed good agreement with fibre widths, particularly the finest categories, determined by image analysis (Akin et al., Table 18.2

Flax fnieness by image analysis and airflow Frequency of occurrence of fibre widths by image analysis

Fibre sample (ISO fineness)a B (21.7) C (23.5) D (28.7) E (32.0) F (33.7) G (39.1) H (46.1) I (50.5) J (72.1) a

10–30 µm

40–100 µm

110–200 µm

210–300 µm

Airflow finenessb

76.3 75.5 65.2 72.3 65.4 65.4 58.1 60.9 46.1

19.6 21.7 28.9 22.3 27.4 26.8 29.7 28.3 36.3

4.1 2.2 5.2 4.2 6.4 7.4 10.3 9.5 13.7

0 0.6 0.7 1.2 0.8 0.4 1.9 1.3 3.9

3.7 4.1 4.6 4.4 5.2 5.1 6.0 6.6 7.4

IFS Standards (and airflow values for fineness) from Institut Francais Textile – Habillement. Modified cotton micronaire method using 5.0 g flax fibres cut to 2.5 cm, which resulted in a reading within the accepted range for the micronaire. Adapted from D.E. Akin et al., Properties of enzymatically retted flax for linen fiber, Book of Papers, Amer. Assoc. Textile Chem. Colorists, 1999, 486–492. b

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Figure 18.1 Light micrograph of processed and cottonised flax fibre bundle (f) with a fragment of shive (s) still remaining attached (↓) owing to insufficient retting. Shive reduces fibre quality and limits usefulness in value-added applications.

1999a) (Table 18.2). Similarly, the use of Laserscan, which depends upon the scattering and diffraction of a laser beam by fibres to give a width distribution, also showed good agreement with the data from graded fibres by IFS (France) and image analysis (Baetens, 1998). The ASTM test method contains two options: (1) airflow resistance and (2) estimated mass per unit length. For calibration of the system, viscose rayon fibres were reduced to 5 cm length with nominal linear densities of 1.1, 1.5 or 3.0 denier and nominal specific fineness index values of 2.55. 2.9 or 4.0. ‘Standard Test Method for Assessing Clean Flax Fibre Fineness’ D-7025-04a was approved in 2004 (ASTM International, 2009).

18.5.4

Shive Content

The presence of non-fibre, trash particles contaminates fibres and is particularly troublesome in high-value products like textiles. The amount of non-fibre contaminants depends upon the quality of retting to a large extent. After retting and subsequent cleaning, shives (i.e. lignified core tissue) and cuticularised epidermis often still remain with the fibre (Figures 18.1 and 18.2). Production efficiency, e.g. spinning of yarn without interruption or fibre loss during needle felt production, and final product quality are both diminished by the non-fibre components. The presence of shives in fibre products creates a major problem, e.g. in composites for the automotive industry (Cescutti and M¨ussig, 2005). Flax fibre bundles are mostly cellulose, i.e. around 65–80%, with other non-cellulosic sugars present (Focher et al., 1992). The shives contain substantially more aromatics and lignin than fibre bundles (Akin et al., 1996), and the different chemistries of these components provide a relatively easy way for differentiation. Table 18.3 shows variations in chemical components of bast fibre bundles and shives. A model was developed using a series of mixtures, with exact proportions (by mass) of ground fibre and shive, which was scanned by near-infrared reflectance spectroscopy (NIRS) (Barton II et al., 2002; Sohn et al., 2004a). Chemometric analysis was performed in Unscrambler software v.9.2 (CAMO, Trondheim, Norway). Spectral data were preprocessed with Savitzky–Golay derivative followed by multiplicative scatter correction. Partial least-squares (PLS) regression and Martens’ uncertainty regression were used to develop a calibration model with a full ‘leave-one-out cross-validation’. Performance of the calibration model was evaluated with a multiple coefficient of determination (R2 ), root mean squared error of calibration (RMSEC)

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Figure 18.2 Light micrograph of processed and cottonised flax fibres (only a few bundles remain) with fragments of cuticle (c) remaining with the fibres. Poor retting results in large fragments of the cuticle remaining with the fibres and fibre bundles. The cuticle is usually present as remnants of the cuticularised epidermis, and large fragments bound to multiple fibre bundles result in poor quality flax.

and root mean squared error of cross-validation (RMSECV). The use of precise proportions of fibre:shive and scanning with NIRS resulted in a rapid, non-destructive method useful for predicting shive in a range of fibres (Table 18.4). ‘Standard Test Method for the Measurement of Shives in Retted Flax’ D-7076-05 was approved in 2005 (ASTM International, 2009).

18.6

Summary of Standards

The four standards approved to date by ASTM International through subcommittee D 13.17 are listed in Table 18.5. The standards for colour, fineness, and shive content were approved initially in the year listed in Table 18.5 based on intralaboratory data and then validated in 2009 with bias and precision statements with interlaboratory data from round robin tests.

18.7

Future Standards

The properties for which standards have already been developed are some of those commonly required for typical fibre applications. Standards for strength and length are two others that require future action. Other methods are proposed in Chapter 13 for testing bast fibres. The general idea of force to break a certain fibre mass could be considered for a variety of methods. Fibre length may be more problematic. While long flax (from longitudinal flax processing) for high-value textiles has a minimum length of about 50 cm, tow or flax Table 18.3

Carbohydrate and aromatic constituents in dew-retted flax fractions

Fraction

Non-cellulose carbohydrates in mg/g

Glucose in mg/g

Aromatics in mg/g

Shives Bast fibre

158 94

237 650

13 Trace

Adapted from J. Sci. Food Agri. 72, 155–165 (1996).

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377

Shive content in processed flax using the NIRS model

Flax fibre and processing conditions

Predicted shive by NIRS in % 2.3 ± 0.6 4.5 ± 1.8 4.0 ± 1.0 18.9 ± 1.8 3.6 ± 0.6

Fibre flax ‘Ariane’ – enzyme retted, commercially cleaned and cottoniseda Fibre flax ‘Ariane’ – dew retted, commercially cleaned and cottonisedb Mature linseed – enzyme retted, commercially cleaned and cottoniseda Mature ‘Ariane’ – enzyme retted, pilot-plant cleaned only Mature ‘Ariane’ – enzyme retted, pilot-plant cleaned, 1 × Shirley Analyzer

a Enzyme retted with Viscozyme plus chelator, cleaned through the unified line and cottonised through the LaRoche system (Ceskomoravsky len, Humpolec, Czech Republic). b After dew retting, fibres cleaned through the unified line and cottonised through the LaRoche system (Ceskomoravsky len, Humpolec, Czech Republic). Adapted from D.D. McAlister III et al. Cotton fibers: properties and interaction with flax fibers in blends: focus on rotor spun yarn, 26th International Cotton Conference Bremen, 2002.

from disordered flax processing, where fibre bundles are non-uniform and non-aligned from the whole plant, could be extremely variable. Methods used for cotton or with newer image analysis systems will probably provide the starting point for fibre length standards in flax (see Chapter 13). Recent attempts have been made to use rapid spectroscopic methods to assess flax fibre quality in place of more time-consuming physical methods. Models using near-infrared reflectance spectroscopy have been used for several parameters, including fibre content in intact stems (Barton II et al., 2002), degree of retting (Archibald and Akin, 2000) and flax content in linen/cotton blended fabrics (Sohn et al., 2005). Near-infrared spectroscopy using particular wavelength ranges has been used to assess flax fibre fineness using calibration data from derivative thermogravimetric analysis and airflow methods (Faughey and Sharma, 2000). While a near-infrared model has been developed and a standard approved to predict shive content in flax fibre, the presence of cuticularised epidermis also contributes impurities in clean fibre. The cuticularised epidermis contains a high level of wax and cuticle along with aromatics (Morrison III and Akin, 2001) and may require another near-infrared model for assessment. A preliminary study was made to establish an NIRS-based method, without calibration but using the flax epidermal layer as a marker, to estimate fibre purity (Sohn et al., 2004b). An index was calculated that may be useful, pending further work, for predicting fibre purity beyond that for shives. Use of spectroscopy and chemometric methods to assess quality offers possibilities of further developing models and standards for rapid and non-destructive assessment of flax fibres. These spectroscopic methods, however, require calibration sets from some other assessment method, e.g. wet chemical, strength, fineness, etc. In addition to development of new calibrations, current work has been undertaken to transfer calibration models to more robust instruments, providing the ability for on-line or at-line assessment and grading of fibre at the plant (de Haseth et al., 2008). These newer methods for fibre analysis along with more standard methods for determining fibre properties provide a basis for an expanded set of standards and ultimately for an objective classification system for flax fibres.

Table 18.5

Flax standards to date under subcommittee D 13.17 of ASTM Internationala

Title of standard

Designation

Approved

Standard Terminology Relating to Flax and Linen Standard Test Method for Color Measurement of Flax Fibre Standard Test Method for Assessing Clean Flax Fibre Fineness Standard Test Method for the Measurement of Shives in Retted Flax

D-6798-02 D-6961-03 D-7025-04a D-7076-05

2002 2003 2004 2005

a

Adapted from Annual Book of ASTM Standards, Textiles Vol. 7, ASTM International, Westshohocken, PA.

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References Akin, D.E. (2005) Standards for flax fiber. Standardization News, 2005(September), 22–25. Akin, D.E., Condon, B., Sohn, M., Foulk, J.A., Dodd, R.B. and Rigsby, L.L. (2007) Optimization for enzyme-retting of flax with pectate lyase. Ind. Crops Prod., 25, 136–146. Akin, D.E., Epps, H.H., Archibald, D.D. and Sharma, H.S.S. (2000) Color measurement of flax retted by various means. Text. Res. J., 70, 852–858. Akin, D.E., Gamble, G.R., Morrison III, W.H., Rigsby, L.L. and Dodd, R.B. (1996) Chemical and structural analysis of fiber and core tissues from flax. J. Sci. Food Agric., 72, 155–165. Akin, D.E., Hardin, I.R., Rigsby, L.L. and Epps, H.H. (1999a) Properties of enzymatically retted flax for linen fiber. Book of Papers. American Association of Textile Chemists and Colorists, Research Triangle Park, NC, pp. 486–492. Akin, D.E., Henriksson, G., Evans, J.D., Adamsen, A.P.S., Foulk, J.A. and Dodd, R.B. (2004) Progress in enzyme-retting of flax. J. Nat. Fibr, 1, 21–47. Akin, D.E., Rigsby, L.L. and Perkins, W. (1999b) Quality properties of flax fibers retted with enzymes. Text. Res. J., 69, 747–753. Antonov, V., Marek, J., Bjelkova, M., Smirous, P. and Fischer, H. (2007) Easily available enzymes as natural retting agents. Biotechnol. J., 2, 342–346. Archibald, D.D. and Akin, D.E. (2000) Use of spectral window preprocessing for selecting near-infrared wavelengths for determination of the degree of enzymatic retting of intact flax stems. Vibr. Spectrosc., 23, 169–180. ASTM International (2009) Annual Book of ASTM Standards. Textiles Vol. 7. ASTM International, Westshohocken, PA. Baetens, E. (1998) Determination of the fibre fineness distribution by Laserscan. 1st Nordic Conference on Flax and Hemp Processing, Tampere, Finland, pp. 81–89. Barton II, F.E., Akin, D.E., Morrison, W.H., Ulrich, A. and Archibald, D.D. (2002) Analysis of fiber content in flax stems by near-infrared spectroscopy. J. Agric. Food Chem., 50, 7576–7580. Beckmann, A. and Kromer, K.H. (1995) Evaluation of test standards for measuring the fibre content and strength values of flax. Zemedelska Technica, 41, 121–124. Cescutti, G. and M¨ussig, J. (2005) Industrial quality management. Kunststoffe Plant Eur., 1, 97–100. de Haseth, J.A., Akin, D.E. and Barton II, F.E. (2008) Sensors and chemometrics for flax fiber quality and for processing. 2008 International Conference on Flax and Other Bast Plants, Saskatoon, Saskatchewan, Canada, 21–23 July, pp. 10–15. Drieling, A., B¨aumer, R., M¨ussig, J. and Harig, H. (1999) Testing strength, fineness, and length of bast fibres. Tech. Tex., 42, 261–262. Epps, H.H., Akin, D.E., Foulk, J.A. and Dodd, R.B. (2001) Color of enzyme-retted flax fibers affected by processing, cleaning, and cottonizing. Text. Res. J., 71, 916–921. Faughey, G.J. and Sharma, H.S.S. (2000) A preliminary evaluation of near infrared spectroscopy for assessing physical and chemical characteristics of flax fibre. J. Near Infrared Spectros., 8, 61–69. Focher, B., Marzetti, A. and Sharma, H.S.S. (1992) Changes in the structure and properties of flax fibre during processing, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F., M. Publications, Belfast, Northern Ireland, pp. 329–342. Hamilton, I.T. (1986) Linen. Textiles, 15, 30–34. INF (2002) Institute of Natural Fibres, Poznan, Poland, Euroflax Newsl., 17(1), pp. 15–16. INF (2004) Institute of Natural Fibres, Poznan, Poland, Euroflax Newsl., 21(1), p. 7. ISO (1980) International Standard (ISO) 2370. Textiles – determination of fineness of flax fibres – permeametric methods. International Organization for Standardisation. McAlister III, D.D., Foulk, J.A., Akin, D.E. and Annis, P.A. (2002) Cotton fibers: properties and interaction with flax fibers in blends: focus on rotor spun yarn. 26th International Cotton Conference Bremen, Faserinstitut, Bremen, Germany, pp. 207–211. Merriam-Webster (1999) Merriam-Webster’s Collegiate Dictionary, 10th edition. Merriam-Webster, Inc., Springfield, MA, USA. Morrison III, W.H. and Akin, D.E. (2001) Chemical composition of components comprising bast tissue in flax. J. Agric. Food Chem., 49, 2333–2338. M¨ussig, J. (2003) EIHA’s classification of hemp fibre by colour grades. 1st International Conference of European Industrial Hemp Association, H¨urth, Germany, 23–24 October.

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Otto, R. and Rohs, W. (1969) Objektive Bestimmung der Feinheit von Bastfasern durch Messung im Lufstrom. Textilindustrie, 71, 344–353. Pfefferkorn, R. (1944) Oregon Fiber Flax for an American Linen Industry. Oregon State College Cooperative Association, Corvallis, OR, USA, 46 pp. Ross, T. (1992) Preparation and spinning of flax fibre, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F., M. Publications, Belfast, Northern Ireland, pp. 275–296. Sharma, H.S.S. and Van Sumere, C.F. (eds) (1992) The Biology and Processing of Flax. M. Publications, Belfast, Northern Ireland, 576 pp. Simor, P. (1965) Faserfeinheit- und Spaltbarkeitmessung an Flachs mit dem Microaire-apparat. Spinner, Weber, Textilveredelung, 29–33. Sohn, M., Barton II, F.E., Morrison III, W.H. and Akin, D.E. (2004a) Prediction of shive content in pilot plant processed flax by near infrared reflectance spectroscopy. J. Near Infrared Spectros., 12, 251–258. Sohn, M., Barton II, F.E., Morrison III, W.H. and Akin, D.E. (2004b) A new approach for estimating purity of processed flax fiber by NIR spectroscopy. J. Near Infrared Spectrosc., 12, 259–262. Sohn, M., Himmelsbach, D.S., Akin, D.E. and Barton II, F.E. (2005) Fourier transform near-infrared spectroscopy for determining linen content in linen/cotton blend products. Text. Res. J., 75, 583–590. van Dam, J.E.G., van Vilsteren, G.E.T., Zomers, F.H.A., Shannon, W.B. and Hamilton, I.T. (1994) Industrial Fibre Crops. Directorate-General XII, Science, Research and Development, European Commission, 247 pp. van Langenhove, L. and Bruggeman, J.P. (1992) Methods of fibre analysis, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F., M. Publications, Belfast, Northern Ireland, pp. 311–327. Van Sumere, C.F. (1992) Retting of flax with special reference to enzyme-retting, in The Biology and Processing of Flax, ed. by Sharma, H.S.S. and Van Sumere, C.F., M. Publications, Belfast, Northern Ireland, pp. 157–198.

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PART V APPLICATIONS: CURRENT AND POTENTIAL

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19 Composites

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19.1 Historical, Contemporary and Future Applications Tuomas H¨anninen and Mark Hughes Department of Forest Products Technology, Aalto University, Helsinki, Finland

19.1.1 Introduction In recent years there has been significant interest in the use of natural fibres as potential reinforcement for both organic and inorganic matrices. Although, as will be discussed later on in this chapter, the use of natural fibres to reinforce composites is not a new concept, the motivation for their use has, perhaps, changed. Before the advent of man-made fibres, in particular glass fibre, natural fibres of both vegetable and mineral origin were the only reinforcement available for fibre-reinforced composite materials. Indeed, the technology associated with vegetable-fibre-reinforced composites was fairly well advanced even as late as the 1940s (Brown, 1947). Of course, the main driver for using natural fibres such as cotton, jute and flax in these early composites was mainly technical, and it is perhaps fair to say that it is only with the developments that have taken place over the past decade or so that environmental concerns and cost have become major drivers in the development of natural-fibre-reinforced composites. The aim of this chapter is briefly to review the contemporary applications for natural-fibre-reinforced composites and to consider some of the potential future application areas for these materials. While we are mainly concerned with the current and future applications, it is nevertheless of interest to explore some of the historical uses for these materials. In the following section, some historical applications for natural-fibrereinforced composites will be briefly introduced; in this chapter an arbitrary distinction has been made to separate historical applications from contemporary ones, and, as many of the more recent developments in natural fibre composites have taken place since the 1980s, Section 19.1.2 dealing with historical aspects spans the era from antiquity to 1980. Contemporary applications (1980 onwards) are considered in Section 19.1.3, while future applications are covered in Section 19.1.4. At the time of writing there is extensive research in certain areas relating to natural fibre composite materials. For instance, the preparation of cellulose nanocrystals (covered in detail in chapter 19.6) for applications including composites, is currently the subject of intensive study within many research groups around the world, and there are great expectations that this Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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will lead to a new era of high-performance bio-based composite materials. It will be interesting to see whether these expectations are realised. Another topic that is also receiving much attention is that of the synthesis of polymers from renewable resources. Again, this area is the subject of intensive research activity, which has already led to commercial polymers from renewable resources such as polylactic acid (PLA). Undoubtedly this is an exciting area of development, and it will be interesting to see the advances made in the next decade or two. In the context of this chapter, natural fibres are defined as mainly lignocellulosic fibrous materials produced by or extracted from plants. Although animal hair, wool, silk and mineral fibres could also be considered as natural fibres, these will be excluded from the scope of this chapter. More details on the structure and the technical use of wool and silk are given in Chapters 11 and 12. Lignocellulosic fibres, derived from both wood and non-wood sources, will be considered.

19.1.2 Historical Applications (–1980) Natural fibres have been used to reinforce building materials since the dawn of civilisation. The earliest applications date back to 10 000 bc in China, where shards of pottery have been found to contain fragments of hemp fibre (Rowell, 2008). During the nineteenth century, before the invention of modern resins and plastics, there were examples of composite materials reinforced with natural fibres. During the 1850s in America, for instance, shellac was being compounded with wood flour to produce union cases used to display early photographs (Stubbs, 2009). At around the same time in France, Francois Charles Lepage combined albumen and wood flour to produce a composite material that he called ‘Bois Durci’ (‘hardened wood’). Lepage patented this material in 1855 and subsequently sold the invention to a Mr A. Lartry, who founded the Societ´e du Bois Durci, which produced decorative items from the material. Bois Durci was manufactured by soaking wood sawdust in diluted albumen and then drying the impregnated material before moulding it under pressure and steam heat (PHS, 2007). In India, the first attempts to utilise non-wood fibre were carried out in 1926 when jute was used to reinforce shellac (Pal, 1984). However, the age of modern composites can be considered to have begun in 1907, when Leo Baekeland patented the first fully synthetic commercial thermosetting resin – Bakelite. Bakelite is brittle, and so early on it was often combined with fillers such as wood flour to improve its properties. In such a form, Bakelite can be thought of as a rudimentary composite. Owing to its good heat resistance and electrical insulation properties, it was often used in radios, telephones and as electrical insulators (Lewark, 2007). Although ‘reinforced’, Bakelite in this form could not truly be regarded as a structural material, however. The first attempts to produce a structural fibre-reinforced polymer matrix composite can be mainly attributed to the work of Norman De Bruyne in the 1930s, although some pioneering work was carried out earlier than this in the 1920s by Messrs Caldwell and Clay, who used natural fibre fabrics to reinforce synthetic resins for airscrews (De Bruyne, 1937). De Bruyne developed a material known as ‘Gordon-Aerolite’. Gordon-Aerolite was a composite consisting of unidirectionally aligned unbleached flax thread impregnated with phenolic resin, and this material was used experimentally to produce a full-scale main wing spar for the ‘Bristol Blenheim’ light bomber and for the fuselage of the Supermarine ‘Spitfire’ fighter aircraft (McMullen, 1984). These latter developments took place during the course of World War II and were in response to a threatened shortage of bauxite for the production of duralumin following the invasion of France. The anticipated cut in the supply of bauxite never occurred, however, and so the research was discontinued (Aero Research Limited, 1945). The properties of Gordon-Aerolite were impressive, with a Young’s modulus of 48 GPa and a tensile strength of 480 MPa claimed for a unidirectional skein of the material (Aero Research Limited, 1945). World War II saw much research into the use of natural fibres of one form or another; however, despite this work, a pilot seat for the ‘Spitfire’ and aircraft drop tanks were the only cellulose fibre-based composite applications that ended up in production (McMullen, 1984).

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Figure 19.1.1

387

Trabant motor car.

Gordon-Aerolite and similar composites were manufactured by a high-pressure moulding technique requiring heat to cure the resin. Such high-pressure laminates were common until the advent of ‘cold-cure’ resins such as unsaturated polyester resins and epoxies, and, although these resins have largely supplanted phenolic resins in many composite applications, high-pressure phenolic laminates reinforced with natural fibre such as cotton and paper are still manufactured. Products such as those manufactured by Tufnol are commercially available and used in certain engineering applications (Tufnol, 2008). Although, as we have seen, some of the early materials research work on natural-fibre-reinforced composites was stimulated by the need to develop new materials for aircraft, more significant applications for naturalfibre-reinforced composites have arisen in the automotive industry, a sector that is nowadays a significant consumer of these materials (see Chapters 3.3 and 19.4). The use of ‘green’ composites in automobiles can be traced back to Henry Ford who, in 1941, tested carbody panels made from a fibre-reinforced soy-protein plastic (Shurtleff and Aoyagi, 2004). The Trabant (Figure 19.1.1) is another example of the early use of natural-fibre-reinforced composites in automotive applications. Introduced in 1958, the Trabant, produced in the German Democratic Republic by VEB Sachsenring Automobilwerke Zwickau, was manufactured using a monocoque construction with the roof, bootlid, bonnet, wings and doors manufactured from a thermosetting phenolic resin reinforced with cotton fibre (Sonntag and Barthel, 2002). Apart from isolated cases such as the Trabant car, after World War II and the commercialisation of glass fibres in the 1940s, the applications for natural-fibre-reinforced composites declined, and it was not until the beginning of the 1970s and the first oil crisis that some limited interest in the use of natural fibres in composites was again shown. During the 1970s, some work was conducted using sisal and jute to reinforce epoxy and polyester matrices with the aim of producing low-cost housing units as well as other common structures (Winfield and Winfield, 1974; Paramasivam and Abdul Kalam, 1974). Parasivam and Abdul Kalam (1974) found that nearly half of the tensile strength of glass fibre–epoxy composites could be achieved with unidirectional sisal-reinforced epoxy. Other experimental jute-fibre-reinforced polyester composites were produced for low-cost housing units as well as for grain silos and fishing boats during this period (Winfield, 1979).

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Later, the utilisation of bagasse/phenolic resin composites was studied by Salyer and Usmani (1982). Corrugated and shingle roofing panels, decorative laminates, counter tops, furniture, insulation and phenolic moulding powder were all considered as potential products. Pilot-scale composite manufacturing to produce material for roofing was undertaken, and its installation in houses in the Philippines, Jamaica and Ghana was carried out (Salyer and Usmani, 1982). Most of the applications considered so far have involved combining natural fibre with some form of organic matrix material. In the 1970s, however, another application area was opened up to natural fibres owing to legislation. Asbestos had previously been used to reinforce cement. However, with the global efforts to reduce or ban asbestos from a wide range of products, natural fibres, in particular wood pulp fibres, were considered as an alternative (Coutts, 2005). The advantages of cement reinforced with natural fibre, when compared with wood, for instance, are improved dimensional stability, moisture resistance, decay resistance and fire resistance. When compared with the unreinforced cement itself, fibre-reinforced cement has improved toughness and ductility, making it far more versatile. Fibre-reinforced cement is nowadays produced in several countries and is used in applications that include cladding for housing and other constructions. With the exception of the Trabant car and high-pressure laminates, of which Tufnol is a good example, as well as the use of wood pulp to reinforce cement, there are relatively few early examples of widespread applications for natural-fibre-reinforced composites prior to 1980. Before the advent of man-made fibres such as glass fibre, natural fibres were the only viable reinforcement available. However, the introduction of glass fibre and the development of cold-cure resins inevitably led to a decline in the applications for natural-fibre-reinforced composites, and it was not until the 1980s that renewed interest was again shown in these materials.

19.1.3

Contemporary Applications (1980 – 2009)

During the 1980s, interest in natural-fibre-reinforced composites was again renewed, with increased research activity into the potential of both wood- and non-wood-fibre-reinforced composites being seen throughout this and subsequent decades. It is worthwhile briefly examining some of the main driving forces that led to the renewed interest in these materials before considering the main current application areas. Broadly, the drivers for using natural-fibre-reinforced composites are either environmental considerations or cost. The environmental impact of humankind’s activities is now becoming one of the major factors influencing our actions, with the need to reduce carbon dioxide emissions being a major issue. Plants sequester CO2 during growth, releasing it again when they decay. Natural fibres therefore ‘trap’ CO2 while they are in use. Of course, the situation is not quite so straightforward, as energy is required to process the fibres into some form in which they can be used in composites, and this will most probably come from the burning of fossil reserves, thereby emitting CO2 . Nevertheless, compared with man-made fibres, natural fibres have a potentially lower environmental burden than their man-made counterparts. Moreover, the ability of natural fibres to biodegrade naturally (or be incinerated for energy recovery) alleviates some of the potential problems associated with the recycling or disposal of man-made fibre-reinforced composites. Glass fibre is by far the most commonly used man-made fibre used in composites today. With a density less than 60% that of glass fibre, natural fibre offers significant potential advantages in terms of mass savings, especially if the fibre volume fraction is appreciable. This has real potential for lower CO2 emissions when composites reinforced with natural fibres replace those reinforced with glass fibre in transportation applications. This is one of the principal reasons for wishing to use natural fibres in transportation, particularly automotive, applications – mass reduction and the consequent improvements in fuel efficiency. Another aspect of the potential reduction in environmental impact that can be achieved through using natural-fibre-reinforced composites is the replacement of existing materials having a higher environmental burden. One particular example is the potential replacement of treated timber with so-called wood plastic composite (WPC) materials. As we will see later, WPCs are frequently used for decking, an application

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in which treated timber is also used. Environmental concerns led to severe restrictions on the use of CCA (copper chrome arsenic)-treated wood in Europe in 2004, with other regions having taken similar steps. In this case we see both a potential environmental benefit – from using a less harmful material – as well as a market opportunity for WPC materials! The fibres are, of course, only part of the composite system, and we must also consider the matrix material (strictly we should consider all materials including additives used to stabilise the polymers or to modify the interfacial properties between fibre and matrix). In the past few years there have been many developments in the area of bio-based resins and plastics. Although a review of these is beyond the scope of this chapter and will be described in more detail in Chapter 19.5, it is worth noting that significant environmental advantages could potentially be obtained through combining natural fibres with bio-based resins or plastics. These would be true ‘green’ composites. Nevertheless, the situation at the moment is that the majority of organic matrices are still based on fossil reserves (be they virgin or recycled). A high percentage of fibre in the composite is advantageous both technically and environmentally, as it will lead not only to improved mechanical properties but also to a reduction in the amount of matrix material needed. As the matrix will almost invariably carry a higher environmental penalty, it is preferable to incorporate as much fibre as possible. As we will see in the following section, obtaining a higher fibre-to-matrix ratio is currently the subject of much research activity. The second major driver is cost. Although fibres specifically grown for technical applications will attract a premium, there are numerous sources of ‘wastes’ that could be utilised more economically. Much of the early research work on wood plastic composites was stimulated by the desire to redirect waste fibre and plastics from landfill to form useful products. Recently, different agricultural wastes have been investigated as potential reinforcing fillers for plastics, which has led to the novel use of natural-fibre-reinforced plastics in aquaculture applications (Marti-Ferrer, 2008). Although covered in greater detail in other chapters, it is appropriate briefly to consider the manufacturing processes for natural-fibre-reinforced composites, as these have a direct bearing upon the products manufactured and therefore the applications. Most current natural-fibre-reinforced composites are manufactured using thermoplastic polymers rather than thermosetting resins (see Chapter 3.3). Depending upon the fibre type, whether it is a long bast fibre such as flax or hemp, whether it is wood fibre or wood flour, the manufacturing process may differ. The long length of bast fibres make these fibres difficult to process by extrusion; however, when formed into comingled felts of natural and synthetic fibres, they may be pressed into sheets and subsequently moulded into three-dimensional components. Extrusion requires that the reinforcing elements be relatively short so that they may be mixed effectively in the extruder. Extrusion is ideal for processes where long lengths of continuous section are required, and so it is no surprise that decking and other similar profiles are manufactured by this process. Current applications for natural-fibre-reinforced composites can be divided into two main areas – construction and automotive, although there are a number of more niche applications such as in the manufacture of musical instruments. Flaxwood guitars are made in Finland from an injection-mouldable composite material based on spruce wood. It is claimed that this composite material has a superior tonal quality to wood, as well as being more stable (Flaxwood, 2005). This is an extremely good example of the development of a high-value application for natural-fibre-reinforced composites. However, the main application area for extruded WPC materials is in construction. Since the 1990s, the market for WPC materials has grown significantly, particularly in applications where the low maintenance and weather resistance of WPCs are of real benefit. The main growth has been seen in the USA. Here, the biggest segment is in decking and railing products, moulding and trim, as well as fencing the door and window components in the residential construction market. Decking alone accounts for about half of the overall WPC volume and has been estimated to claim a $US 4.6 billion share of the US decking market. The share of WPCs in the overall decking market has increased from 4% in 1996 to 14% in 2006. In spite of the declining housing market in the 2000s, the decking market has remained fairly stable because the major part of the demand is generated by remodelling and repair activities (Wood, 2007). In Europe, the market

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Figure 19.1.2 WPC materials – UPM ProFi Deck. WPC materials – UPM ProFi Deck. Reproduced with permission from UPM-Kymmene Group.

has been slower to develop; however, production of decking products has begun, with several companies manufacturing WPC products. Large forest product companies such as the UPM-Kymmene Group have recently entered the market with their ProFi decking products (Figure 19.1.2). The second major application area for natural-fibre-reinforced composites is the automotive sector. At the beginning of the 1990s, the automotive industry was driven by increasing environmental awareness to develop ways to use natural fibre composites. The applications have mainly been in interiors and for nonor semi-structural uses. Bast fibres have predominantly been used in the automotive industry, often using comingled felts of thermoplastic and natural fibre, although some composites combining natural fibre with thermosetting resins have also been used. Early composites that replaced wood fibreboards were found in the Mercedes E-class, where the door panels were made out of a mixture of flax and sisal fibres in an epoxy resin matrix (Brosius, 2006) (see Chapter 19.4). The trend for using natural-fibre-reinforced composites in automotive applications seems to be continuing, with the likes of Lotus Cars recently revealing its ‘Eco Elise’ sports car containing components manufactured from hemp fibre (Pulman, 2008). Almost invariably in these cases the matrix material is a polymer derived from fossil resources, but manufacturers are now working on replacing the matrix with bio-derived polymers. For example, Toyota manufactures components for its Raum model from kenaf-fibre-reinforced polylactic acid (Toyota, 2009). It seems likely that, with the ongoing developments in resins and plastics derived from renewable resources, more advanced natural-fibre-reinforced composites will be produced for the automotive sector in the near future. Whether these components will remain limited to fairly low-load situations, such as interior panels and trim, or whether they will eventually be used in more demanding structural applications

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will most probably depend upon the outcome of current research programmes. Some of the barriers to achieving higher-performing composite materials will be highlighted in the following section. Current applications for natural-fibre-reinforced composites centre on the construction and automotive industries, although there are a number of niche applications where limited volumes of material are used. The two main drivers currently stimulating the use of these materials are the environment and cost. However, there are certain barriers to the further adoption of these materials in more demanding applications (although clearly the potential is there), and these will be discussed in the following section.

19.1.4 Future Applications Natural-fibre-reinforced composites have found commercial application in the construction and automotive industries, and it seems highly probable that they are now well enough established to stay. It is also probable that the current environmental concerns will remain with us and, if anything, will become more acute and provide an ever-stronger driving force for the development of ‘greener’ materials. Against this backdrop, what are the likely developments that will take place in natural-fibre-reinforced composites, and what will the future applications be? This section will explore some of the potential future applications and some of the factors that may drive these changes. It seems likely that, in the existing application areas, namely construction and the automotive sectors, further improvements will continue to be made in the performance of existing products that will expand the areas of application. As we have seen in the previous section, in the automotive sector, developments are already under way to combine natural fibres with matrix polymers derived from renewable resources. These developments will continue to improve the environmental profile of these materials and so extend their utility. One way in which the application areas for natural-fibre-reinforced composites could be extended would be if the performance of the composites were to be improved so as to give them potential as true structural materials. The potential is there; Gordon Aerolite, mentioned above, had enviable mechanical properties. The Young’s modulus of crystalline cellulose has been estimated to be in the region of 135 GPa (Sakurada et al., 1962), and the tensile strength around 10 GPa (Oksman et al., 2006). These values are comparable with those of synthetic polymer fibres such as aramid. With certain natural fibres, particularly bast fibres such as flax and hemp, experimentally determined tensile properties are also impressive. For example, the Young’s modulus of flax has been reported to lie in the range 50–70 GPa, with a tensile strength in the range 500–900 MPa (Ivens et al., 1997). The Young’s modulus of flax is on a par with that of E-glass fibre, although the strength values are more modest compared with glass. So why are natural-fibre-reinforced composites not currently being used in structural applications? Two main reasons would seem to come to the fore – the inherent properties of the fibre and the composite ‘fibre architecture’. Unlike man-made fibre, the structure and morphology of natural fibres is heterogeneous and irregular. Natural and processing-induced defects in natural fibres such as flax reduce the tensile properties of the fibres (Davies and Bruce, 1998), as well as leading to microstructural failure when the fibres are used as composite reinforcement (Hughes et al., 2000). It is probable that little can be done to reduce the occurrence of naturally occurring defects, although processing-induced damage could be reduced with careful handling. The logical step would be to remove the defects from the fibres, and this could be achieved either by isolating the basic fibrous building blocks of the wood cell wall – the cellulose nanocrystals – and using these as the reinforcement, or by reconstituting the fibre. Both these approaches are being investigated. Cellulose nanocrystals are being looked at as potential high-strength, high-stiffness composite reinforcement, while regenerated cellulose could have the potential to reinforce composites. Fibre architecture is a term used to describe the reinforcement geometry (aspect ratio), the orientation of the reinforcement relative to the applied loads, the reinforcement packing arrangement and the volume fraction of the reinforcement. Along with the inherent properties of the reinforcing fibre, fibre architecture strongly influences the performance of a composite material. With man-made fibres there is a wide variety of textiles of differing structures, ranging from unidirectional tapes to

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complex multiaxial or knitted goods. These are available to composite processors, allowing them to produce the high-performance composite materials commonly used in aerospace and other demanding applications. The situation with natural fibres is, however, different. Of course there are woven textiles designed for clothing and other such purposes, but their structure is poorly optimised for composites. Non-woven felts of natural fibre (often combined with thermoplastic fibres which subsequently form the matrix) are frequently used in the preparation of natural fibre composites because of their low cost; however, the random fibre orientation is poorly optimised for high-performance applications. Higher-performing composites reinforced with natural fibres could be produced if appropriate textiles were to be developed. This would also give the processors of composites, currently only able to manufacture with man-made fibres, an alternative to work with, and would thus open up new application areas for natural-fibre-reinforced composites. There have already been some projects to develop textiles suitable for high-performance applications, and at the time of writing there is at least one ongoing project to develop these textiles. The NATEX project (Natural Aligned Fibres and Textiles for Use in Structural Composites Applications) is one such project, funded under the European Commission 7th Framework Programme which aims to develop textiles from natural fibres that are suitable for use as high-strength reinforcement for structural composites (NATEX, 2009). With success and the development of a supply chain to produce natural fibre textiles optimised for composites, it is likely that new, higherperformance applications can be developed for these materials. Possible application areas must surely include those where the low density of the composites would be of advantage, and so it is possible that more structural applications will be seen in the transportation sector, including aerospace, marine transport, rail and road – in short, applications in which fibre-reinforced composites are currently used. It seems unlikely that natural fibre composites will ever compete with ‘exotic’-fibre-reinforced composites, but against glass-fibre-reinforced material the situation might be different. Another issue with using natural fibre as composite reinforcement is that of the polarity of the fibre, making it incompatible with hydrophobic matrices and leading to reduced strength and durability. Chemical modification provides a solution to this problem. In recent studies it has been shown that the most promising approach for modification is to form covalent bonds between the fibres and matrix. Such an effect has been achieved by using maleated and silane coupling agents (John and Anandjiwala, 2008). Positive results have also been achieved with alkali treatment. During alkali treatment, cellulose is mercerised from native, tightly packed cellulose I allomorph to a looser, ‘regenerated’ cellulose II, making the structure more accessible and more susceptible to chemical reactions. Alkali treatment also affects the other components of fibres. The use of coupling agents derived from natural products is an intriguing topic. Lignin, which can be easily recovered from, for example, residues of the papermaking process, has been explored as an adhesion promoter in cottonfibre-reinforced composites (Graupner, 2008). Shellac, chitin and chitosan have also been efficiently used as coupling agents (John and Anandjiwala, 2008). Overcoming some of the problems associated with the lack of compatibility between the fibre and matrix should lead to improved short- and long-term performance, which would then open up new and more demanding application areas for natural-fibre-reinforced composites. Wood plastic composites are generally extruded into linear components of constant cross-section. Injection moulding, on the other hand, allows the rapid manufacture of three-dimensional components and is routinely used to manufacture a whole range of plastic goods (more details about natural fibre composite processing techniques are given in Chapter 19.3). Injection moulding would open up a range of new application areas for WPC materials currently processed by extrusion, and is seen as an area of high growth potential. One of the negative aspects of current WPC materials is their relatively high density compared with that of wood, for example. The density of a typical WPC is about 2.5 times greater than that of spruce softwood. However, foaming offers the possibility of reducing the density of WPCs, which would then help open up new application areas where reduced mass is required, or where a higher specific stiffness is needed. Perhaps one of the most exciting prospects for composites reinforced with natural fibres is that presented by so-called cellulose nanocomposites. Since the beginning of this century, the pulp and paper industry has been undergoing significant change. The production of pulp and paper is moving to Asia and South America, where the abundance and the low cost of the raw material, coupled with low labour costs, make production

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more attractive than in other parts of the world. This has proven to be a severe setback for the economies of countries where the pulp and paper industry has traditionally been important to the national economy, and has stimulated the search for new high-value products to replace pulp and paper. Cellulose nanocrystals are building blocks biosynthesised to provide structural properties in living organisms. By separating cellulose nanocrystals from their natural matrix consisting of lignin and hemicelluloses, they can again be used as reinforcement in composites. Although the first studies on cellulose nanocrystals as reinforcement in composites date back to 1995, no commercial applications have yet been developed (Dufresne, 2008). Nanosized cellulose particles (NCPs) have been seen as one of the solutions to the pulp and paper industry’s problems. At the present time, research has mainly been concentrating on producing NCPs from pulped wood, although they can be produced from almost any fibrous plant material. NCPs can be divided into two categories according to the production method employed. Nanofibrillated cellulose (NFC) is produced by grinding pulp, while microcrystalline cellulose (MCC) is prepared by removing amorphous parts of pulp by severe acid hydrolysis. The production methods of different NCPs will be discussed more thoroughly in Chapter 19.6 of this book. The importance attached to the development of cellulose nanocomposites and other materials and products based on cellulose nanocrystals is exemplified by the creation of the Finnish Centre for Nanocellulosic Technologies in the spring of 2008 to investigate opportunities for nanocellulose as a raw material. One of the major issues to be investigated by the centre, a joint venture between the Technical Research Centre of Finland (VTT), Aalto University, Helsinki and the UPM-Kymmene Group, has been the production of nanofibrils on an industrial scale. Applications of nanocellulose are mainly considered to be in paper and packaging products, although construction, automotive, furniture, electronics, pharmacy and cosmetics are also being considered (TEKES, 2008). The high strength and stiffness as well as the small dimensions of NCPs may well impart useful properties to composite materials reinforced with these fibres, which could subsequently be used in a range of applications. Doubtless, new applications will be found for composite materials that utilise matrices derived from renewable resources. In recent years, much research has been directed at developing resins and plastics from renewable resources, and this has led to the commercialisation of several bio-based polymer systems. PLA, for example, has now been in commercial production for several years and, as mentioned above, is being used by Toyota in the manufacture of automotive components. With further advances in these bio-derived plastics, there will inevitably be new composite materials reinforced with natural fibres, with a range of properties that will make them suitable for particular applications. It might be expected that certain of these bio-derived resins will be more compatible with natural fibres, which will obviate the necessity for adhesion promoters and thereby improve the durability and performance of the composites. Biodegradability may be a specific functionality that will be tailored into future natural fibre composites!

19.1.5

Conclusions

It seems clear that the application of natural-fibre-reinforced composites can do much to reduce the environmental burden of our materials usage by replacing fossil-based materials directly (or by reducing the processing energy required), by reducing the fuel consumption of vehicles incorporating these materials or by replacing materials with a higher environmental impact. The history of the use of natural fibre composites stretches back into antiquity; however, there have been some radical innovations over the past few decades, such as the development of extruded WPC materials, which have now found application in the construction sector. Fibres such as flax and hemp grown for their technical properties are now finding use as composite reinforcement in automotive parts. The future for these materials looks promising, and, with advances in the materials science and technology of these materials, new application areas will undoubtedly be found. The

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most promising applications may well be those where light mass (relative to glass-fibre-reinforced plastics) and ‘green’ credentials are most advantageous.

References Aero Research Limited (1945) A fighter fuselage in synthetic material. Aero Research Limited technical notes. Brosius, D. (2006) Natural fiber composites slowly take root (2 January 2006); available at: http://www. compositesworld.com/articles/natural-fiber-composites-slowly-take-root.aspx (accessed 22 April 2009). Brown, W.J. (1947) Fabric Reinforced Plastics. Cleaver-Hume Press Ltd, London, UK. Coutts, R.S.P. (2005) A review of Australian research into natural fibre cement composites. Cem. Concr. Compos., 27, 518–526. Davies, G.C. and Bruce, D.M. (1998) Effect of environmental relative humidity and damage on the tensile properties of flax and nettle fibers. Text. Res. J., 68(9), 623–629. De Bruyne, N.A. (1937) Plastic materials for aircraft construction. The 615th lecture read before the Royal Aeronautical Society, 28 January 1937, Royal Society of Arts, London, UK. Dufresne, A. (2008) Polysaccharide nanocrystal reinforced nanocomposites. Can. J. Chem., 86, 484–494. Flaxwood (2005) The flaxwood story; available at: http://www.flaxwood.com/about+flaxwood/ (accessed 29 June 2009). Graupner, N. (2008) Application of lignin as natural adhesion promoter in cotton fibre-reinforced poly(lactic acid) (PLA) composites. J. Mater. Sci., 43, 5222–5229. Hughes, M., Hill, C.A.S., S`ebe, G., Hague, J., Spear, M. and Mott, L. (2000) An investigation into the effects of microcompressive defects on interphase behaviour in hemp–epoxy composites using half fringe photoelasticity. Compos. Interfaces, 7(1), 13–29. Ivens, J., Bos, H. and Verpoest, I. (1997) The applicability of natural fibres as reinforcement for polymer composites, in Renewable Bioproducts: Industrial Outlets and Research for the 21st Century, EC-Symposium, 24–25 June 1997, International Agricultural Center (IAC), Wageningen, The Netherlands. John, M.J. and Anandjiwala, R.D. (2008) Recent development in chemical modification and characterization of natural fiber-reinforced composites. Polym. Compos., 29, 187–207. Lewark, B.A., Sr. (2007) Composites: past, present, future: phenolics revisited (6 January 2007); available at: http://www.compositesworld.com/columns/composites-past-present-amp-future-phenolics-revisited.aspx (accessed 22 April 2009). Mart´ı Ferrer, F. (2008) DOLFIN Project: composite with crop waste like a reinforced filler to use in aquiculture, in Proceedings of III International Seminar on Biodegradable Polymers and Sustainable Composites, Valencia, Spain, 3–4 March 2008. McMullen, P. (1984) Fibre/resin composites for aircraft primary structures: a short history, 1936–1984. Composites, 15, 222–230. NATEX (2009) Welcome to NATEX; available at: http://www.natex.eu/ (accessed 30 June 2009). Oksman, K., Matthew, A.P., Bondeson, D. and Kvien, I. (2006) Manufacturing process of cellulose whiskers/polylactic acid nanocomposites. Compos. Sci. Technol., 66, 2776–2784. Pal, P.K. (1984) Jute reinforced plastics: a low cost composite material. Plast. Rubber Process. Applic., 4, 215–219. Paramasivam, T. and Abdul Kalam, P.J. (1974) On the study of indigenous natural-fibre composites. Fibr. Sci. Technol., 7, 85–88. PHS (2007) Franc¸ois Charles Lepage invented Bois Durci, in Plastics Historical Society (July 2007); available at: http://www.plastiquarian.com/lepage.htm (accessed 29 June 2009). Pulman, B. (2008) First official green pictures. Carmagazine (9 July 2008); available at: http://www.carmagazine. co.uk/Green-Cars/Search-Results/Green-First-Pictures/Lotus-Eco-Elise-first-pictures/ (accessed 29 June 2009). Rowell, R.M. (2008) Natural fibres: types and properties, in Properties and Performance of Natural-Fibre Composites, ed. by Pickering, K.L. Woodhead Publishing Limited, Cambridge, UK. Sakurada, I., Nukushina, Y. and Taisuke, I. (1962) Experimental determination of the elastic modulus of crystalline regions in oriented polymers. J. Polym. Sci., 57, 651–660. Salyer, I.O. and Usmani, A.M. (1982) Utilization of bagasse in new composite building materials. Ind. Eng. Chem. Prod. Res. Dev., 21, 17–23.

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Shurtleff, W. and Aoyagi, A. (2004) Henry Ford and his employees: work with soy. A special exhibit – the history of soy pioneers around the world. Unpublished manuscript, Soyinfo Center, Lafayette, CA; available at: http://www.soyinfocenter.com/HSS/henry ford and employees.php (accessed 29 June 2009). Sonntag, W. and Barthel, W. (2002) Kunststoff f¨ur Karosserieverkleidungen, in Proceedings of 4th International Wood and Natural Fibre Composites Symposium, Kassel, Germany, 10–11 April 2002. Institut f¨ur Werkstofftechnik, Universit¨at Gh Kassel, Kunststoff- und Recyclingtechnik, pp. 1-1–1-27. Stubbs, P. (2009) Ambrotype photo in union case. EdnPhoto (June 2009); available at: http://www.edinphoto.org.uk/ 1 early/1 early photography - processes - ambrotype photo in union case - mono.htm (accessed 29 June 2009). TEKES (2009) Suomen nanoselluloosakeskus perustettu (18 February 2008); available at: http://akseli.tekes.fi/opencms/ opencms/OhjelmaPortaali/ohjelmat/NANO/fi/system/uutinen.html?id=3622&nav=Uutisia&arkisto=true (accessed 5 May 2009). Toyota (2009) Toyota Eco-Plastic – first adoption of a plant derived plastic; available at: http://www.toyota.co.jp/ en/environment/recycle/design/recycle.html (accessed 29 June 2009). Tufnol (2008) Composite chemicals; available at: http://www.tufnol.com/tufnol/default.asp (accessed 5 May 2009). UPM (2009) UPM profi; available at: http://w3.upm-kymmene.com/upm/internet/upm profi eng.nsf/sp?open&cid= homepage (accessed 5 May 2009). Winfield, A.G. (1979) Jute reinforced polyester projects for UNIDO/Government of India. Plast. Rubber Int., 4, 23–28. Winfield, A.G. and Winfield, B.L. (1974) Reinforced plastics in low cost housing, in Advances in Chemistry Series, No. 134, Fillers and Reinforcements for Plastics, ed. by Deanin, R.D. and Shott, N.R. American Chemical Society, Washington, DC, USA, pp. 207–218. Wood, K. (2007) Wood-filled composites jump off the deck (12 January 2007); available at: http://www.compositesworld. com/articles/wood-filled-composites-jump-off-the-deck.aspx (accessed 22 April 2009).

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19.2 Design, Material Properties and Databases Erwin Baur and Frank Otremba M-Base Engineering + Software GmbH, Aachen, Germany

19.2.1 Introduction An important potential application of natural fibres is their use as reinforcement for plastics. Such applications are known, for example, in the automotive industry. Before a material is used in a technical application, it has to go through an extensive design process. This chapter describes the key elements of this design process, which includes material selection, the generation of a part geometry and calculation of part performance. These aspects are explained with a special focus on the specific requirements for natural-fibre-reinforced plastics.

19.2.2

Elements of Design

The first and most important information for designers when dealing with natural-fibre-reinforced materials is that we do not need a principally new methodology. The general rules for engineering and material selection are valid, and the engineer dealing with these materials can rely on existing strategies and literature, especially about designing with plastics (Erhard, 2006; Michaeli et al., 1995; Schmitz, 1984; Baur et al., 2007; Ehrenstein, 2007). However, some specific data are required, and well-established tools need to be extended to specific characteristics of natural-fibre-reinforced materials. The following is mainly concerned with design solutions for the classical processes of injection moulding, compression moulding, press forming and – limited owing to the restricted design freedom – extrusion. It is only applicable to natural fibres that are used in one of these processes on an industrial scale. The design process can be divided into three categories: shaping, dimensioning and material selection. It is very important to point out that these three tasks cannot be viewed in isolation. They are interconnected. For example, by changing to a new material with higher mechanical properties, it is possible to reduce the Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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dimensions of a component, but at the same time the shape might have to be adjusted, e.g. by a material-specific rib design. r Shaping. Shaping is the generation of a three-dimensional model of the new part. Shaping is the classical design task that in the end leads to the blueprint of the new product. The result of the shaping process must ensure that the new product fulfils all required functions (like connections, fit with other components, transmission of forces, and much more), but many material-specific and process-related rules must also be considered. These can be as global as ‘avoid undercuts in moulding processes’ or very specific, like ‘for a specific combination of process and material use correct draft angles’. r Dimensioning. This process rules the quantitative fixing of all proportions of the new product. As most plastic parts are of three-dimensional but plane nature, dimensioning is mainly concerned with defining the right wall thickness in relation to the (mostly mechanical) requirements. To find the correct dimensions, complex engineering calculations are necessary. r Material selection. Material costs usually make up more than 50% of the overall production costs. Therefore, material selection is of extreme importance. It may be defined as the selection of the best compromise between technical and economic requirements. It cannot be expected that during material selection one will only have to choose the best solution between different alternatives based on natural fibres. Under normal conditions, natural fibres will have to compete with all other materials that are technically suitable for an application, and only if they are the best solution in all aspects will they have a chance of being selected. Consequently, natural-fibre-reinforced materials need to offer the same level of material information as the designer is used to with other materials. The non-availability of reliable material data for selection and design is without doubt the main handicap for natural-fibre-reinforced materials.

19.2.2.1

Design Tools

In the following, some standard tools will be described that are typically used by designers of plastic components. As mentioned, they are also valid for natural-fibre-reinforced materials, but need to be adjusted and extended. These examples will demonstrate the principal method and strategy. Proper tools are not yet available for all purposes; in many cases designers will have to develop them for each specific new process or material. It is important, though, that such tools are made available for the decision-makers, e.g. in the automotive industry, in order to improve the position of natural-fibre-reinforced materials in early design considerations.

19.2.2.1.1

Design Catalogues

To support the designer in the selection of proper functional elements, design catalogues help to present knowledge from previous projects or applications in an understandable and easy-to-find way. They are compiled on the basis of surveys with experienced engineers. The accumulated knowledge is prepared and listed systematically. The results are tables with all known solutions for a specific problem, usually with graphs and ratings. Typical design catalogues for natural-fibre-reinforced materials cover topics such as: r r r r r

forming; stiffening; connecting; lamination; recycling and disassembly.

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The catalogues have to distinguish between different manufacturing processes. Catalogues for injection moulding are completely different from those for form pressing or extrusion. In many aspects, catalogues will show considerable differences between a thermoplastic version compared with the thermoset version of the same process. Table 19.2.1 shows the catalogue ‘Stiffening of natural-fibre-reinforced components produced in a press forming process’ (Baur et al., 2009).

19.2.2.1.2

Design Rules

During the process of shaping, engineers will have to observe many material- and process-related rules. An experienced designer simply knows these rules and does not have to be reminded. However, whenever engineers work with new materials, like natural-fibre-reinforced plastics, they need support in the form of design guidelines. These are always restricted to a specific combination of material and manufacturing process. They offer a systematic list, ordered by functional or geometric criteria. The rules should be supported by graphical illustrations, showing good and bad examples. Table 19.2.1 shows an exert from a design guide for press forming products (Baur et al., 2009).

19.2.2.1.3

Material Data Sheets

Material data sheets are the classical way to describe the properties of a material. They are needed for material selection, as well as for calculations in the dimensioning process. Unfortunately, compared with other materials like metals, plastics and natural-fibre-reinforced plastics need far more properties. This is due to their complex dependency on temperature, time (creep), ageing and effects that are of little concern for other materials, such as chemical resistance and flammability. In the plastics industry – different to other industries – material data are almost exclusively provided by the material producers (chemical industry). This fact was not well understood by early providers of naturalfibre-reinforced materials, and the availability of material data is still far behind the industrial standards. Comparability of Data. When creating material data sheets, reliability and comparability are of crucial importance. The results of material tests can be influenced by many means, such as the geometry of the test samples, the processing of the test samples, the conditions during the tests (temperatures, speeds see Figure 13.18) and the interpretation of the test results. Differences in just one of these factors will dramatically influence the test results (the same is true of fibre testing – see Chapter 13). If data need to be compared, e.g. for material selection, it is absolutely crucial that all tests are run under the same conditions. Also, they must be documented in order to allow designers to interpret and predict local properties in the final part. For materials that can be processed in standard flow moulding operations, such as injection moulding, an internationally accepted standard introduced by a group of material suppliers can be used for material characterisation. Starting in 1988, they developed a complete system for defining the production and the geometry of test specimens and the testing procedures for the most relevant mechanical, thermal, rheological and electrical characteristics. These procedures have been documented in a series of International Organisation for Standardisation (ISO) standards and form the background of the international plastics database CAMPUS (T¨ullmann et al., 2001). The relevant standards are: r ISO 10350, covering all standards for single-point data; r ISO 11403, covering all standards for multipoint data;

Entire part

Stiffening by design (curved surface)

Corrugation

5

4

Support itself

Like support

Like support

Both sides

None

One side

Visibility

Depth limited by design guideline, height, generally 150 mm max.

Depth limited by design guidelines, generally <10 mm

Depth limited by design guidelines, generally <15 mm

Arbitrary

As needed by design and construction

As needed by design and construction

Smaller than wall E.g. thickness markings

Dimensions

Additional functions and application areas

Favourable

Low

Moderate

High material outlay

Low

Outlay / costs

High

High

Relatively low

Very low

Effectiveness

Not possible for many parts owing to connecting dimensions

Must be planned for the concept

Process permits low elevations only

Comments

Appendix

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Domes

Like support

Like support

Material of the reinforcing element

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Entire part

2

No.

Ribs, elevation

Diagram

Local

Reinforcing Arrangement elements

Main part

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Plastic or metal

Additional layer of same or any material

Planar surface or profile of any material

7

8

9

Tailored blank

Added profile

One side

One side (both sides)

One side

Arbitrary

Arbitrary

Large, free areas

At highly loaded corners and edges

E.g. referencing systems, retainers, hot riveting

Assembly purposes

High

High (on processing)

Favourable owing to dual function (mounting and stiffening)

High outlay

High

High

Limited

High

Joined by pressing together

Joined by pressing together

Ribs can be moulded from thermoplastic polymers

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Add-on ribs, e.g. modular parts

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Table 19.2.2

Relevant properties for characterisation of natural-fibre-reinforced plastics

Property

Standard

Unit

ISO 527-1/-2 ISO 527-1/-2 ISO 527-1/-2 ISO 527-1/-2 ISO 527-1/-2 ISO 179/1eU ISO 179/1eA ISO 179/1eU ISO 179/1eA

MPa MPa % MPa % kJ/m2 kJ/m2 kJ/m2 kJ/m2

ISO 11359-1/-2 ISO 11359-1/-2

E−4 /◦ C E−4 /◦ C W/(m K) Class ◦C ◦C ◦C

Mechanical properties Tensile modulus Yield stress Yield strain Stress at break Strain at break Charpy impact strength (+23 ◦ C) Charpy notched impact strength (+23 ◦ C) Charpy impact strength (−30 ◦ C) Charpy notched impact strength (−30 ◦ C) Thermal properties Coefficient of linear thermal expansion (parallel) Coefficient of linear thermal expansion (normal) Thermal conductivity Burning behaviour (analogue UL) Melting temperature (only thermoplastics) Glass transition temperature (only thermoplastics) Vicat softening temperature B (only thermoplastics)

IEC 60695-11-10 ISO 11359-1/-2 ISO 11359-1/-2 ISO 306

r ISO 3167, describing a multipurpose test specimen; r ISO 254, covering material-related rules for test specimen production. With only minor extensions, they can also be used for natural-fibre-reinforced moulding grades, as the project N-FibreBase showed (Baur, 2004). It is highly recommended to use the same standardisation whenever natural-fibre-reinforced materials are characterised for design purposes. Other processes such as press forming and compression moulding can also be covered, with some modification of the rules for test specimen production. Table 19.2.2 presents a list of relevant properties for natural-fibre-reinforced materials, as developed by N-FibreBase. During the same project, a recommendation for testing natural fibres for industrial use was also developed, and this is described in detail in Chapter 13.

19.2.2.1.4

Databases

The handling of material data is supported by databases. They allow easy access to design data, offer powerful functions for material selection and comparison and enable direct data transfer to calculation software such as FEM. Table 19.2.3 presents some relevant databases.

19.2.3 Material Modelling and Simulation Even though material properties are of extreme importance for the designer, it must be understood that, in an actual part, the same material properties as in a standardised test will not be found. The designer must know that, owing to processing influences, the material properties are different at any geometric point in the part when working with reinforced plastics. Many influences have to be considered. In some cases the same fundamental assumptions that have been made for conventional reinforced polymer compounds can be used with good results, but in some cases they need specific adjustments, which will be demonstrated using the

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Examples of material databases for polymers

Name

Internet page

Comments

CAMPUS Material Data Center

www.CAMPUSplastics.com www.materialdatacenter.com

Biopolymer Database in Material Data Center

www.materialdatacenter.com

PLASPEC Global

www.ptonline.com

N-FibreBase

www.N-FibreBase.net

Driven by international resin producers, free access Contains data about engineering plastics (including CAMPUS data) and engineering tools As a result of a project by Fachhochschule Hannover, Germany, engineering data of biopolymers were collected. They are published in a subsection of Material Data Center Published by Plastics Technology magazine, specifically for the US market. Powered by Material Data Center Specific databases for natural-fibre-reinforced materials and natural fibres

example of anisotropy. The anisotropy of the material, mainly caused by process-induced fibre orientations, has a dramatic influence on mechanical properties. This effect cannot be predicted by material testing, but must be calculated using adequate material models that make it possible to determine the performance of the composite from information about the matrix and fibre material. Engineers can rely on well-established models such as the Halpin–Tsai or Tsai–Wu models for calculation of the mechanical properties of glass-fibre-reinforced polymers (Halpin and Kardos, 1976; Tucker and Liang, 1999). These models describe glass-fibre-reinforced plastics as ideal composites with perfectly round and stiff fibres of defined length. Although not all of these idealisations can be applied to glass fibres, these models are successfully used. They make it possible to determine the elastic modulus from the mechanical properties of the matrix and fibre materials (moduli, Poisson’s ratio, fibre length and content) for unidirectional fibres. With the help of orientation averaging methods (Gusev et al., 2002), the results can also be used for short fibre injection and compression moulding materials if the fibre orientation distribution is known, e.g. from process simulation. The use of these fundamental material models is state of the art in the industry. Commercial simulation software packages make use of these models for the simulation and design of polymer parts. These software tools are not developed for a special kind of material and in principle should also be applicable to natural-fibrereinforced materials. A precondition is that the existing material models are valid for this class of materials, or can be adjusted. Natural fibres show some principal differences from man-made fibres, which makes modelling the compound properties more difficult. There are no elementary fibres of defined diameter as there are for glass or carbon fibres, but fibre bundles of varying dimensions. The fibre bundle diameter may change during processing, and this will influence the properties of the bundles and of the compound. Because of their lower stiffness, bending of the fibres and fibre bundles is a relevant effect. As well as man-made fibres, natural fibres may shorten during processing. Schmitz (2006) examined kenaf-fibre-reinforced PP and detected a significant change in diameter distribution between raw fibre bundles and the fibres/fibre bundles of the injection-moulded part. He found a correlation between diameter and mechanical fibre properties that was quite similar to the well-known Griffith equation for the strength/diameter dependency of glass fibres (Figure 19.2.1). If the diameter change is known, the mechanical properties of the refined fibre bundles can be calculated with this equation. Schmitz also shows that these calculated moduli can be used in the standard Halpin–Tsai and Tsai–Wu models. The models show good predictions of the measured results by using the adjusted modulus, while the standard models do not (Figure 19.2.2). Obviously, small changes of standard models are needed to simulate natural fibres sufficiently. Unfortunately, in practice, fibre bundle splitting is not yet predictable. So the application of the advanced models is still challenging.

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Figure 19.2.1

Correlation of kenaf fibre bundle diameter and modulus.

While the prediction of anisotropic composite properties is the most important issue, there are material models for many other effects such as temperature dependency, creep, etc. The mechanical models seem to be applicable with minor adjustments, as described before, but the processing simulation of natural-fibrereinforced materials remains a major problem. Owing to the low temperature resistance of the natural fibres, composites have to be processed at extremely low temperatures, very close to freezing temperature. It is doubtful that the conventional flow models are valid in this temperature range. Viscosity measurements with natural-fibre-reinforced materials are impossible with standard equipment because of fibre plugging. Also, models for predicting the flow-induced fibre orientation have not been proved with natural fibres yet. Models for determination of shrinkage and warpage work very well with natural-fibre-reinforced materials and need no modification. They make it possible to predict the low shrinkage, one of the advantages of natural-fibre-reinforced plastics (Otremba, 2006). Table 19.2.4 gives an overview of the required data for simulation of injection-moulded parts and the relevance of standard test methods. It can be extended to other flow processes (such as compression moulding), but the press forming method is not covered, even though press forming is the processing method with the highest volumes for naturalfibre-reinforced plastics.

Figure 19.2.2

Measured and calculated Young’s modulus of the compounds.

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405

Rating of material data for simulation Injection moulding flow simulation

Viscosity (temperature, shear velocity)

Problems with standard test equipment, especially for bast-fibre-reinforced plastics Standard methods applicable Standard methods applicable Standard methods applicable Fibre orientation distribution cannot be measured sufficiently for natural-fibre-reinforced plastics

Thermal conductivity (temperature) Heat capacity (temperature) Density (pressure, temperature) Fibre–fibre interaction coefficient

Injection moulding, shrinkage and warpage simulation Thermal conductivity (temperature) Heat capacity (temperature) Density (temperature) Fibre and matrix modulus (temperature) or compound modulus in different sample directions (temperature)

Standard methods applicable Standard methods applicable Standard methods applicable Fibre orientation distribution of samples is not known

Static mechanical simulation Fibre and matrix modulus (temperature) or compound modulus in different sample directions (temperature) Tensile strength of compound

Standard methods applicable Fibre orientation distribution of samples is not known Standard methods applicable

Crash simulation Compound stress–strain diagram (strain rate)

Fibre orientation distribution of samples is not known, but may be neglected in crash simulation Thermal simulation

Thermal conductivity (temperature) Heat capacity (temperature) Density (temperature)

Standard methods applicable Standard methods applicable Standard methods applicable

In press forming, a textile, ‘non-woven’ semi-finished product is shaped and compacted under pressure and temperature in a press. It is a shaping process that involves local elongation and stretching. There is no special process simulation software available, but it is known that processing effects such as local compression or pull-out effects have a significant influence on the properties of the press formed material. Actual research projects show, for example, that process simulation is possible for press-formed materials by using general finite element software. Even crash simulations based on such input have been run (Joas, 2009). Based on material data measured in high-speed tensile tests, PAM-CRASH material cards were generated and head impact simulations were done. Comparison with experimental results showed very good correlation, as seen in Figure 19.2.3. This proves that natural-fibre-reinforced materials can meet the highest engineering requirements if proper measures are taken.

19.2.4

Conclusion

The acceptance of natural-fibre-reinforced plastics in technical applications depends on the availability of material data and specific design information. Such information and adapted methodologies are partially available. However, much development is still necessary before natural-fibre-reinforced plastics can match conventional engineering plastics. In particular, specific material models for processing simulation and

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Figure 19.2.3 Simulation comparison; acceleration time curves; results from the crash simulation with the software PAMR CRASH versus test results determined with a hemispherical test body (Lignoflex
) measured in a high-velocity impact test. E. Baur, N. Graupner, S. Joas and F. Otremba, Press Formed Parts from Natural Fibre-Reinforced Polymers, Kunststoffe International, 03, 24–30, 2009. Adapted with permission from Carl Hanser Verlag GmbH & Co., Germany.

prediction of process-induced fibre orientation need to be developed. Also, action is needed to transfer the existing technology to the relevant designers in the field.

References Baur, E. (2004) N-Fibre Base – Information u¨ ber naturfaserverst¨arkte Kunststoffe. Konstruktion, 7/8, IW 11–13. Baur, E., Brinkmann, S., Schmachtenberg, E. and Osswald, T. (2007) Saechtling Kunststoff Taschenbuch. Hanser Verlag, Munich, Germany. Baur, E., Graupner, N., Joas, S. and Otremba, F. (2009) Press formed parts from natural fibre-reinforced polymers. Kunststoffe Int., March, 24–30. Ehrenstein, G. (2007) Mit Kunststoffen konstruieren. Hanser Verlag, Munich, Germany. Erhard, G. (2006) Designing with Plastics. Hanser Verlag, Munich, Germany. Gusev, A., Heggli, M., Lusti, H.R. and Hine, P.J. (2002) Orientation averaging for stiffness and thermal expansion of short fibre composites. Advd Eng. Mater., 4(12), 931–933. Halpin, J.C. and Kardos, J.L. (1976) Halpin–Tsai equations: a review. Polym. Eng. Sci., 16(5), 344–352. Joas, S. (2009) Berechnung von Formpressteilen aus naturfaserverst¨arkten Kunststoffen. Conference Kunststoffe + Simulation, 27–28 May 2009. Hanser Verlag, Munich, Germany. Michaeli, W., Brinkmann, T. and Lessenich-Henkys, V. (eds) (1995) Kunststoffbauteile werkstoffgerecht konstruieren. Hanser Verlag, Munich, Germany. Otremba, F. (2006) Betrachtungen zum Schwindungs- und Verzugsverhalten von PP-NF, in 4. N-FibreBase Kongress ‘Naturfaserverst¨arkte Kunststoffe (NFK) – Wood-Plastic-Composites (WPC) Bio-Kunststoffe’, H¨urth, Germany, 27–28 June 2006. Faserinstitut Bremen e.V. (FIBRE), M-Base Engineering + Software GmbH, nova-Institut GmbH, H¨urth, Germany. Schmitz, J. (1984) Anleitung zum methodischen Konstruieren von Spritzgiessteilen. PhD Thesis, RWTH Aachen, Germany. Schmitz, M. (2006) Modeling the Mechanical Properties of Natural Fibre-Reinforced Biopolymer, in 8th International Conference on Woodfibre–Plastic Composites, Madison, WI, USA, 23–25 May 2006. Tucker, C.L. and Liang, E. (1999) Stiffness predictions for unidirectional short-fibre composites review and evaluation. Compos. Sci. Technol., 59(5), 655–671. T¨ullmann, R., Kurzknabe, R., Sarabi, B., Laumen, K., Maurer, G. and Baur, E. (2001) CAMPUS – the world standard for plastics. Kunststoffe Plast Eur., 91(10), 10–12.

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19.3 Natural Fibre Composite Processing: A Technical Overview Tim Huber University of Canterbury, Department of Mechanical Engineering, Christchurch, New Zealand

¨ Nina Graupner and J¨org Mussig Hochschule Bremen – University of Applied Sciences, Department of Biomimetics, Bremen, Germany

19.3.1 Introduction In the last decade, a great deal of research has been carried out in the field of fibre-reinforced plastics (see Chapter 3.2). As such composites need different processes to non-reinforced plastics to benefit from their special properties, the processing methods of fibre-reinforced polymers have also undergone a development to keep up with these new materials. Recently, the use of natural fibres as reinforcement has become increasingly important, and, owing to their different properties, the common methods for processing fibre-reinforced plastics have to be adjusted to meet new and more demanding requirements. The most important of these requirements is a reduction in processing temperatures, because of the inability of natural fibres to resist temperatures higher than 150 ◦ C for long processing durations and short-term exposures to temperatures up to 220 ◦ C. Exceeding these temperatures can lead to discoloration, volatile release, poor interfacial adhesion and embrittlement of the cellulose components (Holbery and Houston, 2006). Furthermore, the fibres tend to swell when exposed to humidity, which causes a decrease in mechanical properties in the final composite (Biagotti et al., 2004; Saheb and Jog, 1999). Another problem is a weak adhesion between natural fibre and matrix, but this problem can be reduced by the application of coupling agents or fibre pretreatments (Bledzki and Gassan, 1999; Fowler et al., 2006; George, 2001). The variation in fibre properties can be controlled and predicted when quality management is utilised during fibre pretreatment and processing (Cescutti and M¨ussig, 2005). A common problem, especially for natural-fibre-reinforced thermoplastics, is the development of unpleasant odours during processing (see Chapter 13). The odours can be reduced by using fibres with an optimal degree of retting and low impurities, by reduced processing temperatures and cycle times or by adding odour-reducing additives (Bledzki et al., 2003; Knobelsdorf et al., 2005; Fischer et al., 2008). Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications  C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Figure 19.3.1

Important parameters for choosing the adequate process. Adapted and modified from Breuer & Neitzel, 1997.

Besides their virtue of being an environmentally friendly and an almost carbon-dioxide-neutral material, they have other advantages over traditional fillers and reinforcements, i.e. lower costs and a lower density. They are also less abrasive to processing equipment, and their dust is less irritant to the skin and respiratory tracts than that of conventional reinforcements such as glass fibres (Carus et al., 2008; Anonymous, 2007). Natural fibres can be processed either with thermosetting matrix materials or in combination with thermoplastic polymers and elastomer matrices to form fibre-reinforced composites. On account of the currently more widespread use of thermoplastic and thermoset reinforced composites, this chapter focuses on the production procedures of these processes. A description of natural-fibre-reinforced elastomers is given, for example, in the studies by Abdelmouleh et al. (2007), Akhtar et al. (1986), Ganster and Fink (2006), Geethamma et al. (1995) and Jacob et al. (2004). In Chapter 8, the properties and processing of sisal/natural rubber composites are described in greater detail. The choice of the right processing methods depends on several parameters such as part geometry, complexity, size, the properties of the chosen fibres and polymers, the defined fibre orientation, the quantity and quality of the product and the fact that not all processing methods are applicable to all materials (see Figure 19.3.1) (Grove, 2006). Below, the single processing methods, the processing conditions and required raw materials, the resulting part geometry and the application range will be described for thermosetting and thermoplastic naturalfibre-reinforced plastics. More information on conventional composite processing can be found in work by Ehrenstein (1992), Harper (2006), Michaeli (2006), Neitzel and Mitschang (2004), Breuer and Neitzel (1997), Long (2007), Chung (2003), Chawla (2001), and Peters (1998).

19.3.1.1

Thermosets

In the early years of development, the majority of applications using fibre-reinforced polymers used thermosetting resins as the matrix. These are polymers that result from irreversible chemical reactions of resins called crosslinking or curing. The curing is usually achieved by heat and/or by chemical additives. The most commonly used natural-fibre-reinforced thermosetting materials are epoxy resins, phenolic resins, polyurethanes, polyester, vinyl resins and acrylate resins. The most used fibres are wood, flax, hemp, kenaf, sisal, jute and cotton (Bledzki et al., 1998; Hepworth et al., 2000; Eichhorn et al., 2001). Although many different processes have been developed for glass- or carbon-fibre-reinforced thermosets, only some are relevant for the use of natural fibres. These will be described in this chapter.

19.3.2

Compression Moulding

Compression moulding of natural-fibre-reinforced composites is the most commonly used process within the automotive industry. The components are often two-dimensional but three-dimensional structures with

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a maximum impression depth of up to 15–20 cm are possible. About 35% of the compression-moulded parts for the automotive industry are processed with a thermoset matrix. Compared with compression moulding with thermoplastic matrices, the thermoset composites have better thermal properties but are usually more expensive and are not easily recycled (Carus et al., 2008; Baur et al., 2009). In general, needle felts or fleeces are used as semi-finished products. Wood fibres are often used for reinforcement, but hemp, flax or mixtures from hemp, flax, sisal and kenaf are also chosen for producing semi-finished products. These products are impregnated with a thermoset resin system using a number of different procedures, including spray coating. They are then shaped under specific pressure and temperature into the specific parts. A fibre load of 35–90% is possible. Different compression moulding processes are currently used for composite production. They vary in cycle time, pressure and temperature, depending on the semi-finished part and the resin system used. The pressure is set to approximately 10–11 N/mm2 at press temperatures between 130 and 230 ◦ C and press times ranging from 30 sec to 5 min. The preferred wall thickness of the final parts is between 1.6 and 2 mm (Graupner et al., 2008). An overview of different process parameters and resin systems having decisive influence on mechanical composite characteristics is given in Baur et al. (2009) and Medina et al. (2008). Important form-pressing processes are further described in Chapter 19.4. Compression moulding is most adequate for medium-sized quantities and larger-scaled parts, as the costs for the moulding tools are lower compared with injection moulding. Typical applications, besides the automotive industry, are in the transportation sector. Examples are prototypes of natural-fibre-reinforced seat boxes and seat back cladding in rails. Other uses include suitcases, instrument cases and hard hats (Karus et al., 2006; Fries and M¨uller, 1998; Bhat et al., 2004; Riedel, 2003).

19.3.3 Pultrusion Pultrusion, a neologism created from the words ‘extrusion’ and ‘pulling’, is a process for producing continuous fibre-reinforced plastic profiles with constant cross-sections and forms. Instead of being pushed, as it is done in extrusion, the composite material is pulled through the shaped dye to form the profiles (Wiedmer and Friedrich, 2004). Currently, it is possible to produce standardised I-, T- and U-profiles and simple rods or slats. Pultrusion is a completely automatic, fast and cost-effective production process capable of high-volume production for structural applications (profiles) (Zhu et al., 2004). It also benefits from a small amount of rejections and no need for further additives. For the pultrusion process, reinforcements must be present in yarns, rovings or reels and are doffed from bobbins (Goutianos et al., 2006). The fibres are guided over deflection pulleys through a resin bath to impregnate them fully with the resin. Then the fibres are pulled through a series of tooling, to remove excessive resin and to organise the fibre into the correct shape. After that, the composite is passed through a heated dye to cure the resin. Thereafter the material is cooled down, and then gripped by a take-off unit working with either caterpillar tracks or hydraulic clamps. The endless profiles are then cut to the desired length by a cut-off saw at the end of the pultrusion line (Trevor, 2000). A schematic illustration of the process can be seen in Figure 19.3.2. The application of natural fibres can be done on pultrusion machines, but the natural fibres need a minimum strength in their dried state to avoid tearing. Furthermore, the fibre volume fraction needs to be carefully controlled, and a complete saturation of the fibres should be achievable. As well as vinyl ester, polyester and epoxy resins, biopolymers can be used to produce biocomposites (Gensewich and Riedel, 1999). Owing to the hydrophilic character of the fibres and their resulting swelling when exposed to humidity, the most promising applications appear to be in the furniture and transportation sector, as well as for window- or doorframes (Singh and Gupta, 2005; Mathur, 2006).

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Figure 19.3.2

19.3.4

Schematic of a pultrusion line.

Winding Technique

Another technique using natural fibre yarns or slivers is the winding process to produce a cylindrical hollow structure. The successful fabrication of sisal–epoxy composites is described in Chapter 8.

19.3.5 SMC (Sheet Moulding Compound) For the sheet moulding compound (SMC) technique, glass fibres are normally used as reinforcement for a thermosetting matrix system. It is suitable for two-dimensional parts with a high resistance to mechanical loads. A SMC component part is produced in a two-step process (Sawallisch, 1984). First, a prepreg (a semifinished fibre resin product) is prepared from a thermosetting resin (e.g. unsaturated polyester), reinforcing fibres with a length of 6–20 mm, inorganic fillers and a catalyst. Additional release agents, antishrink agents and thickeners can also be used. A thickener (e.g. MgO) increases the viscosity of the resin, which simplifies its handling and enables mobility of the fibres owing to the rheological characteristics of the ripened resin paste throughout the mould. Prepreg production is usually done by spreading the resin on a thermoplastic backing film, distributing the fibres on the resin and covering them with another resin spread film. A thin sheet of impregnated fibres with a putty-like structure is produced. This can be cut to the necessary size (Kim et al., 1997; van Voorn et al., 2001). Figure 19.3.3 shows a schematic illustration of the process. Following the ripening of the SMC prepregs for a certain time at a specific temperature, the prepregs are ready for compression moulding (Kim et al., 1997). The storage and processing times depend on curing process of the resin system. In the second step – the compression moulding – the sheets (prepregs) can be placed in a mould, which is then closed and heated to temperatures up to 130–160 ◦ C. Placing different sheets in succession is possible, but it is important to control the geometry and the accurate deposition of the different sheets in order to control the fibre orientation in the final component (Sawallisch, 1984; Castro and Griffith, 1989). The piece is exposed to pressure, a distribution of the fibres in the ripened resin paste to the extremities of the mould starts and, under heat, the resin begins to couple. The direction and flow velocity of the resin paste influences the fibre distribution and orientation as the fibres move within the matrix. This in turn influences the direction of the shrinkage appearing during the cooling and hardening of the component and the mechanical properties.

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Figure 19.3.3

411

SMC production process.

Following cooling and hardening, the component can be taken out of the mould and fettled if necessary (Castro and Griffith, 1989; van Voorn et al., 2001). The formed parts have homogeneous characteristics as well as very smooth, hard and paintable surfaces (Kim et al., 1997). The SMC technique for natural fibre composites is a relatively new process, but several studies have already demonstrated its viability. van Voorn et al. (2001) processed a standard SMC resin paste with flax fibres and showed that the SMC product had comparable mechanical properties with glass-fibre SMC. M¨ussig et al. R – a vegetable(2006) investigated a hemp-fibre-reinforced SMC composite for a bus body component. PTP
oil-based thermoset resin – was used as a resin. The results established an application spectrum in automotive applications for interior as well as exterior component parts. Possible applications include wall units, trim components, spoiler, bumper or funnels (Sch¨afer, 1998; Carus et al., 2008).

19.3.6

Resin Transfer Moulding (RTM) and Structural Reaction Injection Moulding (SRIM)

Resin transfer moulding (RTM) is an injection process that operates with low pressures of up to 5 bar. The productivity is low compared with injection or compression moulding, resulting in relatively expensive but high-strength composites. A thermoset resin such as epoxy, polyester, phenolic or acrylic is mixed with a hardener and is injected into a closed cavity consisting of a male and female tool that contains the reinforcing materials in form of felts or fabrics (see Figure 19.3.4) mostly made of flax or hemp fibres (Rouison et al., 2004). Ideally, the resin is dispersed quickly throughout the part. The resin fills the cavity in such a way that the air is slowly pushed out of the mould. The problem of RTM processing of natural fibre felts is that the fibres tend to float and are therefore not fully wetted by the resin or are disarranged. However, this problem may be solved by adjusting the processing properties. Sometimes there are also difficulties in distributing the resin evenly in the reinforcing materials to avoid unwetted fibres, voids or resin pockets. The additional assistance of a vacuum (vacuum-assisted resin transfer moulding – VARTM) can be very helpful in such cases by transferring the resin though the fibre felt or fabric and removing the remaining air. As the moulds are usually not heated, fibre-reinforced or filled resins can be used as a mould material (O’Donnell et al., 2004; Panigrahy et al., 2006). The VARTM process is illustrated in Figure 19.3.5. The moulded part is cured at or

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Figure 19.3.4

RTM process in operation.

slightly above room temperature until the end of the curing reaction. The thickness of the part is controlled by the distance between the tools. The tools can also be heated to improve the curing process (Oksman et al., 2001; Oksman, 2001; Potter, 1997). The use of natural fibres in the RTM process has been analysed by several research groups. Richardson and Zhang (2000) presented a study on a non-woven hemp-fibre-reinforced phenolic composite. Owing to poor clamping at low fibre loads, the reinforcement was displaced during the RTM process. This effect was reduced by increasing the fibre load, but edge flows were also observed. This problem could be solved by using preforms larger than the mould. Although there have been some problems with fibre displacement and edge flows, a majority of researchers have reported a good processability for different fibres and resins. Rouison et al. (2004) reported a good wetting of hemp/kenaf fibres with an unsaturated polyester resin during RTM. Oksman (2001) reported good flow characteristics in the mould and achieved a high fibre load in the composites. Further investigations of natural fibres in the RTM process are described, for example, in O’Dell (1997), S`ebes et al. (2000) and Williams and Wool (2000).

Figure 19.3.5 VARTM process in operation. Adapted from F. Weyrauch, H. Stadtfeld, C. Kissinger, Harzinjektionsverfahren, Handbuch der Verbundwerkstoffe, Carl Hanser, 2004.

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Typical applications for natural-fibre-reinforced RTM composites are medium- and larger-sized parts, such as housings for radar units, as natural fibres do not affect the radiation. Other applications include signposts, boat hulls, speaker boxes and furniture, as well as automotive applications (B¨ottger et al., 2008). In some design and furniture products, the fibres are used rather as an eye-catcher in combination with transparent resins than as reinforcing materials (Carus et al., 2008; Holbery and Houston, 2006; B¨ottger et al., 2008). Structural reaction injection moulding (SRIM) is a resin injection process used in the fabrication of foamed, extensive and thin-walled parts, and is therefore most suitable for interior trim elements (Rudd et al., 1997). The parts, mostly made of foamed polyurethane and needle felts, are processed in a closed cavity. The fabric is placed in the cavity, and polymer resin and hardener are then injected into the heated tool to form the composites in cycle times of 1–2 min. The main difference between this method and RTM is the pressure used, usually reaching up to over 20 bar (Rowell et al., 1997; Anonymous, 2003). A comparison between the flexural properties of sisal/thermoset composites produced by RTM and compression moulding techniques is given in Chapter 8, Table 8.3.

19.3.6.1

Thermoplastics

The use of thermoplastic materials as the matrix for fibre-reinforced composites has increased in the last few years. This is due to their easier processability when compared with thermosets, the fact that they can be easily stored for an almost unlimited period of time and that they are non-toxic and repeatedly meltable (Hepworth et al., 2000). Thermoplastics start to melt at a specific temperature and are, for this reason, able to form composites with reinforcing fibres. Owing to the thermal limits of the natural fibres, only some low-melting thermoplastics may be used. These are mainly polypropylene (PP), polyethylene (PE), polystyrene (PS) and polyvinyl chloride (PVC). There are also some biopolymers such as poly(lactic acid) (PLA), polyhydroxyalkanoates (PHA) and polyhydroxybutyrate (PHB) or lignin that are applied as matrix materials, but only on a small scale. Several kinds of natural fibres are used for reinforcing thermoplastic polymers; most important among these are wood, flax and hemp, but tropical and subtropical fibres such as abac´a, kenaf, sisal and coir are also applied (Carus et al., 2008). Aside from the low processing temperatures, another major problem is the adhesion between fibre and matrix. The matrix materials, for example PP, are often non-polar and hydrophobic, and chemical bonding to the polar and hydrophilic natural fibres is difficult. The adhesion can be significantly improved by the use of coupling agents and/or fibre pretreatments which are used by most manufacturers. The most frequently used adhesion promoter for PP composites is maleic anhydride. The most important production processes will be presented in the next pages.

19.3.7 Injection Moulding Injection moulding is defined as producing parts with thermoplastic material and is one of the most important processing methods for converting thermoplastic materials into all forms of product (Grelle, 2006). In the field of natural fibres, injection moulding is a relatively new processing technique with great potential for the future. A high demand for natural fibre injection-moulded parts is expected in the coming years. It is possible to produce parts with complex three-dimensional structures in very high quantities and in short cycle times. The formed shape is controlled by a constrictive chamber, the mould (Rosato et al., 2000; Holbery and Houston, 2006). The plastic material, often in granule form, is inserted into a heated and rotating screw and melted until it reaches a determined temperature and viscosity (see Figure 19.3.6). This part of the injection moulding process is called plastication. Natural-fibre-reinforced plastics are usually processed at temperatures of 175–190 ◦ C,

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Figure 19.3.6 Injection moulding process. Adapted from G. Ehrenstein (Ed.), Faserverbund-Kunststoffe: Werkstoffe, Verarbeitung, Eigenschaften, Carl Hanser Verlag, 1992.

as higher temperatures might affect the fibres or cause undesired odours. The temperature limits of naturalfibre-reinforced polymers might cause some problems with the flowability of the melt, but they also offer some advantages compared with regular plastics, such as a lower energy demand, shorter cycle times and smaller degradation at in-mould decoration. Furthermore, compared with glass-fibre-reinforced plastics, the natural fibres have a minor distortion and can cause lower abrasion in the moulds or screws, depending on the chemical composition of the fibres. Most granules available today are made with polypropylene as matrix material (Huber et al., 2008; Bledzki et al., 2008; Ortmann et al., 2005). Short fibres/fibre bundles are used in most cases owing to the compounding processes of the granules. They affect the melt flow, and the filling ratios differ from those with glass-fibre-reinforced granules (Fung et al., 2003; Nechwatal et al., 2003). Owing to the hygroscopic character of the fibres, the granules should be dried before feeding into the screw. The melt is transported towards the tip of the screw until there is sufficient bulk of the material to fill the mould (see Figure 19.3.7). The screw then stops rotating and moves forward to push the material through various flow channels in the cavities of the usually preheated mould. Built-up pressures reach 500–2000 bar and are maintained until the melt is cooled and solidified. After cooling of the mould, the plastic parts can be removed.

Figure 19.3.7

Injection moulding process.

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Besides the process parameters, fibre–fibre interactions as well as fibre–matrix interactions play an important role. Sometimes the mechanical characteristics of the composites are worse than those of the pure matrix owing to low interfacial interactions. Other difficulties are presented by fibre agglomerates, which lead to an inhomogeneous fibre distribution in the composite. The constitution of the fibre surface is also very important for stress transfer from fibre to matrix in the flow. However, pretreatments of the fibres, as well as additives, lead to a more homogeneous fibre distribution and improved composite characteristics (Saheb and Jog, 1999; Takase and Shiraishi, 1989). Maleic anhydride is often used as an adhesion promoter for natural-fibre-reinforced polypropylene.

19.3.8 19.3.8.1

Compression Moulding and One-Step Compression Moulding Compression Moulding

Just as for thermoset matrices, thermoplastic compression moulding processes are most efficient for mediumsized quantities and for extensive parts. The main matrix materials used are polypropylene and, mainly in North America, polyethylene (Hull, 2006). Some applications for biopolymers have been developed in the last few years that have great potential for the future. The most commonly used fibres are flax and hemp, jute, kenaf and sisal. A blend of these fibres has proven to be advantageous, as the fine fibre bundles improve the adhesion between fibre bundle and matrix owing to their large surfaces, whereas the coarser fibre bundles provide a good saturation of the felts and avoid resin pockets (Carus et al., 2008). For thermoplastic compression moulding, a semi-finished textile product (mostly needle felts) manufactured from natural fibres and thermoplastic fibres is used (Sch¨ussler, 1998). The natural fibre load ranges between 30 and 60% (by mass), and the mass per unit area ranges between 1000 and 2600 g/m2 . The precut parts are heated to temperatures between 160 and 250 ◦ C for a certain time (from 30 s to 15 min), depending on the materials used. The preheated part is then transported to the press and is compression moulded at temperatures ranging between 10 and 80 ◦ C for 20 s to 4 min. (Hoogen, 1999). Distortions are a common problem in this process.

19.3.8.2

One-Step Processing

In addition to compression moulding, it is possible to produce completed composite parts, for example interior trim parts, including decorating foamed materials and soft-touch surfaces, as well as inserts, in one moulding step without the use of resins or adhesives. The materials are placed in the mould prior to the compression moulding process (Hoogen, 1999). Compression moulding, as well as one-step processing, with thermoplastic matrices is the dominant process within the German automotive industry, although the increase in application has stagnated in later years (see Chapter 3.3 and Carus et al., 2008).

19.3.9

Press Flow Moulding – Direct Long-Fibre Thermoplastic (D-LFT) Process

The D-LFT process is a continuous process. Fibres are present in yarns or rovings and are fed directly with a fibre load ranging from 15 to 45% (by mass) into the heating zone of a twin-screw ‘mixing’ extruder (see Figure 19.3.8). A polymer is simultaneously melted in a second ‘melt’ extruder and added to the fibres at the intake zone of the mixing extruder. The fibres are wetted by the polymer matrix, and the homogenised fibre/polymer mixture is then continuously forced through a slot die to form a plastic strand. The strand can then be cut and placed in a mould to be pressed into the desired shape. Hold-up times in the mixing extruder and

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Figure 19.3.8

LFT processing.

temperatures above 220 ◦ C should be avoided. The main industrial application of this processing technique was the manufacture of an underbody for Daimler in 2006 using abac´a fibre bundles and polypropylene (Blezdki et al., 2008; Scher¨ubl and Hintermann, 2005; Holbery and Houston, 2006; B¨ohm et al.; 2005; Brast, 2001).

19.3.10

Extrusion

In the extrusion process, compounded granules, including the natural fibres and the thermoplastic matrix, are fed into the heating zone of the extruder, where they are heated to a molten and homogenised state by dispersive and distributing mixing and shear heating within the screw of the extruder. The mixture is then forced through a die to form endless structures with a defined cross-section and shape (see Figure 19.3.9). Devolatilisation is also an important step in the extrusion process. Different kinds of extruder, such as singlescrew or planetary extruders, as well as twin-screw extruders, which can be co- and counterrotating, can be used. For the processing of natural fibres, twin-screw extruders are most frequently used (Holbery and

Figure 19.3.9

Extrusion process.

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Houston, 2006; Torres et al., 2003). Typical applications are tubes, profiles and plates fabricated with singleor twin-screw extruders. In general, it is possible to use all kinds of natural fibre injection moulding granules (Nechwatal et al., 2003), but bast- or leaf-fibre-reinforced plastics are hardly used for extrusion applications. The main reason for this is that the necessary requirements of the extruded profiles can be matched by cheaper wood fibres or wood flour. Hence, the only material used in relevant amounts for natural-fibre-reinforced thermoplastics are wood plastic composites (WPCs). The main applications of WPCs are, owing to their wood-like haptics and optics based on the high wood content (50–80%), in the construction sector. They are mainly used for the production of decking, especially in North America, or furniture, but there are also some applications in the automotive industry (Carus et al., 2008; Clemons, 2002). The main difficulties when processing WPCs are that the wood needs to be dried to below 1% of moisture content either in-line or before processing. The moisture content of the wood can vary from about 4 to 10%. Therefore, the fibre dosage has to be controlled carefully when the fibres are dried within the extruder. On the other hand, the pre-dried fibres also need special handling, as they are very hygroscopic, easily inflammable and could cause dust explosions (Pritchard, 2004). Furthermore, the wood starts to blacken at high temperatures; therefore, it should be processed at temperatures of 140–150 ◦ C, although this could cause problems with the flowability of the polymers. The blackening does not so much affect the mechanical properties of the composites, but it does considerably influence colour and odour. The fibre can be degraded or damaged by the shear forces of the screw, which is why only low shear should be applied (Hanawaldt, 2002; Klesov, 2007).

19.3.11

Thermoplast Pultrusion

Pultruded profiles are traditionally manufactured using thermosetting resins, but in recent years the demand for thermoplastic pultruded profiles has increased. Several different thermoplastic pultrusion techniques have been developed. Semi-finished products such as hybrid yarns, preimpregnated tapes and interweaved prepregs can be used. The semi-finished products can be shaped and consolidated in the die with a following cooling zone. The impregnation of the fibres with the matrix takes place by hot melt application or fluidisedbed application. Otherwise, the fused matrix can be screw injected into the resin cavity, which is used in thermosetting pultrusion processes. In this case, the reinforcement is pulled through the melt (van de Velde and Kiekens, 2001; Larock et al., 1989; Devlin et al., 1991; Miller et al., 1998). For natural fibre pultrusion processes, comingled hybrid yarns are used. Friedrich et al. 2007 described a pultrusion process that was carried out with flax and PP. For the experiments, flax and PP were spun to a yarn with a flax fibre content of 30 and 50%. Rectangular profiles were produced by the pultrusion facility shown in Figure 19.3.10. The yarn was stored in a roving holder. The first step was to place the yarn in a preheating chamber, in which it was heated close to the melting temperature of the PP. Following preheating, the yarn was pulled by a pulling device at the end of the production line through the heating cavity. The PP melted and the flax fibres were coated with the matrix. Finally, the profile was pulled through the cooling cavity (Friedrich et al., 2007).

19.3.12

Conclusions

As the interest in fibre-reinforced polymers in general and natural-fibre-reinforced plastics in particular is growing, and as new materials and corresponding processes are constantly being developed, only a selection of important processing techniqes could be presented in this chapter. It seems likely that the well-established processes for natural-fibre-reinforced thermosets will be constantly applied, as much research has already been done in those areas and, with the possible application of bio- or bio-based resins, new biocomposites with very good mechanical properties can be produced.

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Figure 19.3.10 Thermoplastic pultrusion with natural fibres and polypropylene (PP). Adapted from K. Friedrich, M. Evstatiev, I. Angelov and G. Mennig, Pultrusion of Flax–Polypropylene Composite Profiles, Handbook of Engineering Biopolymers: Homopolymers, Blends and Composites, Carl Hanser Verlag, 2007.

The processing of natural-fibre-reinforced thermoplastics, especially injection moulding and extrusion, promises new applications in very high quantities and will probably become increasingly important in the next few years. However, techniques must be developed to reduce fibre damage during processing. As described in Chapter 8, new approaches in composite processing techniques are to reduce the shear forces and to heat-treat the natural fibres. It is essential that, for both thermosets and thermoplastics, the processes developed for non-biological conventional composites are advanced and adapted to the special requirements of natural fibres to produce high-performance composites from renewable resources. This includes the adaptation of processing conditions, the choice of adequate processing techniques for the desired components, considering geometry and quantity, and the choice of the right raw materials.

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Hepworth, D.G., Bruce, D.M., Vincent, J.F.V. and Jeronimidis, G. (2000) The manufacture and mechanical testing of thermosetting natural fibre composites. J. Mater. Sci., 35, 293–298. Holbery, J. and Houston, D. (2006) Natural-fiber-reinforced polymer composites in automotive applications. JOM, (November), 80–86. Hoogen, N. (1999) Flax fiber reinforced PP components in large serie. Kunststoffe Int., 89, 103–106. Huber, T., M¨ussig, J., Baur, E. and Otremba, F. (2008) Natural reinforcement. Kunststoffe Int., 7, 97–101. Hull, J.L. (2006) Compression and transfer moulding, in Handbook of Plastic Processes, ed. by Harper, C.A. John Wiley & Sons, Inc., Hoboken, NJ, USA. Jacob, M., Thomas, S. and Varughese, K.T. (2004) Mechanical properties of sisal/oil palm hybrid fiber reinforced natural rubber composites. Compos. Sci. Technol., 64, 955–965. Karus, M., Ortmann, S., Gahle, C. and Pendarovski, C. (2006) Use of natural fibres in composites for the German automotive production from 1999–2005, in NF – Market Study. nova Institut, H¨urth, Germany. Kim, K.T., Jeong, J.H. and Im, Y.T. (1997) Effect of moulding parameters on compression molding sheet molding compound parts. J. Mater. Process. Technol., 67, 105–111. Klesov, A.A. (2007) Wood Plastic Composites. Wiley-Interscience, Hoboken, NJ, USA. Knobelsdorf, C., L¨utzkendorf, R. and Reussmann, T. (2005) M¨oglichkeiten und Grenzen zur Beeinflussung des Emissionsverhaltens naturfaserverst¨arkter Werkstoffe, in 5th International Symposium on Materials Made from Renewable Resources, naro.tech, Messe Erfurt AG, Erfurt, Germany. Larock, J.A., Hahn, H.T. and Evans, D.J. (1989) Pultrusion processes for thermoplastic composites, in 44th Annual Conference, Composites Institute, The Society of Plastics Industry, 6–9 February. Long, A.C. (ed.) (2007) Composites Forming Technologies. Woodhead Publishing Limited, Cambridge, UK. Mathur, V.K. (2006) Composite materials from local resources. Constr. Build. Mater., 20, 470–477. Medina, L., Schledjewski, R. and Schlarb, A.K. (2009) Process related mechanical properties of press molded natural fiber reinforced polymers. Compos. Sci. Technol., 69(9), 1404–1411. Michaeli, W. (2006) Einf¨uhrung in die Kunststoffverarbeitung. Carl Hanser Verlag, Munich, Germany. Miller, A.H., Dodds, N., Hale, J.M. and Gibson, A.G. (1998) High speed pultrusion of thermoplastic matrix composites. Compos. Part A: Appl. Sci. Mfg, 29A(7), 773–782. M¨ussig, J., Schmehl, M., von Buttlar, H.-B., Sch¨onfeld, U. and Arndt, K. (2006) Exterior components based on renewable resources produced with SMC technology – considering a bus component for example. Ind. Crops Prod., 24, 132–145. Nechwatal, A., Mieck, K.-P. and Reußmann, T. (2003) Developments in the characterization of natural fibre properties and in the use of natural fibres for composites. Compos. Sci. Technol., 63, 1273–1279. Neitzel, M. and Mitschang, P. (eds) (2004) Handbuch der Verbundwerkstoffe. Carl Hanser Verlag, Munich, Germany. O’Dell, J.L. (1997) Natural fibers in resin transfer molded composites, in Proceedings Wood-Fiber-Plastics Composites Symposium. Forest Prod. Soc., Madison, WI, USA, p. 280. O’Donnell, A., Dweib, M.A. and Wool, R.P. (2004) Natural fiber composites with plant oil-based resin. Compos. Sci. Technol., 64, 1135–1145. Oksman, K. (2001) High quality flax fibre composites manufactured by the resin transfer moulding process. J. Reinf. Plast. Compos., 20(7), 621–627. Oksman, K., Wallstr¨om, L., Berglund, L.A. and Dias Toledo Filho, R. (2001) Morphology and mechanical properties of unidirectional sisal–epoxy composites. J. Appl. Polym. Sci., 84, 2358–2365. Ortmann, S., Schwill, R., Karus, M. and M¨ussig, J. (2005) The up-and-coming material group. Kunststoffe Plast. Eur., 95, 23–28. Panigrahy, B.S., Fung, J., Panigrahi, S., Tripathy, A., Rajakumar, B. and Kamal, A. (2006) Flax fibre based composite profiles for construction industries. Paper No. 06–168 at the CSBE/SCGAB 2006 Annual Conference Edmonton, Alberta, Canada, 16–19 July 2006. Peters, S.T. (ed.) (1998) Handbook of Composites. Chapman and Hall, London, UK. Potter, K. (1997) Resin Transfer Moulding. Springer Verlag, New York, NY. Pritchard, G. (2004) Two technologies merge: wood plastic composites. Plast. Additives Compounding, (July/August), 18–21. Richardson, M.O.W. and Zhang, Z.Y. (2000) Experimental investigation and flow visualization of the resin transfer mould filling process for non-woven hemp reinforced phenolic composites. Compos.: Part A: Appl. Sci. Mfg, 31, 1303–1310. Riedel, U. (2003) BioVerbunde im Schienenfahrzeugbau, in 4th International Symposium on Materials Made from Renewable Resources, naro.tech, Messe Erfurt AG, Erfurt, Germany.

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Natural Fibre Composite Processing: A Technical Overview

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Rosato, D.V., Rosato, D.V. and Rosato, M.G. (2000) Injection Molding Handbook, 3rd edition. Springer-Verlag, New York, NY, USA. Rouison, D., Sain, M. and Couturier, M. (2004) Resin transfer molding of natural fiber reinforced composites: cure simulation. Compos. Sci. Technol., 64, 629–644. Rowell, R., O’Dell, J., Basak, R.K. and Sarkar, M. (1997) Applications of jute in resin transfer molding, in Proceedings of International Seminar on Jute and Allied Fibres: Changing Global Scenario, Ijira Association, Calcutta, India, pp. 89–96. Rudd, C.D., Kendall, K.N., Long, A.C. and Mangin, C. (1997) Liquid moulding Technologies: Resin Transfer Moulding, Structural Reaction Injection Moulding and Related Processing Techniques. Woodhead Publishing, Cambridge, UK. Saheb, D.N. and Jog, J.P. (1999) Natural fiber polymer composites: a review. Adv. Polym. Technol., 18, 351–363. Sawallisch, K. (1984) Compounding of sheet molding compound. Polym.-Plast. Technol. Eng., 23, 1–36. Sch¨afer, D. (1998) Einsatz und Potential naturfaserverst¨arkter Kunststoffe in der Automobilindustrie, in G¨ulzower Fachgespr¨ache: Nachwachsende Rohstoffe – von der Forschung zum Markt. FNR, G¨ustrow, Germany. Scher¨ubl, B.R. and Hintermann, M. (2005) Use of natural-fibre reinforced plastics in automobile exteriors, in 8th International AVK–TV Conference, 27–28 September 2005, Arbeitsgemeinschaft Verst¨arkte Kunststoffe – Technische Vereinigung e.V. (AVK-TV), Frankfurt am Main, Germany. Sch¨ussler, A. (1998) Automobilinnenteile aus Naturfaservliesen. Kunststoffe, 88, 1006–1008. S`ebes, G., Cetin, N.S., Hill, C.A.S. and Hughes, M. (2000) RTM hemp fibre-reinforced polyester composites. Appl. Compos. Mater., 7, 341–349. Singh, B. and Gupta, M. (2005) Performance of pultruded jute fibre reinforced phenolic composites as door frame materials in buildings. J. Polym. Environ., 13, 127–137. Takase, S. and Shiraishi, N. (1989) Studies on composites from wood and polypropylene II. J. Appl. Polym. Sci., 37, 645–659. Torres, F.G., Ochoa, B. and Machicao, E. (2003) Single screw extrusion of natural fibre reinforced thermoplastics (NFRTP). Int. Polym. Process., 18, 33–40. Trevor, F. (2000) Pultrusion for Engineers. CRC Press, Boca Raton, FL/Woodhead, Cambridge, UK. van de Velde, K. and Kiekens, P. (2001) Thermoplastic pultrusion of natural fibre reinforced composites. Compos. Struct., 54, 355–360. van Voorn, B., Smit, H.H.G., Sinke, R.J. and de Klerk, B. (2001) Natural fibre reinforced sheet moulding compound. Compos. Part A: Appl. Sci. Mfg, 32(9), 1271–1279. Weyrauch, F., Stadtfeld, H. and Kissinger, C. (2004) Harzinjektionsverfahren, in Handbuch der Verbundwerkstoffe, ed. by Neitzel, M. and Mitschang, P. Carl Hanser Verlag, Munich, Germany. Wiedmer, S. and Friedrich, K. (2004) Pultrusionsverfahren, in Handbuch der Verbundwerkstoffe, ed. by Neitzel, M. and Mitschang, P. Carl Hanser Verlag, Munich, Germany. Williams, G.I. and Wool, R.P. (2000) Composites from natural fibers and soy oil resins. Appl. Compos. Mater., 7, 421–432. Zhu, J., Chandrashekhara, K., Flanigana, V. and Kapila, S. (2004) Manufacturing and mechanical properties of soy-based composites using pultrusion. Compos. Part A: Appl. Sci. Mfg, 35, 95–101.

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19.4 Natural Fibre-Reinforced Polymers in Automotive Interior Applications Eugen Pr¨omper Johnson Controls, Burscheid, Germany

19.4.1 Introduction Materials based on natural resources such as wool, cotton or natural leather have been used as decorative surface materials for interior parts and seats since the beginning of industrial automobile production. Wood plates (chipboards), painted or unpainted, were also used as decorative covering parts in automotive interiors. Since the late 1940s, these parts have become more contoured, meaning that new processes have had to be developed for the use of natural materials. The development of natural fibre (NF) compounds based on wood fibres and coir fibre bundles for these contoured interior parts started in the early 1950s. Decorative door panels and the upholstery of automotive seats were made of these materials, with melamine resins for the wood fibre and latex for the coir bundles. The development of thermosetting resins such as phenolic resins or unsaturated polyester resins in the 1960s led to the development of simple, contoured, compression-moulded parts reinforced with wood or cotton fibres. An increasing amount of natural materials was thus used in the automotive sector during the 1960s and 1970s. At the end of the 1980s, with the development of airbag systems as a safety component of the inner outfit, the requirements of automotive interior parts changed from their being simply decorative to being structural parts with relevant safety features. Head impact functions also had to be fulfilled by sections of the instrument panels. This led to a basic change in the requirements profile for natural fibre compounds. To fulfil the requirements for higher values of mechanical properties, the use of longer natural fibres or fibre bundles with higher mechanical properties was necessary. Also, the resins changed to easily applied and fast reactive polymers, including an optimisation of emissions without styrene, phenol or formaldehyde. The following section describes the development of natural materials for automotive interior applications between 1950 and 2008 from the viewpoint of a tier-1 supplier, as a direct supplier for parts to the automotive industry, with the experience of the German companies Fibrit (Grefrath), Naue (Espelkamp) and Happich (Wuppertal). These companies came together in the mid-1990s to form the company Johnson Controls; the Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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European Headquarters are based in Burscheid, Germany. Besides experience and the results of external institutes, material developments are mostly based on positive experience but also on negative practical experience in the different stages of material or process developments, and on good cooperation between the material supplier and producer. Additionally, besides material costs, increased environmental standards (emissions, odour, waste, recycling) have also heavily influenced the advanced stages of this material class.

19.4.2

Configuration of Natural Fibre Compounds and Requirements

Basically, natural fibre compounds are made from different natural fibres and resins to compound the fibre into an increasingly homogeneous system in a three-dimensional part. The principal possibilities of NF compounds are shown in Table 19.4.1 Details and a basic description of the natural fibres are given in other chapters of this book. The most important fibre properties from the viewpoint of a part producer are permanent constant quality, low emissions and fogging and good mechanical properties. The final properties of an NF compound with thermosetting resin are more influenced by the conditions of the fleece or felt processing than by the mechanical characteristics of a single fibre or fibre bundle (Beckmann and Kleinholz, 1999). Essential for the part properties are the felt density, level of needling and other characteristics. The correlation between the properties of the natural fibre, the fibre felt and the final interior part is shown in Figure 19.4.1. This figure shows how the same properties remain the same for fibres, fleece and final substrate. Other important characteristics change, especially for the function of a trim part. Here, in addition to the properties of fibre and resin, the production parameters of part manufacturing also have a major impact on the final part values. The characteristics of the fibres/fibre bundles have an impact on the production and quality of the fibre/fibre felt (Polyvlies, 2007). Important is the distribution of fibre/fibre bundle length. For flax fibre bundles, the optimum length is between 40 and 80 mm. A homogeneous distribution leads to a uniformly distributed fleece and felt, which is very important for homogeneous resin absorption and thus for homogeneous properties in the final part. The percentage of shives must be below 5%; a higher percentage has a negative impact on the fleece and felt production and the surface character of the product. Most fibre fleece and felts for automotive parts usually consist of a mixture of fine natural fibre bundles such as flax or kenaf and coarser fibre bundles such as hemp or sisal; this leads to regular absorption of the resin in the felt or fleece. For three-dimensional contoured trim parts, the stretching behaviour of the felt is an important criterion.

Table 19.4.1

Principal configuration of NF compounds

Natural fibre/fibre bundles

Polymer

Process

Application

Wood, Flax Hemp, Jute, Kenaf

Thermoplastic: r Polypropylene r Co-polyester

Compression moulding in a cold tool (<50 ◦ C) after preheating (190–220 ◦ C). Injection moulding (195 ◦ C)

Door panel, pillars, bolster, seat back panel (max. temp.: 110 ◦ C)

Wood, Flax, Hemp, Jute, Kenaf, Sisal

Thermosetting: r Melamine r Polyester resin r Acrylic resin r PUR r Epoxy

Compression moulding in a hot tool (140–220 ◦ C, depending on the resin) after impregnation of the fibre felts with resin

Instrument panel, door panel, seat back panel (temp.: >110 ◦ C)

Coir

Elastomer: r Latex

Impregnation of the fibre with resin; forming, vulcanisation

Seat upholsteries

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Figure 19.4.1

425

Properties: natural fibre/fibre bundle → natural fibre felt → door panel substrate.

Here, the fibre bundle formulation and the needling parameters in particular are ultimately essential in determining whether a contoured trim part is feasible with a felt formulation and physics. Much work has to be invested in investigating how the fibre surface reacts to the resin: an ‘open’ surface for the thermosetting resin and good adhesion to thermoplastic matrices. This forms the basis for the mechanical properties of the substrates, such as tensile strength and energy absorption. The process conditions of these materials are basically influenced by the chemistry of the resins. Furthermore, the final properties of a product to fulfil customer standards are affected by the resin; this is shown in particular by the maximum working temperature. For compounds with thermoplastic resins, this temperature is approximately 105–110 ◦ C; for compounds with thermosetting resins, it is up to 130 ◦ C. In principle, the manufacture of NF parts by compression moulding can be divided into four sections: r wet preforming of natural fibre plus resin, followed by compression moulding (temperature: 230–250 ◦ C); r production of needle felts with pretreated fibres (wood plus thermosetting resin), followed by compression moulding (temperature: 220 ◦ C); r production of needle felts, application of reactive polymer resins, followed by compression moulding (temperature: 140–170 ◦ C); r production of a felt or an extrusion sheet with a mixture of natural and thermoplastic fibres, followed by compression moulding (temperature: <40 ◦ C). For a global supplier of automotive interior parts, which produces these parts all over the world, pretreated felts have the advantage of being processed with lower equipment investment. An additional impregnation for the resin means more investment. These producers are interested in buying the pretreated semi-finished product ready for forming and compression. Unfortunately, up to now, serial application is only known for wood fibre felts pretreated with thermosetting resins such as acrylates. Other natural fibre felts with flax, hemp, kenaf or jute have to be impregnated in-line with fast-reacting polymers. Further developments are desirable.

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19.4.3 Natural-Fibre-Reinforced Materials The development of contoured natural fibre parts for automotive applications started post-1947 with the process development of Fibrit wood fibre material for decorative trim parts, based on the paper manufacturing process. The combination of animal fibres such as horsehair together with coir fibre bundles and combined with latex for the rubberised hair for seat pads followed the rubber manufacturing process.

19.4.3.1

Overview: Natural Fibre Materials 1950–2008

A detailed time line is shown in Figure 19.4.2 (Pr¨omper, 2008). Important steps included the first automotive parts in 1954 and the first instrument panel with Fibrit material in 1957. Rubberised hair was primarily produced for upholstery in furniture or train application, and also in the late 1950s for automotive seats. Further developments in these materials and their processing are registered in this timeline. The transfer from a single decorative part for a door panel or an instrument panel to a structural part with higher mechanical properties was necessary owing to higher safety requirements for the integrated airbag lid or for head or side impact requirements. This change can be followed by the development of NF compounds with thermosetting resins in the 1990s. Also shown is the change to emission-optimised resins without styrene or formaldehyde, such as polyurethane or epoxides. Important steps for the Fibrit process in the last 15 years have included the introduction of a formaldehyde-free formulation, by replacing the melamine–formaldehyde resin with acrylicbased resins, and sieve technology with grooved dies in the steel compression mould (further information on the thermosetting matrix materials for natural fibre composites is given in Chapter 19.5). The process for rubberised hair containing a portion of animal fibres reached a huge milestone in 2000, when the laying process changed to a dispersion head/rotary table process with 100% coir fibres, which led to a new trade name: FaserTec (Klusmeier et al., 2006). Table 19.4.2 provides a summary of the different natural fibres/fibre bundles, including their components, main application and most important properties. This table shows the materials and trade names of Johnson Controls (Burscheid, Germany); other major European interiors companies have similar materials based on natural fibres and thermoplastic or thermosetting resins.

19.4.3.2

Natural Fibre (NF) Compounds for Decorative Trim Parts

As described previously, the manufacture of NF compounds for decorative trim parts can be classified into four versions (Beckmann and Kleinholz, 1999; Jacobs, 2006; Pr¨omper, 2008). The Fibrit process takes the wet preforming of slurry with wood fibre to transport the impregnated fibres into the mould. The other three versions require impregnated natural fibre felt, which is compression moulded in a hot or cold tool, depending on the chemical structure of the thermoplastic or thermosetting resin. In times of low-cost materials and processes with low investment expenses, especially the ‘old’ processes such as Fibrit or FaserTec, which starts with fibre preparation followed by impregnation of the fibres/fibre bundles with a special mixture of resins and additives in its own formulations, are still very successful in terms of cost and production. The main reasons for this are the consequences of continuously improving material properties and process conditions over the past years. These factors are also the best premises for future growth and for remaining a cost-effective and efficient interior material.

19.4.3.2.1

Fibrit – Wood Fibre Material with Acrylic Resin

Fibrit is more than 90% based on natural resources. The material is very sustainable, light, recyclable (PTS, 1990) and, because of the preforming step with water, best suited for highly contoured interior parts.

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Figure 19.4.2

427

Timeline of NF materials between 1950 and 2008.

The fibre formulation contains approximately 65% wood fibre, 30% bleached and unbleached cellulosic fibres and less than 5% synthetic process fibres. The chemicals, acrylic-based resins and additives also account for less than 5% of the slurry. As a partner of the chemicals, and the swell medium for the wood fibre, water is very important for the functioning of this process. This swelling of the fibre and the change in the chemical potential of the wood fibre by changing the pH value are the reason why the resins in this process amount to approximately 3%. All other NF compounds require more than 15% and up to 40% resin to achieve good mechanical properties in the final part. The manufacturing process (see Figure 19.4.3) can be divided into two sections: preparing the wood fibre slurry with a material content of 0.5% in water and the three-step forming and pressing process (Figure 19.4.3).

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Table 19.4.2

Applications and properties of current NF in 2008 In production (2008)

Density in g/cm3

Flexural strength in MPa

Flexural modulus in MPa

Impact strength in kJ/mm2

Acrylic resin dispersion

Instrument panel, door panel, air channel, bolster, low- and middle-segment cars

0.75–0.80

30–45

3000–3300

20–30

Needle felt wood fibre

Acrylic resin dispersion

Instrument panel, door panel, seat back panel, low-, middle- and high-segment cars

0.85–0.95

50–60

3000–3800

12–20

Naturfaser–EP

Needle felt flax/hemp fibre bundles

Epoxy resin

Door panel, high-segment cars

0.75–0.85

55–70

4500–5800

25–35

EcoCor

Hemp or kenaf fibre bundles

Polypropylene Seat back panel, fibre door panel, middle-segment cars

0.7–1.0

45–55

2300–2700

25–35

HM PP

Wood powder Polypropylene Door panels, and fibres, granules, inserts, low- and extruded thermoplastic middle-segment sheets cars

1.1

30

2000

4

PP–NF for injection moulding

Fibrowood recyclates

Polypropylene Plastic retainer for granules, seat back panel, all thermoplastic segments

1.14

57

3800

12

FaserTec

Coir fibre bundles

Latex

—

—

—

—

Components Material trade name

Fibre

Resin

Fibrit

Wood fibre

Fibrowood

Seat upholstery, middle- and high-segment cars

Spruce chips are cleaned and defibrated into single fibres in the form of a thermomechanical pulp (TMP). The other fibres, bleached and unbleached cellulose, polyethylene pulp and polypropylene fibre, which are delivered in the form of pressed sheets, are simultaneously opened in a pulper with high mechanical stirring; Fibrit recyclates, scrap or punching waste are also added here. This mixture is poured into a storage tank with cycle water in a solids content of 5%. When adding the chemicals, the pH value has to be fixed exactly and buffered. The cycle water is kept in the light alkaline range with sodium aluminate. All chemicals are added to the mixing tanks, and the pH value is changed to the acid range by adding alum or aluminium polychloride; the reaction time is approximately 20 min. The slurry is then led to the service tank, and the solids content changes to 0.5%; the pH value is in the light acid range. The pressing process starts with the forming station, where the slurry is poured under pressure onto a sieve in the mould, which already nearly has the contour of the finished part. Here, the process water serves as a carrier for the wood fibres to reach the homogeneous forming part in the mould. The solids content is about 20%. This formed part is passed on to the second station where the shaped form is pressed. The water from these two stations is collected in the water cycle. After the press station, the solids content is at 55%. The last step is the compression press at a temperature of 220–250 ◦ C for about 60 s. Compared with felt compression moulding, the Fibrit requires lower press force per square metre, as no material melt flow or three-dimensional stretching is necessary. Twelve single parts can thus be preformed, dried and pressed in

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Figure 19.4.3

429

Process cycle Fibrit HFFS.

one press cycle (60 s), which is used in a current serial application for a total car set of four doorpanels, four door inserts and four bolster lines. After conditioning, the finished part has to be stamped and is then ready for covering, back foaming and final assembly. Current products include instrument panels with integrated airbag lid for the passenger airbag, door panels and air channels.

19.4.3.2.2

Fibrowood – ‘Needled’ Wood Fibre Prepreg with Acrylic Resin

Fibrowood is a wood fibre felt impregnated with acrylic resin and mixed with synthetic process fibres. This material was developed to replace Fibrit where installation of the great plant unit with fibre preparation would not be economically viable. These semi-finished products can be transported as precut felts or in felt rolls. With simple installation engineering, Fibrowood is suitable for worldwide implementation. The properties of Fibrowood depend on the fibre formulation. A great variation is possible with different synthetic fibre fleeces on the back side and with different amounts of synthetic fibres. The process starts with the defibration of wood chips to single fibres. These swelled fibres are sprayed with a final acrylic resin content of between 12 and 15% and then dried. Prepreg production starts with a thin synthetic fleece on which the impregnated wood fibres mixed with synthetic fibres are placed onto a homogeneous semi-finished part, followed by the needling step. Figure 19.4.4 shows the pressing process, in which the precut felt is placed in a hot tool. The press conditions are 35 s at 220 ◦ C, with one or two ventilation intervals to eliminate steam. After acclimatisation, the trim part is ready for assembling. Fibrowood is used as a carrier for door panels, door inserts and seat back panels. The contour of the final part defines the formulation of the material (up to 20% of synthetics) and the density of the needling. Different fibres such as polyester, co-polyester or bicomponent fibres can be used, normally with melting points of approximately 120 ◦ C. A great variation in felt stretching behaviour is possible to manufacture any

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Figure 19.4.4

Fibrowood. Wood fibre felt with acrylic binder.

highly contoured trim part in the compression process. The condition of the back-side fleece can improve the following process steps such as gluing or welding of plastic parts. Fibrowood is a cost-effective alternative, having low-cost equipment and a high variation in physical and process properties. Serial applications are also possible for instrument panels with an integrated airbag lid.

19.4.3.2.3

Needled Natural Fibre Prepreg Coated with Reactive Resins

As the mechanical properties of the wood fibre material are limited, longer natural fibres bundles such as flax, hemp, sisal, kenaf or jute have been used for NF compounds since the late 1980s. Thermosetting resins were used, such as phenol–formaldehyde or unsaturated polyester with styrene, to make the first trim parts. These parts provided higher mechanical properties and higher tensile strength than the wood fibre material, which was required by the increased safety standards for door panels. Figure 19.4.5 presents the process steps for the ‘polyester-reinforced natural fibre mat’ (PNM). The jute needle felt, at that time with an avivage of oil (see Chapter 6.7.1.3) on the fibre surface, was dried with hot air and then impregnated from both sides of the fibre felt with approximately 33% of polyester resin. The impregnated felt was stored in a roll car and transported to the press. The felt was cut and then pressed for 55 s at 130 ◦ C. The time between impregnation and pressing depends on the peroxide catalyst and takes up to 20 min. The final part was punched and then ready for covering. The quality of these materials depended on the uniformity of the fibre felt. Air permeability, which is important for the vacuum covering process, is particularly dependent on the felt properties. Conglomerates of resin in thin areas of the felt could decrease the vacuum and cause the covering process to fail. Additionally, as for the wood fibre felt, needling density is also an important property here to ensure the stretch behaviour of the felt and thus the contour possibilities of the final part. During the 1990s, the emission standards for interior parts increased, and so the demand for styreneor formaldehyde-free resins was postulated. Other natural fibres also came into discussion. One important development step was the use of PUR resin, which came from elastomers in shoe applications. The NF felt with flax and sisal fibres bundles had the major advantage that no oil avivage was used (compared with jute) and impregnation with PUR resins was excellent. The fibre felt is dried at 130 ◦ C with hot air. This process reduces a lot of low-molecular ingredients and thus the fogging value of the final product. The felts were sprayed with the two-component PUR system on both sides and placed directly into the 130 ◦ C hot mould. The press time was approximately 60 s. Plastic retainers could be placed into the hot mould before pressing, eliminating the need to glue them at a later date. This material, called Fibropur, was used for door panels in high-segment cars.

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Figure 19.4.5

431

PNM – polyester-reinforced jute fibre felt.

In recent years, a new impregnation process with a spray technology has led to a switch from PUR resin to epoxy resin. The main advantage of the new process is a reduction in overspray. The authorisation of hemp cultivation in Germany in 1996 was also a new step towards replacing the sisal fibre bundles and led to a flax/hemp (50:50) needle felt. Figure 19.4.6 shows the process cycle of natural fibre with epoxy resin (NF–EP) material. After drying, the felt is sprayed on both sides with 35% of the epoxy resin and then cut. The sections are transported to the press where compression must start within 15 min. After pressing for 50 s at a mould temperature of 150 ◦ C, the parts are stamped and then ready for trimming. Plastic parts such as retainers, staking lines or other functional parts can also be laid into the mould before pressing. The carriers show nearly the same properties as the PUR part. With such a high mechanical property level and good side crash behaviour, this material is useful for middle- and high-segment cars.

19.4.3.2.4

EcoCor – Natural Fibre and Thermoplastic Fibres, Extruded Sheets (HMPP)

The thermoplastic matrix for NF compounds is polypropylene. Other polymers are too heavy or too expensive. Extruded PP sheets with wood powder have been used for a long time, especially in southern Europe, as a very cost-effective trim material. These sheets are limited, however, in their ability to stretch to create contoured trim parts and in their mechanical properties, such as tensile strength and brittleness. To improve these properties, a particular amount of natural fibres is added to the formulation. R process (Renolit, no date). Owing to the low temperature of the Figure 19.4.7 shows the Wood-Stock
mould, ‘one-step processes’ are possible. In one step, the substrate is formed and compressed together with the surface material and bound to the carrier without any glue. Here the extruded sheets are heated up with infrared energy to a temperature of 210–220 ◦ C; in addition, the surface materials, such as textiles, PVC or TPO vinyl, are preheated. Both components are combined and pressed at a mould temperature

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Figure 19.4.6

NF–EP material – flax/hemp needle felt sprayed with epoxy resin.

R below 40 ◦ C. The final product can be finished. Wood-Stock
is used for different trim parts such as door panels or inserts for low- or middle-segment cars. The prefabricated needle felt, consisting of a combination of natural fibres and PP fibres, demonstrates improved stretching behaviour and higher impact strength values. The EcoCor material is a combination of PP fibres with kenaf fibre bundles.

Figure 19.4.7

R Wood-Stock
– extruded PP sheets with wood powder and natural fibres.

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Figure 19.4.8

433

EcoCor – needle felt kenaf fibre bundles and PP fibres.

The EcoCor process is suitable for a ‘one-step process’ (Figure 19.4.8), but two-step processes are also possible for highly contoured parts. Depending on this contour, it may be necessary to preform the felt. The fibre form leads to a more open material structure, enabling vacuum processes. Mould surface temperatures below 40 ◦ C make it possible to place foam pads between the surface material and the carrier, so that soft armrests are feasible. The high content of thermoplastics also makes it possible to improve welding process steps by pressing welding flanges, which can be bent over using hot air. As is contains 50% thermoplastic material, EcoCor is a cost-effective material used for door panels, inserts, seat back panels and trim parts for trunks. The parts can be used for low- or middle-segment cars.

19.4.3.3

Injection Moulding of NF–PP Compounds with NF Recyclates

Injection moulding of NF–PP compounds for plastic parts is not very common in serial application for automotive interior parts. There are two issues that make interior application very difficult: the preparation of the compound and the injection moulding conditions. In both manufacturing steps, a temperature of more than 190 ◦ C is often necessary, which is too much for the thermal stability of natural fibres. This results in decomposition and unpleasant odours, which are unacceptable for interior trim parts. Mixing of the components at a temperature below 130 ◦ C is a successful step in the process, allowing the first serial application in car interiors for a retainer in the seat back panel. Figure 19.4.9 shows the process cycle. The process was developed by Johnson Controls, together with FiberGran (Jakwerth, 2004), to recycle production waste of the Fibrowood process for new car applications. Punching waste from the Fibrowood seat back panel is shredded and then compounded with PP copolymers in a narrow molecular weight distribution. This process is carried out below 130 ◦ C. The material is injection moulded into a seat retainer, which is welded onto the back of the vinyl-covered panel. The production waste is thus used for the same product and the material cycle is closed. Much research was carried out into finding the right flow channel for the injection mould, to reduce the shear strain for the materials. The walls of the retainer were also too thick to keep the melt temperature below 190 ◦ C. With this material, all customer

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Figure 19.4.9

NF material recyclates and PP granules for injection moulding.

requirements, especially for odours and emissions, were fulfilled and serial release was achieved. Other serial applications for this recycling material are being tested.

19.4.3.4

FaserTec – Seat Upholstery Made of Coir Fibre Bundles and Latex

Seat upholstery made of FaserTec demonstrates very good thermal and fluid behaviour for the human body and very good seating comfort owing to its open structure and optimum fluid permeability properties (Klusmeier et al., 2006). The material is light and sustainable and based almost 100% on natural resources; a recycling concept is in serial application. Components include an almost 50:50 ratio of coir fibre bundles and latex. The resin formulation consists of natural latex with additives and a catalyst for the vulcanisation step. Figure 19.4.10 shows the actual FaserTec process cycle on a carousel. A fleece made of recycled synthetic fibres and wool is placed in the tool. The coir fibre bundles are cut and mixed with the latex compound and spread by a robot onto the fleece. The fibre–latex mixture and fleece are than pressed into a contour; the pad is dehumidified in a drying process. After removal, the pads are stamped and ground, and then vulcanised for around 20 min in an autoclave with saturated steam. The temperature increases from 85 to 120 ◦ C, and the boiler pressure from about 0 to 2 bar. The FaserTec pad is then ready for trimming. Today, FaserTec can be produced in different variations (Figure 19.4.11) (Klusmeier, 2008). Serial FaserTec is made up of fleece, latex and coir fibre bundles. Dual FaserTec ensures a soft-touch surface by using different amounts of natural and synthetic latex in the process. Comfort Line FaserTec, with inserts made of natural fibres or with polyester fleece inserts, reduces the mass and the cost, and also ensures a comfortable soft-touch upholstery surface. Current products include seat upholstery for automotive seat back rests. Owing to their higher firmness, FaserTec pads can also be used as module carriers in higher-segment vehicles. Compared with PUR foam, FaserTec makes it possible to fix holders or special modules (e.g. massage modules) in high-comfort seats.

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Figure 19.4.10

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FaserTec process cycle.

19.4.4 Conclusion and Outlook Natural fibre compounds are used for automotive parts where the cost and function of the completed parts fulfil customer specifications (Jacobs, 2006). The current situation in the automotive industry is that the ‘green’ factor is not enough to convince automakers about interior part concepts of renewable resources. Cost efficiency and sustainability plus compliance with technical customer regulations are the main factors for winning concepts. Costs aside, lightweight parts are the next important factor for saving energy during the lifetime of a car. The CO2 balance of an automotive seat shows that more than 70% of CO2 emissions come

Figure 19.4.11

FaserTec – concept comparison of actual developments.

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from fuel consumption. Mass reduction will be one of the major challenges in the development of sustainable cars. NF compounds, as lightweight materials, can help to achieve this goal. Therefore, the actual development of NF compounds starts with mass reduction, by reducing the area mass of natural fibre felts. As in recent years, the side airbag is no longer placed in the door panel; the mechanical properties of these parts can be reduced, so a material with lower physical properties can be used; mass reduction is possible by reducing the felt mass and by reducing the mass and the amount of resins. The next development will be the increased use of NF–PP compounds for injection-moulded parts. Besides the seat retainer, shown previously, wood plastic composites (WPCs) are close to serial applications in car interiors. More research is required to modify hemp, kenaf and flax fibre bundles so that they can also be used in the injection moulding process. This will also make it possible to show the ‘green credentials’ of natural materials in a car interior without covering them with vinyl. This green surface for interior parts is currently in demand by car designers. NF felts preimpregnated with thermosetting resins will definitely increase the usage of this material on account of the cost reduction. The initial steps have been completed, but a great research effort is still needed. The replacement of petrochemical-based resins in NF compounds with biopolymers must offer good thermosetting possibilities. Thermosetting resins based on bio-oils have been introduced and made available. These represent a promising alternative to PUR or epoxy resins. The replacement of PP with thermoplastic biopolymers will be very difficult or impossible in terms of sustainability: biopolymers such as polylactide acid (PLA) or polyhydroxybutyrate (PHB) have densities much higher than those of other common plastics. For sustainability, it is always best to have lightweight parts made of PP rather than any other biopolymer. Meeting the demands of end-customers and legal regulations for CO2 emissions, and in view of the switch from petrochemical-based raw materials to renewable resources, NF compounds will have a promising future as a light and sustainable material for automotive interior parts.

References Beckmann, A. and Kleinholz, R. (1999) Prospects and risks of natural fibres for automotive interior parts, in 2nd International Symposium on Materials from Renewable Resources, Erfurt, Germany, September 1999. Jacobs, W. (2006) Automotive interior parts, decision matrix for material selection, in 9th International AVK (Federation of Reinforced Plastics) Conference on Reinforced Plastics and Thermosets, Essen, Germany, September 2006. AVK-TV, Frankfurt, Germany. Jakwerth, G. (2004) Personal information, FiberGran GmbH & Co. KG, Ostritz/OT Leuba, Germany. Klusmeier, W. (2008) Automobile pads made from FaserTec – a sustainable material with great potential. 8th International CTI (Car Training Institute) Forum: Automotive Seating, Munich, Germany, June 2008. Klusmeier, W., Weing¨artner, A. and Janz, M. (2006) Automobile upholstery from FaserTec – a material with future. 6th Global Wood and Natural Fiber Composites Symposium, University of Kassel, Kassel, Germany, April 2006. Polyvlies (2007) Personal information, Polyvlies, Franz Beyer GmbH & Co. KG, Hoerstel, Germany. PTS (1990) Environmental technical expertise: production of Fibrit N. The Paper Technology Specialist (Papiertechnische Stiftung, PTS), Munich, Germany. Pr¨omper, E. (2008) Natural fibre and wood fibre reinforced compounds for automotive interiors – a success story. International Congress: Raw Material Shift and Biomaterials, nova-Institut, Cologne, Germany, December 2008. R , Renolit Gor S.p.A., Buriasco (TO), Italy. Renolit (no date) Company prospects: Wood-Stock


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19.5 Composites Based on Natural Resources Martien van den Oever and Harri¨ette Bos Wageningen University and Research Centre, Food and Biobased Research, Department of Fibre and Paper Technology, Wageningen, The Netherlands

19.5.1 Introduction Although most composite materials to date have been fully based on petrochemical or mineral sources, composites can also be prepared from natural resources. In this case, both the fibres/fillers as well as the matrix material can be made from renewable resources. In this chapter, an overview of existing and recently developed renewable matrix materials, both thermoplastics and thermosets, is presented. Next, the important issue of the durability of natural fibres is addressed, and some methods for improving the durability are discussed. The second part of the chapter focuses on the variety of composite materials that can be made from renewable resources. We discuss a range of different composites, which can be related to the structure of the reinforcing natural fibre. A schematic representation of the structure of, for instance, flax bast fibres is given in Figure 19.5.1. All bast fibres have a similar structure. The bast fibre bundles can be transformed into needle-punched non-wovens and applied in thermoplastic and thermoset composites (see Chapter 19.3). The single fibre (elementary plant cell) can be obtained during extrusion compounding with thermoplastics, and the resulting material can be applied in injection or compression moulding processes. Single fibres and slightly further refined fibre grades are also used in paper applications. Even further refining of the fibres yields so-called microfibrillar cellulosic fibres with nanodimensions (that is, one dimension is smaller than 100 nm), and these fibres can be processed into nanocomposites (see Chapter 19.6). All these composites are described in this chapter. In the last part of the chapter, panel and board materials are discussed. These can be based on, for instance, the shives. A fairly new development of the all-cellulose composite is also addressed in a separate section.

Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
C 2010 John Wiley & Sons, Ltd

Edited by J¨org M¨ussig

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Figure 19.5.1 Schematic representation of the structure of flax fibre. M.J.A. van den Oever and H.L. Bos, K. Molenveld, Flax fibre physical structure and its effect on composite properties: Impact strength and thermomechanical properties, Die Angewandte Makromolekulare Chemie, 272, 1999, 71–76. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Adapted with permission.

19.5.2 19.5.2.1

Matrix Materials from Renewable Resources Renewable Polymers

Biopolymers are the most abundant organic compounds in the biosphere. A comprehensive overview of the most important biopolymers is given in the eleven volumes of Biopolymers (Steinb¨uchel, 2003). In this chapter we will mainly focus on industrially relevant or promising polymers from renewable resources. Biomass was the first source for the production of polymeric materials. There are a few strategies to make polymers from renewable resources. One approach is to take a naturally occurring polymer such as starch or cellulose and modify it, either chemically or via the use of additives to reach the desired property profile. A different approach is to produce or isolate small molecules from the renewable resource, which can then be polymerised into the desired material. With this last approach, also a mixture of renewable and non-renewable resources can be used, leading to a product that is partially renewable. A variety of examples will be presented in the following sections. 19.5.2.2

Thermoplastics

Cellulose polymers are produced via the modification of the cellulose backbone. Nitrocellulose was one of the first widely used plastics based on renewable resources, for instance as material for films. Owing to its high flammability, it was replaced with cellulose acetate in the 1950s. Cellulose acetate is still used in various applications such as cigarette filters and fabrics. Cellulose polymers are these days mainly applied as fibres, but they are also used in decorative applications owing to their good optical properties (gloss). Some of the cellulose polymers are biodegradable. Being a thermoplastic, it is in principle possible to make a fibrereinforced composite on the basis of cellulose polymers; however, applications with fibre reinforcements are, to our knowledge, not commercially known. Some research results will be presented further on in this chapter.

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Nylon-11 is these days marketed as Rilsan (Arkema, Colombes, France). It is made from castor beans (from the castor oil plant, Ricinus communis) and is a thermoplastic material with a high-performance property profile, used in particular for its resistance to chemicals and fuels and good mechanical properties over a wide temperature range. Rilsan can be reinforced with, for instance, glass or carbon fibres (Arkema, 2009). The thermoplastic processing temperatures of Rilsan lie well above 200 ◦ C, which makes the material less suited for reinforcement with natural cellulose fibres. About 10 years ago, starch plastics (thermoplastic starch, TPS) entered the market. At this moment, this is one of the most important biopolymers. Starch is turned into a thermoplastic material by the addition of a plasticiser. Original applications were, for instance, biodegradable bin-liners for the green bin and mulching film for agriculture. At this time, about 50% of TPS is used as loose-fill foams for packaging applications. TPS can be filled with natural fibres. TPS is rather sensitive to water, and therefore, for some applications, the polymer is blended with biodegradable but petrochemical-based polyesters. An important driver for many of the products where TPS finds application is the fact that it is biodegradable (Bolck, 2006). Around 2002, polylactic acid (PLA), a material that was developed for medical applications in the 1980s (Eenink, 1987; Leenslag, 1987) and has found ample application since (Purac, 2009), was marketed as a polymer for, among others, packaging and fibre applications. PLA can be made from starch, sugar, cellulose or whey. PLA was the first of a new generation of polymers in which building blocks from renewable resources are used to make completely new materials. The properties of PLA are often compared with those of PET. PLA therefore finds application as packaging material, films and food containers, and in disposables (bottles, cups). As PLA has a good properties profile and an acceptable price, and can be processed at relatively moderate temperatures, it is also suited for application as the matrix in fibre-filled composites. Although commercial applications are limited thus far, there are a number of developments ongoing, a few of which will be presented later in this chapter. Another example of thermoplastics from renewable origin is PHA/PHB, a family of materials that are still in a more or less experimental stage. Although in principle materials with a wide range of properties can be made, the application of these materials is still limited owing to their high price. PHA/PHB can be applied, for instance, as (compostable) film, for injection moulding applications, for example bottles, in fireworks, in hinges and in many other applications. Mirel Bioplastics (Cambridge, MA) is presently constructing a plant for the production of these materials (Mirel, 2009). The plant will have commercial products available for customers in the second quarter of 2009 and is designed to produce 50 million kg of PHA/PHB annually. Biomer (Krailling, Germany) markets the Biomer material for injection moulding applications and film casting (Biomer, 2009). Another approach is taken in, for instance, polypropylene terephthalate (PPT, a sister material to PET), where one of the constituting building blocks of an existing material is replaced with a bio-based feedstock. Dupont presently produces the material Sorona from circa 50% renewable feedstock. The dialcohol (PDO) used in the polyester synthesis is produced from starch via fermentation, whereas the diacid is from a petrochemical origin. Another newly developed material is Zytel 610 (Innovations Report, 2009) from Dupont, a nylon 6,10 that is aimed for applications at high temperatures and harsh environments and is partially made from castor oil, which makes it 40% renewable. Many combinations are possible within this approach, and we can expect to see more of these materials entering the market in the coming years (Blaauw et al., 2008). The most recent development is seen in Brazil, where Braskem (Rio Grande do Sul, Brazil) is developing green PE (and PP) from biomass feedstock. Braskem is presently building a plant that will produce HDPE from sugar cane via the production of ethanol (C2 H5 OH). This approach could become very successful, as, by only changing the feedstock, but producing the same building blocks and polymers, the wealth of materials science knowledge that has been built up over the past 50 years can still be used for these new materials (Blaauw et al., 2008). The green PE can of course be used to produce thermoplastic composites, as it has the same properties as standard petrochemical HDPE (Braskem, 2009).

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19.5.2.3

Thermosets

Various parties have tried to develop bioresins based on different feedstocks. Most ofthese resins have disappeared again from the market. Bottlenecks were, among others, the high viscosity and the limited curing rate. Resin systems that are fully based on renewable resources are presently not commercially available. There are, however, a number of resin systems that are partly based on renewable resources. Bioresins can be based on different systems. Four groups can be distinguished: r r r r

resins based on natural (vegetable) oils; furan resins; lactic-acid-based resins; polyurethane resins based on natural polyols.

19.5.2.3.1

Systems Based on Natural (Vegetable) Oils

Their availability, price and functionality make natural oils (triglycerides - see Figure 19.5.2) very well suited for application in resin systems. Triglycerides are esters consisting of glycerol (propane-1,2,3-triol) with three fatty acids (carboxylic acids).

Figure 19.5.2

Chemical structure of a triglyceride.

There exists a wide variety of vegetable oils, which differ in the type of fatty acids. Some of the vegetable oils contain unsaturated fatty acids, which means that they have one or more carbon–carbon double bonds (unsaturated bonds). These bonds can be used for the production of a thermoset material because they can react with a crosslinker. Vegetable oils can be classified as: r drying oils, containing a large number of unsaturated bonds, like linseed oil, which can even react with the oxygen from the air; r semi-drying oils, containing a medium amount of unsaturated bonds, like soybean oil; r non-drying oils, which contain a small number of unsaturated bonds, like rapeseed oil. Most relevant for industrial purposes are the drying and semi-drying oils. The properties of the oils can differ widely, and the properties of the resin are therefore strongly dependent on the type of oil used. Oils often used are: linseed oil, castor oil (which contains reactive hydroxyl groups), sunflower oil, soy oil and tall oil (a byproduct of the paper industry). Compared with thermoset systems such as polyester or epoxy resins, the natural oils are not very reactive. To increase their reactivity, the natural oils are often modified with reactive groups, like epoxide, maleate or acrylate groups. Some examples that are commercially available are: r r r r

epoxidised soybean oil (ESO – see Figure 19.5.3), which contains approximately 3–4 epoxy groups; epoxidised linseed oil (ELO – see Figure 19.5.4), which contains approximately 5–6 epoxy groups; AESO (see Figure 19.5.5), which is ESO additionally modified with acrylic acid; PRIPLAST and PRIPOL, which are polyols and polyesterpolyols, used in the polyurethane (PUR) industry.

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Figure 19.5.3

Chemical structure of ESO.

Figure 19.5.4

Chemical structure of ELO.

441

A traditional application of vegetable oils is in alkyd resins (paint), in which case the triglycerides are broken down into monoglycerides and applied in combination with phthalic anhydride (C6 H4 (CO)2 O). There are a number of groups working on the development of resins based on epoxidised soybean oil (mainly in America) or epoxidised linseed oil (mainly in Europe): the ACRES group of Delaware University, USA, the University of Missouri-Rolla, USA, the Biocomposites Centre, Bangor, UK, A&F WageningenUR, Wageningen, the Netherland, and some others. Linseed oil is more expensive but also more reactive, and can therefore yield products with a higher crosslink density and a higher glass transition temperature (T g ). In Europe, much more so than in America, the developments are focused on products that are fully renewable. These developments are focused on application of the resins as coatings and application as structural resins, including composites reinforced with natural fibres. The resin systems at present contain 25–70% renewable resources. The (semi-commercial) resins are relatively cheap: 2–5 €/kg (Molenveld, 2007). 19.5.2.3.2

Furan Resins

Furfuryl alcohol (IUPAC: 2-furanmethanol) is the main ingredient of traditional furan resins, which are crosslinked with urea, formaldehyde or a mixture of both. These resins are mainly used in the casting

Figure 19.5.5

Chemical structure of AESO.

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Figure 19.5.6

Chemical structure of furfuryl alcohol (IUPAC: 2-furanmethanol); furan prepolymer.

industry. Presently, new applications for furfuryl alcohol (see Figure 19.5.6) are as platform chemicals for the production of environmentally benign polyols, solvents and alternative resins for a variety of applications. The new furanic resins can be 70–100% based on renewable resources and are expected to have a price range of 3–6 €/kg (Molenveld, 2007). 19.5.2.3.3

Lactic-Acid-Based Resins

A new approach is taken by JVS Polymers (Espoo, Finland) in the product Pollit. Starting from renewable building blocks, new polymers are produced (Akesson et al., 2006). Pollit is a thermoset resin based on lactic acid methacrylate. The main ingredients are lactic acid (see Figure 19.5.7), itaconic acid and pentaerythritol. Advantages of this system are: r high percentage of renewable resources used; r good mechanical properties in combination with natural fibres (Akesson et al., 2006). This system can be polymerised in different ways: r at high temperatures (170 ◦ C); r at room temperature with peroxides; r via UV curing. 19.5.2.3.4

Renewable Polyurethane Resins

Polyurethane resins are two-component systems based on diisocyanates and polyols. It is an important class of thermoset materials that can vary in properties from flexible to very rigid. The polyols can be replaced very well with polyols from a renewable origin, and many developments in this direction are ongoing. Examples of renewable polyols are glycerol and sucrose (see Figure 19.5.8). The percentage of renewable ingredients is likely to be around 50% (Molenveld, 2007). 19.5.2.3.5

(Semi-)Commercial Resin Systems

At present there are a number of resin systems on the market, even though some are still produced on a small semi-commercial scale. Examples are: r Envirez 5000, produced by Ashland (Ashland Performance Materials, Dublin, OH, USA). This product is marketed especially for John Deere. The resin is approximately 25% renewable.

Figure 19.5.7

Chemical structure of lactid acid (IUPAC: 2-hydroxypropanoic acid).

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Figure 19.5.8

443

Chemical structure of glycerol (propane-1,2,3-triol); sucrose.

r GREEN BMC, produced by Ashland (Ashland Performance Materials, Dublin, OH, USA) in cooperation with BMCI (Bulk Moulding Compounds Inc., West Chicago, IL, USA). r Ebecryl 860, produced by Cytec Surface Specialties Germany GmbH & Co KG, Hamburg, Germany, is an important ingredient for many resin systems. It is an epoxidised soybean oil acrylate that can be crosslinked with maleic anhydride or styrene. r PTP, produced by B.A.M., Ipsheim, Germany, which is based on epoxidised linseed oil and polycarboxyl anhydrides (Sch¨onfeld, 1996). The resin is presently 65% renewable, but development also to produce the anhydrides from a renewable resource could increase the renewability to 96%. r Cara plastics, which are soybean-based resins developed by the University of Delaware, Newark, DE, USA. r BIOREZ, a newly developed furfuryl-alcohol-based resin produced by TransFurans Chemicals bvba, Geel, Belgium.

19.5.3 Durability of Natural Fibres in Polymer Composites 19.5.3.1

Routes to Overcome Biodegradation

Durability of natural fibre polymer composites in a dry environment does not seem to be an issue. Most European car manufacturers have been using natural-fibre-based composites for inside applications such as door trims since the 1990s (see Chapter 19.4), and no serious claims have been made public so far. For outside applications, the situation is different. The first generation of wood plastic composites (WPCs) using a petrochemical-based thermoplastic like PP or PE has been marketed as highly durable and free of maintenance (Morris and Cooper, 1998). The basic idea was that the natural fibres were encapsulated in a polymer that was resistant to environmental attack and therefore immune to weathering, etc. However, even improved compositions, including coupling agents, preservatives and UV absorbers, have shown that the natural fibres remain sensitive to decay (Westin et al., 2008). Proper encapsulation of the fibres makes them less accessible for biodegradation, but not to such an extent that they become fully durable. For that purpose the fibre itself has to be modified to become more durable. The biodegradation of natural fibres is related to the enzymatic activity of microorganisms. These microorganisms feel at home and become active at elevated moisture contents, while natural fibres easily take up water. The mere fact of water absorption already reduces the strength and stiffness performance of natural fibres. Also, uptake of water causes dimensional changes, thus causing cracks in the composite material and making the fibres more accessible for influences from outside. Biodegradation of the fibres causes further reduction in properties. Protection against biodecay is generally obtained (1) by making the fibres less accessible to enzymes, (2) by changing the substrate specific configuration such that enzymes do not recognise the polysaccharide polymers in the fibre anymore, (3) by removing the components most sensitive

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to biodegradation and (4) by making fibres less hygroscopic. Methods to make natural fibres more durable include: r furfurylation; r acetylation; r heat treatment.

19.5.3.1.1

Furfurylation

Furfurylation of solid wood and natural fibres is a recent development to protect natural fibre material. Wood or fibres are impregnated with furfuryl alcohol (see Figure 19.5.6) or prepolymers of the furfuryl alcohol and cured at temperatures around 180 ◦ C. The resin encapsulates the fibres and at the same time occasionally may change the chemical structure of the fibre by reacting with hydroxyl groups of the lignin in the fibre. The mass percentage gain of resin by the fibres (WPG) is ca. 20% up to as high as 70% (Hoydonckx et al., 2007; Westin et al., 2008; Van der Zee et al., 2007). Soil burial trials have shown that furfurylation of wood fibres decreases the speed of decay significantly (Westin et al., 2008). Companies involved in this technology are WPT, Porsgrunn, Norway (wood treatment), and TFC, Geel, Belgium (supplier of furfuryl-alcohol-based resins).

19.5.3.1.2

Acetylation

Acetylation is one of the most frequently evaluated methods for making natural fibrous materials more durable. Many ways to achieve acetylation of natural fibres have been evaluated (Rowell, 1983). One of the more simple routes comprises soaking of the fibre material in acetic anhydride, heating to a reaction temperature of 100–120 ◦ C for 1–4 h and extraction of unreacted acetic anhydride by applying, for instance, vacuum (Rowell et al., 1986; Rana et al., 1997; Hill et al., 1998; Rana et al., 1999; Zafeiropoulos et al., 2002). Chemical reaction with acetic anhydride modifies the fibre cell wall hydroxyl groups (functional group –OH) with acetyl groups (functional group –COCH3 ) so that they become less accessible to enzymes and become hydrophobic. Acetylation is commercially applied by Titan Wood, Arnhem, the Netherlands, for making solid wood more durable. Prior to further use of the modified fibres, the unreacted acetic anhydride must be removed, as cellulose chains are sensitive to degradation at elevated composite processing temperatures in the presence of acids. The maximum degree of acetylation appears to relate to the chemical composition of natural fibres, being higher for fibres with high lignin content. Acetylation is supposed to occur throughout the fibre (Hill et al., 1998). Typical WPG values are 5–20%. Swelling trials show that acetylation reduces the swelling of particle board from 45% to less than 10% after 5 days of soaking in water (Rowell et al., 1986). Biodegradation studies in horticultural soil confirm the significant protective effect of acetylation; in the longer term, however, biodegradation is not avoided (Hill et al., 1998; Westin et al., 2008).

19.5.3.1.3

Heat Treatment

The process of heat treatment to make woody material more durable is the oldest of the three methods discussed. The process usually consists of the following steps: starting material may be dried or fresh fibre material; high temperature kiln drying at 100–130 ◦ C; intensive heat treatment at 180–250 ◦ C for several hours; cooling down and remoisturing (StoraEnso, 2009). To prevent the fibres from burning, some processes use water vapour, others use nitrogen, others use natural oils like rapeseed, linseed or sunflower oil

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Figure 19.5.9 Chemical structures of furfural (IUPAC: furan-2-carbaldehyde), HMF (IUPAC: 5-(hydroxymethyl)-2-furaldehyde) and vanillic acid (IUPAC: 4-hydroxy-3-methoxy-benzoic acid) released during Plato heat treatment process.

(Rapp, 2001). A variation of the process is the Dutch Plato process, which comprises: hydrothermolysis at 160–190 ◦ C under increased pressure for several hours; drying to a moisture content of circa 10%; curing at 170–190 ◦ C for circa 15 h; conditioning for a few days (Rapp, 2001). Commercial introduction has taken a long time because of complications related to large-scale production at high temperature up to 240 ◦ C; adequate heat transfer; keeping the fibrous material from burning; maintaining fibre strength (Rapp, 2001). R
Currently marketed heat-treated solid wood materials include ThermoWood of StoraEnso, Porvoo, Finland, R
Plato Wood of Plato International, Wageningen, the Netherlands, and RETIwood of RetiTech, Paris, France. Soil burial trials have shown that heat treatment of wood fibres decreases the speed of biodegradation significantly (Westin et al., 2008). The Plato wood process appeared also to be applicable for annual fibres like flax fibre. The process comprises a partial hydrolysis of hemicellulose and lignin in natural fibres into lower-molecular aldehydes (containing a terminal carbonyl group C=O) like furfural and hydroxymethylfurfural (HMF) and benzoic acid derivates like vanillic acid at temperatures around/above 160 ◦ C for approximately 30 min (Figure 19.5.9). After drying, the materials are heated to above 150 ◦ C for circa 2 h in order to achieve curing of the freshly formed components into a water-resistant resin (Pott et al., 2000). This process has been developed for flax R
fibres by Ceres, Wageningen, the Netherlands, and the fibres have been called Duralin . Water absorption trials of Duralin and untreated flax–PP show that the treatment decreases the water diffusion rate in the fibres. As a result, the composite’s mechanical performance is retained for a longer period (Stamboulis et al., 2000). All methods described above appear to improve the durability of natural fibres. A clear comparison between the methods described above and their effect on the durability of natural fibres has thus far not been published. Fire retardancy, ultraviolet stabilisation and colour changes may also play a role in the durability of natural fibre composites. These issues may be solved by using suitable additives in the polymer or resin system.

19.5.4 Thermoplastic Composites Chapter 19.3 gives an overview of composite processing techniques for natural-fibre-reinforced thermoplastics and thermosets.

19.5.4.1

Motivation for Biopolymer Composites

Biopolymers are relatively expensive and therefore not the first choice for fibre-reinforced composites, where performance/price ratio is very important. As a result, commercial applications of biopolymer composites are hardly of importance when considered on the basis of market share. However, the research community has addressed a number of topics that are of importance for applications of biopolymer-based composites. The motivations for the research on biopolymer composites include: fluctuating prices for existing synthetic polymers based on fossil oil; CO2 -neutral production of materials to minimise climate change; the possibility of biodegradation to avoid litter. Cheap fibre sources like, for instance, wheat straw may also act as a filler, thus

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Figure 19.5.10 Flexural modulus of natural fibre–PLA composites: ramie ( ), flax (), cotton (). Dotted line represents flax–PP data. Adapted with permission from M.J.A. van den Oever, B. Beck and J Mussig, Natural Fibre-PLA Composites: Processing and Mechanical Properties, Recent Advances in Research on Biodegradable Polymers and Sustainable Composites Volume 1. Copyright 2009, Nova Science Publishers Inc.

reducing biopolymer costs. However, here it must be considered that natural fibres have a more pronounced negative effect on polymer impact strength than, for example, a conventional filler like chalk (Zuiderdam, 2003; Bos et al., 2006). 19.5.4.2

Polylactic Acid (PLA)

The main biopolymer evaluated as a basis for composites is PLA. PLA has properties similar to PS and PET at room temperature. Natural fibres are well able to improve the mechanical performance of PLA (Figure 19.5.10). A similar level of results is reported by other researchers (Bodros et al., 2007). Adequate fibre–matrix adhesion is important in order to obtain effective reinforcement of a polymer matrix by natural fibres, as has been shown by Bos et al. (2006) for flax–PP composites (Figure 19.5.11). Research




Figure 19.5.11 Flexural strength of flax fibre–PP composites versus mass fraction: kneaded flax–PP (
, ), extruded flax–PP (♦, ); solid symbols represent composites with MAPP coupling agent. Adapted from Composites Part A: Applied Science and Manufacturing, 37, H.L. Bos, J. Mussig and M.J.A. van den Oever, Mechanical properties of short-flax-fibre reinforced ¨ compounds, 1591–1604. Copyright 2006, with permission from Elsevier.

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Figure 19.5.12 Storage modulus of injection-moulded PLA-based materials, as determined in dynamic mechanical thermal analysis applying flexural mode.

efforts to enhance fibre–matrix adhesion in natural fibre–PLA composites have not shown reinforcing effects on PLA thus far (Lee et al., 2004; Lee and Wang, 2006; Huda et al., 2008). PLA in the molten state is sensitive to hydrolysis in the presence of water. Natural fibres are hygroscopic, and it appears to be difficult to feed them completely dried to compounding equipment. It has been shown, however, that moisture in natural fibres entering compounding equipment hardly affects the mechanical performance of the composites produced (van den Oever et al., 2009). PLA is in the glassy state at room temperature and has a T g of around 60 ◦ C. At this temperature, the PLA rapidly decreases in stiffness by 2–3 decades, unless a significant amount of the semi-crystalline PLA is in a crystallised state. Semi-crystalline PLA grades exhibit very slow crystallisation rates, though – too slow to achieve sufficient crystallisation during regular industrial processing. If a sufficient level of crystallinity in PLA can be obtained, for instance by annealing at around 100 ◦ C, the PLA or its composites retain their structural integrity up to higher temperatures (Figure 19.5.12). Therefore, research is currently focusing on suitable nucleating agents for PLA. A few small enterprises are presently offering natural-fibre-reinforced PLA compounds for injection moulding purposes, for example Ficotex (Bremen, Germany) and Kareline (Joensuu, Finland) (Ficotex, 2009; Kareline, 2009).

19.5.4.3

Polyhydroxybutyrate (PHB)

PHB-reinforced natural fibre composites are reported to have good fibre–matrix adhesion and adequate reinforcement levels but a low impact strength. The brittle character can be reduced by using hydroxyvalerate copolymers of PHB; however, the strength and stiffness decrease as a result (Gatenholm et al., 1992; Bourban et al., 1997; Mohanty et al., 2000; Keller, 2003; Bodros et al., 2007).

19.5.4.4

Thermoplastic Starch (TPS)

Although thermoplastic-starch-based materials are very water sensitive, they are evaluated for making fibrereinforced composites. Natural fibre–thermoplastic starch composites generally exhibit good fibre–matrix adhesion, and significant strength improvement is reported (Wollendorfer and Bader, 1998; Curvelo

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et al., 2001; Bodros et al., 2007). To modify processing and durability properties, starch for thermoplastic applications is often blended with other polymers such as polycaprolactone and cellulose-based polymers.

19.5.4.5

Cellulose Acetate (CA)

Cellulose acetate (CA) (acetate ester of cellulose) has an acidic character, and its processing requires a huge amount of stabilisers. Compounding trials with natural fibres and CA have resulted in serious degradation (Wollendorfer and Bader, 1998; Wageningen UR, 1995). Cellulose acetate propionate (CAP), however, can be processed well with natural fibres, and these composites are considered for WPC materials (Westin et al., 2008). Cellulose acetate butyrate (CAB) has been evaluated as a matrix for biodegradable composites, the strength performance largely depending on the reinforcing fibre (Glasser et al., 1999; Gindl and Keckes, 2004).

19.5.4.6

Microfibrillar-Cellulose (MFC)-Reinforced Polymers

A new and popular type of composite is that based on cellulosic microfibrils, which are fibres of cellulosic origin with diameters of less than 100 nm and therefore also called nanofibres (see Chapter 19.6). These fibres are reported to have a stiffness of 110–137 GPa and a strength of up to 2000 MPa (Boldizar et al., 1987). After Boldizar et al. first published their results concerning the production and use of such nanofibres in PP, PE and PS polymer composites in 1987, the number of papers published on cellulosic nanofibres increased tremendously. The routes described to produce cellulosic nanofibres range from mere mechanical refining of wood fibres and bast fibres (Zimmermann et al., 2004; Purz et al., 1998) to elaborate chemical hydrolysis, extraction and purification stages in order to obtain cellulose whiskers from relatively pure cellulose sources such as flax, wood and microcrystalline cellulose (Bhatnagar and Sain, 2005; Oksman et al., 2006), or to obtain nanofibres from less pure cellulose sources such as potato tuber (Dufresne et al., 2000). The energy required to produce microfibrils is estimated from theoretical calculations by Chakraborty et al. (2006) to be 28.7 kW h/kg. STFI-Packforsk (2009), making use of enzymes in the refining process, has reported on its website to be producing MFC at 5 kW h/kg. The sources to obtain cellulosic nanofibres may range from wood pulp prepared for paper production to bast fibres (flax, hemp), potato tuber and swede root. Also, bacterial cellulose is evaluated in composites. Polymers used for evaluating cellulosic nanofibre composites are mostly water soluble, as fibrils of <100 nm diameter easily form hydrogen bonds (‘paper bonds’) upon drying. Therefore, composites are most often produced by the solution casting method. The polymers include starch, hydroxypropyl cellulose (HPC) and PVA. The goal is to obtain stronger composites than with natural fibres having microdimensions. This appears very well possible. Nakagaito and Yano (2004) mention bending strength values of up to 400 MPa for a wet-laid, random, in-plane, cellulosic nanofibre non-woven-like structure from wood pulp bonded by 2–10% of phenol–formaldehyde (PF) resin. A further interesting feature of nanofibre composites is their transparency at relatively high fibre contents (Figure 19.5.13) (see Chapter 19.6, Figure 19.6.7).

19.5.4.7

Liquid Wood

A recent development is that of ‘liquid wood’, defined as material based on underivated biopolymers (Burkhardt-Karrenbrock et al., 2001). One type is based on lignin, natural fibres and natural waxes and was developed at Fraunhofer Institut f¨ur Chemische Technologie, Pfinztal, Germany. It is currently being R . produced commercially by Tecnaro GmbH, Ilsfeld-Auenstein, Germany, under the brand name Arboform
Specific characteristics of Arboform include its low thermal expansion coefficient and its unique looks. It is being used in automotive interiors, electronics, precision products, furniture, consumer items and toys.

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Figure 19.5.13 100 µm.

449

Composite film based on 30% cellulosic microfibrils from softwood pulp in potato starch with a thickness of

A second type is based on natural fibres in a matrix of starch or proteins and other resins and is being produced commercially by Fasalex GmbH, Kopfing, Austria, under the brand names Fasal and Fasalex. The properties of Fasalex come close to those of MDF; however, it absorbs less water in the long term. The liquid wood materials can be injection moulded or extruded using conventional equipment.

19.5.5 19.5.5.1

Thermoset Composites Composites Based on Natural-Oil-Based Systems

Some authors have investigated the properties of thermoset composites with both a renewable matrix as well as renewable fibres. The group of R.P. Wool at Delaware University has been very active in the development of oilbased thermoset resins and focuses, among others, on the development of composites with natural fibres (O’Donnell et al., 2004; Williams and Wool, 2000; Morye and Wool, 2005). Williams and Wool (2000) reported experimental composites with hemp and flax, based on a modified acrylated epoxidised soybean oil (AESO, a commercial product – see Figure 19.5.5), which are crosslinked with styrene and divinylbenzene. They used the RTM technique and found good mechanical properties and a good adhesion between fibres and matrix. The water absorption of these composites, however, was high. Besides composites with bast fibres, the authors also produced particle boards with flax shives as fillers. They concluded that these last materials could provide an interesting alternative to the existing urea–formaldehyde particle boards.

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Morye and Wool (2005) reported on hybrid composites with flax and glass fibres and a soybean oil matrix. They found that, with a proper arrangement of the fibres in the composites, a synergistic effect could be reached, resulting in improved flexural and impact performance of the materials.

19.5.5.2

Composites Based on the PTP System

PTP composites have been under development for some time already (Sch¨onfeld, 1996; Sch¨onfeld, 2000; Sch¨onfeld et al., 2005). M¨ussig et al. (2006) describe an experimental body panel for a passenger bus. They compare the standard glass-fibre polyester SMC body panel with a composite made from hemp and R
having a matrix of PTP . The authors conclude that indeed it is possible to make a large body part with the natural system based on SMC technology; the system can be optimised to such an extent that no changes in the technical procedure for component production from semi-finished products will be required. However, the impact resistance of the experimental body part still needs to be improved further. The stiffness is within the range of the standard, and the mass of the part is significantly lower. The authors furthermore show that it is technically feasible to produce a large part from renewable resources, which has a better environmental profile than the fossil alternative (Schmehl et al., 2008). The component has also been successfully tested in an on-road test of a bus in Braunschweig, Germany (M¨ussig et al., 2007). Boquillon (2006) describes a composite made from hemp and PTP resin. He finds good compatibility between the fibres and the composite and thus improved flexural properties, both stiffness and strength. Boquillon concludes that, for this material, industrial applications can reasonably be foreseen. The PTP resin is presently marketed by Polynt GmbH (Miehlen, Germany), with glass fibres as the reinforcing agent, under the trade name Bio-Dur (Polynt, 2009). From the available literature it can be concluded that fully renewable thermoset composites with both a renewable matrix and renewable fibres are still in an experimental phase, although commercial applications might be expected in the coming years.

19.5.6 19.5.6.1

Panel and Boards Composites with High Fibre Content

Panel and boards consist of about 90% fibrous material glued together by resins, and as such can be defined as a composite. The structural or reinforcing element may be in the form of flakes, chips, strands, particles, fibre bundles or fibres. Wood-based panels have been produced since about a century and find main application in the building and furniture industry. The advantage of such panels over solid wood is that a larger fraction of trees can be used to produce building materials, thus reducing costs. During the 1940s, wood-based particle board was developed in Germany in order to tackle the problem of shortage of lumber for plywood. More recently, variations of these boards have been developed, one of which is the widely used medium-density board (MDF). Most commercial boards are glued with petrochemical-based urea–formaldehyde (UF) resin because it is cheapest. Melamine–formaldehyde (MF) and phenol–formaldehyde (PF), also based on fossil oil, are more durable, but are also more expensive than UF. To obtain a cost-performance optimum, MUF resins with a melamine content in the 1–23% range are used (Alexandropoulos et al., 1998). The structures of these building blocks are presented in Figure 19.5.14. The drawback of formaldehyde-based resins is that formaldehyde is a suspected carcinogenic component and may be released from the board after incomplete curing. Therefore, the maximum exposure limits of humans in the workplace environment to formaldehyde has been reduced during the past 30 years (Alexandropoulos et al., 1998). Reduction in formaldehyde emission is the subject of further study.

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Figure 19.5.14 Chemical structures of fossil-oil-based building blocks for binder systems: urea (IUPAC: diaminomethanal), formaldehyde (IUPAC: methanal), melamine (IUPAC: 1,3,5-triazine-2,4,6-triamine) and phenol.

Also, inorganic binders such as gypsum and cement are being applied to produce panels for building applications.

19.5.6.2

Bio-Based Resins

Bio-based resins are currently not being used in commercial board production, but research on bio-based resin for panel and board applications is ongoing. Castor-oil-based, polyurethane-bonded MDF boards have been studied by de Campos and Rocco Lahr (2004). The furan-based Biorez has been evaluated as binder in panels and boards in the EU 6th Framework project Ecobinders (Ecobinders, 2009). A relatively new development comprises the use of a thermoplastic binder for making MDF boards for applications where thermoforming is of interest (Persson, 1994).

19.5.6.3

Non-Wood-Fibre-Based Panels

It was soon discovered that fibrous materials other than wood could also be used for making panels. The first plant for the production of bagasse-based panels was built in 1920 in the USA (Youngquist et al., 1996). Also, boards based on flax shives have been produced for 50 years already (Linex, 2009). During the past decades, research on non-wood-based panels has multiplied, and fibre sources include: all kinds of straw such as from rice, wheat and soy (Fadl et al., 1977; Norford et al., 1999; Karr et al., 2000; Ye et al., 2005); coir pith (Mallari et al., 1991; Viswanathan and Gothandapani, 1999); sunflower stalks (Khristova et al., 1996; Balducci et al., 2008); vine prunings (Ntalos and Grigoriou, 2002); durian peel (Khedari et al., 2003); cotton stalks (Guler and Ozen, 2004); maize and miscanthus stalks (Balducci et al., 2008). A serious disadvantage of non-wood fibre sources is the usually high silica content of above 1% (Youngquist et al., 1996), as machining properties are believed to be negatively affected by such silica fractions (Torelli and Cufar, 1995; Porankiewicz, 2003). This may keep existing wood-based particle board producers from shifting to such non-wood fibre sources. Whereas the potential availability of such non-wood fibre sources is huge (Youngquist et al., 1996), new players in the market would have to face huge investment costs. Investment costs range from $US 30 million for a particle board plant of 200 000 m3 /year capacity in China (Asia Dekor, 2006) to $US 110 million for a plant of 500 000 m3 /year capacity in Turkey (Kastamonu, 2008). During the past years, a few German enterprises have marketed hemp-shive-bonded boards (Kosche, 2006; Sachsenleinen, 2009).

19.5.6.4

Binderless Panels

Hardboard may be produced from steam exploded wood fibre without binders via a wet process. Bonding between the fibres is achieved by hydrogen bonds (‘paper bonds’) and by the lignin present in the wood,

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Table 19.5.1 Processing conditions used for commercial board manufacture and for experimental non-wood-based binderless boards Reference Wood fibre Wood fibre, 6–9% MC Wood fibre Straw Coir pith Rice straw Bagasse, 7–14% MC Coir dust, 18–20% MC Oil palm fronds, 10.5% MC Miscanthus, 7% MC Kenaf core, 8–13% MC Kenaf core, 10–30% MC Coconut husk, 3–47% MC

PB, commercial MDF, commercial Hardboard, commercial Commercial (Wilson, 1995; Yates, 2006) Narayanamurti et al., 1969 Fadl et al., 1977 Mobarak et al., 1982 Mari, 1996 Suzuki et al., 1998 Salvado´ et al., 2003 Okuda and Sato, 2004 Xu et al., 2006 Van den Oever et al., 2010

Pressure in bar 15–40 20–25 ≤50

Temperature in ◦ C

Pressing time in s/mm

150–200 150–200 200–210

4–13 9–12 100–180

200–260 25–210

120–160

570–860

60–270 150–250 25 250–400 19–146 53 30 60–150

210 175–185 150–250 125–150 195–245 140–200 190 160–200

99–111 195–225 100–200 160 24–120 30–100 40–200

which acts as a natural binder. This process was developed early in the twentieth century. The commercial production process usually consists of three stages, the first comprising high pressure to remove water and bringing the board to the desired thickness, the second comprising water vapour removal and the third comprising final consolidation. Binderless boards based on straw were developed in Sweden in the 1920s (Wilson, 1995; Yates, 2006) and marketed later on, up to the present day, as Stramit (Wilson, 1995; Stramit, 2006; Ekopanely, 2009). The straw-based boards are applied for non-structural separation walls, at the same time providing sound insulation. Many other non-wood fibre sources have been evaluated for binderless boards during the past decades, including: coir pith and dust, rice straw, whole bagasse, oil palm fronds, miscanthus, kenaf core and whole coconut husk. The common idea is that the lignin present in these materials acts as a natural binder. The processing basically consists of bringing the raw material in suitable form for the production of an air-laid semi-finished product that subsequently can be hot pressed into a board. The consolidation conditions required to obtain boards with a suitable properties profile are generally more demanding than for wood-fibre-based boards, either in pressure, temperature or time applied for consolidation (Tables 19.5.1 and 19.5.2). In particular, the pressure and consolidation time applied for the experimental binderless boards significantly exceeds those of commercial board production. The pressing times are expressed in seconds per mm of board thickness because the time required for adequate heat transfer for curing and consolidation throughout the (resin-bonded) boards is proportional to the thickness of the boards. The platen temperatures are often much higher than required for consolidation or curing of the commercial resin bonded board (Alexandropoulos et al., 1998). For instance, lignin starts to flow under pressure in the temperature range 150–180 ◦ C, and urea-based resins usually cure in the temperature range 100–130 ◦ C. The flow characteristics of a particular lignin also depend on the moisture content (MC) in the fibre. For board materials, the bending strength and stiffness are usually expressed as modulus of rupture (MOR) and modulus of elasticity (MOE) respectively (Table 19.5.2). An important indicator of the performance of boards under wet conditions is the thickness swelling (TS) in water of the board. Some of the binderless boards produced on a lab scale perform remarkably well compared with commercially available particle board, MDF and hardboard.

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Table 19.5.2 Physical and mechanical properties of commercially available boards and of experimental non-wood-based binderless boards Density in kg/m3 MOR in MPa MOE in GPa TS after 24 h in % Wood fibre Wood fibre, 6–9% MC Wood fibre Straw Coir pith Rice straw Basasse, 7–14% MC Coir dust, 18–20% MC Oil palm fronds, 10.5% MC Miscanthus, 7% MC Kenaf core, 8–13% MC Kenaf core, 10–30% MC Coconut husk

PB, commercial MDF, commercial Hardboard S1S, commercial Commercial Narayanamurti et al., 1969 Fadl et al., 1977 Mobarak et al., 1982 Mari, 1996 Suzuki et al., 1998 Salvado´ et al., 2003 Okuda and Sato, 2004 Xu et al., 2006 Van den Oever et al., 2010

650–800 650–800 800–1120 400–420 840–1360 885 1340–1360 450–800 800–1200 810–1320 500–1000 300–500 1160–1390

11–18 25–52 21–34 0.2 15–27 16–23 49–63 1–8 2–20 12–56 3–36 2–20 25–48

3–3.5 2.7–4.8 2.2–2.5 0.9–1

11–21 15–41 24 6–58 15–48 9–23 6–34

1.0–7.4 1.6–7.6 0.8–5.5 0.2–2.4 2.9–5.5

3–30 20–81 9–24 9–22

19.5.7 All-Cellulose Composites Usually, composites are composed of fibres and matrix with different chemical structure. As a consequence, the adhesion is often poor and needs to be improved by applying coupling agents. Composites based on cellulosic fibres and a cellulose matrix have been studied to eliminate the adhesion problem (Nishino et al., 2004). At the same time, the recyclability of such monomaterial composites will improve. The manufacturing procedure is based on differences in solubility of native cellulose and activated cellulose. Cellulose can be dissolved in N,N-dimethyl acetamide (DMAc) with a few percent LiCl after it has been activated previously in DMAc for a certain period (Nishino et al., 2004). The evaluated composites include microcrystalline-based (random in-plane) composites with a tensile strength of 240 MPa and a modulus of 13 GPa (Gindl and Keckes, 2005), ramie-fibre-based unidirectional (UD) composites with tensile strengths of 440–540 MPa and a modulus in the range of 20–28 GPa (Nishino et al., 2004; Qin et al., 2008; Soykeabkaew et al., 2008) and man-made cellulose-fibre-based UD composites with a tensile strength of 150–910 MPa and a modulus of 7–23 GPa (Soykeabkaew et al., 2009). Mercerisation of the ready composite film resulted in translucent films, indicating elimination of residual cracks and voids in the composite (Qin et al., 2008). The produced composite films were reported to have a thickness of 0.2–1 mm, although not all studies have reported the film thickness.

19.5.8

Conclusion

The potential of composites based on natural resources has been explored over the past few decades all over the world. A whole range of thermoset and thermoplastic resins have been developed and are being produced on a commercial scale. The prices of the biopolymers, however, are high compared with fossiloil-based polymer systems. This has resulted in a limited number of applications so far, and some producers of biopolymers have had to fold. A further drawback of bio-based resins, also for panel and board applications, is their limited reactivity. These characteristics have limited the application of biocomposites to niche markets so far. Polylactic acid however seems to have become a commodity, with prices dropping below 2 €/kg. Together with an excellent mechanical properties profile, and the ongoing research to improve the high-temperature properties, PLA has the potential to become a competitor of polyolefin-based composites.

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Whereas natural fibres are sensitive to degradation in a humid environment, they are very durable in regular in-house and in-car conditions. The durability of natural fibres can be extended by thermomechanical or chemical modification, and some composites are even tested for outdoor applications. Currently, many national and EU governments are developing policies to replace fossil resources with biobased resources (for instance, the Lead Markets Initiative for Europe), one of the spearheads being bio-based products. A continued development effort is expected to accelerate the introduction and expand the range of biocomposite applications in the years to come.

References Akesson, D., Skrifvars, M., Sepp¨al¨a, J.V. and Walkenstr¨om, P. (2006) Preparation of natural fibre composites from biobased thermoset resins. 27th Risoe International Symposium: Polymer Composite Materials for Wind Power Turbines, Roskilde, Denmark; available at: http://www.risoe.dk/Risoe dk/Home/Conferences/symp27/∼/media/ Risoe dk/Conferences/AFMsymp27/Documents/40 2006 Presentation Aakesson.ashx (accessed 22 April 2009). Alexandropoulos, D., Nakos, P. and Mantanis, G. (1998) European approach to particleboard and MDF adhesives, in Proceedings of 1998 Resin and Blending Seminar, Composite Panel Association (CPA), Charlotte, NC, USA, 10–11 December 1998, pp. 137–146. R
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