Tribology of Natural Fiber Polymer Composites (Woodhead Publishing Series in Composites Science and Engineering) [2 ed.] 0128189835, 9780128189832

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Tribology of Natural Fiber Polymer Composites

Woodhead Publishing Series in Composites Science and Engineering

Tribology of Natural Fiber Polymer Composites Second Edition

Navin Chand Mohammed Fahim

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2021 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-818983-2 (print) ISBN: 978-0-12-819073-9 (online) For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

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Contents

Preface List of abbreviations 1

Natural fibers and their composites 1.1 Introduction 1.2 Sources of natural fibers 1.2.1 Types of plant fibers 1.2.2 Extraction of fibers 1.3 Surface modification of natural fibers 1.4 Chemical treatments 1.4.1 Silane treatment 1.4.2 Acetylation 1.4.3 Isocyanate treatment 1.4.4 Stearic acid treatment 1.4.5 Graft copolymerization 1.4.6 Duralin treatment 1.4.7 Permanganate treatment 1.4.8 Peroxide treatment 1.4.9 Benzoylation treatment 1.4.10 Maleated coupling agent 1.5 Physical treatment 1.5.1 Mercerization 1.5.2 Corona, cold plasma treatment 1.6 Sisal and jute fiber 1.6.1 Thermogravimetric analysis of untreated and treated sisal fibers 1.6.2 Crystallinity of sisal fibers 1.6.3 Thermal properties of sisal-based hybrid fabric 1.6.4 Spectral characterization of untreated and treated sisal fibers 1.6.5 Thermogravimetric analysis of jute fibers 1.6.6 Spectral characterization of untreated and treated jute fibers 1.6.7 Comparative data on sisal and jute fibers 1.6.8 Crystallinity of treated fibers 1.6.9 Spectral characterization of treated fibers 1.6.10 Morphological characterization of treated fibers

xi xiii 1 1 3 5 11 14 15 15 17 17 18 18 20 20 21 21 22 22 22 23 23 23 25 25 27 28 28 28 31 31 32

vi

Contents

1.7

2

Wood 1.7.1 Thermogravimetric analysis 1.7.2 Spectral characterization 1.8 Bamboo 1.8.1 Thermogravimetric analysis 1.8.2 Spectral characterization of untreated and treated bamboo 1.8.3 Crystallinity 1.9 Cotton 1.9.1 Thermogravimetric analysis 1.9.2 Spectral characterization of cotton 1.10 Flax fiber 1.10.1 Thermogravimetric analysis 1.10.2 Spectral characterization of flax fibers 1.11 Mechanical properties of natural fibers 1.12 Natural fiber polymer composites 1.12.1 Thermoset-based composites 1.12.2 Thermoplastic-based composites 1.12.3 Biodegradable polymers-based composites 1.13 Applications of natural fiber composites 1.13.1 Automotive applications 1.13.2 Construction industry 1.13.3 Rural and cottage industry 1.14 Significance and economics of natural fiber polymer composites 1.14.1 Economic aspects of natural fibers 1.14.2 Cultivation of natural fiber as crops 1.15 Sources of further information and advice References

33 33 33 35 35 35 35 38 38 39 40 40 40 40 43 45 47 48 49 49 52 53 53 54 54 55 56

Introduction to tribology of polymer composites 2.1 What is tribology? 2.2 Origin of friction 2.3 Definition of wear and its classification 2.4 How friction and wear are measured 2.4.1 Contact configurations 2.4.2 Operating parameters 2.4.3 Sliding wear test 2.4.4 Abrasive wear tests 2.5 Mechanical characterization of polymer composites 2.5.1 Impact strength of natural/synthetic fiber-reinforced polymer composites 2.6 Tribology characterization of polymer composites 2.6.1 Friction coefficient 2.6.2 Wear formula 2.6.3 Wear mechanism 2.6.4 Effect of operating parameters

61 61 62 66 68 68 68 70 70 72 74 75 75 75 75 76

Contents

2.6.5 Effect of fiber reinforcement 2.6.6 Effect of filler 2.7 Significance of composites in tribology 2.7.1 Polymer matrix composites References

vii

76 77 79 80 83

3

Sisal-reinforced polymer composites 3.1 Sisal fiber 3.1.1 Advantages and disadvantage of sisal fibers 3.1.2 Chemical composition of sisal fibers 3.1.3 Physical structure of sisal fibers 3.1.4 Mechanical properties of sisal fibers 3.2 Sisal polymer composites 3.2.1 Surface modification of sisal fibers 3.2.2 Sisal polyester composites 3.2.3 Sisal epoxy composites 3.2.4 Sisal phenolic composite 3.2.5 Sisal polyethylene composite 3.3 Mechanical properties of sisal polymer composites 3.3.1 Sisal thermoset composites 3.3.2 Sisal thermoplastic composites 3.4 Tribological behavior of sisal polymer composites 3.4.1 Abrasive wear behavior: sisal epoxy composite 3.4.2 Sliding wear behavior: sisal polyester composite 3.4.3 Friction and wear behavior: sisal phenolic composite References

87 87 87 89 89 90 93 93 93 95 96 97 97 97 98 99 99 102 106 109

4

Jute-reinforced polymer composites 4.1 Jute fiber 4.1.1 Advantages and disadvantages of jute fibers 4.1.2 Chemical composition of jute fibers 4.1.3 Physical structure of jute fibers 4.1.4 Mechanical properties of jute fibers 4.2 Jute polymer composites 4.2.1 Surface modification of jute fibers 4.3 Tribological behavior of jute composites 4.3.1 Jute polyester composites 4.3.2 Jute polypropylene composites: effect of coupling agent 4.3.3 Jute epoxy composite: effect of heat treatment References

111 111 112 112 113 113 113 118 123 123 125 129 129

5

Cotton-reinforced polymer composites 5.1 Cotton fiber 5.1.1 Advantages and disadvantage of cotton fiber 5.1.2 Chemical composition of cotton fiber

131 131 131 131

viii

Contents

5.1.3 Physical structure of cotton fiber 5.1.4 Mechanical properties of cotton fiber 5.2 Cotton polymer composites 5.2.1 Cotton polyester composites 5.3 Tribological behavior of cotton polyester composites 5.3.1 Graphite-filled polyester composites 5.3.2 Ultra-high molecular weight polyethylene-filled polyester composite 5.3.3 Lubrication behavior of cotton References

132 134 136 137 146 148

6

Bamboo-reinforced polymer composites 6.1 Bamboo 6.1.1 Advantages and disadvantages of bamboo 6.1.2 Physical properties of bamboo 6.1.3 Chemical composition of bamboo 6.1.4 Mechanical properties of bamboo 6.2 Bamboo polymer composites 6.2.1 Bamboo thermoset composites 6.2.2 Bamboo thermoplastic composites 6.3 Tribological behavior of bamboo and bamboo polymer composites 6.3.1 Abrasive wear behavior 6.3.2 Sliding wear behavior References

163 163 163 164 164 165 166 166 168 169 169 175 176

7

Wood-reinforced polymer composites 7.1 Wood 7.1.1 Advantages and disadvantages of wood 7.1.2 Chemical composition of wood 7.1.3 Physical structure of wood 7.2 Wood plastic composites 7.2.1 Wood flour polyethylene composites 7.2.2 Wood flour polypropylene composites 7.2.3 Mechanical properties of wood flour polypropylene composites 7.2.4 Other polymers 7.3 Tribological behavior of wood flour polymer composites 7.3.1 Abrasive wear behavior 7.3.2 Wear behavior of wood flour epoxy composites References Sources of further information and advice

177 177 177 178 178 179 180 186 187 187 187 187 190 190 191

Industrially significant natural fiber-reinforced polymer composites 8.1 Introduction 8.1.1 Hemp fiber-reinforced polymer composites

193 193 194

8

154 159 160

Contents

ix

8.1.2 Kenaf fiber-reinforced polymer composites 8.1.3 Rice husk filled polymer composites 8.1.4 Date- and oil palm-reinforced polymer composite 8.2 Some critical aspects of machining of natural fiber polymer composites References

195 198 199

9

Green tribology and tribological characterization of biocomposites 9.1 Introduction 9.1.1 Natural fiber composites 9.2 Tribological characteristics of green biocomposites 9.2.1 Nanocellulose fiber-based polymer composites References

207 207 207 208 209 211

10

Molecular dynamics simulation and tribological behavior of polymer composites 10.1 Basic theory of molecular dynamic simulation 10.2 Application of molecular dynamics simulation to understand wear and friction behavior of polymer composites 10.2.1 Potential used in molecular dynamics simulation of polymer composites 10.2.2 Simulation model and protocol 10.2.3 Tribological study References

201 203

213 213 214 214 215 216 217

Appendix: Chemical composition of natural plant fibers

219

Index

221

Preface

Fiber-reinforced polymer (FRP) composites which have been established as one of the most promising modern materials to replace conventional metals and alloys in numerous structural and tribological applications have gained a great deal of attention in the recent years. FRP materials developed using thermoplastic and thermosets such as matrices, natural, and synthetic fibers as reinforcements and organic and inorganic materials as fillers have tremendous potential owing to their high strength-to-weight ratio, tailoring potential, and resistance to wear, corrosion, and impact. Synthetic fibers such as glass, carbon, aramid, etc., have been used largely with both thermosets (epoxy, polyester, phenolics, and vinylester) and thermoplastics (polyetheretherketone, polyethylenes, polyamides, polyimides, polypropylenes, polycarbonates, etc.) to develop FRP materials. Natural fiber-reinforced composites developed using plant fibers obtained from plants such as sisal, cotton, sunhemp, jute, hemp, flax, banana, coir, etc., have attracted the attention of the manufacturers because of their high strength, low cost, and biodegradability. This book focuses on the science and technology of natural fiber polymer composites vis-a`-vis recent developments and advances in the field. The main thrust is on explaining the behavior of these composites while keeping specific application areas in mind. The book particularly covers the potential use of these composites in structural and tribological applications. It consists of seven chapters dealing with the availability and processing of natural fiber composites and their structural, thermal, mechanical, and tribological properties. Details of the chemical composition of natural plant fibers are given in the Appendix at the end of the book which is intended to serve as a quick reference for the readers. The first chapter introduces various types of natural fibers, composites based on them, and their applications in various fields. Natural fibers, extracted from plants (leaves, roots, stems, fruits, seeds, etc.) are noncarcinogenic, lightweight, strong, biodegradable, and economically viable. By virtue of their high performance properties, such as high modulus and tensile strength, they are poised to replace expensive synthetic fiber reinforcements, such as glass, carbon, aramid, organic, and mineral fillers in FRP composites. Plant fibers, which have a long history of conventional and traditional use to make baskets, clothing, sacks, ropes, rugs, etc., have made headway in developing FRP materials for tribological and structural applications. For instance, plant fibers such as kenaf, hemp, flax, jute, and sisal are making their way into the interiors and upholstery of cars. Natural fiber composites of thermoplastics and thermosets have been developed to make door panels, seat backs, headliners, package trays, dashboards, and trunk liners in passenger cars. This has been a direct consequence of the strict environmental legislation that bans

xii

Preface

the use of glass fibers in cars and forces manufacturers to replace existing material with biodegradable components. Natural fibers are biodegradable and, when they are reinforced in biodegradable polymers, the disposal of products based on such biocomposites becomes easy. Since the book is focused on the tribological characterization and applications of natural fiber-based polymer composites, a brief introduction to the subject of tribology is taken up in Chapter 2, Introduction to Tribology of Polymer Composites. This will immensely benefit readers who are interested in natural fibers and composites but are not very familiar with the concepts of tribology. The chapter includes the essentials and basic principles of friction and wear characterization of materials. Chapters 3 7 discuss the progress which has been made in the development, characterization, and property evaluation of various polymer composites based on natural fiber reinforcements in the form of sisal, jute, cotton, bamboo, and wood, respectively. Among the plant fibers available worldwide, these are the ones for which tribological characterization has been carried out and which have been identified as potential substitutes as reinforcements in tribocomposites. Emphasis has been placed on the tribological characterization of the natural fiber polymer composites and their structure property correlation. Three new chapters have been added in the revised edition to include the progresses made in the research and applications of industrially significant natural fibers such as hemp, kenaf, flax, oil palm, date palm, rice husk, and polymer composites based on these fibers. Chapter 8, Industrially Significant Natural FiberReinforced Polymer Composites, deals with the tribological behavior of these tribocomposites. Emphasis is laid on discussing the effect of aging and machining of these composites on the tribological behavior since this aspect is critical to their real life applications in tribologically significant situations. Chapter 9, Green Tribology and Tribological Characterization of Biocomposites, deals with the recent developments in Green Tribology and tribological characterization of pure biocomposites while Chapter 10, Molecular Dynamics Simulation and Tribological Behavior of Polymer Composites, deals with the recent developments in the tribological characterization of polymer nanocomposites using molecular dynamic (MD) simulation which can be extended to natural fiber polymer composites. The authors feel that this book will be specifically useful for industry, researchers, academicians, and students who are associated with research and development in the field of tribology involving natural fibers and their composites. A strong need was felt for such a book because the data available from different sources are scattered. Moreover, no book is available dealing with the tribological behavior of such composites and their structure property correlation. Every effort has been made to include as many references as possible. However, some references may have been inadvertently left out, and it will be appreciated if this is brought to the attention of the authors. Authors

List of abbreviations

AFRP AN BP CCA CF CFRP CMC CMT DCP DSC DTA DTG EDA ELV FM FRP FTIR FWHM GF GFRP HDPE HM-CFRP HS-CFRP IPN iPP LAOW LDPE LLDPE MAH MAPE MAPP MDF MDI/PMDI MDI MDPE MEMS MF MMC MOE MOR

aramid fiber-reinforced plastic acrylonitrile benzoyl peroxide chromated copper arsenate carbon fiber carbon fiber-reinforced plastic ceramic matrix composite compression molding technique dicumyl peroxide differential scanning calorimetry differential thermal analysis derivative thermogravimetry ethylenediamine end-of-life of vehicles flexural modulus fiber-reinforced polymer Fourier transform infrared full width at half maximum glass fiber glass fiber-reinforced plastic high density polyethylene high modulus carbon fiber-reinforced plastic high strength carbon fiber-reinforced plastic interpenetrating network isotactic polypropylene low amplitude oscillating wear low density polyethylene linear LDPE maleic anhydride maleic anhydride-grafted polyethylene maleic anhydride-grafted PP medium density fiber polymeric diphenylmethane diisocyanate methylene diphenyl diisocyanate medium density polyethylene microelectromechanical systems melamine-formaldehyde metal matrix composite modulus of elasticity modulus of rupture

xiv

NFRP PA PAI PCL PE PEEK PEI PET PF PHBV PI PMC PP PPE PPS PS PTFE PVA PVC RH RMT RTM SEM SFRP TG TGA TM TS UCS UD UF UFS UHMWPE UTS WAXD WF WPC WPG XLPE

List of abbreviations

natural fiber-reinforced plastics phthalic anhydride polyamideimide polycaprolactone polyethylene polyetheretherketone polyetherimide polyethylene terephthalate phenol-formaldehyde polyhydroxybutyrate-valerate polyimide polymer matrix composite polypropylene poly(phenylene ether) polyphenylene sulfide polystyrene polytetrafluoroethylene poly(vinyl alcohol) polyvinylchloride relative humidity roller mill technique resin transfer molding scanning electron microscope short fiber-reinforced polymer thermogravimetry thermogravimetric analyses tensile modulus tensile strength unconfined compressive strength unidirectional urea-formaldehyde ultimate flexural strength ultra-high molecular weight polyethylene ultimate tensile strength wide angle X-ray diffraction wood flour wood plastic composite weight percent gain cross-linked polyethylene

Natural fibers and their composites

1.1

1

Introduction

The oft-repeated maxim that there is sufficient for everybody’s need but not for their greed appears to be literally true for the natural resources available in abundance across the world. Had there been an optimum utilization of these resources, the threat to the ecological system together with other environmental-related concerns would not have reached such alarming proportions. Today, with the fast pace of industrialization and large-scale housing construction in urban areas spreading at an ever faster rate, these natural resources are depleted and forest cover is shrinking. There is an urgent need to maintain a balance between growth of human settlement and exploitation of natural resources, in particular the depletion of forests. The harmony between human settlement and natural vegetation, flora and fauna is a law of nature; and the long history of human existence has been a witness to the stark reality that whenever this harmony is disturbed it has given rise to problems of dangerous proportions. Conservation of metals is one such major issue. Metals are extracted from naturally occurring minerals and ores, found in the earth’s crust, through long, tedious, and expensive metallurgical processes. Continuous exploitation of these natural resources has become detrimental to the existence of large reserves. Consequently, concerted efforts are being focused on developing materials that can be a suitable alternative, if not a perfect substitute, for metals and alloys. From time immemorial, metals have been an integral part of human life, ranging from their use in household items to their large-scale applications as building materials in the construction and transport industries, aircraft structures, ship building, and the defense and automobile industries. The replacement of metals would have been a distant dream but for the rapid progress in the development of materials such as glass, polymers, ceramics, synthetic fibers, and numerous organic and inorganic substances which has proved to be a turning point. All these materials have unique property profiles and possess outstanding characteristics. Even more amazing is the fact that when combined together to form composites, they offer a plethora of useful properties. The shortcomings of one ingredient are compensated by other ingredients. For instance, fiber-reinforced polymer composites filled with inorganic fillers have found large-scale applications in almost every field of engineering (Table 1.1) [1]. Composites, defined as multiphase systems that consist of at least two different groups of materials, which are chemically and physically distinct and separated by interfaces, have generated a great deal of interest. Their impact on socioeconomic structures has been so immense that composite technology has Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00001-3 © 2021 Elsevier Ltd. All rights reserved.

2

Tribology of Natural Fiber Polymer Composites

Table 1.1 Applications of composites in different areas of engineering [1]. Field

Composites

Applications

Mechanical engineering

MMCs, CMC, PMCs

Automobile engineering Civil engineering

PMCs, MMCs

Turbines, pump blades, gears, bearings, seals, machine components, cutting tools and tool bits Aircraft components, brakes, and tyres

Medical engineering

PMCs, CMCs

Aerospace engineering

Carbon
carbon composites, PMCs, MMCs PMCs, CMCs

Electrical engineering Miscellaneous

PMCs

PMCs, MMCs, CMCs

Conveyors, radomes, building materials, panels, cabinets, overhead tanks, and storage tanks Biomedical applications, orthopedics, prosthetics, joint prostheses, hip and joint replacements, and dentistry Seals, bearings, and brakes

Semiconductors, piezoelectric transducers, bushes, electrodes, and turbocharger rotors Agricultural and mining equipment, sports equipment, boats, magnetic tape recording, and magnetic optical data storage

become an active area of research and development across the world. No sector has remained untouched by the benefits of composites. From household items, toys, and sports equipments to the construction industry and aircraft structures, composites have occupied every possible inch. Composites consist of one or more discontinuous phases embedded in a continuous phase. The discontinuous and continuous phases are termed as reinforcement and matrix, respectively. The type and reinforcement geometry provide strength to the matrix, and the resultant composite develops properties such as high-specific strength, stiffness, and hardness, which are much more than the individual components. For this reason, composites are often termed as tailored materials because judicious choice of matrix, reinforcing elements, and processing techniques allow properties to be achieved as per the requirement. Nowadays, the field has grown so much that a range of inorganic and organic fibers/fabrics, chemically coated fiber/ fabrics, matrices (polymeric, ceramic), fillers, and a host of processing techniques are available that have rendered possible the realization of any type of composite. You name it and you can find it in the market [2]. As fast as the field of composites has grown, the problems related to them have caught up even faster, one of these being disposal. This is because they contain toxic ingredients and are difficult to recycle. They neither degrade nor decompose on their own and pose a threat to the surrounding ecological system. Due to this issue of environmental pollution, there is an increasing demand to develop composites that are biodegradable. Fibers are an important and integral

Natural fibers and their composites

3

part of the composite industry. Various inorganic (glass) and organic (carbon, graphite, aramid, and polymer) fibers can be used to develop lightweight, high strength, and high modulus polymer composites. Apart from these synthetic fibers, plant-based fibers, now known as natural fibers, are also used to reinforce polymers. A great deal of research is being focused on extracting fibers from plants and modifying them artificially so that they become compatible with the polymeric matrices. Common plant fibers, also known as lignocellulosic fibers, are obtained from plants like sisal, jute, cotton, banana, hemp, ramie, flax, linen, bamboo, wood, coir, etc., some of which are shown in Fig. 1.1. The main advantages of these fibers are that they are biodegradable, economical, and available in abundance, and offer properties comparable with those of synthetic fibers. Another advantage is that they provide employment to thousands of people living in the rural areas where the extraction of fibers has grown into a fullyfledged industry. For instance, the coastal states of India like Kerala, where naturally occurring coconut plantations are abundant, have become major suppliers of coir fibers (Fig. 1.2). When these lightweight, noncarcinogenic plant fibers are reinforced in polymeric matrices or biodegradable polymers, their utility is much increased. Natural fiber-reinforced polymer composites have found large-scale applications in the automotive industry and as building material in low-cost housing, as will be discussed in the subsequent sections.

1.2

Sources of natural fibers

Natural fibers are generally classified in the literature as being derived from plant, animal, or mineral sources according to their origin [3]. Plant fibers are composed of cellulose. Common examples include cotton, linen, jute, flax, ramie, sisal, and hemp. These fibers are extracted from the fruits, seeds, leaves, stem, and skin of plants. Hence they are categorized as seed fiber (collected from seeds or seed cases, e.g., cotton and kapok), leaf fiber (collected from leaves, e.g., sisal and agave), bast fiber or skin fiber (collected from the skin or bast surrounding the stem, e.g., jute, kenaf, hemp, ramie, rattan, soyabean, vine, and banana fibers), fruit fiber (collected from the fruit of the plant, e.g., coconut and coir fiber), and stalk fiber (stalks of the plant, e.g., straws of wheat, rice, barley, and other crops including bamboo, grass, and tree wood). The most widely used natural fibers are cotton, flax, and hemp, although sisal, jute, kenaf, and coir are equally popular. Fibers obtained from animals are composed of proteins. Common examples include silk, wool, mohair, and alpaca. These fibers are obtained from hairy mammals such as sheep (wool), goat (alpaca, cashmere), horse, etc. Silk fibers are obtained from the dried saliva of silkworms during the preparation of cocoons. Fibers are also obtained from bird feathers and are known as avian fibers. Mineral fibers are naturally occurring fibers such as asbestos, while ceramic fibers include glass fibers (glass wool and quartz), aluminum oxide, silicon carbide, and boron

4

Tribology of Natural Fiber Polymer Composites

Figure 1.1 Sources of plant fibers: (A) sisal; (B) cotton; (C) jute; (D) bamboo; (E) wood; (F) coir; and (G) banana.

carbide. Metal fibers include aluminum, brass, steel, etc. Since the main focus here is on plant or vegetable fibers and composites derived from them, more detail on animal (protein) and mineral fibers will not be given as these are beyond the scope of this book.

Natural fibers and their composites

5

Figure 1.2 Landscape showing coconut plantation in the coastal state of Kerala.

1.2.1 Types of plant fibers A general classification of natural lignocellulosic fibers obtained from different parts of plants is given in Table 1.2. Among these, the most widely used plant fibers include cotton, sisal, jute, bamboo, and wood. A brief outline of the sources of these and other fibers is given below. Subsequent sections discuss the methods of extraction for a number of plant fibers, available chemical and physical treatments, thermogravimetric, spectroscopic, structural, morphological, and mechanical properties together with natural fiber polymer composites and their applications [4]. Details of these fibers and their properties are given in the respective chapters devoted to the individual fibers.

1.2.1.1 Jute Jute fiber is produced from plants in the genus Corchorus, family Malvaceae. Jute is a lignocellulosic fiber that is partially a textile fiber and partially wood. It falls into the bast fiber category (fiber collected from bast or skin of the plant). The chemical composition of jute fiber includes cellulose (64.4%), hemicellulose (12%), pectin (0.2%), lignin (11.8%), water soluble (1.1%), wax (0.5%), and water (10%). Jute fiber consists of several cells. These cells are formed out of crystalline microfibrils based on cellulose, which are connected to a complete layer by amorphous lignin and hemicellulose. Multiples of such cellulose and lignin/hemicellulose layers in one primary and three secondary cell walls stick together to form a multiple layer composite. These cell walls differ in their composition (ratio between cellulose and lignin/hemicellulose) and in the orientation of the cellulose microfibrils.

Leaf

Pineapple Sisal Agava Henequen Curaua Banana Abaca Palm Cabuja Albardine Raphia Curaua´

Bast

Hemp Ramie Flax Kenaf Jute Mesta Urena Roselle

Cotton

Seed

Kapok Loofah Milk weed

Pod Fibers

Table 1.2 Classification of natural fibers and their sources.

Coir

Husk

Oil palm

Fruit

Rice Oat Wheat Rye

Hulls

Kenaf Jute Hemp Flax

Core

Wheat Oat Barley Rice Bamboo Bagasse Corn Rape Rye Esparto Sabai Canary grass

Grass/reeds

Wood Roots Galmpi

Other

Natural fibers and their composites

7

1.2.1.2 Sisal Sisal fiber is extracted from the leaves of the plant Agave sisalana. In India four varieties of sisal plants are found—Sisalana, Vergross, Istle, and Natale. Different varieties of plants have different yields of fibers. Leaves from the first two varieties yield more fibers than those from the other two. The fiber content also varies with age and source of the plant. The chemical composition of the leaf is moisture (87.25%), fiber (4%), cuticle (0.75%), and other dry matter (8%). The length of the fiber varies from 60 to 120 mm. The ends of the fiber are broad and blunt. Cells which are angular in shape are normally 500
6000 μm long and 5
40 μm wide, giving a maximum aspect ratio (length/diameter of fiber) of 150. Calcium oxalate crystals are present in parenchyma circular and are often found packed with tiny globules. The sisal leaf contains mechanical, ribbon, and xylem fibers (discussed in detail in Section 1.2.2.1).

1.2.1.3 Cotton Cotton belongs to the botanical genus Gossypium and is a member of the subtribe Hibisceae, family Malvaceae. Cotton fiber grows around the seeds of the plant and consists of pure cellulose. The typical arrangement of cellulose gives cotton fibers strength and absorbent characteristics. The fibers are made up of a number of layers whose structure resembles the coils in a spring.

1.2.1.4 Bamboo Bamboo belongs to the grass family Poaceae, subfamily Bambusoideae, and tribe Bambuseae. It consists of cellulosic fibers embedded in a lignin matrix. Two types of cells exist in bamboo—matrix tissue cells (leptodermous) and sclerenchyma cells which are enveloped in the matrix tissue. Vascular bundles made up of sclerenchyma cells act as reinforcement in bamboo. A vascular bundle is made up of several phloem fibers, and a phloem fiber consists of several layers of pillar fibers. Microfibers in each layer of the pillar fibers are spirally arranged at a fixed spiral angle, which varies for different layers of pillar fibers. The main chemical constituents of bamboo are cellulose, hemicellulose, and lignin. Hemicellulose and cellulose, present in the form of holocellulose in bamboo, contribute more than 50% of the total chemical constituents present in bamboo.

1.2.1.5 Wood Wood is derived from shrubs and trees. It is a heterogeneous, hygroscopic, cellular, and anisotropic solid material composed of cellulose (40%
50%) and hemicellulose (15%
25%) held together by lignin (15%
30%). It is commonly classified as either softwood (e.g., pine) or hardwood (e.g., oak). Softwood species comprise wood cells, mostly of one kind, tracheid, that make the material more uniform in structure than hardwoods. There are no vessels (pores) in softwood. Hardwoods are more or less filled with vessels.

8

Tribology of Natural Fiber Polymer Composites

1.2.1.6 Banana Banana belongs to the genus Musa, family Musaceae. This plant is usually referred to as a tree but is actually a giant herb, whose trunk or stem is composed of overlapping leaf bases, which sheath it. Each stem fruits only once, being replaced by new suckers which, in turn, flower, fruit, and die. It is a tropical plant which grows in sheltered positions. Banana plants are identified by large paddle-shaped leaves with a thick midrib. Due to their length and tendency to eventually droop, the edges of the leaves tend to fray. Banana grows wild as well as being cultivated as a field crop and also as a backyard crop in households. Banana plants are available throughout the tropics from 300 N to 300 S of the equator. The banana fibers, which are used for making products like mats, bags, bins, etc., are extracted from the trunk of the plant. There are a number of Musa varieties cultivated for the fruit, but only four varieties yield fiber— Sentuluvan, Aethalpalal, Rasagatali, and Palayankottai. The Senthulavan variety is red in color and yields a reddish brown colored fiber, while the other three yield white colored fiber. These varieties are mainly cultivated in the Southern Indian belt in Kanyakumari, Kerala, Tamil Nadu, and Tirunelveli. Blending the fiber with silk and polyester creates an interesting and elegant fabric which is a smooth, silky, lightweight, accommodating, and pleasant to work with. Soft fiber of various types is blended with cotton, wool, and synthetic fibers for decorative items like wall hangings, table mats, ladies bags, flower vases, children caps, lamp shades, etc. Moderately soft fiber is used for making single-ply and multiply twines, hessian cloth, linoleum backings, theatrical canvas and utility items like kitbags, tool bags, grocery bags, etc. Coarse fibers are used to make carpets, doormats, lime and cleaning brushes, and cushions.

1.2.1.7 Coir Coir fiber is obtained from the fruits of coconut tree (species Cocos nucifera). The outer layer of the fruit is called the husk. The husk (exocarp) of the coconut consists of a smooth waterproof outer skin (epicarp) and fibrous zone (mesocarp). The mesocarp comprises strands of fibrovascular bundles of coir embedded in a nonfibrous parenchymatous connective tissue. Coir is mainly used to develop an array of products such as doormats, rugs, yardages, floor mats, and geotextiles. As a result, the coir industry has become the largest source of employment in many parts of coastal India, employing nearly 400,000 workers in fiber processing, spinning of coir yarn, and manufacture of coir fabrics. The processing and spinning are spread across the entire coastal belt. Coir has been combined at the fiber stage with materials such as sisal fiber, banana fiber, etc., to increase the strength and pliability of the end product. As reinforcement in polymeric composites, coir fibers have demonstrated a great deal of potential. However tribocharacterization of these coir composites is still in the formative stages.

1.2.1.8 Flax Flax (also known as common flax or linseed) is a member of the genus Linum usitatissimum and belongs to the family Linaceae, which consists of 13 genera and

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9

300 species. The word usitatissimum, which means most useful, is appended because of its traditional usefulness as an agricultural crop. The crop is grown as flax, which is taller and has thinner stems with fewer branches as compared with the other member, linseed, which is characterized by short, thick stems with more branches. Flax is a self-pollinated crop, and is among the oldest of the cultivated plants that grow in temperate regions. Both the seeds and the fibers extracted from this plant have always been in great demand. Flax fibers assume significance as an important textile raw material due to their excellent properties. These fibers belong to the group of bast (stem) fibers together with jute, ramie, and hemp. Flax fibers consist of 70% cellulose and 30% noncellulose compounds such as hemicellulose, lignin, pectin, waxes and fats, mineral salts, natural coloring matter, and watersoluble compounds.

1.2.1.9 Hemp Hemp is a strain of the Cannabis sativa plant species. This is one of the fastest growing plants from which fiber can be extracted for use in a variety of industrial applications. Hemp is a bast fiber and is extracted by separating hurd and bast fiber using decortication. Hemp stalks are water-retted first before the fibers are beaten off the inner hurd either manually (scutching) or by using crushing rollers/brush rollers/hammer-milling, wherein a mechanical hammer mechanism beats the hemp against a screen until hurd, smaller bast fibers, and dust fall through the screen. The plants are cut at 2
3 cm above the soil and left on the ground to dry. The cut hemp is laid in swathes to dry for up to 4 days. This is followed by water retting (the bundled hemp floats in water) or dew retting (the hemp remains on the ground and is affected by the moisture in dew and bacterial action). The quality of hemp fiber depends greatly on whether the crop was grown primarily for textiles. A crop grown specifically for fiber generally yields a higher quality product. The time at which industrial hemp is harvested is also crucial in the quality of the fiber produced. Hemp grown specifically for this purpose is usually harvested 70
90 days after seeding; early in the flowering stage and well before seed is produced. The short fibers produced during the separation process are known as tow and the long fibers are called line fiber. Line fiber can be incredibly long—depending on the height of the plant, a single strand may be up to 5 m in length. Once separated, the line fiber is cleaned and carded to size, cut, and bailed; ready to be further processed and spun. The tow fiber is just compressed and bailed. Hemp tow is used for stuffing or coarse yarn spinning, while line fiber is used for higher-end applications such as clothing fabrics, furnishing, and floor coverings. Hemp is regarded superior to other natural fibers, both in terms of cultivation and application since it requires fewer inputs to grow and less water is needed than cotton. Industrial hemp is pest and disease resistant; partly due to the fact it grows so fast. Low lignin levels enable environmentally friendly bleaching without the use of chlorine. Far more fiber can be harvested from hemp than cotton or flax using the same amount of land. It is a good insulator, possesses antibacterial properties, excellent breathability, high abrasion resistance, and superior UV blocking attributes. Hemp fibers can easily be

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Tribology of Natural Fiber Polymer Composites

blended with other fibers, such as flax, cotton, or silk, as well as polymers to make woven fabrics. Hybrid fabrics comprising fiber glass, hemp fiber, kenaf, and flax has been used to make composite panels for automobiles.

1.2.1.10 Kenaf The botanical name of commonly known kenaf fiber is Hibiscus cannabinus (genus Hibiscus; family Malvaceae). Kenaf is one of the allied fibers of jute and shows similar characteristics. It is an annual or biennial herbaceous plant (rarely a shortlived perennial) growing to 1.5
3.5 m tall with a woody base. Kenaf matures in 100
200 days. The fibers in kenaf are found in the bast (bark) and core (wood). The bast constitutes 40% of the plant. Crude fiber separated from the bast is multicellular, consisting of several individual cells stuck together. The stems produce two types of fiber: a coarser fiber in the outer layer (bast fiber, and a finer fiber in the core. The bast fibers are used to make ropes. The main uses of kenaf fiber have been rope, twine, coarse cloth (similar to that made from jute), and paper. It is also useful as cut bast fiber for reinforcing polymers to form composites.

1.2.1.11 Okra The botanical name of commonly known okra fiber is Abelmoschus esculentus, (family mallow). The plant is cultivated in tropical, subtropical, and warm temperate regions around the world. This is a bast fiber obtained from the stem of the plant and has industrial uses such as the reinforcement of polymer composites.

1.2.1.12 Rice husk Rice husk (RH) is an agricultural waste. It is the outer hard protective covering which surrounds the paddy grain and accounts for 20%
25% of its weight. It is removed during rice milling. During milling of paddy about 20% of the weight of paddy is received as husk. It is formed from hard materials, including opaline silica and lignin. It has been used as building material, fertilizer, insulation material, or fuel. Combustion of rice hulls produces rice husk ash (RHA) which is a potential source of amorphous reactive silica. Most of the ash is used in the production of Portland cement. The ash is a very good thermal insulation material. Rice hulls are also a source of silicon carbide whiskers which are used to reinforce ceramic cutting tools that increases their strength manifold.

1.2.1.13 Date palm and oil palm fibers Date palm (phoenix dactylifera), tree is mostly found in countries such as Iran, Iraq, Saudi Arabia, and the United States (California). Dates are edible fruits, which are consumed as fresh, dried, or in processed forms. Fibers are extracted from the leaves of date palm tree. Date palm fiber mostly consists of cellulose embedded in the lignin matrix. Date palm leaf is separated into two parts (leaflet and rachis). Cellulose and lignin percentage of date palm fiber are comparable to that of coir,

Natural fibers and their composites

11

hemp, and sisal fiber. However water absorption of date palm fiber is quite less due to low cellulose content compared with coir, hemp, and sisal. The cellulose content in date palm fibers is more than that of lignin, which makes it appropriate for use in automotive applications. Date palm fiber has a lower density compared with other natural fibers which renders composites suitable for automotive applications. Although the density of the date palm is lower, the fiber length and diameter are higher than other plant fibers. Oil palm tree is grown in over 42 countries worldwide and its primary use is the production of palm oil. Malaysia is the world’s largest oil palm producer, accounting for 60% of the world’s supply. Hybridization of oil palm fibers with sisal, jute, and glass fibers provides superior mechanical properties, and oil palm-based hybrid composites are suitable for application in biomedical and automotive industries.

1.2.2 Extraction of fibers The way in which fiber is extracted from the plant crop is a critical factor in determining the yield. Harvesting needs to be done methodically so that the production cost is not affected and the fiber quality is not compromised. The basic techniques of extraction of some natural fibers are briefly discussed below.

1.2.2.1 Sisal Sisal fibers are extracted by microbiological retting, by hand scraping or by using a raspador machine. Green leaves cut from the plant are crushed and beaten by a rotating wheel set with blunt knives (a process known as decortication) then held by mechanical means against the scraping action of the blade, which removes cellulosic material. Decorticated fibers are then washed and dried, either in the sun or by hot air. In order to obtain high quality fibers, fiber recovery takes place immediately after harvesting so that the natural gummy material is retained. If this hardens the separation of fibers becomes difficult. A double retting process makes the extraction of fiber easier [5]. In this process, leaves are removed from the tank when the retting is half complete, dried, and retted again after a few months. Superior quality fibers in terms of luster, flexibility, and strength are obtained after the repeated process. Sisal produced in Brazil accounts for about 50% of the world market. On average, each leaf of sisal plant grown in Brazil contains 700
1400 technical fibers of length 50
100 cm and displaying a horse-shoe cross-section. The fibers are extracted using a primitive machine popularly known as a “periquita” [6].

1.2.2.2 Banana Banana fibers, which are concentrated near the outer surface, are extracted by hand scraping, chemically, by retting, or using raspadors. They can also be extracted by boiling leaf sheaths in sodium hydroxide solution. Hand-stripped fibers are generally of better quality than those obtained by raspador. During the monsoon the

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Tribology of Natural Fiber Polymer Composites

stalks are usually plentiful. The extraction of the fiber from the stripped leaf sheath cut to a size of 0.3
0.4 m long and 0.07 m wide is done by hand scraping using 0.15 m long blunt blades on a soft wooden plank. The pith is then removed continuously until the fibers appear clean. Fresh pseudostems yield fibers which are 1.5% of the pseudostem weight [7]. This method of extraction is carried out by artisans in the cottage industry sector. Fiber extraction by machine has not been very successful in terms of fiber quality. The fibers extracted from the stems after harvesting the banana fruits are found to be stronger than those extracted before harvesting the fruits [7]. During fiber extraction, two or three of the outer sheaths are rejected due to the coarse nature of the fibers from these layers. Similarly, the innermost two or three sheaths are rejected since these contain more pulpy materials which make the extraction of good quality fiber quite difficult. After extraction the fibers are washed thoroughly and hung up in sunlight to dry as soon as they are stripped. The drying period depends on the quality of the pulpy material adhering to the strips. When thoroughly cleaned, about 5 h is required for drying on a normal dry day. These fibers are bright in luster and white in color. Insufficient cleaning and washing, and inadequate drying cause degradation of the fibers due to chemical and biological action and they eventually lose strength and luster. Since the stalks are available in abundance during the rainy season, it is not possible to extract the fibers immediately from all of them. Any delay results in poor fiber quality. Besides India and Ecuador, Brazil is one of the largest producers of banana. The most commonly cultivated species in Brazil is Musa cavendishi [6]. The banana plant has a 3
9 m long stem with a diameter between 200 and 370 mm. This stem consists of different layers containing longitudinal fibers (of about 24% pseudostem). The pseudostem is disposed of after harvest and is used for solid mulching. Banana fibers are extracted manually from the pseudostems using a low-cost fiber extraction process; the average fiber yield is only 1%
2% on a dry weight basis. A mechanical decorticator developed for the extraction of fibers such as caraua is reported to yield about 1.75 kg on a dry weight basis per man hour [6].

1.2.2.3 Coir Coir fibers are obtained from coconuts. A fully matured coconut tree can produce 50
100 coconuts per year, and each fruit takes 1 year to ripen. Tender coconuts, harvested after about 6
12 months on the tree, consist of white fibers while fully mature coconuts consist of brown fibers. The fibrous husks of the fully matured coconuts are separated from the hard shell (dehusking). The fibrous husks are then soaked in water to swell the fibers and make them soft. White fibers are extracted from tender coconuts by retting. The fibrous husks are suspended in water for about ten months during which the microorganisms break down the plant tissues and loose fibers are formed. In Kerala, coconuts are gathered every 45 days and the best crop is collected during the March
May period. Dehusking of these nuts is done manually. Separation of the fibers from the husk is carried out by bacteriological treatment called retting and then by beating the husks. Various methods of retting and

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extraction of the fibers exist today [8]. The process consists of soaking the husks in stagnant water for 3
6 weeks. Sometimes the husks are also buried in basin-shaped pits dug on the banks of backwaters, and the necessary water movement is provided by the ebb and flow of water at the top and percolation of the water from the subsoil. Retting depends on temperature, water, rate of removal of the foul water, and the stresses that husks are subjected to during the process. The fiber quality thus depends on retting. In Kerala, the average fiber yield is about 80 g per nut and an average sized fully matured Kerala nut weighs 1.1 kg (in contrast to bigger, better, and heavier nuts in the Philippines, Sri Lanka, etc.) [9]. The better quality of fiber produced in Kerala is attributed to the short interval of time between the plucking, husking, and removal of the husk to the retting areas, together with a longer period of retting (about 6
9 months). A shorter retting period produces fibers of uniform quality irrespective of the seasonal and environmental variations. The other method of extraction of coir fibers is by decorting of dry husks. Depending on the method of extraction of the fiber, three types of fibers can be obtained, namely (1) mat fibers (longest fibers obtained from retting), (2) bristle fiber (extracted from both ripe and dry husks), and (3) mattress fibers.

1.2.2.4 Flax Flax fiber is extracted from the bast or skin of the stem of flax plant. Flax fibers are arranged in the form of thin filaments, grouped in longitudinal slender bundles distributed circularly around a central wooden cylinder. These bundles are fully embedded into an intermediary holding tissue binding them outwardly to the protective outer skin and inwardly to the inner supporting wooden cylinder. The holding tissue is made up entirely of dynamic cells, having semipermeable membranes. The wooden cylinder, the fibers and the outer skin, in contrast, are totally made up of fixed static cells, having normal permeable membranes. Flax fiber is soft, lustrous, and flexible, stronger than cotton fiber but less elastic. The extraction of the fiber occurs in three stages. First the cortex or bark is removed (decortication). The cortex is then scraped to remove most of the outer bark, the parenchyma in the bast layer and some of the gums and pectins. Finally, the residual cortex material is washed, dried, and degummed to extract the spinnable fiber [10]. The best grades of flax are used for linen fabrics such as damasks, lace, and sheeting. Coarser grades are used for the manufacture of twine and rope. Flax fiber is also a raw material for the high quality paper industry for use in printed banknotes and rolling paper for cigarettes.

1.2.2.5 Cotton Cotton fibers are separated from the seed pods using a cotton gin, which is a machine that separates cotton fibers. It uses a combination of a wire screen and small wire hooks to pull the cotton through the screen, while brushes continuously remove the loose cotton lint to prevent jams.

14

1.3

Tribology of Natural Fiber Polymer Composites

Surface modification of natural fibers

Natural fibers biodegrade because organisms identify the carbohydrate polymers (mainly hemicellulose) in the cell wall while their enzyme systems hydrolyze these polymers into digestible units. Moreover, fibers change dimensions with change in the moisture content because the cell wall polymers contain hydroxyl and other oxygen containing groups that attract moisture through hydrogen bonding. Hemicelluloses are mainly responsible for moisture absorption, but the accessible cellulose, noncrystalline cellulose, lignin, and surface of the crystalline cellulose also play a major role. Strength is also lost as the cellulose polymer undergoes degradation through oxidation, hydrolysis, and dehydration reactions. Photochemical degradation takes place primarily in the lignin component (cellulose is much less susceptible to ultraviolet light degradation). The degraded lignin leads to the erosion of poorly bonded carbohydrate-rich fibers from the surface. As a consequence fresh lignin gets exposed to further degradative reactions. Thus the lignin component leads to char formation which then acts as insulation for composite and prevents further thermal degradation. Since the natural fibers are used as reinforcement in polymer matrix composites, their compatibility with the polymeric matrices is a key factor in controlling the mechanical performance of the composites [11]. The interfacial adhesion between the lignocellulosic fibers and the matrix is generally poor because the fiber, as mentioned in the preceding paragraph, is hydrophilic and the matrix is hydrophobic [12]. Hence the composites exhibit poor mechanical properties. The interfacial adhesion can be improved if either matrix or fibers or both can be modified artificially by chemical or physical treatments. This is achieved by several means, including use of additives, grafting of functional groups on the lignocellulosic fibers or coating fibers with additives that carry suitable functional groups, in order to make the fiber surface more compatible/reactive with the matrix material. The various reactive species that can be used for fiber modification include acetic anhydride, n-alkyl isocyanates, styrene maleic anhydride, and silanes. The other method for promoting interfacial adhesion involves the use of additives that act as coupling agents. The additives include polyesteramide polyol, titanates, and chemicals based on trichloro-s-triazine. These coupling agents activate hydroxyl groups or introduce new moieties that can effectively interlock with the matrix. Coupling agents help in the elimination of weak boundary layers, production of a tough and flexible layer, development of a highly cross-linked interphase region with a modulus intermediate between that of the substrate and that of the polymer, improvement of the wetting between polymer and substrate, formation of covalent bonds with both materials, and alteration of acidity of substrate surface [12,13]. A review of the chemical modifications of natural fibers aimed at improving the adhesion with a polymer matrix is given in the following section. Both matrix and fiber modification are effective tools to improve fiber
matrix interaction. However while the former is quite fast the latter is slower.

Natural fibers and their composites

1.4

15

Chemical treatments

1.4.1 Silane treatment Silane is one of the most important members of the reactive species used for fiber modification [12]. Organosilanes are the main group of coupling agents developed to bond polymer to mineral fibers. The organo-functional group in the coupling agent causes the reaction with the polymer, either by copolymerization or the formation of an interpenetrating network (IPN) [14]. This curing reaction of a silane treated natural fiber enhances the wetting of the resin. Silanes are widely used as coupling agents for natural fiber polymer composites [15
18]. They are represented by a general formula, R
(CH2)n
Si(OR0 )3 where n 5 0, 1, 2, 3, OR0 is the alkoxy group that can be hydrolyzed and R is the functional organic group. The mechanism of silanization is shown in Fig. 1.3A.

Figure 1.3 (A) Scheme showing silane treatment of natural fiber [14]; (B) scheme showing acetylation of natural fiber [61]; (C) scheme showing isocyanate treatment of natural fiber [22]; and (D) scheme showing graft polymerization of natural fiber [29].

16

Figure 1.3 (Continued)

Tribology of Natural Fiber Polymer Composites

Natural fibers and their composites

17

Silane coupling agents may reduce the number of cellulose hydroxyl groups in the fiber
matrix interface. In the presence of moisture, the hydrolyzable alkoxy group leads to the formation of silanols. The silanol then reacts with the hydroxyl group of the fiber, forming stable covalent bonds to the cell wall that are chemisorbed onto the fiber surface [16]. Therefore, the hydrocarbon chains provided by the application of silane restrain the swelling of the fiber by creating a cross-linked network due to covalent bonding between the matrix and the fiber. The reaction schemes are given as follows [16]: CH2 CHSiðOC2 H5 Þ3 ! H2 O ! CH2 CHSiðOHÞ3

3C2 H5 OH CH2 CHSiðOHÞ3

Fiber

OH ! CH2 CHSiðOHÞ2 O

Fiber1H2 O

1.4.2 Acetylation In the acetylation technique, the plant fiber is soaked in acetic anhydride with or without an acid as catalyst. Acetic anhydride is preferred to acetic acid because the latter does not react sufficiently with cellulose. However because acetic anhydride is not a good swelling agent for cellulose, the latter is first soaked in acetic acid and subsequently treated with anhydride at higher temperatures for a period ranging from 1 to 3 h. Soaking the fiber in acetic acid accelerates the reaction. Acetylation of the hydroxy group swells the plant fiber cell wall and reduces the hygroscopic nature of the cellulose fiber. The equation of the acetylation process is shown in Fig. 1.3B. Generally, acetylation involves a reaction introducing an acetyl functional group (CH3COO2) into an organic compound. In the case of natural fibers, acetylation causes plasticization of cellulosic fibers. The reaction involves the generation of acetic acid (CH3COOH) as byproduct which must be removed from the lignocellulosic material before the fiber is used. Chemical modification with acetic anhydride (CH3
C (5O)
O
C(5O)
CH3) substitutes the polymer hydroxyl groups of the cell wall with acetyl groups, modifying the properties of these polymers so that they become hydrophobic [18]. The reaction of acetic anhydride with fiber is shown as [18]: Fiber

OH

CH3

Oð 5 OÞ
O
C ð 5 OÞ 1 CH3 ! Fiber
OCOCH3 1CH3 COOH Acetylation reduces the hygroscopic nature of natural fibers and increases the dimensional and thermal stability of composites [17
21].

1.4.3 Isocyanate treatment In the isocyanate treatment technique, NaOH treated fibers are washed and dried. The fibers are then soaked in carbon tetrachloride (CCl4), a catalyst is added, and the mix is stirred well. The reaction is allowed to continue for a considerable amount of time at a temperature slightly higher than room temperature with

18

Tribology of Natural Fiber Polymer Composites

continuous stirring. Fibers are then purified by refluxing and are finally washed with distilled water and oven dried at 100 C [22]. The complete equation is shown in Fig. 1.3C. An isocyanate is a compound containing the isocyanate functional group
N 5 C 5 O, which is highly susceptible to reaction with the hydroxyl groups of cellulose and lignin in fibers. Isocyanate is reported to work as a coupling agent used in fiber-reinforced composites [19,21,23]. The reaction between fiber and isocyanate coupling agent is shown below [24]:

where, R could be different chemical groups (such as alkyl). Isocyanate treatment has been used to improve fiber
matrix adhesion in natural fiber polymer composites [25
27].

1.4.4 Stearic acid treatment Stearic acid treatment involves the reaction of the hydroxyl group of the fiber with the stearic acid group. It is done to make the fiber surface hydrophobic in order to yield better compatibility. This kind of treatment does not significantly affect the fiber strength. Stearic acid (CH3(CH2)16COOH) in ethyl alcohol solution was investigated in fiber treatment by Paul et al. [19] and Zafeiropoulos [28]. It was reported that this treatment removed noncrystalline constituents of the fibers, thus altering the fiber surface topography. Zafeiropoulos [28] also observed that treated flax fibers were more crystalline than the untreated ones and stearation decreased the fiber surface free energy.

1.4.5 Graft copolymerization Graft copolymerization, involving initiation by free radicals, is one of the most common methods used for the grafting of vinyl monomers onto cellulose [18,29
31]. These free radicals are produced as a result of a reaction of the cellulosic chain in a redox system. In this reaction, the oxidation of the anhydroglucose units occurs along the cellulosic chains and macrocellulosic radicals are generated on the surface of the fiber. In the presence of a monomer, the oxidation is lower than in the case of cellulose alone. In this case, the macrocellulose radicals generated by the initiator are used to carry out the graft copolymerization of the polymer and the degradation of the cellulose is reduced. When the cellulose molecule cracks and radicals are formed, the radicals sites are treated with a suitable solution (compatible with the polymeric matrix), for example vinyl monomer, acyrlonitrile, methyl methacrylate, and polystyrene. The resulting copolymer possesses properties

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19

that are characteristic of both a fibrous cellulose and a grafted polymer [29]. The complete mechanism is shown in Fig. 1.3D.

1.4.5.1 Acrylation and acrylonitrile grafting Acrylation reaction is initiated by free radicals of the cellulose molecule. Cellulose can be treated with high energy radiation to generate radicals together with chain scission [13]. Acrylonitrile [AN, (CH2 5 CH
Cv N)] is also used to modify fibers. The reaction of AN with fiber hydroxyl group occurs in the following manner [30]: Fiber
OH1CH2 ; CHCN ! Fiber
OCH2 CH2 CN Graft copolymerization of AN on sisal fibers was studied by Mishra et al. [31] using a combination of NaIO4 and CuSO4 as initiator in an aqueous medium at temperatures between 50 C and 70 C. Reaction medium, treatment time, initiator, AN concentration and even fiber loading influenced the graft effect. It was found that untreated fibers absorbed the most water and 25% AN-grafted sisal fibers absorbed the least water, suggesting that changes in the chemistry of the fiber surface reduced the affinity of fibers to moisture. It was also found that grafting of chemically modified fibers with 5% AN brought a higher increase in tensile strength and Young’s modulus of fibers than grafting with 10% and 25% AN. The explanation for this was that grafting at low concentration of AN may create an orderly arrangement of polyacrylonitrile units. Mishra et al. [31] also concluded that optimum graft yield was obtained with a treatment duration of 3 h.

1.4.5.2 Impregnation In the impregnation technique, the reinforcing fiber/fabric is impregnated with a polymer that is compatible with the polymer matrix [32
34]. Low viscosity polymer solutions are used for impregnation. For instance, one of the methods used to improve specific properties of natural fiber cell material is chemical impregnation under vacuum or pressure. When cellulose fibers are impregnated with a butyl benzyl phthalate plasticized polyvinylchloride (PVC) dispersion, excellent partitions can be achieved in polystyrene (PS). This significantly lowers the viscosity of the compound and of the plasticator and results in cosolvent action for both PS and PVC [34]. Resins or chemicals that are highly reactive to the hydroxyl groups of cellulose, hemicellulose, and lignin components of wood include epoxies, isocyanates, anhydrides, lactones, and diols. Chemical impregnation also has the potential to reduce the susceptibility of wood to biological degradation. Another widely studied system is the crosslinking of wood via impregnation with formaldehyde in the presence of an acid catalyst [35].

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Tribology of Natural Fiber Polymer Composites

1.4.6 Duralin treatment Duralin treatment (Ceves BV, Netherlands) is generally used to remove hemicellulose and lignin from the flax fiber. It involves three steps—hydrothermolysis, drying, and curing. The Duralin process reduces moisture absorption and biological degradation, and increases the fiber yield. The reduced water absorption is attributed to the extraction of hemicellulose, the network formation through crosslinking of the degradation products of hemicellulose and lignin, and the increased crystallinity of cellulose [36].

1.4.7 Permanganate treatment Permanganate induces grafting reactions between the natural fibers and the polymer matrix. Permanganate such as potassium permanganate (KMnO4) roughens the natural fiber surface and produces mechanical interlocks with the matrix similar to alkali treatment. Hence, the interfacial bonding between permanganate treated natural fiber and matrix is improved. Permanganate is a compound that contains permanganate group MnO24. Permanganate treatment leads to the formation of cellulose radical through MnO23 ion formation. Then, highly reactive Mn13 ions are responsible for initiating graft copolymerization as shown below [24]:

Most permanganate treatments are conducted by using potassium permanganate (KMnO4) solution (in acetone) in different concentrations with soaking duration ranging from 1 to 3 min after alkaline pretreatment [19,23]. Paul et al. [19] dipped alkaline treated sisal fibers in permanganate solution at concentrations of 0.033%, 0.0625%, and 0.125% in acetone for 1 min. As a result of permanganate treatment, the hydrophilic tendency of the fibers was reduced, and thus the water absorption

Natural fibers and their composites

21

of the fiber-reinforced composite decreased. The hydrophilic tendency of the fiber decreased as the KMnO4 concentrations increased. However at higher KMnO4 concentrations of 1%, degradation of cellulosic fiber occurred which resulted in the formation of polar groups between fiber and matrix.

1.4.8 Peroxide treatment Peroxide is a molecule with the functional group ROOR containing the divalent ion O
O. Organic peroxides tend to decompose easily to free radicals of the form RO
which then reacts with the hydrogen group of the matrix and cellulose fibers. For example, the peroxide initiated free radical reaction between polyethylene (PE) matrix and cellulose fibers is shown by the following [19,23]: RO
OR
2RO RO 1 PE
HsROH 1 PE RO 1 Cellulose
HsROH 1 Cellulose PE 1 Cellulose
PE
Cellulose Benzoyl peroxide [BP, (C6H5CO)2O2] and dicumyl peroxide [DCP, (C6H5C (CH3)2O)2] are chemicals in the organic peroxide family that are used in natural fiber surface modifications. In peroxide treatment, fibers are coated with BP or DCP in acetone solution for about 30 min after alkali pretreatment [19,37,38]. Peroxide solution concentration was 6% [18] and saturated solutions of peroxide in acetone were used [37,38].

1.4.9 Benzoylation treatment Benzoylation is an important transformation in organic synthesis [39]. Benzoyl chloride is most often used in fiber treatment. It includes benzoyl (C6H5C 5 O) which is attributed to the decreased hydrophilic nature of the treated fiber and improved interaction with the hydrophobic PS matrix. The reaction between the cellulosic hydroxyl group of the fiber and benzoyl chloride is shown below [40]: Fiber 2 OH 1 NaOH ! Fiber 2 O1 Na1 H2 O Benzoylation of fiber improves fiber
matrix adhesion, thereby considerably increasing the strength of the composite, decreasing its water absorption, and improving its thermal stability. NaOH and benzoyl chloride (C6H5COCl) solutions have been used for surface treatment of sisal fibers [40,41]. It is observed that the thermal stability of treated composites is higher than that of untreated fiber composites. The treatment is found to improve the interfacial adhesion of flax fiber and PE matrix [42]. The fiber is initially alkaline pretreated in order to activate the hydroxyl groups of the cellulose and lignin in the fiber; then the fiber is suspended in 10% NaOH and benzoyl chloride

22

Tribology of Natural Fiber Polymer Composites

solution for 15 min. The isolated fibers are then soaked in ethanol for 1 h to remove the benzoyl chloride and finally washed with water and dried in the oven at 80 C for 24 h.

1.4.10 Maleated coupling agent Maleated coupling agents are widely used to strengthen composites containing fillers and fiber reinforcements [43
45]. Maleic anhydride is used to modify fiber surface as well as polypropylene (PP) matrix to achieve better interfacial bonding and mechanical properties in composites [46
48]. The PP chain permits maleic anhydride to be cohesive and produce maleic anhydride-grafted polypropylene (MAPP). Then the treatment of cellulose fibers with hot MAPP copolymers provides covalent bonds across the interface as shown below. The mechanism of reaction of maleic anhydride with PP and fiber can be explained as the activation of the copolymer by heating (170 C) before fiber treatment and then the esterification of cellulose fiber [49]. After this treatment, the surface energy of cellulose fibers is increased to a level much closer to the surface energy of the matrix. This results in better wettability and higher interfacial adhesion of the fiber. MAPP used as coupling agent for the surface modification of natural fibers significantly improves the mechanical properties, such as Young’s modulus, flexural modulus, hardness, and impact strength of plant fiber-reinforced composites [50,51].

1.5

Physical treatment

1.5.1 Mercerization Alkaline treatment or mercerization (ASTM D1695) is defined as the process of subjecting a vegetable fiber to the action of a fairly concentrated aqueous solution of a strong base to produce great swelling with resultant changes in the fine structure, dimension, morphology, and mechanical properties. Sodium hydroxide (NaOH) is the most commonly used chemical for bleaching and/or cleaning the surface of plant fibers and it also changes the fine structure of native cellulose I to cellulose III, with depolymerization and the production of short length crystallites. The basic fiber properties such as strength and elongation at break can be changed by a suitable choice of mercerization parameters. Mercerization is one of the chemical treatments of natural fibers most commonly used to reinforce thermoplastics and thermosets. The important modification resulting from alkaline treatment is the disruption of hydrogen bonding in the network structure, thereby increasing surface roughness. This treatment removes a certain amount of lignin, wax, and oils covering the external surface of the fiber cell wall, depolymerizes cellulose, and exposes the short length crystallites [30]. Addition of aqueous sodium hydroxide (NaOH) to natural fiber promotes the ionization of the

Natural fibers and their composites

23

hydroxyl group to the alkoxide [16]: Fiber
OH1N2 OH ! Fiber
O
Na1H2 O Thus alkaline processing directly influences the cellulosic fibril, the degree of polymerization, and the extraction of lignin and hemicellulosic compounds [52]. In alkaline treatment, fibers are immersed in NaOH solution for a given period of time. Alkaline treatment not only increases the surface roughness resulting in better mechanical interlocking but also increases the amount of cellulose exposed on the fiber surface, thus increasing the number of possible reaction sites [53]. Consequently, alkaline treatment greatly improves the mechanical behavior of natural fibers, especially fiber strength and stiffness, and hence the mechanical properties of the resulting composite [54].

1.5.2 Corona, cold plasma treatment Corona discharge as well as cold plasma treatment is used for surface oxidation activation [54,55]. This process changes the surface energy of the cellulose fibers and, in the case of wood, surface activation increases the amount of aldehyde groups. A variety of surface modifications can be achieved using different gases. Surface crosslinking could be introduced, surface energy could be increased or decreased and reactive free radicals and groups can be produced.

1.6

Sisal and jute fiber

1.6.1 Thermogravimetric analysis of untreated and treated sisal fibers Thermogravimetric analyses (TGA) show that sisal fibers are thermally stable up to 200 C. Mass loss at this temperature is very small. Beyond this temperature mass loss is large. For temperatures above 300 C, considerable mass loss occurs due to cellulose and hemicellulose decomposition. Both unmodified and chemically modified sisal fibers exhibit similar behaviors, with small variations in mass loss percentage (Table 1.3) [56]. Up to 100 C, the mass loss of sisal fibers is related to water loss associated with fiber humidity. Sisal fibers subjected to mercerization and esterification show smaller water loss, indicating their lower hygroscopicity compared with unmodified sisal fibers. Mercerization results in the partial extraction of hemicelluloses, which are highly hydrophilic and are mainly responsible for water absorption in lignocellulosic fibers, as they are more accessible than the cellulose chains of the crystalline regions. Esterification with succinic anhydride substitutes part of the hydroxyls present for ester groups and introduces carboxylic groups. Hydrogen bonds involving the latter and the remaining hydroxyls groups can be generated, thus decreasing the amount of polar groups available to interact with water. These substitutions lower humidity absorption.

Unmodified Mercerized Esterified Ionized Air-treated

Sisal

2.9 1.0 2.4 2.6

100 C

3.3

3.6 1.9

200 C 10.4 9.3 13.9 8.6

300 C 73.6 56.1 62.4 74.0

400 C

Table 1.3 Weight loss obtained from TGA of sisal fibers and crystallinity index [56].

96.4 82.6 90.7 80.0

500 C

98.8 97.0 98.7 95.8

800 C

57 52 51 58

Crystallinity index, Ic (%)

Natural fibers and their composites

25

1.6.2 Crystallinity of sisal fibers Differential scanning calorimetry (DSC) curves of sisal fibers show the first exotherm at 300 C, which is related to the hemicellulose decomposition occurring at that temperature (Fig. 1.4). The second exotherm (between 400 C and 500 C) (Fig. 1.4) is related to the break of the chemical bonds of the protolignin present in the fiber. The mercerized and esterified sisal fibers show a slight decrease in the temperature of the first exotherm. These fiber treatments decrease the degree of crystallinity (Table 1.3), which increases the proportion of the chains with weaker intermolecular interactions (noncrystalline region) and decreases the degradation temperatures. Comparative studies on TGA of sisal fiber and a popular thermoplastic matrix PP in the temperature range 30 C
650 C show that for sisal fiber dehydration as well as degradation of lignin occurs in the temperature range 60 C
200 C and most of the cellulose is decomposed at a temperature of 350 C (Fig. 1.5) [57]. Derivative thermogravimetry (DTG) curves (Fig. 1.6) comprise three regions. Region I contains a peak at 65 C which corresponds to the heat of vaporization of water from the fiber. Region II contains a peak at about 350 C due to the thermal depolymerization of hemicellulose and the cleavage of the glycosidic linkage of cellulose. Region III contains a peak at about 550 C due to the further breakage of decomposition products of region II leading to the formation of tar.

1.6.3 Thermal properties of sisal-based hybrid fabric Table 1.4 shows the thermal properties of natural fabrics based on sisal and other plant fibers such as jute and cotton and bonded by a polymeric resin [58]. The thermal diffusivity and thermal conductivity of sisal fabric are almost the same irrespective of the direction of the heat flux. This indicates thermal isotropy and less thermal distortion for composites fabricated with sisal fibers in comparison to jute or cotton reinforced

Figure 1.4 DSC curve of (A) untreated sisal fiber; (B) esterified sisal; (C) ionized air-treated sisal, and (D) mercerized sisal [56].

26

Tribology of Natural Fiber Polymer Composites

Figure 1.5 Comparative TGA thermogram of sisal fiber and polypropylene [57].

Figure 1.6 DTG curve of sisal fiber and polypropylene [57].

ones, since the mismatch of the thermal properties between fiber and matrix is small. For jute composites, thermal diffusivity and thermal conductivity values were higher than those of the resin matrix, and vary with the direction of the heat flux. The thermal conductivity is higher for both composites in the direction parallel to the fabrics while the thermal diffusivity was higher in the direction perpendicular to the fabrics.

Natural fibers and their composites

27

Table 1.4 FTIR peak assignments of jute fibers [61]. Peak assignment

Wavenumber (cm
1) (untreated jute fibers)

Peak assignment

Wavenumber (cm
1) (acelytated jute fibers)

OH stretching

3200
3600

3471

CH2 and CH3 stretching Carbonyl (CQO) stretching of carboxylic acid or ester H
O
H bending of absorbed water Benzene ring stretching (lignin) Benzene ring stretching (lignin) CH2 bending in lignin CH2 and CH3 bending CH3 bending CH2 wagging (lignin) C
O stretching of acetyl (lignin) C
O
C antisymmetric bridge stretching in cellulose and hemicellulose O
H association in cellulose and hemicellulose C
O stretching in cellulose, hemicellulose, and lignin β-glucosidic linkage Out-of-plane OH bending

2905 1740

C
H stretching (with intermolecular hydrogen bonding) C
H stretching Carboxylic anhydride

2920 1743

C
H bending C
H bending

1384 1248

C
C stretching

1059

1650 1616 1500 1440 1411 1357 1320 1235 1150

1110 1028 879 600

1.6.4 Spectral characterization of untreated and treated sisal fibers Fig. 1.7 shows the Fourier transform infrared (FTIR) spectra for untreated and treated sisal fibers. With mercerization, a reduction of some vibrations with respect to untreated fibers is observed [59]. Thus the band located in the 1245
1268 cm21 region associated with the C
O ring of lignin diminishes, and the band at 1740 cm21 related with carboxylic groups present in pectins and hemicellulose almost disappears. Other important changes are related to the reduction of peaks between 3200 and 3600 cm21 that correspond to the reduction of some hydroxyl groups associated with the vibration modes of chemical substances as carbohydrates and fatty acids present in the fibers. The FTIR spectrum of silanized sisal fibers presents a peak at 840 cm21 corresponding to Si
C, which is characteristic of silanol, thus indicating that the silane has reacted on the fiber surface.

28

Tribology of Natural Fiber Polymer Composites

Figure 1.7 FTIR spectra of (A) silanized; (B) mercerized, and (C) untreated sisal fiber [59].

1.6.5 Thermogravimetric analysis of jute fibers Derivative thermogravimetric curves (Fig. 1.8) and DSC curves (Fig. 1.9) of jute fibers show two exothermic peaks indicating decomposition of jute fibers in two steps. The first decomposition peak at 301 C is attributed to the hemicellulose decomposition and the second exotherm at 361 C is attributed to α-cellulose decomposition. In the acetylated jute fibers the peak for hemicellulose decomposition and cellulose decomposition appears at 373 C. It is inferred that the thermal stability of hemicellulose increases due to acetylation. The decomposition temperature also increases from 363 C to 373 C, indicating increase in the thermal stability of acetylated fibers. Furthermore, the acetylated material does not contribute to char formation. The DSC thermogram of jute fibers indicates the decomposition at 363 C due to the decomposition of α-cellulose. For acetylated jute fibers this peak shifts to 370.8 C.

1.6.6 Spectral characterization of untreated and treated jute fibers The FTIR spectra of untreated and acetylated jute fibers are shown in Fig. 1.10 and band assignments for untreated jute fibers are shown in Table 1.5. Acetylation of jute fibers decreases the peak area at 3450 cm21 indicating protection of free hydroxyl groups by acetyl groups. The band intensity of untreated fibers at 1740, 1357, and 1235 cm21 increases and shifts to higher values in the acetylated fibers of 1751, 1375, and 1241 cm21, respectively.

1.6.7 Comparative data on sisal and jute fibers As mentioned in Section 1.6.1, the thermal stability of mercerized sisal fibers is quite different from that of the untreated fibers. The same result holds true for other

Figure 1.8 DTG thermograms of untreated jute fibers [60].

Figure 1.9 DSC thermograms of untreated jute fibers [60].

Figure 1.10 FTIR spectra of (A) untreated and (B) acetylated jute fibers [60].

30

Tribology of Natural Fiber Polymer Composites

Table 1.5 Thermal properties of lignocellulosic fabrics [58]. Material

Thermal properties

Sisal/cotton Jute/cotton Resin

Direction of the heat flux

Specific heat (J/cm3/ C)

Thermal diffusivity (mm2/s)

Thermal conductivity (W/m/ C)

Parallel Perpendicular Parallel Perpendicular


1.037 0.94 1.068 0.536 0.987

0.178 0.20 0.524 0.677 0.153

0.185 0.19 0.555 0.36 0.15

Table 1.6 Crystallinity index and first exotherm peak temperatures of lignocellulosic fibers as a function of alkali treatment [61]. NaOH (%)

0 0.8 2 4 6 8 30 4M

Crystallinity index (%)

Exotherm peak ( C)

Sisal

Jute

Change in crystallinity index (%)

Sisal

Jute

Shift in exotherm peak temperature ( C)

70.90 71.04 68.93 74.66 79.30 75.11 78.77 73.51

78.47 76.61 83.10 83.06 82.37 82.50 82.50 78.32

7.57 5.57 14.17 8.4 3.07 7.39 3.73 4.81

365.31 356.96 351.75 353.24 347.57 350.74 350.74 348.23

369.48 359.05 354.88 346.55 349.66 351.75 342.36 349.66

4.17 2.09 3.13 2 6.69 2.09 1.01 2 8.38 1.43

natural fibers such as jute. The DSC curves of sisal and jute subjected to mercerization are characterized by one endothermic peak between 70 C and 100 C and two exothermic peaks at higher temperatures [61]. Both the fibers show an endothermic peak at around 80 C, which is due to water desorption. The breaking down of the acetyl group causes the second endothermic peak observed in jute fiber. Beyond this peak, both the fibers show an exothermic peak between 380 C and 400 C. The exothermic peak seen in jute and sisal is more regular in acetylated fibers. The first exothermic peak listed in Table 1.6 reflects the stability of the fibers as a function of caustic soda concentration.

Natural fibers and their composites

31

1.6.8 Crystallinity of treated fibers X-ray diffraction results for alkalized fibers show an overall increase in crystallinity index (Ic) for sisal and jute (Table 1.6). Jute fiber has higher crystallinity index at any of the caustic soda concentrations than sisal fiber.

1.6.9 Spectral characterization of treated fibers The FTIR spectra obtained on the untreated and acetylated fibers reveal the extent of grafting of acetyl groups to fiber cell walls. Table 1.7 shows the characteristic peaks and their assignments. The peak at 3440 cm21 in untreated fibers is attributed to the presence of intermolecular hydrogen bonding which tends to shift to higher absorbency values in acetylated fibers, for example, 3480 cm21 in sisal and jute. The increase in peak intensity at 1743 cm21 in sisal and jute is due to the bonded acetyl group. The increase in absorbency in the region between the 1000 and 1500 cm21 bands shows the increase in O
H stretching, indicating that there has been a reduction in the number of hydroxy groups at the 3400
3500 cm21 band. Chemical reactions occurring during acetylation of the fibers and the presence of a peak in both the fibers at 1740 cm21 are caused by the reaction of the ester groups Table 1.7 FTIR peaks and their assignments for acetylated fibers [61]. Assignments

Sisal (acelytated) wavenumber (cm
1)

Peak assignments

Sisal (untreated) wavenumber (cm
1)

C
H stretching (with intermolecular hydrogen bonding) C
H stretching Carboxylic anhydride C
H bending C
H bending

3423

OH stretching

3327

2920 1740 1384 1250

2883 1724 1623 1506

C
C stretching

1059

C
H symmetrical stretching CQO stretching vibration OH bending of absorbed water CQC aromatic symmetrical stretching HCH and OCH in plane bending vibration In-the-plane CH bending S ring stretching G ring stretching C
O
C asymmetrical stretching C
C, C
OH, C
H ring and side group vibrations COC, CCO, and CCH deformation and stretching C
OH out of plane bending

1423 1368, 1363 1325 1259 1152 1046, 1020 895 662

32

Tribology of Natural Fiber Polymer Composites

present at 1734 cm21 in the untreated fibers with the acetyl groups observed at the former peak. Similarly, the reduction of the intermolecular hydrogen bonding between 3406 and 3471 cm21 confirms the grafting of the acetyl groups on the cellulose structure, thus replacing the hydroxyl groups.

1.6.10 Morphological characterization of treated fibers The surface topography of as extracted jute and sisal is quite rough as seen in the scanning electron micrographs (Figs. 1.11 and 1.12). Sisal and jute comprise bundles of individual cells that have been bound together by lignin-rich, weak intermolecular bonds. Sisal fibers are discontinuous, comprising short lengths joined together end to end, whereas jute fibers are continuous. Fig. 1.11B shows ridges on the surface of clean jute fiber after alkali treatment.

Figure 1.11 Scanning electron micrograph of (A) untreated and (B) alkali-treated jute fiber [61].

Figure 1.12 Scanning electron micrograph of (A) untreated and (B) alkali-treated sisal fiber [61].

Natural fibers and their composites

1.7

33

Wood

1.7.1 Thermogravimetric analysis The TG curve of wood fiber in a nitrogen environment shows a two-step thermal degradation [62]. At 153 C, wood loses 1% weight, and Tmax of the first and second peaks of wood are observed at 385 C and 531.5 C, respectively. In thermal degradation of wood at high temperature, free radicals are produced that accelerate the reactions of the thermal degradation [63,64]. At about 420 C, the carbon backbone of wood decomposes to produce carbon free radicals.

1.7.2 Spectral characterization In the case of wood, infrared spectroscopy is used to determine the structure of wood constituents and chemical changes in wood due to chemical treatments. This technique helps in estimating the lignin and carbohydrate contents in wood. FTIR spectra of two species of wood, that is softwood and hardwood, are shown in Fig. 1.13. In the region between 1800 and 800 cm21 significant changes in the absorbance and shapes of the bands and their positions are observed for two species. The most representative bands in this spectral range are listed in Table 1.8 [65]. The reduced intensity of the band at 1740 cm21 is slightly higher in hardwood than in softwood, probably because the former has a higher number of acetyl groups [66]. The appearance of a doublet at 1610
1595 cm21 for hardwood and a singlet at 1595 cm21 for softwood is attributed to the difference in the guaiacyl content

Figure 1.13 FTIR spectra of softwood (box) and hardwood (aspen) [65].

34

Tribology of Natural Fiber Polymer Composites

Table 1.8 Characteristic IR of the wood samples [65]. Wave number (cm21)

Assignment

1740 1610
1595 1510 1465 1426 1335 1316 1158
1162 898

C
O stretching of acetyl or carboxylic acid C
C stretching of the aromatic ring (lignin) C
C stretching of the aromatic ring (lignin) Asymmetric bending in CH3 (lignin) CH2 bending (cellulose) OH in plane bending (cellulose) CH2 wagging Asymmetric bridge C
O
C stretching (cellulose) Asymmetric, out of phase ring stretching (cellulose)

Table 1.9 Absorbance intensities of IR peaks in softwood [65]. Wave number (cm21)

Band

Assignment

Absorbance intensity average values

1739 1598 1510 1465 1426
1430 1335 1316

C5O C5C C5C C
H C
H2 O
H CH2

Hemicellulose 1 lignin Lignin Lignin Lignin Cellulose Cellulose Cellulose

0.387 0.260 0.317 0.072 0.098 0.066 0.045

between the two species. The lignins of softwood are made up of guaiacyl nuclei, while the lignins of hardwoods consist of a mixture of guaiacyl and syringyl nuclei [67]. The main difference between softwoods and hardwoods is the large amount of metoxyl groups found in hardwoods, which is reflected as a band near 1600 cm21. This is associated either with a relative pure ring stretching mode or with a relative pure ring stretching mode in species strongly associated with the aromatic C
O
CH3 stretching mode [68]. The spectral ratio between absorbing bands at 1595 and 1510 cm21 (both assigned to lignin) in the hardwood is similar, and this is attributed to the predominant syringyl unit, while in the softwood spectra the band at 1510 cm21 is more intense than at 1595 cm21, attributable to a higher content of guaiacyl units (trans-coniferyl alcohol) [27]. The doublet at 1335
1316 cm21 is assigned to the cellulose component, and is related to the crystallized cellulose I and amorphous cellulose content [69,70]. A decrease in the ratio 1335/1316 signifies an increase in crystallinity. Thus it is inferred that the amorphous cellulose content is higher in softwood than in hardwood (Tables 1.9 and 1.10). In the spectral region assigned to cellulose C
O
C bridges, the infrared bands are slightly shifted for each of the wood samples. The band located at 1158 cm21 for softwood is shifted to 1162 cm21 for hardwood. In crystallized cellulose this band is located at 1163 cm21 while for amorphous cellulose it is located at 1156 cm21 [68,70]. These values confirm that hardwood shows a higher crystallized cellulose I content than softwood [66,68,70].

Natural fibers and their composites

35

Table 1.10 Absorbance intensities of IR peaks in hardwood [65]. Wave number (cm21)

Band

Assignment

Absorbance intensity average values

1739 1598 1510 1465 1426
1430 1335 1316

C5O C5C C5C C
H C
H2 O
H CH2

Hemicellulose 1 lignin Lignin Lignin Lignin Cellulose Cellulose Cellulose

0.370 0.287 0.287 0.078 0.132 0.099 0.083

1.8

Bamboo

1.8.1 Thermogravimetric analysis Fig. 1.14 shows the thermogravimetry and differential thermal analysis (TG
DTA) curve of bamboo fiber. The decomposition of bamboo fiber starts from 161 C.

1.8.2 Spectral characterization of untreated and treated bamboo The transmission bands obtained in the FTIR spectra of untreated and treated bamboo (species Bamboosa balcua) and their assignments are given in Table 1.11 [71]. A broad peak due to
OH stretching vibration within the region 3100
3800 cm21 appeared as a consequence of alkali treatment and gave rise to a higher number of
OH groups. The band at 2853 cm21 in the untreated bamboo is attributed to the
OH stretching vibration of inter- and intramolecular H-bonding present among cellulose, hemicellulose, and lignin molecules of bamboo fiber. The band at 1733 cm21 is ascribed to the CO stretching of carboxylic acid or ester present in the bamboo. A small peak at 1636 cm21 is due to absorbed water. A lower shift of the peak 1044.26 cm21 (C
O/ C
C stretching vibration) is attributed to a change in the molecular orientation of fiber present in the untreated bamboo. The strong band at 893 cm21 due to glucosidic linkage in untreated bamboo shifted to higher wave numbers in the alkali-treated fibers. This is due to the rotation of glucose residue around the glucosidic bond. The bands at 680.74 and 669.17 cm21 are due to an out-of-plane bending vibration of an intermolecular H-bonded
OH group. The peak at 680.74 cm21 disappears, probably due to the gradual depletion of hemicellulose with increasing alkali treatment.

1.8.3 Crystallinity X-ray diffraction data for untreated and 10% alkali-treated bamboo samples show that the crystallinity and orientation angle increase due to alkali treatment (Tables 1.12 and 1.13). This is in contrast to the other natural fibers such as sisal which show decrease in crystallinity due to alkali treatment. The full width at half maximum (FWHM) value also indicates a higher crystallinity or better-ordered

36

Tribology of Natural Fiber Polymer Composites

Figure 1.14 TG
DTA curve of bamboo fibers.

Table 1.11 FTIR speaks of untreated and alkali-treated bamboo dust [71]. Peak assignments

Untreated bamboo (cm21)

Alkali treated (10%) cm21

5 OH stretching vibration 5 CH stretching vibration cell/ hemicellulose 5 OH stretching vibration of inter and intramolecular H-bonding .CO stretching of carboxylic acid or ester Absorbed water .C 5 C stretching vibration of lignin Lignin component .CH2 bending in lignin .CH2 and 1 CH3 bending 5 CH bending .CO stretching of acetyl ring Antisymmetric bridge C 5 O 5 C stretching .CO/C 5 C stretching vibration β-glucosidic linkage Out of plane bending vibration of intermolecular H-bonded 5 OH group Torsional vibration of pyranose ring

3300
3800 2921.63

3100
3800 2922.59

2853.17 1733.69 1636.3 1608.34 1464.67
1530.24 1436.71 1419.3 1384.64 1263.43
1339.32 1160.94 1044.26

1733.69 1636.3 1608.34 1464.67
1530.24 1436.71 1419.35 1382.71, 1387.53 1369.21
1320.04 1161.9 1032.33

896.73

896.73

680.74, 669.17 602.64
516.83

689.427, 669.17 605.539
512.08

Untreated bamboo 10% alkali-treated bamboo

Sample

45.57 50.10

43.54 45.84

0.675 0.693

0.667 0.676

Dust

Strip

Strip

Dust

Degree of crystallinity

Crystallinity

54.43 49.90

Strip

56.46 54.16

Dust

Amorphous (%)

Table 1.12 X-ray diffraction data of untreated and alkali-treated bamboo dust and strip [71].

45.57 35.46

Strip

43.54 38.23

Dust

Cellulose I (%)

0 14.64

Strip

0 7.6

Dust

Cellulose II (%)

38

Tribology of Natural Fiber Polymer Composites

Table 1.13 FWHM and orientation factor of untreated and alkali-treated bamboo dust and strip [71]. Sample

Untreated bamboo 10% alkali-treated bamboo

FWHM

Herman’s orientation factor

Strip

Dust

Strip

Dust

5.1482 4.810

5.2294 4.850

0.9433 0.9447

0.9425 0.9444

arrangement of fibers. Orientation factor fx, calculated from the mean orientation angle, increases for the alkali-treated sample.

1.9

Cotton

1.9.1 Thermogravimetric analysis Cotton fibers consist essentially of 95% cellulose I (β-1,4-D-anhydroglycopyranose). The remaining 5% noncellulosic compounds are located primarily in the cuticle and primary cell wall and contain wax, pectic substances, organic acids, sugars, and ash-producing organic salts. After chemical processing, these noncellulosic materials are removed, and the cellulose content of the cotton fibers increases to more than 99%. Primary cell wall, which is less than 0.5 mm thick, consists of around 50% cellulose. Therefore, two cotton fibers that are identical except for having different maturities have different quantities of primary cell wall per unit of mass. Consequently, the amount of the primary cell wall per unit mass is estimated by the measurement of the weight loss as a function of the temperature with TGA. The weight loss of cotton fibers occurs between 130 C and 380 C [72]. This is attributed to cellulose decomposition between 250 C and 350 C. Below 200 C, the weight loss is due to the loss of adsorbed water. Above 200 C, thermal decomposition and depolymerization occur. Between 250 C and 290 C, primary volatile decomposition releases CO2, CO, and H2O consisting of random chain scission in the low-order regions of the cellulose followed by relaxation of the broken chains and dehydration, decarboxylation or decarbonylation of anhydroglucose units. Between 290 C and 310 C, the volatile products include anhydroglucoses (1,6-anhydro-β-D-glucopyranose, 1,6-anhydro-β-D-glucofuranose, and 1,4:3,6-dianhydro-α-D-glucopyranose). Between 310 C and 350 C, additional volatile products are formed by dehydration of the anhydroglucoses (5-hydroxymethyl-2-furfural, 2-furyl hydroxymethyl ketone, and levoglucosenone). The TGA thermogram of cotton fiber is characterized by three regions (Fig. 1.15). Region I located between 37 C and 150 C shows initial weight loss followed by a plateau region between 225 C and 425 C (region II) and region III located between 425 C and 600 C. The first derivative of this thermogram (DTG) reveals

Natural fibers and their composites

39

three peaks at 488 C, 359 C, and 521 C. DSC analysis on the cotton shows peak characteristics with regard to the presence of water and heat of fusion of about 163 J/g [73].

1.9.2 Spectral characterization of cotton The main peaks present in the FTIR spectra of the cotton yarn are characteristic of cellulose, which is the main constituent of cotton (Table 1.14) [74]. The noncellulosic peak at 770 cm21 is attributed to the presence of compounds such as waxes, fatty acids, esters, etc., in the cuticle and primary cell wall.

Figure 1.15 TGA of cotton fibers up to 600 C [72].

Table 1.14 FTIR peaks of cotton [74]. Wavenumber (cm21)

Intensity

Assignment

3370 2900 1740 1590 1375 1270 1010 770 730

Strong Moderate No Weak Weak Moderate Strong No No

3300: O
H stretching 2900: C
H stretching 1740: C 5 O stretching (noncellulosic) 1547: N
H stretching (noncellulosic) 1330: hydroxyl 1250: C
O stretching 1030: unassigned; 1035:
O
stretching 700
900; noncellulosic 700
900; noncellulosic

40

1.10

Tribology of Natural Fiber Polymer Composites

Flax fiber

1.10.1 Thermogravimetric analysis The DTG curve of flax fiber shows removal of water and thermal decomposition of cellulose components [75]. The degradation of lignin occurs within the range 385 C
455 C, with the maximum at 431 C.

1.10.2 Spectral characterization of flax fibers Flax fibers consist of nearly 40 elementary fibers that are bound together by a lignin
hemicellulose matrix. The bulk fiber consists of approximately 70 wt.% cellulose, 18
19 wt.% hemicellulose, 2
2.5 wt.% lignin, and 5
2 wt.% wax. The majority of cellulose lies within the elementary fiber in the form of crystalline and amorphous cellulose [76]. The FTIR spectra of untreated and acetylated flax fiber, both acetylated dew-retted and acetylated green flax, show two new peaks at around 1733 cm21 and 1228
1235 cm21. These two peaks are associated with the C
O stretching (1733 cm21) and C
O stretching (1228
1235 cm21) of the carboxyl group. These peaks indicate the presence of the acetyl groups in the fibers and also that these acetyl groups are involved in an ester bond with the fiber constituents. The peak at around 1600 cm21 is associated with absorbed water in crystalline cellulose rather than with C
C stretching in aromatic rings. Therefore, it appears that the fibers are free of lignin, a compound rich in aromatic rings, or more precisely that there is no lignin at such a level that it could be detected by FTIR. The peaks at 2920 and 2850 cm21 are associated with C
H stretching of nonaromatic compounds.

1.11

Mechanical properties of natural fibers

The mechanical properties of various natural fibers are listed in Tables 1.15 and 1.16. The chemical composition of fibers is given in Table 1.17. From these tables it is clear that the mechanical properties depend on the quality of fibers, their composition, and properties as well as the age of the plant from which the fibers are extracted. Therefore, it is very difficult to get the same mechanical properties after repeat testing. However an immediate conclusion can be drawn that, although the natural fibers may not be as strong as graphite or aramids, flax, jute, bamboo, and hemp fibers have higher moduli (stiffness) than E-glass fibers. Some fibers also have strengths comparable to E-glass fibers. In general, the strength and stiffness of plant fibers depend on the cellulose content and spiral angle which the bands of microfibrils in the inner secondary cell wall make with the fiber axis. The amount of cellulose is closely associated with the crystallinity index of the fiber and the microfibril angle with respect to the main fiber axis. Fibers with high crystallinity index and/or cellulose content have been found to possess superior mechanical properties. Sisal fibers with cellulose content of 67% and microfibril angle of 10
228 have a tensile strength and modulus of elasticity of 530 MPa and 9
22 GPa, respectively. On the other hand, coir fiber with a cellulose

Bamboo (Dendrocalamus strictus)

Cotton Jute Coir Banana Sisal Flax Softwood kraft fiber Mesta Pineapple Kusha grass fiber Palm fiber

UTS (MPa) 500
800 460
533 131
175 529
754 468
640 1100 1000 157.3 413
1627 150.5 98.14 143
263 180
215 43
113

Diameter (μm)


25
200 100
450 80
250 50
200

200 20
80 390 240 80
800 70
1300

Table 1.15 Mechanical properties of some of the natural fibers [4].

0.05 2.5
13 4
6 7.7
20.8 9.4
15.8 100 40 12.62 34.5
82.51 5.69 2.22 9.8
13.3 4.4
6.1

Modulus (GPa)
1.16 15
40 1
3.5 3
7

1.56 1.6 2.12 30.8 3.6
5.1 2.0
2.8 13
20

Elongation (%)
8.1 39
49 11 10
22

9.6 14.8





Microfibril angle (Φ), (degree)

42

Tribology of Natural Fiber Polymer Composites

Table 1.16 Physical and mechanical properties of natural and synthetic fibers [4]. Fiber

Density (g/cm3)

Diameter (μm)

TS (MPa)

TM (GPa)

Elongation at break (%)

Flax Hemp Jute Ramie Sisal Abaca Cotton Coir E-glass Kevlara Carbon

1.5 1.47 1.3
1.49 1.55 1.45
1.5
1.6 1.15
1.46 2.55
17 1.44 1.78

40
600 25
500 25
200
50
200
12
38 100
460

5
7

345
1500 690 393
800 400
938 468
700 430
760 287
800 131
220 3400 3000 3400b
4800c

27.6 70 13
26.5 61.4
128 9.4
22
5.5
12.6 4
6 73 60 240c
425b

2.7
3.2 1.6 1.16
1.5 1.2
3.8 3
7
7
8 15
40 2.5 2.5
3.7 1.4
1.8

a

Kevlar, Dupont, Switzerland. Ultra high modulus carbon fibers. Ultra high tenacity carbon fibers.

b c

Table 1.17 Chemical composition of natural fibers. Fiber

Cellulose (%)

Lignin (%)

Hemicellulose (or pentosan) (%)

Pectin (%)

Ash (%)

Abaca Sisal Henequen Kenaf (core) Jute (core) Fiber flax Seed flax Kenaf (bast) Jute (bast) Hemp Ramie

56
63 47
78 77.6 37
49

7
9 7
11 13.1 15
21

15
17 10
24 4
8 18
24





10



3 0.6
1
2
4

41
48

21
24

18
22




0.8

71 43
47 31
57

22 21
23 15
19

18.6
20.6 24
26 21.5
23

2.3




5 2
5

45
71.5

12
26

13.6
21

0.2

0.5
2

57
77 68.6
91

3.7
13 0.6
0.7

14
22.4 5
16.7

0.9 1.9

0.8


content of 43% and microfibril angle of 30
498 is reported to have a tensile strength and modulus of elasticity of 106 MPa and 3 GPa, respectively. Apart from this, the tensile strength, modulus and percent elongation of natural fibers also depend on the experimental conditions such as fiber length, diameter, and test speed. These aspects have been discussed in detail in the subsequent chapters for individual fibers.

Natural fibers and their composites

43

According to one classification, the mechanical properties of natural fibers have been explained on the basis of reinforcing elements and their elastic moduli [13]. For instance, the elastic modulus of bulk natural fibers such as wood has been estimated to be about 10 GPa. Cellulose fiber with moduli up to 40 GPa can be separated from wood by chemical pulping processes. Such fibers can be further subdivided by hydrolysis followed by mechanical disintegration into microfibrils with an elastic modulus of 70 GPa. The fibers produced commercially by the paper and pulp industry have elastic modulus of this order.

1.12

Natural fiber polymer composites

Fiber-reinforced polymeric composites have high-specific strength and modulus compared with metals. By virtue of these two properties, fiber-reinforced polymer composites were used as structural material in the aerospace industry [1
3]. However over the years these composites have found acceptance in automotive parts, circuit boards, building materials, and specialty sporting equipment. These composites currently available on the market use nondegradable thermoplastics and thermosets as matrices and high strength fibers, such as graphite, aramids, and glass as reinforcement [1
3]. Since composites are made up of two or more dissimilar materials, they cannot be easily recycled, either ending up in landfills or being incinerated after use. Both these disposal alternatives are expensive and wasteful, and may contribute to environmental pollution. This has motivated the search for new fiber-reinforced polymer composites that are environmentally friendly, that is they do not cause much harm to the environment and decompose naturally without releasing pollutants into the environment. The use of biodegradable plant-based “lignocellulosic” fibers has been a natural choice for reinforcing polymers to make them environmental friendly. Plant-based fibers are inexpensive and available in abundance across the world. These fibers are nonabrasive and hence do not cause much damage. They have a hollow and cellular nature and thus perform well as acoustic and thermal insulators [77]. The hollow tubular structure also reduces their bulk density and makes them lighter in weight. Consequently plant-based fibers have been used to reinforce nondegradable thermoplastic polymers such as (PP), high, medium, and low density polyethylenes (HDPE, MDPE, LDPE), nylons, PVC to produce natural fiber composites [78
83]. Plant-based fibers have also been reinforced in thermosetting resins such as epoxy, polyester and polyurethane [84
87]. The bulk of plant-based fiber composites are, however made using wood flour (a waste from sawmills), or wood fiber (obtained from waste wood products) as an inexpensive filler for PP and PVC [88]. These composites, known as WPCs, with wood fiber content ranging between 30% and 70%, are commonly used in outdoor decking, railroad ties, window and door frames, automotive panels, and furniture. They are easy to process using currently available technology and machinery for blending, forming, and processing. Wood fibers and flour have also been used with resins/binders such as polymeric diphenylmethane diisocyanate (MDI or PMDI) to produce various grades of

44

Tribology of Natural Fiber Polymer Composites

medium density fiber (MDF) boards as substitute for wood. Plant-based fibers such as abaca, bamboo, flax, henequen, hemp, jute, kenaf, pineapple, ramie, sisal, etc., that possess good mechanical properties are also being evaluated as low-cost alternative reinforcements in composites. These fibers may not be as strong as graphite or aramids, but flax, jute, bamboo, and hemp fibers have higher moduli (stiffness) than E-glass fibers (Table 1.15) [87,89,90]. Some fibers also have strengths comparable to E-glass fibers. Panels made from such plant-based fibers and PP or other thermoplastics are already in use in many automobiles. All major automobile manufacturers are exploring their use in other interior applications as well [90
94]. Unfortunately, because these composites use nondegradable polymers their end of life disposal is difficult. As a result, significant research efforts are being directed at developing fully biodegradable composites by combining natural fibers with biodegradable resins [95]. These composites can be easily disposed of or composted without harming the environment. A variety of natural and synthetic biodegradable resins are available for use in such composites (Table 1.18) [95]. Most of these resins degrade through enzymatic reactions when exposed to a compost environment or in moist/wet outdoor environments through similar microbial/bacterial attack. The tensile and flexural strengths of these composites are reported to be significantly higher in the grain direction than many wood varieties, even at a low fiber content of 28%. With higher fiber content and better processing, the mechanical performance is improved further for use in noncritical applications, such as secondary structures in

Table 1.18 Biodegradable polymer resins. Natural polymers 1.

2.

3. 4.

Polysaccharides (starch, cellulose, chitin, pullulan, levan, konjac, and elsinan) Proteins (collagen/gelatin, casein, albumin, fibrogen, silks, elastins, and protein from grains Polyesters and polyhydroxyalkanoates Other polymers (lignin, lipids, shellac, and natural rubber)

Manmade polymers 1.

Poly(amides)

2.

Poly(anhydrides)

3.

Poly(amide-enamines)

4.

Poly(vinyl alcohol)

5. 6. 7.

Poly(ethylene-co-vinlyl alcohol) Poly(vinyl acetate) Polyesters [(poly(glycolic acid), poly(lactic acid), poly (caprolactone), poly(ortho esters)] Poly(ethylene oxide) Some poly(urethanes) Poly(phosphazines) Poly(imino carbonates) Some poly(acrylates)

8. 9. 10. 11. 12.

Natural fibers and their composites

45

housing and transportation. A lot of research is being carried out in this area and a concise review has been carried out by Netravali and Chabba [95]. One of the major problems is the cost of the resins. Most biodegradable resins currently cost significantly more—three to five times more—than the commonly used resins such as PP, LDPE, HDPE, and PVC. Moreover, diameters and strengths of plant-based fibers vary significantly with source, age, retting and separating techniques, geographic origin, rainfall during growth, and cellulose/hemicellulose/lignin content. Most cellulosic fibers also swell and lose strength when they absorb moisture and shrink when they lose moisture. Repeated moisture absorption/desorption can significantly lower their strength and affect their bonding to the resin. Both of these factors reduce the strength of the composites significantly. Chemical modifications such as acetylation, silane and other treatments could reduce their moisture sensitivity as discussed in Section 1.3. Apart from the critical problem of moisture absorption, most lignocellulosic fibers cannot withstand processing temperatures higher than 175 C for long durations, which limits their ability to be used with some thermoplastic resins. Notwithstanding these limitations, natural fibers as fiber reinforcement for composites have a long history, primarily because the advantages they offer over manmade fibers, such as low density, low cost, recycling, and biodegradability, are numerous. These advantages mean that natural fibers have the potential to replace glass fibers in composite materials. In particular, the mechanical properties of natural fibers, especially flax, hemp, jute, and sisal, are relatively good, and they may compete with glass fiber in terms of specific strength and modulus [24,96]. However the components of natural fibers, including cellulose, hemicellulose, lignin, pectin, waxes, and water-soluble substances (Table 1.17) [30,97], and their composition may differ with growing conditions and test methods even for the same kind of fiber. A large amount of hydroxyl group in cellulose gives natural fiber hydrophilic properties when used to reinforce hydrophobic matrices, and this results in poor interface and poor resistance to moisture absorption [98]. The low interfacial properties between fiber and polymer matrix often reduce their potential as reinforcing agents due to the hydrophilic nature of natural fibers.

1.12.1 Thermoset-based composites Natural fibers are combined with thermosets such as unsaturated polyester, phenol-formaldehyde, novalac-type phenol-formaldehyde, and epoxy resins to form composite materials (Table 1.19). In thermoset matrix composites, the fibers are impregnated with thermosetting resins and then kept for curing either at room temperature or elevated temperature. These composites with thermosetting matrices are developed using hand lay-up, modified lay-up/press molding, pultrusion, vacuum infusion, and resin transfer molding (RTM), to achieve high performance components. Although epoxy resins have been used to obtain higher quality in the product, unsaturated polyesters have been more popular due to their low cost and adaptability for large composite structures. The incorporation of treated natural fibers compared with untreated fibers results in improved

46

Tribology of Natural Fiber Polymer Composites

Table 1.19 Polymeric matrices used in natural fiber polymer composites. Natural fiber thermosets

Thermoplastics

Cellulose

Epoxy

PP, PE, PA66 PS, PVC

Flax Jute

Epoxy, MF Epoxy, polyester, vinylester, phenolic Epoxy, polyester, novolac, Epoxy, phenolic Phenolic, epoxy, polyester, vinylester Polyster, phenolic Polyester Polyester, phenolic Phenolic Epoxy, polyester Polyester, phenolic Polyester

PP, PE PP, PE

Sisal Kenaf Hemp

Cotton Coir Banana Bagasse Bamboo Pineapple Wood flour/ fiber Ramie Abaca Oil-palm a



Polyester, phenolic

Biodegradable polymers

Polyhydroxybutyrate, Mater-Bi,a polylactic acid, cellulose esters, starch plastics, polycaprolactone, and aliphatic polyesters/ copolyesters

PP, PE, PS PP



PP PP PE PP, PE, PS, PVC PP PHBV


Mater-Bi, Novamont, Italy.

strength in composites [99]. Among thermoset composites, phenol formaldehyde matrix composites show higher strength and modulus than epoxy composites followed by polyester composites. Lignin-rich fiber (e.g., coir) composites show better resistance to weathering when compared with cellulose-rich fiber (e.g., sisal and banana) composites. Lignin, which has lower affinity toward moisture, appears to act as a protective barrier for cellulose microfibrils from moisture absorption. The mechanical properties of some thermoset-based composites are given in Tables 1.20 and 1.21 [100].

Natural fibers and their composites

47

Table 1.20 Mechanical properties of unidirectional and short fiber polyester composites [100]. Fiber (wt.%)

Unidirectional Sisal (40) Banana (30) Coir (30) Chopped random Sisal (25) Banana (25) Coir (25) Fabric Banana
cotton a

Tensile strength (MPa)

Young’s modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

Impact strength (kJ/m2)

129 121 45

8.5 8.0 4

192
56

7.5
4

98a 52a 44a

34.5 43.5 14.0

1.9 2.3 1.4

86.4 92 31.2






30 10 11

27.9
35.9b

3.3

50.6
64b




3.1
7.5b

Impact strength (Charpy) for 0.5 Vf. Depending on type of fiber in the test direction.

b

Table 1.21 Comparative properties of jute
polyester and glass
polyester composites [100]. Properties

Jute
polyester

Glass
polyester

Density (g/cm3) Water absorption (%) after 24 h soaking Thickness swelling (%) after 24 h soaking Flexural strength (MPa) Flexural modulus (GPa) Tensile strength (MPa) Tensile modulus (GPa)

1.25 0.84 0.17 60.12 2.97 44.25 2.88

1.45 0.17 0.12 138.2 4.02 117.4 6.08

Source: IJIRA, Kolkata, India.

1.12.2 Thermoplastic-based composites Natural fibers are compatible and have the potential to be used as reinforcements in thermoplastics (Table 1.19). Plastics are a preferred choice because of the potential to use the conventional processing equipment of thermoplastic-based systems with low maintenance costs as natural fibers are less abrasive. However high processing temperature cannot be used because fibers begin to degrade. This processing factor, therefore, limits the type of thermoplastic matrix, such as PE, PP, and PS, to be used with lignocellulosic fibers. The final properties of the composite are strongly influenced by the mechanical properties and the geometrical characteristics of the reinforcement, the fiber
matrix and the fiber
fiber interactions, distribution and

48

Tribology of Natural Fiber Polymer Composites

orientation of fibers, rheological properties, and the solidification of the melt during processing. Natural fiber reinforcement accelerates the crystallization kinetics of polymer matrices as a consequence of the heterogeneous nucleation effect on natural fibers. The presence of transcrystalline regions on the fiber surface improves the quality of the fiber
matrix interfacial interaction. Generally, a poor adhesion is observed due to the difference, in terms of polarity, of the natural fiber and thermoplastics. Subsequently, stress transfer is improved by pretreating the fiber, the matrix, or both simultaneously using coupling agents or compatibilizing agents that improve the properties of natural fiber-reinforced thermoplastics.

1.12.3 Biodegradable polymers-based composites Common biodegradable plastics are poly(lactic acid), cellulose, esters, starch plastics, poly(caprolactone), and aliphatic polysters/co-polyesters. These polymers have emerged as potential replacements for conventional plastics in different applications. When natural fibers are incorporated in such biodegradable plastics a fully biodegradable biocomposite is realized. The biodegradable polymers are used as matrices in natural fiber-reinforced composites used for making tubes, car doors, interior paneling, sandwich plates, etc. Cellulose-polyhydroxybutyrate (bacteriaproduced polyester) systems show improved mechanical properties. Chemically treated jute fibers reinforced in the same matrix also show improved properties. Surface modified natural fibers and commercial biodegradable plastics can be prepared by extrusion followed by compression molding [101,102]. Some of the main disadvantages of biodegradable polymers are their hydrophilic character, fast degradation rate and low mechanical properties in wet environments. Among the family of biodegradable polyesters, polylactides (i.e., polylactic acids) have been the focus of much attention because they are produced from renewable resources such as starch, biodegradable and compostable, and possess, very low or no toxicity, and high mechanical performance
comparable to those of commercial polymers. However they are expensive. Blending aliphatic polyesters with hydrophilic natural polymers is of significant interest, since it could lead to the development of a new range of biodegradable polymeric materials. However aliphatic polyesters and hydrophilic natural polymers are thermodynamically immiscible, leading to poor adhesion between the two components. Various compatibilizers and additives have been developed to improve their interface. The thermoplastic biopolymers that have been developed primarily for the packaging industry do not have the material properties to meet the matrix system requirements for fiber composite materials. In particular, the high breaking elongation and high processing viscosity prove to be disadvantageous. In contrast with thermoplastics, the development of naturally based thermosets appears to be easy since suitable starting substances can be provided by maleinated triglycerides, epoxidized vegetable oils, polyols, and aminated fats. Petrochemical reagents are still needed to crosslink these monomers and to create and integrate stable molecule sequences. Among these substances, isocyanates, amines, polyols, and polycarboxylic acids are preferred. Research is now, however concentrating on the

Natural fibers and their composites

49

development of an isocyanate from a biological source. Vegetable oil epoxy acrylates and vegetable oil epoxy resins are also being developed. Various combinations have been examined in order to find suitable solutions. In addition, different fillers, for example organic substances (starch, etc.), and inorganic substances (calcium carbonates, magnesium oxides, aluminum phosphates, etc.), have been tested. The latter play an important role as stabilizers or flame retardants.

1.13

Applications of natural fiber composites

Natural fiber polymer composites have numerous applications in almost all fields of engineering. Being cost-effective, they are especially suited for low-cost housing, the building and construction industry (panels, false ceilings, partition boards, etc.), packaging, automobile interiors, and storage devices. They have also emerged as potential candidates as wood substitutes in furniture. For instance, lightweight jute
polyester composites are being used to make chair shells, doors, and partitions, instrument panels, home furnishings, car body panels, etc. Similarly, jute
polypropylene composites have been used for products such as automotive interiors, shipping pellets, bobbins and spools, flowerpots, toys, plastic decking and fencing, furniture, handles, etc. Apart from this, jute-based geotextiles have been developed for applications in prevention of soil erosion, leaching, etc. Natural fibers have also emerged as a potential alternative to wood fiber
plastics, talc- or mica-filled PP, and glass fiber composites in automotive parts. Natural fibers such as flax, hemp, kenaf, jute, sisal, abaca, and banana are also being explored for use as reinforcement in thermosets as well as thermoplastics to develop composites that can be used to make products such as decking, window and door profiles, fencing, siding, railings, furniture, flooring, and marine components. All these products use PP-based composites produced by compression molding or thermoforming extruded sheets or commingled mats of PP and plant fibers [103].

1.13.1 Automotive applications Plant fibers as insulating or damping materials or as fillers or reinforcement in polymeric materials have attracted much attention for automotive applications. Plant fibers are currently used in the interior of passenger cars, truck cabins, and other automotive applications [103]. Besides their use in trim parts such as door panels or cabin linings, plant fibers are used extensively for thermoacoustic insulation. Such insulating materials are mainly based on cotton fibers recycled from textiles and cotton fibers up to 80% by weight. Likewise, coconut fibers bonded with natural latex are used as seat cushions. The ability of plant fibers to absorb humidity increases the comfort value, in contrast to synthetic materials. Door panels in passenger cars made up of flax fiber mat embedded in an epoxy resin reduce weight by about 20% and improve the mechanical properties. The flax/sisal reinforcement allows molding of intricate three-dimensional shapes. Thermoset polymers such as

50

Tribology of Natural Fiber Polymer Composites

PF (phenolic resin) are used as binders for cotton or similar fibers in the production of inner trim parts and insulating or damping materials. These resins provide mechanical properties such as stiffness and strength and are economically viable. The thermosets have a superior thermal stability and lower water absorption than thermoplastics. However due to the demand for recycling and alternative processing techniques, thermosets are being replaced by thermoplastics. Thermoplastics-based plant fiber composites can be easily processed using compression molding. Plant fibers of high quality embedded in thermoset binders prevent fogging better than synthetic materials. Furthermore, natural fibers can normally be processed at low temperatures, below 230 C, that cut down the costs of manufacturing processes requiring higher temperatures. However a major disadvantage associated with the use of natural plant fibers is the variation in fiber quality due to growth conditions, time of harvest, and method of extraction of fibers. The automotive industry has used fiber-reinforced plastic composites to make its products lighter. However glass, carbon and aramid fiber-reinforced polyester, epoxy, and other similar resins are difficult to recycle and hard to dispose of. They do not degrade naturally. Thermoplastics are better as they can be thermally recycled to produce new products. However for a more sustainable future and to meet growing regulatory pressures—of which the most pressing is the European Union’s end-of-life of vehicles (ELV) directive requiring that, by 2015, all new vehicles should be 95% recyclable—a more complete solution is needed. A logical solution is to combine recyclable thermoplastic resins (polypropylene or PP, polyolefin, polyethylene, polyurethane, and polyamide are some of those already used in vehicles) with biodegradable plant-based fibers. Natural fibers have the potential to reduce vehicle weight (up to 40% compared with glass fiber, which accounts for the majority of automotive composites), while satisfying increasingly stringent environmental criteria. The use of natural fibers minimizes harmful pollutants, and their eventual breakdown is environmentally benign. The environmental impacts that remain can be reduced by choosing crops and farming methods that economize on fuel, fertilizer, and pesticide, together with efficient extraction and treatment systems. Natural fibers emit less CO2 when they break down. They are nonirritating and nonabrasive, and do not blunt manufacturing tools or processing equipment. Fiber-producing crops are easy to grow and could take up marginally used agricultural capacity in developed countries. Apart from being environmental friendly, natural fiber reinforcement offers several advantages. For instance, hemp fiber-reinforced phenolic resin composite shows increase in flexural strength from 11 to 25 MPa and improves stiffness by 23% [104]. Impact resistance of unreinforced phenolic, which tends to be brittle, was markedly improved by the hemp reinforcement since the fibers help dissipate impact forces into the matrix. A ductility index improvement from 3.77 to 2.58 also emphasized the rise in toughness. The introduction of the hemp mat reduced the number and sizes of voids formed in the composite during the cure of the thermosetting resin, because the naturally hydrophilic fibers absorb moisture produced by the cure reaction.

Natural fibers and their composites

51

Natural fiber-reinforced plastics (NFRPs) have been used in the production of vehicles for over a decade, Mercedes Benz having set the precedent in 1994 by using jute reinforced plastic for the interior door panels of its E-Class vehicles. Jute, like hemp, grows well in Europe and is one of several agricultural crops having a particularly fibrous bast, or outer sheath to the stems—analogous to tree bark. Because the long, strong bast fibers lie somewhere between wood stocks and E-glass (the most commonly used form of glass fiber) in terms of the mechanical properties, they can substitute for either. Glass fiber substitution, especially for car interior items like door panels, parcel shelves, and headliners where conventional composites represent over-engineered solutions, offers a promising way forward. Vehicle manufacturers and their suppliers who have adopted NFRPs have noted that, in addition to their high strength and stiffness per weight (Table 1.15) and environmental virtues, the materials have other benefits too. These include acoustic insulation, easier health and safety management, rapid production by compression or injection molding, and potentially lower cost.

1.13.1.1 Using hemp fiber as a substitute for aramid in brake pads Brake pads are a complex mixture of organic binder, inorganic powders, synthetic fibers, and metal particles, each of which performs a definite role in the finished product. The raw materials are blended, pressed into shapes and baked at 150 C to make the brake disc pads. Aramid fibers (Kevlar DuPont, Switzerland) were adopted to replace asbestos because of environmental and health concerns. However these fibers are expensive and their decomposition and dissipation into the environment during use is still being investigated. With environment preservation being on the agenda of international organizations and nations, natural and biodegradable replacement has become the order of the day. Natural fibers are a potential replacement for synthetic fibers in brake pads. However natural fibers are expected to perform, if not equally at least close to that of the synthetic fibers. They must facilitate homogeneous mixing of the ingredients and help in producing a composite which should possess sufficient mechanical strength, high wear resistance and thermal characteristics to withstand severe temperature and operating conditions during braking. Among the different natural fibers, such as hemp, jute, flax, cotton, sisal, and kenaf, researchers have found that jute and hemp possess better tribological characteristics. Hemp has the advantage over jute because it can be spun and mixed to produce fiber blends with optimized properties. The ECOPAD [105] research team comprising the team from the University of Exeter’s Advanced Technologies department, X-At, and a consortium of industrial partners representing brake pad manufacturers, suppliers and end-users has developed technical knowhow wherein hemp fibers have been enhanced and processed to replace a significant proportion of synthetic fibers and resins in brake pads. The team showed that new blends consisting of hemp fibers offered the same frictional performance as pads made using aramid fiber. This new material, which avoids the

52

Tribology of Natural Fiber Polymer Composites

use of heavy metals and is based on a tin compound, offers an alternative to lead and antimony friction modifiers which cause health hazards. NFRPs are not limited to nonstructural applications in vehicle interiors. These materials which are comparable to para-aramids for strength and can potentially reduce the weight of automotive composites by 40%, are being investigated for structural applications as well. However composite quality is marred by poor fiber
matrix coupling because naturally hydrophilic fibers do not bond well with thermoplastics and other resins. Low thermal tolerance rules out certain manufacturing processes normally used with composites. Fibers degrade too readily, something that can occur during compounding and molding as well as in service. When they break down, the material may smell unpleasant. Rotting is accelerated by the fibers’ tendency to attract moisture, which causes them to swell. In particular, price, fiber characteristics, and quality may vary substantially, depending on cultivation conditions and agricultural policies. These issues need to be addressed.

1.13.2 Construction industry Natural fiber-based polymer composites have found wide-scale application in the construction sector. For instance, sandwich composite panels manufactured by hand lay-up using natural fiber-based laminate as face material and corrugated sheet as core material are lightweight and have excellent bending stiffness and good thermal and sound insulation. Similarly, hybrid composites developed with glass, sisal, and polyester resin have found use in semistructural applications. Sisal fiber-reinforced polyester composite filled with wollastonite has proved to be a suitable alternative to glass fibers in dough and bulk molding compounds for use in the housing sector. The addition of sisal fibers in wollastonite/polyester mix reduces the brittleness and acoustic-damping coefficient. Sisal-based dough molding compound can be used to develop building materials such as checker floor plate, roof tiles, sanitary-ware, etc. Similarly, jute pultruded doorframes made using woven jute fabric and phenolic resin show no sign of warping, bulging, discoloration, etc. They possess good dimensional stability due to low moisture absorption compared with wooden doorframes. Moreover, they have good electrical insulation and corrosion resistance properties. Jute
coir composite provides an economic alternative to wood for the construction industry [100]. It involves the production of coir
ply boards with oriented jute as face veneer and coir plus waste rubber wood inside. The coir fiber contains about 46% lignin as against 39% in teak wood. Therefore, it is more resistant than teak wood to rotting under wet and dry conditions and has better tensile strength. Natural fiber-reinforced boards can be used in place of wood for partitioning, false ceilings, surface paneling, roofing, furniture, cupboards, wardrobes, etc. A roofing sheet developed using coir and cement has properties close to that of asbestos sheet (Table 1.22).

Natural fibers and their composites

53

Table 1.22 Properties of coir
cement roofing sheet [100]. Property 3

Density (g/cm ) Water absorption after 24 h soaking (%) Thickness (mm) Pitch length (mm) Pitch depth (mm) Weight (kg/m2) Bending strength (MPa) Deflection (mm) Thermal conductivity (kcal/m2/h/ C)

Natural fiber sheet

Asbestos sheet

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

2.0 25 6 146 48 13.50 25
30
0.24

Source: CBRI, Roorkee, India.

1.13.3 Rural and cottage industry Natural fibers and their composites have numerous applications in rural and cottage industries. Many products can be made using these materials, creating large-scale job opportunities for the rural population. These products include fishing baskets, rice strainers, kitchen baskets, cradles, storage boxes, furniture, furnishings, packaging, etc. Other minor products include spa products, belts, slippers, and clothes. Among natural fibers, jute is the leading candidate and the one that has opened new vistas of applications. Apart from conventional uses as ropes, twines, low-cost dartboards, mattresses, carpets, handicrafts, and wire rope cores, jute has entered various diversified sectors such as the paper industry, celluloid products, nonwoven textiles, geotextiles, etc. Geotextiles are lightly woven fabrics made from natural fibers that are used for soil erosion control, seed protection, weed control, and many other agricultural and landscaping uses. They can be used for more than a year, and the biodegradable jute geotextile left to rot keeps the ground cool and is able to make the land more fertile. Diversified by-products which can be cultivated from jute include its use in food, cosmetics, medicine, paints, and other products. Bamboo fabric is soft and possesses natural antibacterial properties. Clothing made from bamboo fiber is popular for various activities, and sheets and towels made from bamboo have become luxury items. The fiber of bamboo has long been used to make high quality handmade paper that is still produced in small quantities.

1.14

Significance and economics of natural fiber polymer composites

Natural fibers offer immense potential as reinforcement in polymers for various industrial applications. Thus they have a larger bearing on the socioeconomic development

54

Tribology of Natural Fiber Polymer Composites

of a country. The ever-increasing demand for environmental friendly products is forcing materials suppliers and manufacturers to consider the environmental impact of their products during processing, recycling, and disposal. Consequently, the use of natural fibers to develop cost-effective and biodegradable composites has generated a great deal of interest. However natural fiber-reinforced polymer composites developed using synthetic polymers, reinforced by either synthetic or natural fibers, respectively, limit the environmental friendliness of the resulting composite due to low biodegradability and problems related to recycling materials. By incorporating natural fibers into biodegradable polymer matrices, biocomposites are developed that offer a feasible solution. Much research is in progress for better and larger scale use of natural fibers such as sisal, jute, flax, or hemp as well as wood fiber/flour, bamboo, cotton, etc. [106]. These fibers have found numerous applications in different industrial sectors, particularly in the automotive sector which demands greater comfort and driving performance, increased safety standards and fuel efficiency and reductions in CO2 emissions. Thus the use of lightweight construction materials in automobiles is a prerequisite. These factors, coupled with other influences such as business competition, low cost, launch of new products, and strict environmental legislation, further drive the use of ecofriendly natural fibers and their composites. As per the new environment laws in various countries, cars and other vehicles have to restrict the amount of waste products. Only 5% of scrapped vehicles can be disposed of by landfill or incineration. As per one estimate, around two million vehicles become defunct in the UK each year. Although 74%
80% of the total weight of a car is recycled, there is greater pressure on the automobile manufacturers to improve the recyclability of new vehicles. Recycling is the only means to end pollution by landfill. Natural fiberreinforced composites that lead to weight savings of about 50%, and cost reduction by approximately 30%, can be extremely helpful in such an endeavor.

1.14.1 Economic aspects of natural fibers Natural fibers are noncarcinogenic, they are also nonabrasive to mixing and molding equipment and this reduces the overall cost (Table 1.23). Two important natural fibers, flax and hemp, that are used in automotive interiors are up to 40% cheaper than standard glass fibers. Secondly, they help in drastic weight reduction of a car which produces fewer emissions and is fuel-efficient (Table 1.16). However the low thermal stability, microbial and fungi resistance, susceptibility to rotting, and hydrophilic character of natural fiber resulting in high moisture uptake, affect the properties of natural fiber polymer composites. The other major concern is their nonuniformity—the variability of their dimensions and mechanical properties. However their disadvantages far outweigh the advantages and, despite the current limitations, further applications for natural fibers are being explored.

1.14.2 Cultivation of natural fiber as crops The natural fiber crop is an alternative crop. Fiber crops such as hemp and flax remove heavy metals such as cadmium, lead, and copper from the soil. The plants

Natural fibers and their composites

55

Table 1.23 Comparison of benefits of natural fibers over synthetic fibers [103]. Advantages

Disadvantages

Low cost Renewable Low density, lightweight

High moisture absorption Poor microbial resistance Local and seasonal quality variations Demand and supply cycles

Low thermal resistance High strength and elasticity modulus Sound abatement capability Nonabrasive Low energy consumption Thermal incineration possible with high energy recovery No residues when incinerated Full safe handling, no skin irritations Can be stored for long periods of time (if prevented from contact with moisture) Crops can be used for cleaning soil Crops recycle CO2 from the atmosphere Fast absorption/desorption of water (6) Biodegradability (6)

retain the soil fertility and have the ability to grow on nutrient-poor lands. These plants reduce soil erosion by covering large zones where few crops can be grown, thus reducing the erosive impact of wind and water. However these fiber crops are not sustainable because their production (especially cotton) requires huge amounts of water, pesticides, fungicides, and herbicides. As monocrops without crop rotation and with tillage, and with the use of commercial fertilizers, herbicides, and pesticides, they contribute to soil-degradation just as other monocultures do. These factors tend to disrupt ecological balances and have detrimental effects on farming and rural economies. Apart from this, rural communities are not in a position to update their technological means to improve production, and that keeps them away from growing sisal as natural fiber crop. This situation can be improved by enhanced productivity and increasing sales volume and prices, developing new uses and costeffective technology for crop production, and by improved fiber extraction techniques and the utilization of by-products and production waste.

1.15

Sources of further information and advice

K. Esau, Anatomy of Seed Plants, second ed., Wiley, New York, 1977. R. Kozlowski and M. Machiewacz-Talarczyk, Inventory of World Fibers and Involvement of FAO in Fiber Research, Institute of Natural Fibers, Poznan, Poland, 2005. M.-S. Ilvessalo-Pfaffli, Fiber Atlas, Identification of Papermaking Fibers, Springer Series in Wood Science, Springer-Verlag, Berlin, 1993.

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R.H. Kirby Vegetable Fibers, Leonard Hill Books Ltd, London, 1963. S.K. Batra, Other long vegetable fibers: Abaca, banana, sisal, henequen, flax, ramie, hemp, sunn and coir, in: M. Lewin (Ed.) Handbook of Fiber Chemistry, third ed., Taylor and Francis, Boca Raton, FL, 2007, p. 453. L.R. Czerniak, R. Kirkowski, R. Kozlowski M. Zimnicwska, Proceedings Symposium on Hemp, Flax and Other Bast Fibrous Plants: Production, Technology and Ecology, Institute of Natural Fibers, 24
25 September, Poznan, Poland, 1998, p. 18. R. Kozlowski, M. Machiewacz-Talarczyk, Proceedings on 8th Pacific Rim Biobased Composites Symp, Kuala Lumpur, 2006, p. 12.

References [1] K. Friedrich (Ed.), Advances in Composites Tribology, Composite Materials Series, vol. 8, Elsevier, Amsterdam, 1993. [2] Recent advances in composite materials, in: Proceedings on Special Symposium on Composites in Intelligent and Sustainable Infrastructure, 20
23 February, New Delhi, (2007). [3] K. Othmer, fifth ed., Encyclopedia of Chemical Technology, vol. 10, Wiley, Hoboken, NJ, 1980, p. 181. [4] N. Chand, P.K. Rohatgi, Natural Fibers and Composites, Periodical Experts, New Delhi, 1994. [5] P.K. Rohatgi, P.D. Ekbote (Eds.), Materials Science and Technology in the future of Madhya Pradesh, Regional Research Laboratory, Bhopal, 1985. [6] K.G. Satyanarayana, J.L. Guimaraes, F. Wypych, Compos. Part A 38 (2007) 1694. [7] H.R. Manersberger (Ed.), Textile Fibers, sixth ed., Wiley, New York, 1954. [8] Coir: its Extraction, Properties and Uses, CSIR Publication, New Delhi (1960). [9] Diamond Jubilee of Coconut Research in India, Souvenir, Central Plantation Crops Research Institute, Kasargod, Kerala (1976). [10] Allam, J. Nat. Fibers 1 (3) (2004) 77. [11] D. Puglia, J. Biagiotti, J.M. Kenny, J. Nat. Fibers 1 (3) (2004) 23. [12] X. Li, L.G. Tabil, S. Panigrahi, J. Polym. Environ. 15 (2007) 25. [13] A.K. Bledzki, J. Gassan, Prog. Polym. Sci. 24 (1999) 221. [14] E. Plueddemann, Silane Coupling Agents, second ed., Plenum Press, New York (1991); K.L. Mittal (Ed.), Silanes and Other Coupling Agents, VSP, The Netherlands. [15] I. Van de Weyenberg, J. Ivens, A. De Coster, B. Kino, E. Baetens, I. Vepoes, Comps. Sci. Technol. 63 (2003) 1241. [16] R. Agarwal, N.S. Saxena, K.B. Sharma, S. Thomas, M.S. Sreekala, Mater. Sci. Eng. A 277 (2000) 77. [17] M.Z. Rong, M.Q. Zhang, Y. Liu, G.C. Yang, H.M. Zeng, Compos. Sci. Technol. 61 (2001) 1437. [18] A. Valadez-Gonzalez, J.M. Cervantes-Uc, R. Olayo, P.J. Herrera-Franco, Compos.: Part B. 30 (1999) 321. [19] Paul, K. Joseph, S. Thomas, Compos. Sci. Technol. 57 (1) (1997) 67. [20] A.S.C. Hill, H.P.S. Abdul Khalil, M.D. Hale, Ind. Crop. Prod. 8 (1) (1998) 53. [21] M.S. Sreekala, S. Thomas, Compos. Sci. Technol. 63 (6) (2003) 861.

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[22] D. Maldas, B.V. Kokta, C. Daneault, J. Appl. Polym. Sci. 37 (1989) 751. [23] K. Joseph, S. Thomas, C. Pavithran, Polymer 37 (1996) 5139. [24] T.W. Frederick, W. Norman, Natural Fibers Plastics and Composites, Kluwer Academic Publishers, New York, 2004. [25] J. George, R. Janardhan, J.S. Anand, S.S. Bhagawan, S. Thomas, Polymer 37 (24) (1996) 5421. [26] C.U. Pittman Jr., G.-R. He, B. Wu, S.D. Gardmer, Carbon 35 (3) (1997) 317. [27] M.S. Sreekala, M.G. Kumaran, S. Joseph, M. Jacob, S. Thomas, Appl. Compos. Mater. 7 (2000) 295. [28] N.E. Zafeiropoulos, Compos. Part A 33 (2002) 1083. [29] Supriya Mishra, PhD thesis, Utkal University, Orrisa, India (2000). [30] A.K. Mohanty, M. Misra, L.T. Drzal, Compos. Interfaces 8 (2001) 313. [31] S. Mishra, M. Misra, S.S. Tripathy, S.K. Nayak, A.K. Mohanty, Macromol. Mater. Eng. 286 (2001) 107. [32] J.M. Felix, P. Gatenholm, J. Appl. Polym. Sci. 42 (1991) 609. [33] A.C. Khazanchi, M. Saxena, T.C. Rao, in: P. Hamelin, G. Verchery (Eds.), Textile Composites in Building Construction, Editions Pluralis, Paris, 1990, p. 69. [34] P. Gatenholm, H. Bertilsson, A. Mathiasson, J. Appl. Polym. Sci. 49 (1993) 197. [35] L.J. Mathias, S. Lee, J.R. Wright, S.C. Warren, J. Appl. Polym. Sci. 42 (1991) 55. [36] D.T. Quillin, D.F. Caulfield, J.A. Koutsky, J. Appl. Polym. Sci. 50 (7) (1993) 1187. [37] M.S. Sreekala, M.G. Kumaran, S. Joseph, M. Jacob, S. Thomas, Appl. Compos. Mater. 7 (2000) 295. [38] M.S. Sreekala, M.G. Kumaran, S. Thomas, Compos.: Part A 33 (2002) 763. [39] S. Paul, N. Puja, G. Rajive, Molecules 8 (2003) 374. [40] J.K. Mattoso, L.H.C. Toledo, R.D. Thomas, S. De Carralho, C.H. Pothen, L. Kala, et al., in: E. Frollini, A.L. Leao, L.H.C. Mattoso (Eds.), Natural Polymers and Agrofibers Composites, IQSC/USP, UNESP and Embrapa Instrumentacao Agropecuaria, San Carlos, 2000, p. 159. [41] K.C. Manikandan Nair, S. Thomas, G. Groeninek, Compos. Sci. Technol. 61 (16) (2001) 2519. [42] Wang, MSc thesis, University of Saskatchwan, Canada (2004). [43] T.J. Keener, R.K. Stuart, T.K. Brown, Compos. Part A 35 (3) (2004) 357. [44] K. Van de Velde, P. Kiekens, Compos. Struct. 62 (2003) 443. [45] G. Cantero, Compos. Sci. Technol. 63 (2003) 1247. [46] J. Gassan, A.K. Bledzki, Compos. Part A 28 (1997) 1001. [47] M. Van den oever, T. Peijs, Compos. Part A 29 (3) (1996) 227. [48] P.V. Joseph, K. Joseph, S. Thomas, C.K.S. Pillai, V.S. Prasad, Compos. Part A 34 (3) (2003) 253. [49] A.K. Bledzki, S. Reihmane, J. Garsan, J. Appl. Polym. Sci. 59 (1996) 1329. [50] S. Mohanty, S.K. Nayak, S.K. Verma, S.S. Tripathy, J. Reinf. Plast. Comp. 23 (6) (2004) 625. [51] S. Mishra, J.B. Naik, Y.P. Patil, Compos. Sci. Technol. 60 (9) (2000) 1729. [52] A. John, M.W. Schroder, M. Futing, K. Schenzel, W. Diepenbrock, Spectrochim. Acta 58A (2002) 2271. [53] A. Valadez-Gonzale, J.M. Cervantes-Uc, R. Olayo, P.J. Herrera-Franco, Compos. Part B 30 (1999) 309. [54] M.N. Belgacem, P. Bataille, S. Sapieha, J. Appl. Polym. Sci. 53 (1994) 379. [55] I. Sakata, M. Morita, N. Tsurata, K. Morita, J. Appl. Polym. Sci. 49 (1993) 1251.

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[56] J.M.F. Paiva, E. Frollini, Macromol. Mater. Eng. 291 (2006) 405. [57] P.V. Joseph, K. Joseph, S. Thomas, C.K.S. Pillai, V.S. Prasad, G. Groenenckx, et al., Compos. Part A 34 (2003) 253. [58] O.L.S. Alsina, L.H. de Carvalho, F.G. Ramos Filho, J.R.M. d’Almeida, Polym. Test. 24 (2005) 81. [59] P. Ganan, S. Garbizu, R. Llauo-ponte, I. Mondragon, Polym. Compos. 26 (2005) 121. [60] A.K. Rana, R.K. Basak, B.C. Mitra, M. Lawther, A.N. Banerjee, J. Appl. Polym. Sci. 64 (1997) 1517. [61] L.Y. Mwaikambo, M.P. Ansell, Angew. Makromol. Chem. 272 (1999) 108. [62] Li, J. He, Polym. Degrad. Stab. 83 (2004) 241. [63] Y. Tsuchiya, K. Sumi, J. Polym. Sci. Polym. Lett. 6 (1969) 356. [64] E. Kiban, J.K. Gillham, J. Polym. Sci. 20 (1976) 2045. [65] X. Colom, F. Carrillo, F. Nogues, P. Garrige, Polym. Degrad. Stab. 80 (2003) 543. [66] K.K. Pandey, J. Appl. Polym. Sci. 71 (1999) 1969. [67] P.A. Evans, Spectrochim. Acta 47A (9/10) (1991) 1441. [68] K.V. Sarkanen, H.M. Chang, TAPPI J. 50 (11) (1967) 572. [69] X. Colom, F. Carrillo, Eur. Polym. J. 38 (11) (2002) 2225. [70] M.L. Nelson, R.T. O’Connor, J. App. Polym. Sci. 8 (1964) 1311. [71] M. Das, D. Chakraborty, J. Appl. Polym. Sci. 102 (2006) 5050. [72] N. Abidi, E. Heguet, D. Ethridge, J. Appl. Polym. Sci. 103 (2007) 3476. [73] L.Y. Mwaikambo, E.T.N. Bisanda, Polym. Test. 18 (1999) 181. [74] V. Fervel, S. Miscler, D. Landolt, Wear 254 (2003) 492. [75] V. Titok, V. Leontiev, L. Shostak, L. Khotyleva, J. Nat. Fibers 3 (1) (2006) 35. [76] N.E. Zafeiropoulos, P.E. Vickers, C.A. Baillei, J.F. Watts, J. Mater. Sci. 38 (2003) 3903. [77] T. Peijs, e-polymers T002 (2002) 1. [78] S. Peterson, K. Jayaraman, D. Bhattacharyya, Compos. Part A 33 (2002) 1123. [79] X.W. Yuan, K. Jayaraman, D. Bhattacharyya, Sisal fiber and its composites: The effects of plasma treatment, ACCM-3 15
17 July, Auckland (2002) 615. [80] L.T. Drzal, A.K. Mohanty, M. Misra, Polym. Prepr. 42 (2001) 31. [81] K. Joseph, C. Pavithran, M. Brahma Kumar, S. Thomas, J. Appl. Polym. Sci. 47 (1993) 1731. [82] S.J. Eichhorn, C.A. Baillie, N. Zafeiropoulos, L.Y. Mwaikambo, M.P. Ansel, A. Dufresne, et al., J. Mater. Sci. 36 (2001) 2107. [83] G.C. Escamilla, J.R. Laviada, J.I.C. Cupul, E. Mendizabal, J.E. Puig, P.J.H. Franco, Compos.: Part A 33 (2002) 539. [84] P. Flodin, P. Zadorecki, in: E. Seferis, B. Stark (Eds.), Composite Systems From Natural and Synthetic Polymers, Elsevier, 1986, p. 59. [85] P. Zadorecki, A.J. Michell, Compos. Sci. Technol. 27 (1986) 291. [86] L. Hua, P. Flodin, T. Ronnhult, Polym. Compos. 8 (1987) 203. [87] D.N. Sahb, J.P. Jog, Adv. Polym. Technol. 18 (1999) 351. [88] Opportunities for Natural Fibers in Plastic Composites, Kline and Company, Inc, Little Falls, NJ (2000). [89] U. Reidel, J. Nickel, Angew. Makromol. Chem. 272 (1999) 34. [90] K. Okubo, T. Fuji, First International Workshop on ‘Green’ Composites, 19
20 November, Tokushima (2002) 17. [91] Y. Yamamoto, et al., First International Workshop on ‘Green’ Composites 19
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[93] D.L. Kaplan (Ed.), Biopolymers From Renewable Resources, Springer, New York, 1998. [94] Wool R.P., Proceedings on ICCE-6 27 June
3 July, Orlando, FL (1999) B45. [95] A.N. Netravali, S. Chabba, Mater. Today 6 (2003) 22. [96] K. Van de velde, P. Kiekens, J. Appl. Polym. Sci. 83 (2002) 2634. [97] R.M. Rowell, R.A. Young, Y.K. Rowell, Paper and Composites From Agro-based Resources, CRC Lewis Publishers, Boca Raton, FL, 1997. [98] V.A. Alvarez, R.A. Ruscekaite, A. Vazquez, J. Compos. Mater. 37 (17) (2003) 1575. [99] L. Uma Devi, S.S. Bhagawan, S. Thomas, J. Appl. Polym. Sci. 64 (9) (1997) 1739. [100] S. Biswas, G. Srikanth, S. Namgia, Proceedings on COMPOSITES 2001, 3
6 October, Tampa, FL (2001). [101] D. Puglia, A. Tomassucci, J.M. Kenny, Polym. Adv. Technol. 14 (11
12) (2003) 752. [102] V.P. Cyras, J.F. Martucci, S. Iannace, A. Vazquez, J. Thermoplast. Compos. 15 (3) (2002) 253. [103] T. Schuh, U. Gayer, in: A.L. Leao, F.X. Carvallo, E. Frollini (Eds.), Lignocellulosic Plastic Composites, UNSEP, Sao Paolo, 1997, p. 181. [104] M. Richardson, Z. Zhang, Reinf. Plast. 4 (2001). [105] ,www.oakdenehollins.co.uk/sti-funded-projects.html. [accessed May 2008]. [106] G. Marsh, Mater. Today 6 (2003) 36.

Introduction to tribology of polymer composites

2.1

2

What is tribology?

We come across tribology in everyday life. Since our childhood we have been taught that during the evolution of man, fire was discovered when two stones were rubbed against each other. Since then the “rubbing” between two surfaces has caused a sort of revolution, and today this has become a full-fledged branch of science, now known as tribology (derived from the Greek word tribos which means rubbing) [1]. The friction between the stones which led to the discovery of fire has had serious implications and bearing on man’s life. Life cannot exist without friction. If there is no friction between ground and feet we cannot walk; vehicles cannot move on the roads if there is no friction between the road and the tyres; we cannot write on paper without friction between the tip of the pen and the paper. An eraser will not rub out the pencil marks on the paper. In fact we cannot hold a pen if there is no friction between pen and fingers. As we grow and learn to drive a vehicle, we realize that almost no friction is required between the machine parts like the piston and cylinder in the engine, the gears and bearings, and numerous other moving parts. Lubricants in the form of engine oil and grease are used to increase the life of a component. Likewise, the face cream we use should be smooth enough to be applied to the face without causing harm to the skin. The tanning cream used for sunbathing consists of nanoparticles of a solid lubricant so that it can be spread and applied smoothly. The friction between hair, shaving, wear due to cutting by teeth, and wear and friction caused at joints in the human body sandblasting, polishing, and grinding are all examples of tribology. In fact, tribology exists everywhere right from the human body to microelectromechanical systems (MEMS), to aircraft components and the Earth’s tectonic plates. Studies of all the three factors mentioned above, namely friction, wear, and lubrication of materials, comprise the science of tribology. On a broader perspective, it deals with studies of the interface between two or more bodies in relative motion, such as in gears, bearings, piston
cylinder assembly, and gyroscopes (Fig. 2.1). The interactions at the interface between two surfaces cause friction and wear of the materials involved. These interactions lead to the transmission of forces and dissipation of mass (wear) and energy (friction). Friction and wear are interrelated in the sense that frictionless processes will not result in wear. On the other hand, increasing friction forces does not always result in increased wear loss. The consumption of energy alters the physical and chemical behavior of the materials, changes the surface topography, and causes generation of loose wear particles (wear debris). Since the consumption of mass as well as energy is involved, the Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00002-5 © 2021 Elsevier Ltd. All rights reserved.

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Tribology of Natural Fiber Polymer Composites

Figure 2.1 Tribological components; (A) ball bearings; (B) roller bearings; and (C) gears.

subject of tribology assumes paramount significance. A significant amount of energy is consumed ultimately in friction processes and a similar amount of energy is lost as a result of wear processes. Thus a scientific effort is required to develop materials and technologies that can control the wear and friction of components and increase their life.

2.2

Origin of friction

One of the most important aspects of tribology, and one that is still being investigated today, is how friction is generated at the atomic level. The genesis of friction between two surfaces in relative motion is the key to finding an answer to the everincreasing problem of huge financial losses due to wear and friction. Friction is closely related to the energy dissipation at the surface. Whenever work is done by a thermodynamic system, heat is generated and this heat is equal to a change in the internal energy of the system plus the work done by the system. This is known as the first law of thermodynamics. In the case of friction, when external work is done by a frictional force it should be equal to the energy dissipated plus the change in the internal energy. Since the change in internal energy is very small compared with energy dissipated, friction is mainly due to the latter. Friction is the measure of energy dissipated, which is caused by chemical and mechanical damage. These damage processes take place in the bulk of the material (plowing) and at the

Introduction to tribology of polymer composites

63

Figure 2.2 Typical friction curve.

interface (adhesion) [2]. Consequently, theories of friction are based on adhesion and plowing. According to an earlier theory of friction based on adhesion alone, it was assumed that asperities of the surfaces in contact form welded junctions that shear during sliding causing friction. Thus friction was thought to be dependent on the actual or real area of contact1 which, in turn, depends on the applied and tangential load. However this theory could not explain satisfactorily the large discrepancy between the theoretical and experimental values of the friction coefficient. Moreover, the reasoning that metals with greater solubility form junctions readily and, thus, have higher wear and friction coefficients did not hold much water because phenomena such as chemisorption and physisorption easily contaminate the surface and the compositions of surface and bulk are quite different. Hence, the adhesion theory was modified using the combined effects of surface (adhesion, asperity deformation) and bulk (plowing) properties. According to the modified theory, the coefficient of friction μ between the sliding surfaces is a combination of asperity deformation μd, plowing by wear particles and hard surface asperities μp, and the adhesion between the flat surfaces μa. The individual contributions of the three mechanisms depend on the contact surface topography, operating conditions, and the type of material. A typical curve between coefficient of friction and sliding distance is shown in Fig. 2.2. From this figure it is clear that initially adhesion does not play any significant role at this stage due to the contaminated nature of the surface. Similarly, asperity deformation is not of much significance because the asperities in contact deform as soon as sliding commences and the surface is easily polished. As sliding progresses, the frictional force increases due to enhanced adhesion and hence a small increase in μ is observed. The slope of this curve would have increased had there been entrapped wear particles, which would have plowed the surfaces. With further increase in sliding duration, μ increased linearly due to plowing by the entrapped wear particles. If the entrapped wear particles are of equal hardness then 1

Real area of contact is the area of asperities in contact.

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Tribology of Natural Fiber Polymer Composites

they can plow both of the surfaces and the plowing will be greater. With further increase in sliding time, μ becomes steady as the number of entrapped particles between the interface becomes constant. The number of entrapped particles leaving the surface becomes equal to the particles leaving the interface. However these mechanisms depend on the experimental conditions and the nature of the materials. The initial increase in the value of μ is known as the static friction coefficient and the steady state is known as the dynamic friction coefficient. The contribution of asperity deformation is substantial for the static coefficient of friction and minimal for the dynamic friction coefficient. Similarly, the contribution of adhesion and plowing is substantial for the dynamic friction coefficient and minimal for the static friction coefficient. Friction force increases due to adhesion of surfaces in contact as a result of welding and formation of junctions. Due to contamination of surfaces, adhesion is not good initially (static friction coefficient) but, as sliding progresses, the deformation of asperities exposes new surfaces and adhesion increases. The dynamic friction coefficient is attributed more to the plowing component of the frictional force. Either the entrapped wear particles or the hard asperities of the surface or both can penetrate the surface. This results in the formation of grooves. However when either of the two surfaces is very hard and smooth, the wear particles slide along the hard surface and no plowing occurs. When the hard surface is very rough, wear particles plow the softer surface and create debris which causes wear of sliding surfaces. This becomes evident when ridges are formed along the sides of plowed grooves when observed through scanning electron microscopy (SEM). If the wear debris is either viscoelastic or plastic material, it sticks to the sliding interface and undergoes repeated deformation, consuming energy. This increases the friction force. Based on the three friction mechanisms, namely asperity deformation, adhesion, and plowing as discussed above, a quantitative treatment is proposed to determine the total friction coefficient. It is assumed that since plowing is a kind of deformation the friction force can be divided into a deformative component and an adhesive component [3]: μf 5 μa 1 μd

(2.1)

where μf is the friction force, μa is the adhesive friction force, and μd is the deformative friction force. The friction coefficient can be expressed analogously by: μ 5 μa 1 μd

(2.2)

As mentioned in the preceding paragraphs, the value of each component depends on the operating conditions, nature of materials, and topography of contact surface. Qualitatively, these factors are represented by the real contact surfaces, the shear strength of the contacts, and the way in which the material is sheared and fractured in and around the contact zone during sliding [4]. In terms of the shear strength of the contacts τ s, the adhesive friction force Fa is given by:

Introduction to tribology of polymer composites

F a 5 τ s Ar

65

(2.3)

where Ar is the real contact surface. Thus the adhesive friction coefficient is given by: μa 5

Fa τ s Ar 5 FN FN

(2.4)

where FN is the normal load. The adhesive friction coefficient in Eq. (2.4) depends on the adhesive interaction (intermolecular interaction between solid state bodies) and the real area of contact. The minimum energy required to separate the two surfaces in contact is given by [5]: Wab 5 γ a 1 γ b 2 γ ab

(2.5)

where γ a and γ b are the surface energies of the separated surfaces and γ ab is the boundary surface energy of the two contacting surfaces. The real area of contact for an elastic contact Ar is approximately proportional to the normal force FN and inversely proportional to the Young’s modulus E. Hence 

E Ar 5 c FN

n (2.6)

where n , 1 and c is a proportionality constant [6]. The more generalized form of Eq. (2.6) is given in terms of the relative contact area, that is, the ratio of the real area of contact Ar and the apparent area of contact Ao. Thus Φ5

  Ar 2 βp 5 1 2 exp E Ao

(2.7)

where β is the roughness coefficient and p is the normal pressure. Eq. (2.7) indicates that the influence of Young’s modulus and the normal pressure is very small compared with that of relative contact area Φ. Using the above substitutions in Eq. (2.4) and replacing Ar by Φ, the equation for the adhesive component of the friction coefficient is given by: τs
 μa 5  p 1 2 exp 2βp=E

(2.8)

Eq. (2.8) indicates that the adhesive component of the friction coefficient can be reduced if the Young’s modulus of the material is increased and the shear strength of the contacts is decreased. The Young’s modulus of a material also influences the depth of penetration which, for elastic deformation, is given by [7]: h 5 chmax n1

p E

n2

(2.9)

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Tribology of Natural Fiber Polymer Composites

where h is the depth of penetration, hmax is the maximum roughness, c is the proportionality constant, and n1, n2 are exponential factors (0 , n1, n2 , 1). Similarly, the depth of penetration for plastic deformation is given by [7]:  h 5 chmax

p σdF

n2 (2.10)

where σdF is the compression yield point or hardness. From Eq. (2.9) it is clear that resistance to deformation is proportional to the depth of penetration. It is also known that friction is energy dissipation which is proportional to mechanical loss factor, tan δ. Thus a relationship is suggested for the deformation component of friction: μd 5 ξ

p E

tanδ

(2.11)

where ξ depends on the characteristic of the surface. Using Eqs. (2.8) and (2.11), the friction coefficient is given by: p τs
 1 ξ μ5  tanδ E p 1 2 exp 2βp=E

2.3

(2.12)

Definition of wear and its classification

Wear is defined as the progressive loss of material from the operating surface of a body as a result of relative motion at the surface [8]. It is caused by disintegration of the interacting components as a result of overstressing of the material in the immediate vicinity of the surface. Wear is not catastrophic but, in most cases, it certainly reduces operating efficiency. It results in dimensional changes of the components or damage to the surface. This causes an associated problem of vibrations and/or misalignments. The propagation of cracks formed at or near the stressed surface may in extreme cases lead to fracture of the component. Components lose their applicability as a result of change in dimensions due to surface damage or wear. In the wear process the amount of material removed is quite small which makes it difficult to detect wear by casual inspection. However the change in shape, like obsolescence and surface damage in some parts, becomes very obvious. The common forms of wear are defined as follows. Adhesive or sliding wear occurs when two bodies slide over each other and fragments are pulled off from one surface, which then adhere to the other surface. These fragments or loose wear particles further come off from the deposited surface and are transferred back to the original surface causing wear in successive cycles. This form of wear is generally termed as adhesive wear. It arises from the strong

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Figure 2.3 Schematic of (A) two- and (B) three-body abrasive wear.

adhesive forces set up whenever atoms come into close contact. During sliding, a small area on one surface comes into contact with a similar patch on other surface; there is a probability, small but finite, that when this contact is broken, break will occur not at the original interface but within one of the materials. Abrasive wear occurs when a rough and hard surface or a soft surface containing hard particles slide on a softer surface and plow a series of grooves in it. Material from the grooves is removed in the form of loose wear particles. Abrasive wear also arises when hard abrasive particles are introduced between sliding surfaces. The two forms of wear, one involving a hard rough surface and the other hard abrasive grains, are generally referred to as the two-body and three-body abrasive wear process, respectively, as shown in Fig. 2.3. Two-body abrasive wear does not occur when the hard, sliding surface is smooth. Similarly, three-body abrasive wear does not occur when the particles in the system are small or when they are softer than the sliding materials. Corrosive wear in material generally occurs when sliding takes place in a corrosive environment. In the absence of sliding, the products of corrosion would form a film on the surfaces, which would tend to slow down or even arrest the corrosion. The sliding action wears the film away for corrosive attack to continue. Erosive wear is defined as the damage produced by sharp particle impingement on the body. This form of wear resembles abrasion in the sense that both types are caused by hard particles. In erosion, the surface roughness produced becomes relatively greater because an impinging particle readily removes material from a low point on the surface. Fatigue wear occurs due to repeated slides or rolling over a track. The repetitive loading/unloading cycles of materials exposed induce formation of loose fragments of surface material or subsurface cracks. This results in breakup of the surface with formation of loose fragments, leaving large pits in the surface. Fretting wear or low-amplitude oscillating wear (LAOW) arises when contact surfaces undergo oscillatory tangential displacement of small amplitude. The motion is so small (per cycle) that it is difficult to anticipate the overall large volume of wear debris produced in the process. When the amplitude is large it is known as reciprocating wear. Cavitation: When a portion of a liquid is under tensile stresses, it boils and the bubbles suddenly collapse to produce a mechanical shock. A solid surface in the neighborhood is damaged by this shock leading to the removal of a particle. This process is called cavitation. It is analogous to surface fatigue wear. Materials

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resistant to surface fatigue wear, namely hard but not brittle substances, are resistant to cavitation. Resistance to corrosion attack by liquid, however, is an additional requirement for cavitation resistance.

2.4

How friction and wear are measured

Tribomachines are used to carry out tribological tests to measure the friction and wear coefficient of a material. The tests mainly comprise of two solid specimens in contact with each other as well as with the surroundings or lubricant. The operating variables act together or individually on the solid bodies in contact, lubricants, etc. Test specimens having simple geometry are generally utilized with well-defined test conditions. One of the moving specimens has a rotating cylinder or disc against solid specimen leading to point, line, or flat contact.

2.4.1 Contact configurations Some of the universally accepted contact configurations discussed below are shown in Fig. 2.4 [9]. These include multiple spheres, crossed cylinders, pin on flat (reciprocating or linear motion; moving pin, moving flat, multiple contact), flat on flat (reciprocating or linear), rotating pins on disc, pin on rotating disc, cylinder on cylinder, cylinder or pin on rotating cylinder, rectangular flat on rotating cylinder, and multiple specimens. The interaction characteristics which describe the test system are the initial contact area and its changes during the test, the initial contact pressure and its changes during the test, and the relative volumes and surface areas of the test specimens. The transmission of mechanical work occurs via the contact area whereas, for the emission of friction-induced acoustic or thermal energy into the environment, the surfaces not actually in contact play an important role.

2.4.2 Operating parameters The major operating parameters used for tribological tests are the type of motion, velocity, load, temperature, test duration, environment, and the counterface roughness. The type of motion is related to the geometry of the system. The four basic types of motion are rolling, sliding, spin and impact, or a combination of these.

Figure 2.4 Contact configurations for tribotests in sliding and abrasive wear modes.

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Load is applied to the test system as a mass (dead weight) by a spring, by hydraulic means or by electromagnetic means. In the case of static loading, the different principles of load application give the same value of the load force FN, while in the case of dynamic loading, due to the different mass
spring damper combinations inevitably involved in the different loading principles, different load
time behavior may result. Therefore, the dynamic load FN(t) behavior can be different as compared with the initial static load FN. The load is measured using force transducers based on strain gauges, inductive elements, or pressure-sensitive detectors evaporated on the surface of the specimen in the contact zone. However measurement of the load by means of an electromechanical force transducer has the great advantage that the FN signal together with the signal of the friction force FF can be directly fed to an electronic divider which gives the actual value of the friction coefficient, μ 5 FF/FN during the test. The actual load FN applied to the sample determines, in connection with the area of contact A, the contact pressure p 5 FN/A. Since the value of A changes due to wear during the test, the value of p may also undergo some changes as a function of time. In the case of polymeric materials, an increase in the applied load usually leads to an increase in the wear and a reduction in the coefficient of friction. These tendencies are influenced by fillers. The changes in the specific wear rate of neat polymers are found if the load leads to temperatures above the glass transition temperature Tg or melting point Tm. An increasing load leads to elastic and plastic deformation at the contacting asperities which then increases the real area of contact. This increase in the real area of contact due to asperity collapse influences the frictional coefficient. Sliding speed is an important operating parameter in tribotesting because it gives the friction-induced power loss of a tribotesting system. It is responsible for frictional heating. An increase in velocity decreases the coefficient of friction and increases the wear rate [10]. However these trends depend on the type of material. Electronic or optoelectronic transducers are used to control the velocity. Temperature of the test gives an idea of the thermal state, in particular the initial stage of the test. To control the temperature, thermocouple and infrared pyrometers are used. Temperature has a significant influence on the properties of physics-based systems. The mechanical properties and damping characteristics in particular change rapidly around the characteristic temperatures such as Tg. Under nondry conditions, however, or when an internal solid lubricant such as polytetrafluoroethylene (PTFE) is used, this effect is not observed, and the coefficient of friction tends to drop with increasing temperature. Thermal softening of polymers can lead to a drop in surface hardness that can lead to increases in the real areas of contact. This can lead to rapid increases both in the coefficient of friction and the wear rate, which can be controlled by fillers. Sliding duration: During tribological testing, operating variables such as load FN, velocity v, and temperature T act as a function of time or duration t. The test duration t depends on the nature of the investigation. The tribotesting operation variables discussed so far refer to the test wherein the triboelements are entirely within a closed specimen chamber. When these elements interact with lubricant

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and different environment, the flow rates also constitute important operating parameters. Counterface roughness: The influence of counterface roughness is complicated by the topographical modification caused either by polymer transfer or by polishing through abrasive action of filler debris.

2.4.3 Sliding wear test A common laboratory device used universally for friction and wear measurement is the pin-on-disc machine. A schematic of the contact configuration of pin on disc is shown in Fig. 2.5. Cylindrical or rectangular pin specimens are held against a rotating steel (or other hard material) disc counterface. The load on the sample is applied by means of a hydraulic system and the pressure is computed by dividing the load by the nominal contact area. The frictional force is measured by means of strain-gauge force transducer. In the dry sliding wear tests, the disc is rotated at a constant speed without any lubrication. Temperature rise during the tests is monitored using a thermocouple inserted into the sample (stationary) very close to the end being abraded. Wear rate is calculated either from weight loss measurements or height loss that is converted into volume loss using density data.

2.4.4 Abrasive wear tests Abrasion resistance of a material is defined as the ability of the material to withstand mechanical action such as rubbing, scraping, or erosion, which tends progressively to remove material from its surface. It is significantly affected by factors such as test conditions, type of abrasive, and development and dissipation of heat during the test cycles. Many different types of abrasion-measuring equipment are available, but the relation between test results and actual abrasion-related wear remains very poor. Nevertheless, the test provides a relative rating of materials when performed under a specified set of conditions.

Figure 2.5 Pin-on-disc contact configuration.

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Tabor abrader: The most widely accepted abrader in the industry is the Tabor abrader [11]. Wheels of varying degrees of abrasiveness are used and the test sample is generally a 4 in. diameter disc or 4 in. [2] plate having both surfaces substantially plane and parallel. A 1.4 in. diameter hole is drilled in the center. The specimen is conditioned using standard conditioning practices prior to testing. To commence testing, the test specimen is rotated on a turn table under a pair of weighted abraded wheels that produces abrasion through slide slip. The turn table is started and an automatic counter records the number of revolutions. Most tests are carried out using at least 5000 revolutions. The specimen is weighed to the nearest milligram and the test results are reported as weight loss in mg/1000 cycles. The grade of abrasive wheel along with the amount of load at which the test is carried out is reported. This machine is very useful for characterizing the abrasion resistance of polymers, composites, blends, and wood plastic composites, used for making furniture and floor tiles, in the laboratory using the procedure described above. ASTM standards: The ASTMD1044 test method is available for estimating the resistance of transparent plastic materials to one kind of abrasion by measurement of its optical effects. The test is carried out in a similar manner to that described above, except that 100 cycles with a 500 g load are normally used. A photoelectric photometer is used to measure the light scattered by the abraded track. The ratio of the transmitted light to that which is diffused by the abraded specimens is recorded as a test result. Volume loss in a flat specimen subjected to abrasion with loose abrasives or a bonded abrasive such as cloth or paper is measured by the ASTMD1242 test method. The above test methods cover the determination of the resistance to abrasion of flat surfaces of plastic materials, measured in terms of volume loss by using the two different abrasion testing methods listed below: G

G

Test method A: loose abrasives; Test method B: bonded abrasives on cloth or paper.

These test methods provide a means of measuring the durability of plastic materials for a given application. The resistance to abrasion or the abrasion loss measured as the loss in volume at 1000 revolutions is calculated as: Volume loss ðm3 Þ 5

w1 2 w2 ρ

(2.13)

where w1 is the initial weight, w2 is the final weight, and ρ is the density of the material being abraded. The abrasive wear resistance of polymeric materials has been determined using a variety of methods. Among the most common are rubbing of a specimen surface against a coated abrasive belt (shown schematically in Fig. 2.6) and the measurement of the comparative wear resistance of flat specimens by lapping them on a lap master plate with abrasive particles of known size. The wear resistance in these tests is affected by the hardness, chemical composition, toughness, and the microstructure of the materials; the size of the specimens; the hardness of the abrasive particles; the geometry and size of the abrasive grains; and the environment.

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Figure 2.6 Schematic of two-body abrasion test apparatus.

Worn surface analysis: Since friction and wear processes are surface phenomena, surface investigation techniques play an important role in analyzing friction and wear mechanisms. Microscopic observations are utilized to study plane surfaces. Quantitative measurements of surface irregularities, for example, steps, scratches, or grooves on smooth surfaces, are carried out with an optical microscope. Peak to valley surface roughness values up to 0.015 μm are measured in this way. The technique also provides a means to view a three-dimensional (3D) picture of the surface roughness. Surface roughness data such as the average surface roughness Ra (center line average) value is determined for different cross-sections of the profile by following an individual interference line with a plantimeter. The application of optical microscopy techniques for investigating the worn surfaces is limited to the study of relatively plane and smooth surfaces because of the poor depth of focus of optical techniques, being only 0.1 μm at a magnification of, say, 500 times. This difficulty has been overcome with the introduction of the SEM. Due to the extremely short wavelength of the electron beam, both high magnification and high depth of focus are obtained at a magnification of 5000 times. The depth of focus is higher than 10 μm. Therefore, the SEM technique is conveniently used to study relatively rough surfaces as shown in the micrographs of worn surfaces. The atomic force microscope (AFM) which gives resolution at atomic scale is also used to study surface roughness and nanotribological characterization.

2.5

Mechanical characterization of polymer composites

Friction and wear are not intrinsic material properties but depend on experimental parameters. Various material properties, such as surface energy, cohesive and

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adhesive strength, interaction, and adhesion with the counterface, ultimate tensile strength, elongation to break, hardness, creep, hydrogen bonding, etc., are responsible for difference in wear behavior [12]. However no direct correlation exists between wear and mechanical properties such as elongation, tensile, flexural, impact, or hardness of a material. In the case of short fiber-reinforced or particulate-filled composites, Ratner
Lancaster plots (wear resistance is directly proportional to the product of tensile strength S and elongation e) generally hold good. Some relations between wear resistance and hardness (H) or products of all three characteristics (S, e, and H) have also been reported. However these relations vary from material to material and are universally invalid. In general, the mechanical characterization of fiber-reinforced composites is carried out using stress
strain curves following ASTM standards, and strength properties such as tensile (ASTM 638), flexural (ASTM 790), and interlaminar (ASTM 2344) strength are determined. These properties are then used to explore the correlation between wear resistance and friction coefficient. In the case of composites, fiber and reinforcements generally increase the loadcarrying capacity and strength, and reduce the extent of interaction of the polymer with the counterface. Fibers are wear resistant and wear preferentially to the matrix. The performance of fiber-reinforced polymers depends on the type of fiber and matrix, concentration, distribution, aspect ratio, alignment, and its adhesion to the matrix. As seen in the following equation, the higher the aspect ratio (l/r, where l and r are the length and radius of the fiber, respectively), the more will be the contact load transferred from the matrix to the fiber and the greater the degree of wear resistance (inverse of wear rate): σf 5

τ2 1 σm r21

where σf is the contact stress, σm is the compressive stress of the matrix in the composite loaded against counterface under a load W, and τ is the tangential stress produced because of the difference between the moduli of matrix and fiber. It is, however, not the case that with increase in the concentration of fibers, the wear resistance increases continuously. In fact, it either deteriorates or becomes constant beyond a typical optimum concentration. Generally, short fibers (0.1
6 mm) approximately in the 20%
30% range are used for reinforcement and reducing wear of thermoplastics. Although continuous fibers reinforced unidirectionally or multidirectionally (2D or 3D composites) in thermosets or thermoplastics definitely enhance the wear resistance of composites very significantly, short fibers are more favored because of the potential for easy, cost-effective, and rapid moldability. Glass fibers (effective in reducing wear but generally affecting μ adversely), carbon and graphite fiber (effective in reducing both, apart from enhancing thermal conductivity), and aramid fibers (effective in reducing both friction and wear) are the most favored fibers in tribocomposites. The performance and stability of fiberreinforced composite materials depend on the development of coherent interfacial bonding between fiber and matrix.

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As discussed in the previous chapter, in natural fiber-reinforced composites there is a lack of good interfacial adhesion between the hydrophilic cellulose fibers and the hydrophobic resins due to their inherent incompatibility. Short, cellulose-based fibers will also tend to agglomerate. The presence of waxy substances on the fiber surface contributes immensely to ineffective fiber to resin bonding and poor surface wetting. Similarly, the presence of free water and hydroxyl groups, especially in the amorphous regions, worsens the ability of plant fibers to develop adhesive characteristics with most binder materials. High water and moisture absorption of the cellulose fibers causes swelling and a plasticizing effect resulting in dimensional instability and poor mechanical properties. Plant fibers are also prone to microbiological attack leading to weak fibers and reduction in their lifespan. Fibers with high cellulose content have also been found to contain high crystallite content. These are the aggregates of cellulose blocks held together closely by the strong intramolecular hydrogen bonds in which large molecules, for example dyes, are not able to penetrate unless the cell wall is swollen. Fibers are, therefore, usually subjected to treatment such as alkalization and acetylation, with or without heat, to first bulk or swell the cell wall to enable large chemical molecules to penetrate the crystalline regions.

2.5.1 Impact strength of natural/synthetic fiber-reinforced polymer composites There are no studies suggesting that impact strength of polymer composites directly influences the tribological properties such as wear and friction performance that are surface-related properties of composites. The exception is the case of erosive wear, in which case high impact strength can be beneficial in resisting the impact of impinging particles. Since natural fiber-reinforced polymer composites are used in automotive applications such as panels and decking, impact resistance is a critical parameter. Consequently, many studies have been carried out to study the impact strength of such composites, and this is discussed in detail in subsequent chapters. A comparison of the impact properties of different natural fiber-reinforced composites such as sisal, pineapple, banana, and coir shows that sisal fiber-based composites possess the highest impact toughness owing to the optimal microfibrillar angle of the fiber (21 degrees for sisal, 12 degrees for banana, 14 degrees for pineapple, and 45 degrees for coir) [13]. The toughness of composites increased with the microfibrillar angle of the fibers and reached a maximum at 15
20 degrees. It then decreased with increasing angle. The optimal microfibrillar angle of sisal fiber leads to better impact resistance with a work of fracture of 98.7 kJ/m2 when the fiber volume fraction was 50%. For the same volume fraction of pineapple fibers, this was 79.5; banana fibers 51.6; and coir fibers 43.5 kJ/m2. The specific work of fracture for natural fiber composites compares well to that of synthetic fiber composites. The specific work of fracture (i.e., toughness per unit density) of 60% volume fraction sisal fiber
polyester composites was 115 kJ/m2 while for ultra-high molecular weight polyethylene (UHMWPE) and E-glass fibers, these values were 125 and 165 kJ/m2, respectively.

Introduction to tribology of polymer composites

2.6

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Tribology characterization of polymer composites

2.6.1 Friction coefficient The friction coefficient μ is defined as the ratio between the friction force FF needed to create sliding and the normal force FN which presses the two bodies against each other: μ5

FF FN

The static friction coefficient μs is measured just before sliding and the dynamic friction coefficient μd is measured during sliding. The latter, widely used as a reference, is a useful parameter.

2.6.2 Wear formula Polymers are viscoelastic plastic materials in which the hardness changes continuously as a function of the indentation time. Therefore, the wear behavior of these materials is not characterized in terms of the wear coefficient defined by Eq. (2.14) since it involves hardness: V5

KLS 3H

(2.14)

where V is the wear volume, L is the load, S is the sliding distance, and H is the hardness. Instead, wear characteristics of polymers and fiber composites are measured using specific wear rate Ko (or wear coefficient) and pv limit. The wear coefficient is defined as Ko 5 V/LS 5 V/pvt where V is the wear volume, v is the sliding speed and t is the sliding duration. Ko is not a dimensionless quantity. The pv limit is used to define the onset of catastrophic failure of polymers and composites due to melting and extrusion. Since the product of the normal load p and the velocity v is not exactly proportional to the temperature rise, the value of the pv limit is not a constant, but varies depending on the specific load and the specific velocity. Therefore, the pv limit has to be specified in terms of a limiting load at a given sliding speed or in terms of a limiting speed at a given load.

2.6.3 Wear mechanism Wear of polymers occurs due to plastic deformation, brittle fracture, and fatigue. Hence, they are reinforced with fibers and the resultant composite supports the dynamic stresses induced by an applied load and the tangential frictional stresses that ultimately prevent wear. Wear of polymer composites is not an intrinsic material property, but depends on the wear mechanism (abrasion, adhesion, fatigue, and chemical degradation) external conditions (temperature, contact pressure, velocity,

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and environment), relative movements (sliding, multiple/single pass, reciprocating, and impacting), contacting materials (roughness and hardness of the metallic counterparts, bonded or loose sharp particles, and third-body interface), and composite materials (polymer matrix, type and structure of reinforcement, filler/matrix interface, and internal lubricants). Triboproperties are also very sensitive to the environmental conditions (such as atmosphere, humidity, and temperature) in addition to the experimental parameters. The influence of some of the operating parameters on the tribological properties of polymer composites has already been discussed in Section 2.4.2. They are reproduced in the subsequent section for immediate reference.

2.6.4 Effect of operating parameters G

G

G

G

Load: In the case of polymeric materials, an increase in the applied load increases the wear and reduces the friction coefficient. An increasing load leads to elastic and plastic deformation at the contacting asperities which then increases the real area of contact. This increase in the real area of contact due to asperity collapse influences the friction coefficient. Sliding speed: An increase in velocity decreases the friction coefficient and increases the wear rate. However these trends depend on the type of material. Temperature: The friction coefficient tends to drop with increasing temperature. Thermal softening of polymers decreases surface hardness and increases the real areas of contact. This can lead to rapid increases in both the friction coefficient and wear. Counterface roughness: There exists an optimum surface roughness of the counterface at which the friction and/or wear rate is minimal. However the results are contradictory because while for some composites the minima of friction coefficient and specific wear rate match the minimum of average surface roughness Ra, for other composites no such matching is found.

2.6.5 Effect of fiber reinforcement Fiber reinforcement in polymer matrices is an effective method of developing, tribologically significant composites. Various types of fiber reinforcement, such as discontinuous or short (chopped) fiber reinforcement, continuous or long (unidirectional) fiber reinforcement, and woven fabric or bidirectional reinforcement, are available, and these offer a great deal of scope to tailor a range of properties. For polymers that possess high-specific wear rates in the unreinforced condition, almost any type of reinforcing fiber results in significant reductions in wear and improvement in mechanical properties. Commonly used synthetic fibers such as glass, carbon, graphite, and aramid are all available as short, long, or fabric for reinforcement in thermosets as well as thermoplastics. Among all these fibers, continuous carbon fibers reinforced in a thermoset resin exhibit excellent tribological properties such as low wear rate in sliding wear mode in severe operating conditions [14
16]. This was attributed to the fact that the high fiber contents that are made possible with thermosetting resins and the preferential load bearing by the fibers ensure that tribological properties are controlled by fiber reinforcement.

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Figure 2.7 Friction and wear of various fiber-reinforced polymer composites in sliding wear mode (p 5 1.5 N/mm, v 5 0.83 m/s, D 5 16 km; blank box denotes parallel, hatched box denotes antiparallel, and solid box denotes normal direction of fibers to the sliding direction) [17].

Besides type of reinforcement, the fiber orientation (parallel, antiparallel, and normal to the sliding plane) also affects the wear of composites in sliding wear mode. In the case of unidirectional carbon fiber composites, the effect is reported to be minimal while, for the cases of carbon, glass, aramid, and steel fiber composites, the effect is reported to be substantial in similar operating conditions. The effect of the type of fiber reinforcement and fiber orientation on specific wear rate and friction coefficient is best summarized in Fig. 2.7 [17]. High strength carbon fiber-reinforced plastic (HS-CFRP) and high modulus carbon fiberreinforced plastic (HM-CFRP) exhibit a lower Ko of the order of 10216 m3/Nm and a low friction coefficient of 0.2 while glass-reinforced and steel-reinforced composites exhibit a high Ko of the order of 10213 m3/Nm and a high friction coefficient of 0.4. The excellent triboproperties of carbon fiber-based composites are due to the better mechanical properties, such as tensile modulus and interlaminar shear strength, and also to the self-lubricating ability and high strength of carbon fibers. Carbon fabric-reinforced thermoplastics such as polyimide and polyetherimide also exhibit better triboproperties as compared with glass fiber reinforcement in different wear modes.

2.6.6 Effect of filler Fillers not only enhance the various mechanical properties of polymers but also strongly influence the wear performance of polymers as well as composites. As

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mentioned in the preceding paragraphs, the tribological characteristics, that is, the friction coefficient and the wear resistance, are not intrinsic properties of a material but depend on the system in which these materials are to function. The friction and wear characteristics of polymeric materials can be improved either by reducing their adhesion to the counterface or enhancing their mechanical properties. Both of these objectives can be achieved by using inorganic/organic fillers. To reduce adhesion, internal lubricants such as PTFE and graphite flakes are incorporated in the polymeric matrix. During sliding, PTFE film is transferred on the counterface and the friction coefficient is reduced [18]. Apart from this, fillers also enhance the thermal conductivity of the polymer composite and hence prevent an increase in the specific wear rate. However in certain cases, fillers decompose and generate reaction products which enhance the bonding between the transfer film and the counterface [2]. Sometimes the fillers decrease the wear resistance because they generate more discontinuities in the material. Table 2.1 summarizes the specific wear rates of various filler modified and/or short fiber-reinforced thermoplastic composites [19]. In recent times, polymer composites containing different fillers and/or reinforcements that are used as sliding elements have found new tribological applications in automobiles. One such example for automotive applications is the manufacture of ball joints in the car chassis. In such an application, the tribocouple is subjected to higher loads and temperatures as high as 120 C [19]. Consequently, the demand for high wear resistance becomes increasingly important. Inorganic particles enhance the mechanical properties of polymers, and this enhancement depends on the size of the particles [20
31]. Nanosized particles increase the mechanical properties significantly. Similarly, fillers enhance the thermal conductivity in composite. As far as the wear resistance is concerned, contradictory results are obtained. For instance, the wear resistance is increased when fillers decompose and generate reaction products which enhance the bonding between the transfer film and the counterface, while other fillers decrease the wear resistance because they generate more Table 2.1 Specific wear rate of various filler modified and/or short fiber-reinforced thermoplastic composites (block-on-ring, pressure 5 2 MPa; v 5 1 m/s, test duration: 8 h against steel counterface) [19]. Material

Vol.%

Specific wear rate, 10215 m3/Nm

PTFE 1 PPS 1 graphite 1 CF PTFE 1 PPS 1 graphite 1 CF PTFE 1 PPS 1 graphite 1 CF PTFE 1 PEEK 1 graphite 1 CF PTFE 1 PEEK 1 graphite 1 CF PTFE 1 graphite 1 Al2O3 PTFE 1 CF 1 Al2O3 PTFE 1 PPS 1 CF PTFE 1 CF PTFE 1 CF 1 bronze

51.6 1 31.8 1 4.8 1 11.8 51.0 1 31.6 1 3.9 1 13.5 51.9 1 32.4 1 2.6 1 13.1 12.4 1 61.8 1 11.7 1 14.1 9.7 1 49.7 1 12.5 1 28.1 76.8 1 19.8 1 3.4 84.1 1 12.6 1 3.3 52.5 1 28 1 19.5 78.6 1 21.4 80 1 10 1 10

1.53 1.40 1.20 4.31 6.33 22.40 1.25 1.69 1.75 0.565

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discontinuities in the material [2]. As seen in Table 2.1, the composite containing PTFFE 1 10 vol.% bronze 1 10 vol.% CF exhibits an excellent wear resistance, mainly because of the better thermal conductivity properties of bronze. In other research papers it has been reported that while CuO and CuS are very effective in reducing the wear rate of PEEK [20] and PA11 [21
23], ZnF2, ZNS, and PbS increased the wear rate of PA11 [24]. The wear resistance of PPS [25
28] could be improved by the addition of CuS, Ag2S, or NiS but was reduced by CaF2, ZnF2, SnS, PbSe, or PbTe. Fine particles seem to contribute better than large particles to property improvement under sliding wear conditions [32
35]. However if the particle size is reduced down to the nanoscale level (,100 nm), the wear performance of these nanocomposites is significantly different compared with that of micron particle-filled systems [36,37]. When these inorganic particles are used in combination with functional fillers such as PTFE powder, graphite flakes (solid lubricants), short glass, and carbon and aramid fibers, polymer composites with high wear resistance can be obtained [38
40]. A combined effect of nanoparticles with short carbon fibers exhibits a significant improvement in the wear resistance of both thermoset (epoxy) as well as thermoplastic (PA66) composites [41].

2.7

Significance of composites in tribology

Advanced materials such as polymer matrix composites (PMCs), metal matrix composites (MMCs), ceramic matrix composites (CMCs), and carbon
carbon composites comprise the four main groups of tribomaterials. Tribomaterials have a unique property profile and possess a spectrum of wear, friction, lubrication, and mechanical properties that are used in designing materials for specific tribological applications as shown in Table 2.2 [42]. The availability of a range of fibrous Table 2.2 Tribological applications of polymers and their composites [42]. Composites

Applications

Tribological trends

Continuous fiberreinforced polymer composites

Aerospace seals, bearings, can operate in high temperature conditions

Unfilled polymers, short fiber-reinforced polymer composites

Seals, gears, slideways, abrasive, and applications bearings

Thin layer composites on metals

Pivot bearings, high pressure conditions

μ . 0.09, Ko . 1028 mm3/ Nmv , 5 m/s, pv , 100 MPa m/s μ . 0.03, Ko . 1027 mm3/ Nmv , 5 m/s, pv , 150 MPa m/s μ . 0.06, Ko . 1029 mm3/ Nmv , 1 m/s, pv , 300 MPa m/s

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reinforcement, fillers, matrices, and processing techniques offers ample scope for tailoring properties in composites as required for a specific application. For instance, if high strength properties are required, such as for structural applications as in aircraft structures, radomes, then composites can be fabricated having high tensile strength and modulus, high impact strength in order to maintain structural integrity, and high abrasion
erosion resistance. For antifriction materials that are used in bearings, apart from structural and dimensional integrity, the material should possess low friction coefficient and low wear. For friction materials that are used in clutches, brakes, etc., strength properties are secondary and tribological properties such as high friction coefficient and low wear are of primary concern.

2.7.1 Polymer matrix composites Among the four groups of tribomaterials PMCs have shown immense potential, mainly because of their self-lubrication properties, lightweight and resistance to wear, corrosion, and organic solvents. In PMCs, the polymeric matrices are generally used as binders and their role is to transfer the stress to the filler/fibrous reinforcement. Engineering polymers such as thermosets (epoxy, polyester, phenolics, vinylesters, etc.) as well as thermoplastics (polyetheretherketone, polyimides, polypropylene, polyethersulfone, etc.) and elastomers (rubber, etc.) are used as matrices to fabricate PMCs. High performance, high temperature thermoplastics are used as matrices for making components which are expected to function under severe operating conditions such as high load, high speed, high temperature, and extreme environmental conditions. Similarly, there is a range of fibrous reinforcement comprising both synthetic (glass, carbon, aramid, graphite, etc.) as well as natural (jute, cotton, sisal, sun hemp, bamboo, etc.) fibers available which can be used individually or in combination. The reinforcement, with or without fillers, enhances the tribological properties of the base matrices significantly as shown in Table 2.3 [43]. PMCs reinforced with discontinuous, continuous, and woven fabric of fibers have always been considered as good structural materials. They are used in low, medium, and high stress structural applications such as boat building, chemical plants, and the automotive and aerospace industries due to their ease of fabrication into complex shapes, high strength to weight ratio, excellent corrosion resistance, good weathering properties, and good thermal and electrical insulation [44]. In low stress applications such as panels, cladding, doors, shower cabinets, small boats, etc., glass fiber-reinforced PMCs act as space filling panels, supporting their own weight but not subjected to any significant external loads. The main design requirements are a good surface finish, good weathering properties, and the ability to withstand small impact damage. In medium stress applications, glass fiber-reinforced PMCs are subjected to significant loads of short duration or low sustained loads, such as in pressure vessels and storage tanks for use in chemical plants, large marine applications such as boats, small hovercraft, and marine sweepers, and land transport applications such as rail coach parts and larger automotive moldings for car bumpers, body panels, and truck cabs. The high stress applications of PMCs are in the aerospace

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81

Table 2.3 Effect of fiber/filler reinforcement on abrasive wear behavior of polymer composites [43]. Resin

Fiber/filler

Wt.%

Wear rate due to fillers

PEI

GF PTFE Graphite and MoS2 Graphite CaCO3 BaSO4 Fly ash Bronze powder PTFE PEEK Al2O3 (120 mesh)

16, 20, and 25 15 15 15 15 and 40 10
60 10
60 10
60 30

Increased


Increased Increased

Increased

0
100 0
100 5
30


Increased Decreased
Decreased then increased

(280 mesh) Glass spheres


10

GF

10, 20, 30, and 40 20
30 30

PI PP

Epoxy PEEK PTFE PTFE 1 PPS

Polydimethyl siloxane PEI UHMWPE PP PET

Quartz TiO2 GF Glass spheres

Decreased Increased up to 20% followed by reduction Decreased Decreased Increased

and defense industries, and typical applications include power boats, gliders which are fabricated by hand lay-up using woven roving polyester or unidirectional glass polyester, helicopter rotor blades, and rocket motor casings which are filament wound with epoxy resin. Besides structural applications much progress, particularly in recent years, has been made in designing PMCs for tribological applications. PMCs are promising as tribomaterials because of their inherent properties such as self-lubrication, low cost, lightweight, quiet operation, better friction properties, ease of fabrication, and resistance to wear, corrosion, and organic solvents. They are used as seals, bearings, gears (low friction, low wear), conveyer belts (low wear), turbine or pump blades (low wear), brakes, tyres (low wear and moderate friction), dental applications (low wear), and hip replacements in which the substitute material should have low wear and low friction coefficient [45]. Based on the type (short fiber, unidirectional long fiber, and woven fabric), content, and orientation (parallel, antiparallel, and perpendicular to sliding direction) of fibrous reinforcement, the wear resistance of composite materials is either enhanced or lowered.

ASTM A514 Specimen travels horizontally (linearly) against a rotating drum counterface Testing can be either abrasive or adhesive

Simulates wear in applications such as conveyor belts

ASTM G99 Specimen is held vertically or horizontally and loaded against a rotating counterface

Contact area of specimen is constant with respect to sliding time

Typical operating parameters include sliding distance, sliding velocity, applied load, wet or dry sliding, and abrasive or adhesive contact condition

Pin on drum

Pin-on-disk/block on disk

Contact area of specimen varies with respect to sliding time Simulates wear in applications such as pulleys and camshafts

Typical operating parameters include sliding distance, sliding velocity, applied load, temperature, and wet or dry sliding

ASTM G77 Specimen is held against a rotating ring or wheel at 90 degrees to the ring or wheels axis of rotation

Block on ring

Simulates wear in applications such as tyres, earrings, and rollers

ASTM G65 Specimen is held against a rotating rubber wheel, while sand is introduced at the rubber interface Abrasive and adhesive testing

Rubber wheel abrasion tester

Force gauge measures tangential force during scratching

A conical stylus is attached to its holder at the cantilever arm. The specimen is loaded normally by placing dead weights onto the stylus holder. Abrasive and adhesive testing

Simulates wear in applications such as sliding window panels and drawers

Specimen is fixed on a motor-driven shaft which rotates at a very low speed

Scratch tester

Specimen is held in a container filled with abrasives while the counterface slides linearly

Linear tribotester

Table 2.4 Common test configurations used for tribological characterization of polymeric composites based on natural fibers.

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83

The addition of high strength and high modulus fibers like glass, carbon, aramid, graphite, etc., along with fillers such as PTFE, MoS2 or graphite powder as second phase materials in polymers results in improved wear, friction, physical, and mechanical properties [46]. For instance, bearings made of fiber-reinforced polymer composites are better than bearings made up of pure polymers since the former perform better when high thermal stability and mechanical strength are required. Appropriate filler along with fiber reinforcement generally reduces the wear rate, increases the thermal conductivity and creep resistance, and modifies the wear mechanism including interfacial wear phenomena. Incorporation of rigid filler into a polymer, which transfers film during sliding wear, reduces the rate of film transfer and its removal. Rigid filler particles protruding from the worn surface, perhaps as film of transferred polymer, act as a lubricant. The wear rate is then controlled by the rate of wear of the filler. Apart from this, filler retards reorientation at the composite interface and suppresses the rate of transfer of film deposition. It also produces local stress intensifications within the transferred layer and produces a more strongly attached transferred film. These two effects of the filler retard the overall rate of transfer wear. Since modes of wear of components depend on the type of application, suitable PMCs must be chosen. For instance, in the case of conveyer belts that are subjected to severe abrasive conditions due to the presence of hard mineral ores, aramid fiber-reinforced rubber composite are used. Similarly, bearings used in mechanical or aerospace applications for high load-carrying purposes are made of short or continuous fiber-reinforced polymer composites. Gears or racks for moving mechanical assemblies are generally made up of short fiber-reinforced and selflubricated thermoplastic blends while multilayer leaf springs, which are usually subjected to fretting-fatigue, are made up of unidirectional continuous carbon fiber
epoxy composites. PMCs reinforced with natural fibers are used in the automotive industries for dashboards, upholstery, vehicle interiors, etc. Natural fibers, in particular jute, cotton, and sisal, are being chemically modified to make polymer composites for such applications (see Section 1.2). In fact, the first tribological application of natural fiber-reinforced polymer composites was reported in the mid20th century with the use of cotton fiber-reinforced phenolic composites in aircraft bearings [47]. Since then, many plant fibers have been studied with a view to ensure their optimum utilization in developing tribocomposites. These natural fiber
polymer tribocomposites are characterized using several test configurations discussed in Section 2.4 and summarized in Table 2.4.

References [1] K.H.Z. Gahr, Microstructure and Wear of Materials, Tribology Series, vol. 10, Elsevier, Amsterdam, 1987. [2] B.J. Briscoe, in: K. Friedrich (Ed.), Advances in Composite Tribology, Composite Materials Series, vol. 8, Elsevier, Amsterdam, 1993, p. 3. [3] N.P. Suh, H.C. Sin, Wear 69 (1981) 91.

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[4] G.M. Bartenev, V.V. Lavrentev, Friction and Wear of Polymers, Elsevier, Amsterdam, 1981. [5] E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965. [6] I.V. Kragelski, M.N. Dobyein, V.S. Kombalov, Friction and Wear: Calculation Methods, Pergamon Press, Oxford, 1982. [7] G. Polzer, F. Meibner, Grundlagen zu Reibung und Verschleib, VEB Deutscher Verlag, Leipzig, 1978. [8] N.P. Suh, Tribophysics, Prentice Hall, Englewood Cliffs, NJ, 1986. [9] H. Czichos, Tribology, A Systems Approach to the Science and Technology of Friction, Lubrication and Wear, Tribology Series, vol. 1, Elsevier, Amsterdam, 1978. [10] P.M. Dickens, J.L. Sullivan, J.K. Lancaster, Wear 112 (1986) 237. [11] V. Shah, Handbook of Plastics Testing Technology, Wiley-Interscience, New York, 1984, p. 73. [12] J. Bijwe, M. Fahim, in: H.S. Nalwa (Ed.), Handbook of Advanced Functional Molecules and Polymers, vol. 4, Gordon and Breach Science Publishers, 2001, p. 265. [13] A.R. Sanadi, S.V. Prasad, P.K. Rohtagi, J. Mater. Sci. 21 (1986) 4299. [14] J.P. Giltrow, J.K. Lancaster, Wear 16 (1970) 359. [15] J.K. Lancaster, Tribology 5 (1972) 249. [16] J.P. Giltrow, Composites 4 (1973) 55. [17] T. Tsukizoe, N. Ohmae, Fibre Sci. Technol. 18 (1983) 265. [18] A.M. Hager, M. Davies, in: K. Friedrich (Ed.), Advances in Composite Tribology, Composite Materials Series, vol. 8, Elsevier, Amsterdam, 1993, p. 107. [19] K. Friedrich, Z. Zhang, A.K. Schlarb, Compos. Sci. Technol. 65 (2005) 2329. [20] S. Bahadur, D. Gong, Wear 154 (1992) 151. [21] S. Bahadur, D. Gong, J.W. Anderegg, Wear 154 (1992) 207. [22] S. Bahadur, D. Gong, Wear 162
164 (1993) 397. [23] S. Bahadur, D. Gong, J.W. Anderegg, Wear 165 (1993) 205. [24] S. Bahadur, A. Kapoor, Wear 155 (1992) 49. [25] Q. Zhao, S. Bahadur, Wear 225
229 (1999) 660. [26] Q. Zhao, S. Bahadur, Wear 217 (1998) 62. [27] L. Yu, S. Bahadur, Wear 214 (1998) 245. [28] C.J. Schwartz, S. Bahadur, Wear 251 (2001) 1532. [29] L. Yu, S. Yang, H. Wang, Q. Xue, J. Appl. Polym. Sci. 19 (2000) 2404. [30] K. Tanaka, in: K. Friedrich (Ed.), Friction and Wear of Polymer Composites, Elsevier, Amsterdam, 1986, p. 137. [31] S. Bahadur, D. Tabor, Wear 98 (1984) 1. [32] K. Friedrich, J. Mater. Sci.: Mater. Med. 4 (1993) 266. [33] X.S. Xing, R.K.Y. Li, Wear 256 (2004) 21. [34] K. Friedrich, in: K. Friedrich (Ed.), Friction and Wear of Polymer Composites, Elsevier, Amsterdam, 1986, p. 233. [35] J.M. Durand, M. Vardavoulias, M. Jeandin, Wear 181
183 (1995) 833. [36] Q. Xue, Q. Wang, Wear 213 (1997) 54. [37] Q. Wang, Q. Xue, H. Liu, W. Shen, J. Xu, Wear 198 (1996) 216. [38] K. Friedrich (Ed.), Friction and Wear of Polymer Composites, Composite Materials Series, vol. 1, Elsevier, Amsterdam, 1986. [39] K. Friedrich (Ed.), Advances in Composite Tribology, Composite Materials Series, vol. 8, Elsevier, Amsterdam, 1993. [40] K. Friedrich, Z. Zhang, P. Klein, Wear of polymer composites, in: G.W. Stachowiak (Ed.), Wear
Materials, Mechanisms and Practice, Wiley, Chichester, 2005, p. 269;

Introduction to tribology of polymer composites

[41] [42]

[43] [44] [45] [46] [47]

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P. Sydenham, R. Thorn (Eds.), Handbook of Measuring System Design, Wiley, New York. Z. Zhang, C. Breidt, L. Chang, F. Haupert, K. Friedrich, Compos: Part A 35 (2004) 1385. K. Friedrich, Z. Lu, R. Scherer, in: Proceedings of International Conference on Advanced Materials, EMRS spring meeting, 1991 sympo. 44, Composite Materials, Strasbourg, France, May 1991, pp. 27
31. J.J. Rajesh, J. Bijwe, U.S. Tewari, J. Mater. Sci. 36 (2) (2001) 351. N. Chand, M. Fahim, An Introduction to Tribology of FRP Materials, Allied Publishers, New Delhi, 2000. J.K. Lancaster, Proc. Inst. Mech. Engrs 182 (1,2) (1967) 33. U.S. Tewari, J. Bijwe, Chapter 5 in: K. Friedrich (Ed.), Advances in Composite Tribology, Composite Materials series, vol. 8, Elsevier, Amsterdam, 1993. J.K. Lancaster, in: K. Friedrich (Ed.), Friction and Wear of Polymer Composites, Composite Materials Series, vol. 1, Elsevier, Amsterdam, 1986, p. 363.

Sisal-reinforced polymer composites 3.1

3

Sisal fiber

Sisal fiber (the name is derived from the botanical name of the plant Agave sisalana) is extracted from the leaves of the plant [1]. This plant is found across the world, mostly in the tropical and subtropical regions, such as north and south America, Africa, West Indies, Brazil, Tanzania, India, and the Far East. It is not systematically cultivated and grows wild along railway tracks and along the hedges of agricultural fields. The plants require soil rich in calcium, magnesium, potassium, nitrogen, and phosphorous. Various varieties of sisal plants are found across the world. For example, in India there are four varieties of sisal plant—Sisalana, Vergross, Istle, and Natale. Different varieties of plants have different yields of fibers. Leaves from the first two varieties yield more fibers than those from the other two. The fiber content also varies with the age and source of the plant. Typical composition of the leaf is moisture (87.25%), fiber (4%), cuticle (0.75%), and other dry matter (8%). On average a single plant gives 5 6 leaves at a time and a single leaf contains 1000 fibers. A single leaf weighs about 600 g and yields 3% fiber by weight. Fibers are extracted by microbiological retting, hand scraping, or by using a raspador machine. Leaves are crushed and then held by mechanical means against the scraping action of the blade, which removes cellulosic material. In order to obtain high quality fibers, fiber recovery takes place immediately after harvesting so that the natural gummy material is retained. If this hardens the separation of fibers becomes difficult. A double retting process makes the extraction of fiber easier [2]. In this process, leaves are removed from the tank when the retting is half complete, dried, and retted again after a few months. Superior quality fibers in terms of luster, flexibility, and strength are obtained after the repeated processes. The length of the fiber varies between 0.6 and 1.2 m. The ends of the fiber are broad and blunt. Cells which are angular in shape are normally 500 6000 μm long and 5 40 μm wide giving a minimum aspect ratio (length/diameter of fiber) of 150. Calcium oxalate crystals are present in parenchyma circular and are often found packed with tiny globules [3]. The sisal leaf contains fibers as shown schematically in Fig. 3.1A and B. Commercially important fibers are extracted from the periphery of the leaf.

3.1.1 Advantages and disadvantage of sisal fibers G

G

Sisal is one of the most widely used natural fibers and is very easily cultivated. It possesses high strength, durability, ability to stretch, affinity to dyes, and resistance to deterioration in salt water.

Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00003-7 © 2021 Elsevier Ltd. All rights reserved.

88

Tribology of Natural Fiber Polymer Composites

Figure 3.1 (A) The transverse section of sisal leaf shows phloem and xylem fibers; (B) longitudinal section shows parenchyma. G

G

G

G

Sisal fibers are smooth, straight, coarse, and inflexible. It can be used alone or blended with wool or acrylic. It has short renewal times and grows wild in the hedges of fields and railway tracks. Nearly 4.5 million tons of sisal fibers are produced every year throughout the world. Sisal represents 2% of the world’s population of plant fibers.

Sisal-reinforced polymer composites

G

G

89

No pesticides or chemical fertilizers are used in sisal production. Sisal cultivation as a fiber crop does not cause environmental degradation.

A disadvantage of the sisal fibers is that they absorb air humidity causing expansion and contraction of products made from sisal fibers. They are also prone to microbial attack when wet and degrade when exposed to heat, light, and ultraviolet radiation.

3.1.2 Chemical composition of sisal fibers The chemical composition of sisal fibers includes 78.8% cellulose, 8% lignin, 10% hemicellulose, 2% waxes, and about 1% ash by weight [1]. These constituents are described in detail in the Appendix. The length of sisal fiber is between 1.0 and 1.5 m, and the diameter is about 100 300 μm. The fiber is a bundle of hollow subfibers (Fig. 3.2A and B). Their cell walls are reinforced with spirally oriented cellulose in a hemicellulose and lignin matrix. The composition of the external surface of the cell wall is a layer of lignaceous material and waxy substances that bond the cell to its adjacent neighbors. A sisal fiber in cross-section is built up of about 100 fiber cells (each with a length of 2 5 mm). The fiber cells are linked together by means of middle lamellae, which consist of hemicellulose, lignin, and pectin (Fig. 3.2B). The fiber cell consists of a number of walls built up of fibrillae. In the outer wall (primary wall) the fibrillae have a reticulated structure. In the outer secondary wall (S1), which is located inside the primary wall, the fibrillae are arranged in spirals with a spiral angle of 40 degrees in relation to the longitudinal axis of the cell. The fibrillae in the inner secondary wall (S2) have a sharper slope, 18 degrees. The innermost wall (the tertiary wall) is thin and has a reticulated arrangement of fibrillae. The fibrillae are, in turn, built up of microfibrillae with a thickness of about 20 nm. The microfibrillae are composed of cellulose molecular chains with a thickness of 0.7 nm and a length of a few micrometers (the degree of polymerization for sisal is about 25,000). Cellulose is a hydrophilic glucan polymer consisting of a linear chain of 1,4β-bonded anhydroglucose units, and this large amount of hydroxyl groups makes sisal fiber hydrophilic (see Appendix).

3.1.3 Physical structure of sisal fibers Sisal fiber has a real density of 1.45 g/cm3, an apparent density of 1.20 g/cm3, and its porosity is 17%. Moisture regain is 11% at 65% relative humidity (RH) and 32% at 100% RH [1]. The fiber is moderately crystalline. The spiral angle around the fiber axis is 20 25 degrees [1]. Sisal fiber of diameter 21.5 3 1023 cm shows elastic strain of 8.7%. These fibers are more compressible in the transverse direction than in the longitudinal direction. Sisal fibers fade rapidly when exposed to sunlight because of the presence of lignin. The swelling of these fibers is related to their moisture regain value. Plant fibers with high moisture regain have low porosity, which does not appear to change significantly with swelling due to moisture absorption. Indirect drying causes greater reduction in density, strength, and rigidity

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Tribology of Natural Fiber Polymer Composites

Figure 3.2 (A) Cross-section of sisal fiber; (B) model of unit cell of sisal fiber.

of these fibers as compared with direct drying, indicating that the molecular arrangement is less ordered in indirectly dried fibers.

3.1.4 Mechanical properties of sisal fibers The stress strain curve for a typical sisal fiber having an initial modulus of 3.86 GPa and tensile strength of 168.9 MPa is shown in Fig. 3.3 at a crosshead speed of 0.02 m/min [4]. The strength and stiffness of plant fibers depend on the cellulose content and the spiral angle that the bands of microfibrils in the inner secondary cell wall make with the fiber axis [5]. Thus, the tensile properties of sisal fiber are not uniform along its length [6]. The root or lower part has low tensile strength and modulus but high fracture strain. The fiber becomes stronger and

Sisal-reinforced polymer composites

91

Figure 3.3 A typical stress strain curve of sisal fiber.

Table 3.1 Properties of sisal fibers obtained from different sources [7]. Density (kg/m3)

TS (MPa)

TM (GPa)

Maximum strain (%)

1450 1450

604 530 347 500 400 450 530 450

9.4 15.8 9.4 22 14 16 21 9 20 7 13 17 22 7 13

3 7 5 3.6 5.1 5 14 4 9 3.64 5.12 4 9

1030 1410 1400 1450

640 600 700 700 630 700

Diameter (mm) 50 200 50 300

100 300 100 300

stiffer at mid-span and the tip has moderate properties. The mechanical properties of sisal fibers obtained from different sources are shown in Table 3.1 [7]. The tensile strength and percent elongation at break decrease while tensile modulus increases with fiber length. With increasing speed of testing, both tensile modulus and tensile strength increase, but elongation does not show any significant variation. However, at a test speed of 500 mm/min, the tensile strength decreases sharply. This is attributed to the internal structure of the fiber, such as cell structure, microfibrillar angle (20 25 degrees), and defects. In rapid mechanical testing, the fiber behaves like an elastic body, that is, the crystalline region shares the major applied load resulting in high values of both modulus and tensile strength. When the testing speed decreases, the applied load is borne increasingly by the amorphous region. However, at very slow test speeds, the fiber behaves like a viscous liquid. The amorphous regions take up a major portion of the applied load giving a low fiber modulus and a low tensile strength. However, at very high strain rates

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Table 3.2 Mechanical properties of sisal fibers with different age at different temperature [10]. Age of plant (months)

36 60 84 108

Degradation in property (%)

Toughness per unit volume (MJ/m3) Ambient temperature (30 C)

At 100 C

4.8 5.5 6.0 7.4

4.1 4.3 4.7 5.2

14.58 21.81 21.66 29.72

303 300 280 339

32.96 40.94 44 41.65

21 22 17 21

19.23 24.13 50 43.24

Tensile strength (MPa) 36 60 84 108

452 508 500 581 Tensile modulus (GPa)

36 60 84 108

26 29 34 37

(B500 mm/min), the sudden fall in tensile strength occurs due to the presence of imperfections in the fiber causing immediate failure. The microfibrillar angle and the number of strengthening cells in the sisal fibers do not influence the tensile modulus and strength. As the test length increases, the number of weak links or imperfections also increases which reduces the tensile strength. However, with increasing fiber length sisal offers a higher resistance to applied stress due to larger numbers of oriented cellulosic fibers. This probably also accounts for the higher modulus of the fibers at longer test lengths. The tensile strength, modulus, and toughness (defined as energy absorption per unit volume) of sisal fiber depend on plant age as well as temperature. The relative effect of plant age on the mechanical properties is less prominent at 100 C than at 30 C. This is attributed to the more intense removal of water and/or other volatiles (at 100 C) originally present in the fibers, which otherwise act as plasticizing agents in the chains of the cellulose macromolecules. Interestingly, at 80 C both tensile strength and modulus decrease with age of the plant. This trend is different to testing at 100 C (Table 3.2) [9].

3.1.4.1 Thermal properties of sisal-based fabric The chemical structure of sisal fibers does not change below 200 C while the degree of crystallinity increases. A slight weight loss (B2%) below 200 C is probably caused by the evaporation of water absorbed by sisal fibers (around 100 C),

Sisal-reinforced polymer composites

93

substances of low boiling point and others that can be decomposed below this temperature. However, the large amounts of cellulose, hemicellulose, and glucans are not lost. The thermal behavior remains the same between 150 C and 200 C. Hence, thermal treatment of sisal fiber can be carried out below 200 C (Table 3.3) [10].

3.2

Sisal polymer composites

3.2.1 Surface modification of sisal fibers The hydroxyl groups that occur throughout the structure of sisal fibers make them hydrophilic. Consequently, when sisal fibers are used to reinforce hydrophobic polymer matrices the resulting sisal polymer composites have poor interface and poor resistance to moisture absorption [10]. The hydrophilic sisal fibers absorb a large amount of water in the composite, leading to failure by delamination. However, better wetting and chemical bonding between fiber and matrix can remove this limitation. The effect of surface modification techniques such as alkali treatment, H2SO4 treatment, both acid and alkali treatment, benzol/alcohol dewax treatment, acetylation, thermal treatment, alkali thermal treatment, and thermal/ alkali treatment on tensile properties of sisal fibers is shown in Table 3.4 [8]. Thermal treatment at 150 C increases the strength and modulus properties because of the increased crytallinity (from 62.4% for untreated to 66.2% for 150 C/ 4 h treated) of sisal fibers. However, as the temperature reaches 200 C, the tensile properties decrease considerably as a result of the degradation of fibers. Acetylation reduces the moisture content from 11% to 5.45% and hence the tensile strength of acetylated sisal fiber is reduced from 445 to 320 MPa, caused by the loss of the hemicellulose in the fiber during acetylation [11]. Similarly, the coupling agents such as N-substituted methacrylamide, gamma-methacryloxypropyl trimethoxy silane, neopentyl(dially)oxy,tri(dioctyl)pyro-phosphate titanate, and neopentyl(diallyl)oxy,triacrylzirconate significantly reduce the moisture absorption by providing hydrophobicity to the surface via long-chain hydrocarbon attachment. In addition, these coupling agents penetrate the cell wall through surface pores and deposit in the interfibrillar regions and on the surface, restricting further ingress of moisture [12]. The hydroxyl groups attached to the glucose units of the cellulose, hemicellulose, and lignin component of sisal fiber react with these coupling agents in the presence of moisture.

3.2.2 Sisal polyester composites The properties of sisal-reinforced polyester composites are improved when sisal fibers are suitably modified with surface treatment. The modified interphase is less stiff than the resin matrix and provides a deformation mechanism to reduce interfacial stress concentration. Further, it also prevents fiber/fiber contacts and hence removes the sources of high stress concentrations in the final composites [13]. By

Untreated 150 C/4 h treated 200 C/0.5 h treated

Sisal fiber

2 2

2

61

Weight loss (%)

69 69

Temperature ( C)

First stage of decomposition

300

302 297

Start temperature

Temperature ( C)

327

334 333

End temperature

Second stage of decomposition

Table 3.3 Thermogravimetric data of heat treated sisal fibers [9].

23

23 32

Weight loss (%)

478

478 478

Temperature ( C)

92.7

91.8 93.2

Weight loss (%)

Third stage of decomposition

Sisal-reinforced polymer composites

95

Table 3.4 Effect of fiber surface treatment on tensile properties of sisal fibers [10]. Surface treatment

Tensile modulus 3 103 (g/tex)

Elongation at break (%)

No treatment Alkali treated Acetylated Thermal

1.18 0.53 0.35 1.22

2.5 7.5 8.3 3.5

Table 3.5 Effect of surface treatments of sisal fibers on the properties of sisal polyester composites (fiber content 50 vol.%) [13]. Property

Untreated

N-substituted methacrylamide treated

Silane treated

Titanate treated

Zirconate treated

Density (g/cm3) Void content (%) Tensile strength (MPa) Elongation (%) Tensile modulus (GPa) Energy to break (MJ/m2) 3 105 Flexural strength (MPa) Flexural modulus (GPa)

0.99 16.11 29.66

1.05 5.88 39.48

1.12 3.01 34.14

1.02 12.55 36.26

1.00 13.30 34.69

9.52 1.15

9.75 2.06

5.71 1.75

8.00 1.67

9.51 1.39

7.96

11.06

4.78

8.60

10.13

59.57

76.75

96.88

75.59

72.15

11.94

15.35

19.42

15.13

14.46

improving interfacial adhesion, the moisture-induced degradation of composites is reduced. Treated fiber composites absorb moisture at a slower rate than their untreated counterparts, probably because of the formation of a relatively more hydrophobic matrix interface region by co-reacting organo-functionality of the coupling agents with the resin matrix. Table 3.5 shows that the strength retention of surface-treated composites is higher than that of composites containing untreated sisal fibers. N-substituted methacrylamide treated sisal fiber-reinforced polyester composites generally exhibit better mechanical properties. Sisal fibers treated by Nsubstituted methacrylamide and silanes have relatively lower void contents compared with other fiber-treated composites.

3.2.3 Sisal epoxy composites Silane treatment improves the adhesion and moisture resistance properties of sisal fibers. The treatment of sisal fibers in silane, preceded by mercerization,

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Tribology of Natural Fiber Polymer Composites

Table 3.6 Mechanical and physical properties of sisal epoxy composites (fiber volume fraction B40%) [14]. Material/ treatment

Compressive strength (MPa)

Flexural strength (MPa)

Flexural modulus (GPa)

Relative density

Epoxy resin Untreated/dry Mercerized/dry Silane/dry Untreated /wet Mercerized/wet Silane/wet

120.0 148.0 183.1 184.8 62.53 75.83 98.53

95.0 266.5 262.1 244.5 221.7 200.7 237.2

3.1 15.93 17.63 17.36 9.15 10.12 12.33

1.17 1.14 1.24 1.25

Water absorbed (%) after 72 h

15.6 9.0 5.0

provides improved wettability, mechanical properties, and water resistance of sisal epoxy composites (Table 3.6) [14]. Mercerization improves wettability of the fibers because of the increase in density of the composite. The additional sites of mechanical interlocking as a result of treatment lead to improvement of interfacial bonding and promote resin/fiber interpenetration at the interface. The high hydrophobic resin pick-up also accounts for the reduction in water absorption and hence improved mechanical properties under wet conditions. Although treatment of sisal fibers in silane preceded by mercerization produces very little change in the mechanical properties of dry composites, mechanical performance under wet conditions, and hence water resistance, can be improved. The treatment in 100% silane produces fibers that are almost hydrophobic. This may be a result of improved interfacial bonding arising from the use of the silane. Water molecules at the interface tend to replace the resin fiber covalent bond by weaker hydrogen bonds, hence silane plays an important role in reducing water absorption in cellulosic fiber-reinforced composites. It is an established fact that sisal fibers have a central hollow region, the lumen, which gives access to water penetration by capillarity, especially when composites have high fiber content [15]. So, although silane treatment can create a hydrophobic fiber surface, it is not possible to prevent water from entering the composite by capillary action, as long as the fiber ends are exposed. Thus, for practical purposes it may be necessary to seal off the external surfaces by water repellents so as to keep water uptake in the composite to a minimum.

3.2.4 Sisal phenolic composite The cohesiveness between rigid sisal fiber and brittle phenol formaldehyde (phenolic resin) is improved when sisal fiber is treated with silane that enhances the mechanical properties of the composites. Water resistance of the composites can be improved when sisal fiber is treated with silane and grafting [16].

Sisal-reinforced polymer composites

97

3.2.5 Sisal polyethylene composite Sisal fiber treatment by an alkali, isocyanate, peroxide, or permanganate improves, although to different extents, the tensile properties of short sisal fiber-reinforced polyethylene (PE) composites for both randomly and unidirectionally oriented fibers [17]. The treatment causes fiber fibrillation, which increases the effective surface area available for wetting by the matrix resin. The hydrophilic nature of sisal fibers is reduced by treating the fiber surface with a derivative of cardanol because of the linkage of the long-chain structure of cardanol to the cellulosic fibers. The technique, known as isocyanate treatment, makes sisal fibers compatible with the PE matrix, thus resulting in a strong interfacial bond between these two constituents [17]. Similarly, peroxide can graft PE onto the cellulose surface. The peroxideinitiated free radicals react with the PE matrix as well as the cellulose fibers. The tensile strength of the composites increases with peroxide concentration up to a critical level depending on the fiber content and then becomes constant. The existence of a critical concentration of peroxide suggests that the peroxide-initiated grafting reactions terminate when the fibers are covered with grafted PE and excess peroxide causes some crosslinking of the PE molecules themselves [17]. Grafting between sisal fibers and PE matrix can also be achieved using permanganate which roughens the fiber surface and produces mechanical interlocks with the matrix [17]. However, the permanganate concentration is a critical factor in order to achieve the desired mechanical properties in the composite. It has been observed that the tensile strength reaches a maximum at a permanganate concentration of 0.055% and then decreases sharply with further increase in permanganate concentration. This is caused by the degradation of cellulosic fibers at high permanganate concentration. Apart from fiber treatment, compatibilizers such as maleic anhydride have been used to modify thermoplastics such as polypropylene (PP) and PE to derive matrices such as MAPP and MAPE to improve the fiber/filler matrix interface in sisal as well as other natural fiber-reinforced thermoplastic composites.

3.3

Mechanical properties of sisal polymer composites

3.3.1 Sisal thermoset composites The tensile strength and modulus of sisal fiber-reinforced polyester composites increase linearly with fiber volume fraction Vf. When Vf # 40%, impact strength also increases linearly with Vf. The toughness of composites increases with the microfibrillar angle of the fibers and reaches a maximum at 15 20 degrees. It then decreases with increasing angle [18]. Since the optimal microfibrillar angle of sisal fiber is 21 degrees, this leads to better impact resistance. The fiber surface treatment has a strong effect on the impact behavior of the sisal fiber-reinforced polyester and epoxy composites, and the effects are different for different matrices. Since fiber pull-out is the major contributor to the energy absorption, increased fiber-tensile strength promoted by thermal treatment increases the impact performance of the

98

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composites. Water absorption in sisal fiber composites is mainly caused by the sisal fibers and leads to a very poor interface between the sisal fiber and the matrix. Different matrix systems have different interface characteristics. Generally, water absorption in sisal polyester composite is two to three times more than that in sisal epoxy composite, and this leads to their different impact properties. For sisal epoxy composite, the impact strength improves with water absorption as a result of an acceptable level of interface debonding, but for sisal polyester composites, the impact strength decreases through the complete destruction of the interface.

3.3.2 Sisal thermoplastic composites 3.3.2.1 Sisal polyethylene composite The tensile properties of short sisal fiber-reinforced PE composites show a gradual increase with fiber length reaching a maximum at about 6 mm (12.5 MPa) and then decreasing (10.24 MPa) at 10 mm [19]. This is attributed to the fact that long fibers tend to bend or curl during molding,1 which causes a reduction in the effective length in a particular direction and hence decreases the mechanical properties. In contrast, unidirectional short fibers achieved by extrusion enhance the tensile strength and elastic modulus of the composites along the axis of fiber alignment compared with randomly oriented fiber composites. Different processing methods lead to different extents of fiber damage, different fiber length distributions, and hence different mechanical properties. Fiber length, orientation, volume fraction, and fiber surface treatment also affect the dynamic mechanical properties of sisal fiber-reinforced PE. The addition of 10% short sisal fibers into low density polyethylene (LDPE) increases the storage moduli and loss moduli of the composites which then become steady at higher volume fraction [20]. Both storage and loss moduli of randomly oriented composites are intermediate between those of longitudinally and transversely oriented composites. The influence of fiber length indicates that a critical length of 6 mm is needed to obtain maximum dynamic moduli [20 23]. This suggests that a critical length exists for maximum stress transfer between fiber and matrix. The storage and loss moduli of the isocyanate-treated composites are higher than those of the untreated composites as a result of the improved fiber/matrix interface adhesion. Both the tensile properties and the dimensional stability of sisal fiber-reinforced composites are strongly affected by boiling water immersion. The ageing properties of untreated and isocyanate-treated sisal fibers show that treated composites exhibit better mechanical properties and dimensional stability as compared with untreated composites as a result of the existence of an effective interfacial bond between fiber and matrix [21]. 1

Melt mixing (MM) and solution mixing (SM) methods were used for processing samples. In MM, fiber was added to a melt of thermoplastic and mixing was done in a mixer at a particular temperature and speed. The hot mix was extruded using an injection molding machine as rods. In SM, fibers were added to a viscous solution of thermoplastics and transferred to a tray and kept in a vacuum oven.

Sisal-reinforced polymer composites

99

The mechanical properties of sisal- and glass fiber-reinforced LDPE hybrid composites improve with increasing volume fraction of glass fibers, presumably due to the superior properties of glass fibers as shown in Table 3.7 [2]. A positive hybrid effect has been observed for all mechanical properties except elongation at break. This effect is a consequence of increased fiber dispersion and orientation with increasing volume fraction of glass fibers. Similarly, alkali treatment of sisal fibers slightly improves the mechanical properties of a 50:50 sisal/glass-reinforced PVC hybrid composite (Table 3.8). However, the improvement obtained is less than 10%. Water absorption of the composite reduces from 11.6% to 3.1% compared with the nonhybridized sisal fiber composite. For another hybrid composite based on sisal/glass fibers and PVC hybrid, a positive hybrid effect is observed for the flexural modulus and unnotched impact strength, but a negative hybrid effect for the flexural strength may be due to the poor interface between sisal, glass fibers, and PVC matrix [24]. Also, since the composites are immersed in water, the latter had a detrimental effect on the fiber/ matrix interface leading to reduced properties.

3.4

Tribological behavior of sisal polymer composites

3.4.1 Abrasive wear behavior: sisal epoxy composite Tribological evaluation of sisal fiber-reinforced epoxy composites in abrasive wear mode assumes significance because of the fact that components made from this material, such as car interiors and upholstery, are subjected to severe abrasive conditions during regular use. Abrasive wear tests on such a composite compression molded using epoxy resin/sisal fibers (20/80 wt.%) were performed using grade 400 emery paper embedded with silicon carbide particles (grit size B23 μm) in a single-pass condition [25]. The embedded hard SiC particles abraded the test sample and led to a weight loss which was then used to calculate specific wear rate (Ko) using the formula Ko 5 W/ρLD, where ρ is the density of sample, L is the applied load, and D is the sliding distance. The specific wear rates as a function of applied load for pure polysulfide-modified epoxy and other composites are shown in Fig. 3.4. The specific wear rate decreased with applied load for all materials. The specific wear rate for the composite in which the fibers were oriented longitudinal (parallel) to the sliding direction was the highest. However, it was still lower than pure epoxy. When sisal fibers are aligned parallel to the sliding direction, there is maximum contact area during sliding as compared with the case when fibers are oriented transverse (antiparallel) to the sliding direction. Consequently, severe damage is caused throughout the length of the fibers, and wear is caused by the removal of debris consisting mainly of pieces of fibers broken as a result of microcracking, microcutting, and microplowing of fibers. In contrast, when fibers are oriented transverse (antiparallel) to the sliding direction, these processes occur across the diameter of the fibers which are eventually broken with repeated sliding. Hence,

0.3

0.27

0.24

0.21

0.18

0.15

0.09

0.06

SFRP

1

2

3

4

5

6

7

0.2

0.16

0.14

0.10

0.08

0.06

0.04

0.02

Glass

0.2

0.3

0.5

0.6

0.7

0.8

0.9

1

VSFRP

1

0.8

0.7

0.5

0.4

0.3

0.2

0.1

VGFRP

SFRP, sisal fiber-reinforced plastics; GFRP, glass fiber-reinforced plastics. Note: Values in parentheses are properties of randomly oriented composites.

GFRP

Sisal

Hybrid designation

Volume fraction of fibers

12.37 (7.25) 15.97 (7.72) 16.68 (7.83) 16.92 (7.97) 17.6 (8.11) 17.76 (8.26) 19.98 (8.7) 20.98 (9) 22.94 (9.28)

TS (MPa) 133.3 (130) 160.8 (147.3) 171.4 (152) 187.5 (160) 192.3 (175.2) 196.5 (189.6) 200 (190.3) 210 (194.6) 220 (200)

TM (GPa)

Table 3.7 Mechanical properties as a function of GFRP in sisal/glass hybrid composites [22].

8.8 (13.55) 7.64 (12.52) 7.56 (11.79) 7.16 (10.89) 6.9 (10.39) 6.6 (6.8) 5.88 (6.0) 5.62 (9.56) 4.81 (9.37)

Elongation at break (%)

86.83

81.33

79.78

73.23

72.16

71.32

70.45

60.66

52.57

Tear strength

64

63

60

57

54

52

50

47

43

Hardness

Sisal-reinforced polymer composites

101

Table 3.8 Properties of 50:50 sisal/glass hybrid composites containing alkali-treated and untreated sisal fiber [22].

Untreated Treated

TS (MPa)

TM (GPa)

Elongation at break (%)

L

R

L

R

L

R

17.76 19.66

8.26 8.89

196 210

189 200

6.6 5.2

10.12 9.9

Tear strength (N/mm)

Hardness (Shore D)

73.23 78.47

57 62

L, Longitudinal oriented; R, random oriented.

Figure 3.4 Specific wear rates of sisal epoxy composites [25].

wear rate is lower. The wear rate is further lowered when the fibers are oriented perpendicular to the sliding direction. Since only the fiber ends come in contact with the grits, not much damage is caused and they offer a high resistance to abrasion. The specific wear rate of the four materials followed the order: pure epoxy . parallel . antiparallel . perpendicular. The higher wear resistance (reciprocal of specific wear rate) of the composites compared with pure epoxy was attributed to the enhanced mechanical properties (tensile strength and elongation) of the composites due to fiber reinforcement and improved fiber matrix interfacial strength. The enhanced fiber matrix interfacial bonding increases the resistance of composites to any damage. The scanning electron micrograph of the worn surface of pure epoxy in Fig. 3.5A shows the plastic deformation and microplowing of brittle matrix during wear. The micrograph (Fig. 3.5B) of the worn surface of the composite containing fibers oriented parallel

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Tribology of Natural Fiber Polymer Composites

Figure 3.5 Scanning electron micrographs of worn surface of (A) epoxy matrix, (B) P-type, (C) AP-type, and (D) N-type sisal epoxy composites [25].

to the sliding direction shows fibrillation and long broken pieces of sisal fibers as wear debris. The micrograph (Fig. 3.5C) shows the worn surface of the composite containing fibers oriented antiparallel to the sliding direction. Fibers have been damaged but not as much as in the previous case. Wear debris consists of broken fiber pieces few in number. For composites containing fibers perpendicular to the sliding direction, almost no damage to the fibers is seen and the worn surface shows a smooth topography (Fig. 3.5D).

3.4.2 Sliding wear behavior: sisal polyester composite Short sisal fibers having a very high aspect ratio (diameter 200 400 μm, length 6 mm) were incorporated into unsaturated polyester resin to prepare sisal polyester composites. Sliding wear behavior of these composites was evaluated on a pin-ondisk sliding wear machine against stainless steel (EN-32) counterface at a constant sliding distance of B6 km, sliding velocity B1.75 m/s, and applied load 10 100 N [26]. Fig. 3.6 shows the specific wear rate of all the composites as a function of load. Pure polyester exhibited the lowest wear rate of all the composites. This was

Sisal-reinforced polymer composites

103

Figure 3.6 Specific wear rate of sisal polyester composite in sliding wear mode [26].

attributed to the poor bonding between sisal fibers and polyester that led to the easy detachment of the sisal fibers from the matrix during repeated sliding. However, when the fibers were treated with silane the specific wear rate decreased considerably, showing improvement in the fiber matrix interfacial bonding. In general, an increase in the weight percentage of sisal fibers increased the load-carrying capacity and hence the pv limit2 of the polymer. Composites containing higher concentration of fibers exhibited higher wear rate. The specific wear rates of all the materials followed the order, SP27 . SP42 . SP27silane . SP10 . pure polyester (subscript denotes fiber concentration). Fig. 3.7 shows the friction coefficients μ of all the materials as a function of sliding duration at an applied load of 20 N. The friction coefficient initially increases and then becomes steady. The addition of sisal fiber increased the coefficient of friction of polyester resin. The neat polymer showed a friction coefficient B0.50 while the composites containing 10 wt.% sisal fiber and 27 wt.% sisal fiber showed friction coefficients of 0.60 and 0.80, respectively. The silane treated sisal fiber composite showed invalid friction coefficient ( . 1) under the same sliding conditions. However, when the applied load is increased from 20 to 80 N, the friction coefficient of silane treated sisal fiber composite reduces considerably and matches that of the pure polyester (0.50). The higher friction coefficient of the pure polymer and composites is attributed to the swelling of the polymer due to frictional heat and roughening of the counterface caused by the sisal fibers which act as abrasives when exposed from the matrix during repeated sliding. Fig. 3.8 shows the increase in the temperature of the counterface, and hence the interface, during friction tests at different loads. The counterface temperature increased with increase in applied load. 2

The point at which load sensitivity of wear at a particular sliding speed for a material suddenly increases is defined as the pressure velocity (pv) limit of that material.

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Tribology of Natural Fiber Polymer Composites

Figure 3.7 Coefficient of friction of sisal polyester composite in sliding wear mode. (From top to bottom: SP27silane, SP27, SP10, and neat polyester) [26].

Figure 3.8 Temperature rise of counterface with increase in load in sliding wear tests [26].

Overall friction process can be summarized as: the frictional heat generated during sliding increased the temperature of the composite. This led to the softening of the matrix which allowed greater surface contact between pin and disc. The increase in the real area of contact increased the coefficient of friction. However, with the addition of sisal fiber, the real area of contact reduced due to a discontinuous phase that further increased the friction coefficient. Due to frictional heat, wear debris consisting of sisal fibers decomposed and acted as abrasives that led to a

Sisal-reinforced polymer composites

105

Figure 3.9 Scanning electron micrographs of worn surface of sisal polyester composites: (A) polyester; (B) 10 wt.% sisal; (C) 27 wt.% sisal; (D) 42 wt.% sisal; (E) silane-treated 27 wt.% sisal [26].

further increase in friction coefficient. These mechanisms are manifested in the micrographs of worn surfaces as discussed in the following section. The scanning electron micrograph (Fig. 3.9A) of the worn surface of polyester at 30 N applied load shows the plastic deformation of brittle polyester due to frictional heat generated at the interface. The micrograph in Fig. 3.9B shows the worn surface of the

106

Tribology of Natural Fiber Polymer Composites

composite containing 10 wt.% sisal fiber. The wear debris mainly consisted of polymer matrix, due to microplowing, and very few pieces of broken fibers. The micrograph in Fig. 3.9C shows the worn surface of the composite containing 27 wt.% sisal fiber. The worn surface shows fiber matrix debonding with repeated sliding which resulted in wear debris consisting of large pieces of fibers. Hence, this composite has higher wear rate. The micrograph in Fig. 3.9D shows the worn surface of the composite consisting of 42 wt.% sisal fibers. Since the composite has a high concentration of sisal fiber, the higher fiber/fiber interaction led to a poor bonding with the matrix and formation of wear debris consisting of broken fiber pieces that accumulated in the wear track and roughened the counterface leading to high wear and friction. The micrograph in Fig. 3.9E shows the worn surface of silane-treated sisal polyester composite containing 27 wt.% sample.

3.4.3 Friction and wear behavior: sisal phenolic composite Nonasbestos fiber-based brake pads, linings, couplings, etc., have been in demand because of environmental and human health concerns [27,28]. The alternatives to asbestos fibers include mineral fiber, metal fiber, and artificial polymer fiber such as glass fiber, Al2O3 fiber, carbon fiber, steel fiber, aramid fiber, and their combinations [29]. Sisal fibers are a good alternative because they are biodegradable, inexpensive, lightweight, and exhibit high specific mechanical performance. However, since the temperature of friction surface during, braking could go up to 750 C, sisal fibers can easily decompose. Furthermore, its poor wettability, incompatibility with resins, and high moisture absorption restrict its use [30]. However, with improvement in its chemical and structural properties, sisal fiber-reinforced friction composites exhibit properties equivalent to the commercial friction composites. For instance, a sisal fiber-reinforced phenolic composite slider prepared using silane treated sisal fiber and molded at a temperature of 160 C and pressure of 20 MPa shows that, with increasing sisal fiber contents, the friction coefficient increases at 150 C but, when the surface temperature of the iron disk increases to 250 C and 350 C, the friction coefficient rise to a maximum with 20 wt.% sisal fiber and then begin to decrease. The wear rate decreases with the addition of sisal fibers and drops to a minimum with 20 wt.% addition of sisal fiber. However, when the surface temperature of the friction disk increases, the wear rate increases significantly. This is attributed to the fact that high friction force is caused by the tensile failure strength of sisal fibers [31]. When the fiber is compressed elastically by rigid counterface asperity due to the compression of cell vacuums in the sisal fiber structure, the sisal fiber attains its stress limit and is ruptured by the higher friction force (Fig. 3.10). The friction force is a combination of mechanical force and molecular attraction between the surfaces in contact. The mechanical force between the composites and the counterface is generated by the plowing action of rigid asperity and the tensile force of the sisal fiber is caused by the same asperity. When the rigid asperities deform the matrix, the entire strain force accumulates in the sisal fibers. This strain force acts on the stressed regions of the matrix and greater friction power is

Sisal-reinforced polymer composites

107

Figure 3.10 Scanning electron micrograph showing tensile failure of sisal fiber during sliding [31]. Table 3.9 Composition of sisal fiber sliders (wt.%) [31]. Symbol Resin Fiber Copper Barite Felspar ZnO Friction powder

Sb2S3 Clay

S1 S2 S3 S4 S5

2 2 2 2 2

15 15 15 15 15

10 15 20 25 30

10 10 10 10 10

15 15 15 15 15

10 10 10 10 10

3 3 3 3 3

5 5 5 5 5

Balance

consumed. In contrast, when the rigid asperity deforms the sisal fiber, tensile fracture occurs and friction power is consumed. Thus, the braking reaches its maximum. When the sisal fiber content is more, a larger proportion of fibers is exposed on the friction surface, which increases the real area of contact between the elastic fibers and the rigid asperities, and hence the friction coefficient increases. When the contents of other fillers remain fixed (Table 3.9), the composites wear rate remains at a minimum when the ratio of resin sisal fiber is fixed at 3:4 (resin 15 wt. % and fiber 20 wt.%). When the proportion of resin to sisal is not optimized, more resin will be exposed to the counterface and hence will be easily abraded. Similarly, if the matrix content is less, then enough resin will not be available to bind the fibers and other fillers and debonding will take place. The interface temperature is another parameter which has a detrimental effect on the friction and wear behavior of sisal composites. For temperatures higher than 250 C, sisal decomposes and loses weight as a result of which the friction coefficient decreases and the wear rate increases (Figs. 3.11 and 3.12). The wear behavior at 150 C is dominated by microcutting of the fiber and fatigue cracking of the matrix (Fig. 3.13). When the surface temperature is higher than 250 C, sisal fiber

108

Tribology of Natural Fiber Polymer Composites

Figure 3.11 Variation of friction coefficient of composites with sisal fiber content [31].

Figure 3.12 Variation of wear rate of composites with sisal fiber content [31].

Figure 3.13 Scanning electron micrograph showing fatigue cracking of matrix during sliding [31].

Sisal-reinforced polymer composites

109

Table 3.10 Sisal fiber and fibers used in commercial disk brake pads [31]. Symbol

Fiber

Length/diameter (mm)

C1 C2 C3 SF

Asbestos Glass Steel/mineral Sisal

2 5/0.01 0.03 2 5/0.01 0.03 5/0.008 0.05 (steel fiber), 2 5/0.02 0.04 (mineral fiber) 10 15/0.2 0.3 (in composites)

decomposes into carbon powder and appears on the friction surface at 350 C. However, the friction coefficient does not decrease because of the abrasive action of carbon powder. The powder neither adheres to the counterface nor does it form a transfer film as graphite does. The undecomposed parts of sisal fibers retain the strength of composites. When compared with commercial disk brake pads reinforced with different fibers (Table 3.10) [31], sisal fiber-reinforced material has a relatively higher wear rate beyond 250 C; however, it has the best antithermaldecay property.

References [1] N. Chand, P.K. Rohtagi, Natural Fibers and Composites, Periodical Experts, New Delhi, 1994. [2] K.G. Satyanarayana, A.G. Kulkarni, P.K. Rohtagi, J. Sci. Ind. Res. 40 (1981) 222. [3] C.G. Jarman, S. Mykoluk, L. Kennedy, A.J. Canning, Trop. Sci. 19 (4) (1977) 173. [4] P.K. Rohatgi, P.D. Ekbote (Eds.), Materials Science and Technology in the future of Madhya Pradesh, Regional Research Laboratory, Bhopal, 1985. [5] N.G. Paul, V. Chatterjee, S.S. Bhattacharya, Text. Trends (1972) 1. [6] K.C. Dhyani, N.G. Paul, Jute Bull. 36 (1974) 1. [7] Y. Li, Y.-W. Mai, L. Ye, Comp. Sci. Technol. 60 (2000) 2037. [8] G.C. Yang, H.M. Zeng, J.J. Li, N.B. Jian, W.B. Zhang, Acta Sci Nat Univ Sunyatseni 35 (1996) 53. [9] N. Chand, S.A.R. Hashmi, J. Mater. Sci. 28 (1993) 6724. [10] G.C. Yang, H.M. Zeng, W.B. Zhang, Cellulose Sci. Technol. 3 (1995) 15. [11] N. Chand, S. Verma, A.C. Khazanchi, J. Mater. Sci. Lett. 8 (1989) 1307. [12] B. Singh, M. Gupta, A. Verma, Polym. Compos. 17 (1996) 910. [13] J.K. Kim, S. Lu, Y.-W. Mai, J. Mater. Sci. 29 (1994) 554. [14] E.T.N. Bisanda, M.P. Ansell, Compos. Sci. Technol. 41 (1991) 165. [15] S.K. Pal, D. Mukhopadhyay, S.K. Sanyal, R.N. Mukherjee, J. Appl. Polym. Sci. 35 (1988) 973. [16] G.C. Yang, H.M. Zeng, J.J. Li, Fiber Reinf. Plast. Compos. 3 (1997) 12. [17] K. Joseph, S. Thomas, Pavithran, Polymer 37 (1996) 5139. [18] J.E. Gordon, G. Jeronimidis, Philos. Trans. R. Soc. Lond. Ser. A 294 (1980) 545. [19] K. Joseph, S. Thomas, C. Pavithran, M. Brahmakumar, J. Appl. Polym. Sci. 47 (1993) 1731. [20] K. Joseph, S. Thomas, C. Pavithran, J. Reinf. Plast. Compos. 12 (1993) 139. [21] K. Joseph, S. Thomas, C. Pavithran, Compos. Sci. Technol. 53 (1995) 99.

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[22] G. Kalaprasad, S. Thomas, C. Pavithran, N.R. Neelakantan, S. Balakrishnan, J. Reinf. Plast. Compos. 15 (1996) 48. [23] G. Kalaprasad, S. Thomas, C. Pavithran, J. Mater. Sci. 32 (1997) 4261. [24] G.C. Yang, H.M. Zeng, N.B. Jian, J.J. Li, Plast. Ind. 1 (1996) 79. [25] N. Chand, U.K. Dwivedi, Polym. Compos. 28 (4) (2008) 437. [26] N. Chand, U.K. Dwivedi, Polym. Compos. 29 (3) (2008) 280. [27] W. Osterle, M. Griepentrog, T. Gross, Wear 251 (2001) 1469. [28] B.D. Garlanda, I.J. Beyerleinb, L.S. Schadlera, Compos. Sci. Technol. 61 (2001) 2461. [29] S. Narayanan, L.S. Schadler, Compos. Sci. Technol. 59 (1999) 2201. [30] H. Larbig, H. Scherzer, B. Dahlke, R. Poltrock, J. Cell. Plast. 34 (1998) 361. [31] X. Xin, C.G. Xu, L.F. Qing, Wear 262 (2007) 736.

Jute-reinforced polymer composites 4.1

4

Jute fiber

Jute has always been identified with the Indian subcontinent, which is one of the largest global producers of jute. Over the years, jute has become an integral part of the socioeconomic life of India. Right from its domestic use as household items, it has a long tradition of use in multiple applications in agriculture, cottage industry, the paper and pulp industry, building construction, manufacturing, and the automobile sector as raw material for car interiors. In recent years, their excellent compatibility with engineering polymers has opened new potential applications as triboefficient composites for specific applications. Biodegradability is its biggest asset. Jute fibers possess immense potential as fibrous reinforcement in polymeric composites. The fibers are extracted from jute plants in the genus Corchorus, family Malvaceae [1]. Jute fibers, extracted from bast or skin of the plant, are off-white to brown in color, and 1 4 m long. The cultivation of jute requires a warm and wet climate, with temperatures ranging from 20 C to 40 C and relative humidity of 70% 80%. Jute requires 5 8 cm of rainfall weekly and extra during the sowing period. The two varieties of jute are white jute (Corchorus capsularis) and Tossa jute (Corchorus olitorius). The latter is soft, smooth, and stronger than the former. White jute as well as Tossa jute is cultivated in Bengal (India and Bangladesh), which is the largest global producer of jute. Jute has a long history of use in industrial applications. The use of natural fibers in the automobile industry to make cars lighter dates back to 1940. These fibers were found to have the potential to replace glass fiber banned by the European Union in automobiles. Natural bast fibers such as flax, jute, and hemp are the best alternative fibers for automobile interior production because their ductile and specific stiffness are an advantage during side impacts. Among these fibers, jute is an economically viable option since it is the cheapest plant fiber with very high tensile strength. Today jute is recognized more as a wood fiber than as a traditional textile fiber. As wood fiber, jute has entered various diversified sectors, where natural fibers are the preferred choice. Among these industries are paper, celluloid products (films), nonwoven textiles, composites (pseudo-wood), geotextiles, and construction fabric. In the cottage industry sector, jute is used to make cloth for wrapping bales of raw cotton, sacks, and coarse cloth. The fibers are also woven for use as furnishings. White jute is fast replacing the synthetic materials in many of these uses owing to its biodegradable nature. The fibers are either used alone or blended with other types of fibers to make twine and rope. Jute mats are used in land restoration to prevent flood erosion. Jute is also used to make containers for plant saplings that Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00004-9 © 2021 Elsevier Ltd. All rights reserved.

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Tribology of Natural Fiber Polymer Composites

can be planted directly without disturbing the roots. It is worth mentioning that jute is the major crop among those able to prevent the deforestation caused by industrialization.

4.1.1 Advantages and disadvantages of jute fibers G

G

G

G

G

G

Jute fiber is biodegradable, recyclable, and eco-friendly. It is the cheapest of the plant fibers that are procured from the bast or skin of the plant’s stem. It can be grown in 4 6 months with a huge amount of cellulose being produced from the parenchyma of the jute stem. It has high tensile strength and low extensibility, and it ensures better breathability of fabrics. It is totally free of any narcotic element or odor. It possesses good insulating and antistatic properties as well as low thermal conductivity and a moderate moisture regain.

The disadvantages of jute fibers include moisture absorption, poor drapability and crease resistance, brittleness, fiber shedding, and fade on exposure to sunlight. Its strength decreases when wet, and it is vulnerable to microbial attack in humid climates.

4.1.2 Chemical composition of jute fibers Climatic conditions, age, and the digestion process influence not only the structure of the fibers but also the chemical composition. A general chemical composition of jute fibers is given in Table 4.1. Jute fibers contain 60% 64% cellulose, 14% 16% pentason, 12% 14% lignin, and other components like fats, pectin, ash content, moisture, etc. The details of these constituents are given in the Appendix. The fiber thickness varies between 40 and 80 µm, which leads to a variation in the tensile strength between 1000 and 480 MPa. Jute fibers can withstand up to 100 C in air without any decomposition.

Table 4.1 Chemical composition of jute fiber [2]. Component

Content (%)

Cellulose Hemicellulose Pectin Lignin Water soluble Wax Water

64.4 12.0 0.2 11.8 1.1 0.5 10.0

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4.1.3 Physical structure of jute fibers Jute fiber (transverse section of Corchorus olitorius stem shown in Fig. 4.1A) consists of several cells. These cells are formed out of cellulose-based helical crystalline microfibrils, with helical angles ranging from 20 to 30 degrees, which are connected to a complete layer by amorphous lignin and hemicellulose [2]. Multiple layers of such cellulose lignin/hemicellulose in one primary and three secondary cell walls stick together to form a multiple layer composite as shown in Fig. 4.1B. These cell walls differ in their composition (ratio between cellulose and lignin/ hemicellulose) and in the orientation (spiral angle, Table 4.2) of the cellulose microfibrils. The characteristic values for these structural parameters vary from one natural fiber to another as well as by physicochemical fiber treatments such as mercerization or acetylation. These values for jute fibers are shown in Table 4.2. The spiral angle of the fibrils and the cellulose content generally determine the mechanical properties of the cellulose-based natural fibers [3].

4.1.4 Mechanical properties of jute fibers Table 4.3 [2] lists the mechanical properties of jute fibers. The tensile properties of jute fibers depend strongly on their diameter and on the temperature and the length of heating time as shown in Tables 4.4 4.6. The tensile strengths of ethylenediamine (EDA) and acetylated jute fibers are shown in Table 4.7 [4]. Jute fibers are in general suitable for reinforcing plastics due to their relatively high strength and stiffness and low density (Table 4.3).

4.2

Jute polymer composites

The strength properties of jute fibers and their compatibility with polymers have led to the fabrication of a spectrum of composites including jute epoxy, jute polyester, jute phenol formaldehyde, and jute polypropylene for uses such as low cost housing elements, silos for grain storage, and small fishing boats [5]. Table 4.8 [6] lists some of the mechanical properties of these composites. Up to 40 wt.% of jute can be easily incorporated in either epoxy or polyester resin. For instance, the addition of about 25 wt.% fibers increases both tensile strength and modulus by more than 100% and causes a marginal increase in flexural modulus and strength and no change in compressive strength. However, the high resin consumption and absorption and desorption of moisture by the composites is a serious limitation. The composites are susceptible to temperature and water, and hence show a decrease in both tensile strength (up to 24%) and modulus (up to 25%). Furthermore, these composites fail at 45 degrees to the loading direction during compressive tests due to shear failure and fiber failure. The strength properties of these composites degrade due to weathering in dry and wet conditions (Table 4.9) [7]. This is attributed to the fact that high shrinkage stresses are produced in the composite due to moisture causing surface crazing and

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Figure 4.1 (A) Transverse section of jute stem (Corchorus olitorius); (B) schematic showing unit cell of jute fiber [4].

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Table 4.2 Structural parameters of jute fiber [2]. Cellulose content (wt.%)

Spiral angle (degrees)

Cross-sectional area, 3 1022 (mm2)

Cell length, L (mm)

L/D ratio

61

8.0

0.12

2.3

110

Table 4.3 Mechanical properties of jute fiber [2]. Diameter (µm)

UTS (MPa)

Tensile modulus (GPa)

25 200

460 533 2.5 13

Elongation (%)

Specific strength (MPa)

Specific modulus (GPa)

Microfibril angle (degrees)

1.16

340

42.7

8.1

Table 4.4 Mechanical properties of jute fibers as a function of fiber diameter [4]. Fiber diameter (µm)

Tensile strength (MPa)

Tensile modulus (MPa)

28.13 35.16 35.17 56.27 36.90 19.60 22.60 38.20 59.50 44.50 53.20 25.12 47.20 50.49 26.60 27.38 42.50 28.13 25.90

590 480 330 240 250 1210 1090 120 200 400 330 400 180 440 210 380 270 220 690

45.0 35.0 31.0 17.5 25.3 26.7 54.2 9.4 15.23 19.90 16.80 18.20 30.10 39.10 18.30 32.60 21.40 15.00 46.80

fiber polymer interfacial debonding. However, the deterioration of strength properties of composites can be prevented by coating the jute fibers with lignin and EDA before incorporation into the matrix [4]. The fiber coating drastically reduces the resin wastage during fabrication of treated fiber-reinforced composite, and resin consumption reduces to half of that required for untreated fibers. Similarly, EDA treatment of fibers reduces moisture absorption by composites. The treatment does

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Table 4.5 Tensile strength and tensile modulus of jute fibers heated at various temperatures for 2 h in vacuum [4]. Temperature ( C)

Tensile strength, 3 103 lb/in.2

Tensile modulus, 3 106 lb/in.2

Unheated 100 (in air) 150 200 250 300

50 48 45 54 45 15

3.8 4.2 5.4 4.3 2.6 4.1

Table 4.6 Tensile strength and modulus of jute fibers heated at 200 C in vacuum for various duration [4]. Duration of heating

Tensile strength, 3 103 lb/in.2

Tensile modulus 3 106 lb/in.2

1 2 4 6 8

44 54 57 37 33

3.9 4.3 4.3 4.7 3.7

Table 4.7 Tensile strength of EDA-treated jute fibers [4]. Samples

Tensile strength, 3 103 lb/in.2

Untreated 20 % EDA 40 % EDA 60 % EDA Acetylated

50 40 43 24 15

not adversely affect the tensile strength and modulus of jute polyester composites which were found to be 143.5 MPa and 6.9 GPa and 153.4 MPa and 8.4 GPa for treated and untreated fibers, respectively. Moisture absorption of jute polymer composites under constant humidity and ambient conditions is controlled by the impregnated fiber phases: with increasing volume fraction of jute fiber the moisture absorption levels increase. Hybrid composites developed using jute (up to 20%) and glass fibers (up to 40%) in polyester resin show increased tensile strength and modulus. Tensile strength and modulus of the weathered hybrid composites containing epoxy decrease by 5% 23% and 10% 15%, respectively, while with polyester resin these properties change

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Table 4.8 Mechanical properties of jute polymer hybrid composites [6]. Polymer matrix

Epoxy

Polyester

Fibre reinforcement Jute (wt%)

E-glass (%)

32.9 18.0 20.0 14.4 0 0 21.8 10.1 0 0 100 0

0 40.0 30.0 40.0 68.2 0 0 38.5 69.1 0 0 100

UTS (MPa)

TM (GPa)

104 157.0 143.0 238.0 429.0 59.0 84.0 200.0 391.0 37.0 441.5 3270.0

15.0 25.4 22.7 30.6 41.3 3.6 12.2 18.2 38.8 4.1 25.5 68.7

Table 4.9 Physical properties of jute fiber and glass fiber-reinforced composite [7]. Property

Bulk density (kg/m3) Fiber content (%) Water absorption 25 C (%) 24 h 3 days 7 days Water absorption, 100 C, 1 h (%) Flexural strength (MPa) Dry 24 h water soaking 3 days soaking 7 days soaking Tensile strength (MPa)

Unweathered sheet

Weathered sheet

Jute fiberreinforced

Glass fiberreinforced

Jute fiberreinforced

Glass fiberreinforced

1150 12 15

1300 28 32

1025

1250 30 35

2.34 2.88 3.87 3.08

1.03 1.17 1.27 1.05

3.23 4.16 5.07 3.90

1.28 1.69 1.97 1.34

23.00 32.00 42.60 34.00 24.20

107.40 123.20 135.20 74.10 76.00

11.60 28.20 19.60 19.10 9 20.6

103.30 99.20 98.80 64.90 63.00

marginally (0.5% 1.1%). The longitudinal compressive strength of the hybrid composite is strongly influenced by the thickness of the core (jute) and shell (glass) reinforcement. For E-glass/jute fiber/epoxy unidirectional hybrid composite, no

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buckling is observed during longitudinal and transverse compressive strength tests. When the diameter/time (d/t) ratio increases, the specimen fractures prematurely by fiber buckling. For small d/t ratios fiber kinking is observed in the jute core [7].

4.2.1 Surface modification of jute fibers As mentioned in the preceding sections, moisture absorption of jute fibers is a major limitation in developing composites with better mechanical properties. This limitation can be dealt with by modifying the fiber with chemical treatment. It has been observed that the surface modification of jute fibers due to coupling agents such as polyesteramide polyol, acrylic acid, silanes, and polyvinyl acetate (discussed in the subsequent paragraphs) causes noticeable improvements in the characteristic values of the composites, depending on the fiber and the matrix and on the type of surface treatment used. These chemical treatments are discussed in the subsequent section.

4.2.1.1 Silanization In unmodified jute epoxy and jute polyester composites, the moisture content at equilibrium increases with increasing fiber content [8 10]. However, the water repellence improves in jute polyester composites, treated with polyvinylacetate. Composites with silanized jute fibers showed about 20% lowered moisture at equilibrium [8]. Silane treatment reduces the amount of hydroxyl groups available on the jute fibers and free to bind moisture. Due to the surface treatment with silanes, these composites show increased (B30%) static characteristic values compared with unmodified composites at standard humidity (Fig. 4.2). The tensile strength of the silanized fiber composites is nearly independent of the moisture content of the

Figure 4.2 Tensile strength and modulus of silane-treated jute composites [12].

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composites. Unmodified jute epoxy composites reach only 65% of the values of the dry strength at the maximum moisture content of 5.2 wt.%. Similar results have been obtained on composites without coupling agents [11]. For unidirectional jute epoxy (fiber content, 33 vol.%) and jute polyester (fiber content 22 vol.%) composites, which were stored for 2 h in boiling water, tensile strength decreases from 10% to 16% while tensile modulus decreases from 13% to 25%. The improved moisture resistance caused by the application of the coupling agent is due to an improved fiber matrix adhesion. The coupling agent builds chemical bonds (silanol bonds) and hydrogen bonds, which reduce the moisture caused by fiber matrix debonding. The tensile modulus of unmodified jute epoxy composites follows the same trend as that for modified composites (Fig. 4.2), but moisture influence is distinctly clear when the coupling agent is used. This is reflected by the decrease in the tensile modulus of the jute fibers with increasing moisture content. Tensile strength of the fibers does not change with changing moisture contents. In contrast to the results of tensile tests, no such influence of humidity is observed for flexural strength and modulus by silane application [12]. The drop in the flexural strength of the silanized jute epoxy composites starts for a fiber concentration of 20% or higher. This is due to the fact that in the case of tensile stress the entire cross-sectional area of a material is loaded homogeneously while in flexural loadings only a marginal zone is affected. For jute fibers, under flexural stresses, the fibrillar orientation of 8 degrees causes a nearly transversal loading to the fibrils. Hence, the hydrophilic properties of hemicellulose and lignin influence the properties of the composites as they do under tensile stress. The fatigue strength is also influenced (Fig. 4.3). The silane-modified composites at a moisture content of 4.5 wt.% show nearly the same curve as the unmodified composites at standard humidity (i.e., 21 wt.% moisture).

Figure 4.3 Fatigue tests of jute-reinforced epoxy composite at different moisture content [12].

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4.2.1.2 Mercerization Similar to silane treatment, alkali (NaOH) treatment improves the crystallinity in the jute fibers and increases its modulus. The strength of the fibers improves while elongation to break reduces after treatment (Table 4.10). The flexural strength of the alkali-treated jute fiber-reinforced vinylester composites improves from 199.1 to 238.9 MPa, the modulus improves from 11.89 to 14.69 GPa, and the interlaminar shear strength increases from 0.238 to 0.2834 MPa. The load displacement curves of the composites containing untreated and alkali-treated jute fibers are shown in Fig. 4.4 [13].

4.2.1.3 Acetylation Jute composites based on acetylated jute fibers possess improved mechanical properties, the degree of improvement depending on the jute fiber content and the Table 4.10 Mechanical properties of untreated and alkali-treated jute vinylester composites [13]. Jute (wt. %) 0 8

15

23

30

35

Type of fiber

Modulus (GPa)

Flexural strength (MPa)

Breaking energy (J)

Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h Untreated Treated 2 h Treated 4 h Treated 6 h Treated 8 h

2.915 4.220 3.446 4.205 3.967 3.130 5.544 6.024 6.539 5.546 5.337 7.355 8.065 9.384 8.542 7.132 10.030 10.990 12.850 12.490 11.170 11.890 12.700 14.690 14.890 12.320

120.70 106.30 96.27 121.20 101.80 93.97 128.60 134.70 146.50 121.50 127.60 145.70 157.70 172.70 155.40 145.80 180.60 189.40 218.50 195.90 197.50 199.10 205.20 238.90 232.00 204.20

0.8227 0.2948 0.2497 0.3634 0.2270 0.2488 0.3399 0.3530 0.4016 0.2569 0.3351 0.3531 0.4048 0.4198 0.3553 0.3762 0.4799 0.4816 0.5061 0.4319 0.5042 0.5543 0.4570 0.5695 0.5678 0.5099

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Figure 4.4 Load displacement curves of untreated and alkali-treated jute composites: (A) vinylester resin, 23 wt.% fiber; (B) untreated; (C) 4 h NaOH-treated, 35 wt.% fiber; (D) untreated; (E) 4 h NaOH-treated [13].

duration of the treatment [14]. The tensile and flexural strengths of composites consisting of untreated and acetylated jute fibers are shown in Fig. 4.5 [14].

4.2.1.4 Graft copolymerization Several studies have been conducted to study the effectiveness of maleic anhydride polypropylene (MAH PP) copolymer as a coupling agent. For instance Mieck et al. [15] have reported an increase in shear and tensile strengths of about 100% and 25%, respectively, for flax PP composites, when the coupling agent was applied to the flax fibers before the composite was processed. These values depended on the grafting rate and on the average molar mass of the graft copolymer as well as on the application parameters. Similar increases in strength properties were obtained with a PP matrix modified with MAH [16]. The acetic anhydride groups of the MAH coupling agent led to hydrogen as well as chemical bonds with the hydroxyl groups of the flax fiber, anchored strongly by the coupling agent onto the fiber surface. Further, the long PP chain of the MAH PP coupling agent led to an adaptation of the very different surface energies of the matrix and reinforcement fiber, which allowed a good wetting of the fiber by the viscous polymer. The improved wetting can increase strength by an increased work of adhesion. Improved tensile and impact properties have also been determined for MAHmodified PP composites reinforced with wheat straw fibers [17]. The chemical

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bonding between the anhydride and the hydroxyl groups caused a better stress transfer from the matrix into the fibers leading to a higher tensile strength. The influence of coupling agent (MAH PP) on the creep behavior of jute PP composites shows that, in comparison to composites with untreated fibers, the strain in the outer fiber is reduced by about 20% 25% through the use of a coupling agent (Fig. 4.6). Jute PP composites also show distinct impact behavior with and

Figure 4.5 Tensile and flexural strength of untreated and acetylated jute fiber composites; prefix C denotes untreated and prefix A denotes acetylated composites [14].

Figure 4.6 Creep behavior of untreated and treated jute PP composite [23].

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123

without coupling agent (MAH PP). This is attributed to the fact that the damage initiation can be shifted to higher forces with a strong fiber matrix adhesion, as composites with a weak fiber matrix adhesion at smaller forces break down at a load perpendicular to the fiber. For these materials it could also be shown that the dissipation factor at the composites with MAH PP-modified jute fibers with 0.52 J is clearly smaller than those of the composites with unmodified jute fibers with a loss-energy of 0.76 J (each at an impact energy of 1.5 J). When the composites have no coupling agent, a part of the impact energy is degraded in the fiber matrix interface, for example by debonding and friction effects. Under these test conditions, multiple impact load as shown in Fig. 4.7 leads to a decrease in loss energy until the third impact. For composites modified with MAH PP, after the third impact only slight damage occurs while for composites without coupling agent damages are not controlled.

4.3

Tribological behavior of jute composites

4.3.1 Jute polyester composites Adhesive wear behavior of jute polyester composites for bearing applications has been studied against a steel counterface [18]. The friction coefficient attains a steady state after a running-in period of about 20 min when the tests are carried out in dry conditions at low and high energy values (pressure velocity product) of 0.61 and 1.65 MPa/ms, respectively. The variation of friction coefficient with the three fiber orientations (transverse, longitudinal, and normal) with respect to sliding directions is shown in Fig. 4.8. The wear behavior in the three fiber orientations at low and high pv limits is shown in Fig. 4.9. An increase of fiber volume fraction increases the friction coefficient of the composite and decreases its wear rate at

Figure 4.7 Effect of fiber treatment on the loss energy and damping of jute PP composite [23].

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Figure 4.8 Friction coefficient of jute polyester composite at high and low pv limit [18].

Figure 4.9 Wear rate of jute polyester composite at high and low pv limit [18].

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125

both low and high values of pv limit when the fibers are oriented normal to the specimen surface [18].

4.3.2 Jute polypropylene composites: effect of coupling agent The mechanical and thermal properties of MAH PP solution treated jute fiberreinforced PP composites (CT) and MAH PP melt mixed jute fiber-reinforced PP composites (MT) are listed in Table 4.11 [19]. The tensile strength of the MAH PP-treated composite is higher compared with the MAH PP melt mixed jute fiber-reinforced PP composite. The improvement in the tensile strength is due to the improved fiber matrix adhesion and improved stress transfer through MAH PP to the fibers. The improved adhesion results from the presence of MAH PP with increased polarity, which reacts with the hydroxyl groups present on the fiber surface [19]. The improved tensile strength due to the coupling agent is also due to the reduction in fiber pull-out and less fiber matrix debonding. MT composite showed less tensile strength than untreated composite due to the formation of microcavities (weak sites) during melt mixing at 170 C inside the jute fibers as a result of the desorption of water and decomposition of volatiles present in the jute fibers. Fig. 4.10A shows the tensile fractured surface of the unmodified jute fiber PP composite. Jute fibers debonded easily and pulled out from the PP matrix indicating poor interfacial adhesion between jute fibers and PP. Holes are created in the PP matrix during fiber pull-out (Fig. 4.10B). Tensile fracture occurs in PP due to stretching of molecular chains and the subsequent formation and coalescence of voids in the bulk of the composite material. Fig. 4.10C exhibits the fractograph of MT composite which shows reduction in fiber pull-out due to increased adhesion. Fig. 4.10D shows fiber pull-out and inner cell structure of the fiber in the longitudinal direction. The fractograph, Fig. 4.10E, shows twist in the fibers during melt mixing on a two-roll mill and internal defects created by removal of volatiles and moisture from the fibers. Fig. 4.10F shows the micrograph of tensile fractured samples of CT jute PP composite. The addition of MAH PP also increases the elongation of the jute fiber PP composite because there is a good adhesion between fiber and matrix, which gives better orientation of fibrils in the direction of deformation. The acid anhydride groups of the MAH PP coupling agent form the chemical bond with the hydroxyl Table 4.11 Mechanical and thermal properties of untreated (UT) and treated jute fiberreinforced PP composites. Properties

UT

MT

CT

Hardness (shore D) Tensile strength (MPa) Elongation (%) Melting point Tm ( C)

65 33.78 8.1 161.73

70 31.2 6.5 163.45

68 35.55 10.1 163.33

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Figure 4.10 Micrographs of tensile fractured samples of (A) unmodified jute PP; (B) holes due to fiber pull out in untreated jute PP; (C) MT jute PP; (D) fiber pull-out in MT jute PP; (E) fiber twist; and (F) CT jute PP composite [19].

groups of the lignocellulosic fiber [8,20 22]. The anhydride rings of MAH PP are covalently linked with the hydroxyl groups of the jute fibers to form ester linkage. The long chains of PP in MAH PP are compatible with neat PP matrix. Hence it lowers the surface tension of the fibers and increases its wettability with the PP matrix. The formation of ester linkage between the anhydride group of MAH PP and hydroxyl groups of the natural fiber is quite established [8,20 22]. The

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127

improved structural integrity of fiber composite is reflected in the differential scanning calorimetry (DSC) thermogram that shows that MAH PP-treated jute fiber PP composite exhibits better thermal stability than untreated jute PP composite (Fig. 4.11). Fig. 4.12 shows the abrasive wear behavior of untreated jute fiber-reinforced PP composites, MAH PP solution-treated jute fiber-reinforced PP composites and

Figure 4.11 DSC curves of untreated (UT) and treated jute composites [19].

Figure 4.12 Specific wear rate of untreated and treated jute fiber-reinforced PP composite [19].

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MAH PP melt-treated jute fiber-reinforced PP. Specific wear rate is maximum for untreated jute fiber PP composite due to the weak interfacial bond between the jute fibers and the matrix. Jute fibers are detrimental to the abrasive wear resistance of PP which exhibits a far greater energy to fracture and can thus accommodate plastic strain generated under the abrasive conditions to a greater extent. However, when treated with MAH PP solution, the composite shows better wear resistance due to the improved fiber matrix adhesion. The addition of coupling agent to the composite during melt mixing gives better wear resistance as compared with the MAH PP solution treated jute fiber-reinforced PP composite, because of the modification of the matrix during melt mixing of MAH PP at 170 C. Abrasive particles cause plastic deformation of the PP matrix (Fig. 4.13A). In the microplowing process, plowed material forms ridges. The scanning electron micrographs of the worn surface of untreated composite (Fig. 4.13B) shows debonding, fiber microcutting, and microplowing. Abrasive particles easily remove the fragmented fibers as wear debris from the surface unlike MT and CT composites (Fig. 4.13C and D). Wear of fiber-reinforced polymer composites proceeds in four steps: matrix wear, fiber wear, fiber fracture, and fiber matrix interfacial debonding. When a MAH PP coupling agent is used, interfacial debonding

Figure 4.13 Micrograph of worn surfaces of (A) pure PP, (B) untreated jute PP, (C) CT jute PP, and (D) MT jute PP composite [19].

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Figure 4.14 Abrasive wear behavior of heat-treated jute epoxy composites [24].

between jute fibers and PP is greatly increased and hence it dominates over the other mechanisms.

4.3.3 Jute epoxy composite: effect of heat treatment The abrasive wear behavior of heat-treated jute epoxy composites is shown in Fig. 4.14. Heat treatment of jute fibers increases the wear rate of composites, probably due to the removal of moisture from fibers and creation of defects at the fiber matrix interface [19]. The weakened interfacial bond strength decreases the resistance of composites to any damage. Some uncured resin is cured due to heat treatment and that increases the brittleness of the epoxy matrix. This increase in brittleness leads to easy matrix cracking during abrasion causing higher wear rate.

References [1] N. Chand, P.K. Rohtagi, Natural Fibers and Composites, Periodical Experts, New Delhi, 1994. [2] K.-P. Mieck, A. Nechwatal, C. Knobelsdorf Melliand, Textilberichte 11 (1994) 892. [3] T.M. Maloney, in: S.M. Lee, R.M. Rowell (Eds.), International Encyclopedia of Composites, VCH, New York, 1995. [4] M.K. Sridhar, G. Basavarajappa, S.G. Kasturi, N. Balasubramanian, Ind. J. Text. Res. 7 (1982) 87. [5] K.G. Satyanarayana, K. Sukumaran, P.S. Mukherjee, C. Pavitran, S.G.K. Pillai, Cem. Concr. Compos. 12 (1990) 117.

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[6] H. Wells, D.H. Bowden, I. MacPhail, P.K. Pal, Proceedings on 35th Annual Technical Conference, Society of the Plastics Industry, Section 1-F (1980) 1. [7] S.S. Labana, W.K. Plumerer, W.J. Burlant, Polym. Sci. Eng. 12 (1) (1972) 34. [8] J. Gassan, A.K. Bledzki, Polym. Compos. 18 (2) (1997) 179. [9] M.A. Semsarzadeh, Polym. Compos. 7 (2) (1986) 23. [10] M.A. Semsarzadeh, A.R. Lotfali, H. Mirzadeh, Polym. Compos. 5 (2) (1984) 2141. [11] A.N. Shan, S.C. Lakkard, Fiber Sci. Technol. 15 (1981) 41. [12] S.K. Pal, D. Mukhopadhyay, S.K. Sanyal, R.N. Mukherjee, J. Appl. Polym. Sci. 35 (1988) 973. [13] D. Ray, B.K. Sarkar, A.K. Rana, N.R. Bose, Bull. Mater. Sci. 24 (2) (2001) 129. [14] A.K. Rana, B.C. Mitra, A.N. Bannerjee, J. Appl. Polym. Sci. 72 (1999) 935. [15] K.P. Mieck, A. Nechwatal, C. Knobelsdorf, Angew. Makromol. Chem. 225 (1995) 37. [16] J. Gassan, A.K. Bledzki, Compos.: Part A 28A (1997) 1001. [17] M. Avella, R.dell’Erba R, Proceedings on Ninth International Conference on Composite Materials, Vol. II, Madrid, 9 (1993) 864. [18] A.A. El-Sayed, M.G. El-Sherbiny, A.S. Abo-El-Ezz, G.A. Aggag, Wear 184 (1995) 45. [19] N. Chand, U.K. Dwivedi, Wear 261 (2006) 1057. [20] G. Cantero, A. Arbelaiz, F. Mugika, A. Valea, I. Mondragon, J. Reinf. Plast. Comp. 22 (2001) 321. [21] J. Rout, M. Mishra, S.K. Nayak, S.S. Tripathy, A.K. Mohanty, S.K. Verma, Int. J. Plast. Technol. 5 (2002) 55. [22] J. Gassan, A.K. Bledzki, Polym. Compos. 13 (1997) 179. [23] A.K. Bledzki, J. Gassan, Prog. Polym. Sci. 24 (1999) 221. [24] N. Chand, U.K. Dwivedi, M.K. Sharma, T.S.V.C. Rao, Proc. 40th Intl. Symp. on Macromolecules—MACRO 2004, Paris (2004).

Cotton-reinforced polymer composites 5.1

5

Cotton fiber

Cotton (derived from the arabic word al qutun) is an important agricultural crop belonging to the genus Gossypium, subtribe Hibisceae, and family Malvaceae [1]. It is the main source of clothing and means of livelihood for a large population across the world. Cotton cultivation needs a long period of growth with plentiful sunshine and water; the fibers are harvested in dry weather. These climatic conditions are native to tropical and warm subtropical latitudes in the Northern and Southern hemispheres, the United States being the largest cotton-growing nation. The American cottons have 26 chromosomes while Asian and African cottons have only 13. Four species of Gossypium account for practically all the world’s supply of cultivated cotton [2]. These are: Gossypium hirsutum: These varieties of Central American origin constitute 87% of the world’s production. Their maximum height is 1.8 m. Gossypium barbadense is believed to have originated in Peru. Its height is between 1.8 and 4.6 m. This species includes the Sea Island and Pima S-2 cottons, and some of the Egyptian varieties. Gossypium arboreum includes the tree cottons found in Nigeria and the native cottons of India and Pakistan. It grows as tall as 4.6
6.1 m. Gossypium herbaceum has an average height of 1.2
1.8 m and is slow yielding. It has short fibers.

5.1.1 Advantages and disadvantage of cotton fiber Cotton fibers have high strength and durability and possess absorbency. G

G

Cotton is a biodegradable fiber. It can be easily blended with other fibers.

The only disadvantage is that cotton cultivation requires a high input of chemical fertilizers and insecticides and plenty of water.

5.1.2 Chemical composition of cotton fiber Cotton grows around the seeds of the cotton plant (see Fig. 1.1b). It consists of pure cellulose of high molecular weight, typically $ 6 3 105, comprising long chains of D-glucose units joined by β-1,4 glycosidic links. The rigidity of the cellulose structure in cotton, conferred by the anhydroglucopyranose units with their β-1,4 glycosidic (COC) links and intermolecular hydrogen bonding, makes cotton Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00005-0 © 2021 Elsevier Ltd. All rights reserved.

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resistant to environmental extremes. The typical arrangement of cellulose gives cotton fibers a high degree of strength and absorbency. A single cotton fiber contains more than 20 layers of cellulose coiled as springs. The chemical composition of cotton includes cellulose 82.7%, hemicellulose 5.7%, pectin 5.7%, water soluble 1.0%, wax 0.6%, and water 10%. The degree of polymerization (Pn) of cotton fiber is around 7000. Details of these constituents are given in the Appendix.

5.1.3 Physical structure of cotton fiber Cotton fibers vary considerably in their gross morphology, namely convolutions, cell wall thickness, cross-sectional shape, etc., and in their fine structure, namely fibrillar orientation, reversals, the packing density of microfibrils, etc., from variety to variety and species to species. Table 5.1 shows the dimensional and structural characteristics of different varieties of cotton [3,4]. The two varieties, namely G. arboreum and G. herbaceum cottons, are short and coarse and have few convolutions and structural reversals per unit length as compared with G. hirsutum and G. barbadense cottons. G. arboreum and G. herbaceum cottons have a significantly higher percentage of fibers with a circular cross-section than G. hirsutum and G. barbadense cottons [5]. These species also vary in the convolution angle, spiral angle, and X-ray angle. For instance, the cell wall of G. herbaceum cotton is composed of finer micropores and smaller crystallites than the cell wall of G. hirsutum cotton [6]. In general, cotton fiber of all varieties has a ribbon-like shape with twists or convolutions at regular intervals. The molecular chains aggregate in an extended and nonfolded form into elementary fibrils that combine to form a microfibril. The fibrils are 4
10 nm wide and Bmm long [7
9]. There is no lattice coherence along the elementary fibril and it breaks down after every 50 nm. Consequently the fibril contains mismatched crystal blocks with the same axial orientation of the cellulose chains but different orientation of the a- and c-axes [10]. The microfibrils are arranged as a helix in concentric cylindrical growth layers. These fibrils combine to form bundles (macrofibrils) of larger diameter due to physical coalescence as a result of the reduced surface free energy. They are interconnected and have widths around 100 nm [11]. The helix angle is constant throughout the cross-section and along the length for all cottons. The apparent variations in different varieties are attributed to the superposition of the convolutions and the helical angles [7
9]. However, the sense of the helix reverses from 30 to 100 times in the fiber. This reversal frequency primarily depends on the variety of cotton and the growth conditions [12]. The crystalline orientation has been shown to be high at the reversal points. Cotton is essentially crystalline; only about one-third of the total molecules constitute the amorphous phase. The disorder is mainly due to small crystalline units that are randomly packed. The structure of cotton is considered to be paracrystalline. Some physical characteristics of the cotton fibers, including the mean fiber length, linear density, convolution angle1 and moisture absorption, are 1

During drying from a swollen cellular tube to the collapsed-fiber form, in-built strains and stresses are locked-in in the fibers. In fact, the asymmetry of mechanical strains during drying is thought to be responsible for the typical convoluted structure of the fiber.

Length (mm)

10
25 10
25 20
32 35
45

Cotton species

G. arboreum G. herbaceum G. hirsutum G. barbadense

17
25 17
30 16
20 14
18

Width (µm)

40.5 34.8 15.3 18.2

Round 48.0 51.5 67.6 65.3

Elliptical 11.5 13.7 17.1 16.5

Flat

Avg. cross-sectional shape (75% mature)

30
60 30
50 50
75 30
55

Convolutions (cm)

Table 5.1 Dimensional and structural characteristics of different cotton species [13].

2
6 2
5 10
27 12
20

Reversals (cm)

5.1
11.8 6.2
8.9 8.8
12.3 3.9
8.5

Convolution angle

29.6
39.8 34.4
37.5 35.4
38.0 28.2
35.0

Spiral angle

22.6
35.0 26.5
31.8 28.6
34.6 22.8
30.5

X-ray angle

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Table 5.2 Some physical characteristics of two important species of cotton [13]. Cotton species

Mean fiber length (cm)

Linear density (mtex)

Convolutions/ cm

Convolution angle

Moisture regain

G. hirsutum G. herbaceum

2.775 2.310

168.80 179.82

80 60

13 340 11 400

6.410 6.765

Table 5.3 Degree of crystallinity and crystallite orientation of two species of cotton [13]. Cotton species

Degree of crystallinity

Crystallite orientation X-ray angle

G. hirsutum G. herbaceum

0.685 0.730

40%

50%

75%

Herman’s orientation factor

38.7 43.5

34.0 37.5

22.5 24

0.592 0.547

provided in Table 5.2 [3]. The data on the degree of crystallinity and crystalline orientation are provided in Table 5.3 [3].

5.1.4 Mechanical properties of cotton fiber The stress
strain curve of the cotton fiber is very similar to that of a glassy solid. However, the elongation at break is relatively large for the former compared with that of the latter. Under tensile loading, the dominant feature is the splitting of the structure along its length due to the fibrillar nature of the fiber [14]. This splitting occurs between fibrils and the break adjacent to a reversal, causing a tear that develops along the fiber and follows the helical path of the fibrils around the fiber. In cotton fibers, since the interaction between the chain molecules is strong, the elasticity of cotton is dominated by changes in internal energy. Crystallinity, crystal size, and the links between crystalline units offer high correlation with Young’s modulus in the fiber-axis direction [15]. The crystallites take part in the deformation even at low strains during measurement of the modulus at room temperature. A good correlation also exists between birefringence and crystalline orientation [7
9]. Mature fibers exhibit higher elastic modulus than less mature fibers. The strength of cotton fiber is inversely proportional to the X-ray orientation angle, which in turn is a measure of the convolution and the spiral angle [16]. However, the strength
orientation correlation is relatively poor [17,18]. These correlations decrease with increasing test length, which indicates that the strength of cotton fibers is determined partly by the orientation and partly by the presence of weak places along the fiber length. The decrease of strength with increasing number of reversals was higher for highly oriented samples. The strength of cotton is linearly

Cotton-reinforced polymer composites

135

related to its molecular weight. However, a paracrystalline-lattice distortion adversely affects the strength [19]. The elongation at break for cotton fiber also shows a good correlation with the X-ray angle [7
9], increasing with the increasing angle. Since crystals are elastic up to only relatively low elongations, beyond which plastic deformation takes place, the elongation of cotton depends partly on the alignment of the fibrils in the direction of stretch and partly on their deformation. The mechanical properties of cotton fiber, including the stiffness, strength, elongation at break, toughness, and bundle strength are listed in Table 5.4 [13]. These properties are discussed in the subsequent section.

5.1.4.1 Stiffness The stiffness of cotton fiber depends on the molecular as well as the crystalline orientation. At very low strain rates (below 0.5%), a linear stress
strain curve is obtained [15]. The average molecular orientation fmol is related to the crystallite and amorphous orientation in terms of the following relation [20]: fmol 5 Xfc 1 ð1
Xfa Þ where X is the degree of crystallinity and fc and fa are the Herman’s orientation factors for the crystalline and amorphous regions, respectively. The results for cotton are listed in Table 5.5 [13]. The data show that the amorphous regions in cotton are in an oriented state and their orientation is same as that of the crystalline phase. Thus the degree of crystallinity is not a critical factor in determining the fiber Table 5.4 Mechanical properties of two species of cotton [13]. Cotton species

Average Young’s modulus (gf/tex)

Average tenacity (strength) (gf/tex)

Elongation (%)

Bundle strength (gf/tex)

Toughness index (gf/ tex)

G. hirsutum G. herbaceum

383.01 488.39

22.65 22.52

6.93 5.00

48.24 48.78

0.92 0.62

Table 5.5 Details of three compatibilizers [24]. Compatibilizer

Temperature ( C)

Reaction time (min)

MAH (phr)

DCP (phr)

Graft content (wt.%)

Intrinsic viscosity (dL/g)

C1 C2 C3

120 140 150

5 5 5

7 7 7

0.3 0.7 0.7

0.84 1.72 2.14

0.82 0.68 0.57

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Tribology of Natural Fiber Polymer Composites

modulus. Firstly, the amorphous regions will be in the glassy state and therefore rigid. Secondly, they have high orientation, close to that of the crystalline phase.

5.1.4.2 Strength of single fibers and bundle strength The strength of cotton fibers depends on the molecular weight, orientation, number of reversals, and gauge length of the test specimen [16]. The bundle strength of cotton is higher than the single fiber strength (Table 5.4). This difference is attributed to the difference in the gauge lengths; zero for the bundle strength measurement and 1 cm for single fiber strength measurements. Bundle strength shows good correlation with X-ray orientation and maturity coefficient. Thus, an increase in molecular weight and crystallite orientation leads to an increase in the bundle strength of cotton. However, single fiber strength does not show any correlation with orientation. This is attributed to the fact that as the gauge length increases the correlation with orientation decreases and the reversal frequency starts to dominate. For average fineness, the breaking strength values show more scatter. It has been observed that the breaking load of fibers increases with increasing linear density up to the average linear density of the cotton and then becomes steady. Consequently, values of breaking strength obtained are much lower than the average, indicating that the distribution of linear density influences the breaking strength. The breaking of fibers involves the rupture of molecules, that is, the tensile failure occurs when hydrogen bonds are overcome. In cellulosic fibers, overcoming hydrogen bonds provides the measured strength values. Once hydrogen bonds are overcome, the stress can concentrate on fibrils close to the reversals and breakage occurs [21].

5.1.4.3 Elongation at break and toughness The elongation at break shows a good correlation with toughness. It also shows good correlation with X-ray angle, linear density, and the maturity coefficient. Similarly, the toughness shows good correlation with the X-ray angle. Thus the main factors that make elongation high are: low crystalline orientation, low linear density, and low maturity coefficient. The toughness is determined by the elongation of the cotton fiber.

5.2

Cotton
polymer composites

It has already been mentioned in Chapter 1, Natural Fibers and Their Composites of this book that plant fibers are currently used in the interior of passenger cars and truck cabins as trim parts, door panels, or cabin linings. Composites based on plant fibers are also used extensively for thermoacoustic insulation. Such insulating materials are mainly based on cotton fibers recycled from textiles and comprise cotton fibers up to 80% by weight. Polyester is frequently used to develop cotton fiberreinforced polymer composites since polyester is inexpensive, easily available as a

Cotton-reinforced polymer composites

137

liquid, easily processed and cured, and possesses good mechanical properties when reinforced with fibers and fillers. Polyesters are suitable for a variety of applications and are adaptable to the fabrication of structures of complex and intricate shapes.

5.2.1 Cotton
polyester composites Polymer composites based on cotton fibers have gained significant importance both in technical applications, such as the automotive industries, and in terms of strength requirements [22,23]. However, the main disadvantage of these composites is the lack of good fiber
matrix interfacial adhesion, which adversely affects the properties of composites. The poor adhesion between fiber and matrix is due to the hydrophilic nature of the former, which if decreased either by chemical modification or the use of a compatibilizer can greatly enhance the composite properties. The compatibilizers strongly affect the mechanical properties as discussed in subsequent sections.

5.2.1.1 Effect of cotton content on mechanical properties The addition of cotton fibers in polymers decreases yield stress, increases Young’s modulus, and decreases elongation at break and impact strength. The decrease of yield stress with increasing fiber content (Fig. 5.1) is attributed to the poor adhesion between the two phases (i.e., fiber and matrix). However, a significant increase of Young’s modulus (Fig. 5.2) is due to the significantly high Young’s modulus of cotton fiber [24] that increases the stiffness of the composites as the fiber content increases. The reduction of matrix amount as fiber loading is increased contributes to the decrease of impact strength because the matrix is primarily responsible for the absorption of the impact energy. The incorporation of cotton fibers reduces the 30

Yield stress (MPa)

Co

C1

C2

C3

25

20

15

0

10

20

30

40

50

Fibre content (wt%)

Figure 5.1 Influence of fiber content as well as three compatibilizers on yield stress of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite) [24].

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Tribology of Natural Fiber Polymer Composites

2600 Tensile stress (MPa)

Co

C1

C2

C3

2200 1800 1400 1000 600 200 0

10

20

30

40

50

Fibre content (wt%)

Figure 5.2 Influence of fiber content as well as three compatibilizers on tensile modulus of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite) [24].

elongation at break of the composites. The significant reduction in mechanical properties at high fiber content is attributed to the presence of many fiber ends in the composites, which could cause crack initiation [25]. All the mechanical properties discussed in the preceding paragraphs improve with the use of compatibilizers. These agents modify the interface by interacting with both the fiber and the matrix, thus forming a link between the two phases of the composite. This is attributed to the ability of the maleic anhydride (MAH) to react with the hydroxyls of the cotton fibers and the compatibility of the grafted copolymer bionolle chains with the main bionolle phase. The yield stress of the composites with 50 wt.% fiber content increases to 23.9 and 26.4 MPa with the addition of lowest and highest compatibilizers (Table 5.5), respectively, compared with the value of 19.7 MPa for the noncompatibilized composite. Similarly, the addition of the compatibilizers leads to a slight improvement in Young’s modulus (Fig. 5.2) and a significant increase in impact strength (Fig. 5.3). The elongation at break is not affected significantly by the addition of the compatibilizer. A slight improvement in Young’s modulus (Fig. 5.4) is obtained when the compatibilizer content is increased. The impact strength also increases with increasing compatibilizer content (Fig. 5.5), the effect of the compatibilizer becoming more pronounced with increased fiber content. The addition of compatibilizer reduces the water uptake of composites (Fig. 5.6) due to the formation of covalent bonds between the functional groups of MAH and the hydroxyl groups at the surfaces of cotton fibers [26]. With the increase in the compatibilizer content, less water is absorbed. Since there are more functional MAH groups as compatibilizer content increases, so more bonds are formed between matrix and fibers. The same conclusion is drawn from the use of different grafting content compatibilizers. Thickness swelling of cellulosic materials occurs when the cell wall is

Cotton-reinforced polymer composites

139

120

Impact strength (J/m)

Co

C1

C2

C3

100 80 60 40 20 20

30

40

50

Fibre content (wt%)

Figure 5.3 Influence of fiber content as well as three compatibilizers on impact strength of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite) [24].

4000 Young’s Modulus (MPa)

3500 5 phr

3000 2500

10 phr

2000

15 phr

1500 1000 500 0 0

20

30

40

50

60

Fibre content (wt%)

Figure 5.4 Influence of fiber content as well as compatibilizer content on Young’s modulus of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite) [24].

bulked by water. Composites with compatibilizer show lower thickness swelling compared with noncompatibilized composites (Fig. 5.7). Figs. 5.8 and 5.9 show the modulus of rupture (MOR), modulus of elasticity (MOE), and tensile strength of cotton stalks
polyester composites containing different ratios of fibers that were milled using different screen sizes [27]. These properties depend on fiber size as well as fiber content. MOR increases on increasing

140

Tribology of Natural Fiber Polymer Composites

120

-

Impact strength (J/m)

5 phr 100 10 phr 15 phr

80 60 40 20 0

20

30

40

50

60

Fibre content (wt%)

Figure 5.5 Influence of fiber content as well as compatibilizers content on yield stress of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite) [24].

Water absorption (%)

10

control

C1 10 phr

C2 10 phr

C3 10 phr

8 6 4 2 0 3

6

9

12

14

Immersion time (days)

Figure 5.6 Influence of compatibilizers and their content on moisture absorption characteristics of cotton fiber-based composites containing 50 wt.% fiber [24].

fiber content up to 25%. Tensile strength of the composites also decreases at the lower fiber content (15%
20%). A critical fiber content is required before the strength of the composites becomes greater than that of the polymer matrix. Maximum MOR is achieved for the composite containing 25% cotton stalk fibers milled using a 0.2 cm screen. Tensile strength of the composites exceeds that of neat polyester only on using 20%
25% cotton stalk fibers milled using a 0.35 cm screen. Water absorption and thickness swelling decrease with decreasing fiber size and fiber content.

Cotton-reinforced polymer composites

141

10

Thickness swelling (%)

no compatibiliser 8 with compatibiliser 6 4 2 0

20

60 Fibre content (wt%)

Figure 5.7 Histogram showing thickness swelling of cotton fiber-based composites with and without compatibilizer (C2; 15 phr) [24].

5.2.1.2 Effect of esterified cotton fibers on mechanical properties The infrared (IR) spectra of the esterified cotton fibers show a carbonyl absorption peak at 1725 cm21 due to the presence of the bark,2 which contains waxes, resins, starches, and a high percentage of tannic acid (Fig. 5.10). The intensity of this peak increases as a result of esterification. No improvement in MOR, MOE, and tensile strength of the composites could be achieved due to esterified fibers (Table 5.6) [27]. In fact, there was a slight decrease in the values of MOR and MOE. Notwithstanding the lower strength properties obtained using esterified fibers, esterification is still preferred because it leads to an increase in the fiber
matrix interaction that could compensate for the lower fiber content and, at sufficiently high ester content, could result in higher strength properties. Although water absorption increases, thickness swelling decreases because of the introduction of the ester groups into the cell wall polymers. Esterification prevents further swelling caused by water absorption.

5.2.1.3 Effect of hybridization 5.2.1.3.1 Cotton/ramie
polyester composites The tensile strength of plain weave ramie/cotton hybrid polyester resin matrix composites increases significantly for the composites with high fiber volume fractions [28]. However, no linear relationship between the tensile strength and the volume fraction of fibers exists. The volume fraction of longitudinal ramie fibers controls the overall tensile properties. The transverse cotton fibers (along the tensile axis) do not contribute significantly to the tensile properties. Table 5.7 shows the effect of different (0/90) fabric stacking sequences on the tensile strength of the polyester/ 2

Bark present in the cotton contains a high percentage of UV-absorbing groups that increases the UV resistance of cotton
polyester composite.

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Figure 5.8 Influence of fiber content and fiber size on (A) MOR and (B) MOE of cotton stalk
polyester composites [27].

ramie
cotton composites. (0) direction indicates that the ramie fibers are aligned along the test direction, whereas (90) direction is referred to whenever the ramie fibers are perpendicular to the test direction. The tensile strength obtained is greater than that of the matrix, and an increase of up to 243% is achieved.

Cotton-reinforced polymer composites

Figure 5.9 Influence of fiber content and fiber size on tensile strength of cotton stalk
polyester composites [27].

Figure 5.10 Comparative FTIR spectra of (A) untreated and (B) MAH esterified cotton stalks [27].

143

144

Tribology of Natural Fiber Polymer Composites

Table 5.6 Effect of esterification of cotton fibers on mechanical properties (average values) of cotton stalks
polyester composites (fiber wt.% B25) milled fiber size (0.2 cm) [27]. Fiber

MOR (MPa)

MOE (GPa)

Tensile strength (MPa)

Water absorption (%)

Thickness swelling (%)

Cotton stalks Esterified cotton stalks

29.73 25.30

1.25 1.16

10.42 9.44

12.52 15.21

4.34 2.25

Table 5.7 Tensile stress of the cotton
ramie hybrid composites with (90/0) configurations [28]. Material

Total volume fraction of fibers (%)

Ramie Vf % (parallel) to the test direction

Stacking sequence

σ (MPa)

Fabric I

49.7 6 0.7 55.3 6 1.2 55.3 6 1.2 57.2 6 3.4 45.3 6 3.1 49.3 6 3.4 49.3 6 3.4 50.9 6 3.4 54.1 6 1.8

12.9 6 0.4 19.2 6 0.6 9.6 6 0.6 14.8 6 1.8 12.7 6 2.3 18.5 6 2.5 9.2 6 2.5 14.3 6 2.6 19.5 6 1.3

0/90 0/90/0 90/0/90 0/90/0/90 0/90 0/90/0 90/0/90 0/90/0/90 0/90

46.3 6 2.7 61.3 6 4.1 43.5 6 3.6 60.8 6 3.0 56.2 6 3.4 61.4 6 4.1 43.2 6 6.2 54.0 6 3.3 55.2 6 4.0

60.2 6 1.4 60.2 6 1.4 60.9 6 1.3 52.9 6 0.5

28.9 6 1.0 14.5 6 1.0 21.9 6 0.9 20.6 6 0.4

0/90/0 90/0/90 0/90/0/90 0/90

85.0 6 9.3 51.7 6 1.9 70.3 6 11.7 60.5 6 4.3

58.0 6 0.6

22.5 6 0.5

0/90/0/90

68.9 6 3.5

Fabric II

Fabric III

Fabric IV

5.2.1.4 Cotton/kapok
polyester composites Polymer matrix composites (PMCs) developed using kapok/cotton fabric as reinforcement and isotactic polypropylene (iPP) and MAH grafted polypropylene as matrices exhibit better mechanical properties [29]. Untreated kapok/cotton fabric
iPP shows less stress resistance compared with composites with mercerized kapok/cotton fabric. Mercerization forms a ductile interface and good fiber
matrix interfacial adhesion. Weathered kapok/cotton fabric
iPP composite has low interfacial bond

Cotton-reinforced polymer composites

145

Figure 5.11 Effect of chemical treatment and weathering on tensile strength of kapok/cotton (kc)
iPP composites [29].

strength and low stress resistance. The tensile strength of iPP matrix-reinforced with untreated kapok/cotton fabric is much higher than that of composites reinforced with alkali-treated fabric and acetylated fabric. Both alkali treatment and acetylation reduce the crystalline cellulose, resulting in low tensile strength of the composites (Fig. 5.11). Kapok/cotton
MAH
iPP composites have the lowest tensile strength compared with other composites, and this is due to the brittle characteristics of the MAH
iPP matrix causing poor load transmission. The stiffness or modulus of the untreated iPP composites increases with an increase in fiber volume fraction and then decreases beyond about 23% fiber volume fraction. On the other hand, the addition of the same amount of fiber causes a significant increase in the initial tensile modulus of the MAH
iPP-reinforced with untreated kapok/cotton fabric. It also has the highest stiffness properties at higher fiber content (Fig. 5.12). The flexural properties of iPP and MAH
iPP
kapok/cotton fabric composites are shown in Figs. 5.13 and 5.14 together with glass fiber
MAH
iPP composites. The increase in the cellulose reinforcements on the iPP- and MAH
iPP-based composites shows a trend similar to that of the increases in their flexural strength and modulus. The maleated polypropylene fiber-reinforced composite gives superior flexural properties compared with the conventional polypropylene composites. MAH
iPP
kapok/cotton-reinforced composite shows an increasing trend of toughness with an increase in fiber volume fraction. The overall toughness (represented by work done, J) is, however, much lower than that of the fiber-reinforced iPP composite (Fig. 5.15). This is mostly due to the brittle characteristics of the MAH
iPP matrix. The slight increase in toughness of the acetylated fabric composite is attributed to the plasticization effect of the acetylation process on the fibers.

146

Tribology of Natural Fiber Polymer Composites

Figure 5.12 Effect of chemical treatment and weathering on tensile modulus of kapok/cotton (kc)-iPP composites [29].

Figure 5.13 Comparison of flexural strength of cotton/kapok (kc) and glass fiber-reinforced iPP composites [30].

5.3

Tribological behavior of cotton
polyester composites

Tribological applications of unidirectional cotton fiber-reinforced polyester composite, as bearings in conjunction with water lubrication and cooling, are quite old [30]. However, published work on the friction and wear behavior of cottonreinforced polymer composites is scarce. One important work on the effect of

Cotton-reinforced polymer composites

147

Figure 5.14 Comparison of flexural modulus of cotton/kapok (kc) and glass fiber-reinforced iPP composites [29].

Figure 5.15 Comparison of toughness (represented by work done) of treated and untreated cotton/kapok (kc) fabric-reinforced iPP composites [29].

orientation of cotton fibers on friction and wear behavior of polyester composites in sliding wear mode is a pioneering study which showed that the fiber diameter affected the friction coefficient µ of composites [31]. For pins sliding in the direction perpendicular to sliding (normal, N) a small increase in µ accompanied the increase in fiber volume fraction Vf, whereas in the longitudinal (L) and transverse (T) directions µ slightly decreased initially and then attained a steady value, essentially constant for values of Vf . 0.15. This was

148

Tribology of Natural Fiber Polymer Composites

obviously due to the larger area of exposed cotton fiber in the case of the L and T directions than in the N direction. When the percentage of cotton fibers increases in the area of contact, the friction coefficient tends to decrease. The decrease in µ in the L and T directions was due to the easy detachment of fibers from the bulk of the composite, in contrast with the N direction in which fibers are less exposed and their detachment is difficult. With respect to the L and T directions, the latter showed a lesser decrease in µ compared with those in the former. This is probably because, for the fibers to be pulled out of the surface, the frictional forces in the T direction must overcome an additional resisting force to deformation exerted by the polyester matrix backing up the fibers along their length, which does not exist in the L direction. The specific wear rate of all the three samples decreased initially with the increase in Vf and then became almost constant for Vf . 0.15 (Fig. 5.16). The highest wear rate occurred in the L direction of sliding for the same reason discussed above. Additionally, the improvement in the mechanical properties of the polymer also contributes to this factor (Table 5.8). The formation of a fiber-rich composite surface in the N and T directions and, to a smaller extent, in the L direction indicates that cotton particles act as lubricant in decreasing the wear rate (Fig. 5.17). The diameter of the cotton fibers was either 0.3 or 0.45 mm, which increased by a factor of 2
3 during sliding. Some of them acquired an oval shape with the major axis being oriented in the direction of sliding. Above the pv limit, the behavior was quite different. In the N direction µ increased with increasing Vf then attained a steady value at Vf 5 0.15, whereas µ remained almost constant throughout in the L and T directions. The values of µ determined were always smaller than the corresponding values below the pv limit. The specific wear rate decreased in all three cases with increasing Vf then became constant for Vf . 0.15 irrespective of the fiber diameter df. At any given Vf the N direction of sliding showed the lowest specific wear rate followed by the T and L. In the N direction of sliding, the long cotton fibers extending through the matrix prevented the catastrophic failure of the frictional heat softened polyester at the sliding surface. This is in contrast to the higher specific wear rate observed in the T and L directions where the fibers extending parallel to the sliding surface are easily pulled out from the softened matrix. The micrograph of the worn surface of a specimen (Vf 5 0.127, df B0.3 mm) after testing in the N direction above the pv limit showed smudging of the surface with deformed cotton fiber that was much greater than that observed below the pv limit.

5.3.1 Graphite-filled polyester composites Cotton-reinforced polyester composites are used for making fabric bearings. Hence, the friction and wear performance (low friction coefficient and low wear rate) of such antifriction composites should be very high. In view of this, efforts have been made to reduce the friction coefficient of cotton-filled composites. Graphite acts as a solid lubricant due to its lamellar crystal structure. When used as a solid lubricant in cotton
polyester composites, the tribological properties are improved significantly due to the lubricating action of the layer-lattice structure of graphite.

Cotton-reinforced polymer composites

149

Figure 5.16 Friction coefficient of cotton
polyester composites as a function of fiber volume fraction for (A) normal, (B) longitudinal, and (C) transverse orientation of fibers. Dashed lines are for speed 32 m/s while solid line shows data at a sliding speed of 10 m/s [31].

Neat polyester

6.11 101.83 44.75 2476.09 2.375 53.88 53.88 702.06 11.86 44.22

Property

Energy absorbed for impact per unit width (kg m/s2) Flexural strength at mid-point (μm/mm) Maximum strain at mid-point (μm/mm) Modulus of elasticity at bending (N/mm2) Strain energy density at maximum stress (N/mm2) Tensile strength at the point of break (N/mm2) Tensile strength at yield (N/mm2) Modulus of elasticity (N/mm2) Strain of fracture (%) Plain strain fracture toughness (N/m3/2)

Table 5.8 Mechanical properties of cotton
polyester composites [32].

22.27 103.10 50.185 2727.3 2.759 71.75 71.56 1122.13 8.65 61.54

df 55 0.3 mm Vf 5 0.127

Reinforced polyester

97.1 142.02 63 4251.61 3.56 117.51 108.05 1235.21 11.29 113.489

Vf 5 0.275 34.33 116.1 59.48 2773.49 2.923 87.03 76.3 1109.54 10.69 78.122

df 5 0.45 mm Vf 5 0.167

105.37 143.17 65.35 3774.85 3.37 116.48 110.28 1256.79 10.65 112.76

Vf 5 0.286

Cotton-reinforced polymer composites

151

Figure 5.17 Wear rate of cotton
polyester composites for (A) normal, (B) longitudinal, and (C) transverse orientation of fibers. Dashed lines are for speed 32 m/s while solid line shows data at a sliding speed of 10 m/s [32].

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Tribology of Natural Fiber Polymer Composites

However, the friction and wear of graphite-filled composites depend on the concentration of graphite powder as observed in the pin-on-disk sliding wear tests [32]. The specific wear rate of pure polyester reduces with the increased load, and it fails at a smaller pv value (0.597 MPa pressure; 2.22 m/s sliding speed) (Fig. 5.18). However, at higher loads the wear rate increases due, obviously to the greater frictional heat which softens the matrix. The transfer film of polyester improves the wear performance. Simultaneously, the increase in interface temperature causes a deterioration in the mechanical properties and the load-carrying capacity beyond B0.6 MPa pressure. When the polyester was reinforced with cotton waste, the pv limit increased to B1.2 MPa. However, it was accompanied by a higher friction heat and interface temperature that led to higher wear rates. When these composites were filled with graphite (5 phr), the wear performance (reciprocal of specific wear rate) was much better than that of the cotton
polyester composite. The specific wear rate was almost steady in the graphite-filled cotton
polyester composite which could be tested even up to B1.6 MPa. With increased concentration of graphite, the wear rate reduced, indicating the lubrication effect of graphite. The addition of graphite also reduced the friction of the cotton
polyester composite (Fig. 5.19). The friction coefficient also decreased with increasing graphite content. With frictional heat, thermal softening of the composite caused a larger contact between the specimen pin and sliding disc and hence a larger friction coefficient value [33,34]. In contrast, the friction coefficient increased with cotton reinforcement, probably due to the reduced area of sliding contact between pin and counterface. It is proposed that resin-bonded cotton fibers increase the friction coefficient while the loose cotton fibers might align in the direction of motion and hence reduce frictional force [35].

Figure 5.18 Specific wear rate of graphite-filled cotton-reinforced polyester composites [32].

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Figure 5.19 Coefficient of friction of graphite-filled cotton-reinforced polyester composites [32].

Figure 5.20 Rise in temperature of counterface disc with increasing load during sliding wear tests [32].

Operating parameters such as load and temperature had contrasting influence on the friction behavior. For instance, the cotton
polyester composite showed a higher friction coefficient and corresponding higher sliding interface temperature. Although the temperature increased with load (Fig. 5.20), the friction coefficient reduced, which is possibly due to the deterioration of fiber bonding with increased temperature, leading to the easy pull-out of fibers aligned in the sliding direction. In the case of graphite-filled cotton fiber-reinforced polyester composite, the

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temperature of the contact surface drastically reduced due to the reduced frictional heat as a consequence of the lubrication effect of graphite. Moreover the higher conductivity of graphite increased the heat dissipation and prevented accumulation of heat at the contact.

5.3.2 Ultra-high molecular weight polyethylene-filled polyester composite The incorporation of a small fraction of ultra-high molecular weight polyethylene (UHMWPE) particles improves the wear resistance of base polymers like polypropylene, polyester, etc. [35,36]. The wear characteristics of cotton
polyester composites measured on a pin-on-disk machine show that among neat polyester, cotton waste
polyester composite and cotton waste
UHMWPE-filled polyester composite, neat polyester possesses a low pv limit and low wear resistance (Fig. 5.21). Cotton waste-reinforced polyester composites show a better structural integrity and a higher pv limit. When filled with UHMWPE particles, the pv limit of cotton
polyester composite is not adversely affected. Furthermore, when UHMWPEmodified polyester resin is used as matrix, not only is the pv limit increased, but also the wear resistance increases as well. The increase in the concentration of UHMWPE reduces the wear rate even at higher loads, possibly due to the lubricating effect of UHMWPE. For the same reason, UHMWPE-filled cotton fiberreinforced polyester composite exhibited low friction coefficient which decreased further with the increase in the UHMWPE content (Fig. 5.22). The value of µ which increased with cotton reinforcement was reduced to below 0.40, nearly half of that of a polyester resin and nearly one-third of that of a cotton
polyester composite. The increased friction coefficient on cotton reinforcement is due to the

Figure 5.21 Specific wear rate of UHMWPE-filled cotton-reinforced polyester composites [35,36].

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Figure 5.22 Coefficient of friction of UHMWPE-filled cotton-reinforced polyester composites [35,36].

Figure 5.23 Coefficient of friction of UHMWPE-filled cotton-reinforced polyester composites as a function of load [35,36].

decrease in the area of contact between pin and counterface. The cotton fibers resist frictional heat more than the polyester resin and therefore offer a resistance to sliding movement that resulted in the increased friction coefficient of the cotton
polyester composite. Fig. 5.23 shows the effect of applied load on the friction coefficient of polyester, cotton
polyester, and UHMWPE-filled cotton
polyester composite. The friction

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coefficient increased with load for pure polyester. The cotton
polyester composite showed higher friction coefficient as well as higher sliding surface temperature. The temperature increased with load but the friction coefficient reduced and this was attributed to the deterioration of fiber binding with increased temperature that led to the easy pull-out of fibers and their subsequent alignment in the sliding direction. The addition of UHMWPE changed the trend and, due to the lubrication effect, the additional UHMWPE content in cotton
polyester composite resulted in a reduction in the friction coefficient. The increased load further reduced the value of μ due to the formation of a transfer layer of UHMWPE on steel disc. Worn surfaces when observed under scanning electron microscope (SEM) revealed the morphological changes of studied materials subjected to sliding wear. The worn surface of cotton-reinforced polyester composites showed maximum damage to the surfaces (Fig. 5.24). Cotton fibers along with the polyester matrix were damaged under sliding action. Cavities were formed due to the removal of material. Bigger cavities were expected in this case because cotton fiber may not allow small debris to be removed easily; instead, the combined cotton
polyester lumps were removed leaving behind larger cavities. The addition of UHMWPE in the cotton
polyester composite modified the wear, and the worn surfaces showed smooth topography and less material removal (Figs. 5.25 and 5.26). SEM examination also revealed the incomplete wetting of cotton fiber with polyester resin and therefore an air entrapped region is observed at certain places (Fig. 5.27). Loose cotton and fragments were observed in a cavity, which was formed during sample preparation. The high viscosity of the unsaturated polyester may be attributed to such pockets wherein resin could not penetrate sufficiently to embed properly in the bundles of cotton fibers. The addition of UHMWPE in

Figure 5.24 Micrograph of worn surfaces of cotton
polyester composites [35,36].

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Figure 5.25 Worn surface of UHMWPE (13.97 vol.%)-filled cotton
polyester composites [35,36].

Figure 5.26 Magnified view of the worn surface of UHMWPE (13.97 vol.%)-filled cotton
polyester composite [35,36].

cotton
polyester composites provided microdots having excellent wear resistance properties and protected the composite from further wear (Fig. 5.28). The maximum load was carried by UHMWPE particles and hence an increased volume percent of UHMWPE significantly reduced wear rate as well as friction coefficient.

Figure 5.27 Worn surface of UHMWPE (14.19 vol.%)-filled cotton fiber-reinforced polyester composite [35,36].

Figure 5.28 Schematic showing formation of friction dots of UHMWPE during sliding wear tests [35,36].

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159

5.3.3 Lubrication behavior of cotton The lubrication mechanism of cotton fiber can be understood better using a cotton transfer film in a steel-on-steel contact on a ball-on-disc machine [37]. The film is formed during the sliding wear. The friction coefficients of steel-on-steel measured in the presence and absence of a cotton transfer film are shown in Fig. 5.29 for

Figure 5.29 Coefficient of friction versus time with and without transferred cotton film at different RH conditions [37].

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relative humidity (RH) of 30%, 50%, and 75%. The friction coefficient is smaller for the experiments with a cotton film, confirming the lubricant effect of these films. At a low RH of 30%, the friction coefficient for the steel
steel contacts was high (0.6
0.8) and very unstable. The friction coefficient of the steel/cotton
steel contact was unstable at the beginning of the experiment, then assumed a constant value of about 0.4. In the presence of a transfer film, the friction coefficient was about 0.3 during the first third of the run, then it rose sharply to the value typical for a steel
steel contact. At a high RH of 75%, the friction coefficient of the steel
steel contact was between 0.5 and 0.6, but in the presence of a transfer film it was as low as 0.25. This value was stable during the first 50 s, then the friction coefficient rose sharply to the steel
steel value. In addition, changes of roughness of the sliding track were hidden by the presence of the cotton film which increased the apparent roughness of the disc both inside and near the wear track. The wear rate generally increased with the RH, but the dispersion of the results also increased. The lubricant effect may be due to the cellulose itself or the wax, or due to a synergistic effect of the two. However, the formation of the transfer film is not an intrinsic property of cellulose but is the result of complex phenomena. The transfer film is composed of fragmented cotton fibers, which agglomerate and adhere to the metal. Fragmentation is essentially a mechanical phenomenon and therefore depends on the resistance of the fibers and of the yarn. In principle, weak fibers favor fragmentation and film formation compared with stronger fibers. On the other hand, adhesion is strongly affected by surface chemical effects: the presence of specific chemical components within the fibers may promote adhesion and cohesion of cotton fiber fragments, thus facilitating the formation of the transfer film as well as its tribological properties. The presence of wax in cotton is a prerequisite for the formation of a transfer film [38,39].

References [1] I.V. de Gury, J.H. Carra, W.R. Goynes, The Fine Structure of Cotton, Dekker, New York, 1973. [2] N. Chand, P.K. Rohtagi, Natural Fiber and Composites, Periodical Experts, New Delhi, 1994. [3] S.M. Betrabet, K.P.R. Pillay, R.L.N. Iyenger, Text. Res. J. 33 (1963) 720. [4] S.M. Betrabet, R.L.N. Iyenger, Text. Res. J. 34 (1964) 46. [5] B.M. Petkar, P.C. Oka, V. Sundaram, Proceedings on 18th Joint Technological Conference of ATIRA, BTRA and SITRA, Vol. 28 (1977) 1. [6] K.L. Datar, S.M. Betrabet, V. Sundaram, Text. Res. J. 43 (1973) 718. [7] R. Meredith, Text. Prog. 7 (4) (1975). [8] J.W.S. Hearle, R. Greer, Text. Prog. 2 (4) (1970). [9] J.O. Warwicker, R. Jeffries, R.L. Colbran, R.N. Robinson, The Cotton Silk and Man Made Fibers Research Association, Shirley Institute, Manchester, 1966. [10] A. Peterlin, P. Ingram, Text. Res. J. 40 (1970) 353.

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[11] H.H. Dolmetsch, H. Dolmetsch, Text. Res. J. 39 (1969) 568. [12] H. Wakeham, T. Radhakrishnan, G.S. Vishwanathan, Text. Res. J. 29 (1959) 450. [13] V.B. Gupta, A.V. Manohar, B.C. Panda, in: P.W. Harrison (Ed.), Proceedings on 63rd Annual Conference of the Textile Institute, IIT Delhi (January 18
23) (1979). [14] W.E. Morton, J.W.S. Hearle, Physical Properties of Textile Fibers, 2nd edition, The Textile Institute and Heinemann, Manchester and London, 1975. [15] I.M. Ward, Mechanical Properties of Solid Polymers, Wiley, London, 1971. [16] S.P. Rowland, M.L. Nelson, C.M. Welch, J.J. Hebert, Text. Res. J. 46 (1976) 194. [17] L. Rebenfeld, W.P. Virgin, Text. Res. J. 27 (1957) 286. [18] A. Rajagopalan, N.B. Patil, V. Sundaram, Proceedings on 15th Joint Technological Conference of ATIRA, BTRA, SITRA (1974). [19] A. Vishwanathan, Cellulose Chem. Technol. 9 (1975) 103. [20] R.J. Samuels, Structured Polymer Properties, Wiley, New York, 1974. [21] A.V. Tobolsky, H.F. Mark, Polymer Science and Materials, Interscience, New York, 1971. [22] A.K. Bledzki, J. Gassan, Prog. Polym. Sci. 24 (1999) 221. [23] A.K. Mohanty, M.A. Khan, G. Hinrichsen, Compos. Sci. Technol. 60 (2000) 1115. [24] V. Tserki, P. Matzinos, C. Panayiotou, J. Appl. Polym. Sci. 88 (2003) 1825. [25] A.K. Mohanty, M.A. Khan, G. Hinrichsen, Compos.: Part A 31 (2000) 143. [26] J. Gassan, A.K. Bledzki, Compos.: Part A 28A (1997) 1001. [27] M.L. Hassan, A.M.A. Nada, J. Appl. Polym. Sci. 87 (2003) 653. [28] C.Z. Paiva Junior, L.H. de Carvalho, V.M. Fonseca, S.N. Monteiro, J.R.M. d’Almeida, Polym. Test. 23 (2004) 131. [29] L.Y. Mwaikambo, E. Martuscelli, M. Avella, Polym. Test. 19 (2000) 905. [30] J.K. Lancaster, in: A.D. Jenkins (Ed.), Polymer Science, Vol. 2, Amsterdam, North Holland, 1972, p. 959. [31] A.M. Eleiche, G.M. Amin, Wear 112 (1) (1986) 67. [32] S.A.R. Hashmi, U.K. Dwivedi, N. Chand, Wear 262 (11
12) (2007) 1426. [33] P.V. Vasconcelos, F.J. Lino, A.M. Baptista, R.J.L. Neto, Wear 260 (2006) 30. [34] A.M. Hagger, M. Davis, in: K. Friedrich (Ed.), Advances in Composite Tribology, Elsevier, Amsterdam, 1993, p. 107. [35] S.A.R. Hashmi, U.K. Dwivedi, N. Chand, Tribol. Letts 21 (2) (2006) 79. [36] S.A.R. Hashmi, S. Neogi, A. Pandey, N. Chand, Wear 247 (2001) 9. [37] V. Fervel, S. Mischler, D. Landolt, Wear 254 (2003) 492. [38] B.C. Jiang, Tribol. Trans. 34 (3) (1991) 369. [39] H.P. Stout, Wear 15 (1970) 149.

Bamboo-reinforced polymer composites

6.1

6

Bamboo

Bamboos belong to the grass family Poaceae, subfamily Bambusoideae, and tribe Bambuseae. Bamboos are perennial grasses and about 1000 species grow across the world in diverse climates, from cold mountains to hot tropical regions. However they are not native to Europe, North Africa, Western Asia, Northern North America, most of Australia, and Antarctica. In Southeast Asia there are 220 species of bamboo and in West Malaysia alone there are 70 species. In India there are about 80 species of which two of the most important are Dendrocalamus strictus and Bambusa bambos. The woody stems of bamboos, which are called culms, rise from the underground woody rhizomes. These shoots appear above ground as tender, pointed cones with imbricate sheaths. They elongate rapidly and reach their full height in 2 4 months. The number of culms depends on the kind of bamboo, the size and vigor of the clump, and the amount of rainfall. In mature, well-grown clumps 10 20 culms are produced in a year. Bamboos flower very infrequently at intervals of 25 50 years or more. Excessive hot weather promotes flowering. A bamboo clump does not usually produce any additional culms for a year before it flowers. The strength properties of bamboo are strongly influenced by the species, age, moisture content, and position along the culm, and the exterior is hard and tough. Bamboo is made up of two fibrous layers of which the outer layer contains twice as much fiber as the inner layer and is hence stronger. Bamboo is as strong as timber in compression and very much stronger in tension.

6.1.1 Advantages and disadvantages of bamboo Bamboo is a solid material that possesses attractive strength properties by virtue of which it is used in a variety of applications from household items, handicrafts, and furniture in building construction, low cost housing, and concrete reinforcement. Some of the major advantages of bamboo are: G

G

G

G

Bamboo has a very high strength and hence can be effectively utilized for many loadbearing applications. It is available in abundance. It is biodegradable. Bamboo fibers can be used to make fabrics that are very soft and possess antibacterial properties.

Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00006-2 © 2021 Elsevier Ltd. All rights reserved.

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G

G

G

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Seasoned or weathered bamboo is very hard, mature stems being very strong. It is light and very tough and hence can be used for making houses, fences, rafts, bridges, furniture, scaffolding, beams, columns, etc. It can be substituted for steel reinforcing rods in concrete construction.

A disadvantage of bamboo is that it is infested by wood boring insects. It needs to be kept dry or else treated with preservatives. Moreover, it absorbs moisture and is vulnerable to heat, light, and ultraviolet radiation. Apart from this, bamboo cultivation is invasive and spreads through roots and rhizomes. It is difficult to completely remove bamboo grove.

6.1.2 Physical properties of bamboo Bamboo is a lignocellulosic material, in which cellulosic fibers are embedded in a lignin matrix. Two types of cells exist in bamboo, leptodermous (matrix tissue) cells and sclerenchyma cells, which are enveloped in the matrix tissue. Vascular bundles made up of sclerenchyma cells act as reinforcement in bamboo. A vascular bundle is made of several phloem fibers, and a phloem fiber consists of several layers of pillar fibers. Microfibers in each layer of the pillar fibers are spirally arranged at a fixed spiral angle, which varies for different layers of pillar fibers. A vascular bundle is composed of several right-handed spiral phloem fibers [1].

6.1.3 Chemical composition of bamboo The main chemical constituents of bamboo are cellulose, hemicellulose, lignin, and water. Hemicellulose and cellulose are present in the form of holocellulose in bamboo, which contributes more than 50% of the total chemical constituents present. Lignin acts as a binder for the cellulose fibers and also behaves as an energy storage system (Table 6.1) [1,2]. Details of these constituents are given in the Appendix. Table 6.1 Chemical composition of bamboo [1]. Constituent

(%)

FT-IR peak assignment

Wavenumber (cm21)

α-cellulose

48.2

2921.63

Holocellulose

73.3

Lignin

2.14

QCH stretching vibration cell/hemicellulose QCH stretching vibration cell/hemicellulose Lignin component

Other components Pentosan Alcohol-benzene solubility Hot water solubility Ash Moisture

20.3 2.2 7.7 2.3 11.7

2921.63 1464.67 1530.24

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6.1.4 Mechanical properties of bamboo Bamboo culm (stem) has a unique structure and resembles that of a unidirectional fiber-reinforced composite with many nodes along its length. It consists of cellulose fibers, oriented along the bamboo culm, embedded in a ligneous matrix. However very few efforts have been focused on the extraction of the fibers from bamboo. Because of the nonavailability of fibers, only a few studies are available on the properties of these fibers and their use as reinforcement for polymers. Notwithstanding this fact, several forms of bamboo, such as the whole bamboo, sections, strips, and fibers, can be used for reinforcement with various thermoplastic and thermoset polymers. The structural variation in bamboo with cross-section and height shows that the fraction of cellulosic fibers varies from 15% 20% to 60% 65% while the tensile strength and modulus vary from 100 to 600 MPa and from 3 to 15 GPa, respectively [3]. This is attributed to the relative fraction of fibers in the specimen. The same variation has been found for bamboo fiber bundles of 1 2 mm diameter. Bamboo fibers have immense potential as reinforcement. However extraction of bamboo fibers is a tedious process. One of the most common techniques used is delignification. In this technique, lignin is dissolved in sodium hydroxide (NaOH) solution and then the cellulosic fibers are extracted [4]. NaOH dissolves lignin by breaking it into smaller segments. A very strong NaOH solution together with a long soaking time dissolves the lignin. The fibers are then separated using chemical as well as mechanical processes. Among these processes, two popular techniques are the compression-molding technique (CMT) and the roller mill technique (RMT). The tensile strength of the fibers obtained by the two techniques is shown in Fig. 6.1. The strength of fibers varies with fiber diameter (Fig. 6.2A).

10 CMT 8 frequency

RMT 6 4 2 0 300

400

500

600

700

800

900

1000

Tensile strength (MPa)

Figure 6.1 Statistical distribution of the tensile strength of the bamboo fibers obtained from compression-molded technique (CMT) and roll mill technique (RMT) [4].

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Avg Tensile strength (MPa)

(A)

1000

(D)

800 600 400 200 0

0.15

0.2

0.25

Fibre diameter (mm) (B) 180

Flexural strength (MPa)

160

Flexural modlus, x 10 GPa

140 120 100 80 60

(E)

40 20 0 cmt1525

rmt1525

Tensile strength (MPa)

(C) 25 control 20

silane treated

15 10 5 0

0

10

20

30

50

Fibre loading (phr)

Figure 6.2 (A) Tensile strength of the fibers with different diameters of fibers [4]; (B) flexural properties of composites prepared using fibers of different diameter [4]; (C) tensile strength of untreated and silane-treated bamboo fiber-reinforced natural rubber composites [8]; (D) tensile modulus of untreated and silane-treated bamboo fiber-reinforced natural rubber composites [8]; and (E) elongation to break of untreated and silane-treated bamboo fiber-reinforced natural rubber composites [8].

6.2

Bamboo polymer composites

6.2.1 Bamboo thermoset composites Short bamboo fiber-reinforced polyester composites having a volume fraction of 25% bamboo fibers exhibit flexural strengths (75 175 MPa) that are significantly higher than that of polyester (20 MPa) [4]. The flexural moduli of composites with large diameter fibers increase monotonically with increase in the volume fraction of fibers. However flexural moduli of composites with small diameter fibers remain the same as

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Table 6.2 Bamboo fiber-reinforced polyester composites prepared using different diameters of fibers [4]. Mechanical treatment

Volume fraction of bamboo fibers (%)

Average diameter of bamboo fibers (mm)

CMT

8 10 15

0.45

RMT

20 15

0.15 0.25 0.4 0.15 0.25

that of the polyester. Both the larger diameter fibers as well as smaller diameter fibers lead to the same level of reinforcement as far as the flexural strength of the composite is concerned. However the flexural modulus of composites containing finer size fibers is considerably less than that of those made with coarser fibers. For unidirectional bamboo fiber-reinforced polyester composites (Table 6.2) fiber configuration and volume fraction affect the properties of the composites (Fig. 6.2B). In the case of epoxy-based composites, the mean tensile strength of bamboo strip-reinforced epoxy composites has been found to be around 203 MPa [5]. Similarly, for bamboo fiber epoxy composites (fiber diameter: 0.5 0.8 mm) with different stacking patterns in a laminate, the strength varies from 260 to 390 MPa.

6.2.1.1 Surface modification of fibers The poor adhesion between resin matrix and bamboo leads to debonding of the composites on ageing. Interfacial adhesion is improved by chemical treatment. For instance, polyesteramide polyol used as an interfacial agent that improves interfacial adhesion and hence the mechanical properties of the composites [6]. Both tensile strength and flexural strength of treated bamboo epoxy and bamboo polyester composites show significant improvement. The attachment of polyesteramide polyol organofuctionality onto the fiber surface is stable because of the hydrogen bonding between the functional group of polyesteramide polyol and the surface reactive protons of bamboo fiber.

6.2.1.2 Mercerization Alkali treatment of bamboo improves the mechanical properties of bamboo-based polymer composites. The infrared (IR) spectrum of untreated and alkali-treated bamboo samples shows that the peak area due to OH stretching vibration within the region 3100 3800 cm 1 increases considerably with increase in alkali concentration [7]. The alkali treatment increases the number of OH groups. The effect of alkali treatment on bamboo has been discussed in Chapter 1, Natural Fibers and Their Composites.

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6.2.1.3 Silanization Silane treatment improves the interfacial bonding between bamboo fibers and rubber, which ultimately influences the mechanical properties [8]. As seen in Fig. 6.2C, the tensile strength of bamboo fiber-reinforced natural rubber deteriorates with bamboo loading due to poor adhesion of bamboo fibers of different shapes and sizes with the rubber matrix. However composites containing silane-treated bamboo fibers show better strength than untreated bamboo fiber-reinforced rubber. In contrast to tensile strength, tensile modulus of same composites increases with increasing fiber loading (Fig. 6.2D). The elongation to break exhibits the same results as that of tensile strength and decreases rapidly with increasing fiber loading (Fig. 6.2E). With increase in fiber loading, molecular motion of the polymer matrix is restricted. The silane-treated bamboo fiber rubber composite shows lower elongation than the untreated bamboo fiberreinforced composite. The silane coupling agent improves the surface functionality of the bamboo fibers and enables the fibers to bond chemically with the rubber matrix. The treatment provides better wetting and dispersion of fibers in the rubber matrix.

6.2.2 Bamboo thermoplastic composites Bamboo fiber-reinforced polymer composites should be lightweight and possess good weathering ability, good design and manufacture flexibility, and medium strength for indoor applications such as in the furniture and construction industries. The interaction between the polypropylene (PP) matrix and bamboo fiber is promoted by using maleated PP (s-MAPP and m-MAPP) [9]. The endothermic peak in the differential scanning calorimetry (DSC) curve of PP, s-MAPP, and m-MAPP at 169 C, corresponding to the melting of its α-crystalline phase, is accompanied by a second endothermic peak at 152 C, 150 C, and 150 C, respectively, for composites. This indicates that a β-phase structure is formed in all the three kinds of bamboo fiber-filled PP composites. The DSC curves obtained at a cooling rate of 10 C/min show exothermic peaks at 107 C, 109 C, and 116 C, respectively, for pure PP, s-MAPP, and m-MAPP which shift to 110 C, 114 C, and 117 C for composites based on them. The higher cooling crystallization temperature Tc values of the composites indicate a rapid crystallization rate of the composites due to the nucleation effect of the bamboo fiber. Bamboo fibers act as a nucleating agent for PP, s-MAPP, and m-MAPP in the composites. This is confirmed by wide angle X-ray diffraction (WAXD) patterns that show three strong equatorial α-form peaks of PP or PP sequences at (110), (040), and (130) [9]. The pattern of the pure PP and PP composites indicates a new peak located at 2θ 5 16 which is the characteristic single β-form peak (300). The WAXD pattern of bamboo shows no peak between 2θ 5 10 and 20 .

6.2.2.1 Effect of maleated PP on mechanical properties of composites Bamboo fiber-reinforced maleated PP composites having bamboo fibers of different sizes (,500 μm, 500 850 μm, 850 μm to 1 mm, and ,2 mm) show different mechanical properties [10]. The tensile strength and modulus of bamboo-reinforced

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169

Figure 6.3 (A) Tensile modulus and (B) tensile strength of bamboo fiber-reinforced pure polypropylene composites (solid circles) and bamboo fiber-reinforced MAPP composites (triangles) [10].

MAPP increase with increasing content up to 65 wt.% (Fig. 6.3A and B). A tensile modulus of 3.4 GPa is obtained for 50 wt.% filled PP while for 50 wt.% MAPP, it is around 4 GPa. Similarly, tensile strength of 50 wt.% bamboo MAPP composite is 36 MPa while for PP composites, tensile strength decreases slightly. Subsequent to the improvement of adhesion between the bamboo fiber and the polymer matrix, tensile strength and modulus also improve with increasing maleic anhydride (MAH) content. The situation of poor wetting on the surface of bamboo fiber by unmodified PP due to different surface energies between the fibers (hydrophilic) and the PP matrix (hydrophobic) is improved in the case of MAPP because of the formation of hydrogen bonds in the interfacial region; for instance between the hydroxyl ( OH) group of cellulose or lignin in bamboo fiber with the anhydride groups in the MAPP matrix. MAPP can crystallize on the bamboo surface, and the bamboo fiber acts as both a reinforcing agent and a nucleator for MAPP. This surface crystallization contributes to the better interface adhesion in bamboo PP MAPP composite. With increasing size of bamboo fibers, both tensile strength and tensile modulus decrease considerably, probably due to the fact that at the same composition a smaller fiber has a relatively larger surface area which results in better contact between fiber and matrix.

6.3

Tribological behavior of bamboo and bamboo polymer composites

6.3.1 Abrasive wear behavior 6.3.1.1 Effect of fiber orientation and abrasive size Abrasive wear of bamboo stem (Phyllostachys pubescens) depends on the influence of orientation with respect to the abrading surface and the abrasive particle size

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[11]. The vascular bundles contained in sclerenchyma cells are reinforcers of bamboo. These vascular bundles are made of many phloem fibers, which may be considered as fibers in general composites. The fiber (vascular bundle) orientation with respect to the abrading surface has a significant influence on abrasive wear of a bamboo stem. Normally oriented specimens (N-type) give a better abrasion resistance than parallel-oriented (P-type) and antiparallel-oriented (AP-type) specimens. The surface layer of a bamboo stem has higher abrasion resistance than the inner layer. Abrasion depths of all specimens increase as abrasive particle size increases. The surface morphologies of P- and AP-type specimens abraded against abrasives containing quartz sand (0.45 0.90 mm diameter) show that wear debris is generated due to microcutting and microcracking by hard abrasive particles. The magnitude of microcutting and microcracking damage to the P-type specimen is less compared with AP-type specimens. The worn surface of the N-type specimen shows that the vascular bundle fibers protrude on the matrix when a certain depth of the matrix tissue is removed. Microcracks observed on the abraded surface of the bundle fibers suggest abrasive wear due to ductile delamination. These microcracks propagate slowly causing small pits. The microcutting and microcracking damage to the abraded surface of the matrix tissue between the bundles is less than that of the P- and AP-type specimens but greater than that of the P-type specimen. When the specimens are abraded by abrasives of higher size (0.25 0.45 mm diameter) similar features are obtained, although they caused less abrasion damage. An increase in the bundle fiber content improves the elastic modulus and longitudinal tensile strength of bamboo stem. Yakou and Sakamoto [12] measured the hardness of Phyllostachys pubescens and demonstrated that the average hardness of the fiber and matrix tissue decreased continuously from the outside surface toward the inside surface. Therefore, it is considered that the abrasion resistance of the surface layer of a bamboo stem (N-type specimen) is higher than that of its inner layer (P-and AP-type specimen) due to higher vascular fiber content and greater hardness. Because of the lower cleavage strength and transverse tensile strength of a bamboo stem, abrasive particles easily cut the surface layer of P- and AP-type specimens to generate grooves, microcracks and brittle rupture. Since the vascular bundles have higher ductility and strength than the matrix tissue, the former gives better abrasion resistance than the latter.

6.3.1.2 Abrasive wear behavior of bamboo (Dentrocalamus strictus) The same results regarding the effect of vascular bundle orientation of bamboo in abrasive wear mode have been found for a different species [13]. The abrasive wear rate of bamboo depends on the applied load as well as on the three orientations of the vascular bundles, namely parallel, antiparallel, and normal to the sliding direction. The wear rate increases with applied load. When the normal load on the grits increases, the load is distributed to a larger number of surface asperities and the latter deform. The grits penetrate deep, causing increased wear. Severe deformation of the interfacial bonding between the lignin matrix and the vascular bundles causes high wear. The specific wear rate of bamboo for three orientations of vascular bundles follows the trend (Fig. 6.4): normal , antiparallel , parallel.

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Figure 6.4 Specific wear rate of bamboo specimens in different orientation [12].

The wear rate of bamboo normal to the fiber direction (N) is lower than that in the transverse fiber direction (P) and even lower than in longitudinal fiber direction (AP) at different applied loads. Normally oriented bamboo exhibits maximum wear resistance. The long fibers are well embedded deep in the matrix which offers the greatest resistance to the abrasives. The vascular fibers are oriented normal to the sliding direction, and only a cross-section of fibers comes in contact with the grits. Normally oriented vascular bundle fibers prevent the penetration of the abrasive and resist the movement of abrading particles. In the P-type sample, ploughing is higher and a fibril is completely removed. Since the vascular fiber bundles are in the parallel direction, the probability of the grits remaining in contact with the fibers is quite high. In the APtype sample, elongation of fibrils followed by microcutting takes place. However the fibers still remain in the lignin matrix, unlike those of the P-type sample in which severe damage is caused along the entire length of the fiber. When the abrasive grit size increases, the wear of bamboo also increases (Fig. 6.5). In the case of coarser abrasives, the grits penetrate deep and hence a large portion of material is removed leaving behind large cavities. When the load remains the same, the effective pressure on individual grits increases with coarser abrasive particles, as the load is shared by a lesser number of grits. When the grits are finer in size, the effective pressure on each individual grit is less. As a result, the entire load is distributed on a large number of grits, which do not contribute much in the way of material removal. However when the load is increased, the effective stress on each individual abrasive particle reaches a level where the grits shear the surface. Scanning electron micrographs of worn surfaces of P-, AP-, and N-type samples show that when fibers are parallel to the sliding direction the entire fiber length is abraded, peeled off, and removed during abrasion (Fig. 6.6A). The lignin matrix between the fibers is removed and fibrils are exposed and are then removed in the successive cycles of abrasion. Wear tracks are formed due to microploughing and damage caused to vascular fibers. In the AP-type sample, the abrasive particles slide perpendicular to the fiber alignment, which causes microcutting of fibers. Fig. 6.6B clearly shows the wear track and broken pieces of fibers on it. These pieces align themselves in the sliding direction. In the case of the

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Figure 6.5 Effect of grit size (400B23 μm; 320B36 μm; 180B78 μm; and 120B116 μm) on abrasive wear rate of bamboo [12] (L 5 longitudinal, T 5 transverse).

Figure 6.6 Scanning electron micrograph of worn surface of (A) P-type sample, (B) AP-type sample, and (C) N-type sample [12].

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N-type sample, cells of fiber are oriented normal to the sliding direction, hence pulverized cross-sections of vascular fiber bundles are clearly seen (Fig. 6.6C).

6.3.1.3 Effects of vascular fiber content The vascular fiber content of bamboo (Phyllostachys pubescens) significantly affects the abrasive wear behavior (Fig. 6.7) [14]. Measurement is carried out using a stereoscope with a computer image manipulation system. The wear volume of the bamboo specimen depends on the fiber content as well as the sliding velocities (1.68, 2.35, and 3.02 m/s) and abrasive particle size (0.104 0.214, 0.214 0.420, and 0.420 0.840 mm). The wear volume increases with increasing sliding velocity and increasing abrasive particle size. The tensile strength and elastic modulus of the bamboo stem increase with the vascular fiber content. Both the tensile strength and tensile modulus, as well as the impact strength of the bamboo stem, are approximately proportional to its vascular fiber content (Table 6.3 and Fig. 6.8). The tensile strength of the bamboo with vascular fiber content of 28.9 and 34.4 vol.% was higher by 74.5% and 153.3% than that with vascular fiber content of 23.1 vol.%, respectively. The elastic modulus of the bamboo with vascular fiber content of 28.9 and 34.4 vol.% was higher by 24.2% and by 88.7% than that with vascular fiber content of 23.1 vol.%, respectively. The

Figure 6.7 Effect of the vascular fiber content on the wear volume of bamboo specimens [13].

Table 6.3 Impact strength of bamboo stem [14]. Fiber content (vol.%)

Impact strength (kJ/m2)

23.1 28.9 34.4

72.8 90.7 117.5

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Figure 6.8 Tensile strength and modulus of bamboo as a function of vascular content [15].

increase of strength properties results in better abrasive wear resistance. Yakou and Sakamoto [12] measured the hardness of Phyllostachys pubescens and considered that the reason for the higher abrasive wear resistance of vascular fibers compared with the matrix tissue is the higher hardness of the former than that of the latter. Thus the better mechanical properties of the bamboo with higher vascular content result in a higher abrasive wear resistance.

6.3.1.4 Effects of rough morphology Abrasive wear depends on the mechanical properties such as tensile strength, impact strength, hardness, and fracture toughness and on the surface roughness. The total friction force generated at the contact surface consists of friction force acting on the smooth parts of the surface and on the rough parts. The surface of bamboo stem is rough, and the rough parts encourage abrasive particles to roll near the protruding fiber ends and reduce the damaging action on the vascular fibers and matrix tissue. The general size of the fiber spacing along the sliding direction is 0.5-mm or more while the size of the abrasive particles of 0.104 0.214 mm and 0.214 0.420 mm is less than that of the fiber spacing. The size of the abrasive particles of 0.420 0.840 mm is close to that of the fiber spacing. Thus the rough surface morphology occurs if the spacing of the asperities is more than that of the abrasive particles.

6.3.1.5 Effects of contact configuration The topography of the worn surface created by the large grit size is more rough than that created by the smaller grits. The force acting on the sliding surface of a unit area due to abrasive particles results from the so-called “centrifugal” force of the grits [14]. This force, related to the density and the velocity of the grits, increases with the sliding velocity because the density of grits remains constant. The average shear stress on the unit surface apparent contact area is the resultant of

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shearing forces generated by all contacting grits in contact with the surface in this unit area. Likewise, the average compressive stress on the unit surface apparent contact area is the resultant of the normal force generated by all contacting grits in this unit area. When the abrasives are smaller, a larger number of abrasives contact the surface on a unit area compared with the case with larger sized abrasives. Therefore, the real shear stress and real normal load acting on the surface due to a larger abrasive particle are higher than those due to a smaller abrasive particle. Consequently, larger abrasive particles accelerate rupture of the surface layer causing larger wear volume [14]. The higher centrifugal force of the abrasive material means higher sliding velocity and a higher wear volume of the bamboo stem.

6.3.2 Sliding wear behavior Sliding wear of bamboo (Phyllostachys pubescens) stem depends on normal load and sliding velocities (Fig. 6.9) as well as on the relative orientation of bamboo fibers with respect to the sliding direction [15]. The wear volume increases as the normal load and velocity are increased. The N-type specimens give the best wear resistance. Temperature rises due to the interfacial friction are quite high under high load, particularly at high sliding velocity. The material transfer phenomenon from the bamboo to the iron counterface occurs due to adhesion. In the initial stage, transferred material forms some patches on the counterface. As sliding progresses, transferred material patches extend along the sliding direction. When the interfacial contact reaches a steady state, the adhesion of transfer film also becomes steady. However since the transferred material film is not continuous it is detached. This transferring detaching process results in adhesive wear of bamboo. The wear debris of bamboo specimens is either finely divided powder or large particles.

Figure 6.9 Effect of fiber orientation on abrasive wear behavior of bamboo specimen at a sliding speed of 0.42 m/s. Same trend follows at higher sliding speed of 0.84 m/s [15].

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References [1] R.M. Badhwar, K. Kadambi, Indian For. 82 (1956) 524. [2] S. Jain, R. Kumar, U.C. Jindal, J. Mater. Sci. 27 (1992) 4598. [3] S. Amada, Y. Ichikawa, T. Munekata, Y. Nagase, H. Shimizu, Compos.: Part B 28B (1997) 13. [4] A.P. Deshpande, M. Bhaskar Rao, C. Lakshmana Rao, J. Appl. Polym. Sci. 76 (2000) 83. [5] F.G. Shin, W.P. Zheng, M.W. Yipp, J. Mater. Sci. 24 (1989) 1481. [6] M. Saxena, V. Sorna Gowri, Polym. Compos. 24 (2003) 428. [7] M. Das, D. Chakraborty, J. Appl. Polym. Sci. 102 (2006) 5050. [8] H. Ismail, S. Shuhelmy, M.R. Edyham, Eur. Polym. J. 38 (2002) 39. [9] Y. Mi, X. Chen, Q. Guo, J. Appl. Polym. Sci. 64 (1997) 1267. [10] X. Chen, Q. Guo, Y. Mi, J. Appl. Polym. Sci. 69 (1998) 1891. [11] J. Tong, L. Fien, J. Li, B. Chen, Tribol. Int. 28 (5) (1995) 323. [12] T. Yakou, S. Sakamoto, Jpn. J. Tribol. 38 (4) (1993) 491. [13] N. Chand, U.K. Dwivedi, S.K. Acharya, Wear 262 (2007) 1031. [14] J. Tong, Y. Ma, D. Chen, J. Sun, L. Ren, Wear 259 (2005) 78. [15] J. Tong, R.D. Arnell, L. Ren, Wear 221 (1998) 37.

Wood-reinforced polymer composites

7.1

7

Wood

Wood is a natural composite obtained from shrubs and trees. It is a heterogeneous, hygroscopic, cellular, and anisotropic material, composed of fibers of cellulose (40% 50%) and hemicellulose (15% 25%) held together by lignin (15% 30%) [1]. Details of these constituents are given in the Appendix. A tree grows in height as well as in diameter by the formation of new layers which envelope the entire stem. This leads to growth rings known as annual rings, if the change in season is annual. Thus the age of a tree can be estimated by the number of annular rings. In regions where there is no seasonal difference, growth rings are not distinct. Within a growth ring, the part nearest to the core is more open textured and lighter in color than that nearer to the outer portion of the ring. The inner portion is formed early in the season, when growth is comparatively rapid, it is known as early wood or spring wood. The outer portion is the late wood or summer wood, being produced in summer. In some species such as white pines different parts of the ring are not distinct and the wood is very uniform in texture. In contrast, in hard pines the late wood is very dense and darker colored than the lighter colored early wood. The properties of wood depend on the particular tree from which it has been obtained. Similarly, the density of wood depends upon its source.

7.1.1 Advantages and disadvantages of wood Wood has been an integral part of human civilization ever since the evolution of man, and it has a great bearing on the socioeconomic life of human populations across the world. Over the ages, large-scale multiple applications of wood have been so immense that this has led to overexploitation of the material, causing depletion of forests and posing a threat to the very existence of the ecological system. Some of the advantages of wood include: G

G

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Wood is a natural composite. It is an anisotropic, hard, and tough solid material that possesses high load-bearing capacity. It is noncorrosive and biodegradable. Even the smallest part of wood has significant use. It has multiple applications in almost every field of engineering, from household items, kitchenware, furniture, doors, windows, frames, ridges, bridges, structural, and building material to its use as reinforcement in composites.

Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00007-4 © 2021 Elsevier Ltd. All rights reserved.

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A major disadvantage of this material is its porous nature. Wood absorbs moisture if it is not used with proper care and preservatives. It has poor resistance to abrasion and delamination, and it is also vulnerable to heat, light, and exposure to ultraviolet rays.

7.1.2 Chemical composition of wood Wood consists primarily of cellulose, hemicellulose, and lignin. Softwoods and hardwoods are slightly different in chemical composition and react differently with certain chemicals. Softwood consists of about 43% cellulose, 28% hemicellulose, and 29% lignin. Hardwood consists of about 43% cellulose, 35% hemicellulose, and 22% lignin. A typical wood fiber wall contains four main layers as shown in Fig. 7.1, the primary wall and three layers of secondary wall: the outer (S1), middle (S2), and inner (S3) layers. The thickness of the layers, other than the S2 layer, remains relatively constant from one fiber to another. Therefore the difference between a thin-walled spring wood and a thick-walled summer wood fiber is due to the difference in the thickness of the S2 layer. Fibers in wood are bound together by lignin. When the wood is chemically pulped, the middle lamella is softened and the lignin is removed in order to separate the fibers. The fibrils in the primary wall are irregularly oriented.

7.1.3 Physical structure of wood Wood is commonly classified as either softwood or hardwood. The wood from conifers (e.g., pine) is called softwood, and the wood from trees such as oak is called

Figure 7.1 Model representing unit cell structure of wood [1].

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hardwood. Hardwoods are not necessarily hard and softwoods are not necessarily soft. In coniferous or softwood species the wood cells are mostly of one kind, tracheids, and as a result the material is much more uniform in structure than that of most hardwoods. There are no vessels or pores in softwood. In contrast, the hardwoods are filled with vessels. When the larger vessels or pores are localized in the part of the growth ring formed in spring, thus forming a region of more or less open and porous tissue, then it is known as a ring-porous species (e.g., chestnut, mulberry, and oak). The rest of the ring, produced in summer, is made up of smaller vessels and a much greater proportion of wood fibers which give strength and toughness to wood. In diffuse-porous woods the pores are scattered throughout the growth ring instead of being collected in a band or row (e.g., basswood, birch, buckeye, maple, poplar, and willow). Water in wood is stored in the cell walls, in the protoplasm of the cells, and as free water in the cell cavities and spaces. When wood is dried it loses the water stored in the protoplasm, cell cavities, and spaces but still retains from 8% to 16% of that in the cell walls. Even oven-dried wood retains a small percentage of moisture [1]. The water content makes the wood softer and more pliable.

7.2

Wood plastic composites

Wood flour (WF)/fiber-reinforced polymer composites are commonly known as wood plastic composite (WPC). It is composed of wood fibers recovered from sawdust (and other cellulose-based fiber fillers such as peanut hulls, bamboo, straw, digestate, etc.) and waste plastics including high density polyethylene (HDPE) and polyvinyl chloride (PVC). The powder is mixed and then extruded to the desired shape. Additives such as colorants, coupling agents, stabilizers, blowing agents, reinforcing agents, foaming agents, and lubricants are used to tailor the end-product for a specific application. A major advantage of WPC over wood is the ability of the material to be molded to any intricate shape. WPCs are environmentally friendly and require less maintenance than other alternatives such as wood that has been treated with preservatives. The chemical preservatives with which wood is impregnated (of which chromated copper arsenate is currently the most widely used due to its excellent fungicidal and insecticidal properties) give rise to serious concerns due to their adverse effect on the environment. WPCs have excellent resistance to cracking and splitting, and they behave like wood although they are not as rigid. The material is also sensitive to staining from a variety of agents due to its porous nature. Similar to WPCs, so-called engineered wood composed of WF/fibers is used for veneers, such as plywood. These have various advantages that include: G

G

They can control deforestation because, instead of cutting down trees, wood fibers and WF obtained from wood wastes can be used to develop WPCs having strength and other properties equivalent to wood. They are manmade and can be designed to meet application-specific performance requirements.

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G

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Large panels of engineered wood may be constructed from small trees. Small pieces of wood and wood that has defects can be used in many engineered wood products, especially particle- and fiber-based boards. Treated engineered wood products are often stronger and less prone to humidity-induced buckling than equivalent solid woods.

However engineered wood products, made by using wood fiber and a binder, are not completely biodegradable due to the use of nonbiodegradable binders such as urea-formaldehyde (UF), phenol-formaldehyde (PF), melamine-formaldehyde (MF), and methylene diphenyl diisocyanate (MDI) or ethyl carbanate (urethane) resins.

7.2.1 Wood flour polyethylene composites As mentioned in the preceding section, the use of WF as fillers for plastics is growing rapidly [2,3]. As per one estimate, North America is a well-established consumer of WPCs, especially for decking/flooring applications. Polyethylene (PE) accounted for 70% of WPCs used in North America; PVC accounted for 18% and polypropylene for 11%. During 2002, the North American plastics industry consumed about 2.5 million tons of fillers of all types. Of this total, about 180,000 tons was accounted for by cellulosics and “natural fibers.” The same report claimed that every 1% conversion of calcium carbonate, talc, or glass to wood accounted for 50 million pounds of filler or 100 million pounds of compounded plastic product. Trex Co (of Winchester, VA) dominates the US market for WPC planking, with a 70% market share. It claimed that WPCs accounted for about 6% of the US planking market. The upsurge in growth is mainly due to the replacement of treated lumber products. WPC planking typically consists of 47% polymer, 47% wood fiber, and 6% additives. The breakdown of this additive market is estimated as: 35% colorants (both organic and inorganic); 30% lubricants; 20% heat and light stabilizers; 10% coupling agents; and 5% others (including rheology control agents and softeners) [3]. Chemical modifications of the polymeric matrix are required to improve the interfacial adhesion between wood fiber/flour and polymer as discussed in the following sections.

7.2.1.1 Effect of compatibilizing agents The interface compatibility between lignocellulosic fillers and thermoplastic polymers such as linear low density polyethylene (LLDPE), HDPE, etc., in WPCs is improved by the incorporation of compatibilizing agents such as maleated polypropylene (MAPP) and maleated polyethylene (MAPE) in polyethylene (LDPE and HDPE) composites. The tensile strengths of the composites made of lignocellulosic filler polyethylene decrease with increasing filler loading, due to the poor interfacial bonding and the presence of agglomerate fillers. The weak bonding obstructs the stress propagation and causes the tensile strength to decrease as the filler loading increases [4]. However, when a compatibilizing agent (MAPP and MAPE) is used, tensile strength of the composite is improved (Fig. 7.2A and B) due to the better compatibility and wetting of the matrix polymer. Compared with MAPE,

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MAPP is less effective with the PE matrix because of the incompatibility between the PP backbone of MAPP and the PE matrix polymer in the composites [5]. For instance, the impact strengths of the MAPP-incorporated composites decrease whereas those of the MAPE-incorporated composites are almost the same or slightly increased (Fig. 7.3). Ionomers have also been used as compatibilizing agents for polyethylenes. Ionomers based on copolymers of acids and olefin monomer units are amphiphilic and can be tailored to be compatible with both the matrix and the wood, hence acting as coupling agents [6]. WPCs using blends of HDPE and

Figure 7.2 (A) Effect of fiber content on the tensile strength of wood flour (WF) filled LDPE composites [5]; (B) effect of compatibilizing agents on the tensile strength of WFfilled LDPE composites containing 30 wt.% of WF [5].

Figure 7.3 Impact strength of WF LDPE composites with and without compatibilizing agents [5].

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poly(ethylene-co-methacrylic acid) ionomers as matrices containing 4% of the sodium ionomers deform and break in a manner similar to the straight blend of HDPE and maple wood. With the increase in ionomer content, more compression-related failure is observed. Ionomers improve modulus of elasticity (MOE) at low ionomer contents (Fig. 7.4A). Similarly, modulus of rupture (MOR) of composites depends on the moduli of the ionomers. The zinc composites exhibit higher MOR values than the sodium composites (Fig. 7.4B). Apart from this, the strain capacities of the ionomers also affect the impact properties (Fig. 7.5). The notched izod strength of the composites with sodium ionomers attains higher values than the zinc ionomers, especially at the high ionomer contents. Because of the sufficient interfacial bonding and tough nature of the ionomers, a higher MOR is achieved with the more rigid matrix.

Figure 7.4 The effect of ionomer content on (A) MOE and (B) MOR of the wood plastic composites [6].

Figure 7.5 Effect of ionomer content on impact strength of wood plastics composites [6].

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7.2.1.2 Effect of cross-linking Cross-linked polyethylene (XLPE) possesses better physical properties than general PE. Cross-linking with water is achieved by grafting PE with silane followed by hydrolysis to Si OH groups and subsequent condensation to form Si O Si bonds. This process proceeds through free radical initiators and subsequently condenses through water, leading to the formation of cross-linking. Since wood composite is usually used outdoors and under environmental ageing conditions, hence the sunlight and moisture would further promote the water cross-linking reaction of wood fiber LLDPE composite. Fourier transform infrared (FTIR) spectra for 30 wt.% wood fiber-reinforced composite for various water cross-linking times show changes in IR absorption peak intensities due to actual changes in the chemical composition of composite (Fig. 7.6) [7]. Composites treated with water crosslinking reaction exhibit better tensile strength than untreated composites (after 4 h water cross-linking treatment, the tensile strength of 30 wt.% wood composite increases from 14.7 to 27.5 MPa, which corresponds to an 87% increase). The increase in tensile strength is due to the cross-linking network formation between the fibers and the polymer matrix, that is LLDPE polymer chains. However elongation of composite reduces, due to the cross-linking network (chemical bonding) between the flexible LLDPE matrix and the stiff wood fiber or between the LLDPE intermolecular chains. Longer water cross-linking time significantly improves the flexural strength (from 11.2 to 26.6 MPa) and flexural modulus (from 213.8 to 614.6 MPa). However, longer water cross-linking time shows no effect on the impact strength of wood composites. Cross-linking reaction limits the shear deformation behavior of the polymer matrix and slightly decreases the impact strength of WF-reinforced LLDPE composites. However, cross-linking reaction also strengthens the interface between WF and LLDPE matrix; this helps the impact energy transfer from matrix to reinforcement, leading to an increase in the impact strength.

Figure 7.6 FTIR spectra of 30 wt.% wood fiber-reinforced LLDPE composite for various water cross-linking durations [7].

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Besides cross-linking with water, several techniques have been developed to obtain cross-linked PE, such as peroxide cross-linking, irradiation, and silane-crosslinking. However, both peroxide and irradiation cross-linking techniques are costly and pose a risk of precuring. Moreover, there exists a thickness limitation in radiation- cross-linking [8]. In contrast, the silane cross-linking technique does not suffer from high investment cost, and the ethylene vinyl silane copolymer can be processed and shaped using conventional thermoplastic processing equipment and subsequently cross-linked after the processing steps. In the case of silane-grafted HDPE, wood could be incorporated in the cross-linked network since the silanol groups react with hydroxyl groups in wood as well as with other hydroxyl groups grafted on the PE backbone [9]. It has been shown that the creep response during short-term loading decreases upon silane cross-linking of the composites. Moreover, the toughness of the silane cross-linked composites remains much higher than that of the noncross-linked ones. The flexural strength of neat HDPE also remains higher than the XLPE samples.

7.2.1.3 Effect of weathering Surface oxidation occurs due to exposure for both the neat HDPE and the WF HDPE composites. The surface of the WF HDPE composites oxidizes to a greater extent than that of the neat HDPE [10]. The addition of WF to the HDPE matrix results in more weather-related damage. Neat HDPE may undergo crosslinking in the initial stages of accelerated weathering. However, WF reduces the ability of HDPE to cross-link, resulting in HDPE chain scission that dominates the initial weathering stage. The absorbed moisture influences the mechanical properties of WF HDPE composites. The absorption of water in WPCs is strongly influenced by the fiber weight fraction, the type of matrix, the level of interfacial bonding, the overall temperature, and the presence of a compatibilizer [11]. Fig. 7.7 shows the variation in water uptake. With prolonged exposure, water penetrates

Figure 7.7 Water absorption curves for the WF-filled HDPE composites [11].

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deeper into the material, reaching particles embedded some distance from the surface. The flexural strength and modulus of the WPCs with absorbed moisture (Figs. 7.8 and 7.9) fall as the moisture content increases. The reduction in flexural strength and modulus is due to plastification of the matrix associated with disruption and fracture of the Van der Waals forces between the chains of the HDPE. Similarly, the disruption of the highly ordered hydrogen bonds in the structure of the wood weakens the resistance of the wood to applied stress, resulting in a loss of strength and stiffness within the reinforcement.

Figure 7.8 Influence of water absorption on the flexural modulus of wood flour-filled HDPE composites [11].

Figure 7.9 Influence of water absorption on the flexural strength of wood flour-filled HDPE composites [11].

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7.2.2 Wood flour polypropylene composites Wood modifies the crystalline structure of polypropylene (PP) and induces transcrystallization [12]. The orientation, orientation distribution, and possible aggregation of wood particles all add to the structural variety of WF PP composites. Wide angle X-ray diffraction shows that the WF is characteristic for the cellulose I form and does not change during compounding. PP block copolymer crystallizes in the α-form. Compounding does not change the crystal modification of PP, and the addition of functionalized polymers does not affect the crystalline morphology. Cellulose nucleates PP and induces transcrystallinity [12]. Nucleation decreases spherulite size and increases crystallization temperature, leading to higher crystallinity and thicker lamellae [13]. Fig. 7.10 shows the stress strain curves of composites containing 50 wt.% WF at 0.1 MAPP/wood ratio [14]. The composite containing the high molecular weight coupling agent deforms more than twice as much as the other two composites. The low strength of the neat composite is due to the easy debonding of the fibers. This leads to the free deformation of the matrix until the voids merge to form catastrophic cracks. The small molecular weight MAPP creates the same interfacial adhesion as the other compound, but it results in the lowest deformability. At high MAPP content, the smaller number of entanglements per molecule decreases the deformability of the matrix and the interphase, which leads to premature failure and smaller strength.

Figure 7.10 Stress strain curves of wood flour-filled PP composites containing 50 wt.% wood flour at 0.10 MAPP/wood ratio (A) without MAPP; (B) high MAH content (3.5 wt.%); and (C) low MAH content (1 wt.%) [14].

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7.2.3 Mechanical properties of wood flour polypropylene composites 7.2.3.1 Effect of compatibilizing agents The mechanical performance of WF PP composites is influenced by the interface modifications [15]. Esterification of WF with maleic anhydride (MAH) or with the addition of a compatibilizing agent [maleic anhydride polypropylene copolymer (PP MAH)] modifies the polymer filler interaction. Mechanical properties (except Young’s modulus) are not improved either by the WF chemical modification or by the use of PP MAH. However, compatibilization methods improve the dispersion of the WF in the PP matrix. Although creep behavior of composite samples is improved by the addition of PP MAH, the composites prepared from MAH-treated WF exhibit larger deformations. In contrast to interface modification, bulk treatment such as low dose gamma irradiation improves the mechanical properties of polypropylene filled with WF [16].

7.2.3.2 Effects of mercerization and ageing Mercerization with 10% NaOH and functionalization with a copolymer affect the mechanical properties of WF PP [17]. When MAPP is added, it slightly enhances both properties for all fiber contents. In contrast, mercerization weakens fibers and degrades mechanical properties. It produces irregularities in the wood fiber which increase the water uptake and consequently degrade the composite.

7.2.4 Other polymers Other than PE and PVC, high temperature and biodegradable polymers that are used in WPCs are poly(phenylene ether) (PPE), poly(vinyl alcohol) (PVA) modified by phthalic anhydride (PA), polycaprolactone (PCL) blended with starch, etc. However, they are less popular.

7.3

Tribological behavior of wood flour polymer composites

7.3.1 Abrasive wear behavior The heterogeneous tissue morphology due to the differences in the shape and growth of the cells in wood leads to a heterogeneous structure that affects the physical, mechanical, and machining properties of wood. The heterogeneity also produces irregular high stress abrasion properties [18]. In three-body or low stress abrasion, the microscopic tissue of wood is affected because each loose abrasive grain works independently during abrasion [19]. The array direction of the annual ring and fiber affects the abrasion in directions both parallel and perpendicular to

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the friction surface. Abrasion done on the axial (A), radial (R), and tangential (T) sections produces different results as discussed below.

7.3.1.1 Effect of abrasive grain size A size effect [20] was observed in the case of softwood as well as hardwood. In the case of hardwood, the authors observed that the wear rate is constant when grain size is approximately .100 μm, and the critical grain size effect is also observed, as in the softwood. However, wear rate in the case of other hardwoods (sawagurumi and kiri) increases significantly at a specific abrasive grain size, but an irregular dependence also appears with the change of the abrasive grain size. This was attributed to the size of the vessel tissue. The irregular dependence on the abrasive grain size in hardwood may be peculiar to the surface tissue rubbed in the axial section. It was also observed that for the softwood, the critical grain size effect appears in the radial (R) and tangential (T) sections in spite of the different tissue on each friction surface. Similarly, in the case of hardwood, the same critical grain size effect in the R and T sections as in the axial section was observed. However, wear rate in the rubbed sawagurumi and kiri specimens increased noticeably at a particular abrasive grain size, and the irregular dependence appears with the change in the grain size. The abrasive grain size at which the wear rate increases irregularly corresponds to the mean vessel size. The authors proposed a wear mechanism according to which, when the abrasive grain of a size smaller than that of the vessel pore is pushed into the vessel tissue, the cutting depth of the abrasive grains becomes small due to a number of abrasive grains in contact. However, the load applied to one abrasive grain becomes greater when the specimen is in contact with the large abrasive grain size because there are few abrasive grains in contact. At this time, abrasive grains of equivalent size, which have been in contact with the vessel tissue, are pushed into the vessel pore, and the cutting depth of the other abrasive grains increases due to the decreasing numbers abrasive grains in contact with the material surface. In contrast, the load applied to one abrasive grain increases more when in contact with the larger abrasive grain size because there are fewer abrasive grains in contact. However, the abrasive grain in contact with the vessel tissue is not pushed into the vessel pore because the size of the abrasive grain is much bigger than that of the vessel pore. Therefore, the cutting depth of the abrasive grain may not become so much larger than that of the equivalent vessel size. In terms of the cutting depth of the abrasive grain, the abrasive wear process for the hardwood seems to generate significant abrasions on the equivalent vessel size.

7.3.1.2 Effect of counterface material It has been observed that the wear volume, W, on the axial section of katsura wood depends on the counterface materials which are made of PE, polyether ether ketone (PEEK), and polyamidimide (PAI) [21]. In the case of the PE counterface material, W increases linearly with increase in sliding distance L in the tangential and radial

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directions. Moreover, in the case of the PEEK counterface material, W also increases linearly with increases in L, but W is larger than that of the PE counterface material. However, W for the PAI counterface material, the material with the highest yield stress, is smaller than that of the PEEK counterface material. These results indicate that the katsura wood wear volume initially increases with the yield stress of counterface material, but then starts to decrease. Specific wear rate Ko in the tangential and radial sections is larger than that in the axial section. The abrasion results for the tangential section show that Ko is greatest at a yield stress of approximately 20 MPa and tends to decrease gradually with a larger yield stress. This tendency is also apparent in the radial section. The above results reveal that a peak Ko value exists for all abrasion directions. Consequently, the authors proposed a mechanism of three-body abrasion between the specimen and various counterface materials. The peaks in three-body abrasion for both the katsura wood specimen and the plastic specimens are believed to be closely related to the yield stress of the material. For wood specimens, the peak occurred when the yield stress of the counterface material was approximately twice as large as that of the katsura wood, and the peak on the plastic specimen occurred when the yield stress of the counterface material was approximately the same as that of the specimen. The difference in the results between the katsura wood and the plastic material was attributed to the change in embedding balance of the loose abrasive grains, which was assumed to be affected by the porous wood structure.

7.3.1.3 Effect of anisotropy structure of wood In two-body abrasion, the wood becomes more difficult to abrade as the yield stress and density of the wood increase. It has been reported that the two-body abrasion wear rate of wood increases with grain size as the pressure gets higher, and that the critical grain size effect is not observed during the abrasive wear of various woods. In the case of three-body abrasive wear, properties of various wood samples rubbed with loose abrasive grains were studied using katsura wood (density 5 0.47 g/cm3). This specimen of wood was used to investigate the influence of the anisotropy structure of wood on its abrasive wear behavior. Additional wood samples were used to investigate the influence of various wood and counterface material combinations. Table 7.1 shows the properties of these additional specimens. The hardwood specimens were kiri, sawagurumi, and katsura and the softwood specimens were sawara, yezomatsu, momi, and kuromatsu. The various counterface materials used were coniferous woods (sawara and kuromatsu), PP, PE, acrylic resin (PMMA), aluminum (Al), iron (Fe), cast iron (Fe-C), and copper (Cu). It was observed that W in the radial and tangential sections increases linearly, and W in the radial section is higher than that in the tangential section. The value of W along the longitudinal direction on the tangential and radial sections tends to increase with pressure. However, the value of W in the tangential and radial sections decreases with increasing pressure, and W at higher applied surface pressures becomes smaller than that in the longitudinal direction. These results indicate that the three-body abrasive wear rate on the axial section increases with increasing

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Table 7.1 Properties of different types of wood [21]. Specimen

C or H

Density, ρ (g/cm3)

Yield stress, σy (MPa)

Kiri (Paulownia tomentosa Steud.) Sawara (Chamaecyparis pisifera Endl.) Yezomatsu (Picea jezoensis Carr.) Sawagurumi (Pterocarya rhoifolia Sieb. and Zucc.) Momi (Abies firma Sieb. and Zucc.) Katsura (Cercidiphyllum japonicum Sieb. and Zucc.) Kuromatsu (Pinus thunber gii Parl.)

H C C H

0.29 0.34 0.36 0.39

21 29 37 42

C H

0.45 0.45

47 50

C

0.55

55

C, Softwood; H, hardwood.

applied surface pressure, but that the wear rate on the tangential and radial sections does not always depend on the applied surface pressure in the same way. The authors suggested that the three-body abrasion of katsura wood is affected by the cutting action of loose abrasive grains in addition to the yield stress of the material. They also suggested that the specimens with a higher yield stress show little abrasion with all wood, plastic, and metal counterface materials.

7.3.2 Wear behavior of wood flour epoxy composites WF-filled epoxy composites exhibit specific wear rate (Ko) of the order of 10 10 m3/Nm in abrasive wear mode and B10 14 m3/Nm in sliding wear mode [22]. Composites containing 40 wt.% WF exhibit the lowest specific wear rate in abrasive wear mode and those containing 20 wt.% WF exhibit the lowest specific wear rate in sliding wear mode. This was attributed to the fact that in abrasive wear mode, the wear debris consisting of mainly WF particles was maximum for 10 wt. % composite and minimum for 40 wt.% composite. In sliding wear mode, the exposed WF particles caused maximum roughening of the steel counterface in the case of composites containing a higher concentration of WF particles. Hence, they exhibited a higher value of specific wear rate.

References [1] K.U. Leistikow, T. Koln (Eds.), The Wood Book, Taschen America, 2002; NIIR Board of Consultants & Engineers (Eds.), The Complete Technology Book on Wood and its Derivatives, National Institute of Industrial Research, New Delhi, 2005. [2] D.P. Kamdem, H. Jiang, W. Cui, J. Freed, L.M. Matuana, Compos. Part A 35 (2004) 347. [3] Modern Plastics International, 33 (1) (2003) 30.

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[4] H.-S. Yang, H.-J. Kim, J. Son, H.J. Park, B.J. Lee, T.S. Hwang, Compos. Struct. 63 (3 4) (2004) 305. [5] H.-S. Yang, M.P. Wolcott, H.-S. Kim, S. Kim, H.-J. Kim, Compos. Struct. 79 (2007) 369. [6] T. Li, N. Yan, Compos. Part A 38 (2007) 1. [7] C.-F. Kuan, H.-C. Kuan, C.-C.M. Ma, C.-M. Huang, Compos. Part A 37 (2006) 1696. [8] N.C. Liu, G.P. Yao, H. Huang, Polymer 41 (12) (2000) 4537. [9] M. Bengtsson, P. Gatenholm, K. Oksman, Compos. Sci. Technol. 65 (10) (2005) 1468; M. Bengtsson, K. Oksman, Compos.: Part A 37 (2006) 752. [10] N.M. Starka, L.M. Matuana, Polym. Degrad. Stab. 86 (2004) 1. [11] S.-H. Huang, P. Cortes, W.J. Cantwell, J. Mater. Sci. 41 (2006) 5386. [12] J. Karger-Kocsis, J. Varga, in: J. Karger-Kocsis (Ed.), Polypropylene: An A Z Reference, Kluwer Academic Publishers, Dordrecht/Boston/London, 1999, p. 348. [13] J. Varga, J. Mater. Sci. 27 (10) (1992) 2557. [14] L. Danyadi, T. Janecska, Z. Szabo´, G. Nagy, J. Mo´czo´, B. Puka´nszky, Compos. Sci. Technol. 67 (13) (2007) 2838. [15] A.J. Nun˜ez, P.C. Sturm, J.M. Kenny, M.I. Aranguren, N.E. Marcovich, M.M. Reboredo, J. Appl. Polym. Sci. 88 (2003) 1420. [16] C. Albano, J. Reyes, M. Ichazo, J. Gonza´lez, M. Brito, D. Moronta, Polym. Degrad. Stab. 76 (2002) 191. [17] G. Cantero, A. Arbelaiz, F. Mugika, A. Valea, I. Mondragon, J. Reinf. Plast. Compos. 22 (1) (2003) 37. [18] T. Ohtani, T. Yakou, S. Kitayama, J. Wood Sci. 48 (2002) 264. [19] T. Ohtani, K. Kamasaki, C. Tanaka, Wear 255 (2003) 60. [20] H. Sin, N. Saka, N.P. Suh, Wear 55 (1979) 163. [21] T. Ohtani, K. Kamasaki, C. Tanaka, Precis. Eng. 28 (2004) 73. [22] U.K. Dwivedi, N. Chand, Polym. Compos. (2008). Available from: https://doi.org/ 10.1002/pc.20548.

Sources of further information and advice A.J. Stamm, Wood and Cellulose Science, Roland, New York (1964). Wood Handbook, Forest Products Laboratory, USDA Forest Service, Madison, WI (1999).

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8

Hemp, kenaf, flax, oil palm, date palm, and rice husk-based composites

8.1

Introduction

Keeping in mind the environmental conservation as a top priority, scientists across the world are attempting to find industrial applications for plant fibers and their residues [1]. Tribology is a cross-disciplinary area which involves inputs from various disciplines and heavily relies on materials and composites which can effectively perform in tribology-related situations. An elaborate review on the state-of-the-art on the tribological behavior of natural fiber-reinforced composites has been recently published by Omrani et al. [2]. The hydrophilic nature of fibers used as reinforcements in plastics is a major concern. The moisture content can affect the mechanical properties and interface bonding. Therefore, in order to achieve good mechanical and tribological properties, the moisture absorption capability of natural fibers needs to be drastically reduced. In an overview on tribology of natural fiberreinforced polymer composites, it has been suggested that treatment of fibers influence both mechanical as well as tribological behaviors of the composites [3]. Treating the natural fibers stabilizes the fiber matrix interfacial bonding which enhance the load carrying ability of the fiber under mechanical and tribological loadings. Natural fiber polymeric composites exhibit high friction coefficient. Graphite as solid lubricant for such composites may reduce the friction coefficient of the composite and maintains high wear characteristics. On the other hand, using water as lubricant for natural fiber/polymer composite may deteriorate the composite strength. It has been reported that a reduction of friction coefficient can also be achieved when polyester was reinforced with coir, betel nut, and oil palm fibers by about 30%, 46%, and 60%, respectively [4 6]. This is attributed to the modification that took place on the counterface wear track and the topographical composite surfaces, that is, the role of this modification in the roughness of film transfer generation on counterface. In other words, the presence of natural fibers on the composite surface smoothens the film transfer on the counterface, which leads to decrease in the interlock between the asperities in contact leading to low friction coefficient. However, the friction coefficient is still high for such materials to be used in tribological applications such as bearing, sliding, and bushes. Moreover, the low friction coefficient may sacrifice the wear performance of the material. The Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00008-6 © 2021 Elsevier Ltd. All rights reserved.

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addition of cotton and sisal fibers to polyester composites significantly increased the friction coefficient by 46% and 50% compared with the neat polyester. This was due to the low sensitivity of the cotton fiber to heat of friction as compared with polyester resin which offered higher resistance to sliding movement. The two possible solutions to overcome the high friction coefficient of the polymeric composites based on natural fibers, is either by introducing solid lubricants to the composites or to operate the composites under wet contact conditions. The subsequent section deals with the tribological properties of natural fiber polymer composites.

8.1.1 Hemp fiber-reinforced polymer composites Tribological properties of hemp fiber-reinforced plant-derived polyamide 1010 biomass composites have been extensively investigated by Nishitani et al. [7]. These authors studied the chemically treated fiber on the tribological properties of biomass composites. Hemp fibers were surface treated by two surface treatment methods: (1) alkali treatment by sodium hydroxide solution and (2) surface treatment by silane coupling agents. Three types of silane coupling agents, namely aminosilane, epoxysilane, and ureidosilane were used. These HF/PA1010 biomass composites were extruded using a twin extruder and were injection molded. The mechanical and tribological properties were evaluated by the ring-on-plate type sliding wear test. It was found that tribological properties of HF/PA1010 biomass composites improved with the surface treatment by the silane coupling agent. This was attributed to the change in the mode of friction and wear mechanism by the interfacial adhesion between fiber and matrix polymer according to the type of silane coupling agent used. In particular, the ureidosilane coupling agent showed the best improvement effect for the tribological properties of these biomass composites. The µ L curves of pure plant-derived PA1010 (100%) and various surface-treated HF/ PA1010 biomass composites showed different behaviors according to the type of materials. The µ of pure PA1010 increased to about 0.3 immediately and stabilized up to 200 m, and then µ gradually increased up to about 0.8 from L 5 200 400 m, after which the value became stable after L 5 400 m. This was attributed to the increase in the real contact area between the pure PA1010 sample and counterface of the carbon steel (S45C) ring caused by the softening of the polymeric sample surface, due to frictional heat, that led to full contact between the test specimen and metal counterface. The results also showed an abrupt increase in wear. On the other hand, the µ L curves of various HF/PA1010 biomass composites demonstrated different behaviors. The µ of these biomass composites increased abruptly soon after the initial run, then gradually rose up to about 0.6 at L 5 300 m, after which it stabilized with or without surface treatment by various silane coupling agents. This was probably due to the change in the mode of friction mechanism when filled with HF. The contact areas between the polymer and the metallic counterface reduced by introducing discontinuous phases [8]. However, the µ L curves shifting to the steady-state period was slightly different for different surface treatment. The µ and Vs of pure plant-derived PA1010 (100%) improved when filled with HF and surface treated by the combination of NaOH alkali treatment and various silane coupling

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agents. However, the effect of the filling of HF on the degree of reduction in µ was higher than that of the type of surface treatment by the silane coupling agent. The µ of various surface-treated composites decreased in the following order: PA1010 .. HF-S2 (epoxysilane) . HF-S1 (aminosilane) . HF (untreated) . HF-S3 (ureidosilane). On the other hand, the Vs of the composites showed different behavior from that of µ, decreasing in the following order: PA1010 . HF . HF-S1 . HF-S2 . HF-S3. Unlike µ, the effect of the type of surface treatment by the silane coupling agent was higher than that of the filling of HF. Ureidosilane coupling agent showed the best improvement effect on the tribological properties, both µ and Vs, of HF/ PA1010 biomass composites. These positive results were attributed to the change in the mode of friction and wear mechanism by the type of silane coupling agent. The coupling agent not only led to a better interaction and interphase adhesion between the hemp fiber and polymer, but also helped in homogeneous dispersion of fiber in the composites. Surface treatment by silane coupling agent prevented the debonding, breakage, and detachment of fibers as confirmed by the morphology of worn surface. The limiting pv value improved when filled with hemp fiber, surface treated by silane coupling agent, and increased volume fraction of fiber.

8.1.2 Kenaf fiber-reinforced polymer composites Kenaf fiber as reinforcement can reduce the wear rate of polymer composites. For instance, irrespective of fiber orientation, it has been found that kenaf fibers improved the tribological properties of epoxy. However, the decrease in the wear rate of the epoxy was more when the fiber orientation was normal to the sliding direction [9]. This was attributed to the fact that the ends of the fibers were exposed to the sliding counterface. Therefore pulling out or detachment of fibers from the matrix was almost diffcult leading to a lower wear rate [10]. On the other hand, debonding occurred at a higher applied load (70 N). Although high thermomechanical deformation deteriorated the interface between the fibers and the matrix, there was no major pull-out of fibers. Microcracks generated on the worn surface at very high applied load (100 N) propagated due to the high side force causing increase in the wear rate at higher applied load. Narish et al. [11] investigated the wear and frictional properties of chemically treated kenaf fiber polyurethane composites under dry and wet contact conditions for parallel, antiparallel, and normal fiber orientation. There was significant improvement in wear resistance when neat polyurethane was reinforced with kenaf fibers. However, fiber orientation influenced wear performance and followed the order: antiparallel . normal . parallel. Predominant wear mechanisms for antiparallel were detachment of fibers and plastic deformation; whereas parallel orientation of fibers led to micro- and macrocracking, detachment of fibers, microdelamination, and plastic deformation. Studying the effects of moisture on wear of same composites, it was found that specific wear was lower in dry contact condition, for both parallel-oriented fiber composites and antiparalleloriented fiber composites for loads up to 60 N [12]. Under higher applied loads in

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wet contact, specific wear in antiparallel- and normal-oriented fiber composites was lower. Coefficient of friction under wet sliding showed a reduction of more than 90% for all fiber orientations indicating that the presence of water cooled the interface temperature, reducing friction and wear. Chin and Yousif [9] have found insignificant effect of applied load (30 100 N) and sliding velocity (1.1 3.9 m/s) on the specific wear rates (Ws) of kenaf fiber-reinforced epoxy (KFRE) composite. Ws of KFRE composites exhibited steady state after about 3 km sliding distance. Increase of applied load slightly increased the friction coefficient and the interface temperature. The worn surface revealed that the fiber ends are still well adhered in the matrix and there was no sign of debonding or pullout at lower applied load (50 N). At higher applied load (70 N), the worn surface showed debonding of fibers and this was due to the high thermomechanical loading which rendered the material removal from the resinous regions and weakened the interfacial area. However, there was still no sign of pull-out of fibers. At higher applied load of 100 N, microcracks became clear on the surface due to the high side force indicating the high wear resistance in the rubbing zone. Chin and Yousif [10] have found that the presence of kenaf fibers in the composite reduced the frictional coefficient of epoxy from 0.75 to 0.56. Nirmal et al. [13] investigated the effects of using kenaf fibers as particle fillers in epoxy composites under dry contact test on a pin-on-disk machine. The composite was subjected to 5 30 N applied loads, at a velocity of 2.83 m/s. Various weight percentages of kenaf particles (5% 20%) composites were used alongside neat epoxy for comparison. It was found that all kenaf fiber-reinforced epoxy composites showed lower wear rate and friction as compared with neat epoxy. Also, 15 wt.% kenaf fiber-reinforced epoxy composites showed the best performance, whereby wear rate and friction improved by 67% and 56%, respectively. The superior performance of 15 wt.% kenaf fiber-reinforced epoxy composites was attributed to large amounts of back transfer film on the composite surface, especially at higher loads. This transfer film protected the rubbing surface from further wear. Roughness measurements on the test samples of 15 wt.% kenaf fiberreinforced epoxy composites showed highest surface roughness, increment of 48.2%, as compared with the virgin composite before testing. Investigations have also been done to explore the role of kenaf fibers as a fibrous reinforcement in developing a hybrid composite [14,15]. Natural fiber is good in ductility and bending moment whereas synthetic fiber is good in flexural strength, means it is good in transverse load capability. Combining these materials to form hybrid polymer composites can provide more advantages. However, developing such hybrid composites containing natural fiber face some difficulties like delamination of fabric layer due to over curing and polymer chain scissions. Hence, placement of natural fabrics and synthetic fabrics in the hybrid composite is critical to the end performance. The stacking sequence controls the overall property. For instance, flexural strength of flax and kenaf is very low, which implies that it is not suitable for any transverse load application. A hybrid of glass and any natural fiber (flax or kenaf) may increase its transverse as well as longitudinal strength. But, flexural extension of natural fiber is more compared with synthetic fibers, which implies that it has more ability to absorb sudden transverse load than synthetic

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fiber. Hardness of synthetic fiber is more in comparison to natural fiber (flax and kenaf). The carbon fiber has 76.6% more hardness than natural fiber, and this may be due to the less binding capacity between fiber and epoxy. Low impact strength of natural fiber reduces its application in any high and frequent impact zone in automotive or nautical industries. The natural fiber has low natural frequency. For carbon fiber, natural frequency is 28.9% more than kenaf fiber. Thus synthetic fibers have more stiffness than natural fibers. A comparison of low stress abrasive behavior of flax/kenaf/glass/carbon fiber hybrid polymer composite consisting of bidirectional kenaf and flax fabric (379.84 and 268.07 gsm, respectively) and woven roving glass and carbon fiber of 300 gsm reinforced in epoxy is given in Table 8.1 [14]. The specific wear rate of long kenaf fiber polyester composite (KPEC) and kenaf epoxy composite (KEC) obtained in dry sliding conditions at a constant velocity of 1.4 m/s showed same wear characteristics [16]. A comparison of the tribological characteristics of oil palm fiber/epoxy (OPF/E) and kenaf fiber/epoxy (KF/E) composites in terms of their fiber composition and temperature has yielded some important observations [17]. For example, the COF decreased with higher temperatures for both composites due to the existence of a thin lubricating layer of molten resin and low shear strength at a higher temperature. The results also showed that OPF/E with 70 wt.% fiber had a higher COF. However, interestingly, this was contrary to the KF/E composite where the highest COF was for 30 wt.% fiber. The reason for this phenomenon could be explained based on the hardness of the composites related to fiber composition. Lower hardness increased the contact area, resulting in a higher COF. With increase in interfacial surface temperature, the composite lost its hardness property. The wear rate of the OPF/E and KF/E composites tested at different temperatures for each fiber composition increased as temperatures increased for both composites at all compositions. However, the wear rate of the OPF/E composite was shown to be lower than the KF/E composite at high temperatures. This is consistent with the increased softening of materials at a high temperature. Moreover, the wear rate significantly increased at a certain temperature as the transition from mild to severe wear was realized. In the mild regime, volume loss due to wear increased linearly with the temperature indicating that wear progresses under steady-state conditions. At higher temperatures, wear rates were no longer a linear function and increased abruptly for all fiber compositions. Within a certain Table 8.1 Specific wear rate of different composites [14]. Composite type

Load (N)

Sliding distance (m)

RPM (rev/ min)

Specific wear rate 3 1023 mm3/ Nm

Hardness (BHN)

Glass Carbon Kenaf Flax

25.5 25.5 25.5 25.5

1260 1260 1000 1000

125 125 125 125

22.28 18.76 16.83 20.81

53 62 23 14

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temperature range, wear was initially mild but became severe when the tests were run with increasing temperatures. For the OPF/E composite, it was found that increasing fiber composition and temperature led to severe wear. For the KF/E composite, severe wear occurred earlier with the low fiber composition which meant that the high fiber composition could result in better wear performance. This mild severe wear transition has been further confirmed by worn surface morphology which showed microcrack, fine grooves, and debonding mechanisms. At high temperatures, predominant wear mechanisms were delamination, broken fibers, torn fibers, fractured fibers, or resinous regions. However, there are differences in terms of fiber composition where the high fiber content indicates severe wear mechanism for the OPF/E composite while the KF/E composite showed that higher fiber content could withstand severe wear mechanism.

8.1.3 Rice husk filled polymer composites Various studies have been done on exploring a variety of applications for rice husk as a filler material [18,19]. In one such study it has been found that tribological properties of rice husk-reinforced epoxy composites are better than neat polymer. Results revealed that embedding rice husk can improve the tribological properties by reducing the wear rate for all weight percentage of rice husks. In addition, there is an optimum weight percentage for rice husk content where the wear rate is minimum. The optimum point is 10%. Consequently, increasing the amount of fibers more than 10% has a reverse effect on wear rate and tends to increase in wear rate [20]. The tribological properties of brake pads by using rice straw (RS) and rise husk dust have been investigated. The tribological properties were significantly improved by the addition of RS and rice husk dust in the composites and it was concluded that these composites can be effectively used in brake pad formulations [21]. The effects of different fiber weights and surface treatment on wear properties of rice husk-reinforced PVC (RFRPVC) composite were studied by Kapur et al. [22]. The 10, 20, and 30 wt.% percentage fibers were used and results were compared with neat PVC. Foaming agent was used in the composite to reduce the density. Wear was found to be the highest for PVC sample, whereas lowest for the RFRPVC composite with 30 wt.% percentage-treated fiber. Also, in the case of the RFRPVC composites, higher fiber weight content (up to 30 wt.%) gave better wear resistance. The worn surface study revealed that huge microchips were evident in the PVC sample. The treated rice husk composites showed much less damage. Rice husks can also be carbonized by mixing with phenolic resin in an inert gas environment, forming rice husk ceramics or “RH ceramics.” Friction was higher in wet contact condition compared with dry conditions. A similar observation was seen for the wear rate as well. Photomicrographs showed much thicker transfer and silicarich transfer film was developed during dry sliding. This layer is hydrophilic and induces low friction between mating surfaces. On the other hand, for wet sliding, the silica wear debris generated is washed away, forming a much thinner transfer layer consisting mostly of carbon, which induces more friction. Under both conditions, the friction and wear values were considered low for composites, overall less

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than 0.2 and 1.0 3 1028 mm2/N, respectively [23]. In another study friction and wear properties of polyamide (PA66) filled with rice bran ceramics (PA66/RBC) and glass beads (PA66/GBs) were investigated [24]. It was reported that pure PA66 showed higher friction and wear compared with PA66/GB and PA66/RBC, particularly at low sliding speeds. At higher sliding speeds, there was not much difference in friction between the three tested materials. Wear was also significantly higher for pure PA66, as high as 1 3 1027 mm2/N. For both PA66/RBC and PA66/GB, the Ws were less than 1 3 1028 mm2/N. SEM observations showed that for pure PA66, large wear particles were observed at low sliding speeds, caused by friction-induced surface breakage of the material. At higher sliding speeds, plastic flow is more evident. For the PA66/RBC and PA66/GB on the other hand, observations were similar, at low speeds, and smaller wear particles than pure PA66 was seen. At higher sliding speeds, voids can be seen due to RBC or GB removal at the rubbing interface. This confirms that reinforcing PA66 with RBC or GB is effective, particularly at low sliding speed.

8.1.4 Date- and oil palm-reinforced polymer composite The palm tree stem is covered with a mesh made of single fiber. These fibers create a natural woven mat of crossed fibers of different diameters. Several studies have been done on using different parts of date palm for composite materials [25 36]. However, very less has been reported on the tribological characterization of date palm fiber-reinforced polymer composites. The effects of date palm leaf as reinforcement on polyvinylpyrrolidone polymer matrix composite have been investigated. It was observed that the wear rate decreases by increasing the weight percentage of date palm leaves up to 26 wt.% of date palm leaves fiber. The minimum value of wear rate was observed when 26 wt.% of date palm leaves fiber was embedded into the matrix and thereafter, the wear rate increases with increasing the date palm leaves’ content. In addition, the optimum value of weight percentage of date palm leaves to have a minimum value of COF was found to be 26 wt.%.This can be attributed to the proper combination of various properties including tensile properties, hardness, toughness, and above all the fiber matrix adhesion. However, at higher fiber contents, above the 26 wt.%, internal slippage of chain molecules was increased that led to lower wear resistance and consequently, increased weight loss at higher fiber contents. The friction coefficient decreased with increasing applied load and the weight loss of the composites increased as the applied load increased for all sliding speeds for both the neat polyvinylpyrrolidone and its composites. At higher normal load, temperature at the contact surface between composites and counterface is also increased, and it caused micromelting and mechanical deterioration. Thus the friction coefficient decreased. Higher weight loss at higher load is due to the deeper grooving and more material removal from the sample with increasing load [37]. Tribological properties of polyester composite (OPRP) reinforced with oil palm fiber improved by about three to four times compared with neat polyester. In addition, the friction coefficient of polyester composite was less by about 23% than that of the neat polyester [38]. The effect of solid lubricant on

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tribological properties of date palm leaves/epoxy has been investigated. Solid lubricant as well as fibrous reinforcements helped to decrease the friction coefficient of the epoxy. In addition, embedding 3 wt.% graphite as solid lubricant caused reduction in the friction coefficient significantly when compared with date palm leaves/ epoxy composites where the graphite/date palm leaves/epoxy has 10% lower COF than epoxy while date palm leaves/epoxy showed 7% reduction in COF [39]. The tribological properties of polyester composites reinforced by woven glass-reinforced polyester (WGRP) have also been compared with seed oil palm-reinforced polyester (S-OPRP) composites. Glass fiber-reinforced polymer composites have better COF and wear rate than seed OPRP composites. However, the S-OPRP composite reinforced by 35 vol.% of seed oil palm exhibited a promising wear result. Studies on worn surfaces showed that the wear mechanism of S-OPRP composites were microcracks, deformation, and pulled-out of fibers while abrasive wear was dominant mechanism for the WGRP composites [39]. Oil palm biomass fibers-based hybrid biocomposites and those developed using polyester as a base matrix have been explored for their tribological characteristics [38,40]. It has been reported that specific wear rate (WS) was higher for neat polyester compared with OPRP composite at various sliding distances, and at higher applied loads; the specific wear rate was decreased in OPRP by three or four times. As sliding velocity increases, specific wear rate decreases at higher loads for OPRP composite; whereas for neat polyester, as sliding velocity increases, specific wear rate also increases at all applied loads. This was due to the addition of oil palm fibers, and polyester resin composites possess high strengths and also, it was clearly understood that at longer sliding distance and higher applied loads, the oil palm fibers assisted in the protection of exposed rubbing layer of the composite during sliding. There was 5% 23% reduction of friction coefficient which occurred at higher loads in oil palm fiber-reinforced composites. Parting, splitting, and bending of fibers were the commonly noticed phenomenon in the wear mechanism of oil palm fiber-reinforced composites. Yousif and Nirmal [41] studied the tribological performance of palm fiber polymeric composites aged in various solutions. The palm fibers were chemically treated with 6% sodium hydroxide (NaOH) solution for 24 h which could help interfacial bonding strength between the fiber and matrix observed from SEM images, and subsequently the composite samples were immersed in water, salt water, diesel, petrol, and engine oil for a period of 3 years for testing. Treated oil palm fiber-reinforced composite immersed in water showed approximately 18% lower WS, whereas the treated oil palm fiber-reinforced composite submerged in diesel, petrol, and engine oil showed similar trend whereby steady state of WS was seen at 5 km. This could be attributed to the fibers which were immersed in 50% salt water experiencing cell wall densification, making them weak in the resin. A higher specific wear was noticed in the petrol immersed-treated oil palm fiber-reinforced composite compared with those immersed in diesel and engine oil. The absorption rate of petrol was found to be 0.5% higher than that of diesel and engine oils, which absorbed only 0.2%. Furthermore, the presence of engine oil at the interface during sliding would lower the thermomechanical loading for this composite. The lower absorption rate (0.2%) of diesel showed a significant increase in the friction

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coefficient for treated oil palm fiber-reinforced composite. Frictional performance for all the five immersed solutions followed the order of diesel . engine oil . water . petrol . salt water, where diesel showed superior frictional performance. Friction performance improved by approximately 43.2% when immersed in diesel. This finding demonstrates that aged treated oil palm fiber-reinforced composite could be used in applications where ageing of the composite would take place, such as diesel tanks, oil tank containers, nonstructural bearing, and sliding materials subjected to tribology loading conditions. Fazillah et al. [42] compared tribological characteristics of oil palm fiber/epoxy and kenaf fiber/epoxy composites under dry sliding condition for 30, 50, and 70 wt.% fibers content. At high temperature and reduced friction, a thin lubricating layer consisting of molten resin is formed. It was indicated that friction showed a decreasing trend as temperature increased for both types of composite. The temperature range for testing was 23 C, 40 C, 100 C, and 150 C. The hardness of oil palm fiber-reinforced composite and kenaf fiber-reinforced composite plays a vital role in the friction behavior of composites; and the Oil Palm fibre reinforced epoxy (OPRE) showed a significant increase with 70% and that of kenaf fiber-reinforced composite was 30%. The wear rate on the other hand, showed increasing trend as temperatures increases, for both composites. This was due to the reduction in hardness at higher temperatures for both composites. However, at higher fiber content, kenaf fiber-reinforced composite showed better wear resistance than oil palm fiber-reinforced composite. This was due to the transition from mild to severe wear, which occurred at a much lower wt. % of fibers in the case of kenaf fiber-reinforced composite. With regards to the natural fiber/polymer composites, the tribological behavior of untreated oil palm fiberreinforced polyester (U-OPFFP) composites was studied under dry/wet adhesive mode [5]. Under dry conditions, the untreated fiber/polyester exhibited poor wear characteristics leading to debonding the oil palm fibers during the sliding, especially under severe conditions. The high interface temperature during dry adhesive condition led to softening of the polyester region and then pullout of fiber from the bulk to the surface. Presence of the water assisted to absorb the frictional heat resulting in lower material removal and lower friction coefficient compared with the dry contact conditions behavior.

8.2

Some critical aspects of machining of natural fiber polymer composites

Machining of natural fiber-reinforced plastic (NFRP) composites is required to achieve the desired geometrical standards for industrial parts. Since these composites contain natural fibers that are heterogeneous with high transversal flexibility having a cellulosic structure in the form of a cellulose microfibrils along the fibers axis, a lot of machinability issues crop up [43 49]. These composites involve a multiscale heterogeneity from microscopic elementary fiber scale to the overall macroscopic NFRP composite scale. A tribomechanical investigation of the local

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behavior of flax fiber-reinforced polypropylene composites at a geometric contact scale has been reported to understand this multiscale behavior [50]. A triboindenter was used to conduct both nanoindentation and scratch tests on NFRP composites surface by identifying the elementary flax fibers and the polypropylene matrix separately at microscale. The indentation traces obtained on both elementary flax fibers and PP matrix at the same applied load (500 µN) showed different elastic modulus for flax fibers and PP matrix. It was mainly attributed to the heterogeneous cellulosic structure of flax fibers because the nanoindentation response of flax fibers significantly depends on the nanoindentation location inside the fiber cross-section. Moreover, the elastic modulus of flax fibers decreases significantly by increasing the penetration depth mainly due to the major contribution from the amorphous noncellulosic constituents of flax fibers at this contact scale. Increasing the cutting depth during the indentation generates elastic modulus values near to those of hemicellulose and lignin. For PP matrix, no such variation was noticed because of its homogeneity. A drastic increase of the elastic modulus was observed when the tip radius was increased from 40 to 100 nm and from 100 to 400 nm. However, the elastic response of PP matrix is not affected by changing the geometric contact scale which showed that multiscale cellulosic structure of natural fibers significantly affects their mechanical properties. Scratching PP matrix generated high plastic deformation, while scratching flax fibers cross-sections seems to generate a material removal without high plastic deformation. Thus scratching PP matrix involves a plowing mechanism while for flax fibers material shearing seems to be the predominant mechanism. The difference in the mechanisms that occurred when scratching flax fibers and PP matrix can be due to the high plasticity of PP matrix compared with flax fibers. The friction coefficient increases slightly with the sliding speed increase. Flax fibers friction seems not to be affected by the applied load during scratching while PP matrix friction decreases slightly by scratching load increase. The nanoscopic friction behavior of flax fibers and PP matrix is different from their microscopic friction behavior. In contrast to microscale behavior, PP matrix generates more friction than flax fibers at nanoscale. This friction difference between flax fibers and PP matrix is reduced at low sliding speed when increasing the applied load because the PP friction coefficient decreases slightly by the load increase, while the flax fibers friction coefficient increases by the load increase at low sliding speed. The multiscale friction difference can be due to the activated friction mechanisms at nanoscale which are not similar to that of microscale. Adhesion is the predominant friction mechanism at nanoscopic contact scale. For PP matrix, the friction force due to adhesion increases by increasing the applied load (i.e., increasing the contact area). However the friction coefficient is not significantly affected by the applied load. Therefore increasing both the friction force and the normal force does not affect significantly the friction coefficient. For flax fibers, due to the cellulosic structure of flax fibers at this scale level, the adhesive friction presents the mechanism of energy dissipation that is due to both breaking strong adhesive bonds between the contacting surfaces and the adhesion hysteresis [51]. The adhesion hysteresis strongly occurs for heterogeneous surfaces [51]. This is the case of flax fibers cross-section that have cellulose microfibrils (1 4 nm)

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embedded in noncellulosic polymers (hemicellulose and lignin). The combination of these two adhesion friction mechanisms can explain the increase of friction coefficient for flax fibers when increasing the applied load.

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[16] N.A. Nordin, F.M. Yussof, S. Kasolang, Z. Salleh, M.A. Ahmad, Wear rate of natural fibre: long kenaf composite, in: The Malaysian International Tribology Conference 2013, MITC2013, 2013. [17] F.F. Shuhimi, M.F.B. Abdollah, M.A. Kalam, M. Hassan, A. Mustafa, H. Amiruddin, Tribological characteristics comparison for oil palm fibre/epoxy and kenaf fibre/epoxy composites under dry sliding conditions, Tribol. Int. 101 (2016) 247 254. Available from: https://doi.org/10.1016/j.triboint.2016.04.020. [18] S. Tamba, I. Cisse, F. Rendell, R. Jauberthie, Rice husk in lightweight mortars, in: 2nd International Symposium on Structural Lightweight Aggregate Concrete, 2000, pp. 117 124. [19] P. Mishra, A. Chakraverty, H.D. Banerjee, Studies on physical and thermal properties of rice husk related to its industrial application, J. Mater. Sci. 21 (1986) 2129 2132. [20] S. Majhi, S. Samantarai, S. Acharya, Tribological behavior of modified rice husk filled epoxy composite, Int. J. Sci. Eng. Res. 3 (2012) 1 5. [21] I. Mutlu, Investigation of tribological properties of brake pads by using rice straw and rice husk dust, J. Appl. Sci. 9 (2009) 377 381. [22] T. Kapur, T.C. Kandpal, H.P. Garg, Electricity generation from rice husk in Indian rice mills: potential and financial viability, Biomass Bioenergy 10 (5 6) (1996) 393 403. [23] N. Chand, M. Fahim, P. Sharma, M.N. Bapat, Influence of foaming agent on wear and mechanical properties of surface modified rice husk filled polyvinylchloride, Wear 278 279 (2012) 83 86. [24] K. Shibata, T. Yamaguchi, K. Hokkirigawa, Tribological behavior of polyamide 66/rice bran ceramics and polyamide 66/glass bead composites, Wear 317 (1 2) (2014) 1 7. [25] K.M. Zadeh, I.M. Inuwa, R. Arjmandi, A. Hassan, M. Almaadeed, et al., Effects of date palm leaf fiber on the thermal and tensile properties of recycled ternary polyolefin blend composites, Fibers Polym. 18 (7) (2017) 1330 1335. [26] M.H. Gheith, M.A. Aziz, W. Ghori, N. Saba, M. Asim, et al., Flexural, thermal and dynamic mechanical properties of date palm fibres reinforced epoxy composites, J. Mater. Res. Technol. 8 (1) (2019) 853 860. [27] M.M. Haque, M. Hasan, M.S. Islam, M.E. Ali, Physico-mechanical properties of chemically treated palm and coir fiber reinforced polypropylene composites, Bioresour. Technol. 100 (20) (2009) 4903 4906. [28] K. Alzebdeh, M. Nassar, H. Al Rawahi, N. Al-Hinai, Characterization of mechanical properties of date palm fronds reinforced composites: a comparative evaluation, in: ASME 2016 International Mechanical Engineering Congress and Exposition, American Society of Mechanical Engineers, 2016. [29] K. Al-Kaabi, A. Al-Khanbashi, Hammami, date palm fibers as polymeric matrix reinforcement: DPF/polyester composite properties, Polym. Compos. 26 (5) (2005) 604 613. [30] A. Dehghani, S.M. Ardekani, M.A. Al-Maadeed, A. Hassan, M.U. Wahit, Mechanical and thermal properties of date palm leaf fiber reinforced recycled poly (ethylene terephthalate) composites, Mater. Des. 52 (2013) 841 848. [31] X. Li, L.G. Tabil, S. Panigrahi, Chemical treatments of natural fiber for use in natural fiber reinforced composites: a review, J. Polym. Environ. 15 (1) (2007) 25 33. [32] H. Ibrahim, M. Farag, H. Megahed, S. Mehanny, Characteristics of starch-based biodegradable composites reinforced with date palm and flax fibers, Carbohydr. Polym. 101 (2014) 11 19. [33] N. Benmansour, B. Agoudjil, A. Gherabli, A. Kareche, A. Boudenne, Thermal and mechanical performance of natural mortar reinforced with date palm fibers for use as insulating materials in building, Energy Build. 81 (2014) 98 104.

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[34] T. Alsaeed, B.F. Yousif, H. Ku, The potential of using date palm fibres as reinforcement for polymeric composites, Mater. Des. 43 (2013) 177 184. [35] S. Hegazy, K. Ahmed, S. Hiziroglu, Oriented strand board production from watertreated date palm fronds, BioResources 10 (1) (2015) 448 456. [36] H. Hosseinkhani, M. Euring, A. Kharazipour, Utilization of date palm (Phoenix dactylifera L.) pruning residues as raw material for MDF manufacturing, J. Mater. Sci. Res. 4 (1) (2014) 46 62. [37] J.R. Mohanty, S.N. Das, H.C. Das, Effect of fiber content on abrasive wear behaviour of date palm leaf reinforced polyvinylpyrrolidone composite, ISRN Tribol. 2014 (2014) 453924. [38] B. Yousif, N. El-Tayeb, The effect of oil palm fibers as reinforcement on tribological performance of polyester composite, Surf. Rev. Lett. 14 (2007) 1095 1102. [39] B. Yousif, Replacing of glass fibres with seed oil palm fibres for tribopolymeric composites, Tribol. Mater. Surf. Interfaces 2 (2008) 99 103. [40] H.P.S. Abdul Khalil, M. Jawaid, A. Hassan, M.T. Paridah, A. Zaidon, Oil Palm Biomass Fibres and Recent Advancement in Oil Palm Biomass Fibres Based Hybrid Biocomposites, Intech, Rijeka, 2012. [41] B.F. Yousif, U. Nirmal, Wear and frictional performance of polymeric composites aged in various solutions, Wear 272 (1) (2011) 97 104. [42] F. Fazillah, M. Fadzli, B. Abdollah, A. Kalam, M. Hassan, H. Amiruddin, Tribological characteristics comparison for oil palm fibre/epoxy and kenaf fibre/epoxy composites under dry sliding conditions, Tribol. Int. 101 (2016) 247 254. [43] J.P. Davim, P. Reis, Damage and dimensional precision on milling carbon fiber- reinforced plastics using design experiments, J. Mater. Process. Technol. 160 (2005) 160 167. Available from: https://doi.org/10.1016/j.jmatprotec.2004.06.003. [44] F. Chegdani, S. Mezghani, M. El Mansori, A. Mkaddem, Fiber type effect on tribological behavior when cutting natural fiber reinforced plastics, Wear 332 333 (2015) 772 779. Available from: https://doi.org/10.1016/j.wear.2014.12.039. [45] F. Chegdani, S. Mezghani, M. El Mansori, On the multiscale tribological signatures of the tool helix angle in profile milling of woven flax fiber composites, Tribol. Int. 100 (2016) 132 140. Available from: https://doi.org/10.1016/j.triboint.2015.12.014. [46] F. Chegdani, S. Mezghani, M. El Mansori, Experimental study of coated tools effects in dry cutting of natural fiber reinforced plastics, Surf. Coat. Technol. 284 (2015) 264 272. Available from: https://doi.org/10.1016/j.surfcoat.2015.06.083. [47] F. Chegdani, S. Mezghani, M. El Mansori, Correlation between mechanical scales and analysis scales of topographic signals under milling process of natural fibre composites, J. Compos. Mater. 51 (2017) 2743 2756. Available from: https://doi.org/10.1177/ 0021998316676625. [48] C. Baley, Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase, Compos. - Part A Appl. Sci. Manuf. 33 (2002) 939 948. Available from: https://doi.org/10.1016/S1359-835X(02)00040-4. [49] A. Lefeuvre, A. Bourmaud, L. Lebrun, C. Morvan, C. Baley, A study of the yearly reproducibility of flax fiber tensile properties, Ind. Crop. Prod. 50 (2013) 400 407. Available from: https://doi.org/10.1016/j.indcrop.2013.07.035. [50] F. Chegdani, Z. Wang, M.E. Mansori, T.S. Satish, Multiscale tribo-mechanical analysis of natural fiber composites for manufacturing applications, Bukkapatnam, Tribol. Int. 122 (2018) 143 150. Available from: https://doi.org/10.1016/j.triboint.2018.02.030. [51] M. Nosonovsky, B. Bhushan, Multiscale friction mechanisms and hierarchical surfaces in nano- and bio-tribology, Mater. Sci. Eng. R Rep. 58 (2007) 162 193. Available from: https://doi.org/10.1016/J.MSER.2007.09.001.

Green tribology and tribological characterization of biocomposites

9.1

9

Introduction

The concept of Green tribology was first suggested by Zhang [1] and Jost [2] endorsed it at 4th World Tribology Congress in 2009 [3]. It is an emerging field and now it has included energy conservation, emission reduction, minimization of wear and friction, wind turbines, smart coatings, and fundamentals within its ambit. Green tribology can be regarded as a subdiscipline of tribology within the general concept of tribology, but it focuses on environmental and biological impacts. Hence, it is defined as “the science and technology of research on the tribological theories and technologies, and the practices related to a sustainable society and nature,” and might be termed as “tribology for sustainability” or “sustainable tribology.” This classical definition has been quoted widely [4 7]. Wood [4] illustrated seven distinctive principles of green tribology, namely: (1) minimization of friction and wear; (2) natural and biodegradable lubrication; (3) use of sustainable chemistry and engineering; (4) biomimetic approaches; (5) surface texturing; (6) real-time monitoring; and (7) sustainable energy applications. Among these factors, sustainable energy application, that is, sustainable development through sustainable design, durability, and life quality assume lot of significance.

9.1.1 Natural fiber composites In order to cut down on the dependency on fossil fuels for material requirements, natural- or plant-based materials are looked up to as potential alternatives for conventional synthetic materials. Plant-based materials are not only lighter in weight, cost effective, but also are most importantly biodegradable and recyclable. Hence, the research and application of natural fiber polymeric composites for tribocomponents have been quickly developed. A review of natural fibers and their use in automotive industry to achieve a green technology target in manufacturing of cars specifically has been published [8]. It is a fact that owing to the lighter weight of plant-based fibers and bioderived resins the automotive industry can take advantages of using these materials. Currently, most composites in the market are focused with long-term durability design while using nondegradable polymeric resins such as epoxies and high-strength fiber such as glass. All these materials prove to be a good characteristic of composite but still lack in environmental concern. Most of the polymers and synthetic fibers are derived from petroleum, a nonreplenishable commodity. The current research work is thus focused on obtaining a biocomposite, based on biodegradable raw materials, cost reduction, and the maintenance of the Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00009-8 © 2021 Elsevier Ltd. All rights reserved.

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manufacturing process based on the current scenario. Although the automotive industry is moving fast in their way to expand green technology in composites, however producing the energy efficient and durable biocomposites is the biggest challenge. In the recent years, there has been a surge in interest of ecofriendly materials and technology in industrial sectors and hybrid biocomposite from natural resources. In the last two decades, significant advances in the development of biodegradable polymers have been made [9]. Basically, production of polymers which can biodegrade is generally obtained via polymerization of agricultural-based raw materials. Various forms of biodegradable polymer materials have been implemented and thus proven to have potential uses in industries. Many of these polymers are well suited for adhesive applications such as environmental-friendly packaging, recyclable envelope adhesives, carpet backing, and many other products that are eventually destined for the municipal waste disposal facility. Flax and hemp which can be the source for biofibers have potential as a raw material which are of great importance for the production of various types of composites used in automotives, building materials, packaging, papers, and furniture industries [10]. Biofibers are used as viable alternative to synthetic fibers as reinforcement in plastics because of low cost, lightweight, good mechanical performance, and biodegradable properties. In fact, a better performance can be created using biofiber with biodegradable resins. While in automotive industry, biocomposites have a vast number of applications [11]. Natural fiber composites are often poorer in properties, mostly mechanical, when compared with synthetic fiber composites. A possible solution to this issue is the use of natural fiber/synthetic fiber combination in polymer hybrid composites. Although the biodegradability of the composites is compromised by synthetic fibers, this is compensated by the improvement in their mechanical and physical properties. Hybrid composites use more than one kind of fibers in the same matrix and the idea is to get the synergistic effect of the properties of both fibers on the overall properties of composites. There has been a significant increase in research on natural fiber/synthetic fiber hybrid composites in the recent years. Natural fibers are mostly hybridized with glass fibers because of their comparable properties and low cost. Some studies however have been done on hybridization of natural fibers with more expensive carbon and aramid fibers. There is a considerable improvement in mechanical properties of these composites following hybridization, especially when synthetic fiber plies are used as skin and natural fiber plies are used as core. Various natural fiber surface treatments have been used to improve their interfacial adhesion with the matrices and, hence, their mechanical properties.

9.2

Tribological characteristics of green biocomposites

The uses of natural fiber-reinforced composites in day-to-day life range from smallscale household components such as table tops, chairs, bookshelves, panels, false ceilings, and garden furniture to advanced industrial applications such as structural

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components in drainage cover, building industry, and interior components in the automobile. A substantial number of studies have been performed on developing biocomposite materials with natural fibers and biopolymers such as starch [12], polylactide (PLA) [13], polycaprolactone (PCL) [14], and poly(3-hydroxybutyrateco-3-hydroxyvalerate) (PHBV) [15]. The development of green biocomposites is highly beneficial to various industrial sectors. Natural fibers from plants such as cotton, bamboo, wood, flax, hemp, abaca, kenaf, banana, sisal, oil palm, and jute have been combined with synthetic fibers like glass, carbon, aramid, etc., to tailor desired properties in the hybrid composites. There have been very few studies on tribological characterization of green biocomposites made using natural fiber-reinforced biopolymers [16]. Poly(lactic acid) (PLA) is a biodegradable polymer which can be synthesized by ring-opening polymerization or the condensation polymerization process of lactic acid monomer obtained through the fermentation of dextrose from starch feedstock [17]. High molecular weight PLA is a colorless, glossy, and stiff thermoplastic polymer with properties similar to those of polystyrene. PLA can be degraded by a simple ester bond and the presence of enzymes is not essential to catalyze this hydrolysis. The rate of degradation depends on the size and shape of the article, the isomer ratio, and the temperature of hydrolysis [18]. PLA also exhibits properties such as high strength, superior modulus, biodegradability, and easy processing [18]. However due to its relatively high cost PLA cannot compete with conventional and economical polymers. To reduce its cost and produce a reasonably priced composite, PLA can be combined with inexpensive fillers such as natural fiber. Besides PLA, other biodegradable polymers are poly(butylene succinate) (PBS) [19], polyhydroxyalkanoates (PHA), and poly(hydroxybutyrate-co-valerate) (PHBV). However the present level of production and high cost restrict the industrial applications of biocomposites. In addition, its hydrophilic properties make it a challenge to design component and products that will retain their mechanical integrity and tribological properties despite outdoor applications. The inconsistency in the quality of natural fiber, flammability, moisture absorption, and expansion are the main shortcomings in natural fiber composite products. There are also significant problems in the fiber matrix interaction which reduce the mechanical and tribological properties of the product. Surface-modified jute fiber composites have higher wear resistance when compared with untreated jute fiber/PLA. Among all the treatments applied, silane-treated jute fiber/PLA shows the highest wear resistance. This is due to the strong interaction in the silane-treated jute fiber/PLA as compared with other treatments in the composites’ adhesion and tensile test results [20].

9.2.1 Nanocellulose fiber-based polymer composites Cellulose nanofiber (CNF) is a new type of natural nanoscale fiber, made purely from cellulose molecules. These fibers have shown remarkable mechanical properties compared with other natural fibers as well as glass and carbon fibers [21 23]. In an extensive work on tribological characterization of such composites, the silylated and nonsilylated CNF aerogels were used as reinforcements and a biobased

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epoxy as the matrix materials for making CNF/epoxy nanocomposite using an improvised liquid composite molding (LCM) process [24]. Tribological investigations, such as the friction coefficient and wear behavior of these CNF-based composites were studied under various sliding speeds and loads. It was observed that the coefficient of friction (COF) decreases with increasing volume fraction of nanocellulose in the bioepoxy for all the three normal loads during the low sliding speed of 0.15 m/s. The reduction in COF was found to be significant for larger normal loads. The reduction in friction was also substantial when the volume fraction of nanocellulose was increased from 0% to 0.9% than 0.9% to 1.4%. Neat bioepoxy showed better tribological performance at lower normal loads and the 1.4% composite showed better tribological performance at higher normal loads due to the presence of nanocellulose on contact surfaces. It was also observed that for a given volume fraction, the wear rate increased with increasing normal load while it decreased with increasing volume fraction of the cellulose in the bioepoxy matrix. Furthermore, the wear rate decreased with increasing sliding speed. The volume loss in nanocellulose/ bioepoxy composites is significantly less than that of the neat bioepoxy. This is because the incorporation of nanocellulose in the bioepoxy matrix effectively improved the mechanical and tribological properties of bioepoxy due to the enhancement in strength properties and the ability of nanocellulose fibers to resist the bending force [25]. Better wear resistance was also attributed to the higher elastic modulus of the bioepoxy matrix reinforced by 1.4% CNFs. In addition, the cellulose fibers play a significant role in enhancing fracture toughness of polymer matrices through several energy absorbing mechanisms, such as fiber pullout, fiber fracture, and fiber-bridging [26]. These mechanical factors have significant effect on the tribological properties of nanocellulose/bioepoxy composites. The photomicrographs of worn surface showed that the surface damage was controlled by the addition of nanocellulose to the bioepoxy matrix. This reduced the wear volume loss and hence the nanocomposites showed improved wear resistance when compared with the neat bioepoxy materials as there were no fibers to support the matrix of neat bioepoxy. Hence, the important reason for better wear resistance of the composites is the load bearing ability of CNFs in the matrix. Moreover, by increasing the content of CNF, more fibers can shoulder the load which tends to decrease the wear rate at higher volume fraction of CNFs. There were cracks on the surface of composites tested with higher load while no crack can be found on the surface of composites with low loads. Consequently, the wear rate is greater at the higher applied load due to the deterioration of materials in the presence of cracks. The bonding between nanocellulose and bioepoxy was broken at higher normal load, therefore CNFs were not able to carry the load and hence the load bearing ability of CNFs decreased and it tended to have higher wear rates at higher normal loads. The neat bioepoxy shows a reduction in COF with increasing sliding speed. For the 0.9 vol.% nanocellulose/bioepoxy composite, the COF decreased with increasing sliding speed up to 0.25 m/s and then it further increased with the increasing sliding speed till 0.35 m/s. These variations are significantly less at the 10 N normal load. Despite having an increasing COF with the increasing speed, the COF of nanocomposite is less than that of the neat bioepoxy. The 1.4 vol.% nanocellulose/bioepoxy composite exhibits different

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tribological behavior at low, medium, and high normal loads. Tribological behavior of the silylated CNF composites showed lower COFs and wear volumes than the neat bioepoxy due to the formation of a transfer film on the mating surfaces, which led to a decrease in the “direct contact” of the composite with the asperities of the hard metallic counterface. At higher loads, the COF is decreased for the composites due to the higher wear rates and more exposed CNF fibers on the surfaces. No consistent correlation between the sliding speed and COF/volume loss was observed.

References [1] S.W. Zhang, Current industrial activities of tribology in China, in: Plenary Lecture to the China International Symposium On Tribology (CIST2008), Beijing, 2008. [2] 30th Anniversary and ‘Green Tribology’—Report of a Successful Chinese Mission to the United Kingdom, 7 14 June 2009. Tribology Network of Institution of Engineering and Technology, London, 2009. [3] S.W. Zhang Tribological application in China and green tribology. Lecture to the DIUS (Department for Innovation, Universities and Skills) of UK government (8th June), and to the Green Tribology Seminar (9th June, Institution of Engineering and Technology), London, 2009. [4] H.P. Jost Green tribology—a footprint where economics and environment meet, in: Address to the 4th World Tribology Congress, Kyoto, 2009. [5] M. Nosonovsky, B. Bhushan (Eds.), Green tribology: Biominetics, Energy Conservation and Sustainability, Springer, Berlin, 2012. [6] R. Chattopadhyay, Green Tribology, Green Surface Engineering and Global Warming, ASM International, Materials Park, 2014. [7] R.J.K. Wood, Green tribology at sea, in: The 5th World Tribology Congress, September 8 13, 2013, Torino, Italy, 2013. [8] E. Assenova, V. Majstorovic, VenclA, M. Kanadeva, Green tribology and quality of life, Int. J. Adv. Qual. 40 (2012) 26 32. [9] S. Karlsson, A.C. Albertsson, Biodegradable polymers and environmental interaction, Polym. Eng. Sci. 38 (1998) 1251 1253. [10] M.J. John, S. Thomas, Biofibres and biocomposites, Carbohydr. Polym. 71 (2008) 343 364. [11] J. Njuguna, P. Wambua, K. Pielichowski, K. Kayvantash, Natural fibre-reinforced polymer composites and nanocomposites for automotive applications, Cellulose fibers: bioand nano-polymer composites, Springer, Berlin Heidelberg, 2011, pp. 661 700. [12] S. Shibata, Y. Cao, I. Fukumoto, Flexural modulus of the unidirectional and random composites made from biodegradable resin and bamboo and kenaf fibres, Compos. Part. A: Appl. Sci. Manuf. 39 (4) (2008) 640 646. Available from: https://doi.org/ 10.1016/j.compositesa.2007.10.021. [13] A. Arbelaiz, B. Ferna´ndez, A. Valea, I. Mondragon, Mechanical properties of short flax fibre bundle/poly (ε-caprolactone) composites: influence of matrix modification and fiodi content, Carbohydr. Polym. 64 (2) (2006) 224 232. [14] K. Oksman, M. Skrifvars, J.F. Selin, Natural fibres as reinforcement in polylactic acid (PLA) composites, Compos. Sci. Technol. 63 (9) (2003) 1317 1324. Available from: https://doi.org/10.1016/S0266-3538(03)00103-9.

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[15] R. Bhardwaj, A.K. Mohanty, L. Drzal, F. Pourboghrat, M. Misra, Renewable resourcebased green composites from recycled cellulose fiber and poly (3-hydroxybutyrate-co3-hydroxyvalerate) bioplastic, Biomacromolecules 7 (6) (2006) 2044 2051. [16] B.P. Chang, H. Md Akil, M.H. Zamri, Tribological characteristics of green biocomposites, in: M. Jawaid, et al. (Eds.), Green Biocomposites, Green Energy and Technology, Springer International Publishing AG, 2017. Available from: http://doi.org/10.1007/ 978-3-319-46610-1_7. [17] L. Suryanegara, A.N. Nakagaito, H. Yano, The effect of crystallization of PLA on the thermal and mechanical properties of microfibrillated cellulose-reinforced PLA composites, Compos. Sci. Technol. 69 (7) (2009) 1187 1192. [18] D. Garlotta, A literature review of poly (lactic acid), J. Polym. Environ. 9 (2) (2001) 63 84. [19] H.S. Kim, B.H. Lee, S. Lee, H.J. Kim, J.R. Dorgan, Enhanced interfacial adhesion, mechanical, and thermal properties of natural flour-filled biodegradable polymer biocomposites, J. Therm. Anal. Calorim. 104 (1) (2011) 331 338. Available from: https:// doi.org/10.1007/s10973-010-1098-9. [20] N. Ramli, N. Mazlan, Y. Ando, Z. Leman, K. Abdan, A. Aziz, Natural fiber for green technology in automotive industry: a brief review, IOP Conf. Ser. Mater. Sci. Eng. 368 (2018) 012012. [21] M. Henriksson, L.A. Berglund, P. Isaksson, T. Lindstroth, T. Nishino, Cellulose nanopaper structures of high toughness, Biomacromolecules 9 (6) (2008) 1585. [22] S.-Y. Lee, S.-J. Chun, I.-A. Kang, J.-Y. Park, Preparation of cellulose nanofibrils by high-pressure homogenizer and cellulose-based composite films, J. Ind. Eng. Chem. 15 (1) (2009) 50 55. [23] M. Henriksson, L.A. Berglund, Structure and properties of cellulose nanocomposite films containing melamine formaldehyde, J. Appl. Polym. Sci. 106 (4) (2007) 2817 2824. [24] B.B. Barari, Experimental and Numerical Characterization of Scalable Cellulose NanoFiber Composite (Theses and Dissertations), The University of Wisconsin-Milwaukee, 2016. [25] H. Alamri, I.M. Low, Microstructural, mechanical, and thermal characteristics of recycled cellulose fiber halloysite epoxy hybrid nanocomposites, Polym. Compos. 33 (4) (2012) 589 600. [26] I.M. Low, J. Somers, H. Kho, I. Davies, B. Latella, Fabrication and properties of recycled cellulose fibre-reinforced epoxy composites, Compos. Interfaces 16 (79) (2009) 659 669.

Molecular dynamics simulation and tribological behavior of polymer composites

10.1

10

Basic theory of molecular dynamic simulation

Molecular dynamics (MD) is a computer simulation in which the time evolution of a set of interacting atoms is traced by integrating their equation of motion. The integration is performed by numerically solving the Newton’s second law of motion for individual atoms. The interactions between these atoms are defined by empirical potential fields. The force (f) acting on any atom k, by surrounding atoms, can be given as [1] fk 5 2 @Ek =@rk

(10.1)

where, Ek and rk are the potential energy and the position of atom k, respectively. Potential energy of atoms is obtained from an empirical potential field. Using this force, the acceleration of the atom can be calculated using fk 5 mk ðd2 rk =dt2 Þ 5 mk ak

(10.2)

where, mk, rk, and ak are the mass, the position, and the acceleration of atom k, respectively. The system of atoms is allowed to move under these accelerations for a specific time period called time-step, which is generally in the order of a femtosecond (10215 s). The new positions and velocities of the atoms at the end of a time-step are obtained using a numerical integration algorithm such as Velocity Verlet algorithm [1]. Temperature is controlled by modifying the velocities of atoms, while pressure is controlled by adjusting the size of the simulation domain. Temperature of a system of atoms is defined as the average kinetic energy of the atoms. In a constant temperature MD simulation, the average value of temperature over a time period is kept constant by appropriately scaling the velocities of atoms, which can be achieved by using a Berendsen barostat and Nose´Hoover barostat [1]. In both these systems pressure is adjusted by changing the dimensions of the simulation domain. Most of the experiments are conducted under constant temperature and pressure (isothermalisobaric), which can be simulated in MD using the NPT ensemble, where P and T stand for fixed pressure and fixed temperature, respectively. Tribology of Natural Fiber Polymer Composites. DOI: https://doi.org/10.1016/B978-0-12-818983-2.00010-4 © 2021 Elsevier Ltd. All rights reserved.

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Tribology of Natural Fiber Polymer Composites

Application of molecular dynamics simulation to understand wear and friction behavior of polymer composites

Very few microscopic studies have been conducted on the tribological properties of polymer composites using MD simulations. MD simulation has been recently used to explain the enhancement of mechanical and tribological properties of grapheme-filled polymer composites at atomistic level [2]. A layer model containing iron atoms as the top nanorod and the grapheme-reinforced polymer composites as the core was constructed to study the tribological properties of polymer composites. Friction and wear processes were assessed by sliding the top iron nanorod on the surface of the polymer matrix. The interaction energy between graphene and the polymer matrix, the angle, bond, and kinetic energy of the polymer chains of the polymer composites monitored during simulation were used to arrive at different conclusions related to mechanical and tribological properties. A three-layer molecular model has also been used to understand the tribological properties of carbon nanotubes (CNTs)/polymer composites [3]. The atom movement velocities, atom concentration, peak temperature, and average cohesive energy in the friction interface region between polymer matrix and iron layers were calculated to explain the molecular mechanisms. Using MD simulation for graphene/polyethylene (PE) composites it has been shown that the toughness increases significantly with an increase in the volume fraction of graphene sheets by examining the interaction strength between polymer chains and graphene [4]. MD simulations also showed that noncovalently functionalized graphene surfaces may affect the thermal conductance owing to the heat transferability between paraffin and graphene by its high-frequency bondstretching vibrations [5].

10.2.1 Potential used in molecular dynamics simulation of polymer composites In general, a condensed-phase optimized molecular potential for atomistic simulation studies (COMPASS) forcefield, which is the first ab initio forcefield that enables accurate simultaneous predictions of a broad range of molecules and polymers, is used for MD simulations of polymer-based composites. In the COMPASS forcefield [6], the total potential energy is composed of both valence and nonbonded interaction terms. The valence expressions consist of diagonal (bond stretching, bond angle bending, torsion angle rotations, and Wilson out-of-plane angle potentials) and off-diagonal cross-coupling (interactions between diagonal terms) terms. The nonbonded interactions include Van der Waals (vdW), hydrogen bond energy, and long range electrostatic interactions (Coulombic functions). The vdW energy is represented by a sum of repulsive and attractive LennardJones terms [7]. This forcefield has been used to describe the mechanical properties of graphene and polymers [8,9].

Molecular dynamics simulation and tribological behavior of polymer composites

215

10.2.2 Simulation model and protocol The simulation model consists of a unit cell of a certain size containing a singlelayer graphene and an empty unit cell in the same size for comparison purpose. The graphene is edge-functionalized by hydrogen element in order to reach a better mechanical property [10]. A polymer chain composed of monomer units are packed into the simulation cells with a predefined density. Various available softwares can be used for packing processes. The molecules in a cell are built with a Monte Carlo style, by minimizing close contacts among atoms, while keeping a realistic distribution of torsion angles. To obtain a global minimum energy configuration, a geometry optimization is first performed using the method of conjugate-gradient [11] with convergence criteria of 0.00001 kcal/mol. The two unit cells are then allowed to equilibrate over NPT simulations (isothermalisobaric ensemble) at a room temperature of 298K and atmospheric pressure of 101 KPa for 2 ns with a time step of 1 fs. Equilibrated molecular systems of the pure polymer matrix and graphene/polymer composites can be obtained after a geometry optimization. These simulation processes are aimed to remove internal stresses in the polymer matrix. After the equilibration process, the constant-strain minimization method is then applied to the system to calculate the Young’s modulus and shear modulus of the two polymer composites. In addition, a potential cut-off radii is applied in the calculation of the nonbonded LJ interaction. A thermostat [12] and barostat algorithm [13] are applied in the system temperature and pressure conversion. The Ewald summation method [14] with an accuracy of 0.001 kcal/mol is used in calculation of the electrostatic interactions. Periodic boundary conditions are adopted in all the molecular models in MD simulations. In order to study the tribological properties, a three-layer model is constructed in which the middle layer is polymer matrix (with and without graphene), while the bottom and top layer are of iron nanorods. Then, a geometry optimization with an energy convergence tolerance of 1.0e24 kcal/mol and a force ˚ is applied for finding a global miniconvergence tolerance of 0.0001 kcal/mol/A mum energy configuration for the two models. To further equilibrate the molecular systems, a five-cycle annealing process is then followed under the constant volume and the constant temperature (NVT ensemble) from 150K to 350K for 200 ps. In the annealing process, the polymer chains of the composites can be further relaxed in a wide temperature range to obtain a local energy minimum. To fulfill the friction process, a shear loading is then employed to the upper iron nanorod by moving it with a desired speed for a specified time under a normal loading. The temperature and time step of the MD simulations are preset. During the MD simulations, the forces in the sliding direction and the trajectories of atoms of the upper iron nanorod and polymer matrix are calculated and recorded to investigate the friction coefficient and abrasion loss. The friction coefficient can be expressed as: µ 5 FN/FT, (where, µ is the friction coefficient; and FN and FT are the normal force and friction force, respectively). The worn polymer molecules can be determined by recognizing the molecules which are moved out of the polymer matrix. Then, the abrasion rates can be calculated by the ratio of the numbers of the worn molecular to the total molecular of the polymer matrix. To elucidate the results of the MD simulations,

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the interaction potential energy of the interfacial region between the polymer matrix and graphene reinforcement, the angle, bond, and kinetic energy of the polymer matrix are calculated. The stresses in the x-axis direction are calculated based on the virial stress. Finally, the strainstress curves of the pure polymer matrix and graphene/polymer composites are obtained. The constant strain approach is adopted by applying constant strains in the six directions of x, y, z, yz, xz, and xy of the two molecular models. Every strained structure is then optimized to keep the cell parameters fixed. The stresses in all directions are calculated based on the virial stress definition. The elastic 6 3 6 stiffness (Cij) and compliance (Sij) matrices are then built up from a linear fit between the applied strain and resulting stress.

10.2.3 Tribological study In order to investigate the tribological properties of polymer composite the middle part of the layer structure is subjected to a shear loading by the upper nanorod by moving it for a specified time at a specified temperature (NVT) and a specified velocity under a specified normal loading. During the MD simulations the forces in the sliding direction and the trajectories of the atoms of the polymer composites are calculated and recorded to determine the friction coefficient and abrasion loss. The molecules reduced from the model lattices are recognized as the worn polymer molecules from the polymer matrix during the simulations. The abrasion rates are then given by the ratio of the numbers of the worn molecular to the total molecular of the polymer matrix. In order to explore the mechanism on the improvement of the tribological properties of polymer composites, the interaction potential energy of the interfacial region between the graphene sheet (GS) reinforcement and polymer matrix are calculated. Then, the variation of the interaction energy between the GS and polymer matrix of the graphene/polymer composites is plotted. The radial distribution function (RDF) spectra between iron element and C atoms of the polymer chains of the pure polymer matrix and graphene/polymer composites are then calculated and the space range is used as the potential interactions of the polymer matrices. A molecular model of NBR matrix incorporated by nanofillers has also been modeled on similar lines as discussed in previous section [15]. Based on the MD simulation and experimental works, it has been shown that because of the strong vdW adsorption forces, NBR chains can be efficiently restricted around the surface of nano-ZnO particle, leading to low wear and friction.

10.2.3.1 Nanoscratching MD simulations have also been carried out to understand the nanoscratching of a polystyrene polymer [16]. In order to generate the amorphous PS specimen a large number of PS single chains were randomly arranged in a rectangular box by using the Packmol software [17]. MD model of nanoscratching of PS consisted of a PS specimen of full atomic representation and a virtual spherical probe. The bottom of the specimen is fixed. Prior to nanoscratching, the probe with a radius of 1 nm is

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217

initially placed above the surface of the PS specimen to avoid interaction between the probe and specimen. The nanoscratching process consists of two stages; indentation followed by scratching. In the indentation stage, the probe penetrates into the PS specimen along negative Y direction with a constant velocity. Then the probe scratches along negative X direction with the same constant velocity in following scratching stage. Adaptive intermolecular reactive bond order (AIREBO) potential was used to describe the atomic interactions of PS specimen [18]. To exclude the adhesion effect on the nanoscratching process, the virtual spherical probe is modeled by a strong repulsive potential [19]. In such way the adhesion between the probe and the PS specimen is eliminated, because the force is zero until the probe starts to contact with the PS specimen. The MD simulation of nanoscratching of PS consisted of two processes, as relaxation followed by nanoscratching. Two conditions are satisfied at the end of the relaxation process; temperature fluctuates around the desired value, and pressure approaches 0 bar. It may be achieved either by energy minimization by keeping the potential energies of atoms relaxed to their local minimum values at 0K using the conjugate gradient method or NPT dynamic relaxation wherein the system is heated to the desired temperature of 10K from 0K under 0 bar using the NoseHoover method in the isothermalisobaric NPT ensemble. Nanoscratching process is then performed by indenting and scratching the PS specimen using the spherical probe. The three force components acting on the probe, as scratching force along X direction, indentation force along Y direction and lateral force along Z direction change and depend on the stress states. The morphology of machined surface of the PS specimen gives an idea of nanoscratching.

References [1] D.C. Rapaport, The Art of Molecular Dynamics Simulation, Cambridge University Press, Cambridge, 1995. [2] S.J.V. Frankland, V.M. Harik, G.M. Odegard, D.W. Brenner, T.S. Gates, The stress strain behavior of polymer nanotube composites from molecular dynamics simulation, Compos. Sci. Technol. 63 (2003) 16551661. [3] Y.L. Li, S.J. Wang, B. Arash, Q. Wang, A study on tribology of nitrile-butadiene rubber composites by incorporation of carbon nanotubes: molecular dynamics simulations, Carbon 100 (2016) 145150. [4] J. Liu, J. Shen, Z. Zheng, Y. Wu, L. Zhang, Revealing the toughening mechanism of graphene polymer nanocomposite through molecular dynamics simulation, Nanotechnology 26 (29) (2015) 291003. [5] Y.L. Li, S.J. Wang, E.Q. He, The effect of sliding velocity tribological properties polymer/carbnanotube composites, Carbon 106 (2016) 106109. [6] D. Rigby, H. Sun, B.E. Eichinger, Computer simulations of poly(ethylene oxide): force field PVT diagram and cyclization behavior, Polym. Int. 44 (1997) 311. [7] T.A. Halgren, The representation of van der Waals (vdW) interactions in molecular mechanics forcefields: potential form, combination rules, and vdW parameters, J. Am. Chem. Soc. 114 (20) (1992) 78277843.

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[8] L. Wang, L. Duan, Isothermal crystallization of a single polyethylene chaininduced by graphene: a molecular dynamics simulation, Comput. Theor. Chem. 1002 (2012) 5963. [9] S.C. Shiu, J.L. Tsai, Characterizing thermal and mechanical properties of graphene/ epoxy nanocomposites, Compos. Part B Eng. 56 (2014) 691697. [10] Q.X. Pei, Y.W. Zhang, V.B. Shenoy, A molecular dynamics study of the mechanical properties of hydrogen functionalized graphene, Carbon 48 (3) (2010) 898904. [11] B.T. Polyak, The conjugate gradient method in extrermal problems, USSR Comput. Math. Math. Phys. 9 (4) (1969) 94112. [12] H.C. Andersen, Molecular dynamics at constant pressure and/or temperature, J. Chem. Phys. 72 (1980) 23842393. [13] H.J. Berendsen, J.P.M. Postma, W.F. van Gunsteren, A. DiNola, J. Haak, Molecular dynamics with coupling to an external bath, J. Chem. Phys. 81 (8) (1984) 36843690. [14] P.P. Ewald, Ewald summation, Ann. Phys. 369 (1921) 253. [15] Y. Li, Q. Wang, S. Wang, A review on enhancement of mechanical and tribological properties of polymer composites reinforced by carbon nanotubes and graphene sheet: molecular dynamics simulations, Compos. Part B: Eng. 160 (2018) 348361. [16] Y. Kai Du, J. Tang, Zhang, T. Sun, Molecular dynamics modelling and simulation of mechanical nanoscratching of polystyrene, Int. J. Nanomanuf. 9 (1) (2013) 98107. [17] L. Martı´nez, R. Andrade, E.G. Birgin, J.M. Martı´nez, Packmol: a package for building initial configurations for molecular dynamics simulations, J. Comput. Chem. 30 (13) (2009) 21572164. [18] S.J. Stuart, J.A. Harrison, A.B. Tutein, A reactive potential for hydrocarbons with intermolecular interactions, J. Chem. Phys. 112 (14) (2000) 64726486. [19] C.L. Kelchner, S.J. Plimpton, J.C. Hamilton, Dislocation nucleation and defect structure during surface indentation, Phys. Rev. B 58 (17) (1998) 11085.

Appendix: Chemical composition of natural plant fibers

Cellulose is the essential component of all plant fibers. It is a linear condensation polymer consisting of D-anhydroglucopyranose units (often abbreviated as anhydroglucose units or even as glucose units for convenience) joined together by β-1,4-glycosidic bonds. It is thus a 1,4-β-D-glucan. The pyranose rings are in the 4c1 conformation, which means that the CH2OH and OH groups, as well as the glycosidic bonds, are equatorial with respect to the mean planes of the rings [1]. The Haworth projection formula of cellulose is given by [2]. The molecular structure of a cellulose is responsible for its supramolecular structure and this, in turn, determines many of its chemical and physical properties. In the fully extended molecule, adjacent chain units are oriented by their mean planes at an angle of 10 degrees to each other. Thus the repeating unit in cellulose is the anhydrocellulobiose unit and the number of repeating units per molecule is half the degree of polymerization (DP). This may be as high as 14,000 in native cellulose, but purification procedures usually reduce it to something in the order of 2500. The DP shows that the length of the polymer chains varies depending on the type of natural fiber. The mechanical properties of a natural fiber depend on its cellulose type, because each type of cellulose has its own cell geometry and the geometrical conditions determine the mechanical properties. Solid cellulose forms a microcrystalline structure with crystalline as well as amorphous regions. Naturally occurring cellulose crystallizes in monoclinic sphenodic structures. The molecular chains are oriented in the fiber direction. The geometry of the elementary cell is dependent on the type of cellulose. Hemicellulose comprises a group of polysaccharides (excluding pectin) that remains associated with the cellulose after lignin has been removed. The hemicellulose differs from cellulose in three important aspects. Firstly, they contain several different sugar units whereas cellulose contains only 1,4-β-D-glucopyranase units. Secondly, they exhibit a considerable degree of chain branching, whereas cellulose is a strictly linear polymer. Thirdly, the DP of native cellulose is 10 to 100 times higher than that of hemicellulose. Unlike cellulose, the constituents of hemicellulose differ from plant to plant.

220

A1

Appendix: Chemical composition of natural plant fibers

Chemical structure of hemicellulose

Lignins are complex hydrocarbon polymers with both aliphatic and aromatic constituents [2]. Their chief monomer units are various ring-substituted phenylpropanes linked together. Structural details differ from one source to another. The mechanical properties are lower than those of cellulose. At the value of 4 GPa, the mechanical properties of isotropic lignin are distinctly lower than those of cellulose [3].

A2

Chemical structure of lignin

Pectin is a collective name for heteropolysaccharides, which consist essentially of polygalacturon acid. Pectin is soluble in water only after a partial neutralization with alkali or ammonium hydroxide [3]. Waxes make up the part of the fibers which can be extracted with organic solutions. These waxy materials consist of different types of alcohols, which are insoluble in water as well as in several acids (palmitic acid, oleaginous acid, and stearic acid) [3]. In addition to these components, jute fiber also consists of nitrogenous matter and traces of pigments like b-carotene and xanthophyll.

References [1] T.P. Nevell, S.H. Zeronian, Cellulose Chemistry and its Applications, Wiley, New York, 1985. [2] G.E. Kritschewsky, Chemische Technology von Textilmaterialien, Legprombitisdat, Moskau, 1985. [3] J. Gassan A.K. Bledzki 7th Internationales Techtexil Symposium 1995, Frankfurt, 20 22 June 1995.

Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A Abraders, 71 Abrasive wear, 67, 67f, 68f bamboo, 170 173, 175f sisal-epoxy composites, 99 102 testing, 70 72, 99 wood-plastic composites, 179 187 Acetylation, 17, 28, 31 32, 45, 74, 93, 113, 120 121, 144 145 Acrylation, 19 Acrylonitrile grafting, 19 Additives, 14, 48, 179 180 Adhesion theory of friction, 62 66 Adhesive friction coefficient, 64 65 Adhesive wear, 66 67, 123 125, 175 Alkaline treatment, 22 23 Animal fibers, 3 4 Anisotropy structure of wood, 189 190 Applications of composites, 2t, 10 11 automotive, 10 11, 49 52, 74, 78 79 construction, 1 2, 49, 52 engineering, 2t rural and cottage industries, 53 Asperity deformation, 62 64 ASTM standards, 71 73 Automotive industry, 3, 50, 207 208 Avian fibers, 3 4 B Bamboo, 3, 7, 35 38, 38t, 80 81, 163, 179, 208 209 abrasive wear, 170 173 advantages and disadvantages, 163 164 chemical composition, 164, 164t contact configuration, 174 175 crystallinity, 35 38 fiber orientation, 169 170 maleated PP effects, 168 169

mechanical properties, 165, 167, 173 174 mercerization, 167 physical properties, 164 silanization, 168 sliding wear, 175 spectral characterization, 35 surface modification, 167 surface morphology, 174 thermogravimetric analysis, 35 tribological behaviour, 169 175 vascular fiber content, 173 174 Bamboo thermoplastic composites, 168 169 Bamboo thermoset composites, 166 168 Banana, 3, 4f, 6t, 8, 11 12, 45 46, 47t, 49, 74, 208 209 extraction, 11 13 Bast fibers, 3, 5, 9 10, 51 Benzoylation, 21 22 Biodegradability, 45, 53 54, 55t, 111, 207 209 Brake pads, 51 52, 107 109, 109t, 198 199 C Carbon fibers, 76 77, 106, 196 197, 209 211 Cavitation, 67 68 Cellulose, 3, 5, 7 11, 14, 17 23, 25, 27t, 28, 31 35, 34t, 35t, 38 43, 45 46, 46t, 48, 74, 89 93, 97, 112 113, 115t, 131 132, 160, 164 165, 178, 186, 201 203, 209 211 Chemical composition bamboo, 164, 164t cotton, 131 132 jute, 112 sisal, 89

222

Chemical composition (Continued) wood, 178 Chemical treatments, 15 23, 33, 118, 145f, 146f, 167. See also Mercerization acetylation, 17 acrylation, 19 acrylonitrile grafting, 19 benzoylation, 21 22 Duralin, 20 graft copolymerization, 18 19, 121 123 impregnation, 19 isocyanate, 17 18 maleated coupling agents, 22 permanganate, 20 21 peroxide, 21 silane, 15 17 stearic acid, 18 Classification of natural fibers, 3, 6t Coir, 3, 4f, 8, 45 46, 74, 193 194 extraction, 12 13 Compatibilizing agents cotton, 137 145, 139f wood plastic composites, 179 187, 182f Construction industry, 1 2, 49, 52 Contact configurations, 68, 68f, 70, 174 175 Corona discharge, 23 Corrosive wear, 67 Cottage industries, 11 12, 53, 111 112 Cotton, 3, 7, 9 10, 13, 38 39, 83, 131, 193 194 advantages and disadvantages, 131 chemical composition, 131 132 compatibilizers, 137 145, 139f crystallinity, 135 136 elongation at break, 136 esterified fibers, 141 extraction, 11 13 lubrication behaviour, 159 160 mechanical properties, 134 141, 135t physical structure, 132 134 spectral characterization, 39 stiffness, 135 136 strength of fibers and bundles, 136 thermogravimetric analysis, 38 39 Cotton polyester composites, 137 160, 149f, 150t, 151f, 156f cotton-kapok, 144 145 cotton-ramie, 141 143 graphite filled, 148 154

Index

tribological behaviour, 146 160 ultra high molecular weight polyethylene filled, 154 158 wear rate, 151f Counterface materials, 188 190 Counterface roughness, 68, 70, 76 Coupling agents, 14 15, 18, 22, 47 48, 93 95, 118 119, 121 123, 125 129, 179 182, 186, 195 MA-PP/MAH-PP, 121 123, 125 129 silane, 168, 194 195 Crosslinked polyethylene, 97, 183 184 Crystallinity, 35 38 bamboo, 35 38 cotton, 134 141 sisal, 25 Cultivation of natural fibers, 9 10 D Duralin treatment, 20 Dynamic friction coefficient, 63 64, 75 E E-glass fibers, 40 44, 74 Economics of natural fiber polymer composites, 53 55 Energy consumption, 55t Engineered wood products, 180 Engineering applications, 2t Erosive wear, 74 Esterification cotton, 144t sisal, 23 Extraction of fibers, 3, 11 12, 50 banana, 8 coir, 8 cotton, 7 flax, 8 9 jute, 5 6, 111 113 sisal, 7 F Fatigue wear, 67 68 Fiber orientation, bamboo, 169 170, 175f Fillers, tribological effects, 77 79 Flax, 3, 8 10, 13, 40 45, 49, 51, 53 55, 111 112, 196 197, 208 209 extraction, 11 13 spectral characterization, 33 34

Index

thermogravimetric analysis, 33 Free radicals, 18 19, 21, 23, 33, 97, 183 Fretting wear, 83 Friction, 61 62 adhesion theory, 62 63 adhesive coefficient, 64 65 asperity deformation, 62 64, 106 107 energy consumption, 55t fiber reinforcement effects, 76 77 filler effects, 77 79 friction coefficient, 75 measurement, 70 origin, 62 66 ploughing component, 171 sliding distance curve, 63 64 Fruit fibers, 3 G Glass fibers, 3 4, 11, 45, 49 52, 77, 99, 106, 111 112, 116 118, 207 208 Gossypium sp., 7, 131 Graft copolymerization, 18 20 Graphite fibers, 73 Graphite filled cotton-polyester composites, 152 H Heat treatments, 129 Hemicelluloses, 5, 7 9, 19 20, 23, 25, 27 28, 40, 45, 89, 92 93, 113, 119, 131 132, 164, 177 178, 201 203 Hemp, in brake pads, 51 52 I Impact strength, 22, 74, 97 99, 137 138, 139f, 173 174, 180 181 Impregnation, 19 Inorganic fillers, 1 2 Insulation, 10, 14, 49 52, 80 81, 136 137 Interfacial adhesion, 14, 21 22, 74, 93 95, 125, 137, 144 145, 167, 180, 186, 194 195, 207 208 Ionomers, 181 182, 182f Irradiation crosslinking, 184 Isocyanate treatments, 16f, 17 18, 97 98 J Jute, 111 113 acetylation, 120 121

223

advantages and disadvantages, 112 applications, 111 112 chemical composition, 112 extraction, 111 mechanical properties, 113 morphological characterization, 32 physical structure, 113 spectral characterization, 28, 31 32 surface modification, 118 123 thermogravimetric analysis, 28 Tossa jute, 111 white jute, 111 Jute epoxy composites, 118 119, 129 Jute polyester composites, 123 125 Jute polymer composites, 113 123 moisture absorption, 112, 116 118 strength properties, 113 116 Jute polypropylene composites, 49, 125 129 K Kapok, 3, 144 145 Kerala, 3, 5f, 8, 12 13 L Leaf fibers, 3 Lignins, 5, 7 10, 14, 18 23, 33, 40, 45, 89, 112 113, 164 165, 177 178, 201 203 Lignocellulosic fibers, 3, 5, 14, 23, 30t, 43, 45, 47 48, 125 127 Load, 69 70, 75 76, 91 92, 122 123, 136, 148 152, 170 173, 188, 193 196 Lubrication, 61 62, 70, 79 80, 152 behaviour of cotton, 159 160 M MA-PP/MAH-PP, 121 123, 125 129, 187 Maleated coupling agents, 22 Maleated PP effects on bamboo, 168 169 Matrix, 2, 7, 14, 17, 20, 22, 48 49, 73, 93, 104 106, 118, 164 Measurement of friction and wear, 68 72 abrasive wear tests, 70 72 ASTM standards, 71 contact configurations, 68, 68f counterface roughness, 70, 76 load, 69, 76

224

Measurement of friction and wear (Continued) operating parameters, 68 70, 76 sliding duration, 69 70 sliding speed, 69, 76 sliding wear tests, 70 temperature, 69, 76 worn surface analysis, 72 Mechanical properties, 40 43, 90 93, 97 99, 113, 134 141, 165, 168 169, 187 bamboo, 165 and cellulose, 180 cotton, 134 141 filler effects, 77 79 jute, 113, 115t reinforcement effects, 76 77 sisal, 90 93 sisal polymer composites, 97 99 wood-plastic composites, 179 187 Mercerization, 22 23, 120, 167, 187 bamboo, 167 jute, 120 sisal, 23 32, 93 97 wood-plastic composites, 179 187 Metal conservation, 1 2 Metal fibers, 3 4, 106 Mineral fibers, 3 4, 15, 106 Moisture absorption jute, 112, 116 118 wood, 178 179 Morphological characterization jute, 32 sisal, 32 Multiphase systems, 1 2 N Natural fiber polymer composites, 5, 15, 18, 43 49, 46t applications, 5, 49 53 bamboo, 166 169 cotton, 136 145 economics, 53 55 jute, 113 123 sisal, 93 97 thermoplastic based, 47 48 thermoset based, 45 46 wood, 187 190

Index

Natural fibers, 1 animal, 3 4 categories, 3 chemical treatments, 15 22 classification, 3, 6t, 43 cultivation, 54 55 extraction, 11 13 physical treatment, 22 23 plant, 3, 5, 50 sources, 3 13 surface modification, 14 O Operating parameters, 68 70, 76, 153 154 P Pectin, 5, 8 9, 13, 27, 45, 89, 112, 112t, 131 132 Permanganate treatment, 20 21 Peroxide crosslinking, 183 184 Peroxide treatment, 21 Photochemical degradation, 14 Physical structure bamboo, 164 cotton, 132 134 jute, 113 sisal, 89 90 wood, 178 179 Physical treatments, 5, 14, 22 23 Plant fibers, 3, 4f, 5 11, 17, 22, 25 26, 40 42, 49 50, 74, 83, 89 90, 111 112, 136 137, 193 194 Plasma treatment, 23 Ploughing component in friction, 171 Pollution, 2 3, 53 54 Polymer matrix composites (PMCs), 14, 80 83, 144 R Ramie, 3, 8 9, 43 44, 141 143 Recycling, 45, 49 50, 53 54 Reinforcement materials, 2 effect on wear and friction, 77 Resins biodegradable, 44 45 impregnation, 19 Rural industries, 53

Index

S Seed fibers, 3 Silane grafted HDPE, 184 Silane treatments, 15 17, 16f, 95 96, 118 120, 168 bamboo, 168 jute, 118 123 sisal epoxy composites, 95 96 Silk fibers, 3 4 Sisal, 3, 5, 7 advantages and disadvantages, 87 89 chemical composition, 89 crystallinity, 25 esterification, 23 extraction, 7, 11 isocyanate treatments, 17 18 leaf composition, 87 mechanical properties, 90 93 mercerization, 23, 28f, 120 morphological characterization, 32 physical structure, 89 90 spectral characterization, 27 28 surface modification, 93 thermal properties, 25 26, 92 93 thermogravimetric analysis, 23 24 Sisal epoxy composites, 95 96 abrasive wear, 99 102 Sisal phenolic composites, 96 friction and wear behaviour, 106 109 Sisal polyester composites, 93 95 sliding wear, 102 106 Sisal polyethylene composites, 97 99 Sisal polymer composites, 93 97 mechanical properties, 97 99 tribological behaviour, 99 109 Sisal thermoplastic composites, 98 99 Sisal thermoset composites, 97 98 Skin fibers, 3 Sliding distance curve, 63 64 Sliding duration, 63 64, 69 70, 75, 102 103 Sliding speed, 69, 76, 148 152, 151f, 198 203, 209 211 Sliding wear, 66 67, 78 83, 175 bamboo, 175 sisal polyester, 102 106 Sliding wear tests, 70, 104f Spectral characterization bamboo, 35 cotton, 39

225

flax, 40 jute, 28 sisal, 27 wood, 33 34 Stalk fibers, 3, 139 140 Static friction coefficient, 63 64, 75 Stearic acid treatments, 18 Strength. See also Mechanical properties cotton fibers and bundles, 156 157 impact strength, 74 jute, 111 112 tensile strength, 19, 40 42, 52, 72 73, 79 80, 90 92, 97 98, 101 102, 111 119, 116t, 118f, 121 122, 125, 139 140, 165, 166f, 168 170, 174f, 180 181 Surface modification, 14 bamboo, 167 jute, 118 123 sisal, 93 Surface morphology, bamboo, 174 T Tabor abraders, 71 Tailored materials, 2 Temperature in test systems, 68 69 Tensile strength, 19, 40 42, 52, 72 73, 79 80, 90 92, 97 98, 101 102, 111 119, 116t, 118f, 121 122, 125, 139 140, 165, 166f, 168 170, 174f, 180 181 Testing. See Measurement of friction and wear Thermal properties, sisal, 25 26, 92 93 Thermogravimetric analysis bamboo, 35 cotton, 38 39 flax, 40 jute, 28 sisal, 23 24 wood, 33 Thermoplastic composites, 77 78, 78t bamboo, 168 169 sisal, 98 99 Thermoset composites, 45 46 bamboo, 166 168 sisal, 97 98 Tossa jute, 111 Tribology, definition, 61 62

226

U Ultra high molecular weight polyethylene filled composites, 74, 154 158 V Vascular fiber content of bamboo, 173 174 W Waxes, 8 9, 39, 45, 89, 141 Wear, 61 62, 66 72, 75 76. See also Abrasive wear adhesive wear, 66 67, 123 125, 175 cavitation, 67 68 corrosive wear, 67 definition, 66 68 erosive wear, 67, 74 fatigue wear, 67 68 fiber reinforcement effects, 76 77 filler effects, 77 79 fretting wear, 67 measurement, 70 mechanisms, 72, 75 76, 83, 188, 194 196, 199 201 sliding wear, 66 67, 70, 76 77, 77f, 83 wear coefficient, 68, 75 Weathering of wood, 184 185 White jute, 111 Wood, 7, 33 34, 177 179

Index

advantages and disadvantages, 177 178 anisotropy structure, 189 190 chemical composition, 178 growth rings, 177 physical structure, 178 179 spectral characterization, 33 34 thermogravimetric analysis, 33 water storage, 179 Wood flour epoxy composites, 190 Wood flour polyethylene composites, 180 185 Wood flour polypropylene composites, 186 Wood-plastic composites, 179 187 abrasive wear, 187 190 compatibilizing agents, 180 182, 187 counterface materials, 188 189 crosslinked polyethylene, 183 184 engineered products, 180 mechanical properties, 187 mercerization, 187 silane grafted HDPE, 184 tribological behaviour, 187 190 weathering, 184 185 zinc composites, 181 182 Worn surface analysis, 72 Z Zinc composites, 181 182