Short fibre polymer composites

Short fibre filled polymer composites form a relatively new family of materials,yet they are already well established in many applications. There is a vast range of materials in this category, some offering unique properties, some simply competing with other materials because of their relatively low cost. Their potential advantages are far from being fully realized and we anticipate continued growth in their use for many years to come. Research into these materials is crucial to their development and exploitation and will be for many years to come. Research continues into the design of short fibre reinforced composites and into the fundamental mechanisms that govern their behaviour, and also into methods of fabrication that will not only produce the required shape but will also result in the optimal properties being achieved.

Short fibre-polymer composites Short fibre-polymer composites Edited by S K DE and J R WHITE WOODHEAD PUBLISHING LIMITED CAMBRIDGE ENGLAND Published by Woodhead Publishing Limited, Abington Hall, Abington, Cambridge CBl 6AH, England First published 1996, Woodhead Publishing Limited 1996, Woodhead Publishing Limited Conditions of sale All rights reserved No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher While a great deal of care has been taken to provide accurate and current information, neither the authors, nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused, or alleged to be caused, by this book British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 85573 220 Cover design by the ColourStudio Typeset by Vision Typesetting, Manchester, England Printed by Galliard (Printers) Ltd, Great Yarmouth, England Contents Preface Contributors Survey of short fibre-polymer composites ix xi J R WHITE AND S K DE 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Introduction Composition Morphology of short fibre reinforced polymers Mechanics of short fibre reinforced polymers Measurement of fibre orientation distribution Properties of fibre reinforced polymers Conclusions References Short fibre filled thermoplastics 14 17 18 19 21 J R WHITE Introduction Materials Fabrication - injection moulding Properties Effect of fabrication on properties Conclusions References 21 Thermosetting short fibre reinforced composites 54 2.1 2.2 2.3 2.4 2.5 2.6 21 24 35 44 50 50 S B WlLKlNSON AND J R WHITE 3.1 Introduction 3.2 Curing characteristics 3.3 Thermosetting resin types 54 54 56 Contents vi 3.4 3.5 3.6 3.7 3.8 3.9 3.10 Fabrication methods Reinforcing fibres Fibre orientation Fillers and other additives to thermosets Residual stresses Properties of short fibre reinforced thermosets Conclusions References Short fibre-thermoplastic elastomer composites 61 64 66 70 73 77 81 81 84 G B N A N D A N D B R GUPTA 4.1 4.2 4.3 4.4 Introduction Classification of TPEs Short fibre-elastomer composites Parameters influencing the characteristics of short fibre-polymer composites 4.5 Short fibre-thermoplastic elastomer composites (SF-TPE) References Composites of polychloroprene rubber with short fibres of poly(ethy1ene terephthalate) and nylon 84 85 87 89 96 113 116 M ASHIDA 5.1 5.2 5.3 5.4 5.5 Introduction Preparation of composites Mechanical and viscoelastic properties Effect of absorbed water Dynamic fatigue of composites Acknowledgement References Properties and processing of short metal fibre filled polymer composites 116 116 117 131 135 142 142 144 D M BlGG 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 Introduction Metal fillers Polymers Electrical properties Thermal properties Mechanical properties Processing Effect of environment on properties References 144 146 149 149 156 156 162 165 166 Contents Electrically conductive rubber and plastic composites with carbon particles or conductive fibres vii 168 P B J A N A , A K MALLICK A N D S K DE 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 Introduction Percolation phenomena in conductive polymer composites Mechanism of electrical conduction Effects of processing factors on electrical properties Effects of polymer matrices on conductive network formation Effects of the types of filler, their geometry and morphology Effect of structural deformation on electrical resistivity Hydrostatic pressure effect on resistivity Temperature effects on volume resistivity Galvanomagnetic properties of conductive rubber composites EM1 shielding effectiveness References Design and applications of short fibre reinforced rubbers 168 168 170 172 175 175 176 177 177 179 182 188 192 M C H LEE 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 Introduction Materials Sample preparation Test procedure Stress-strain behaviour of unidirectional short fibre reinforced elastomer composites Fracture morphology, fibre configuration and deformation mechanism Stress-strain equation for unidirectional short fibre reinforced elastomer composites Modulus properties of unidirectional short fibre reinforced elastomer composites and applications Concluding remarks References Design considerations and end-use applications of short fibre filled rubbers and thermoplastic elastomers 192 193 193 194 195 196 202 202 207 208 210 A P FOLD1 9.1 9.2 9.3 9.4 9.5 Index Introduction 210 21 Background Technical/engineering basics: effect of fibres on matrix properties 218 Applications: proven and potential 226 Summary 253 References 254 257 Preface Short fibre filled polymer composites form a relatively new family of materials, yet they are already well established in many applications There is a vast range of materials in this category, some offering unique properties, some simply competing with other materials because of their relatively low cost Their potential advantages are far from being fully realized and we anticipate continued growth in their use for many years to come Research into these materials is crucial to their development and exploitation and will be for many years to come Research continues into the design of short fibre reinforced composites and into the fundamental mechanisms that govern their behaviour, and also into methods of fabrication that will not only produce the required shape but will also result in the optimal properties being achieved The book reflects this and is offered as an introduction to anyone starting out in research into short fibre-polymer composites It also provides the background and bibliography for further reading It is intended to be of value to industrial technologists who are working with these materials and are seeking further insight into their manufacture and behaviour It is also meant to inspire materials users to consider new applications for these composites - even perhaps to formulate new ones with different combinations of properties A special feature of the book is that it includes significant discussion on rubber-matrix fibre composites, an important sub-class of short fibre reinforced composites that is often neglected in reviews of polymer composites We are indebted to the contributors of the chapters for their co-operation We are grateful to them for letting us perform some editorial interventions with the aim of adding to the coherence of the text: we accept full responsibility if we have erred in this task! We wish to acknowledge also our students and colleagues at the Indian Institute of Technology, Kharagpur and the University of Newcastle upon Tyne who are responsible for stimulating and maintaining our interest in the area of short fibre filled polymer composites We are especially grateful to those students who have conducted research in this area under our supervision and whose work forms an important part of the chapters which we have authored X Preface This book would not have appeared without the efforts of Patricia Morrison and her colleagues at Woodhead Publishing We are indebted to our wives (Deya and Li Tong) for their expert technical help and advice during the preparation of the manuscript as well as for their patience and understanding Finally we are thankful to our children (Barna, Dominique, Michelle, Francine and Christopher) for their cheerful acceptance of one more task competing for our attention S K De J R White Short fibrepolymer composites 246 tread -belts - carcass plies - bead inner Ii sidewall er filler carcass turnup (b) \bead filler 9.21 Typical components of a radial tyre Design and end-use applications of short fibre filled rubbers 247 pull into the inner liner This usually manifests itself in its milder form as a visible indentation on the inside of the tyre; in its severe form, the carcass cords actually pull through the inner liner, thereby breaching its integrity To avoid this, short fibre reinforcement, with the fibre direction perpendicular to the carcass cords, is w r y beneficial Relatively small amounts, 5phr or less, of staple or floc (e.g inexpensive nylon) can completely eliminate the pull-through In some cases, even the thickness of the inner liner can be reduced without the threat of cord strike-through Both abrasion and cut resistance could be improved by incorporating the fibres in the tread pattern but this is not practical with current manufacturing methods The fibres would have to be perpendicular to the surface of the tread to be effective for improved tread wear and parallel to it for increased penetration resistance No efficient method is available for producing the precursor of the tread (sometimes called the ‘camelback’) with such fibre orientations, nor would the orientation remain undisturbed during the moulding and curing process ’ Belts d o not need fibre reinforcement nor is there any practical means of achieving it, especially since the rubber surrounding the belt cords is so thin To increase the lateral integrity of the belt, it is essential to orient the fibres perpendicular to the belt cords Otherwise, the fibres would cause additional heat generation without significant modulus improvement in the circumferential direction Further information on tyre components is provided by Goettler and Swiderski.2 Prevorsek and co-workers4’ undertook a study to measure the heat generation rate of rubber reinforced with short nylon and polyester They found, as expected, lower heat generation with the lower modulus fibre at fixed strain amplitude What makes their paper interesting is that they developed a three dimensional finite element analysis technique Very little has been reported on the use of short fibre reinforcement in the exterior panels of automobiles General Motors did some theoretical investigations, reported by Kia.48 Glass was used as the reinforcing fibre The main problem was that the fibres close to the surface left a visually perceptible mark, called the readout The readout is basically deformation of the surface with a texture similar to that of the underlying fibres The magnitude of the readout depends on the thickness of the fibre With glass fibres of less than 10-20 pm in diameter, the readout did ‘not cause cosmetic problems’ 9.4.7 Miscellaneous end uses Medium to light coated fabrics, including articles formed from them (e.g diaphragms), can benefit from short fibre reinforcement The objective is to reduce the end count of expensive twisted cords or strands in the fabric and substitute short fibre reinforcement in the sheeting material to close the gaps so generated A combination of coated fabric and hose resulted in flexible couplings of low pressure, large diameter pipes, such as used in sewage lines The flangeless cast 248 Short fibre-polymer composites iron pipes are joined by rubber sleeves which are secured by stainless steel hose clamps on the respective pipe ends These couplings - having to withstand some hydrostatic pressures and the resultant pull-out forces, as well as the lateral shear forces caused by pipe misalignment - are thick walled Yet, during installation this thick sleeve has to be stretched over the pipe, an especially difficultjob in cold eath her.^' The sleeve could be made lighter, thinner and still easily manageable with short fibres incorporated circumferentially The same kind of clamps could be used, but a size smaller The use of fibre reinforcement in dock fenders and methods to fabricate them has been discussed by Goettler and Swiderski.28 This is an ideal application because the manufacturing method lends itself to easy implementation of the preferred fibre orientation, i.e around the circumference of a tubular fender Sheet roojing, which usually is an unvulcanized EPDM compound or similar, can benefit greatly from short fibre reinforcement Such roofing material is generally applied to industrial buildings with large roof areas Therefore, just the placement of large, long pieces of rubber sheeting can put considerable stress on the sheeting In addition, any large unevenness in or on the roof will allow the unvulcanized material to creep, to the point that it will fail Tensile reinforcement with short fibres is a practical solution to these problems A similar but often overlooked solution is in green (1.e uncured) rubber stock that is so weak that its work-up on the mill becomes impossible without reinforcement Here the fibre reinforcement is just a temporary processing aid Typical examples are: more than 10 times improvement in green yield strength of a Sol50 SBR-NR stock with phr nylon or polyester staple of mm l e r ~ g t h ~ , ’ ~ Seals and gaskets are a potentially large market for short fibre reinforcement The ‘border’ between seals and gaskets is somewhat blurred Usually, seals are relatively soft, pliable, mainly elastomeric, have three dimensional, sometimes intricate shapes, and are used around moving parts Gaskets are harder, contain less than 20% elastomeric ‘binder’ and are usually flat, being used between flat flanges, engine blocks, etc Both are pre-manufactured off-site to the desired shape (Sealants, discussed below, sometimes called formed-in-place gaskets, perform similar functions to gaskets but they are dispensed in a viscous liquid form at the site of use and harden later.) Seals and gaskets are not only important in fluid transfer and chemical/petroleum manufacturing, but also play a very important role in automotive engineering5’ and many other industries and uses too numerous to list Their functions are also numerous: they have to seal, i.e prevent fluids (liquids, gases and vapours) from leaking between two, not perfectly mating, surfaces; they have to allow motion between these surfaces whether caused by temperature variation or by vibration or by rotating shafts; they have to maintain resilience for long periods of time and under adverse ambient conditions; they have to distort under pressure to conform to the irregularities of the mating surfaces yet resist ‘extrusion’ under pressure; and they have to withstand chemical attacks and extreme temperatures Fortunately, individual seals and gaskets not have to meet all these requirements, just most of them - Design and end-use applications of short fibre filled rubbers 249 What short fibre reinforcement offers to seals and gaskets is excellent creep resistance, especially under elevated temperatures (only with heat resistant fibres: p-aramid, glass, asbestos) Gaskets are made with two methods: compressed (C) gaskets which are calendered from a solvent-based mixture, and beater addition (BA) gaskets which are produced on machines similar to those of paper-making technology Originally, asbestos was the main fibre choice for gasket reinforcement, but recently, health considerations have necessitated other choices Cellulosic fibres provide sufficient strength and modulus for ambient temperature service but the fibres degrade at 200 "C Glass has the thermal stability and the high strength/modulus but it tends to break up in high shear mixing, thus reducing its reinforcing efficiency Beside the relatively high price, the disadvantage of p-aramid is its tendency for high static electricity build-up (a consideration to be taken very seriously when working with flammable solvents) Frances has presented a detailed paper on the replacement of asbestos in gasket application^.^' He found that among the three parameters used in judging gasket performance (tensile strength, measured according to ASTM F152; gas sealability, ASTM F37 Method B; creep relaxation or stress retention, modified ASTM F38), creep-relaxation was the most critical He concluded that p-aramid pulp at greater than 5% levels far exceeded the sealability requirement ofboth BA and C asbestos gaskets; that about % p-aramid pulp was needed to achieve the tensile strength (- 23 MPa) of the high performance C asbestos gaskets (BA asbestos gaskets are 13 MPa); that % p-aramid was needed to equal the stress retention performance of C asbestos gaskets at 300 "C,and less than 3% to match the BA asbestos gasket performance Based on these studies, he concluded that p-aramid could replace asbestos economically in the high performance, high price C gasket market, yet economically it could not compete with asbestos in BA gaskets A manufacturingguide is available for using Kevlar" pulp in gasket ~heeting.'~ As mentioned above, seals are usually used on contact with moving, rotating parts It is therefore important not only for the seal to exhibit high abrasion resistance, but also to impart little wear to its mate Watson and ~ o - w o r k e r s ~ ~ * ~ ~ studied these effects in conjunction with short p-aramid fibres and found that 15 phr Kev1ar.Y.pulp in a neoprene compound reduced the coefficient of friction from 0.55 to 0.25 and increased the NBS abrasion resistance from 243% to 436% in the on-end orientation direction In the ASTM D-3702 thrust washer test, 10 wt% lOmm long Kevlarg staple in a copolyester resin resulted in a weight loss of only 0.7 mg of the steel washer.53With thermoplastic nylon 6,6,20 wt% of 6mm Kevlar-49 staple produced a wear factor reduction from 917 to 239, a coefficient of friction decrease from 0.435 to 0.390, and a steel washer wear from 0.2 to