Pultrusion for Engineers

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PFEPR 9/22/2000 7:18 PM Page i

Pultrusion for engineers

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Pultrusion for engineers Edited by Trevor F Starr

Cambridge England

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Published by Woodhead Publishing Limited, Abington Hall, Abington Cambridge CB1 6AH, England www.woodhead-publishing.com Published in North and South America by CRC Press LLC, 2000 Corporate Blvd, NW Boca Raton FL 33431, USA First published 2000, Woodhead Publishing Ltd and CRC Press LLC © 2000, Woodhead Publishing Ltd The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from the publishers. The consent of Woodhead Publishing and CRC Press does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing or CRC Press for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN 1 85573 425 7 CRC Press ISBN 0-8493-0843-7 CRC Press order number: WP0843 Cover design by The ColourStudio Typeset by Best-set Typesetter Ltd., Hong Kong Printed by TJ International Ltd, Cornwall, England

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For Mary, though an inadequate recognition for over 20 years support and encouragement in the activities of Technolex

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Contents

Preface Contributors Pultrusion terminology Pultrusion and associated companies

xi xiii xviii xxx

1

Composites and pultrusion trevor f starr and jaap ketel

1.1 1.2 1.3 1.4

Composites Pultrusion Summary References

1 8 17 17

2

The pultrusion process david shaw-stewart and joseph e sumerak

19

2.1 2.2 2.3 2.4 2.5

Machine design and operation Tooling and allied design Conclusion Summary References

19 49 64 65 65

3

Profile design, specification, properties and related matters david evans

66

Introduction Common pultrusion materials Profile: design Profile: specification and production Profile: property prediction Process characteristics

66 66 67 78 84 90

3.1 3.2 3.3 3.4 3.5 3.6

1

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Contents

3.7 3.8

Conclusion References Appendix: important standards

90 91 92

4

Thermoset resins for pultrusion ben r bogner , walt v breitigam , mike woodward and kenneth l forsdyke

97

4.1 4.2 4.3 4.4 4.5

Polyester and vinyl ester resins Epoxy resins for pultrusion Acrylic resins for pultrusion Phenolic resins for pultrusion References

97 125 147 155 171

5

Reinforcements for pultrusion james v gauchel, luc peters and trevor f starr

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5.1 5.2 5.3 5.4 5.5

Introduction Fibre manufacture and characteristics Reinforcement handling Summary References

175 175 195 196 196

6

Pultrusion applications – a world-wide review trevor f starr

197

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17

Introduction Airport Cableways Cooling towers Fencing Flooring and walling systems Kolding bridge Leisure Optical fibre tension/support member Railways Rock-soil support applications Selection – customer profiles Stagings and walkways The ‘Eyecatcher’ Troll Phase One Vehicle body panels Water and sewage treatment plant

197 198 200 204 204 207 209 212 214 215 218 220 222 222 225 227 228

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Contents

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6.18 6.19

Conclusion References

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7

Infrastructure – a positive market brian wilson

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7.1 7.2 7.3 7.4 7.5 7.6 7.7

Introduction Design considerations Machining, fastening and finishing systems Cleaning, inspection and repair procedures Case-history applications Conclusions References

231 232 234 241 244 261 262

8

The future – beyond 2000 w brandt goldsworthy

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8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

Introduction Machine, die and profile design, size and capacity issues Reinforcement and related issues Matrix issues Potential for fibre architecture development Profile development – issues and potential Application potential Processing potential Summary

264 264 275 278 281 286 291 298 300

Index

301

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Preface

There are many people who must with sincere gratitude be acknowledged in the preparation of this book. There are simply too many to name, from Peter whose enthusiasm for reinforced plastic composites converted me 30 years ago from metallurgy, through many other friends and colleagues from that now world-wide industry, to those who are also chapter authors. That is of supreme importance, for without their respective contributions and support, even the somewhat lengthy gestation of this book – and for which apologies are due – would have been impossible. All have played a part in enriching my life and not least in respect also of the technology of composites, which for each of us continues to offer a future that is not just exciting but also unquantifiable. Pultrusion is just one sector of that composites industry which has steadily developed and grown over the last 60 years so that in total it now produces annually some 5.5 million tonnes of saleable moulded composite product valued at around US$140 billion. Authoritative forecasts see those figures approaching 7 million tonnes and US$203 billion just a few years into the 21st century. Pultrusion now commands a respectable share of that total market, although that attributable to North America continues to surpass by several factors that for Europe or indeed any other heavily industrialised world region. These high-performance close-dimensioned composites products, or pultruded profiles known generically as pultrusions and available in both standard and often complex custom-moulded form, are now offering increasing and serious competition to reinforced concrete, steel, aluminium and other metal-based as well as timber and thermoplastic cross-sectional products, manufactured in either discrete or continuous length. Like composites generally, pultrusions have already confirmed their worth, their costeffectiveness and their excellent and consistent mechanical, physical and environment-resistant properties. Moreover, these profiles in both small and massive section are now available from stock from a wealth of large, medium and even small concerns who, at considerable capital investment, xi

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have taken the decision to specialise in the technology and application of pultrusion. The publication of this book is therefore timely. I hope it is a volume that also meets the objectives of being readable, of interest and value, not just to those who practise the technology of pultrusion or who have concern generally with composites but, more importantly, to the many professional disciplines, civil and structural engineers, architects, designers, specifiers and purchase managers, who over recent years have made or should now be making, increasing and better use of pultrusions. Being well illustrated and with case-history examples extending over a wide engineering spectrum, the volume is equally addressed to the university student and graduate requiring a condensed authoritative description of the theory and practice of pultrusion technology. If all these aims are realised, then those who in their separate ways have contributed to the pages that follow will feel more than justified for the time they have devoted to that endeavour. Most have a day-to-day concern with pultrusion, but without question all are fired by the challenge they are being increasingly offered. This needs just one illustration. The high-performance demands of the civil engineering infrastructure market, which were previously the province of steel or reinforced concrete, are now being increasingly answered to advantage by the pultrusion industry. It is, however, also a situation demanding greater potential user attention and this book is seen as complementary to that task. An improved awareness, leading to better recognition and then acceptance of what composites – and in turn therefore, pultrusions – have as unique engineering materials to offer, has over recent years become a growing directive of the composites industry. Without those three vital education stages, that opportunity cannot ever be fully realised. Through wider specification and performance databases, their particular moulding or fabrication route and their product characteristics, pultrusions undoubtedly lead in this important and vital task. Finally, the interest and support of the publisher and his staff must receive a grateful acknowledgement from all who have been concerned in its preparation. Trevor Starr C Eng FIM

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Contributors

Ben R Bogner Ben is a Research Associate at the BP Amoco Research Center in Naperville, Illinois, where he has worked for the past 13 years. He has over 25 years’ experience in developing composite materials with a special emphasis on pultrusion, other composite processing techniques and composite testing. Ben holds a Master’s degree in chemical engineering from Illinois Institute of Technology and is a Registered Professional Engineer (PE) in the State of Illinois and a Chartered Engineer (C Eng MIsntE) in the United Kingdom. He holds six patents and has presented over 25 papers on the use of composite materials and composite processing. His current research work includes the creep testing of fibreglass structures and pultruded beams, the establishment of test standards for buried composites structures and finally the development of composite materials for infrastructure application. Walt V Breitigam Walt, an MSc graduate in organic chemistry from the Ohio State University, is a Senior Staff Research Chemist in the Resins Department at the Westhollow Technology Center of the Shell Chemical Company located in Houston, Texas. In this position he has been involved in new product development and customer technical support since 1976, and a responsibility that has been focused on epoxy resins and their curing agents, epoxy vinyl esters, bismaleimides and waterborne resins for adhesive, structural and composite application. In addition to numerous patents and publications on the use of these thermoset resins, Walt, who remains an active member of SAMPE, SPI, CFA, TAPPI and ASTM, is the recipient of two best paper awards, one from SAMPE and the other from an SPI RP/CI conference. David Evans As Operations Director for Creative Pultrusions Inc, Dave has been closely involved with a wide spectrum of pultrusion activity – for example, product xiii

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and system development and quality control – for the last 27 years. He currently oversees the operation of Creative’s facility at Roswell in New Mexico and is also Vice-Chairman of the Pultrusion Industry Council, of which he has been a member since its inception. In addition for the last six years, Dave has chaired some portion of the Technical/Environmental Committee working on the preparation of pultrusion standards and other authoritative publications. Ken L Forsdyke Chartered Chemist, Fellow of the Institute of Materials and Member of the Royal Society of Chemistry, Ken has spent 37 years in the polymer industry. After some 16 years in thermoplastics, he moved to thermosets and control of a technical services laboratory for phenolic resins. Here he was responsible for the development of low-temperature cure phenolic resins and catalysts for composites, insulation and floral foam systems. Of the grades developed for all the modern composites processing techniques, phenolic pultrusion was one of the first. This led to a specialisation in composite materials generally and in 1990 Ken founded FORTECH, a consultancy practice that, with world-wide clients and expert witness experience, embraces all aspects of thermosetting composites processing and application, but which retains a special affection for phenolics. FORTECH acts as Secretariat of the UK Composites Processing Association and Ken is an active member of the committee of the South Wales Polymer Group of the Institute of Materials. James V Gauchel For the past 32 years Dr Gauchel has been concerned with the process and performance aspects of composite material systems. His experience ranges from the development of new resins and fibres for high-performance US Navy and other military applications, to the optimisation of commercial building materials for domestic and international housing schemes. Since 1980 Jim has been the pultrusion process expert for Owens Corning in North America. In that position he has developed improved products and process, work that has resulted in the authorship of over 20 technical papers and 8 US patents, two of which are directly related to improved pultrusion processing techniques. W Brandt Goldsworthy Often called with affection the ‘grand-daddy’ of pultrusion, this biography is also proud to recognise that during 1999 Brandt was invested as a Knight of the Order of the Crown by the Belgium monarchy. This accolade recog-

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nised not just Brandt’s contribution to the growth of composites technology in Belgium, but his lifelong involvement in the field of artificial materials such as his invention in 1949 of a continuous, automated composite manufacturing process called ‘pultrusion’. Then there was the fibreglass surfboard, other customer recreation products, such as fibreglass fishing rods, to say nothing of his contribution to the design and development of the first fibreglass automotive body and the use of composites in airplane fuselage construction. Since completing his education in Mechanical Engineering at the University of California, some 60 years ago, Brandt has authored more than 112 publications, been awarded 50 US patents many with international recognition and received some 21 industry awards. Jaap Ketel After studying structural engineering in the 1960s, Jaap continued with Business Administration and then majored in Industrial Marketing, before spending four years in Australia selling epoxy-glass pipe systems worldwide. Following a serious car accident in 1977, he established his own trading company Ketech BV to sell Koch fibreglass pipe systems and Hobas centrifugally cast pipes throughout Europe and the Caribbean, a business that was disposed of in 1985 in favour of a composites consultancy practice which specialised in both composite pipe systems and pultrusion technology. The latter soon became the European Pultrusion Technology Association (EPTA) and with offices in the Netherlands currently has an active membership of some 100 companies in 30 countries world-wide. Luc Peters After gaining experience in the manufacture of paper towels and associated materials, Luc joined Owens Corning’s European Research & Development Laboratory at Battice in Belgium, where he was responsible for the development of specific software for the testing of composites. He then worked as technical support engineer concerned with preforming, centrifugal casting, resin transfer and sheet/bulk (SMC/BMC) moulding and pultrusion, composites fabrication processes. Successful programmes led to the qualification of Owens Corning reinforcements in numerous applications ranging from SMC vehicle body panels, bumper beams and lighting poles. Luc has contributed to a number of technical papers concerned with all those processes, and with reinforcement fabrics and pultrusion. David Shaw-Stewart David studied mechanical engineering at Loughborough University followed by a year’s postgraduate course at Imperial College. While working

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in the machine tool industry, a further year’s study at Cranfield University led to an MSc. After working at Ferranti Limited in the burgeoning numerical control industry, he joined the ‘brain drain’ to California, gaining experience in the aerospace, toy and electronic industries before finally joining Goldsworthy Engineering in 1969 as a project engineer responsible for the design of advanced composites processing machinery. On returning to England in 1973 he was a co-founder of Pultrex Limited, where he assumed responsibility for the design and development of the company’s complete range of pultrusion and filament winding equipment. He has served two terms as Chairman of the European Pultrusion Technology Association, a position relinquished in 1998. Trevor F Starr Metallurgical graduate, Chartered Engineer and Fellow of the Institute of Materials, Trevor has for over 30 years had a close involvement with the world-wide composites industry, particularly through the UK-based consultancy practice, Technolex, which he founded in 1978. In addition to wideranging client assignments, Technolex has compiled several directories and databooks covering the raw materials used by the composites industry and has also been proud over the past decade to prepare three editions of a statistical and authoritative profile of the global composites industry for Elsevier Science. A well-known speaker at many international conferences, in 1985, Trevor was largely instrumental in establishing what has become known as the World Composites Institute and for nearly 10 years after its inception in 1989, Technolex acted as Secretariat to the major authoritative body of the UK composites industry, the Composites Processing Association. Joseph E Sumerak A graduate of Case Western Reserve University, Joe’s introduction to pultrusion was in 1975 as Process Engineer with the Glastic Company which he left five years later to establish Pultrusion Technology Inc, suppliers of processing machinery, tooling and characterisation instruments. Since then over 150 turnkey projects have been completed world-wide. In 1992 Pultrusion Dynamics Inc was founded to pursue advanced research topics in the area of heat transfer and pultrusion process modelling, work which has led to new techniques for off-line and on-line process optimisation. In 1997 Pultrusion Dynamics was acquired by Creative Pultrusions Inc to strengthen, through their Tecnology Center, the sale of pultrusion equipment, tooling, technology products and services. As President of the Pultrusion Dynamics Division and Vice President of Technology for Creative Pultrusions, Joe writes and lectures widely on pultrusion processing topics.

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Brian Wilson Employing knowledge and expertise gained from over 32 years in the composites and aerospace industry with such major companies as Rockwell, Brunswich & Aerojet, civil and mechanical engineering graduate Brian Wilson established the Wilson Composite Group in 1988. Under his guidance as President, that company offers expert witness and consultancy in manufacturing technology to the composites industries throughout the United States, Europe and Japan. Theirs is a particular specialisation in the infrastructure market sector, in pultrusion and in the organisation of highpowered seminars dealing with topics of vital importance to the future of the composites industry. These attract captains of the now international composites industry as both speakers and delegates. Mike Woodward Graduate in Chemistry from Liverpool (John Moores) University, Mike spent almost 20 years working on fire-retardant additives for polyester resin and associated projects as part of the Research and Development Department of Imperial Chemical Industries. This eventually brought him into contact with the MODAR methylmethacrylate–urethane resin project and its early commercialisation for ‘closed-mould’ and pultrusion fabrication techniques, an involvement which ultimately resulted in a responsibility for technical support throughout the UK, Italy and Benelux countries. Since the purchase by Ashland Composite Polymers of the MODAR business in 1993, Mike has added to that responsibility all European countries and in turn a much closer concern with the manufacture and application of pultrusion technology.The latter has brought with it an emphasis on fire, smoke and toxicity and the associated European standards and testing procedures.

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Pultrusion terminology

This glossary was initially compiled for the use of the Composites Processing Association, by Trevor Starr when Secretariat of that UK authoritative body. It has subsequently been developed and also substantially employed in three editions of Composites – A Profile of the Reinforced Plastics Industry, Markets and Suppliers (Elsevier Advanced Technology). It is reproduced with full permission and acknowledgement. This basic explanation of the more common terms specifically employed by the pultrusion sector of the composites industry aims to supplement or explain the descriptions to be found elsewhere in the text.

Matrix and related Accelerator

A chemical, usually zinc or cobalt ‘soaps’, or tertiary amines and sometimes called a promoter, which is added in small percentage to a mixture of thermoset resin and catalyst to speed up the curing reaction at room temperature. Elevated temperature cure does not require the addition of an accelerator. May be premixed by the resin manufacturer.

B-stage

A partial cure stage, where the resin matrix is solid but still flexible and workable.

Catalyst

An active reagent often called the hardener, promoter or curing agent which causes thermoset-based matrix resins to cure. They are typically organic peroxides in the form of a paste or liquid dispersions in a plasticiser, and added in small percentage by the fabricator.

Fillers

These usually consist of fine inert powders which are inorganic in nature (marble and silica flours, aluminium oxide and silicate, talc and pumice), added in limited

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percentage to the matrix to reduce costs. Under certain circumstances they may also enhance certain mechanical and physical properties. Fillers should not be confused with pigments or fire-retardant additives, although they may colour the resin and be advantageous to both properties. In certain specific applications, the filler addition may be considerably increased to the total exclusion of reinforcement in, for example, the manufacture of simulated marble and polymer– concrete products. Fire retardancy

There are two basic methods by which the fire performance of composites is enhanced. Either the thermoset or thermoplastic matrix may be chemically modified, or inorganic or organic fire-retardant additives, typically in the form of fine powders, may be incorporated in the formulation. Although a wide variety of fireretardant gel-coats and matrix resin systems are available to satisfy different authoritative fire performance standards, it is not unusual for the thermoset fabricator to formulate his or her own special-purpose grades and then supply respective fire hardness data. On the other hand, phenolic resins are intrinsically firehard, requiring no modification or additives to achieve a very high standard of fire performance. As a consequence their smoke and smoke toxicity are much reduced in comparison with other thermosets and most thermoplastics.

Gelcoat

Typically a thin (0.40–0.90 mm) layer of unreinforced, normally polyester resin modified by the supplier to alter the rheological properties and applied by brush or spray directly to the released mould-tool surface. Gelcoats must be allowed to polymerise fully before the thermoset-based laminate is constructed. They enhance the surface sealing of the glass fibre reinforcement and therefore typically also provide the decorative finish to the moulded component. As they are more often than not pigmented, they provide, as well as this self-decorated colour, a hard surface, resistant to weather, chemical-corrosion attack, etc, to the extent dictated by the grade or type employed. Optically stabilised and fire-retardant grades with enhanced resistance to ultraviolet radiation attack, and

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Pultrusion terminology respectively, fire, are also readily available. Thermoplastic-based composites are not gel-coated, but may be decorated by a post-moulding paint treatment, as indeed may also thermoset-based components particularly if they employ phenolic resins.

Inhibitors

Chemical additives employed to prolong the storage or shelf-life of thermoset resins, disregarding whether the resin is accelerated or non-accelerated.

Matrix

The component of a composite which surrounds the reinforcement. In other words, the unchanged, unalloyed portion of the homogeneous composites moulding, in which the fibrous reinforcement is encapsulated. It gives solid form to the finished component and confers its durability to the strength properties of that reinforcement.

Ortho-, iso- and tere-phthalic acid

Chemical terms used to classify unsaturated polyester resins, where the latter is superior to the former.

Polymerisation

A crosslinking process, building long-chain molecules.

Pot-life

A time in minutes, which denotes the length of time (at a particular temperature and accelerator addition condition), that a catalysed thermoset resin can be processed. In other words, with increasing polymerisation of the system from a liquid to a solid, the viscosity increases to a point at which the resin can no longer be worked, nor effectively wet-out the reinforcement.

Thermoplastic

Matrices that are capable of being repeatedly softened – and therefore reworked – by an increase in temperature, being restored to the original condition when the temperature is reduced.

Thermoset

Matrices which once changed by polymerisation from the initial viscous liquid condition, become an irreversible, infusible and environmentally resistant insoluble solid.

Mechanical and physical properties Anisotropic

Not isotropic, having different properties along axes in different directions.

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Delamination

The failure of a laminate caused by the splitting, separation, or other loss of bond between respective layers of reinforcement or plies, caused by the action of some imposed load or stress.

Elastic limit

Denotes the highest stress, or load, that a laminate is capable of sustaining without permanent stress remaining once that load is released. The elastic limit is said to have been exceeded when the load is sufficient to cause permanent deformation or damage. Composites behave somewhat differently from, for example, metals, in respect of the stress/strain relationship.

Heat distortion

The temperature at which a standard test bar deflects a specific amount under a given load. In other words it delineates the temperature that must not be exceeded by a particular resin system.

Isotropic

Having uniform mechanical properties in all directions.

Modulus

A measure of the stiffness or rigidity of a material which is independent (E-modulus) of the geometric shape of the component. The numerical value is obtained by dividing the stress by the strain, when a specimen is loaded within its elastic limit. The terms tensile and flexural modulus relate to the type of stress applied.

Specific modulus

The modulus value divided by specific gravity or density in consistent units.

Specific strength

The ultimate tensile strength (UTS) divided by specific gravity (or density) in consistent units which take into account the effect of gravity.

Specific stress

The ratio of the force to the mass per unit length and equal to the stress per unit density.

Production techniques and materials Barcol

See Hardness.

Composite

A material consisting of two or more different constituents which retain their identity, when combined together to provide properties unobtainable with either constituent separately.

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Core

A range of materials consisting of foam, honeycomb, balsa wood or bonded fibre fabrics employed to form the central portion of sandwich constructions. Core materials are employed in a cost-effective way to add stiffness to a laminate at a low weight increase compared to employing additional laminate thickness.

Cure

A layperson’s term for the time/temperature related molecular crosslinking process – known more correctly as polymerisation – which changes a thermosetting resin from the liquid to solid state, following chemical activation by a catalyst, and an irreversible reaction possibly promoted by the addition of an accelerator. No by-products are formed during the formation of these long molecular chains.

Exotherm

A term applied to the heat which is evolved during the polymerisation of a thermoset. Care has to be taken to ensure that this approximates to that which at the same time is lost to the surrounding tool and environment. If not, then there is a danger that in overheating, the polymerising composite will exceed the combustion temperature of the resin. In other words it is essential to achieve a careful balance between such factors as catalyst and accelerator addition and type, tool or environmental temperature, and the moulding mass.

Fabricator

A composites component manufacturer or moulder.

Flow

The movement of the thermoset or thermoplastic resin matrix during moulding.

FRP

An acronym which more correctly stands for fibre reinforced polymer, but is often taken to mean fibreglass reinforced plastic.

Gel-time

The number of minutes, following catalyst (and if applicable accelerator) addition to the thermosetbased gel-coat or matrix, before it assumes a soft-gel condition, a term which is applied to the initial jellylike condition consisting of a network of solid aggregates held in a liquid phase. Not to be confused with pot-life, the gel-time is equally dependent on the type and percentage of those additions, as well as on the temperature at which the system is held.

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GFRP

An acronym for glass fibre reinforced plastic.

GMT

An acronym for glass mat thermoplastics, a classification for glass fibre reinforced thermoplastic sheets which in a process akin to steel-pressing, can be stamped at a high compressive pressure and speed to the required 3D shape.

GRP

An acronym, glass reinforced plastic, which more correctly should be glass fibre-reinforced polymer.

Hardening time

The time in minutes from the appearance of a soft-gel polymerised condition (see gel-time) with the matrix resin, to the point where it has become sufficiently solid to allow the moulding to be withdrawn from the mould.

Hardness

Determined by a hand-held indenter, the numerical Barcol Hardness value provides some indication of the all-important cure condition of all thermoset resin composites with the exception of those based on phenolic systems.

Honeycomb

A structure, typically of continuous hexagonal-shaped cells formed, for example, from paper, aluminium and other metal foils, and used as a core material in sandwich constructions. Different cell sizes and overall thicknesses (and also of the web) are available.

Laminate

Although typically applied to the total thickness of a composites moulding, it more correctly applies just to the moulded assembly of plies (i.e. the fibre reinforced portion) when manufactured by hand, spray, cold/ warm press, resin injection or vacuum bagging/autoclave techniques. Strictly speaking, the laminate does not include the gel-coat, flow-coat or any other feature of the overall composites construction.

Lay-up

The description of the components and arrangement of the reinforcement in a laminate.

PMC

An acronym for polymer matrix composites.

Post-cure

The additional processing of a composites component at an elevated temperature (typically 40–80 °C for several hours, in the case of glass/polyester-based formulations) to ensure a complete theoretical develop-

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Pultrusion terminology ment of the molecular crosslinked structure or cure (polymerisation), of the resin matrix. By this means the full mechanical and physical properties applicable to the particular resin matrix employed are attained.

Post-forming

A process whereby thermoplastic-based composites are locally reheated and subjected to an additional forming process not possible in the original tooling. Certain thermoset-based systems can also undergo a degree of similar re- or further working, given that the initial polymerisation has not proceeded beyond the B-stage.

Prepregs

Not to be confused with premix compounds, these are another form of pre-impregnated ready to mould materials covering a variety of roving, unidirectional, knitted or woven fabrics, compounded with an optimum quantity of an equally wide variety of thermoset and now, but to a lesser degree, thermoplastic resin matrices. They tend to be more sophisticated Bstaged compounds than premixes, used therefore for higher-performance application and while many are moulded by hot-press, compression moulding techniques, many others are more suited to autoclave fabrication. Although capable of handling and shipment they have a limited storage, useful moulding life.

Release agent

Aqueous or solvent-based polymers, applied by brush or spray to the mould-tool surface, which on drying act as parting-agents to ensure a clean, easy and undamaged removal of the moulding. They may also be waxed-based materials, or pure waxes, applied by hand with a cloth. Alternatively the high-capital fabrication techniques may for example employ zinc and other stearates added as a powder to the uncured resin fraction of the composite, such that they migrate to the mould-tool surfaces through the action of the heat applied to polymerise the resin.

Resin-rich

A term that denotes that the whole or a local area of a moulding does not contain the specified quantity of reinforcement. In other words, a situation where there is an excess of resin over the requisite resin : reinforcement ratio required to give optimum laminate properties.

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RP

An acronym for reinforced plastic, the term by which composites were originally known.

RTP

An acronym for fibre reinforced thermoplastics. Although all versions are eligible to be classified as composites, as far as the composites industry itself is concerned, the use of the word composites tends largely to be restricted to any fibrous addition longer than 6 mm irrespective of matrix.

Wet-bath

A suitably sized tank holding liquid resin through which the reinforcement passes and during which that reinforcement becomes impregnated with that resin matrix.

Wet-out

The speed with which the matrix is completely absorbed by the fibre reinforcement. In other words the speed with which that matrix completely replaces the air from around each filament, any void within that reinforcement, or other part of the composites formulation.

Reinforcement Aramid

A high-strength, high-modulus fibre of highly oriented polyamide (nylon) incorporating an aromatic ring structure. Trade names are Kevlar and Nomex.

Binder

An emulsion or powder coating employed to bind the random chopped strands together in the manufacture of for example, chopped strand mat.

Carbon

A fibre reinforcement offering improved modulus and stiffness over glass, but typically at some 10 times (minimum) the cost. Cheaper versions at only five times the cost, or even less, are becoming available. A wide variety of grades (standard, intermediate, high and ultra-high modulus) are produced by the pyrolysis in an inert atmosphere of organic precursor fibres such as rayon and polyacrylonitrile (PAN).

Catenary

A defect in a roving or tow, caused by uneven tension in the filaments or strands, resulting in some fibres hanging below the remainder when the tow or roving is stretched horizontally.

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CFRP

An acronym standing for carbon fibre reinforced polymer.

C-glass

A glass fibre formulation designed to have optimum corrosion resistant properties. The material should not be confused with CemFil AR or other alkali-resistant glass fibre formulations developed for the reinforcement of cement and gypsum.

Chopped strand Mat (CSM)

The glass fibre reinforcement most typically employed in the manufacture of commercial composites. Essentially it is an open-textured, chemically bonded, nonwoven fabric composed of a random distribution and orientation of chopped glass fibres, normally 5 cm long, although shorter fibre length versions are available. CSM is sold by weight per unit area. For example a 450 mat weighs 450 g per metre square.

Combined, blended or knitted fabrics

This group of reinforcement materials classifies those where two or more types of standard reinforcement are employed together in the form of a pre-prepared, blanket-type material. They are used principally for advanced, specialised and high-duty applications. It is feasible to blend or mix together, glass, carbon, aramid and other fibres to form an even wider variety of combined reinforcement.

Continuous filament (or strand) mat (CFM)

A reinforcement somewhat similar to chopped strand mat but, as the name implies, consisting of long continuous lengths of glass fibre overlaying each other in a totally random swirl-like pattern to form a more open textured, stronger reinforcement. Unlike chopped strand mat, it is difficult to handle and can only be employed in closed-mould fabrication techniques such as cold/warm-press, resin-injection moulding and pultrusion.

Continuous filament yarn

The fibre formed when two or more continuous filaments are blended together into a single continuous strand.

Count

A number indicating the mass per unit length, or length per unit mass, of a yarn.

Coupling and sizing agents

Complex chemical coatings applied during the manufacture of glass (and other) fibres to protect, size and/or lightly bind the individual filaments together and to

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subsequently promote or couple the adhesion between those filaments and the surrounding matrix. Some types of fibre may undergo an alternative post-fibre manufacturing treatment (but without a coating), which has the same objective. None of these treatments should be confused with the term Binder. Denier

The weight in grams of 9000 metres of roving, tow, yarn or strand.

E-glass

A glass fibre formulation based on calcium alumina borosilicate, but having a maximum alkali (combined sodium and potassium oxide) content of 1%, originally developed to have high resistivity and therefore suitable for electrical laminates, which has become the standard reinforcement for the vast majority of commercial components manufactured by the composites industry.

End

An individual roving, tow, thread, yarn or filament, especially in the warp direction.

Fibre

Material in the form which has a high length-tothickness ratio and is characterised by flexibility and fineness.

Fibreglass

The generic name for typically glass fibre reinforced/ polyester resin composites, although more correctly it refers to glass fibre insulation material.

Filament

A single fibre of indefinite length.

Fill

The end running across the width of a woven fabric, also called the weft.

Pick

An end in the weft (fill) direction.

Preforms

A handleable but open form of reinforcement, preshaped to the approximate contour and required thickness of the finished component, and typically composed of chopped fibres bound together with a binder soluble in the thermoset resin system to be employed. Preforms are used principally in resin-transfer moulding (RTM), as they permit accurate placement of the reinforcement which may also vary locally across the whole component.

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Reinforcement

Refers to that unchanged, unalloyed portion of the homogeneous composites moulding, which being of a fibrous nature, adds strength to the matrix in which it is subsequently encapsulated.

Roving

An untwisted assemblage of strands.

S (& R)-glass

In comparison with E-glass, these are highperformance glass formulations offering fibre with a superior modulus. They are therefore more specifically employed for the manufacture of composites for the aerospace and other advanced market sectors.

Spun yarn

A yarn consisting of fibres of regular or irregular lengths, usually bound together by twist.

Staple fibre

Fibres cut or broken into predetermined lengths, typically 30–480 mm.

Strand

An untwisted, compact bundle of filaments.

Strand count

Denotes the number of strands in a plied yarn, or roving.

Surface tissue

Although not strictly a reinforcement, these highly calendered bonded glass or polyester fibre tissues (or veils), can be incorporated at the gelcoat–laminate interface to enhance the environmental and chemical resistance of the former and hence the total moulding. To further advantage they reduce the possible incidence of reinforcement showing through that gelcoat.

Tex

The weight in grams of 1000 metres of roving, tow, yarn or strand.

Tow

A loose bundle of filaments, substantially without twist.

Unidirectional

A term where all the reinforcement is aligned in the same direction.

Warp

The end running lengthwise in a woven fabric.

Weft

The end running across the width of a woven fabric, often termed the fill.

Woven fabric

A wide range of weave patterns is available for composites reinforcement, such as plain, satin, leno and crowsfoot in a wide variety of weights, woven from yarns or fibres.

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Woven roving

Fabrics woven from roving and available in a range of weights, styles and grades, which can be considered as heavyweight reinforcement.

Yarn

A twisted bundle of strands.

Yield

The length of material equivalent to unit weight.

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Pultrusion and associated companies

This listing is not definitive and considers only those whose website address details have been provided, largely but not entirely, through the European Pultrusion Technology Association. COMPANY

WEBSITE

Ahlstrom Glassfibre Oy Axel Plastics Research Labs, Inc Blagden Cellobond BP Amoco Chemical Composites Worldwide, Inc Creative Pultrusions International Ltd Dow Deutschland Inc DSM•BASF Structural Resins BV European Pultrusion Technology Association Exel Oy (Kivara Factory) Fiberline Composites A S Fibreforce Composites Ltd Fibrmat Ltd Fibrolux GmbH Ghent University, Textile Department James Quinn Associates Ltd Martin Pultrusion Group Inc Menzolit-Fibron GmbH Neste Chemicals Technology Centre Nioglas, SL Owens Corning Inc Pas-Gon FRP Products Pera Technology Centre Powertrusion 2000 International Inc PPG Industries UK Ltd Pultrusion Dynamics Inc Reichhold A S Respia Ltda

www.ahlstrompapergroup.com www.axelplast.com www.cellobond.com www.amoco.com www.compositenews.com www.creativepultrusions.com www.dow.com www.dsmresins.com www.pultruders.com www.exel.fl www.fiberline.com www.fibreforce.u-net.com www.fibrmat.com/fibrmat/ www.fibrolux.com www.textiles.rug.ac.be www.users.rapid.co.uk/quinn www.net-ohio.nal/pultruder www.menzolit-fibron.de www.neste.com www.danigraf.com/nioco www.owenscorning.com www.pas-gon.co.il www.pera.com www.powertrusion.com www.ppg.com www.na-ohio.net/puldyn/ www.reichhold.com www.respla.cl

xxx

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Pultrusion and associated companies Skloplast A S Strongwell, Inc Technical Fibre Products Ltd Top Glass SpA Velio A S Vink N V

xxxi

www.conjuntim.sk/sklopast/ www.strongwell.com www.cropper.com/lfp.him www.topglass.it www.velio.com www.vink.com

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1 Composites and pultrusion TREVOR F STARR AND JAAP KETEL

1.1

Composites Introduction

‘Composites’ are arguably the world’s oldest materials that humans have made to answer their particular needs. The Bible, Exodus, Chapter 5, records the use of straw in the making of clay bricks and there is equally good evidence that the ancient Egyptians knew how to spin crude glass fibres and form them with naturally occurring resins into decorative articles. Both examples record utilising the principle of all composite materials, the ability to strengthen – or reinforce – a weaker or brittle material, by the simple addition of another having a fibrous nature (Fig. 1.1). Much later in history came ‘wattle and daub’, a loose or interwoven collection of either twigs (or, later, formed timber laths) coated with clay (but, eventually, plaster), which was extensively employed in the construction of walls and ceilings. Then came another composite, wrought iron, where the elongated fibrous slag inclusions effectively strengthened the surrounding pure iron matrix, by reducing its otherwise high ductility. Much more recently, and still an important and very necessary construction material, there is reinforced concrete. Here the internal critically designed, interlaced ‘fibrous’ network of steel rods, enhances the mechanical properties of the surrounding cement, sand and aggregate matrix, whose factory or on-site cast shape provides the desired building or construction component. Now, to some irony, that steelwork is beginning to meet a serious competitor in the form of a lightweight, non-corrosive reinforcement network constructed instead from ‘rebars’ based on profiles produced by pultrusion. That fabrication technique is just one of several well-established ways by which today’s ‘composites’ – formerly called reinforced plastics, and often known with growing generic inaccuracy as ‘Fibreglass’ or by such acronyms as ‘GRP’ (glass reinforced polymer) and ‘FRP’ (fibre reinforced polymer) – can be critically manufactured, or moulded, to shape. 1

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Pultrusion for engineers Reinforcement (indicative)

Elongated fibrous slag

Twigs or laths

Matrix

FRP composites

Pure iron (‘ferrite’)

Wrought iron

Clay or plaster

Wattle & daub

1.1 Typical composite materials

The wide application and track-record success of these composites, the development and commercialisation of which began barely 60 years ago, are therefore founded on the age-old, well-established and recognised technology of all composite materials: on the use of a fibrous reinforcement to markedly improve the properties of the matrix in which that reinforcement is distributed and contained. The result is a homogeneous – but totally unalloyed – mixture of normally only two, completely dissimilar materials which confer their distinct properties to each other without the loss of separate identity or characteristic. Thus in the case of these reinforced plastics, or composites, the unchanged fibrous reinforcement adds its immense strength to the durable, chemical and corrosion-resistant properties of a surrounding polymeric matrix which may be either thermoset or, increasingly over recent years, thermoplasticbased. Their steady and continuous development now boasts world-wide industrial importance and virtually continuous positive growth, typically well in excess of the respective country’s gross domestic product.

Market and application Although trading fluctuations obviously occur, that remarkable growth pattern is suitably exemplified for the United States in Table 1.1. Further, the discipline of composites technology, whether through manufacture or product acceptance example, has extended to virtually every corner of the world. The total 1998 output of that world-wide industry has been estimated as 5.5 ¥ 106 tonnes, valued at US$143 ¥ 109, rising respectively to 7.0 ¥ 106 tonnes and US$205 ¥ 109 by 2005.1 To provide just a number of diverse examples from a selection of well over 50 000 distinct components that have been identified, glass fibre reinforced thermoset-based composites,2 can be used for the moulding of modular designed construction-site housing and offices, as well as for per-

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Composites and pultrusion

3

Table 1.1. Growth of the composites industry in the USA over 25 years (tonnes ¥ 1000) 1983

1985

1988

1990

1995

1998

Finished product output

870

1006

1206

1168a

1440

1633

% growth from previous year

+2.5

+3.0

+4.8

+1.3

+4.3

+5.1

a

Growth fell in 1989 (-4.1%) and 1991 (-8.4%). Source: Composites Institute, The Society of the Plastics Industry, USA.

manent accommodation such as can be found, for example, near the Arctic Circle in Canada’s Frobisher Bay, or for smoothly contoured enclosures on tracked vehicles in the Antarctic. Equally they may find application for temporary–permanent concrete shuttering in the Far East; for switch-gear cabinets and railway carriage components throughout Australia; or in the assembly of complex medical equipment manufactured in the United States; for ducting, chemical processing equipment and building cladding panels in the UK or for pleasure craft plying the Mediterranean or for pipes and water storage tanks in the Middle East. When joined in that same quantification by thermoplastic-based versions, composites can be employed for automotive, truck and bus components irrespective of their country of manufacture; for a wide variety of armour and defence equipment produced by European suppliers, or as a final example, for the control surfaces ‘carried’ around the world by aircraft. This can all be further confirmed by Table 1.2 which demonstrates the current percentage share breakdown over the nine market classifications which for a number of years have been employed to quantify the US composites industry.

Reasons for using composites There are many reasons for the wide acceptance of composites by the professional architect, civil and consulting engineer, designer, purchase manager, specifier and other disciplines serving, for example, the aerospace, agricultural, defence, domestic, engineering, industrial, infrastructure, leisure and marine market sectors. The following can be considered as the ‘standard’ properties, typically exhibited by these FRP composites; the majority are equally applicable to pultruded profiles: • •

high strength at low weight; moulding to close dimensional tolerances, with their retention under inservice conditions;

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Table 1.2. Percentage market share in the USA for 1998 Aerospace/military: Cargo containers, control surfaces, internal fittings, window masks, galley units and trolleysa

0.6

Appliance/business: Covers, enclosures and fittings, frameworks and panelling

5.5

Construction: External and internal cladding, pre-fabricated buildings, kiosks, enclosures, structural and decorative building elements, bridge elements and sections, quay facings, signposts and street furniture

20.8

Consumer products: Components for domestic and industrial use, sanitary ware, sporting goods, swimming pools, notice boards, theme park items

6.3

Corrosion-resistant equipment: Chemical plant, linings, oil industry components, pipes and ducts, grid flooring, staging and walkways, process and storage vessels

11.8

Electrical/electronic: Internal and external aerial components and fittings, generation and transmission components, insulators, switch boxes, distribution poles and posts, ladders and cableways

10.0

Marine: Canoes, boats, yachts, workboats, window masks and internal/external fittings for ferries and cruise liners, buoys, lifeboat and rescue vessels, surf and sailboards

10.1

Transportation: Automotive, bus, camper, truck and vehicle components and fittings generally, land and sea containers, seating, railway signalling components, enclosures

31.6

Unclassified: Not otherwise classified

3.3

a Examples are indicative only. Source: Composites Institute, The Society of the Plastics Industry, USA.

• • • • • • • • •

good impact, compression, fatigue and electrical properties; ability to reduce part assembly markedly; excellent environmental resistance; ability to fabricate massive one-piece mouldings; proven in-service track record; low-to-moderate tooling costs; cost-effective manufacturing processes; ability to build in, ex-mould tool, both colour and texture decoration; particularly attractive in-service life costs.

The following additional properties can readily be provided by reinforcement and/or matrix alteration, chemical addition or other formulation, material or fabrication alteration: • • •

excellent chemical and corrosion resistance; high ultraviolet radiation stability; good-to-excellent fire hardness;

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Composites and pultrusion • • • • • •

5

good structural integrity; good thermal insulation; ability to attenuate sound; respectable abrasion resistance; ready bonding to dissimilar materials; medium-to-high productivity rates.

However, above all these properties, which are clearly attractive to the user, composites have one unique ability that is not possessed to the same degree by any other competitive material employed for the types of application examples quoted. The physical, mechanical and cost-effective properties of any reinforced plastic composite can be ‘tailored’ over a wide range to fully equate with the performance specification demanded. As already intimated in the above listing, this is simply a question of reinforcement, matrix or fabrication change, chemical or other addition. Those choices are often related in turn to the eventual application, the related environment and the call-off quantity, as well as the complexity and size of the required component. That uniqueness does, it needs to be admitted, also have one serious drawback in the particular context of securing potential customer recognition and ultimately new, formerly unrealised orders. Although employing first-quality raw materials, equipment, fabrication techniques and procedures having the backing of authoritative Standards and Codes of Practice, as far as the finished product supplied to the customer is concerned, there are, for obvious reasons, very few ‘standard’ composites. One of the few composite product exceptions are pultruded profiles, particularly if the special-purpose custom-moulded profiles are largely excluded. Indeed, the ability to supply the design engineer with comprehensive property data for a carefully identified profile section has been a major parameter in the acceptance of this form of composite moulded product by the professions and the other disciplines identified. Certainly, even the initial interest for the civil engineering/infrastructure, let alone the growing development of that whole market sector, would not have otherwise been possible. Nevertheless, there are several other additional disadvantages that can limit the use of composites, and these must be recognised: • • • •

poor ductility, particularly when compared with metals and considering those composites that are thermoset-based; low stiffness in comparison to many traditional and/or competitive materials; temperature is limited, with few exceptions, to be not in excess of 150 °C; limited recycling ability even when thermoplastic-based.

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Fabrication techniques Composite fabrication techniques range in nature from the low capitalintensive, high-labour content, to the exact opposite of high-capital, lowlabour content technique. In total, there are around 20 well-established techniques, and with only limited restriction it can be feasible to select that process that best satisfies the quantity, economics, dimensional, shape complexity and mechanical/physical property performance specifications demanded by the customer for a particular component or application. This wide fabrication ability applies particularly to those techniques that principally employ just glass fibre reinforcement and a thermoset matrix. Here typically both the composite and the finished component are manufactured and then moulded at the same time. While the latter situation applies to pultrusion (and also to another capital-intensive technique, filament-winding), other capital-intensive techniques, such as compression moulding at high pressure and elevated temperature, more usually employ a pre-compounded or prepared but uncured (i.e. not yet polymerised) composite. Thermoplastic-based composites are also more usually precompounded in this way, although in their case polymerisation will have already occurred. Collectively the diversity of fabrication techniques can easily complicate the correct understanding of what is in any case a complex technology,3 and this needs a brief review to complement earlier paragraphs and later chapters on reinforcement and matrix.

Contact moulding This simple but effective process on which the composites industry was founded continues to be very extensively employed, even though it is highly labour-intensive and therefore, by inference, prone to product quality problems. However, it remains ideal for prototypes, large one-piece components and those required in limited quantity. Open tooling, usually itself a composites fabrication, is employed, into which the reinforcement – in both mat and fabric grades – is laid together with the matrix resin. The two are then immediately consolidated by hand into a ‘laminate’, an action that, in removing unwanted air, ensures a total ‘wet-out’ of the reinforcement with the resin. Catalyst additions to the resin promote the polymerising liquidto-resin molecular structure crosslinking change which is common to all thermoset resins, following which the moulded component is stripped from the mould tool, trimmed or otherwise finished and subsequently despatched to the customer. It is worth emphasising that the application of a ‘resin-rich’ gelcoat to the tool surface prior to laying-in the reinforcement, provides on the working-

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face of the moulding, an environmentally resistant, coloured and decorative surface which, in accurately replicating the mould-tool surface, can also be low profile textured. In other words, there is no need for a postmoulding paint or other surface treatment although such can be applied depending on customer requirements. Like most composites, laminate thickness is in principle controlled by both the weight and type of reinforcement employed, with the mechanical and physical properties similarly governed as well as by the resin matrix and the resin : reinforcement ratio achieved. As an alternative to using mats or fabrics, the reinforcement – initially in the form of continuous rovings – can be mechanically chopped and ‘spraydeposited’ with the resin onto the open mould tool surface. Although specifically developed to facilitate the manufacture of massive mouldings, such as for example yacht hulls, process quality is even more dependent on operator skill than the more common contact moulding process just described. Resin injection Often referred to as resin-transfer moulding (RTM) or resin infusion, this ‘first-level’ capital-intensive manufacturing technique has increased in popularity over recent years. The process employs a matched male–female tool set (again typically of glass fibre/polyester or vinyl ester composites construction) and is less sensitive to the vagaries of contact moulding. It also benefits from the advantage of being able to utilise a much wider range of reinforcement successfully and, as the name implies, once the reinforcement has been placed in position and the tool closed, the matrix resin is injected at ‘low’ pressure into the tool cavity. After cure (polymerisation) the tool is opened, the moulding removed and, once trimmed and otherwise finished, is shipped to the customer. The process offers many other advantages over contact moulding. Not only is the mould-tool surface replicated on both surfaces but like all closed-mould techniques, there is a much improved control of the important resin : reinforcement ratio and thus the component thickness, weight and ultimately finished composite properties. However, the prime benefit is the marked improvement in production economics. Given a sufficient volume, RTM – or its variants, of which there are an increasing number – can be readily adapted even for quite massive components into a semiautomated process using multiple tooling. Compression moulding A variety of low-to-high pressure, cold-to-elevated temperature, compression moulding techniques of increasing sophistication and therefore capital

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investment, is available to the composites fabricator. Like contact moulding and RTM, at least one cold/warm-press moulding produces the composite laminate at the same time as the moulding but most employ an already prepared but uncured reinforcement/matrix compound covered by such terms as low-pressure moulding compound (LPMC), sheet moulding compound (SMC) or bulk moulding compound (BMC). There is also a precompounded range of thermoplastic-based composites, generically known as glass mat thermoplastics (GMT) finding very promising and rapidly growing automotive, bus and truck application. Disregarding the weightsaving and corrosion-resistant advantages, this material’s attraction to many customers within those markets is that the material is handled very much as if it were sheet steel. Although composites tooling is suitable for the lowerpressure processes covered by this general heading, expensive matched metal tools are more usual, and this certainly applies for the GMTs and other long-fibre composites where moulding is respectively simply a stamping operation, or a ‘high’-pressure injection process. Finally there are the SMC and BMC compounds where both high pressure and temperature also apply. Filament winding As the name implies, the process involves the winding of a continuous, but pre-wet-out reinforcement around some form of cylindrical mandrel. Although still widely employed for the manufacture of both small and large diameter pipes, the high hoop-stress acceptance that is a feature of this wound reinforcement has resulted in the process being developed for a wide variety of pressure vessels and tanks. That acceptance, enhanced by winding at selective, computer-controlled angles with a wide variety of reinforcement types, is, when coupled to the use of resin systems of a chemicalresistant nature, ensuring a very promising future for the process. One practical disadvantage of the process, applying to any component other than an open tube, is the limitation on the component design caused by the need to remove the mandrel on which the product is formed, as a final stage. However, the clever use of inflatable, sectional and otherwise demountable mandrels, largely overcomes the problem.

1.2

Pultrusion Background and history

Although Sir Brandt Goldsworthy is undoubtedly recognised as the ‘granddaddy’ of pultrusion technology, who in the 1950s was one of the first to develop and improve machines for what is basically a simple composite manufacturing technique, there remains some debate regarding those pro-

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files that saw initial commercial acceptance. Some would list the wide variety of rectangular or triangular slot-wedges, the insulation component used in the manufacture of commutators, while others would favour transformer spacers, fishing and other solid rod blanks, kite rods, tent poles, cycle flag masts, as well as profiles satisfying the decorative needs of the building or housing trades, and other equally popular constant cross-section shapes produced in massive quantity. Nevertheless, all possessed at least some structural capability and exhibited close dimensional tolerance. In addition, they confirmed an ability to employ a range of simple roving or perhaps fine-weave fabrics based on glass fibre, a material that was then, and will certainly be in the future, the reinforcement of choice used predominantly with the three thermoset matrices then available: unsaturated polyester, vinyl ester and epoxy resin. Further, even in those days the pulling of several small sectioned profiles at the same time from a multiple die was not unusual. Contrary to today’s practice, some of the early machines (Fig. 1.2) were

1.2 Vertical pultrusion machine design: a fibre collimation, b pultrusion die, c control panel, d puller devices, e final production stage – profile machining

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of a vertical rather than the horizontal design now universal. Many were also of the intermittent pull type, again in comparison to the steady continuous pull of today. Indeed the use of a farm tractor as a horizontal pulling mechanism was not unknown! However, there is also a need to recognise that by the very early 1960s at least one major facility, the Megajoule Bank at the UK Culham Laboratory and part of a project to produce electrical energy from thermal nuclear fusion, employed a close assembly of hand-fabricated small-section composite ‘rods’ within its overall concrete construction. Fabricated simply from glass fibre rovings impregnated with polyester resin, this complex internal structure very successfully answered the problem of otherwise providing a reinforcing structure where the use of conventional steel ‘rebars’ was totally impractical. Sadly, no photographic or other record of this progenitor use of the pultruded profile elements, only relatively recently seeing growing commercial importance for the structural reinforcement of concrete, can be located. The importance to the past, the present and the future of the pultrusion industry of channel and related pultruded sections for the fabrication of a wide variety of ladder assemblies must also be recorded (Fig. 1.3). Offering electrical insulation, light weight, high strength and long life whatever the environment, these profiles based on the total length pultruded daily can be adjudged the foundation of today’s pultrusion industry. Indeed they are profiles that have been recognised by the European Pultrusion Technology Association (EPTA) as the pultrusions of the millennium. Through a number of emerging market applications, interest in the capital- and material-intensive fabrication technique of pultrusion grew

1.3 Ladder constructed from pultruded profile sections

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steadily from the 1960s onwards and was accompanied by major process, reinforcement and matrix developments emanating largely, but not exclusively, within the United States. That situation in respect of market, application and technology typically continues to this day. However, although this all occurred at a time when composite manufacturing development emphasis, funded by the US Government Department of Defense, was mainly centred on the aerospace and aircraft industries, pultrusion was afforded minimal interest. Compared, for example, with tape-laying and filament-winding techniques, pultrusion was generally considered to be a lower level of manufacturing technology, capable only of providing, primarily, components for commercial application. Consequently, the technology, tooling and product initiation funding, all essential to develop the process, its recognition and acceptance, had to be provided by the pultrusion fabrication companies themselves, or in turn by their customers who were beginning to purchase the increasing variety of component profiles by then being pultruded. Some 50 years later, most of those companies have emerged as the majors who, as illustrated by the many case-history studies discussed in later chapters, remain dedicated and committed to this still-developing technology and its steadily enlarging market. Interest and involvement quickly expanded world-wide, initially throughout the USA, the UK and the rest of Europe, then Australia and the Far East, India and SE Asia generally and, much more recently, China and the Gulf States of the Middle East. Some 150 companies in 28 different countries are now believed to be engaged in the technology, and in-house, academic and government-funded research continues apace, particularly since the attributes of the process have been increasingly seen to closely match many of those demanded for the civil engineering-infrastructure sector. The early technology through the 1950s, 1960s and 1970s concentrated principally on shapes with longitudinal continuous fibre placement, in glass matrixed with unsaturated polyester resin. A typical selection of these standard, close dimensioned angles, channels, I-beams and rod sections is shown in Fig. 1.4. During those years came the ability to pultrude accurately aligned hollow tubulars up to, say, 100 mm (4≤) in diameters at different wall thickness, a process that called for a marked development in tooling techniques. However, it was not until the late 1970s and early 1980s that the technology was usefully extended to include other reinforcement varieties such as mats, simple and complex fabrics, as well as surfacing cloths, all leading to today’s use of many complex mats, knitted, stitched, uniaxial, multiaxial and combination materials, perhaps positioned so that their warp is angular to the pulling direction. Commensurate with that was the introduction of

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D

B=H T R r

2–48

20–200 2–12 2&7 4

B 25–200 H 20–200 T 2–12 R 2&7 r 4

d 5–107 D 8–114

H 30–50 B 30–50 d 16–30

B H T1 T2

20–108 25–360 2.5–18 2.5–18

B H T1 T2

8–180 20–300 2.5–18 2.5–18

B H T1 T2

60–90 60–72 6–11 6–10

B 8–65 H 8–51

T B

8–65 20–300

B 25–100 H 30–100 T1 3–8 T2 2–8

All dimensions in mm; size ranges are indicative only

1.4 Selection of standard profiles (Courtesy, Fiberline Composites AS)

carbon- and aramid- (Kevlar) based reinforcements, or their hybrids with glass. Finally, but certainly nowhere near the end of the potential material, process or technology development, a filament-winding capability was added to the pultrusion process during the 1970s. This enabled hoop and helical strength to be readily provided to tubular pultruded components. Then in the early 1980s, two new matrices were added to the composites armoury and ultimately these became available for pultrusion. Phenolic systems were the first, and were initially catalytically cured but they are now, for pultrusion, principally thermally or otherwise polymerised.

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Although resol and novolak condensation products of phenol and formaldehyde were arguably the world’s earliest moulding material, their application to ‘long-fibre’ composites fabrication only became possible following development of what became known as the ‘second-generation’ phenolics. The other ‘new’ resin system for pultrusion, which was fully commercialised at around the same time, was based on the chemistry of a urethane methacrylate system dissolved in methyl methacrylate monomer and described more simply as a modified acrylic resin from which the initial trade name ‘Modar’ was derived. Modar and phenolic systems joined an ever-increasing range of polyester, vinyl ester and epoxy derivatives and, more recently, positive moves to employ certain thermoplastic polymers, such as polypropylene, as pultrusion matrices. As the sophistication of the whole technology and the tooling design steadily improved, so did the size and complexity of both the standard and the rapidly growing number of special ‘custom-moulded’ profiles, designed and specified to meet a customer’s particular needs and performance requirements. Some examples of the latter are shown in Fig. 1.5, with others being illustrated in later chapters. All this was clearly closely related to an expansion of the pultrusion market-place, and one of many developments was an increase in the number, variety and complexity of the edge shapes that the building industry had been using ever since the process was first commercialised. Principal among these were pultruded window lineals aimed – along with the competitive extruded U-PVC (polyvinyl chloride) sections which had also been establishing themselves in the market – at displacing the established wooden and aluminium forms. By the late 1970s and early 1980s, the civil engineering profession began to see composites generally as a material of opportunity for the infrastructure market place. As well as offering a way of eliminating corrosion, there was interest in the side benefits of reduced construction and improved lifecycle costs which stemmed from the light-weight, high-strength and lowmaintenance requirement properties of these composites. Further, through increasing and improving track record example, these homogeneous combinations of fibrous reinforcement contained within some form of polymeric matrix confirmed other important features such as good structural integrity and fatigue resistance. With time, then, pultruded profiles were correctly becoming classified as structural elements. However, the level of this developing interest was not sufficient to create high product volumes since the initially foreseen applications were chiefly for secondary structures which did not require product certification to the major building codes. Then there was the issue of cost, seen as a major stumbling block in the development of infrastructure applications. It was the incorrect perception of the civil engineering industry that

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All dimensions mm

T 4 6 B 4 6 t 3 3

B X H/L/T 25x120/50/6 30x120/50/6 40x120/50/6

1.5 Some examples of complex, custom-moulded profiles (Courtesy, Fiberline Composites A/S)

composites such as pultruded profiles had originated from a high-cost aircraft/aerospace/defence process. In addition, any mention of carbon fibre as the sole reinforcement specifically applicable to the infrastructure application only served to increase that same concern. However, in the 1990s, and with the advent of larger, higher-pulling capacity machines, the available cross-section profile size increased significantly, allowing primary structure demands to be seriously considered, an advance that more recently has also been aided by major reductions in the cost of carbon fibre. Thus the infrastructure market-place is in an excellent position now to become an important and major composites customer.

The pultrusion market-place The pultrusion sector obviously features within the trading statistics published by the world-wide composites industry (as shown by Tables 1.1 and 1.2), but has yet to be separately classified. Historically, in 1960 some 20 US pultruders were reported4 as having produced around 4500 tonnes of pul-

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truded profile product. Otherwise accurate, authoritative statistical data are difficult to obtain and are extremely limited, with the exception of one recent study5 commissioned by EPTA whose purpose is to allow the membership to judge the industry’s growth and potential. What is very clear, however, is that the pultrusion sector has grown and continues to grow annually at a rate that well exceeds even the average 3–5% typically experienced by the remainder of the world-wide composites industry. Indeed figures as high as 12% per year are seemingly not inaccurate, meaning a doubling of the output tonnage on well under a 10year cycle. A brief review of the major end-markets that will enjoy this growth is therefore deemed both necessary and valuable as an introduction to the case-history and future market opportunity comments presented in greater detail through later chapters. Construction In terms of volume, construction is the largest market that can with confidence be seen as offering the greatest opportunity to pultrusion well into the future. However, it has always been seen as a difficult market to penetrate, particularly as Codes of Practice and Product Specifications are rarely written with composites in mind. There is therefore a clear requirement for greater user education through track-record, case-history example and the better application as well as amplification of the existing pultruded profile design data. Corrosion resistance In terms of future growth this market is seen as an excellent ‘number two’, and one of the principal reasons is the fact that in many situations a composite is the best material to employ. Consequently, given an additional structural requirement as in the case of walkways, fencing, stairs, ladders and staging (Fig. 1.6), pultruded profiles become very much the preferred answer. They have the additional advantages of light-weight and easy and cheaper shipment and installation. Electrical Cable tray support members (Fig. 1.7) and ladders have always been significant markets for pultruded profiles and no decline is foreseen. However, to both must be increasingly added such items as transmission poles and towers, which find pultrusions of benefit for similar reasons to the corrosion-resistant application, as well as their electrical insulating qualities.

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1.6 Walkway structure and stairs constructed entirely from pultruded profiles (Courtesy, EPTA)

Marine Although sea, fresh water and fungoid/biological-resistant components required for such uses as marine and marina structures, piers, docks and quays could justifiably feature under the construction heading, this whole waterfront market is, as to be detailed later (Chapters 6 and 7), already receiving close attention through the installation of trial constructions. While the United States leads in this work, the results that on first indication show high promise must result in a major world-wide expansion. Transportation Pultruded profiles have already found use for example in bus luggage racks, and exterior panelling, while composites generally have over recent years been increasingly and successfully considered for many transport applications, whether road, rail or sea. The latter examples are now legion from window masks, structural elements, seats, partitions, cargo containers and even the filament winding of the whole envelope of rail rolling-stock. Given close collaboration therefore with vehicle builders and users – and not excluding the transport infrastructure requirement – pultruded profiles can with confidence be expected to find a growing market acceptance.

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1.7 Pultruded profile cable way (Courtesy, Technolex)

1.3

Summary

The technology of pultrusion has been and, on all the available evidence, is likely to remain, a very attractive application and growth sector of the whole composites industry. Over the years it has developed strongly from its conception and birth, but there are seemingly many developmental areas still open to the process which over the years will enable fresh market challenges to be realised. Moreover pultruded profiles are already recognised as a high-quality engineering–industrial product capable of satisfying a wide range of high-performance, structural element requirement.

1.4

References

1. Starr, Trevor, Composites – A Profile of the International Reinforced Plastics Industry, Markets and Suppliers, 3rd edn, Elsevier, Oxford, 1999 ISBN 1-85617354-2.

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2. © UK Composites Processing Association 1998. It should be noted that certain sections of this paragraph draw heavily on material first written by the chapter author, T F Starr, when Secretariat of that association (1989–1997), but later assigned to the association for publicity purposes. 3. The following are recommended for further reading: Murphy, John, Reinforced Plastics Handbook, 2nd edn, Elsevier, Oxford, 1998 ISBN 1-85617-348-8. Hancox, Neil L and Mayer, Rayner M, Design Data for Reinforced Plastics – A Guide for Engineeers and Designers, Chapman & Hall, London, 1994 ISBN 0-412-49320-9. 4. Mayer, Raymond, Handbook of Pultrusion Technology, Chapman & Hall, London, 1960 (out of print). 5. EPTA, Pultrusion Tomorrow – A Market Research Study, European Pultrusion Technology Association, Leusden, The Netherlands, 1998.

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2 The pultrusion process DAVID SHAW-STEWART AND JOSEPH E SUMERAK

2.1

Machine design and operation Introduction

Pultrusion is the only truly continuous processing technique for the moulding of reinforced plastics, now more commonly called composites. The process is characterised by a low labour content and a high raw material conversion efficiency for manufacturing profiled shapes (sometimes referred to as lineals), at an attractive cost and consistent quality, all typically without the need for any secondary finishing steps prior to product shipment. The process has acquired maturity, is now practised world-wide and has become very competitive in the supply of a wide range of crosssection shapes in a variety of composites formulations. This chapter comprehensively considers each of the machine, tooling and process elements that constitute pultrusion technology, from handling and impregnation of the reinforcement, through die design and cure, to the continuous pulling and final product take-off arrangements. Like many fabrication processes, however, many minor and major production variations exist, together with several value-added post-pultrusion functions, and each will be reviewed as appropriate.

Outline process description An overview of the pultrusion process and the equipment employed is shown in Fig. 2.1, in principle a simple process to produce in continuous length composite profiles to the desired close dimensioned cross-section. In more detail, the reinforcement fibre materials – or reinforcement ‘pack’ (sometimes also called the fibre architecture) in the form of continuous strands (rovings) or plys (mats, fabrics and veils) – are held on creel racks and fed continuously through a guiding system prior to being impregnated with the desired liquid matrix resin. This reinforcement pack is then 19

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Pultrusion for engineers Resin impregnation and fibre guiding Creel

Controls

Fibres Die

Cut-off saw Reciprocating pullers

Profile

Take-off

2.1 The pultrusion machine

gathered – or ‘collimated’ – in a progressive manner into a preformed shape which roughly matches that of the desired finished profile, before entering the heated curing die. On die exit, that resin-impregnated pack has been changed by polymerisation into a fully shaped and solid profile which must then cool sufficiently before being gripped by the continuous, and typically reciprocating, pulling mechanism. Finally a diamond tipped flying cutoff saw cuts the finished profile into the required lengths, which are then handled by a take-off system. The line speed of the process, typically of the order of 1 m (3.3 feet) per minute, can be determined as the throughput rate that results in a reinforcement pack that exhibits optimum resin wet-out as well as resulting in a high degree of cure to produce a composite material of the highest possible integrity and maximum reproducible physical and mechanical properties. Having an overall length of rarely less than 12 m (40 feet) but often 18 m (60 feet) or even longer, most pultrusion machines typically comprise a number of distinct sections which are bolted together to form the complete system. Therefore, although the final format can be one continuous machine, in certain instances a space may be left between individual units, for example, die and pull-off units, to allow the addition and/or removal of other in-line process operations as and when required. These may be a pullwinder, a continuous profile surface coater or perhaps equipment to perform some specialised on-line machining operation. Pultrusion machines are usually classified by the width and height of the ‘pulling envelope’ and also, as shown by Table 2.1, by the pulling force that the machine can exert on the profile being manufactured. However, it is important to note that the pulling envelope, whose height can often be increased by simply fitting a larger-capacity cut-off saw, does not necessarily mean that a profile to that enlarged size can be pultruded.

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The pultrusion process

21

Table 2.1. Pultrusion machine size range Pulling force, kg (lbf)

Pulling envelope Width, mm (in)

Height, mm (in)

3 000 (6 600)

250 (10.0)

4 000 (8 800)

300 (12.0)

125 (5.0) 160 (6.4)

6 000 (13 200)

500 (20.0)

160 (6.4)

8 000 (17 600)

750 (30.0)

230 (9.2)

12 000 (26 400)

1000 (39.4)

230 (9.2)

24 000 (52 800)

1300 (51.0)

350 (14)

Machine and process detail Reinforcement material supply Regardless of whether glass, carbon, aramid or a hybrid of any two or even all three fibre types is used, the reinforcement pack comprises a form of roving with perhaps the addition of mat and/or other ‘fabric’ forms as dictated by the profile specification. Most of the strength of a pultruded profile, and certainly in the longitudinal direction (and whose presence also allows the profile to be pulled into, through and out of the die), is derived from this continuous unidirectional fibre roving, most usually of glass. Each package of roving, of which there may be many, is supplied in a plastic sleeve weighing around 20 kg called the ‘cheese’. These are located on what is known as the creel rack such that the separate roving fibres are fed from a centre-pull arrangement from the cheese before being brought together to the end of the rack through a system of typically ceramic guidance ‘eyes’ or plates fabricated from steel, Teflon (polytetrafluoroethene, PTFE) or a high-quality polyethylene such as ultra-high molecular weight polyethylene (UHMWPE). These guidance arrangements clearly impose some tension on the fibre feed; however, too little can result in sagging which promotes tangling while too much can result in breakage as well as incomplete wetout at the later impregnation stage. More delicate fibres such as carbon, which could otherwise be damaged by the twist that can develop, employ an external unwind creel on a simple cardboard core which is allowed to rotate freely as the fibre feeds to the pultrusion machine. Although large, the creel rack is often designed to be a mobile unit equipped with wheels, and as such this offers the advantage of being quickly exchanged with another when there is a change in the production run necessitating a different reinforcement pack. Other types of reinforcement used in pultrusion to provide transverse

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and additional mechanical improvement as specified – for example shear strength – are supplied in the form of a wide variety of mats, woven, knitted, multiaxial, stitched or combination ‘fabrics’. Supplied in rolls cut to the required width, all these materials are held on the creel rack (Fig. 2.2) by means of horizontal or vertical rotating spindles, according to the required orientation of that additional reinforcement within the profile. Their guidance arrangements ready for impregnation and die entry typically duplicate that employed for rovings. The optimum supply, handling and guidance management of the reinforcement are clearly important manufacturing issues and include the joining of a spent cheese or other fabric package, to a new one. Being like string, rovings can be carefully knotted but other reinforcements depend on some form of splicing that may include hand or machine sewing, stapling or adhesive bonding. The technique of splicing is a critical operator skill which can make the difference between a routine process maintenance procedure and one that results in scrap, line stoppage and down-time. It is also necessary to consider what effect the splice ply thickness might have, especially with thin wall profiles. Matrix resin impregnation It is clearly of vital importance that this reinforcement pack is completely wet-out or impregnated by the chosen matrix resin if the full mechanical

2.2 Typical reinforcement rack arrangement

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and physical performance of the profile is to be realised. In other words and like any composite, to achieve a high-quality finished product then an optimum reinforcement to resin ratio must apply, where the profile is also both free of voids (caused by any entrapped air) as well as one which is neither glass- nor resin-rich. At the same time it is at the impregnation stage that any resin additives, pigment and optical stabilisers, release agent and perhaps also mineral fillers have to be introduced. Although the achievement of an optimum glass (or any reinforcement) fibre to resin matrix ratio typically causes little complication, any filler addition (e.g. materials that are substantially inorganic), whether added for physical property modification or simply to reduce the profile cost, or whether to enhance fire retardancy (e.g. aluminium trihydrate), simply exacerbates the problem of optimum impregnation. For example, although typically finely ground, any filler must be kept in suspension by slow but constant agitation within the wet-bath, or in the reservoir feeding any resin pumping arrangement. It is also necessary here to add that the catalyst addition necessary to promote the polymerisation of the resin will be added either manually as the wet-bath is charged or recharged, or preferably by an automatic dispensing system permitting the percentage to be readily adjusted commensurate with resin type, the environmental and the die temperatures. Most commercial pultruders employ one of two open resin wet-bath impregnation techniques, although the alternative pressure injection of the requisite quantity of matrix resin directly into the die in which the profile is being shaped and cured is finding increasing favour. There are two reasons for this. The first reason is that die impregnation is to be preferred when employing multiaxials and similar sophisticated reinforcements in order to retain that multiaxiality in its initial and required format as it passes through the machine die. The second reason is environmental, i.e. styrene is not lost to the surrounding factory atmosphere, but its use is also being dictated by environmental issues limiting the emission of the styrene and other monomer solvents typically present in the matrix resin (Chapter 4). However, open wet-bath techniques are much cheaper to install and operate, whether of the ‘dip’ or ‘through’ type, both of which can show some design variation. In the simplest ‘dip’ form, the reinforcement passes under either one or two bars that are both kept submerged with the required liquid resin. By adding a third – or more bars – then an ‘up-and-over’ path for the reinforcement is created as it passes through the bath. This action tends to flatten out the reinforcement on the alternate surfaces which, in driving out any air entrapped within the fibres, aids impregnation. However, although also aiding the impregnation of higher viscosity resins, increasing the number of these bars can raise the friction forces and in turn the pulling-load, up to

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a point where fibre damage or, at worst, reinforcement breakage occurs. Any resin carried forward by the reinforcement and ultimately squeezed out by the later action of the collimation plates or finally on die entry is recirculated back to the resin bath by means of a drip collection tray. In ‘through’ designs, the reinforcement is guided through the wet-bath by means of holes in typically two guidance plates which may in fact equally form each end of the bath. To advantage over the simplest ‘dip’ technique, these plates also allow a limited amount of forming or gathering of the reinforcement both prior to and during impregnation. ‘Through’ designs are also favoured for those reinforcements more prone to breakage (particularly when ‘softened’ by impregnation) or for overcoming build-up of the reinforcement around bars of the simple dip-type. In a further variation such ‘through’ plates can be assembled into one (Fig. 2.3) or several ‘dipper’ units, each of which can be individually immersed, or retracted. A typical dipper unit of this type can have virtually any number of slots or holes as required to allow the requisite number of rovings or alternatively the more complex reinforcement forms previously noted, to pass suitably through the bath. Another clear advantage of this type of ‘through’ impregnation arrangement is the ability by lifting just one dipper during production to add extra reinforcement or to untangle any knots which may have developed. Finally, by lifting these dippers out of the resin, dry reinforcement eventually passes into the die, effectively ending the production run, and an alternative to draining the resin bath. Wet-baths are charged with resin either manually or by means of an automatic pump with a level control. Although the complexity of the latter is complicated by the need to add a solvent-flush cleaning system, the maintenance of a constant resin level does aid the wet-out efficiency. Nevertheless as already intimated, the most environmentally friendly way is to impregnate the reinforcement by resin injection direct into the profile forming die. Dry reinforcement, whatever its nature, is also easier to

Collimation plates

Supports Die Profile

Dipper Fibres

Resin Resin drip tray

2.3 Typical wet-bath resin impregnation

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Resin injection Heaters Dry reinforcement

Profile

Injection cavity

Die

2.4 Typical resin-injection die impregnation

handle than when impregnated; increasingly so the heavier the reinforcement. Thin fabrics, or tissues and veils, are typically troublesome since they have a tendency to readily collapse especially when in a vertical format or when handled individually. A properly designed and controlled die-impregnation system, such as illustrated by Fig. 2.4, will expose only very small quantities of the matrix resin and/or monomer solvent to the atmosphere during production. In fact reductions of up to 90% and better can readily be obtained. The entrapped air passes out through the dry and completely consolidated reinforcement pack as it enters the die, and the injected resin pressure (5–30 bar; 60 to 400 psi) is adjusted commensurate with the type and quantity of reinforcement being employed as well as to minimise the amount of excess resin issuing from the die. Although there are some limitations to this procedure particularly where thick sections using highfibre volume fractions are concerned, it has been found practical in those situations to undertake the injection in either single or multiple stages prior to the actual die entry.1 Penetrating a compacted fibre structure with perhaps a highly filled resin is obviously a difficult task. The injection chamber geometry, length, taper and number and location of injection ports, plus the control of the pumping equipment involved, are all variables that make this method of impregnation a much more critical – and expensive – alternative to any open bath technique. The time of resin-to-fibre exposure prior to cure is in addition clearly substantially reduced and if the whole system is not suitably controlled then very inferior profiles can result. On the other hand, when properly designed, controlled and maintained, consistently high mechanical and physical profile properties do result. Reinforcement forming Another pultrusion art is the method by which either the dry or impregnated reinforcement pack is arranged or gathered together – collimated –

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prior to die entry. That complexity also depends on the nature of that pack and the type of profile being produced. Solid shapes constructed solely from unidirectional roving fibres require just two or perhaps three forming plates again fabricated from steel, Teflon (PTFE) or polyethylene (UHMWPE) and containing successively smaller holes through which the impregnated rovings pass to both remove excess resin and pre-shape them to near-die dimensions. As the reinforcement pack becomes more complicated, initially with the addition of either continuous filament mat (CFM) or chopped strand mat (CSM) but later with other more sophisticated fibre forms, then this collimation process obviously becomes increasingly more involved. The same is true as the complexity and/or dimensions of the profile increase, even if the reinforcement pack consists of just rovings. While the final forming plates must provide an optimum distribution and packing of the reinforcement pack as it enters the die cavity, the holes and/or slots through which the pack passes must also be carefully and progressively sized to remove any excess resin. At the same time these slots or holes must impose no undue tension on that reinforcement sufficient to cause damage or, at worse, breakage. The latter can be particularly apposite with CSM materials unless well supported by rovings or other reinforcement, or otherwise secured in place by for example on- or off-line stitching. Tubular products, to take another example, are even more difficult, requiring the addition of a mandrel to form the internal shape. This mandrel must be held in the correct alignment to the heated curing die and must also be long enough to allow the reinforcement pack to be formed onto it before die entry. Typically, a supporting framework is used, which locates the die at one end and holds the fixed end of the mandrel at the other, while also providing the location for the reinforcement forming guides.2 Ideally, this framework should be designed to be quickly demountable from the pultrusion machine, thus allowing for initial set-up and final clean-down to be carried out away from the machine. The reinforcement of a pultruded tube is normally based on an internal mat which is allowed to overlap slightly as a means of preventing the development of a structural weak point along the length of the tube. A core of unidirectional fibres is then commonly applied over this internal mat, followed by a final overlapping outer mat. To ensure that no wrinkling of the reinforcement occurs as it passes into the die, it is essential that these two mats have the same path length in relation to the rovings. This is achieved by a ‘constant velocity’ system typically applied where mats and fabrics generally are concerned. Here the inner mat goes through a series of forming plates spread over a distance, while the outer mat goes from the flat shape to the complete overlap required, in a short distance using a specially shaped guide.

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A multicell profile resembling a series of adjoining rectangular boxes requires an even more substantial framework to support the requisite number of mandrels, the die and the larger number of forming guides required for the complicated reinforcement guiding and forming, particularly for the profile webs formed between the mandrels. Here the supporting framework will usually comprise a removable or ‘mobile bed’ which allows for the set-up and eventual clean-down to be done well away from the production area. Such an arrangement is essential where so much die, mandrel, forming and guidance complexity is involved, and indeed is also of positive value in the manufacture of many profiles, whether or not employing mandrels. Balanced profile construction and symmetry are, finally, two additional principles critical to forming guide design. In order to control flatness and straightness in a profile, it is desirable wherever possible to design the construction so that a uniform number of plies – or equal amounts of reinforcement – are present in each section of the profile. At the same time because heavy woven fabrics or multistitched materials present problems of permeability and therefore wet-out, an alternative is the use of a greater number of thinner plies, although this is a solution that may exacerbate the collimation process. Dies and die heaters Pultrusion dies are usually about 1 m (40 inches) long, a length often governed by the capacity of the tool shop surface grinding machine. Generally, they are manufactured from two or more pieces of close tolerance machined tool steel forming both upper and lower tool-halves which when fitted together, create a parallel cavity showing a tolerance of 0.05 mm (0.002 inches) or better along its whole length. Suitable tool steels (e.g. AISIP20) exhibit a hardness of 30RC, are easy to machine, grind and hand polish to the high surface finish required for the tool cavity which, for the benefit of long-term wear resistance and die friction reduction, is normally hard-chrome plated (0.04 mm, 0.001 6 inches thick) and diamond polished. However, an optimum adhesion between plate and tool steel is essential because any reinforcement and/or resin seizure within the die cavity can either locally or totally strip away the plating. Consequently the alternative is to employ tool steel with a high chromium content, which is subsequently surface or through hardened. Although this can be a viable means of enhancing wear resistance, pultrusion dies can be subjected to high and cyclic physical and thermal stresses, causing hardened tools to crack readily in those areas where, for example, thin tool sections apply. Obviously die life and refurbishing costs are important factors when costing a pultrusion operation but both are impossible to quantify accu-

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rately beyond the need to typically re-polish and/or re-plate the tool cavity after the production of between 30 000 and 50 000 m (100 000 and 165 000 feet) of manufactured profile. Generally, dies making solid profiles comprising solely unidirectional fibres will show a longer life than those where the profile incorporates mat or other forms of reinforcement. In addition, the wear tends to occur at just two locations, die entry and at a point along the cavity, usually one-third of its length, where the resin cures or polymerises. Consequently, by designing a symmetric die, it is practical virtually to double the die life by changing the die exit for the die entry point. Die design Although die design is one of the major topics to be discussed in much greater detail later in this chapter, the following comments are considered relevant in this initial overall machine and process review. The common use of split die sections or multipiece dies as already described, means that no matter how good their fit, the respective tool cavity parting lines can create, on the outer surface of the pultrusion, just visible longitudinal ‘ribs’. Nevertheless, even if these ribs cannot in effect be hidden by altering, through changes in die design, their exact location on the profile cross-section, they may be sufficiently intrusive only in for example the case of rods, tubes and perhaps other profiles, which may have to undergo some later post-pultrusion operation. Although the use of onepiece gun-drilled, honed, plated and polished dies can clearly overcome the problem, this manufacturing procedure is totally unsuited where small cavities (e.g. 50%

Shrinkage, % volume

6–12

1–6

Mould release effectiveness

Good

Fair

Typical processing rates

0.6–1.5 m/min

7–10 cm/min

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Thermoset resins for pultrusion

135

degree of profile consolidation), the net shrinkage is not nearly as important as the shrinkage profile. Polyester resins gel first with continued expansion, followed later by rapid shrinkage, whereas epoxies undergo volumetric shrinkage before gelation, followed by a continuous slow rate of shrinkage. Thus, an epoxy resin cannot simply be directly substituted for either a polyester or vinyl ester resin formulation running under the same die and process conditions. Increasing additions of particulate and fibrous fillers can be employed to reduce this total volumetric shrinkage progressively as indeed will higher reinforcement loadings. In turn, therefore, all three alter the shrinkage profile. Within the pultrusion die, the presence of fillers reduces the volumetric shrinkage level prior to gelation of the resin. This factor, along with the contribution of increased thermal expansion pressure from a higher level of filler or reinforcement loading, results in less reduction in the pressure within the die, allowing the product to process without the appearance of resin sloughing on the finished profile surface. Cure conversion As already suggested by Table 4.22, polyester and vinyl ester resins reach a gel state at a much lower level of cure conversion than do epoxy resins, some 10–30% compared to 40–60%. This difference is important because, as discussed in section 2.2, the location and forces developed in the resin gel zone of the pultrusion die are, as also discussed later, critical to a successful pultrusion operation. By varying the temperature along the length of the die, the location of the gel zone can be altered. At the same time, the cure kinetics of the epoxy resin system can be adjusted by proper materials selection, to impact on the location of this zone. With epoxy resins, more so than with the polyester and vinyl esters, the cure rate and pot-life are usually directly dependent. To reduce the internal forces in the die, a short gel zone and a fast cure with the gel occurring early on in the cure conversion are desirable features. The neat epoxy resin system cure kinetics can be increased by the addition of accelerators and/or die temperature, although the latter is preferred because of pot-life consideration. Finally, both the reinforcement and any filler addition make a significant contribution to the resulting cure kinetics, the cure temperature exotherm and the gel characteristics of any epoxy resin system. Line speed Resin selection always plays a very significant factor in any pultrusion market opportunity, but particularly in the case of epoxies. Their process-

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ing efficiency as determined by the production line speed is, as shown by Table 4.22, totally different, and overlooking all other factors the ¥10 factor difference directly impinges on the cost of the finished profile. Obviously the need for faster epoxy resin pultrusion has prompted the development of systems with improved cure rates. Die cavity pressure and associated conditions There is clearly a close interaction among a number of pultrusion process variables, and as a consequence several statistical experimental techniques have been employed in an attempt to optimise epoxy resins for more efficient processing without any loss in finished product performance. Some of these variables, such as fibre volume, line speed and die temperature, are easy to control directly while others, such as pressure and frictional forces within the die, are not. Epoxy resins exhibiting a desirable low melt viscosity are particularly sensitive not just to the usual cure time and temperature relationship but in the context of pultrusion, also to the pressure conditions imposed by the die cavity.55 In order to obtain high-quality epoxy-based profiles it is desirable to apply pressure at about the same time as the resin gels. Although the accurate measurement of pressure within a pultrusion die cavity is extremely difficult, work by Sumerak,56 Fanucci & Nolet,57 Moschair et al50 and others is worth summary not just in the context of the epoxies, but in terms of pultrusion matrix systems generally. The internal dynamics encountered by a resin system when passing through a pultrusion die is illustrated by Fig. 4.12 and the related description is centred on three basic zones: zone 1 where viscous shear forces apply, zone 2 where those forces are cohesive and zone 3 where sliding friction forces only are applicable. The resin-impregnated reinforcement pack at room temperature enters the die in zone 1 and as a result of an immediate temperature increase, thermally expands to cause a rise in hydraulic pressure. Viscous shear forces are generated in the front portion of this zone which result in greater pulling loads. As the now shaped but uncured profile moves into the gelation zone 2, resin cure begins and this changes the previous viscous liquid into a nonflowing, sticky gel and ultimately a crosslinked rubber-like material. As the level of cure proceeds towards vitrification, volumetric shrinkage takes place, and this causes a reduction in the pressure forces and the subsequent release of the now formed profile, from the internal wall surfaces of the die cavity. Through zone 3 only minor frictional forces exist as the finished product is finally pulled through the die. Both the cross-section of the profile and the line speed have a direct and important effect on the distribution – and the shape – of the forces present

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Gel zone Strip heater Die

Solid phase

Liquid phase

Die

Zone 1 Viscous shear forces

Zone 3 Sliding friction forces Zone 1 Cohesive forces

4.12 Dynamic zones within a pultrusion die

in gel zone 2. Cohesive forces occur in this zone until sufficient cure has been developed to cause the resin to become rubbery, a change that adds substantial frictional forces, thus further increasing the load required to pull the profile through and from the die. With progressive curing of the resin, the accompanying volumetric shrinkage reduces these frictional forces. Consequently, the cohesive and frictional conditions within the gel zone of the die have the greatest effect on the magnitude and changes in the pulling load. However, as line speed increases, the pressure profile moves further downstream into the gel zone, and this zone will move to occupy a larger surface area of the die.56 The net result is an increase in the internal forces, causing high pull loads and possible deterioration of the surface quality of the profile. This build-up of internal die pressure is closely dependent on the processing rate and while the initial hydraulic pressure increase is related to thermal expansion, the pressure loss is due to volumetric cure of the resin system. The thermal properties of the steel die can also contribute a relatively small effect on pressure reduction. Insufficient pressure promotes poor surface quality, known as sloughing, and perhaps in association, product performance problems. Insufficient resin cure and low shrinkage result in the development of excessive pull loads.

Typical epoxy resins for pultrusion Over the past 15 years, the most studied epoxy resins for pultrusion have principally been based on liquid BPA-based or modified BPA-based

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systems, cured with either amines, amines plus accelerators or acid anhydrides with accelerators.57,58 Some typical commercial examples from among many others are described by Table 4.23 which in addition to detailing a number of non-resin related components added primarily as process-

Table 4.23. Formulation and mechanical/physical performance – glass fibrea reinforced epoxy resin pultrusions Component

parts by weightb Ac

EPON® resin 9310 EPI-CURE® curing agent 9360 EPON® resin 9405 EPON® resin 9420 EPI-CURE® curing agent 9470 EPI-CURE® curing agent accelerator 537 Mold Wiz Internal Release 1846e ASP 400Pf VYBARTM 825g EPON® Resin 9302 EPI-CURE® curing agent 9350 EPON® resin 9500 EPI-REZ® curing agent 9550 Glass content %wt. Moisture absorption % wt.i Flexural strength (Mpa) 23 °C dry 93 °C dry 93 °C wetj 149 °C dry Flexural modulus (Gpa) 23 °C dry 93 °C dry 93 °C wetj 149 °C dry Short beam shear (Mpa) 23 °C dry 93 °C dry 149 °C dry a

Bc

Cc

D

Ed

100 33 100

0.67 0.67 20

28 2 0.65 20

100 32 2 0.65 20

10 3 100 3

0.7 10

100 33 81 0.3 592 351 48.2 41.3 68.9 48.2 34

80 0.5 558 517 207 269 48.2 48.2 41.3 34.5 62.0 48.2 13.8

80

75h

83

682

1019h

1102

303 48.2

41.3h

48.2

34.5 55.1 41.3 13.8

65.5h

Glass fibre, PPG Hybon 2079, 112 yield, matrix resin as shown. All systems may not still be commercially available. c One-zone die temperature control @ 200 °C. d Two-zone die temperature control @ 190 °C. e Ex. Axel Plastics Research Labs, Inc. f Clay filler ex. Engelhard Corp. g Mould release additive ex. BARCEO. h Product is a 2.54 cm diameter rod, running @ 10.16 cm/min from a one-zone die temperature @ 150 °C. i Samples immersed in water for 14 days. j Samples tested at 93 °C after immersion in 93 °C water for 14 days. b

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ing and system cost reduction aids, provides an outline of the respective mechanical and physical performance for glass reinforcement-based pultrusions. It should, however, be noted that all those values must be considered as indicative; they are provided only for guidance as variations in profile size and line speed and including even minor formulation changes can affect the resulting physical properties. Nevertheless flexural property values obtained with A, B and C are typical of a conventional unidirectional flat epoxy glass fibre composite, with E being the optimum; none of the values under D can be compared directly because of the significant difference in product geometry, but the overall sensitivity to both moisture and temperature are clear. However, the modulus property is less sensitive to the combination of moisture and elevated temperature and to elevated temperature only. Approximately 85% of the 23 °C (74 °F) modulus value is retained when tested under wet 93 °C (200 °F) conditions. The dry 149 °C (300 °F) flexural modulus value is about 70% of its value at 23 °C (74 °F). Under short beam shear testing, A provided the best overall retention at elevated temperature, 70% at 93 °C (200 °F) and around 50% at 149 °C (300 °F). Indeed of the five epoxy resin systems evaluated, A provides the best overall performance under hot and hot/wet environments, a situation generally confirmed and recognised by the pultrusion industry. Comparable carbon fibre epoxy pultrusion data is reproduced in Table 4.24 and although the data for C are limited, all three conditions provide very good flexural performance. At 97% both A and B retain a high level of flexural modulus under all test conditions, and this retention is somewhat better than the 70% value (Table 4.23) obtained with the glass fibre profiles when tested under the same condition. On the other hand, the flexural strength of the carbon fibre profiles is sensitive to both temperature and moisture, a conclusion also found with the corresponding glass fibre profiles. Under dry and elevated temperature testing, there is good retention of strength – 83% for A and 62% for B – up to 121 °C (250 °F), but above that and irrespective of dry or wet conditions, the temperature has a more degrading effect on the flexural strength as shown by retention values in the range of 60% for A, but only 25% for B. Up to 121 °C (250 °F) shear performance whether for A or B is similar, with both providing lower performance values with increasing temperatures. At 149 °C (300 °F) A is clearly superior, providing a retention of about 50% compared with the 23 °C (74 °F) value. Under the test conditions considered by Table 4.24, the carbon fibre profile performance is irrespective of the epoxy resin system, retained well up to about 121 °C (250 °F), even in the presence of moisture, with A proving the best overall. Pultruded carbon– epoxy composite performance data obtained with

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Table 4.24. Formulation and mechanical/physical performance – carbon fibrea reinforced epoxy/amine system pultrusions Parts by weightb

Component

Ac EPON® resin 9310

100

EPI-CURE® curing agent 9360

33

Bc

EPON® resin 9405

100


28

EPI-CURE® curing agent accelerator 537

Cd

0.67

EPON® resin 9302

100


3

Mold Wiz internal release 1846e

0.67

0.65

VYBARTM 825f

0.67 3

ASPTM 400Pg

15

Fibre content (%wt)

63

63

Moisture absorption (%wt)h

0.56

0.64

Glass transition temperature °Ci

164

164

15 60

Flexural strength (MPa)

23 °C 93 °C 93 °C 121 °C 121 °C 149 °C

dry dry wetj dry wet dry

1283 1124 1076 1069 807 772

1276 1117 897 793 317 372

1128

Flexural modulus (GPa)

23 °C 93 °C 93 °C 121 °C 121 °C 149 °C

dry dry wetj dry wetk dry

124 124 124 117 117 117

124 124 117 117 117 110

120

dry dry dry dry

83 62 48 41

83 55 41 28

72

Short beam shear (MPa) 23 °C 93 °C 121 °C 149 °C a

Hercules AS4 W-12K carbon fibre not specified. Matrix as shown. All systems may not still be commercially available. c One-zone die temperature @ 200 °C. Pull rate 30.48 cm/min. d One-zone die temperature @ 149 °C. Pull rate 12.7 cm/min. e Internal mould release by Axel Plastics Research Labs, Inc. f Mould release additive from BARCEO. g Clay filler from Engelhard Corp. h Samples conditioned by immersion in 93 °C water for 14 days. i Measured by dynamic mechanical analysis. j Tested in 93 °C water after immersion in 93 °C water for 14 days. k Tested at 121 °C in water as soon as possible after immersion in 93 °C water for 14 days. b

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an anhydride cured system, both with (B) and without (A) a small per cent silica filler additive, are provided in Table 4.25. It can be seen that overall B gave the best performance, and although post-curing did improve the shear strength by about 15%, it had no effect on the flexural properties.

Processing comment It is not the purpose of this section of Chapter 4 to provide comprehensive pultrusion processing and associated detail for epoxy resin systems beyond that already provided. That information and related recommendations can be readily obtained from the manufacturers and suppliers of the wide diversity of resins now available world-wide. However, it is judged appropriate briefly to review several processing parameters that relate to the earlier question of improving the process dynamics and reducing the finished

Table 4.25. system

Pultruded profile performance – carbon fibre epoxy/anhydride

Component

System parts by weight A

B

EPON® resin 826

2000

2000

Methyl tetra-hydrophthalic anhydride

1832

1832

200

200

Di-glycidyl ether of 1,4-butanediol LT-1 Carnauba wax

a

Benzyldimethylamine

36

36

20

20

Precipitated silicab

2.5

Carbon fibre content % volume

c

60

60

930

1211

23 °C after 2 h @ 177 °C

836

1230

Flexural modulus (GPa) 23 °C no post-cure 23 °C after 2 h @ 177 °C

158 160

194 194

72 79

74 88

Flexural strength (MPa) 23 °C no post-cure

Short beam, shear strength (MPa) 23 °C no post-cure 23 °C After 2 h @ 177 °C a

Mould release additive from Ceara. Filler added basis %wt of total weight of resin, curing agent and diluent. c Seventy-seven ends of 140 yield carbon fibre, nominal tensile modulus 344 GPa. Composite Institute of the Society of the Plastics Industry. Reprinted with permission. b

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profile cost. Indeed several of these developments may ultimately prove of importance in other thermoset resin pultrusion processing. Fibre tension The mechanical properties of a pultruded shape are highly dependent on the parallel alignment of the fibres as they pass through guide and resin impregnation stages, and indeed until sufficient resin matrix cure to secure them into the desired position has been achieved within the die cavity. Several workers59 have demonstrated simple low-cost tensioning devices comprising little more than flexible polyethylene tube and eyelets, which are effective in controlling fibre alignment with the minimum of damage to individual fibres, and as a result have offered improvements in the epoxy composite flexural strength by some 47% and in the case of the flexural modulus by 21%. Radiofrequency (RF) preheat It is clear from earlier discussion and well confirmed in practice that the difficulties of pultruding epoxy-based profiles increase with the size and complexity of the profile. Cross-sections greater than 1.27 cm (0.5 inch) tend to generate a relatively high level of internal stress, which can lead to the development of cracks within the finished product. The effect of cure and degree of cure, together with the non-homogeneous exothermic nature of the cure from the surface to the centre-line, and to which must be added the heat transfer problems, all contribute to that problem and the challenge of its resolution. Several processing techniques have already been found effective in reducing the difficulty of pultruding thicker epoxy-based profiles. Very careful control of the degree of cure, the point of cure initiation and the peak exotherm temperatures, are all somewhat obvious keys to success, and here the use of multizone die heating is particularly important. Another successful technique, however, is to employ an RF energy source to pre-heat the resin-impregnated reinforcement, immediately before die entry. As a consequence, the resin begins to gel earlier in the die, and because the centre-line and surface temperatures therefore move closer together, the peak cure exotherm is reduced, resulting in a much more homogeneous cure condition throughout the profile and thus in turn a marked reduction in the residual internal stress. To additional advantage, lower pull loads and higher line speeds may also result. Even so, there are also potential disadvantages in employing RF heating. There can be difficulty in controlling an optimum temperature which balances processing efficiency relative to the pot-life of any resin recycled to the reinforcement impregnation wet-bath. Furthermore, even short dura-

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tion production line stoppages can, owing to prolonged RF heating, initiate resin gel either prior to die entry, or too early within the die. Ultrasonic activation An ultrasonically activated pultrusion die, applying vibratory energy resonating glass and carbon fibre epoxy-based profiles at 15 kHz, has been shown60 to improve reinforcement wet-out and as a consequence permit higher fibre loadings within the profile, together with a reduction in the pulling-force and in turn, therefore, faster line speeds. Microwave energy activation Microwave-assisted pultrusion at 2450 MHz generated from a single mode cavity, in the form of a cylinder surrounding a pultrusion die made of polytetrafluoroethylene or borosilicate glass, has been shown to be effective in the manufacture of solid glass fibre epoxy rod of

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