Pultrusion for Engineers

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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

175

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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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

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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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xxvii

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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Pultrusion terminology

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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2

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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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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Pultrusion for engineers

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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Pultrusion for engineers

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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9

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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Pultrusion for engineers

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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11

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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Pultrusion for engineers

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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13

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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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. <6 mm, 0.236 inch diameter) are concerned or where owing to the profile specification – as for example in the case of epoxy matrices – there may be a need to open the tool frequently and remove any blockage that may have occurred. In the same way, the several parts of any multipiece die must continue to fit together accurately after many dismantling and cleaning cycles. Three well-tried techniques exist, with the simplest being dowel pins spaced at intervals along the die length and with high-tensile steel screws to hold the separate tool sections together. A more positive system employs, instead of dowels, ‘keys’ fitting into keyways machined in opposing tool sections and the final choice, which provides for the most accurate registration between respective tool sections, involves machining matching registers on adjoining tool surfaces. Optimum die design depends on a number of factors. Some relate to the die manufacturing process, others to the ultimate use of the die as part of an efficient and effective pultrusion line. In terms of tool manufacture one golden rule is never to have sharp corners, as these readily fill with cured resin and are not automatically ‘cleaned’ by the abrasive action of the rein-

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forcement pack. Where possible, a minimum radius of 0.5 mm (0.02 inches) is desirable and parting-lines and radiused corners within a cavity should never coincide. This is best illustrated (Fig. 2.5) by a simple U-section channel profile die constructed from four tool sections, which also illustrates the use of matching registers. Above all in importance, however, is the necessary dimensional calculation to allow for the resin shrinkage which occurs on polymerisation from the liquid to the solid condition. This differs from resin system to resin system and, in addition, the degree of shrinkage can be affected by the reinforcement, its type, orientation and fibre volume. Unsaturated polyesters, for example, exhibit a much higher shrinkage than epoxy, but ‘low-profile’ or shrink-reducing additives can provide an effective answer. Typically, a simple unidirectional glass fibre roving/polyester resin rod will shrink diametrically by some 2% and if close dimensional tolerances are specified for the finished product, then this allowance must be built into the dimensions of the tool cavity. Resin shrinkage, when coupled to certain shape characteristics of the profile, can have other disadvantageous effects. For example, a U-section channel profile of the type produced from tooling shown in Fig. 2.5, will invariably exhibit ‘toe-in’ distortion of the two flanges unless this is allowed for by either designing the die with a 1–2° ‘toe-out’, or adding a post-sizing stage immediately after die exit. Finally, there is considerable debate within the pultrusion industry regarding the best way to run multiple lines of simple profiles, such as rods and a frequent operational requirement. The question is whether to use a number of single cavity dies, or whether to use a multicavity design. With the often more expensive single cavity option, a die that ceases to function properly can be quickly removed and replaced while all the other dies are

Channel cavity Keyways

Upper part of die

Side spacer part

Lower part End view

2.5 Optimum joint arrangement, four piece channel die

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still running, whereas with a multicavity die, all lines must be stopped until the problem is resolved. Die mounting Pultrusion machine operation can exert considerable force on even the most simple of dies as most of the pulling force – which today can be in excess of 12 tonnes – is transmitted through the die to the machine bed. A strong mounting arrangement is therefore vital because it is not uncommon for all the holding-down screws to shear off, should the reinforcement lock in the die. Consequently, a common practice is to have key slots machined across the base of the die and for the die to be positioned on a series of rigid bars, locating in this slot, as a means of transferring the pulling load into the pultrusion machine bed in an optimum fashion. The die is then held down by high-tensile screws going into the bar after passing, either through holes in the die, or originating from a top mounting clamp bar, all as shown in Fig. 2.6. A second support bar locates the other end of the die, and this may be rigidly fixed to the machine bed, or be adjustable for height. Additionally the die bed – or die table – can be designed to adjust for height, to ensure that the profile is always in critical and correct alignment with the remainder of the pultrusion machine. Finally, and as now considered, the die mounting can be incorporated into the die heating arrangements.

2.6 Keyway and bar die location to machine bed

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Die heating The industry uses three main types of heating media, electric resistance, hot oil and steam, each of which can be applied to the die in a variety of different ways. Possibly the most common is the single or double, electrically heated and thermostat controlled platen, with the die or dies located on or between them, to provide a stable high thermal mass, and probably also separated along the die length, into heating and cooling zones. If applicable, the upper platen is usually arranged to be easily lifted to allow for die removal and replacement. However, these platens can equally be raised to the required temperature by either steam or hot oil, although neither is as convenient as electricity, nor do they typically demonstrate the same critical temperature control which may sometimes be required for certain resin systems. Nevertheless, in addition to the useful thermal mass they provide, another advantage with platens, whatever their heating system, is a fast set-up time compared with the alternative use of individual electrical heaters that must be connected and then later disconnected after each production run. Equally steam or oil heating can be applied directly to the die through a series of interconnected passageways formed within the die and this arrangement can be more efficient than a final cooling platen, for an exit zone that needs to run at a lower temperature. Even so there are certain profiles, particularly those demonstrating a deep section, where the precise control of heating becomes essential on all four die surfaces. This situation is best satisfied, for example, by fitting additional side heater elements to the die even if clamped between upper and lower platens. Alternatively it is also common for dies to be fitted with a complete set of electrical resistance heaters, which can number eight or more. There are several types available with the simplest being in the form of flat strips bolted or otherwise positioned on the die with suitable clamps. These are, however, not always the most efficient, as it can be difficult to achieve the close surface and therefore thermal contact required. A much better engineered system employs cartridge heaters located in holes machined in the die, or better still aluminium or steel heater blocks with their own built-in cartridge or other types of electrical element, critically fitted internally within the die. Because such arrangements markedly improve the heat transference, there is the pronounced improvement in temperature control within each zone and/or die area demanded by complex profiles and/or those employing a sophisticated high-performance specification. In terms of temperature control, the minimum two zone system will employ individual controllers, each actuated by a thermocouple probe – typically those designated ‘J’ and ‘K’ are most common for pultrusion –

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positioned in the appropriate die zone. Non-symmetrical profiles can, however, require more selective temperature zoning, making additional controllers essential and a topic covered in greater detail later. Clearly the control of heat flow and its distribution through the whole die assembly is of vital importance in optimising the actual profile pultrusion process. Consequently, it is now possible to achieve an optimum heater selection, die position and temperature control, etc, through a finite element analysis (FEA) program.3 With this approach, the thermal environment within the die can be critically modelled relative not only to the applied heat sources, but also to the conductive and convective losses, as well as the thermal contribution arising from the exothermic reaction within the steadily moving profile as it cures or polymerises from the liquid to solid condition. Alternative heating and cooling systems Both radio frequency (RF) and microwave heating find use in pultrusion, either as a means of pre-heating the reinforcement pack and/or the matrix resin, or for curing the profile. Although both offer considerable benefits, the means of sensing and controlling the temperature is, however, complicated simply because conventional thermocouples cannot be employed within the surrounding heating chambers necessary as part of either system. Pre-heating of the impregnated reinforcement pack through contact with a radiused plate usually set to a temperature of 60 °C (140 °F) and, prior to die entry, offers several advantages. By reducing the resin viscosity, the fibre wet-out is improved as this allows an easier flow of resin around each fibre and in turn causes a reduction in the pulling forces owing to the lowered viscous shear effect when stripping excess resins from the reinforcement pack at die entry. However, the main benefit is an improvement in production line-speed because the additional heat input shortens the cure time. Using RF as a pre-heating system involves passing the impregnated fibres through the RF chamber between tuned electrodes. The fibres pass through as a bundle, but do not need to be precisely contained. For simple symmetrical shapes such as rods, the RF tuning is simple. For shapes such as channels or window lineals, the design and use of the electrodes are more complicated. When RF is used for curing, the impregnated fibres pass through a shaped die positioned within the chamber which is tuned to the fibre volume passing through. However, the die has to be made of a material that is ‘transparent’ to RF radiation, and although PTFE is suitable, it is subject to high wear.

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Post-die sizing After the profile has been formed and then polymerised within the die, it usually exits as a solid, although perhaps only 90% cured. As it is also hot, it remains briefly in a somewhat pliable condition permitting a post-die ‘sizing’ operation using typically a cooled aluminium block to ensure, for example, that the correct angularity of the whole or sections of the profile is achieved. Continuous pulling systems As the name of the process implies, the formed composite profile is pulled through the die, the pulling force being that level of machine tensile force that must by some means be provided, to sustain continuous and steady motion of the product at a specified speed. Before considering the variety of pulling systems employed, it is necessary to summarise the factors that affect the magnitude of that pulling force or, conversely, the pulling resistance: • • • • •

•

Collimation resistance in the creels, resin baths and material preforming areas prior to die entry. Bulk compaction of fibre and resin to the final net cross-section at the die entry. Viscous drag of the wetted material sliding against the die surface in the pre-reaction region. Thermal expansion forces as the material heats to and beyond the peak exotherm level. Resin de-bonding forces at the die interface, where the adhesive tendency of the resin film is overcome by the cohesive attraction to the moving crosslinking resin and fibre substrate. The mechanical drag of material against the die surfaces as shrinkage of the composite material proceeds during cure.

The relative magnitude and axial position of these resistance components is related to the fibre and mineral filler solid content, the fibre architecture, resin viscosity, resin reaction rate, resin shrinkage behaviour, die surface condition and die geometry factors such as entry and exit tapers. As a consequence of their number, no reliable method exists for predicting the pulling resistance of a given profile geometry. However, once they are all known and fixed for a particular composite, profile and production set-up, then because they are also stable parameters, if the pulling force is continually measured and recorded it provides a clear indication of process stability.

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Two distinct versions of pulling system are employed, the continuous tractor and the reciprocating puller, and variations of both exist. Although in common pultrusion use, many of the former continuous tractors, which can be further classified as of the ‘belt’ or ‘cleated’ design (Fig. 2.7), have been adapted from mechanisms primarily intended as haul-off units for thermoplastic extrusions. Most belt-type machines have two belts, an upper and a lower usually of reinforced rubber with multiple V-grooves on their inner side which make contact with two horizontally aligned drive rollers powered by a DC motor and a gearbox. The outer, working face of the belt is typically composed of urethane pads (Shore Hardness 90–95) which are bonded to the belt and are either flat or specially shaped to grip the profile. Overall, this puller design tends to be restricted to simple profiles demanding lower pulling forces such as bar stock or small diameter tubulars where these pads would, for example, be a half circle in shape. To achieve a clamping force between the two belts they also run on a series of ‘idler’ rollers positioned between the outer drive rollers that create a number of pinch points. Finally, the distance between the belts is adjustable to suit different profile heights. A cleated type of continuous tractor puller uses two or more sets of upper and lower roller chains running side by side and in the machine direction, fitted with attachment points that mate with a series of transverse bars, or

Vertical adjustment Backing rollers

Profile

Drive rollers Continuous belt

Belt type tractor puller Vertical adjustment

Urethane pads not shown

Backing support bars-

Alternative to backing rollers as in belt type shown

Profile

Side views

Drive sprocket wheels Chains

Cleats

Cleat type tractor puller

2.7 Belt and cleat type continuous tractor puller units

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cleats. Each chain runs around a pair of sprockets, one of which is powered by the drive system, while the other has a means of tensioning the chains. Additional intermediate sprockets or rollers support the chains along their length. Each cleat, like the belt type of continuous tractor, is fitted with flat or suitably shaped urethane pads to grip the profile, and with the upper and lower chains capable of being adjusted to the requisite profile height. Pneumatic or hydraulic cylinders with respective critical pressure control circuits, are used to generate the necessary clamping force.Again the drive is usually by a DC motor and gearbox, but as a result of the potential for cleat-type pullers to accept larger profiles and greater pulling loads, a closed loop tachogenerator is usually included to give more precise speed control. One of the disadvantages of both continuous tractor designs is that the chains – or the belt – are not continuously supported or restrained where they run parallel to the profile. Consequently they only fully grip the profile at each sprocket or roller pinch point. A more uniform clamping force along that length of the chain – or belt – can, as included in Fig. 2.7, be generated by the use of hardened steel support ‘ways’ rather than sprockets or rollers. Another major disadvantage of particularly the cleated tractor design is the need for a large number of gripping cleats, which need to be regularly replaced as they wear or have to be changed when a different profile is to be manufactured. Replacement is both time-wasting and also adds to the production costs, but as will be described in more detail later, the disadvantages do not apply with the reciprocating types of puller which, although they are typically larger, require only one pair of gripper pads per profile for each puller. Although more complex in its operation, the alternative reciprocating type of puller also offers a number of distinct advantages particularly where the larger, more complex design of profile applies. As shown in Fig. 2.8 most employ two puller units to give a continuous hand-over-hand pulling motion. Because the backward (or ‘open’) return movement for each of the pullers is faster than the initial forward (or ‘closed’) pulling movement, time provided to both grip and pull together is critical at the point where one puller takes over from the other. Two forms of drive system are used for reciprocating machines, either mechanical or hydraulic, and Table 2.2 provides a comparison of their respective features. In the first case, there are four common mechanical drive arrangements, either a continuous single or double drive chain, a rack and pinion arrangement or finally a recirculating ballscrew, all suitably coupled to a gearbox driven by a variable speed DC motor. Where hydraulics are used, each puller is fitted with a separate cylinder. Of them all, mechanical or hydraulic, the recirculating ballscrew is the most efficient, giving the most rigid and accurate performance.

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2.8 Reciprocating puller machine

Table 2.2. Comparison of mechanical and hydraulic reciprocating puller mechanisms Mechanical

Hydraulic

Cost

Both very similar, although a high specification hydraulic system can cost more than an equivalent mechanical system

Number of components

More

Less – requires only a cylinder to move the puller platen

Maintenance

More as a result of the number of bearings, clutches, gears, etc

Less – requires only attention to seals, filters, control valves, etc

Cleanliness of operation

Better

More work – oil leaks can be serious with safety complications

Speed control

Easy to achieve

More complicated

Noise

Minimal

Higher – sound deadening enclosures on the pump unit may be necessary to satisfy health and safety regulations

Power consumption

Efficient, typically 5.5 kW

Typically almost double – but can be as high as 16 kW

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With the continuous chain, in order to move forward, each of the two pullers attaches itself in sequence to that chain. At the end of the forward feed stroke and on release from the chain, the puller is in turn returned to the start position by either a subsidiary pneumatic or hydraulic cylinder. That mechanical–pneumatic (or hydraulic) complication can, however, be overcome by employing two chains, one for each puller, and with the forward and backward motion then controlled through a system of reverse motion clutches. In the third mechanical drive option, each puller is fitted with a gear rack separately driven by a pinion through forward and reverse motion clutches that are in turn powered from the main gearbox, motor drive unit. In the fourth case, each puller is driven by a recirculating ballscrew and nut with forward and reverse movements again controlled by clutches. The use of positive toothed clutches for the forward motion guarantees that the two pullers will run at precisely the same speed. As already considered under both continuous puller designs, the importance of consistent and critical speed control is essential whatever the chosen reciprocating drive system. With the mechanical designs this is normally answered by the use of a tachogenerator closed loop feedback device to the drive motor, ensuring better than 0.06 m/min (2.4 inches/min) under varying load conditions. However, because profiles can seize or jam in the die, it has to be possible for the drive motor to exert maximum torque at zero or low speed and this is the reason why DC motors are favoured. Although vectored AC inverter drives can run under similar conditions, there are cost implications. Hydraulic drive reciprocating machines use a suitably sized cylinder for each puller, which is part of a total system comprising electric motor-driven high-pressure pump, reservoir, control valves and probably a cooling system for the hydraulic oil. Although such a total system can be used in addition to operate other machine functions, such as raising and lowering the clamp units as well as the die table and the resin bath, the precise speed control demanded is more complicated than with any mechanical puller drive arrangements. This is appropriate when the variable load requirements of a pultrusion machine are taken into consideration. The latest proportioning control valves, being more robust and reliable, are an improvement on the earlier servo valves, and most commercially available hydraulic machines are now also fitted with closed-loop speed control arrangements using some form of positional feedback fitted to each puller cylinder. Gripping arrangements – reciprocating pultrusion machines Two of the most important features of any pultrusion machine, irrespective of the form and design of the drive system, are the method and material

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used for gripping the profile. These become even more critical, if more than one profile is being pulled, because any slippage will result in a difference in the final cut profile length, and hence in the production of costly scrap. Research has established a clear relationship between the clamping force that must be applied to the profile if the maximum pulling force of the machine is to be employed. (For example, the use of a gripper pad material having a coefficient of friction (m) equal to 0.4, means that the clamping force is 2.5¥ the pulling force.) Choice of gripper pad material is therefore critical and although urethane with a Shore A Hardness between 90 and 95 is often preferred, hardwood blocks machined to the profile shape are sometimes chosen even though their coefficient of friction is lower. Rubber, particularly any suitably hard black type, is rarely recommended as this usually results in black marks appearing on the profile. Whatever the gripper material, however, it must clearly be capable of withstanding typical operating temperatures approaching 100 °C (220 °F) over a long period. Most reciprocating machines are therefore supplied with a set of flat gripper pads based on steel or aluminium plates to which a minimum of 6 mm thick urethane has been adhesive bonded to the working face. The overall plate design is arranged to provide an easy fit to both the upper and lower surface of each puller. As necessary, and as noted earlier, the urethane can be machined to suit the profile shape, but it can equally be cast in a form that closely matches a suitable part of the profile cross-section. Alternatively, and in the case of, say, a channel section, flat gripper pads can be machined to suit the ‘outer’ and ‘inner’ faces of the channel. This is judged a suitable location to describe the procedure for starting a reciprocating machine. Initially the dry reinforcement pack correctly feeding through guidance and forming plates is clamped within or otherwise secured to the second puller unit. When that puller reaches the end of its stroke, it is retracted, fast, and the pack is then repositioned for another stroke. At a suitable point the resin impregnation is begun and when cured profile reaches the first puller, it is then gripped and moved forward in the normal manner as the several functions, impregnation, die temperature and profile speed for example, are optimised. Clamping systems – reciprocating pultrusion machines There are a number of different clamping systems used on reciprocating type pultrusion machines. Most versions have a plate, or platen, which is also the base of the puller unit and the downward acting clamping system is mounted above and from this platen, although some earlier machines had upward clamping systems. Using a plate, or platen, as the main moving base provides a rigid reference surface for the fitting and alignment of the

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gripper pads. The precise alignment of profiles, from the die through the pulling system, is of vital importance. Figure 2.9 shows a basic puller unit with a die set type of clamping system. The upper clamping platen locates on the four corner posts, with one or more hydraulic cylinders operating the clamping motion. This type of arrangement can be reversed by having the upper platen pulled down by the four corner posts that locate in the base platen. In an alternative arrangement, a single short-stroke hydraulic cylinder is fitted with a clamping foot, mounted on a supporting bridge that is also adjustable for height. On wide machines it is feasible to mount several clamps of this type across the machine bed. Hydraulic cylinders actuate quickly and at high force and it is therefore essential to satisfy health and safety regulations by their optimum guarding. One somewhat safer arrangement is to employ a ‘floating’ upper clamp plate activated by an air operated bladder, providing a short 5 mm (0.2 inches) but equally high-force movement. A typical set-up can generate a clamping force of some 11 tonnes and the floating motion ensures a uniform clamping force in all directions. It is also an arrangement that can be particularly important when running several profiles at the same time. The clamp foot can be positioned in a manner that allows the height to be simply adjusted manually by means of a handwheel. In another arrangement the clamping unit can be mounted on a bridge which can be adjusted for height, or equally positioned across such a bridge to suit different profile centre-line distances. The use of two or more of these clamp units mounted

Hydraulic cylinder/cylinders Fixed top plate

Clamping plate

Guide posts

Lower plate is puller platen

2.9 Die set type clamping arrangement

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on each puller also enables profiles of different heights to be started, stopped and run simultaneously. Cut-off saw The final section of a pultrusion machine is the ‘flying’ cut-off saw, usually a free-standing unit that can be either directly attached to the puller section or moved away to provide space for an additional in-line process such as powder coating. All commercially made machines use such a ‘flying-saw’, meaning that during the cutting operation the saw table moves at the same speed as the pultruded profile. When the cut is initiated typically in one of three different ways, a clamping system holds the profile to the saw table and the table then moves together with the profile, usually under the assistance of a pneumatic cylinder. The amount of permitted movement along the saw bed has to be designed to be sufficient to allow the cutting cycle to be completed even at the maximum pultrusion machine speed.After completing the cut, the clamps release and the saw unit retracts to its starting position. The saw can be initiated either manually to a pre-programmed profile length set at the control panel and measured by an encoder whose driving wheel runs on the profile or by means of target switches or sensors actuated by the profile. Manual cutting signalled by push button/s located adjacent to that machine stage or more rarely on the main control panel, is, however, only really suitable where the profile length is not critical. When running multiple profiles, puller slippage can result in cutting to incorrect length and a problem solved by using separate sensors for each profile arranged so that the cut is initiated by the last sensed profile. Additional features may enable, for example, batch quantities of a certain length of profile to be programmed into the overall machine control. A variety of saw positions relative to the profile centre-line can apply but each one will typically employ a diamond-tipped saw rotating at a peripheral speed of about 2500 m/min (8200 ft/min). Although an overhead – or even below machine bed – ‘plunge-cut’ position can apply, both of these tend to suffer from the disadvantage of offering a limited cutting envelope. One preferred overhead system, therefore, has the saw travelling on a beam, where the distance traversed brings the saw centre-line to the far side of the profile cutting envelope. This arrangement also has the advantage of being able to accommodate two saw heads when there is the need, for example, to cut two parallel running profiles into perhaps randomly different lengths. Finally, the saw can be arranged to move both vertically as well as across the machine bed. For environment, if not good housekeeping reasons, optimum dust extraction from the whole flying-saw area is essential. Two versions are common, a standard dust duct-extraction arrangement, or the use of a wet-

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cut system. In the latter, water containing a small percentage of soluble oil to prevent rust is pumped from a tank and, after use, is returned to the tank where the dust is allowed to settle out in two troughs. Although it offers several advantages, not least longer saw life and more total easier dust collection, wet-cutting is easier to operate with overhead saw arrangements as the motor and drive unit are positioned above both the water spray and drain-off areas. Nevertheless, safety with the best possible guarding for operative protection is an all-important factor in the whole flying-saw arrangement. Take-off systems As the profiles are sawn to length, they need to be supported in some manner and several arrangements are common. The simplest approach is a flat table whose surface is positioned just below where the profile exits the saw, but it does require continual operator attendance. That need can be answered by a pneumatic cylinder operated pusher arm, synchronised with the saw unit, which moves the profiles sideways on the table. Instead of pushing the profiles in this way, a tilting table can be used such that the profiles slide slowly and steadily into some form of collecting trough. Even more sophisticated is the fitment of a standard industrial roller conveyor to provide easier movement of the profiles than on a flat table. A powered rubber-covered roller can also be used to move the profiles away from the saw, or some or all of the conveyor rollers can also powered and probably by the same motor unit. Taking that arrangement further, the roller conveyor can also be tilted. Such a standard powered roller conveyor can also be fitted with pneumatically operated lifting arms located between the rollers below their working surface and at intervals of approximately 800 mm. These arms deposit the profiles in a trough alongside the unit, with the total system operated from the main pultrusion machine control panel. When the saw cut has been completed and the saw clamps released, the powered roller is switched on to take the profile away from the saw and to roll it on to the conveyor rollers. The length of roller operation time is programmable and as soon as the motor stops, the lifting arms are operated on a preprogrammed timed up and down motion. This can all be finally developed into an automatic take-off and stacking system, but that is only viable where very large volumes of the same profile are concerned. Pultrusion machine controls Most commercially made pultrusion machines now have their control panels mounted in pendant units, as shown by Fig. 2.10. The ability to rotate

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2.10 Typical pendant control panel fitted with six temperature controllers

this display and monitor or alter the controls from either side of the machine improves the ergonomics, particularly when several machines – which also do not therefore need to be ‘handed’ – are located side by side. Without overlooking other machine control functions already described, there are three principal pultrusion machine control functions, temperature, machine operation and control and finally monitoring and data logging which demand fuller description. Temperature Because most pultrusion machines use electric resistance die heating, temperature control is usually a simple ‘on–off’ function using either contactors or solid-state relays (SSRs), operated in turn by carefully positioned thermocouple-fed temperature controllers. Contactors are, however, far from ideal: not only are they noisy but in order to optimise their life they are normally switched at a minimum frequency of once every 30 seconds, a timing detrimental to the critical temperature control often necessary. Consequently, the general trend is towards the use of non-contact SSRs set to switch at two-second intervals, a short time span which gives more precise control of temperature, particularly when using the latest microprocessor temperature controller. An even better, although more expensive, option having the additional disadvantage of requiring more electric cabinet space, is the use of thyris-

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tors. Under the control of a variable voltage output emanating from the respective temperature controller, these constantly vary the voltage supplied to the chosen electrical resistance heaters. With the advent of microprocessors, temperature controllers have reduced considerably in size, while increasing in control capability and speed. Internal switches and software enable controllers to be configured for a wide range of options which now typically include: • • • • • • •

the supply voltage; a temperature display in either °F or °C; the type of thermocouple type employed; output for contactor or SSR switching; minimum output switch time; high and low temperature alarms; and finally self-tuning functions.

The latter, known also as an auto-tune function, works by first observing the temperature response of a die during the heating-up stage and then altering the internal control circuit to suit the conditions observed. In this manner, the temperature overshoot that can occur with some heating systems is substantially reduced. Many controllers also now include fuzzy logic circuits which employ different algorithms to adapt the control parameters to suit the mass of die and profile being heated. For external control and data logging, temperature controllers can be fitted with RS 485 computer communications, while for normal manual control, push buttons are used to raise and lower the desired temperature or ‘set point’, with both actual and this set-point temperature displayed. Touch-screen displays are another alternative. Here, the temperature control functions are executed within either a main PLC/PC (a computer dedicated to controlling the machine operation) or by communicating with a centrally located unit. The screen then has a page, or a several page, display which shows all the heater zone temperatures and additionally provides an ability to adjust the set points, as required. All the control functions and options that are available through each individual control unit are also available within an integrated touch screen type of display system. Finally under this heading, and because die temperature, profile cure rate – polymerisation – and line speed are so closely related, there is need to consider the process dynamics or reaction kinetics that are governed by the exothermic reaction of unsaturated thermosetting resin with styrene (or other monomers) which is in turn driven by a temperature-dependent catalytic process. Overall this is the reactivity of the resin and the cross-section thickness of the profile. At any combination of all these parameters, the process will not just achieve an optimum equilibrium but consistent properties. Such a balance between energy input and cure development for a

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Pultrusion for engineers Thermal environment

Ambient or preheated material zone

Cool zone

Energy flux into material + exothermic heat generation

Energy flux into material

Zone 1 heater

Conductive & convection heat dissipation

Zone 2 heater Separation – profile from die cavity

Liquid

Collimation resistance

Viscous drag

Gel

Adhesive debonding

Solid

Thermal expansion v volumetric shrinkage

Mechanical sliding friction

Pulling resistance Otherwise known as Die Dynamics

2.11 Internal polymerisation balance

given line speed is illustrated by Fig. 2.11. While it is in general desirable to have polymerisation proceed to completion within the die at the highest practical line speed, the optimum position of that reaction will differ, depending upon the resin characteristics and the profile geometry. Unfortunately, there is no current means to continually measure the state of cure of a profile exiting the die. The best method is to intermittently measure the position of the reaction occurring within the die under constant process conditions. Since cure is in effect initiated by input die heat and proceeds with internal exothermic heat generation, the change in temperature within the composite can be measured and related to the axial die position as an indication of progress of the cure. This is achieved by a ‘process exotherm test’ which employs a thin thermocouple inserted into the moving material stream at the die entry, producing a temperature record with time and therefore line speed, as illustrated by Fig. 2.12. This shows the profile of the die temperature together with the resulting material temperature and there are four major features of interest, viz the material entry temperature, the point where the reaction initiates (where there is a change in the slope of the temperature curve caused by a rise of the material temperature relative to the die temperature), the position of the peak exotherm and the material exit temperature. The thicker the product then owing to the rate of heat transfer into what is an insulating

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392 °F

45

Peak exotherm temperature

200°C

Material temperature Die temperature 302 °F

150 °C Die heater set-points

Temperature 212 °F

122 °F

100°C

50 °C

Peak exotherm position Control thermocouples

20 7.8

40 15.8

60 23.5

80 31.5

100 cm 39.4 in

Die position

2.12 Process exothermal result

composite material, the more different is this exotherm behaviour. However, by using this process exotherm technique, it is possible to establish the ideal conditions for resin reaction rate, die temperature set points and line speed; in other words conditions that yield the highest productivity, the lowest scrap rate and the best product quality over a long continuous period. Since pultrusion is a continuous equilibrium-based process, it is reasonable to assume that a measurement at a point in time is representative of the continuous process environment as long as process parameters and input materials are consistent. This is the reason why procedures for correct catalyst and resin mixing, knowledge of the resin reactivity, as well as the consistency of temperature controllers and machine speed are all important. Batch-to-batch variations in resin reactivity and/or variations in line speed or temperature settings can easily throw the process into an unbalanced state where without good troubleshooting based on regular process exotherm evaluation, high levels of scrap result. Machine operation and control After temperature, the main control function of a pultrusion machine is concerned with the actual mechanism employed to pull the profile through the die, and related to that, the speed of its operation which obviously

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governs the speed the profile is actually pulled through the die. As shown by Table 2.3, there is a close relationship between the control of this function and the type of pulling mechanism and in turn its complexity. For a continuous tractor type of puller, the minimum requirements are just ‘start’, ‘stop’ and ‘speed control’, while for a reciprocating puller system with continuous hand-over-hand motion, there are additional functions necessary to sequence the pullers, together with the clamping and unclamping of the profile gripping mechanisms. Like temperature control, once again all these machine functions can be operated from the control panel and/or input into an attached PLC/PC. Finally, an ‘emergency stop’ system consisting of highly visible buttons, grab bars or wires, to both stop the machine and kill the electric power, is essential on all machines in addition to a number of other normal operation ‘stop’ buttons located around the machine. On earlier machines the speed control was a simple rotary knob with an analogue read-out. Recent developments in digital control systems have, however, led to the use of digital speed displays in either metres or feet per minute, adjusted by a rotary control or alternatively by ‘increase’ and ‘decrease’ push buttons. Additionally it is desirable to be able to switch more rapidly to a temporary low-speed mode, should a problem arise. With reciprocating puller machines, it is important to be able to open and close the puller clamping mechanism quickly, particularly during start-up operations, although on simple continuous tractor type machines, manual operation of the upper of the two tracks is adequate. Two further reciproTable 2.3. Pulling systems and control functions Function

Reciprocating pullers

Continuous tractor pullers

Essential functions Emergency stop Start Stop Speed control Clamp on/off

Yes Yes Yes Yes Yes

Yes Yes Yes Yes Yes, with manual adjust

Non-essential functions (but highly desirable) Automatic purge Tandem pull Fast retract Clamp pressure Low speed switch Start-up mode Programmable cut Length

Yes Yes Yes Yes Yes Yes Yes Yes

Yes Yes Not applicable Not applicable Yes Yes Not applicable Yes

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cating type machine controls are also important, ‘tandem pull’ and ‘fast retract’. The first permits both pullers to be operated at the same time; a useful facility when extra force is required to pull a profile that is beginning to seize within the die. As the name implies, ‘fast retract’ brings both pullers back to their starting positions at the maximum speed possible. The clamping pressure applied to the profile by the reciprocating pullers as they are moving forward is another parameter which can feature as a machine control function. For example, an ability to adjust the clamping pressure, is particularly useful when making thin-walled tubes which can easily be seriously deformed by applying excessive force. When a pultrusion machine operates at a constant speed and for several hours with well-stabilised die temperatures, that steady state can allow small particles of resin to stick to the cavity surfaces. By stopping, or ‘purging’, the machine for a few seconds, the temperature profile within the die is altered, causing those small particles to be drawn out when the machine restarts. Some reciprocating type pultrusion machines therefore offer up to three automatic purge options based on either the number of reciprocating cycles, a measurement of the length of profile produced, or finally by means of an adjustable time basis. (The last two are also applicable to continuous tractor type machines.) The length of time that the machine is stopped on such a purge cycle is yet another variable that can be controlled and is typically within the range of 10–15 seconds. Monitoring and data logging Production variables of the whole pultrusion process, together with a batch number record, can readily be monitored instantly and/or logged, to give different levels of information as may be required during a particular working shift, for historical, R&D, quality control or other reasons. They are suitably classified as production information as well as force measurement, and most commercially available pultrusion machines now have comprehensive production monitoring, through a machine control or an external PC, of a range of parameters which will typically include: • • • • • • •

running time, with reset at the beginning of a working shift, the day or a new production run; total length of profile produced; total number of profile lengths produced; temperature zone record including tolerances achieved and alarm conditions to quantify against specific logged points; pulling force and alarm conditions; machine speed; and clamping forces.

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Although the measurement and recording of the actual pulling force (typically in kilograms or pounds) are not critical to the performance of the whole pultrusion operation, it is an important parameter because it illustrates changes in the pulling resistance, and hence in for example, the frictional drag on the reinforcement pack, the drag or viscous shear in the resin impregnation and forming operation, or finally the viscous shear and friction forces within the die. Those changes during a production run, or from a set of previous standards for a particular profile design and specification, are likely to indicate some malfunction or latent problem. Suitable methods of measurement include analogue, bargraph or digital displays of the drive motor current consumption for mechanically driven machines, or the system/cylinder pressures when hydraulic driven machines apply, or otherwise from the output of a load cell secured to a particular machine item. Load cells can directly measure the force being applied by the pullers, the direct push/pull load on a reciprocating platen, or alternatively can log the torque reaction. However, a load cell positioned on the die mounting table is the only way of accurately measuring the actual load within the die.

Process enhancements As intimated at the start of this chapter, the basic pultrusion process is open to a number of unique additional processes. In addition, for example, to the radio frequency and microwave pre-heating already mentioned, those that are positioned prior to die entry include: • • • • • •

on-line reinforcement forming processes such as braiding and circumferential winding, known as pullwinding; encapsulation of core materials; and the on-line printing of veil or tissue materials, while those that apply after die exit include: piercing, routing, grinding and sanding, etc; on-line powder coating, or other finishing treatment; and on-line printing for product identification.

All add to the whole pultrusion process capability, although one or two must be considered as speciality developments which have required a great deal of engineering just to satisfy a particular client’s needs. Typically they also demand a great deal of manufacturing attention, and may perhaps even form the weakest link in the whole process line, leading to frequent and total process shut-down or the generation of scrap material even though the profile is, for the most part, acceptable. However, like that now described, they add important benefits.

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The pultrusion process Counter-rotating winding heads Mandrel and support

49

Fibre spools

Heated die

Pullwound tube

Layer a. fibres

Layer b. wound fibres

Layer c. fibres

Layer d. wound fibres

Layer e. fibres

2.13 Twin-spool pullwinder arrangement

Pullwinding Although the more complex reinforcements (which can also be formed online as a stitched assembly) find their particular place when the highest profile strength or performance is demanded, pullwinding is another optional on-line equipment process designed to provide an equivalent level of reinforcement capability when the profile is tubular in nature. As shown by Fig. 2.13, a typical stand-alone pullwinder has two counterrotating heads, each fitted with a number of fibre spools. A mandrel is supported through the centre of the machine and aligned with the die, and the arrangement illustrated is capable of producing a tube with five distinct fibre layers. The inner (a) is a conventional layer of unidirectional fibres directly on the mandrel, which is then over-wrapped (b) by the first winding head. A second unidirectional layer (c), follows before over-wrapping (d) with the second head and then applying a final outer layer (e) consisting again of unidirectional fibres. To ensure that the total fibre quantity, as well as the orientation or wind angle of the wrapped fibers, remains consistent and as specified, the winding heads are precisely controlled relative to the linear motion.

2.2

Tooling and allied design Introduction

The tooling necessary for successful pultrusion is best considered as being either of a primary or secondary nature, with the former differentiated by the actual die – or tool cavity – which critically defines the profile shape by which the process is accomplished.

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It is necessary now to consider in greater detail the design, manufacture and use of this all-important primary die component of the complete pultrusion machine. The secondary tooling hardware components necessary to manufacture a pultruded profile effectively, such as resin bath guiding and forming guides, post-sizing devices, puller blocks, clamping arrangements and cut-off saw, have all been sufficiently dealt with. However, one additional explanation is essential. The term secondary only truly applies when all those items have been specifically designed and manufactured for employment with a particular die. When any of these components are designed solely for general use and are not therefore specific to an individual profile, then they are not considered as forming any part of the whole tooling package.

Pultrusion die: design and fabrication In effect the pultrusion die is a pressure reaction vessel in which the chosen thermoset matrix resin and reinforcement pack are successively consolidated, heated and finally cured by molecular crosslinking, into the solid composites profile of the desired dimensions and cross-section. With profile production continuous and often on a 24 hours-per-day, 7 days-per-week basis and a reinforcement fibre package that is abrasive with matrices whose chemical composition is often acidic, the combined result is that processing conditions place very severe in-service life demands on the design and fabrication of the pultrusion die. All those conditions will now be addressed and although much of that description is very heavily oriented to thermoset matrices because they remain the most common, with the single exception of the curing stage where there are distinct differences, much the same can apply when employing a thermoplastic-based matrix. Although equally applicable to the production of standard profiles, to the benefit of the reader there is similar descriptive orientation to the custommoulded profile supply situation. Design considerations Of all the design and manufacturing considerations that apply, the prime essential towards the eventual quantity production of a totally successful profile is the employment, through either a well-established pultrusion company or an independent design house, of a designer and tool supplier team who jointly possess an intimate knowledge of the pultrusion process. Nothing is to be gained from the conversion of a customer’s design by a tooling company having little or no knowledge of the process. A tooling project then begins with a close evaluation of the profile geometry presented by the customer for conversion to pultrusion. Frequently it

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is necessary to modify that design to exclude features that would introduce either tooling fabrication or processing difficulties. These might include excessively sharp corners, undercuts, thin section details or areas prone to poor surface replication. However, after several design iterations it should be possible to provide the customer with a computer-aided drafting (CAD) drawing of the proposed profile for approval, which should ultimately form a comprehensive visual and documented record of the tool design and manufacturing history. With the final profile shape and dimensions approved, it now becomes necessary to quantify relative to the chosen reinforcement-matrix specification, the percentage shrinkage or other dimensional movement that can be confidently forecasted to occur during the process. This is a critical step and indeed different values may apply in the horizontal or vertical planes of the profile, particularly where there are major thickness changes or where the cross-sectional shape is complex. In addition, there may all too frequently be an angular shrinkage or ‘toe-in’ at intersecting surfaces such as exhibited by even the simplest of channel sections. Finally, and before any steel is cut, all these movements must be related to the overall dimensional tolerances, degree of profile twist and distortion, etc, acceptable to the customer. Failure to take them all sufficiently into account will later be paid for by wasted product and costly rework to the die. Once all these factors are known and agreed, the design can more critically consider the overall die construction, the alignment and fastening arrangements for the respective die segments or components, as well as eventual fixture of the die to the machine bed. At some appropriate stage, the design team will in addition need to address such matters as die mass, zone heating/cooling arrangements, thermal balance and transfer, and other features of the pultrusion process that enable processing speed and quality to be optimised. Die cavity In order for the cured profile to be drawn through the die, the cavity must be of a constant cross-section over the majority of its length, and particularly so over the entire area where the temperature conditions are sufficient to initiate the curing reaction. Should the process be stopped for a period of time and omitting the cool entry end, then that ‘curing area’ has the potential to extend over approximately 90% of the die length. Consequently, any use of expanded entry tapers and radius geometries at the die mouth have usually to be limited to the first 25–75 mm (1–3 inches) of the die cavity. If not, and without critical control of the process temperatures, should cure extend into any tapered entry area, then the production of cured profile exhibiting a larger cross-section than the remaining die would

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clearly prevent any further forward motion of the profile through the die. However, relief tapers positioned towards the die exit and certainly beyond the mid-point of the die where the profile should have substantially cured, do lower the frictional drag and thereby assist in reducing the required pulling force. Die length Although factors of cost and frictional drag cannot be overlooked, generally the longer the die the better, as longer dies permit more precise temperature zoning and in turn faster process speeds. Lengths of between 900 and 1000 mm (36–40 inches) are typical with a maximum limit of perhaps 1500 mm (60 inches) when economically justified by very high-volume production call-offs. For reasons of economy of supply or as a result of the profile geometry, it can, however, be practical to employ shorter dies, even as short as 200 mm (8 inches) if the requisite heating and cooling conditions can be adequately accommodated. On the other hand process speed is severely constrained with such short dies and their real benefit is in prototyping to provide proof of concept or dimensional qualification, rather than full-scale production. Die construction With the exception of seamless ‘gun-barrel’ tooling, early pages in this chapter indicated that the use of several segments was typical for the construction of a pultrusion die. The precise machining, alignment and subsequent secure fixing of these die components together have been seen as all-important. As the profile cures, an internal die pressure ranging from 3.5 to 13.8 bar (50–200 psi) can be generated by a combination of the fibre ‘spring’ forces and the thermal expansion of the reacting matrix. Consequently, it is essential to retain cavity parallelism and tool ‘tightness’ if witness lines on the profile surface, twist and other dimensional distortion are to be prevented. Each method for ensuring this optimum alignment and die tightness, by the use of precision dowels, keyways or full length offsetting and interlocking die pieces, has a logical place in die design and has associated economic ramifications. Steel selection Alloy steel is the favoured material for die construction and those grades that on the basis of their machinability, toughness, hardness, polishability, wear resistance, durability and economy have been shown to give the best results for the pultrusion process exhibit the typical compositions shown in

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Table 2.4. Although it is generally agreed that the higher the hardness the greater the tool life, a pultrusion die also requires surface lubricity, release characteristics and corrosion resistance, properties which may only be achieved with surface treatment. Accordingly, the majority of pultrusion dies combine a 30–40RC hard steel with a hard chrome (70RC) surface plate 0.025–0.075 mm (0.001–0.003 inches) thick. An alternative treatment to enhance the surface hardness is ion nitriding, while other pultruders choose to use a through-hardened steel of around 55–60RC. Nevertheless, none of these are without problems. While surface treatment is unnecessary with a hardened die, release difficulties and chemical corrosion pitting can, as already noted, be encountered. Hardened steels are also more difficult to repair because the weld restoration techniques employed may limit the rework which is feasible, owing to the possibility of brittle fracture or fatigue failure in thin sections or around bolt threads. Plating, on the other hand, can all too readily strip from the die cavity as a result of inferior application or profile seizure. Die fabrication sequence While the necessary die fabrication stages do not differ markedly from those common to mould-tool making, their brief review highlights several important differences and illustrates the precise care which, for reasons already presented, is essential. Figure 2.14 shows the complete process of die fabrication in a project schedule format. Although the critical lead-time functions will depend on the complexity of the die and the capacity and productivity of the tool-making shop, deliveries as short as 3 weeks to as long as 20 weeks can apply.

2.14 Die fabrication in project schedule format

0.35

0.12

1.0

1.5

0.40

1.0

P20 HH

A2

D2

H13

440C stainless

17.5

5.0

12.0

5.0

0.50

1.7

1.0

Cr

—

0.40

0.35

1.0

1.5

0.80

0.90

Mn

0.50

1.3

1.0

1.0

0.25

0.40

0.20

Mo

4.5

1.0

0.45

0.50

1.0

0.50

0.30

Si

0.50Ni

1.0V

0.80V

1.0V

4.0Ni

0.40V

Other

56

47

30/60

30/60

40

30

28

Rc

VG

VG

E

E

VG

G

F

Weara

Resistance

AISI = American Iron & Steel Institute. Compare % alloy elements for other country specifications. HH = High hard. Rockwell (Rc) hardness values show as received and heat treated values for A2 and D2. a Crossed cylinder wear test. b Charpy C-notch impact test. c Based on 50 ¥ 152 ¥ 1000 mm3 (2≤ ¥ 6≤ ¥ 40≤) ground top and bottom/saw cut sides steel segment.

0.40

P20

C

Weight % alloy elements

4140

AISI specification

P

F

P

F

F

VG

G

Toughnessb

2.50

2.10

2.30

1.85

1.60

1.25

1.0

Unit Costc

54

Table 2.4. Alloy tool steels for pultrusion

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Steel procurement Unless already undertaken by the steel supplier, the requisite die pieces cut from plate or bar are first ground square and parallel to provide accurate registration for all subsequent machining and grinding. It is clearly vital that the outside surfaces are square and parallel to the eventual die cavity. Rough machining Using that registration, the requisite portion of the die cavity is typically machined to within 0.5 mm (0.020 inches) of the net dimensional shape as appropriate to each steel block or segment constituting the whole tool set. With proper cutting tools, a large amount of steel can be removed fairly rapidly, although if the quantity removed is large relative to the width and depth of the block being machined, stress relieving to protect die component alignment and profile tolerances is desirable prior to finishing stages. Each steel grade has a specified stress relief heating cycle that will prevent distortion during subsequent heat treating or other operations. Further, if the die is to be case or through-hardened, both treatments must be undertaken after rough machining and before final surface grinding. It must also be recognised that both can cause a level of distortion sufficient to make it necessary to have to ‘square’ the steel again or even leave a larger final grinding allowance than the recommended 0.5 mm (0.020≤). Surface grinding In this operation, a precision linear reciprocating grinding machine equipped with an abrasive wheel formed to the die contour imposes the appropriate final shape of the die cavity into the respective die block – or segment – components. This grinding wheel is formed with a diamond stylus using either a diaform template tracing machine, or an automatic CNC (computer numerical control) wheel dressing function as available on some computer-based machine tools. Steady linear grinding gradually achieves the final shape of each die cavity segment, by removing between 0.002 5 mm and 0.025 mm (0.000 1 and 0.001 inches) per pass. It is critical that the die parting line surfaces are also maintained or ground flat and parallel to the cavity at this time. On completion of this grinding operation, the surface of the cavity should exhibit a 0.006 4 mm (25 microinch) finish, somewhat rougher than that demanded for successful profile processing. In addition, while flat surfaces (i.e. those that will eventually be parallel to the pultrusion machine bed) ground in this manner will show only linear machining marks along the die length, other surfaces ground by the edge of the contoured abrasive wheel will have circular machining marks typically oriented

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transverse to the die length and therefore the pulling direction. Both must be polished out for successful pultrusion. Assembly and finish machining After all the die segments have been ground to size, they are assembled and firmly clamped together before being further machined as necessary to accommodate such items as precision dowell pins, cartridge and strip heaters, thermocouples, forming guide mounting threads and finally socketheaded cap screws or as otherwise required to fasten the die together securely and eventually to the pultrusion machine bed. If keyways or similar locking alignment devices are used, then their provision may have already been completed during the previous machining and grinding stages. It is important at this point to recognise that because the die will be broken into its separate components, cleaned and then reassembled frequently throughout its life, these operations are part of the overall design consideration. Whenever possible, therefore, fastening systems, for example, are best oriented from just one die surface such that unnecessary movement of what are typically heavy die segments is avoided. Cavity modification That work is then completed by forming the entry end detail, the simplest of which is the provision of a constant radius around the entire profile perimeter on that face of the die. A typical entry radius is 6 mm (0.25 inch), a size that does not over-stress the fibres constituting the reinforcement pack and has the effect of also gradually stripping away the surplus resin when employing wet-bath impregnation. Many pultruders also favour in addition to an entry radius at the die mouth a continuing but short entry taper which imposes a steadily increasing compressive force on the still unformed profile, rather than an abrupt consolidation. Both assist in removing any entrapped air, and also result therefore in a reduction in the incidence of voids within the profile. Another benefit of an entry taper is a reduction in the die wear which almost always initiates from this point of the cavity as well as from the corners and the extremities of the profile geometry. If the die cavity is of a geometry that would allow either end of the die to be the entry, then double the die-life can often be realised. In this situation, the entry end radius and taper geometry must obviously be provided on both ends of the die. However, if the die can only be operated in one direction, then the exit radius can be as small as 1.5 mm (0.062 inch) and necessary simply to prevent the possibility of a final sharp edge scraping the finished profile as it exits the die.

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Polishing After all these detail modifications are complete, the cavity is ready to be polished, a time intensive procedure where the surface finish is improved from the as-ground quality of 0.006 4 mm (25 microinches) to at least as good as the 0.002 5 mm (10 microinches) suitable for most materials processed by pultrusion. Some pultruders, however, specify an even smoother 0.001 mm (4 microinches) as required for the processing of difficult materials, of which epoxy can be one. Polishing is achieved by using a sequence of abrasive papers starting at 320 grit and moving progressively down to 600 grit, and then perhaps even diamond paste compounds to achieve the required final smoothness. During the process and as a means of achieving the best release characteristics, it is important that all the polishing is done in the eventual pulling direction. It is also vital to avoid rounding-off the sharp parting line edges of the die segments or otherwise a pronounced witness line will be created on the pultruded profile. A quantitative assessment of the degree of smoothness achieved is obviously essential and here the use of surface profilometers is recommended. They are inexpensive and in providing a critical assurance of the polished quality, are preferred to optical evaluation procedures which, although useful, can only measure the uniformity of the polish. As nothing is to be gained from over-polishing the cavity, the die specification should clearly indicate the surface finish demanded prior to plating, after which the cavity must again be polished.

Plating The die cavity is now ready for electrolytic plating with, as has already been noted, a hard chrome that will accurately replicate the cavity substrate rather than providing any levelling or masking of defects. It is important, therefore, that the plating company works closely with the toolmaker. The plater must understand, for example, that any scratches or gouges that occur in the cavity area as a result of handling will have a serious effect on the die’s ability to produce pultruded profiles successfully. In addition, the uniformity of that plate both along and across the cross-section surfaces of the whole die cavity will be critical in determining the processing behaviour, the degree of wear and eventual in-service life. Any chrome build-up on sharp corners or deficiency in sharp female radii are characteristics that can cause problems to the toolmaker and the end user: both can be alleviated somewhat with proper anode design and carefully controlled plating cycles. Another unacceptable aspect of plating is

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edge roughness or a practice termed ‘masking’, when the chrome is plated exactly to the cavity edge. Much more desirable is to allow the chrome to ‘flow’ over the edge and onto the parting line surface of each die segment and from where this excess chrome can be later removed by the toolmaker to leave a sharp and smooth edge to the cavity. This finishing operation is typically known as ‘sparking’ the parting lines. Finally, the plated cavity is repolished with particular attention given to any areas of pitting, roughness or chipping which might cause a later problem. Defects within the plate that occur in the critical area of resin gellation can, for example, result in profile adherence to the die and thus, cosmetic defects – or worse – on the profile surface. Again chrome chipping on the parting line edges can cause glass abrasion during processing which will not only be revealed as defects on the profile, but will prompt early die wear in this area. Final assembly Once all the dowels, keys, fasteners, etc, which have been lubricated for ease of maintenance are assembled to the die, then the heater elements, thermocouples, platens, etc, can be introduced, making the whole die assembly ready to manufacture its first trial pultrusions. Figure 2.15 shows the end view of a typical multipiece pultrusion die of considerable complexity with the respective items highlighted for clarity.

2.15 Pultrusion die end view

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Additional die design and fabrication considerations Mandrels for hollow profiles The above die design and fabrication sequence descriptions solely addressed solid shapes, whereas hollow section profiles are far from uncommon and the inclusion of more than one hollow section within the profile geometry is certainly also frequently necessary. As outlined in the first part of the chapter their production requires the use of a mandrel – or mandrels – typically rigidly fixed to a support bracket fastened to the entry end of the die and cantilevered forward through the whole length of the die cavity. A substantial length of the mandrel must also extend backwards from this support, simply to accommodate material forming. In other words, the profile is formed around, cured and finally drawn over this stationary mandrel. A typical mandrel configuration and mounting arrangement are shown in Fig. 2.16, from which it is clear that there are several special requirements of that whole design needing consideration.Their fabrication, however, does not differ in substance from that already described. Sizing the mandrel length is critical and closely related to the reinforcement pack employed. Too short a mandrel will introduce fibre placement problems, while too long a mandrel will present support and alignment problems as well as increasing tooling costs, even though less expensive materials can be used for that part of the construction that is outside the die cavity area. Mandrel length also frequently exceeds grinding machine capacity, making the use of assembled designs employing welding or pinning techniques necessary. Furthermore, the mandrel support must not interfere with the reinforce-

Inner mat guiding and folding Die

Unidirectional fibres

Mandrel support Mandrel

Supporting framework bars Outer mat Constant velocity mat folder

2.16 Mandrel and support design

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ment input paths and one feature of the design that demands considerable pultrusion rather than tool-making experience. That support must also allow for the very critical centring and concentricity adjustment, although once centred at start-up, the steadily forming profile will, if properly balanced and given optimum and consistent reinforcement wrap-over, serve to keep the mandrel in its optimum position. This is especially true of small mandrels where no amount of support or alignment precision can overcome the inherent longitudinal flexibility of the mandrel. Providing an adequate entry-end mandrel geometry reduction is often required as this aids the correct forming of the reinforcement totally free of wrinkling, both around the mandrel and against the die cavity surface. A transition from the reduced to the final geometry occurs gradually as the reinforcement pack enters the die. An exit-end relief taper is also advantageous since during the matrix curing stage there will be shrinkage onto the mandrel which otherwise would cause excessive drag. The exact location, size and degree of these transition tapers can only be determined from knowledge of the cure characteristics of the particular resin being processed. The same considerations equally apply to profiles with multiple and adjacent mandrels such as those necessary in the production of multicellular profiles for beams, planking and similar applications. In this case, however, it is also necessary to provide additional separation between each mandrel to accommodate the reinforcement forming arrangements but without creating any excessive drag. Because mandrels have to be supported in a cantilever fashion and are therefore lengthy, weight reduction is often necessary and this is typically achieved by boring large internal weight-relief holes or by initially employing tubular materials whenever practical in the construction. Consideration must also be given to the critical matter of the thermal influence of the mandrels on the processing parameters. Consequently the problem is overcome by incorporating active heating elements such as internally located electric cartridge heaters, by circulation of steam or hot oil or with the use of static heat pipes (gas filled conductive heat exchangers) fitted into the mandrel. While effective in transferring heat from the centre of the mandrel into the profile as an aid to through-cure, all these solutions clearly exacerbate other described problems surrounding the use of mandrels generally.

Pultrusion die: further considerations and use Wear considerations No matter how well a pultrusion die is designed, fabricated, plated and polished, the continual abrasive action of fibre dragging against the cavity

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surfaces will eventually cause wear to occur. The goal is to extend the life of the die as long as possible through the initial selection of materials, by the use of surface treatments and with the thoughtful design of die entry geometry. It is not always possible, however, to fault any of these parameters when premature wear does occur. The pultruder must take responsibility for careful fibre alignment during forming and the use of the correct widths of input mats and fabrics. Any improper forming or excessive fibre packing, especially at the extremes of the profile section, can rapidly degrade the surface of the die cavity. Consequently it is extremely difficult to predict or guarantee die life, although given favourable conditions the production of some 33 000–150 000 m (100 000–500 000 feet) of profile is not atypical before some either minor or major rework of the die becomes essential. As already intimated, profiles with high fibre volume fractions will dramatically reduce these levels of life expectancy, although even in that situation the use of a soft thermoplastic surface veil, as a less abrasive shield to the reinforcement fibres adjacent to the die surface, can effect a dramatic improvement. Die cavity surface treatments As the surface condition of the die cavity obviously plays a large part in the successful use and life of a pultrusion die, some amplification of this whole topic seems appropriate. As previously mentioned, problems encountered with chrome plating include poor uniformity of coverage, poor adhesion and the need for secondary steps to complete the die after plating. Also because it is an electrolytic process, the deposition of chrome will favour areas of high current density such as sharp corners. Conversely, female sections with small radii are difficult to chrome plate and unfortunately these areas often occur at the ends of the profile where excess glass is likely to provide a very high abrasive stress. Careful anode design and moderate plating rates can help improve uniformity of deposition. Today the common practice is to rely upon hard chrome plating to impart a surface to the substrate steel that provides the functions of wear resistance, corrosion resistance, lubricity and release characteristics. While it is possible to pultrude with an unplated surface, the longevity of the die and the incidence of surface defects on the profile from release problems can quickly justify the expense of chrome plating, even if only of a ‘flash chrome’ nature where a thickness of 0.012 mm (0.0005 inch) or under applies. Such a treatment is in fact eminently suitable for round profiles and those of a low fibre volume or containing high weight additions of soft filler materials, which totally shrink away from the die wall. However, as already noted

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the general practice that provides a good balance in wear resistance, uniformity and cost is hard chrome deposited at a thickness of 0.025–0.05 mm (0.001–0.002 inch). The plate can be applied heavier, and it is the practice of some pultruders to apply 0.05–0.1 mm (0.002–0.004 inch) even though this may mean that it is then necessary to regrind the cavity to restore dimensional tolerances owing to the excessive local chrome build up that can occur. Although chrome plating will accurately replicate the polished substrate, it will not fill pits or scratches. It is also known to exhibit a microcrack structure and as a result of the plating treatment this can extend into the substrate metal, providing a pathway for corrosive attack and thus a mechanism for mechanical failure from the compressive and shear stresses experienced during pultrusion. One partial answer to this problem is to employ a selective plating procedure that deposits a very dense chrome exhibiting a much finer microcracked structure. There are also processes that will infuse polymeric materials into the microstructure as a means of reducing corrosive penetration and these have the further advantage of enhancing the surface lubricity and thus improving the profile release properties. However, the real effectiveness of these typically proprietary alternatives has yet to be reported in the literature. There is then the difference in hardness between the substrate metal and the hard chrome, which is believed to work against both the optimum adhesion and retention of the plate to the substrate. Indeed there is some evidence that by employing a harder die steel, the life of the chrome plating can be extended. Here pre-hardened steels, heat treatment during tool manufacture or, as also described and again considered later, ion nitriding are all possible solutions. Nevertheless it does needs to be recognised that the hardest practical substrate retaining some rework possibilities is around 55RC. This still leaves a substantial spread between the hard chrome at 70RC and the substrate. As a final comment, methods of toughening the substrate steel by such techniques as cryogenic treatment are also believed to have merit in addressing this problem. As a way of eliminating the plating uniformity problem, some pultruders have found the answer in nickel or nickel/chrome electroless plating. Nickel certainly provides a ‘softer’ interface between the die cavity surface and the hard chrome, and although nickel by itself does not have sufficient hardness for long-term durability, its interfacial presence does appear to reduce the level of mechanical strain that would otherwise apply. Finally, there are a number of other surface treatments that have been extensively evaluated such as titanium nitride and other diamond-like coatings. While typically they are very hard, because in practice their thickness can only be of the order of a few micrometres, they are rapidly stripped

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from the cavity surface by the action of the abrasive conditions that apply. Another treatment is ion impingement to diffuse nitrogen into the structure of the hard chrome plate. Although employing high-energy sources requiring vacuum chambers which without rotational arrangements limits treatment to line-of-sight application, this technique is beginning to show some potential. Nevertheless, these sophisticated surface enhancements often come at a substantial price disadvantage, so once again the issue of extending die life is reduced to a balance of cost, function and the use of good manufacturing and die maintenance practices. Rework procedures Fortunately most dies can be repaired effectively and at reasonable cost if the onset of wear is detected early enough. It is therefore imperative that the pultrusion company establishes a practice of frequent and regular die inspection. Where a cavity has been hard chromed, the simple action of wiping a copper sulphate solution over the surface will identify any loss of chrome by turning the substrate steel a copper colour. If that is the sole damage, then it is a simple and relatively inexpensive procedure to strip the die of the remaining chrome, to polish the cavity and replate the die. Such a procedure can be repeated numerous times to extend the die life, but it must also be acknowledged that some surface defects that can occur with time, such as chemical pitting, may eventually preclude this simple restoration treatment. In that situation, and if the profile tolerances and/or geometry allow, the die surface may be restored by grinding the cavity as little as 0.25 mm (0.010 inch) deeper to eliminate all visible defects, prior to replating. Where die maintenance procedures have been overlooked – and this is too frequently the norm – damage will extend beyond simple chrome loss and pitting to severe wear of the substrate material. When this applies, the repair procedure comprises stripping the chrome, followed by a weld repair of the worn area/s and then regrinding the cavity prior to polishing, plating and otherwise as initially manufactured. On the other hand, dies that have been ion nitrided or through-hardened are much more difficult to repair in this fashion owing to the possibility of weld porosity. However, as the repair work required steadily becomes more extensive, then a point is reached where it is simply a question of an economic decision whether to restore or totally replace the die. Even so, rework is often chosen regardless of cost simply because in those cases where wear has disrupted production, time becomes of greater importance. Generally most rework is much quicker to complete then the fabrication of a new die.

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2.3

Conclusion

As expressed later in Chapter 8, there is little doubt that pultrusion machines will steadily get bigger. With increasing confidence in the ability of composites materials generally to offer the performance required, for example, by major structural applications, there will therefore be no real limit to the profile size and complexity which it will be practical to manufacture. At the same time and most significant will be a better understanding and control of the whole process. Consequently the following predictions regarding various aspects of the whole pultrusion process can be made with some confidence.

Materials As has been seen and as is the province of Chapter 5 to clarify, many types of glass, carbon, aramid and hybrid reinforcement are already used, but the use of multiaxial, braided and specially formatted reinforcements will markedly increase, particularly for high-performance profiles. Commensurate with all this, resin and catalyst systems will continue to be developed to produce better performance, better quality profiles, at faster production speeds.

Machines and machine controls Although it is unlikely that there will be any radical changes in the overall design of the pultrusion machine, developments in process control will influence the type of control system used. It is likely, therefore, that some dedicated industrial PC control will increasingly feature, allowing machines to be programmed to suit specific profile requirements using an expanded version of for example the already existing ‘Pultext’ software program.4 At start-up, such programs will set and continue to control the whole process and production parameters in optimum fashion. In other words, if the speed is increased or other change/s is imposed, then all the remaining production parameters will adjust through that closed control loop, to new optimum set points. Such a system will also data log those conditions and retain them for beneficial use on subsequent production runs.

Saw-cut and take-off systems Here there will certainly be advances in the handling of profiles after the cut-off operation, particularly for high-volume products. Automatic labelling, bar coding and automated bunding are areas open for greater process efficiency.

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65

Summary

While basically a simple composites fabrication technique the two parts of this chapter have shown that there is a great deal of precise process and tooling detail – as well as expertise – associated with the successful manufacture of pultruded profiles. Although with some justification pultrusion is often referred to as a high-capital-intensive technique, it should more correctly be classified as a material-intensive technique. Compared to every other well-established composites fabrication technique, it rapidly consumes the two raw materials, reinforcement and resin in large quantity to produce in a continuous manner a consistent, first-quality engineering material. As further chapters will confirm, over many market and application areas, pultrusions now offer considerable opportunity for future use and growth. Whether in respect of materials and machinery, the whole process is also open to considerable development and pultrusion is one composites fabrication technique that will with confidence on all available evidence, still be extensively practised as humans move from the 21st to 22nd century.

2.5

References

1. Peters, L, Badding, P, Gauchel, J V, Beckman, J J, ‘High pressure resin injection die technology for emission control in the pultrusion process’, EPTA 3rd World Conference, June 1996. 2. Shaw-Stewart, D E, ‘Production of the pultrusion process’, 35th Annual RP/CI, SPI Conference, January 1980. 3. Sumerak, J E, ‘Pultrusion die design optimisation opportunities using thermal finite element analysis techniques’, 49th Annual RP/CI, SPI Conference, Cincinnati, February 1994. 4. Pultext pultrusion programming language, Pultrex Limited, Brunel Road, Clacton-on-Sea, Essex, CO15 4LU, UK.

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3 Profile design, specification, properties and related matters DAVID EVANS

3.1

Introduction

Futurists have claimed that in general our technology is advancing so rapidly that we now live in a virtually new world every 11 months. While for the majority such a statement may be related only to computers, the reality is that changes are indeed everywhere, from medical and farming practices to materials selection and technology. The first design manual for structural plastics was published by the American Society of Civil Engineers in 1979,1 and in conjunction with other confirmatory evidence this had a large effect in moving ‘plastics’ from a cheap, kitchenware and children’s toy material into a serious design option for a wide variety of engineering markets and applications. However, unlike other ‘plastics’ which have also eventually found industrial, engineering and structural uses, reinforced plastic composites have increasingly confirmed that they possess a unique ability to be critically designed for optimum finished product performance, whether that be based on physical, mechanical, environmental or long in-service economic assessment. Increasingly, particularly since 1980, one of the preferred composites fabrication processes is pultrusion.

3.2

Common pultrusion materials

It is estimated that over 85% of the pultrusions currently manufactured world-wide are a composite of E-glass fibre, typically continuous unidirectional glass rovings or continuous strand (or filament) mat (CFM), but at times chopped strand mat (CSM), ‘encapsulated’ within a resin matrix – often in a fire-retardant grade – of either ortho- or, more often, iso-phthalic unsaturated polyester or vinyl ester. The balance is composed of other E or even A-glass woven fabrics and reinforcements as well as those based on carbon or aramid fibres and their respective hybrids with yet another option, three further thermosetting resins based on acrylic, epoxy or phe66

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nolic chemistry. All these many choices receive in-depth consideration in Chapters 4 and 5 and it is worth emphasising that as profile performance demands increase, there will be a continuing move away from what can currently be termed the 85% E-glass–polyester resin norm. In fact, recent marked reductions in the cost of certain grades of carbon fibre can for example be confidently expected to promote a pronounced increase in its use as a pultruded profile reinforcement. Finally, and parallel to other composites technology development and commercialisation, there is a slow but steadily increasing interest in the use of thermoplastic matrices as typified by polypropylene.

3.3

Profile: design

Pultruded profiles can be readily classified as standard or custom.

Standard profiles This term is usually reserved for those profiles (Fig. 3.1) that are classified as ‘standard’, simply because they are all manufactured by virtually every pultrusion company around the world and with a commonality that also reflects their replication of similar timber, aluminium or steel sections. As a result they clearly find extensive use, although it must be noted that the dimensional (Table 3.1), physical and mechanical properties while exhibiting close batch-to-batch consistency from any one manufacturer, may not necessarily exactly duplicate those for an otherwise totally identical profile from another manufacturer. Equally many other ‘standard’ profiles are available than the typical selection of bar, hollow, rectangular, round, channel, angle, I and H-sections illustrated. Manufacturers’ catalogues and technical literature should therefore always be consulted.

Custom profiles As the name implies, this term is usually applied to those profiles that are designed, specified and manufactured for a particular customer, who in turn may also own the tooling. Occasionally, but typically only after many years of production, such profiles may eventually find their way into a manufacturer’s standard profile catalogue. Although the ‘standards’ through their typically long case histories have very clearly proved many of the preferred attributes of design and specification, it is the custom profiles that in terms of shape complexity, dimensional tolerance and specification continue to develop both the process and technology of pultrusion. As a clear demonstration of what can now be successfully pultruded, four examples in volume production are now described

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Pultrusion for engineers A Round 10 mm ≤ A ≤ 80 mm 0.4” ≤ A ≤ 3.15” A Square 10 mm ≤ A ≤ 80 mm 0.4” ≤ A ≤ 3.15”

A

A

B

Rectangle A = 2B 10 mm ≤ A ≤ 120 mm 0.4” ≤ A ≤ 4.75”

A

B

Flat A = 10B 10 mm ≤ A ≤ 240 mm or more 0.4” ≤ A ≤ 10.0” or more

A Tube 10 mm ≤ A ≤ 200 mm or more 0.4” ≤ A ≤ 8.0” or more

C

A Box beam, square 20 mm ≤ A ≤ 200 mm or more 0.8” ≤ A ≤ 8.0” or more

A C

A

C

A

Box beam, rectangle, A = 2B 20 mm ≤ A ≤ 320 mm or more 0.8” ≤ A ≤ 12.5” or more

3.1 Selection of standard profiles (Courtesy, European Pultrusion Technology Association)

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‘U’ channel, square 20 mm ≤ A ≤ 200 mm 0.8” ≤ A ≤ 8.0”

B C

A

B C

‘U’ channel, A = 2B 20 mm ≤ A ≤ 200 mm or more 0.8” ≤ A ≤ 8.0” or more

A H section beam 50 mm ≤ A ≤ 320 mm or more 2” ≤ A ≤ 12.5” or more

C A

A C B

I section beam A = 2B 50 mm ≤ A ≤ 480 mm or more 2” ≤ A ≤ 19.0” or more

A

L section, equal leg 20 mm ≤ A ≤ 200 mm 0.8” ≤ a ≤ 8.0”

A C

A

B C

L section, unequal leg, A = 2B 20 mm ≤ A ≤ 200 mm 0.8” ≤ A ≤ 8.0”

A T section 20 mm ≤ A ≤ 200 mm or more 0.8” ≤ A ≤ 8.0” or more

B C B

A C

3.1 Continued

Z section, A = 2B 20 mm ≤ A ≤ 200 mm 0.8” ≤ A ≤ 8.0”

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Table 3.1. Indicative dimensions standard glass/polyester profiles and tolerances Section

Dimension A

Dimension B

Dimension C

Round

0.85–27.5 0.035–1.10 8.0–25.4 0.30–1.0 8.0–25.4 0.30–1.0 6.35–44.0 0.25–1.75 6.35–44.0 0.25–1.75 40.0–51.0 1.6–2.0 11.65–300.0 0.45–11.75 8.0–102.0 0.30–4.0 25.4–83.0 1.0–3.25 20.0–94.0 0.80–3.70

— — 8.0–25.4 0.30–1.0 2.75–25.4 0.10–1.0 1.60–9.50 0.06–0.35 — — 20.0–25.4 0.80–1.0 3.80–51.0 0.15–2.0 25.4–203.0 1.0–8.0 25.4–44.0 1.0–1.75 16.0–95.25 0.60–3.75

— — — — — — — — 1.25–2.54 0.05–1.0 3.0–6.0 0.12–0.24 1.75–4.0 0.65–0.16 7.0–9.5 0.275–0.375 3.18–5.0 0.125–0.20 3.45–5.2 0.13–0.20

Direction

Dimension

Tolerance

Thickness (mm)

<5 5 <0.2 0.2 <3 12 25 50 100 >100 <0.12 0.5 1.0 2.0 4.0 >4.0 <3 6 >6 <0.12 0.24 >0.24

±0.25 ±0.35 ±0.01 ±0.014 ±0.15 ±0.2 ±0.25 ±0.35 ±0.5 ±0.75 ±0.006 ±0.008 ±0.01 ±0.014 ±0.02 ±0.03 ±3 ±6 ±10 ±0.12 ±0.24 ±0.4

(mm) (inches) Square (mm) (inches) Rectangle (mm) (inches) Flat (mm) (inches) Tube (mm) (inches) Box beam/ (mm) rectangle (inches) U channels (mm) (inches) I & H sections (mm) (inches) L angles (mm) (inches) T section (mm) (inches)

Thickness (inches) Width and depth (mm)

Width and depth (inches)

Length (mm)

Length (inches)

NB Many other standard profiles are available and tolerances may differ between suppliers, who should also be contacted in respect of other dimensional tolerances, e.g. concavity, convexity, twist and straightness. Courtesy, Fiberforce Composites Ltd.

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3.2 Profile for camper construction (Courtesy, Holiday Rambler Inc)

and illustrated; many other suitable examples appear elsewhere in this book. Baggage door This component (Fig. 3.2) is only one of a large number of custom (and standard) profiles now being employed by many of the world’s established vehicle manufacturers whether for motor homes, campers, buses or trucks. This particular one-piece assembly, which might also act as an air conditioner duct, an electric light support and associated conduit, combines what was previously a multiplicity of sub-parts. Unit assemblies of this type are very common in pultruded profiles generally, their application not only reducing assembly time but also contributing to further weight savings beyond the use of separate profiles. Window lineals Profiles for window and door frame construction (Fig. 3.3), typically exhibit as here, highly complex and close-fitting cross-sections. Over recent years they have become increasingly common because their use totally eliminates the need for the thermal breaks normally associated with aluminium versions. Furthermore, and also neglecting their more optimum weatherresistant properties, the structural integrity of the composite profile offers strong competition to U-PVC extrusions. Indeed, the use of in effect standard profiles to internally strengthen the latter as a post-extrusion operation, is not unknown.

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Pultrusion for engineers Double glazed window unit

68 mm 2.687”

68 mm 2.687” Pultruded profiles

NB The respective profiles ‘click’ together on assembly.

3.3 Typical mullion/transom window frame lineal (Courtesy, Inline Fiberglass Inc)

3.4 Airfoil section (Courtesy, Cofimco USA)

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Airfoil section While impossible to classify as a high-energy wind generator, many of these small windmill units (Fig. 3.4) find excellent application as water pumps or otherwise in the generation of a domestic or farm electricity supply, typically in very isolated locations. Replacing the former three-piece spliced design which was susceptible, to rainwater, ice and snow attack, their efficiency depends on a carefully designed pultruded profile which must not degrade, twist or fail in any mode. That has all been successfully achieved with virtually no maintenance input. Modular building panel There is a slow but steadily growing use in the pultruded manufacture of modular panels (Fig. 3.5) which interlock or can be otherwise assembled into complete structures suitable for any number of applications. Typical is the housing of service company plant or, at the other end of the spectrum, electronic early-warning equipment. Little or no supporting framework is

3.5 Building constructed from pultruded profiles

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necessary and the lightweight property of each panel aids shipment to site, reduces the site and foundation work necessary, as well as enabling the building to be easily erected and if required, later dismantled. Finally, such panels can be readily foam-filled to enhance thermal insulation and the excellent environmental/weather resistance of composites has a direct influence on the low long-term maintenance input required.

The design process In addition to providing an excellent illustration of the design flexibility possible with pultrusion, none of these four profiles replicates an already existing design manufactured in an alternative material. That is important because such a procedure invariably leads to failure with pultruded composites owing to their anistropic nature. In other words, every opportunity must be taken to optimise the particular benefits and unique advantages offered not just by composites, but also by pultrusion. Indeed, in the case of the window lineal an attempt to include screw assembly pockets in the design identical to their aluminium counterparts proved much less effective and moreover significantly increased production difficulties and cost. Consequently it was necessary to develop alternative and, to ultimate advantage, more successful assembly methods. Taking a completely different example, the pultruded box beam structurally outperforms a conventional steel I-beam primarily because the composite version negates the web/ flange shear associated with the conventional configuration. In other words, very close liaison between manufacturer and customer is vital at every stage during the design and specification of a custom moulded profile, ideally starting as soon as the profile is conceived. Only in this way will the total advantages of an optimum combination of composite material properties and the pultruder’s expertise be realised in the most costeffective manner possible. Equally, that is a process that must also include, as now discussed, the resolution of such closely related matters as production rates, part-to-part reproducibility, surface finish, appearance, durability and last, but certainly not least, the agreed dimensional tolerances of the profile.

Profile thickness, production rates and part-to-part reproducibility Although the upper limit will slowly grow commensurate with further thermoset resin and cure developments, pultrusions are already produced in a variety of section thickness typically ranging from 2 to 30 mm (0.08–1.2 inches). However, it is essential to recognise that because the rate of matrix

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polymerisation (cure) is influenced by the profile section thickness, there is, at least theoretically, a maximum production speed possible for a particular profile at a particular matrix/reinforcement specification. Not unexpectedly then, those interrelating factors also show a very close relationship to the eventual profile price per unit length, per batch quantity. For this reason, the traditional practice with, say, aluminium and steel sections of simply increasing the thickness of any part of the cross-section as a ready means of enhancing the structural capability is not necessarily the best answer where pultrusion is concerned. If an increase in section depth is not practical, then the only alternative usually possible is a total re-design of the whole profile. Further, although thermally optimised die-heating systems2 can assist in the successful manufacture of ‘unbalanced’ profiles, ideally the design, if not also the reinforcement pack, should always be uniform about the centre-line or otherwise ‘balanced’, typically about the neutral axis. In the same fashion, the use of fillets across section changes as a way of providing additional strength, to prevent corner cracking or other in-service failure at that radius, is counter-productive with a pultruded profile. Owing to their shape and size, fillets can only be created by additional reinforcement which may be prone to cracking and thus create stress risers completely negating, therefore, the effect of the additional cross-sectional area of that fillet. At the same time, a fillet area may prove difficult to ‘fill’ with reinforcement, leading as shown by Fig. 3.6, to the possibility of resin richness and thus weakness at that part of the profile section. Moreover, resin richness can cause resin pickup on the surface of the die cavity leading to profile seizure within the die or, at worst, serious spalling damage to the cavity. Consequently, when designing pultruded profiles, every attempt must be made to maintain the same part thickness throughout the section change at the radii to avoid production or later in-service problems. However, and as also illustrated by Fig. 3.6, pultrusion in comparison with the traditional material forms offers the distinct benefit of providing a homogeneous and consistent level of ‘continuous’ reinforcement through such a ‘blended radii’ cross-section change. In effect, therefore, a stronger section change is generated than would otherwise result. Variations in the wall thickness across the profile section can also affect both the tolerance level of all the remaining dimensions and the part-topart variances. Furthermore, since it is an inherent property of the pultruded profile for dimensions to shrink by up to 2% as it cures, exits the die and cools, problems such as linear part distortion – or twist – can also occur in any direction, particularly if care has not been taken in the profile design and reinforcement pack specification. The distribution of the latter and its

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Bad

Better

Better

Best

R = 1.5 mm (0.06”) R = 1.5 mm (0.06”) R = 1.5 mm (0.06”)

R = 0.25 mm (0.01”)

R = 1.5 mm + thickness of profile (0.06”+ T )

Reinforcement follows through the radius

Resin-rich area – can occur as a result of inferior tool design, and also at internal radii, particularly if attempting to create a fillet (see text); placement of rovings at radii can prevent such problems.

3.6 Treatment at pultruded profile radii (Courtesy, Fibreforce Composites Ltd)

volume fraction are also important, as is the cure regime chosen for the profile. The latter is in turn related to the particular grade and type of thermoset resin employed but, as already suggested, equally to the die temperature and length (Chapter 2.2), as well as the selected pulling speed. For obvious reasons this dimensional shrinkage and possible distortion must also be taken into consideration in the design of the tooling itself, if the desired profile dimensions, angularity and configuration are to be achieved. For example, in the case of even a simple channel section where ‘closure’ across the open end can occur, it is typically necessary to machine the tooling with a 2° outward taper to compensate. One over-riding and closing comment under this sub-heading is essential. All the above is based on the still normal practice of using platen or cartridge-type heaters to raise the die temperature as necessary to initiate the cure. Even with close microprocessor control, it is difficult to achieve

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that level of critical temperature control which could resolve some of these noted design and associated problems. Consequently, this is another reason for the development and recent commercialisation of thermally optimised die systems,2 whereby the profile cure cycle can be modelled and continuously monitored and controlled in such a way that it is now practical to pultrude perfect non-uniform cross-section profiles at a steady rate of reproduction.

Surface finish, part appearance and durability Unlike the majority of composite fabrication processes, under normal operating conditions the pultrusion process produces profiles with a surface that is relatively fibre-rich. For many applications and for a variety of reasons that circumstance is unacceptable. Although visually rarely not too detrimental, the eventual in-service environmental exposure can lead to a condition known as fibre-blooming, which, if severe, can ultimately result in serious deterioration of the composite structure itself. However, by completing the outer layers of the reinforcement pack with a polyester or polypropylene-based surfacing veil – or tissue – a resin-rich surface results which not only improves the depth of colour and overall appearance, but also enhances the weather/environmental performance. The action of the veil is to ‘push’ the actual reinforcement pack very slightly below the external surfaces. Further enhancement of the profile in respect of corrosion resistance is provided by the use of A and C-glass tissues together with resin matrices such as vinyl esters which are specifically formulated for that market sector. Dyed and printed veils can also be employed to further improve the surface appearance and colour depth of profiles used for decorative application or, taking another example, to provide a finish which replicates timber when the profile is to be employed in furniture manufacture. A final tissue version based on carbon will produce surface conductive profiles for static grounding applications. Like other composite components which frequently use an unreinforced resin gel-coat to provide not just a decorative, but also an environmentally resistant surface to protect the underlying reinforcement, pultruded profiles can be finished on-line with a variety of surface treatments. Typical in the manufacture of ski-poles, is on-line epoxy powder coating but urethanebased paint or other treatments applied using secondary off-line stations after the cutting-to-length operation are far from unusual. In addition to improving the visual appearance, such treatments often enhance the ultraviolet resistance under outdoor exposure conditions. In a more recent window lineal development, co-extrusion with vinyl polymers creates a sufficiently thick coating to allow separate profiles to be ultrasonically welded into larger assemblies.

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Finally, and further illustrating the versatility of pultruded profiles, it is clearly practical to enhance the stiffness of any profile by the use and/or specialised placement of suitable fibres – such as carbon – within the initial reinforcement pack. Otherwise the addition of a shallow rib/s or other section changes can add rigidity to a profile.

Dimensional tolerances Through organisations such as the Pultrusion Council of the Composites Industry3 and the European Pultrusion Technology Association (EPTA),4 which support the world-wide pultrusion industry, standard dimensional tolerances have been established by the industry and accepted by such authorities as the American Society for Testing Materials (ASTM). Table 3.1 in conjunction with Fig. 3.1 has already indicated the nature of the dimensional information now widely available. Typically, however, these dimensional tolerances are fairly general and do not necessarily reflect the best that can actually be achieved by the process. Tighter tolerances can therefore apply but as this can add to the overall cost, it needs to be a matter for discussion and agreement between supplier and customer. These meetings should ensure that the end use requirements fall within the range which are reproducible by the process for a particular profile and specification.

3.4

Profile: specification and production

Like any quality product and as already intimated there is a need for early discussion and close agreement between the pultruded profile supplier and the customer. This is equally true for all reinforced plastic composites as well as pultrusions and is particularly apposite when, like pultruded profiles, the composite material itself is to be formed and the eventual product manufactured, at the same time. Further, although the guidelines that follow are more applicable to custom-moulded rather than standard profiles, certain agreed customer approvals are of course just as desirable for the latter. That requirement is best answered by relating the purchase order to a detailed and agreed profile production-supply specification whose clauses quantify those parameters that are judged important to both supplier and customer. As relevant, these clauses should in turn make close reference to the many internationally recognised standards now published covering composites technology generally and pultruded profile manufacture in particular. Commensurate with the expansion of the pultrusion market, moves to enlarge the latter category have been put in hand over recent years. One important example is the attempt by the European pultrusion industry

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through EPTA to persuade the CEN (Comité Europèen de Normalisation) to compile a standard5,6 which should provide a reliable level of performance, so that ‘industrial users and design engineers would gain confidence in using pultruded profiles’. It was judged that ‘not only were pultrusions important in their own right, but also because their off-the-shelf availability often provides a first entry point for experience of composites’. Discussion and compilation began in December 1996 and the very recently published CEN Standard has the number EN 13706. In addition and although far from a definitive list, other authoritative American (API, ASTM and MIL), French (NF), German (DIN), Japanese (JIS) and UK (BS) specification standards covering the reinforced plastics and composites sector and of particular relevance to pultrusion, are provided through an Appendix to this chapter. Many are achieving, or have already achieved either European (CEN) or international (ISO) acceptance. Although the clauses of this recommended production-supply specification may well differ from one purchase order to another, the following text outlines what should at least be considered during discussion before being rated of importance to a particular supply. Clauses typically fall into one of four classifications, each detailed below, namely product specification, physical properties, mechanical properties, and quality procedures. It is a guide offered without warranty of any kind, either expressed or implied.

Product specification It should now be clear that the composite from which the profile will be pultruded is formulated from three main constituents: matrix resin, reinforcement and additives. Each is open to total and careful selection, both necessary and vital if the required mechanical performance and environmental resistance are to be fully satisfied. Although the use of thermoplastic matrices is growing, thermosetting systems such as unsaturated polyester, vinyl ester, epoxide, phenolic and acrylic resins currently predominate. All can be purchased in a variety of distinct grades of perhaps different viscosity, ‘toughness’, environmental resistance or, with the exception of the phenolics which are firehard (i.e. they exhibit optimum fire, smoke and toxicity performance), fire retardancy. All also typically permit some selective ‘chemical’ or additive modification by the user, but again with the obvious exception of the phenolics, the use of hydrated alumina to provide the necessary degree of fire retardancy. In terms of the reinforcement, the basic choice rests between glass, carbon, aramid (or much more rarely another thermoplastic-based fibre) or their respective hybrids. That however, is far from the complete picture. Glass fibre is for example available in six formulations (A, C, E, R, S and ECR) and there is then the exact make-up of the reinforcement

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pack as well as its volume fraction relative to the whole profile, to consider. Like the matrix, all receive more detailed consideration in Chapters 4 and 5. The additives within the composite may also have a profound influence on the finished properties of the profile, as already explained in the context of fire retardancy. However, depending on their weight/volume percentage addition, other typically inorganic fillers such as talc and kaolin can, given compatibility with the chosen resin matrix, enhance the compressive strength in the same way that glass, polyester or other tissues overlaying the reinforcement can improve the surface properties and environmental resistance. Finally, ultraviolet or optical stabilisers may be formulated as a part of the matrix supply or added separately just before use together with pigments and release agents. In fact all these options and the need for optimum selection and identification, are steadily leading the pultrusion industry into drawing up an internationally recognised classification system based on a six or more letter and/or digit code. The position of each would have a particular significance in comprehensively describing the composite formulation, etc, employed for the profile. One proposal is quoted in literature4 published by the EPTA. Here UP.E5.PF for example would refer to a profile comprising an unsaturated polyester matrix with approximately 50% axial E-glass reinforcement, having a corrosion-resistant polyester veil, and supplied in a fire-retardant grade.

Physical properties A listing of the physical properties which should be critically defined and agreed would include the following. It is worth emphasising that considered collectively, this stage in the whole supply process should be viewed as equally vital to ensuring a guarantee of the mechanical performance, all as a further part of the suggested profile production-supply specification: • • • • • • • • • • •

chemical resistance; coefficient of thermal expansion; cross-section dimensional tolerance; electrical conductivity; fire-retardant performance; glass to resin ratio; hardness; longitudinal dimensional, twist and warp tolerance; thermal conductivity; ultraviolet resistance; water absorption.

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Mechanical properties Like the above, this further alphabetical listing is in many respects no different from that which should be discussed and agreed in any material component-supply situation: • • • • • • • •

compressive strength; elongation; fatigue resistance; flexural modulus; flexural strength; impact resistance; tensile modulus; ultimate tensile strength.

Quality procedures Although agreement on the quality procedures relative to a manufacturing process is far from unusual in an agreed specification document between supplier and customer, the inclusion of control/approval procedures covering the raw materials employed is certainly not typical. Nevertheless, in a process where the material and the product are being manufactured at the same time, it has to be strongly recommended that the customer, irrespective of any process control measures or finished product physical or mechanical evaluation, also ensures that only the very highest raw material standards apply. It is important to have an early and agreed definition of all quality issues and to understand the required level of performance as well as the causes for finished product rejection. Once a comprehensive supply specification is in place, the ability to distinguish causes of defects and appropriate corrective responses is critical to controlling scrap levels to the benefit of both supplier and customer. Matrix Thermoset resins should typically be checked in respect of such properties as acid number, monomer content, reactivity (i.e. gel- and cure-time, plus peak exotherm temperature) and viscosity. A visual assessment of colour and the presence of contamination and gel particles can usefully be included. Reinforcement In terms of the reinforcement, the properties that should be assessed will largely be dictated by the type/s of reinforcement that it is intended to

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employ in the profile. Rovings are for example typically checked for what is known as yield, the presence of knots and fuzz and perhaps also tensile strength, whereas for mat-type materials their weight per unit area, moisture and binder content (and perhaps type) are the important parameters. Fillers The demonstration of a minimum moisture content is also vital for any fillers that are to be added to the formulation. The particle size distribution of that filler can also frequently be another critical property. Ancillary materials Last but not least in the context of raw materials, the customer should ensure that all the ancillary materials that are to be employed – catalysts, release agents, pigments, etc – satisfy, as already recommended, the most exacting standards. Tooling Where custom-moulded profiles are concerned, the tooling is, as has been seen, frequently owned by the customer who should therefore ensure that the selected tool steel and the internal finishing treatment, usually polished hard chrome, conforms to that which will provide the longest possible inservice life. Process As far as the actual pultrusion process is concerned, the monitoring, control and recording of the listed parameters below are essential in ensuring the highest product standard, at the lowest possible scrap levels. It needs to be recognised that unlike those competitive products that can be readily recycled by, for example, a re-melting or re-moulding process, the thermosetbased composite suffers the disadvantage of being irreversible. Defective material or product cannot therefore be recovered. • • • • •

A high standard of raw material storage, distribution and control to the shopfloor. Optimum supplier housekeeping to avoid product contamination. Regular visual checks to creel, guidance and forming plates. Regular resin impregnation monitoring including wet-bath temperature. The careful exercise of all mandatory health and safety regulations.

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•

83

The comprehensive monitoring and recording of all production parameters, including catalyst addition, release agent percentage, die temperature profile, line speed and pulling force. Subsequent finished product mechanical and physical evaluations as already considered.

All the above are further enhanced for the customer if the supplier makes use of a comprehensive batch code system which relates raw material and process conditions to the date and time of profile manufacture. When recorded in some form of computer, card or machine-production log book, an obvious link is established to the agreed production-supply specification and moreover in a manner that enables such records to be employed by the supplier to provide the customer with a Certificate of Compliance for every batch of profiles manufactured for them. At the same time such detailed records provide important data for subsequent production runs of the same profile and the management and engineering, with critical information enabling continual assessment of the whole efficiency of the process and the performance of the shopfloor personnel.

Defect identification Comprehensive day-to-day production records also assist in both tracing the cause and implementing immediate rectification of profile defects.These can usually be classified under one of three headings: composition, process parameters or processing procedures. Composition Defects that can be traced to the reinforcement-matrix formulation of the profile can, for example, include such problems as resin reactivity, incomplete resin mixing, contamination, moisture content and low mat fibre binder, and are typically resolved through incoming or in-process raw material testing procedures. Process parameters In this case defects can result, for example, from too high (or low) a die temperature or too fast a line speed, in other words problems typically identified by process exotherm testing methods as described in Chapter 2.2. Processing procedures Suitably indicative of the several problems under this heading are the reinforcement forming arrangements prior to die entry, impregnation

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difficulties, inferior die design and profile gripping systems, all of which are open to systematic resolution by respective design improvement. In addition the obvious relationships between safety, shopfloor housekeeping, hygiene and the positive attitude of production staff to the overall company success are worth emphasis. Matters such as machine guarding, electrical grounding, elimination of debris, proper lighting, the use of safety glasses and shoes and, as a final example, correct training in the handling and use of hazardous materials such as the catalysts employed are all important factors in maintaining the scrap and waste levels as low as possible.

3.5

Profile: property prediction

What is known as the Rule of Mixtures7 can, using for example the following two equations and authoritative fibre and matrix properties (Tables 3.2 and 3.3), provide some guidance to the designer on the expected performance of any reinforced plastic composite. Longitudinal: Xc = VrXr + VfXf Transverse:

(A)

1 Vr Vf = + Xc X r X f

(B)

where Xc = desired property (tensile strength or modulus, flexural strength or modulus) Vr = volume of resin (%) Xr = property of resin (relative to Xc) Vf = volume of fibre (%) Xf = property of fibre (relative to Xc) However, the accuracy of these calculations is very closely dependent on the actual placement, type and alignment of the reinforcement within the finished laminate, and although applicable to all reinforced plastic composites, this is clearly particularly apposite in the context of pultruded profiles.

Table 3.2. Typical reinforcement fibre properties7 Property

E-glass

S-glass

Aramid

Carbon

Density (g/cm3)

2.6

2.49

1.47

1.77

Tensile strength (MPa)

3450

4585

2750

1900–3100

Tensile modulus (GPa)

72.4

86.9

62.0

Elongation to break (%)

4.8

5.4

2.3

227–379 0.5

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Profile design, specification, properties and related matters Table 3.3.

85

Typical thermoset matrix resin properties7

Property

Polyester

Vinyl ester

Epoxy

Density (g/cm )

1.13

1.12

1.28

Tensile strength (MPa)

77

81

76

Flexural strength (MPa)

123

138

115

3

Flexural modulus (GPa)

2.96

3.72

3.24

Elongation at break (%)

4.5

5.0

6.3

Heat distortion temperature (°C)

71

104

165

For example, if Equation A is applied to a polyester resin-based pultruded bar having a 55% by volume E-glass content, then the calculated longitudinal tensile strength ((3450 MPa ¥ 0.55) + (77 MPa ¥ 0.45)) would be 1932 MPa, whereas the published values for profiles pultruded to this specification are only in the range of 1100 MPa. In other words, the optimum or true fibre alignment required for the calculated result of these Rule of Mixture equations (A and B) to be correct and guaranteed can rarely, if ever, be achieved in production. Furthermore, the normal use of the stacked plies consisting of rovings, mats, woven, stitched or other reinforcements found in the majority of pultruded profile laminates, would distort these formulae even further. Consequently a more accurate method for estimating the properties of a pultruded composite laminate is necessary. Equation C employs a similar type of equation but instead of calculating the properties for the reinforcement and the matrix element of the composite, it estimates the performance of each individual layer or ply within the laminate constituting the profile (Table 3.4): Xc = TlXl + TmXm

(C)

where Xc = desired property (tensile strength or modulus, flexural strength or modulus) Tl and Xl = thickness and property of laminate/reinforcement type (there can be more than two reinforcement/laminate types in a pultruded profile) Tm and Xm = thickness and property of laminate/reinforcement type (there can be more than two reinforcement/laminate types in a pultruded profile) For example, a profile laminate 3 mm thick, consisting of two external plies of 300 g/m2 chopped strand mat (1.016 mm thick, or 33.9% of total laminate) and with a central longitudinal roving core (1.984 mm thick, or 66.1% of total laminate), would show a predicted ((0.661 ¥ 1100 MPa) + (0.339 ¥

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Table 3.4. Typical laminate mechanical properties8,9 Roving/ polyester

Woven roving/ polyester

SMCa polyester

Glass content (%wt)

50–75

45–60

20–35

Density (g/cm3)

1.6–2.0

1.5–1.8

1.8–1.85

Tensile strength (MPa)

410–1180

230–240

50–90

Tensile modulus (GPa)

21–41

13–17

Flexural strength (MPa)

690–1240

200–270

140–210

9

Compressive strength (MPa)

210–480

98–140

240–310

a

Sheet moulding compound, a preimpregnated short (50-mm) chopped glass fibre material, formulated with polyester resin and fillers, etc, for moulding by a high-pressure, evaluated temperature (hot-press) technique between steel tools.

1.85 MPa)) longitudinal tensile strength of 727.73 MPa. As this estimated value falls within the range of standard published values for this type of laminate construction, Equation C does therefore provide a useful pultrusion design aid, and this in turn demands additional input data: • •

Woven roving or stitched, off-axis fibre can be applied at the reinforcement manufactured thickness Because of their respective reinforcement constructions, neither chopped strand mat (CSM) nor continuous filament mat (CFM) exhibits a true tensile strength prior to resin impregnation and eventual cure, and it is therefore usual to consider them as high fibre-loaded SMC materials (see Table 3.4), and for CSM to apply these typical thicknesses: 150 g/m2 300 g/m2 450 g/m2 600 g/m2

= = = =

0.254 mm 0.508 mm 0.762 mm 1.016 mm

A development of this procedure is illustrated by Table 3.5, which determines the amount of reinforcement required to fill a channel section 10 cm wide ¥ 5 cm deep at a constant thickness of 5 mm, while at the same time allowing the approximate tensile modulus to be calculated. Appropriate values are entered into columns 1–4 inclusive and 6. This allows column 5 to be calculated. Using a compressibility volume fraction for each material (column 8) as determined from Table 3.6, it is then possible to calculate the respective values for columns 9 and 10. The sum of the layer cross-sectional areas is then compared with the cross-sectional area (CSA) of the profile. The reinforcement layers or weights are adjusted to give 100% die fill.

UD Carbon

Second

Third

113 600

1200

3

2

4800

Mass (g/m2)

No of ends

Tex (g/km or mg/m) 1

0.3

0.3

4

Effective width

Material/Fabric

Rovings

1082.4

180

360 1.8

2.55

2.55

6

5a 542.4

Density (g/cc)

Weight (g/m)

Fibre

453.89

100.0

141.18

212.71

7b

Volume (cc/m)

0.66

0.28

0.62

8

Local volume fraction

Considers a channel section, 10 cm wide ¥ 5 cm deep ¥ 0.5 cm thick, CSA = 10 cm2. * Global volume fraction of fibre volume is volume of fibre in the layer relative to the total volume. a Column 1 ¥ column 2 ∏ 1000, or column 3 ¥ column 4. b Column 5 ∏ column 6. c Column 7 ∏ column 8. d Column 7 ∏ CSA ∏ 100. e Column 10 ¥ column 11 ¥ column 12.

Totals

Roving

CFM x 2

First

Type

Layer

Table 3.5. Tabulation to establish profile construction and tensile modulus9

9.99

1.52

5.04

3.43

9c

Layer CSA (mm2)

0.1

0.1412

0.2127

10d

Fibre Global Vf*

2.3

70

70

11

Fibre modulus (GPa)

1

0.375

1

12

b

41.60

23

3.71

14.89

13e

Tensile modulus (GPa)

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Table 3.6. Compressibility of reinforcement – volume fraction9 Reinforcement

Fibre volume fraction (%)

E-glass roving E-glass chopped strand mat (CSM) E-glass continuous filament mat (CFM) E-glass woven roving Unidirectional carbon cloth ±45° Carbon fabric Kevlar® fabric

0.62 0.38 0.28 0.58 0.66 0.56 0.68

Table 3.7. Fibre efficiency factors (b)9 Reinforcement

b

Unidirectional Bidirectional (woven roving) Chopped strand mat

1.0 5 0.375

Column 11, the fibre modulus, can then be completed for each layer plus in column 12 an efficiency value (b) which, as shown by Table 3.7, depends on the directionality of the fibres in the particular layer. The final column 13 – the moduli of the individual layers – can then be completed, the sum of which is a reasonable estimate of the tensile modulus of that particular profile, when employing the chosen reinforcement pack. Although Equation C in particular is useful for profile feasibility studies or for other purposes such as the design of localised stiffeners or tensile flanges, the fact has to be recognised that whatever theoretical approach is employed, a pultruded profile is a composite and therefore extremely sensitive to the location of the reinforcement. Consequently, by far and away the best practice is to design around known performance values for a given configuration or composite loading. These days most pultrusion manufacturers have available a comprehensive profile/performance database such as the already referenced Design Guide7 and these can usefully provide the designer with a very respectable level of performance predication for a particular section. Similar shaped and dimensioned profiles, or even an epoxy adhesive bonded mock-up comprising, say, two or more standard and perhaps also machined profiles, can be very suitably employed to characterise and confirm the design and specification of a potential profile. The structural loading acceptance can equally be established in this way with respectable accuracy.

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Table 3.8. Recommended mechanical properties for pultruded structural shapes3 Propertya

Test method

Minimum value

Tensile strength (MPa) Longitudinal Transverse

ASTM D638

206 45

Tensile modulus (GPa) Longitudinal Transverse

ASTM D638

15.9 5.5

Flexural strength (MPa) Longitudinal Transverse

ASTM D790

206 45

Flexural modulus (GPa) Longitudinal Transverse

ASTM D790

10.3 4.8

Compressive strength (MPa) Longitudinal Transverse

ASTM D695

206 45

ASTM D2344

20.7

Apparent horizontal shear (MPa) Longitudinal b

Barcol hardness

ASTM D2583

50

Water absorption (%wt)

ASTM D570

0.7 max

Density (g/cm3)

ASTM D792

1.6–1.9

Glass content (%wt)

ASTM D2584

50 ± 5

a Values derived from a limited series of ‘round-robin’ testing of stock standard structural shapes, 6 mm thick. No emphasis was placed on glass loading or placement; however, the testing fell within a very close range. b The use of surface veils – tissues – can affect the Barcol value. Phenolic-based composites can also give false readings.

This practice can minimise the initial tooling investment in a much more comprehensive way than is possible with prototype tooling, where the latter is often just a shorter version of the production tool, saving basically only the cost of the tool steel. Even prototype tools have to be machined, polished and plated, operations that account for the bulk of the tooling costs. In addition and owing to their shorter length, they may not produce totally satisfactory trial profiles for evaluation, and an eventual increase in their length with tool extension pieces is also certainly not to be recommended. As already referenced, additional design and mechanical property reference data are also provided through the two authoritative associations which very actively support the pultrusion industry (Table 3.8).3,4

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3.6

Process characteristics

As a continuous machine production process, pultrusion with its low direct labour requirement of often below 10% demonstrates an important advantage over many alternative composite fabrication techniques, even when they are capital-intensive. If, as is usual, the cost of manufacture inclusive of wages and benefits is related to the quantity of finished product then, as the latter increases, the allocation of those costs for a given length of pultruded profile decreases. This has the obvious potential to allow for either a price reduction to the customer or an additional profit margin to the pultruder. However, unlike other composite fabrication techniques the process allows for additional ways of enhancing productivity beyond that optimum level initially thought practical for a particular profile in respect of die design and its operating parameters such as pulling speed. In other words, because of the nature of the process, enhanced productivity can frequently be secured whenever found feasible by running multiple streams of product to the fullest extent of the available machine envelope capacity. For example, a single 100 mm (4 inch) wide profile pulled on a machine with a 600 mm (24 inch) envelope is totally inefficient. Adding a second line results in a 100% improvement in output with an important 50% reduction in the allocated labour content per unit length. Further lines have successively greater impacts, although problems of additional material handling, maintenance and inspection, etc, can eventually seriously degrade those gains. The pultrusion process also benefits from what is virtually a total conversion of the matrix and reinforcement entering the die, into a cured composite and moreover a finished product demanding even at worst very limited secondary finishing. Compared with other composite fabrication processes then, the process provides a very high percentage yield from those two raw materials. Although resin and fibre waste does occur at both startup and shut-down, and during any production run owing to processing and specification-related causes, the proportion in comparison to finished product and these other techniques is much less. Applying diligent control, thoughtful process designs and proper scheduling assist in keeping the total waste to better than 3% and often as low as 1% where long production runs are feasible. Levels approaching 10% only apply where short runs are essential, or for infrequently produced custom profiles and those which are required to meet tight tolerance requirements.

3.7

Conclusion

Pultruded profiles are increasingly becoming a more viable structural material for a wide variety of applications. Much of that recognition and accep-

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tance by the engineer and designer has accrued from the better knowledge, understanding and use of the many process and related parameters reviewed by this chapter. Further explanation and confirmation of this structural viability and wide application spectrum is the purpose of the following chapters, which collectively enable the confident prediction that the pultrusion industry will continue to grow in the foreseeable future by as much as 15% per annum. Indeed there is potential for even that figure to be exceeded as a number of developments now in hand are commercialised. The design freedom of pultrusion is a major part of the reason for its success, coupled to the unique properties of the reinforced plastic composite and the ever-expanding skills of the many companies who remain excited by the technical and market challenge offered by the process. From its humble beginnings as a means to produce kite rods and fishing rod blanks, to the current position where entire composite housing units, railcar bodies and bridges are being constructed from basically simple pultruded profiles, there is clear demonstration that for the engineer it is the composites process of the future.

3.8

References

1. Structural Plastics Design Manual, FHWA-TS-79-203, US Government Printing Office, Washington DC, 1979. 2. TOPDIETM Pultrusion Dynamics Technology Center, Oakwood Village, OH, USA. 3. Pultrusion Industry Council of the Composites Institute, Recommended Specification for Materials Used in Pultruded Shapes, Society of the Plastics Industry, USA (now merged with the Composites Fabricators Association’s Pultrusion Growth Alliance, also of the USA). 4. EPTA, Standard for Pultruded Composite Structural Profiles, European Pultrusion Technology Association, Leusden, The Netherlands, 1998. 5. Sims, Graham D, ‘Standardisation of structural pultruded composites’, Composites: Design Data and Methods, Centre for Materials Measurement & Technology, National Physical Laboratory, Teddington, UK. 6. Sims, Graham D, ‘Pulling together on European standards for pultruded profiles’, Composites: Design Data and Methods, Centre for Materials Measurement & Technology, National Physical Laboratory, Teddington, UK. 7. Creative Pultrusions, The Pultex® Pultrusion Design Manual, Creative Pultrusions Inc, Alum Bank, PA, USA. 8. Fibreglass Ltd, Design Data Fibreglass Composites, Fibreglass Limited, St Helens, UK (as this authoritative publication is now no longer published, a suitable alternative reference is provided by reference 9). 9. Quinn, J A, Composites – Design Manual, James Quinn Associates Ltd, Liverpool, UK.

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Appendix: important standards General ASTM D-2563-70

Standard recommended practice for classifying visual defects in glass reinforced plastic laminate parts.

Pultrusion – directly related API Spec 11C-88 ASTM A-1405 ASTM D-3647-84 ASTM D-3917-84 ASTM D-3918-80 ASTM F-1092 BS 6128: Part 3–81 BS 6128: Part 5–81

BS 6128: Part 7–81

MPI-P-79C(2)-64

Specification for reinforced plastic sucker rods. Ladder rail specification, underwriter laboratories. Standard practice for classifying reinforced plastic pultruded shapes according to composition. Standard specification for dimensional tolerance of glass-reinforced plastic pultruded shapes. Standard definitions of terms relating to reinforced plastic pultruded products. Fiberglass handrail specification. Industrial laminated rods and tubes based on thermosetting resins specification for round pultruded rods. Industrial laminated rods and tubes based on thermosetting resins: specification for rectangular pultruded rods. Industrial laminated rods and tubes based on thermosetting resins: specification for hexagonal pultruded rods. Plastic rod and tube, thermosetting, laminated.

Pultrusion – indirectly related BS 6128: Part 1–81 DIN 40618-06.71 JIS K 6914-77 JIS K 7011-89

Industrial laminated rods and tubes based on thermosetting resins: classification and methods of test. Laminated products; laminated moulded tubes of paper-base laminate or fabric-base laminate. Laminated thermosetting tubes. Glassfibre reinforced plastics for structural use.

Raw materials ASTM D-1763-81 BS 3396: Part 3–87

Specification for epoxy resin. Woven glass fibre fabrics for plastic reinforcement – specification for finished fabrics for use with polyester resin systems.

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BS 3496-73

Specification required for E-glass fibre chopped strand mat for the reinforcement of polyester resin system.

BS 3496-89

Specification for E-glass fibre chopped strand mat for reinforcement of polyester and other liquid laminating systems.

BS 3691-69

Specification for glass fibre roving for the reinforcement of polyester and epoxide resin systems.

BS 3749-74

Specification for woven roving fabrics of E-glass fibre for the reinforcement of polyester resin systems.

ISO 3672/1-79

Plastics – Unsaturated polyester resins – Part 1: Designation.

ISO 3673/1-80

Plastics – Epoxide resins – Part 1.

ISO 3342-88

Textile glass – Mats – Determination of fracture strength by traction.

JIS K 6919-92

Liquid unsaturated polyester resin for reinforced plastics.

JIS R 3411-84

Textile glass chopped strand mats.

JIS R 3412-84

Glass rovings.

JIS R 3417-84

Woven roving glass fabrics.

MIL-G-9084C-70

Glass cloth, finished for resin laminates.

MIL-M-15617A(1)-75

Mat, fibrous glass for reinforcing plastics.

NF B 38-205-83

Textile glass – fabrics – basis for a specification.

NF B 38-301-78

Textile glass – mats for reinforcement (made from chopped or continuous strand) – basis for specification.

Terminology BS 1755: Part 1 – 1982

Glossary of terms used in the plastics industry.

BS 1755: Part 2 – 1974

Glossary of terms used in the plastics industry.

ISO 472-88

Plastics – Vocabulary.

ISO 6355-89

Textile glass – Vocabulary.

Testing – composites generally ASTM D-256

Test methods for impact resistance of plastics and electrical insulating materials.

ASTM D-570

Test method for water absorption of plastics.

ASTM D-635

Test method for rate of burning and/or extent and time of burning of self-supporting plastics in a horizontal position.

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ASTM D-638-89

Standard test methods for tensile properties of plastics.

ASTM D-696

Test method for coefficient of linear thermal expansion of plastics.

ASTM D-790-86

Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials.

ASTM D-792

Test methods for specific gravity and density of plastics by displacement.

ASTM D-2343

Test methods for tensile properties of glass fiber strands, yarns and rovings used in reinforced plastics.

ASTM D-2344

Apparent interlaminar sheer strength of parallel fiber composites by short-beam method.

ASTM D-2583

Test method for indentation hardness of rigid plastics by means of Barcol impressor.

ASTM D-2584

Test method for ignition loss of cured reinforced resins.

ASTM D-2734

Test methods for void content of reinforced plastics.

ASTM D-3846

Test method for in-plane shear strength of reinforced plastics.

ASTM D-4357

Specifications for plastic laminates made from woven roving and woven yarn glass fabrics.

ASTM E84

Test method for surface burning characteristics of burning materials.

BS 2782: Pt 10-M1006-77

Methods of testing plastics: glass reinforced plastics: measurement of hardness by means of a Barcol impressor.

BS 2782: Pt 10 Method 1008A-D

See ISO 3597

BS EN ISO 527 1997

Part 4. Plastics – determination of tensile properties. Test conditions for isotropic and orthotropic fibre reinforced plastic composites. Part 5. Plastics – determination of tensile properties. Test conditions for unidirectional fibre reinforced plastic composites.

DIN EN60-11.77

Glass reinforced plastics: determination of the loss on ignition.

EN ISO 14,125 1998

Fibre reinforced plastic composites – determination of flexural properties.

EN ISO 14,129 1999

Fibre reinforced plastic composites – determination of the in-plane shear stress/shear strain, including the inplane shear modulus and strength by the ±45° tension test method.

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EN ISO 14,130 1997

Fibre reinforced plastic composites – determination of apparent interlaminar shear strength by short beam method.

ISO 15,310 1999

Fibre reinforced plastic composites – determination of in-plane shear modulus by plate twist.

ISO 2114-74

Plastics – unsaturated polyester resins – determination of acid value.

ISO 25356-74

Plastics – unsaturated polyester resins – measurement of gel time at 25 °C.

ISO 2555-89

Plastics – resins in the liquid state or as emulsions or dispersions – determination of apparent viscosity by the Brookfield test method.

ISO 3268-78

Plastics – glass reinforced materials – determination of tensile properties.

ISO 3597-77

Textile glass reinforced plastics – determination of mechanical properties on rods made of rovingreinforced resin. Part 1. General considerations and preparation of rods. Part 2. Determination of flexure strength. Part 3. Determination of compressive strength. Part 4. Determination of interlaminar shear strength.

ISO 8515

Textile glass reinforced plastics – determination of compression properties parallel to the laminate.

ISO 4585-89

Textile glass reinforced plastics – determination of apparent inter-laminate shear properties by shortbeam test.

NF T 51-514-78

Unsaturated polyester resins – conventiona determination of reactivity at 80 °C.

Testing – pultruded profiles ASTM D-3914-84

Standard test method for in-plane shear strength of pultruded glass reinforced plastic rod.

ASTM D-3916-84

Standard test method for tensile properties of pultruded glass fiber reinforced plastic rod.

ASTM D-4385-84a

Standard practice for classifying visual defects in thermosetting reinforced plastic pultruded products.

ASTM D-4475

Standard test method for apparent horizontal shear strength of pultruded reinforced plastic rods by the short-beam method.

ASTM D-4476-85

Standard test method for flexural properties of fiber reinforced pultruded rods.

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ASTM D-5028

Standard test method curing properties of pultrusion resins by thermal analysis.

ASTM D-5117-90

Standard test method for dye penetration of solid fiberglass reinforced pultruded stock.

ISO/CD 1268-8

Fibre reinforced plastics – preparation of test plates, pultrusion.

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4 Thermoset resins for pultrusion BEN R BOGNER, WALT V BREITIGAM, MIKE WOODWARD AND KENNETH L FORSDYKE

There are literally hundreds of thermosetting resins now available to the composites industry.1 The formulation of many has been specifically designed to meet the particular performance requirements demanded by the fabrication technique to be employed, and/or the market or application to be satisfied. Pultrusion is no exception and as a consequence, there are polyester and vinyl ester, epoxide, acrylic and phenolic-based thermoset systems specific to that technology. Owing to that importance, this has required these four predominant systems to be reviewed in turn by an authority in each field, those authors being in the order listed above.

4.1

Polyester and vinyl ester resins

The pultrusion process began with, and continues to be, a process dominated by the thermosetting resins, unsaturated polyester and vinyl ester. Together, these two resin systems offer a combination of unique properties that allow for the production of strong rigid parts at fast production rates. This section outlines their manufacture and testing and compares the types of each now available and their respective properties. Finally, after reviewing why both resin systems find use in manufacturing a finished composite profile, the section concludes with typical formulation details which describe any additives that may, in conjunction with the reinforcement, also be included. Suitable examples of the latter are release agents to reduce the frictional forces within the die (which would otherwise cause both die and product damage), plus pigmentation, possibly fire-retardant modifiers and other additives whose sole purpose is to enhance the surface finish of the pultruded profile.

Unsaturated polyester resins Of the two, it is the unsaturated polyester (USPE) resins that are most widely employed, simply because they demonstrate a combination of prop97

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erties that are not only well suited to the pultrusion process but, through chemical and additive alteration, permit ready and critical modification to answer specific process and finished product performance demands. Their low viscosity, for example, promotes quick and total reinforcement wet-out, and their high polymerisation reactivity allows for a fast but totally controllable cure within the pultrusion die. Of supreme importance, the latter defines much of the commercial success of the process. Nevertheless, many of those advantages are duplicated by the somewhat more expensive vinyl esters, which in return offer improved toughness and better caustic and chemical environmental resistance. Consequently, both matrix systems process well, to offer in comparison to timber, aluminium, steel, thermoplastic or reinforced concrete profiles, severe competition when judged as applicable on a life-cycle, environmental or structural basis. Types of unsaturated polyester resins Unsaturated polyester resins are all based on the use of an unsaturated monomer within a polymer backbone, formed by reacting diacids and glycols and dissolving the eventual resin in a reactive diluent, typically styrene monomer. All commercial polyesters use either maleic anhydride or fumaric acid as the unsaturated monomer and the three major types that result are orthophthalic anhydride polyesters, isophthalic acid polyesters or terephthalic acid polyesters. These three basic types can all be manufactured in an identical reactor vessel but, as described later, each exhibits its own set of specific properties. Unsaturated polyesters offer high strength and resiliency. They can be specially formulated for a wide range of demanding physical and mechanical property applications, including good resistance both to the weather and a variety of chemical media. Compared with many other thermosetting and thermoplastic resins, they can to distinct advantage be pigmented, mineral-filled and fibre-reinforced while still in the liquid state. Moreover, these unsaturated polyesters can be very successfully employed for a wide range of fabrication or moulding technique from labour to capitalintensive, including as well as pultrusion, hand and spray lay-up, casting, resin transfer, sheet, bulk and injection moulding, and filament winding. Collectively they are – and remain, like glass fibre – the work-horses of the reinforced plastics, composites industry. Overview – production and use of unsaturated polyester resins The manufacture of a polyester resin is based on a reversible condensation reaction known as polyesterification, with water produced as a by-product.

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Almost all polyester resins are formulated with an excess of hydroxyl groups: •

•

•

As commercial esterification proceeds in a heated reactor, the reaction rate can be increased by raising the temperature of the reactor,2 improving the agitation, and also by using an efficient method of water removal. Removing by-product water from the reaction shifts the reaction equilibrium toward the polyester end product. At the end of the reaction, the polyester is blended with (i.e. dissolved in) inhibited styrene or some other reactive monomer, prior to being despatched to the customer. Small percentage additions of promoters, otherwise called accelerators and usually added by the resin manufacturers, plus catalysts which are typically added immediately before component fabrication are then used to initiate a free radical reaction. During this reaction the styrene – or other reactive monomer – crosslinks with the unsaturated functional groups present as part of the polyester molecule, to form a solid irreversible thermoset polymer. As shown in Table 4.1 a number of controllable variables, such as the use of different difunctional acids, different glycols and different levels of styrene, are the means of modifying the final properties of the polyester resin. Production equipment

The following equipment is required to manufacture unsaturated polyester resins: • • • • •

reactor with agitator, heater, thermometers, inert gas sparge, raw material addition ports and sample taps; partial condenser; total condenser; distillate receiver; blending tank for thinning the finished resin. Reactor vessels

On the quality control and resin development laboratory scale, these are as shown by Fig. 4.1 typically 2- or 4-litre glass vessels, with larger pilot production reactors then ranging from 20 litres (5 gallons) to 400 litres (100 gallons). The following parameters are typical for a pilot plant reactor: •

Variable-speed turbine blade agitator to maintain a tip speed between 1 and 5 m/s (200–1000 ft/min).

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Table 4.1. Unsaturated polyester building blocks3,4 Building block

Ingredient

Characteristics

Unsaturated anhydrides and dibasic acids

Maleic anhydride

Low cost, moderately high heat deflection temperature (HDT) Highest reactivity (crosslinking), higher HDT, more rigidity, slower forming a polyester

Fumaric acid

Saturated anhydrides and dibasic acids

Phthalic (orthophthalic) anhydride

Isophthalic acid

Adipic acid, azelaic acid, and sebacic acid

Chlorendic anhydride Terephthalic anhydride Tetrachlorophthalic anhydride Glycols

Propylene glycol

Dipropylene glycol Diethylene glycol Hydrogenated bisphenol-A Tetrabromobisphenol-A

• • • • •

Lowest cost, moderately high HDT; provides stiffness, high flexibility, and tensile strength High tensile and flexural strength, better chemical and water resistance, high HDT Flexibility (toughness, resilience, impact strength); adipic acid is lowest in cost of flexibilising acids Flame retardance High heat deflection, high strength Flame retardance Low cost, good water resistance and flexibility, compatibility with styrene Flexibility and toughness Greater toughness, impact strength and flexibility Corrosion resistance, high HDT, high flexibility and tensile strength Flame resistance

Mechanical foam breakers attached to the top of the agitators. Baffles on the reactor wall to promote turbulence. An adequate heating capacity, say 100 W/l (75 MBtu/h), to allow rapid heating and shorter processing times. Raw material addition ports within the reactor and sampling taps in both the reactor and the distillate receiver. The provision of a gas sparge, through which nitrogen or some other inert gas containing less than 20 ppm (and preferably less than 10 ppm) oxygen, can be bubbled through the reaction mixture to prevent colourproducing oxidation reactions. Towards the end of the reaction, this sparge facility also helps removal of the last traces of water.

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Thermometer Steam in Water out

Packed column partial condenser Steam out Motor

Total condenser Water in

Gas sparge

4-neck resin flask

Receiver

Thermometer Electric mantle Agitator

4.1 Two-litre bench scale reactor for unsaturated polyester resin production

In full-scale commercial production (see Fig. 4.2) the polyesterification reactor should ideally be constructed of materials that will not corrode or contaminate the reaction mixture. This is typically satisfied by the use of glass-lined, 316 stainless steel. Further, that reactor should be designed to agitate the ingredients thoroughly, in order to prevent the formation of hot spots from discolouring the finished resin, as well as promoting contact between the powdered acid and liquid glycol. Partial condensers The purpose of the partial condenser fitted to the top of the reactor vessel is to separate water from the glycol, but also to return the higher-boiling components to the reaction. The efficiency of this water separation is governed by the size of the condenser, the efficiency of the heat transfer of a surrounding jacket through which hot water and/or steam passes, but overall by the respective vapour-to-liquid mass transfer rates. Employing two different atmospheric pressure conditions, Table 4.2 lists the boiling points for some of the common glycols used in the manufacture

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Pultrusion for engineers T Thermocouples

Baffled shell and tube partial condenser

Water out

Motor

Total condenser

Water in

Gas sparge Charge port

Sample tap

Distillate receiver

Turbine agitator

To thinning tank

Drain line

4.2 Commercial reactor for unsaturated polyester resin production

Table 4.2. Boiling points of pure components Boiling point (°C) 1 atm

345 kPa gauge

Water

100

148

Propylene glycola

189

215

198

230

Ethylene glycol Dipropylene glycol

a

230

285

Diethylene glycola

245

310

Neopentyl glycol

210

Sublimes

a The glycols most often used for unsaturated polyesters formulated for pultrusion.

of unsaturated polyester resin. To maintain separation efficiency while at the same time avoiding flooding, the temperature profile of this partial condenser must be carefully monitored at, as a very minimum, the top, middle and bottom of the column. However, by increasing the monitoring a more accurate assessment of the temperature profile results and this offers a much greater degree of control over the quality of the finished resin. Ideally,

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the temperature at the exit from the partial condenser should remain slightly above the boiling point of water, respective to the reactor operating pressure. As the reaction progresses, changes occur in the composition and temperature of the vapours rising from the reactor. Therefore, there may be an additional need to adjust the partial condenser column conditions, so as to maintain sufficient separation. For example, careful monitoring is particularly important when employing a low molecular weight glycol such as propylene glycol because without optimum control, the molecular weight of the finished resin and in turn a number of moulding and application properties can vary. Finally, as the reaction end point nears, either increasing the temperature of the surrounding jacket or if using pressure-rated equipment, by applying a vacuum, enhances removal of the evolved water. Higher sparge rates towards the end of the reaction will also reduce the time to reach the chosen end point. Blending tanks Using in preference to the reactor itself, the hot resin is then blended into the styrene monomer in a separate blending tank of the following typical specification. In order to allow for a 50% by volume non-volatile dilution, the vessel must have a capacity of at least twice that of the reactor. In addition to being equipped with baffles to aid mixing and an agitator capable of handling viscous liquids it must be capable of being heated and cooled and, finally, vented through a condenser to prevent any vapour emission. Unless located close to the reactor, the tank and transfer lines should also be capable of being heated. The use of a separate blending tank also immediately frees the reactor for the next polyester resin batch. Styrene is the most common reactive diluent used to produce unsaturated polyesters. Blending polyesters into styrene requires some care as styrene has a boiling point of 145 °C (295 °F) and many polyesters must be heated above that temperature to keep them suitably fluid until blended in this fashion. In fact prior to dilution, some of the higher molecular weight polyesters must be held at near 180 °C (355 °F) if they are to flow through the transfer lines to the blending tank, although this is rarely, if ever, necessary for those grades that are suitable for pultrusion. Processing details Three distinct procedures are suitable for the manufacture of unsaturated polyester resins.The first or one-stage method is the quickest, most common method of producing orthophthalic acid-based pultrusion polyester resins, and involves adding all the raw materials to the reactor at the same time.

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Applicable to the manufacture of both maleic and phthalic anhydridebased resins, the result is a short, single reaction which takes place at a maximum reaction temperature of 230 °C (450 °F), with completion determined firstly by molecular weight, but also by viscosity and the acid value number. The most critical part of the whole reaction is the final hours of the reaction. The second alternative is a two-stage procedure (explained in Table 4.6 later) which becomes more essential when producing higher-quality resin systems such as those formulated with isophthalic or terephthalic acid. There is better control of the polymer structure, and this leads to resins with higher heat deflection temperatures, excellent physical properties and better mechanical property retention after exposure to corrosive environments. The final method is a modification of this two-stage procedure, which although shortening the reaction time, actually results in polyesters exhibiting many of the same physical properties as those of the complete two-stage procedure.The second stage begins after the isophthalic or terephthalic acid has undergone half-ester formation, which occurs after at least 50% of the theoretical reaction water has been collected and when a sample of the reaction mixture forms a clear ‘pill’ when cooled to room temperature. When more than 50% of the water from the first-stage of the reaction has been collected, the unsaturated acid is added, at what is effectively an earlier stage than the long processing – or reaction – time that might otherwise apply. Various studies have indicated that the best balance of processing time and resin properties is obtained by introducing this second-stage component when between 65 and 90% of the theoretical distillate has been collected. As a result, the overall production time may be shortened by up to several hours. The use of a number of catalysts to accelerate the first-stage atmospheric esterification reaction between the acids and glycols of the resin formulation, has been studied, but their careful selection is necessary because some can darken the resin beyond an acceptable level. As the esterification reaction progresses, both the molecular weight and the viscosity of the resin increase, while the hydroxyl number and the acid value – or acid number – both decrease. Either of those four values can therefore be used to determine when that reaction equates with the optimum properties specified for a particular resin grade relative to the conditions under which it will be fabricated (or moulded) and then later employed. Quality control Viscosity and acid number are the two tests commonly used in polyester resin manufacture. Both can be evaluated quickly and accurately, allowing

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their use as a control variable under the conditions described for example in the following Standard documentation: Viscosity – SPI Resins Technical Committee Test Procedures AF-145 and Acid Number – ASTM D1045.5 Molecular weight and hydroxyl number are difficult to measure quickly and are therefore unsuitable for quality control, but rather as a means of characterising the resin after processing is complete. Vapour-phase osometry (ASTM D3592) and the hydroxyl number as described in ASTM D2849 determine the molecular weight. The sum of the acid and hydroxyl number is known as the ‘end group number’. When particular resin formulations are regularly produced in the same reactor, it is frequently practical to compile a graph that relates one or more of these resin properties, acid and hydroxyl value, viscosity and molecular weight, to process or reaction time. In other words, once compiled, a rapid determination of both viscosity and acid number, enables the reaction to be critically monitored. This is well illustrated by Figs. 4.3 and 4.4 which record the changes in acid and hydroxyl numbers, viscosity and molecular weight during the final hours of producing a polyester resin under full two-stage process conditions. It must, however, be emphasised that different reaction vessels, different formulations and processing conditions will produce different results, even though the overall trends will be the same.

30

20

90 Hydroxyl number, solid basis

Acid number, solid basis

40

80 Second stage processing

70 60 50 40

10

Acid number Hydroxyl number

30

2

6 10 Processing time (h)

12

4.3 Acid and hydroxyl numbers during final hours of processing

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Pultrusion for engineers Viscosity

2300

U

O

K

Molecular weight (average)

Gardener–Holdt viscosity (60NVM)

W

Molecular weight

2000

1500

1000

Second stage processing

2

6 10 Processing time (h)

14

4.4 Viscosity and molecular weight during final hours of processing (NVM = non-volatile material)

Vinyl ester resins The vinyl ester resins that are specially formulated for use in the pultrusion process are produced from an epoxy backbone that has been reacted with acrylic monomers to form an unsaturated resin, or one which like the unsaturated polyesters is capable of being crosslinked. Types of vinyl ester resin There are two major types of vinyl esters with diglycidylether bisphenol-A resins constituting the first.6,7 These liquid epoxy resins are reacted with methacrylic acid and then dissolved in styrene monomer. The second major type is based on epoxy novolak resins and methacrylic acid, also finally dissolved in styrene. Small amounts of polyester vinyl esters are manufactured, but are seldom used in the pultrusion process. Vinyl esters have been used in the manufacture of pultruded profiles since the introduction of these resin systems to the market in the early 1970s.8 Epoxy vinyl esters are best known for their good corrosion resistance, particularly the novolak vinyl ester types with their high strength and excellent toughness. A third type, the urethane vinyl esters, have found utility in pultrusion because they exhibit a lower viscosity and can

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therefore be highly filled with aluminium trihydrate to provide good flame resistance.9–11 Overview – production and use of vinyl ester resins Epoxy vinyl esters are now produced by over 30 companies world-wide, using reaction vessels very similar to those necessary for the unsaturated polyesters, although construction in 304 stainless steel with a glass liner, or 316 without a liner are more typical. However, because no water or other condensation product is formed during the reaction, a partial condenser is not required. The majority of the vinyl esters consumed by the composites industry and including fabrication by pultrusion, use diglycidylether bisphenol-A (DGEBA) as the backbone of the resin (Fig. 4.5). This polymer has a good combination of chemical resistance, toughness and heat deflection temperature. Such resins have a typical epoxy equivalent weight (EEW) of between 180 and 220, but other molecular-based weights do find application when special properties are required. Vinyl ester production begins by pumping the requisite epoxy resin into a reactor vessel, as shown diagrammatically in Fig. 4.6, after which a stoichiometric quantity of methacrylic acid is added, prior to raising the temperature. This causes the epoxide rings of the resin to react with the acid group of the methacrylic acid to form an ester, which is not unlike a polyester simply because there is more than one ester linkage per molecule. However, the ester linkage is more protected on a vinyl ester than on a polyester, and in addition there are fewer ester linkages to be chemically attacked. Reaction times usually vary from 1.5 to over 6 h. However, while the total reaction time can be short, the reactor cycle times may vary from 5 h to 15 h owing to the heating and cooling of the reaction products. For example, the epoxy/acid reaction is exothermic (-110 kJ/gmol), requiring the vessel to be cooled once the reaction reaches temperature. The temperature may range between 80 and 135 °C (175 and 275 °F) but as shown by Table 4.3 which details reactor conditions and then Table 4.4 which in association outlines vinyl ester formulations suitable for pultrusion, many plants operate at around 110–120 °C (230–250 °F), a lower temperature which offers a measure of safety from a runaway exotherm reaction.

4.5 Chemical structure of bisphenol-A vinyl ester

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Pultrusion for engineers Motor

Solid epoxy resin from storage

N2

N2

Styrene from storage

Diglycidylether bisphenol-A Steam and CW

Steam and CW

Methacrylic acid

110–115 °C

(CW = cold water)

Reactor

To storage tank

Styrene dilution Tank

Product to warehouse

4.6 Vinyl ester production

Table 4.3. Reaction conditions and times for a DGEBA-based vinyl ester Temperature (°C) Reaction time (h) Batch cycle (h) % conversion of methacrylic acid % selectivity of epoxy resin % overall yield

110–120 4 6.5 99 100 +99

Table 4.4. Vinyl ester resin formulations for pultrusion Constituents

Moles

Moles

Moles

Epoxy resin, EEW 188

1

2

2.5

Bisphenol-A (Mol wt 228)

—

2

1

Methacrylic acid (Mol wt 86.1)

1

2

3

Hydroquinone (Mol wt 110)

0.0016

0.0007

0.0007

Mono-tertiary-butylhydroquinone (Mol wt 166)

—

0.0007

0.0007

In other words, if the epoxy/acid reaction is not properly cooled on termination, then the usual result is an unsaleable gelation of the unfinished resin within the reactor.Vinyl esters, owing to their more reactive base polymers, can also self-polymerise – or gel – much easier than polyesters prior to the styrene monomer dilution, and consequently the quantity of inhibitor added to prevent this pre-gelation is often about twice that used for polyesters. Hydroquinone, p-benzoquinone, hindered phenols and hindered quinones are the more common free radical inhibitors used with vinyl esters.

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After the vinyl ester reaction is complete, the resin is cooled and dropped into a thinning – or blending – tank where the styrene monomer is added, together with additional inhibitors if these are necessary. The resin is then pumped to a holding tank or into drums for shipment. For resin grades destined for pultrusion, the styrene content typically varies from 35 to 45 wt%. Bisphenol-A type vinyl esters generally have clear casting elongations of about 5% with heat deflection temperatures in the 100–110 °C (210–230 °F) range, a balance of properties that results in the composites structure having good toughness and higher glass transition temperatures (Tg). Quality control Standard quality control tests for liquid vinyl esters are similar to those for unsaturated polyesters: non-volatile materials (NVM),12 SPI Gel Time, SPI Exotherm Test, Brookfield Viscosity and finally a determination of the molecular weight by high-pressure liquid chromatography. First-quality manufacturers can be expected to monitor all the resins they supply for consistency, and indeed many supply a certificate of analysis with each batch of resin. Customers are always recommended, however, to institute their own internal quality audit, relating that certificated data to their own evaluations. If these are plotted and monitored against time/delivery, the result forms an important part of their own overall quality control programme. Although slightly less important, the same principles should always apply for the unsaturated polyesters.

Pultruded profile property comparison – unsaturated polyesters and vinyl esters Tables 4.5 to 4.16 and Fig. 4.7 summarise the results of a major study to compare, from resin formulation to finished properties, the use of several typical but laboratory-manufactured unsaturated polyesters in the production of pultruded profiles. A commercial isophthalic acid-based polyester resin (Resin A) and a commercial bisphenol-A vinyl ester resin (Resin B) were included in those evaluations to provide control and comparative data. Both A and B are employed in high volume for pultruded products in the United States. The findings provide useful matrix resin comparative interacting property data.

Resin matrix details The three laboratory-made polyester resin formulations, TG-270, TG-271 and TG-272 are described by Table 4.5. Although they all exhibit an iden-

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Table 4.5. Trial unsaturated polyester resin formulations TG-270

TG-271

TG-272

Propylene glycol, mol (PG)

2.45

2.45

2.45

Diethylene glycol, mol (DEG)

1.00

1.00

1.00

Purified isophthalic acid, mol (PIA)

1.00

1.50

2.00

Maleic anhydride, mol (MAN)

2.00

1.50

1.00

Catalyst,a wt% of first-stage charge

0.10

0.10

0.10

Styrene monomer, wt%

36

36

35

a

Fascat 4100, trademark of Elf Atochem.

Table 4.6. Two-stage procedure employed for manufacturing trial isophthalic acid-based unsaturated polyesters: TG-270, TG-271, TG-272 Stage 1 1. Glycols added to reactor and heated to 150 °C (300 °F). 2. Purified isophthalic acid added and inert gas sparge commenced. 3. Heating continued to a maximum of 205 °C (400 °F), and held at that temperature until the prepolymer reaction achieves an acid number of less than 30. 4. Prepolymer cooled to below 160 °C (325 °F). 5. Glycol loss measured and replaced with propylene glycol. Stage 2 6. Maleic anhydride added and reactor contents reheated to a maximum temperature of 230 °C (450 °F) and held at that temperature until the polymer reaction achieves an acid number of less than 15. 7. Polymer cooled below 205 °C (400 °F) and then inhibited with 100 ppm hydroquinone. 8. Hot blended with styrene and further inhibited with a 50 ppm addition (based on resin solids) of p-benzoquinone.

tical glycol content, the purified isophthalic acid/maleic anhydride (PIA/MAN) molecular ratio was altered to provide different reactivities. The first, TG-270, is a high reactivity resin which offers pultruders the opportunity of a high pulling speed and good physical properties, whereas TG-271 is a medium reactivity system comparable to the commercial isophthalic polyester (Resin A). As shown by the MAN content, TG-272 is a lowreactivity resin, the peak exotherm temperature decreasing substantially with a reduction in the ratio of maleic anhydride. All were produced using the same two-stage cook procedure as detailed by Table 4.6. The final liquid, resin properties including exotherm temperature and SPI gel times are reproduced in Table 4.7.

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Thermoset resins for pultrusion Table 4.7.

Final resin properties

Non-volatile material (NVM) (wt%) Brookfield viscosity, cP (mPa s) Acid number (mg KOH/g) SPI gel test: Gel time (min : s) Cure time (min : s) Peak exotherm, °C (°F)

Table 4.8.

111

TG-270

TG-271

TG-272

64.6 634 12.9

64.0 542 13.7

65.1 758 13.1

4 : 15 6 : 23 222 (432)

5 : 12 8 : 05 205 (402)

6 : 29 10 : 51 167 (333)

Clear cast resin properties Polyesters TG-270

TG-271

TG-272

Resin Ab

Vinyl estera Resin Bb

Flexural strength at break ASTM D790

psi MPa

20 800 144

23 100 159

18 400 127

16 000 110

18 000 124

Flexural modulus ASTM D790

psi MPa

599 000 4 130

560 000 3 860

571 000 3 940

430 000 2 960

450 000 3 100

Tensile strength at break ASTM D638

psi MPa

10 600 73

11 600 80

10 800 74

11 000 76

11 800 81

Tensile modulus ASTM D638

psi MPa

560 000 3 790

571 000 3 860

3 940

Elongation ASTM D638

%

2.3

2.5

2.2

4.6

5.0

Deflection temperature ASTM D648 @ 264 psi

°C °F

99 210

78 172

60 140

71 160

101 214

490 000 3 380

Casting conditions: 100 parts resin + 1 phr benzoyl peroxide + 1 phr styrene; post-cured at 16 h @ 57 °C (135 °F), 1 h @ 82 °C (180 °F), 1 h @ 104 °C (220 °F) and finally 1 h @ 120 °C (248 °F). a 45% styrene content. b From supplier’s literature.

Properties The clear cast, unreinforced resin data of Table 4.8 shows that increasing the PIA/MAN ratio level lowers the heat deflection temperature. Furthermore, both the commercial polyester (A) and vinyl ester (B) demonstrated higher elongation values than the laboratory prepared polyesters, although that higher value was not duplicated in the finished pultruded composite. TG-270 had a deflection temperature of 99 °C (210 °F) and at 2.3% an elongation higher than the glass fibre reinforcement, which as a consequence would result in the reinforcement failing under high stress conditions, before the resin matrix.

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Pultrusion for engineers Formulation and pultrusion conditions

As shown in Table 4.9 these three experimental unsaturated polyester resins (TG-270; 271; 272) plus the commercial isopolyester and vinyl ester resins (A and B) were formulated with an internal release agent, a filler and organic peroxide catalysts, as later employed in the manufacture of pultruded test bars. In order to maintain a similar, although not identical, reactivity level between all five, the vinyl ester was formulated with twice the catalyst level to that used for the isopolyesters. Table 4.10 details the respective reactivities as established by their gel and cure properties. As will be seen, TG-271 and the commercial isopolyester had high exotherms with TG-270 reacting faster than the vinyl ester. Compared with TG-270, the

Table 4.9. Trial pultrusion formulations Polyesters

Resin Internal release agenta Clay fillerb Organic peroxidec Organic peroxided

Vinyl ester

TG-270

TG-271

TG-272

Resin A

Resin B

100 1 30 1 0.25

100 1 30 1 0.25

100 1 30 1 0.25

100 1 30 1 0.25

100 1 30 2 0.25

a

Zelec UN, trademark of EI DuPont de Nemours & Co. ASP-400, clay filler, product of Engelhard, Minerals & Chemicals Division. c Perkadox 16, bis-4-t-butylcyclohexylperoxydicarbonate, trademark of Akzo Chemie America. d USP245, 2,5-dimethyl-2,5-bis(2-ethyl-hexanoylperoxy)hexane, product of US, Peroxygen Division, Witco Chemical Corp. b

Table 4.10.

Gel and cure properties of trial pultrusion resin formulations

Brookfield Viscosity @ ambient, RVT, Spindle #5

10 rpm cP/mPa s 100 rpm cP/mPa s

SPI gel-test

Gel-time (min) Cure-time (min) Peak exotherm

Unsaturated polyesters

Vinyl ester

TG-270

TG-271

TG-272

Resin A

Resin B

2800

2400

3000

3600

1600

1740

1580

1900

3016

1280

1.0 1.6 213 °C 415 °F

1.8 2.7 159 °C 318 °F

1.6 3.1 140 °C 284 °F

1.7 2.4 180 °C 356 °F

1.7 2.4 217 °C 423 °F

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reactivities of TG-271 and TG-272 result in a reduction in line speed which can be employed. Each formulation was then employed to produce on a commercial pultrusion machine a number of standard 124 ¥ 3 ¥ 910 mm (4.875 ¥ 0.125 ¥ 36 cubic inches) test bars. Using one central layer of roving with continuous strand mat to the upper and lower surfaces and careful production control, a common glass fibre content of 55% by weight was achieved. To prevent premature cure, the first 150 mm (6 inches) of the die were left unheated, while the remaining 760 mm (30 inches) were electrically heated on just the top and bottom surfaces. The next section of the die was controlled at a temperature of 120 °C (250 °F) while the final section was set at 135 °C (275 °F). Stable temperatures were typically achieved after pulling 5 m (15 feet) and the steady-state conditions described by Table 4.11 were easily maintained at a line speed of 380 mm/min (15 inches/min). Physical properties – pultruded test bars When evaluated on a dynamic mechanical analyser under ‘E’ flexural loss modulus conditions, and as shown by Table 4.12, the glass transition point (Tg) of the finished pultruded test bars was only significantly different for two of the three laboratory manufactured resins TG-270 and TG-272, respectively higher and lower by around 30% than the average value for TG-271, and resins A and B. However, as shown by Table 4.13, all five test bars exhibited statistically equivalent tensile and flexural properties. Indeed the differences between all the polyesters resins – both commercial and laboratory fabricated – and the vinyl ester are insignificant in the fibre direction. Further, because the glass fibre reinforcement at its elongation of 2% carries the load until

Table 4.11.

Pultrusion conditions Unsaturated polyesters

Vinyl ester

TG-270

TG-271

TG-272

Resin A

Resin B

Line speed (in/min) (mm/min)

15 380

15 380

15 380

15 380

15 380

Peak exotherm °C °F

193 379

201 394

172 342

193 379

189 372

Peak exotherm position cm inch Exit temperature °C °F

54.6 21.5 167 333

58.2 22.9 155 311

64.3 25.3 150 302

48.3 19.0 158 316

55.1 21.7 149 300

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Pultrusion for engineers Table 4.12. Glass transition temperatures, pultruded samples Polyesters

°C °F

Table 4.13.

Vinyl ester

TG-270

TG-271

TG-272

Resin A

Resin B

145 293

98 208

77 171

118 244

120 248

Tensile and flexural properties of pultruded samples Unsaturated polyesters

Vinyl ester

TG-270

TG-271

TG-272

Resin A

Resin B

Tensile modulus ASTM D638 Parallel direction

psi ¥ 1000 MPa

3 480 24 000

3 560 24 500

3 310 22 800

3 900 27 500

3 640 25 100

Tensile modulus Transverse direction

psi ¥ 1000 MPa

1 030 7 100

1 320 9 100

1 320 9 100

940 6 500

1 590 11 000

Tensile strength ASTM D638 Parallel direction

psi ¥ 1000 MPa

58.2 398

57.0 401

56.3 393

62.4 388

62.6 430

Tensile strength Transverse direction

psi ¥ 1000 MPa

9.2 63

8.8 61

9.6 66

9.4 65

9.9 68

% Elongation

Parallel Transverse

1.9 1.8

1.8 1.8

1.9 2.1

1.8 1.7

1.9 1.7

Flexural modulus ASTM D790 Parallel direction

psi ¥ 1000 MPa

1 810 12 500

1 820 12 500

1 500 10 300

1 790 12 300

1 880 13 000

Flexural modulus Transverse direction

psi ¥ 1000 MPa

1 160 8 000

1 200 8 300

1 070 7 400

1 300 9 000

1 490 10 300

Flexural strength ASTM D790 Parallel direction

psi ¥ 1000 MPa

64.7 446

66.6 459

58.6 404

56.4 389

58.6 404

Flexural strength Transverse direction

psi ¥ 1000 MPa

27.8 192

27.8 192

33.5 231

28.5 196

33.5 231

failure, a cast resin elongation greater than 2% (see Table 4.8) is not required. In other words the elasticity of the glass controls the elongation of the pultruded composite. In the transverse direction, the tensile strengths were all around 20% of that measured parallel to the glass fibres, a finding that is common for pultruded profiles. Property directionality, as a result of the pulling direction and in turn the alignment thereby imparted to

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the reinforcement, can be very pronounced for pultruded profiles, although transverse properties are equally dependent on the nature of the contained reinforcement. Finally, it should be noted that the vinyl ester did not exhibit a higher composite tensile strength or higher composite tensile elongation.

Factors affecting line speed The highest possible line speed commensurate with optimum surface finish, mechanical, physical and dimensional tolerance properties of the manufactured profile is clearly an important factor in ensuring economic viability for the whole pultrusion process. Several studies have shown the importance of die temperature, catalyst level (see later), filler content and resin type on the speed at which profiles formulated with a particular resin can be pulled.13 Isophthalic acid polyesters tend to pull faster than vinyl ester resins both under laboratory conditions and in commercial practice. The reason is the higher reactivity and the harder cured (polymerised) surface of the isophthalics when they leave the die.14 Vinyl esters, probably because or the presence of secondary hydroxyl groups and a residue of the oxirane ring opening with methacrylic acid, are therefore slightly more difficult to process than unsaturated polyester resins.15 In their classic report McQuarrie and Hickman16 show methods to monitor the pull speeds of a variety of resin systems. Pull rates are determined by the fastest pull speed at which the peak exotherm occurs within the die length. This is a reasonable standard, since a profile with a peak exotherm outside the die, tends to have a dull soft surface that can readily blister, after leaving the die. Figure 4.7 compares the maximum pull speeds of the TG-270 isopolyester with the commercial vinyl ester, resin B, clearly confirming that an unsaturated isophthalic polyester can be pulled faster than a vinyl ester at an equivalent catalyst level.

Comparison – isophthalic and orthophthalic polyester resin In another study, where the only formulation difference was the aromatic acid employed, an isophthalic polyester (TG-270C) was compared under pultrusion conditions with an orthophthalic-based system (TG-322). Their respective resin, clear casting and finished profile (composite) properties are shown in Tables 4.14, 4.15 and 4.16, and the last indicates that the isophthalic polyester (TG-270C) produced slightly higher tensile and flexural properties compared with the orthopolyester (TG-332). However, more importantly, TG-270C also exhibited a higher composite glass transition

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Pultrusion for engineers 1200

Line speed, mm/min

1000 800 600 400 200 0 Vinyl ester, 1% Cat

Isopolyester, 1% Cat

Vinyl ester, 2% Cat

Isopolyester, 2% Cat

Resin/catalyst

4.7 Effect of catalyst concentration on line speed

Table 4.14.

Resin properties

Non-volatile material (NVM), wt% Brookfield viscosity, cP/mPa s Acid number, mg KOH/g SPI gel-test, Gel-time (min : s) Cure-time (min : s) Peak exotherm °C °F

TG-270C Isopolyester

TG-322 Orthopolyester

64.5 1548 15.2 4 : 51 6 : 48 229 444

63.0 204 16.1 6 : 40 9 : 09 217 423

temperature (Tg) which therefore permits exposure to a higher application temperature before the onset of degradation. UV resistance Pultruded composite profiles are often exposed to a variety of external environmental condition, and although low water absorption and ambient temperature properties are typical, adequate UV resistance can be a problem. Unless steps are taken to enhance that resistance, the profile surface can be degraded with exposure of the reinforcement, which if serious leads to a reduction in the level of structural acceptance.

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Thermoset resins for pultrusion Table 4.15.

117

Properties of clear cast resins

Flexural strength at break (ASTM D790) psi MPa Flexural modulus (ASTM D790) psi MPa Deflection temp. @ 264 psi (ASTM D648) °C °F

TG-270C Isopolyester

TG-322 Orthopolyester

13 300 92 534 000 3 682 99 210

13 200 91 530 000 3 654 92 198

Formulation: 100 parts resin, 1 phr benzoyl peroxide, and 1 phr styrene. Cure conditions for the clear castings: 16 h at 57 °C (135 °F), 1 h at 82 °C (180 °F), 1 h at 104 °C (220 °F) and 1 h at 120 °C (248 °F).

Table 4.16.

Properties of pultruded specimens (50 wt% glass) TG-270C Isopolyester

TG-322 Orthopolyester

psi MPa psi MPa

4 380 000 30 200 1 460 000 10 000

4 170 000 28 700 1 080 000 7 400

Tensile strength at break (ASTM D638) Parallel psi MPa Transverse psi MPa

78 300 540 10 300 71

71 600 494 10 900 75

1.9 2.0

1.9 2.0

1 930 000 13 300 1 440 000 9 900

1 780 000 12 300 1 170 000 8 100

67 300 437 27 800 192

59 100 407 26 200 181

151 304

125 257

Tensile modulus (ASTM D638) Parallel Transverse

Elongation at break (ASTM D638) Parallel % Transverse % Flexural modulus (ASTM D790) Parallel Transverse

psi MPa psi MPa

Flexural strength at break (ASTM D790) Parallel psi MPa Transverse psi MPa Glass transition point (Tg)a

a

°C °F

Tg tested on TA Instruments dynamic mechanical analyser using E≤ flex loss modulus.

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Four pultrusion resins, a phthalic anhydride (PAN) polyester, an isophthalic polyester, a terephthalic acid (TA) polyester and a vinyl ester were tested in order to establish which would offer the best UV resistance.17 After pultrusion, the flexural and tensile properties of the pultruded composites were tested. Because, as has been seen, the glass fibres control the physical properties of the pultruded bar, no differences were noted in the strength properties. The samples were cut into sections and placed into three different standard UV accelerated test units for 1000 hours: QUV (with a ‘B’ bulb), xenon and carbon arc. Figure 4.8 illustrates the gloss retention and Figure 4.9 the resulting yellowness index for the four matrix resins, with the clear conclusion that the use of optical stabilisers to enhance the level of UV resistance is often highly desirable. Common and well-tried additives are 4-hydroxy-4-methoxybenzophenone (‘Cyasorb UV-9’ from Cytec) and 2-(2-hydroxy-5-methylphenyl) benzotriazole (‘Tinuvin P’ from Ciba), added to the pultrusion formulation at 0.25–1.0 wt% based on resin. Typically vinyl esters use additive levels at the higher end of this range, polyesters at the lower end.

Chemical resistance of pultruded profiles Pultruded profiles are often selected not just for their structural performance and ease of assembly, but because of their chemical resistance.18,19 Unsaturated polyesters and vinyl ester resins are resistant to salt solutions,

200

Percent loss of gloss

QUV ‘B’ bulb Carbon arc Xenon 100

0 PAN resin

Isopolyester

TA polester

Vinyl ester

4.8 Loss of gloss due to exposure. Exposure: QUV ‘B’ bulb, carbon arc and xenon. Time = 1000 h

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600 QUV ‘B’ bulb 500

Percent increase

Carbon arc 400 Xenon 300

200

100

0 PAN resin

Isopolyester

TA polyester

Vinyl ester

4.9 Yellowness index of pultruded composites. Exposure: QUV ‘B’ bulb, carbon arc and xenon. Time = 1000 h

hydrogen sulphide vapours and many acids that would severely attack, for example, steel and aluminium. Vinyl esters offer additional resistance to caustic solutions. Numerous studies have shown the proven performance of polyesters and vinyl esters in corrosive environments.20,21 As a consequence, pultruded profiles are becoming increasingly selected for applications such as marinas, wastewater treatment plants, cooling towers, scrubbers, chemical plants and pulp and paper mills.

Polymerisation (resin cure) within the pultrusion die It should by now be well appreciated that both polyester and vinyl ester resins are transformed – or cured – from the liquid to the solid state by what is known as addition polymerisation.22 That time–temperature sensitive mechanism, which in effect means that all of the monomer in the resin has been reacted, is promoted by the addition of a small weight percentage of a catalyst, but at a rate that is also affected by small weight percentage accelerator and inhibitor additions typically made by the resin manufacturer. As far as pultruded profiles are concerned, this change to a hard thermoset resin takes place within the die through which the reinforcement and matrix are steadily pulled to create the finished profile. A wide variety of organic peroxide catalysts are available for polyester and vinyl ester thermosets, but some are specific to the pultrusion process. For example in the case of the unsaturated polyesters, high reactivity

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peroxides are used to adjust the gel-time in the die and are called ‘kicker’ peroxides, but although they decompose rapidly at high temperatures, the quantity of free radicals they provide are not usually in sufficient quantity to provide for a good cure. Consequently lower reactivity peroxides or ‘finishing peroxides’ as they are known are often included, and suitable ‘kickers’ and ‘finishers’ are listed in Table 4.17. Taken together and correctly formulated, they can provide a fully cured composite condition within 30 to 90 seconds. Vinyl resins are almost always cured with heat-activated peroxide catalysts, which generate sufficient ‘free radicals’ to cause the double bonds in the resin and monomers to crosslink. However, it is also vital to select a catalyst (and at a respective level) that will not cause the resin to cure at near ambient temperatures for at least 24 hours, but that will cure rapidly at the die temperature, normally between 90 and 160 °C (195–320 °F). Any catalyst system must also decompose rapidly at that temperature so all of the resin and monomer are thermoset in the resulting composite. Because there is an obvious relationship between the degree of cure and the amount of unreacted styrene monomer in the finished profile, it is pos-

Table 4.17.

Selected pultrusion peroxides and relative reactivities

Chemical name

Kicker peroxides Bis(4-t-butylcyclohexyl) peroxydicarbonate Methyl isobutyl ketone peroxide Medium reactivity peroxides t-Butyl peroxy-2-ethylhexonate Dibenzoyl peroxide Finishing peroxides 1,1-Bis(t-butylperoxy) cyclohexane 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane t-Butyl peroxybenzoate

Trade name

Active oxygen content (%)

Critical temp. (°C)

Pot-life at 20 °C with 1–2% catalyst in resin

Perkadox 16

3.8

45

2 days

Trigonox HM

8.8

50

6 hours

Trigonox 21 Lucidol CH-50X

7.0 3.3

60 70

9 days 10 days

Trigonox 22-B-50 USP245

6.2

70

21 days

6.7

70

21 days

Trigonox C

8.1

80

30 days

Lucidol is a trademark of Elf Atochem. Trigonox is a trademark of Akzo Nobel. USP245 is a product of US Peroxygen Division, Witco Chemical Corp.

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sible and important that by a combination of peroxides and accelerators, any residual monomer is lower than 2.5% and preferably nearer 0.01%.23 For example, 1% bis (4-t-butylcyclohexyl) peroxydicarbonate (Perkadox 16) and 1% t-butyl peroxybenzoate (Trigonox C) can, when employed in a typical pultrusion thermoset, result in a residual styrene level of 0.7%. However, if the Trigonox C is replaced with 1% t-butyl peroxy-2ethylhexonate (Trigonox 21), the residual styrene has been demonstrated to drop to around 0.2%. Viewed another way, optimum catalyst selection affords one of the best methods of improving pultrusion line speeds, but is equally affected by changes in the thermoset employed and in profile shape.24

Polyester and vinyl ester pultrusion additives As must already be appreciated, most pultrusion resin formulations include, together with the reinforcement pack, several other additives included for a specific and frequently vital purpose.

Internal release agents As discussed elsewhere, internal release agents prevent the pultruded composite from adhering to the surfaces of the die cavity during the polymerisation process, as the profile is pulled steadily through the die.Without these additives, commercial pultrusion would be difficult because the thermoset resins employed in the process are inherently good adhesives which therefore need to be restrained from bonding to the internal surfaces of the die, to in turn reduce the force needed to pull the profile through the die. The requirements for an efficient internal release agent are excellent compatibility with the chosen resin matrix, and in as minimal addition as is practical commensurate with a complete elimination of die adhesion and suitable reduction in the pulling force necessary. At the same time preferred release agents must not degrade the physical or surface properties of the composite, nor should they cause problems with secondary operations such as painting. A good release agent must enhance the surface finish and appearance and equally prevent any build-up of cured resin particles. Taking these factors into account, the effectiveness of a release agent can be evaluated by measuring the pulling force required to pull the profile through the die, and most commercial pultrusion machines are able to control and record that force. One study25 comparing the effectiveness of a range of release agents demonstrated that the pulling force for a particular profile formulation and construction could be reduced by around 50%, but at the same time made more critical because some agents could in fact degrade the flexural properties of the finished component.

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A list of common release agents is provided by Table 4.18. Linear aliphatic acid phosphates (example: Zelec UN) are often used because the acid group bonds to the metal surface while the associated linear aliphatic group provides the needed slippage or surface lubrication. Release agents of this type do not significantly alter the cure conditions or the physical properties, nor do they typically interfere with any secondary finishing operation such as painting. Other release agents that do not cause problems with secondary operations are fatty amine compounds (INT-PS 125 from Axel) which permit painting with minimum surface preparation. However, internal release agents based on dimethyl silicones can cause severe paint adhesion problems and any form of silicone is therefore not usually recommended as release agents for pultruded composites. When employing unsaturated polyesters the usual level of release agent addition is one part per hundred parts of resin, whereas up to double that quantity applies for the vinyl esters. For complex, highly dimensional, difficult to pultrude profiles – such as complex, narrow section window framing – the use of a combination of release agents is not unusual. Low-profile additives These additives are blended into the resin matrix simply to enhance the surface finish of the profile. The term derives from the sheet moulding compound process which is frequently employed for the fabrication of external automotive body panels where an attractive, smooth, ‘low-profile’ Class ‘A’ surface finish is essential. During the copolymerisation of a thermoset resin with crosslinking monomers, a small percent of volume shrinkage occurs. In the case of pul-

Table 4.18.

Common pultrusion release agents

Release agent – Type

Trade name (source)

Comments

Aliphatic acid phosphate

Zelec UN (Stephen Chemical)

General-purpose release agent. Efficient.

Fatty amine mixtures

INT-PS 125 (Axel)

Lower pulling forces required.

Zinc stearate

—

Low cost internal release agent; lower efficiency.

Aliphatic ester

Mol-Gard-X (Lilly Industries)

General-purpose release agent. Good release with fewer secondary bonding problems.

Polyamines

Tech-lube 500 (Technick)

Good release with fewer secondary bonding problems.

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trusion this causes the profile to pull away from the die cavity surfaces and a mechanism that also tends to cause the resin to shrink around the reinforcement, with the result that a slightly pronounced or exposed fibre pattern can be apparent at the surface. In other words, the finished profile surfaces do not completely reproduce the high-quality polished working surfaces of the die cavity, and there can be some related dimensional loss where tight, high-tolerance profiles are concerned. If pronounced, this shrinkage can also cause warpage and other distortion of the profile as well as voids and internal cracks, all of which are detrimental. The use of low-profile additives, typically thermoplastic polymers dissolved in styrene monomer, overcomes production problems of this nature. The degree of improvement in shrinkage control is dependent on the structure of the thermoplastic employed, its molecular weight, the percentage addition, plus the structure and type of thermoset resin used. With proper attention to all these factors, it is practical to pultrude virtually nil-shrink profiles. Table 4.19 compares some of the more common thermoplastic polymers that can be considered as suitable for the pultrusion process. They differ somewhat from those used in sheet moulding compounds because many of Table 4.19.

Low-profile additives for pultrusion

Type of resin

Percent solids

Commercial designation (corporate source)

Comments

Polystyrene

100

MR 63004 (Ashland Chemical)

Easily pigmented. Low cost.

Neulon Additive 5000 (Union Carbide) LP -90 (Ashland Chemical & Union Carbide)

Excellent surface finish.

Polyvinyl acetate

35 40

Polyester–urethane hybrids

50

Uralloy 2020 (Ashland Chemical)

Low profile, good dimensional tolerances.

Polyethylene

40

Microthene FN 510 (Millennium Petrochemical)

Low shrink.

100

EAB-551 (Eastman Chemicals)

Improved processing and better part surfaces.

35

Aropol Q-701C (Ashland Chemical)

Good surface finish and good weathering resistance.

Cellulose acetate butyrate Polymethyl methacrylate

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the latter contain carboxyl groups that can cause an undesirable thickening of the thermoset resin particularly when impregnation of the reinforcement occurs in a wet-bath prior to die entry. Fillers and pigments Most pultrusion resin formulations include both fillers and pigments.26 The former have several important functions to fulfil – they can reduce the exotherm temperature as the resin polymerises; being generally low-cost materials they can offer a useful profile cost reduction; they can improve the interlaminar shear property; they can enhance the electrical properties and finally the fire retardance, or more specifically the flame resistance. Pigments, on the other hand, simply allow the pultrusion manufacturer the opportunity to alter the surface colour and appearance, although on occasion they may also act in part as a filler. However, both fillers and pigments need to be selected with care because in addition to affecting the cure conditions, they can have abrasive tendencies. For example, the silicas and titanium dioxide should be avoided, with ‘softer’ fillers such as calcium carbonate, talcs and clays being favoured along with such pigments as zinc oxide. Where the flame resistance of the profile needs enhancing as in the case of, for example, most thermosets other than those based on phenolic chemistry, then this can be provided by the addition of fillers such as aluminium trihydrate (ATH) and zinc borate. As considered further in Section 4.3, the particle size of the filler is also of vital importance, as a small particle exhibits a much larger surface area per given weight. Consequently, because both the catalyst and the release agent can be absorbed onto the surface of the filler, the requisite quantity respectively to promote the cure and to lubricate die and profile as it passes through the die may not be available other than at increased additions when employing fillers exhibiting a small particle size. Silanes, or silicon-based chemicals, are occasionally added to enhance the ‘coupling’ or bonding of the thermoset-based resin to both the reinforcement and also the filler addition. They bond to the silicas or the silicates within both of those materials, and at the same time possess organic functionalities that react with either the vinyl ester or polyester resin being employed in the pultruded profile formulation. In most cases these coupling agents are present in very low amounts and are typically no more than 0.01–0.3 wt% of the resin content. However, glass fibre reinforcement often has a coating of a silane coupling agent as part of its manufacturing process, so a further silane addition may not necessarily improve the reinforcement/resin adhesion, and therefore the mechanical strength properties, even though their addition will

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improve the adhesion to clays and silicate fillers.27 Better adhesion between both resins and filler will improve the physical properties, particularly those of pultruded composites exposed to wet environments. One study28 showed that the use of a silane, 3-methacryloxypropyl trimethyloxysilane increased the wet strength of a pultruded composite by over 50% compared with the control formulation without silane. Finally, the electrical resistance or dielectric property of pultruded profiles can also be improved with the use of these agents.

Conclusions and final comments Unsaturated polyesters and vinyl esters are the most widely used resins for the pultrusion process. Resin selection is critical to the final properties of the pultruded profile, such as environmental resistance, thermal properties, surface appearance and physical strength, and equally in the context of processing parameters. The latter include such factors as the speed of reinforcement wet-out, the gel-to-cure of the resin (and therefore the whole profile) within the die, pulling speed and force. In turn these can also be linked to surface appearance and environmental resistance. However, it must not be overlooked that the resin is only one part of the total pultruded composite. Consequently it must complement all the other materials employed in the total pultrusion formulation if good processing characteristics are to be achieved along with a finished composite that fully meets the user’s needs and the demanded performance.

4.2

Epoxy resins for pultrusion Introduction

Thermosetting epoxy resins are among other high-performance applications, now well known for their usefulness as a matrix resin in the manufacture of fibre reinforced plastic composites. Developed in the late 1940s, they became commercially available around 1950 and constitute a very versatile family of resins which range in physical form from low-viscosity liquids to solid materials exhibiting molecular weights from less than 500 to over 3000. Together with a choice of co-reactants or curing agents, and other modifiers, this permits a wide latitude in their selection for any given application. In the context of pultrusion, suitable epoxy resins began appearing in the mid-1980s, and now increasingly compete with the wide spectrum of thermoset resins also considered by this chapter, as well as the more recent but still very limited introduction of other thermosets such as the polyimides and the thermoplastics, as typified by polyphenylene sulphide (PPS) and

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polyetheretherketone (PEEK).29 Specifically, epoxy resin systems are selected for pultrusion when the very highest level of toughness, fatigue and creep performance are demanded together with solvent, chemical and electrical resistance, plus elevated temperature property retention.30 For example, epoxy resin systems are frequently the preferred pultruded profile matrix for aerospace and defence applications incorporating carbon and aramid reinforcements.31–34 Finally, and in addition to their recognised adhesive qualities to a broad range of substrates which therefore includes the fibre reinforcement, these epoxies demonstrate a 100% polymerisation reactivity free from any by-products. There are a number of well recognised publications35–39 which address the general features and performance characteristics of epoxy resin systems in detail. Resin selection is normally a balance between two factors, processing and end-use performance. Among the former parameters are viscosity, reactivity, working time (or pot-life), cure (or polymerisation) conditions, the dimensional shrinkage that accompanies polymerisation and finally the health, safety and environment aspects which are considered in-depth in a Suppliers of Advanced Composite Materials Association (SACMA) publication.40 It is also worth noticing that epoxy resin systems are available for pultrusion that do not contain the volatile styrene monomer reactant and solvent present in both unsaturated polyester and vinyl ester resins. As shown by Table 4.20, epoxy resins also find wide application as surface protective coatings, for casting and to meet the needs of additional plastic industry sectors as well as the structural and electrical industries. Indeed the domestic US epoxy resin market which as published by the Society of the Plastics Industry (SPI) totalled 220 000 tonnes (480 million pounds) in 199641 is almost equally divided between protective coatings and these structural end uses. Table 4.20. Market distribution, domestic US epoxy resin sales 1996 End use

%

Adhesives and bonding

9.5

Aggregate, flooring and paving

7.4

Printed circuit boards

13.4

Protective coatings

53.2

Structural FRP

7.2

Tooling, casting and moulding

3.7

Unclassified

5.6

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Epoxy resin-based composite components fall under the ‘structural FRP’ category, although because of the historical domination30 of polyester and vinyl ester systems, only a small portion will also have been manufactured by pultrusion. Furthermore both those competitors are not only often more amenable to the pultrusion process, but can provide an acceptable performance over a relatively wide, generally less severely demanding requirement.42 At the same time, although a relatively large number of commercial epoxy resins, co-reactants and modifiers are readily available from a number of manufacturers, the choice of those that can be potentially or actually employed for pultrusion is limited by reason of viscosity or some other property parameter.

Epoxy chemistry and manufacture In its broadest sense the term epoxy resin connotes any polymeric material containing one or more epoxide (or oxirane) groups as defined by the generic chemical structure described by Fig. 4.10. In many cases, R or R¢ denotes a hydrogen atom, as when the epoxide groups in a molecule – and often referred to as the ‘functionality value’ – are in the terminal position on the polymer. When an epoxy has two epoxy groups per molecule, it may be referred to as a two (or di) functional resin, whereas multifunctional resins clearly contain more than two epoxy groups per molecule. The total concentration of epoxy groups on a weight basis is expressed by a parameter commonly known as the ‘weight per epoxy’ (WPE) or the ‘epoxy equivalent weight’ (EEW), otherwise known as the epoxy molar mass (EMM). Because the weight per epoxide equals the grams of resin to provide 1 molar equivalent of epoxide, these three terms are interchangeable. Chemical structures representing the various classes of commercial epoxy resin products can be found illustrated in other publications.37,39 In general, then, commercial epoxy resins are compounds or mixtures of compounds containing more than one epoxide group per molecule, or a combination of epoxide groups and hydroxyl groups. Before these reactive intermediates, as they are known, can be fabricated into a useful product, like any thermoset they must be ‘cured’ or crosslinked by polymerisation

4.10 Generic epoxide chemical structure

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4.11 BPA-based epoxy: manufacturing chemistry and general molecular structure

into a three-dimensional, infusible molecular network through the use of co-reactants, otherwise known as curing agents. This crosslinking can be achieved in basically two ways, through either the epoxide or hydroxyl groups, by: • •

a direct, catalytic promoted homo-polymerisation molecular coupling; or employing a reactive intermediate.

Glycidation is the most common way of introducing epoxy functionality into the resins and resin intermediates.39 By far the most common epoxy resins are those based on the glycidation of 2,2-bis(4-hydroxyphenyl) propane (commonly called bisphenol-A, or BPA) with epichlorohydrin. These resins are di-functional because they theoretically contain two epoxy groups per molecule as illustrated by the general structure for BPA-based epoxies of Fig. 4.11. Commercial epoxies of this type show values for n of between 0 and 25 and as indicated by the properties shown in Table 4.21, a number related to the resin’s molecular weight and often employed in resin selection for a given application. For example, resins where n < 1 are liquid at room temperature, whereas a value >2 typifies those that are solid under that condition. An additional group of epoxy resins within the BPA-based products are those utilising tetra-bromo-bisphenol A as the starting material and offering some degree of fire retardancy.

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Thermoset resins for pultrusion Table 4.21.

129

Properties of commercial grades of BPA epoxy resins

Average molecular weight

Average weight per epoxide

Approx. value of n

Viscosity Pa s @25 °C

350

182

0

380

188

0.12

14

8

600

310

0.9

Semi-solid

900

475

2.0

Solid

1400

900

3.7

Solid

2900

1750

9.0

Solid

3750

3200

11.9

Solid

Novolacs are a starting source for phenolics which can be glycidated in the same way as the BPA-based resins, to give multifunctional (n > 2) epoxy resins. The novolacs are made from phenol or alkylated phenols with formaldehyde. The lowest molecular weight oligomer in this family is difunctional and commonly called the di-glycidyl ether of bisphenol-F, which is a resin liquid at room temperature. On the other hand, under that condition, the multifunctional epoxy novolacs are usually highly viscous liquids or solids and, once cured, have the potential to provide a polymeric material with a high degree of crosslink density, which can enhance both the elevated temperature performance and the chemical resistance. Epoxy resins made by the glycidation of monofunctional phenols, and mono- or multifunctional alcohols, are often used as modifiers for the di- and polyfunctional epoxy resins. This class of resins are usually lowviscosity liquids at room temperature and can therefore provide a way of reducing the viscosity of a chosen more viscous system, as well as enhancing certain performance properties. However, such modification opportunity may be limited by adverse effects as, for example, a reduction in modulus and the temperature and chemical resistance owing to a lowered crosslinked density of the cured polymer. Finally, resin systems based on non-aromatic alcohols also offer the advantage of good weathering stability. Epoxies can also be manufactured from starting materials other than those containing hydroxyl functional moieties, and examples are primary and secondary amines.37,39 Yet another route is the epoxidation of olefins37,39 such as vegetable and mineral oils or polymers of the Diels–Alder condensation products of butadiene as well as coupled Diels–Alder products from butadiene, acrolein and six-membered cyclic olefins and derivatives, all employing peracetic acid as the epoxidation agent. However, to produce useful epoxies the olefins must have more than one double bond and

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because the resulting resins generally are of low viscosity and contain no aromatic moieties, the choice of co-reactants is more limited than varieties made via epichlorohydrin. Nevertheless such resins do offer good weather stability and electrical properties. Glycidyl esters ranging from aliphatic to aromatic, and mono- to polyfunctional and prepared by the glycidation of organic acids,37,39 are also available, although because these systems can be degraded by hydrolysis they do not find application in certain chemical environments.

Epoxy curing agents Although as described later other methods of cure are practical, there are three main chemical co-reactants – amines, acid anhydrides and Lewis acids – usually employed by the composites industry to promote the cure of epoxy resins. Their reaction mechanisms are well described in the literature37,39 with the cure temperature generally providing a good indication of the potential end-use temperature of the fully polymerised resin. In other words, room temperature cured systems are only really suitable for low-to-moderate temperature environments, whereas those cured at, say, 121 °C (250 °F) or even higher exhibit a much improved temperature performance. Whatever the reactive addition, it is usual to employ what is known as a two component system where the resin is normally designated as Part A and the curing agent (with or without accelerator) is Part B. Amines Typically the most widely employed epoxy resin curing agent, suitable amines contain a plurality of primary, or primary and secondary active amine hydrogens in combination with a di-epoxide, that together react to produce a crosslinked polymer network. Mixed primary and secondary amines are applicable for room temperature cure, whereas the aromatic varieties are more generally employed for elevated temperature cure. Theoretically, in both cases only the stoichiometric quantity of amine as calculated by the following equation, will ensure optimum processing characteristics and final cured resin properties: Molecular weight of amine / no. of amine hydrogens ¥ 100 = Parts by weight ( pbw ) of WPE of epoxy resin amine 100 pbw, resin However, a preferred balance between these two important parameters is more typically achieved by experimental determination of an ideal

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production stoichiometry, using as the starting point such a calculated value. Anhydrides Owing to their more complex curing chemistry and the need for an accelerator to be present, second in line to the amines as co-reactants and curing agents come the cyclic acid anhydrides. In addition to that accelerator which may be either water, alcohol, a tertiary amine, metallic salts, imidazoles, or Lewis acids and bases,37,39 an elevated temperature is normally necessary if cure is to be initiated. Like the amines, once again the stoichiometric quantity of anhydride as calculated by the following equation may not provide the optimum production value with a commercial epoxy resin: Moleclar weight of anhydride / no. of anhydride groups ¥ 100 = Parts by weight (pbw ) of WPE of epoxy resin anhydride 100 pbw of resin Owing to the fact that the anhydride can undergo a competing chemical reaction with any hydroxyl group present, experience has shown that the preferred value determined by experiment is usually around 85–100% of the calculated theoretical stoichiometry. Lewis acids Typified by boron trifluoride and its complexes, these Lewis acid catalytic epoxy curing agents homo-polymerise the epoxide group rather than the co-reaction mechanism of reactive intermediates that is applicable for the amines and anhydrides. Given careful acid selection, the cure can either proceed very rapidly at room – or even lower – temperature or, alternatively, provide a long room temperature pot-life, ultimately requiring a moderate to elevated temperature to initiate the cure reaction. Indeed, even when incorporated directly with the resin in what is then effectively a onecomponent system, such latent cure additions, as they are known, can offer a shelf-life of between 6 and 12 months. Although not applicable to pultrusion this is, for example, a useful and essential property for reinforcements that are preimpregnated with, say, epoxy resin prior to elevated temperature and high-pressure moulding to shape. The mono-ethylamine complex of boron tri-fluoride (BF3–MEA) is one common example of such a latent curing agent and since the agent’s function is primarily catalytic, stoichiometric considerations are not applicable. Additions in the range of 3–10 pbw based on 100 pbw of resin are usual.

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Although epoxy resins cannot be cured by the free radical chemistry used for unsaturated polyesters and vinyl ester, several non-traditional mechanisms for polymerising epoxy resins are practical and worth comment because they have been, or could be, incorporated into the pultrusion process. Two such methods are cationic photo-cure43,44 and microwave energy; the former can be employed with chemically based mechanisms given that the epoxy contains moieties or components capable of free radical activation. It is possible to generate a reactive cationic curative by the photo-decomposition of a suitably selected diazonium or sulphonium salt to produce a Lewis acid (BF3, PF5, etc) acting as a catalytic curing agent.45,46 Under pultrusion conditions, microwave energy has also been shown47,48 to provide selective enhancement of the rate of cure of a glass fibre–epoxy profile where a BF3–MEA addition had been made to the resin system. Finally, certain selected epoxy resin systems have the potential to be partially cured (known as B-staged) by the pultrusion process and then postformed into the finished and cured component using other moulding techniques.49

Material selection for epoxy pultrusion On a general performance basis, the epoxy resin matrix is selected for composites manufacture in preference to the polyesters and vinyl esters, principally because of the superior chemical and temperature resistance up to 150 °C (300 °F). Both can be important in pultruded profile applications, but equally attractive are the improved physical properties, fatigue, creep resistance and toughness36 as well as a better electrical insulating quality and the maintenance of these advantages under hot/wet environments typified by water saturation up to 121 °C (250 °F). However, that selection is also strongly influenced by processability factors and cost, where the latter takes into account not just the material but also the issue of process dynamics, and a topic therefore considered later. Matrix resin The degree of crosslinking obtained with a BPA-based epoxy resin system with a given curing agent tends to decrease with increasing molecular weight of the resin. Thus, smaller WPE value resins will generally give improved temperature and chemical resistance. The use of multifunctional resins will further increase the thermal and chemical resistance. Flexibility and toughness are benefits of those resins having higher molecular weights

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or WPE values within any given classification. Finally, however, the lower resin viscosity characteristics demanded for preferred pultrusion reinforcement wet-out mean the selection of a low WPE liquid system or alternatively one that has been suitably modified by blends with other resins or modifiers. Reinforcement All forms of fibre reinforcement are suitable for epoxy pultrusion although generally speaking only those such as S-glass, carbon and aramid, which offer a higher-performance commensurate with that of the matrix, are selected. Once again, however, a key consideration is to ensure that the fibre has been given the optimum surface sizing chemistry designed for good compatibility with epoxy resin systems. Curing agents In this case the selection must be based on those agents that provide both the relatively long pot-life and elevated temperature cure activation requirements of the pultrusion process. These needs, when added to the highest performance properties that are assumed by the requirement first to specify an epoxy matrix-based composite for the profile, essentially limit the choice to a small number of amines, particularly the aromatic versions or the anhydrides. Where a conventional pultrusion process applies, all are good candidates for use. Fillers The inorganic particulate fillers such as calcium carbonate, silica, aluminium trihydrate and the alumino-silicates (clays), which are widely used for the polyesters, vinyl esters and modified acrylic resin systems, are equally suitable for those that are epoxy-based. However, clay fillers at additions of between 10 and 30 pbw have in particular been shown to enhance the chemical resistance and dielectric properties of a fully cured epoxy-based composite as well as offering an improved surface finish. Mould release Owing to the dynamic process conditions of pultrusion, internal mould release additives are formulated directly into the resin system to assist the release of the cured resin from the internal die wall surface. Although the metallic stearates and phosphate esters that are typically employed for polyester and vinyl ester resin pultrusion are equally suitable for the

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epoxies, preferred release agents include synthetic polyethylene waxes and a propriety material from Axel Plastics Research Laboratories, Inc coded Mold WIZ-INTTM #1846. All tend to contaminate the profile surface; a factor therefore that must be taken into consideration in respect of any post-pultrusion operation such as painting or adhesive bonding.

Process dynamics There is a need to recognise that epoxy resin systems behave differently in the pultrusion process from either the polyesters or the vinyl esters. Both systems have, as clearly shown by Table 4.22, inherent resin properties that are essentially direct opposites and that therefore affect the respective process dynamics: in other words, characteristics that combine to make the use of epoxy resins for pultrusion more challenging. Therefore questions of shrinkage, reactivity and cure which collectively affect and control the conditions within the die cavity and in turn the production parameters, now demand some analysis. Indeed with a growing interest in the use of epoxy systems for pultrusion, recent years have seen numerous scientific studies in the area of process control, especially with regard to the use of analytical modelling techniques.50–54 Shrinkage Polyester and vinyl ester resins undergo a volumetric cure shrinkage nearly twice that of the epoxies with, as a direct consequence, the need for higher pulling forces for the latter and, as a general rule, the restriction of much slower line speeds. However, in terms of the quality of the finished profile surface (measured as dull, sloughed or peeling) and product performance (measured as the Table 4.22. dynamics

Relative comparison of resin properties affecting pultrusion

Polyester/vinyl esters

Traditional epoxies

Viscosity (mPa s)

Low (500–2000)

High (3000+)

Cure-rate

Fast

Slow

Gel-time

Short (seconds)

Long (minutes)

Conversion @ gelation

10–30%

>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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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

EPI-CURE® curing agent 9470

28

EPI-CURE® curing agent accelerator 537

Cd

0.67

EPON® resin 9302

100

EPI-CURE® curing agent 9350

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 <10 mm (0.4 inch) diameter. The work has clearly indicated the viability of producing with a very limited choice of resin systems, fully cured rods up to 10 mm diameter at line speeds of 80 cm/min (30 inches/min). Both of those parameters show a distinct improvement on other epoxy-based pultruded profile production conditions. Furthermore, when using a travelling microwave arrangement, it would seem possible to dispense with a die and otherwise create a ‘fluid’ die condition. Process combinations Using only standard but carefully controlled pultrusion conditions, Breitigam and others49 have proved the ability to fabricate a B-staged carbon fibre stitched preform for later processing based on a selected multifunctional solid glycidyl amine epoxy resin employing a solid multifunctional aromatic amine curing agent. In addition to this optimum selection of a latent resin system, the pultrusion process was designed to quench – by rapid cooling – the chemical reaction of the product immediately following die exit. After storing at 5 °C (40 °F) for up to six months, it was found practical to mould and cure this material under autoclave conditions to result in a product having properties identical to conventional composite moulding techniques. Post-cure Although a relatively high degree of cure can be mathematically predicted when a composite profile exits the die, Long et al,61 Kershaw et al62

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and others have confirmed that the mechanical properties of glass, carbon and aramid, epoxy resin profiles, can be significantly improved by an on-line post-cure treatment, such as 200 °C (400 °F) for 4 min. For example, the shear, flexural and interlaminar fracture toughness were all improved, although at temperatures above 230 °C (450 °F) the values are degraded as a result it was judged of damage at the resin–fibre interface. This development suggests marked process benefits for epoxy-based pultrusions as traditionally high-performance fibre composite profiles based on epoxy resin are processed with the inclusion of an off-line post-cure of up to perhaps 2 h at 177 °C (350 °F).

Pultruded epoxy-based profile examples Several examples of pultruded profiles fabricated with epoxy resins are illustrated by Figs. 4.13–4.17, and these represent a wide range of products using both glass and carbon fibre reinforcement, from recreational, through industrial to aircraft/aerospace applications. While their selection tends to

4.13 Prototype missile case (Courtesy: Strongwell, Inc)

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4.14 Prototype missile case (Courtesy: Strongwell, Inc)

4.15 Prototype ‘J’ stiffener (Courtesy: Goldsworthy Associates)

confirm the current cross-section limitation, their application also very clearly confirms that even where the required quantities are low, the resulting high performance makes their fabrication by pultrusion and from epoxy resin systems not just essential but highly viable.

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4.16 Prototype air foil (Courtesy: Compositek)

4.17 Prototype chair spring (Courtesy: George Morrell Company)

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Conclusion Over the past 15 years, a number of epoxy resin systems have been developed and introduced as viable candidate resins for use in the pultrusion process to fabricate composite structures. The availability of such epoxy resins offers both the pultruder and the end user a choice of considering this potentially low-cost fabrication process to make composite products for use in high-performance areas such as aircraft/aerospace and selected industrial and recreational applications. Epoxy resin composites reinforced with glass, carbon and aramid fibre have all been pultruded successfully. Much development work has been accomplished in the area of mathematical modelling and experimentation to improve the process efficiency of epoxy resin systems and the quality of the final pultruded product. The mechanical properties of pultruded composite products have been found to compare favourably with composites made by more traditional fabrication methods such as autoclave cure. Although other epoxy resins are available, there are three systems that can now be considered as those of choice for ease of use under conventional pultrusion conditions: •

System 1 – EPON® resin 9310/EPI-CURE® curing agent 9360/EPICURE® curing agent accelerator 537. Finds application in those areas where high temperature (150 °C; 300 °F) mechanical properties are demanded along with an excellent broad range of chemical resistance and electrical properties. • System 2 – EPON® resin 9500/EPI-CURE® curing agent 9550 finds application where more moderate to low temperature (<100 °C; 210 °F) are needed along with good durability and toughness. • System 3 – EPON® resin 826/MTHPA/BDMA offers a long pot-life with good composite mechanical properties up to 121 °C (250 °F) along with good chemical resistance and electrical properties.

4.3

Acrylic resins for pultrusion

In 1985 the Mond Division of the UK company ICI Ltd introduced the world-wide composites industry to a new range of modified acrylic thermoset liquid resins given the trade name MODAR®. Eight years later the business was purchased by Ashland Composite Polymers, with production now centred in France and employing fabrication techniques such as resin transfer moulding (RTM), wet press and vacuum bag moulding but in particular pultrusion. The current range of resins finds increasing acceptance for a wide range of applications covering the automotive, communications, construction, electrical, mass transit, offshore and other industries.

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Chemistry MODAR resins are basically a urethane–methacrylate pre-polymer dissolved in methyl methacrylate monomer (MMA). Curing, or polymerisation from the liquid to the solid state, is achieved in the same way as an unsaturated polyester, in other words via a free-radical crosslinking molecular process initiated by decomposing an organic peroxide either thermally or chemically with a cobalt or amine accelerator. Consequently, and again in similar manner to the polyesters, no aggressive acid or alkaline compounds are employed in processing, and no by-products are produced during the curing process.

Features The basic features of all MODAR resins are their low viscosity and high reactivity, two properties of particular importance in the context of composite processing by pultrusion. As a result these resins not only wet-out the reinforcement readily but allow respectable line speeds to be achieved under typical die length and operating conditions. In other words, using MODAR systems very high rates of productivity can be achieved, when manufacturing profiles that are totally free from internal stress, central or at worst surface cracking within thicker sections. As far as the finished profile is concerned the main attributes are a highquality surface finish, particularly when using the low-profile grade 826HT, and a toughness which allows the full potential benefit of the reinforcement to be attained. That property is also assisted by an excellent compatibility between the resins and the sizing and coupling agents applied by the reinforcement manufacturer, plus the already noted intrinsic wet-out property. Mechanical performance can also be classified as very good, while at the same time MODAR systems filled with non-toxic aluminium trihydroxide particulate have also become recognised as offering outstanding fire, smoke and toxicity (FST) performance. Owing to their low viscosity and in conjunction with commercially available dispersing agents, MODAR systems can easily accept even the very high loadings of aluminium trihydroxide needed to meet the most stringent fire requirements. That FST property with be discussed again in more detail below.

Grade availability Four distinctly different grades of MODAR resin exist and their liquid resin and cast unreinforced resin properties are summarised respectively in Tables 4.26 and 4.27. The 826HT low-profile grade has been specially developed for pultrusion and is able to provide an excellent balance in respect of optimum surface finish and productivity, freedom from cracking, dimen-

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sional stability and last, but by no means least, good chemical resistance. However, it is necessary to note that, because the depth of pigmentation is compromised by the low-profile action of this system, dark pigmentation finishes cannot be achieved. The 855 grade is best described as a tough resin in view of the relatively high extension to break characteristic coupled to good modulus values. In summary it can be considered the system to employ for applications demanding pigmentation and optimum mechanical properties. On the other hand 865 is of value when high filler loadings are required, whereas 835S which exhibits a very low viscosity is in combination with dispersing agent BYK W966, the system to employ for high filler loadings of typically aluminium trihydrate. Consequently 835S is the resin-of-choice for fire-retardant applications.

Process parameters Although the pultrusion processing of MODAR resins differs little from that of the unsaturated polyesters and vinyl esters, minor differences are

Table 4.26.

MODAR® liquid resin properties

Viscosity @ 25 °C (cP)

826HT

835S

855

865

135

55

750

130

Density (G/ml)

1.12

1.08

1.11

1.07

Solids content (%)

60

50

60

50

Table 4.27.

MODAR® properties of cast unreinforced resin

Properties

MODAR 855

MODAR 865

MODAR 835S

Barcol hardness Flexural strength (MPa) Flexural modulus (GPa) Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%) Impact strength (un-notched Charpy) (KJ/m2) Heat distortion temperature (°C)

55 153 3.7 88 3.1 4.6 23 117

40 215 3.4 79 2.7 6.5 22 107

40 132 2.9 56 2.2 6.5 30 83

MODAR 826HT is not included because the low-profile action gives irregular voiding in castings, and hence erratic and potentially low mechanical property figures.

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important and the respective parameters therefore demand a brief review. Resin formula and catalyst level A typical formula for pultrusion processing is given by Table 4.28. The use of a dual catalyst system will be noticed which is a factor of real importance if fully cured, crack-free profiles pulled at an optimum line speed are to result. Typical catalyst systems comprise a mixture of t-butyl perbenzoate (TBPB) and bis-4-t-butyl cyclohexyl peroxydicarbonate (BCPHC) although two precautions are essential. The use of TBPB is limited to between 1 and 2 parts by weight (pbw) on resin, with the BCHPC limited at less than 1.4 pbw and it is also vital that the latter is dissolved in twice its own weight of solvent monomer either styrene or methyl methacrylate (MMA). In practice these catalyst additions must be optimised against line speed, cross-section thickness and complexity of the profile. Reinforcement Like any composite component or profile, the type, configuration and volume fraction of the reinforcement are chosen to equate with the enduse mechanical performance requirements demanded. MODAR pultrusion resins can be successfully processed with a wide variety of glass, carbon and aramid-based reinforcements ranging from rovings and mats to more sophisticated grades and including also surfacing veils, although heavily PVC-sized mats and tissues are not compatible and must therefore be avoided. Fillers and pigmentation High-fibre loadings and/or the use of tissues and veils can enhance the surface appearance of a profile and this, in addition to other property improvement, is good reason for incorporating (as suggested by Table 4.28) mineral fillers such as Microdol Extra or OmyaCarb 5ML, or similar parTable 4.28.

Typical MODAR® pultrusion formula

MODAR resin

100 pbw

Inorganic filler

0–200 pbw

Trigonox C

1.0–2.0 pbw

Perkadox 16

0–1.4 pbw

PAT 654 internal release agent

1.0 pbw

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ticulate material, within the profile formulation. Coarse grades must, however, be avoided. Not only can they have the reverse effect, but they create much higher pulling forces than those of smaller particle size. Where fire retardancy is a specific requirement, then aluminium trihyroxide or trihydrate has, as already discussed, to be the chosen filler addition. All standard polyester pigment pastes are compatible with MODAR resins, although the earlier note regarding the difficulty of achieving a good depth and eveness of colour with the 826HT grade needs re-emphasis. Certain pigments can also affect the stability of the wet-bath and whatever the required resin addition, trial evaluations are always recommended prior to full-scale production runs. Release agents The use of ‘internal’ release agents added to the pultrusion formulation is standard practice. They migrate to the profile surface to act as a lubricant at the die cavity-profile interface. A number of acidic-free materials such as PAT 654 or MoldWiz INT54 are compatible with MODAR resins and additions of 1 pbw on resin are usual. Impregnation MODAR resins can be readily processed on standard, conventional pultrusion machines, and no modification is required, whether employing wetbath or die-injection reinforcement impregnation methods. However, like the polyesters and vinyl esters, it has been found preferable when employing a wet-bath to limit the distance between the die entry and the bath so as to reduce as much as possible the evaporation of the solvent MMA monomer. In addition, both the bath and that space should ideally be enclosed by an overhead ducting equipped with low-level ventilation to remove MMA vapour from the factory environment. The expected wet-bath pot life of unpigmented, filled MODAR systems, even with the maximum catalyst levels, is normally around 24 h at 20 °C (68 °F). At higher ambient temperatures, or to extend the pot life, up to 300 ppm of inhibitor can be added, although the use of the latter will effectively decrease the line speed even if the die temperature is also increased as a means of speeding up the cure reaction. Die conditions Work has shown that the optimum die length is 1 m, with the die cavity chrome plated to a hardened tool steel to provide a long working life. Although it is usual to employ a cure temperature of 140 °C (285 °F),

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the allowable working tolerance can be considered as 120–180 °C (250– 355 °F). Pulling speed For simple, typically standard rather than custom-moulded lightly mineralfilled profiles, a line speed approaching 6 m (20 feet) per minute can often be attained owing to the high reactivity of these methacrylate-type resins. This is considerably better than competing thermosets, although with increasing complexity of the profile cross-section, and equally as either the filler or reinforcement loading is increased, there is a gradual and steady reduction in the attainable line speed. Surface quality Although the low-profile MODAR 826HT is the resin of choice for optimum surface quality, achieving that property for a pultruded profile is closely related to the applied pulling force. There is a certain minimum force that will result in the highest possible surface quality free of sloughing or other visually inferior conditions which, owing equally to an unsuitable line speed or die temperature/profile, may for example be related to boiling or evaporation of the MMA monomer within the die prior to cure.

Mechanical properties of pultruded profiles As already indicated, the mechanical properties of MODAR pultruded profiles are dominated by the reinforcement within the profile, its volume fraction and distribution. In addition, those same properties are influenced not just by the mineral filler loading but also the inherent toughness of the MODAR matrix. At the same time the excellent wet-out of the reinforcement, resulting from the low viscosity of those resin systems, coupled to an excellent fibre sizing compatibility ensure that the full potential benefit of the glass reinforcement is attained. Tables 4.29 and 4.30 provide a brief but comprehensive summary in confirmation of those statements.

Fire retardancy The ability of these intrinsically low-viscosity MODAR resins to be highly mineral filled is taken to advantage in the manufacture of fire-retardant pultruded profiles employing non-toxic aluminium trihydrate (ATH), which then demonstrate excellent low smoke and low-toxicity gas emission (FST)

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Thermoset resins for pultrusion Table 4.29.

153

Mechanical properties, MODAR® 826HT pultruded profiles

Property

Glass content 66.5% v/va Longitudinal

27% v/v filled

39% v/v filled

Longitudinal

Transverse

Longitudinal

Transverse

Tensile strength (MPa)

1060

389

43

512

49.7

Tensile modulus (GPa)

30

18

12

29

15

Extension (%)

—

2.2

1.6

2.0

1.6

Flexural strength (MPa)

—

348

141

494

162

Flexural modulus (GPa)

—

11

9.3

17

9.5

a

Unfilled system, glass only, others are lower glass content and aluminium trihydrate filled 150 phr.

Table 4.30.

Effect of MODAR® resin grade on mechanical properties

Glass construction 48% v/v CFM/roving/CFM

Filler loading (phr) 0

Property

Flexural strength

MODAR 826HT (MPa) 533

MODAR 855 (MPa) 549

56% v/v roving

30

Flexural strength

900

1042

65% v/v roving

0

Tensile strength

1152

1230

65% v/v roving

10

Tensile strength

1054

1094

properties in comparison to even the very best fire-retardant unsaturated polyester, vinyl ester and epoxy systems. While that filler-to-fire performance relationship is obvious, the glass content, its distribution and the profile thickness are also related, and as a consequence, specific recommended ATH loadings to meet any particular fire standard, is controlled by individual circumstances. As a guide, Table 4.31 details the minimum ATH loadings (phr) to both MODAR 826HT and 835S needed to satisfy the various European fire standards by a 4 mm (0.16 inch) thick pultrusion produced at 40% by volume glass in a mat/roving/mat configuration and where both mats are at a density of 450 g/m2 (1.5 oz/ft2). Several smoke and toxicity results are also included.

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Table 4.31. ATH loadings (phr) to meet certain fire standards plus associated smoke and toxicity results Fire test condition

BS 476 Parts 6 & 7 Class 1 Class 0

Minimum ATH (phr)

Smoke & toxicity results French NFF 16-101

90 170

— F0

French NFP 92-501 M1

150

F0

German DIN 4102 B1

150

German DIN 5510 S4/SR2/ST2

100

Underwriter’s Lab. 94-V0

90

BS 6853 App. B.5.2 3 m cube A0 (on)

A0 (off)

— 2.26

— 2.50

NES 713 Toxicity Index

— 1.2

For ATH loadings up to 150 phr, the recommendation is for a grade demonstrating an average particle size of 6–7 mm, such as Alcan FRF80 or FRF85. Under these conditions a 2% by weight addition on the filler of dispersing agent BYK W996 produces a marked reduction in dispersion viscosity and hence easier processing. With ATH loadings in excess of 150 phr, it has been shown desirable to choose a finer grade down to 2 mm, such as Alcan SF2 or Martinal ON901, and in addition to increase the quantity of dispersing agent BYK W966 to 3% by weight, if the pulling force is to be kept as low as possible.

Case history The ability of MODAR resins to provide high productivity, excellent fire safety, very good mechanical performance and a high quality of profile surface finish has resulted in their successful use for several major projects. These include cable trays in the Channel Tunnel,63 for blast relief panels for the Troll offshore project, as rockbolts for rock support systems, as the tension/support member for optical fibres, as ‘third rail’ covers and finally as camouflage rods for NATO tents. Some of these receive more detailed consideration in Chapter 6.

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155

Phenolic resins for pultrusion Introduction

Phenolic resins are the oldest totally artificial polymers. Developed in the last decade of the nineteenth century, phenolic resins found application in a phenomenal range of products and industries. They still do today, the range including brake linings and shoe soles, tyres and electric plugs, felts and foundry sand binders, plywood adhesives and varnish. Among the traditional uses of phenolics used in solution form, were the laminates made by the impregnation of cotton fabrics, other cloths or paper, followed by drying and a partial cure with the latter completed in a high-temperature, high-pressure press. These laminates were, and still are, used for both mechanical and electrical applications, the paper laminate being the original printed circuit boards which started to develop as solid-state electronics replaced valves. The cloth-reinforced laminates were the forerunners of what would be described today as true composites; anisotropic materials where the manufacturer can identify the direction of greatest stress in the end application and can build the material by orientation of the reinforcing fibres in the cloth, to cope with that stress. While cotton cloth/phenolic laminates still find many uses, cotton has been replaced with glass or carbon fibre-based materials in many of today’s ‘high-technology’ applications.To complete the picture a phenolic impregnated reinforcing cloth (known generically as a ‘prepreg’) is often cured to shape in an autoclave rather than pressed into flat sheets in a press. In other words the oldest artificial polymer has been brought up-to-date in many respects. Nevertheless, until the early 1980s it was always necessary to cure phenolics at an elevated temperature, normally between 150 and 180 °C (300 and 360 °F). However, development work at that time64 had the objective of producing grades that would enable phenolic resins to be employed for composites using exactly the same fabrication techniques that are used for unsaturated polyester resins; in other words the ability to achieve a full cure at room temperature by the use of catalysts. The latter aim was never fully realised; most post-1980 phenolic composite laminating resins required a cure temperature of at least 60 °C (140 °F) if there was to be full property development. This extensive work did not neglect the particular requirements of the pultrusion industry and the development of phenolic resins suitable for the process is the subject of this section. However, before looking at the technology of phenolic pultrusion, a comment must be made as to the reason why there should be a desire to use phenolics in the composites industry at all. Being condensation poly-

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mers they are difficult to use in many processes and many preconceived ideas, built on sound reasoning for polyester or epoxy resins, need to be discarded. There remains, however, a major driving force for the use of phenolics and that is their exceptional performance in fire. There is no other organic matrix resin which produces as little smoke as a cured phenolic resin under fire conditions and the resistance to fire of these materials is exceptional. It follows therefore that in a wide range of applications where safety from fire risk is critical, phenolic resins have a vital part to play. They have found this application area in underground railways, hospitals, offshore installations, aircraft interiors, schools and many other places where fire safety is paramount. Furthermore, these resins display an excellent high-temperature mechanical property retention, up to 200 °C (400 °F).

Chemistry of phenolic resins The generic name ‘phenolic resins’ covers a wide range of materials made by the condensation polymerisation of a phenol with an aldehyde or, occasionally, a ketone. Taking ‘a phenol’ as meaning a hydroxy substituted aromatic ring and ‘an aldehyde’ as an aliphatic compound with a termination —CH2O, it will be seen just how wide that range is, both in theory and in practice. It is not the intention in this section to list all the available types of resin, some of which would, and many of which would not, lend themselves to the pultrusion process. Most, but not all, of the resins met in the pultrusion processes are made simply from phenol (C6H5OH) and formaldehyde (CH2O), although another group of resins will be discussed later in the section. The basic process for the manufacture of these resins needs to be understood, along with the chemistry by which the cross linking or ‘curing’ process is achieved. It is the curing process which produces the heat and fire resistant material which is sought after and a general understanding of the chemistry and its principal by-product will explain some of the inherent problems which have had to be overcome in producing phenolic resins which are suitable for pultrusion. Phenolic resins fall, effectively, into two categories known as ‘novolaks’ and ‘resols’. In general novolak resins have a molar excess of phenol to aldehyde and cannot undergo the crosslinking reaction without the addition of more aldehyde. Resols, on the other hand, have the necessary excess molar proportion of aldehyde to phenol required for self-crosslinking and are generally the preferred species for pultrusion. Simplifying the chemistry and omitting for clarity a wide range of intermediate stages and side reactions, phenol (which is normally a liquid in its slightly impure industrial form) is blended in a stirred reaction

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vessel with formalin, a solution of formaldehyde in water. An alkaline catalyst, commonly sodium hydroxide, is then introduced to promote a first-stage reaction known as ‘methylolation’ (Fig. 4.18), which produces a range of methylolphenols. Although an acid catalyst could, theoretically, be employed and often is for the production of novolaks, acids tend to cause a methylolation reaction which is too fast for commercial resol production. In the second stage, or condensation reaction, the methylolphenols produced in the first stage, along with residual phenol and formaldehyde, polymerise through the formation of -CH2- or methylene links and the emission of water as a by-product (Fig. 4.19). The two chemical stages are a single production process. When the desired average molecular weight has been achieved, the reaction rate is reduced by both lowering the temperature and, often, neutralisation of the catalyst with a suitable organic acid. However, resins designed for thermally cured fabrication processes (150–180 °C; 300–360 °F) are not normally neutralised, the residual catalyst from the production reaction assisting later with the speed of cure. As the final part of the resin production, most of the residual formaldehyde and much of the water is distilled off under vacuum, and the remaining phenolic resol resin is a viscous liquid ranging in colour from straw to dark brown.

OH

OH

OH

OH

CH2OH + CH2O

fi

CH2OH

+

+

CH2OH Phenol

Formaldehyde in excess

CH2OH

Various mono- and dimethylol phenols

4.18 Methylolation reaction

OH

OH CH2OH

OH

OH

OH CH2

CH2OH

CH2OH

OH

OH

OH CH2OH

CH2OH

H2 O etc CH2OH

CH2OH

CH2OH H2O

Various methylolphenols

Dimer

Methylolphenol

4.19 Condensation reaction

+

H2 O

Phenolic resin chains

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Curing of phenolic resins When heated, this resin will undergo further condensation polymerisation to produce a three dimensional macromolecule which is ‘thermoset’, that is to say does not melt below the temperature at which thermal decomposition starts to occur. This process is commonly referred to as the ‘curing reaction’. The curing reaction is illustrated in Fig. 4.20. The thermal cure reaction normally takes place at 150–180 °C (300–360 °F) but the cure of phenolic resins can be accelerated by the use of catalysts. Mineral acid catalysts will produce cure at room temperature but the reaction tends to be difficult to control and the acids unpleasant to handle. The commercial systems available are based on organic acids and cure at slightly elevated temperatures, normally 60–70 °C (140–160 °F). However, although acid cure can be and is used commercially in pultrusion, it is thermal cure or thermal cure accelerated by alkaline conditions which is most commonly used. The thermoset molecule which results from the curing of phenolic resins is more thermally stable than that of most other thermosetting systems. A fully cured phenolic does not start decomposition until at least 300 °C (570 °F). When decomposition is initiated, e.g. in a fire, phenolics are known for their very low toxic emissions and tendency to char giving off mainly carbon dioxide, carbon monoxide and water and leaving a carbon residue if the fire is not fully supplied with oxygen. Although acid cure systems were initially and indeed are still employed

OH

OH

OH CH2OH

OH

OH

OH

OH

OH

OH

CH2OH

OH

OH

CH2OH

OH CH2OH

CH2OH

CH2OH etc

HOCH2 CH2OH

CH2OH OH

OH

OH

OH

3 H2O

CH2OH

‘Crosslinked’ or ‘cured’ phenolic resin HOCH2

CH2OH OH

OH

OH

OH

Phenolic resin chains

4.20 Curing reaction

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commercially in pultrusion, it is thermally cured systems or, as will be seen later, those whose thermal cure is accelerated by alkaline conditions, that are now most common.

Early phenolic pultrusion developments Driven by the thinking of the established reinforced plastics industry, initial attempts to pultrude phenolic resins were based on the acid catalysed systems developed for hand-lay and other traditional polyester processes. The theory was that, if the pultrusion die could be kept below 100 °C (212 °F), preferably around 80 °C (175 °F) to allow for exotherm, the water present would not boil and there would be no real problem in making phenolic matrix pultrusions. However, the cure in a pultrusion die has to take place rapidly, normally in about one minute for the process to be economically viable. Using acid catalysed systems available at the time, it proved impossible to achieve this rate of cure at 80 °C (175 °F) with a system which also had an acceptable pot life in the resin bath. Die temperatures were increased with lower catalyst levels to overcome the bath life problem and, even with the water being above boiling point, using the longest dies available, a low pull speed and a carefully developed temperature profile, product was made. The process had been proved as possible but it was now essential to make it commercial. The bath life problem could be overcome by the use of injection pultrusion. The resin and acid catalyst were metered to a static mixer mounted directly on top of the pultrusion die and there was no bath to cure prematurely. Acid cure, injection pultrusion was the first successful commercial process and is still used today. However, a major problem was soon encountered. The acids used in the catalysis of phenolic resols for low temperature cure are strong organic acids. Even with chromium plated dies, attack soon became obvious on the die itself and large areas of chromium plate have been known to detach from the die bore. Whether a lateral thought or simply a return to basic phenolic technology, it was soon realised that although pultrusion was a process normally employing catalysed resin systems, this did not mean that it was essential to adopt the same approach with phenolics. In traditional applications phenolics had been cured without any catalyst addition at temperatures well within the range of the majority of pultrusion dies. Resol resins with heat hardening times (HHT) of 5–6 min at 130 °C (270 °F) were well known and, using the ‘half the time for a 10 °C (20 °F) rise in temperature’ rule, these would clearly cure within a few seconds at 180 °C (360 °F), a temperature that had already been evaluated for acid cure systems. Such traditional resols were, however, typically more viscous than the phenolics designed

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for acid cure and the difficulty of obtaining an optimum reinforcement wetout was seen as a potential problem. Furthermore, because it is common practice to add filler to pultrusion resins to improve processing and surface finish, if this proved necessary with the phenolics, then the resin viscosity would pose an even more serious problem. Trials on thermally cured systems soon showed promise, however, and they have since become the most commonly used for phenolic pultrusion. In parallel with the development of systems based on phenol/formaldehyde resins, research was being carried out on further alternatives based on a resorcinol-modified resin employing paraformaldehyde as the curing agent. This type of phenolic chemistry had been known and used for many years in the wood adhesive and laminated wood industries. Phenolic resins of this type contain lower levels of water (typically 6% before reaction) compared to acid cure systems (ca. 13% catalysed) and similar levels to thermal cure systems. Commercial resorcinol-modified resins are formulated as two liquid components with viscosities similar to thermal cure grades.The components are mixed in approximately 3 : 1 ratio.65 Bath life presents no problem if temperature controlled and an unfilled blended resin typically illustrates a viscosity of between 2500 and 3000 centipoise (cP).

Raw materials for phenolic pultrusion Resin systems As has been seen, there are three principal groups of phenolic resin commercially available for pultrusion: acid cure, thermal cure and those that are resorcinol-modified. Although all three have undergone development beyond the outline already provided, their principal differences remain unchanged. New catalysts offering a very much extended ambient temperature bath life which only become really active at the elevated die temperature have been developed for the acid system. These are normally esters that yield acids when heated. While some also claim less die attack, it is generally accepted that the acid system still needs very careful process control and the use of additives if serious die erosion is to be avoided. However, their low viscosity usually around 400–700 cP (although some versions are as high as 2000 cP) does have the potential to improve reinforcement wet-out and that is a distinct advantage when the use of mineral fillers such as aluminium trihydrate or neutral clays applies. Alkaline fillers must obviously be avoided when using acid catalysed systems. Thermal cure resins remain the most popular even though being generally more ‘condensed’ than the acid cure type, they exhibit higher viscos-

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ities. In addition they tend to be supplied at lower water contents which again influences the viscosity. However, although that can range between 800 and 6000 cP, between 2000 and 5000 cP is much more typical and because thermally cured resins demonstrate an unlimited bath life, any impregnation problem can be easily resolved by warming the resin bath to a temperature offering a reduced viscosity. A further development has been the use of low percentage additions of alkaline oxides or hydroxides or what have become known as active fillers. Because their careful use accelerates the cure at elevated die temperatures without shortening the bath life to any real extent, they can offer a major advantage in the pultrusion of thermal cure phenolics. The resorcinol-modified systems continue to be available and pultrusions made using this system have found particular application in the reinforcement of laminated timber structural members. Reinforcements Early attempts to pultrude glass-phenolic profiles often resulted in products with poor mechanical properties even when the cross-section appearance of the product seemed completely satisfactory. The reason was shown to be an inferior compatibility of the rovings and other reinforcement with the matrix resin, owing to the fact that the majority of the surface treatments that had been applied to the basic fibre were specifically designed for use with polyester resins. These sizing treatments are applied to aid the filaments’ interfacial bonding to the surface of the surrounding matrix resin. Development work by the glass reinforcement manufacturers, often in close cooperation with the resin manufacturers, resulted in sizing/coupling materials more compatible with phenolic resins and as a direct result good mechanical properties can now be achieved. However, it remains important that care is taken by pultruders when selecting glass reinforcement for phenolic pultrusion. Generally a glass sized with epoxy-silane is likely to work well with all types of phenolic while an amino-silane may give acceptable results with thermal and alkaline systems but not with acidcatalysed resins. The surface condition of any pultruded profile can be improved by the use of a surfacing veil or tissue. For phenolics, those based on glass, whether continuous or principally of a non-woven type, are to be preferred over those based on polyester which will seriously impair the fire and particularly the surface spread of flame performance. Other fibre reinforcements have been used with phenolic pultrusion but with mixed results. Carbon, with a surface treatment formulated to be compatible with epoxy resins, can be pultruded successfully with phenolic and

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some excellent results have been achieved commercially. Aramids are much more difficult to wet-out than either carbon or glass. The chemistry of natural fibres such as jute or hemp has many similarities to that of phenolic resins and, although the use of these naturally occurring and environmentally friendly products as reinforcing fibres has yet to be fully developed, phenolics may well prove to be the ideal matrix resins for them. Fillers The use of fillers in polyester pultrusion is common practice. Fillers can improve surface finish, impart special properties such as fire retardance or electrical properties and/or simply reduce the cost of the product. However, one thing which all fillers do to a resin system is increase the viscosity in the bath and for that reason the use of fillers with phenolics, many of which are of inherently higher viscosity than other resin systems, is less common. Inherent fire retardance is the raison d’être for the use of phenolic resins and hence fire retardant fillers such as alumina trihydrate are not necessary to achieve a high fire rating from the product. However, small amounts may be added to improve surface finish if required. Care must also be taken with fillers which are alkaline or react with acids when the acid cure system is used. Clearly calcium carbonates (‘chalks’ or ‘whitings’) are unacceptable with an acid cure system although they are used fairly commonly with polyesters. Other additives While there are a number of other additives which may assist phenolic pultrusion, the one that is essential to the processes is an internal release agent. Early work on phenolic pultrusion used zinc stearate and although this can be effective to a limited extent, the surface of the finished product always has a smeared appearance. The fact that phenolic resins suitable for pultrusion have water as the only solvent makes the physical chemistry, which determines the compatibility of the release agent with the resin, very different from polyesters or other systems containing organic solvents or diluents. Some specialist release agent manufacturers have therefore developed materials suitable for this unusual aqueous environment. They function well and die sticking is not a recognised problem. Other additives that can prove useful for phenolic pultrusion are those that reduce the effective viscosity of either filled or unfilled systems and hence improve the wet-out of the reinforcing fibre pack. However, care must always be taken when using any additive that it does not compromise the fire and particularly the low smoke and smoke toxicity performance of phenolics. Very low levels (<0.5%) of some additives which offer excellent

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processing benefits, can increase the smoke emission in sensitive fire tests such as those for BS6853 to the point where the finished phenolic profile is no longer acceptable for use in specific applications. One company which has been successful in the pultrusion of acid catalysed phenolic profiles has developed an additive or additive package which effectively reduces the corrosive effect of the acid on the pultrusion die.

The phenolic pultrusion process Although the principles of pultrusion for phenolic resins are identical to those for other thermoset resins, there are specific points of difference and these must be clearly understood. If a resin bath process is used, serious consideration should always be given to using a surrounding heating or cooling jacket. For example when employing acid-catalysed resins, the working life of the bath can often be sufficiently extended by cooling to make the process more commercially viable. Alternatively with a thermally cured system, whether or not mineral filled or even accelerated with an alkaline filler, it can be found helpful to raise the bath temperature in order to lower the resin viscosity as a means of assisting the impregnation of the reinforcement pack. When acid catalysts are used, it is essential that the resin bath is constructed from a suitable acid-resistant stainless steel such as Grade 316 and, although not strictly necessary for thermally cured systems, the presence of water in the resin makes suitable stainless steel the preferred material in overcoming premature bath failure, whatever the chosen phenolic matrix. The same applies for the metering pump, static mixer and other metal components which are necessary when employing direct die injection for impregnation. Pump seals, glands and all other ancillary fittings should also be very carefully selected and specified to minimise any corrosion problem throughout the total system. The die for phenolic pultrusion with acid catalyst may be hardened stainless steel or chromium plated if an effective corrosion prevention package can be developed. It is important to remember that, whilst chromium plate is not itself attacked by the acids used for curing the phenolic resins, almost all such plate has a degree of porosity. Under the high temperature and pressure conditions present in the die during processing, the acid eventually attacks the steel behind the plating and the plating may be loosened and stripped from the die. For thermal and alkaline assisted curing and the curing of the resorcinol modified resins, a normal chromium plated finish is sufficient. The one major difference in dies for phenolic resins is that they should be longer than would normally be used for polyester and most other pul-

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trudable resins. The author prefers a minimum die length of 1.5 metres although pultrusion is possible with dies of one metre. There are two reasons for this. The first is that for the achievement of optimum mechanical properties in the finished profile, the time in the die needs to be at least one minute for most systems. It follows, therefore, that for economically viable pultrusion speeds (1 m.min-1 or greater) the die needs to be an absolute minimum of one metre. However, the temperature profile in the die is very important for the effective pultrusion of phenolics and three zones are normally regarded as necessary. To achieve this, without the die temperature simply averaging out, length is essential. It is usually advantageous also to have an unheated, or even cooled front end to the die. This prevents heated material returning to the bath and reducing the life of the mix therein by increasing the temperature. Add all these requirements and the die is about 1.5 m. long. Temperature control is very important and controllers should be of at least ‘proportional’ quality. Simple ‘on/off’ systems cause heat surges in the die and these are not to be advised. The temperature profile should, after the initial unheated or cooled short length, rise to ca. 120 °C (250 °F) for the first zone, rise again to ca. 180 °C (360 °F) for the central zone and fall to ca. 160 °C (320 °F) for the final, exit zone. These figures are not to be taken in any way as absolute and resin suppliers, when sufficiently experienced in supplying the pultrusion industry, will recommend processing conditions for their own products. The question of ‘what happens to the water?’ is frequently asked. It is important that this does not escape back towards the die entry and a high glass loading and cool zone at the face will prevent that happening. In the die where the temperature first exceeds 100 °C (212 °F) the water is totally trapped and doubtless cannot vaporise fully. This creates die pressure but, unlike non-aqueous systems, the pulling force is not high. In fact it is often found to be lower than for polyester through the same die. The theory for this is that the steam actually forms a thin layer against the die and hence produces a gaseous film and little friction. Ultimately most of the water escapes as steam forward with the product but some is entrapped in the product in the form of spherical microvoids. The quantity of these voids, which are not crack initiators as is entrapped air, but spherical and in the matrix rather than at the resin/reinforcement interface,66 is very dependent on the resin system used. Provided that the resin has been through the correct cure cycle within the die, there should be no evidence of steam damage to the surface or visible voids within the product. Undercured materials, which have not reached their ultimate mechanical properties, can blow apart on exit from the restrictions of the die as a result of superheated steam within and it is clear when all is not well with the process parameters.

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Surface finish with the right release agent and a surfacing veil on the product should be of a very good quality. A leading pultruder in the field has produced automotive quality finish on pultruded sections using an acid cure system.

Properties of phenolic matrix pultrusions Fire and heat resistance properties As already stated, it is the intrinsic and exceptional fire, smoke, low smoke toxicity (FST) and heat resistant properties of the phenolic composite that are so attractive to the pultruded profile customer. For example in comparison with other resin matrices, phenolic composites, containing less glass than they would if manufactured as a pultruded profile, have been shown67 to lose very little of their mechanical properties up to 200 °C (390 °F). The very high heat distortion temperature attainable (Tg is quoted between 180 °C (360 °F) and 240 °C (465 °F) for resin and reinforcement combinations) makes phenolic pultruded profiles particularly appropriate for use where fire is a serious risk. Furthermore, a correctly formulated phenolic–glass resin composite will have no problems in meeting the requirements of, for example, BS476, Parts 6 & 7 (‘Method of Test for Fire Propagation for Products’ and ‘Method for Classification of the Surface Spread of Flame’) to give a UK Building Regulation Class 0 fire rating. Similarly, such a composite can confidently be expected to meet the Class I requirement of the ASTM E84 ‘tunnel test’, the French Ml and FI standards, the German BI standard and the very stringent smoke and toxic emission requirements of, say, BS6853, 1998 Category Ia, ‘Fire Precautions in the Design & Construction of Railway Passenger Rolling Stock’. Although it is not yet possible to confirm classifications under the new ‘Reaction to Fire’ Euroclassification system, Table 4.32 provides a good indication of the fire and smoke properties that can be expected from a typical phenolic pultrusion. However, it has to be re-emphasised that small additions to the overall formulation, such as an incorrect choice of release agent, may make a considerable difference to the fire performance and especially the smoke and smoke toxicity generation of these phenolic composites. Most pultruded profiles do not equate to the size and form of sample demanded by the test procedure and it should be remembered that it will probably be a composite formulation, rather than a product, which is, in practice, being evaluated. Nevertheless, on all the available evidence the expectation is that phenolic pultrusions will fall into the highest Euroclassification which does not involve non-combustibility inclusive, where applicable, of the smoke and

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Table 4.32. profiles

Typical fire and smoke properties for phenolic/glass pultruded

Typical resulta

Test

Requirement

BS 476, Part 6

I < 12, i < 6

I = 5.2, i = 0.3

BS 476, Part 7

Flame spread <165 mm

Class 1

UK Building Regulations 1985

BS 476, Pt.6 + Pt.7, Class 1

Class 0

BS 6853, 1998 (3 m cube test, Cat. 1a)

Ao (on) < 2.6, Ao (off) < 3.9

Ao (on) = 1.07, Ao (off) = 1.20

NES 713 combustion toxicity

0.42

OSU Test: Total heat release Peak heat release

<65 kW min/m2 <65 kW/m2

6–25 kW min/m2 39–48 kW/m2

ATS 1000.001 Vertical Flammability Smoke Generation 1.5 min Smoke generation 4.0 min

<6≤ <100 <200

0.24≤ 5 25

ASTM E84 flame/smoke

15/20 or less

NF F 16-101 flame/smoke

M1/F1

ASTM D2863-77 oxygen index

>65%

Pittsburg University, Toxicity Test

>95

a

Properties will vary with the resin system and glass content of the profile.

flaming droplet options. In other words such structural components will continue to perform their role well after many other materials, including some metals, have either burnt away or failed structurally as a result of loss of mechanical properties at elevated temperatures. Mechanical properties Mechanical properties of phenolic resin pultrusions are of the same order as those for polyesters and vinyl esters at normal ambient temperatures with the advantages mentioned above becoming important at elevated temperatures. Quoted properties vary considerably and great care must be taken when looking in the literature to ensure that the glass used in the work reported was truly compatible with the resin system. Some comparative work on resin systems is carried out using the same glass with the inten-

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tion of making a real comparison of the matrix resins. This is, of course, not valid if the glass and resin are not compatible. Table 4.33 gives some typical properties. Miscellaneous physical properties Phenolic resin based materials, from moulding powders (‘Bakelite’) to the well known brown phenolic/paper printed circuit board, have been associated traditionally with their excellent electrical insulation properties. Thermal cure and resorcinol based resin systems continue to exhibit good electrical resistance properties but care must be taken in associating all phenolic systems with similar properties. Systems cured with acid catalysts or containing metal salts or oxides as cure accelerators may not exhibit the same electrical properties. Certainly the electrical arc resistance of many of these materials is less good than their thermal cure cousins. The reason for this is simple. The high ionic material content of the cured resin, resulting from catalyst residues, increases the conductivity. If an arc strikes across the surface, the resin carbonises leaving a conducting track. It is this carbonisation of the surface which helps to give phenolics their excellent resistance to fire but does not help the material’s electrical arc properties. Water absorption of the acid cure systems is also higher than the thermal cure materials and, although this has very little effect on the mechanical properties of the finished composite,69 it could cause deterioration in electrical resistive performance. Table 4.34 shows some miscellaneous electrical and other physical properties for phenolic/glass pultruded profiles. Colour The colour of phenolic resins is brown. Those made with an acid catalyst will be closer to orange and age to orange brown and will be virtually opaque. Those made from thermal cure phenol/formaldehyde systems are brown and are also virtually opaque. The resorcinol modified resin based pultrusions are also brown but suggest a degree of transparency in the resin compared to the other systems. There have been attempts to colour phenolic resin systems and, whilst a short term pigmentation is possible, none seen by the author are stable in daylight and they also darken at elevated temperatures. To achieve coloured finishes on phenolic pultrusions it is necessary to post finish with a suitable paint system. Systems are available based on two pack urethane or acrylic chemistry which do not damage the fire performance to any noticeable extent and these should be used when colour is needed.

260

950–1000

*Properties are measured in the 0° (i.e. axial) direction unless otherwise stated.

1.8

19

Mixed glass (64% wt)/acid cure resin system

330

2.9–3.0

Unidirectional glass (65% wt)/acid cure resin system

36

Flexural strength (MPa)

520

565

Elongation at break (%)

Mixed glass (64% wt)/thermal cure resin system

Tensile modulus (GPa) 1100

Tensile strength (MPa)

Unidirectional glass (75% wt)/resorcinol-modified resin system

Property*

9.5

31–34

23

46

Flexural modulus (GPa) 39

In-plane shear stress (MPa)

30

45

Short-beam shear stress (MPa)

168

Table 4.33. Typical mechanical properties of phenolic/glass pultruded profiles

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Table 4.34. Miscellaneous physical properties of thermal cure phenolic/glass pultruded profiles Insulation resistance after water immersion

1 ¥ 108 W

Axial breakdown voltage in oil at 90 °C (25 mm)

35–45 kV

Transverse electrical strength in oil at 90 °C

7.5 MV/m

Dielectric breakdown

20–30 kV

Water absorption

24 hours = 0.5% 7 days = 1.0%

Relative density

1.5–2.0

Tg

220–250 °C (430–480 °F)

Properties will vary with the resin system and glass content of the profile.

Recommendations for manufacturers of suitable paints should be obtained from the resin supplier.

Applications The majority of the commercially pultruded phenolic profiles manufactured today employ either the thermal cure or resorcinol-modified resins. In addition, because of the relative difficulties in the pultrusion of phenolic resins compared with most other thermoset matrices, profile application growth has tended to be limited to those areas where the FST or elevated temperature properties are of direct advantage. Nevertheless, it is important to recognise that these very applications constitute pioneering market sectors for composites generally and as a consequence phenolic pultrusions can rightly claim to have advanced the cause of the composites industry as a whole. Markets such as chemical plants, ships, offshore drilling rigs, coal and other deep mining installations have all traditionally been the domain of metals where the construction materials are concerned. Other areas of high risk-to-life in the case of fire, such as underground and surface railways, failed, until the late 1970s, to realise the extent of that risk. Historically, these markets often used flammable materials in both the train construction and the underground infrastructure but many corrective steps, often involving phenolic-based composites, have since been taken in many countries. The UK has in many ways been the leader in this material translation. In order to restrict the fire, smoke and smoke toxicity performance of the materials of construction employed by a particular industry or market sector, there are usually two choices: to revert to non-combustible

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materials such as metals and concrete or to consider the very best of those which, while not strictly non-combustible, do undoubtedly exhibit extremely good fire performance. The motivation to take the latter course is often long-term economic. Metals are heavy. In transport applications this means the use of more energy to move a vehicle and energy is expensive. At the same time the heavier the vehicle then the lower the load capacity. In static applications such as the grid flooring employed in chemical plants or offshore installations there is the additional continuing problem of corrosion with most affordable metal solutions. Weight is clearly also a vital factor in offshore rig construction. In fact a lightweight structure will often save money overall owing to the ability to employ lighter constructions for both foundations and supporting structures. Pultrusions, together with filament-wound components, are the commercial composites industry’s structural products. Furthermore, because they are readily available in standard ‘off-the-shelf’ forms such as channels, angles, I-beams, box and tubular sections, they are better understood by those used and accustomed to the proven traditional alternatives. Combining this with the light weight and high strength of composites results in the ‘perfect material’ for many applications, at least until the question of what would happen in a fire is added to the equation. Phenolic pultruded profiles (Fig. 4.21) may resolve that fire performance problem to provide the engineer, designer or specifier with the ultimate lightweight structural material solution which, even after fire exposure, may well continue to carry a large proportion of the original load. Applications to date have been in grid floor, cable trays, hand rails, ladders, third rail covers and other areas where the longevity and light weight of composite materials needs to be combined with the structural strength of pultrusions and the fire properties of phenolics. This is a rapidly expanding market area. The international interest in environmental matters, including, particularly, energy efficiency, increases in magnitude daily. This, in turn, leads to demands for light weight and long life for transport systems.

Conclusion The phenolic pultrusion process is now established and most major pultrusion companies around the world are offering pultruded phenolic profiles. Both standard, off the shelf, sections and bespoke profiles are available and there is no longer reason for an engineer considering the enormous advantages of composite structural members to be deterred if fire is an important consideration in the application concerned. If, however, ‘noncombustible’ materials are specified by regulation, no organic material can

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4.21 Standard and custom, acid-catalysed phenolic resin/glass pultruded profiles (Courtesy, Fiberline A/S, Kolding, Denmark)

be used. The regulating body, however, may be interested in learning about phenolic composites and phenolic pultrusions in particular!

4.5

References

1. Starr, T F, Thermoset Resins for composites – Directory and Databook, Woodhead Publishing, Cambridge, 1998. 2. Amoco Chemicals, Processing Unsaturated Polyesters Based on Amoco Isophthalic Acid, Bulletin IP-43c, Amoco Chemicals, Chicago, IL, 1995. 3. Amoco Chemicals, How Ingredients Influence Unsaturated Polyester Properties, Bulletin IP-70b, Amoco Chemicals, Chicago, IL, 1990. 4. Boeing, H V, Unsaturated Polyesters: Structure and Properties, Elsevier, Amsterdam, 1964. 5. ASTM, Standards are issued yearly; plastic testing standards are in Volumes 8.01, 8.02, 8.3 and 8.4, American Society for Testing and Materials, West Conshohocken, PA. 6. Fekete, F, Plant, W J & Keenan, P J, (H H Robertson Co), Great Britain Patent Application GB 660520, 1966. 7. Fekete, F et al (H H Robertson Co), United States Patent 3301743, 1966.

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8. Meyer, R W, Handbook of Pultrusion, Chapman & Hall, London, 1985. 9. Orton, M L & Charles, S C, ‘Polymerizable composition’, European Patent Application EP 234 692, 1987. 10. Orton, M L & Spurr, W I, ‘Polymerizable urethane compositions’, European Patent Application EP 189 987, 1986. 11. Ashland Chemical, Modar 865 Modified Acrylic Resin, International Version, Ashland Chemical, Columbus, OH, 1995. 12. Composites Institute, Resin Technical Committee Test Procedures, Society of the Plastics Industry, New York, 1986. 13. Herbert, The Influence of the Process Parameter on the Mechanical Properties of Pultruded GRP-profiles, European Owens-Corning Fiberglas, Battice, Belgium, 1989. 14. Amoco Chemicals, Isopolyester Pultrusion Resins Offer High Strength and Fast Pull Speeds, Bulletin IP-89c, Amoco Chemicals, Chicago, 1994, pp. 7–9. 15. Nelson, D, Reaction Polymers: Chemistry – Technology – Applications – Markets, edited by Gum, Riese & Ulrich, Hanser Publishers, Munich, 1992, pp. 465–466. 16. McQuarrie, T S & Hickman, J H, ‘The pultruder’s handbook to resin selection’, SPI Composites Institute Annual Conference, Cincinnati, OH, 3 Feb, 1987. 17. Bogner, B R & Borja, P P, Ultraviolet Resistance of Pultruded Composites, JEC, Paris, 1994. 18. Fibergrate, Fiberglass Grating: Characteristics, Applications and Engineering Data, Fibergrate Corporation, Dallas, TX, 1993. 19. MMFG, Corrosion Resistance Guide: Industrial Products, MMFG (Strongwell), Bristol, VA, 1996. 20. Hollaway, L, Pultrusion, RAPRA Review Report, Pergamon Press, Oxford, 1989. 21. Top Glass, A Story of Pultrusion, Top Glass SpA, Piotello, Milano, 1993. 22. Ashland Chemical, The Pultrusion Process: Markets and Products, Ashland Chemical, Columbus, OH, 1997. 23. Groenendaal, N, The Pultrusion Process, Selection Criteria for the Cure System, Akzo Chemicals, Deventer, The Netherlands, 1996. 24. Martinez, E C, ‘Vinyl ester resin pultrusion’, Dow Plastics – Thermoset Polymers, Dow Chemical, Midland, MI, 1995. 25. Bryan, P C, ‘Factor affecting pulling force in pultrusion of unidirectional composites: a statistical investigation’, Section 4-A, SPI Composites Institute Annual Conference, 1989. 26. Atkins, K E & Rex, G C, ‘Internal pigmentation of low profile composites, Part III’, Section 13-D, SPI/Cl Annual Conference, Cincinnati, OH, 1994. 27. Plueddemann, I P & Stark, G L, ‘Surface modification of filler and reinforcements in plastics’, Section 4-C, SPI RP/C Conference, 1977. 28. Dow Corning, A Guide to Dow Corning Silane Coupling Agents, Dow corning, Midland, MI, 1990, p. 17. 29. Leonard, L, Continuous Processing Advanced Composites, July/August, 1988, pp. 28–35. 30. Martin, J D & Sumerak, J E, ‘Pultrusion’, Engineered Materials Handbook, Vol. I Composites, ASM International, 1987, pp. 533–543. 31. Breitigam, W V & Ulbrich, P W, ‘A non-MDA epoxy resin system for pultrusion’, 45th Annual Conference and Exhibit of the SPI Composites Institute, The Society of Plastics Industry, Inc, 1990, pp. 81–87.

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32. Meade, L E, Manufacturing Development of Pultruded Composite Panels, NASA Contractor Report 181780, prepared for National Aeronautics and Space Administration, Langley Research Center, Contract NAS1-15069, 1989. 33. Krolenski, S & Gutouski, T, ‘Effect of the automation of advanced composite fabrication process on part cost’, 18th International SAMPE Technical Conference, Vol. 18, 1986, pp. 83–89. 34. Liskey, A K, Pultrusion on a Fast Track, 3rd edn, Advanced Materials and Process, February, 1989, pp. 31–35. 35. Murphy, J, Reinforced Plastics Handbook, 1st edn, Elsevier Advanced Technology, Oxford, 1994, pp. 14–24. 36. Lubin, G, Handbook of Composites, Van Nostrand Reinhold Company, New York, 1982, pp. 19–272, 478–490. 37. May, C A, ‘Epoxy resins’, Engineered Materials Handbook, Vol 1 Composites, ASM International, 1987, pp. 66–77. 38. Fiberglass Pipe Handbook, 2nd edn, Fiberglass Pipe Institute, Composites Institute of the Society of the Plastics Industry, 1992, pp. 7–28. 39. May, C A (ed), Epoxy Resins Chemistry and Technology, 2nd edn, Marcel Dekker, Inc, New York, 1988. 40. Safe Handling of Advanced Composite Materials, Suppliers of Advanced Composite Materials Association, 3rd edn, Arlington, VA, April 1996. 41. SPI Committee on Resin Statistics, The Society of Plastics Industry, New York, 1996. 42. Pudgeon, C D, ‘Polyester resins’, Engineered Materials Handbook Volume 1 Composites, ASM International, 1987, pp. 90–96. 43. Schlesinger, S I, US Patent No. 3 708 296, 1973. 44. Schlesinger, S I, ‘Photocure of epoxy resins’, Photographic Sci. Engr, 18(4), 387, 1974. 45. Crivello, J V, US Patent Nos 4 069 055; 4 069 056; 4 058 400; 4 058 401, 1978. 46. Smith, G H, Belgium Patent No. 828 841, 1975. 47. Bush, S F & Methven, J M, ‘Selective enhancement of specific reaction pathways using microwave energy’, IChemE Res. Event – Eur. Conf. Young Res. Chem. Eng. 1st, Volume 1, 1995, pp. 565–567. 48. Methven, J M, ‘A comparison of various microwave applicators for the continuous manufacture of thermoset composites’, 39th International SAMPE Symposium and Exhibition (Moving Forward with 50 Years of Leadership), 1994, pp. 25–33. 49. Breitigam, W V et al, US Patent No 5 098 496, 1992. 50. Moschair, S M, Reboredo, M M, Larrondo, H & Vazquez, A, ‘Pultrusion of epoxy matrix composites: pulling force model and thermal stress analysis’, Polym. Compos. 17(6), 850–858, 1996. 51. Chachad, Y R, Roux, J A, Vaughan, J G & Arafat, E S, ‘Effects of pull speed on die wall temperatures for flat composites of various sizes’, J. Reinf. Plast. Compos. 15(7), 718–739, 1996. 52. Villiappan, M, Roux, J A & Vaughan, J G, ‘Temperature and cure in pultruded composites using multi-step reaction model for resin’, J. Reinf. Plast. Compos. 15(3), 295–321, 1996. 53. Chachad, Y R, Golestanian, H, Valliappan, M, Roux, J A & Vaughan, J G, ‘Temperature and cure characterisation of pultruded composites and kinetic analysis of epoxy resin’, 38th International SAMPE Symposium and Exhibi-

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54.

55. 56.

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62.

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64.

65.

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tion (Advanced Materials: Performance through Tec), 1993, pp. 1275– 1290. Trivisano, A, Maffezzoli, A, Kenny, J M & Nicolais, L, ‘Mathematical modelling of the pultrusion of epoxy-based composites’, Adv. Polym. Technol. 10(4), 251–264, 1990. Lam, P W K & Piggott, M R, ‘The durability of controlled matrix shrinkage composites’, J. Materi. Sci., 25, 1197–1202, 1990. Sumerak, J E, ‘Understanding pultrusion process variables for the first time’, 40th Annual Conference, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, January 1985. Fanucci, J P & Nolet, S C, ‘Measurement of pressure and temperature distributions inside a pultrusion die’, HTD (Am. Soc. Mech. Eng.), 132 (Transp. Phenom. Mater. Process.), 113–118, 1990. Kiernan, D & Tessier, N, ‘Modification of epoxy resins for improved pultrusion processing’, 40th Annual Conference, Reinforced Plastics/Composites Institute, The Society of Plastics Industry, Inc., 2G, 1985, pp. 1–6. Arnold, E S M & Tessier, N J, ‘A simple, low-cost, tensioning device for pultruding composite materials’, 32nd International SAMPE Symposium and Exhibition (Adv. Mater. Technol. ’87), 1987, pp. 1299–1308. Tessier, N, Madenjian, A, Kiernam, D & Moulder, G, ‘Ultrasonically activated pultrusion dies’, 31st International SAMPE Symposium and Exhibition (Mater. Sci. Future), 1986, pp. 21–31. Long, E R, Jr, Long, S A T, Collins, W D, Gray, S L & Funk, J G, ‘Effects of postcuring on mechanical properties of pultruded fiber reinforced epoxy composites and the neat resin’, SAMPE Q. 20(3), 9–16, 1989. Kershaw, J A, Tulig, T L & Yamashita, G I, ‘Pultrusion processing of epoxies into advanced composite structures’, COGSME Fabricating Composites Conference Proceedings, EM 86-711, 1986. Whitaker, W A & Johnston, A, ‘Pultruded cable tray in the Channel Tunnel and other enclosed situations’, Paper 13-c, 48th SPI RP/CI conference, Cincinnati, 1993. Forsdyke, K L, ‘Developments in the applications technology of phenolic GRP’, Paper 15, British Plastics Federation (BPF), Reinforced Plastics (RP) Congress, 1982. Dailey, T H & Shuff, J, ‘Phenolic resins enhance public safety by reducing smoke, fire and toxicity in RP’, Paper 15-D, Society of the Plastics Industry (SPI), Composites Institute (CI) Conference, 1991. Vaughn, J & Lackey, E, ‘Processing tips for phenolic pultrusion’, Composites Fabricators Association (CFA) Composites ’98, Paper 31, 1998. Forsdyke, K L, ‘Phenolic GRP for underground safety’, Paper 13-C, SPI/Cl Conference, 1990. Charalambides, M N & Williams, J G, ‘Fracture toughness characterisation of phenolic resin & its composites’, PhD Thesis, Imperial College of Science, Technology & Medicine, London. Forsdyke, K L, ‘The effect of water absorption on the mechanical performance of glass reinforced, phenolic matrix composites’, Paper 16, BPF, RP Congress, 1992.

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5 Reinforcements for pultrusion JAMES V G AUCHEL, LUC PETERS AND TREVOR F STARR

5.1

Introduction

The reinforcements employed for the pultrusion process are now as varied as the standard, structural and custom-moulded profiles made by the process. Historically, combinations of glass fibre rovings, chopped strand and continuous filament (strand) or swirl mats have made up the reinforcement package. Today those still commonly employed and productattractive materials are being joined by a wide variety of other glass-based reinforcements such as tows and yarns, fabrics, stitched complexes, tissues, ‘papers’ and surfacing veils.1 At the same time and although more expensive, carbon and aramid fibres, often supplied in identical reinforcement and allied grades or forms, or equally as hybrids with glass, are beginning to find steadily growing acceptance. To that list must then be added boron, other organic and inorganic fibres – and even steel – all of which are beginning to find some application, albeit still very limited, when their cost performance is also essential for a specific application, or to meet a specific performance. Reinforcements serve multiple functions in both pultruded parts and in the production process itself. Besides being the primary load-bearing entity in the profile, certain reinforcements can enhance functional performances such as electrical conductivity, radar cross-section or thermal performance. In the process, the reinforcement allows the profile to be pulled through the die, acting as a load transfer medium as well as the source of bulk which allows the die to be continuously and uniformly filled.

5.2

Fibre manufacture and characteristics

Although glass fibre in its myriad forms finds over 95% use in the production of pultruded profiles, there is a need to describe the process by which both aramid and carbon fibres are manufactured. As already intimated, the developing interest in their use has to be recognised and at the same time 175

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all three reinforcements do offer specific properties which are normally translated through to the type of application for which those reinforcements are employed.

Aramid fibre Developed in 1965 by Du Pont, aromatic ether amide fibre, trade name Kevlar®, has ever since been considered as an alternative to glass in certain composites applications. Several grades are available and two other manufacturers, Akzo and Teijin, have for some years supplied material under the respective trade names Twaron® and Technora®. Typically, that interest stems from their respectable high-strength, medium-to-high modulus and very low-density (1.39–1.44) properties. In addition, aramid fibres are fire-resistant, perform well at both low (-40 °C; -40 °F) and high temperatures (180 °C; 355 °F), and are insulators of both electricity and heat, as well as being resistant to organic solvents, fuels and lubricants. Composites made from aramid reinforcement also usefully show a linear stress/strain relationship which is between that of glass and carbon (Fig. 5.1) but, more importantly, aramid fibre is highly tenacious, showing nothing of the brittleness exhibited by the other two. Further, the strength : weight and stiffness : weight performance of aramid fibre is a primary advantage; thus the high-modulus varieties can provide higher specific modulus than some carbon fibre grades and certainly all glass fibres. If specific strength is the criterion, then aramid out-performs not just glass, but also those carbon fibre grades that exhibit high strength (Table 5.1). It is in fact the combination of their density, high tensile properties and high tenacity that make aramids highly suitable for use as armour or ballistic protection, whether as a fibre alone or as a composite. With the one application exception where material suitability is the most important arbiter, the high price structure has been the sole reason for the slow growth and limited annual consumption currently amounting to no more than some 40 000 tonnes per annum of which perhaps only 20–25% Table 5.1. Index of tensile strengths for various fibres Fibre

Index

Kevlar

100

Carbon

74

E-glass

46

Nylon

45

Steel

19

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is employed for composites. Growth is, however, expected to pick up to around 5% per annum over the next few years. In addition, that price disadvantage is also compounded by a much greater difficulty in cutting, trimming or otherwise machining not just the reinforcement itself, but also the finished moulding, whatever the fabrication technique. As a consequence, but again with that one exception, aramid

Kevlar 49 Resin-impregnated strand

Carbon HT Resin-impregnated strand

Carbon HT

Kevlar 49 Yarn, twist added

Kevlar 29 Yarn, twist added

‘E’-glass Resin-impregnated strand Stress

‘E’-glass roving

Strain

5.1 Stress – strain relationship for aramid, carbon and glass fibres

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fibre is typically employed by the composites – and pultrusion – industry as a hybrid in an increasingly wide variety of specialist reinforcement grades and fabrics with glass, or to a much lesser extent with carbon.

Carbon fibre Like aramid, and irrespective of the composite property benefits it can confer, price has equally been disadvantageous to carbon fibre. However, that situation has begun to change as a result of important and recent production process developments which are now seeing some grades fall dramatically in price, typically by as much as 60%. Consumption, typically still only some 15 000 tonnes per annum by the composites industry, must as a result rise by several factors over the next few years, as the use of even small percentage additions of carbon fibre to an otherwise glass reinforced profile can dramatically improve the stiffness performance. When applied for example to an H- or I-section beam, that improvement is of importance to the civil and corrosion engineer. Neglecting the low filament property, ‘fibrous’ carbon used for such applications as insulation and gland packing, the standard, intermediate high and ultra-high modulus (respectively SM; IM; HM; UHM) carbon fibre varieties (Table 5.2) suitable for composites reinforcement,2 are manufactured from one of two raw materials: polyacrylonitrile (PAN) fibres or coal tar pitch. While numerous manufacturing patents exist, many of which are now out of date or have been superseded, it is important to recognise that the respective manufacturing descriptions that follow are indicative only. Too many variations and on-going developments exist and a more definitive review is not the province of this book. One has clearly resulted in the major price reduction already mentioned. This has the potential to open up many

Table 5.2. Typical properties, carbon fibre for composites Fibre

Density (g/cm3)

Modulus (GPa)

Strength (GPa)

Specific modulus (Mm)

Specific strength (Mm)

PAN HM 45

1.9

441

1.76

23.2

0.093

PAN HMS

1.86

370

2.75

19.9

0.148

PAN T300

1.76

230

3.2

13.1

0.182

PAN T650/42

1.79

290

4.83

16.2

0.270

Pitch P25W

1.9

160

1.4

8.4

0.074

Pitch P55

2.0

380

2.1

19.0

0.105

Pitch P100

2.15

724

2.2

33.6

0.102

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new markets and new opportunities, particularly for those pultruded profiles employing a glass–carbon hybrid reinforcement or alternatively, a low percentage carbon addition to enhance very cost-effectively specific areas of performance such as the section modulus of a structural beam. With minor exceptions, PAN-based fibres exhibit the highest strengths and elongation-to-break and are the most widely produced and used, partly because they were commercially developed before the pitch-based versions. Nevertheless, compared even with the number and range of aramid, glass and hybrid reinforcements now available, careful selection of the many grades of PAN and pitch-based carbon fibres now available is, as shown by Table 5.2, clearly essential. In addition Table 5.3 illustrates the typical classifications and ranking descriptions that can apply. Originally, filament diameters between 6.5 and 8.5 mm were typical, but values between 5 and 8 mm are now more popular. Manufacture, PAN-based carbon fibre The starting point, or precursor, is the manufacture of the acrylonitrile fibre in a reaction between propylene, ammonia and air typically at 450 °C (840 °F), at a pressure of nearly 4 bars, and in the presence of a catalyst such as bismuth phosphomolybdate. The acrylonitrile is then polymerised in a very carefully controlled reaction that is promoted by suitable initiators, molecular chain transfer and retarding agents, prior to being dissolved in a solvent such as dimethyl-formamide to form a ‘dope’. In a manner similar to many thermoplastic fibres this dope is then spun at high pressure through spinnerets into filaments, to produce a ‘tow’ of continuous filaments numbering between 1000 and 320 000 before being washed, dried and wound on to spools or, for the heavier tows, crimped and laid into boxes.

Table 5.3. Classification and ranking of carbon fibre for composites application By grade

Each grade classified by modulus and ranked by

Standard modulus (SM): <265 GPa, or High strength/high strain (HT)

Filament/tow & tex

Intermediate modulus (IM): 265–320 GPa

Modulus

High modulus (HM): 320–440 GPa

Elongation

Ultra-high modulus (UHM): >440 GPa

Specific modulus Specific strength

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Input From creels for ‘fine’ tows From boxes for ‘heavy’ tows

Oxidation oven

Untreated HS Untreated IM fibre fibre Untreated HM Untreated VHS fibre fibre Sizing

1

2

3

Carbonisation furnaces 123 Oxidised acrylic fibre

4

Surface treatment

HS, VHS, IM & HM fibres Surface treated and sized Unsized fibres

Graphitisation furnace 4

5.2 Flow diagram of PAN carbon fibre plant

Conversion into carbon fibre is, depending on the grade, a two- or threestage process and diagrammatically illustrated in Fig. 5.2. The first stage is oxidation which, although many variations exist, is principally done under tension and in air at 200–300 °C (390–570 °F), to convert the PAN polymer into a ‘ladder’ structure which can be best described as a series of ring molecules joined together. These form the basis of the carbon structure of the final fibre. Carbonisation in nitrogen at 1000–1500 °C (1830–2730 °F) follows to produce all the standard and intermediate modulus grades, with further heating under an argon atmosphere at 2000–3200 °C (3630–5800 °F) to produce the high and ultra-high modulus grades. Manufacture, pitch-based carbon fibre Although the pitch precursor is considerably cheaper than polyacrylonitrile, the cost of molecular weight selection, cleaning and converting it into a suitable highly anisotropic or ‘mesophase’ single crystal form offsets the lower material cost. Together with certain proprietary additives and solvents, the pitch is then heated and agitated at between 250 and 400 °C (480 and 750 °F) under inert gas conditions for typically 20 h until ‘mesophase spherules’ form, the insoluble portion of which is then melt-spun into filaments. These filaments are subsequently treated to make them infusible by carefully controlled heating in an oxidising atmosphere at up to 300 °C (580 °F) prior to carbonisation in nitrogen or argon at between 1000 and 3000 °C (1830 and 5440 °F). Finally, whether PAN or pitch-based, the resulting tows are in a similar manner to glass fibre, surface treated or ‘sized’ in a variety of ways to protect them during subsequent forming operations such as beaming and weaving, as well as to enhance the compatibility and eventual bond to the thermoset or thermoplastic matrix into which, in whatever form, they will be compounded.

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Glass fibre Glass fibre has a long history, far removed from one of the strongest and versatile of materials in the form that it is known today. Phoenician and Egyptian artisans were the first to draw molten glass into a fibrous form, which was then used to enhance the beauty of glassware or otherwise was compounded with naturally occurring resins into useful and decorative artefacts, the precursor of today’s composites. Figure 5.3 illustrates many of the glass fibre reinforcements of interest to the composites and pultrusion engineer. The most common are direct and single end roving, conventional or multi-end roving, bulk roving, continuous filament (or swirl) mat, woven roving, chopped strand mat, stitched complexes and surfacing veil. In offering a specific and possibly unique property, each format readily allows the reinforcement package to be precisely constructed or tailored, commensurate with both the cross-section and complexity of the profile, as well as the particular application, its performance and economics. As shown by Table 5.4, which indicates the range of compositions applicable for the manufacture of glass fibre reinforcement, glass is first formed by blending and then melting together a number of common indigenous inorganic ores. Glass types A, C and D do not find as much application for composites manufacture, and certainly not for pultrusion, as do the remainder. Table 5.5 provides an indication of the respective physical and mechanical properties of the seven glass formulations shown in Table 5.4. Mono-filaments of glass fibre are formed by drawing (or ‘spinning’) the

5.3 Glass fibre reinforcements used for pultrusion

4–5

Other

ZnO

TiO2

BaO2

0.7

1.5

<0.5

K2O

1.3

8–8.5

Na2O3

22–23

0.2–0.3

0.2–0.3

0.1–0.2

5–5.5

2–3

13–14

73–74

D Hi-dielectric

Fe2O3

F

14.2

2.5

MgO

B2O3

0.6

10.0

Al2O3

65–66

72.0

SiO2

CaO

C Chemical resistant

A ‘Window’ glass – high alkali

0–1

0.25–1

0.25–1

0.05–0.5

0–0.7

5–11

0–5

15–25

12–16

52–56

E ‘Electric’ glass

2–5

0–3

0.25–1

0.25–1

0.05–0.5

0.5

18–25

10–15

52–56

ECRTM Special E-glass

6

9

25

60

R High dielectric glass

0.2

0.27

0.21

0.01

10.27

0.01

24.8

64.2

S High strength glass

182

Table 5.4. Typical glass fibre formulations (% by weight)

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183

Typical glass fibre physical and mechanical properties ECRTM

A

C

D

E

Density (g/cm3)

2.46

2.46

2.14

2.57

2.71

2.55

2.47

Fibre modulus (GPa)

73

74

55

72.5

72.5

86

88

2500

UTS (MPa)

2760

2350

Strain to failure (%)

2.5

2.5

Refractive index MoH hardness

1.451

1.47

R

S

3400

3400

4400

4600

2.5

2.5

3

3

1.555 6.5

1.541

1.524 6.5

UTS ultimate tensile strength, MoH Moh’s Hardness.

molten glass at high speed through electrically heated platinum–rhodium ‘bushings’, in effect a resistance-heated plate having 400, 800 1600 or more nozzle-like orifices or holes. The filament diameter, which can for composites reinforcement manufacture vary between 3 and 24 mm (0.000 12 and 0.000 95 inch), but more typically between 9 and 15 mm (0.000 35 and 0.000 59 inch), is determined not just by the size of the holes in that bushing, but by the temperature, viscosity, the rate of spinning and cooling, and the specific glass formulation. Although each filament – or thread – may be some 2 mm (0.08 inch) in diameter as it emerges from the ‘bushing’, the high-spinning – in effect the take-off or winding speed of perhaps 200 km/h (125 mph) – stretches each viscous thread by up to 40 000 times its original length. Immediately after spinning, each filament is coated or ‘sized’ with special surface agents to bind them together into ‘bundles’, but also to offer both lubrication and abrasion resistance during subsequent post-spinning operations. As for carbon fibre, other parts of that coating promote the optimum chemical coupling of that filament bundle irrespective of its format, to either the thermoset or thermoplastic matrix which will complete the formation of the composite itself. This is also irrespective of the fabrication technique employed, whether contact moulding, filament winding or pultrusion, to name just three. Filament ‘bundles’ which may comprise several hundreds of filaments are then formed into strands prior to conversion into rovings or yarns and, as shown schematically by Fig. 5.4, into the many other reinforcement forms already mentioned. In a sense, therefore, the glass fibre industry manufactures and supplies two distinct types of product to the composites industry. There are the various types of roving that go directly to the customer from the glass forming – or spinning – process, and the remainder that have undergone some form of post-spinning, beaming, plying, fabric weaving, assembly or

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conversion process to make them suited for the specific requirements of the customer. Although requiring, particularly at the initial spinning stage, highly capital-intensive manufacturing equipment, consumption tonnages are with few exceptions typically sufficient to permit for each a wide distribution of production costs. The consequence is a range of highly costeffective, distinct and specialised reinforcements each ideally suited to the particular requirements of the multiplicity of well-established fabrication processes now employed by the composites industry. Each of those reinforcements is now discussed in the context of its specific use in the pultrusion process. However, it needs emphasising that in each case and whatever the fabrication process, the reinforcement is classified not just by its composition and its specific format, but also by the surface treatment applied commensurate with the requisite matrix resin. There is need to remember that the latter may be as diverse as, for example, unsaturated polyester and phenolic. All this has to be borne in mind by the pultrusion engineer when selecting the preferred reinforcement to meet the functional needs of each profile application. However, while that choice has obvious cost implications, there is equally a need to consider the particular pultrusion process being employed to manufacture the profile. In other words, for example, is the reinforcement pack to be wet-bathed or will that resin impregnation stage take place within the die cavity itself?

Raw material storage hoppers

Process operations after filament forming Finished products Melting furnace Holding furnace & forehearth

Raw material blending

Bushing Sizing & coupling

Binder

Continuous filament mat

Twisting Calendering

Chopping

Tissues & veils

Chopped strands

Roving

Spun roving Yarns

Milling

Milled fibres

Weaving & related processes Chopped strand mat

Woven roving

Plying, beaming, coating, texturising

Uni-, bi- & tri-axial materials Fabrics

Binder Combination reinforcements

5.4 Processes for manufacturing glass fibre formats

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Rovings Glass fibre rovings, which might be said to resemble coarse string, are also available as hybrids with other filaments such as carbon or aramid. These will have been blended into the product at some forming stage, to vary the subsequent profile performance both between and within each category of composition. Another roving hybrid is formed by the blending of polypropylene filaments with the glass, and a format that offers obvious attractions when the matrix is thermoplastic-based and more often than not also polypropylene. Indeed these polypropylene filaments may constitute the only matrix material necessary for successful thermoplastic-based profiles to be produced. Conventional, assembled or multistrand roving (Fig. 5.5) frequently termed multi-end, is a popular roving that is ‘assembled’ from a number of filament bundles or forming packages, into a tow of the appropriate tex, textile terms that are described in the glossary of Pultrusion Terminology. Glass roving weights typically range from 600 to 18 900 tex, with fibre diameters between 7 and 24 mm (0.000 28–0.000 95 inch), although for pultrusion, the most favoured filament diameter is 17 mm (0.000 67 inch) compared with the smaller diameter of 7–9 mm (0.000 28–0.000 35 inch) which is often preferred when employing carbon or aramid fibre. Conventional rovings are most commonly used in those applications where large volumes of reinforcement, formed from longitudinal ‘layers’ of roving plies running through sometimes massive thicknesses of profile, provide a suitable mechanical property. Equally, they are eminently suit-

5.5 Conventional or assembled roving

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able for filling space at the low glass volumes which otherwise can result in the more resin-rich cross-sections that are prone to exotherm cracking through the heat that is typically released by a thermoset resin matrix as it polymerises. Such rovings also allow higher and more diverse filler loadings of the matrix. When married to a low glass content the result is a very costeffective functional performance of particular benefit in applications such as electrical strain rods, dead-end insulators or ‘hot-stick’ profiles: in other words, probes and tools for handling live electrical power sources whose manufacture is superior to those employing a direct or single end roving as the primary longitudinal reinforcement. Conventional rovings also offer a somewhat limited but still useful degree of reinforcement at 90° to the profile direction without the use of any reinforcement specifically placed in that direction. These added transverse properties arise from the catenary or drape which is an inherent property of this form of reinforcement and which occurs as a result of differential fibre length produced within the tow as it is assembled or manufactured. As a consequence, slight misalignment of the roving package occurs as a result of the pulling force applied to the profile during processing. This slightly off-axis component creates enough fibre ‘bunching’ to produce a limited degree of transverse strength in an otherwise predominantly longitudinally biased profile. Conventional or assembled rovings are also the format employed in the weaving of woven roving and in the manufacture of complex stitched and other reinforcement formats which are finding increasing pultrusion application. Nevertheless, the final selection of a conventional versus a direct roving is usually based on the tex availability and the degree to which the catenary and drape are important in the weaving, stitching or other reinforcement assembly process. However, single or direct end roving (Fig. 5.6), which is produced directly from the fibre forming process, remains the most commonly employed reinforcement for pultrusion. After the sizing and coupling agent surface treatment, the 1000 to 4000 filaments created from the bushing are gathered into a tow and wound directly into a stand-alone package called a ‘cheese’ – for that is what it resembles – ready for delivery to the customer. The typical tex range for single end rovings is 1200–9600, at fibre diameters ranging from 14 to 34 mm (0.000 55 to 0.001 34 inch). Normally the lower the tex, the smaller the filament diameter. Two process attractions of single end roving are its low catenary and low fuzz, a term by which loose and broken fibres are known. Very obviously, it can therefore pass more readily through the guidance and forming devices and, for similar reasons, offers a more reproducible mechanical performance, whether used in the normal unidirectional ‘as supplied’ direction, or when used in the manufacture of stitched and woven fabrics. In addition there are components such as sucker rods for the oil industry and ground

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5.6 Single end of direct roving

anchors, where this longitudinal tensile property of the single end or direct roving, is much more critical and thus a dominant factor. Normally selection of the optimum grade of single end reinforcement is dictated by the size and shape of the profile to be manufactured. The product thickness, the number of roving plies to give the fibre coverage required across each ‘layer’ and the number of ‘layers’ to create the required profile thickness dominate the selection process for each profile. If what is known as full coverage is required, the thickness of each layer has also to be matched to the tex of the reinforcement, such that within each layer the optimum reinforcement : matrix resin ratio is both achieved and maintained. This is all ensured by the use of mass balance principles, or preferably by means of computer design programs now available from the reinforcement supplier. The latter calculate the correct number and size of roving plies or layers required for a given design and cross-section. Other methods of reinforcement selection, based on the mechanical performance required, are considered by Chapters 2 and 3 while further comments regarding fibre architecture generally, and the future development of pultrusion reinforcements are offered in Chapter 8. Finally, bulky or texturised glass rovings (Fig. 5.7) are speciality forms of roving designed principally to improve the processing of those profiles which in certain profile cross-section areas demand, for example, a total filling of the die impractical with mats or any other form of roving. In other words, such speciality rovings have the ability to fill corners in complex shapes, or alternatively to ‘clean’ the die and prevent the formation or build-

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5.7 Bulky or texturised roving

up of resin-rich areas which would cause local spalling and eventual disruption of the process. Because of their fluffy nature, these bulky or texturised rovings also improve the reinforcement wet-out properties under low-pressure injection die conditions. At the same time they provide a further slight improvement in the transverse properties of the profile in comparison to conventional or assembled rovings. However, there is a need to emphasise that this transverse property improvement is much less than that which results from the use of mats and stitched fabrics within the reinforcement pack. With carbon fibre rovings, a similar effect to the bulked or texturised glass formats is available from ultra-high filament count tows. Carbon fibres with filament levels in the 48 000–300 000 range tend to have similar attributes to their glass counterparts without going through a texturising or bulking process, a phenomenon related to the characteristics of the precursor fibre used to form these high-filament count carbon grades.

Mats, complex reinforcements and veils Mats, roving and yarn fabrics, stitched and other complex reinforcements, plus veils, are widely and increasingly employed in the pultrusion process to develop those structural and functional attributes that cannot be achieved using roving reinforcement alone. These attributes centre around the ability of mats to develop off-axis structural performance at a lower reinforcement volume and at thereby normally reduced profile cost compared with that which would otherwise be required at a given cross-section

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or profile quality. The quality includes both visual and non-visual functions such as improved corrosion resistance. Finally, it is usual for complex reinforcement forms as well as veils, to be commercially available in either glass, carbon, aramid fibre or their hybrids and at the same time to be of multiple construction and, therefore, as required to provide the particular and unique properties specified for a particular application. Mats Most non-woven mats, such as either the continuous filament (CFM) variety composed of long, continuous lengths of fibre strands which overlie each other in a totally random swirl pattern (Fig. 5.8) or the open-textured, chemically bonded chopped strand mat (CSM) of randomly distributed typically 5 cm (2 inch) long fibres, are exclusively made of glass. Both are supplied in roll form typically at 300 and 450 g/m2 (1.0 and 1.5 oz/ft2), and the former constitutes the most common mat reinforcement used for pultrusion. It is manufactured either by the continuous belt process indicated in Fig. 5.4, or by a patented drum winding and stretching, the Modigliani Process. Whatever the manufacturing method, the mat’s randomly distributed continuous fibres are positioned such that a finite percentage of the rein-

5.8 Continuous filament (strand) or swirl mat

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forcement is aligned transverse to the remainder. This alignment varies from product to product, with axial-to-transverse stiffness ratios varying from between 5 : 1 for a highly machine directionally aligned mat to 4 : 5 for the most common pultrusion grades. Although not the easiest reinforcement to handle, the benefits of providing strength and stiffness in the transverse as well as the longitudinal pulling direction, together with an improvement in the shear strength of the profile, are obvious. For the same reason there is an improvement in both bearing strength and resistance to notch sensitivity. The coarse, typically Eglass fibre bundles that make up the mat also provide a highly porous ‘fabric’ which wets-out very readily. To additional advantage, this bulk nature limits the development of resin-rich areas in the profile, as well as reducing the glass fraction required for the processing of a specific crosssection. In its fully compressed state within the pultrusion die, a 300 g/m2 continuous filament mat will occupy a thickness of approximately 0.5 mm at a glass volume fraction of 11%. For a roving to occupy the same thickness, its volume fraction would be closer to 58%. Although not specifically designed for pultrusion, nor as beneficial to it, chopped strand mats do find some application given care in their selection. Being largely hand lay-up, contact-moulding materials, they are, for example, not designed to handle the stresses associated with pulling the mat from the creel stand and through the die. This is particularly applicable with wet-bath impregnation techniques, when the wet and therefore weak mat can completely fall apart even before it enters the die. However, because their use is often related to specific needs, such as improved surface quality or corrosion resistance, they can ideally be ‘carried’ and therefore supported throughout the process by the other reinforcement forms present, such as continuous filament mat. Alternatively, and in a way ideally, they constitute part of, for example, a complex mat-roving stitched ‘fabric’ construction. Clearly, they are also easier to employ in flat or gently curved profiles, rather than in complex cross-sections perhaps also exhibiting marked changes in thickness. Finally in dealing with mats, brief mention must be made of a wet-laid mat product not unlike thick paper, which although suffering similar disadvantages to chopped strand mat, does find some pultrusion application to add bulk and to perform in the manner of tissues or veils. Fabrics and stitched complexes As their growing use and discussion in Chapter 8 clearly indicates, fabrics and stitched complexes are certain to become the next generation reinforcements for the pultrusion process. This will very largely be dictated by the high-performance applications for which pultrusion profiles are already

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being considered and employed, whether in secondary or primary structures. In other words, the use of fully computerised designed and manufactured reinforcement packs must steadily become the norm. Consisting of specific stacking sequences of rovings, mats, fabrics and veils, their use will become inevitable as pultrusion competes ever more successfully with, for example, timber, aluminium, structural and stainless steel, and even reinforced concrete sections. Nominally this is nothing new, because fabrics have for many years been used as pultrusion reinforcement, although mostly in an inefficient manner. This results from the difficulties of first slitting and then accurately processing the fabric through the often complex guides demanded by the type and design of profile for which this form of reinforcement is so essential, without any loss of fibre or critical alignment. Speciality fabrics such as narrow tape fabrics, woven to the exact design width required, and also braided tapes and fabrics with no cut edges, have eliminated many of these processing problems but their continuing high cost makes them an unattractive answer for all but the most high-value applications. A new process in which 1–2% of a thermoplastic fibre is woven into the fabric seems to be another promising means of answering some of these production problems, as well as allowing lower costs to be realised. The thermoplastic addition allows the fabric to be heat-set not just to prevent the loss of edge fibres, but to retain the desired fibre alignment both before, during and after wet-out, even at the high pulling loads typically experienced with larger and more complex, sophisticated profiles. These fabrics provide promise for the future, both in terms of a stand-alone reinforcement and as components in stitched complexes, or in the growing use of thermoplastic matrices. Stitched complexes of the type illustrated in Fig. 5.9 have also been available to the pultrusion industry for many years. These multi-layer reinforcement packages were developed for the aerospace and highperformance marine industry, as a way of guaranteeing the structural performance of laminate constructions which would otherwise be manufactured by labour-intensive processes. Although initially interested, their acceptance by the pultrusion industry would have been quicker and more comprehensive had their cost performance been better. While acceptable to other users, the manufacturers of these stitched reinforcements had not sufficiently studied the particular needs and differences of the pultrusion industry. However, the demise of the Western Hemisphere’s aerospace programme and the stagnation in the marine pleasure craft industry which began in the mid-1990s have caused both suppliers and users to reconsider their respective positions. Together they have recognised that the pultrusion industry has a large unrealised potential in growing markets such as the rebuilding of the infrastructure.

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5.9 Stitched complex

Veils As in any composites fabrication process, veils (also known as tissues) are employed directly from the roll and, like all reinforcement other than rovings, typically split beforehand to the desired width. Principally, veils enhance the quality of the surface finish and in turn therefore the environmental/chemical resistance of the profile. However, in pultrusion they serve another function, offering marked but low-cost protection to the die cavity from the possible scarring action of the reinforcement pack as it passes through the die. Although glass and carbon-based veils are used to satisfy special applications and functional needs, spun-laced polyester and spunwoven polyester grades are more common. As the pultrusion process slowly but surely changes from open wet-bath to closed die-injection impregnation as a means of improving the shop-floor environment, then the permeability properties of the veil system will become much more important. Finally, veils can be supplied printed with a variety of decorative effects which can faithfully reproduce that effect on the surface/s of the profile. This still relatively new technique is opening up additional markets for pultruded sections, such as in the reproduction of timber sections for quality furniture manufacture.

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Concluding comments In conclusion then, new reinforcements have been and are being developed which will better meet the ‘fitness for use’ requirements of the pultruder, who is demanding better process and also more controlled off-axis performance. At the same time improvements in efficiency with an eye towards product line consolidation have allowed the stitched complex manufacturer, for example, to reduce his or her conversion costs and subsequently to produce a more cost-effective range of pultrudable reinforcements. This whole changing process will undoubtedly accelerate. At the same time, the pultruder has been able to quantify his or her needs and design requirements more critically for this whole range of new reinforcement products, and moreover to better translate that information to the respective fabric weavers and converters. One of these needs was establishing the bulk or thickness requirements for specific fabric complexes as the complexes are processed through the pultrusion die. This is illustrated in Fig. 5.10 which plots the difference in thickness versus applied die pressure for a series of stitched fabrics, data that readily allow the pultruder to determine the thickness, glass fraction and eventual structural performance that each of these competitive fabric systems will deliver in a pultruded section. From this, a value can be assigned and finally an assessment made of the cost effectiveness of any particular reinforcement. Once theoretical, but now beginning to be confirmed in practice, this 0.90

0.80

0.70

CFM 8643

mm 0.60

0.50

DBM 1715 CDM 2145 DBM 1708 A70.00.03 unidirectional rovings + CSM Woven roving 800 CDB 340

0.40

0.30 0

0.5

1.0

1.5

2.0

Compacting pressure (Bar)

5.10 Product thickness v compaction pressure curves: mat, woven roving and complex reinforcement

2.5

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Table 5.6. Theoretical relationships between pressure and volume of layer: characteristic equations for compressibility of typical reinforcing materials Material

Equation

E-glass – chopped filament mat (CFM)

Vf% = 8.56 + 15.49P 0.5

E-glass – chopped strand mat (CSM)

Vf% = 20 + 14.69P 0.5

E-glass – roving

Vf% = 32 + 23.96P 0.5

E-glass – woven roving

Vf% = 40 + 14.37P 0.5

Unidirectional carbon

Vf% = 34 + 25.55P 0.5

±45 ° carbon fabric

Vf% = 35 + 16.29P 0.5

Courtesy, Owens Corning & J A Quinn.

same basic understanding can, as shown by Table 5.6, equally be applied with similar benefit to the more standard or common types of reinforcement used by the pultrusion industry. There is, for example, a series of simple formulae that can be employed to predict the shape of the thickness pressure curves, and Fig. 5.11 indicates how these equations compare with actual measured data. 70 Theoretical: roving Theoretical: woven roving Theoretical: A70.00.30

60

Woven roving 800 50

A70.00.30 unidirectional rovings + CSM Theoretical: CSM

40 V f (%) Theoretical: CFM 30 CFM 8643 20

10

0

0.5

1.0

1.5

2.0

2.5

Compaction pressure (Bar)

5.11 Theoretical and actual, volume fraction v compaction pressure curves: mat, roving, woven roving and complex reinforcement

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As both theory and databases improve, the designer of pultruded profiles will steadily become more accurate in matching the capability of the pultruded part to the needs of the application, thus making it an ever-more cost-effective process to employ compared with traditional materials and manufacturing processes.

5.3

Reinforcement handling

In composites manufacture, all reinforcements demand to be handled with great care if their ability to strengthen the otherwise weak and brittle thermoset resin matrix, or toughen a thermoplastic matrix, is not to be impaired. This is particularly apposite for reinforcements that, like the pultrusion process, pass through or are otherwise guided and channelled into a forming die which in itself can impose a severe strain. Although protected by a polythene or similar wrapping, dropped, dented or cut roving ‘cheeses’ will, for example, result in tangling or fuzz, which all too obviously can only exacerbate any problem arising from that situation. All types of roving should be pulled from the ‘cheese’ when held in an upright position only on the reinforcement creel. While those based on glass are usually pulled from the centre, an external feed arrangement is not unknown and is more typical where delicate fibre handling is essential. Carbon and aramid-based rovings for example are pulled from the ‘cheese’ in this way, ideally using in addition a ‘lazy-susan’ type take-off. Thereafter, it is recommended that the first guide-eye is always placed in a position that minimises the amount of friction generated as the fibre moves down and exits the creel rack. For a centre-pulled direct roving this might be 8–15 cm (3–6 inches) directly above the centre of the ‘cheese’. Ideally, and irrespective of fibre type and grade, all guidance devices down to the pultrusion machine should then be of ceramic, polished steel, chrome steel or carbon black filled polyethylene. To reduce the potential for fibre abrasion and to minimise differential tension between individual strands, it is essential that these are kept clean and free of fuzz. A comparison of centre or external pull from a glass roving ‘cheese’ shows that the former minimises packaging waste and, in allowing simpler creel handling designs with simpler tension devices which have less or even no moving parts, the overall result is improved operating efficiencies at lower maintenance costs. In addition, it is also easier to move through the creel rack to attach new ‘cheeses’ or transfer from ‘cheese’ to ‘cheese’.These are major production advantages, but are accompanied by the development of a twist in the roving, something that does not happen with an external pull arrangement. However, although the whole ‘cheese’ handling, creel take-off and tensioning system can be designed to provide a centre-pull twist free delivery, such arrangements are typically complex and are not jus-

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tified when related to the value they bring to the finished profile. Finally, pulling simultaneously from both the centre and externally is possible, but rare and only recommended and warranted where the creel size is insufficient to handle the size and number of roving ends required for the application. The handling of every other form of reinforcement, typically from rolls of material split to the desired width, is somewhat easier and more straightforward. The reinforcement is typically fed from the roll in the same plane as the pultrusion machine, to pass over simple rollers and/or bars by which the material is fed down to the guidance, forming or collimation plates and impregnation arrangement. Somewhere within that whole system will be simple tensioning devices to control the unwinding of the reinforcement from the roll.

5.4

Summary

Reinforcements in the pultrusion process are literally the backbone that makes the pultruded profile perform. Profiles, whether replicating standard sections in other materials, for use as hollow or structural elements or in a wide multiplicity of custom-moulded form, are becoming as varied in format as the imagination of the designer, and as simple or as complex as required by the application. It is the skill of the designer and pultrusion engineer in understanding the interaction of reinforcement selection with the pultrusion process and with the performance of the parts produced by the process that separates the successful pultrusion company from the remainder. As the pultruder’s needs for more and different types of reinforcement format grow, suppliers to the industry must be flexible and proactive in developing systems that meet the present and future needs of this challenging and exciting industry.

5.5

References

1. Starr, T, Glass-fiber Directory and Databook, 2nd edn, Chapman & Hall, London, 1997, ISBN 0-412-78370-3. 2. Starr, T, Carbon and High Performance Fibers – Directory and Databook, 6th edn, Chapman & Hall, London, 1995.

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6 Pultrusion applications – a world-wide review TREVOR F STARR

6.1

Introduction

The technical development, commercialisation and market growth of pultruded profiles from kite rods and electrical slot wedges, through ladders and simple cross-sections to true structural elements of increasingly massive section is one of the success stories of the now mature composites industry. There are many reasons. Product criticality is the main reason, whether in respect of the superior and consistent mechanical–physical property performance or close dimensional tolerances, coupled to an excellent surface finish that can be attained whatever the profile complexity. Both the cost-effective and high environmental/chemical-resistant benefits must be added, which typically accrue from the correct formulation, specification and use of composites irrespective of the fabrication technique employed. All are features ably demonstrated by the profiles whose selection and following description also highlights in many other positive ways why pultruded profiles are of increasing attraction to the designer, specifier, consulting engineer, architect or purchase manager. The offshore oil/gas industry application alone confirms why timber, metal, thermoplastic and sometimes reinforced concrete are in their respective ways often no longer the optimum material answer for the manufacture of engineering sections or profiles to be used as structural elements. With such a diversity of case-history excellence, any selection, which must perforce be limited, is also invidious. Many companies other than those gratefully acknowledged for the provision of the illustrations could, through other very suitable example, have equally been featured. At the same time any suggestion of an importance in the order of listing has been countered by recourse to classifications based simply on the alphabet. However, even that classification employing named or market-application examples was not without difficulty. With justification, several could feature again under a different heading. 197

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6.2

Airport

The lightweight, structural, electrical resistant and easily erected properties of pultruded profiles come to the fore in the design of a wide range of aerial and lighting support assemblies which play their equal part in securing the safety of air travel. In addition their ability to be readily designed and constructed on-site from the wide selection of standard glass–polyester rather than custom-moulded profiles now available, plus built-in self-colour decoration and low maintenance benefits, all add further advantages. As a further important consideration, should the worst happen, then the flexible nature of pultruded profiles’ individual structural elements presents a much lower hazard than would otherwise be the case for a steel or even aluminium alternative, the design of which also had to allow for insulators of one type or another. Such constructions typically satisfy the Federal Aviation Authority specification AC 150/5345-45A and, although sufficiently rigid to accept strong winds and heavy ice-loads, are frangible as stipulated by ICAO ADM6. As a result of, for example, the patented cross-bar joint-bonding method developed by the Finnish pultruder Excel Oy, there is no twisting, swaying or oscillation within the structure, which in the case of approach lighting support masts (Fig. 6.1) would otherwise cause serious distortion of the light beam and its pattern presented to the pilot. In service since 1990, well over 50 complex, successful installations have now been completed by that same company who can add to all those advantages the clear finding that there is no need for periodic realignment and/or tightening of the fixing system that are found necessary with certain types of aluminium masts. Many of the same advantages apply to the growing use of pultruded profiles for the construction of heavy duty, maintenance-free airport fencing systems that to vital importance are also radar transparent. At the same time they do not disrupt the optimum working of the Instrument Landing System (ILS). One fence design by the Belgium company Bekaert employs an internal fixing system that prevents easy removal of the separate panels from which it is constructed and, being free of cross bars and topped with a sharp sawtoothed finish, makes for an extremely difficult obstacle to climb (Fig. 6.2). The manufacturer claims that around 100 m (325 ft) of fence can be erected by two men per day and that the design can be readily up-graded in those areas where there may be exposure to jet engine blasts of 300 km (185 miles) per hour. While the support elements typically embedded in concrete might well employ a vinyl ester as the matrix, a good-quality isophthalic polyester has been found eminently suitable for the remainder of the structure, all items being solely glass fibre reinforced.

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6.1 Typical airport approach lighting assemblies (Courtesy, Exel Oy, Finland)

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6.2 Pultruded profile airport fencing system (Courtesy, Bekaert Composites, Belgium)

6.3

Cableways

The electrical resistance properties for which composites are generally noted has prompted the extensive use of pultruded profiles in the design of cableway support or cable tray systems. These range from those assembled from rod or tube with suitable jointing pieces (Fig. 6.3) to a wide range of custom-moulded design to satisfy in an optimum manner the weight, size and number of electrical, telephone and perhaps even optical data transmission lines that are required to be securely carried over what may be both a long and undulating distance. Additional properties such as special environmental, fire or chemical-corrosion resistance may also be very frequently specified. Among what are perhaps scores of pultruded cableway designs, the work of the Italian pultrusion company Top Glass SpA and the UK company Fibreforce Ltd is worthy of note as joint suppliers meeting the exacting standards of the cable trays employed in the Channel Tunnel. The main profiles produced for this project are shown in Fig. 6.4 with Fig. 6.5 illustrating one manufactured length of the five-way design and its installation with other cable tray designs within the tunnel.

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6.3 Cableway assembled from pultruded profiles and additional moulded fixtures

Five-way cable tray 450X100 mm2 (17.7” X 4”)

Two-way cable tray 600X200 mm2 (24” X 7.8”)

For telephone, sound, radio and fibre optic cables located in service tunnel

For high-power cables in main tunnels

6.4 Channel Tunnel cable way profiles (Courtesy, Top Glass SpA)

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6.5 Channel Tunnel profiles and installation (Courtesy, Top Glass SpA)

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Pultrusion applications – a world-wide review Table 6.1.

203

Mechanical and fire performance, Channel Tunnel cable tray profile

Fire properties

Limit

Result

BS 476 Pt 6 Fire Propagation

I<6 I < 12

0.5 11.8

BS 476 Pt 7 Surface Spread of Flame

Class 1

Class 1a

BS 6853 Smoke Evolution (Rolling Stock Design)

A0 On < 4 Off < 6

2.3 2.5

BS 6853 Temperature Index (Flammability)

>350 °C (>660 °C)

>350 °C (>660 °C)

NFF 16–101 Fire Behaviour

M2 FO

M1 FO

NES 713 Toxic Fume Index

<5 (Ideal Cat 1)

1.2

Toxic gas exclusions/limits

No halogens <100 ppm SO2 <10 ppm NOx

None present Not present Not present

Mechanical properties

Longitudinal

Transverse

Flexural strength (MPa)

535

155

Flexural modulus (GPa)

17.6

12.2

Tensile strength (MPa)

470

46.8

Tensile modulus (GPa)

22.6

12.0

Impact strength (J/m)

4440b 4858c

— —

a

Zero burn. b Notched. c Unnotched. Matrix: MODAR 826HT 100 pbw Antimony trihydrate 170 pbw Data: Ashland Chemicals Inc.

Glass: CFM/Roving/CFM 39% v/v (49%w/w)

Neglecting for a moment the fire performance, the environmental conditions within the tunnel are particularly severe, from the wind speed of 360 km/h (225 mph) created by the frequent passage of trains, to an ambient temperature varying between 10 and 40 °C (50–105 °F) at a humidity level of 100%. In addition steady seawater ingress into the tunnel was another condition that could be accommodated more readily by a composite pultrusion than by galvanised mild steel or exotic grades of stainless steel which were competitive materials considered for the project by Transmarche-Link (TML) and its subcontractors Balfour Kilpatrick and Spie Batignolies. There was concern with both the latter over a high corrosion rate which over a long period of time would lead to unacceptable replacement costs and major rail service disruption. The stringent fire performance requirements were answered by the use

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of an aluminium trihydrate-filled MODAR methacrylate resin matrix (826 HT) as shown – together with mechanical property data – in Table 6.1. In total, the production involved well over 400 km (250 miles) of pultrusion, corresponding to more than 3600 tonnes (7.9 million lb) in weight.

6.4

Cooling towers

Compared with the traditional materials, timber, steel and reinforced concrete formerly employed, the performance advantages of low-weight, high-strength, excellent electrical, corrosion and environmental resistance certainly apply in the construction of cooling tower structures.1 They are applications offering a tremendous development and sales potential for nothing more than, in effect, standard-section glass fibre–polyester resin formulated pultruded profiles. When Barrick Goldstrike Mines, Inc, for example, faced the need to cool water associated with its mining operation in the high desert region of northern Nevada, the optimum environmental answer satisfying the extreme weather conditions of the site was a massive, competitively priced, on-site erected, pultruded construction some 14 m (46 ft) tall, spanning 330 m (1100 ft) and over 16 m (54 ft) wide, manufactured by the American company Bedford Reinforced Plastics. Twenty-nine flat bed trucks were employed to ship to a tight schedule the 278 tonnes (612 000 lbs) of square, angle, channel, deck board and structural members, whose combined length totalled more than 120 000 m (400 000 ft). To the benefit of fabrication and installation lead times, the thousands of bolt connecting holes required were machined prior to shipment, using a computer-controlled router to ensure dimensional consistency and accuracy. The tower (Fig. 6.6) consists of two banks of 10 cells, each capable of reducing the temperature of over 356 000 l (65 000 US gallons) of water per minute from 55 °C (130 °F) down to a temperature equating at all times with that of the local Humboldt River to which it is returned. Making this mandatory regulation even more of a challenge, the temperature of the region varies from -30 °C in winter to 24 °C in summer and while that volume is not a huge amount of water for a tower to handle, it is this thermal duty that has made the project and its composites method of construction unique.

6.5

Fencing

Natural hedges of bush, tree and other foliage make ideal division for the containment of farm and other animals but their costly maintenance, slow growth and inability to be relocated make the use of fencing materials essential. However, most timber, reinforced concrete and metal-based alter-

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6.6 Water cooling tower structure (Courtesy, Bedford Reinforced Plastics)

natives present, in addition to cost, distinct disadvantages such as slow installation which has given rise to the growing and effective use of electric fence systems by the farming industry. These can be quickly moved and although the more cost-effective answer in competition to these traditionals, most commercial systems still continue to employ extruded thermoplastic in the form of small diameter rods or alternative profile sections. Even if those designs contain chopped glass fibre and are of larger section, their major disadvantages are typically limited temperature resistance and insufficient toughness, with the latter property degraded in-service by ultraviolet (sunshine) attack. Taken together they impair their attraction as a sound long-term capital investment for the farmer. On the other hand pultruded glass fibre/polyester fence post rod, 10 mm (0.4 inch) or 13 mm (0.5 inch) in diameter (Fig. 6.7), incorporating several proprietary fence-wire attachment and other devices, overcomes all these disadvantages without introducing any further constraint to a more widespread use than a higher cost (but much better capital investment) in comparison to the thermoplastic alternative. Not only is the former thermoset, with a typical di-electric strength of 12 kV/mm, one of the best insulators, but the material is totally impervious to moisture and is not affected by the extremes of ambient temperature, super-phosphates and chemical sprays. Furthermore, the glass–polyester composite will not readily support the

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6.7 Pultruded fence post system (Courtesy, Pultron Composites Ltd, New Zealand, and Tern Engineering Pte Ltd, Singapore)

Table 6.2. Mechanical properties, pultruded fence posts Tensile strength

400 MPa

Tensile modulus

25 GPa

Flexural strength

450 MPa

Flexural modulus

25 GPa

Compressive strength

200 MPa

Courtesy: Tern Engineering Pte Ltd, Singapore.

growth of lichens or for example moss, and if fire performance is an additional requirement in locations where the occurrence of fire is a possibility, then a phenolic-based, glass fibre pultrusion can readily provide an immediate, low-cost answer. This enhanced toughness (Table 6.2) has also permitted the successful extension of these pultruded fence post systems to the containment of even the largest and heaviest animals in the wild, ranging upwards from kangaroos to elephants. For example, while a fence height of perhaps less than 1 m (3.3 ft) is suitable with three horizontal wires for most farm animals, certain predators require around 1.25 m (4 ft) with perhaps seven wires, whereas a three-wire arrangement, nearly 2 m (6 ft) high is necessary for

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elephants: overall an effective, increasingly recognised and valuable new fencing system based on the very simplest of pultruded profiles. There is a clear difficulty in many instances in segregating fence, walkway and staging systems, whether employed for agriculture, chemical processing, water filtration and treatment, or some other use. Reference should therefore be made to later sections dealing specifically with stagings and walkways.

6.6

Flooring and walling systems

The above comment is equally true in connection with flooring, walling and allied systems employing pultruded profiles. These modular, often selfjointing, industrial systems typified by Figure 6.8, are ideal for a wide range of filtration, water-treatment, chemical and corrosive environments. As a result, a number of proprietary systems (e.g. Bio-Planktm from Bi Original as in Figure 6.9 or FlowGRIPtm from Redman Fisher Engineering) are now available. Indeed, their design and development often originates from companies who do not pultrude, thus allowing them the competitive purchase of profiles in specific custom shapes and dimensions, as well those classified as standards.The American concerns, Strongwell and Creative Pultrusions, together with the UK company Fibreforce and Bekaert in Belgium, are among many manufacturers world-wide from which such profiles are

6.8 ‘Bio-Plank’TM modular flooring units employed in water treatment/filtration plant (Courtesy, Fibreforce Ltd)

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sourced. A typical example of this ‘innovative application of GRP pultrusion technology’, as patented by Redman Fisher Engineering (FlowGRIP) employs a 0.5 m (20 inch) wide by 40 mm (1.6 inch) deep integrally coloured interlocking plank with a built-in sealing tongue and groove forming a ‘solid’ surface. Otherwise the design may be slotted to allow rapid drainage, or the respective profile components coated with an anti-skid texture. Designed to give maximum rigidity at minimum weight such pultruded profile constructions are rapidly replacing timber, steel and reinforced concrete. The low shipment weight and ease of erection onto a minimum of foundation work, and an ability, for example, to accept a loading of 13 kN/m2 on a raised 200 m2 (2150 ft2) platform are very desirable attributes, to say nothing of their virtual elimination of an on-going maintenance requirement. Another major advantage is the non-sparking characteristic of the pultruded composite. Many industrial environments cannot tolerate the generation of sparks, and the offshore industry market is therefore of particular and growing importance to the pultrusion industry. If nothing else, the replacement of corroded steel components at offshore production platforms is not only expensive but typically requires a temporary halt to operations through the danger of sparks, etc, during the necessary on-site welding. Room for heavy lifting gear may be another constraint, answered in the case of a 12 ¥ 6 m2 (40 ¥ 20 ft2) composite well-bay platform installed by four men in just two days compared with the eight workmen in five days required for the steel alternative. There is much market-application interaction. Nevertheless it is obvious through many of this chapter’s application examples, that machine-made

6.9 ‘Bio-Plank’TM employed as walling (Courtesy, Bi Original)

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pultruded profiles are steadily opening-up the way for new design thinking that places great benefit on the consistent quality and predictable engineering properties for which such profiles are now increasingly recognised. It is appropriate to include in this particular application review comment pertinent to both fire and component costs. Fire performance receives consideration in other chapters but it is worth reporting that taking just one example, pultruded flat panels successfully passed jet-fire tests conducted by Shell Expro at the British Gas Test Centre. During this tough 2 hour test, the panels were exposed to a high-pressure gas jet burning at around 1200 °C (2200 °F) which was estimated to have consumed some 18 tonnes (40 000 lbs) of gas. As additionally described later, further offshore tests have also confirmed the ability of the composite assemblies to withstand the effects of an offshore explosion. However, virtually ever since their introduction, composite components have been criticised – and indeed too frequently rejected – on account of their admittedly higher supply cost when compared with the other materials with which they daily compete. Any assessment based solely on the supply cost is very seriously in error: a situation that is now being increasingly recognised and accepted. A much more accurate and therefore correct assessment is the in-service life cost, which takes into account such important matters as shipment, installation and the maintenance, environmental cost savings, all of which can be appreciable. Finally, any comment regarding flooring systems cannot overlook the now extensive use of gratings fabricated from pultruded profiles which are typically available from a large number of companies world-wide in 1 m2 (11 ft2) modules and which have a visual appearance not dis-similar from the flooring shown in Fig. 6.8. A reproduction of actual point-load deflections for a typical pultruded grating design is given by Fig. 6.10 and most grating suppliers are in a position to provide similar and additional data including, for example, uniform and linear load conditions. Among all the other examples mentioned, pultruded floor members are finding a growing agriculture acceptance where – in addition to surround fencing – there is a rapid build and later teardown requirement for farm animal containment units of wide dimensional and weight acceptance variation having long-term resistance to what is typically a severely corrosive environment.

6.7

Kolding bridge

Referenced elsewhere in other chapters, the two UK bridges, Aberfeldy and Bonds Mill, employing pultruded profiles have since their construction in the early 1990s become famous. Through their success, they have become recognised as definite percursors for this type of application. However, their combined effect on that market potential has, even though the short

210

6.10 Point loading data for pultruded grating (Courtesy, Fiberline Composites A/S, Denmark)

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span of Bonds Mill carries road traffic, been somewhat limited simply because custom rather than standard profiles were employed in their construction. It can be argued therefore that the other bridge examples discussed in Chapter 7, but in particular the Kolding bridge2 over a main, double railway track in Denmark, have, as a result of their use of standard, off-the-shelf, glass fiber–polyester resin profiles, had a much greater effect in demonstrating the way forward for this sector of the composites industry (Fig. 6.11). Its success as a 40 m (130 ft) long light vehicle and pedestrian bridge, is a positive example of the increasing penetration by composites generally, of the civil engineering–infrastructure requirement. In addition, Kolding bridge, designed and built by the Danish pultrusion company Fiberline Composites A/S, has secured a number of historical firsts. Not only is it the first for Scandinavia, and also the first to span a railway line and a busy one at that, but it is one of the biggest examples of its type with the performance specification demanding not just safe and quick installation but requiring a

6.11 The Kolding, pultruded profile bridge (Courtesy, Fiberline Composites A/S, Denmark)

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minimum of on-going long-term maintenance. Weighing less than 12 tonnes (26 500 lbs) which is less than half that of a similar steel design, the loadcarrying capacity of 500 kg/m2 (100 lbs/ft2) allows for snow-clearing vehicles, etc, of up to 5 tonnes (11 000 lbs) to pass at the same time. The maximum allowable wheel pressure is 1.8 tonnes (4000 lbs). On-site assembly, which took two nights during weekends when the train traffic was at its lowest, was assisted by the in-factory assembly of three main bridge units – an 18.5 m (60 ft) tower, a 27 m (88 ft) deck section and a 13 m (43 ft) deck section, both 3 m (10 ft) wide – using stainless steel bolts at every connection. Special attention was given to these fixing points to ensure they could withstand both the static and dynamic forces, as well as creep. Sufficient transverse as well as longitudinal performance in each of the required polyester-based profile sections was guaranteed by the careful placing, as necessary within the fibre architecture, of glass fibre woven roving, combination and biaxial materials in addition to unidirectional rovings. It must, however, be noted that certain parts of the wall sections, seen typically perforated in Fig. 6.11, are not pultrusions but rather flatplate glass–polyester mouldings chosen for their cost-effectiveness in this largely non-structural area. Although considerable structural design, development and evaluation work was undertaken well before construction commenced, with the latter continuing as part of the overall quality control procedure, the completed bridge is being carefully monitored by 28 stategically located strain gauges. The results are already providing a wealth of data of value to other designers, constructors and engineers and particularly in the context of a second bridge that Fiberline is hoping to construct in Switzerland. Finally, Table 6.3 provides some comparable cost estimates between pultruded composites, steel and reinforced concrete.

6.8

Leisure

Providing confirmation that pultruded profiles are beginning to penetrate every facet of twenty-first century life and will increasingly do so is ideally exemplified by the extensive use of profiles in the construction of a spectacular re-creation of Italy’s Lake Como as the frontage to the new Bellagio Hotel and Resort in Las Vegas, Nevada (Fig. 6.12). Spanning 400 m (1320 ft) overall and with a capacity of 113 million litres (30 million US gallons), this ‘lake’ and huge fountain display feature a water, light and musical show each night, vying with the many other visual attractions to be found around this leisure city. Although invisible from the surface of the lake, some 182 tonnes (400 000 lbs) of black pigmented structural pultruded profile manufactured by the American pultruder Bedford Reinforced Plastics, supports some

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Table 6.3. Unit breakdown comparing Kolding bridge construction costs, pultrusions, steel and reinforced concrete Composite

Steel

Reinforced concrete

Engineering

60a

30

Foundations

60

75

90

120

20

90

Production assembly

60

90

Installation

30

60

90

Surface treatment

10

30

15

Materials

22

Nil

Other

30

40

40

Totals

370

345

347

a

Includes development costs.

6.12 Section of pultruded profile sub-structure in Lake Como feature at Bellagio Hotel and Resort, Las Vegas (Courtesy, Bedford Reinforced Plastics, USA)

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3500 m2 (38 000 ft2) of composite grating supplied through Seasafe of Lafayette, Louisiana and Grating Systems of Ogden, Utah. That grating is employed in turn to support the entire fountain and lighting display, and this continuously submerged pultruded-grating substructure, typically 0.6 m (2 ft) below the lake surface, exhibits a length of around 800 m (2600 ft), at a maximum width of just over 3.6 m (12 ft) and a height of almost 4 m (13 ft). At that depth and because of the black pigmentation, none of that substructure is visible when the fountain and light-show are not operating and the lake is caused to model the usual placid appearance of Lake Como. Claimed to be the largest pultruded structure yet built, the basic construction is, with the exception of the grating, clearly not too dissimilar from the cooling tower described earlier and also attributed to Bedford. Steel, although considered initially for the construction, was eventually discounted principally on account of weight and speed of installation rather than as a result of the long-term non-corrosive nature of the chosen pultruded glass fibre – isophthalic polyester resin composite. Heavy-lift cranes with lengthy booms would have been essential with a steel version, a point firmly demonstrated by Bedford’s with a 1 m (40 inch) length of 0.45 m (18 inch) pultruded I-beam easily handled by two people, which in steel would otherwise have only been moved by mechanical means. The whole installation demanded just the use of fork-lift trucks, saving an estimated US$400 000 on heavy-duty cranage, a situation that adds strong emphasis to earlier comment that the manufactured cost of pultruded profile – and indeed composites generally – forms only a small part of the whole inservice costing equation.

6.9

Optical fibre tension/support member

For all their telecommunication advantages over the traditional copper versions, the friability of optical fibres together with their potential for surface and other damage which destroys or impairs transmission quality have demanded the development of a suitable support or, in other words, core member. This not only offers the required protection but acts in accepting all the tension, fixture and other installation or in-service loads that would otherwise be imposed on the optical fibre ‘bundle’. To satisfy other parameters, that tension member was required to be light weight, tough and flexible, available in small diameters and in virtually continuous length, totally resistant to any environment in which the optical fibre cable might be located and moreover be of the lowest possible cost at the highest possible surface quality (Fig. 6.13). Pultruded support members in diameters between 0.5 and 8 mm (0.02 and 0.3 inch) either in the form of solid rod or hollow tube and based on glass roving reinforcement in a MODAR 826HT acrylic resin matrix have answered this performance specification in an optimum manner and are

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6.13 Tension/support member for optical fibres (Courtesy, IPT, Top Cable, Austria)

Table 6.4.

Typical specification for tension member

Fibre type

E-glass

Fibre content

80% ± 2% w/w

Specific weight

2.1 g/cm3

Tensile strength

1400 N/mm2

Tensile modulus

50 000 N/mm2

Flexural strength

1850 N/mm2

Compressive strength

950 N/mm2

Elongation

2.2%

Water absorption (168 h @ 70 °C)

0.02%

Temperature stability

-50 to +120 °C

Data: IPT, Top Cable, Austria.

therefore now manufactured in increasing quantity by several large global concerns. Typical tension member properties are listed in Table 6.4.

6.10

Railways

As the development and growth of railway, mass-transit and ‘people-mover’ systems generally accelerate world-wide, so will their utilisation of pul-

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truded profiles. Three specific examples from among many have been chosen as suitable illustrations of that growing infrastructure market. Their order has been based solely on an alphabetical listing rather than any attempt to suggest their respective importance. However, in every case the success of the application has been linked among other advantages, to the combination of excellent electrical and environmental resistance properties.

‘METEOR’ guide bar METEOR is the fourteenth RATP line of the Paris underground system, connecting the southern part of the city with Gennevilliers. It has been designed for automatic train operation and continues the use of the pneumatic guide wheel and electrical power collection arrangements which over the years have proved highly successful and are now a famous part of the whole Paris metro and RATP system. Insulated portions of that guidance system are as illustrated in Fig. 6.14, provided by individual 18 m (60 ft) lengths of a pultruded profile manufactured by Doneco Celtite Profilex (DCP) of Villers-Saint-Paul, France. These insulating guide bar sections are mechanically fixed at 1.8 m (6 ft) centres, to the traditional guidebars/collection arrangement, and their 260 ¥ 90 mm2 (10 ¥ 3.5 inch) cross-section is detailed in Fig. 6.15. Satisfying the French fire and smoke resistance classification M1F1, the requisite profile has a

6.14 Pultruded insulated METEOR guide rail (Courtesy, DCO, France)

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260 mm

90 mm

6.15 Cross-section, METEOR guide rail pultrusion (Courtesy, DCP, France)

glass fibre content of 55% in an aluminium trihydrate-filled MODAR methacrylate resin matrix. At some 33 kg/m (22 lbs/ft), the profile is stated to be one of the heaviest currently manufactured in Europe.

Overhead catenary support A wide range of catenary support designs can be noted world-wide. Most are based on galvanised steel constructions which therefore demand the use of often complex insulation arrangements to support the high-voltage overhead electric power supply cable. However, the growing use of pultruded profile mast and tower assemblies for the industrial and domestic supply of electricity is now being duplicated by the railways for new constructions or over refurbished lines. While the extent to which the glass fibre, typically isophthalic polyesterbased profile alternative will be employed in the total construction, the additional advantages, which include light-weight, ease of erection and ability to withstand high wind speed loadings, are slowly being recognised and accepted as the preferred alternative to steel. A typical overhead catenary support arrangement is illustrated in Fig. 6.16.

Third-rail protective cover Although extensively employed world-wide as a means of collecting the necessary high and often DC voltage for both under- and above-ground railway systems, the third-rail arrangement for all its other advantages clearly presents a particular danger to both maintenance and all operating staff. With lines frequently passing domestic gardens and open farming land for considerable distances, it is surprising that accidental or farm-stock death is not much greater than it is. However, that risk has been recognised in the building of many new mass transit systems by the installation of third-rail covers whose careful design

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6.16 Overhead catenary support arrangement (Courtesy, Top Glass SpA, Italy)

protects those crossing the tracks but in a manner that does not impede in any way the electrical power collection. The ability to form strong, complex shapes in continuous length by pultrusion, to offer high strength, ease of shipment to and handling on-site of finished easily fixed profiles with high electrical and excellent environmental resistance, is now an accepted – and growing – market opportunity. Many suitable designs exist, that shown in Fig. 6.17 being very typical.

6.11

Rock-soil support applications

Pultruded tubes and solid bar in diameters typically <60 mm (1.6 inch) are, in addition to finding some application in for example hand-rail support systems, being increasingly employed as the basis of a variety of devices designed to improve the stability of both rock and soil faces before, during

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6.17 Pultruded third rail cover (Courtesy, European Pultrusion Technology Association)

and after tunnel excavation or other major earthwork. In other words, the permanent but also temporary installation of these devices sometimes in conjunction with concrete injection into those surfaces has the objective of preventing – or at worst limiting – the potential for collapse of any treated surface along existing weak planes or otherwise through surfaces that have been weakened by the tunnelling or excavation operation (Fig. 6.18). Compared with steel or even the thermoplastic versions which are suitable for some operations, pultruded pipe or rod exhibits a number of distinct advantages, one of which is a combination of mechanical and physical properties of benefit to their on-site installation. Ease of installation plays its obvious place in enhancing levels of safety and also the speed and costeffectiveness with which the whole work progresses. Pultruded pipe and rod also shatter more readily and with less danger than steel in any subsequent blasting procedure necessary to extend the treated tunnel surfaces or other excavation. In addition, however, to their long-term corrosion resistance, is the very high shear strength property which particularly assists for example in the pre-consolidation of the tunnel drive face or in the strengthening of the tunnel or other surface by in effect creating reinforcing ‘rings’ around that surface. Finally, because the elastic modulus is lower than that of steel, support devices based on pultrusions typically offer a much closer fit to the mechanical characteristics of the rocky mass.

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6.18 Installation of pultruded rock support devices (Courtesy, ATP srl, Italy)

To date more than 2000 km (1250 miles) of pultruded pipe and rod have for example been employed in Italy during tunnel works such as required by the new high-speed railway line between Rome and Florence. The supplier, ATP srl, lists a wide range of different pultruded tube or rod designs ranging from smooth surfaced tube, to those whose outer surface has been machined or roughened in some way to assist in the adhesion between device, any injected concrete and the excavated surface. Other threaded versions can after installation be fitted with external plates enabling the device to be tensioned in position.

6.12

Selection – custom profiles

There is a clear need in any review of pultrusion applications to reaffirm the importance of custom-moulded profiles, not just to that industry but to the customer at large. Like other comment within this chapter the number of custom mouldings now available world-wide that could be selected for illustration is legion and as a consequence there is a need to emphasise that Fig. 6.19 gives solely a small typical selection. Collectively they also encompass a dimensional range which can equally be considered as very indicative of current production. However, as also confirmed by Fig. 6.19 the vast majority of the custommoulded profiles currently pultruded could equally be classified under one,

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a

d

b

e

c

f

221

6.19 Selection of custom mouldings. (a) Channel profile 25 ¥ 95 ¥ 2.8 mm3 (10≤ ¥ 3.75≤ ¥ 0.11≤); (b) connection profile 130 ¥ 110 ¥ 4 mm3 (5≤ ¥ 4.3≤ ¥ 0.16≤); (c) corner profile 90 ¥ 52 ¥ 3 mm3 (3.5≤ ¥ 2≤ ¥ 0.12≤); (d) guide rail profile 100 ¥ 40 ¥ 10 mm3 (4≤ ¥ 1.6≤ ¥ 0.4≤), 5 mm (0.2≤) thickness (e) handrail profile 100 ¥ 81 ¥ 3 mm3 (4≤ ¥ 3.2≤ ¥ 0.11≤), 53 mm (21≤) diameter; (f) insulator profile 180 ¥ 25 mm2 (7≤ ¥ 1≤) (Courtesy, Nordic Supply Composites A/S, Norway)

or more, of the headings employed by this chapter. For example the channel profile is typically the cover to a cable tray or duct with the connection profile employed in vehicle construction, as is the corner profile. The handrail profile is obviously a special employed for that application, while the insulator profile has much the same use as the profile shown in Fig. 6.14. Finally, it is important to emphasise that like the vast majority of pultrusions generally and whether or not mineral filled, a glass fibre reinforced isophthalic polyester composition is and is likely to remain the most usual.

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6.13

Stagings and walkways

It is clear from much of the preceding market-application analysis that it is the structural properties of pultruded profiles that are particularly attractive to the designer and engineer. In other words pultruded profiles can be treated in exactly the same manner as, for example, steel sections, but gain advantage through their lightweight, high-strength and corrosion-resistant nature. Although they cannot be welded, the fixing and adhesive bonding techniques reviewed in the following chapter provide a very suitable alternative. As a consequence therefore pultrusions have found ready application in the construction of all forms of staging, stairways and walkways, uses that have clear links to fencing, railings and many of the other market areas considered by this chapter. They are, as has already been mentioned, obviously all inter-related. In addition every construction dimension from small to massive and whether required by the chemical processing or leisure industry to highlight just two market sectors, could be exemplified with Fig. 6.20 selected simply because it is so typical of the vast majority. Formed of standard off-the-shelf, self-coloured sections, the staging in Fig. 6.20 is supported on similar standard I-beams of suitable cross-section all demanding nothing more sophisticated in terms of the profile formulation than glass fibre reinforcement and a good-quality isophthalic polyester resin as the matrix. The only changes that may on occasion be required are the use of a vinyl ester matrix to further enhance the corrosion resistance or, as the profile dimensions increase commensurate with a higher structural loading, the possible use of a small quantity to carbon fibre laid down together with the glass in certain areas of the profile,3 typically in an I-beam, within the upper and lower flanges.

6.14

The ‘Eyecatcher’

A further example of the structural load-bearing acceptance for which pultruded profiles are now accepted is the residential/office building constructed for display during a 1999 Swiss building exhibition and dubbed the ‘Eyecatcher’. This five-story vision for the building industry (seen in part view in Fig. 6.21) employs simply flat, U- and I-profiles joined as required to form beams using a two-component UV-resistant epoxy adhesive which in turn were bolted together into sub-units to facilitate disassembly and later reconstruction when the structure will be employed as a permanent office building. Although in many ways therefore a trial or prototype construction, the Danish company Fiberline Composites has designed and constructed a building offering good thermal properties free of either cold or warm

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6.20 Typical pultruded staging and walkway (Courtesy; Bekaert Composites)

‘bridges’, which integrates the façade in a load-bearing construction of a building which is visible both internally and externally. In order to achieve an insulation value equivalent to a 300 mm (12 inch) wall, certain translucent panels of the façade form a sandwich construction filled with a superinsulating material trade named ‘Aerogelen’. Once again as confirmed by Table 6.5 the material combination of unsaturated polyester resin and glass fibre results in highly suitable mechanical properties for this type of structural application. To enhance their weatherability, the individual profiles employed a surfacing tissue and factors such as low weight, the ability to machine the profiles quickly and cleanly as required by the overall design ensured low assembly costs in comparison to traditional materials. Over the years the building and construction industry has made extensive use of composites approaching, if not for some countries exceeding, a

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6.21 The ‘Eyecatcher’ residential/office building (Courtesy, Fiberline Composites A/S, Denmark)

Table 6.5. Construction and associated data, ‘Eyecatcher’ structure Weight

10 tonnes (22 000 lbs) total

Height

14.5 m (48 ft)

Ground area

10 ¥ 12 m2 (33 ¥ 40 ft2)

Load-bearing capacity

3 kN/m2, with balcony at 4 kN/m2

Allowable deflection

L/200 (horizontal) and L/350 (vertical)

Courtesy: Fiberline Composites A/S, Denmark.

consumption level of 20% on the total annual finished product output. Among that massive tonnage now approaching a world-wide total of around 1 million tonnes (2.2 ¥ 109 lbs), have been many attempts to produce low-cost housing units, some of which have employed or have considered

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the use of pultruded profiles. There seems little doubt therefore that with the advent of the ‘Eyecatcher’ and for example the building described in Chapter 3 that this form of structural use for pultruded profiles will become common-place in the future.

6.15

Troll Phase One

The Norwegian Troll Phase One gas field project remains one of the largest European offshore developments that has made an extensive, successful use of composites (Fig. 6.22). Pultrusions manufactured in the UK by Fibreforce Limited were used in a totally typical manner for walkways, handrails and other structures required to bear heavy weight. However, incorporated within the design of an associated on-shore structure, there is a unique blast relief panel cladding system, constructed from a pultruded GRP profile manufactured by the same company. This system was required to have a fast opening response time of less than 25 milliseconds, to significantly reduce the peak overpressure generated within the affected area and in turn, therefore, reducing the need for additional structural reinforcement to ensure survival of the building. An aluminium construction had been considered, but after initial evaluation the two extrusions that would have been necessary required welding, a process judged too costly and time consuming. The use of composites provided, not unexpectedly, both an efficient manufacturing process and an optimum answer to these seven design and performance specification criteria: • • • • • • •

the lowest number of component parts; a long maintenance free in-service life; the lowest total installed costs; a minimum of base debris should the blast system be activated; flexible production in respect of a variable panel length; flame spread characteristics meeting Class 1, BS476 Part 1; and the minimum possible weight for fastest possible blast response time.

However, pultrusion was not the immediate composites fabrication choice. Hand-lay or contact-moulding had eventually to be rejected as unattractive for a production requirement of 30 000 m (98 400 ft), given the additional demands of minimum thickness and weight. Likewise another composites alternative, resin-transfer moulding (RTM), was not judged ideal to manufacture over the required volume a sufficiently reproducible, quality product at the necessary width of 448 mm (17.5 inches), particularly where a variable length of up to 7 m (23 ft) also applied. Using pultrusion, however, it was possible to provide a suitably structural profile, typically 3.5 mm (0.14 inch) thick and weighing 3.7 kg/m (2.5 lbs/ft), manufactured at some

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6.22 Blast relief cladding panel: Norwegian Troll Project, Phase One (Courtesy, Fibreforce Ltd)

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6.22 Continued

1500 m (4900 ft) per week and exhibiting the low flame-spread, low-smoke, low-toxicity fire hardness properties provided by an antimony trihydratefilled MODAR® methylmethacrylate–urethane matrix. However, the major advantage was the ability to incorporate in the profile design at a point of localised thickness reduction, a passive hinge of glass mat and KevlarTM reinforcement, where the latter is oriented perpendicular to the longitudinal axis of the cladding panel. When blast activated, the panel first rotates around and then shears along the deliberately weakened section, but is restrained from catastrophic failure by the presence of the KevlarTM fibres.

6.16

Vehicle body panels

The use of composites, moulded to shape by a variety of fabrication techniques, is now well established across the total car, bus and truck spectrum – and indeed including all forms of railway vehicle, cargo and ship application. In terms of the first three, those components range from decorative trim, through under-body and ancillary equipment mouldings, to structural elements and every form of external body panel. Overall it is one major composites success story, which is far from finished. However, although pultrusion delivers a product that, while perhaps complex in cross-section, is

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6.23 Pultruded bus panel

also continuous and straight and a design situation that must impose some obvious limitations to the use of the process within this whole market area (i.e. vehicle and related ‘body’ panels) where wrap-around, double curvature panels are more typical. Nevertheless two examples follow which demonstrate something of the available opportunity. Figure 6.23 illustrates a pultruded body/window mask panel from a bus following certain post-pultrusion treatment such as machining the window opening, edge finishing and painting. Obviously the use of pultrusion for profiles of this type is only cost-effective if the call-off quantity allows the considerable tooling cost to be suitably amortised. Therefore if the same basic panel can be employed for more than one location on the vehicle then those numbers are considerably improved. The other examples shown diagrammatically in Fig. 6.24 permit through their design as cladding for refrigerated trucks quicker amortisation because they (and like configurations) may have additional application in the lining of cold stores and clean rooms.

6.17

Water and sewage treatment plant

Traditionally two-thirds of the construction costs of water and sewage treatment plants lie in the civil engineering works which are static, inflexible and require continuous on-going maintenance. At a competitive cost, pultruded structures allow the minimum of site preparation, are fast to assemble,

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15mm (5.9”)

3 mm (0.12”)

250 mm (9.8”)

215mm (8.45”)

14 mm (0.55”)

320 mm (12.6”)

3mm (0.12”)

10 mm (0.39”)

6.24 Pultruded cladding panels for refrigerated truck construction

readily allow a variety of different size housings, enclosures, stagings, walkways and footbridges to be rapidly constructed which moreover are easy to adapt or resite at a later date. However, further illustration simply to repeat much of what has already been described seems superfluous, but this final list summarising the prime benefits of pultruded profiles to the consulting engineer involved with water and sewage treatment plant design is judged worth emphasising: • • • • •

The non-sparking property demanded by many industrial environments. A design deflection typically 1/200 of the span. A maximum span acceptance, typically 1700 for 5 kN/m of load. A low weight, allowing even large sections to be moved by hand. A fire performance which can readily meet Class I of BS476, Part 7, or better.

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6.18

Pultrusion for engineers A thermal expansion similar to that of steel. Resistance to a wide range of environments and hazardous chemicals. Excellent electrical insulation.

Conclusion

Written by authorities within the industry, every chapter in this book rightly concludes on an upbeat note not just for composites, but particularly for the secure, promising and developing future of the pultrusion sector. These few examples from thousands provide adequate case-history support in confirmation, but one important factor is still lacking. Before pultruded profiles are purchased for whatever market and application, in increasing quantity, there must be better recognition and acceptance of their properties, benefits and advantages by the architect, consulting engineer, designer, specifier or purchase manger. That need for education has over the years failed until recently to be suitably addressed. The author of this chapter as editor of the whole book trusts that these pages will have the direct effect of therefore prompting interest not just in pultrusion, but in composites generally.

6.19

References

1. Christopher, W, & Carlson, P E, ‘Design considerations for a fiberglass, field erected, closed circuit cooling tower’ CTI Journal, 18(2) 52–61. 2. Andresen, F R, & Thorning, H, ‘Composite bridge based on standard profiles & elements’, Reinforced Plastics Asia ’97, Singapore, 1997. 3. Barefoot, G, Sitton, D, Smith, C, & Witcher, D, ‘Innovation of pultrusion structural shapes for infrastructure’, Paper 24E, International Composites Expo, 1997.

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7 Infrastructure – a positive market BRIAN WILSON

7.1

Introduction

Perhaps as far back as 1985, it began to be recognised that the composites industry was in a position to begin to answer the component demands of what has become known as the infrastructure market. In competition then with steel, reinforced concrete and other more traditional materials, composites were at last acknowledged as being able to satisfy the critical requirements of the civil engineer. That was not unexpected. Virtually ever since the birth of the reinforced plastics composites industry around 40 years earlier, the building and construction sector has been a valued customer. Today this sector typically consumes world-wide some 20% of its annual output, and at a total of some 1.1 million tonnes a very clear recognition of the value being increasingly placed on these unique engineering materials. Composed of a fibrous reinforcement within an initially liquid thermoset resin that is readily polymerised into a highly environmentally resistant, usable solid, reinforced plastics, composites offer many attractions. High strength, low weight and durability predominate, but their specification can be easily and economically tailored to satisfy a wide low- or high-performance spectrum. Equally, there is a wide variety of labour- to capital-intensive fabrication techniques available for their cost-effective moulding into small-to-massive, simple-tocomplex shapes. Finally, a steadily growing track record has provided confirmation of the ability of these composites to provide a maintenance-free, optimum in-service life. Pultruded composite profiles, with their excellent structural integrity, were therefore soon identified as having particular interest and relevance to this growing and virtually unquantifiable civil engineering, infrastructure market application, typically defined as the market-applications sectors listed in Table 7.1. That opportunity and the manner in which it is being increasingly realised are the subjects of this chapter. Case-history examples will describe and clearly emphasise the way that pultruded profiles are beginning to answer 231

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Pultrusion for engineers Table 7.1. The civil infrastructure market-place1 •

Bridges

•

Electric power generation

•

Ports, harbours and waterways

•

Rail and mass transit

•

Roads and highways

•

Solid waste disposal

•

Telecomunications

•

Water and wastewater treatment

that sector’s highly specialised and particular needs. Collectively, they also indicate something of the prestige contracts now in hand, as well as the important part being played by the academic community in this whole building-construction market expansion. Other case-history and application examples within this volume simply serve to complement and confirm that picture. Such an appraisal would, however, be seriously incomplete without some thoughts on the design, cleaning and care of pultruded structural assemblies or on the machining, fastening, finishing and repair of the profiles from which they are constructed. While very relevant to the infrastructure market, such data are of course of equal importance to the majority of pultrusion applications; those paragraphs precede the case histories. This chapter deliberately does not duplicate process history, material properties, selection and specification, equipment, tooling, nor the methodology and technology of the pultrusion process; there is a listing, by web-site address only, of the principal pultrusion companies world-wide at the beginning of the book. All are involved in the production of both standard and custommoulded structural and other profile elements.

7.2

Design considerations

Because both the material and the moulded product are formed at the same time, virtually all composites possess a unique ability. In other words, by matrix, reinforcement and fabrication technique selection, the mechanical, physical, environmental and economic performance of the finished composite can be critically adjusted, or ‘tailored’, to meet the customer’s needs fully. That ability, which with some irony is the key to much of the success of composites, does have one serious drawback for the design engineer: there are few ‘standard’ materials and this has a direct bearing on the limited

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Infrastructure – a positive market B

B

b y

D X

233

X

D

d X

X

Tf

Y X

X

D

Tw b

Y =D 2 Area = BD I xx ¢

y B

Y =D 2 Area = BD - bd

BD 3 = 12

I xx ¢

BD 3 - bd 3 = 12

Area = BD - b (Tw )

X yy ¢ I = Moment of Inertia Y = Distance to neutral axis Neutral Axis X – X

D - 2D 2Tf + bT w2 2DB - 2b (D - Tw ) =B 2

Y xx ¢ =

2TfD 3 + bTw3 - Area(D - Y xx ¢ )2 3 BD 3 - b 3 (D - Tw ) = 12

I xx ¢ = I yy ¢

7.1 Areas, moments of inertia and neutral axis examples

quantity of published performance data currently available, even though computer software,2 is slowly beginning to reduce the problem. Pultruded profiles are completely different. Here there is now a wealth of ‘standardised’ mechanical and physical property data, all supported with manuals,3 nomograms and other expected design aids. These are freely available to the design engineer from all the major manufacturers. That is now being increasingly supported by specialised computer software4 also produced by the pultruded profile manufacturer. To further design advantage, all the ‘standard’ angle, round, flat, hollow and I-beam profiles produced by the pultrusion industry conform to recognised section properties. As exemplified by moments of inertia, areas, neutral axis position, stress and deflection of beams (Fig. 7.1), all are well accepted by the engineering profession for any cross-section product produced in continuous lengths. Much of the same increasingly applies to the ‘custom-moulded’ profiles. In other words, the structural, load-carrying performance for a particular design and dimensioned pultruded profile is similar, even if not in fact virtually identical, from manufacturer to manufacturer. Moreover, and to importance, that performance is directly comparable with the performance of steel, aluminium, timber and other sectional products.

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7.3

Machining, fastening and finishing systems

In a chapter devoted to the use of pultruded profiles for the type of structural assembly demonstrated by the infrastructure application, it is relevant to consider the ways in which profiles can first be machined and then securely fastened together. Further, and although not a regular requirement, profiles or such assembled structures may subsequently require to be finished in some way. As indicated earlier, other associated in-use procedures such as cleaning and profile repair are also discussed in this brief review.

Machining Like composites and other materials suitable for the manufacture of profile sections, pultruded elements can be machined by drilling, grinding, milling, punching, routing, sanding, sawing, shearing, tapping, threading and turning. However, as optimum results demand slightly different procedures, their outline explanation is essential. Obviously every recognised machining safety precaution must always be observed but owing to their flexibility, it is very important that pultruded profiles are well supported and fixed. If not, then chipping or at worst warping and distortion around or even beyond the machined area can occur. While tungsten carbide-tipped tools are a suitable choice irrespective of the machining operation, diamond-tipped tools are always to be preferred, although neither should employ either an excessive force or too high a cutting speed. Any heat build-up will cause the matrix resin to soften which, as well as exacerbating the cutting load/speed conditions, will prevent the production of a cleanly cut, milled or otherwise machined area or surface. Consequently, even with diamond-tipped tools, water lubrication has to be strongly recommended as this also has the additional advantage of limiting the airborne evolution of cutting dust and debris. While this dust and debris is in most cases non-toxic and basically non-injurious to health, it can cause mild-to-serious skin irritation. Barrier creams are usually sufficient to prevent that irritation even in those people having a very sensitive skin. Lubricants other than water are not typically recommended, since in combination with glass/resin debris, a damaging abrasive can be created if the build-up becomes severe. Most tungsten and diamond-tipped tool suppliers can supply grades/types specifically designed for the machining of composites. Likewise, the supplier’s cutting speed and operating recommendations should be adhered to at all times if perfect results are to be obtained at minimum effort and expense. The optimum conditions for grinding are very similar, whether by hand or machine. Light pressures, coarse grit wheels or papers and good water lubrication are all important if clean cuts and a perfect finish are to result.

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Although drilling is to be preferred to punching, thin composite sections can be successfully and cleanly punched as long as the shape or diameter is small. Punched areas larger than 3.25 cm2 (0.2 inch2) or thicker than 2.5mm (0.1 inch) are only really practical where phenolic-based profiles are concerned. Even here, specially shaped punches that permit only a small portion of the punch head to penetrate the profile at any one time are recommended. In addition, the punch should also be equipped with a stripper device so that removal does not cause any damage to the material around the punched area. A similar operation, shearing, is however not recommended unless it is undertaken in the direction of the continuous reinforcement within the profile. Again, this must only be done with a blade that progressively enters the profile. In spite of the fact that pultruded studs and nuts can be purchased, and are indeed successfully employed, any threading of composites even in thicker section can only be recommended when the applied load is low or where an additional adhesive bond is to be employed. Threading has the obvious disadvantage of cutting into the continuity of the reinforcement to leave localised machined areas which, because they then also tend to be resin-rich, demonstrate poor shear strength. Where practical, a coarser thread will help, but the amount of improvement is small. Finally, and although its application to pultrusion is limited, these comments cannot conclude without noting that composites can be successfully cut and profiled to shape, using a high-pressure water jet of around 150 mm (0.039 inch) in diameter.

Assembly Pultruded profile elements can be successfully fixed and secured together into assemblies of vastly different size, complexity and structural loadcarrying ability, by virtually the same techniques that are used for timber, concrete, steel or aluminium sections (Fig. 7.2). However, there is one particular exception which also applies to concrete and timber. Pultrusions cannot be welded together, but may otherwise be mechanically or adhesively bonded, and whichever procedure is chosen, some preliminary work has typically to be undertaken on the respective profile elements. While for adhesive bonding this may be as limited as localised mechanical (or even hand) abrasion, drilling or other machining operations, possibly involving the use of jigs to ensure optimum profile-to-profile registration will obviously usually apply for mechanical fixing.

Mechanical fastening Seven different mechanical fixing procedures can apply: nailing, screwing, bolting, tapping, riveting, the use of threaded metal inserts and the use of

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7.2 Bolted assembly of massive pultruded structure (Courtesy, EPTA)

other specially designed fixing-security devices. The correct and optimum choice depends on a variety of circumstances. Nailing can be satisfactory but only where the profile is being secured to a material such as timber that will accept and retain the nail. If the nail is hardened and tempered, then it is practical simply to drive in the nail as long as the profile thickness does not exceed 1.6 mm (1/16 inch). Above this, then the profile will need to be predrilled to take the nail. It is therefore impractical – in fact it is impossible – to nail together two profile elements successfully or indeed to nail a profile to an opposing composites structure or component.

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As a simple, but more expensive, alternative to nailing, and also suitable for creating a secure profile-to-profile fixing where high strength is not required, self-tapping screws as long as they are not of the ‘lag’ type. Their performance can in fact be cost-effectively enhanced by the additional use of an adhesive over the bond area. In this situation the screw can be considered as holding the opposing elements together until the adhesive has cured. Bolted fixings using conventional nuts, bolts and washers are, however, the most favoured method, particularly where it may be necessary to demount or alter the constructed assembly at some later stage. Nevertheless, it is also not unusual to again apply an adhesive at the interface. Such an application enhances the overall integrity of the joint, particularly if it is subjected to vibration, fatigue or alternating load condition. The effectiveness of that adhesive is improved where the opposing surfaces are correctly prepared to provide an optimised intimate contact. In addition, the higher the torque level applied to the fixture the better, as this helps to overcome a common problem around bolted composite connections. High, but localised, stress levels around the bolt hole can otherwise be sufficient to cause premature component failure, particularly in thin sections. As an alternative to the use of a separate nut or perhaps internally threaded insert, it is occasionally practical to produce a threaded hole in the opposing profile as long as the precautions already explained are fully recognised. The properties of tapped composites are very inferior to even those of aluminium, and this is particularly true if an adhesive is not employed, in effect, to lock the bolt or other fixing device in place. Although composite surfaces do not deform in the same way as, say, steel or aluminium, metal rivets can be employed to provide a very effective fixing. These are available in a variety of lengths and head sizes, and possess the attractions of low cost and assembly speed. This offsets the fact that although not a typical requirement, such joints are not readily parted. While the use of washers positioned under both rivet ‘heads’ can overcome problems of the rivet penetrating the composite surface, rivets cannot generally be considered really suitable for profiles greater than 4 mm (0.16 inch) in thickness. Finally come fasteners and threaded inserts. Over recent years, many of the major companies involved in the supply of special fastening or fixing devices have designed versions specifically for composites. Many are equally suitable therefore for pultruded profiles. Although they cannot be introduced into the profile during the moulding stage, as may apply in other composite fabrication techniques, their location into a suitably dimensioned drilled and/or tapped hole is a simple and quick operation. It is often a feature of their design that they are securely locked into place without the need for adhesive once the opposing male connection is completed. Fixing

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arrangements of this type are most commonly employed where there is a frequent need to dismount the profile-to-profile fixture. A fixing arrangement employing steel bar, bolts and washers to join three square section tubular shapes together is illustrated in Fig. 7.3. A variety of similar 90° and smaller angled joint designs using this type of fastener arrangement is clearly possible. In all cases where mechanical fixtures apply, it is clearly essential to ensure that they are as resistant to the environment to which they will be exposed as is the matrix employed to manufacture the profile composite. The same consideration applies to any adhesive employed. It is unacceptable for the joints to fail the structure if the profiles forming that structure remain in an in-service condition.

Snap-fit joints These comments cannot conclude without reference to the recent development of a fixing system where, as a feature of their design, opposing profiles simply mate or snap together to form a high-performance secure joint but one also offering a degree of lateral flexibility. Such ‘snap-fit’ systems

Threaded insert

Steel dowel or bar

Fixing bolt

7.3 Proprietary fastener arrangement for hollow tubulars (Courtesy, Fiberforce Composites Ltd)

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offer obvious advantage in the erection of a structure but even more importantly are, as shown by Fig. 7.4, capable of distributing the ‘point’ load stress which can develop at a joint, over a larger area. Some post-pultruded machining of both the male and female profiles is, as in this illustrated design, usually necessary and this may be undertaken either off-line or online by means of, for example, the multi-axis CNC machining centre discussed in Chapter 8. There are of course the machining cost implications to consider and another drawback is the need to ensure that the reinforcement pack employed enables the highest possible interlaminar shear strength to be retained within both opposing profiles. As referenced here later and in Chapter 8, such a structural assembly arrangement was initially employed for the construction of high-voltage

Male Section

Female Section

7.4 ‘Snap-fit’ pultruded joint concept (Designed by Goldsworthy Associates, Inc for Ebert Composites)

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electricity transmission towers. Their well-demonstrated success under high-duty, adverse conditions augers well for a growing use of this type of pultruded profile fixture. If nothing else, they preclude the necessity for secondary adhesives needing expensive pre- or post-bonding treatments. Many other snap-fit joint designs will no doubt be developed to satisfy specific circumstances.

Adhesive bonding Although adhesives can, even under very adverse conditions, provide extremely strong and very durable bonds between adjacent composite pultruded profiles, their successful application requires a great deal more care than any mechanical system. In addition to the thorough surface abrasion, cleaning and perhaps priming necessary, the chemistry of those adhesives adjudged the optimum (typically, for performance, toughened acrylic, epoxide or vinyl-oxirane based) rarely, if ever, provides an instantaneous bond. The opposing profiles must usually be mechanically held firmly together until the structure is at least capable of accepting a self-load condition. This can take several hours, and can be a serious drawback to the use of adhesives. One answer, however, may be a fixture somewhat akin to, but simpler and cheaper than, the interlocking profiles used in the transmission tower construction example. Such a connection would offer a level of interlocked bonding sufficient to provide an initial degree of self-load acceptance while an adhesive bond develops. There then frequently remains the need to mix and apply the adhesive correctly, as well as the problem of a perhaps limited usable pot-life. Adhesives in the form of a thin ‘film’ are available and overcome some of these deficiencies but there can still be the requirement to apply heat in order to develop an optimum bond condition. This creates obvious difficulties away from the factory unit in a situation typically applicable for on-site assembly. Nevertheless, adhesives are, for every type of application, receiving more and more development attention, and their use can be expected to increase even when they have to be employed under conditions that today might be judged difficult or entirely unsuitable. Finally, it is important to recognise that joint designs can be subjected to tension, compression, shear and peel loads, or frequently some changing combination of all four. Adhesives work best when the stresses are at right angles to the plane of the joint, or in other words where a pure compression or tension load applies. They may also be suitable under shear conditions or when the stress is in a direction parallel to the plane of the joint. However, adhesives are not ideal where a peel stress applies, such as where the loading is at an angle intermediate between the tension/compression

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and shear load condition. In this case adhesive joints may need to be supplemented with a mechanical fixing.

Finishing treatments More often than not, a simple localised corrosion protection application of an epoxy-based paint over the metal components of the fixing provides a very adequate finishing treatment. There may, however, be occasions where the whole structure, even though composed of ‘decorated’ profiles resistant to the application environment, may need to be painted after assembly to enhance that condition. In most cases, acrylic lacquers, even vinyl or oilbased paints, can be effective finish treatments, given care over their selection and particularly in their application. Polyester and obviously the even higher-quality epoxide-based systems will ensure more superior environmental protection. Nevertheless, whatever material is chosen, there will be a need first to remove all traces of the release agent (added to provide die lubrication) from the surfaces to be painted. Hand abrasion followed by a light solvent clean is usually sufficient but a very low-profile sand or gritblasting will ensure a longer-lasting interfacial key between paint and profile surface and is also usually quicker when the whole surface of a large structure needs to be treated in this fashion. Finally, where an enhanced level of ‘mechanical’ and/or environmental protection is required, each joint can be over-laminated by hand with a ‘bandage’ of glass reinforcement bonded with a suitable thermoset resin. Particular examples are where the joints are subject to a regular or continual attack – if only from water – and although rarely visually appealing, such a bandage application can prove very cost-effective in the context of the life of the complete structure. In addition there is typically some enhancement of the overall structural integrity, although against that, any later dismantling of the structure will be more difficult, if not impossible.

7.4

Cleaning, inspection and repair procedures

Correctly specified composites, manufactured under high-quality standards, are noted for their excellent environmental behaviour and even in the long term typically demand the minimum of maintenance attention. Nevertheless, regular inspection of any assembled profile structure is strongly recommended. Depending on the number of years in service, this may necessitate nothing more than a simple washing treatment with detergent and water. Indeed routine cleaning, which may equally employ a combination of three described techniques, can also be considered as a preventative maintenance measure.

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Cleaning Given that water and detergent may not readily remove any accumulated dirt on the profile surfaces, or as deposited at each joint between individual profiles, then the choice rests between solvent and steam cleaning. The first has many advantages. There is no capital equipment required and although labour-intensive, the technique is quick and effective even though the additional use of mild abrasive cloths may be necessary in areas where the dirt is stubborn and ingrained. Unfortunately the evolution of potentially toxic vapours and the danger of fire with certain solvents are distinct disadvantages particularly in confined areas. This careful solvent selection, and sparing use, may well be critical. Steam cleaning is, however, also not without problems. The most critical is the temperature and pressure of the steam, with 120 °C (250 °F) and 5.8bar (85-psi) being considered very much the respective maxima. Even so, the steam jet must be applied carefully to any joint, to reduce the incidence of any looseness that might otherwise develop.

Inspection Heavily loaded or stressed structures usually benefit from an occasional or even regular NDT (non-destructive testing) inspection to check for external or internal crack development within the profiles, or at the fixtures. In most cases, however, a thorough visual examination which specifically concentrates on the joint areas is sufficient. Typical problems are mechanical looseness and/or adhesive debonding, although associated profile damage at or adjacent to the fixing area/s can occur in addition to mechanical, abrasive, impact, environmental, ultraviolet or other surface degradation. All can be readily rectified as part of a planned maintenance programme.

Repair In most cases, and given the exercise of a regular inspection regime, any repair will typically be of a minor nature and therefore easily undertaken at low cost. However, even in the worst situation following failure of a profile or a fixture as a result of, for example, accidentally exceeding the design load, replacement of either is rarely difficult even though adjacent areas of the structure may need to be temporarily supported in some way. The procedure is clearly no more difficult than removing and rewelding in place any part of a steelwork structure as a result perhaps of corrosion failure. That type of failure is much less likely to happen with a composites profile structure although even if optimally specified there may, with time, be the

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development of ‘fibre-blooming’ on the profile surface. Unless very severe, when the reinforcement fibres start to become exposed at the surface, this ‘blooming’ is typically characterised by a white surface appearance caused by exposure to the environment and/or ultraviolet radiation attack. In other words, it indicates some degradation of the matrix resin chosen when the profile was manufactured. ‘Blooming’ should not be confused with fading or colour change of the pigmentation added to the profile formulation for decorative reasons; strong, primary colours, reds, blues and greens are particularly prone to colour change, with whites, unless optically (ultraviolet) stabilised, tending to turn yellow. The incidence of ‘blooming’ can be reduced or totally prevented in three ways, and the first is the obvious use of a more environmentally resistant resin in the profile formulation. Further improvement can be gained through the use of a final tissue ‘cover’ to the reinforcement within the profile, causing the surface resin richness to be enhanced. Finally, a suitable post-moulding paint treatment could have been applied initially, although this solution is best considered as an in-service surface treatment or repair after ‘blooming’ has occurred. The preparation and application procedures employed in restoring the surface appearance and condition in this way typically match those already described, although when applied under inservice conditions, the use of a resin rather than paint coat treatment is much less practical. In any situation where the reinforcement has become exposed, a successful repair over what will typically be a more localised area can only be completed with the application of a thin layer of catalysed resin matching or compatible with that of the matrix used initially for the profile. In this situation, the degraded area must first be more heavily abraded under dry conditions, to remove the loose fibres and then wiped clean of dust. Should a portion of a profile become chipped, flaked or broken during service, but not otherwise demonstrate failure, then repair which would typically be for visual effect, can be readily undertaken with polyester resin putty paste. The latter, which is available in unreinforced and reinforced grades, is widely used in the car repair and do-it-yourself businesses. Given close adherence to the manufacturer’s recommendations in respect of catalyst addition, mixing and application, it can be highly effective in treating such damaged areas, even at quite thick sections. It is, however, rarely if ever load-bearing, and demands good surface preparation if an optimum bond to the damaged surface is to be ensured. Finally, there may be those cases where even though the profile has been seriously damaged, its total replacement is not practical. In other words, repair must restore the structural integrity and this is normally achieved by splicing in a new piece of composites profile bonded in place with an adhesive specifically formulated to accept what will be high interfacial loads.

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These additional profile sections may require machining prior to bonding, in order to develop the essential optimum fit between the damaged area and the splice. Where a splice is not possible owing to the design of the structure and the load or the environmental conditions to which it is subjected, an alternative is to overwrap the area with a composites laminate. This repair would then resemble the ‘bandage’ sometimes applied as previously described to enhance the structural integrity of a joint.

7.5

Case-history applications

Disregarding destruction through ‘local’ wars, civil disobedience and natural disasters, there is now no country where many aspects of the infrastructure as defined by Table 7.1 do not need urgent restoration or preferably replacement. Taking just the example of the United States, the annual cost for construction repair and renovation work alone has been estimated at some US$95 billion and this figure is expected to rise over US$100 billion in the early years of the twenty-first century. In other words the traditional materials of concrete, timber and steel formerly utilised for these infrastructure applications are, or have reached, their in-service life spans. To fail as Busel1 describes it, to restore this ‘network of construction that ties a nation together and provides it with the ability to deliver goods and services’ must, if not immediately, have a seriously adverse affect on the further growth, development and wealth of society generally. While the reinforced plastic composite is far from the sole ‘modern’ material better able to meet that challenge, that ability has already received discussion elsewhere in this book, with pultrusions as now exampled, ideally placed to accept a very respectable proportion of that marketapplication opportunity.

Bridge decking Although a new application which can be considered as still under development, the use of composites for the replacement of corroded and deteriorated steel bridge deck plates is accelerating, particularly in the United States and Europe.5 Employed on lift and swing bridges typically over river and estuary crossings, the steel decking which is often in the form of welded grating can corrode in as little as five years. The common refurbishment material is reinforced concrete6 but as this has the disadvantage of significantly increasing the weight of that section of a bridge that is required to move for the passage of ships and other vessels, the use of lightweight, corrosion-resistant pultruded planking is now finding favour. Various design solutions are being investigated, but the most typical employs these basically simple ‘planks’ as the top and bottom surfaces of a structural open-cell sandwich construction also based on com-

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posites. This use of composites imposes no limitations on the original lifting or swing mechanism. Indeed those capacities can be reduced to obvious energy-saving advantage, in addition to enabling the size and number of lifting bridge counter-weights to be reduced.

Bridge maintenance enclosures This new application originated in the UK and utilises modular pultruded plank sections, as cost-effective cladding to either a portion or the entire underside of road or rail bridges. As illustrated in Fig. 7.5, an enclosed space is created which permits maintenance workers to pass through the bridge substructure and gain safe, easy and uninterrupted access for inspection, painting and general repair. There is no scaffolding requirement and this becomes an important advantage over wide rivers, channels and estuaries or where the road or rail traffic across or under the bridge is heavy. At the same time, the enclosure offers both excellent environment and corrosion protection to the main bridge structure as well as a protected location for electricity, gas, telephone, water and sewage services. Finally, the enclosure can smooth out turbulent wind patterns across the bridge section and is typically more aesthetically pleasing (Fig. 7.6) than a bare bridge structure, particularly for those constructions carrying service lines.

Complete bridge structures Before massive pultruded structural members can be applied to ever-larger bridge constructions, many smaller, low-load-carrying designs must be fabricated. These will then need to be carefully monitored in order to confirm Safety walling e.g. pultruded planking

Road surface

Bridge structure

Services Composite cladding e.g. pultruded sheet or planking

Protected maintenance access

7.5 Drawing of typical bridge maintenance enclosure (Courtesy, Technolex, based on Maunsell Ltd)

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7.6 Pultruded enclosure cladding, second Severn crossing (Courtesy, Maunsell Ltd)

through in-service application the excellent lightweight, easy erection, lowmaintenance, long-life, creep, environment and corrosion resistance properties already claimed. That essential track-record is already being acquired – and fast. A growing number of pultruded profile demonstration bridges, both pedestrian and vehicular, have already been erected by consortia of civil engineering design companies, state highway departments, ministries of transport and construction companies, many with the close involvement of the civil engineering department of a local university or another academic body heavily committed to composites research and development.7,8 Typical US examples are the Wick Wyre Run Bridge, Delaware, a pedestrian bridge in the Daniel Boone National Forest, Kentucky, a truss bridge providing wheel-chair access in Washington DC and the vehicular bridge at Clear Creek in Bath County, Kentucky (Fig. 7.7). Others have been installed at a number of National Parks in the United States. Many of these examples are the closest that any installation currently comes to what could be termed a ‘production item’. Considered together, they form a standard series of designs offering loading and unsupported span length differences, for installation in what are frequently very wet conditions. A high level of corrosion resistance is in fact a major reason why the National Parks choose pultruded profile construction. The Parks authority has every confidence that the installation will totally eliminate the

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7.7 Clear Creek Bridge in Bath County, Kentucky

former short-term repair or replacement that had been necessary when employing, for example, timber. Another primary attribute is their light structural weight, which eases the related problems of delivery and erection to what are frequently remote locations. Access can often be difficult even for a helicopter and then the only alternative is for volunteers to carry in the bridge components by hand. To this list must be added examples that were designed, developed and installed by commercial enterprises, including the now very well-known 60 m (200 ft) long pedestrian bridge over the River Tay at Aberfeldy in Scotland and, in England, the Bonds Mill vehicular and pedestrian basculelifting bridge (Fig. 7.8). Other valuable examples are a number of allcomposites bridges, albeit not necessarily involving pultruded structural elements, which have been built since around 1990 in the People’s Republic of China. It is important to emphasise that although the Aberfeldy and Bonds Mill bridge examples both employed specially designed custom-moulded profiles, the success of the vast majority so far constructed has typically required nothing more than the optimum selection from the wide range of standard profiles normally available ex-stock from all pultrusion companies. That use of simple angles, channels, I-beams and perhaps hollow sections is all too apparent from some of the accompanying illustrations. One final and excellent example in that context is the pedestrian, cycle and light vehicle bridge (Fig. 7.9) erected over a main railway line near Kolding in Denmark. As far as the future is concerned, that standard profile

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7.8 Bonds Mill, bascule lifting bridge, Shropshire, UK (Courtesy, Vetrotex/Maunsell Ltd)

7.9 Pedestrian, cycle and light vehicle bridge, Kolding, Denmark (see also Fig. 6.11) (Courtesy, Technolex)

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construction principle is all-important, and the reason why more comprehensive details regarding this Kolding bridge were featured in Chapter 6. Further, the in-service monitoring of that bridge will, with other examples, undoubtedly provide data vital to the development of this application and the simple, albeit lightly loaded structural elements employed, into the much more massive, but still standard, profiles of the future. Spans will become longer and loadings heavier, and those profiles will also be supplemented by special design to satisfy particular situations. One new unique double webbed, shear reinforced, structural beam (Fig. 7.10) for bridge application, with overall dimensions of 305 mm (12 inches) for the top flange and 915 mm (36 inches) deep, has already been designed by Dr Abdul Zurieck of the Georgia Technical University and pultruded by the Strongwell Corp. Other profile designs are in hand and their fullscale production only awaits the results of the comprehensive testing under trial bridge conditions that will initially be necessary.

Crash barriers Many designs of crash barrier exist. Perhaps the most typical is the ‘W’ shaped galvanised rolled-steel version employed at just above ground level,

7.10 Double web, shear reinforced, structural beam (Courtesy, Georgia Technical University and Strongwell Corp.)

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along many kilometres of the central reservation (median) of the world’s motorways, freeways and autobahns. In combination, the cross-section and the fixture design are successful in deflecting and slowing down impacting vehicles, preventing both their entry into on-going traffic lanes or catapultlike return in the original direction of travel. There is no reason why both items could not be fabricated as pultruded profiles. Indeed, the necessary research and development has already been completed, with trial installations in place in order to gain the essential operational experience (Fig. 7.11). Of importance, an over-sophisticated specification is not seen as necessary. Glass, rather than carbon fibre, reinforcement in a matrix of high-grade ultraviolet resistant unsaturated polyester is more than adequate. While both items would be considerably more expensive than the current galvanised steel versions, this massive civil engineering infrastructure opportunity accurately illustrates the need, when considering any composites solution, to base the decision on a critical assessment of the long-term in-service life costs. Under that analysis the benefits of light weight, high strength, application fitness, corrosion-resistance, low maintenance, easy shipment and rapid installation exhibited by composites become all too apparent, valid and useful. All that remains is to gain the recognition and acceptance of the road traffic engineer.

Curtain walling Otherwise described as ‘close out’ walls formed between the vertical structural floor columns for the exterior of buildings under construction, these curtain walls are frequently fabricated from reinforced concrete. As a consequence, they are heavy and difficult to handle, particularly above the tenth floor of high-rise buildings. Installation has resulted in fatal injuries. The use of composites provides an ideal solution as exemplified by a filled core sandwich construction, developed in Japan through the 1990s. This strong, lightweight and easily fixed panel (Fig. 7.12) is based on outer laminate skins which, depending on the specified structural integrity, can utilise either glass or carbon reinforcement, or a hybrid of both. Again like the crash barrier example, a matrix of UV-resistant high-grade polyester would be perfectly acceptable as well as essential in a fire-retardant version. An alternative composites answer is the fabrication by pultrusion of narrow interlocking panel sections. These impact-resistant structural elements have the advantage of quicker, safer installation and their modular design equally answers many identical building and other applications.

7.11 Pultruded crash barrier installation under evaluation (patented structure, University of Wisconsin)

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7.12 Typical composite curtain wall installation (Courtesy, N T Adco Ltd, Japan)

Mine shaft shoring Although well developed through the 1990s and finding extensive worldwide application, the use of pultruded rods for the construction of a shoring and strengthening system for mine and tunnel shafts has received very little publicity. It deserves to be better known, if only in respect of an ease of handling and installation. In principle, it is a system of non-corrosive, environmentally resistant, lightweight machined-to-length tie rods, available in several diameters and post-pultruded threaded either along the whole length or at their outer ends only. As with steel, that machining operation can, if necessary, be undertaken without difficulty or the need for specialised equipment on site. In use, respective lengths are locked together into the requisite design pattern, by means of special ‘locking fixtures’ which may also be machined from pultruded bar or other standard profiles as dictated by application circumstances.

Piles The use of both reinforced concrete and steel tubular piles has been shown over recent years to be highly successful for either land or marine-based

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installations. With the growing use of composites providing more optimum corrosion resistance for a wide variety of marine applications, the development and commercialisation of composite piles in both tubular and flat plate form is of growing interest.9 That is particularly apposite where the latter can be designed as a pultrusion, readily inter-locking and enhancing the overall structural integrity of the construction. Such a construction is frequently employed for coffer dams to provide a dry, safe working location typically below the normal water level and from which bridge, pier and deck structures can be constructed. While a lightweight, high-strength composite pile may perhaps be judged as offering only minor benefit, the same certainly does not apply for the enhanced corrosion resistance which is looked upon as a very significant advantage for any pile that has to be driven into water. Seawater and freshwater environments are severely corrosive in terms of both chemical and water organism attack.As a consequence, composite piles are being increasingly employed to form the first part of the support structure for piers and docks. However, for land-based installations the same does not yet apply. Although other composite properties are far from unattractive, premature failure by corrosion is less valid in this situation and unlikely therefore to be such a limiting factor in any life-service quantification. At the same time, both concrete and steel versions are usually of substantial cross-section, and overall remain the preferred product. Furthermore, even though the installation of composites piling typically consumes less energy and is therefore cheaper, the present pile-driving equipment has been designed to handle these traditional versions at impact forces totally unsuitable for composites. Although composite piles are currently often manufactured by filamentwinding and other well-established fabrication techniques, tubular, plate or sheet-like forms are more suited to pultrusion (Fig. 7.13). If nothing else, the economics of the process are attractive compared, for example, with filament-winding and a strong market is therefore envisaged. The clear expectation is that the pultruded product, particularly when in a tubular shape, will comprise a core of low-value recycled material such as flake polyethylene from waste milk or detergent bottles, over which the outer protective composite ‘skin’ will be pultruded.

Piers, jetty and dock constructions Commensurate with the growing use of composites for piles as already described, the same use for the total construction of all types of pier, jetty, dock and marine constructions is axiomatic. Several structures of this type have already been built, albeit largely as demonstration units. One important example is the Advanced Waterfront Technology Test Site10 (AWTTS)

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7.13 Typical installation of pultruded FRP sheet piling

located at Port Hueneme, California (Fig. 7.14). Designed and constructed by the US Navy Facilities Engineering Services Center (NFESC) with the assistance of the US Army Corps of Engineers Construction Engineering Research Laboratory, this comprehensively monitored trial dock installation located at the harbour entrance is 46 m (150 ft) long and 5.5 m (18 ft) wide. Typical spans are at 3 m (10 ft), although several are double that length. Carbon fibre tendons prestress the concrete decking over one of these 6 m (20 ft) spans, with a second at that length made entirely of composites and based on pultruded box sections. In addition to other composite items, test plaques and components – which include competitive materials such as epoxy-coated steel rebars – the facility supports a catwalk or working platform fabricated from a number of pultruded profiles such as handrails, gratings and other structural members. Overall the whole construction is providing direct and valuable in-service comparison of a variety of materials and their respective repair techniques, employed in marine constructions of this type. Several years will elapse before the results are known but the clear objective is to establish optimum design and specifications to ensure the totally successful use of composites. With their extensive utility service supply systems (water, steam, sewerage, electrical power and lighting), and use of pipes, conduits and hangers as well as structural elements, naval waterfront environments of this type offer the composites engineer wide evaluation opportunity.

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7.14 Demonstration dock installation at Port Hueneme

Rails, steps, staging and walkways Offering a cost-effective, corrosion-free alternative to the former steel or timber versions, many pultruded profile walkways, stagings, platforms and ladder/step assemblies have been constructed world-wide since 1980. They were one of the earliest success stories for pultruded profiles, providing years of maintenance-free in-service life in locations as diverse as chemical refineries and processing plants, storage tank farms, large industrial buildings, aircraft maintenance hangars, helicopter and offshore platforms. In fact the list is virtually endless and one reason for this extensive use is that the applicable codes of practice or material placement regulations were and remain easy to achieve without question or difficulty. Of further importance, the vast majority of the requirements covered by this application classification can be simply constructed from the wide variety of universally available, differently dimensioned channels, angles, small I-beams, squares and round tubulars. These standard profiles from a world-wide industry typically employ just longitudinal reinforcement in an unsaturated polyester matrix. Should any particular environmentally resistant conditions apply, then all that is normally necessary is a change to a more suitable matrix. Any structural enhancement is answered equally easily by use of a more sophisticated reinforcement pack. Similar to the pedestrian bridges mentioned earlier, a typical walkway and guard-rail construction, with or without ladder access, can be assembled using nuts and bolts (perhaps machined from pultruded bar stock where corrosion conditions are severe) or, still somewhat less conventionally although the procedure is growing, by adhesive bonding. Irrespective

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of size and location, erection is normally simple and quick and rarely requires the use of lifting gear.

Reinforcing bars Many pultrusion companies around the world now supply glass/polyester or glass/vinyl ester ‘rebar’ in diameters of between 3 mm (0.125 inch) and 25 mm (1.0 inch) for the reinforcement of concrete.11 Their design typically incorporates a variety of post-pultruded machined surface ‘ribs’ or ‘nodules’ which effectively replicate the steel version. Although initially more expensive than the traditional reinforcement material, their use totally overcomes what can be serious long-term water and alkaline attack. Other composite ‘rebar’ versions, also based on a standard pultruded rod, employ composite ‘coatings’ to produce the effect of a mechanical ‘key’ to the concrete.12 Figure 7.15 shows the ‘C’-shaped cross-section composites rebar, C-BARTM, based on pultruded bar that has been developed as a proprietary product by the American company, Marshall Industries. Such composite pultruded reinforcing bars may also be used as prestressing or post-tensioning members in concrete slabs and bridge decks. Here, tapered compression locking devices are used to maintain the tension within the composite member. Further, tensioning members of this type may employ carbon rather than glass reinforcement in order to minimise the potential for long-term creep.

7.15 Composite re-bar (C-BarTM) based on pultruded profiles (Courtesy, Marshall Industries & Reichhold Inc)

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Signs and signposts The traditional materials typically employed for this application include steel, aluminium, timber and concrete. However, all suffer from certain constraints. Steel is heavy and requires constant expensive maintenance to limit corrosive attack from the local environment, aluminium is attractive to the thief as a resale material, suitable timber is now in short supply but even so suffers exactly the same disadvantages as the utility poles considered later, while concrete, as well as being heavy and thus more difficult to ship and install, is an unattractive material where visual impact is required. As a consequence, composites are increasingly satisfying this market sector and some of those requirements can be fabricated as a pultrusion. For example, the use of profiles as the structural support member minimises weight during shipment and erection, and completely overcomes not just the corrosion problem but also the potential for ground-water contamination from the toxic coatings and impregnating materials used with timber. Equally there is a minimum painting and repair requirement, for an artefact that has no resale or real recycling value. Finally, however, there is one all-important advantage that can be readily built-in to a composite signpost or similar structure. While the impact properties for a fibreglass post are excellent, the support structure can be designed to fail by shear at a particular point above ground level. So effective is this critical shear in minimising damage to both the vehicle and its occupants, that Europe and the United States are beginning to demand that this feature is a mandatory requirement of the overall design.

Toll booths and other housings With worsening road congestion, both the United States and Europe have, or are planning, to construct more toll roads. Typical examples are the Dulles Airport Toll Way in Washington, DC, several highways down the eastern border of Southern California and a proposed Midlands Link motorway in the UK. Until preferred automated and electronic systems for collection of the toll fee are introduced, there will be a continual need to house the collecting agents suitably, as well as to replace and refurbish the many hundreds of booths already in service. Typically using contact-moulding techniques, the composites industry has already proved its ability to prefabricate a wide variety of low-cost housing and service enclosures, including toll booths. Their lightweight, easyto-handle, prefabricated sections employing glass reinforcement and unsaturated polyester resin as materials of choice, which can be readily brought to site and installed, have already confirmed the economic viability of this form of construction.

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Nevertheless, by employing pultruded profiles in their construction,13 their excellence can be further enhanced, particularly in respect of production speed, structural integrity, installation and application suitability. In addition, and as demonstrated by a number of prototypes and full-scale production units designed for domestic, office and factory accommodation (Fig. 7.16), their size can be readily increased on a modular basis. Therefore as far as the infrastructure is concerned, pultrusion is a material construction technique that offers the composites industry an unquantifiable opportunity, not least of which would be the logical development of a modular design of composite-pultruded unit to satisfy the continuing, frequently desperate need for at least basic human accommodation.

Transmission towers The service and utility companies have for some years employed pultruded profiles for a variety of applications, including the obvious replacement of timber poles and low-voltage electricity distribution towers. One particular attraction is the high di-electric resistance of the composite profile, but ease and rapidity of shipment and installation, and the high strength to weight properties of composites generally, are others. Consequently the development of a pultruded profile design for the erection of towers (or pylons) carrying high-voltage power lines was only a matter of time and very appropriately involved the ‘father of pultrusion’,

7.16 Environmental test facility constructed from pultruded elements

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Brandt Goldsworthy (Goldsworthy & Associates). Based on glass reinforced profiles compounded with unsaturated polyester or, depending on location within the tower, epoxy matrix resin, the overall patented design is actually unique in that neither bolts, nuts nor adhesives are required. As described earlier in this chapter (Fig. 7.4), the tower component profiles simply snap-fit together and demonstration structures 26 m (84 feet) high (Fig. 7.17) manufactured by Strongwell Ebert – a joint venture between the Ebert Composites Corporation and the Strongwell Corporation – installed on a Southern Cailfornia Edision 230 kV power line, have been certified for use by all the major American electric power companies. The multiyear, multimillion dollar development programme to design a composite lattice tower was based on some preliminary concepts by Ebert Composites, while the funding was provided by or through development contracts between Ebert, several utilities such as Southern California Edison and the federal government. Ebert then engaged Goldsworthy &

7.17 Demonstration high-voltage transmission towers (Courtesy, Strongwell Ebert)

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Associates, Inc, to assist in the engineering design and development of the bolt-less, snap-fit interlocking structural system, with subsequent patents owned and controlled by Ebert. It is of importance to emphasise that even though a portion of the whole tower construction became disconnected during one of the many certification tests that were undertaken, the final load acceptance was well in excess of that which otherwise caused failure of competitive bolted steel and galvanised designs. The use of composites also brings another very important benefit to power supply. Because these pultruded transmission towers are nonconductive, the use of expensive insulators for the mounting of the highvoltage power lines is no longer necessary. As a result, the frequent cleaning or even replacement of these insulators – particularly in salt or industrial environments – no longer applies. The maintenance costs are therefore much reduced, particularly in respect of difficult locations where low component shipment and ease of installation are also advantageous savings to offset the higher initial cost of the pultruded design.

Utility poles As already noted, there are millions of utility poles in use all over the world, to support many forms of telephone, power and utility cable. Each utilises either a cast (and often reinforced) concrete structure, a metallic tubular form or timber for its construction. Frequent replacement, particularly of the timber versions through rot or insect attack, but also of others through general corrosion, is far from unusual. With maintenance and replacement being expensive, alternative material versions were increasingly demanded by the utility companies. That need is now increasingly being satisfied by the technology of pultrusion,14 in the typical form of thin-walled composite tubular shapes, perhaps also employing, as in Fig. 7.18, snap-fit connection devices to aid assembly and installation. Where necessary these can after installation be filled with a lean concrete mix, foam or even granulated recycled waste plastics to prevent buckling. Either way, all these versions overcome two serious deficiencies with timber. There is now a scarcity of suitable timber, and deepening environmental regulations restrict the use of cost-effective preservative treatments such as creosote, which leach out toxic products capable of contaminating ground-water supplies. That picture needs completion, however, with the note that by using a column-wrapping technique, composites are also being employed to refurbish deteriorated timber poles. The wrapping of a thin composite ‘bandage’ around the underground plus for a short distance, the above-ground portion of the pole, is proving to be a cost-effective procedure to provide around 10 years’ additional in-service life for poles which are in a reasonable condition. It also helps to alleviate the environmental problem.

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7.18 ‘Tower top’ utility pole (Courtesy, Strongwell Ebert)

7.6

Conclusions

Over the coming years, composites – and particularly pultruded profiles – can with confidence be expected to play an important and increasing part in supplying the needs of the civil engineering infrastructure market. The quoted case histories, a multitude of other prototype and in-service examples and finally the wide-ranging research and development15 now in hand provide ample confirmation. Indeed pultruded profiles are fast becoming an increasingly viable structural material for a wide diversity of application beyond just the needs of the infrastructure. There is therefore every confidence that the pultrusion industry will continue to grow in the foreseeable future by well over 10% per annum with the potential for even that figure to be exceeded as a number of developments now in hand are commercialised. Perhaps more than any other sector of the whole composites industry, that whole opportunity offers the greatest challenge and excitement to the many world-wide

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pultrusion companies now involved. Their number must also increase with time. Their future is bright.

7.7

References

It is important to note that as far as the civil engineering infrastructure portion of this chapter is concerned, the following is only a very small collection of conference papers that could have been provided. A comprehensive, definitive listing was not seen as the purpose of either this chapter nor the whole book. Readers wishing to research further are recommended to examine in particular the published proceedings of recent International Composite Expo and Composites Fabricators Association (CFA) conferences. 1. Busel, J P, ‘FRP composites used for the civil infrastructure in the USA’ Composites in Infrastructure Conference, EPTA, Paris, 1999. 2. ‘Laminate Analysis Program’, a composite materials design tool. Anaglyph Ltd, London (other software programs are available). 3. Fibreforce, Design Manual – Engineered Composite Profiles, Fibreforce Composites Limited, Runcorn, UK (many similar manufacturer published manuals are available). 4. Creative Pultrusions, The Pultex® Pultrusion Design Manual, Creative Pultrusions Inc, Alum Bank, PA USA. 5. Lopz-Anido, R, Troutman, D L & Busel, J P, ‘Fabrication and installation of modular FRP composite bridge deck’, Paper 4-A, International Composites EXPO ’98, Nashville, 1998. 6. Holloway, L C & Leeming, M B, Strengthening of Reinforced Concrete Structures, Woodhead Publishing, Cambridge, 1999, ISBN 1 85573 378 1. 7. Johnansen, G E, Wilson, R J, Roll, F, Gaudini P G & Gray, K, ‘Design and construction of FRP pedestrian bridges – reopening the Point Bonita Lighthouse Trail’, Paper 3-F, International Composites EXPO ’97, Nashville, 1997. 8. Johansen, G E, Wilson, R J, Guest, J, Wallace, S, Jaskovich, G and Eriksson, M, ‘Design and construction of FRP pedestrian bridges subjected to extreme snow load conditions’, Paper 25-C, International Composites EXPO ‘99, Cincinnati, 1999. 9. Lampo, R, Maher, A, Busel, J & Odello, R, ‘Design and development of FRP composite piling systems’, Paper 16-C, International Composites EXPO ’97, Nashville, USA, 1997. 10. Odello, R J, Warren, G E, Hoy, D & Busel, J P, ‘Composite elements for navy waterfront facilities’, Paper 16-E, International Composites EXPO ’97, Nashville, 1997. 11. McClaskey, C R, ‘FRP composite rebars come of age: a rational strategy for commercialisation’, Paper 25-D, International Composites EXPO ’97, Nashville, 1997. 12. Kumar, S V, Thippeswamy, H K & Gangarao, H V S, ‘McKinleyville Bridge: construction of the concrete deck reinforced with FRP rebars’, Paper 6-F, International Composites EXPO ’97, Nashville, 1997. 13. Starr, T F, ‘RP and housing: an examination’, Paper 22-C, 38th Annual RP/CI, SPI Conference, Houston, 1984.

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14. Miller, K, Misel, J & Sumerak, J, ‘Pultruded utility pole application presents significant technological challenges’, Paper 30, Composites Fabricators Association, Composites ’98, San Antonio, 1998. 15. PIC, Load Resistance Design Manual, Pultrusion Industry Council (PIC) of th Composites Institute, the Society of the Plastics Industry, USA, in conjunction with the American Society of Civil Engineers, (NB folowing the merger of the Composites Institute with the Composites Fabricators Association (CFA), the PIC is now merged with Pultrusion Growth Alliance of the CFA). Early paragraphs on ‘Machining, Fastening & Finishing’ draw heavily on a Fabrication & Repair Manual published by Strongwell Inc., 400 Commonwealth Avenue, Bristol, VA 24201, USA.

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8 The future – beyond 2000 W BRANDT GOLDSWORTHY

8.1

Introduction

Even though the pultrusion industry has experienced dramatic growth over the last decade, it remains a job-shop industry, producing a great variety of standard and custom profiles over a broad spectrum of commercial and industrial applications. The increasing realisation that composites are not exotic speciality materials, but just another basic engineering material, is bringing about a transition to that industry. The entry of pultrusion as a high-quality engineering material processing technology into the area of structural profiles for civil engineering and infrastructure use is driving this change. In all probability, then, the next century will see the advent of fully integrated, highly capitalised captive pultrusion plants producing high tonnage commodity structural items, in much the same manner as was demonstrated by the growth of the steel and aluminium structural profile business. That will be both accompanied by, as well as demand, a growing awareness, recognition and acceptance of, the use of fibre reinforced–polymer matrix composites generally.

8.2

Machine, die and profile design, size and capacity issues

While maintaining its basic characteristics, the pultrusion machine of the future will probably vary rather dramatically from currently operated equipment. The machines in use today are designed for maximum flexibility so that the custom pultruder can produce almost any required shape within the dimensional window of that machine. This need for flexibility is reflected in both the initial cost and the operating efficiency – a ‘jack of all trades, master of none’ syndrome. The advent of essentially single product, indefinite run pultrusion plants, will therefore permit machine design at lowest cost and maximum efficiency. This, in combination with the need for 264

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increasingly massive structural profiles (Fig. 8.1) and large load-bearing panels (Fig. 8.2 and 8.3), will drive the size of the machines on which they are pulled, far beyond any in current use: in other words, high-volume applications that will demand dedicated facilities having the capability to pultrude large structural elements.

Machine size One of the tremendous advantages of the pultrusion process is that as long as it can be surrounded by a die, there is no theoretical limit to the size of profile that can be produced. This stems from the fact that there are virtually no pressure requirements in the pultrusion process, whereas profile production in traditional materials, demands very high rolling or extrusion pressures. Both limit the maximum product size achievable. Today, machines that will comfortably pull 150 ¥ 150 cm (60 inch ¥ 60 inch), or for example, 240 ¥ 37.5 cm (95 inch ¥ 15 inch) sized profiles already exist, while machines having a capacity of 300 ¥ 120 cm (120 inch ¥ 48 inch) are currently in design.

Reinforcement impregnation However, as profiles grow even larger, dramatic changes in machine design, configuration and function will be essential. Many will be inter-related and

8.1 Six-cell beam, 90 cm2 outside envelope (Courtesy, Goldsworthy Associates, USA)

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8.2 Load-bearing bottom rail for six-car Union Pacific Railroad autohauler (Courtesy, Alcoa/Goldsworthy, USA)

8.3 Union Pacific Railroad auto-hauler (Courtesy, Alcoa/Goldsworthy, USA)

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can be expected to be major in nature. For example, owing to the considerable and economically unacceptable quantity of scrap raw material – particularly reinforcement – that is initially produced, current start-up techniques urgently need development and change. Further, the use of closed reinforcement impregnation or, in other words, liquid matrix resin-injection direct into the die cavity, as a means of controlling or even eliminating volatile emissions, is almost certainly the way future pultrusion machines will operate. Even if not ultimately mandatory, it is a highly desirable change in comparison with the open wet-bath techniques, which are still widely practised. But the more complex the profile and the heavier the profile section (Fig. 8.4), then the more difficult the injection die design and function become. Already pultrusions are exhibiting wall thickness well in excess of 25 mm (1 inch) and although the clear trend and need are upwards, the wet-out of this mass at high reinforcement loadings, and in often complex configurations, presents a real challenge. Because some form of standard or universal answer, capable of accommodating wide varieties of both shape and thickness, is seemingly beyond development, the design of the injection die system will therefore have to be approached on a case-by-case basis.

Guidance tooling Equally challenging is the design of the guidance tooling between the reinforcement supply racks and the die. Thicker and thicker wall sections, will require literally hundreds of plies of reinforcement, often of different forms and probably also of different fibre types, all of which have to be precisely guided and positioned into their final shape, prior to die entry (Fig. 8.5). In all probability, this may ultimately mean that the final forming to shape of mega-ply profiles will have to take place within the die. Only in this way will there be sufficient ply separation for optimum reinforcement wet-out. In many cases this pre-die entry section will also be required to satisfy techniques for accomplishing such functions as off-axis reinforcement forming, tacking or stitching, all aimed at ensuring that the reinforcement pack arrives at the die in precisely the manner dictated by the ultimate structural requirements of the product being pultruded. As will be reviewed in greater detail later, matrix and reinforcement changes will themselves probably also change the basic configuration of the pultrusion machine, as well as in the operation of its various functions. If reinforcement forms can be developed to withstand the hydraulic shear forces which are particularly high at the forming die entry, then very much higher running speeds – perhaps upwards of 30 m/min (100 ft/min) – can be seen as theoretically possible, when for example employing thermoplastic matrices. Both those changes, production speed improvement and a much

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8.4 Massive 12 m (40 ft) truss section assembly, US Navy, designed for 1800 tonne loading (Courtesy, Ebert Composites Inc/Goldsworthy, USA)

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8.5 Complex reinforcement guidance system for 67.5 cm (26.5≤) wide, Darrieus wind turbine blade (Courtesy, Flowind/Goldsworthy, USA)

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greater use of the thermoplastic matrix, are seen as major pultrusion industry objectives.

Profile shape, size and complexity Tradition suggests that structural profiles, particularly for civil engineering and infrastructure use, are I-beams, H-beams, channels, angles, tubes or other hollow sections. However, these are not necessarily optimum shapes, but are the only profiles that can be readily rolled on a steel rolling mill or otherwise formed in metals. Composite structural profiles do not demonstrate the same constraints. They can be shaped and be of a optimum dimension not only to carry the desired structural and other loads, but also to allow as part of the design a multi-functional capability such as in-built jointing techniques, or the provision of attachment methods that do not require the drilling of bolt holes (Fig. 8.6 and 8.7). In addition, they can incorporate raceways, or fluid and air conduits, all features open to on-going development whether of the pultrusion machine or its related tooling. One example of a current multifunctional design is the Union Pacific Railroad (USA) auto-hauler (Figs. 8.2 and 8.3), the construction of which employs large structural profiles having an integrally pultruded snap-fit joining edge (Fig. 8.8). Another important and vital facet of the composites business – and irrespective of fabrication process – is the well-established ability to create forms and shapes by simply building-up the reinforcement plies and directly moulding these to the desired shape. As a result, machining other than minor trimming or drilling has always been seen as virtually unnecessary as far as composites are concerned. This is completely different from traditional materials where parts are brought to the desired shape, contour or finished state, by forming and/or machining processes. However even today, and increasingly in the future, situations apply where the machining of composites and pultruded profiles to a specific contour or otherwise is really the only economically feasible – and sensible – approach to meet an assembly problem or the final requirements of a component. It is an opinion that does not overlook the perhaps large quantity of generated waste – or the associated problems of health and disposal – nor the fact that composites tend to destroy conventional machining tools.

On-line machining A huge step forward in bringing pultruded profiles more competitively into the market-place against traditional materials would result from the installation of a five axes or more multiaxis machining centre (Figs. 8.9 and 8.10)

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8.6 Dovetail interlock for electrical transmission pole (Courtesy, Composite Power)

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8.7 Patented snap-together pultrusions employed for electrical transmission tower construction (Courtesy, Ebert Composites, Inc)

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8.8 Integral snap-fit joint profiles, Union Pacific auto-hauler (Courtesy, Alcoa/Goldsworthy, USA)

8.9 Five-axis cell for synchronous in-line machining of pultruded profiles (Courtesy, Ebert Composites, Inc, USA)

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8.10 Diagrammatic impression of CNC 22-axis machining centre (Courtesy, Goldsworthy Associates)

operating in-line with the pultrusion machine. This would not only cut the product to length as at present, but undertake and complete any finishing operation which, owing to complexity, intricacy or position, was much more logical – or otherwise impossible to undertake – than simply by pultrusion die forming. Such a technique not only improves the process economics by eliminating costly secondary manufacturing operations, but allows whole new design vistas, such as high load-bearing snap joints and tapered sections, as well as an infinite variety of other component configurations not open to an as-shaped profile. A classic current example is the development of the solid fishing-rod blank. In the beginning, the tendency was to develop automated equipment to produce a tapered form by stepping-off reinforcement material plies to achieve the taper. This demanded very expensive equipment, which exhibited poor reliability and slow production rates. Moreover, the result was a product that was barely acceptable from an aesthetic standpoint. Finally, the traditional approach of taking a piece of solid bar stock and simply running it through a centreless grinder to achieve the taper was reexamined. This not only cut the manufacturing and capital equipment cost

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dramatically, but produced a product in one simple finishing operation, having a beautiful finish and optimised structural characteristics. There will be other related and marked machine changes, including the ability to print and/or apply a decorative finish coat on-line and perhaps even some limited component sub-assembly. Overall, the objective will be the quicker, cheaper manufacture of totally finished on-line profiles. As will be seen later, much will also depend on reinforcement and matrix changes and development.

Sensing and machine control Finally, there will be associated advances in sensing and machine control techniques, which will permit such precise in-line process monitoring that the need for quality control and inspection after the final cut-off – or machining/assembly/finishing operation – will be virtually eliminated.

8.3

Reinforcement and related issues

The strength member of a composite structural element is the reinforcement, a very fine filamental material that has been traditionally made from glass, carbon, aramid and other synthetic, or even natural fibres. Following the lead of the textile industries, these various reinforcements have (see also Chapter 5) been provided in the form of roving, tow, yarn, mat and woven fabrics. More recently, other forms of ‘broadgood fabrics’, with specific fibre architectures (Fig. 8.11) and offering production and finished property gains, have been stitched, knitted or otherwise ‘bonded’ together to provide multilayered, controlled, off-axis reinforcements in the crosswise ‘Y’ direction. To particularly enhance omnidirectional properties, a range of woven and braided three-dimensional (3D) reinforcements are now becoming increasingly available, some of which are also able to provide an integrated skin–core sandwich structure or, taking another example, as a means of enhancing interlaminar shear. Unfortunately, most of these 3D weaving techniques are relatively slow and the resulting product is therefore too expensive for many commercial applications. Nevertheless, because those property benefits are real and of increasing interest to pultruders, a great deal of research to find an inexpensive solution to this problem can confidently be expected, although by inter-plying the reinforcement prior to resin wet-out, the pultrusion process by itself provides some opportunity to produce pseudo-3D structures. Even so, an innovative way of introducing ‘Z’ – or in other words, ‘through the laminate’ – direction fibres would greatly enhance profile properties and thus broaden the ability of the process to satisfy ever-more stringent structural applications.

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Pultrusion for engineers Non-woven Plain weave

Four shaft or crowfoot satin weave 3D weave 0°

Multiaxial fabric

-45° 90° +45° Quadriaxial cross-section

8.11 Typical fabric, multi-layered and 3D reinforcement weaves and non-woven constructions (Courtesy, Devold ATM, CS-Interglas and Goldsworthy Associates)

However, whatever the form and type of reinforcement employed, it must possess the ability to withstand distortion – or at worst, destruction – caused by the viscous shear forces within the die, which are generated by the initially liquid matrix resin. Unfortunately these rise almost exponentially with production speed, and therefore constitute a new challenge to production speed improvement that was formerly held by the matrix. Improvements in thermoset resin-curing technology and, in addition, the ever-growing move to replace that traditional matrix with thermoplastic polymers, have largely removed cure rate as the pacing factor. However, unless techniques can also be developed that allow exact net resin ratio pultrusion, that potential reinforcement failure mode will remain dominant. Other production benefits become possible with the advent of thermoplastic matrices, as well as thermoplastic-based reinforcement fibres. In addition to the potential of higher production speeds, there is the chance to create many new and unique forms of reinforcement by, for example, thermal tacking, all at considerably lower cost than weaving and braiding. Taken together then, there will be an opportunity to bring composite products generally into an ever-more favourable competitive position, in comparison to traditional materials.

Hybridised reinforcement and tissues Because the airframe/aerospace industry demands optimum strength-toweight performance, those market sectors have placed a great deal of emphasis on the use of high-modulus fibres, typified by carbon. This has led

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to an actual but totally unnecessary division of the composite industry into two camps: the so-called ‘advanced’ sector employing high-modulus fibres, as opposed to the ‘commercial’ sector where glass and other lower-modulus fibres predominate. However, pultrusion and civil engineering structures in particular will almost certainly expand the use of hybridised reinforcements, that is, a mix of both high- and low-modulus fibres. As a means of achieving specific properties in various parts of a pultruded profile, it is very clearly an ideal technique and one open to considerable development and exploitation. A suitable example would be an I-beam in a civil engineering structure where the section modulus is dramatically improved by the use of a thin ply of carbon fibres on the upper and lower surfaces, while the major mass of the beam comprises less expensive glass fibres. Further, combining synthetic or natural fibres with glass reinforcements can equally enhance the local performance of the profile, in respect of, for example, abrasion resistance or some other specific and desired component performance. To a limited degree the technique is already in use today, as exemplified by the now almost universal practice of using synthetic fibre veils – or tissues – on the surface of a profile to enhance ultraviolet resistance. These veils, with their ability to be printed with permanent labelling or in a broad range of colours and patterns, offer in addition another form of product decoration and/or identification.

‘Smart’ structures and other developments Another area that will undoubtedly be exploited from its current modest beginnings is the development of ‘smart structures’. The ability to pultrude optical fibres, piezoelectric materials or scannable grids as an integral part of the finished product will permit critical service-life monitoring of the state-of-health of structures assembled from these profiles. Any unexpected overload condition or structural weakness will thereby be immediately identified and accurately located, well before catastrophic failure, or other serious problem occurs. This capability can be expected to bring about a whole new era in public safety, particularly as it relates to the civil infrastructure market-place. The future will also bring new types of special reinforcing fibres, not just thermoplastic or even based on synthetics, all able to perform a whole spectrum of specific requirements. This development will be driven by factors ranging from the need for alkali resistance, through recycling and into the ever-present requirement for lower and lower reinforcement and hence profile costs. One good example of this trend is the development in Russia of basalt fibre. Basalt has the potential to become a low-cost fibre with essentially the same composite properties as glass, but with a considerably higher alkaline resistance. These and similar developments will continue to

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contribute to the enhancement of the pultrusion industry, both in ease of processing and in final product properties and costs.

8.4

Matrix issues Thermoset systems

Even today, the thermoset resin matrix materials available to the composites industry permit product properties to be tailored to meet virtually any electrical, chemical, environmental or mechanical/physical endproduct performance. This applies equally to the manufacture of pultruded profiles and a unique ability that can only extend as new thermosets are commercialised.

Thermoplastic systems However, although a much broader spectrum of thermoplastic polymers all in quantity production exists, these were until relatively recently ignored by the composites industry, even though possessing the potential to enhance the ‘tailoring’ ability described above. For all their attraction, processing in conjunction with long-fibre reinforcement presented a number of problems. Following recent development, that is at last beginning to change and because that drawback has been easier to overcome by pultrusion, that sector of the composites industry is leading the way. Strangely enough in view of their typically low softening or melting temperature, a pultrusion interest in the use of thermoplastics was initially prompted by the need for a very high-temperature-resistant composite. Development work in the use of common polymers is now accelerating quickly and will continue to grow. Indeed, it should from the beginning have been apparent that thermoplastic matrices were better candidates for producing composites structures. The very definition of a composite is the use of an immensely strong fibrous material to carry the load within a weaker surrounding matrix. However, in order to make them commercially handleable, those fine reinforcing filaments have to be bundled together into some form of roving, tow or yarn strand. Unfortunately, it has never been possible to develop a bundling technique where every single filament is perfectly collimated and in equal tension. Therefore, the result is a few filaments in tension, but with the vast majority hanging as a ‘catenary’ on those tensioned filaments. Even so, the use of this type of reinforcement bundle, encapsulated in a thermoset matrix such as unsaturated polyester resin and exhibiting a typical elongation of only 1.0–1.5%, gave birth to the composites industry. Consequently, for the rein-

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forcement truly to take up the load there has to be some failure of the matrix: in other words, therefore, the production of a structural material that even if only exhibiting microcracks has in a sense partially failed before it actually begins to accept the design loads intended for the component! Thermoplastic matrices on the other hand naturally have elongations of 7 or 8% or greater and moreover exhibit a totally different stress–strain relationship (Fig. 8.12). Therefore when reinforced thermoplastic composites are loaded and as the matrix elongates, the reinforcement takes up the load free of this premature microcracking and moreover with complete elimination of the otherwise stress–strain curve plateau. In addition, this high elongation property results in a toughness that makes composites much more abuse-tolerant. As already indicated and providing certain remaining reinforcement and processing problems can be resolved, thermoplastics also have the potential for markedly increasing pultrusion running speeds by several orders of magnitude. Thermoplastic matrices are in most cases also considerably less expensive than the thermosets and have the additional and marked advantage of being totally recyclable in both directions. In other words, very low-cost recycled thermoplastic resins can be used as matrix materials and at the end of its useful life, the eventual product can be simply chopped up and made into very worthwhile long-fibre compression moulding compounds for a myriad of other uses. Finally, they offer a much better working environment through the elimination of emissions and the sticky mess associated with most thermoset composite processing.

Stress

Tested under different environmental conditions

Reinforced thermosets show a linear relationship

Strain

8.12 Stress–strain curves for long fibre reinforced thermoplastic (nylon 66) (Courtesy, LNP Engineering Plastics)

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For all these reasons, it would appear that thermoplastics might become the dominant matrix for all composite applications and, in particular, for pultrusion. However, no material meets every requirement. For example, the infrastructure and civil engineering uses that are now being contemplated for pultruded profiles often have special requirements that cannot be met by most thermosets and thermoplastics. One of the most important is fire safety. Although fire standards have not yet been mandated for many applications of this type, it is a foregone conclusion that pultruded composites must ultimately conform to increasing severe safety codes.

Phenolic matrix The only matrix materials currently available that will comfortably meet all the flame spread, smoke generation and toxicity requirements are the phenolics (Fig. 8.13). Strangely enough, these are the oldest resins known to the plastics industry and were really the first systems used for creating composites. In spite of this, they virtually disappeared with the advent of unsaturated polyesters and other polymerisation reaction resins such as vinyl esters and epoxides.As discussed in Section 4.4, their intrinsic fire resistance is the cause of their still recent resurgence, and it can be confidently expected that their growing use and importance will continue. Nevertheless, phenolics would appear to have some very substantial disadvantages as pultrusion matrices because of three inherent problems. First,

800

Epoxy

700 600 Optical density

Polyester 500 400 300 200 100

Phenolic 2

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8.13 Smoke generation – matrix resin comparison

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they are what are known as condensation rather than polymerisation reaction resins and in a continuous, essentially no-pressure process such as pultrusion, the evolution of water during cure strongly suggests the possible occurrence of almost insurmountable void problems within the profile. However, in actual practice, this water is evolved as steam at the exit end of the die, with the result that phenolic pultrusions show only a slightly greater potential for voids than competitive thermoset matrices. The second problem was the initial use of acid catalysts to initiate this condensation reaction. These had the propensity for unacceptable damage to both the die as well as the reinforcement, but as again discussed in Section 4.4, this is a problem now eliminated by the development of catalysts of a non-acid type. The phenolic resin also has an even lower elongation than polymerisation reaction matrices such as polyester, to result in a product that is harder and less ductile. This shortcoming can however, be overcome by alloying the phenolic with polyvinyl butyral (PVB) or nylon, although there is an obvious trade-off in terms of optimised fire standards when toughening the finished laminate in this way.

Copultruded matrices One final matrix-related area that has currently seen little application but which promises new, large potential applications is the use of copultruded systems, where two compatible matrices having different end-product properties are pultruded together into a totally integrated profile. The copultrusion of different but compatible matrices, combined with different fibre systems, would for example enable different but ‘local’ property regions to be generated within one integrated profile component. The process will become commonplace in the future because even on current evidence it is clearly feasible to construct a structural profile formed of highvalue fibres embedded within an equally load-carrying matrix, but where one face or area demonstrates very high abrasion – or perhaps impact – resistance, formed from a suitable alternative reinforcement contained within, say, an elastomeric matrix compatible with the load-carrying one.

8.5

Potential for fibre architecture development

As has already been confirmed by case-history and related examples, the primary difference between composites and traditional materials is the ability of the former to employ a fibre architecture that permits the laminate, and thus the moulded product, to exhibit specific load-carrying capability. Other materials are typically isotropic in nature with essentially equal properties in all directions.

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Although pultrusion offers a particular ability in developing and enhancing this fibre architecture advantage, relatively little has been done to create fibre designs where, for example, the plies are not all in the same plane, but where they are shifted off-axis in order to enhance a specific property such as interlaminar shear. Alternatively, by changing the plane of the reinforcements, the latter could also be transposed to transverse shear. By simply shifting the reinforcement planes in this way, the effect is to create a Z-axis and if undertaken in conjunction with in-plane, off-axis reinforcement developed by orienting the plies in relation to each other, a fibre structure is achieved normally only practical – if indeed at all – by stitching multiple axis plies together. Obviously developing an optimum fibre architecture for any given structural member is not only complex, but is also quite likely to require within a single structural element the use of a wide variety of different reinforcement forms, such as stitched and woven fabrics, roving or tows, and even braids. Until now, the practice of fibre architecture has typically been based on employing reinforcement forms that were easy to pultrude, rather than as dictated by the optimum properties required for the specific structure. Under these conditions, it becomes obvious that fibre architecture design and also the necessarily associated guidance tooling design are totally independent in the accomplishment of a proper or optimum fibre architecture. This mandates that tool designer and structural engineer work as a team from product conception. That repeats earlier recommendations found for example in Section 2.2. The current constraint in fibre architectural development is the form of reinforcement that is indeed available and from which more optimum pultrusion ‘fabrics’ can be woven, or otherwise constructed. However, as the optimisation of mechanical properties becomes more and more the primary driver – as it will – it can be expected that other forms of reinforcement will be developed. For example, techniques are already being tested for providing Z-axis enhancement by incorporating multiple pre-manufactured composite ‘pins’ into the specific type of fabric plies being laminated. It would be a large step forward in fibre architectural development, therefore, if some form of Z-axis reinforcement, other than 3D weaving could be developed. Three-dimensional weaving and braiding works well to bring about desired properties but in most cases it is far too expensive to permit its application to civil engineering structures. The most difficult problem to overcome in looking at specific composite structural shapes is to carry the off-axis loading that results in interlaminar shear or tensile loading. An economical solution would probably be the greatest single advance in the use of pultruded profiles for primary structures. There are, however, new techniques in current development that do have the potential for solving this problem in a manner that retains economic viability. Certainly,

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the ultimate achievement of this type of reinforcement will bring composite design approaches much closer to those used in the design of traditional materials. Nevertheless, it needs to be kept in mind that the fibre architecture might not be chosen solely on the basis of providing optimum properties. In some cases, the formability of the reinforcement may become more critical than its ultimate physical properties. For example, tubular net fabrics have poor reinforcing capabilities but can readily conform to almost any difficult shaping requirement. This capability, in combination with other strength member types of reinforcement, may result in a product with adequate properties while providing ease of processability and therefore lower costs. Any fibre architecture design must always consider all these factors. Pultruded three- and four-cell airfoils (Fig. 8.14) are, for example, frequently of hybridised carbon/E-glass construction in order to obtain critical stiffness. In all such cases, the fibre architecture for the stiffness member and that for the flexible face will obviously be totally different in nature. Pultrusion of ‘living hinges’ on structures, for example, also becomes possible with this type of technology. As described earlier, the move to thermoplastic resin matrices also allows the development of many new reinforcement forms. These offer acceptable economics, simply because the reinforcement can be more readily and continuously manipulated at high production speeds, into almost any plane or

8.14 Pultruded carbon/E-glass hybridised airfoils (Courtesy, Goldsworthy Associates)

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position, prior to a simple heat-tacking operation to create the most optimum desired reinforcement form. Such complex fibre architecture would be extremely difficult, if not impossible, to achieve with the use of thermoset matrices. Additionally, pre-made reinforcement forms could also be combined in the guidance tooling, and again secured by heat tacking to provide almost any conceivable form of fibre architecture. The design of fibre architecture is of course not only concerned with properties, but on many occasions with the geometry of the part. Examples of geometry-satisfying but not pultruded composite components are rightor sharp-angle web-to-flange configurations, where the load tolerance of the part is enhanced by providing relatively generous radii between the flange and the part. This requires a space-filling, formable reinforcement best accomplished in most cases by incorporating pure unidirectional roving or tows. In other areas, such as tapered or stepped-thickness sections, it may enhance the processing as well as the properties to use different thicknesses of the same form of reinforcement in the make-up of the fibre architecture. This can be feasible with pultrusion (Fig. 8.15), and another possibility that has already gained some, albeit limited, commercialisation is to incorporate relatively low-density core type materials in order to increase the section

8.15 Stepped thickness, thicker reinforcement for stronger highstressed side-wall panel areas, Union Pacific auto-hauler (Courtesy, Alcoa/Goldsworthy)

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modulus, while keeping both component weight and sale costs within acceptable limits. As the entry into primary civil and infrastructure profiles with optimised load-carrying capability further develops, changing fibre architectures at very high reinforcement concentrations and having fibres oriented on the load paths, will become common. Producing off-axis multiple ply materials is currently done by weaving, braiding and stitching techniques, with the latter approach becoming the most favoured as it provides the greatest flexibility in orientation and does not distort the fibre strands. But in almost every civil and infrastructure application, economics is the primary driving factor. In looking at optimum fibre architecture – and clearly there is a close relationship with the matrix employed – it will therefore be necessary to keep in mind that there are often several ways of achieving the same end result. Rovings or tows are the basic forms of reinforcement, and pultrusion frequently allows their direct and entire use in producing the desired fibre architecture, as opposed to woven fabrics or stitched off-axis fabrics. Not only will the cost be dramatically reduced thereby, but also because the reinforcing elements are perfectly straight, the finished profile properties will probably be substantially increased. This is yet another classic example of the need for the tool designer, the process engineer and the stress analyst to be working together during the initial concept development stages. Again as already mentioned, another important consideration is the need to ensure that both the chosen architectural form and the reinforcement from which it is constructed are capable of surviving without distortion, deterioration or even destruction during processing. The handling and abuse suffered by the reinforcement while traversing the area between supply spool and forming die can be quite severe. Then there are the hydraulic loads imposed during impregnation, or the viscous shear generated at forming die entry. Any one of these may reach a level high enough by itself to dictate the choice of both reinforcement and the form of its architecture. Although one effective answer is to sandwich the weaker fibres between plies that are less subject to failure, it is clearly apparent that it is not just simply a question of matching the fibre architecture to the respective load diagrams. It is also paramount to ensure that the design requirements as well as the processing and economics targets are all equally satisfied. The inter-dependence of all these parameters and their correct choice can therefore be critical, particularly so in respect to the all-important process economics. For example, another prime way to achieve optimum pultrusion economies is to create the architectural or reinforcing fibre form in-line on the pultrusion machine. This can be done in a number of ways. In-line braiding and circumferential fibre winding (Fig. 8.16) are techniques

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8.16 Improved process economics from fibre architecture employing winding wheels (Courtesy, Goldsworthy Associates)

that have already been exploited, but there is still a great deal of room for additional development. Collectively, all these described techniques will undoubtedly become more prevalent as captive pultrusion begins to prevail over custom pultrusion. Where a pultrusion line is dedicated to a single product, in-line production of the fibre architecture, even by extravagant in-line fibre arrangement and placement methods, becomes a viable operation owing virtually entirely to the overall savings in material cost that are possible.

8.6

Profile development – issues and potential

If optimum results are to be obtained in the designing of composite materials and components, then most of what applies in the use of traditional materials needs to be disregarded. The background reasoning has already been reviewed, but the degree of difficulty in this task is more readily appreciated by setting aside the idea that virtually every structure is stiffness critical. This idea stems from the fact that in employing the moduli of traditional materials as a baseline, there was no need to evaluate the potential advantages of designing with a lower moduli material. However, because of the property differences between composites and these perhaps more familiar materials, that does become a necessary thinking process. The

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same would equally apply in an environment where composites are the dominant materials of construction. There would then be difficulty in translating designs into, for example, aluminium and steel. The differences that must be learnt encompass not only the different properties, but also those of the processing technologies, plus the design freedoms that result from these differences. For example, continuous shapes that can be produced in steel are limited simply because of the high shaping pressures required. Conversely, pultrusion being essentially a non-pressure process offers virtually no constraints to the complexity of the shapes that can be produced (Fig. 8.17). As noted earlier, it is therefore perfectly feasible to design-in multiple functions in a structural pultruded member, additional to pure load-carrying capability. This attribute results in lowering the cost of the resulting product by eliminating the need for the assembly of multiple components. The convenience and simplicity of such synergistic designs are well illustrated by top hat-stiffened panels and J-stiffened shapes (Fig. 8.18), which to advantage replace welding and the use of rivets with simple one-piece profiles. The economic advantages of being able to design multiple functions into a single pultruded composite profile, as opposed to attempting to accomplish the same in traditional materials, is further confirmed (Fig. 8.19) by a 16.5 m (55 ft) long structural beam. Not only does this carry major loads of the assembly of which it is a part, but it also provides for the location and

8.17 Window lineals and other complex profiles (Courtesy, Goldsworthy Associates)

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8.18 J-stiffened shapes integrating multiple functions in a single structural element (Courtesy, Goldsworthy Associates)

8.19 Cross-section 16.5 m (55 ft) pultruded structural beam (Courtesy, Alcoa/Goldsworthy)

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attachment of the shear panels that are an additional part of the structure. On the other hand, fabrication in, say, aluminium was not possible simply because a sufficiently large extrusion press does not exist. Consequently, had the use of aluminium – or perhaps even steel – been essential for some reason, then manufacture would have required four 16.5 m (55 ft) long seam welds, an obviously expensive solution in comparison with pultrusion, at least given the respectable number beams required. It is therefore the pultruded profile designer’s responsibility not only to shape and engineer the structural member to best carry the loads, but also to incorporate all additional features and functions that are practical, such as, for example, locating or indexing fixtures, guide-ways or ducting. That best-practice approach permits the whole design to achieve the lowest possible final cost, commensurate with the final specification and the quantity requirement. Even neglecting the much more attractive, shipment, erection and long-term maintenance, long-term life costs that typically apply for composites generally – and particularly for pultruded profiles – that procedure ensures for the eventual customer a very competitive result in comparison to traditional materials. Further, the pultrusion process exhibits the valuable ability to combine the basic reinforcement and matrix continuously with other materials. Some mention has already been made of this method of creating a product with integrated properties and features never before achievable. This starts with hybridisation of the basic reinforcing materials, such as the incorporation of carbon fibres in strategic locations where necessary to achieve higher section modulus values, or the simpler use of foams, balsa and other lowdensity core materials. Other hybridisation possibilities of importance to the designer include the use of metallic grids, wires and bar for shielding conductive structures, or to provide a host of other special product performance requirements such as incorporating, optical fibres in the fibre architecture pack to create ‘smart’ structures.

Design aids Statements such as the above flag up a feature of composite materials design that now demands comment. It is both a blessing and a curse. Although the ability to ‘tailor’ the composites component performance very critically through both specification and fabrication technique choice is now increasingly recognised and accepted as unique and a marked benefit in comparison with traditional materials, it is also a practice difficult to define by mechanical property tables, nomograms or other design engineering aids. The situation presents the composites engineer with a severe problem. The almost limitless component properties that are practical by employing composites, whether or not formed by pultrusion, mean that the compila-

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tion of a totally comprehensive mechanical/physical property database equivalent to that of the traditional materials is a task that will be continuing almost indefinitely. However, this must certainly not imply any lack of such background information, quite the reverse, although it does need to be appreciated that there is also a huge amount of data that cannot readily be reduced to a tabular, graphical or other guidance form. To a degree this has created something of a dilemma in promulgating standards and the other well-known techniques used by the engineering fraternity. Equally, it is a situation that has had a dramatic impact on the current development and acceptance of composites structures by almost every engineering discipline. It is therefore a pertinent situation to consider when looking into the future of the pultruded profile market. However, there is, as suggested below, a relatively simple, straightforward, workable solution to this seemingly unfathomable problem. Basically, the procedure is really no different from other traditional design techniques. Of further importance it is an approach equally applicable to the design of all composites moulded components, as well as pultruded profiles. The essential steps are as follows: • • •

• • •

To determine the load magnitude and directions for the specific structural element involved. To formulate the fibre architecture that will carry these loads in an optimum fashion. To decide which composites processing technique is likely to achieve the desired performance and economics, even though this may not always be pultrusion. To prepare test specimens by, for example, pultruding the chosen fibre architecture and fibre type with the selected matrix resin. To evaluate the physical/mechanical properties of those test specimens. To employ that property data in design software, even where that has been developed for traditional materials.

Experience shows that the result does indeed validate the design to within a perfectly acceptable 1 or 2% tolerance. In today’s environment, computer modelling is typically the first step in evaluating the validity of a new concept or structural design. However, without this recommended approach, the assumptions employed in the model are very likely to be far from valid, with the result that a promising new technology (e.g. the use of pultruded profiles for a particular application) is rejected simply as a result of spurious modelling. The primary consideration in designing with these non-traditional composite materials is the need to eliminate established patterns of design thought and to examine the problem with a completely open mind. This is something easily stated but difficult to accomplish for most engineers,

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whatever their discipline; however, it is seen as essential, if the growth opportunity open to composites in general and specifically pultruded profiles is to be accelerated and secured. As in no time in the past, design in these materials mandates the need to have the respective engineering and manufacturing inputs co-operating in the product design from conception right through to ultimate fabrication. If economic- and performance-feasible composite components and structures are to be achieved, it is no longer possible for engineering to complete its task in isolation and then simply hand-over the results to those responsible, for manufacture. While this approach has always been bad practice, it was workable with traditional materials where the end product was achieved by machining or otherwise forming the material into the finished product. With composites – where more often than not the material is being made at the same time as the product – processing is such an important, even vital, part of the whole design process that it has to be integrated if satisfactory results and product economics are to be achieved.

8.7

Application potential

Categorically, pultruded products are already part of the well-established penetration of composites in many marketing areas. That diversity is well illustrated by Fig. 8.20–8.23 inclusive, and the following list also graphically illustrates the progress that has been made, particularly since 1980.

8.20 Glass reinforced pultruded automotive rear suspension link rods (Courtesy, Delphi Automotive Systems)

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8.21 Primary pultruded structural elements in 10 ¥ 4.88 m2 (32 ¥ 16 ft2) short-span bridge at Laurel Lick, USA (Courtesy, Creative Pultrusions & West Virginia University)

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8.22 Pultruded slats and gratings for animal pens (Courtesy, Goldsworthy Associates)

8.23 Ocean pier, pultruded grating to accept 14 tonne, 3-axle truck (Courtesy, IKG Industries)

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Although on the basis of earlier chapters clearly very much an outline, the total constitutes not only a substantial body of products, but also serves well to provide sufficient and simple illustration as to the most probable future opportunities for pultruded profiles. There will be a broadening of the scope within each sector and again as already suggested many high-volume opportunities yet to be developed and commercialised. • • • • • • • • • • • • • • •

Agriculture – fencing and staging, grape stakes, irrigation flumes and plant tenting arches. Aircraft – flooring beams, structural elements. Automotive – sundry components. Building materials – window lineals, doors, door frames, panels and structural elements. Civil structures – bridges and bridge and high-rise building components. Communications – microwave towers and antennae. Consumer goods – tool handles and furniture. Corrosion-resistant applications – tankage, grating, hand-rails, ducting and staging. Electrical/electronic – hot-sticks, ladders, printed circuit boards, strain insulators, cable trays, switch gear springs and third-rail covers. Industrial – bearings, tubing and processing tanks, wiper blades. Marine – boat hull and superstructure members, fenders and sheet piling, and pier decks. Space – satellite and launch vehicle structures. Sporting goods – fishing rods, golf shafts, hockey stick handles and tent poles. Transportation – railcar, buses and all forms of ‘people mover’ components. Utilities – transmission towers, distribution poles and sub-station structural and associated elements.

Nevertheless, it is difficult to quantify or place any real limitation on the future markets and applications for pultruded profiles. To advantage, there is, as should be obvious, much reinforcement matrix processing–market application inter-relationship. In reality, the opportunities are so numerous that it might be simpler to identify those areas where pultrusions could never apply! In other words, any attempt to predict the future is at best perilous in such a rapidly developing and changing technology. However, there are specific areas of application and specific products within those areas that are worth extended comment. There will be, more extensively than in the past, markets that have never been exploited by traditional materials, those created by substitution for traditional materials, and yet other areas where pultrusion will become the actual enabling technology, thereby creating entirely new products and opportunities, where a

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market does not currently exist. The unique properties and processing of composites generally allow this type of exploitation. At the same time, both approaches raise a number of important factors that must equally receive associated consideration, because they demand greater recognition and acceptance by either the pultruder, the eventual customer or his or her professional adviser. Pultruded profiles are linear in nature and in searching for new, or future, applications, a respectable starting point is the linear nature of many everyday items or structures. For example, many furnishing units are composed of, or based on, an assembly of straight-line elements, whether they are table tops, cabinet shelves, door or window frames, floor or ceiling mouldings. Indeed a building is typically constructed from straight structural elements, typically reinforced concrete beams or slabs, both of which could be replaced by a single-piece pultruded panel having a low-density core in addition to provide thermal and acoustic efficiency. As has already been indicated (Chapters 3 and 6, Fig. 8.24)), such a panel could also be readily moulded with interlocking edge members capable of carrying the structural loads of the building and to provide ease of construction. Another related example is the unacceptable use of steel for the construction of high- and mid-rise buildings in areas of frequent seismic activity. When the ground movement is vertical, the stress risers created, for example, by two high-modulus steel members being hard line-welded are simply not capable of carrying the induced loads. However, relatively

8.24 IBM building

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low-modulus but high-strength structural members fabricated by pultrusion would attenuate this energy. Further, if the joint design has sufficient flexibility to allow those loads to be redistributed throughout the structure, then such a building construction would demonstrate a seismic resistance many orders of magnitude better than the welded steel alternative. Proof of this concept is already provided by the current growth in the use of composites to either repair deteriorating reinforced concrete structures, or improve their seismic resistance in some ‘add-on’ fashion as in column wrapping. In the final analysis, this has to beg the question why reinforced concrete was employed in the first place, and certainly to emphasise that the same mistakes should no longer be repeated! These are matters of not just better economics and a very much longer in-service life, but importantly they also bear directly on public safety. For these reasons there should be a concentrated drive by the political community, the building authorities and relevant professional disciplines to see that such jeopardy is removed. In another way, they are classic examples of the design and other problems where, in addition to building codes, industry standards, special interest groups and building trade unions all constitute seemingly impenetrable barriers to securing the tremendous market potential which undoubtedly exists for composites in civil engineering and infrastructure applications. That potential is both near and long term but because it represents such huge individual markets and such a diverse number of applications, it is difficult to focus on any single application or product area.As always, the problems involved in exploiting this situation are not so much technical as that of educating the civil engineering community. Like other professional disciplines, they have to be moved away from their traditional thought process, from the use of the conventional with which they are comfortable.Also they have to be made aware of the product and its spectrum of benefits before they can accept and ultimately employ pultrusions to advantage in solving their day-to-day needs. This situation is somewhat perplexing to the pultrusion manufacturer, for the gains in converting an existing structural element to composites are, through an extensive track record, now so obvious that there should be no hesitation in making that conversion. A prime example of this is the electrical industry’s reluctance to convert steel transmission towers to composites even though all the technical problems have been overcome and all the proof-of-concept testing completed (Fig. 8.7). Pure logic seems to say that it is totally illogical to hook 500 kV to a steel tower when a high-strength, environmentally resistant, nonconductive composite profile can do the same job, better! The fact that pultrusion is the only profile-producing technique that does not require high processing pressure can also be expected to be increasingly important in the future. As a consequence, pultrusion machinery is

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relatively light weight and capable therefore of being transported. In other words a capital- and material-intensive composites process, which readily permits, as suggested by Fig. 8.25, mobility in profile production. The possibility of hauling a pultrusion machine into orbit to produce the structural elements required for space station and power generation platform construction has already been demonstrated. Together with alternative earthbound industrial and habitat application opportunities, these examples are the most likely introductory candidates in what could become a burgeoning mobility area and a major infrastructure market of the future for pultrusion to secure. Another opportunity not open to traditional materials and processing techniques is the laying of pipe continuously from a totally mobile production platform such as a ship. It is impossible to lay steel pipe in deep water simply because the catenary weight of the pipe between the ship laying the pipe and the sea bottom is sufficient to cause the pipe to fracture. Irrespective of diameter and wall thickness, that does not apply for pultruded pipe simply because such pipe exhibits nearly neutral buoyancy in seawater. In addition, the production process is continuous, eliminating the costly process of joining discrete lengths, as with steel and producing joints that are very vulnerable owing to the imposed tensile loads.

8.25 Potential pultrusion machine for building space structures in orbit (Courtesy, Goldsworthy Associates)

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Pultrusion would therefore provide an ideal solution and laying such a pipeline from Algiers to France could, for example, also change the entire economy of the European nations by providing low-cost energy. Similarly, a pipeline from the Venezuelan oil fields to Curaçao would dramatically lower the cost of transporting oil to that major refinery. Further, modern oil exploration techniques have found huge reserves under deeper and deeper water and those reserves can only be brought on-line by the ability to lay deep-water pipe. Finally, such optimum mobility clearly permits pultrusion manufacturing plants to be built on-site and particularly in remote areas where the shipment costs of finished profiles is both prohibitive and time-consuming.

8.8

Processing potential

Turning now to applications related to processing changes, it is necessary first to emphasise that little change to the basic characteristics of the pultrusion machine is anticipated, although many different configuration changes will undoubtedly apply. Opinions already expressed make it clear that these will be particularly apposite in respect of thermoplastic matrix pultrusion. Initial developments already point up the theoretical capability of pultruding at speeds orders of magnitude greater than those currently feasible with thermoset systems. One possible configuration envisions a preheating oven of some 60 or perhaps even 90 m (200–300 feet) in length, upstream of the basic pultrusion machine. The increased residence time that this would allow would bring the thermoplastic polymer to the die already at the melt temperature, even though it is perhaps travelling at upwards of 30 m/min (100 ft/min). The purpose of the die would then be simply to shape and chill the chosen reinforcement/matrix system, in order, in effect, to freeze the composite into the desired profile shape. The heating technique employed for that oven could also prove to be critical. The use of electron beam, radiofrequency, or microwave and induction pre-heating techniques would, if employed in conjunction with thermoplastics that have been particularly optimised for one of these techniques, also of themselves result in marked reductions in processing time. However, as already considered, the ability to achieve these kinds of running rates is strangely enough not limited specifically by the matrix system, but rather by the ability of the reinforcement pack to withstand both the initial handling and the viscous shear and other loads generated at die entry or later in the process. Because some of these loads increase exponentially with speed, new forms of reinforcement, again as discussed earlier, have to be a primary area for future development and ultimately use. These must include the use of thermoplastic-based reinforcement fibre,

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as typified by the co-mingled polypropylene/glass varieties now being introduced by the major glass fibre manufacturers. Their use will, in addition, help to resolve the desirable manufacture of complex multi-ply reinforcements by heat-tacking oriented plies together with, for example, a heated ‘sheep’s foot’ roller. Because composites were largely born and nurtured in the airframe/aerospace industry, achieving optimum physical properties from the laminate has always been the driving force. As the surface area of the pultruded profile becomes ever larger, the probability is that the formability of the reinforcement pack will become more important than attaining the absolute optimum in respect of laminate properties. In these cases, the development of knitted fabrics as reinforcement would, although not answering every need, certainly enable the glass content to be varied over the area of the profile, permiting simpler in-feed tooling. It would also overcome the current need for a large number of space-consuming reinforcement supply racks. Moves such as these would have a direct effect on lowering manufacturing costs. A further extension of processing technology is the ability to pultrude continuous helical shapes. This could promote such application advances as grid-stiffened skin aircraft fuselages and the high-speed overlaying of cylindrical concrete structures offering better performance in seismic resistance or load-carrying rehabilitation. Many applications will require process developments other than those already discussed, which while interesting have as yet received little development attention. For example, other than to demonstrate its feasibility, commercial progress in respect of curved pultrusion has been limited, although as a process it offers the capability to open up entirely new fields of end product and thus application. Indeed the possibilities are so numerous as to prohibit detailed listing, but some of the obvious applications are in large diameter pipe and tankage, furniture elements, plus numerous sporting goods components, together with agricultural, vehicle and structural industrial building elements. Based on just the last two decades of world-wide pultrusion track record, there are undoubtedly many developments that will occur as the process continues to evolve.These will provide entirely new dimensions to the scope of applications. Future trends will almost certainly be in the direction of integrating multiple properties within a single element. A great deal of work is already going on in this area, but the possibilities have hardly been scratched. Pultrusion lends itself to this type of processing, owing to the practicality of feeding in various sections of the desired profile through separate injection dies, combining compatible resins to produce multiproperty as well as multicoloured profiles. This applies not only to the resin systems, but equally to

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the fibre types and the ability, for example, to feed low-density cores, printed papers, fabrics, veils and even metallics to achieve the desired property mix or aesthetics. This ability to achieve property and colour mixes within a single profile opens up marketing possibilities in virtually every application category, and in particular sectors which have never been considered as pultruded profile markets.

8.9

Summary

As most of the problems involved in getting new materials accepted involve changing traditions, improved success in the future could well benefit from a shift from the current defensive approach to one that is more proactive. In other words, instead of justifying the use of pultruded profiles on the basis of property comparison with traditional materials, there is need to emphasise that the latter were the only materials available at the time the product was conceived. Further, sufficient time was elapsed in the service life of such constructions, to prove in comparison to composites that they have very pertinent shortcomings. Many telling examples exist; some have been quoted. In summation then, the future success of these pultruded composite profiles and the speed at which they will further evolve are almost solely dependent on the composite industry’s ability to change the education of those in government authority and the graduate training of the professional architect, designer and engineer, rather than further technological development by either the composites or the pultrusion industry. What are needed are ‘decision makers’, with a thorough understanding not only of composites, but of pultruded profiles. Both must no longer be conceived as exotic alternatives having limited and expensive usage areas. Rather what is offered is just another structural material that happens to have a much greater scope of viable application, than the traditionals such as timber, steel, aluminium and reinforced concrete. That will ensure that composites, and particularly pultruded profiles (currently the fastestgrowing segment of that industry), will be the dominant structural material of the future. All of this serves to substantiate the belief that the pultrusion industry has only just begun its upswing on the evolutionary curve; it has every indication of continuing to be one of the fastest-growing industries in the global economy and will accelerate this growth pattern throughout the foreseeable future.

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Index

accelerator, def, xviii acrylic resins, 147–54 adhesive bonding, 240–1 anisotropic, def, xx applications, 71–3, chapters 6, 7 & 8 acrylic-based profiles, 154 airports, 198–200 bridges, 209–12, 244–9 buildings, 74, 222–5, 257–8 cableways, 200–3 cooling towers, 203–4 crash barriers, 249–50 curtain walling, 250 electrical, 215–18, 258–60 epoxy-based profiles, 144–6 fencing, 204–7 flooring systems, 207–9 future potential, 291–8 infrastructure, chapter 7 leisure, 212–14 mining, 252 optical fibre/tension member, 214–15 oil, gas and off-shore industry, 225–7, 252–5 phenolic-based profiles, 169–70, 280 railways, 215–18, 270 rebars, 256 rock-soil supports, 218–20 signposts, 257 stagings and walkways, 222, 255–6 towers, 258–60, 272 vehicle body panels, 227–30 water cooling towers, 203–4 water front, 252–5 water treatment and allied, 228 aramid, def, xxv fibre, 176–8 authors, xiii–xvii B-stage, def, xviii Barcol, hardness, def, xxi Bellagio Hotel, 212–14 binder, def, xxv

CFRP, def, xxvi CFM, see continuous filament mat C-glass, def, xxvi CSM, see chopped strand mat carbon, def, xxv fibre, 178–80 manufacture, 179–80 catalyst, addition, 120, 130, 133 def, xviii catenary, def, xxv chopped strand mat, application def, xxvi clamping arrangments, 38–9 cleaning, 241 collimation, 20, 25–7, 267–70 combined, blended or knitted fabrics, application def, xxvi composite, def, xxi composites, introduction, 1 applications, 2, 4, 15 & 16 fabrication techniques, 6–8 properties, 3–5 compression moulding, 7 computer aids, software, see design software contact moulding, 6 continuous filament mat, application def, xxvi continuous filament yarn, application def, xxvi continuous strand mat, see continuous filament mat contributors, see authors core, def, xxii count, def, xxvi coupling agents, def, xxvi creel racks, 19 cure, def, xxii custom profiles, 14, 220–1 cut-off saw, 40–1 defect, identification, 83–4 delamination, def, xxi

301

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302

Index

denier, def, xxvi design, development, 289–91 manual, 66 process and considerations, 73–8, 232 software, 88, 233, 289–91 die, cavity, 51 design, 27–30, 50–4 development, 264–75 fabrication, 54–9 heating, 27, 31–2 materials, 52–3 mounting, 30 plating, 27–8, 57–8, 61 pressure conditions, 44, 50, 136–7 reworking, 63 dimensional tolerances, 70, 78 E-glass, def, xxvii education, 230, 300 elastic limit, def, xxi end, def, xxvii epoxy resins, 125–47 EPTA, see European Pultrusion Technology Association European Pultrusion Technology Association, 10, 80 exotherm, def, xxii fabricator, def, xxii fabrics, 190–1 3D, 281–6 development, 281–6 fastening, adhesive, 240–1 mechanical, 235–8 snap-fit joints, 238–40 fibre, def, xxvii architecture, development, 281–6 fibreglass, def, xxvii filament, def, xxvii filament winding, 8 fill, def, xxvii fillers, application, 26, 82, 124, 133, 150–1, 162 def, xviii finishing treatments, 241 fire, hardness, 152–4, 152–4, 165–6, 280 retardancy, def, xix flow, def, xxii FRP, def, xxii gelcoat, def, xix gel-time, def, xxii GFRP, def, xxiii glass fibre, 181–93 manufacture, 182–4 properties, 183 GMT, def, xxiii Goldsworthy, Brandt, 8, chapter 8 GRP, def, xxiii

gripping arrangements, 37–9 guidance, fibre, see collimation hardening time, def, xxiii hardness, Barcol, def, xxi heat distortion, def, xxi honeycomb, def, xxiii impregnation, development, 265–7 inhibitor, def, xx isotropic, def, xxi jointing, see fastening Kolding bridge, 209–12, 248 laminate, def, xxiii lay-up, def, xxiii Lewis acids, 131 line speed, 20, 115, 136, 152 low profile additives, 122–3 machine, controls, 41–8 design and operation, chapter 2 developments, 63, 264–75 machining, 234 mandrels, 26–7, 59–60 market, 2, 14, 231 future, chapter 8, 291–8 materials, composites, xxi–xxv matrix, def, xx, see also thermoset and thermoplastics matrix, development, 278–81 impregnation, 22–5 shrinkage, 29 mats, 189–90 microwave heating, 32, 143 Modar resins, 147–54 modulus, xxi ortho-, iso and tere-phthalic acid, xx peroxides, 120 phenolic resins, 155–71, 280 pick, def, xxvii pigments, 124, 150–1 plating, 27–8, 57–8, 61 PMC, def, xxiii polishing, 57 polyester resins, unsaturated, 97–106 polymerisation, 119–21 balance, 43–5 def, xx epoxy resins, 130–2 phenolic resins, 158 polyester resins, 112, 116, 119–21 vinyl ester resins, 119–21 post-cure, 143–4 def, xxiii

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Index post-forming, def, xxiv pot-life, def, xx preforms, def, xxvii prepregs, def, xxiv process, characteristics, 90 development, 64, chapter 8 economics, 5, 74, 90–1, 231, 298–300 potential, 298–300 processing, acrylic based profiles, 149–54 epoxy based profiles, 132–7 phenolic based profiles, 163–5 polyester & vinyl ester based profiles, 112–18, 121–5 production techniques, composites, xxi–xxv profiles, applications, 15 & 16, see applications chemical resistance, 118 cost-effectiveness, 209, 211–12 et al custom, 14, 67 design, chapter 3 development, 264–75, 286–9 future opportunity, chapter 8 mechanical properties, chapter 3, 117, 138, 140, 141, 152–3, 168 physical properties, 80, 113–15, 138, 140, 167, 169 service life, 197–8 et al sizing, 33 specification, chapter 3 standard, 12, 67, 233 supply cost, 209 et al property, floor loading, 210 prediction, 84–9 pulling systems, 33–7 pultrusion, companies, websites, xxx–xxxi council USA, 78 development, chapter 8 equipment, 19–21, see machine history, 8–14 process conditions, 112–13 pultrusions, see profiles pullwinding, 49 quality control, 47–8, 81–4 R-glass, def, xxviii RP, def, xxv RTM, see resin injection RTP, def, xxv radio frequency heating, 32, 142 reinforcement, 21, chapter 5 architecture or pack, 19 def, xxv–xxix handling, 21–2, 25–7, 193–6 development, 275–8 release agent, 121, 133, 151 def, xxiv repair, 241–4 resin, impregnation, 22–5

303

manufacture, epoxy, 127–30 phenolic, 156–7, 160–1 polyester, 99–106 vinyl ester, 107–9 property comparison, 109–11, 115–16, 117, 129, 134, 149 resin injection, 7 resin-rich, def, xxiv roving, 185–8 def, xxviii S-glass, def, xxviii shrinkage, 122, 134 sizing agents, def, xxvi smart structures, 277 snap-fit joints, 238–40, 273 specification, 78–80 specific modulus, def, xxi specific strength, def, xxi specific stress, def, xxi spun yarn, def, xxviii standards, authoritative, 79, 92–6 standard profiles, 12, 233 staple fibre, def, xxviii stitched complexes, 190–1 strand, def, xxviii strand count, def, xxviii surface tissue, 192–3 def, xxviii take-off arrangements, 41 terminology, def, xviii–xxix tex, def, xxviii thermoplastic, def, xx resin, developments, 278–81 thermoset, def, xx development, 278–81 resins, chapter 4 tissues, see veils two, def, xxviii ultrasonic activation, 143 ultra-violet resistance, 116 unidirectional, def, xxviii veils, 192–3, 276–7 vinyl ester resins, 106–9 warp, def, xxviii website addresses, xxx–xxxi weft, def, xxviii wet-bath, 23–4 def, xxv wet-out, def, xxv woven fabric, def, xxviii woven roving, def, xxix yarn, def, xxix yield, def, xxix

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