Introduction to Automotive Composites

An Introduction to Automotive Composites

Editors: Nick Tucker & Kevin Lindsey

Rapra Technology Limited

An Introduction to Automotive Composites

Editors: Nick Tucker and Kevin Lindsey

rapra TECHNOLOGY

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2002 by

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2002, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library. Cover micrograph: Glass fibre mat taken by Kevin Lindsey. Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologize if any have been overlooked.

ISBN: 1-85957-279-0

Typeset, printed and bound by Rapra Technology Limited Cover printed by The Printing House, Crewe, UK

Contents

Introduction .......................................................................................................... 1

1

1

Who is this Book For? ........................................................................... 6

2

About Warwick Manufacturing Group .................................................. 6

3

About the Authors ................................................................................. 7

4

Acknowledgements ................................................................................ 7

What are Composites? .................................................................................... 9 1.1

Introduction ........................................................................................... 9

1.2

Why Composites have Useful Properties ................................................ 9

1.3

Why Add Fibres to the Matrix? ............................................................. 9 1.3.1

The Importance of Fracture Toughness .................................... 10

1.4

Performance of the Reinforcement ....................................................... 11

1.5

Categories of Composites .................................................................... 13

1.6

1.5.1

Natural Composites ................................................................. 14

1.5.2

Biocomposites .......................................................................... 17

1.5.3

Carbon-Carbon Composites .................................................... 17

1.5.4

Ceramic Matrix Composites (CMC) ........................................ 18

1.5.5

Metal Matrix Composites (MMC) .......................................... 19

1.5.6

Polymer Matrix Composites .................................................... 21

Why are Polymer Matrix Composites the Most Popular? .................... 21

References ..................................................................................................... 22 2

Polymer Chemistry and Physics ..................................................................... 23 2.1

Why do I Need to Know About Polymer Chemistry? .......................... 23

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An Introduction to Automotive Composites

2.2

What is a Polymer? .............................................................................. 24 2.2.1

Basic Description ..................................................................... 24

2.2.2

Branching and 3D Networks ................................................... 25

2.2.3

Copolymers ............................................................................. 26

2.3

The Consequences of Being Large ........................................................ 28

2.4

Chain Entanglement ............................................................................ 29

2.5

Intermolecular Forces .......................................................................... 29 2.5.1

Polar Interactions ..................................................................... 30

2.5.2

Other Interactions .................................................................... 31

2.6

Crystalline versus Amorphous Polymers .............................................. 31

2.7

Orientation .......................................................................................... 34

2.8

Property Variation with Temperature .................................................. 35

2.9

Flow Characteristics ............................................................................ 36

2.10 Commodity versus Engineering........................................................... 37 2.11 What do Trade Names Mean? ............................................................ 37 References ..................................................................................................... 38 3

ii

Composite Ingredients .................................................................................. 39 3.1

Introduction ......................................................................................... 39

3.2

Thermoplastic versus Thermoset.......................................................... 39

3.3

Thermosets .......................................................................................... 40 3.3.1

Thermoset Characteristics........................................................ 41

3.3.2

Polyester Resins ....................................................................... 42

3.3.3

Catalysts and Accelerators ....................................................... 43

3.3.4

Inhibition ................................................................................. 44

3.3.5

Other Admixtures .................................................................... 44

3.4

Thermoplastics .................................................................................... 44

3.5

Reinforcements .................................................................................... 46

Contents

3.6

3.5.1

Reinforcements, not Fillers or Extenders ................................. 46

3.5.2

Fibre Types .............................................................................. 47

3.5.3

Fibre Loading and Matrix Interaction ..................................... 51

3.5.4

Coupling Agents and Size Formulations .................................. 51

Core Materials ..................................................................................... 52

References ..................................................................................................... 57 4

General Properties of Composites - Stiffness, Strength and Toughness .......... 59 4.1

Introduction ......................................................................................... 59

4.2

How to Obtain Material Property Data ............................................... 60

4.3

Introduction to Methods of Obtaining Material Property Data ........... 61

4.4

Analytical Expressions for Deriving Composite Mechanical Properties.. 61 4.4.1

Continuous Random Reinforced Fibre Composites ................. 62

4.4.2

Woven Fibre Composites ......................................................... 63

4.4.3

Laminated Composites ............................................................ 63

4.4.4

Prediction of Strength .............................................................. 63

4.5

Laminate Analysis Software ................................................................. 64

4.6

Properties of Short Fibre Reinforced Composite Materials .................. 64 4.6.1

Stiffness of Short Fibre Composites ......................................... 65

4.6.2

Strength of Discontinuous Fibre Composites ........................... 66

4.7

Energy Absorption and Toughness ....................................................... 66

4.8

Composite Test Methods ..................................................................... 67

4.9

4.8.1

Determination of Composite Properties by Testing .................. 68

4.8.2

Reasons for Mechanical Testing ............................................... 70

The Effect of Manufacturing Methods ................................................. 71 4.9.1

Resin Transfer Moulding ......................................................... 71

4.9.3

Laminated Material ................................................................. 71

4.9.4

Compression Moulding ........................................................... 72

References ..................................................................................................... 74 iii

An Introduction to Automotive Composites

5

How Can We Use Composites in Car Manufacture? ..................................... 77 5.1

5.2

5.3

5.4

Introduction ......................................................................................... 77 5.1.1

What do We Mean by Composites? ......................................... 77

5.1.2

What Are Structural and Semi-Structural Elements? ................ 78

The Drivers for the Use of Composites ................................................ 79 5.2.1

Where Should We Save Weight? ............................................... 80

5.2.2

How Do We Save Weight? ....................................................... 81

5.2.3

Better Use of Existing Materials ............................................... 81

5.2.4

Alternative Lightweight Materials ........................................... 81

5.2.5

Alternative Manufacturing Methods ........................................ 82

Examples of Composites in the Automotive Industry .......................... 83 5.3.1

Composite Modular Front Ends .............................................. 83

5.3.2

Front Fenders (and Other Skin Panels) .................................... 84

5.3.3

Tail Doors ................................................................................ 85

5.3.4

Side Doors ............................................................................... 86

5.3.5

Fascias ..................................................................................... 87

5.3.6

Seating ..................................................................................... 88

Summary .............................................................................................. 89

Reference ...................................................................................................... 89 6

Manufacturing with Thermoset Composites ................................................. 91 6.1

iv

Manufacturing Methods ...................................................................... 91 6.1.1

Mixing reinforcement with resin .............................................. 91

6.1.2

Impregnating the reinforcement with resin .............................. 91

6.1.3

Consolidating the Product ....................................................... 92

6.2

Contact moulding ................................................................................ 92

6.3

Resin Infusion under Flexible Tooling (RIFT) ...................................... 93 6.3.1

The Advantages of RIFT .......................................................... 94

6.3.2

The Disadvantages of RIFT ..................................................... 95

Contents

6.4

Pre-pregging (autoclaving) ................................................................... 95 6.4.1

6.5

6.6

Compression moulding ........................................................................ 97 6.5.1

Injection moulding ................................................................... 98

6.5.2

Materials ............................................................................... 100

Resin Transfer Moulding ................................................................... 108 6.6.1

Low Pressure Hot Press ......................................................... 111

6.7

Structural Reaction Injection Moulding (SRIM) ................................ 111

6.8

Filament Winding .............................................................................. 112

6.9

Pultrusion .......................................................................................... 114 6.9.1

7

Consumable items in vacuum bag processing .......................... 95

Applications. .......................................................................... 116

Manufacturing with Thermoplastic Composites ......................................... 119 7.1

Thermoplastic Characteristics ............................................................ 119

7.2

Long Fibre - GMT ............................................................................. 120

7.3

7.2.1

Introduction ........................................................................... 120

7.2.2

Properties ............................................................................... 121

7.2.3

Processing .............................................................................. 122

7.2.4

Uses ....................................................................................... 123

7.2.5

Tooling .................................................................................. 124

Short fibre - Injection Moulding ........................................................ 124 7.3.1

Making composites by co-injection ........................................ 125

7.3.2

The Co-injection moulding process with Fibres ..................... 126

7.3.3

Co-Injection moulding stages [1-3, 9] .................................... 129

7.3.4

The Materials ........................................................................ 130

7.3.5

Moulding Procedure .............................................................. 130

7.3.6

The influence of Moulding Conditions on the dual injection process .................................................................... 131

7.3.7

Examples of 2K mouldings .................................................... 131

References ................................................................................................... 133

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An Introduction to Automotive Composites

8

9

Economics of Composites Manufacture ...................................................... 135 8.1

Introduction ....................................................................................... 135

8.2

Commercial Composite Cost Analysis ............................................... 135

8.3

Comparison of Materials ................................................................... 137

8.4

Parts Integration and Modules ........................................................... 139

What to do with Composites at the End of Vehicle Life .............................. 141 9.1

Introduction ....................................................................................... 141 9.1.1

Material Breakdown of a Typical Current Vehicle ................. 142

9.2

Recycling and Recovery from ELV ..................................................... 143

9.3

End-of-Life Vehicles ........................................................................... 144

9.4

Mechanical Recycling ........................................................................ 144 9.4.1

Thermoplastics ...................................................................... 144

9.4.2

Thermosets ............................................................................ 147

9.5

Chemical Recycling ............................................................................ 148

9.6

Thermal Conversion Technologies ..................................................... 150 9.6.1

Pyrolysis ................................................................................ 151

9.6.2

Hydrogenation ....................................................................... 151

9.6.3

Gasification ........................................................................... 152

9.7

Energy Recovery ................................................................................ 152

9.8

Automotive Shredder Residue ............................................................ 153

9.9

Creation of a Recycling and Recovery Infrastructures ....................... 154

9.10 Design For Disassembly and Recycling .............................................. 154 9.11 Developing Recyclate Markets ........................................................... 155 9.12 Developing a Recycling and Recovery Infrastructure ......................... 155 9.13 Conclusion ......................................................................................... 156 References ................................................................................................... 157

vi

Contents

10 The Future of Composites ........................................................................... 159 10.1

The Advantages of Composites ....................................................... 159

10.2

Future Vehicle Manufacture - Hypercars ......................................... 160

10.3

How Do We Achieve 300 mpg Plus? ............................................... 160

10.4

What is the Technical Feasibility? ................................................... 161

10.5

Economic and Market Environment ............................................... 161

10.6

What Stops us From Building Hypercars Now? .............................. 162

10.7

Costs ............................................................................................... 163

10.8

Summary and Comment on the Feasibility of Hypercars ................ 163

10.9

What Will the Next Generation be? ................................................ 164

10.10 Composite Front Ends .................................................................... 164 10.11 Doors and Tail Doors ..................................................................... 165 10.12 Bonnets and Front Zones of Vehicles .............................................. 166 10.13 Structures ........................................................................................ 166 10.14 Summary ......................................................................................... 167 References ................................................................................................... 168 11 Design Guidelines for Composites............................................................... 169 11.1

Why Composites? ........................................................................... 169

11.2

Choice of Materials ......................................................................... 171

11.3

Getting What You Want .................................................................. 171

Bibliography ...................................................................................................... 173 Abbreviations and Acronyms ............................................................................. 177 Contributors ...................................................................................................... 181

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An Introduction to Automotive Composites

viii

Introduction Leaving aside the business of bricks, straw, and industrial relations mentioned in the Old Testament (Exodus 5), because on the technical side it is probably related with the need to dry thick sections evenly rather than structural considerations. It is arguable that the history of artificial composites started with the invention of drawn glass fibre by the Egyptians in about 1500BC. Moving on into what is now Greece, Athenian Hoplites (about 500BC) used resin bound linen to make the Lineothorax, a tough and light cuirass type body armour. Across the world, in late medieval Japan, lacquered Chinese water buffalo hide and papier-maché was used to a visually dramatic similar effect. There followed a bit of a developmental quiet period, in terms of our subject matter, until the end of the 19th century when the motor-car, or at least Gottleib Diamler’s hot-rod tricycle was conceived. In the late 1930s the story really begins to take off. At this time Henry Ford attempted to use Soya oil to produce a phenolic resin and thence to produce a wood filled composite material for car bodies (Figure 1). Meanwhile, in the UK, there were discussions as to whether the early glass fibres were more appropriately employed in the manufacture of soft furnishings, and perhaps you mightn’t do better sticking with fibres about which more was known such as flax - a flax reinforced Spitfire fuselage was made in the 1940s at Duxford, Cambridgeshire.

Figure 1 Henry Ford and his composite car

1

An Introduction to Automotive Composites In the 1950s when glass fibre reinforcement material and cold setting polyester resins became commercially available, this put the manufacture of compound curved streamlined automotive bodies into the reach of low volume, low capital companies. The first use of composites by a high volume manufacturer was probably the 1954 Singer Hunter (Figure 2).

Figure 2 1954 Singer Hunter - GRP bonnet and side valences (Source: The North American Singer Owners Club)

The early models featured Glass Reinforced Plastic (GRP) bonnet and side valences, made by Marston Excelsior Ltd. of Wolverhampton: GRP was chosen to reduce the weight of the vehicle. By the beginning of the 1960s the low volume car producers were producing structural monocoques in hand laid GRP – examples are the Lotus Elite, and the Rochdale Olympic (Figure 3). These craft level wet hand lay up methods were the mainstay of composite production throughout the nineteen sixties, limiting their application to low volume high value specialist sports car manufacture - for example the Reliant Scimitar (Figure 4). The interest of the volume automotive industry in composites is a more recent tale linking chemistry, materials science, manufacturing technologies, technology transfer and entrepreneurship over the last half century or so. Since the 1950s Rover Group has

2

Introduction

Figure 3 The hand laid Rochdale Olympic was a composite monocoque (Source: www.rochdale-owners-club.com)

Figure 4 The Reliant Scimitar (GTE SE6a shown here) had a hand laid body supported by a steel chassis (Source: Nick Tucker)

produced a composite rich demonstrator vehicle about once a decade, possibly the high water mark of these efforts was a composite monocoque Mini that actually went into production in Chile. However, composites have not been widely adopted in high volume car manufacture until relatively recently.

3

An Introduction to Automotive Composites

Figure 5 The Pontiac Fiero – mass production composite intensive body (Source: www.pontiacfiero.com) In the late 1980s the Pontiac Fiero (Figure 5) laid a good claim as the first mass production composite intensive car body. The Fiero had a space frame chassis and a body using a number of different types of composites. The high performance (and cost) image of composites has lead to amusing spin offs such as the manufacture of polypropylene mouldings for the juvenile market that look ‘just like carbon fibre’. These articles are of course limited to non-structural applications such as air ducts and trim pieces. It is now the right time to reconsider the role of composite materials in volume car manufacture. Some of the reasons for this are technical; such as the availability of appropriate manufacturing technologies like resin transfer moulding and structural reaction injection moulding. Some are marketing considerations - in a very competitive market, the struggle to add more features to cars without returning to the bulk and girth and fuel consumption of the 1950s American saloon (Figure 6) (the current VW Golf weighs about 11/2 times the mark 1 model). There are also other more worthy reasons, encapsulated in the following quote from a particularly far sighted senior manager at Rover-BMW group: ‘Currently the whole of the motor industry has a major focus on weight and weight reduction. The main reasons for this are first, as car manufacturers, we have a concern

4

Introduction

Figure 6 The 1955 Ford LaTosca concept car – lightweight not first on the list of design priorities (Ford Motor Company)

for... the effect the motor vehicle has on the environment and second, there is legislation to ‘encourage’ us in this direction... To make a body shell as light as possible, low density, high specific property materials should be used, which could lead to the use of polymer matrix composites for the entire body structure.’ D.A.Jeanes ‘The Virtuous Weight Spiral’ a paper given at the IBCAM Vehicle Technology Conference Advanced Strategies for Vehicle Mass Optimisation, 1997. This belief provided the justification for the Warwick Manufacturing Group (WMG) to run a sequence of ‘composites awareness’ courses, championed by David Lusty during 1998-1999 for interested Rover and BMW Group designers and manufacturing engineers. It was felt that whilst the volume car industry had a clear and demonstrable facility in design and manufacture with metal and plastic, there was not yet a widespread culture of working with so called ‘advanced materials’, and in particular polymer matrix composites. The courses were aimed at inoculating Rover Group with an appreciation of composites, materials properties, manufacturing technologies, and the wider implications of composites use. This, in turn, would make the incorporation of composites into automotive manufacturing, a more comprehensible option for the company. In the late 1990s, Rover Group (moving later into the BMW Group phase) was working very closely with researchers at the Warwick Manufacturing Group at the University of Warwick. The collaboration (first known as EPIC – Engineering Polymers Integrated

5

An Introduction to Automotive Composites Capability), and then SALVO – Structurally Advanced Lightweight Vehicle Objective) had the aim of providing information on new materials, manufacturing technologies, and facilitating the integration of such materials and technologies into volume automotive manufacturing in the new millennium. The courses were written and run by researchers from Warwick Manufacturing Group and engineers from Rover Group. The course was supported by a ‘sourcebook’ This book covered the areas of the lectures given during the course in some depth and also provided references and select bibliographies to allow the participants to gain further insight into the subject. In the light of continued interest in composites as suitable materials for high volume production cars the information, the book has been rewritten from original source material for a wider audience.

1 Who is this Book For? Curiosity may kill cats, but it is the life-blood of innovative engineering, says Professor Ashby (Who he? See the bibliography). This book is aimed at people who need to innovate in the use of materials in car manufacture. This includes car industry workers in manufacturing and design, as well as students who propose to study or work in the automotive industry. Choice of materials in industry is never merely a technical matter; cost factors of raw materials, manufacture and end of life disposal must also be considered. The book should enable the reader to appreciate something of the benefits and difficulties of working with composites in the mass market. The reader should ideally have some knowledge of elementary chemistry, materials science and the automotive industry, but if you haven’t, don’t lose heart: the authors have tried to introduce these areas in as painless a way as possible.

2 About Warwick Manufacturing Group Since it was founded in early 1980s by Professor S.K. Bhattacharyya, WMG has adopted a partnership approach, involving industry closely in the delivery of its technology transfer, training and research programmes. Typical of this approach is the joint effort between Rapra Technology Ltd and the Warwick Manufacturing Group to run the EPSRC/DTI funded Faraday Plastics Partnership. Faraday Partnerships are part of a Government initiative to bring industry, particularly SMEs, and academia closer together – to accelerate technology transfer and create a more competitive industrial base.

6

Introduction The Group enjoys long-term relations with over 500 companies world-wide. Knowledge transfer programmes include international, (e.g., ERDF), national, (e.g., Teaching Company Schemes), and regional programmes, (e.g., Regional Challenge), with a particular emphasis on aiding SMEs and supply chains. WMG has pioneered new ways of working with companies to define and deliver post experience education. Currently, over 4000 people a year from over 400 companies are involved in training aimed at revitalising management of manufacturing and process based companies. The Group has a strong track record in awareness activities. Each year over 3500 visitors are received from industry and professional bodies in the UK and abroad. WMG is recognised as a DTI Advanced Manufacturing Technology/Computer Integrated Manufacturing Centre. Since WMG also runs a number of full and part time masters degrees, it is uniquely positioned to run courses for partner companies which can present the current state of the art in manufacture, with specific elements covering future developments specifically relating the needs of partner companies.

3 About the Authors This book was written and compiled by a group of engineers, scientists and technologists working for the Warwick Manufacturing Group of the University of Warwick, and various high, medium and niche volume car manufacturers. The contents are based on research, notes, academic papers and technical reports written by the group.

4 Acknowledgements By way of expressing our debt of gratitude, the editors would like to acknowledge the contributions of our co-authors and a considerable group from behind the scenes, amongst whom are: Frances Powers, our divine Rapra editor Claire Griffiths, our editorial assistant Steve Barnfield who typeset the book and designed the cover Sally Humphreys, who originated the project at Rapra Caroline Barlow who produced the index David Lusty, who championed the Rover composite courses …and our partners and families who, on the whole, coped magnificently.

Nick Tucker and Kevin Lindsey March 2002

7

An Introduction to Automotive Composites

8

1

What are Composites? David Britnell, Rebecca Cain and Nick Tucker

1.1 Introduction This chapter introduces composite materials by discussing why they exhibit their desirable properties of lightness, stiffness, strength, and toughness. This leads into a discussion of the various types of composite, and why polymer matrix composites predominate.

1.2 Why Composites have Useful Properties The materials scientist’s definition of a composite says that a composite material contains a chemically and/or physically distinct phase distributed within another continuous phase and exhibits properties that are different from both of these. Historically, composite materials have been used for centuries in one form or another, for example, straw reinforced mud bricks, paper and concrete. Wood and bone are an example of naturally occurring composites. Artificial composites are based on fibrous materials bound together with resins, one of the earliest synthetic types being glass reinforced polyester. Composites are especially attractive for their high specific strength (stress/density or σ/ρ) and modulus (stress/stiffness or σ/Ε) and low density (mass/volume or m/v). This has led to great interest in their research and development in the past few decades, particularly for aero applications where additional payload capacity and engine efficiency are paramount. In more recent times the automotive industry has shown interest in structural (primary load carrying members) composite materials for similar reasons, although the effects of exhaust emissions on the environment has provided the impetus for the lightweight vehicle of the future. Composites also offer numerous advantages over many traditional materials like metals, including low density and enhanced corrosion and temperature capability.

1.3 Why Add Fibres to the Matrix? Some monolithic (consisting of a single material) materials that are isotropic (having uniform physical properties in all directions) and homogeneous, such as thermosetting

9

An Introduction to Automotive Composites polymers, e.g., epoxies, polyesters, and phenolics, intermetallics, e.g., titanium aluminide and molybdenum disilicide, and ceramics, would make ideal engineering materials in terms of strength or wear resistance, but often exhibit brittle behaviour and low fracture toughness (as, for example, does window glass) leading to catastrophic failure. In other words, such materials do not deform with a large degree of plasticity at room temperature. Failure typically occurs by propagation of cracks nucleated from processing defects such as pores or from surface abrasions such as scratches or machining marks. Failure of brittle materials has been well characterised after the Griffith [1, 2] theory of 1920 which identified that the strength of a material was dependent on the size of the flaws it contained.

1.3.1 The Importance of Fracture Toughness The resistance of a material to crack propagation is described by its fracture toughness (Kc) and is the value of critical stress necessary to propagate a crack. The lower the value of this parameter the more brittle the material. It is thus possible to enhance the ultimate tensile strength of a material (UTS) by either changing the critical flaw size through processing, for example, or by changing the fracture toughness via microstructural modifications to form a composite structure or toughening mechanism. Various toughening techniques have been utilised including phase transformation, microcrack, particle toughening and toughening by the addition of fibres and/or whiskers. Most of these are limited to specific materials or chemistries. The most versatile method by far of improving the fracture toughness is through the addition of fibres. Glass and polyester, for example, both have a Kc of ~ 0.7 MPam1/2 whereas a 50% glass filled polyester composite has a fracture toughness of ~ 50 MPam1/2. The mechanism by which the fibres toughen polymeric matrices is shown in Chapter 4. Fibrous composites may be manufactured using a vast range of matrices and reinforcements and their properties can be altered substantially by changes in constitution and quantity of the respective components. When selecting materials for use in composites, of prime importance, are thermal expansion matching, density, creep behaviour, modulus and the chemical compatibility between the fibre and matrix and also the composite components and the environment. Of additional importance are the ability of the reinforcement to withstand the processing conditions and the ability of the matrix to infiltrate the fibres adequately. One of the most significant tests of composite quality is that the composite does not fail in a catastrophic manner but gracefully, in a tough manner, much as metals do. Figure 1.1 is a schematic stress/strain plot illustrating the desired failure characteristics of a composite in contrast to a brittle monolithic material.

10

What are Composites?

Figure 1.1 Schematic stress/strain plot for a composite with a high fracture toughness and a brittle material with a low fracture toughness

It can be seen in Figure 1.1 that initially, there is a linear portion to the curve and a subsequent load drop corresponding to the matrix cracking. It should be noted that this does not result in failure of the composite. As the stress is increased the matrix continues to crack and in so doing absorbs energy. Progressively more and more of the load is transferred to the fibres until at the maximum stress (σmax) they carry all of the load. After this maximum the fibres begin to break but continue to dissipate energy by pulling out of the matrix, resulting in a work of fracture many times that of its monolithic counterpart, as previously noted. The way in which load is transferred between the matrix and fibre in the composite is a very important parameter in attaining composite toughness.

1.4 Performance of the Reinforcement Composites may be broadly classified according to the nature of the matrix phase - for example, metal matrix composite (MMC), polymer matrix composite (PMC) or ceramic matrix composite (CMC). The scope of this discussion will be limited to those composites that have brittle polymer matrices. Reinforcements may be in the form of single crystal whiskers, platelets, long fibres, short fibres, small particles or precipitates (or a combination of any of these) which are typically supported in a less brittle matrix. Materials in the fibrous form exhibit much greater strengths than in any other form, and furthermore the smaller the diameter of the fibre the greater the strength [3], see Figure 1.2.

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An Introduction to Automotive Composites

Figure 1.2 Reducing fibre diameter increases tensile strength (after [3])

This may be explained first by preferred orientation produced during drawing or spinning of fibres and secondly by considering failure as a statistical process induced by flaws (briefly mentioned in the introduction). Quite simply, the smaller the fibre the less likelihood of it containing a critical flaw. In view of this fact, it is crucial to composite integrity that the surface of the fibre be protected from damage during fabrication. Hence, a sizing agent of some description is applied, which not only must be compatible with the fibre but also with the matrix material for optimum bonding. In polymer composites this protective coating is the foundation of the fibre/matrix interface which has a very important function and is highlighted in more detail in a following section. The purpose of the matrix is to allow the fibres to be formed into a useful shape, to transport load to the fibres throughout the composite (via an interface) and to protect them from damage and corrosion. Fibrous reinforcements are available in a variety of diameters but generally fall into the range of 5 to 100 μm. Reinforcements are available in the form of long or short fibres in a variety of architectures including continuous and discontinuous mats and woven rovings as well as chopped and continuous strands or laminates. The diameter and length very much dictate the format of reinforcement (and properties). Large diameter fibres, such as silicon carbide or sapphire monofilaments are formed into shape by filament winding for example, where the bend radius on the fibre is shallow. Small diameter fibres such as glass can easily be formed directly into complex moulds as mats or rovings or if discontinuous may be injected into the cavity. Fibres will generally fall into four categories; glass, ceramic, metal or polymer, as do matrices. The processing route of the composite 12

What are Composites? is usually dictated by the matrix material. Production of polymer composites invariably involves low temperature/low pressure compression and injection of resin into a cavity whereas metal and ceramic matrix composites require high pressure and/or temperature sintering or melting operations.

1.5 Categories of Composites This section introduces some of the various types of composites that exist today. Composite materials are not new or even a human invention, most natural biological materials are composites. Examples include bone, horn, insect shells, plants, teeth and wood. The microscopic structure of these biomaterials are generally very complex and relatively little is still known about them. The mechanical properties of these materials, however, are known and compare very favourably with artificial composites. In some cases the strength to weight ratio of natural composites is better than for some artificial composites. Nature has, however, had millions of years to get the design of these composites right. Artificial composites, like straw bricks and concrete, on the other hand have only been around for hundreds, or at most a few thousand years, with the high performance artificial composites only being developed over the last three to four decades. Composites may be further categorised by their constituent materials or by the processing routes, in some instances these two nomenclatures are confused so that a material is generally referred to by it’s processing route and vice versa. This section will concentrate on the processing routes and the overall processing technologies, rather than details of the constituent materials (see Chapter 3). It would be useful at this stage to aid the discussion by introducing the generic polymer types. •

Thermoplastic composites (compared to thermoset composites): can be remelted without damage to the material have higher thermal expansion coefficients generally higher material costs generally have lower working temperatures generally have poorer solvent resistance can be economically recycled (although the reinforcements may present problems)

•

Thermoset composites (compared to thermoplastic composites): can’t be remelted without damage to the material have lower thermal expansion coefficients generally lower material costs

13

An Introduction to Automotive Composites generally have higher working temperatures generally have better solvent resistance can be recycled (economics of which are somewhat questionable)

1.5.1 Natural Composites Most of the structural elements found in nature are composites. Examples are wood, horn, and the shells of crustacea. Animal composites have reinforcements made from polysaccharides or calcium salts, matrix materials are often proteins, e.g., collagen, elastin and keratin in bones and teeth or complex phenolic compounds in hard insect shells. Bone has brittle high modulus apatite crystals as reinforcement, bound together with ductile, organic collagen fibres. The bones of birds attain the stiffness required to support the muscle loads required for flight without weighing too much, by means of a honeycomb cored structure. In the plant world cellulose and lignin predominate. Bamboo has a cellulose matrix, with silica reinforcement giving high rigidity and high impact strength; properties exploited in the early days of aircraft manufacture and to date by the makers of kites, and as scaffolding. The microscopic structure of these materials is always complex, and when compared to artificial equivalents the mechanical performance is usually very good. Note that the world’s largest aircraft, the Hughes Corporation ‘Spruce Goose’ was built in the 1940s of wood.

Figure 1.3 The Hughes Corporation Spruce Goose (Source: Brian Lockett) 14

What are Composites? Natural composite structures demonstrate an efficiency of design and manufacture that as workers with artificial composites we do well to study and if possible, emulate. Consider wood as a structural composite: trees can resist the elements for centuries, requiring low energy inputs for growth and maintenance, further at the end of their useful life recycling routes are efficient and well established. The plus points for wood can be summarised as: •

Low energy production methods – just grow it!

•

High work of fracture – weight for weight, as good as ductile steel and better than some artificial composites.

•

Tensile strength – up to five times the strength of steels in common use

•

Resistant to stress concentrations – careful nailing does not measurably reduce the strength.

•

Recyclable – as a natural material it is easy to recycle on the compost heap.

The minus points for wood are: •

Slow to grow – cycle times are in the order of decades, therefore careful long-term planning is essential.

•

Requires seasoning – after felling wood must be dried at a controlled rate to avoid cracking.

•

Limited scope for producing shaped articles without fabrication – perhaps ship’s masts are the only example.

•

Labour intensive – requires skilled craftsmen for best results.

•

Suffers long-term creep – will distort under load, particularly in wet conditions.

•

Prone to rot – may start recycling too early.

In automotive applications, wood has been used, albeit in near homeopathic quantities, as burr walnut and black ash veneer for interior trim. In structural applications, the Morris Minor Traveller (Figure 1.4) sported an external ash wood space frame, strangely redolent of the 1950s Ford ‘Woody’, the structural nature of the frame allowed the possibility of an MOT failure due to rot: note that in the visually similar Mini Countryman, the ash frame was not structural. In the niche market, Morgan Cars (Figure 1.5) use a structural ash frame for their sports cars. However, it is unlikely that wood will feature as a structural element in modern high production volume cars.

15

An Introduction to Automotive Composites

Figure 1.4 Morris Minor Traveller – external ash wood space frame (Source: Alix Powers-Jones)

Figure 1.5 The Morgan – internal ash wood space frame (Source: The Morgan Motor Company)

16

What are Composites?

1.5.2 Biocomposites Biocomposites include natural structural materials such as wood, but can also include artificial composites made with synthetic resins and reinforcing fibres such as jute, banana fibre, coconut fibre, and bamboo fibre. Rice husks can be used as the basis for the manufacture of silicon carbide whiskers, and research is being conducted in the use of spider silk as a fibrous reinforcement. The strength of spider silk is in a similar range to that of aramid, e.g., DuPont Kevlar, fibres. Biocompatible bone substitutes (Hapex; Smith & Nephew) are made by producing a polyethylene-hydroxy-apatite composite. This material has similar mechanical properties to bone, and is sufficiently near to bone in a biological sense for the surrounding bone to grow into the artificial material. Applications include long life artificial hip joint mountings and artificial inner ear bones which can restore hearing to the profoundly deaf. Biocomposites are attractive because of the low environmental impact of their product life cycle, they can be locally produced, and they do not add to the net level of CO2 in the atmosphere; this is because they can be composted at the end of their life. As yet however, biocomposite materials have made limited commercial impact due to a number of factors, such as: •

High cost of matrix materials

•

Concern over the controllability of the degradation process

•

Uncertain raw material properties

•

Disappointing material properties of composites cf. theoretical predictions

•

Uncertain economics

•

Difficulties in breaking into the mature artificial composites market

Mercedes, Daimler Chrysler and BMW, use a flax reinforced polypropylene for some non-structural interior door panels, but as yet the performance of these materials is insufficient for structural parts, and the fossil origin matrix is not biodegradable. The emphasis of research and development in the past half century has been on artificial fibres and resins, and a matching effort will be required to advance structural biocomposites into commercial availability. The driver for this is the attractive proposition of being able to compost the materials at the end of life.

1.5.3 Carbon-Carbon Composites Carbon-carbon composites are made from carbon fibres embedded in a carbon or graphite matrix. Graphite does not melt, it sublimes, passing directly from the solid phase into 17

An Introduction to Automotive Composites the gaseous. This physical change occurs at about 3500 °C. Carbon-carbon composites have a density of one-quarter that of steel, a high specific heat capacity and thermal stability in a non-oxidising atmosphere up to 2500 °C (mechanical properties tending to fall off after this point). In an oxidising atmosphere, the material burns to produce extraordinarily expensive CO2. The matrix is insinuated into the reinforcing material either as a liquid or by deposition of a chemical vapour. The material is densified in a controlled atmosphere at high temperature to produce the composite material. Due to the wide range of reinforcement configurations and processing techniques, the range of properties available in carboncarbon composites is very large. World production in 1995 amounted to about 500 tonnes, most of which was used for aircraft disc brakes. Figures quoted by Kelly [4] show that the carbon-carbon disc brakes used on Concorde save about 600 kg compared to the steel equivalent. When used in racing motorcycle brakes there is some difficulty in getting the brakes hot enough to work, and it is the practice to do the first lap or so with the brakes on to bring them up to operating temperature. It is reported that the operating range of such disc brake is from a lower limit of 200 °C to an upper limit of 400 °C, it is therefore unlikely that such materials would be satisfactory for road use. In the Russian Federation, the NIIGrafit company exploit the biologically inert nature of carbon-carbon composites to make wound dressings. Other applications include throats, nozzles and thrust tubes for rocket motors. The high cost and exotic performance conditions of carbon-carbon composites are likely to limit the automotive application of these materials.

1.5.4 Ceramic Matrix Composites (CMC) Ceramic matrix composites are reinforced for fundamentally different reasons to those for polymer and metal matrices. Whereas polymers and metallics are reinforced for increased strength and modulus, ceramics are reinforced to increase their toughness and damage tolerance. Consider the strength of bone china: the Wedgewood company used to demonstrate the compressive strength of its bone china tea cups by balancing a London Transport Routemaster bus (weighing approximately 11,000 kg) on six tea cups (Figure 1.6). However, the reader will be familiar with the effect of rapid sequential compressive and tensile loadings on ceramics (as for example by dropping a teacup onto a hard surface). So, ceramic materials suffer in structural applications not from intrinsic weakness, but from their low flaw tolerance and brittle nature, otherwise ceramics are ideally suited for

18

What are Composites?

Figure 1.6 The compressive strength of ceramics (Source: Josiah Wedgewood and Sons, Ltd.)

work in high temperature and chemically aggressive environments. The properties can be improved by the addition of reinforcements; typical reinforcements used are either continuous fibres or discontinuous reinforcements of fibres, whiskers or particulates. Since the 1950s attempts have been made to develop whisker-reinforced ceramics (often called CERMETs). Examples are Noroc-33 developed in the 1970s by the Norton Company in USA comprising a silicon carbide matrix reinforced with silicon nitride particles. The heat resisting tiled surface of the space shuttle is made from cermet tiles. Other applications are cutting tool inserts, and a number of other aerospace applications that are still in the R&D phase. As with carbon-carbon composites, the expense and difficulty of manufacturing are likely to limit the automotive application of these materials. However, CMC cutting tool inserts, with their tolerance of high cutting speeds are in common use in the metal shaping processes.

1.5.5 Metal Matrix Composites (MMC) MMC are produced by the addition of reinforcing particles to molten metal (often aluminium), by the infiltration of molten metal into a particulate or fibrous preform (usually silicon carbide), or heating a mixture of reinforcement and powdered metals. MMC out

19

An Introduction to Automotive Composites perform metals in terms of wear resistance, better elevated-temperature properties and creep resistance. They also have low coefficients of thermal expansivity. They suffer from high costs, complex fabrication methods, and limited service experience. Applications tend towards the exotic; the antenna booms on the Hubble Space Telescope are made of a MMC. They make a good candidate for power electronics packaging due to high thermal conductivity, low thermal expansivity, and good mechanical properties. If hybrid electricalinternal combustion power trains become popular, designers may well specify MMC for power train power electronics. The usual mode of failure for high power semiconductors is the detachment of the semiconductor element from its heat sink. The electronics then overheat and fail. This mode of failure is due to strain caused by a mismatch in coefficients of thermal expansion between the semiconductor elements and the heat sink material (usually an aluminium extrusion). If low coefficient of thermal expansivity MMC is used, the probability of this mode of failure is much reduced, with a consequent increase in reliability. MMC are beginning to make an appearance as disc rotors for high performance motorcycle brakes, and on concept cars. The desired properties for this application are good thermal conductivity, and wear resistance coupled with light weight.

Figure 1.7 Light weight front corner module with Metal Matrix Composite brake disc (Source: www.delphi.com)

20

What are Composites?

1.5.6 Polymer Matrix Composites Polymer matrix composites amount to 75% of the world composite market by value or by tonnage. The technology for manufacturing both raw materials and finished articles is the most mature of all the composites. This is why the rest of this book will emphasise polymer composites, the reasons for the maturity of the polymer composite market will be discussed in the next section.

1.6 Why are Polymer Matrix Composites the Most Popular? ‘Composites have turned out to be a fantastic technological success and a total financial failure to date’ says John DeVault, President of Hercules Inc., the major US producer of carbon fibres and advanced composite components [5]. The question is why are some parts of the composite market relatively underdeveloped. Marsh [6] attributes the low growth of the advanced composites market to a mis-match in the efforts put in to the areas shown in Figure 1.8.

Figure 1.8 Components needed for use of composites in manufacturing

Comparatively large investments have been made in materials development, and industrial scale methods to manufacture the materials and articles made from the materials. Developments in these two areas have not been matched by the introduction of products requiring the use of advanced materials. Large companies, such as Hercules have invested large sums of money in advanced composite development, but have yet to see significant returns as the market is still not

21

An Introduction to Automotive Composites mature. Even so, we are however not actually in a period of decline, but a period of consolidation where the initial take up of advanced composites will be slow, but as more and more successful products enter the market, the materials market slims down to a smaller number of key materials, and designers gain a better appreciation of the capabilities of these materials, the rate of up take will increase. The comparative success of advanced polymer composites compared to the non-polymer end of the market may be because the manufacture of polymer composites by craft methods, e.g., canoes by hand lay-up, was already well established, and the methods for making advanced polymer composites are usually developments of pre-existing manufacturing techniques, whereas the manufacturing technologies for the non-polymer matrix composites are novel, and often do not relate to any existing technique. Hoover [7] says that the argument that the high cost of advanced composites (MMC in this case) limits their use, and that limited use and low production volumes in turn keep the price high is simplistic, and that even if production volumes are raised, prices will not fall. The reason for this is that the relatively underdeveloped production methods do not show sufficiently large economies of scale to show significant reductions in price. It is therefore likely that polymer matrix composites will continue their domination of the market in the medium term at least.

References 1.

A.A. Griffith, Philosophical Transactions of the Royal Society, 1921, 163.

2.

J.E. Gordon, The New Science of Strong Materials or Why You Don’t Fall Through the Floor, Pelican, Harmondsworth, UK, 1984.

3.

F.O. Anderegg, Industrial Engineering Chemistry, 1939, 31, 37.

4.

Concise Encyclopedia of Composite Materials, Ed., A. Kelly, Pergamon Press, Oxford, 1989.

5.

Wall Street Journal, 26th August, 1992.

6.

E. Marsh, A Profile of the International Advanced Composites Industry, Elsevier Advanced Technology, 1994.

7.

W.R. Hoover, Proceedings of the 5th Annual ASM/ESD Advanced Composites Conference, Dearborn, MI, USA, 1989, 211.

22

2

Polymer Chemistry and Physics Robert Coates

Synthetic polymers, or plastics as they are commonly referred to, are a relatively new class of materials. Although first developed toward the end of the nineteenth century, when much to the relief of the African Elephant population (in 1864, 8000 elephants were slaughtered to provide ivory for the billiard tables of the UK alone), Parkesine or Celluloid began to replace ivory for billiard balls, and the market for elephants foot umbrella stands began it’s terminal decline. The chemistry of these materials really wasn’t well understood until as late as the 1920s when Staudinger, building on the earlier work of Pickles, proposed and tested the hypothesis that substances (his colleagues referred supportively to his work as ‘schmierenchemie’ or greasy chemistry) such as cellulose, starch and rubber were in fact long chain molecules [1]. The cut-off of key natural raw materials during the Second World War helped spur on the technology to develop synthetic replacements for materials such as rubber. So-called Buna (styrene-butadiene copolymer) rubbers were developed almost overnight [2] as replacements for natural rubbers in what is the most commonly used automotive composite part, the tyre. In the 1950s, a now well established plastics industry turned toward consumer markets as an outlet for its products [3]. Advancement in technology and consequent volume growth in the market for replacements of wood and metal components, and items that could not be made from anything else, e.g., soft contact lenses, have been significant and steady up to the present day.

2.1 Why do I Need to Know About Polymer Chemistry? What is all this to us though? First, macroscopic properties of (unfilled) polymers are often determined by their chemical structure. Therefore to understand why polymers have certain properties it is necessary to have an appreciation of molecular structure. For example, it is well known that Nylon (DuPont’s trade name for what is chemically polyamide) absorbs water resulting in an increase in volume. This can be attributed directly to the presence in the chemical structure of amide groups which have a strong affinity for water molecules. Second the functionality of composites is dependent on the molecular interaction of the fibrous reinforcement and the matrix. To gain an appreciation of this and why we can 23

An Introduction to Automotive Composites happily use chemically near identical materials for nasty stretchy handle supermarket carrier bags, and inextensible rigging lines on a hang glider, we need to know a bit about the business of molecules.

2.2 What is a Polymer? 2.2.1 Basic Description The word ‘polymer’ is derived from the Greek word ‘poly’ which means many, and ‘mer’ which, in modern Greek means a place. Therefore a polymer is ‘many places’. Likewise, ‘mono’ means one, which leads to monomer or ‘one place’. Monomers are the building blocks of polymers. Taking polyethylene (PE) as an example: the monomer unit is ethylene, which can be designated by the following molecular structure (Figure 2.1), where ‘C’ represents carbon and ‘H’ is hydrogen.

Figure 2.1 Structure of polyethylene

In polyethylene, a large number of ethylene monomer units are joined by changing the double bond between the carbon atoms to a single bond and creating a new single bond between monomer units. An actual polymer molecule of polyethylene has far too many units to show in a diagram, so it is often represented by the notation in Figure 2.2.

24

Polymer Chemistry and Physics

Figure 2.2 Notation for large molecules where ‘n’ designates the number of monomer units (which can be in the range 101 - 106). This large linear section of a polymer molecule is known as the polymer backbone.

2.2.2 Branching and 3D Networks Polymers are not always linear long chain-like molecules as suggested by the previous description. Sometimes there are chains attached to the backbone chain, these chains may be comparable in length to the backbone itself. This phenomenon is termed branching (see Figure 2.3). Some polymers, like polyethylene, can be made in linear, e.g., high density PE (HDPE) or branched versions, e.g., low density PE (LDPE). The determining factor for the occurrence of branching is the method of manufacture, called ‘synthesis’.

Figure 2.3 A branched polymer

Sometimes, both ends of the branch chains are attached to the backbone chains of separate polymer molecules. If enough branch chains are attached to two polymer molecules, it can happen that all of the polymer backbone chains in a sample will be attached to each other

25

An Introduction to Automotive Composites in one huge network. In this case the sample is in fact a single molecule. Polymers like this are termed crosslinked polymers (see Figure 2.4). Most thermoset polymers are of this type. The degree of crosslinking has a large effect on the mechanical and physical properties of the polymer, lightly crosslinked polymers are rubbery in nature. As the degree of crosslinking increases, the thermal stability of the polymer increases, the original vulcanisation process discovered by Charles Goodyear in 1839. Crosslinking means that chemical bonds must broken if the material is to melt. In other words the chemical structure of a thermoset is damaged, usually irreversibly by melting. Note that the crosslinking process can continue at a slow rate. This is most commonly seen in under bonnet rubber hoses, over the life of the car at typical underbonnet temperatures of up to 140 °C, rubber hoses gradually cross link into hard brittle structures. This is not a problem until the car owner removes a hose during maintenance or repair, and then tries to seal it with a hose clip upon replacement. This is an example of the main distinction that can be made between thermoset polymers (in the fully cured state) and thermoplastics. Further differences between thermoplastics and thermosets are covered in more depth in Chapter 2.

Figure 2.4 A crosslinked polymer

2.2.3 Copolymers Polyethylene is produced from only one type of monomer and is therefore termed a homopolymer. However, many polymers are produced from more than one type of monomer. These are termed copolymers. There are a number of different types of structures of copolymers which derive from the method of synthesis. These are described by the following structures where A and B are different monomers (Figure 2.5).

26

Polymer Chemistry and Physics

Figure 2.5 Examples of copolymer structures

Commercial copolymers are often produced to generate materials with the benefits of two or more kinds of polymer (see Figure 2.5) For examples, polypropylene (PP) can be bought in a range of versions. Both block copolymers and random copolymers of polypropylene (PP-copol) with polyethylene are commonly used in applications where greater impact properties than those of PP homopolymer are required. Acrylonitrile butadiene styrene (ABS) is another classic copolymer. This is comprised of three different momomers (those in the name) and is therefore known as a terpolymer (ter being latin for three). In Figure 2.5 the following copolymers are referred to:

Table 2.1 Copolymers – types and properties Type of copolymer

Abbreviation

Full name

Properties

PP copol

Polypropylene copolymer

High impact properties

Alternating

SMA

Poly(styrene-comaleic anhydride)

High heat resistance, Low CLTE, good solvent resistance

Block

SEBS

Styrene-poly(ethylenebutylene)styrene

Hard degradation resistant elastomer

Graft

Hivalloy

High value alloy

Random

CLTE: coefficient of linear thermal expansion

27

An Introduction to Automotive Composites

Blends and Alloys Alloying and blending are further methods of combining two or more polymers to produce a single material with improved physical properties. However, they are distinct from copolymerisation in that combination of the two polymer types is done at the macroscopic level and not at the molecular level. Combination of the two polymers is usually achieved by a physical mixing stage after polymerisation. ‘Blend’ is the term usually used for a combination of miscible polymers and ‘alloy’ is the term usually used for immiscible polymer combinations. So, for example, we can mix a rubber compound with a brittle epoxy resin to produce a tougher resin system, we’ll know that the rubber has been added, because the resin will have a milky appearance due to the tiny blobs of rubber suspended in the resin. The difference between alloys and blends is that alloys tend to have the characteristic of a homogeneous material, often with improved properties over the original polymers. Blends tend to be an average of the original polymers with remaining characteristics of the original materials. Alloys, at the microscopic level, have phases of one polymer dispersed in the other polymer (almost like oil droplets dispersed in water). To produce a usable alloy there must be a degree of compatibility between the polymer types. This is usually achieved by adding an additional material which bridges across the polymer interface and bonds the two phases together (very much like a surfactant does with oil and water mixtures). This phenomenon is also the basis of coupling fibre reinforcements to polymer matrices in polymer composites. This is covered in Chapter 2.

2.3 The Consequences of Being Large Large molecules (sometimes called macromolecules) such as polymers have unusual properties due to their size. So the behaviour of polymers in solid and liquid phases is different to solids and liquids that are more familiar, like steel and water. These properties contribute to the behaviour of polymers, and account for wide variation of mechanical properties over a relatively narrow range of temperatures. If we take the polyethylene structure described earlier and examine the physical properties of molecules of varying length (or molecular weight) as shown in Table 2.2. Molecules change from gases through liquids to solids as molecular weight increases. The reasons for this are explained in Section 1.4.

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Polymer Chemistry and Physics

Table 2.2 Variation of properties with molecular weight Value of ‘n’ (number of repeat units)

Molecular Weight / gmol-1

Physical State at 23 °C

1

~30

Gas

6

~170

Liquid

35

~1000

Grease

140

~4000

Waxy

250

~7000

Hard wax

430

>12000

Resinous

3570

~100,000

Hard plastic

2.4 Chain Entanglement In many cases polymer chains are flexible and tend to twist and wrap around each other, so the polymer molecules collectively form a complex tangled arrangement. When a polymer is in the molten state, the chains act like tangled-up spaghetti. If you try to pull out any one strand of it slides out with ease. When a polymer is in the solid state, it acts more like a ball of tangled string. Here it is much more difficult to pull out a single strand. This phenomenon greatly contributes to the strength of many polymers.

2.5 Intermolecular Forces All molecules, both small ones and polymers attract other through electrostatic interactions. With large molecules (and water!) it is possible that due to the shape of the molecule, even though the molecule has no over all charge (it’s not an ion), the charge is not evenly distributed. So, one end of the molecule may be slightly less or more negative than the other. This is known as polarity; polar molecules stick together better than nonpolar molecules. For example, water and methane have similar molecular weights. Methane’s weight is sixteen and water’s is eighteen. Methane is a gas at room temperature, and water is a liquid. This is because water is very polar, and the forces of attraction associated with polarity are enough to bind the molecules together as a liquid, whilst methane (non-polar but otherwise very similar) is a liquid. Intermolecular forces also affect polymer molecules. With polymers however, these forces are greatly compounded. Crudely speaking, the bigger the molecule, the more molecule there

29

An Introduction to Automotive Composites is to exert an intermolecular force. Even when only weak van der Waals forces are at play, they can be very effective in binding different polymer chains together. This means that flow properties of these materials are unlike those of simpler liquids, for example, water. For a fuller description of these intermolecular forces see Section 4.1.1.

2.5.1 Polar Interactions Atoms of all elements have a characteristic called ‘electronegativity’ which can be defined as the degree of attraction or affinity of electrons to atoms in a particular element. Atoms of each element have different levels of electronegativity. A covalent bond between two different atoms has an uneven distribution of electrons due differences in electronegativity of the atoms. Such a bond is considered polar, it has a small negative charge on one atom and a small positive charge on the other. Electrostatic attraction can occur between two polar bonds leading to intermolecular interactions. Since polarity results in molecular attraction, most polymers with moderate to high polarity are also crystalline. Crystallinity is explained later in Section 1.6. Hydrogen bonding is the result of polarity and its effect on hydrogen atoms. This type of bonding requires a high degree of electronegativity, and only fluorine, nitrogen, oxygen, and possibly chlorine will form hydrogen bonds. The hydrogen atom is covalently bonded to one molecule and hydrogen bonds to an atom of one of the above elements on another molecule. Hydrogen bonds are much weaker than covalent bonds (such bonds join monomer units) but are the strongest of the polar interactions. Polarity affects polymer properties in more ways than just crystallinity. Since water molecules are polar, they are more strongly attracted to polar polymer molecules. This results in increased water absorption in polymers with high polarity. The small water molecules act to dissociate the much larger polymer molecules from each other through secondary bonding to them. This relative displacement of molecules reduces their influence on each other. This results in significant property and dimensional changes. Polarity affects thermal properties of polymers by raising the melting point and the glass transition temperature. This occurs due to additional attraction from polarity helping to restrict molecular mobility. It requires additional energy to overcome this additional attraction and dissociate the molecules from each other. Solvent resistance is improved due to the tighter, more ordered molecular structure. This resistance can be counteracted somewhat by the use of polar solvents, which would dissociate molecules with a similar mechanism to water. As might be expected, polarity does influence the electrical properties of the polymer due to unbalanced electrical charges in the molecules. The physics of this characteristic are

30

Polymer Chemistry and Physics complex. The electrical properties are dependent on the freedom of the dipoles to align themselves with an electrical field. This freedom is very dependent on the degree of crystallinity and the temperature. With adequate mobility, the dipoles will align and the polymer will have a high dielectric constant. If crystallinity is sufficient to prevent mobility, the dielectric constant will be low. Polarity can lower the electrical resistance of a polymer by several orders of magnitude. Water absorption due to polarity creates an additional drop in resistivity. However, polymers such as Nylon 6/6 that have high polarity and water absorption are still inherently excellent insulators. The organic composition still has a much greater influence on a polymers electrical properties than even the combined effect of polarity and water absorption.

2.5.2 Other Interactions Induction forces are caused by the influence of permanent dipoles in certain areas of a molecule on local areas of surrounding molecules that are inherently non-polar. The dipoles exert a pull on the electrons and nuclei of the surrounding molecules, causing their electrical charges to become unbalanced, resulting in what are called ‘induced’ dipoles. The force between the permanent dipole and the induced dipole is called induction force. Induction forces are weak in comparison to primary bond forces. Dispersion forces are the result of temporary or instantaneous changes in electrical charge due to fluctuations in the configuration of the atoms caused by their natural tendency to vibrate and rotate. This causes disturbances in the electron clouds of neighbouring atoms. At any point in time, the average of these forces is zero. This pulling and pushing effect of the atoms on each other results in attraction referred to as ‘dispersion’ forces. Dispersion forces are inherent in all molecules but are the only major intermolecular forces between non-polar molecules. Dispersion forces dominate all intermolecular bonding except where hydrogen bonding is present.

2.6 Crystalline versus Amorphous Polymers In common with many materials, polymers can be crystalline or amorphous in structure. The degree of crystallinity can be influenced by mould temperature. If the melt is allowed to cool slowly, then the resulting moulding will be highly crystalline, and will shrink appreciably more than a rapidly cooled amorphous moulding. Mechanical properties will also vary with degree of crystallinity. It is therefore important to understand exactly what is going on here. Molecular chains of predominantly carbon to carbon single bonds such as in polyethylene tend to be very flexible and fold up on themselves or with other chains and form very

31

An Introduction to Automotive Composites tightly packed and ordered areas called crystals. Intermolecular interactions stabilise the crystal structures. For polyethylene, the chains stretch for about 10 nm before they fold. Polymer chains can also stack, such a structure is termed a lamella. Levels of crystallinity can vary from zero to near 100%. Because of the size of most polymer molecules, only sections of molecules are mobile enough to form crystal sites. In most crystalline polymers there remains an amorphous phase. Polymers which exhibit some degree of crystallinity are termed semi-crystalline.

Figure 2.6 Crystallinity in polymers - spherulite formation

Time and temperature during processing are major influences on the degree of crystallinity a specific polymer can attain. If a crystalline polymer is taken above its melting point and cooled very slowly, more and larger crystals are formed. If that polymer is cooled quickly from above its melting point, the crystals are smaller and rarer. The level of crystallinity of a polymer has a major effect on its properties. Polymers in which crystals are a dominant feature in the structure are classified as ‘crystalline’ polymers, and include: polyethylene, polypropylene, acetals, polyamides (except polyterethalamide - amorphous polyamide), most thermoplastic polyesters, and in some cases polyvinyl chloride. Crystalline and amorphous polymers tend to each have distinctive property profiles. The similarity of polymers within each classification is much greater than that between

32

Polymer Chemistry and Physics

Table 2.3 The effect of crystallinity Amorphous Materials

Crystalline Materials

Low - 0.4% to 0.8%

Moderate to high - 1.5% to 3.0%

Molecules do not pack tightly upon cooling. Shrinkage is dominated by coefficient of thermal expansion and contraction.

When molten, polymer is amorphous. Upon cooling, molecules pack tightly with crystal growth. Shrinkage is due to both thermal contraction and crystal formation.

Optical

The loosely packed random molecules do not refract much light, allowing good transparency and up to 92% light transmittance.

Crystal sections of molecules refract light rays, preventing a clear path of transmission, resulting in translucency or opacity except when highly oriented or in thin sections.

Thermal Processing

Softening is gradual as heat is applied. Large groups of molecules become mobile. The resin becomes rubbery with increasing mobility. Softening increases until the resin is processable.

Heat will cause some softening due to molecular mobility of the amorphous regions. When the crystal melting point is reached, the structures break down and the chains become very mobile. Processing must occur above the melting point.

Chemical Resistance

Attacked by many common solvents such as: petrol, mineral spirits, acetone, and toluene. Solvents etch the surface forming microscopic stress cracks. Any stress on the material, even residual moulding stresses will cause the cracks to propagate through the resin resulting in fracture type failure.

Resistant to most common solvents. Solvents may still etch the surface, which is low in crystal sites. When stress is applied, the stress cracks can't propagate past the internal crystal sites, which act to interrupt the propagation. The resin will not fail until very close to its normal failure stress. This behaviour is much more characteristic under short term loading.

Wear Resistance

Random molecular structure at the surface causes good conformance by deformation to mating surfaces. This causes friction to be high resulting in poor lubricity and higher wear. Amorphous polymers often have to be modified by adding small particles of a crystalline polymer to be useable in wear or other dynamic applications.

Surface and subsurface structure is very ordered. Deformation and conformance to mating surface is low. Crystal sites offer more resistance to sections being ‘torn’ loose. This results in low friction and good wear properties. Polyamides, polyesters, and acetals make up the majority of wear applications due to combined wear and temperature resistance.

Mould Shrinkage

33

An Introduction to Automotive Composites them. Although polymers within each classification are sometimes interchangeable in an application, polymers from the other classification are rarely interchanged. Table 2.3 shows some general effects of crystalline and amorphous structure on enduse properties for non-filled, non-reinforced polymers. For other properties affected by crystallinity the relationships are not as diverse or as well understood. Strength properties depend on many factors in addition to physical structure. It is known that crystallinity increases the modulus and lowers the elongation at yield of a particular polymer, yet other polymers of both classifications may exhibit the same properties independent of crystallinity.

2.7 Orientation Orientation is another form of physical polymer structure. It is more ordered than amorphous but less ordered than crystalline. Orientation is generally induced in a polymer during its processing, for example, Dyneema gel spun polyethylene, used as a composite reinforcement, and as rigging cord on hang gliders. The molecules are forced into a structure where the chains are somewhat stretched and uncoiled and lie in the direction they are stressed. The reader can try this by cutting a strip from a black plastic bin liner; this strip can be stretched to several times it’s original length. The stretching must be done slowly, as the process is rate dependent, the molecules must have time to unwind. You will note that the highly orientated bin liner is stronger than it’s unaligned parent material. Orientation can be in one direction (uniaxial), in two directions (biaxial), or theoretically in all three directions (triaxial). Three dimensional orientation would be due to stretching and uncoiling only. This is rarely seen. In crystalline polymers the crystal sites tend to align with the stress also. Orientation requires a high degree of molecular mobility and significant orientation can only occur above the glass transition temperature or in the fluid state. It can be reversed by heating the polymer in an unstressed state to a temperature where the molecules become mobile enough to return to random order. Some polymers, liquid crystals, possess the peculiar ability to maintain their orientation even in the liquid state. At present these materials are still very high cost, with consequent limited applications. Should you wish to investigate the effects of orientation, take an empty crisp packet and place in the oven at about gas mark three (160 °C). With care the packet can be shrunk to a fully detailed miniature, as the molecules return to their unstressed state. This bias within a moulded part, fibre, or film has a significant effect on its properties. In films and fibres it can be an advantage, in moulded parts it generally is not as it causes shrinkage to differ between the flow direction and the transverse direction.

34

Polymer Chemistry and Physics

2.8 Property Variation with Temperature Consideration of service temperature is particularly important for polymers and polymer matrix composites. All polymers exhibit a wide variation of mechanical and physical properties over a relatively small range of temperatures, such as that expected in automotive applications. Aside from the combustion chambers, a components may see service temperatures ranging from 140 °C under the bonnet to –45 °C for a car parked out doors in a Finnish winter. The mechanical properties of polymers tend to reduce with increasing temperature; in particular there is usually a marked reduction over a fairly small temperature range. This leads to a fairly well defined maximum service temperature. There are two main measures that are used to specify the service temperature. The heat distortion temperature (HDT) and the Vicat softening point measure directly the change in mechanical properties as the temperature increases and either measure may be used as a good indicator of the service temperature of the polymer. It should be noted that this temperature is usually significantly lower than any melting points quoted. Another significant temperature is that of glass transition (Tg) (see Figure 2.7), it is strictly speaking a measure of the temperature where there is a step change in the stiffness of the polymer characterised by a change of a glassy structure to a rubbery structure. This is usually at a similar temperature to the HDT. The Tg is sometimes (mistakenly) described as a ductile/brittle transition temperature due mainly of the proximity of the temperature of the two phenomena in most polymers, it must be remembered that this is not correct for all polymers.

Figure 2.7 Variation of modulus with temperature (Tg: glass transition temperatur Tm: melting temperature)

So in summary, the large size of polymer molecules, and the entangled nature of the material at the molecular level means that polymers do not have a sharp melting point as do for example, ice and the metals. At a low enough temperature, the typical polymer

35

An Introduction to Automotive Composites will be a hard brittle material. As the temperature rises the polymer softens becoming tough and rubbery. Eventually, swapping modulus for viscosity the polymer melts to a viscous fluid. Note that this explanation is approximately true for thermosets and thermoplastics, but in the case of thermosets when melting occurs irreversible chemical damage to the polymer network occurs. These changes in properties are of obvious importance in specifying composite articles. For example, for structural elements it is important to ensure that the polymer matrix is in it’s glassy brittle state at service temperatures, however, if it is tyres we are talking about, the rubbery zone is preferable. What is happening at the molecular level during the glass transition? As the polymer is heated, it expands and allows greater freedom of movement for the molecules. As mentioned previously, there may also be a significant mechanical properties change, at around this temperature. There are also chemical consequences to this change. As the temperature of a thermoset polymer moves towards the glass transition region, the mobility of any residual reacting species is increased. Consequently the probability of reacting species coming into contact is increased and the reaction proceeds further towards completion than would otherwise be possible below the glass transition temperature. This is the reason for so called ‘post cure’, sometimes added as an extra phase in manufacturing to make sure that a consistent maximum of properties are obtained.

2.9 Flow Characteristics Although the joins between each molecular building blocks in thermoplastic polymers are strong (covalent type), they do allow the individual components to rotate, and consequently, the chains are flexible, and are normally entangled probably resembling in some sense a plate of spaghetti or noodles. The chains are weakly attached to one another (van Der Waals forces) but can slide over one another. This means that the polymers form viscous liquids upon melting. If the polymer flows, the net effect is to untangle the molecular spaghetti into strands with a common orientation. The molecules can now flow past each other more easily and the viscosity declines yet further. This process is known as ‘shear thinning’ and is an important effect in that must be taken into account in the manufacture of polymeric articles. Shear thinning of fluids is the reason why we should not stir non-drip paints, if we do, the paint will shear thin and become ordinary dripping paint. So the shear thinning effect means that polymer melts do not behave as predicted by simple Newtonian viscosity (so they are non-Newtonian fluids), and if we are to make processing predictions based on viscosity measurements we must make sure that the viscosity is measured at an appropriate shear rate. In other words, the runniness of polymer solutions and melts depends on the vigour with which the liquid is stirred.

36

Polymer Chemistry and Physics What this means in terms of composite manufacture is that it is intrinsically difficult to inject thermoplastic melts over long fibre reinforcements, as is done with thermoset resins, and other methods of manufacture should be used.

2.10 Commodity versus Engineering Thermoplastics are presently used in much higher volumes than thermosets, because they are cheaper and easy to use in mass production. In the early days of plastics, thermosets (Bakelite!) dominated the market. In the last three decades, advancements in thermoplastic technology have resulted in thermoplastics with most of the properties of thermosets (including high price) and few of the drawbacks. Thermoplastic polymers are generally classified as either commodity or engineering types. This is as much an economic, as a technical distinction. Commodity usually refers to the polyolefin types which are less expensive than engineering thermoplastics and have generally poorer physical properties. They are largely used in the packaging industry. Engineering thermoplastics are more expensive and their use is only justified in more demanding applications. In other words, you have to have a good engineering reason for specifying an engineering polymer. Table 2.4 lists some common commodity and engineering thermoplastics.

Table 2.4 Engineering and commodity thermoplastics Commodity Thermoplastics

Engineering Thermoplastics

Polyethylene (PE)

Polyamide or Polyamide (PA)

Polypropylene (PP)

Polyester (PET/PBT)

Polyvinylchloride (PVC)

Acrylic (PMMA)

Polystyrene (PS)

Polycarbonate (PC)

PET: poly(ethylene) terephthalate PBT: poly(butylene) terephthalate PMMA: polymethyl methacrylate

2.11 What do Trade Names Mean? The commonly used trade names of polymers names tell us nothing about the properties of what is in the bag or drum. Each company uses subtly different methods formulation

37

An Introduction to Automotive Composites and synthesis to manufacture their generic type of material, therefore the processing and material properties will also vary. For example, the term polyurethane (PU) covers a wide range of polymers with very wide ranging properties. The name only refers the linkage between mer units, it implies nothing about the rest of the backbone or the degree of cross-linking and branching. In reality, PU can vary from rigid thermoset materials to elastomeric thermoplastics. This also true for polyamides and polyesters. Even for polypropylene where there is less possibility for variation in the chemical structure (in comparison to polyurethanes, polyesters or polyamides) there is enough variability in the chemical structure of different grades leading to a wide range in physical properties. So, use the following check list when shopping: •

Base Polymer + Filler Type and Level, e.g., > PP GF30 < means polypropylene homopolymer with 30% glass fibre (according to BS EN ISO 11469: 2000 [4])

•

Whether material is thermoplastic or thermoset if appropriate (thermoplastic in this case)

•

Material Supplier, e.g., DSM

•

Material Trade-name, e.g., FIBERFIL

•

Material Grade Reference, e.g., J-60/30/E

References 1.

J.M.G. Cowie, Polymers: Chemistry and Physics of Modern Materials, 2nd Edition, Blackie, Glasgow, UK, 1991.

2.

J.I. Kroschwitz, Concise Encyclopedia of Polymer Science and Engineering, Wiley-Interscience, New York, USA, 1998.

3.

E.G. Couzens and V.E. Yarsley, Plastics in the Service of Man, Penguin Books, Harmondsworth, UK, 1956.

4.

BS EN ISO 11469: 2000, Plastics—Generic Identification and Marking of Plastics Products, 2000.

38

3

Composite Ingredients Kevin A. Lindsey, Neil Reynolds and Nick Tucker

3.1 Introduction This chapter examines some of the commonly available materials used in the manufacture of composites. In an industry as intensely price driven as the automotive industry, this means that our choice of matrix resins and reinforcements will be fairly limited, and it is with this cheering thought in mind that representative materials have been selected for closer scrutiny.

3.2 Thermoplastic versus Thermoset Polymers are traditionally classified into thermoplastic or thermosetting. This distinction is somewhat artificial, as a given polymer can often be processed to produce either a thermoset or a thermoplastic end product. Thermoplastics are fully polymerised in their raw state (as supplied). There is essentially no chemical reaction involved in processing. The use of thermoplastics involves a physical processing step (melting). Application of heat will result in softening or melting at which time the material will flow and can be formed or moulded into the desired shape. Cooling of the material returns it to its former solid state, locking in any dimensional or shape changes. This cycle is reversible and can be repeated many times, or until the cumulative effects from the thermal cycling and high shear during processing start to degrade the polymer. It should be noted that this process can therefore occur anytime after initial forming. This may limit the application of thermoplastic matrix composites if they are likely to be subject to high temperature during service, perhaps under the bonnet or manufacture in passage through the paint oven. However, the ease of processing compared to thermosets, no chemical reactions to worry about, make thermoplastic matrix materials favourite for high volume applications. Thermosets are not fully polymerised in their raw state. They are usually in a solid or resinous liquid state prior to use. Most thermosets require the use of an extra component to achieve cure, this often termed a catalyst or curing agent. Application of heat and pressure will cause the polymer to first go through a softening stage during which it will flow easily and can be impregnated into long fibre reinforcements followed by a chemical

39

An Introduction to Automotive Composites reaction completing the polymerisation – ‘curing’ or ‘crosslinking’. Most thermoset polymers have a highly crosslinked structure when cured and therefore can no longer be made to flow. Re-application of heat only degrades the resin. In some limited cases thermoset polymers (for example acrylic-based adhesives) produce largely linear polymer molecules when cured which can therefore be processed thermally after cure. Thermoset composites are used therefore for the most structurally demanding applications, particularly if high temperatures are involved. In choosing a resin system we must consider the suitability of the physical and chemical properties of the resin for the chosen processing route, matching to the reinforcements and also how the properties of the cured resin will suit the end use of the composite article. Factors to consider are: •

Resin viscosity - sufficiently low to penetrate the reinforcement.

•

Size of moulding - cure reactions are often exothermic (give out heat), therefore if the moulding is thick, it may be that the rate of cure of the moulding must be reduced to prevent an uncontrolled rise in temperature during the curing process which may be sufficient to actually damage the moulding.

•

Speed of reaction - has an important bearing on the rate of article production.

•

Compatibility with reinforcement - the resin must wet and adhere to the reinforcement.

•

Service temperature - Over some range of temperatures polymers go through a transition in properties from brittle and glassy to soft and rubbery (glass transition or Tg). It is usual to use polymers in structural applications on the hard and brittle side of this transition. Note also that in cold conditions the glassy nature of polymers may result in an unacceptable loss of product toughness.

•

Moisture Level - some polymers are not as satisfactory as others in wet conditions, one of the reasons to use vinylester rather than polyester resins. Moisture ingress into a laminate can lead to severe loss of properties.

…and finally to the budding composites engineer, the best of luck.

3.3 Thermosets Some of the more common thermoset resins are listed in Table 3.1.

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Composite Ingredients

Table 3.1 The joy of thermosets Thermoset Type

Common Uses

Alkyd (polyester)

Automotive body panels and fender/wing walls (SMC - sheet moulding compound), tool housings (BMC - bulk moulding compound) brackets, industrial equipment housings, coatings.

Epoxy

Coatings, casting compounds, encapsulating for electrical components, laminates, and adhesives.

Phenolic

Electrical switch housings, relays, laminates, adhesives (plywood, particle board), handles (cooking pots and pans), knobs and electrical motor components.

Polyurethane

Sealants, adhesives and coatings. Automotive body panels (Reaction Injection Moulding), foams.

Urea and Melamine Formaldehyde

Electrical breakers, receptacles, closures, knobs and handles, appliance components, adhesives, coatings and laminates.

3.3.1 Thermoset Characteristics Thermosets can provide a class ‘A’ surface finish (usually means high gloss, ripple free – but there is no widely recognised standard of assessment) probably because with low initial viscosity the resin readily wets and conforms to the mould surface. These systems are suitable for use to produce large and structural components (strong crosslinked chemical structure) while making use of lightweight - low cost tooling (low initial viscosity). The disadvantages may be in process control due to complicated chemistry with variation in properties across a component, due to variation in the chemical reaction during manufacture. The factors that started off the popular rise of composite articles were the availability of reasonably cheap glass fibre, and the development of cold cure polyester resins. These resins had the advantage over older phenolic systems in that they did not require high pressure matched dies during processing, hence the fibres were not damaged to the same extent during processing. These resins are still the dominant force in volume manufacturing because they are cheap and reliable. It is therefore worth examining these materials in some detail.

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An Introduction to Automotive Composites

3.3.2 Polyester Resins Polyester resins are one of those polymers with a foot in both thermoset and thermoplastic camps. In the saturated form, when all spare bonds on the carbon backbone are capped by hydrogen atoms, we have unreactive thermoplastic polyester (Terylene or Dacron, so redolent of the 1970s, are examples). Two types of thermoset polyester resins are available: Orthophthalic – the oldest of the polyesters, but still available and cheap Isophthalic - showing improved properties in terms of chemical and water resistance, as well as mechanical strength. Thermoset polyester resins are usually supplied in the form of polyester dissolved in styrene monomer. It is the styrene that gives these systems their distinctive sharp and pungent smell. The human nose can detect the monomer down to a few parts per million. This, coupled with the probability that styrene is a carcinogen, means that the viability of processes that expose workers to high styrene levels is decreasing. Figure 3.1 shows a schematic diagram of the role of styrene in the polyester curing process. The styrene forms connecting bridges between polyester molecules, causing the material to change from a runny liquid to a hard brittle solid. The finished moulding will still smell of styrene, because the reaction doesn’t go all the way. As the reaction proceeds, the liquid becomes more viscous, and it becomes progressively more difficult for the

Figure 3.1 Crosslinking in polyesters

42

Composite Ingredients reacting species to come into contact. This means that a finished moulding will undergo a period of maturation after demoulding, and may not develop full mechanical properties until days or even weeks after manufacture. This maturation can be speeded up by the process of post curing – heating the finished moulding to above the glass transition temperature (Tg) in an oven (incidentally removing the smell, and causing slight matrix shrinkage). Two mechanisms are involved, first the removal of the volatile styrene monomer by diffusion, and second, by expanding the molecular lattice it is possible that residues of unreacted species will be able to make contact and react. Although this process ensures a low odour product with maximum properties, the cost of an extra manufacturing stage means that post cure is not often used in high volume automotive manufacture. The control of this reaction is fundamental to successful manufacture of composite articles. The resin can be made to cure in a controlled fashion at temperatures between room temperature and 150 °C. The curing reaction is exothermic, heat is evolved during the linking process. It is therefore important to consider the surface to volume ratio of the moulding. If a moulding is made too thick or the cure reaction is pushed too fast, there is a real possibility that thermal runaway will occur and the moulding will be damaged or may even catch fire. The control of this chemical system is achieved by adding a number of other ingredients, leading to a resin that can be stored at room temperature for several months and then initiated into a reaction changing it from liquid to solid in a matter of minutes.

3.3.3 Catalysts and Accelerators The polyester curing reaction proceeds slowly at room temperature, thus any polyester system will have a shelf life (usually about a year), and ordering must be matched to production for this reason. It is easy to tell if a resin has gone past its sell-by date because it will have turned from a mobile liquid to a rubbery solid (known as ‘gelling’). For processing purposes, the reaction rate must be speeded up, there are two traditional ways of speeding up a chemical reaction. •

Catalysts – the addition of a chemical species that isn’t consumed in the reaction. Catalysts (usually organic peroxides) work by providing a site for the reacting species to come together, thus increasing the probability of the reaction occurring.

•

Heat – putting energy into the system causes the molecules to move faster, increasing the probability of an impact, and also the probability of a resulting reaction.

Let us first consider the effect of these factors on the base resin formulation. If a catalyst is added to the formulation the reaction will go faster, but only after the resin has been heated up. This may be satisfactory if the moulding is made with a process allowing heating of the resin, but not so good if it is desired to work at room

43

An Introduction to Automotive Composites temperature. For low temperature reactions an accelerator is added to the resin. Chemically these are cobalt soaps or tertiary amines. Resin formulations often have an accelerator added during manufacture (pre-accelerated). This does not reduce the shelf life of the resin beyond commercial acceptability, but it does mean that on addition of the catalyst at the point of manufacture, heat is not required to speed up the reaction.

3.3.4 Inhibition The gel time can be extended by the addition of an inhibitor. These are often copper salts, and ‘poison’ the reaction, slowing down the rate, or in the case of overdose stopping it altogether. Why, when working under the usual discipline of maximising production would anybody consider doing this? It may be required if a thick laminate is to be produced. An uninhibited reaction may exhibit the thermal runaway mentioned in Section 1.3.2. The problem can also be addressed by the addition of fillers or by designing a cored structure.

3.3.5 Other Admixtures It is also possible to alter the nature of the formulation by the addition of fillers. Alumina trihydrate (Al2O3.3H2O) can be used to produce a fire retardant formulation. The resistance to fire is due to a number of mechanisms, the most important of which is altering the ratio of that which burns to that which does not burn in favour of that which does not burn. When the material is heated the alumina gives up its water of hydration, quenching the combustion reaction by lowering the temperature, and provide an inert atmosphere of steam. It may also be that the alumina undergoes a transient increase in surface area, as it gives up its water, providing a site for adsorption for the free radicals of the combustion process. If increased toughness is required in the final laminate, plasticisers can be added to the formulation to produce a more leathery (less prone to propagating cracks, but less stiff) final product.

3.4 Thermoplastics There are a number of thermoplastic matrix materials suitable for use in automotive applications, examples would be polymers such as polyamide 12 (PA12) or polypropylene (PP).

44

Composite Ingredients When choosing a thermoplastic composite material system, there are several points to consider. Of course, as previously stated, for the automotive industry, cost must be the main parameter distinguishing one material from another. Care must be taken to select the material carefully, as the cost of the final article will not simply involve the materials cost. Factors such as processibility of the material can add or subtract from the cost of any given component. Also, suitability to application is particularly important when using thermoplastics, as the material system within an automotive structure has to endure both mechanical loading and (in some cases, severe) heat loading. Not only are there sources of heat energy from within the vehicle itself (engine and associated exhaust, etc.), but problems can arise from the heat energy build-up from a vehicle exposed to sunlight over long periods of time. With this in mind, a comparison between PA12 and PP will give an idea of how best to select a thermoplastic material system. Firstly, examine a few selected material properties of the two –(Table 3.2).

Table 3.2 Selected material properties PA and PP Property

PA12

PP

Density, (g/cm )

1-1.2

0.9-1.2

Modulus (GPa)

2. 3

~1.2

Strength (MPa)

10

30

Melting Point (°C)

180

160

220 ± 15

200-260

3

Processing Window (°C)

The densities of the two are similar, so weight would not be a factor. In fact, the choice of which material to use would depend on cost and temperature considerations. The main benefit from using a PA12-based composite material would be due to the need for a component to retain structural properties under heating (80+ °C). A PP-composite component simply could not do this. However, for components away from sources of heat energy, PP would be the material of choice for a variety of reasons. The material is low cost, easy to process, and has sufficient mechanical properties to result in semi-structural components when combined with a reinforcement. Of particular interest in this comparison exercise is the processing window. As stated earlier, in order to mould the matrix material into the required geometry, a thermoplastic composite needs to be heated until the matrix is molten. To achieve this

45

An Introduction to Automotive Composites with a PP composite requires heating in the region from 200-250 °C, whereas with PA12, one would require much tighter heating (220 °C ± a few degrees) and transfer control (before material freezes-off) to ensure a successful moulding. Processing the material at too high a temperature will result in polymer degradation, whilst processing at too low a temperature will yield a badly moulded article, essentially, the matrix would still be frozen and unable to flow. The costs associated with the level of process control needed for PA12 would add significant costs to the final component. Conversely, a material as easily processed as PP will result in a very cost-effective component. Because of these factors, PP-composite components have already found their way into a host of semi-structural automotive applications: bumper armatures, load floors, frontend assemblies, and even an entire boot door.

3.5 Reinforcements 3.5.1 Reinforcements, not Fillers or Extenders Within a long fibre composite the fibres are the main load carriers, this is because they can offer a continuous controlled orientation load path. In short fibre composites the orientation of the fibres is not in the direct control of the designer and the level of reinforcement is lower, but the role of the reinforcement is still to improve the mechanical properties of the article. This improvement will certainly be in stiffness, and if the ingredients are chosen correctly, tensile strength as well. The difference between a filler or extender and a reinforcement is that a reinforcement will improve the mechanical properties of the finished article. Reinforcement materials include high strength and high modulus (stiffness) glasses, carbon, aramid, boron, silicon carbide, polyethylene, and natural fibres. Issues of cost and availability mean that glass fibres are the most commonly used materials. Cost and availability of the various fibre types is subject to change associated with new technical developments, and economics, therefore a range of fibres are included in the chapter. The strength properties of composites are mainly determined by fibre strength, similarly for stiffness. The fibres are pre-treated with sizing materials to prevent damage to individual fibres during handling and subsequent processing, and may also be treated with coupling agents to promote adhesion to the matrix material. The fibres may be woven into cloth, bonded together into continuous filament mat (CFM), by means of a thermoplastic binder, or loosely twisted into bundles (tows) for use in filament winding or pultrusion. To achieve a desired product property mixtures of fibre type may be used (aramid/carbon fibre, for example).

46

Composite Ingredients

3.5.2 Fibre Types Glass Some authorities [1] claim the ancient Egyptians as the inventors of glass fibres, making vessels by wrapping drawn glass fibres around clay formers in the XVIIth dynasty, or in metric figures 1500 BC. However, in modern times glass fibres were drawn in Germany at the turn of the last century. Glass fibres are now established as a low cost reinforcement material with adequate mechanical properties. They are produced by melting sand (SiO2) with various fluxes and stabilisers. The furnace melt is allowed to flow by gravity at a carefully controlled temperature through a bushing (think of a large high tech watering can rose with a couple of hundred very tiny holes in it). The emerging glass is gathered up and drawn down to be picked up on a rotating collet and are so drawn into fine fibres. At this point the fibres have an ultimate tensile strength of 3.5 GPa. The size reduction during the drawing process allows the molecular structure to settle down into a regular and micro-crack free formation: this is the reason for the high strength of the fibres. The diameter of the filaments is usually in the range 5 to 24 μm. Between drawing and winding the fibres are usually passed over a sizing pad, to be coated in an aqueous solution of chemicals to protect the fibres from abrasion and cause them to stick together to avoid problems with static electricity. These strands of fibres are subsequently processed in several ways. Rovings consist of parallel strands virtually free from twist wound onto a single removable core of approximately 20 kg arranged for internal or external unwinding. The textile industry unit of Tex is often used to define the amount of glass in a bundle. Tex is the mass in grams of 1000 metres of a bundle of fibres. Typical Tex values are 1200, 2400 and 4800. Rovings can be chopped into short lengths, usually 3, 6, 12, 25 or 50 mm to produce chopped strand or short fibre reinforcements. Single stands or rovings of small amounts of strands can be twisted and doubled into textile yarns, continuous filament mats, woven, and non-crimp fabrics (stitch bonded). A variety of grades of glass have been developed for differing purposes: •

E-CR (CR = corrosion resistant) glass is a boron free formulation with high resistance to acid corrosion.

•

S Glass is a calcium alumino silicate glass with high strength stiffness and cost.

•

E Glass has good electrical resistance as the formulation resists leaching in wet conditions, but usually used for reinforcing polymers due to cost.

47

An Introduction to Automotive Composites It is worth considering some of the other fibre reinforcements (see Table 3.3), as the current prices are subject to change, and therefore these materials may well become available to the automotive industry in the medium to long-term.

Table 3.3 Common fibre mechanical properties Material

Tensile Strength (GPa)

Tensile Modulus (GPa)

Specific Density

Specific Strength (GPa)

E Glass

3.4

22.0

2.60

1.31

Carbon

3.5

230.0

1.75

2.00

Kevlar

3.6

60.0

1.44

2.50

Carbon Fibre Carbon fibre has the highest stiffness to weight ratio of any structural material. It was originally developed in 1963 at the Royal Aircraft Establishment in Farnborough. Carbon fibres can be produced from many man-made fibres and some natural fibres [(consider early work by Swann (before Edison)] on carbonised cellulose for light bulb filaments), however, few materials produce fibres with useful mechanical properties. During the search for high modulus, high strength fibres in the 1960s many workers arrived at the conclusion that the in-plane stiffness of the graphite structure was very high, and that it would result in stiff fibres if carbon fibres could be produced with a favourably orientated structure. Note that strictly speaking the structure of carbon fibre is not entirely equivalent to graphite, and therefore the material should be known as carbon fibre, rather than graphite. Courtaulds determined that degradation of polyacrilontrile (PAN) fibres produced a graphite like structure. These fibres became known as ‘Courtelle’ fibres. The process of carbon fibre production is essentially that the PAN fibres are stretched in hot water to provide a high tenacity fibre. The white fibres are then carbonised (turning black in the process) at a relatively low temperature. They are subsequently heated progressively in absence of oxygen to a maximum temperature between 1100 and 3000 °C to convert the structure into one characterised by aligned covalently bonded basal planes of carbon atoms along the axis of the fibres. The mechanical properties (tensile strength and modulus) depend upon the this structure, the type of fibre initially used and the graphitisation temperature. It was found that the strength-temperature plot peaked at about 1800 °C while the modulus-temperature plot continued to rise beyond 3000 °C (Table 3.4).

48

Composite Ingredients

Table 3.4 Comparison of carbon fibre and high tensile steel Material

Tensile Strength (GPa)

Tensile Modulus (GPa)

Specific Density

Specific Strength (GPa)

Standard Grade Carbon Fibre

3.5

230.0

1.75

2.00

High Tensile Steel

1.3

210.0

7.87

0.17

Unlike glass, carbon fibres are not isotropic and are relatively soft in the transverse direction. The complex production route means that this material is expensive. However, one of the more unlikely side effects of the collapse of the Iron Curtain, is that factories from former Soviet bloc countries are now able to sell into the international market. Generally, these materials are somewhat coarser than the western equivalent, but are somewhat cheaper. These low cost fibres are the most likely option for the use of carbon fibre in automotive structural applications. On the distant research horizon are carbon fibres made from the most recently discovered allotrope of carbon Buckminsterfullerene – the so-called ‘Bucky tubes’, unfortunately currently limited to about 20 μm in length.

Aramids Commonly known as Kevlar (or Twaron, or Technora), Dupont developed this aramidbased organic fibre in 1969; it became commercially available in 1972. Covalently bonded, aligned, organics such as aramids offer great potential to produce materials that are light, tough and strong in tension, however difficulties in manufacturing are likely to ensure that these materials will remain a high cost option for the foreseeable future. However, the past three decades have taken aramids from a top-secret strategic material, to a tyre reinforcement. In the Dupont version of this material, the family of products includes Kevlar, Kevlar 29 and Kevlar 49. Kevlar is made in large quantities for reinforcement of tyres, hoses and belts. Kevlar 29 is used for rope cables, ballistic devices, coated fabrics. Kevlar 49 is a material with a high modulus of elasticity designed specifically for reinforcing plastics in the

49

An Introduction to Automotive Composites aerospace industries. In general, these aramid fibres are high strength, high modulus, and low density. They are not however very resistant to UV light, and are also prone to taking up water from the atmosphere, but in the case of water absorption, without loss of properties.

Gel spun polyethylenes These materials are sold under the trade name of Dyneema. Chemically similar to the stuff supermarket carrier bags are made from, but with higher molecular orientation. The orientation of the molecules is achieved by drawing out the fibres as they solidify. High tensile strength, and modulus, better chemical and moisture resistance than aramids, but not tolerant of high temperatures, if heated beyond a critical temperature the orientation and consequently the mechanical properties are lost. This limits the choice of processing methods for this material, as well as end use.

Boron Old in terms of advanced materials, being developed in the early 1960s and consists of boron deposited by chemical vapour deposition around a 12 μm tungsten core. Very high material properties (six times the stiffness of glass) are matched to very high material costs. Vacuum bagged boron epoxy composites have been used to repair impact damage and fatigue cracks in aeroplanes. However, in terms of price, performance and toxicity, this material usually loses out to carbon fibre.

Natural Fibres These are available in abundance and are very inexpensive, (hence the common use of such materials by the packing industry), though mechanical properties are low with respect to structural applications they can be used as low cost fillers or to assist energy management applications. Again, as with Aramid fibres moisture uptake can be a problem, and unlike Aramid fibres natural fibres are biodegradable. When density is taken into consideration the performance of these fibres potentially matches that of existing materials (see Table 3.5). Fibres can be obtained from many plants, these fibres include jute, flax, hemp and kenaf. The physical properties of these fibres are variable not only between species but also as a result of growing conditions, age of the plants and fibre extraction process.

50

Composite Ingredients

Table 3.5 Comparison of fibre properties

Flax

Specific Density

Tensile Strength (GN/m2)

Specific Tensile Strength

Tensile Modulus (GN/m2)

Specific Tensile Modulus

1.2

2.0

1.6

85.0

71

Hemp

0.895

25.1

Kenaf

1.191

60.0

E Glass

2.6

3.5

1.35

72.0

28

Kevlar 49

1.44

3. 9

2.71

131.0

91

Carbon

1.75

3.0

1.71

235.0

134

Biocomposites are attractive because of the low environmental impact of their product life cycle; they can be locally produced, and they do not add to the net level of CO2 in the atmosphere; this is because they can be composted at the end of life. These fibres are gaining acceptance in the automotive industry, with Mercedes, Daimler Chrysler and BMW using polypropylene reinforced with flax as interior body panel materials.

3.5.3 Fibre Loading and Matrix Interaction Loading of the individual fibres depends upon sufficient adhesion between the matrix and the filaments. This is achieved through the addition of coupling agents into the size solution, which is added to fibres at the end of production. These coupling agents normally constitute 1%-2% of the size solution which in turn makes up about 1% of the fibre weight. A further additive often applied to fibres is a binder to allow preforming this is often a thermoplastic resin, allowing the fibres to be preformed under the application of heat and pressure.

3.5.4 Coupling Agents and Size Formulations Fibre reinforcements for composites, tend to be brittle and susceptible to damage. Just handling a glass fibre can reduce it’s strength by up to 40%. In order to protect the fibres during weaving and production of glass fibre mat, the fibres are coated with a protective layer called a size, which forms a thin layer around the glass fibres (20 nm to 100 nm, approximately 1% of the mass of the fibres). Most of the size (95%) is a low molecular weight polymer. The remainder of the size is a mixture of lubricants and plasticisers and the last 1% to 2% of the size is a coupling agent.

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An Introduction to Automotive Composites Coupling agents are required to form a chemical bond between the chemically dissimilar glass fibres and the matrix resin. The coupling agent acts as the load transfer agent between the matrix and the glass fibres. Making sure that the coupling agent is as efficient as possible is important to ensure that the composite performs to its maximum strength and stiffness. Before the coupling agent can produce the required chemical bond, the size formulation must be removed. Removal of the size is usually carried out in situ when the composite is manufactured. The size formulation is designed to be compatible with and dissolve in the matrix resin, leaving the coupling agent able to join with the matrix and the resin. This leads to size formulations being specifically designed for the given matrix resin. Typical coupling agents used are based on organofunctional silanes (normally called silane coupling agents). These molecules have the necessary types of reaction sites that allow bonding to both the glass fibres and the matrix resin. Significant work in the past has concentrated on developing coupling agents for a range of different resins, and there now exists a range of coupling agents for all of the most usual resins encountered.

3.6 Core Materials The final ingredient of composite structures is the core. According to Gordon [2], the first use of a cored sandwich material in aerospace was balsa wood skinned with birch wood in the 1930s with the de Havilland Comet, designed by Edward Bishop (and later in his de Havilland Mosquito).

Figure 3.2 The De Havilland Mosquito - an early example of cored composite structure (Source: Jim Stockwell)

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Composite Ingredients The need for cores is explained by the desirability of light and stiff structures. Also, when working with exothermic thermoset systems, it is necessary to limit the surface to volume ratio of the mouldings to avoid thermal damage during the cure process. In a beam under load, the main work of resisting the load is done by the material at the surface of the structure, therefore if we can tailor the properties of our beam it may be possible to dramatically reduce the weight without compromising its mechanical performance. In composite terms we can use our fibre reinforced laminate on the outside, and something in the middle to hold the skins apart. As the beam bends and it resists the load, the core will be put into shear, so the properties required for a core material are low density and shear strength. The candidate materials for this role are: •

Rigid polymer foams

•

Honeycombs

•

Syntactic foams – hollow spheres set in resin

•

Balsa Wood

The definition of an ideal core material that could be used in automotive composite applications would be a low cost, low density material (by low density, we mean that the density of the core material is greatly lower than that of the outer composite material skin) that significantly improves the mechanical properties of a component without incurring weight or cost penalties. The introduction of such a lightweight material into a composite material system usually results in a sandwich material system. Traditionally, composite sandwich systems have been considered the domain of high performance application sectors such as the

Figure 3.3 A thermoplastic sandwich panel (Source: Neil Reynolds)

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An Introduction to Automotive Composites aerospace, motorsport and marine industries. However, they are currently the topic of much research regarding their use in high-volume automotive applications due to falling material prices, as well as entirely novel material systems being offered by suppliers. This means that the automotive engineer could feasibly justify the use of a sandwich material system for an application. The drivers behind using a core material to create a sandwich component would be: •

enhanced component flexural stiffness due to increased section;

•

same flexural stiffness with reduced component weight;

•

reduced component costs;

•

shortened cycles times;

•

insulation: sound (better Noise-Vibration-Harshness properties), and heat (thermal insulation);

•

in thermosetting applications, can reduce risk of runaway exotherm, which can pose a fire hazard;

•

good impact properties;

•

parts integration.

The first two items on the above list are the main considerations for the engineer. These define that the primary application for the core material will be a sandwich beam/panel (Figure 3.4) in a flexural loading.

Figure 3.4 Sandwich beam (Source: Neil Reynolds)

By displacing the composite skins further away from the neutral axis of a beam/panel the cross-section and so flexural stiffness of a component is increased. This is ideally done with the addition of minimum weight. However, a caveat to this last statement would be

54

Composite Ingredients that the ability of the core material to transfer shear stresses through the beam thickness to the composite skins is of prime importance, as is the core material’s ability to withstand compressive loading (crushing). Therefore, the mechanical properties of the selected core material should not be too low, and the performance of any given core material is directly related to its density. Consequently, the engineer must trade-off this property-density problem in order to optimise for a given application. One such example of this trade-off is in a simple box-section, where the core material is effectively air (disregarding the webbing or sidewalls connecting the upper and lower skins); here the beam properties are enhanced through increased component section, but the shear forces are not transmitted and localised crushing of the section is not inhibited. The core materials that are available are isotropic cellular materials (foams) and anisotropic cellular materials (honeycombs, balsa). Foam cores are polymeric in nature (thermoplastic or thermosetting), and some examples are polyurethane (PU, thermosetting), polyvinyl chloride (PVC, thermoplastic or thermosetting), expanded polypropylene (EPP, thermoplastic).

Figure 3.5 Sandwich cross section (Source: Neil Reynolds)

Honeycombs, however, are made from a range of materials, including thermoplastic and thermosetting polymers as well as paper and aluminium. Gordon [2] claims the invention of the honeycomb core to George May, a circus proprietor in 1943. In Table 3.6 some representative mechanical data are given for core materials, other materials are included for density comparison only. Note that the figures in the table are for worst- and best-case-scenarios for each material type, and also measured in the optimum direction of the anisotropic honeycomb and balsa materials. For example, the off-axis properties of a honeycomb material are greatly reduced (in some cases much lower than half of the axial properties).

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An Introduction to Automotive Composites

Table 3.6 Representative mechanical data for core materials Density (g/cm3)

Compressive Modulus (MPa)

Shear Modulus (MPa)

Polymeric foams

0.02-0.30

10-400

3-290

Honeycombs

0.03-0.20

130-1700

41-330

Balsa Wood

0.10-0.25

2240-7720

108-312

‘E’ Glass

2.60

—

—

Thermoset resin

1.20

—

—

Thermoplastic resin

0.90

—

—

1.10-2.00

—

—

Material Type

Glass Reinforced Polymers

The different core materials result in unique system properties and require greatly differing manufacturing techniques. The anisotropic core materials and thermosetting foams are largely brittle in nature and demand that little or no damage can be introduced in the manufacturing process, meaning that possible complexity of form of the final article is low, whereas the thermoplastic honeycombs and foams can be thermoformed at any stage of the manufacturing process, resulting in a part with some degree of complexity within it. However, the more brittle cores generally yield stiffer sandwich structures – good for flexural rigidity. The thermoplastic cores can offer a very tough response, resulting in sandwich structures optimised for energy absorption. From these statements it can be seen that, combined with the choice of appropriate facing materials, a core material will allow the engineer a high level of design freedom, resulting in an optimised component. A representative manufacturing route for thermosetting cores with thermosetting skins would be resin transfer moulding (RTM), where the core would simply be included within the mould along with the skin fibre reinforcement. Upon introduction of the matrix to the mould, the resin seeps into the outer pores of the core material (in the case of foams) or impregnates the glass scrim on the surface of the honeycomb, which forms a strong mechanical bond in the skin-core interface. A typical method used to manufacture thermoplastic composite sandwich components is an adapted compression-moulding cycle, whereby heated molten composite skins are wrapped around a cold thermoplastic core, and the whole arrangement is compression moulded in one shot. Here, the heat energy from the molten skins is sufficient to cause local melting of the core material, allowing core thermoforming and a large degree of chain entanglement at the core-skin interface. Note that for the thermoplastic process, a single polymer system is preferred (improved bonding), whereas with a route such as RTM, the choice of core is more flexible. 56

Composite Ingredients The cost benefits of using core materials with composites can be summarised as follows. The costs of many traditional core materials (EPP) is now falling, along with the introduction of new, cheap core alternatives, e.g., PP honeycomb. By using a low-density core material, the amount of more expensive, more dense composite material used in a component will fall, so not only will the part be lighter, but also cheaper as well. This reduction in the amount of composite material will also reduce cycle time through lower cure time for thermoset articles, or shorter heating and cooling times for thermoplastic processing routes. Current examples of automotive applications for sandwich components using core materials are: sandwich panel boot load floors (PP-honeycomb, PP-glass laminate facings), thermoset sandwich beam A-pillar (as in the new Aston Martin Vanquish), sandwich bumper beams.

Figure 3.6 The Aston Martin Vanquish (Source: The Ford Motor Company)

References 1.

Concise Encyclopedia of Composite Materials, Ed., A. Kelly, Pergamon, Oxford, UK, 1994.

2.

J.E. Gordon, Structures or Why Things Don’t Fall Down, Pengiun, Harmondsworth, UK, 1978.

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An Introduction to Automotive Composites

58

4

General Properties of Composites - Stiffness, Strength and Toughness Rebecca Cain, Martyn K. Pinfold and Kevin A. Lindsey

4.1 Introduction This chapter is devoted to the understanding of the mechanical properties of composites. This introductory text cannot hope to explain all of the intricacies of the mechanical properties of composites, rather it aims to give a broad understanding that can be developed through the references and extra reading sources supplied. In common with all other engineering materials, the properties of composites depend upon both their chemical make up, e.g., matrix and reinforcement properties, and their processing conditions. The effect of processing is discussed in more detail in Chapters 6 and 7, so this section deals with the effect of the constituents on the mechanical properties in particular continuous and chopped fibre reinforced composites. Where composites differ from the engineering metals is that for the metals a number of types are defined (for example a 6000 series aluminium alloy). In addition, within these broad families particular grades are defined, e.g., 6063, with set chemical compositions. Further to this, for a defined set of processing conditions (such as an extrusion with a T6 temper for aluminium) the properties are known and are enshrined in an international standard allowing the user to have some certainty of the mechanical properties that are to be expected. Composites do not have the well known, established properties associated with metals. There is therefore no single reference or standard for the mechanical properties of composites. This in part is one of the difficulties of designing with composites. This is also one of composites greatest attractions: the ability to change or tailor the mechanical properties of the composites to what is required for the job in hand. This chapter ends with a short exposition into the methods used for the structural analysis of composites. The input into these techniques is a ready supply of the appropriate mechanical properties. This chapter highlights some of the methods by which the mechanical properties of composites may be elucidated, from both mathematical prediction and experiments.

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An Introduction to Automotive Composites

4.2 How to Obtain Material Property Data Before any time or effort is spent in gaining the material properties of composites it is worthwhile reflecting upon what properties are required. Only the macro properties of composites will be discussed, since it is these properties that are generally used in engineering calculations (though an understanding of how the properties are derived may aid the application of the composites). In general, the stiffness (Young’s modulus) and strength are required. For composites, there is the added complication of directionality, the properties may be different in different directions. This behaviour is known as anisotropy. Figure 4.1 [1] illustrates four possible fibre orientations, all of which will exhibit different mechanical properties of the composite according to the axis along which they are measured. Generally, composite materials may be described as orthotropic which means the properties vary along three principal (orthotropic) directions, x, y and z, which are at 90° to each other.

Figure 4.1 Fibre orientation is the key to composite properties. a) uni-direction, b) random, c) orthogonal directions and d) multiple directions. After Askeland [1].

For engineering metals, it is only usually the stiffness (Young’s Tensile Modulus, E), strength (σ), strain (ε) and Poisson’s ratio (ν) that are required in order to define the metal sufficiently in order to carry out a mathematical analysis. It is usually implied that for metals the remaining properties (such as shear modulus, G) can be calculated from the Young’s modulus (E) and Poisson ratio (ν). For composites this is not generally possible due to the inhomogeneous composition on a microscopic scale. Hence, a set of such data is required for each orthotropic direction.

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General Properties of Composites - Stiffness, Strength and Toughness

4.3 Introduction to Methods of Obtaining Material Property Data There are three ways in which the composite material properties may be found. The first, and most obvious method, is to ask the material supplier for the information. However, the properties supplied by some material suppliers are incomplete and may not relate precisely to the composite that will be produced. Often only one Young’s modulus and one Poisson’s ratio are supplied. Another method of determining the material properties is to manufacture a sample of the material and to determine the properties experimentally. Given the incomplete properties supplied by some of the material suppliers and the expense and time of mechanical testing there is a need to quickly obtain an estimate of the mechanical properties of the composites. Such a technique exists and is known as ‘micromechanics’. Micromechanics is a method by which the required material properties may be mathematically derived given the constituent properties, the fibre orientation and the required fibre volume fraction. This chapter will discuss obtaining material data by experimental and micromechanical means.

4.4 Analytical Expressions for Deriving Composite Mechanical Properties Micromechanical prediction of the material properties of various types of fibre reinforced composites is not an exact science. Even with the simplifying assumptions the micromechanics theories can predict the material properties of a unidirectional composite within ‘acceptable engineering accuracy’ when compared to the available experimental data, even though most of the assumptions upon which the micromechanics theory is based are violated by the real material. Deriving the stiffness of composites (or more strictly, a single layer of a unidirectional composite commonly known as a lamella or a ply) is simply estimated using the following equations. The Young’s modulus (E) of one of these layers measured along the fibre axis can be described by the rule of mixtures (known as the Voigt expression). It can be seen from Equation 4.1 that the stiffness of the composite is equal to that in the matrix and fibres combined and their relative proportions. Ec1 = VfEf + (1- Vf) Em

(4.1)

where E is Young’s modulus, V is volume fraction and c, f and m refer to composite, fibre and matrix, respectively, subscript 1 refers to the longitudinal principal direction [2]. When the material is measured transversely, however, the modulus is reduced and may be described by Equation 4.2, known as the Reuss expression. This is one of a number of such equations and a review of such is shown in [3].

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An Introduction to Automotive Composites Ec2 = {Vf / Ef + 1- Vf / Em}-1

(4.2)

where the symbols have the meanings given previously, subscript 2 refers to the transverse principal direction. In both instances the effects of Poisson expansion/contraction are assumed to be negligible and stress transfer across the fibre/matrix interface is assumed to be perfect, i.e., it is presumed that the fibre is completely bonded to the matrix. This is not necessarily true and there are many equations that take into account these factors [3]. A unidirectional (UD) composite in which the fibres are arranged in the ‘x’ direction, and ‘z’ is taken through the thickness of the material is said to be transversely isotropic. The properties of the material through the thickness are thus taken to be the same as those transverse to the ply such that: Ey = Ez, Gxy = Gxz, νxy = νxz. A unidirectional composite material can therefore be characterised by just 6 elastic constants, i.e., Ex, Ey, Gxy, Gyz, νxy, νyz.

4.4.1 Continuous Random Reinforced Fibre Composites In a random fibre reinforced composite in which the fibres are assumed to be random in the ‘xy’ plane then Ex = Ey although in practice there is often a preferential fibre direction due to the manufacture of the fibre mat such that there may be a 10% difference in Ex and Ey. Precise prediction of the inplane stiffness of composites is difficult and many numerical expressions exist, one of the best known is that derived by Krenchel [4]: Ec1 = ηVfEf + (1- Vf) Em where the symbols are the same as those used previously with η being known as the efficiency factor which varies with different fibre architectures. It can be seen that the the Krenchel equation is simply the Voigt equation used previously, modified with a efficiency factor (η). The value of the efficiency factors have been calculated for a range of reinforcement types. It was found for planar random reinforcements (such as those in composite produced from random fibre mats that η=0.375. For unidirectional composites η is equal to 1. For a 0/90 composite (with equal fibres in each direction) η= 0.5. This again is an approximation, but it is useful to gain a first indication of the stiffness of the composites.

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General Properties of Composites - Stiffness, Strength and Toughness

4.4.2 Woven Fibre Composites Calculating the material properties for woven composites is more difficult than for the non-woven composites discussed previously. Instead of micromechanics, a finite element (FE) route has been tried to create detailed models of the micro geometry of the weave pattern using FE analysis. It would appear that the only data readily available concerning woven material are the fibre volume fraction, weave pattern and constituent properties. Detailed information on the weave pattern (used to create the analytical models) is not readily available from material suppliers. Thus the use of such analytical models is not practical in a commercial design environment. Relatively simple methods such as laminate analysis, which only use the uni-directional properties determined from the constituent properties, are therefore required to calculate the mechanical properties of the woven material.

4.4.3 Laminated Composites It is difficult to give typical properties of a laminated composite material, as there is a large number of possible laminate combinations. The properties of laminated structures may be built up from constituent layer properties using classical laminate analysis. For a review of these theories see [5]. Calculations of laminate properties are automated in many composite programs such as CoDA [6].

4.4.4 Prediction of Strength The static strength of a composite can be predicted by micro-mechanics, though it is less well understood than that of the elastic properties and is ‘difficult’ [7]. Hashin [8] stated that the prediction of composite strength is of such difficulty that with the present state of knowledge ‘it does not seem possible to predict a coherent, reasonably rigorous development’. It is reported that at present no satisfactory micro-mechanical models exist to predict the strength of a composite from the properties of the constituent materials [9]. The models that do exist give results that are at least 50% too high [10]. Thus most composite structures are certified by test rather than by analysis, with matters unlikely to change unless more realistic failure theories are developed [11, 12]. Due to the above problems the prediction of the strength of composites has not been included here. It is possible that when more test data becomes available to validate the existing micromechanical predictions these properties will then be given.

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4.5 Laminate Analysis Software The equations given in Section 4.4 are simple and give a good first approximation but there are some inaccuracies, especially for the determination of the transverse properties and strengths. Results obtained from experimental tests can be used as a check of the accuracy of the micro-mechanical theories. This often results in the use of empirical factors such that the calculated predictions match more closely the experimental data. Such empirical factors need to be determined for the material being considered, and may vary from material to material. To determine the material properties of composites with fibre arrangements other than UD, researchers often use either detailed analytical models of the fibre and resin micro geometry or equations which use the UD micromechanical equations as a starting point. If the designer uses micromechanics to determine the required properties then the fibre arrangement and fibre volume fraction are parameters that should be determined to suit the particular components requirements. The designer however, may be unlikely to have knowledge of the micromechanical equations necessary to determine the required material properties. Thus software such as CoDA [6], LAP [13] or LAMICALC [14] allows the designer to undertake an initial design study. This software allows an engineer to define the lay up of the composite and will determine the overall composite properties from micromechanical equations that have been validated against test data and in some cases include a correction factor. It is apparent that the micromechanical prediction of the material properties of various types of fibre reinforced composites is not an exact science. If micromechanical equations are used to predict the composite material properties then these equations only predict the properties that may be obtained. Many researchers, such as Smith [9], state that for final design purposes the theoretical estimates of moduli should not be regarded as a substitute for reliable test data, because of the possible variation caused by the manufacturing process and the limiting assumptions upon which the micromechanical analysis is based. The best that may be achieved from such micromechanical predictions is an estimate of the actual composite properties. The theoretical estimates of the properties may however be useful for evaluating initial designs and studying the influence of the various parameters upon the overall performance of the composite component. Note that in rigorous mathematical terms the values given by some of the equations at the extremes of fibre volume fraction, i.e., 0 or 1.0 are not as they should be, but the values given for practical values of the fibre volume fraction are acceptable.

4.6 Properties of Short Fibre Reinforced Composite Materials The preceding analysis of composites has concentrated on the continuous fibre unidirectional or random fibre composites. Discontinuous fibre composites are most

64

General Properties of Composites - Stiffness, Strength and Toughness often used in injection moulding, where the action of the screw in the injection moulding barrel breaks down the fibres to lengths between 1 mm and 10 mm. Discontinuous fibres are also found in some compression moulded composites such as glass mat thermoplastic (GMT) and sheet moulding compound (SMC). For SMC and GMT compression mouldings short glass fibres are used where greater flow lengths are required. Continuous fibres may impede flow, whereas the shorter fibres will be transported along with the matrix to the extremities of the mould, thus giving higher quality mouldings.

4.6.1 Stiffness of Short Fibre Composites In composites containing discontinuous reinforcements, stresses are transferred from the matrix to the fibres by shear forces at the fibre/matrix interface. The fibres may still carry a high proportion of the load on the composite, but there is a section at both ends of each fibre where the fibre tensile stress is still building up to it’s maximum value and in which region it’s reinforcing efficiency is diminished. In general this leads to short fibre composites being less stiff than continuous fibre composites. For most practical composites, the stiffness of the short fibre composites is equal to that of a continuous fibre composite if the fibres are of the order of only 1 mm long (Figure 4.2). It should be noted that in most practical short fibre composites, such as injection moulded reinforced thermoplastics or dough moulding compounds the fibres will usually not be random but be slightly aligned by the fabrication process. This should be taken into account during detailed design of components.

Figure 4.2 Fibre length effects

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An Introduction to Automotive Composites

4.6.2 Strength of Discontinuous Fibre Composites As highlighted in section 4.6.1, discontinuous fibre composites are loaded, through or across the fibre matrix interface via shear forces, leading to an ineffective length of individual fibres at each end. Not only does this lead to a decrease in stiffness of the composites (when compared to continuous fibre composites) but also suffer a reduction in strength. As a guide, approximately 95% of the continuous fibre composite strength can be achieved in a short fibre composite with fibres only 8 mm10 mm long (Figure 4.2). One of the limiting factors on strength of short fibre composites is the fibre / matrix interface strength. The higher the interfacial shear strength the more rapid the build up of stress at the end of fibres therefore a stiff/strong composite requires stiff/strong interfaces. Though as indicated previously stiffness and strength do not increase with fibres above 8 mm long, the impact properties of a material are improved through using longer fibres. It is generally accepted that a fibre length of approximately 20 mm is required in order to gain the impact properties of a continuous fibre composite (this should be regarded as a rule of thumb – the exact length depend on many things such as geometry of the specimen, fibre/matrix interface strength etc).

4.7 Energy Absorption and Toughness In section 4.6.2 on short fibre composites, it was noted that the strength of the composites can be improved with a stronger/stiffer interface. It is not always desirable for the fibre and matrix to be strongly bonded and some reasons for this will be discussed. Fibre debonding and pull out is one of the important energy absorbing mechanisms during composite fracture. Figure 4.3 illustrates a UD composite which has undergone multiple fibre fracture and debonding in and around a matrix crack. Note that the matrix has failed in a brittle manner without yielding. It can also be seen that the fibres have pulled out of their ‘sockets’. This mechanism increases the fracture toughness by dissipating energy. If the fibre/matrix interface is too strong, then it is possible that the composite would crack in a single fracture rather than the crack being diverted by the fibres. This single failure mechanism would reduce the toughness of the composite significantly. Other mechanisms operate in composites based on ductile matrices (such as thermoplastic polymers and metals) to dissipate energy. In most instances polymer matrix composites fail by delamination/debonding, an inherently tough mechanism. At present there exist no analytical means of deriving the energy absorbtion of composite structures. This topic remains the subject of research by academics and the major auto manufacturers alike.

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General Properties of Composites - Stiffness, Strength and Toughness

Figure 4.3 Multiple fracture and fibre pull out in a uni-directional composite. After Aveston and others [15]

4.8 Composite Test Methods It has already been stated that the micromechanics approach to mechanical property determination will only ever yield an approximation to the actual properties. To finalise designs and to ensure the accuracy of the design predictions it is always advisable to carry out mechanical tests to confirm the initial predictions of the material properties. In general, test methods for composite materials are less well developed than for metals for instance, especially in the through thickness or ‘z’ direction. This section defines some of the tests that are carried out to determine these values. As has been discussed previously, the material properties of composites are not only affected by the chemical make up (fibres, resins etc) but also by the processing route and conditions. It is therefore important that the material properties that are gathered not only reflect the materials to be used but also the proposed processing route. At the end of this section there is a short introduction into how different processing routes for the composites can affect the mechanical properties gained from the composites.

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An Introduction to Automotive Composites It should be noted that this section deals with how to get the material properties that are used as an input to the design process, rather than testing the final components to check whether the design process has yielded the correct predictions. It is wise to carry out some check tests on the final component.

4.8.1 Determination of Composite Properties by Testing Composite materials are both inhomogeneous (properties are positionally dependent) and anisotropic (properties are orientationally dependent) in comparison to metallic materials that are homogeneous and isotropic. This means that determination of their mechanical properties is rather more complex. The elastic properties of a material are a measure of its stiffness and may be used to predict the behaviour under load. The elastic properties of the composite are determined by the elastic properties and volume fraction of the constituents (fibre and matrix) and additionally by the orientation of the reinforcement. It has been stated previously that composite materials are orthotropic and it follows therefore, that a single value for strength or moduli would be misleading unlike a metal for which properties are well known. There are nine independent elastic constants to describe the elastic behaviour of such a material in contrast to a truly isotropic (properties the same in all directions) material for which there are only three. Strength characterisation requires measurement of at least 5 independent parameters (assuming transverse isotropy). Elasticity theory can be used to determine the elastic constants given sufficient information, however, the strength of a composite material is affected by a number of parameters including fibre orientation, volume fraction of fibres, the amount of porosity, fibre length, lamination lay up and environmental effects. All of this means that each material and it’s properties are unique. Consider the unidirectional fibre composite in Figure 4.4. The properties measured parallel to the loading direction i.e. in the fibre direction would be dominated by the strength of the fibres since they are oriented in the load axis. In the transverse direction, however, the fibres take little of the load and the matrix material dominates the properties. When random or non-unidirectional laminated fibre lay ups are used, this large change in properties is reduced and the modulii and strength values in the two directions begin to converge. The composite is thus said to be quasi-isotropic, which should not be taken to mean true isotropy.

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General Properties of Composites - Stiffness, Strength and Toughness

Figure 4.4 A unidirectional composite – transverse and longitudinal loading

The design of a test specimen (and the selection of an appropriate standard) to evaluate a material will ensure that the sample fails in the desired mode for the intended application. A poorly designed compression test, for example, may actually fail in tension and hence produce meaningless data. The anisotropy exhibited by composite materials and the range of failure mechanisms has led to the development of numerous test piece geometries. Differences in fibre lay-up, fibre length and orientation may necessitate differences in the geometry of the specimen to achieve the same failure mode. Generally consulting the appropriate testing standard will readily determine which geometry is suitable for a given composite type. The high stiffness of reinforcements used in composite materials causes them to be vulnerable to bending stresses. The majority of test methods available at the current time are concerned with the in-plane characterisation of composites, but as discussed previously these materials display property variations about three symmetrical axes. Composite materials are finding much greater usage in structural applications, which has resulted in increased section thickness. Structures must hence be designed and analysed for out -ofplane as well as in-plane loading and as such material property data in three dimensions must be utilised. A review by Broughton and Sims [16] on through-thickness tests for polymer composites discusses the problems associated with the testing of thick laminates. Their evaluation found none of the tests or test piece geometry’s to be ideal. It is expected that further development of these methods will be required before the results may be used with confidence.

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An Introduction to Automotive Composites It should be remembered that test data relates only to the conditions under which it was collected and that the failure of an actual composite structure may be more complex due to mixed mode loading for example. It may be necessary therefore, to undertake both detailed analysis and in-situ material measurements of real components.

4.8.2 Reasons for Mechanical Testing There are three broad categories of tests each with its own specific purpose: 1. Determination of properties for use in analysis 2. Research and Development tests (R&D) 3. Quality Control tests (QC) The properties measured in each of these tests are a function of the specific circumstances under which the test was conducted. It is important to remember this fact when determining the reasons for doing the test and to understand the limitations of the test method. Typically, scientific testing will elucidate information on the intrinsic properties of the material whilst QC testing determines the integrity of the material or of a component. R&D work may necessitate the need for both types of tests. Evaluation may be further subdivided into destructive, e.g., tension, compression, shear, and nondestructive, e.g., acoustic emission, ultrasound, x-ray testing. Generally, QC testing of manufactured components for example, suggests the need for non-destructive characterisation where the part would either pass or fail. Determination of material properties for say, design, demands destructive testing, such as the tensile test where the material is pulled in tension until it separates into two parts to determine its strength. To characterise a material completely is an enormous undertaking since the range of properties to describe a material is extensive, e.g., strength in shear, tension and compression, density, stiffness, hardness, impact, creep, fatigue, corrosion, etc. In addition, these properties will change with temperature and perhaps strain rate, particularly for polymeric composites. For isotropic materials characterisation may be achieved by UD testing, however, for anisotropic and orthotropic materials, such as composites, only multidirectional characterisation is appropriate. The testing regime, therefore, must be suitable for the material and the intended application of the data since testing is both expensive and time consuming. A frequently used and quoted method of assessing composite materials is the flexural test (also referred to as bend testing). Flexural properties are not, however, intrinsic material properties, i.e., not one of the nine elastic constants and five strengths to describe the material, and is most often used for quality control purposes. This test has been used widely because it is extremely easy to perform and is also inexpensive.

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4.9 The Effect of Manufacturing Methods Contained in this section is a brief review of the different moulding techniques (the details of which are covered in Chapters 6 and 7) and how the material properties are affected by the different techniques. When analysing composite components it must be remembered that the properties of the composite are dependent upon both the fibre and matrix constituent properties, the relative amounts of each and the orientation of the fibres. Defects and local variation in properties caused by processing are usually neglected in an analysis as they cannot be identified and quantified before manufacture. An analyst will often assume that unless the composite component has specifically been designed to have different material properties in different areas, then the properties will be constant throughout the component. This makes the component much easier to model analytically as different properties do not have to be attached to different areas of the model. The validity of this assumption depends very much upon the manufacturing process for the composite component.

4.9.1 Resin Transfer Moulding In a resin transfer moulding (RTM) the glass mat is first heated and pressed into a mould to create a preform. Resin is then injected or transferred into the mould to create the composite component. As the glass mat is pressed into the mould the arrangement of the fibres may be distorted from that originally designed. Rudd and co-workers [17] and Karbhari and coworkers [18] state that a frequently encountered problem in RTM is the movement of the fibre reinforcement during either preforming of the fibre mat or injection of the resin. Such a distortion of the fibre alignment or arrangement may give rise to resin or fibre rich areas as well as areas containing different directions of fibres than those originally designed. This may thus cause different material properties to be observed in these areas. Rudd [17] suggests that the manufacturing process of preforming should be modelled to determine the material properties after preforming, and that these properties, not those obtained from test samples, should be used in any subsequent analysis. Rudd describes the use of software to perform a simulation of the preforming process with the objective of providing material data.

4.9.3 Laminated Material Much of the published literature concerning the finite element analysis (FEA) of components manufactured from composite materials only considers laminated materials

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An Introduction to Automotive Composites used in aerospace applications. In these materials each ply or lamina is usually considered to consist of a UD fibre reinforced material with the material properties being obtained either from manufacturers data sheets, experiments or micromechanical calculations. Each ply is then stacked at a predetermined fibre reference angle to form a laminated material consisting of many plies. Many commercial FEA programs have composite elements that allow a number of plies of material to be modelled. The number and orientation of the plies is obviously important as this will determine the overall properties of the laminated composite. If the lay up of the plies is known then the overall composite properties may be determined by using software such as CoDA [6]. When modelling a laminated material the thickness and material properties of each individual ply as well as its lay up angle and orientation relative to the other plies and the global co-ordinate system will have to be specified by the analyst. Thus, much more material data needs to be known and specified when modelling a laminate. As the orientation of the individual plies of material needs to be carefully modelled then local co-ordinate systems may be required to model the correct orientation angle relative to the component geometry. It is possible that during manufacture some of the plies may be misaligned. The FEA is thus modelling the predicted and designed fibre lay up rather than what might actually be manufactured and some sensitivity studies may be required. The analysis needs to represent the ply orientation as it actually exists in the component under consideration. This may not be an easy requirement when modelling doubly curved surfaces.

4.9.4 Compression Moulding Components may be manufactured from a compression moulded composite material such as glass mat thermoplastic (GMT) or SMC. GMT usually consists of continuous random glass fibres in a polypropylene matrix, whilst SMC usually consists of 25 mm long random glass fibres in a polyester matrix with a calcium carbonate filler. In the compression moulding process heated blanks, or charges, of material are placed in a pre-heated mould. The mould is then closed which forces the charge material to flow over the surface of the mould to form the required component. During this process the fibres will tend to flow with the matrix. The glass will therefore be redistributed during the flow and this means that the fibre orientation as well as the fibre concentration or volume fraction, will tend to vary throughout the component [19, 20, 21]. This redistribution of the glass can result in a variation of the properties of the material throughout the moulded component. The amount of variation will depend upon the length of flow, the component geometry and the initial position of the charge. If the flow path of the material is long enough then the initially random fibres may become significantly aligned in some areas as they are dragged along with

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General Properties of Composites - Stiffness, Strength and Toughness the flow. The flow of the material means that components manufactured in this manner often show measured materials data different to the manufacturers data sheet, and a variation in properties throughout, if the influence of the flow during moulding is neglected. The effect of the processing of the material upon the fibre orientation and its resultant effect causing a variation of the material properties needs to be considered before undertaking a structural analysis. If the material properties vary throughout the component then the model may need to be broken down into a number of regions with different properties assigned to each region. Local co-ordinate systems may be required to model fibre directions for each region. The model is therefore more complex and time consuming to create. The two methods of obtaining this detailed material property information would appear to be either from tests or from computer modelling of the compression moulding process. The modelling of the flow of the composite material during the moulding process has been studied by a number of researchers and suggests that flow modelling simulation software should be used to provide the resultant material properties for a structural analysis. The study of this process has led to the development of software such as PLASTEC [22] and CADPRESS [23] which models the compression moulding process and provides the varying material properties for a subsequent structural analysis. Such compression moulding analysis software is not cheap and does add another analysis step to the design process, however it may be the quickest and easiest way to obtain the required material properties for a structural analysis. Use of such software does of course require factors such as the material data, mould geometry, moulding conditions and charge position to be known and controlled during manufacture. In a flow analysis programme such as PLASTEC the material properties are determined from the fibre orientation resulting from the flow of the material. By using micromechanical equations the material properties can be predicted for each element. The flow analysis software typically uses thin shell elements and therefore the through thickness properties cannot be predicted. This may not be important if the component consists of a relatively thin sheet of material. However, if the component consists of relatively thick sections then the accuracy of the flow analysis data may be in some doubt. There would appear to be limitations in the flow simulation software that is currently available. The software currently does not consider variations in the fibre concentration and the importance and significance of considering this variation in fibre concentration is not currently understood. In addition the flow analysis currently only determines the variation in the moduli and not the strength variation throughout.

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References 1.

D.R. Askeland, The Science and Engineering of Materials, Brooks/Cole Engineering Division, Monterey, CA, USA, 1984, p.519.

2.

T-W. Chou, Microstructural Design of Fiber Composites, Cambridge University Press, Cambridge, 1991.

3.

M.K. Pinfold, Composite Mechanical Properties for use in Structural Analysis, University of Warwick, April 1995. [PhD Thesis]

4.

H. Krenchel, Fibre Reinforcement, Akademisk Forlag, Copenhagen, Denmark, 1964.

5.

Z. Hashin, ASME Journal of Applied Mechanics, 1983, 50, 481.

6.

CoDA, Composites Design Analysis, Anaglyph Ltd, Suite 33, 10 Barley Mow Passage, London, W4 4PH.

7.

L.A. Carrlsson, J.W. Gillespie, J.M. Whitney and R.L. McCullough, Delaware Composite Design Encyclopedia, Micromechanical Materials Modelling, Volume 2, Technomic Publishers, Lancaster, PA, 1990.

8.

Z. Hashin, Theory of Fibre Reinforced Materials, NASA Technical Report No. ANSI-8818, Accession No. N72-21932, 1970.

9.

C.S. Smith, Design of Marine Structures in Composite Materials, Elsevier, London, 1990.

10. Private communication, Dr W. Broughton, Division of Materials Applications, National Physics Laboratory, Teddington, Middlesex. 11. M.J. Owen, Proceedings of the 2nd International Conference on Composite Structures, Paisley, UK, 1983, Ed., I. Marshall, Applied Science Publishers Ltd., p.21. 12. L.J. Hart-Smith, Composite Structures, 1993, 25, 3. 13. LAP – Laminate Analysis Program, Anaglyph Ltd, Suite 33, 10 Barley Mow Passage, London, W4 4PH. 14. LAMICALC – Dr K. Stellbrink, Composite Software, Drosselweg 7, D-71126 Gaufelden 2, Germany. 15. J. Aveston, G.A. Cooper and A. Kelly, Proceedings of the Conference on Properties of Fibre Composite, NPL IPC Science and Technical Publications, 1971.

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General Properties of Composites - Stiffness, Strength and Toughness 16. W.R. Broughton and G.D. Sims, An Overview of Through Thickness Test Methods for Polymer Matrix Composites, NPL Report No. DMM (A) 148, 1994. 17. C.D. Rudd, V. Middleton, M.J. Owen, A.C. Long, P. McGeehin and L.J. Bulmer, Composites Manufacturing, 1994, 5, 3, 177. 18. V.M. Karbhari, S.G. Slotte, D.A. Steenkamer and D.J. Wilkins, Composites Manufacturing, 1992, 3, 3, 143. 19. H.T. Kau, Polymer Composites, 1987, 8, 2, 82. 20. V.K. Stokes, Proceedings of Antec ’91, Montreal, Canada, 1991, 2102. 21. T. Hirai, Proceedings of the 6th International Conference and the 2nd European Conference on Composite Materials, Eds., F.L. Matthews, N.C. Buskell, J.M. Hodgkinson and J. Morton, Elsevier Applied Science, London, 1987, Volume 1, 121. 22. L.G. Reifschneider and H.U. Akay, PLASTEC Prediction of Stiffness and Thermal Expansion Coefficients from Fibre Orientation Analysis Results, Technalysis, Inc., 7168 Zionsville Road, Indianapolis, IN 46268, 1990. 23. CADPRESS, Compression Moulding Simulation, The Madison Group, Polymer Processing Research Corporation, 565 Science Drive, Madison, WI 53711, USA.

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How Can We Use Composites in Car Manufacture? Kevin A. Lindsey and Nick Tucker

5.1 Introduction 5.1.1 What do We Mean by Composites? We shall define and redefine the idea of a composite material from various viewpoints as we progress through this book. As a starting point, if we mix differing materials, the resulting mixture may exhibit material properties somewhere in between those of the individual ingredients. If we glue fibrous materials into bunches or sheets, the mechanical performance of such materials is very high, especially considering their low density. Why should this be? Let us consider why this should be the case from first principles.

5.1.1.1 How Strong Should a Material Be? We know that materials are constructed from atoms. The atoms may be grouped together into clumps called molecules, or arranged in orderly ranks as crystals. These groupings are bound together by chemical bonds. The nature of these bonds is very well understood, having been studied intently by chemists and physicists over the past few centuries. The strengths of these bonds are very well quantified and we judge the measurements to be true because the same values are used to underpin calculations about the speed and efficiency of chemical reactions. So it is possible to calculate a theoretical strength for a material from the known strength of the bonds holding the fundamental particles together. However when we make samples of a material and test the strength by snapping them the results are disappointing, the values are always well below our theoretical calculations.

5.1.1.2 Why Aren’t They That Strong? We should consider why a bit of material breaks where it does. If we apply a load to our test piece it will snap at the weakest place. Even if we have done our very best to produce a beautifully smooth polished uniform piece there will still be a point of weakness. In the

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An Introduction to Automotive Composites ultimate case this will not be some small blemish in the surface of the material, but a dislocation of a few layers of atoms. It is from this atomic level defect that a crack can start to propagate causing the complete failure of our specimen.

5.1.1.3 What Can We Do to Overcome This Problem? If we draw out a material into a fibre it is possible to measure the strength at certain thicknesses as we progressively draw the fibre thinner and thinner and longer and longer. The results of this experiment show that as the fibre gets thinner, the nearer we get to our calculated theoretical value. The reason for this is that the drawing process removes the molecular scale defects that are the origin of our fatal cracks as J.E. Gordon says ‘think of it like this; a fibre made of a single strand of atoms must have the theoretical strength, or none at all’ [1]. Thus at a fibre thickness for glass of about 2.5 μm, the strength of a newly stretched glass fibre is quite close to the theoretical maximum.

5.1.1.4 What’s This to Us? Even though the individual fibre has a tensile strength of near it’s theoretical maximum, it is still by itself, not a terribly strong thing. It is also prone to losing its strength by damage to the surface, even touching a strand with a greasy finger can do it. The reason that this curious property increase is more than a laboratory curiosity is because when such fibres are treated to protect them against surface damage and combined into felts, wovens and bonded fabrics it is possible to bind them into composite materials which have the potential to exhibit on the large scale, the excellent mechanical properties of the reinforcements seen in individual fibres.

5.1.1.5 So What Are Composites? Composites exploit the good mechanical properties of small diameter fibres by binding them into a mass with some form of glue (often a thermosetting polymer). The properties of this material will a combination of the properties of the ingredients for example, the strength and stiffness of the fibres and the chemical resistance (protecting the fibres) of the polymer glue or matrix.

5.1.2 What Are Structural and Semi-Structural Elements? Throughout this book there are a number of instances of structural and semi-structural components. Structural applications include the primary structure that would be

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How Can We Use Composites in Car Manufacture? equivalent to the longitudinals, bulkheads and monosides of current vehicle build technology. Semi-structural panels include front ends (radiator closure panels), reinforcement panels for closures and possibly roof panels and any additional horizontal panels. This concept assumes that the primary structure will maintain the body stiffness and strength with additional hang-on vertical panels such as fenders and rear quarters.

5.2 The Drivers for the Use of Composites As with any material used in the automotive industry, there are two major drivers for the use of composites; weight and cost. As is also the case with the automotive industry, the need to reduce manufactured cost is paramount, and overwrites any technical consideration. The effect of this is considered in Chapter 8. Currently the whole of the motor industry has a major focus on weight and weight reduction. The reason for weight reduction the concern for the environment and, in particular, the effect the motor vehicle has on the environment. Reducing vehicle weight improves fuel efficiency, thus reducing the environmental burden of the motorcar. The requirement for cost is at first sight more obvious – we would all like cheaper cars – but the effect of cost on the vehicle cost is more complex than may be first imagined and is discussed in Chapter 8 on economics. In Europe, the European Commission, Council and Parliament have requested a CO2 reduction target, which states that by 2005 the weighted fleet average of CO2 emissions for European new car sales must be less than 120 g/km. The emission of CO2 is directly proportional to fuel consumption, and 120 g/km equates to about 56 mpg. The auto industry has put in a counter offer of 140 g/km by 2008 with a further reduction by 2011. To meet even the reduced targets typical volume car manufacturers will have to improve fuel economy by between 25% and 40%. In the short term, fiscal measures to force the automotive companies to meet the CO2 target are not likely if the automobile companies can show that they will reach the required targets. If companies do not meet the voluntary 2008 requirements it is likely that at this point some sort of fiscal measures will be applied, forcing customers to pay more for the same vehicles. Whatever measures are taken, CO2 emissions will be an extremely important competitive issue for the next decade and the companies that win will be those that manufacture the most fuel efficient vehicles. There are a number of ways to reduce fuel consumption - such as improving powertrain efficiency, reducing rolling resistance of tyres and improving aerodynamics as well as

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An Introduction to Automotive Composites saving weight. All of these must be addressed. It has been shown that changes in these give varying effects on fuel consumption. Powertrain efficiencies give the greatest single benefit, but the industry is keen to continue to give similar or improved vehicle performance, whilst still meeting the emissions requirements, and therefore weight reduction is a good option to consider. It has been shown that a 10% reduction in weight can give approximately a 5% reduction in fuel consumption (depending upon size of vehicle). As part of the very conventional looking P2000 family of vehicles, the Ford Motor Company is developing a hybrid power train sport utility vehicle claiming that an aluminium body in white, frame, suspension and closures provide major weight savings of as much as 50 percent compared with conventional designs. To make a lighter body shell, the lightweight and high mechanical performance of polymer composites make them an attractive choice for the entire body structure. This concept, discussed in more detail in later chapters, is the far future, but composites are already in use in a number of applications. This is the subject of the following sections.

5.2.1 Where Should We Save Weight? Having established that we need to reduce vehicle weight, the best areas for vehicle mass savings need to be identified. We need to consider the vehicle as a whole. If we save weight on the body then we can reduce engine size and can therefore save weight on the powertrain. In turn we can then save weight on the chassis. By reducing weight of powertrain and chassis components we can make further savings on the body. It is considered, that for every 1 kg of weight which can be saved on a component, at the concept design stage of a vehicle, it is likely there is an opportunity to save a further 0.3 kg elsewhere, due to this ‘knock-on’ effect. In order to ensure that vehicle stability is not compromised, it is essential to ensure that the centre of gravity does not become higher as we make weight savings. For example, if the weight of (low down) chassis components were reduced, without making any other reductions, then the centre of gravity would obviously move up. When considering weight reduction then the body is a good place to start. The weight of the body, including the trim and hardware, accounts for about half the vehicle weight; but it has been shown in the study done by the Rocky Mountain Institute in the States, on so called ‘hypercars’ (Figure 5.1), that in the extreme case, if we save approximately 60% on the body weight, then we could save about 60% on the total vehicle weight, the additional mass savings being due to knock on effects.

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Figure 5.1 Amory Lovins concept Hypercar (Source: The Hypercar Centre)

5.2.2 How Do We Save Weight? There are a number of ways of reducing vehicle weight. These are: •

Better use of existing materials.

•

Use of alternative lightweight materials

•

Use of alternative manufacturing methods.

5.2.3 Better Use of Existing Materials Simply reducing the amount of material in a part, by making it smaller or thinner will obviously reduce vehicle component mass. This can be achieved by ensuring that the design is optimised and that it is not ‘over-engineered’ in any area. Using existing materials with new manufacturing processes, e.g., the use of laser welded blanks in vehicle bodies, to enable us to put the thickness where we want it and use thinner gauges where this is unnecessary can also save weight. Another example may be the hydroforming of steel tube to replace fabricated box sections.

5.2.4 Alternative Lightweight Materials We can use alternative materials, e.g., we can replace the steel currently used in the body with high strength grades, saving about 10% on the weight component to component. It

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Figure 5.2 ULSAB vehicle (Source: American Iron and Steel Institute)

is considered that up to 60% of the steel in a body could be replaced with high strength steel; this could give a weight saving of up to about 20 kg. It has been shown in the UltraLight Steel Auto Body (ULSAB) – see Figure 5.2 (run by an international consortium of sheet steel producers) programme that by careful use of high strength steels, together with tailored blanks and hydroformed components that the weight of a conventional vehicle body structure could be reduced by 66 kg. Replacing one material with another is not always straightforward, and there are many factors to consider; such as the cost (both piece cost and tooling), performance (both static and dynamic and durability). The performance at environmental extremes must also be considered, one factor that is growing in importance is the environmental impact associated with using the material, considering aspects such as its recyclability, as well as analysing energy utilisation from ‘cradle to grave’.

5.2.5 Alternative Manufacturing Methods Already mentioned is the use of tailored laser welded blanks and hydroforming in current vehicle monocoque bodies, but also the vehicle body could be built differently for example an aluminium space-frame structure clad with aluminium or composite panels could give weight benefits. It is usually assumed that a monocoque structure will be heavier than its space frame equivalent. We could consider greater integration of components, particularly in considering greater use of plastics and composites. A more modular approach to building vehicles may increase the opportunities here.

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5.3 Examples of Composites in the Automotive Industry There are already a number of composites used in the automotive industry. A few examples of which follow. These are mainly from a European standpoint although examples of the US industry where some composites are widely used are also included.

5.3.1 Composite Modular Front Ends The first composite front end was introduced in 1987 on a Peugeot 405, manufactured from sheet moulding compound (SMC). Others, e.g., Peugeot 605 and Citroen XM (Figure 5.3), also in SMC, followed this.

Figure 5.3 Citroen XM – a composite front end (Source: Citroen)

The one-piece composite front end replaces a number of steel panels, which make up the front end of a more traditional vehicle body and this gives a weight reduction. It is also possible to integrate other parts reducing weight further, for example by eliminating additional brackets and fixings etc. More recently composite front ends have been produced from thermoplastic composites, for example the Audi A4 (Figure 5.4) and the Volkswagen Golf and Polo. Also a further development has been the Ford Focus (Figure 5.5), which has a polymer composite/steel hybrid front end.

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Figure 5.4 Audi A4 – thermoplastic composite front end (Source: Audi)

Figure 5.5 Ford Focus – hybrid front end (Source: The Ford Motor Company)

5.3.2 Front Fenders (and Other Skin Panels) Most front fenders are in steel and this is the most cost effective material currently for high volume applications. There are a few vehicles that have aluminium bodies, such as the Audi A8 and the Honda NSX, and there are other vehicles such as those in the Land Rover range that have aluminium skins on a steel inner structure. The use of aluminium can reduce the weight of a fender to about half that of the similar steel component, but at a significant increase in the cost (about twice the cost). Composites can be used though these so far have been mainly confined to SMC. There are SMC fenders as used on the

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How Can We Use Composites in Car Manufacture? Lincoln Continental. This uses a new flexible grade of SMC, perhaps to attempt to compete more directly with injection moulded parts. Other composites used for fenders are those based on reinforced reaction injection moulding (RRIM). There is structural reaction injection moulding (SRIM) as used on vehicles such as the Pontiac Fiero, GM’s ‘F’ Car (Camaro & Firebird), and the GM APV. For most fenders, however, the materials of choice seem to be thermoplastic injection mouldings rather than composite panels. Thermoplastic injection moulded panels, e.g., as used in the US since 1988 on GM’s ‘C’ & ‘H’ cars, also used on the Chrysler LH, and on GM’s Saturn, and in Europe on the Mercedes 500E, Renault Clio 16v, Renault Megane Scenic and Rover Freelander (Figure 5.6).

Figure 5.6 Land Rover Freelander – thermoplastic injection moulded fenders (Source: Land Rover)

5.3.3 Tail Doors There are a number of composite tail doors on vehicles currently, such as the Citroen AX, BX, ZX and Xantia, PSA’s MPV, Fiat’s Tempra Estate, Regata and Tipo, the Renault Espace (Figure 5.7), all of which are in SMC, and then there’s the Vauxhall Tigra (in this case the outer panel only), as mentioned earlier. To date most of the composite examples are SMC - this is predominantly due to the stiffness of the material (and its similar Coefficient of Linear Thermal Expansion (CLTE)

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Figure 5.7 Renault Espace – Composite tailgate (Source: Renault, France)

to steel). However there are issues with the use of SMC such as the difficulty in achieving a truly ‘A’ class finish. In the US there is considerable experience of the use of SMC for body panels and have developed the various techniques to improve the surface finish, such as in-mould coating and moulding with vacuum. It is still debatable whether the surface finish meets the expected quality aspirations that volume manufacturers are striving for and which are expected by discerning customers. An exception to SMC is the Tigra tail door, which has a polypropylene outer panel as mentioned earlier; however this has a double skin steel construction underneath and is unlikely therefore to save much weight. There have also been a number of developments to investigate thermoplastic tail doors in the industry and the new Mercedes A Class (Figure 5.8) has a thermoplastic tail door. This component has a glass mat thermoplastic composite (GMT) inner structure bonded to a Noryl GTX outer panel (similar material to that used on the Freelander. It is estimated that an average size SMC tail door could save about 3 kg over a steel assembly, but the combination of a thermoplastic injection moulded outer on a GMT inner could save about 4 kg.

5.3.4 Side Doors In considering side doors, many of the same comments apply as those mentioned with reference to tail doors. There are a number of composite doors fitted to vehicles - again mainly SMC, for example those with composite inners and outers are GM’s Corvette,

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Figure 5.8 Mercedes A Class - thermoplastic tail door (Source: Mercedes-Benz)

Camaro and Firebird, all in SMC and then there are a few with plastic outers on a steel structure such as the Pontiac Fiero, Mazda AZ-1, Renault Espace in SMC and BMW Z1 and Saturn, both with injection moulded outer panels. The requirements for a side door are more structurally demanding than a tail door, with the requirement for side intrusion. It is perhaps unlikely that in the short-term that we can make a viable door without some form of metal reinforcement (be it steel, aluminium or perhaps even magnesium). As mentioned for tail doors, there is a possibility of using thermoplastics for door outers with a reinforced composite inner, but this time with a metallic frame structure. It is estimated that weight savings of up to 5 kg per door may be achieved with this approach, with a steel frame. In the long-term perhaps an all-composite side door may be possible, giving further weight savings.

5.3.5 Fascias Most of the weight saving opportunities on fascias revolves around changing the material of the cross-car beam and/or integration of this, and other components such as the air ducting, etc., with the fascia, or fascia armature.

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An Introduction to Automotive Composites For example, the cross-car beam on the Ford Ranger pick-up is in SMC which gives a saving of 2.3 kg. The fascia for the Dodge Dakota comprises of three injection mouldings in PC/ABS. This replaces the cross-car beam and weight savings of 2.5 kg are obtained. The design also gives a 50% saving on parts count and defrost performance is said to be improved by 30%. Armatures in magnesium have also been demonstrated integrating the cross-car beam. Savings of up to 5 kg are said to be achievable. Fiat have used this technology on the Fiat Marea.

5.3.6 Seating The seating in a vehicle is another area worthy of mention when we are considering weight saving opportunities, since this typically weighs 50-60 kg for a saloon car (Figure 5.9). Audi have announced a polymer composite/metal hybrid seat that claims to save approximately 5 kg per seat. This follows previous work where Mercedes claimed up to a 3 kg weight saving through the use of polymer composites.

Figure 5.9 Reco seat - a WMG development model for composite seating (Source: WMG, Warwick, UK)

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5.4 Summary Composite materials are already being used in various forms throughout the automotive industry, from sheet moulding compound (SMC) fenders to thermoplastic composite tail doors. The use of composites has been driven by the requirement to save weight and also by the reduction in investment costs associated with composites. Future economic and environmental pressures will tend to increase the use of low-density materials and composites in particular.

Reference 1.

J.E. Gordon, New Science of Strong Materials or Why Things Don’t Fall Through the Floor, Penguin, London, UK, 1991,

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6

Manufacturing with Thermoset Composites Andrew J. Hulme, Mark R. Johnson and Nick Tucker

6.1 Manufacturing Methods 6.1.1 Mixing reinforcement with resin All composites manufacturing processes involve mixing the matrix material with the reinforcing material. It is possible to accomplish this by mixing fibres into the matrix and then using some method of pumping to form the finished article from the mixture. However, as we begin to add significant quantities of reinforcement to our resin, the viscosity of the mixture rises steeply, and we are unable to pump our slurry into the mould tooling. This limits us to about 20 vol% for these methods. Typical applications of this idea are thermal injection moulding of composite articles, reinforced reaction injection moulding (RRIM), and dough and bulk moulding (DMC and BMC). Note that in this type of process we have no direct control over where the reinforcement ends up in our finished product.

6.1.2 Impregnating the reinforcement with resin If we consider the other approach of insinuating the matrix into a preformed mat of reinforcement, the picture is much more interesting. Given a suitably thin (probably thermoset) resin, we can readily imagine pumping it through a densely packed reinforcing mat, and achieving a thorough uniform penetration or wet out. It is at this point that we are able to exploit the low viscosity of thermoset resin systems. Note that as we heat and pump such a system it will initially decline in viscosity because the thinning of the liquid associated with the physical process of heating and shearing will, in the early stages, happen quicker than the rise in viscosity associated with the chemical reactions of polymerisation. We are therefore in a good position to approach the theoretical maximum reinforcement loading (in the region of 70 vol% for continuous filament glass mat – 100 vol% glass would be like a window pane). If we consider packing orientated glass fibres together, there will come some point where we cannot compress the fibre mass any further without damaging the fibres and reducing the strength of the finished composite - it is this point that gives us maximum material strength, and probably our lightest component. However it should be noted that as we

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An Introduction to Automotive Composites approach this theoretical maximum, the resin rich outer surface of the finished article disappears, leaving us with a product that is prone to damage by ingress of moisture. Any weight saved by working at this level of reinforcement will be lost by the need to add a protective coating of paint to the finished product.

6.1.3 Consolidating the Product To make our composite article we must also have a consolidation phase. In this part of the operation the reinforcement is packed to the highest practical density. This is either done before resin impregnation, as with closed mould processes such as resin transfer moulding (RTM) or structural reaction injection moulding (SRIM), of after resin saturation as in filament winding or pultrusion.

6.2 Contact moulding The factors that started off the popular rise of composite articles were the availability of reasonably cheap glass fibre, and the development of cold cure polyester resins. These events coincided in the early 1950s. Polymer matrix composite (PMC) manufacture had of course already been used on both sides of the Atlantic in the manufacture of radomes (Figure 6.1).

Figure 6.1 A Gloucester Meteor Radome (Source: Nick Tucker)

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Manufacturing with Thermoset Composites (by resin injection in some cases), but the popularisation of the cause did not occur until a little later, when the low capital costs of start up (bucket, brush, roller, shed) lead to a golden age of low volume kit car manufacture. The process is simple. We make a master pattern (‘buck’) of the article we wish to make, this master can be a wooden frame, covered in wire mesh, with the final surface finish being plaster. The surface is then treated to make it impermeable (cellulose acetate being one option). A gel coat of neat polyester resin is then painted on to a thickness of about 0.5mm. As a rule of thumb, the next layers of glass fibre and polyester resin should be laid up about twice as thick as the finished mounding is expected to be. It may be necessary to add stiffening ribs to the mould. The glass is rolled into place with a roller to consolidate it and remove any trapped air. Once the mould has cured it can be parted from the buck, it can be polished (the standard of surface finish achieved here reflects the finish of the final product) and coated with a release agent. The mould is then gel coated (a slightly thinner coat than for the mould) – too thick a coat will lead to crazing on the moulding surface, too thin a coat will lead to fibre witness on the surface and the risk of fibre delamination due to water ingress. After the gel coat has dried to a tacky finish, the fibreglass is laid up on top of it. Resin is brushed in and stippled and rolled as before to produce a well-consolidated composite (say 30 vol% reinforcement. This method clearly suits the craftsman, and is very suitable for short production runs, however, increasing concerns with the level of styrene fumes evolved by this method are likely to lead to a decline in its popularity.

6.3 Resin Infusion under Flexible Tooling (RIFT) Increasingly strict legislation to limit styrene emissions (in this case from polyester resin systems) has been the key factor in promoting the increased use of resin infusion under flexible tooling (RIFT). Variations on this theme include vacuum assisted resin injection (VARI) and Seemans Composites Resin Infusion Process (SCRIMP). RIFT uses tooling very similar to that used in contact moulding. The mould is release treated, and gel coated, and dry reinforcing fibre is laid up in the mould. A flexible membrane is then laid the mould and sealed at the edges. The mould cavity is then evacuated, and the pressure of the atmosphere is then used to infiltrate the resin through the reinforcement (see Figure 6.2). The requirements for successful RIFT processing are a low viscosity resin system with a long pot life (time to gelation).

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Figure 6.2 Schematic diagram of RIFT

This method was first described in 1950 as the Marco method where resin is drawn from a moat surrounding the mould, in 1972 the Lotus Group patented VARI, in this case the resin is poured into the mould before closure. In 1991, Seeman attempted to gain UK and European patents for the SCRIMP process, which has the novel aspect of using a mesh to distribute resin within the tool. These methods are routinely used to manufacture boats up to 28 metres in length. However, RIFT methods are really a bit slow for high volume automotive use.

6.3.1 The Advantages of RIFT •

It is a closed mould process

•

Styrene emission reductions of up to 90% are claimed

•

Improved working environment

•

Relatively cheap set up costs

•

Existing moulds can be adapted

•

Large mouldings can be produced (28 m boat hulls)

•

Heating and ventilation costs are reduced

•

Labour savings of over 50% over hand lay-up are claimed

•

The quality and reproducibility of mouldings are enhanced as are the mechanical properties of the mouldings produced.

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The laminating process can be integrated. Cores, stiffening, inserts, etc., can be moulded with the parent laminate

•

Most resins and reinforcements can be used.

6.3.2 The Disadvantages of RIFT •

There is an apparent add-on cost of moulding disposables which must be balanced out against labour savings, reduced styrene levels, and improved working environment.

•

New techniques and skills must be learnt and a different approach to moulding shop management implemented with emphasis on preparation.

•

The question of waste disposal.

•

Resin and reinforcement specification is important to maximise potential benefits of the process. Compaction due to the vacuum reduces thickness increasing fibre content and strength but reducing stiffness for a given lay-up, consequently components must be designed (or redesigned) to maximise the benefits of the process and the laminate properties available. (based on ‘Vacuum Injection - a low cost entry into closed mould processing. The hand lay up moulder’s answer to the styrene issue?’ J. Nixon, Scott-Bader Ltd).

6.4 Pre-pregging (autoclaving) Prepregs are reinforcement materials prepared by the PRE imPREGnation of the reinforcement by partially polymerised resin. The builder then has only to cut the desired shape from the prepreg, drape it over a form, and apply heat and pressure to consolidate and cure the finished article. This method produces high quality, void free, components, and is popular with the aerospace industry who have learnt to cope with its slow cycle times. It may take a month to produce a single helicopter rotor blade.

6.4.1 Consumable items in vacuum bag processing The high quality of prepreg mouldings is achieved at the expense of a significant amount of in-process waste. This limits it’s application to ultra high performance niche vehicles, e.g., Formula 1 racing cars.

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An Introduction to Automotive Composites Release agent - allows the release of the cured prepreg component from the tool. Peel ply - Optional layer allowing free passage of volatiles and excess matrix during the cure. Can be removed easily after cure to provide a bondable or paintable surface. Bleeder fabric - Usually made of glass fabric felt and absorbs the excess matrix. The matrix flow can be regulated by the quantity of bleeder, to produce composites of known fibre volume. Release film - This layer prevents further flow of matrix and can be slightly porous (with pin pricks) to allow the passage of only air and volatiles into the breather layer above. Breather fabric - Provides the means to apply vacuum and assists removal of air and volatiles from the whole assembly. Thicker breathers are needed when high autoclave pressures are used. Vacuum bag and sealant - Provides a sealed bag to allow removal of air and consequent formation of a vacuum. The advantages of prepregging are: •

Good control of fibre orientation using uni-directional prepreg

•

High temperature properties available from thermoset resins

•

Inexpensive versatile tooling concepts

•

Resin formulation and impregnation is the prepreg manufacturers responsibility

•

Chemical process control by end user is limited to setting cycle times and temperature profiles

The disadvantages of prepregging are: •

Expensive in raw materials both prepreg and throw away breather, vacuum bagging etc.)

•

Prepreg has a limited shelf life

•

Refrigeration of raw materials is required

•

Expensive and time consuming hand lay-up is required

•

Workers are exposed to reactive chemicals

•

Long energy intensive cure cycles (may be multiple cycles)

•

Multiple autoclavings may be necessary to allow the build of different functional layers in the component, e.g., heater elements.

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Figure 6.3 Schematic of the autoclaving method

6.5 Compression moulding A vertical press is used (see Figure 6.4). The mould is opened and a pre-measured amount of material is placed into the fixed female half of the mould. Material charging is often carried out by hand, with the next charge being stored at elevated temperature to reduce cycle times. The mould is closed, forcing the material to fill the mould cavity. Good surface finish and dimensional tolerances are achieved by adding a slight excess of material to ensure that there is sufficient moulding pressure. The mould is left closed long enough for the material to heat up and the curing reaction to take place. Once this has occurred then the mould is opened and the part removed. The advantages of compression moulding are: •

Simple process.

•

Inserts can easily be placed into mouldings.

•

Large components can be moulded.

•

Damage to reinforcement fibres is kept to a minimum.

•

Mould press can be relatively simple, especially for small moulds.

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Figure 6.4 Schematic of a compression moulding press (Source: Industrial Dielectrics (UK) Ltd.)

The disadvantages of compression moulding are: •

Labour intensive process, with hand loading and removal of material. Mould flash has to be removed.

•

Cure time depends on wall thickness. A thicker section will take longer to heat up, hence the time until the material solidifies will be longer.

•

Cold material does not flow as well and causes wear on the tool. Pre-heating of material helps it to flow more readily.

6.5.1 Injection moulding For thermosets the process is similar to injection moulding of thermoplastics (see chapter 7), the main difference being that the injection barrel is run at lower temperature 30 – 60 oC and the mould is heated to promote cure, rather than cooled to freeze the mould contents – see Figure 6.5. A reciprocating screw plasticises the material through conductive and frictional heating. The heating lowers the viscosity of the resin making flow easier. The temperature has to be kept below a critical level to prevent the curing reaction taking place. Thermoset resins ‘going off’ in the barrel of an injection moulder can be disastrous - the machine must be taken out of service, and the barrel dismantled

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Figure 6.5 Schematic of a thermoset injection moulding machine (Source: Industrial Dielectrics (UK) Ltd.)

for cleaning. Material is injected into the mould by the screw moving forward. Cycle times are shorter than compression moulding since the material is preheated uniformly in the barrel. This process is more suited to large production runs of smaller mouldings. Advantages: •

Shorter cycle times than compression moulding.

•

Consistent moulding results.

•

The shot weight of each moulding can be accurately controlled.

Disadvantages: •

Material wears the screw and barrel much more quickly than thermoplastics.

•

Material is wasted in the form of sprues and runners.

•

Flow and weld lines often show up in mouldings, especially where material has to flow around cores in the mould.

•

Moulding material must have consistent flow properties to ensure that mould quality remains constant.

•

Reinforcement fibres are damaged due to the high shear experienced in the injection barrel. This limits the process to materials containing shorter fibre lengths (5 – 10 mm).

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High machine cost when compared with a compression moulding machine. A facility is also needed to force material into the barrel as it is not free flowing granules. This usually takes the form of hydraulic ram known as a ‘stuffer’.

To reduce the effect of weld lines in mouldings with cores the two moulding processes are combined for injection-compression moulding. This operation is carried out on a modified injection moulding machine. The material is injected into a mould that is not fully closed. Once the shot has been injected then the mould is fully shut, compressing the material into the mould and eliminating weld lines.

6.5.2 Materials Dough Moulding Compound (DMC) and Bulk Moulding Compound (BMC) These take the form of a dough-like mixture of thermoset resin, short glass fibre reinforcement and mineral fillers. It is usually supplied as pre-measured ‘cheeses’ that can be easily used in injection moulding machines, although the material is equally suited to compression moulding when supplied in rope form. DMC is made in a Z-blade mixer (Figure 6.6) where unsaturated polyester resin dissolved in styrene is mixed with calcium carbonate to make the dough. A catalyst (organic

Figure 6.6 Preparation of dough moulding compound (DMC) using a Z-blade mixer (Source: DCB Mouldings) 100

Manufacturing with Thermoset Composites Figure 6.7 DMC manufacture (Source: DCB Mouldings) peroxides) is needed in small amounts to activate the curing reaction when heat is applied. Short glass fibres, typically around 6mm in length are added to the mix to give a fibre volume fraction of around 15%-20%. Other materials can be added to give different properties (Table 6.1) to the material: •

Magnesium oxide increases the resin viscosity resulting in mouldings with better surface finish.

•

Alumina tri-hydrate (ATH) increases fire resistance by decomposing when heated to release water.

•

Zinc stearate is used as a mould release and can be added to moulding compounds in small quantities to aid component removal.

•

Coloured pigments can be easily added to provide decorative finishes.

Thermoplastic additives such as polyvinyl acetate or PMMA may be used to improve surface finish. These melt in the hot tool and create a resin rich region on the surface of the moulding.

Table 6.1 Typical Properties of DMC Density

1.6-2.0 g/cm3

Tensile Strength

20-40 MPa

Flexural Strength

80-115 MPa

Flexural Modulus

7-12 GPa

Heat Distortion Temperature Thermal Conductivity Coefficient of Thermal Expansion

200 °C 0.4 W/m °C 20 x 10-6 m/m °C

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An Introduction to Automotive Composites Reaction inhibitors, e.g., hydroquinone, can be used to improve the stability of the material. At room temperature the activated resin will have a finite shelf life before it begins to cure, as the temperature increases then the shelf life is rapidly reduced. Adding an inhibitor prevents the reaction occurring until the material is above a threshold temperature, usually around 80 oC. Carbon black can be used to create an electrically conductive material for screening and earthing purposes. Advantages of DMC are: •

Good mechanical strength and stiffness.

•

Low density.

•

Hard wearing.

•

Can achieve good surface finish.

•

Good dimensional stability at elevated temperatures.

•

Good chemical stability and electrical resistance.

Example mouldings include electrical connectors, e.g., fuse box housings, relay bases, under bonnet covers and manifolds (Figure 6.8) where higher temperatures are experienced. Headlamp reflectors are injection moulded in DMC, a resin rich surface on the reflector gives a ‘class A’ finish onto which a metallic coating can be deposited (Figure 6.9).

Figure 6.8 Example DMC mouldings - fuseboxes and distributor caps (Source: Industrial Dielectrics (UK) Ltd.)

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Figure 6.9 headlamps from DMC (Source: Industrial Dielectrics (UK) Ltd.)

Figure 6.10 Ford inlet manifold (Source: Industrial Dielectrics (UK) Ltd.)

At the end of their life DMC can be recycled in a number of ways, it can be ground down to a powder which can be used as an inert filler to replace calcium carbonate in DMC production. The material can be broken down chemically or thermally to recover the glass and filler, with an additional product of an oily substance that can be refined back into new resins

SMC Sheet Moulding Compound (SMC) as the name suggests is supplied in sheet form. The basic ingredients are the same as with DMC, however the glass fibres used are longer, usually around 25mm. Material is made continuously and rollers are used for the mixing 103

An Introduction to Automotive Composites

Figure 6.11 Manufacture of SMC pre-preg (Source: DCB Mouldings)

rather than Z-blades. Material is produced between plastic backing sheets, to prevent styrene evaporation and to enable the material to be stored on a roll without the layers sticking together. The material properties can be varied to suit the application and processing by using the same additives as with DMC. The longer fibres give SMC higher mechanical strength and stiffness. However, processing is limited to compression moulding, injection moulding has to be ruled out as fibre damage caused by the screw is too great. Some short fibre SMC is produced for injection moulding but this is really a half way material between DMC and SMC. A sheet of SMC is placed into the open mould, the sheet may be cut into shape to suit the moulding or extra layers of material added where needed. The longer fibres mean that SMC does not flow as easily as BMC. This limits the kind of mouldings that can be achieved with SMC. Deep bosses and ribs may only be filled with resin. Careful tool design is required to ensure that glass fibres reach the full depth of bosses. SMC has a higher strength and stiffness in comparison with DMC, due to the longer fibre lengths. SMC also has similar electrical and chemical resistance properties to BMC SMC has a cheaper component cost than steel for up to 300,000 parts per year (European SMC alliance). However, SMC components are only seen on 10%-15% of vehicles produced in the region of 100,000 per year (Reinforced Plastics, May 1998). 104

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Table 6.2 Typical properties of SMC Density

2.0-2.8 g/cm3

Tensile strength

30-200 MPa

Flexural modulus

10-20 GPa

Heat distortion temperature

200 °C

Max continuous working temperature

150 °C

Thermal conductivity Coefficient of thermal expansion

0.4 W/m °C 20 x 10-6 m/m/°C

Advantages of SMC are: •

Low density.

•

Creep resistant.

•

High tolerance mouldings can be produced.

•

Large area mouldings can be produced.

•

High strength and stiffness.

•

Environmental resistance.

•

Vibration sound damping.

•

Parts integration, the number of mouldings to make up a component is far less than the steel equivalent.

•

Cheaper than steel pressings even for medium annual volumes.

Disadvantages of SMC are: •

Deep ribs and bosses can be difficult to fill with reinforcement.

•

Surface can suffer from small pinholes and blisters. Rework to fill holes is often carried out to obtain a ‘class A’ finish.

•

High tooling cost.

Typical applications for SMC components tend to be large single piece mouldings. Examples include electrical switch boxes, railway carriage panels, domestic external meter boxes (gas and electricity). In the automotive industry, European manufacturers Renault and Citroen produce SMC body panels, tailgates and bonnets (Figure 6.12). Other popular

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Figure 6.12 Citroen Xantia tailgate, produced by Menzolit Fibron

automotive applications include sun-roof assemblies, rear bumper beams for sports utility vehicles (SUV), truck cab panels and spoilers. Removable roof mouldings are also popular for off road vehicles. The mouldings are stiff and lower weight than steel panels, making removal and fitting easier (Landrover Freelander, Vauxhaull Frontera) are examples of this. Under bonnet applications include valve covers (Figure 6.13) and an SMC cam box. Where stiffness at elevated temperature is needed and extra sound deadening is provided.

Figure 6.13 Rover engine valve cover moulded by Sertec-PMC

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Figure 6.14 A Rolls Royce cam box (Source: WMG)

A cam cover (Figure 6.14) has been made which takes full advantage of the properties of SMC. A stiff, creep resistant moulding was made to very tight tolerances. The component also has the advantage of damping the gearbox vibrations and withstanding the harsh seawater environment. In the USA, SMC automotive components are more widespread. Both Ford and GM produce a liner box for their sports utility vehicles. The SMC liner is dent and corrosion resistant, it can be repaired easily with filler. Using SMC instead of steel gives a weight saving of 30%. The Budd Company have produced a SMC front end moulding for the Ford Taurus in one piece with all fixing inserts included (Figure 6.15). Another high profile moulding they produced

Figure 6.15 SMC front end moulding for the Ford Taurus (Source: The Budd Company)

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Figure 6.16 Two-piece SMC windscreen surround, replaces a multi-part steel pressing (Source: The Budd Company)

was the windscreen surround for the Plymouth Prowler sports car (Figure 6.16). The moulding is both stiffer and lighter than the equivalent steel component, saving around 35% in weight.

6.6 Resin Transfer Moulding First described by Mr J. Rees in 1957 RTM has proved an attractive first step into the production of closed mould composite articles (by closed mould, we mean a mould with a cavity - geometrically similar to those used in thermal injection moulding). RTM machines usually inject long pot life resins at low pressures (less than 1 MPa) and slow speeds, producing high quality mouldings. The low injection pressures mean that tooling can be composite, and consequently the whole process is low capital cost. Single pot resin systems are often used. Such systems (usually epoxies or bismaleimides) are thermally activated and as the name implies do not require any mixing of ingredients on site. These systems are particularly popular with aerospace end users because there is no possibility of error due to on site mixing of components. Injectors for such systems are little more than heated plenum chambers. The resin is introduced to the chamber and injected into the mould by means of air pressure. More sophisticated multistream systems are available whereby the resin is mixed with its catalyst just before it enters the mould. Unfortunately, the low injection rates mean that the resin/catalyst stream must pass through a static mixer (a chamber with a convoluted passage) in order to make mixing complete. It is necessary to flush this chamber with solvent at frequent intervals to avoid blockages with cured resin.

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Figure 6.17 A polyester system RTM machine (Plastech Megaject) (Source: Nick Tucker)

The single pot system (Figure 6.18) uses a premixed resin. The curing reaction will only go at an appreciable rate above a certain temperature. Thus it may be necessary to store the resin at low temperatures to get a tolerable shelf life. The pail unloader has a heated platen which melts the layer of resin adjacent to it. This liquid resin is then pumped into the mould cavity. The mould has been previously treated with a non-silicone release agent and loaded with the preformed fibre reinforcement. Prior to injection the mould is evacuated to assist the filling process. Filling may take from five minutes up to an hour depending on the fibre loading and part geometry. It may also be necessary to control mould venting to make sure that all parts of the moulding are filled with resin Depending on the resin system, the curing process may take from 30 minutes up to 4 hours for a bismaleimide system. It is possible to preheat the resin to lower the viscosity and reduce the injection time, but it should be noted that this will reduce the time taken for the resin to set (gel time). As a rule of thumb, chemical reactions double in speed for every 10°C rise in temperature. The twin pot system (Figure 6.18) uses two component resins, comprising the resin and the curing agent. These components are stored separately and mixed in precise ratios just before injection.

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Figure 6.18 Single and dual stream RTM machines

In the twin component system shown, (Figure 6.17) two reciprocating pump cylinders are mechanically linked, altering the mechanical advantage of the linkage alters the dispensed ratios of resin and curing agents. The two separate streams are mixed in a labyrinthine passage (called a static mixer) just before injection into the mould. Note that an air and solvent flush is needed to purge this passage between shots. Using this method, the onus for correct mixing of the components has passed from the resin manufacturer to the end user. However, most modern machines provide the facilities for recording a complete process history for inclusion in quality control records. Injection pressure for both systems are up to 5 bar (1 MPa), injection rates depend on the resistance to flow experienced in the mould. Typical RTM components include body panels on the Aston Martin Vanquish, high capacity van roofs and spoilers, including the Rover Coupé. Advantages of RTM are: •

Short simple cure cycles

•

Precise control of fibre volume

•

Complex components easily manufactured

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Manufacturing with Thermoset Composites •

No need for refrigeration (twin pot systems)

•

High quality low voidage parts

•

Minimal amount of waste

•

Good dimensional control of finished product

•

Capability of producing net shape components

Disadvantages of RTM are: •

Difficult to incorporate purely unidirectional plies

•

How large can parts be? - may need multiple injection ports

•

Resins may need to be mixed on site

•

More difficult to guarantee correct mixing of components

•

Tooling (double sided, solid) may be expensive compared to contact moulding (still cheaper than steel pressing)

•

Preforms may have to be made as a separate process

6.6.1 Low Pressure Hot Press This process is similar to RTM. Glass mat reinforcement is laid in the tool but the resin is poured into the tool. The tool is then closed and the resin is forced into the glass fibres and is cured. An example of a low pressure hot pressing is the MGF Hardtop.

6.7 Structural Reaction Injection Moulding (SRIM) Reaction injection moulding (RIM) is a faster more sophisticated version of RTM. RIM systems have two components that are mixed and then co-react, usually with the addition of heat, to form a solid cured resin. RIM technology was developed for the polyurethane industry (PURIM). Polyurethanes have very fast cure times (approximately 30 seconds) that necessitate the two components be kept separate until just before injection. Injection pressures are in the order of 15 MPa and mould temperatures are approximately 50 ° C. Low injection pressures allow lower tooling costs. Further developments of RIM include RRIM. In RRIM short (approximately 10 mm) glass fibres are mixed with the resin. This forms a short fibre reinforced component. Due to the abrasive nature of the glass fibres internal surface of mix heads have to be hardened. Injection pressures are slightly higher than un-reinforced PURIM. Examples of RRIM include MGF and MG RV8 bumpers, many US bumpers and lower volume finishers such as Defender wheel arch eyebrows.

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An Introduction to Automotive Composites RIM technology can be used to inject over preplaced reinforcement (the so called ‘preform’). This technique is known as structural reaction injection moulding (SRIM). The stages of manufacture are: •

Fibre reinforcement is placed in the mould and the mould closed.

•

Two resin components are mixed just before injection into the fibres at pressures around 3 MPa.

SRIM technology allows high fibre reinforcement content. SRIM is not limited to PU, other resins that can be used are epoxies, vinylesters and polyesters. Current applications of SRIM include undertrays. Injection pressures are higher (say 10-20 MPa), injection rates are faster, and cycle times are quicker - using a PU matrix, a shot can be fired and demoulded in less than one minute. The design of the machine also does away with the need to solvent flush between shots (a drawback of RTM). A precision engineered mix head allows jets of the component materials to be fired into one another at high speed (Reynolds number of greater than 300). Residence time within the mixing head is of the order of 20 ms, therefore very fast reacting chemical systems can be used. Possible chemical systems are: •

Polyurethane

• • • •

Polyurethane/polyureas Epoxy Acrylics Nylons

Disadvantages of RIM are the high capital cost of plant (equivalent to thermal injection moulders) and higher cost of tooling - higher than RTM, but not as high as thermal injection moulding

6.8 Filament Winding Filament winding is a method of fabricating reinforced plastic objects by the means of applying continuous reinforcement around a rotating mandrel. This method provides an excellent way of achieving localised reinforcement in the required regions by varying the fibre build-up and placement according to given geometric parameters. As the filament winding machine is often computer numerically controlled (CNC) controlled the fibre can be placed accurately allowing increased performance with a minimal weight increase.

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Manufacturing with Thermoset Composites Filament winding is by far the best process for providing optimal strength and stiffness, although it is limited to circular sections without undercuts. It is possible to fabricate ‘T’ pieces if the filament winder has enough axes of movement. This process is most often used for the manufacture of items such as high performance pressure vessels, golf club shafts, high-pressure tubing and aerofoil sections for use in the aerospace industry. Advanced Composite Products and Technologies of California are filament winding driveshafts for personal and commercial motor vehicles. The mandrel can be integrated into the design of the product as is the case with pressure vessels, or it is removed after the cure of the material takes place (golf club shafts). The material used for filament winding is a reinforcing fibre with either a thermoset or thermoplastic matrix impregnated into it. The reinforcing fibres can be any of the following, although glass and carbon fibres are by far the most prevalent: •

Glass fibre (‘E’ or ‘S’)

•

Aramid, e.g., Kevlar (Dupont) or Twaron (Akzo Nobel)

•

Carbon fibre (graphite)

•

Boron

•

Silicon carbide

•

Alumina

The matrix material is most often a thermoset resin such as epoxy or polyester used either ‘wet’ or ‘prepregged’. With a prepreg material the fibre is received at the factory with the matrix material already impregnated into the fibre. Prepreg allows faster fabrication and more accurate fibre placement although fibre/resin choice is limited. With the wet system the fibre ‘tow’ or filament is run through a resin bath which applies the resin to the previously dry fibre. Wet winding is cheaper than prepreg, and the choice of fibres and resin systems is much greater, although it is slower, messier and accurate resin content of the matrix is difficult. Thermoplastic filaments are now appearing in the market place. These are costly and often use polyether ether ketone (PEEK) or PA (Nylon) for the matrix. With a thermoset system the component is cured after winding, thermoplastic filament winding machines employ a heated head that melts the matrix before it is applied to the mandrel, and it cures ‘in process’. Thermoplastic prepregs are currently almost exclusive to the aerospace industry. To establish a filament winding facility there are a minimum of three pieces of equipment that are required. The first is a filament winding machine (Figure 6.19); this can have between 2 and 7 axes of movement, and between one and ten spindles for mounting the mandrels. An oven is also required for curing the product after winding, unless the fibre being utilised in the process has a thermoplastic matrix. If a ‘wet’ system is being used a resin bath is required for impregnation of the fibre. The remaining part of the plant is a

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Figure 6.19 Filament Winding Machine (Source: Dräger Safety UK Limited, Blyth, Northumberland)

creel. This is used to provide the accurate delivery and tension of the fibre. The tension that is placed upon the fibre acts to consolidate the composite.

6.9 Pultrusion Pultrusion is a method of producing continuously reinforced profiles. Resin impregnated bundles of glass fibre are pulled through a heated die, curing the material and producing the rigid profile. A schematic of the process is shown in Figure 6.20.

Figure 6.20 Schematic of pultrusion process (Source: Dräger Safety UK Limited, Blyth, Northumberland)

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Manufacturing with Thermoset Composites E-Glass fibre rovings, Continuous Filament Mat (CFM), Woven Mat and Chopped Strand Mat (CSM) can be used individually or in combinations to give the required reinforcing properties. Carbon and aramid fibres can also be used for high quality applications. Generally polyester and vinyl ester resins are used, but phenolics, epoxies and acrylics can be used. Thermoplastic pultrusion is also available using glass fibres pre-coated with thermoplastic, or co-mingled with thermoplastic fibres. A good surface finish is achieved by using a polyester tissue as a surface veil. This gives a resin rich surface and protects the fibres below. Coloured or printed decorative veils can be used with clear resin systems. The reinforcement is threaded through a series of guides that first pass the material through a resin bath, where it is saturated with resin. On leaving the resin bath the reinforcement passes through further guides to squeeze out excess resin and to funnel the material into the die entrance (Figure 6.21). This procedure is important, it ensures high fibre volume fractions of 70% and prevents excess resin causing high pressure within the die, leading to breakdown. The die is usually around a metre long, made from hard alloy steel and the moulding surface is polished and has a hard chrome plating. Hollow sections can be produced by using a mandrel within the die, this has to be held in position from outside the die entrance. In some cases dies are designed to allow the resin to be injected into the profile. This is a much cleaner option, but less popular.

Figure 6.21 Glass rovings and mat pass through guides that squeeze out excess resin, before the material enters the die

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An Introduction to Automotive Composites The die is heated to 120-170 oC to ensure that the profile is cured by the time it exits the die. The time to cure depends on the wall thickness of the pultrusion, the resin system and the temperature. Typical pull speeds are in the region of 0.5-2.5 metres per minute. The process is driven by pulling the profile through the die. This is done using either a caterpillar track puller or using a pair of reciprocating gripper feet. The required length can be cut off after the puller. Pultruded profiles can be simple small diameter (6mm) rod up to very large I beams 609 mm x 1905 mm with a 190 mm wall thickness. Complex hollow sections with multiple, different shape cavities are possible. Typical wall thicknesses are in the region of 2–20 mm. Pultrusions can be joined by a variety of methods. Frameworks can be screwed, bolted or riveted together, often pieces are adhesively bonded. Reinforcement plates can be added to ensure the stress is distributed correctly across the joint. Structures can also be created by bonding profiles into cast aluminium or composite lugs. Advantages of pultrusion are: •

High strength to weight ratio.

•

Elastic behaviour up to failure.

•

Excellent fatigue resistance.

•

Excellent corrosion and electrical resistance.

•

Low cost process for runs of over 5000 metres.

Disadvantages of pultrusion are: •

High capital cost for machinery and tooling.

•

Set-up times are long. Reinforcement needs to be threaded up by hand. Large profiles will have hundreds of roving ends to thread.

•

Tooling wear is high due to the glass fibres being pulled across the die surface.

•

Maximum strength is only achieved in the pulling direction.

6.9.1 Applications. Railings for walkways and ladders on drilling rigs and frameworks in chemical factories are made from pultrusions. Their excellent chemical resistance can withstand the harsh offshore environment. Boat builders also use pultrusions for strengthening spars and yacht masts.

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Manufacturing with Thermoset Composites The power generation industry use pultruded rods as armatures in switchgear. Tool handles for power workers are also made from pultrusions for added safety. The rail industry also use pultrusions where high power electricity are involved. In the USA, the majority of ladders are made from pultruded box sections. The ladders are as lightweight as aluminium, but there is no risk of electrical conduction if the ladder rests on power cables. Most houses in the UK have a pultruded tube, used to protect the gas supply pipe that enters the building. Automotive applications are not as common due to the nature of pultrusions, i.e., long and straight. Commercial vehicle trailers often have a pultruded frame to support the glass fibre panels. The door panels on US fire engines are pultruded as they are lightweight and unaffected by the water. Chevrolet have reduced the weight of the tailgate for their pickup truck by using a pultrusion(Figure 6.22). The pultrusion also has the advantage of being cheaper to produce than the equivalent steel component for the volumes required.

Figure 6.22 Chevrolet pickup tailgate by Creative Pultrusions (Source: Creative Pultrusions, Inc., Alum Bank, PA, USA)

Drive-shafts and axles can be produced with 60% weight savings using pultrusions. Spicer and Strongwell have produced a single piece drive shaft for a GM pickup truck (Figure 6.23). The pultrusion has a combination of carbon and glass fibres in vinyl ester resin and is bonded to aluminium end caps. The design is simplified to a single component with excellent corrosion resistance and additional vibration damping.

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Figure 6.23 A single piece drive shaft for a GM pickup truck (Source: Strongwell (MMFG), USA)

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7

Manufacturing with Thermoplastic Composites Neil Reynolds, Mark W. Pharaoh, Amrick R. Singh

The chemical stage of processing thermoplastic articles takes place during manufacture of the raw materials, This involves the production of the raw materials in a controlled environment (the ‘reactor’) followed by the addition of admixtures during a compounding/ extrusion and pellet forming phase. The subsequent article manufacturing process is therefore relatively simple. The material only requires subsequent heating to soften or melt it, which allows it to flow such that it can be moulded and cooled to produce the required shape. The rate of cooling of a semicrystalline material such as polypropylene (PP), Nylon and polyester will affect the ultimate properties by varying the degree of crystallinity. Therefore, parts are normally designed to have a constant wall thickness throughout with the consequence that the rate of cooling is therefore uniform. The shelf life of a thermoplastic is not normally of concern as they are stable when stored in the appropriate conditions. The conditions that may cause degradation are UV light, humidity, ozone, excess temperature and chemical attack.

7.1 Thermoplastic Characteristics Unreinforced thermoplatics can provide a class ‘A’ surface finish, though problems remain with reinforced parts due to varying coefficients of thermal expansion and the effects of flow orientation. Short cycle times are available through compression moulding, though tooling costs are greater than for low pressure liquid thermoset moulding. Materials such as PP are also difficult to join through adhesive bonding and painting is only possible when using modified grades. Creep remains a problem with thermoplastics and applications currently remain limited to non-structural and semistructural parts. Base thermoplastics can also be modified to allow different properties to be obtained such as adding rubbers to improve impact properties or the addition of carbon to improve electrical conduction.

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7.2 Long Fibre - GMT 7.2.1 Introduction Glass Mat Thermoplastic (GMT) covers a family of materials that use a variety of matrixes: PP, PET, and Nylon, with a mat like glass fibre architecture. They are formed via compression moulding and use only a small amount of material flow, compared to injection moulding. Because only a small amount of material flow is required very long fibre lengths can be used; between 12mm and continuous, depending on application and mould complexity. Injection moulded parts rarely have over 3mm fibre lengths. The longer fibre lengths in compression moulded parts makes them a more structural alternative. The mats are made with a variety of architectures, but all have a random or pseudorandom format. Commonly the continuous fibre architecture is used as this offers better impact behaviour. Chopped fibre arrangements are generally used when there is a need to fill a complicated structure. Another family of materials that fall into this category are the continuous aligned fibre materials. The constituents are the same: PP, PET, and Nylon matrices; but the fibres, typically glass, carbon or Kevlar are aligned into either a continuous tape or a woven mat. The schematic fibre architectures shown below are for the product Plytron (manufactured by Borealis) and the product Twintex (manufactured by Saint Gobain Vetrotex International). Unidirectionally reinforced Plytron tape is laid-up on a ply-byply basis to manufacture a laminate material. The Twintex fabric is woven from a yarn that contains both PP and glass fibres - this arrangement is termed commingled. The weave type determines the properties of the final consolidated laminate.

Figure 7.1 PP-glass thermoplastic laminate schematic (Source: Neil Reynolds)

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Figure 7.2 Twintex balanced weave (Source: Neil Reynolds)

Figure 7.3 Twintex 4:1 weave (Source: Neil Reynolds)

7.2.2 Properties The mechanical properties of these materials are dominated by the fibres used and the fibre architecture. The more aligned the fibres, the higher the strength and modulus, but piece complexity is compromised. Table 7.1 lists typical properties for PP/glass-based materials. Some property variation is usually present in random fibre GMT due to fibre orientation during material flow.

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Table 7.1 Material Type/Supplier

Reinforcement Regime

Glass Weight Fraction

Tensile* Modulus (GPa)

Tensile* Strength (MPa)

3-D random

20-50% w.f.

3-7

60 - 120

Individually orientated UD plies

65% w.f.

11 - 28

178 - 720

Woven PP/Glass commingled yarn

60-75% w.f.

14 - 38

320 - 800

E-Glass Fibre (Vetrotex)

—

100% w.f.

73

3400

PP

—

0% w.f.

1.6

38

GMT (GE Plastics) Plytron laminate (Borealis) Twintex (Vetrotex)

*Based on manufacturers data.

7.2.3 Processing Processing of GMT is relatively simple and is easily automated. Both flow forming and stamp forming grades are available, with the flow forming method being used for more complicated shapes and in very much greater volumes. An example of the high level of part complexity achievable is shown in Figure 7.4, a GMT bulkhead component.

Figure 7.4 GMT bulkhead component (Source: Neil Reynolds)

The GMT blanks are produced as large sheets, about 3-7mm thick, by the material supplier. The blanks are easily shipped and have an indefinate shelf life, as well as being unaffected by the environment. The blanks are then cut to shape prior to heating in an oven to melt

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Manufacturing with Thermoplastic Composites the PP. During the melting process the material lofts to more than twice its original volume. Once the material is melted, it can be moulded. The blank shape and positioning of the lofted material in the mould can affect the mould filling and final properties of the part, as fibre flow to the extremities of the mould is vital. Packages are available for predicting part performance and optimum blank arrangement, making the technique easily reproducible. The aligned fibre materials have a number of different processing routes available, although due to the nature of the aligned fibres, ribs and bosses cannot be moulded. Vacuum-bagging and simple stamp forming techniques are often used as the materials can be formed at low pressures. A relatively new technique gaining favour as it offers high processing speed, is membrane, or pressure forming. The technique heats the aligned fibre lay-up in a vacuum between two formable membranes. The whole frame can then be either vacuum formed over a plug, or pressure formed into a cavity. Once the part has been released, the membranes can be reused.

7.2.4 Uses Uses for GMT are very varied, from purely aesthetic parts such as cam covers to semistructural parts like load floors, and safety-critical parts such as bumper beams Figure 7.5, crush cans and front ends. The parts can sometimes be let down visually, due to read through of the fibres on the surface and sink marks due to PP rich areas. Those parts used visually are generally grained to hide these flaws or covered with either fabric or carpet for interior use.

Figure 7.5 GMT bumper (Source: Neil Reynolds)

Developments with the aligned fibre material have shown the possibility to produce parts with exceptional strength and modulus to weight ratios, in the form of load floors and bumper armatures. The limiting factor at present is process speed, although the part cycle time is coming down.

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An Introduction to Automotive Composites The fabrication of parts with hybrid structures can show advantages for certain applications. These hybrids can be either GMT/Plytron, GMT/foam and aligned fibre/ foam. Both show some structural advantages, but detailed costing would need to be carried out to assess potential cost savings to go along with the potential weight savings.

7.2.5 Tooling Processing pressures for GMT are around 15 MPa, which for medium sized parts requires a machine of about 1500 tonnes clamping pressure. The tooling required is different to injection moulding tooling, as the shear edges are vertical with a precision 0.025 mm, so that the full pressure of the press is applied to the material and not onto the stops. Prototype tooling routes are available using metal spray, concrete and resin arrangements. These are capable of producing up to about 100 parts, the precise number of parts available from this kind of short run tooling depends on the complexity of the moulding and the care taken by the operator.

7.3 Short fibre - Injection Moulding Thermoplastic injection moulding (TIM) is one of the most common routes for processing of thermoplastics, although the majority of articles made by this method are unreinforced, it is gaining currency as a method for manufacturing composite articles. The polymer granules are fed into the end of a heated extruder/injector barrel. In the barrel, a homogenous high viscosity melt is produced. The melt is then injected into a chilled, closed mould. Components can be produced in cycle times of approximately 1 to 3 minutes depending upon size. Injection pressures of 200 MPa and temperatures of 250° C are common. High injection pressures lead to high tooling costs, several hundred thousand pounds being typical for a bumper. Examples of injection moulding include: bumpers, grilles, engine protection panels, badges, finishers, plastic headlamp lenses, body panels on the GM Saturn, and vertical panels on the BMW Z1. To maintain the close production tolerances demanded by today’s precision components, an accurate knowledge of the behaviour of the material is needed. This includes material shrinkage and differential shrinkage behaviour, the effect of fillers and the reaction of the material to the processing parameters of the injection moulding process. It is usually possible to predict with a fair degree of accuracy the shrinkage (the difference between mould and part dimensions) to be expected, and although the degree and orientation of shrinkage can to some degree be confirmed with simple test specimens, care must be taken in using this information to predict the behaviour of production mouldings. The

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Manufacturing with Thermoplastic Composites strength of the injection moulding process is its ability to produce mouldings that are often of a highly complex nature, this means that the flow regime within the mould, the consequent distribution of pressures during mould filling, and the time taken for the varying thicknesses of the moulding to freeze are all highly dependent on the mould geometry. All these factors have effects on the shrinkage of the polymer and may result in the moulding will still be unusable because it has warped to an unacceptable degree. There may be several reasons and remedies for this: the gate is in the wrong position, the mould cooling system inadequate or because of a failure to fully optimise machine parameters. With partially crystalline, fibre reinforced thermoplastics in particular, the tendency of these materials to warp must be taken into account from the designing of the part right through to the processing stage, with appropriate steps taken to counteract it. The important point to consider here is not so much the absolute degree of shrinkage as the size of the possible range between longitudinal and transverse shrinkage. Crystalline plastics tend to shrink more than amorphous plastics since crystallisation brings with it a reduction in volume. These crystallisation conditions depend on the processing parameters during manufacture, especially the mould temperature. Shrinkage is less in the direction of orientation of the molecules and reinforcing materials than perpendicular to the direction of orientation. The difference in shrinkage in these two directions provides information on the tendency to warp. These differences can be particularly great in glass reinforced, partially crystalline materials such as PBT and PA6 and part design plays a dominant role as regards possible tolerances. There is a tendency for shrinkage to increase as wall thickness and effective pressure increase. Warping can be limited by altering the mould to take advantage of this fact by having moulds of varying wall thicknesses to compensate for known shrinkage factors. As stated above shrinkage is dependant on effective pressure. Since pressure is normally lower away from the gate than it’s vicinity the shrinkage will be greater. The greater the difference in pressure between the end of the flow path and the gate, the more a centrally gated part will warp. It can be concluded therefore that by using a high injection speed and as a low a holding pressure as possible warpage can be reduced. Since the fibres in glass fibre reinforced materials are orientated predominately in the direction of flow and superimposed on molecular orientation, the shorter the fibre is, the more isotropic the product. High precision moulded parts can be manufactured using extremely short glass fibres (100 μm) or amorphous materials filled with glass spheres.

7.3.1 Making composites by co-injection Generally speaking, the higher the level of reinforcement added to the injection mixture, the lighter and stiffer, and cheaper the resultant moulding. We are therefore encouraged

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An Introduction to Automotive Composites to maximise the level of reinforcement, however, before we reach the ultimate limiting factor of poor flow qualities there is another significant problem that must be addressed, that of fibre witness. The use of highly reinforced materials alone tends to compromise the surface finish of the moulding because the fibres ‘witness’ through onto the surface. To avoid the need to compromise with surface finish, the co-injection moulding process can be used. This process uses fibre filled (and conventional) core materials with a thin external coating of unreinforced polymer by injecting two different polymers into the same mould cavity. The outer, primer material could be for example, a conductive primed automotive trim moulding for in-line electrostatic painting. The inner layer can be a highly reinforced material to provide the mechanical strength of the article. There is a considerable manufacturing cost reduction through the co-injection moulding process as compared external steel body panels. This is due to a reduction in both tooling and production costs. The process uses a built in compatibilisation system, coating thermoplastic components in the mould with a primer. This technique has been designed to meet industrial painting requirements half way with [2-6]. The materials are in a layered combination with skin layer and a core layer to achieve an appropriate mix of properties relevant to the component requirements. The selected materials would ideally adhere to each other [7-8]. However, there are many polymer combinations that do not adhere to each other and may or may not meet the engineering requirements of two-layer or co-injection moulding, presenting poor or no adhesion between the different materials; these are called incompatible thermoplastics [10]. For example, a Nylon skin would not normally bond to a polypropylene core material. A commercial product must have a surface with a paintable thermoplastic skin and the core material needed to be a low-cost, lightweight and tough. When moulding incompatible materials, interfacial adhesion needs to be promoted between the skin and core materials, so compatibilisers (molecules with heads and tails soluble in different polymers, e.g., maleic anhydride) are needed to promote adhesion. It should be noted that compatibilisers are not the only important factors in achieving good interfaces between the co-injected polymers or polymer composites. The processing conditions, such as machine setting parameters, i.e., injection speed, holding pressure, melt temperature, the amount of skin and core, and rheological properties of the materials, are also major parameters.

7.3.2 The Co-injection moulding process with Fibres The co-injection moulding process with fibrous and modified cores and incompatible skins with compatibilisers all have to go through the same fundamental injection moulding cycle: these are as follows [9] (see also the cycle diagrams in Figure 7.6).

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Figure 7.6 The injection moulding cycle

1. The locking device of the machine closes the mould. Only one half of the mould moves. 2. The polymeric resin in the form of pellets is fed from the hopper to the flights of a rotating screw. Rotation of the screw conveys the material forward along the barrel where it is transformed to a viscous melt through a combination of external heat sources and the mechanical action of the screw. 3. The molten material accumulates at the front of the screw as it retracts. Once an adequate amount of material has accumulated, the injection phase is activated. 4. The screw moves forward like a ram and causes the melt to flow through the flow channels of the mould, (i.e.; the sprue, the runner and the gate), into the mould cavity, pushing the air in front of the material into the cavity. The time taken for this operation is called injection time. (The air is vented out eventually through the tool, if it is correctly designed.) 5. As soon as the melt strikes the relatively cool surface of the mould it begins to solidify and contract. After the mould cavity is filled, additional material is forced into the cavity under pressure. This compensates for the shrinkage, which results from the solidification of the molten polymer. 6. The screw maintains pressure until the gate(s) freeze(s). The time for this operation is the holding time. 7. The screw-back starts, but the mould remains closed until the moulding is cool enough to be extracted (cooling cycle).

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An Introduction to Automotive Composites 8. The machine opens the mould allowing the removal of the moulding from between the two parts. 9. The ejector mechanism of the machine lifts the finished moulding out of the mould cavity. The machine is now ready to start the next cycle of operations. How does co-injection moulding (also known as 2k or dual injection moulding process) differ from this conventional injection moulding process? Dual injection, as might be expected, involves the injection of two materials through a special nozzle (Figure 7.7)[9],

Figure 7.7 Nozzle and the two barrels of the co-injection machine (Source: Battenfeld))

Figure 7.8 Schematic diagram of dual injection (Source: Battenfeld))

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Manufacturing with Thermoplastic Composites making a sandwich construction consisting of a structurally enhanced and modified fibrous substrate, (i.e., PP, material A) covered by a skin suitable for painting, (i.e., Nylon (PA), material B). This overcomes the conflict of mechanical performance versus cosmetic finish mentioned previously. The relative amounts of skin and the substrate can be varied to different produce different skin:core ratios giving optimum skin thickness or overlap sealing edges. Materials can either be injected simultaneously or sequentially; sequential injection is shown in Figure 7.8 [2-6].

7.3.3 Co-Injection moulding stages [1-3, 9] In detail, the co-injection process consists of the following stages: Stage 1 The mould is closed hydraulically, both injection units are charged with polymer, and screws are retracted. The valve is shut. Stage 2 The valve is opened to the first injection unit, which injects a partial charge of skin (non-flammable) material into the mould. Stage 3 Next the valve is opened to the second injection unit, which injects core material into the mould, forcing the skin polymer outwards. The mechanism is analogous to blowing up a balloon. So with two polymer charges, provided the conditions of polymer temperature, mould temperature, injection rate and other variables are controlled, the core polymer forces the skin to the edges of the mould (Stage 3) laying down a uniform layer as it goes without bursting through. Stage 4 The full injection pressure of the second injection unit is applied to pack the mould and ensure a good surface finish Stage 5 The valve is again opened to the first injection unit and a sufficient amount of skin polymer is injected into the mould cavity to clear the sprue of core polymer. If the small additional charge were not injected, foam structure would be revealed on cutting the sprue from the moulding, and residual core polymer in the valve channel would appear in the skin of the next moulding, causing a poor surface finish on a subsequent moulding part. Stage 6 In the final stage, the valve unit is closed and full clamp pressure is held for a few seconds. The core polymer cools to give a uniform cellular structure encapsulated with in a thin skin of polymer.

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Figure 7.9 The Battenfeld single/dual thermoplastic injection moulding machine (Source: Battenfeld))

Figure 7.9 shows a Battenfeld co-injection moulding machine series BM-2000/630 +630BK [9] of the sort used for dual injection.

7.3.4 The Materials Structural composite mouldings using long glass fibres are possible with this process (for this process ‘long’ means an initial length of about 20 mm). There is a important requirement when moulding with long glass fibre polymers, such materials requires careful injection moulding, make sure that glass breakage is minimised. In reality, it is very likely that the glass fibre length is reduced, but by processing the fibres correctly and optimising the control can result in moulding achieving capable of limited engineering structural application [1-5].

7.3.5 Moulding Procedure The combinations of each skin and core are simultaneously injected at different volumes, various trials are required until optimisation of the conditions for the skincore are obtained. To achieve the optimum condition of the moulding components, firstly, the metering strokes of skin and core polymer melts are varied until the flow front of the core polymer flows close to the edge of the moulding with no core breakthrough. Switch-over points for the injecting polymers and the injection speeds of skin (A) and core (B) are varied to adjust the flow front of core polymer and avoid the core breakthrough.

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Figure 7.10 An example of the skin-core structure (Source: Battenfeld))

7.3.6 The influence of Moulding Conditions on the dual injection process It is necessary to understand the influential conditions, such as, injection speeds, melt temperatures, tool temperatures, length of simultaneous phases and compatibiliser on skin and core thickness formation. Injection speed is one of the most effective factors in controlling the process. The effect is quite similar to that of melt temperature. Skin-core melt temperatures play an important role in fibrous skin formations and core penetration. In the case of increasing of skin melt temperature, lower skin viscosities ensures that the skin material flows more easily than the core, lower melt temperatures lead to a higher skin thickness and core breakthrough. Cogswell [11] quantifies this effect in terms of the ratios of component viscosity and centre line velocity: N = 1−

1 tanh(0.4 log10 M ) 2

where N is centreline velocity ratio, and M is the ratio of component viscosities, however, it should be noted that this equation was derived from consideration of a simple cylindrical geometry.

7.3.7 Examples of 2K mouldings The following components with electrostatic surfaces by incorporating short and long glass/ carbon fibres as well as incorporating compatibilisers into the core material for adhesion are shown as example of this process. Figure 7.11 shows a Wheel trim (sectioned to show the skin-core structure). Figure 7.12 and Figure 7.13 shows the fender mouldings (made by Magna (Exteriors) UK before and after painting on a standard production line [1-5].

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Figure 7.11 Wheeltrim moulding (Source: Warwick Manufacturing Group)

Figure 7.12 The Magna Fender - before in-line painting (Source: A.R. Singh)

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Figure 7.13 The Magna Fender - after in-line painting (Source: A.R. Singh)

References 1.

G.F. Smith, R.A. Easterlow and A. Singh, Warwick Manufacturing Group Inhouse Research.

2.

D.F. Oxley and D.J.H. Sandiford, Plastics & Polymers, 1971, 39, 142, 288.

3.

D.J. Stafford, Proceedings of PENTEC 72, The Plastic Institute, London, UK, 1972, Paper No.11.

4.

Encyclopaedia of Polymer Science and Engineering, Volumes 12-13, Eds., H.F. Mark and J.I. Kroschwitz, John Wiley & Sons, Inc., New York, 1991.

5.

A. Magalheas, Adhesion of PP on PA6 with PP-MA Compatibiliser for Rover Fenders, DSM Research, The Netherlands, 1997.

6.

P. Somnuk, Experimental Study of Simultaneous Co-injection Moulding Process, University of Warwick, 1995. [Ph.D. Thesis]

7.

R. Seldén, Journal of Injection Moulding Technology, 1997, 1, 4, 189.

8.

The ICI Sandwich Moulding Process, General description and applications, ICI Publication, 1981.

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An Introduction to Automotive Composites 9.

Battenfeld’s Operating Instructions for BMT-1100/2x300 Co-Injection Moulding Machine, Battenfeld GmbH.

10. H. Eckardt, How to develop a successful co injection application, Fourteenth annual Structural foam conference and parts competition, The Society of the Plastics Industry, Inc., 1986. 11. N. Cogswell, Polymer Melt Rheology – a Guide for Industrial Practice, George Goodwin Ltd, London for The Plastics and Rubber Institute 1981. (UMI out-ofprint books on demand edition, 1993).

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8

Economics of Composites Manufacture Gordon F. Smith

8.1 Introduction The aim of this section is to give the general outline for cost analysis within the commercial environment and to give general guidelines for cost analysis of composite components. The general costs of composite components versus steel will be given along with specific examples of cost analysis automotive components. The economics of material choice will also be discussed with an understanding of why parts integration tends to be requisite for economic manufacture of composite components. This argument is extended to include the ability to manufacture modules, and the pros and cons of internal manufacture versus bought in components

8.2 Commercial Composite Cost Analysis This section will outline a typical method of commercial cost analysis for automotive composites and defines some of the terms that are used to describe cost analysis. In general, the process steps in defining the component cost are as follows •

Define financial targets for total vehicle variable cost, etc., (see Figure 8.1), internal rate of return (IRR).

•

Propose product cost targets.

•

Demonstrate technology options - brainstorm different technology ideas.

•

Agree routes/assess risk.

•

Obtain quotes from various suppliers using a standard format.

•

Analyse quotes by different suppliers/different process and material choices.

If the quotes obtained are too high, a cost down process will occur. Product change requests will be raised and further quotes will be obtained from suppliers.

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Figure 8.1 Breakdown of the selling price of a vehicle

Figure 8.2 Breakdown of the total investment cost

In general two costs are assessed for each component, investment cost and piece cost. Both of these are broken down into a number of different components as shown in Figures 8.1 and 8.2 for piece cost and investment cost, respectively. The cost break

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Economics of Composites Manufacture down shown is for a generalised vehicle although the method can be applied to particular components. When comparing different materials such as composite versus steel it must be remembered that different overhead rates may apply, due to, for example, a different amount of assembly floor space being required. To carry out complete and comprehensive cost analyses all of the factors should be assessed, though on a direct swap of one component against another a majority of additional costs, such as fixed overhead, may remain constant. Similar to the piece costs, investment cost should include manufacturing costs, assembly tooling costs [Body In White (BIW) costs] and any dedicated tooling used on moulding machines, tooling for robotic manipulators and handling equipment. The actual moulding machine costs are not included for components that are bought in to the company, the moulding machine costs are included in the running costs of the machine (see later).

8.3 Comparison of Materials Where a direct part for part replacement is undertaken it is usual to neglect additional costs such as infrastructure costs and sales and administration overheads, since it is assumed that these costs are the same for all materials choices. By making the same component out of different materials it is likely that each component will have a different piece cost and investment cost. This section shows how piece cost and investment cost affect the profitability by the use of composites for automotive components. One of the most important steps in the development of cost analysis is the identification of all of the process steps required for the manufacture of the components. An example shown in Table 8.1 is the manufacture of a load floor from Glass Mat Thermoplastic (GMT). From each of the operations listed in Table 8.1 there will be an associated tooling cost (investment cost) and piece cost. The piece cost is made up from both machine running costs and labour costs (where manual input is required). For each material of choice, all of the process steps must be included. For a typical vehicle assembly area, gauging the correct costs for assembly of components to include all the associated overheads is quite often difficult. In a comparison of steel versus composites there are certain generalities that may be put forward; tooling (investment) costs are much lower for composites than steel and piece prices tend to be higher (due mainly to higher material costs). Typical material costs are shown in Table 8.2. For all materials there is significant variation in piece cost depending upon actual composition. This is further complicated for composites with different proportions of

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Table 8.1 Moulding operations for a GMT floor pan Operation number

Process

Method

10

Cut and prepare material

Manual load and trim

20

Press/forming

Vertical moulding press

30

Transfer

Robot operation

40

De-flash/trim/inspection

Manual operation

50

Stillage

Manual operation

60

Removal from stillage

Manual operation

61

Bonding

Robot operation

70

Routing

Robot operation

80

Transfer/unload

Robot operation

110

Inserts

Dedicated machine

120

Transfer/unload

Manual operation

Table 8.2 Typical material costs Cost per kg (£)

Density (kg m-3)

Modulus (GPa)

Steel

0.3–0.5

7800

210

Aluminium

0.6–1.1

2600

70

SMC

1.20–1.60

1200

12

GMT

2.6–3.2

1200–1400

6–10

RTM composites

2.2–4.00

1200–1600

8–12

Material

both fibres and fillers. For a large tool such as a fender there may be up to a 60% saving on tooling costs. As a general rule, for composites the total selling costs are approximately 3 to 4 times the total material cost. Figure 8.3 shows a typical cost comparison break-even graph for an SMC fender versus steel. The ordinate shows the total cost that is made up from the investment cost plus a variable cost to give the total cost. Total cost increases linearly with volume since the piece cost remains constant. The lesser slope represents the lower piece cost of the steel as compared to the SMC component, whereas the lower investment cost of the SMC component is evident from the interception of the Y-axis from the two data sets.

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Figure 8.3 Break-even graph for a fender in SMC or steel

The lines cross (the break-even point) at a total volume of 250,000 units (in this example). This is typical for a one-on-one replacement of a steel component by a composite one. This type of analysis tends to lead to the conclusion that composites can be economically used for low volume, or niche vehicles, for example Lotus Cars. This break-even point is not constant over time as more cost effective tools and methods will be developed and introduced through the article life cycle.

8.4 Parts Integration and Modules The previous sections have dealt with one-on-one parts replacement, and analysis has shown that composites only tend to be economic at low volumes of, say, less than 50,000 units per year. Previous examples of composites in this book have outlined the use of a composite taildoor on the Mercedes A Class, whose production volume would be in excess of 200,000 units per year. It is therefore pertinent to understand how such a component can be cost effective.

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An Introduction to Automotive Composites Composite components, through the use of flexible moulding technologies, can incorporate a number of different design features into the same component. Through the parts integration this allows parts savings that in turn produce significant cost savings, by the deletion of manufacturing and assembly costs. As highlighted previously, to understand the comparisons between components made from different materials full costs of the component including manufacture and assembly is required. The next stage from the integration of composite components is the use of modules. In this instance not only are many functions included in the same component, but also components are put together to form a multifunctional module that has been assembled away from the production line. This gives the original equipment manufacturers (OEM) the benefits of having a smaller number of components to handle on the assembly line and the associated cost savings that it provides. The negative side of modules is the change in profit associated with module manufacture away from the OEM to the supplier. The debate about the use of modules in OEM continues.

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9

What to do with Composites at the End of Vehicle Life Vannessa Goodship

9.1 Introduction Automotive recycling can be dated back to Henry Ford. The wooden packing crates his components were delivered in were used to make the floor panels of the Model T Ford [1]. The motor industry and environmental awareness have both grown considerably since those times. Recent years have seen a rise in the importance of environmental issues in both the public and political arenas. Industry has been increasingly under pressure to improve its environmental performance especially in relation to landfill avoidance issues. The European Community has targeted the high profile automotive industry in an effort to remove end-of-life vehicles from the waste stream. In 1997, the EU commission for the environment published the first draft of its ‘End-Of–Life (ELV) Vehicle Directive’. In May 2000, the 15 EU states finalised the bill in which the disposal cost of all cars will ultimately fall at the feet of the manufacturers. As well as introducing this ‘producer responsibility’, the Directive also states how the disposal may take place. The percentage of UK ELV sent to landfill will reduce from 25%, to a maximum of 15% of the vehicle weight in the year 2005 and a maximum of 5% landfilled by the year 2015. A further 10% of the ELV can be incinerated if latent energy is recovered. The remaining 85% of the vehicle must be recycled or recovered to comply with the EC Directive [2]. This is summarised in Table 9.1.

Table 9.1 Planned recycling targets in European end-of-life vehicle directive Target year

2006

2015

Reuse and recycling of material

≥ 80%

≥ 85%

Thermal energy conversion

≤ 5%

≤ 10%

Recovery

≥ 85%

≥ 95%

Landfill

≤ 15%

≤ 5%

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An Introduction to Automotive Composites The automotive industry has declared recycling non-competitive and a number of collaborative groups have been established. In the UK, the most significant are Automotive Consortium On Recycling and Disposal (ACORD) and the Consortium for Automotive REcycling (CARE). These two groups encompass collaboration from the entire ELV chain. Recycling automotive plastics is a massive issue, to give it some perspective in the UK alone there were 1.8 million vehicles scrapped in 1998 [3]. In Europe, it is estimated that by the year 2005, 10.5 million vehicles (approximately 10.5 million tonnes of automotive material) will require disposal [4]. In order for industry to meet the stated landfill levels, it will be necessary to look at methods of removing ELV material from the waste stream and reusing, recovering and recycling it. In order to do this, it is first necessary to know the average vehicle material content and percentage break down

9.1.1 Material Breakdown of a Typical Current Vehicle The material composition of a typical Ford vehicle is given in Table 9.2 [1].

Table 9.2 Constituents of typical Ford vehicle Metallic - ferrous

68%

Metallic - non ferrous

8%

Glass

3%

Plastics

8.0%

Coatings

2.0%

Fluids

2%

Miscellaneous (trim, textiles, natural fibres, etc.)

4.0%

Tyres/Rubber

5.0%

Plastics, elastomers and polymer composites account for approximately 13% of a vehicles total weight, which is currently the second largest group of materials being used in vehicle construction after metals. Since metals have a well developed recycling system, plastics and other non metallic materials are the obvious target for further landfill avoidance initiatives.

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9.2 Recycling and Recovery from ELV The plastic content of cars can vary considerably from model to model. In the current BMW 3 series, around 160 kg (11.6%) of plastic is currently used [5]. Such quantities are likely to increase as the trend is to move towards weight reduction of vehicles for reduced exhaust emissions and fuel economy. It is thought that future vehicle designs will use more plastic panels, powertrain components and trim parts. Therefore, in the future, the post consumer plastic waste stream from vehicles is also set to enlarge. At current levels, if this is equated with the 10.5 million tonnes of ELV requiring disposal each year, then approximately 1.2 million tonnes of plastic materials will require recycling, recovery or disposal per year, in order to keep this material from entering landfill. This means a vast expansion of current recycling activities is required. All thermoplastic, thermoset, elastomeric and their associated composites have the potential to be removed from the waste stream by being recycled or recovered into high quality valuable materials. The key to success being the ability to create a balance between technology, economy and the environment to create a sustainable marketplace. Possible recycling steps can be split into four types: •

Reuse, e.g., a bumper is removed and used on another vehicle

•

Mechanically recycled, the material is shredded and reprocessed

•

Chemical recycling, the polymer is turned to basic monomer units

•

Energy recovered, the materials are incinerated

There are distinct differences between re-use, recycling and energy recovery (incineration). Re-use utilises the material or part in its original form. Mechanical recycling reuses the material, but to create a new component. Chemical recycling returns the polymer back into monomer building blocks for the creation of new materials. Recovery concentrates on reusing the oil or hydrocarbons contained within the waste plastic or recovering the latent energy contained within the plastic through incineration. There are many technologies for recycling and recovering plastics that are currently in existence. Some are in use by industry and capable of processing large quantities of material in a cost effective manner, whilst others currently exist in laboratories and are as yet, not cost effective processes.

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An Introduction to Automotive Composites A fully sustainable infrastructure for the recycling and recovery of plastics will only occur when it is economically viable to do so and the demand is created for the end product materials. The legislative restrictions placed upon the motor industry have meant that it is left to drive the initiative to create new markets for its by-products.

9.3 End-of-Life Vehicles At the end of a vehicles life, a dismantler will reclaim all valuable metallic and nonmetallic parts for reuse or recycling. After dismantling the vehicle is shredded and the remaining automotive shredder residue (ASR) is generally landfilled. The dismantling stage is arguably the most cost effective time to remove polymeric components, e.g., dashboards or bumpers, as once the remaining hulk is shredded, the various polymer fractions in the shredder residue require sorting from both themselves and the other shredder fractions before further material can be recovered. ASR will be discussed in more detail later but first some of the issues of mechanical recycling and reprocessing will be considered.

9.4 Mechanical Recycling

9.4.1 Thermoplastics Mechanical recycling is the most common recycling method used today. Materials are shredded, ground or flaked and then reprocessed. Most thermoplastic processors will have in-house granulation facilities as generally process scrap and rejects can be re-fed into the production machines with little problems providing attention is paid to cleanliness. Since mechanical recycling can be used to recycle both factory waste and post consumer materials, it means that, in the case of the automotive industry, both the vehicle manufacturers and their suppliers can help to recycle polymeric-based materials. Automotive waste plastics are not the only waste plastics on the market and often waste from one waste stream finds re-use in another. In the case of the Chrysler Composite Concept Vehicle (CCV) see Figure 9.1, waste from the packaging stream in the form of old plastic drink bottles was used in conjunction with virgin glass-fibre filled material, to produce body panels [6]. To achieve high quality recyclates, it is generally necessary to sort the polymeric and composite waste into generic types, prior to mechanically recycling. The main difference between factory waste and post-consumer waste is that with post-consumer waste it is much harder to identify the various mixtures of plastics that are in the scrap. For this

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Figure 9.1 Chrysler Composite Vehicle (Source: http://popularmechanics.com)

reason plastics used for packaging must be labelled with a series of codes that allow the material to be readily identified. Turn over any plastic bottle or container and you will see a generic code similar to the one shown in Figure 9.2. This takes some of the difficulties away in the sorting process and similar marking of parts is recommended in most ‘Design for Recycling’ guidelines.

Figure 9.2 An example of the coding system used to mark plastics packaging for ease of recycling

Mechanised sorting technologies utilise differences in material properties such as density, surface properties, conductivity or colour. A widely established method for polyolefin separation is the float-and-sink process which works on the principle that less dense polyolefin materials float whilst other denser fractions sink (see Figure 9.3). With water as a separation medium a polyolefin fraction of 98 wt% purity can be achieved [7].

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Figure 9.3 Schematic of a continuous float-sink separator (Source: J. Brandrup, M. Bittna, W. Michaeli and G. Menges, Recycling and Recovery of Plastics, page 256. © Carl Hanser Publishers, Munich, 1996.)

Other sorting techniques include X-ray sorting, triboelectrification, near infra red (NIR) spectroscopy and flotation. For the advantages, disadvantages and suitability for use the reader is referred to Pascoe [8]. A detailed breakdown of these processes is beyond the scope of this section and the reader is referred to Brandrup [7] for a deeper analysis. Sorting helps ensure compatibility of the materials with themselves and the recycling processes. It also helps provide repeatability of recyclate properties. Blends of incompatible materials can be processed in the presence of compatibiliser additives, however this technology often requires trial and error in determining the exact quantities of additive required and the cost of the compatibiliser can be prohibitive [9, 10, 11]. In order for recyclates to be used for higher added value applications, it may be necessary to add other materials or additives to them. Two examples of this have already been mentioned, the CCV body panels were made from drinks containers (PET) blended with glass fibre materials. The addition of compatibilisers to increase the properties of mixed plastics is the other. A third strategy is to add stabilisers to mop up degradation products caused by processing and to replace additives lost through initial processing. According to Herbst and Pfaender [12], this can increase long-term stability, UV light stability and economic value of recyclate materials. One problem specific to composites is that the act of regrinding and/or recompounding reduces the residual fibre length with a corresponding effect on mechanical performance. For recycling glass mat reinforced thermoplastics, the Bondpress process seems the most

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What to do with Composites at the End of Vehicle Life promising, allowing full recycling of GMT material for compression moulding retaining 90% of the original mechanical strength. The process works by reducing the size of the parts to a minimum of 50 mm. These sections can then be heated and pressed [13]. A summary of other available processes is provided by Mattus and Neitzel [13]. A staged breakdown of the recycling of mixed plastic waste could include some or all of the following stages: •

grinding

•

washing

•

separation

•

regranulation

•

compounding: this could include blending with other virgin or recyclate materials, or adding additives such as compatibilisers, reinforcement or heat and light stabilisers

•

reprocessing: for example injection moulding

The strategies mentioned so far are used for thermoplastic recyclates, however there have been several successful processes developed to mechanically recycle thermoset materials.

9.4.2 Thermosets Thermoset materials for many years were considered to be non-recyclable since they do not re-melt like thermoplastic materials. To overcome this challenge a series of companies and trade associations across the globe were set up to develop recycling methods for these materials. These groups were known as ERCOM (Germany), VALOR (France), SMC Alliance (USA) and the FRP Forum (Japan). The process developed in Germany and patented by BASF is known as particle recycling for material re-use of SMC. A closed loop system was proposed and set up, with the first recycled parts being used in 1992. A brief overview of the process involves the following stages: The waste SMC and BMC material is hammer milled and then separated. The coarse material is returned to the process and the finer material is sieved to give several grades of a fibrous reinforcement. These graded fibres are then sent to the manufacturers of virgin reinforced plastics. They are able to use between 10% and 30% of the recyclate fibre in place of traditional fibres without any adverse effect on quality. In fact they claim a 10% weight saving [14]. SMC, BMC, printed circuit boards and thermoplastics glass reinforced plastic (GRP) can all be recycled by this system [7]. The American based SMC Automotive Alliance is helping to develop commercial processes for turning composite scrap into raw materials [15]. However currently the best approach to recycling SMC is to grind it up into filler and fibre fractions for use in new markets.

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An Introduction to Automotive Composites Thermosets can successfully be used as a functional filler for thermoplastics [16], this process physically and chemically modifies the waste and combines it with polymer to produce enhanced material properties. The modifications are necessary to optimise the bond between the recyclate and the virgin polymer. This work is carried out as part of RRECOM, Recycling and REovery from COMposite Materials, a UK based alliance of 16 companies and two universities, Brunel and Nottingham. The focus so far has been on mechanical recycling of polymeric materials. Mechanical recycling is usually best suited to single type polymers or polymer streams that can be easily sorted. Economics is usually the determining factor for sorting as the cost of washing and sorting can make recycling expensive. More difficult multi-material waste streams, whilst useable as plastic lumber type products or in building aggregates [7], offer little in the way of material properties or added value quality products. Thermoset scrap can be used as powdered materials held by adhesive resins, as fillers in thermoplastics or as part of a new uncured thermoset matrix. The glass reinforcement can generally be recovered for re-use. However, there are other methods available for recycling and recovery using chemical means. One of these is chemical recycling where the polymer is broken down into molecules that can be easily separated from impurities.

9.5 Chemical Recycling Since this type of recycling is very much on chemistry rather than engineering, only brief details of some of the areas under development will be discussed. More details of the process can be found in the reference sources. Meszaros [17] splits the depolymerisation routes into two types (Figure 9.4).

Figure 9.4 Routes to depolymerisation [17]

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What to do with Composites at the End of Vehicle Life The depolymerisation of polymers allows the recovery of the original monomer from which new material can be made. It is especially suited to condensation polymers such as PET, PMMA, PA and PU as the process is simply the reverse of the one used to create them. The route for PMMA is shown in Figure 9.5. A process to depolymerise PMMA was developed and patented in Germany as far back as 1949 [18]. The process floats PMMA on the surface of a lead bath at temperatures of around 550 °C in an oxygen free environment. Lead contamination in the recovered PMMA monomer severely limits application for this process. An alternative and more environmentally friendly method has been developed using a twin-screw extruder, which heats material beyond its depolymerising temperature. The monomer is recovered as a gas and condensed into a barrel [19] however this process is still at a laboratory stage. In 1997, ICI Acrylics and Mitsubishi Rayon announced a joint venture to develop more efficient depolymerisation technologies for PMMA and were offering to take back scrap PMMA [20]. The PMMA is crushed and thermally treated at 500 °C to produce MMA monomer.

CH3 CH2

C

CH3 CH2

COOCH3

C

CH3 CH2

COOCH3

C COOCH3

CH3 CH2

C

CH3

+ CH2

COOCH3

C COOCH3

Figure 9.5 Depolymerisation of PMMA

The depolymerisation of Polyamide 6 has also been well researched. In 1997, DSM Chemicals (North America) and Allied Signal commissioned a world scale facility to depolymerise Polyamide 6 from carpet waste [21]. It opened in 1999 and 25% of these depolymerised monomer building blocks are used to make new Polyamide 6 compounds under the tradename AKULON ReCap with a particular focus towards their use in automotive applications [22]. The recovery of polyester materials such as PET can be achieved using hydrolysis. Again it is the reverse of the reaction (polycondensation) used to make polymer in the first place. Dupont hold a patent on one such process that converts the polyester back to carboxylic acid [23]. Hydrolysis was also the preferred method of breakdown for two pilot plants for the breakdown of PU foams. The two companies involved in this case being Bayer (Germany) and General Motors (USA) [7]. The PU reaction is slightly more complicated than the one for PET as there may be isocyanates or ureas also present depending on the application. This can produce mixtures of substances that may require further separation.

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An Introduction to Automotive Composites A pilot plant was opened in the UK by ICI in 1998 to look at the feasibility of chemically recycling polyurethane, their method being a process called split-phase glycolysis [24]. Like hydrolysis the method is complicated by the presence of urea and isocyanurate. Separation can be avoided in this case however by further chemical reactions. The basic chemistry involved in these reactions is presented by Ehrig [25] for those who require further details. As can be seen in Figure 9.6, a variety of recycling methods can and are used for polyurethane recycling.

Figure 9.6 Recycling routes for polyurethanes [7]

9.6 Thermal Conversion Technologies Thermal processing can be defined as the conversion of solid wastes into conversion products with a release of heat energy. It can serve two purposes: volume reduction and energy recovery. Thermal depolymerisation is a ‘Route A’ process (see Figure 9.4). Category definitions usually stem from consideration of process air requirements: •

Pyrolysis: thermal processing in absence of oxygen

•

Hydrogenation: pyrolysis but in a high hydrogen or carbon monoxide environment.

•

Gasification: partial combustion in which a fuel is deliberately combusted with less than stoichiometric amounts of air.

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What to do with Composites at the End of Vehicle Life Compared to direct combustion, these methods can offer significant benefits. They are environmentally cleaner process routes producing significantly lower emissions. The reduction in density enables cost reduction for subsequent transport and handling and there is also an increase in energy density. Conversion to fuel results in higher overall efficiencies than standard combustion.

9.6.1 Pyrolysis There are two types of pyrolysis techniques, both carried out in an oxygen depleted environment. Low-temperature pyrolysis, also known as ‘cracking’ is a depolymerisation technique. The main products produced include potential feedstocks for polyolefin production as well as higher boiling liquids and waxes. Whilst laboratory cracking of simple polymers is widely understood, mixtures of polymers including associated fillers, additives and other impurities make commercial scale-up more difficult. BP Chemicals developed this technology on a pilot plant in Grangemouth, UK. High-temperature pyrolysis can be used for co-mingled wastes at temperatures in excess of 600 °C and is many used for waste that is so highly mixed or contaminated that combustion is not an option due to the pollution hazards that combustion products may pose. Pyrolysis produces 5-20 times less gas than standard combustion techniques offering reduction in the costs of scrubbing as well as concentrating and binding pollutants in the coke residue for easier disposal. There are a number of different methods and many pilot plants have been in operation. The reader is referred to Brandrup [26], for a detailed discussion.

9.6.2 Hydrogenation This is a similar process to pyrolysis but in this process the mixed plastic waste (MPW) or composite material is heated with hydrogen. As the molecules are cracked, (the process is often termed hydrocracking), they are saturated with the hydrogen molecules to produce a saturated liquid and gaseous hydrocarbons. The pressure of the hydrogen must be sufficient to suppress repolymerisation or undesirable coking products. In 1992, a coal-oil plant in Bottrop, Germany, successfully trialed hydrogenation technologies with plastic waste. Some restrictions on feedstocks were necessary to optimise the efficiency of the plant, thus close collaboration with suppliers is essential.

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9.6.3 Gasification In this route B process (Figure 9.4), the bulk of the carbon present in the waste feedstock is converted to gas leaving a virtually inert ash residue for disposal. This is the result of partial combustion of the feedstock with oxygen in the form of air, steam, pure oxygen, oxygen enriched air or carbon dioxide. This method is favoured for fuel gas production since a single gaseous product is formed at high efficiency without requiring expensive and potentially dangerous air separation plants. The synthetic gas produced can be classified according to its composition, heat value and application. Relatively high temperatures are needed (800-1600 °C) depending on the gases used. There are a variety of different variations on this process all derived from the original processes developed for coal and oil. The reader is referred to Brandrup [27] for further details of these process variations.

9.7 Energy Recovery There are a variety of methods used for energy recovery. One, pyrolysis has already been discussed. Incineration, production of waste-derived fuel, and gas recovery from landfill site emissions are three examples of energy recovery in operation. The amount of energy recovery depends on the calorific values of the fuels that are used. Values quoted in the literature, tend to give an average calorific value of mixed plastic waste as 35 MJ/kg and composites 20 MJ/kg. When compared to paper (16 MJ/kg) and organic waste (3 MJ/kg), it can be seen that plastics give a relatively high energy return when incinerated. Energy recovery from waste has tended to be much more common in mainland Europe and the USA than in the UK. For example, Luxembourg uses around 75% of its municipal waste for energy generation whilst the UK uses only 6% [28, 29]. This difference is mainly due to the acceptance and relative cheapness of landfill as a waste disposal option in the UK. However the increasing expense and restrictions on landfilling should see a trend towards alternative disposal methods. Many countries throughout Europe use municipal solid waste combustors with state-ofthe-art energy recovery and flue gas cleaning technology to produce a high percentage of domestic electricity requirements. Energy recovery is even more efficient if the combustor is linked to a municipal localised heating system for the supply of hot water and process steam. In Paris, France, residential buildings in some areas are equipped with combustors. Thereby domestic waste is incinerated locally and used to provide low cost heating for the residents.

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What to do with Composites at the End of Vehicle Life MPW can also be used as a secondary fuel source in cement kilns and steel foundries. Ground plastic waste is added to the main fuel source and can replace up to 16% by weight of the primary fuel. This secondary fuel source has been found to reduce the NOx and SOx emissions and it is estimated that up to 40% of the primary fuel can be replaced by MPW in the future. A variety of recycling methods and options have now been discussed. These options can now be applied to one of the biggest issues surrounding landfill avoidance, that being the disposal of ASR.

9.8 Automotive Shredder Residue The constituents of ASR are variable. A typical composition of ASR is given in Table 9.3.

Table 9.3 Typical constituents of European ASR [28, 29] Category

Range (wt%)

Metal

2-17

Wire

1-11

Rubber

7-29

Foam

1-10

Wood

2-14

Plastic

7-13

Textile

10-28

Paper/card

3-6

Miscellaneous combustibles

9-16

Miscellaneous non-combustibles

3-19

Although studies have shown ASR can be incinerated without harm to the environment, it’s calorific value is only 0.19 KJ/Kg [30]. This is due to the large non-combustible component that still needs disposal after combustion. For this reason most research has gone into separation of ASR for further recycling. One of the first countries to tackle to issue of ASR landfill was Japan. A series of plants designed to sort and reprocess automotive shredder residue are up and running or in the pipeline by automotive manufacturers. Toyota opened an ASR recycling

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An Introduction to Automotive Composites plant in 1998 and an 87% recycling efficiency for end-of-life vehicles was attained as of October 1999 [30]. BMW are working with a company in Belgium on a plant along similar lines [5]. In the UK, a major study is underway backed by the CARE group to also look at the technical feasibility of recovering recyclable materials from shredder residue. Processing and encapsulating ASR untreated, using conventional plastic processes has been achieved by researchers at University of Warwick [31]. However, this work has not been taken up commercially. The American Plastics Council (APC) has initiated a feasibility study to look at using ASR as a coke replacement in steel blast mill furnaces. This process requires the ASR to be pre-treated to reduce halogen and heavy metal content [32]. The variety of materials present in ASR mean it is likely that future disposal will incorporate many of the different processes and methods for recycling that have been outlined in this chapter. In order to do so, it is clear there must be co-operation through the entire vehicle design to disposal cycle for effective solutions to be found. For this an infrastructure is required.

9.9 Creation of a Recycling and Recovery Infrastructures Any successful sustainable technology must balance issues of technology, economy and environment. For the vehicle industry it means solutions for the following: •

Develop cost effective sorting and recycling technologies and processes, capable of creating quality materials from materials found in ELV.

•

Design parts, which can be easily disassembled (design for disassembly; DFD).

•

Design parts, which can be easily recycled (design for recycling; DFR).

•

Create recyclate markets, both within and external to, the vehicle industry.

•

Create an ELV recycling infrastructure, allowing post consumer waste to move from dismantlers to recycling centres in an effective manner.

As well as these, it is necessary to educate and inform all those involved in vehicle manufacture how they can aid in reaching recycling targets.

9.10 Design For Disassembly and Recycling A successful strategy for disassembly is the easy and cost-effective removal of parts. If a part is difficult to remove, it is likely to be left on the vehicle shell to become shredder

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What to do with Composites at the End of Vehicle Life residue. If removal is labour and time intensive, the cost effectiveness of recycling falls. Therefore parts should be designed with disassembly in mind. Design for Recycling guidelines produced by Ford include guidance on the following: •

Quick and easy removal of parts and fluids for cost effectiveness.

•

Parts marking: the benefits of easy identification of generic material types were discussed earlier in the chapter, it is a practice proven successful in the packaging industry.

•

Material selection: this includes usage of materials that can be recycled, avoiding mixtures of incompatible materials and reducing unique material types, all of which make the recycling process easier. By reducing the number of different plastics used, segregation becomes easier and more efficient. Design for Manufacture (DFM) can both aid and hinder Design for Disassembly (DFD) and the recycling process. What is good for one is not necessarily good for the other. For example, a part that is easy to fit may not necessarily be easy to dismantle. It may be necessary to compromise so both can be satisfied.

•

Materials of risk to health and the environment: how to eliminate them or best deal with associated hazards.

9.11 Developing Recyclate Markets Both applications and consumers must be found for recyclate materials, in order to create demand and allow recycled material to have economic value in the marketplace. If the associated value of the material is sufficient, then recycling of the material will be both cost effective and sustainable. Whilst many applications for these recycled materials may be found within the Motor Industry, it is likely that other markets can be found for specific recycled materials.

9.12 Developing a Recycling and Recovery Infrastructure In order for materials to be successfully recycled and recovered, an associated infrastructure needs to be in place to ensure all areas of the recycling loop are working in harmony. This will include removing materials from the ELV, sorting and cleaning them, and then transport to recycling centres. Material processing will need to produce

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An Introduction to Automotive Composites recycled materials of constant properties and quality and be capable of transporting this material from the processors onward back into the manufacturing sequence. Infrastructures of this type exist, for example in dealing with municipal waste, and once set up in the motor industry, can help to make the landfill avoidance of ELV materials, a cost effective and sustainable process.

9.13 Conclusion The EC directive for ELV defines the Motor Industry’s responsibility for ‘ELV collection, recycling and recovery objectives’. Target materials for recycling and recovery include thermoplastic, thermoset plastics and composites. These materials collectively contribute 11%-19% of the total vehicle weight dependent on the model. These materials can be recycled directly by reusing the component, mechanically recycled by physically grinding or powdering the material prior to reprocessing, chemically recycled by breaking the polymer units into smaller units for use as monomer or fuel, or energy recovered using incineration. New and cost effective recycling techniques are continually developing pushed by environmental legislation and concern. The trend towards plastic manufacturers themselves, marketing grades containing recycled materials, takes much of the emphasis away from the designers to find ways of incorporating materials and composites. The lack of knowledge on consistency of quality and properties, often a cause for resistance to switch to recyclates, is removed. New potential uses for recyclates can also be more easily identified. Designing components for disassembly and recycling and setting up an infrastructure capable of handling the post consumer materials are issues that must and are being addressed. As the European automotive recycling legislation is enforced the Motor Industry is set to drive the ELV disposal programme into the future. As a footnote, the key waste management steps as illustrated by Ford are shown in Table 9.4.

Table 9.4 A progressive approach to waste management Step 1

Step 2

Step 3

Avoid waste by correct production process selection.

Residues which cannot be avoided must be reduced in terms of quality and toxicity.

Residual materials are recycled wherever possible.

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References 1.

C.J. Stroud in Plastics in Automotive Engineering, VDI Verlag, Dusseldorf, Germany 2000.

2.

CARE mini conference. Motor Heritage Centre, Warwick, UK, 1997.

3.

ACORD, Second Annual Report, Society of Motor Manufacturers and Traders Ltd., London, UK, 1999.

4.

Proposal for a European Parliament and Council Directive on End of Life Vehicles, Brussels, 10/07/97.

5.

D. Urth, Presented at the 8th Plastics Recycling Conference, Krefeld, Germany, 1999.

6.

V. Freeman, The Times, 1997, No. 66078, 45.

7.

J. Brandrup, M. Bittner, G. Menges and W. Michaeli, Recycling and Recovery of Plastics, Carl Hanser Verlag, 1996, Chapter 3.

8.

R.D. Pascoe, Sorting of Waste Plastics for Recycling, Rapra Review Report, Rapra Technology, Shrewsbury, UK, 2000, 11, 4.

9.

S. Datta and D.J. Lohse, Polymeric Compatibilisers: Uses and Benefits in Polymer Blends, Carl Hanser Verlag, Germany, 1996.

10. C.G. Hagberg and J.L. Dickerson, Proceedings of SPE Antec ’96, Indianapolis, 1996, Volume I, 288. 11. J. Grenci, S. Dey, C. Jacob, A. Patel, S.S. Dagli, S.H. Patel and M. Xanthos, Proceedings of Antec ’93, New Orleans, LA, 1993, Volume I, 488. 12. H. Herbst and R. Pfaendner, Proceedings of the Rapra Conference, Plastics Reborn in 21st Century Vehicles, Nuneaton, UK, 1999, Paper No.3. 13. V. Mattus, M. Neitzel, R. Dittmann, H. Hoberg and H. Wallentowitz, Kunstoffe Plastics Europe, 1998, 88, 1, 18. 14. M. Newborough, Thermal Depolymerisation of PMMA, Polymer Recycling Network, Seminar Proceedings, 2000, University of Warwick. 15. K. Jost, Automotive Engineering, 1995, 103, 8, 40.

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An Introduction to Automotive Composites 16. M.J. Bevis, C.E. Bream, P.R. Hornsby, K. Taverdi and K.S. Williams, Proceedings of SPE Antec ’98, Atlanta, GA, USA, 1998, Volume III, 2932. 17. M.W. Meszaros, Proceedings of Plastics Recycling: Survival Tactics thru the 90s, Schaumburg, IL, USA, 1993, 73. 18. K. Graser, R. Hoock and D. Urth in Plastics in Automotive Engineering, VDI Verlag, Dusseldorf, Germany, 2000. 19. K. Breyer and W. Michaeli, Proceedings of SPE Antec ’98, Atlanta, GA, 1998, Volume III, 2942. 20. Ends Report, 1997, 275, 27. 21. T. Brand, Chemical Marketing Reporter, 1997, 252, 26, 1. 22. Reuse/Recycle, 2000, 30, 5, 38. 23. J.L. Harvie and S.M. Heppell, inventors; E.I. DuPont de Nemours and Company, assignee; US Patent 5886057, 1999. 24. Ends Report, 1998, 284, 20. 25. R.J. Ehrig, Plastics Recycling: Products and Processes, Carl Hanser Verlag, 1992. 26. J. Brandrup, M. Bittner, G. Menges and W. Michaeli, Recycling and Recovery of Plastics, Carl Hanser Verlag, 1996, 434. 27. J. Brandrup, M. Bittner, G. Menges and W. Michaeli, Recycling and Recovery of Plastics, Carl Hanser Verlag, 1996, 455. 28. T334 Environmental Monitoring and Control, The Wastes Block, Open University, Milton Keynes, UK, 1992. 29. OECD, Washington Waste Minimisation Workshop, Volume 1, Five Waste Streams to Reduce, 1995, Washington DC, USA, 1996. 30. Y. Tsujita and T. Kajiwara in Plastics in Automotive Engineering, VDI Verlag, Dusseldorf, Germany, 2000. 31. R.L. Cain, V. Goodship, J.C. Love, G.F. Smith and N. Tucker, Polymer Recycling, 2000, 5, 2, 63. 32. D. Barnet, et al, Proceedings of ARC ’99, Dearborn, MI, USA, 1999, 299.

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10

The Future of Composites Jason Rowe

The first part of this section deals with the future of vehicles in general and describes the future of vehicle manufacturing in light of the use of composites. Some of the ideas presented may not be feasible in the near term due to the underlying technology not being available or economical with current processing technology. These ideas serve as a way of extending the thinking processes that are required for new vehicle development. The final part of this section shows some of the near future work that could form part of the next generation of vehicles.

10.1 The Advantages of Composites In most cases polymer matrix composites (PMC) are in competition against existing metal components. In the case of automotive applications this means steel and aluminium. The advantages of steel are cost, strength, and a route for recycling that is an integral part of the manufacturing process, 50% of manufactured steel finds its way back to the steel works as scrap [1]. The disadvantages are the very high cost of plant and tooling and the limits of ductility. The case for aluminium is also constrained by the relatively high material cost. However, this is usually off-set by an additional weight saving potential. The ductility implication of metals mean that complex shapes must be made as fabrications. Making a similar component from PMC is characterised by cheap plant, but expensive materials, which allow complex shapes to be made as single articles. Whilst PMC are usually accepted for low volume applications, they can only overcome their perceived disadvantages for large scale manufacture by being used to make a more sophisticated product. Köster [2] describes the Chrysler Corporation ‘Composite Intensive Vehicle’ (CIV) which achieved a 106:1 reduction in body costs, and quotes figures showing that a CIV can achieve a 30% fuel consumption improvement over a steel monocoque equivalent. More recently (1997), the Chrysler Corporation has produced pilot runs of the Chrysler Composite Concept Vehicle (CCV) using glass reinforced PET as the main structural material (see Table 10.1).

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Table 10.1 Comparison of Dodge Neon and the CCV (Reinforced Plastics, Dec 1997) Weight

Chrysler CCV

Chrysler Dodge Neon

400 kg

2,000 kg 2

185,000 m2

Size of manufacturing plant

28,000 m

Time to build

6.5 h

19 h

Cost of new plant

$300 million

$900 million

Number of Parts

1,100

4,000

10.2 Future Vehicle Manufacture - Hypercars Moving further into the future, perhaps the way ahead is a complete composite vehicle, for which the term Hypercar was coined. A Hypercar is defined by it’s performance. Not performance in the sense of 0-60 mph times and top speed, though these must remain within current boundaries, but performance in terms of fuel efficiency. The Rocky Mountain Institute (RMI) have put together a comprehensive study of fuel efficient vehicles. In the US the big three automakers have joined together in a Partnership for a New Generation of Vehicles (PNGV) to develop cars capable for 80 mpg, about 3 times the current Corporate Average Fuel Economy Standards (CAFE) requirements. The RMI asserts that this goal is too conservative and that greater than 300 mpg is achievable from today’s technology.

10.3 How Do We Achieve 300 mpg Plus? The short answer to build fuel efficient vehicles is to make them lighter (also give them lower drag factors, lower rolling resistance). The core thesis of the work at RMI is that by reducing the weight of the body, the mass of the power train and ancillary equipment, etc. can also be reduced in mass. Again by reducing the other component’s mass you can reduce the body mass and so forth. This concept is called mass decompounding. To make a body shell as light as possible, low density, high specific property materials should be used, which leads to the use of polymer matrix composites for the entire body structure. The material of choice (by RMI) is carbon fibre, epoxy matrix composite, various forms of which are well characterised in the aerospace industry.

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The Future of Composites For the power train, a series hybrid drive system appears to be the most fuel efficient. The proposed design is a fuel efficient petrol engine that provides electrical power for a set of electric motors to drive the road wheels. Also there are the associated charge storage devices and load levelling devices to allow performance boosts as and when required. The combination of a low drag coefficient, ultralight body with a hybrid electric power train provides the technology required to deliver the proposed fuel economy.

10.4 What is the Technical Feasibility? Another tenet of the RMI theory is the proposition that all of the technology required to build a Hypercar is available and it is possible to ‘bolt’ different technologies together to form the complete concept. According to RMI high speed RTM is able produce the composite body shell to the required dimensions and accuracy. This point is concurred since Ford already uses RTM for a number of applications from high roof model Transit vans through to high class panels for the Aston Martin DB7 and latterly with Lotus sourcing the body panels for the new Elise from RTM. So technical feasibility is there to produce composite components from RTM, though see the later section on costs.

10.5 Economic and Market Environment The ‘need’ for a change to the Hypercar philosophy may be forced upon the industry by a radical change in the fuel consumption requirements. Also, a change in manufacturing technologies and processes may be forced by a major change in the cost base of vehicle manufacture. RMI suggest both of these are happening now. To understand whether the automotive industry is in an era of change it is worth understanding what some of the distinguishing features of this state of change are. From Utterback ‘Managing the Dynamics of Innovation’ [3], this state of change is characterised by the rapid emergence of a range of new and varied product that meet customer requirements with a range of innovative solutions to customers problems or at a much reduced cost. The first reaction of the dominant industry is an incremental improvement in its current technology that staves off competition for a longer period. The first point, are new designs emerging? In the industry at present there are a number of different vehicle are being developed, e.g., Chrysler Pronto, Pronto Spyder and CCV, which have shown a design evolution, GM Ultralight, EV and Speedster, and Ford AIV. Each vehicle has a different design solution and a dominant product has not become apparent. The second point: incremental improvements in current technology to oppose the threat (e.g., high strength steel and ULSAB) are bound to continue to occur!

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An Introduction to Automotive Composites In deciding which concept will become the dominant design, history may again help. From Utterback [4], ‘Starting a firm with a new product in a new industry involves much borrowing of diverse elements from others and synthesising them creatively. We have seen that the winners are often the ones that are most experimental and flexible in matching the early forms of product with unexpected demands and opportunities and think through the development of their innovation in the most thorough and systematic way’.

10.6 What Stops us From Building Hypercars Now? There are a number of ‘stoppers’ that make the development of composite vehicle more difficult: •

Volume manufacture

•

Tooling assumptions (soft tooling)

•

Design complexity

•

Design for energy absorption

•

Computer aided engineering (CAE) capability

•

Component quality

•

Performance feel

•

Fit and finish

•

Robust supply chain

•

Recycling

•

Risk

Soft tooling is not an option for high volume manufacture in terms of quality and also cost (though they are cheap, they wear out, therefore you need more). Another key stopper in the past has been the inability to design for energy absorption during crash. Lotus has, however, engineered niche vehicles with composite crash structures from carbon and glass reinforced thermosetting materials, e.g., Elise I and II plus several others for major European OEM. These materials are, however, constrained to the lower volume end of the market so that Lotus (as well as other OEM and consortium’s, e.g., CRACTAC), are investigating energy absorbing materials with thermoplastic matrices for higher volume applications. Thermoplastic matrices will also improve the recyclability of the part.

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10.7 Costs The cost argument can often be misused. Judging the economic viability on a part substitution basis is misleading particularly if the analysis is based on part manufacture only. Cost analysis should reflect two aspects: •

The total cost of introducing the part including secondary operations, painting and assembly, and

•

The additional design and performance benefits of utilising the new material, i.e., the potential of taking out weight, cost or structure elsewhere in the vehicle.

By replacing the entire vehicle you can ‘tunnel through the cost barrier’ thus making the entire vehicle cheaper. However, material selection must be undertaken on the basis outlined above as piece costs alone for current composites panels show that thermoset composites may not be economically viable. This is born out by reference to GM Ultralight (a concept car made from expensive prepreg aerospace materials), where a carbon fibre composite bodied car had a material cost alone for the body shell of $13,000 [5]. The target cost for this complete body shell was less than $1,000. RMI figures (Costing the Ultralight in Volume Production: Can Bodies in White be Cost Effective, Mascarin, Dieffenbach, Brylawski, Cramer and Lovins, 1996), are shown in Table 10.2. This gives break even volumes (compared to steel) of 0.15 million vehicles when comparing manufacturing costs, and 0.29 million vehicles when lifecycle costs (including costs of disposal, and vehicle use) are taken into account.

Table 10.2 Piece costs for Hypercar manufacture Steel

Hypercars using composite tooling

Total manufacturing piece cost

$1,755

$2,483

In use total

$2,341

$2,719

$350 million

$240 million

Investment cost

10.8 Summary and Comment on the Feasibility of Hypercars All products and processes are continually at risk from step changes in technology or a change in the cost base of manufacture. In most cases the risk is negligible to the dominant

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An Introduction to Automotive Composites industry due to the prevailing economic and social climate. There are a number of automakers who are experimenting with a range of new vehicle concepts. The driving force for this change may be environmental pressure to produce highly fuel efficient vehicle the 300 mpg vehicle. Although car makers are constantly seeking ways to improve fuel efficiency, the car buying public is not keen to embrace fuel economy as a major factor for new car choice. Though auto manufacturers are keen to start reducing vehicle masses that are rising due to safety requirements and customer delights. Cost always is a prime consideration. The Hypercar principle would need to be able to deliver a substantially lower cost, with improved fuel economy as an added bonus. Costs of the Hypercar may be reduced to allow significant break even volumes as shown by Mascarin [5], though these would need to be reworked on a specific design to gain confidence in the projected costs. One final point not considered is safety requirements. Proposed safety regulations may mean that we have to build vehicles in a different way. The technical direction chosen by RMI, (RTM carbon fibre composites), may not be best suited to high volume production, with other routes such as thermoplastic stamping on aluminium or steel spaceframe being equally viable. RTM type technology is ideally suited to low volume manufacture and the distributed risk of low volume manufacture may be the key to use RTM technology.

10.9 What Will the Next Generation be? Predicting the long-term future is difficult and may indeed include Hypercars of one shape or form. In the shorter term there are a number of applications of composites that have been driven through weight savings, cost savings and quality improvements.

10.10 Composite Front Ends The first composite front end was introduced in 1987 on Peugeot 405, manufactured from SMC. This was followed by others, e.g., Peugeot 605 and Citroen XM - also in SMC. The one piece composite front end replaces a number of steel panels which make up the front end of a more traditional vehicle body and this gives a weight reduction. It is also possible to integrate other parts reducing weight further, for example by eliminating additional brackets and fixings, etc. The composite front end also gives a number of other benefits:-

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The Future of Composites Since the single composite component replaces several steel pressings the accuracy of the front end is improved thus improving fit and finish of components on the front end of the vehicle. This, together with the ability to integrate other components such as the battery tray, ducting, etc., reduces the overall parts count. There is a reduction in cost (both tooling investment and piece cost) due to this integration. It is considered that repair costs can also be reduced. The front end is obviously resistant to corrosion, and the requirement to paint ‘black-out’ in the front grille ‘see-through’ area is eliminated. In 1991 VW took the step to introduce a front end in GMT (glass mat thermoplastic) for the Golf. This gave further weight savings over SMC. Subsequently GMT has been used for front ends on VW Polo, Audi A4 and more recently on Audi A3. Weight savings of approximately 25% are achievable with SMC over steel. GMT parts can be approaching half the weight of the similar steel component, so on a front end typically 2-3 kg saving is possible. GMT was developed in the late 1970s, initially being used for items such as undertrays. Its use has been growing ever since then (particularly in the German market) and new manufacturing plants have been set up in Europe. The areas where it is likely to continue to increase in use are those that could be termed ‘semi-structural’ body applications. However due to problems of glass ‘read-through’ it can not be used for visible components, unless these are grained; and ‘A’ class is out of the question. It is likely that the use of composite front ends in one form or another will form part of all OEM long range vehicle plans. The latest developments include the use of steel pressings in the mould. These hybrid technologies exploit the best of both materials technologies in terms of cost, weight and recyclability, to offer a unique design solution. Bayer initially developed the technology using a polyamide material and the injection moulding process. The Ford Focus uses such a part.

10.11 Doors and Tail Doors Similar to the front ends in the section above, the first applications used thermoset composites, particularly SMC and it’s variants. As thermoplastic composites have improved in mechanical performance, these materials have come to take over from the thermoset

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An Introduction to Automotive Composites composites. There appears to be a major drive for the use of thermoplastic, due to the greater weight savings and potential to recycle when compared to thermoset composites. The greatest obstacle to thermoplastic materials are their high expansion characteristics. Although raw material manufacturers have managed to make improvements thermoplastic materials suitable for closure applications still have greater CLTE values than steel, aluminium or thermosetting composites (RTM and SMC). The implication of greater expansion manifests itself when OEM try to control flush and gap margins of adjacent panels – OEM will not stand for large panel gaps, panels out of flush or panels bowing. Clearly the challenge for thermoplastic materials is either to manage the expansion through clever design or to reduce the CLTE. The future use of thermoplastics relies on this challenge being met by both the OEM and the raw material manufacturers. Lotus, on the other hand, has demonstrated the use of thermosetting composites for closures (in fact the whole body of the new Elise II is a combination of RTM and SMC panels). Flush and gap margins are on a par with conventional metal bodied vehicles. These technologies are, however, only suited to the lower volume end of the market.

10.12 Bonnets and Front Zones of Vehicles Future of bonnets and front zones of vehicles will be driven by the need to meet future crash regulations. Current steel structures may not be able to meet the proposed legislation. The requirements may mean that the front sections of the vehicle be deformable to reduce pedestrian impact forces whilst maintaining the energy management required during front end crash. This is a demanding role for the materials that make up this front end section. The use of thermosetting composites as energy absorbing structures, most notably by Lotus, has been touched upon earlier in this document. The use of thermoplastic composites in this area is actively being pursued by a number of OEM and first tier suppliers. The driving force for this activity has to be the additional cost and weight savings possible through the use of thermoplastics, as well as the increased recyclability of the material.

10.13 Structures Examples of true structural applications of composite materials in the automotive industry are difficult to find. This is because OEM have a concern over damage assessment. Once a composite part has been in an impact it is difficult to assess whether there is any

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The Future of Composites internal damage. The parts can be destructively tested but this defeats the object of not having to replace the part. The industry is currently developing non-destructive testing (NDT) solutions such as infrared photography and neural networks. Despite this raw material manufacturers are making significant progress in developing structurally integral materials: •

Carbon fibre/thermoplastic co-mingled tube has been developed for applications such as roll-over bars, suspension sub-frames, side impact beams and chassis longerons. The process is similar to steel hydroforming but has the advantage of low cost tooling and very significant weight savings. A roll-over bar for a bus in this material has already been produced, replacing an 18 kg steel version with a composite part weighing under 3 kg.

•

A thermoplastic polyurethane/glass pultrusion technology may provide a solution for side impact beams. Profiles in this technology exhibit outstanding stiffness and strength characteristics at a specific gravity much lower than aluminium.

•

A PA12 liquid monomer for use in the RTM process has been developed. Thermoplastic RTM opens up the possibility of using lower cost carbon fibre reinforcements, which will give higher stiffness, and conductivity for electromagnetic induction (EMI) screening. This technology will also enable recyclable crash structures.

10.14 Summary Currently, for a one to one swap of panels, composite components tend to be more economic at low volumes. The use of composites therefore tend to lend themselves for use in derivatives or niche vehicles. It is only when a complete redesign is possible or is required (for example to meet future crash regulations) that the composites tend to become cost effective at higher volumes. Other applications of composites include the roof panels or exterior skins of certain vehicles. Again these may be for a variety of derivatives off of the same platform, giving an economic viable route to product differentiation. An alternate approach is to use the low investment necessary for composite panels to allow for re-skinning of current vehicles, again leading to enhanced customer satisfaction through product differentiation. It is true that to gain all the benefits afforded by composites in the use for structures a redesign of vehicle may be required, but significant opportunities exist for the replacement of current panels to give weight and cost savings and also possibly to increase customer

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An Introduction to Automotive Composites satisfaction. Short-term applications are likely to be the closure panels where integration of components and features not only give the weight and cost savings but also manufacturing enhancements due to reduced number of components. Composite use on our current vehicles looks set to increase substantially (market trends suggest up to 10% growth per year in automotive markets) and the use of such components will give the OEM a customer benefit that will be hard to ignore. The successful exploitation of composite materials may well give motor manufacturers the edge they require to stay ahead of the marketplace and it is up to each OEM to ensure they remain at the forefront of this technology.

References 1.

J.E. Gordon, The New Science of Strong Materials or Why You Don’t Fall Through the Floor, Penguin, Harmondsworth, UK, 1984.

2.

J. Köster, Composites Manufacturing, 1990, 1, 2.

3.

J.M. Utterback, Mastering the Dynamics of Innovation, Harvard Business School Press, Havard, USA, 1994.

4.

J.E. Gordon, The New Science of Strong Materials or Why You Don’t Fall Through the Floor, Penguin, Harmondsworth, UK, 1984, p.216.

5.

A.E. Mascarin, Mechanical Engineering, 1992, May, 64.

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11

Design Guidelines for Composites Kevin A. Lindsey

Throughout this book there have been numerous examples of composite manufacturing techniques, different composite materials and the properties and costs associated with each of these. With all of these disparate materials and techniques, the choice is bewildering for any would-be designer. It is not the role of an introductory text to discuss in depth the detailed answers to a given design brief. The aim of this chapter is therefore to give an approach to the choice of materials and processes and to provide an insight into some of the factors that should be considered when composites are to be specified for use for an engineering application.

11.1 Why Composites? ‘Why composites’ is a question that appears early on, and continues throughout this book. The reader will now be aware of a series of answers, from various viewpoints, to this seemingly innocuous question. This chapter re-phrases that question in the form in which the practising automotive engineer is most likely to hear it: Why should we use composites instead of metals? The short answer to this question is: only when there is a benefit to doing so. This chapter discusses how and why such a benefit can be assessed, remembering that our fashionably high-tech composites will not be the correct solution for all eventualities. Excluding the exotica of the Formula 1 industry (see Figure 11.1) and various ‘Supercars’ one of the prime motivations of all automotive companies is the reduction of the cost base. As highlighted previously, the weight of vehicles is becoming more important, however there are very few vehicle programmes that will allow a cost increase even for a weight reduction. Composites can reduce the weight of vehicles though this has to be evaluated in terms of cost also. In the section on economics and in the introduction, the use of composites to replace metallic components has been shown to be cost effective. An example in the cost savings of composites is through the use of GMT for the front end of Audi and VW vehicles. Here the ability of composites to allow parts count reduction reduced the cost of the

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An Introduction to Automotive Composites

Figure 11.1 Formula 1 - the cutting edge of composites (Source: Ford Motor Company)

complete assembly. This is quite often the case, for a straight one on one swap, the composite component is seldom cheaper, it is only utilising one of the specific advantages of composites, e.g. moulding of complex features, that a cost saving will be accomplished. As discussed in the section on economics, care has to be take to understand exactly what is meant by cheaper. In the above example for the GMT front end, we are implicitly referring to a high volume vehicle and effectively ignoring the investment cost and just relating to the piece cost. This approach, however, does ignore one of the benefits of the use of composites - low investment costs. The low investment costs needs further expansion, it must be remembered that the investment costs are low when compared to the tens to hundreds of millions of pounds spend on the tooling for high volume steel bodied vehicles. In this instance low investment does not mean zero investment and may mean that substantial sums are required to purchase the level of tooling that would be required for the vehicle (more of this later). The lower investment is (generally) coupled with higher piece cost hence the break even scenarios highlighted in the section on economics. In summary, the major forces for the use of composites are the saving of weight, but this must be done economically. The economics may be such that the composite component is always cheaper than the steel component, e.g., GMT front end, or only viable up to a given volume. Of course the ultimate incorporation of many components into a single moulding or module is the complete composite vehicle as discussed in Chapter 10. This is a special case and hopefully the potential benefits of this route will some day come to fruition.

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11.2 Choice of Materials The subject of material choice is the mainstay of many a design lecture and is indeed an important aspect of gaining the correct design and costs. The factors for consideration for materials choice are listed elsewhere, however it is worthwhile to dwell on this subject. One of the important aspects of material choice, the cost, has already been mentioned. In order to gain a definitive cost, a material must be known and also the amount of material to be used must also be known. This would lead us on to the mechanical design of the component. In general the stiffness of the composites would be a factor of 20 less than that of steel, whereas the strength may only be, at worst, a factor of 2 or 3 lower than that of steel (for some composites the strength may be greater than that of steel or aluminium). This means that, in general, a design with a composite component is stiffness limited, that is, if it is stiff enough then it is usually strong enough, at least on a macroscopic scale. Of course to regain the extra stiffness that is lost from the material the composite is usually thicker than the equivalent steel panel. At this point it is worth considering what type of loads are to be applied to the composite. If pure tensile then the composite must be thicker than the steel component in the ratio of the modulii. This would create an inordinately thick panel. Most loads do tend to have a bending component, in resisting such loads a typical 0.8 mm thick steel panel would be equivalent to a 3 mm thick composite panel (depending upon modulus of the composite), which is in turn equivalent to a 1.2 mm thick aluminium panel. This leads to a general rule of thumb that for bending, the composite panels tend to be 40% lighter than steel and similar to aluminium. More detailed analysis would be required either using some basic hand calculations or FEA. One area where it is critical to carry out some detailed analysis is in the local stresses around fixing points and the like. In these regions the local stress may exceed that of the material. Also metals tend to have a larger strain to failure than composites and it is usually this point that would lead to a premature failure of structure that would pass initial design calculations. The last area to be careful of in design with composites is the failure by an unpredicted mode. Although composites may have a very high tensile strength, the shear strength and in particular the inter laminar shear strength tends to be relatively low. In this instance the low shear strength may cause premature failure. This is especially the case in thick sections and in sandwich panels.

11.3 Getting What You Want By the time you are ready to talk delivery dates and order volumes with suppliers, you will have evaluated a few materials, checked out the economics and found that the material properties that you require (probably from either data sheets or micromechanical estimates) are satisfactory. The composite would not only have the correct mechanical properties, it would also have the correct temperature performance and whatever other features are required for the application.

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An Introduction to Automotive Composites There are still a few hurdles to overcome. Although the material has been chosen, it is now a case of defining the material so that the supplier of the material can produce precisely what is required and will meet all of the specifications. Again for metals it is simple to call up the relevant international standard, which would adequately define the yield strength etc. As discussed elsewhere in this book such a standard does not exist for composites. Also composite properties are determined by the processing conditions. For GMT there may exist up to a 40% variation in the properties at different points on the components depending upon resin flow within the tool. Similarly for the thermoset composites the moulding conditions would determine the state of cure of the resin and hence the mechanical properties of the composite. For this a simple way out is to define the required mechanical properties and leave it to the supplier to achieve what is required. By this point you would probably have been working with the supplier in order to understand any of the potential pit falls in the materials and these factors would have been taken into account in the final calculations of the materials. This will also allow the supplier to make the necessary adjustments to the process in order to achieve what is required. As always, continued monitoring of components during manufacture is required to ensure that the successful design remains so throughout the product’s life.

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Now read on…

Here is a selection of background reading. Composites are a fairly stable technology, and a lot of the knowledge available from textbooks is likely to have a good long shelf life. Material and equipment suppliers are the best sources of information for current best practice. Academic papers are the best source of very up-to-date or specialised information on new materials and processing techniques. Getting hold of these books and papers is easy if you have access to an academic library, and should be possible from the better public libraries. If not, you can pay, and order materials direct from the British Library at Boston Spa: Customer Services The British Library Document Supply Centre Boston Spa Wetherby West Yorkshire LS23 7BQ United Kingdom There is a lot of information available from the internet. We have not included electronic sources in this list because web sites tend to change with great rapidity, and much of the information is fairly low grade, it is however good for manufacturer’s data. You will just have to get in there and surf with your favourite search engine, (e.g. www.google.com).

What are composites? K.K. Chawia, Ceramic Matrix Composites, Chapman and Hall, London, UK, 1993. J.E. Gordon, The New Science of Strong Materials or Why You Don’t Fall Through the Floor, Penguin, Harmondsworth, UK, 1984. D. Hull and T.W. Clyne, An Introduction to Composite Materials, Second Edition, Cambridge University Press, Cambridge, UK, 1996.

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Automotive Composites B.Z. Jang, Advanced Polymer Composites: Principles and Applications, ASM International, Materials Park, OH, USA, 1994. C.D. Rudd, Composites for Automotive Applications, Rapra Review Report No.126, Volume 11, Number 6, 2000. M.M. Schwartz, Composite Materials Handbook, Second Edition, McGrawHill, New York, NY, USA, 1992.

Polymer Chemistry and Physics G. Challa, Polymer Chemistry: an Introduction, Ellis Horwood, New York NY, USA, 1993. E.G. Couzens and V.E. Yarsley, Plastics in the Service of Man, Penguin Books, Harmondsworth, UK, 1956. J.M.G. Cowie and J. McKenzie Grant, Polymers: Chemistry and Physics of Modern Materials, 2nd Edition, Blackie, Glasgow, UK, 1991. A.J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, London, UK, 1987. Concise Encyclopedia of Polymer Science and Engineering, Ed., J.I. Kroschwitz, Wiley, New York, NY, USA, 1998. J.W. Nicholson, The Chemistry of Polymers, The Royal Society of Chemistry, Cambridge, UK, 1991. G. Odian, Principles of Polymerisation, 3rd Edition, John Wiley & Sons, New York, NY, USA, 1991.

Composite Ingredients M.F. Ashby and D.R.H. Jones, Engineering Materials 1 - An Introduction to their Properties and Applications, 2nd Edition, Butterworth-Heinemann, Oxford, UK, 1996. MF Ashby and DRH Jones, Engineering Materials 2 - An Introduction to Microstructures, Processing and Design, 2nd Edition, Butterworth-Heinemann, Oxford, UK1994.

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Types of Hot Runner Systems D.R. Askeland, The Science of Engineering Materials, 3rd SI Edition, Chapman and Hall, London, UK, 1996. J.E. Gordon, Structures or Why Things Don’t Fall Down, Penguin Books, Harmondsworth, UK, 1978. Concise Encyclopedia of Composite Materials, Ed., A. Kelly, Pergamon, Oxford, UK, 1994.

General Properties of Composites Engineered Materials Handbook, Volume 1, ASM International, Materials Park, OH, USA, 1987. D. Hull, An Introduction to Composite Materials, Cambridge University Press, Cambridge, UK, 1981. V. John, Testing of Materials, Macmillan, London, UK, 1992. Handbook of Polymer-Fibre Composites, Ed., F.R. Jones Longman Scientific and Technical, Harlow, UK, 1994. Concise Encyclopedia of Composite Materials, Ed., A. Kelly, Pergamon Press, Oxford, UK, 1994. P.K. Mallick, Fiber-Reinforced Composites, Materials, Manufacturing and Design, Marcel Dekker Inc., New York, NY, USA, 1988.

Composite Processing Prepreg Technology, Publication No. FGU265, Ciba Composites, Duxford, UK, 1995. C.W. Macosko, RIM: Fundamentals of Reaction Injection Moulding, Carl Hanser Verlag, Munich, Germany, 1989. C.D. Rudd, A.C. Long, K.N. Kendall and C.G.E. Mangin, Liquid Moulding Technologies, Resin Transfer Moulding, Structural Reaction Injection Moulding and Related Processing Techniques, Woodhead Publishing Ltd., Cambridge, UK, 1997. Crystic Polyester Handbook, Scott Bader Company, Wellingborough, UK, 1994.

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Automotive Composites

Recycling J. Denault, C. Robert, J. Michaud and M.N. Bureau, Recycling Technologies for Continuous Glass Fibre/ Polypropylene Composites, Proceedings of Polymer Composites ’99, Quebec, Canada, 1999, p331. R.A. Denison, Environmental Life-cycle Comparisons of Recycling, Landfilling and Incineration: A review of recent Studies, Annual Review of Energy and the Environment, 1996, 21, 191. K. Graser and R. Hoock, Recyclates in Automobile Construction, Kunstoffe Plast Europe, 1995, 83, 3, 15. V. Goodship, An Introduction to Polymer Recycling, Rapra Technology Ltd, Shrewsbury, UK, 2001. P. Schaefer, Recycling of Post Consumer Composites in Europe, Proceeedings of the International Composites Expo ’98, Nashville, TN, 1998, Session 8-B. R. Waite, Household Waste Recycling, Earthscan Publications, London, UK, 1995.

Composite Markets E. Marsh, A Profile of the International Advanced Composites Industry, Elsevier Advanced Technology, Oxford, UK, 1994.

176

Abbreviations and Acronyms

ABS

Acrylonitrile-butadiene-styrene

ACORD

Automotive Consortium on Recycling and Disposal

APC

American Plastics Council

ASR

Automotive Shredder

ATH

Alumina tri-hydrate

BIP

British Industrial Plastics

BIW

Body in white

BMC

Bulk moulding compound

CAE

Computer aided engineering

CAFE

Corporation for Automotive Fuel Recovery

CARE

Consortium for Automotive Recycling

CCV

Chrysler Concept Vehicle

CCV

Composite Concept vehicle

CFM

Continuous filament mat

CIV

Composite intensive Vehicle

CLTE

Coefficient of linear thermal expansivity

CMC

Ceramic matrix composite

CNC

Computer numerically controlled

CRACTAC

Crashworthy Automotive Structures from Thermoplastic Composites

CSM

Chopped strand mat

CVD

Chemical vapour deposition

D&D

Design and Development

DFD

Design for Disassembly

DFM

Design for Manufacture

DFR

Design for Recycling

DMC

Dough moulding compound

DTI

Department of Trade and Industry

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An Introduction to Automotive Composites E-CR

Corrosion resistant glass

ELV

End of life vehicle

EMI

Electromagnetic induction

EPIC

Engineering Polymer Integrated Capability

EPP

Expanded polypropylene

EPSRC

Engineering and Physical Sciences Research Council

ERDF

European Development Fund

EU

European Union

FEA

Finite Element Analysis

GMT

Glass mat thermoplastic

GRP

Glass reinforced plastics

HDPE

High density PE

HDT

Heat distortion temperature

IRR

Internal rate of return

LDPE

Low density PE

MFG

Manufacturing

MMA

Methyl methacrylate

MMC

Metal matrix composite

MPW

Mixed Plastic Waste

NDT

Non destructive testing

OEM

Original Equipment Manufacturers

PA

Polyamide

PAN

Polyacrylonitrile

PBT

Poly(butylene) terephthalate

PC

Polycarbonate

PE

Polyethylene

PEEK

Polyether ether ketone

PET

Polyethylene terephthalate

PMC

Polymer matrix composite

PMMA

Polymethylmethacrylate

PNGV

Partnership for a New Generation of Vehicles

PP

Polypropylene

PS

Polystyrene

PU

Polyurethane

178

Abbreviations and Acronyms PURIM

Polyurethane reaction injection moulding

PVC

Polyvinylchloride

QC

Quality Control

R&D

Research and Development

RIFT

Resin infusion under flexible tooling

RIM

Reaction injection moulding

RMI

Rocky Mountain Institute

RRECOM

Recycling and Recovery from Composite materials

RRIM

Reinforced reaction injection moulding

RTM

Resin transfer moulding

SALVO

Structurally advanced lightweight vehicle objective

SCRIMP

Seemans composite resin infusion process

SEBS

Styrene-poly (ethylene-butylene) styrene

SMA

Poly (styrene-co-maleic anhydride)

SMC

Sheet moulding compound

SME

Small and Medium Sized Enterprises

SRIM

Structural reaction injection Moulding

Tg

Glass transition temperature

TIM

Thermoplastic injection moulding

UD

Unidirectional

ULSAB

Ultralight Steel Auto Body

UTS

Ultimate tensile strength

UV

Ultraviolet

VARI

Vacuum assisted resin injection

VW

Volkswagen

WMG

Warwick Manufacturing Group

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180

Abbreviations and Acronyms

Contributors

Dr David J. Britnell Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK Rebecca L. Cain Advanced Technology Centre Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK

Mark R. Johnson Advanced Technology Centre Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK Dr Kevin A. Lindsey Gibbs Technology Limited Nuneaton Warwickshire CV11 4LY UK

Robert Coates Lotus Engineering Hethell Norwich NR14 8EZ UK

Dr Mark W. Pharaoh Advanced Technology Centre Warwick Manufacturing Group University of Warwick Coventry CV4 7AL

Dr Vannessa Goodship Advanced Technology Centre Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK

Dr Martyn K. Pinfold Advanced Technology Centre Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK

Dr Andrew J Hulme Rapra Technology Limited Shawbury Shrewsbury Shropshire SY4 4NR UK

Neil Reynolds Advanced Technology Centre Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK

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An Introduction to Automotive Composites Jason Rowe Lotus Engineering Hethell Norwich NR14 8EZ UK Amrick R. Singh Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK Dr Gordon F. Smith Advanced Technology Centre Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK Dr Nick Tucker Advanced Technology Centre Warwick Manufacturing Group University of Warwick Coventry CV4 7AL UK

182

Main Index

Index

2K moulding 128, 131 see also co-injection moulding

A accelerators 43–44 ACORD (Automotive Consortium On Recycling and Disposal) 142 acrylics 37 acrylonitrile butadiene styrene (ABS) 27 adhesion 51, 126 admixtures 44 Advanced Manufacturing Technology/ Computer Integrated Manufacturing Centre 7 advanced materials 5, 21–22 aerospace applications 9, 18, 19, 20, 52, 95, 108, 160 air ducts 4 alkyd (polyester) resins 41 alternating copolymers 27 alternative materials 81–82 alumina fibres 113 alumina trihydrate (ATH) 44, 101 aluminium 80, 82, 84, 138, 159, 164, 167, 171 amorphous polymers 31–34, 125 anisotropic materials 55, 56, 60, 68, 69, 70 aramid fibres 49–50, 113 ASR (automotive shredder residue) 144, 153–154 Aston Martin Vanquish 57 ATH (alumina trihydrate) 44, 101 Audi A4 83, 84

autoclaving (pre-pregging) 95–97 Automotive Consortium On Recycling and Disposal (ACORD) 142 automotive industry 83–89, 142 see also cars; vehicles automotive shredder residue (ASR) 144, 153–154

B backbone chains 25 balsa wood 52, 53, 55–56 Battenfeld single/dual thermoplastic injection moulding machine 130 beams 53, 54–55 bending stresses 69, 70, 171 biocomposites 17, 51 biodegradation 50 bleeder fabric 96 blends 28, 146, 147 block copolymers 27 BMC (bulk moulding compound) 91, 100–103, 147 BMW 3 series 143 boat manufacture 94 body shells 5, 160, 161, 163 body structure 5, 80, 81–82, 160 Bondpress process 146–147 bond strength 77 bone substitutes 17 bonnets 2, 166 boron fibres 50, 113 box-sections 55, 117 break-even point 139, 163 breather fabric 96

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An Introduction to Automotive Composites brittle materials 10–11, 18 buckminsterfullerene 49 Budd Company 107–108 bulkhead component 122–123 bulk moulding compound (BMC) 91, 100–103, 147 bumpers 111, 123, 131–133, 138–139 Buna rubbers 23

C CAE (computer aided engineering) 162 calcium alumino silicate glass 47 calcium carbonate 100, 103 calorific values 152, 153 carbon black 102 carbon–carbon composites 17–18 carbon dioxide emission reduction 79 carbon fibre composites 120, 160, 162, 163, 164, 167 carbon fibres 4, 48–49, 51, 113 carbonised cellulose 48 carcinogens 42 CARE (Consortium for Automotive REcycling) 142, 154 cars bodies 2, 4, 5, 80, 81–82 composite material use 77–89 historical development 1–5 see also vehicles catalysts 43–44, 100–101 CCV (Chrysler Composite Concept Vehicle) 144, 145, 146, 159–160 cellular materials 55–56, 129 Celluloid 23 centre of gravity 80 ceramic matrix composites (CMC) 11, 13, 18–19 CERMETs 19 CFM (continuous filament mat) 46, 115 chain entanglement 29, 36 change 161, 163

184

charge storage devices 161 chassis 3, 4, 80 chemical bonds 30, 77 chemical recycling 143, 148–150 chemical resistance 33, 78, 104 chemical structure 23–26, 77 Chevrolet tailgate 117 Chile 3 China 1 chopped strand mat (CSM) 115 Chrysler Composite Concept Vehicle (CCV) 144, 145, 146, 159–160 Chrysler Dodge Neon 160 Citroen Xantia 106 XM 83 closure panels 166, 168 CMC (ceramic matrix composites) 11, 13, 18–19 CNC (computer numerical control) 112 coatings 12, 142 coefficient of linear thermal expansion (CLTE) 85, 166 coefficient of thermal expansion 20, 85, 101, 105, 119, 166 co-injection moulding 125–133 colour 101, 115 combustion 44, 150–151, 152, 153 commodity thermoplastics 37 compatibility 10, 40, 126, 131, 146 competitive market 4 composites advantages 159–160 automotive use 83–89 awareness courses 5–6 categories 13–21 classification 11 definition 9, 77–78 design guidelines 169–172 drivers for automotive use 79–82 future development 159–168 historical use 1–5, 9

Index ingredients 39–57 mechanical properties 59–75, 171 structural applications 69, 78–79, 166–167 test methods 67–70 versus steel 135, 137–139, 159, 163, 164–165, 166, 170–171 compression moulding effect on mechanical properties 72–73 long fibre GMT 120 short fibre composites 65 thermoplastic composites 56, 119 thermoset composites 97–98, 100, 104 vertical press 97, 98 compression tests 69 compressive loading 55 compressive modulus 56 compressive strength 18–19 computer aided engineering (CAE) 162 computer numerical control (CNC) 112 Concorde 18 consolidation phase 92, 93 Consortium for Automotive REcycling (CARE) 142, 154 consumables, vacuum bag processing 95–96 contact moulding 92–93 continuous filament mat (CFM) 46, 115 continuous random reinforced fibre composites 62, 120, 122 cooling rate 119, 125 copolymers 26–27 core materials 52–57, 126 Corporate Average Fuel Economy standards (CAFE) 160 corrosion resistance 47, 116, 165 cost analysis 135–140, 163 cost factors 6, 79–80, 82, 164–165, 169– 171 core materials 57 recycling 154, 155–156 repair 165

thermoplastic selection 45–46, 159 coupling agents 46, 51–52 courses, composite awareness 5–6 Courtelle fibres 48 covalent bonds 30 cracking (pyrolysis) 151 crack propagation 10–11, 66–67, 78 crash structures 162, 166, 167 creep resistance 20, 105, 107, 119 crosslinking 26, 40 crushing 55 crystallinity 31–34, 119, 125 CSM (chopped strand mat) 115 curing polyester resins 42–43, 93, 94, 101, 102, 116 thermosets 39–40, 93, 96, 98, 109, 172

D Daimler, Gottlieb 1 Daimler tricycle 1 damage tolerance 18–19 debonding 66 De Havilland Mosquito 52 delamination 66 density 5, 9, 160 core materials 53, 54, 57 DMC 101, 102 energy conversion 151 fibres 48, 49, 50–51 plastics sorting 145–146 polyethylenes 25 SMC 105 thermoplastics 45, 56 thermosets 56, 92 depolymerisation 148–150, 151 design for disassembly and recycling 154– 155, 156 evolution 161–162, 167

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An Introduction to Automotive Composites guidelines 169–172 laminate analysis software 64, 72 materials choice 171 optimisation 81 destructive testing 70 dies 115–116 disassembly 144, 154–155, 156 disc brakes 18, 20 discontinuous fibre composites 64–65 dislocations 78 dispersion forces 31 disposal 6, 141–142, 163 door panels 17, 51 doors 85–87, 106, 117, 139, 165–166 dough moulding compound (DMC) 65, 91, 100–103 drag coefficient 160 drawing 47, 78 drive shafts 117, 118 DTI 6, 7 dual injection moulding 125–133 dual stream resin systems 108, 109–110 ductility 159 Dyneema, gel spun polyethylenes 34, 50

E economics composites manufacture 135–140 market environment 21–22, 143, 144, 155, 161–163, 168, 169–171 education 5–7 efficiency factors 62 E glass 47–48, 51, 56, 115, 122 Egypt 1, 47 elastic constants 62, 68 elasticity theory 68 elastic properties 68 see also modulus; stiffness elastomers 143 electrical properties DMC 102

186

polymers 30–31 thermoplastic composites 119, 167 electrical resistance 47, 102, 104, 116 electricity production 152 electric motors 161 electromagnetic induction (EMI) screening 167 electronegativity 30 electrostatic interactions 29, 30, 131 elevated-temperature properties 20 emissions 79, 151, 152 end of life disposal 6, 141–142, 163 End-Of-Life (ELV) Vehicle Directive 1997 141, 156 end-of-life vehicles (ELV) 141–158, 163 energy absorption 66–67, 162, 166 conversion 141, 150–152 recovery 143, 150, 152–153 Engineering Polymers Integrated Capability (EPIC) 5–6 engineering thermoplastics 37 environmental impact biocomposites 17, 51 material selection 155 motor vehicles 5, 79, 82, 141, 164 epoxy resins 41, 160 Engineering and Physical Sciences Research Council (EPSRC) 6 ethylene 24 European Commission 79, 141, 156 exothermic processes 40, 43, 53 extenders 46

F failure characteristics 10–11, 69–70, 171 Faraday Plastics Partnership 6 fascias 87–88 fatigue resistance 116 FEA (finite element analysis) 63, 71–72, 171

Index feasibility 161 fenders 84–85, 131–133, 138–139 fibreglass 93 fibre pull out 66–67 fibre reinforcement 9–11 continuous random fibre composites 62, 120, 122 short fibre composites 64–66 fibres categories 12–13 debonding 66 diameter 11–12 length 12, 64–66, 146 mechanical properties 47–51, 78 orientation 12, 46, 48, 60, 68–69, 72– 73, 91, 96, 125 pull out 66–67 strength 46, 78 surface protection 12 types 47–51 witness 123, 126 filament winding 112–114 fillers 44, 46, 103, 148 finite element analysis (FEA) 63, 71–72, 171 fire retardants 44 flaws 12 flax reinforcement door panels 17, 51 mechanical properties 51 Spitfire fuselage 1 flexural modulus 101, 105 flexural stiffness 54, 56 flexural strength 101 flexural test 70 float-and-sink process 145–146 flow characteristics thermoplastics 36–37, 119, 120, 126, 130, 172 thermosets 98, 99, 104 flow forming 122 flow modelling software 73

fluids 36, 142, 155 foam automotive shredder residue 153 cores 53, 55–56, 124, 129 polyurethane 149 Ford, Henry 1, 141 Ford Focus 83, 84, 165 hybrid power train sport utility vehicle 80 inlet manifold 103 LaTosca 5 material breakdown 142 recycling guidelines 155, 156 Taurus 107 Woody 15 Formula 1 industry 169, 170 fracture toughness 10–11, 66–67 France 147, 152 front ends 83–84, 164–165, 169 front zones 166 fuel efficient vehicles 79–80, 160–161, 164 fuels from waste 152, 153 fumes 42–43, 93, 94

G gasification 150, 152 gearboxes 107 gelling 43, 109 gel spun polyethylenes 34, 50 Germany 47, 147, 165 glass grades 47–48 in vehicles 142 glass fibres 2, 46, 47–48, 101 Egyptians 1, 47 filament winding 113 production 47 soft furnishings 1 glass mat thermoplastic (GMT)

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An Introduction to Automotive Composites front ends 165, 169 load floors 137–138 long fibre 120–124 mechanical properties 121–122 processing 71, 72, 111, 122–123, 172 recycling 146–147 short fibre 65 taildoors 86 tooling 124 uses 123–124 glass reinforced plastic (GRP) 2 glass reinforced polyester 9, 10, 101 glass reinforced polymers 56, 144, 146, 162 glass transition temperature 35–36, 40, 43 glass weight fraction 122 GM pickup truck 117, 118 Ultralight 163 GMT see glass mat thermoplastic graft copolymers 27 granulation 144, 147 graphite 17–18, 48, 113 see also carbon fibres Greece 1 grinding 146–147 GRP (glass reinforced plastic) 2

H hand lay-up 2, 3, 92–95, 96 HDPE (high density polyethylene) 25 headlamp reflectors 102–103 health risks 42, 155, 156 heat distortion temperature (HDT) 35, 101, 105 heating localised heating systems 152 reaction increase 43, 109 hemp 50–51 Hercules 21

188

high density polyethylene (HDPE) 25 high-temperature properties 20 high-temperature pyrolysis 151 high tensile steel 49 high volume manufacture 2–6, 159, 162, 164, 170 historical overview 1–5, 9 Hivalloy 27 hollow sections 115, 116 honeycomb cores 53, 55–56 hoses 26 hot pressing, low pressure 111 Hughes Corporation ‘Spruce Goose’ 14 hybrid structures 124, 161, 165 hydraulic rams 100 hydrocracking 151 hydroforming 81, 82 hydrogenation 150, 151 hydrogen bonding 30 hydrolysis 149–150 hypercars 80–81, 160–164

I impact properties 119, 167 incineration 141, 143, 152, 153 induction forces 31 inhibitors 44, 102 injection moulding co-injection 125–133 cycle 127, 129 hybrid process 165 long fibre composites 130 pressure 110, 111, 112, 125 reprocessing 147 sequential injection 128, 129 short fibre composites 65, 104, 120, 124–133 thermoplastic composites 120, 124– 133 thermoset composites 98–100, 104 inlet manifolds 103

Index innovation 6, 161 interfacial shear strength 66 intermolecular forces 29–30, 32 internal rate of return (IRR) 135 investment cost 136, 137, 163, 170 isocyanurate 149–150 isophthalic polyesters 42 isotropic materials 55, 62, 68, 70 ivory 23

Japan 1, 147, 153

loading 51, 54, 55, 69, 171 load levelling devices 161 localised heating systems 152 long fibre composites 46, 103–104, 120– 124, 130 longitudinal loading 69 Lotus Cars 2, 139, 166 low density polyethylene (LDPE) 25 low pressure hot pressing 111 low-temperature pyrolysis 151 low volume manufacture 2, 159, 162, 167 Luxembourg 152

K

M

kenaf 50–51 Kevlar 48, 49, 51, 113, 120 kit car manufacture 93 knowledge transfer programmes 7 Krenchel equation 62

macromolecules 28–29 magnesium 88 magnesium oxide 101 manufacture economics 135–140 polymer matrix composites 22 thermoplastic composites 119–134 thermoset composites 91–118 manufacturers, car disposal costs 141 manufacturing methods alternative 82, 161 development 21 effects on mechanical properties 67, 71–73, 172 see also processing Marco method 94 marine engines 107 market environment 21–22, 143, 144, 155, 161–163, 168, 169–171 marketing 4 mass decompounding 160 mass production 4 material property data 60–73, 171 materials choice 6, 135, 137–139, 155, 163, 169, 171

J

L lamellar structure 32, 61, 72 laminate analysis 63 software 64, 72 laminated composites 53, 63, 95, 120, 122 manufacturing method effects 71–72 mechanical properties 68, 171 landfill avoidance 141, 142, 143, 144, 153, 156 Land Rover Freelander 85 large molecules 28–29 laser welded blanks 81 LDPE (low density polyethylene) 25 legislation 5, 144, 156, 166 lifetime 141–158, 163 lightweight materials 53, 81–89 Lineothorax 1 load floors 123, 137–138

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An Introduction to Automotive Composites development 21–22 economic comparison 137–138 parts marking 155 vehicle composition breakdown 142 matrix interaction 51 maximum service temperature 35 maximum stress 11 mechanical properties 59–75, 171 analytical expressions 61–62 core materials 53–56 fibres 47–51, 78 GMT 121–122 processing effects 67, 71–73, 172 recycled materials 146–147 temperature effects 20, 35–36, 48, 171 test methods 67–70 mechanical recycling 143, 144–148, 150 mechanical testing 67–70 melamine formaldehyde 41 melting 35, 39, 45 membrane forming 123 Mercedes A Class 86, 87, 139 metal matrix composites (MMC) 11, 13, 19–20, 22, 87 metals 59, 60, 142, 153, 169, 171–172 micromechanics 61, 63, 64, 67, 73, 171 microscopic structure, natural composites 13, 14 microstructural modifications 10 miles per gallon (mpg) 79, 160–161 Mini 3 Mini Countryman 15 mixed plastic waste (MPW) 151, 153 mixtures 61, 108 MMC (metal matrix composites) 11, 13, 19–20, 22, 87 modules 82, 139–140, 170 front ends 83–84 modulus 9, 45, 171 compressive 56 flexural 101, 105 GMT 121, 123

190

shear 56 temperature effects 35–36, 48 tensile 48, 49, 51, 122 testing 68–69 see also Young’s modulus moisture 40, 50 molecular structure 77 polyester resins 42 polymers 23–25, 28–29, 31–34, 36 molecular weight 28–29 monocoques 2, 3, 82, 159 monomers 24, 26–27, 148–150 Morgan Cars 15, 16 Morris Minor Traveller 15, 16 moulding effects on mechanical properties 71– 73, 172 moulding size 40, 43, 125 polyester resins 42–43, 93, 94 temperature 31, 125, 131 mpg (miles per gallon) 160–161, 164 MPW (mixed plastic waste) 151, 153

N natural composites 9, 13, 14–17 natural fibres 50, 142 networks, polymers 25–26 NIIGrafit company 18 non-destructive testing 70, 167 non-Newtonian fluids 36 Noroc-33 19 Norton Company 19 nozzles 128 Nylon 23, 31, 113, 120

O odour 42–43, 93, 94 off road vehicles 106 optical properties 33 organic peroxide catalysts 43, 100–101

Index organofunctional silanes 52 orientation fibres 12, 46, 48, 60, 68–69, 72–73, 91, 96, 125 polymer structure 34, 36, 125 original equipment manufacturers (OEM) 140, 165, 166, 168 orthophthalic polyesters 42 orthotropic composites 60, 68, 70 overhead costs 137

P PA see polyamide packaging coding system 145, 155 painting surfaces 126, 129, 131–133 PAN (polyacrylonitrile) 48 Parkesine 23 particle recycling 147 particle reinforcement 19 Partnership for a New Generation of Vehicles (PNGV) 160 parts integration 139–140, 165, 168, 170 patents 94 PBT (poly(butylene) terephthalate) 37 PC (polycarbonate) 37 PE see polyethylene PEEK (polyether ether ketone) 113 peel ply 96 PET (poly(ethylene) terephthalate) 37, 120, 146, 149, 159–160 Peugeot 405 83 phenolic resins 1, 41 physical state 29 pickup trucks 117, 118 piece cost 136, 137, 163, 170 plastics 23, 82 automotive shredder residue 153 coding system 145 filament winding 112 material selection 155 recycling 142, 143, 151, 156

see also polymers ply see lamellar structure Plymouth Prowler 108 Plytron 120, 122, 124 PMC see polymer matrix composites PMMA (polymethyl methacrylate) 37, 101, 149 PNGV (Partnership for a New Generation of Vehicles) 160 Poisson ratio 60, 61, 62 polarity 29, 30–31 polyacrylonitrile (PAN) 48 polyamide (PA) 23, 37, 113, 165 polyamide 6 149 polyamide 12 44–46, 167 poly(butylenes) terephthalate (PBT) 37 polycarbonate (PC) 37 polyester 9, 37, 149 polyester resins 41, 42–43 cold setting 2, 92 moulding 93, 94, 100–101 pultrusion 115 polyether ether ketone (PEEK) 113 polyethylene (PE) 37 gel spun 34, 50 structure 24–25, 28–29, 32 poly(ethylene) terephthalate (PET) 37, 120, 146, 149, 159–160 polymer matrix composites (PMC) 5, 11, 21–22, 92, 159, 160 manufacturing methods 22 processing 13 world market 21–22 polymers alloying 28 amorphous 31–34 blending 28 branching 25–26 chains 24–26, 29, 31, 34, 36 chemistry and physics 23–37 crosslinking 26 crystalline 31–34

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An Introduction to Automotive Composites description 24–25, 24–28 foams 53, 55–56, 124, 129, 149 mechanical properties 35–36 see also plastics polymethyl methacrylate (PMMA) 37, 101, 149 polyolefins 145, 151 polypropylene (PP) 37, 119 copolymers 27 flax reinforced 17 GMT 120–123 material selection 44–46 mouldings 4, 86 polystyrene (PS) 37 poly(styrene-co-maleic anhydride) (SMA) 27 polyurethane (PU) 38, 41, 111, 112, 149– 150, 167 polyvinyl acetate 101 polyvinylchloride (PVC) 37 Pontiac Fiero 4, 85 post curing 36, 43 post experience education 7 power trains 20, 79, 160, 162 PP see polypropylene preferred orientation 12 preforming 71, 112 pre-pregging 95–97, 104, 113, 163 presses 97, 98, 124 pressure, injection moulding 110, 111, 112, 125 pressure forming 123 processing 12–13 effects on mechanical properties 67, 71–73, 172 GMT 122–123 processing window 45–46 thermoplastics 39 thermosets 39–40, 45–46 see also manufacturing methods properties see mechanical properties protective coatings 12

192

PS (polystyrene) 37 pultrusion 114–118, 167 PU (polyurethane) 38, 41, 111, 112, 149– 150, 167 PVC (polyvinylchloride) 37 pyrolysis 150, 151, 152

Q quality control (QC) tests 70 quasi-isotropic composites 68

R radomes 92 random copolymers 27 random reinforced fibre composites 62, 68, 120, 122 Rapra Technology Ltd 6 raw materials 6, 96, 119, 167 reaction injection moulding (RIM) 85, 91, 111–112 reaction rate 40, 43, 109 recovery 142, 143–144 infrastructures 144, 154, 155–156 recycling 82, 141–158, 162, 166 DMC 103 infrastructures 144, 154, 155–156, 159 recyclate markets 154, 155, 156 Recycling and REcovery from COMposite Materials (RRECOM) 148 reinforced reaction injection moulding (RRIM) 85, 91, 111 reinforcements 46–57 compatibility 10, 40 performance 11–13 resin impregnation 56, 91–92, 95–97 types 11 release agents 96, 109 Reliant Scimitar 2, 3

Renault Espace 85, 86 repair costs 165 research and development (R&D) tests 70 resin infusion under flexible tooling (RIFT) 93–95 resins 39–40 impregnation 56, 91–92, 95–97 resin transfer moulding (RTM) 4, 161, 164 effect on mechanical properties 71 thermoplastic composites 167 thermoset composites 56, 108–111, 112, 138 reuse of materials 141, 142, 143, 156 Reuss expression 61–62 RIFT (resin infusion under flexible tooling) 93–95 RIM (reaction injection moulding) 85, 91, 111–112 risk 135, 162, 163 Rochdale Olympic 2, 3 rolling resistance 160 roll-over bars 167 Rover 2–3, 5, 6 valve covers 106 Rover–BMW Group 4–5 rovings 47, 115 RRECOM (Recycling and REcovery from COMpostie Materials) 148 RRIM (reinforced reaction injection moulding) 85, 91, 111 RTM see resin transfer moulding rubbers 23, 119, 142, 153 Russian Federation 18, 49

S safety requirements 164 SALVO (Structurally Advanced Lightweight Vehicle Objective) 6 sandwich material systems 52, 53–57, 129, 171

sealants 96 seating 88 SEBS (styrene-poly(ethylene–butylene) styrene) 27 Seemans composites resin infusion process (SCRIMP) 93 selling price 136–137 semiconductors 20 semi-crystalline polymers 32, 119 semi-structural elements 78–79, 119, 123, 165 separation 145–146, 147 sequential injection 128, 129 service temperature 35, 40, 105 shear modulus 56 shear strength 66, 171 shear stresses 55 shear thinning 36 sheet moulding compound (SMC) 65, 72, 83, 84–89, 103–108, 138–139, 147, 164–165 shelf life 98, 102, 119, 122 short fibre composites 46, 64–66, 101, 124–133 shrinkage 33, 124–125 side doors 86–87 side valences 2 silane coupling agents 52 silicon carbide 19, 113 silicon nitride particle reinforcement 19 Singer Hunter 2 single pot resin systems 108, 109, 110 sizing 47 agents 12, 46 size formulations 51–52 skin–core structure 131 skin panels 84–85 SMA (poly(styrene-co-maleic anhydride)) 27 SMC (sheet moulding compound) 65, 72, 83, 84–89, 103–108, 138–139, 147, 164–165

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An Introduction to Automotive Composites softening 39 soft tooling 162 software flow modelling 73 laminate analysis 64, 72 solid waste combustors 152 solvent resistance 30, 33 sorting techniques 145–146, 147, 148, 153 sound insulation 54, 105 soya oil 1 space frames 4, 15–16, 82, 164 specialist sports car manufacture 2 specific density 48, 49, 51, 167 specific strength 9, 48, 49 specific tensile modulus 51 specific tensile strength 51 spider silk 17 spin offs 4 Spitfire fuselage 1 split-phase glycolysis 150 sports car manufacture 2, 108 sports utility vehicles (SUV) 80, 106, 107 SRIM (structural reaction injection moulding) 4, 85, 111–112 stabilisers 146 stamp forming 122, 123 static strength 63 steel car bodies 81–82, 83, 84, 126 high tensile 49 versus composites 135, 137–198, 159, 163, 164–165, 166, 170–171 stiffness 9, 60, 61, 167, 171 flexural 54, 56 short fibre composites 65, 66 SMC 104, 105 stiffness/weight ratio 48 testing 68, 69 stirring 36 strength 45, 60, 77–78, 91, 159, 167 discontinuous fibre composites 66

194

fibres 46, 78 GMT 121, 123 prediction 63 SMC 104, 105 strength/temperature plots 48 testing 68–69 stress/density see specific strength stress/stiffness see modulus stress/strain plots 10–11, 171 structural elements 9, 78–79, 166, 166– 167 thermosets 40, 41 wood 15–16 Structurally Advanced Lightweight Vehicle Objective (SALVO) 6 structural reaction injection moulding (SRIM) 4, 85, 111–112 structure see chemical structure; molecular structure styrene, polyester curing 42, 93, 94, 100 styrene–butadiene copolymer 23 styrene-poly(ethylene–butylene) styrene (SEBS) 27 supercars 169 suppliers 135, 144, 151, 162, 171–172 surface finish moulding 86, 93, 97, 101, 102, 126 painting 126, 129, 131–133 polyester resin 115 thermoplastics 119 thermosets 41 SUV (sports utility vehicles) 80, 106, 107 syntactic foams 53 synthesis 25

T tail doors 85–86, 106, 117, 139, 165–166 technology transfer 2, 6–7, 135, 161–162 Technora 49 temperature heat distortion 35, 101, 105

moulding 31, 125, 131 property variation 20, 35–36, 48, 171 pyrolysis 151 service 35, 40, 105 working temperature 105 tensile modulus 48, 49, 51, 122 tensile strength 10, 11–12, 48, 49, 51, 78, 101, 105, 122, 171 terpolymers 27 test methods 67–70 test piece geometries 69 textiles 142, 153 Tex values 47 thermal conductivity 101, 105 thermal conversion technologies 141, 150–152 thermal expansion coefficients 20, 85, 101, 105, 119, 166 thermal injection moulding 91 thermal insulation 54 thermal processing 33 thermal runaway 43, 44, 54 thermoplastic composites 13 long fibre 120–124 manufacturing processes 37, 56, 119– 134 short fibre 124–133 thermoplastic injection moulding (TIM) 85, 124–133 thermoplastics 39–40, 44–46 characteristics 119 commodity versus engineering types 37 density 45, 56 filaments 113 flow characteristics 36–37, 119, 120, 126, 130 mechanical recycling 143, 144–147 versus thermosets 39 thermoset composites 13–14, 162, 163, 166 manufacturing processes 56, 91–118

structural applications 40, 41 thermosets 9–10, 39–44 characteristics 41 crosslinking 26 density 56 flow characteristics 98, 99, 104 mechanical recycling 143, 147–148, 166 uses 41 versus thermoplastics 39 TIM (thermoplastic injection moulding) 85, 124–133 tolerances 97, 105, 107, 124 tooling costs 137–138, 159, 162, 163, 167, 170 thermoplastic composites 124, 126 thermoset composites 93–95, 96, 105, 111, 116 toughening 10 toughness see fracture toughness tows 46, 113 T pieces 113 trade names 37–38 training 5–7 transverse properties 61–62, 69 trim pieces 4 Twaron 49, 113 twin pot system 109–110, 111 Twintex 120–121, 122 tyres 23, 49, 142

U ultimate tensile strength (UTS) 10, 47 UltraLight Steel Auto Body (USLAB) 82 unidirectional (UD) composites laminated materials 72, 120, 122 mechanical properties 61, 62, 63, 64, 66–67 testing 68–69, 70 University of Warwick 5–7

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An Introduction to Automotive Composites urea 149–150 urea formaldehyde 41 USA 107, 117, 147, 160

V vacuum assisted resin injection (VARI) 93 vacuum bag processing 95–96, 123 valve covers 2 van der Waals forces 30, 36 VARI (vacuum assisted resin injection) 93 Vauxhall Tigra 85, 86 vehicles dismantling 144, 154 end of life 141–158 future development 159–168 material breakdown 142 parts integration and modules 139– 140, 165, 168, 170 parts marking 155 selling price 136–137 weight reduction 2, 4–5, 79–89 see also cars vibration damping 105, 107 Vicat softening point 35 viscosity 36–37, 40, 41, 91, 101, 109, 131 Voigt expression 61, 62 volume fraction 61, 64, 68 vulcanisation 26 VW Golf 4

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W warping 125 Warwick Manufacturing Group (WMG) 5–7 waste disposal 95, 141–156 water absorption 30, 31 wear resistance 20, 33 weave patterns 63, 121 Wedgewood 18, 19 weight 4, 79–80, 116 reduction 2, 4–5, 79–89, 159, 160– 161, 164, 169–170 wheel trims 131, 132 whisker-reinforced ceramics (CERMETs) 19 wood 14–15 aerospace applications 52 automotive applications 15–16 automotive shredder residue 153 wood filled composite 1 working temperature 105 woven fibre composites 63, 115, 120–121

Y Young’s modulus 60, 61–62

Z Z-blade mixer 100 zinc stearate 101

ISBN: 1-85957-279-0

Rapra Technology Limited Rapra Technology is the leading independent international organisation with over 80 years of experience providing technology, information and consultancy on all aspects of rubbers and plastics. The company has extensive processing, analytical and testing laboratory facilities and expertise, and produces a range of engineering and data management software products, and computerised knowledge-based systems. Rapra also publishes books, technical journals, reports, technological and business surveys, conference proceedings and trade directories. These publishing activities are supported by an Information Centre which maintains and develops the world’s most comprehensive database of commercial and technical information on rubbers and plastics.

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