Design and Manufacture of Structural Composites 0128191600, 9780128191606

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Design and Manufacture of Structural Composites

About the Series The Woodhead Publishing Series in Composites Science and Engineering is a suite of professional reference books providing comprehensive coverage of recent developments in composite materials research for application now and in the future.

Editor-in-Chief: Professor Costas Soutis, Head of Aerospace Engineering, University of Manchester, UK

Series Editors: Professor Adrian Mouritz, Executive Dean, RMIT, Australia Professor Suresh Advani, Assoc. Director, Centre for Composite Materials, Univ. Delaware, USA Professor Bodo Fiedler, Director Inst. Plastics and Composites, TU Hamburg, Germany Professor Leif Asp, Division of Materials and Computational Mechanics, Chalmers, Sweden Professor Yuris A. Dzenis, R. Vernon McBroom Professor of Engineering, Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, USA Professor Chun H. Wang, Head of School of Mechanical Engineering, UNSW, Australia

Woodhead Publishing Series in Composites Science and Engineering

Design and Manufacture of Structural Composites Edited by

Lee Harper Associate Professor University of Nottingham, United Kingdom

Mike Clifford Associate Professor University of Nottingham, United Kingdom

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2023 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions . This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-12-819160-6 (print) ISBN: 978-0-12-819161-3 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Gwen Jones Editorial Project Manager: Zsereena Rose Mampusti Production Project Manager: Prem Kumar Kaliamoorthi Cover Designer: Matthew Limbert Typeset by STRAIVE, India

For Oscar.

Contributors

Remko Akkerman University of Twente, Enschede, The Netherlands Ali Aravand School of Mechanical and Aerospace Engineering, Queen’s University Belfast, Belfast, United Kingdom Edward Archer Engineering Research Institute, Ulster University, Belfast, United Kingdom Anais Barasinski Universite de de Pau et des Pays de l’Adour, E2S UPPA, CNRS, IPREM, Pau, France Michael Bogdanor Composites Manufacturing and Simulation Center, Purdue University, West Lafayette, IN, United States James Broughton School of Engineering, Computing and Mathematics, Oxford Brookes University, Oxford, Oxfordshire, United Kingdom Shuai Chen Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom Francisco Chinesta PIMM, Arts et Metiers Institute of Technology, Paris, France Mike Clifford Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom Monali Dahale Engineering Research Institute, Ulster University, Belfast, United Kingdom Cody Dempsey Engineering Research Institute, Ulster University, Belfast, United Kingdom Pierre Dumont Universite Lyon, INSA-Lyon, CNRS, LaMCoS, UMR5259, Villeurbanne, France Andreas Endruweit Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom

xiv

Contributors

Anthony Favaloro Hexagon Manufacturing Intelligence, West Lafayette, IN, United States Christoph Frommel German Aerospace Center (DLR e.V.), Center for LightweightProduction-Technology (ZLP), Augsburg, Germany Chady Ghnatios Mechanical Engineering Department, Notre Dame UniversityLouaize, Zouk Mosbeh, Keserwan, Lebanon Johnathan Goodsell Composites Manufacturing and Simulation Center, Purdue University, West Lafayette, IN, United States Sebastiaan Haanappel AniForm Engineering B.V., Enschede, The Netherlands Tobias Haase German Aerospace Center (DLR e.V.), Center for LightweightProduction-Technology (ZLP), Augsburg, Germany Steven Hancock InCA Technology GmbH, Bern, Switzerland Lee Harper Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom Pascal Hubert Werner Graupe Chair on Sustainable Composites Manufacturing, Mechanical Engineering, McGill University, Montreal, Canada David Hughes Department of Engineering, School of Computing, Engineering and Digital Technologies, Teesside University, Middlesbrough, United Kingdom Lars Larsen German Aerospace Center (DLR e.V.), Center for LightweightProduction-Technology (ZLP), Augsburg, Germany Patrick de Luca ESI Group, Parc Icade, Sevilla Building, Saarinen, Rungis, France Florian Martoı¨a Universite Lyon, INSA-Lyon, CNRS, LaMCoS, UMR5259, Villeurbanne, France Alistair McIlhagger Engineering Research Institute, Ulster University, Belfast, United Kingdom Geoffrey Neale Engineering Research Institute, Ulster University, Belfast, United Kingdom Laurent Orgeas Universite Grenoble Alpes, CNRS, Grenoble INP, 3SR Lab, Grenoble, France

Contributors

xv

Andrew Parsons Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom Helena Perez-Martı´n School of Engineering, Institute for Materials and Processes, The University of Edinburgh, Edinburgh, United Kingdom Steve Pickering Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom Prasad Potluri Department of Materials, Northwest Composites Centre, University of Manchester, Manchester, United Kingdom Kevin Potter Aerospace Engineering Department, University of Bristol, Bristol, United Kingdom Calvin Ralph Engineering Research Institute, Ulster University, Belfast, United Kingdom Dipa Ray School of Engineering, Institute for Materials and Processes, The University of Edinburgh, Edinburgh, United Kingdom Sree Shankhachur Roy Department of Materials, Northwest Composites Centre, University of Manchester, Manchester, United Kingdom Thomas Turner Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom Michael Vistein German Aerospace Center (DLR e.V.), Center for LightweightProduction-Technology (ZLP), Augsburg, Germany Mark Willmeroth German Aerospace Center (DLR e.V.), Center for LightweightProduction-Technology (ZLP), Augsburg, Germany Fei Yu Department of Composite Structures, Aircraft Strength Research Institute of China, Xi’an, China

Preface

Fibre-reinforced polymer composites are the preferred design solution for a broad range of high-performance applications. Clear examples of their success are illustrated by the growing demand in the aerospace, marine, civil infrastructure, and automotive sectors to save weight, reduce costs, and lessen harmful CO2 emissions. Material costs have steadily declined over recent years, but manufacturing challenges continue to be the major barrier preventing wider adoption, particularly in highvolume applications. Recyclability is of growing importance due to increasing societal pressure on the industry to become more sustainable, with end-of-life disposal of structures being a particular concern. This book provides engineers with an overview of the main manufacturing challenges encountered when processing fibre-reinforced composite materials. Composites are unique in that the material is created at the same time as the structure, resulting in very close links between the constituents, the manufacturing process, and the resulting mechanical performance. This book takes an in-depth look at material choices and the intermediate steps required to convert fibre/matrix combinations into finished products. It provides an insight into recent developments for each of the manufacturing processes covered, addressing automation, design, cost, rate, mechanical performance, and sustainability. The book is designed primarily as a teaching resource; the content has evolved from courses given by the authors to students of mechanical engineering and materials science, at both undergraduate and postgraduate levels. It also draws upon experience gained during research projects and from leading industry experts in the field. The text will provide undergraduate students and non-specialists with a valuable introduction to composites manufacturing techniques, and it will also help experienced composite engineers to determine the most suitable manufacturing routes and to understand the challenges associated with the production of high-performance composite components. The content is divided into five key sections. The first section gives a general introduction to the principles of composite materials and introduces the constituents commonly used to produce structural fibre-reinforced polymers. The book looks exclusively at polymer composites and does not consider other matrices, such as metals or ceramics. The second section looks at the initial steps required to combine these constituents into useful intermediate forms for subsequent manufacturing. Here, there is a close link between the material specification and the process design. The third section covers a range of moulding processes to consolidate and/or cure the constituents into a rigid material in the shape of the final geometry. The fourth section considers the digital design process, using current computer-aided engineering tools to design and manufacture virtual components before time and money are invested to

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Preface

produce physical artefacts. Lay-up design is not addressed here, as it is assumed that the composite designer will have already determined the correct location for the stiff fibres, at the right orientation and at the appropriate volume fraction, to meet the inservice demands. The final section addresses the current sustainability issues facing the composites industry. A high degree of in-process waste, issues with disassembly, lack of infrastructure, and limited closed-loop recycling options jeopardise the future use of fibre-reinforced composites for high-volume mainstream applications. A range of fibre recovery and conversion processes are presented to help build a circular economy. The editors acknowledge the support of the many contributors to this book, which has been our pleasure to produce. All authors have also contributed to the success of the Engineering and Physical Sciences Research Council’s (EPSRC) Future Composites Manufacturing Research Hub (EP/P006701/1). The Hub is a national project to engage academics from across the UK to deliver a step-change in the production of polymer matrix composites, working with a wider network of international academic partners and industry. This book goes some way towards summarising the research being conducted within the Hub, which is helping to drive the development of automated manufacturing technologies to deliver components and structures for future demanding applications.

Introduction Lee Harper and Mike Clifford Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom

1.1

1

What is a composite?

Fibre-reinforced composites offer significant performance advantages over other materials. Their high specific stiffness and strength enable the creation of innovative engineering structures and can lead to environmental benefits, such as fuel savings via reduced mass for transport applications. In addition, their construction enables the creation of multi-functional structures, delivering transformative design opportunities. A composite material consists of two or more constituents, typically a stiff, brittle fibre surrounded by a matrix. The arrangement and type of reinforcement dominate the mechanical properties of the composite, with the matrix material responsible for transferring stress from fibre to fibre whilst maintaining the fibre spacings and orientations. The matrix also provides vital protection to the fibres from abrasion and the environment. The combination of the two constituents results in a new material with remarkable anisotropy, sometimes with mechanical properties (e.g., tensile strength) that vary by orders of magnitude depending on the orientation and volume content of the reinforcement. Engineering composites are typically built up from individual fibre plies at different orientations, which are laminated layer by layer to create the final structure. Fig. 1.1 provides a summary of the different structural levels and corresponding length scales within fibre-reinforced composites. The synergistic effect of adding stiff fibre reinforcement to a matrix is a result of the local load sharing that occurs at the fibre–matrix interface, which is key to understanding the mechanical behaviour of composite materials. The elastic strain is typically constant within the fibre and matrix phases of a simple unidirectional (UD) composite, when the load is applied parallel to the fibres (isostrain). The local stress however, varies according to the fibre distribution, as the total load sustained by the composite is distributed between both phases. The Young’s modulus is therefore typically calculated as a weighted average from the moduli of the two constituents, based on the fibre volume content. This is a useful first estimation of the elastic ply properties and is the basic building block for determining the in-plane stiffness of more complex laminates. In this book, we focus exclusively on polymer matrix composites. Composites based on metals and ceramic materials exist, but the applications and manufacturing challenges for these are very different. The interested reader is referred to Ref. [1] to learn more about these materials. The choice of constituents to form a fibre-reinforced polymer composite can be selected to achieve a particular balance of properties, including structural, electrical, thermal, and tribological. This book only discusses Design and Manufacture of Structural Composites. https://doi.org/10.1016/B978-0-12-819160-6.00009-3 Copyright © 2023 Elsevier Ltd. All rights reserved.

4

Design and Manufacture of Structural Composites

Fig. 1.1 Diagram showing the multi-scaled nature of fibre-reinforced composite materials.

the structural behaviour of these materials, but it has been shown that optimised composites tend to provide superior performance in all of these areas. It is worth keeping in mind when designing fibre-reinforced composite structures that the material is often created at the same stage as the component. The properties are therefore dependent on the manufacturing process, which controls the placement of the fibres. This makes computer-based design somewhat challenging for the beginner who may be more familiar with designing with metals, where material properties are similar in all directions. Consequently, inexperienced designers can often be too conservative, opting for composite layups with lower, quasi-isotropic properties to offer a greater factor of safety. This approach negates one major advantage of composite structures, as the strong, stiff fibres are not primarily aligned in the direction of the applied loads, therefore producing a suboptimal layup. This book does not cover laminate design in detail (readers can get a more in-depth account from Refs. [2,3]), but instead focuses on the effect of the manufacturing process on the structural performance. Fig. 1.2 summarises the key materials and manufacturing processes discussed in this book. Polymers are generally split between thermoset and thermoplastic materials, with the different fibre formats classified as either continuous or discontinuous. Clearly the choice of polymer and fibre architecture strongly influences the manufacturing route, dictating the intermediate material format, the preforming route, and the final moulding process. What is not clear from Fig. 1.2, however, is the motivation for using these different manufacturing processes and how they influence the cycle time, cost, and mechanical performance of the moulded component. This is partly captured in Fig. 1.3, which shows a comparison of Takt time and tensile strength for a range of common composite manufacturing processes. Broadly speaking, mechanical performance is directly proportional to cycle time, which in turn is proportional to component cost. Components for structurally demanding applications require careful placement of the reinforcing fibres to ensure they are perfectly aligned with the principal loads. Components manufactured by autoclave-cured prepregs are the industry benchmark in terms of mechanical performance. The cycle time can be extremely long due to lengthy cure times, along with expensive intermediate materials and high labour content, which impact the component cost. A step-change in both cycle time and cost can be achieved if discontinuous fibre reinforcements are used,

Structural Composites

Thermoset Processes

Matrix

Fibre Architecture

Discon nuous Fibres

Intermediate

Moulding Compound

Preforming Process

Hand Layup

Moulding Process

Thermoplas c Processes

Mat / Veli

Diaphragm Forming

Compression Moulding

Con nuous Fibres

Dry tex le

Prepreg

Discon nuous Fibres

Tow / Tape

Thermoforming

3D Weaving

Braiding

Autoclave

Liquid Moulding

Filament Winding / Pultrusion

Injec on Moulding Pellets

Con nuous Fibres

Organosheet

Mat/Veil

Thermoforming

AFP / ATL

Injec on Moulding

Injec on Overmoulding

Tow / Tape

AFP / ATL

Thermoforming / Stamping

Autoclave

Fig. 1.2 Flow diagram showing different material options and processing routes for manufacturing structural fibre-reinforced composites.

6

Design and Manufacture of Structural Composites

Fig. 1.3 Comparison of cycle time and mechanical performance for a range of composite manufacturing processes. All materials are based on carbon fibre composites (unless otherwise stated), where tensile strength is normalised assuming a 50% Vf. Fabric-based composites are based on 0/90 degrees fibre architectures.

as these materials are more amenable to automation and less precision is required during manufacture. The mechanical performance is compromised, however, and can be an order of magnitude lower than continuous fibre composites, with much greater variability. Manufacturing process selection is typically performed at the detailed design stage, which means critical issues relating to the manufacturing processes are not identified until it is too late, making redesign difficult and costly [4]. Figs 1.2 and 1.3 demonstrate that process selection is complex, depending on many factors. A concurrent engineering approach is required to consider the manufacturing process early during the product development stage, to reduce product development times and production costs, whilst avoiding quality issues.

1.2

Composites manufacturing

Fibre-reinforced composites are widely used in a variety of engineering applications, encompassing a broad range of fibre and matrix combinations and providing a multitude of component design and manufacturing options. The advantages of fibrereinforced polymer composites are clear, but manufacturing complexities associated with the precise placement of the fibres and ensuring integrity of the polymer matrix can yield comparatively low production rates and high manufacturing costs. Compared to steel, for example, where process selection is largely based on either casting or fabrication from wrought products, there are many more options for manufacturing fibre-reinforced composite structures, as shown in Fig. 1.2. Process selection is typically based on the chosen fibre/resin combination, the target production volume, mechanical performance expectations, and cost. Manufacturing research is therefore key to further exploitation of composites in existing sectors (aerospace, automotive,

Introduction

7

marine, and defense) and more widespread adoption in emerging sectors such as civil infrastructure, renewable energy, and rail. Composite manufacturing processes have evolved in response to new material developments and the drive from new applications. Early processes relied on fabrics created using textile weaving looms that were invented during the first industrial revolution. Material waste and touch labour were high, as fibres were placed by hand into a mould before liquid resin was applied with a brush and spread out with a roller. Wet layup, as it became known, was first introduced in the 1930s, following the development of glass fibres by Owens Corning (1935), unsaturated polyester by Carlton Ellis (1936), and the first epoxy resin by Paul Schlack (1938). The inception of cross-linkable liquid resins enabled manufacturing innovations to continue in the 1950s, with the development of pultrusion, vacuum bag infusion (see Chapter 10), and large-scale filament winding (Chapter 7). Pultrusion is commonly used to manufacture linear components with a constant cross-section, including tent poles and window profiles. It is a continuous process that is easily automated, offering low cost for high volumes. Similarly, filament winding was developed to manufacture open cylinders and became the basis for large-scale rocket motors that propelled the first human space flight in 1961 [5]. Vacuum infusion, on the other hand, was used to produce larger shell structures, such as boat hulls, where the resin content could be controlled more accurately than with wet layup. Consequently, the marine sector became the largest consumer of composite materials in the 1960s. In 1953, a full composite body went into production for the Chevrolet Corvette using wet layup [6,7]. The surface finish was generally poor by today’s automotive standards and the mechanical performance was somewhat compromised by the moderate fibre volume fraction, high void content, and poor control and repeatability of the fibre architecture. The body for the third-generation Corvette was produced using matched metal dies in 1968, which improved the surface finish and reduced cycle times. This was a significant advancement in fibreglass production, and it laid the foundation for the development of compression moulding (Chapter 11). Sheet moulding compounds (SMC) and bulk moulding compounds (BMC) emerged as the dominant processes for producing composites for the automotive industry. Compounds of this type typically contain chopped glass fibres and resin, which flow simultaneously under pressure, in a hot mould tool actuated by a press. The introduction of compression moulding enabled complex parts to be formed in less than 10 min, expanding composites into applications where production volumes could approach 100,000 parts per annum. In 1961, the first carbon fibre was patented, and it became commercially available in 1963, shortly followed by the first high modulus fibres in 1964. Carbon fibres dramatically improved the specific stiffness of thermoset composites, with a fibre modulus approximately 2.5 times higher than E-glass fibres at the time (172 GPa was quoted as the fibre modulus of Union Carbide’s rayon-based Thornel 25 in 1964 [8]). Carbon fibres expanded the use of composites into high-performance structures for aerospace, defence, and motorsports, but the fibre cost was prohibitively expensive, so carbon was reserved for niche applications where production volumes remained low. Manufacturing processes were conservative and slow to ensure that

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Design and Manufacture of Structural Composites

the high-value carbon fibres were positioned accurately whilst minimising material waste. High-volume processes such as compression moulding were avoided due to the limitations on performance from the discontinuous fibres. Therefore, processing carbon fibres initially required significant levels of touch labour. Pre-impregnated carbon fibre materials were developed in the late 1960s using high-molecular weight epoxies to maximise mechanical performance. Preimpregnation meant that the resin flow distance was minimised during cure to reduce the risk of dry spots in the final moulding. Prepregs require a lengthy cure cycle of several hours in an autoclave (Chapter 9), further restricting the application of carbon fibres for mainstream applications at that time. The 1981 McLaren MP4/1 was the first Formula One car to use a carbon fibre monocoque [9,10], which was designed by McLaren engineers following a visit to the Rolls-Royce factory to see prototype fan blades being manufactured for the RB211 jet engine. Whilst the application of carbon fibres remained niche, the potential for these high-stiffness fibres had been demonstrated and the benefits were clear for a wide range of applications. However, there was still the need for precise fibre deposition methods that could be automated to increase production volumes. In the following years, many variants of existing fibre conversion and moulding processes were developed, offering incremental improvements on previous processes, including resin transfer moulding (Chapter 10). An important milestone came in 1988 when the first automated fibre placement (AFP) machine became commercially available from Hercules, and was adopted by Boeing, Lockheed, and Northrop [11]. AFP (discussed in Chapter 6) is considered to be a form of additive manufacturing, depositing pre-impregnated fibre tows onto the surface of a tool using a gantry robot. It is commonly used for large, single curvature components in the aerospace sector, such as the fuselage, wing skins, or spars and was certified to produce components for the Boeing 787 Dreamliner that entered service in 2011. AFP machines are most efficient when producing long components of several metres, enabling the gantry to reach peak velocity before decelerating at the end of the course. They are unsuitable for producing small, complex components with double curvature, as access is often restricted due to the size of the deposition head and the deposition rate is severely affected by the stop–start nature, as courses become shorter. This brief history has broadly presented the most important technological advancements in the composite sector over the last century, introducing the main production processes discussed in this book. Following on from AFP, three-dimensional (3D) printing is now emerging as a viable solution for producing structural composites of the future. In 2014, Markforged launched the first 3D printer suitable for processing continuous carbon fibres [12]. This is potentially a game-changer, as it enables components to be manufactured out of an autoclave and without a mould tool, offering more sustainable manufacturing. Precise placement of continuous fibres enables the architecture to be optimised to suit critical load paths, providing more design flexibility to avoid black metal designs. 3D printing is still very much at the development stage for producing structural fibre-reinforced composites, with deposition rates significantly lower than those expected in the aerospace sector, for example. For this reason, 3D printing is not discussed in detail in this book, but it is a disruptive technology that will benefit many sectors.

Introduction

1.3

9

Where is the industry heading?

1.3.1 Automation Figures indicate that the compound annual growth rate (CAGR) for the composites sector will be 3.3% up to 2024, with an anticipated worldwide market valued at £85 billion [13]. Clearly, demand for composites is strong, but without new step-changing technologies there is a danger that this growth will stagnate. The composites sector requires repeatable, high-quality components produced at rate and the only viable solution is to reduce the dependency on touch labour and increase levels of automation. Robots are typically taught by an operator to repeat functions, which is effective when materials and structures are consistent, but is more challenging for composites because of the inherent variability caused by the complex mesoscopic fibre architecture. Automation is readily used for producing metallic tools, for example, where lights-out production can be implemented using fully autonomous CNC machines with automatic tool changers. However, the application of automation to replicate the dexterity of a laminator to place individual plies into a mould tool has been less successful. Many components have been inadvertently designed as black metal parts, with the assurance that they can be created by hand layup at low volumes. Composite design principles need to be reformed if higher volumes are to be achieved, as small changes to the geometry, ply layup, or material format may enable automation to be implemented more readily. Automating any kind of composite manufacturing process requires some degree of flexibility to account for material variability. Intelligent robots can provide this by updating the trajectory of the end-effector using data collected from in-process sensors. This makes it possible to accurately measure and record the influence of each action whilst monitoring the development of defects in the layup. The component can therefore be produced automatically without the need for intensive testing or non-destructive evaluation, as the physical layup can be subsequently compared to a digital twin to confirm that the fibre architecture will meet in-service requirements. This concept is the main element of the Industry 4.0 approach, using machine learning to create intelligent production processes, enabling better decision making towards risk reduction. Put simply, the objective of Industry 4.0 is to create ‘smart factories’. An example of creating an optimised laminate using an Industry 4.0 approach is the fibre patch placement process developed by Cevotec [14]. Rather than continuous full coverage plies, small patches of spread tow are placed over the tool surface so that the fibres are aligned with the primary load paths. The optimum deposition strategy is determined by simulation, and the placement is automated using robots, adopting a vision system to confirm the orientation and location of each patch. Feedback is then used to adjust the end effector to ensure correct placement [15], which highlights the connectivity amongst stress analysis, quality assurance, and manufacturing through process simulation. Extreme or hyper automation is pushing boundaries towards the hypothetical limit of 100% autonomy. The Industry 4.0 vision for composites manufacturing uses smart machines, storage systems, and production facilities to autonomously exchange

10

Design and Manufacture of Structural Composites

information and coordinate tasks. Hence, significant effort is required to network machines along the production line. Mobile communication technology can be used to conduct operations more efficiently using a combination of autonomous mobile robots (AMR), collaborative robots, digital twins, augmented reality, and asset condition monitoring. Data can be stored on the cloud and can be used to immediately feed forward or feed back information to other machines in the line. For example, fibre orientation information from the preforming step can be fed forward to the infusion process to compensate for local perturbations in the fibre architecture, thereby enabling a self-adapting production process. Alternatively, the same information can be fed back to the AFP machine to change the fibre deposition process to avoid these fibre perturbations in subsequent preforms. Ultimately, the adoption of Industry 4.0 tools and techniques has the potential to significantly reduce composite part cost and waste whilst improving component quality. The automation and sensing technology required to implement Industry 4.0 already exists, but the challenge is integrating this technology within existing organisations and supply chains within the composites sector.

1.3.2 Sustainability Composites are making aircraft more fuel efficient, increasing the range of electric vehicles, and providing huge wind turbine blades to generate electricity to power our homes. As interest in composites manufacturing grows, so too should our attention towards sustainability and impact on the environment. Whilst composite materials can increase the lifespan of products exposed to harsh conditions by decades and reduce their environmental impact when in-service, processing these materials at the end of their life is difficult. It is estimated that just 2% of fibre-reinforced composites produced globally are recycled (indicated by Fig. 1.4), which leaves almost 10 Mt

Fig. 1.4 An overview of materials recycling in 2020. The value at the end of each bar represents the total mass of recycled material in Mt.

Introduction

11

(megatonnes) of material destined for landfill each year [16]. Separating out the constituent materials is difficult and there are a number of technological challenges that need to be overcome to enable this. Current recycling techniques often degrade the performance of the constituent materials, reducing their value and offering limited options for future applications. Currently, more than 80% of composites are made from materials that are derived from non-renewable sources [17], making them unsustainable. Whilst plant-based natural fibres and biological resins have been developed to address the sustainability issues of fibre-reinforced composites, the mechanical performances of these materials are variable and at least an order of magnitude lower than current synthetic fibre solutions. It is questionable whether the performance of these biomaterials will ever meet the expectations for high-performance structural applications in the aerospace sector; therefore, the sustainability of existing materials needs to be improved. The global demand for carbon fibres is expected to reach 117 kt (kilotonnes) in 2022 [18]. The environmental footprint of these fibres is largely due to the high energy consumption required to convert the precursor material into filaments with highly aligned basal planes, which accounts for 90% of the total environmental impact [6,7]. The embodied energy during the production of virgin fibres is somewhere between 171 and 771 MJ/kg [19,20], compared with 13–80 MJ/kg for steel [21,22]. The embodied energy for carbon fibre is simply too high to justify sending this material to landfill. Therefore, the development of fibre recycling systems is essential for recovering the residual fibre value from end-of-life waste. Thermal fibre recovery systems have been commercialised, including the plant developed by Gen2 Carbon, Birmingham, United Kingdom (formerly ELG Carbon Fiber Ltd.), but the main barrier to adopting these materials is the lack of suitable downstream manufacturing routes that can accommodate these recyclates. The low repeatability of recycled products is also a challenge because there is currently no infrastructure in place to sort waste streams based on the origin of the virgin fibre. The current market for recycled carbon fibre is therefore based upon low-value non-structural applications, with further research required in composites manufacturing to enable a true cradle-to-cradle loop for these high-value materials.

1.3.3 Challenges 1.3.3.1 Aerospace The aerospace sector was an early adopter of composites, and it is a sector that continues to grow. There has been a dramatic increase in the amount of advanced carbon composites in recent generations of aircraft structures to reduce mass and combat excessive CO2 emissions. The Boeing 787 Dreamliner uses carbon fibre composites for the fuselage and wings, which accounts for 32 tons of material per airframe [23]. By 2021, just over 1000 787 Dreamliners had been produced, with the largest volume completed in 2019 at 14 per month [24]. The UK’s Aerospace Technology Institute (ATI) has set targets for additional weight reductions of 35% by 2035 and has anticipated a 40% increase in required production rates to meet future demand [25]. Airlines are looking to replace ageing

12

Design and Manufacture of Structural Composites

aircraft fleets with the latest, most fuel-efficient models and it is anticipated that up to 40,000 new aircraft will be required to meet demand in the short-to-medium-term (2025–38) [26]. It is estimated that replacements for single-aisle regional planes, such as the Airbus A320, will require 100 wing sets per month. A manufacturing stepchange is therefore required to increase the number of wing sets produced by an order of magnitude per month, moving away from labour-intensive processes and shifting towards higher levels of automation. When Airbus decided to fabricate the A350 twin-aisle aircraft from carbon fibre composites back in 2004, much of the decision-making was driven by the manufacturing technologies available at the time. The long, moderately contoured surfaces of the wing structure were a good fit for AFP, since these systems are adept at consistently laying large amounts of pre-impregnated material over large areas. The epoxy matrix however requires a long autoclave cure cycle to achieve a critical porosity level of less than 1%. This is not sustainable going forward and the industry is looking for new materials and methods to enable higher rates of manufacturing, better control of processing conditions and lower cost, suitable for more integrated complex geometries. In the longer term towards 2050, new aircraft configurations will be required to further reduce fuel burn and carbon emissions, with greater levels of integration between the powertrain and the airframe. Novel airframe configurations currently being considered include the blended wing body [27], the double-bubble fuselage [28], the strut-braced wing [29], and the box-wing aircraft [30]. These new designs will all require innovative composite manufacturing solutions to produce high-quality airworthy components at rate. The UK’s FlyZero project is investigating zero-carbon emission commercial flight and has concluded that liquid hydrogen technology is the most viable fuel for the future for regional, narrow-bodied, and mid-sized aircraft [31]. Unlike kerosene, hydrogen cannot be stored in the wing so large insulated composite tanks are required, which present new challenges in terms of packaging, materials, maintenance, and sustainability. By 2030, entirely new aviation markets are expected that will exploit electrification and autonomy in the urban airspace. The increase in online shopping and ‘on-demand delivery’ has seen the unmanned aerial vehicle (UAV) market grow, with the Federal Aviation Administration (FAA) estimating more than 800,000 drones to be flying business-related missions by 2023 in the United States. The payload of a drone has a huge impact on flight time and power usage, and they are likely to be entirely battery powered or have hybrid powertrains. These vehicles have a similar architecture to helicopters to enable vertical take-off and landing (VTOL), but are significantly smaller. Therefore, composites are expected to play a critical role in the design and manufacturing of these structures. Thermoplastic composites can be rapidly manufactured using short fibre injection moulding, a process that has been used in the automotive sector for 30 years. However, this alone offers limited structural performance. Thermoplastic injection overmoulding is an emerging composites manufacturing method that combines continuous fibre reinforcement with injection-moulded fibre-filled polymers. This offers high levels of functional integration, net-shape manufacturing, and low cycle times coupled

Introduction

13

with high levels of automation, which is ideal for producing the volumes expected for future UAV applications. Personal flying taxis are not currently in service, but there are a large number of aerospace and technology companies exploring this space, with forward orders already being placed for electric VTOL craft. The vision is to relieve ground-based commuter congestion with airborne commuting in large metro areas. Current barriers include aviation airworthiness and safety, which will be governed by similar federal aviation regulations to light-lift helicopters. Composites are certain to be used for their high specific properties, but aerospace-quality components will be required at automotive volumes, which is difficult to achieve with current out of autoclave composite processes.

1.3.3.2 Renewable energy Wind turbine blades are constantly pushing the limits on size and require advanced composite materials, designs, and manufacturing. The wind energy market has long been the largest volume user of glass fibre, but has recently overtaken the aerospace sector as the largest global consumer of carbon fibre, accounting for 40% of total production. Power output from a turbine is proportional to the blade length squared, but the weight of a blade is proportional to the blade length cubed. In 2021, Vestas in Denmark announced a 15 MW turbine with 115 m long blades for offshore applications [32], making carbon fibre composites the only possible material choice to produce a cost-effective blade of this length. Blades are typically manufactured as two outer skins that are adhesively bonded together about a central box spar. Prepreg and vacuum-assisted resin transfer moulding (VARTM) have emerged as the two most common processes, with the majority of small blade manufacturers adopting the lower-cost VARTM option. Larger blades are manufactured from prepreg, which is more repeatable and offers optimised resin ratios and higher mechanical performance. Large blades tend to use the heaviest possible fabric or prepreg plies to achieve manufacturing efficiency, but this can affect in-plane properties and increase delamination, as thick composite laminates have an increased likelihood of hidden flaws. Fabric overlaps, where individual rolls of fabric end, can cause stress concentration effects. Fibre waviness, large-scale porosity, resin richness, and variable degrees of cure are also a concern. Blade manufacturers are moving away from marine-based open mould and resin infusion technologies, towards aerospace technologies such as automated tape laying (ATL) to produce large-scale prepreg blades. Material handling and deposition are therefore big challenges at this scale, as large volumes of temperature-sensitive material need to be deposited quickly over such long distances (100 m).

1.3.3.3 Energy storage According to the Hydrogen Council, more than 30 countries have hydrogen roadmaps, with the total investment in this renewable energy source expected to be £220 billion by 2030. According to the European Hydrogen Roadmap, fuel cell vehicles (FCV) could account for one in 22 passenger vehicles and one in 12 light commercial

14

Design and Manufacture of Structural Composites

vehicles by 2030 [33]. Consequently, the storage of hydrogen for transportation applications is a big challenge across all links of the supply chain, starting with production, distribution networks, and filling stations, but particularly in the vehicles themselves. Hydrogen is the lightest molecule and 1 kg of gas occupies more than 11 m3 at room temperature, at atmospheric pressure [34]. Onboard vehicle storage is therefore challenging, as pressures of 700 bar are typically required to increase the storage density. Hydrogen is also very difficult to contain, as the molecules are the smallest of any element and can permeate through polymeric or non-metallic materials very easily. Wet or prepreg filament winding is commonly used to lay down impregnated carbon or glass fibre tapes to produce composite tanks, which are then cured in an autoclave. Current composite storage tanks typically have an aluminium liner to prevent gas leakage, but research is being conducted to produce linerless vessels, using modified resins filled with nanoparticles to reduce the gas permeability [35]. These offer the largest weight savings over steel vessels, but removing the liner also makes it possible to produce conformal shapes, offering better packaging for automotive and aerospace applications. There are durability issues with linerless vessels, however, caused by delamination from matrix microcracking or static fatigue, which is sensitive to the resin distribution during manufacture. Composite vessels therefore require very tight process control, as variability may present serious safety issues. Linerless vessels are also challenging to manufacture in large volumes, as removable mandrels are required to facilitate the filament winding process. If FCVs are accepted by the general public, there is currently no way to produce linerless vessels to meet the anticipated demand.

1.3.3.4 Automotive The use of fibre-reinforced composites has already transformed the aerospace market, with potential for a similar technology shift in the automotive sector, offering more design flexibility, durability, low weight, and corrosion resistance. The BMW iProgramme was the first attempt by an OEM to use carbon fibre composites to produce a monocoque for a mainstream vehicle, which has yet to be equalled. The BMW i3 was launched in 2013 [36] and sold more than 220,000 units over 9 years [37], the highestselling composites road car to date. The monocoque was produced using high-pressure resin transfer moulding (HP-RTM), using a patchwork of simple non-crimp fabric (NCF) preforms. Despite the perceived success, these volumes are still relatively low in automotive terms. Interestingly, BMW has not developed a composites-intensive replacement for the i3, but instead has opted for multi-material hybrid architectures (aluminium, carbon fibre composite, and high-strength steel), which create additional challenges in joining and managing stress transfer between parts. Compression moulding of sheet moulding compounds (SMC) offers one of the fastest routes for manufacturing composite components from thermoset matrices, accounting for 70% (by mass) of composites used in the automotive industry [38]. Typical cycle times vary between 3 and 5 min depending on part thickness, but this is an order of magnitude higher than the cycle time for a metal stamped part. Even if snap curing resins are used to decrease the cross-linking time, the failure strength and material variability are major concerns for structural applications. Co-moulding

Introduction

15

of prepreg and SMC hybrid structures is under investigation to reduce the performance gap between continuous and discontinuous fibre composites whilst maintaining the low cycle time advantage of compression moulding [39,40], but this is challenging for deep-draw parts requiring significant out-of-plane charge flow. There is rising demand for electric vehicles (EV) as consumers are actively looking to reduce spending on fuel and lower emissions. According to the International Energy Agency, 6.5 million EVs were sold in 2021 [41]. The key to enhancing the energy efficiency of these vehicles is to reduce their weight, using fibre-reinforced composites to offset heavier items such as copper stator windings, magnets, and lithium-ion batteries. There is demand for robust battery enclosure systems that can meet stringent mechanical and impact requirements as well as fire, smoke, and toxicity performance to protect vehicle occupants in the event of a battery fire. Integrated composite structures could offer a step-change for electric powered vehicles [42], as structural batteries could yield significant weight savings and increase range whilst requiring significantly fewer parts. The electric vehicle industry is making great strides with this technology, with others likely to follow suit, but current composite manufacturing processes are again unlikely to meet global demand.

1.4

Summary

In this chapter, we have considered the historical development of manufacturing processes for fibre-reinforced composites. We have observed how the materials have entered mainstream production, including the automotive, aerospace, energy storage, and renewable energy sectors, replacing metallic structural elements. The use of composites in many industries continues to expand. With the emergence of new manufacturing processes, the sector is in a healthy position to tackle major challenges, such as how to scale up production volumes through increased use of automation and how to sustainably address the recycling of end-of-life structures on an industrial scale.

References [1] 16 – Metal matrix, fibre–metal and ceramic matrix composites for aerospace applications, in: A.P. Mouritz (Ed.), Introduction to Aerospace Materials, Woodhead Publishing, 2012, pp. 394–410, ISBN 9781855739468, https://doi.org/10.1533/9780857095152.394. [2] T.W. Clyne, D. Hull, An Introduction to Composite Materials, Cambridge University Press, 2019. [3] R.M. Jones, Mechanics of Composite Materials, CRC Press, 2018. [4] A. Hambali, et al., Composite manufacturing process selection using analytical hierarchy process, Int. J. Mech. Mater. Eng. 4 (1) (2009) 49–61. [5] B. Wilson, Rocket motors help winding take-off, Reinf. Plast. 40 (9) (1996) 72–80. [6] R. Glass, Exploring composites and their modern uses, Reinf. Plast. 64 (5) (2020) 271–275.

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[7] S. Sajan, D. Philip Selvaraj, A review on polymer matrix composite materials and their applications, Mater. Today Proc. 47 (2021) 5493–5498. [8] B.A. Newcomb, Processing, structure, and properties of carbon fibers, Compos. A: Appl. Sci. Manuf. 91 (2016) 262–282. [9] D. Mathijsen, McLaren: 40 years in the lead in automotive carbon fiber, Reinf. Plast. 65 (6) (2021) 34–37. [10] G. Savage, Formula 1 composites engineering, Eng. Fail. Anal. 17 (1) (2010) 92–115. [11] A. Brasington, et al., Automated fiber placement: a review of history, current technologies, and future paths forward, Compos. Part C Open Access 6 (2021) 100182. [12] MarkForged develops 3D printer for carbon fibre, Reinf. Plast. 59 (1) (2015) 12, https:// doi.org/10.1016/j.repl.2014.12.027. [13] A.E. Krauklis, et al., Composite material recycling technology – state-of-the-art and sustainable development for the 2020s, J. Compos. Sci. 5 (1) (2021) 28. [14] Cevotec, Reinf. Plast. 61 (5) (2017) 268, https://doi.org/10.1016/j.repl.2017.07.068. [15] J.P. Snudden, B. Horn, C. Ward, K. Potter, K. Drechsler, Effects of automated patch placement on the mechanical performance of reformed NCF carbon fibre, in: 17th European Conference on Composite Materials 2016: Munich, Germany, 2016. [16] F. Reux, JEC OBSERVER: Current Trends in the Global Composites Industry 2020–2025, 2021. https://www.jeccomposites.com/press/the-jec-observer-current-trends-in-theglobal-composites-industry-2020-2025/. [17] J.J. Andrew, H.N. Dhakal, Sustainable biobased composites for advanced applications: recent trends and future opportunities – a critical review, Compos. Part C Open Access 7 (2022) 100220. [18] I. Bianchi, et al., Environmental impact assessment of zero waste approach for carbon fiber prepreg scraps, Sustain. Mater. Technol. 29 (2021) e00308. [19] A. D
er, et al., Modelling and analysis of the energy intensity in polyacrylonitrile (PAN) precursor and carbon fibre manufacturing, J. Clean. Prod. 303 (2021), 127105. [20] R.J. Tapper, et al., An evaluation of life cycle assessment and its application to the closedloop recycling of carbon fibre reinforced polymers, Compos. Part B Eng. 184 (2020) 107665. [21] K. Wiesen, M. Wirges, From cumulated energy demand to cumulated raw material demand: the material footprint as a sum parameter in life cycle assessment, Energy Sustain. Soc. 7 (1) (2017) 13. [22] H.C. Kim, T.J. Wallington, Life-cycle energy and greenhouse gas emission benefits of lightweighting in automobiles: review and harmonization, Environ. Sci. Technol. 47 (12) (2013) 6089–6097. [23] V. Giurgiutiu, Introduction, in: V. Giurgiutiu (Ed.), Structural Health Monitoring of Aerospace Composites, Academic Press, Oxford, 2016, pp. 1–23 (Chapter 1). [24] J. Hardiman, Boeing Wants to Go Back to Pre-Pandemic 787 Production Rates, in Simple Flying, 2021. https://simpleflying.com/boeing-pre-pandemic-787-production-rates/. [25] Aerospace Technology Institute (ATI), Accelerating Ambition – Technology Strategy, Aerospace Technology Institute (ATI), 2019, p. 48. https://www.ati.org.uk/wp-content/ uploads/2021/08/ati-technology-strategy.pdf. [26] Airbus.com, Airbus Forecasts Need for Over 39,000 New Aircraft in the Next 20 Years, 2019. https://www.airbus.com/sites/g/files/jlcbta136/files/ddb21038f51e72d719328fe93921e1fc_ EN-Global-Market-Forecast-2019-2038.pdf. [27] L. Wang, N. Zhang, H. Liu, T. Yue, Stability characteristics and airworthiness requirements of blended wing body aircraft with podded engines, Chin. J. Aeronaut. 35 (6) (2022) 77–86, https://doi.org/10.1016/j.cja.2021.09.002.

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[28] T.R. Brooks, B.D. Smith, Aerostructural design optimization of the D8 aircraft using active aeroelastic tailoring, in: AIAA Scitech 2020 Forum, 2020. [29] Y. Ma, S. Karpuk, A. Elham, Conceptual design and comparative study of strut-braced wing and twin-fuselage aircraft configurations with ultra-high aspect ratio wings, Aerosp. Sci. Technol. (2022) 107395. [30] V. Cipolla, et al., A DoE-based approach for the implementation of structural surrogate models in the early stage design of box-wing aircraft, Aerosp. Sci. Technol. 117 (2021) 106968. [31] Nigel Town, E.B, UK Capability in Zero-Carbon Aircraft Technologies, Aerospace Technology Institute (ATI), 2022. https://www.ati.org.uk/wp-content/uploads/2022/01/ FZO-IST-REP-0047-UK-Capability-and-Opportunities_v1.0.pdf. [32] G. Nehls, Vestas unveils 15-MW offshore wind turbine with 115.5-meter-long blades, Composites World (2021). https://www.compositesworld.com/news/vestas-unveils-15mw-offshore-wind-turbine-with-1155-meter-long-blades. [33] G. Gardiner, Composites end markets: fuel cells and batteries (2022), Composites World (2021). https://www.compositesworld.com/articles/composites-end-markets-fuel-cellsand-batteries-2022. [34] J. Andersson, S. Gr€onkvist, Large-scale storage of hydrogen, Int. J. Hydrogen Energy 44 (23) (2019) 11901–11919. [35] M. Legault, The first commercial type V composite pressure vessel, Composites World (2012). https://www.compositesworld.com/articles/next-generation-pressure-vessels. [36] BMW, i3 makes its world premiere, Reinf. Plast. 57 (5) (2013) 7. [37] A. Nedelea, BMW i3 production reportedly coming to an end in July, InsideEVs (2022). https://insideevs.com/news/563722/bmw-i3-production-ends-2022/. [38] A.D. Evans, et al., Flow characteristics of carbon fibre moulding compounds, Compos. A: Appl. Sci. Manuf. 90 (2016) 1–12. [39] D.M. Corbridge, et al., Compression moulding of composites with hybrid fibre architectures, Compos. A: Appl. Sci. Manuf. 95 (2017) 87–99. [40] J. Wulfsberg, et al., Combination of carbon fibre sheet moulding compound and prepreg compression moulding in aerospace industry, Procedia Eng. 81 (2014) 1601–1607. [41] A. Rybczynska, Global EV Sales on Track to Hit Record 6.3 Million in 2021: BNEF, Bloomberg, 2021. https://www.bloomberg.com/press-releases/2021-12-15/global-evsales-on-track-to-hit-record-6-3-million-in-2021-bnef. [42] S. Kalnaus, et al., Multifunctional approaches for safe structural batteries, J. Energy Storage 40 (2021) 102747.

Reinforcing fibres Andrew Parsons Composites Research Group, Faculty of Engineering, University of Nottingham, Nottingham, United Kingdom

2.1

2

Introduction

Composites are widely used commodity materials. In the broadest sense, they are the combination of two or more materials that have notably different chemical or physical properties, which are blended together to create a material with properties unlike any of the constituents. By using fibres with high aspect ratio, a greater blending of properties can result, particularly in terms of strength. The most commonly known manmade example of a fibre-reinforced plastic (FRP) is fibreglass, though perhaps the most ubiquitous naturally occurring example is wood. FRPs fulfil the role of lightweight structural materials by virtue of their relatively low densities (compared to metals and ceramics) providing high specific properties. FRPs normally comprise a minimum of three components: a reinforcing fibre; a polymer matrix and a tailored coupling agent to provide intimate contact between the fibre and matrix. To be effective, there must be a strong interfacial bond between the fibre and matrix to transfer external loads between the two constituents because the fibres are not always directly loaded. The assumption of a strong interface can be used to develop a prediction of composite properties using a volume-averaged approach. When loading occurs along the axis of the fibre and the interface is perfect, the fibre and matrix must experience equivalent strain. Stress in the elastic region must therefore be distributed between the matrix and fibre based on area fraction (equivalent to volume fraction in a continuous fibre composite) and elastic modulus. This leads to the rule of mixtures equation:
E ¼ ηθ ηL Vf Ef + 1  Vf Em

(2.1)

where Ef and Em are the fibre and matrix modulus values, respectively; Vf is the fibre volume fraction; and ηθ and ηL are the Krenchel orientation factor and the length correction factor, respectively (both ¼ 1 in the case of a continuous unidirectional reinforcement). Because typical fibre modulus values can be 10 to 100 (or more) times those of matrix polymers, it can be seen that fibres are the main load-carrying element in an FRP. In high-performance composites (fibre content of more than 50% by volume), the matrix material is there more or less as a means of maintaining the position of the fibres (preserving ηθ) and protecting them from damage to achieve an optimum mechanical result. In lower-performance composites (lower fibre content of less than Design and Manufacture of Structural Composites. https://doi.org/10.1016/B978-0-12-819160-6.00008-1 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Design and Manufacture of Structural Composites

30% by volume), the matrix can provide a significant contribution to properties, particularly in compression. Fibres used to reinforce composites are high-aspect-ratio materials, even at their shortest lengths. This results in anisotropic component properties and provides a wealth of opportunities in the creation of FRP products, enabling reinforcement to be used efficiently without wasting mass. However, this requires highly knowledgeable design engineers to make the best use of the materials. Therefore, it is rarely achieved, resulting in overly conservative designs to account for the anisotropy and intrinsic levels of material variability incurred through complex manufacturing processes.

2.2

Types of fibre reinforcement

There are a wide variety of different fibre materials used in FRP. In terms of volume, by far the most common is glass fibre [1]. However, with an increasing drive for mass reduction as a means of reducing energy use, carbon fibre is in increasing demand, particularly for structural parts. A breakdown of the global market for fibre use in structural composites is provided in Fig. 2.1. The drive for sustainability also pushes the development of other fibre types. This section will consider the most common materials used as reinforcement in FRPs.

Fig. 2.1 Global structural composites market in 2019. Data from K. Pulidindi, S. Mukherjee, Structural Composites Market Size by Matrix (Polymer, Wood, Metal), By Reinforcement Materials (Glass Fiber, Carbon Fiber, Aramid Fiber), by Sector (Construction & Infrastructure, Energy, Transportation [Automotive, Aerospace & Defense]), Industry Analysis Report, Regional Outlook, Application Growth Potential, Price Trends, Competitive Market Share & Forecast, 2020–2026, 2020.

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2.2.1 Inorganic fibres 2.2.1.1 Glass fibre The most common type of fibre reinforcement is glass fibre, an amorphous, inorganic material made predominantly from silica and alumina, combined with other elements to adjust properties and suppress the melting point for easier manufacturing. Electrical glass (commonly referred to as E-glass) has been produced commercially as a continuous fibre since 1938 and remains the most common type of glass fibre in use for FRP. Glass fibres have relatively high tensile strength (2000–5200 MPa) but are limited by the presence of any surface cracks and so must be protected during handling. Glass fibre modulus values are relatively modest (70–90 GPa), yielding similar specific modulus values to aluminium. However, considerable effort has been put into improving this aspect. Continuous, aligned glass fibres are in general produced using large-scale melting furnaces that refine a mixture of oxides before transporting them to a gravity-driven fibre drawing station called a bushing. These bushings are made from a platinum/rhodium alloy with hundreds, or even thousands, of holes from which continuous glass fibres are drawn. Immediately after drawing, a protective coating (sizing) is applied to the fibres before they are either chopped or wound onto cardboard tubes under controlled tension and dried (see Fig. 2.2). Typical filament diameters are 5–20 μm and can vary substantially based on manufacturing conditions (hydrostatic pressure, temperature, bushing hole size, drawing speed). Whilst glass fibre properties are higher for smaller diameters due to their crack-limited strength, the filament diameter may be driven by economics because a greater mass of material can be produced per time with a larger filament. Whilst E-glass remains the main glass fibre type in use, several iterations of glass design have produced more specialised glasses with higher properties. S, R, and T glasses are variants with lower modifier content developed in the latter part of the 20th century that provide higher strength and modulus, but cost about 10 times as much as E-glass [2]. Pure quartz can also be drawn into a high-strength fibre with good thermal resistance and high elongation to failure, but requires very high processing temperatures (>2300°C) and specialised techniques.

Fig. 2.2 Schematic showing continuous filament glass fibre production.

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Design and Manufacture of Structural Composites

Growth in the wind energy sector since 1995 has driven the need for low-cost, highmodulus fibres to stiffen turbine blades. This includes the creation of Owens Corning’s H glass [3] and Nippon Electric’s Glass Innofibre [4]. Whether these will complement or compete with carbon fibre is yet to be seen, as production costs are heavily dependent on platinum prices, whilst new high-performance fibres require higher and higher manufacturing temperatures. For example, E-glass is produced at around 1260°C, whilst S glass fibres are drawn at 1565°C. Higher manufacturing temperatures mean more frequent replacement of bushings and the need for more thermally stable platinum alloys. High-performance fibres are also starting to make use of rare-earth materials that can both increase the modulus and reduce forming temperatures (the Innofibre requiring only 1300°C, for example), but these are in high demand in many different applications and will likely add significant cost to a commodity material. Perhaps of particular significance, glass fibres are radio-opaque, meaning they can be used in infrastructure without blocking communications. This has become increasingly important since the United States passed the Composite Standards Act in 2020 to support the use of composite materials in infrastructure projects. Consequently, composites are being employed in construction for bridges and facades in both Europe and the United States, frequently using glass reinforcement.

2.2.1.2 Basalt fibre Basalt fibres are a particular form of glass fibre characterised by a relatively high iron content. Basalt fibres have been known for as long as E-glass, but were not considered as a possible reinforcement material for composites until the 1960s, where they were overshadowed by S-type glasses. Basalt requires similar forming temperatures to S glass and is more aggressive to bushings whilst providing mechanical properties somewhere between E and S glass fibres. However, basalt fibre has proved to be a useful high-temperature material and a potential replacement for asbestos. It has also recently found a particular niche in rebar for concrete reinforcement, thanks to its high specific properties in relation to steel and good resistance to the strongly alkaline conditions of cement. Whilst basalt is a single component material, the composition can vary significantly between mining sites and so consistency of product can be difficult without careful monitoring [5].

2.2.1.3 Ceramic fibres There are a range of crystalline fibres that are used for reinforcement, including boron, silica, silicon carbide, zirconia, and alumina. They can be produced in continuous or short form and provide very high temperature resistance. They are typically used in non-polymer matrix composites and so will not be greatly discussed here. Boron/ epoxy tapes are used in aerospace to repair aluminium structures, but the large diameter of the fibres (100–200 μm) and resulting high bending stiffness compared to carbon fibres mean they are more commonly used as short fibres in metal matrix composites.

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2.2.2 Organic fibres 2.2.2.1 Carbon (graphitic) fibre Carbon fibre reinforcements are synonymous with high-performance FRP composites. Their high strength, very high modulus, and low density provide extremely high specific properties. They are essentially highly aligned forms of graphite and have been in commercial use since the 1960s. Carbon fibres are produced by heating polymer fibres in an oxygen-free environment until they convert to almost entirely carbon (92%–99%, depending on grade). The polymer fibres are first stretched to provide molecular alignment and are stabilised at 200–300°C before conversion to carbon (see Fig. 2.3). The most common precursor material is polyacrylonitrile (PAN), representing 96% of fibre production, though fibres can also be produced from pitch (a petroleum derivative) and rayon (insoluble cellulose). Rayon fibres are of low mechanical properties and not greatly in use whilst mesophase pitch can provide extremely high modulus values (K-1100 fibre reported up to 965 GPa [6]). However, the commercial use of pitch-based fibres is limited by cost, with a very low strain to failure at around 0.4% making them difficult to handle and process. They are, however, being considered for use in space applications for thermal management due to their extremely high thermal conductivity [7]. The process of manufacture is extremely important to the final properties of carbon fibres. After the majority of the fibre is converted to carbon by oxidation, the ‘graphitisation’ process completes the conversion and induces microstructural changes. The resulting fibre structure is dependent on the final temperature of the process. Higher temperature treatment results in a higher carbon content and a slightly higher density (1.75–1.80 g cm3). The increasing temperature also increases order in the graphitic crystal phase and increases the size of the crystals, resulting in an increase in modulus, as shown in Fig. 2.4. Increases in strength, however, are restricted by the development of voids at higher temperature. Tensile strength in fibres is flaw-limited, and so the tensile strength falls away at higher graphitisation temperatures. By converting fibre at 1500°C the strength can be maximised, whilst the modulus continues to rise beyond this point. This results in two primary classes of carbon fibres: Type I high-modulus fibres produced at >2500°C and Type II highstrength fibres produced at between 1200 and 1700°C. In a drive to reduce costs, there is also a third class of fibre produced in the range 1000–1200°C that provides lower, but still significant, mechanical properties and is referred to as Type III. Carbon fibre filament diameters are in the order of 5–10 μm. Carbon fibres are also conductive, which can provide benefits in certain applications, but can also present difficulties in manufacturing when generating dust. Due to carbon fibre costs, their main areas of application are in the aerospace industry, where weight is of critical importance. However, demand from the wind industry recently overtook the aerospace sector, as the need for cleaner energy sources drives the creation of increasingly large wind turbine blades [8]. Sporting goods and highend automobiles are also significant areas of use for carbon fibres, where the market can withstand high price points.

Fig. 2.3 Schematic and images showing carbon fibre production.

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Fig. 2.4 Representation of the effect of graphitisation temperature on the tensile modulus and tensile strength of PAN-based carbon fibres.

Carbon fibres can be made more cost effectively by increasing throughput during production in an approach similar to what was seen with glass fibres. Whilst glass fibres are used in mainly stiffness-driven applications, carbon fibres are frequently used for their strength. It has been well known since the Griffiths publication 100 years ago [9] that smaller fibres are stronger than large diameter fibres due to having a lower density of flaws. As such, it is preferable to maintain a small filament diameter and to increase the number of filaments within the tow to make carbon fibres more cost effective. Carbon tow sizes range from 1k, 3k, 6k, 12k, 24k, and now 50k. A cost model developed for carbon fibre production demonstrates that precursor costs are the most significant factor in overall price, contributing up to 65% of the overall cost [10]. A 3k carbon fibre tow would cost $35/kg by that model whilst a 50k tow would cost $11/kg. This is largely in relation to spinneret size in the PAN process, where the capital equipment and factory space are similar irrespective of the tow number. Cost savings plateau at the 50k tow level through this process. Aerospace uses the smaller, more-expensive tow sizes in non-crimp fabrics whilst wind and automotive compete for the large tow sizes. Aerospace fibres also have more strict requirements for tolerance, certification, and traceability, which add further cost. Alternative precursors are also being considered as routes to more affordable carbon fibres as well as potentially offering more sustainability. PAN costs in the order of

26

Design and Manufacture of Structural Composites

$3–9/kg, whereas PE and lignin cost $1–3/kg. Textile-grade PAN has been converted into high filament count tows in the low modulus range, with the potential to be converted into very high filament count tows (300–610k). Polyethylene (PE) has been considered as a carbon fibre precursor since the late 1970s, with tensile strengths reported up to 2.6 GPa and a tensile modulus of 200 GPa [6,11]. However, the required investment costs have remained too high for commercial development due to the need for large-scale sulphonation [6]. Lignin has been investigated as a precursor due to its relative abundance and solvent-free spinning processes. However, graphitisation of pure lignin has so far only provided limited mechanical properties of up to 1 GPa tensile strength and around 50 GPa tensile modulus [6,12]. Lignin has been blended with other polymers with some success, for example a 70/30 mix of hardwood lignin and pyrolyzed fuel oil attained a tensile modulus of 100 GPa [12]. It has also been blended with PAN as a means of incorporating renewable materials into the carbon fibre process to achieve fibres in the lower modulus range. For example, a blend of PAN with 25% lignin produced a carbon fibre with a tensile strength and modulus of 2.24 and 217 GPa, respectively [6]. With a rise in biomanufacturing, there are even approaches being considered for the use of algae and yeast processes to create the necessary precursors for carbon fibre manufacture. Combining this approach with renewable energy sources could possibly turn the production of carbon fibre into a CO2 sink [13]. Energy use is high in the production of carbon fibres, both for precursor and carbonisation processes, and the incorporation of both renewable energy sources as well as more efficient heating methods such as microwaves will be important in making carbon fibre production more sustainable.

2.2.2.2 Aramid fibre Aramid is perhaps one of the best-known polymer-reinforcing fibre types, with versions including the ‘bulletproof’ material Kevlar and the heat-resistant Nomex. Aramid fibre was developed in the 1970s and is a highly orientated polymer of extremely high strength, in the order of 3 GPa. It is very resistant to impact and abrasion, even at low temperatures, and is used in composites for vehicle crash structures, helicopter blades, and other leading edges on aircraft and sports equipment. The compressive properties are poor, however, and Aramid is also very difficult to cut, which can cause manufacturing difficulties. It must also be protected from ultraviolet radiation and will absorb moisture (up to 6%) if not encapsulated with a suitable matrix. Aramid is sometimes hybridised with carbon fibres to provide composites with enhanced impact resistance or resistance to vibration during cutting operations.

2.2.2.3 Natural (cellulosic) fibres There is an increasing interest in the use of plant-derived natural fibres in polymer composites, principally from a sustainability standpoint. They are of low density and so provide relatively high specific strength and modulus but as a crop are variable

Reinforcing fibres

27

in quality, can absorb water (and in so doing, swell), and pose difficulties in creating good interfacial bonding. There is also a ceiling in processing temperature because natural fibres begin to degrade above 200°C. Bast fibres such as hemp and flax can provide specific fibre properties on a par with E-glass, although they are discontinuous filaments and must be converted to yarn or warp-spun if intended for use in continuous fabrics. Their sustainability credentials, low cost, and low abrasion in processing have led to the widespread use of natural fibres by all major automotive manufacturers to create door panels, seatbacks, shelving, and other shell structures [14]. Natural fibres also see significant use in the construction industry for lightweight structural components such as flooring and door/window frames, but are not yet used for primary structures [15].

2.2.2.4 Other polymer fibres Commodity polymers such as PE and polypropylene (PP) are readily converted into continuous filaments. They can be stretched to provide molecular alignment and to increase fibre strength. With careful processing, these enhanced fibres can then be fused together to form what is referred to as a self-reinforced polymer composite – a material with the same chemical composition of the matrix and reinforcing fibre [16]. Self-reinforced polymers have near perfect interfacial bonding with no need for coupling agents, exhibit very good impact resistance, and are of very low density. However, they have relatively low maximum working temperatures. They have been used for personal protection equipment, body armour, luggage, and automotive underbody panels.

2.3

Fibre coatings

Essential aspects of fibres used for reinforcement are the condition and chemistry of their surface; for a fibre reinforcement to be successful, there must be a strong interface between the fibre and matrix. Furthermore, the fibres must survive and maintain their strength during downstream processing, as they are converted into fabrics and subsequently composites. Coatings are added during the fibre manufacturing phase to protect them during this time, but this can also affect the interphase region between the fibre and the surrounding matrix, and therefore the final properties of the composite. For most thermosetting resins, manufacturing temperatures are not an issue for fibre coatings. However, when using high-temperature thermoplastics, special consideration of the temperature resistance of the coatings must be made. Glass fibre surfaces are very smooth and are reliant on chemical bonding. This is usually achieved by treating the fibre surfaces with a silane compound to form a covalently bonded coating with specific functional groups selected to interact with a particular resin. Normally this silane is incorporated as a part of the sizing solution applied immediately after fibre drawing, but may be added at a later stage of the production process. The coupling agent only forms a small proportion of the sizing solution, which also includes film formers, lubricants, emulsifiers, and other additives. It is

28

Design and Manufacture of Structural Composites

very important to select the correct glass fibre product to match the matrix resin of interest because the functional groups can vary. There is an extremely broad range of silanes available, but the majority of commercial systems are focused on three types: aminopropyltriethoxysilane (APTES), used as a thermoplastic compatible coupling agent; γ-methacryloxypropyltrimethoxysilane (MPTMS), used as the principal coupling agent for polyester resins; and γ-glycidoxypropyltrimethoxysilane (GPTMS), used for epoxies [17]. The graphitisation process used in the manufacturer of carbon fibres and the resulting progression to very high (99%) carbon composition renders the fibre surfaces relatively inert, with few functional groups for bonding. To make the fibre surface more compatible, oxidative treatments are applied that generate functional groups with which coupling agents can interact. The treatments also generate increased roughness of the fibre surface, which provides additional mechanical interlocking at a cost of some fibre strength by the introduction of flaws. Balancing this loss of fibre properties against effective adhesion is a key field of research, with a variety of approaches under consideration [18]. Natural fibres have relatively rough surfaces, meaning that mechanical interlocking can be achieved easily, but fibres tend to be hydrophilic as well as containing fats and waxes that result in poor wetting and poor interfacial adhesion. Treatments with alkali can help to remove the fats and waxes and coupling agents are typically used to create a more suitable surface chemistry for matrix bonding. The most common treatment is the use of maleic anhydride (MA) with a suitable grafted matrix functional group. Low-cost natural fibres are often combined with low-cost PP and so a MAPP coupling agent is used. Natural fibres are strongly associated with the wider textile industry, where newer enzyme treatments are potentially offering more environmentally friendly alternatives to MAPP in the near future. Aramid fibres have smooth surfaces and generally have poor direct adhesion to polymer matrices. Similar approaches have been tried as with carbon fibres, generating surface roughness for mechanical interlocking or chemical and coupling agent treatments to make the surface chemistry more compatible.

2.4

Fibre forms, nomenclature, properties, and testing

Reinforcing filaments provide a wide range of mechanical properties and behaviours but in order to be usefully applied, they must be converted into forms that can be handled and used in manufacturing processes. This will be covered in more detail in Chapter 4 – Intermediates.

2.4.1 Finished fibre forms Initial products from fibre manufacturing include various types of bundled filaments. With glass and carbon, many filaments are brought together after manufacturing and bound together by surface coatings to form stable and resilient bundles variously called strands, tows, and rovings. The terms are somewhat interchangeable but

Reinforcing fibres

29

Fig. 2.5 Different finished fibre forms using continuous filaments.

fundamentally they all refer to parallel bundles of continuous filaments without twist. ‘Strand’ is more commonly used in reference to glass fibre and tow is more common to carbon. ‘Roving’ can refer to a combination of strands or tows brought together to form a larger ‘bundle’ (an assembled roving), though it can also refer to a bundle drawn directly (a direct roving) (see Fig. 2.5). These continuous materials can be chopped at the point of manufacture to produce short lengths of discontinuous material, in the order of centimetres. Conversely, discontinuous natural fibres can be converted by carding into parallel filament bundles called slivers, which are loosely equivalent to strands and tows and which are normally converted to yarns by spinning to provide more stability and strength. Yarns are distinct in that they have a degree of twist, either S or Z (see Fig. 2.5). Twist provides a means of controlling the compaction of fibres and provides resistance to damage when processed through tight bends for aggressive processes such as weaving. Yarns can be assembled into threads as a means of balancing out twist by combining yarns twisted in either direction. The twisting process is commonly used with natural fibres, polymers, and glass in the production of textile fabrics. However, for continuous filaments, twist imparts a helical pathway to the individual filaments and results in a reduction in axial mechanical properties; therefore, it is not common with carbon fibres. Weaving methods incorporating flat spread tows of carbon have been developed that avoid the need for twist. Strands, tows, rovings, and yarns are defined in terms of their values of tex or denier. Tex is the mass in grammes of a length of 1000 m, whereas denier is the mass in grammes per 9000 m. Assembled rovings and yarns will also have a count of the number of assembled strands or tows and in the case of yarns their degree of twist. Filaments of different types can be combined to form commingled products. Typically, these are for thermoplastic matrix composites, for example E-glass/PP. These ensure that the reinforcement and matrix are already carefully intermingled so as to improve wet out and part quality when consolidated. Quality can be variable, depending on the process used to intermingle the filaments.

30

Design and Manufacture of Structural Composites

2.4.2 Textile materials The step beyond bundle forms is textile forms, where tows or yarns are combined to produce sheet materials. These will typically be in roll form, a metre or more wide and tens of metres in length. Textiles are a means of setting fibres in desired orientations whilst creating an easy-to-handle material that can be cut and stacked as required. Textiles are defined by standards (e.g., BS 3396 for yarn-based glass fabrics, BS 3749 for woven glass rovings) and require traceability through the supply chain. Continuous filament textiles are covered in detail in Chapter 4 – Intermediates.

2.4.3 Discontinuous fabrics A chopped strand mat is a material in which short lengths of fibre strand are bound together in a random distribution using polymer solutions, powders, or stitching to form a sheet. Such sheets can be obtained in a range of weights from 225 to 1200 gsm, though they can include significant amounts of binder material (on the order of 10% by mass) and can be extremely variable in terms of both thickness and uneven distribution of strands. They are usually made of E-glass, are low cost, and easy to form but the lack of directional orientation and low final fibre volume fraction (< 40%) means that they are mainly used for semi-structural panels. Very thin versions (30 gsm) can be used as surfacing tissue – a resin-rich surface to provide a smooth finish over coarser fibre reinforcement beneath. Chopped strands are also commonly processed into bulk and sheet moulding compounds (BMC and SMC) for high-throughput part production with less-demanding mechanical requirements. With the ongoing activities in recycling of composites (see Section 2.5), quasi-UD fabrics are becoming available [19]. By passing recovered discontinuous fibre suspensions through orientation-selective processes, the discontinuous fibres are majority aligned along a single axis. These materials combine relatively high mechanical properties with ease of forming. As discussed in Section 2.1, fibres have high aspect ratios even at their shortest lengths. Because filament diameters are in the order of 10 μm, they have aspect ratios of 100 at lengths of just 1 mm. Therefore, fibre lengths must become very short before ηL in Eq. (2.1) falls significantly below 1. The concept of a critical length Lc, a minimum length of fibre to which full stress transfer from matrix to fibre can be achieved, arises from the theories of Cox [20] and Kelly-Tyson [21]. Provided the fibre length remains above this value, aligned but discontinuous materials can achieve properties that are competitive with their continuous fibre equivalents [22].

2.4.4 Properties and testing Definition of the mechanical properties of fibre reinforcements can be made at multiple length scales from single filaments to strands to full textiles. The selection of properties to measure will depend on the design intent and models utilised. Single filament measurements will provide material properties to support microscale models

Reinforcing fibres

31

Table 2.1 Indicative filament properties. Material E-Glass High-performance glass Basalt Boron Carbon (PAN) Carbon (Pitch) Aramid Natural fibre

Cost (£/kg)

Density (g/cm3)

Tensile strength (MPa)

Tensile modulus (GPa)

1–2 12–20

2.54 2.5–2.62

3400 3400–5200

72 72–93

1–4 2300 12–83 12–83 11–25 0.2–2.4

2.7 2.4–2.6 1.76–2.1 2.0–2.2 1.45–1.47 1.2–1.6

4000–4800 3000–5000 1800–7000 1400–3000 3000–3600 400–1500

89–100 390–400 230–540 140–965 60–180 5–130

whilst textile properties provide information related to behaviour during forming. A wide range of international testing standards exists to cover these different conditions. Representative properties for a range of filaments are provided in Table 2.1. Care must be taken when comparing properties from different sources because strength depends on the filament diameter and gauge length used for the test. For brittle fibres such as glass and carbon, strength is a statistical distribution and is often expressed in terms of the Weibull strength; this is reported as the most probable strength value based on a large batch of test specimens (30 +). The Weibull theory states that the failure strength for brittle materials is dependent on the test volume, as the probability of a critical flaw increases with the size of the specimen, hence the importance of the filament diameter and gauge length used in the test. Similarly for ductile polymer filaments, properties can be affected by test speeds. For glass fibres, the sonic modulus (NOL TR 65 87) is suggested to be the best predictor of subsequent laminate modulus. Individual filament testing is a difficult, time-consuming, and laborious process and so dry and impregnated tow testing methods have been developed and standardised. Bundles of filaments provide a much broader sample (typically 100 times as many filaments as testing in a monofilament study) but also exhibit damage from friction, lack of perfect alignment, and indirect loading. As such, they do not represent pure filament properties, but are more representative of the behaviour experienced within a laminate.

2.5

Sustainability, recycling, and reuse

The high specific properties offered by composite components have driven their use in a range of sectors to provide lightweight solutions. In aerospace in particular, the proportion of composite components has increased in each generation of aircraft, with 50% of the weight of the Boeing 787 Dreamliner and 53% of the Airbus A350 XWB being composite material [23]. As green energy dominates the global agenda, composite wind turbines are seeing explosive growth both in size and number [8].

32

Design and Manufacture of Structural Composites

With explosive growth in production, the need to recycle becomes increasingly important. It is expected that 6000–8000 aircraft will reach their end of life in 2030, with each containing 20 t of carbon fibre. Wind turbine waste is predicted to overtake aero waste by 2035. This is in addition to the waste generated at the point of manufacture; cutting waste can be up to 40%. For dry fibre processes, it is relatively straightforward to recover and reuse the waste, but this is not the case with pre-preg materials. The drive towards sustainability has also seen increasing legislation coming into place to reduce, reuse, and recycle. In the automotive sector, for example, European Union directives (ELV 2000/53/EC) have required that 85% of a vehicle (by weight) must be reused or recovered since 2006 and 95% since 2015. This has been a determining factor in the increased use of natural fibres and bioresins in automotive composites, with manufacturers actively developing natural fibre components since the 1990s. Daimler Chrysler spent around $1.5 billion on environmental initiatives in 2000 and Toyota claimed the first 100% natural composite product in 2003 for a mainstream automotive application, which was a kenaf/PLA spare tire cover [14]. The European Composites Industry Association (EuCIA) has developed a cradleto-gate composite manufacturing tool as a means to assess the environmental impact of composite components (https://ecocalculator.eucia.eu). With a lot of composite waste, the difficulty in separating, cleaning, and dismantling old parts means that little is cost effective beyond energy recovery – that is, burning. However, because most matrix polymers and fibre precursors are still based on fossil fuel resources, this is not sustainable in the long term. In particular, carbon fibres represent a resource that should be recovered for reuse. The level of embodied energy is very high and the amount of CO2 generated per mass of material is also large relative to other alternatives (see Table 2.2). There are a small number of carbon fibre recycling plants worldwide, but the capacity is much lower than the current virgin fibre demand. Commercial capacity for recycling is in the order of 10 kt per annum versus an expected production demand of 194 kt for CFRP in 2022 [24]. Table 2.2 Embodied energy and CO2 produced for different fibre types.a Material Carbon fibre (high strength) Carbon fibre (ultrahigh modulus) Glass fibre Basalt fibre Polyester resin Epoxy resin Steel Aluminium a

Values from CES EduPack.

Embodied energy (MJ/kg)

CO2 generated during production kg/kg

270–300

19–21

910–1000

65–72

49–54 0.85–0.95 68–75 122–135 31–81 190–215

2.8–3.1 0.05–0.06 2.4–2.7 6.3–6.9 2.3–5.9 12–15

Reinforcing fibres

33

Whilst they represent the majority of composite reinforcement (approximately 90% at the time of writing), the value of glass fibres at present is too low for recycling to be considered economical and the consequential knock down in properties using existing recovery processes is prohibitive. However, should future generations of glass fibres incorporate rare earth elements and improved recovery methods provide better property retention, their value may increase sufficiently to support recycling. The waste stream from wind turbines is also expected to be easy to separate, reducing overall costs. Intermediate approaches are used in the meantime, such as incorporating waste glass FRP into the cement clinker process, partially replacing fuel and raw material. Recovery of carbon fibre materials is presently undertaken commercially through mechanical and thermal processes. Mechanical processing, crushing, and grinding is a down-cycling approach providing short fibre and particulate materials for lower-grade applications. Thermal processing separates the matrix and fibre components by burning, with the potential to recover clean fibres at length scales in the order of centimetres. With appropriate process control, recovered fibre properties can be close to those of virgin fibres [24], but are in a low-density, fluffy form that must be further processed, as discussed in detail in Chapter 21. Recycling processes can use as little as 5% of the energy of virgin carbon fibre production and recycled fibres are sold at around half the price of virgin fibre. However, the biggest barrier for uptake lies in market confidence, standardisation, and certification.

2.6

Summary

It continues to be the case that glass fibres are the most common reinforcing fibre for polymer composites and that carbon is the preferred choice in more demanding applications. However, the decreasing costs of carbon fibres are increasing their market share and the particular alkali resistance of basalt is also carving out a significant niche [25]. The increasing demand for sustainable materials is also moulding the shape of the reinforcements sector, with successful recycling routes set to have a major impact over the next decade. Natural fibres are seeing increasing use as a part of that narrative, but seem unlikely to progress beyond semi-structural applications due to their relatively low specific properties and concerns over repeatability.

References [1] E. Witten, V. Mathes, M. Sauer, M. K€uhnel, Composites Market Report 2018, AVK – Industrial Association for Reinforced Plastics eV, 2018. [2] N.M. Demina, Current compositions for processing high-strength high-modulus continuous glass fiber (review), Fibre Chem. 48 (2) (2016) 118–124. [3] A. Berthereau, Race for always higher glass modulus materials, in: Aachen Reinforced! Glass & Carbon Fibres, 2021. Online (Hosted by Institut f€ ur Textiltechnik of RWTH Aachen University, Aachen, Germany). [4] H. Li, New high-strength and high-modulus fiber glasses, in: Aachen Reinforced! Glass & Carbon Fibres, 2021. Online (Hosted by Institut f€ ur Textiltechnik of RWTH Aachen University, Aachen, Germany).

34

Design and Manufacture of Structural Composites

[5] T. Dea´k, T. Cziga´ny, Chemical composition and mechanical properties of basalt and glass fibers: a comparison, Text. Res. J. 79 (7) (2009) 645–651. [6] B.A. Newcomb, Processing, structure, and properties of carbon fibers, Compos. A: Appl. Sci. Manuf. 91 (2016) 262–282. [7] C.L. Bertagne, T.J. Cognata, R.B. Sheth, C.E. Dinsmore, D.J. Hartl, Testing and analysis of a morphing radiator concept for thermal control of crewed space vehicles, Appl. Therm. Eng. 124 (2017) 986–1002. [8] https://www.statista.com/statistics/702145/global-carbon-fiber-consumption-share-byend-use-market/. [9] A.A. Griffith, G.I. Taylor, VI. The phenomena of rupture and flow in solids, Philos. Trans. R. Soc. Lond. Ser. A 221 (582–593) (1921) 163–198. [10] S. Nunna, P. Blanchard, D. Buckmaster, S. Davis, M. Naebe, Development of a cost model for the production of carbon fibres, Heliyon 5 (10) (2019) e02698. [11] S.M. Aldosari, M. Khan, S. Rahatekar, Manufacturing carbon fibres from pitch and polyethylene blend precursors: a review, J. Mater. Res. Technol. 9 (4) (2020) 7786–7806. [12] S. Wang, J. Bai, M.T. Innocent, Q. Wang, H. Xiang, J. Tang, M. Zhu, Lignin-based carbon fibers: formation, modification and potential applications, Green Energy Environ. 7 (4) (2021) 578–605. [13] U. Arnold, A. De Palmenaer, T. Br€uck, K. Kuse, Energy-efficient carbon fiber production with concentrated solar power: process design and techno-economic analysis, Ind. Eng. Chem. Res. 57 (23) (2018) 7934–7945. [14] B.C. Suddell, A.D.A.S. Rosemaund, Industrial fibres: recent and current developments, in: Proceedings of the Symposium on Natural Fibres, Rome, 2008. [15] K.L. Pickering, M.G.A. Efendy, T.M. Le, A review of recent developments in natural fibre composites and their mechanical performance, Compos. A: Appl. Sci. Manuf. 83 (2016) 98–112. ´ . Kmetty, T. Ba´ra´ny, J. Karger-Kocsis, Self-reinforced polymeric materials: a review, [16] A Prog. Polym. Sci. 35 (10) (2010) 1288–1310. [17] J.L. Thomason, Glass fibre sizing: a review, Compos. A: Appl. Sci. Manuf. 127 (2019) 105619. [18] M. Sharma, S. Gao, E. M€ader, H. Sharma, L.Y. Wei, J. Bijwe, Carbon fiber surfaces and composite interphases, Compos. Sci. Technol. 102 (2014) 35–50. [19] G. Gardiner, Revolutionizing the Composites Cost Paradigm, Part 1: Feedstock, Composites World, 2020. [20] H.L. Cox, The elasticity and strength of paper and other fibrous materials, Br. J. Appl. Phys. 3 (3) (1952) 72–79. [21] A. Kelly, W.R. Tyson, Tensile properties of fibre-reinforced metals: copper/tungsten and copper/molybdenum, J. Mech. Phys. Solids 13 (6) (1965) 329–350. [22] S. Yarlagadda, J. Deitzel, D. Heider, J. Tierney, J.W. Gillespie Jr., Tailorable universal feedstock for forming (TUFF): overview and performance, in: SAMPE North America, Charlotte, NC, 2019. [23] R. Slayton, G. Spinardi, Radical innovation in scaling up: Boeing’s Dreamliner and the challenge of socio-technical transitions, Technovation 47 (2016) 47–58. [24] J. Zhang, V.S. Chevali, H. Wang, C.-H. Wang, Current status of carbon fibre and carbon fibre composites recycling, Compos. Part B Eng. 193 (2020) 108053. [25] A.E. Krauklis, C.W. Karl, A.I. Gagani, J.K. Jørgensen, Composite material recycling technology – state-of-the-art and sustainable development for the 2020s, J. Compos. Sci. 5 (1) (2021) 28.

Resins for structural composites
rez-Martı´n Dipa Ray and Helena Pe School of Engineering, Institute for Materials and Processes, The University of Edinburgh, Edinburgh, United Kingdom

3.1

3

Introduction

The matrix material in a composite plays a key role in binding and protecting the reinforcement fibres from the environment. Stress is transferred from the matrix to the load-bearing fibres and hence an effective bond is essential at the fibre/matrix interface. The type of matrix determines the maximum service temperature, moisture and chemical resistance, and thermal stability. It also provides a composite with its toughness, impact and damage tolerance, and abrasion resistance. Polymeric matrices used in high-performance structural composites can be either thermosets or thermoplastics. Thermosets are generally low-molecular-weight, lowviscosity resins that are converted into non-recyclable three-dimensional (3D) cross-linked structures that are infusible and insoluble. The process of cross-linking is called ‘curing’, which results from chemical reactions that are driven by heat, supplied by external sources or by the exothermic heat liberated during the curing reaction. The liquid resin first turns into a semi-solid gel and then finally into an infusible solid on completion of curing. The formation of a cross-linked structure provides the inherent rigidity for thermoset matrices. Hence, they are the most widely used matrix candidate in structural composite applications. Thermoplastics, on the other hand, are high-molecular-weight polymers that can be melted, shaped, and then cooled. As thermoplastics do not cross-link, they may be subsequently reheated, reshaped, and welded, which are key characteristics in the context of a circular economy. In recent years, the use of high-performing thermoplastics, such as polyether ether ketone (PEEK) and polyether ketone ketone (PEKK), has risen in advanced composite applications. This chapter focuses on both families of resins, highlighting the key options for structural applications within the field.

3.2

Thermosetting resins

Thermosetting resins are viscous materials commonly obtained from petroleum-based resources. The presence of various functional groups makes these materials chemically reactive, making it possible to convert them into polymers [1]. More sustainable natural options are also being derived from plant-based sources, having major applications in paints, adhesives, and coatings, but these are rarely used in structural composites due to their poor mechanical performance [2]. Therefore, synthetic resins Design and Manufacture of Structural Composites. https://doi.org/10.1016/B978-0-12-819160-6.00001-9 Copyright © 2023 Elsevier Ltd. All rights reserved.

36

Design and Manufacture of Structural Composites

(epoxy, phenol-formaldehyde, benzoxazine, etc.) dominate the global market for structural applications, which is the main focus of this chapter.

3.2.1 Epoxy Epoxy resins have established themselves as the most common matrix material for high-performance composites. Epoxy resins are a class of monomeric or oligomeric materials that further react to form cross-linked thermosetting polymers, offering an excellent combination of strength, chemical and solvent resistance, good adhesion to a broad range of reinforcing fibres, and low shrinkage on cure. Epoxy resins are used for a wide range of applications in the composites industry, from commodity products to aerospace structures to structural adhesives [1,3–5].

3.2.1.1 Epoxy chemical structures Epoxy resins are compounds, or mixtures of compounds, that are characterised by the presence of one or more epoxide or oxirane groups (Fig. 3.1) [3]. By far the most commercially significant ones are obtained by glycidation of bisphenol A with epichlorohydrin, producing diglycidyl ether of bisphenol A (DGEBA). Epoxy prepolymers are formed by reacting DGEBA with bisphenol A (Fig. 3.2). This is a difunctional epoxy, as two epoxide end groups can react with curing agents. ‘N’ denotes the number of polymerised subunits and is typically in the range from 0 to 25. This forms the most widely used epoxy type. Glycidyl amine-based epoxy resins are produced by reacting aromatic amines with epichlorohydrin, which contain higher functionality (such as three or four reactive epoxide groups). The most important glycidyl amine is tetraglycidyl methylene

Fig. 3.1 Chemical structure of an epoxide group.

CH3 O

CH3

O CH3

O

+

HO

OH CH3

O

CH3

CH3 O O

O

O CH3

OH

O CH3

Fig. 3.2 Epoxy prepolymers are formed by reacting DGEBA with BA.

O n

Resins for structural composites

37

Fig. 3.3 Chemical structure of tetraglycidyl methylene dianiline (TGMDA).

dianiline (TGMDA) (Fig. 3.3), which is the base resin used in the majority of highperformance composites. The high functionality (four) results in high reactivity and provides cross-linked structures that exhibit a high cross-link density, excellent chemical and thermal resistance, high glass transition temperature (Tg), and outstanding mechanical properties. Trifunctional epoxies such as the triglycidyl derivative of p-aminophenol (TGAP), shown in Fig. 3.4, are also available. It is possible to tailor the cross-link densities and hence the mechanical properties by using blends of di-, tri-, or tetrafunctional epoxies. Stiffness can be adjusted by tailoring the cross-link densities. Suppliers often mix different epoxies to produce resins with desired levels of strength, stiffness, and toughness. Such blends also enable the viscosity and elevated-temperature performance to be tuned for the resulting composite material. High-performance epoxies can also be formulated as B-staged systems, where the reaction between the resin and the curing agent is only partially complete. When this system is reheated at higher temperatures, the cross-linking reaction is completed and the system fully cures. B-staged resins are typically one-part systems and therefore do not require mixing prior to use. Table 3.1 shows the mechanical properties of some commonly used high-performance epoxy resins.

3.2.1.2 Epoxy cure mechanism Epoxy resins undergo reactions with curing agents to form a cross-linked, 3D infusible network. The curing mechanism is based on the opening of the epoxide ring and reaction with the curing agent. The unfavourable bond angles of the epoxide group make them chemically reactive with a variety of reactants that can easily open the ring to form cross-linked structures [1,3,4]. Cross-linking can only proceed by two types of curing mechanism: direct coupling of the resin molecules by a catalytic homopolymerisation, or coupling through a reactive intermediate. Reactions to cure low-molecular-weight epoxy resins occur with the epoxide ring. The capability of this ring to react with a variety of reactants via different paths makes epoxy resins versatile to work with. The chemistry of most curing agents currently used with epoxy resins is based on polyaddition reactions that result in coupling as well as cross-linking. The

Fig. 3.4 Chemical structure of triglycidyl derivative of p-aminophenol (TGAP).

38

Design and Manufacture of Structural Composites

Table 3.1 Mechanical properties and viscosities of some commonly used high-performing unreinforced epoxy resins. Building blocks

Viscosity

CYCOM® 890 Aromatic glycidyl derivative, mono component

Viscosity @ 120°C 0 h– 30.5 mPa s 2 h– 37.4 mPa s

HexFlow® RTM 6 Premixed, mono component epoxy system

Viscosity @120°C 0 h– 33 mPa s 2 h– 59 mPa s

EPIKOTE™ System 600 Polyfunctional, unmodified epoxy resin system based on methylene dianiline and aromatic amine

Viscosity @50°C 4900  1500 mPa s

Mechanical properties Tensile: 70 MPa Tensile Modulus: 3.1 GPa Flexural: 139 MPa Flexural modulus: 3.2 GPa Fracture toughness, KIC: 0.9 MPa m1/2 Tensile: 75 MPa Tensile modulus: 2.89 GPa Flexural: 132 MPa Flexural modulus: 3.3 GPa Fracture toughness, KIC: 0.6 MPa m1/2 Fracture toughness (GIC): 89 J/m2 Tensile strength: 75  10 MPa Tensile modulus:3.07 GPa Flexural strength: 130 MPa Flexural modulus: 2.95 GPa Fracture toughness, KIC: 0.7 MPam1/2 Fracture toughness (GIC): 210 J/m2

Tg (Dry)

Refs.


210°C

[6]


215°C

[7]


208°C

[8]

curing agents generally contain active hydrogen (e.g. polyamines, polyacids, polyphenols, etc.), as shown in Fig. 3.5 [3]. Epoxy resins and curing agents contain more than one reaction site per molecule, which can form 3D networks during curing. The specific reactions of the various curing agents with epoxide groups will not be discussed in detail in this chapter, but the

Resins for structural composites

39

Fig. 3.5 Reaction of epoxide group with curing agents containing active hydrogen.

main focus is to discuss the influence of different curing agents on the properties of the composites. Amines are very common curing agents for epoxy resins [3], reacting with the epoxide ring by an addition reaction without the formation of by-products. Amines can be aliphatic polyamines or their derivatives, modified aliphatic amines and aromatic amines. The reactivity of an epoxide with an amine depends on the influence of the associated steric and electronic factors. The hydroxyl groups might also play an important role in the epoxide-amine reaction. Aliphatic amines are highly reactive and produce an exothermic reaction during curing, enabling epoxies to be processed at room temperature. However, post-cure cycles are often carried out for such composites to improve their high-temperature performance. Aromatic amine curing agents generally require a high temperature for curing and impart high-temperature performance to the composites, such as diaminodiphenyl sulfone (DDS) that is widely used in high-performance epoxy composites. An extensive list of amine hardeners for epoxy resin is given in Ref. [4], but a selection is presented in Fig. 3.6. The final properties of the cured epoxy resin or epoxy composite depend on the degree of cross-linking obtained during cure. The degree of cross-linking is a function of the stoichiometry of the epoxy resin and curing agent, and the extent of the reaction between them. Anhydride curing agents are also used to cure epoxy resins [3,4]. They generally require higher temperatures, longer durations to achieve full cure, and sometimes an additional catalyst to increase the cure rate. They yield good high-temperature properties, chemical resistance, and electrical properties. One anhydride group reacts with one epoxy group during cure, but anhydride curing agents are susceptible to moisture uptake, which can inhibit the cure reaction.

Fig. 3.6 Aliphatic and aromatic curing agents used with epoxy resins.

40

Design and Manufacture of Structural Composites

3.2.1.3 Epoxy curing agent selection The mechanical properties of an epoxy resin are influenced by the curing agent according to the reaction conditions, leading to the formation of different network structures. The selection of an appropriate curing agent is crucial to achieve the best performance for a particular application. For example, different amine curing agents can result in different glass transition temperatures (Tg) for an epoxy resin, as shown in Table 3.2 [9]. Curing agents with rigid aromatic structures (e.g. DDS) need high curing temperatures and yield higher Tg, leading to the highest possible mechanical and thermomechanical performance. A higher Tg permits a higher service temperature. Table 3.3 shows an example of the same bisphenol A-based epoxy resin reacted with three different curing agents yielding different Tg values. Hence, the service temperature of the same resin can vary significantly by using different curing agents. Reaction speed plays a significant role in the selection of the curing agent. If a high production rate is the main criterion, fast-reacting curing agents, such as aliphatic amidoamines, are suitable candidates. The reaction rate can be measured from the change in viscosity. Table 3.4 shows an example where three different curing agents with the same epoxy resin show three very different reaction rates [9]. Different curing/post-curing cycles influence the type of network formation in the resin, which controls the resulting Tg and mechanical properties. ‘Working life’ is another important point as that controls how long the liquid resin flows to wet out the fibres before gelation starts. Reaction time is determined by how fast the curing agent reacts with the epoxy resin and ‘working life’ depends on the reaction time. In the case of epoxy prepregs, unlike liquid resins, storage life and storage conditions are more important to avoid premature cross-linking. The service environment is Table 3.2 Different amine curing agents lead to very different glass transition temperature (Tg) with the same epoxy resin [9]. Amine curing agent

Tg (°C)

Amidoamines Ethyleneamines Cycloaliphatic Aromatic

40–100 110–125 145–175 160–220

Table 3.3 Three curing agents reacted with the same bisphenol A-based epoxy resin yield different Tg values [9]. Curing agent

Tg (°C)

Diethylene triamine Diaminocyclohexane Diaminodiphenylsulfone

115 158 190

Resins for structural composites

41

Table 3.4 Three curing agents reacted with the same epoxy resin show different reaction rates [9]. Curing agent

Time (h) to reach 10,000 centipoise viscosity @ 40°C

Amine terminated Secondary amine Primary amine

3.6 4.5 5.9

therefore another consideration before selection of the curing agent. Environmental conditions, such as high humidity, can significantly affect the chemical structure. In this case the curing agent should not be moisture-sensitive, otherwise this can lead to loss in properties over time.

3.2.1.4 Epoxy resin selection Resin properties, such as viscosity, cure cycle, gel time, Tg, and mechanical properties, vary from one application to another, depending on the manufacturing process used and service conditions required. Short Takt times are essential in the automotive industry to achieve high production volumes, therefore a very fast cure cycle is needed to produce finished components in 2–3 min [10]. The viscosity of the resin is critical in this case, as the resin must flow to fill the mould cavity before gelation occurs. Resins with high Tg are preferred, enabling demoulding of the part at higher temperatures to facilitate rapid cycle times. In the renewable energy (wind and tidal turbines) and marine sectors, extra-large (
80 m in length) composite structures have very different resin property requirements. Liquid resins with room- or low-temperature cure cycles are used, which have low viscosities to aid the infusion of such large structures. Low cure temperatures along with low exotherms are desirable, especially for thick parts. Epoxy prepregs are also used, with typical cure schedules of 8–10 h at 70°C, or 4–6 h at 80°C [10]. Different curing agents can be used with the same resin, as discussed above, to suit the processing of such large structures. The ‘pot life’ at room temperature and ‘working life’ should naturally be longer for such large structures. In aerospace, the key requirements are high hot/wet mechanical performance, high hot/wet Tg, fire-smoke toxicity (FST) resistance, and toughness, where both the resins and curing agents should fulfil these requirements. The toughness of epoxy resins is often increased by incorporating a thermoplastic phase [11]. The non-recyclable nature of thermosetting resins is now a big concern for the environment, contributing largely to landfill problems. At the same time, synthetic resins are produced from petroleum-based resources, which is causing depletion of fossil fuels. Both problems have severe impacts on the environment and necessary actions are needed to initiate a gradual transition towards more environmentally friendly options. The focus of composites research is therefore shifting significantly towards a circular economy. New sustainable epoxies are now being developed from renewable resources, which are briefly discussed below.

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Design and Manufacture of Structural Composites

3.2.1.5 Bio-derived epoxies There is growing interest in creating epoxy resins and hardeners to lower the impact on the environment without compromising performance. Through green chemistry, sustainable raw materials, and efficient manufacturing, new green molecules are being created and sustainable resins are being developed. Vegetable oils such as soybean, linseed, canola, castor, Karanja, hemp, cottonseed, rapeseed, and palm have been used in many research studies as precursors to develop bio-based resins. There are several published works highlighting the synthesis and development of such vegetable oil-based epoxy resins [12]. Nikafshar et al. [13] synthesised a bio-based epoxy resin from vanillin, which can produce composites with competitive advantages over a DGEBA baseline. However, the major drawback of these bio-based epoxy resins is the poor mechanical properties, preventing them from competing with petroleum-based epoxy resins such as DGEBA. A calcium nitrate solution was used as an inorganic accelerator to accelerate the curing reaction of the bio-based epoxy resin, which reduced curing times as well as improved the mechanical properties such as tensile strength, pull-off strength, and Izod impact strength. The thermal and mechanical properties were compared with the DGEBA-based epoxy resin. It was hypothesised that the inorganic accelerator influenced the mechanical performance of the epoxy matrix through inducing changes in physico-chemical events that occurred during the curing process. Various bio-based curing agents have also been developed [12], which are yet to be widely used in industrial applications. However, with the current thrust on sustainability, such green curing agents will become more popular in coming years. A new series of ‘green’ epoxy curing agents has already been developed [14]. Some commercially available bio-epoxies are given in Table 3.5 with key characteristics. The properties of the bio-based epoxies in Table 3.5 are comparable to their synthetic counterparts, but cure cycles are generally longer and the Tg is lower. Moreover, not many studies have reported the long-term performance of these bio-epoxies or their composites, such as fatigue life, hot–wet properties, water absorption behaviour, etc. Further research is therefore required to establish their long-term suitability for structural applications.

3.2.1.6 Recyclable epoxies Despite epoxies being the most desirable and highest-performing matrix materials for structural composite applications, they are non-sustainable in nature and are generating huge quantities of landfill waste as components come to the end of life. To improve the sustainability of epoxies, new technologies are emerging for large-scale applications, such as re-formable thermoplastic epoxies and thermoset epoxies with reversible or cleavable dynamic cross-links. Thermoplastic epoxies: For the case of thermoplastic (TP) epoxies, liquid monomeric resin is impregnated into the reinforcement fabrics and polymerised in situ during ‘curing’ when linear molecules are formed devoid of any cross-links. Depending on the time and temperature of ‘curing’, the molecular weight changes. An interesting development in thermoplastic epoxies was reported by Imanishi et al. [17], where difunctional epoxy resin and difunctional phenolic compounds were mixed at a stoichiometric ratio, keeping the amount of catalyst fixed to 2 phr. The formulated

Table 3.5 Commercial bio-epoxies and their properties. Trade name

Green content

SR InfuGreen 810 Green Epoxy systems for Injection and Infusion

38% of carbon from plant origin

Pro-set Bioepoxy M1049 M2048 (Infusible grade)

36% bio-based

Viscosity

Properties

Refs.

(20 % mPa s) @ 15°C–2200 @ 20°C–1200 @ 25°C–750 @ 30°C–470 @ 40°C–210 Curing cycles: 24 h @40°C–8 h @80°C 480 mPa s @25°C At 20°C, vacuum pressure for 12 h after infusion + the resin and hardener mix will initially cure to a brittle ‘B’ stage after 24 h at room temperature + post curing at 50°C for 16 h

Tensile Modulus–2.6–3.0 GPa Strength–60–69 MPa %Elongation–5.9%–9.5% Flexure Modulus–2.6–3.0 GPa Strength–101–113 MPa Tg–72–96°C (different curing agents and curing conditions) Tensile Modulus–3.4–3.8 GPa Strength–60–65 MPa %Elongation–2.8%–3.5% Flexure Modulus–4.2–4.5 GPa Strength–96–104 MPa Tg–65–86°C (different curing agents and curing conditions)

[15]

[16]

44

Design and Manufacture of Structural Composites

resins were polymerised in situ without cross-linking during curing. Glass fabrics were preimpregnated with liquid resins, dried, and consolidated into laminates by in situ polymerisation under heat and pressure. The solvent resistance of thermoplastic epoxy matrices would be very different to conventional cross-linked thermoset epoxies. TP epoxies would be more susceptible to solvent attack like any other thermoplastic polymer. This dissolution behaviour is not desirable for many structural applications, but may be beneficial in the recycling process, where dissolution of the matrix would enable the recovery of the continuous reinforcement in its original form. Epoxies with cleavable amine hardeners: New dynamic or cleavable hardeners are emerging for epoxy resins that can create reversible cross-links in the polymer matrix or the cross-links can be broken when required. This interesting development was published by Ruiz de Luzuriaga et al. [18], where the authors reported the use of exchangeable disulphide cross-links in epoxy resins. Fibre-reinforced polymer composites were produced with the ‘dynamic’ epoxy resin and good mechanical properties were reported for the epoxy composites whilst showing potential for reprocessability, repairability, and recyclability. These composites were easily synthesised from readily available materials and could therefore be easily implemented for applications within the transportation, energy, or construction sectors, amongst others. Research is ongoing to develop new ways to synthesise cleavable bonds in cured epoxies to improve the recyclability of carbon fibre/epoxy composites [19,20], but further research is required to understand the resulting static and dynamic mechanical properties.

3.2.1.7 Powder epoxy Powder epoxies are extensively used for coatings, but are quite new in the field of fibre-reinforced composites [21–26]. Liquid resins require long flow path lengths for large parts, posing the risk of uneven or incomplete wetting, with added problems associated with thermal runaway for thick sections. Powder epoxies can be a potential solution to these concerns, as shown in Fig. 3.7 [23]. Commercially available powder epoxies for composite applications have a melting point around 60°C, an infusion temperature around 120°C, and a curing temperature around 160°C. The melt viscosity is as low as 1 Pa s at the infusion temperature, which enables the resin to permeate effectively into fibre preforms and produce consolidated, but uncured semi-pregs [24]. The uncured semi-pregs are easy to handle, can be stored at room temperature, can be assembled into large structures of complex shape, and cured or co-cured later. This offers significant benefits for producing large structures, where co-curing leads to seamless joints. Toughened grades of powder epoxy exist and these can be used to produce composites with toughened zones in selected areas or layers [27].

3.2.2 Unsaturated polyester Unsaturated polyester resins (UPR) are used extensively in structural applications, but have limited use in high-performance areas. UPR has lower mechanical properties, inferior weathering resistance, high cure shrinkage, but lower cost compared to epoxies. The chemical structure of UPR is shown in Fig. 3.8.

Resins for structural composites

45

Patm

Laminate

HTcond

Heated tool

Heat Source

Heat +

Pressure

Fig. 3.7 A schematic showing the manufacturing of 60 plies glass fibre/powder epoxy thick laminate. ´ Bra´daigh, Reproduced with permission from J.M. Maguire, P. Simacek, S.G. Advani, C.M. O Novel epoxy powder for manufacturing thick-section composite parts under vacuum-bag-only conditions. Part I: through-thickness process modelling, Compos. Part A Appl. Sci. Manuf. 136 (2020) 105969, doi:10.1016/j.compositesa.2020.105969.

O O H

O O

*

O

*

O

O O

* denotes reactive groups

*

O

O

*

O

n=5-6

Ester groups

Fig. 3.8 Chemical structure of an unsaturated polyester resin.

O

H

46

Design and Manufacture of Structural Composites

The common reactants used in producing UPR are diols, diacids, and unsaturated anhydrides. These ‘unsaturations’ introduced in the backbone chain (dC]Cd) take part in free radical curing and provide the sites where cross-linking occurs. Styrene monomer is used as a reactive diluent with UPR, which gets chemically bonded in the network structure during curing. The proportion of unsaturated anhydrides can be varied to tailor the degree of cross-linking. The ester linkage in the backbone chain is prone to hydrolytic degradation and leads to poor weathering resistance.

3.2.3 Vinyl ester Vinyl ester (VE) resins are produced from the reaction of epoxy-terminated molecules with unsaturated acids. The chemical structure of VE is shown in Fig. 3.9. Like unsaturated polyester resins, styrene is used with VE as a reactive diluent that gets chemically bonded within the 3D network during curing. VE resins are considered to have intermediate performance in between UPR and epoxy resins.

3.2.4 Benzoxazine Benzoxazine (BZ) resin is another high-performance thermosetting resin that is used as a matrix material for advanced polymeric composites. It is polymerised via a ringopening reaction of cyclic benzoxazine monomers synthesised by the Mannich reaction from formaldehyde, phenol or phenol derivative, and primary amine [28]. A polymerisation of benzoxazine resin by ring opening is shown in Fig. 3.10. BZ resins are important because they exhibit: (i) near-zero volumetric change upon curing, (ii) high fire, smoke, and toxicity properties, and (iii) release no major reaction Ester groups

Fig. 3.9 Chemical structure of vinyl ester resin.

O

O

*

* *

O

O

O

OH

O OH n=1-2

*

denotes reactive groups

Fig. 3.10 A polymerisation of benzoxazine resin by ring-opening reaction.

*

Resins for structural composites

47

by-products during polymerisation, although some minor components can be produced due to the thermal cleavage or evaporation of monomer and solvent molecules [28,29]. BZ resins have high resistance to moisture, chemicals, and other corrosive materials, and thus are suitable for use in extreme environments. They can be stored at room temperature unlike high-performing single-component epoxies, which have to be stored in a freezer at 26°C to prevent any undesired curing reaction. Blending of BZ with epoxy resin is feasible because the ring-opening reactions of BZ produce phenolic hydroxyl groups, which can react with epoxy resins and provide additional cross-linking points in the resulting network structure [30,31]. This increase in cross-link density increases the Tg and strongly influences the mechanical and thermal properties. In addition to epoxy, BZ resins can also be blended with other polymers such as bismaleimide, polycarbonate, polypyrrolidone, and polycaprolactone to tailor their properties [28]. Although BZ resins offer several advantages, disadvantages include: (i) high brittleness, (ii) requirement of high cure temperatures, (iii) difficulties in preparing films or complex structures. The brittleness of BZ resins is often reduced by incorporating a thermoplastic phase [32–34]. New generations of BZ resins combine excellent stiffness and high-temperature performance with outstanding hot/wet strength. Dry Tg can be around 240°C and hot/wet Tg can be around 160°C. The flexural strength and flexural modulus are approximately 150 MPa and 4 GPa, respectively, but these vary depending on the grade of the resin. There are possibilities to develop bio-based BZ resins to minimise environmental impact and maximise the use of natural resources [35]. Eco-friendly BZs can be synthesised from various plant-derived phenols. Phenolic substances are abundantly available in the plant kingdom [36], such as arbutin [37,38], chavicol [39], pyrogallol [40] thymol [41,42], and vanillin [43,44]. They can be extracted from plants and bridged with various aromatic and aliphatic amines to produce benzoxazines. The abundance of plant-derived phenols makes the synthesis of bio-BZ a feasible and economically viable approach. BZ resins find applications in the aerospace sector for interior panels and bulkheads, the rail industry for interior components, the oil and gas industry for risers and down-hole plugs, and the consumer electronic industry for printed circuit boards, etc. Further examples of the application potential of BZ resins in the space and aerospace sectors can be found in the literature [45–48].

3.2.5 Bismaleimide Polybismaleimides (BMI) are high-performance thermosetting addition-type polyimides, exhibiting high strength and rigidity at elevated temperatures, long-term heat and oxidative stability, excellent electrical properties, and relatively low moisture absorption. They exhibit outstanding dimensional stability, superior high-temperature performance, and high chemical resistance including hydrocarbons, alcohols, and halogenated solvents. Due to their high long-term creep resistance, they can replace metals in many structural applications [1,49]. The chemical structure for BMI is shown in Fig. 3.11.

48

Design and Manufacture of Structural Composites

Fig. 3.11 General structure of a bismaleimide monomer, R ¼ alkyl or aryl.

Bismaleimide monomers are synthesised by reacting a primary diamine with maleic anhydride in the presence of acetic anhydride and sodium acetate, to affect the dehydration and cyclisation of the resulting maleamic acid [49]. The BMI structure can be tailored by using different monomers, which change the Tg and properties of the resins. Representative examples of some monomers (R) are shown in Table 3.6. Monomers exhibit high melting temperatures (Tm) (Fig. 3.11 and Table 3.6), which is typically due to their crystalline nature resulting from the strong interaction between carbonyl groups in the imide rings. This introduces severe processing challenges [49]. Low-melting amorphous resin blends are often used to improve processability [50]. Bismaleimides (BMIs) cure through addition reactions in the temperature range of 175–190°C. Further post-curing at 230–245°C can develop high-temperature properties [1]. The versatile maleimide functional groups are able to undergo a range of useful, addition-type chemical reactions including ene-Alder or Diels-Alder, Michael addition, and free-radical reactions depending on the types of reactants present in the system [49]. These chemical reactions do not produce volatiles, thus minimising the formation of voids in the composites, which is an added advantage. Polymerisation occurs via thermally induced free radical reactions at high temperatures (above 180°C) between the double bonds of adjacent molecules, producing highly cross-linked structures. The high cross-link density leads to the excellent elevated temperature performance of these resins, but also makes them brittle with low fracture toughness. Brittleness is common in highly cross-linked thermosets such as multifunctional epoxies, but is made worse in BMIs due to the presence of polar carbonyl groups, which cause ordered stacking of the polymer chains with less possibility for energy dissipation [51]. BMIs are often blended with reactive co-monomers such as vinyl and allyl compounds, allyl phenols, isocyanates, and aromatic amines. These blends are usually easier to process and have improved toughness and flexibility. The cross-link density of the network is reduced by introducing such monomers, improving the energy-absorbing ability, hence the impact performance. However, this leads to a decrease in Tg of the cured resin and a reduction in modulus. BMIs have been blended with thermoplastic polymers such as polyetherimides [52], poly(ether ether ketone)s [53], and poly(ether sulfone)s [54] to improve toughness. Thermoplastic toughening is a proven concept for brittle thermosets and has been employed in BMI resins by the leading composite materials suppliers, Hexcel [55] and Cytec [56]. BMI resins can be combined with other high-performance thermosetting resins to further enhance properties. With careful selection of stoichiometries and processing conditions, such combination can lead to the formation of co-polymers or interpenetrating networks that outperform both of the individual constituents. Combinations include BMI/epoxy, BMI/cyanate ester, and BMI/benzoxazine

Table 3.6 Representative monomers and their properties [49]. -R-

Melting point (°C)

Cure onset (°C)

Cure peak (°C)

Cured Tg (°C)

155–168

174

235

342

212

217

236

313

158–163

203

302

312

104

198

211

288

239

–

252

–

163

–

154

–

50

Design and Manufacture of Structural Composites

[49]. BMI-based composites can be processed by autoclave curing, filament winding, and resin transfer moulding. There are several ongoing studies to develop sustainable BMI resins [57–66]. This research is likely to grow in view of the increasing importance of sustainability. Highperformance thermosetting resins, depending on the chemistry, can be reacted or blended with each others to tailor properties [67–82]. This also opens up opportunities for introducing green content in synthetic resins. However, the end of life of these nonrecyclable thermosetting resins needs to be planned at the very beginning whilst designing or synthesising the material.

3.2.6 Toughened thermosets The rigid and highly cross-linked structure of thermoset resins results in limited toughness and ultimately a brittle failure. Their inherent brittleness makes them notchsensitive and prone to impact damage, causing delamination and transverse cracking, which is particularly disadvantageous for structural applications. Many works have been carried out on improving the fracture toughness of thermoset resins and their composites over the years and various approaches have been adopted. Toughened grades of thermoset resins are also commercially available for structural composite applications [83]. The most common way of toughening a thermosetting resin is by the addition of a tougher second phase, such as a thermoplastic polymer or rubber. The second phase remains dispersed in the thermoset matrix and helps with energy absorption, but this might cause a reduction in the initial elastic stiffness of the matrix. The stiffness reduction is nominal [84–86] when incorporating a high-modulus thermoplastic toughener, such as polyether sulphone (PES), but the reduction in stiffness for rubber-toughened systems can be significant. Rubber tougheners can be in the form of liquid rubber [87–90] or rubber nanoparticles [91–93], which are dispersed in the resin system before composite manufacturing commences. Simple mixing of a thermoplastic polymer with a thermosetting resin can lead to a significant increase in viscosity of the system, which can adversely affect manufacturing. Hence, a preferred route is to add the toughening thermoplastic in the interlaminar region in the form of particles, fibres, micro veils, or nanofibrillar mats. In many cases, the thermoplastic polymer dissolves in the liquid thermosetting resin and phases out during curing, forming a second phase that acts as an energy-absorbing region [94,95]. However, in some cases the thermoplastic phase remains undissolved in the liquid resin, acting as a crack arrester during crack propagation, as shown in Fig. 3.12 [96,97]. All these mechanisms have been found to successfully reduce the inherent brittleness of thermosetting resins and improve their toughness.

3.3

Thermoplastic resin systems

Thermoplastic polymers are materials that become soft and mouldable at higher temperatures, and solidify upon cooling. This potentially enables repair, welding, and a route for recycling, qualities that thermoset resins do not possess [98–103]. Although

Resins for structural composites

51

Fig. 3.12 SEM images of nylon 6 toughened samples: (a) side views of the sample toughened with PA6-d13r (top: crack path after the propagation, bottom: the area near the crack front) and (b) fractured surface of the sample toughened with PA6-d16a. Reproduced with permission from W.-T. Wang, H. Yu, K. Potter, B.C. Kim, Effect of the characteristics of nylon microparticles on Mode-I interlaminar fracture toughness of carbonfibre/epoxy composites, Compos. Part A Appl. Sci. Manuf. 138 (2020) 106073, https://doi.org/ 10.1016/j.compositesa.2020.106073.

the advantages of thermoplastics are well established, their high-melt viscosities require high-processing temperatures and pressures, which makes them highly cost-prohibitive. However, with the increasing interest in recyclability, thermoplastic composites are becoming more important in this growing field. There are several thermoplastic polymers that are used as matrices in composites, but this section covers only a selection of them for structural applications. Polyaryletherketones (PAEKs) are a family of thermoplastic polymers that exhibits high stiffness, strength, and high-temperature performance, which are considered to be the most important thermoplastic matrices for high-performance structural applications. These polymers have high viscosities, need high temperature and pressure for processing, and are very expensive. At the other end of the spectrum are in situ polymerisable liquid thermoplastic resins, which have recently been identified as an emerging solution for improving the processability of thermoplastic composites reinforced with continuous fibres. Liquid thermoplastic resins enable manufacturing of composites via conventional resin transfer moulding (TP-RTM). Moreover, these liquid thermoplastic resins are monomeric or oligomeric in nature, and therefore exhibit very low viscosities (1–10 mPa s) compared to traditional thermoplastic polymers or traditional thermosets. This ensures good wetting of the fibres, resulting in high-quality parts for structural applications. In situ polymerisation takes place during composites manufacturing and the end product is a recyclable thermoplastic

52

Design and Manufacture of Structural Composites

composite material. Caprolactam monomer leads to anionic polyamide 6 (APA6) and monomeric acrylic resins are liquid thermoplastics that have been found to be suitable for manufacturing high-performance structural composites via in situ polymerisation. PAEKs, APA6, and acrylic resins are all discussed in this chapter.

3.3.1 PEEK and PEKK 3.3.1.1 Chemical structure Poly aryl ether ketones (PAEKs) are a family of high-performance thermoplastic polymers, commonly combined with carbon fibre to produce composites for demanding applications, such as structural components for aerospace or for medical applications [99,101,104,105]. They are distinguishable from other thermoplastics by their hightemperature stability, mechanical strength, chemical and flame resistance, low density, and low production of toxic fumes when burned. The molecular backbone of different PAEKs dictates differences in properties, as these chains consist of aromatic rings connected by alternating ether and ketone groups. The ratio and order of the ether and ketone groups are therefore responsible for the naming convention of molecules within the PAEK family, as shown in Table 3.7. Ether linkages are flexible and provide chain mobility, in turn allowing for melting and processability. Ketone linkages, whilst being a stiffer functional group, share the same angle as the ether linkage and are compact, permitting some chain rotation. This allows for packing of chains without the need for identical sequencing, resulting in easy crystallisation [108]. PEEK is the most widely used polymer from the PAEK family, consisting of purely para-linkages, resulting in a highly linear backbone and thus easy chain packing. Crystallisation kinetics of PEEK are therefore fast, and crystallinities of up to approximately 40% can be achieved [108,109]. The bulky aromatic rings, along with the ether-to-ketone ratio, give rise to the high Tg and Tm of approximately 143°C and 343°C, respectively [108,110,111]. This may vary depending on the molecular weight, polydispersity, or potential impurities present in the polymer. PEKK, another polymer from the PAEK family, may be expected to have a higher melting point than PEEK due to a higher ketone content. However, it is not quite so simple, due to the existence of PEKK isomers. This material can be synthesised by combining diphenyl ether with terephthalic acid (T) or isophthalic acid (I) [106], resulting in either a para- or a meta-substitution. Varying amounts of these isomers in a backbone chain result in different grades of PEKK, which are often classified with their T/I ratio. This is the ratio of para- to meta-substitutions, or of terephthalate (T) to isophthalate (I) content. A higher T/I ratio results in linear chains, benefitting crystallisation kinetics and resulting in higher melting temperatures; whereas lower T/I ratios offer more flexible chains and lower melting temperatures, which benefits processability and manufacturing, but at the expense of the achievable crystallinity and slower crystallisation kinetics due to the introduction of symmetry defects [112,113]. Table 3.8 draws a comparison between PEEK and different grades of PEKK, and shows that whilst PEKK has a higher ketone content, the T/I ratio plays an important role in the melting temperature.

Table 3.7 Polymer repeat units for PEEK, PEK, PEKEKK, and PEKK [106,107]. Structure

Name

Ketone (%)

PEEK

33

PEK

50

PEKEKK

60

PEKK

67

54

Design and Manufacture of Structural Composites

Table 3.8 Comparison of Arkema products: PEEK and several PEKK grades [110]. PEKK

Glass transition temperature (°C) Melting temperature (°C)

PEEK

60/40

70/30

80/20

143 343

160 305

162 332

165 358

3.3.1.2 Crystallisation and morphology The characteristic properties of PEEK and PEKK are also dictated by the molecular arrangement during crystallisation. At a molecular level, polymers are arranged in the same unit cell structure, shown in Fig. 3.13a. This is the sole unit cell form for PEEK and is known as Form 1, whereas for PEKK there exists a second, less-stable form. Form 2 is favoured by cold or solvent crystallisation methods, and also becomes present with increasing chain stiffness and decreasing chain mobility, qualities that are the result of the higher ketone content in PAEKs [98,106,114–116]. A higher T/I ratio in PEKK results in an increase in the presence of this second form as well, where the stiffer para-linkages are predominant in the backbone chain. Upon melting of a sample that contains both forms, crystal structures containing Form 2 will melt at a lower temperature due to it being less thermally stable. Several projections of this unit cell have been proposed, as shown in Fig. 3.13b and c [112,114].

Fig. 3.13 Crystal packing models of (a) Form 1 (two-chain orthorhombic), (b) Form 2 (onechain orthorhombic), and (c) Form 2 (two-chain orthorhombic). Reproduced with permission from R.M. Ho, S.Z.D. Cheng, B.S. Hsiao, K.H. Gardner, Crystal morphology and phase identifications in poly(aryl ether ketone)s and their copolymers. 1. Polymorphism in PEKK. Macromolecules 27 (1994) 2136–2140, https://doi.org/10.1021/ ma00086a023.

Resins for structural composites

55

O

Amorphous region between lamellae

O O

Crystal lamellae

O O

O O O

O

O

O

O O

Crystal lamellae

O

O

Fig. 3.14 Crystalline morphology schematic, showing lamellae (crystal) and amorphous regions of a spherulite. Magnitude increases left to right [107].

PEEK and PEKK chains are arranged into unit cells when crystallising, which form individual lamellae, as shown in Fig. 3.14. An amorphous zone exists between the lamellae, made up of chain ends and entangled segments. Lamellae grow into sheaf-like structures, which further develop in and out of the plane to form spherulites [108,111,117,118]. This process is commonly referred to as primary crystallisation, and covers sporadic nucleation of spherulites through to growth impingement, where two different spherulite fronts meet and impinge. Parallel to this, a secondary crystallisation mechanism takes place, where the amorphous parts between lamellae further crystallise [99,102,119–121]. These two crystallisation processes are not independent, even though they occur in parallel. In order for secondary crystallisation to take place, there must be established lamellae with amorphous material between them [119,122]. Crystallinity influences the properties of PEEK and PEKK. Typically, a high crystallinity is desirable, as this offers the best mechanical performance and chemical resistance. The extent to which crystallinity develops is reliant on the thermal history of the material during processing: The slower the polymer is cooled, or the longer it is held at an adequate processing temperature, the more the crystalline domains develop. The lower bulkiness of PEEK enables faster crystallisation than in the case of PEKK, whose different grades result in different chain linearity and different crystallisation capabilities. The higher the T/I ratio, the more linear the PEKK molecule is and therefore the faster it is able to crystallise. This comes at the expense of a higher viscosity and higher processing temperatures needed [102,107].

3.3.2 Anionic polyamide 6 (APA6) High-molecular-weight polyamides, such as polyamide 66 (PA66), polyamide 6 (PA6), or polyamide 12 (PA12), are popular as composite matrices in engineering applications. They are generally available containing short fibre reinforcements that can be used directly for injection moulding or other shaping processes. With continuous fibre reinforcement, in situ polymerisable caprolactam is becoming popular to manufacture anionic polyamide 6 composites (APA6). The in situ polymerisation temperature is around 160°C, which is much lower than the melting temperature of

56

Design and Manufacture of Structural Composites

PA6 (
225°C). In addition, caprolactam has a much lower viscosity that PA6, which facilitates fibre wetting and rapid manufacturing. The anionic ring opening polymerisation of caprolactam into high-molecularweight polyamide-6 (PA-6) takes place via a catalysed reaction performed at 130–170°C [4]. A catalyst/initiator (typically sodium caprolactamate) and an activator (hexamethylene-1,6-dicarbamoylcaprolactam, caprolactam magnesium bromide) are used in the polymerisation reaction. Final conversions of up to 99.3% can be obtained in 3–60 min, depending on the type and amount of activator and initiator [123]. The chemical structure of a caprolactam monomer and typical catalyst/initiator/activators are shown in Fig. 3.15, with the anionic polymerisation reaction shown in Fig. 3.16. The reaction consists of three steps: (i) dissociation of the initiator (in most cases a metal caprolactamate) or ‘anion formation’, (ii) ‘complex formation’ between the

Fig. 3.15 The chemical structures of reactants: (a) caprolactam, (b) N-acetylcaprolactam, (c) hexamethylene-1,6-dicarbamoylcaprolactam, (d) sodium caprolactamate, and (e) caprolactam magnesium bromide [123].

Fig. 3.16 Anionic polymerisation of ε-caprolactam into polyamide 6 using hexamethylene1,6-dicarbamoylcaprolactam as activator and caprolactam magnesium bromide as initiator [123].

Resins for structural composites

57

initiator and the activator, and (iii) anionic ‘polymerisation’ where an anion is regenerated after every monomer is added to the growing chain [123,124]. The reaction rate primarily depends on the combination of the initiator and activator, but is also influenced by the polymerisation temperature. The processing temperature of APA6 is typically below the melting temperature (Tm) and crystallisation temperature (Tc) of the final polymer (Tm ¼ 220°C, Tc ¼ 185°C), hence both the polymerisation and crystallisation occur simultaneously [125]. The processing temperature significantly influences the reaction rate [126]. At temperatures below 130°C, the crystallisation proceeds very quickly and there is a possibility that the reactive groups get trapped inside the growing crystals before undergoing any polymerisation. This leads to a low conversion rate, leading to poor mechanical properties [124,127]. Whereas, when the starting temperature is higher, say above 180°C, the reacting mixture heats up and exceeds the melting temperature of the PA-6 polymer (Tm ¼ 220°C). This results in a very different polymer morphology with bigger crystals. When processing takes place around 145–150°C, higher levels of crystallinity (50%) can be attained [128]. Higher crystallinity leads to high mechanical properties, improved chemical resistance, and higher moisture or water resistance [129]. The crystallinity can be further enhanced by the annealing process, which is often carried out with semi-crystalline thermoplastics. The mechanical properties of the resulting PA6 polymer are very promising, with a Young’s modulus of 4.2 GPa, a maximum strength of 96 MPa, and an elongation at break of 8.5% [125]. However, processing of APA-6 needs some special attention as described below. The caprolactam monomer needs to be stored under dry atmospheric conditions to avoid the monomer flakes sintering together, due to sublimation and recrystallisation in the presence of moisture. The reactants are heated, melted, and mixed under liquid nitrogen in a heated static mixer, and then dispensed into a heated tool containing the reinforcement. Such resin transfer moulding systems are generally expensive because they require heating and pumping of the components above the melt temperature at all times whilst eliminating moisture in the system. In a recent work, an inexpensive but effective RTM system has been described in detail for polymerising an APA6 system, as shown in Fig. 3.17 [130]. The reactants exhibit very low water-like viscosities during processing and are very sensitive to pressure change. Whilst APA-6 has many excellent properties, there are some challenges associated with its processing, especially for large industrial-scale applications. Moisture: The anionic polymerisation can be very easily terminated in the presence of moisture [131]. This requires the precursor materials, the mould, and interconnecting resin transfer moulding components to be degassed and/or purged with inert gas. This becomes more challenging when infusing an APA-6 mixture into a fabric, as the fabric needs to be dried in advance to prevent premature termination. For large-scale manufacturing with this system, a method is needed for vacuumdrying multiple preforms at once. The raw materials are also hygroscopic in nature, which means that they need to be stored in a way that minimises their exposure to moisture. Storage under a nitrogen blanket, under vacuum, or in the presence of desiccant are needed, which all add cost.

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Design and Manufacture of Structural Composites

Stirrer motor Recirculation port

N2 Stirrer

Union cross

Ceramic heater

Mixing head

Fig. 3.17 RTM machine components designed for APA-6 system. ´ Reproduced with permission from J.J. Murray, C. Robert, K. Gleich, E.D. McCarthy, C.M. O Bra´daigh, Manufacturing of unidirectional stitched glass fabric reinforced polyamide 6 by thermoplastic resin transfer moulding, Mater. Des. 189 (2020) 108512, https://doi.org/10.1016/ j.matdes.2020.108512.

Quality control: The quality of the product can be difficult to control due to the aforementioned challenges relating to moisture during manufacturing, but the mechanical performance of the final polymerised product is also heavily dependent on the moisture content at any given time. This is a characteristic of most polyamides, especially for PA-6, which can absorb up to 9.5 wt.% moisture [131]. Heating and sizings: Unlike room-temperature infusible resins, the APA-6 system requires heating of the raw materials and equipment. Temperatures of over 69°C are required for the raw materials whilst temperatures of 150–200°C are required for the mould for polymerisation to occur. This adds to the expense and complexity of the process. Corrosion: The catalysts used for this type of polymerisation are highly corrosive and can lead to issues with equipment when precautionary design measures are not in place. This becomes apparent when using valves and components that contain standard O-ring/sealing materials. Many of the common materials used for this purpose such as nitrile, EPDM, and FKM are not suitable, as they corrode and/or swell in the presence of catalysts and caprolactam. Perfluoroelastomer (FFKM) is the only known sealing material that can handle these conditions; however, it is much more expensive in comparison to standard sealing materials. Low viscosity: Whilst the low viscosity of the raw materials and pre-polymerised mixture offer many clear advantages, this also means that low back pressures naturally result in the mould, which are insufficient to wet out reinforcements. To overcome this, a flow-restricting device at the outlet can be useful. Vacuum assistance is also recommended to prevent void formation and to improve wet-out.

Resins for structural composites

59

3.3.3 Liquid acrylic resins Liquid acrylic resins are another type of infusible thermoplastic resin suitable for producing large composite structures at room temperature. These monomeric liquid thermoplastic resins have low viscosities (100 mPa s), which facilitate the impregnation of reinforcement without the application of any significant pressure. The bulk polymerisation of acrylic resins was first studied in the 1940s, but only a small number of studies have reported on the reactive processing of acrylic composites [132–136]. With growing interest in infusible thermoplastics, a new liquid acrylic resin system has been introduced by Arkema, called Elium® [137]. The suitability of this system has recently been demonstrated for the production of large-scale components by resin infusion [18,19,134,135,138], including possibilities for end-of-life composites recycling [20,136]. Acrylic resins for composite matrices are based on acrylic homopolymers or copolymers, as shown in Fig. 3.18. Different grades of Elium® resins are available on the market, as presented in Table 3.9 [137]. Acrylics polymerise by free radical polymerisation via three steps: (i) First, initiation starts by radical peroxides (initiators or catalysts), which generate free radicals on dissociation in the reaction environment. These free radicals then attack the monomers and open up their double bonds (unsaturation), forming reactive radical sites on the monomers; (ii) Second is the propagation step, where the initiating monomer molecules with reactive free radical sites attack other monomer molecules and the chain propagates via covalent bonding; (iii) Finally, the termination step occurs by recombination in the presence of another radical or a termination agent to bring an end to the chain growth. Process parameters such as initiator concentration, temperature, and pressure significantly influence the rate of free radical polymerisation and consequently the resultant molecular weight, molecular weight distribution, and Tg [139,140]. Understanding their effects on the resulting acrylic polymer is an important step in manufacturing in situ polymerised acrylic matrix composites. The effect of different initiator concentrations on various properties of acrylic resins is shown in Table 3.10.

Fig. 3.18 Chemical structure of an acrylic: (a) homopolymer and (b) copolymer.

Table 3.9 A comparative table of properties of different grades of acrylic resins used in composite applications (Elium® resins). ®

Elium RT150

®

Elium 180

Elium® 188, Elium® 188 O

Elium® 280

Elium® C595 E

®

Commercial name

Elium 150

New name assignation Recommended process Viscosity at 25°C (m Pas) Process temp. (°C) Process time @ 25°C (min) Initiator/catalyst

E–150

E–RT150

E–180

E–188O

E–280

E–C595E

Infusion

Injection

Infusion; injection

–

100

100

Infusion; injection 100

100

–

Pultrusion; wet compression; winding 500

15–25 55–60

15–25 55–60

20–60 45–60

20–60 45–60

20–115 –

Adjustable

Dibenzoyl peroxide 76

Dibenzoyl peroxide 76

Dibenzoyl peroxide 66

Dibenzoyl peroxide 66

–

Peroxide blend

76

66

3.3

3.3

3.17

3.17

3.3

3.17

6 130

6 130

2.8 116

2.8 116

6 –

– –

–

–

3.83

3.83

–

–

130

130

111

111

–

–

3.25

3.25

2.91

2.91

–

–

Tensile strength (MPa) Tensile modulus (GPa) Elongation (%) Compressive strength (MPa) Compressive modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa)

´ Bra´daigh, D. Ray, Continuous fibre-reinforced thermoplastic acrylic-matrix composites prepared by liquid resin infusion – a review, Reproduced with permission from W. Obande, C.M. O Compos. Part B Eng. 215 (2021) 108771, https://doi.org/10.1016/j.compositesb.2021.108771.

Table 3.10 Influence of various initiator concentrations on polymerisation behaviour of liquid acrylic resins. Polymerisation

Rheology

Transition

Decomposition

Initiator content (wt %)

T100% (°C)

Tonset (°C)

Tmax (°C)

2ΔH (J/g)

Tgel (°C)

Tg (°C)

Tonset, Peak 1 (°C)

Tonset, Peak 2 (°C)

0.8 1.2 1.6

199.3 122.4 163.9

75.9 57.9 74.0

82.6 70.4 81.8

129.4 150.7 210.7

87.0 84.4 87.5

116.2 123.4 123.3

113 115 108

298 302 326

´ Bra´daigh, D. Ray, Continuous fibre-reinforced thermoplastic acrylic-matrix composites prepared by liquid resin infusion – a review, Reproduced with permission from W. Obande, C.M. O Compos. Part B Eng. 215 (2021) 108771, https://doi.org/10.1016/j.compositesb.2021.108771; O.A. de Raponi, L.C.M. Barbosa, B.R. de Souza, A.C. Ancelotti Junior, Study of the influence of initiator content in the polymerization reaction of a thermoplastic liquid resin for advanced composite manufacturing, Adv. Polym. Technol. 37 (2018) 3579–3587.

62

Design and Manufacture of Structural Composites

Despite having several advantages, there are some challenges associated with this resin. Polymerisation condition: In situ polymerisation should be carried out in a closed environment in the absence of air, as oxygen adversely affects the polymerisation reaction. The presence of air will cause variation in chain length and molecular weight distribution, leading to polymerised products showing variation in their properties. The reliability of the product will consequently be reduced. Exotherm: The exotherm released during the polymerisation of acrylic resins is a concern for thick composite parts. The generation of high exotherm temperatures can lead to boiling of the monomers during infusion, causing voids within the composites that are detrimental for structural applications. Some specific low-exotherm grades of acrylic resin are recommended for thick parts (available in the market), or exothermic control agents can be used. Odour: This resin has a strong odour of acrylic monomer and is unpleasant to work with. It is highly volatile and strong extraction is needed during working with this resin.

3.4

Resin characterisation

This section discusses characterisation techniques that are used to determine the properties of both the liquid resin and the corresponding cured material. The principles of these techniques will not be discussed, but their application to measure the resin properties in the context of composite manufacturing and performance will be. More detailed descriptions of the characterisation techniques can be found in Ref. [141].

3.4.1 Rheometry The flow and curing characteristics of liquid thermosetting resins can be investigated by rheometry [142]. An oscillating parallel plate rheometer is generally used where a small amount of liquid resin (mixed with catalyst) is placed between the parallel plates (Fig. 3.19). The change in viscosity can be measured as a function of temperature at a fixed shear rate, or as a function of shear rate at a fixed temperature, to investigate shear thinning [143]. The temperature at which the resin viscosity drops to a minimum value indicates the suitable temperature for liquid resin infusion, providing the best conditions for the resin to permeate into the reinforcement fabric. The liquid resin starts to cure over time at a fixed temperature and gets converted into a semi-solid gel, before finally becoming a rigid cross-linked solid [144]. The time available before the start of the cure reaction indicates the processing window, as shown in Fig. 3.20. The maximum time available corresponds to the lowest processing temperature (T5) for liquid resin infusion, whereas the rate of curing reaction is highest at the highest processing temperature (T1). The shear storage modulus (G0 ), shear loss modulus (G00 ), and viscosity are generally plotted as a function of temperature. The crossover point of the shear storage

Resins for structural composites

63

Fig. 3.19 A parallel plate rheometer.

106 105

viscosity η* [Pas]

104

T1

3

T2

2

T3 T4 T5

10 10

101 T1>T2>T3>T4>T5 100 10–1 10–2 10–3 103

104

time t [s]

Fig. 3.20 Isothermal curing of thermosetting resins at different isothermal temperatures [144].

modulus and shear loss modulus curves indicates the gel point [145], as the resin undergoes the curing reaction (Fig. 3.21). Ideally, the test should be terminated just after gelation before the formation of the cross-linked solid to avoid any damage to the machine. In the case of thermoplastic polymers, change in shear viscosity is often measured as a function of shear rate at a

64

Design and Manufacture of Structural Composites

106

Approximate gel point

Moduli G', G" [Pa]

106

105 G’ 5

10

G” η* 104

104

Complex viscosity η* [Pas]

107

Minimum viscosity

103

103 80

100

120

140

160

Temperature T [C]

Fig. 3.21 Determination of gel point as a crossover point of shear storage and shear loss modulus. Gel point is the practical limit for processing. Redrawn after J. Gotro, Rheology of Thermosets Part 3: Controlled Strain Measurements, 2014. https://polymerinnovationblog.com/rheology-thermosets-part-3-controlled-strain-measurements/.

fixed temperature (at or above the melting temperature) or as a function of temperature at a fixed shear rate [146]. Fig. 3.22 shows a generic representation of the reduction in viscosity as a function of shear rate for different grades of PEEK. The viscosity profile generated by this technique provides a guideline for processing these high-viscosity materials for large industrial applications.

3.4.2 Differential scanning calorimetry The curing reaction of a thermosetting resin, or the exotherm released during curing, can be measured using differential scanning calorimetry (DSC) [141]. A representative DSC graph is shown in Fig. 3.23. Curve 1 shows enthalpy ΔH released during heating or curing of a thermosetting resin. If a partially cured sample is run in the DSC, the residual curing reaction takes place (Curve 2) and the exotherm released (ΔHR) is used to calculate the degree of cure in the starting material. The degree of cure is calculated using the following equation: α¼

  ΔH R  100 ΔH T

(3.1)

where ΔHT is the enthalpy released during 100% curing of the sample. A similar residual exotherm is released during a post-curing cycle and the exothermic enthalpy can

Resins for structural composites

65

100000 PEEK Grade 3

Shear viscosity (Pa-s)

PEEK Grade 2 PEEK Grade 1

10000

1000

100

10 0.01

0.1

1

10

100

1000

Shear rate (s–1)

Fig. 3.22 Shear viscosity of various PEEK grades at 380°C. Molecular weight increases with the PEEK grade number. Redrawn after M. Yuan, J.A. Galloway, R.J. Hoffman, S. Bhatt, Influence of molecular weight on rheological, thermal, and mechanical properties of PEEK, Polym. Eng. Sci. 51 (2011) 94–102, https://doi.org/10.1002/pen.21785.

Heat Flow End Down

Cure exotherm measured from peak area

DH

Curve 1

DHR Curve 2 Curve 3

–40

300 Temperature (°C)

Fig. 3.23 Epoxy curing exotherm results measured in DSC.

66

Design and Manufacture of Structural Composites

be used to measure the degree of cure that happened during the standard cure cycle before post-curing. The absence of any residual curing (exothermic) peak on a DSC run confirms complete curing of the sample (Curve 3). This is often used as a quality control tool to ensure complete curing of thermosetting resins. However, only small resin samples (5–15 mg) are tested in the DSC, which may not be representative of the exotherm generated from the bulk resin during composite processing, but the results are still indicative to optimise manufacturing. For thermoplastic polymers or thermoplastic composites, DSC is used to determine the effect of cooling rate on the crystallinity development in the structure. This approach is applicable to all semi-crystalline thermoplastic matrices such as PEEK, PEKK, or polyamides. A typical cycle is shown in Fig. 3.24, where a material sample is held above the melting temperature, then cooled at the designated cooling rate, followed by a heat scan to determine the crystallinity development during cooling. Fig. 3.25 provides an example of a heat scan for carbon fibre/PEKK after being exposed to different cooling rates. At the low cooling rate (R1), the sample shows a typical melting endotherm near the melting temperature. Whereas for faster cooling rates (e.g. R4), a cold crystallisation peak develops, indicating that there is more amorphous content present in the sample, which crystallises during this heating scan. The sample therefore contains a lower overall crystallinity due to the faster cooling rate. The level of crystallinity developed in a thermoplastic composite during manufacturing directly influences the mechanical performance. The degree of crystallinity can be calculated using the following equation [147]: χ¼

ΔH m  ΔH cc  100 α  ΔH 100%

(3.2)

where χ is the percentage crystallinity, Δ Hm is the melting enthalpy, Δ Hcc is the cold crystallisation enthalpy, α is the weight fraction of the matrix content (if the material is not a composite, α ¼ 1), and Δ H100% is the theoretical melting enthalpy of a 100% crystalline polymer. Δ Hm and Δ Hcc can be obtained by calculating the area under the respective peaks from the DSC heat scan; Δ H100% can be obtained from the literature. 400 350

Temperature (°C)

Fig. 3.24 Example of a sample’s thermal history undergoing a DSC crystallisation analysis. The dotted line indicates the cooling rate segment of the cycle, which will vary depending on the analysis.

300 250 200 150 100 50 0 0

10

20

30

Time (min)

40

50

Resins for structural composites

67

Cooling rates (R1