Composite is a broad term covering a range of materials, from those used to build boat hulls all the way up to military fighter jets. For the highest performance composites, the raw materials employed are called prepreg.
This is a woven or unidirectional fabric of fibres pre-impregnated with thermosetting resin. The fibres are likely to be any advanced fibre such as carbon, Kevlar or indeed glass. The resin such as epoxy or phenolic will have a high viscosity, therefore a high molecular weight and excellent mechanical properties.
It is B-staged, which means the resin is catalysed and continuously curing, and needs to be stored in a freezer to slow down the curing reaction. If left outside a freezer their life is typically only 28 days.
To manufacture a composite component, the prepreg is cut into shapes that suit the component, laid up to the required thickness, placed in a vacuum bag and then cured under heat and pressure in an autoclave. They deliver the highest quality in terms of fibre volume fraction (>55%), void content (<0.5%) and mechanical strength.
The downside of these materials is that they are expensive (around £40 per sq m), have a short life (typically one year in a freezer) and, with recent industry consolidation, are available only with large minimum order quantities (25-50sq m) and long delivery times (more than three months).
racket and cork
Large consumers such as the aerospace and automotive industries can cope with these constraints, but for research organisations, particularly universities and SMEs, it can lead to large amounts of waste prepreg. Often only a few metres of material are required, leaving the remainder of the roll to sit in a freezer until it expires.
Prepreg waste derives from two main sources:
- Offcuts from manufacture as a result of cutting irregularly shaped plies. It is estimated that 30-50% of prepreg is wasted during production, which is 3,000 tonnes of carbon fibre and 15,000 tonnes of glass fibre waste a year in the UK.
- Rolls which have exceeded their out-life (number of days outside a freezer) or gone past their use-by date.
Offcuts contain uncured resin. This is treated as contaminated waste and is expensive to dispose of. These prepreg offcuts generally have the plastic film removed, are stacked into large lumps, placed in a vacuum bag and cured off in an autoclave prior to disposal in landfill.
Research has demonstrated that carbon epoxy prepreg rolls retain their mechanical properties well beyond their end of life, although they are not appropriate for certified applications such as aerospace. The roll can be returned to the manufacturer for relifing, where it is assessed with various analytical techniques and returned with a new use-by date. But in most cases these rolls sit in the freezer until an appropriate use arises or an opportunity to donate the roll to another cause presents itself. For example, universities and particularly Formula Student racing teams are always looking for free prepreg material.
Recycling is another option using the continuous pyrolysis process developed by West Midlands-based ELG Carbon Fibre, although careful process control is required to prevent fibre deterioration. The resulting prepreg rolls are likely to have imperfections including fibre angle variations and damaged areas.
But ELG is currently accepting waste for closed loop recycling for customers who want the recyclate back afterwards. In this way the provenance of the recyclate can be guaranteed, and its value is greater than more generic recycled carbon fibre from rolls and end-of-life components. Supply chain security remains a major obstacle for scale up. Beyond recycling, the only remaining option is disposal via landfill which is becoming increasingly expensive.
Sustainability was a major theme in the 2016 UK composites strategy. It discusses the need to incorporate circular economy principles by keeping these materials in use at their highest value for as long as possible. It identifies a key target of reducing composite waste by considering sustainability when developing materials and the importance of developing the market for recyclates.
Other aspects of interest are the potential to bring industrial biotechnology into the sector to assist with enhancing the properties of natural fibre composites and developing new resins designed for recycling.
Unused rolls of prepreg represent an opportunity to prevent the waste of valuable and energy-intensive material by making them available to other users. In response to this, a website has been established by the Composite Systems Innovation Centre at University of Sheffield in partnership with a local digital development company, Razor.
Prepreg is a small percentage of the over-all market for composite materials, so it is worth looking at the wider industry and particularly end-of-life options for composite components. As such materials have become cheaper, many new applications have appeared in products with a life of less than 10 years, such as sporting goods and cars. So large volumes of carbon fibre products are now coming to their end of life and, with legislation such as the End-of-Life Vehicles Directive, it is necessary to recycle.
Recycled carbon fibres are an order of magnitude more valuable than glass fibres, making them a more attractive target for recycling.
Recycling of carbon fibre is the focus of ongoing research and there are a number of options under development, of which only pyrolysis is running commercially. In this process, components are crushed or shredded and put through a furnace to burn off the resin, leaving intact fibres albeit with reduced mechanical properties. Ideally, the resin would provide all the energy for the process although this is not yet possible.
Variations on pyrolysis use either a fluidised bed or microwaves to heat the material. These claim energy savings and reduced fibre damage, although all pyrolysis processes require careful control to retain the original properties of the fibre.
Other recycling routes include mechanical size reduction using a hammer mill. This leaves flakes of fibre and matrix that can either be burnt for energy recovery or used as a filler in moulding compounds. Research is investigating how these recycled fillers can actually improve the fracture toughness of composite materials by arresting cracks.
Of great interest are novel solvolysis-type processes which break down the resin into lower molecular weight chemicals using supercritical solvents such as water, ethanol and propanol. This enables value to be extracted from both the resin and the fibre without use of high temperatures or forming char on the fibre.
Finding uses for the recycled carbon fibre is the subject of intense research focused on powdered material typically 500 microns, short fibres of around 6mm and long fibres greater than 100mm. The powder and short fibres can be compounded with thermoplastic such as polypropylene or nylon, then extruded, chopped and injection moulded.
This has the potential to manufacture recycled carbon components that are cost-competitive with virgin glass fibre. The longer fibres are more challenging but thin veil, which is either oriented or random, can be manufactured using a variation of the paper-making process. More recently carding has been employed to make recycled carbon semi-aligned mats that can either be prepregged or commingled with thermoplastic, presenting a material that could be a promising route to use of carbon fibre recyclate at high volume.
It’s an exciting time for the recycling of composites, and the market is likely to grow in line with the increase in composite use we have seen during the past two decades and predicted demand.
An online tool for materials reuse
The Composites.Exchange website has been developed to address the high-performance end of the composites market. It is a tool designed to help sharing of these high-value materials and prevent them having to be recycled or landfilled.
From a research perspective, the ability to use a wide range of fibres and matrices is desirable because it helps to determine and model the effect of fibre and matrix on the performance of the composite in a wide range of static and dynamic conditions. A typical test panel requires approximately 1sq m of material, so purchasing a whole role is extravagant.
The website aims to give users rapid access to a library of shared materials which may include complete rolls or offcuts of prepreg and ultimately help them to find a use rather than ending up in landfill. It is also hoped that the site will facilitate collaboration between users and be a simple tool to assist with material management.
carbon epoxy woven
James Meredith is lecturer in composites in the Department of Mechanical Engineering at Sheffield University