Capturing carbon fibre

Lou Reade reports.

Plastics recycling continues to hit new highs, but this only applies to thermoplastics. Because they are easily re-melted, the process of separating the thermoplastic matrix from any fibre reinforcements is relatively straightforward. This is not the case for thermoset composites. Because the cross-linked matrix cannot be re-melted, the separation process is far more difficult. Little wonder, then, that carbon fibre composites have traditionally been sent to landfill. But this is beginning to change, as a number of techniques to recycle carbon fibre composites have been developed. With carbon fibre costing around £10,000 per tonne, anything that can be done to find a cheaper source of material will be welcome. At the same time, growing use by industries such as wind power and automotive means that even more carbon fibre will be consumed in future. Relight my fibre There are two elements to a carbon fibre composite: the carbon fibre itself; and the thermoset matrix - usually epoxy resin - in which it is trapped. “In carbon fibre recycling, you may accept that you can’t get anything useful back from the epoxy resin,” says Steve Pickering, associate professor in mechanical engineering at Nottingham University’s department of mechanical, materials and manufacturing engineering. “You get rid of it thermally, and recover the expensive bit – the fibre.” Composite parts – be they aircraft wings or F1 car bodies – are shredded, and heated in the absence of oxygen (pyrolysis). This breaks down the epoxy resin, leaving the carbon fibres behind. The fibre usually accounts for around two-thirds of the weight of a component. It is likely that parts made using recycled fibres will have lower mechanical properties than the original components. “Nobody is saying that you would take carbon fibres from a component, and use them in the same application again,” he says. “You’re not going to see wings recycled into wings.” The reason for this is simple: when the parts are shredded, the continuous carbon fibres are broken into short lengths, so the composite no longer exhibits the high strength and stiffness of the original material. There is already a healthy market for this sub-millimetre length ‘milled carbon’ – an anti-static additive for thermoplastics is just one application. The rising availability of recycled milled carbon reduces the need for ‘virgin’ material. “We’re looking at ways to process these short carbon fibres into composite materials with good properties – but they’ll never be as good as ‘virgin’ components,” he says. He adds that ready availability of cheaper recycled carbon fibre could make the material available for new applications. “Right now, you can’t always justify the use of carbon fibre in aircraft interiors. But that might change if the cost came down,” he notes. As a member of the Technology Strategy Board-funded Affordable Recycled Carbon Fibre (AFRECAR) project, Pickering produced some demonstrator technologies for this year’s JEC Composites show – including an interior panel for an aircraft. And there are plenty of potential applications in other industries – which explain Ford’s presence as an AFRECAR partner. Others include Advanced Composites Group (now UMECO), Boeing, Toho Tenax and wind turbine producer Vestas. Many variants of the pyrolysis technique are being investigated, in the hope that they could be used to reprocess carbon fibre composites cheaply and efficiently. Pickering’s own approach is a thermal process-based on fluidised beds to try and remove the epoxy matrix. But it’s not all at the laboratory scale. Several commercial operations (including ELG Carbon Fibre in the UK) are already recycling carbon fibre, using a variety of pyrolysis techniques. And aircraft giant Boeing has recently invested £0.6 million per year in strategic research collaboration with Nottingham University to take carbon fibre recycling further. “They want to see how they could re-use carbon fibre from within their manufacturing processes, as well as from end-of-life products,” states Pickering. The two organisations have worked together since 2006. The initial funding will last for three years, though the intention is to make it a rolling programme. “We’re currently looking at ways of getting the carbon fibre back from the composite,” he adds. “The next issue is to find ways of using this recycled material in practical applications – because it’s so different to the virgin material.” Less random, more tandem One technique being investigated at Nottingham is how to make the orientation of these short fibres less ‘random’. With continuous fibre, the greater orientation means that fibre strands can pack together closely – giving a fibre volume fraction (the amount of fibre in the component) of around 60%. Randomly oriented fibres, such as the recycled CF, will not pack together so closely. Pickering is investigating how the orientation of the short fibres could be improved – which would again help to produce stronger components. He is using ‘papermaking’ techniques to disperse recycled fibres in a slurry, which is then passed through a wire mesh to create a non-woven fabric. In order to raise the volume fraction, he is dispersing the fibres in liquid and looking to manage its flow in order to align the fibres. “We’re making some progress in the lab, but it’s still early days,” he says. And, in common with other researchers, he is also investigating how chemicals – rather than pyrolysis – could be used to recover the carbon fibre. “A number of teams, including ours, are using supercritical fluids to dissolve out the resin and extract the carbon fibre,” he continues. A supercritical fluid is a special state of matter, which blurs the boundary between gas and liquid. This often makes it an effective solvent: supercritical carbon dioxide, for example, is used to decaffeinate coffee beans. For his part, Pickering is using supercritical propanol to extract carbon fibre. Although using supercritical fluids to recover carbon fibre is still at a very early stage, he says that it has potential advantages over pyrolysis. For example, it allows the recovery of some chemicals from the epoxy resin. “With pyrolysis, there’s also some degradation in the recovered carbon fibres in terms of strength and stiffness compared to virgin material,” he says. “Using supercritical fluid, the indication is that there is less degradation.” www.nottingham.ac.uk