Sky-high ambitions for carbon fibre re-use

It is now possible to recover carbon fibre from composites and use it again in new composites, with minimal loss of properties – it is even happening in the aerospace industry. While most re-use tends to occur in less demanding applications than the original ones, carbon fibre from a high-performance aerospace component has been trial recycled into an aerospace component for the same aircraft. Currently, world production of carbon fibre is around 25,000 tonnes per year, but that is growing. Not only are aerospace companies using more carbon fibre in civilian and military aircraft, but demand from the makers of upmarket cars, bicycles and sports equipment is on the increase as well. Tony Carroll, development manager of Milled Carbon, describes carbon fibre as being extremely expensive and in short supply. Until now, recycling of carbon fibre components has been regarded primarily as a problem incurring costs, but Carroll takes a different view, seeing it as a valuable resource. Speaking at the recent Resource Efficiency KTN conference, ‘Designing out waste’, he recalled how Milled Carbon began life as “a meeting between two people in a pub” in 2003, but had now grown to 10 people, with headquarters at Henley in Arden, and production facilities in West Bromwich and Wednesbury. Clearly, the company has found the right brew with its suite of processes, which involve breaking down the resin with the assistance of heat. Offcuts and components that have failed inspection are treated by pyrolysis – ie, heating in the absence of air under tightly controlled conditions, so that the carbon is not combusted. By contrast, end-of-life components are treated in a fluidised bed technology, where fibres and resin are separated at high temperatures, energy is extracted from the polymer and the fibres are left in a clean condition, but with slightly reduced properties. Meanwhile, research into suitable processes is being conducted in partnership with the University of Nottingham, which is leading the HIRECAR – High value composite materials from REcycled CARbon fibre - collaborative programme. According to Carroll, there is a component under test in an F18 fighter plane that is made from carbon fibre recycled from scrap from F18 production. Research using F18 materials shows that fibres recovered and cleaned with alcohol in an ultrasonic bath for 45 sec are almost as good as those originally supplied, both in terms of strength and ability to bond to binders. However, the main markets for recycled carbon fibre are presently seen in components with less demanding mechanical requirements, which can function with chopped or milled carbon fibre, as opposed to long lengths. The main Milled Carbon products are chopped fibres in 1mm to 12mm lengths, and 50 micron to 500 micron milled fibre. Typical applications are moulded non-critical components for airliners where weight saving is still very important, such as in seats and cabin fittings, or to provide electromagnetic shielding to plastic enclosures, or provide anti-static properties to integrated circuit trays. But the scope for broadening its use is highly encouraging. Research is under way, for example, into using recycled carbon fibres in ceramic brake disks, spun yarn and bipolar plates in fuel cells. The company is also working with Nottingham University on breaking down composite matrix materials, using microwaves and radio frequency heating. The main obstacle is that optimum process conditions require having some knowledge of the fibre matrix, which is often a commercial secret that manufacturing companies wish to keep to themselves. There is also a requirement to ensure carbon fibre components made with different matrix materials are kept separate and Carroll stresses the need to educate recyclers on this point. Pointers * Carbon fibre recovered from scrap and from end-of-life components is already being supplied as chopped or milled product * Suitable applications include chopped fibre reinforced plastic components and those requiring electrical conductivity to provide electromagnetic shielding or anti static properties * Research is underway on how to recycle carbon fibre into more mechanically demanding applications