Growing use of long fibre-reinforced polymer (FRP) composites is increasing the need to develop recycling routes and supply chains for these materials. Several solutions now exist to recycle carbon fibre waste back into high value products, and research is ongoing to improve the value of the recyclate, says trade association Composites UK (Hemel Hempstead, Herts / UK; www.compositesuk.co.uk) in a new report. Composites are by nature strong, durable and non-homogeneous, which makes them inherently difficult to recycle and commercialisation of recycling routes has been challenging, says the report.

The global market for composite products is expected to reach USD 95 bn (EUR 86 bn) by 2020, an increase of 40% from 2014. Inevitably, this results in more waste from manufacturing and creates an increasing challenge to develop economically sustainable end-of-life recycling routes, says the report, which focuses mainly on the recycling of FRP composites with a thermoset polymer matrix but also covers thermoplastic composites. End-of-life waste for carbon fibre-reinforced polymers (CRP) is still small, but production waste can represent 30-50% of production volumes where prepreg processes are used, resulting in an estimated 2,000-3,000 t/y of CRP waste in the UK alone. Glass reinforced polymers (GRP) manufacturing waste has been estimated at about 15,000 t/y in the UK (approximately 10% of manufactured parts, estimated to be 150,000 t in 2014 for the UK and Ireland). GRP end-of-life waste is estimated at around 50,000-60,000 t/y, says Composites UK.

A supply chain for recycling CRP using a pyrolysis process is now becoming established, led by ELG Carbon Fibre (Coseley, West Midlands / UK; www.elgcf.com) in the UK. Intermediate products with recycled carbon fibre have been developed for milled and chopped applications in thermoplastic and thermoset compounds and coatings, non-woven textiles and 3D preforms, says Composites UK. “The next challenge is for increased take up of these intermediates in high volume applications,” it adds.

GRPs remain more difficult to recycle economically, and the most promising route for end-of-life applications is to co-process with refuse-derived fuel in cement kilns. This process is available in Germany, and cement kiln co-processing and energy from waste solutions are starting to become available in UK. Waste management consultant Agecko (Easingwold / UK; www.agecko.com) has worked with Composites UK members to progress this process, as well as direct re-use of GRP waste in temporary road surfaces on landfill sites. In addition, regrind from manufacturing waste can be incorporated into various processes including spray-up, casting and moulding, providing some reinforcement value from short fibres or flakes, says the report. “Some companies operate this in-house, though grinding to fine filler is not commercially viable because of the very low cost of virgin fillers,” it adds.

Other processes being developed for recycling FRPs include chemical processes that can recover value from the resin, such as a solvolysis chemical/thermochemical process used for recycling CRP using a heated solvent or solvent mixture to break the resin into lower molecular weight chemicals. Variants of pyrolysis can also be used, for example within a fluidised bed or using microwave energy.

The energy demand for chemical processes is relatively high, although it may be offset by the potential to gain value from the resin as well as fibres, notes the report. Pyrolysis is in the intermediate range in terms of energy demand, but only requires about 10% of the energy input required to produce virgin carbon fibre. Mechanical grinding uses very little energy in comparison, but produces a lower value product.

Thermoplastic composites can be shredded and recycled by melting, although the existing supply chain is limited. Epoxy resins that are easier to recycle have been developed, where the composite can be degraded in low temperature chemical processes to release fibres and downgraded resins suitable for use as thermoplastics or adhesives. Dry fibre waste has been frequently overlooked, says the report, but adds up to around 15,000 t in the UK, from the manufacture of raw fibres and intermediates to offcuts in ply cutting. Some textile products manufactured from dry carbon fibre waste are commercially available and small but increasing amounts of glass fibre waste are used in infrastructure/timber replacement products.

As well as looking at the recycling of FRP waste, other wastes from the composites industry are being tackled, including clean fibres and consumables. For example, recycling routes are being developed for breather fabrics, films and silicone release paper. Another route involves mixing glass fibre manufacturing waste with waste plastics to make rail and coastal infrastructure products, Composites UK said.

Stella Job, supply chain and environmental manager at Composites UK and one of the report’s authors, commented, “It is important for Composites UK as a trade association to work with our own industry and the waste management sector to minimise any negative environmental impact and gain the best value from our waste. We are committed to supporting new processes and supply chains which will help us achieve this.”

The Composites UK report, entitled “Composites recycling – where are we now?”, incorporates data compiled by the Exhume (efficient X-sector use of heterogeneous materials in manufacturing) research partnership between the UK universities of Birmingham, Cranfield, Exeter and Manchester, and funded by the UK's Engineering and Physical Sciences Research Council (EPSRC).

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