Composites are designed to be stiff, strong and durable, which enables light weight structures for transport and critical technologies like wind turbine blades and high pressure tanks. It also means they last a long time. Wind turbine blades are routinely recertified for extended lifetimes, composite boats and ships are often refurbished for reuse and roofing and cladding materials can have three or more times the lifetime of steel alternatives. But the same excellent properties make composites inherently resistant to being broken down for recycling.
It is sometimes said that composites can’t be recycled but this is not the case. Both carbon (CFRP) and glass fibre (GFRP) composites are being recycled commercially, and have been for over a decade. Supply chains for commercial recycling routes are still limited, processes are generally not closed-loop and economic viability is challenging, but solutions have emerged and continue to be improved for gaining value from end of life composite materials, contributing to a circular economy.
CFRP scrap typically goes through a partial pyrolysis process which decomposes the resin, leaving clean carbon fibres. These short fibres can be used in injection moulding, or made into fabrics, either commingled with thermoplastic or ready to be infused with resin to make new parts. Several other fibre recovery processes are in development or commercial at small scale, as well as short fibre alignment technology which offers a higher value product.
Glass fibres are lower in value, so a different approach is taken. In some cases, ground GFRP is used in-house in new products. GFRP scrap can be co-incinerated in cement kilns, where the polymer is burnt for energy, and the glass (aluminosilicate) and any calcium carbonate filler are recycled into cement clinker, displacing mined resources. Increasingly, GFRP waste is being sent either to energy from waste or to cement kilns instead of landfill. Work is under way to commercialise thermal processes for GFRP, which becomes more viable with economies of scale.
Natural fibre composites can be ground and used as filler, but as they are first-generation bio-derived, it makes sense to reclaim the embodied energy by combustion. Thermoplastic composites can be shredded and compounded for re-use in injection moulding.
Useful references
A description of composite end of life solutions is given in chapter 8 of Composites UK’s Sustainability of FRP Composites – A Good Practice Guide, 2022. (Download link via a short survey). Chapter 2 introduces design for circularity principles, chapter 4 addresses use of recycled materials, and chapter 9 covers policy and regulation.
The FRP Circular Economy Study, 2018, assessed the best way forward for disposal/recycling of larger scale fibre reinforced polymer waste in terms of cost and environmental impact in the UK.
Composites Recycling – Where are we now?, 2016, produced by Composites UK with the universities of Birmingham, Nottingham and Manchester gives more details of the recycling processes available for carbon and glass fibre waste.
Current commercial solutions, available in the UK, for reuse and recycling of composites and composite manufacturing scrap can be found at What Can I Do with my Waste?
Commercially available materials, supplied by Composites UK members, with recycled or bio-based content are listed at Materials with Bio/Recycled Content.