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Closed Loop Recycling of Carbon Fibre Reinforced Plastic

Carbon fibre reinforced plastics are strong, lightweight, chemically resistant, and are essential materials for a more sustainable future. However, their complex nature makes them very difficult to recycle, meaning most of the 200,000 tonnes made each year, will end up in a landfill. To address this, we have developed a closed-loop recycling process.

Carbon Fibre Lab Work | University of Birmingham

Discover our work on Carbon Fibre reinforced plastics.

Carbon Fibre Lab Work | University of Birmingham

Transcript

Transcript

We produce 200,000 tons of carbon  fibre reinforced plastic every year. These materials can be used to store hydrogen,  a clean fuel for a Net Zero  future, in tanks like this.

However, they often end up in landfill. This isn't sustainable, so instead we  can recycle it in a reactor - breaking  down the plastic to recover valuable  chemicals and clean carbon fibres. We carefully measure out the right amount of  solvents - here, we are using acetone, water and  acetic acid, all of which are considered non-toxic  and green as they can be made from biomass. We pour this solvent mixture  into a reactor which already  contains the carbon fibre reinforced plastic. The reactor lid is then  placed on top of the reactor. We check it to make sure that it  is sitting in the right position. Once the lid is fitted correctly,  clamps are mounted onto each side to  hold the lid down onto the reactor unit.

A torque wrench is then used to  tighten the bolts in the clamps,  meaning the reactor is fully sealed  and can withstand the pressure inside. For safety reasons we operate the  reactor inside a fume cupboard. The reactor is heated to 260° and held  at that temperature for two hours. After this time, we loosen the screws taking  care not to damage the clamps or the reactor lid. After taking off the clamps, we can remove the lid  and take a look inside the reactor. Here, we can see a dark liquid  containing the valuable organic chemicals. This is a mixture of the solvents and monomers  which come out of the composite material.

We can recover these for later separation,  reusing the solvents in further reactions and  processing the monomers into new plastics. As well as these organic products we can  also recover long, strong carbon fibres. These long fibres can be wound onto  a pin or a mandrel - the larger the  waste composite which was put into the  reactor, the longer these fibres will be. By converting waste composites into  reusable carbon fibres and monomers,  we prevent this crucial material from going to  landfill, and make composites more sustainable.

  • Whole CFRP pipe section, before recycling

    Carbon fibre reinforced plastics (CFRPs) are made of two main components: the carbon fibres which provide strength and stiffness, and a plastic which holds the fibres together. Currently, about 200,000 tonnes of CFRP are made each year, and that amount is only expected to grow. As they are so strong and chemically inert, they can be used to store and transport hydrogen; a fuel which is zero-emission at the point of use, and therefore essential to the decarbonisation of our energy system. CFRP pipes, a section of which is shown in Figure 1, can be made out of carbon fibres and a type of plastic called polyamide-6 (PA6).

Figure 1: Whole CFRP pipe section, before recycling.

Unfortunately, the growing amount of CFRPs will only lead to an increasing amount of waste and it is, therefore, essential that a closed-loop recycling process can be developed. Currently, mechanical recycling (typically shredding and sieving) results in very short, low-value fibres and a resin-rich fraction, thus significantly downgrading the material. Other commercial processes involve pyrolysis, a high temperature, energy intensive process which can recover carbon fibres, but often the plastic is lost. That’s where our research in the School of Chemical Engineering comes in: Assistant Professor Dr Matt Keith and undergraduate researcher Preston Sweeney have developed a medium-temperature process which is able to recover both carbon fibres, and a chemical called ε-caprolactam. This is the monomer to PA6, meaning that when it is purified, it can potentially be reused to make virgin-quality plastic.

The process is based on solvolysis. Here, a solvent mixture consisting of water, acetone, and acetic acid was used to depolymerise the plastic. These chemicals were chosen because they are safe, non-toxic, cheap, and can be made from renewable resources. The solvent composition, reaction time, temperature, and pressure were optimised in a small reactor, and then the process was scaled up so that a whole pipe section could be recycled. The long carbon fibres recovered are much more valuable than short fibres; they can be used to make stronger composite materials and so avoid being downcycled. Initial strength tests of the carbon fibres showed that they were similar to virgin fibres, while chemical analysis of the solvent showed high concentrations of ε-caprolactam, which you can see in the diagram below.

Diagram of closed loop recycling of CFRPs

Figure 2: Diagram of closed loop recycling of CFRPs

What Next?

This project has demonstrated that a CFRP can be recycled with long fibres, and valuable chemicals, recovered. Preston has since returned to his undergraduate degree and is currently undertaking a placement at Valero in Pembrokeshire. Dr Matt Keith, meanwhile, is pursuing further research looking at the downstream processing of CFRPs, including how the organic products can be separated and re-used. He plans to make secondary composite materials, test them, and further prove out this technology to drive greater circularity within the CFRP industry. 

Contact us

  • Dr Matt Keith

    Assistant Professor

    Profile of Matt Keith, Chemical Engineering, Univeristy of Birmingham