Recycling carbon fibre composites
Dr Stephen Pickering at the University of Nottingham, UK, looks at the mechanisms available to close the loop on carbon fibres.
Carbon fibre’s mechanical properties make it one of the best structural materials for use where low weight is required. Consequently, it is increasingly used in the aerospace industry where weight reduction lessens fuel consumption and emissions. New aircraft now being developed contain 50% carbon fibre composite by weight.
In wind turbines, carbon fibre is becoming a popular choice. In these and many other structural applications, carbon fibre is used with epoxy, a thermosetting resin, as the matrix.
Recycling such composites is not straightforward – thermosetting resins cannot be remoulded and so the material cannot be recycled in the same way as thermoplastics. Furthermore, the high volume fraction of carbon fibre reinforcement makes reprocessing problematic.
Most waste ends up in landfill, yet there are strong incentives to recycle carbon fibre composites – it is a valuable material with prices starting at £10,000/t, and carbon fibre manufacture is energy intensive, requiring more energy per tonne than the production of aluminium from ore. Quantities of waste are increasing from manufacturing operations and from components now reaching their end-of-life.
Mechanical or thermal
As thermosetting polymers cannot be remoulded, their recycling processes are based on mechanical options, in which the waste is reduced in size by comminution to produce fibrous or powdered materials that can be used as fillers, or thermal processes, where the polymer is removed to leave a clean carbon fibre recyclate with good mechanical properties (see image below).
If done with care, the mechanical recycling produces fibre rich material fractions that can be used to provide reinforcement in new composites. However, the carbon fibre is coated in polymer residues and it is difficult to incorporate these in new composites and achieve full reinforcement benefit.
In thermal recycling several types of process are under development. Pyrolysis involves heating in the absence of air. At high temperature, in excess of 400-500°C, the epoxy resin decomposes into shorter chain organic molecules, usually in gaseous form, but a proportion of pyrolytic char forms and remains on the fibres, making them more difficult to process.
The gaseous products can potentially be recovered and used as chemical feedstock, but in most practical pyrolysis processes they have limited value and are burned to provide energy. Contamination of the recovered carbon fibre with pyrolitic char can be avoided if the atmosphere during pyrolysis is controlled to allow oxidisation of the char. Pyrolysis where the heat is provided by microwaves also results in a clean carbon fibre recyclate.
A fluidised bed process has also been developed where the polymer is removed from the scrap composite in a bed, fluidised by air, operating at about 550°C. As the atmosphere is oxidising there is no pyrolitic char on the surface of the carbon fibres, which are carried away in the gas stream and separated in a cyclone. The polymer is fully oxidised and there is potential for energy recovery. This process is particularly robust in dealing with components that may contain mixtures of different material and other contaminants. Any organic material is oxidised and separated from the fibres. Metallic materials remain in the fluidised bed.
Thermal fluid processes involve processing the waste composite at high pressure and in a fluid at temperatures of 200-300°C. The fluid may be a supercritical alcohol, such as propanol. Under these conditions, the epoxy resin breaks down into more elementary materials, typically phenol-based compounds, which have potential for reuse as chemical products. High quality clean carbon fibres can be recovered after processing.
In the pyrolysis or thermal fluid recycling processes, the physical form of the recovered fibres is basically that of the material fed in, but with the polymer removed. If waste prepreg materials are processed it may be possible to recover the carbon fibre in the form of a textile, in which the fibres are continuous, ready for re-impregnation. However, in most cases, the waste is in a variety of physical forms and so a short fibre product could be produced by either chopping the waste before processing or by chopping the carbon fibre recyclate after recycling.
The fibre products are in a fluffy form (see image, top, left) as there is no longer any size holding the individual fibres together in bundles. The fluidised bed process also produces a ‘fluffy’ recyclate where the carbon fibre appears as individual filaments with a distribution of fibre length.
The mechanical properties of recycled carbon fibre are generally good. Measurements of stiffness show that all the recycling processes yield recycled fibres with a stiffness similar to that of virgin fibre. However, there is some degradation in tensile strength. The thermal fluid processes only show a reduction of a few per cent. The pyrolysis processes show slightly more strength reduction – up to five per cent. However, thefluidised bed process gives a loss of 25-50%. This is due to the increase in mechanical agitation and the oxidising atmosphere. The process is particularly suitable for contaminated end-of-life waste which may not be appropriate for other methods.
The electrical conductivity of the recycled fibre is similar to that of virgin fibre and analysis of the surface chemistry shows that after recycling active oxygenated species remain on the surface and the recycled fibres bond well to epoxy resin.
Carbon fibre is commonly milled to fibre lengths of less than one millimetre to make a conducting filler that can be compounded with thermoplastics for use in electromagnetic shielding. It is relatively easy to mill any form of recycled carbon fibre for these applications. Alternatively, short carbon fibres of a few millimetres in length can be converted into a non-woven tissue and used as an electrically conducting surfacing layer in polymer composites. As the fibres in the recyclate are individual filaments not bound together with size, they can easily be dispersed in the water-based processes used to make tissue and non-woven fabrics.
For structural composite applications, continuous fibre or chopped fibre bundles are normally required. Recycled carbon fibre is not in the same form and so alternative methods of processing are needed, particularly if high fibre volume fraction applications are sought.
For low value applications where fibre volume fractions of around 10% are required, the fibres can be incorporated into a bulk moulding compound. These are usually products in which the polymer is highly filled and the recycled fibres can be mixed into the compound by direct blending. Higher fibre volumes (20-30%) are used in sheet moulding compounds (SMCs). These are conventionally produced by chopping continuous fibre tows and spreading them on a resin base. The fluffy discontinuous recycled carbon fibres are difficult to disperse uniformly and an easier form to use is a non-woven mat.
Fibre volume fractions up to 40% can be achieved by compression moulding these mats, but at the highest volume fractions high moulding pressures are required to compress the random fibre structure and significant fibre breakage occurs, limiting the mechanical properties that can be achieved. The random mat reduces the amount of flow that can be achieved during moulding, requiring a material that is much closer to net shape than is traditional in SMC moulding.
Recycled carbon fibre has successfully been compounded into thermoplastics at up to 30% fibre volume fraction. When compounding with polypropylene, a coupling agent such as maleic anhydride grafted polypropylene is needed to achieve a good bond. A key issue is feeding the recycled fibre into the compounding machine. The fluffy recycled carbon fibre cannot easily be processed and an intermediate form is needed for commercial operation.
The best structural properties can be gained with fibre volume fractions in the region of 60%. These are usually achieved by moulding prepreg made from continuous fibres in a highly aligned form. As the fibres are aligned, low moulding pressures, such as those used in an autoclave, can generate high fibre volume fractions. If recycled fibre is to be used for these structural materials, then fibre alignment techniques must be developed. While recycled carbon fibre is unlikely to be used in primary aircraft structures, there are many less critical applications where the fibres could contribute to reduced weight.
Dr Stephen Pickering
Carbon fibre recycling activities
Commercial carbon fibre recycling operations are in the early stages of development. In Germany, Hadeg Recycling GmbH and CFK Valley Stade Recycling GmbH and Co KG are developing operations.
Adherent Technologies Inc, based in the USA, is probing thermal fluid processes for recycling carbon fibre
composites. In the UK, Recycled Carbon Fibre Ltd in Birmingham has recently commissioned a continuous pyrolysis process line.
At the University of Nottingham, UK, the AFRECAR Project (Affordable Recycled Carbon Fibre) is investigating methods for producing aligned forms of recycled carbon fibre, for applications in the automotive and aerospace industries. The project is also researching supercritical fluid processing as a means of providing high quality recycled carbon fibre as well as the potential for useful chemical products from the polymer. The project is funded by the Technology Strategy Board and involves collaboration with Boeing Company, Ford Motor Company, Advanced Composites Group, Technical Fibre Products Ltd, Milled Carbon Ltd and Toho Tenax Europe GmbH.
Websites: Hadeg Recycling GmbH, CFK Valley Stade Recycling GmbH, Adherent Technologies Inc, Recycled Carbon Fibre Ltd, AFRECAR