ACTIONS

CARBON FIBRES RECYCLING

Fibre reclamation consists on recovering the fibres from the CFRP, by employing an aggressive thermal or chemical process to break-down the matrix (typically a thermoset); the fibres are released and collected, and either energy or molecules can be recovered from the matrix. Fibre reclamation may be preceded by preliminary operations, e.g. cleaning and mechanical size-reduction of the waste.
Fibre reclamation processes are particularly suitable to CFRPs: carbon fibres have high thermal and chemical stability, so usually their excellent mechanical properties are not significantly degraded. Generally, the rCFs have a clean surface and mechanical properties comparable to the virgin (v-) precursors (Fig. 1); nevertheless, some surface defects and strength degradation have also been reported.
After reclamation, the recycled fibres are usually re-impregnated with new resin to manufacture recycled CFRPs. In addition, rCFs have also been used in non-structural applications.

Fig. 1 – Mechanical properties of recycled carbon-fibres and their virgin precursors. (a) Young’s modulus. (b) Strength. (c) Interfacial shear strength with epoxy resin.

Pyrolysis
Pyrolysis, the thermal decomposition of organic molecules in an inert atmosphere, is one of the most widespread recycling processes for CFRP. During pyrolysis, the CFRP is heated up to 450 °C to 700 °C in the absence of oxygen; the polymeric matrix is volatilised into lower-weight molecules, while the CFs remain inert and are eventually recovered. In the UK, the Milled Carbon Group started developing a pyrolysis process for CFRP in a pilot-plant in 2003, and finally upgraded to ‘‘the world’s first commercial scale continuous recycled carbon fibre operation” and formed Recycled Carbon Fibre Ltd. Their process is implemented as a semi-open continuous belt furnace with controlled atmosphere to avoid char formation; it complies with all legislation on the treatment (post-combustion) of off-gases, and the resin’s calorific value is recovered and fed back in the process. The company has successfully reclaimed fibres from virtually all types of waste; the large dimensions and continuity of the furnace belt allow for entire out-of-date pre-preg rolls to be recycled while maintaining the architecture of the reinforcement. The group recently launched Green Carbon Fibre Ltd. for commercialisation of recycled products (e.g. milled and chopped fibres or pellets).
The Japan Carbon Fiber Manufacturers Association (JCMA) started working on CFRP recycling in 2006. JCMA currently runs a pyrolysis plant, but details on the process itself and mechanical properties of fibres have not been disclosed. In the USA, Materials Innovation Technologies RCF (MIT-RCF) was created by the homonymous ‘‘advanced-materials solutions developer” company, which had started recycling CFRP in 2008 using an undisclosed pyrolysis process. Their approach includes a preliminary step of chopping the feedstock to a consistent length; after pyrolysis, an in-house developed manufacturing process (three dimensional engineered preforming, 3-DEP) proved to be particularly suitable for re-manufacturing.
In Germany, CFK Valley Stade Recycling GmbH & Co. KG uses a continuous pyrolysis process (complemented with an oxidation step for char removal) developed together with the Technical University of Hamburg-Harburg and ReFiber ApS. The process is suitable for several types of CFRP waste, and the main products comprise milled fibres, chopped fibres, and textile products.
In Italy, Karborek S.p.a. uses a combined pyrolysis and upgrading (in oxygen) patented process to recycle the fibres and avoid char formation; although fibre-length is preserved during reclamation, Karborek’s main products are milled and chopped rCFs, as well as blended non-woven veils with carbon and thermoplastic fibres.
A variation of the pyrolysis process, using a continuous micro- wave approach has been implemented by Firebird Advanced Materials Inc., in the USA.
In Germany, HADEG Recycling Ltd. (working in collaboration with the Technical University of Hamburg-Harburg) reclaims CFs by pyrolysis, and also commercialises unprocessed manufacturing remainings (dry CF rovings or fabrics, and uncured pre-preg cut-offs).

Oxidation in fluidised bed
Oxidation is another thermal process for CFRP recycling; it consists in combusting the polymeric matrix in a hot and oxygen-rich flow (e.g. air at 450 °C to 550 °C). This method has been used by a few researchers, being the fluidised bed process (FBP) the most well-known implementation. FBP has been developed and implemented by Pickering et al. (2000) at the University of Nottingham. During recycling, CFRP scrap (reduced to fragments approximately 25 mm large) is fed into a bed of silica on a metallic mesh. As the hot air stream passes through the bed and decomposes the resin, both the oxidised molecules and the fibre filaments are carried up within the air stream, while heavier metallic components sink in the bed; this natural segregation makes the FBP particularly suitable for contaminated EoL components. The fibres are separated from the air stream in a cyclone, and the resin is fully-oxidised in an afterburner; energy-recovery to feed the process is feasible.