Bernd Gulich of STFI (Germany) reminded us that all commercial carbon fibre composites use filament yarns or tows often in woven structures for maximum strength and stiffness. Production of these structures results in short carbon fibre waste which can be converted into nonwovens with different but useful properties. Furthermore, post-consumer carbon composites are now becoming available and these can be pyrolysed to release pure carbon fabrics which can be shredded and also converted to nonwovens. Unlike the rigid woven structures these nonwovens can be moulded into complicated shapes. While the composite strengths are low compared with virgin fibre products, the resulting mouldings are very lightweight compared with glass reinforced plastics and therefore suitable for use in non-structural components of cars and planes. At present, long reclaimed carbon fibres are chopped to about 60mm length and carded into waddings for stitch-bonding or needling. The technology is difficult because the dust created may well be harmful and is certainly capable of shorting out any unprotected electrical circuits.
Mario Löhrer of RWTH Aachen University (Germany) was also using rejects from carbon fibre composite production but only in the form of staple fibre or rovings which could be short-cut and air-laid into nonwovens. These would be used to produce back rests for the front seats of cars in order to demonstrate the usefulness of the technology.
So far, discontinuous air-laying of the output of a Trützschler fine-opener had been used to make isotropic waddings for impregnation and this was now being scaled up into a continuous process. The system appeared to do little damage to the fibres and little dust was produced outside the air-layer. An in-situ polymerisation impregnation system was also being developed. Here the carbon fibre nonwoven was coated with lauro-lactame monomer, an activator and a catalyst, and this was polymerised and cured with UV and heat to form the composite in one continuous operation.