The demand for carbon fibre is increasing, mostly because of the need for light-weight construction is growing. For electric cars and energy-efficient vehicles, light-weight carbon composites are soon indispensable. It is not only their weight that is the decisive factor but also that the material is extremely robust. In the future, lots of applications will follow, for which high-performance material will be needed. Have there been about 74 000 tons of carbon fibres produced worldwide in 2017 it will be probably 120 000 tons in 2022 if the experts are to be believed.
One problem that still no solutions exists for, is the recycling of carbon composites respectively the retrieval of the valuable fibres. Experts believe that in 10 to 15 years latest waste to a large extent will occur when today's products will have reached the end of their life cycle - such as the carbon body of the BMW i3 or the fuselage and wings of the Airbus A350 or the rotor blades of wind energy plants. There is still a lack of recycling technologies in industrial scale.
But the shortage of large-scale recycling solutions is also a chance. Those who quickly bring the relevant recycling know-how to application maturity can open up an interesting market. The potential for marketing fibre cuttings from composite production in Europe alone is estimated at more than €300 million - at a market price of €20 per kilo. Fibres from recycled composites would be added to that.
Organo sheets - fibres on a staple
But something is happening. German textile research has developed some fascinating technologies lately, that could be used for commercial carbon fibre recycling in the future. Using recycled carbon fibres (rCF) or fibre cutting leads to one problem: it is lots of different lengths one has to deal with. To process the fibre fragments into new composite components with homogenous features is quite a challenge - especially regarding stability.
The ITA has managed to find a solution to that problem. They developed a process where the fibres are combined with layers of paper and nonwovens. The sequence of layers can be flexibly adapted to various lengths of fibres, which means 97 per cent of the rCF can be processed to so-called organo sheets with fully homogenous mechanical features. The surface of those sheets can be refined like metal sheets and then processed further on.
rCF-Tapes formable as you like
The DITF managed to combine rCF with polyamide fibres in a way to produce nonwoven tapes, that can be further processed into larger elements and construction components with a tape-weaving machine. The advantage of that combination with polyamide is that the components can be warmed and then formed any way you like. In that process, the rCF and the polyamide fibres together are laid down neatly aligned in staples. When the finished tapes are warmed, the plastic melts. Pressing the tape into a mould, the rCF can move in the form without losing its orientation.
Some basic problems had to be solved in the first step. Initially, a plant had to be built to provide a neat fibre orientation and stretching of the tapes with. Next, the scientists looked for the right tools to use for forming the tapes in a three-dimensional way. Another question that had to be clarified, was how to warm the tape optimally to get semi-finished products from highly oriented rCF. Meanwhile, the process is that good, that complex geometries can be realised in a thermoforming or tape laying processes without draping it additionally.
Finishing the surface on the point
Currently, the ITV deals with surface treatment of rCF in a project still running. Normally the surface of carbon fibres has to be modified before those can be wrapped in the matrix material. Thereto a coating, the so-called sizing is typically applied. On one hand, that facilitates the processing of the fibres on the machines because their sliding behaviour is optimized. And on the other hand, the adherence of fibre and matrix material is improved. The ITV now has developed a process to coat rCF-fibres with various lengths automatically with the sizing to functionalise them deliberately for a special purpose. The combination of rCF and polypropylene as matrix material that is used quite often is in the focus of that project.
Fungi grow on CFRP and clear away the surrounding plastic. © Hohenstein Institut für Textilinnovation
Moulds expose fibres
While the three described projects start with the processing of existing rCF that is already existing, the Hohenstein-Institutes in Bönnigheim go for the recycling of carbon composites. They are developing a biotechnical process where moulds "eat" the plastic part of the composites - the matrix. That way the fibres are exposed gently. Though recycling procedures for rCF are already in use, a method to separate the fibres from the matrix in composite materials and thereby keep the fibres long and in high-quality is missing so far. The biotechnology invented in Bönnigheim could be a way out of that. Fungi have various biological degradation mechanisms for plastics. At the institutes, moulds of the genera Aspergillus, Cladosporium and Chaetomium are examined in order to find out about their potential to degrade epoxies. After the first tests have shown which fungi are most effective to break down plastics, the principle is now supposed to be tried out in testing plants.
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