The use of carbon fibre reinforced polymers (CFRP) is increasing every day. This type of material have been used in aerospace and automotive industries (amongst others) for years, but now the cost of manufacturing components made of carbon fibre is becoming more accessible for mass production and more companies are introducing CFRP parts in their products because of their good mechanical properties, energy absorption capability and low weight. However, since a large increment in the production is observed, companies need to be aware of the different recycling techniques that are currently available for these materials.
Nowadays there are different ways to recycle composite materials and some of them are more developed than others. However, the use of recycled carbon fibres (rCF) is not that common in industry, mainly because of the lack of confidence in their performance when compared to virgin carbon fibres (vCF). In addition, there is a clear disadvantage: rCFs cannot be used for the same applications as what they were originally designed for. Because of this, I want to introduce some of the recycling techniques which are currently available for composites.
The first technique is “mechanical grinding”. This process consists of crushing components into very small pieces. Then, the resulting product can act as reinforcement or filler. Nevertheless, the mechanical properties of the reinforcement made of rCFs are considerably worse than the properties of vCFs, due to some issues caused by a poor adhesion between these rCFs and the new resin. In addition, the use of recycled fillers is also limited because they also deteriorate the mechanical properties of the new component if the percentage of filler exceeds certain amount. On the other hand, these products can be used for producing energy, since they have a high content of resin.
Other techniques can be classified as thermal processes. They are not useful for recovering resin products, since they are subjected to very high temperatures and they produce gases (methane, carbon dioxide…):
- The first thermal process is pyrolysis, which can be conducted with or without oxygen. This technique produces contaminated fibres which require a post-heating process. That extra treatment also deteriorates the properties of the fibres in a very severe way.
- A variation of pyrolysis is fluidised-bed process. This technique uses a bed which is then fluidised using really hot air. This method is relatively quick and the resulting fibres can be bonded to polymer matrices quite easily. The best part of this process is that it can be used to recycle composites from contaminated materials, sandwich panels with foam cores and components with metal inserts. On the other hand, the mechanical properties seem to be deteriorated even more than with standard pyrolysis.
- Micro-wave assisted pyrolysis uses microwaves to heat the component very fast, saving energy. Unfortunately, applications of this technique are not showing great results so far.
Finally, we have solvolysis, which is a chemical process that uses a solvent to deteriorate the matrix. One of its advantages is the great amount of possibilities that it offers in terms of solvents, temperature, pressure and catalysts. In addition, when compared to pysolysis, this process requires a lower temperature to degrade the resin.
To conclude, I would like to say that even though these techniques are still being improved, there is a clear debate regarding the actual benefits of recycling carbon fibre composites. Why is that? Well, basically some studies have revealed that for certain applications, the cost of using rCFs is even higher than the cost of vCFs. Besides, some of the processes produce hazardous substances for the environment. However, in my opinion, looking into this topic in order to improve the way this kind of material is recycled should be a must if we really want to have a sustainable future. For more information, I highly recommend the publication by Oliveux et al: “Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and reuslting properties”.