The use of composite materials in the aeronautic industry has been increasing since (in the 70s in the North American market and the 80s in Europe) they started to be used in commercial aviation as a substitute for classic materials such as metals. The reason for their use was largely because of their capacity to reinforce in preferential directions, their high rigidity, specific resistance and their enhanced fatigue and corrosion behaviour. Currently, the main reasons to justify their use in this sector are to do with reducing the structural weight of the aircraft, the reduction in the number of parts needed in its assembly (fewer zones for riveting) and, finally, the reduction of maintenance operations over the useful lifespan of the craft.
But there are certain limitations in their use such as the high costs of the raw material and of labour for the manufacture of large parts, the need for long periods of development, together with the complexity associated with its design, the difficulties in obtaining certifications for the necessary materials. On balance, between the advantages and disadvantages that these materials have, their current application in a commercial aircraft involves 20% of its weight. However, future tendencies point to an important increase in this percentage, enabling reductions in both weight and cost of aircraft, enhanced safety conditions and reduced environmental impact.
Considering that the average life of a plane is about 20 years and that parts made from composite materials are not repaired but replaced, the aeronautics sector is finding a huge quantity of waste material on its hands a solution for which has to be found. This is because the only currently available way to treat this type of materials is by dumping them in authorised dumps where they are still accepted. In fact, in the aeronautics market, future tendencies already point to the substitution of thermostable materials by other kinds of materials such as thermoplastics or GLARE®-type plastic/metal hybrid materials, although the reasons that justify this change are based more on economic (automation of the process, lowering labour costs, mass production, obtaining materials with enhanced mechanical properties, etc.) than on criteria of recyclability.
Also, carbon fibre is used as a reinforcement element only in high mechanical exigency applications and where the price may not be a very decisive factor. The mass production market or the automotive one cannot entertain materials with such high costs. Thus, the quest for an alternative to the treatment of carbon fibre from disused aeronautic components targets their reuse as reinforcement in the form of short fibre for new sectors that can take on both its properties and its cost.
It is in this context that this INASMET-Tecnalia research project arises the principal aim being to perfect a recycling technique that enables, on the one hand, the obtaining of carbon fibre from waste components and, on the other, the study and evaluation of the possibilities of its reuse as an element of reinforcement in new applications.
There were three techniques considered for the recovery of carbon fibre for reuse. In the first place, a chemical process based on nitric acid with which the resin is dissolved and carbon fibre is obtained after several washings with acetone and water. As a second alternative, a thermal process of pyrolysis in a controlled atmosphere of argon is used whereby the resin is eliminated at a temperature in which the carbon fibre remains unaltered. And, as a third and final alternative, the possibility of incinerating this type of waste material has been evaluated for its energetic potential. These three experimental techniques were applied to one wing of a mini-aircraft built at INASMET-Tecnalia. Aeronautic companies such as Airbus and Boeing have expressed interest and concern regarding the environmental impact of dismantling composite material parts in the sector.
The second objective of the project was to find a potential application for the recycled carbon fibre. This was combined in different percentages - 10 and 30% - with two different commercial thermoplastic resins widely used in the automotive sector - polypropylene and polyamide. Various mechanical properties have been determined with the aim of evaluating the effect of the addition of the recycled carbon fibre. In each and every case it was shown that there was a significant enhancement in the properties in reinforcing the thermoplastic with recycled fibre using any of the techniques but, above all, by using the fibre obtained by chemical digestion.
Looking at economic criteria and taking into account that the starting material can be considered as waste material without additional cost, the costings should be carried out on the basis of the treatment used. With the chemical method, the cost of the nitric acid has to be taken into account, the time needed for the washing of the fibre (it has to be washed three times in water and a final wash in acetone), the drying time and the time for the treatment of the residual by-products – nitric acid with resin dissolved in it. As regards the thermal method, the fibre is obtained practically clean after the combustion of the resin. Some carbon specks appear on the surface of the fibre, eliminated with a light shaking. It is a more rapid process than the chemical digestion.
Regarding environmental criteria, although the chemical technology turns out to be technically and economically viable, for the recycling of composites in the aeronautics sector there are difficulties from an environmental perspective. The fact that working with toxic chemical products – as is the case with nitric acid - they have to be heated to be more effective, and this involves high-level safety measures. On evaluating a recycled process, it is necessary to evaluate if the desired outcome – in this case, recovery of recycled carbon fibre – is going to generate more residues of a more toxic nature than the initial material, and this effect is notable in the chemical technology alternative which is why its large-scale use is not justified.
As a main conclusion of the study it can be said that, although the three alternatives ways studied for the recovery of carbon fibre are each technically viable, environmentally the only method that can be considered for the recovery of carbon fibre on a large-scale is that of pyrolysis technique, and that the quality of the fibre obtained enables its use as reinforcement in applications that to date have not been considered due to the high costs involved.