The difficult task of recycling the carbon fibers of cars and airplanes

This article delves into the difficulties, complexities, and opportunities when recycling the carbon fibers present in vehicles such as cars or airplanes.

The use of carbon fibers has increased significantly in recent years. In 2017 around 70,500 metric tons were consumed worldwide. The increase in demand over the next decade is a fact, with forecasts ranging from 140 Mt for 2020 to 175 Mt for 2025. These high estimates of carbon fiber consumption suggest that, in the future, a large amount of carbon will be generated. waste rich in these materials.

Carbon fiber is used almost exclusively to manufacture composite materials. Composite materials reinforced by fibers are the most important from the technological point of view, and among them are those reinforced with carbon fiber. They have a high resistance to fatigue and high rigidity at low and high temperatures and, simultaneously, a low density, so they have a very good strength-to-weight ratio.

This type of materials is used more and more frequently, especially in applications that need resistant and lightweight materials. They are used, for example, in the aeronautical and defense industry (sectors that consume 36% of the total) followed by the automobile industry (24%), wind energy (13%), sports and leisure (13%), construction ( 5%) and other diverse uses (9%).

To give some representative example of the wide use of these materials in different industries, the Airbus A380 stands out: 40% of its total weight corresponds to carbon fiber composite materials.

Another significant example, in this case in the automotive sector, is the BMW i3 model, totally electric, whose body is made of these materials.

BMW i3 / Pixabay

With prices ranging from 30 to 58 euros per kilogram of carbon fiber, there is a clear need to develop a circular economy model to recover them from waste and reintroduce them as secondary raw materials instead of dumping them in landfills or burning them in Incineration facilities (the two most used options currently).

Recycling strategies

Their properties (stability and resistance) make these materials difficult to recycle. So far, three major groups of recycling technologies have been developed: mechanical, chemical and thermal processes.

• Mechanical recycling methods consist of reducing the particle size of the composite materials by cutting and grinding. In the early stages, the metals are removed and grinds are continued to reduce the particle size below 50 mm. Finally, a classification is made by particle size. The resulting material is usually used as charges, in low percentages, for new materials, although it could also be used as a fuel, due to the high calorific value of the resins. The main disadvantages of this method are the extensive use of energy and the scarce utility of the resulting materials to obtain a mixture of fibers and resins.
• The chemical recycling processes are based, above all, on the solvolysis that degrades the polymers that form the composite materials to recover the monomers. They are processes made with supercritical fluids at high pressures and through the use of organic solvents. The monomers can be reused in the industry to manufacture more polymers, contributing to the circular economy and improving the environment. This prevents the synthesis of new monomers, reducing the environmental impact of this industry. The biggest disadvantage of this method is that the process involves a large investment cost, the use of hazardous chemicals and too high a cost in cleaning the fibers obtained from it.
• Regarding thermal methods, pyrolysis stands out, which involves subjecting these materials to relatively high temperatures, decomposing the polymers and transforming them into gases and liquids that can be used for the generation of energy. On the other hand, fibers are obtained, which are coated with a solid carbonaceous product (char). The main disadvantages of this method are, on the one hand, the extensive use of energy and, on the other hand, that the fibers obtained do not present good properties, largely due to its fouling with carbonaceous products derived from heat treatment.

There are mixed strategies that combine two or more types of methods to improve the performance and the results of the process, such as the thermochemical procedures developed by a spin-off of the CSIC, TRC SL. In this case, after the treatment, combustible gases and liquids and totally clean fibers are recovered that maintain most of their physical properties and, therefore, can be used for the manufacture of new composite materials.

Carbon fibers suitable for reuse.

Advantages of the circular economy

The good results obtained with this technology are reflected in the properties of the new materials manufactured with recycled carbon fibers. In addition, these are cheaper (15-23 € / kg) than virgin fibers.

The materials made with recycled fibers, for example by the vacuum resin infusion technique, have physical properties that make them suitable for the manufacture of new pieces of composite materials for various sectors, such as automotive and 3D manufacturing. .

These properties may, with a high probability, be improved thanks to the new studies being carried out. The demand for carbon fibers grows every year and the recycled carbon fibers are highly competitive materials.

In summary, using recycled fibers, and in some cases, monomers reduces costs and energy and clearly contributes to favor the circular economy of such important sectors as the aeronautical industry, wind energy or the automotive industry.

Recycled Materials, National Center for Metallurgical Research (CENIM-CSIC) and Olga Rodríguez Largo, National Center for Metallurgical Research (CENIM-CSIC)

This article was originally published in The Conversation.