Composite material Recover continuous carbon fibers from composites

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Researchers at the Fraunhofer Institute for Short-Term Dynamics, Ernst Mach Institute, EMI have developed a technology that allows continuous carbon fibers to be recovered from composites without any loss in material quality.

Carbon fiber-reinforced composites, so-called composites, are particularly strong and lightweight, making them preferred materials in many industries. However, the challenge of disposal and recycling of these high-performance materials is significant. In previous recycling processes, the fiber-reinforced composites are shredded, which leads to shortened fibers and thus to downcycling. The research team at Fraunhofer EMI has now developed a process in which fibers from used composites are efficiently processed for reuse without impairing their mechanical properties.

A carbon fiber composite material consists of fiber bundles embedded in a polymer. This allows the fibers to be held together, define the geometry of a component, and protect the fibers from environmental influences. There are two types of plastic in which the fibers can be embedded: thermosetting composites consist of a non-meltable matrix, meaning they cannot be reprocessed. They behave like an adhesive that hardens and forms a permanent solid bond. Thermoplastic composites, on the other hand, can be melted and reprocessed. However, thermosets are easier to process and are therefore more frequently used in the industry.

Peeling-based recycling of wound structures

Researchers at Fraunhofer EMI use a high-performance laser to selectively remove the fiber reinforcement of thermosetting composites. This process is particularly relevant for hydrogen pressure vessels, where a carbon fiber bundle is continuously wound around a plastic shell to ensure they are particularly stable and can withstand high operating pressures of up to 700 bar (10,2 psi). With this recycling process, the thermosetting matrix surrounding the carbon fibers can be efficiently removed through local pyrolysis, while the carbon fibers themselves remain almost intact. "The special feature of this process is that we implement the pyrolysis of the matrix and the unwinding of the fibers simultaneously, as quickly as possible and without damaging the carbon fibers," explains project manager Dr. Mathieu Imbert.

We have found a very good compromise between process efficiency and recyclate quality. Our results show that the recovered continuous fibers exhibit the same high-performance characteristics as new fibers, making the process particularly attractive.

Dr. Mathieu Imbert

The challenge lies in defining the optimal process window: the matrix decomposes at 572°F to 1112°F, while the fibers can be damaged from around 600 degrees Celsius. "We have found a very good compromise between process efficiency and the quality of the recyclate. Our results show that the recovered continuous fibers exhibit the same high-performance characteristics as new fibers, making the process particularly attractive," says Dr. Imbert.

Economic plus ecological benefits

The process offers not only ecological advantages but also economic potential for recycling companies. The local heat input and simultaneous removal of the continuous fiber bundle save the long pyrolysis time and correspondingly high process costs that the thick-walled hydrogen tanks usually cause. The laser-assisted recovery also requires only about one-fifth of the manufacturing energy of new fibers. The project will continue until the end of 2025 and is part of the DigiTain project, funded by the Federal Ministry for Economic Affairs and Climate Action.

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