High performance carbon fiber reinforced plastics (CFRP) have firmly established themselves in modern airplanes. Repairs, however, are very laborious and often even impossible. Most of the time the entire component has to be replaced. The PYCO Research Division at the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam, Germany, has developed a simple, cost-effective and energy-efficient way to make sustainable repairs. Moreover, entire components can be completely recycled in a process in which the expensive carbon fibers are reclaimed. Researchers will present their developments at JEC World in Paris.
Composites made from crosslinked polymers – so-called thermosets – are reinforced with carbon, glass or natural fibers. Their rich spectrum of properties have increased their importance in aerospace, the automotive industry, wind power generation, shipbuilding, railway construction, building construction, and civil engineering. Yet, even the best material can become damaged or show wear and tear. Engineers must then decide whether the defective area should be painstakingly and expensively patched, or whether the entire component has to be replaced. “Repairing and recycling polymer-based composites are inseparably linked to resource efficiency and sustainability”, explains Dr. Christian Dreyer, who leads the Research Division Polymeric Materials and Composites PYCO at the Fraunhofer IAP. “Finite resources increase the importance of sustainable management and the use of recyclable and repairable polymer materials”, says Dreyer.
The researchers have therefore developed a process for repairing and chemically recycling fiber-reinforced thermosets. These are especially used as matrix resins in composites for high-stress components. The crosslinked polymers form a very rigid structure that gives the component its shape. But it is precisely this structure that creates a problem when it comes to repairing or recycling the component. Unlike thermoplastics, once thermosets are cured, it is very difficult to chemically decompose them.
“We have developed a fast and gentle way for a chemical recycling which allows highly crosslinked plastics to be broken down into their basic elements”, Dreyer explains. This development enables a component to be completely recycled or to be repaired locally. The resin matrix is gently removed from the defective area without significantly impacting the mechanical properties of the reinforcement fiber. The exposed fibers are then refilled with repair resin and cured. The new process is also setting the standard for recycling. Until now discarded components have been either incinerated or shredded to be used as fillers. The Fraunhofer researchers have the crucial advantage when it comes to chemical recycling: the often expensive reinforcement fibers are reclaimed alongside the decomposed polymer matrix. Due to the limited size of the components, the fibers are no longer continuous filaments. Nevertheless there are many applications that use fibers measuring up to several centimeters.
Professor Alexander Böker, who heads up the Fraunhofer IAP explains: “This recycling process is of particular interest to companies since the matrix material can also be recycled on an industrial scale. This allows sufficient quantities of new ‘recycling thermosets’ to be synthesized. The Fraunhofer Pilot Plant Centre for Polymer Synthesis and Processing PAZ – a joint initiative of the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam-Golm and for Microstructure of Materials and Systems IMWS in Halle/Saale – enables us to manufacture these resins in industry-relevant quantities