The repair of fiber composite lightweight components, such as in wings, fuselage sections, tail surfaces and doors of aircraft, is time-consuming and cost-intensive and requires several work steps. Usually, the damaged area is restored using a complex wet lamination process or by applying fiber-reinforced plastics (FRP) or aluminum structures, so-called doublers, to the surface. However, these variants take a long time to cure and require additional adhesives. Researchers at the Fraunhofer IFAM have now developed a repair patch made of dynamic polymer networks - experts also call them vitrimers - that shortens the previously lengthy, time-consuming repair process to 30 minutes. The special feature of the new material, which is based on benzoxazines - a new class of thermoset polymers: The polymerized plastic does not melt and does not otherwise behave like a classic resin system in the wet lamination process. Due to the dynamic cross-linking processes of the polymer, the material can be heated locally. When heated, the cured patch adapts to the area to be repaired. At room temperature, the polymer exhibits duromeric properties, resulting in a non-tacky and storage-stable patch. This saves energy as the patch can be stored at room temperature without cooling, reducing storage costs.

The patch is applied to the lightweight component to be repaired by means of pressure and thermally induced exchange reactions, enabling rapid repair with final strength being achieved within 30 minutes. There is no need to handle reactive hazardous substances, as is necessary with conventional resin systems. The vitrimeric properties allow the patch to be removed without leaving any residue if required. "With our adhesive-free, storage-stable fiber-reinforced patch, direct repair of damaged composites and hybrid structures is possible. Thanks to the vitrimeric nature of the polymer, the patch behaves like a conventional thermoset composite during storage, but can be joined easily and cleanly by simple heating without the need for additional adhesives," explains Dr. Katharina Koschek, Head of the Adhesive Bonding and Polymeric Materials division at Fraunhofer IFAM in Bremen.

Longer service life thanks to resource- and energy-efficient repair of lightweight structures

The innovative material is characterized by its high mechanical strength and thermal stability, making it particularly suitable for mobility applications such as automotive and rail vehicle construction as well as in aviation. It can be shaped and has self-healing properties. At the end of its life, it can be recycled, as the polymer network can be dissolved and both the fibers and the polymer system can be reused. "Conventional duromers cannot be subsequently shaped and are not recyclable. Our benzoxazine-based vitrimers, on the other hand, combine all of these properties. The convertible material covers many aspects of the sustainable use of plastics in terms of the circular economy," emphasizes the researcher. "Through repair and reuse, it extends the service life of lightweight constructions and helps to reduce the amount of new raw materials used." Another advantage is that it can be combined with other materials and is therefore also suitable for integration into metallic structures such as steel.

Patient-specific shaping and fitting of prostheses and orthoses

The flexibility of benzoxazine-based vitrimers opens up potential applications in various sectors - even outside of the mobility industry: in orthopaedics, the thermoformable plastic can be used to create individually adaptable orthoses and prostheses in the future. At present, a great deal of manufacturing effort is required for the precise production of lightweight aids, as conventional fiber composite materials only allow a small amount of post-processing after the resin has hardened. "Prostheses are custom-made for patients. However, the aids do not always fit. Minimal inaccuracies of fit or physiological changes lead to the prosthesis or orthosis causing pain for the patient and counteracting the therapy. Until now, new prostheses have had to be made, which can take up to several months due to the demand and the time-consuming manual work involved in orthopaedics," explains Dr. Koschek.

The use of thermoformable materials could avoid the need to remake a medical aid. In the CFKadapt project, researchers at Fraunhofer IFAM, together with REHA-OT Lüneburg Melchior und Fittkau GmbH, E.F.M. GmbH and the Leibniz Institute of Polymer Research Dresden (IPF), have developed a new, highly customizable fiber-plastic composite material based on dynamic polymer networks. The main difference to commercial matrix systems for orthopaedic aids made of fiber composites is the possibility of post-processing and modelling the new material at the corresponding pressure or support points for dynamic adaptation to the patient and their changing needs in the course of therapy. The trick: the new polymer-fiber composite mix can be heated locally and individually adapted. "The advantages lie in the great freedom of design and layout as well as in the significant reduction of waste during production and a longer service life of the aids, as they can be continuously adapted during therapy. For those affected, one thing counts above all else - getting a precisely fitting orthopaedic aid as soon as possible," summarizes Koschek. The standardized production of components with subsequent individual adaptation also results in cost benefits and an efficient production process in the long term.