Fascination with technology New class of fiber-reinforced plastics

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Fraunhofer-Institut für Fertigungstechnik und Angewandte Materialforschung (IFAM)

Wuxi InfiMotion Technology Co., Ltd.

In our "Fascination with Technology" section, we present impressive projects from research and development to designers every week. Today: a plastic patch that significantly speeds up and simplifies previously complex repair processes on damaged aircraft lightweight components.

The repair of fiber composite lightweight components, such as in wings, fuselage sections, tail surfaces and doors of aircraft, is time-consuming and costly and requires several steps. Usually, the damaged area is restored using the complex wet lamination process or by surface-applied fiber composite plastics (FCP) or aluminum structures, called doublers. However, these variants need to harden for a long time and require additional adhesives.

Repair shortened to 30 minutes

Researchers from the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research IFAM have now developed a repair patch made of dynamic polymer networks—experts also call them vitrimers—which reduces the previously time-consuming, elaborate repair 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 also does not 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. The cured patch fits the area to be repaired in the heated state. At room temperature, the polymer exhibits thermoset properties, leading to an adhesive-free and storage-stable patch. This saves energy, as the patch can be stored at room temperature without cooling, hence reducing storage costs.

No more handling of hazardous materials

The patch is applied to the lightweight component to be repaired through pressure and thermally induced exchange reactions, enabling a quick repair with the final strength reached within 30 minutes. The handling of reactive hazardous substances, as required with classic resin systems, is eliminated. The vitrimer properties allow residue-free removal of the patch if needed.

"With our adhesive-free, storage-stable fiber-reinforced patch, a direct repair of damaged composite materials and hybrid structures is possible. Thanks to the vitrimer nature of the polymer, the patch behaves like a conventional thermoset composite during storage, but can easily and cleanly be joined by simple heating, without the need for additional adhesives," explains Dr. Katharina Koschek, Head of Adhesive Bonding and Polymer Materials at Fraunhofer IFAM in Bremen.

Ideal for mobility applications

The innovative material is characterized by high mechanical strength and thermos-tability, making it particularly suitable for mobility applications such as automotive and rail vehicle construction as well as aviation. It is deformable and has self-healing properties. At the end of its life, it can be recycled, as the polymeric network is solvable and both the fibers and the polymer system are reusable.

"Conventional thermosets cannot be subsequently deformed and are not recyclable. Our benzoxazine-based vitrimers, on the other hand, combine all these properties. The changeable material covers many aspects of sustainable use of plastics in terms of circular economy," emphasizes the researcher. "By repairing and reusing, it extends the lifespan of lightweight constructions and contributes to reducing the use of new raw materials." Another advantage: It can be combined with other materials, hence also suitable for integration into metallic structures such as steel.

Use in orthopedics

The flexibility of benzoxazine-based vitrimers opens up application possibilities in various industries—also beyond the mobility industry: In orthopedics, individually adaptable orthoses and prostheses can be realized in the future using the thermoformable plastic. Currently, a very high level of manufacturing effort has to be made to produce lightweight aids that fit perfectly, as the conventional fiber composite materials only allow a limited extent of rework after the resin has hardened.

"Prostheses are tailor-made for patients. However, the aids do not always fit. Minimal fit inaccuracies or physiological changes cause pain to those affected by the prosthesis or orthosis and counteract the therapy. So far, new prostheses had to be created, which due to the demand and the elaborate manual work in orthopedics can take up to several months," explains Dr. Koschek.

Highly adaptable fiber-plastic composite material

The use of thermoformable materials could bypass the need to remake a medical aid. In the CFKadapt project, the researchers at Fraunhofer IFAM, together with REHA-OT Lüneburg Melchior and Fittkau GmbH, E.F.M. GmbH and the Leibniz Institute for Polymer Research Dresden (IPF), have developed a new, highly adaptable fiber-plastic composite material based on dynamic polymer networks.

The essential difference to commercial matrix systems for orthopedic aids made of fiber composites is the possible reworking and modeling of the new material at the appropriate pressure or support points for a dynamic adaptation to the patient and their changing needs during therapy. The trick: The new polymer-fiber composite mix can be locally heated and individually adjusted.

"The advantages lie in the great design and layout freedom as well as the significant reduction of scrap in production and a longer service life of the aids, as these can be continuously adapted in therapy. For those affected, one thing counts above all else—getting a perfectly fitting orthopedic aid as soon as possible," concludes Koschek.