Researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM developed a new polymer patch. The thermoformable, recyclable plaster is pressed onto the damaged area and cures completely in just 30 minutes. The innovative fibre-reinforced plastic is so versatile that it can be used in a wide range of industries, from aviation to orthopaedics.
Repairing lightweight composite parts - think aircraft wings, fuselage sections, tail surfaces and doors - is a time-consuming and costly process with multiple work steps. Engineers usually repair the damaged area using a laborious wet lamination process. Or by applying fibre-reinforced polymers (FRPs) or aluminium structures, known as doublers, to the surface. However, these methods involve long curing times and require additional adhesives. Researchers at Fraunhofer IFAM developed a polymer patch of dynamic polymer networks. This makes it possible to shorten the lengthy and laborious repair process to just 30 minutes.
Innovative material
The material is based on benzoxazines, a new class of thermosetting material known as a thermoset. What makes this material unique is that the polymerised plastic does not melt or otherwise behave like a traditional resin system used in wet lamination. The dynamic cross-linking processes of the polymer patch make it possible to heat the material locally. When heated, the fully cured patch adapts to the repair site. At room temperature, the polymer exhibits thermosetting properties. As a result, the patch is non-sticky and can be stored in a stable condition. This saves energy, as the polymer plaster can be stored at room temperature and does not require refrigeration, thereby reducing storage costs.
Use polymer plaster
Polymer plaster is applied to the lightweight part to be repaired using pressure and thermally induced exchange reactions. It enables fast repairs, with full cure within 30 minutes. Working with reactive hazardous materials, as required with traditional resin systems, is not necessary. The vitrimeric properties allow the polymer plaster to be removed when needed, without leaving residues.
Direct repairs
“Our adhesive-free, storage-stable, fibre-reinforced patch enables immediate repairs to damaged composite materials and hybrid structures. As the polymer is naturally a vitrimer, the patch behaves like a conventional thermosetting composite material during storage. But it also bonds easily when simply heated, without the need for additional adhesive.” So says Dr Katharina Koschek, head of the Adhesive Bonding and Polymeric Materials department at Fraunhofer IFAM in Bremen.
Use in aerospace
The innovative material has high mechanical strength and thermostability. This makes it particularly suitable for mobility applications such as automotive, railway infra and in aviation. It can be deformed and has self-healing properties. At end-of-life, it can be recycled, as the polymer network can be dissolved and both the fibres and polymer system can be reused. "Conventional thermosets cannot be deformed afterwards and are not recyclable. In contrast, our benzoxazine-based polymer plasters have all these properties. The versatile material covers many aspects of the sustainable use of polymers with a view to the circular economy," says Koschek. "Through repair and reuse, it extends the life of lightweight structures and helps to reduce the consumption of new raw materials." Another advantage is that it can be combined with other materials, making it suitable for integration into structures made of metals such as steel.
Use in orthopaedics
Its versatility opens up opportunities for applications in various industries, even orthopaedics. Here, for example, the thermoformable material could be used to make individually customisable orthoses and prostheses in the future. Currently, customisation of lightweight devices such as these is a very labour-intensive process, as conventional composite materials do not allow many adjustments once the resin is cured. "Prostheses are custom-made for individual patients. But they don't always fit. The smallest imperfection in the fit or physiological change can mean the prosthesis causes pain or discomfort to the patient and disrupts their treatment. This used to mean making a new prosthesis, and because of the requirements and amount of detailed manual work involved in orthopaedic work, this can take several months," Koschek explains.
Collaboration
The use of thermoformable materials could eliminate the need to remake these types of medical devices. In the CFCadapt project, researchers from Fraunhofer IFAM collaborated with orthopaedic and prosthetics company REHA-OT Lüneburg Melchior und Fittkau GmbH, E.F.M. GmbH and the Leibniz Institute of Polymer Research Dresden (IPF) to develop a new fibre-reinforced polymer based on dynamic polymer networks that can be modified in various ways. The main difference between the new material and commercial matrix systems for orthopaedic devices made of fibre composites is the ability to re-adapt and remould the new material to the appropriate pressure or support points. This allows dynamic adaptation to the patient and their changing needs during the course of treatment.
Featured image: The mouldable, recyclable polymer plaster can take on any desired shape and size (Fraunhofer IFAM)
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