Polymer patch made of dynamic polymer networks

Scientists at the Fraunhofer Institute for Production Technology and Advanced Materials IFAM have developed a new polymer patch that can significantly speed up and simplify the previously labor-intensive, expensive and time-consuming repair processes of damaged lightweight aircraft components.

The thermoformable, recyclable repair patch is pressed onto the damaged area and cures completely in just 30 minutes. The innovative fiber-reinforced plastic is so versatile that it can be used in a wide range of industries, from aerospace to orthopedics.

Repairing lightweight fiber composite components, such as those used in aircraft wings, fuselage sections, tail surfaces, and doors, is a time-consuming and expensive process that involves many work steps. The damaged area is typically repaired by a laborious wet lamination process or by applying fiber-reinforced polymers (FRP) or aluminum structures known as doublers to the surface. However, these methods require long cure times and additional adhesives.

Scientists at Fraunhofer IFAM have developed a repair patch made of dynamic polymer networks, also known in the industry as vitrimers, that shortens the previously long and tedious repair process to just 30 minutes.

What’s truly unique about this innovative material – which is based on benzoxazines, a new class of thermosetting materials known as thermosets – is that the polymerized plastic doesn’t melt or behave differently than traditional resin systems used in wet lamination.

Dynamic polymer cross-linking processes allow local heating of the material. The fully cured patch conforms to the repair site when heated. At room temperature, the polymer is thermoset, so the patch is non-sticky and stable during storage. This saves energy because the patch can be stored at room temperature and does not require refrigeration, reducing storage costs.

The patch is applied to the lightweight part requiring repair using a pressure and heat exchange reaction. It allows for a quick repair, fully curing within 30 minutes. There is no need to work with reactive hazardous materials, which is necessary with traditional resin systems. The vitrimer properties allow the patch to be removed when necessary, without leaving any residue.

“Our adhesive-free, storage-stable, fiber-reinforced patch enables direct repairs of damaged composite materials and hybrid structures. Because the polymer is a vitrimer by nature, the patch behaves like a conventional thermoset composite material during storage, but also bonds cleanly and easily simply by heating, without the need for additional adhesives,” explains Dr. Katharina Koschek, Head of the Adhesive Bonding and Polymeric Materials section at Fraunhofer IFAM in Bremen.

Aviation applications: resource-saving and energy-efficient repairs of lightweight structures extend service life

The innovative material is characterized by high mechanical strength and thermal stability, making it particularly suitable for mobile applications such as automotive and rail vehicle engineering and aviation. It can be deformed and has self-healing properties. It can be recycled after reaching the end of its service life, as the polymer network can be dissolved and both the fibers and the polymer system can be reused.

“Conventional thermosets cannot be deformed and are not recyclable. In contrast, our benzoxazine-based vitrimers demonstrate all these properties. This versatile material encompasses many aspects of the sustainable use of polymers with a view to the circular economy,” Koschek emphasizes.

“By repairing and reusing it extends the life of lightweight structures and helps reduce the consumption of new raw materials.” Another advantage is that it can be combined with other materials, making it also suitable for integration with structures made of metals such as steel.

Application in orthopedics: Individual shaping and adjustment of prostheses and orthoses for individual patients

The versatility of benzoxazine-based vitrimers opens up potential applications in a variety of industries, even beyond the mobility sector. In orthopedics, for example, the thermoformable material could be used to realize individually adjustable orthoses and prostheses in the future. Currently, custom-made lightweight devices like these are a very labor-intensive process, because conventional fiber composite materials do not allow for much adjustment once the resin has cured.

“Prosthetics are custom-made for individual patients. However, they don’t always fit. The slightest imperfection in fit or physiological change can mean that the prosthetic or orthotic is causing pain or discomfort for the patient, interfering with their treatment. Previously, this meant that a new prosthetic had to be made, and due to the demands and amount of detailed handwork involved in orthopedics, this could take several months,” Koschek explains.

The use of thermoformable materials can eliminate the need for remanufacturing of this type of medical aid. In the CFKadapt project, scientists from Fraunhofer IFAM joined forces with the orthopedic and prosthetic company REHA-OT Lüneburg Melchior und Fittkau GmbH, EFM GmbH and the Leibniz Institute for Polymer Research in Dresden (IPF) to develop a new fiber-reinforced polymer that is based on dynamic polymer networks and can be tailored in various ways.

The key difference between the new material and commercial matrix systems for orthopedic devices made of fiber composites is the ability to re-adapt and mold the new material to the appropriate pressure or support points, allowing for dynamic adjustments to the patient and their changing needs during treatment. The trick is that the new fiber-polymer composite blend can be locally heated and individually adjusted.

“The benefits lie in the great freedom of design and configuration, the significant reduction in waste during production, and the longer service life of these devices, because they can be continuously adjusted during treatment. For patients, the most important factor is that they can receive an individually fitted orthopedic device as quickly as possible,” says Koschek.

Standardization of the production of individual components with the possibility of subsequent individual modification can also bring financial benefits and increase the efficiency of the production process.