The requirement for multipurpose materials, that meet high quality standards in terms of specific properties and processability across all industries, has not onlyexisted since the industrial revolution. Particularly since the digital revolution, the demand for high-performance materials and strength has been growing. High stiffness, temperature and chemical resistance, low density and good processability in established processes with coexisting reparability and recyclability of the components are becoming increasingly important, for example, in aviation. With the introduction of a new material family of polymer-based vitrimers by Leibler et al. in 2011, the requirements mentioned are to be met even better than before. Vitrimers combine the advantages of thermosets and thermoplastics and open up completely new possibilities for Fiber Polymer Composites (FRPC).
Polymer-based vitrimers are characterized by the fact that they show similar specific properties as thermoset matrix systems below the glass transition temperature. Above Tg, the chemical polymer bonds can reorganize themselves, thus enabling processability in thermoforming and welding processes. In the project AIRPOXY (www.airpoxy.eu), Institut für Verbundwerkstoffe (IVW) is responsible for researching the processability of carbon fiber reinforced vitrimer polymers (3R-CFRPC) as well as for developing process concepts for the industrial production of aerospace components. The focus of IVW's research work is on optimization and development of adapted thermoforming, welding and repair processes for 3R-CFRPC.
The R&D work on thermoforming processes is divided into continuous and quasi static process design. Continuous process design allows the fully automated production of continuous profiles. Impregnated and cross-linked single layers are used as input material, which are stacked according to the target thickness of the component and fed to the pressing process. This allows unlimited freedom of the target geometry, which can also be implemented with regard to differences in thickness perpendicular to the process direction without changing the input materials. After the stacked individual layers are inserted in a positioned manner into the tool, they are pre-consolidated by applying pressure as well as temperature, then formed into the target geometry according to interval feed and tool geometry. At the end of the process, the continuous profile is cooled down below Tg in the cooling area of the thermoforming tool with simultaneously applied consolidation pressure to prevent reshaping when leaving the thermoforming tool. If a flat plate tool is used, flat 3R-CFRPC semi-finished products can also be produced in the continuous thermoforming process – those can, for example, be formed in subsequent processes due to their reprocessing properties.
More complex 3D part geometries can be produced by a quasi-static thermoforming process adapted to the 3R-CFRPC. After heating over Tg, e.g. in an infrared radiator field, the part is transported to the thermoforming tool in a press, which forms the 3R-CFRPC upon being closed. After cooling below Tg, the thermoforming tool is opened and the component can be removed.
Within Airproxy project, the bonding and welding processes are also considered for 3R-CFRPC. IVW is developing the application of energy-efficient and non-contact induction welding as a weight-neutral joining method for 3R-CFRPC.
The unlimited reprocessability property of vitrimers also offers the possibility to easily repair defects by stimulating the damaged area by temperature and pressure. This could also be carried out on already installed components, associated with enormous cost-saving potential in the aviation industry, for example. IVW is developing several concepts for repairing different types of damage, e.g. delaminations, detached welded and glued joints or the repair of air inclusions, which have been created e.g. during the RTM process.
Further research topics of the project consortium are the optimization of the Vitrimer resin formulation, its processability in the RTM process, investigations for component testing and structural health monitoring as well as adhesive joining. In parallel, process simulations of all investigated processes are also being prepared. The knowledge gained will be validated by the production and testing of aerospace demonstrator components.
The AIRPOXY consortium consists of a multidisciplinary team of 11 partners from 6 EU countries.
The project is funded from the European Union's Horizon 2020 Research and Innovation Program (Grant Agreement No 769274).