High production rates, short cycle times, as well as robust processes are the prerequisites for an efficient production of high-performance components made of fiber reinforced polymer composites (FRPC), especially carbon fiber reinforced polymer composites (CFRPC). FRPCs based on thermosets provide high service temperatures and integrally manufactured components in small and medium quantities. FRPC based on thermoplastic polymers however, are characterized by high production quantities, weldability and uncomplicated recycling. The development of the vitrimers - a completely new family of plastics - by Montarnal et al. in 2011, opens up completely new possibilities in regards to use, processing and recycling of FRPC by combining the advantages of thermosets and thermoplastics.
Within the framework of the AIRPOXY project (ThermoformAble, repaIrable and bondable smaRt ePOXY based composites for aero structures), the possibilities and achievable property advantages by using vitrimers in the field of CFRPC will be researched in greater detail. Aim of this EU-funded project is to demonstrate exemplarily - by means of CFRPC components from the aviation sector - that also production and maintenance costs can be significantly reduced. For this purpose, carbon fiber reinforced epoxy resin-based vitrimers are to be developed as a matrix polymer and the processability is to be investigated. At room temperature, vitrimers as well as thermoplastics are fully cross-linked. At temperatures above the glass transition temperature (Tg), vitrimers FRPC can be thermoformed like thermoplastics. When vitrimers are heated, the covalent bonds detach and re-crosslink during cooling. If the vitrimer FRPC is deformed before re-crosslinking, the deformation is retained after cooling. Hence, vitrimers combine the advantages of common thermosets such as high stiffness, chemical resistance and low creep with thermoplastic properties, such as reversible formability, weldability, and recyclability. Three possible processing methods along the process chain for the production of vitrimer CFRPC components are being investigated in detail at Institut für Verbundwerkstoffe GmbH.
For the production of flat semi-finished goods, individual layers of carbon fiber fabric prepregs with fully and partially cured vitrimers are processed into continuous vitrimer CFRPC sheets - using a continuous compression molding technology by simultaneous application of consolidation pressure and temperature.
Further processing of these semi-finished materials and subsequent component manufacture are carried out using the thermoforming process. Due to the specific forming mechanisms of vitrimer CFRPC, a completely new process approach has to be developed, which is nevertheless comparable to the established thermoforming of thermoplastic CFRPC. This primarily concerns temperature management and associated exposure times, in order to enable the vitrimer the reorganization of the dynamic bonds during the forming process. The corresponding process windows will be developed to ensure a reliable processing of the vitrimer CFRPC during thermoforming.
The dynamic, covalent bonds also offer the option of using vitrimer CFRPC by thermal joining processes. For this, research is being carried out to determine if induction welding developed at IVW is suitable as a joining technology and if new concepts for component repair can be developed on this basis.
Further research topics of the project consortium are the optimization of the vitrimer resin formulation, its processability in the RTM process, investigations on component testing, and structural health monitoring as well as adhesive joining. The knowledge gained will be validated by manufacturing and testing demonstrator components from the aviation sector.
The AIRPOXY consortium consists of a multidisciplinary team of 11 partners from six EU countries.
AcknowledgementsThe project is funded from the European Union's Horizon 2020 Research and Innovation Program, Grant Agreement No 769274.
Dipl.-Ing. Stefan Weidmann
Institut für Verbundwerkstoffe GmbH
Phone: +49 631 2017 383