Material Bonded Metal-FRP-Joints by Continuous Hybrid Profile Pultrusion

Lightweight design of complex structures is crucial for saving energy and, finally, CO2 emissions in transportation. For that reason, beside the mainly metallic primary structures (chassis), e.g. in the automotive industry, fiber reinforced plastics (FRP) are used for secondary structures in rising amounts to match the requirements of the e-mobility.
In order to realize effective FRP lightweight design in load-bearing structural components, a cooperation between Fraunhofer IWU and Leibniz IPF has developed hybrid profile components that can be produced in large volumes in future using a modified, continuous pultrusion process. The profiles consist of an FRP base structure and a metallic core, which increases the ductility of the resulting component. Furthermore, metal inserts are partially placed at the surface to integrate additional functionalities into the component structure, e.g. sliding properties.
In order to achieve a high bond strength between metal and FRP, pre-coated metal coils are fed into the pultrusion process. A special two-stage cross-linking powder coating developed at the IPF was used for the metal coating, which has a latent reactive adhesive function when cured. This powder coating was originally developed for material-bonded metal-plastic joints produced with thermoplastics via injection molding at temperatures above 200 °C. By adding dendrimers to the powder coating, the powder coating has been adapted to bond with the epoxy matrix of the FRP during the pultrusion process, which took place at temperatures between 150 and 190 °C. In the boundary layer between the pre-coated metal and FRP, covalent bonds were formed through interfacial reactions, which resulted in a material bond between the two components. In this way, a very high bond strength was achieved. The integration of the bond formation directly into the pultrusion process also resulted in savings in additional process steps, energy, cleaning and pre-treatment chemicals and waste water. This was due to the elimination of subsequent bonding processes and the complex pre-treatment processes required for this and ultimately increased efficiency in the manufacture of metal-FRP composites.