Recycling of carbon fibers from carbon fiber reinforced thermoset polymers by use of sub- and supercritical fluids

The recycling process chain of carbon fiber reinforced polymers (CFRP) recycling consists of the preparation of the recyclables, the fiber-matrix separation, the textile processing of the recycled carbon fibers (rCF) and the re-infiltration of the rCF-semi-finished product. In the present study, the separation step is investigated since it has major impact on the mechanical properties, surface properties and the processability of the rCF as well as on the energy balance of the CF recycling process. The investigated separation process is based on the solvolysis of a thermoset matrix material by two different sub- and supercritical fluids (SCF). Pure water and a water/ethanol-mixture are used as solvents. Treatment time and temperature, which have major impact on the efficiency of the process, were varied. The rCF were examined by Scanning-Electron-Microscopy (SEM) and Atomic-Force-Microscopy (AFM) for matrix residues and possible degradations of the fiber surfaces. The chemical composition of the rCF surfaces is characterized via X-ray-Photoelectron-Spectroscopy (XPS) and compared to an electrochemically oxidized unsized virgin carbon fiber (vCF). The mechanical properties of the rCF were characterized via single-fiber-tensile-tests. The study shows that the rCF properties depend on the solvent composition and treatment conditions. For optimized parameters rCF with properties comparable to vCF were obtained.