Requirements for the Chemical Recycling of Fibre Reinforced Polymers

The increasing use of fiber reinforced polymers (FRP) in lightweight construction is presenting new fundamental challenges for the recycling industry. In Germany alone, approx. 225 kt of glass fibre reinforced plastic (GFRP) material was produced in 2019, compared to approx. 141 kt of carbon fibre reinforced plastic (CFRP) worldwide, which has to be recycled even after the end-of-life. Main focus is principally on epoxy resin and carbon fibers (used for example in wind power plants and in electro mobility). Apart from pyrolysis, which currently is the only industrially used process and is mainly applied to carbon fiber reinforced polymers, chemical processes are being researched for the recycling of reinforcing fibres and matrix. The ability to fully recover both fiber and matrix by using solvents at the same time is one of the main advantages of chemical solvent processes. The challenge in chemical recycling is the selectivity of the respective solvents when applied to the variety of material combinations as matrix or fiber reinforcement in composite materials. Aim of this research work is the investigation and definition of requirements for a fiber recovery from the epoxy matrix with subcritical and supercritical water. In the chemical recycling of GFRP and CFRP waste, the epoxy resin is converted into a liquid phase, while the fibers remain as a solid residue. Due to the chemical stability of epoxy resins, their chemical recycling by solvolysis requires high temperatures and high pressures; the process conditions can be defined as subcritical or supercritical depending on the physical state of the solvent during the process. Especially the use of supercritical fluids is considered promising for the recycling of FRP. Supercritical fluids have low viscosity, a high mass transfer behaviour and, most important, a high diffusion coefficient, which leads to a high solvability. The typically used solvents water or alcohols are considered to be environmentally friendly. The results of the research show that the amount of resin remaining decreases with increasing temperature and processing time, while temperature having the greater influence on resin removal.