Evolution of pyrolysis char during the recovery of carbon fiber reinforced polymer composite and its effects on the recovered carbon fiber

Pyrolysis emerges as a highly promising technique for the reclamation of carbon fibers from waste carbon fiber reinforced polymer (CFRP) composites. The properties of the recovered fiber are profoundly influenced by the removal of pyrolysis char. This study delved into the evolution of pyrolysis char during the pyrolysis recovery of CFRP composite and its subsequent impact on the mechanical properties of the recovered fiber. The findings illustrated that the evolution of pyrolysis char could be categorized into two distinct stages: formation and polymerization. Throughout the pyrolysis process, the epoxy resin underwent initial decomposition into aromatic compounds, followed by cleavage into light alkanes. The emergence of pyrolysis char was attributed to the epoxy resin's depolymerization, which led to the formation of defective carbon crystals and oxygen-containing structures. Following the cracking of epoxy resin, the polymerization of pyrolysis char occurred, facilitating the elimination of unsaturated oxy-containing structures and defective carbon crystals, which reduced the pyrolysis char content while elevating its graphitization degree. Furthermore, raising the pyrolysis temperature or prolonging the duration accelerated the polymerization process of pyrolysis char. Notably, the oxidation of pyrolysis char caused a degradation of the tensile strength of the reclaimed fiber. Compared to pyrolysis duration, the impact of pyrolysis temperature on the tensile strength of recovered fiber was more pronounced, due to the reduced pyrolysis char at elevated temperatures. The conclusions provided some valuable insights for refining the pyrolysis recovery of carbon fibers with superior mechanical properties from waste CFRP composites.