Thermal degradation behavior and decomposition mechanism of thermoset plastic for carbon fiber-reinforced plastic recycling under varied process conditions

Thermosetting plastics, widely used across industries, are infusible and insoluble cross-linked polymers, challenging to recycle and reuse upon disposal. To address this, efforts have been directed toward recycling waste thermoset plastics, a process akin to upcycling. This study primarily aims to establish an environmentally friendly upcycling process for the thermochemical recycling of end-of-life (EoL) thermoset plastics, ensuring the preservation of their technical, economic, and environmental benefits. We investigated the thermal decomposition of diglycidyl ethers of bisphenol A epoxy (EP) cured with diethylenetriamine (DETA) and diaminodiphenylsulfone (DDS) in a superheated steam (SHS) atmosphere. The reaction mechanism entails the homolytic decomposition of thermoset polymer chains, with subsequent saturation of the resulting radicals through dehydrogenation from the SHS. A prominent macroscopic mechanism for thermally decomposing EP using an amine curing agent encompasses the initial ester bond cleavage, chain-end scission, and cyclization reactions under SHS influence. Introducing benzene rings rendered EP/DDS decomposition significantly tougher than EP/DETA. Under SHS conditions, the epoxy decomposition rate surpassed 95 % irrespective of the amine curing agent. Additionally, the mechanical properties of recycled carbon fiber (rCF), retrieved from carbon fiber-reinforced plastic (CFRP) pyrolysis using the optimized SHS method, demonstrated a tensile strength of approximately 80 % of the as-received carbon fiber (As-CF).