Towards recycling of waste carbon fiber: Strength, morphology and structural features of recovered carbon fibers

Carbon fiber is one of the most widely used materials in high demand applications due to its high specific properties, however, its post-recycling properties limit its use to low performance applications. In this research, the carbon fiber recovering is examined using two methods: two-step pyrolysis and microwave-assisted thermolysis. The results indicate that the fibers recovered by pyrolysis show reduced surface and structural damage, maintaining the original mechanical properties of the fiber with losses below 5%. The fibers recovered by microwaves undergo significant surface changes that reduce their tensile strength by up to 60% and changes in their graphitic structure, increasing their degree of crystallinity by Raman index I_D/I_G from 1.98 to 2.86 and their amorphous degree by I_D"/I_G ratio from 0.411 to 1.599. Recovering fibers from microwave technique is 70% faster compared to two step pyrolysis, and provides recycled fibers with superior surface activation with the presence of polar functional groups -OH, -CO, and -CH that react with the epoxy matrix. The thermal, morphological, structural and mechanical characterizations of the recovered fibers detailed in this work provide valuable findings to evaluate their direct reuse in new composite materials.