The effect of sizing and surface oxidation on the surface properties and tensile behaviour of recycled carbon fibre: An end-of-life perspective

Carbon fibre is not widely recycled due to the inconsistent tensile properties of the recycled carbon fibre (rCF). It is unclear whether the surface treatments for virgin carbon fibre (vCF) have an impact on the quality of rCF. This paper presents an experimental investigation into the effect of epoxy sizing and electrochemical oxidation for vCF on the tensile and surface properties of rCF recycled by pyrolysis. Carbon fibre with sizing, or surface oxidation, or both, experience a reduction in tensile strength by 44.6%, 43.7% and 50.1%, respectively, during the recycling process, compared to 19.8% for un-treated fibre. Surface-treated carbon fibres exhibit more surface defects (I_D/I_G from 1.6 to 2.7 from Raman spectroscopy) and a smaller average crystallite size (from 7.6 nm to 6.2 nm from X-ray diffraction) after recycling. The results indicate that the interactions between the sizing agent and surface functional groups contribute to the down-cycling of carbon fibre.