Atmospheric-Pressure Plasma-Induced Surface Modification of Recycled Carbon Fibers for Improved Interfacial Properties in Epoxy Composites

The recycling of carbon fiber-reinforced polymer (CFRP) is essential for reducing environmental pollution and improving resource efficiency. However, carbon fibers recovered during the recycling process are typically obtained in the form of short fibers, and the removal of sizing leaves the fiber surface unsized, resulting in deteriorated interfacial properties. In this study, an atmospheric-pressure plasma (APP) treatment applicable to continuous processing was employed to modify the surface characteristics of recycled carbon fiber (rCF) and promote more favorable interactions with an epoxy matrix. The surface characteristics of plasma-treated rCF were analyzed using temperature-programmed desorption, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, and single-fiber contact angle measurements. Furthermore, the mechanical performance of recycled carbon fiber-reinforced epoxy composites, hereafter denoted as rCFRP, fabricated with the treated fibers was evaluated through tensile and flexural testing. APP treatment increased the content of oxygen-containing groups on the rCF surface, with the O1s/C1s ratio increasing from 0.15 to 0.39 and the polar component of the surface free energy increasing from 14% to 32% which may have contributed to more favorable interactions with the epoxy matrix. The tensile and flexural strengths of the rCFRP composites increased by up to 59% and 58%, respectively, compared with those of the untreated rCFRP composites. This study demonstrates that APP treatment is an effective approach for surface modification of rCF and suggests its potential applicability to the continuous manufacturing of rCFRP composites.