Physical properties of recycled carbon fibers from carbon fiber reinforced plastics by various recycling methods

Abstract

The use of composite materials incorporating carbon fibers (CFs) is steadily increasing. Given the high energy costs associated with manufacturing CFs, the need for recycling has become critical not only for environmental sustainability but also for economic viability. Although several recycling methods have been developed recently, there remains a significant gap in the integrated analysis of the properties of recycled CFs (r-CFs). Such an analysis is essential to maximize the usability of r-CFs recovered through various recycling techniques. Unlike previous studies that focused on individual recycling methods, this study systematically compared pyrolysis, supercritical fluid (SCF), superheated steam (SHS), and chemical oxidation under standardized conditions to elucidate direct correlations between process parameters and r-CF characteristics. Therefore, in this study, we analyzed the physical properties of r-CFs recovered through various recycling processes, focusing on surface morphology, chemical functionality, mechanical properties, and interfacial adhesion with polymer resins. The recycling processes included thermal and chemical oxidation methods, with a particular emphasis on preserving CF mechanical integrity while enhancing interfacial compatibility with polymer matrices. The organic residues and surface composition of r-CFs were analyzed through XPS, SEM, and AFM, and their electrical conductivity and tensile strength were measured. Additionally, the interfacial shear strength (IFSS) between epoxy resin/r-CFs and polyamide 6/r-CFs was evaluated. As a result, it was confirmed that chemical oxidation method was found to effectively introduce oxygen functional groups while minimizing structural degradation, thereby maintaining the mechanical and electrical properties of r-CF better than the recycling method using high temperature and high pressure. In particular, the chemical oxidation method facilitates improved interfacial adhesion with epoxy resin due to the synergy between surface roughness and chemical bonding, while its effect on PA6 adhesion is comparatively limited due to the high viscosity of the resin. These findings contribute to establishing sustainable recycling strategies for CFRP waste and advancing circular economy principles in the composites industry.