Enhanced Interfacial Properties of High-Modulus Carbon Fiber Reinforced PEKK Composites by a Two-Step Surface Treatment: Electrochemical Oxidation Followed by Thermoplastic Sizing

A two-step surface treatment was conducted on high strength and high modulus carbon fibers (HSHM-CFs) to enhance the interfacial properties of polyetherketoneketone (PEKK) matrix composites. In the two-step surface treatment, the surfaces of HSHM-CFs were electrochemically oxidized followed by sizing treatment. Thermoset epoxy (EP) sizing agents and thermoplastic polyetherimide (PEI) sizing agents were utilized for comparison. Results showed that PEI sizing agents had better wetting effects than EP sizing agents. The sizing process led to increases in fiber strength due to healed surface flaws and inhibited micro-crack initiation. Although EP sizing agents were incompatible with PEKK matrix, the interlaminar shear strength (ILSS) of PEKK matrix composites reinforced by EP-sized fibers still increased, compared with that by untreated fibers, due to hydrogen bond and weak chemical reactions between HSHM-CFs and PEKK matrix. The strong interaction between PEI sizing agents and HSHM-CFs, diffusion and entanglement as well as the non-covalent π-π interaction between PEI sizing agents and PEKK matrix resulted in highest ILSS value of PEI-sized HSHM-CF/PEKK composites.