As a key component of composite materials, the interface quality is crucial for determining the mechanical properties of composites. Carbon fiber sizing treatment significantly enhances the fiber-matrix interface, a process extensively utilized in the carbon fiber industry. This study synthesized an environmentally friendly waterborne polyurethane sizing agent and investigated the impact of molecular weight, a critical factor, on composite performance by varying the soft segment type in the polyurethane. This research provides insights into cost-effective and eco-friendly surface treatment methods for carbon fibers and the design of robust interface structures.

In this work, the trend in the performance of carbon fiber (CF) and its composite during self-polymerization of polydopamine (PDA) at carbon fiber surface was investigated by varying the self-polymerization time of dopamine in an aqueous solution. Research has shown that the PDA coating elevated the surface roughness and polarity of the inert fiber. The tensile strength of single carbon fiber was significantly improved, especially after 9 h of polydopamine self-polymerization, increasing by 18.64% compared with that of desized carbon fiber. Moreover, the interlaminar shear strength (ILSS) of CF-PDA9-based composites was 35.06% higher than that of desized CF-based composites. This research will provide a deep insight into the thickness and activated ingredients of dopamine oxidation and self-polymerization on interfacial compatibility of carbon fiber/epoxy resin composites.