Waterborne Polyurethane/Nano‐SiO₂ Hybrid Organic–Inorganic Sizing Agents for Enhanced Mechanical Properties of Carbon Fibers/Epoxy Composites

The bond strength between carbon fiber (CF) and sizing agent and the surface roughness of CF play a vital role in improving the interfacial properties of composite materials. In order to solve this problem, a new multiscale enhancement strategy is developed. Through the hydrolysis reaction of anhydrous silica sol in self‐emulsifying highly hydrophilic waterborne polyurethane (WPU) emulsion, the generated crosslinked network of SiOSi bonds generates hydrogen bond interaction between CF surface and WPU, which enhances the bonding strength of sizing agent to CF. At the same time, the nano‐SiO 2 generated by hydrolysis further improves the interfacial stress transfer of the composites through the interlocking mechanism. The surface roughness, O/C ratio, and compatibility with the resin matrix of the CF after sizing are significantly improved, and the interlaminar shear strength and flexural strength of the corresponding composites are increased by 53.06 and 50.89%, respectively. In addition, the sizing agent prepared by this method overcomes the shortcomings of poor storage stability of traditional inorganic nanomaterial‐modified sizing agent. In short, this work provides assistance for the development and application of high‐performance sizing agents.