Reactive Ionic Liquid‐Modified Bio‐Based Waterborne Polyurethane Sizing for Improved Interfacial Compatibility in Carbon Fiber/Epoxy Composites

Waterborne polyurethane (WPU) is widely employed as a carbon fiber (CF) sizing agent; however, the intrinsic polarity mismatch between conventional WPU and epoxy resin (EP) often restricts interfacial adhesion and limits stress transfer efficiency in carbon fiber/epoxy (CF/EP) composites. In this work, a reactive ionic liquid (IL) was incorporated into a bio‐based WPU sizing system to modulate interfacial interactions and enhance compatibility with the EP matrix. The introduction of IL provides additional polar interaction sites within the sizing layer while maintaining the dispersion stability of WPU. Remarkable improvements in interfacial and mechanical properties were achieved. At an optimized WPU:IL solid ratio of 1:0.75, the interfacial shear strength (IFSS) increased from 56.15 MPa for unsized composites to 105.91 MPa, corresponding to an enhancement of 88.62%. Meanwhile, the ILSS and flexural strength reached 86.02 and 1810 MPa, representing improvements of 80.22% and 58.77%, respectively. Thermal analysis indicated that the incorporation of IL did not disrupt the fundamental curing chemistry of the EP system but contributed to a more favorable interfacial network integration. The significant improvement in mechanical properties is attributed to the synergistic effect of improved molecular affinity and localized network reinforcement at the fiber–matrix interface, which promotes more efficient stress transfer and suppresses interfacial debonding. This study provides an effective and sustainable strategy for designing high‐performance WPU sizing systems for advanced CF/EP composites.