Simple Flexible-Rigid Compatible Interface Design Based on Waterborne Polyurethane and Its Mechanism in Carbon Fiber Composites

The interface is critical to the performance of carbon-fiber-reinforced polymer composites (CFRPs). This work presents a “rigid-flexible” hybrid sizing agent, where amino-functionalized silica (NH2–SiO2) microspheres are dispersed within a waterborne polyurethane (WPU) matrix, designed to enhance the fiber–matrix interface. The WPU ensures excellent compatibility and adhesion with the epoxy resin, while the NH2–SiO2 provides mechanical reinforcement and active sites for chemical bonding with the matrix. Treated fibers retained high single-filament strength (3.35 GPa). The interlaminar shear strength (ILSS) of the composite reached 84.61 MPa, representing a 23.5% improvement over the composite sized with neat WPU. Fractography revealed a transition from fiber-dominant pull-out to resin-rich failure, indicating a shift to a stronger interface. Based on these indirect observations, the enhancement mechanism is proposed to involve covalent/hydrogen bonding facilitated by the amino groups and a synergistic “rigid-particle-reinforcement/flexible-polymer-bridging” effect at the interface. This strategy offers a pathway to surpass the limits of conventional sizing for high-performance CFRPs.