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 (NH 2 –SiO 2 ) 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 NH 2 –SiO 2 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.