Composites With Epoxy‐Modified Waterborne Polyurethane Sizing Agents Enhance Mechanical and Thermal Properties

ABSTRACT The interface is critical to the mechanical properties of carbon fiber‐reinforced polymer (CFRP) composites. This research introduced a range of epoxy‐modified waterborne polyurethane (WEPU) sizing agents to improve both the mechanical performance and interfacial properties of composites. The investigation focused on the effect of different sizing agent concentrations and the amount of epoxy resin added on the mechanical properties of the composites. In particular, 1.5% concentration of the WEPU‐4% agent provided the most significant improvement in CFRP interfacial strength. Carbon fiber (CF) treated with this agent exhibited a tensile strength of 3.27GPa, an increase of 6.58%. The contact angle between the fiber and epoxy was reduced from 54° to 48°, improving the wettability of the resin on the CF surface. In addition, the interlaminar shear strength (ILSS) of the composites increased to 79.09 MPa, an improvement of 37.29%. This study provides an efficient and effective approach to WPU modification that significantly improves the interfacial performance at the CF–resin interface. The sizing agent synthesis is straightforward and efficient, in line with contemporary principles of environmental sustainability.