Hyperbranched bio-based waterborne polyurethane sizing agent: A ZnO-tartaric acid chelation strategy for synchronous interfacial enhancement and UV protective performance in carbon fiber/epoxy composites

Bio-based waterborne polyurethane (WPU) has been instrumental in enhancing the adhesion between carbon fibers (CF) and the epoxy resin matrix (EP). However, conventional waterborne polyurethane sizing agents are susceptible to photochemical oxidation and degradation under prolonged ultraviolet (UV) exposure, leading to a gradual decline in interfacial properties. This instability limits their effectiveness in extreme environments. In this study, a novel carbon fiber sizing agent based on hyperbranched bio-based waterborne polyurethane zinc (WPUZ) was developed. By incorporating tartaric acid (TA) and zinc oxide (ZnO) through a chelation strategy, the CF/EP composites achieved simultaneous interfacial reinforcement and UV protection. Fourier transform infrared (FTIR) spectroscopy confirmed the formation of stable zinc carboxylate (-COO - Zn 2+ ) bonds between TA and ZnO, preventing ZnO nanoparticle aggregation and improving dispersion within the waterborne polyurethane matrix. Mechanical testing revealed that WPUZ-treated CF/EP composites exhibited significant improvements, with interfacial shear strength (IFSS), interlaminar shear strength (ILSS), and three-point bending strength increasing by 93.92 %, 46.00 %, and 53.42 %, respectively. Furthermore, UV aging tests demonstrated that after one week of xenon lamp exposure, the ILSS of WPUZ-treated composites remained above 92 %, far exceeding the performance of untreated and pure WPU-sized composites. These results highlight the effectiveness of WPUZ in enhancing both mechanical durability and UV resistance in CF/EP composites.