Synthesis of hyperbranched polyurethane sizing agent with high-solid content via self-catalytic method for improving interfacial adhesion of CF/PA6 composites

To improve the unsatisfactory interfacial adhesion of carbon fiber/polyamide-6 (CF/PA6) composites, a hyperbranched waterborne polyurethane (HWPU) sizing agent with high-solid content was designed. As a carboxy chain extender for HWPU, carbic anhydride-tris(hydroxymethyl)methyl aminomethane (CA-THAM) with a self-catalytic capacity was synthesized to accelerate the chain extension. The synthesized hyperbranched polyurethane exhibited low viscosity, along with a strong double-electron layer conformation after dispersing in water. The electrostatic repulsion between the latex particles effectively prevented agglomeration, delivering a high solid content of 58% for the sizing agent. The influence of different sizing agent concentrations on the wettability of CF as well as the interfacial properties of composites was systematically studied. Compared with unsized CF/PA6 composites, the interlaminar shear strength and flexural strength of CF-1.5 wt%/PA6 composites were up to 60.3 MPa and 508.4 MPa, with an increment of 50.3% and 27.7%, respectively. Furthermore, the interfacial strengthening mechanism of sizing agent was discussed. The preparation of this high-solid content sizing agents was facile and environmentally friendly, which would hopefully reduce costs during the transportation.