Bio-Based Waterborne Polyurethane Sizing Agent Containing Strong Adhesion Catechol Structure for the Interfacial Enhancement of Carbon Fiber/Polyamide 6 Composites

Herein, a bio-based polyurethane (PU) was synthesized using L-lysine diisocyanate and castor oil as raw materials. Following the salt formation reaction and subsequent emulsification process, a waterborne sizing agent was successfully obtained. To enhance the adhesion strength between the carbon fibers (CF) and sizing agent, 3,4-dihydroxybenzoic acid containing a catechol group was chosen as the neutralizing agent and introduced into the PU. The particle size of the PU sizing agent was 124.3 nm, indicating its suitability for long-term storage. After treating with 1.0, 1.5, and 2.0 wt% concentrations of the sizing agent, the CF referred to as CF1, CF2, and CF3, respectively, were used to prepare CF/polyamide 6 (PA6) composites. The sizing agent considerably improved the interfacial bonding between the CF and the PA6 matrix, with the CF2/PA6 composites showing the greatest enhancement. Specifically, the flexural strength, interlaminar shear strength and interfacial shear strength of the treated composites were enhanced by 28.3%, 47.5%, and 52.3%, respectively, compared with those of the unsized composites. The interfacial failure mechanism of the composites under stress was elucidated via fracture morphology characterization. These findings offer a valuable reference for the future development of novel waterborne PU sizing agents.