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

ABSTRACT 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.