Molecularly engineered high-solid-content waterborne polyurethane for interfacial tailoring and intrinsic UV resistance of CF/PA6 composites

A novel environmentally friendly waterborne polyurethane (WPU) sizing agent was developed in this study for interfacial tailoring of carbon fiber (CF) reinforced polyamide 6 (PA6) composites (CF/PA6 composites), with the aim of simultaneously enhancing their mechanical properties and UV resistance, thereby achieving structural-functional integration. The sizing agent was molecularly designed with multi-hydroxyl side groups and UV-resistant chain segments, and synthesized via a facile two-step organic route, endowing it with excellent interfacial reactivity and intrinsic UV resistance. Regulating the hydrophilic group content enabled the preparation of WPU sizing agent with a high solid content of 56%, which exhibited high coating efficiency, environmental benignity, yellowing resistance, and storage stability, indicating considerable potential for industrial-scale production. Relative to unsized CF/PA6 composites, the flexural strength, interlaminar shear strength (ILSS), and interfacial shear strength (IFSS) of WPU sized CF/PA6 composite were enhanced by 61.5%, 57.0%, and 71.2%, reaching 715 MPa, 62.8 MPa, and 74 MPa, respectively, and the retention rate of mechanical properties after UV irradiation was as high as 98%. Furthermore, molecular dynamics (MD) simulations revealed microscopic interlocking between WPU and PA6, leading to an increased interfacial coupling thickness. This study offers new insights into the development of high-performance thermoplastic resin-based composites and the microscopic understanding of their interfacial tailoring mechanisms.