Super-Robust Mechanical and Self-Healing Properties of Waterborne Polyurethane Composites

Waterborne polyurethane (WPU) with self-healing properties is highly desirable for sustainable development. However, balancing the mechanical and self-healing properties of WPU is challenging. In this study, ultrahigh mechanical and self-healing performance was simultaneously achieved by introducing dopamine-modified iron-oxide nanoparticles (Fe₃O₄@DA NPs) to a WPU matrix consisting of soft poly(tetramethylene ether) glycol segments and hard isoforone diisocyanate and dimethylolpropionic acid segments. Consequently, the tensile strength and toughness of the WPU/Fe₃O₄@DA composite increased to 63.6 MPa and 179.35 MJ·m^–3, respectively; these values correspond to improvements of 276% and 403% over those of pure WPU (16.9 MPa and 44.50 MJ·m^–3, respectively). The proposed composite also exhibited excellent self-healing ability at room temperature; that is, the stress and strain self-healing efficiencies after 6 h at 60 °C increased by 93.5% and 100%, respectively. This performance was achieved because the Fe₃O₄@DA NPs, which acted as reinforcing fillers, interacted strongly with the hard WPU segments via the covalent reaction of the DA amine groups with the WPU-matrix isocyanate groups, remarkably enhancing mechanical properties. Moreover, catechol–Fe(III) coordination bonds formed between the DA and Fe₃O₄ NPs, which promoted relaxation behavior and, thus, enhanced the self-healing efficiency. The approach presented in this study constitutes an effective strategy for developing an efficient self-healing polymer with high mechanical strength, which can endow self-healing coatings and adhesives with long service life in severe environments.