Self-Healing Waterborne Polyurethane Coatings Based on Dynamic Covalent Borate Esters with Boron–Nitrogen Internal Coordination

Waterborne polyurethane (WPU) coatings frequently experienced surface and interior damage during long-term use, leading to small cracks and significantly diminishing their protective and aesthetic properties, thereby reducing their service life. To address these challenges, we synthesized a borate ester-bonded compound with boron–nitrogen internal coordination (MN). Subsequently, a self-healing WPU coating was successfully prepared using polytetrahydrofuran (PTMEG-1000), diphenylmethane diisocyanate (MDI), a hydrophilic chain extender of 2,2-bis(hydroxymethyl)propionic acid (DMPA), and a borate-ester chain extender of MN. The results indicated that the storage stability of WPU emulsions containing MN was enhanced, maintaining stability without stratification for up to 30 days and remaining stable after centrifugation. The incorporation of MN into WPU films conferred an 83% self-healing ability, a tensile strength of 24.2 MPa, and significantly improved thermal stability. Additionally, WPU paint films containing MN exhibited superior water resistance, with no visible degradation after 24 h of immersion and a water absorption rate of only 10.0%. Moreover, the mechanical properties of WPU paint films with MN were also enhanced, demonstrating a hardness of 5H, an adhesion grade of 0, an impact resistance exceeding 50 cm, and a flexibility of 1 mm. At the same time, WPU paint films containing MN exhibited self-healing capabilities, effectively repairing cross-scratches. This study provided an approach for the development of WPU coatings technology, which demonstrated that self-healing WPU has potential applications in aerospace coatings, the automotive industry, and household products.