Click reaction benzimidazole into waterborne polyurethane toward anti-flashing rust, self-healing, robust coatings

Waterborne coatings have emerged as a promising alternative to traditional solvent-based coatings due to their environmentally friendly and easy application characteristics. Nevertheless, the application of these coatings is still limited by significant challenges such as the occurrence of flash rust phenomenon and the difficulty of reconciling mechanical properties with self-healing functions. Here, we report a novel self-healing waterborne polyurethane coating that utilizes coordination chemistry to achieve anti-flashing rust, mechanical robustness, and preserve self-healing and adhesion capabilities. The waterborne coating was designed by attaching 2-(2-benzimidazolyl) ethanethiol (BET) onto the polyurethane side chain. The conjugated benzimidazole forms a coordination structure with the tinplate substrate, effectively preventing the occurrence of flashing rust from the metal substrate during the film-forming process. Furthermore, BET forms a Zn(II)-ligands cross-linking structure with the exogenous zinc ions through coordination bonds, resulting in a distinctive sparse inside and dense outside structure due to the inhomogeneous distribution of zinc ions within the coating, thereby leading to high surface hardness (4H), and holding strong adhesion (similar to 2.0 MPa). In addition, the coordination between Zn2+ and BET provides extra interchain forces, resulting in a high self-healing efficiency (95.9%). Our work proposes a novel strategy to address the contradiction of simultaneously achieving anti-flashing rust and high hardness, without compromising adhesion and self-healing capabilities, thus offering the potential for expanding the commercial applications of waterborne polymer coatings.