Scratch-healable automotive clearcoats based on disulfide polyacrylate urethane networks

Abstract

We report a novel scratch-resistant and healable automotive clearcoat that can not only protect the substrate surface but also facilitate scratch repair. A self-healable disulfide polyurethane diol adduct (DSPUDA) was synthesized via step-growth polymerization with bis(4-hydroxyphenyl) disulfide, isophorone diisocyanate, and poly(tetrahydrofuran). The reversibility of bond cleavages and associations of the synthesized DSPUDA was quantified through nuclear magnetic resonance analysis, and its healing properties were confirmed using a universal testing machine and a rheometer. The material properties of clearcoats were tuned by varying the amounts of the DSPUDA added to prepared control clearcoats. The scratch resistances of the resultant clearcoats were assessed using an Amtec-Kistler car-wash tester and a gloss-meter, and their scratch-healing performances were evaluated using a micro-scratch tester and an optical microscope. In addition, the healing efficiency was quantitatively scrutinized using a nano-scratch tester and an atomic force microscope. The thermal properties of the clearcoat samples specifically, the thermal decomposition temperature, glass-transition temperature, and degree of crosslinking were characterized by thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis, respectively. The mechanical properties of the clearcoat surfaces were investigated using nano-indentation and pendulum hardness tests. The healing efficiency and mechanical properties of the prepared clearcoats exhibited an inverse trade-off relationship. Nevertheless, optimizing the proportion of reversible disulfide bonds of the DSPUDA in the polyacrylate-based urethane networks not only resulted in superior scratch healing and scratch resistance but also maintained sufficient mechanical properties for the coatings to be used as automotive clearcoats.