Transparent and Thermally Stable Urushiol Films by Fast Thiol–ene Curing

Abstract

Lacquer is well recognized in the coating industry thanks to its excellent performance, artistic value, and renewable properties. However, there exists difficulty in curing thick lacquer film due to its dark color attributed to the phenol oxidation and the inertness of internal double bonds that are persistent in the cured films, which also reduces the thermal stability of the obtained film. Meanwhile, the slow manual curing process also limits its largescale commercial use. To address the issues, in the current study, urushiol−urethane prepolymers were synthesized, characterized, and adopted as the foundation for transparent and thick lacquer film preparation through a fast and green thiol−ene click reaction with polymercaptan. By this mild and readily implemented method, flexible and transparent networked films based on renewable urushiol can be obtained with almost no film shrinkage. Moreover, the cross-linking density, modulus, and hydrophobicity of the films are also alterable, as validated by the differential scanning calorimetry, contact angle, and rheological measurements. More interestingly, the functionalization by mercapto groups not only stabilizes the double bonds remaining in the cured urushiol films but also endows the films with a good hydrophilicity. The high moduli of these films can be retained after repeated distortion under high temperatures, exhibiting an excellent thermal stability, which is incomparable with those of traditional lacquer films. Moreover, the urushiol-urethane prepolymer was demonstrated to well copolymerize with styrene as a macro-cross-linker for good film formation, exploring the application generality of this approach.