UV–thermally dual-curable 1K clearcoat via urethane and radical reactions
Soeun Kim, Kyu Cheol Lee, Seung Man Noh
Abstract
Conventional polyurethane-based automotive clearcoat systems have mainly consisted of a polyol and an isocyanate crosslinker, requiring a metal catalyst and high-temperature curing conditions. Herein, we report a UV–thermally dual-curable 1K clearcoat that contains isocyanate crosslinkers blocked by methacrylate-functionalized pyrazole (BIPyA-3) and a photoactive benzophenone containing a polyol (BP–polyol). The blocked isocyanate can form a urethane bond with the polyol after deblocking, and the self-initiation of methacrylate in the blocking agent can participate in the radical reaction. In addition, under UV irradiation, the benzophenone group can generate radicals that can propagate the methacrylate functional group of BIPyA-3 as well as form CC bonds with the polyol, thereby increasing the overall crosslinking density in the clearcoat. Rheological behaviors of the novel clearcoat were investigated by oscillatory rheometry under combined thermal and UV curing conditions. The thermal and surface mechanical properties of the films cured under different conditions—(i) thermal, (ii) thermal–UV, (iii) UV–thermal, and (iv) UV—were compared, and their relationship with the overall crosslinking density was investigated. This novel dual-curable, urethane–methacrylate clearcoat cured via a UV–thermal process can not only contribute to enhancing the surface hardness of clearcoats without modifying their chemistry but also effectively consume unreacted methacrylate functional groups in the cured films.