Rational design of a room‐temperature curing method, based on the epoxy–thiol click reaction, for <scp>UV</scp>‐curable hard coatings with ultrahigh strength and adhesion

Abstract In recent years, there has been a growing demand for UV‐curable hard coatings because they offer several advantages, for example, lower energy consumption and the absence of volatile organic compound emissions. Anionic UV curing with photobase generators (PBGs), such as epoxy–thiol cross‐linking, enables curing under ambient conditions. However, designing a room‐temperature photoanionic curing system for epoxy–thiols remains challenging due to the complex effects of resin combinations on the cured products, thus making it difficult to prepare films with high hardness. Herein, we present a controlled reaction design for an anionic UV‐curing system by combining the chemical structures of epoxy resins and multifunctional thiols. The anionic UV‐curing system using PBGs that generate organic superbases demonstrated UV‐delayed curability, as evidenced by FT‐IR and photorheological analyses. To achieve high hardness, it was necessary to create the thiols with ultrarigid structures; epoxy resins with a bisphenol F structure were optimal for reacting with thiols for ultrarigid structures. This combination afforded an indentation hardness of 262 MPa with an epoxy conversion rate of 77% even at room temperature.