Delayed-onset UV-curable adhesives with long-term storage stability by Michael addition reaction between multifunctional acrylate and acetoacetic acid esters using photobase generators

In recent years, there has been a growing demand for UV-delayed curable adhesives that bond lightproof adherends. Anionic UV-curing with photobase generators (PBGs) is promising as a delayed-onset UV-curing system because of its slow termination reaction without concerns pertaining to metal corrosion. However, one-component anionic UV adhesives with high reactivity at room temperature and long-term storage stability are challenging to develop. Herein, we describe an anionic UV-curing system based on the Michael addition of acetoacetic acid esters (AAs) and acrylates with trans-o-coumaric acid (Cou)-type PBGs that release organic superbases. The AA/acrylate curing system demonstrated UV-delayed curability, as evidenced by FT-IR studies, at an exposure dose of 1600 mJ/cm² under 365 nm LED-UV irradiation. Intriguingly, the delayed-onset adhesive, composed of bifunctional rigid AA, multifunctional acrylate, and Cou-type PBG, bonded lightproof copper substrates with a lap shear strength of 6.1 MPa. Furthermore, this one-component UV adhesive showed excellent storage stability; the solution state was retained for >14 days at 40°C. This long-term storage stability is due to the appropriate acidity of the active methylene protons of the AAs—stability is retained as long as no superbase is produced from the Cou-type PBG after UV irradiation.