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

Abstract

AbstractIn 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.