Photoinduced Redox Initiation System for Delayed-Onset Radical UV-Curable Adhesives on Opaque Substrates

Conventional radical ultraviolet (UV) curing requires optically transparent substrates owing to the rapid termination of propagating radicals, limiting its use in opaque or geometrically complex assembly cases. We report a photoinduced redox initiation system for delayed-onset radical UV-curable adhesives combining a coumarin-based photobase generator (Cou-TMG) with dilauroyl peroxide (LP). Upon UV irradiation, Cou-TMG releases 1,1,3,3-tetramethylguanidine, which reacts with LP through a redox process that continuously generates radicals, sustaining polymerization after irradiation stops. Optimized photobase generator loading and irradiation dose provide an open time sufficient for substrate lamination (low C═C conversion immediately after UV irradiation) and endow substantial postpolymerization capability (higher C═C conversion after 1 h), contrasting conventional radical UV curing systems. The delayed-onset, radical UV-curable adhesives are applicable to various multifunctional acrylates. Specifically, trimethylolpropane triacrylate-based adhesives achieve a high lap-shear strength of 4.4 MPa on aluminum, and the present system further bonds Cu, Fe, polycarbonate, and glass substrates.