Shrinkage-resistant ultraviolet-curing coatings

An ultraviolet-curable liquid coating composition which possesses minimal substrate deformational shrinkage on cure is disclosed in which (A) a cationically curable polyepoxide having a 1,2-epoxy equivalency of at least 1.3 is used in admixture with (B) an acrylate-functional compound which is either (1) a monoacrylate-functional compound carrying at least two primary hydroxy groups; or (2) an epoxy-functional monoacrylate in admixture with a compound carrying a plurality of primary hydroxyl groups. An acrylate-functional liquid (C), preferably a monoacrylate, is used to provide coating viscosity, and a catalyst component capable of initiating, on ultraviolet exposure, both the conventional free-radical polymerization of ethylenically unsaturated monomers, and the cationic polymerization of polyepoxides in the presence of a compound carrying the primary hydroxy group. This catalyst component usually comprises a radiation sensitive aromatic onium salt capable of releasing a Lewis acid when exposed to radiant energy, together with an effective amount of a ketonic free radical photoinitiator when the onium salt is ineffective to perform this function. This provides a two stage cure in which shrinkage is minimized to control curling and cupping of easily deformable coated substrates.