Photopolymerizable epoxide coating compositions containing titanium dioxide pigment and method of polymerization using same

In the method of producing a glossy adherent white coating upon a substrate, the steps comprising:

• A. admixing to form a polymerizable epoxide composition:
• (i) 54.7-89.7 percent by weight of a cationically polymerizable material consisting at least principally of an epoxide prepolymer;
• (ii) 10-45 percent by weight of titanium dioxide pigment;
• and
• (iii) 0.3-10.0 percent based upon the weight of said polymerizable material, of a photoinitiator component relatively stable to ambient white light but readily decomposable upon exposure to ultraviolet light to provide a Lewis acid to effect polymerization of said cationically polymerizable material, said photoinitiator component including:
• a first photoinitiator compound comprising an aryldiazonium salt wherein the aryldiazonium group is selected from the class consisting of 2,5-diethoxy-4-p-tolylmercaptobenzene diazonium, 4-chloro-2,5-dimethoxybenzene diazonium, 2,5-dimethoxybenzene diazonium, 2,5-dimethoxy-4-p-tolylbenzene diazonium and 2,5-dibutoxy-4-benzamidobenzene diazonium, and mixtures thereof, and wherein the salt group is selected from the class consisting of hexafluorophosphates, fluoroborates, fluoroantimonates and fluoroarsenates, and mixtures thereof;
• and a second photoinitiator compound relatively stable to ultraviolet radiation having a wavelength of at least 355 nanometers and decomposable by ultraviolet radiation having a wavelength of 200-355 nanometers;
• B. applying said polymerizable composition to a substrate to develop a coating of the desired thickness thereon;
• and
• C. exposing said coating to ultraviolet radiation to produce photolysis of said photoinitiator component to generate a Lewis acid and polymerize the epoxide composition within one minute to a hard glossy surface which is tack-free.