Process for epoxy phosphate coating resins

Abstract

The invention is an improved process for preparing epoxy resin/phosphoric acid reaction products having high enough phosphomonoester contents to be water-dispersible (as base salts) and low enough free acid contents to yield water-resistant cured coatings. The phosphoric acid is provided as an acid source material deriveable by the reaction of a hydroxylic P--OH functionality-limiting agent (blocking agent) with phosphoric anhydride as such or as a reaction product thereof with less than six molecular proportions of water. A resin solvent, which may also function as the P--OH blocking agent but otherwise is inert and acceptable as part of the final coating resin formulation, is employed as the reaction medium. The epoxide/acid reaction is allowed to proceed, effectively in the absence of water, until the content in the reaction mixture of species indistinguishable from H.sub.3 PO.sub.4 by base-titration has dropped to a minimum. Then enough water to hydrolyze the phosphopolyesters present is added and the hydrolysis is carried out, thereby cleaving the ester groups formed from the blocking agent and generating the P--OH groups needed for salification. The disclosed resin compositions are of particular utility for coating the interiors of beverage cans.