Coating composition catalyzed with phosphonic acid derivative-#2

A thermosetting coating composition adapted for low temperature bake applications which, exclusive of pigments, solvents and other nonreactive components, consists essentially of:

• (A) a bifunctional copolymer bearing hydroxy functionality and pendent epoxy functionality, having a number average molecular weight (Mn) of between about 1500 and about 10,000 and a glass transition temperature (Tg) of between about -25.degree. C. and about 70.degree. C., said copolymer consisting essentially of (i) between about 5 and about 25 weight percent of monoethylenically unsaturated monomers bearing gylcidyl functionality and between about 5 and about 25 weight percent of monoethylenically unsaturated monomers bearing hydroxy functionality, with the total of said gylcidyl and hydroxy functional monomers being not greater than about 30 weight percent of the monomers in said bifunctional copolymer and (ii) between about 90 and about 70 weight percent of other monoethylenically unsaturated monomers;
• (B) A reactive catalyst comprising at least one phosphonic acid derivative having the formula:
• ##STR3## wherein n=1 to 2 and R is selected from the group consisting of alkyl, cycloalkyl and aryl radicals;
• (C) an amine-aldehyde resin crosslinking agent;
• and
• (D) up to about 45 weight percent based on the total weight of (A), (B), (C), and (D) of a hydroxy functional additive having a number average molecular weight (Mn) of between about 150 and about 6000,
• said phosphonic acid derivative being included in said composition in an amount sufficient to provide between about 0.8 and about 1.4 equivalents of acid functionality for each equivalent of pendent epoxy functionality on said bifunctional copolymer, and said amine-aldehyde resin crosslinking agent being included in said composition in an amount sufficient to provide at least about 0.4 equivalents of nitrogen cross-linking functionality for each equivalent of hydroxy functionality included in said composition as a group on an initial component thereof or generated in situ or as a result of esterification of said pendent epoxy functionality of said bifunctional copolymer during cure of said coating composition.