A thermosetting coating composition with enhanced flexibility and impact strength and excellent weatherability adapted for use as an automotive topcoat which upon curing forms a hard, glossy, durable coating. The coating composition comprises:-
(A) a modified hydroxy acrylic copolymer which is the stepwise reaction product of:-
(i) a copolymer bearing pendant hydroxyl groups and having a number average molecular (M n ) of between about 1000-5000, (ii) a dicarboxylic acid anhydride comprising at least about 50 percent by weight of an alkyl hexahydrophthalic anhydride, and (iii) a monoepoxide functional monomer comprising at least about 90 percent by weight of a C 2 -C 10 monoepoxide functional monomer; (B) an amine-aldehyde or polyisocyanate crosslinking agent; (C) optionally a hydroxy functional additive, and (D) solvent. The anhydride is reacted in an amount sufficient to esterify at least about 30 percent of the pendant hydroxyl groups of copolymer (i) thereby generating carboxyl functionality, substantially all of which is thereafter reacted with the monoepoxide functional monomer. The crosslinking agent is included in the composition in an amount sufficient to provide at least about 0.5 equivalent of crosslinking functionality per equivalent of hydroxy functionality included in the composition either as a hydroxyl group of the modified hydroxy acrylic copolymer or as a hydroxyl group of the optional hydroxy-functional additive. (ii) dicarboxylic acid anhydride comprising at least 50 percent by weight of an alkyl hexahydrophthalic anhydride, wherein (i) and (ii) are reacted in an amount sufficient to esterify from 30 to 100 percent of the hydroxyl groups of the copolymer, thereby generating pendant carboxyl groups; and (iii) a monoepoxide functional monomer comprising at least 90 percent by weight C - C 10 monoepoxide functional monomers, wherein the monomer is reacted with the pendant carboxyl groups of the polymer reaction product of (i) and (ii) so as to provide at least 1.0 equivalent of epoxide groups per equivalent of the pendant carboxyl groups generate pendant hydroxyl groups; (B) an amine-aldehyde or polyisocyanate crosslinking a gent; (C) 0-50 weight percent based on the total weight of (A), (B), (C) and (L) of a hydroxy functional additive having a number average molecular weight (L n ) of from 150 to 6000, preferably 500-2500; and (D) solvent. The amine-aldehyde crosslinking agent is included in the composition in an amount sufficient to provide at least 0.60, preferably from 0.75 to 3.75, equivalents of nitrogen crosslinking functionality for each equivalent of hydroxyl functionality included in the composition either as a hydroxyl group on the modified hydroxy acrylic copolymer or as a hydroxyl group on the hydroxy functional additive. The polyisocyanate crosslinking agent is included in an amount sufficient to provide 0.5 to 1.6 reactive isocyanate groups for each equivalent of hydroxy functionality included in the composition as an hydroxyl group on either the said copolymer or said additive. The composition preferably contains at least 60% by weight and preferably at least 70% by weight of non-volatile solids. In addition, the coating composition of this invention may include additives such as catalysts, antioxidants, U.V. absorbers, flow control or wetting agents, antistatic agents, pigments, plasticizers,