High solids urethane coatings with enhanced flexibility and impact strength

A high solids coating composition which contains greater than about 60 percent by weight nonvolatile solids and which expect for pigments and other nonreactive components, consists of:

• (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 weight (M.sub.n) of between about 1000-5000, said copolymer being prepared from (a) between about 5 and about 40 weight percent of monoethylenically unsaturated hydroxy alkyl ester having formula CH.sub.2 =CR.sub.1 --COOR.sub.2 wherein R.sub.1 is H or an alkyl group and R.sub.2 is a hydroxy alkyl group and (b) between about 95-60 weight percent of other monoethylenically unsaturated monomers;
• (ii) dicarboxylic acid anhydride comprising at least about 50 percent by weight of an alkyl hexahydrophthalic anhydride, wherein (i) and (ii) are reacted in an amount sufficient to esterify between about 30 and bout 100 percent of said pendant hydroxyl groups of said copolymer thereby generating pendant carboxyl groups;
• and
• (iii) a monoepoxide functional monomer comprising at least about 90 percent by weight C.sub.2 -C.sub.10 monoepoxide functional monomers, wherein said monomer is reacted with said pendant carboxyl groups of the polymer reaction product of (i) and (ii) so as to provide at least about 1.0 equivalent of expxide groups per equivalent of said pendant carboxyl groups to generate pendant hydroxyl groups;
• (B) polyisocyanate crosslinking agent having two or more reactive iscocyanate groups per molecule;
• (C) up to about 50 weight percent based on the total weight of (A), (B), (C), (D) and (E) of a hydroxy functional additive having a number average molecular weight (M.sub.n) of between about 150 and about 6000;
• (D) solvent;
• and
• (E) up to 15 weight percent based on the total weight of (A), (B), (C), (D) and (E) of a flow control additive,
• said polyisocyanate crosslinking agent being included in said composition in an amount sufficient to provide between about 0.50 and about 1.6 reaction isocyanate groups for each equivalent of hydroxyl functionality included in said composition either as a hydroxyl group on said modified hydroxy acrylic copolymer or as a hydroxyl group on said hydroxy functional additive.