| abstract |
The present invention relates to polyurethane powder coating compositions containing A) a hydroxyl-functional binder component, which is solid below 40° C. and liquid above 130° C., has a content of carboxyl groups (calculated as COOH; molecular weight 45) of 0.5 to 2.0 wt. %, and contains at least one polymeric polyol having an OH number of 15 to 200 mg KOH/g and a number average molecular weight of 400 to 10,000 and B) a polyaddition compound which contains uretdione groups and optionally free isocyanate groups, is solid below 40° C. and liquid above 125° C., and is prepared from aliphatic and/or cycloaliphatic diisocyanates, C) at least one zinc compound as hardening catalyst, and D) a compound which has a number average molecular weight of 200 to 5000 and contains groups reactive to carboxyl groups, wherein i) components A) and B) are present in amounts such that 0.8 to 2.2 isocyanate groups of component B) are present for each hydroxyl group of component A), ii) the term isocyanate groups of component B) means the sum of isocyanate groups present in dimeric form as uretdione groups and free isocyanate groups, iii) components A) and D) are present in amounts such that 0.8 to 2.0 groups of component D) which are reactive to carboxyl groups are present for each carboxyl group of component A), iv) when aliphatic and/or cycloaliphatic dicarboxylic acids having 4 to 20 carbon atoms and/or aliphatic hydroxyl carboxylic acids having 4 to 18 carbon atoms are present as component A2), the amount of components A2) and D) in the total amount of components A), B) and D) is less than 10 wt. % and v) the amount of component C) in the total amount of components A) to E) is 0.05 to 5 wt. %. |