Powder coating compositions and their use for coating heat resistant substrates

A powder coating composition, which may be cured at temperatures of 120.degree. to 150.degree. C. to provide a coating that is non-yellowing, elastic, and resistant to solvents and chemicals, comprising

• A) a binder component containing one or more copolymers which are prepared from olefinically unsaturated monomers, contain epoxy groups and optionally hydroxyl groups and have an epoxy equivalent weight of 365 to 2840, a glass transition temperature Tg of 20.degree. to 100.degree. C. and a weight average molecular weight of 1500 to 30,000,
• B) a curing component having a melting point or range of 40.degree. to 130.degree. C. and containing one or more compounds selected from (i) (cyclo)aliphatic dicarboxylic acids having 4 to 20 carbon atoms, (ii) monomeric and/or polymeric anhydrides of such dicarboxylic acids, (iii) polyol-modified polymeric anhydrides of such dicarboxylic acids, and (iv) aliphatic hydroxycarboxylic acids having 4 to 18 carbon atoms and
• C) one or more polyaddition compounds which contain uretdione groups, have a melting point or range of 40.degree. C. to 125.degree. C. and are prepared from (cyclo)aliphatic diisocyanates,
• provided that components A), B) and C) are present in proportions such that there are 0.5 to 1.5 carboxyl and/or anhydride groups of component B) and 0.1 to 1.2 uretdione groups of component C) for each epoxy group of component A).