Powder coating compositions and their use for coating heat-resistant substrates

A powder coating composition that can be hardened without splitting off blocking agents to form highly crosslinked, non-yellowing, flexible coatings which comprises

• A) a resin mixture which is solid below 30.degree. C. and liquid above 120.degree. C., containing
• A1) a polyisocyanate component which has an NCO content of 5.0 to 23% by weight, an average NCO functionality of at least 2.1 and a content of monomeric diisocyanates having a molecular weight of less than 300 of less than 0.5% by weight and which contains one or more lacquer polyisocyanates having free isocyanate groups bound to primary and/or secondary carbon atoms and
• A2) a polyol component containing one or more polyhydroxyl compounds having an OH number of 30 to 200 and comprising a member selected from the group consisting of polyhydroxy polyurethanes, polyhydroxy polyacrylates and polyhydroxy polyesters, wherein at least 50% of the hydroxyl groups present in component A2) are bound to secondary or tertiary carbon atoms and wherein components A1) and A2) are present in amounts sufficient to provide an equivalent ratio of isocyanate groups to hydroxyl groups of 0.6:1 to 1.2:1, and optionally
• B) catalysts for the reaction between hydroxyl groups and isocyanate groups.