Polyurethane powder enamels free of cleavage products as well as enamel coatings produced from such powders

Claim 1

A process for preparing polyurethane powder enamles free of cleavage products selected from monoalcohols, primary or secondary amines, having good storage stability, which are curable above 120° C., and which are linear polyisocyanate addition products which are irreversibly, partially or completely, blocked and which contain uretidinedione groups, characterized in that, a. linear isophorone diisocyanate-diol polyaddition adducts which are irreversibly, partially or completely, blocked with terminal monoalcohols or monoamines and which contain uretidinedione groups, whose uretidinedione groups are essentially free from isocyanurate groups and are derived from isophorone diisocyanate which can be more than 98% decomposed back into isophorone diisocyanate by heating, having the following formula: ##STR7## wherein R= ##STR8## X= ##STR9## R'=a divalent aliphatic, cycloaliphatic, aromatic or araliphatic hydrocarbon radical of a diol, n>1, R"=a monoalcohol radical or primary or secondary monoamine radical having 1-30 C-atoms, having an NCO/OH ratio or uretidinedione dimer of isophorone diisocyanate to diol of 1:0.5 to 1:0.9, preferably 1:0.6 to 1:0.8, a uretidinedione/diol molar ratio between 2:1 and 6:5, preferably 3:2 to 5:4, a molecular weight between 550 and 4300, preferably between 1500 and 2000, and a melting point of ≧70° C. to ≦130° C., preferably 80° C. to 120° C., b. polyhydroxy compounds having an OH functionality of ≧3.4 to ≦7, preferably 3.7 to 4.5, an average molecular weight of 2000 to 5000, preferably 2500 to 4500, a hydroxyl number between 30 and 100 mg KOH/g, preferably between 40 and 80 mg KOH/g, a viscosity of <100,000 mPa s preferably <40,000 mPa s at 160° C. and a melting point of ≦70° C. to ≦120° C., preferably 75° C. to 100° C., and c. the additives customary in polyurethane chemistry, are reacted, wherein the molar ratio of a. and b. is selected so that the NCO/OH ratio is 0.8:1 to 1.2:1.