been reached excess monomeric isocyanate is subsequently separated off by short path evaporation if the catalyst is volatile enough the reaction mixture can be freed from the catalyst in the course of the separation of monomer in this case there is no need to add catalyst poisons in principle a broad palette of isocyanates is suitable for the preparation of polyisocyanates containing uretdione groups in accordance with the invention isophorone diisocyanate ipdi hexamethylene diisocyanate hdi 2 methylpentane diisocyanate mpdi 2 2 4 trimethylhexamethylene diisocyanate 2 4 4 trimethylhexamethylene diisocyanate tmdi norbomane diisocyanate nbdi methylenediphenyl diisocyanate mdi and tetramethylxylylene diisocyanate tmxdi are used with preference very particular preference is given to ipdi and hdi the reaction of these polyisocyanates carrying uretdione groups to give powder coating hardeners a containing uretdione groups includes the reaction of the free nco groups with hydroxyl containing monomers or polymers such as polyesters polythioethers polyethers polycaprolactams polyepoxides polyesteramides polyurethanes or low molecular mass di tri and or tetraalcohols as chain extenders and if desired monoamines and or monoalcohols as chain terminators and has already been described on many occasions ep 669 353 ep 669 354 de 30 30 572 ep 639 598 or ep 803 524 all incorporated herein by reference preferred powder coating hardeners a containing uretdione groups have a free nco content of less than 5 by weight and a uretdione group content of from 6 to 18 by weight calculated as c 2 n 2 o 2 molecular weight 84 this range for the uretdione group content includes all specific values and subranges therebetween such as 8 10 12 14.