A highly reactive polyurethane powder coating composition, which contains
uretdione groups that are hardenable at low temperatures, contains A) at
least one uretdione-containing powder coating hardener based on
aliphatic, (cyclo)aliphatic or cycloaliphatic polyisocyanates and
hydroxyl-containing compounds, the hardener having a melting point of
from 40 to 130.degree. C., a free NCO content of less than 5% by weight,
and a uretdione content of 6-18% by weight; B) at least one
hydroxyl-containing polymer having a melting point of from 40 to
130.degree. C., and an OH number of between 20 and 200 mg KOH/g; C)
0.001-3% by weight of at least one tetraalkyl ammonium carboxylate
catalyst, based on a total amount of components A) and B). Components A)
and B) are present in a ratio so that for each hydroxyl group of
component B) there is from 0.3 to 1 uretdione group of component A).