Powder coating crosslinking agents containing uretdione groups and free isocyanate

A process for the preparation of a polyaddition compound which is solid below 40° C. and liquid above 125° C. and has

• a) an average isocyanate functionality of 2.0 to 8.0, and a content of free isocyanate groups (calculated as NCO;
• molecular weight=42) of 2.1 to 6.0 wt. %,
• b) a content of uretdione groups (calculated as C2N2O2;
• molecular weight=84) of 10 to 19 wt. %,
• c) a content of urethane groups (calculated as —NH—CO—O;
• molecular weight=59) of 10 to 17 wt. % and
• d) a content of ester groups —CO—O (calculated as —CO—O;
• molecular weight=44) and/or carbonate groups —O—CO—O (calculated as —CO—O;
• molecular weight=44) of 1 to 17 wt. %,
• which comprises reacting
• A) a uretdione polyisocyanate which is free from isocyanurate groups and optionally
• B) up to 10 wt. %, based on the total weight of components A) and B), of another diisocyanates with
• C) a polyhydroxy compound having an average molecular weight of 134 to 1200 and containing ester groups and/or carbonate groups, other than hydroxypivalic acid neopentylglycol ester, optionally
• D) up to 70 wt. %, based on the total weight of components C) and D), of another polyhydroxy compound which is free from ester groups and carbonate groups and has a molecular weight of 62 to 400, and optionally
• E) up to 20 wt. %, based on the total weight of compounds C), D) and E), of a compound containing one isocyanate-reactive group,
• at an equivalent ratio of isocyanate groups to groups which are reactive towards isocyanates of 1.2:1 to 1.8:1.