The present invention relates to polyaddition compounds, which are solid below 40° C. and liquid above 125° C., are prepared by the reaction of an isocyanate component containing uretdione groups with a polyol containing ester and/or carbonate groups, other than hydroxypivalic acid neopentylglycol ester, and have 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. %. The present invention also relates to a process for the preparation of these polyaddition compounds to two-component polyurethane powder coating compositions containing these polyaddition compounds as the crosslinking agent and isocyanate-reactive compounds as the binder.