The invention relates to polyisocyanate adducts obtainable by reacting at least one polysiloxane of the structure (I), at least one polyisocyanate having at least three NCO groups, at least one (meth)acrylate of the structure (II), where U is one of the following groups: (III), and optionally a compound reactive with isocyanates. The indices m, n, x and y assume values of 0 to 100. In addition, t=2 to 30, s=1 to 400 and i=1 to 5. The radicals identified by the letter R are suitable organic groups. ASi is an OH—, NH2— or CO2H— group. In addition, m+n+x+y≧3. The invention further relates to a process for preparing the polyisocyanate adducts and to a radiation-curable coating composition comprising at least one radiation-curable polymer as a binder and at least one inventive polyisocyanate adduct. The invention further relates to the use of the inventive polyisocyanate adducts as an additive.