Process for preparing allophanate-containing actinically curable polyurethane prepolymers with increased resistance

Abstract

The present invention relates to a process for preparing low-viscosity, allophanate-containing and NCO-free polyurethane prepolymers curable by actinic radiation, and to the products obtainable by this process. The prepolymers have residual monomer contents of less than 0.5% by weight and an NCO content of less than 1% by weight, and are prepared by a process in which A) diisocyanates, B) hydroxy-functional compounds containing groups which react with ethylenically unsaturated compounds on exposure to actinic radiation, and C) polyester, polyether or polycarbonate polyols having a functionality of at least 1.9 and a number-average molecular weight (M) of at least 1000 g/mol, D) optionally in the presence of a catalyst, are reacted to form NCO-group-containing prepolymers having radiation-curing groups, which are subsequently reacted E) optionally after addition of monofunctional isocyanates in the presence F) of an allophanatization catalyst to give allophanate-containing prepolymers, the ratio of NCO groups of the compounds from A) to the OH groups of the compounds from B) and C) being 1.45:1.0 to 1.1:1.0 and the ratio of the OH groups from B) to the OH groups from C) being 8.0:1 to 1.5:1.