BLOCKED POLYURETHANE IMPACT MODIFIERS FOR IMPROVING THE IMPACT STRENGTH OF LOW-TEMPERATURE CURING SINGLE-COMPONENT EPOXY ADHESIVES

The invention relates to an impact modifier which is an isocyanate-terminated polymer, the isocyanate groups of which are partially or completely blocked by reaction with a blocking agent, obtainable by A) reacting a) two or more polyols comprising a1) polytetramethylene ether glycol and a2) at least one hydroxy-terminated polybutadiene, wherein the total proportion of polytetramethylene ether glycol and hydroxy-terminated polybutadiene is at least 95 wt.-%, relative to the total weight of polyols, and b) at least one acyclic aliphatic or cycloaliphatic polyisocyanate in the presence of c) at least one catalyst while forming an isocyanate-terminated polymer, and B) reacting the isocyanate-terminated polymer obtained in step A) with d) at least one blocking agent selected from the group consisting of phenols, 2-benzoxazolinones and pyrazoles in the presence of e) at least one organic bismuth catalyst while forming the partially or completely blocked isocyanate-terminated polymer. The use of the impact modifier in epoxy resin adhesives results in good impact strength even at low curing temperatures.