RIGID POLYURETHANE FOAMS WITH REDUCED SHRINKAGE

A rigid polyurethane foam obtainable by mixing a) isocyanates, b) compounds having groups which are reactive toward isocyanates, c) a blowing agent, d) a catalyst, e) one or more foam stabilizer, and optionally, f) further additives to form a reaction mixture, applying the reaction mixture to a reinforcing material and curing the reaction mixture, wherein the isocyanates (a) have a viscosity of not more than 600 mPas at 25° C. and the compounds (b) having groups which are reactive toward isocyanates comprise (b1) 45-70 wt-% (based on the total weight of (b) of an aromatic polyester polyol having a functionality of 2.5 or less and a hydroxyl number of more than 220 mg KOH/g (b2) 20-40 wt.-% (based on the total weight of (b) of a polyether polyol having a functionality of 4 or more and a hydroxyl number of more than 400 mg KOH/g, and (b3) more than 15 wt.-% (based on the total weight of (b) one or more low molecularweight chain extender and/or one or more crosslinker and/or one or more o-toluene diamine initiated polyether polyol ("TDA polyol"), and wherein the blowing agent comprises 1-chloro-3,3,3-trifluoropropene shows low thermal conductivity values and little shrinkage and its liquid reaction mixture fast wetting and penetration into a stack of several glass fiber mat layers.