Process for producing rigid polyurethane foams

The present invention relates to an aromatic pol- yetherol (b1) obtainable by condensation of aromatic al- cohol (b11) and aldehyde (b12) wherein the aromatic al- cohol (b11) comprises a mixture, consisting of at least one alkoxylation product (bill) of an aromatic alcohol, the alkoxylation product (b111) having a hydroxyl value of 110 to 500 mg KOH/g and at least one alkylated aromatic alcohol (b112) which may be alkoxylated and wherein the alkyl group is bonded to the aromatic ring, the alkyl group having 4 to 18 carbon atoms, wherein the molar ratio of the alkoxylation product (b111) of the aromatic alcohol and the alkylated aromatic alcohol (b112) is in the range of 5 : 1 to 2 : 1 and the average OH-functionality of the polyetherol (b1) is 2 to 8. Further, the present in- vention relates to a method of producing a polyurethane rigid foam comprising mixing (a) polyisocyanate, (b) at least one compound having at least two hydrogen atoms reactive towards isocyanates, (c) optionally flame retard- ant, (d) blowing agent, (e) catalyst and (f) optionally fur- ther additives, to form a reaction mixture and reacting the reaction mixture to obtain the polyurethane based rigid foam, wherein the compound reactive towards iso- cyanates (b) comprises an aromatic polyetherol (b1) and a rigid polyurethane foam obtained by such a method.