Method for producing a rigid polyurethane foam

A process for producing a rigid polyurethane foam or a rigid polyisocyanurate foam, the process comprising reacting a mixture consisting essentially of a) at least one polyisocyanate;

• b) a polyol composition (PZ) comprising at least one compound having at least two hydrogen atoms reactive toward isocyanate groups and at least one dicarboxylic diester of formula (I):
• wherein:
• R1 and R2 are each independently H or an alkyl moiety having from 1 to 6 C atoms;
• and
• R3 and R4 are each independently an alkyl moiety having from 1 to 18 C atoms;
• and wherein at least one of said at least one compound having at least two hydrogen atoms reactive towards isocyanate groups is a polyether alcohol (P1) with a hydroxyl number in the range of 350 to 600 mgKOH/g and with a functionality in the range of 3.5 to 5.5, in an amount in the range of from 35 to 55% by weight based on the weight of component b),
• c) a foam stabilizer;
• and
• d) at least one blowing agent,
• forming a rigid polyurethane foam or a rigid polyisocyanurate foam,
• wherein the at least one blowing agent is water or water in a mixture with at least one other blowing agent selected from the group consisting of propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, cyclohexane, dimethyl ether, methyl ethyl ether, methyl butyl ether, methyl formate, acetone, 1,3,3,3-tetrafluoropropene, 1,1,1,2,3-pentafluoropropene, 1,1,1,4,4,4-hexafluorobutene, and 1-chloro-3,3,3-trifluoropropene,
• insofar as water is used as the sole at least one blowing agent, a content of the water is from 1 to 10% by weight, based on a total weight of the polyol composition (PZ), and insofar as water is used in the mixture with the at least one other blowing agent selected from the group consisting of propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, cyclohexane, dimethyl ether, methyl ethyl ether, methyl butyl ether, methyl formate, acetone, 1,3,3,3-tetrafluoropropene, 1,1,1,2,3-pentafluoropropene, 1,1,1,4,4,4-hexafluorobutene, and 1-chloro-3,3,3-trifluoropropene, the content of the water is from 0.1 to 5% by weight, based on the total weight of the polyol composition (PZ),
• a quantity of the at least one dicarboxylic diester of formula (I) used is in the range from 0.5 to 5.0% by weight, based on the weight of the polyol composition (PZ), and
• said reacting is conducted in the absence of a fluoroalkane blowing agent.