Composition that is suitable for producing polyurethane foams and that contains at least one nucleating agent

A method of producing polyurethane or polyisocyanurate foams, the method comprising:

• (a) forming a composition (Z1) by adding a surfactant TD, having an HLB value below 6 and no silicon atom, to a composition (ZP) comprising at least one polyol and at least one catalyst that catalyzes formation of a urethane, urea or isocyanurate bond;
• and
• (b) reacting the composition (Z1) with at least one polyisocyanate, to obtain a polyurethane foam or a polyisocyanurate foam,
• wherein:
• from 0.05 to 10 parts by mass of the surfactant TD are added per 100 parts by mass of the composition (ZP);
• the reacting occurs in the presence of at least one nucleating agent selected from the group consisting of a perfluorinated hydrocarbon, an ether having at least one perfluorinated hydrocarbyl moiety and a ketone having at least one perfluorinated hydrocarbyl moiety, and a blowing agent, wherein the nucleating agent differs from the blowing agent;
• and
• the nucleating agent and the blowing agent are mixed to obtain a composition (Z2) that is added to the composition (Z1) to obtain a composition (ZS) before the reacting with the at least one polyisocyanate, wherein after composition (Z2) is added to composition (Z1) and prior to the reacting with the at least one polyisocyanate, the composition (ZS) is stabilized such that said composition (ZS) is clear and devoid of any droplet formation or clouding as perceivable by the naked eye, and
• wherein the surfactant TD is isotridecanol or isononanol.