Process for preparing rigid and flexible polyurethane foams

Process for preparing a rigid polyurethane foam having a core density of 3-27 kg/m3, said process comprising:

• reacting a reaction system comprising
• (a) a polyisocyanate;
• (b) a first isocyanate reactive compound having an average equivalent weight of at most 374 and an average number of isocyanate-reactive hydrogen atoms of from 2 to 8;
• (c) a second isocyanate reactive compound having an average equivalent weight of more than 374 and an average number of isocyanate-reactive hydrogen atoms of from 2 to 6;
• and
• (d) water;
• wherein said reaction system is reacted in the presence of
• (i) a first catalyst comprising a tin salt of a monocarboxylic, saturated, aliphatic acid having 2-12 carbon atoms;
• and
• (ii) a second catalyst comprising a sodium and/or potassium salt of a protic acid, the acid having at least 2 acidic hydrogen atoms and a pKa in water 2-10 (catalyst 2), and wherein 10-90% of the acidic hydrogen atoms have been replaced by said sodium and/or potassium ions;
• wherein said first catalyst and said second catalyst are each present in an amount of from 0.1-5% by weight, relative to the total amount of ingredients used to prepare said foam, and wherein the ratio of said first catalyst to said second catalyst is from 30:70 to 95:5.