Process for preparing rigid and flexible polyurethane foams

Process for preparing a rigid polyurethane foam by reacting

• a polyisocyanate (1),
• a polyether polyol (2) having a hydroxyl number of at least 150 mg KOH/g and an average nominal hydroxyl functionality of from 2 to 8,
• a polyether polyol (3) having an average equivalent weight of 1000-3000, a hydroxyl number of from 10 to less than 150 mg KOH/g, an average nominal hydroxyl functionality of 2-3 and the structure
• HO--(EO).sub.x --(PO).sub.z --(EO).sub.y --X(--O--(EO).sub.y --(PO).sub.z --(EO).sub.x H).sub.n
• wherein EO is an ethylene oxide radical, PO is a propylene oxide radical x=1-15, y=0-6, z is such so as to arrive at the above equivalent weight, n is 1-2 and X is a hydrocarbon radical having 2-10 carbon atoms or a radical having the formula --CH.sub.2 --CH.sub.2 --(O--CH.sub.2 --CH.sub.2).sub.1-2 --, and
• water
• wherein the amount of polyisocyanate (1), polyol (2), polyol (3) and water is 55-80, 3-20, 10-30, and 2-6 parts by weight respectively per 100 parts by weight of polyisocyanate (1), polyol (2), polyol (3) and water and wherein the reaction is conducted at an isocyanate index of 102-200 and wherein the polyisocyanate is reacted with one or more isocyanate-reactive compositions comprising one or more of the aforementioned polyol (2), polyol (3) and water and not comprising compounds which have a primary, secondary or tertiary nitrogen atom.