Pur-/pir rigid foams containing polyester polyols with reduced functionality

A multistage process for producing a polyester polyol PES-B comprising:

• a) completely reacting at least one carboxyl compound selected from the group consisting of i) polyfunctional carboxylic acids and ii) polyfunctional hydroxy-reactive carboxylic acid derivatives selected from the group consisting of carbonyl chlorides, alkyl carboxylates, hydroxy-functional carboxylic acids, lactones and carboxylic anhydrides with at least one polyol containing 2-8 hydroxy groups to afford a polyester polyol PES-A,
• wherein in step a) the ratio of the molar amount of employed hydroxyl groups [n(OH)a] to the molar amount of carboxyl-equivalent groups [n(carboxy)a] is:
• n(OH)a/n(carboxy)a>1, and
• b) subsequently reacting the polyester polyol PES-A obtained in step a) with a monofunctional alcohol to afford the PES-B,
• wherein the ratio of the molar sum of the hydroxy groups n(OH)reactant of all reactant molecules employed in steps a) and b) to the molar sum of the employed carboxyl-equivalent groups n(carboxy)reactant is:
• n(OH)reactant/n(Carboxy)reactant>1.