Process for producing a polyurethane foam and polyurethane foam obtainable therefrom

1. A process for producing a polyurethane foam, comprising the following steps:

providing a mixture in a mixing head, where the mixture comprises:

A) a component reactive towards isocyanates;

B) a surfactant component;

C) a blowing agent component selected from the group consisting of linear, branched or cyclic C 1 - to C 6 -alkanes, linear, branched or cyclic C 1 - to C 6 -fluoroalkanes, N 2 , O 2 , argon and/or CO 2 ;

wherein the blowing agent component C) is present in the supercritical or near-critical state; and

D) a polyisocyanate component;

wherein:

the prevailing pressure after the mixing of components A),B),C), and D) is from ≧80 bar to ≦150 bar,

the proportion of the blowing agent C) in the reaction mixture comprising components A),B),C), and D), is in the range from ≧3% by weight to≦60% by weight,

the temperature of the mixture is in the rang of from ≧20 °C and ≦80 °C, and the relative proportion by weight Υof surfactant component B) in the mixture comprising components A),B) and C) is from ≦0.05 to ≧0.3, based on the entire composition;

discharging the mixture comprising components A), B), C), and D) from the mixing head wherein, during the discharge of the mixture, the pressure prevailing in the mixture is lowered to atmospheric pressure;

whereby the produced polyurethane foam has (i) an average pore diameter of from ≧10 nm to ≦10000 nm and (ii) a bimodal cell size distribution, wherein one maximum of the cell size distribution is in the range from ≧10 nm to ≦500 nm and a further maximum of the cell size distribution is in the range from ≧1 μm to ≦500 μm.

2. The process according to claim 1 , wherein means have been arranged in the mixing head in order to increase the resistance to flow during the discharge of the mixture comprising components A), B), C) and D).

3. The process according to claim 1 , wherein component A) comprises a polyester polyol having a hydroxy number from ≧200 mg KOH/g to ≦600 mg KOH/g and a short-chain polyol having a hydroxy number of ≧800 mg KOH/g.

4. The process according to claim 1 , wherein the surfactant component B) comprises a polyethylene oxide-polyether having oligodimethylsiloxane end groups, wherein the number of the dimethylsiloxane units is ≦5.

5. The process according to claim 1 , wherein the HLB value of the surfactant component is from ≧10 to ≦18.

6. The process according to claim 1 , wherein the surfactant component B) is present in a form covalently bonded to a compound reactive towards isocyanates or to a polyisocyanate.

7. The process according to claim 1 , wherein the polyisocyanate component D) comprises monomeric and/or polymeric diphenylmethane 4,4′-diisocyanate.

8. The process according to claim 1 , wherein the amounts present of components A), B), C), and D) are as follows:

A) from ≧25% by weight to ≦35% by weight; and

B) from ≧4% by weight to ≦15% by weight; and

C) from ≧5% by weight to ≦40% by weight; and

D) from ≧30% by weight to ≦60% by weight.