Polyurethane foam

1. A microcellular polyurethane foam, obtained by reacting:

i) an isocyanate-terminated prepolymer obtained by reacting a polyisocyanate with a first polyester, which is formed from:

a) a dimer fatty acid; and/or

b) a dimer fatty diol;

ii) a second polyester, which is formed from:

a) a dimer fatty acid; and/or

b) a dimer fatty diol; and

iii) a chain extender;

wherein:

i) said first and second polyesters may be the same or different;

ii) said foam retains at least 80% of its initial tensile strength after being subjected to hydrolysis for 2 weeks; and

iii) the molar ratio of the chain extender to second polyester reacting to form the microcellular polyurethane foam is in the range from 1 to 10:1.

2. The foam of claim 1 , wherein the foam retains at least 90% of its initial tensile strength properties after being subjected to hydrolysis for 2 weeks.

3. The foam of claim 1 , wherein the foam retains at least 30% of its initial tensile strength properties after being subjected to hydrolysis for 4 weeks.

4. The foam of claim 3 , wherein the foam retains at least 40% of its initial tensile strength properties after being subjected to hydrolysis for 4 weeks.

5. The foam of claim 1 , wherein the foam retains at least 60% of its initial elongation at break properties after being subjected to hydrolysis for 2 weeks.

6. The foam of claim 1 , wherein the foam comprises:

i) a density in the range from 0.35 to 0.9 gcm −3 ;

ii) a tensile strength greater than 30 kgcm −2 ;

iii) an elongation at break of greater than 200%;

iv) a tear strength greater than 1.2 kNm −1 ; and/or

v) an impact resilience less than 45%.

7. The foam of claim 1 , wherein the dimer fatty acids and/or dimer fatty diols of each of said first and second polyesters have a trimer content of 2 to 15% by weight.

8. The foam of claim 1 , wherein the dimer fatty acids and/or dimer fatty diols of each of said first and second polyesters have a trimer content of 5 to 15% by weight.

9. The foam of claim 1 , wherein each of said first and second polyesters is additionally formed from a non-dimer dicarboxylic acid.

10. The foam of claim 9 , wherein the non-dimer dicarboxylic acid comprises adipic acid.

11. The foam of claim 1 , wherein the chain extender is a diol having an aliphatic linear carbon chain comprising 1 to 10 carbon atoms.

12. The foam of claim 1 , wherein the foam comprises:

i) a hardness in the range from 20 to 60 Shore A;

ii) a tensile strength in the range from 35 to 80 kgcm −2 ;

iii) an elongation at break of greater than 250%;

iv) a tear strength in the range from 2 to 8 kNm −1 ; and/or

v) an impact resilience in the range from 10 to 35%.

13. The foam of claim 1 , wherein the first polyester and/or second polyesters are obtained from the reaction consisting of:

i) dimer fatty acid;

ii) adipic acid; and

iii) diethylene glycol.

14. The foam of claim 1 , wherein the first polyester is obtained from the reaction product of a reaction consisting of:

a) dimer fatty acid;

b) adipic acid; and

c) diethylene glycol;

wherein

the ratio of dimer fatty acid, adipic acid, and diethylene glycol is in the range of 0.3 to 0.7:0.3 to 0.7:1.0 to 3.0 by weight.

15. A process for forming a microcellular polyurethane foam, obtained by reacting:

an isocyanate-terminated prepolymer obtained by reacting a polyisocyanate with a first polyester, which is formed from:

a) a dimer fatty acid; and/or

b) a dimer fatty diol;

a second polyester, which is formed from:

a) a dimer fatty acid; and/or

b) a dimer fatty diol; and

iii) a chain extender;

wherein:

i) said first and second polyesters may be the same or different; and

ii) said foam retains at least 80% of its initial tensile strength after being subjected to hydrolysis for 2 weeks.

16. A shoe sole comprising a microcellular polyurethane foam, obtained by reacting:

i) an isocyanate-terminated prepolymer obtained by reacting a polyisocyanate with a first polyester, which is formed from:

a) a dimer fatty acid; and/or

b) a dimer fatty diol;

ii) a second polyester, which is formed from:

a) a dimer fatty acid; and/or

b) a dimer fatty diol; and

iii) a chain extender;

wherein:

i) said first and second polyesters may be the same or different; and

ii) said foam retains at least 80% of its initial tensile strength after being subjected to hydrolysis for 2 weeks.