Method for producing a polyurethane foam by microcasting
A polyurethane foam based on MDI (diphenylmethane diisocyanate) and on a polyol with an ethylene oxide content of greater than 50%, is made by mixing, under pressure, to form a foaming liquid precursor of polyurethane foam: a first reactive liquid, referred to as liquid A, comprising (a) a urethane prepolymer based on a first portion of the MDI and on a first portion of said polyol, and (b) the second portion of the MDI in the free state, said prepolymer being dissolved in this second portion of MDI and a second reactive liquid, referred to as liquid B, comprising the second portion of said polyol and water as foaming agent. The amount of said polyol in the liquid B represents between 25% and 75% by weight of the total of said polyol; this method is advantageously used for casting a polyurethane foam into the cavity of a tire casing.
1. A method for manufacturing a polyurethane foam based on diphenylmethane diisocyanate and on a polyol having an ethylene oxide content of greater than 50% by weight comprising the step of:
mixing a first reactive liquid and a second reactive liquid, under pressure, to form a foaming liquid precursor of polyurethane foam,
wherein the first reactive liquid comprises (a) a urethane prepolymer based on a first portion of the diphenylmethane diisocyanate and on a first portion of the polyol and (b) a second portion of the diphenylmethane diisocyanate in the free state, said prepolymer being dissolved in the second portion of diphenylmethane diisocyanate in the free state,
wherein the second reactive liquid comprises the second portion of the polyol and water as foaming agent,
wherein the amount of the polyol in the second reactive liquid is between 25% and 75% by weight of the total of the polyol,
wherein the first and second reactive liquids are mixed using high-pressure counter-current, and
wherein the mixing pressure is greater than 50 bar.
2. The method according to claim 1 , wherein the amount of the polyol in the second reactive liquid is between 30% and 70% by weight of the total of the polyol.
3. The method according to claim 2 , wherein the amount of the polyol in the second reactive liquid is between 35% and 55% by weight of the total of the polyol.
4. The method according to claim 1 , wherein the amount of water in the second reactive liquid is between 2 and 10% by weight.
5. The method according to claim 4 , wherein the amount of water in the second reactive liquid is between 3 to 8% by weight.
6. The method according to claim 1 , wherein the hydroxyl number of said polyol is between 10 and 180 mg KOH/g.
7. The method according to claim 6 , wherein the hydroxyl number of said polyol is between 20 and 120 mg KOH/g.
8. The method according to claim 1 , wherein the number-average molecular weight Mn of the polyol is between 1000 and 12 000 g/mol.
9. The method according to claim 8 , wherein the number-average molecular weight Mn of the polyol is between 2000 and 8000 g/mol.
10. The method according to claim 1 , wherein the molar ratio of diphenylmethane diisocyanate to the polyol is between 2 and 10.
11. The method according to claim 10 , wherein the molar ratio of diphenylmethane diisocyanate to the polyol is between 3 and 6.
12. The method according to claim 1 , wherein the NCO index during foaming is between 0.7 and 1.3.
13. The method according to claim 12 , wherein the NCO index during foaming is between 0.8 and 1.2.
14. The method according to claim 1 , wherein the weight ratio of the second reactive liquid to the first reactive liquid is greater than 0.25.
15. The method according to claim 14 , wherein the weight ratio of the second reactive liquid to the first reactive liquid is between 0.3 and 1.0.
16. The method according to claim 1 , wherein diphenylmethane diisocyanate is the only diisocyanate used, or, if several diisocyanates are used, diphenylmethane diisocyanate constitutes the predominant diisocyanate by weight.
17. The method according to claim 1 , wherein the diphenylmethane diisocyanate is 4,4′-diphenylmethane diisocyanate.
18. The method according to claim 1 , wherein the content of ethylene oxide of the polyol is greater than 70%.
19. The method according to claim 18 , wherein the content of ethylene oxide of the polyol is greater than 90%.
20. The method according to claim 1 , wherein a total flow rate of the first and second reactive liquids, during mixing, is between 0.1 and 1 liter/min.
21. The method according to claim 1 , wherein the mixing pressure is greater than 100 bar.
22. The method according to claim 1 further comprising the step of casting the foaming liquid precursor of polyurethane foam into the cavity of a tire casing rotating on itself to form and deposit in situ the polyurethane foam inside said cavity.
23. The method according to claim 22 , wherein the polyurethane foam thus formed in the cavity of the tire casing has an apparent core density, measured according to standard ISO 845, of between 0.020 and 0.070 g/cm 3 .
24. The method according to claim 23 , wherein the polyurethane foam thus formed in the cavity of the tire casing has an apparent core density, measured according to standard ISO 845, of between preferably within a range from 0.030 to 0.060 g/cm 3 .