Vacuum-supported method for the production of polyurethane foam

1. A method for the production of a polyurethane foam, comprising the steps of:

providing an isocyanate-reactive component A comprising a polyol component A1 which further comprises a physical blowing agent T, wherein the physical blowing agent T is present in the isocyanate-reactive component A in the form of an emulsion with the polyol component A1 constituting the continuous phase and droplets of the physical blowing agent T the dispersed phase of the emulsion, wherein the average size of the droplets of the physical blowing agent T is ≧0.1 μm to ≦20 μm, the droplet size being determined by using an optical microscope operating in bright field transmission mode, wherein the polyol component A1 comprises:

A1a: a polyether polyol with a hydroxyl number of ≧15 mg KOH/g to ≦550 mg KOH/g and a functionality of ≧1.5 to ≦6.0 obtained by the addition of an epoxide to one or more starter compounds selected from the group of carbohydrates and/or at least difunctional alcohols; and

A1b: a polyether polyol with a hydroxyl number of ≧100 mg KOH/g to ≦550 mg KOH/g and a functionality of ≧1.5 to ≦5.0 obtained by the addition of an epoxide to an aromatic amine; and

A1c: a polyester polyether polyol with a hydroxyl number of ≧100 mg KOH/g to ≦450 mg KOH/g and a functionality of ≧0.1 to ≦3.5 obtained by the addition of an epoxide to the esterification product of an aromatic dicarboxylic acid derivative and an at least difunctional alcohol;

combining at least the isocyanate-reactive component A and an isocyanate component B, thereby obtaining a polyurethane reaction mixture;

providing the polyurethane reaction mixture in a cavity; and

reducing the pressure within the cavity to a pressure lower than ambient pressure, wherein the pressure is reduced by ≧1 mbar up to ≦900 mbar;

wherein the cavity is ventilated to ambient pressure before the gel time of the polyurethane reaction mixture is reached.

2. The method according to claim 1 , wherein the pressure within the cavity is reduced before the polyurethane reaction mixture is provided in the cavity.

3. The method according to claim 1 , wherein the pressure within the cavity is reduced after the polyurethane reaction mixture is provided in the cavity.

4. The method according to claim 1 , wherein the pressure is reduced by ≧50 mbar to ≦300 mbar.

5. The method according to claim 1 , wherein the cavity is ventilated to ambient pressure when 60 to 99% of the gel time of the polyurethane reaction mixture is reached.

6. The method according to claim 1 , wherein the polyurethane reaction mixture has a gel time of ≦50 seconds.

7. The method according to claim 1 , wherein before ventilating to ambient pressure, the step of reducing the pressure within the cavity to a pressure lower than ambient pressure is conducted in such a way that after an initial reduction of the pressure, the pressure is allowed to rise as a consequence of an expansion of the polyurethane reaction mixture.

8. The method according to claim 1 , wherein before ventilating to ambient pressure, the reduced pressure is kept constant.

9. The method according to claim 1 , wherein the pressure within the cavity is adjusted to different levels at different cavity areas by using two individually operatable vacuum systems.

10. The method according to claim 7 , wherein the pressure level within different cavity areas is adjusted, wherein the pressure level within a cavity area having a first shape is adjusted to a first pressure level, wherein the pressure level within a cavity area having a second shape is adjusted to a second pressure level, wherein the first shape is different than the second shape, and wherein the first pressure level is different than the second pressure level.

11. The method according to claim 9 , wherein the physical blowing agent T is present in the isocyanate-reactive component A in the form of an emulsion with the polyol component A1 constituting the continuous phase and droplets of the physical blowing agent T the dispersed phase of the emulsion, wherein the average size of the droplets of the physical blowing agent T is ≧0.1 μm to ≦15 μm, the droplet size being determined by using an optical microscope operating in bright field transmission mode.

12. The method according to claim 9 , wherein the polyol component A1 further comprises:

A1c′: a polyester polyol with a hydroxyl number of ≧100 mg KOH/g to ≦450 mg KOH/g and a functionality of ≧1.5 to ≦3.5 obtained by the esterification of a polycarboxylic acid component and a polyalcohol component, wherein the total content of aromatic dicarboxylic acid derivatives employed in the esterification, based on free aromatic dicarboxylic acids, is ≦48.5 mass-%, based on the total mass of polyalcohol component and polycarboxylic acid component,

and/or

A1d: a polyether polyol with a hydroxyl number of ≧500 mg KOH/g to ≦1000 mg KOH/g and a functionality of ≧1.5 to ≦5.0 obtained by the addition of an epoxide to an aliphatic amine and/or a polyfunctional alcohol, and/or

A1e: a di-, tri- or tetrafunctional aminic or alcoholic chain extender or cross-linker.

13. The method according to claim 1 , wherein the physical blowing agent T is selected from the group consisting of hydrocarbons, halogenated ethers, perfluorinated hydrocarbons with 1 to 6 carbon atoms and mixtures thereof.

14. The method according to claim 9 , wherein the mass ratio of A1:T is ≧5:1 to ≦12:1.

15. The method according to claim 9 , wherein the polyol component Al has a viscosity according to EN ISO 3219 at 20° C. of ≧1000 mPas to ≦18000 mPas.

16. The method according to claim 1 , wherein the isocyanate-reactive component A further comprises:

A2: water;

A3: at least one stabilizer selected from the group of polyether polydimethylsiloxane copolymers; and

A4: at least one catalyst selected from the group consisting of triethylenediamine, N,N-dimethylcyclohexylamine, tetramethylenediamine, 1-methyl-4-dimethylaminoethylpiperazine, triethylamine, tributylamine, dimethylbenzylamine, N,N′N″-tris-(dimethylaminopropyl)hexahydrotriazine, dimethylaminopropylformamide, N,N,N′,N′-tetramethylethylenediamine, N,N,N′, N′-tetramethylbutanediamine, tetramethylhexanediamine, pentamethyldiethylenetriamine, tetramethyldiaminoethyl ether, dimethylpiperazine, 1,2-dimethylimidazole, 1-azabicyclo[3.3.0]octane, bis(dimethylaminopropyl) urea, N-methylmorpholine, N-ethylmorpholine, N-cyclohexylmorpholine, 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, triethanolamine, diethanolamine, triisopropanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, and dimethylethanolamine.

17. The method according to claim 1 , wherein the isocyanate component B comprises:

B1: at least one isocyanate selected from the group consisting of toluylene diisocyanate, diphenylmethane diisocyanate, polyphenylpolymethylene polyisocyanate, xylylene diisocyanate, naphthylene diisocyanate, hexamethylene diisocyanate, diisocyanatodicylclohexylmethane, and isophorone diisocyanate; and/or

B2: an isocyanate-terminated prepolymer obtained from at least one polyisocyanate B1 and at least one isocyanate reactive compound selected from the group consisting of:

A1c′: a polyester polyol with a hydroxyl number of ≧100 mg KOH/g to ≦450 mg KOH/g and a functionality of ≧1.5 to ≦3.5 obtained by the esterification of a polycarboxylic acid component and a polyalcohol component, wherein the total content of aromatic dicarboxylic acid derivatives employed in the esterification, based on free aromatic dicarboxylic acids, is ≧48.5 mass-%, based on the total mass of polyalcohol component and polycarboxylic acid component;

A1d: a polyether polyol with a hydroxyl number of ≧500 mg KOH/g to ≦1000 mg KOH/g and a functionality of ≧1.5 to ≦5.0 obtained by the addition of an epoxide to an aliphatic amine and/or a polyfunctional alcohol; and

A1f: a polyether carbonate polyol with a functionality of ≧1.5 to ≦8.0 and a number average molecular weight of ≧500 g/mol to ≦10000 g/mol.

18. The method according to claim 1 , wherein the cavity into which the polyurethane reaction mixture is provided is a refrigerator insulation frame.