Polyamide membrane with improved hydrolytic stability and method for production thereof

1. A method for production of a hydrolytically stable polyamide membrane, the method comprising:

a) preparing a homogeneous solution containing 10–90% by weight of an aliphatic polyamide in 90–10% by weight of a solvent system, wherein the solution of polyamide and solvent system has a critical demixing temperature and a solidification temperature, and shows a miscibility gap below the demixing temperature in the liquid state, and wherein the solution contains an antioxidant agent as a stabilizer for the polyamide, the antioxidant agent being essentially insoluble in the solvent system at or below the demixing temperature;

b) shaping the solution into a shaped object in a die at a die temperature, the die temperature being above the critical demixing temperature;

c) cooling the shaped object with a cooling medium conditioned to a cooling temperature below the solidification temperature, at such a cooling rate that a thermodynamic non-equilibrium liquid-liquid phase separation into a polymer-rich phase and a polymer-poor phase occurs, followed by solidification of the polymer-rich phase to a membrane structure when the temperature falls below the solidification temperature; and

d) removing the solvent system from the shaped object, wherein the antioxidant is a sterically hindered phenol selected from the group consisting of pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (CAS no. 6683-19-8), 3,3′,3′,5,5′,5′-hexa-tert-butyl-α,α′,α′-(mesitylene-2,4,6-triyl)tri-p-cresol (CAS no. 1709-70-2) and N,N′-hexane-1,6-diylbis(3-(3,5-di-tert-butyl-4-hydroxyphenylpropionamide)) (CAS no. 23128-74-7).

2. The method according to claim 1 , wherein a mixture of a compound A and a compound B is used as the solvent system; wherein compound A is a solvent for the polyamide, and wherein compound B is selected from the group consisting of a non-solvent for the polyamide, a solvent that dissolves the polyamide but dissolves the polyamide at a temperature that is at least 50° C. higher than a temperature at which compound A dissolves the polyamide, and a swelling agent for the polyamide.

3. The method according to claim 1 , wherein the solvent system is water soluble.

4. The method according to claim 1 , wherein water is used as the cooling medium.

5. The method according to claim 1 , wherein the polyamide has an average molecular weight between 20000 and 60000 Daltons.

6. The method according to claim 1 , wherein the polyamide permits sup erheated steam sterilization of the membrane structure at temperatures of at least 130° C. without any change in the membrane structure.

7. The method according to claim 1 , wherein the polyamide comprises polyamide-6.

8. The method according to claim 1 , wherein the stabilizer is first mixed with the polymer in the form of a dry blend and melted along with the polymer, before the homogeneous solution with the solvent system is prepared.

9. The method according to claim 1 , wherein a concentration of the antioxidant agent is between 0.01 and 5% by weight relative to the polyamide.

10. The method according to claim 9 , wherein the concentration of the antioxidant agent is between 0.05 and 2% by weight relative to the polyamide.