Patent ID: 7429343
Filing Date: 2008-09-30
Classification: B01D,B29C

Abstract:
1. Process for producing an integrally asymmetrical hydrophobic membrane consisting of at least one polyolefin, the membrane having a sponge-like, open-pored, microporous support structure and a separation layer with a denser structure compared to the support structure, the process comprising at least the steps of: a) preparing a homogeneous solution from a system comprising 20-90% by weight of the at least one polyolefin and 80-10% by weight of a solvent for the at least one polyolefin, wherein the system at elevated temperatures has a range in which it is present as a homogeneous solution, on cooling has a critical demixing temperature, below the critical demixing temperature in the liquid state of aggregation has a miscibility gap, and has a solidification temperature, b) rendering the solution to form a shaped object, with first and second surfaces, in a die at a die temperature above the critical demixing temperature, c) cooling the shaped object by contacting the shaped object with a liquid cooling medium that does not dissolve or react chemically with the at least one polyolefin at temperatures up to the die temperature, the liquid cooling medium being conditioned to a cooling temperature below the solidification temperature, at such a rate that a thermodynamic non-equilibrium liquid-liquid phase separation into a high-polymer-content phase and a low-polymer-content phase takes place and solidification of the high-polymer-content phase subsequently occurs when the temperature of the shaped object falls below the solidification temperature, and d) optionally removing the low-polymer-content phase from the shaped object, wherein a solvent for the at least one polyolefin is selected for which, on cooling at a rate of 1° C./min, the demixing temperature of a solution of 25% by weight of the at least one polyolefin in the solvent is 10 to 70° C. above the solidification temperature.