Highly asymmetric, hydrophilic, microfiltration membranes having large pore diameters

A method to prepare an asymmetric hydrophilic integral sulfone polymer membrane, the membrane comprising a microporous skin and a porous support, the microporous skin possessing a high density of skin pores, the skin pores having an average diameter of from about 0.1 micron to about 10 microns, and the porous support comprising an asymmetric region of gradually increasing pore sizes, to an average diameter of from about 5 to about 1000 times the diameter of the skin pores, comprising:

• providing a casting dope comprising between about 9% and 12% by weight of a sulfone polymer and between about 3% and 15% by weight of a hydrophilic polymer, dissolved in a solvent;
• casting the dope to form a thin film;
• exposing the thin film to a gaseous environment, including water vapor at a relative humidity of between about 50% and 80% for between about 2 seconds and 20 seconds;
• and
• coagulating the film in a water bath having a temperature between about 20° C. and 70° C. and recovering the membrane.