ULTRAFILTRATION MEMBRANE

The invention relates to a hollow fiber membrane for ultrafiltration, which is formed from a hydrophobic aromatic sulfone polymer and at least one hydrophilic polymer and has an open-pore separating layer on the lumen side, an adjoining supporting layer with asymmetric, sponge-like pore structure without finger pores and, adjoining the supporting layer, in the direction toward the outer surface, an outer layer. The separating layer has a separating limit between 20 000 and 200 000 daltons, a thickness of not more than 10% of the membrane thickness and an essentially isotropic pore structure. The size of the pores in the supporting layer initially increases up to a zone with maximum pore size and then decreases in the direction toward the outer layer, the outer layer has a thickness of from 10 to 30% of the wall thickness and an essentially isotropic pore structure. The mean size of the pores in the outer layer is greater than in the separating layer, but smaller than in the support layer. The wall thickness of the hollow fiber membrane is in the range from 100 to 450 mum. The invention further relates to a process for producing these hollow fiber membranes.