Mechanically stable ultrafiltration membrane, and method for producing same

A method for producing an ultrafiltration membrane, comprising:

• (a) applying a first polymer solution to a support layer, to form a first polymer layer which penetrates at least 25% into the support layer, thereby forming a coated support layer;
• (b) contacting the coated support layer from (a) with a gas, which comprises a nonsolvent, relative to the polymer in the first polymer solution, leading to a depth of penetration of the nonsolvent from an atmosphere formed by the nonsolvent-containing gas into the applied first polymer solution of at least 5.0% but less than 80%, based on a total depth of the applied first polymer solution, wherein the gas comprises from 1.0 g/m3 to 20 g/m3 of the nonsolvent;
• (c) applying a second polymer solution to the first polymer layer, to form a second polymer layer on the first polymer layer of the coated support layer, thereby forming a multiply coated support layer;
• and
• (d) introducing the multiply coated support layer into a precipitating bath, which comprises a precipitant, relative to the polymer in the first and/or second polymer solution,
• wherein the first and second polymer layers comprise cellulose derivates,
• wherein the support layer is a nonwoven web, a woven fabric or an open microfilter membrane, and
• wherein after (d) the first and second polymer layers, taken together, have a complex structure wherein the pores initially widen from a first main surface within the second polymer layer, the pores in the interior of the membrane then taper on entry into the first polymer layer, and the pores in the course of the structure widen again toward a second main surface.