Microporous polyvinylidene fluoride membrane

Hydrophobic hollow-fiber membrane made from a vinylidene fluoride polymer, wherein the hollow-fiber membrane has a wall with a wall thickness, an outer surface on its outer side and an inner surface on its inner side and facing its lumen,

• wherein the hollow-fiber membrane has a continuous skin on its inner surface and a continuous skin on its outer surface,
• wherein pores are formed in the skin of the inner surface and in the skin of the outer surface, and
• wherein the hollow-fiber membrane, adjacent to the skin of the inner surface, has a supporting layer having a microporous, sponge-like, open-pored structure that is substantially isotropic across the wall thickness, the supporting layer extending over at least 80% of the wall thickness and comprising pores having an average diameter of less than 1 μm,
• and wherein
• the vinylidene fluoride polymer forming the structure of the hollow-fiber membrane has a weight-average molecular weight Mw in the range from 550 000 to 700 000 daltons and a polydispersivity, given by the ratio of the weight average MW and the number average MN of the molecular weight, from 3 to 7,
• the pores in the skin of the outer surface and the pores in the skin of the inner surface have a closed perimeter in a plane of the skin, and the pores in the skin of the outer or the inner surface have a ratio of their longitudinal extension in the direction of the longitudinal axis of the hollow-fiber membrane to their transverse extension in the circumferential direction of the hollow-fiber membrane of a maximum of 10,
• the porosity of the hollow-fiber membrane lies in the range from 50 to 90 vol. %, the wall thickness in the range from 50 to 300 μm, and the diameter of the lumen in the range from 100 to 500 μm,
• a transmembrane flow for isopropyl alcohol in the range from 2 to 8 ml/(cm2·min·bar), the hollow-fiber membrane has a maximum separating pore diameter dmax in the range from 0.3 to 0.7 μm, determined according to a bubble point test method DE-A-36 17 724;
• and
• the pores in the skin of the inner surface are smaller than the pores of the skin of the outer surface and the pores in the skin of the outer surface are larger than the pores in the supporting layer.