Porous membrane of vinylidene fluoride resin and process for producing the same

A process for producing a hollow yarn porous membrane of vinylidene fluoride resin for water filtration, comprising:

• providing a composition by adding 70-250 wt. parts of a plasticizer and 5-80 wt. parts of a good solvent for vinylidene fluoride resin to 100 wt. parts of (A) a vinylidene fluoride resin having a weight-average molecular weight of at least 200,000 and a ratio of weight-average molecular weight/number-average molecular weight of at least 2.5, or (B) a vinylidene fluoride resin comprising 2-75 wt. % of a first vinylidene fluoride resin having a weight-average molecular weight of 400,000-1,200,000 and 25-98 wt. % of a second vinylidene fluoride resin having a weight-average molecular weight of 150,000-600,000 giving a ratio of the weight-average molecular weight of the first vinylidene fluoride resin/the weight-average molecular weight of the second vinylidene fluoride resin of at least 1.2;
• melt-extruding the composition to form a hollow yarn film,
• cooling the hollow yarn film preferentially from an outer surface thereof within a cooling medium at 5-120° C. to form a hollow yarn solid film,
• extracting the plasticizer, and
• then stretching the hollow yarn film.