Vinylidene fluoride resin porous membrane, manufacturing method therefor, and method for manufacturing filtrate water

Abstract

A porous membrane of vinylidene fluoride resin, including a 10 μm-thick portion contiguous to one surface thereof which comprises network resin fibers having an average diameter of at most 100 nm and shows a porosity A1 of at least 60% as measured by a focused ion beam-scanning electron microscope, and showing a surface pore size of at most 0.3 μm on said one surface thereof. The porous membrane has a treated water side surface layer showing a small surface pore size suitable for water filtration treatment and formed of extremely thin network resin fibers giving an extremely high porosity, thus showing an excellent minute particle-blocking performance and also extremely good anti-soiling resistance and regeneratability. The porous membrane is produced by a process including: melt-extrusion of a composition obtained by adding to vinylidene fluoride resin of a large molecular weight a relatively large amount of a polyester plasticizer which is mutually soluble with the resin and provides the resultant mixture with a crystallization temperature that is substantially identical to that of the vinylidene fluoride resin alone to form film, followed by cooling from one side of the film to solidify the film, extracting the plasticizer and stretching the membrane after the extraction while partially wetting a surface portion thereof.