Microporous polyolefin film, separator for battery, and production processes therefor

A battery separator comprising:

• a polyolefin microporous membrane;
• and
• a porous layer on one or more sides of the polyolefin microporous membrane,
• wherein:
• the porous layer includes a water-soluble resin or a water-dispersible resin and a heat-resistant particle,
• a variation range of an F25 value in a width direction of the polyolefin microporous membrane is not greater than 1 MPa,
• an average thickness T(ave) of the porous layer is from 1 μm to 5 μm, and
• the F25 value is a value obtained by dividing a load at 25% elongation of a sample of the laminated polyolefin microporous membrane as measured with a tensile testing machine by a cross-sectional area of the sample.

A method of producing a polyolefin microporous membrane comprising:

• (a) preparing a polyolefin resin solution by melt-kneading a polyolefin resin and a forming solvent;
• (b) forming an unstretched gel sheet by extruding the polyolefin resin solution and cooling the extrudate;
• (c) stretching the unstretched gel sheet in a longitudinal direction by passing the unstretched gel sheet between two or more pairs of longitudinal stretching rollers, wherein the pairs of rollers have different peripheral speed ratios, whereby a longitudinally stretched gel sheet is obtained, wherein each pair of longitudinal stretching rollers includes a longitudinal stretching roller and a nip roller contacting the longitudinal stretching roller in parallel, and wherein a contact pressure of the nip roller against the longitudinal stretching roller is not less than 0.05 MPa and not greater than 0.5 MPa;
• (d) stretching the longitudinally stretched gel sheet in a transverse direction by holding the longitudinally stretched sheet with clips having an inter-clip distance of not greater than 50 mm at a tenter exit, whereby a biaxially stretched gel sheet is obtained;
• (e) extracting a forming solvent from the biaxially stretched gel sheet and drying the biaxially stretched gel sheet;
• and
• (f) heat-treating the sheet after the drying to obtain the polyolefin microporous membrane.