Method of producing composite porous membrane

1. A method of producing a composite porous membrane, the composite porous membrane comprising:

a porous membrane A formed of a polyolefin-based resin; and

a porous membrane B containing a heat-resistant resin laminated to the porous membrane A,

wherein the porous membrane A satisfies formulas (A) to (C), the composite porous membrane satisfies formula (D), and the composite porous membrane satisfies formulas (E) and (F):

thickness of porous membrane A<10 μm  formula (A);

0.01 μm ≦average pore diameter of porous membrane A≦1.0 μm  formula (B);

30% ≦porosity of porous membrane A≦70%  formula (C);

thickness of entire composite porous membrane≦13 μm  formula (D);

peel strength at interface between porous membrane A and porous membrane B≧1.0 N/25 mm  formula (E);

20≦Y−X≦100  formula (F), and

wherein X is a gas permeation resistance (seconds/100 ccAir) of porous membrane A, and Y is a gas permeation resistance (seconds/100 ccAir) of the composite porous membrane,

the method comprising the steps of:

(i) applying a heat-resistant resin solution onto a base film, followed by passing the coated film through a low-humidity zone having an absolute humidity of less than 6 g/m 3 , and then through a high-humidity zone having an absolute humidity of 6 g/m 3 or more and 25 g/m 3 or less to form a heat-resistant resin membrane on the base film; and

(ii) bonding together the heat-resistant resin membrane formed in step (i) and the porous membrane A formed of a polyolefin-based resin, followed by converting the heat-resistant resin membrane into the porous membrane B by immersion in a solidification bath, and washing and drying the same to obtain the composite porous membrane.

2. The method according to claim 1 , wherein the base film is peeled off after the composite porous membrane is obtained in step (ii).

3. The method according to claim 1 , wherein the base film is a polyester-based film or polyolefin-based film having a thickness of 25 to 100 μm.

4. The method according to claim 1 , wherein in step (i), the time of passage through the low-humidity zone is 3 seconds or more and 20 seconds or less, and the time of passage through the high-humidity zone is 3 seconds or more and 10 seconds or less.

5. The method according to claim 2 , wherein the base film is a polyester-based film or polyolefin-based film having a thickness of 25 to 100 μm.