Composite porous membrane and method of producing the same

1. A battery separator composite porous membrane comprising a porous membrane B comprising a heat-resistant resin that is free of water-soluble polymers and laminated on a surface of a polypropylene resin of an outermost layer of a porous membrane A comprising at least one layer, wherein the heat-resistant resin being free from water-soluble polymers, wherein at least one of the outermost layers comprises the polypropylene resin, the composite porous membrane satisfying Equations (A) to (D):

0.01 μm≦Average pore size of porous membrane A≦ 1.0 μm  (A);

30%≦Porosity of porous membrane A≦ 70%  (B);

Peeling strength at interface between porous membrane A and porous membrane B≧ 1.0 N/25 mm  (C); and

20 ≦Y−X≦ 100  (D)

wherein X is an air resistance (sec/100 cc Air) of the porous membrane A, and Y is an air resistance (sec/100 cc Air) of the composite porous membrane.

2. The composite porous membrane according to claim 1 , wherein the porous membrane A is a laminate of three layers of polypropylene/polyethylene/polypropylene.

3. The composite porous membrane according to claim 1 , wherein the heat-resistant resin is a polyamide-imide resin, polyimide resin, or polyamide resin.

4. The composite porous membrane according to claim 3 , wherein the heat-resistant resin is a polyamide-imide resin having a logarithmic viscosity of not less than 0.5 dl/g.

5. A method of producing the composite porous membrane according to claim 1 , comprising steps (i) to (iii):

Step (i): coating a heat-resistant resin solution onto a substrate film, and then passing the substrate film through a low humidity zone at an absolute humidity of less than 6 g/m 3 followed by a high humidity zone at an absolute humidity of 6 g/m 3 to 25 g/m 3 , thereby forming a heat-resistant resin membrane on the substrate film,

Step (ii): providing a porous membrane A comprising at least one layer, wherein at least one of its outermost layers comprises a polypropylene resin; and

Step (iii) laminating the heat-resistant resin membrane formed in Step (i) on a surface of the polypropylene resin of the outermost layer of the porous membrane A of Step (ii), and then converting the heat-resistant resin membrane into a porous membrane B by immersion in a coagulation bath, followed by washing and drying, thereby obtaining a composite porous membrane.

6. The method according to claim 5 , further comprising peeling the substrate film after obtaining the composite porous membrane in Step (iii).

7. The method according to claim 5 , wherein the substrate film is a polyester-based film or a polyolefin-based film with a thickness of 25 to 100 μm.

8. The method according to claim 5 , wherein an amount of linear oligomers on the surface of the substrate film is 20 μg/m 2 to 100 μg/m 2 .

9. The method according to claim 5 , wherein, in Step (i), a time of passage through the low humidity zone is 3 seconds to 20 seconds, and a time of passage through the high humidity zone is 3 seconds to 10 seconds.

10. The composite porous membrane according to claim 2 , wherein the heat-resistant resin is a polyamide-imide resin, polyimide resin, or polyamide resin.

11. A method of producing the composite porous membrane according to claim 2 , comprising steps (i) to (iii):

Step (i): coating a heat-resistant resin solution onto a substrate film, and then passing the substrate film through a low humidity zone at an absolute humidity of less than 6 g/m 3 followed by a high humidity zone at an absolute humidity of 6 g/m 3 to 25 g/m 3 , thereby forming a heat-resistant resin membrane on the substrate film,

Step (ii): providing a porous membrane A comprising at least one layer, wherein at least one of its outermost layers comprises a polypropylene resin; and

Step (iii): laminating the heat-resistant resin membrane formed in Step (i) on a surface of the polypropylene resin of the outermost layer of the porous membrane A of Step (ii), and then converting the heat-resistant resin membrane into a porous membrane B by immersion in a coagulation bath, followed by washing and drying, thereby obtaining a composite porous membrane.

12. A method of producing the composite porous membrane according to claim 3 , comprising steps (i) to (iii):

Step (i): coating a heat-resistant resin solution onto a substrate film, and then passing the substrate film through a low humidity zone at an absolute humidity of less than 6 g/m 3 followed by a high humidity zone at an absolute humidity of 6 g/m 3 to 25 g/m 3 , thereby forming a heat-resistant resin membrane on the substrate film,

Step (ii): providing a porous membrane A comprising at least one layer, wherein at least one of its outermost layers comprises a polypropylene resin; and

Step (iii): laminating the heat-resistant resin membrane formed in Step (i) on a surface of the polypropylene resin of the outermost layer of the porous membrane A of Step (ii), and then converting the heat-resistant resin membrane into a porous membrane B by immersion in a coagulation bath, followed by washing and drying, thereby obtaining a composite porous membrane.

13. A method of producing the composite porous membrane according to claim 4 , comprising steps (i) to (iii):

Step (i): coating a heat-resistant resin solution onto a substrate film, and then passing the substrate film through a low humidity zone at an absolute humidity of less than 6 g/m 3 followed by a high humidity zone at an absolute humidity of 6 g/m 3 to 25 g/m 3 , thereby forming a heat-resistant resin membrane on the substrate film,

Step (ii): providing a porous membrane A comprising at least one layer, wherein at least one of its outermost layers comprises a polypropylene resin; and

Step (iii): laminating the heat-resistant resin membrane formed in Step (i) on a surface of the polypropylene resin of the outermost layer of the porous membrane A of Step (ii), and then converting the heat-resistant resin membrane into a porous membrane B by immersion in a coagulation bath, followed by washing and drying, thereby obtaining a composite porous membrane.

14. The method according to claim 6 , wherein the substrate film is a polyester-based film or a polyolefin-based film with a thickness of 25 to 100 μm.

15. The method according to claim 6 , wherein an amount of linear oligomers on the surface of the substrate film is 20 μg/m 2 to 100 μg/m 2 .

16. The method according to claim 7 , wherein an amount of linear oligomers on the surface of the substrate film is 20 μg/m 2 to 100 μg/m 2 .

17. The method according to claim 6 , wherein, in Step (i), a time of passage through the low humidity zone is 3 seconds to 20 seconds, and a time of passage through the high humidity zone is 3 seconds to 10 seconds.

18. The method according to claim 7 , wherein, in Step (i), a time of passage through the low humidity zone is 3 seconds to 20 seconds, and a time of passage through the high humidity zone is 3 seconds to 10 seconds.

19. The method according to claim 8 , wherein, in Step (i), a time of passage through the low humidity zone is 3 seconds to 20 seconds, and a time of passage through the high humidity zone is 3 seconds to 10 seconds.