Separator for electric storage device

1. A separator for an electric storage device, comprising a microporous membrane comprising a polypropylene resin (A) and a thermoplastic elastomer (B), wherein the microporous membrane has the following regions:

(i) a polymeric matrix comprising the polypropylene resin (A) and the thermoplastic elastomer (B);

(ii) fibrils comprising the polypropylene resin (A) and extending in a Machine Direction (MD) of the microporous membrane from the polymeric matrix; and

(iii) pores between the fibrils; and

wherein a weight ratio of the polypropylene resin (A) to the thermoplastic elastomer (B) is 99.9:0.1 to 80:20,

wherein the region of the thermoplastic elastomer (B) is oriented along the MD of the microporous membrane, and

wherein a Melt Flow Rate (MFR) of the microporous membrane is 1.5 g/10 minutes or less, or wherein a Melt Flow Rate (MFR) of the polypropylene resin (A) is 1.0 g/10 minutes or less, or wherein the thermoplastic elastomer (B) forms a region in which a ratio (MD/TD) of a length in the MD to a length in a Transverse Direction (TD) is 1.5 or more and 10.0 or less;

and wherein the separator has a thickness from 11 to 16 μm, and

wherein a porosity of the microporous membrane is 37% or more and 51% or less.

2. The separator according to claim 1 , wherein the thermoplastic elastomer (B) is free of propylene.

3. The separator according to claim 1 , wherein the thermoplastic elastomer (B) is present in the polymeric matrix, but is not present in the fibrils.

4. The separator according to claim 1 , wherein, in the case of a crystalline, thermoplastic elastomer in which a hard segment of the thermoplastic elastomer (B) is crystalline, a crystalline melting point peak of the hard segment is 50 to 110° C., or in the case of an amorphous, thermoplastic elastomer in which a hard segment of the thermoplastic elastomer (B) is amorphous, a glass-transition temperature of the hard segment is 30 to 110° C.

5. The separator according to claim 1 , wherein a Melt Flow Rate (MFR) of the thermoplastic elastomer (B) is 2.0 g/10 minutes or more.

6. The separator according to claim 1 , wherein the thermoplastic elastomer (B) is ethylene/α-olefin copolymer.

7. The separator according to claim 1 , wherein the separator has an air permeability resistance of 100 to 500 seconds/100 ml, provided that the air permeability resistance is a value obtained by multiplying an actual measured air permeability resistance of the separator by 14 μm after dividing the actual measured air permeability resistance by a thickness of the separator.

8. The separator according to claim 1 , wherein the separator has a puncture strength of 400 gf or more, provided that the puncture strength is a value obtained by multiplying an actual measured puncture strength of the separator by 14 μm after dividing the actual measured puncture strength by a thickness of the separator.

9. The separator according to claim 1 , wherein a Melt Flow Rate (MFR) of the microporous membrane is 0.1 g/10 minutes or more.

10. The separator according to claim 1 , wherein a Melt Flow Rate (MFR) of the polypropylene resin (A) is 0.1 g/10 minutes or more.

11. The separator according to claim 1 , wherein the separator comprises a microporous multi-layered membrane in which the microporous membrane comprises the polypropylene resin (A) and the thermoplastic elastomer (B).

12. The separator according to claim 11 , wherein a density of the polyethylene for the microporous membrane comprising polyethylene as a major component is 0.96 g/cm 3 or more, and a Melt Flow Rate (MFR) of the microporous membrane comprising polyethylene as a major component is 0.6 g/10 minutes or less.

13. The separator according to claim 11 , wherein the separator comprises not only the microporous membrane comprising the polypropylene resin (A) and the thermoplastic elastomer (B), but also the microporous multi-layered membrane and a microporous membrane comprising polyethylene as a major component are laminated.

14. The separator according to claim 11 , wherein the maximum puncture strength is 20 gf/μm or more.

15. The separator according claim 11 , wherein a puncture strength of the microporous membrane has at least two maximum values in a depth-strength curve of the puncture test.

16. The separator according to claim 13 , wherein the maximum puncture strength is at least one of the at least two maximum values in the depth-strength curve.

17. The separator according to claim 13 , wherein a Melt Flow Rate (MFR) of the microporous membrane is 1.4 g/10 minutes or less, or 0.1 g/10 minutes to 1.4 g/10 minutes.

18. The separator according to claim 11 , wherein an average longest pore diameter of the microporous membrane is from 100 nm to 2000 nm.

19. The separator according to claim 11 , wherein a pore diameter ratio (a/b) of an average longest pore diameter (a) of the microporous membrane to a pore diameter (b) perpendicular to the average longest pore diameter (a) is from 1.5 to 30.

20. The separator according to claim 11 , wherein longest pore diameters of the microporous membrane are arranged in one direction.

21. The separator according to claim 11 , wherein the maximum puncture strength is 50 gf/μm or less.

22. The separator according to claim 11 , wherein the microporous membrane comprises the polypropylene resin, as well as at least one selected from the group consisting of:

a copolymer having a branched chain; and

a copolymer having a conjugated double bond.

23. The separator according to claim 19 wherein a Melt Flow Rate (MFR) of the polypropylene is 1.0 g/10 minutes or less.

24. The separator according to claim 19 , wherein a Melt Flow Rate (MFR) of the microporous membrane is 1.5 g/10 minutes or less.

25. The separator according to claim 19 , comprising:

the polypropylene; and

a thermoplastic elastomer being free of propylene.

26. The separator according to claim 19 , wherein the separator comprises a microporous multi-layered membrane in which the microporous membrane comprising a polypropylene as a major component and a microporous membrane comprising a polyethylene as a major component are laminated.

27. The separator according to claim 1 , wherein the separator has a single-layer structure or a three-layer structure.

28. The separator according to claim 1 , wherein the thickness of the separator is 14 to 16 μm.

29. The separator according to claim 1 , wherein the microporous membrane is made by extrusion through a circular-die.