Multi-layer microporous film

1. A multilayer microporous membrane comprising:

i. a first layer comprising ≦20.0 wt % polymethylpentene, based on the weight of the first layer; <20.0 wt % polypropylene based on the weight of the first layer and further comprising ≧60.0 wt % polyethylene based on the weight of the layer, wherein the polyethylene comprises a first polyethylene having an Mw<1.0×10 6 and a second polyethylene having an Mw≧1.0×10 6 , the polypropylene is an isotactic polypropylene having an Mw>0.8×10 6 , and the polymethylpentene has an Mw of from 1.0×10 4 to 4.0×10 6 and a Tm≧200° C.; and

ii. a second layer comprising 10.0 wt % to 40.0 wt % polymethylpentene based on the weight of the second layer, 21.0 wt % to 35.0 wt % polypropylene based on the weight of the second layer, and ≧20.0 wt % polyethylene based on the weight of the second layer, wherein the polyethylene comprises a first polyethylene having an Mw<1.0×10 6 and a second polyethylene having an Mw≧1.0×10 6 , the polypropylene is an isotactic polypropylene having an Mw>0.8×10 6 , and the polymethylpentene has an Mw of 1.0×10 4 to 4.0×10 6 and a Tm≧200° C.;

the second layer having a composition that is not the same as that of the first layer and the second layer having a thickness ≦2.0 times that of the first layer.

2. The membrane of claim 1 , wherein the membrane has a meltdown temperature ≧175.0° C. and an NEA≦4.0 seconds.

3. The membrane of claim 1 , wherein the membrane further comprises a third layer, the second layer being located between the first and third layers, wherein the total amount of polypropylene and polymethylpentene in the second layer is ≦65.0 wt % based on the weight of the second layer.

4. The membrane of claim 1 , wherein (i) the first, second, and third layers have a combined thickness ≧10.0 μm, (ii), the second layer's thickness is ≦ each of the first and third layer's thicknesses, and wherein and the third layer has substantially the same composition and thickness as the first layer.

5. The membrane of claim 4 , wherein the membrane has a normalized air permeability ≦30 seconds/100 cm 3 /μm, a normalized pin puncture strength in the range of ≧1.4×10 2 Mn/μm, a meltdown temperature ≧180.0° C., and an NEA≦2.0 seconds/μm.

6. The membrane of claim 5 , wherein (i) the first layer and third layers comprise 75.0 wt % to 85.0 wt % of the first polyethylene and 15.0 wt % to 25.0 wt % of the second polyethylene based on the weight of the layer; (ii) the second layer comprises 10.0 wt % to 15.0 wt % of the polymethylpentene, 21.0 wt % to 35.0 wt % of the polypropylene, 25.0 wt % to 35.0 wt % of the first polyethylene, and 25.0 wt % to 35.0 wt/o of the second polyethylene, the weight percents being based on the weight of the second layer; (iii) the first polyethylene has an Mw of 1.0×10 5 to 0.90×10 6 , an MWD of 3.0 to 10.0, and a terminal unsaturation amount <0.20 per 1.0×10 4 carbon atoms; (iv) the second polyethylene has an Mw of 1.0×10 6 to 5.0×10 6 and an MWD of about 4.0 to about 15.0; (v) the polymethylpentene has a Tm of 220.0° C. to 230.0° C. and an MFR of 10.0 dg/min to 40.0 dg/min; and (vi) the polypropylene has an Mw of 0.8×10 6 to 2.0×10 6 , a Tm≧160.0° C., a ΔHm≧90.0 J/g, and an MWD of about 2.5 to about 6.0.

7. A battery separator film comprising the microporous membrane of claim 1 .

8. A method of producing a microporous membrane comprising:

(a) forming a first mixture comprising a first diluent and a first polymer blend comprising ≦20.0 wt % polymethylpentene based on the weight of the first polymer blend;

(b) forming a second mixture comprising a second diluent and a second polymer blend comprising 10.0 wt % to 40.0 wt % polymethylpentene based on the weight of the second polymer blend, 21.0 wt % to 35.0 wt % polypropylene based on the weight of the second layer, and ≧20.0 wt % polyethylene based on the weight of the second layer;

(c) forming a third mixture comprising a third diluent and a third polymer blend comprising ≦20.0 wt % polymethylpentene based on the weight of the third polymer blend;

(d) producing a sheet comprising a first layer comprising the first mixture, a third layer comprising the third mixture, and a second layer comprising the second mixture, the second layer being located between the first and third layers, the first mixture having a substantially different composition from each of the first and third mixtures and the second layer having a thickness ≦2.0 times that of the first and third layers; and

(e) removing at least a portion of the first, second, and third diluents from the sheet.

9. The method of claim 8 , wherein the first and third mixtures are substantially the same mixture.

10. The method of claim 8 , wherein the first, second, and third diluents are substantially the same diluent.

11. The method of claim 8 , wherein the second polymer blend comprises <40.0 wt % polypropylene and further comprises ≧20.0 wt % polyethylene based on the weight of the second polymer blend.

12. The method of claim 8 , wherein the total amount of polypropylene and polymethylpentene in the second polymer blend is ≦65.0 wt % based on the weight of the second polymer blend.

13. The method of claim 8 , wherein the polyethylene comprises a first polyethylene having an Mw<1.0×10 6 and a second polyethylene having an Mw≧1.0×10 6 , the polypropylene is an isotactic polypropylene having an Mw>0.8×10 6 , and the polymethylpentene has an Mw of 1.0×10 4 to 4.0×10 6 and a Tm≧200° C.

14. The method of claim 13 , wherein (i) the first polymer blend and third polymer blend comprise 75.0 wt % to 85.0 wt % of the first polyethylene and 15.0 wt % to 25.0 wt % of the second polyethylene based on the weight of the polymer blend; (ii) the second polymer blend comprises 10.0 wt % to 15.0 wt % of the polymethylpentene, 21.0 wt % to 35.0 wt % of the polypropylene, 25.0 wt % to 35.0 wt % of the first polyethylene, and 25.0 wt % to 35.0 wt % of the second polyethylene, the weight percents being based on the weight of the second polymer blend; (iii) the first polyethylene has an Mw of 1.0×10 5 to 0.90×10 6 , an MWD of 3.0 to 10.0, and a terminal unsaturation amount <0.20 per 1.0×10 4 carbon atoms; (iv) the second polyethylene has an Mw of 1.0×10 6 to 5.0×10 6 and an MWD of about 4.0 to about 15.0; (v) the polymethylpentene has a Tm of 220.0° C. to 230.0° C. and an MFR of 10.0 dg/min to 40.0 dg/min; and (vi) the polypropylene has an Mw of 0.8×10 6 to 2.0×10 6 , a Tm≧160.0° C., a ΔHm≧90.0 J/g, and an MWD of about 2.5 to about 6.0.

15. The method of claim 8 , further comprising cooling the sheet following step (d).

16. The method of claim 8 , further comprising stretching the sheet in at least one direction before and/or after step (e).

17. The method of claim 16 , wherein the stretching is conducted before and after step (e), and the stretching after step (e) is conducted while the sheet is exposed to a temperature of 120° C. to 132° C.

18. A microporous membrane made by the method of claim 8 .

19. A battery comprising an anode, a cathode, an electrolyte, and at least one separator located between the anode and cathode, the separator comprising:

i. a first layer comprising ≦20.0 wt % polymethylpentene, based on the weight of the first layer, wherein the first layer comprises <20.0 wt % polypropylene and ≧60.0 wt % polyethylene based on the weight of the layer; and

ii. a second layer comprising 10.0 wt % to 40.0 wt % polymethylpentene based on the weight of the second layer, 21.0 wt % to 35.0% polypropylene based on the weight of the second layer, and ≧20.0 wt % polyethylene based on the weight of the second layer;

the second layer having a composition that is not the same as that of the first layer and the second layer having a thickness ≦2.0 times that of the first layer.

20. The battery of claim 19 and a load electrically connected to the battery.

21. The battery of claim 19 , wherein the electrolyte contains lithium ions.

22. The battery of claim 21 , wherein the battery is a lithium ion secondary battery used as a power source for an electric vehicle or hybrid electric vehicle.

23. The multilayer microporous membrane of claim 1 , wherein the second layer has a thickness ≦1.1 times that of the first layer.

24. The multilayer microporous membrane of claim 1 , wherein the second layer has a thickness ≦1.0 times that of the first layer.

25. A multilayer microporous membrane comprising:

i. a first layer comprising ≦20.0 wt % polymethylpentene, based on the weight of the first layer; and

ii. a second layer comprising 10.0 wt % to 40.0 wt % polymethylpentene based on the weight of the second layer, 21.0 wt % to 35.0 wt % polypropylene based on the weight of the second layer, and ≧20.0 wt % polyethylene based on the weight of the second layer;

the second layer having a composition that is not substantially the same as that of the first layer and the second layer having a thickness ≦2.0 times that of the first layer.