Microporous Multilayer Membrane, System And Process For Producing Such Membrane, And The Use Of Such Membrane

1 . A process for producing a microporous membrane, comprising the steps of:

(a) combining a polyolefin composition and at least one diluent to form a mixture, the polyolefin composition comprising at least a first polyethylene having a crystal dispersion temperature (T cd ) and polypropylene;

(b) extruding the mixture through an extrusion die to form an extrudate;

(c) cooling the extrudate to form a cooled extrudate having a first area;

(d) orienting the cooled extrudate in at least a first direction by about one to about ten fold at a temperature of about T cd +/−15° C.; and

(e) further orienting the cooled extrudate in at least a second direction by about one to about five fold at a temperature about 10° C. to about 40° C. higher than the temperature employed in step (d) to a second area at least ten fold larger than the first area.

2 . The process of claim 1 , further comprising the steps of:

(f) removing at least a portion of the solvent or diluent from the cooled extrudate to form a membrane;

(g) orienting the membrane to a magnification of from about 1.1 to about 2.5 fold in at least one direction; and

(h) heat-setting the membrane to form the microporous membrane.

3 . The process of claim 1 , wherein said step of orienting the cooled extrudate in at least a first direction utilizes a roll-type stretching machine.

4 . The process of claim 1 , wherein said step of orienting the cooled extrudate in at least the first direction utilizes a tenter-type stretching machine.

5 . The process of claim 1 , wherein said step of further orienting the cooled extrudate in at least a second direction utilizes a tenter-type stretching machine.

6 . The process of claim 1 , wherein the polyolefin composition comprises a high density polyethylene and polypropylene.

7 . The process of claim 6 , wherein the polyolefin composition further comprises an ultra high molecular weight polyethylene.

8 . The process of claim 6 , wherein the polyolefin composition comprises at least about 30 wt. % high density polyethylene and at least about 30 wt. % polypropylene.

9 . The process of claim 8 , wherein the polyolefin composition comprises at least about 20 wt. % ultra high molecular weight polyethylene.

10 . The process of claim 1 , wherein the polyolefin composition comprises:

(i) at least 20 wt. % of the first polyethylene resin, the first polyethylene having an Mw of from 2×10 5 to 9×10 5 and an MWD of from 3 to 50;

(ii) from 5 wt. % to 60 wt. % of a polypropylene having an Mw of from 6×10 5 to 4×10 6 , an MWD of from 3 to 30, a heat of fusion of 90 J/g or more; and

(iii) from 0 wt. % to 40 wt. % of a second polyethylene having an Mw of from 1×10 6 to 5×10 6 , an MWD of from 3 to 30, the weight percentages based on the weight of the polyolefin composition.

11 . A microporous membrane comprising polyethylene and polypropylene and having a thickness fluctuation standard deviation in at least one planar direction of≦0.7 μm and a melt down temperature≧150° C.

12 . The microporous membrane of claim 11 , wherein the membrane has a capacity recovery ratio≧73%.

13 . The microporous membrane of claim 11 , wherein the membrane has an MD 105° C. heat shrinkage≦3.5% and a TD 105° C. heat shrinkage≦5%.

14 . The microporous membrane of claim 11 , wherein the polyethylene comprises a first polyethylene having an Mw<1×10 6 and a second polyethylene having an Mw≧1×10 6 .

15 . The microporous membrane of claim 11 , wherein the polypropylene has an Mw≧1×10 4 and a heat of fusion≧90 J/g.

16 . The microporous membrane of claim 11 , wherein the membrane has an electrolytic solution absorption speed≧3.5.

17 . The microporous membrane of claim 11 , wherein the membrane has a thickness variation after heat compression≦10%.

18 . The microporous membrane of claim 11 , wherein the membrane has an air permeability after heat compression≦600 seconds/100 cm 3 .

19 . The microporous membrane of claim 11 , wherein the membrane has a maximum shrinkage in the molten state≦25%.

20 . The microporous membrane of claim 14 , wherein the membrane comprises:

(i) from 20 wt. % to 80 wt. % of the first polyethylene, the first polyethylene resin having an Mw of from 2×10 5 to 9×10 5 and an MWD of from 3 to 50;

(ii) from 5 wt. % to 60 wt. % of polypropylene having an Mw of from 6×10 5 to 4×10 6 , an MWD of from 3 to 30, a heat of fusion of 90 J/g or more; and

(iii) from 0 wt. % to 40 wt. % of the second polyethylene, the second polyethylene having an Mw of from 1×10 6 to 5×10 6 , an MWD of from 3 to 30, a heat of fusion of 90 J/g or more, percentages based on the mass of the membrane.

21 . A battery comprising an anode, a cathode, and electrolyte, and at least one separator located between the anode and the cathode, the separator comprising polyethylene and polypropylene and having a thickness fluctuation standard deviation in at least one planar direction of≦0.7 μm and a melt down temperature≧150° C.

22 . The battery of claim 21 , wherein the battery is a lithium ion secondary battery.

23 . The battery of claim 21 , wherein the separator comprises:

(i) from 20 wt. % to 80 wt. % of the first polyethylene, the first polyethylene resin having an Mw of from 2×10 5 to 9×10 5 and an MWD of from about 3 to 50;

(ii) from 5 wt. % to 60 wt. % of polypropylene having an Mw of from 6×10 5 to 4×10 6 , an MWD of from 3 to 30, a heat of fusion of 90 J/g or more; and

(iii) from 0 wt. % to 40 wt. % of the second polyethylene, the second polyethylene having an Mw of from 1×10 6 to 5×10 6 , an MWD of from 3 to 30, a heat of fusion of 90 J/g or more, percentages based on the mass of the membrane.

24 . The battery of claim 21 , wherein the separator has a melt down temperature≧160° C.

25 . The battery of claim 21 used as a power source for an electric vehicle or hybrid electric vehicle.