Methods of producing and using microporous membranes

1. A method of producing a microporous membrane comprising:

(a) stretching a multi-layer extrudate in at least one of a machine direction (MD) or traverse direction (TD),

the extrudate comprising at least first and second layers,

the first layer comprising a first polyolefin and at least a first diluent, and

the second layer comprising a second polyolefin and at least a second diluent, the second polyolefin comprising polypropylene in an amount in the range of from 25.0 wt. % to 35.0 wt. % based on the weight of the second polyolefin, the polypropylene having an Mw>0.9×10 6 and a ΔHm ≧100.0 J/g;

(b) removing at least a portion of the first and second diluents from the stretched extrudate to produce a dried membrane having a first length along MD and a first width along TD;

(c) stretching the membrane in MD from the first length to a second length larger than the first length by a first magnification factor in the range of from about 1.1 to about 1.5 followed by stretching the membrane in TD from the first width to a second width that is larger than the first width by a second magnification factor in the range of from about 1.1 to about 1.3, wherein the TD dry stretching magnification factor is less than the MD dry stretching magnification factor; and then

(d) reducing the second width to a third width, the third width being in the range of from the first width to about 1.1 times larger than the first width,

such that the membrane has a TD 105° C. heat shrinkage of 0.5% or less, a 130° C. heat shrinkage of 8.0% or less in at least one planar direction, and a TD maximum shrinkage in a molten state of 11.0% or less.

2. The method of claim 1 , further comprising removing at least a portion of any volatile species from the extrudate after step (b).

3. The method of claim 1 , wherein

(i) the first polyolefin comprises a first polyethylene in an amount in the range of from 90.0 wt. % to 99.0 wt. % and a second polyethylene in an amount in the range of from about 1.0 wt. % to 10.0 wt. %, the weight percents being based on the weight of the first polyolefin, the first polyethylene having an Mw≦1.0×10 6 and the second polyethylene having an Mw>1.0×10 6 ;

(ii) the second polyolefin comprises the polypropylene in an amount in the range of 25.0 wt. % to 35.0 wt. %, and further comprises (i) a first polyethylene having an Mw≦1.0×10 6 in an amount in the range of 60.0 wt. % to 90.0 wt. % and (ii) a second polyethylene having an Mw>1.0×10 6 in an amount in the range of from 0.0 wt. % to 10.0 wt. %, the weight percents being based on the weight of the second polyolefin;

(iii) the first diluent is present in the first layer of the extrudate in an amount in the range of from about 25.0 wt. % to about 99.0 wt. % based on the weight of the combined weight of the first polyolefin and the first diluent; and

(iv) the second diluent is present in the second layer of the extrudate in an amount in the range of from about 25.0 wt. % to about 99.0 wt. % based on the combined weight of second polyolefin and second diluent.

4. The method of claim 1 , wherein the extrudate further comprises a third layer comprising a third polyolefin, the third polyolefin comprising a first polyethylene in an amount in the range of from 90.0 wt. % to 99.0 wt. % and a second polyethylene in an amount in the range of from about 1.0 wt. % to 10.0 wt. %, the weight percents being based on the weight of the third polyolefin, the first polyethylene having an Mw≦1.0×10 6 and the second polyethylene having an Mw>1.0×10 6 .

5. The method of claim 1 , wherein

(i) the extrudate is a three-layer extrudate;

(ii) the second layer is located between the first and third layers and is in planar contact with the first and third layers;

(iii) the first polyolefin and the second polyolefin are the same polyolefin;

(iv) the second layer has a thickness of 5.0% to 15.0% of the extrudate's total thickness; and

(v) the first and third layers have the same thickness, the thickness of the first and third layers each being in the range of 42.5% to 47.5% of the extrudate's total thickness.

6. The method of claim 1 , wherein the first and second diluents are independently selected from one or more of nonane, decane, decalin, and liquid paraffin.

7. The method of claim 1 , wherein the stretching of step (a) is conducted by simultaneously stretching the extrudate in MD and TD.

8. The method of claim 7 , wherein during step (c) the MD stretching is conducted before the TD stretching, wherein the TD stretching is conducted to a second magnification factor in the range of from 1.15 to 1.25, wherein the first magnification factor is >the second magnification factor, and wherein

(i) the MD stretching is conducted while the membrane is exposed to a first temperature in the range of Tcd−30.0° C. to about Tm−10.0° C. and

(ii) the TD stretching is conducted while the membrane is exposed to a second temperature that is higher than the first temperature but lower than Tm; and

wherein the reducing of step (d) is conducted while the membrane is exposed to a temperature ≧the second temperature.

9. The method of claim 8 wherein the third width is in the range of 1.0 to 1.05 times the first width.

10. The method of claim 8 , wherein the first magnification factor is in the range of 1.2 to 1.4.