Microporous membranes, methods for making these membranes, and the use of these membranes as battery separator films

1. A microporous membrane comprising polymethylpentene, polypropylene, and polyethylene, and having a meltdown temperature≧180.0° C., a normalized air permeability≦75.0 seconds/100 cm 3 /μm, and a pin puncture strength≧0.90×10 2 mN/μm, wherein the polymethylpentene, polypropylene, and polyethylene are present in an amount of 5.0 wt. %≦polymethylpentene≦25.0 wt. %, 5.0 wt. %≦polypropylene≦25.0 wt. %, and 50.0 wt. %<polyethylene≦90.0 wt. %, the weight percents being based on the weight of the membrane, the amount of polymethylpentene in wt % is ≧the polypropylene amount in wt. % and the polymethylpentene and polypropylene are present in the membrane in a combined amount≧25.0 wt. %, based on the weight of the membrane.

2. The microporous membrane of claim 1 , wherein the membrane has a TD heat shrinkage at 105° C.≦5.0%, a TD heat shrinkage at 130° C.<20.0%, a TD heat shrinkage at 170° C.≦40.0%, a thickness≦30.0 μm, and a porosity of 20% to 80%.

3. The microporous membrane of claim 1 , wherein the polymethylpentene has a Tm≧200.0° C., and an MFR≦80.0 dg/min.

4. The microporous membrane of claim 1 , wherein the polypropylene is an isotactic polypropylene having an Mw≧6.0×10 5 , an MWD≦8.5 and a ΔHm≧90.0 J/g.

5. The microporous membrane of claim 1 , wherein the polyethylene has an Mw of 1.0×10 5 to 2.0×10 6 and a Tm≧130.0° C.

6. The microporous membrane of claim 1 , wherein the polymethylpentene has an MFR of 22.0 dg/min to 28.0 dg/min and a Tm of 223.0° C. to 230.0° C.

7. The microporous membrane of claim 1 , wherein the polyethylene has an amount of terminal unsaturation≦0.14 per 1.0×10 4 carbon atoms.

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