Microporous membranes, methods for making such membranes, and the use of such membranes as battery separator film
View Patent ↗The invention relates to microporous membranes having high meltdown temperature, low shutdown temperature, and resistance to heat shrinkage at elevated temperature. The membranes can be produced by stretching a sheet comprising polymethylpentene, polyethylene, and diluent, and then removing the diluent. The membranes can be used as battery separator film in, e.g., lithium ion batteries.
1. A membrane comprising a polymer mixture comprising:
(a) polymethylpentene having a Tm≧200.0°C., and an MFR≦80.0 dg/min; (b) a first polyethylene having an Mw<1.0 ×10 6 , an MWD≦15.0, an amount of terminal unsaturation ≦0.20 per 1.0 ×10 4 carbon atoms and a Tm≧131.0°C.; and (c) a second polyethylene having a Tm<131.0° C., wherein the membrane contains ≦1.0 wt. % of polyethylene having a terminal unsaturation amount >0.20 per 1.0 ×10 4 carbon atoms and (i) is microporous; (ii) has a meltdown temperature ≧180.0°C.; (iii) has a shutdown temperature ≦131.0°C.; and (iv) has a 170° C. TD heat shrinkage ≦30.0%.
2. The membrane of claim 1 , which is free of polyethylene having a terminal unsaturation amount >0.20 per 1.0 ×10 4 carbon atoms.
3. The microporous membrane of claim 1 , wherein the polymethylpentene is contained in an amount of 5.0 wt % to 15.0 wt %, based on the weight of the membrane.
4. The microporous membrane of claim 1 , further comprising polypropylene in an amount of 5.0 wt % to 20.0 wt %, based on the weight of the membrane.
5. The microporous membrane of claim 1 , wherein the membrane has a 105° C. TD heat shrinkage ≦5.0%, a 130° C. TD heat shrinkage ≦20.0%, a normalized pin puncture strength ≧80.0 mN/μm, a thickness ≦30.0 μm, a porosity of 20% to 80%, and a normalized air permeability ≦280.0 seconds/100 cm 3 /μm.
6. The microporous membrane of claim 1 , wherein the membrane comprises micropores and microfibrils, the microfibrils comprising a substantially single phase of the polymethylpentene, the first polyethylene, and the second polyethylene.
7. The microporous membrane of claim 4 , wherein (i) the polypropylene is an isotactic polypropylene having an Mw≧6.0 ×10 5 , an MWD≦6.0, and a ΔHm≧90.0 J/g; and (ii) 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.
8. The microporous membrane of claim 1 , wherein (i) the first polyethylene has an amount of terminal unsaturation ≦0.14 per 1.0 ×10 4 carbon atoms; and
(ii) the second polyethylene has a Tm of 115.0° C. to 130.0° C., an Mw of 5.0 ×10 3 to 4.0 ×10 5 , and an MWD of 1.5 to about 5.
9. The microporous membrane of claim 6 , wherein the microfibrils comprise >90.0 wt % of a single polymer phase of the polymethylpentene, the first polyethylene, and the second polyethylene, based on the weight of the microfibrils, and wherein the membrane contains ≧1.0 wt % of phase-separated polymer, based on the weight of the membrane.
10. A battery separator film comprising the microporous membrane of claim 1 .