Polyolefin Microporous Membrane

Abstract

Disclosed is a polyolefin microporous membrane including a multilayer film having two or more layers. In this polyolefin microporous membrane, at least one surface layer has a thickness of not less than 0.2 μm but not more than 5 μm and contains inorganic particles, while at least one layer contains a polyethylene and has an air permeability of not less than 50 second/100 cc but not more than 1000 second/100 cc and a puncture strength of not less than 3.0 N/20 μm.

1 . A polyolefin microporous membrane comprising a laminated film having two or more layers in which the surface layer on at least one side has a thickness of 0.2 μm or more and 5 μm or less and contains inorganic particles and at least other one layer contains polyethylene, and which has a gas transmission rate of 50 sec/100 cc or more and 1000 sec/100 cc or less and a piercing strength of 3.0 N/20 μm or more. 
   
   
       2 . A polyolefin microporous membrane according to  claim 1  which comprises a laminated film having three or more layers. 
   
   
       3 . A polyolefin microporous membrane according to  claim 1  or  2 , wherein the surface layer on at least one side having a thickness of 0.2 μm or more and 5 μm or less contains 5% by mass or more and 60% by mass or less of inorganic particles and 5% by mass or more and 95% by mass or less of polypropylene. 
   
   
       4 . A polyolefin microporous membrane according to  claim 1  or  2 , wherein the surface layer on at least one side having a thickness of 0.2 μm or more and 5 μm or less contains 5% by mass or more and 60% by mass or less of inorganic particles, 5% by mass or more and 90% by mass or less of polypropylene, and 5% by mass or more and 90% by mass or less of polyethylene. 
   
   
       5 . A polyolefin microporous membrane according to  claim 1  or  2 , wherein the surface layer on at least one side having a thickness of 0.2 μm or more and 5 μm or less contains 5% by mass or more and 60% by mass or less of inorganic particles, 20% by mass or more and 90% by mass or less of polypropylene, and 5% by mass or more of polyethylene, and at least other one layer contains 30% by mass or more of polyethylene and contains no inorganic particles. 
   
   
       6 . A polyolefin microporous membrane according to any one of  claims 1 - 5  which has a total thickness of 2 μm or more and 50 μm or less. 
   
   
       7 . A polyolefin microporous membrane according to any one of  claims 1 - 5  which has a total thickness of 5 μm or more and 30 μm or less. 
   
   
       8 . A polyolefin microporous membrane according to any one of  claims 1 - 7 , wherein the ratio of breaking strength in MD to breaking strength in TD is 0.1 or more and 8.0 or less. 
   
   
       9 . A polyolefin microporous membrane according to any one of  claims 1 - 8 , wherein the inorganic particles are oxide or nitride of one of silicon, aluminum and titanium. 
   
   
       10 . A polyolefin microporous membrane according to any one of  claims 1 - 9  which is obtained through a step of kneading a polyolefin resin, inorganic particles and a plasticizer in the amounts satisfying the following formula (1):
   0.6≦weight of plasticizer/(oil absorption for plasticizer×weight of inorganic particles×density of plasticizer)×100≦1.2  (1)   
   
   
       11 . A polyolefin microporous membrane according to any one of  claims 1 - 10  which is obtained by forming a laminated sheet by co-extrusion method, subjecting the sheet to biaxial stretching, and extracting the plasticizer. 
   
   
       12 . A separator for lithium ion batteries which uses the polyolefin microporous membrane according to any one of  claims 1 - 11 .