Microporous Polyolefin Membrane, Its Production Method, And Battery Separator

Abstract

A microporous polyolefin membrane comprising a polyethylene resin, and polypropylene having a weight-average molecular weight of 6×10 5 or more and a heat of fusion of 90 J/g or more (measured by a differential scanning calorimeter), a fraction having a molecular weight of 1.8×10 6 or more being 10% or more by mass of the polypropylene.

1 - 4 . (canceled) 
     
     
         5 . A microporous polyolefin membrane comprising at least one polyethylene, and at least one polypropylene having a weight-average molecular weight of 6×10 5  or more and a heat of fusion of 90 J/g or more, a fraction of said polypropylene having a molecular weight of 1.8×10 6  or more being 10% or more by mass of the polypropylene. 
     
     
         6 . A method for producing a microporous polyolefin membrane comprising the steps of
 (1) extruding a molten blend of a polyethylene resin, polypropylene having a weight-average molecular weight of 6×10 5  or more and a heat of fusion of 90 J/g or more (measured by a differential scanning calorimeter), a fraction having a molecular weight of 1.8×10 6  or more being 10% or more by mass of said polypropylene, and a membrane-forming solvent from a die,   (2) cooling the resultant extrudate to a gel-like sheet, and   (3) conducting the stretching of said gel-like sheet and the removal of said membrane-forming solvent one or more times in an arbitrary order.   
     
     
         7 . The method for producing a microporous polyolefin membrane according to claim  2 , wherein said step (3) is removing said membrane-forming solvent after the stretching of said gel-like sheet, stretching said gel-like sheet after the removal of said membrane-forming solvent, or conducting the stretching of said gel-like sheet and the removal of said membrane-forming solvent in this order and further stretching said gel-like sheet. 
     
     
         8 . A battery separator formed by a microporous polyolefin membrane comprising at least one polyethylene, and at least one polypropylene having a weight-average molecular weight of 6×10 5  or more and a heat of fusion of 90 J/g or more, a fraction of said polypropylene having a molecular weight of 1.8×10 6  or more being 10% or more by mass of the polypropylene. 
     
     
         9 . The microporous polyolefin membrane of claim  1 , wherein said a polypropylene has a weight-average molecular weight of about 8×10 5  or more. 
     
     
         10 . The microporous polyolefin membrane of claim  1 , wherein said fraction of said polypropylene having a molecular weight of 1.8×10 6  or more is at least 15% or more by mass. 
     
     
         11 . The microporous polyolefin membrane of claim  1 , wherein said polypropylene has a heat of fusion of 95 J/g or more. 
     
     
         12 . The microporous polyolefin membrane of claim  1 , wherein said polypropylene comprises 20 to 80% by mass of the combined masses of the polyethylene and polypropylene. 
     
     
         13 . The microporous polyolefin membrane of claim  1 , wherein said at least one polyethylene resin is ultra-high-molecular-weight polyethylene, a polyethylene other than the ultra-high-molecular-weight polyethylene, or a mixture of ultra-high-molecular-weight polyethylene and polyethylene other than the ultra-high-molecular-weight polyethylene. 
     
     
         14 . The microporous polyolefin membrane of  claim 9 , wherein said ultra-high-molecular-weight polyethylene preferably has a weight-average molecular weight of about 5×10 5  or more. 
     
     
         15 . The microporous polyolefin membrane of  claim 10 , wherein said ultra-high-molecular-weight polyethylene has a weight-average molecular weight of about 1×10 6  to about 15×10 6 . 
     
     
         16 . The microporous polyolefin membrane of  claim 11 , wherein said ultra-high-molecular-weight polyethylene has a weight-average molecular weight of about 1×10 6  to about 5×10 6 . 
     
     
         17 . The microporous polyolefin membrane of  claim 9 , wherein said ultra-high-molecular-weight polyethylene comprises an ethylene homopolymer, or an ethylene.α-olefin copolymer containing an amount of an α-olefin other than ethylene. 
     
     
         18 . The microporous polyolefin membrane of claim  1 , wherein said at least one polyethylene resin has a weight-average molecular weight of 2×10 5  or more, and Mw/Mn of 5-300. 
     
     
         19 . The microporous polyolefin membrane of  claim 9 , wherein said polyethylene composition comprises (a) polyethylene having a weight-average molecular weight of 5×10 5  or more, and (b) at least one selected from the group consisting of polyethylene having a weight-average molecular weight of 1×10 4  or more and less than 5×10 5 , polybutene-1 having a weight-average molecular weight of 1×10 4  to 4×10 6 , a polyethylene wax having a weight-average molecular weight of 1×10 3  to 1×10 4 , and an ethylene.α-olefin copolymer having a weight-average molecular weight of 1×10 4  to 4×10 6 . 
     
     
         20 . The microporous polyolefin membrane of  claim 15 , wherein said polyethylene composition comprises polyethylene having a weight-average molecular weight of 5×10 5  or more, and polyethylene having a weight-average molecular weight of 1×10 4  or more and less than 5×10 5 . 
     
     
         21 . The microporous polyolefin membrane of  claim 16 , wherein said polyethylene having a weight-average molecular weight of 1×10 4  or more and less than 5×10 5  is high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, or linear low-density polyethylene. 
     
     
         22 . The microporous polyolefin membrane of  claim 17 , wherein said polyethylene composition comprises polyethylene having a weight-average molecular weight of 5×10 5  or more, and high-density polyethylene having a weight-average molecular weight of 1×10 4  or more and less than 5×10 5 . 
     
     
         23 . The microporous polyolefin membrane of claim  1 , wherein the microporous polyolefin membrane has a porosity of 25-80%, an air permeability of 20-2,000 seconds/100 cc, a prick resistance of 2,000 mN/20 μm or more, and a meltdown temperature of 170° C. or more. 
     
     
         24 . The method of claim  1 , wherein the microporous polyolefin membrane has a porosity of 25-80%, an air permeability of 20-2,000 seconds/100 cc, a prick resistance of 2,000 mN/20 μm or more, and a meltdown temperature of 170 to 190° C.