Multilayer separator exhibiting improved strength and stability

Abstract

A multi-layer microporous battery separator which comprises: a high molecular weight polypropylene layer having a melt flow index of ≦1.2 measured at layer; a polyethylene layer; and a high molecular weight polypropylene layer having a melt flow index of ≦1.2 measured at layer. The resulting microporous battery separator which is formed by a dry stretch process produces the microporous battery separator which has a porosity of ≦37% while maintaining a gurley from 13-25 seconds and a thickness of ≦25 microns.

1 . A multi-layer microporous battery separator comprising: 
 a high molecular weight polypropylene layer having a melt flow index (MFI) of ≦1.2 measured at layer;    a polyethylene layer;    a high molecular weight polypropylene layer having a melt flow index of ≦1.2 measured at layer;    which forms a microporous battery separator by a dry stretch process, where said microporous battery separator has a porosity of ≦37% while maintaining a Gurley from 13 to 25 seconds and a thickness of ≦25 microns.    
   
   
       2 . The multi-layer microporous battery separator according to  claim 1 , where said microporous battery separator exhibits an increase 5% or more in mixed penetration strength compared to a tri-layer dry-stretched microporous battery separator of the same thickness.  
   
   
       3 . The multi-layer microporous battery separator according to  claim 1 , where the net shrinkage of said microporous battery separator is less than 5% measured for 6 hours at 105° C.  
   
   
       4 . The multi-layer microporous battery separator according to  claim 1 , where the ionic resistance of said microporous battery separator is less than 2.5 ohms-cm 2 .  
   
   
       5 . The multi-layer microporous battery separator according to  claim 1 , where the polyethylene layer is a high density polyethylene.  
   
   
       6 . A multi-layer microporous battery separator comprising: 
 a tri-layer dry-stretched microporous battery separator having an outer polyolefin layer, an inner polyolefin layer and an outer polyolefin layer and an overall thickness of ≦25 microns;    said outer polyolefin layers are a high molecular weight polypropylene having a melt flow index of ≦1.2 measured at layer;    said inner polyolefin layer is a polyethylene; and    where said tri-layer dry-stretched microporous battery separator exhibits an increase 5% or more in mixed penetration strength compared to a tri-layer dry-stretched microporous battery separator of the same thickness.    
   
   
       7 . The multi-layer microporous battery separator according to  claim 6 , where said microporous battery separator exhibits a porosity of ≦37% while maintaining a Gurley from 13 to 25 seconds.  
   
   
       8 . The multi-layer microporous battery separator according to  claim 6 , where the net shrinkage of said microporous battery separator is less than 5% measured for 6 hours at 105° C.  
   
   
       9 . The multi-layer microporous battery separator according to  claim 6 , where the ionic resistance of said microporous battery separator is less than 2.5 ohms-cm 2 .  
   
   
       10 . A multi-layer microporous battery separator comprising: 
 a tri-layer dry-stretched microporous battery separator having an outer polyolefin layer, an inner polyolefin layer and an outer polyolefin layer;    said outer polyolefin layers are a high molecular weight polypropylene;    said inner polyolefin layer is a polyethylene; and    where said tri-layer dry-stretched microporous battery separator has a thickness of ≦25 microns, a porosity of 37% or lower while maintaining a Gurley from 13 to 25 seconds, and exhibits a 5% increase in mixed penetration strength compared to a tri-layer dry-stretched microporous battery separator of the same thickness.    
   
   
       11 . The multi-layer microporous battery separator according to  claim 10 , where the net shrinkage of said microporous battery separator is less than 5% measured for 6 hours at 105° C.  
   
   
       12 . The multi-layer microporous battery separator according to  claim 10 , where the ionic resistance of said microporous battery separator is less than 2.5 ohms-cm 2 .  
   
   
       13 . A process of for the preparation of a multi-layer microporous battery separator comprising the steps of: 
 providing a polypropylene having a MFI ≦1.0 measured at pellet before processing and a polyethylene;    extruding said high molecular weight polypropylene to form a precursor polypropylene film;    extruding said polyethylene to form a precursor polyethylene film;    laminating said precursor polypropylene films to each side of said precursor polyethylene film to form a non-porous tri-layer precursor;    annealing said non-porous tri-layer precursor;    stretching said non-porous tri-layer precursor to form a stretched microporous tri-layer film;    allowing a relaxation of said stretched microporous tri-layer film to form a microporous tri-layer film; and    where net stretch of said microporous tri-layer film is less than 90%.    
   
   
       14 . A battery separator made of a microporous polyolefin having an overall thickness of ≦25 microns where the net shrinkage of said separator is less than 5% measured for 6 hours at 105° C.  
   
   
       15 . The battery separator according to  claim 14  having a porosity of 37% or lower.  
   
   
       16 . The battery separator according to  claim 15  having a Gurley from 13 to 25 seconds.  
   
   
       17 . The battery separator according to  claim 14 , where said separator has a porosity of ≦37% while maintaining a Gurley from 13 to 25 seconds.  
   
   
       18 . A multi-layer battery separator made of a microporous polyolefin having an outer layer of a high molecular weight polypropylene layer having a melt flow index of ≦1.2 measured after processing at said outer layer.  
   
   
       19 . The multi-layer battery separator according to  claim 18 , where said separator has a porosity of ≦37% while maintaining a Gurley from 13 to 25 seconds.  
   
   
       20 . The multi-layer microporous battery separator according to  claim 19 , where the ionic resistance of said microporous battery separator is less than 2.5 ohms-cm 2 .