Microporous high density polyethylene film and preparing method thereof

Abstract

The present invention discloses a microporous high-density polyethylene film for a battery separator, and process for preparing the same. The microporous high-density polyethylene film according to the present invention comprises of high-density polyethylene with weight average molecular weight of 2×10 5 ˜4×10 5 , containing not more than 5 wt % of molecules with molecular weight of 1×10 4 or less and not more than 5 wt % of molecules with molecular weight of 1×10 6 or more, and has the properties of puncture strength of 0.22 N/μm or more, Darcy's permeability constant of 1.8×10 −5 Darcy or more, and shrinkage of 4% or less in machine and transverse direction, respectively. Particularly, the microporous high-density polyethylene film has excellent extrusion-compoundability and stretchability with high productivity and enhances performances and stability of the battery using the film.

1 . A process for preparing microporous high-density polyethylene having puncture strength of 0.22 N/μm or more, Darcy's permeability constant of 1.8×10 −5  Darcy or more, and shrinkage of 4% or less in machine and transverse direction, respectively, which comprises the steps of 
 melt-extruding a composition consisting of 20˜50 wt % of high-density polyethylene with weight average molecular weight of 2×10 5 ˜4×10 5 , containing not more than 5 wt % of molecules with molecular weight of 1×10 4  or less and not more than 5 wt % of molecules with molecular weight of 1×106 or more (Component I), and 80˜50 wt % of diluent (Component II) to form a sheet;    stretching the sheet at a temperature range where 30˜80 wt % of the crystalline portion of the sheet melts, by using simultaneous stretching with a tenter type stretcher, in a stretching ratio of 4 times or more in machine and transverse direction, respectively, with total stretching ratio of 25˜50 times to form a film;    cooling the film, just upon the completion of stretching, under the condition of retaining the tensile strength applied during the stretching,;    extracting the diluent from the resultant film; and    heat-setting the film at a temperature range where 10˜30 wt % of the crystalline portion of the film melts.    
   
   
       2 . A process for preparing microporous high-density polyethylene according to  claim 1 , wherein Component I comprises not more than 2 wt % of comonomer which is selected from the group consisting of propylene, butene-1, hexene-1 and 4-methylpentene-1.  
   
   
       3 . A process for preparing microporous high-density polyethylene according to  claim 1 , wherein Component II is paraffin oil having kinetic viscosity of 20˜200 cSt at 40° C.  
   
   
       4 . A process for preparing microporous high-density polyethylene according to  claim 1 , wherein the cooling temperature at the cooling step is higher than ambient temperature but lower than the crystallization temperature of the stretched film, and the cooling time is from 5 to 30 seconds.  
   
   
       5 . A process for preparing microporous high-density polyethylene according to  claim 1 , wherein the residual amount of diluent in the film at the extraction step is not more than 1 wt %.  
   
   
       6 . A microporous high-density polyethylene film prepared by a process according to  claim 1 , which has puncture strength of 0.22 N/μm or more, Darcy's permeability constant of 1.8×10 −5  Darcy or more, and shrinkage of 4% or less in machine and transverse direction, respectively.  
   
   
       7 . A microporous high-density polyethylene film prepared by a process according to  claim 2 , which has puncture strength of 0.22 N/μm or more, Darcy's permeability constant of 1.8×10 −5  Darcy or more, and shrinkage of 4% or less in machine and transverse direction, respectively.  
   
   
       8 . A microporous high-density polyethylene film prepared by a process according to  claim 3 , which has puncture strength of 0.22 N/μm or more, Darcy's permeability constant of 1.8×10 −5  Darcy or more, and shrinkage of 4% or less in machine and transverse direction, respectively.  
   
   
       9 . A microporous high-density polyethylene film prepared by a process according to  claim 4 , which has puncture strength of 0.22 N/μm or more, Darcy's permeability constant of 1.8×10 −5  Darcy or more, and shrinkage of 4% or less in machine and transverse direction, respectively.  
   
   
       10 . A microporous high-density polyethylene film prepared by a process according to  claim 5 , which has puncture strength of 0.22 N/μm or more, Darcy's permeability constant of 1.8×10 −5  Darcy or more, and shrinkage of 4% or less in machine and transverse direction, respectively.