Microporous polyolefin film with improved meltdown property and preparing method thereof

Abstract

The present invention is related to microporous polyolefin films that may be used for battery separators and the methods of manufacturing the same. These microporous polyolefin films are characterized by being manufactured in a method comprising the steps of melt-extruding a composition, comprised of 20-50 weight % of a resin composition, comprised of 90-98 weight % of polyethylene (Component I) having a weight average molecular weight of 2×10 5 -4×10 5 and less than 5 weight % of molecules of which molecular weight is less than 1×10 4 and less than 5 weight % of molecules of which molecular weight is greater than 1×10 6 , and 2-10 weight % of polypropylene (Component II) of which weight average molecular weight is 3.0×10 4 -8.0×10 5 and the peak of the melting point is higher than 145° C., and 80-50 weight % of a diluent (Component III), to mold in the form of sheets; stretching the above sheets to mold in the form of films; extracting the diluent from the above films; and heat-setting the above films. They are also characterized by having a puncture strength of greater than 0.14 N/μm, Darcy's permeability constant of greater than 1.5×10 −5 Darcy, closing temperature of microporous films of lower than 140° C., and melt-down temperature of higher than 160° C. They can enhance the performance and stability of batteries using them as well as the productivity of microporous films owing to their high thermal stability and superior extrusion compoundability and physical properties.

1 . A method of manufacture of microporous polyolefin films having a puncture strength of greater than 0.14 N/μm, Darcy's permeability constant of greater than 1.5×10 −5  Darcy, closing temperature of microporus films of lower than 140° C., and melt-down temperature of higher than 160° C. comprising the steps of: 
 melt-extruding a composition, comprised of 20-50 weight % of a resin composition, comprised of 90-98 weight % of polyethylene (Component I) having a weight average molecular weight of 2×10 5 ˜4×10 5  and less than 5 weight % of molecules of which molecular weight is less than 1×10 4  and less than 5 weight % of molecules of which molecular weight is greater than 1×10 6 , and 2-10 weight % of polypropylene (Component II) of which weight average molecular weight is 3.0×10 4 ˜8.0×10 5  and the peak of the melting point is higher than 145° C., and 80-50 weight % of a diluent (Component III), to mold in the form of sheets;    stretching said sheets to mold in the form of films;    extracting said diluent from said films; and    heat-setting said films.    
   
   
       2 . Microporous polyolefin films in  claim 1 , characterized by that said microporous polyolefin films have a weight average molecular weight of 2.0×10 5 ˜4.5×10 5  and the peak of the melting point of higher than 145° C.  
   
   
       3 . The method of manufacture of microporous polyolefin films in  claim 1 , characterized by that said Component I is homopolyethylene; or ethylene co-polymer containing less than 20 weight % of alpha-olefin having 3-8 carbon atoms, or the mixture of said homopolyethylene and said ethylene co-polymer.  
   
   
       4 . The method of manufacture of microporus polyolefin films in  claim 1 , characterized by that said Component II is homopolypropylene, which is a polypropylene having the peak of the melting point of higher than 145° C.; random polypropylene containing alpha-olefin having 3-8 carbon atoms or their mixtures; polyprophylene containing alpha-olefin having 2-8 carbon atoms or co-polymer resin using the combination of said alpha-olefin; or the mixture of said homopolypropylene, said random polypropylene, and said polypropylene.  
   
   
       5 . The method of manufacture of microporous polyolefin films in  claim 1 , characterized by that the ratio of said Component I and said Component II is 95-97 weight % to 3-5 weight %.  
   
   
       6 . The method of manufacture of microporous polyolefin films in  claim 1 , characterized by that said Component III is one or more components selected from aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, paraffin oil, etc.; phthalic acid esters such as dibutyl phtalate, dioctyl phthalate, etc.; aromatic others such as diphenyl ether, etc.: fatty acids having 10 to 20 carbon atoms such as stearic acid, oleic acid, linoleic acid, linolenic acid, etc.; fatty acid alcohols having 10 to 20 carbon atoms such as stearic acid alcohol, oleic acid alcohol, etc.; and one or more fatty acid esters in which one or more fatty acids selected from saturated and unsaturated fatty acids having 4 to 26 carbon atoms in the fatty acid group such as palmitic acid mono-, di-, or tri-ester, stearic acid mono-, di-, or tri-ester, oleic acid mono-, di-, or tri-ester, linoleic acid mono-, di-, or tri-ester, etc. are ester-bonded with alcohols having 1 to 8 hydroxy radicals and 1 to 10 carbon atoms.  
   
   
       7 . The method of manufacture of microporous polyolefin films in  claim 1 , characterized by that said step of stretching is done within a temperature range at which 30-80 weight % of the crystal portion of said Component I is melted in the machine and transverse directions greater than 3 times each at the total ratio of stretching of 25-50 times.  
   
   
       8 . Microporous polyolefin films manufactured according to said method of manufacture in  claim 1 .  
   
   
       9 . Microporous polyolefin films manufactured according to said method of manufacture in  claim 2 .  
   
   
       10 . Microporous polyolefin films manufactured according to said method of manufacture in  claim 3 .  
   
   
       11 . Microporous polyolefin films manufactured according to said method of manufacture in  claim 4 .  
   
   
       12 . Microporous polyolefin films manufactured according to said method of manufacture in  claim 5 .  
   
   
       13 . Microporous polyolefin films manufactured according to said method of manufacture in  claim 6 .  
   
   
       14 . Microporous polyolefin film manufactured according to said method of manufacture in  claim 7.