Polyethylene microporous film for a rechargeable battery separator and a method of preparing the same

Abstract

Disclosed are a polyethylene microporous film and a method of preparing the same. The polyethylene microporous film, which has a laminated structure comprising B layer/A layer/B layer, prepared by melt-mixing a polyethylene and an aliphatic hydrocarbon solvent together at controlled mixing ratios to separately form an A layer and a B layer having different porosities, and then coextruding the A and B layers, thus exhibiting excellent mechanical properties, such as strength and elongation, and high-temperature stability. Therefore, the polyethylene microporous film is suitable for use in a rechargeable battery separator.

1 . A polyethylene microporous film for a rechargeable battery separator, comprising: 
 an A layer formed by melt-mixing 20 to about 40 wt % of a polyethylene with 60 to about 80 wt % of an aliphatic hydrocarbon solvent;    B layers formed by melt-mixing 45 to about 65 wt % of a polyethylene with 35 to about 55 wt % of an aliphatic hydrocarbon solvent, the B layers are laminated on both surfaces of the A layer; and    the A layer and B layers are coextruded to form a laminated structure comprising B layer/A layer/B layer.    
     
     
         2 . A polyethylene microporous film for a rechargeable battery separator, comprising: 
 an A layer formed by melt-mixing 12.8 to about 64.9 wt % of a polyethylene, 0.1 to about 7.2 wt % of a thermoplastic resin incompatible with the polyethylene, and 35 to about 80 wt % of an aliphatic hydrocarbon;    B layers formed by melt-mixing 20 to about 65 wt % of a polyethylene with 35 to about 80 wt % of an aliphatic hydrocarbon solvent, the B layers are laminated on both surfaces of the A layer; and    the A layer and B layers are coextruded to form a laminated structure comprising B layer/A layer/B layer.    
     
     
         3 . The polyethylene microporous film as set forth in  claim 1 , wherein the polyethylene is selected from the group consisting of high density polyethylene having an average weight molecular weight ranging from 100,000 to 500,000, ultrahigh molecular weight polyethylene having an average weight molecular weight ranging from 1,000,000 to 5,000,000, and a mixture of thereof.  
     
     
         4 . The polyethylene microporous film as set forth in  claim 3 , wherein the mixture comprises 60 to about 80 wt % of high density polyethylene having an average weight molecular weight ranging from 100,000 to 500,000, and 20 to about 40 wt % of ultrahigh molecular weight polyethylene having an average weight molecular weight ranging from 1,000,000 to 5,000,000.  
     
     
         5 . The polyethylene microporous film as set forth in  claim 1 , wherein the polyethylene has a melt index of 1 g/10 min or less.  
     
     
         6 . The polyethylene microporous film as set forth in  claim 2 , wherein the thermoplastic resin incompatible with the polyethylene is selected from the group consisting of copolyester and nylon 6.  
     
     
         7 . The polyethylene microporous film as set forth in  claim 2 , wherein the polyethylene microporous film has a first melting temperature ranging from 125 to 145° C., and a second melting temperature ranging from 175 to 235° C.  
     
     
         8 . The polyethylene microporous film as set forth in  claim 1 , wherein the aliphatic hydrocarbon solvent is selected from the group consisting of nonane, decane, undecane, dodecane, and liquid paraffin oil.  
     
     
         9 . The polyethylene microporous film as set forth in  claim 1 , wherein the polyethylene microporous film is 3 to about 50 μm thick.  
     
     
         10 . The polyethylene microporous film as set forth in  claim 1 , wherein the A layer is 1 to about 20 μm thick.  
     
     
         11 . The polyethylene microporous film as set forth in  claim 1 , wherein the B layer is 1 to about 10 μm thick.  
     
     
         12 . A method of preparing a polyethylene microporous film for a rechargeable battery separator comprising the steps of: 
 melt-mixing 20 to about 40 wt % of a polyethylene with 60 to about 80 wt % of an aliphatic hydrocarbon solvent to form an A layer, melt-mixing 45 to about 65 wt % of a polyethylene with 35 to about 55 wt % of an aliphatic hydrocarbon solvent to form B layers, coextruding the A layer and B layers to laminate B layers on both surfaces of the A layer, and then cooling the coextruded layers, to form a gel composition having a laminated structure comprising B layer/A layer/B layer;    biaxially stretching the gel composition, to prepare a film;    extracting the aliphatic hydrocarbon solvent from the A layer and B layers of the film using an organic solvent to remove the aliphatic hydrocarbon solvent, to prepare a microporous film; and    heat-treating the microporous film at a temperature not greater than the melting temperature of the polyethylene.    
     
     
         13 . A method of preparing a polyethylene microporous film for a rechargeable battery separator comprising the steps of: 
 melt-mixing with 12.8 to about 64.9 wt % of a polyethylene, 0.1 to about 7.2 wt % of a thermoplastic resin incompatible with the polyethylene, and 35 to about 80 wt % of an aliphatic hydrocarbon solvent to form an A layer, melt-mixing 20 to about 65 wt % of a polyethylene with 35 to about 80 wt % of an aliphatic hydrocarbon solvent to form B layers, coextruding the A layer and B layers to laminate B layers on both surfaces of the A layer, and then cooling the coextruded layers, to form a gel composition having a laminated structure comprising B layer/A layer/B layer;    biaxially stretching the gel composition, to prepare a film;    extracting the aliphatic hydrocarbon solvent from the A layer and B layers of the film using an organic solvent to remove the aliphatic hydrocarbon solvent, to prepare a microporous film; and    heat-treating the microporous film at a temperature not greater than a melting temperature of the polyethylene.    
     
     
         14 . The method as set forth in  claim 12 , wherein the biaxial stretching is conducted at a ratio ranging from 4×4 to 8×8 at 105 to about 125° C.  
     
     
         15 . The polyethylene microporous film as set forth in  claim 2 , wherein the polyethylene is selected from the group consisting of high density polyethylene having an average weight molecular weight ranging from 100,000 to 500,000, ultrahigh molecular weight polyethylene having an average weight molecular weight ranging from 1,000,000 to 5,000,000, and a mixture of thereof.  
     
     
         16 . The polyethylene microporous film as set forth in  claim 15 , wherein the mixture comprises 60 to about 80 wt % of high density polyethylene having an average weight molecular weight ranging from 100,000 to 500,000, and 20 to about 40 wt % of ultrahigh molecular weight polyethylene having an average weight molecular weight ranging from 1,000,000 to 5,000,000.  
     
     
         17 . The polyethylene microporous film as set forth in  claim 2 , wherein the polyethylene has a melt index of 1 g/10 min or less.  
     
     
         18 . The polyethylene microporous film as set forth in  claim 2 , wherein the aliphatic hydrocarbon solvent is selected from the group consisting of nonane, decane, undecane, dodecane, and liquid paraffin oil.  
     
     
         19 . The polyethylene microporous film as set forth in  claim 2 , wherein the polyethylene microporous film is 3 to about 50 μm thick.  
     
     
         20 . The polyethylene microporous film as set forth in  claim 2 , wherein the A layer is 1 to about 20 μm thick.  
     
     
         21 . The polyethylene microporous film as set forth in  claim 2 , wherein the B layer is 1 to about 10 μm thick.  
     
     
         22 . The method as set forth in  claim 13 , wherein the biaxial stretching is conducted at a ratio ranging from 4×4 to 8×8 at 105 to about 125° C.