Polyolefin/filler films having increased WVTR and method for making

Abstract

Films, made of polyethylenes and fillers, and articles made therefrom greater WVTR than previously available films based on conventional Ziegler-Natta based polyethylenes. The polyethylenes are produced in a metallocene-catalyzed production process. The films may be made by a cast film process, and may be made in widely varying filler content, generally polyethylene to filler ratios of 30/70 to 70/30. The metallocene based polyethylenes when combined with filler also permit the extrusion of thinner films leading to lighter weight and softer films. The m-polyethylenes utilized for making such films typically have a Composition Distribution Breadth Index above 50%, a M w /M n below 3, and a M z /M w below 2.

We claim:  
     
       1. A process for making a polyolefin film having high WVTR, comprising: 
       a) mixing at least one polyethylene with a filler in a ratio of 10/80 to 80/20;  
       b) extruding a film from the polyethylene/filler mixture of a);  
       c) stretching the film to a sufficient amount to achieve a WVTR at least 10% higher than the WVTR described by the equation:  
       
         
           WVTR=−10,900+320 (filler weight %)  
         
       
       wherein said polyethylene has a Mw/Mn less than 3, and a CDBI greater than 50%. 
     
     
       2. The process of claim  1 , and wherein said filler is CaCO 3  and said filler is present in said mixture from 30-60% based on the total polyethylene content, and wherein said stretched film has a WVTR at least 20% higher than the WVTR of the equation in claim  1 . 
     
     
       3. The process of claim  1  wherein said polyethylene has a M w /M n  less than 2.5 and a CDBI greater than 65%, and wherein said filler is CaCO 3  and is present 30-50% based on the total polyethylene content, wherein and drawdown is at least 30% less than the drawdown expressed by the equation of claim  1 , and where said stretched film has a WVTR at least 30% higher than the WVTR of the equation in claim  1 . 
     
     
       4. A polyethylene film having a high WVTR, made by a process comprising: 
       a) combining a polyethylene having Mw/Mn less than 3 with a CaCO 3 , in a weight ratio of from 3:1 to 1:3;  
       b) extruding said combination into a film having a thickness between 2-4 mils;  
       c) stretching said film to a degree sufficient to achieve a WVTR of at least 10% greater than the WVTR described by the equation:  
       
         
           WVTR=−10,900+320 (CaCO 3  weight %).  
         
       
     
     
       5. A method for producing a polyolefin film comprising: 
       a) adding a filler to at least one polyethylene, where said polyethylene has a Mw/Mn of up to 2.5, at a filler to polyethylene ratio of 3:1-1:3;  
       b) forming a film from the polyethylene/filler of a) wherein said film, after stretching in the TD in a ratio from the initial dimension of 2:1-4:1 has a WVTR at least 20% greater than the WVTR described by the equation:  
       
         
           WVTR=−10,900+320 (weight % filler).  
         
       
     
     
       6. A method of making a filled polyethylene film having a thickness up to 3 mils, comprising: 
       a) forming a film from a polyethylene having a Mw/Mn less than 2.5, combined with CaCO 3  in a ratio polyethylene filler 3:1-1.3, wherein said film has a thickness, as extruded up to 3 mils.  
     
     
       7. The method of claim  6 , wherein said film has a thickness as extruded up to 2.5 mils. 
     
     
       8. The method of claim  7  wherein said method additionally comprises stretching said film by at least 2:1 and heat setting said stretch by heating said stretched film at a temperature below the softening temperature of the stretched film. 
     
     
       9. A polyethylene film having a high WVTR made by a process comprising: 
       a) combining a polyethylene having a Mw/Mn less than 3 and a CDBI greater than 60% with a filler,  
       b) forming a film from the combination of a),  
       c) stretching said film of b) to a sufficient degree to achieve a WVTR of at least 10% greater than the WVTR described by the equation:  
       
         
             WVTR =10,900+320 (filler weight %).