Biaxially stretched film

Abstract

A simultaneously biaxially stretched polypropylene film having unique properties is disclosed. The properties result from the unique method of making the film and include: a tensile elongation to break in the first stretch direction of at least 110%; a tensile volumetric energy to break in the first stretch direction of at least 18,000 in-lb/in 3 ; and, a variability in film thickness along the first stretch direction, as measured by the standard deviation relative to the average, of less than 10%.

What is claimed is:  
     
         1 . A simultaneously biaxially stretched polypropylene film having: 
 a. a first stretch direction and a second stretch direction;    b. a tensile elongation to break in the first stretch direction of at least 110%;    c. a tensile volumetric energy to break in the first stretch direction of at least 18,000 in-lb/in 3 ; and    d. a variability in film thickness along the first stretch direction, as measured by the standard deviation relative to the average, of less than 10%.    
     
     
         2 . The film of  claim 1 , having a tensile elongation to break in the first stretch direction from about 112% to about 164%.  
     
     
         3 . The film of  claim 1 , having a tensile volumetric energy to break in the first stretch direction from about 18,500 in-lb/in 3  to about 28,200 in-lb/in 3 .  
     
     
         4 . The film of  claim 1 , having a variability in film thickness along the first stretch direction, as measured by the standard deviation relative to the average, from about 2.4% to about 9.4%.  
     
     
         5 . The film of  claim 1 , having a variability in film thickness along the second stretch direction, as measured by the standard deviation relative to the average, of less than 10%.  
     
     
         6 . The film of  claim 5 , having a variability in film thickness along the second stretch direction, as measured by the standard deviation relative to the average, from about 2.2% to about 8.1%.  
     
     
         7 . The film of  claim 1 , having a film thickness between about 0.020 to 0.064 mm.  
     
     
         8 . The film of  claim 1 , having a tensile elongation to break in the second stretch direction less than the tensile elongation to break in the first stretch direction.  
     
     
         9 . The film of  claim 8 , having a tensile elongation to break in the second stretch direction less than or equal to about 55%.  
     
     
         10 . The film of  claim 8 , having a tensile elongation to break in the second stretch direction greater than or equal to about 34%.  
     
     
         11 . The film of  claim 1 , having a tensile volumetric energy to break in the second stretch direction less than the tensile volumetric energy to break in the first stretch direction.  
     
     
         12 . The film of  claim 11 , having a tensile volumetric energy to break in the second stretch direction less than or equal to 17,400 in-lb/in 3 .  
     
     
         13 . The film of  claim 11 , having a tensile volumetric energy to break in the second stretch direction greater than or equal to 9,200 in-lb/in 3 .  
     
     
         14 . The film of  claim 1 , having a stretch ratio in the first stretch direction smaller than the stretch ratio in the second direction.  
     
     
         15 . The film of  claim 1 , having a stretch ratio in the first stretch direction smaller than the uniaxial natural stretch ratio.  
     
     
         16 . The film of  claim 1 , having a stretch ratio in the second stretch direction larger than the uniaxial natural stretch ratio.  
     
     
         17 . The film of  claim 1 , wherein the first stretch direction and the second stretch direction are substantially perpendicular.  
     
     
         18 . The film of  claim 17 , wherein the first stretch direction is the machine direction (MD) and the second stretch direction is the transverse direction (TD).  
     
     
         19 . The film of  claim 1 , wherein the polypropylene is isotactic polypropylene.  
     
     
         20 . A tape comprising a film backing having first and second major surfaces and an adhesive layer on at least a portion of one of said first and second major surfaces, wherein said film backing comprises the simultaneously biaxially stretched polypropylene film of  claim 1 .  
     
     
         21 . The tape of  claim 20  further comprising a release layer on the surface of the film backing opposite the surface having the adhesive layer.