Method of stretching film and such film

Abstract

A method of stretching films in which all or a portion of the width of the film is cooled during or just after stretching so as to improve the uniformity of the film. The includes stretching a polymeric film in a tenter that grasps the film with a plurality of clips along the opposing edges of the film and propels the clips to thereby stretch the film. The tenter includes driven clips and idler clips, with at least one idler clip between respective pairs of driven clips. The cooling is done so as to improve the uniformity of the clip spacing relative to the spacing obtained at otherwise identical process conditions without such cooling.

1 . In a method of stretching a polymeric film comprising the steps of grasping the film with a plurality of clips along the opposing edges of the film and propelling the clips to thereby stretch the film, wherein the plurality of clips includes driven clips and idler clips, with at least one idler clip between respective pairs of driven clips, the improvement comprising: 
 a) heating the polymeric film to a sufficiently high temperature to allow a significant amount of stretching without breaking; and    b) imparting a machine direction cooling gradient to at least a portion of the width of the stretched film in an effective amount to reduce the value of idler clip lag from the value of idler clip lag in the absence of said cooling.    
   
   
       2 . The method of  claim 1 , wherein step b) includes actively cooling the opposed edge portions of the film.  
   
   
       3 . The method of  claim 1 , wherein step b) includes actively cooling the center portion of the film.  
   
   
       4 . The method of  claim 1 , wherein step b) includes actively cooling substantially the entire width of the film.  
   
   
       5 . The method of  claim 1 , wherein step b) includes cooling at least a portion of the film by at least 3° C.  
   
   
       6 . The method of  claim 1 , wherein the method further includes propelling the clips through a stretch section in which the film is stretched and subsequently through a post-stretch treatment section, and wherein step b) is performed in at least one of the stretch section and the treatment section.  
   
   
       7 . The method of  claim 1 , wherein the method includes biaxially stretching the film.  
   
   
       8 . The method of  claim 1 , wherein the method includes simultaneously biaxially stretching the film by propelling the clips at varying speeds in the machine direction along clip guide means that diverge in the transverse direction.  
   
   
       9 . The method of  claim 1 , wherein the film comprises polypropylene.  
   
   
       10 . The method of  claim 9 , wherein the method includes stretching the film to a final area stretch ratio of from 16:1 to 100:1.  
   
   
       11 . The method of  claim 9 , wherein step a) comprises heating the film to from 120 to 165° C.  
   
   
       12 . The method of  claim 11 , wherein step b) includes forcing cooling air onto the film, wherein the cooling air is at least 5° C. cooler than the film.  
   
   
       13 . In a method of stretching a polymeric film comprising the steps of grasping the film with a plurality of clips along the opposing edges of the film and propelling the clips to thereby stretch the film, wherein the plurality of clips includes driven clips and idler clips, with at least one idler clip between respective pairs of driven clips, the improvement comprising: 
 a) heating the polymeric film to a sufficiently high temperature to allow a significant amount of stretching without breaking; and    b) imparting a machine direction cooling gradient to at least a portion of the width of the stretched film in an effective amount to improve the downweb caliper uniformity relative to the downweb caliper uniformity in the absence of said cooling.    
   
   
       14 . The method of  claim 13 , wherein step b) includes actively cooling the opposed edge portions of the film.  
   
   
       15 . The method of  claim 13 , wherein step b) includes actively cooling the center portion of the film.  
   
   
       16 . The method of  claim 13 , wherein step b) includes actively cooling substantially the entire width of the film.  
   
   
       17 . The method of  claim 13 , wherein the method further includes propelling the clips through a stretch section in which the film is stretched and subsequently through a post-stretch treatment section, and wherein step b) is performed in at least one of the stretch section and the treatment section.  
   
   
       18 . The method of  claim 13 , wherein the method includes simultaneously biaxially stretching the film by propelling the clips at varying speeds in the machine direction along clip guide means that diverge in the transverse direction.  
   
   
       19 . The method of  claim 13 , wherein the film comprises polypropylene.  
   
   
       20 . The method of  claim 19 , wherein the method includes stretching the film to a final area stretch ratio of from 16:1 to 100:1.  
   
   
       21 . The method of  claim 19 , wherein step a) comprises heating the film to from 120 to 165° C.  
   
   
       22 . The method of  claim 19 , wherein step b) includes forcing cooling air onto the film, wherein the cooling air is at least 5° C. cooler than the film.