Heat shrinkable polyester film

Abstract

The present invention provides a heat shrinkable polyester film whose hot water shrinkage along its main shrinkage direction is equal to or greater than 20% when the film is put in hot water of 70° C. for 5 sec; 35% to 55% when put in hot water of 75° C. for 5 sec; and 50% to 60% when put in hot water of 80° C. for 5 sec, wherein the compressive strength of the label formed by the film satisfies a following expression (1): Y>X 2.2   (1) where y denotes compressive strength (mN) and x denotes film thickness (μm).

1. A heat shrinkable polyester film whose hot water shrinkage along its main shrinkage direction is equal to or greater than 20% when the film is put in hot water of 70° C. for 5 sec; 35% to 55% when put in hot water of 75° C. for 5 sec; and 50% to 60% when put in hot water of 80° C. for 5 sec, wherein the compressive strength of a label formed by the film satisfies a following expression:
   y> 1 . 1 x 2.2    
 
       where y denotes compressive strength (mN) and x denotes film thickness (μm). 
     
     
       2. A heat shrinkable polyester film according to  claim 1 , wherein the film has a thickness distribution of 6% or less. 
     
     
       3. A label for a bottle which is produced by a heat shrinkable polyester film according to  claim 1 . 
     
     
       4. A heat shrinkable polyester film according to  claim 1 , wherein the compressive strength of the label formed by the film satisfies a following expression:
   y>1.1x 2.2      
       where y denotes compressive strength (mN) and x denotes film thickness (μm). 
     
     
       5. A heat shrinkable polyester film according to  claim 1 , wherein the thickness of the film between 10 μm and 200 μM. 
     
     
       6. A heat shrinkable polyester film according to  claim 5 , wherein the thickness of the film is between 20 μM and 200 μM. 
     
     
       7. A heat shrinkable polyester film according to  claim 1 , wherein the polyester has an acid component and a diol component, wherein the main acid component is terephthalic acid, mixed with 14 mol % or more of isophthalic acid and less than 5 mol % of an adipic acid, and wherein the main diol component is an ethylene glycol, mixed with 5 mol % to 15 mol % of butanediol. 
     
     
       8. A heat shrinkable polyester film according to  claim 7 , wherein the polyester contains less than 4 mol % diethylene glycol. 
     
     
       9. A method of producing a heat shrinkable polyester film consisting of:
 a) drying a polyester material;  
 b) melting the polyester material;  
 c) extruding it into a film at a temperature of 200° C. to 300° C. and rapidly cooling the film to obtain an undrawn film;  
 d) drawing the undrawn film at a temperature equal to or greater than Tg−5° C. and less than Tg+15° C. along a first direction by a drawing factor of 1.05 or more;  
 e) pre-heating the film;  
 f) drawing the film along a second direction (perpendicular to the first direction) by a drawing factor of 4.5 or more (first drawing step);  
 g) drawing the film further at a temperature of 65° C. to 85° C. along the second direction by a drawing factor of 1.05 or more (second drawing step),  
 
       wherein Tg is the glass transition temperature of the film. 
     
     
       10. The method of  claim 9 , wherein the drawing temperature in step d) is 70° C.-90° C. 
     
     
       11. The method of  claim 9 , wherein the drawing factor instep d) is 1.05 to 1.20. 
     
     
       12. The method of  claim 9 , wherein the drawing factor in step d) is 4.7 to 5.2. 
     
     
       13. The method of  claim 9 , wherein the drawing is a biaxial drawing in which the film is drawn in the first direction and then drawn with a tenter in the second direction. 
     
     
       14. The method of  claim 13 , wherein the biaxial drawing is a successive biaxial drawing. 
     
     
       15. The method of  claim 13 , wherein the biaxial drawing is a simultaneous biaxial drawing. 
     
     
       16. The method of  claim 9 , wherein the first direction is the longitudinal direction (extrusion direction) and the second direction is the transverse direction (perpendicular to the extrusion direction). 
     
     
       17. A heat shrinkable polyester film produced by the method of  claim 9 . 
     
     
       18. A heat shrinkable polyester film according to  claim 17 , wherein the thickness of the film is between 10 μM and 200 μM. 
     
     
       19. A heat shrinkable polyester film according to  claim 17 , wherein the thickness of the film is between 20 μM and 200 μM. 
     
     
       20. A heat shrinkable polyester film according to  claim 17 , whose hot water shrinkage along its main shrinkage direction is equal to or greater than 20% when the film is put in hot water of 70° C. for 5 sec; 35% to 55% when put in hot water of 75° C. for 5 sec; and 50% to 60% when put in hot water of 80° C. for 5 sec, wherein the compressive strength of a label formed by the film satisfies a following expression:
   y>1.1x 2.2    
 
       where y denotes compressive strength (mN) and x denotes film thickness (μm).