Low-shrinkage polyester film and preparation thereof

Abstract

A biaxially oriented polyester film simultaneously satisfying the following formulas (I) to (III): |S.sub.180 -S.sub.120 |≦54 (I) T.sub.S.sup.MD ≧100 (II) Q.sub.120 ≦0.7 (III) wherein S 180 is shrinking stress (g/mm 2 ) of the film in the machine direction at 180° C., S 120 is shrinking stress (g/mm 2 ) of the film in the machine direction at 120° C., T S MD is shrinkage initiation temperature (°C.) of the film in the machine direction, and Q 120 is shrinkage (%) of the film in the machine direction after 5 hours' treatment at 120° C., is described. The film has a good dimensional stability due to its low shrinkage, particularly in the machine direction, and substantially free from defect such as wave and wrinkle. It is prepared by biaxially stretching, heat-setting and relaxing an extruded polyester film under particular conditions so as to obtain a biaxially oriented polyester film having these characteristics.

What is claimed is: 
     
       1. A biaxially oriented .[.polyester.]. .Iadd.polyethylene terephthalate .Iaddend.film simultaneously satisfying the following formulas (I) to (III):   |S.sub.180 -S.sub.120 |≦54        (I)       T.sub.s.sup.MD ≧100                                 (II)       Q.sub.120 ≦0.7                                      (III)     Wherein S 180  is shrinking stress (g/mm 2 ) of the film in the machine direction at 180° C., S 120  is shrinking stress (g/mm 2 ) of the film in the machine direction at 120° C., T s   MD  is shrinkage initiation temperature (°C.) of the film in the machine direction, and Q 120  is shrinkage (%) of the film in the machine direction after 5 hours' treatment at 120° C.   
     
     
       2. A film according to claim 1, wherein the shrinkage (Q 180 ) of said film in the machine direction after 30 minutes' treatment at 180° C. is not more than 1.3%. 
     
     
       3. A film according to claim 1 or claim 2, wherein the shrinkage (Q 180 ) of said film in the machine direction after 30 minutes' treatment at 180° C. is not more than 0.5%. 
     
     
       4. A process for producing a biaxially oriented .[.polyester.]. .Iadd.polyethylene terephthalate .Iaddend.film simultaneously satisfying the following formulas (I) to (III):   |S.sub.180 -S.sub.120 |≦54        (I)       T.sub.s.sup.MD ≧100                                 (II)       Q.sub.120 ≦0.7                                      (III)     wherein S 180  is shrinking stress (g/mm 2 ) of the film in the machine direction at 180° C., S 120  is shrinking stress (g/mm 2 ) of the film in the machine direction at 120° C., T s   MD ) is shrinkage initiation temperature (°C.) of the film in the machine direction, and Q 120  is shrinkage (%) of the film in the machine direction after 5 hours' treatment at 120° C., which comprises the steps:   extruding a polyester at a temperature from 270° to 300° C. into an amorphous sheet,   stretching the sheet in the machine direction by a stretch ratio from 2.5 to 4.5 times at a temperature from 80° to 120° C.,   stretching the film in the transverse direction by a stretch ratio from 3.3 to 4.5 times at a temperature from 80° to 140° C.,   heat-setting the film at a temperature form 225° to 260° C. for from 1 sec to 10 min while permitting the film to relax in the transverse direction by a factor from 1 to 15% in a heat-setting zone,   further permitting the film to relax in the machine direction or the transverse direction or both directions by a factor from 0.01 to 10% in a cooling zone at a temperature not exceeding 180° C., and   winding up the biaxially oriented film thus heat-set.   
     
     
       5. A process according to claim 4, wherein the taken-up film is further subjected to off-line heat treatment at a temperature from 120° to 180° C. for from 10 sec to 10 min.