Patent ID: 9080027
Filing Date: 2015-07-14
Classification: C08J

Abstract:
1. A heat-shrinkable polyester film made of a polyester resin comprising 50% by mole or more ethylene terephthalate and containing at least 7 mol % of at least one monomer capable of forming an amorphous component in the whole component of the polyester resin, wherein the following requirements (1) to (5) are satisfied: (1) a glycerin shrinkage rate in the width direction is not more than 10% when the film is treated for 10 seconds in glycerin at 80° C.; (2) a glycerin shrinkage rate in the longitudinal direction is not less than 0% and not more than 10% and a glycerin shrinkage rate in the width direction is not less than 20% and not more than 40% when the film is treated for 10 seconds in glycerin at 100° C.; (3) a glycerin shrinkage rate in the width direction is not less than 40% and not more than 70% when the film is treated for 10 seconds in glycerin at 120° C.; (4) a shrinkage rate of the film in the width direction after aging for 672 hours under an atmosphere of a temperature of 60° C. and a relative humidity of 40% is not more than 1.5%; and (5) the number of initial breakages, which is the number of samples are broken at not more than 5% elongation when a tensile test is repeated 10 times in the film longitudinal direction using a tensile tester with a distance between chucks being kept at 100 mm after aging for 672 hours under an atmosphere of a temperature of 60° C. and a relative humidity of 40%, is not more than 2 times, wherein the heat shrinkable polyester film is prepared by (a) a lengthwise drawing step of lengthwise drawing a undrawn film so as to provide a total lengthwise draw ratio of not less than 2.8 times and not more than 4.5 times by lengthwise drawing the film in the longitudinal direction at a draw ratio of not less than 2.2 times and not more than 3.0 times at a temperature of not less than Tg and not more than (Tg+30° C.) (first-stage lengthwise drawing) and then lengthwise drawing the film in the longitudinal direction at a draw ratio of not less than 1.2 times and not more than 1.5 times at a temperature of not less than (Tg+10° C.) and not more than (Tg+40° C.); (b) an annealing step of relaxing the film after lengthwise drawing at a relaxation rate of not less than 10% and not more than 50% in the longitudinal direction while heating the film in the width direction using an infrared heater; (c) an intermediate heat treatment step of heat-treating the film after annealing for a time period of not less than 1.0 seconds and not more than 9.0 seconds at a temperature of not less than 130° C. and not more than 190° C. with both edges in the width direction being held by clips in a tenter; (d) a forced cooling step of actively cooling the film after the intermediate heat treatment until the surface temperature becomes a temperature of not less than 80° C. and not more than 120° C.; (e) a transverse drawing step of drawing the film after forced cooling at a draw ratio of not less than 2.0 times and not more than 6.0 times in the width direction at a temperature of not less than (Tg+10° C.) and not more than (Tg+50° C.); and (f) a final heat treatment step of heat-treating the film after transverse drawing for a time period of not less than 1.0 seconds and not more than 9.0 seconds at a temperature of the transverse drawing temperature+not less than 15° C. and not more than 40° C. with both edges in the width direction being held by clips in a tenter.