Patent ID: 8685305
Filing Date: 2014-04-01
Classification: B29C,B29K,B65D,Y10T

Abstract:
1. A process for producing a heat-shrinkable polyester film comprising the following steps (a) to (e) with respect to an undrawn polyester film prepared from a polyester resin component comprising (i) a polyester resin with ethylene terephthalate as the main constituent and (ii) at least 13 mo1% of at least one monomer component capable of forming an amorphous component in the polyester resin component: (a) a lengthwise drawing step of drawing the undrawn film at a temperature of not less than 75° C. and not more than 100° C. by not less than 1.1 times and not more 1.8 times in the longitudinal direction; (b) an intermediate heat treatment step of heat-treating the film after the lengthwise drawing at a temperature of not less than 110° C. and not more than 150° C. for not less than 7 seconds and not more than 28 seconds in a state of holding both ends in the width direction by clips in a tenter; (c) a forced cooling step of positively cooling the film after the intermediate heat treatment till the surface temperature becomes a temperature of not less than 70° C. and not more than 90° C.; (d) a transverse drawing step of drawing the film after the forced cooling at a temperature of not less than 65° C. and not more than 90° C. by not less than 3.5 times and not more than 5.0 times in the width direction; and (e) a final heat treatment step of heat-treating the film after the transverse drawing at a temperature of not less than 80° C. and not more than 100° C. for not less than 5 seconds and not more than 30 seconds in a state of holding both ends in the width direction by clips in a tenter, wherein the heat-shrinkable polyester film satisfies the following requirements (1) to (4): (1) the hot-water shrinkage in the longitudinal direction is not less than −2% and not more than 4% when the heat-shrinkable polyester film is treated in hot water of 80° C. for 10 seconds; (2) the hot-water shrinkage in the width direction is not less than 50% and not more than 80% when the heat-shrinkable polyester film is treated in hot water of 95° C. for 10 seconds; (3) the number of initial breakages, which is the number of breakages before 5% extension, is not more than 7 when the heat-shrinkable polyester film is repeatedly subjected to a tensile test 10 times in the longitudinal direction using a tensile tester with a distance between chucks of 100 mm after being aged in an atmosphere of 85%RH at 30° C. for 672 hours; and (4) the perpendicular tear strength to the longitudinal direction per unit thickness is not less than 300 N/mm and not more than 410 N/mm after the heat-shrinkable polyester film is shrunk by 10% in the width direction in hot water of 80° C.