Thermally shrinkable polyester film, method of manufacturing the same, and packed product using the same

摘要

There is provided a thermally shrinkable polyester film where the opening ability along the perforation is very good. A thermally shrinkable polyester film comprising polyester resin in which ethylene terephthalate is a main constituting component and not less than 13 molar % of one or more monomer component(s) which can become non-crystalline component is/are contained in the total polyester resin components, wherein the film has specific thermally shrinking characteristics and specific mechanical characteristics after thermal shrinking treatment.

主权项

1. A thermally shrinkable polyester film, comprising:
polyester resin comprising ethylene terephthalate as a main component and not less than 13 molar % of one or more monomer components which can become non-crystalline wherein the thermally shrinkable polyester film fulfills the following requirements (1) to (4): (1) the difference in the thermal shrinking rate in hot water in the widthwise direction of the film when treated in hot water at 80° C. for 10 seconds and for 3 seconds is 3% to 15%; (2) the thermally shrinking rate in hot water in the widthwise direction and in the lengthwise direction when treated in hot water at 90° C. for 10 seconds are 40% to 80% and 0% to 12%, respectively; (3) the right-angle tear strength in the lengthwise direction per unit thickness after shrunk to an extent of 10% in the widthwise direction in hot water at 80° C. is 180 N/mm to 310 N/mm; and (4) the tensile breaking strength in the lengthwise direction is 90 MPa to 300 MPa, wherein the thermally shrinkable polyester film is prepared by steps (a) to (g): (a) a longitudinal stretching step where the non-stretched polyester film is stretched in 2.2-fold to 3.0-fold in a lengthwise direction at the temperature of Tg to (Tg+30° C.) and then stretched in 1.2-fold to 1.5-fold in a lengthwise direction at the temperature of (Tg+10° C.) to (Tg+40° C.) whereupon the longitudinal stretch is done to make the total stretching magnification 2.8-fold to 4.5-fold; (b) an annealing step at a film temperature of (Tg+10° C.) to (Tg+40° C.) where the film after the longitudinal stretching is subjected to relax of 20% to 50% in the lengthwise direction together with heating in the widthwise direction using an infrared heater; (c) an intermediate thermal treating step where the film after the annealing is subjected to a thermal treatment for 1.0 second to 9.0 seconds at the temperature of 130° C. to 190° C. in a state of being held by clips at the both ends in the widthwise direction in a tenter; (d) a natural cooling step where the film after the intermediate thermal treatment is passed through an intermediate zone where it is shut out from the heating zone of each step and no positive heating operation is conducted whereupon a natural cooling is done; (e) a compulsory cooling step where the film after the natural cooling is positively cooled down until the surface temperature reaches 80° C. to 120° C.; (f) a lateral stretching step where the film after the compulsory cooling is stretched in 2.0-fold to 6.0-fold in a widthwise direction at the temperature of (Tg+10° C.) to (Tg+40° C.); and (g) a final thermal treating step where the film after the lateral stretching is subjected to a thermal treatment for 1.0 second to 9.0 seconds at the temperature of 80° C. to 130° C. in a state of being held by clips at both ends in the widthwise direction in a tenter.