Recently, in applications such as label package doubling as a protection of a glass bottle and a PET bottle etc. and display of articles, cap sealing and accumulation package, there have been widely used drawn films (so-called heat-shrinkable films) composed of a polyvinyl chloride resin, a polystyrene resin, a polyethylene resin or the like. Of these heat-shrinkable films, a polyvinyl chloride film has problems that heat resistance is low, and it generates hydrogen chloride gas in incineration and causes dioxin. A polystyrene film has problems that it is inferior in chemical resistance, as well as an ink with a special composition needs to be used in printing, it requires high temperature incineration and generates a lot of black smoke accompanied by an abnormal odor. Therefore, as a shrink label, there has been widely used a polyester-based heat-shrinkable film which is high in heat resistance, easy to incinerate, and excellent in chemical resistance, and the use amount tends to increase being accompanied by an increase in turn volume of PET containers.
Further, as a heat-shrinkable film, a film is generally utilized which greatly shrinks in the width direction in terms of handleability in label production. Hence, the conventional heat-shrinkable polyester film has been produced by drawing at a high ratio in the width direction in order to exhibit a sufficient shrinkage force in the width direction at heating.
However, there is a problem that the heat-shrinkable polyester film, when stored in an outside warehouse which is not temperature-controlled during a hot summer, results in the film shrinking (so-called natural shrinkage), leading to a reduction in product width. There is another problem that when the heat-shrinkable polyester film is stored in a hot warehouse similarly, the shrinkage rate in the main shrinkage direction decreases, so that the film cannot be attached aesthetically and efficiently unless temperature conditions for causing heat shrinkage are changed when attaching the film by heat-shrinking the same to a container such as a PET bottle after label-cutting. Therefore, the heat-shrinkable polyester film is generally stored at a low temperature of not more than 25° C. However, storage at a low temperature during a summer or the like is not desirable in terms of delivery, costs and recent environmental issues.
It is reported in Non-Patent Document 1 that the dimensional stability (so-called natural shrinkage rate) under storage at 30° C. or 40° C. is improved if heat treatment conditions after drawing are optimized such that the temperature is in a range of 80° C. to 95° C. and the relaxation rate in the width direction is not less than 10%. However, a change in shrinkage rate after storage is unknown, and it is unknown whether a natural shrinkage rate and a change in shrinkage rate are satisfactory when the film is stored at a higher temperature (e.g. 60° C.).