Heretofore, polyvinyl chloride was used for a heat-shrinkable film for e.g. a shrink package, but in recent years, a block copolymer made of a vinyl aromatic hydrocarbon and a conjugated diene or its resin composition has been used. There are various processes for producing the heat-shrinkable film, and as the simplest process, a production process called tubular method may be mentioned.
Patent Document 1 discloses that beyond expectation, it is extremely difficult to produce a heat-shrinkable film by using the above block copolymer made of a vinyl aromatic hydrocarbon and a conjugated diene or its resin composition by means of a tubular method, and the temperature control in a stretching step is to be precisely carried out so as to stabilize forming. On the other hand, Patent Document 2 discloses that some kinds of block copolymers made of a vinyl aromatic hydrocarbon and a conjugated diene are suitable for the tubular method.
The tubular method is a process for producing a heat-shrinkable film by extruding a molten resin from a ring-shaped die to obtain a tube, controlling the tube at a proper temperature, and then injecting air into the tube so as to expand it (hereinafter referred to as “bubble formation”) for stretching in a direction perpendicular to the flow direction of the film. A power required for stretching the film by the bubble formation can be determined from the difference between the outside pressure of the bubble and the inside pressure for forming the bubble by the injection of the air into the tube.
Specifically, compressed air is injected into the above expanded tube by a needle to form a bubble, a cooling temperature of the bubble is controlled or the position of a pinch roll for pinching the downstream side of the bubble is further adjusted, whereby the internal pressure of the bubble is adjusted so as to stabilize the bubble. However, the tube is continuously extruded, therefore there is a case where the thickness or diameter of the tube slightly changes and further the resin temperature slightly changes with time (hereinafter generally referred to as “variable factors in production”), whereby abnormal expansion (hereinafter referred to as “excess expansion”) or the like locally occurs at the time of bubble formation, and therefore it will be difficult to stably produce the film.
In order to prevent the occurrence of the excess expansion, it has been desired to develop a material which is less influenced by the variable factors in production while it is not so much required to seriously control e.g. the amount of air to be injected or the resin temperature.
Patent Document 1: JP-A-50-6673
Patent Document 2: JP-A-07-216186