The present invention relates to a heat shrinkable film prepared by a tubular process from an ethylene polymer and a process for preparing the same.
Hitherto, a polyvinyl chloride film, a polypropylene film and a polythylene film are known as heat shrinkable films.
A heat shrinkable polyvinyl chloride film has an excellent characteristic such that it shows a good shrinkability at a relatively low temperature, e.g. in the vicinity of 100.degree. C., but has the disadvantages that an additive such as a plasticizer or a heat stabilizer added to the resin may cause a hygienic problem and also that the film generates hydrogen chloride upon the destruction treatment by fire of the used film.
A heat shrinkable polypropylene film has a good heat shrinkability because of being subjected to biaxial stretching, but has the disadvantages that it reveals only at a relatively high temperature of 100.degree. to 140.degree. C. and also that the heat seal strength is not so large.
On the other hand, a heat shrinkable film made of ethylene polymers such as a polyethylene shrinkable film has been widely employed in shrink packaging, since it is inexpensive and has a large heat seal strength. The heat shrinkable polyethylene film has been prepared by a so-called inflation process which is one of tubular processes and in which a molten tube extruded from a die is blown-up as it is by the process of an internal gas. In such a conventional process, an effective molecular orientation does not take place, and, therefore, a known heat shrinkable film of ethylene polymers has not always satisfactory characteristics. For instance, the film strength is small and the elongation is large. Also, the film shows a high heat shrinkage percent at a temperature near the melting point of the polymer, but the heat shrinkable temperature range is narrow, and also the close adhesion of the film to articles to be wrapped is not sufficient due to small shrinkage stress. Although it is theoretically clear that the mechanical properties of the film are improved by stretching the film, it has been considered to be difficult to highly stretch a film of ethylene polymers by conventional processes so as to impart the heat shrinkability to the film.
In order to eliminate the disadvantages of heat shrinkable polyethylene films, there are proposed a process as disclosed in Japanese Patent Publication Nos. 18893/1962 and 9086/1964 in which a polyethylene film is irradiated with ionizing radiation so as to cause intermolecular crosslinking and then stretched with heat, and a process as disclosed in Japanese Patent Publication No. 20432/1973 in which a resin irradiated with radiation or a mixture of resins containing the irradiated resin is subjected to the formation of film followed by stretching. However, these processes have the disadvantages that the radiation treatment increases the cost, and that the heat sealability of the film is bad, and that it is impossible to recover the waste film. Also, a low density polyethylene shrinkable film is poor in transparency.
Accordingly, it is an object of the present invention to provide a heat shrinkable film of ethylene polymers which can reveal an excellent heat shrinkability and a large shrinkage stress.
A further object of the invention is to provide a heat shrinkable film suited for use in shrink wrapping, which can closely adhere to articles to be wrapped and which has excellent transparency, heat sealability and impact resistance.
Another object of the invention is to provide a process for preparing a heat shrinkable ethylene polymer film having excellent heat shrinkability and shrinkage stress by means of biaxial stretching.
These and other objects of the present invention will become apparent from the description hereinafter.