Heat shrinkable film and process for preparing the same

Claim 1

A process for preparing a heat shrinkable ethylene polymer film which comprises the steps of (1) introducing a non-stretched tubular film of an ethylene polymer having a density of 0.91 to 0.93 g./cm. 3 at 25° C. into a tubular biaxial stretching apparatus, a film obtained by melt-extruding said ethylene polymer and cooling the extrudate to solidify without stretching it showing a tensile strength at 300% elongation of not less than 15 kg./cm. 2 at a temperature of 15° C. below the melting point of said ethylene polymer, (2) biaxially stretching said non-stretched tubular film to give a biaxially oriented film by means of an internal gas pressure at a temperature within the orientable temperature range under the following conditions: (a) the surface temperature of the film at a starting point of expansion being within the range of 15° to 25° C. below the melting point of said polymer, (b) the film in a stretching zone extending from the starting point to a finishing point of expansion having a temperature gradient such that the surface temperature of the film at a position of 1/4 to 1/3 the length of the stretching zone from the starting point is maximum, and the difference between the maximum surface temperature and the surface temperature of the film at the starting point being not more than 5° C., (c) the temperature drop of the film from the maximum temperature position to the finishing point being within the range of 15° to 20° C., and (d) the film being cooled while traveling from the finishing point to a distance of 1.0 to 1.5 times the vertical distance of the stretching zone so that the surface temperature of the film drops by 30° to 40° C., and (3) recovering the resulting biaxially oriented film.