Orientation of tubular polypropylene film by heating with - in orientation temps

摘要

Permits reduction to a minimum of the damage caused to the optical props. of the film and yields a film having a combined contraction along and across the tube of less than 25%. Process comprises heating the film at a temp. within orientation temp. limits, stretching the film biaxially by radial expansion and linear extrusion of the tube, and cooling the film when the tube is at its max. expansion, (a) the fall in temp. of the film is limited to a value not more than 15 degrees C. from the initial radial expansion to a point where the tube achieves a diameter corresponding to max. expansion; (b) the centripetal force developed by the bubble is limited to a value not exceeding 225 kg./cm2. before cooling. Pref. the temp. of the expanded film is kept above 130 degrees C. so as to obtain a heat-stable film. When the normal orientation temps. for polypropylene are 140-158 degrees C., the min. temps. to which the film may fall during orientation and before cooling, are 130-143 degrees C. The fall in temp. of the film is controlled by heating means located to the exterior of the tube, and in which circulates a fluid heated to 50-155 degrees C.