ES372802A1 - Preparation of Films of Polypivalolactone - Google Patents

Abstract

Polypivalolactone films are made by meltextruding the polymer into air through a die having a longitudinal slot extrusion orifice, drawing out the extruded melt at a drawdown ratio between 5 :1 and 500 :1 (preferably between 10 :1 and 200 :1) to form a film which is rapidly cooled to a temperature below 150 C., preferably below 100 C., in a sufficiently short time to produce a transparent film. The solidified transparent film is then stretched in one or more directions at a temperature between 20 and 200 C. (preferably between 125 and 200 C.) at such a stretch ratio that the elongation at break of the film after stretching, measured in the direction(s) of stretching and at room temperature is 10 to 200%, preferably 40 to 80%. The film is then annealed at a temperature between 110 and 220 C., preferably between 150 and 200 C., without applying stretch. The film may be allowed to shrink freely or to a certain extent, but is advantageously kept at a constant length during the heat-setting or annealing step. Thereafter the film is further stretched in one or more directions at a temperature between 100 and 220 C. and at a stretch ratio such that the elongation at break of the film after stretching, measured in the direction(s) of stretching and at room temperature is between 5 and 70%. In the example, a polypivalolactone having an intrinsic viscosity (determined in benzyl alcohol at 150 C.) of 1À10 decilitres per gram is melt-extruded at 285 C. through a die having an extrusion slot 15À7 mm. long and 0À2 mm. wide to form a film which is cooled and solidified by being passed through water at 15 C., the level of the water being 5 cm. below the die. The film is taken up at a speed of 6 m/min. and the draw-down ratio applied to the film during its passage from the die to the take-up roll is 26À5 : 1 (draw-down ratio 26À5:1). The clear 30 micron thick film obtained is stretched longitudinally at a temperature of 175 C. and a stretch ratio of 4 : 1 which imparts to the film elasticity and an elongation at break of about 70% when further stretched longitudinally at 175 C. The film is heat set at 180 C. for 8 minutes while held at constant length and is then stretched again at 185 C. and a stretch ratio of 1À5 : 1 whereby there is imparted to the film an elongation at break of 15% and a shrinkability of 35% at 180 C. By subsequent heat relaxation of the film at 180 C. free of tension the elongation at break of the film is increased to about 70%.