GB873220A - Polypropylene films and methods for their production - Google Patents

Abstract

A process is described for making a polypropylene film of substantially uniform thickness in which at least 75% by weight of the polypropylene is insoluble in boiling n-heptane, and in which the polypropylene shows preferential molecular orientation (as determined by X-ray diffraction) with the molecular chain axes in the plane of the film, the molecules of polypropylene in the amorphous regions of the film being sufficiently free from internal strains so that the film has a linear shrinkage of not greater than 6% when heated for 1 min. at 120 DEG C., and the degree of molecular orientation being such that the film has a breaking tensile stress of at least 18,000 p.s.i., in any direction in the plane of the film, when measured on a specimen prepared by using a stamping cutter according to B.S.S. 903 Part A2 (1956) type C specimen at 20 DEG C. in two directions substantially at right angles substantially simultaneously. In the process, polypropylene film, sheet or tube in which at least 75% by weight of the polypropylene is insoluble in boiling n-heptane, made by melt-extrusion and quenching, is passed through a narrow zone in which it is heated rapidly to a temperature in the range 120 DEG -170 DEG C., at which it may be uniformly drawn but below the crystalline melting point of the polypropylene, and is freely drawn during or immediately after such passage in at least the direction along the length of the film and, before, during, or after such drawing, is stretched transversely such that the area of the undrawn film is increased 16-100 times, and is then heattreated at from 75 DEG -170 DEG C., preferably 145 DEG -160 DEG C., the degree of crystallinity of the film before drawing and the drawing temperature, rate and ratio and heat-treatment conditions being so chosen that the product conforms to the above criteria regarding linear shrinkage and breaking tensile stress. By rapid heating is meant heating by, e.g., infra-red radiation for at most 5 seconds, and by free drawing is meant drawing between a feeding means and a haul-off means. During heat-treatment the film should not be allowed to shrink in any direction by more than 15%. Alternatively, a two-stage heattreatment may be used, the film being allowed to shrink in the second stage, which may be effected at from 110 DEG -130 DEG C. Undrawn tubular film may be drawn in two directions substantially at right angles substantially simultaneously, and the heat-treatment stage may be effected in a continuous process in which the film is prevented from lateral contraction by internal pressure and from longitudinal contraction by means of feed and haul-off rollers. Alternatively, during heat-treatment, the diameter of the inflated tabular film is allowed to shrink while longitudinal contraction is substantially prevented. Specification 840,191 is referred to.