Patent GB-922288-A (GB)
Title: Polypropylene films and methods for their preparation
Inventor: BUTEUX RICHARD HAROLD BARCLAY
IPC: B29C44/00
Filed: 1960-10-26
Abstract: A voided polypropylene film is made by drawing a polypropylene film through a neck at a rate sufficient to cause the formation of voids, between a feed and hard-off means which apply a draw ratio (as defined below) to the film whereby the neck is localized.  A neck is a visible deformation in the film through which it draws. Draw ratio is defined as the ratio of the initial length to the drawn length when the film has been drawn through a spontaneously produced neck which produced voids, and is somewhat higher than the natural draw ratio, which is the same ratio where the film draws through a spontaneously produced neck and does not produce voids.  "Drawing" herein means drawing through a spontaneously produced neck and "stretching" herein means stretching with or without the formation of a neck.  The film to be drawn may be made by melt-extrusion and quenching, and the draw ratio may be 5,5:1-10:1, preferably 5,5:1-9:1 and especially 6,5:1 to 8:1.  Film may be drawn longitudinally by bringing it near to, or in contact with, a surface heated to at least 5 DEG C. above room temperature, preferably between 15 DEG C. above room temperature and 100 DEG C., especially at either 40 DEG -60 DEG C., or 60 DEG -80 DEG C., or 80 DEG -100 DEG C.  Thus in Fig. 1, film 1 is drawn <PICT:0922288/IV(a)/1> <PICT:0922288/IV(a)/2> <PICT:0922288/IV(a)/3> between rolls 2 and 3 while heated by the heatsource 5, which localises the neck.  A low power heat source 4 may also be used, which serves to warm a larger area of the film, the position of which is controlled by idler rolls 6 which may be cooled if desired.  The heat-source may be an infra-red heater or bar heater, or the film may be heated by ultrasonic radiation, dielectric heating or hot air.  Most conveniently, film is drawn longitudinally by bringing it near to or into contact with a heated roller over which it passes, the film drawing through a neck immediately before or as it comes into contact with the heated roller.  Thus in Fig. 4, film 21 is drawn between capstan rollers 22 and 25, passing over heated roller 23 and idler roller 24, the neck forming at 26.  Film may be drawn in this way at from room temperature to 100 DEG C. at a draw ratio of 6:1-8:1 preferably at from 30 DEG -85 DEG C. at a draw ratio of 6,3:1 to 7,8:1.  In a modification, a draw ratio of 5,5:1-6:1 is used, and under these conditions, heat is not used to localize the position of the neck.  Drawing may be effected while the film is immersed in a liquid, e.g. water at up to 25 DEG C.  In this way, a translucent film is obtained, of uniform appearance, with a reduced number of voids.  Alternatively, film may be drawn laterally. Thus in Fig. 2 film 11 is drawn laterally by the clips 14 of the stenter frame while passing between rolls 12 and 13, a y-shaped heater 15 serving to localize the neck.  The y-shaped heater can be replaced by a heater in the position of the foot of the y.  After the drawing step the film may be further stretched in a direction transverse to that of drawing, suitably at 80 DEG -160 DEG C. or 130 DEG -155 DEG C., e.g. in air at a draw ratio of 1,8:1-2,3:1, or with the film immersed in a liquid, at a draw ratio of 1,5:1-3,5:1, especially 2:1-3:1, suitably at 40 DEG -100 DEG C., and preferably at 45 DEG -65 DEG C.  Film stretched while immersed in a liquid assumes a matt appearance.  Alternatively, after the drawing step the film may be re-stretched in the same direction at a stretch ratio of 1,1:1-1,5:1, suitably at 50 DEG -160 DEG C., and especially at a total stretch and draw ratio of 8:1-10:1 at 80 DEG -140 DEG C.  After stretching, the film may be heated and allowed to shrink.  Shrinkage may be effected at 80 DEG -150 DEG C., with slight tension on the film so as to avoid cockling and buckling.  Films so made are oriented, have voids in at least a principal direction of orientation in their plane, and have a density of less than 0,80 g./cc.  Such film may have a density of less than 0,7 g./cc., a silvery or metallic appearance, and a longitudinal break strength of at least 50,000 p.s.i.  The films are useful in the manufacture of electrical tape, magnetic recording tape, computer tape, video tape, cable lapping tape, adhesive tape, textile ribbon, typewriter ribbon, conveyer belting and webbing.