Patent GB-917903-A (GB)
Title: Polypropylene films
Inventor: STEAD BRIAN DOUGLAS
IPC: B29C55/28
Filed: 1960-09-22
Abstract: <PICT:0917903/IV(a)/1> A biaxially oriented isotactic polypropylene film which shows substantial molecular orientation in its plane and a substantially greater degree of orientation in the longitudinal direction than in the transverse direction as shown by birefringence measurements in and out of the plane of the film is made by stretching a substantially unoriented isotactic polypropylene film to a greater extent in the longitudinal direction than in the transverse direction at a temperature below its melting point.  The out of plane birefringence of the film may be at least 5 X 10-3 preferably at least 10 X 10-3, and the in plane birefringence may be at least 4 X 10-3, preferably at least 10-3.  The longitudinal breaking strength of the film may be at least 35,000 psi, preferably at least 40,000 psi and the transverse breaking strength at least 15,000 psi, while the tensile modulus should be greater than 4 X 105 psi and the elongation at break in the longitudinal direction no more than 40%. Stretching of the film in the two directions may be carried out substantially simultaneously.  Thus a tubular film made by melt-extrusion and quenching may be inflated to increase its length and diameter, the longitudinal stretching being enabled to exceed the transverse stretching by applying increased stretching forces between a restrictive means through which the tube of film is fed on one side of the expanding bubble of the tube and a haul-off means at the other side. Stretching may be effected at 120 DEG -170 DEG C., and especially at 155 DEG -168 DEG C.  Alternatively, the film may be stretched successively in two directions.  Thus a film made by melt-extrusion and quenching may be stretched more in the longitudinal direction than in the transverse direction by a first stretch in one of these directions at 155 DEG -168 DEG C., and by a second stretch in the other direction at 155 DEG -168 DEG C., if the film has not been allowed to cool between stretching steps; or between 160 DEG -168 DEG C., if such cooling has occurred, provided that the second stretch ratio is less than the first stretch ratio, or at 163 DEG -168 DEG C. if such cooling has occurred and the second stretch ratio is greater than the first.  Film which has been biaxially stretched as above may be stretched again in the longitudinal direction to an extent sufficient to produce a greater degree of orientation in the longitudinal direction.  A heat setting step may precede said further stretching.  The further stretch ratio may be in the range 1,05 to 1,8:1, or 1,05 to 2:1 where the biaxially stretched film has not been heat set, and is preferably 1,2:1 to 1,6:1 and desirably 1,5:1 to 1,6:1.  The total degree of longitudinal stretching imparted to the film in the biaxial stretching and further longitudinal stretching steps should be at least 6 times, preferably 8-13 times, while the degree of stretching in the transverse direction should be 3-10 times.  After stretching, the film may be heat set, suitably at 100 DEG -165 DEG C. and especially at 145 DEG -160 DEG C.  Shrinkage may be permitted during the heat setting or a separate shrinkage step may be applied after heat setting. Specifications 840,191 and 873,220 are referred to.