GB849436A - Synthetic polymer films - Google Patents

Abstract

<PICT:0849436/IV(a)/1> Isotactic polypropylene film is continuously stretched laterally, preferably to between 4 and 10 times its initial width, to cause molecular orientation at a temperature between the crystalline melting point of the polymer and a temperature 30 DEG C. below that point. The crystalline melting point is defined as the temperature at which all crystallites in the polymer have disappeared when a thin film of the polymer is slowly raised in temperature and examined under the microscope in polarised light. In an example, a flat film of isotactic polypropylene having a crystalline melting point within the range 170 DEG to 175 DEG C., a viscosity of 7,7 X 105 poises at 190 DEG C, and containing 0,8% of amorphous polymer soluble in diethyl ether, is laterally stretched at 150 DEG C. to 8 times its original width by means of the apparatus shown in the drawing. In this apparatus the film 1 is fed under tension over the idler roller 2 to the pulleys 3 and 4 which are disposed at an angle to each other and mounted on mutually inclined shafts. They are of equal size and are symmetrically disposed with respect to each other as in the case of an object and its mirror image. The film first contacts the peripheries of the pulleys where they are closest together. The margins of the film are gripped in the grooves of the pulleys by the endless belts 5 and 6 for half a revolution of the pulleys. The film is then released from the grooves at the points where the peripheries of the pulleys are furthest apart and passed over the cooled idler roller 7. The lower halves of the pulleys are immersed in ethylene glycol maintained at 150 DEG C. in the bath 8. Specifications 867,532 and 840,191 are referred to.