Patent GB-1141367-A (GB)
Title: Method and apparatus for orienting films made from thermoplastic polymers
IPC: B29C55/06
Filed: 1965-05-05
Abstract: 1,141,367. Uniaxially oriented films. O. B. RASMUSSEN. 29 April, 1966 [5 May, 1965], No. 19056/65. Heading B5B. A method for producing a high degree of unidirectional molecular orientation in films made from organic thermoplastic polymers by stretching the films at elevated temperatures, is characterized in that the film is longitudinally stretched to an extent not more than 75% of n, where n is the maximum draw ratio, defined as the draw ratio necessary to get maximal orientation, and simultaneously or subsequently producing fine longitudinal folds in the film, to such an extent that the width of the film is reduced by an amount of more than 1: #n, and thereafter longitudinally stretching the film at a temperature in or slightly below the melting range of the polymeric material to a degree of orientation which is, or is close to, the maximum, and simultaneously compressing the longitudinal folds and thereby tending to flatten the film. The reduction of the film width may be accomplished by producing wavy longitudinal folds, the height of which is preferably 2 to 4 times the thickness of the film. Such longitudinal folds may be produced simultaneously with the initial stretching by accomplishing the latter by means of two sets of rollers, the axes of which intersect at an angle of 70 to 85‹. In apparatus as shown, a film 1 is preheated by passage under a heated roller 2 and over another heated roller 3, both of which are provided with means not shown for control of the temperature, and both of which are driven in order to prevent uncontrolled stretching of the film. The film is preheated to a temperature or at slightly below the melting range of the film material, after which it passes to a set of rollers 4, said rollers being kept pressed against one another and at least one of the rollers preferably being coated with a heat resistant rubber, for example a nitrile rubber. Both rollers may be provided with means not shown for heating and for temperature control, and both are preferably driven. From rollers 4, the film is passed under a roller 5 at an angle of 70 to 85 degrees to the original longitudinal direction, and then over a roller 6 to a set of rollers 7 adapted to be driven at greater peripheral speed than the set of rollers 4. Thus rollers 7 together with rollers 4 constitute the means for carrying out the initial stretching of the film and also for producing the fine longitudinal folding of the film by the oblique taking off from the set of rollers 4. Rollers 5 and 6 are provided with means for heating and for temperature control, not shown, and may be replaced by an oven through which the film is led. As shown, endless rubber belts 8 are supported by rollers 7, said belts being driven by two sets of rollers, each consisting of rollers 9 and 10, at a peripheral speed and with a compression causing the two belts to be in a stretched state at their zone of contact in the nip of rollers 7. These rollers may thus impart a compression to the folded film to such an extent that a flattening of the folds occurs by flowing of the film material. The final stretching of the film to obtain a degree of orientation at or slightly below the maximum is at least initiated during this compression between the belts and is effected by carryng the film through a set of rollers 11 co-operating with the set of rollers 7 and driven at the necessary higher peripheral speed. Rubber belts 8 must be sufficiently elastic and heat resistant and are suitably made of nitrile rubber. The belts may be heated by heaters 12 with temperature control means 13. Alternatively, if rollers 7 are made with a perfectly even surface, rubber belts 8 may be omitted. In this case the rollers coming into contact with the hot film may be coated, e.g. with a silicone coating, to prevent the film from adhering to the roller surfaces. Alternatively, the longitudinal folds may be produced by means of two sets of intercombing rotating discs, each set of which is mounted upon a common shaft. Specified materials include isotactic and syndiotactic polypropylene, and high density polyethylene both of which may be stretched to about ten times the original length. In an example, a substantially unoriented film of isotactic polypropylene is provided with pin point holes and stretched at 135‹ C., the film direction being changed at an angle of 80 degrees. Initial stretching is effected to a ratio of 6: 1 and final stretching to a ratio of 1À5: 1, i.e. a total stretch ratio of 9: 1 without ruptures occurring. Such film normally ruptures at a stretching ratio of about 7: 1 or in the absence of pin holes at a ratio not exceeding 9À5: 1.