GB1113136A - Process and apparatus for the production of oriented films of thermoplastics material - Google Patents

Abstract

1,113,136. Biaxially-oriented films. KALLE A.G. 19 May, 1966 [19 May, 1965], No. 22360/66. Heading B5B. A process for the production of biaxially oriented film of thermoplastics material, in which the film is stretched in tubular form, comprises feeding said film from a conveyer system on to an expanding device located within the tubing and withdrawing the film from said expanding device, by means of a takeup conveyer system, the tubing being shaped by the said expanding device into a triangular or polygonal cross-section and being stretched while preserving the same cross-sectional shape. Apparatus as shown (Fig. 1) includes a feed conveyer system (rolls 2) and a take-up conveyer system (rolls 3) spaced along a conveyer path for the tubing 1, and an expanding device 4 located in the path of the tubing between said conveyer systems and arranged for insertion within the tubing, the conveyer systems being capable of being driven so that the takeup system runs at a speed higher than the feed system and the expanding device comprises a plurality of spaced strip-like supports extending in the direction of the conveying path of the tubing and so arranged that by engagement with the interior of the tubing they can shape it into a triangular or polygonal cross-section, said supports as shown diverging from one another in order to effect transverse stretching of the tubing. Thus in Fig. 1, the longitudinal stretch of the tubing is determined by the ratio V 2 to V 1 , where V 2 is the speed at which rolls 3 are driven and V 1 is the speed at which rolls 2 are driven. Similarly, the extent of transverse stretch is governed by the ratio E 2 to E 1 , this being the ratio of the distances of the guide elements from one another. In the apparatus of Fig. 2, pairs of conveyer belts 5, 5a, 5b shape the tubing into a cross-section with straight boundary lines and impart to it a speed V 1 . Supporting means 6, 6a, 6b determine the increase in the breadth of the film, while conveyer belts 7, 7a, 7b rotating at a speed V 2 greater than V 1 contribute the forces required for stretching and, by means of the ratio oftheir speed to the speed V 1 , determine the increase in length of the film. The stretched tubing is divided into flat webs by means of the doublecutting elements 8, 8a, 8b and fed by the guide rollers 9, 9a, 9b to reeling devices (not shown). Friction between the film web and supporting means may be minimized by constructing the sliding surfaces of supporting means 6, 6a, 6b from porous material through which air can be arranged to escape so that the sliding friction is reduced. Alternatively, a system of rotating chains from the belts or rolls may be used for supporting the tubing during stretching. In Fig. 2a, the supporting means are replaced by roller conveyers 10, 10a, 10b relative movement between the tubular film and supporting means being largely avoided. The apparatus of Fig. 3 enables sequential stretching with a specific increase in area to be effected by differentiating the speed of the rotating conveyer elements. Pairs of conveyer belts 11, 11a, 11b impart a speed V 1 to the tubing. Conveyer belts 12, 12a, 12b can be driven at a speed V 2 greater than V 1 so that it is possible to effect longitudinal stretching. Owing to the increase of the distance between the supporting and conveyer belts 12, 12a, 12b, the tubing is stretched in the transverse direction. Finally, the tubing can be stretched again in the longitudinal direction by driving the conveyer belts 13, 13a, 13b at a speed V 3 higher than that of 12, 12a, 12b. The apparatus of Fig. 4 offers advantages in the economy of space required. Supporting and conveyer belts 14, 14a, 14b arranged in the interior of the tubing shape it into a substantially triangular crosssection and impart to it a speed V 1 in the upward direction. By inverting the tubing and passing it downwards at the speed V 2 over concentrically arranged strip-shaped supporting and conveyer belts 15, 15a, 15b, the distances of which from one another increase in the direction of movement, the tubing is stretched longitudinally and transversely. In this ease, the belts of the conveyer elements define two coaxial prisms, the conveyer assembly supplying the film being inside the assembly drawing off the film. The apparatus of Fig. 5 enables a tubular film to be stretched in such a manner that the stretching ratios in the direction of movement and at right angles thereto are equal at any time during stretching. When conveyer belts 16, 16a, 16b are moved at a speed V 1 and the conveyer belts 17, 17a, 17b at a speed V 2 , the ratio of the speeds V 2 to V 1 indicates the entire stretch in the direction of movement. The entire stretch in the transverse direction depends on the shape of the expanding device. When the ratio of the speeds V 2 to V 1 is equal to the stretching ratio in the transverse direction, it is possible with this apparatus not only to perform simultaneous stretching in both stretching directions, but it is also ensured that the increments of stretching in the direction of movement and perpendicular thereto are equal at any time during stretching. In general, instead of the pairs of squeeze rollers shown, rotating conveyer elements in the form of chains, endless belts, or rollers may be used. Materials specified include polypropylene which may be stretched at temperatures from 150‹ to 165‹ C. and superficial stretching ratios of 40 to 60, polyethylene films of density 0À918 may be advantageously stretched at from 90‹ to 95‹ C. to 9 to 16 times the original surface area, and polyethylene terephthalate films may be stretched at from 80‹ to 90‹ C. at superficial stretching ratios between 12 and 20.