Patent GB-1307177-A (GB)
Title: Method of preparing a uniaxially stretched film having a large width
IPC: B29C55/04
Filed: 1971-08-06
Abstract: 1307177 Uniaxially stretched films POLYMER PROCESSING RESEARCH INSTITUTE Ltd and NIPPON PETROCHEMICALS CO Ltd 6 Aug 1971 [12 Aug 1970] 36987/71 Heading B5B [Also in Division D1] A process in which a film of a thermoplastic resin is stretched to produce a uniaxially stretched film comprises passing said film from feed means to take-up means; between said feed means and said take-up means, forming corrugations in said film running longitudinally of said film to decrease the width of the film progressively in the direction of passage of said film from the original width to an apparent width which is approximately equal to the original width multiplied by the reciprocal of the square root of the stretch ratio; heating said film by contacting a corrugated surface thereof with a heating fluid; and uniaxially stretching the heated film to produce a substantially flat, uniaxially stretched film. The distance between the last member of said feed means contacted by said film and the point of initiation of said stretching should be sufficient to maintain the difference in length between the selvages of the film and the centre of the film within 1%, and said distance is suitably from two to three times the original width of said film. The heating fluid may be hot air, as in the apparatus of Fig. 1, or hot water, as in the apparatus of Fig. 3. The film may be made of polyvinyl alcohol, polyethylene, polypropylene, a polyamide or a polyester. Where a polyvinyl alcohol film is used, the heating fluid is suitably a concentrated aqueous solution of an inorganic sulphate solution at a temperature of about 100‹ C. The film may be corrugated mechanically, using a wavy slit 6. Stretching may be effected in two stages; thus in the stretching of polyethylene a first stage stretching to a draw ratio of 9 to 10 using hot air temperatures of about 105‹ C. is preferred, followed by second stage stretching carried out between roll 10 and pinch rolls 13, using a hot air temperature of 125‹ to 127‹ C. In the stretching of polyvinyl alcohol films, where, for example, an ammonium sulphate solution is used for coagulation, the ammonium sulphate entrained by the film surface must be removed, and a method is described for effecting this. The process of the invention enables uniaxially stretched films having little or no stretching in the transverse direction to be obtained, even when using films of relatively large width. Films so made are easier to fibrillate. Polypropylene film is preferably treated in the hot air chamber of Fig. 1, using a two-stage stretching process. Polyvinyl alcohol film is similarly treated, but the completely dried stretched film must be humidified by passage through hot water at 70‹ to 75‹ C. just before the pinch rolls at the film feeding side of the film splitter to bring it into a slightly elastic state so as to avoid breakage occurring during splitting. In the case of an elastic stretched film, such as polyamide film, the heat treatment carried out is sufficient to promote crystallization and splitting may be carried out in a cold state. In the apparatus of Fig. 1, the roll of film 1 is stretched between guide roll 4 and guide rolls 7, 8, 9 and 10, and is corrugated by passage through the wavy slit 6, better seen in Fig. 2 (not reproduced). As shown, provision is made for the circulation of hot air on either side of the film. The film leaves the chamber through take-up pinch rolls 13 and passes to the next processing stage. In Fig. 3, a raw material film 51 is passed through pinch rolls 52 and 52<SP>1</SP>, and is then led vertically downwards through a bath 55 of hot water in a tank 54. After heating and stretching, a flat film 57  is formed, which moves upwards past guide roll 56 through pinch rolls 58 and 58<SP>1</SP>, where liquid entrained on the film is squeezed off. The film is then passed through friction rolls 59 and 59<SP>1</SP> and thence is passed for subsequent treatment. The initial contact of film 51 with the bath 55 is substantially at that point 53 where the film has been corrugated by passage through a wavy slit.