Patent GB-852701-A (GB)
Title: Improvements relating to the production of inflated tubular films
IPC: B29C48/03
Filed: 1958-02-25
Abstract: <PICT:0852701/IV(a)/1> Inflated tubular films are produced from polyvinyl chloride or copolymers thereof by mixing the polymer or co-polymer in particulate form with 5-25% by wt. (based on the weight of the mixture) of a solvent boiling at 110 DEG -220 DEG C. which dissolves the polymer at elevated temperatures but neither dissolves nor softens it at room temperature, in a closed storage chamber which can be maintained at super- or sub-atmospheric pressure, feeding the mixture continuously to a screw type extruder the inlet of which communicates with the bottom of the chamber and is connected directly thereto, the extruder being jacketted along its length by a series of heating zones, each maintained at a higher temperature than the preceding zone, thus establishing a temperature gradient within the extruder increasing in the direction of travel of the polymer-solvent mixture, the first zone being maintained at not less than 100 DEG C. and the last at not more than 180 DEG C.  The interior of the storage chamber is maintained at a pressure such that undue evaporation of the solvent from the heated polymer-solvent mixture does not occur and the mixture is continuously extruded in the form of a rod or ribbon which is comminuted and thereafter fed to a separate apparatus for melt-extruding the mixture to form an inflated tubular film. Alternatively the mixture is extruded from the first extruder through an annular extrusion orifice to form a tubular film which is then passed directly through a heated evaporating chamber in the form of a long shaft through which a current of heated air is passed in counter-current to the direction of travel of the tubular film, which is simultaneously inflated by maintaining within it a bubble of air at superatmospheric pressure.  As shown, a suitable apparatus for the production of tubular films by the second method comprises a closed main container 1, a hopper 2 mounted thereon and a feed screw 3 which coacts with an auxiliary feed screw 4.  The shaft 20 of the feed screw 3 rotates in the bearing 21 provided with a gastight running gland 22, and has a stirrer 14 rigidly mounted on it.  A reflux condenser 15 is mounted on the container 1, which can be placed under pressure or vacuum through connections 16.  The extruder casing is provided with a series of jackets 5-9 corresponding with an intermediate zone A (which may be a cooling zone) and heating zones B-E, each jacket being provided with inlet and outlet ducts communicating with independent heat-exchange medium circulation systems.  The polymer-solvent mixture is forced by the extrusion screw 3 into a die head 10 provided with an annular extrusion orifice 23 and encased in a thermal insulating jacket 24.  The extrusion head is provided with a duct 25 for the introduction of air or other gas at superatmospheric pressure into the interior of the tubular film extruded through the orifice 23 and with a duct 26 for the discharge of the air or other gas containing solvent vapour evaporated from the inner surface of the tubular film.  The inflated film is then drawn downwards through a long evaporating chamber 11, heated by a jacket 31 through which a heat-exchange medium is passed.  A stream of preheated air is passed through the chamber 11 in countercurrent to the direction of travel of the tubular film in order to evaporate solvent from the outer surface of the film, the air entering the chamber 11 through the inlet tubes 17 and being discharged through the outlet tubes 19, which may communicate with the pipe 26.  The film is then drawn by means of a pair of pinch rolls (not shown) through a confining tube 12 which is of smaller diameter than the chamber 11 and is surrounded by the jacket 27 through which a coolant is circulated. The tube 12 functions as a ring gauge to control the degree of inflation of the film.  The film material may be polyvinyl chloride of molecular weight about 45,000 or higher or a copolymer of at least 85% of vinyl chloride and at most 15% of vinylidene chloride, vinyl acetate, vinyl methyl ketone, methyl methacrylate or methacrylic acid.  The solvent may be dipropyl or dibutyl ether, ethylene glycol mono- or di-acetate or mono- or di-propionate, mixed ethylene glycol esters, toluene, xylene, monochlorbenzene, monochlortoluene, dichlortoluene or tetra- or deca-hydronaphthalene. The comparatively low extrusion temperature at which the film can be produced, due to the presence of a solvent, enables films free from colour and odour to be produced.