Patent GB-808343-A (GB)
Title: Method and apparatus for manufacturing sheeting, film or film base of organic high polymer material
IPC: B29C48/91
Filed: 1955-03-09
Abstract: Films of predetermined thickness are made by depositing a film-forming composition containing a volatile solvent on a first casting wheel to form a film of a thickness (when partially cured) less than, but at least half of, the predetermined thickness, this film formation being effected at a greater speed than would be used in forming the film of predetermined thickness directly, stripping the film from the casting wheel and partially curing it by evaporation of a portion of the volatile solvent, passing the partially cured film to at least one further casting wheel, the diameter of each further casting wheel being less than that of the first casting wheel, depositing further film-forming composition containing a volatile solvent on the partially cured film while it is passing over and in contact with each of the further casting wheels so as to bring the thickness of the film (when fully cured) to the predetermined thickness, and fully curing the film, the curing being effected by evaporation of substantially the whole of the remaining volatile solvent from the film.  The apparatus used comprises a first casting wheel with associated feed hopper, at least one further casting wheel of diameter smaller than that of the first casting wheel, a drying chamber interposed between each pair of wheels, a feed hopper associated with each of the further casting wheels and a further drying chamber adapted to receive the film from the last wheel, the wheels being rotated continuously and means being provided for passing the film formed on one wheel through a drying chamber to the next wheel.  In an example, a solution 3 (Fig. 1) of cellulose acetate and triphenyl phosphate in a mixture of methylene chloride, cyclohexane and methyl alcohol is spread from a hopper 2 on to the peripheral surface of a casting wheel 4 to form a film 8 of wet thickness 0.019 inch which sets upon the wheel and is stripped therefrom at 9.  The film is drawn through the drying chamber 11 and festooned over rolls 12-14, some of which may be idler rolls and others positively driven, under roll 16 and over a second casting wheel 17 which is similar to but of smaller diameter than wheel 4. More dope 19 of the same composition supplied to pressure feed hopper 18 through conduit 21 (Figs. 1 and 2) is spread on to the film 8 to a wet thickness of 0.010 inch as it passes over the casting wheel 17. The pressure hopper 18 is preferably positioned with its extrusion lips 0.020 to 0.20 inch from the periphery of the wheel 17 and it or its extrusion slot may be inclined at an angle between about 10 and 50 degrees to the vertical on the film approach side.  The freshly formed composite film 25 is drawn and festooned over the rolls 27-29 <PICT:0808343/IV (a)/1> <PICT:0808343/IV (a)/2> and then drawn from the terminal roll 31 over a series of rolls in the drying chamber 35 and collected on the wind-up roll 42.  The finished sheet is 0.0056 inch thick.  The drying chambers 11, 35 are provided with inlets and outlets 36-39 for the heated air used as an evaporative medium for the volatile solvent.  The pressure hopper 18 (Fig. 2) comprises two thick wall sections 45, 46 secured together by bolts 47 and enclosing spaces 48, 49 for containing the dope, which is continuously supplied under pressure through the conduit 21 and extruded through the slot 51 having rounded outer edges 52.  The space 54 between the hopper 18 and the wheel 17 is substantially enclosed by baffles 55, 56 to minimize the formation of slugs of dope on the film during extrusion. The temperature of the hopper is controlled by circulating a heat exchange medium through ducts such as 61, 62. The two- or multistage casting process of the invention renders it practicable to operate the casting wheels at a higher peripheral speed than usual and thus increase the output of the apparatus, and the final film has less tendency to curl than films produced with one casting wheel only. Wheel 4 is usually over 10 feet and not greater than 30 feet.  Wheel 17 is preferably at least 6 inches but generally less than 10 feet in diameter.  If a third or further layers of dope are to be applied the subsequent wheels in the series may have the same diameter as wheel 17, and the third layer may be applied on the same side of the film as the second layer or on the opposite side.  The dope used in the second or further steps of the process may be the same as or differ from that used in the first step.  A number of additional examples are given all using dopes containing cellulose acetate but otherwise differing in composition.  Specifications 640,730, 640,731 and 687,728 are referred to.