GB855216A - Improvements relating to the coating of moving webs - Google Patents

Abstract

855,216. Coating by extrusion; laminates. DU PONT DE NEMOURS & CO., E. I. Oct. 29, 1958 [Nov. 8, 1957], No. 34689/58. Class 140. The coating of a moving web of paper or film with an aqueous high viscosity solution of a water-permeable organic colloid is effected by extruding the coating on to the web, travelling at a higher speed of the order 2 to 2000 times that of the extrudate, at a temperature above that at which the coating sets but below 105‹ F. and at a sub-atmospheric pressure below the " static equilibrium pressure " and between the (lower) " air occlusion pressure " and the (still lower) " apparent equilibrium pressure " whilst continuously removing water vapour from the coating zone, the condensible vapours in the zone consisting substantially entirely of those evolved from the solution. The abovementioned pressures are respectively that at which the coating liquid normally boils at the operating temperature, that at which air begins to be trapped beneath the coating, and that to which the surrounding atmosphere can, it is found, be reduced, depending on the conditions of extrusion, without boiling of the coating liquid. The process is applicable to the coating, for photographic purposes, of film base and papers with dispersions or emulsions, in water or ethanol, of hydrophilic colloids, such as gelatin, albumen, agar-agar, polyamides, polyvinyl alcohol, partly hydrolized polyvinyl esters, and polyvinyl acetals, as such or containing silver halides, hardeners, waterproofing materials, light-filtering dyes and pigments, magnetizable materials or phosphors ; it is also applicable to the application of adhesives and surface layers to textiles and paper. The process can be used with materials having viscosities generally within the range 1000 to 100,000 centipoises and solids content between 20 and 65%. Web speeds up to 2400 feet per minute may be employed. The carrier liquid is stated to separate on the surface of the coating and may be removed by evaporation in the applied low pressure, with or without radiant heating as by electrically heated panels, or by mechanical means such as sponge-faced rolls or air-knives. As shown, a web 2 is coated with a ribbon 15, extruded from a heated or cooled extrusion hopper 7 having an adjustable nozzle, while passing over a coating roller 6. The coated web passes over a roller 8 which is arcuately adjustable about the point 17 on roller 6. To reduce trapping of air beneath the coating the ribbon 15 is arranged to contact the web below the point 17, and, according to the speed of coating the angle theta between the nozzle and the coated web may be adjusted for optimum working between 178 and 184 degrees. The apparatus shown is enclosed in an evacuable chamber having inlet and outlet sealing rolls for the web or enclosing the supply and take-up reels therefor. The hopper 7 may be designed for multiple coating as described in Specification 800,527. Examples describe the coating of gelatino silver halide emulsion on to a gelatinecoated cellulose acetate web, the coating of a similar emulsion on to a polyethylene terephthalate web precoated with a vinylidenemethacrylate-itaconic acid copolymer itself coated with gelatine, the simultaneous coating of a polyethylene terephthalate web with (a) an aqueous silver halide gelatine emulsion and an anti-abrasion gelatin solution; and (b) coloured aqueous silver halide polyvinyl acetal emulsions, and the coating of a cellulose acetate web with an aqueous silver halide polyvinyl alcohol emulsion. Specifications 703,802, 705,950, 718,422, 755,151, 758,401, 799,618 and U.S.A. Specifications 2,139,628, 2,538,257, 2,681,294 and 2,765,492 also are referred to.