GB663645A - Improvements in or relating to the production of moistureproof sheet wrapping materials - Google Patents

Abstract

663,645. Coating webs. BRITISH CELLOPHANE, Ltd. Oct. 6,1949 [Oct. 22, 1948], No. 27564/48. Class 140. A moistureproof layer of vinylidene chloride acrylonitrile copolymer is applied to the surface of a solid film or films of water-sensitive, organic, non-fibrous material by introducing an aqueous dispersion of the copolymer into a nip formed between two surfaces of the film or films, and separating the surfaces, each with its coating, in a time less than the time of imbibition of water from the aqueous dispersion through either of the two surfaces. The time of imbibition, i.e. the finite time which elapses before the film has imbibed sufficient water to cause it to suffer an appreciable change in physical properties, may be determined by bringing two similar films together at a nip formed between two horizontal rollers, at which nip a small pool of the dispersion is maintained, passing the coated films in contact to the nip of a pair of rollers situated six inches below the first pair, and there separating them, the speed of withdrawal of the films is increased until the appearance of the coating changes from rough to smooth, the calculated time of passage of the films between the two nips at this critical speed being taken as the time of imbibition. The films may be of regenerated cellulose, lowly substituted cellulose ethers or esters, polyvinyl alcohol, alginic acid derivatives, or protein such as gelatine or casein. The vinylidene chloride acrylonitrile copolymer dispersion may be made as in Specification 655,732 ; its copolymer content may range from 17 to 65 per cent, and its viscosity should not exceed 25 centipoises at 20‹ C. The ratio of monomeric vinylidene chloride to monomeric acrylonitrile in the mixture from which the copolymer is formed may be up to 96: 4, a ratio of 92: 8 being preferred. The dispersion may be applied to the film surfaces at the nip between two rollers, the gap between which is adjustable, the speed of rotation of the rollers being regulated, and the dispersion being added in controlled amounts so as to form a small reservoir of liquid at the nip, small rubber dams or air jets preventing the liquid from flowing past the edges of the films. The coated surfaces are separated on emerging from the nip. As shown in Fig. 3, regenerated cellulose film 31 from a roll 32 is passed between the nip 33 of rollers 34, 35, into which an aqueous dispersion of a vinylidide chloride acrylonitrile polymer is fed from a tap 41, escape of the fluid past the edges of the film being prevented by air jets 42. The film then passes, coated side outermost, around the rollers 36, 37 and 38, (the axes of the rollers 36 and 38 being at 45 degrees to the vertical, and that of roller 37 vertical) and, with the uncoated side outermost, around the roller 35 to the nip 33, where the second side is coated, through a heating device, which fuses the copolymer coating, and under a roller 39, to be collected on a roll 40. In another form, the web is passed through a drying- chamber after each pass through the nip, and finally through a heating chamber, and expander and tension-regulation rollers, and tracking devices for the edges of the web are provided. One side of each of two sheets of different materials, e.g. regenerated cellulose and alginic acid derivative, or regenerated cellulose and gelatine, may be coated simultaneously by passage through a common nip.