Patent GB-721305-A (GB)
Title: Improvements in or relating to the manufacture of moisture-proof sheet wrapping materials
Inventor: BERRY WILLIAM
IPC: B29D7/01
Filed: 1952-04-04
Abstract: The process of the parent Specification for making moisture-proof sheet wrapping material from an aqueous emulsion of vinylidene chloride polymer or copolymer is modified in that the emulsion contains 50-70 per cent by weight of solid polymer or copolymer and that the base sheet upon which the emulsion is deposited and from which the formed sheet is subsequently stripped is a sheet of a water-insoluble, non-fibrous cellulose derivative or polyamide having a maximum water-absorption, when saturated with water at 25 DEG  C. under normal atmospheric pressure, of 1-20 per cent by weight, based on the weight of the dried sheet.  The aqueous emulsion may be of a copolymer of vinylidene chloride and acrylonitrile, vinyl chloride, ethyl acrylate, methyl methacrylate or a mixture of these substances. Preferred co-polymers are prepared by polymerizing mixtures containing 80-98 per cent of monomeric vinylidene chloride.  The aqueous emulsions of vinylidene chloride polymer or copolymer obtained vary from a thin white liquid latex at 50 per cent polymer concentration to a thick creamy paste at 70 per cent polymer concentration.  If desired, a plasticizer such as dibutyl phthalate or a chlorinated diphenyl may be present.  Preferably, the base sheet on which the emulsion is deposited has a smooth surface and shows a variation of the diffusion coefficient with water-content so that a short lag of about one second occurs before extraction of water begins. Preferably, also, the deposited layer, after stripping from the base sheet, is dried and consolidated by heat, but the heat treatment may be effected before stripping if desired.  The base sheet may be made of cellulose acetate, nitrate or aceto-butyrate and the degree of substitution may be from 1 to 3.  It may be of cellulose triacetate plasticized with 25 parts of diethyl phthalate per 100 parts of cellulose acetate.  A sheet of ethyl or benzyl cellulose or of a polyamide may be used.  Prior to use, the base sheet may be dried, e.g. by exposure to hot dry air.  The base sheet may be in the form of an endless band which, after the deposited layer has been stripped therefrom, is dried by heating and then cooled ready for reuse.  The process of forming the polyvinylidene chloride sheet may be carried out in a continuous manner.  In an example, a mixture of vinylidene chloride, acrylonitrile, the sodium salt of a condensation product of formaldehyde and naphthalene sulphonic acid, ammonium persulphate, sodium metabisulphite and water is heated under reflux and then cooled.  The aqueous emulsion thus obtained is spread on to a sheet of plasticized cellulose acetate, previously dried to 0.5 per cent water-content. The water of the emulsion is wholly absorbed by the base sheet and the deposited layer of copolymer is dried and consolidated by heating, first at 120 DEG C. for 2 minutes and then by exposure to radiant heat from an electric fire at a distance of 6 inches for 30 seconds.  The film is then stripped from the base sheet.  In another example, a mixture of vinylidene chloride, methyl methacrylate, the sodium salt of a condensation product of naphthalene sulphonic acid and formaldehyde, ammonium persulphate, sodium metabisulphite, sodium dodecyl toluene sulphonate, chlorinated diphenyl and water is heated under reflux and then cooled.  The aqueous emulsion so prepared is spread on to a sheet of ethyl cellulose (30 per cent ethoxy content) which has previously been dried to 0.5 per cent water content.  The water of the emulsion is absorbed by the base sheet and the deposited layer of copolymer is stripped off and then dried and consolidated by heating to give a clear self-supporting film.