GB669907A - Improvements in or relating to the manufacture of moisture-proof sheet wrapping materials - Google Patents

Abstract

An aqueous emulsion of vinylidene chloride polymer or copolymer is applied to a base sheet of hydrophilic, water-insoluble, non-fibrous organic material in a layer of such thinness that the water of the emulsion is substantially wholly absorbed by the base sheet and the dispersed polymer or copolymer deposited on the sheet in the form of a layer, and the layer is stripped from the base sheet to form a sheet of moisture-proof wrapping material. The deposited layer is preferably dried and consolidated by heat either before or after stripping. The base sheet may be in the form of a sheet, a continuous length or an endless band, thus making the process discontinuous or continuous. Preferably, it has a smooth surface and shows a variation of diffusion coefficient (as defined in the Specification) with water content to ensure a short time lag before extraction of the water of the emulsion begins. The preferred basesheet is a film of regenerated cellulose, but other materials such as lowly substituted cellulose esters and ethers, alginic and albuminous materials and wholly or superficially saponified cellulose acetate may be utilized. The regenerated cellulose base sheet should preferably have a water content (based on the dry sheet) of between 4 per cent and 8 per cent. The relative thicknesses of the regenerated cellulose basesheet and of the emulsion layer should be such that when absorption is complete the water content of the regenerated cellulose sheet should be between 15 and 30 per cent (based on the oven dry weight), thus facilitating stripping. The base sheet may be a thick film as cast or a laminated sheet. The preferred copolymers for the emulsion contain between 80 and 98 per cent monomeric vinylidene chloride, the remaining ingredient or ingredients being vinyl chloride, styrene, acrylonitrile or an alkyl acrylate or alkacrylate in which both the alkyl group of the ester group and that of the acid group comprise not more than 4 carbon atoms. The stripped, dried film may be cold stretched if desired. In an example (all parts by weight) an aqueous emulsion is prepared from 9 parts of vinylidene chloride, 1 part of acrylonitrile, 1 part of a 20 per cent aqueous solution of the sodium salt of C12-C18 alkyl sulphates, 0.06 part of ammonium persulphate, 0.03 part of sodium metabisulphite and 12.5 parts of water. In the cold emulsion (after polymerization at 39 DEG C.) a laminated sheet of regenerated cellulose (0.006 inch thick and 4 per cent water content) is immersed for 1 second, withdrawn and allowed to drain for 15 seconds. The deposited layer is stripped off and dried and consolidated by radiant heat to give a clear moisture-proof film 0.0008 inch thick. Specifications 654,342 and 655,732 are referred to.