GB821930A - A process for the manufacture of a coated sheet material suitable for use for packing purposes and in moulds - Google Patents

Abstract

A material suitable for use for packing purposes and in the construction of moulds is manufactured by coating paper or cardboard, e.g. Kraft paper, upon one surface with a swollen water-insoluble layer of alginic acid or of a compound thereof, and upon the other surface with a layer which is impermeable or substantially impermeable to water and water vapour. The impermeable layer may be, e.g. a synthetic resin such as polyethylene, polyvinyl chloride, polystyrene, polyvinylidene chloride and mixed polymerization products, a mixture of polyethylene with paraffin wax, chlorinated rubber, or bitumen. If polyethylene is used, it may be bonded to the paper or cardboard by heat sealing, or may be spread on to the paper in the molten condition. The alginate layer may be applied by treating the paper with a solution of a water-soluble compound of alginic acid, e.g. an alkali metal or ammonium salt thereof, and converting it to an insoluble compound by subsequent treatment, e.g. with a salt of an alkaline earth metal or heavy metal (Ca, Ba, Sr, Zn, Cu, Al, Cd, Co, Cr, Ni and Mn are named). It is advantageous to use a hygroscopic salt, e.g. CaCl2, or to add a hygroscopic substance, e.g. glycerine or glycol, to the alginate solution. The alginate may also be rendered insoluble by treatment with an acid in solution or as a vapour. Alternatively, the paper or cardboard may be first treated with a solution of the precipitating agent (e.g. CaCl2) and subsequently sprayed with a solution of soluble alginate. In another method, the paper is treated with a solution which contains alginate derivatives, metal salts which yield water-insoluble metal alginates and nitrogencontaining substances, e.g. ammonia or aliphatic amines, which form complexes with said metal salts. In this process the complex-forming substance is removed, e.g. by heat. The alginate layer may be modified with e.g. starch, dextrin, cellulose ethers, mucilages, gum tragacanth, protein hydrolysates, pigments or powdered metals. The application of the alginate layer may be delayed until shortly before the material is used, or until after it has been made into bags, moulds or the like.