GB400589A - Improvements in the production of sheet materials - Google Patents

Abstract

Solutions of cellulose derivatives are extruded into coagulating baths containing softening-agents for the cellulose derivatives, and the films, foils, or sheets produced are subjected during or continuously with their production to transverse tension, and at least part of the softening-agents are afterwards removed from the products. The cellulose derivative may be cellulose acetate or other cellulose ester or ether. The transverse tension may be such as to prevent shrinking, or it may produce an increase in width. The products may also be subjected to longitudinal stretching. The solvent or softening-agent employed in the coagulating bath may be chloroform, ethylene or methylene chloride, tetrachlorethane, ethylene methylene dioxide, diacetone <PICT:0400589/IV/1> <PICT:0400589/IV/2> <PICT:0400589/IV/3> <PICT:0400589/IV/4> alcohol, acetone, glycol mono-acetate, methyl glycol mono-acetate, an acetin, dioxane, diethylene - dioxide - dicarbinol - diacetate, ethyl lactate, diethyl tartrate, formic, acetic, or lactic acid. Softening-agents which are not true solvents such as diethyl phthalate, and the mono-methyl isomeric xylene sulphonamides may be used undiluted. The solvent substances are diluted with water, alcohol, glycol, glycerol, benzene, toluene, xylene, petrol, benzine, kerosene, carbon tetrachloride, trichlorethylene, tetrahydro- or dekahydro-naphthalene. The coagulating bath may also contain inorganic or organic salts, sugars, cellulose degradation products or other substances which diminish the solubility of the solvent in the non-solvent of the coagulating medium. The film-forming solution may be made with acetone, methylene ethylene dioxide, or methyl ethyl ketone as solvent, and pigments, dyestuffs, or resins may be added. The stretching force may be prevented from operating on the extruded sheet until after a certain amount of coagulation has taken place. The excess of softening agent is removed after the stretching operation by evaporation, or washing in water or a salt solution, or other non-solvent for the cellulose derivative or in a more dilute solution of softening agent. If desired, the longitudinal stretching device may contact with the washed and dried portion of the film, the fully set film transmitting the applied tension back to the soft or plastic part. The film may subsequently be polished with glycerine, or coated with a higher fatty ester of cellulose, or nitrocellulose with or without gums and resins, or printed, or embossed. As shown in Figs. 1 and 3, the film 9, taken from the coagulating bath containing softening agents, is passed between a series of pairs of nip rollers 14 which are positively driven and geared together. Successive pairs of rollers may rotate at progressively increasing speeds so that the film is stretched longitudinally between successive pairs. The first pair may rotate at a greater peripheral speed than the rate of production of the film so that the film is stretched while passing through the coagulating bath. The pairs of rollers following the first pair are inclined to the transverse direction of the film which is thus tensioned laterally and stretched. As shown in Figs. 4 and 5, the film-forming solution is deposited from a hopper 6 on to a drum 7 partly immersed in a coagulating bath 8 containing softening agent. The film is stripped and passed round rolls 10, 11 to the stretching device which consists of pairs of endless bands 20 which nip the film 9 at the edges. Contact is assisted by pressure rolls 22. The bands are inclined so as to stretch the film laterally and they may also draw the film forward at a greater speed than the peripheral speed of the drum 7 in order that the film may be stretched longitudinally. The stretching device may be enclosed in a casing in order that the foil withdrawn from the coagulation bath in a soft condition may be maintained in this condition during the stretching process.