FR759706A - Improvements in the manufacture of films, sheets and the like containing cellulose derivatives - Google Patents

Abstract

When coagulating cellulose ester and ether films, sheets, foils &c., transparent, smooth and easily detachable products are obtained by employing a coagulation bath with which a solvent or latent solvent of the film-forming solution has only a limited miscibility, and removing the film from the bath whilst it still contains a substantial proportion of such solvent or latent solvent. The liquid used for the preparation of the film forming solution may be a solvent, or a mixture of solvents, or a solvent mixture, although true solvents are preferred. If acetone or other water-miscible solvents are employed, the coagulation bath conveniently comprises a hydrocarbon; if a halogenated hydrocarbon or other hydrophobe solvent is used, an aqueous coagulation bath may be employed. The miscibility of the solvent with the coagulation bath may be still further reduced by dissolving in the latter salts, sugars &c., according to Specification 343,062, or by adding to the bath a proportion of the solvent; on the other hand the miscibility of the solvent with the coagulation bath may be increased somewhat by adding to the latter a substance miscible with both, and if desired the miscibility may increase during the travel of the film through the bath as by adjusting the composition of the latter. The band, drum &c. on which the film is carried may be positioned in proximity to the orifice at which the film is formed or at some distance therefrom; the coagulation bath may contact with both sides of the film as in Specification 405,692, for example, by extruding the film into the coagulation liquid; or the film may be extruded on to a supporting band, drum &c. on which it is spread by means of doctor blades &c. or by utilising the rear lip of the extrusion slit, or by means of a roller. A convenient apparatus comprises a drum, the lower part of which dips into the coagulation liquid, the pin-forming solution being applied by means of a doctor blade to the upper descending half of the drum; the film may be dried on the ascending side by means of a heated segment. The solvent carried by the film may be removed by evaporation as by drying on a smooth surface, or by washing, the rate of removal of the solvent being preferably restrained, either in a separate bath or by means of a layer of liquid on top of or below the coagulation bath, through both of which the film passes in succession. A stretch may be applied to the film during its formation, for example, as described in Specification 400,589. If the solvent retained in the film is more volatile than the non-solvent of the coagulation bath, a milk white film is obtained by simple drying. Specifications 340,324, 341,148, 356,343, 375,423, 375,424 and, in the Provisional Specification, 405,676 also are referred to.