Patent GB-412601-A (GB)
Title: Improvements in the manufacture of films, sheets and similar materials
IPC: C08L1/08
Filed: 1932-11-02
Abstract: When coagulating films, foils, sheets &c. using a solution of cellulose acetate or other suitable base in an organic solvent, the solvent content of the coagulation-medium at the film-forming orifice is arranged to be substantially greater than that at the point of <PICT:0412601/IV/1> exit of the film, such increase of solvent content being effected by means of a counter-current movement of the coagulation-medium relative to the film.  The increase of solvent content is of the order of 7 per cent or more, for example, of 10--30 per cent.  By employing a medium containing a substantial proportion of solvent during the early stages of the travel of the film, a transparent product is obtained; this is especially the case if the proportion of solvent is high, or if the medium contains salts, sugars &c.  Conditions may be such that the film leaving the coagulation-medium is devoid of solvent or contains only a small proportion; the coagulated film may be simply dried or it may be washed with water or salt solution and then dried, and in order to preserve the transparency of the product washing should be carried out in such a manner as to obviate too rapid penetration of the non-solvent of the washing bath and these should be present during the drying, solvents less volatile than the non-solvent liquid. Preferably aqueous coagulation-media are employed, for example, solutions of salts such as calcium chloride or sodium bisulphite, and the coagulation-medium may contain a proportion of solvent, swelling agent or plasticizer, which in the case of a solvent may be the same as that contained in the film-forming solution; and the coagulation-medium may contain other additions such as glycol, glycerol and sugars as described in Specification 406,985.  The base of the film-forming solution may be cellulose acetate, nitrate or other cellulose ester or ether, or vinyl acetate &c.  In examples, a solution of cellulose acetate in acetone containing a plasticizer is extruded into 25--35 per cent sodium bisulphite solution, the coagulation-medium moving at such a speed that on withdrawal it contains 15 per cent acetone; a solution of cellulose acetate in diacetone alcohol is extruded into an aqueous diacetone alcohol solution containing initially 5 per cent diacetone alcohol and finally 12--15 per cent.  Any suitable counter-current apparatus may be employed, the film being extruded either horizontally or upwardly or preferably downwardly.  Apparatus comprising a vessel divided by one or more partitions into two or more compartments of substantially uniform cross-section and with means for causing a current of coagulation-medium to flow with substantially the same velocity on both sides of the film, is especially suitable.  An example of such apparatus is shown.  In this, a vessel 8 is divided into vertical compartments 8<a>, 8<b> communicating at the bottom by a removable partition 9 which may be adjusted in height as by means of fins 14 carried by the partition and engaging with the slotted lid 15; the lower edge of the partition is rounded and polished to permit of the free passage of the film, or a roller 12 may be provided at this point; the width of the vessel 8 is approximately equal to that of the film, and the other lateral dimension of the compartments should be small in order that the coagulation-medium is brought into close contact with the film and so economically employed; perforated pipes &c. 16 on opposite sides of the issuing film serve for the supply of coagulation-medium, and for its withdrawal there is provided at the film-shaping device 10 a perforated pipe 17 arranged to one side or a collector surrounding the device.  To obviate escape of solvent from the coagulation-medium when open vessels are employed, a suitable non-volatile liquid may be floated on the bath as described in Specification 411,094, partitions being provided if necessary to prevent the liquid contacting with the film or from being carried away.  The film-shaping device 10 may be immersed in the bath, or it may be situated above its surface as described in Specification 407,351. The film may be supported near to or actually at the shaping device 10 by means of a roller, and such roller may be wholly or partly immersed in the coagulation-medium; provision may be made to supply coagulant to both sides of the film as is described in Specification 405,692, but preferably when a partly-immersed roller is employed and the film contacts first with the non-immersed portion, means such as a squeegee are provided to render that portion of the roller free of coagulant. Specifications 375,423, 375,424, 405,676, 406,337, and 407,673 also are referred to.