Patent GB-357552-A (GB)
Title: Improvements in and relating to the manufacture of cellulose acetate
IPC: C08B3/28
Filed: 1930-03-19
Abstract: <PICT:0357552/III/1> Apparatus for the recovery of acetic acid from a cellulose acetate acetylation dope by vacuum distillation comprises a still 1 fitted with an inlet 1, sludge discharge 5, agitating gear 4 and a heating-jacket 6, and connected to a copper condenser 7 and earthenware receiver 8 which is connected to a vacuum pump by a pipe 21.ALSO:<PICT:0357552/IV/1> <PICT:0357552/IV/2> Cellulose acetate is recovered from its solution in acetic acid in a form readily amenable to purification by washing and without the production of fines, by lowering the acetic acid content by incorporation of aqueous precipitant in such a manner that the solution will shape readily between shaping-elements, having moistened water absorbent surfaces and, so shaped, will yield a porous opaque precipitate when submerged in aqueous precipitant, shaping the so-prepared solution and submerging in an aqueous precipitant preferably in excess.  The invention is applied particularly to a ripened acid dope from which a substantial proportion of acetic acid has been recovered by distillation under reduced pressure after neutralization of the acidic catalyst; it may also be applied to acetic acid solutions of waste cellulose acetate.  The shaping elements may comprise rollers of soft wood or bands or rollers surfaced with chrometanned leather or cotton filter cloth.  Generally, the amount of precipitant added should be such as to lower the concentration of the solvent aqueous-acetic acid to 60-80 per cent, too small an addition will result in imperfect shaping and adhesion to the rollers, &c. while too large an addition will lessen the strength of the shaped product and cause a tendency to fines; the Specification gives data showing the effect of cellulose acetate concentration and the presence of salts upon the amount of precipitant to be incorporated in the solution.  In an example, a primary acid dope prepared by acetylating 170 lbs. of cotton with 600 lbs. glacial acetic acid, 400 lbs. acetic anhydride and 18 lbs. sulphuric acid is hydrolysed by an addition of 6-10 per cent of water in the form of 30 per cent aqueous acid followed by standing at 30-35 DEG  C. until the product is soluble in acetone and plastic in chloroform. The warm, viscous dope was then transferred to a still fitted with agitating gear and heating-jacket. Finely-ground sodium acetate in quantity sufficient to neutralize the sulphuric acid was gradually and thoroughly incorporated with the dope and the still then closed and put under a vacuum of 2-3 in. mercury.  Heat was then applied by water &c. at 90 DEG  C. circulated through the jacket and heating and stirring maintained until 40-50 per cent of the acetic acid had been distilled off.  Water was then cautiously incorporated with the viscous still residue to bring the cellulose acetate to the verge of precipitation and the prepared solution while still warm was fed between moistened rollers or bands to form a ribbon-like product which was then immersed in water.  The Specification gives further examples of the treatment of particular dopes.  Fig. 1 shows suitable apparatus for the distillation step comprising a still fitted with an inlet 1, discharge 5, agitating gear 4 and heating-jacket 6, and connected to a copper condenser 7 and earthenware receiver 8 which is connected to a vacuum pump by a pipe 21.  The ribboning apparatus may comprise a feed tank 9, Fig. 2, discharging into a hopper 11 leading to rollers 12, 13 of soft wood rotating in a precipitating bath 16.  An endless belt 15 supports the ribbon and delivers it to a belt 19 which delivers the product to a soaking bath.  The endless belt supporting the shaped ribbon may pass upwardly through an inclined trough down which a stream of water is discharged.  The dried products are amenable to grinding and may be used for the preparation of solutions for artificial silk, films, lacquers, &c.  According to the second Provisional Specification, the diluted dope may be run into flat-bottomed vessels or trays containing or submerged in water.