GB461593A - Improvements in and relating to processes for the manufacture of foils or films of cellulose tri-acetate - Google Patents

Abstract

<PICT:0461593/IV/1> <PICT:0461593/IV/2> An acetylation solution of cellulose tri-acetate, in which the catalyst has been neutralized immediately after completion of acetylation, is cast in an aqueous precipitating bath containing 7--20 per cent of acetic acid, and the film or foil thereby obtained is then washed and dried. Cellulose acetates containing an acetyl content of not less than 59 per cent calculated as acetic acid are used. Preferably the solution is subjected to stabilization before being used for film formation, in order to maintain the solution in liquid condition and avoid premature coagulation. Stabilization may be effected by heating the solution for several hours at temperatures above 20 DEG C. and preferably at 80--100 DEG C. after neutralizing the catalyst. The precipitating bath is preferably kept below 30 DEG C. Slit-casting apparatus may be used and the film-forming solution may contain more than 10 per cent of cellulose tri-acetate. The film may be stretched during coagulation. Softening agents such as triphenyl phosphates or tributyl phosphates may be added to the film-forming solution in amount up to 100 per cent of the cellulose acetate. In an example, an acetylation solution containing sulphuric acid as catalyst is treated for neutralization of the catalyst and the excess of anhydride is eliminated by addition of methylene chloride or formic acid. The viscosity of the solution is controlled by heating to 30--50 DEG C. during filtration, de-aeration, and casting. The solution is extruded under pressure through a slit into a cooled coagulation bath. After the first washing, the film is partially dried, washed again, and finally dried. Softening agents such as glycerine, di-glycerine, and glucose, may be added to the washing baths. The film may be washed with methyl alcohol, benzol or other organic liquid prior to the drying by heat. As shown in Figs. 1 and 2, a film is formed by casting the cellulose ester solution from the device 10 on to roller 9 in the precipitating bath 1 containing 7--20 per cent acetic acid solution. The precipitated film passes over rollers 11--14, then into a washing bath 2 over rollers 15--19, into a second washing bath 3, and then into a drying chamber 4 over heated drying rollers 25--31. Regulated quantities of air are passed through chamber 4 by means of a fan. The partially dried film then passes from roller 32 into a bath 5 in which acid brought to the film surface by the drying process is washed out, and then into a bath 6 containing water or methyl alcohol. This bath may also contain a softening agent and dissolved pigment. The film then passes to the drying chamber 7 and finally to a frictionally driven winding device 8. If desired, the washing liquid may flow in counter current to the film; the individual baths may be arranged one below another and connected with the precipitating bath by pipes and valves.