Patent GB-207562-A (GB)
Title: Improvements in or relating to the manufacture of cellulose acetates
IPC: D01F2/30
Filed: 1922-05-24
Abstract: Cellulose acetates having viscosity characteristics of an order higher than the acetates prepared by the processes of Specifications 6463/15, 14101/15, 100,009, and 101,555, and more especially having viscosities double or more of such acetates, are produced generally by increasing the proportion of acetic acid or other solvent in the reaction mixture to six times or more the weight of the cellulose or its near conversion product taken. Employing such larger proportion of acetic acid &c. and using sulphuric acid or other strong condensing agent, while maintaining the conditions as regards strong cooling both of the reaction mixture and in the early part of the acetylation as defined in Specifications 6463/15 and 14101/15, acetates having double or higher viscosities are obtained; alternatively, acetates having viscosities up to double and beyond are obtained if the proportion of acetic acid &c. is increased up to 8 to 20 or more times the weight of the cellulose and provided that the temperature of the reaction mixture prior to the acetylation is selected so that the quantity of heat required to raise the mixture from this temperature to the mean reaction temperature is greater than in the process of Specifications 6463/15 and 14101/15. A compensation is to be made in cases where very large proportions of acetic acid and very small proportions of condensing agent are employed, in which cases the pre-cooling may be dispensed with, and in extreme cases the temperature of the reaction mixture may be slightly raised. When using weak or very weak condensing agents, acetates of the higher viscosity characteristics are obtained while employing the increased proportions of acetic acid, provided that the conditions are selected in such a manner that after the initiation of the reaction the temperature remains substantially constant or rises only within narrow limits; the temperature of the reaction in such cases varies with the character of the condensing agent and the proportion employed, and it may be necessary in some cases substantially to raise the temperature of the reaction mixture. It is preferred to use a restricted quantity of acetic anhydride, preferably not more than three times the weight of cellulose taken; suitable proportions are 1.8-2.5 times such weight. The action may be carried out in suspension, in which case the conditions above enumerated are observed, and in addition a suitable proportion of a non-solvent such as benzol added to the mixture. Suitable strong condensing agents are sulphuric acid and chlorsulphonic acid; examples of weak condensing agents are nitrosulphonic acid, pyrosulphuryl and sulphuryl chlorides, bisulphates containing free sulphuric acid, ammonium bisulphate, sodium bisulphate containing no free sulphuric acid; and of very weak condensing agents, ammonium, copper, methylamine and zinc sulphates.  The acetic acid may be replaced wholly or in part by other solvents or diluents such as chloroform, tetrachlorethane, carbon tetrachloride, trichlormethylene, or benzol.  In carrying out the process the cellulose may be introduced into a mixture of the acetic acid, acetic anhydride and condensing agent, or the latter preferably diluted with acetic acid or acetic anhydride may be added subsequently; alternatively, the cellulose may be treated with acetic acid and the condensing agent, and the anhydride subsequently added-this modification is particularly advantageous when sulphuric acid is employed in proportions of 3 per cent or less of the weight of the cellulose; or the cellulose may first be treated with acetic acid, the acetic anhydride then added, and subsequently the condensing agent.  The primary cellulose acetates may be employed directly in the manufacture of artificial silk, celluloid-like masses, films, varnishes, etc.; or they may be first subjected to a further or secondary treatment or reaction in order to modify their solubility characteristics; suitable processes for this purpose are described in Specifications 20977/11 and 20852/12; and such further treatment may be carried out in the original reaction mixture or on the isolated products, and in all cases any excess of acetic anhydride must be destroyed as by the addition of lactic acid &c., and if in order to quicken the reaction the temperature is raised say to 50-100 DEG C. then the condensing agent present must first be neutralized to neutral sulphate or neutral salt. Specifications 2026/07, 14271/10, and 24382/10 also are referred to.