Patent GB-814824-A (GB)
Title: Purification of solutions of carboxylic acid esters of cellulose
IPC: C08B3/22
Filed: 1956-06-27
Abstract: Solutions of carboxylic acid esters of cellulose in water-immiscible solvents are purified by subjecting the ester solution to solvent extraction with an aqueous medium in which is dissolved an electrolyte, sufficient of the water-immiscible solvent being present to prevent precipitation of the cellulose ester.  Water-soluble materials are thereby removed from the cellulose ester solution and dissolve in the aqueous electrolyte solution.  The process is applicable to cellulose esters of fatty acids containing 2-4 carbon atoms in the molecule, especially cellulose acetates having an acetyl value above 60 per cent, reckoned as acetic acid, and acetone-soluble cellulose acetates. It is also applicable to cellulose benzoate.  The term "water-immiscible solvent" is used to include solvents which are soluble in water to an appreciable but small extent only.  The solvent may be methylene chloride, tetrachloroethane, trichlorethane, chloroform, nitromethane, m-cresol, cyclopentanone, pyrrole, furfural or trichlorethanol.  The electrolyte dissolved in the aqueous medium is preferably a salt having at least one inorganic component and especially a salt forming polyvalent (including divalent) ions in aqueous solution, preferably polyvalent anions or better still both polyvalent anions and polyvalent cations.  The valency of the anion or of both the anion and the cation should preferably be higher than 2. The concentration of the electrolyte in the aqueous extracting medium may be about 0.1 to 1 gram-ion per litre, assuming the electrolyte to be completely ionized.  The electrolyte may be ammonium dihydrogen phosphate, sodium citrate, aluminium sulphate, ammonium acetate, magnesium acetate, calcium acetate, magnesium sulphate, sodium acetate, sodium sulphate, calcium chloride or aluminium chloride.  The aqueous extraction medium may be water, or a mixture of water with a water-miscible organic solvent such as methanol, ethanol, n-propanol, iso-propanol, ethyl acetate, pyridine, acetone, or methyl acetate.  In the preferred embodiment of the invention, the solution of the cellulose ester in the water-immiscible solvent is formed by esterifying cellulose with a carboxylic acid anhydride in the presence of sufficient of a water-immiscible solvent to dissolve the cellulose ester formed. There may be present during the extraction with the aqueous medium a small proportion (preferably about 0.001 to 0.1 per cent by weight of the cellulose ester solution) of a surface-active agent to reduce the tendency of the solution to adhere to solid surfaces with which it comes into contact.  The surface-active agent may be sodium lauryl sulphate, alkyl-benzene sodium sulphonates, alkyl-naphthalene sulphonates, polyoxyethylene esters of fatty acids or alkyl aryl polyethylene glycol ethers.  In an example, a cellulose esterification solution which has been diluted with further methylene chloride to give a solution containing 11 per cent of cellulose acetate, 16.3 of acetic acid, 0.14 of water and 0.045 of perchlorate ions is extracted with five times its volume of a 2.5 per cent aqueous solution of magnesium sulphate.  The extracted solution contains only 0.30 per cent of water, whereas when the extraction is carried out with water containing no electrolyte or other solute the extracted solution contains 0.94 per cent of water.  Specifications 779,802 and 789,210 are referred to.