Patent GB-605912-A (GB)
Title: Improvements in the manufacture and purification of cellulose esters of higher fattyacids
IPC: C08B3/22
Filed: 1946-01-23
Abstract: The purification of higher fatty acid esters of cellulose which are contaminated with free higher (i.e. normally solid) fatty acid comprises the extraction of the free higher fatty acid by means of a solvent comprising an ether or hydrocarbon or mixture of hydrocarbons of boiling point between 60 DEG  and 200 DEG  C.  Examples of esters which may be purified by this process are those such as cellulose laurate, stearate and palmitate, mixed esters comprising radicles of these acids together with formyl, acetyl, propionyl, and butyryl radicles, and also cellulose ether esters of higher fatty acids.  The extent of the contamination is considerable (20-30 per cent of the weight of the ester) and this cannot be distilled off and is difficult to extract.  Most solvents are unsuitable since the extracted acid is usually contaminated with small amounts of impurities of a cellulosic nature, so that if the recovered acid be converted to its anhydride and used again, the resulting ester suffers from lack of clarity.  If an ether be selected as solvent, it should preferably be an unsubstituted lower aliphatic ether such as diethyl ether (which, however, must be used between 15-30 DEG  C.). It is generally more convenient to employ a mixture consisting mainly or wholly of aliphatic hydrocarbons of boiling point between 70 DEG and 150 DEG  C., such as petrol of any ordinary grade.  The ester to be purified should be given a series of washes with fresh solvent substantially free from acid.  The first few washes may, however, be carried out using a systematic extraction in which the solvent is used to extract successive batches of ester of increasing fatty acid content, either batchwise or continuously in counter-current. The amount of solvent used in each extraction may be 1.5 to 2.5 times the weight of wet crude ester.  After substantially all the free acid has been removed, the ester may be freed from solvent by, for example, heating in a current of air.  The solution of free acid usually contains small quantities of coloured and metallic substances which may be removed before recovering the acid by an adsorbing agent, e.g. fuller's earth.  The solvent is then distilled off in an enamel-lined still, the last traces being removed by live steam.  The pure dry molten acid may be run off and converted, if desired, to the anhydride for further esterification.  In an example, 74 parts by weight of acetone-soluble cellulose acetate is added to a solution of 88 parts of stearic acid in 433 parts of ethylene dichloride containing as catalyst 0.2 parts of perchloric acid or 0.18 parts of zinc chloride or 0.36 parts of hydrogen chloride.  When a clear solution is obtained, the cellulose acetate stearate is precipitated by the addition of boiling water (which also vaporizes the ethylene dichloride) and the precipitate drained until a wet crude ester containing 1 part of solid to 4 of liquid is obtained.  The ester is then washed batchwise with petrol with stirring at 50 DEG  C., each wash lasting for 15 minutes.  The petrol used in the first wash has already been used for five previous washes, the second batch four times, and so on, the sixth and succeeding washes being performed with petrol which is initially free from stearic acid.  After the tenth wash, the ester is dried in a current of heated air.  The extracts from the first, seventh, eighth, ninth and tenth washes are combined, passed through a bed of fuller's earth and distilled from an enamel still.  The last traces of petrol are removed by the injection of live steam into the still base.  The stearic acid residue is sufficiently pure to be converted into stearic anhydride for further esterification. Specification 543,311 is referred to.