Patent GB-635668-A (GB)
Title: Improvements in the manufacture of cellulose esters
IPC: C08B3/22
Filed: 1948-02-24
Abstract: A solution of an organic ester of cellulose is filtered through a layer of needle-like crystals of magnesium sulphate or a sulphate of an alkaline-earth metal, during the manufacture or purification of the cellulose ester.  The solution of the ester, e.g. a carboxylic ester, may result from the esterification of cellulose in presence of sulphuric acid as catalyst and of a solvent of the ester.  After completion of esterification part or all of the sulphuric acid catalyst may be neutralized by means of a compound of magnesium.  The cellulose ester may be ripened in solution and prior to filtration, the solution may be diluted, preferably with an aqueous solution of the organic acid corresponding to the cellulose ester.  The acicular magnesium sulphate crystals may be formed in the solution by rapidly cooling the solution from a temperature above 60 DEG  C. to a temperature below 50 DEG  C. and then allowing the solution to stand, preferably for several hours, so as to give time for the crystals to grow. The requisite cooling may be effected by the addition of a cooled diluent, e.g. having a temperature of -15 DEG  to +20 DEG  C. When the acicular magnesium crystals have reached the desired size, the solution is passed through a filter, e.g. a plate and frame filter press, provided with fabric or fine wire mesh cloths or screens. The magnesium sulphate crystals are deposited on the cloths or screens to form the desired filter layer or bed.  The process may be applied to the production of cellulose acetate, propionate, butyrate, and other cellulose carboxylates.  In an example, cotton linters are pretreated with glacial acetic acid, sulphuric acid, and water, and are then esterified with a mixture of acetic anhydride, acetic acid, and sulphuric acid, the initial temperature being -10 DEG  C. and the peak temperature being 35 DEG  C. Water is added to convert the remaining anhydride to acetic acid, and most of the sulphuric acid is neutralized with a solution of magnesium acetate in aqueous acetic acid.  Sufficient water is added for ripening, which is effected at 100 DEG  C.  A further amount of magnesium acetate is added and the solution is diluted and cooled to just below 50 DEG  C. by adding 60 per cent of its weight of 40 per cent aqueous acetic acid previously cooled to -12 DEG .  The solution is allowed to cool further to 35 DEG  C. and allowed to stand for 6 hours to allow needle-like crystals of magnesium sulphate to form.  The solution is then filtered through stainless steel wire screen.  The cellulose acetate is precipitated from the filtered solution by adding a large amount of water and is washed neutral with demineralized water.  The product has an acetyl value of 56 per cent (calculated as acetic acid).  It dissolves in 95 per cent aqueous ethyl alcohol and in 95 per cent aqueous acetone to give solutions of very high clarity. Moulded products of high clarity and freedom from haze may be obtained.ALSO:A solution of a cellulose ester is filtered by passing it through a layer of needle-like crystals of an alkaline-earth metal sulphate or magnesium sulphate.  The solution may be an esterification solution and the magnesium or calcium sulphate may be formed during neutralisation of sulphuric acid used as catalyst. Acicular magnesium sulphate crystals may be formed in such a solution by cooling the solution rapidly from a temperature above 60 DEG  C. to one below 50 DEG  C. and then allowing the solution to stand, preferably for several hours, so as to give time for the crystals to grow.  The solution containing the crystals is then passed through a filter e.g. a plate and frame filter press, provided with fabric or fine wire mesh cloths or screens.  The crystals are deposited on the cloths or screens to form the desired filter layer or bed.