Patent GB-674993-A (GB)
Title: Cellulose esters
IPC: C08B3/16
Filed: 1949-10-11
Abstract: Mixed cellulose esters of two mono-carboxylic acids are made by the action on the cellulose of the anhydrides of the acids in the presence of an acid esterification catalyst by a process wherein the cellulose is pretreated with the more reactive esterifying acid and 1-10 per cent (on the weight of the cellulose) of the esterification catalyst, and the pretreated cellulose is then reacted firstly with the anhydride of the less reactive esterifying acid in the presence of sufficient of an inert solvent to dissolve the mixed cellulose ester eventually formed, and secondly, after the reaction has proceeded so far that the evolution of heat has substantially ceased, with the anhydride of the more reactive esterifying acid, the reaction being continued until a substantially clear solution is formed.  The process is particularly intended for making mixed cellulose esters of acetic acid or propionic acid and a less reactive monocarboxylic acid containing at least 0.8, and especially one or more esterifying groups corresponding to the less reactive acid for each anhydro-glucose unit of the cellulose.  Examples of mixed cellulose esters which may be made according to the invention are cellulose acetate propionate, cellulose acetate n - butyrate, cellulose propionate n-butyrate, cellulose acetate acrylate, cellulose acetate methacrylate, cellulose acetate crotonate and cellulose propionate crotonate.  The cellulose may be in the form of cotton linters or a purified wood pulp and the amount of the esterifying acid used for the pretreatment is preferably not more than the weight of the cellulose.  The preferred esterification catalyst is sulphuric acid.  An inert diluent may be mixed with the esterifying acid used in the pretreatment of the cellulose and may be a solvent for the mixed cellulose ester, for example nitromethane, nitroethane, nitropropane, nitrobenzene, chlorobenzene or methylene chloride, nitromethane being preferred. The pretreatment is preferably carried on for 0.25-2 hours at 30-50 DEG  C.  In the manufacture of cellulose acetate crotonate the amount of crotonic anhydride employed is preferably 1.5-3 times the weight of the cellulose, and the duration of the first stage of the esterification may be 30-50 minutes.  The second stage of the esterification is then carried out by adding acetic anhydride, preferably in amount 1-3 times the weight of the cellulose, after pre-cooling it, e.g. to 0-10 DEG  C.  It is usually desirable that the total amount of both anhydrides should be somewhat in excess of that required to esterify all the hydroxyl groups of the cellulose.  If it is desired to obtain an ester free from, or containing only a very small proportion of, free hydroxyl groups, e.g. less than 0.15 hydroxyl group for each anhydro-glucose unit of the cellulose, the primary ester may be precipitated by means of water after hydrolysing unreacted anhydride and neutralizing the catalyst, e.g. by means of an excess of aqueous sodium carbonate solution.  The solution is preferably diluted with about its own volume of acetone before the addition of water, as the ester is then precipitated by the water in a useful soft fibrous form.  If a product having a higher content of free hydroxyl groups is required, the ester may be ripened in the solution, preferably at 35-85 DEG  C., after adding more than enough water to hydrolyse any remaining anhydride, but not sufficient to precipitate any of the ester, and before the catalyst is neutralized; the ripened ester may be precipitated by means of water, if desired after adding acetone. In an example (in which all the parts are by weight) 100 parts of cotton linters are pretreated by stirring for about 2 hours at 25 DEG C. with 100 parts of glacial acetic acid and 3 parts of sulphuric. The mixture cooled to about 7 DEG  C. is then mixed with a mixture of 300 parts crotonic anhydride and 300 parts nitromethane cooled to about 5 DEG  C.  The temperature of the whole is allowed to rise in 15 minutes to 36 DEG  C., at which temperature it is maintained for about 25 minutes.  Acetic anhydride (150 parts) cooled to 10 DEG  C. is then added over about 15 minutes and the reaction is continued for a further 80 minutes, the mixture being cooled sufficiently to prevent the temperature from rising above about 38 DEG  C.  The mixture is then cooled to 25 DEG  C. and unreacted anhydride is hydrolysed by adding enough cold aqueous acetic acid.  Part of the reaction mixture is separated and the sulphuric acid catalyst contained therein is neutralized with an excess of aqueous sodium carbonate, acetone in amount equal to the volume of the reaction mixture is added and the cellulose acetate crotonate is precipitated by the addition of water.  The cellulose acetate crotonate obtained contains 1.01 crotonyl groups, 1.88 acetyl groups and 0.11 free hydroxyl group for each anhydro-glucose unit. The remaining portion of the reaction mixture is ripened at 60 DEG  C. for about 4 hours and the mixed ester precipitated after neutralizing the acid catalyst and adding acetone as above.  The ester contains 1.01 crotonyl groups, 1.73 acetyl groups and 0.27 free hydroxyl group for each anhydro-glucose unit.  Examples are given also of the preparation of cellulose acetate n-butyrate, and acetate propionate.  Specification 578,504 is referred to.