United States Patent Office Patented Feb

2,923,706 United States Patent Office Patented Feb. 2, 1960 2 : ployed as catalyst, and the whole of this amount is uni formly distributed throughout the cellulose in the course 2,923,706 of a pretreatment in which the main pretreatment agent MANUFACTURE OF CELLULOSEESTERS OF . is an aliphatic acid containing up to 4 carbon atoms in ALPHATIC ACDS the molecule, especially acetic acid. It has been found Norman B. Campbell, Drummondville, Quebec, and Lau that amounts of sulphuric acid below 1%, if introduced rent Berthiaume, Marcotte Village, Quebec, Canada, into the cellulose in this way, are capable of catalysing assignors to Canadian Celanese Limited, Montreal, the esterification so effectively that smooth substantially Quebec, Canada, a body politic and corporate gel-free solutions are readily obtained. At the same time, No Drawing. Application June 22, 1955 O as a result of the small, amount of sulphuric acid present, Serial No. 517,372 the esterification temperature can be allowed to rise above 50 or 60° C. without causing undue degrada 6 Claims. (CI. 260-229) tion of the cellulose molecule with consequent loss of viscosity. Indeed it is possible to use temperatures This invention relates to the manufacture of cellulose 5 so high that the esterification can be completed in a esters of aliphatic acids, especially esters of lower ali matter of minutes, e.g. in 5-15 minutes without causing phatic acids (i.e. aliphatic acids containing 2-4 carbon undue Viscosity loss; this is of course a great advantage, atoms in the molecule) such as cellulose acetate by a since it enables esterification to be carried out as a process of the solution type in which a lower aliphatic continuous process. - acid, e.g. acetic acid, is used to dissolve the cellulose 20 In describing the invention in more detail it will be ester as it is formed and sulphuric acid is used as cata convenient to refer particularly to the production of lyst. cellulose triacetate. It will be understood, however, that When cellulose acetate, for example, is made by proc the information given will apply also mutatis mutandis esses of this type, the ester as first obtained normally to the production of cellulose esters of other aliphatic contains a proportion of sulphate groups which, unless 25 acids, especially cellulose propionates, butyrates, acetate substantially completely removed, render it unstable propionates and acetate butyrates in which the cellulose towards heat. If an acetone-soluble cellulose acetate is is substantially completely esterified in the sense of con required, the cellulose acetate first formed (known as taining substantially less than 0.25, for instance less than the primary cellulose acetate) is partly hydrolysed so as 0.05 free hydroxyl groups for each anhydroglycose unit. to split off a proportion of its acetyl groups and give 30 It will be understood also that the cellulose triacetate or it an acetyl value of say 52.5-56.5% reckoned as acetic other ester first produced can if desired be ripened to acid. In this process, which is termed ripening, most or modify its solvent properties, giving for example an substantially all of the sulphate groups are also split off, acetone-soluble cellulose acetate. • so greatly increasing the stability of the cellulose acetate. The sulphuric acid is uniformly distributed throughout On the other hand, when it is desired to produce a 35 the cellulose in admixture with glacial or highly concen cellulose triacetate (using the term in its usual connota trated aqueous acetic acid as the main pretreatment acid. tion to include cellulose acetates of acetyl value of about If desired, part of the acetic acid, but preferably not 59-62.5%) and especially cellulose triacetate having an more than half, may be replaced by acetic anhydride. acetyl value closely approaching the maximum of 62.5%, The acid mixture may be sprayed onto the cellulose, e.g. between 61 and 62.5%, little or no hydrolysis of 40 which is then preferably stirred or tumbled throughout acetate groups is permissible, and thus the concomitant the pretreatment, or the cellulose may be immersed in splitting off of sulphate groups does not take place, or the acid mixture. These operations are preferably carried at least not to the same extent. In consequence the pro out at 25-40 C. The cellulose onto which the pre duction of a heat-stable cellulose triacetate of acetyl treatment acids have been sprayed or which is immersed value approaching the maximum is a matter of greater in the acids is held for a time sufficient to allow uniform difficulty. 45 penetration of the pretreatment acids to all parts of the One method by means of which the proportion of sul- ". cellulose. Usually this penetration will be completed phate groups in cellulose acetate can be reduced is to in an hour or less, though the pretreatment may be car employ only a small quantity of sulphuric acid as cata ried on for a longer time if desired. lyst in the production of the cellulose acetate. It is The uniform penetration of the cellulose by the pre found, however, that when acetic acid is used as the 50 treatment acids may be facilitated and speeded up by. solvent in the acetylation, reduction of the amount of first soaking the cellulose in an aqueous liquid, e.g. water } catalyst used tends to prevent a smooth gel-free solu or a dilute aqueous acetic acid, which may contain part tion being obtained. For this reason it is usual to em of the Sulphuric acid to be used in the pretreatment and ploy quite a high proportion of sulphuric acid, for ex acetylation. After such a treatment with an aqueous ample about 10-16%. (Here and throughout this speci 55 liquid the cellulose may if desired be squeezed, so as to. fication amounts of reagents and catalyst used in the - remove part of the liquid before adding the pretreat esterification of cellulose are expressed in terms of the ment acids. Similarly when the cellulose has been dry weight of the cellulose unless otherwise stated.). - soaked in the pretreatment acids it may be squeezed before It is an object of the present invention to provide a the acetylation proper so as to reduce the amounts of method of acetylating cellulose using acetic acid to dis 60 acetic acid and Sulphuric acid on the cellulose to the de solve the cellulose acetate formed and also a very small : sired values. amount of sulphuric acid as catalyst, in which a clear The amount of Sulphuric acid employed in the pre Substantially gel-free solution is obtained. treatment and thus in the subsequent acetylation, includ It is a further object of the invention to provide a ing any remaining from a preliminary treatment with an method whereby a cellulose triacetate of good heat-sta 65 aqueous liquid as described above, may be as low as bility can be obtained, especially a cellulose triacetate 0.01% and is preferably between about 0.02% and 0.75% having an acetyl value well above 61% and even above of the dry weight of the cellulose, amounts between 62% reckoned as acetic acid. - - about 0.02% and 0.5% being particularly useful. Other objects of the invention will appear hereinafter. It has been found that the viscosity of the cellulose In the esterification of cellulose in accordance with the 70 triacetate obtained is directly dependent on the rate of invention only a small amount of Sulphuric acid is em acetylation, regardless of the combination of acetylation 2,923,708 3 4. temperature and amount of sulphuric acid employed. EXAMPLE I The amount by which the temperature of the acetylation 15 parts of wood pulp were soaked for 1 hour at 25 mixture rises depends of course on a number of factors C. in water containing sulphuric acid in concentration including the amounts of acetic acid and anhydride pres 0.023%, and the pulp was then first rinsed and afterwards ent in relation to the amount of cellulose, the presence soaked in 98% aqueous acetic acid containing Sulphuric or absence of small amounts of water in the pretreated acid in concentration 0.023%. After about 1 hour the cellulose when it is brought into contact with the acetyla bulk of the liquid was run off and the pulp Squeezed tion liquor, and the degree of cooling or heating, if any, until it contained about 3 times its dry weight of liquid. employed during the acetylation, as well as the amount This concluded the pretreatment. The pretreated pulp of sulphuric acid present; thus the viscosity of the prod O was then added to about 215 parts of a mixture of acetic uct can be controlled for example by so choosing the anhydride and acetic acid containing 96% of anhydride, initial temperature of the acetylation liquor that the tem and the mixture was stirred until a clear solution had Eure during the acetylation rises to the appropriate formed. The reaction conditions were varied so as to evel. allow the temperature to rise to different levels in differ The relatively high acetylation temperatures used in 15 ent runs. From the clear solution the triacetate was accordance with the invention have important advan precipitated by mixing in dilute acetic acid in the known tages. One of these is that at these temperatures acetic way, and the precipitate was washed and dried. Three acid dissolves cellulose triacetate more readily than at typical runs gave the figures shown in Table I. In all the temperatures usually employed, and also gives solu cases the cellulose triacetate had a good heat stability, tions having a lower viscosity at any particular concen 20 even though it was given no separate stabilisation treat tration.

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