Cellulose esters hydrolysis

Cellulose triacetate is obtained by the esterification of cellulose (qv) with acetic anhydride (see Cellulose esters). Commercial triacetate is not quite the precise chemical entity depicted as (1) because acetylation does not quite reach the maximum 3.0 acetyl groups per glucose unit. Secondary cellulose acetate is obtained by hydrolysis of the triacetate to an average degree of substitution (DS) of 2.4 acetyl groups per glucose unit. There is no satisfactory commercial means to acetylate direcdy to the 2.4 acetyl level and obtain a secondary acetate that has the desired solubiUty needed for fiber preparation. 

Fig. 7. Combined sulfur during preparation of cellulose acetate hydrolysis of sulfate and esters (6). Acetylation schedule A, mixer charged with linters and acetic acid B, minor portion of catalyst added C, began cooling to 18°C D, acetic anhydride added and continued cooling to 16°C E, significant portion...

Barrier Layers. Depending on composition, barrier layers can function simply as spatial separators or they can provide specified time delays by swelling at controlled rates or undergoing reactions such as hydrolysis or dissolution. Suitable barrier materials include cellulose esters and water-permeable polymers such as gelatin and poly(vinyl alcohol) (see Barrier polymers). 

C. Schwarzinger, I. Tanczos and H. Schmidt, Pyrolysis gas chromatography/mass spectrometry and thermally assisted hydrolysis and methylation (THM) analysis of various cellulose esters, J. Anal. Appl. Pyrol., 58 59, 513 523 (2001). 

General protease, a-amylase, and exoglucanase activities were estimated using hide powder-, amylose-, and celliilose-azure substrates, respectively, as described earlier (49). Here, standard curves were developed for the hydrolysis of each azure-linked substrate by standard enzymes of known activity. By this method, one cellulose-azure hydrolysis unit corresponds to one filter paper unit, one unit of hide powder-azure activity corresponds to the hydrolysis of 1.0 nmole of iV-benzoyl-L-tyrosine ethyl ester (BTEE) per min, and one amylose-azure unit of activity corresponds to the hydrolysis of 1.0 nmole of maltose from starch per 30 min. 

Most commercial preparations of cellulose esters still follow, basically, the methods described by Franchimom and Miles—esterification with sulfuric acid catalyst followed by hydrolysis. The principal steps in this process are shown in Fig. I. 

Benzyl Alcohol Phenylmethanol or Hydroxytol-uene (called l -Oxy-T-methyl-benzol and benzyl chloride with Na or K carbonate in soln or by other methods. There are two commercial products technical grade and "FFC (free from chlorine) grade(Ref 3). Toxicity and fire hazard are discussed in Ref 5. It is used extensively in many branches of the chem industries, such as, for the manuf of esters(acetic, benzoic, sebacic, etc), as a solvent for cellulose esters ethers, etc. During WW I, it was used in a "dope for airplane fabrics... 

Most experiments were performed with cotton or cotton linters as highly crystalline celluloses. Table I shows conditions leading to complete dissolution. A minimum amount of an acid which forms a cellulose ester (sulfuric or trifluoromethylsulfuric acid) (Entries 5 7-14) is necessary for the reaction. The dissolution is accelerated by a temperature increase (Entries 10-12 13, 14) and leads to water-soluble cellulose acetate hydrogensulfate. Whereas this primary hydrolysis can be achieved within 1-5 min, the deesterification and complete hydrolysis of the soluble cellulose derivative proved to be much more difficult. This is in contrast to the generally accepted view that the main resistance to the hydrolysis of cellulose lies in the crystalline nature or low accessibility determining the heterogeneous first step of the reaction. 

Cellulose esters (e.g., cellulose triacetate, cellulose diacetate, cellulose propionate, and cellulose butyrate) are prepared by initially treating cellulose with glacial acetic acid (or propionic acid and butyric acid) followed by the corresponding acid anhydride with a trace of strong acid as a catalyst in chlorinated hydrocarbon. Complete esterification reactions result in the formation of a triester, which undergoes water hydrolysis to form a diester. Cellulose acetate alone or in combination with cellulose triacetate or cellulose butyrate is used as a semipermeable membrane for osmotic pumping tablets, primarily in controlled release systems. The permeability of the membrane can be further modulated by adding water-soluble excipients to the cellulose esters. 

In this book, he emphasized the importance of the microscopic and the submicroscopic structure of fibrous high polymers. The reactions of cellulose with water, aqueous alkalis, organic bases, ammonia, and strong salt solutions were all stressed. Special attention was given to various types of cellulose esters, to cellulose xanthate, and to the cellulose ethers. The oxidation of cellulose under a variety of conditions was described, as were the hydrolysis reactions. The latter included discussions on reversion and on the kinetics of acid hydrolysis. It is interesting to note that Heuser, who earlier had criticized the terms hydrocellulose and oxycellulose, and had... 

The possibility of prejaiing a cellulose ester and the simplest at lenecarboxylic acid-propiolic acid has l n inv gated. The data obtained (57) show that cellulose propiolates of hi degree of sul titu-tion cannot be prepared the reaction d cellulose with propiolyl chloride. The highest DS obtained was 0.12, probably because of the extreme instability of the acylating reagent used to dfect hydrolysis. 

This class of ceUulose derivatives has attracted the attention of investigators because of their specific properti, primarily tl ir increased hydrophobicity. There is, however, a seriom dmwback, tyjrical rf most silicon-containing cellulose esters prepared by the action of trialkyl-chlorosilanes, namely, the low stability of tl C-O-Si bond to hydrolysis. One way of overcoming this difficulty is to increase the length of the organosilicon radical. 

The tin content of the tributylstannylcarboxymethyl cellulose ester is practically unchanged on boiling in water for one hour. When subjected to hydrolysis with 0.1 Af solutions of HCl and NaOH for 1 h at 20 C, the tributylstannyl residues of this ester are completely split off. Thus, polymeric acylates ctf trialkyltin, like low-molecular-weight or-ganotin compounds of this type, are unstable to the action of aqueous solutions of acids and alkalis. 

The solubility restrictions that apply to the manufacture of the mixed esters are the same as those for the cellulose acetate, in that no soluble products are obtained by partial esterification. Hydrolysis of the esters in acid solution, however, yields uniform products showing gradually changing physical properties with increasing free hydroxyl content. The exact ratio of hydrolysis of acetyl to hydrolysis of propionyl or butyryl groups depends upon the composition of the hydrolysis solution. Thus, a cellulose acetate propionate hydrolyzed in acetic acid solution will retain a higher proportion of acetyl groups than would the same cellulose ester hydrolyzed in propionic acid. 

Polymer chemists have found the fully acetylated product, called a trlester, to be of limited value in the plastics and coatings industries. Some free hydroxyl groups along the polymer chain are necessary to effect solubility, flexibility, compatibility, toughness, etc. Acylation to a predetermined degree less than the triester is not feasible if a uniform, soluble product is desired. Therefore, the cellulose is fully acylated and then hydrolyzed back to the desired hydroxyl level. The hydrolysis reaction is relatively slow and may be controlled and terminated as required. Commercial cellulose esters may contain up to 5% by weight of hydroxyl obtained in this manner. 

It is required (usually as a condition of warranty) to minimize the rate of hydrolysis of the cellulose acetate ester. Cellulose acetate hydrolysis reduces the useful life of the membrane by increasing the flux and reducing the rejection of the membrane. 

Ethylcellulose. The cellulose ethers obtained by the alkylation of cellulose are important plastic materials. TTiey are more stable than the cellulose esters and are incapable of undergoing hydrolysis, which makes them more resistant to acid and alkalies. The alkyl ethers are, however, soluble to a considerable extent in waier and alkalies, more so than benzyl-cellulose but less than cellulose acetate. 

The most prominent cellulose ester produced on the industrial scale is cellulose acetate. The reaction is usually performed with acetic anhydride and with sulfuric acid as a catalyst. To minimize heterogeneities, acetylation is allowed to run nearly to completion, and subsequently partial ester hydrolysis is initiated by the addition of water until a desirable solubility is achieved that corresponds to a DS of about 2.5. Such higher acyl homologues as propanoyl or butanoyl exhibit more thermoplastic properties. Many specialized esters such as chiral (-)-menthyloxyacetates, furan-2-carboxylates, or crown-ether-containing acylates have been prepared on the laboratory scale and characterized by NMR spectroscopy. Various procedures have been applied, using anhydrides and acyl chlorides as acylating agents in combination with such bases as pyridine, 4-dimethylaminopyridine (DMAP), or iV,iV -carbonyldi-imidazole. The substitution pattern of cellulose acetates has also been modified by postchemical enzymatic deacetylation. Cellulose 6-tosylates have been used as activated intermediates for nucleophihc substitution to afford 6-amino-6-deoxy, 6-deoxy, or 6-deoxy-6-halo-celluloses. ... 

In the presence of water (moisture) the reverse reaction can occur and is indeed favored at ordinary conditions for organic acids. Consequently, cellulose esters from organic acids like acetic acid and higher homolog are prepared only by the removal of water as it is formed. The resulting product is moisture sensitive, the degree of which decreases with the progressive hydrocarbon nature of R. Therefore while cellulose acetate (R = -CH3) is susceptible to hydrolysis, cellulose propionate (R = -CHj-CHj-CHj) and cellulose butyrate [R = -(CHjlj-CHj] are hydrophobic, b. The formation of cellulose ethers, unlike that of cellulose esters, is not reversible. Cellulose ethers are therefore less sensitive to hydrolysis than cellulose esters. 

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