Cellulose acetate preparation

Evidence for a chemical linkage between cellulose and mannan, at least in softwood pulps, continues to accumulate. In this connection, Steinmann and coworkers have observed that, during the normal preparation of cellulose acetate, a considerable part of the xylan may be lost (30 to 60%), whereas the mannan content is not greatly affected. These authors suggest that this may indicate that the mannan is bound to the anhydroglucose chain. According to these workers, certain differences in properties of cellulose acetate prepared from different samples of cellulose can be explained if a mannan-to-cellulose bond actually does exist. However, it must be realized that no definitive proof has been presented of a mannan-to-cellulose linkage in any preparation of wood cellulose. 

The first organic ester of cellulose was cellulose acetate, prepared by Schutzenberger in 1865 by heating cotton and acetic anhydride to about 180 °C in a sealed tube until the cotton dissolved (J ). In 1879, Franchimont acetylated cotton at lower temperatures with the aid of a sulfuric acid catalyst (2). Miles, in 1903, described the preparation of partially hydrolyzed cellulose acetate, which was easily distinguished from the fully acetylated... 

DMSO/TBAF is highly efficient as a reaction medium for the homogeneous esterification of cellulose by transesterification and after the in situ activation (see below) of complex carboxylic acids. The acylation using acid chlorides and anhydrides is limited because the solution contains a certain amount of water caused by the use of the commercially available TBAF trihydrate and the residual moisture in the air-dried polysaccharides. Nevertheless, this system has shown a remarkable capacity for the esterification of lignocellulosic mataials, for example, sisal fibres, which contain about 14 per cent hemicellulose [28]. The DS values of cellulose acetate prepared from these fibres with acetic anhydride in mixtures of DMSO/TBAF were found to decrease with increasing TBAF concentration from 6 to 11 per cent (Table 16.4), due to the increased rate of hydrolysis both of the anhydride and the ester moieties. 

Cao, Y., Wu, J., Meng, T., Zhang, J., He, J., Li, H. Zhang, Y. (2007). Acetone-Soluble Cellulose Acetates Prepared by One-Step Homogeneous Acetylation of Comhusk Cellulose in an Ionic Liquid l-Allyl-3-Methylimidazolium Chloride (AmimCl), Carbohydrate Polymers, 69, pp. 665-672... 

CH2CI2. A colourless liquid with a chloroform-like odour b.p. 4I°C. Prepared by heating chloroform with zinc, alcohol and hydrochloric acid manufactured by the direct chlorination of methane. Decomposed by water at 200°C to give methanoic and hydrochloric acids. Largely used as a solvent for polar and non-polar substances, particularly for paint removal (30%), dissolving cellulose acetate and degreasing (10%). It is more stable than carbon tetrachloride or chloroform especially towards moisture or alkali. It is somewhat toxic. U.S. production 1981 280000 tonnes. 

McjC = CHCOCH3. Colourless liquid b.p. 129"C, with a strong peppermint-like odour. Prepared by distilling diacetone alcohol in the presence of a trace of iodine. Converted to phorone by heating in propanone with dehydrating agents such as sulphuric acid. It is a solvent For cellulose acetate and ethyl-cellulose and other polymers. 

Highly colored, they have been used to dye cellulose acetate (552) and acrylic fibers (553). Cationic dyes prepared from 2-azothiazoles by simple alkylation on the ring nitrogen (552) have been used increasingly with the introduction of polyacrylonitrile fibers with basic sites that can be colored with such dyes (554). 

Cellulose Acetate, Propionate, and Butyrate. Cellulose acetate is prepared by hydrolyzing the triester to remove some of the acetyl groups the plastic-grade resin contains 38 to 40%... 

The predominant cellulose ester fiber is cellulose acetate, a partially acetylated cellulose, also called acetate or secondary acetate. It is widely used in textiles because of its attractive economics, bright color, styling versatiUty, and other favorable aesthetic properties. However, its largest commercial appHcation is as the fibrous material in cigarette filters, where its smoke removal properties and contribution to taste make it the standard for the cigarette industry. Cellulose triacetate fiber, also known as primary cellulose acetate, is an almost completely acetylated cellulose. Although it has fiber properties that are different, and in many ways better than cellulose acetate, it is of lower commercial significance primarily because of environmental considerations in fiber preparation. 

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. 

The solution process consists of four steps preparation of cellulose for acetylation, acetylation, hydrolysis, and recovery of cellulose acetate polymer and solvents. A schematic of the total acetate process is shown in Figure 9. 

Fig. 12. Cellulose acetate cigarette filter tow (a) as supplied and (b) "opened" for cigarette-filter rod preparation.

Membrane stmcture is a function of the materials used (polymer composition, molecular weight distribution, solvent system, etc) and the mode of preparation (solution viscosity, evaporation time, humidity, etc). Commonly used polymers include cellulose acetates, polyamides, polysulfones, dynels (vinyl chloride-acrylonitrile copolymers) and poly(vinyhdene fluoride). 

Cellulose acetate [9004-35-7], prepared by reaction of cellulose with acetic anhydride, acetic acid, and sulfuric acid, is spun into acetate rayon fibers by dissolving it in acetone and spinning the solution into a column of warm air that evaporates the acetone. Cellulose acetate is also shaped into a variety of plastic products, and its solutions are used as coating dopes. Cellulose acetate butyrate [9004-36-8], made from cellulose, acetic anhydride, and butyric anhydride in the presence of sulfuric acid, is a shock-resistant plastic. 

Cellulose Deriva.tives, Cellulose can be derivatized to make both water-soluble gums and hydrophobic polymers. The preparation of the hydrophobic cellulose esters (qv), cellulose acetates and cellulose nitrates, has already been mentioned. The water-soluble cellulose derivatives are cellulose ethers (qv). 

Cellulose acetate [9004-35-7] is the most important organic ester because of its broad appHcation in fibers and plastics it is prepared in multi-ton quantities with degrees of substitution (DS) ranging from that of hydrolyzed, water-soluble monoacetates to those of fully substituted triacetate (Table 1). Soluble cellulose acetate was first prepared in 1865 by heating cotton and acetic anhydride at 180°C (1). Using sulfuric acid as a catalyst permitted preparation at lower temperatures (2), and later, partial hydrolysis of the triacetate gave an acetone-soluble cellulose acetate (3). The solubiUty of partially hydrolyzed (secondary) cellulose acetate in less expensive and less toxic solvents such as acetone aided substantially in its subsequent commercial development. 

Cellulose esters of aromatic acids, aUphatic acids containing more than four carbon atoms and aUphatic diacids are difficult and expensive to prepare because of the poor reactivity of the corresponding anhydrides with cellulose Httle commercial interest has been shown in these esters. Of notable exception, however, is the recent interest in the mixed esters of cellulose succinates, prepared by the sodium acetate catalyzed reaction of cellulose with succinic anhydride. The additional expense incurred in manufacturing succinate esters is compensated by the improved film properties observed in waterborne coatings (5). 

Mixed cellulose esters containing the dicarboxylate moiety, eg, cellulose acetate phthalate, have commercially useful properties such as alkaline solubihty and excellent film-forming characteristics. These esters can be prepared by the reaction of hydrolyzed cellulose acetate with a dicarboxyhc anhydride in a pyridine or, preferably, an acetic acid solvent with sodium acetate catalyst. Cellulose acetate phthalate [9004-38-0] for pharmaceutical and photographic uses is produced commercially via the acetic acid—sodium acetate method. 

Starting cellulose, prepared by deacetylation of commercial, medium viscosity cellulose acetate (40.4% acetyl content). 

Cellulose chloroacetates (30) and aminoacetates (30,31), acetate sorbates (32), and acetate maleates (33) have been prepared but are not commercially important. These esters are made from hydrolyzed cellulose acetate with the appropriate anhydride or acid chloride in pyridine. 

Mixed esters containing the dicarboxylate moiety, eg, cellulose acetate phthalate, are usually prepared from the partially hydroly2ed lower aUphatic acid ester of cellulose in acetic acid solvent by using the corresponding dicarboxyhc acid anhydride and a basic catalyst such as sodium acetate (41,42). Cellulose acetate succinate and cellulose acetate butyrate succinate are manufactured by similar methods as described in reference 43. 

Other mixed esters, eg, cellulose acetate valerate [55962-79-3] cellulose propionate valerate [67351-41-17, and cellulose butyrate valerate [53568-56-2] have been prepared by the conventional anhydride sulfuric acid methods (25). Cellulose acetate isobutyrate [67351-38-6] (44) and cellulose propionate isobutyrate [67351-40-0] (45) have been prepared with a 2inc chloride catalyst. Large amounts of catalyst and anhydride are required to provide a soluble product, and special methods of delayed anhydride addition are necessary to produce mixed esters containing the acetate moiety. Mixtures of sulfuric acid and perchloric acid are claimed to be effective catalysts for the preparation of cellulose acetate propionate in dichi oromethane solution at relatively low temperatures (46) however, such acid mixtures are considered too corrosive for large-scale productions. 

Cellulose Acetate. Almost all cellulose acetate, with the exception of fibrous triacetate, is prepared by a solution process employing sulfuric... 

Several derivatives of cellulose, including cellulose acetate, can be prepared in solution in dimethylacetamide—lithium chloride (65). Reportedly, this combination does not react with the hydroxy groups, thus leaving them free for esterification or etherification reactions. In another homogeneous-solution method, cellulose is treated with dinitrogen tetroxide in DMF to form the soluble cellulose nitrite ester this is then ester-interchanged with acetic anhydride (66). With pyridine as the catalyst, this method yields cellulose acetate with DS < 2.0. 

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...

Cellulose acetate with improved solubiUty properties can be prepared from low quaUty wood pulps by multistage addition of the components (97) or by intermpting the reaction in the early stages, filtering, and continuing the acetylation with fresh reactants (98,99). 

Low viscosity cellulose acetate is used in lacquers and protective coatings for paper, metal, glass, and other substrates and as an adhesive for cellulose photographic film because of its quick bonding rate and excellent bond peel strength (135) (see Coatings). Heat-sensitive adhesives for textiles have also been prepared from cellulose acetate (136). Extmded cellulose acetate film makes an excellent base for transparent pressure-sensitive tape (137) (see Adhesives). 

Desalination membranes with improved, rigid, and stable surfaces have been prepared from cellulose acetate propionate (170). These films are generally more resistant to hydrolysis than those from cellulose acetate. 

Cellulose Diacetate. When preparing cellulose diacetate for dyeing, strong alkahes must be avoided in the scouring of acetate because the surface of the cellulose acetate would be saponified by such treatment. Many fabrics tend to crease and therefore requke open-width handling. Scouring is frequendy carried out on a jig or beam using 1.0 g/L of surfactant and 0.5—1.0 g/L tetrasodium pyrophosphate for 30 min at 70—80°C. 

The earliest preparation of cellulose acetate is credited to Schiitzenberger in 1865. The method used was to heat the cotton with acetic anhydride in sealed tubes at 130-140°C. The severe reaction conditions led to a white amorphous polymer but the product would have been severely degraded and the process difficult to control. Subsequent studies made by Liebermann, Francimont, Miles, the Bayer Company and by other workers led to techniques for controlled acetylation under less severe conditions. 

Triphenyl phosphate is a crystalline solid which has less compatibility with the polymer. This may be expected from solubility parameter data. It is often used in conjunction with dimethyl phthalate and has the added virtues of imparting flame resistance and improved water resistance. It is more permanent than DMP. Triacetin is less important now than at one time since, although it is compatible, it is also highly volatile and lowers the water resistance of the compound. Today it is essential to prepare low-cost compounds to allow cellulose acetate to compete with the synthetic polymers, and plasticisers such as ethyl phthalyl ethyl glycollate, which are superior in some respects, are now rarely used. 

Hydroxy-containing polymers such as poly(methyl-methacrylate-co-hydroxyethyl methacrylate) [65,66] or secondary cellulose acetate [67,68] were used for this purpose. Vanadium (V) 8-hydroxy quinoline-hydroxy-ethyl methacrylate adduct, prepared by condensation of the latter with a VOQ2OH complex, is polymerized to... 

FIGURE 6-2 Steps in the preparation of an ainperometric enzyme electrode with simple enzyme immohilization hy trapping between an inner cellulose acetate and outer collagen membrane, cast on the electrode body. (Reproduced with permission from reference 1.)... 

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