Dissolution of Cellulose

Cellulose (VIII) is spun into fiber or cast into film by using a chemical reaction to convert it into a soluble xanthate derivative (Turbak, 1988). This is achieved by treating cellulose with 18-20% aqueous sodium hydroxide solution at 25-30°C for about 0.5-1 h. Much of the sodium hydroxide is physically absorbed into the swollen polymer some of it may be in the form of cellulose alkoxides. The excess alkali is pressed out of the cellulose pulp and the mass aged to allow oxidative degradation of the polymer chains to the desired molecular weight. The alkali cellulose is then treated with carbon disulfide at about 30° C and the resulting mass dissolved in dilute sodium hydroxide to form the sodium 

An alternate procedure used in a few specialty applications is the cuprammonium process. This involves stabilization of cellulose in an ammonia solution of cupric oxide. Solubilization occurs by complex formation of cupric ion with ammonia and the hydroxyl groups of cellulose. Regeneration of cellulose, after formation of the desired products, is accomplished by treatment with acid. The main application of the cuprammonium process is for the synthesis of films and hollow fibers for use in artificial kidney dialysis machines. The cuprammonium process yields products with superior permeability and biocompatibility properties compared to the xanthation process. Less than 1% of all regenerated cellulose is produced by the cuprammonium process. 

A more recent process for producing regenerated cellulose involves the use of V-methyl-morpholine-A-oxide, a highly polar solvent, at about 130°C to dissolve cellulose [Rosenau et al., 2001], About 5% of all regenerated cellulose is produced by this method. 

Cellulose acetate is the most important ester derivative of cellulose. It is produced by acetylation of cellulose using acetic anhydride in acetic acid in the presence of a strong acid catalyst (usually sulfuric acid). In Eq. 9-29 the symbol is a general means of representing a polymer molecule minus the functional group of interest and ( )—OH specifically 

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In any solvent system, the essential factors required for dissolution of cellulose include adequate stabihty of the electrolyte/solvent complex cooperative action of the solvated ion-pair on hydrogen bonding of cellu-... 

The dissolution of cellulose and other plant-based material in ILs has been studied by Moyna and Rogers [64-66]. Here, the application of NMR excluded fhe need for solvenf suppression techniques. In combination with and C1/ C1 relaxation measurements, it was found that dissolution of cellulose in [C4CiIm]Cl was promoted by the nonhydrated chloride ions, breaking up the hydrogen bonding networks via the cellulose hydroxyl groups. 

The nature of solubility and the strength of solvent. According to present views the dissolution of cellulose esters consists in the separation of the chain-molecules under the specific influence of the solvent until all bonds between the chains disappear. The macromolecules of polymer can slide apart even at the swelling stage. Because of their elasticity, it is possible that the chains can be pushed aside in certain places or along the entire molecules. Further, a shortening of chains occurs, but being a secondary effect it proceeds slowly. 

Miles and Milboum [20] and Trogus [5] have revised Sapozhnikov s chart so as to indicate areas rich in water and most abundant in sulphuric acid, in which cellulose undergoes swelling at the time of nitration, the area richest in sulphuric add where dissolution of cellulose prior to nitration proceeds and the zone defining mixtures being used in practice (Figs. 123 and 124). 

Ideally, dissolution of cellulose in the amine N-oxide is supposed to be an entirely physical process without any chemical changes of pulp or solvent. However, in real-world processes there are several chemical processes observed, which cause formation of appreciable amounts of byproducts. A strong discoloration of the solution due to chromophore formation has been observed, which is accompanied by degradation of both the solute cellulose and the solvent NMMO at the elevated process temperatures, which in turn can provoke very severe effects, such as degradation of cellulose, temporary or permanent discoloration of the resulting fibers, decreased product... 

Swatloski, R.P., S.K. Spear, J.D. Holbrey and R.D. Rogers, Dissolution of Cellulose with Ionic Liquids, Journal of the American Chemical Society, 124, 4974—4975 (2002). 

Baker, T. J., Schroeder, L. R., and Johnson, D. C. (1978). Dissolution of cellulose in polar aprotic solvents via formation of methylol cellulose. Carbohydr. Res. 67 C4-C7. 

A detailed study has been made of the action of pure triduoroacetic acid on cellulose and cellobiose (and their acetates). Dissolution of cellulose occurs, and swelling takes place with rupture of hydrogen bonds and with micellar dispersion esteridcation takes place without occurrence of degradation, the cellulose being fully recovered on hydrolysis of the triduoro-acetylated product. It appears that there is a more rapid rate of triduoro-acetylation of primary than of secondary alcohol groups. 

One of the barriers to cellulose hydrolysis by cellulases is the high crystallinity of cellulose in its native form. The regeneration of cellulose after dissolution in ionic liquids results in cellulose of amorphous form. Microcrystalline cellulose was found to be hydrolysed 50-90 times faster by cellulases following regeneration after dissolution in [C,mim][Cl] or [Amim][Cl] [182], which indicates that dissolution of cellulose in an ionic liquid may be useful as a pretreatment method. 

Among the various solvents suggested for dissolution of cellulose, cuprammonium solution is recommended as a general solvent having 15 0.1 g/1 Cu, 200 5 g/1 NHj and less than 0.5 g/1 nitrous oxide. The fluidity in this solvent of the solution of cotton is given by... 

Zhu S, Wu Y, Chen Q et al (2006) Dissolution of cellulose with ionic liquids and its application a mini-review. Green Chem 8 325-327... 

Dissolution of cellulose has three major purposes. The first is to prepare regenerated and man-made cellulose fibers or films from cellulose solutions at the industrial level. Environmentally friendly and cost-profitable systems to dissolve and regenerate cellulose are now required. The second purpose is to use cellulose solutions as homogeneous reaction media during chemical modifications, which have been investigated at the laboratory level. The last one is to analyze cellulose samples. Molecular mass and molecular mass distribution studies using cellulose solutions are included in this category. Numerous cellulose solvents have, therefore, been developed and studied for these purposes. 

One possible explanation for these different modes of cellulose depolymerization in the same species of wood is that the cellulolytic enzyme molecules of Poria monticola are smaller than those of Polyporus versicolor and for that reason would be able to penetrate and act in regions of the fine structure of the fibers that are not accessible to those of the latter fungus. This hypothesis has led to efforts (as yet incomplete) to determine the molecular size of the cellulolytic enzyme proteins of these two organisms. Another possible explanation is that the initial dissolution of cellulose and other cell-wall polysaccharides is accomplished by catalysts that are not enzyme proteins and therefore could be substantially smaller in molecular size. Halliwell (21) has described experiments on the... 

Conventionally produced cellulose powders (microcrystalline cellulose) consist of irregularly shaped fibrous particles of limited use for column chromatography. Beaded cellulose is prepared by dissolution of cellulose powder in a suitable solvent, followed by droplet formation in a suspension medium, and subsequent solvent extraction or crosslinking. Cellulose triacetate and tricarbamate derivatives are useful as low-cost sorbents for the process-scale separation of enantiomers (section 10.4.2). 

Physicochemical studies of other aliphatic, carboxylic esters of polysaccharides include (a) assessment of the gel-permeation properties of the series cellulose propionate-cellulose heptanoate, (b) dissolution of cellulose acetate phthalate, (c) monolayer properties of amylose acetate, and (d) melting and transition temperatures of the series amylose acetate-amylose hexanoate. Amylose acetate assumes a helical conformation at air-water interfaces and in oriented films, and the X-ray-analysis data for mannan acetate imply a threefold screw-axis... 

Physicochemical studies of the solution properties of amylose benzoate by light-scattering, osmometric, and viscosimetric techniques showed that the molecules behave as coils. Factors affecting the rate of dissolution of cellulose acetate phthalate in aqueous solution are important from the point of view of use as an enteric-coating material. The removal of the benzoyl group during methanolysis is slow and concurrent with the methanolysis, " whereas deacetylation precedes methanolysis. 

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