Acetylation of cellulose

Safety precautions Before this experiment is carried out. Sect. 2.2.5 must be read as well as the material safety data sheets (MSDS) for all chemicals and products used. 

25 ml of 80% acetic acid at 60 °C are carefully stirred into one half of the primary solution in order to destroy the excess acetic anhydride care must be taken that there is no precipitation of cellulose acetate during this addition.The solution is held at 60 °C for another 15 min, then poured into a 11 beaker 25 ml of water are carefully stirred in. After the addition of another 200 ml of water, the cellulose triacetate precipitates as a white, crumbly powder.The product is filtered from the dilute acetic acid, slurried with 300 ml of distilled water, and the supernatant liquid decanted after 15 min this procedure is repeated until the washings give a neutral reaction.The polymer is dried as far as possible by suction or centrifugation and then in the oven at 105 °C.Yield about 7 g of cellulose triacetate.The product is soluble in methylene chloride/methanol (volume ratio 9 1), but practically insoluble in acetone. 

Into the other half of the primary solution, 50 ml of 70% acetic acid and 0.14 ml of concentrated sulfuric acid are slowly added with stirring at 60°C, in order to bring about partial saponification of the cellulose triacetate. The stoppered bottle is held at 80°C for 3 h and then worked up as described for cellulose triacetate. Yield 6-6.5 g. Cellulose 2,5-acetate (acetyl group content 40%) is soluble in acetone and methylene chloride/methanol (volume ratio 9 1). 

The progress of the partial saponification can be checked by a simple solubility test. About 1 ml of the solution is withdrawn and the cellulose acetate precipitated with water. The small sample is quickly washed free of acid and dried as much as possible by pressing between two filter papers. Some fibers of the still damp material are placed in a test tube with 15-20 ml of toluene/ethanol (volume ratio 

1 1) and heated to boiling in a water bath. If the fibers go into the solution, then after about 15 min the whole charge can be worked up as described above. 

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An apparatus for the continuous fibrous acetylation of cellulose in benzene has been described (68,69). The process involves continuous activation, acetylation, partial saponification of the resulting triacetate, and drying of the product. 

Recent Developments. A considerable amount of cellulose acetate is manufactured by the batch process, as described previously. In order to reduce production costs, efforts have been made to develop a continuous process that includes continuous activation, acetylation, hydrolysis, and precipitation. In this process, the reaction mixture, ie, cellulose, anhydride, catalyst, and solvent, pass continuously through a number of successive reaction zones, each of which is agitated (92,93). In a similar process, the reaction mass is passed through tubular zones in which the mixture is forced through screens of successively small openings to homogenize the mixture effectively (94). Other similar methods for continuous acetylation of cellulose have been described (95,96). 

High temperature acetylation of cellulose above 50°C produces cellulose acetate from low purity wood pulp cellulose in shorter reaction times. In a high temperature method recently disclosed (102), cellulose reacts with 200—400% acetic anhydride in the presence of <5% acid catalyst at 68—85°C for 3—20 min. After the acid catalyst is neutralized with magnesium acetate, the cellulose acetate is hydrolyzed at 120°C for two hours (103). Several modified catalyst systems have been developed for acetylation of cellulose above 90°C (89,90). 

The presence of free sulphuric acid in rayon-spinning baths limits application of the austenitic steels, but they are used for acetylation of cellulose in the acetate process. They are also used for dissolving and spinning solutions in the cuprammonium processes. 

Recently, use of LiCl/DMAc and LiCl/l,3-dimethyl-2-imidazolidinone as solvent systems for acetylation of cellulose by acetic anhydride/pyridine has been compared. A DS of 1.4 was obtained the substituent distribution in the products synthesized in both solvents was found to be the same, with reactivity order Ce > C2 > C3. Therefore, the latter solvent system does not appear to be better than the much less expensive LiCl/DMAc, at least for this reaction. It appears, however, to be especially efficient for etherification reactions [178]. It is possible, however, that the effect of cellulose aggregation is more important for its reaction with the (less reactive) halides than with acid anhydrides this being the reason for the better performance of the latter solvent system in ether formation, since it is more efficient in cellulose dissolution. 

Callow, H.J. (1951). Acetylation of cellulose and lignin in jute. Journal of the Indian Chemical Society, 28(11), 605-610. 

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... 

The manufacture of cellulose acetate involves the acetylation of cellulose from cotton linters or wood pulp by acetic anhydride and acetic acid. Production started about 30 years ago, and early products that were developed include safety photographic films, airplane dopes, and acetate fabrics. It is now also produced in the form of sheeting, rods, and tubes and is widely used as a molding compound. 

Acetylation of cellulose to the triacetate has been carried out without breaking down of the structure with acetic anhydride containing pyridine to help open up the cell wall structure and to act as a catalyst (71). This led Stamm and Tarkow (72) to test the liquid phase reaction on wood. High dimensional stabilization without break down of the structure was obtained, but excessive amounts of chemical were used. They hence devised a vapor phase method at atmospheric pressure that proved suitable for treating veneer up to thicknesses of 1/8 inch. Acetic anhydride pyridine vapors generated by heating an 80-20% mixture of the liquids were circulated around sheets of veneer suspended in a box lined with sheet stainless steel. Hardwood veneer,... 

N-Acetyl glucosaminidase, V, 61 Acetyl phosphate, III, 237 Acetyl propionyl. See 2,3-Propanedione. Acetyl value, determination of, for starch acetates, I, 290 Acetylation, of cellulose, I, 311 rates for various cellulosic fibers, V, 122, 123... 

Halohydrocarbons, as solvents for acetylation of cellulose, I, 313 Haptenes, dextrans as, II, 215 pneumococcus polysaccharide as, II, 221... 

Cellulose triacetate (CTA) Polymer with complete acetylation of cellulose that is hydrophilic, but suffers from very narrow acceptable temperature (<30°C), and pH (4-6 nominal, 2-9 occasional) ranges. This material is also highly susceptible to microbial attack. (In-filco Degremont is the only manufacatuer using this material as of this publication.)... 

Because acetylation of cellulose proceeds in a heterogeneous system, the reaction rate is controlled by the diffusion of the reagents into the fiber structure. The quality of the cellulose raw material used for acetate rayon is... 

The acid-catalyzed acetylation of cellulose proceeds according to the following reaction formula ... 

Fig. 12.10. Process vessel for acetylation of cellulose. (Courtesy Celanese Fibers Co.)...

The mixture of unreacted acid, water, and anhydride is fed to a still, which yields dilute acetic acid overhead and an anhydride-acetic acid mixture at the bottom (see Fig. 12.11). Conditions are controlled in such a way that the raffinate is about 40 percent anhydride and 60 percent acetic acid. As already mentioned, this is the desired ratio for the reaction mixture used for acetylation of cellulose. 

Miyamoto et al. [ 164,165] conducted homogeneous acetylation of cellulose in a 10% LiCl-dimethylacetamide solvent. The 6-OH group was about twice as reactive as the 2-OH group for DS up to 1.2, and the factor was reduced to 1.4 for higher DS samples. 

Miyamoto et al. [165] observed a more uniform acetylation among different hydroxyl groups in LiCl dimethylacetamide (DMAC) as compared to heterogeneous reactions (Table 3). Cellulose dissolved in DMSO-PF is known to form methylol derivatives, especially for the 6-OH group. Acetylation of cellulose in this system [174-176] was shown to occur preferentially at the methylol hydroxyl group generated in situ. 

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