Cellulose reaction with acetic anhydride

Clement and Riviere [59] also reported that cellulose acetate or a mixed ester — a nitrate-acetate — can be obtained by reacting cellulose nitrate with acetic anhydride, acetic acid, and sulphuric acid. According to more recent contributions, e.g. Wolfrom, Bower and Maker [60], the reaction should be performed as follows Cellulose nitrate is dissolved in the cold in a little sulphuric add and acetic anhydride, the surplus of acetic anhydride is then hydrolysed also in the cold, and cellulose acetate is extracted with a suitable solvent, such as chloroform. Other methods of acetylating nitrocellulose consist in reduction, for instance with zinc and hydrogen chloride, which entails denitration of the ester, followed by acetylation with acetic anhydride. All these reactions are carried out in the same vessel. Further, it is possible to synthesize mixed esters, cellulose nitrate-acetates, by subjecting cellulose to the action of a mixture that includes nitric acid, acetic add and acetic anhydride in the presence of sulphuric acid (Kruger [61]). The use of a large amount of nitric acid favours the formation of nitrocellulose only. Mixed esters are formed... 

Bonding of chemicals to wood cell wall components--cellulose, hemicellulose and lignin—can change the physical and chemical properties of the wood. For example, reaction of southern pine with simple epoxides results in a modified wood which is resistant to attack by subterranean termites in laboratory tests (1). Wood modified with acetic anhydride, dimethyl sulfate, 0-propiolactone and epoxides are highly resistant to attack by microorganisms in standard soil block laboratory tests (2,3). Southern pine modified by reaction with acetic anhydride and propylene and butylene oxides has a reduced tendency to swell in the presence of water (4). 

The acetylation reaction of cellulose is often prepared by forming a solution in a mixture of acetic anhydride and sulfuric acid. This results in the formation of a triacetate. When a lower degree of esterification is desired, the triacetate is partially hydrolyzed. A two-step procedure is needed, because it is not possible to control the degree of esterification in the reaction with acetic anhydride and sulfuric acid. In a typical process, dry cellulose is pretreated with 300 parts acetic anhydride, one part sulfuric acid, and 400 parts methylene chloride. The reaction mixture is agitated while the temperature is maintained at 25-35 for 5-8 eight hours. By the end of that period all the cellulose... 

Cellulose secondary acetate fibres are manufactured from cotton linters by steeping in glacial acetic acid and sulphuric acid-catalysed reaction with acetic anhydride. The reaction is exothermic and the final product in a maximum of 20 hours is cellulose triacetate, which is converted to secondary acetate by adding sufficient water. The hydrolysis is stopped when 1/6 of the acetate groups have been randomly changed to hydroxyl groups. The precipitated polymer flakes are dissoluted in acetone containing small amounts of water or alcohol. The chemical formula of cellulose triacetate and the diacetate fibre production chart are shown in Fig. 4.5. 

The production of cellulose diacetate requires three steps. A preliminary treatment of cellulose with glacial acetic acid for 1 or 2 h is followed by reaction with acetic anhydride with a trace of sulfuric acid as a catalyst. In order to keep the temperature of the reaction at about 50°C, methylene chloride is refluxed. A typical charge would be the following [4] ... 

About half of the wodd production comes from methanol carbonylation and about one-third from acetaldehyde oxidation. Another tenth of the wodd capacity can be attributed to butane—naphtha Hquid-phase oxidation. Appreciable quantities of acetic acid are recovered from reactions involving peracetic acid. Precise statistics on acetic acid production are compHcated by recycling of acid from cellulose acetate and poly(vinyl alcohol) production. Acetic acid that is by-product from peracetic acid [79-21-0] is normally designated as virgin acid, yet acid from hydrolysis of cellulose acetate or poly(vinyl acetate) is designated recycle acid. Indeterrninate quantities of acetic acid are coproduced with acetic anhydride from coal-based carbon monoxide and unknown amounts are bartered or exchanged between corporations as a device to lessen transport costs. 

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. 

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. 

In the fibrous acetylation process, part or all of the acetic acid solvent is replaced with an inert dilutent, such as toluene, benzene, or hexane, to maintain the fibrous stmcture of cellulose throughout the reaction. Perchloric acid is often the catalyst of choice because of its high activity and because it does not react with cellulose to form acid esters. Fibrous acetylation also occurs upon treatment with acetic anhydride vapors after impregnation with a suitable catalyst such as zinc chloride (67). 

Sulfuric acid reacts with acetic anhydride to form acetylsulfuric acid (79). This reaction is favored by low temperature and high anhydride concentration. In cellulose acetylation, probably both sulfuric acid and acetylsulfuric acid exist and react with cellulose to form cellulose sulfate acid ester. 

Solution Process. With the exception of fibrous triacetate, practically all cellulose acetate is manufactured by a solution process using sulfuric acid catalyst with acetic anhydride in an acetic acid solvent. An excellent description of this process is given (85). In the process (Fig. 8), cellulose (ca 400 kg) is treated with ca 1200 kg acetic anhydride in 1600 kg acetic acid solvent and 28—40 kg sulfuric acid (7—10% based on cellulose) as catalyst. During the exothermic reaction, the temperature is controlled at 40—45°C to minimize cellulose degradation. After the reaction solution becomes clear and fiber-free and the desired viscosity has been achieved, sufficient aqueous acetic acid (60—70% acid) is added to destroy the excess anhydride and provide 10—15% free water for hydrolysis. At this point, the sulfuric acid catalyst may be partially neutralized with calcium, magnesium, or sodium salts for better control of product molecular weight. 

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. 

Acetylation rates have also been studied by Centola37 who treated natural and mercerized ramie fibers for varying times with acetic anhydride and sodium acetate and examined the reaction products chemically and by X-ray diffraction. The reagent was considered to penetrate into the interior of fibers. A heterogeneous micellar reaction was believed to occur that converted a semi-permeable elastic membrane around the micelles into the triacetate. The rate of acetylation of mercerized ramie was observed to be faster than that of unmercerized fiber. Centola concluded that about 40 % of the cellulose in native ramie is amorphous and acetylates rapidly. 

Cellulose acetate and triacetate may be used as plastics or spun into fibers for textiles. They are made by the reaction of cellulose with acetic anhydride. 

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,... 

The industrial preparation of cellulose diacetate employs acetic anhydride with sulphuric acid as catalyst. The reaction is conducted at low temperature and cellulose starts to dissolve in the acetylation bath as the reaction progresses. The reaction is conducted until practically full acetylation. The homogeneous solution obtained is then hydrolysed to reduce the DS to 2.4. Precipitation in dilute acetic acid, then washing with water and finally drying produce cellulose acetate flakes. 

We met cellulose, the bulk polysaccharide of woody plants, in Chapter 49. It is a strong and flexible polymer but no use for making fabrics or films as it cannot be processed. One solution to this problem is to carry out chemical reactions that transform its properties. Acid-catalysed acetylation with acetic anhydride gives a triacetate with most of the free OH groups converted into esters. 

The preparation of the first starch acetate, as well as the first cellulose acetate, was announced by SchUtzenberger in 1865. These acetates were prepared by heating the carbohydrates in acetic anhydride to about 140-160 . Further examination of this reaction has been made by Traquair who found that on heating starch to 90° with acetic anhydride a derivative of low acetyl content (1-4%) is obtained which is capable of forming clear, somewhat elastic films. This starch acetate, termed Feculose, was produced commercially for a time, being sold for use as a thickening agent and as a size for textiles and paper. 

Various details of procedure are known for the esterification of cellulose with acetic anhydride in acetic acid. solvent. The factors of pretreatment, catalyst concentration, acetylation temperature and time of reaction are kept in balance in order that products of satisfactory appearance and the desired range of viscosity will be obtained. Modifications of this general formula usually involve the use of different solvents to replace part or all of the acetic acid. 

All esterification processes with acetic anhydride yield the fully esterified ester as the first soluble product. If the reaction medium is a solvent for the triester a solution is obtained, and if a reaction which retains the fiber structure is employed, samples taken at intervals are all insoluble in solvents until complete esterification is attained. The process in this way differs from nitration, in which soluble, partially esterified products are obtained by adjustment of the concentration of the nitration acids. In the etherification of cellulose, the ethers (e.g., methylated cellulose) prepared by partial substitution are also soluble products, exhibiting continuous, gradual changes in solubility characteristics with increasing substitution. 

The anhydride method of acetylation gives an acid by-product that results in an acidic condition in the wood and a loss of 50% of the reaction chemical. These by-products must be removed to prevent degradation. Acetic acid, the by-product of acetylation with acetic anhydride, is virtually impossible to remove completely from wood. This results in a product that smells of acetic acid, acid conditions that catalyze the removal of more acetyl groups, acid hydrolysis of cellulose fibers which results in strength losses over a long term, and acid corrosion of metal fasteners used in the wood product. 

Among the esters with organic acids, cellulose acetate is the most important one. Cellulose acetate has numerous uses such as manufacture of yarn, photographic films, lacquers, etc. Cellulose can be acetylated starting with lower levels of esterification up to the formation of cellulose triacetate. The acetate is obtained from the reaction of cellulose with acetic anhydride, usually in the presence of a catalyst such as H2SO4 or HCIO4. Numerous industrial procedures are known for this process [43]. 

Derivation Reaction of purified cellulose with acetic anhydride in the presence of sulfuric acid as catalyst and glacial acetic acid as solvent, followed by very slight hydrolysis. 

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