Derivatives of Cellulose

Many derivatives of cellulose have been synthesized over the years [12–14]. These include esters of both organic and inorganic acids, ethers, and various graft copolymers. Only some of them, however, achieved commercial importance. One of the earliest commercial esters of cellulose was cellulose nitrate. It was originally prepared as an explosive (guncotton) in the middle of the nineteenth century, and later as a medical aid (collodion, for covering wounds). Later films from cellulose nitrate were used in photography, called celluloid. Nitrocellulose was also probably the first successful commercial plastic, used to form many articles. Today it is generally displaced by other materials. Cellulose nitrate, however, is still being used in some surface finishes, though here too it is gradually being displaced. Cellulose is nitrated by mixtures of nitric and sulfuric acids. The type of acid mixture used depends on the intended products. For the preparation of plastic grade materials, 25% of nitric acid is combined with 55% of sulfuric acid and 20% water. The dried cellulose is soaked for 20–60 min at 30–40C. There is little change in appearance as the structure of the cellulose is maintained. The bulk of the acid is then removed, usually by spinning in a centrifuge and the remaining acid washed out with copious amounts of water. The product is often bleached with sodium hypochlorite and washed. The degree of nitration is controlled by reaction conditions and particularly by the amount of water in the nitrating bath. Products with 1.9–2.0 nitrate groups per each glucose unit are used in plastics and lacquers. Some materials, however, with a nitrate content as high as 2.0–2.4 groups per each glucose have been used in some lacquers. The higher nitrate content of 2.4–2.8 groups per each glucose is in materials intended for use as explosives. The esterification reaction can be illustrated as follows:

The molecular weight of cellulose nitrates used in plastics and lacquers is usually reduced. This is done by heating the slurry of the polymer in water at about 130–160C for up to 30 min under pressure. Cellulose acetate was also prepared originally in the nineteenth century. Commercial develop ment, however, started early in twentieth century. In the 1920s acetate rayon and acetate fibers were developed and cellulose acetate became an important molding material. At about the same time cellulose lacquers were also developed. Today, however, many of these materials have been replaced by other polymers. The acetylation reaction of cellulose is often prepared by forming a solution in a mixture of acetic anhydride and sulfuric acid. This results in 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, 1-part sulfuric acid, and 400 parts methylene chloride. The reaction mixture is agitated while the tempera ture is maintained at 25–35C for 5–8 h. By the end of that period, all the cellulose is dissolved and the cellulose triacetate has formed in the solution. Partial hydrolysis is accomplished by adding to the methylene chloride solution aqueous acetic acid (50%). The solution is then allowed to stand to reach the desired degree of hydrolysis. This usually takes about 72 h at room temperature. Sulfuric acid, still present from acetylation, is then neutralized by addition of sodium acetate and most of the methylene chloride is distilled off. The partially hydrolyzed cellulose acetate is then precipitated by addition of water and washed thoroughly. The washing also includes a 2-h wash with very dilute sulfuric acid to remove hydrogen sulfate esters that cause polymer instability. The process can be illustrated as follows:

Cellulose triacetate is also prepared by a heterogeneous process in the presence of benzene, a non-solvent. The triacetate that forms in both processes is hard to mold, but it can be used in films and fibers. The diacetate is more suited for plasticization and molding.

Many other esters of cellulose were prepared at various times, including some mixed esters. Various cellulose acetate-butyrates are manufactured today and are perhaps the best known of the mixed esters. They are synthesized in the same manner as cellulose acetate. Mixed anhydrides are used in esterification reactions catalyzed by sulfuric acid. The products are then slightly hydrolyzed. The butyric groups enhance flexibility and moisture resistance. The materials have the reputation of being tough plastics and are used in such applications as tool handles. Lower molecular weight grades are also used in surface finishes. Several cellulose ethers are also prepared commercially. The original patents for preparation of cellulose ethers date from 1912. In spite of that, cellulose ethers never attained the industrial importance of cellulose esters. The ethers are prepared by reacting alkali cellulose with an alkyl halide or with an epoxide:

Typical commercial ethers are methyl, ethyl, hydroxyethyl, hydroxypropyl, carboxy-methyl, aminoethyl, and benzyl. Ethyl cellulose is used industrially as a plastic similarly to cellulose acetate. The water-soluble ethers, like methyl, carboxymethyl, and hydroxyethyl, are used as thickeners in foods and in paper manufacturing. Cellulose can be reacted with acrylonitrile to form a cyanoethyl ether. The Michael condensation takes place with alkali cellulose:

Cyanoethylated cellulose does not appear to be used commercially in any quantity. Very stable silyl ethers form when cellulose is treated with trimethyl chloro-silane or with bis (trimethylsilyl)-acetamide [15]:

Some interesting approaches to cellulose modification are possible via formations of double bonds in the glucopyranosine unit at the 5,6 positions [16]. This is accomplished by dehydro-halogenating a previously formed 6-iodocellulose:

The resultant unsaturated compound can be converted into a number of derivatives. Examples of some of them are:

Other compounds that can be added across the double bonds are carbon tetrachloride, phosphorus trichloride, and methyl alcohol. Many graft copolymers of cellulose were reported. In wood, cellulose is present with lignin, a natural phenolic polymer, described in Sect. 8.3. Kobayshi and coworkers [16] grafted phenolic resins onto cellulose to form an artificial wood polymer.

The reaction was catalyzed by a complex of iron with N, N-ethylene bis (sallicylidene amine) as well as a horseradish peroxidase enzyme to carry out oxidative coupling of phenols using hydrogen peroxide as an oxidizing agent. The product is a new type of plastic.

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عوامل تضعف القدرات الإدراكية لدى الإنسان

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دراسة بريطانية: الأطعمة النباتية عالية الجودة تقلل خطر فشل القلب

7 أطعمة يومية "تسمم" جسمك بالبلاستيك دون أن تشعر

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الآخبار التكنلوجية

اكتشاف أثري في أوريغون الأمريكية قد يعيد كتابة تاريخ البشرية

اكتشاف كائن بحري مدرع بلسان "حديدي" في بحر اليابان

نظام كوكبي "مقلوب" يحير علماء الفلك

بسبب الذكاء الاصطناعي.. هذه الوظائف "ستختفي" بعد أشهر قليلة ؟!

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مواضيع ذات صلة

Structures and Chemistry of Proteins

Proteins

Polyisoprene

Lignin

Regenerated Cellulose

Cellulose

Starch

Hemicelluloses

Dendrimers and Hyperbranched Polymers

Poly (arylene ether) s and Poly (arylene ether ketone) s

Double-Stranded Polymers

Cardo Polymers