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one of the most common natural polymers (a polysaccharide); the principal component of the cell walls of plants, ensuring mechanical strength and elasticity in plant tissues. Thus, the cellulose content is 97–98 percent in the hairs of cottonseeds, 75–90 percent in the stalks of bast crops (flax, ramie, jute), 40–50 percent in wood, and 30–40 percent in bulrushes, grasses, and sunflowers. It is also found in some lower invertebrates.
Cellulose serves mainly as a structural material in plants, hardly participating at all in metabolism. It is not broken down by the ordinary enzymes (amylase, maltase) of the mammalian gastrointestinal tract. Cellulose is decomposed into D-glucose by the action of the enzyme cellulase, released by the intestinal microflora of herbivorous animals. The biosynthesis of cellulose occurs with the participation of an activated form of D-glucose.
Preparation. Techniques for separating cellulose from natural materials are based on the action of reagents that dissolve or break down the noncellulosic constituents of plant tissues (proteins, fats, waxes, resins, and lignin and other polysaccharides occurring with cellulose). The techniques employed depend on the plant material and the intended use of the cellulose. The principal processes are the soda, sulfite, and sulfate. In the soda process, or digestion, the plant material is treated with a dilute solution of sodium hydroxide under pressure and then is bleached with such oxidizing agents as sodium hypochlorite. This process is used mainly for obtaining cotton pulp. With sulfite digestion, the plant material is treated under pressure with aqueous solutions of the bisulfite of calcium, magnesium, sodium, or ammonium, the solutions containing small amounts of free sulfur dioxide. In the sulfate process, the plant material is treated under pressure with an aqeuous solution containing sodium hydroxide and sodium sulfide. The sulfite and sulfate processes are used for obtaining cellulose from wood. Cellulose is separated from straw in a method involving consecutive treatment by aqueous solutions of sodium hydroxide and chlorine.
Structure and properties. Cellulose is a white, fibrous material with a density of 1.52–1.54 g/cm3 (at 20°C). It is soluble in cuprammonium solution, in aqueous solutions of quaternary ammonium bases, in aqueous solutions of complexes of the hydroxides of polyvalent metals (Ni, Co) with ammonia or ethylenediamine, and in an alkaline solution of a complex of iron (III) with sodium tartrate. Cellulose is also soluble in solutions of nitrogen dioxide in dimethylformamide and in concentrated phosphoric and sulfuric acids (dissolution in acids being accompanied by the breakdown of the cellulose).
Cellulose macromolecules are composed of D-glucose units linked together by β-1,4 glycosidic linkages in linear, unbranched chains:
The average degree of polymerization varies within wide limits. For viscose fiber, for example, polymerization involves 300–500 units, while in cotton and bast fibers the figure is 10,000–14,000 (determined viscometrically or ultracentrifugally). Cellulose exhibits significant polydispersity, the nature of the distribution of molecular weight depending on the starting materials and the method employed in obtaining cellulose.
Cellulose is usually classified as a crystalline polymer. It is characterized by polymorphism, that is, the existence of a series of structural (crystalline) modifications differing in crystal lattice parameters and certain physical and chemical properties; the principal modifications are cellulose I (native cellulose) and cellulose II (hydrated cellulose).
Cellulose has a complex supramolecular structure. It is made up of microfibrils, each of which comprises several hundred macromolecules and has the shape of a spiral (thickness, 35–100 angstroms; length, 500–600 angstroms and more). The microfibrils join to form larger structures (300–1500 angstroms), which have different orientations in various layers of the cell wall. The fibrils are cemented by a matrix consisting of other polymer materials of a carbohydrate nature (hemicellulose, pectin) and protein (extensin).
The glycosidic linkages between the monomer units of cellulose macromolecules are readily hydrolyzed by acids, which is the cause of the degradation of cellulose in a water medium in the presence of acid catalysts. The product of the complete hydrolysis of cellulose is glucose; this reaction forms the basis of the industrial production of ethyl alcohol from cellulose-containing raw materials. Partial hydrolysis occurs, for example, during the separation of cellulose from plant materials and during chemical treatment. The incomplete hydrolysis of cellulose, carried out in such a way that only structural segments having a low degree of order are degraded, is used to obtain a microcrystalline form of cellulose that occurs as a loose, snow-white powder.
In the absence of oxygen, cellulose is stable up to 120°–150°C; with a further increase in temperature, native cellulose fibers are degraded, while fibers of hydrated cellulose undergo dehydration. Above 300°C, the fibers undergo graphitization (carbonization); this process is used in the production of carbon fibers.
Cellulose is readily esterified and alkylated as a consequence of hydroxyl groups in the monomer units of the macromolecules. These reactions are commonly used in the industrial production of ethers and esters of cellulose. Cellulose reacts with bases; its reaction with concentrated solutions of sodium hydroxide, which leads to the formation of alkali cellulose (the mercerization of cellulose), is an intermediate step in the production of cellulose esters and ethers. Most oxidizing agents nonselectively oxidize the hydroxyl groups of cellulose to aldehyde, ketone, or carboxyl groups; only a few of the agents (for example, periodic acid and its salts) oxidize selectively, that is, oxidize OH groups at specific carbon atoms. Cellulose undergoes oxidative degradation in the production of viscose (a step in the aging of alkali cellulose). Oxidation also occurs during the bleaching of cellulose.
In order to eliminate some of the less desirable features of cellulose fibers (low elasticity, lack of resistance to microorganisms, flammability) and to impart valuable new properties, cellulose materials are modified through graft polymerization or through treatment by polyfunctional compounds, such as epoxy compounds and methylol derivatives of urea. In this way, wrinkle-resistant fabrics are produced from cellulose fibers (mainly, cotton fibers), as are ion-exchange, nonflammable, hemostatic, and bactericidal materials.
Use. Cellulose is used in making paper, cardboard, and various artificial fibers, namely hydrated cellulose fibers (viscose fibers, cuprammonium fibers) and cellulose ester fibers (acetate fibers, triacetate fibers). Cellulose is also used in making films (cellophane), plastics, and lacquers. Native cellulose fibers (cotton and bast), as well as artificial cellulose fibers, are commonly used in the textile industry. Cellulose derivatives, mainly esters and ethers, are used as thickening agents in printing inks and as sizing and dressing preparations and suspension stabilizers in smokeless powder. Microcrystalline cellulose is used as a filler in pharmaceuticals and as a sorbent in the branches of chromatography concerned with preparation and analysis.
REFERENCESNikitin, N. I. Khimiia drevesiny i tselliulozy. Moscow-Leningrad, 1962.
Kratkaia khimicheskaia entsiklopediia, vol. 5. Moscow, 1967. Pages 788–95.
Rogovin, Z. A. Khimiia tselliulozy. Moscow, 1972.
Tselliuloza i ee proizvodnye, vols. 1–2. Moscow, 1974. (Translated from English.)
Kretovich, V. L. Osnovy biokhimii rastenii, 5th ed. Moscow, 1971.
L. S. GAL’BRAIKH and N. D. GABRIELIAN