Cellulose is mainly used to producepaperboard andpaper. Smaller quantities are converted into a wide variety of derivative products such ascellophane andrayon. Conversion of cellulose fromenergy crops intobiofuels such ascellulosic ethanol is under development as arenewable fuel source. Cellulose for industrial use is mainly obtained fromwood pulp andcotton.[6] Cellulose is also greatly affected by direct interaction with several organic liquids.[10]
Cellulose was discovered in 1838 by the French chemistAnselme Payen, who isolated it from plant matter and determined its chemical formula.[3][11][12] Cellulose was used to produce the first successfulthermoplastic polymer,celluloid, by Hyatt Manufacturing Company in 1870. Production ofrayon ("artificialsilk") from cellulose began in the 1890s andcellophane was invented in 1912.Hermann Staudinger determined the polymer structure of cellulose in 1920. The compound was first chemically synthesized (without the use of any biologically derivedenzymes) in 1992, by Kobayashi and Shoda.[13]
Cellulose has no taste, is odorless, ishydrophilic with thecontact angle of 20–30 degrees,[14] is insoluble inwater and most organicsolvents, ischiral and isbiodegradable. It was shown to melt at 467 °C in pulse tests made by Dauenhaueret al. (2016).[15] It can be broken down chemically into its glucose units by treating it with concentrated mineral acids at high temperature.[16]
Cellulose is derived fromD-glucose units, whichcondense through β(1→4)-glycosidic bonds. This linkage motif contrasts with that for α(1→4)-glycosidic bonds present instarch andglycogen. Cellulose is a straight chain polymer. Unlike starch, no coiling or branching occurs and the molecule adopts an extended and rather stiff rod-like conformation, aided by the equatorial conformation of the glucose residues. The multiplehydroxyl groups on the glucose from one chain formhydrogen bonds with oxygen atoms on the same or on a neighbour chain, holding the chains firmly together side-by-side and formingmicrofibrils with hightensile strength. This confers tensile strength incell walls where cellulose microfibrils are meshed into a polysaccharidematrix. The high tensile strength of plant stems and of the tree wood also arises from the arrangement of cellulose fibers intimately distributed into thelignin matrix. The mechanical role of cellulose fibers in the wood matrix responsible for its strong structural resistance, can somewhat be compared to that of thereinforcement bars inconcrete,lignin playing here the role of thehardened cement paste acting as the "glue" in between the cellulose fibres. Mechanical properties of cellulose in primary plant cell wall are correlated with growth and expansion of plant cells.[17] Live fluorescence microscopy techniques are promising in investigation of the role of cellulose in growing plant cells.[18]
A triple strand of cellulose showing thehydrogen bonds (cyan lines) between glucose strandsCotton fibres represent the purest natural form of cellulose, containing more than 90% of thispolysaccharide.
Compared to starch, cellulose is also much morecrystalline. Whereas starch undergoes a crystalline toamorphous transition when heated beyond 60–70 °C in water (as in cooking), cellulose requires a temperature of 320 °C and pressure of 25MPa to become amorphous in water.[19]
Several types of cellulose are known. These forms are distinguished according to the location of hydrogen bonds between and within strands. Natural cellulose is cellulose I, with structures Iα and Iβ. Cellulose produced by bacteria and algae is enriched in Iα while cellulose of higher plants consists mainly of Iβ. Cellulose inregenerated cellulose fibers is cellulose II. The conversion of cellulose I to cellulose II is irreversible, suggesting that cellulose I ismetastable and cellulose II is stable. With various chemical treatments it is possible to produce the structures cellulose III and cellulose IV.[20]
Many properties of cellulose depend on its chain length ordegree of polymerization, the number of glucose units that make up one polymer molecule. Cellulose from wood pulp has typical chain lengths between 300 and 1700 units; cotton and other plant fibers as well as bacterial cellulose have chain lengths ranging from 800 to 10,000 units.[6] Molecules with very small chain length resulting from the breakdown of cellulose are known ascellodextrins; in contrast to long-chain cellulose, cellodextrins are typically soluble in water and organic solvents.
The chemical formula of cellulose is (C6H10O5)n where n is the degree of polymerization and represents the number of glucose groups.[21]
Plant-derived cellulose is usually found in a mixture withhemicellulose,lignin,pectin and other substances, whilebacterial cellulose is quite pure, has a much higher water content and higher tensile strength due to higher chain lengths.[6]: 3384
Inplants cellulose is synthesized at theplasma membrane by rosette terminal complexes (RTCs). The RTCs arehexameric protein structures, approximately 25nm in diameter, that contain thecellulose synthase enzymes that synthesise the individual cellulose chains.[29] Each RTC floats in the cell's plasma membrane and "spins" a microfibril into thecell wall.[citation needed]
RTCs contain at least three differentcellulose synthases, encoded byCesA (Ces is short for "cellulose synthase") genes, in an unknownstoichiometry.[30] Separate sets ofCesA genes are involved in primary and secondary cell wall biosynthesis. There are known to be about seven subfamilies in the plantCesA superfamily, some of which include the more cryptic, tentatively-namedCsl (cellulose synthase-like) enzymes. These cellulose syntheses use UDP-glucose to form the β(1→4)-linked cellulose.[31]
Bacterial cellulose is produced using the same family of proteins, although the gene is calledBcsA for "bacterial cellulose synthase" orCelA for "cellulose" in many instances.[32] In fact, plants acquiredCesA from the endosymbiosis event that produced thechloroplast.[33] All cellulose synthases known belongs toglucosyltransferase family 2 (GT2).[32]
Cellulose synthesis requires chain initiation and elongation, and the two processes are separate.Cellulose synthase (CesA) initiates cellulose polymerization using asteroid primer,sitosterol-beta-glucoside, and UDP-glucose. It then utilisesUDP-D-glucose precursors to elongate the growing cellulose chain. Acellulase may function to cleave the primer from the mature chain.[34]
Cellulose is also synthesised bytunicate animals, particularly in thetests ofascidians (where the cellulose was historically termed "tunicine" (tunicin)).[35]
Cellulolysis is the process of breaking down cellulose into smaller polysaccharides calledcellodextrins or completely intoglucose units; this is ahydrolysis reaction. Because cellulose molecules bind strongly to each other, cellulolysis is relatively difficult compared to the breakdown of otherpolysaccharides.[36] However, this process can be significantly intensified in a propersolvent, e.g. in anionic liquid.[37]
At temperatures above 350 °C, cellulose undergoesthermolysis (also called 'pyrolysis'), decomposing into solidchar, vapors,aerosols, and gases such ascarbon dioxide.[42] Maximum yield of vapors which condense to a liquid calledbio-oil is obtained at 500 °C.[43]
Semi-crystalline cellulose polymers react at pyrolysis temperatures (350–600 °C) in a few seconds; this transformation has been shown to occur via a solid-to-liquid-to-vapor transition, with the liquid (calledintermediate liquid cellulose ormolten cellulose) existing for only a fraction of a second.[44] Glycosidic bond cleavage produces short cellulose chains of two-to-sevenmonomers comprising the melt. Vapor bubbling of intermediate liquid cellulose producesaerosols, which consist of short chain anhydro-oligomers derived from the melt.[45]
Continuing decomposition of molten cellulose produces volatile compounds includinglevoglucosan,furans,pyrans, light oxygenates, and gases via primary reactions.[46] Within thick cellulose samples, volatile compounds such aslevoglucosan undergo 'secondary reactions' to volatile products including pyrans and light oxygenates such asglycolaldehyde.[47]
Hemicelluloses arepolysaccharides related to cellulose that comprises about 20% of the biomass ofland plants. In contrast to cellulose, hemicelluloses are derived from several sugars in addition toglucose, especiallyxylose but also includingmannose,galactose,rhamnose, andarabinose. Hemicelluloses consist of shorter chains – between 500 and 3000 sugar units.[48] Furthermore, hemicelluloses are branched, whereas cellulose is unbranched.[citation needed]
Cellulose is soluble in several kinds of media, several of which are the basis of commercial technologies. These dissolution processes are reversible and are used in the production ofregenerated celluloses (such asviscose andcellophane) fromdissolving pulp.[citation needed]
The most important solubilizing agent is carbon disulfide in the presence of alkali. Other agents includeSchweizer's reagent,N-methylmorpholineN-oxide, andlithium chloride indimethylacetamide. In general, these agents modify the cellulose, rendering it soluble. The agents are then removed concomitant with the formation of fibers.[49] Cellulose is also soluble in many kinds ofionic liquids.[50]
The history of regenerated cellulose is often cited as beginning with George Audemars, who first manufactured regeneratednitrocellulose fibers in 1855.[51] Although these fibers were soft and strong -resembling silk- they had the drawback of being highly flammable.Hilaire de Chardonnet perfected production of nitrocellulose fibers, but manufacturing of these fibers by his process was relatively uneconomical.[51] In 1890, L.H. Despeissis invented thecuprammonium process – which uses a cuprammonium solution to solubilize cellulose – a method still used today for production ofartificial silk.[52] In 1891, it was discovered that treatment of cellulose with alkali and carbon disulfide generated a soluble cellulose derivative known asviscose.[51] This process, patented by the founders of the Viscose Development Company, is the most widely used method for manufacturing regenerated cellulose products.Courtaulds purchased the patents for this process in 1904, leading to significant growth of viscose fiber production.[53] By 1931, expiration of patents for the viscose process led to its adoption worldwide. Global production of regenerated cellulose fiber peaked in 1973 at 3,856,000 tons.[51]
Regenerated cellulose can be used to manufacture a wide variety of products. While the first application of regenerated cellulose was as a clothingtextile, this class of materials is also used in the production of disposable medical devices as well as fabrication ofartificial membranes.[53]
Thehydroxyl groups (−OH) of cellulose can be partially or fully reacted with variousreagents to afford derivatives with useful properties like mainly celluloseesters and celluloseethers (−OR). In principle, although not always in current industrial practice, cellulosic polymers are renewable resources.
Cellulose acetate and cellulose triacetate are film- and fiber-forming materials that find a variety of uses. Nitrocellulose was initially used as an explosive and was an early film forming material. When plasticized withcamphor, nitrocellulose givescelluloid.
A commercial thermoplastic used in coatings, inks, binders, and controlled-release drug tablets,[56] also employed in the production of oleogels and bioplastics[57]
Often used as itssodiumsalt, sodium carboxymethyl cellulose (NaCMC)
E466
The sodium carboxymethyl cellulose can becross-linked to give thecroscarmellose sodium (E468) for use as adisintegrant in pharmaceutical formulations. Furthermore, by the covalent attachment of thiol groups to cellulose ethers such as sodium carboxymethyl cellulose, ethyl cellulose or hydroxyethyl cellulosemucoadhesive and permeation enhancing properties can be introduced.[62][63][64] Thiolated cellulose derivatives (seethiomers) exhibit also high binding properties for metal ions.[65][66]
Fibres: Cellulose is the main ingredient oftextiles.Cotton and synthetics (nylons) each have about 40% market by volume. Otherplant fibres (jute, sisal, hemp) represent about 20% of the market.Rayon,cellophane and other "regeneratedcellulose fibres" are a small portion (5%).[citation needed]
Consumables:Microcrystalline cellulose (E460i) and powdered cellulose (E460ii) are used as inactivefillers in drug tablets[68] and a wide range of soluble cellulose derivatives, E numbers E461 to E469, are used as emulsifiers, thickeners and stabilizers in processed foods. Cellulose powder is, for example, used in processed cheese to prevent caking inside the package. Cellulose occurs naturally in some foods and is an additive in manufactured foods, contributing an indigestible component used for texture and bulk, potentially aiding indefecation.[69]
Building material: Hydroxyl bonding of cellulose in water produces a sprayable, moldable material as an alternative to the use of plastics and resins. The recyclable material can be made water- and fire-resistant. It provides sufficient strength for use as a building material.[70]Cellulose insulation made from recycled paper is becoming popular as an environmentally preferable material forbuilding insulation. It can be treated withboric acid as afire retardant.[citation needed]
The majorcombustible component of non-foodenergy crops is cellulose, withlignin second. Non-food energy crops produce more usable energy than edible energy crops (which have a largestarch component), but still compete with food crops for agricultural land and water resources.[72] Typical non-food energy crops includeindustrial hemp,switchgrass,Miscanthus,Salix (willow), andPopulus (poplar) species. A strain ofClostridium bacteria found in zebra dung, can convert nearly any form of cellulose intobutanol fuel.[73][74][75][76]
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